Fred Anne L Katell

Philipp, Asma Axel Tam

Laura

Amanda Nicolo

Adelais

Adama

Arthur

Riya Sophie

Stephanie

Feng

Ben

Virgile Ambroise

Miguel, Morane, Trang, Lien, Thu, Loan, Julien Olga Robinson

Dimitri

Na Karel

Minh Khoai

Chi Tu

Tuong Linh

thank you so much for your help during the time of trouble. I would like to thank Lucile, the person I shared the lab with during these three years, thank you for all the help and support and for being super proper and neat all the time, it was really fun working along side with you. And to Maxence, thank you for being such a hard-working intern. To the people I spent these three years with: to Benjamin for all the laughter and the inside jokes, to Fatima for being such a sweet sweet person, to la petite Marie for always telling me that I am awesome, to Arnaud for being the feisty person that I love, to Guillaume for all the dinner parties, to Clémence for always being there for me through thick and thin, to María mi madré for your unconditional love, to Khaled for your endless inspiration, to Alice for your ironic cheerfulness, to Salomé

Acknowledgement

The work in this thesis was performed at the Laboratoire de Chimie Inorganique (LCI) at the I hereby thank Corine Mathonière and Lionel Salmon for having accepted the work of this thesis as the reviewers. I also would like to thank Antoine Tissot and Anne Bleuzen for accepting to be parts of my jury. I greatly thank Laurent Lisnard for being my comitté de suivi de thèse and for thoroughly following my work during my PhD. I thank you all for the valuable evaluation and assessment, and the insightful discussion during the thesis defense.

To my thesis supervisors: Talal and Laure, thank you for having entrusted me with the project and for your insight and support that have helped me until the very end. I'm very grateful for the opportunity and I am forever indebted to you, for you are always resourceful, inspiring, and understanding. Thank you Talal, for remembering me as one of your students during my master in Vietnam. And Laure, for being more than a supervisor to me all this time, I will always remember all the time we shared together, inside and outside of work.

I would like to thank Sandra and Mauri, the people who set the first stone of my studies. I am very honored to be able to continue their works. Many many thanks to the greatest magnetic engineer in the world: Eric Rivière, thank you for such kindness, enthusiasm, and skills. A lot of thanks to François Brisset for all the EDS analysis (and for keeping my samples there when I forgot to collect them). I also would like to thank Christian Herrero, for his EPR measurements which started completely out of the blue. I would like to thank the rest of the ICMMO staff for making my experience here unforgettable.

Abstract

The research work in this thesis is focused on the preparation and the study of the physical properties of switchable nanocrystals based on spin crossover and photomagnetic materials that are potential candidates as nanosensors.

The spin crossover nanocrystals belong to the coordination network family based on the Fe(II)

Hoffman chlatrates [Fe(pyrazine)][Pt(CN) 4 ]. The study is focused on controlling the size of self-standing nanorods and on the investigation of their thermal bistability in relation with their environment and their interaction with guest molecules. We show that the growth of the rod-shaped nanocrystals is obtained in the presence of preformed 2-3 nm nanoparticles of the Prussian Blue Analogue CsNiCr(CN) 6 that is thought to direct anisotropic growth of the objects. The formed rods can be stable for a long time due to their negative charge. They possess a rather wide thermal hysteresis centered around room temperature. The width and position in temperature of the thermal hysteresis depend on the nature of the environment of the nanocrystals.

With ammonium salts having long alkyl chains (such as dodecyl trimethylammonium bromide or hexyl trimethyl ammonium bromide), the thermal hysteresis becomes narrower and shifts to low temperature. While with shorter alkyl chains, the shift of the hysteresis is smaller without much change in its width. When the rods are embedded in a rigid silica matrix, the transition is shifted to very low temperature and the thermal hysteresis vanishes. The insertion of I 2 molecules leads a thermal hysteresis centered at a higher temperature with a slight widening.

The photomagnetic nanoparticles belong to the bimetallic Prussian Blue Analogues (PBAs) family based on CoFe. This type of materials may undergo a thermal transition from the paramagnetic Co II Fe III state to the diamagnetic Co III Fe II one. The high temperature state can then be restored at low temperature by light illumination. We focused on three types of CoFe PBAs nanoparticles containing the Cs I (12 nm) and the Rb I alkali ions with two different sizes 30 and 70 nm for the latter. The size of the objects was controlled by different parameters such as the nature of the alkali ion, the concentration of the precursors and their stoichiometry.

The combination of different techniques such as electronic and infra spectroscopies together with Electron Spin Resonance Spectroscopy and X-ray powder diffraction allows following the evolution of the nanoparticles' composition with time in solution at room temperature. The evolution study leads to the general conclusion that the electron transfer phenomenon from Co II to Fe III occurs in solution during several days before the objects reach their thermodynamic stable state that was found to depend on the nature of the alkali ion and on the size of the objects. The (photo)magnetic behavior was then investigated in the solid state for different evolution times of the three types of particles, which allowed proposing a qualitative mechanism of their formation in solution.

Résumé

Les travaux de recherche de cette thèse concernent la préparation et l'étude des propriétés physiques de nanocristaux commutables basés sur des matériaux de type transition de spin et photomagnétiques potentiellement candidats en tant que nanocapteurs.

Les nanocristaux à transition de spin appartiennent à la famille des réseaux de coordination basés sur les chlatrates de Hoffman [Fe(pyrazine)] [Pt(CN) 6 ]. L'étude se concentre sur le contrôle de la taille des nano-bâtonnets et sur l'étude de leur bistabilité thermique en relation avec leur environnement et de leur interaction avec les molécules invitées. Nous montrons que la croissance des nanocristaux en forme de bâtonnets est obtenue en présence de nanoparticules préformées de 2 à 3 nm de l'analogue du bleu de Prusse CsNiCr(CN) 6 , censées diriger la croissance anisotrope des objets. Les nano-bâtonnets formés sont stables pendant une longue période en solution en raison de leurs charges négatives. Ils possèdent une hystérèse thermique assez large centrée sur la température ambiante. La largeur et la position en température de l'hystérèse thermique dépendent de la nature de l'environnement des nanocristaux. Avec des sels d'ammonium à longue chaîne (tels que le bromure de dodécyl triméthylammonium ou le bromure de d'hexyl triméthyl ammonium), l'hystérèse thermique se rétrécit et se décale vers les basses températures. Avec des chaînes alkyles plus courtes, le décalage de l'hystérèse est plus petit sans que sa largeur ne change beaucoup. Lorsque les nanobatônnets sont enrobés dans une matrice de silice rigide, la transition est déplacée à très basse température et l'hystérèse thermique disparaît. L'insertion de molécules de I 2 conduit à une hystérèse thermique plus large centrée sur une température plus élevée que la température ambiante.

Les nanoparticules photomagnétiques appartiennent à la famille des réseaux bimétalliques d'analogues du bleu de Prusse (ABP) basés sur le CoFe. Ce type de matériaux peut subir une transition thermique de l'état paramagnétique Co II Fe III à l'état diamagnétique Co III Fe II .

L'état de haute température peut ensuite être rétabli par un éclairage lumineux à basse température. Nous nous sommes concentrés sur trois types de nanoparticules de CoFe contenant les ions alcalins Cs I (11 nm) et Rb I avec deux tailles différentes : 30 et 80 nm pour ces dernières. La taille des objets était contrôlée par différents paramètres tels que la nature de l'ion alcalin, la concentration des précurseurs et leur stoechiométrie. La combinaison de différentes techniques, telles que les spectroscopies électronique et infrarouge, associée à la spectroscopie par résonance paramagnétique électronique et à la diffraction des rayons X sur poudre permet de suivre l'évolution de la composition des nanoparticules avec le temps en solution à la température ambiante. L'étude de l'évolution conduit à la conclusion générale que le phénomène de transfert d'électrons de Co II à F e III se produit en solution pendant plusieurs jours avant que les objets n'atteignent leur état thermodynamique stable qui dépendait de la nature de l'ion alcalin et de la taille des objets. Le comportement (photo)magnétique a ensuite été étudié à l'état solide pour différents temps d'évolution des trois types de particules, ce qui a permis de proposer un mécanisme qualitatif de leur formation en solution.

Introduction and Objective

loop centered at room temperature [2] but also they have the ability to accept various guest molecules within their pores that can modify their spin cross-over behavior. [3][4][5][6] • The Charge-Transfer of CoFe Prussian Blue Analogues that may possess two different electronic states corresponding to different oxidation states of the CoFe species: Co II Fe III and Co III Fe II depending on the external perturbation. It may be possible to switch form one state to another through electron transfer by light irradiation [7][8][9] or by variation of temperature. The charge-transfer also induces a spin cross-over on Co, which can be detected by a change in volume or in red-ox potential.

Overview

This thesis contains four chapters:

Chapter 1 is dedicated to the state of art of the spin cross-over charge transfer phenomena focusing on the two targeted materials of this thesis, the Hofmann-like clathrate coordination network in Section 1.2 and the CoFe Prussian Blue Analogues in Section 1.3.

Chapter 2 concerns the preparation and the investigation of the behavior of nanorods of the Fe II pz[Pt(CN) 4 ] Hofmann-like clathrates stabilized in the presence of nanoparticles of the Prussian Blue Analogues CsNiCr(CN) 6 : the method of synthesis, the characterization of the objects, the method of controlled-growth, the influence of environment (inorganic and organic)

and guest molecules on the spin cross-over behaviors.

Chapter 3 deals with the nanoparticles of CoFe Prussian Blue Analogues. We study the effect of the alkali ions Cs I and Rb I on their formation in solution using different techniques and then their magnetic properties in solid state when recovered either pure or in the presence of organic surroundings.

Finally, Chapter 4 is a general conclusion as well as some perspectives.

1.1 Spin Cross-Over Phenomenon The spin cross-over (SCO) phenomenon was discovered by Cambi and coworker in 1930s when they were studying iron(III) dithiocarbamate complexes on which they observed for the first time the anomalous magnetic properties. [10] But it did not attract intensive attention until 1964 when an SCO complex of [Fe(phen) 2 -(SCN) 2 ] was synthesized by Baker et al. [11] where they observed absolute zero, the entropy term TDS vanishes and the free energy equals to the entropy term DG = DH. At sufficiently low temperatures, the enthalpy still dominates and the contribution of the entropy term is negligible. Consequently, the condition of stability is decided by the difference in energy of the phases, the more stable state being the state of lower energy and in HS ⌦ LS transitions, generally, it is the LS state. [START_REF] Konig | [END_REF] At transition temperature T c , the free energies of the two phases are equal, DG=0! DH=T DS. And at sufficiently high temperature, the entropy term is the dominant factor. DH and DS may be composed of several contribution:

DH=DH el + DH intra.vib + DH lat.vib (1.2) DS=DS el + DS intra.vib + DS lat.vib (1.3)
where "el" refers to the electronic enthalpy/entropy and "intra.vib" and "lat.vib" respectively refer to the intramolecular and lattice vibrational characteristics accompanying the spin transition. It is well established that DH el > 0 on going from LS to HS and as DH el is the largest contributor to DH in Equation 1.2, this is implied that DH > 0. Gütlich also found that DS > 0 on going from LS to HS state. [START_REF]G ütlich in Metal Complexes[END_REF] DS el may contain contributions from both spin and orbital degeneracy DS spin el and DS orb el . However, in the case of spin transition between 1 A 1 and 5 T 2 , DS orb el is often partially or completely removed [START_REF]G ütlich in Metal Complexes[END_REF][START_REF] Konig | [END_REF] and thus:

DS u DS spin el =Rln w HS w LS (1.4)
where w HS and w LS are the number of spin configurations for the HS and LS state respectively.

For instance, for compounds of Fe II , DS spin el = Rln 5 = 13.4 J K -1 mol -1 .T h e s t u d i e s t o measure the values of the entropy changes in LS ⌦ HS transitions for some complexes of Fe II are described in Table 1.1.

Compound T c DH DSM e t h o d R e f (K) (kJmol -1 )( J K -1 mol -1 )
[Fe(phen) Apparently, in Equation 1.1, as long as the temperature is low enough (DH > TDS), the LS state is more stable than the HS state. Evidently, the driving force of a spin transition of LS ⌦ HS is the gain in entropy which is provided partly by the change in total spin and by another part by the associated change in molecular configuration. In another word, when the entropy term is in domination with increasing temperature causing DG < 0, HS state becomes more favored.

Pressure-Induced Spin Cross-Over

Most often, SCO is considered to be a thermally-induced phenomenon. However other physical conditions can be a trigger to induce and/or to modify the spin transition. First of all, the synthesis process of the compounds has a strong effect on the spin transition behavior. Many factors can be of an alternation on the SCO phenomenon such as crystal quality, particle sizes or defects occurred during sample preparation (stirring speed, precipitation, grinding, drying, etc...). These factors, however are usually unpredictable.

Among these stimuli, the pressure-induced SCO has been under the most studies [START_REF] Ksenofontov | Gütlich in Spin Crossover in Transition Metal Compounds III[END_REF]. The effect of applied pressure was first studied by Ewald et al. on the family of Fe III dithiocarbanate complexes in solution. [34] The LS 2 T 2 state has a smaller molecular volume than the HS 6 A 1 state and will be more and more favored with increasing pressure. [35] It was pointed out by E. König that the strong dependence of the ligand-field strength on the metal-donor atom distances and the resulting large difference in metal-ligand bond lengths between the two spin states leads to the change of entropy, which is the driving force for the thermal spin transition. [36] As a matter of fact, the fundamental cause for the influence of pressure on the SCO phenomenon is the large difference in the metal-donor atom bond lengths with Dr HL =r HS -r LS ⇡ 0.1 and 0.2 Å for Fe III and Fe II SCO molecules respectively. [START_REF] Ksenofontov | Gütlich in Spin Crossover in Transition Metal Compounds III[END_REF] The presentation of pressure effect on the LS and HS potential wells of Fe II is showed discontinuous transition in Figure 1.7b emerges from the presence of strong cooperativity (first order transition). The situation in between of (a) and (b), as a matter of fact, does exist. The transition with hysteresis in Figure 1.7c occurs when there's a particularly high cooperative interactions where the temperature of LS!HS transition on heating is higher than that of HS!LS transition on cooling. This type of transition has come to be acknowledged as one of the most remarkable aspects of SCO phenomenon. This leads to the bistability of the system and therefore a memory effect. As already mentioned, bistability refers to the ability of a system to be observed in two different electronic states in a certain range of some external perturbation (most often temperature). [51] The potential of this aspect of SCO in devices for storage, memory and display has been highlighted by Kahn et al. [52] and has attracted much attention of research in this area. This type of transition will be the center of this thesis and will be more detailed below. The two-step transition in Figure 1.7d is comparatively rare and have origins in several sources. The most obvious is the presence of two lattice sites for the complex molecules. The retention of a significant high spin fraction (Figure 1.7e) at low temperatures may also arise from various sources (domain size, defects,...).

Spin Cross-Over Materials

SCO phenomenon has been observed in all metal ions of first transition series with electronic configuration 3d 4 -3d 7 [START_REF] Garcia | Gütlich in Spin Crossover in Transition Metal Compounds II[END_REF]. However, the most common SCO compounds are those found in 3d 6 systems with Fe II complexes made up the largest number. The only other 3d 6 example with SCO behavior is Co III which often adopts LS state due to its high ligand field strength reducing its spin pairing energy. [START_REF] Garcia | Gütlich in Spin Crossover in Transition Metal Compounds II[END_REF] In the case of Fe II ions, the ligand field strength is weaker so by using different type of ligands to appoint either LS or HS systems, Fe II ions can fulfill the requirements for SCO in a certain degree of tunability of ligand field strength hence the large number of SCO Fe II compounds. While in the the case of Fe III ions with the configuration 3d 5 , the ions are smaller and the LS configuration is favored. Then SCO compounds can be further categorized in two different class based on the nature of molecular interactions: (1) mononuclear metal complexes and (2) polynuclear metallic compounds.

Mononuclear Spin Cross-Over Complexes

The majority of mononuclear SCO materials are of [FeN 6 ] 2+ systems. Among many ligands, only for 4-R-1,2,4-triazoles, isoxazoles, and 1-alkyl-tetrazoles (Figure 1.8) has the Fe II N 6 SCO chromophore, which is required to generate the SCO phenomenon, has been found to consist of six chemically identical heterocyclic ligands. These spin transition materials are particularly interesting since only a single N-donor ligand is involved in the synthetic procedure.

Common bidentate ligands are often the derivatives of 2,2'bipyridine (bipy) and 1,10-phenanthroline (phen) while the tridentate ligands are derivatives of 2,2':6',2"-terpyridine this can not explained simply by ligand field theory as py has a stronger ligand field than that of pz. The 3-D Fe(pz)[M 0 (CN) 4 ] complexes have a more rigid structure creating an internal pressure, hence, might be responsible for the effective stronger ligand field at Fe II sites.

The family of SCO Hofmann clathrates compounds contains of 2-dimensional (2-D) and ] network [20] It has been pointed out by Bonhommeau et al. [2] that the spin transition of these compounds are strongly dependent on the water content in the pores of the 3D network as showed in Other guests: Other guests other than solvent and halides have been reported to induce different changes in the SCO phenomenon of SCO compounds. For example, on these uptake and release of guest molecules in SCO materials, gas molecules like CO 2 ,N 2 , and O 2 induced no spin conversion, while five or six membered ring molecules like pyrazine, benzene, pyrrole,

Spin State Dependent Host-Guest Interaction

The adsorption of gases like N 2 and O 2 in the porous framework of Fe(pz)[Pt(CN) 4 ] has been observed to favor the HS state than the LS state at 298K. [87] Hence, adsorption and desorption of these gases is temperature dependent, since on cooling the system, desorption of gas occur upon transition from HS to LS and this behavior can be related to the pore size of the network. For CO 2 , adsorption is favored in the HS state at higher pressures, in agreement with the enthalpy changes. This could possibly due to the high porosity of the HS phase and higher pressure is needed to optimize the structural rearrangement of the system, while at lower pressure, adsorption is higher in the LS state of the system. The authors speculated that this behavior at low pressure could be due to the "sandwiching" of these guest molecules between pyrazine pillars giving more optimal pz•••pz separation.

Spin transition changes are more pronounced when the guest molecules are vapors or liquids (ethanol, methanol, acetone, acetonitrile and toluene) due to a greater host-guest interactions and energetics of the system. On adsorption and desorption of acetonitrile, the spin transition does not follow the same path and provides the memory effect.

Based on these experimental observations, it was concluded that it's not only the changes in ligand field around the metal center caused by inclusion of guest molecules in the porous network but rather temperature dependent host-guest interaction energies determines the spin transition temperature. By choosing the appropriate spin state of the system at comparable temperature and pressure, selective adsorption or desorption of guest molecules can be obtained. This property of these porous frameworks can be exploited in sepration of mixture of gases. prepared by reverse micro-emulsion were found to have a partial and gradual SCO without hysteresis, [9] 4nm NPs embedded in chitosan beads reveal a large thermal hysteresis close to room temperature. [90] It has also been observed that partial washing of the surfactant have a tremendous effect on the SCO properties. [92] From these examples, it is important to note the importance of carrying out studies on NPs that are fully characterized in order to compare not only the respect of sizes but also the influence of shape as well as the environment since even traces of surfactants or solvent can have a tremendous effect on the SCO behaviors of these compounds. Ideally, self-standing particles without any stabilizer or surfactant should be targeted to avoid these effects, and moreover, to study these effects systematically.

A first experimental demonstration has been reported by our group in 2011 [93] ] nanoplatelets surrounded by calixarene and silica of variable thickness. STEM-EELS profile of the composition obtained across a particle surrounded by 2nm of silica. [63,93] This could highlight the importance of tuning the thickness and rigidity of the matrix to efficiently enhance the SCO phenomenon. Several theoretical investigations into this aspect have carried out by the group of Boukheddaden. [94,95] They have illustrated an electroelastic model of which the internal pressures in the core and the shell depending on their respective thickness was calculated using 2D core-shells. It has been suggested by the authors that acoustic impedance mismatch between the core and the shell maybe detrimental to the cooperativity.

A study on the microcrystals of [Fe(phen) 2 (NCS) 2 ] by Tissot et al. shows that the effect of surface is negligible as compared with NPs of extended coordination polymers where the environment at the surface is strongly modified. [96] The purpose of this work was to rationalize the theoretical work by Muraoka et al. [97] based on the Ising model specifying the edge effects for particles embedded in polymers. In their experiments, microcrystals of Fe II (phen) 2 (NCS) 2 with the size of 1700 ⇥ 1700 ⇥ 160 nm 3 were prepared using sudden precipitation method. These particles were then embedded in polyethylene glycol (PEG), polyvinylpyrrolidone (PVP), glycerol, nujol, and eicosane as dispersing media and the SCO cooperativity was compared with bare micro-particles of the same size. Samples with PEG showed similar behavior as bulk due to the weak interaction between the PEG macromolecule and the microcrystals. Sample with heated PEG gave a rather broad transition which was related to the fact that on heating, the melted PEG absorbed by anchoring at the surface, which then leads to denser composite and increasing interfacial effects. Similar behavior can be observed on the samples with PVP compared to that of heated PEG. Remarkably, samples with glycerol exhibits a wide hysteresis with relatively gradual conversion in cooling mode and a steeper transition in heating mode, which was not present in the bulk compound (Figure 1.30). Indeed, when going from HS to LS state, the viscous matrix becomes glassy and exerts an increasing negative pressure that delays the SCO and shifts the descending branch to lower temperatures. [96,98] The volume of the LS particles is smaller than the original cavity, and the particles are ultimately free from the interaction. This would explain why the ascending curve is very similar to that of the bulk and this cooperativity may thus be considered as "matrix-induced".

Another experimental demonstration of the crucial role of the matrix is the use of polysaccharides (chitosan and alginate) as hard templates to confine the growth of 3-5nm The strong cooperativity observed on these ultra-small NPs might be stemmed from such connectivity. However, by using alginates as matrix instead of chitosan, these cooperativity 1.3. Photo-Induced Magnetic CoFe PBAs magnetic properties were also intensively studied and the ordering temperatures of "bulk" PBA compounds (often made of sub-micronic interacting particles) was tuned by controlling the nature and strength of exchange interaction between the two metal centers of the M'-CN-M pairs, [START_REF] Gadet | [END_REF][110][111][112] while a large variety of stimuli able to tune their magnetism have merged along the years, ranging from light, pressure, magnetic and electric fields and guest/ion inclusion [113][114][115][116] have been pursued over the years. The vast properties of this class of materials has been widely reported such as proton conductivity, [117] proton-to-electron charge transfer, [118] and photocatalysis water splitting. [119] 1.3.2 Magnetic Properties of Prussian Blue Analogues

1.3.2.1 Long-Range (Anti) Ferromagnetic Interactions in Prussian Blue

Analogues

The group of Verdaguer et al. [120] has rationalized the magnetic behavior of these systems using the Goodenough-Kanamori rules and the orbital model developed by Kahn. Because of the cubic structure of the systems, the local symmetry of the metal ions can be approximated to be octahedral with the d-orbitals having t 2g and e g symmetries. The nature of the exchange coupling responsible of the magnetic properties can then be considered as the sum of two contributions one antiferromagnetic due to the overlap between orbitals of the same symmetry (t 2g -t 2g ) and one ferromagnetic due to the orthogonality of the orbitals (t 2g -e g ). [120,121] In this way, several PBAs are known with long range ferro-or ferri-magnetic behavior below a Curie temperature (T c ) were prepared and studied. Some of them show an ordering only below low temperatures (T c < 50 K), [122][123][124][125][126][127][128][129][130] but larger ordering temperatures (50 K < T c < 300 K) have been found too. [131][132][133] As it turns out, the specific magnetic properties of PBAs are not only dependent on the nature of the metals and the stoichiometry but some evidence exists that they might also be dependent on the specific grain size and shape of the involved material.[134-138]

1.3.2.

Changes in Magnetization upon External Stimuli

PBAs are versatile materials because they are switchable. The phenomenon was first reported by Hashimoto et al. in 1996 on the CoFe based networks. [139] The behavior of this system will be detailed below.

Other PBAs switchable materials were also discovered. The Fe

II [Cr III (CN) 6 ] 2/3 • 5H 2 O
network shows a decrease in magnetization below T c 21K when it is irradiated with 360 -450 nm light. [140] This was explained by a mechanism in which the ferromagnetic coupling between Fe II and Cr III has changed due to the fact that the spins of both metal ions were not perfectly aligned after the irradiation. More recently, the studies based on the determination of the crystalline field parameter of Co ions by X-ray Absorption Spectroscopy (XAS) have made better progress in understanding the origin of the diamagnetic pairs. [155,156] Measurements have showed, for the first time, that the crystal field parameter of Co II ions varies because of the different geometries of Co II -NC-Fe IIII while that of Co III ions remains invariable thanks to its t 2g 6 e g 0 configuration highly stabilized in octahedral symmetry (Figure 1.36). magnetic behaviors based on the size/nature of the cations selected. The host-guest interaction studies have been carried out using the "bare" FepzPt@CsNiCr compound and some selected cation-recovered counterparts doped with I 2 to examine combined effects of guest and matrix.

Size studies are carried out using DLS and TEM (JEOL JEM-1400 at 120kV). Structure studies are carried out using X-ray diffractometer (Panalytical X'Pert Pro MPD) and FT-IR spectroscopy. Chemical composition is studied using EDS (ZEISS Supra 55VP). Magnetic properties of the compounds is studied on SQUID magnetometer (Quantum Design MPMS 5).

Synthesis and

.2.1 Energy Dispersive Spectroscopy Analysis

The composition of FepzPt@CsNiCr and CsNiCr were studied by EDS (Table 2.2). The analysis showed CsNiCr nearly preserved its composition (Cs 0.5 Ni[Cr(CN) 6 ]) in the final rods.

The ratio of CsNiCr to Fe obtained from the analysis is ⇠ 1:8 which is also the ratio used as precursors for CsNiCr and Fe. As explained later on, this indicated that the CsNiCr amount needed to obtain stable nanorods in FepzPt@CsNiCr is rather small.

Cs

X-Ray Powder Diffraction Patterns

FepzPt@CsNiCr and CsNiCr powders were studied on XRPD as shown in 4.7±0.7 nm (here the experimental broadening can be neglected), slightly bigger than the size observed on TEM and DLS which is usually the case on our PBA NPs and may be attributed to higher diffraction intensity for the larger coherent length of the samples.

The caculated lattice constant is a = 10.52±0.06 Å which is close to the value reported by Prado et al. [175] The coherent domain sizes of the FepzPt@CsNiCr NPs calculated on the peaks at (001), ( 110), ( 111), (200), and (002) are reported in Table 2.

Infra-red Spectroscopy

The two compounds were studied by IR spectroscopy using KBr pellet (Figure 2.9). The spectra of the FepzPt@CsNiCr NPs had an intense vibration at 2172 cm -1 which has been assigned for the n(C ⌘ N) of Fe-Pt linkages. Two weak shoulder peaks were observed on FepzPt@CsNiCr around 2147 cm -1 and 2132 cm -1 which have been assigned for the non-bridging Pt -CN and Cr -CN respectively. The peaks in the region between 1500 -800 cm -1 belong to pyrazine while the peak at 1729 cm -1 belongs to the n(C = O) of acetone still presents in the NPs after drying at room temperature. The peaks at 805 cm -1 was assigned to the g(C-H) of pyrazine ring that links to the Fe II HS indicating the compound is at HS state. [176] Figure 2.9: Infra-red spectrum in KBr pellet of FepzPt@CsNiCr NPs at room temperature (inset: Infra-red spectrum in KBr pellet of CsNiCr CTA NPs)

A summary of relevant frequencies observed on these spectra is presented in Table 2.4.E v e n though it is difficult to identify the CsNiCr in FepzPt@CsNiCr, there are a few bands of the

) qT= g 2 8 ⇥ s(s + 1) (2.2)
where g Ni = 2.28 and g Cr = 1.99 as previously reported by Prado et al. [START_REF] Prado | [END_REF] and s Ni = 3/2 and s Cr = 1/2. We have q M T value of Ni-Cr estimated to be 3.14 (Equation A.2). With 1/8

CsNiCr to Fe, 0.39 emu.K/mol should be subtracted from the q M T of FepzPt@CsNiCr at low temperature making this value 0.6 emu.K/mol instead, indicating an almost complete transition. Below 80 K, there is a sharp increase reaching a maximum with the value q M T ⇡ 3.2 emu.K/mol at 22K then below 22K sharply decreased indicating the presence of Ni-Cr pairs that interact ferromagnetically. In heating mode from 5 K, the transition occurs at 297 K revealing a hysteresis width DT = 7 K. The transition is less abrupt than in cooling mode with T c " = 302 K.

A summary of these two scanning is presented in Table 2.5. Upon fast cooling, the hysteresis shifted to lower temperature as well as opened wider than in slow cooling. The compound also have a lower g HS at low temperature. And the q M T of CsNiCr is also higher than that in slow cooling due to the fact that CsNiCr might be easily damaged when staying in high temperature longer time in the slow cooling phase. Even though fast cooling appears to have wider hysteresis, the slow cooling is considered to reveal the "true" hysteresis. This phenomenon is due to a kinetic effect upon fast cooling. All measurements were then performed at 1 K/min on the other samples to avoid these effects.

Scan-rate T c # (K) T c " (K) T 1/2 (K) DT( K ) g HS (%) TEM grids were taken at one hour, three hours, 18 hours as well as DLS measurement. After 18 hours, the solution has a yellow-orange color, similar to that of FepzPt@CsNiCr.

1 (K/
After 18 hours, the compound was collected by centrifugation (900 0rpm, 4 o C, 45 minutes).

The yellow precipitate obtained was washed once with water and once with acetone before being dried under vacuum for several hours. The yellow-orange powder was then finely ground for other characterization (EDS, XRPD, IR, SQUID). A thermochromic effect was observed The powder of FepzPt@CsNiCo was studied on XRPD as shown in Figure 2.17.T h e compound has a similar pattern as its CsNiCo counterpart and also fitted well with the simulated diffractogram proposed by Real et al. [20] The peaks corresponding to CsNiCo NPs could not be observed on the pattern of FepzPt@CsNiCo due to the broadening of the peaks and the weak proportion of CsNiCo in the rods. However, the broadness and weakness of some peaks (100, 200 and 002) observed on FepzPt@CsNiCo could due to the overlapping with the planes (200), (220), and (400) of CsNiCo. This could also be a factor that made the calculated sizes of FepzPt@CsNiCo smaller than that of FepzPt@CsNiCr (Table 2.9). The length of the Fe-pz-Fe bond in FepzPt@CsNiCo showed to be the same as its CsNiCr counterpart while the length between Fe-NC-Pt showed to be about 0.03 Å greater than that in FepzPt@CsNiCr.

Compound

Infra-red Spectroscopy

The powder of CsNiCo@CTA was studied in IR spectroscopy using KBr pellet as showed in The powder of FepzPt@CsNiCo was studied on FT-IR spectroscopy as showed in Figure 2.19.

The spectrum shows an intense and asymmetric peak at 2171 cm -1 . As described for CsNiCr, sizes of the NPs increase. FepzPt without CsNiCr with the largest grain size (in most cases not relevant since Scherre's law cannot be reliable above 100 nm) again emphasize the crucial role of CsNiCr as a size-controller. The Fe-pz-Fe and Fe-NC-Pt bond lengths of these compounds reveal little difference and are similar to those of the bulk.

FepzPt@(CsNiCr) 1/16 with larger size, this band decreased. The band around 805 cm -1 of the g(C-H) links to the Fe II HS was observed on CsNiCr-contained compound. On the blank, this band is a shoulder next to another band at 812 cm -1 which might be the Fe II LS -CH linkages. Other than these, the three compounds show similar spectra in IR spectroscopy (Figure A.9) indicating the three compounds have rather similar structures and also similar to that of the bulk.

Magnetic Properties

The powder of blank FepzPt, FepzPt@(CsNiCr) 1/16 , and FepzPt@(CsNiCr) 1/14 were studied on SQUID magnetometer with the same sequence used for FepzPt@(CsNiCr) 1/8 (Figure 2.26).

The qT=f (T) plot of the blank compound (FepzPt produced without PBA) has a square shaped hysteresis indicating a cooperative transition between T c # = 282 K and T c " = 303 K with a hysteresis of DT = 21 K. FepzPt@(CsNiCr) 1/16 retained a similar shape of hysteresis loop as FepzPt with T c # = 294 K and T c " = 305 K (slightly higher than that of the bulk) with a hysteresis width of 11 K. FepzPt@(CsNiCr) 1/4 on the other hand, has the hysteresis completely shifted to lower temperature. The compound had a hysteresis loop of DT' 8K with T c # = 277 K and T c " = 285 K and a a hysteresis loop of DT = 8 K. All the compounds have the q M T values at the range of 3.51-3.67 emu.K/mol at 350 K which in agreement with their Fe II HS state. FepzPt reference without CsNiCr, the q M T decreased to 0.2 emu.K/mol suggesting a complete transition. FepzPt@(CsNiCr) 1/16 with only 1/16 CsNiCr to Fe also shows an almost complete transition at low temperature, reaching minimum of q M T = 0.62 emu.K/mol. FepzPt@(CsNiCr) 1/4 with 1/4 CsNiCr to Fe, indeed is the least complete with minimum value of q M T = 1.85 emu.K/mol obtained at around T = 180 K. The FepzPt@(CsNiCr) 1/8 indeed s an intermediate value with minimal q M T = 1.0 emu.K/mol at 168K. Considering 1 Cr-Ni pair has q M T ' 3.14 emu.K/mol. This minimum q M T value is expected to be around 0.785 emu.K/mol for (CsNiCr) 1/4 , 0.392 emu.K/mol for (CsNiCr) 1/8 , and 0.196 emu.K/mol for (CsNiCr) 1/16 if all Fe II are low spin considering only the CsNiCr contribution. From these values, a remaining fraction of HS Fe II can be deduced corresponding to 12.2% for (CsNiCr) 1/16 , 17.6% for (CsNiCr) 1/8 , and 30.1% for (CsNiCr) 1/4 . Below 60K where ferromagnetic coupling between Ni and Cr is observed with a strong increase of q M T that reaches a maximum around 14.4 emu.K/mol, 3.2 emu.K/mol and 1.6 emu.K/mol, for (CsNiCr) 1/4 , (CsNiCr) 1/8 , and (CsNiCr) 1/16 , respectively. The temperatures at which the temperatures are observed are slightly different, at 37 K for (CsNiCr) 1/4 , 23 K for (CsNiCr) 1/8 , and 22 K for (CsNiCr) 1/16 . However as CsNiCr PBA can be damaged by the pre-heating treatment at 400 K (linkage isomerism), these maxima values are not totally reliable. Moreover saturation effects are probably present as the measurements were done at 5000 Oe. Relevant values obtained from qT=f (T) plot are summarized in Table 2.14. Apart from FepzPt, the three compounds containing CsNiCr all followed certain trends. As the CsNiCr content increases, the hysteresis is shifted to lower temperature as well as a narrowing down of the hysteresis loop. When there is more CsNiCr present on the rods, the transition is less complete with a HS fraction that increases. The shape of the hysteresis is also affected with increasing the CsNiCr content , the transition goes from an abrupt to a more gradual conversion. Interestingly, the two compounds with 1/16 and 1/8 of CsNiCr have their hysteresis loop centered at room temperature similar to that of reference FepzPt . So a compromise is found between highly stable sub-100 nm rods while keeping the cooperativeness of the SCO phenomenon. The shell of CsNiCr particles exerts itself a pressure effect on the FepzPt core and this will be discussed later on in this chapter. as at very low temperature, the color of the compound changed from yellow to pink/orange color, similar to that of the bare compound.

Influence of Organic Cations

Strategy and Synthesis

To study the matrix effect, the negatively charged NPs of FepzPt@CsNiCr were recovered with different ammonium salts in order to observe the impact on the SCO behavior when there a constant amount of cations is added on the surface of the NPs. There are several advantages of using these cations:

• The NPs will be fully recovered due to electrostatic interactions

• There is no change in structure, the NPs have the same size and defects after recovery

• There is no change in the ligand field around the Fe II sites (in most cases the polar head is the same)

• A systematic study can be carried out in function of length of the apolar part of the cations.

The recovery was executed by adding 100 ml of 18-hour as-made NPs drop-wise into 100 ml of the solvent mixture 80% water/ 20% Acetone at 2 o C containing 6 mM of cation salt under moderate stirring. After addition, the solution was kept at low temperature until deposited (usually 1 hour to 6 hours depending on the cations). At the same time, a small amount of as-made NPs was kept under the same conditions (by adding 100ml of only the 80% water/ 20%

Compound Cs

Ni Cr Fe Pt NPs 0.05 0.12 0.1 1 1.1 NPs@DDTMA 0.03 0.12 0.1 1 1.1 NPs@HTMA 0.03 0.12 0.1 1 1.1 This strategy based on electrostatic interactions was thus an optimal way to insure a constant amount of cation and see systematic effects of tiny quantities of organic matter on the SCO phenomenon.

XRPD were registered (Figure A.13) and both samples reveal similar patterns as the original bare FepzPt@CsNiCr nanorods. It is important to mention that we performed all experiments several times but on a same batch of nanorods with all the cations to avoid any effect of a slightly different size distribution.

From these data, we confirmed that there is as expected no significant change in the structure of the NPs upon recovering with both long chain cations, and that the matrix can be considered as a sub-monolayer or organic cations that compensate the charge of the CsNiCr shell.

transition temperatures but also probe the new effects of the matrix on the cooperativity of the spin transition.

Synthesis

The four samples bare NPs, NPs@DDTMA, NPs@HTMA and NPs@EV have been doped with

Energy Dispersive Spectroscopy Analysis

EDS analysis were performed on the four samples, confirming the presence of I 2 in the compounds as showed in Table 2.29. Unfortunately the increase of Pt is not reliable due to of a line that gives big discrepancies (to be done in a close future). Nevertheless, the expected ratio on the original compound is close to 1 between Fe and Pt, and here all ratios I/Fe are close to 0.85-1. As expected, the amount of CsNiCr in these compounds are still the same as in the non-iodinated compounds. From the analysis, and having in mind that EDS has a rather large uncertainty, NPs@HTMA•I 2 is the most doped with 1 I to 1.6 Pt while NPs@EV•I 2 is the least doped with only 0.85 I. In the literature [3,4], the bulk was reported to contain maximum 1P tt o1I .

Compound

Infra-red Spectroscopy

The doped compounds have been investigated by IR spectroscopy as shown in Figure 2.39.

After doping with I 2 , a shift of the band around 2174 cm -1 is observed for all samples which is associated with the n(C⌘ N) of Pt and Fe, shifting from ⇠ 2174 cm -1 to ⇠ 2180 cm -1 .T h i si s due to the change in the oxidation state of the Pt centers Pt II ! Pt IV upon doping with I 2 . temperatures, but the g HS are also much lower than those of the un-doped compounds.

Table 2.22 shows a summary of all the compounds after doping with I 2 .

Among these compounds, the doped compounds coated with DDTMA and EV have the most shifted transition, with 100 K shift for DDTMA and 94 K shift for EV. This shift is expected for full insertion of iodine (Pt/I=1) on the bulk compound. The bare NPs and HTMA despite having the larger content in iodine as stated by EDS, seem to be less strongly affected as they are only shifted by 78 K for both compounds. DDTMA-coated particles is the most strongly affected by iodine insertion, not only because of the highest shift of its transition, but also by the hysteresis widening that is 8.5 K wider. All the other samples also opened a wider hysteresis: 8 K wider for the bare NPs, 2.5K wider for NPs@HTMA, and 7.5 K for NPs@EV The sample of NPs@EV•I 2 , even though less shifted than the sample of NPs@DDTMA•I 2 , has the highest transition temperature. This study reveals that the hysteresis in one specific matrix may be tuned towards higher temperatures by controlled insertion of iodine but also may modify to a more abrupt hysteresis which can be beneficial for storage purpose. similar polar heads, that would not to affect too much the ligand field around the FeII sites already covered partly by a shell of CsNiCr particles. By using trimethyl alkyl ammoniums with short chains (2 C and a terminal group X = Br, CH 3 or OH), the separating layer is expected to be weakly organized apart from the case where hydrogen bonding takes place.

Compound T c # (K) T c " (K) DT( K ) T 1/2 (K)
Terminal OH groups were expected to play a role, but here it seems to be minor as with all types of terminal groups, the SCO is only very weakly shifted as compared to the bare rods (that have Cs + as counteraction). It means that all cations have about the same weak interactions with the particles. When increasing the length of the alkyl chains above C6 as with HTMA and DDTMA, van der Waals interactions are favored and play a role on the organization of the intermediate organic layer, that may become more rigid and enhance this negative pressure effect, leading to a shift to lower temperatures and slight decrease of the hysteresis width of the FepzPt cores. As the amount is systematically controlled, it is thus possible to have a fine-tuning of the hysteresis temperature by the effect of the matrix. This study should be completed with other longer alkyl trimethylammoniums in the future to establish a relationship between the length of the alkyl chain and the shift of temperature.

The ethyl viologen is a case apart, as it induces probably a very different orientation of the molecule in between the nanorods, and may even lay almost parallel to the surface of the particles as compared to long alkyl chains that will be perpendicular and interdigitated. This may explain a comparable cooperativity between the bare particles and the EV-recovered particles that are very closely connected. Methyl viologen and other derivatives will be studied and confirm this hypothesis. Furthermore, even if the amount of organic matter is around 1% in weight, it would be interesting to promote the transformation of EV to its radical counterpart (by light ideally) and see if it affects the cooperativity at room temperature.

Different thermochromic compounds may be obtained (as the radical EV •+ is blue and may combine to that of the SCO coumpound).

Finally, the 3 nm silica shell appears as the most rigid matrix, with a strong negative pressure effect, that still does not suppress completely the hysteresis due to the rather big size of the rods.

Regarding the effects of guest inclusion, the differences in the shifts between the compounds are not fully understood. As expected, all iodine insertion leads to important change in the ligand field due to oxidation to Pt IV . Surprisingly, the bare particles seem to have an equivalent shift as those covered with HTMA, while the most organized matrix (DDTMA) favors a higher shift and larger hysteresis once doped, close to the effect on the EV sample.

These samples would need further characterization (by XRPD to correlate and ICP) that have not be possible in the present work. The EV form (ion or radical) should also be checked

(EPR).
counterparts, which is different from the constant hysteresis width that has been reported on the bulk by Real et al. [20] In all cases, the iodine insertion (that does not affect the CsNiCr behavior) induces a contraction due to HS to LS transformation (that corresponds to more than 10% change in volume). If the particles do transform, it means that the cations do have enough mobility to enable this transformation. Once doped, the particles are "freed" from the negative pressure and recover a hysteresis shape closer to that of the bulk. The most intriguing factor is that the widening and change in shape is not completely correlated to the shift of the T 1/2 . Interestingly, in the case of the most rigid silica shell matrix, the strain imposed by the silica is so important that the conversion from HS to LS by iodine insertion is not obtained.

All these results underline the nice but complex interplay of matrix and guest effects that may be a way (in close or far future) to trigger the cooperative SCO at the optimal temperature of the targeted application by tailoring the environment (matrix/guest) of the SCO particles.

Introduction

The CoFe containing Prussian Blue Analogs are fascinating systems because they undergo electron transfer coupled spin crossover that can be induced thermally and upon light irradiation. These systems have been investigated in the "bulk" form and it has been found that, depending on the nature and the amount of alkali ions inserted in the tetrahedral sites of the face centered cubic structure, two different electronic states may be stabilized at room temperature either Co II Fe III or Co III Fe II . If the room temperature state is the low spin diamagnetic Co III Fe II one, its transformation to the high spin paramagnetic state by light irradiation at low temperature depends on the nature and the concentration of the alkali ions within the network. When the paramagnetic Co II Fe III state is the stable one at room temperature, a thermal electron coupled transfer spin crossover may occur leading to the low spin state that may then be retransformed to the high spin state at low temperature by light;

it may also remain in the high spin state over the whole temperature range.

The nature of the state at room temperature that is responsible of the overall behavior depends on the approach used to prepare the compound. Since the reaction is carried out by mixing a ferricyanide salt with [Co II (H 2 O) 6 ] 2+ in water, obtaining the high spin Co II Fe III or the low spin Co III Fe II or a mixture depends on the synthetic details. For example, performing the synthesis by a very quick addition of the two precursors in the absence or presence of Rb I

or Cs I will have an influence on the size of the objects and the electronic state of the compound.

Objectives

We are interested in obtaining relatively small nanoparticles with low size distribution for their integration in electronic devices. Objects with small sizes can be obtained with a very quick addition of the two precursors that privilege quick nucleation. However, this approach has dramatic influence on the insertion of the alkali ions within the primary nuclei and on the electron transfer between Fe III and Co II during the reaction. The objects obtained by this quick addition approach may be electronically different from those obtained by a very slow addition.

In this chapter, we investigate the kinetic of formation of nanoparticles of CoFe PBA obtained by a quick addition of the precursors in the presence of Rb I and Cs I and for different concentration of the precursors. We propose a model of the electron transfer during the first steps of nucleation and the subsequent growth process. We also study the magnetic and photomagnetic behavior of the as-prepared objects.

CsCoFe

colorless after the washing. The precipitate had a deep purple color. The precipitate was dried under vacuum for several hours. In order to prepare a powder of the particles at t0, the aqueous purple solution was quickly precipitated by the CTABr solution, centrifuged, washed and dried. This powder samples is noted t0 even though the process takes at least 15 minutes.

Thus, the t0 samples n the solid state has already evolved while the solution t0 samples used for the UV-Visible and EPR studies have only few second age.

Another compound noted CsCo II Fe II was prepared to be used as a reference compare the difference between the characteristics of Co brown yellow color formed. The compound was collected using centrifugation (9000 rpm, 20 minutes) and washed one time with distilled water and one time with acetone and then dried under vacuum, the powder was ground to fineness for other studies. This compound is hereby called CsCo II Fe II . The second method is the same as for CsCoFe NPs described above only

using K 4 [Fe(CN) 6 ] • 2H 2 O instead of K 3 [Fe(CN) 6 ].
The compound is collected after being recovered with CTABr. This compound is hereby noted CsCo II Fe II .

Characterization

Transmission Electronic Microscopy Imaging and DLS Measurement

Solutions containing the objects were casted on TEM grids for t0 and t60h. The images showed the presence of objects in the 13 nm average. The analysis of the size gave the following values: 13.1±1.5 nm and 12.3±1.9 nm for t0 and t60h respectively (Figure 3.2).

We have noticed that better quality TEM images for small particles (less than 20 nm) are obtained when Dimethyldioctadecylammonium bromide (DODABr) was used to recover the particles instead of CTABr. We, thus, recovered the CsCoFe NPs with DODABr for TEM studies after one hour in solution. The as-made CsCoFe DODA NPs were redispersed in chloroform and casted on a TEM grid. The TEM imaging showed that the shape and size of the particles are well-preserved even after being recovered with DODABr and redispersed in chloroform with a size of 11.0±1.5 nm. The size of CsCoFe NPs observed with DLS remained unchanged during the 60 hours time evolution and was found equal to 15.6 nm (Figure 3.1)i n fair agreement with TEM images if one considers the hydrodynamic diameter observed in DLS. In order to qualitatively analyze the electronic spectra and get insight into the evolution in solution and the composition of the t0 and t60h species, we first recorded the spectra of the different reference compounds. We will, in the following restrict the analysis of the spectra to the 800-300 nm spectral region.

The UV-Vis spectrum of the CsCo II Fe II NPs (with concentration of 1 mM for Co II ), spectra to the t0 and t60h ones, we recorded the spectra of the supernatant solutions after recovering the nanoparticles at t0 and t60h. The spectra at t0 and t60h corrected from the amount of [Fe III (CN) 6 ] 3-are showed in Figure 3.5. The corrected t60h spectrum has a large band centered at 525 nm with a minimum 3.2. CsCoFe located at only few nanometers from the maximum of the band of the t0 spectrum; one can thus assume that the t60h spectrum corresponds to nanoparticles containing a large contribution from one species that would be Co III Fe II . However, one cannot exclude the presence of residual Co II Fe II . At t0, the spectrum can be considered as due to contributions from Co II Fe III and Co III Fe II species and in this case some Co II Fe II pairs must be present at the interface of the two former ones.

K 3 [Fe III (CN) 6 ], K 4 [Fe II (CN) 6 ]
The quantification fo the contribution of the Co II Fe II species is difficult from the electronic spectra because it is clearly small. Based on EPR studies of the t0 and t60h solutions (see below), we estimated the contribution of the Co II Fe II to be around 10%, this value must be taken with caution because the signature of these species may be different from the two spectroscopies and a lower value can be justified. For the sake of simplicity, we assumed a contribution of 10% for the Co II Fe II species in the electronic spectra that was subtracted from the time dependent spectra. The t60h spectrum corrected from 12% [Fe III (CN) 6 ] 3-and the 10% Co II Fe II species is given in Figure B.2, where one notices the difficulty of determining the exact amount of Co II Fe II to be removed.

The corrected (from 13% of ferricyanide and 10% of Co II Fe II ) time dependent spectra are depicted in Figure 3.6. These spectra are consistent with and evolution from a species that has a major contribution of the Co II Fe III species to one with a major of Co III Fe II one. It is not easy to quantify the contribution of the different species. However, a reasonable assumption is that the t60h correspond to almost the pure Co III Fe II species because the band at 395 nm has almost completely disappeared. For such a case, one can do assumptions on the contribution of the Co III Fe II species to the t0 spectra. Considering different contributions, it appears that the spectra at t0 cannot have a contribution larger than 40% of the t60h one otherwise negative absorbance values appears. This leads to the conclusion that the t0 spectra has at least 60% contribution of the high spin Co II Fe III species. However, larger contribution up to 100% is possible. In order to get a better insight into the evolution of the electronic spectra, we assumed a contribution of 35% of the t60h spectrum in the t0 one. Thus, subtracting this contribution from all the spectra allows to have the evolution of the Co II Fe III fraction from t0 to t60h.

One way to check whether the spectrum at t0 corresponds to a pure Co II Fe III is to compare it to that of a reference where pure Co II Fe III species are present. The spectrum of CoFe 0.66 nanoparticles was recorded (Figure 3.7), it shows mainly two bands at the same energy as the t0 CsCoFe spectrum (520 and 398 nm). But, unfortunately, it is not possible to have information on the relative intensity of the bands because of the diffusion background due to the relatively large size of the CoFe 0.66 nanoparticles (around 200 nm and aggregating in solution). The full characterization of these NPs will be discussed later at the section below.

CsCoFe

thus not push further the reasoning on the mechanism of the electron transfer here.

Energy Dispersive Spectroscopy Analysis

Electron Dispersive Spectroscopy (EDS) allows estimating the relative percentages of the metallic atoms Co, Fe, Cs. The results shown in Table 3.2 are consistent with the presence of some Fe(CN) 6 vacancies that are slightly larger at t0 than at t60h, which is consistent with the presence of a slightly larger amount of ferricyanide in the t0 supernatant solution. The amount of Cs I present within the nanoparticles increased with time and corresponds almost to the amount of ferricyanide (considering their charge) inserted during the time evolution in solution. However, the percentage of inserted ferricyanide is around 4% (compared to Co), which is only a small fraction (20%) of the atoms present in the nanoparticles' surface.

Sample Co Fe Cs

CsCoFe 0min 1 0.86 0.5 CsCoFe 60h 1 0.92 0.7 CsCo II Fe II 1 1 1.6

X-Ray Powder Diffraction

The XRPD patterns of CsCo II Fe II and CsCoFe NPs recovered at t0, t1h and t60h correspond to the expected face-centered cubic (fcc) structure. The calculated cell parameters are given in Table 3.3. The NPs' size determined from the width of the peaks are in the range expected from TEM studies. However, because of the objects are made from a mixture of different species that usually contribute to distort and widen the peaks, one cannot discuss further these values. Nevertheless, it is possible to draw some conclusions by comparing the cell parameters at different times. At t0, the peaks can be considered as a sum of two contributions even though it is difficult to separate them. The cell parameter values reported were determined from the 2θ value corresponding to the maxima of the peaks and correspond thus to the larger cell parameter value (10.18 Å) (Figure 3.8 and Table 3.3). With time, it appears clearly that the peaks become more symmetric and shift to higher 2θ values (smaller cell parameter) (Figure 3.9).

At t1h and t60h, we obtain the same cell parameter leading to the conclusion that most of the transformation occurred within the first hour. Even though, the size of these objects cannot be accurately determined from the XRPD patterns, the larger size at t60h is consistent with the inclusion of ferricyanide and Cs I with time. The possibility of simple growth of the objects can be excluded because the solution did not contain any free Co II species ([Co II (H 2 O)] 2+ ) as The same calculation of the different contributions can be carried out for t60h (cell parameter equal to 10.02 Å). We find the following percentages for the different species assuming again 10% of Co II Fe II : 0.83{Co III Fe II }0.07{Co II Fe III }0.1{Co II Fe II }. This is consistent with the UV-Visible study where the spectra at t60h suggest the absence of the Co II Fe III species.

In summary of this part, the XRPD studies are consistent with the UV-Visible data and confirm that the as-prepared NPs have a major contribution of the high spin Co II Fe III species and that this contribution transforms almost completely with time to give the low spin species

Co III Fe II .

Infra-red Spectroscopy

The infra-red spectroscopy study may be valuable to get information on the contribution of the different species. However, it is not simple to obtain quantitative information because the intensity of the asymmetric vibration of cyanide in the 2000-2200 cm -1 region is very different for the different species. In particular, when cyanide is linked to Fe III , the intensity of its vibration band is almost ten times weaker than when its is linked to Fe II . In addition, depending on the nature of the species the intensity may vary by a factor of 2 or 3. In general, when Fe II containing species are present, they dominate the spectra.

The IR spectra of the samples at t0 and t60h (Figure 3.10) are dominated by the band around 2110 cm -1 that corresponds to the Co II Fe III species. However, one can observe that around 2160 cm -1 , the t0 sample has a band almost absent in the t60h one. This band is assigned to cyanide bridging the Co II Fe III species. In addition, a very weak band can be observed around may have a slightly different structure than the classic one. The spectrum of the CoFe 0.66 nanoparticles that have almost pure Co II Fe III species is shown in Figure 3.10 (right). In addition to the main band at 2160 cm -1 due to the Co II Fe III species, one observes two other bands at 2120 and 2090 cm -1 assigned to Co III Fe II and Co II Fe II respectively. Moreover, a very weak band is present at 2192 cm -1 . Very recently, Bleuzen and coworkers, on the basis of an extensive X-ray Absorption Spectroscopy studies at the K-edge of Co and Fe, suggested the presence of a "distorted" Co II Fe III structure in Rb containing nanoparticles that is responsible of this high energy band. [173] For the Rb containing particles, the vibration energy was observed at 2190 cm -1 as in the alkali free network. But for the Cs containing nanoparticles, this vibration energy is lower (2172 cm -1 ). If this high energy band is due to a distorted network present at the particles' surface as suggested by Bleuzen in the Rb I containing particles, the degree of distortion would be the same for the alkali free network (same vibration energy at 2190 cm -1 ). But the distortion would be much weaker for the present Cs I containing particles. This is a reasonable assumption because Cs I has a larger size than Rb I and fits better in size the tetrahedral site of the fcc structure. Actually, one may suggest that for the Rb I 5.5 nm particles reported by Bleuzen, the outer shell where the distortion is located there is a deficiency of Rb I in the tetrahedral sites responsible of the distortion as in the free alkali objects. The intensity ratio between the nanoparticles and the aqueous Co II solution is 0.75 correlated to the difference in their magnetic susceptibilities at T = 10K and indicating that all the Co II species present in the CsCo II Fe II nanoparticles contribute to the EPR signal. This observation confirms that the volume and surface Co II ions (roughly 15% of the total ions) of the CsCo II Fe II nanoparticles have the same EPR spectra. The spectra also show the absence of free [Co II (H 2 O)] 2+ in the nanoparticles dispersion as for the CoFe 0.66 nanoparticles confirming that the amount of Co II ions within the nanoparticles is the same as that used for their formation. One can, then, conclude that Co II (Fe III ) 6 species are EPR silent while Co II (Fe II ) 6

ones have a well defined EPR signal corresponding to octahedral Co II molecules. It is, however, not straightforward to conclude whether Co II (Fe III ) x (Fe II ) 1-x species will be EPR active or not. One may assume that the presence of one Fe III ion as a first neighbor of Co II (x = 5) would make the signal disappear. However, this may not be the case because the relatively weak exchange coupling interaction in the Co II -NC -Fe III pairs thought to be in the range 2-4 cm -1 , so that a signal may be still present at T = 10K. Nevertheless, we assume that the presence of more than two Fe III ion bridged to Co II will rub away the EPR signal, but we have no evidence for such behavior.

The EPR spectrum of an as-prepared (t0) aqueous dispersion of CsCoFe nanoparticles was an EPR signal (Table 3.5). Upon irradiating the frozen solution (T = 10 K) for 10 minutes, the intensity of the signal decreased by 74% indicating that only 26% of the Co II ions remain active after irradiation (Figure 3.15). The original signal (same intensity and shape) is restored when the solution is heated to 110 K and then cooled down back to 10 K. The same experiment is performed for the t60h solution. The intensity of the signal before irradiation is the same as for the t0 solution (Figure 3.15 and Table 3.5). The irradiation of the solution leads to a decrease of the signal intensity of only 21% instead of the 74% observed for the t0 solution. Here also, the original signal was recovered after relaxation at 150 K.

Magnetic Studies

The magnetic studies were carried out on two samples that were recovered after about 1 hour after mixing the precursors. They, thus, have composition closer to the t60h sample than to the t0 one that, as stated above, is very difficult to obtain in powder form. One sample was recovered with CTA, noted CsCoFe CTA, and the other was recovered with the organic polymer polyvinylpyrrolidone (PVP), noted CsCoFe PVP that allows diluting the objects in the solid state. The comparison between the (photo)magnetic behavior of the samples permits studying the effect of dilution particularly on their dynamic properties before and after irradiation.

For CsCoFe CTA, the data are given per mol because it was possible to compute a formula based on the EDS studies of the CTA and the PVP samples and on the assumption that Cs I is present only in the tetrahedral sites while CTA + act as counter cations; the formula is:

(CTA) 0.2 Cs 0.7 [Co(Fe(CN) 6 ) 0.9 ] • 2H 2 O. The samples were mechanically blocked in order to prevent orientation with the applied magnetic field. The PVP containing samples were in the form of very thin films as transparent as possible in order to maximize the light induced transformation, while the CTA containing sample was in a powder form.

For CsCoFe CTA, the qT value at room temperature (0.15 emu.K/mol) is very weak and indicates that a very small number of paramagnetic ions are present within the nanoparticles consistent with small amounts of Co II Fe II and Co II Fe III species (for Co II , S = 3/2, qT=3 emu.K/mol and for Fe III , S = 1/2, qT = 0.4 emu.K/mol) (Figure 3.17). Upon cooling down, the qT product is almost constant and increases slightly at low temperature to reach a value close to 0.77 emu.K/mol at T = 6 K, then it decreases. The increase of qT and the presence of a maximum is consistent with some exchange coupling among paramagnetic ions within the NPs, which can only be due to Co II Fe III species, but in small quantity. The sample was illuminated (635 nm) at T = 10 K, the temperature decreased to 2 K and the qT measured (Figure 3.17).

Upon heating up, a maximum in the qT curve is observed at T = 14 K, qT then decreases and reaches a value of 2.2 emu.K/mol at T = 50 K where it levels slightly until 80 K (qT = 1.9 emu.K/mol) and then decreases more quickly and reaches the value before irradiation at T = 150 K that corresponds the the relaxation temperature (Figure 3.17). This is the behavior expected for the light induced diamagnetic Co II Fe II to magnetic Co II Fe III species. The behavior of qT between 2 and 20 K is due to the presence of a magnetic order within the sample. Above 20 K, the sample is in the exchange coupled paramagnetic state and above 150, it relaxes to its original state before irradiation. The thermal dependence of qT after relaxation was found identical to the original curve indicating that the process is fully

CsCoFe

percentage of Co III Fe II (76%) species after one hour found in the case of a ferromagnetic exchange coupling is closer to the values estimated from X-ray powder diffraction and electronic spectroscopic data. Moreover, a rate of transformation of 26% seems more reasonable than 65% because the penetration of light inside the powdered sample is not very high. However, the case of an antiferromagnetic coupling cannot be completely discarded. An independent technique giving the light induced rate of transformation is needed to be able to discriminate between the two mechanisms.

In order to get insights into the nature of the magnetic order of the nanoparticles embedded in CTA, we carried out alternating current dynamic susceptibility studies in the dark and after light irradiation. A small (3 Oe) alternation magnetic field with different frequencies is applied to the sample and the thermal dependence of the in-phase (q 0 ) and out-of-phase (q 00 ) susceptibility signals are measured. The presence of a q 00 signal and its thermal dependence give information on the magnetic order or the blocking of the magnetization within the sample.

Before irradiation, no out-of-phase signal was observed at T larger than 2 K consistent with the absence of magnetic order or blocking of the magnetization as one would have expected from the absence of a hysteresis loop and a maximum in the field cooled magnetization curve.

After irradiation at T = 10 K, a temperature dependent out-of-phase signal is observed (Figure 3.20). The maximum of the q 00 = f (T) curve at the lowest available frequency (0.1 Hz) is 11.2 K very close to the maximum of the ZFC curve (12 K). The thermal dependence of q 00

= f (T) was not analyzed in details but the very small variation of the temperature maxima of the curves with frequency (1.2 K) between 300 and 0.1 Hz indicates a magnetic order of the type spin glass rather than a blocking of the magnetization of superparamagnetic objects. This type of order can be due to dipolar interaction between the not strictly identical objects within the sample.

In order to investigate the effect of dilution on the magnetic behavior and to answer the question whether individual objects present a blocking of their magnetization, we prepared a sample containing the CsCoFe nanoparticles diluted in PVP: CsCoFe PVP. The measurements were made on a highly diluted thin film that gives a weak response so that no quantitative analysis can be made. The thermal dependence of the qT product before and after irradiation and after relaxation has the same behavior as for the concentrated CsCoFe CTA sample indicating that dilution in PVP keeps the integrity of the nanoparticles and their overall magnetic behavior (Figure 3.21).

The ZFC and FC magnetization curves recorded with an applied of 50 Oe were recorded after irradiation at different power values (P) of the laser (635 nm) (Figure 3

UV-Vis Spectroscopy

The formation of the RbCoFe NPs was followed kinetically by UV-Vis spectroscopy (Figure 3.29 left). At t0, the spectrum has a broad band in the 450-550 nm region as for the CsCoFe NPs assigned to the charge transfer of the Co II Fe III species, but we have already seen that in the same region a band due to the formation of Co III Fe II species is also present.

RbCoFe

amount of ferricyanide found in the supernatant solution correspond to the amount of vacancies extracted from the EDS studies as for CsCoFe. Moreover, for the free alkali CoFe 0.66 nanoparticles, the amount of ferricyanide found in the supernatant solution is 33% of the total concentration of ferricyanide used in full agreement with the EDS results. The spectra of CoFe 0.66 and RbCoFe at t0 and t60h are depicted in Figure 3.30 (right). We observe that, at t0, the intensity of the band at 525 nm is larger than that of CoFe 0.66 and the intensity of the band at 390 nm is weaker suggesting the presence of some Co III Fe II species at t0. The evolution at t60 suggests that part of the Co II Fe III species transformed to Co III Fe II .

X-Ray Powder Diffraction Studies

XRPD patterns of RbCoFe powder collected at 1h and 60h by centrifugation are presented in and small amount of Co III Fe II , and may be Co II Fe II . For t60h, we observe a main peak at hkl values corresponding to a cell parameter of 10.28 Å as at t1h and for CoFe 0.66 , but also peaks at larger 2θ values (particularly for the (400) reflection) that correspond to a smaller cell parameter that may correspond to the Co III Fe II species. The intensity of these peaks is much weaker than the main ones, which may be due to a small amount of the second species but also to the fact it is not well crystallized in comparison to the first one. At any rate, the observation of two peaks at different 2θ values is consistent with the presence of two different well-defined networks corresponding to the two different species within the nanoparticles suggesting that objects have a core-shell heterostructure. We tentatively tried to determine the value of the cell parameter of the minority species and the size of the corresponding network using the 400 0 reflection (Figure 3 The intriguing result is the decrease in size between t1h and t60h (see Table 3.8) suggesting the occurrence of other phenomena in solution. One possibility could be the formation of smaller particles having larger content in Rb and a smaller size that would be 3.3. RbCoFe irradiation, which correspond to the relaxation temperature. Above T = 172 K, the q M Ti s identical to the one before irradiation showing the thermal crossover. It is interesting to notice that the q M T values of the curve after irradiation (in the heating mode) are the same at 250 and 115 K suggesting that the amount of matter that underwent the thermal crossover and the light induced electron transfer is the same. This means that light did not transform Co III Fe II species that would have been present at room temperature but only those that underwent the thermal electron transfer. This may be due to the fact that at room temperature there was almost no Co III Fe II or very little to detected.

In order to investigate the thermal bistability of the sample, we compare the q M Tc u r v e si n the cooling and heating modes before and after irradiation (Figure 3.36). The curves in the cooling modes before irradiation and after relaxation are superimposible. The same thermal hysteresis is observed before and after irradiation with T down and T up equal to 190 and 200 K respectively.

RbCoFe t60h

The same procedure was applied on the t60h sample (Figure 3.37). The same general behavior is observed as for the sample at t1h, but with an important difference concerning the amount of high spin species present at high temperature and those that underwent a thermally induced electron transfer. The q M T value at room temperature is equal to 1.4 emu.K/mol corresponding to around 47% of the sample being in the high spin state, while it was about 82% at t1h. This is qualitatively consistent with the infra-red data where the amount of high spin species decreases with time. The amount of low spin species is thus much larger at t60h than at t1h. After thermal crossover, about 37% of the sample is the high spin state (q M T= 1.22 emu.K/mol); thus around 10% of the high spin species underwent a thermal electron transfer, so that the sample has now an appreciable amount of low spin species larger than at t1h. From the q M T=f (T) curves in Figure 3.37, one observes that the values of q M Ti nt h e heating mode after irradiation around 120 K are almost the same as that at room temperature indicating that light induced electron transfer only in the amount of matter that underwent the thermal electron transfer and that the low spin species that were present at high temperature were not light active at low temperature. This is in contrast with our observation in the CsCoFe NPs where light induced electron transfer occurred for the species that were high spin at room temperature. This is may be related to the concentration of alkali ions and their nature. Indeed, the Rb I cations included in the fcc structure is smaller than that of the Cs I in the CsCoFe NPs. This interesting aspect was not investigated further here. However, Talham and coworkers suggested the presence of mixed high spin/low spin phase that has a cell parameter around 10.12 Å in addition to the classical high spin phase that present different thermal and light induced electron transfer. [185] But, we do not observe such Considering the EDS, TEM, and electronic spectroscopy results, one can suggest that the observed evolution of the electronic spectra is due to the relatively small size of the particles together with the flexibility of the network that allows a good diffusion of the Rb I ions relatively deep into the nanoparticles allowing the occurrence of the electron transfer process with time in solution; the charge is compensated by the addition of [Fe(CN) 6 ] 3-species present in solution at the particles surface. The surface to volume atomic ratio for 30 nm particles is 10%. It is, thus, not unreasonable to assume a reconstruction of the surface of the particles in water because of the lability of the Co II coordination bonds. Consequently, this leads to the conclusion that at t0 the surface of the particles is much richer in Co II ions than in the inert Co III ones. However, it is not clear whether an electron transfer occurs at the surface of the particles during this reconstruction. One may assume that it is not the case because the surface Co II ions would still have coordinated water molecules that prevent reaching the right electrochemical potential for the electron transfer process to occur. Based on this analysis, the surface of the particles would have mainly high spin Co II Fe III species.

Infra-red Spectroscopy:

The IR spectra of RbCoFe2 t0, t24h and t60h are shown in Figure 3. Table 3.15: Sizes and cell parameter of RbCoFe2 obtained from XRPD patterns at t0, t24h and t60h (Average size and lattice constant were calculated using only the information of the peaks (200, 220, 400) which corresponding to Co III Fe II species since (200 0 ,2 2 0 0 ,4 0 0 0 )b e l o n gt oa n o t h e rs p e c i e s )

Electron Paramagnetic Spectroscopy Studies

Unfortunately, we did not measure the t0 sample but only the t60h one (Figure 3.46). The spectra are almost identical to those of CsCoFe NPs. From the spectra before irradiation, we extracted the contribution of the paramagnetic Co II ions that was found equal to 11% of the total amount of Co in the sample consistent with the infra-red data. Irradiation leads to a

Conclusion

In this work, we have presented two types of coordination network at nanoscales.

The first coordination network SCO Fe II Hoffman clathrates of formula Fe(pz)[Pt(CN) 4 ] possessed a thermochromic effect and a rather wide hysteresis centered at room temperature.

The growth of our nanorods of FepzPt@CsNiCr was successfully controlled by the ⇠3n ms e e d s of CsNiCr PBAs. The obtained nanorods had a well-controlled size of 33.1±7.3 nm in width and 85.6±26.7 nm in length and were stable for a rather long period of time when kept at low temperature. Other than EDS, there is no other technique could be used to detect the presence of CsNiCr in the rods. Magnetic properties measured by a SQUID magnetometer showed an abrupt transition with a hysteresis of 7 K width centering around room temperature. The influence of different content of CsNiCr has been investigated. The results showed that as the amount of PBA seeds increased, the size of the nanorods decreased, both width-wise and length-wise. When there is no colloidal CsNiCr present in the solution, well-controlled nanorods were not obtained. Not only that, the outer CsNiCr indeed had some effect on the SCO phenomenon of the nanorods, as seen when decreasing the amount of CsNiCr precursors, in this case the transition was more complete as well as more abrupt.

However, to obtain very controlled nanorods, at least 1/8 CsNiCr to Fe should be used.

Another type of PBA seeds could be used for controlled-growth of the nanorods: CsNiCo

PBAs of ⇠2 nm. The compound exhibits thermochromic effect as well as an abrupt transition centering at room temperature, similar to its CsNiCr counterpart.

The nanorods were introduced to different environments to observe the change in their magnetic properties: the silica matrix and the organic cations. By embedding the nanorods in a silica matrix using sol-gel method, we obtained a gradual conversion instead of an abrupt one as previously observed on the matrix free NPs. The hysteresis of silica-coated nanorods shifted to a lower temperature and the hysteresis narrowed down to very small width which is in agreement as what has been previously reported by our group. [93] A similar phenomenon has been observed on the nanorods recovered with organic cations with long chain (DDTMA and HTMA), while with shorter chain or aromatic rings (BETA, HETA, CETA, and EV), the hysteresis only slightly shifted to a lower temperature, but, the hysteresis width was preserved.

On the other hand, insertion of I 2 on some of the samples (bare NPs, NPs@DDTMA, NPs@HTMA, NPs@EV) led to a wider hysteresis centering above ambient temperature.

All these studies reveal the importance of the fine-tuning of the environment for various cooperative effects of Hofmann clathrates at the nanoscale. By both engineering the matrix and guest, the SCO hysteresis and temperature may be optimized depending on the characteristics of the targeted materials.

  Institut de Chimie Moléculaire et des Matériaux d'Orsay (ICMMO), Université Paris-sud 11, Université Paris-Saclay. I would like to thank University of Science and Technology of Hanoi, Campus France and Université Paris-sud 11 to support me financially during these three years of PhD. I would like to express my deepest gratitude to the director of the LCI: professor Talal Mallah. I also would like to extend my thanks to the administrative and financial staff of Université Paris-Saclay and Université Paris-sud 11, without whom this thesis would not be possible.

vi 3 . 1 3 . 4 4 . 1 1 . 6 7 ± 1 . 2

 31344116712 Introduction and Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2 Objectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 CsCoFe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1 Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2 Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3 Electron Paramagnetic Resonance Spectroscopy . . . . . . . . . . . . . . . 3.2.4 Magnetic Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 RbCoFe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1 RbCoFe 70 nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2 RbCoFe2 30nm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . viii Contents Discussion and Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Perspectives and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bibliography A NPs of FepzPt@CsNiCr B NPs of Co-Fe PBAs ix List of Figures 1.1 Schematic of material exhibits a change in physical properties in response to external stimuli such as light (hn), temperature (T) electric field (E), and pressure (P) .... 1.2 Magnetic moment vs. Temperature for [Fe(phen) 2 (SCN) 2 ]( •), [Fe(bipy) 2 (SCN) 2 ] (x), and [Fe(phen) 2 (SeCN) 2 (•)][11] ........................... 1.3 Simplified Tanabe-Sugano diagram of Fe II [24] (bottom) and electronic distributions of the two ground states for Fe II in an octahedral complex[23] (top) ......... 1.4 Adiabatic potentials wells for HS ( 5 T 2g ) and LS ( 1 A 1g ) states of Fe II compounds along the metal-ligand direction r(Fe-L). The condition for SCO phenomenon is DE 0 HL =k B T[23] ...................................... 1.5 Schematic representation of pressure affecting on LS and HS potential wells of Fe II [33] Illustraiton of LIESST and reverse-LIESST in Fe II SCO complexes. Straight lines are optical transitions. Straight arrows are the mechanism of these effect. Curly arrows are the non-radiative relaxations. [49] . . . . . . . . . . . . . . . . . . . . . . 1.7 Represenation of the principal types of spin transition curves HS fraction (g HS ) vs temperature (T): a) gradual; b) abrupt; c) with hysteresis; d) two step and e) incomplete[50] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.8 Monodentate ligands for the synthesis of Fe II SCO complexes[54] . . . . . . . . . . . 1.9 Bidentate and Tridentate ligands for Fe II SCO complexes[55] . . . . . . . . . . . . . 1.10 New ligands for synthesizing Fe II SCO complexes (adapted from [57]) . . . . . . . . 1.11 4-R-1,2,4,-triazole and [Fe II (R -trz) 3 ] n (adapted from [62]) . . . . . . . . . . . . . . 1.12 q M T vs T plot for [Fe(NH 2 trz) 3 ]Br 2 • H 2 O in cooling (•) and warming (N)m o d e s [ 62] 1.13 Perspective view of 2D Hofmann clathrate Ni(NH 3 )[Ni(CN) 4 ] • 2G (left), Fe(py) 2 [Ni(CN) 4 ] (middle), and Fe(pz)[Pt(CN) 4 ] • H 2 O (right)[58] . . . . . . . . . . 1.14 Structures of pyrazine and pyridine . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.15 Thermal magnetic susceptibilities of 2D Fe(py) 2 [M(CN) 4 ]( 4) and 3D Fe(pz)[M(CN) 4 ] • 2H 2 O( N) coordination networks with M = Ni II ,P d II ,P t II [20, 58] 1.16 Effect of water content on the SCO behavior of Fe(pz)[Pt(CN) 4 ] compound[2] . . . . 1.17 Structure of azpy and bpac molecules . . . . . . . . . . . . . . . . . . . . . . . . . . 1.18 Structure of Fe(azpy)[Pt(CN) 4 ] • nH 2 O (Pt = pink, Fe = orange, N = blue, C = black, O = red) and its magnetic properties before (•) and after (•) dehydration[82] x List of Figures 1.19 Structure of 3D Fe(bpac)[Pt(CN) 4 ] coordination polymer (left) and magnetic properties of Fe(bpac)[Pt(CN) 4 ](1/2bpac) • nH 2 Ow i t hn=1( •) and n = 0 (•) (right)[83] 1.20 X-Ray crystal structure of Fe 2 (azpy) 4 (NCS) 4 with and without EtOH (left) and its magnetic properties (right)[85] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.21 Structure of Fe(pz)[Pt(CN) 4 • X] (X = Cl, Br, I) (left) and the magnetic behavior of Fe(pz)[Pt(CN) 4 • I] (right)[3] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.22 Control of transition temperature of I 2 -doped compounds of Fe(pz)[Pt(CN) 4 • (I) n ] (n = 0 -10) with blue = 0 to red = 10[4] . . . . . . . . . . . . . . . . . . . . . . . . 1.23 Proposed mechanism of iodien migration induced by spin transition[4] . . . . . . . . 1.24 Magnetic behavior of Fe(pz)[Pt(CN) 4 ] upon doped by CS 2 (purple) and benzene (yellow)[5] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.25 Enhanced bistability of Fe(pz)[Pt(CN) 4 ] as an effect of thiourea guest molecules[6] . 1.26 Synthetic scheme of the sequential assembly of Fe(pz)[M(CN) 4 ] (M = Ni, Pd, Pt) thin film (left) and temperature dependence of the normalized Ramin intensity ratio (I(norm) = I(1025cm -1 )/I(1230cm -1 )) for Fe(pz)[Pt(CN) 4 ]p o w d e r( ⌅) and film samples (•) upon cooling and heating[88] . . . . . . . . . . . . . . . . . . . . . . 1.27 TEM images of Fe(pz)[Pt(CN) 4 ] nanocrystals with the sizes of 230 ⇥ 230 ⇥ 55 (a) and 61 ⇥ 61 ⇥ 21 (b) nm (left) and their magnetic properties (2 and 3, respectively) in comparison with the bulk (1) (right)[89] . . . . . . . . . . . . . . . . . . . . . . . 1.28 HADDF-STEM images and size distribution of Fe(pz)[Pt(CN) 4 ] of cubic NPs of 7.nm and rod-like NPs of 14.7 ± 2.3 nm in length and 12.1 ± 2.1 nm in width (left) and their magnetic properties (1 and 2, respectively) in comparison with the bulk (right)[9] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.29 Thermal evolution of qT for Fe(pz)[Pt(CN) 4 ] nanoplatelets surrounded by calixarene and silica of variable thickness. STEM-EELS profile of the composition obtained across a particle surrounded by 2nm of silica.[63, 93] . . . . . . . . . . . . . . . . . . 1.30 Experimental data obtained on microcrystals of [Fe(phen) 2 (NCS) 2 ]e m b e d d e di n eicosane, nujol, and glycrol[96, 98] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.31 The use of Prussian Blue as blue paint in Starry Night (1889) by Van Gogh . . . . . 1.32 Scheme of unit cell of a Prussian Blue Analogues. Left: alkali free (x = 0). Right: with alkali in the tetrahedral sites (x = 1). . . . . . . . . . . . . . . . . . . . . . . . 1.33 Interconversion between the diamagnetic (Fe II LS -CN-Co III LS ) and paramagnetic (Fe III LS -CN-Co II HS ) electronic configurations due to thermally and photo-induced metal-tometal electron transfer processes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.34 Field-cooled magnetization versus temperature curves at H = 5 G before (•) and after (•) red light irradiation. The magnetization enhanced by illumination (hn) can be removed by thermal treatment (D) above 150 K.[139] . . . . . . . . . . . . . . . . xi List of Figures 1.35 Schematic representation of compounds with different composition: without alkali (C 0 ), with Rubidium (Rb 1.8 ) and with Cesium (Cs 3.9 ) and their magnetization curves before and after irradiation as the function of temperature (four hour irradiation time, hn = 750 ± 50 nm)[148] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.36 Qualitative position of the redox potential of the Co III/II and Fe III/II couples and qualitative energy position of the Co III Fe II and Co II Fe III states for the ACoFe Analogues (A = Na, Rb and Cs)[155] . . . . . . . . . . . . . . . . . . . . . . . . . . 1.37 Potential Energy Diagram of the electronic states of the Co-NC-Fe pair involved in the photo-excitation process[150] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.38 Temperature dependence of the magnetization of Na 0.34 Co 1.33 [Fe(CN) 6 ].4.5H 2 O PBAs before and after irradiation[157] . . . . . . . . . . . . . . . . . . . . . . . . . . 1.39 TEM images of CoFe analogues prepared by (a) co-precipitation and (b) water-in-oil microemulsion method[167] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.40 Left: TEM image of a powdered Co II /Fe III xerogel showing NP crystalline structure of 8-10nm. Right: Field-cooled (FC) and zero-field-cooled (ZFC dc magnetization vs temperature curves at H = 100 Oe (a) and FC (•) and ZFC (•) hysteresis loops at T = 5K (b) and T = 13K (c)[162] . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.41 (a) TEM image of Rb 0.9 Co 4 [Fe(CN) 6 ] 2.9 • 6.6H 2 O NPs of 13.0 ± 3.2nm and (b) temperature dependence of the low field (100G) susceptibilities of ZFC dark (⌥), FC dark (⌅), and FC light (⇤) for different sizes of RbCoFe NPs[172] . . . . . . . . . . . 1.42 (a) HRTEM micrographs and (b) Temperature dependence of the magnetization of the PBA-silica nanocomposite before (•) and after (•) irradiation[164] . . . . . . . . 1.43 Proposed structure for the PBA particles of RbCoFe (100-150nm), CoFe (200-250nm), NanoRbCoFe (5.5nm) and NanoCoFe (5.5nm

1. 1 . 1

 11 Introduction of Spin Cross-Over Phenomenon 1.1.1.1 Discovery of Spin Cross-Over and Ligand Field Theory

Figure 1 . 5 .

 15 Figure 1.5. When pressure is applied, the relative vertical displacement of the potential wells is increased due to a slight decrease in bond length. Increasing pressure will be favoring the LS state of the molecule, therefore shifting the spin transition to a higher temperature since pressure increases the zero point energy difference DE 0 HL by the work term pDV 0 and decrease the activation energy DW 0 HL which favors the LS state.

Figure 1 . 16 .

 116 Figure 1.16. When water molecules are present, the spin transition is incomplete and gradual while dehydrating the samples by heating at 420 0 C for 30 minutes would lead to a complete and abrupt spin transition with a square shaped of 24K width hysteresis loop centering at room temperature.

Figure 1 . 23 :

 123 Figure 1.23: Proposed mechanism of iodien migration induced by spin transition[4]

on the 10 ⇥

 10 10 ⇥ 5n m 3 nanoplatelets of Fe(pz)[M(CN) 4 ] (M = Pt, Ni) that were surrounded by a coating of variable nature without any contamination by the surfactant. The results show that, by coating with an altered calixarene leads to full washing of the surfactant while remaining crystalline while growing different thickness of a silica shell with the same process would lead to different SCO behaviors (Figure 1.29). It has been observed that only the sample coated by 2nm thickness of SiO 2 exhibited SCO behavior with a hysteresis loop while calixarene coated and 4nm SiO 2 coated samples, even though exhibited spin transition, didn't have or had a very small hysteresis.

Figure 1 . 29 :

 129 Figure 1.29: Thermal evolution of qT for Fe(pz)[Pt(CN) 4] nanoplatelets surrounded by calixarene and silica of variable thickness. STEM-EELS profile of the composition obtained across a particle surrounded by 2nm of silica.[63, 93]

  Hofmann clathrates of Fe(pz)[M(CN) 4 ] (M = Ni, Pd, Pt) particles.[99] The authors observed on the samples of Fe(pz)[Ni(CN) 4 ] NPs within chitosan a thermal hysteresis loop. The NPs located in the chitosan pores are covalently linked to the amino groups of the chitosan itself, hence, might involve in the hydrogen bonding network via the remaining solvent molecules.

Figure 1 . 36 :Figure 1 . 37 :

 136137 Figure 1.36: Qualitative position of the redox potential of the Co III/II and Fe III/II couples and qualitative energy position of the Co III Fe II and Co II Fe III states for the ACoFe Analogues (A = Na, Rb and Cs)[155]

  Characterization of FepzPt@CsNiCr Nanorods 2.2.1 Synthesis and Study of the Colloidal Solution 2.2.1.1 Synthesis This synthesis was first studied and introduced by Dr. Mauricio Lopez-Jorda and extensively studied during this thesis. The NPs were made in two steps using solvent of a mixture of (100-x)% water/x% acetone (xA) at 2 o C as the solvent cooled with x= 20 -40. As a first step, the nanoparticles of CsNiCr PBAs were prepared. 50 ml of 80W/20A containing CsCl 4 mM (33.7 mg) and NiCl 2 2m M (23.8 mg) was under vigorous stirring for 30 minutes in an ice bath, maintaining at low temperature (2 o C). After 30 minutes, 50 ml of solution containing K 3 [Cr(CN) 6 ] 2 mM (32.5 mg) was added swiftly into the CsNi mixture resulting in a colorless solution. The solution was then stirred vigorously for one hour. After that, 50ml of the colloidal of CsNiCr solution was used for growing the Hofmann-like clathrates and the rest was recovered with CTABr by adding the solution drop-wise into 50 ml of solution containing CTABr 6 mM using peristaltic pump (1.5 ml/min). The CTA-recovered compound was centrifuged (9000 rpm, 4 o C, 20 minutes) then washed one time with water (⇠200 ml) and one time with acetone (⇠200 ml) before being dried under vacuum. The obtained powder was collected for other studies. 50 ml of CsNiCr was diluted by adding another 50 ml of the mixture of water and acetone. 100 ml of solution containing pyrazine C 4 H 4 N 2 (pz) at a concentration of 100 mM (800.9 mg) was added swiftly to CsNiCr under vigorous stirring. There is no color change at this point. The mixture was stirred for 30 minutes, and 100 ml of 4 mM solution of Fe(BF 4 ) 2 • 6H 2 O (135 mg) was added swiftly to CsNiCr still under vigorous stirring. The color changed to very light yellow once Fe 2+ was added. After 30 minutes more, 100 ml 6 mM solution of K 2 [Pt(CN) 4 ] • 3H 2 O (258.9 mg). The color changed to bright yellow after 15 minutes of stirring. The solution was stirred for three hours then kept in the fridge for 18 hours. TEM time (Figure 2.7 right) suggested that after 3 hours the reaction reached completion. There is no indication of whether Fe(BF 4 ) 2 and K 2 [Pt(CN) 4 ] is still present in the solution since their extinction coefficient are too low to be observed (Figure A.2). This transition at 446nm could be assigned for a charge transfer of Fe -Pt indicating the formation of the Fe -NC -Pt linkages with extinction value e = 2071.

Figure 2 . 7 :

 27 Figure 2.7: UV-Vis spectra of FepzPt@CsNiCr (left) and the evolution at 446 nm (right) during 18 hours

Figure 2 . 8 .

 28 FepzPt@CsNiCr is rather corresponding to the simulated diffractogram proposed by Real et al.[20] (Figure A.3). Most of them present in the simulated pattern double peaks except for(111) reflection. No clear identification of the PBA fcc can be done due to the low ratio of CsNiCr and broadness of the peaks that are perfectly coincidental. The average size of of coherent domain in CsNiCr NPs calculated using Scherrer's equation (D = kl/bcosθ)i s⇠

3 .

 3 These peaks are chosen since they are correspond to single reflection. The mean size of these NPs as estimated from XRPD patterns are close to the size observed on TEM indicating that the NPs are mono-crystalline. For FepzPt@CsNiCr pattern that is dominated by the tetragonal Hofmann clathrate structure the calculated lattice constants are a = b = 7.42 Å and c = 7.21 Å. These values are close to the bulk reported by Real et al.[20] a corresponds to the distance between the two Fe atoms on the [Pt(CN) 4 ] 2-planar, while c give the Fe-Pt-Fe distances. Using this value, the length of Fe-NC-Pt linkage was deduced to be 5.10 Å which is corresponding to the value of a mixture of Fe II HS and Fe II LS which could be due to the fact that there is acetone present in the structure after drying at room temperature.

Figure 2 . 8 :

 28 Figure 2.8: XRPD patterns of FepzPt@CsNiCr NPs and CsNiCr NPs (⇤ indicating the peaks of substrate or CTA)

Figure 2 . 13 :

 213 Figure 2.13: Thermal susceptibility of the scan-rate dependence FepzPt@CsNiCr at 1 K/min (-)and10K/min (•)i nc o o l i n g( -)a n dh e a t i n g( ---)m o d eu n d e rH=5 0 0 0O e

  6 ]2m M (44.0 mg) (pre-cooled to 2 o C) was added swiftly into the Cs-Ni mixture resulting in a colorless solution. The solution was then stirred vigorously for one hour at 2 o C. After stirring, a small amount was taken out for DLS and TEM grid. 50 ml of as-made CsNiCo solution was then diluted in 50 ml of solvent. 100ml of solution containing pyrazine pz 100mM (800.9 mg) was added swiftly to CsNiCo under vigorous stirring. There is no color change at this point. After the mixture was stirred for 30 minutes, 100 ml of solution containing FeBF 4 4 mM (135mg) was added swiftly to CsNiCr still under vigorous stirring. The color change to very light yellow once Fe 2+ was added. The mixture was continued under stirring for another 30 minutes. After than, 100 ml solution containing K 2 [Pt(CN) 4 ] • 2H 2 O 6mM (258.9mg). The color change to bright yellow after 10 minutes of stirring. The solution were stirred for three hours then kept at low temperature for 18 hours.

Figure 2 . 16 :

 216 Figure 2.16: XRPD patterns of NPs of CsNiCo CTA and CsNiCr CTA

Figure 2 . 17 :

 217 Figure 2.17: XRPD patterns of NPs of FepzPt@CsNiCo and FepzPt@CsNiCr

Figure 2 . 18 .

 218 Figure 2.18. At high frequency, CsNiCo showed two distinctive peaks: a strong peak at 2180 cm -1 assigned for the n(C ⌘ N) of Ni-NC-Co pairs, and a medium peak at 2135 cm -1 assigned for the n(C ⌘ N) of the terminal Co -CN on the surface of the NPs. At low frequency, CsNiCo only had two peaks of the n(M -CN): a strong peak at 454 cm -1 and a shoulder peak at 419 cm -1 .

Figure 2 . 18 :

 218 Figure 2.18: IR spectra of CsNiCo CTA and CsNiCr CTA in KBr pellet

Figure 2 . 24 :

 224 Figure 2.24: XRPD patterns of FepzPt, FepzPt@(CsNiCr) 1/4 , FepzPt@(CsNiCr) 1/8 , and FepzPt@(CsNiCr) 1/16

  (a) Blank FepzPt (b) FepzPt@(CsNiCr) 1/16 (c) FepzPt@(CsNiCr) 1/4 (d) Comparison

Figure 2 . 26 :

 226 Figure 2.26: Thermal susceptibility of FepzPt with different CsNiCr content in cooling (-) and heating (---) mode in comparison with FepzPt@(CsNiCr) 1/8 at scan rate = 1 K/min and under H = 5000 Oe

Figure 2 . 30 :

 230 Figure 2.30: Thermal susceptibility of NPs@SiO 2 in cooling (-) and heating (---) mode at scan rate = 1 K/min under H = 5000 Oe

Figure 2 . 32 :

 232 Figure 2.32: IR spectra in KBr pellet of FepzPt@CsNiCr recovered with DDTMABr (left) and HTMABr (right) in comparison with the bare NPs and the cations

I 2

 2 in order to switch the transition at higher temperature. The samples after being in the vacuum oven (120 o C, 30 minutes). They have been kept inside an open vial in a desiccator together with a jar containing a few gram of iodine. Vacuum was created gently inside the desiccator and then let under vacuum over night. The powder of the four compound would turn deep purple within a few hours of exposure. This color change is irreversible. The powder was then characterized (IR, EDS, SQUID) without any further treatment.

Figure 2 . 39 :

 239 Figure 2.39: IR spectra in nujol of the bare NPs, NPs@DDTMA, NPs@HTMA and NPs@EV before and after doped with I 2

Figure 2 .

 2 Figure 2.40 shows the thermal evolution of the q M T of the I 2 -doped NPs on SQUID magnetometer scanning in the window of 400K to 5K in both cooling and heating modes. By doping with I 2 , the compounds not only switch their transition temperature to very high

Figure 2 . 40 :

 240 Figure 2.40: Thermal susceptibility of bare NPs, NPs@DDTMA, NPs@HTMA and NPs@EV before and after doped with I 2 in cooling (-) and heating (---) mode at scan rate = 1 K/min under H = 5000 Oe

Figure 3 . 3 :

 33 Figure 3.3: UV-Vis kinetic studies during 60 hours of CsCo III Fe II NPs in water

Figure 3 . 4 .

 34 Figure 3.4.T h eC s C o II Fe II nanoparticles have the most intense spectrum with one main bandthat has a maximum absorbance at 373 nm (DO = 1.9) and a minimum at 302 nm (DO = 0.61). The CsCoFe spectra at t0 and t60h shows an evolution of their intensities in this region that excludes a large contribution of the Co II Fe II species to the studied nanoparticles. One cannot, however, exclude a contribution of few percents of the objects. The next intense spectrum is that of the [Fe III (CN) 6 ] 3-that has clearly contributions to the t0 and t60h ones.The spectrum of [Fe II (CN 6 )] 4-is the less intense one,[180] (Figure B.1) and, as we will see below after subtracting the contribution of the [Fe III (CN) 6 ] 3-species, can be considered as not

Figure 3 . 4 :

 34 Figure 3.4: Comparison of CsCo III Fe II with CsCo II Fe II ,K 3 [Fe III (CN) 6 ]1 m M ,a n dK 4 [Fe II (CN) 6 ]1 m Mb y UV-Vis spectroscopy

Figure 3 . 5 :

 35 Figure 3.5: UV-Vis spectra of the supernatant of CsCo III Fe II NPs in water (left) and the spectra of CsCoFe in water at time 0 and 60h with supernatant spectra subtracted (right)

Figure 3 . 8 :Figure 3 . 9 :

 3839 Figure 3.8: XRPD pattern of CsCoFe and CsCo II Fe II NPs recovered with CTABr

  study shows that high spin to low spin transformation is fast during the first 30 minutes (Figure3.3). Therefore, the XRPD pattern of the "t0" sample was not really taken at t0, and an evolution of part of the high spin Co II Fe III species already occurred. Actually, the XRPD pattern of a sample recovered after one hour, gives a cell parameter almost identical to that of one recovered after 60 hours, showing that most of the transformation occurred within the first hour.

Figure 3 . 10 :

 310 Figure 3.10: IR spectra of CsCoFe NPs at t0 and t60h (left) and CoFe 0.66 NPs (right) in nujol

Figure 3 .

 3 Figure 3.14: X-band perpendicular mode EPR spectra at 10K, 30K and 50K of CsCo II Fe II in water (left) and aqueous solution of [Co II (H 2 O)] 2+ at 10K (right). Condition: Amplitude = 8MA, Gain = 34G, Microwave = 1mW, Frequency = 9.63GHz

  recorded at T = 10 K (Figure 3.15). A main rather isotropic signal very similar to that of CsCo II Fe II is observed at g = 4.30 very close to g = 4.36 (CsCo II Fe II ) and another very weak anisotropic resonance at g = 2.39. The relative intensity of the signal in comparison to that of [Co II (H 2 O)] 2+ gives a ratio of 0.1 indicating that 10% of the Co II ions present in the NPs show

Figure 3 .

 3 Figure 3.31 and Table 3.8.

Figure 3 . 31 : 66 Figure 3 . 32 :

 33166332 Figure 3.31: XRPD patterns of RbCoFe collected at t1h and t60h in comparison with CoFe 0.66

  core and a Co II Fe III shell with around 25 nm thickness, which leads to 65 nm particles. For such objects, the 12 nm core contributes only to 4% of the total volume of the 65 nm particle, confirming the qualitative electronic spectra that after 60h a very small amount of transformation occurred. Moreover, the fact that the values of the cell parameters of the main peaks at t1h and t60 are the same is consistent with a spatially located evolution compatible with the formation a core-shell heterostructure. The transformation mechanism involves the contribution of Rb I ions present in solution. The flexibility of the network and the presence of vacancies may allow the diffusion of Rb I cations inside the NPs because their size is smaller than the tetrahedral sites. The presence of Rb I favors the electron transfer and thus the formation of the Co

Figure 3 . 40 :

 340 Figure 3.40: UV-Visible spectra of RbCoFe2 during 60 hours (left) and in comparison with RbCoFe at 0min (-) and 60 hour (---) (right)

Figure 3 . 41 :

 341 Figure 3.41: UV-Vis spectra of the supernatant of RbCoFe2 NPs in water (left) and the spectra of RbCoFe2 in water at time 0 and 60h with supernatant spectra subtracted (right)

  42.T h es p e c t r as h o wt h eevolution already observed for RbCoFe2 where at t0 it is dominated by the Co II Fe III species while at t60h by the Fe II based species. The spectra at t24h shows an intermediate situation.

Figure 3 . 42 :

 342 Figure 3.42: IR spectra of RbCoFe2 at time 0, 24h and 60h in nujol

Figure 3 . 45 :

 345 Figure 3.45: XRPD patterns of (400) reflection of RbCoFe2 powder collected at t24h and t60h in the range of 32-37 2θ (deg)

Figure 3 . 53 :

 353 Figure 3.53: UV-Vis spectra of Rb6CoFe2 during 60h (left) and the Absorbance/Time at 520 nm of Rb6CoFe2 in comparison with RbCoFe2 (right)

Figure 3 . 54 :Figure 3 .

 3543 Figure 3.54: UV-Vis spectra of Rb6CoFe2 in water at time 0 and 60h with supernatant spectra subtracted (right)

Figure B. 3 :

 3 Figure B.3: FT-IR spectra of CsCo III Fe II collected at 0 min and 60h in nujol

Figure B. 4 :

 4 Figure B.4: FT-IR spectra of CsCo II Fe II in KBr pellet

Figure B. 13 :

 13 Figure B.13: UV-Vis spectra of Rb6CoFe2 in comparison with Rb6CoFe2

  .36 Cation of group 3 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.37 IR spectra in KBr pellet of FepzPt@CsNiCr recovered with BPTA in comparison with the bare NPs and BPTABr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . spectra in nujol of the bare NPs, NPs@DDTMA, NPs@HTMA and NPs@EV before and after doped with I 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.40 Thermal susceptibility of bare NPs, NPs@DDTMA, NPs@HTMA and NPs@EV before and after doped with I 2 in cooling (-) and heating (---) mode at scan rate = 1 K/min under H = 5000 Oe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Field cooled magnetization before and after irradiation and Zero-Field cooled magnetization after irradiation for CsCoFe CTA; B = 50 Oe . . . . . . . . . . . . . . . . 3.19 Magnetic hysteresis loop (left) and magnetization curves (right) before and after irradiation and after relaxation for CsCoFe CTA (the continuous lines are a guide for the eye) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.20 Thermal dependence of the in-phase (left) and out-of-phase magnetic susceptibility for CsCoFe CTA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.21 Thermal dependence of the qT product before and after irradiation and after relaxation for CsCoFe PVP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.22 ZFC (•) and FC (N) magnetization after irradiation at different power and after relaxation for CsCoFe PVP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Co[Fe(CN) 6 ] 0.9 • 2H 2O ) before and after irradiation and after relaxation . . . 3.49 Comparison of the q M T=f (T) curves after irradiation in heating mode for t0 and t60h of RbCoFe2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.50 Schematic representation of the evolution of the Co II Fe III pairs into Co III Fe II pairs in the studied samples (Co II Fe II was not taken into account) . . . . . . . . . . . . . Vis spectra in molar extinction of K 4 [Fe II (CN) 6 ] and K 3 [Fe III (CN) 6 ] . . . . . . B.2 UV-Vis spectra of the supernatant-subtracted CsCoFe at 60h with 0.1mM of CsCo II Fe II subtracted . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.3 FT-IR spectra of CsCo III Fe II collected at 0 min and 60h in nujol . . . . . . . . . . . B.4 FT-IR spectra of CsCo II Fe II in KBr pellet . . . . . . . . . . . . . . . . . . . . . . . . Co II Fe III 0.66 in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.6 X-band perpendicular mode EPR spectra at 10K of CsCoFe at time 0 in comparison with CsCo II Fe II in water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O] 6 ] 2+ in water. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cr and Pt to 1 Fe (error: 20%) . . . . . . . . . . . . . . . . . . . . . 2.17 Summarized of the value from the thermal susceptibilities of FepzPt@CsNiCr recovered with DDTMA and HTMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Size and Bravais lattice values obtained from XRPD patterns of CsCoFe CTA NPs and CsCo II Fe II . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

	List of Figures List of Tables		List of Figures
	3.41 UV-Vis spectra of the supernatant of RbCoFe2 NPs in water (left) and the spectra A.11 z potential measurement of FepzPt@CsNiCo NPs after 18h . . . . . . . . . . . . . . 2.16 EDS analysis of FepzPt@CsNiCr and FepzPt@CsNiCr in DDTMA and HTMA with
	2.38 Thermal magnetic susceptibilities of NPs recovered with cation of group 3 in cooling of RbCoFe2 in water at time 0 and 60h with supernatant spectra subtracted (right) 3.42 IR spectra of RbCoFe2 at time 0, 24h and 60h in nujol . . . . . . . . . . . . . . . . . 3.43 Deconvolution of IR spectra of RbCoFe2 at t0 and t60h . . . . . . . . . . . . . . . . 3.44 XRPD patterns of RbCoFe2 powder collected at time 0 and 60h . . . . . . . . . . . 3.45 XRPD patterns of (400) reflection of RbCoFe2 powder collected at t24h and t60h in A.12 FT-IR spectroscopy of FepzPt@CsNiCo and CsNiCo CTA in KBr pellet . . . . . . . A.13 XRPD patterns of FepzPt@CsNiCr, FepzPt@CsNiCr@DDTMA, and FepzPt@CsNiCr@HTMA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.14 TEM image and size distribution of FepzPt@CsNiCr 30A . . . . . . . . . . . . . . . A.15 TEM image and size distribution of FepzPt@CsNiCr 40A . . . . . . . . . . . . . . . ratio of Cs, Ni, 2.18 EDS results of FepzPt@CsNiCr and FepzPt@CsNiCr in BPTA, BETA, CETA and List of Tables HETA with ratio of Cs, Ni, Cr and Pt to 1 Fe (error: 20%) . . . . . . . . . . . . . .
	(-) and heating (---) mode at scan rate = 1 K/min under H = 5000 Oe . . . . . . the range of 32-37 2θ (deg) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.46 X-band ? mode EPR spectra of 200mL RbCoFe 1h (-) and 60h (---) in water at A.16 TEM image of FepzPt@CsNiCr 50A, 60A, 70A . . . . . . . . . . . . . . . . . . . . . 2.19 Summarized of the value from the thermal susceptibilities of FepzPt@CsNiCr recov-A.17 Illustration of possible interfacial interaction of FepzPt@CsNiCr . . . . . . . . . . . ered with BPTA, BETA, CETA and HETA . . . . . . . . . . . . . . . . . . . . . . . 2.39 IR 3.15 X-band perpendicular mode EPR spectra at 10K before and after irradiation by red light (l = 635 nm) of CsCo III Fe II NPs in water. Condition: Amplitude = 8MA, 3.30 UV-Vis spectra of the supernatant of RbCoFe NPs in water (left) and the spectra of RbCoFe in water at ah and 60h with supernatant spectra subtracted (right) . . . . . 3.31 XRPD patterns of RbCoFe collected at t1h and t60h in comparison with CoFe 0.66 . 3.32 PXRD patterns of (400) reflection at 33-38 2θ (deg) of RbCoFe at t60h . . . . . . . 3.33 FT-IR of CoFe 0.66 , RbCoFe at t1h and t60h in nujol . . . . . . . . . . . . . . . . . . 3.34 Deconvoluted infra-red spectra in the 2000-2200 region at t1h and t60h for RbCoFe 3.35 X-band ? mode EPR spectra of 200mL RbCoFe 1h (-) and 60h (---) in water at 10K before and after (•) irradiation by red light (l = 635nm). Condition: Amplitude = 8MA, Gain = 34G, Microwave = 1mW, Frequency = 9.63GHz . . . . . . . . . . . 10K before and after (•) irradiation by red light (l = 635nm). Condition: Amplitude = 8MA, Gain = 34G, Microwave = 1mW, Frequency = 9.63GHz . . . . . . . . . . . 3.47 Thermal magnetic susceptibility of RbCoFe2 collected at t0 (Rb 0.2 Co[Fe(CN) 6 ] 0.85 • 2H 2 O) before and after irradiation and after relaxation . . . . . . . . . . . . . . . . 3.48 Thermal magnetic susceptibility of RbCoFe2 collected after 60 hours 1.1 Results of Calorimetric and DSC Measurements on Solid SCO Complexes of Fe II .. 1.2 DH and DS estimated by DSC for Fe(pz)[M(CN) 4 ] network[20] . . . . . . . . . . . . 2.1 Summary of size from TEM images of the nanorods collected at different time . . . 2.20 EDS results of FepzPt@CsNiCr and FepzPt@CsNiCr in BPTA, BETA, CETA and HETA with ratio of Cs, Ni, Cr and Pt to 1 Fe (error: 20%) . . . . . . . . . . . . . . 2.21 EDS analysis of I 2 -doped samples (*Pt is not reliable in these measurements)(error: 20%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.1 UV-B.5 X-band perpendicular mode EPR spectra at 10K, 30K, 50K, and 100K of 2.2 EDS analysis of CsNiCr NPs and FepzPt@CsNiCr NPs (error: 20%) . . . . . . . . . 2.3 Estimated estimated correlation length of FepzPt@CsNiCr and CsNiCr CTA for dif-ferent reflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 IR frequencies of FepzPt@CsNiCr NPs and CsNiCr NPs in comparison with the bulk[78](s: strong; m: medium; sh: shoulder; w: weak; n: stretching; d: in-plane 2.22 Summary of the relevant values from the thermal susceptibilities of the bare NPs, NPs@DDTMA, NPs@HTMA, NPs@EV after doped with I 2 . . . . . . . . . . . . . . 3.1 Summary of sizes of CsCoFe NPs obtained from TEM and DLS . . . . . . . . . . . . 3.2 EDS analysis of CsCoFe NPs (error: 20%) . . . . . . . . . . . . . . . . . . . . . . . (Rb 0.6 3.51 Comparison of the magnetic behavior after irradiation at T = 10 K for t1h samples of CsCoFe and RbCoFe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.7 X-band perpendicular mode EPR spectra at 10K of CsCoFe at time 0 in comparison with [Co[H 2 B.8 DLS z potential measurement of RbCoFe NPs at 1h and 60h . . . . . . . . . . . . . bending; g: out-of-plane bending) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 Summary of relevant values obtained from thermal susceptibility of FepzPt@CsNiCr at two different scan-rates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3 3.4 Contribution of each species obtained from the deconvolution of IR spectra of CsCoFe 3.52 Comparison of the magnetic behavior after irradiation at T = 10 K for of RbCoFe B.9 Different TEM images of RbCoFe of different samples . . . . . . . . . . . . . . . . . 2.6 Summary of mean sizes of FepzPt@CsNiCo in comparison with FepzPt@CsNiCr . . NPs in the 200-2200 cm -1 at t0 and t60h . . . . . . . . . . . . . . . . . . . . . . . . and RbCoFe2 at t1h and t0 respectively . . . . . . . . . . . . . . . . . . . . . . . . . 3.53 UV-Vis spectra of Rb6CoFe2 during 60h (left) and the Absorbance/Time at 520 nm of Rb6CoFe2 in comparison with RbCoFe2 (right) . . . . . . . . . . . . . . . . . . . 3.54 UV-Vis spectra of Rb6CoFe2 in water at time 0 and 60h with supernatant spectra subtracted (right) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.55 Proposed model of RbCoFe2 NPs at t0, t24h and t60h . . . . . . . . . . . . . . . . . A.1 Color evolution of FepzPt@CsNiCr . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.2 UV-Vis spectra of Fe(BF 4 ) 2 and K 2 [Pt(CN) 4 ] . . . . . . . . . . . . . . . . . . . . . . A.3 Simulated diffractogram of FepzPt bulk by Real et al. . . . . . . . . . . . . . . . . . A.4 IR spectrum of CsNiCr CTA in pellet . . . . . . . . . . . . . . . . . . . . . . . . . . B.10 Thermal magnetic susceptibility of CoFe 0.66 (K 0.03 Co[Fe(CN) 6 ] 0.66 • 6H 2 O) before 2.7 EDS analysis of CsNiCo NPs and FepzPt@CsNiCo NPs (*The Pt content is overes-3.5 Quantification from EPR spectra of CsCoFe in solution at 10K before and after (-) and after (•) irradiation by red light in cooling (-) and heating (---) mode . . timated)(error: 20%) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B.11 DLS z potential measurement of RbCoFe2 NPs at time 0 and 60h . . . . . . . . . . 2.8 Size and Bravais lattice from XRPD patterns of CsNiCo NPs in comparison with 3.6 Summary of size of RbCoFe and CoFe 0.66 obtained from TEM and DLS . . . . . . . B.12 FT-IR spectra of KCoFe and RbCoFe collected at 1h and 60h in nujol . . . . . . . . B.13 UV-Vis spectra of Rb6CoFe2 in comparison with Rb6CoFe2 . . . . . . . . . . . . . . B.14 DLS measure and TEM image of Rb6CoFe2 NPs . . . . . . . . . . . . . . . . . . . . CsNiCr NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.7 EDS analysis of CoFe 0.66 and RbCoFe NPs (error: 20%) . . . . . . . . . . . . . . . 2.9 Size and Bravais lattice from XRPD patterns of FepzPt@CsNiCo NPs in comparison 3.8 Size and cell parameters values obtained from XRPD patterns for RbCoFe and with FepzPt@CsNiCr NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . CoFe 0.66 NPs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.10 Summary of relevant values obtained from thermal susceptibility of FepzPt@CsNiCo 3.9 Contribution of each species obtained from the deconvolution of infra-red spectra of NPs in comparison with FepzPt@CsNiCr NPs . . . . . . . . . . . . . . . . . . . . . . RbCoFe nanoparticles in the 2000-2200 cm -1 region at t1h and 60h . . . . . . . . . . 2.11 Summary of mean size of (CsNiCr) 1/16 , (CsNiCr) 1/8 , amd (CsNiCr) 1/4 obtained from TEM images . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.10 Quantification from EPR spectra of RbCoFe as as-made NPs in water at 10K before 3.36 Thermal magnetic susceptibility of RbCoFe collected at t1h (Rb 0.16 Co[Fe(CN) 6 ] 0.82 • 2H 2 O) before and after irradiation, and after relaxation . . . . . . . . . . . . . . . . 3.37 Thermal magnetic susceptibility of RbCoFe collected at t60h 2.12 EDS analysis of FepzPt with different CsNiCr content (error: 20%) . . . . . . . . . and after irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.5 DLS measurement of FepzPt@(CsNiCr) 1/16 NPs during 18 hours . . . . . . . . . . . A.6 z potential measurement of FepzPt@(CsNiCr) 1/16 NPs during 18 hours . . . . . . . 2.13 Correlation length and bond length obtained from XRPD patterns of FepzPt, 3.11 Relevant values obtained from magnetic measurements of RbCoFe NPs at t1h and
	2.6 TEM images and size distribution of FepzPt@CsNiCr at 1h, 3h, 18h and 48h . . . . Gain = 34G, Microwave = 1mW, Frequency = 9.63GHz . . . . . . . . . . . . . . . . (Rb 0.25 Co[Fe(CN) 6 ] 0.85 • 2H 2 O) before and after irradiation and after relaxation . . A.7 DLS measurement of FepzPt@(CsNiCr) 1/4 NPs during 18 hours . . . . . . . . . . . FepzPt@(CsNiCr) 1/16 , FepzPt@(CsNiCr) 1/8 , and FepzPt@(CsNiCr) 1/4 (*Sizes are
	3.16 Evolution of the oxidation state upon irradiation highlighting the surroundings of 3.38 DLS size measurement of RbCoFe2 NPs at time 0 and 60h . . . . . . . . . . . . . . A.8 z potential measurement of FepzPt@(CsNiCr) 1/4 NPs during 18 hours . . . . . . . . indicative as Scherrer's law does not apply above 100nm) . . . . . . . . . . . . . . .
	3.39 TEM image and size distribution of RbCoFe2 at t0, t18h, and t60h . . . . . . . . . . A.9 FT-IR spectroscopy of FepzPt, FepzPt@(CsNiCr) 1/4 , (CsNiCr) 1/8 , and (CsNiCr) 1/16 2.14 Summary of relevant values obtained from thermal susceptibility of FepzPt with
	3.40 UV-Visible spectra of RbCoFe2 during 60 hours (left) and in comparison with and CsNiCr CTA in KBr pellet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . different CsNiCr content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
	RbCoFe at 0min (-) and 60 hour (---) (right) . . . . . . . . . . . . . . . . . . . . . A.10 DLS measurement of FepzPt@CsNiCo NPs during 18 hours . . . . . . . . . . . . . . 2.15 EDS analysis of FepzPt@CsNiCr@SiO 2 (error: 20%) . . . . . . . . . . . . . . . . . .
	xv xvii	xviii	xvi

). RbCoFe and NanoRbCoFe with Co III Fe II core and Co II Fe III shell. CoFe and NanoCoFe with Co II Fe II core and Co II Fe III shell[173] . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 DLS measurement of CsNiCr NPs in 3 cycles . . . . . . . . . . . . . . . . . . . . . . 2.2 z potential measurement of CsNiCr NPs in 3 cycles . . . . . . . . . . . . . . . . . . . 2.3 DLS measurement of FepzPt@CsNiCr at 1 hour (black), 3 hours (red), and 18 hours (blue) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 z potential measurement of FepzPt@CsNiCr at 1 hour (black), 18 hours (red), and 48 hours (blue) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.5 TEM image and size distribution of CsNiCr NPs . . . . . . . . . . . . . . . . . . . . 2.7 UV-Vis spectra of FepzPt@CsNiCr (left) and the evolution at 446 nm (right) during 18 hours . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.8 XRPD patterns of FepzPt@CsNiCr NPs and CsNiCr NPs (⇤ indicating the peaks of substrate or CTA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xii (-) and heating (---) mode at scan rate = 1 K/min under H = 5000 Oe . . . . . . 23.1 DLS size measurement of CsCoFe NPs at t0 (top) and t60h (bottom) . . . . . . . . 3.2 TEM images and size distribution of CsCo III Fe II NPs . . . . . . . . . . . . . . . . . 3.3 UV-Vis kinetic studies during 60 hours of CsCo III Fe II NPs in water . . . . . . . . . 3.4 Comparison of CsCo III Fe II with CsCo II Fe II ,K 3 [Fe III (CN) 6 ] 1mM, and K 4 [Fe II (CN) 6 ] 1mM by UV-Vis spectroscopy . . . . . . . . . . . . . . . . . . . . . . 3.5 UV-Vis spectra of the supernatant of CsCo III Fe II NPs in water (left) and the spectra of CsCoFe in water at time 0 and 60h with supernatant spectra subtracted (right) .

3.6 Time dependent spectra corrected from the contribution of ferricyanide and Co II Fe II species (left) and spectra to which 35% of the spectra at t60h were subtracted (left) 3.7 Solution electronic spectrum of CoFe 0.66 nanoparticles . . . . . . . . . . . . . . . . . 3.8 XRPD pattern of CsCoFe and CsCo II Fe II NPs recovered with CTABr . . . . . . . . 3.9 PXRD patterns of (400) reflection at 33-38 2θ (deg) of CsCoFe at t0, t1h, t60h . . . 3.10 IR spectra of CsCoFe NPs at t0 and t60h (left) and CoFe 0.66 NPs (right) in nujol . 3.11 Deconvoluted IR spectra of CsCoFe NPs at t0 (left) and t60h (right) in nujol . . . . 3.12 Deconvolution of IR spectra of CsCoFe in KBr pellets at t60h . . . . . . . . . . . . . 3.13 Low frequency IR spectra of CsCo III Fe II collected at t0 and 60h in nujol . . . . . . . 3.14 X-band perpendicular mode EPR spectra at 10K, 30K and 50K of CsCo II Fe II in water (left) and aqueous solution of [Co II (H 2 O)] 2+ at 10K (right). Condition: Amplitude = 8MA, Gain = 34G, Microwave = 1mW, Frequency = 9.63GHz . . . . . . . . . . . interfacial Co II ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.17 Thermal dependence of the qT product before and after irradiation after relaxation (left) and zoom of the qT curve before irradiation (right), for CsCoFe CTA . . . . . xiv List of Figures 3.18

3.23 Magnetization (left) and hysteresis loop (right) versus field before and after irradiation and after relaxation at T = 200 K . . . . . . . . . . . . . . . . . . . . . . . . . . 3.24 Thermal dependence of the out-of-phase magnetic susceptibility for CsCoFe PVP after irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.25 DLS size measurement of RbCoFe NPs at 1h and 60h . . . . . . . . . . . . . . . . . 3.26 DLS size measurement of Co II Fe III 0.66 NPs . . . . . . . . . . . . . . . . . . . . . . . 3.27 TEM image and size distribution of CoFe 0.66 NPs . . . . . . . . . . . . . . . . . . . 3.28 TEM images of (A) RbCoFe at t1h and (B) RbCoFe at t60h and histogram of size distribution of (A) and (B) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.29 UV-Vis spectra of RbCoFe during 60 hours (left) and comparison with CoFe 0.66 , K 4 [Fe(CN) 6 ], and K 3 [Fe(CN) 6 ] (right) . . . . . . . . . . . . . . . . . . . . . . . . . . t60h . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.12 Sizes of RbCoFe2 NPs obtained from TEM and DLS . . . . . . . . . . . . . . . . . . 3.13 EDS analysis of RbCoFe2 at t0 and t60h (error: 20%) . . . . . . . . . . . . . . . . . 3.14 Contribution of each species obtained from the deconvolution of infra-red spectra of RbCoFe2 nanoparticles in the 2000-2200 cm -1 region at t0 and t60h . . . . . . . . . xix Chapter 1

Table 1 . 1 :

 11 2 (SCN) 2 ] 176.29 8.60 ± 0.14 48.78 ± 0.71 cal [30] [Fe(phen) 2 (SeCN) 2 ] 231.26 11.60 ± 0.44 51.22 ± 2.33 cal Results of Calorimetric and DSC Measurements on Solid SCO Complexes of Fe (SCN) 2 ] and [Fe(phen) 2 (SeCN) 2 ][30] pointed out that, in Equation 1.3 only one third of total entropy change DS on passing from LS to HS state, stems from DS el and about two thirds originate from the changes in phonon modes with about 50% of the total DS vib accounted for by changes in metal-ligand stretching frequencies.

	[30]

II

The values of entropy change vary between 48 -86 J K -1 mol -1 , this is considered more than enough for a change in spin state alone.

[START_REF] Konig | [END_REF]

A study on detailed analysis of the vibrational contributions DS vib has been carried out by Sorai and Seki on the systems of 1.1. Spin Cross-Over Phenomenon [Fe(phen) 2

Table 1 .

 1 II (CN) 4 ]} n where M II is Ni, Pd or Pt and L is an N-donor monodentate ligand with x = 2 or bis-monodentate rod-like ligand with x = 1. The structure is consistently made up of parallel stacking of 2-D {Fe II [M II (CN) 4 ]} n square planar grids in which Fe II atoms are occupied by four [M II (CN) 4 ] 2-planar units via the N-bond and the axial positions are occupied by the two L ligands, defining 2-D if L is monodentate or 3-D if L is bis-monodentate.

	3-dimensional	(3-D)	cyanide-bridged	bimetallic	compounds	formulated	as
	{Fe

II (L) x [M These compounds have the transition temperature above 200K with hysteresis ranging from 10K to 20K and thermochromic effect. The enthalpy and entropy variations, DH and DS calculated by Differential Scanning Calorimetry (DSC) for the compounds of Fe(pz)[M(CN) 4 ] network are showed in Table 1.2. Fe(pz)[Ni(CN) 4 ] Fe(pz)[Pd(CN) 4 ] Fe(pz)[Pt(CN) 4 ] 2: DHa n dDS estimated by DSC for Fe(pz)[M(CN) 4

  Environmental Effects on Spin Cross-Over PhenomenonMatrix and interface can play a major role in SCO behaviors in SCO compounds, especially in those of nano-scales. The NPs of Fe(pz)[Pt(CN) 4 ] of Hofmann clathrate family have been reported to exhibit intriguing phenomenon regarding this effect: while the NPs of 7nm

	The first strategies have been reported in Bousseksou's group with the nanopatterning directly
	on substrates. Sequential assembly or layer-by-layer assembly techniques have been used for
	the fabrication for thin films of the 3D network of Fe(pz)[M(CN) 4 ] network by Cobo et al.[88]
	The 3D network was prepared by dipping alternatively the gold substrate into a solution of the
	precursors (Fe(BF

1.2.4 Nanomaterials of 3D Hofmann-like Clathrate Fe(pz)[M(CN) 4 ]:

Examples of Strategies to Reduce the Grain Size

As mentioned above, there are various synthetic methods that have been reported to be successfully used to downscale these materials and still preserve their magnetic properties. 4 • 6H 2 O) and (TBA) 2 M(CN) 4 with M = Ni, Pd, Pt and TBA = tetrabutylammonium) and pyrazine in ethanol multiple times, with rinsing with ethanol between each time. The gold substrate was first functionalized with mercaptopyridine group which was used as anchoring group. Then on this, a monolayer of 4,4'-azopyridine was built. 1.2. SCO Hofmann-like Clathrate Coordination Network 1.2.5

  1.3. Photo-Induced Magnetic CoFe PBAs are replaced by stronger field -NC ligands resulting in the increase of the crystal field parameter of Co. The redox potential of the Co III /Co II couple becomes inferior to that of Fe III (CN) 6 /Fe II (CN) 6

	and	electron	transfer	occurs	spontaneously
	Co II HS -NC -Fe III LS ! Co III LS -NC -Fe II LS explaining the formation of the diamagnetic pairs.[144,
	150]

Table 2 . 2 :

 22 EDS analysis of CsNiCr NPs and FepzPt@CsNiCr NPs (error: 20%)

Table 2 . 3 :

 23 Estimated estimated correlation length of FepzPt@CsNiCr and CsNiCr CTA for different reflection

Table 2 .

 2

8: Size and Bravais lattice from XRPD patterns of CsNiCo NPs in comparison with CsNiCr NPs

Table 2 .

 2

			Size (nm)			Fe-Fe Fe-Pt
	(001) (110) (111) (200) (002)	( Å)	( Å)
	FepzPt@CsNiCo 43.7	47.4	27.3	37.4	32.7	7.42	5.13
	FepzPt@CsNiCr 60.5	42.8	33.0	49.6	34.5	7.43	5.10

9: Size and Bravais lattice from XRPD patterns of FepzPt@CsNiCo NPs in comparison with FepzPt@CsNiCr NPs

Table 2 .

 2 13: Correlation length and bond length obtained from XRPD patterns of FepzPt, FepzPt@(CsNiCr) 1/16 , FepzPt@(CsNiCr) 1/8 , and FepzPt@(CsNiCr) 1/4 (*Sizes are indicative as Scherrer's law does not apply above 100nm)

	Compound			Peaks			Fe-Fe Fe-Pt
		(001)	(110) (111) (200) (002)	( Å)	( Å)
	FepzPt@(CsNiCr) 1/4	58.4	39.4	32.1	40.2	34.0	7.44	5.10
	FepzPt@(CsNiCr) 1/8	60.5	42.8	33.0	49.6	34.5	7.43	5.10
	FepzPt@(CsNiCr) 1/16	60.5	67.7	37.7	61.5	38.6	7.43	5.11
	FepzPt	122.7* 104.9* 46.9	56.5	94.4	7.47	5.16

Table 2 .

 2

16

: EDS analysis of FepzPt@CsNiCr and FepzPt@CsNiCr in DDTMA and HTMA with ratio of Cs, Ni, Cr and Pt to 1 Fe (error: 20%)

Table 2 .

 2 21: EDS analysis of I 2 -doped samples (*Pt is not reliable in these measurements)(error: 20%)

		Cs	Ni	Cr Fe Pt*	I
	NPs@DDTMA•I 2 0.02 0.13 0.08 1	1.6 0.95
	NPs@HTMA•I 2	0.02 0.13 0.09 1	1.6	1.0
	NPs@EV•I 2	0.02 0.15 0.09 1	1.6 0.85
	NPs•I 2	0.02 0.12 0.08 1	1.6	0.9

Table 2 .

 2

	Bare NPs•I 2	370	383	13	376.5
	NPs@DDTMA•I 2	376.5	390.5	14	383.5
	NPs@HTMA•I 2	368.5	377.5	9	373
	NPs@EV•I 2	384.5	398	13.5	391.2

22: Summary of the relevant values from the thermal susceptibilities of the bare NPs, NPs@DDTMA, NPs@HTMA, NPs@EV after doped with I 2

  II Fe II pairs to those of Co III Fe II and Co II Fe III in different studies. The compound was made by two different methods. The first method is co-precipitation: 50 ml of distilled water containing CsCl (10mM) and CoCl 2 • 6H 2 O (5mM) was added drop-wise to a 50 ml aqueous solution of K 4 [Fe(CN) 6 ] • 2H 2 O (5mM) using a peristaltic pump (1.5 ml/minutes) while stirring vigorously. Upon adding, a suspension with a

Table 3 .

 3

2: EDS analysis of CsCoFe NPs (error: 20%)

  III Fe II }0.45{Co II Fe III }0.1{Co II Fe II }. The calculations were made assuming the following cell parameters for the different species: 9.95, 10.32, 10.4 Å for Co III Fe II ,C o II Fe II , and Co II Fe III , respectively. These figures are not far from those found from the UV-Visible spectroscopy studies in solution. The ratio of the low spin to high spin species is found equal to 1, while the UV-Vis spectroscopy study suggested that it should lower than 0.7. Thus, the percentage of the high spin species at t0 is smaller than what would be expected from the solution study. This is not surprising because it is extremely difficult to capture to high spin species in the solid state because it takes some time (few minutes) to obtain a precipitate as explained in the section devoted to the synthesis. And, the solution

	3.2. CsCoFe
	Compound	Peak (nm)		d	a
		(002) (022) (222) (004)	(nm)	( Å)
	CsCoFe CTA t0	11.66 8.41	7.89	6.09	8.5±2.0	10.18
	CsCoFe CTA t1h	10.24 9.14	8.36	8.72	9.12±0.5 10.03
	CsCoFe CTA t60h 14.48 15.18 13.88 15.57 14.77±0.6 10.02
	CsCo II Fe II	13.24 10.71 21.68 24.39 17.5±5.7 10.32

Table 3 . 3 :

 33 Size and Bravais lattice values obtained from XRPD patterns of CsCoFe CTA NPs and CsCo II Fe II

Table 3 . 5 :

 35 Quantification from EPR spectra of CsCoFe in solution at 10K before and after irradiation As we stated above, if we assume that only the Co II ions surrounded by diamagnetic Fe II (CN) 6 are EPR active, the decrease of their amount upon irradiation means that they are

	Compound CsCoFe dark CsCoFe irradiation % Decrease
		(mM)	(mM)	(%)
	CsCoFe t0	0.1	0.026	74
	CsCoFe t60h	0.098	0.077	21

Table 3 . 6 :

 36 Summary of size of RbCoFe and CoFe 0.66 obtained from TEM and DLSEnergy Dispersive SpectroscopyEnergy Dispersive Spectroscopy was used to estimate the relative concentration of the metallic atoms within the samples prepared without the presence of Rb and the t1h and t60h for the Rb containing NPs. The sample prepared in the absence of Rb I has the composition expected for the neutral lacunar PBA with 1 Co for 0.66 Fe noted CoFe 0.66 in the following (Table3.7).The Rb containing samples are less lacunar and have the same Fe/Co ratio (within experimental errors). The amount of Rb seems to be slightly larger for the sample recovered at t60h. Moreover, K is found in the t60h sample that seems to be absent in the t1h one. For t1h, if we assume that the sample is made of pure Co II Fe III species, the charge balance gives -0.3 (+2-3x0.82+0.16). For t60h, one obtains -0.3 if K is excluded and -0.15 considering K.These values are satisfactory given the error in the EDS measurements. Since the NPs are negatively charged (negative zeta potential), some of the Rb I ions are not located in the tetrahedral sites of the fcc structure but acts as counter cations. The amount of Rb I cations within the structure is thus very low. The RbCoFe NPs would thus have an electronic structure very close to those of CoFe 0.66 . The major effect of Rb I is to reduce the size of the objects during the synthesis (from around 180 nm to around 70 nm) and leads to charged NPs that have some colloidal stability in solution, while the Rb I free NPs are neutral and thus form aggregate and eventually precipitate.

	3.3. RbCoFe
	Sample	TEM Size (nm) DLS Size (nm)
	RbCoFe t1h	70.8±7.3	68.5
	RbCoFe t60h 70.9±10.9	68.5
	CoFe 0.66	182.3±44.0	190

.22)

. The FC curves at two different P values (the one for P = 20 mW/cm 2 is not shown) indicate that the amount of

Table 3 . 7 :

 37 EDS analysis of CoFe 0.66 and RbCoFe NPs (error: 20%)

  .32). A cell parameter value of 9.95 Å was found and a size of 12 nm. While the size extracted from the reflections corresponding to the main peaks was found equal to 62 nm. The cell parameter value corresponds to that of the pure Co III Fe II network. These results are consistent with the presence of two separated networks within the nanoparticles corresponding to Co II Fe III and Co III Fe II with 65 and 12 nm sizes respectively. It is, thus, possible to suggest that at t60 the nanoparticles are made of a 12 nanometer Co III Fe II

  III Fe II species.

	Compound		Peak (nm)		D	a
		(200)	(220)	(400)	(420)	(nm)	( Å)
	RbCoFe 1h	83.89	84.87	86.93	88.03	85.9±1.6	10.28
	RbCoFe 60h 70.00	69.51	65.90	60.90	66.6±3.6	10.28
	CoFe 0.66	209.80 169.81 173.94 176.13 182.4±16.0 10.28

Table 3 . 8 :

 38 Size and cell parameters values obtained from XRPD patterns for RbCoFe and CoFe 0.66 NPs

Table 3 .

 3 14) that most of the Co II Fe III species that constitute the major contribution of the t0 spectrum evolved to Co III Fe II . The amount of Co II Fe II species is around 10%. A small amount of distorted Co III Fe II is present (3-4 %), that belong probably to the 10% surface atoms.

		Position (cm -1 ) t0 (%) t60h (%)
	Co II Fe III	2159	75	16
	Co II Fe III (distorted)	2187	4	3
	Co II Fe II	2092	14	10
	Co III Fe II	2120	7	71

Table 3 .

 3 14: Contribution of each species obtained from the deconvolution of infra-red spectra of RbCoFe2 nanoparticles in the 2000-2200 cm -1 region at t0 and t60h

Table 3 .

 3 15).

	Peak	RbCoFe2 0min	RbCoFe2 24h	RbCoFe2 60h
		d (nm)	a ( Å)	d (nm) a ( Å)	d (nm) a ( Å)
	(200)	36.49	10.28	26.23	10.26	25.44	10.19
	(200 0 )	-	-	-	-	19.99	9.99
	(220)	29.28	10.28	24.26	10.23	-	-
	(220 0 )	-	-	-	-	18.09	9.97
	(400)	29.99	10.27	26.36	10.25	26.36	10.24
	(400 0 )	-	-	10.51	10.01	21.28	9.95
	Average 31.9±3.2 10.27	25.6±1.0 10.25	25.9±0.5 10.21

t60h (Average size and lattice constant were calculated using only the information of the peaks (200, 220, 400) which corresponding to Co III Fe II species since (200 0 , 220 0 , 400 0 ) belong to another species) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PBAs compounds obtained from XRPD at plane (400,400 0 ) . . . . . . . . . . . . . . stretching of Cr-C bonds at low frequencies that are characteristic for CsNiCr as the two intense peaks of 494 and 380 cm -1 which were reported by Prado et al. [175] Those were not observed on the bulk but could be observed as weak or shoulder peaks on FepzPt@CsNiCr due to the small amount of CsNiCr in FepzPt which is coherent with EDS analysis.

Group

Bulk FepzPt [78] Table 2.4: IR frequencies of FepzPt@CsNiCr NPs and CsNiCr NPs in comparison with the bulk [78](s: strong; m: medium; sh: shoulder; w: weak; n: stretching; d:i n -p l a n eb e n d i n g ;g: out-of-plane bending)

STEM-EELS and High Resolution TEM Studies

To have a proper description of these heterostructures, STEM-EELS and High Resolution TEM studies (HRTEM) were carried out by A. Gloter at LPS on some nanorods (during M. Lopez Jorda's postdoc). As shown on Figure 2.10 The STEM-EELS mappings of N, Fe, Ni and Cr show a uniform distribution of these elements over the whole rod, with slightly more heterogeneity in the Ni and Cr mapping (that may also be due to damage).

2.3.

Influence of the Nature of the PBA Nanoparticles: FepzPt@CsNiCo low temperature, reaching a minimum of q M T = 0.5 emu.K/mol at 5 K. Upon heating up from 5 K, an abrupt transition occurs around 299K with T c " = 304 K, revealed a rather wide hysteresis of DT = 11 K centered at the same temperature (299 K) as FepzPt@CsNiCr. As observed in Figure 2.20 (right), the hysteresis loop of FepzPt@CsNiCo has a similar shape as that of FepzPt@CsNiCr, with an abrupt transition upon cooling while a less abrupt one upon heating. Most interestingly, both compounds have the same transition temperatures around room temperature. 2.10. The magnetic behavior of FepzPt@CsNiCo NPs is rather similar to that of FepzPt@CsNiCr NPs (in term of transition temperature, shape of hysteresis, thermochromic behavior). Still FepzPt@CsNiCo exhibits a 4 K-wider hysteresis as well as a lower g HS at low temperature, which indicates that the transition is slightly more complete than that of FepzPt@CsNiCr. We have attributed this small difference to the fact that FepzPt@CsNiCo NPs have a larger size (length and width) than FepzPt@CsNiCr NPs which is expected to impact the cooperativity of the hysteresis. The slightly smaller thickness of the PBA shell of CsNiCo could also reinforce this cooperativity as compared to CsNiCr counterpart. However, the main conclusion of this subchapter is that provided that tiny nanoparticles of PBA are used, they seem to guarantee the stabilization of the FepzPt nanorods with no significant change in the nature of the spin transition that remains centered around room temperature.

Even if not developed further in this manuscript, attempts have been done with several other PBA nanoparticles that are larger size. The self-stabilization of well controlled was not possible in this case, emphasizing the importance of ultra-small nanoparticles below 5 nm.

Compound

Width (nm) Length (nm) DLS (nm)

FepzPt@(CsNiCr)

X-Ray Powder Diffraction Pattern

The four compounds were studied on XRPD as shown in Figure 2.24. The four patterns are similar to each other and also similar to the bulk. Among them, FepzPt without CsNiCr is the most compatible with the simulated pattern. The most noticeable difference on these patterns are those imbalance of the three sets of double peaks at (200,002), (210,102), and (211,112). As observed on the pattern of FepzPt, (002), (102), and (112) are more intense than their twins while in the compounds contained CsNiCr, it is the opposite. The peaks of ( 200) and (211) in those compounds with CsNiCr are coincidental around the position of the peaks of ( 220) and (400) of CsNiCr which might be the factor that made these peaks more intense. Still this is rather unlikely since peaks of CsNiCr NPs are expected to be very broad and of low intensity.

Sizes and Bravais lattice constants of these compounds have been deduced from the XRPD patterns as shown in Table 2.13. The calculated coherence lengths are rather in good range with the size determined by TEM indicating all of them except FepzPt without CsNiCr are rather mono-crystalline. The mean size is purely indicative since depending on the reflections, the coherent domain size is more related to length or width of the nanorods.

As the content of CsNiCr increases, the grain sizes of the NPs decrease. determined by TEM indicating all of them are rather mono-crystalline. As the content of CsNiCr increases, the grain as the NPs@SiO 2 has an average size of 39.7 ± 6.7 nm in width and 104.5 ± 22.25 nm in length.

The silica thickness is certainly of a few nm but a STEM-EELS mapping should be done to confirm it. However, the band of at 805 cm -1 of C-H of pz linked to Fe II HS is still visible.

Discussion and Conclusion

New self-standing SCO nanoparticles FepzPt@CsNiCr have been successfully prepared by a simple and straightforward route. The size of the NPs can be simply controlled by controlling the amount of CsNiCr seeds or adjusting the acetone content in the solvent (reported in Annex A). All the NPs possess a rod-shape with a width around 25 -40 nm while the length varies from 30 to 120 nm. The content of PBA nanoparticles tunes the stability and average size of the rods. By using 1/8 equivalent of CsNiCr to FepzPt, a compromise is found with well-individualized NPs that exhibit a 8 K-wide hysteresis loop centered around room temperature in the powder. Using CsNiCo instead of CsNiCr allows stabilizing similar types of nanorods with a slightly greater size resulting in a more open hysteresis centered also at room temperature.

The environmental effects have been studied on these nano-objects as self-standing nanorods below 100 nm FepzPt@CsNiCr appeared as ideal candidates to finely control the matrix effects. Still it turned out to be more delicate than predicted as the amount of CsNiCr that tend to attach to the surface does plays a role on the cooperativity of the SCO phenomenon and has to remain strictly constant when comparing the different matrices. Formation of the FepzPt cores with more CsNiCr impacts the rod sizes but a range was found in which it is not strongly affected (between 1/4 and 1/8 Cs per Fe). In this range, a progressively denser layer of ultra-small CsNiCr particles is formed on the lateral faces of the crystalline rods. To our opinion, the shell is not continuous. The roughly estimated amount of CsNiCr NPs per rod for ratio 1/8 is 230, that is about 45-55 CsNiCr particles of 3 nm per side, with a separation of 5-6

nm. When a ratio of 1/4, about a hundred of particles per face are present with interdistance of 3 nm. The presence of these CsNiCr particles modifies the environment of the Fe II sites located at the surface and slightly delays the nucleation by exerting a small negative pressure on the SCO core, that increases with the proportion of such CsNiCr particles. It is not be comparable to the pressure of a continuous shell. A shell containing defects as probably the case of large core-shells reported by Talham et al., [174] where a thick PBA shell does not impede the SCO of the RbCoFe core and only delays it, still with Here each 3 nm particle of CsNiCr may match every two cells (21 Å = 10.5*2) with 2 planes of Fe-Pt separated by 22 Å (0.74*3) along the c direction, thus applying a local strain on these planes. The more the amount of attached particles, the more important the strain and delay of the SCO. The contraction to LS of the SCO core is slightly impeded as if CsNiCr layer was acting as a spring when the layer is more compact. This is why we chose to study intermediate coverage where self-standing nanorods are obtained without impacting too much the cooperativity.

Once this compromise was found, the important point has been to systematically recover these rods with a constant composition of the heterostructures by using cations that have rather 3.10).

Magnetic Studies

RbCoFe t1h

The magnetic studies have been performed on RbCoFe collected after 1h (Figure 3.36)i nt h e range of 250 K to 10 K. The sample was first measured by cooling down from 250 K to 2 K and then by heating up. It was then cooled down to 10 K irradiated and the qT measured in the heating mode up to 250 K. Then, it was cooled down to 2 K. We present, first the two curves measured in the heating mode before and after irradiation (Figure 3.36, top right). We, then, comment all the curves.

The q M T value at T = 250 K (2.76 emu.K/mol) is very slightly lower to the value expected for non-interacting Co II (S = 3/2) and Fe III (S = 1/2, g = 2) ions estimated be equal to 3. Energy Dispersive Spectroscopy:

EDS analysis gives the results presented in Table 3.13 at t0 and t60h. The difference in the Fe and Rb percentages is significant and cannot be due to experimental errors. We observe an increase of the Fe and Rb concentration with time. However, the charge balance does not lead to zero. Nevertheless, even though the absolute percentages are not accurate, the evolution is real and is larger in this case than for RbCoFe. It indicates an addition of ferri(o)cyanide species at the surface of the objects (their size is too large to diffuse into the particles) and inclusion of Rb I within the nanoparticles during time evolution. The amount of Rb I cations included during the time evolution is larger for the present NPs than for RbCoFe; it can be due to the smaller size of the particles. The present particles have an edge size (28 nm) that is less than half of RbCoFe (65-70 nm) so that the time necessary to reach the center of the particles is much shorter for the former than for the latter.

Compound

Co Fe Rb

RbCoFe2 0min 1 0.82 0.2 RbCoFe2 60h 1 1 0.6

UV-visible Spectroscopy Studies:

The formation of the NPs was followed by UV-Vis during 60 hours as showed in Figure 3.40.

An evolution is observed as for the CsCoFe and RbCoFe cases. But it is closer to the Cs I containing particles in the sense that a large change occurs with time. At t0, the spectrum is very close to that of RbCoFe and that of the CoFe 0.66 indicating that the particles have a majority of the high spin Co II Fe III species. After 60 hours, the spectrum is closer to that of the CsCoFe at 60h indicating a change larger than that for RbCoFe at 60h. However, the band at 390 nm has not completely disappeared (as for CsCoFe) indicating that more high spin species remain after 60 hours.

The concentration of ferricyanide in the supernatant solutions after collection of the particles by centrifugation was found larger at t0 (20% of the total ferricyanide concentration) than at

RbCoFe

decrease of 40% similar to what was found for the Cs I containing nanoparticles (Table 3.16).

Magnetic Studies

RbCoFe2 t0:

The magnetic properties of RbCoFe2 t0 were studied in the same conditions as for the other compounds (Figure 3.47). The q M T value at room temperature (3.12 emu.K/mol) is only slightly lower from the one expected for a pure high spin state (3.34 emu.K/mol) based on the following formula: Rb 0.2 Co[Fe(CN) 6 ] 0.85 • 2H 2 O. One computes that around 93% of the sample is in the high spin state, which is largest value among all the samples studied (apart from CoFe 0.66 of course). Upon cooling, a thermal spin crossover takes place with T down around 212 K. The transition is complete at 170 K and reaches a q M T value equal to 2.54 emu.K/mol corresponding to a transformation of about 30% of the high spin species. q M T increases very slowly upon cooling until 40 K where it increases more abruptly and reaches a maximum at 12 K with a value of 5.1 emu.K/mol. The abrupt increase is due to the onset large magnetic correlation due the large amount of high spin species that remains after the

Perspectives and Future Work

Self-standing nanorods FepzPt@CsNiCr shall be studied with other trimethyl alkyl ammoniums with alkyl chains between C6 and C20 to complete the study and try to establish a relationship between the alkyl chain lengths and SCO critical temperatures. Switching characteristics may thus be fine-tuned in a predictable way depending on the selected cation.

Other alkylviologens may be of interest to confirm the large hysteresis observed with EV, and switching of the alkylviologens to the radical form would be of interest to probe the effect of the redox state on the SCO as well as thermochromic properties. Other activatable polymers XRPD pattern of FepzPt@CsNiCr, FepzPt@CsNiCr@DDTMA, and FepzPt@CsNiCr@HTMA showing no change in structure upon coated the NPs with cations.