Dr Isabelle Gautier-Luneau

Michael Holzinger

Rajaa

Lucy

Matthew

Deborah

Youssef

In order to complete my thesis, I have luckily received generous evaluation, advice and support from many people. I

CHAPTER 1 GENERAL INTRODUCTION

This thesis focuses on the use of TiO2 nanoparticles (NPs) modified with a [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridine) photosensitizer (PS) and a functional complex / organic entity for applications in photocatalysis and photocurrent generation. TiO2 is a semiconductor which has been extensively investigated in photocatalysis and photodegradation of organic pollutants in water. Here we propose to use TiO2 NPs (i) as a platform to immobilize different entities proximally, and (ii) as an electron relay between immobilized redox PS and an electron acceptor (A) to obtain a PS/TiO2/A hybrid triad. In this thesis, [Ru(bpy)3] 2+ is employed as the visible PS and a metal complex is used as the electron acceptor. [Ru(bpy)3] 2+ is one of the most investigated redox PS, notably because of its good absorption in the visible domain, long-lived excited state and reversible redox properties in both oxidation and reduction. In this chapter we will first highlight some important properties of TiO2 semiconductor and [Ru(bpy)3] 2+ PS that are relevant to our studies. Afterwards, some examples of hybrid systems containing TiO2, [Ru(bpy)3] 2+ and a functional entity for photocatalytic applications will be presented.

I.1. Semiconducting TiO2 nanoparticles

TiO2 is probably the most studied semiconductor, particularly in the field of photocatalysis. One of the earliest works on TiO2 as a photocatalyst was dated back in 1921 by Renz 1 . The author observed a color change of TiO2 powder from white to a darker color when TiO2 was irradiated with sunlight in the presence of glycerol. In this section we will highlight some of the most important properties of TiO2 semiconductor, together with a few relevant works using TiO2 as a photocatalyst. We will not discuss the quantum confinement effect of TiO2 as the NPs mentioned in this section are sufficiently large. For more information on the properties and applications of TiO2 NPs, the reader is kindly referred to recent reviews by Burda 2 , Henderson 3 and Fujishima 4 .

I.1.1. Crystal phases

Three main types of TiO2 crystal phases are often mentioned in literature namely anatase, rutile and brookite. They can be described as distorted TiO6 octahedra with different symmetries or arrangements (Figure I-1). In the anatase structure, two TiO6 octahedra are connected via a common edge, whereas in rutile and brookite structures both corner and edge are shared between two TiO6 units. As a result, the three structures differ in Ti-O bond length which plays a critical role in their electronic properties. The phase transformation in the bulk TiO2 has been extensively studied by Diebold. 5 The author concluded that rutile is the only thermodynamically stable phase, while anatase and brookite are metastable and can be transformed to rutile when heated. However, the phase stability of TiO2 NPs heavily depends on the particle size. For example, in the range of 325-750 0 C, anatase is the most stable phase for particles smaller than 11 nm, brookite is the most stable phase for particles with diameter from 11 nm to 35 nm, and rutile is the most stable phase for particles larger than 35 nm. 6

I.1.2. Photophysical properties of TiO2 particles

The electronic structure of a semiconductor consists of a valence band (VB) and a conduction band (CB). They are separated by a bandgap where there are no electron states.

The bandgap thus corresponds to the energy difference between the upper edge of the VB and the lower edge of the CB. In the ground state, all the electrons are located in the VB. Upon absorption of a photon with energy higher than the bandgap, an electron can be promoted to the CB, leaving a positively charged hole in the VB. Consequently, the material becomes conductive due to the electrons in the CB. 4 When the electron and hole recombine, a photon can be emitted. The positions of the CB and VB of several semiconductors are shown in The bandgap of bulk TiO2 depends on the crystal phase. Anatase and rutile phases show a bandgap Eg = 3.2 eV and 3.0 eV, respectively. 3 The bandgap in anatase and rutile phases is also indirect and direct, respectively. The bandgap in brookite is reported in the range of 3.1-3.4 eV 8 and is indirect 2 . A direct bandgap corresponds to the promotion of an electron from the VB to the CB without changing its momentum, whereas an indirect bandgap requires a change in the electron momentum. Consequently, when the electron and hole recombine, a photon is directly emitted in the rutile phase, while in the anatase phase the electron needs to pass through an intermediate state to transfer the momentum to the crystal lattice. The intermediate state is also referred as "electron trap" or "hole trap", which is located inside the bandgap at energy levels higher or lower than the Fermi level EF = (ECB + EVB)/2, respectively. The traps are induced by crystal defects, which create extra states inside the bandgap. 2 In the anatase phase, the recombination between CB electrons and trapped holes to emit photons is conventionally referred as "type 1" emission, while the recombination between trapped electrons and VB holes is called "type 2" emission. Both type 1 and type 2 emissions depend strongly on the energy of the defect sites, meaning the way to synthesize TiO2 particles. Usually the emission wavelength maximum is observed at ~ 550 nm for anatase phase and ~ 840 nm for rutile phase. Direct observations of CB electrons, trapped electrons and trapped holes can be achieved with a variety of spectroscopic techniques. For example, infrared (IR) spectroscopy can detect the CB electrons, and electron paramagnetic resonance (EPR) spectroscopy can be used to observed trapped holes in oxygen atoms (denoted as O -) and trapped electrons in Ti atoms (denoted as Ti 3+ ). 2 Dynamics of these charge carriers has also been studied with transient absorption spectroscopy (TAS). Tamaki et al. 10 reported that upon the bandgap excitation, photo-induced holes are trapped within 100 fs at sites near the surface of anatase TiO2 NPs, while photoinduced electrons are first trapped at shallow sites near the surface before relaxing into deeper trapping sites in the bulk within ~ 500 ps. Kinetics of the charge recombination is complicated. An early work by Gratzel et al. 11 in 1987 showed that the radiative recombination occurs within 30 ns for anatase TiO2 colloid with diameter smaller than 12 nm.

Recombination rate also depends on the size of TiO2 NPs and the number of e -/h + pairs that have been generated, meaning the power of the excitation light. For example, the recombination rate was estimated at 1 × 10 11 cm 3 .s -1 for 2 nm anatase NPs and 3 × 10 7 cm 3 .s -1 for 27 nm NPs. 3 Another approach to study the charge recombination process was reported by Colbeau-Justin et al. 12 using time-resolved microwave photoconductivity method. The authors concluded that there is a competition between a fast recombination process and a fast trapping process of a part of the charge carriers in anatase. The trapping process reduces the number of holes, thus increases the lifetime of the electrons on the CB. All of these information are important for the applications of TiO2 as a photocatalyst. Generally, a fast excitation rate and a slow charge recombination rate are required so that the electrons and holes will have enough time to react with substrates in solution.

I.1.3. TiO2 as a photocatalyst

As mentioned above, the photoexcitation of TiO2 results in the free electrons and holes as well as O -radicals, which are all reactive. They offer TiO2 capability to reduce or oxidize a variety of organic and inorganic species on the surface of the NPs. We choose three types of reactions, namely water oxidation, CO2 reduction and decomposition of aqueous pollutants, where TiO2 NPs have been tested as a photocatalyst.

 Water oxidation reaction

Light-induced water splitting reaction consists of two half-reactions: oxidation of water to O2 at the photoanode and reduction of water to H2 at the photocathode. As shown in Figure I-2, it is thermodynamically more favorable for the positively charged holes on the TiO2 CB to oxidize water than for the CB electrons to reduce water. The water (or OH -) oxidation by VB holes can occur as follows:

4 h + + 2 H2O  O2 + 4 H +
The first attempt to construct a photoelectrochemical cell for water splitting was proposed in 1972 by Fujishima and Honda using a n-type TiO2 electrode as photoanode and a

Pt counter electrode. 13 Under UV light, the holes generated on the VB of TiO2 are used to oxidize water whereas the electrons on the CB are collected through an external circuit to the Pt electrode where protons are reduced to form H2. Despite the fact that the process is highly thermodynamically favorable, no publication has shown direct experimental evidence for the water oxidation reaction on TiO2. It has been proposed that the required nucleophilic attack of water to a trapped surface hole is not achieved. 3 The mechanism of the water oxidation reaction by TiO2 photocatalyst was investigated by Kavan,Gratzel et al. 9 using anatase TiO2(101) and rutile TiO2(001) single crystal deposited onto an electrode. An anodic current was observed when the electrodes were irradiated with UV light at zero bias. Cyclic voltammograms were taken in the dark for the electrodes before and after they were exposed to UV irradiation. The authors observed an irreversible cathodic peak in the CVs of both anatase and rutile electrodes after UV light exposure, which was attributed to the reduction of a surface-bound peroxide intermediate. However, no exhaustive water oxidation experiment was mentioned.

 CO2 photoreduction

Although many works on the photoreduction of CO2 have been made based on TiO2, they require a co-catalyst or thermal activation of TiO2 and not the direct photoexcitation of TiO2. 3 The only publication using TiO2 as a photocatalyst to reduce CO2 was conducted by Anpo et al. 14 The authors studied anatase TiO2 NPs and rutile NPs in the presence of CO2 + H2O gas mixture. UV irradiation of the solid TiO2 NPs at 275 K resulted in the formation of CH4 as the main product together with traces of C2H4 and C2H6 in the gas phase. These gas products were not present without the UV light. The photocatalytic activity of TiO2 was found to depend on the crystal phase and particle size of TiO2 NPs, as well as the ratio of CO2/H2O in the gas phase. Table I-1 summarizes the amount of CH4 produced and some relevant characteristic parameters of these TiO2 photocatalysts. The anatase phase was found to be more catalytically active than the rutile phase for small NPs. Increasing the particle size to several hundred nanometers lead to the decrease in the amount of CH4, probably due to the reduced surface area and, consequently, reduced number of available sites for CO2 molecules to be adsorbed.

 Decomposition of aqueous pollutants

A wide variety of aqueous pollutants can be decomposed by TiO2 particles under UV irradiation, such as alkanes, aliphatic alcohols, carboxylic acids, alkenes, aromatics, polymers, surfactants, herbicides, pesticides and dyes. 4 The decomposition of pollutants occurs via the formation of highly oxidative hydroxyl (OH  ) and hydroperoxyl (HO2  ) radicals in water following the UV irradiation of TiO2 (Scheme I-1). These radicals can oxidize organic compounds to CO2 and H2O.

Scheme I-1. Generation of OH  and HO2  radicals in water by photoinduced e -and h + on TiO2

Only UV light and O2 are required for this type of photocatalysis; the reactions can occur at room temperature. However, the quantum yield and efficiency depend on many factors such as the crystal phase and size of TiO2 photocatalyst, concentration, pH, light intensity, etc. 4 For example, when studying the quantum yields of liquid-solid photocatalytic reactions using TiO2 slurry catalysts, Serpone et al. 15 found that the maximum quantum yield was obtained with low light intensity and high substrate concentration. The maximum quantum yield of 14 % was reached at 365 nm for phenol degradation using Degussa P25 TiO2 NPs (rutile/anatase = 3/1, d = 25 nm) at pH 3.

To summarize, although TiO2 shows some photocatalytic activities, it only works under UV light and the selectivity is low. The visible domain of the solar spectrum is not efficiently utilized. To address the former issue, an additional PS capable of absorbing visible light can be anchored on TiO2 NPs to better harvest the solar light. A suitable PS is [Ru(bpy)3] 2+ , which will be described in Section I.2. Meanwhile the selective issue can be solved by incorporating a more efficient and selective catalyst onto the surface of TiO2 NPs. This TiO2/catalyst system will be mentioned in Section I.3.

I.2. Ru(II) tris-bipyridine: a prototypical photosensitizer

A PS is a molecule capable of absorbing photons, leading to an excited state which is able to interact with its surrounding media. The [Ru(bpy)3] 2+ (Figure I-4) has been one of the most studied PS in the last decades due to its excellent properties such as chemical stability, redox reversibility, long-lived excited state and strong luminescence. In this section we will discuss in detail about these properties, which are fundamental for further studies in this thesis.

I.2.1. Electrochemical properties

The redox properties of [Ru(bpy)3] 2+ complex was first studied with cyclic voltammetry by Matsumoto et al. 16 in MeCN solution containing tetraethylammonium perchlorate (TEAP)

as electrolyte (Figure I-5). In the anodic part, a reversible one-electron oxidation wave is observed at E1/2 = 1.25 V vs NHE which is attributed to the metal oxidation to Ru III species with a low spin 4d 5 configuration. 17 In the cathodic part, three successive one-electron reduction processes are observed, with the first one centered at around -1.40 V vs NHE. All the added electrons are localized on the bpy ligands. 17 No ligand dissociation has been reported. Therefore, the [Ru(bpy)3] 2+ PS exhibits rich and reversible redox properties. To characterize the excited state, it is important to introduce two parameters namely lifetime  and emission quantum yield . They are defined by the following equations: 18 The Dexter mechanism, also known as the exchange mechanism, involves a double electron exchange between the donor and the acceptor molecules. An electron moves from the LUMO of the donor to the LUMO of the acceptor, accompanied by a simultaneous electron transfer from the HOMO of the acceptor to the HOMO of the donor. This electron exchange requires a strong overlap between the orbitals of the donor and the acceptor, hence it strongly depends on the nature of the linker connecting the two entities. The rate constant of the Dexter energy transfer process kDexter depends on the distance r between the donor and the acceptor as where K is related to specific orbital interactions, J is the normalized spectral overlap integral between the emission spectrum of the donor and the absorption spectrum of the acceptor, and  is the attenuation factor which is specific for each linker.

The Forster mechanism, also called Coulombic mechanism, is a long range mechanism, which means that there is no requirement for a physical contact between the donor and the acceptor. In this context, the electronic nature of the linker between the donor and the acceptor, if present, does not affect the energy transfer rate. The rate constant of the Forster energy transfer process kForster is expressed by Equation I-4:

22 6 D Forster D J k r    (Eq I-4)
where D and D are the emission quantum yield and luminescence lifetime of the donor, r is the distance between the donor and the acceptor.

I.2.3.2. Photo-induced electron transfer mechanisms

Electron transfer reactions involve an exchange of electron(s) between two molecules in which one is oxidized while the other is simultaneously reduced. Photo-induced electron transfer reactions are only possible when the redox potentials of the two molecules are well aligned so that the overall process is thermodynamically favorable (G < 0). The process is governed by the redox potential of the excited state of the PS and the oxidation / reduction potential of the electron donor / acceptor, respectively. The redox potential of the PS excited state is estimated by the Rehm-Weller equation: 24 where E * 1/2 is the reduction or oxidation potential of the excited state, E1/2 is the reduction or oxidation potential of the ground state and E00 is the difference in energy between the zeroth vibrational states of the ground and excited states. The values of E1/2 can be experimentally determined from a cyclic voltammetry experiment. The zero-zero excitation energy E00 is usually approximated by the emission energy Eem of the compound at 77 K (Figure I-9). As an approximation, E00 value can be estimated from the emission energy at room temperature in the case of [Ru(bpy) where h is the Planck's constant, c is the speed of light in vacuum, e is the charge of an electron, em,77 K and em,RT are the emission wavelength maximum at 77 K and room temperature, respectively. In this equation, G represents the free enthalpy of the reaction, Eox(D) and Ered(A) are the oxidation potential of the donor and the reduction potential of the acceptor respectively, s is the solvent dielectric constant, D r  and A r  are the ionic radii and RC is the center-to-center separation distance between the donor and the acceptor. E00 is the singlet or triplet state energy determined by the emission maximum at 77 K. The third term represents the Coulombic interaction between the two charged moieties. The last term corrects the difference in the ion-pair solvation in which the redox potentials were measured. This expression can be also written in a more simplified form as follows: Grafting a metal complex onto a surface can be achieved by modifying the complex ligands with suitable anchoring groups, which depend on the nature of the surface. Table I-2 summarizes some common anchoring groups reported in literature for several surfaces such as TiO2, Au, SiO2, ZrO2 and ITO. The surfaces can be either a macroelectrode or NPs. In this thesis, we focus on using the TiO2 NPs as a platform to immobilize different species (PS, D, A). Although some early works about grafting [Ru(bpy)3] 2+ PS on TiO2 NPs employed carboxylic acid, 34 in recent years the phosphonic acid has emerged as a more suitable anchoring group. Since the pioneering work by Gao et al. 26 in 1996, numerous publications have been made using phosphonic acid to graft different species onto TiO2

NPs. 30 Herein, we summarize some critical details for the grafting of a species onto TiO2 NPs using the phosphonic acid groups.

On a computational point of view, the binding mode between -P(O)(OH)2 and TiO2

surface is still on debate. Using phosphonic acid as a model to study the grafting on anatase and rutile TiO2, Luschtinetz et al. 35 concluded that the bidentate mode was more stable than the tridentate one. However, Nilsing et al. 36 claimed that the most stable mode of binding was the monodentate when they used another computational method. It is also experimentally challenging to distinguish between the three binding modes.

On an experimental point of view, a recent publication by Hirsch et al. 27 compares the adsorption process of different derivatives of phosphonic acid or carboxylic acid on TiO2

anatase NPs (d ~ 34 nm). Thermogravimetric analysis (TGA) was employed to quantitatively determine adsorption parameters. As shown in Table I-3, the grafting density is slightly lower when phosphonic acid is used to graft the species on TiO2 instead of carboxylic acid. However, the phosphonic acid groups exhibit significantly stronger bonds with the surface as indicated by the adsorption constant Kads and free energy G. Negligible differences are found when a phenyl or a hexyl chain is grafted on TiO2 NPs. This work provides experimental evidence for the advantages of using phosphonic acid as anchoring group compared with carboxylic acid.

I.2.5. Photo-induced charge transfer processes on TiO2 NPs sensitized with [Ru(bpy)3] 2+

In this section we will highlight some relevant studies on the kinetics of photo-induced charge transfer processes on TiO2 NPs modified with [Ru(bpy)3] 2+ PS (denoted as TiO2/Ru II ).

The TiO2/Ru II NPs can be either dispersed in colloidal solutions or deposited on a nanocrystalline TiO2 thin film electrode. The carboxylic or phosphonic acid is used as anchoring group. For a comprehensive overview in this topic, the reader is kindly referred to several excellent reviews for studies in colloidal solutions 37,38 and on mesoporous thin film electrodes 2,38

I.2.5.1. In colloidal solutions

Scheme I-2. Photosensitizers described in Section I.2.5.1.

One of the pioneering works on the kinetics of photo-induced charge transfer processes in TiO2/Ru II colloidal solution was published by Desilvestro et al. 34 The authors grafted

[Ru(dcb)3] 2+ (dcb=2,2'-bipyridine-4,4'-dicarboxylic, Scheme I-2a) on anatase TiO2 NPs (d = 6 nm) and studied the charge transfer in aqueous solution at pH 2 by means of time-resolved emission and transient absorption spectroscopies (TAS). Following an excitation at 530 nm, TiO2/Ru II* exhibited a rate of electron injection to TiO2 of 3.2 × 10 7 s -1 . The acidic media of 2.5 < pH < 6 allowed for the existence of the complex. In comparison, a mixture of [Ru(bpy)3] 2+ (Scheme I-2b) and TiO2 NPs did not show any quenching of the excited state Ru II* in acidic solution. At pH 10 [Ru(bpy)3] 2+ could be physisorbed to TiO2 due to electrostatic interaction between the cation dye molecules and negatively charged surface.

The electron injection rate was significantly slower, at only 1.5 × 10 5 s -1 . The recombination rate of (e -)TiO2 and Ru 3+ was similarly slow in both cases, around 4 × 10 5 s -1 . The work emphasized the importance of covalently linking the dye to TiO2 semiconductor to enhance the electron injection to TiO2. with kb ~ 10 5 -10 6 s -1 . In the presence of phenothiazine (PTZ) as an electron donor, the oxidized dye was efficiently reduced at a rate of kD = 3 × 10 8 s -1 before the back electron transfer with (e -)TiO2. The charge separated state (e -)TiO2 and PTZ + remained stable in a millisecond lifetime.

The nature of photo-induced, trapped electrons on anatase TiO2 sites (Ti 3+ ) has been investigated by Rittmann-Frank et al. 40

I.2.5.2. On TiO2 mesoporous thin film electrodes

Scheme I-3. Photosensitizers described in Section I.2.5.2.

The breakthrough in mesoporous, nanocrystalline TiO2 thin film electrodes sensitized with a metal complex dye is the report by O'Regan and Gratzel in 1991, 41 where they deposited a monolayer of [Ru(dcb)2(µ-(CN)Ru(CN)(bpy)2)2] 6+ complex (Scheme I-3a) on 10µm thick layer of TiO2 NPs. The modified electrode combined the high area surface of the film and excellent UV-vis absorption properties of the dye to yield an incident photon-tocurrent efficiency (IPCE) of > 80 % and light-to-electric energy conversion of > 7 %, together with great stability. The rate of charge injection was estimated to exceed 10 12 s -1 in this photovoltaic cell, while that of complex decomposition was smaller than 2 × 10 4 s -1 .

Since the O'Regan and Gratzel's paper, there have been numerous extensive studies on the photo-induced kinetics of dye-sensitized solar cells (DSSC). Wang et al. 42 constructed a series of conducting, rigid-rod linkers to immobilize [Ru(bpy)3] 2+ on TiO2 NPs thin films to study the dependence of charge transfer processes on the distance between Ru 2+ sites and the semiconductor. The interfacial electron transfer was strongly pH-dependent: kinj > 10 8 s -1 (pH 1, unable to be time-resolved) and kinj = 1×10 7 s -1 (pH 11). The injection quantum yield was found to decrease as the distance increased. Back electron transfer was, on the contrary, independent of the linker length, remaining at ~ 5×10 5 s -1 . The size of TiO2 NPs has been shown to be critical for the kinetics of electron injection. 47 Using N3 dye anchored on a 15-µm thick layer of TiO2 NPs with various diameters, the authors studied the photo-induced charge separation and recombination processes by means of time-resolved emission spectroscopy and femtosecond TAS. The injection rates lied on ps-ns time scale for all the sizes, but the injection efficiency significantly decreased from ~ 90 % (diameter < 50 nm) to 30-70 % (diameter > 50 nm). The drop was attributed to the dye aggregation in the space between the large particles. This work highlights the importance of using small TiO2 NPs (d < 50 nm) to achieve good injection efficiency.

Gillaizeau

To summarize, the photo-induced charge transfer processes on TiO2/Ru II dyads have been well studied in colloidal solution and on mesoporous thin film electrodes. In recent years the phosphonic acid to anchor Ru dyes on TiO2 NPs has gained more attention due to its higher stability than carboxylic acid. However, to the best of our knowledge, there is no publication on the photo-induced charge transfer process in solution when the phosphonic acid is used. We are interested in this topic as it is fundamental for further charge transfer studies in this thesis when another component like an electron donor or acceptor is also immobilized on TiO2/Ru II NPs.

I.3. Incorporation of a catalyst on TiO2/Ru II nanoparticles

As mentioned at the end of Section I.1, the photocatalytic reactions induced by TiO2 suffer from low yields and low selectivity. Therefore, a catalyst can be anchored on TiO2 NPs for obtaining desired products. For example, a hybrid system comprising metal oxide NPs functionalized with the enzyme carbon monoxide dehydrogenase (CODH) and [Ru(bpy)3] 2+

visible PS was synthesized for CO2 photoreduction (Scheme I-5). 48 Under irradiation at  > 420 nm, the Ru II /TiO2/CODH hybrid system produced CO as the only product. The ability to store multiple reduction equivalents in the [Cr(ttpy)2] 3+ complex in homogeneous solution and on the triad will be described.

Chapter 4 will show the electrocatalytic and photocatalytic activity for CO2 reduction using [Mn(ttpy)(CO)3Br] catalyst in homogeneous solution. The Mn catalyst and [Ru(bpy)3] 2+

PS are then both anchored on TiO2 NPs to form the Ru II /TiO2/Mn I triad. The triad is tested in the photocatalytic CO2 reduction under visible irradiation, in the presence of a sacrificial electron donor. Some mechanistic studies will also be described.

Finally, in Chapter 5 we will present the use of the oxidative equivalents stored on the Ru III sites of the photoinduced charge separated state (e -)TiO2/Ru III for the polymerization of pyrrole units. The synthesis of a hybrid system consisting of a [Ru(bpy)3] 2+ PS bearing two pyrrole moieties immobilized on TiO2 via phosphonic acid groups will be discussed. excité du [Ru(bpy)3] 2+* à la bande de conduction de TiO2 se produit dans l'échelle de temps de la nanoseconde pour former l'état à charge séparée (e -)TiO2/Ru III , alors que la recombinaison de charge se produit à l'échelle de la milliseconde. L'identification de l'électron sur les sites Ti 3+ de surface de la matrice TiO2 a été faite par spectroscopie RPE. La longue durée de vie de l'état à charge séparée (e -)TiO2/Ru III offre la possibilité d'engager l'électron ou le "trou" dans des réactions redox prédéfinies, qui seront discutées dans les chapitres suivants.

II.1. Introduction

Despite the various works on the photo-induced charge transfer processes of nanocrystalline TiO2/Ru II electrodes and TiO2/Ru II colloidal solution mentioned in Section I.2.5, to the best of our knowledge there are no investigations of the kinetics of a [Ru(bpy)3] 2+ dye grafted on TiO2 NPs via a phosphonic acid group in solution. We are interested in the colloids as they resemble the media in photocatalytic studies. Herein, we will present the kinetics of photo-induced charge transfer processes of a phosphonic-derivatized [Ru(bpy)3] 2+ dye immobilized on TiO2 NPs. The phosphonic anchoring group is chosen due to its good affinity with metal oxide surfaces and better stability than carboxylic group as described in Chapter 1. The asymmetric complex bearing only one phosphonic acid group prevents the possible binding to two adjacent nanoparticles, thus it may be potentially helpful for the charge injection to TiO2 semiconductor and simplifies the kinetics studies. It has also been demonstrated that additional phosphonic groups decrease the rate of photo-induced electron injection to TiO2 as mentioned in Chapter 1. 3 A long alkyl chain separating the [Ru(bpy)3] 2+ photosensitizer and TiO2 substrate may reduce both electron injection and back electron transfer rates between the PS and TiO2. 4 It is however noted that the Ru-complex with such additional methylene groups may be less electrochemically stable than that bearing a 4,4'-(PO3H2)2-bpy ligand under an applied potential of 1.5 V vs Ag/AgCl for 16 hours. 3 In order to study the effect of the phosphonic functional group on the electrochemical The extinction coefficient for the MLCT transition exhibits a minor variation:  = 13000 M -1 .cm -1 for [Ru(bpy)3] 2+,5 and 14100 M -1 .cm -1 for [Ru-PO3Et2] 2+ .

II.2. Phosphonate-derivatized

After photoexcitation at 450 nm, the complexes exhibit a strong emission, with maxima at 608 nm for [Ru(bpy)3] 2+ and about 615 nm for the other two complexes. The maximum of emission of [Ru(bpy)3] 2+ is in accordance with literature. 6 The shape of the emission spectrum is broad and identical for all the complexes. The result indicates that the phosphonate group has negligible effect on the emission of this photosensitizer. The substituted alkyl chain is an electron donating group to the bpy ligand. A similar red shift in the emission spectrum has been reported for the methylene group attached to [Ru(bpy)3] 2+ complex. 3 Maestri et al. 7 explained the shift as follows: after the photo-induced MLCT process, the HOMO (t2g) metal orbital is destabilized causing the HOMO-LUMO gap to reduce. Therefore the emission maximum is shifted to longer wavelengths. II-1).

b) Time-resolved emission spectroscopy

The time-resolved emission of Ru This lifetime is in accordance with the one recorded by time-resolved emission spectroscopy.

II.2.3. Electrochemical properties

The and [Ru-PO3Et2] 2+ complexes exhibit in the anodic part a one-electron reversible oxidation wave centered on the metal at E1/2 = 0.91 V. In the cathodic part, they show three successive one-electron reduction processes centered on the bpy ligands at E1/2 = -1.66, -1.86 and -2.12 V. The reduction waves are reversible, with the first one producing [Ru II (bpy -)(bpy)2] + denoted hereafter as Ru I .

In the anodic scan, the Ru II complexes bearing a dimethyl bipyridine group show a shift to lower oxidation potentials compared with the [Ru(bpy)3] 2+ reference in accordance with the complex bearing an electron-donating substituted group on bpy ligands. 3 In the cathodic scan, the third reduction potential of the functionalized complexes is significantly shifted to a more negative potential (-2.12 V compared with -2.07 V) whereas the first two reduction potentials are comparable with that of [Ru(bpy)3] 2+ . This change could be explained by the charge localization on the bpy ligand, while the metal oxidation state remains unchanged. 5 Since the dimethyl is an electron donating group, the first and second reduction processes likely occur on the un-substituted bpy ligands, and the last reduction is on the bpy bearing the dimethyl group. To study the TiO2/Ru II NPs, we first prepared the analogous SiO2/Ru II as a reference. In this system there are no charge injections to the SiO2 due to an extremely negative potential of the conduction band (CB) of SiO2. The SiO2 NPs was synthesized from tetraethyl orthosilicate (TEOS) precursor following the Stober's sol-gel procedure 11 with some modifications. The procedure is schematically presented in Scheme II-3. The sonication was employed instead of mechanical stirring in order to obtain a higher dispersity. 12 The rate of addition of NH3 catalyst has been proved to determine the NPs size. 12 With NH3 dropwise addition, the NPs possess a nearly spherical shape, as shown in the Transmission Electron The synthesis of TiO2 NPs also followed a sol-gel procedure in an acidic condition with titanium(IV) isopropoxide (TTIP) as precursor. 13 The synthesis route is schematically demonstrated in Scheme II-4. TiO2 exhibits three main crystalline phases: anatase, rutile and brookite. The synthesis was carried out with a high water-to-titanium ratio, aiming at obtaining uniform, sub-20 nm anatase TiO2 particles. 13 The slow addition of HCl catalyst has been shown to be critical for obtaining nanometer-sized particles. The anatase phase of TiO2 was chosen in this study for (i) being a better photocatalyst than rutile, 14 and (ii) having less types of surface charge traps than P25 Degussa TiO2 NPs, facilitating the study of charge transfer mechanisms on/through the semiconducting particles. It should however be noted that the P25 TiO2 comprising ~75 % anatase and 25 % rutile has been demonstrated as the best TiO2 photocatalyst. 15 The anatase TiO2 was readily formed by annealing the as-synthesized powder at 450 0 C for 2 hours. For a comparative study, we also prepared the anatase/rutile mixed phase TiO2 by annealing at 700 0 C. It has been reported that the anatase to rutile phase transformation occurs at around 700 -800 0 C 16 because the rutile is more thermodynamically stable than anatase. 17 37.9 0 , 48.1 0 , 54.0 0 and 55.0 0 which could be ascribed to the (101), ( 103)+(004)+( 112), ( 200), ( 105) and (211) lattice planes of the anatase phase, respectively. The sample annealed at 700 0 C exhibits, beside the mentioned peaks of anatase, additional peaks at 2 = 27.4 0 , 36.1 0 , 41.2 0 , 54.3 0 and 56.6 0 (marked with asterisks). These peaks could be assigned to the (110), ( 101), ( 111), ( 211) and ( 220) lattice planes of the rutile phase, respectively. Integrating the area under the anatase and rutile characteristic peaks shows the ratio of anatase/rutile = 8/2. In both samples, the characteristic peak of brookite (30.8 0 ) is not observed.

The crystallite size of the NPs can be estimated from their XRD patterns using the Scherrer equation:

17 0 0.9 ( ) cos d       (Eq II-2)
where  is the X-ray wavelength (1.5406 Åfor CuK), 0 is the instrumental line broadening (0 = 0.12 rad),  (rad) is the line broadening at half of the maximum intensity (FWHM) and  (degree) is the Bragg angle. The XRD peaks become narrower when the size is larger. The crystallite size is, therefore, estimated as 10 nm for anatase TiO2 NPs. For anatase/rutile TiO2

NPs, the sizes of anatase and rutile domains are 91 nm and 204 nm, respectively. 3 and -14.4 (Hz) for SiO2 and TiO2 sensor, respectively. Using the Equation II-3, one can calculate the increased mass of 23 ng.cm -2 (SiO2) and 255 ng.cm -2 (TiO2). The surface coverage is then calculated as  = 2.3 × 10 -11 mol.cm -2 and 2.5 × 10 -10 mol.cm -2 for SiO2 and TiO2, respectively, provided that the mass of coupled solvent is neglected. They are equal to 0.1 molecule.nm -2 for SiO2 and 1.5 molecules.nm -2 for TiO2. The contact angle of the modified surfaces with water is found to be quite identical: (24.4 ± 1.6) 0 for SiO2/Ru II and (22.4 ± 1.7) 0 for TiO2/Ru II , implying a similar outer layer. It also suggests a similar degree of solvation for both films. Given the initial surfaces having a contact angle of ~10 0 (SiO2) and ~4 0 (TiO2), the modified ones become clearly more hydrophobic, due to the bpy ligands. The experiments prove that the dye adsorption onto SiO2 and TiO2 surfaces could be achieved under ambient conditions within several hours. The TiO2 surface coverage is an order of magnitude higher than SiO2 as the mass of coupled solvent is expected to be the same for both films. It could be explained by the higher stability of the Ti-O-P bond than Si-O-P one. 19

II.3.3. Dye sensitization with [RuP] 2+ on colloidal nanoparticles

The [RuP] 2+ -dye sensitization of TiO2 and SiO2 NPs was achieved by mixing the particles and the complex in ethanol/acetone (8/2 v/v) solution at room temperature. The NPs were then centrifuged and washed with the solution multiple times until the ungrafted complex in the supernatant solution cannot be detected anymore with UV-vis spectroscopy.

By measuring the UV-vis absorbance of the solution after each centrifugation, we estimate the Ru II loading as 0.19 mmol.g -1 TiO2 (both anatase and anatase/rutile) and 0.02 mmol.g -1 SiO2

(< 10 nm) NPs. It is in good agreement with the QCM-D experiment on the flat surfaces where the surface coverage on TiO2 is about ten times higher than SiO2.

The main advantage of the heterogenization of the photosensitizer complex on NPs is their high surface area and high dispersibility in a variety of solvents. However, both the intact and sensitized NPs with diameter less than 10 nm show rapid precipitation in many solvents such as MeCN, ethanol, acetone, DMF and water. Large aggregates of bare NPs were observed with SEM and TEM. The precipitation prohibits the formation of a stable colloid for further studies.

As a comparative study, we then employed commercially available nanopowders of SiO2 (d < 25 nm) and anatase TiO2 (d < 20 nm) produced by Aldrich, which show better dispersibility in MeCN and water. After the dye sensitization with [RuP] 2+ , the particles still retain their dispersibility, forming a colloid stable for several days. The loading of Ru II on these NPs is estimated as 0.21 mmol.g -1 TiO2 and 0.076 mmol.g -1 SiO2. Thanks to the great dispersibility of these NPs, they were chosen for further studies in this thesis. [START_REF]mmol.g -1 (about 3800 Cr III molecules per particle or 2 Cr III molecules per nm 2 ). C6H5-CH2-PO3H2/TiO2/[Cr2P] 3+ (C6H5-CH2-PO3H2 = benzylphosphonic acid, BPA). Similar to the case of TiO2/[Cr2P] 3+ , but a mixture of [Cr2P] 3+ and BPA was used instead of References[END_REF] The spectral changes are characteristic of the grafting of [RuP] 2+ on TiO2 surface.

In the case of SiO2/Ru II , a strong, broad Si-O stretching band at about 1100 cm -1 precludes the detection of the phosphonate surface linkage, which is expected at ~1150 cm -1 . 21

However the spectrum of SiO2/Ru II displays a broader signal in this region compared to that of naked SiO2 NPs. The P=O stretch at 1250 cm -1 partially remains on SiO2/Ru II . The grafting of [RuP] 2+ is then expected to be more stable on TiO2 than on SiO2. Samples were measured in KBr pellets at room conditions.

II.3.5. Electrochemical properties a) [RuP] 2+ grafted on FTO

The electronic communication between grafted [RuP] 2+ and a metal oxide electrode was studied using a Fluorine-doped Tin Oxide (FTO) coated glass electrode. FTO was chosen because (i) the phosphonic group is suitable to be grafted on the surface, 22 and (ii) it is rather redox inactive in the range of Ru A quasi-reversible reduction peak at -1.25 V associated by an oxidation on the reversed scan at -0.65 V was detected. This system is attributed to the reduction of phosphonate shows that the charge injection to the CB occurs at around -0.6 V vs SCE (-0.9 V vs Ag/AgNO3 0.01 M). 25 In the CV of TiO2/Ru II NPs, the peak due to the reduction of TiO2 NPs is so intense that the detection of a small quantity of [Ru(bpy)3] + centered at -1.6 V, if any, is unfeasible.

a) UV-vis absorption spectroscopy and emission spectroscopy

The UV-vis absorption of SiO2/Ru II and TiO2/Ru II NPs was investigated in solid state using an integration sphere. The powders were dispersed in a KBr pellet as a transparent substrate in the UV-vis region. Both SiO2/Ru II and TiO2/Ru II NPs show a similar broad emission spectrum after being excited at 450 nm. While the emission peak of the former (612 nm) is comparable to the [Ru-PO3Et2] 2+ complex (615 nm), that of the latter is red-shifted to 627 nm. Since the Ru II loading on TiO2 is 3 times higher than on SiO2 NPs, the redshift could be a consequence of a selfquenching process between adjacent Ru II species on the same NP. Using Equation II-1, the oxidation potential of grafted Ru II* excited state is calculated as -1.11 V for SiO2/Ru II and -1.00 V for TiO2/Ru II . free complex in solution, which may be a consequence of an additional energy transfer deactivation pathway between adjacent Ru II centers on the NPs. The time-resolved luminescence decay shows that the energy quenching is more pronounced for TiO2/Ru II than SiO2/Ru II . It is probably due to the higher loading of Ru II on TiO2 than on SiO2: about 2 molecules/nm 2 TiO2 surface area compared with 0.5 molecules/nm 2 SiO2 surface area.

Comparing the lifetimes of TiO2/Ru II* state with or without BPA spacers, one can observe a significant drop in the lifetime value (32 to 8 ns) associated with an increase in the fractional amplitude. It could be explained by the separation of grafted Ru II species favoring the electron injection to TiO2 and diminishing the energy transfer pathway. than on TiO2 (0.21 mmol.g -1 ). The charge injection rates of TiO2/Ru II NPs to TiO2 are significantly lower than that reported for a phosphonic-derivatized Ru-dye immobilized on a nanocrystalline TiO2 electrode, 27 but they are in accordance with previous works in colloidal solutions. 28,29 The rates of energy transfer are comparable to a Ru-carboxylic dye grafted on a TiO2 thin film electrode. 30 In the presence of a lateral spacer, the electron injection is significantly enhanced in terms of kinetics and percentage. where D is the quantum yield of the donor, n is the refractive index of the solvent,  is the orientation factor (usually equal to 2/3 for a chromophore free to rotate in solution) and J (M -1 .cm 3 ) is the overlap integral between the absorbance and emission spectra. The overlap integral J is thus determined to be 4.4 × 10 -15 M -1 .cm 3 and the Forster distance Ro is 1.9 nm. The FRET efficiency and Ru-Ru distance on SiO2 and TiO2 are summarized in Table II-6. Excitation light was chosen at 532 nm and a UV filter was placed in front of the lamp to avoid direct bandgap excitation of TiO2 NPs.

We first investigated the TA spectrum of [RuP] 2+ complex grafted on SiO2 NPs and compare it with the spectrum of [Ru-PO3Et2] 2+ free complex which has been presented in

                (Eq II-11)
This equation is conveniently fit with a power function.

KWW cr KWW KWW KWW KWW KWW k k                                   (Eq II-12)
The recombination kinetics is estimated as (140 ± 30) s -1 . This result suggests the recombination kinetics 5 orders of magnitude slower than the kinetics of electron injection to TiO2. It also shows that a longer linker between the [Ru(bpy)3] 2+ PS and phosphonic goup reduces the charge recombination rate more significantly than shorter linkers: for example without methylene groups the [Ru(dtb)2(dcb)] 2+ grafted on TiO2 thin film exhibits recombination kinetics of ~1 × 10 3 s -1 . 32 Such a very slow recombination process allows efficient charge injection and utilization of the photo-generated CB electrons and oxidized "hole" dye molecules on TiO2 surface in successive redox reactions.

II.3.7. Electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy has been employed to identify the sites of photo-induced trapped electrons on TiO2 NPs as they are paramagnetic. Upon photon absorption at 455 nm by the [Ru(bpy)3] 2+ dye, an electron is injected to the CB of TiO2 and then trapped in Ti 3+ sites, which will be called here "electron traps". The charge separated state (e -)TiO2/Ru 3+ was then created and directly observed with the EPR spectroscopy at 20 K.

We first examined different irradiation methods:

-Ex situ irradiating an EPR tube containing TiO2/Ru II colloid in water or MeCN by a 455 nm LED for 2-3 hours, then freezing the tube with liquid nitrogen -In situ irradiating an EPR tube containing the powder or colloid of interest by immersing an optical fiber inside

Our experiments show that the ex situ irradiation method does not produce enough paramagnetic species to be observed. It is probably due to charge recombination between (e -)TiO2 and Ru 3+ sites occurring when the tube is transferred from the dewar to the EPR cavity. In situ irradiation with a fiber immersed inside the tube containing TiO2/Ru II in powder form or in colloid does not improve the signal. Comparing the powder form and colloid, we found that the colloid gives better signal, possibly due to its better ability to diffuse light throughout the sample. Therefore, a colloid in MeCN and in situ irradiation from the wall of the EPR cavity have been chosen for further studies. MeCN solvent was used in order for the EPR experiment to be consistent with previously described characterizations.

Details of the experimental setup can be found in the Experimental Section. sites, 34,35 while the electrons on the CB of TiO2 are EPR silent. 36 There are also no trapped holes (usually denoted as O -centers in literature) 35 detected in this study, proving that the electron transfer from Ru 2+* to TiO2 generates the signal instead of electron promotion from the VB to the CB of TiO2. The signal of Ru 3+ is, however, not detectable. This can be explained by the fact that its EPR spectrum is around 70 times broader than the spectrum of Ti 3+ (~3500 G for Ru 3+ compared with ~50 G for Ti 3+ ), so the EPR signal corresponding to Ru 3+ should appear roughly 70 × 70 = 4900 times smaller than the EPR signal corresponding to Ti 3+ . Therefore, the Ru 3+ signal is not detectable. The signal reproducibility upon changing temperature proves that the electron transfer process is reversible.

In radicals. 37 In our process it may come from the photocatalytic degradation of ethanol 38,39 as it may not be completely removed by heating in an oven at 80 0 C.

II.4. Conclusion

In The results indicate a nanosecond injection rate from Ru II* to TiO2 with ~ 90 % yield, followed by a millisecond charge recombination rate, thus proving an efficient, long-lived charge separated state. The addition of lateral spacers to separate neighboring Ru II species shows a marked increase in the kinetics and the yield of electron injection to TiO2, which is attributed to the reduced energy transfer between the photosensitizers. The FRET study also reveals an average distance of 2.5 nm (SiO2/Ru II ) and 1.8 nm (TiO2/Ru II ) between neighboring Ru II species on the surface. Finally, the photo-induced charge separated state (e -)TiO2/Ru 3+ has been directly observed in situ by EPR spectroscopy. The mechanistic studies in this Chapter serves as a fundamental basis for the following chapters where additional redox active components are anchored on the TiO2 NPs.

CHAPTER III CHROMIUM(III) BIS-TOLYLTERPYRIDINE COMPLEXES AS ELECTRON ACCEPTORS

Abstract

In this chapter, the rich electrochemical and photophysical properties of [Cr(ttpy)2] 3+ (ttpy = 4'-(p-tolyl)-2,2':6',2''-terpyridine) complex will be discussed. Different redox states of [Cr(ttpy)2] n+ (n = 3, 2, 1, 0) complex have been studied with UV-vis absorption spectroscopy and Electron Paramagnetic Resonance (EPR) spectroscopy. DFT calculations show that all the reduction steps occur on the ttpy ligands. The [Cr(ttpy)2] 3+ complex can be doubly reduced to [Cr(ttpy)2] + by [Ru(bpy)3] 2+ photosensitizer under visible irradiation in presence of triethanolamine (TEOA), showing its ability to store two reduction equivalents on the Cr complex. This multi-electron storage ability can also be achieved with a mixture of TiO2/Ru II nanoparticles (NPs), [Cr(ttpy)2] 3+ and TEOA in solution. Furthermore, the doubly reduced [Cr(ttpy)2] + complex shows some activity as a catalyst for the proton reduction reaction in a non-aqueous solution. Afterwards, the [Cr(ttpy)2] 3+ alone or in association with the [Ru(bpy)3] 2+ complexes are grafted on TiO2 NPs via phosphonic acid as anchoring group to form TiO2/[Cr2P] 3+ dyad and Ru II /TiO2/[Cr2P] 3+ triad structures. In contrast to the free [Cr(ttpy)2] 3+ complex in solution, the Ru II /TiO2/[Cr2P] 3+ triad does not show the ability to store electrons on the [Cr2P] 3+ units under continuous irradiation in the presence of TEOA. It may be due to accelerated kinetics of back electron transfer between grafted transient [Cr2P] 2+ and Ru 3+ species in close proximity at the surface of the NPs.

Résumé

III.1. Introduction

III.1.1. Chromium(III) polypyridyl complexes

Among the polypyridyl complexes of first-row transition metals, Cr(III) complexes still remain underexplored in comparison with notably Co(III) and Fe(II) complexes. 1 Herein, the photophysics andelectrochemistry of [Cr(tpy)2] 3+ (tpy = 2,2':6',2''-terpyridine) and[Cr(bpy)3] 3+ (bpy = 2,2'-bipyridine) complexes will be briefly summarized to emphasize its rich properties. For more detail the reader is kindly referred to recent reviews by Kane-Maguire 2 and Ford 3 .

The hexacoordinated Cr(III) complexes possesses an [Ar]3d 3 configuration of the metal ion and an octahedral symmetry. 2 A simplified Jablonski energy diagrams of relevant energy levels is shown in Scheme III-1. In the ground state, the three electron occupy three t2g orbitals rendering the spin state S = 3/2. The quartet ground state is hence denoted 4 A2g. Upon photon absorption, an electron can be promoted from a t2g orbital to one of the two unoccupied eg orbitals without changing its spin state. Depending on the orientation of the t2g and eg orbitals involved in this electron promotion, two energy levels are formed, namely 4 T2g and 4 T1g. The former has a lower energy level. As the electron is relaxed to the lowest energy level 4 T2g, it can return to the ground state by fluorescence emission or non-radiative decays. complexes, also exhibit a similar trend. The authors proposed that due to a more open structure in the tpy complexes, the metal core is more exposed to solvent molecules which can enhance the non-radiative relaxation of its excited state. Consequently, the emission lifetime is lower than in the bpy complexes. In addition, quenching of the 2 Eg states of [Cr(bpy)3] 3+* by I -, Fe 2+ or O2 has also been shown to be efficient. The bimolecular quenching rate is ranging from 10 7 M -1 .s -1 (for O2) to 10 9 M -1 .s -1 (for I -).

Furthermore, homoleptic Cr(III) bipyridyl and terpyridyl complexes also show rich electrochemical properties. For example, Wieghardt and co-worker 5 reported that [Cr(tpy)2] 3+

shows four reversible one-electron reduction processes. Based on density functional theory (DFT) calculations supported by a wide range of spectroscopic techniques, they attributed all the four added electrons to ligand-reduction processes while Cr remains +3. The study suggests the possibility to accumulate multiple electrons on the complex, which is a crucial step towards multi-electron catalysis such as proton or CO2 reduction. The complex has also been proposed as a potential candidate for the water reduction because water molecules, in solid state, are found to stay close to the Cr(III) core of the [Cr(tpy)2] 3+ complex encompassed by two orthogonal tpy ligands. 6 However, this proposed catalytic reaction has not been published yet.

An important and recent progress in this field is the successful synthesis of heteroleptic Cr(III) polypyridyl complexes. 7,8 It opens a new route to functionalize the tpy ligands with suitable anchoring groups to graft the complex onto surfaces or to synthesize dyads in order to explore its capability of storing multiple charges at a single Cr(III) site.

III.1.2. Accumulation of multiple redox equivalents in molecularsemiconductor hybrid systems

In the search for sustainable energy sources to replace traditional but polluted fossil fuels, hydrogen produced by proton reduction reaction arises as an absolutely clean source.

Besides, the capture and conversion of the infamous greenhouse gas CO2 into more valueadded energy sources has also received particular attention in recent years. However, both the water reduction to form H2 and CO2 reduction using molecular catalysts require the accumulation of more than one electron on the catalytic sites. For instance the proton reduction to H2 is a two electron reduction process. We are interested in molecular systems capable of accumulating multiple redox equivalents under visible light. In this part we will highlight the challenges associated with the multiple charge accumulation processes and some recent advances in the field, with a focus on a novel class of molecular-semiconductor hybrid systems. Detailed reviews on the multi-charge storage and catalytic applications can be found in literature. 9,10 According to Hammarstrom,9 The charge accumulation faces several major challenges such as: (i) efficient PS to harvest light and to selectively inject the photo-excited electron to A, (ii) suitable energy levels of the components, (iii) sufficient long lifetime of the charge separated state D + -PS-A -, (iv) chemically stable A -and D + species. Therefore, it is challenging to design purely molecular systems for the charge accumulation. Redox relays are sometimes required to suppress the back electron transfer steps. The competing back electron transfer processes and the difficulties to synthesize these supramolecules are of the main barriers.

Meanwhile, molecularsemiconductor (TiO2) hybrid systems have several advantages over the purely molecular systems such as: (i) easier synthesis of individual components, (ii) long-lived charge separated state (e -)TiO2/Ru 3+ (see Chapter 2), (iii) TiO2 as a scaffold for grafting a wide range of complexes. This chapter is hence dedicated to this hybrid material class (two examples have already been described in Section I.3).

a) Accumulation of multiple oxidative equivalents

The first example for the accumulation of multiple oxidative equivalents using a regenerative PS and a multi-electron donor has been reported by Hammarstrom and coworkers 11 . The authors covalently linked a [Ru(bpy)2(CN)] 2+ PS to an oligo(triarylamine) (OTA) electron donor, and the Ru-OTA complex was grafted onto TiO2 nanoparticles (NPs) (Scheme III-3). After the first laser pulse excitation at 510 nm, the Ru 2+* state injects an electron to TiO2 to form (e -)TiO2/Ru 3+ within 5 ps. A single electron transfer from OTA regenerates the Ru 2+ dye in less than 1 ns. The charge separated state (e -)TiO2/Ru II -OTA + is then formed with ~ 100 % yield. Afterwards, the second laser pulse at 480 nm excites the Ru 2+ dye and another electron is injected to TiO2, followed by the rapid reduction of Ru 3+ by OTA + ( << 15 ns). The resulting multiple charge separated state, (2e -)TiO2/Ru 2+ -OTA 2+ , has been shown to be formed with remarkable nearly 100 % yield and attributed to the efficient separation between the A sites (TiO2) and D sites (OTA). The photo-induced electron transfer is schematically described in Scheme III-3.

Scheme III-3. Accumulation of two oxidative equivalents on OTA site of TiO2/Ru-OTA NPs which is mentioned in reference 11. Note: OTA + and OTA 2+ oxidative charges are delocalized over the whole OTA unit.

The pioneering work by Hammarstrom's group has motivated other researchers to develop this D-PS-A design for catalytic applications. For example, T. J. Meyer and coworkers reported a PSmolecular catalyst dyad anchored on TiO2 thin film electrode for water oxidation in aqueous solution 12 (Scheme III-4a). They covalently grafted a Rua II -Rub II -OH2 dinuclear complex on mesoporous TiO2 thin film electrode, where Rua II acts as a visible light PS and Rub II -OH2 is a known pre-catalyst for water oxidation to produce O2. 13 Steadystate and transient spectroscopic studies have been conducted to probe the formation of transient species under irradiation and their associated electron transfer kinetics. After the first laser pulse, the Rua II PS is excited and quickly injects an electron to TiO2 film in less than 20 ns, followed by the electron transfer from Rub II site to transiently form (e -)TiO2/Rua II -Rub III -OH species with ~ 10 % quantum yield. When a bias is applied to pre-oxidize the system to form TiO2/Rua II -Rub III -OH, the irradiation excites an electron of the Rua II PS, followed by electron injection to TiO2. Subsequently, another mono-electron transfer occurs between Rub III -OH and Rua III species to form Rua II and Rub IV =O within 20 ns and < 15 % yield. The work has provided experimental spectroscopic evidence for sequential mono-electron transfer steps towards the accumulation of two oxidative equivalents in Ru IV =O centers, which is the catalytic center for water oxidation reaction 13 . Scheme III-4a schematically illustrates the electron transfer events.

As a comparison, T. J. Meyer and co-workers also applied the similar concept but a Rua II PS and Rub II molecular catalyst have been co-loaded on TiO2 thin film electrode (Scheme III-4b) instead of being covalently linked together. 14 The first photoexcitation and electron injection also occurs at less than 20 ns. Lateral hole hopping process between Rua III and Rub II -OH2 to yield Rua II and Rub III -OH is accomplished within hundreds of microseconds, which is significantly lower than the covalently linked Rua-Rub-OH2 complex. However, when the Rub II -OH2 is pre-oxidized to Rub III -OH by an applied bias, no signal of Rub IV =O has been detected when the Rua/Rub molar ratio is around 1.8:1. It is attributed to the increased recombination rate between (e -)TiO2 and oxidized Rua III or Rub III -OH species. Increasing the Rua/Rub ratio to 6.4:1 has indeed allowed for the formation of the desired Rub IV =O species.

b) Accumulation of multiple reductive equivalents

On the other hand, efforts have been made to accumulate multiple reductive equivalents under visible light. From a purely molecular approach, Wasielewski and co-workers 15 made the first report using perylene bis(carboxyimide) (PBDCI) as an electron acceptor connected with two porphyrins (P2H) as PS (Scheme III-5). Under laser excitation, both P2H units can be simultaneously excited to the singlet state, then sequentially transferring an electron to PBDCI moiety to achieve a doubly reduced species, PBDCI 2-. The resulting charge separated state, + H2P-PBDCI 2--P2H + , shows a relatively short lifetime of ~ 5 ns. Interestingly, this work emphasizes the role of excitation light intensity for the charge accumulation on the PBDCI unit.

Scheme III-5. Molecular structure of H2P-PBDCI-P2H mentioned in reference 15

To date, most of the works rely on purely molecular systems. 9,10,16,17 In this chapter we will first discuss the electrochemical and photophysical properties of [Cr(ttpy)2] 3+ (ttpy = 4'-(p-tolyl)-2,2':6',2''-terpyridine) complex. The ttpy instead of tpy ligand has been chosen for the convenience of subsequent functionalization of the methyl group with phosphonic acid to be anchored on TiO2. Multiple electron storage induced by visible light will also be mentioned. The electrocatalytic activity of the homogeneous complex towards proton reduction will be addressed as well. Afterwards, the building and characterization of TiO2/Cr 3+ and Ru 2+ /TiO2/Cr 3+ NPs following the PS-TiO2-A design will be thoroughly described.

III.2. Chromium(III) bis-tolylterpyridine complexes in solution III.2.1. Synthesis

The synthesis of [Cr(ttpy)2] 3+ complex with ClO4 -counter anions is depicted in Scheme III-7. Starting material has been chosen to be CrCl2 thanks to the lability of the metal ion. Indeed, Cr 3+ is known to be inert, which makes it difficult for complexation. After the complexation between Cr 2+ salt and ttpy ligand under Ar atmosphere, an excess amount of NaClO4 was added to precipitate the complex out of water/ethanol solvent. The reaction mixture was then bubbled with air to yield the desired product [Cr(ttpy)2] 3+ . It has been shown that this strategy may prevent the dissociation of the ttpy ligand in the presence of water, as water can coordinate with the Cr 3+ metal core. 19 As the sample is paramagnetic, it cannot be investigated with 1 H NMR spectroscopy. The complex shows no oxidation waves when the potential is swept until 1.5 V vs Ag/AgNO3 0.01 M. In the cathodic scanning, it shows four successive one-electron reduction processes at E1/2 = -0.47, -0.85, -1.35 and -2.25 V. All the waves show peak-to-peak splitting Ep around 60 -70 mV, indicating electrochemically reversible processes. The redox potentials are in accordance with literature. 7 The analogous [Cr(tpy)2] 3+ complex also shows comparable redox potentials: E1/2 (V) = -0.44, -0.86, -1.36 and -2.28 vs Ag/AgNO3 0.01 M reference electrode. [START_REF]mmol.g -1 (about 3800 Cr III molecules per particle or 2 Cr III molecules per nm 2 ). C6H5-CH2-PO3H2/TiO2/[Cr2P] 3+ (C6H5-CH2-PO3H2 = benzylphosphonic acid, BPA). Similar to the case of TiO2/[Cr2P] 3+ , but a mixture of [Cr2P] 3+ and BPA was used instead of References[END_REF] Although the ttpy ligand in this studies contains tolyl substituting group at the para position which is a weak electron donating group, its effect on the redox potentials is negligible. It can be seen that the  spin number remains around 3 in all the series, suggesting the oxidation state of Cr atom is always +3. The  spin number slightly increases from the +3

to -1 oxidation states, i.e. up to four-electron reduction. Furthermore, the excited states of [Cr(ttpy)2] 0 and [Cr(ttpy)2] -1 complexes have also been investigated. The result suggests that all four added electrons are mainly localized on the ttpy ligands. Therefore, the electronic structure of [Cr(ttpy)2] n+ complexes at ground states can be described as:

n = 2 [Cr III (ttpy)(ttpy -)] 2+ n = 1 [Cr III (ttpy -)2] + n = 0 [Cr III (ttpy -)(ttpy 2-)] 0 (ground state) n = -1 [Cr III (ttpy 2-)(ttpy 2-)] 1-(ground state)
The result of these ground states are in accordance with Wieghardt et al. 5 on analogous complexes [Cr(tpy)2] n+ (tpy = 2,2':6',2"-terpyridine) using the B3LYP functional calculation. to the intraligand charge transfer between the tolyl and tpy groups. The transition at 767 nm is assigned to the charge transfer between tolyl and metal center. Therefore, the overall broad absorption band at 800 nm is a consequence of different charge transfer processes which finally leads to the charge transfer between ttpy ligand and Cr metal center. The whole process is thus described as ligand-to-metal charge transfer (LMCT). The calculation also shows a strong electronic transition at 532 nm (18800 cm -1 ) which can be assigned to the strong absorption peak at 500 nm. between the tolyl and tpy moieties. Therefore, the whole process can also be described as a LMCT transition. Moreover, this calculation also shows a strong electronic transition at 485 nm (20600 cm -1 ) and 559 nm (17900 cm -1 ) which can be assigned to the strong absorption peak at 524 nm and 560 nm.

b) Steady-state and time-resolved emission spectroscopy

The steady state luminescence emission of [Cr(ttpy)2] 3+ complex in MeCN solution under Ar has been studied by irradiating the sample at 370 nm. This wavelength was chosen as it is the absorption maximum of the complex. The emission spectrum is shown as dotted line in Figure III-6, in comparison with the absorption spectrum of the complex (solid line).

The emission, whose maximum is at 770 nm, is very weak. Its quantum yield at room temperature is estimated to be less than 10 -3 and attributed to the relaxation of the 2 Eg state to the 4 A2g ground state. 22 The shoulder at around 730 nm could be due to the relaxation of the From the diagram one can calculate the oxidizing power of Cr

III.2.4. Electron paramagnetic resonance spectroscopy

Since the [Cr(ttpy)2] n+ (n = 3, 2, 1) complexes are all paramagnetic, we employed electron paramagnetic resonance (EPR) spectroscopy to study changes in spin state when the Considering stepwise monoelectronic reductions of the original complex [Cr(ttpy)2] 3+ , the expected spin state of these complexes is as follows: [Cr(ttpy)2] 3+ : S = 3/2; [Cr(ttpy)2] 2+ : S = 1; [Cr(ttpy)2] + : S = 1/2 and [Cr(ttpy)2] 0 : S = 0. Therefore, we should expect that all the complexes except the last one are paramagnetic and detectable by EPR spectroscopy.

However, the [Cr(ttpy)2] 2+ complex is EPR silent when an X-band EPR spectrometer is used to record the signal. A similar study for [Cr(tpy)2] 2+ complex also shows no EPR signal, but its spin state of S = 1 has been confirmed by SQUID (Superconducting Quantum Interference Device) measurement. 5 The [Cr(ttpy)2] 3+ complex (S = 3/2) exhibits a very broad EPR

 Photoreduction of [Cr(ttpy)2] 3+ by TEOA under continuous visible irradiation

In The inset shows a plateau after ~ 10000 s, suggesting the transformation has been complete.

Based on the absorbance at 723 nm we estimate the second reduction step to occur with around 80 % yield.

 This quenching rate is close to the limit of the diffusion rate of species in solution, which is usually around 10 10 M -1 .s -1 . In literature, a similar result was obtained for the quenching of [Cr(ttpy)2] 3+* by a mononucleotide base. 26 The authors recorded KSV constants ranging from 1.7 × 10 4 with guanine to 10

0 0 1 [ ] 1 [ ] SV q K TEOA k TEOA        (Eq III-2)

 Photoreduction of [Cr(ttpy)2] 3+ by [Ru(bpy)3] 2+ and TEOA under continuous visible irradiation

The previous section shows that [Cr(ttpy)2] 3+ complex can be excited by visible light and then be photoreduced. However it is not a good PS due to its low absorbance in the visible region. Therefore, the quest for a better PS is necessary. A recent publication has proposed to substitute the ttpy ligand with a strong electron donor group to shift the absorption of the Cr(III) complex to the visible domain. 22 In our study we focus on the Afterwards, the band at 800 nm gradually disappears while a peak at ~ 720 nm and a shoulder at ~ 560 nm are formed and increase in intensity. These changes correspond well to the [Cr(ttpy)2] + species. During the experiment, the peak at 450 nm, which is assigned to the It is clear that in both cases, the products are the same. The reaction ( 1) is more energetically favorable. The [Ru(bpy)3] 2+ complex also absorbs more than the [Cr(ttpy)2] 3+ complex in the visible region due to higher spectral coverage and extinction coefficient.

Therefore, we assume that the reaction ( 1) is the main quenching mechanism. However, to verify whether the reaction (2) can occur or not, we need to find a non-photoactive, redox chemical to replace [Ru(bpy)3] 2+ . This chemical should also have a similar redox potential as the Ru 3+ /Ru 2+ couple (E1/2 = 0.94 V). Triphenylphosphine (PPh3) with Epa (POPh3/PPh3) = 0.91 V 28 in MeCN solution has been chosen since it does not absorb in the visible region.

 Photoreduction of [Cr(ttpy)2] 3+* by PPh3 under continuous visible irradiation

The monoelectronic photoreduction of [Cr(ttpy)2] 3+ complex by PPh3 under visible In our study, when [Ru(bpy)3] 2+ complex is used instead of PPh3 (reaction 2 mentioned above), the quenching of [Cr(ttpy)2] 3+* by [Ru(bpy)3] 2+ can then be possible.

 Quenching of [Ru(bpy)3] 2+* excited state by [Cr(ttpy)2] 3+

In order to study the kinetics of the quenching reaction between [Ru(bpy)3] 2+* and [Cr(ttpy)2] 3+ (reaction 1 above), the Stern-Volmer experiment has been carried out. 19 A linear plot of 0/ vs the concentration of [Cr(ttpy)2] 3+ was obtained upon excitation at 460 nm and the quenching constant kq was found to be ~ 3 × 10 9 M -1 .s -1 . As there are no overlaps between the emission spectrum of [Ru(bpy)3] 2+* and the absorption spectrum of [Cr(ttpy)2] 3+ , the quenching reaction cannot occur via an energy transfer mechanism. Therefore, we attribute this quenching process to an electron transfer mechanism. When [Cr(bpy)3] 3+ was used instead of [Cr(ttpy)3] 3+ , a very similar kq value was found (kq = 3.3 × 10 9 M -1 .cm -1 ). 29 This quenching constant is one order of magnitude higher than the constant for the quenching of It can be seen that the monoexponential function is a better fitting model. The kinetics of back electron transfer is estimated to be (3.2 ± 0.1) × 10 6 s -1 .

 Mechanism of the photoreduction of [Cr(ttpy)2] 3+ in the presence of [Ru(bpy)3] 2+

and TEOA

Since TEOA, a sacrificial electron donor, is present in the exhaustive photoreduction experiment mentioned above, there are two possible photoreduction pathways for the quantitative photogeneration of [Cr(ttpy)2] + complex:

- M -1 .s -1, 34 (in deaerated aqueous solution, CTEOA ~ 0.1 M). Converted to the pseudo first-order kinetics, this rate is comparable to the back electron transfer rate (3.2 × 10 6 s -1 ). As the amount of TEOA in the exhaustive photoreduction experiment is about 1000 times higher than [Cr(ttpy)2] 3+ , it is statistically more likely that the [Ru(bpy)3] 3+ complex is reduced by TEOA than by [Cr(ttpy)2] 2+ , suggesting that step 4 is more likely to occur than step 3.

Therefore, the back electron transfer is short-circuited by TEOA, allowing [Cr(ttpy)2] 2+ complex to be reduced again.

After the [Cr(ttpy)2] 2+ singly reduced complex is formed, it can quench the [Ru(bpy)3] 2+* state via either a reductive quenching process (step 5) or an oxidative quenching process (step 6). Although the reductive quenching reaction is more energetically favourable (G5 = -0.84 eV compared with G6 = -0.25 eV), as we observed the gradual formation of [Cr(ttpy)2] + species, the oxidative quenching reaction should be kinetically faster.

Furthermore, the TEOA sacrificial agent is known to efficiently reduce [Ru(bpy)3] 3+ PS (step 4) to sustain the step 6.

 Conclusion for the photoreduction mechanism of [Cr(ttpy)2] 3+ by [Ru(bpy)3] 2+

and TEOA:

To summarize, [Cr(ttpy)2] 3+ complex can be reduced twice to form [Cr(ttpy After 3 hours under the -1.2 V bias, the CV recorded with a clean carbon disk electrode was very similar to that obtained before the experiment, except for declines in intensity of the redox waves. The CV recorded with the carbon plate WE produced unstable responses. After 3 hours, the surface of the carbon plate was covered with a visible layer that was not soluble in MeCN. In the absence of TFA, there is no such layer covering the WE when an exhaustive electrolysis of [Cr(ttpy)2] 3+ complex was conducted at a similar potential. It is hence suspected that during a long time under the -1.2 V bias and in the presence of H + , the [Cr(ttpy)2] + complex is partly decomposed, releasing a ttpy ligand that is then deposited onto the WE. This layer blocks the conductivity of the electrode so that the catalytic reaction is stopped. In fact, this electrocatalytic experiment is not reproducible as sometimes the WE was blocked quickly within 1 or 2 hours at the -1.20 V bias. The suspected ttpy layer is always observed.

In an attempt to get insight into the catalytic mechanism, we first created [Cr(ttpy We also attempted to use pulsed EPR technique to study the samples mentioned above.

The experiment is based on the following assumptions: (i) if another paramagnetic complex formed during the catalytic process has a similar continuous EPR spectrum as [Cr(ttpy)2] + , it may show changes in spin relaxation time; (ii) if decoordination of ttpy ligands occurs, interaction between the S = 1/2 system and the 14 N atoms should change as well, which can be detected by the HYSCORE method; (iii) if a hydride complex is an intermediate for H2

production, interaction between the S = 1/2 system and protons should be enhanced in the HYSCORE spectrum. Therefore, we recorded the spin relaxation time and HYSCORE spectra (Figure III-19) for 2 samples: [Cr(ttpy)2] + complex alone and the complex with 1 equivalent of TFA. However, the two samples show completely similar spin relaxation and HYSCORE spectra, except for the reduced intensities as seen in the continuous X-band EPR spectra. where hydrogen atom is linked to the Cr core is not proved. Moreover, the released ttpy ligand is neutral in charge at -1.2 V bias, thus it is electrochemically inactive and cannot be involved in the proton reduction reaction. Therefore, we tend to attribute the catalytic process to a ligand-based reduction process where a protonated terpyridine complex, [Cr III (ttpyH)(ttpy -)] 2+ , can be an intermediate.

III.3. [Cr(ttpy)2] 3+ complex immobilized on TiO2 nanoparticles

After studying the [Cr(ttpy)2] 3+ complex in homogeneous environment, we immobilized it on TiO2 NPs to study photo-induced charge transfer processes. Using TiO2 NPs as a support can be more convenient in photocatalytic studies than using an electrode due to higher loading of the catalyst. Another advantage of this TiO2 NPs support over an electrode surface is that it is also easier to study the evolution of photo-induced paramagnetic species by EPR spectroscopy. Similar to Chapter 2, the phosphonic acid was used as anchoring group to the TiO2 surface.

III.3.1. Synthesis

The synthesis route to ttpy-PO3Et2 and ttpy-PO3H2 ligands, their complexation with Cr 2+ UV-vis absorbance of supernatant solution after each centrifugation step, we estimate the loading of [Cr2P] 3+ complex to be about 0.20 mmol.g -1 , which corresponds to about 3800

[Cr2P] 3+ molecules per particle or 2 molecules per nm 2 of surface.

III.3.2. Electrochemical properties

III.3.2.1. ttpy-PO3Et2 ligand and [Cr(ttpy-PO3Et2)2] 3+ complex in solution

The electrochemical behaviors of the ttpy-PO3Et2 ligand and corresponding

[Cr(ttpy-PO3Et2)2] 3+ complex have been characterized with cyclic voltammetry in order to study the effect of the phosphonate group. Comparing ttpy and ttpy-PO3Et2, the phosphonate derivatization increases the reduction potential of the ttpy ligand: from -2.35 V for ttpy to -2.13 V for ttpy-PO3Et2. The phosphonate substituent induces an electron withdrawing effect on the -conjugated system, thus making its reduction easier. The ttpy-PO3Et2 ligand also shows a more reversible behavior than the ttpy ligand.

The [Cr(ttpy-PO3Et2)2] 3+ complex exhibits a very similar CV as the [Cr(ttpy)2] 3+ complex appears at a potential similar to the analogous [Cr(ttpy)2] 3+ complex rather than the free ttpy-PO3Et2 ligand. Besides, no signals of the free ttpy-PO3Et2 ligand at -2.13 V are detected.

III.3.2.2. Grafting [Cr2P] 3+ complex on ITO

We first attempted to study the unprotected [Cr2P] 3+ complex directly with cyclic voltammetry to compare with the results mentioned above. However, the [Cr2P] 3+ complex (with ClO4 -as counteranion) is not soluble in a variety of non-aqueous solvents like MeCN, DMF and CH2Cl2. We therefore chose to anchor the complex onto an ITO electrode to study its electrochemical properties. where  (mol.cm -2 ) is the surface coverage of the complex. For the first reduction step at -0.42 V, a linear plot between anodic peak current Ipa and scan rate v is obtained (Figure

III-21b

). This linear relationship suggests the electronic communication between anchored

[Cr2P] 3+ and ITO electrode is kinetically controlled, thus proving the successful grafting of

[Cr2P] 3+ onto the ITO support.

III.3.2.3. Grafting [Cr2P] 3+ complex on TiO2 NPs

We first attempted to characterize the electrochemical behavior of TiO2/[Cr2P] 3+ NPs in colloidal solution using C or Pt disk electrode as WE. However our attempts were not successful due to low conductivity of TiO2 NPs and probably low surface coverage. employed in an analogous EPR experiment for the free complex (see Section III.2.4).

Therefore, the local concentration on the NP surface is the main cause for this EPR line. This assumption is supported by the estimated loading of 2 [Cr2P] 3+ molecules per nm 2 . The high local concentration of paramagnetic [Cr2P] 3+ complex provides evidence for the successful grafting of [Cr2P] 3+ on TiO2 NPs.

We then prepared TiO2/{[Cr2P] 3+ +BPA} (BPA = benzylphosphonic acid) NPs containing 2 % Cr and 98 % BPA as lateral spacers. With this dilution, the average distance between Cr sites is expected to significantly increase. complexes are estimated to be 60 µmol.g -1 (~ 1000 molecules per NP, 0.6 molecule per nm 2 ) and 6 µmol.g -1 (~ 100 molecules per NP, 0.06 molecule per nm 2 ), respectively. Further details can be found in the Experiment Section.

III.4.2. Photophysical properties a) UV-vis absorption spectroscopy and emission spectroscopy

Similar to previous UV-vis absorption experiments, the Ru 2+ /TiO2/[Cr2P] 3+ NPs were also mixed with KBr and pressed into a pellet for measurement. As we are interested in the possibility to store multiple electrons at the Cr sites, the molar ratio of Ru to Cr species was set at 10 so that the electron supply from [RuP] 2+ PS is not a limiting factor for the [Cr2P] 3+ reduction. BPA, which is optically transparent in the visible region, was also included as lateral spacer to enhance the photo-induced electron injection rate as shown in Chapter 2.

Another reason for using BPA is to separate the paramagnetic Cr 3+ sites from each other for the EPR characterization, which will be discussed later in this section.

b) Emission spectroscopy

The emission of Ru 2+ /TiO2/[Cr2P] 3+ triad has been recorded in steady state after photoexcitation at 450 nm.

                (Eq III-8)
From the KWW and KWW parameters, one can estimate a representative charge recombination rate kcr by applying the Equation III-9. 45 The KWW fitting parameters (KWW and KWW) as well as the recombination rate kcr are collected in Table III-7 and compared to those obtained with the TiO2/Ru 2+ dyad.

KWW cr KWW KWW KWW KWW KWW k k                                  
(Eq III-9) triad compared with that of TiO2/Ru 2+ dyad was surprising to us, as the presence of an electron acceptor like [Cr2P] 3+ should scavenge the injected electron on the CB of TiO2, preventing back electron transfer between (e -)TiO2 and Ru 3+ species. However, as we have assigned the signal decay at 450 nm to the (e -)TiO2 disappearance, two assumptions should be considered in the triad case: (i) the charge recombination process between (e -)TiO2 and Ru 3+ is accelerated, or (ii) another electron transfer pathway besides the charge recombination process to consume these trapped electrons. The charge recombination rate between two species should only depend on their nature, distance and the solvent, thus the first assumption is not sensible. As the lifetime of the charge recombination is very long (cr = (kcr) -1 = 7.1 ms),

[Cr2P] 3+ should be able to scavenge the electrons on TiO2, supporting the second assumption.

Therefore, we tend to attribute this two-fold increase in kcr value to the additional electron transfer between (e -)TiO2 and [Cr2P] 3+ to transiently form the Ru 3+ /TiO2/[Cr2P] 2+ charge separated species. The rate of electron transfer between (e -)TiO2 and [Cr2P] 3+ should be in the same range with the recombination rate between (e -)TiO2 and Ru 3+ .

Theoretically, such a transient charge separated state like Ru 3+ /TiO2/[Cr2P] 2+ can also be achieved by lateral electron transfer between Ru 2+* and Cr 3+ species. To verify whether it is formed via the "through particle" or "on particle" mechanism, a redox-inert substrate like SiO2 NPs can be employed so that only the latter mechanism can govern the electron transfer event. However, the surface coverage of phosphonic anchoring group on SiO2 surface has been proved to be much lower than on TiO2. Further reductions in the concentration of adsorbates like that will make it very challenging to characterize with the TA technique.

Besides that, the rate of photo-induced electron injection from Ru 2+* to TiO2 is very fast (kinj ~ 10 7 -10 9 s -1 , see Chapter 2). The quenching of [Ru(bpy)3] 2+* by [Cr(ttpy)2] 3+ in solution has been shown to be 3.0 × 10 9 M -1 .s -1 . 19 Taking into account the concentration of [Cr(ttpy)2] 3+ complex in the range of 10 -5 M, the pseudo first-order quenching constant will be ~ 3 × 10 4 s -1 , which is significantly smaller than the injection rate. Besides, as the complexes are fixed on TiO2 surface and the concentration of Cr 3+ is only 2 %, the chance for electron transfer to occur between Ru 2+* species and underlying Ti 4+ sites should be significantly higher than between Ru 2+* and Cr 3+ . Therefore, it is reasonable to assume the "through particle" charge transfer as the main mechanism for the Ru 3+ /TiO2/[Cr2P] 2+ triad.

III.4.3. Electrochemical properties a) [Cr2P] 3+ and [RuP] 2+ co-immobilized on ITO electrode

In a similar fashion as the previous section, we attempted to study the electrochemical [Ru(bpy)3] 2+ complex in solution. It is therefore concluded that the photoreduction rate is reduced when immobilized Ru 2+ dye is used instead of [Ru(bpy)3] 2+ complex in solution. We assume that this rate decline is due to the poorer contact between the colloid and the [Cr(ttpy)2] 3+ homogeneous complex, therefore anchoring both complexes on TiO2 NPs may accelerate this rate. This assumption will be checked and discussed in the next section.

III.4.5. Photoreduction of immobilized

b) Charge transfer reactions monitored by UV-vis spectroscopy

In order to compare with the photoreduction experiment described in the previous section, we conducted a similar experiment using Ru 2+ /TiO2/[Cr2P] 3+ triad instead of TiO2/Ru 2+ and [Cr(ttpy)2] 3+ mixture, in the presence of TEOA. However, no spectral changes are observed. The photoreduction of immobilized [Cr2P] 3+ is therefore deemed unsuccessful.

c) Charge transfer reactions monitored by EPR spectroscopy

In order to understand why the photoreduction of immobilized [Cr2P] 3+ in the triad is not accomplished, a light-coupled EPR experiment has been carried out at low temperature and in the absence of TEOA sacrificial donor to study the formation of photo-induced paramagnetic species. Our research hypothesis is that if [Cr2P] + doubly reduced complex is formed, it can be observed by EPR as in the case of [Cr(ttpy)2] + complex (S = 1/2). The spectrum before irradiation seems to suggest an EPR-silent system, although

[Cr2P] 3+ molecules is expected to absorb in a wide range of magnetic field in a similar fashion as [Cr(ttpy)2] 3+ free complex. However, the amount of grafted [Cr2P] 3+ in this study (~ 3.6 × 10 -5 M) is about two order of magnitude lower than in the experiment with Cr free complex (~ 10 -3 M). Due to its very broad spectrum, the signal of grafted [Cr2P] 3+ on TiO2 may be too small to record. With this degree of Cr complex dilution of only 2 %, the broad line centered at g ~ 2 due to high local concentration of paramagnetic Cr sites is also not observed.

Under continuous irradiation at 455 nm, the trapped electrons on TiO2 are quickly created. They are characterized by EPR lines at g = 1.990 and g// = 1.959. 46,47 No trapped holes (usually denoted as O -species in literature) 47 are detected in this triad, indicating that the trapped electrons are due to electron injection from Ru 2+* PS rather than direct bandgap excitation of TiO2 semiconductor. Similar to the TiO2/Ru 2+ dyad, four small EPR lines (ratio = 1:3:3:1) separated by 23 G are also detected (marked with asterisks). They are ascribed to  CH3 radicals 48 which have possibly been produced by the photo-degradation of remaining ethanol adsorbed on TiO2. 49,50 Meanwhile, the expected signal of sites is high than in homogeneous solution, thus this back electron transfer may be enhanced.

Therefore, unfortunately we are not able to observe the electron accumulation in [Cr2P] 3+ sites.

III.5. Conclusion

In initial complexes, thus preventing the [Cr2P] 2+ complex to be further reduced.

In the next chapter we will present a study on another complex as an electron acceptor that undergoes a coupled chemical reaction after being reduced. The coupled reaction may reduce the back electron transfer kinetics.

CHAPTER IV

Ru(II)/TiO 2 /Mn(I) TRIAD FOR PHOTOCATALYTIC CO 2 REDUCTION

IV.1. Introduction

In recent decades, the rise of the CO2 content in the Earth atmosphere has become a major concern for human beings. As a greenhouse gas, it has been identified as a main factor contributing to the global warming 1 and sea level rise 2 . Chemical conversion of CO2 into more value-added products such as CO and HCOOH provides a means to reduce its atmospheric content and to recycle CO2 emitted from coal burning factories. However, the process is energy consuming as the O=C=O bonds are quite stable (around 800 kJ.mol -1 ).

Table IV-1 shows the standard reduction potentials for several important products. 3 The potentials represent equilibrium potentials that are required for the reaction to occur, but usually a higher potential is necessary to achieve a significant reaction rate. In this chapter we will mainly focus on the photocatalytic CO2 reduction, with an emphasis on molecular and hybrid systems to tackle the issue. The reader is kindly referred to recent reviews by Perutz et al. 1 , Ishitani et al. 5,6 , Rieger et al. 7 and Fontecave et al. 8 for more information about homogeneous and hybrid systems for the electro-and photo-catalytic CO2 reduction.

Since the pioneering works by Lehn and co-workers on the photocatalytic 9 and electrocatalytic 10 CO2 reduction using [Re(bpy)(CO)3Cl] catalyst, numerous publications on homogeneous catalysts have been made. The general strategy is to covalently link a photosensitizer (PS) and a molecular catalyst together for CO2 reduction under visible light.

This approach has been extensively summarized in the aforementioned reviews. It, however, often requires challenging multi-step synthesis to reach the desired supramolecules.

In recent years, an emergent approach is to immobilize the molecular PS and catalyst onto the surface of another material such as semiconductors, nanoparticles (NPs), electrodes and metal-organic framework (MOF) 5 to form a new hybrid system. We are interested in the systems containing NPs as they provide a good scaffold with a great surface area to accommodate the molecular PS and catalyst. Furthermore, semiconducting NPs such as TiO2 can also act as an electronic component for the electron transfer reaction between the PS and the catalyst centers. Finally, this approach usually requires significantly less complicated synthesis than the supramolecular approach. This introduction section will highlight recent developments in this class of hybrid materials.

Cowan and co-workers 11 immobilized [Ru(bpy)3] 2+ PS and Ni(cyclam) 2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) catalyst on ZrO2 for photocatalytic CO2 reduction to CO (Scheme IV-1). The system Ru 2+ /ZrO2/Ni 2+ (Ru:Ni = 2.6:1 % mol) produceed both CO (14.0 ± 1.4 µmol.g -1 ) and H2 (58.1 ± 9.1 µmol.g -1 ) after 7 hours of visible irradiation in the presence of ascorbic acid as a sacrificial electron donor. It corresponded to the maximum turnover number (TONmax) for CO equal to 4.8, while the selectivity of CO2 over H2 was still poor.

Increasing the irradiation time lead to decrease in activity, possibly due to deactivation pathway via the formation of Ni I (cyclam)-CO species. Under the same conditions, a mixture of the two complexes in solution exhibited negligible CO production (TONmax (CO) ~ 0.2 after 7 hours). After excitation of the Ru 2+ complex, transient absorption spectroscopy (TAS)

showed that Ru 2+* excited state was partly reductively quenched by ascorbic acid to form Ru + species, which then transferred an electron to the Ni 2+ sites. The rate of electron transfer between Ru + and Ni 2+ immobilized on TiO2 was estimated as 7.7 × 10 3 s -1 , whereas the rate between the two complexes in solution was significantly slower (10 s -1 ). This enhanced electron transfer rate in the Ru 2+ /TiO2/Ni 2+ triad was proposed to be critical for the higher photocatalytic activity of the triad than the complex mixture in solution. Since ZrO2 is redoxinactive, the charge transfer events in Ru 2+ /ZrO2/Ni 2+ could only occur via electron transfer between neighboring Ru 2+ and Ni 2+ species on surface, called "on particle" mechanism. This study proved that the immobilization of the PS and catalyst on surface increased the electron transfer kinetics by eliminating the complex diffusion in solution. In order to improve the photocatalytic efficiency of TiO2/Re I system, Kang and coworkers 14 immobilized an organic dye and [Re(bpy)(CO)3Cl] catalyst on TiO2 NPs (Scheme IV-3). In contrast to the Reisner's work on TiO2/Re I where Re complex acted as both PS and catalyst, the Re species in this study only operated as a catalyst. Since photocatalytic CO2 reduction in homogeneous solution using the [Re(bpy)(CO)3Cl] suffers from short durability due to the catalyst degradation, fixing it on a surface was expected to reveal its intrinsic photocatalytic activity. Under visible irradiation ( > 420 nm), the hybrid system exhibited remarkable TONmax (CO) > 435 versus the Re catalyst in DMF solution and in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial electron donor.

The system also reached good selectivity for CO, since H2 content was less than 5 % of CO content, and no formic acid was detected with the HPLC analysis of the liquid phase. Mechanistic studies have also been discussed in the three publications mentioned above.

After the first monoelectronic reduction step on the metal center, the Br -leaves and a Mn 0 -Mn 0 dimer is formed. Reduction of the dimer generates the catalytic active species

[Mn(bpy)(CO)3] -where the pentacoordinated Mn -I center offers a vacancy for a CO2 adduct.

The reduction steps can be summarized as follows:

[Mn I (bpy)(CO)3Br] + e - [Mn I (bpy -)(CO)3Br] - [Mn I (bpy -)(CO)3Br] - [Mn 0 (bpy)(CO)3] + Br - [Mn 0 (bpy)(CO)3]  1/2 [Mn 0 (bpy)(CO)3]2 1/2 [Mn 0 (bpy)(CO)3]2 + e - [Mn -I (bpy)(CO)3] -

The first example concerning [Mn(bpy)(CO)3Br] complex immobilized on TiO2 thin film electrode has been reported by Reisner and co-workers. 19 They functionalized the bpy ligand with two phosphonic acid groups (called [MnP]) to anchor on the TiO2 surface. The TiO2/[MnP] modified electrode was used for the electrocatalytic CO2 reduction in MeCN/H2O

(95/5, v/v) solution. The reason to deposit the complex on a mesoporous TiO2 thin film was to achieve higher loading and better electrical conductivity between the electrode and the catalyst than the free complex in solution. Under an applied potential at -1.7 V vs Fc + /Fc (-1.6

V vs Ag/AgNO3 0.01 M) during 2 hours, the modified electrode produced CO as the main product with TONmax = 112. H2 was also detected at ~ 20 % yield of CO, while HCOOH was not detected. The intermediate [Mn 0 (bpy)(CO)3]2 dimer was observed with UV-vis absorption spectroscopy although the Mn complex has been fixed on the surface. The authors attributed this observation to two possible mechanisms: (i) the lability of the phosphonic acid anchoring groups that the complex could be temporarily desorbed, forming a dimer then being readsorbed, and/or (ii) the local concentration of the Mn complex was high enough to be dimerized.

Recent works by Marc Bourrez [START_REF]mmol.g -1 (about 3800 Cr III molecules per particle or 2 Cr III molecules per nm 2 ). C6H5-CH2-PO3H2/TiO2/[Cr2P] 3+ (C6H5-CH2-PO3H2 = benzylphosphonic acid, BPA). Similar to the case of TiO2/[Cr2P] 3+ , but a mixture of [Cr2P] 3+ and BPA was used instead of References[END_REF] and Matthew Stanbury 21 , former PhD students in our CIRe laboratory have focused on a novel catalyst [Mn( 2 -ttpy)(CO)3(MeCN)] + (ttpy = 4'-(ptolyl)-2,2':6',2''-terpyridine) for CO2 reduction to CO and/or HCOOH. The complex showed better electrocatalytic CO2 reduction efficiency (TONmax (CO) = 13) than the previously reported [Mn( 2 -tpy)(CO)3(MeCN)] + (tpy = 2,2':6',2''-terpyridine) complex by Kubiak et al. 22 (TONmax (CO) ~ 4). In this chapter we will briefly present the characterization, electroand photo-catalytic CO2 reduction using a similar complex, [Mn( 2 -ttpy)(CO)3Br], in homogeneous solution. This chapter focuses on the immobilization of this Mn complex on TiO2 NPs via a phosphonic acid anchoring group to form TiO2/Mn I dyad, and subsequently the co-immobilization of a [Ru(bpy)3] 2+ -based PS and Mn complex onto TiO2 to yield

Ru II /TiO2/Mn I triad. Scheme IV-5 shows the structures of the dyad and triad that will be presented in this chapter. The photocatalytic CO2 reduction using the triad will also be described. = -1.44 V is assigned to the leaving of Br -and pyridyl coordination to form a tridentate linkage between the ttpy ligand and Mn metal ion (reactions 5 and 6). It is followed by the release of a CO group and the formation of a dimer through metal-metal bonding (reaction 7).

On the reverse scanning, the cathodic peak at -1.44 V is associated with an anodic peak at -0.89 V which corresponds to the oxidation of the Mn 0 -Mn 0 dimer (reaction 8). Another irreversible oxidation peak is observed at 0.27 V due to the oxidation of the metal center in the tridentate complex (reaction 9). This peak is not observed if the CV is first swept in oxidation (Figure IV-1a).

(

) [Mn I ( 2 -ttpy)(CO)3Br] + e - [Mn I ( 2 -ttpy -)(CO)3] + Br - Epc1 = -1.44 V (6) [Mn I ( 2 -ttpy -)(CO)3] +  [Mn 0 (ttpy)(CO)3] (7) [Mn 0 (ttpy)(CO)3]  1/2 [Mn 0 (ttpy)(CO)2]2 + CO (8) 1/2 [Mn 0 (ttpy)(CO)2]2 + MeCN  [Mn I ((ttpy)(CO)2(MeCN)] + + e -E = -0.89 V 5

IV.2.4. Electrocatalytic CO2 reduction

The electrocatalytic CO2 reduction experiment has been conducted using In order to study the reduction products of CO2, a potential of -1.7 V was applied for comparison with previous studies. CO gas is produced as the only reduction product with TONmax (CO) = 12 after 4 hours and 100 % Faradaic yield while no H2 or HCOOH is formed. reduction products like CO or HCOOH are formed via a two-electron, two-proton reduction process.

In our series of photocatalytic CO2 reduction experiments, the molar ratio of Mn:Ru was varied to determine the optimal value, which will be used to graft the complexes on TiO2

NPs. It is an advantage to work with NPs as the PS/catalyst ratio can be easily controlled, in comparison with supramolecules where PS and catalyst molecules are linked to each other, usually, at 1:1 ratio. It is noted that the excitation wavelength has to be carefully chosen to minimize the photo-induced decarbonylation of the [Mn(ttpy)(CO)3Br] complex as this side reaction can occur under room light. 27 In our study, the result obtained with the ratio of Mn:Ru = 1:1 is in agreement with a previous work 21 (TONmax (HCOOH) = 11, TONmax (CO) = 8) where the sample was monochromatically irradiated at 480 nm instead of 470  40 nm. We thus conclude that irradiation at 470  40 nm can selectively excite Ru 2+ PS in the same way as the monochromatic irradiation at 480 nm. Interestingly, this photocatalytic reduction process in DMF/TEOA mixed solvent produces a mixture of CO and HCOOH while the electrocatalytic reduction using [Mn(ttpy)(CO)3Br] in MeCN only produces CO. This observation is in accordance with literature for the electrocatalytic and photocatalytic CO2 reduction using [Mn(bpy)(CO)3Br] complex in MeCN and DMF/TEOA solvent, respectively. 17

The table also shows that decreasing the Mn:Ru ratio leads to higher total TON as expected due to higher chance for each Mn I molecule being reduced twice by two Ru I molecules. The best Mn:Ru ratio is determined at 1:10 where the total TON reaches the highest value of 50 in this series, although the selectivity of HCOOH over CO at about 1.6:1 is not high.

IV.3. TiO2/Mn I dyad and Ru II /TiO2/Mn I triad nanoparticles

IV.3.1. Synthesis

 Synthesis of ttpy-C3-PO3H2 ligand

To graft the [Mn(ttpy)(CO)3Br] complex on TiO2 NPs, we chose phosphonic acid as anchoring group. As presented in Chapter 3 for the [Cr(ttpy)2] 3+ complex bearing two phosphonic acid groups, the ttpy-PO3H2 ligand cannot be isolated from DMSO for characterization. Therefore, we employed a new synthesis route to ttpy-(CH2)3-PO3H2

(denoted as ttpy-C3-PO3H2) ligand that has three methylene units as a bridge between the ttpy ligand and the phosphonic functionality (Scheme IV-9). In the last step, after the solvents were evaporated, there was still an unknown amount of HBr side product in the sample. This new ligand shows good solubility in DMSO. The ligand was characterized by mass spectroscopy and NMR ( 1 H, 31 P) spectroscopy before deposition on TiO2 NPs. Further details can be found in the Experimental Section.

Scheme IV-9. Synthesis route to ttpy-C3-PO3H2 ligand

 Synthesis of TiO2/Mn I dyad NPs

We first attempted to synthesize a new complex [Mn(ttpy-C3-PO3H2)(CO)3Br] by mixing Mn(CO)5Br and the ttpy-C3-PO3H2 together in the same way we synthesized the parent complex [Mn(ttpy)(CO)3Br]. However, this approach was not successful due to the poor solubility of the metal precursor in DMSO, and the presence of HBr that may protonate the tpy ligand. Working with mixed solvent like DMSO/diethyl ether or DMSO/acetone did not solve the issue. Therefore, we employed a two-step synthesis: first, the ligand was grafted on TiO2 NPs in DMSO solution, followed by the complexation reaction between Mn(CO)5Br

and TiO2/ttpy-C3-PO3H2 NPs in acetone (Scheme IV-10). A similar "complexation on surface" strategy has been realized for the immobilization of [Re(bpy)(CO)3Cl] on SiO2 NPs via amide linkages 28 , and the immobilization of [Mn(bpy)(CO)3Br] on either a metal-organic framework (MOF) 29 or a periodic mesoporous organosilica (PMO) 30 substrate. After each step, modified TiO2 NPs were washed with copious amounts of corresponding solvents to remove the excess amount of the ligand, metal precursor and HBr, prior to being dried under vacuum. Loading of Mn I species on TiO2 is estimated to be ~ 0.20 mmol.g -1 (around 3800 molecules per particle or 2 molecules per nm 2 ) by measuring the absorbance of washing solutions after each centrifugation step. It is important to note that in the second step of our procedure, acetone was used instead of diethyl ether because the ether was so volatile that it was completely evaporated during the necessary centrifugation steps after the grafting process.

Scheme IV-10. Synthesis route to TiO2/Mn I NPs  Synthesis of Ru II /TiO2/Mn I (Mn:Ru = 1:10) triad NPs

We prepared the Ru II /TiO2/Mn I triad with the ratio of Mn:Ru = 1:10 based on the optimization of the Mn:Ru molar ratio in the photocatalytic CO2 reduction described above.

The complex ratio was roughly controlled by using an amount of [RuP] 2+ that was 10 times greater than the amount of Mn(CO)5Br precursor. Further detail can be found in the Experimental Section. Since there are much less Mn species, grafting Mn first ensures that they can be better distributed on the TiO2 NPs. After the two-step synthesis of TiO2/Mn I NPs, Ru 2+ PS was then grafted as shown in Scheme IV-11. Loading of Mn I and Ru II complexes are roughly estimated to be 14 µmol.g -1 and 140 µmol.g -1 , respectively, by measuring the absorbance of washing solutions after each centrifugation step. The loading correspond to: (i)

for Mn I : 270 molecules per NP or 0.14 molecules per nm 2 , (ii) for Ru II : 2700 molecules per NP or 1.4 molecules per nm 2 .

Scheme IV-11. Synthesis route to Ru II /TiO2/Mn I NPs

IV.3.2. Infrared spectroscopy

IR characterization has been employed to prove that the Mn complex was successfully anchored on TiO2 by focusing on the CO stretching bands. It is important to note that the mode of coordination of the ttpy ligand to the Mn I metal ion depends on the solvent used in the complexation reaction. Acetone has been shown to form a tridentate homogeneous complex, [Mn( 3 -ttpy)(CO)2Br], 23 which can be distinguished from the bidentate one based on the missing of the peak at ~ 2040 cm -1 . In our case, since the peak at 2022 cm -1 is identical to the parent bidentate complex [Mn( 2 -ttpy)(CO)3Br], we conclude that the immobilized Mn I complex retains the bidentate linkage even though the "complexation on surface" step has been carried out in acetone. (

) [Mn 0 (ttpy-C3-PO3H2)(CO)2(MeCN)]  [Mn I (ttpy-C3-PO3H2)(CO)2(MeCN)] + e - Epa = -1.16 V 16
The first reduction process is centered on the ligand as for the complex in solution, and should lead to the release of Br -(reaction 14). Subsequently, an intramolecular charge transfer process induces the formation of Mn 0 complex (reaction 15). While the Mn 0 complex in solution undergoes dimerization after releasing CO (equation 7), we suppose that the immobilized Mn 0 complex on FTO stays on a monomeric form (equation 15). We have no experimental evidence for the release of CO when the complex is grafted on FTO. On the reverse scan, the oxidation peak at -1.16 V is attributed to the oxidation of the Mn 0 complex (equation 16).

Based on the first reduction peak at -1.43 V, a linear relationship between the cathodic peak current Ipc and scan rate v has been obtained (Figure IV-5b), proving the successful grafting of the Mn I complex onto the electrode. The surface coverage  of Mn I complex can be estimated to be Mn = 1.9 × 10 -10 mol.cm -2 (see Experimental Section for the calculation)

or about 1.1 molecules per nm 2 . This number is in line with the loading of Mn I on TiO2 NPs

(2 molecules per nm 2 ) shown in Section IV.3.1. The  value also suggests that the Mn I complex molecules stay sufficiently far from each other on the FTO surface, supporting our assumption that the dimerization should be prohibited.

b) Mn I complex grafted on TiO2 NPs

Afterwards, we investigated the CV of TiO2/Mn I NPs in solid state using a microcavity Therefore, the anodic peak at 0.86 V is assigned to the oxidation of Mn I metal center to Mn II (reaction 17) in a similar way to the homogeneous complex in solution (Eox1 = 0.62 V, Section IV.2.2, reaction 1), although the potential has been shifted to a considerably higher value than that of the free complex [Mn I ( 2 -ttpy)(CO)3Br] in solution.

The oxidation process is irreversible since the electron transfer reaction may lead to a ligand exchange process between Br -and MeCN solvent (reaction 18). The second oxidation peak associated with the fac to mer configuration transformation of the Mn complex and subsequent oxidation of the mer Mn I complex in solution are not observed for the FTO/Mn I electrode. A reason for this phenomenon could be the geometric constraints of the immobilized Mn species precluding the fac to mer transformation.

In the cathodic region of TiO2/Mn I (Figure IV-6b), the first reduction peak appears at -1.11 V associated with an oxidation peak on the reverse scan at -1.01 V. These two redox peaks are not observed for TiO2/ttpy-C3-PO3H2 NPs (Figure IV-7b), thus it must be due to the reduction of the Mn I complex. Similar to the FTO/Mn I modified electrode, we also assume that the formation of dimer should be prohibited once the complex has been anchored on a surface. Therefore, we attribute the two peaks at -1.11 V and -1.01 V to the formation and re-oxidation of [Mn 0 (ttpy-C3-PO3H2)(CO)2(MeCN)] species in the same way as FTO/Mn I electrode (reactions 14-15 and reaction 16, respectively).

IV.3.4. Photophysical properties a) UV-vis absorption spectroscopy

The UV-vis absorption spectra of TiO2/Mn I dyad and Ru II /TiO2/Mn I triad are shown in (dotted line) NPs. Loadings of Mn I complex on TiO2/Mn I and Ru II /TiO2/Mn I are estimated to be ~ 0.20 mmol.g -1 and 14 µmol.g -1 , respectively.

b) Emission spectroscopy

The emission spectra of TiO2/Mn I dyad and Ru II /TiO2/Mn I triad NPs have been

c) Transient absorption spectroscopy

After light excitation, the charge separated state Ru III /(e -)TiO2/Mn I is created. Transient absorption (TA) experiments have been carried out to study the absorption spectra of these transient species and the kinetics of charge recombination. Similar to Chapter 3, low concentration of the colloidal solution is required to avoid as much light scattering as possible. As a result, the full TA spectra from 400 nm to 700 nm need to be accumulated for at least 20 minutes to obtain a good signal-to-noise ratio. However, we encountered timeinduced precipitation phenomenon, thus the result is not reliable. Therefore, we only focus on the signal decay at 450 nm to compare with the signal of TiO2/Ru II at the same wavelength (Section II.3.6c). The decay at 450 nm for the Ru II /TiO2/Mn I triad is shown in It can be seen that both KWW parameters are very similar to those of TiO2/Ru II NPs.

The KWW value is far from 1, justifying the use of the stretch exponential function. The charge recombination kinetics can be calculated as follows 32 :

11 1 1 1 KWW cr KWW KWW KWW KWW KWW k k                                   (Eq IV-1)
The rate of charge recombination is then estimated to be ~ 150 s -1 , which is identical to that of TiO2/Ru II (140 s -1 ). This result suggests that the recombination of (e -)TiO2 and Ru III is not affected by the presence of Mn I complex on the surface, implying that the Mn I complex cannot be reduced by the electrons injected to TiO2. It is in agreement with thermodynamic considerations where E(Ti 4+ /Ti 3+ on TiO2) = -1.0 V and E(Mn I /Mn 0 ) = -1.43 V. The electron transfer between (e -)TiO2 and Mn I , if occurs, would be endergonic by 0.43 eV.

IV.3.5. Photocatalytic CO2 reduction

The photocatalytic CO2 reduction using Ru II /TiO2/Mn I triad NPs has been carried out in the same conditions as in Section IV.2.5. After 16 hours under irradiation at 450 nm, the triad photocatalyst produces HCOOH as the only product with the yield of 390 µmol.g -1 NPs, corresponding to TONmax (HCOOH) = 27 vs Mn I catalyst. The experiment was reproduced twice to confirm the result. Increasing the irradiation time to 60 hours results in less formic acid (TON = 25) due to its degradation. Although the total TON is lower than that of the free [Ru(bpy)3] 2+ and [Mn(ttpy)(CO)3Br] complexes in solution (TONtotal = 50), the selectivity of HCOOH over CO is significantly improved for the triad.

A control experiment using TiO2/Ru II instead of Ru II /TiO2/Mn I does not produce CO or HCOOH, which confirms that the Mn I complex acts as the catalyst. When TiO2/Ru II NPs and free Mn I complex are used, we obtained the similar results as for the free complexes in homogeneous solution. When SiO2/Ru II and free Mn I complex are used, the amounts of CO and HCOOH are significantly reduced. These experiments suggest that TiO2 has an active role in the photocatalytic mechanism, and the immobilization of the Mn I complex is related to the enhanced selectivity of HCOOH over CO. It is important to note that the Mn 0 -Mn 0 dimer, which has been proposed to be the photocatalytically active species for the [Mn(bpy)(CO)3Br] complex in solution 17 , was not able to formed when the Mn complex was anchored on the framework. The increased activity for UiO-67/Mn I was attributed to the isolation of the Mn I active sites on the framework, which stabilized the catalyst and prevented dimerization of the singly reduced Mn 0 species.

Scheme IV-12. Proposed mechanism for the photocatalytic CO2 reduction using UiO-67/Mn I catalyst in ref 29

Our results also highlight the role of semiconducting TiO2 NPs as a substrate compared to SiO2. In literature, Li and co-workers 28 immobilized [Re(bpy)(CO)3Cl] catalyst on SiO2

NPs via amide linkages for photocatalytic CO2 reduction in the presence of [Ru(bpy)3] 2+ PS.

The SiO2/Re I system only showed photocatalytic activity similar to the free complex in solution (TONmax (CO) ~ 12).

The proposed mechanism of the photocatalytic CO2 reduction shown in Scheme IV-8 requires that the Mn I complex needs to be reduced twice to form the catalytically active species Mn -I . For the mixture of [Ru(bpy)3] 2+ and [Mn(ttpy)(CO)3Br] complexes in solution, the reduction of Ru 2+* excited state by BNAH occurs first to yield Ru + species, which can reduce the Mn complex twice. In the case of Ru II /TiO2/Mn I triad, there are two reduction pathways to reduce Mn I to Mn 0 : (i) electron injection from Ru II* to TiO2 followed by electron transfer to Mn I , then Ru III is regenerated by BNAH (reactions 20, 21 and 22); or (ii) Ru II* is reductively quenched by BNAH first to form Ru I , then Ru I transfers an electron to Mn I via lateral transfer on surface and/or injects an electron to TiO2 (reactions 23, 24 and 25). Thermodynamically, the electron transfer between (e -)TiO2 and Mn I in the first pathway (reaction 21) is not favorable. The TAS results mentioned in Section IV.3.4 also show that the injected electrons on TiO2 CB are not consumed by Mn I species. Therefore, the second pathway is more feasible to reduce the Mn I complex. In this pathway, TiO2 seems to be inactive, which is in accordance with a previous publication 12 . TiO2 only acts as a substrate so that the Ru II PS and Mn I catalyst molecules are arranged sufficiently close to each other to enhance the charge transfer kinetics. Since the molar ratio of Mn:Ru is equal to 1:10, the chance that a Mn I molecule is surrounded by several Ru II molecules is expected to be high.

However, our experiments using (TiO2/Ru II + free Mn I ) and (SiO2/Ru II + free Mn I ) photocatalytic systems resulted in significantly less products for the latter system (Table IV-4), suggesting that TiO2 may have an active role but it is not well understood yet.

In contrast with the electrocatalytic CO2 reduction experiment, the second reduction step to generate Mn -I complex has not been observed for the grafted Mn complex on FTO by cyclic voltammetry. However, the photocatalytic active species can be the singly reduced

b) Ru II /TiO2/Mn I triad and BNAH as electron donor

In a previous study 21 on homogeneous Mn I complex, the first reduction step leads to Mn 0 -Mn 0 dimer which is EPR-silent due to the antiferromagnetic coupling of two Mn 0 (S = 1/2) species. In this study, since the Mn I complex has been anchored on TiO2 surface, the dimerization cannot occur, hence the expected singly reduced Mn 0 complex (S = 1/2) may be detectable with the EPR technique.

We carried out the EPR investigation of the Ru II /TiO2/Mn I triad in MeCN solution containing BNAH (0. It can be seen that the two samples exhibit two EPR lines at g = 1.990 and g// = 1.960 corresponding to trapped electrons in Ti 3+ sites. 33,34 As shown in Chapter 2, they are attributed to the injected electrons from Ru 2+* to TiO2. Based on the line at g// = 1.960, the amount of trapped electrons in both samples is about equal, which is due to the same amount of Ru II species and the same time of signal accumulation.

Beside the signal of Ti 3+ , an intense signal at 3441 G has been detected in both samples.

As it is similar in shape and position to the BNAH radical obtained under O2, we also attribute this signal to the BNAH radical. However, since there are no O2 in this case, the BNAH radical must be a consequence of photo-induced electron transfer reaction from BNAH to Ru II* excited state to form BNAH + and Ru I species. If Mn I is redox-inactive, the signal of BNAH radical should be identical in both cases due to the same amounts of Ru II and TiO2. In fact, the signal intensity for Ru II /TiO2/Mn I triad is significantly greater than for TiO2/Ru II dyad. Meanwhile, there are no reasons for the reductive quenching of Ru 2+* by BNAH to be accelerated in the triad. Therefore, the increase in intensity of the line at 3441 G may imply a superimposition of two paramagnetic species.

To further study the signal centered at 3441 G, pulsed EPR technique was employed. 55 Mn isotope (I = 5/2), which shows a quadrupolar interaction at 55 MHz. This interaction will spread the HYSCORE signal of Mn in Mn 0 complex on a very broad range, making it undetectable. Therefore, the presence of monomeric Mn 0 complex is not observed by the EPR technique. It is also important to note that the doubly reduced species [Mn -I (ttpy)(CO)2] -in homogeneous solution has been shown to exhibit a broad EPR line spread over ~ 3000 G 21 , which is characteristic of an unpaired electron on the metal center. However, this doubly reduced species is not observed in our study for Ru II /TiO2/Mn I triad. From the electrochemistry and EPR results, we propose that the grafted Mn I in the triad undergoes the one-electron reduction pathway, which has been proved for the analogous [Mn(bpy)(CO)3Br] complex anchored on the MOF named UiO-67 (Scheme IV-12). 29 In this scenario the catalytically active species is thought to be [Mn 0 (ttpy-C3-PO3H2)(CO)2] (Scheme IV-13)

where a coordination site is ready for the formation of a CO2 adduct 5 .

Scheme IV-13. Proposed catalytically active species [Mn 0 (ttpy-C3-PO3H2)(CO)2] for the Ru II /TiO2/Mn I triad

IV.4. Conclusion

In this chapter, firstly, the electrochemical properties of [Mn I ( 2 -ttpy)(CO)3Br] complex were investigated. The complex showed promising electrocatalytic activity for CO2 reduction with TONmax (CO) = 12, which could occur via one-or two-electron pathway mechanism. In the presence of [Ru(bpy)3] 2+ as a PS and BNAH as a sacrificial electron donor, the Mn complex could be reduced and the mixture showed photocatalytic activity for the CO2 reduction to produce both CO and HCOOH under irradiation at 470 nm. These results are in accordance with the analogous [Mn I ( 2 -ttpy)(CO)3(MeCN)] + complex which has been already

studied by former PhD students in the CIRe group.

Afterwards, a terpyridine-based ligand bearing a phosphonic acid group was synthesized and adsorbed on TiO2 NPs. The [Mn I ( 2 -ttpy-C3-PO3H2)(CO)3Br] complex on TiO2 (called TiO2/Mn I ) was obtained by coordination chemistry between Mn(CO)5Br

precursor and TiO2/ttpy modified NPs. The grafting process was proved by IR, UV-vis absorption as well as cyclic voltammetry. Subsequently, [Ru(bpy)3] 2+ was also immobilized on TiO2/Mn I NPs to form a Ru II /TiO2/Mn I triad. The ratio of Mn:Ru was chosen to be 1:10 as it was the optimal condition for the photocatalytic CO2 reduction using the two homogeneous complex in solution. The Ru II /TiO2/Mn I triad NPs showed good photocatalytic activity for the CO2 reduction: TONmax (HCOOH) = 27 over 16 hours of irradiation with 100 % selectivity for HCOOH. Although the total TON was lower than that of the two homogeneous complexes in solution (TONtotal = 50), the selectivity for HCOOH was greatly improved.

A reason for this improved photocatalytic behavior using the triad is thought to be related to the immobilization of Mn I catalyst. In homogeneous solution, the reduction of Mn I complex may follow the two-electron pathway: first, it undergoes monoelectronic reduction and dimerization to form Mn 0 -Mn 0 , which is subsequently reduced again to yield the catalytically active species Mn -I . When the Mn I complex is immobilized, the dimerization step is assumed to be prohibited, thus the monomeric paramagnetic Mn 0 species (S = 1/2) is expected. Unfortunately, its presence was not confirmed by EPR, possibly due to a very broad signal of the Mn 0 quadrupolar interaction in the HYSCORE spectrum. The Mn -I species were not observed by X-band EPR technique. Based on the electrochemistry and EPR results, we propose that the reduction of the immobilized Mn I complex in Ru II /TiO2/Mn I triad may follow the one-electron pathway without the formation of Mn 0 -Mn 0 dimer, hence the catalytically active species is assumed to be [Mn 0 (ttpy-C3-PO3H2)(CO)2]. Under visible light and in the presence of BNAH as an electron donor, Ru II is reduced to Ru I , followed by electron transfer from Ru I to Mn I to form the Mn 0 species. The electron transfer process probably occurs between neighboring Ru I and Mn I species on the surface of TiO2, and not through TiO2 conduction band. The specific role of TiO2 scaffold compared to SiO2 is not yet fully understood.

V.1. Introduction

Photoactive hybrid nanocomposites are promising materials for a wide range of applications from photocatalysis to photoelectrodes. Engineering the interface between organicinorganic components has important impact on the material properties, thus it is critical for every application. Notably, coating of an organic polymer on TiO2 nanomaterials may change its hydrophobic / hydrophilic property, stability or electrical conductivity. In the latter case, the use of pyrrole, vinyl or styrene monomers offer an advantage to easily control the polymerization process, which can be initiated by TiO2 band-gap excitation. The resulting material consists of TiO2 nanoparticles (NPs) embedded in a conducting polymer network.

This section is focused on the preparation methods of these materials, their characterization and applications after deposition on transparent electrodes in photo-to-current energy conversion.

Strandwitz et al. 1 studied the in situ photopolymerization of pyrrole in mesoporous TiO2 thin film. First, a layer of TiO2 (thickness ~ 0.5 µm) was dip-coated onto a Fluorine-doped Tin Oxide (FTO) electrode. The electrode was then immersed into an aqueous solution of pyrrole and methyl viologen (MV 2+ ) under 365 nm irradiation. This TiO2 band-gap excitation promotes electrons to the conduction band (CB) and leaving a hole on the valence band (VB).

The electron is then transferred to MV 2+ , while pyrrole is subsequently oxidized by the holes.

In turn, the oxidized pyrrole initiates the oxidative polymerization of pyrrole to form polypyrrole. The photopolymerization process and mechanism of pyrrole oxidative polymerization are summarized in Scheme V-1. Pore size between TiO2 NPs coated on FTO was monitored by N2 adsorption isotherms. After 24 hours of irradiation the average pore diameter slightly decreases from 4.7 nm to 4.3 nm, which equals to ~ 20 % pore filling by the polypyrrole film. This poorly efficient coverage is probably due to several reasons such as: (i) the competitive UV absorption of polypyrrole, thus blocking the light to reach TiO2; (ii) polypyrrole acting as an energy barrier for other pyrrole monomers in solution to be in contact with the electrode and then be oxidized; and (iii) decomposition of polypyrrole due to photocatalytically active TiO2 NPs. The aforementioned works requires UV light to initiate the photopolymerization process since TiO2 does not absorb visible light. However, such condition may eventually lead to destruction of the polymer as TiO2 can photodegrade organic materials under UV light. 3 Incorporating pyrrole and [Ru(bpy)3] 2+ photosensitizer (PS) allows the polymerization to occur under visible irradiation. One of the pioneering works in this field has been done by Deronzier et al. 4 The authors covalently linked the PS and pyrrole via alkyl chains, named

[Ru(bpy-pyr)3] 2+ (Scheme V-3). The complex can be electropolymerized and deposited on a Pt electrode in either MeCN or H2O solvent, although the polymerization kinetics is much slower in H2O. In air-saturated aqueous solution, the complex can be photopolymerized under visible irradiation ( > 405 nm), forming a film in less than 30 s. The film is electroactive in the anodic region in both MeCN and H2O solvents (with suitable electrolytes); however, it is not in the cathodic region in aqueous solution due to its hydrophobicity. The authors later proposed a mechanism 5 for the photopolymerization process (Scheme V-3). The critical steps are the reductive quenching of [Ru II (bpy-pyr)3] 2+* excited state by O2 to generate Ru III species (step 2), followed by an electron transfer between the pyrrole moiety and Ru II leading to a radical cation on pyrrole moiety (step 3). This radical then initiates the polymerization (step 4).

= -1.73, -1.93 and -2.16 (V) are assigned to the reduction of the complex localized on its three bpy ligands since they are in accordance with the [Ru(bpy)3] 2+ complex. 8 The weakly electron-donating -CH2-group on a bpy ligand shifts all the potentials to more negative values than that of [Ru(bpy)3] 2+ . Table V- nm ( = 13900 M -1 .cm -1 ) is similar to that of [Ru(bpy)3] 2+ at 450 nm ( = 13000 M -1 .cm -1 ) 8 .

After excitation at 450 nm, the [Ru-pyr] 2+ complex exhibits a broad emission spectrum with a maximum at 621 nm, which is characteristic of Ru 2+* 3 MLCT state. Both visible absorption and emission wavelength maxima are only slightly red-shifted compared to those of [Ru(bpy)3] 2+ . The shifts are in accordance with literature for a series of [Ru(bpy)3] 2+ linked to two or three pyrrole groups via alkyl chains 10 and show that the substitution only has a minor influence on the electronic structure of the ground state and excited state of the complex. Using the simplified Rehm-Weller equation (Equation V-2), the redox potential of grafted Ru III /Ru II* in TiO2/[Ru-pyr] 2+ is estimated at -1.16 V vs Ag/Ag + 0.01 M, which is equal to that of the free complex [Ru-pyr] 2+ in solution. The result confirms that the complex reduction power has been retained on the NPs. s -1 , which is comparable to that of TiO2/Ru II NPs (3.0 × 10 7 s -1 ). All the lifetimes and kinetic rates of TiO2/[Ru-pyr] 2+ are in line with those of TiO2/Ru II NPs (Table V-3) since the pyrrole moieties do not interact with [Ru(bpy)3] 2+* centers as previously observed. In the meantime, the rate of energy transfer process is only increased by 3 times, thus making it two order of magnitude lower than the injection rate in the nanocomposite obtained due to its broad EPR signal (see Section II.3.7). However, a radical is detected for pyrrole which suggests an efficient reduction of Ru 3+ by pyrrole moieties even at 20 K. EPR line width Hp-p has been shown to decline with temperature for an electrochemically synthesized film, staying at less than 0.15 G at 100 K. 14 In this case the large line broadening value (Hp-p = 9 G) suggests a decrease in polaron mobility due to polypyrrole immobilization. 15 In order to get more insight into the isotropic g value of 2.0037, DFT calculations have been conducted by Dr. Jean-Marie Mouesca (INAC/SyMMES) using the SAOP (Statistical Average of Orbital Potentials) exchange-correlation potential. 16 First, as in our system there is a long alkyl chain attached to the nitrogen atom of pyrrole, we performed g-value calculations for pyrrole and pyrrole-CH3 radical cations (Table V -5) to study the effect of the electrondonating methyl group. To simplify the task, we only examined monomer, dimer and trimer radical cations. The calculated g-values are presented in Table V-6. In both pyrrole and pyrrole-CH3 cases, the g-values of all the cations slightly increase as the polymer chain propagates. In literature, an electrochemically synthesized polypyrrole film also exhibits a slight increase in the g-value when the synthesis time is prolonged. 17 Nevertheless, the differences between pyrrole and pyrrole-CH3 cations are insignificant. It can be concluded that the introduction of the methylene group has negligible effects on the isotropic g-values of the cations. Since these g-values of pyrrole and pyrrole-CH3 cations cannot explain the experimental g = 2.0037, we attempted to investigate the pyrrole=O species (Table V-5). This species is a side product between oxygen and corresponding radical cations 17 (Scheme V-10). As the methylene group has almost no effects on the g-values of the cations, we chose the pyrrole=O molecule without the methylene group to study in order to reduce the calculation complication. Interesting, the g-value of the dimer cation of this species (2.00399, Table V-6) is very close to the experimental value. The result suggests that the EPR line centered at g = 2.0037 may be due to the formation of the pyrrole=O dimer as a side product. where k is the Boltzmann's constant, T the absolute temperature and  the solvent viscosity (0.341 mPa.s for MeCN at RT) 18 . The diffusion coefficient D can be calculated from the correlation function g(). For a monodisperse colloid this correlation function is given by:   V.4.5. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) is a surface sensitive technique which allows for qualitative and quantitative determination of elemental compositions on the surface. In principle it can detect all the elements with Z = 3 (Li) and above. The chemical state of an element, which depends on its oxidation state and its bonding with surrounding atoms, can be distinguished by change in binding energy.

In collaboration with Dr. Anass Benayad (LITEN, CEA-Grenoble), we have conducted a comparative XPS study for TiO2/Ru II , TiO2/[Ru-pyr] 2+ and TiO2/poly(Ru-pyr) NPs. The main purpose was to record any changes in N, Ru and P peaks before and after the photopolymerization and to quantitatively determine the atomic concentration of the elements of interest. TiO2/Ru II was used as reference to monitor spectral changes associated with the incorporation of the pyrrole function. Spectral calibration was based on the Ti2p3/2 peak at 457.9 eV, as this peak of TiO2 support remains unchanged for all the samples. Firstly, XPS survey spectra have been recorded for the three samples (Figure V-13).

The presences of Ti, O, N, C, and P elements are clearly observed. The Ru3d signal appears at the binding energy very close to that of C1s signal. Other elements are not detected. In an XPS study of polypyrrole, Joo et al. 14 have also shown that the C1s core level signal could be decomposed into three Gaussian lines. The line due to C-N linkages appeared at the binding energy higher than the line due to C-C linkages. The line at the highest binding energy was characterized by a vastly greater FWHM compared to the other two lines due to C-C and C-N linkages. It was attributed to disorders in the polypyrrole chains such as interchain connections and chain terminations. In our study, the line at ~ 287.5 eV remains almost similar in all three samples and its FWHM is comparable to the other peaks, it is not expected to be a consequence of the polymer chain disorders since the amount of the disorders should increase with respect to the irradiation time. Therefore, we attribute this line to the contaminated -C=O/COO containing species on the particle surface. molecules, while the one at 530.3 eV can be attributed to the oxygen atoms in phosphonate groups. The line at highest binding energy shows a very small contribution to the O1s signal and could be ascribed to hydroxyl groups on the TiO2 particle surface. 21 TiO2/[Ru-pyr] 2+ spectrum displays the same Gaussian lines after deconvolution. However, after photopolymerization only two lines at lower binding energies remain. The disappearance of the sub-oxides is explained by the coating polypyrrole network on the particle surface. The result for relevant elements is shown in Figure V-19. Firstly, it is noted that the Ru and P contents remain the same after photopolymerization (0.2 % and 0.9 % respectively), indicating the PS is stable in the experimental conditions. Ti content is also kept unchanged at ~ 19 %. Slight decreases in contents of N (bpy) from 1.6 % to 1.2 % and C (C-N) from 8.0 % to 6.4 % may suggest a small degree of decoordination of the bpy ligands and/or a buried photosensitizer inside polypyrrole. The decoordination of the bpy ligands from [Ru(bpy)3] 2+ has been known to occur under prolonged irradiation time. 22 However, taken into account the small growth in N (pyrrole) from 1.4 % to 1.6 % while the shape of Ru3d spectra is the same, the buried PS inside the polymer chains is probably the main reason for the declines in N (bpy) and C(C-C) contents during the photopolymerization. achieved by applying a bias between the cathode and anode immersed in a solution of charged particles in a polar solvent. Under the bias the positively charged particles migrate to the cathode while negatively charged ones are drifted to the anode. The method has found various applications in depositing charged particles onto a conducting substrate 23 or metal organic framework (MOF) thin films 24 for electronic component fabrication or surface coating. The deposition process can be conveniently controlled and reproducible by manipulating the electric field between the electrodes, the colloid concentration and the time of deposition. 23 In this study we found this method suitable for our positively charged nanoparticles / nanocomposites which could be coated on the cathode. Acetonitrile was chosen as solvent due to its rather large electrochemical active window, which makes it chemically inert in the presence of strong oxidants or reductants such as [Ru(bpy)3] 2+* , ppyr n+ and (e -)TiO2 species.

Moreover, the solvent should not have too high or too low dielectric constant to favor the EPD process. 23 The solvent should also be sufficiently polar to well suspend the NPs. The deposition was performed under Ar to avoid any undesired side reactions.

To prepare the TiO2/poly(Ru-pyr) nanocomposite-coated FTO electrode, we employed two strategies which are named after the sequential steps for a comparative study: contents remain almost the same while those of Ti and P markedly decrease. Since the XPS technique is highly surface sensitive to only a few nm depth while primary TiO2 NPs are ~ 25 nm in diameter, the result suggests that TiO2, together with the phosphonic anchoring group, may be buried inside the polypyrrole network. The increase in C (C-C bonds) content, which is present in the C11 chain linking [Ru(bpy)3] 2+ PS and pyrrole moiety, supports this explanation. Moreover, the EPD technique does not lead to the degradation of the complex despite a high applied electric field (2.4 × 10 3 V.m -1 ) during 1 hour of the deposition process. 10.5 µA.cm -2 , EPD-Photo: 7.2 µA.cm -2 ) compared with FTO/TiO2/Ru II electrode (5.5 µA.cm -2 ). The result proves the importance of using conducting polypyrrole layer for this light-to-electricity energy conversion application. It is noted that the Photo-EPD strategy allows for a photocurrent 30 % greater than the EPD-Photo strategy.

Photocurrent stability has also been investigated under prolonged visible irradiation (Figure V-32b). After 16 minutes, the electrode fabricated by the Photo-EPD method shows ~ 60 % decrease in current magnitude, while that fabricated by the EPD-Photo method exhibits only 40 % decrease. The latter case is similar to the case of FTO/TiO2/Ru II electrode.

Therefore, the Photo-EPD fabricated electrode allows for greater but less stable photocurrent than the EPD-Photo fabricated one. As the Photo-EPD method induces a less homogeneous surface, the aggregates may be desorbed more easily than the EPD-Photo electrode during the irradiation time. V.

Conclusion

The complex [Ru-pyr] 2+ where a [Ru(bpy)3] 2+ PS is covalently linked to two pyrrole units has been systematically characterized in solution for its electrochemical and photophysical properties. The complex can be electropolymerized onto an ITO electrode.

Immobilization of the complex on TiO2 NPs with phosphonic anchoring group has been achieved via a stepwise approach. First, bpy-PO3H2 ligand was allowed to chemically adsorbed on TiO2, before surface complexation between TiO2/bpy NPs and [Ru(bpy-pyr)2Cl2] was conducted. The resulting hybrid system, called TiO2/[Ru-pyr] 2+ , has also been characterized by electrochemical and photophysical methods. Under visible irradiation the [Ru(bpy)3] 2+ PS is excited and injects electrons to the CB of TiO2 with the rate of 2.4 × 10 7 s -1 , which is comparable to the system without pyrrole function, i.e. TiO2/Ru II .

In air-saturated MeCN solution, the TiO2/[Ru-pyr] 2+ hybrid NPs shows its ability to photopolymerize the pyrrole units under visible irradiation. The nanocomposite exhibits faster electron injection to TiO2 and energy transfer between grafted neighboring Ru II* species than TiO2/[Ru-pyr] 2+ NPs. XPS spectroscopy reveals that the PS conserves its coordination sphere after the polymerization and a polypyrrole film covers the particle surface. However, the Photo-EPD electrode exhibits a less stable photocurrent, which could be related to the desorption of the immobilized NPs from FTO substrate.

GENERAL CONCLUSIONS AND PERSPECTIVES

General conclusions

In this thesis, we used TiO2 nanoparticles (NPs) as a redox-active substrate to in solution to [Cr(ttpy)2] + in the presence of TEOA. The photoinduced electron transfer occurs through the CB of TiO2. TiO2 therefore acts as an electron relay between the Ru 2+* as an electron donor and Cr 3+ as an electron acceptor.

We also aimed to utilize the positive charge on the oxidized Ru 3+ in the photoinduced charge separated state (e -)TiO2/Ru 3+ for the oxidative polymerization of pyrrole. We synthesized a hybrid system containing TiO2 NPs as a scaffold and an immobilized of TEOA as a sacrificial electron donor. The enhanced photocurrent for the nanocomposite is attributed to the better conductivity and nanostructuration of the polypyrrole network.

In the case of [Mn(ttpy)(CO)3Br] anchored on TiO2/Ru II NPs, the injected electrons on TiO2 by Ru 2+* cannot be transferred to the Mn I sites because the reduction power of (e -)TiO2

is not sufficient to reduce Mn I . In the presence of a sacrificial electron donor like 1-benzyl-1,4-dihydronicotinamide (BNAH) and under visible light, the Ru 2+* excited state is first reductively quenched by BNAH to form Ru + , which has a stronger reduction power than Ru 2+* . Therefore, the Ru + can reduce Mn I to Mn 0 by a lateral electron transfer on the surface of TiO2 NPs and not through the CB of TiO2. Therefore, TiO2 may not participate in the electron transfer process leading to Mn 0 . We propose that the singly reduced Mn 0 complex is the catalytically active species on the Ru II /TiO2/Mn I triad for the photoreduction of CO2.

Photocatalytic CO2 reduction under visible light using the triad and BNAH as a sacrificial electron donor produces HCOOH as the only product with TONmax = 27 after 16 hours. The role of TiO2 scaffold in this photoinduced electron transfer process is not yet fully understood, however it seems to have a more active role than just a redox-inert scaffold like SiO2.

Perspectives

Different publications have emphasized the interest of using polypyridyl complexes of Cr(III) in photoredox catalysis. For instance, recent works have proved that [Cr(bpy)3] 3+based complexes can accelerate the Diels-Alder cycloaddition under visible irradiation and in the presence of O2, due to its high photooxidation power. 1,2 In this case, [Cr(ttpy)2] 3+ may be beneficial as the redox potential of its excited state (1.14 V vs Ag/AgNO3 0.01 M) is higher than that of [Cr(bpy)3] 3+ (0.89 V 3 ). Also, our work showed that the lifetime of [Cr(ttpy)2] 3+* excited state is long enough for electron transfer reactions. Moreover, with the phosphonic acid derivatization we can immobilize the complex on NPs for the easier separation of the catalyst after the reactions.

An older publication also reported the possible reduction of CO2 to HCHO by a polyvinyl film bearing [Cr(tpy)2] 3+ (tpy = 2,2':6',2"-terpyridine) units as an electrocatalyst. 4 In this thesis, we studied the electroreduction of H + to H2 from trifluoroacetic acid (TFA) in MeCN catalyzed by [Cr(ttpy)2] 3+ . It will be interesting to test the triad Ru 2+ /TiO2/Cr 3+ in the photocatalytic proton reduction as well. In order to regenerate the Ru 2+ PS an electron donor like TEOA or ethylenediaminetetraacetic acid (EDTA) is usually used in non-aqueous and aqueous media, respectively. However, they only work in basic solutions. Meanwhile, TFA is a relatively strong acid (pKa = 12.8 in MeCN 5 ). Therefore, another electron donor and/or proton source should be employed to drive the photocatalytic proton reduction using this triad.

In the photocatalytic CO2 reduction experiments using Ru II /TiO2/Mn I triad, the use of electron donor BNAH has several disadvantages as recently described. 6 Firstly, the reductive quenching of Ru II* excited state by BNAH is not total. Secondly, after formation of Ru I and BNAH + , a fast back electron transfer between them occurs which prevents the utilization of the charge on Ru I reduced species. Thirdly, (BNA)2, the oxidation product of BNAH, is more efficient than BNAH to quench the Ru II* excited state and can terminate the catalytic cycle.

Therefore, the photocatalytic result may be enhanced nanocomposite and deposited on an electrode for proton reduction. In this way, the loading of these catalysts on a geometrical electrode area should be higher than simply grafting them on a flat electrode surface.

if 1,3-dimethyl-2-phenyl-2,3-dihydro- 1H-benzo[d]-imidazole (BIH) or 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H- benzo[d]imidazole (BI(OH)H) ( Scheme

Chemicals

Solvents: Acetonitrile (CH3CN), dichloromethane (CH2Cl2) and ethanol (C2H5OH, all purchased from Fisher, HPLC grade), dimethyl sulfoxide (DMSO, Acros, anhydrous 99.7%), N,N-dimethylformamide (DMF, Acros, anhydrous 99.8%), ethylene glycol (Prolab), chloroform (CHCl3, Carlo Erba, HPLC grade), diethyl ether (Aldrich, 99.8%), hexane (95%), pentane (Carlo Erba), methanol (SDS anhydrous, analytical grade) and acetone (Aldrich, 99.5%) were used as purchased without any further purifications. Dry solvents were obtained by distillation under Argon. Distilled water was prepared with a Milli-Q system.

Reagents: all reagents have been used without further purifications, unless otherwise stated.

Nanoparticles (NPs): commercially available TiO2 (anatase, d < 25 nm, Aldrich), TiO2

(Degussa P25) and SiO2 (d < 20 nm, Aldrich) were used as received.

Others: Column chromatography was carried out on silica gel 60 (Merck, 70-230 mesh). Thin layer chromatography (TLC) was performed on plates coated with silica gel 60 F254.

Apparatus and techniques

Nuclear magnetic resonance (NMR): 1 H NMR, 13 C NMR and 31 P NMR spectra were recorded with a 400 or 300 MHz Bruker spectrometer at room temperature (RT). Chemical shifts in the 1 H NMR spectra were referenced to residual solvent peaks. Coupling constants (J) and the chemical shifts () were shown in Hz and ppm, respectively. The abbreviation for the characterization of the peaks are as follows: s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, dd = doublet of doublet, dt = doublet of triplet.

UV-visible absorption spectroscopy (UV-vis):

UV-vis spectra of a homogeneous solution or a colloid in a conventional quartz cell were recorded with a Varian Cary 300 or a MCS 500 UV-vis-NIR Zeiss spectrophotometer. To record spectra of a powder or a thin film on electrode, a Perkin Elmer Lambda 650 spectrometer and an integration sphere were used.

The NPs of interest were mixed with KBr, then a hydraulic press was used to make a pellet for the experiments.

Luminescence: Emission spectra were recorded with a Fluoromax-4 (Horiba Scientific) in a quartz cuvette with 4 transparent faces. Samples were purged with Ar for 15 minutes prior to experiment. The luminescent quantum yield  was calculated as follows: Time-resolved emission spectroscopy: Spectra were recorded with a time-correlated single photon counting (TCSPC) after the samples were excited at 400 nm by a picosecond Nd:YAG laser. The decay of [Ru(bpy)3] 2+* state at 610 nm was measured by a PicoHarp 300 TCSPC detector (PicoQuant). The fitting of the luminescent decays was performed with a FluoFit software (PicoQuant). In case of a colloid, light scattering was taken into account by subtracting the decay at 400 nm from the decay at 610 nm prior to the fitting.

Transient absorption spectroscopy: Spectra were acquired using a LP920K system (Edinburgh Instruments) equipped with a nanosecond Nd:YAG laser (Brilliant -Quantel).

Pump excitation of the sample was achieved by a third harmonic (355 nm) or a second harmonic (532 nm) module. A Xe900 pulsed Xenon lamp was used to generate a probe light.

The photons were dispersed using a monochromator, transcripted by a R928 (Hamamatsu) photo-multiplicator, and recorded on a TDS 3012C (Tektronix) oscilloscope. All samples were purged with Ar for 15 minutes prior to the measurement.

Electrochemistry: All electrochemical measurements were performed in a standard where Epa and Epc are anodic and cathodic peak potentials respectively.

The surface coverage  (mol.cm -2 ) of a redox complex grafted on an ITO or FTO electrode is calculated as follows:

Q nFA 
where Q (C) is the integrated area under the oxidation or reduction peak , n is the number of electron transfer, F = 96500 C.mol -1 the Faraday constant, and A (cm 2 ) the apparent electrode area.

Electrocatalysis: Experiments were carried out at RT under Ar (for proton reduction) or CO2/CH4 (95/5 v/v) mixture (for CO2 reduction) in a sealed conventional three-electrode cell.

The solvent was MeCN + TBAPF6 (0.1 M), to which water may be added in some CO2 reduction experiments. Reference and counter electrodes were Ag/AgNO3 0.01 M and Pt plate, respectively. Working electrode was a carbon plate (8 cm 2 ) which had been cleaned by diamond paste and washed with ethanol prior to experiment. In a normal setting with 3 electrodes and 10 mL solution, the headspace volume was measured to be 170 mL. During the experiments, at each time interval a 100 µL sample was taken from the headspace gas using a gas tight injection syringe. Gas products were analyzed with the gas chromatographs described below. At the end of the experiments, a liquid sample was taken for HCOOH analysis.

Quartz crystal microbalance with energy dissipation (QCM-D): All QCM-D experiments were run at a flow rate of 10 L/min, using SiO2-or Ti-coated quartz crystals (Q-Sense). All solutions were kept in a thermomixer (Ependorf) for a constant temperature of 21 0 C during the experiment. A pretreatment of the surface before measurement is required to improve the surface cleanliness. The SiO2-coated sensor was sonicated in a 2 % SDS aqueous solution for 20 minutes, washed with water, then exposed to UV-O3 treatment for 30 minutes.

The Ti-coated sensor was rinsed in Hellmanex 1 % solution for 30 minutes, washed with water, dried and rinsed in pure ethanol for 10 minutes, then exposed to UV-O3 treatment for 2 hours to oxidize the surface to TiO2. Finally, both sensors were rinsed again in ethanol for 20 minutes, dried under N2 before a QCM experiment.

Photocatalytic CO2 reduction: A glass tube (12.5 mL) was charged with a [Ru(bpy)3] 2+ photosensitizer complex and a catalyst in DMF/TEOA mixture (5 mL, CTEOA = 1 M). The PS and catalyst can be homogeneous or anchored on TiO2 NPs. The mixture was sonicated for a few minutes prior to be purged with Ar (20 mins) and then CO2/CH4 (95/5 v/v) mixture (20 mins). The dead volume was estimated at 7.5 mL. Afterwards, the tube was irradiated with a Xenon lamp (4 cm apart) in the presence of a UV filter and a 450 nm bandpass filter in order to selectively excite the PS. The light power is estimated to be 0.3 mW.cm -2 . During the experiment, at each time interval a 100 µL sample was taken from the headspace gas using a gas tight injection syringe. Gas contents were analyzed by GC which is described below. At the end of the experiment, a liquid sample was taken to analyze HCOOH content by HPLC.

Gas chromatography: Gas samples (100 µL) were taken from the headspace of the tube containing the reaction mixture. If the gas phase only contains H2, the analysis was run with a Perkin Elmer Autosystem XL gas chromatograph equipped with a 5 Å molecular sieve column (oven temperature = 32 0 C) and a thermal conductivity detector (TCD), using argon as the carrier gas. If the gas phase contains both CO and H2, the analysis was conducted with a Perkin Elmer Clarus 500 gas chromatograph equipped with a PDID detector and a 30 m Carboplt 1010 column (Antelia) and 560S mass spectrometer using a TurboMass 5.4.2 program. The gas contents were calculated against 5 % CH4 as internal reference in the gas phase. The results were compared to a standard gas mixture of CO, H2 and CO2 (Air Liquid).

Helium was used as the carrier gas. If the gas phase only contains H2, the analysis was run on Transmission electron microscopy (TEM): Experiments were conducted in collaboration with Dr. Jean-Luc PUTAUX (CERMAV) using a Phillips CM200 Cryomicroscope operating at 80 kV under RT and high vacuum. A small volume of colloid was deposited onto a glow-discharged copper grid and the solvent was evaporated.

X-ray photoelectron spectroscopy (XPS): Experiments were conducted in collaboration with Dr. Anass BENAYAD (LITEN/CEA Grenoble). The XPS analyses were performed with a ULVAC PHI 5000 VersaProbe II spectrometer using AlKα X-ray radiation (1486.6 eV).

The residual pressure inside the analysis chamber was 7 × 10 -8 Pa. A fixed analyzer pass energy of 23 eV was used for core level scans, leading to an overall energy resolution of 0.6 eV. Survey spectra were captured at a pass energy of 117 eV. All spectra were referenced against an internal signal, typically by adjusting the Ti2p 3/2 level peak at a binding energy of 457.9 eV. The XPS spectra were fitted using Multipak V9.1 software in which a Shirly background is assumed and the fitting peaks of the experimental spectra are defined by a combination of Gaussian (80%) and Lorentzian (20%) distributions.

Synthesis

General conditions:

In case of air-sensitive syntheses, a Schlenk technique or a glovebox was used. The flasks and other glassware were dried in oven overnight prior to the syntheses. Loadings of metal complexes on NPs were roughly estimated by measuring the UV-vis absorbance of supernatant solutions after centrifugation which indicates the amount of unadsorbed complexes.

Ligands and complexes

[Ru(bpy)2(dmbpy)](PF6)2. A mixture of [Ru(bpy)2Cl2] (52 mg, 0.10 mmol) and 4,4'dimethyl-2,2'-bipyridine (dmbpy, 20.3 mg, 0.11 mmol, 1.1 equiv) in ethylene glycol (anhydrous, 5 mL) was refluxed at 120 0 C in 2 hours under Ar. The solution color gradually changed from purple to dark red. After the mixture was allowed to cool to RT, an excess amount of saturated solution of KPF6 in water was added, resulting in immediate red precipitates. The powder was filtered, then solubilized in acetone to remove the excess KPF6 salt. Subsequently, the solution was evaporated to give red crude product. Small amounts of acetone and diethyl ether were used to solubilize the crude product and then evaporated several times to completely eliminate water. Finally a red fine powder was obtained (70 mg, 80 % yield). [Cr(ttpy-PO3H2)2](ClO4)3 ([Cr2P] 3+ ). In a glove box, ttpy-PO3H2 (30 mg, 0.077 mmol) was dissolved in 5 mL DMSO. To this was added CrCl2 (4.5 mg, 0.037 mmol) dissolved in 2 mL H2O. Upon addition, the solution turned dark purple in color. A further 40 mL of H2O was added and the solution stirred for 1 h at 25 0 C. Three spatula-fulls of LiClO4 were added but no precipitation was observed. The purple solution was removed from the glove box and stirred for 12 h at RT whilst exposed to air. The resulting dark yellow solution was filtered to afford the product [Cr2P] 3+ as a pale yellow solution. The solvent was evaporated under vacuum to afford a grey/yellow solid (25 mg, 60 %). [Cr(ttpy)Cl3]. The complex has been synthesized following reported procedures. 2,3 To 10 mL EtOH was added anhydrous CrCl3 (0.19 g, 1.17 mmol) then 4'-tolyl-2,2':6',2"-terpyridine (ttpy, 0.48 g, 1.49 mmol). The mixture was heated to reflux then granulated zinc (42 mg, 0.64 mmol) was added. The mixture color turned to khaki green. It was stirred vigorously for 12 h at 80 °C. The khaki green precipitate was filtered, washed twice with EtOH and dried under high vacuum (0.45 g, 79 %).1 H NMR cannot be done because the product is paramagnetic.

[Cr(ttpy)(OTf)3] (OTf = CF3SO3 -). The complex has been synthesized following reported procedures. 2,3 To [Cr(ttpy)Cl3] (0.20 g, 0.42 mmol) was added CF3SO3H (3.68 g, 24.5 mmol) giving a yellow/brown solution which was stirred for 12 h at RT. The reaction was cooled to 0 °C and 30 mL Et2O was added. The solid produced was separated by filtration, washed with Et2O and the resulting brown solid dried under high vacuum (0.21 g, 59 %). 1 H NMR cannot be done because the product is paramagnetic.

[Cr(ttpy)(ttpy-PO3H2)](PF6)3 ([Cr1P] 3+ ). The complex has been synthesized following reported procedures. 2,3 Precursors [Cr(ttpy)(OTf)3] (25 mg, 0.029 mmol) and ttpy-PO3H2 (10.5 mg, 0.027 mmol) were dissolved in 8 mL degassed MeCN plus 8 mL degassed DMSO and the brown solution was stirred at 80 0 C under an Ar atmosphere for 48 h. The solution was cooled to RT and an excess amount of KPF6 in water added. No obvious precipitate was formed, so the solution was filtered to remove any insoluble impurities prior to be used for grafting [Cr1P] 3+ onto ITO electrodes or TiO2 NPs.

ttpy-(CH2)3-PO3H2.

The synthesis requires three consecutive steps as follows:

(i) Step 1: Synthesis of [3-(4-formyl-phenoxy)-propyl]-phosphonic acid diethyl ester: 4-hydroxybenzaldehyde (200 mg, 1.637 mmol) was dissolved in 5 mL of acetonitrile before addition of K2CO3 (340 mg, 2.457 mmol, 1.5 equiv) and diethyl(3bromopropyl)phosphonate (0.35 mL, 1.802 mmol, 1.1 equiv). The reaction was heated at 50°C overnight. After cooling to RT, 50 mL of water were added and the solution was extracted with 50 mL of CH2Cl2 (3 times). The organic layer was washed with brine before being dried with MgSO4. After filtration and evaporation, the crude was purified by chromatography on silica gel with ethyl acetate as eluent to give 389 mg (1.295 mmol) of the pure compound as a white solid with 79 % yield. 190.8, 163.9, 132.1, 130.1, 114.8, 67.9, 67.7, 61.8, 61.7, 23,3, 22.6, 21.4, 16.6, 16.5. 31 (ii) Step 2: Synthesis of [3-(4-[2,2';6',2'']-terpyridin-4'-yl-phenoxy)-propyl]-phosphonic acid diethyl ester

The aldehyde (558 mg, 1.858 mmol) was dissolved in 12 mL of methanol before addition of 2-acetylpyridine (0.42 mL, 3.1716 mmol, 2 equiv), KOH (208 mg, 3.1716 mmol, 2 equiv) and 12 mL of NH4OH (30%). The reaction was heated at 50 °C during 3 days. After cooling to RT, the solvent was evaporated and the crude was purified by chromatography on silica gel. The elution started with AcOEt and went on with MeOH 2%, 5% before finishing with a mixture of AcOEt/MeOH/Et3N (94/5/1). The crude compound was dissolved in EtOH and precipitated with addition of water. After filtration, the compound was recrystallized twice with a mixture of AcOEt/Cyclohexane to give 209 mg (0.415 mmol) of a white powder with a 22 % yield. 9, 156.3, 155.7, 150.0, 149.0, 137.4, 130.9, 128.8, 124.0, 121.7, 118.7, 115.1, 67.7, 67.6, 61.8, 61.7, 23.2, 22.9, 21.8, 16.7, 16.6. 31 (iii) Step 3: Synthesis of [3-(4-[2,2';6',2'']-terpyridin-4'-yl-phenoxy)-propyl]-phosphonic

22%

To a solution of bromotrimethylsilane (TMSBr, 0.315 mL, 2.383 mmol, 10 equiv) in 5 mL of anhydrous CH2Cl2 under inert atmosphere, 120 mg of the phosphonic ester dissolved in 5 mL of CH2Cl2 was added dropwise in 5 minutes at RT. After two hours of reaction, 1 mL of a mixture acetone/water (1/1) was added. After one more hour, the solvent was evaporated and the crude was triturated in 20 mL of hot ethanol. After cooling to RT, the compound was filtrated and dried. A yellow powder was obtained (126 mg). The yield cannot be determined because of the unknown amount of HBr in the molecule. 2, 152.1, 151.2, 150.1, 146.7, 141.6, 128.6, 128.2, 126.0, 123.0, 119.2, 115.2, 67.8, 67.6, 24.6, 23.2, 22.8. 31

Nanoparticles and their surface modifications

SiO2 nanoparticles. The ultrasound-assisted synthesis of SiO2 NPs follows the Stober's method 5 with some modifications 6 . To a mixture of 50 mL ethanol and water was added 4 mL tetraethyl orthosilicate (TEOS), then sonicated for 2 h. Ammonia was added to the mixture at different feed rates to achieve >100 nm NPs (dropwise) or <10 nm NPs (0.05 mL.min -1 ). A milky gel was formed after 5 hours of sonication. It was separated from the solution by centrifugation, washed with ethanol, dried in an oven at 80 0 C overnight and calcined at 250 0 C for 2 hours to yield a white powder.

TiO2 nanoparticles. The synthesis of anatase TiO2 NPs follows a modified sol-gel process. 7 To a flask containing 50 mL pure ethanol was dropwise added 6 mL (0.02 mol) titanium isopropoxide (TTIP), then the mixture was vigorously stirred for 30 minutes to obtain a milky solution. A solution of 2 mL H2O/HCl (3/1, v/v) was added dropwise to the TTIP mixture.

After 2 hours the solution became clear, and allowed for peptization by refluxing it at 70 0 C for 24 hours, followed by aging at RT overnight. The nanoparticles were then separated and Abstract: This thesis aims to investigate the possibility of using TiO2 nanoparticles (NPs) as a platform to immobilize proximal coordination complexes that can interact with each other by photoinduced electron transfer. We have studied hybrid nanomaterials combining [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridine) as a photosensitizer and [Cr(ttpy)2] 3+ or [Mn(ttpy)(CO)3Br (ttpy = 4'-(p-tolyl)-2,2':6',2''-terpyridine) as electron acceptors. To immobilize the various complexes on the surface of TiO2, a phosphonic acid functional group was introduced on one of the bipyridines of the [Ru(bpy)3] 2+ center and on the terpyridines of the [Cr(ttpy)2] 3+ complex. Under visible light, the TiO2/Ru II colloid undergoes a photo-induced charge transfer process leading to a long-lived charge separation state (e -)TiO2/Ru III , which makes it possible to be engaged in successive oxidation or reduction reactions. In particular, the visible irradiation of the TiO2/Ru II colloid in the presence of [Cr(ttpy)2] 3+ and triethanolamine (TEOA) as a sacrificial electron donor allows the two-electron reduction of [Cr(ttpy)2] 3+ . Subsequently, the [Cr(ttpy)2] 3+ complex has been immobilized on the TiO2/Ru II NPs to form a Ru II /TiO2/Cr III assembly in which the photoinduced electron transfer processes were investigated. In order to propose a system for the photocatalytic reduction of CO2, the [Mn(ttpy)(CO)3Br] and [Ru(bpy)3] 2+ complexes were co-immobilized on TiO2 NPs following a chemistry on surface approach to form a Ru II /TiO2/Mn I triad. Under irradiation at 470 nm, this system exhibits excellent selectivity towards HCOOH as the only product of CO2 photoreduction in DMF/TEOA solvent mixture, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH) as a sacrificial electron donor. Another hybrid system linking a [Ru(bpy)3] 2+ unit to two pyrrole functions and being immobilized on TiO2 has also been synthesized and studied. Under visible light, the transient (e -)TiO2/[Ru-pyr] 3+ species induce the polymerization of pyrrole to form a TiO2/poly(Ru-pyr) nanocomposite. The nanocomposite deposited on an electrode generates, in the presence of TEOA, a stable anodic photocurrent of more than 10 μA.cm -2 . All the results show that TiO2 NPs can be used to associate different complexes in a close environment by limiting the interactions in the ground state but allow photoinduced electron transfer processes between them. Depending on the redox potentials of the different components, the electron transfer takes place either through the semiconducting NPs or on the surface.

Remerciement: Cette thèse a été financée par le LabEx grenoblois Arcane (ANR-11-Labx-003-01).

Figure I- 1 .

 1 Figure I-1. Representation of TiO2 crystal phases: (a) anatase: tetragonal, a = 3.785 Å, c = 9.513 Å; (b) rutile: tetragonal, a = 4.593 Å, c = 2.959 Å); (c) brookite: orthorhombic, a = 9.181 Å, b = 5.455 Å, c = 5.142 Å. Adapted from reference 2

Figure

  Figure I-2.

Figure I- 2 .

 2 Figure I-2. Positions of the upper edge of the valence band (green) and the lower edge of the conduction band (red) of several semiconductors in contact with aqueous electrolyte at pH 1. Adapted from reference 7

2 Figure I- 3 .

 23 Figure I-3. Representation of the photon absorption and emission of TiO2 in anatase and rutile phases. Straight and wavy lines indicate radiative and non-radiative decays, respectively. Adapted from reference 2

Figure I- 4 .

 4 Figure I-4. Molecular structure of [Ru(bpy)3] 2+ photosensitizer

Figure I- 5 .

 5 Figure I-5. Cyclic voltammogram (CV) of [Ru(bpy)3] 2+ in MeCN + 0.2 M TEAP (tetraethylammonium perchlorate) solution. Adapted from references 16,17

Figure I- 6 .

 6 Figure I-6. (a) Simplified molecular orbital diagram of octahedral Ru(II) polypyridine complexes with arrows indicating electronic transitions occuring in the UV-vis region. 18 (b) Simplified Jablonski diagram indicating radiative and non-radiative decays of the excited states of Ru(II) polypyridine complexes. 20 MLCT = metal-toligand charge transfer, MC = metal-centered transition, LC = ligand-centered transition, GS = ground state.

  kr and knr are the rate constants of radiative and non-radiative decays, respectively. The UV-vis absorption and emission spectra of [Ru(bpy)3] 2+ are shown in Figure I-7 together with proposed assignments for the electronic transitions. 17 In the absorption spectrum (Figure I-7a), the strong band at 285 nm is assigned to spin allowed LC    * transition. The two broad bands at 240 nm and 450 nm are attributed to the MLCT d   * transitions. The shoulders at 322 and 344 nm are assigned to the MC d  d transitions. Excitation of [Ru(bpy)3] 2+ in any of its absorption bands leads to the emitting 3 MLCT state. This emission is centered at around 600 nm where its energy, intensity and lifetime are dependent on the temperature and the solvent (Figure I-7b). In deaerated acetonitrile and at room temperature the lifetime of this luminescence is estimated to be around 1 µs with a quantum yield of em= 0.06.

Figure I- 7 .I. 2 . 3 .I. 2 . 3 . 1 .

 723231 Figure I-7. (a) Electronic absorption spectrum of [Ru(bpy)3] 2+ in alcoholic solution. (b) Emission spectrum of [Ru(bpy)3] 2+ in alcoholic solution at 77 K (solid line) and room temperature (dashed line). Adapted from reference 18 I.2.3. Quenching of the [Ru(bpy)3] 2+* excited state In the presence of suitable electron donors (D) and acceptors (A), the 3 MLCT excited state of [Ru(bpy)3] 2+ can take part in bimolecular reactions resulting in the quenching of its luminescence. The quenching reaction is governed by the nature of the quenchers. It can occur via an energy transfer reaction, oxidative or reductive quenching reaction as follows: [Ru(bpy)3] 2+* + Q  [Ru(bpy)3] 2+ + Q * Energy transfer [Ru(bpy)3] 2+* + Q  [Ru(bpy)3] 3+ + Q - Oxidative quenching [Ru(bpy)3] 2+* + Q  [Ru(bpy)3] + + Q + Reductive quenching

Figure I- 8 .

 8 Figure I-8. Illustration of the Forster and Dexter energy transfer mechanisms 23

Figure I- 9 .

 9 Figure I-9. Electronic configuration of the [Ru(bpy)3] 2+ complex in ground state and excited stateAfter calculation of the potentials of all the redox couples which could be involved, a first approximation of the exergonicity of the photo-induced electron transfer process can be evaluated using the derived equation by Weller:25

  Figure I-10 shows some structures mentioned in this publication. Table I-3 summarizes important adsorption parameters such as monolayer grafting density , adsorption constant Kads and free binding energy G.

Figure I- 10 .

 10 Figure I-10. Structures of some derivatives of phosphonic anc carboxylic acids anchored on TiO2 NPs 27Table I-3. Calculated adsorption parameters: monolayer grafting density , adsorption constant Kads and free binding energy G mentioned in reference 27

  using X-ray Absorption Spectroscopy. A [Ru(dcb)2(NCS)2] 2+ dye (Dye N3, Scheme I-2d) was anchored on 20-nm anatase TiO2 colloidal solution. The trapping sites, located on the surface, were shown to be completely populated within 70 ps. The lifetime of the trapped electrons was found to be slightly higher for dye-sensitized TiO2 NPs than bare TiO2, lying on a nanosecond time scale. Two types of surface trapping sites were distinguished as sixfold-and fivefold-coordinated Ti 3+ sites located at energies of 1.6 eV and 1.2 eV below the CB respectively.

  photoexcitation of the dye-sensitized film, the electron injection which occurred directly from the1 MLCT delocalized excited state was too fast to be observed, suggesting a sub-hundred femtosecond process. The injection from the 3 MLCT state of the attached bpy was also not detectable. The slower component of the injection occurred from the thermalized triplet excited state in a multiexponential fashion with time constants ranging from 1 to 50 ps. This slower process results from an excited state localized on a bpy ligand of the Ru(II) dye that is not attached to the TiO2 surface. This electron needs to be first transferred to the attached bpy ligand by interligand charge transfer (ILCT) and then injected to TiO2. ILCT efficiency is claimed to control the triplet channel of electron transfer from the dye. It can be changed by chemical modification of the bpy ligand and by the solvent. Scheme I-4 illustrates the discussed charge injection pathways from the N3 dye to TiO2 film. Therefore, localization of the initially photoexcited electron from the MLCT state and its orientation with respect to the semiconductor surface play an important role in the kinetics of electron injection.

  Scheme I-5. (a) Structure of the Ru II /MOx/CODH hybrid system: MOx is either TiO2 (P25), anatase TiO2, rutile TiO2, ZnO or SrTiO3 NPs; D is a sacrificial electron donor (2-(N-morpholino)ethanesulfonic acid. (b) CO production under visible irradiation using various MOx substrates. From a molecular approach, Durrant, Reisner et al. 49 have proposed to graft [Ru(bpy)3] 2+ as a PS and cobaloxime as a catalyst on TiO2 NPs (Scheme I-6) for proton reduction reaction. This reaction requires two electrons to produce H2. Using transient absorption spectroscopy and time-resolved emission spectroscopy, the authors showed that the electron transfer from the Ru II* excited state and Co III site occurs through the conduction band (CB) of TiO2. The long-lived charge separated state Ru III /(e -)TiO2/Co III (1/2 ~ 400 ms)allowed the Co III sites to be reduced by the CB electrons of TiO2. However, there were no experimental evidences for the accumulation of two electrons on the Co III site, although it

  Scheme II-1. Synthesis procedure for [Ru(bpy)2(bpy-PO3Et2)] 2+ ([Ru-PO3Et2] 2+ ) and [Ru(bpy)2(bpy-PO3H2)] 2+ ([RuP] 2+ )

  Scheme II-2. Synthesis of [Ru(bpy)2(dmbpy)](PF6)2

Figure II- 1 .

 1 Figure II-1. (a) UV-vis spectra and (b) emission spectra of [Ru(bpy)3] 2+ (black), [Ru(bpy)2(dmbpy)] 2+ (red) and [Ru-PO3Et2] 2+ (blue) in MeCN under Ar. Emission spectra were recorded in a fluorescent cuvette after 450 nm light excitation.

Figure II- 2 .

 2 Figure II-2. Time-resolved emission spectra of (a) [Ru(bpy)3] 2+* , (b) [Ru(bpy)2(dmbpy)] 2+* and (c) [Ru-PO3Et2] 2+* in MeCN under Ar. The samples were excited at 400 nm using a picosecond pulsed laser, while the emitted photons were collected at 610 nm using a TCSPC photometer. The red curves represent the monoexponential fitting of the decays

c)

  Transient absorption spectroscopy TAS was employed to study the UV-vis absorption of photoexcited [Ru-PO3Et2] 2+* species. Following the excitation by a nanosecond pulsed laser at 532 nm, the complex shows a bleaching at 450 nm which recovers to the initial value after ~2000 ns (Figure II-3a). The bleaching is associated with an increased absorbance at 370 nm. The TA spectra are characteristic of the photoexcited state [Ru III (bpy -)(bpy)(bpy-PO3Et2)] 2+* . The bleaching at 450 nm is fit with monoexponential function to yield a lifetime of 931 ± 5 ns (Figure II-3b).

Figure II- 3 .

 3 Figure II-3. (a) TA spectra of [Ru-PO3Et2] 2+ complex in Ar-saturated MeCN solution at various time intervals after the sample was photoexcited by a 532 nm nanosecond pulsed laser. The data points at 530 nm have been removed to avoid light scattering from the 532 laser. (b) Signal bleaching at 450 nm (black) fit with a monoexponential function (red) and the fitting residual.

Figure II- 4 ,

 4 Figure II-4, while the redox potentials are collected inTable II-2. Both [Ru(bpy)2(dmbpy)] 2+

Figure II- 4 .

 4 Figure II-4. Cyclic voltammograms of (a) [Ru(bpy)2(dmbpy)] 2+ and (b) [Ru-PO3Et2] 2+ (0.5 mM) in MeCN + 0.1 M TBAPF6. WE = C disk (d = 3 mm), CE = Pt, RE = Ag/AgNO3 0.01 M, v = 100 mV.s -1

Figure II- 5 .

 5 Figure II-5. EPR signals and simulations of (a) [Ru(bpy)3] 3+ and (b) [Ru(bpy)3] + produced by exhaustive electrolysis of 1 mM [Ru(bpy)3](PF6)2 in MeCN + 0.1 M TBAPF6. The signals were recorded with an X-band EPR spectrometer (f  9.65 GHz, 2 mW) at 10 K (signal a) and 60 K (signal b). They were accumulated in 80 s.

Scheme II- 3 .Figure II- 7 .

 37 Scheme II-3. Synthesis procedures of SiO2 NPs with various sizes

Figure II- 8 .

 8 Figure II-8. (a) TEM and (b) FE-SEM micrographs of SiO2 NPs with diameter < 10 nm

Scheme II- 4 .

 4 Scheme II-4. Synthesis procedures of TiO2 NPs with anatase and/or rutile phases In collaboration with Prof. Isabelle Gautier-Luneau (Institut Néel -Grenoble, CNRS), the morphology of the two TiO2 NPs were studied with X-ray diffraction (XRD) and compared with the standard values recorded in JCPDS No. 21-1272 (anatase), JCPDS No. 21-1276 (rutile) and JCPDS No. 29-1360 (brookite). XRD patterns of the two TiO2 samples are presented in Figure II-9. The sample annealed at 450 0 C shows XRD peaks at 2 = 25.3 0 ,

Figure II- 9 .

 9 Figure II-9. XRD patterns of TiO2 NPs annealed at (a) 450 0 C and (b) 700 0 C. The black and blue numbers indicate anatase and rutile lattices, respectively. The asterisks mark the peaks of the rutile phase.

Figure

  Figure II-10 shows the FE-SEM micrographs of the synthesized anatase and anatase/rutile TiO2 NPs. The primary particle size is estimated to be <10 nm for anatase TiO2 and 30-40 nm for anatase/rutile TiO2. The increased size of TiO2 NPs due to thermal treatment is in accordance with literature. 17 The aggregation of the NPs offers larger crystallite domains than individual particle size. The aggregation precludes statistical studies of size distribution.

Figure II- 10 .II. 3 . 2 .

 1032 Figure II-10. FE-SEM micrographs of TiO2 NPs annealed at (a) 450 0 C and (b) 700 0 C II.3.2. In situ observation of [RuP] 2+ dye adsorption on surface Before grafting the [RuP] 2+ photosensitizer on SiO2 and TiO2 NPs, we studied the dye adsorption on their analogous flat surfaces to determine the dye concentration, temperature and time required, as well as surface coverage. The adsorption is conveniently monitored by Quartz Crystal Microbalance with Energy Dissipation (QCM-D) technique, which is based on the change in vibrational frequency of the quartz crystal upon the adsorption of an analyte.

Figure II- 11 .

 11 Figure II-11. QCM-D responses for the adsorption of [RuP] 2+ complex on (a) SiO2 and (b) TiO2 coated sensors. Red and blue lines represent changes in frequency and dissipated energy, respectively, at various overtones. All experiments were run at 21 0 C.

Figure

  Figure II-12. (a) FT-IR spectra of: (1) [RuP] 2+ , (2) anatase TiO2, (3) TiO2/Ru II , (4) the subtraction of signal (2) from (3). (b) IR spectra of: (1) [RuP] 2+ , (2) SiO2, (3) SiO2/Ru II , (4) the subtraction of signal (2) from (3).

  anchoring group. For instance the CV of unprotected [RuP] 2+ in MeCN:H2O (95:5 v/v) mixture shows an irreversible reduction peak at -1.20 V (Figure II-13c) that is not present in the CV of protected [Ru-PO3Et2] 2+ complex (see Figure II-4b). In literature, when monolayers of n-alkane phosphonic acids are anchored on gold, a desorption peak was observed at -1.26 V vs SCE. 23 For [RuP] 2+ grafted on FTO, the reduction of phosphonate accompanied by an oxidation peak at -0.65 V shows better stability of the phosphonate group on metal oxide than gold.

Figure

  Figure II-13. (a) CV of FTO/Ru II electrode, recorded in MeCN + 0.1 M TBAPF6 under Ar, v = 100 mV.s -1 . (b) Ipa-v plot of Ru III /Ru II oxidation peak shows a linear relationship, R 2 = 0.999. (c) CV of [Ru-PO3H2](PF6)2 in

Figure II- 14 .

 14 Figure II-14. (a) CVs of SiO2/Ru II (solid line) and bare SiO2 (dotted line). (b) CVs of TiO2/Ru II (solid line) and bare TiO2 (dotted line) in the cathodic region. (c) CVs of TiO2/Ru II (solid line) and bare TiO2 (dotted line) in the anodic region. The CVs were recorded with a microcavity Pt electrode (d = 50 µm) in MeCN + 0.1 M TBAPF6 under Ar, v = 100 mV.s -1 .

Figure II- 15 .

 15 Figure II-15. CVs of bare graphite electrode (a) and a TiO2 film deposited on graphite electrode (b) at pH 6. The CVs are taken from reference 25

Figure

  Figure II-16. (a) Solid state UV-vis and (b) emission spectra of SiO2/Ru II and TiO2/Ru II NPs.

Figure II- 17 .

 17 Figure II-17. Time-resolved emission spectra of (a) SiO2/Ru II , (b) TiO2/Ru II and (c) Ru II /TiO2/BPA (10% Ru II and 90% BPA as spacer) in MeCN under Ar. The samples were excited by 400 nm picosecond pulsed laser, emitted photons were recorded at 610 nm using a TCSPC photometer. The red curves represent the monoexponential fitting of the decays

Figure

  Figure II-18. Overlap between the absorption (blue solid line) and emission (red broken line) spectra of

Section II. 2 .

 2 2c. The transient spectrum of SiO2/Ru II* 20 ns after excitation at 532 nm is shown in Figure II-19a. Signal bleaching at 450 nm is observed which is similar to the TA spectrum of [Ru-PO3Et2] 2+ free complex (see Section II.2.2). The bleaching is assigned to the absorption of the ground state of [Ru(bpy)3] 2+ core which has been depopulated upon absorption of 532 nm photons. Attempts to record the full TA spectra after the laser excitation are not successful due to precipitation of the NPs over a time period longer than 15 minutes.

Figure

  Figure II-19b shows the time profile of the absorbance change of SiO2/Ru II 20 ns after the laser excitation. The signal has been averaged from 40 measurements due to weak absorption of the NPs. The signal recovers to the initial value within 10 µs. The signal was fit with a monoexponential function to yield a lifetime of 3630 ± 480 ns. It is surprisingly four times longer than the lifetime of SiO2/Ru II* state recorded with the time-resolved emission spectroscopy (838 ns). However, this experiment is reproducible to give similar lifetimes regardless of the colloid concentration.

Figure II- 19 .

 19 Figure II-19. (a) TA spectrum of SiO2/Ru II NPs in MeCN under Ar 20 ns after excitation by a nanosecond pulse laser at 532 nm. The data point at 530 nm has been removed to eliminate the scattering of excitation light by the NPs. (b) Signal bleaching at 450 nm (black) fit with a monoexponential function (red) and the fitting residual. The signal has been averaged from 40 measurements.

  Figure II-20b presents the -ln(OD/ODmax) vs t plot and the fitting curve.

Figure II- 20 .

 20 Figure II-20. (a) TA spectrum of TiO2/Ru II NPs in MeCN under Ar 20 ns after excitation by a nanosecond pulse laser at 532 nm. Inset shows the decay at 450 nm. (b) -ln(OD/ODmax) versus t plot (black line) obtained at 450 nm and fit with a power function (red line).

Figure II- 21

 21 Figure II-21 presents its EPR signal, as well as the signal simulation of the trapped electrons on Ti 3+ anatase sites. The signal remains stable for a few minutes after the light is switched off. It is also reproducible when the sample is raised to room temperature, then frozen again and irradiated with the laser. The electron traps are characterized by two g values: g = 1.990 and g// = 1.959 as shown by the simulation in Figure II-21. The g values have been corrected with 2,2-diphenyl-1-picrylhydrazyl (DPPH) reference (g=2.0036). As a comparison, the EPR signal of naked anatase TiO2 NPs remains unchanged under the 455 nm irradiation. Therefore, the recorded signal is attributed to the trapped electrons on Ti 3+

  Figure II-21 one can observe the presence of additional four EPR lines (1:3:3:1 ratio) separated by 23 G, which are marked by asterisks. The lines are assigned to  CH3

Figure II- 21 .

 21 Figure II-21. EPR signal of photo-induced charge separation on TiO2/Ru II colloid (10 mg/mL) in MeCN glass (red line) and the simulation of trapped electrons on Ti 3+ sites of anatase TiO2 (black line). Asterisks show the signal of  CH3 radicals. The sample was irradiated in situ by a 455 nm LED. The spectrum was taken at 20 K with an X-band EPR spectrometer (f = 9.65208 GHz, 2 mW, 5 G modulation), accumulated for 30 minutes.

  Chapter 2 we have reported the study of a [Ru(bpy)3] 2+ -based photosensitizer bearing a phosphonic group to be grafted onto the surface of TiO2 and SiO2 NPs. The phosphonate-derivatized precursor [Ru-PO3Et2] 2+ has been extensively characterized in MeCN solution using electrochemistry, UV-vis spectroscopy, luminescent emission spectroscopy and time-resolved emission spectroscopy. The results show that it retains excellent properties of the prototype [Ru(bpy)3] 2+ photosensitizer after ligand modifications. Functionalization of NPs with [RuP] 2+ dye has been proved to be more efficient on TiO2 than SiO2, reaching the [RuP] 2+ loading of 0.21 mmol/g TiO2 and 0.076 mmol/g SiO2 NPs. The successful grafting of [RuP] 2+ on SiO2 and TiO2 surfaces has been proved by the QCM-D and FT-IR experiments. After being anchored, the [RuP] 2+ complex shows a similar oxidation potential as the [Ru-PO3Et2] 2+ free complex in solution. The wavelength of emission maximum is almost unchanged. The kinetics of visible light-induced electron transfer from Ru II* units to TiO2 and energy transfer from Ru II* to Ru II immobilized on the surfaces of TiO2 and SiO2 have been studied in colloids with time-resolved emission spectroscopy and TAS.

  Dans ce chapitre, les propriétés électrochimiques et photophysiques du complexe [Cr(ttpy)2] 3+ (ttpy = 4'-(p-tolyl)-2,2':6',2''-terpyridine) sont décrites. Différents états redox du complexe [Cr(ttpy)2] n+ (n = 3, 2, 1, 0) ont été étudiés par spectroscopie d'absorption UV-vis et spectroscopie par résonance paramagnétique électronique (RPE). Les calculs DFT confirment que toutes les étapes de réduction se produisent sur les ligands ttpy. Le complexe [Cr(ttpy)2] 3+ peut être doublement réduit sous la forme [Cr(ttpy)2] + par irradiation dans le visible en présence de triethanolamine (TEOA) comme donneur d'électron sacrificiel. La réaction est accélérée en ajoutant le [Ru(bpy)3] 2+ dans le milieu. La réaction de photoréduction à également lieu entre les nanoparticules hybride TiO2/Ru II et le complexe [Cr(ttpy)2] 3+ libre en solution en présence de TEOA. En outre, le complexe [Cr(ttpy)2] + doublement réduit présente une activité catalytique vis à vis de la réduction des protons en solution organique. Les complexes de [Cr(ttpy)2] 3+ seul ou avec le [Ru(bpy)3] 2+ ont aussi été immobilisées à la surface des nanoparticules de TiO2 pour former des systèmes de dyade hybride TiO2/[Cr2P] 3+ et de triade Ru II /TiO2/[Cr2P] 3+ . Contrairement aux systèmes précédents, la triade Ru II /TiO2/[Cr2P] 3+ ne permet pas de stocker des électrons sur les unités [Cr2P] 3+ sous irradiation continue en présence de TEOA. Cela peut être dû à une cinétique rapide du transfert d'électrons en retour entre les espèces transitoire [Cr2P] 2+ et Ru III greffées, à proximité en surface des nanoparticules.

The 4

 4 Scheme III-1. Simplified Jablonski energy diagram for octahedral Cr(III) polypyridyl complexes. IC = internal conversion, ISC = intersystem crossing, BISC = back intersystem crossing, F = fluorescence, P = phosphorescence. Solid arrows indicate processes involving photon absorption or emission, while dotted arrows show processes that do not involve the photon absorption and emission. Adapted from reference 2

  Scheme III-2. Generic scheme for a photo-induced, sequential charge transfer process leading to multiple charge accumulation in D-PS-A triad. Solid blue and black arrows indicate transformations involving a photon or an electron, respectively. Dashed arrows indicate possible charge recombination pathways. Adapted from reference 9

Scheme III- 4 .

 4 Scheme III-4. Multiple oxidative equivalents accumulated on single molecular site for photocatalytic water oxidation: (a) [Rua-Rub-OH2] 4+ PS-catalyst assembly immobilized on mesoporous TiO2 thin film electrode mentioned in reference 12; (b) Rua/TiO2/Rub-OH2 PS/TiO2/catalyst assembly (Rua:Rub = 6.4:1 % mol) mentioned in reference 14.

  Scheme III-6. Structure of Ru II /TiO2/Co III (Ru:Co = 3:1 % mol) mentioned in reference 18

Scheme III- 7 .

 7 Scheme III-7. Synthesis procedure for [Cr(ttpy)2] 3+ complex

Figure III- 1 .

 1 Figure III-1. CVs of [Cr(ttpy)2] 3+ (1 mM, black line) and ttpy ligand (1mM, green line) recorded in MeCN + 0.1 M TBAPF6. WE = C disk (d = 3 mm), v = 100 mV.s -1

Figure III- 2

 2 Figure III-2 shows the  and  spin numbers of these complexes. In all the cases the  spin is ~3 while added electrons are sequentially localized on the two tpy ligands. Therefore, the authors concluded that the oxidation state of Cr remains +3. For the neutral, triply reduced complex [Cr(tpy)2] 0 (Figure III-2d), since the Cr III is not reduced, the third electron is thought to be delocalized over two reduced (tpy -) ligand. This could be a reason for a relatively facile reduction step from [Cr(tpy)2] + to [Cr(tpy)2] (E1/2 = -1.36 V). When the fourth electron is introduced (Figure III-2e), all the electrons are localized on the tpy ligands.However, the two ligands differ in spin state: one singlet (tpy 2-) 2-and one triplet (tpy  ) 2-.

Figure III- 2 .

 2 Figure III-2. Calculated Mulliken spin density distribution in [Cr(tpy)2] n+ complexes: (a) n = 3, (b) n = 2, (c) n = 1, (d) n = 0 and (e) n = -1. Adapted from reference 5

Figure III- 3 .

 3 Figure III-3. UV-vis absorption spectra of various oxidation state of [Cr(ttpy)2] n+ : (a) n=3, (b) n=2, (c) n=1, (d) n=0. Spectra were recorded by exhaustive electrolysis of [Cr(ttpy)2] 3+ in (a, b and c) MeCN + 0.1 M TBABF4 or (d) MeCN + 0.1 M TBAClO4 under Ar at suitable potentials

Figure III- 4 .

 4 Figure III-4. Calculated electronic transitions for [Cr(ttpy)2] 2+ singly reduced complex. GS = ground state For the [Cr(ttpy)2] + doubly reduced complex, [Cr III (ttpy -)2] 2+ , calculations have been focused on the transition at 767 nm, which can be assigned to the experimental absorption peak at 723 nm. Electronic transitions at this wavelength are shown in Figure III-5. The calculated transition is a combination between processes (a) and (b) with equal contribution. In the [Cr(ttpy)2] + ground state, the two electrons are localized on the two tpy ligands and equally distributed between the tpy (a) and tolyl (b) parts. The process (a) corresponds to a fully LMCT transition: (tpy)  d. The process (b) corresponds to the intraligand CT

Figure III- 5 .

 5 Figure III-5. Calculated electronic transitions for [Cr(ttpy)2] + doubly reduced complex.

Figure III- 6 .

 6 Figure III-6. UV-vis absorption spectrum (solid line) and emission spectrum (dotted line) of [Cr(ttpy)2] 3+ complex in MeCN under Ar. The redox potential of excited state E(Cr III* /Cr II ) can be estimated by assuming an energy diagram similar to that of [Cr(diamine)3] 3+ complexes as shown in literature. 2 The diagram is shown in Scheme III-8.

4 Figure III- 7 .

 47 Figure III-7. Decay of [Cr(ttpy)2] 3+* excited state recorded at 770 nm in MeCN under Ar after excitation at 400

Figure

  Figure III-8. (a) TA spectra of [Cr(ttpy)2] 3+ complex in MeCN under Ar at different time intervals after laser excitation at 355 nm. Arrows indicate temporal spectral changes. (b) TA decay at 550 nm. Excitation was done by nanosecond laser pulses at 355 nm. Red line represents a monoexponential fitting with lifetime of (295 ± 4) ns.

  original complex [Cr(ttpy)2] 3+ is reduced. All samples have been prepared by exhaustive electrolysis of [Cr(ttpy)2] 3+ (1 mM) in MeCN + 0.1 M TBAPF6 solution under Ar at suitable potentials: -0.7 V for n = 2, -1.1 V for n = 1, -1.6 V for n = 0. The spectra of [Cr(ttpy)2] 3+ and [Cr(ttpy)2] + detected with an X-band EPR spectrometer are shown as black lines in Figure III-9.

Figure III- 9 .

 9 Figure III-9. EPR spectra of [Cr(ttpy)2] n+ at various oxidation states: (a) n = 3, (b) n = 1 which were produced by exhaustive electrolysis of 1 mM [Cr(ttpy)2] 3+ in MeCN + 0.1 M TBAPF6. They were recorded with an X-band EPR spectrometer (f  9.6553 GHz) with following parameters: (a) 20 mW, 10 K, modulation: 10 G, accumulation time: 50 mins; (b) 0.02 mW, 20 K, modulation: 2 G, accumulation time: 70 mins. Red line represents the simulation of [Cr(ttpy)2] + complex.

  order to study the reactivity of [Cr(ttpy)2] 3+* excited state, we have carried out the exhaustive photoreduction experiment using an excess amount of TEOA (300 equivalents) under continuous visible irradiation. Triethanolamine (TEOA) is known as a sacrificial electron-donating quencher. Changes in UV-vis absorption spectra during the irradiation period are shown in Figure III-10.

Figure

  Figure III-10. UV-vis absorption spectra of [Cr(ttpy)2] 3+ complex in Ar-saturated MeCN solution containing 300 equivalents of TEOA during 400 s (a) and 200 min (b). They were taken at different time intervals under continuous visible irradiation (400 nm <  < 750 nm). Arrows indicate temporal spectral changes during the irradiation. Inset in graph (a) shows temporal changes in the absorbance at 800 nm, which represents the formation of [Cr(ttpy)2] 2+ species. Inset in graph (b) shows temporal changes in the absorbance at 570 nm, which represents the formation of [Cr(ttpy)2] + species. During the first 400 s (Figure III-10a), the peaks at 800 nm and 500 nm are gradually formed and increased while the band at ~ 370 nm decreases. These changes corresponds to the [Cr(ttpy)2] 3+  [Cr(ttpy)2] 2+ reduction. The inset shows temporal change in absorbance at 800 nm. A plateau is observed after ~250 s, indicating a complete chemical transformation.Taking into account the absorbance of the newly formed [Cr(ttpy)2] 2+ at 800 nm as its characteristic peak, we estimate the transformation to occur with a quantitative yield.Meanwhile, a blank experiment has also been carried out in dark, which shows no changes in the absorption spectrum of [Cr(ttpy)2] 3+ .

  Upon the TEOA addition, the emission maximum of [Cr(ttpy)2] 3+* is gradually reduced (Figure III-11a) under continuous light excitation at 360 nm. The peak position and shape remain unchanged. When a picosecond pulsed laser was used to excite the sample, the lifetime  of [Cr(ttpy)2] 3+* was found to decline. A plot of 0/ vs [TEOA] is shown in Figure III-11b. The experimental data is fit with the Stern-Volmer equation 25 :

where 0 

 0 and  are the lifetime of [Cr(ttpy)2] 3+* in the absence and presence of TEOA quencher respectively (0 = 295 ns from the TAS result), KSV the Stern-Volmer constant and kq the quenching constant. The linear fitting is shown as the red line in Figure III-11b.

Figure

  Figure III-11. (a) Quenching of the emission of [Cr(ttpy)2] 3+ (1.3 × 10 -5 M) excited state upon increasing concentration of TEOA in Ar-purged MeCN solution. The sample was excited at 360 nm. (b) Stern-Volmer plot for the quenching of [Cr(ttpy)2] 3+* by TEOA. The sample was excited by a 400 nm picosecond pulsed laser and emitted photons were recorded at 770 nm.As predicted by thermodynamics, TEOA reductively quenches the [Cr(ttpy)2] 3+* excited state. Since the UV-vis absorption spectrum of TEOA and the emission spectrum of [Cr(ttpy)2] 3+* do not overlap, the quenching pathway should be only via electron transfer.From the Stern-Volmer plot one can extract KSV = 850 M -1 and hence kq = 3 × 10 9 M -1 .s -1 . The high quenching constant indicates that TEOA is a good quencher for the [Cr(ttpy)2] 3+* state.

[

  Ru(bpy)3] 2+ PS as it possesses great properties of a PS: high absorbance in visible region ( = 13000 M -1 .cm -1 at 450 nm), strong emission and long lifetime ( = 608 ns in MeCN under Ar, see Chapter 2). Moreover, its photochemical and redox properties have been well established. 27 The photoreduction experiment of [Cr(ttpy)2] 3+ has been performed in MeCN solution under Ar in the presence of [Ru(bpy)3] 2+ as additional PS and TEOA as sacrificial electron donor. Temporal UV-vis spectral changes are shown in Figure III-12a during the irradiation period. First, the 800 nm band is formed (black arrow) associated with a peak at 500 nm, which is characteristic of [Cr(ttpy)2] 2+ formation. This step seems to finish within 10 s where the next absorption spectrum is taken, showing a very fast rate. Without the [Ru(bpy)3] 2+ PS the [Cr(ttpy)2] 2+ formation is accomplished only after 250 s under the same irradiation conditions (Figure III-10a).

1

  MLCT absorption maximum of [Ru(bpy)3] 2+ PS, remains almost unaffected. The experiment clearly demonstrates that [Cr(ttpy)2] 3+ complex can be doubly reduced to [Cr(ttpy)2] + in the reaction conditions, and the [Ru(bpy)3] 2+ PS has been well regenerated by the sacrificial agent TEOA. Prolonged irradiation does not lead to the formation of [Cr(ttpy)2] 0 species.

Figure

  Figure III-12b presents the temporal absorbance change at 720 nm (red line), which is the UV-vis signature of the [Cr(ttpy)2] + species. A plateau has been reached after about 4000 s, signaling the double reduction reaction has been completed with 100 % conversion for the [Cr(ttpy)2] 3+ to [Cr(ttpy)2] + transformation. Without the [Ru(bpy)3] 2+ PS, the reaction has been completed after ~ 10000 s with 80 % conversion. Therefore, the use of this PS accelerates the photoreduction reaction as expected.

Figure

  Figure III-12. (a) UV-vis absorption spectra of a mixture of [Ru(bpy)3] 2+ (4 × 10 -5 M), [Cr(ttpy)2] 3+ (2 × 10 -5 M) and TEOA (1.5 × 10 -2 M) in Ar-saturated MeCN solution. The spectra were taken every 100 s under continuous irradiation at 400 nm <  < 750 nm. Arrows indicate the formation of the [Cr(ttpy)2] + species. (b) Comparison of temporal changes in the absorbance at 720 nm in the presence (red line) and absence (black line) of [Ru(bpy)3] 2+ complex.As the irradiation light is between 400 and 750 nm where both [Ru(bpy)3] 2+ and [Cr(ttpy)2] 3+ complexes absorb the light, hence both are excited. The thermodynamically possible electron transfer reactions between the two complexes are as follows:

  irradiation has been monitored by spectrophotometry. Changes in UV-vis absorption spectra are shown in Figure III-13.

Figure

  Figure III-13. (a) UV-vis absorption spectra of a mixture of [Cr(ttpy)2] 3+ and PPh3 (1000 equivalents) in Arsaturated MeCN solution. The spectra were taken every 60 s under irradiation at 400 nm <  < 750 nm. Arrow shows the disappearance of the [Cr(ttpy)2] 3+ complex. (b) Zoom-in from 400 nm to 900 nm to show the formation of the [Cr(ttpy)2] 2+ complex indicated by arrows.During the irradiation, the absorption maximum of [Cr(ttpy)2] 3+ complex at 370 nm gradually decreases accompanied by an increase of a peak at ~ 500 nm and a broad band centered at 800 nm. These changes correspond to the formation of the [Cr(ttpy)2] 2+ species.The experiment can be explained by the following reaction:

[

  Cr(ttpy)2] 2+* by PPh3. Therefore, in our study the reaction 1 between [Ru(bpy)3] 2+* and [Cr(ttpy)2] 3+ is probably the main quenching mechanism in our study as it is more favorable than reaction 2 in terms of both thermodynamics and kinetics.As another approach to confirm the reaction 1, we have conducted a flash photolysis experiment for the mixture of [Cr(ttpy)2] 3+ and [Ru(bpy)3] 2+ complexes. Transient absorption (TA) spectral changes are shown in Figure III-14a after the sample has been excited at 532 nm where only [Ru(bpy)3] 2+ species absorb. A negative OD value is observed at ~ 450 nm which corresponds to the absorption maximum of [Ru(bpy)3] 2+ ground state. A concomitant positive OD band centered at ~ 520 nm is recorded, which can be due to the absorption of the photogenerated [Cr(ttpy)2] 2+ complex (see Figure III-3). After 1700 ns all the OD values return to zero. Therefore, the spectral changes are attributed to the back electron transfer reaction between the transient [Cr(ttpy)2] 2+ and [Ru(bpy)3] 3+ species leading to [Cr(ttpy)2] 3+ and [Ru(bpy)3] 2+ .

FigureFigure

  Figure III-14. (a) TA spectra of [Cr(ttpy)2] 3+ and [Ru(bpy)3] 2+ mixture (molar ratio = 1:1) in MeCN solution under Ar at different time intervals after the sample was excited by a 532 nm nanosecond pulsed laser. (b) Plot of 1/OD vs  at 520 nm (red), fit with a linear function (black).

  Pathway 1: [Ru(bpy)3] 2+* state is first quenched by [Cr(ttpy)2] 3+ (oxidative quenching) -Pathway 2: [Ru(bpy)3] 2+* state first quenched by TEOA (reductive quenching)(Note : E(Ru 3+ /Ru 2+ ) = 0.94 V, E(Ru 2+ /Ru + ) = -1.67 V, E(Ru 2+* /Ru + ) = 0.37 V, E(Ru 3+ /Ru 2+* ) = -1.10 V, E(Ru 3+ /Ru 2+* ) = -1.10 V, E(Cr 3+ /Cr 2+ ) = -0.47 V, E(Cr 2+ /Cr + ) = -0.85 V, Epa(TEOA + /TEOA) = 0.42 V vs Ag/AgNO3 0.01 M). For the formation of [Cr(ttpy)2] 2+ , the two proposed pathways are concerned with whether the first quenching step of [Ru(bpy)3] 2+* excited state is by [Cr(ttpy)2] 3+ (pathway 1)or TEOA (pathway 2). Comparing the step 2 of these pathways, it is clear that the quenching reaction by [Cr(ttpy)2] 3+ is thermodynamically more favourable as the quenching reaction by TEOA is slightly endergonic. The quenching rate of [Ru(bpy)3] 2+* by [Cr(ttpy)2] 3+ is very high (kq = 3 × 10 9 M -1 .s -1 ) as mentioned above. Meanwhile, the reductive quenching of [Ru(bpy)3] 2+* by TEOA is not detected.32 Together with our calculated G values, we conclude that the pathway 1 is more favorable than pathway 2 in terms of both thermodynamics and kinetics.In the pathway 1, after the formation of [Cr(ttpy)2] 2+ , the back electron transfer reaction between [Cr(ttpy)2] 2+ and [Ru(bpy)3] 3+ (step 3, G3 = -1.41 eV) is even more favorable than the quenching reaction (G2 = -0.63 eV). This back electron transfer step has been proved to be very fast by the flash photolysis experiment mentioned above. Since we observe the net formation of [Cr(ttpy)2] 2+ and then [Cr(ttpy)2] + , we assume that TEOA efficiently scavenges the [Ru(bpy)3] 3+ molecules (step 4) to prevent this back electron transfer reaction. In literature, the reduction rate of [Ru(bpy)3] 3+ by TEOA is between 6 × 10 6 M -1 .s-1, 33 or 2 × 10 7

  photoreduction of [Cr(ttpy)2] 3+ occurs via the pathway 1 where the first reduction step is the reductive quenching of [Ru(bpy)3] 2+* by [Cr(ttpy)2] 3+ . The back electron transfer reaction between photogenerated species [Ru(bpy)3] 3+ and [Cr(ttpy)2] 2+ is short circuited by the excess amount of TEOA, which efficiently reduces [Ru(bpy)3] 3+ to regenerate the [Ru(bpy)3] 2+ PS.

Figure III- 16 .

 16 Figure III-16. (a) CV of 1 mM [Cr(ttpy)2] 3+ in the presence of TFA (40 equivalents, 40 mM) (red line) and CV of 40 mM TFA alone (grey line). (b) CVs of 1 mM [Cr(ttpy)2] 3+ upon the addition of TFA. Experiments were carried out in MeCN + 0.1 M TBAPF6 solution under Ar. WE = C disk (d = 3 mm), v = 100 mV.s -1 In order to study the change in electrochemical behavior of the [Cr(ttpy)2] 3+ complex in more detail, we recorded its CVs in the presence of different amounts of TFA (Figure III-16b). The first and second redox waves remain unchanged, suggesting completely reversible reduction until the [Cr(ttpy)2] + complex. After that, two new irreversible reduction waves is formed around -1.0 V and -1.2 V, which may indicate the formation of an intermediate for the electrocatalytic process. The new peak is associated with a complete disappearance of the third redox wave. When the amount of TFA is more than 20 equivalents, the new reduction peak at -1.0 V seems to merge with the reduction peak of Cr 2+ /Cr + couple.

Figure III- 17 .

 17 Figure III-17. Electrocatalytic proton reduction by [Cr(ttpy)2] 3+ (0.5 mM) in the presence of TFA (50 eq) as proton source at the bias of -1.20 V. Solution: 0.1 M TBAPF6 in MeCN, WE = C plate (8 cm 2 ).

  )2] + complex by exhaustive electrolysis of the [Cr(ttpy)2] 3+ complex in MeCN, then added different amount of TFA. The samples were characterized by EPR spectroscopy and UV-vis spectroscopy (Figure III-18). Upon the addition of 1 equivalent of TFA, the EPR signal of [Cr(ttpy)2] + is gradually reduced by half. Up to 50 equivalents of TFA the [Cr(ttpy)2] + EPR signal completely disappears. No other signals have been found in other region of the X-band spectrum. The result simply suggests the consumption of [Cr(ttpy)2] + species to form EPRsilent species. It is important to note that the [Cr(ttpy)2] 2+ in solution is EPR silent although its spin number is S = 1. The changes in the UV-vis spectra of [Cr(ttpy)2] + species upon the TFA addition correspond to the formation of [Cr(ttpy)2] 2+ . The EPR and UV-vis spectra suggest that during the reduction of H + to H2, the [Cr(ttpy)2] + species are oxidized to [Cr(ttpy)2] 2+ rather than [Cr(ttpy)2] 3+ .

Figure III- 18 .

 18 Figure III-18. (a) Changes in the EPR spectrum of [Cr(ttpy)2] + upon addition of TFA: no TFA (black), 0.5 eq TFA (red), 1 eq TFA (blue) and 50 eq TFA (dashed line). (b) Changes in the UV-vis spectrum of [Cr(ttpy)2] + upon addition of TFA.

Figure III- 19 .

 19 Figure III-19. HYSCORE spectra of [Cr(ttpy)2] + complex (1 mM): (a) in the absence of TFA, (b) in the presence of 1 equivalent of TFA, recorded with delay d1 = 136 nsBased on the results mentioned above, the mechanism for this electrocatalytic proton reduction process is still not clear. As the HYSCORE spectra of [Cr(ttpy)2] + complex in the presence and absence of TFA are identical, formation of a hybrid complex [Cr III (H)(ttpy -)] 2+

  Scheme III-10. Synthesis route for [Cr(ttpy-PO3Et2)2] 3+ and [Cr2P] 3+ complexes

  Figure III-20 presents their CVs in comparison with the CV of the ttpy ligand alone. Their redox potentials are collected in Table III-5 in comparison with the redox potentials of ttpy ligand and [Cr(ttpy)2] 3+ complex.

Figure III- 20 .

 20 Figure III-20. CVs of ttpy (green), ttpy-PO3Et2 (blue) and [Cr(ttpy-PO3Et2)2] 3+ (red) in MeCN + 0.1 M TBAPF6 under Ar. WE = C disk (d = 3 mm), v = 100 mV.s -1

  Figure III-21a presents the CV of ITO/[Cr2P] 3+ modified surface. The half-wave potentials are collected in Table III-6 in comparison with the redox potentials of the [Cr(ttpy-PO3Et2)2] 3+ precursor.

Figure III- 21 .

 21 Figure III-21. (a) CV of ITO/[Cr2P] 3+ modified electrode (area = 1.2 cm 2 ) recorded in MeCN + 0.1 M TBAPF6, v = 100 mV.s -1 . (b) Ipav plot is fit with a linear function (R 2 = 0.9999).

  Therefore, we employed a microcavity Pt electrode (d = 50 µm) to study the electrochemical properties of TiO2/[Cr2P] 3+ in powder form. Figure III-22 shows the CVs of the TiO2/[Cr2P] 3+ NPs and bare TiO2 NPs where the potential was swept from 0 to -2 V in anhydrous MeCN solution. The first reduction step to form [Cr2P] 2+ at around -0.4 V is not observed. It could be due to the fact that the [Cr2P] 3+ molecules bearing two phosphonic acid groups are anchored on two TiO2 NPs, thus are buried inside the TiO2 matrix, making it difficult to be in contact with the Pt electrode.As the redox potentials of grafted [Cr2P] 3+ on TiO2 NPs are difficult to measure, the potentials of ITO/[Cr2P] 3+ are taken for further studies.

Figure III- 22 . 1 III. 3 . 3 .

 22133 Figure III-22. CVs of TiO2/[Cr2P] 3+ NPs (solid line) and TiO2 (dashed line) recorded in solid state using a microcavity Pt electrode in MeCN + 0.1 M TBAPF6 under Ar, v = 100 mV.s -1

Figure III- 23 .

 23 Figure III-23. Solid state UV-vis absorption spectra of TiO2/[Cr2P] 3+ (black line) and TiO2 NPs (dashed line) The modified TiO2/[Cr2P] 3+ NPs shows absorption maximum at 580 nm whereas the bare TiO2 NPs do not absorb in the visible range. The precursor [Cr2P] 3+ complex in solid state also exhibits this absorption maximum, which is not present in the spectra of [Cr(ttpy)2] 3+ complex or ttpy-PO3H2 ligand (Figure III-24). Therefore we attribute the 580 nm absorption peak to the immobilized [Cr2P] 3+ complex on TiO2.

Figure III- 24 .III. 3 . 4 .

 2434 Figure III-24. Normalized UV-vis absorption spectra of [Cr2P] 3+ complex (black), [Cr(ttpy)2] 3+ complex (blue) and ttpy-PO3H2 ligand (red) recorded in solid state by mixing them with KBrThe TiO2/[Cr2P] 3+ NPs do not emit light in the 600 -900 nm range after photoexcitation at 580 nm or 450 nm where the anchored [Cr2P] 3+ complex shows some degree of absorption but not TiO2 scaffold. The weak luminescence of [Cr(ttpy)2] 3+* excited state seems to be completely quenched after being grafted onto TiO2 NPs.

  Figure III-25b shows its EPR response. No signals are observed at other regions of this X-band spectrum. It can be seen that although the broad EPR line is still observed, sub-structures of individual [Cr2P] 3+ sites start to be detectable. A sharp line at 3440 G and two maxima at ~ 3200 G and 2960 G are similarto the signal of [Cr(ttpy)2] 3+ free complex. However, the highest absorption peak of the free complex at ~ 1400 G is not observed in the NPs.

Figure III- 25 . 2 )

 252 Figure III-25. EPR spectra of TiO2/[Cr2P] 3+ colloid in MeCN (C = 6 g.L -1 ) with (a) 100 % [Cr2P] 3+ and (b) 2 % [Cr2P] 3+ + 98 % BPA spacers. They were taken with an X-band EPR spectrometer (f = 9.6538 GHz, 10 G modulation, 20 mW) at 10 K during 84 s.

  Figure III-26 presents the absorption spectrum of Ru 2+ /TiO2/[Cr2P] 3+ triad, compared with those of bare TiO2 NPs and TiO2/[Cr2P] 3+ dyad.

Figure III- 26 .

 26 Figure III-26. Solid state UV-vis spectra of TiO2/[Cr2P] 3+ (black line), Ru 2+ /TiO2/[Cr2P] 3+ (Ru:Cr:BPA = 20:2:78, % mol) (red line) and TiO2 NPs (dashed line)The double peak at around 450 nm is attributed to the 1 MLCT absorption of [Ru(bpy)3] 2+ -type PS as seen in Chapter 2. The small absorption maximum at 580 nm is ascribed to the grafted [Cr2P] 3+ species as shown in Section III.3. Only changes in the intensities of the two peaks are observed when the molar ratio of Ru:Cr is changed. The presence of both complexes on the NPs after thorough washing is an evidence for the successful co-grafting process.

  photoexcitation at 450 nm. Figure III-27 shows its spectrum in comparison with the spectrum of TiO2/Ru 2+ dyad. Both spectra have the same broad, featureless shape, thus it is attributed to the 3 MLCT emission of [RuP] 2+ PS. The peak position of the triad (624 nm) is slightly blueshifted to that of the dyad (630 nm). The blue-shift has been shown to be due to the separation of the Ru 2+ PS by BPA lateral spacers. No emission at 770 nm by [Cr2P] 3+* excited state has been observed.

Figure III- 27 .

 27 Figure III-27. Normalized emission spectra of Ru 2+ /TiO2/[Cr2P] 3+ triad (Ru:Cr:BPA = 20:2:78, % mol) (red line) and TiO2/Ru 2+ dyad (black line) recorded in MeCN under Arc) Transient absorption spectroscopyWe initially aimed to use TA technique to record TA spectra of Ru 2+ /TiO2/[Cr2P] 3+ at different time intervals after laser excitation. In order to avoid as much light scattering as possible and maintain a sufficiently stable colloid, the triad concentration was kept very low at 0.04 g.L -1 in MeCN solution. Consequently, the full TA spectra need to be accumulated over at least 20 mins to obtain a good signal-to-noise ratio. During this time, however, the NPs begin to precipitate. Therefore, we only focus on the signal decay at 450 nm, which has been ascribed to the charge recombination between (e -)TiO2 and Ru 3+ species (see Chapter 2). The decay (Figure III-28a) is very similar in shape as the same decay of TiO2/Ru 2+ dyad (Figure II-20), although in this triad it seems to recover to zero faster than in the dyad.

Figure III- 28 .

 28 Figure III-28. TA measurement for Ru 2+ /TiO2/[Cr2P] 3+ triad (Ru:Cr:BPA = 20:2:78, % mol, C = 0.04 g.L -1 ) recorded in MeCN under Ar 20 ns after excitation at 532 nm: (a) Signal decay at 450 nm, (b) -ln(OD/ODmax) versus t plot (black line) obtained at 450 nm and fit with a power function (red line). Similar to Chapter 2, as the charge recombination process should take into account the diffusion of both trapped electrons and oxidized PS, its kinetics should be fit with the Kohlrausch -Williams -Watts (KWW)'s stretched exponential equations (Equations III-7 and III-8). 44 Figure III-28b shows the fitting (red line) according to the KWW model. The KWW fitting parameter is close to that obtained with TiO2/Ru 2+ dyad and far from 1, which justifies the stretch model.

  behavior of [Cr2P] 3+ and [RuP] 2+ complexes co-grafted on ITO electrode prior to study the Ru 2+ /TiO2/[Cr2P] 3+ triad. An ITO surface was immersed into a solution containing both the complexes with the same concentration of 1 mM for 20 hours in dark, then washed thoroughly before measured with cyclic voltammetry. Its CV is shown in Figure III-29a. In the anodic part, a reversible wave at E1/2 = 0.90 V is attributed to the Ru 3+ /Ru 2+ couple. In the cathodic part, three successive reversible waves are detected, corresponding to the reduction of Cr 3+ to Cr 2+ , Cr + and Cr 0 respectively. This CV can be considered as the superimposition of ITO/[RuP] 2+ and ITO/[Cr2P] 3+ CVs. Based on the Ru 3+ /Ru 2+ wave at ~ 0.9 V and the Cr 3+ /Cr 2+ wave at ~ -0.48 V, the surface coverage  of the complexes is calculated to be (2.0 ± 0.2) × 10 -12 mol.cm -2 for [RuP] 2+ and (5.0 ± 0.5) × 10 -12 mol.cm -2 for [Cr2P] 3+ . The higher loading of [Cr2P] 3+ compared with [RuP] 2+ is probably due to the presence of two phosphonic anchoring group on [Cr2P] 3+ while [RuP] 2+ has only one group. Also based on the two aforementioned redox waves, a plot of Ipav (Figure III-29b) has been obtained for both Ru and Cr species. Linear fittings are well achieved for both species, proving the two complexes have been successfully grafted on ITO.

Figure

  Figure III-29. (a) CV of ITO/{Ru 2+ +[Cr2P] 3+ } electrode recorded in MeCN + 0.1 M TBAPF6 under Ar, v = 100 mV.s -1 . (b) Ipav plot based on the Ru 3+ /Ru 2+ (red squares) and Cr 3+ /Cr 2+ (blue circles) peaks, together with the linear fitting (R 2 = 0.999 for Ru 3+ /Ru 2+ and 0.992 for Cr 3+ /Cr 2+ ).

Figure III- 30 .

 30 Figure III-30. (a) UV-vis spectra of a mixture of [Cr(ttpy)2] 3+ free complex (C = 57 µM) and TiO2/Ru 2+ NPs (C = 0.25 g.L -1 ) in Ar-saturated MeCN solution containing 300 equivalents of TEOA. The spectra were taken every 300 s under irradiation at 400 nm <  < 750 nm. They have been corrected to exclude the light scattering effect of NPs. Arrows indicate the spectral changes during the experiment. (b) Evolution of [Cr(ttpy)2] + complex (measured at 723 nm) during the experiment. By tracking the 723 nm peak, one can calculate the concentration of the [Cr(ttpy)2] + complex as a function of irradiation time (Figure III-30b). After 3000 s the conversion from [Cr(ttpy)2] 3+ to [Cr(ttpy)2] + is only 60 %. As a comparison, when [Ru(bpy)3] 2+ is used instead of TiO2/Ru II , the conversion reaches ~ 90 % after the same time. The concentration of Ru 2+ PS in both cases is comparable: ~ 5.3 × 10 -5 M in this dyad sample and 4 × 10 -5 M for

  Scheme III-13. Energy diagram of Ru 2+ /TiO2/[Cr2P] 3+ triad and the expected photo-induced electron transfer cascade: (1) photo-excitation of Ru 2+ dye, (2) electron injection to the CB of TiO2, (3) sequential electron transfer to reduce [Cr2P] 3+ to [Cr2P] 2+ and then [Cr2P] + , (4) regeneration of Ru 2+ PS by TEOA, (5) back electron transfer between (e -)TiO2 and Ru 3+ species, and (6) back electron transfer between [Cr2P] 2+ or [Cr2P] + and Ru 3+ species. Dashed rectangle surrounds the triad components.

  Figure III-31 shows the EPR signals of the triad in MeCN solution before and after irradiation.

Figure III- 31 .

 31 Figure III-31. EPR spectra of Ru 2+ /TiO2/[Cr2P] 3+ NPs (20% Ru, 2% Cr and 78 % BPA) before and after in situ irradiation by a 455 nm LED, together with simulation of trapped electrons in anatase TiO2 NPs. The spectra were recorded with an X-band EPR spectrometer (f = 9.65 GHz, 20 mW) at 10 K. The asterisks mark peaks of  CH3 radicals.

Scheme IV- 1 .

 1 Scheme IV-1. Photocatalytic CO2 reduction using Ru 2+ /ZrO2/Ni 2+ NPs mentioned in reference 11. AA = ascorbic acid Since [Re(bpy)(CO)3Cl] complex can act as both PS and catalyst, Reisner and coworkers have studied the photocatalytic CO2 reduction using a related complex [Re(bpy)(CO)3(3-picoline)] + bearing two phosphonic acid groups (denoted as [ReP] + ) which was immobilized on TiO2 NPs. 12 The chemical structure of TiO2/[ReP] + NPs and the proposed catalytic mechanism are shown in Scheme IV-2. Under irradiation the [ReP] + photocatalyst was reduced by triethanolamine (TEOA), a sacrificial electron donor, to form [ReP] which could react with CO2. Optimal loading of the Re complexes on TiO2 NPs was found to be 0.02 mmol.g -1 . In DMF/TEOA mixed solvent, TiO2/[ReP] + NPs showed TONmax (CO) ~ 50 after 24 hours of visible irradiation. In comparison, in the absence of TiO2, the [Re(bpy)(CO)3(3-picoline)] + homogeneous complex could only produce a small amount of CO (TONmax ~ 6) during the same period. The mixture of TiO2 and [Re(bpy)(CO)3Cl] catalyst without phosphonic acid groups in solution also produced the same amount of CO as the Re catalyst alone. By transient absorption measurements, the authors attributed the enhanced photocatalytic activity of TiO2/[ReP] + to the better stabilization of the catalytically active species [ReP] by TiO2 surface: the lifetime of TiO2/[ReP] was estimated to be more than 1 swhile that of reduced [Re(bpy)(CO)3(3-picoline)] in solution was around 60 ms. The longer lifetime of this intermediate species allowed for higher chances to react with CO2 and to undergo the required second reduction step to release CO. However, the second reduction step was not observed by TAS. In this study, TiO2 was considered inactive in the electron transfer mechanism; it only acted as a scaffold to stabilize the intermediate.

  Scheme IV-3. Photocatalytic CO2 reduction using Dye/TiO2/Re I NPs mentioned in references 14 and 15The search for other complexes that can replace the prototypical but expensive [Re(bpy)(CO)3Cl] complex is of great interest. Analogous complexes, [Mn(bpy)(CO)3Br] and [Mn(dmbpy)(CO)3Br] (dmbpy = 4,4'-dimethyl-2,2'-bipyridine) (Scheme IV-4a and b), were first reported by Chardon-Noblat, Deronzier and co-workers in our CIRe group for electrocatalytic CO2 reduction.16 In MeCN solution and 5 % H2O as proton source, the complexes produced CO as the only reduction product with TONmax = 13 for [Mn(bpy)(CO)3Br] and TONmax = 34 for [Mn(dmbpy)(CO)3Br]. Recently, Ishitani et al.17 reported the photocatalytic CO2 reduction using [Mn(bpy)(CO)3Br] as catalyst, [Ru(dmbpy)3] 2+ as PS and 1-benzyl-1,4-dihydronicotinamide (BNAH) as sacrificial electron donor in DMF/TEOA (4/1, v/v) mixed solvent. Under continuous irradiation at 480 nm to selectively excite the PS, HCOOH was produced with high TONmax (149) and selectivity (85 %) over CO and H2. The activation pathways of the catalyst in the electrocatalytic or photocatalytic studies seem to have a great influence on the selectivity of the CO2 reduction reaction.

Scheme IV- 5 .

 5 Scheme IV-5. Structure of (a) TiO2/Mn I dyad and (b) Ru II /TiO2/Mn I triad

7 . 2 Figure IV- 1 .

 721 Figure IV-1. CVs of [Mn(ttpy)(CO)3Br] complex (C = 1 mM) recorded with C disk electrode (d = 3 mm) in MeCN + 0.1 M TBAPF6 under Ar, v = 100 mV.s -1 : (a) reductive scanning first, (b) oxidative scanning first

Figure IV- 2 .

 2 Figure IV-2. UV-vis absorption spectra of [Mn(ttpy)(CO)3Br] complex in MeCN solution in air.

[

  Mn(ttpy)(CO)3Br] catalyst and H2O as proton source. First, the CVs of the complex recorded in neat MeCN + 0.1 M TBAPF6 under Ar (black line) and in MeCN + 5 % H2O under CO2 (red line) are shown in Figure IV-3. We can see that the current is improved at both reduction peak, with the catalytic current occurring on the second reduction. It suggests that the electrocatalytic CO2 reduction may occur at both potentials, i.e. a mix of one-electron and two-electron mechanisms. A similar tendency has already been observed for [Mn(bpy)(CO)3Br] 22 , [Mn(bpy)(CO)3(MeCN)] 21 and [Mn(phen)(CO)3Br] 18 complexes.However, when [Mn(ttpy)(CO)3(MeCN)] was used, only increase in the current magnitude at the second reduction peak (-1.62 V) was reported.21

Figure IV- 3 .

 3 Figure IV-3. CV of [Mn(ttpy)(CO)3Br] complex (C = 1 mM) in MeCN + 0.1 M TBAPF6 + H2O (5 %, v/v) under CO2 (red line), in comparison with the CV recorded under Ar in neat MeCN (black line), v = 100 mV.s -1 , WE = C disk (d = 3 mm) IV.2.5. Photocatalytic CO2 reduction in presence of [Ru(bpy)3] 2+

Figure IV- 4

 4 shows the IR spectra of the parent complex [Mn(ttpy)(CO)3Br] (black line), bare TiO2 NPs (blue line) and TiO2/Mn I dyad (red line) in solid state in the CO vibration region. It can be seen that the peak at 2022 cm -1 due to symmetric CO vibration remains unchanged, while the two other peaks seem to merge with each other to form a broad band centered at ~ 1930 cm -1 . The merging of two antisymmetric CO stretching bands after the Mn I complex has been grafted on TiO2 surface is consistent with literature for Re 12 -and Mn 19 -tricarbonyl bipyridine complexes immobilized on TiO2 NPs. The experiments prove that the Mn complex has been successfully grafted.

Figure IV- 4 .

 4 Figure IV-4. Solid state FT-IR spectra of [Mn(ttpy)(CO)3Br] complex (black line), bare TiO2 NPs (blue line) and TiO2/Mn I NPs (red line) in the CO stretching region.

  Pt electrode. Due to different scale of current intensity, the CV is split into two parts corresponding to the anodic region (Figure IV-6a) and cathodic region (Figure IV-6b). For comparison, the CVs of TiO2/ttpy-C3-PO3H2 NPs are also shown in Figure IV-7.

Figure IV- 6 .

 6 Figure IV-6. CVs of TiO2/[MnP] NPs (solid lines) recorded in solid state using a microcavity Pt electrode in MeCN + 0.1 M TBAPF6 under Ar, v = 100 mV.s -1 : (a) anodic region, (b) cathodic region. Dashed line shows the CV of bare TiO2 NPs recorded under the same conditions.

Figure IV- 7 .

 7 Figure IV-7. CVs of TiO2/ttpy-(CH2)3-PO3H2 NPs recorded in solid state using a microcavity Pt electrode in MeCN + 0.1 M TBAPF6 under Ar, v = 100 mV.s -1 : (a) anodic region, (b) cathodic region In the anodic region of TiO2/Mn I (Figure IV-6a), only a broad, irreversible oxidation peak centered at Eox = 0.86 V is observed. Bare TiO2 NPs are not conductive in anodic potential scanning, and TiO2/ttpy-C3-PO3H2 NPs also do not show any oxidation peaks as expected (Figure IV-7a). Therefore, the anodic peak at 0.86 V is assigned to the oxidation of

( 17 )

 17 fac [Mn I ( 2 -ttpy-C3-PO3H2)(CO)3Br]  fac [Mn II ( 2 -ttpy-C3-PO3H2)(CO)3Br] + + e - (18) fac [Mn II ( 2 -ttpy-C3-PO3H2)(CO)3Br] + + MeCN fac [Mn II ( 2 -ttpy-C3-PO3H2)(CO)3(MeCN)] 2+ + Br -

Figure IV- 8 ,Figure IV- 8 .

 88 Figure IV-8, in comparison with TiO2 absorption spectrum. In solid state, TiO2/Mn I NPs do not exhibit the absorption maximum at 420 nm like the free Mn I complex in solution. It only shows the extension of the absorption band into the visible region.When both Ru II and Mn I complexes are grafted onto TiO2 with molar ratio of Mn:Ru = 1:10, the absorption maximum at around 450 nm is attributed to the 1 MLCT absorption band of Ru II PS. The absorption band of Mn I overlaps to that of Ru II , which is much more intense in the visible region. A small peak at around 575 nm has also been detected, which is not present in the UV-vis spectrum of TiO2/Ru II dyad (Figure II-16) or the [Mn( 2 -ttpy)(CO)3Br] complex (Figure IV-2).

  recorded in colloidal solution following excitation at 450 nm. The TiO2/Mn I dyad does not show any emissions in the range between 500 and 850 nm. The emission spectrum of Ru II /TiO2/Mn I triad is shown in Figure IV-9 with a maximum of emission at 618 nm. It totally resembles the emission spectrum of TiO2/Ru II (Figure II-16), hence it is attributed to the 3 MLCT luminescence of Ru II* complex.

Figure IV- 9 .

 9 Figure IV-9. Emission spectrum of Ru II /TiO2/Mn I colloidal solution after excitation at 450 nm.

  The positive OD has also been observed in the TiO2/Ru II case and ascribed to the transient absorption of (e -)TiO2. Hence, the decay corresponds to the charge recombination between Ru III and (e -)TiO2. Since the charge recombination between these species should take into account the diffusion of both trapped electrons and oxidized PS, the Kohlrausch -Williams -Watts (KWW)'s stretched exponential equation 31 is used to extract the kinetics of charge recombination, as previously described in Section II.3.6. Figure IV-10b shows the data fit by the KWW's equation. The KWW parameters (KWW and KWW) are collected in Table IV-3.

Figure IV- 10 .

 10 Figure IV-10. (a) TA decay at 450 nm for Ru II /TiO2/Mn I triad (Ru:Mn = 10:1); (b) -ln(OD/ODmax) versus t plot (black dots) obtained at 450 nm and fit with a power function (red line). The experiment was carried out in MeCN under Ar following excitation at 532 nm by a nanosecond pulsed laser.

  Note: TON is calculated against the amount of Mn I catalyst Our photocatalytic CO2 reduction results underline the importance of anchoring both PS and molecular catalyst on TiO2 surface in terms of product selectivity. A previous study by Perutz, Reisner and co-workers 12 concerning the immobilization of [Re(bpy)(CO)3(3picoline)](PF6) catalyst on TiO2 NPs has also emphasized this importance in terms of the amount of the reduction product. The authors showed that the TiO2/Re I catalyst produces more CO than the free Re I complex in solution with the same concentration: TONmax (CO) = 48 for TiO2/Re I compared with TONmax (CO) = 6 for the homogeneous Re I in solution.In a slightly different approach, another study by Kubiak, Cohen et al.29 also showed enhanced selectivity towards HCOOH when [Mn(dcbpy)(CO)3Br] (dcbpy = 5,5'dicarboxylic-2,2'-bipyridine) complex was anchored on a metal-organic framework called UiO-67. In the presence of [Ru(dmb)3] 2+ (dmb = 4,4'-dimethyl-2,2'-bipyridine) as PS, BNAH as sacrificial electron donor and DMF/TEOA mixed solvent, the UiO-67/Mn I photocatalyst produced TONmax (HCOOH) = 110 and TONmax (CO) = 5 after 18 hours of irradiation at 470 nm, corresponding to the selectivity of 95 % for HCOOH. Meanwhile, under the same conditions, the free [Mn(dcbpy)(CO)3Br] catalyst mixed with [Ru(dmb)3] 2+ PS in solution only produced TONmax (HCOOH) = 57 and TONmax (CO) = 5 after 18 hours, corresponding to the selectivity of 92 % for HCOOH. A mechanism was proposed in this work to explain the formation of HCOOH as the main product (Scheme IV-12). It was based on the formation of singly reduced, coordinately unsaturated [Mn 0 (bpy)(CO)3] and, subsequently, [(bpy)(CO)3Mn I -OC(O)H] adduct as intermediate species.

(

  Note: E(Ru III /Ru II ) = 0.92 V; E(Ru II /Ru I ) = -1.67 V; E(Ru III /Ru II* ) = -1.10 V; E(Ru II* /Ru I ) = 0.31 V; E(Ti 4+ /Ti 3+ on TiO2) = -1.0 V; E(Mn I /Mn 0 grafted on TiO2) = -1.11 V; Eox(BNAH + /BNAH) = 0.27 V vs Ag/AgNO3 0.01 M).

  complex [Mn 0 (ttpy-C3-PO3H2)(CO)2] anchored on TiO2 NPs in a similar way to what has been proposed by Kubiak, Cohen et al. 29 for UiO-67/Mn I system (Scheme IV-Before mimicking the conditions applied in the photocatalytic CO2 reduction, we first investigated the magnetic behavior of the sacrificial electron donor BNAH alone in MeCN solution. As expected, the sample does not show any EPR signal when the measurement was performed in dark. Under continuous irradiation at 455 nm, it shows a thin EPR line centered at g ~ 2 if the sample has been prepared under O2 (Figure IV-11, red line). If the sample has been prepared under Ar, no EPR signals have been recorded (Figure IV-11, black line). Therefore, the organic radical BNAH + is obtained by photooxidation of BNAH under visible irradiation and in the presence of O2. This experiment shows the importance of the complete removal of O2, otherwise this g ~ 2 line may interfere with the desired signals of other transient species.

Figure IV- 11 .

 11 Figure IV-11. EPR spectra of BNAH (0.1 M) in MeCN prepared under O2 (red line) and under Ar (black line), irradiated in situ by a 455 nm LED. The spectra were recorded with an X-band EPR spectrometer (f = 9.655 GHz, 2 mW, 5 G modulation) at 10 K, accumulated for 10 mins.

  1 M) in dark and under 455 nm irradiation. The EPR tube containing this sample has been carefully prepared in dark under Ar before being frozen in liquid nitrogen. As a comparison, we also prepared another tube containing TiO2/Ru II dyad instead of the triad, and the same amount of BNAH and MeCN as for the first tube. The Ru II content in both samples is approximately equal. As expected, both samples are EPR-silent when they are measured in dark since Mn I (3d 6 ) and Ru II (4d 6 ) are diamagnetic. Under 455 nm irradiation, photo-induced paramagnetic species can be detected by EPR. Their EPR signals under 455 nm irradiation (Figure IV-12) have been accumulated over the same time period.

Figure IV- 12 .

 12 Figure IV-12. EPR spectra of TiO2/Ru II dyad (black) and Ru II /TiO2/Mn I triad (red) colloidal in MeCN (CNPs = 6 g.L -1 ) and 0.1 M BNAH, irradiated in situ by a 455 nm LED. The samples were prepared in an Ar glovebox.Both spectra were recorded with an X-band EPR spectrometer (f = 9.654 GHz, 2 mW, 5 G modulation) at 10 K, accumulated for 55 mins.

Figure

  Figure IV-13a shows the field-swept spectra of BNAH + radicals alone prepared under O2 (red) and the radicals in the presence of Ru II /TiO2/Mn I triad prepared under Ar (black). Both samples were irradiated in situ at 455 nm. The two spectra are very similar, indicating that most of the signal in the triad + BNAH sample is due to BNAH + radicals. In addition, HYSCORE spectrum of the triad + BNAH sample was also recorded (Figure IV-13b). Spots on the diagonal line centered at 15 MHz are attributed to proton hyperfine coupling of the BNAH + radicals, while a weak spot at 3.6 MHz is assigned to distant Mn atoms. No Mn hyperfine couplings were observed. It is however not in contradiction with the calculations of hyperfine interactions by Dr. Jean-Marie Mouesca (CEA Grenoble / INAC / SyMMES) for

Figure

  Figure IV-13. (a) Pulsed EPR field-swept spectra of BNAH + radicals alone (red) and Ru II /TiO2/Mn I triad + BNAH + (black). (b) HYSCORE spectrum of Ru II /TiO2/Mn I triad + BNAH + mixture. The samples were irradiated in situ by a 455 nm LED.

Scheme V- 1 .

 1 Scheme V-1. (a) Mechanism of pyrrole oxidative polymerization. (b) In situ photopolymerization of pyrrole on mesoporous TiO2 film. Adapted from reference 1. The LUMO level of pyrrole cannot be determined electrochemically.Well-defined nanostructured composites of TiO2 NPs and various polymers such as polystyrene (PS), poly(methyl methacrylate) (PMMA) and poly(N-isopropylacrylamide) (PNIPAM) have been reported by Wang et al.2 following a surface-initiated photopolymerization method. TiO2 NPs were first modified with 3-(trimethoxysilyl)propyl methacrylate (MPS) which not only formed Si-O-Ti linkages with TiO2 but also possessed terminal C=C bonds. Under UV light electrons and holes are created on the CB and VB of TiO2, respectively. The holes are then transferred to the C=C bonds to generate radicals which can initiate the oxidative polymerization process. The covalent bonding between MPS and TiO2 ensures that the polymer is bound to the TiO2 surface. However, the polymer degradation due to photocatalytically active TiO2 under UV light has not been mentioned. Scheme V-2 summarizes this mechanism. The TiO2/polymer core/shell NPs with precise, tunable architecture are versatile building blocks for numerous potential applications in

1

 1 collects the redox potentials of [Ru-pyr] 2+ and [Ru(bpy)3] 2+ for comparison.

Figure V- 1 .a

 1 Figure V-1. CV of [Ru-pyr] 2+ (1 mM) in MeCN + 0.1 M TBAP under Ar. WE = C disk (3 mm), v = 100 mV/s

Figure V- 2 .Figure V- 3

 23 Figure V-2. Electropolymerization of [Ru-pyr] 2+ (0.6 mM) on an ITO electrode (v = 100 mV/s, in MeCN + TBAP 0.1 M under Ar). (a) 15 CVs obtained during the polymerization, with vertical arrows indicating the changes in its CV. (b) CV of the ITO/poly(Ru-pyr) electrode after transferred to a complex-free solution. V.2.3. Photophysical properties Figure V-3 presents the UV-vis absorption and emission spectra of [Ru-pyr] 2+ complex compared with those of [Ru(bpy)3] 2+ reference. The absorption maxima at 460 nm and 286 nm of [Ru-pyr] 2+ are assigned to the singlet metal-to-ligand charge transfer ( 1 MLCT) and ligand-centered (LC) transition, respectively. The extinction coefficient of [Ru-pyr] 2+ at 460

Figure V- 3 .

 3 Figure V-3. (a) UV-vis absorption spectra and (b) emission spectra of [Ru(bpy)3] 2+ (black) and [Ru-pyr] 2+ (red) complexes in MeCN under Ar. Emission spectra were recorded after 450 nm light excitation. Similar to Section II.2.2, using [Ru(bpy)3] 2+ as reference ( = 0.062) 11 , the emission quantum yield  of [Ru-pyr] 2+ is calculated as 0.043 (see Experimental Section for the calculation), which is about 70 % of that of [Ru(bpy)3] 2+ .

Figure V- 4 .

 4 Figure V-4. Time-resolved emission decay of [Ru-pyr] 2+* excited state in MeCN under Ar. The sample was excited at 400 nm using a picosecond pulsed laser, and the emitted photons were collected at 610 nm using a TCSPC photometer. The red curve represent the monoexponential fitting of the decays Table V-2. Summary of the photophysical properties of [Ru-pyr] 2+ and [Ru(bpy)3] 2+ complex

7 .

 7 estimated that E (Ru II* /Ru I ) = 0.31 V vs Ag/Ag + 0.01 M. Taking into account the oxidation potential of the pyrrole monomers (E ~ 0.84 V), the photo-induced electron transfer reaction between [Ru(bpy)3] 2+* and pyrrole moieties would be endergonic by around 0.52 eV. TiO2/[Ru-pyr] 2+ NPs is schematically summarized in Scheme V-Similar to Chapter 3, a bpy ligand bearing a phosphonic group (denoted bpy-PO3H2) was prepared in the first step. The use of TMSBr as deprotecting agent of the -P(O)(OEt)2 group lead to the formation of HBr by-product, which could not quantitatively separated from the desired product. Our attempts to use this ligand for the complexation with [Ru(bpy-pyr)2]Cl2 (where bpy-pyr = 4-methyl-4'-[11-(1H-pyrrol-1-yl)undecyl]-2,2'-bipyridine) to form [Ru(bpy-pyr)2(bpy-PO3H2)] 2+ were not successful due to the H + in HBr initiating the oxidative polymerization of pyrrole moieties. The polypyrrole film could be easily observed in a NMR tube.Therefore, we employed a stepwise approach as we did in Chapter 3 for the synthesis of TiO2/[Ru-pyr] 2+ NPs. First, the bpy-PO3H2 ligand was chemically adsorbed onto anatase TiO2 colloidal aqueous solution at room temperature, followed by alternate centrifugationwashing steps with copious amount of water to remove unadsorbed ligands and HBr from the solid phase. In the second step, [Ru(bpy-pyr)2]Cl2 was added to a suspension of TiO2/bpy NPs in ethanol / acetone (8/2 v/v) solution under Ar, and the reaction mixture was refluxed overnight. The modified NPs were then separated and washed by alternate centrifugationwashing steps in ethanol / acetone mixture, and dried in an oven to yield a dark red powder.The complexation between TiO2/bpy and [Ru(bpy-pyr)2]Cl2 was quantitatively achieved as the [Ru(bpy-pyr)2]Cl2 complex cannot be detected by UV-vis spectroscopy in the washing solution after the complexation. The loading of [Ru-pyr] 2+ on anatase TiO2 is estimated as 0.20 mmol.g -1 (about 3800 molecules per NP or 2 molecules per nm 2 ) by measuring the UVvis absorbance of supernatant solution after each centrifugation step, which is similar to that obtained for TiO2/Ru II (see Section II.3.3).

Figure V- 5 .

 5 Figure V-5. CV of TiO2/Ru II (black) and TiO2/[Ru-pyr] 2+ (red) NPs recorded in solid state with a microcavity Pt electrode (d = 50 µm) in MeCN + TBAPF6 0.1 M under Ar, v = 100 mV/s. V.3.3. Photophysical properties  UV-vis-NIR spectroscopy Figure V-6a shows the UV-vis-NIR spectrum of TiO2/[Ru-pyr] 2+ NPs in solid state using an integrating sphere. A broad peak at ~ 460 nm was recorded, which is similar to the initial [Ru-pyr] 2+ complex. The peak is attributed to the 1 MLCT absorption band of [Ru(bpy)3] 2+ core. The LC transition is not clearly observed due to the strong absorption of TiO2 at wavelengths below 390 nm. In colloidal solution of MeCN under Ar, TiO2/[Ru-pyr] 2+ NPs display a broad, featureless emission spectrum (Figure V-6b) after light excitation at 450 nm. The emission spectrum is similar in shape as those obtained with [Ru-pyr] 2+ and TiO2/Ru II colloid, thus it can be reasonably assigned to the emission of the [Ru(bpy)3] 2+* excited state. The emission maximum at 625 nm is slightly red shifted compared to the initial complex [Ru-pyr] 2+ (621 nm). A similar red shift after immobilizing the Ru complex on TiO2 has been described for TiO2/Ru II (see Section II.3.6).

Figure V- 6 . 3 . 6 . 4 ,

 6364 Figure V-6. (a) Solid state UV-vis-NIR spectrum of TiO2/[Ru-pyr] 2+ NPs (solid line) and TiO2 NPs (dashed line) meaured in solid state. (b) Emission spectrum of TiO2/[Ru-pyr] 2+ colloid in MeCN under Ar, excited at 450 nm.

Figure V- 7 .

 7 Figure V-7. Time-resolved emission decay of TiO2/[Ru-pyr] 2+* NPs in MeCN under Ar, excited at 400 nm by a picosecond pulse laser and recorded at 610 nm. Red line shows a biexponential fitting of the decay.

  Scheme V-8. Schematic illustration of the photopolymerization process

Figure V- 8 .

 8 Figure V-8. Solid state UV-vis absorption spectra of TiO2/[Ru-pyr] 2+ (black line), TiO2/poly(Ru-pyr) (red line) and naked TiO2 NPs (dashed line) b) Emission spectroscopy The changes in emission spectrum of TiO2/[Ru-pyr] 2+ during the photopolymerization process are shown in Figure V-9. After light excitation at 450 nm, all the samples after 0 h, 1 h and 2 h of irradiation exhibit a similarly broad signal indicative of [Ru(bpy)3] 2+* state.Emission intensity of the nanocomposite decreases by around 50 % after 1 hour and 60 % after 2 hours of photopolymerization. The emission maximum is slightly red-shifted from 625 nm to 631 nm after the two hours.

Figure V- 9 .

 9 Figure V-9. Steady-state emission spectra of TiO2/[Ru-pyr] 2+ colloid (C = 0.05 g.L -1 ) in MeCN after 0h, 1h and 2h irradiation at 450 nm (black, red and blue lines respectively). Signals were recorded under an Ar atmosphere.

Figure V -

 V Figure V-10. Time-resolved emission decays of TiO2/[Ru-pyr] 2+* after (a) 0h, (b) 1h and (c) 2h irradiation at 450 nm. Red curves represent the biexponential fitting of the decays.

Figure V -

 V Figure V-11. EPR signal of the photo-induced polymerization of TiO2/[Ru-pyr] 2+ colloid (6 g.L -1 ) in MeCN glass (black line), together with the simulation of trapped electrons on Ti 3+ sites of anatase TiO2 (red line) and the simulation of polypyrrole (blue line). The sample was irradiated in situ by a 455 nm LED from the cavity window. The spectrum was accumulated for 1 hour at 20 K with an X-band EPR spectrometer (f = 9.6532 GHz, 0.2 mW, 5 G modulation)

4 .

 4 Scheme V-10. Formation of pyrrole=O cations

  A and B are constants in the monoexponential model, n the refractive index of dispersant, 0 the laser wavelength used in the DLS measurement and  the scattering angle. The DLS measurements have been conducted at 23 0 C in MeCN solution for samples before and after 1 or 2 hours of photopolymerization. Their correlograms are shown Figure V-12. All the signals are well fit with a monoexponential function indicating monodispersecolloids. The hydrodynamic diameter is thus determined to be 354 nm, 537 nm and 437 nm for the colloids after 0, 1 and 2 hours of irradiation, respectively. The result corresponds to a 50 % increase in size after 1 hour and 25 % increase in size after 2 hours of irradiation compared to the initial size. It is expected that the size of NPs also undergoes a similar change as the hydrodynamic diameter. Such large increases are expected to be significantly higher than the experimental uncertainties. It is a bit surprising that the average size of the NPs increases during the first hour of irradiation and decreases after. In any cases, the result suggests that pyrrole polymerization may mainly occur between the [Ru-pyr] 2+ species on the same particle rather than connecting the particles together, since in the latter case the diameter should be increased much more significantly.

Figure V -

 V Figure V-12. DLS correlograms of TiO2/poly(Ru-pyr) colloids in MeCN after 0 h (red), 1h (green) and 2 h (blue) of photopolymerization.

Figure V -

 V Figure V-13. XPS survey spectra of (a) TiO2/Ru II , (b) TiO2/[Ru-pyr] 2+ and (c) TiO2/poly(Ru-pyr) NPs

Figure V -

 V Figure V-14. Ti2p XPS core level spectra of (a) TiO2/Ru II , (b) TiO2/[Ru-pyr] 2+ and (c) TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines

Figure V -

 V Figure V-15 presents the Ru3d and C1s spectra of the three samples. The Ru3d spectra are fit with two Gaussian lines which peak at 280.7 eV and 284.8 eV corresponding to the 5/2 and 3/2 states, respectively. 19 The part of the Ru3d3/2 spectrum that overlaps with the C1s spectrum has been subtracted prior to C1s peak fitting. The C1s spectra are not symmetric, which are then decomposed into three Gaussian lines at around 284.4 eV, 285.7 eV and 287.5 eV. The three component lines are assigned to C1s core signal in C-C, C-N and contaminated C=O/COO bonds, respectively. They remain almost unchanged in both peak position and FWHM during the photopolymerization.

Figure V- 15 .

 15 Figure V-15. C1s and Ru3d (5/2 and 3/2) XPS core level spectra of (a) TiO2/Ru II , (b) TiO2/[Ru-pyr] 2+ and (c) TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines

Figure V -

 V Figure V-16 presents the N1s XPS spectra of the three samples. In TiO2/Ru II , the spectrum is clearly fit with one Gaussian line peaked at 399.8 eV and FWHM = 1.62 eV. It is reasonably assigned to the =N-group in bpy ligands as they are all alike. The peak is in accordance with literature concerning the N atoms in bpy ligands being coordinated with Rh(I) metal center. 20 Deconvoluting the N1s spectra of TiO2/[Ru-pyr] 2+ and TiO2/poly(Ru-

Figure V -

 V Figure V-16. N1s XPS core level spectra of (a) TiO2/Ru II , (b) TiO2/[Ru-pyr] 2+ and (c) TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines The P2p XPS spectra of the three samples are shown in Figure V-17. The spectrum of TiO2/Ru II is deconvoluted into two lines which peak at 132.4 eV and 133.3 eV, which are assigned to the P2p3/2 and P2p1/2 states, respectively. Introduction of the pyrrole moieties has minor influence on the peak positions (132.2 and 133.1 eV) while the FWHM remains the same. The peak position and FWHM of both P2p3/2 and P2p1/2 states are unchanged during the photopolymerization. It is hence concluded that the phosphonate anchoring groups are unaffected by the photopolymerization.

Figure V -

 V Figure V-17. P2p XPS core level spectra of (a) TiO2/Ru II , (b) TiO2/[Ru-pyr] 2+ and (c) TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines

Figure V -

 V Figure V-18 shows the O1s XPS spectra of the three samples. The spectrum of TiO2/Ru II can be decomposed into three Gaussian lines which peak at 529.1 eV, 530.3 eV and 531.6 eV. The most intense line at the lowest binding energy is assigned to O 2-anions in TiO2

Figure V -

 V Figure V-18. O1s XPS core level spectra of (a) TiO2/Ru II , (b) TiO2/[Ru-pyr] 2+ and (c) TiO2/poly(Ru-pyr). Right column: (a1), (b1) and (c1) are zoom-in spectra in the 534 -528 eV range of the spectra (a), (b) and (c), respectively. The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines Quantification of percentage atomic concentrations was achieved by integrating the areas under corresponding XPS peaks and correcting the areas with relative sensitivity factors.

Figure V -

 V Figure V-19. Atomic concentration of the elements in TiO2/Ru II , TiO2/[Ru-pyr] 2+ and TiO2/poly(Ru-pyr) NPs. Solid lines are associated with the left y-axis while dashed lines are linked to the right y-axis. Error = ± 0.1 %

  (i) Photo-EPD strategy: TiO2/[Ru-pyr] 2+ precursor was suspended in air-saturated MeCN solution (C = 0.75 g.L -1 ) and irradiated at 450 nm during 2 hours. The photopolymerization lead to TiO2/poly(Ru-pyr) nanocomposite. Afterwards it was transferred to an Ar-saturated MeCN solution (C = 0.3 g.L -1 ) where a potential of 120 V was applied between two FTO electrodes placed 0.5 cm apart in 1 hour for the EPD. This EPD bias is equivalent to an electric field of 24 × 10 3 V.m -1 .(ii) EPD-Photo strategy: TiO2/[Ru-pyr] 2+ precursor was first deposited on a FTO electrode by EPD (Eapplied = 24 × 10 3 V/m) in 1 hour. All the parameters and deposition conditions were kept the same as in the Photo-EPD strategy. The electrode was then washed and immersed into an air-saturated MeCN solution. Surface photopolymerization was achieved by irradiating the electrode at 450 nm. After certain irradiation time intervals the electrode was transferred to an electrochemical cell containing MeCN + 0.1 M TBAPF6 in order to characterize the photopolymerization process by cyclic voltammetry.

Figure V- 20 .

 20 Figure V-20. CVs of FTO/TiO2/[Ru-pyr] 2+ electrode recorded in MeCN + 0.1 M TBAPF6 under Ar. The FTO/TiO2/[Ru-pyr] 2+ modified electrode was therefore subjected to a photopolymerization experiment. It was irradiated at 450 nm in an air-saturated MeCN solution. After certain irradiation time, it was transferred to another electrochemical cell containing 0.1 M TBAPF6 electrolyte in MeCN to record CVs (Figure V-21). After 3 minutes of irradiation, the oxidation peak at ~ 0.9 V due to Ru III /Ru II and pyrrole monomers already decreases in intensity, suggesting the consumption of pyrrole monomers. The emergence of a new oxidative peak at 0.43 V is ascribable to polypyrrole oxidation, which is in accordance with literature. 25 Increasing the irradiation time shows almost no changes in the CVs.

Figure V -

 V Figure V-21. CV of a FTO/TiO2/[Ru-pyr] 2+ electrode under 450 nm irradiation, recorded in MeCN + 0.1 M TBAPF6. Irradiation was performed with a Xe lamp + UV filter + 450-nm bandpass filter. Light power was estimated to be 0.2 W.cm -2 . The UV-vis-NIR spectrum of FTO/TiO2/poly(Ru-pyr) surface is shown in Figure V-22.The absorption of FTO substrate has been subtracted. The broad peak at 420 nm, assigned to

Figure V -

 V Figure V-22. UV-vis-NIR absorption spectrum of FTO/TiO2/poly(Ru-pyr) modified electrode after photopolymerization. Absorption of FTO bare electrode has been subtracted. V.5.3 X-ray photoelectron spectroscopy of FTO/TiO2/poly(Ru-pyr) electrode

Figure V -

 V Figure V-23. XPS survey spectrum of FTO/TiO2/poly(Ru-pyr) electrode fabricated with the Photo-EPD method High resolution XPS characterizations are then focused on the regions corresponding to the elements of interest.Figure V-24 presents their Ti2p spectra. They are well fit with two

  Figure V-24 presents their Ti2p spectra. They are well fit with twoGaussian lines which peak at 457.9 eV and 463.7 eV. The lines are assigned to Ti2p3/2 and Ti2p1/2 states, 19 respectively, of surface Ti 4+ ions. Similar peak position and shape for the two samples shows that Ti 4+ remains intact after the EPD process.

Figure V -

 V Figure V-24. Ti2p XPS core level spectra of (a) TiO2/poly(Ru-pyr) and (b) FTO/TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines

Figure V -

 V Figure V-25 presents the C1s and Ru3d XPS spectra of the nanocomposite and the film. The Ru3d peak of FTO/TiO2/poly(Ru-pyr) can be deconvoluted into two Gaussian lines which peak at 280.1 eV and 284.3 eV. They are ascribed to the Ru3d5/2 and Ru3d3/2 states, respectively. 19 Both peak positions are shifted ~ 0.5 eV to lower binding energies compared to the Ru3d5/2 and Ru3d3/2 peaks of TiO2/poly(Ru-pyr) NPs (280.7 and 284.8 eV). The line shape remains unchanged, indicating the coordination sphere of [Ru(bpy)3] 2+ is conserved after the EPD process. The part of Ru3d3/2 state at 284.3 eV that overlaps with the C1s signal has been subtracted before the deconvolution of the C1s peak. The C1s peak is then decomposed into three Gaussian lines at 284.2 eV, 285.6 eV and 287.5 eV, which are similar to the TiO2/poly(Ru-pyr) nanocomposite. The three lines are thus assigned to C-C, C-N and COO bonds. The strong increase in COO species may stem from impurities on the FTO substrate.

Figure V- 25 .

 25 Figure V-25. C1s and Ru3d (5/2 and 3/2) XPS core level spectra of (a) TiO2/poly(Ru-pyr) and (b) FTO/TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines

Figure V -

 V Figure V-26. N1s XPS core level spectra of (a) TiO2/poly(Ru-pyr) and (b) FTO/TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines

Figure V -

 V Figure V-27 displays the O1s XPS spectra of the two samples. On FTO/TiO2/poly(Ru-pyr) the O1s is evidently more asymmetric on the higher binding energy side. The spectrum is decomposed into three lines at 529.4 eV, 530.4 eV and 531.7 eV. Meanwhile the spectrum of TiO2/poly(Ru-pyr) can only be deconvoluted into two lines at 529.2 eV and 530.6 eV, which are ascribed to O 2-in TiO2 and O atoms in phosphonate group, respectively. Hence for the film, the two lines at lower binding energies is probably contributed by not only O 2-(TiO2) and O (phosphonate) but also O 2-(SnO2) which cannot be distinguished from the formers.

Figure V -

 V Figure V-27. O1s XPS core level spectra of (a) TiO2/poly(Ru-pyr) and (b) FTO/TiO2/poly(Ru-pyr). The red lines represent the fitting obtained by the sum of the Gaussian peaks in blue lines Quantitative determination of percentage atomic concentration of some elements in TiO2/poly(Ru-pyr) NPs and FTO/TiO2/poly(Ru-pyr) thin film is collected in Table V-7. After the EPD of the nanocomposite onto FTO, Ru, N (bpy), N (pyrrole) and C (C-N bonds)

V. 5 . 4

 54 Photo method (Figure V-29) were studied for comparison. Micrographs taken from a large scale (Figures a) show aggregates with several micrometers in diameter in both surfaces. EPD-Photo electrode shows additional much smaller particles surrounding the big aggregates.It should be noted that these small particles are also present on the Photo-EPD electrode but their amount is drastically reduced. In both electrodes, only a part of the FTO electrode is covered by the nanocomposite. This observation is in agreement with the XPS survey spectrum of FTO/TiO2/poly(Ru-pyr) electrode which shows a significant signal of Sn 4+ cations.

Figure V -

 V Figure V-28. SEM micrographs of FTO/TiO2/poly(Ru-pyr) electrode prepared by the Photo-EPD strategy. (a) and (b) Topography of the surface recorded by an ETD detector. (c) Chemical contrast recorded by a CBS detector.

Figure V -

 V Figure V-29. SEM micrographs of FTO/TiO2/poly(Ru-pyr) electrode prepared by the EPD-Photo strategy. (a) and (b) Topography of the surface recorded by an ETD detector. (c) Chemical contrast recorded by a CBS detector.

Figure V -

 V Figure V-30 shows the AFM images of the electrodes.

Figure V - 6 V. 5 . 5

 V655 Figure V-30. AFM micrographs of (a) blank FTO, (b) FTO/TiO2/Ru II , (c) FTO/TiO2/poly(Ru-pyr) prepared by the Photo-EPD strategy, and (d) FTO/TiO2/poly(Ru-pyr) prepared by the EPD-Photo strategy. Surface RMS roughness are 7.4, 19, 8.8 and 5.5 nm, respectively. In the absence of polypyrrole, the FTO/TiO2/Ru II electrode shows a very rough surface with large aggregates (Figure V-30b), where its root-mean-square (RMS) roughness strikingly increases from 7.4 nm for blank FTO (Figure V-30a) to 19 nm. In the presence of polypyrrole, both FTO/TiO2/poly(Ru-pyr) electrodes made by the Photo-EPD and EPD-Photo methods exhibit RMS roughness comparable to the blank FTO (8.8 and 5.5 nm for the Photo-EPD electrode and EPD-Photo electrode, respectively). The EPD-Photo electrode also shows relatively uniform aggregates with diameter of ~ 100 nm and a more homogeneous surface than the Photo-EPD one. Therefore, the polypyrrole is proved to allow for better nanostructuration of the NPs as evidenced by smaller, more homogeneously distributed

Figure V- 31 .

 31 Figure V-31. Electron transfer mechanism for the photocurrent generation using FTO/TiO2/Ru II or FTO/TiO2/poly(Ru II -pyr) electrodes under visible irradiation. CB and VB are conduction band and valence band of TiO2, respectively The photocurrent generation of the electrodes is presented in Figure V-32a. Both FTO/TiO2/poly(Ru-pyr) electrodes show significantly enhanced photocurrents (Photo-EPD:

Figure V -

 V Figure V-32. (a) Photocurrent generation and (b) photocurrent stability of various electrodes. Experiments were conducted in MeCN + 0.1 M TBAPF6 solution under visible irradiation, in the presence of 15 mM TEOA as sacrificial electron donor. Eapplied = 0.12 V vs Ag/Ag + 0.01 M. Light power was estimated as 1.3 W.cm -2 . The notions (1) and (2) represent the Photo-EPD strategy and EPD-Photo strategy respectively.

Film

  photopolymerization on the surface (EPD-Photo method). The presence of non-homogeneous distribution of Ru II species and micrometer-sized aggregates on the electrodes are confirmed by SEM images in both cases. AFM images show that the presence of polypyrrole allows for better nanostructuration of the NPs than without the polypyrrole. Both Photo-EPD and EPD-Photo electrodes exhibit enhanced photocurrent under visible irradiation compared with FTO/TiO2/Ru II where polypyrrole is absent. This observation proves the importance of the polypyrrole coating layer to increase the conductivity and charge injection rate to TiO2 semiconducting particles. The current magnitude produced by the Photo-EPD electrode is 30 % greater than that of the EPD-Photo electrode. It probably indicates a more efficient deposition process of the NPs already embedded in a polypyrrole network and/or faster charge injection kinetics of the electrode made by the Photo-EPD method. Meanwhile, the electrode made by the EPD-Photo may only have the polypyrrole occurring at the outermost layer.

1 )Scheme 1 :

 11 Scheme 1: Structures of BIH and BI(OH)H electron donorsAnother perspective from this thesis is the incorporation of the [Mn(ttpy)(CO)3Br] catalyst to the TiO2/poly(Ru-pyr) nanocomposite, then this hybrid system can be deposited on a FTO electrode. This electrode can be employed for the photoelectrocatalytic reduction of CO2. Similarly, [Cr(ttpy)2] 3+ can also be incorporated in the TiO2/poly(Ru-pyr)

  where I is the integrated area under the emission peak, Abs is the absorbance at the excitation wavelength, n is the refractive index of the solvent, and the subscript ref indicates a reference with known luminescent quantum yield. The reference is chosen to be [Ru(bpy)3](PF6)2 with ref = 0.062 in MeCN under Ar.1

  photocatalysis was analyzed using a Shimadzu liquid chromatograph LC-10AS equipped with an Alltech Select degassing system, a Perkin Elmer Series 200 UV-vis detector and a Perkin Elmer polypore H column (220 mm, 10 µm pore size, 20 µL injection volume), with the column eluent containing 10 mM H2SO4 in H2O. Both chromatographs were checked by injecting an aqueous solution of HCOONa (0.01 M) prior to the experiments.

  1 H NMR (400 MHz, CD3CN): ppm) = 8.49 (d, J = 7.9 Hz, 4H), 8.35 (s, 2H), 8.06 -8.01 (m, 4H), 7.73 (t, J = 5.0 Hz, 4H), 7.53 (d, J = 5.8 Hz , 2H), 7.41 -7.35 (m, 4H), 7.23 (d, J = 5.1 Hz , 2H), 2.53 (s, 6H). Electrochemistry (WE = C disk, RE = Ag/AgNO3 0.01 M, electrolyte = MeCN + 0.1 M TBAPF6, v = 100 mV.s -1 ): E1/2 (V) = 0.91, -1.66, -1.86, -2.12. bpy-PO3Et2. The synthesis of this ligand requires two steps. In the first step, lithium diisopropylamine (LDA) was prepared by dissolving diisopropylamine (DIPA, 630 l, 1.29 mmol) in 3 mL THF under Ar in an oven-dried flask. The solution was cooled to -40 0 C, stirred for 30 minutes before slowly adding n-butyllithium solution (2.5 M in hexane, 0.58 mL, 1.29 mmol). The LDA solution was kept stirring for one hour while the temperature was raised to 0 0 C, forming a pale green solution. In another oven-dried flask, a solution of 4,4'-the glove box and compressed air was bubbled through for 30 mins upon which a color change from purple to dark yellow was observed. The solution exposed to air was left stirring for 12 h. The bright yellow suspension was filtered and the resulting yellow solid recrystallized from a solution of H2O and EtOH and dried under high vacuum to give the product as a dark yellow solid (41 mg, 38 %). Electrochemistry (WE = C disk, RE = Ag/AgNO3 0.01 M, electrolyte = MeCN + 0.1 M TBAPF6, v = 100 mV.s -1 ): E1/2 (V) = -0.34, -0.77, -1.30, -2.21. 1 H NMR cannot be done because the product is paramagnetic.

1 H

 1 NMR cannot be done because the product is paramagnetic. UV-vis: λmax (MeCN) = 370 nm (ε = 36500 M -1 .cm -1 ). Electrochemistry: CVs cannot be done directly due to its poor solubility in any solvent except for DMSO. Instead, its redox potentials were studied by grafting [Cr2P] 3+ on a ITO electrode and used it as WE: E1/2 (V) = -0.42, -0.79, -1.32. (WE: area = 1.2 cm 2 , RE = Ag/AgNO3 0.01 M, electrolyte = MeCN + 0.1 M TBAPF6, v = 100 mV.s -1 ).

  potentials of [Cr1P] 3+ were studied by grafting it on a ITO electrode and used it as WE: E1/2 (V) = -0.44, -0.81, -1.34. (WE: area = 1.2 cm 2 , RE = Ag/AgNO3 0.01 M, electrolyte = MeCN + 0.1 M TBAPF6, v = 100 mV.s -1 ).

  (300 MHz, CDCl3): ppm) = 9.85 (s, 1H),7.80 (d, J = 8.7 Hz, 2H), 6.96 (d, J = 8.7 Hz, 2H), 4.10 (m, 6H), 2.18-1.86 (m, 4H), 1.30 (t, J = 7.0 Hz, 6H). 13 C NMR (75 MHz, CDCl3): ppm) =

  P NMR (121 MHz, CDCl3): ppm) = 31.2. MS-APCI: m/z = 301 [M+H] +

  P NMR (162 MHz, CDCl3): ppm) = 31.6. MS-APCI: m/z = 504 [M+H] +

1 H

 1 NMR (400 MHz, [D6]DMSO): ppm) = 9.02 (d, J = 8.0 Hz, 2H), 8.94 (d, J = 4.8 Hz, 2H), 8.86 (s, 2H), 8.44-8.40 (m, 2H), 8.05 (d, J = 8.7 Hz, 2H), 7.87-7.84 (m, 2H), 7.19 (d, J = 8.8 Hz, 2H), 4.16 (t, J = 6.3 Hz, 2H), 2.02-1.92 (m, 2H), 1.76-1.68 (m, 2H). 13 C NMR (100 MHz, [D6]DMSO): ppm) = 160.

  P NMR (162 MHz, [D6]DMSO): ppm) = 25.9. MS-MALDITOF: m/z = 448 [M+H] + dmbpy-pyr. The synthesis of this ligand consists of two steps. In the first step, a lithium diisopropylamine (LDA) was prepared by dissolving diisopropylamine (DIPA, 630 l, 4.07 mmol) in 5 mL THF under Ar. The solution was cooled to -20 0 C, stirred for 30 minutes before slowly adding n-butyllithium solution (2.5 M in hexane, 1.85 mL, 4.07 mmol). The LDA solution was kept stirring for one hour. In another oven-dried flask, a solution of 4,4'dimethyl-2,2'-dipyridine (dmbpy, 500 mg, 2.714 mmol) in 20 mL dried THF was prepared under Ar and cooled to -70 0 C. The LDA solution was added dropwise to the dmbpy solution at -70 0 C, and the mixture was stirred for 1 hour. In the second step, 1-(10bromodecyl)pyrrole (Br-pyr, 776 mg, 2.714 mmol) was added to the mixture, then the temperature was slowly raised to RT overnight. The reaction was stopped by adding water, then extracting with CH2Cl2 (x3), dried under vacuum, and the product was separated by chromatography (CH2Cl2/Ethyl acetate, 98/2 v/v) to yield a white powder (270 mg, 26 %).

  J = 8.4 Hz, 2H),2.50 (s, 3H). FT-IR (KBr pellet, C=O stretching):  (cm -1 ) =2022, 1948, 1916.

  washed in pure ethanol by centrifugation(1 hour, 5000 rpm). They were subsequently dried in an oven at 80 0 C overnight and calcined at 450 0 C for 2 hours to yield a white powder (276 mg). Titania NPs with a mixed anataserutile (80% -20%) phase were achieved by calcination of the anatase TiO2 NPs at 700 0 C instead of 450 0 C for 2 hours. TiO2/Ru II and SiO2/Ru II . In a typical experiment, a suspension of the TiO2 NPs (anatase, 50 mg) or SiO2 (d < 20 nm, 50 mg) in ethanol/acetone (8/2, v/v) was sonicated for 20 minutes. Subsequently, the [RuP] 2+ complex (14.2 mg, 14 mol) was added to the suspension, and the only [Cr2P] 3+ . In a typical experiment for 10% spacer + 90% [Cr2P] 3+ , a mixture of BPA(1.35 mol) and [Cr2P] 3+ (1.5 mol) in 25 mL DMF/DMSO (8/2 v/v) solution was used to graft onto anatase TiO2 NPs (50 mg), yielding 54.3 mg product. The loading of [Cr2P] 3+ on TiO2 is estimated to be 0.03 mmol.g -1 (about 570 Cr III molecules per particle or 0.3 Cr III molecules per nm 2 ). Ru 2+ /TiO2/[Cr2P] 3+ (Ru:Cr = 1:1). A suspension of TiO2 (anatase, 50 mg) in DMF/DMSO (8/2, v/v) solvent and a mixture of [Cr2P] 3+ (5.7 mg, 5 mol) and [RuP] 2+ (5.1 mg, 5 mol) in the same solvent were sonicated for 20 minutes. They were then mixed together and stirred vigorously for 60 hours in dark. Afterwards, the NPs were separated by centrifugation, washed in DMF/DMSO, DMF and ethanol, respectively, and dried in an oven overnight to yield an orange powder (54.7 mg). Both complexes are expected to be grafted quantitatively. The loadings of [Cr2P] 3+ and [Ru-PO3H2] 2+ are estimated to be 0.10 mmol.g -1 (about 1600 molecules per particle or 1 molecule per nm 2 ). Ru 2+ /TiO2/[Cr2P] 3+ and BPA spacers. Similar to the Ru 2+ /TiO2/[Cr2P] 3+ triad but the [Cr2P] 3+ content was reduced. In a typical experiment for 20 % Ru, 2 % Cr and 78 % spacer, we used a mixture as follows: TiO2 (50 mg), [Cr2P] 3+ (0.3 mg, 0.3 µmol), [Ru-PO3H2] 2+ (3It has been synthesized following a two-step procedure. First, the ttpy-(CH2)3-PO3H2 ligand (4.5 mg, 10 µmol) and TiO2 anatase NPs (50 mg) were solubilized in 25 mL DMSO. The mixture was stirred at RT for 20 h in dark. Afterwards, the modified NPs were separated by centrifugation and washed multipletimes with DMSO and acetone to remove unadsorbed ligands. In the second step, the NPs were mixed with Mn(CO)5Br (2.8 mg, 10 µmol) in 25 mL acetone and refluxed at 45 0 C for 3 h in dark. Finally, the TiO2/Mn I NPs were centrifuged and washed thoroughly with acetone prior to be dried under vacuum to yield a pale yellow powder (40.7 mg). Loading of Mn I on TiO2 is estimated to be ~0.20 mmol.g -1 (about 3800 Mn I molecules per particle or 2 Mn I molecules per nm 2 ). FT-IR (KBr pellet, C=O stretching):  (cm -1 ) = 2022, 1900It has been synthesized by stepwise grafting the Mn and Ru complexes on TiO2 anatase NPs (50 mg), respectively. The grafting procedure for Mn I is similar to that described for TiO2/Mn I NPs in the above paragraph, except that the contents of ttpy-(CH2)3-PO3H2 and Mn(CO)5Br have been reduced to 0.7 µmol (ie. 0.31 mg for ttpy-(CH2)3-PO3H2 and 0.19 mg for Mn(CO)5Br). After being washed thoroughly with acetone and dried under vacuum, the TiO2/Mn I NPs were mixed with [RuP] 2+(7.1 mg, 7 µmol) in 25 mL H2O. The mixture was stirred at RT for 20 h in dark. Afterwards, the modified NPs were separated by centrifugation and washed thoroughly with water, then acetone to remove water from the particles. Finally, the acetone solvent was evaporated to yield a pale orange powder (23.6 mg).TiO2/[Ru-pyr] 2+ . This modification of anatase TiO2 NPs requires two steps. In the first step, a suspension of the NPs (50 mg) in distilled water was sonicated for 20 minutes, before addition of bpy-PO3H2 (38 mol, 11.7 mg) and water to make a total volume of 25 mL. The mixture was vigorously stirred at RT overnight to functionalize the surface with bpy ligands. The NPs were then excessively washed with water and separated by centrifugation (4000 rpm, 45 minutes) until the solution turns from acidic to neutral pH, and no bpy-PO3H2 was spectroscopically detected from the supernatant solution. The particles were then washed two times with ethanol prior to their complexation. In the second step, a suspension of TiO2/bpy NPs in ethanol/acetone (8/2, v/v) was sonicated for 20 minutes, before the addition of [Ru(dmbpy-pyr)2]Cl2 (15.3 mol, 14.6 mg) and the mixed solvent to a total volume of 25 mL. The mixture was refluxed for 60 hours. The TiO2/[Ru-pyr] 2+ NPs were washed in ethanol/acetone and ethanol excessively, separated by centrifugation (4000 rpm, 45 minutes) and dried in an oven overnight to yield a dark red powder (44.4 mg). Loading of [Ru-pyr] 2+ is estimated to be ~0.20 mmol.g -1 (about 3800 molecules per particle or 2 molecules per nm 2 ). Electrochemistry (in MeCN + 0.1 M TBAPF6, WE = Pt microcavity electrode, v = 100 mV.s -1 ): E1/2 = 0.82 V (irreversible).

  Ce mémoire vise à montrer l'intérêt de nanoparticules (NPs) de TiO2 comme plateforme pour immobiliser dans un environnement proche des complexes de coordination pouvant interagir par transfert d'électron photoinduit. Nous nous sommes intéressés à l'étude de nanomatériaux hybrides associant le complexe [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridine) comme photosensibilisateur aux complexes [Cr(ttpy)2] 3+ ou [Mn(ttpy)(CO)3Br] (ttpy = 4'-(p-tolyl)-2,2':6',2''-terpyridine) comme accepteurs d'électrons. Pour immobiliser les différents complexes à la surface du TiO2, une fonction acide phosphonique a été introduite sur une des bipyridines du centre [Ru(bpy)3] 2+ et sur la terpyridine des complexes [Cr(ttpy)2] 3+ . L'étude des processus de transferts de charges photo-induits sous irradiation en lumière visible sur le colloïde TiO2/Ru II montre que l'état à charges séparées (e -)TiO2/Ru III possède une longue durée de vie, ce qui rend possible l'utilisation des charges dans des réactions successives d'oxydation ou de réduction. Notamment l'irradiation du colloïde TiO2/Ru II en présence de [Cr(ttpy)2] 3+ et de triéthanolamine (TEOA) comme donneur d'électron sacrificiel permet la réduction à deux électrons du [Cr(ttpy)2] 3+ . Par la suite, le complexe [Cr(ttpy)2] 3+ est immobilisé sur les NPs de TiO2/Ru II pour former un assemblage Ru II /TiO2/Cr III au sein duquel les processus de transfert d'électrons photo-induits sont étudiés. De manière à proposer un système pour la réduction photocatalytique du CO2, le complexe [Mn(ttpy)(CO)3Br] a été co-immobilisé avec le [Ru(bpy)3] 2+ suivant une approche de chimie sur surface pour former le colloïde Ru II /TiO2/Mn I . Ce système présente une excellente sélectivité vis-à-vis du HCOOH comme seul produit de la photoréduction du CO2 en présence de 1-benzyl-1,4dihydronicotinamide (BNAH) comme donneur d'électron sacrificiel. Un système hybride associant le [Ru(bpy)3] 2+ portant des fonctions pyrroles et immobilisé sur TiO2 a également été synthétisé et étudié. Sous irradiation lumineuse, le transfert de charge (e -)TiO2/[Ru-pyr] 3+ permet d'induire la polymérisation du pyrrole. Le nanocomposite TiO2/poly(Ru-pyr) obtenu et déposé sur une électrode génère, en présence de TEOA, un photocourant anodique stable de plus de 10 μA.cm -2 . L'ensemble des résultats montre que les NPs de TiO2 peuvent être un moyen d'assembler des complexes dans un environnement proche en limitant les interactions à l'état fondamental, mais permettant des transferts d'électron photoinduits entre eux. Suivant les potentiels redox des différents composants, les transferts d'électron ont lieu soit via la nanoparticule soit en surface de celle-ci.

Table I - 1 .

 I1 Summary of the photocatalytic reduction of CO2 using TiO2 NPs mentioned in reference 14

	Catalyst *	Crystal	Particle	Surface	CO2	Bandgap	Amount of CH4
		phase	size	area	adsorbed	(eV)	(µmol.h -1 .g -1 )
			(nm)	(m 2 .g -1 )	(µmol.g -1 )
	JRC-TIO-2	Anatase	400	16	1	3.47	0.03
	JRC-TIO-3	Rutile	30-50	51	17	3.32	0.02
	JRC-TIO-4	Anatase	21	49	10	3.50	0.17
	JRC-TIO-5	Rutile	640	3	0.4	3.09	0.04

* Japan TiO2 reference catalyst

  3] 2+* excited state. The Rehm-Weller equations can be re-written as

	follows:
	E	III (Ru /Ru ) II*		E	III (Ru /Ru ) II		III (Ru /Ru ) II  ,77 K em em 1240 ,RT () hc E e nm  	(Eq I-7)
	E	II* (Ru /Ru ) I	II (Ru /Ru )+ I  ,77 K em hc E e  II I (Ru /Ru )+ em 1240 ,RT () E nm	(Eq I-8)

4. Anchoring functional groups to graft a metal complex onto a surface

		G   	* ( ) nF E D 1/2 ox  1/2 ( ) red E A   or	G   	* ( ) nF E D E A 1/2 1/2 ( ) ox red   	(Eq I-10)
	where	* 1/2 ()

ox ED or I.2.

Table I - 2 .

 I2 Common anchoring groups for various surfaces

	Surface	Anchoring groups	References
	TiO2	Phosphonic acid R-PO3H2	26
		Carboxylic acid R-COOH	27
		Catechol Ph-(OH)2	27
	Au	Thiol R-SH	28
	SiO2	Silane R3-SiH	29
		Phosphonic acid R-PO3H2	30
	ZrO2	Phosphonic acid R-PO3H2	26
		Silane R-SiH3	31
	ITO	Phosphonic acid R-PO3H2	32
		Carboxylic acid R-COOH	33

  2+* was efficiently quenched by electron injection to TiO2 colloids with kinj > 5 × 10 7 s -1 and > 80% efficiency. Back electron transfer occurred in a microsecond time scale,

Argazzi et al. 39

reported a long-lived charge separated state when they irradiated TiO2/[Ru(bpy)2(dcb)] 2+ (Scheme I-2c) NPs in aqueous solution at pH 2. The emission of [Ru(bpy)2(dcb)]

  The resulting TiO2/[Ru-pyr] 2+ NPs can be photopolymerized under visible light to form a nanocomposite TiO2/poly(Ru-pyr) where a polypyrrole network is formed around the TiO2 NPs. In this photopolymerization process, TiO2 acts as an electron acceptor for [Ru(bpy)3] 2+* excited state, and the electrons injected on TiO2 are scavenged by O2. Afterwards, the

	Abstract
	In this chapter we will present the electrochemical and photophysical properties of a
	[Ru(bpy)3] 2+ photosensitizer (bpy = 2,2'-bipyridine) bearing a phosphonate group.
	Afterwards, the phosphonate group is converted to phosphonic acid group to anchor on TiO2
	nanocomposite is deposited onto an electrode for light-to-electricity energy conversion and SiO2 nanoparticles. Photo-induced charge and energy transfer processes in TiO2/Ru II and
	application. SiO2/Ru II colloids under visible light have been studied by time-resolved emission
	spectroscopy and transient absorption spectroscopy. The electron injection from [Ru(bpy)3] 2+*
	excited state to the conduction band of TiO2 occurs in nanosecond time scale to form the
	charge separated state (e -)TiO2/Ru III , whereas charge recombination is in millisecond time
	scale. The trapped electrons on the Ti 3+ sites of TiO2 are evidenced by EPR spectroscopy
	under continuous irradiation. The efficient, long-lived charge separated state (e -)TiO2/Ru III
	CHAPTER II
	PHOTO-INDUCED CHARGE TRANSFER
	PROCESSES ON Ru(II) TRIS-BIPYRIDINE
	SENSITIZED TiO 2 NANOPARTICLES

provides good opportunities to harvest the charges for subsequent redox reactions, which will be discussed in the remaining chapters.

Résumé

Dans ce chapitre, nous présentons les propriétés électrochimiques et photophysiques d'un nouveau complexe du [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridine) portant une fonction d'ancrage de type phosphonate sur une des bipyridines. Par la suite, la fonction phosphonate est hydrolysée en acide phosphonique pour permettre l'immobilisation des complexes du ruthénium sur des nanoparticules de TiO2 et SiO2. Les processus de transfert de charge et d'énergie induits par irradiation lumineuse dans les systèmes hydrides colloïdes TiO2/Ru II et SiO2/Ru II sous lumière visible ont été étudiés par spectroscopie d'émission résolus en temps et spectroscopie d'absorption transitoire en régime nanoseconde. L'injection d'électrons de l'état

Table II -

 II

				em,	, ns (%) b	
				nm a
	[Ru(bpy)3] 2+	286 [75000]	451 [13000]	608	757 ± 9	0.062
	[Ru(bpy)2(dmbpy)] 2+	286 [82000]	454 [14000]	614	986 ± 12	0.070
	[Ru-PO3Et2] 2+	286 [80000]	454 [14100]	615	895 ± 10	0.068

1. Summary of photophysical properties of [Ru(bpy)3] 2+ , [Ru(bpy)2(dmbpy)] 2+ and [Ru-PO3Et2] 2+ : MLCT absorption peak abs, peak of luminescent emission em, lifetime  of the photoexcited Ru 2+* species and the emission quantum yield . LCCT abs, nm [, M -1 .cm -1 ] 1 MLCT abs, nm [, M -1 .cm -1 ] a Excitation at 450 nm b Excitation by a picosecond pulsed laser at 400 nm All measurements were conducted in MeCN solution under Ar at room temperature.

Table II -

 II is the Planck constant, c the speed of light, em,77 K and em,RT the wavelength at emission maximum at 77 K and room temperature, respectively. The redox potential of Ru III /Ru II* is determined to be -1.10 V vs Ag/AgNO3 0.01 M for both [Ru-PO3Et2] 2+ and [Ru(bpy)3] 2+ complexes. It can be concluded that [Ru-PO3Et2] 2+ complex retains beneficial electrochemical and photoredox properties of the [Ru(bpy)3] 2+ complex.

	where h II.2.4. Electron paramagnetic resonance spectroscopy
	The electron paramagnetic resonance (EPR) spectroscopy is a convenient technique for
	qualitative and quantitative studies of paramagnetic species, notably free radicals and
	transition metal complexes. [Ru(bpy)3] n+ (n = 3, 2, 1) complexes were chosen as a reference
	for subsequent studies of the [Ru-PO3Et2] 2+ grafted on SiO2 or TiO2 NPs. The complexes
	were electrochemically synthesized by exhaustive electrolysis of [Ru(bpy)3] 2+ in MeCN + 0.1
					featured at 2590 G corresponding to g = 2.695 In literature the
	EPR signal of [Ru(bpy)3](PF6)3 at 77 K is characterized with g = 2.65 and g// = 1.14. 9 The
	latter g component is not observed in our experiment as it is broad and weak in intensity. The
	signal is typical for a change in the oxidation state of a transition metal. It is then ascribed to a
	Ru II  Ru III oxidation.			Eox		Ered (1)	Ered (2)	Ered (3)
	[Ru(bpy)] 2+,5 When singly reduced to [Ru(bpy)3] + , the complex shows a much narrower signal 0.94 -1.67 -1.83 -2.07
	[Ru(bpy)2(dmbpy)] 2+ (Figure II-5b) featured by g = 2.002 and g// = 1.984, which is ascribed to an electron 0.91 (0.06) -1.66 (0.06) -1.86 (0.06) -2.12 (0.08)
	[Ru-PO3Et2] 2+ localized on a bpy ligand, [Ru II (bpy -)(bpy)2] + . The signals are also in line with previous 0.92 (0.06) -1.67 (0.06) -1.86 (0.06) -2.12 (0.08)
	studies. 8-10
	In this study, [Ru-PO3Et2] 2+ is used as a photosensitizer to inject an electron to the
	conduction band (CB) of TiO2 NPs under visible irradiation, forming (e -)TiO2 and [RuP] 3+ .
	Therefore, the redox potential of [RuP] III/II* is of great interest to study the photo-induced
	electron transfer process. From the redox potentials of the ground state and the emission
	spectra of the complexes, one can estimate the redox potential of Ru III/II* by using the
	E	III (Ru /Ru ) II*		E	III (Ru /Ru ) II		III (Ru /Ru ) II  ,77 K 1240 ,RT em em hc E e  	(Eq II-1)

2. Summary of electrochemical properties of [Ru(bpy)3] 2+ , [Ru(bpy)2(dmbpy)] 2+ and [Ru-PO3Et2] 2+ complexes E1/2 (Ep), V simplified Rehm-Weller equation (Equation II-1): M TBAPF6 solution under Ar. The applied potentials were 1.1 V to form [Ru(bpy)3] 3+ and -1.65 V to form [Ru(bpy)3] + . EPR signals of [Ru(bpy)3] n+ (n = 3, 1) complexes are presented in Figure II-5. The starting complex [Ru(bpy)3] 2+ possesses a low spin d 6 electronic configuration, 5,8 making it EPR silent. When oxidized to [Ru(bpy)3] 3+ , it is transformed to a d 5 configuration and showing a broad signal over ~ 2000 G. The signal of [Ru(bpy)3] 3+ (Figure II-5a) is

Table II -3 summarizes

 II the [RuP] 2+ dye loading values on commercially available SiO2 and anatase TiO2 NPs. The loadings per particle and per nm 2 are roughly estimated by assuming the diameter of TiO2 and SiO2 as 25 nm and 20 nm, respectively. For more detail the reader is kindly referred to the Experimental Section. The loadings per nm 2 for both kinds of NPs are in agreement with the loadings obtained with flat surfaces of SiO2 and TiO2

described in Section II.3.2. Table II-3. Loading

  of [RuP] 2+ dye on commercially available SiO2 and TiO2 (anatase) NPs.

Loading (mmol.g -1 ) Molecules per particle Molecules per nm 2

	SiO2/Ru II NPs	0.076	460	0.5
	TiO2/Ru II NPs	0.21	4000	2.0
	II.3.4. Infrared spectroscopy
	Figure II-12 presents the FT-IR spectra of SiO2/Ru II (a) and TiO2/Ru II NPs (b), in
	comparison with the phosphonic-derivatized dye, [RuP] 2+ , and naked SiO2 and TiO2 NPs. The
	complex spectrum shows the P=O stretch at 1250 cm -1 , which disappears in the spectrum of
	TiO2/Ru II . A shoulder at ~1150 cm -1 for TiO2/Ru II (shown as a peak in the subtracted
	spectrum 4) is attributed to the bidentate surface linkage of -P(O)(O -)2.

Figure II-13a presents the

  III /Ru II and Ru II /Ru I potentials.

	[Ru-PO3Et2] 2+ complex in MeCN solution (
	CV of the FTO/Ru II electrode. Reversible waves at E1/2 = 0.93 V and -1.76 V (E = 60 mV)
	are attributed to Ru III /Ru II and Ru II /Ru I couples, which is in accordance with the protected

see Section II.2.3). A linear relationship between the

  anodic peak current Ipa vs scan rate v (Figure II-13b) for the Ru III /Ru II peak proves that the complex has been successfully grafted onto the electrode.

on TiO2 and SiO2 NPs

	Our attempts to characterize the electrochemical properties of SiO2/Ru II and TiO2/Ru II
	in colloidal solution with C, Pt disk electrodes or C sponge electrode were unsuccessful due to
	low conductivity of TiO2 and SiO2 and low coverage of the redox active species on the
	electrode surface. Therefore, SiO2/Ru II and TiO2/Ru II NPs were examined in powder form by
	cyclic voltammetry using a microcavity Pt working electrode (d = 50 µm). In the anodic
	portion, the SiO2/Ru II exhibits a reversible redox wave at E1/2 = 0.92 V (Ep = 60 mV)
	(Figure II-14a), whereas the TiO2/Ru II shows a quasi-reversible wave at E1/2 = 0.98 V (Ep =
	130 mV) (Figure II-14c). The waves can be attributed to the Ru III /Ru II couple. The redox
	potential of Ru II complex anchored on SiO2 is similar to that of [Ru-PO3Et2] 2+ free complex in
	MeCN and FTO/Ru II electrode, while there is a shift to a more positive oxidation potential
	when the complex is anchored on TiO2. Positive shifts of the redox potential of Ru III /Ru II
	complexes grafted on a nanocrystalline TiO2 thin film versus free complexes in solution have
	also been reported for phosphonate 3 -and carboxylate 24 -derivatized complexes.
	In the cathodic portion of SiO2/Ru II NPs (Figure II-14a), the Ru II /Ru I peak at ca. -1.7 V
	of the precursor [Ru-PO3Et2] 2+ is not observed. In the case of TiO2/Ru II (Figure II-14b), the
	direct charge injection to the CB of TiO2 starts at about -1.0 V vs Ag/Ag + 0.01 M. A similar
	CV for TiO2 film deposited onto a graphite electrode (Figure II-15)

Figure II-16a presents

	their absorption after subtracting the
	absorbance of KBr + NPs. The peaks at ~450 nm of both functionalized NPs can be attributed
	to the 1 MLCT of [Ru(bpy)3] 2+ , as the [Ru-PO3Et2] 2+ shows a similar peak. The LCCT peak at
	286 nm is evidenced for the SiO2/Ru II NPs. For TiO2/Ru II NPs, the absorption overlap
	between Ru II dye and TiO2 in the UV region makes it impossible to observe the LCCT at 286
	nm for the dye. In fact, the absorption of TiO2 NPs starts at ~392 nm (dotted line). This
	absorption of TiO2 allows for the bandgap calculation:	E	g	1240 1240 3.2 (eV) 392 g  	. The
	value is in accordance with the bandgap of bulk anatase TiO2, since quantum confinement
	effects which cause larger bandgaps only take place where the diameter of the particles is
	below 1 nm. 26
	The emission of grafted Ru II* excited state was normalized and shown

in Figure II-16b.

b) Time-resolved emission spectroscopy

	In order to gain more insight into this emission shift, a mixture of [RuP] 2+ (10%) and
	benzylphosphonic acid (BPA, 90%) was immobilized onto TiO2 NPs. The BPA acts as a
	lateral spacer to separate the Ru II species on the particles. The resulting NPs, denoted as
	[TiO2/Ru II + spacers], display an emission peak at 615 nm (Figure II-16b) similar to that of
	SiO2/Ru II and [Ru-PO3Et2] 2+ . Hence, the red-shift emission of TiO2/Ru II NPs could be
	attributed to the enhanced energy transfer between neighboring Ru II* and Ru II species.
	Our attempts to measure the luminescence quantum yield of SiO2/Ru II and TiO2/Ru II
	colloids in MeCN were not successful. In order to measure it correctly, the absorbance of Ru II
	species grafted on the NPs at 450 nm needs to be approximately equal to that of
	[Ru(bpy)3](PF6)2 reference. However, the light scattering, which gradually raises the
	absorbance as the probe light is swept from long to short wavelengths, strongly affects the
	Ru II absorbance at 450 nm. Mathematical attempts to subtract this light scattering effect from
	the real absorption of Ru II species are not efficient. The effect makes it impossible to correctly
	determine the amount of Ru II anchored on SiO2 and TiO2 NPs, thus the quantum yields are
	not measured.
	The time-resolved emission spectra of SiO2/Ru II* and TiO2/Ru II* were recorded after
	excitation by a 400 nm pulsed laser. The decays of the Ru II* excited states are presented in
	Figure II-17. The signal of SiO2/Ru II NPs (black line) can be fit with a monoexponential
	decay to give a lifetime of 838 ns, which is comparable to that of the [Ru-PO3Et2] 2+ complex
	in Ar-saturated MeCN solution (907 ns). The signal of TiO2/Ru II (red line), however, can only
	be fit with a biexponential function to give two lifetime values: 366 ns and 32 ns. The
	fractional amplitudes of these lifetimes are 12% and 88%, respectively. The [TiO2/Ru II +
	spacers] NPs also exhibit a biexponential decay and two lifetimes with corresponding
	fractional amplitudes: 488 ns (2%) and 8 ns (98%). Hence, the short lifetime component of
	TiO2/Ru II* NPs is attributed to the electron injection from the Ru II* to the CB of TiO2, which
	is not observed for SiO2/Ru II NPs. The photo-induced electron transfer process from the Ru II*
	state to TiO2 is not total, as evidenced by the presence of a second component in the decay.
	This longer component still remains shorter than the excited state lifetime of [Ru-PO3Et2] 2+

in Table II-4.

	Using SiO2/Ru II lifetime as a reference where there are no electron injections, the
	kinetics of electron injection from Ru II* to TiO2 substrate, kinj, can be calculated by Equation
	II-5:
	k inj		2 1/ 11 SiO Ru  	(Eq II-5)
	where 1  and		2 / SiO Ru	(s) are the short lifetime of TiO2/Ru II* and the lifetime of SiO2/Ru II* ,
	respectively. Similarly, the kinetics of energy quenching of Ru II* by neighboring Ru II , kq,
	referenced to [Ru-PO3Et2] 2+ free complex in solution, can be estimated by Equation II-6:
	k q	2 [] 32 11 2 Ru PO Et   		(Eq II-6)
	where 2  and		2 [] Ru PO Et 32 		(s) are the long lifetime of TiO2/Ru II* and the lifetime of [Ru-
	PO3Et2] 2+ , respectively. The kinetics values, together with the photophysical properties of the
	NPs, are collected

Table II -4. Summary

 II of photophysical properties of SiO2/Ru II and TiO2/Ru II colloids in MeCN

1 MLCT abs, nm em, nm , ns (%) a kinj, s -1, b kq, s -1, c

	[Ru-PO3Et2] 2+	450	615	895 ± 10	-	-
	SiO2/Ru II NPs	450	612	838 ± 43	-	7.6 × 10 4
	TiO2/Ru II NPs	450	627	366 ± 7 (12%)	3.0 × 10 7 1.6 × 10 6
				32 ± 0.6 (88%)
	Ru II /TiO2/BPA	450	615	488 ± 33 (2%)	1.2 × 10 8 9.5 × 10 5
	(Ru:BPA = 1:9)			8 ± 0.4 (98%)
	a Emission lifetime recorded at 610 nm after excitation by 400 nm nanosecond pulsed laser
	b Kinetics of electron injection from Ru II* to TiO2
	c Kinetics of energy transfer from Ru II* to a neighboring Ru II
	As seen in Table II-4, the rate of energy transfer on SiO2/Ru II NPs is significantly
	slower than on TiO2/Ru II due to lower loading of the [RuP] 2+ dye on SiO2 (0.076 mmol.g -1 )

Table II-5 summarizes the

	kinetics of electron injection and kinetics of energy transfer for similar systems in the
	aforementioned works.

Table II -5. Summary

 II of kinj and kq values obtained in literature for similar systems

Table II -6. Forster

 II resonance energy transfer: lifetime, efficiency and the distance between neighboring Ru II

	species on TiO2 and SiO2 surfaces
		, ns	Efficiency	r (Ru-Ru), nm
	[Ru-PO3Et2] 2+	895
	SiO2/Ru II NPs	838	0.06	2.5
	TiO2/Ru II NPs	366	0.60	1.8
	Ru II /TiO2/BPA	488	0.45	2.0
	It is noted that the energy quenching of SiO2/Ru II* is drastically lower than TiO2/Ru II* ,
	which is explained by a greater distance between the Ru II sites. As expected, the addition of
	BPA spacers reduces the FRET efficiency. It is however still substantially higher than that of
	SiO2/Ru II . The Ru II loading on the surfaces is estimated to be 0.076 mmol.g -1 for SiO2/Ru II
	and 0.02 mmol.g -1 for [TiO2/Ru II + spacers]. The higher Ru II loading but lower FRET
	efficiency and greater Ru-Ru distance of SiO2/Ru II suggest then a more uniform distribution
	of [RuP] 2+ on SiO2 than on TiO2.
	It should also be mentioned that taking into account the calculated average distances,
	the Dexter quenching mechanism cannot be totally excluded. Statistically speaking, some
	pairs of complex molecules can be in contact and their orbitals may overlap to exchange
	electrons. However,

the distances calculated by the FRET method are in line with the surface loading of Ru II species (see Section II.3.3): 0.5 molecules/nm 2 for SiO2/Ru II and 2 molecules/nm 2 for TiO2/Ru II . c) Transient absorption spectroscopy

	TA measurements were performed in order to study the absorption spectrum of photo-
	excited state of SiO2/Ru II* and TiO2/Ru II* and the kinetics of charge recombination. For that
	reason we chose to work with MeCN solvent under Ar to avoid any photocatalytic reactions
	involving the injected electrons on the CB of TiO2 or the oxidized Ru 3+ species. Diluted
	suspensions of NPs in MeCN were required to reduce light scattering effect by the NPs.

Table III - 1 .

 III1 Redox potentials of [Cr(ttpy)2] 2+ complex and ttpy ligand In order to get insight into the charge distribution during subsequent reduction steps, DFT calculations have been performed by Dr. Jean-Marie Mouesca (CEA Grenoble/ INAC/ SyMMES/ CAMPES). The GGA (Generalized Gradient Approximation) exchangecorrelation potential was used to obtain the Mulliken charge and spin population distribution over the [Cr(ttpy)2] n+ (n = 3, 2, 1, 0, -1) complexes. The spin populations on Cr atom compose of  and  spins in its d orbitals which are aligned with or against an external magnetic field, respectively. The  spin number partly quantifies how much the electron(s) localized on the ttpy ligand will spread onto the Cr empty d orbitals. The Mulliken charge and spin populations on Cr of different oxidation states are summarized in TableIII-2.

	E1/2 (Ep), V	Ered (1)	Ered (2)	Ered (3)	Ered (4)
	[Cr(ttpy)2] 3+	-0.47 (0.07) -0.85 (0.07) -1.35 (0.07) -2.25 (0.07)
	ttpy	-2.35 (0.09)	-	-	-

Table III - 2 .

 III2 Mulliken charge and spin populations on Cr atoms of [Cr(ttpy)2] n+ complexes. GS = ground state,

	ES = excited state
	Complex oxidation state	Spin state	Mulliken (Cr) Charge population	Mulliken (Cr) Spin population	 spin (Cr) population	 spin (Cr) population
	+3	3/2	1.22	3.06	3.62	0.79
	+2	1	1.17	2.50	3.41	1.08
	+1	1/2	1.14	2.11	3.25	1.26
	0	0 (GS)	1.12	1.10	2.80	1.77
		1 (ES)	1.17	2.26	3.31	1.19
	-1	1/2 (GS)	1.14	1.55	3.00	1.55
		3/2 (ES)	1.18	2.48	3.41	1.08

Table III -

 III The complex oxidation states have been converged from GGA solutions mentioned above before using the SAOP method to ensure the continuity between the GGA and SAOP calculations. The results are summarized in TableIII-4.

	[Cr(ttpy)2] 3+	370 [36500], 441 [3500], 475 [1500]
	[Cr(ttpy)2] 2+	500 [7700], 584 [2600], 800 [2800]
	[Cr(ttpy)2] +	524 [20200], 560 [19500], 723 [5000]
	[Cr(ttpy)2] 0	520 [37500], 571 [33600], 670 [19900], 776 [10500]
	Time-Dependent (TD) DFT calculations have also been carried out by Dr. Jean-Marie

3. UV-vis extinction coefficients of [Cr(ttpy)2] n+ : n = 3, 2, 1, 0 Complex abs, nm [, M -1 .cm -1 ] Mouesca for the assignment of the UV-vis absorption peaks of the [Cr(ttpy)2] n+ complexes to corresponding electronic transitions. The SAOP (Statistical Average of Orbital Potentials) exchange-correlation potential 21 , which is especially suited for the TD-DFT calculations, has been used.

Table III - 4 .

 III4 Experimental and calculated electronic transition energies for [Cr(ttpy)2] n+ (n = 2, 1) complexes, together with the assignment of their electronic transitions

	Complex Experimental	Experimental	Calculated	Assignment
		abs (nm)	energies (cm -1 )	energies (cm -1 )
	[Cr(ttpy)2] 2+	500	20000	18800
		584	17120
		800 (broad)	12500	13000 and 12100	LMCT
	[Cr(ttpy)2] +	524	19100	20600
		560	17900	17900
		723	13800	13000	LMCT
	For the [Cr(ttpy)2]

2+

singly reduced complex, [Cr III (ttpy)(ttpy -)] 2+ , calculations have been performed for transitions at 828 nm and 767 nm, which are in accordance with the broad absorption band centered at 800 nm. Electronic transitions at these wavelengths are shown in Figure III-4. It is surprising that in the ground state of [Cr(ttpy)2] 2+ , the electron is localized on the tolyl substituting group rather than the tpy part. The transition at 828 nm corresponds

  is the Planck constant, c the speed of light, e the charge of an electron, em,77 K and em,RT (nm) the wavelength at emission maximum at 77 K and room temperature, respectively.

	E	III* (Cr /Cr ) II		III (Cr /Cr ) II  ,77 K em em 1240 ,RT () hc E e nm  		E	III (Cr /Cr ) II	(Eq III-1)
	where h Taking into account E(Cr III /Cr II ) = -0.47 V, the redox potential E(Cr III* /Cr II ) is thus estimated
	as 1.14 V, making the excited state Cr III* a strong oxidant. The result is in line with literature
	for the same complex (E(Cr III* /Cr II ) = 1.06 V) 22 . A previous work 2 on [Cr(bpy)3] 3+ in aqueous
	solution showed E(Cr III* /Cr II ) = 1.44 V vs NHE (about 0.89 V vs Ag/AgNO3 0.01 M), hence
	the [Cr(bpy)3] 3+* is a weaker oxidant than [Cr(ttpy)2] 3+* .
	Time-resolved emission spectroscopy was also carried out to study the emission decay
	E		em E 	E

III* state, E(Cr III* /Cr II ), as follows: III* II III II (Cr /Cr ) (Cr /Cr ) in nanosecond time scale. The decay recorded at 770 nm is shown in Figure III-7. It is well fit with a biexponential function (red curve), giving a short lifetime of 3 ns and a long lifetime

5. Photoreduction of [Cr(ttpy)2] 3+ in the presence of TEOA

	Its EPR signal shows anisotropy of the g-values: g = (2.043, 2.043, 1.955). No clear evidence
	for the hyperfine coupling to the 53 Cr (I = 3/2) naturally occurred isotope is observed. The g-
	values are slightly smaller than that of [Cr(tpy)2] + . 5 It is important to note that the anisotropy
	of the g-values of [Cr(tpy)2] + are more pronounced: g = (2.0537, 2.0471, 1.9603) than
	[Cr(ttpy)2] + .

spectrum showing multiple signals spread over more than 6000 G. The spectrum of [Cr(ttpy)2] 3+ is in accordance with literature for an analogous [Cr(tpy)2] 3+ complex.

23

Amongst the redox states, the doubly reduced [Cr(ttpy)2] + complex is of great interest.

The EPR experiments clearly show that different oxidized states of [Cr(ttpy)2] n+ (n = 3, 2, 1) possess unique EPR signature. This method can be very helpful to identify the oxidation state of the Cr complex when it is photoreduced by TiO2 or TiO2/Ru II NPs, which will be discussed in Sections III.3 and III.4.

III.2.

Quenching of [Cr(ttpy)2] 3+* state by TEOA studied by Stern-Volmer experiment

	To get insight into the quenching mechanism, we conducted a Stern-Volmer
	experiment. In terms of thermodynamics, the oxidizing power of [Cr(ttpy)2] 3+* excited state
	(E(Cr 3+* /Cr 2+ ) = 1.14 V, see Section III.2.3b) is strong enough to oxidize TEOA
	(E(TEOA + /TEOA) = 0.42 V vs Ag/AgNO3 0.01 M) 24 as follows:
	[Cr(ttpy)2] 3+ + h  [Cr(ttpy)2] 3+*
	[Cr(ttpy)2] 3+* + TEOA  [Cr(ttpy)2] 2+ + TEOA +
	TEOA

+  degraded products

The photoinduced electron transfer reaction between Cr 3+* and TEOA is largely exergonic (G = -0.72 eV). Without light this reaction is not favourable since the oxidizing power of [Cr(ttpy)2] 3+ ground state (E(Cr 3+ /Cr 2+ ) = -0.47 V) is much lower than the redox potential of TEOA.

6. Photoreduction of [Cr(ttpy)2] 3+ in the presence of [Ru(bpy)3] 2+ and TEOA

	deactivation pathway for [Cr(ttpy)2] 3+* excited state involving static quenching due to a
	nucleotide-bound Cr(III) complex. In our study, the quenching of [Cr(ttpy)2] 3+* by TEOA
	seems purely dynamic.
	To summarize, the photoreduction of [Cr(ttpy)2] 3+ by TEOA leads to the formation of
	[Cr(ttpy)2] + after two electron transfer steps under continuous irradiation. The reaction occurs
	through the formation of the transient [Cr(ttpy)2] 2+ species, suggesting that this species can
	also be photoactive. On a thermodynamic point of view, [Cr(ttpy)2]

3

with thynine base. Moreover, at high nucleotide concentrations, the Stern-Volmer plot shows an upward curvation indicating an additional + can reduce any species with a potential higher than -0.85 Vvs Ag/AgNO3 0.01 M (around -0.30 V vs NHE).

III.2.

Table III -5. Redox

 III

potentials of ttpy and ttpy-PO3Et2 ligands, [Cr(ttpy)2] 3+ and [Cr(ttpy-PO3Et2)2]

3+ complexes E1/2 (Ep), V Ered (1) Ered (2) Ered (3) Ered (4)

	ttpy	-2.35 (0.09)	-	-	-
	ttpy-PO3Et2	-2.13 (0.07)	-	-	-
	[Cr(ttpy)2] 3+	-0.47 (0.07) -0.85 (0.07) -1.35 (0.07) -2.25 (0.07)
	[Cr(ttpy-PO3Et2)2] 3+ -0.34 (0.05) -0.77 (0.06) -1.30 (0.08) -2.21 (0.08)

  complex, showing four successive one-electron reduction processes centered at E1/2 = -0.34, -0.77, -1.30 and -2.21 V. Based on the electrochemical properties of [Cr(ttpy)2] 3+ complex, these half-wave potentials are thus attributed to [Cr(ttpy)2] 3+ /[Cr(ttpy)2] 2+ ,

	[Cr(ttpy)2] 2+ /[Cr(ttpy)2] + , [Cr(ttpy)2] + /[Cr(ttpy)2] 0 and [Cr(ttpy)2] 0 /[Cr(ttpy)2] -redox couples,
	respectively. The added electrons are localized on the ligands while the oxidation state of Cr
	remains +3. These potentials are slightly positively shifted compared with those obtained for
	[Cr(ttpy)2] 3+ complex. This shift is a consequence of the electron withdrawing phosphonate
	group making it easier to reduce. The fourth reduction process of [Cr(ttpy-PO3Et2)2] 3+

Table III - 6 .

 III6 Redox potentials of ITO/[Cr2P] 3+ electrode and [Cr(ttpy-PO3Et2)2] 3+ complex in MeCN + 0.1 M

	TBAPF6 under Ar
			E1/2 (Ep), V	Ered (1)	Ered (2)	Ered (3)
			ITO/[Cr2P] 3+	-0.42 (0.02) -0.79 (0.03) -1.32 (0.03)
			[Cr(ttpy-PO3Et2)2] 3+ -0.34 (0.05) -0.77 (0.06) -1.30 (0.08)
	The CV of ITO/[Cr2P] 3+ electrode shows three successive one-electron reduction steps,
	which are centered at E1/2 = -0.42, -0.79 and -1.32 V. These values are similar to those
	obtained for the [Cr(ttpy-PO3Et2)2] 3+ complex, therefore the three redox waves are attributed
	to [Cr2P] 3+ /[Cr2P] 2+ , [Cr2P] 2+ /[Cr2P] + and [Cr2P] + /[Cr2P] 0 couples, respectively. No signs of
	phosphonic desorption are detected. The surface coverage  is estimated at (6.0 ± 0.1) × 10 -11
	been proved as follows: 43
	I	p	22 4 nF RT  vA		(Eq III-6)

mol.cm -2 (see Experimental Section for the calculation). This relatively low  value is in the same range with previously reported values for complexes bearing ttpy phosphonic ligands anchored on ITO surface.

[39][40][41][42]

For a kinetically controlled electron transfer process between a grafted redox species and the electrode, a linear relationship between peak current intensity Ip and scan rate v has

Table III - 7 .

 III7 KWW fitting parameters (KWW and KWW) and charge recombination rate kcr between (e -)TiO2 and Ru 3+ species in Ru 2+ /TiO2/[Cr2P] 3+ triad (Ru:Cr:BPA = 20:2:78, % mol), in comparison with TiO2/Ru 2+ dyad

		KWW (µs)	KWW	kcr (s -1 )	cr = (kcr) -1 (ms)
	Ru 2+ /TiO2/[Cr2P] 3+	1400 ± 150	0.46 ± 0.01	300 ± 30	3.3
	TiO2/Ru 2+	3770 ± 800	0.51 ± 0.01	140 ± 30	7.1
	At first, the two-fold increase in charge recombination kinetics of Ru 2+ /TiO2/[Cr2P] 3+

Cr2P] 3+ and [RuP] 2+ co-immobilized on TiO2 NPs

	Our attempts to record the CVs of Ru 2+ /TiO2/[Cr2P] 3+ NPs with a microcavity Pt
	electrode have not been successful as for the TiO2/[Cr2P] 3+ dyad. Two kinds of triads have
	been measured: (i) Ru:Cr = 1:1 (% mol) without BPA spacers, and (ii) Ru:Cr:BPA = 20:2:78
	(% mol). The very low surface coverage as shown with ITO electrode (about 10 times lower
	for the Ru-Cr mixture compared with Cr or Ru alone) makes it difficult to record the CV.
	Therefore, the redox potential of grafted [Cr2P] 3+ /[Cr2P] 2+ couple is assumed to be
	similar to that of ITO/[Cr2P] 3+ electrode (-0.42 V) for subsequent studies in this chapter.
	III.4.4. Photoreduction of [Cr(ttpy)2] 3+ free complex by TiO2/Ru 2+ NPs
	In a systematic study, we first focus on the photoreduction of [Cr(ttpy)2] 3+ free complex
	by TiO2/Ru 2+ dyad before studying the Ru 2+ /TiO2/[Cr2P] 3+ triad. We conducted UV-vis
	absorption measurements with similar experimental conditions as in Section III.2.6 where
	[Ru(bpy)3]

). b) [2+ PS was used instead of TiO2/Ru 2+ NPs. The absorption spectra during the irradiation time are shown in Figure III-30a. It is important to note that the TiO2/Ru 2+ colloid alone does not show the 1 MLCT absorption band of [Ru(bpy)3] 2+ with a maximum at 450 nm as in the solid state. Only a shoulder emerges at the foot of the TiO2 absorption band. The first reduction step to form [Cr(ttpy)2] 2+ complex is usually characterized by a broad band centered at 800 nm; however it is not observed in this case as for the photoreduction experiment of [Cr(ttpy)2] 3+ by [Ru(bpy)3] 2+ in solution (see Section III.2.6). The second reduction step to form [Cr(ttpy)2] + complex is clearly evidenced by the spectral changes indicated by black arrows: the evolution of two absorption maxima at 500 nm and 580 nm, accompanied by the characteristic peak at 723 nm. The experiment proves that the two-electron reduction of [Cr(ttpy)2] 3+ to [Cr(ttpy)2] + is achievable with immobilized Ru 2+ PS on TiO2.

Cr 3+ complex in Ru 2+ /TiO2/[Cr2P] 3+ triad a) Energy diagram

	We first examine the energy levels associated with the electronic components in
	Ru 2+ /TiO2/[Cr2P] 3+ triad. Scheme III-13 describes the redox potentials of Ru 3+ /Ru 2+ ,
	Ru 3+ /Ru 2+* , Cr 3+ /Cr 2+ , Cr 2+ /Cr + and the CB of TiO2, which are relevant to the photo-induced
	electron transfer cascade in this study. Under visible irradiation, Ru 2+ PS is excited (step 1)
	and can inject electrons to the CB of TiO2 (step 2). The electron injection can occur from
	either

1

MLCT state ("hot injection") or

3

MLCT relaxed state of the Ru 2+ PS. This injection step has been described in detail in Chapter 2. Afterwards, in terms of thermodynamics, the electrons on the CB of TiO2 can reduce [Cr2P] 3+ complex twice to form [Cr2P] + (step 3).

Regeneration of Ru 2+ PS is achieved by sacrificial reductant TEOA (step 4). Possible back

  2+ is EPR-silent despite its S = 1 spin state. Therefore, the EPR signal may correspond to either Ru 3+ /(e -)TiO2/[Cr2P] 3+ or Ru 3+ /(e -)TiO2/[Cr2P] 2+ transient species. Low temperature could inhibit the reduction of [Cr2P] 3+ ; however it allows for the detection of transient species. At room temperature the trapped electrons on TiO2 will not be observable. The use of TEOA as sacrificial electron donor to compete with the back

	electron transfer pathways is vital for multi-electron accumulation on [Cr2P] 3+ sites. 16
	Nevertheless, it may pose problems in this EPR experiment because its oxidized species,
	TEOA + , may cause a high radical signal at g ~ 2 as well.
	In summary, despite the possible formation of [Cr(ttpy)2] + complex in solution by
	TiO2/Ru II NPs under visible irradiation and in the presence of TEOA, the quantitative
	photoreduction of grafted [Cr2P] 3+ in Ru 2+ /TiO2/[Cr2P] 3+ triad is not observed. It could be
	due to the difficulty to short-circuit the back electron transfer processes between
	photogenerated Ru 3+ and [Cr2P] 2+ complexes on the TiO2 surface by TEOA. Thermodynamic
	considerations have already shown that the back electron transfer between [Cr(ttpy)2] 2+ and
	[Ru(bpy)2]

[Cr2P] + (S = 1/2 ) at g ~ 2 is not detected. Even if the singly reduced [Cr2P] 2+ species is formed, it is not expected to be EPR-active in this X-band frequency as the analogous [Cr(ttpy)2] 3+ in solution is remarkably more favorable than the reduction of [Ru(bpy)3] 3+ by TEOA (-1.41 eV compared with -0.52 eV), which is a necessary step to further reduce [Cr(ttpy)2] 2+ species to [Cr(ttpy)2] + . When both [RuP] 2+ and [Cr2P] 3+ complexes are grafted on TiO2 with a molar ratio of 10:1, the chance that a [Cr2P] 3+ site is surrounded by [RuP] 2+

  Chapter 3 the electrochemical and photophysical behaviors of [Cr(ttpy)2] 3+ complex have been extensively studied using electrochemistry, steady-state and transient absorption and emission spectroscopies. The complex shows rich electrochemical properties with five RuP] 3+ and [Cr2P] 2+ transient species to regenerate [RuP] 2+ and [Cr2P] 3+

	reversible redox states, [Cr(ttpy)2] n+ (n = 3, 2, 1, 0, -1). DFT calculations demonstrate that all
	the added electrons are localized on the ttpy ligands, thus the oxidation state of Cr always
	remains +3. The four complexes corresponding to n = 3, 2, 1, 0 exhibit different fingerprints
	in UV-vis absorption and EPR spectroscopy, which is convenient to identify them in a
	reaction mixture.
	The [Cr(ttpy)2] 3+ complex has found applications in photo-induced multiple electron
	storage and electrocatalysis in homogeneous environments. In the presence of another PS like
	[Ru(bpy)3] 2+ and a sacrificial electron donor like TEOA, the [Cr(ttpy)2] 3+ can be doubly
	reduced with 100 % conversion. In addition, the [Cr(ttpy)2] 3+ complex also shows
	electrocatalytic activity towards H2 production in non-aqueous solution. It is, however, not a
	very good electrocatalyst due to possible decoordination of ttpy ligands under a prolonged
	bias.
	Immobilization of [Cr(ttpy)2] 3+ on ITO electrode and TiO2 NPs was achieved by
	functionalizing the complex with two phosphonic anchoring group, which is denoted as
	[Cr2P] 3+ . The ITO/[Cr2P] 3+ electrode exhibits three successive reduction waves at potentials
	comparable to those of [Cr(ttpy)2] 3+ , proving that the Cr complex retains rich electrochemical
	properties after being grafted. In contrast to the [Cr(ttpy)2] 3+ complex in solution,
	TiO2/[Cr2P] 3+ NPs show no emission after being photo-excited. The evidence for successful
	grafting on TiO2 is a broad EPR line centered at g ~ 2, which is characteristic of closely
	packed paramagnetic species. The broad EPR line can still be observed at lower [Cr2P] 3+
	concentrations, suggesting that the [Cr2P] 3+ sites likely stay close to each other on the TiO2
	surface.
	Finally, both [RuP] 2+ PS and [Cr2P] 3+ complex have been immobilized on ITO
	electrode and TiO2 NPs to study photo-induced charge transfer events and the possibility of
	accumulating multiple electrons on [Cr2P] 3+ sites. The electrochemical behavior of
	ITO/{[RuP] 2+ +[Cr2P] 3+ } electrode is a superimposition of ITO/[RuP] 2+ and ITO/[Cr2P] 3+

signals, thus proving the successful grafting process. A similar superimposition is also observed in the UV-vis absorption spectrum of Ru 2+ /TiO2/[Cr2P] 3+ triad, which comprises of TiO2/Ru 2+ and TiO2/[Cr2P] 3+ spectra. However, the multiple charge storage on [Cr2P] 3+ sites is not achieved. It may be due to accelerated kinetics of the back electron transfer reaction between grafted [

Table IV - 1 .

 IV1 CO2 reduction potentials vs NHE in aqueous solution, 1 atm gas pressure and 1 M solutes 3

	Reactions	E 0 (V) vs SHE E 0 (V) vs Ag/AgNO3 0.01 M *
	CO2 + e - CO2 -	-1.90	-2.44
	CO2 + 2H + + 2e - HCOOH	-0.61	-1.15
	CO2 + 2H + + 2e - CO + H2O	-0.53	-1.07
	2CO2 + 2H + + 2e - H2C2O4	-0.49	-1.03

* Calculated by subtracting 0.54 V from E 0 vs SHE

4

Table IV -

 IV 2 presents the results obtained with different Mn:Ru ratio. For the experiments to be comparable, we only changed the catalyst amount while keeping the Ru PS amount fixed so that light absorbance by the PS remains the same in all the series. The results were obtained after 16 hours of irradiation at 470  40 nm and corresponded to the maximum of production.

Table IV -

 IV

	Ratio of Mn:Ru a	HCOOH n (µmol) TONmax b	n (µmol)	CO	TONmax b	TONtotal c
	1:1	9.8	14	5.6		8	22
	1:5	3.9	28	1.4		10	38
	1:10	2.2	31	1.3		19	50
	5:1	71.4	20	18.9		5	25
	10:1	88.9	13	35.0		5	17

2. Results of photocatalytic CO2 reduction using a mixture of [Mn(ttpy)(CO)3Br] catalyst and [Ru(bpy)3](PF6)2 PS in 5 ml DMF solution containing TEOA (1 M) and BNAH (0.1 M) under CO2. Irradiation was achieved by using a Xe lamp (0.3 mW.cm -2 , 5 cm apart), a UV-hot filter and a 470  40 nm bandpass filter. The results were obtained after 16 hours of irradiation. a Ratio of [Mn(ttpy)(CO)3Br] : [Ru(bpy)3] 2+ where Ru(II) concentration is unchanged (0.14 mM) b Turnover number (TON) versus Mn catalyst c TONtotal = TONmax (HCOOH) + TONmax (CO)

Table IV - 3 .

 IV3 KWW fitting parameters (KWW and KWW) and charge recombination rate kcr between (e -)TiO2 and Ru 3+ species in Ru II /TiO2/Mn I triad (Ru:Mn = 10:1 % mol), in comparison with TiO2/Ru II dyad

		KWW (µs)	KWW	kcr (s -1 )
	Ru II /TiO2/Mn I	3870 ± 460	0.55 ± 0.01	150 ± 15
	TiO2/Ru II	3770 ± 800	0.51 ± 0.01	140 ± 30

Table IV

 IV

-4 summarizes the aforementioned results.

Table IV - 4 .

 IV4 Results of photocatalytic CO2 reduction using Ru II /TiO2/Mn I triad, TiO2/Ru II dyad + free Mn I in solution, and SiO2/Ru II dyad + free Mn I complex in solution. The DMF solution contained TEOA (1 M) and BNAH (0.1 M). Irradiation was achieved by using a Xe lamp (0.3 mW.cm -2 , 5 cm apart), a UV-hot filter and a 470  40 nm bandpass filter. The results were obtained after 16 hours of irradiation.

				HCOOH	CO
	System	nRu (µmol)	nMn (µmol)	(µmol) n	TONmax	(µmol) n	TONmax	Total TON
	Ru II /TiO2/Mn I	0.3	0.03	0.8	27	0	0	27
	TiO2/Ru II + free Mn I	0.3	0.03	1.1	38	0.2	6	44
	SiO2/Ru II + free Mn I	0.3	0.06	0.5	9	0.2	3	12
	Ru II + Mn I in solution	0.7	0.07	2.2	31	1.3	19	50

  As mentioned in the previous chapter, the redox potential of Ru II* /Ru I can be calculated from the redox potential of Ru III /Ru II and the emission maximum of Ru II* by the simplified

	[Ru-pyr] 2+	460 [13900]	286 [80000]		621	0.043	670 ± 32
	[Ru(bpy)3] 2+	450 [13000]	285 [87000]		608	0.062 11	757 ± 9
	a Excitation by a picosecond pulsed laser at 400 nm
	All measurements were conducted in MeCN solution under Ar at room temperature.
	Rehm-Weller equation (Equation V-1):
	E	II* (Ru /Ru ) I	E	II (Ru /Ru )+ I	,77 K hc em	e	E	II (Ru /Ru )+ I	,RT 1240 em

Table V - 3 .

 V3 Summary of the photophysical properties of TiO2/[Ru-pyr] 2+ compared with [Ru-pyr] 2+ and

	TiO2/Ru II
		1 MLCT abs (nm) em (nm)	 (ns)	kinj, s -1	kq, s -1
	TiO2/[Ru-pyr] 2+	460 a	625	365 ± 8 (14%)	2.4 × 10 7 1.2 × 10 6
				40 ± 1 (86%)
	[Ru-pyr] 2+	460	621	670	-	-
	TiO2/Ru II	450	627	366 ± 7 (12%)	3.0 × 10 7 1.6 × 10 6
				32 ± 0.6 (88%)

Table V -

 V

	4). As the
	irradiation time is increased, both rates are enhanced but the injection rate increases faster
	than the energy transfer rate (6 times compared with 3 times after 2 hours).
	By assuming Forster resonance energy transfer (FRET) model and applying Equations

V-5, 6 and 7 (see Section II.3.6 for

  more detail), the FRET efficiency and the average distance between Ru 2+ species, r(Ru-Ru), on surface was estimated. In this calculation the orientational factor  was assumed to be 2/3, which is generally accepted for a species in solution able to rotate freely. Before irradiation, the distance is about 2.0 nm, which is in accordance with the estimated loading of ~ 2 [Ru-pyr] 2+ molecules per nm 2 of TiO2 surface mentioned above. During the irradiation, the distance is gradually reduced. All the relevant photophysical properties of the nanocomposite are summarized in TableV-4.

Table V - 4 .

 V4 Summary of the photophysical properties of TiO2/poly(Ru-pyr) after 0h, 1h and 2h irradiation at 450

	nm
		em, nm	, ns (%)	kinj, s -1	kq, s -1	FRET	r (Ru-Ru),
						efficiency	nm
	0 h	625	365 ± 8 (14 %)	2.4 × 10 7 1.2 × 10 6	0.46	2.0
			40 ± 1 (86 %)
	1 h	625	251 ± 6 (9 %)	4.6 × 10 7 2.5 × 10 6	0.63	1.8
			21 ± 0.4 (91 %)
	2 h	631	178 ± 6 (2 %)	1.4 × 10 8 4.1 × 10 6	0.73	1.6
			7 ± 0.1 (98 %)
		From the Table V-4, one can notice that the electron injection from Ru 2+* to TiO2 is
	enhanced in both kinetics and efficiency as the irradiation time is prolonged. After 2 hours of
	photopolymerization, the rate is increased by 6 times and efficiency reaches 98 %. Since the
	distance between [Ru-pyr] 2+ and TiO2 surface is fixed during the irradiation time and the
	electron injection follows a tunneling mechanism, this rate increase suggests a more
	conducting environment around the nanocomposite. Therefore the polypyrrole proves to be
	useful to ameliorate the conductivity.

Table V - 5 .

 V5 Chemical structures of monomer, dimer and trimer radical cations of pyrrole, pyrrole-CH3 and

	pyrrole=O species
	Pyrrole	Pyrrole-CH3	Pyrrole=O
	Monomer
	cation
	Dimer
	cation
	Trimer
	cation

Table V -6. g

 V -values of monomer, dimer and trimer radical cations of pyrrole calculated with DFT

	g-value
	Monomer cation	2.00242	2.00239	2.01124
	Dimer cation	2.00254	2.00251	2.00399
	Trimer cation	2.00257	2.00251	2.00294

Table V - 7 .

 V7 Atomic concentration of some elements in TiO2/poly(Ru-pyr) NPs and FTO/TiO2/poly(Ru-pyr)

	electrode. Error = ± 0.1 %
	Atomic concentration (%) N(bpy) N(pyr)	P	Ru	C(C-C) C(C-N)	Ti
	TiO2/poly(Ru-pyr)	1.2	1.6	0.9	0.2	17.5	6.4	19.6
	FTO/ TiO2/poly(Ru-pyr)	1.1	1.7	0.5	0.2	23.9	6.2	4.9

  The phosphonic acid group was used to graft the complexes on TiO2 NPs, thanks to the stability of the Ti-O-P linkages. The grafting process was notably followed by quartz crystal microbalance with energy dissipation. Under visible irradiation, [Ru(bpy)3] 2+ is excited and injects an electron to the conduction band (CB) of TiO2 to form a long-lived charge separated state (e -)TiO2/Ru 3+ , which allows for the utilization of the charges in subsequent charge transfer reactions. For example, adding [Cr(ttpy)2] 3+ complex in a solution containing the TiO2/Ru 2+ NPs and triethanolamine (TEOA) as a sacrificial electron donor generates the doubly reduced [Cr(ttpy)2] + species under visible light. The reaction proceeds stepwise through the formation of (e -)TiO2/Ru 3+ .

	immobilize various complexes including [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridine) as a
	photosensitizer (PS) and [Cr(ttpy)2] 3+ or [Mn(ttpy)(CO)3Br] (ttpy = 4'-(p-tolyl)-2,2':6',2"-
	terpyridine) as an electron acceptor. However, when both [Cr(ttpy)2] 3+ and [Ru(bpy)3] 2+ complexes are grafted on TiO2, the back
	electron transfer reaction between transient Ru 3+ and Cr 2+ to regenerate the initial species
	Ru 2+ and Cr 3+ may occur very fast, so that a further reduction to [Cr(ttpy)2] + cannot be
	obtained, even in the presence of TEOA. In contrast, (e -)TiO2 can doubly reduce [Cr(ttpy)2] 3+

  This polymerization process cannot occur by exhaustive electrolysis since TiO2 is not conductive enough. Therefore, in this photopolymerization process TiO2 acts as an electron acceptor for Ru 2+* to form the transient Ru3+ , which has a lifetime long enough for the oxidation of pyrrole. After two hours of irradiation, the [Ru(bpy)3] 2+ PS does not show any degradation. As an application, the nanocomposite deposited on a FTO electrode can generate an anodic photocurrent two times higher than the electrode covered with TiO2/Ru II NPs under visible light and in the presence

	[Ru(bpy)3] 2+ -based PS bearing two pyrrole moieties. Under visible irradiation and in the
	presence of O2, the [Ru(bpy)3] 2+ PS in the TiO2/[Ru-pyr] 2+ hybrid NPs is excited, injects an
	electron to TiO2 to form a charge separated state (e -)TiO2[Ru III -pyr] 3+ . The Ru III centers are

then reduced by pyrrole to generate Ru II and a pyrrole radical cation. This radical then initiates a photopolymerization process on TiO2 surface to form a nanocomposite TiO2/poly(Ru-pyr) where a polypyrrole network surrounds the NPs. Meanwhile, the injected electrons on TiO2 are efficiently scavenged by O2.

  One should subtract 0.090 V from them to convert them against Fc + /Fc reference. Half-wave potential (E1/2) and peak-to-peak splitting (Ep) are

	calculated as follows:
	1/2 E		2 EE pa 	pc	,	p   E	E	pa		E	pc
	three-electrode cell at RT under a continuous Ar flow or in a glovebox. Electrolytes were used
	without further purifications: tetrabutylammonium perchlorate (TBAP, Fluka),
	tetrabutylammonium hexafluorophosphate (TBAPF6, Aldrich). A silver/silver nitrate in
	CH3CN (Ag/AgNO3 0.01 M) and a platinum (Pt) coil were used as reference electrode and
	counter electrode, respectively. A carbon disk electrode (CHI Instrument) or a home-made Pt
	microcavity electrode was used as working electrode. Before experiment, the C disk electrode
	was polished using diamond paste and cleaned with ethanol. Cyclic voltammograms (CV)

were recorded with a CHI 630 potentiostat (CH Instrument), or a Biologic SP300 potentiostat (Science Instruments). All reported potentials are against the Ag/AgNO3 0.01 M reference electrode, unless otherwise stated.

  = 8.75-8.73 (m, 4H), 8.68 (d, J = 8.0 Hz, 2H), 7.92- 7.88 (m, 4H), 7.38-7.35 (m, 2H), 7.02 (d, J = 8.5 Hz, 2H), 4.17-4.09 (m, 6H), 2.19-2.09 (m, 2H), 2.02-1.94 (m, 2H), 1.34 (t, J = 7.0 Hz, 6H).

1 H NMR (400 MHz, CDCl3): ppm) 13 C NMR (100 MHz, CDCl3): ppm) = 159.

H NMR cannot be done because the product is paramagnetic. Electrochemistry: CVs cannot be done directly due to the poor background signal of DMSO. Instead, the redox

H NMR (400 MHz, CDCl3):  (ppm) = 8.57 (t, J = 4.0 Hz,

2H), 8.28 (s, 2H), 7.17 (d, J = 8.0

Acknowledgements

Abstract

Herein we present the electrocatalytic and photocatalytic reduction of CO2 to CO and HCOOH using a Mn(I) tricarbonyl complex, [Mn(ttpy)(CO)3Br] (ttpy = 4'-(p-tolyl)-2,2':6',2''-terpyridine), as pre-catalyst and [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridine) as photosensitizer. The photocatalytic CO2 reduction experiments were performed under irradiation at 470  40 nm in DMF/TEOA (TEOA: triethanolamine, CTEOA = 1 M) solution mixture containing 1-benzyl-1,4-dihydronicotinamide (BNAH, 0.1 M) as sacrificial electron donor and different ratios of [Mn(ttpy)(CO)3Br]:[Ru(bpy)3] 2+ (Mn:Ru). The optimization of the Mn:Ru ratio shows that the 1:10 ratio offers the best performance in terms of total turnover number (TONmax (CO) = 19, TONmax (HCOOH) = 31) after 16 hours of irradiation. Afterwards, both complexes are grafted on TiO2 nanoparticles via phosphonic acid anchoring groups to form a triad structure (denoted as Ru II /TiO2/Mn I ) with a Mn:Ru ratio estimated around 1:10. Under similar experimental conditions, the Ru II /TiO2/Mn I nanoparticles produce HCOOH as the only product with TONmax (HCOOH) = 27. The enhanced selectivity towards HCOOH when using grafted complexes compared with the mixture of homogeneous complexes in solution could be related to the mechanism of the first reduction process of [Mn(ttpy)(CO)3Br] and hence the catalytically active species. We propose that a doubly reduced Mn -I complex is the active species in homogeneous solution, whereas a singly reduced Mn 0 complex is the active species in Ru II /TiO2/Mn I nanoparticles.

Résumé

Dans ce chapitre, nous présentons les résultats de la réduction électrocatalytique et photocatalytique du CO2 en CO et HCOOH en utilisant un complexe du Mn(I) tricarbonyl [Mn(ttpy)(CO)3Br] (ttpy = 4'-(p-tolyl)-2,2':6',2''-terpyridine) comme pré-catalyseur et le complexe [Ru(bpy)3] 2+ (bpy = 2,2'-bipyridine) comme photosensibilisateur. La réduction photocatalytique du CO2 sous irradiation à 470  40 nm dans le DMF/TEOA (TEOA: triethanolamine, CTEOA = 1 M) et contenant le 1-benzyl-1,4-dihydronicotinamide (BNAH, 0,1 M) en tant que donneur d'électrons sacrificiel a été menée en variant le rapport de concentration [Mn(ttpy)(CO)3Br] : [Ru(bpy)3] 2+ (Mn:Ru). Les résultats montrent qu'un rapport Mn:Ru de 1:10 offre les meilleures performances en termes de TON total: TONmax (CO) = 19, TONmax (HCOOH) = 31 après 16 heures d'irradiation. Les deux complexes [Mn(ttpy)(CO)3Br] et [Ru(bpy)3] 2+ ont été greffés sur des nanoparticules de TiO2 via des groupements d'ancrage du type acide phosphonique portés par les ligands polypyridiniques pour former une structure triade (Ru II /TiO2/Mn I ). Le ratio Mn:Ru a été fixé sur les nanoparticules, proche de 1:10. Dans des conditions similaires à celles de la réduction photocatalytique du CO2 en utilisant des complexes homogènes [Ru(bpy)3] 2+ et [Mn(ttpy)(CO)3Br] en solution, les nanoparticules hybrides Ru II /TiO2/Mn I produisent HCOOH comme seul produit avec un TONmax (HCOOH) = 27. La sélectivité accrue vers HCOOH lors de l'utilisation de Ru II /TiO2/Mn I par rapport au mélange des complexes en solution pourraient être liés au mécanisme du premier processus de réduction de [Mn(ttpy)(CO)3Br] et donc à l'espèce catalytiquement active. Nous proposons que le complexe Mn -I doublement réduit soit l'espèce active en solution homogène, alors qu'un complexe Mn 0 mono réduit pourrait être l'espèce active sur les nanoparticules Ru II /TiO2/Mn I .

IV.2. Mn(I) tolylterpyridine tricarbonyl complex in solution

IV.2.1. Synthesis

The synthesis of [Mn( 2 -ttpy)(CO)3Br] complex was performed following a previous publication by our CIRe group. 23 A simple one-step synthesis was carried out between Mn(CO)5Br and ttpy ligand in diethyl ether to yield [Mn( 2 -ttpy)(CO)3Br] as the only product (Scheme IV-6). The bidentate coordination was confirmed in the aforementioned study. The presence of three CO groups is proved by infrared (IR) spectroscopy in solid state showing three intense peaks at 2022, 1948, 1916 cm -1 . They correspond to symmetric (2022 cm -1 ) and antisymmetric (1948 and 1916 cm -1 ) CO stretching bands. The peaks are slightly blueshifted compared with the analogous complex [Mn( 2 -ttpy)(CO)3(MeCN)] + ( (CO) = 2043, 1962, 1939 cm -1 ) 23 where Br -is replaced by MeCN solvent molecule. The peak positions and the blue-shift tendency when Br -is substituted by MeCN have also been reported for the analogous [Mn( 2 -tpy)(CO)3Br] 22 and [Mn(phen-dione)(CO)3Br] 24 (phen-dione = 1,10phenanthroline-5,6-dione) complexes.

Scheme IV-6. Synthesis route to [Mn( 2 -ttpy)(CO)3Br] complex

IV.2.2. Electrochemical properties

The electrochemical properties of [Mn( 2 -ttpy)(CO)3Br] complex has been studied by cyclic voltammetry in MeCN + 0.1 M TBAPF6 solution under Ar, using a carbon disk electrode as working electrode (WE). Since the CVs are similar to those of [Mn( 2 -ttpy)(CO)3(MeCN)] [START_REF]mmol.g -1 (about 3800 Cr III molecules per particle or 2 Cr III molecules per nm 2 ). C6H5-CH2-PO3H2/TiO2/[Cr2P] 3+ (C6H5-CH2-PO3H2 = benzylphosphonic acid, BPA). Similar to the case of TiO2/[Cr2P] 3+ , but a mixture of [Cr2P] 3+ and BPA was used instead of References[END_REF]21 , [Mn(bpy)(CO)3Br] [START_REF]mmol.g -1 (about 3800 Cr III molecules per particle or 2 Cr III molecules per nm 2 ). C6H5-CH2-PO3H2/TiO2/[Cr2P] 3+ (C6H5-CH2-PO3H2 = benzylphosphonic acid, BPA). Similar to the case of TiO2/[Cr2P] 3+ , but a mixture of [Cr2P] 3+ and BPA was used instead of References[END_REF] and [Mn(phen)(CO)3Br] 18 complexes, our CV interpretation is based on the explanations in these previous works without further investigation.

In the anodic scanning (Figure IV-1a), two irreversible oxidation peaks are detected at 0.62 V and 0.78 V, which is in accordance with Marc Bourrez's study on the same complex. [START_REF]mmol.g -1 (about 3800 Cr III molecules per particle or 2 Cr III molecules per nm 2 ). C6H5-CH2-PO3H2/TiO2/[Cr2P] 3+ (C6H5-CH2-PO3H2 = benzylphosphonic acid, BPA). Similar to the case of TiO2/[Cr2P] 3+ , but a mixture of [Cr2P] 3+ and BPA was used instead of References[END_REF] The author attributed the two peaks to two oxidation steps as follows:

(1) fac [Mn I ( 2 -ttpy)(CO)3Br]  fac [Mn II ( 2 -ttpy)(CO)3Br] + + e - Epa1 = 0.62 V Scheme IV-8. Proposed mechanism for the photocatalytic CO2 reduction using [Mn(ttpy)(CO)3Br] catalyst.

Adapted from reference 21

Upon the excitation at 450 nm, [Ru(bpy)3] 2+ PS is selectively excited and singly reduced by the sacrificial electron donor BNAH. Since the reduction power of BNAH, Eox = 0.27 V vs

Ag/AgNO3 0.01 M (converted from Eox = 0.57 V vs SCE in MeCN 5 ) is stronger than that of TEOA (Eox = 0.42 V 26 ), BNAH can reductively quench the Ru 2+* excited state (E(Ru II* /Ru I ) = 0.31 V). The [Ru(bpy)3] + , in turn, can reduce [Mn(ttpy)(CO)3Br] twice to form the catalytically active species [Mn -I (ttpy)(CO)2] -. In terms of energy, since the reduction power of Ru + is E(Ru II /Ru I ) = -1.67 V while the two reduction steps of the Mn complex occur at -1.44 V and -1.67 V, the Ru + complex can reduce the Mn complex twice. The doubly reduced species [Mn -I (ttpy)(CO)2] -has a free coordination site to form an adduct with CO2. It has been proposed that TEOA also participated to the formation of the Re-CO2-TEOA intermediate complex when a similar catalyst, [Re(bpy)(CO)3Cl], was used. 5 Finally,

IV.3.3. Electrochemical properties a) Mn I complex grafted on FTO electrode

Before studying the electrochemical properties of TiO2/Mn I NPs, we first examined the electrical communication between the grafted Mn I complex and a metal oxide electrode. FTO was chosen for this study because ITO showed significant degradation when the potential was swept to -2 V. The Mn I complex was deposited onto the FTO electrode following the same procedure as for TiO2/Mn I NPs synthesis. The CV of FTO/Mn I modified electrode is presented in Figure IV-5a. The first cathodic peak appears at Epc = -1.43 V, which is at the same potential as the first cathodic peak of the free complex in solution. However, on the reverse scan the associated anodic peak appears at a very different potential (Epa = -1.16 V) than the peak recorded for the homogeneous complex in solution (-0.89 V, Section IV.2.2), which has been assigned to the oxidation of the Mn 0 -Mn 0 dimer formed after the first reduction step. Since the Mn I complex is immobilized onto the FTO surface, the formation of Mn 0 -Mn 0 dimer should be prohibited. This assumption is supported by the lack of a second reduction process which is assigned to the reduction of the Mn 0 -Mn 0 dimer in solution at -1.67 V. Scanning to potentials more negative than -2 V leads to the degradation of the FTO surface itself.

Therefore, we tend to attribute the redox waves for FTO/Mn I electrode to the following processes:

Abstract

In this chapter, [Ru(bpy)3] 2+ photosensitizer has been functionalized with a pyrrole unit and a phosphonic acid group to anchor on TiO2 NPs to form a hybrid system called TiO2/[Ru-pyr] 2+ . The electrochemical and photophysical properties of this system have been studied. Photo-induced electron injection from [Ru(bpy)3] 2+* unit to TiO2 to yield (e -)TiO2/[Ru III -pyr] 3+ occurs in nanosecond time scale, which is followed by the reduction of Ru III by pyrrole moiety. A transient charge separated state (e -)TiO2/[Ru II -pyr + ] 3+ is then formed. In the presence of O2 as an electron scavenger for the injected electrons in TiO2 NPs, the positively charged pyrrole moieties induce oxidative polymerization to yield TiO2/poly(Ru-pyr) nanocomposite. Afterwards, the nanocomposite has been deposited onto a FTO electrode using an electrophoretic deposition process. These electrodes behave as efficient photoanode under visible irradiation in the presence of triethanolamine as sacrificial electron donor. The nanostructuration of the polypyrrole network enhances both the homogeneity of surface deposition and photocurrent intensity compared to the results obtained with a FTO/TiO2/Ru II electrode. In this chapter we will present the incorporation of a pyrrole-containing [Ru(bpy)3] 2+ complex covalently grafted on TiO2 NPs. The aim of this design is to prepare a photoactive hybrid nanocomposite capable of polymerization under visible light, thus avoiding possible detrimental effects of TiO2 on organic components under UV light. It is hence expected that the formation of polypyrrole will increase the conductivity of the resulting nanocomposite material after deposition on an electrode. The resulting hybrid system, called TiO2/[Ru-pyr] 2+ (Scheme V-5) has been designed following the Electron Acceptor -PS -Electron Donor architecture, in which the [Ru(bpy)3] 2+ PS is anchored on TiO2 surface to facilitate the photoinduced electron transfer process as presented in Chapter 2. In this structure TiO2 acts as the electron acceptor for [Ru(bpy)3] 2+* and pyrrole as electron donor to regenerate the PS by reducing the Ru III species. First, the synthesis, electrochemical and photophysical properties of the complex in solution will be discussed. Afterwards, grafting of the complex onto TiO2

Résumé

NPs together with its properties will be presented. The photopolymerization process will be addressed in detail via various characterization techniques. Finally, deposition of the nanocomposite onto a FTO surface and its photo-to-current energy conversion will be shown, highlighting the role of polypyrrole. The synthesis of bpy-pyr ligand 4 and [Ru(bpy-pyr)2Cl2] complex 7 (bpy-pyr = 4-methyl-4'-[11-(1H-pyrrol-1-yl)undecyl]-2,2'-bipyridine) have been done in a similar manner reported in literature with some modifications (see Experimental Section for more details).

Incorporating another bpy ligand was achieved by refluxing [Ru(bpy-pyr)2Cl2] and 4,4'dimethyl-2,2'-bipyridine (dmbpy, 1 equiv) in H2O/ethanol mixture (2/8 v/v) overnight under Ar. The as-synthesized product was allowed for anion exchange with excess amount of KPF6 to yield [Ru(dmbpy)(bpy-pyr)2](PF6)2 complex which is hereafter denoted as [Ru-pyr] 2+ (Scheme V-6). We chose the dmbpy ligand because in the next step a phosphonic group will be incorporated into this ligand as anchoring group to TiO2 NPs. Therefore the study on [Ru-pyr] 2+ complex can be compared with that grafted on TiO2 NPs. V, which is shifted by 100 mV compared with [Ru(bpy)3] 2+ , 8 is attributed to the oxidation of pyrrole and Ru III /Ru II couple, since their oxidation potentials are known to be indistinguisable. 4,6 The bpy-pyr ligand oxidation has been reported to be irreversible at potentials around 1 V. 9 In our case, the Ipa/Ipc ratio is larger than unity, showing that this oxidation is also not reversible. In the cathodic part, three reversible waves centered at E1/2 Scheme V-7. Synthesis procedure of TiO2/[Ru-pyr] 2+ NPs.

V.3.2. Electrochemical properties

The electrochemical properties of TiO2/[Ru-pyr] 2+ NPs were studied in solid state using a microcavity Pt electrode (d = 50 µm). to be consistent with the photopolymerization study mentioned above, although at 20 K it is not expected to react with other species. For g-value calculations, 2,2-diphenyl-1picrylhydrazyl (DPPH) (g = 2.0036) has been used as reference. Simulations of (e -)TiO2 and conducting species in polypyrrole are also shown in red and blue lines respectively. The simulation of trapped electrons in anatase TiO2 has been done according to published gvalues: g = 1.990 and g// = 1.960. 12 It should be noted that only the trapped electrons are observable by EPR spectroscopy, while CB electrons are EPR silent. 13 Similarly, the simulation of conducting species in polypyrrole chain (polarons) has been performed by g = 2.0037 (line width Hp-p = 9 G). It should be noted that this g-value is higher than those recorded by electrochemical or chemical synthesized polypyrrole films (g ~ 2.0025). 14,15 Comparing the signal and the simulations, we suggest the formation of a charge separated state (e -)TiO2/Ru II -pyr + . Under He atmosphere and at such a low temperature, there is no oxidative quencher for the electrons in TiO2, thus they are trapped at defect sites on TiO2 surface and detectable by EPR. The resulting [Ru(bpy)3] 3+ species is probably undetectable The two strategies are schematically depicted in Scheme V-11. The characterization of the photopolymerization of TiO2/poly(Ru-pyr) nanocomposite has been thoroughly discussed in Section V.4. This section focuses on the photopolymerization on FTO/TiO2/[Ru-pyr] 2+ surface following the EPD-Photo strategy. Comparative studies for the two strategies will also be mentioned in terms of the film characterization and their application as photoanode under visible irradiation. ). The wave at ~ 0.9 V, which is assigned to the oxidation of Ru II and pyrrole, is not completely reversible (Ipa/Ipc >> 1). It is similar to the [Ru-pyr] 2+ complex without phosphonic groups in solution (see Section V.2.2). If the electropolymerization occurred on the FTO/TiO2/[Ru-pyr] 2+ modified electrode, we would expect the emergence of a new oxidative peak around 0.5 V corresponding to the polypyrrole moieties, 25 and an increase in the peak intensity due to higher conductivity. The CVs in Photocurrent measurement: A standard electrochemical cell with modified surfaces as working electrode were used. A 250 W Xenon lamp was employed to irradiate the electrode, with a fixed distance of 4 cm between the surface and the head of the optical fiber. A CHI 600 potentiostat was run in chronoamperometry mode to record the photocurrents. During the experiment, the light was alternatively switched on and off by placing a metallic plate over the source outlet. The energy of incident light is estimated to be 1.0 W.cm -2 (without filter), or 1.0 mW.cm -2 (UV hot filter). removed from the glove box and bubbled through with air for 2 h which produced an orange suspension. The solid was separated by filtration and dried under high vacuum to afford desired product as an orange solid (0.66 g, 61 %).

Elemental Analysis (EA): Calc. for C44H28N6Cr: C, 52.99; H, 2.83; N, 8.43. Found: C, 52.17; H, 3.73; N, 8.64. UV-vis: λmax (MeCN) = 368 nm (ε = 36500 M -1 .cm -1 ). Electrochemistry (ii) Step 2: Synthesis of diethyl [4-(2,2':6',2''-terpyridin-4'-yl)

phenyl]phosphonate

To a degassed mixture of 4′-(4-bromophenyl)-2,2′:6′,2″-terpyridine (0.97 g, 2.50 mmol) and [1,1-bis-(diphenylphosphino)ferrocene]dichloropalladium(II) (0.10 g, 0.13 mmol) was added 25 mL anhydrous toluene then Et3N (0.28 g, 2.75 mmol) and diethylphosphite (0.38 g, 2.76 mmol). The orange solution was stirred for 48 h at 90 °C under an Ar atmosphere. The solvent was removed in vacuo and the residue recrystallized from MeCN three times and dried under high vacuum to give a purple solid (0.63 g, 56%) EA: Calc. for C25H24N3O3P: C, 67.40; H, 5.44; N, 9.43. Found: C, 67.71; H, 5.37; N, 9.74 [Ru(dmbpy-pyr)2]Cl2. To an oven-dried round-bottom flask was added RuCl3.3H2O (56 mg, 214 mol) and dried DMF (5 mL). The mixture was stirred and heated to 110 0 C, then the bpy-pyr ligand (167 mg, 429 mol, 2 equiv) was added. It was refluxed for 3 hours. After that, it was cooled to RT, then added 30 mL diethyl ether leading to the precipitation of the product. The solid was subsequently filtered, rinsed with 50 mL acetone/diethyl ether (1/4, v/v) to yield a dark purple solid (120 mg, yield: 59 %). (ttpy, 120 mg, 0.37 mmol) were dissolved in 30 mL diethyl ether. The resulting solution was refluxed for 3 h, and allowed to cool down to RT before the orange solid was filtered. Excess [Mn(CO)5Br] precursor was eliminated by stirring the solid in 50 mL diethyl ether for 30 mins, then the remaining solid was filtered and washed with diethyl ether. The solid product was dried under vacuum for 2 h, yielding a pale yellow powder (170 mg, 85 %). 7.96 (m, 1H), 7.75 (m, 1H), 7.65 (m, 1H), 7.42