Yiming Liang

Maxim N Sokolov

Maxim A Mikhaylov

Helen Ibrahim

Michel Goldmann

Sylvie Choua

Nolwenn Le Breton

Corinne Boudon

Vasilica Badets

Antoine Bonnefont

Laurent Ruhlmann
email: lruhlmann@unistra.fr

A 3D electropolymerized thin film based on an isoporphyrin and on a

Keywords: electropolymerization, organic-inorganic hybrid composites, molybdenum(II) halide cluster, porphyrinoids, isoporphyrin, thin films, photocurrent generation

établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The combination of organic and inorganic building blocks in hybrid systems often outcomes in synergism of characteristic properties. Inorganic part can be polyoxometalate which are a structurally diverse family of anionic metal-oxygen cluster anions constituted of metal elements in their highest oxidation states finding applications in analytical chemistry, medicine, homogeneous and heterogeneous catalysis, electronics and materials science [START_REF] Long | Polyoxometalates: building blocks for functional nanoscale systems[END_REF].

Other type of cluster can be chosen as the inorganic part such as the pyridine end-decorated Mo(II) halide cluster ((Bu4N)2[{Mo6I8}(OOC-C5H4N)6]), possessing remarkable photophysical properties [START_REF] Volostnykh | Hybrid organicinorganic supramolecular systems based on a pyridine end-decorated molybdenum(II) halide cluster and zinc(ii) porphyrinate[END_REF].

In this octahedral halide cluster of molybdenum(II) with the general formula [{Mo6(μ3-X)8}L6] n (X = Cl -, Br -, I -; L= inorganic or organic ligands; {Mo6(μ3-X)8} 4+ is the cluster core), the metal atom is associated with eight strongly bonded inner ligands X and six labile apical ligands L.

Under excitation from the UV light to the green spectral regions, the clusters exhibit a broad red-NIR luminescence both in solution and solid phases. Cluster of the type {Mo6X8} 4+ (X=Cl, Br, I) with L being O-donor, N-donor, C-, P-, and S-donor ligands have been reported [START_REF] Volostnykh | Hybrid organicinorganic supramolecular systems based on a pyridine end-decorated molybdenum(II) halide cluster and zinc(ii) porphyrinate[END_REF][START_REF] Sokolov | Highly luminescent complexes [Mo6X8(n-C3F7COO)6] 2-(X = Br, I)[END_REF][START_REF] Mikhailov | Complexes of {Mo6I8} with nitrophenolates: synthesis and luminescence[END_REF][START_REF] Felip-León | Superior performance of macroporous over gel type polystyrene as a support for the development of photo-bactericidal materials[END_REF][START_REF] Beltrán | A photobleaching resistant polymer supported hexanuclear molybdenum iodide cluster for photocatalytic oxygenations and photodynamic inactivation of Staphylococcus aureus[END_REF][START_REF] Mikhailov | Synthetic tuning of redox, spectroscopic, and photophysical properties of {Mo6I8} 4+ core cluster complexes by terminal carboxylate ligands[END_REF][8][9][10].

The hybrid systems formed by the supramolecular interaction constructed via metal-ligand coordination of pyridine end-decorated Mo(II) halide (X=I, polyiodometalate PIM) clusters and porphyrin have been also reported by Sokolov group. However, large excess porphyrin was taken in order to be able to have six coordinated zinc porphyrins. Indeed, this excess is needed to induce coordination due to the steric hindrance as well as the low formation constant between the pyridine subunits of PIM(py) [START_REF] Beltrán | A photobleaching resistant polymer supported hexanuclear molybdenum iodide cluster for photocatalytic oxygenations and photodynamic inactivation of Staphylococcus aureus[END_REF] and the zinc porphyrin.

To expand the practical applications of this type of cluster, their association to a visiblelight photosensitizer is still a prerequisite. Because of better ability due to better capacity for development and use, covalent cluster-porphyrin polymers would be better suited for building functional devices with photovoltaic properties. Moreover, our group has taken advantage of the reactivity of oxidized porphyrins to develop an easy and original method of porphyrin electropolymerization. Despite the fact that the formation of radical cation was sufficient to perform mono-substitutions such as the formation of the monosubstituted 5-ZnOEP-(meso-bpy) + porphyrin (bpy = 4,4'-bipyridine) when using ZnOEP (zinc--octaethylporphyrin), the electropolymerization failure can be attributed to the presence of the bulky ethyl groups influencing the reaction kinetics. Without a doubt, the nucleophilic attack on the -radical cation is slower than the attack on the dication in the case of zinc--octaethylporphyrin (ZnOEP). Accordingly, if the characteristic time of the potential scan during the electropolymerization is shorter than that of the nucleophilic attacks onto the p-radical cation porphyrin, the substitution will not occur and the radical cation will be reduced during the cathodic scan. An E1(E2CNmesoE3CB)n mechanism has been proposed in the case of 5-ZnOEP(meso-bpy) + porphyrin where the double-oxidation step at the beginning (steps E1E2n) allows the electro-generation of the dication porphyrin and then the copolymer formation [START_REF] Ruhlmann | A polycationic zinc-5,15-dichlorooctaethylporphyrinate-viologen wire[END_REF][START_REF] Ruhlmann | Self-oriented polycationic copolymers obtained from bipyrinium meso-substituted-octaethylporphyrins[END_REF].

In line with this strategy, we have reported a straightforward and efficient electropolymerization procedure [START_REF] Xia | Bis-porphyrin copolymers covalently linked by pyridinium spacers obtained by electropolymerization from β-octaethylporphyrins and pyridyl-substituted porphyrins[END_REF] leading to mixed porphyrin-POM copolymer films, involving either Anderson, Dawson or Lindquist type of POMs, with remarkable photoelectrocatalytic properties under visible light illumination [START_REF] Huo | Synthesis and characterization of Lindqvist-type polyoxometalate-porphyrin copolymers[END_REF]. One noteworthy characteristic of POMs is their ability to act as electron transfer relay, which can promote the transport of photo-generated electrons, avoiding the hole-electron recombination, and subsequently improve the photoelectric conversion efficiency. POMs can be used for constructing hybrids inorganic-organic systems, in which the organic dyes act as the electron donors and the POMs act as electron acceptors. It combines the advantages of the high stability of POM clusters with good finely-controllable structure of the organic subunit [START_REF] Strong | A New Class of Functionalized Polyoxometalates: Synthetic, Structural, Spectroscopic, and Electrochemical Studies of Organoimido Derivatives of [Mo6O19] 2[END_REF][START_REF] Zhang | Theoretical studies on metalloporphyrin-polyoxometalates hybrid complexes for dye-sensitized solar cells[END_REF]. In this case, the associated POM-cation copolymer of porphyrin could be prepared by exchanging the PF6 -counter ions with Keggin type polyoxometalate [START_REF] Hao | supramolecular assemblies obtained by large counteranion incorporation in a well-Oriented polycationic copolymer[END_REF]. Such porphyrin-POM electrostatic assemblies could potentially be involved in photoinduced electron transfers from the porphyrin antenna (donor of electron) to the viologen or pyridinium spacers and to the POM (acceptors of electron). These materials are thus promising for the development of photoelectric devices where the type of POMs can be modulated easily. Even though the outcomes or efficiencies of these hybrid materials have not reached yet a sufficient level, the original designs and innovative synthesis route are still more than inspiring. Furthermore, our group has shown that -radical cation porphyrins are powerful electrophiles which can rapidly react with nucleophiles to form isoporphyrin intermediates [START_REF] Giraudeau | Electrosynthesis and characterization of symmetrical and unsymmetrical linear porphyrin dimers and their precursor monomers[END_REF].

Most isoporphyrins reported until now show a high tendency to decompose either by ring opening or by rearomatization to regenerate the initial porphyrin. In the case of -octaethyl porphyrins, the corresponding isoporphyrins are not stable, due to the loss of a proton from the saturated meso-carbon atom whereas in the case of tetraphenylporphyrin (TPP), the corresponding isoporphyrin is reasonably more stable due to the substitution of the meso-carbon atoms by a phenyl group [START_REF] Gong | Nitrooctaethylporphyrins: synthesis, optical and redox properties[END_REF][START_REF] Xie | Stable isoporphyrin chromophores: synthesis[END_REF].

Recently, our new strategy led to the formation of stable isoporphyrins integrated in the copolymers. Our group has shown that stable isoporphyrins copolymers could be obtained during porphyrin electro-oxidation when sterically less hindered porphyrin is used, such as the zinc-meso-5,15-ditolyl-porphyrin (ZnT2P) which presents only two meso positions occupied by one substitutable proton at positions C10 and C20 (Scheme S1). This was performed in the presence of dipyridyl ligand 1,1"-(1,3-propanediyl)bis-4,4'-bipyridinium hexaflurophosphate salt ((bpy-(CH2)3-bpy + 2PF6 -). The existence of the -cation radical of the isoporphyrin was proved by electron spin resonance spectroscopy. Further oxidation of such copolymer at higher applied potential forms a porphyrin containing copolymer with double viologen spacers, namely poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ (top of Figure 6) [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF].

In this work, the electropolymerization of a 3D hybrid copolymer material is achieved by oxidation of porphyrin such as zinc-5,15-ditolyl-porphyrin (ZnT2P) as organic part in the presence of a pyridine end-decorated molybdenum(II) halide cluster ((Bu4N)2[{Mo6I8}(OOC-C5H4N)6]) (PIM(py)6) as inorganic part. Such a combination is very inviting because molybdenum(II) clusters also possess remarkable photophysical properties.

The formation of these copolymers on ITO electrode has been monitored in-situ by Electrochemical Quartz Microbalance (EQCM). The copolymer was also characterized UV-Vis-NIR spectroscopy, X-ray photoelectron spectroscopy, electrochemistry, ESR (Electron Spin Resonance), AFM and electrochemical impedance spectroscopy (EIS). The photocurrent generation has been investigated under visible-NIR or only NIR light illumination.

The electrochemical impedance spectroscopy properties and the photovoltaic performance of this new 3D hybrid copolymer will be compared with 1D copolymer (poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ ).

Experimental

Reagents and apparatus

All solvents were of reagent grade quality and used without further purification.

The free base 5,15-bis(p-tolyl)porphyrin (H2T2P) [22 -24 ] was synthetized according to published procedures from 2,20-dipyrromethane and p-tolualdehyde [START_REF] Brückner | Novel and improved syntheses of 5,15-diphenylporphyrin and its dipyrrolic precursors[END_REF]. The free base was then metallated with Zn(OAc)2 in THF to give zinc-5,15-bis(p-tolyl)porphyrin (ZnT2P).

The pyridine end-decorated molybdenum(II) halide cluster PIM(py)6 (Figure S1) was synthetized according to published procedures [START_REF] Volostnykh | Hybrid organicinorganic supramolecular systems based on a pyridine end-decorated molybdenum(II) halide cluster and zinc(ii) porphyrinate[END_REF].

Voltammetric data have been recorded with a standard three-electrode system using a PARSTAT 2273 potentiostat. The electrolyte was CH3CN/1,2-C2H4Cl2 (3/7) containing 0.1 mol L -1 of tetrabutylammonium hexafluorophosphate (NBu4PF6). Single-side coated indium-tinoxide (ITO, SOLEMS, 25-35 Ω/cm 2 ) electrodes with a surface area of 1 cm 2 were used as working electrode, and a platinum wire as auxiliary electrode. The reference electrode was the saturated calomel electrode which was electrically connected to the solution by a junction bridge filled with electrolyte. The electrochemical impedance spectrum (EIS) measurements were carried out at sinusoidal voltage excitation with amplitude of 10 mV in a frequency range from 100 kHz to 100 mHz. The ITO electrodes were also used to obtain UV-vis spectra of the electrochemically deposited copolymers on an Agilent 8453 spectrophotometer.

A QCA-922 (SEIKO EG&G instrument) system combined with Versa STAT 3 was used for simultaneous electrochemical quartz crystal measurement (EQCM) and cyclic voltammetric measurements. The electrochemical cell was assembled in a glove box using an ITO AT-cut quartz crystal resonator (mirror finished, resonant frequency: 9.08 MHz ± 50 kHz, A = 0.2 cm 2 , SEIKO EG&G., LTD) as working electrode, a platinum wire as counter electrode, and a Ag/AgCl wire as a quasi-reference electrode. The solution used for the electropolymerization here is the same as the one we used for electropolymerization of the copolymers with the larger ITO electrode. Iterative scans were conducted at a scan rate of 100 mV•s -1 at room temperature with simultaneous recording of the quartz resonance frequency. The change of the quartz resonance frequency (Δf) was converted into the mass change (Δm) on the ITO-coated quartz during iterative cycling by applying Sauerbrey's equation (eq 1): ∆f = -2f0 2 ∆m/A(µ.ρ) 1/2 (1

)
where f0 is the resonant frequency of the fundamental mode, ρ is density of the crystal (2.684 g/cm 3 ), A is working area (0. Atomic force micrographs (AFM) measurements have been conducted directly on the ITO surfaces using a Dimension 3100 (Veeco) in the tapping mode under ambient conditions. Silicon cantilevers (Veeco probes) with a spring constant of 300 N/m and a resonance frequency in the range of 120-139 kHz have been used. The scanning rate was 1.0 Hz.

Photoelectrochemical measurements were performed in a three-electrode electrochemical cell using glass-ITO electrodes covered by the copolymer thin film as photocathode, Pt plate as counter-electrode and Pt wire as pseudo-reference electrode. The photoelectrochemical responses of the copolymer were obtained by on-off light illumination in 5 mM I3 -/ 0.5 M I - aqueous solution using a 300 W Xe arc lamp. I3 -and I -anions act as redox mediator. The scheme of the experimental set-up can be found in Scheme S3. Water filter was used to avoid IR radiation and long pass filters either at λ > 385 nm or λ > 800 nm were used to keep only visible-NIR or NIR radiations, respectively. Current-potential curves measurements were recorded in the dark and under illumination with PARSTAT 2273 potentiostat using linear sweep voltammetry at 10 mV s -1 . EIS spectra were obtained under illumination by applying sinusoidal modulation of the potential (10 mV over the Open Circuit Photopotential) at frequencies from 10 -1 to 10 5 Hz.

Results and Discussion

Redox and optical properties of pyridine end-decorated molybdenum(II) halide cluster PIM(py)6

The electrochemical behaviour of (Bu4N)2[Mo6I8(COOpy)6] (PIM(py)6) (Figure 1A) has been studied by cyclic voltammetry. Three different working electrodes have been used to study PIM(py)6 in solution: a glassy carbon electrode (GC, d = 3 mm, A = 0.283 cm 2 ), a platinum electrode (Pt, A = 1.0 cm 2 ) and an indium tin oxides electrode (ITO, A = 1.0 cm 2 ).

PIM(py)6 2-showed similar redox behaviour on GC and Pt electrodes as shown in Figure 1 and Figure S1 respectively. One reversible oxidation appeared near 1.32 V. The Ipa/Ipc is close to 1 and it was assumed to be the oxidation of {Mo6I8} 4+ /{Mo6I8} 5+ according to the literature [START_REF] Kirakci | A highly luminescent Hexanuclear molybdenum cluster -A promising candidate toward photoactive materials[END_REF].

The small pre-peak (a) observed in Figure 1C is due to the adsorption of PIM(py)6 on the Pt electrode. It corresponds to the oxidation of the adsorbed PIM(py)6. In the case of Pt electrode for instance, the peak current a increase linearly with the scan rate v, as shown in Figure 1D, which indicates that this wave is not controlled by the diffusion. It proves that this peak corresponds to the oxidation of the absorbed PIM(py)6. In contrast, Ipa or Ipc (anodic or cathodic peak current) of wave I increases linearly with the square root of the scan rate showing that wave I corresponds to the diffusion-controlled redox process.

However, in the case of ITO electrode, the anodic peak current Ipa is always higher than the cathodic peak current Ipc, that means the cluster {Mo6I8} 5+ is not reduced completely on the reverse sweep as shown in Figure S1B. The potential of the oxidation corresponding of the step I is also measured at lower potential (i.e. 1.09 V). It may be due to the absorption of the oxidized compound on the surface of the ITO electrode. In this case, it must be noticed that additional reduction peak is observed at -0.96 V (peak * ) in the cathodic domain. This additional peak may be due to the irreversible reduction of the adsorbed compound.

When a higher potential was reached in the anodic domain in the case of GC electrode as shown in Figure 1E, one additional irreversible peak II is observed at 1.96 V. It may be due to the decomposition of the cluster core due to the fully oxidation of the starting Mo II atoms [27-29].

Moreover, one irreversible peak (peak III) is also observed at -1.57 V and corresponds to the irreversible reduction of {Mo6I8} 4+ /{Mo6I8} 3+ (Figure 1F).

(d = 3mm) (A) -1.1 V to 2.2 V (B) 0.0 V to -1.8 V. v = 100 mV s -1 .
The optical spectrum of PIM(py)6 is presented in Figure 1B. With the increase of concentration of PIM(py)6, the absorption intensity also increases as expected from the Beer-Lambert law.

The absorption bands in UV region is due to the π-π* transitions in the aromatic terminal ligands. The broad and featureless absorption wavelength longer than 350 nm may due to the {Mo6I8} 4+ core-centered d-d transitions [START_REF] Mikhailov | Synthetic tuning of redox, spectroscopic, and photophysical properties of {Mo6I8} 4+ core cluster complexes by terminal carboxylate ligands[END_REF]. The result is consistent with the data reported in the literature . The energy gap calculated from optical spectrum at 402 nm (3.08 eV)

between HOMO and LUMO orbitals is close to the value calculated from the electrochemistry:

2.89 eV on GC electrode (Eox -Ered= 1.32 -(-1.57) = 2.89 eV).

Formation of 3D electropolymerized thin film based on a isoporphyrin and on a pyridine end-decorated molybdenum(II) halide cluster

The molecular precursors porphyrins zinc-5,15-bis(p-tolyl)porphyrin (ZnT2P), as well as the pyridine end-decorated Mo(II) halide cluster ((Bu4N)2[{Mo6I8}(OOC-C5H4N)6]) (PIM(py)6) are depicted in Scheme S1. The syntheses of the isoporphyrin copolymer was achieved via our previously reported electropolymerization method [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF]. It relies on the addition of hexapyridyl ligand to an electro-generated π-radical cation porphyrin produced by iterative voltammetric scans (Scheme 1). The electrosyntheses of the (iso)porphyrin copolymers poly-ZnT2isoP • -PIM(py + )6 requires the attack of a pyridyl ligand to an electro-generated -radical cation porphyrin produced by oxidation. Thus, the pyridine end-decorated molybdenum halide cluster (PIM(py)6) can combine with ZnT2P porphyrin through the pendant pyridyl ligands in order to form pyridinium linkers between porphyrin macrocycles. The zinc-meso-5,15ditolylporphyrin (ZnT2P) presents only two meso positions occupied by one substitutable proton at positions C10 and C20. Additionally, all the  positions of ZnT2P are occupied only by protons. Such porphyrin will provide better accessibility for the PIM(py)6 (Scheme 1).

Electrosynthesis of Zn and Mg meso-triarylphosphonium porphyrin were as well reported from Zn or Mg 5,10,15-triarylporphyrin indicating a nucleophilic substitution is mainly at the meso and not at the β position [START_REF] Dime | Electrosynthesis and X-ray Crystallographic Structure of Zn-II meso-triaryltriphenylphosphonium porphyrin and structural comparison with Mg-II meso-triphenylphosphonium porphine[END_REF]. Furthermore, the electrochemical oxidation of pyridin-2-ylthiomeso substituted Ni(II) porphyrins afforded oxidative C-N fusion of pyridinylsubstituted porphyrins again at the meso position giving cationic and dicationic pyridinium-based products [START_REF] Berthelot | Oxidative C-N fusion of pyridinyl-substituted porphyrins[END_REF].

As already reported, for zinc 5,15-ditolyl porphyrin (ZnT2P), the nucleophilic attack appears to be allowed only at the 10 as well as the 20 meso positions of the porphyrin [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF]. It should also be noted that in the absence of nucleophile, the electrogenerated radical ZnT2P +• may undergo a radical coupling reaction and the formation of one dimer with C-C bond formation through meso-meso, meso-β or β-β link between two porphyrins. Actually, previous works in the literature using the 5,15-bis(3,5-di-tert-butylphenyl) porphyrin presented only the formation of a mixture of meso-meso and meso-β linked dimers. These dimers showed a splitting of the Soret band of the optical spectra [START_REF] Ogawa | One-pot electrochemical formation of meso, meso-linked porphyrin arrays[END_REF][START_REF] Ogawa | Completely regioselective synthesis of directly linked meso meso and meso,  porphyrin dimers by one-pot electrochemical oxidation of metalloporphyrins[END_REF][START_REF] Dime | Control over the oxidative reactivity of metalloporphyrins. Efficient electrosynthesis of meso, meso-linked zinc porphyrin dimer[END_REF].

However, when potential iterative scans are performed between 0.9 V and -1.1 V vs. SCE using ZnT2P alone (in the absence of PIM(py)6) an absence of deposit or the formation of a film is observed onto the ITO electrode signifying that the kinetic of radical coupling reaction is relatively slow. It must be noted that the formation of dimer and eventually small oligomer which are soluble cannot be completely excluded. However, no change of the optical spectrum of the solution was observed.

During the oxidation of ZnT2P in the presence of PIM(py)6 between -1.1 V and +0.9 V vs.

SCE (just after the first porphyrin-based oxidation potential), the π-radical cation ZnT2P +• is produced when reaching +0.9 V. It must be noted that at applied potential of +0.9 V, the reversible oxidation of PIM(py) [START_REF] Beltrán | A photobleaching resistant polymer supported hexanuclear molybdenum iodide cluster for photocatalytic oxygenations and photodynamic inactivation of Staphylococcus aureus[END_REF] not yet occurs because the oxidation potential is 1.09 V vs.

SCE. Significant changes of the cyclic voltammogram are observed during iterative scans of the electropolymerization process. As shown in Figure 2A, in the anodic part, the intensity of the oxidation peak corresponding to the first oxidation of the porphyrin ZnT2P continuously increased during the electropolymerization which is correlated to the increase of the film thickness. This also shows the formation of poly-ZnT2isoP • -PIM(py + )6 copolymer.

The first scan toward negative potentials presents no signal. Starting with the second scan, in the cathodic potential range, the quasi-reversible reduction wave appeared near -1.0 V vs. SCE (Figure 2A) has been attributed to the reduction on the electrogenerated pyridinium units introduced between two porphyrins. The continuous increase of these reduction waves indicates a regular growth of the copolymer film onto the ITO working electrode.

It should be noted that when the iterative scan is conducted between -1.1 V and 1.6 V, a partial decomposition of the film is observed (Figure S2B). This is probably due to the second irreversible oxidation of the PIM(py)6 cluster (Figure 1E, peak II).

The formation of poly-ZnT2isoP • -PIM(py + )6 by electropolymerization with iterative scans between -1.1 and +0.9 V has been also monitored in-situ by EQCM (Figure 2B).

Besides, the trace of the first scan in Figure 2B shows a significant decrease of the resonance frequency and thus an increase of the deposited mass at the first oxidation of the porphyrin; i.e.

electropolymerization occurs upon the formation of the radical cation porphyrin ZnT2P +• in the presence of PIM(py)6.

The resonance frequency displays a significant decrease meaning an increase of the deposited mass during the electropolymerization as shown in Figure 2B. The mass of the copolymer film increases with the number of iterative scan n (Figure 2C).

Note that a change of slope is observed for n > 12. It might be related to a transition of the pure 3D architecture of the copolymer film to the copolymer with defects and with the incorporation of supporting electrolyte, monomers as well as solvent molecules.

Assuming that the repeat unit is [3ZnT2P-PIM 2-(py + )6] 2TBA + .6PF6 -in the case of the pure 3D material, the calculated surface coverage Γ is 1.74 × 10 -10 mol.cm -2 after only one scan between -1.1 V and +0.9 V. The theoretical values of mass change can be calculated and are given in Table S1 in the case of the formation of 1D or 3D copolymer.

After one scan, Δm = 0.93 µg.cm -2 of poly-ZnT2isoP • -PIM(py + )6 are deposited. This experimental value is close to the theoretical mass deposited (i.e. 1.06 µg.cm -2 ) calculated by using the anodic charge (Q = 1.73.10 -4 C) as well as the molecular mass matching to the 3D copolymer (5337.2 g.mol -1 ). Furthermore, it is more distant to the value obtained in the case of the 1D copolymer (i.e. 2.18 µg.cm -2 ). These results may suggest the formation of the 3D copolymer. The yield of the formation of the such copolymer can be estimated and is given in Table S2. The calculated yield is 88 % for n = 1 and it decreases with the number of iterative scans giving 39 % for n = 25 scans. This can be correlated to the increase of the resistance when the thickness of the film increases.

By the integration of the pyridinium reduction wave, the value of Δmcalc = 2.15 µg.cm -2 of copolymer deposited is calculated after 3 iterative scans assuming that a 3D copolymer is formed. This reduction corresponds to the 6 electrons process per copolymer unit. The value is in reasonable agreement with the measured value which is 2.79 µg.cm -2 (Table S2).

Scheme 1. The electropolymerization process of poly-ZnT2isoP • -PIM(py + )6. a in 1,2-EtCl2/CH3CN (7/3). b Copolymers obtained by iterative scan between -1.1 V and +0.9 V vs. SCE onto ITO. c Copolymers obtained by iterative scan between -1.1 V and +0.9 V vs. SCE in DMF. d Copolymers obtained by iterative scan between -1.1 V and +1.6 V vs. SCE onto ITO.

UV-visible-NIR spectra on ITO electrodes coated with the copolymer poly-ZnT2isoP • -PIM(py + )6 have been measured at various thickness (Figure 3A). The absorption intensity of the isoporphyrin increases linearly with iterative scan number (inset of Figure 3A).

A typical UV-visible-NIR spectrum of poly-ZnT2isoP • -PIM(py + )6 obtained after electropolymerization between -1.1 and +0.9 V exhibited a broad split Soret absorption band at λ = 433 nm and λ= 475 nm. The bands were red-shifted by 18 and 60 nm respectively as compared to the ZnT2P monomer (Table 1). The visible bands (Q bands), observed at 561 and 645 nm, are also red-shifted by 15 and 61 nm compared to ZnT2P and an additional band at 875 nm in the NIR region is detected. These electropolymerized films have been found to be fully soluble in DMF. Analysis of poly-ZnT2isoP • -PIM(py + )6 in solution by UV-Vis-NIR absorption spectroscopy revealed distinct spectrum than the one recorded on the solid film (Figure S3). The NIR band is not detected and the red-shift of the Soret band is lower.

The superposition of the UV-visible-NIR absorption spectra of ZnT2P in solution and of the copolymer poly-ZnT2isoP • -PIM(py + )6 deposited on ITO are shown for comparison in Figure S3. It reveals that for poly-ZnT2isoP • -PIM(py + )6 the Soret band, attributed to the isoporphyrinbased π-π* electronic transition, is much broader and is split with significant red-shift in the case of the film deposited onto ITO electrode compared to the starting monomer ZnT2P in DMF solution (Table 1).

The red shift of the Soret (B) and the Q bands might result from the presence of the electronwithdrawing pyridinium groups as well as from the nonplanar conformation of the isoporphyrin.

Optical red shifts induced by the non-planarity of the macrocycle are well documented [START_REF] Fajer | Structural effects in chemistry and biology[END_REF][START_REF] Shelnutt | Nonplanar porphyrins and their significance in proteins[END_REF][START_REF] Barkigia | and -octamethyl-5,10,15,20-tetraphenylporphinato)zinc(II)[END_REF][START_REF] Sparks | Metal dependence of the nonplanar distortion of octaalkyltetraphenylporphyrins[END_REF] and have been rationalized by a larger destabilization of the highest occupied molecular orbitals (HOMOs) relative to the lowest unoccupied molecular orbitals (LUMOs) resulting in smaller HOMO to LUMO gaps [START_REF] Senge | ,18-octaethylporphyrinato)nickel(II)d correlation of nonplanarity with frontier orbital shifts[END_REF][START_REF] Souza | Electronic, spectral, and electrochemical properties of (TPPBrx)Zn where TPPBrx is the dianion of bbrominated-pyrrole tetraphenylporphyrin and x varies from 0 to 8[END_REF][START_REF] Anderson | Super-charged Porphyrazines: synthesis and physical properties of octacationic tetraazaporphyrins[END_REF][START_REF] Bernard | Redox properties of porphycenes and metalloporphycenes. A comparison with porphyrins[END_REF].

These changes can also be understood taking into account the presence of intra-and intermolecular exciton-coupling between the porphyrin macrocycle within the copolymer [45-

46].

The additional bands at 475 and 875 nm are attributed to the isoporphyrin structure present in the copolymer as already observed. [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF][START_REF] Abhilash | NO2-Mediated meso-hydroxylation of iron(III) porphyrin[END_REF] Interestingly, the broadening and the splitting of the Soret band as well as the presence of one additional band in the NIR between 750 nm and 1000 nm are expected to be advantageous to photovoltaic applications by extending the domain of solar light absorption. Poly-ZnT2isoP • -PIM(py + )6 isoporphyrin radical film was found to be highly stable in the presence of oxygen (still stable even after one year).

To confirm the presence of the organic radical, poly-ZnT2isoP • -PIM(py + )6 films onto ITO were prepared (25 iterative scans between -1.1 V and 0.9 V, v = 100 mV.s -1 ). Poly-ZnT2isoP • -PIM(py + )6 was then removed from ITO using DMF and this procedure was repeated three times. The obtained solutions were degassed in order to remove dioxygen and transferred to a capillary. The ESR spectrum shown in Figure 3B reveals the presence of the radical centered isoporphyrin (g = 2.0026, linewidth = 6G) [START_REF] Inisan | Electrooxidation of porphyrin free bases: fate of the p-cation radical[END_REF].

Increasing the concentration of the deposited copolymer on the ITO glass leads to an enhancement of the ESR intensity. Usually, isoporphyrin or radical cations porphyrins are reactive and undergo facile degradation. In this study, poly-ZnT2isoP • -PIM(py + )6 radical film is unusually stable for more than one year at the solid state (in air). This stability may be due to the electron-withdrawing groups which considerably facilitate the reduction and stabilize the radical cation. It also may be due to the delocalization of the radical through the porphyrin as well as intermolecular ππ -stacking and π -dimerization between macrocycles.

The deposited film was also studied by scanning atomic force microscopy (AFM) (Figure 3C).

Tightly packed coils of the copolymer appear on the surface with an average diameter of ca.

20-50 nm, the height being around 5.0 nm for the film obtained after 1 iterative scan between -1.1 V and +0.9 V (Figure 3C). The rms surface roughness of the two films have been estimated at 20.0 nm for poly-ZnT2isoP • -PIM(py + )6, (calculated from an area of 0.08 µm 2 ). The copolymer obtained after higher iterative scan number exhibited comparable morphology but showed in several positions some aggregation of the coils accompanied by a larger value of the rms surface roughness (400 nm for n = 15). The formation of coil aggregates might be related to the change of slope of the deposited mass, observed from EQCM measurements in Figure 2C.

The copolymer films were also investigated by X-Ray photoelectron spectroscopy (Figure S4). When the potential limit chosen is 1.60 V (and not 0.9 V), two oxidation waves appear at 1.10

V and 1.38 V as shown in Figure S5. It can be explained by the formation of isoporphyrin cation via oxidation of the isoporphyrin radical followed by the oxidation of the porphyrin which can be formed after the release of the proton. Nevertheless, the deposited film becomes weak and less colored probably indicating partial degradation of the poly-ZnT2isoP • -PIM(py + )6 film at this potential.

Impedance and photoelectrochemical properties

Figures 5A and 5D show the typical current-potential curves of poly-ZnT2isoP • -PIM(py + )6 (n = 1 or 3 scans) thin films on ITO electrodes obtained in 5 mM I2 / 0.5 M I -aqueous solution in the dark and under illumination. The potential axis in the cyclic voltammograms is referred to the equilibrium potential of I3 -/I -redox couple in solution. On bare ITO electrode (Figure 5A, grey curve), the I -oxidation and I3 -reduction kinetics is slow and almost no Faradaic current is observed between -0.3 V and 0.2 V. When the ITO is modified by the poly-ZnT2isoP • -PIM(py + )6 copolymer thin film, significant I -oxidation and I3 -reduction current can be seen in the voltammogramms measured in the absence of illumination (dashed curves in Figure 5A and Figure 5D). This demonstrates that the poly-ZnT2isoP • -PIM(py + )6 copolymer thin film on the ITO electrode acts as an electrocatalyst of I -oxidation and I3 -reduction. Similar behaviour was reported with poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ copolymer thin films [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF].

A typical current-potential curve measured under visible-NIR light illumination (using a long pass filter λ > 385 nm) is displayed in Figure 5A (green full line) for the poly-ZnT2isoP • -PIM(py + )6 copolymers obtained for n = 3 scans. The open circuit photopotential under illumination is around 0.1 V and the short-circuit current is of ca. -130 µA cm -2 revealing electron injection from the copolymer to I3 -in solution under illumination. For a number of electropolymerization potential scans n < 5, isoporphyrin copolymers prepared with PIM(py)6

exhibited better photocurrents than the ones prepared with the double viologen spacer under the same conditions of electropolymerization (Figure 6B). This can be attributed to the much larger amount of isoporphyrin deposited on the ITO electrode during the initial electropolymerization cycles when PIM(py)6 is used instead 1,1"-(1,3-propanediyl)bis-4,4'-bipyridinium hexafluorophosphate salt (bpy + (CH2)3-bpy + 2PF6 -), as confirmed by the higher intensity of the Soret Band of poly-ZnT2isoP • -PIM(py + )6 compared to poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+

(Figure 6A).

The photocurrent response of poly-ZnT2isoP • -PIM(py + )6 in the potentiostatic mode at 0 V under on-off light illumination cycles for various n is shown in Figures 5B and5C. For all measurements, reversible rise and decay of the photocurrent in response to the on/off illumination step can be observed. The photocurrent exhibits an optimum value as a function of n and the copolymer film thickness, the best performance being obtained for n = 3 potential scans. Similar behaviour has been previously reported for other "linear" isoporphyrin copolymers, such as poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ with double viologen spacer [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF].

Note however, that larger of number electropolymerization cycles (around n = 10-15) were needed to reach the optimum in the photocurrent for the poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ copolymer [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF].

To determine the contribution of NIR absorption to the overall photocurrent, the NIR photoresponse was investigated using a longpass filter λ > 800 nm. The typical results are plotted in Figure 5D for the copolymer poly-ZnT2isoP • -PIM(py + )6 thin film obtained after only one electropolymerization potential scan. Reproducible and stable photocurrent of ca. -8 µA cm -2 is observed under on/off illumination cycles with the NIR radiation. In general, the ratio of the photocurrent in NIR region to the overall (visible + NIR region) photocurrent (for example -58 µA cm -2 for n = 1 electropolymerization cycle) is around 12% and the open circuit photovoltage is of ca. 50 mV under NIR illumination.

The contribution is even higher in the case of poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ where the ratio of the photocurrent in NIR region to the overall is 26% (Figure S6). It can be explained by the more intense NIR band in the case of poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ [START_REF] Boudiaf | Stable isoporphyrin copolymer: electrochemical mechanism and behaviour and photovoltaic properties[END_REF]. Scheme 2 represents the energy diagram corresponding to the photoreduction of I3 -on the copolymer poly-ZnT2isoP • -PIM(py + )6. In the former research of our group [14a,49 ] , the inclusion of polyoxometalates (POMs) was found to be beneficial to the photocurrent due to the enhanced electron transfer kinetics from the excited porphyrin of the copolymers to the pyridinum giving the radical pyridyl (py • ), which in turn can give one electron to the POM moieties. This cascade of electron transfer allows to avoid the back recombination between the radical cation porphyrin and the radical pyridyl (py • ).

The HOMO level energies of the poly-ZnT2isoP • -PIM(py + )6 have been estimated from the first oxidation potential observed in the cyclic voltammograms, while the energy level of the dipyridinium species is given by the potential of the first and second reduction peaks (Table 2).

The LUMO levels of the excited poly-ZnT2isoP • -PIM(py + )6 species can be roughly estimated by subtracting the excitation energy of the Soret or Q bands from the HOMO level energies.

The energy level corresponding to the band in the NIR region has also been indicated in the energy diagram. The energy level of the PIM(py)6/PIM(py)6 -at -3.78 eV can be calculated from the potential of reduction of PIM(py)6 (-4.50 -(-0.72) = -3.78 eV).

Under illumination under short-circuit current conditions, the photon absorption by the (iso)porphyrin entities generates an electron hole pair in the copolymer. The electron is transferred from the excited isoporphyrins radical subunit, to the pyridinum subunit which is reduced to form the radical pyridyle py • (dotted red arrow in Scheme 2). The pyridinium acts as a relay for the electron transfer. Then, the radical pyridyle py • in turn can reduce I3 -into I - which is reoxidized at the counter electrode. The oxidized isoporphyrin is regenerated by an electron transfer from the ITO substrate.

As the energy level of the NIR band associated to the isoporphyrin radical subunit is lower than the energy level of the couple Py + /Py  , after non radiative relaxation, the electron also could be transferred directly from NIR energy level to the level I3 -/I -to reduce I3 -to I -, which could explain the photocurrent generated by copolymer poly-ZnT2isoP  -PIM(py + )6 under NIR illumination. In such a case, PIM(py)6 is not mediating the electron transfer.

However, it should be noticed that the PIM(py)6 can also accept electron from the excited isoporphyrins radical subunit giving the reduced form PIM(py)6 -but only in the case of the levels S2 (Soret bands) as well as the more energetic level S1 corresponding to the Q band as shown in Scheme 2 (blue arrows). In such a case, the reduced PIM(py)6 -can in turn give one electron to the pyridium subunit to form the radical pyridyle Py  . Finally, the Py  can exchange electron with I3 -giving I -which is reoxidized at the counter electrode.

It must be mentioned that PIM(py)6 can also be directly excited using visible light, its UVvisible spectrum exhibiting broad band at 402 nm corresponding to the {Mo6I8} 4+ core-centered d-d transitions. The excited PIM(py)6 becomes a good electron donor and can give the electron to the pyridinium group as reported in Figure S8. However, the intensity of this band is low compared to isoporphyrin moieties and the contribution to the overall photocurrent is expected to be small.

Conclusion

In summary, 3D copolymer films have been prepared by the electro-oxidation of 5,15ditolylporphyrin (ZnT2P) in the presence of polyiodometalate with 6 pendant pyridyl ligands (Bu4N)2[Mo6I8(pyCOO)6] (PIM(py)6). Electrogenerated radical cation porphyrin ZnT2P +• is a powerful electrophile which can rapidly react to form copolymer containing stable isoporphyrins (poly-ZnT2isoP  -PIM(py + )6). The existence of the cation radical of the isoporphyrin was proved by electron spin resonance spectroscopy. The achieved copolymers were characterized by UV-Vis-NIR spectroscopy, Atomic Force Microscopy and X-ray photoelectron spectroscopy. Electrochemical properties of these copolymers have been discussed and studied.

The photocurrent measurements under visible-NIR light irradiation show that poly-ZnT2isoP  -PIM(py + )6 thin films exhibit significantly performance even under only NIR illumination.

However, the photovoltaic performances reach an optimum depending on the number of electropolymerization potential scan n. The best performances are obtained for n = 3. Therefore, the control of the thickness of the copolymer films is of great importance to optimize the generation of photocurrent under visible illumination. Furthermore, the results proved that the NIR band played an important role for photocurrent measurement where the contribution is noticeable.

Poly-ZnT2isoP  -PIM(py + )6 in the solid state could be stored for over several months and even one year without any degradation under ambient conditions in air. Further investigations are underway in order to understand more about this notable stability. Other type of stable isoporphyrin radical are in preparation in our laboratory.

characterization of the copolymers as well the EIS and photovoltaic measurements. Maxim N.

Graphical abstract

First example of formation of a 3D hybrid organic-inorganic copolymer based on an isoporphyrin and on a pyridine end-decorated molybdenum(II) halide cluster. The electrical properties and the photovoltaic performances have been investigated under visible-NIR light irradiation.

Figure 1 A

 1 Figure 1 A) Structures of [Mo6I8(pyCOO)6] 2-(PIM(py)6). B) UV-vis spectra of PIM(py)6 with different concentration in 7/3 1,2-dichloroethane / acetonitrile. C) Cyclic voltammograms of PIM(py)6 (0.25 mmol L -1 ) recorded between -1.1 V and +1.6 V vs. SCE on Pt electrode with increasing scan rate in a 1,2-C2H4Cl2/CH3CN (7/3) solution in the presence of NBu4PF6 (0.1 mol L -1 ). D) Peak "a" current versus the scan rate. E) and F) Cyclic voltammograms of cluster complexes of PIM(py)6 in 7/3 1,2-EtCl2/CH3CN solution in the presence of NBu4PF6 (0.1 mol L -1 ) on glassy carbon electrode (d = 3mm) (A) -1.1 V to 2.2 V (B) 0.0 V to -1.8 V. v = 100 mV s -1 .

Figure 2 .

 2 Figure 2. A) Cyclic voltammograms recorded during 25 iterative scans conducted between -1.1 V and +0.9 V/SCE and B) -1.1 V and +1.6 V vs. SCE in a 1,2-C2H4Cl2/CH3CN (7/3) solution of ZnT2P (0.25 mmol L -1 ) in the presence of PIM(py)6 (0.25 mmol L -1 ) and NBu4PF6 (0.1 mol L -1 ). WE: ITO. S = 1 cm 2 . v = 100 mV s -1 . Blue curve: first scan (n=1). Red curve: final scan (n = 25). Red curve: highest reduction current. B) Consecutive cyclic voltammograms (first 25 scans) between -1.1 and 0.9 V and electrochemical quartz crystal microbalance measurements (Δm) for the first 25 scans during the formation of poly-ZnT2isoP • -PIM(py + )6 obtained by the electropolymerization of 0.25 mmol L -1 ZnT2P with 0.25 mmol L -1 PIM(py)6 (0.25 mmol L -1 ) in 1,2-C2H4Cl2-CH3CN (7/3) in the presence of 0.1 mol L -1 NBu4PF6. Working electrode: ITO (A = 0.2 cm 2 ) deposited on a 9.08 MHz AT-cut quartz crystal. v = 100 mVs -1 . C) Mass change (Δm) of the first 25 scans calculated from Sauerbrey's equation versus the number of scan n.

3. 3 Figure 3 .

 33 Figure 3. A) UV-visible-NIR absorption spectra of poly-ZnT2isoP• -PIM(py + )6 onto ITO with different numbers of iterative scans between -1.1 and +0.9 V vs. SCE (v = 100 mV s -1 ). Only one side of the slide is covered by ITO. Inset: plot of the absorbance measured at λ = 433 nm versus n the numbers of iterative scans. B) ESR spectrum of poly-ZnT2isoP -PIM(py + )6 in DMF at room temperature. The solution of poly-ZnT2isoP -PIM(py + )6 was prepared by washing with 160 μL of DMF three covered ITO obtained using 25 scans between -1.1 V and 0.9 V, v = 100 mV s -1 . C) Left: tapping mode AFM topography of poly-ZnT2isoP • -PIM(py + )6 (deposition between -1.1 V and +0.9 V vs. SCE on ITO, n = 1). Right: section analysis of the aggregate marked by a white line.

3 . 4 6

 346 The analysis of the survey spectra of poly-ZnT2isoP • /PIM(py)6 confirms the presence of the isoporphyrin radical subunits (Zn 2p3 at 1022.3 eV, N 1s, and C 1s peaks), and the presence of the PIM cluster subunits (Mo 3d and I 3d). Furthermore, the signals for F 1s (686.6 eV) is also detected and corresponds to the incorporated counterion PF6 -to equilibrate the pyridiniums charges. The C1s peaks are composed of three signals at 289.4 eV corresponding to the -C(O)OH-in the PIM(py + )6, 286.3 eV and 285.4 eV attributed to homo and hetero (connected to nitrogen) carbon atoms respectively. The N1s peaks reveal the presence of two chemically different nitrogen atoms. The contributions at 401.3 eV and 398.5 eV are attributed to the iminic nitrogen and to the reduced pyridinium respectively. Finally, the peak localizated at 394.3 eV might result from the Molybdenum 3p 2/3 orbital. Redox properties of the 3D copolymer poly-ZnT2isoP • -PIM(py + )Electroactive copolymers deposited by cyclic voltammetry (n iterative scans) on ITO surfaces have been characterized by electrochemical methods. After electropolymerization, the electrode was then removed from the electrochemical cell, washed with CH3CN and used as working electrode in a clean electrolytic solution containing only the solvent and the supporting electrolyte (Figure4and FigureS5). The cyclic voltammetry curves shown in Figure4Ahave been recorded with copolymers deposited on the ITO electrode surface after various number of scans (n = 1, 3, 5, 10, 15, 20 cycles) between -1.1 V and +0.9 V. Two successive waves are observed for poly-ZnT2isoP • -PIM(py + )6 during the cathodic scan (Table2 and Figure4), the first being irreversible (peak b) and the second being quasi-reversible (peak c). The first irreversible wave (peak b) is detected at -0.72 V and is very close to the second reduction process (peak c). Peak b corresponds probably to the irreversible reduction of the pyridinium connected to the isoporphyrin close to the pendant hydrogen atom. It generates one radical which reacts rapidly via radical coupling with the radical present onto the isoporphyrin (formation of the C-C bound). The peak d is due to the oxidation of resulting compound obtain after the radical coupling. In peak d the C-C bound is broken to regenerate the starting isoporphyrin radical. Peak c is attributed to the quasi-reversible reduction of the second pyridinium connected to the isoporphyrin.

Figure 4 .

 4 Figure 4. Cyclic voltammograms of poly-ZnT2isoP  -PIM(py + )6 obtained with PIM(py)6 and ZnT2P after A) n = 1, 2, 3, 5, 10, 15 and 20 scans and B) after 10 scans between -1.1 V and +0.9 V in CH3CN/1,2-C2H4Cl2 (3/7) with 0.1 M NBu4PF6. WE: ITO. S = 1 cm 2 , v = 100 mV s -1 .

Figure 5 .

 5 Figure 5. A) Current-potential curves of poly-ZnT2isoP  -PIM(py + )6 (obtained with n = 3 iterative scans between -1.1 V and +0.9 V thin films on ITO electrodes obtained in 0.5 M I -/ 5 mM I2 aqueous solution in the dark or under visible illumination. B) and C) Photoelectrochemical responses of poly-ZnT2isoP  -PIM(py + )6 films obtained with n = 1, 2, 3, 5, 10, 15, 20 or 25 iterative scans. Measurements have been done under on-off light illumination from a 300 W Xe arc lamp (with λ> 385 nm long pass filter) in I3 -5 mmol L -1 and I -0.5 mol L -1 aqueous solution. BIAS potential: 0.00 V vs. OCP. D) Current-potential curves of poly-ZnT2isoP  -PIM(py + )6 (obtained with n = 1 scan between -1.1 V and +0.9 V thin films on ITO electrodes obtained in 0.5 M I -/ 5 mM I2 aqueous solution in the dark, under visible-NIR illumination or under NIR illumination.

Figure 6 .

 6 Figure 6. A) UV-visible-NIR absorption spectra of poly-ZnT2isoP • -PIM(py + )6 compared to poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ onto ITO obtained after n = 3 iterative scans between -1.1 and +0.9 V vs. SCE (v = 100 mV s -1 ). Only one side of the slide is covered by ITO. B) Photoelectrochemical responses of poly-ZnT2isoP • -PIM(py + )6 and of poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ obtained with n = 3 iterative scans.Measurements have been done under on-off light illumination from a 300 W Xe arc lamp (with λ > 385 nm long pass filter) in I3 -5 mmol L -1 and I -0.5 mol L -1 aqueous solution. BIAS potential: 0.00 V vs. OCP.

Figure 7

 7 Figure 7 shows the EIS spectra obtained for poly-ZnT2isoP • -PIM(py + )6 and poly-ZnT2isoP •bpy 2+ -(CH2)3-bpy 2+ under illumination, using a perturbation of 10 mV over the open circuit photopotential. Experimental data are represented by circles while the solid lines correspond to the fit obtained using the equivalent circuit shown in Figure S7. The general behaviour of the impedance spectra exhibited by the poly-ZnT2isoP • -PIM(py + )6 and poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ copolymer are similar. The Nyquist diagrams of the copolymers prepared with one electropolymerization cycle (Figure 7A) possess one semicircle at high frequency followed by a straight line at low frequency, which can be well described by a charge transfer resistance Rct and a Warburg impedance in parallel with the interfacial capacitance Ci. For n = 1, the charge transfer resistance of the poly-ZnT2isoP • -PIM(py + )6 copolymer (ca. 270 Ω) is significantly lower than the one of poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ (ca. 680 Ω). This difference is in

Figure 7 .

 7 Figure 7. Nyquist curves of poly-ZnT2isoP • -PIM(py + )6 and of poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ (obtained with iterative scans between -1.1 V and +0.9 V) on ITO electrodes obtained in 0.5 M I -/ 5 mM I2 aqueous solution under light irradiation. A) n = 1 scan, B) n = 20 scans.

Figure 8 .

 8 Figure 8. Plot of the Rct and Rfilm of the thin films versus n (number of iterative scan, v = 100 mV s -1 , between -1.1 V and +0.9 V). A) poly-ZnT2isoP • -PIM(py + )6, B) poly-ZnT2isoP • -bpy 2+ -(CH2)3-bpy 2+ .

Scheme 2 .

 2 Scheme 2. Schematic illustration of the energy level diagram for poly-ZnT2isoP  -PIM(py + )6 showing electron transfer processes in H2O containing I3 -5 mmol L -1 and I -0.5 mol L -1 (Py + = pyridinium, Porp = porphyrin) under light irradiation.

  Sokolov and Maxim A. Mikhaylov have prepared the PIM(py)6 and also participate to the discussion of this work. Helen Ibrahim and Michel Goldmann have done the AFM measurements. Sylvie Choua and Nolwenn Le Breton have done the ESR measurments and analysis. Antoine Bonnefont, Corinne Boudon and Vasilica Badets contributed to the analysis of the EQCM, EIS as well as photocurrent measurements. They also helped to write the manuscript. All authors discussed the results and commented on the manuscript.

Table 1 .

 1 UV-visible spectral data for ZnT2P in CH3Cl, PIM

(py)6, poly-ZnT2isoP • -PIM(py + )6 in

	poly-ZnT2isoP • -PIM(py + )6 b	433, 475	561, 645, 875
	poly-ZnT2isoP • -PIM(py + )6 c	417	547, 644
	poly-ZnT2P-PIM(py + )6 d	434	567, 607
				1,2-EtCl2,
	on optical transparent ITO electrodes. Under bracket: molar extinction coefficient (ε /10 3 L.M.cm -1 ).
	Compound	Soret band/nm	Q bands/nm	π-π * Band/nm
	ZnT2P	415 (405.4)	546 (18.9), 584 (4.9)
	PIM(py)6 a			220 (170.4), 246
				(109.0), 344 (11.0),
				402 (8.0)

Table 2 .

 2 Electrochemical Potentials in V vs. SCE were obtained from cyclic voltammetry in 1,2-C2H4Cl2 with 0.1 mol L -1 TBAPF6. Scan rate = 100 mV s -1 . Working electrode: ITO, S = 1 cm 2 . The given half-wave potentials are equal to E1/2 = (Epa -Epc)/2. Under bracket: △Ep = |Epa-Epc|.b Potentials in V vs. SCE were obtained from cyclic voltammetry in CH3CN/1,2-C2H4Cl2 (3/7) with 0.1 mol L -1 TBAPF6. Scan rate v = 100 mV s -1 .

	data for ZnT2P, PIM(py)6, and poly-ZnT2isoP • -PIM(py + )6 in EtCl2/CH3CN (7/3)
	solution. Working electrode: ITO.
	Ring oxidation	Reduction of the spacer	Ring reduction
	Compounds	(py + , double viologen or
		PIM)

a c if using GC electrode. Nearly the same potential is measured if using Pt electrode. d if using GC electrode. e only observed in the case of the ITO electrode.

Acknowledgements

We thank CNRS, the Université de Strasbourg (France), Université Paris-Descartes (Paris, France), for funding of this work. We also thank the Université de Strasbourg for Labex CSC (Chemistry of Complex Systems) which has also supported one part of this research as well as the CSC (China Scholarship Council) grant for the Ph.D. grant of Yiming Liang.

Appendix A. Supplementary data

 S1-S2).

Author contributions

Laurent Ruhlmann conceived the idea, and initiated the study. He wrote the manuscript with the assistance of all the co-authors. Yiming Liang carried out the preparation and