Prof Smail Triki

Garcia Yann

Mathieu Marchivie

Dr Mathieu Gonidec

Prof. Corine Mathoniere

Dr Patrick Rosa

Dr Cédric Desplanches

Dr Elizabeth Hillard Thanks

Thi Marlène

Thiet

Margaux Liza

Wenbin Guo

During these three years, I was lucky to meet, collaborate and communicate with wonderful people on scientific and personal levels. Without their help, support and guidance, this thesis work wouldn't be completed, thus I would like to thank all of them. First of all, I would like to express my heartfelt thanks to my supervisors Guillaume CHASTANET and Philippe GUIONNEAU for their continuous support, guidance, encouragement and trust.

During these three years, they taught and trained me this project and provided me a tremendous amount of advices in every scientific aspect of this thesis work. Their dynamic and optimistic attitude, passion for research, research ethics, patience and motivation

Table of contents

and more significant and urgent. Among these fields, spin crossover (SCO) materials have attracted great interest owing to applications in molecular electronics, as data treatment unit, sensor, actuators and/or smart pigments. Thermal, pressure, magnetic-and electric-fields, light, chemical controls can be used as stimuli to induce the spin state switching in SCO materials.

Light is a crucial tool to promote the spin switching in materials as it can operate at ultrafast timescale (femtosecond). Light might be used to promote a photo-reaction on the ligand that will affect the ligand field and the spin state. This can be achieved through Ligand-Driven Light-Induced Spin Change (LD-LISC) and Light-Driven Coordination-Induced Spin-State Switching (LD-CISSS). Light can also be used to bring energy and heat the sample, called the photothermal effect. Light can be tuned to excite and populate states involving the metal center according to the Light-Induced Excited Spin-State Trapping effect (LIESST). Through the population of excited states, a metastable state (hν-HS*) can be trapped at low temperature.

In order to use SCO devices operating at ambient conditions, the spin switching of materials should be effective around room temperature. Dozens of SCO materials have been reported with the thermal spin transition temperature (T1/2) around or above ambient conditions. However, the relaxation temperature (hν-HS* LS) that measures the limit temperature at which the photo-induced state is erased within few minutes, named T(LIESST), is usually below 50 K, which is severely restricted its application. It is still a great challenge to establish the structure-properties relationships corresponding to the LIESST process, though this approach is crucial to discover SCO materials with a high relaxation temperature T(LIESST).

The fundamental understanding of the LIESST effect must be yet deeply completed prior to any rational design of any efficient material.

To understand the relationship of relaxation temperature between thermal and light-induced SCO, the database of T(LIESST) vs T1/2 was created and developed by my host group. It was evidenced a linear relationship between these values as T(LIESST) = T0 -0.3 T1/2. In order to enhance the T(LIESST), T0 should increase. It was proposed that this could be achieved through the increase of the distortion of the coordination polyhedron. The crucial role of variation of trigonal distortion (ΔΘ) on T(LIESST) was confirmed in complexes with bidentate ligands.

Later, based on photocrystallographic studies of two polymorphs of [Fe(PM-BiA)2(NCS)2] complex (PM-BiA = N-2'-pyridylmethylene)-4-(aminobiphenyl)), both the variation of angle distortion (ΔΣ) and bond distortion (Δζ) play a crucial role on T(LIESST). The role of the coordination sphere distortion on the lifetime of the hν-HS* implies that three-dimensional potential wells must be used, to describe the anisotropic breathing of the coordination sphere.

Guided by these valuable works, T(LIESST) can be enhanced by increasing the distortion of the coordination sphere. We focus this work on the synthesis of new compounds with ligands designed to promote strong distortion of the coordination sphere. The first strategy is to promote the steric strain close to the coordination sphere by halogen substitution from para, meta, to The spin crossover (SCO) phenomenon has been discovered more than 90 years ago [1][2] and is increasingly envisioned to take part in devices in molecular electronics, as data treatment unit, sensor, actuators and/or smart pigments. [3][4][5][6][7][8][9][10][11] This has led to a huge interest of the scientific community in coordination chemistry, materials science and physical chemistry aspects of SCO during the last 40 years. In this part, however, we will only present some basic concepts of SCO.

ortho

What drives the occurrence of SCO?

In perfectly octahedral ligand field, the five d orbitals of a transition metal ion are split into two subsets of orbitals, t2g (made of dxy, dyz and dzx) and eg (made of dz and dx -y ). The t2g orbitals are basically non-bonding and are therefore at lower energy than the anti-bonding eg orbitals.

The splitting between the two sets is referred to as ligand field splitting, 10Dq. This ligand field strength depends on both the particular set of ligands and the given metal ion. The population of these two sublevels by electrons is driven by the difference between ligand field splitting energy (Δ = 10Dq) and the interelectronic repulsion energy (Π). Metals with electronic configurations d 4 to d 7 can adopt either high-spin (HS, when Δ is weaker than Π) or low-spin (LS, when Δ is greater than Π) electronic ground states. Under certain circumstances, when the energy difference between the HS and LS states becomes comparable with the thermal energy kBT, a switching from one configuration to the other may occur, leading to the spin crossover (SCO) phenomenon. As a result of the SCO, there are several changes of physical properties arising from the change of electronic configuration. The most obvious modifications concern the metal spin state, the metal-ligand bond lengths and the magneto-optical properties, together with variations in vibration (molecules and network) and thermodynamic characteristics. Notably, the LS ion forms bond-lengths typically 10% shorter than those of the HS ion, bond vibrations differ in both spin states and LS give more coloured and less "magnetic" samples than HS. All these changes can be used to probe the SCO which opens up the use of a very wide range of characterisation techniques 12 , as detailed below.

What perturbation can induce SCO?

Temperature is the most straightforward perturbation used to promote SCO. Temperature, in a schematic way, brings energy to the electrons to populate the eg level leading to a LS to HS switch upon warming. The entropy in HS state is higher than that in LS state due to the large spin multiplicity at HS state. The change in spin state, can also be promoted by the magnetic field that favors the HS state. 13 However, it requires very high magnetic field (20-100 T), which makes this stimulus rarely used and barely investigated. 13 Typical Fe-N bond lengths in FeN6 environments are around 2.0 Å and 2.2 Å in the LS and HS states, respectively, providing a noticeable change upon SCO corresponding to an increase of the metal coordination sphere volume from LS to HS. 14 Applying pressure is therefore another important stimulus that favors the state of lower volume that is the LS state. [15][16][17] Light represents also a crucial tool to promote the spin switching through different means. 18 First, light might be used to promote a photo-reaction on the ligand that will affect the ligand field and the spin state. This can be achieved through Ligand-Driven Light-Induced Spin Change (LD-LISC) [19][20] and Light-Driven Coordination-Induced Spin-State Switching (LD-CISSS) 21 . Second, light can also be tuned to excite and populate states involving the metal center. Through the population of excited states, a metastable state can be trapped at low temperature. This is the case of the Light-Induced Excited Spin-State Trapping effect (LIESST) 22 that will be detailed in the following section I.2. Finally, light can be used to bring energy and heat the sample, promoting photothermal effect. [23][24][25] While LD-LISC and phototothermal effects might be achieved at room temperature, obtaining LIESST at such temperature is strongly challenging.

Though the most intensively studied, temperature is therefore far to be the only route to induce the SCO in a compound. With in view applications using SCO devices running at ambient conditions, the use of high-pressure, magnetic-fields or light-irradiation as stimuli are to be envisaged, though with unequal perspectives. Note also that getting ambient conditions SCO can also take the road of electric or chemical controls. 26 Which techniques to record SCO?

At SCO, the magnetic moment changes between two different spin states that can be easily discriminated using magnetic measurements. In the particular case of Fe 2+ based compounds, SCO occurs between a diamagnetic (LS) and a paramagnetic (HS) states, providing an important magnetic contrast. Hence, the measurement of the magnetic susceptibility as a function of temperature, χ (T), becomes a technique of the first choice to characterize the occurrence of SCO. The magnetic susceptibility is indeed dependent on the value of spin state, expressed as χT = Cg²S(S+1) with C = NAβ 2 /3kB, NA, β, kB, g being the Avogadro number, the Bohr magneton, the Boltzmann constant, and the Lande factor, respectively. Comprehensive surveys of the techniques and computational methods used in magnetochemistry were given by Palacio 27 and Kahn 28 . Another usual method to determine the spin state, as well as the oxidation state of iron-based compounds is 57 Fe Mӧssbauer spectroscopy. [29][30] The latter is actually the true method to obtain the absolute HS/LS ratio within a compound since the electron density at the nucleus strongly differ in HS and LS. Isomer shift σ and the quadrupole splitting ΔEQ 30 , are consequently different for the HS and LS states, and increase from the LS to HS switch.

Along SCO, the population/depopulation of the antibonding HS as function of the temperature, leads to strong differences in bond lengths and bond vibrational characteristics.

Therefore, vibrational spectroscopic techniques are very useful, such as infrared 31 or Raman spectroscopies 32 . The impressive variation of bond lengths can be accurately probed using diffraction technics, preferentially using Single-crystal X-ray diffraction (SCXRD) 14,33 instead of Powder X-ray diffraction (PXRD) 14 , through at least the crystal structure determination in both LS and HS states and with the possibility to explore the full phase diagrams. For instance, structural "movies", determined by variable temperature SCXRD, provide many information from atomic to material scales, including variations of geometry and volume of the coordination sphere, changes of ligand conformation, modification of intermolecular interactions, rearrangement of molecular packing, symmetry breaking and so on. 14,34,35 The change of electronic configuration at the SCO also affects the absorption properties, leading to color change upon switching. Therefore, absorption spectroscopies and optical reflectivity are important to trigger thermo-, piezo-and photo-chromism of the SCO materials. [36][37] Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES) provide information essentially about geometry and oxidation states, which have been used to study the structural and electronic changes occurring during SCO. [38][39] These methods become relevant to get information notably when the SCO sample is not available in the crystalline state.

In the solid state, the thermodynamic rules the behavior of the materials. The LS state is enthalpically favored whereas the HS state is entropically favored. Therefore, calorimetric measurements are crucial to determine the enthalpy and entropy changes. The enthalpy change ΔH = HHS -HLS is typically around 10 to 20 kJ mol -1 , and the entropy change ΔS = SHS -SLS is of the order of 50 to 80 J mol -1 K -1 . 40 It is important to highlight that the SCO process is entropydriven. In fact, only around 25 % of the entropy gain from LS to HS change arises from the change in spin multiplicity, ΔSspin = Rln((2SHS+1)/(2SLS+1)) (13.38 J mol -1 K -1 for Fe 2+ ) while 75% comes from vribrations (molecules, phonons). 41 It is interesting to note that, in the last decades, if the large panel of approaches to probe the spin crossover allowed a deep investigation and therefore a significant step forward in the understanding of the SCO phenomenon, conversely, the requirements of the investigation of the SCO phenomenon allowed to push the fontiers of some characterization technics. It is for example the case of High-Pressure or Low Temperature SCXRD. 34,[42][43] What are the spin crossover features?

Temperature is the main perturbation used to induce the SCO. The feature of the SCO gives information on how the SCO-active centers interact with each others. Indeed, the organization of molecules in a crystal lattice and the supramolecular interactions between them play a key role in the overall behavior of the solid. In fact, the volume change associated with the spinstate switch -change which is actually anisotropic -propagates in the solid via intermolecular interactions. These interactions, between the molecules in the crystal lattice, can be schematized, in a first approach, by springs (Figure I.2). The strength of these springs determines the cooperativity of the solid, that is to say how collective the propagation of the anisotropic volume change of the coordination sphere up to macroscopic scales is. [44][45][46] In a simple view, the smaller the intermolecular distances, the higher the spring force, the higher the cooperativity. Cooperativity, when it is strong, then looks like a domino effect (ferroelasticity): the spin change of a metallic center propagates to the neighboring centers. It is still not clear which kinds of intermolecular interactions contribute to strong cooperativity, however, covalent bonds in polymeric system, hydrogen bonding, and π---π contacts seem to be essential. It should be noted that the intermolecular interactions are not isotropic in the three directions of the crystal lattice, this being able to generate competitions of elastic interactions and give rise to antiferroelasticity. The multi-scale view of SCO (as seen in the following section I.1.2) is therefore important to understand the propagation of the variation in volume from the coordination sphere to the macroscopic solid and to explain the observed behaviors.

The spin crossover in the solid state is therefore strongly influenced by cooperativity. Figure I. 2 shows schematically the temperature evolution of the HS fraction for different levels of cooperativity. The switching temperature, denoted T1/2, is defined as the temperature at which half of the metal centers has switched. To date, several very different SCO shapes have been observed in the solid state, as listed below.

Gradual SCO (Figure I.2a) are characterisitic of weakly cooperative compounds. Such SCO is characterized by the evolution of the HS fraction that covers a wide temperature range, often up to 100 K. It is similar to the features observed in solution with a Boltzmann distribution of the spin state. For example, structural analysis from 120 K to 430 K of [FeL(ClO4)2] (L = 1,4,7tri(1-amionophenyl)-1,4,7-triaza cyclononane) that exhibits a gradual SCO, suggests the continuous variation of average bond lengths and distortion within the FeN6 polyhedron and the absence of direct short contacts between adjacent [FeL] 2+ ions. 47 It is however crucial to note that one given bond length can adopt only two values, the one of the HS state and the one of the LS state. The observation of continuous feature modifications, thus the determination of intermediate bond length values, only reflects an average of the bond lengths at the macroscopic sample scale. This is by the way efficiently used to determine the HS/LS ratio from Fe-N bond lengths. [48][49] Abrupt and hysteretic SCO (Figures I.2b Such strong cooperativity might be associated with structural phase transition. This lead to the saillant concept of bistability. Bistability refers to the ability of a system to be observed in two different electronic states at the same condition for a given external perturbation, depending on the history of the compound. [50][51] . This aspect has driven many research with the aim to use SCO materials as memory elements. [7][8][9][10][11]15 Stepped SCO (Figure I.2d and I.2e) is associated with the appearance of an intermediate situation with a partial conversion of the metal centers, some remaining HS at low temperature for example. Usually, it occurs when different SCO-active sites are present in the network, either before the occurrence of the spin switch or during the SCO through the appearance of symmetry breaking. Stepped SCO have been reported in a vast variety of SCO materials made of Fe 2+ 52-53 , Fe 3+ , 54 Co 2+ 55 , or Mn 3+ 56 ions. Real et al. firstly suggested the presence of "negative cooperativity" to explain the stepped behavior of the dinuclear compound [Fe(bt)(NCS)2]2bpym (bt = 2,2'-bi-2-thiazoline and bpym = 2,2'-bipyrimidine). 57 Symmetry breaking is mainly explained as the result of competing ferro-elastic and antiferro-elastic interactions. [58][59] As already mentioned, the coordination sphere volume change is not isotopric, nor the network of intermolecular interactions. This might lead to elastic competition promoting and ordering of intermediate phases with HS-LS periodic orders, in different ratio, on each step.

This can end up with very peculiar cases such as Devil's staircase ( compound with one metal site, the partial conversion is then due to a perturbation from the crystal packing and comes down to consider the remaining HS site as a disorder on the LS sites. 62 The interesting point for this kind of SCO is the possible presence of hidden hysteresis.

For example, photoinduced hidden hysteresis was reported in Hofmann-like spin-crossover metal-organic frameworks 63 and molecular complexes 64 .

I.1.2. Multiscale view of SCO: from the coordination sphere to the crystal scale

From the structural point of view, the most obvious modification accompanying the spin crossover concerns the reversible shortening of the metal-ligand bond lengths from HS to LS states. The latter was noticed and thouroughly studied in the early stage of the SCO phenomenon. [65][66][67] It has been shown that this internal modification of the coordination sphere has repercussions to all larger physical scales, up to the macroscopic one. 34,68 Consequently, in order to understand how the SCO induces structural changes and, conversely, how structural properties can influence the SCO properties, one needs to adopt a multiscale view of the SCO. 14,[34][35] In other words, determining the bond lengths variations at the SCO is mandatory but largely insufficient to describe the mechanism of the SCO occurring in the crystalline materials. Indeed, the interplay between SCO and structural properties appears so complex that it is still under investigation. Since we aim to draw structure-properties relationships in this work, in particular to penetrate the crucial parameters that govern the light-induced SCO, the structural description will tend to encompass a multiscale view. The latter means that all physical scales must be described: the geometry of metal-ligand coordination sphere, the conformation of molecules, the topology of intermolecular interactions, the rearrangement of molecular packing including phase transition, the coherent domain scale though the microstructure of crystals (or polycrystals) and, when possible, the macroscopic description of the crystal itself. By the last two decades, the SCO community has sometimes attempted to propose a full description of the structure-properties relationships. 68 This very rich field has given rise to the definition of numerous structural parameters nowadays used as daily tools such as the distortion and the volume of the coordination sphere 14,[69][70] as well as some crystal packing or phase diagram features 35,71 . This race to the ultimate multiscale understanding of SCO was yet on course at the time this work started. Many structural features regarding the multiscale view had been reported, as very briefly recall below with the choice to underline those that will be determined and used along this work.

The coordination sphere modifications at the SCO include, from LS to HS, an increase of the metal-ligand bond lengths by an average value of 0.2 Å in the case of the Fe 2+ N6 surrounding (on which this work is focusing, as already stated) and an increase of the deviation from the ideal octahedron, meaning an increase of the distortion. The whole results in an increase of the volume of the coordination sphere (ΔVp) by around 25% for Fe 2+ N6. 34 It is interesting to note that this volume variation appears always of the same amplitude, whatever the SCO complex involved. It is much more intricate to propose a unique tendency for the distortion since it strongly depends on which type of distortion it is referred to (see below); to some extend the distortion of the coordination sphere may appear crucial for the SCO features and at the same time typical for a given compound. The amplitude of the average bond-lengths variation at the SCO is very similar from one compound to another -note that this is true if one pays attention to separate pure thermal effects from SCO effects -and therefore it is hardly a discriminating parameter to account for various SCO features. 48 In addition, the bond-lengths variations are strongly anisotropic which makes the isotropic description (average bond-lengths and volume) rather insufficient.

The reflection on the distortion feature is tricky to drive since it strongly depends on the way the distortion is defined. Across the investigation of the distortion of the coordination sphere in SCO compounds, four parameters were finally selected 14 , among many others in the fields of possibilities 70 .

These parameters all account for a different but complementary point of view. The angle distortion Σ is defined by the sum of the deviations to 90° of the 12 cis ligand-metal-ligand angles 72,70 The trigonal distortion Θ is defined as the sum of the deviations from 60° of the 24 possible θ angles, where θ is defined as the ligand-metal-ligand angle measured on the projection of the two triangular faces of the octahedron projected along its pseudo-threefold axes on the medium plane containing the metal ion 73 The length distortion ζ means the sum of the deviations from the average metal-ligand bond length (Eq I.3), which carries a mixture of distortion and elongation information. 74 The value of the shape parameter, S, is another way to quantify the deviation from the ideal octahedron (Oh) and takes part of a global description of symmetries. 75 The values of ζ, Θ, Σ and S are zero for a perfect octahedron. These values can be calculated using the software OctaDist 76 (https://octadist.github.io) based on development made in the "Switchable Molecules and Materials" group at ICMCB and S can be calculated by the shape software. 77 These parameters have HS and LS values that, in fact, are different from one compound to another. Consequently, these parameters appear essentially a relevant feature for a given compound and are therefore eligible for drawing structure-properties relationships. As it will be described in the next part of this chapter, the trigonal distortion, Θ, has been directly correlated to some SCO features and is now an integrated tool within the study of the SCO phenomenon. 73,[78][79][80][81][82] These four parameters will be used and commented when relevant in this work; it is worth mentioning already that their discussion requires an accurate determination of the crystal structures at least in both HS and LS states, or better at many temperatures. As a general comment, it is clear that the structure-properties interplay should be always, when possible, envisaged through variable temperature investigation as recently demonstrated 48 , this remark is valid for all physical scales, listed below. The molecules, of course, are affected by the modifications of the coordination sphere at the SCO. If the latter are relatively easy to quantify and classify since the events (elongation, distortion) are the same in nature even though different in amplitude, on the other hand the effects of the SCO at the scale of the molecular complex are almost impossible to rationalize because they appear rather different from one compound to another. The response of the molecule to the coordination sphere modification goes from a simple "absorption" of the volume change to a strong molecular deformation involving ligand significant movement. The most spectacular effects, reported so far, are conformational modifications and even reversible bond breaks. 83 For example, the deformation of ligands suggests the conformational change as cis-trans [20][21] , anti-gauch 84 , and angular Jahn-Teller distortion in bpp family 77 . By extension, it can be included at this scale the order-disorder of molecules together with anions or solvent entities also. For example, the order-disorder transitions of anion leading to cooperative SCO 86 and 87 .

were found in [Fe(DAPP)(abpt)][ClO4]2 (DAPP = bis(3-aminopropyl)(2-pyridyl-methyl)amine, abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole) 85 , [Fe{3-bpp-(5-C6H3OH)2}2](ClO4)2•H2O •2acetone (3-bpp-(5-C6H3OH)2 = 2,6-bis[5-(2 -phenol)pyrazol-3-yl]-pyridine)
[Fe(picpzpz)2][BF4]2•MeOH (picpzpz = 3-(2-pyrazinyl)-1-[(2-pyridyl)methyl]pyrazole)
Modifications of the conformation of the ligands, anion, and even solvent appear quite common in SCO compounds and may be linked to some specific behaviours. For example, the hysteretic SCO behaviour is driven by the modification of the conformation of the triflimide from a syn

Σ = Σ i=1-12 |90 -α i | / ° Eq I.1 Θ = Σ i=1-24 |60 -i | / ° Eq I.2 ζ = Σ i=1-6 |(Fe-L i ) -<Fe-L>| / Å Eq I.3
conformation to an intermediate conformation in [Fe(qsal-I)2]NTf2 (qsal-I = (N-8-quinolyl)-5-I-salicylaldiminate). 88 Elsewhere, the conformational changes of the flexible alkyl chains at the SCO impact the magnetic response in [Fe(C10-pbh)2] (C10-pbh = (1Z, N E)-4-(decyloxy)-N -(pyridin-2-ylmethylene)-benzo-hydrazonate). 84 Actually, the subtle relationships between the molecule geometry and the SCO features represents one of the possible way for the rational design of the SCO complexes.

It is worth noting that the molecular shape can inhibit the SCO in some cases. As suggested by Halcrow "if the difference in shape between the high and low spin forms of a material is too large, it will freeze the compound in its high spin state". 78 This is because a solid lattice cannot accommodate the structural rearrangement that would be required for the transition to proceed. 68 The typical example is the HS compound [Fe(bpp)2](PF6)2. 78 The intermolecular interactions can be significantly affected by the SCO and, conversely, the topology of the interactions between metallic centers may strongly influence the SCO features. 14,68 As a general tendency, strong intermolecular interactions favor an abrupt SCO and large hysteresis. The whole difficulty lies in the characterization of what is a "strong" interaction and accounting for all interactions. In essence, many different kind of intermolecular interactions can be found in SCO molecular crystals including hydrogen bonding, π-π contacts, chalcogen bonding, halogen bonding and even atom-atom contacts (as hydrogen-hydrogen ones). This diversity and the huge amount of interactions develop intricated topologies and make very difficult to describe and therefore rationalize the link between SCO features and intermolecular interactions. By the way, the description of intermolecular interactions constitutes in itself, a research field in molecular materials. Many attempts over the years were made to propose a general view of intermolecular interactions in SCO compounds 67,70 , but it is probably the new description proposed by the Hirshfeld surfaces through fingerprints representations 89 that provides the better tool so far to summarize the intermolecular interactions, notably in SCO compounds as recently shown. 90 The description of intermolecular interactions is indeed crucial since they can be viewed as sources of cooperativity. [42][43][44] Therefore, to account for that, their discussions through fingerprints will be used, when possible, in this work. The details on the Hirshfeld surfaces and fingerprints calculation process can be seen in Appendix I.

To illustrate the complexity of the interplay between intermolecular interactions and SCO features, it can be noted that SCO can be prohibited when the intermolecular environment is too congested, though intermolecular interactions are too dense [91][92] or, on the contrary, when the intermolecular interactions are too weak [93][94] .

The molecular packing scale is of course very important in the description of a crystalline compound. As for intermolecular interactions, the features of the crystal packing can drastically, or only smoothly change in parallel to the SCO. When discussing crystal packing in view of correlating to SCO features, many doors suddenly open. 93 One is polymorphism, i.e. the possibility for the same molecule to crystallize within different crystal packings including different symmetries. Largely encountered in the SCO literature, one of our goal in this work will be to use this property to discuss structure-properties relationships. This physical scale also accounts for phase transitions and it has been demonstrated how the phase diagrams can be tricky in SCO compounds 34 . The literature on this subject is very dense reporting for instance symmetry breaking associated to spin crossover. 55,95 The unit-cell dimensions and symmetry can summarize part of the information of the crystal packing. Unit-cell modifications associated to SCO is one of the crucial parameters, probably the first one, to determine when studying SCO compounds. It has been done since the early stage. [64][65][66] If the absolute value of the unitcell has a poor meaning in the objective of rationalizing the SCO features, the relative variation from HS to LS is clearly meaningful. When this work started, the relative unit-cell volume decrease from HS to LS was known to lay between 1 and 10 % based on the existing literature. 14,34 However, it has already been quoted a long time ago that a fine description must focus on the unit-cell parameters that reflects the anisotropy of the unit-cell modifications, rather than the volume. 68 It has been shown that the anisotropy of the unit-cell modifications is a common feature as illustrated, for instance, in triazole based SCO compounds where one parameter can be modified by more than 10% while others are scarcely changed. 96 It is worth noting that a fine unit-cell description, when talking about thermal SCO must be obtained from more than the sole HS and LS unit-cells, respectively, at high and low temperature but from a variable temperature study, including many temperature points on both sides of the SCO. 68,71 This will be done in this work, when possible, and will allow to separate pure thermal from pure SCO effects.

The microstructure scale in a crystal means the scale of the coherent domain, i.e. the smallest part that indeed is formed by a monodomain in wich the unit-cell only repeat by translation, without break of any kind (Figure I.4). Ideally, a single-crystal is one monodomain but, in practice, this is never the case. A real monocrystal is made by a huge number of coherent domains slightly misaligned like fragments in a regular mosaic. 97 When the coherent domains are seriously misaligned and rotated then the sample is a polycrystal, in other words a powder.

Though it must be handle with care, the size and morphology of coherent domains can be extracted from the Bragg peaks morphologies both in single-crystal and in powders. In the former, the degree of mosaicity is estimated through a number denoted as the mosaicity of the crystal and in the latter the average size and morphology of the domains, then called crystallites, are estimated. Of crucial importance in other fields like metallurgy, the investigation of microstructural parameters in SCO materials is still in its infancy. In some specific cases, it has however been clearly demonstrated that the microstructure is strongly affected by the SCO; the coherent domain volume following the unit-cell deep modifications 98 and some tentative to correlate SCO features to microstructure have been attempted 99 . Overall, the determination of microstructural properties offers probably one of the best way to investigate the mechanical fatigability of the SCO materials, i.e. the resilience of the SCO phenomenon to a large number of events. This approach has already shown that in some cases there is a significant mechanical fatigability both in powder 98 and in single-crystals 100 . However, this field is to explore yet and a mettalurgic approach of SCO material represents one of present road to follow. Of course, this approach challenges also the huge field of nanocrystals. Again, even though the literature of nanomaterials in SCO is very dense 101 and will not be discussed here since out of range, it is clear that structural information on nanocrystals are rare and constitute one of the challenges included in the microstructure global one.

The upper and ultimate scale is the macroscopic one, i.e. the crystal scale. It is less in the field of competence of the structural description but, anyway, it is clear that this scale is affected by the SCO through the modification of the colour, the sample volume, the facets and surface aspects and the mechanical parameters. All the lower scale transformation translates up to the volume expansion of the macroscopic sample, due to the elastic interactions between its constituent molecules 14 , which sometimes may lead to irreversible cracks of the samples.

Conversely, the shape of the macroscopic sample may strongly affect the way the SCO starts and propagates along the sample as shown by microscopic early studies. 102 The links between the macroscopic scale, such as the morphology of the crystal, and the SCO features is currently under investigation. 42,[103][104][105] Above is a very brief account of the multiscale view of the SCO consequences and interplay with the structural features. In this work, since structure-properties relationships are targeted mainly at the scales going from the metal coordination sphere to the crystal packing, the microscopic and macroscopic scales will not be commented much anymore and the focus is therefore made on the information coming from the X-ray diffraction investigation, i.e. the crystal structures and the panel of included informations. and I.5b tha report the evolution of the absorption spectra as a function of light irradiation at 20 K and the corresponding Jablonski-Perrin scheme. 108 At 20 K, the ground state is the LS state ( 1 A1). Irradiation into the 1 T1, 1 T2 transitions at 514 nm allows the population of excited states. From these excited states, two inter-system crossings involving the triplet 3 T1 state, lead to the population of the metastable hν-HS* state 5 T2. An irradiation at 820 nm, into the 5 T2 5 E1 transition of the hν-HS*, leads to the reverse-LIESST and allows to erase the hν-HS* state and recover the LS state.

I.2. The

Many studies have been devoted to the understanding of the photoswitching process. [109][110][111][112][113][114] Tribollet et al. have shown, for the compound [Fe(phen)3](BF4)2 (phen = 1,10-phenanthroline) in solution, that the LIESST process involves a cascade of excited states including the MLCT charge transfer bands. 113 Thus, the population of the 1 MLCT state is followed by a first non- Recently, time-resolved photocrystallography studies have been carried out by the team of Collet et al. [115][116][117][118] These authors also confirmed the presence of several steps during photoswitching (Figure I.5d). Based on the structural changes, they first observed the excitation of Fe 2+ ions at constant unti-cell volume. This conversion is done in few ps (1). Then, an expansion of the crystal lattice is observed, over a period of few ns (2). This expansion of the lattice, along with the heat generated by the light pulse, causes other molecules to switch into the hν-HS* state. This increase in the HS fraction occurs in few s (3). Therefore, the LIESST process involves an elastic step that drives the expansion or not of the lattice. This expansion depends on the elastic strength and also the size of the particles and can drive the occurrence of nonlinear effects. 117 Therefore, the elastic interactions are of importance in the photoswitching process. Finally, the hν-HS* state reached being metastable, the latter will relax towards the stable LS state in few ms (4). The population of state hν-HS* therefore involves nonequilibrium processes and thermal effects, leading to non-linear effects. Indeed, ultrafast crystallography showed that a single photon can transform more than 7 molecules. 117,119 Let us mention that the reverse-LIESST process has also been elucidated using time-resolved spectroscopy. 120 -A region governed by the tunnelling effect, at low temperature, where the relaxation process is independent on temperature;

I.2.2. The lifetime of the photo-induced state

-A region where the relaxation is thermally activated, at higher temperature, where the relaxation process is temperature dependent. The relaxation constant then follows an Arrhenius type law. increases in the thermally activated part. 108,123 The role of cooperativity on the lifetime of the photoinduced state has also been identified.

In noncooperative systems (solution, gradual SCO), the relaxation kinetics follow an exponential decay (Figure I.7a) that can be modelled by the equation I. 4 where k0 is the relaxation rate in the tunneling regime. [124][125] However, it often happens that the experimental behavior cannot be described by a simple exponential, but rather by a distribution of exponential laws. This is the case, for example, with diluted systems for which the presence of defects due to dilution causes a stretching of the relaxation curve. This deviation can also come from local inhomogeneities of the coordination sphere. To deal with this type of behavior in "stretched exponential", Hauser introduced a Gaussian distribution of the activation energy, Ea. 124 with Eq I.4 In the case of cooperative materials, we have mentioned that the presence of strong intermolecular interactions generates non-linear phenomena in particular responsible for the abrupt nature of the transitions. This "domino" effect due to cooperativity also impacts the hν-HS* LS relaxations as illustrated in Figure I.7b. At short timescale, relaxation is slowed down compared to a simple exponential while it is accelerated at longer times. The explanation put forward for this sigmoidal behavior of relaxation curves is that, initially, the energy cost to grow LS germs into a HS network is not favorable. As these germs appear, the "domino" effect sets in as a reflect of the internal pressure exerted by the LS sub-network on the HS network. 126 Relaxation is therefore self-accelerated. In general, the more the internal pressure increases, the more the hν-HS* LS relaxation accelerates.

To account for this sigmoidal behavior, Hauser evidenced that the zero point energy difference varies depending on the HS fraction. 126 This behavior results in a change in the activation energy during relaxation. Cooperative interactions tend to stabilize the HS state.

At the onset of relaxation, therefore, the activation energy is maximum. During relaxation, the LS state population destabilizes the HS state by internal pressure resulting in a decrease in activation energy. This can be described by equations I.5-7 where α(T) is the self-acceleration factor.

Eq I.5

Eq I.6

and Eq I.7

Unlike a simple exponential behavior, the relaxation constant depends not only on the temperature, but also on the LS fraction present in the material at time t. The parameter characterizing the cooperativity in this type of equation is the additional activation energy Ea* linked to the self-acceleration term α(T) (Eq I.7). The activation energy Ea therefore decreases as the LS fraction increases. The dependence of Ea on LS or HS varies according to equations I.8 and I.9. 124 Eq I.8

Eq I.9

I.2.3. T(LIESST): a tool to compare compounds

Since the LIESST effect occurs in the solid state at the femtosecond timescale, in a reversible way, there is a strong need to increase the lifetime of the photoinduced state in order to achieve the photoswitching at room temperature. The first attempt at correlating the photoinduced properties of spin crossover compounds was proposed by Herber et.al. from the monitoring of NCS -vibrations using IR spectroscopy. 127 However, it was actually Hauser et al.

who proposed the first relationship. 124 In 1991, on the basis of a set of measurements of relaxation constants in the tunneling region, kHL (T 0 K), Hauser observed a direct correlation of relaxation at T 0 K with the equilibrium temperature T1/2. This relationship is known today as the "inverse energy gap law". Since is related to T1/2, the higher T1/2, the higher , the lower Ea, the faster the relaxation. Hauser et al. also evidenced that an increase in the bondlenghts variations upon SCO can increase the activation energy. 108 The main drawback of this study is that Hauser was only interested in diluted systems, without studying the effects of cooperativity. Moreover, this work was based on the determination of the relaxation constants of the tunnel region, without taking into account the thermally activated zone. This is why in 1998, Létard et al. introduced the T(LIESST) experiment. 128 This experiment measures the limit temperature for photo-inscription, T(LIESST), as an element of comparison. One of the advantages of this approach is that it takes into account all the relaxation processes (tunnel effect, thermally activated regime), whatever the kind of compound (cooperative or not). To this aim, a measurement of the variation in magnetic response is carried out using the SQUID magnetometry, in a relatively modest time scale (a few hours). The experiment consists in measuring the thermo-induced spin crossover down to 10 K. At this temperature, an appropriate irradiation is swichted on until photo-saturation of the magnetic signal (meaning it does not evolve anymore upon further irradiation). The irradiation is then switched off and the temperature is slowly increased at 0. As this limit temperature is dependent on relaxation, the measurement speed was set at 0.3 K/min and is kept constant for all compounds in order to obtain a rigorous comparison between the different materials studied. 18,[129][130][131][132] Typically, a measurement in a SQUID magnetometer takes a dozen hours of recording to estimate the T(LIESST), the time resolution of the device being of the order of a minute. As we can see from such database, most of compounds exhibits T(LIESST) lower than 100 K. Up to now, the "bpp family" based on the tridentate ligands 3-bpp = 2,6-bis(pyrazol-3yl)pyridine and 1-bpp = 2,6-bis(pyrazol-1-yl)pyridine is providing T(LIESST) frequently above 100 K. [133][134][135] This is illustrated by a recent report of a T(LIESST) of 120 K in this family. 136 Another high T(LIESST) value at 108 K related to the photo isomerization of NCS -into SCN - was reported in a polymorph of trans-[Fe(abpt)2(NCS)2] (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4-triazole). 137 To the best of our knowledge, the highest T(LIESST) known so far is 135 K in the molecular complex [Fe(L)(CN)2]•H2O (L = 2,13-dimethyl-6,9-dioxa-3,12,18triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaene). 138 This compound presents a modification of the metal coordination number upon spin crossover, going from 7 in the HS state to 6 in the LS state, with the coordination/decoordination of a Fe-O bond and a change of the ligand conformation. 83 Let us notice that the relaxation temperature in systems involving concomitantly spin transition and electron transfer can be much higher than in purely SCO

I.2.4. How to increase T(LIESST)?

There is a first obvious tool to play on T(LIESST) that is T1/2. According to the "inverse energy gap law" already mentioned, increasing T1/2 decreases T(LIESST). This consists in moving the vibrational potential wells vertically in Figure I.6. Playing on the counter-ion or the number of solvated molecules for example, the switching temperature can be tuned, consequently affecting T(LIESST). This allows to move along one given T0 line, for a given molecule. However, this does not allow to move from one T0 line to another. To go from one line to the other at constant T1/2, so to increase T0, the denticity of the ligand seems to be a crucial parameter. This vertical displacement in the diagram seems to be linked to the rigidity of the ligands and to the deformation of the coordination sphere at the transition. Thus, compounds based on monodentate ligands are lying on T0 = 100 K while many of the bidentate ligands based complexes sit on T0 = 120 K, and tridentate ligands based complexes sit on T0 = 150 K; the highest value being attributed to macrocyclic complexes (T0 = 180 K).

Photocrystallography allowed to go beyond this vision based on the denticity of the ligands.

The first crystal structure of a sample in the hν-HS* state was obtained on the prototype compound [Fe(phen)2(NCS)2] in 2001 140 , followed by many others. One of the first hypothesis was that T(LIESST) was strongly correlated to the bond length change associated to SCO. Indeed, increasing this bond length change consists in separating the vibrational potential wells horizontally (in Figure I.6). However, this one-parameter description suffers from experimental evidences that other parameters than the bond length variation had to be taken into account. Indeed, studies using phosphorus ligand-based complexes that present larger metal-ligand bond-length variations [141][142] , have shown some photoswitching effects without any clear improvement compared to nitrogen-based ligands. 143 The most promising parameter that would favor long-lived metastable states is suspected to be the distortion of the coordination sphere. This follows from the non-isotropic bond-length variation upon the spin crossover (cf. multiscale view of SCO). 34,35 From structure-properties relationships, it appeared that T(LIESST) increases with the distortion of the coordination sphere. 73 A screening of some Fe 2+ SCO complexes clearly demonstrated that, provided similar bond lengths and crystal packings, the trigonal distortion of the coordination polyhedrons linearly varies with T(LIESST) (Figure I.10). For this family of compounds, this structureproperties relationships incidentally anticipated that the highest T(LIESST) should be around 125 K with ΔΘ = 145°. 73 This empirical relationship has been confirmed so far, and the highest T(LIESST) for an Fe 2+ molecular complex corresponds indeed to the largest observed polyhedron trigonal distortion ΔΘ. 73 Moreover, Hayami et al. suggested large molecular distortions around metal ions with the bending mode play an important role in increasing energy barrier. 144 They also suggested both stretching and bending modes must be considered when designing molecular compounds with high T(LIESST). Interestingly, theoretical calculation on the trigonal distortion latter confirmed the crystallography observation and notably the crucial role of Θ on T(LIESST). 74,145 The above results give a clear guideline for the design of SCO materials and reciprocally it validates the relevance of the T(LIESST) measurement. One of the most speaking example that illustrates the role of the coordination sphere distortion on the T(LIESST) is the study of the two polymorphs of the mononuclear [Fe(PM-BiA)2(NCS)2] complex (PM-BiA = N-2'-pyridylmethylene)-4-(aminobiphenyl)). 74,94,146 Polymorph I-S-BiA (S referring to the NCS co-ligand) exhibits a T(LIESST) at 78 K, the and II-S-BiA are 0.5 Å and 0.3 Å, respectively. For a given ΔrHL, the increase of Δζ results in an increase of the energy barrier. As a consequence, the photoinduced hν-HS* can be better stabilized at high temperature in polymorph I-S-BiA than in polymorph II-S-BiA, since the thermal energy needed to overcome the energy barrier during the relaxation from hν-HS* to LS is higher. 74 The role of the coordination sphere distortion in the lifetime of the hν-HS*implies that the single configurational coordinate with two simple two-dimensional potential wells (Figures I.5

I.3. Motivation of the PhD work

From the quick overview reported above on the parameters influencing the lifetime of the photoinduced state, it appears clearly that one has to work on the distortion of the coordination sphere. To this aim, a path that comes directly to mind is to use synthetic chemistry in order to introduce some steric strains close to the coordination sphere. In addition, as related above, polymorphism has clearly demonstrated that the crystal packing can influence the coordination sphere distortion and cooperative interactions can stabilize the hv-HS*. Therefore, our first chosen strategy was to synthesize new ligands to promote either steric strain close to the coordination sphere. And the other strategy was to regulate the intermolecular interactions by using polymorphs and solvatomorphs. To do so, we focused our attention on the [Fe(PM-L)2(NCE)2] family that has been synthesized first more than 20 years ago in the group. 69 This family of ligands was initially designed to promote π-π interactions between the aromatic plateaus. However, additionally to these desired interaction, strong S---H and H---H interactions are also present, building a dense network of intermolecular interactions and leading to a wide range of cooperativity and polymorphism (Figure I.14). 34 Polymorphism is the possibility for the same chemical entity to crystallise by adopting different crystal structures.

Polymorphs have strictly the same chemical formula but are distinguished by theircrystal packing and therefore, potentially, by their physical properties. From the chemical point, polymorphs may be prepared by changing the reactions conditions, including the temperature, solvent, and pressure. The two polymorphs of [Fe(PM-BiA)2(NCS)2] discussed above (Figure I.11) also belong to this family. More recently it has been shown that polymorphism can also be found in [Fe(PM-PeA)2(NCSe)2], where polymorph I-Se-PEA (Se referring to the NCSe coligand) shows abrupt SCO with a large hysteresis associated with a structural phase transition 90 , while polymorph II-Se-PEA displays a gradual SCO 147 . In addition to polymorphism, structural phase transitions with change of crystal symmetry can also occur in this family. Notably, the latter occurs in a non classical way in [Fe(PM-PEA)2(NCS)2] 148 and in polymorph I-Se-PEA of [Fe(PM-PeA)2(NCSe)2 90 since there is an increase of symmetry when lowering the temperature from P21/c in the HS state to Pccn in the LS state. Not fully explained so far, this unusual feature in the whole field of molecular crystals illustrates the diversity and the complexity of behaviors that can be encountered in this SCO family. The present work takes part to a larger project aiming at the design of SCO compounds with

high T(LIESST) values. Our main goal is to focus on a deep and comprehensive understanding of the relevant parameter(s) that can allow to reach high T(LIESST). The general idea is to synthetize and carefully study new compounds presenting high distortion

of the coordination sphere. Our playground is the above [Fe(PM-L)2(NCE)2] family since taking advantage of the well documented existing structure-properties relationships and the diversity of behaviors that it exhibits we will try to design new and optimized compounds.

This family has been shown to be extremely rich in terms of behaviors and in terms of possible functionalization (Figures I. 13 and I.14). It will be used as a platform to promote high distortion of the coordination sphere through the substitution in ortho position of the imine part (Figure I.15). Since the steric strain can prevent the coordination to occur, halogen substituent of various atomic radius will be studied. Finally, different positions of the substitution will be explored to relax progressively the steric strain. We expect that the shorter distance of Fe---X will favor a stronger distortion of coordination sphere. To follow this strategy we have decided to work with halogen functionalization, especially from the small atomic radius of the Fluor atom to the large one of Bromine atom. We have thus played on three different levers. First, by moving the halogen group from para, to meta and ortho positions, the steric strain should induce a significant increase of the distortion of the coordination sphere, which can favor a long lifetime of the photoinduced state. Second, the delicate influence of intermolecular interactions on the distortion of poyhedron can be explored when polymorphism appears in one of these complexes. In addition, the introduction of different halogen groups is expected to strongly affect the intermolecular interactions due to the difference in electronegativity.

Halogen atoms have already been introduced in several families of SCO complexes. For , where dbftpy, dctpy and dftpy are the di-bromo, di-chloro and di-fluoro substituted tripyridine ligands in ortho positions. [154][155] Halogen-Halogen interactions might also promote cooperative transition, like in the [Fe(qsal)2][(C6F3I3)X] compounds. [156][157] In this manuscript, we will describe the synthesis, cristallographic and (photo)magnetic studies of new compounds of the studied family based on ortho, meta and para fluoro and bromo substituted PM-zXA ligands.

In Part II, synthesis, structure, magnetic and photomagnetic properties of new complexes with fluorine substitution at ortho, meta, and para position will be described. The structural properties from the scale of the coordination sphere to crystalline stacking will be displayed.

In Part III, new compounds with bromo-substituted ligands showing polymorphism will be described and compared. Especially, high T(LIESST) and unique unit-cell volume change in one polymorph will be shown and the photo-crystallography of this compound will be also explored via PXRD at low temperature, giving rise to the structure-properties relationship. The structural analysis in a multiscale approach will be discussed and compared in all polymorphs to draw structure-properties relationships.

Finally, the deep examination of the relevant parameters to high In this part, we will described the synthesis of the fluorine-substituted ligands PM-zFA and the related [Fe(PM-zFA)2(NCS)2] complexes. Fluorine functionalization was first explored due its small size. Indeed, Fluor is one of the smallest atom of the periodic table. Therefore, a substitution using this atom might promote strain close to the coordination sphere without preventing this coordination, especially with a functionalization in ortho position of the aniline ring. Moreover, Fluor possesses a large electronegativity which may provide F---H hydrogen bonding in the solid, which could help to bring cooperativity in the materials.

Targeting the above expectations, three ligands with different mono-substituted fluoroaniline groups were first synthesized and then three corresponding iron complexes were crystallized and studied. Their magnetic properties as well as their crystal structures, when accessible, are described, using the multiscale approach described in Part I.1.2.

Part

Synthesis of N-(2'-pyridylmethylene)-4-fluoroaniline (PM-pFA)

A solution of 4-fluoroaniline (0.94 ml, 10 mmol) in diethylether (50 ml), was added to pyridine-2-carbaldehyde (0.95 ml, 10 mmol) in diethylether (50 ml). Then magnesium sulfate (5g, MgSO4) was added in order to absorb the reaction water and the yellow solution was stirred overnight under argon atmosphere. The solution was filtered and the magnesium sulfate washed with CH2Cl2 (5 ml), in order to remove all the products. The solvent was removed under vacuum by rotary evaporator. The ligand was recrystallized in a mixture of CH2Cl2 (10 ml) and petroleum ether (30 ml). After one day at -25 the bulk-yellow crystal was filtrated and recrystallized one more time.

The crystals obtained upon recrystallization were good enough to record single-crystal Xray diffraction (SCXRD) data and to determine the crystal structure of PM-pFA ligand. At 300 K, the PM-pFA ligand crystallizes in the monoclinic system with space group P21/m (Table II.1). All atoms site on the general Wyckoff position (4f) except for C13, C16, C18, C21 and

Synthesis of N-(2'-pyridylmethylene)-3-fluoroaniline (PM-mFA)

Using the same procedure, ligand PM-mFA was synthesized using 3-fluoroaniline. After evaporation of diethylether, only light yellow oil was formed. Different solvents were used for crystallization, however nor single-crystals nor powder were obtained.

Synthesis of N-(2'-pyridylmethylene)-2-fluoroaniline (PM-oFA)

Using the same procedure, the ligand PM-oFA was synthesized using 2-fluoroaniline. The ligand was recrystallized in a mixture of diethyl ether (10 ml) and petroleum ether (30 ml).

After one day at -25 , the colorless solid was filtrated and recrystallized one more time. The quality of the crystals was not good enough to record data and determine the crystal structure.

The powder X-ray diffractogram (Figure II.2) strongly differs from the PM-pFA one, suggesting a different crystal structure.

II.2.2. Synthesis of the [Fe(PM-zFA)2(NCS)2] complexes

All the solvents were treated in order to remove the oxygen in solution before use. Methanol was distillated while diethylether, acetonitrile and dichloromethane were frozen and unfrozen in liquid nitrogen three times. The reactions and crystallizations were performed under argon atmosphere.

A first step consisted in preparing the Fe(NCS)2 reactant. 1 mmol of FeSO4.7H2O and 2 mmol of KNCS were dissolved in 20 ml of methanol each, in presence of few ascorbic acid.

After stirring 2 hours, the resulting solution is filtered under nitrogen atmosphere to remove the K2SO4 formed during the reaction.

As a common method for the synthesis of complexes, we used the layering technique for crystallization, where two solvents containing ligand and metal salt, respectively, diffuse and crystallize slowly at the intermediate layer.

Synthesis of [Fe(PM-oFA)2(NCS)2]

Owing to the very soluble nature of PM-oFA, different solvents were tried for crystallization, including diethylether, dichloromethane, methanol, and chloroform. However, no crystals were formed. After evaporating the solvents (diethylether and methanol), dark solids were obtained,

Synthesis of [Fe(PM-mFA)2(NCS)2]

In this case, 2 ml of diethylether containing 0.2 mmol of ligand (the oily ligand being taken using a syringe) is layered on top of 2 ml of the Fe(NCS)2 methanol solution (0.05mmol/ml) and blue needle crystals were formed in a dark blue solution at 25 after one week, later referred to as I-mFA hereafter.

Synthesis of [Fe(PM-pFA)2(NCS)2]•nCHCl3

When 2 ml of the freshly prepared Fe(NCS)2 solution (0.05mmol/ml) is layered on top of 2 ml of chloroform containing 0. shows the thermal evolution of the χMT product (χM being the molar magnetic susceptibility and T the temperature). The χMT value at 320 K of 3.37 cm 3 K mol -1 as expected for a HS Fe 2+ .

Upon cooling, the χMT product gradually decreases to reach a value close to 0.51 cm 3 K mol -1 below 70 K, suggesting a LS state with the presence of some paramagnetic impurity or some HS residue. So complex I-oFA shows a gradual and slightly incomplete SCO and a thermal switching temperature around 215 K. Since the crystal structure is missing, we suppose that there is no solvent in powder samples and the formula weight of 572.43 g/mol is adopted for the data treatment.

An irradiation at 830 nm was applied for 1h at 10 K, leading to about 20 % of photoswitching. The T(LIESST) measurement performed after this photo-excitation leads to a T(LIESST) value of 55 K. The χMT value at 300 K of 3.45 cm 3 K mol -1 , agrees well with a HS Fe 2+ ion (g = 2.14). Upon cooling, the χMT product gradually decreases to reach a value close to 1.91 cm 3 K mol -1 at 150 K, showing a large plateau between 30 K and 150 K. The decreasing value of χMT corresponds to ~50% of χMT value at 300 K. The slight decrease of χMT below 20 K may be due to the zero-

Crystallographic investigation of I-mFA

SCXRD data were recorded at 100 K, 200 K and 300 K and the crystal structure determination and refinements were performed using the Olex2 software 9 . The detailed experimental and structural data are shown in Table II.2.

At 300 K, the complex I-mFA crystallizes in a monoclinic system with C2/c space group.

The a parameter of 50.7003(10) Å is unusually large for Fe 2+ molecular complexes and even in the general context of small molecule based crystals. This gives rise to a huge unit-cell volume of 10444.1(3) Å 3 (Table II.2). There are two independent crystallographic Fe 2+ ions in the asymmetric unit with all atoms on the general Wyckoff position (8f), labelled hereafter Fe1 and

R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2
Upon slow cooling to 100 K, the crystal-packing of I-mFA remains in the same C2/c space group. Compared to 300 K, all the unit-cell parameters become smaller (Table II.2). There are also two molecules in the asymmetric unit with all atoms on the general Wyckoff position (8f).

As seen in Figures II. 7a and 7b, the Fe-N bond distances in Fe1N6 are longer than that in Fe2N6.

The average value of Fe-N distances (<Fe-N>) are 2.171(4) Å and 1.960(4) Å for Fe1 and Fe2, respectively, which are the typical values for HS and LS Fe 2+ ions, respectively. This means a mixture of HS-LS motifs in the crystal packing at 100 K with a 50% ratio. molecule describing less linear NCS -groups in Fe2. However, they are bending towards each other leading to a shorter S3---S4 (6.058 Å) distance in Fe2 compare to S1---S2 (7.859 Å) in Fe1 motif. In order to describe the N-Fe-N angles, here, σ1 is defined with nitrogen coming from imine groups, and σ2 with nitrogen coming from pyridine. In addition σ3 is defined as the dihedral angle between the phenyl and pyridine rings of one ligand, in order to show its possible change in planarity. These definitions of σ1, σ2 and σ3 apply in all the thesis. The deviation of σ1 and (SC)N-Fe-N(CS) angles from the expected 90° in a perfect octahedron clearly shows that Fe1 is much more distorted than Fe2. On the other hand, the difference in σ2 values translates the cis conformation of the ligand in Fe2 compare to the trans one in Fe1. In the trans stereoisomer Fe1, the planarity of the ligands described by σ3 is higher than in Fe2 to avoid short atom-atom contacts. Moreover, in Fe2, one ligand presents a σ3 of 89.12° illustrating the orthogonality between the fluoro-phenyl and pyridine rings. Apparently, this high bending of NCS -does not favor the inactive of SCO. The ligand filed is depending not only on the bending

Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family

of NCS -but also on the conformation of PM-mFA ligands. Here, the small value of σ3 in Fe1 site indicates that planarity of the ligands favors the weak ligand filed.

Table II At 100 K, the average Fe1-N distance (<Fe1-N) becomes 2.171(4) Å, which slightly decreases compared to 2.180(5) Å at 300 K. The distortion parameters ζ and Σ decrease a little while Θ and shape increase a little from 300 K to 100 K. The tiny variation of distortion parameters result from the small variation in σ1, σ2 and σ3, which agrees well with the observed small RMSD value. Concerning Fe2, all distortion parameters (ζ, Σ, Θ, shape) are smaller at 100 K than at 300 K, owing to SCO. Indeed, the Θ value is 133° at 100 K, leading to the variation of trigonal distortion angles (ΔΘ) of 66°. This variation is mainly due to the decrease of (SC)N-Fe-N(CS) angle, which corresponds to the motion of NCS -groups (Figure II.7b).

All these motions have necessary an impact on the intermolecular interactions. Figures II.9a

and b, show the fingerprints and Hirshfeld surfaces 11 calculated by the CrystalExplorer software 12 . There are huge differences in the fingerprint shapes of the two molecules within the packing. At 300K, the ratio of F---H in Fe1 molecules (13.3%) is much higher than in Fe2 II.2) that is quite uncommon for this family of molecular complexes. andd). The ratio of the selected S---H, F---H and C---C distances in Fe1 are higher than in Fe2. Compared with fingerprints at 300 K, the variation of S---H in Fe1 fingerprint decrease by 1.1% while it increases by 0.9% in Fe2, which might be due to the motion of the NCS -groups in Fe2 at SCO. The ratio of F---H in the fingerprints slightly decrease from 13.3% to 12.9% for Fe1 and from 8.6% to 8.1% for Fe2.

Structure-properties relationship of I-mFA

Two crystallographically independent molecules are present in complex I-mFA, with totally different ligand conformations. Indeed, one of the peculiar point of this compound, from the structural point of view, is the half-half mixture of cis and trans molecular complexes within the crystal packing. This feature is quite rare in Fe 2+ SCO compounds. Upon thermal treatment:

Fe1(trans) is spin crossover inactive while Fe2(cis) shows a complete SCO. The fact that Fe1

does not undergo a SCO can be linked to the huge distortion of the Fe1N6 coordination sphere at 300 K. The whole results in an incomplete SCO, as confirmed by magnetic measurements,

Crystallographic investigation of I-pFA

The structure of I-pFA was determined by SCXRD at 300 K and 100 K. Due to the large unit-cell volume and the weak intensity of SCXRD peaks, the final structural refinement is good but not excellent as shown by relatively high values of wR2. As seen in Table II.4, complex I-pFA crystallizes in the trigonal system with space group R-3 at 300 K. All atoms site on the general Wyckoff position (18f). One can notice the huge unit cell with a volume of 30177( 9) Å 3 , which already makes this compound as quite peculiar in this family of SCO complexes and, as a general matter, in Fe 2+ SCO complexes.

At 300 K, there are two independent Fe II.5), respectively, which are typical for Fe 2+ in HS state, agreeing with the magnetic measurement. Moreover, the RMSD value resulting from the overlay of Fe1 and Fe2 molecules is high (0.333 Å), the main difference originates from the bending and orientation of NCS - groups. In the asymmetric unit, the two chloroform are partially occupied with crystallographic occupancies of 0.62 and 0.31, respectively. Since all atoms site on the general Wyckoff position, the average value of solvent per molecular complex is 0.47, which is the average value of 0. Table II

Structure-properties relationship of I-pFA

Compound I-pFA exhibits a two-step spin crossover that is consistent with the presence of two Fe 2+ crystallographic sites within the crystal packing. The global feature of the SCO curve is gradual with steps at 100 K and 140 K. Since a partial HS to LS conversion for Fe1 (~65% LS) and a fully LS state for Fe2 at 100 K, the two steps character of the curve could have its origin in the different interaction networks adopted by each Fe 2+ site, the solvent molecules being suspected to play an important role here. The two disordered chloroform molecules present in the asymmetric unit develop rather different connections within the crystal packing.

The C(53)HCl3 one only connects with Fe2 while the crown-shaped C(54)HCl3 is linked to both Fe1 and Fe2 via S---H and Cl---H, respectively. When decreasing the temperature, Fe2 is more likely to experience the spin crossover first with a huge modification of S4---H53 distance.

After that, Fe1 starts to switch upon further cooling, accompanied with the modification of Cl5---Cl6 and Cl6---H41. However, the gradual character of both steps, does not provide such clear picture of the steps.

Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family

81

From 300 K to 100 K, the solvent is partially lost from 0.47 to 0.32 occupancy per iron within the crystal. This loss may be related to the experimental condition. Indeed, the possible loss of solvent at 300 K during the measurement as well as the nitrogen flow that cool the crystal during collecting data at 100 K might be responsible of this loss of solvent. It is therefore hard, if not imprudent to draw any conclusions from that loss. The incomplete SCO observed during This gives some clues on the conformation of the ligands in complex I-oFA. If we only look at the spin crossover temperatures, we expect an increase with a weakening of the coordination sphere distortion. Since complex I-oFA show the same switching temperature than complex I-mFA, at first sight, the distortion should be the same in both. However, if we look at the position of these compounds on the T(LIESST) vs T1/2 database, we mentioned that complex I-oFA lies on the T0 = 120 K line while complexes I-mFA and I-pFA lay on T0 = 100 K line. If the hypothesis of the increase of T0 related to an increase of ΔΘ is true, therefore one may expect a higher variation of the trigonal distortion in I-oFA than in the other compounds. It could mean that the strategy of promoting the steric strain by adding substituents close to the coordination sphere is successful. However, without crystal structure we cannot be sure. Moreover, we cannot exclude ligand rotation to increase Fe---F distance in I-oFA to minimize the steric strain. In complex I-pFA, the pyridine rings are in trans conformation in both Fe 2+ sites. If we compare with the Fe1 site of complex I-mFA that presents also a trans conformation of the pyridine groups, the distortion parameters are not changed. It means that this conformation allows the fluorine groups to be far enough from the coordination sphere to do not affect the distortion and the switching properties. The differences in behavior might come first from the incomplete SCO observed in complex I-mFA and the different crystal packings, especially because of the presence of chloroform solvent molecules in complex I-pFA. It is important to notice that spin-crossover compounds with high symmetry of R-3 are not common. For example, the molecular complexes [Fe(ptz)6(BF4)2] (ptz = 1-propyl-tetrazole) 15 and

[Fe(btzb)3](PF6)2•solv (btzb = 1,4-bis(tetrazol-1-yl)butane; solv = 0.5 MeOH or H2O) 16 crystallize in the trigonal system.

Moreover, complex I-pFA can be viewed as a nanoporous molecular materials since the 1D pore along the c axis is larger than 10 Å. Nanoporous materials are usual found in the metalorganic frameworks or covalent-organic frameworks. Actually, the SCO affected by the solvent have already been reported in many framework compounds. [17][18][19][20][21][22][23][24] For example, the abrupt SCO demonstrates the pore expansion owing to the water adsorption results in a progressive stabilization of the HS state by the analysis of the structural and dynamical properties of the framework, with a consequent decrease of T1/2. 20 The nanoporous metal-organic framework [Fe2(azpy)4(NCS)4] (guest) (azpy is trans-4,4-azopyridine) displays the reversible uptake and release of guest molecules, causing substantial changes in the local geometry of the iron(II) centers. 24 Nanoporous molecular materials have been reported in some organic molecules, where only weak non-covalent interactions exist instead of the directional covalent or coordination bonds. [25][26][27][28][29] It is very interesting that complex I-pFA display SCO with the nanoporosity and Especially, in complex I-mFA with an incomplete SCO, the nearer F---Fe distance in Fe1 leads to a stronger distortion that favors the HS state compare to the farer F---Fe distance in Fe2 that leads to a weaker distortion that favors the occurrence of SCO. Moreover, complex I-pFA provides a rare example of nanoporous structure with the high symmetry (R-3). Unfortunately, the structure of complex I-oFA is unknown due to the absence of single crystal sample even if we can suspect a higher variation of the distortion parameters in this compound compared to the other ones.

Two interesting and unprecedented phenomena occur here. Firstly, the coexistence of cis and trans conformations of the pyridine groups in two different Fe 2+ sites in complex I-mFA.

Secondly, an unusual high symmetry is observed in complex I-pFA together with nanoporosity. adapting the literature procedure. 1 Scheme III.1. Reaction scheme of the N-(2'-pyridylmethylene)-n-bromoaniline synthesis (n = 2,3,4).

• Synthesis of N-(2'-pyridylmethylene)-4-bromoaniline (PM-pBrA)

A solution of 4-bromoaniline (1.72 g, 10 mmol) in diethylether (50 ml), was added to pyridine-2-carboxaldehyde (0.9512 ml, 10 mmol) in diethylether (50 ml). Then magnesium sulfate (5g, MgSO4) was also added in order to absorb the reaction water. After that, the yellow solution was stirred overnight under argon atmosphere. The solution was filtered and the magnesium sulfate washed with CH2Cl2 (5 ml), in order to remove all the products. The solvent was removed under vacuum on rotary evaporator. The ligand was recrystallized in a mixture of CH2Cl2 (10 ml) and petroleum ether (30 ml). After one day at -25 , the pale-yellow crystals were filtrated and recrystallized one more time.

The crystals obtained upon recrystallization were good enough to record SCXRD data and to determine the crystal structure of the ligand (Table III.1 and Figure III.1). At 300 K, the PM-pBrA crystallizes in the monoclinic system with space group P21/a, which agrees with the recently reported result. 2 The synthesis protocol in this literature is different from the method described above, where the ethanol solution containing reactants was refluxed at 60 °C for 6 hours and crystals were grown through a slow evaporation of an ethanol solution at 25 . The dihedral angle between phenyl ring and pyridine ring is only 17.94° within the ligand. This

R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2

Synthesis of N-(2'-pyridylmethylene)-3-bromoaniline (PM-mBrA)

Using the same procedure, ligand PM-mBrA was synthesized. Only petroleum ether was used for recrystallization because the ligand is very soluble in polar solvents. The crystals are colorless flakes. It is difficult to perform the SCXRD because the crystal can slowly dissolve in the oil used for mounting the crystals. Actually, the crystals of PM-mBrA can only be stable at low temperature. After removing the ligand from the fridge, it will quickly turn into liquid at room temperature.

Synthesis of N-(2'-pyridylmethylene)-2-bromoaniline (PM-oBrA)

Using the same procedure, ligand PM-oBrA was synthesized. However, it leads to a yellow oil whatever the crystallization method used.

Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 95

III.2.2. Synthesis of the [Fe(PM-zBrA)2(NCS)2] (z = o, m, p) complexes

All the solvent were treated in order to remove the oxygen in solution before use. Methanol was distillated while diethylether and dichloromethane were frozen and unfrozen in liquid nitrogen three times. The reactions and crystallizations were performed under argon atmosphere.

A first step, common to all the complexation reactions, consisted in preparing the Fe(NCS)2 reactant 1 mmol of FeSO4.7H2O and 2 mmol of KNCS were dissolved in 20 ml of methanol each, in presence of few ascorbic acid. After stirring 2 hours at 25 , the resulting solution is filtered to remove the K2SO4 formed during the reaction.

As a common method for the synthesis of the various complexes, the layer by layer technique is often used for crystallization, where two solvents containing ligand and metal salt, respectively, diffuse and crystallize slowly at the intermediate layer. Details for each compound (-o, -m, -p ones) are given hereafter.

Synthesis of [Fe(PM-oBrA)2(NCS)2]

Owing to the very soluble nature of PM-oBrA, different solvents were used for the layering technique, including diethylether, dichloromethane, methanol, and chloroform. However, no crystals were formed and the color of solution went dark blue. Upon slow evaporation of these solutions, a powder is obtained, related to compound I-oBrA. Elemental analysis gives: C: The dropwise addition of 2 ml of Fe(NCS)2 methanolic solution (0.05mmol/ml) into 2 ml of dichloromethane containing 0.2 mmol of ligand, followed by the addition of 6 ml of diethylether, leads to a mixture of dark block and needle crystals after one week at 25 . This The magnetic measurements were performed on a powder sample, using a SQUID magnetometer (see Appendix 3). Figure III. 3 shows the thermal evolution of the χMT product.

The χMT value at 300 K of 3.75 cm 3 K mol -1 , indicating the fully HS state. Upon cooling, the χMT product gradually decreases to reach a value close to 0.48 cm 3 K mol -1 below 50 K, suggesting a LS state. No thermal quenching could have been observed. This compound exhibits a gradual and almost complete SCO, with a switching temperature around 220 K. An irradiation at 830 nm was applied for 1h at 10 K, leading to less than 30 % of photoswitching.

The T(LIESST) measurement performed after this photo-excitation gives a T(LIESST) value of 50 K. Both T1/2 and T(LIESST) are very similar to the ones observed in I-oFA. Since the crystal structure is missing, we suppose that there is no solvent in I-oBrA and the formula weight of 694.25 g/mol is adopted for the data treatment. Table III.2. Crystal data and structure refinement data for polymorph I-mBrA.

Empirical formula C26H18Br2FeN6S2 Formula weight / g mol -1 694.

R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2
There are one and a half molecules in the asymmetric unit, where Fe2 atom sits on the special Wyckoff position (4e) while Fe1 sits on the general Wyckoff position (8f). For In many cases, when structural phase transition and SCO occur, both event are concomitant. [7][8][9] However, they can also occur at different temperatures. For example, the spin crossover and the structural order-disorder phase transition from R-3 to P-1 are in fact two distinct phenomena in [Fe(ptz)6](BF4)2. 10 In complex [{Fe(3-bpp)(NCS)2}2(4,4'bypiridine)]•2CH3OH, a symmetry breaking occurs around 161 K in the HS region. 11 In polymorph I-mBrA, the symmetry breaking therefore drives the compound from a C2/c space group with two crystallographically independent Fe 2+ sites at 300 K to a P21/c space group with three independent molecules at 80 K, without any spin state changes. This indicates the structural phase transition here is probably a pure thermal effect for this compound. The high distortion of the coordination sphere is probably responsible for the absence of SCO in this polymorph. The structural transition weakly affects the packing and has almost no effect on the coordination sphere distortion, keeping it locked in HS.

If we compare this compound with complex I-mFA, there are huge differences regarding first the ligand conformation. In complex I-mFA the pyridine groups are either in cis

Crystallographic investigation of II-mBrA

Data were recorded and the crystal structures were determined at 150 K and 300 K by SCXRD. As seen in Table III The intermolecular interactions were explored with the fingerprints and the Hirshfeld

Structure-properties relationship of II-mBrA

From the structure described above, polymorph II-mBrA crystallizes in the same space group C2/c in HS and LS with a half Fe 2+ molecule in the asymmetric unit. It exhibits a gradual spin crossover at T1/2 = 163 K. This spin crossover is associated to very small modifications of the coordination sphere since the angular and trigonal distortions vary only of 20° and 40°, respectively. This might be at the origin of the impossibility to record a long-lived HS* induced by light or thermal quenching.

Regarding the intermolecular interactions, the S---H ones are not obvious, compared with the other compounds, even if they contribute to around 24 % in fingerprints, the shortest S---H contacts are larger than 3.0 Å. The Br---H and πphenyl---πphenyl contacts are much weaker than H---π interactions. The weak modification of interactions at SCO results in the gradual SCO.

In this respect, this polymorph adopts a fairly widespread behavior in Fe 2+ complexes with a structure-properties relationship based on smooth structural modifications leading to a gradual SCO. [12][13] Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 114

III.3.4. Discussions on I-mBrA and II-mBrA

As described above, the two polymorphs crystallize in the same space group C2/c at 300 K.

However, complex I-mBrA does not exhibit any SCO while it is experiencing a structural transition upon cooling with a symmetry breaking that leads the compounds into a P21/c space group. On the contrary, complex II-mBrA is SCO active and down 150 K it does not exhibit any structural phase transition.

At 300 K, there are two crystallographically independent molecules with different ligand conformations in complex I-mBrA, one with Br next to S atom (Fe2) and the other one with Br far from S atom (Fe1). In complex II-mBrA there is only one Fe site that is rather similar to the Fe2 site in complex I-mBrA. Despite this similarity, there are huge differences between the corresponding molecular geometries at 300 K as revealed by the overlay (Figure III.17). Firstly, the distortion of the coordination sphere in polymorph I-mBrA is much stronger than that in II-mBrA, in agreement with the stabilization of the HS state and the absence of SCO event in complex I-mBrA. Secondly, the bending of NCS -groups lead to a large angle of (SC)N-Fe-N-(CS) in both polymorphs (around 105° in I-mBrA and 97° in II-mBrA) at 300 K (Tables III.3

and 5) but show a clear difference between them. The higher bending in complex I-mBrA can be correlated with the higher distortion in this polymorph. Rodríguez-Jiménez found that in these complexes the HS state is favored when the Fe-NC-S angle is lower than 154°. 14 Large bending of thiocyanate ligand can increase the distortion of FeN6 polyhedron and then favor the HS state. In the literature, four complexes with Fe-N-CS lower than 159° are reported to show SCO. In these complexes, the thiocyanate ligands are all in trans configuration. In polymorph I-mBrA, the angles around 158° at 300 K are close to the 154° value proposed by Rodriguez-Jimenez, and the compounds is SCO inactive in agreement with such value. However, in polymorph II-mBrA the Fe-NC-S angle is of 145° and we could expect a HS compound too, which is not the case. Indeed, despite the huge bending of the NCS group, the global distortion of the coordination sphere is week (Θ < 200° in II-mBrA), leading to the occurrence of SCO while it was not expected if we would have only looked at the Fe-NC-S angle.

If we look at the Br---Fe distances in both polymorphs at 300 K (Table III.6) we see that in polymorph I-mBrA the bromine are at 6.99 Å and 6.08 Å from Fe1 center and at 5.95 Å from Fe2 center. In polymorph II-mBrA where the iron site resembles to Fe2 of polymorph I-mBrA, the Br---Fe distance is of 5.97 Å, similarly to the Fe2 site in polymorph I-mBrA. If we compare these distances to the ones in complex I-mFA (around 5.65 Å in Fe1 that is SCO inactive and 6.27-6.53 Å in Fe2 that is SCO active), we can expect Fe2 in polymorphs I-mBrA and Fe1 in II-mBrA to be SCO inactive with the argument that the shorter the distance, the higher the steric strain, the higher the distortion, the more the HS state is stabilized. The occurrence of SCO in polymorph II-mBrA clearly invalidates this hypothesis. Moreover, the bigger Halogen---Fe distances in Fe1 site of polymorph I-mBrA that does not experience any SCO is opposite to the observation made in I-mFA. Moreover, if we look at the atomic radius of fluorine (0.64 Å) and bromine (1.14 Å) 15 , we see that the difference of 0.5 Å between both halogens is not recovered in the three compounds. Indeed, the mean variation from fluorine to bromine in these compounds is around 0.3 Å, possibly indicating a more constrained coordination sphere in the bromine derivatives. This is the case for polymorph I-mBrA but not for II-mBrA. We can see from the distortion parameters and that the SCO active sites have very similar values while the SCO inactive sites have also similar values, higher than for the SCO active sites. Therefore, the origin of SCO activity might come from an upper scale than just the coordination sphere.

For instance, the density of polymorph I-mBrA (1.606 g cm -3 ) is larger than 1.545 g cm -3 in polymorph II-mBrA, which may indicate that the more dense packing in polymorph I-mBrA may restrict the molecular geometry and then block the HS state. We cannot really compare with the density of complex I-mFA (1.456 g cm -3 ) since the smaller fluorine group might intrinsically reduce the density. Interestingly, I-pBrA shows the highest T(LIESST) obtained so far of the PM-L family (Table I.1). [17][18] This characteristic represents a singular feature of this compound that we tried to understand from the crystallographic point of view.

Crystallographic study of I-pBrA

Variable temperature single-crystal X-ray diffraction (SCXRD) was performed from 80 K to 350 K, including 80, 100, 120, 150, 170, 180, 200, 250 and 300. The thermally quenched Q-HS* has been measured at 80 and 10 K, the latter was performed in collaboration with Dr.

Sébastien Pillet in the University of Lorraine (Nancy, France). At all the measured temperatures I-pBrA crystallizes in the monoclinic system with the space group of P21/n (Table III.7).

In the asymmetric unit, all atoms sit on the general Wyckoff position (4e) and there are two

R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2
At 80 K, all the coordination sphere parameters become very close in both sites (Table III

.8).

This is particularly the case for σ3: 67° and 55° in Fe1 and 56° and 57° for Fe2. The The thermally quenched structure was also determined at 80 K. The compound crystallizes in the same monoclinic space group P21/n with two molecules in the asymmetric unit (Table III In order to evidence the variation of the ligand geometries at SCO, the superposition of This is evidenced by the evolution of the dihedral angle between the pyridine and bromophenyl groups of the same ligand, σ3. In Fe1, from HS to LS, σ3 varies of -8° and +2° for the two ligands while in Fe2 site, it varies of -23° and +27° (Table III features to be stepped. 19 Obviously, it is not the case for polymorph I-pBrA and additionally, it somehow rules out the influence of Σ on the T(LIESST) as already suspected. 20 Thanks to the fact that the HS and Q-HS* structures are similar and that we have access to the LS and Q-HS* structures at the same temperature, 80 K, we can calculate the variation of the distortion parameters avoiding the effect of thermal contraction (Table III.9). One can immediately see that Fe2 experiences much bigger variations with a huge of 122°. However, the difference between Fe1 and Fe2 does not induce any step in the SCO features. Moreover, with a substitution in para position, we do not expect to exert a priori any steric strain on the coordination sphere and to increase the distortion. We have therefore explored the origin of these huge at a bigger scale within the crystal, starting by the intermolecular contacts.

Part The variation of ligand geometry can influence the intermolecular interactions and conversely, changes in intermolecular interactions can influence the ligand geometry. 19,21 We analyzed the intermolecular interactions using the fingerprint derived from the Hirshfeld surface in order to understand the origin of the rotation of the ligands and the high distortion of the coordination sphere.

Unit-cell parameters as a function of temperature of I-pBrA

As a further consequence of all these rearrangements in the ligand conformation and atomatom contacts the unit cell is strongly modified at SCO in a unique so far manner. Indeed, the unit-cell volume increases from HS to LS (Figure III.31): from 300 K to 170 K, the unit-cell volume decreases gradually due to thermal contraction while it increases abruptly from 170 K to 160 K at the SCO, this behavior being reversible upon warming. It is worth to mention that, to the best of our knowledge, it is the first time that the thermal SCO of a molecular complex corresponds to an increase of the unit-cell volume from HS to LS. 19,22,23 . In the SCO temperature region, the increase of volume from HS to LS results in fact from the strong anisotropy of the unit-cell modifications. As shown in Figure III.32, when cooling from 150 K to 200 K, the value of a decreases by 4.40% while c and β increase by 6.55% and 0.83%. If the anisotropy of the unit-cell modifications due to the SCO is a usual fact, it may be the first time that a LS to HS conversion results in a negative expansion. 19,22,23 The latter amplitude is accurately determined by comparing the volumes of the HS and LS unit-cells at the same temperature, the HS one is obtained by thermal quenching and the LS one by slow cooling. At

Powder X-Ray diffraction Photo-crystallography of I-pBrA

To have better insights on the photo-induced process from LS to h -HS*, photocrystallographic investigations were performed. However, the ease with which the HS is quenched below 80 K adds extra difficulties to perform the SCXRD photocrystallography study at very low temperature. For technical reasons, it is easier with PXRD. Indeed, with our From these diffractograms, the unit cell parameters can be extracted using the Le Bail method. 24 (Table III A one step SCO with two very different Fe 2+ sites. Basically, SCO in molecular complexes is greatly related to the crystallographically independent iron molecules in the asymmetry unit.

Since differences in the coordination sphere geometry lead to differences in the ligand-field, it is consequently expected different spin crossover temperatures. Moreover, the environments around each iron sites can be very different leading to various cooperativity around each site.

Here, despite the presence of two very different iron sites in polymorph I-pBrA and very different modifications of intra-and intermolecular interactions involving each site, only one step is observed in the SCO curve. There are few similar examples. For instance,

[Fe(LH iPr )2(NCS)2] (LH iPr = N-2-pyridylmethylene-4-isopropylaniline) shows an abrupt spincrossover (SCO) at T1/2↓ = 154 K on cooling and at T1/2↑ = 167 K on heating, with a 13 K thermal hysteresis, while two distinct molecules are observed. 25 Another example is found in a linear trinuclear triazole-based complex with the central site different from the two external sites of the linear trimeric unit. The three Fe 2+ centers undergo a very sharp SCO at 318 K, probably due to the strong ferro-elastic interactions between the three metal centers. 26 On the contrary, dinuclear complex {[Fe(bt)(NCS)2]2(bpym)} with one crystallographic iron sites shows two-stepped SCO, due to antiferro-elastic interactions. 27 The type of SCO is related to the crystallographic sites but is determined by the antiferro-elastic and ferro-elastic interactions. So it is reasonable to assume that the ferro-elastic interactions exist between Fe1 and Fe2 in polymorph I-pBrA. Note that it is more common that one iron site breaks into more sites in multistep SCO complexes. [28][29][30] A huge reversible rotation of ligand from LS to HS* as shown in the structure in HS, Q-HS* and LS states: one phenyl ring in Fe2 rotates by around 67° at SCO. Such an impressive rotation occurs in the solid state and was definitely demonstrated to be reversible. Rare but similar, though smaller, ligand rotations have been reported in some cases for SCO compounds but the concomitance of the rotation and the SCO was not fully demonstrated. 25,31,32 Based on the results of powder X-ray diffraction photo-crystallography (Table III sphere distortion is that the chemical substitution in para position of the phenyl group is not expected to bring any steric strain enhancing this distortion. This is strongly informing us on the intermolecular origin of this distortion, as discussed for polymorphs I and II in the [Fe(PM-BiA)2(NCS)2] system [34][35] . One may immediately correlate the strong ligand rotation in Fe2 to this high distortion. The fact that several polymorphs of this compound are available will allow us to discuss deeper in the next sections. The negative expansion of unit-cell volume at SCO in polymorph I-pBrA is very unusual.

Normally, the unit-cell volume decreases from HS to LS based on the natural breathing of FeN6 polyhedron. The variation of volume of FeN6 polyhedra from HS to LS are around 25%, which lead to a contraction of the unit-cell volume in the range 2-10%, as so far observed. 23 As seen in Table III.12, polymorph II-pBrA crystallizes at 350 K in the same space group as polymorph I-pBrA, P21/n, which remains at 120 K. However, it was impossible to thermally quench the Q-HS* and only the LS state was observed at 10 K (as seen in Appendix 5).

Similarly to I-pBrA, all atoms sit on the general Wyckoff position (4e) and there are two In order to check whether the order-disorder of the phenyl ring is coupled with the SCO, the structures at 200 K, 250 K, 300 K and 350 K were analyzed carefully, especially the dihedral angle and the ratio of the disordered phenyl rings. As seen in Table III.13, the disorder of phenyl ring exists only above 200 K. From 350 K to 300 K, the occupancy and the dihedral angle of disordered phenyl rings slightly change, while occupancy and the dihedral angle of disordered phenyl rings decrease a lot from 300 K to 250 K. Assuming that the dihedral angle at 350 K corresponds to the HS state, then the HS fraction can be calculated, labeled HS fraction1 in The variations of the angular and the trigonal distortions for Fe1N6 / Fe2N6 are 26/17° (ΔΣ) and 107/95° (ΔΘ) from 350 K to 120 K, respectively (Table III

Structure-properties relationship of II-pBrA

According to the ΔΘ values (107/95° for Fe1N6/Fe2N6, compared to 97/123° in I-pBrA, the expected T(LIESST) would be 65 K. However, there is no LIESST effect from the photomagnetic measurements. Moreover, the thermal quenched structure at 10 K is fully LS, agreeing with the presence of a very short-lived metastable state. Several hypothesis can be made.

The absence of LIESST may be related to the continuous variation of ligand geometry.

Based on the photocrystallography result of polymorph I-pBrA, the h -HS* has a similar structure as the thermally quenched Q-HS*. The energy provided by the presence of the abrupt ligand geometry variation upon HS* LS relaxation, might contribute to increase the lifetime of this metastable state. In polymorph II-pBrA, the continuous variation of ligand geometry induces a much less additional energy that destabilizes the HS* and increases its relaxation, preventing any observation.

Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 157

Another explanation is based on the fact that we cannot be sure that the photo-induced h -HS* has the same disordered structure than the HS one. If the h -HS* state is not disordered the variation in the distortion parameters might be much smaller, strongly decreasing the lifetime of the metastable HS* state.

Another hypothesis is that the model linking T(LIESST) and ΔΘ is not complete. Indeed, in polymorph II-pBrA, the coordination sphere parameters are very close from the Fe1 site in polymorph I-pBrA with Δ = 0.278/0.268 Å for both sites compared to Δ = 0.284/0.354 Å in polymorph I-pBrA. We can notice that, in addition to lower ΔΘ values, one Δ is also much lower. This parameter that measures the anisotropy of elongation has been highlighted during the study of polymorphs I-BiA and II-BiA of [Fe(PM-BiA)2(NCS)2] (cf. Part I.3). [34][35] It is therefore possible that the "simple" trigonal distortion parameter is not sufficient to describe and predict the T(LIESST) behavior.

SCO coupled to order-disorder transition is very common 38 and the disorder can appear in ligand [39][40][41] and anion [42][43] . Usually, the disorder occurs at high temperature and the order at low temperature owing to the fact that high entropy and symmetry states are favored at high temperature. 44 In summary, polymorph II-pBrA shows a gradual SCO coupled with an order-disorder transition. The corresponding modification of the intermolecular interactions and crystal packing is also continuous and polymorph II-pBrA shows the usual decrease of unit-cell volume from HS to LS.

III.4.3. Polymorph [Fe(PM-pBrA)2(NCS)2]-III: III-pBrA

This polymorph was obtain in too small quantities to be able to perform any magnetic characterizations. Moreover, it was always obtained concomitantly with polymorph I-pBrA and the crystals have to be manually selected. Unfortunately, the crystals are very small and the ratio of polymorph III-pBrA is very low in the mixture. We therefore perform all our study on single crystals. Data were collected at 300, 220, 210, 205, 150, 90 K and at 10 K after a thermal quenching. The thermally quenched measurement at 10 K was performed in collaborating with Dr Sébastien PILLET (University of Lorraine, Nancy, France). Additionally to the structure refinement, the unit-cell parameters as function of temperature were also measured by SCXRD.

R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2

Crystallographic study of III-pBrA

Polymorph III-pBrA crystallizes in the triclinic system with the space group P-1 at 300 K and 150 K (Table III The first thing that should be notice is that Fe1 and Fe2 sites are rather different from each other and resemble to the corresponding ones in polymorph I-pBrA. The overlay of Fe1 and Fe2, exhibits a huge RMSD value of 0.968 Å (Figure III.49d). This is also illustrated by the difference in σ3, the dihedral angle between the bromophenyl and the pyridine rings (Table III process. Note that the unit-cell parameters at 70 K is not precise because of the damage of crystal during increasing temperature from 10 K to 70 K. 9) LS

Structure-properties relationship of III-pBrA

Compared to polymorph I-pBrA, the T(TIESST) is much lower in polymorph III-pBrA.

This might first come from the higher T1/2 temperature for polymorph III-pBrA that is expected to decrease the relaxation temperature T(LIESST) (or T(TIESST)).

Structure of polymorph IV-pBrA

SCXRD data were recorded and crystal structures were solved at 300 K and 85 K. Since polymorph IV-pBrA does not exhibit SCO, the evolution of the unit-cell parameters as function of temperature was not measured.

As seen in Table III.18, polymorph IV-pBrA crystallizes in the same space group as polymorphs I-pBrA and III-pBrA, P21/n, at both 300 K and 85 K. Contrarily to the other polymorphs described before, however, there is only one Fe 2+ site in the asymmetric unit ), the bending of the NCS -groups (lower than 152°) tend to favor the HS. 14 We are here at this limit. By the overlay of molecules at 300 K and 85 K, there is almost no difference with RMSD of 0.051 Å.

(
Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family The S---H bonding in polymorphs II-pBrA and III-pBrA are shorter than that in I-pBrA.

The contacts of S---H in polymorph IV-pBrA are weakest. This corresponds to the observed result that S-H are important for the SCO occurrence in PM-L family. 22,45 The short H---H contacts occurs in polymorphs I-pBrA and II-pBrA and the shortest value of 1.957 Å is owing to the disorder of phenyl ring. The inter-and intramolecular H---H contacts play an important role for the rotation of phenyl ring in I-pBrA. The weak π---π contact only occurs in polymorph I-pBrA. Compared to other compounds in PM-L family 22,45 , the absence of π---π contact may be due to the occurrence of short contact of two phenyl rings forbidden by large atomic radius of Bromine at para position. The atom-atom contacts could be associated to the unique properties in polymorph I-pBrA, including H---H, S---S and π---π. Especially, the phase transition of phenyl rings in these polymorphs may result from the modification of H---H contacts at SCO.

Therefore, these four polymorphs differ from each other by the way the geometry of their ligands changes upon cooling and by their crystal packings. Additionally, upon spin crossover in I-pBrA, II-pBrA and III-pBrA, the intermolecular interactions change in different ways.

These three polymorphs display one step SCO, despite the presence of two strongly different Fe 2+ sites. It's like the SCO of one site forces the second one to switch concomitantly. This has been already reported even if rather rarely [25][26] like in the [Fe(1-bpp)2] x[Co(terpy)2]1-x(BF4)2

(1-bpp = 2,6-bis[pyrazol-1-yl]pyridine) solid solutions where the SCO on the iron site drives the SCO on the cobalt site. [47][48] This can be easily understood in compounds with strong cooperative interactions but it's more surprising in poorly cooperative compounds like in polymorph II-pBrA.

Polymorphism is very common in crystalline materials [49][50] and SCO materials with polymorphism have been deeply reviewed by Tao et al. 51 Since SCO phenomenon is a multiscale phenomenon from the coordination sphere to material, slight variations in one of these scale can change the magnetic properties greatly. Polymorphism provides platforms to compare and contrast the structures at all scales, which can help understanding the true structural-property relationship.

It is common that materials display two or three polymorphs with different magnetic The positive and negative expansions of unit-cell parameters can be observed in the temperature range of SCO, owing to the anisotropic variation of the coordination sphere at SCO.

For example, a colossal NTE is driven by SCO in [Fe(bpac)(Au(CN)2)2]·2EtOH (bpac=1,2bis(4'-pyridyl)acetylene) with the flexible framework. 75 Accompanied with contraction of the coordination sphere at SCO, the unit-cell volume decreases from HS to LS. For example, unitcell volume decreases by 80 Å 3 and 232 Å 3 in Fe(phen)2(NCS)2 76 , and [Fe(Htrz)2(trz)](BF4) 77 , respectively. The unit-cell volume change about -1 to -10 % from HS to LS. [22][23] Here, the exotic Geometrically flexible networks in compounds will facilitate the negative thermal expansion (NTE). 78 The NTE effect in ZrW2O8-type solids can be well explained by the rotations of rigid polyhedra units in rigid unit modes. Structural flexibility is a key factor for NTE effect. [Cu3(btc)2] (btc=1,3,5-benze-netricarboxylate), a metal-organic framework that consists of dicopper tetracarboxylate "paddlewheels" and aromatic ring motifs shows the large negative thermal expansion. 79 Regarding the population of the HS* metastable state it was only possible to observe it in polymorphs I-pBrA and III-pBrA. Indeed, the HS* was accessible efficiently using thermal quenching for both polymorphs. However, light irradiation was only possible on polymorph I-pBrA (there was not enough compound of III-pBrA to make any measurement) and it was shown to be only 65 % efficient. The main hypothesis to explain the difficulty to obtain the light-induced phase is that at 10 K, the energy needed to promote the rotation of the ligand is too high. Therefore, the photoswitching is slow and not totally efficient. This has already been reported on other compounds such as the [Fe(3-bpp)2][Au(CN)2)]2 that experiences a twinning and reconstructive phase transition in temperature that is almost impossible to induce by light (another phase is partially obtained). T(LIESST) of the "bidentate" family and one of the highest among SCO compounds. [17][18] .

Concerning polymorph III-pBrA, it lies on the same line than I-S-BiA, a compound belonging to the PM-L family. Therefore, these two polymorphs are remarkable since they do not fit the T0 = 100 K line where all the other compounds of this family lie. 36 More details on this aspect will be discussed in Part IV.

III.5. Conclusion

In idea was to study the steric strain induced on the coordination sphere by these various substitutions with the expectation of changing the coordination sphere distortion. Unfortunately, the complexes with ligands substituted in ortho did not crystallize preventing crystallographic investigations. The origin of polymorphism and the differences between them has been deeply investigated and described and the respective parts. Here, these ten compounds will be compared with the other members of the PM-L family with a focus on the coordination sphere scale to see if any correlation with the T(LIESST) can be confirmed or evidenced.

Then, novel solvatomorphs [Fe(PM-pXA)2(NCE)2].S (E = S and Se, S = Solvent, X = F and Br) will be introduced, though not fully described since there are still under investigation.

These results will be compared to the literature, especially the one focused on SCO complexes based on at least one bidentate ligand. This will give more impact on the correlation, if any. This discussion will be performed, separating compounds with structural transition at the SCO (phase transition, symmetry breaking, order-disorder…) from compounds without.

This should allow to discuss separately the effect of crystal packing change upon SCO on the photo-induced hν-HS* state lifetime.

Finally, a comprehensive discussion on the T(LIESST) and the lifetime of the photo-induced state using a multi-scale view will be presented, similarly to the thermal SCO. This description will be strengthened using data from the literature to tend toward a complete overview.

All this part should allow to draw new trends in the chemical design of SCO with enhanced lifetime of the hν-HS* metastable state. Two hypothesis can be proposed to explain this result.

IV.2. T(LIESST

The first possibility is that for polymorphs I-pBrA and III-pBrA, the metastable states HS* correspond to the HS states observed at room temperature, with the ligand rotation along the HS* to LS relaxation. In polymorph II-pBrA, it is possible that at low temperature, the hv-HS* is not similar to the HS state at high temperature and does not present any disorder. Therefore, it is possible that the ΔΘ obtained from the thermal spin crossover does not correspond to the true ΔΘ between the non-disordered hv-HS* and the LS states. Then, it leads to a difficult comparison of this compound with the others. The second possible explanation is that ΔΘ is not the unique parameter to correlate with the T(LIESST). Indeed, in II-pBrA both ΔΘ and Δ are smaller than in I-pBrA while in III-pBrA only ΔΘ is smaller (Table IV As discussed above, S-PEA, for example, experiences a structural phase transition together with the spin crossover that does not fit the T(LIESST) vs ΔΘ trend. We suppose it's coming from the occurrence of the phase transition that introduces an additional energy barrier or because the hν-HS* is different from the HS one, leading to a pseudo value of ΔΘ from hν-HS* to LS. During the PhD work, many other compounds were synthesized by introducing various solvents in the structure. The idea was, for a given ligand, to regulate the intermolecular interaction and see how it influences the distortion of the coordination sphere. This work, done for "bpp" compounds for example, generally allows to move along one T0 line of the T(LIESST) vs T1/2 database. [12][13] Since the pFA and pBrA-based compounds seem to be very sensitive to the environment we expected to move not only along one T0 line but also to move from one line to another. This would strongly favor the idea that T(LIESST), as for S-PEA, is influenced by the molecular packing, i.e. intermolecular contacts.

Table IV The synthesis and single-crystal structure data can be seen in Appendix 6 and 7. Unfortunately, these compounds were not fully characterized due to very peculiar behaviors that deserve long and detailed analysis. Moreover, not all the LIESST measurements were performed on these samples due to restriction time. The new compounds additionally obtained in this work clearly show that, even if the coordination sphere distortion parameters are prone to give high T(LIESST), symmetry changes (order-disorder, twinning, symmetry breaking) can strongly affect the lifetime of the photoinduced state. This is true as soon as we only discuss the variation of these structural parameters between the HS and LS phases. Without more photo-induced crystal structures we cannot totally exclude that the estimation of the distortion parameters is not accurate in such compounds experiencing significant packing modifications.

IV.2.4. Comparison with SCO complexes based on bidentate ligands

We have extended our comparison to other SCO complexes based on bidentate ligands.

Table IV.4 summarizes the compounds studied, as well as compounds obtained during this PhD work, separating those experiencing some drastic changes in the crystal packing. We removed compounds with poor LIESST effect or with T(LIESST) too close to the hysteresis loop and consequently more sensitive to dynamical effects. [31][32] From these 32 compounds the average ΔΘ and Δ values can be calculated at 115° and 0.21 Å, respectively. The detailed analysis of the influent parameters on lifetime of the hν-HS* metastable state allows us to draw a general picture for this lifetime, as a mirror of the multiscale view of the thermal spin crossover, as proposed in the following chapter.

IV.3. Lifetime of the metastable HS* state -a multiscale view

From the above analysis of data, including other information from the literature, we can discuss more accurately what influences T(LIESST) and more precisely the lifetime of the photo-induced state. This lifetime has been deeply studied by Andreas Hauser in the 80's and 90's. [33][34][35] As already introduced in Part I.2, the inverse energy gap law links T1/2 to the relaxation rate at T 0 K. 33 In a classical 2D view of the vibrational potential wells, it corresponds to a variation of the activation energy through a vertical displacement of the potential wells. The higher T1/2, to lower the activation energy, the faster the relaxation. In addition, Hauser described that the higher the horizontal displacement of the potential wells (that is the higher the average elongation of bond lengths), the higher the activation energy, the slower the relaxation. 34 As mentioned in Part I.2, this view has been shown to be no more valid, or at least not sufficient to describe some behaviors. Finally, Hauser deeply investigated the influence of cooperativity on the relaxation features, that is the isothermal evolution of the HS* fraction as function of time. 35 He showed that cooperative interactions lead to self-accelerated relaxation (see Part I.2.2). While cooperativity and T1/2 are thermodynamic parameters that account for the collective behavior of SCO, only the average elongation of bond lengths is a molecular parameter, related to the coordination sphere. For a given elongation of bond lengths, cooperativity can influence the lifetime. This is a first indication that the lifetime of the photoinduced state is sensitive not only to the coordination sphere but also to its environment within the crystal: everything being equal, an increase of the sole cooperativity parameters fastens the relaxation.

Later on, the introduction of the trigonal distortion evolution at SCO, ΔΘ, and the supposed effect of denticity on the T(LIESST), brought the attention on the coordination sphere as a key scale to control the lifetime of the photo-induced state and the increase of T(LIESST). 3,36,37 This PhD work is based on this assumption, with a focus on coordination sphere distortion to go toward a 2-parameters (at least) description of the HS* lifetime. Along this work, compared to about 30 bidentate-based complexes from the literature, a much more complicated view has been evidenced that is detailed below as a multiscale view.

short irradiation, no changes in the coordination geometry are observed and T(LIESST) = 36 K while for the FeN5S situation T(LIESST) rises at 108 K. 21 This high T(LIESST) is associated to weak ΔΘ (49 °) and Δζ (0.10 Å). This unexpectedly high T(LIESST) value with such low parameters might come from the additional energy needed to promote the thermalisomerization.

This also agrees with the recent study made on tripodal ligands-based complexes, showing that, at least thermal spin crossover, occurs along a coupled breathing/torsion reaction coordinate, moving the complex from octahedral LS state to trigonal prismatic HS state. 41 This analysis, unfortunately has some drawbacks. The main one is that, in most cases the a reorientation of the linear acetonitrile was reported to contribute to a temperature-dependent lattice activation barrier to the spin-transition associated to a higher T(LIESST) compared to other solvatomorphs. 44 Obviously, such changes in intermolecular interactions might change cooperativity.

Another approach to tune the cooperativity is the metal dilution. By substituting the Fe 2+ ion by SCO-inactive cation, the cooperativity pathways are cut and some internal pressure is exert.

The bigger the SCO-inactive ion, the higher the "negative" internal pressure, the more the HS state is stabilized. Solid solutions are generally made with, Ni 2+ , Co 2+ , Zn 2+ , Mn 2+ , and Cd 2+ in order of increasing ionic radius. For a given amount of non-SCO active metal, the bigger the ion the more T1/2 is decreased. 45 The effect of metal dilution on T(LIESST) has been studied on several compounds and especially on [FexM1-x(phen)2(NCS)2] with phen = 1,10-phenantholine (M = Ni 2+ , Zn 2+ , Mn 2+ and Cd 2+ ). 46 It was reported that substitution of Fe 2+ ions by Ni 2+ ions induces a decrease of T(LIESST) while with Zn 2+ , no change in T(LIESST) was observed and for Cd 2+ a 40 K increase of T(LIESST) was noticed. 47 This effect was also reported on relaxation kinetics. 48 These studies and others clearly show that these solid solutions do not follow the "inverse energy gap law" nor the T(LIESST) vs T1/2 trend, indicating that T(LIESST) is indeed dependent on parameters external to the coordination sphere. 49 Molecular rearrangement and symmetry breaking Several kinds of structural rearrangements may occur and influence the lifetime of the photo-induced state.

As we have seen in I-pFA, and in various solvatomorphs, some disordered crystallization solvent molecule create different intermolecular interaction networks around the various Fe 2+ sites there are interacting with. However, in the temperature range of study, no clear ordering of solvent was observed. However, in the literature, several compounds are reported to experience order-disorder of solvent or counter-ion upon SCO, but studies of the metastable HS* state in these system remains rare. 50 Let us note that the order-disorder hysteretic behaviour in the tridentate ligand-based complex [Fe(H4L)2] [ClO4]2•H2O•2(CH3)2CO is supposed to be responsible of the high T(LIESST) value of 104 K. 51 In S-pFA.C9H12 a symmetry breaking has been observed leading to a two-step character of the SCO. The hv-HS*, even if incompletely populated, exhibits a two-step T(LIESST) curve, without clear influence of the symmetry breaking on its lifetime. Symmetry breaking due to elastic competition is increasingly reported in the last ten years, especially on 2D and 3D coordination polymers like the Hoffman-like structures. [52][53][54] Subtle effects might be observed with some regime change in the relaxation rate but without information on the h -HS* structure

Part IV -T(LIESST): A Close and Comprehensive View it is often difficult to discuss in detail. 55 In compound [FeH2L 2-Me ][ClO4]2, symmetry breaking occurs at around 120 K, which leads to two different LS phases and two iron sites appear at 100 K with the doubling of a value. 56 The relaxation from hv-HS* to LS occurs through an intermediate LS high-symmetry phase, which suggests dynamical separation of spin and lattice degrees of freedom in the relaxation process from the photo-induced state. 57 In the {Fe[H2B(pz)2]2L}family symmetry breaking only occurs with L = phen (compound 26 of Table IV.4) leading to a different and lower T(LIESST) of 44 K than with L = bpy without symmetry breaking (T(LIESST) = 52 K). 16 63 However, a recent report shows relatively opposite observations. Reducing the size of [Fe(pz)Pt(CN)4] particles induces a drastic lowering of the relaxation rate after photoexcitation. 64 Up to now, this nanoscale is poorly explored in terms of lifetime of the photoinduced state, as well as the coherent domain scale still unexplored. In summary, the energy barrier from hv-HS* to LS associated to the relaxation temperature can be influenced by the anisotropic distortion of the coordination sphere, the variation of ligand conformation, the modification of interactions, the molecular rearrangement as well as symmetry breaking. In other words, the relaxation from hv-HS* to LS can also be viewed as a multiscale process as shown in Figure IV.13, similar to the multiscale view of thermal SCO. 65 The important impact of such view is that it emphasizes the few amount of photo-induced structures to evidence the real modifications upon light-irradiation and relaxation. Moreover, it clearly shows that, even if everything seems to be controlled at the coordination sphere level, all the chemical design of this sphere might be maximized or minimized by all the other physical scales. In forthcoming research, the target will be to make the synergy between coordination sphere, ligand conformation and symmetry breaking during the LIESST process to reach the maximum of relaxation temperature.

The study of this thesis was devoted to the fundamental understanding of the lifetime of the photo-induced state LIESST effect. The target was to attempt to establish reliable structureproperties relationships to evidence the chemical parameters to play with in order to reach high relaxation temperature T(LIESST). This work was based on previous results from the literature presenting the distortion of the coordination sphere as a crucial parameter to account for.

However, it was not clearly evidenced if it was an angular distortion or an anisotropy of elongation. Moreover, polymorphism might induce very different features of this coordination sphere distortion. Therefore, we explore these aspects through the synthesis of new complexes.

To increase the distortion of the coordination sphere, different chemical strategies were attempted, including steric strain at molecular scale, polymorphism and solvent effect at supramolecular level. New ligands and complexes were then prepared, belonging to the "PM-L family" [Fe(PM-L)2(NCE)2] where PM-L is a bidentate Schiff base ligand formed by the condensation of pyridine carbaldehyde with a substituted aniline. These ligands were designed to promote steric strain close to the coordination polyhedron thanks to halogen substitution on the aniline group. Structures and (photo)magnetic properties were analysed in the multiscale view, from the coordination polyhedron to the crystal packing, through the analysis of the intermolecular interactions. Ten new compounds were synthesized and deeply investigated.

Thirteen solvatomorphs were also obtained but not fully characterized yet.

All these new compounds from the [Fe(PM-L)2(NCE)2].S family, were compared with the other members of this family already known for many years, but also with compounds from the literature based on bidentate ligands. This work allow exotic and significant clues can be drawn.

Both trigonal distortion and anisotropic elongation are relevant parameters towards high T(LIESST)

. Indeed, we demonstrated that the trigonal distortion, thought to be responsible of the increase of the hν-HS* lifetime, is most probably not sufficient to increase T(LIESST). The coordination sphere seems to be both distorted and anisotropically elongated to stabilize this HS* state. T(LIESST) up to 107 K was evidenced on this new compound with both high trigonal distortion and anisotropic elongation. Several compounds stand above the usual trend for this family, showing that the strategy to promote steric strain close to the coordination sphere was successful, at least partially. From the comparison of more than thirty compounds we also evidenced that, whatever the chemical design made on the coordination sphere, the upper scales can strongly come into play and perturb the coordination sphere scale. Indeed, crystal packing and intermolecular interactions can maximize or minimize the effect of the coordination sphere distortion on the T(LIESST). This led to the multiscale view of the T(LIESST).

The multiscale view of T(LIESST).

The study of polymorphism and compounds with symmetry changes during the spin crossover provided crucial information. The anisotropic breathing of the coordination sphere can strongly affect the motion of ligands, the intermolecular interactions and the crystal packing. As a feedback, all the scale of the material can act as a perturbation on the lower scale. This perturbation can add an additional energy to overcome during the hν-HS* to LS relaxation process, therefore enhancing the initial coordination polyhedron distortion or, on the contrary, it can provide additional relaxation pathways that minimize the effect of coordination sphere distortion. Therefore, thinking about increase T(LIESST), not only starts on the design of the coordination sphere but also on accounting for all the physical scales. These modifications can be the source of exotic behaviours. Incidentally, this clearly put in light that the investigation of SCO materials requires a detailed analysis of the unit-cell parameters as function of the temperature, far beyond the sole HS and LS crystal structures. Overall, this result opens new perspectives, even for applicative targets, since we clearly evidence here, probably for the first time, that the volume of the sample does not necessarily decrease from HS to LS, but can on the contrary increase, depending on the compound and even on the polymorph.

Unique thermal expansion of unit-cell volume phenomena

These conclusions are giving important information for the design of high T(LIESST) materials. Regarding the coordination sphere scale, a useful chemical strategy will play not only on the trigonal distortion but also on the elongation of the polyhedron.

-The trigonal distortion is linked to the anisotropic coordination sphere hidden in the shape of a molecule. The steric strain, as a useful method, can promote the trigonal distortion of the coordination sphere. The rigidity and flexibility of ligand must be considered when introducing the steric hindrance. The steric strain in molecular structure may be destroyed owing to the conformational change of ligand. Moreover, the molecular anisotropy and shape are mainly determined by the type of ligands, but also related to the intermolecular interactions. The subtle influence of interactions in polymorphs or solvatomorphs can also regulate the trigonal distortion.

-The anisotropic elongation of the coordination sphere seems more complicated than trigonal distortion to chemically design. One can link the elongation of metal-ligand bond length to the Jahn-Teller effect. Metal ions like Mn 3+ can be a good candidate.

Working on heteroleptic compounds and inducing steric strain through one ligand substitution could help to locally break the symmetry of the molecule and favor some elongation directions. Finally, considering complexes with spin switching associated to the coordination change, such as the coordination/decoordination in macrocyclic complexes, might be a fruitful strategy to follow. Indeed, creating or removing a metalligand bond upon thermal stimulus could strongly enhanced the anisotropic elongation of the coordination sphere. Systems exhibiting such behaviors are compounds with very high T(LIESST) around 130 K.

As we have discussed, T(LIESST) is however not only determined by the coordination sphere scale. One has to account for the upper scales. However, controlling intermolecular interactions in the three directions of space around a strongly unsymmetrical complexes is highly challenging. This is even more challenging when thinking that this scales can help or not the stabilization of the hν-HS* state. Intuitively, if molecular rearrangements increase the density of vibrational states and phonons, one may expect the hν-HS* to LS relaxation to be favored. On the contrary very abrupt changes associated with symmetry breakings, with strong energy differences in the LS and HS crystal packings, might favor the hν-HS* and high

T(LIESST).

A recent work was focused on the calculation of the lattice energy evidencing that cooperative spin crossover are associated to high energy changes of this energy from HS to LS. 1 Such difference in lattice energy should be taken into account while discussing the lifetime of the hν-HS* state in the solid state. Several strategies might be proposed.

- -Symmetry breaking can regulate the lattice energy and then affect the energy barrier from hν-HS* to LS. However, promoting such symmetry breaking is challenging. Again flexible ligands may help to provide antiferro-elastic directions in the network leading to elastic competition. This might be "easier" in 3D networks in which only one or two directions are tunable. However, this should be associated to the coordination sphere distortion too. Highly deformable networks should be considered to both promote high coordination sphere anisotropic distortion and symmetry changes. Unfortunately, at this level of investigation, knowing if these symmetry changes will stabilize or destabilize the hν-HS* state is difficult. Generalizing the investigations on the calculations of lattice energy could provide in the future more insights.

A very schematic view can be that every structural change (trigonal distortion, elongation, change of conformation, symmetry changes, twinning…) is a step in the activation energy stairlike pathway. Each step can increase the energy barrier and stabilize the hν-HS* state (Scheme 1). However, sometimes another path comes to life and the energy is split and the activation energy is not increased (Scheme 1). Moreover, the counterpart is that a network that needs high energy to be deformed won't allow to reach the hν-HS* using light at low temperature. One has to balance all these scales and to make them act in a synergetic way in order that deformation of the coordination sphere is not absorbed by the crystal packing but instead enhanced.

Scheme 1. Scheme of the relationship at and above the molecular scales. The variation of scales above the molecule in the solid state can destabilize or stabilize the hν-HS* state, leading to decrease and increase the activation energy associated to the ultimate relaxation temperature T(LIESST), respectively.

Another interesting aspect of the multiscale view of T(LIESST) is that it evidences some missing parts. Indeed, no investigations on the fatigability of the light-induced and relaxation processes have been made. Information on the coherent domain scale are missing. This would be very interesting for any application involving light-induced process in order to estimate the lifetime of the whole device in terms of cycles. Moreover, very few studies are dealing with nanoparticles, and they currently are contradictory. Again, since thin films or nanoparticles are envisioned in electronic devices, one has to investigate such aspects in the future.

This work has therefore provided new perspectives in terms of chemical design of switchable compounds exhibiting long-lived photo-induced states.

The shape index of Hirshfeld surface is defined in terms of principle curvatures k1 and k2 (eq.3).

The maps of shape index on the Hirshfeld surface can be used to identify complementary hollows (red) and bumps (blue) where two molecular surfaces touch one another. These patterns of red and blue triangles on the shape index surface are diagnostic for close C/C interplanar contacts.

eq.3 New spin-crossover compounds including polymorphism for a multiscale approach towards high T(LIESST)

Nouveaux composés à conversion de spin et polymorphisme pour une approche multi-échelle vers les hautes T(LIESST)

Unité de recherche

  .............................................................................................. PART I -.................................................................................................................................... Nowadays, Material Science is growing very fast, driven by technical and engineering demands. The research and development of new materials with desired properties become more

  position at the molecular scale based on the bidentate ligand. The second one is related to the subtle change of the intermolecular interactions in polymorphism or solvatomorphism. Part I displays some fundamental knowledge on SCO and LIESST effect. In Part II, the synthesis, crystallography and (photo)magnetic studies of new molecular compounds with fluorine from para, meta, to ortho position are presented. In Part III, polymorphs are characterized and the intermolecular interactions are analyzed. The deep examination of the relevant parameters to high T(LIESST) as discussed in Part IV brings new perspectives on

Figure I. 1 Figure I. 1 .

 11 Figure I.1. Schematic picture of low spin and high spin states in octahedral Fe 2+ ions under strong and weak ligand fields.

  and I.2c) can be observed in cooperative compounds leading to a switching event occurring in a complete mode within few Kelvin (< 10K). A hysteresis may appear as shown in Figure I.2c for very strongly cooperative systems.

Figure I. 2 .

 2 Figure I.2. Schematic spin crossover curves as function of temperature. The springs represent the interactions that can reflect the cooperativity. The color of spheres represents the independent crystallography sites.

(Figure I. 3 ,

 3 Eq I.1).

(Figure I. 3 ,

 3 Eq I.2). The Θ value indicates the deviation in the coordination polyhedron geometry from a perfect octahedron (Oh) to a trigonal prismatic structure (D3h).

Figure I. 3 .

 3 Figure I.3. Definitions of the angles used to calculate the distortion parameters Σ and Θ for a six-coordinated metal site. Adapted from ref. 78.

Figure I. 4 .

 4 Figure I.4. Scheme of the multiscale view of the structural modifications from the polyhedron to crystal along the spin crossover process. Adapted from ref. 34.

radiative relaxation 1

 1 MLCT 3 MLCT in less than 100 fs. The second non-radiative relaxation to state 5 T2 involves two processes. The first one concerns the intersystem transition between the triplet state and the quintuplet state in about 200 fs. The second process is the de-excitation of the quintuplet state, via a cascade of vibrational levels, to the lowest level (vibrational cooling). This step is completed in less than 9 ps (Figure I.5c) and is effective thanks to the electron-phonon coupling.

Figure I. 5 .

 5 Figure I.5. Single-crystal UV-Vis spectra (a) and virabtional potential wells for LIESST and reverse-LIESST processes in [Fe(ptz)6](BF4)2 (b); Adapted from ref. 108. Energy levels and non-radiative relaxations path identified for the complex [Fe(phen)3] 2+ (c); Adapted from ref. 113. Schematic drawing of the photoswitching dynamics in the solid state: HS molecules (red circles) generated within 1 ps by laser pulse in the cold lattice with mainly LS molecules (blue circles) (d); Adapted from ref. 115.

  Much work has been devoted to the study of relaxation processes from the metastable HS* state to the LS state, independently on how this HS* is reached (by light irradiation or thermal quenching). The mechanism explaining the relaxation process involves the nonadiabatic multiphonon theory between the two spin states.121 Each spin state is represented by a harmonic potential well. The horizontal separation between these two wells, QHL, is linked to the variation of the metal-ligand bond length during the SCO. The vertical arrangement of the two wells corresponds to the zero point energy difference separating the lowest vibrational energy levels of the LS and HS* states. The value of is assimilated to the enthalpy change, H, associated with the spin crossover at zero temperature (Figure I.6a). The HS* LS relaxation is therefore a function of the overlap between the different wavefunctions of the vibrational levels of the HS* and LS states. Two regions of relaxation were determined as a function of temperature and population of these vibrational levels 121-122 (Figure I.6):

Figure I. 6 .

 6 Figure I.6. Simplified two-dimensional potential-energy diagram of HS* and LS states (a) and Arrhenius plots for various compounds of the isothermal hν-HS* LS relaxations at different temperatures (b). Adapted from ref. 108.

Figure I. 7 .

 7 Figure I.7. Isothermal hν-HS* LS relaxations at different temperatures in the noncooperative compound [Zn1-xFex(mepY)3(tren)](PF6)2 (a) and in the cooperative [Fe(ptz)6](BF4)2 (b); Adapted from ref. 124 and 126, respectively.

  3 K/min. When reaching the thermally activated region of the relaxation, the photo-induced hν-HS* state relaxes to the LS state and the magnetic hν-HS* is erased and the LS state is recovered (Figure I.8). The T(LIESST) temperature is defined as the inflexion point of this curve recorded in the dark and is determined by the minimum of the ∂(χMT)/∂T curve. Far to be an absolute meaningful value, the T(LIESST) temperature is a simple and quick way to compare compounds in regards to their interest for light-induced SCO.

Figure I. 8 .

 8 Figure I.8. The schematic picture of the relaxation temperature measurement T(LIESST). Insert up: the defined T(LIESST) temperature as the minimum of the ∂(χMT)/∂T curve. Adapted from ref. 128.

Figure I. 9 .

 9 Figure I.9. T(LIESST) vs T1/2 database in spin crossover complexes with the different types of ligands. The region in gray corresponds to a T(LIESST) temperature inferior or at least equal to T1/2 that might lead to hidden phases; Adapted from ref. 129.

  photocrystallography, meaningful clues were proposed, as discussed in the following section.

Figure I. 10 .

 10 Figure I.10. The modification of the trigonal distortion from HS to LS, denoted ΔΘ for Fe 2+ complexes belonging to the same family [Fe(L)2(NCE)2] with L a bidentate ligand and X = S or Se, is compared to their T(LIESST) (circle) and T1/2 (square) values. The zone where a LIESST effect may be observed is thus delimited (zone 3). This correlation is made feasible since all the compounds have similar metal ligand bond lengths and relatively close crystal packings. Adapted from ref 73.

  Photocrystallographic studies have been performed on these two polymorphs. As seen in Figure I.11c, if only the average metal-ligand bond length <Fe-N> is considered, the variation of <Fe-N> in polymorph I-S-BiA and II-S-BiA is almost similar. In the isotropic elongation view, the different values of T(LIESST) can not be explained. If the anisotropic elongation (ζ) or the angular distortion (Σ) is considered, one can clearly see the huge differences between the polymorphs. For example, the variation of ζ from LS to hν-HS* at 20 K in polymorph I-S-BiA

and I. 6 )

 6 is no more valid. Therefore, a three-dimensional view must be used, going from isotropic breathing of the coordination sphere towards anisotropic breathing (Figure I.12). At this stage, the most relevant distortion parameter among the above parameters has not been clarified, if any.

Figure I. 11 .

 11 Figure I.11. The magnetic and photomagnetic curves of polymorphs I-S-BiA (a) and II-S-BiA (b) and selected distortion parameters of the coordination sphere of the two polymorphs (c). Insert (a) and (b): the packing of polyhedra in the two polymorphs. Insert (c) up: the molecular structure of [Fe(PM-BiA)2(NCS)2]. Adapted from ref 74 and 94.

Figure I. 12 .

 12 Figure I.12. Sketch of the LS (blue) and HS (red) potential wells in the anisotropic breathing model, i.e. the elongation and distortion. Adapted from ref 111 and 18.

  (Figure I.13). Over 16 compounds are reported in literature (Table I.1) with NCS -or NCSe -counteranions, leading to various SCO features and T(LIESST) values.

Figure I. 13 .

 13 Figure I.13. Examples of compounds belonging to [Fe(PM-L)2(NCE)2] family. PM = N-(2' -Pyridylmethylene), AzA = 4-(Phenylazo)aniline, BiA = 4-(Aminobiphenyl)aniline, DiyA 4-(Phenylbutadiyne)aniline, PEA = 4-(naphthalene-1-ethynyl)aniline, FIA = 4-(2-amino)aniline, FIA = 4-(2-amino)aniline, NEA = 4-(Aminoterphenyl)aniline and TheA = 4-(Thienylethynyl)aniline. Adapted from ref 69.

Figure I. 14 .

 14 Figure I.14. Molecular structure and magnetic results of the selected compounds belonging to [Fe(PM-L)2(NCE)2] family. Adapted from ref 69, 90, 149-150.

Figure I. 15 .

 15 Figure I.15. Schematic picture of the designed PM-zXA (z = o, m, p; X = F, Cl, Br, I) ligands.

  example, Luo et al. found that the spin transition temperatures T1/2 and the width of thermal hysteresis loops show a positive correlation with the electronegativity of the halogen atomsin complexes [Fe(4-X-L)3](BF4)2 (L = N-benzyl-4-amino-1, 2, 4-triazole and X = H, F, Cl, Br and I). 153 Damrauer et al. reported the stabilization of the HS state of terpyridine based complexes introducing steric strain with halogens. It has been shown that the position of the halogen can influence the lifetime of photo-excited states in MLCT based compounds of [Fe(dftpy)2] 2+ , [Fe(dctpy)2] 2+ and [Fe(dbtpy)2] 2+

II. 2 .II. 2 . 1 . 8 Scheme II. 1 .

 22181 Scheme II.1. Reaction scheme of the N-(2'-pyridylmethylene)-n-fluoroaniline ligands (n = 2, 3, 4).

F2

  (2e). The special Wyckoff position (2e) leads to only a half molecule B in the asymmetric unit. Then, there are one and a half molecules in the asymmetric unit, where C15, C17, N3 and N4 atoms are disordered and half occupied in molecule B (Figure II.1). The dihedral angles between the phenyl and pyridine rings are only 5.54° and 0.73°, respectively, leading to the Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family nearly planar geometry of the two molecules. In both molecules, the intraligand H---H distances are less than 2.2 Å (Figure II.1a and b). It is worth to mention that the intermolecular hydrogen bonding N---H (2.834 Å for N1---H11 and 2.895 Å for N2---H20 Å) and F---H contacts (2.488 Å for F2---H13 and 2.747 Å for F1---H2) play an important role in the layer by layer packing of molecules in the mode B/AA/B along the b axis (Figure II.1c). The comparison between the experimental powder X-ray diffraction (PXRD) pattern and the simulated PXRD pattern from the SCXRD data shows strong differences (Figure II.2). This indicates that the main phase of the crystalline powder has a different crystal structure from the single-crystal used to determine the crystal structure. This may be due to the possible polymorphism in ligand.

Figure II. 1 .

 1 Figure II.1. Molecule A (a), molecule B (b), and the crystal packing (c) in the PM-pFA ligand at 300 K. The N---H and F---H hydrogen bonding are represented by the blue and green dashed line, respectively. Thermal ellipsoids are depicted at the 50 % probability level.

Figure II. 2 .

 2 Figure II.2. Experimental and simulated X-ray powder diffraction patterns of PM-pFA ligand (red and black) and PM-oFA ligand (blue) at 300 K. The simulation is done from the experimental single-crystal XRD data.

Figure II. 3

 3 Figure II.3 Thermal dependence of the χMT product of I-oFA recorded at 0.4 K/min (in settle mode) under 20 kOe of applied magnetic field, before irradiation (■), under irradiation ( ) and after irradiation in the dark (■).

Figure II. 4 .

 4 Figure II.4. Experimental X-ray powder diffraction pattern of complex I-oFA.

  photoswitching. The T(LIESST) measurement performed after this photo-excitation leads to a T(LIESST) value of around 32 K.

Figure II. 5 .

 5 Figure II.5. Thermal dependence of the χMT product of I-mFA recorded at 0.4 K/min (in settle mode) under 20 kOe of applied magnetic field, before irradiation (■), under irradiation ( ) and after irradiation in the dark (■).

Fe2. 60 Figure II. 6 .

 606 Figure II.6. Fe1 (a), Fe2 (b) in the asymmetric unit and their overlay with RMSD value (c) of the complex I-mFA at 300 K.

Part II -

  Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family 62 By the superposition of the structurally independent molecules at 100 K and 300 K (Figures II.7c and 7d), the RMSD value is only 0.071 Å for Fe1 in agreement with the absence of SCO on this site. Regarding Fe2, the RMSD value is 0.233 Å, owing to the atomic displacements of NCS -groups, corresponding to the SCO activity of this site. The crystal structure was also determined at 200 K to further examine the plateau seen on the magnetic curve. The average value of Fe-N distances are then 2.171(5) Å and 1.983(5) Å for Fe1 and Fe2 molecules, respectively, which also indicates the fully HS state of Fe1 and ~ 90% LS of Fe2 at 200 K leading to a crystal with roughly 55% HS and 45% LS entities.

Figure II. 7 .

 7 Figure II.7. Fe1 (a) and Fe2 (b) molecules in the asymmetric unit at 100 K, and the overlay of Fe1 (c) and Fe2 (d) between 300 K (red) and 100 K (blue) with the corresponding RMSD (Å) values (b) of the complex I-mFA.

Figure II. 8 .

 8 Figure II.8. Experimental and simulated X-ray powder diffraction patterns of complexes I-mFA. The simulation pattern (red) comes from the experimental single-crystal XRD data at 300 K.

( 8 . 6 %

 86 ), meaning stronger F---H hydrogen bonding around Fe1 molecules (Figure II.9c). An important contribution to the intermolecular interactions comes from the S---H bonding, with rather similar contributions around both molecules (25.5% for Fe1 and 22.0% for Fe2). The ratio of the C---C distances, that usually corresponds to the π---π interaction, are similar and around 3.4 Å in both molecules. Moreover, no F---F and F---S short contacts can be evidenced in the fingerprints.

Figure II. 9 .

 9 Figure II.9. The fingerprints and Hirshfeld surface for Fe1 (a) and Fe2 (b) molecules and selected contacts in fingerprints around both sites (c) of the complex I-mFA at 300K.

Figure II. 10 .

 10 Figure II.10. Pictures of the hydrogen bonding around Fe1 (a), Fe2 (b) molecules, the weak π---π interaction (c) between Fe1 and Fe2 evidenced by the shape index of Hirshfeld surface 11 , and the crystal packing (d) of the complex I-mFA at 300 K.

Figure II. 11 .

 11 Figure II.11. Pictures of the crystal packing the complex I-mFA at 300 K. Black sphere and red bold line means the inversion center and 21 axis, respectively. Fe2N6 polyhedra are in blue.

Figure II. 12 .

 12 Figure II.12. The fingerprints and Hirshfeld surface for Fe1 (a) and Fe2 (b) and the selected contacts in fingerprints Fe1 (c) and Fe2 (d) of the complex I-mFA at 100K.

Figure II. 13 .

 13 Figure II.13. The interactions around Fe1 (a) and Fe2 (b), molecular packing at ab plane (c), and 3D packing (d) of the complex I-mFA at 100K. The molecules with blue octahedral are Fe2.

  with a switching temperature T1/2 of 215 K. Moreover, a weak LIESST effect (<10%) was recorded leading to a T(LIESST) value around 32 K. This places this compound on the T0 = 100 K line of the T(LIESST) vs T1/2 database (Figure I.9 in Part I), as most of the PM-L compounds, except I-S-BiA. 1,3 Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family II.3.3. [Fe(PM-pFA)2(NCS)2]•nCHCl3 (I-pFA) Magnetic properties of I-pFA Magnetic measurements were performed on polycrystalline samples, using a SQUID magnetometer (Appendix 3). Figure II.14 reports the thermal evolution of the χMT product of I-pFA. The χMT value at 300 K of 3.31 cm 3 K mol -1 agrees well with a HS Fe 2+ ion (g = 2.10). Upon cooling, the χMT product decreases to reach a value of 0.31 cm 3 K mol -1 at 10 K, indicating a gradual and almost complete SCO, without any hysteretic behavior. In the differentiate curve (Figure II.14), the presence of two maxima indicates a two-step character of the spin crossover, with switching temperatures close to 140 K and 100 K. This behavior is reproducible.

Figure II. 14 .

 14 Figure II.14. Thermal dependence of the χMT product of I-pFA recorded at 0.4 K/min (in settle mode) under 10 kOe of applied magnetic field. The dotted curve in (a) is the derivative of the SCO curve, whose maxima indicate the switching temperatures. The green curve stands for the T(TIESST) (a) and T(LIESST) (b) measurements described in the text.

  2+ complexes and two chloroform molecules in the asymmetric unit (Figure II.15). The two Fe 2+ molecular complexes are labelled Fe1 and Fe2 hereafter. Both molecules are made of two thiocyanate groups adopting a cis configuration with two other bidentate ligands coordinated to the Fe 2+ ions. These two ligands adopt a conformation with the pyridine groups in trans position, similar to the Fe1 site in complex I-mFA. The NCS -groups are bent in Fe1 and Fe2, with S1-NC-Fe1, S2-NC-Fe1, S3-NC-Fe2 and S4-NC-Fe2 angles of 150.25°, 177.37°, 161.29°, and 158.84°, respectively. The average Fe-N bond lengths (<Fe-N>) are 2.164(5) Å and 2.165(6) Å for Fe1 and Fe2 (Figure II.15 and Table

58 Figure II. 15 .

 5815 Figure II.15. The Fe1 (a), Fe2 (a) and two chloroform molecules (c, d) in the asymmetric unit, and the overlay of two Fe 2+ complexes (e) in complex I-pFA at 300 K. The red and pink represent Fe1 and Fe2 molecules, respectively. Upon cooling at 100 K, complex I-pFA remains in the same R-3 space group with smaller unit-cell parameters compared to the ones at 300 K (Table II.4). There are still two Fe 2+ complexes with similar conformations of the complexes as the ones at 300 K and two partially occupied chloroform molecule sites in the asymmetric unit, with one chloroform refined without anisotropy. As seen in Figure II.16a and TableII.5, the average values of Fe-N distances are 2.032(6) Å and 1.970(6) Å for Fe1 and Fe2 molecules, respectively, which suggests a partial HS to LS conversion for Fe1 (~65% LS) and a fully LS state for Fe2. This is reasonably consistent with the χMT value (0.85 cm 3 K mol -1 ) that suggest 25% HS from magnetic measurement.

Figure II. 16 .

 16 Figure II.16. Fe1, Fe2 and two chloroform molecules in the asymmetric unit at 100 K (a), and the superposition of Fe1 (left), Fe2 (right) molecules between 300 K (red) and 100 K (blue) with RMSD values (b) for complex I-pFA.

Figure II. 17 .

 17 Figure II.17. Experimental and simulated X-ray powder diffraction patterns of complex I-pFA. The simulation pattern (black) comes from the experimental single-crystal XRD data.

. 5

 5 reports the calculated distortion parameters for FeN6 polyhedra at 300 K and 100 K. At 300 K, the angular (Σ) and trigonal (Θ) distortions for both Fe1 and Fe2 are around 85° and 237°, respectively. There are no huge differences in the bond distortion (ζ) and shape values of Fe1N6 and Fe2N6. At 100 K, Σ and Θ are 64° and 180° for Fe1, and 57° and 172° for Fe2, respectively, which corresponds to the fully LS state for Fe2 and incomplete LS state for Fe1. The variation of ζ, Σ, and Θ are very close for both Fe 2+ sites, but different enough to delay the SCO of Fe1 compare to Fe2. As show in Figures II.18a and b, there are no huge differences in the shape of fingerprints of the two Fe molecules. In the Hirshfeld surfaces, the short contacts (red spots) are mainly around NCS -groups and pyridine ring, which agrees well with the high ratio of S---H interactions in fingerprints, above 20 % (Figure II.18e). Another short contact is the F---H interactions that contribute to around 15 % to the fingerprints. The ratio of C---C interactions, which usually corresponds to π---π interaction, are very similar for both molecules and around 6 %, The sharp stripe around 1.0 Å suggests possible H---H contacts with H coming from pyridine. Compared to the shape of fingerprint of complex I-mFA, the contacts appear at longer distances, even above 2.8 Å (di and de) in fingerprints, which results from the S---H and F---H with H coming from the solvent. It means the the solvent tends to separate the Fe 2+ sites with Fe---Fe distances of 13.197 Å (Fe1---Fe1) and 12.305 Å (Fe2---Fe2) in I-pFA compared to 10.935 Å (Fe1---Fe1) and 11.355 Å (Fe2---Fe2) in I-mFA. Concerning the solvent sites, there are huge differences in the fingerprints of the two chloroform molecules (Figures II.18c and d). The shortest contact for C(53)HCl3 is S4---H53 (2.813 Å) while the shortest one for C(54)HCl3 is Cl5---Cl6 (2.139 Å). Three C(54)HCl3 solvents linked together, form a crown ring. The ratio of Cl---H contact is near 50% in both solvent. The calculated Hirshfeld surfaces and fingerprints of the sample cooled at 100 K are shown in Figure II.19. Compared with 300 K, there are no huge differences in the shape of fingerprints for Fe1, Fe2 and solvent molecules. The shortest contacts of Cl5---Cl6 and S4---H53 are 1.913Å and 2.432 Å, respectively, shorter than at 300 K. The ratio of F---H and S---H interactions are almost the same as that at 300 K, however, the shape of S---H is different in Fe1. The ratio of C---C slightly decreases. The sharp stripe below 1.0 Å suggests the possible H---H contacts, where hydrogen atoms come from pyridine rings.

Figure II. 18 .

 18 Figure II.18. The fingerprints and Hirshfeld surfaces for Fe1 (a), Fe2 (b), the C(53)HCl3 (c), the C(54)HCl3 solvents (d), and selected contacts in fingerprints of Fe1 (e) and Fe2 (f) the complex I-pFA at 300 K.

Figure II. 19 . 79 (

 1979 Figure II.19. The fingerprints and Hirshfeld surfaces for Fe1 (a), Fe2 (b), C(53)HCl3 (c), C(54)HCl3 solvent (d) and selected contacts in fingerprints of Fe1 (e) and Fe2 (f) of the complex I-pFA at 100 K.

Figure II. 20 . 6 Figure II. 21 .

 20621 Figure II.20. View of the crystal packing enhancing the solvent position along c axis. Sixnumber rings of Fe2 with six C(53)HCl3 (a), six-number rings of Fe2 with three C(54)HCl3 (b), six-number rings of Fe1 with six C(54)HCl3 (c), and the linkage of Fe 2+ molecules arrangement (d) of the complex I-pFA at 300 K.

  the T(LIESST) measurement (Figure II.14b) could have its origin in a partial loss of solvent upon time. Regarding the the metastable HS* state, it can be partially observed by thermal quenching and by LIESST effect, leading to T(TIESST) and T(LIESST) values of 57 and 55 K, respectively. No clear steps in the T(TIESST) or T(LIESST) curves could be observed as a reflect of the two-step character of the thermal SCO. This places this compound on the T0 = 100 K line of the T(LIESST) vs T1/2 database (Figure I.9 in Part I), as most of the PM-L compounds, except I-S-BiA. Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family II.4. Discussion on the structure-properties relationship of [Fe(PM-zFA)2(NCS)2] Table II.6 summarizes the magnetic and structural features of the three fluorinated PM-L based complexes I-oFA, I-mFA and I-pFA. The structure of I-oFA is unknown due to the absence of single-crystal samples. Complexes I-mFA and I-pFA crystallize in different space groups with rather distinct unit-cell parameters, both exhibiting large unit-cell volume compared to usual ones for this family of compounds and both having a crystal packing based on two structurally independent iron sites. I-mFA shows an incomplete SCO around 215 K involving only one of the iron sites (Fe2), the other one remaining HS even at low temperature. Complex I-pFA displays a gradual two-step SCO with switching temperatures around 140 K and 100 K, one site being fully LS and one only partially at 100 K. As mentioned before, in complex I-mFA, the pyridine rings are in cis configuration in Fe2 site and in trans configuration in Fe1 site. Let us notice that this is the first observation of a cis conformation of the pyridine units in this PM-L family. This cis ion displays a spin crossover. The other site exhibiting a usual trans conformation of the pyridine rings remains HS at low temperature. A closer look at the coordination sphere of these two sites shows that all the distortion parameters of the SCO active site in HS are smaller than the inactive site. This corroborates again the impact of very high distortion on the stabilization of the HS state. Since the conformation of the ligands are strongly different in both sites, one can wonder about the influence of the fluorine group on this distortion. For the SCO inactive site Fe1, the fluorine groups are at 5.654 Å and 5.650 Å from the Fe 2+ center. For the SCO active Fe2 site, the fluorine groups are at 6.526 Å and 6.273 Å from the Fe 2+ center, almost 1 Å farer than in Fe1. Therefore, even in meta position the fluorine atoms may exert a steric strain on the coordination, in that case preventing the SCO to occur. Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family

A 2 , 6 -

 26 fluorophenyl group might have prevent such rotation. Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family 84

  of complex [Fe(bpp)(H2L)](ClO4)2•1.5C3H6O (bpp = 2,6-bis(pyrazol-3-yl)pyridine, H2L = 2,6bis(5-(2-methoxyphenyl)pyrazol-3-yl)pyridine) turns into fully HS when losing one-third of acetone. The new [Fe(bpp)(H2L)](ClO4)2•C3H6O complex can replace acetone by MeOH and H2O to form [Fe(bpp)(H2L)](ClO4)2•1.25MeOH•0.5H2O through a single crystal-to-single crystal process. 18 Another example is related to the reversibility of single crystal-to-single crystal transformation by reabsorption of MeOH in complex {Fe(3-CNpy)2(CH3OH)2/3[Au(CN)2]2} (3-CNpy = 3-CNpyridine), where the hysteresis loop disappears when absorbing the MeOH molecules. 19 Another famous example is observed in {Fe(pz)[Pt(CN)4]}, a 3D metal-organic framework upon water adsorption. Paesani et al.

  Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family 85 high symmetry. This sample also shows large holes within the crystal packing. While these pores are here filled by the chloroform molecules, it however opens interesting perspectives in the context of the scarcity of molecular complexes combining nanoporosity and SCO. This is especially true since, as shown here, chloroform may be quite easy to remove. So, one study in perspective of this work concerns the remove of the solvent or its exchange in a single crystalto-single crystal way. This could bring interesting features in the context of mixing SCO and MOF properties. In summary, three complexes were synthesized and characterized in the [Fe(PM-zFA)2(NCS)2] system (z = o, m, p). From the structural analysis of complexes I-pFA and I-mFA, the angle distortion of the coordination sphere do really enhance from I-pFA to I-mFA.

4 . 92 III. 2 .

 4922 Discussions on I-mBrA and II-mBrA ..................................................................... III.4. Characterizations of four [Fe(PM-pBrA)2(NCS)2] polymorphs ............................. III.4.1. Polymorph [Fe(PM-pBrA)2(NCS)2]-I: I-pBrA ....................................................... III.4.2. Polymorph [Fe(PM-pBrA)2(NCS)2]-II: II-pBrA .................................................... III.4.3. Polymorph [Fe(PM-pBrA)2(NCS)2]-III: III-pBrA ................................................. III.4.4. Polymorph [Fe(PM-pBrA)2(NCS)2]-IV: IV-pBrA ................................................. III.4.5. Discussions on the four pBrA polymorphs............................................................. III.5. Conclusion ................................................................................................................... Bibliography ......................................................................................................................... Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 91 III.1. Introduction As shown in Part. II, the distortion of the coordination sphere can be promoted by changing the position of fluorine atom from para to ortho position in complexes I-pFA and I-mFA. Since the atomic radius of fluorine is the smallest in the halogen serie, bigger halogen atoms, such as Cl, Br and I, could be used to explore the steric strain effect. Since the chlorine substituted PM-zClA (z = o, m, p) ligands are explored by our collaborators, Prof. Anuj K Sharma from the University of Rajasthan (Rajasthan, India), we haven't focus on it. In addition, the atomic radius of iodine may be too large to coordinate with Fe 2+ ions, especially for PM-oIA. So, here we selected the bromine substitution PM-zBrA (z = o, m, p) ligands to study the steric strain effect. Rich polymorphism appears in these systems, where the distortion of the coordination sphere are subtly regulated by the intra-and inter molecular interactions. In this part, the synthesis of the PM-zBrA (z = o, m, p) ligands and the crystallization of the [Fe(PM-zBrA)2(NCS)2] complexes are firstly introduced. Then the characterizations of seven new compounds are described which includes the two polymorphs of [Fe(PM-mBrA)2(NCS)2] and four polymorphs of [Fe(PM-pBrA)2(NCS)2]. Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family Synthesis of the substituted ligands and the [Fe(PM-zBrA)2(NCS)2] (z = o, m, p) complexes III.2.1. Synthesis of the PM-zBrA ligands The Schiff base ligands (PM-zBrA) were synthesized from the condensation of the 2pyridine-carbaldehyde with the corresponding bromine substituted aniline (Scheme III.1)

  + H2O Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 93 leads to the nearly planar geometry of ligand, leading to a short intraligand H6---H8 contact (2.074 Å). It is worth to mention that the intermolecular Br---Br contacts (3.576 Å) play an important role in the molecular packing. The comparison between the experimental X-ray diffraction patterns and the simulated one by single-crystal structure (Figure III.2) indicates that the polycrystalline compound is homogenous and identical to the single-crystal one.

Figure III. 1 .

 1 Figure III.1. Crystal structure of the PM-pBrA ligand at 298 K with the molecule in the asymmetric unit (a), the crystal packing along the b axis (b), and the crystal packing along the c axis (c). Thermal ellipsoids are depicted at the 50 % probability level.

Figure III. 2 .

 2 Figure III.2. The experimental powder diffraction pattern and simulated one from the singlecrystal data of PM-pBrA ligand at 300 K.

98 III. 3 . 2 (

 9832 mixture corresponds to I-pBrA and II-pBrA. Polymorph II-pBrA has never been obtained without polymorph I-pBrA. When 2 ml of acetonitrile containing 0.2 mmol ligand are slowly dropped into 2 ml of Fe(NCS)2 methanolic solution (0.05mmol/ml) at let for one week at 60 , a mixture of dark blue crystals of I-pBrA and III-pBrA are obtained. Polymorph III-pBrA has never been obtained without concomitance of polymorph I-pBrA. Moreover, it was obtained in very small quantities preventing any CHNS or SQUID measurements. When 2 ml of Fe(NCS)2 methanolic solution are added dropwise (0.1mmol/ml) into 2 ml of acetonitrile containing 0.4 mmol of ligand, the dark block crystals of IV-pBrA are formed after one week at 25 . Elemental analysis gives: C: 44.94, H: 2.58, N: 11.79 close to the theoretical values C: 44.98, H: 2.61, N: 12.11 (Appendix 4). Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 97 The powder of polymorphs I-pBrA and II-pBrA can also be prepared in the following procedures. When 5ml CH3CN containing 1mmol ligand are added quickly into 15 ml of Fe(NCS)2 methanolic solution (0.05 mmol/ml) during stirring under argon. After reaction one night at 25 , the solid were flitrated and washed by the cool diethylether, naturally drying in the hood. The products is polymorph I-pBrA. When 10ml CH3OH containing 1 mmol ligand are added quickly into 10 ml of Fe(NCS)2 methanolic solution (0.05 mmol/ml) during stirring under argon. The methanol solution containing reactants was refluxed at 25 for 2 hours, then 5ml diethylether were added after cooling down to room temperature. The solid were flitrated and washed by the cool diethylether.The products is polymorph II-pBrA. Elemental analysis gives: C: 44.97, H: 2.45, N: 11.85 close to the theoretical values C: 44.98, H: 2.61, N: 12.11 (Appendix 4). Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family Characterizations of the [Fe(PM-zBrA)

Figure III. 3 .Figure III. 4 .

 34 Figure III.3. Thermal dependence of the χMT product of I-oBrA recorded at 0.4 K/min (in settle mode) under 20 kOe of applied magnetic field, before irradiation (■), under irradiation ( ) and after irradiation in the dark (■).

Figure III. 5 .

 5 Figure III.5. Thermal dependence of the χMT product of polymorph I-mBrA recorded at 0.4 K/min (in settle mode) under 10 kOe of applied magnetic field.

Part III -

  Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 101 simplicity, Fe1 and Fe2 based complexes are labelled Fe1 and Fe2, hereafter. As seen in Figure III.6 the thiocyanate groups are in a cis configuration and PM-mBrA are also in cis, where two the pyridine rings are in trans conformation. By the superposition of Fe1 and Fe2, one can notice the difference in ligand orientation. Indeed, the two equivalent Br3 atoms point toward the NCS(3) -with the Br3---S3 distance of 5.670 Å in Fe2. In Fe1, the Br1 atom is pointing away from the NCS(2) -while the Br2 atom is pointing toward the NCS(1) -leading to a long Br1---S2 distance of 9.039 Å and a short Br2---S1 distance of 5.219 Å. The average value of Fe-N distances in both molecules are 2.164(2) Å and 2.156(5) Å, respectively, which are the typical value for HS Fe 2+ ion. The thiocyanate groups are not linear with angles of 158.70° (Fe2-NC-S3), 157.44° (Fe1-NC-S2), and 158.74° (Fe1-NC-S1). The bending of NCS -groups gives rise to the large value of N1-Fe1-N6 (105.96°), and N7-Fe2-N7 (104.24°).

Figure III. 6 .

 6 Figure III.6. The two crystallographically independent Fe1 (a) and Fe2 (b) molecules and their superposition of Fe1 (red) and Fe2 (yellow) (c) in polymorph I-mBrA at 300 K. The unit of the Fe-N distances is in angstrom.

Figure III. 7 . 103 Figure III. 8 . 9 , 104 Figure III. 9 . 105 Figure III. 10 .

 710389104910510 Figure III.7. The Fe1 and Fe2 packings along a direction (a), the shape index in Hirshfeld surface for π---π interactions between Fe1 and Fe2 (b) and between Fe1 and Fe1 (c) in I-mBrA at 300 K. The black circles stand for the position of π---π contact.

Part III -

  Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 106 SCXRD data allow to simulate to PXRD pattern at 300 K to compare with the experimental one (Figure III.11). One can clearly see the good agreement observed between both diffractograms indicating that the phase characterized by magnetic measurements corresponds to the crystalline phase studied in SCXRD. Therefore, this compound does not show any SCO down to 10 K, while it presents a symmetry breaking upon cooling.

Figure III. 11 .

 11 Figure III.11. Experimental (red) and simulated (black) powder X-ray diffraction patterns of complex I-mBrA.

III. 3 . 3 .

 33 conformation in Fe2 or in trans conformation in Fe1 whereas in polymorph I-mBrA both Fe 2+ sites have their prydine groups in trans. In complex I-mFA, "trans-Fe1" does not experience Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 107 any spin crossover so as "trans-Fe1" and "trans-Fe2" sites in polymorph I-mBrA. It looks therefore consistent. The bromine groups in polymorph I-mBrA are between 5.95 Å and 6.99 Å away from the Fe 2+ center which is farer than in complex I-mFA where the fluorine are at 5.65 Å from the "trans-Fe1" center. This might be explained by the bigger size of the bromine compare to the fluorine. Polymorph [Fe(PM-mBrA)2(NCS)2]-II: II-mBrAMagnetic properties of II-mBrAMagnetic measurements were performed on a polycrystalline sample, using a SQUID magnetometer (see Appendix 3). Figure III.12 reports the thermal evolution of the χMT product.The χMT value at 300 K of 3.64 cm 3 K mol -1 agrees well with a HS Fe 2+ ion (g = 2.22). Upon cooling, the χMT product gradually decreases to reach a value close to 0 cm 3 K mol -1 below 100 K (0.03 cm 3 K mol -1 exactly), indicating a gradual and complete spin crossover, without any hysteretic behaviour. The transition temperature is about T1/2 = 163 K. The occurrence of a SCO on this sample is strongly different from what was observed on polymorph I-mBrA.

Figure III. 12 . 1

 121 Figure III.12. Thermal dependence of the χMT product of polymorph II-mBrA recorded at 0.4 K/min (in settle mode) under 10 kOe of applied magnetic field.

Final

  diff. peak/hole / e Å -3 2.09/-0.61 1.83/-0.59 R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2 Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 109 All atoms sit on the general Wyckoff position (8f) except Fe 2+ (4e position), so there is only half a molecule in the asymmetric unit. The thiocyanate group and PM-mBrA are in a cis configuration, where the two pyridine rings are in trans conformation (Figure III.13). The ligand conformation is similar to Fe2 in polymorph I-mBrA with the Br atoms pointing toward the S with S1---Br1 of 6.893 Å (at 300 K). The Fe-NC-S angle is 145.5° and the (SC)N-Fe-N(CS) angle is 97.86°. The bending of NCS -groups gives rise to the large S1---S1 distance of 7.767 Å. The average Fe-N bond length (<Fe-N>) is 2.154(5) Å at 300 K (Figure III.13), whichis the typical for and Fe 2+ in its HS state, agreeing with the magnetic measurement.

Figure III. 13 .Figure III. 14 .

 1314 Figure III.13. One full iron molecule in polymorph II-mBrA at 300 K (a), 150 K (b) and the superposition of the molecular structures (c). The hydrogen atoms are omitted for clarity and the red and blue stand for 300 K and 150 K, respectively.

112 Figure III. 15 .

 11215 Figure III.15. The calculated Hirshfeld surfaces of Fe1 at 300 K (a) and 100 K (b), and selected hydrogen bonding in fingerprints at 300 K (c) and 100 K (d), and the πphenyl---πphenyl interaction (e) of II-mBrA.

Figure III. 16 .

 16 Figure III.16. The H---π contacts at 300 K (a) and 150 K (b), and three-dimensional packings at bc plane at 300 K (c) and 150 K (d) in polymorph II-mBrA. The molecules in green stand for the 21 axis along b direction.

Figure III. 17 .

 17 Figure III.17. Superposition of the molecular complex corresponding to the Fe2 site in polymorph I-mBrA (orange) and the molecular complex in polymorph II-mBrA (green) at 300K.

III. 4 . 2 (Figure

 42 Figure III.18 reports the thermal behavior of the χMT product of polymorph I-pBrA,recorded on an assembly of crystals. The χMT value at 300 K of 3.98 cm 3 K mol -1 agrees well with HS Fe 2+ ion (g = 2.30). Upon cooling, the χMT product decreases rapidly below 175 K to reach 0.07 cm 3 K mol -1 below 125 K which agrees with a purely LS state. Upon warming, the curve describes a hysteresis loop. This hysteresis is slightly temperature scan rate dependent (Figure III.18a). Extrapolating the evolution of the switching temperatures at infinitely slow scan rate leads to T1/2↓= 170 K and T1/2↑= 181 K, and an 11 K large hysteresis loop.

Figure III. 18 . 16 Figure III. 19 . 4 K

 1816194 Figure III.18. χMT as a function of the temperature for polymorph I-pBrA at different scan rates (a), the evolution of the switching temperature T1/2 in warming and cooling modes as function of the temperature scan rate (b). The red right lines are the linear regression fits.

Figure III. 20 .

 20 Figure III.20. (left) χMT as a function of temperature for polymorph I-pBrA showing the T(TIESST) curve ( ), the LIESST effect under irradiation at 650 ( ) and 830 nm ( ) and the subsequent T(LIESST) curves ( and after 650 and 830 nm irradiation respectively), recorded at 0.4 K/min. (Right). χMT as a function of time at 10 K, under irradiation at 650 ( ) and 830 nm ( ) with around 5 mW/cm².

  crystallographically independent iron complexes in the asymmetric unit, which are labelled Fe1 and Fe2 (Figure III.21). The thiocyanate groups and PM-pBrA are in a cis configuration, where two pyridine rings are in trans form. At 300 K, the Fe-N-CS angles are 160.79° (Fe2-N-CS4), 175.85° (Fe2-N-CS3), 159.63° (Fe1-N-CS1), and 170.57° (Fe1-N-CS2) (Table III.8). It results in (SC)N---Fe---N(CS) angles in Fe1, and Fe2 of 94.4° and 96.5°, respectively, which are smaller than that in polymorph II-mBrA. The most salient difference between both molecules can be seen comparing σ3 values that probe the dihedral angle between the phenyl and pyridine rings within the same ligand. In Fe1, the σ3 values are of 75° and 52° while they are of 79° and 30° in Fe2. The geometry of the ligands are then clearly different, as illustrated by the large RMSD value of 0.650 Å in the superposition of molecules (Figure III.21d

Figure III. 21 .

 21 Figure III.21. The two independent molecules in the asymmetry unit at 300 K (a), 80 K (b), and 80 K in the Q-HS* (c) of polymorph I-pBrA. Superpositions of Fe1 and Fe2 molecules at 300 K (d), 80 K (e), and 80 K in the Q-HS* (f).

molecules are shown in Figure III. 22 .

 22 From HS to LS, the Fe1 molecule does not change too much (RMSD = 0.374 Å), except the change due to the contraction from the spin crossover. On the contrary, Fe2 experiences a huge change with a RMSD of 0.734 Å. In Fe2, SCO is accompanied by a motion of NCS -but overall by a huge rotation of the bromophenyl groups.

124 Figure III. 22 . 125 Figure III. 23 . 126 Figure III. 24 .

 124221252312624 Figure III.22. Superposition of Fe1 and Fe2 between HS and LS (a), Q-HS* and LS (b), Q-HS* and HS (c) of polymorph I-pBrA.

128 Figure III. 25 . 129 Figure

 12825129 Figure III.25. The selected contacts in the fingerprints at 300 K (a), 80 K (b), and quenched 80 K (c) for polymorph I-pBrA, corresponding to the HS, LS, and Q-HS* states.

Figure III. 26 . 131 Figure III. 27 .

 2613127 Figure III.26. The evidence for short H45---H33 (a), S3---S3 (b) contacts and πpy•••πpy interactions (c) in the Hirshfeld surface in polymorph I-pBrA at 300 K.

Figure III. 28 .

 28 Figure III.28. The Fe2 packing at (10-1) by H---H and πpy•••πpy interactions (a) and 3D structure (b) of polymorph I-pBrA at 300 K.

  and d). Due to the rotation of phenyl ring and atomic displacement of NCS -at SCO, the Fe2---Fe2 distance (10.712 Å) decreases and the Fe2-Fe2-Fe2 angle (155.83°) increases along the b direction, compared with 11.402 Å and 138.58° at 300 K. This leads to a more linear packing of Fe2 in LS state. Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 132 Regarding Fe1 at 80 K, it also become more linear along the b direction with the Fe1---Fe1 distance (10.746 Å) and the Fe1-Fe1-Fe1 angle (154.16°), compared with 11.219 Å and 143.78° at 300 K. Moreover, owing to the elongation of πpy•••πpy from 3.978 Å (300 K) to 5.310 Å (80 K), the corresponding Fe2---Fe2 distance increases from 9.131 Å to 9.607 Å. The overall packing therefore looks strongly modified from HS to LS, reflecting the modification of the intermolecular interactions itself in connection with the ligand rotation.

Figure III. 29 .

 29 Figure III.29. The Fe1 and Fe2 packing along b direction (a), an inverted symmetry of four molecules (b), the arrangement of Fe2 at (101) (c), (10-1) (d), and crystal packing (e) of polymorph I-pBrA at 80 K.

Figure III. 30 .

 30 Figure III.30. Thermal dependence of the H---H distances around Fe1 (a) and Fe2 molecules (b) with values extracted from Table III.10 in polymorph I-pBrA.

135 Figure III. 31 .

 13531 Figure III.31. Temperature dependence of the unit-cell volume in polymorph I-pBrA, determined by single-crystal X-ray diffraction.

Figure III. 32 .

 32 Figure III.32. Temperature dependence of the unit-cell parameters in polymorph I-pBrA, determined by SCXRD. The down (up) arrow means the cooling (warming) mode.

Figure III. 33 .

 33 Figure III.33. Thermal variation of S---S and πpy---πpy distances around Fe2 molecules (a) and the structural overlay of the LS and Q-HS* lattices (b) in polymorph I-pBrA.

  Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 137 collaborator Dr. Sébastien Pillet (University of Lorraine, Nancy, France), the low temperature region accessible was 10 to 80 K under a helium flux. This helium flux immediately quenches the HS state. The fact that T(TIESST) is much higher than 80 K, prevent recording the h -HS* to LS relaxation and therefore the fully LS state at 10 K needed to further excite by light. Hence, the PXRD experiments at 15 K using light irradiation at 810 nm to convert from LS to h -HS*, were performed to explore the variation of unit-cell volume in the light-induced regime. The data were analyzed by the Le Bail method using Fullprof. 24 (Figure III.34a) and the results are gathered in Table III.11. Figure III.34b shows the peak index at 100 K and 300 K, where the main different peaks are (14-1) at 2θ = 18.05° and (24-2) at 2θ = 21.3° for LS, (320) at 2θ = 18.7° and (32-3) at 2θ = 22.5° for HS.

Figure III. 34 . 138 Figure 139 Figure III. 35 .

 3413813935 Figure III.34. Diffractogram of polymorph I-pBrA after 16 h of irradiation at 15K analyzed by Le Bail method (a) and the simulated from SCXRD data and experimental PXRD patterns with peak index at 300 K and 100 K (b).

. 11 )

 11 . As seen in Figure III.36, the value of a decreases by 0.6 Å while c and β increases by 1.2 Å and 1.0° from 15 K (hν-HS*) to 100 K (LS). This strongly anisotropic variation of unit-cell parameters during the hν-HS* to LS relaxation process is quite similar to the one observed for the thermal SCO. So the increase of the unit-cell volume at the thermal SCO is also observed from hν-HS* to LS as seen in Figure III.37. Considering the fact that the unit-cell parameters of hν-HS* at 15 K are quite similar to the quenched one at 10 K, Q-HS*,it could be reasonable to assume that hν-HS* and Q-HS* can have a similar crystal structure at 10 K. In this case, the same single-crystal to single-crystal transformation as the one observed from HS to LS may also occur from hν-HS* to LS. In fact, the 75% of hν-HS* at 10 K clearly witnesses the photo-driven rotation of the phenyl ring and the resulting unit-cell contraction from LS to hν-HS* (expansion from hν-HS* to LS relaxation), which is the reverse behavior of all other known so far SCO molecular compounds. The exotic and innovative character of this polymorph is highlighted again here.

Figure III. 36 .

 36 Figure III.36. Temperature dependence of the unit-cell parameters showing the thermal SCO (empty circles) as well as the photo-excitation from LS (red triangle) to hν-HS* (blue triangle) in polymorph I-pBrA, determined by PXRD.

Figure III. 37 .

 37 Figure III.37. Temperature dependence of the unit-cell parameters showing the thermal SCO (empty square) as well as the photo-excitation from LS (red star) to hν-HS* (blue star) in polymorph I-pBrA, determined by PXRD.

. 11 )

 11 photo-isomers. At 10 K, this huge rotation in solid needs a lot of energy, since it implies huge

Figure III. 38 . 20 Figure III. 39 .

 382039 Figure III.38. T(LIESST) vs ΔΘ in the [Fe(PM-L)2(NCE)2] family. The red sphere on the line represents for polymorph I-pBrA

III. 4 . 2 . 146 Figure III. 40 .

 4214640 Figure III.40. Thermal dependence of the χMT product of polymorph II-pBrA, recorded at 1 K/min under an applied magnetic field of 15 kOe.

  crystallographically independent molecules in the asymmetric unit (Figure III.41a). The average Fe-N bond lengths (<Fe-N>) are 2.140(5) Å and 2.144(7) Å at 350 K for Fe1 and Fe2 entities, which is slightly smaller than the typical value of HS in this family, which is possible due to the partial damage of crystal at high temperature. It is worth to mention that one phenyl ring in Fe2 molecules is disordered between two positions with the occupancy of 0.79/0.21. The dihedral angle of two disordered phenyl ring is 68°. That recalls the value of 67° observed for the rotation of the phenyl ring in I-pBrA. The Fe-N-CS angles are 171.22° and 165.47° in Fe1, and 170.50° and 172.71° in Fe2, suggesting the bending of NCS -groups. By the overlay Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 147 of Fe1 and Fe2 molecules, the main difference in Fe1 and Fe2 comes from the disordered phenyl ring with RMSD of 0.710 Å (Figure III.41c).

Figure III. 41 .

 41 Figure III.41. The Fe1 and Fe2 molecules in the asymmetric unit at 350K (a), 120K (b), and their overlay at 350K (c) and 120K (d) of polymorph II-pBrA. The red and pink stand for molecules at 350 K while the blue and light blue stand for molecules at 120 K.

Figure III. 42 . 150 Figure

 42150 Figure III.42. Superposition of the experimental curve from magnetic measurements and the simulated one based on the dihedral angle of the disordered phenyl ring from SCXRD data ( ) of polymorph II-pBrA.

Figure III. 43 .

 43 Figure III.43. The molecular overlay between 350K and 120K of polymorph II-pBrA.

Figure III. 44 .

 44 Figure III.44. Experimental (black) and simulated (red) from SCXRD data powder X-ray diffraction patterns of complex II-pBrA.

152 Figure III. 45 . 154 Figure III. 46 . 155 Figure III. 47 . 156 Figure III. 48 .

 15245154461554715648 Figure III.45. Selected contacts at 350K (a) and 120K (b) for polymorph II-pBrA.

. 15 )Figure III. 49 .

 1549 Figure III.49. The Fe1 and Fe2 molecules in the asymmetric unit and their superposition at 300 K (a, d), 90 K (b, e), the Q-HS* at 10 K (c, f) of polymorph III-pBrA.

Figure III. 50 .Figure III. 51 .Figure III. 51 .Figure III. 52 .Figure III. 53 .Figure III. 54 .

 505151525354 Figure III.50. The molecular overlay between 300 K(red) and 90 K(blue) (a), between 90 K and quenched 10 K (green) (b), and between 300 K and quenched 10K (c) of polymorph III-pBrA.

Figure III. 57 .

 57 Figure III.57. Temperature dependence of the unit-cell parameters in polymorph III-pBrA, determined by SCXRD. The down (up) arrow means the cooling (warming) mode.

III. 4 . 4 .

 44 Figure III.59a reports the thermal behavior of the χMT product of polymorph IV-pBrA. The χMT value at 300 K of 3.82 cm 3 K mol -1 agrees well with HS Fe 2+ ion (g = 2.30). Upon cooling, the χMT product increases gradually till 15 K and then decreases to reach 4.59 cm 3 K mol -1 at

Figure III. 59 .

 59 Figure III.59. Thermal dependence of the χMT product of polymorph IV-pBrA (a) and the positive Weiss temperature in 1/χM vs T (b), recorded at 1 K/min under an applied magnetic field of 5 kOe.

Figure III. 60 )

 60 with all atoms sitting on the general Wyckoff position (4e). The unit-cell parameters and volume appear thus quite different from other polymorphs. The average Fe-N bond lengths (<Fe-N>) are 2.174(6) Å and 2.168(7) Å at 300 K and 85 K, respectively, which are typical values for HS Fe 2+ , agreeing with the absence of SCO. The Fe-N-CS angles are 156.25° and 155.33° at 300 K, suggesting an important bending of NCS -groups. As already mentioned (Part. III.3.4

Final 31 Figure III. 60 .

 3160 Figure III.60. The molecule in the asymmetric unit at 300 K (a), 85 K (b) and the molecular overlay between two temperatures (c) of polymorph IV-pBrA.

Figure III. 61 .

 61 Figure III.61. The molecular overlay between polymorphs IV-pBrA and I-pBrA (a), and between polymorphs IV-pBrA and III-pBrA (b).

Figure III. 62

 62 Figure III.62 shows the fingerprints and Hirshfeld surfaces. There are no differences in the fingerprint shapes at 300 K and 85 K. (Figures III.62c and d). An important contribution to the intermolecular interactions comes from hydrogen bonding of S---H (S2---H6, 3.043Å) and Br---H (S1---H4, 3.089 Å). Other interactions include the S2---Br2 (3.730 Å), H21-H4 (2.314 Å) and H14---πphenyl (2.868 Å). The ratio of the selected S---H, Br---H, S---Br and H---H contacts are nearly the same, meaning weaker modification of intermolecular interactions from 300 K to 85 K. There is no short S---S contacts.

Figure III. 62 .

 62 Figure III.62. The Hirshfeld surface at 300 K (a), 85 K (b), and the selected contacts in fingerprints at 300 K (c), 85 K (d) of polymorph IV-pBrA.

Figure III. 63

 63 Figure III.63 shows the intermolecular interactions around Fe1. Along b direction, adjacent molecules are linked by S2---H6 bonding with 21 symmetry (Figure III.63a). The S2---Br2 and H21---H4 connect molecules along [101] direction. The shortest Fe---Fe distance is 7.757 Å, where the intermolecular interaction is H14---πphenyl with distance 2.868 Å (Figure III.63b). The 2D packing of molecules at ab plane is shown in Figure III.63c, where the next-nearest Fe---Fe distance is 9.463 Å. The crystal packing can be viewed as Fe1 pseudo-layers alternating along c axis (Figure III.63d). The packing at 85 K is very similar to 300 K and there is no molecular rearrangement between 300 K and 85 K. So, polymorph IV-pBrA shows a high spin state without the variation of ligand conformation.

Figure III. 63 . 4 . 5 .

 6345 Figure III.63. The interaction around Fe1 at bc plane (a), the H---π interactions (b), the packing at ab plane (c), and crystal packing (d) of polymorph IV-pBrA at 300 K.

Figure II. 64 .

 64 Figure II.64. Molecular overlay between I-pBrA and II-pBrA at HS of Fe1 (a) and Fe2 (b), and at LS of Fe1 (c) and Fe2 (d) with the RMSD values. One part of the disordered phenyl rings is omitted.

Figure II. 65 .Figure

 65 Figure II.65. Molecular overlay between I-pBrA and III-pBrA at HS of Fe1 and Fe2 (b), at LS of Fe1 and Fe2 (b), and at Q-HS* of Fe1 and Fe2 (c) with the RMSD values.

Figure II. 66 .

 66 Figure II.66. Structural overlay between I-pBrA (pink and blue) and II-pBrA (red and green) at HS (a) and LS (b).

  behaviors. For example, two polymorphs have been observed in many compounds including [Fe(phen)2(NCS)2] (phen = phenanthroline)52 , [Fe(bt)2(NCS)2] (bt = 2, 2'-bithiazoline)53 , [Fe(PM-BiA)2(NCS)2]35 , {Fe(abpt)2[N(CN)2]} (abpt = 4-amino-3,5-bis(pyridin-2-yl)-1,2,4triazole)54 , [Fe(abpt)2(tcnoet)2] (tcnoet = 1,1,3,3-tetracyano-2-ethoxypropenide anion)55 , [Fe{(pz)3CH}2](NO3)2 ((pz)3CH = tris(pyrazol-1-yl)methane)56 , {Fe[H2B(pz)2]2(phen)} (H2B(pz)2 = dihydrobis(1-pyrazolyl)borate)57 , Fe[(p-IC6H4)B(3-Mepz)3]2 (((p-IC6H4)B(3-Mepz)3 = (p-iodophenyl)tris(pyrazolyl)borate) 58 , [Fe(CNPh-bpp)2](BF4)2 (4'-(4"'yl)pyridine) 60 , [Fe((pz)3CH)((3,5-Me2pz)3CH)](BF4)2 (3,5-Me2pz = (3NCS)2(cddt)]•2CH3OH (cddt = 2-chloro-4,6-bis(dipyrid-2-ylamino)-1,3,5-triazole) 63 , [Fe(NCS)2(bdpp)] (bdpp = 4,6-bis(2',2''-pyridyl)pyrazine) 64 , [Fe(abpt)2(NCSe)2] 65 , [(TPA)Fe(TCC)]PF6 (TCC = 3,4,5,6-tetrachlorocatecholate dianion) 66 . Three polymorphs have been observed in some cases including [Fe(bpy)2(NCS)2] (bpy = 2,2'-bipyridine) 67 , fac-[Fe(dppa)2(NCS)2] 68 , [Fe(sal2-trien)][Ni(dmit)2] (sal2-trien = salicylaldehyde and triethylenetetramine, dmit = 2-thioxo-1,3-dithiole-4,5-dithiolate dianion) 69 , [Fe(PM-PeA)2(NCSe)2] 70 , [FeL2](BF4)2 (L = 4-ethynyl-2,6-bis(pyrazol-1-yl)pyridine) 71 .SCO materials with four polymorphs were only observed in very few compounds, [Fe(abpt)2(NCS)2] 72 , [Fe(tpa)(NCS)2] (tpa = tris(2-pyridylmethyl)amine)73 . A long-lived photoinduced metastable state with T(LIESST) of 110 K is observed in polymorph D of [Fe(abpt)2(NCS)2], which is associated with the linkage isomerization between NCS -and SCN -.74 Here, the observed polymorphism in PM-zBrA family suggests the flexibility of the ligand.The variety of intermolecular interactions and the way they are changing along the SCO led to various behaviors of the unit-cell volume along the polymorphs (Figure II.67). A general feature for the spin crossover compounds is that from HS to LS, the volumes of the coordination sphere and of the unit-cell decrease. The unit-cell parameters can evolve anisotropically but, in general at the end, the unit-cell volume decreases. Here, it's the first time that polymorphs allow the observation of positive, zero and negative expansions of unit-cell volume at SCO.

  behavior of the negative expansions of unit-cell volume from HS (including the hν-HS* and Q-Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 185 HS) to LS in polymorph I-pBrA is unprecedented, which results from the molecular rearrangements at SCO associated to the variation of molecular geometries in space, especially the phenyl ring rotation. Moreover, the zero thermal expansion in polymorph III-pBrA can also be associated to the rotation of phenyl ring.

Figure II. 67 .

 67 Figure II.67. Variety of unit-cell volume dependences on temperature for polymorphs I-pBrA, II-pBrA and III-pBrA.

  Here, in polymorphs I-pBrA and III-pBrA, the conformation changes of ligands (variation of σ3) at SCO indicates the flexibility of ligands and the networks constructed by the short contacts of S---Br and S---S may be also flexible. Hence, the unusual global volume variations in these two polymorphs may provide interesting clues for the design of materials with NTE effect.Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA FamilyAs often observed for polymorphism, the behaviors of the four polymorphs are strongly different (TableIII.21). I-pBrA and III-pBrA show abrupt and hysteretic SCO at different temperatures, correlated to ligand rotation, while polymorph II-pBrA experiences a gradual SCO together with and order-disorder transition and polymorph IV-pBrA remains HS.

  Concerning the T(LIESST) or T(TIESST) values, we can only discuss I-pBrA and III-pBrA on the T(LIESST) vs T1/2 database 36 (Figure II.68). As already mentioned, polymorph I-pBrA falls on the T0 = 150 K line while polymorph III-pBrA falls on T0 = 120 K line. Having a bidentate ligand-based complex on the T0 line gathering most of the tridentate-based compounds is remarkable. It makes polymorph I-pBrA the compound with the highest

Figure II. 68 .

 68 Figure II.68. The highlight of position of polymorphs (red sphere) in the T(LIESST) vs ΔΘ in the [Fe(PM-L)2(NCS)2] family (a) and the database of T(LIESST) vs T1/2 (b).

  conclusion, we have explored the functionalization of the N-(2'-pyridylmethylne)-aniline ligand by bromo groups in ortho, meta and para positions in this section. With these ligands we have synthesized seven new compounds, including: complex I-oBrA, polymorphs I-mBrA and II-mBrA, polymorphs I-pBrA, II-pBrA, III-pBrA and IV-pBrA. The driving idea was to study the steric strain induced on the coordination sphere by this various substitutions with the expectation of changing the coordination sphere distortion. Unfortunately, the complex with ligands substituted in ortho did not crystallize preventing crystallographic investigations, despite a SCO at 220 K and a T(LIESST) value at 50 K. The characteristics of the spin crossover and the T(LIESST) temperatures are reported in TableIII.21.Polymorphs in this family revealed a wide variety of behaviors with order-disorder transition coupled to SCO (II-pBrA), abrupt and hysteretic transitions (I-pBrA, III-pBrA), gradual SCO (II-mBrA) and even the absence of any spin change (IV-pBrA, I-mBrA). In polymorph I-pBrA a great T(LIESST) value of 107 K was recorded, one of the highest reported and associated with a huge distortion of the coordination sphere. From the multiscale description of the structures of each compound, some crucial clues to understand the observed properties were highlighted, as it will be deeply discussed in part IV.Polymorphs I-pBrA and III-pBrA display the thermal induced the excited spin state trapping effect with T(TIESST) at 109 K and around 70 K, respectively. Considering the same space group and the similar ligand conformation between the HS and Q-HS*, the relaxation from the Q-HS* to LS really corresponds to the rotation of phenyl ring. Polymorphs II-pBrA shows the order-disorder transition of phenyl ring coupling with SCO while LIESST and TIESST effects are absent. Generally, the dynamics of molecular geometry at SCO, including the motion of ligands, the rotation of ligand, the order-disorder transition, the other conformational changes, can affect the thermally quenching effect. Our results provide a platform to explore the molecular dynamics at thermal quenching. Moreover, two crystallographically independent iron sites in polymorphs I-pBrA, II-pBrA and III-pBrA exhibit one step thermal SCO. This is not due to the cooperativity because II-pBrA shows a gradual SCO. Since SCO is concomitant with the transition of ligands in these Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family 189 polymorphs, spin state of one iron site might be driven and dominated by the other one associated to the transition of ligands. Last but important, unique thermal expansion phenomena were recorded. Indeed, polymorph I-pBrA shows a negative expansion of the lattice at SCO and III-pBrA seems to do not exhibit any unit-cell volume change at the SCO. Therefore, it should be taken into account that the volume modification associated to the SCO can be negative, zero or positive depending on the compound but independently of the sense of the conversion (HS to LS or LS to HS). The negative and zero expansion of unit-cell volume can open new avenues in the possible application of the SCO phenomenon. Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family IV.1. Introduction In Parts II and III of the manuscript, we have explored the functionalization of the N-(2'pyridylmethylne)-aniline ligand by fluoro and bromo groups in ortho, meta and para positions. With these ligands we have synthesized ten new compounds: complexes I-oFA, I-mFA and I-pFA, with the fluorine and complex I-oBrA, polymorphs I-mBrA and II-mBrA, and polymorphs I-pBrA, II-pBrA, III-pBrA and IV-pBrA with the bromine ligands. The driving

) and the coordination sphere IV. 2 . 1 .

 21 Comparison of the [Fe(PM-zXA)2(NCS)2] compounds Table IV.1 summarizes relevant information for the following discussion on the ten new compounds: complexes I-oFA, I-mFA and I-pFA with the fluorine substitution, and complex I-oBrA, polymorphs I-mBrA and II-mBrA, and polymorphs I-pBrA, II-pBrA, III-pBrA and IV-pBrA with the brome-substituted ligands. The structure of I-oFA and I-oBrA are unknown due to the absence of single-crystal samples.

T(

  LIESST) database (Figure IV.1). 1 Compounds I-mFA and I-pFA fall on the T0 = 100 K line, I-oFA, I-oBrA and III-pBrA are lying on the T0 = 120 K and I-pBrA fits above the T0 = 150 K. If the hypothesis on influence of an increasing trigonal distortion to increase T0, we can therefore expect the following evolution in : I-mFA ≈ I-pFA < I-oFA ≈ I-oBrA < III-pBrA < I-pBrA. Unfortunately the crystal structure of I-oFA and I-oBrA are unknown and therefore their trigonal distortion are not accessible. According to the values reported in Table IV.1, this trend is observed. Therefore, we can expect a higher variation of the trigonal distortion in complex I-oFA than in I-mFA and I-pFA. It could mean that the strategy of promoting the steric strain by adding substituents close to the coordination sphere is successful.

Figure IV. 1 .

 1 Figure IV.1. The positions of the new compounds obtained in this work (green spheres) and described in parts II-III in the T(LIESST) vs T1/2 database.

. 1 )Figure IV. 2

 12 Figure IV.2 reports the T(LIESST) as function of both ΔΘ and Δ in a 3D plot for I-mFA, I-pFA, I-pBrA and III-pBrA. It appears clearly that the highest T(LIESST) are found for high values of both ΔΘ and Δ . However, with values of ΔΘ = 107° and Δ = 0.28 Å, we could expect T(LIESST) above 50 K for II-pBrA while we did not observe anything. Therefore, the hypothesis that the ΔΘ obtained from the thermal SCO is much bigger than the ΔΘ between LS and hv-HS* might be right. Without the photo-induced structure, it remains a hypothesis.

Figure IV. 2 .

 2 Figure IV.2. T(LIESST)/T(TIESST) as function of ΔΘ and Δ for the four compounds reported in Table IV.1 in a 3D view (a) and in projection on the (ΔΘ,Δ ) plane (b).

Figure IV. 3 .

 3 Figure IV.3. T(LIESST) as function of ΔΘ (a) and Δ (b) for the PM-L family. Black square and blue sphere represent the compounds reported inTable IV.2 and Table IV.1, respectively.

Figure IV. 4 .

 4 Figure IV.4. Projection on the (ΔΘ, Δ ) plane of the T(LIESST) values. The red lines stand for the average ΔΘ (90°) and Δ (0.21 Å) values.

Figure IV. 5 .

 5 Figure IV.5. T(LIESST) as function of Δshape for the PM-L family. Black square and blue sphere represent the compounds reported inTable IV.2 and Table IV.1, respectively.

. 3 and

 3 Figure IV.6 summarize various compounds [Fe(PM-pXA)2(NCE)2].S (E = S and Se, S = Solvent, X = F and Br) and their characteristics further obtained along this work.

  As a general observation (Figure IV.6), all the solvatomorphs made with the p-BrA ligand exhibit gradual spin crossover and the solvent entities are disordered in the HS and LS states.On the contrary, p-FA-based compounds present a wide variety of behaviors, with HS compounds (S-pFA.C8H10 and Se-pFA.C8H10), symmetry breaking with order-disorder transition (S-pFA.C9H12), rotation of methyl in mesitylene (Se-pFA.C9H12) or twinning (S-pFA.C7H8 and Se-pFA.C7H8). Moreover, most of the switching phenomena occur with hysteresis.

Figure IV. 6 . 4 K 1 .

 641 Figure IV.6. Thermal dependence of the χMT product of the new compounds in the family of [Fe(PM-pXA)2(NCE)2].S (E = S and Se, S = Solvent, X = F and Br), recorded at 0.4 K/min (in settle mode), before irradiation (■), under irradiation ( ), after irradiation in the dark (■), and the thermal quenching (■) measurements.

Figure IV. 7 .

 7 Figure IV.7. The reversible symmetry breaking with twin domains in the Q-HS* at 10 K (a) and a tentative phase diagram of the symmetry breaking at the SCO (b) in compound S-pFA.C7H8

Figure IV. 8 .

 8 Figure IV.8. T(LIESST) as function of ΔΘ (a), Δ (b) and ΔShape (c) for the PM-L family. Blue sphere represent for the compounds reported inTable IV.1 and Table IV.3.

Figure IV. 9 .

 9 Figure IV.9. T(LIESST) as function of ΔΘ and Δ for the whole new PM-L family. The red lines stand for the average ΔΘ (95°) and Δ (0.21 Å) values.

Figure IV. 10 7 .

 107 Figure IV.10 shows the evolution of T(LIESST) as function of ΔΘ, Δ and ΔShape. One can clearly see that none of the compounds experiencing a structural rearrangement upon SCO fits the linear trend proposed by Marchivie et al. of T(LIESST) vs Δ . 3 This is also the case for the other parameters. If we consider only the compounds without any structural rearrangement upon SCO (compounds 1 to 21), most of them are aligned along the T(LIESST) vs Δ line. Only [Fe(phen)2(NCSe)2] (6) 15 , [Fe(H2B(pz)2)2bpy] (10) 16 and [Fe(DPyDT)2(NCS)2]0.5CH3O (20) 26 are clearly misaligned. A deeper look at these compounds might help to understand this fact. Regarding the evolution of T(LIESST) as function of Shape, more compounds are misaligned compare to the trend found in Figure IV.7. It indicates that the Shape parameter and Δ are not strictly equivalent. When Shape account for all the modifications, Δ only focus on one aspect, giving more insight on the molecular geometry. Finally, concerning Δ , the cloud of points prevent any tentative of correlation.

Figure IV. 10 .

 10 Figure IV.10. T(LIESST) as function of ΔΘ (a), Δ (b) and ΔShape (c). The half-filled blue squares correspond to compound experiencing significant structural rearrangement upon SCO. The labels correspond to the compounds listed in Table IV.4.

Figure IV. 11 .

 11 Figure IV.11. T(LIESST) as function of ΔΘ and Δ for compounds without any structural rearrangement at SCO (a) and including compounds experiencing structural rearrangement upon SCO (green circles in b). The red lines stand for the average ΔΘ (115°) and Δ (0.21 Å) values.

ΔΘ

  and Δζ values are calculated using the HS and LS values, at different temperatures, instead of the LS and HS* values at the same temperature. This is particularly important when dealing with symmetry changes in the materials. Indeed, as discussed above, compounds with symmetry changes are more prone to stand out of the trend. This is particularly the case of [Fe(PM-PEA)2(NCS)2], [Fe(pic)3]Cl2.EtOH or S-pFA.C7H8 and Se-pFA.C7H8 as discussed above. For these compounds, nothing insures that the LS ↔ HS* and the LS ↔ HS transition have the same structural changes. Therefore, using the ΔΘ and Δζ values calculated from the LS ↔ HS transition might be wrong, explaining why these compounds do not fit the trend. This latter aspect points out the fact that the lifetime of the HS* state might be influenced by other aspects than just the coordination sphere geometry and the cooperativity. Indeed, cooperativity does not necessarily implies symmetry changes. Therefore, looking at upper scales of the materials and their influence on the HS* lifetime is important.Conformational changeWhen studying the structural movies of thermal spin crossover, clear motions of ligands have been observed. In this current work, I-pBrA and III-pBrA clearly show a rotation of the bromophenyl rings, both upon LS ↔ HS* and LS ↔ HS transitions. However, despite similar rotation, its impact on the coordination sphere and T(LIESST) (on T(TIESST) more precisely) is different since I-pBrA shows a T(TIESST) of 107 K and III-pBrA of 70 K. The same effect does not gives the same result in that case. This effect can be related to LD-LISC phenomenon (Ligand-Driven Light-Induced Spin Change) where a change in ligand conformation, triggered by light, changes the ligand field and the subsequent SCO properties.42 However, in the present compounds, the change in conformation is not decorrelated from the SCO and it is therefore difficult to disentangle both effects.In the literature, two different thermal spin transitions have been observed in compound [Fe(n-Bu-im)3tren](PF6)2 [(n-Bu-im)3(tren) = n-butylimidazoltris(2-ethylamino)amine] depending on the sweeping rate of the temperature.43 Two different low spin structures are obtained upon fast cooling at 4 K/min or slow cooling at 0.1 K/min, labeled LS1 and LS2, respectively. Irradiation from the LS2 state leads to a T(LIESST) around 50 K while irradiating from LS1 leads to a T(LIESST) of 80 K.Delgado et al. found that an irradiation from LS1 at 10 K leads to a short-lived hv-HS* while irradiating at 80 K leads to a long-lived hv-HS* state, with a relaxation 40 times slower than that recorded at 80 K but obtained from irradiation at 10 K. The structures of hv-HS* states were obtained by irradiation at 10 K and 80 K using synchrotron single-crystal X-ray diffraction. The difference in lifetime was attributed to the promotion of a disorder in conformation of the butyl chains of the ligand when irradiating at 80 K while this disorder is not observed under irradiation at 10 K.Intermolecular interactionsSince both the volume of the molecule and the ligand conformation can change upon SCO, the intermolecular interactions can be strongly affected. Reversely, as we have described in I-pBrA and III-pBrA, short intermolecular H---H interactions might drive ligand conformation changes. Therefore, there could be subtle feedback effects. Such feedback effect might be viewed as cooperativity. Sometimes, changes in intermolecular interactions are driven by solvent reorganization. In compound Se-pFA.C9H12 ([Fe(PM-pFA)2(NCSe)2].C9H12) the methyl group of the mesitylene solvent rotates by 30° from HS to LS. It is not clear yet if the 32 K large hysteresis that is observed nor if the recorded T(LIESST) and T(TIESST) around 100 K, are driven by this conformational change. Up to now, the crystal structures of the Q-HS* and hν-HS* are not determined. Regarding the value of 151 °, one may expect much higher T(LIESST), so probably, the conformational change of mesitylene affects the lifetime of the HS*. In the compound [FeL2][BF4]2•MeCN with L = 4-{isopropylsulfanyl}-2,6-di{pyrazol-1-yl}-pyridine,

  Symmetry breaking from hv-HS* to LS also occurs in other compounds, including polymorph C of trans-[Fe(abpt)2(NCS)2] 20 , [FeH2L 2-Me ] [XF6]2 (L = bis{[(2-methylimidazol-4-yl)methylidene]-3-ami-nopropyl}ethylenediamine, X = P and As) 58- 59 , [FeLeq(bppa)].MeOH (Leq = {3,3'-[1,2-phenylenebis(iminomethylidyne)]bis(2,4pentanedionato)(2-)-N,N',O2,O2'}, bppa = 1,3-bis(4-pyridyl)propane) 60 . At upper scale, some twinning may happen, as observed in S-pFA.C7H8. The occurrence of light-induced twinning remains rather rare in the literature. Compound [Fe(3-bpp)2][Au(CN)2]2 shows a two steps thermal hysteresis of 45 K, accompanied by a reversible reconstructive structural transition and twinning. The low conversion of LIESST as well as unexpected T(LIESST) values are attributed to the high energy cost of this twinning that requires huge motion of molecules. 61 An unpublished results from Triki et al. has shown the reversible photoinduced twinning that leads to a competition of long-range and short-range interactions during the relaxation process. 62 The number of structures in the HS* state remains relatively rare yet to afford a clear picture of the influence of such structural rearrangement on the lifetime of the HS* state. Nanoscale Finally, to complete the multiscale view of T(LIESST), we have to mention the few studies made on nanoparticles. A first report was made by Neville et al. on nanoparticles of [Fe(NCS)2(bpe)2]. It was shown that, down to 80 nm, the T(LIESST) temperature does not change.

Figure IV. 13 .

 13 Figure IV.13. Multiscale view of the relaxation temperature of T(LIESST) showing events or features linked to T(LIESST). Adapted from ref. 64.

  has been evidenced. Polymorph I-pBrA shows a reverse expansion of the lattice volume, i.e. an expansion from HS to LS, and polymorph III-pBrA exhibits a zero expansion of the unit-cell volume change at the SCO. Spin switch in polymorphs I-pBrA to III-pBrA are coupled to strong motion of ligands. Therefore, it should be taken into account that the volume modification associated to the SCO can be negative, zero or positive depending on the compound but independently of the sense of the conversion (HS to LS or LS to HS).

  Molecular reorganization, caused by the conformational change or the order-disorder transition of ligands, can strongly influence the modification of intra-and intermolecular interactions. If flexible ligands are introduced, able to change of conformation, this might introduce more phonon and can disfavor the hν-HS* state. Therefore, flexible ligands should be associated with the promotion of strong intermolecular contacts in order to propagate the change to the whole packing and induce a strong energy difference.

Final 8 Figure

 8 photocristallographiques de deux polymorphes du complexe [Fe(PM-BiA)2(NCS)2] (PM-BiA = N-2'-pyridylméthylène)-4-(aminobiphényle)), l'anisotropie de l'élongation de la liaison métal-ligand (Δζ) a été identifiée comme un facteur important.7 Le rôle de la distorsion de la sphère de coordination dans la durée de vie du hν-HS* implique qu'une représentation de la spectroscopie des niveaux vibrationnels en fonction de l'élongation de la liaison métal-ligand n'est plus suffisante. Il faut utiliser une vision tridimensionnelle, allant d'une respiration isotrope de la sphère de coordination vers une respiration anisotrope (FigureR1).8

Figure R2 .

 R2 Figure R2. Dépendance thermique du produit χMT du polymorphe I-pBrA (haut) montrant les courbes de relaxation après trempe thermique ( ), pendant l'irradiation à 830 nm ( ) et la courbe ( ), enregistrées à 0.4 K/min. Dépendance thermique de volume de maille cristalline (bas) montrant la conversion thermo-induite ( ) ainsi que la photo-excitation depuis l'état BS (étoile rouge) vers l'état hv-HS* (étoile bleue).

Figure R3 .

 R3 Figure R3. T(LIESST) en fonction de ΔΘ and Δ pour les composés ne présentant pas de changements structuraux (a) et ceux avec changements structuraux cerclés de vert (b). Les lignes rouges représentent les valeurs moyennes de ΔΘ (115°) et Δ (0.21 Å).

Light-Induced Excited Spin State Trapping effect I.2.1. The photoswitching process

  Among the various ways of using light to induce a spin crossover, the Ligth-Induced Excited Spin-State Trapping (LIESST) effect is probably the most studied. This effect is reversible, occurs in the solid state and allows the trapping of a metastable state using light irradiation within few femtoseconds. The lifetime of the photo-induced state is strongly temperature dependent and is easily observed at cryogenic temperatures (below 50 K). The current work of this thesis is focused on this lifetime and more details on this phenomenon are given below.

The LIESST effect has been first reported for the solid state in 1984 on the [Fe(ptz)6](BF4)2 (ptz = 1-propyl-1H-tetrazole) SCO compound.

106

Later, Hauser et al. evidenced the reversibility of this phenomenon, called reverse-LIESST. 107 This photoswitching effect is based on the excitation by light of metallic d-d or metal-ligand charge transfer (MLCT) transitions. Hauser et al. deeply studied this effect using absorption spectroscopy as summarized in Figures I.5a

Table I .1. The

 I

	summary of the SCO charateristics of the reported compounds in the
	family of [Fe(PM-L)2(NCE)2] (E = S and Se) when this work starts.
	Compound	Kind of SCO	T1/2↓	T1/2↑	ΔT	T(LIESST)	Ref.
			/ K	/ K	/ K	/ K
	[Fe(PM-HA)2(NCS)2] No SCO, HS					68
	I-[Fe(PM-BiA)2(NCS)2] Hysteretic	168	173	5	78	69,94
	II-[Fe(PM-BiA)2(NCS)2] Gradual	190			34	69,94
	[Fe(PM-NEA)2(NCS)2] Gradual	210			58	149
	[Fe(PM-TeA)2(NCS)2]•	Incomplete	125				69
	0.5CH3OH
	I-[Fe(PM-TheA)2(NCS)2] Gradual	208			no	69
	[Fe(PM-PEA)2(NCS)2] Hysteresis *	188	228	40	64	150
	I-[Fe(PM-PEA)2(NCSe)2] Hysteresis *	266	307	41	no	90
	II-[Fe(PM-PEA)2(NCSe)2] Gradual	245			no	149
	[Fe(PM-DiyA)2(NCS)2] No SCO, HS					68
	[Fe(PM-AzA)2(NCS)2] Gradual	189			46	48,69
	[Fe(PM-FIA)2(NCS)2] Gradual	165			64	150
	[Fe(PM-FIA)2(NCSe)2] Gradual	236			35	150
	[Fe(LH iPr )2(NCS)2]	Abrupt	154	167	13	Not reported 151
	[Fe(LH C6 )2(NCS)2]	Gradual	117			Not reported 152
	[Fe(LH C16 )2(NCS)2]	Gradual	176			Not reported 152
	* SCO hysteresis associated with a structural phase transition.

Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM- zFA Family II.1. Introduction .................................................................................................................... II.2. Synthesis of the fluorine-substituted ligands and of the [Fe(PM-zFA)2(NCS)2] complexes ................................................................................................................................ II

  .2.1. Synthesis of the fluorine-substituted ligands .............................................................

	T(LIESST) on all
	compounds with bidentate ligands will be discussed in Part IV, giving rise to new perspectives
	of the relaxation temperature T(LIESST).
	II.2.2. Synthesis of the [Fe(PM-zFA)2(NCS)2] complexes ...................................................

II.3. Characterizations of the [Fe(PM-zFA)2(NCS)2] complexes ........................................

	II.3.1. [Fe(PM-oFA)2(NCS)2] (I-oFA) .................................................................................
	II.3.2. [Fe(PM-mFA)2(NCS)2] (I-mFA) ................................................................................
	II.3.3 [Fe(PM-pFA)2(NCS)2]•nCHCl3 (I-pFA) ....................................................................

II.4. Discussion on the structure-properties relationship of [Fe(PM-zFA)2(NCS)2] ........ Bibliography ...........................................................................................................................

	Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine
	Substitution of PM-zFA Family
	II.1. Introduction
	SCO materials belonging to the [Fe(PM-L)2(NCE)2] family (E = S and Se) display rich
	structural (polymorphism and symmetry breaking), magnetic and photomagnetic behaviours,
	providing interesting clues for structural-properties relationship in SCO. 1-6 More than 16
	compounds have been reported so far in this family. Since we strongly suspect the relaxation
	temperature T(LIESST) to be greatly related to the distortion of the coordination sphere 7 , our
	target is to chemically promote such distortion and see if it increases the relaxation temperature
	T(LIESST). The PM-zXA (z = o, m, p; X = halogen atoms) ligands have been designed for that
	purpose by introducing the halogen group on a phenyl ring.

Table II .1. Crystal data and structure refinement data for PM-pFA ligand

 II

		at 300 K.
	Empirical formula	C12H9FN2
	Formula weight / g mol -1	200.21
	Temperature / K	300
	Radiation MoKα / Å	λ = 0.71073
	Crystal size / mm 3	0.28 × 0.24 × 0.20
	Crystal system	monoclinic
	Space group	P21/m
	a / Å	4.720(2)
	b / Å	31.460(14)
	c / Å	9.939(5)
	α / °90
	β / °91.582(17)
	γ / °90
	Volume / Å 3	1 475.3(12)
	Z-formula	6
	ρcalc / g cm -3	1.352
	μ / mm -1	0.095
	F(000)	624.0
	2θ range for data collection / °2.59 -52.728
	Index ranges	-5 ≤ h ≤ 5, -38 ≤ k ≤ 39, -12 ≤ l ≤ 12
	Reflections collected	17490
	Independent reflections	3062 [Rint= 0.0510, Rsigma = 0.0488]
	Data/restraints/parameters	3062/0/233
	Goodness-of-fit on F 2	1.136
	Final R indexes [I>=2σ (I)]	R1 = 0.0647, wR2 = 0.1701
	Final R indexes [all data]	R1 = 0.1265, wR2 = 0.1953
	Largest diff. peak/hole / e Å -3	0.19/-0.19

57 II.3. Characterizations of the [Fe(PM-zFA)2(NCS)2] complexes II.3.1. [Fe(PM-oFA)2(NCS)2] (I-oFA)

	Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine
		Substitution of PM-zFA Family
	Although the crystal structure remains unknown, magnetic measurements of I-oFA were
	performed on a powder sample, using a SQUID magnetometer (see Appendix 3). Figure II.3
		2 mmol of ligand, dark block crystals are formed in a dark blue
	solution at 25	after two days, later referred to as I-pFA hereafter. Elemental analysis gives:
	C: 52.37, H: 3.08, N: 13.42 close to the theoretical values C: 50.62, H: 2.96, N: 13.38, calculated
	based on the formula [Fe(PM-oFA)2(NCS)2].0.47CHCl3 (Appendix 4).

Table II . 2 .

 II2 Crystal data and structure refinement data for complex I-mFA.

	Empirical formula		C26H18F2FeN6S2
	Formula weight / g mol -1		572.43
	Temperature / K	300	200	100
	Radiation MoKα / Å		λ = 0.71073
	Crystal size / mm 3		0.24×0.06×0.04
	Crystal system		monoclinic
	Space group		C2/c
	a / Å	50.7003(10)	49.936(2)	49.534(5)
	b / Å	9.45650(10)	9.3384(4)	9.2969(9)
	c / Å	21.8685(4)	21.7673(9)	21.725(2)
	α / °90
	β / °95.0490(10)	94.5107(12)	94.624(3)
	γ / °90
	Volume / Å 3	10 444.1(3)	10 119.1(7)	9 972.1(17)
	Z-formula		16
	ρcalc / g cm -3	1.456	1.503	1.525
	μ / mm -1	0.779	0.804	0.816
	F(000)	4 672.0	4672.0	4 672.0
	2θ range for data collection / °3.226 -51.362	3.754 -52.744	3.762 -52.742
	Index ranges
	Reflections collected	38 172	99 655	97 200
		9 936	10 341	10 212
	Independent reflections	[Rint = 0.0975,	[Rint= 0.1094,	[Rint = 0.1053,
		Rsigma = 0.0758]	Rsigma= 0.0694]	Rsigma = 0.0637]
	Data/restraints/parameters	9936/0/667	10341/0/655	10212/0/655
	Goodness-of-fit on F 2	1.014	1.083	1.022
	Final R indexes [I>=2σ (I)]	R1 = 0.0689,	R1= 0.0614,	R1 = 0.0607,
		wR2 = 0.2068	wR2= 0.1578	wR2 = 0.1373
	Final R indexes [all data]	R1 = 0.1479,	R1= 0.1146,	R1 = 0.0991,
		wR2 = 0.2794	wR2= 0.1825	wR2 = 0.1556
	Largest diff. peak/hole / e Å -3	1.27/-1.29	1.94/-0.72	3.52/-0.92

-61 ≤ h ≤ 61, -11 ≤ k ≤ 11, -26 ≤ l ≤ 26 -62 ≤ h ≤ 62, -11 ≤ k ≤ 11, -27 ≤ l ≤ 27 -61 ≤ h ≤ 61, -11 ≤ k ≤ 11, -27 ≤ l ≤ 27

Table II .5. The

 II

	Compound		I-pFA.
	Temperature / K	300	300	100	100
	Fe 2+ site	Fe1	Fe2	Fe1	Fe2
	<Fe-N> / Å	2.164	2.165	2.032	1.970
	ζ / Å	0.379	0.409	0.198	0.074
	Σ / °85	84	64	57
	Θ / °237	234	180	172
	Shape b	1.473	1.284	0.830	0.607
	(SC)N-Fe-N(CS) / °98.43	103.34	93.78	95.77
	σ1 / °78.22	78.73	82.88	84.72
	σ2 / °166.52	167.48	171.40	176.81
	σ3 / °46.81 /	52.49 /	50.96 /	60.19 /
		38.28	45.22	42.09	48.99

coordination sphere parameters of complex I-pFA. σ1: N-Fe-N with N from imine; σ2: N-Fe-Nwith N from pyridine; σ3: dihedral angle of phenyl and pyridine in one ligand. a values obtained using the OctaDist program (https://octadist.github.io). b values obtained using the SHAPE program (ref.10).

Table II .

 II

	Compound	I-oFA		I-mFA			I-pFA
	SCO type	Gradual	Gradual and incomplete	Gradual and two steps
	T1/2 / K	215		215				140/100
	T(LIESST) / K	55		32					55
	T(TIESST) / K	no		no					57
	Space group			C2/c				R-3
	Temperature / K no single	300		100		300		100
	Fe 2+ site	crystal by SCXRD	Fe1	Fe2	Fe1	Fe2	Fe1	Fe2	Fe1	Fe2
	Conformation of		trans	cis	trans	cis	trans	trans	trans	trans
	pyridine
	Spin state		HS	HS	HS	LS	HS	HS	65%LS 100%LS
	<Fe-N> / Å		2.180	2.160 2.171 1.960	2.164	2.165	2.032	1.970
	ζ / Å		0.405	0.32	0.388 0.058	0.379	0.409	0.198	0.074
	Δζ / Å		Fe1 : 0.017, Fe2 : 0.262	Fe1 : 0.181	Fe2 : 0.335
	Σ / °92	71	91	41	85	84	64	57
	ΔΣ / °Fe1 :1, Fe2 : 30		Fe1 : 21	Fe2 : 27
	Θ / °231	199	237	133	237	234	180	172
	ΔΘ / °Fe1 : -6, Fe2 : 66		Fe1 : 57	Fe2 : 62
	Shape (S)		1.529	0.977 1.596 0.387	1.473	1.284	0.830	0.607
	ΔS		Fe1 : -0.067, Fe2 : 0.590	Fe1 : 0.643	Fe2 : 0.677
	F---Fe / Å		F1-Fe1: 5.654, F2-Fe1: 5.650,	F1-Fe1: 7.059 F3-Fe2: 7.052
	(300 K)		F3-Fe2: 6.526, F4-Fe2: 6.273	F2-Fe1: 7.031 F4-Fe2: 6.945

6

Selected relevant structural and magnetic data of I-oFA, I-mFA and I-pFA.

Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family

These two structural aspects never appeared yet in [Fe(PM-L)2(NCE)2] family, not even in the larger Fe

2+

family based on bidentate ligands. Part II -Promoting Distortion of the Coordination Polyhedron by Steric Strain: Fluorine Substitution of PM-zFA Family Part III -III.1. Introduction .................................................................................................................. III.2. Synthesis of the substituted ligands and the [Fe(PM-zBrA)2(NCS)2] (z = o, m, p) complexes ................................................................................................................................ III.2.1. Synthesis of the PM-zBrA ligands ............................................................................ III.2.2. Synthesis of the [Fe(PM-zBrA)2(NCS)2] (z = o, m, p) complexes ........................... III.3. Characterizations of the [Fe(PM-zBrA)2(NCS)2] complexes (z = m, o) .................. III.3.1. [Fe(PM-oBrA)2(NCS)2] (I-oBrA)............................................................................. III.3.2. Polymorph [Fe(PM-mBrA)2(NCS)2]-I: I-mBrA ....................................................... III.3.3. Polymorph [Fe(PM-mBrA)2(NCS)2]-II: II-mBrA .................................................. III.3.

Table III . 1 .

 III1 Crystal data and structure refinement data for PM-pBrA ligand.

	Empirical formula	C12H9BrN2
	Formula weight / g mol -1	261.12
	Temperature / K	298
	Radiation MoKα / Å	0.71073
	Crystal size / mm 3	0.08 × 0.24 × 0.12
	Crystal system	monoclinic
	Space group	P21/a
	a / Å	11.6167(5)
	b / Å	4.8924(4)
	c / Å	19.5165(3)
	α / °90
	β / °106.7795(5)
	γ / °90
	Volume / Å 3	1061.97(10)
	Z-formula	4
	ρcalc / g cm -3	1.633
	μ / mm -1	3.835
	F(000)	520.0
	2θ range for data collection / °6.542 -54.982
	Index ranges
	Reflections collected	18180
	Independent reflections	2 439 [Rint = 0.0412, Rsigma = 0.0249]
	Data/restraints/parameters	2439/0/136
	Goodness-of-fit on F 2	1.136
	Final R indexes [I>=2σ (I)]	R1 = 0.0382, wR2 = 0.1252
	Final R indexes [all data]	R1 = 0.0528, wR2 = 0.1446
	Largest diff. peak/hole/e Å -3	0.44/-0.37

-15 ≤ h ≤ 14, -6 ≤ k ≤ 6, -25 ≤ l ≤ 25

  of the freshly prepared Fe(NCS)2 solution (0.05mmol/ml) are slowly dropped into 2 ml of dichloromethane containing 0.2 mmol of ligand, dark block crystals are formed in a dark blue solution at 25 after two days, leading to polymorph II-mBrA. Elemental analysis gives: C: 0.11, H: 1.63, N: 0.63, which does not coorespond to the theoretical values C: 44.98, H: 2.61, N: 12.11 (Appendix 4). The values are very small at two measurements and it is not clear why it is so low value in this complex.

	Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism:
	Bromine Substitution of PM-zBrA Family
	If 2 ml Synthesis of [Fe(PM-pBrA)2(NCS)2]: polymorphs I-pBrA, II-pBrA, III-pBrA and
	IV-pBrA
	Depending on the reaction conditions, single crystals of four polymorphs can be obtained
	when reacting the PM-pBrA ligand to a solution of Fe(NCS)2, later referred to as I-pBrA, II-
	mBrA.
	If 2 ml of diethylether containing 0.2 mmol of ligand, are slowly dropped into 2 ml of the
	Fe(NCS)2 methanol solution (0.05mmol/ml), blue needle crystals are formed in a dark blue
	solution at 25 after one week, leading to polymorph I-mBrA. Elemental analysis gives: C:
	96

52.32, H: 2.93, N: 13.14 larger than the theoretical values C: 44.98, H: 2.61, N: 12.11. As seen in Appendix 4, the results of elements were different at two measurements. It was not obvious to obtain something relevant, maybe because of the presence of Br that are influencing the measurements and datatratements.

Synthesis of [Fe(PM-mBrA)2(NCS)2]: polymorphs I-mBrA and II-mBrA

Depending on the reaction conditions, two polymorphs were obtained separately when reacting the PM-mBrA ligand to a solution of Fe(NCS)2, later referred to as I-mBrA and II-44.49, H: 2.48, N: 11.46 close to the theoretical values C: 44.98, H: 2.61, N: 12.11 (Appendix 4). pBrA, III -pBrA and IV-pBrA. When 2 ml methanol containing 0.2 mmol ligand are added dropwise into 2 ml of Fe(NCS)2 methanolic solution (0.05mmol/ml), blue needle crystals are formed after one week at 25 , leading to I-pBrA. Elemental analysis gives: C: 44.93, H: 2.57, N: 11.79 close to the theoretical values C: 44.98, H: 2.61, N: 12.11 (Appendix 4).

Table III .3. The

 III

	Compound			I-mBrA
	Temp. / K	300	300	80	80	80
	Fe 2+ site	Fe1	Fe2	Fe1	Fe2	Fe3
	<Fe-N> / Å	2.164	2.156	2.168	2.164	2.172
	ζ / Å	0.436	0.337	0.391	0.395	0.399
	Σ / °94.2	90.4	94.68	90.10	94.77
	Θ / °254	239	251	232	248
	(SC)N-Fe-N(CS) / °105.96	104.32	105.27	103.74 105.60
	Shape	1.764	1.596	1.765	1.539	1.739
	σ1 / °76.11	76.51	75.76	75.96	75.24
	σ2 / °163.41	163.95	164.24	163.27 163.36
	σ3 / °48.01 /	43.99 /	48.62 /	44.80 /	50.08 /
		42.75	43.99	39.78	40.82	38.08

coordination sphere parameters of polymorph I-mBrA. σ1:N-Fe-N with N from imine; σ2:N-Fe-Nwith N from pyridine; σ3,dihedral angle of phenyl and pyridine in one ligand. a values obtained using the OctaDist program (https://octadist.github.io). b values obtained using the SHAPE program (ref.6).

  .4, polymorph II-mBrA crystallizes in the same C2/c space group at 300 K and 150 K.TableIII.4. Crystal data and structure refinement data for polymorph II-mBrA.

	Empirical formula	C26H18Br2FeN6S2
	Formula weight / g mol -1	694.25
	Temperature / K	300	150
	Radiation MoKα / Å	λ = 0.71073
	Crystal size / mm 3	0.28 × 0.22 × 0.16
	Crystal system	monoclinic
	Space group	C2/c
	a / Å	11.9502(6)	11.6370(10)
	b / Å	15.4615(7)	15.2426(12)
	c / Å	16.4078(10)	15.9001(13)
	α / °90	90
	β / °100.622(3)	98.492(4)
	γ / °90	90
	Volume / Å 3	2 979.7(3)	2 789.4(4)
	Z-formula	4
	ρcalc / g cm -3	1.548	1.653
	μ / mm -1	3.354	3.582
	F(000)	1376.0	1376.0
	2θ range for data collection / °5.844 -52.744	5.94 -48.806
	Index ranges
	Reflections collected	7868	4137
	Independent reflections	3033 [Rint = 0.0477, Rsigma = 0.0408]	2205 [Rint = 0.0204, Rsigma = 0.0292]
	Data/restraints/parameters	3033/0/168	2205/0/168
	Goodness-of-fit on F 2	1.089	1.083

-14 ≤ h ≤ 14, -19 ≤ k ≤ 19, -20 ≤ l ≤ 16 -12 ≤ h ≤ 12, -17 ≤ k ≤ 17, -18 ≤ l ≤ 18

Table III . 5 .

 III5 The coordination sphere parameters in polymorph II-mBrA. : N-Fe-N with N from imine; σ2: N-Fe-Nwith N from pyridine; σ3: dihedral angle of phenyl and pyridine in one ligand. a values obtained using the OctaDist program (https://octadist.github.io).

	Compound		II-mBrA
	Temp. / K	300	150
	Fe 2+ site	Fe1	Fe1
	<Fe-N> / Å	2.154	1.993
	ζ / Å	0.359	0.083
	Σ / °74.25	54.0
	Θ / °196	156
	(SC)N-Fe-N(CS) / °97.86	93.71
	Shape	0.943	0.481
	σ1 / °81.80	86.61
	σ2 / °165.94	174.34
	σ3 / °42.00	46.05

σ1b values obtained using the SHAPE program (ref.6).

Table III .

 III

	Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism:
		Bromine Substitution of PM-zBrA Family
	Compound Fe 2+ site	Fe---X / Å	SCO activity / °	/	°Fe 2+ site	Fe---X / Å	SCO activity	/ °	/	°I-mFA
	Fe2 6.27 / 6.53 active 71 199 Fe1	5.65	inactive 92 231
	I-mBrA Fe1 6.08 / 6.99 inactive 94 254 Fe2	5.95	inactive 90 239
	II-mBrA		-		Fe1	5.97	active	74 196
			117

6. Selected parameters in complexes I-mBrA, II-mBrA and I-mFA at 300 K.

Table III . 7 .

 III7 The crystal data and structure refinement data in polymorph I-pBrA at selected temperatures (full table with complementary temperatures inAppendix 5)

	Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism:
			Bromine Substitution of PM-zBrA Family
	Empirical formula			C26H18Br2FeN6S2
	Formula weight / g mol -1			694.25
	Temperature / K	300	200	150	80	80	10
	Radiation MoKα / Å			λ=0.71073
	Crystal size / mm 3			0.20×0.10×0.10
	Crystal system			monoclinic
	Space group			P21/n
	a / Å		16.7235(7) 16.6521(3) 15.8370(6) 15.8078(8) 16.5266(5) 16.5140(4)
	b / Å		21.3275(9) 21.2327(3) 21.0664(8) 20.9483(12) 21.0835(7) 20.9864(5)
	c / Å		16.7294(7) 16.5891(3) 17.6888(7) 17.6727(10) 16.4474(5) 16.3611(5)
	α / °90
	β / °109.419(2) 109.0200(10) 109.710(2) 109.6700(15) 108.843(2) 108.785(3)
	γ / °90
	Volume / Å 3	5627.4(4) 5545.17(16) 5555.7(4)	5510.8(5)	5423.8(3) 5368.2(3)
	Z-formula				8
	ρcalc / g cm -3	1.639	1.663	1.660	1.674	1.700	1.723
	μ / mm -1		3.551	3.604	3.597	3.627	3.685	3.724
	F(000)		2752	2752	2752	2752	2752	2752
	2θ range for data		4.628-	3.866-	2.994-	4.246-	3.268-
	collection /	°2.984-52.744	50.09	56.34	55.112	52.744	54.206
	Reflections collected	11404	9758	13364	12698	11087	9428
	Independent Reflections(Rint,Rsigma)	6078 (0.0687, 0.0827)	7807 (0.0209, 0.0299)	7689 (0.0597, 0.0786)	8823 (0.0617, 0.0457)	6126 (0.0689, 0.1184)	8599 (0.0352, 0.0260)
	Goodness-of-fit on F 2	1.012	1.029	1.026	1.063	0.978	1.069
	Final R indexes	R1=0.0681	R1=0.0452	R1=0.0679	R1=0.0544	R1=0.0619	R1=0.0385
	[I>=2σ (I)]	wR2=0.139	wR2=0.114	wR2=0.160	wR2=0.115	wR2=0.129	wR2=0.091
	Final R indexes [all data]	R1=0.1546 wR2=0.1734	R1=0.0590 wR2=0.1228	R1=0.1303 wR2=0.2019	R1=0.0957 wR2=0.1379	R1=0.1364 wR2=0.1553	R1=0.0435 wR2=0.0934
	Largest diff. peak/hole / e Å -3 0.89/-1.06	1.71/-1.47	1.79/-0.98	2.57/-1.18	2.39/-0.97 2.53/-0.98

). The average <Fe-N> bond lengths are 2.183(5) Å and 2.178(6) Å in Fe1 and Fe2 entities at 300 K, respectively, in agreement with two HS Fe 2+ ions.

  .7 and Figure III.21c) The ligand conformations are very similar to that at 300 K with σ3 values of 66° and 46° for Fe1 and of 79° and 22° in Fe2 (TableIII.8). The overlay of these two

	molecules is very similar to the one obtained at 300 K, with a RMSD value of 0.723 Å (Figure
	III.21f). The average <Fe-N> bond lengths are 2.182(6) Å and 2.171(6) Å for Fe1 and Fe2
	centers, respectively, indicating that the HS state can be fully quenched, in agreement with the
	magnetic measurements.
	Table III.8. The coordination spheres parameters in polymorph I-pBrA at the selected
			temperature.
	Temperature / K	300	300	80	80	80quench	80quench
	Fe 2+ site	Fe1	Fe2	Fe1	Fe2	Fe1	Fe2
	<Fe-N> / Å	2.183(5) 2.178(6) 1.953(5) 1.955(5) 2.182(6) 2.171(6)
	ζ / Å	0.425	0.450	0.085	0.043	0.369	0.397
	Σ / °70.54	84.07	59.54	56.28	69.71	86.61
	Θ / °265.41	269.23	167.56	159.26	263.00	281.43
	Shape b	1.653	1.726	0.729	0.653	1.636	1.884
	SC-N-Fe / °159.63 /	160.79 /	169.88 /	169.10 /	154.81 /	160.63 /
		170.57	175.85	172.95	170.28	168.79	174.92
	(SC)N-Fe-N(CS) / °94.38	96.50	88.08	87.73	93.84	95.65
	σ1 / °89.18	80.94	99.05	99.21	82.88	80.00
	σ2 / °164.32	165.86	175.92	176.05	171.40	167.16
	σ3 / °75.01 /	79.62 /	66.95 /	56.25 /	66.49 /	78.77 /
		52.67	30.26	55.49	57.20	45.96	22.46

σ1: N-Fe-N with N from imine; σ2: N-Fe-Nwith N from pyridine; σ3: dihedral angle of phenyl and pyridine in one ligand. a values obtained using the OctaDist program (https://octadist.github.io). b values obtained using the SHAPE program (ref.

6)

  III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family Table III.9. Polyhedron distortion parameters of polymorph I-pBrA at various temperatures.

	Temperature / K	<Fe1-N>	<Fe2-N>	ζ / Å	ζ / Å	Σ /	°(Fe1N6)	Σ /	°(Fe2N6)	Θ /	°(Fe1N6)	Θ /	°(Fe2N6)
	(spin state)	/ Å	/ Å	(Fe1N6)	(Fe2N6)
	350 (HS)	2.190(5) 2.180(6) 0.431	0.437	71.11	84.83 265.56 268.96
	300 (HS)	2.183(5) 2.178(6) 0.425	0.450	70.54	84.07 265.41 269.23
	250 (HS)	2.187(4) 2.179(5) 0.419	0.426	70.54	85.90 269.82 272.25
	200 (HS)	2.188(4) 2.181(4) 0.408	0.419	70.26	86.98 268.65 277.45
	180 (HS)	2.188(7) 2.173(6) 0.373	0.468	71.59	87.94 267.19 281.13
	170 (HS)	2.186(8) 2.178(8) 0.382	0.424	70.58	87.68 267.99 281.21
	150 (LS)	1.956(6) 1.955(5) 0.101	0.037	57.89	54.63 162.73 154.39
	120 (LS)	1.952(7) 1.950(7) 0.091	0.045	60.18	57.18 168.97 160.73
	100 (LS)	1.958(6) 1.958(5) 0.119	0.053	59.69	56.28 167.56 159.26
	80 (LS)	1.953(5) 1.955(5) 0.085	0.043	59.54	56.28 166.39 158.65
	80 (Q-HS*)	2.182(6) 2.171(6) 0.369	0.397	69.71	86.61 263.00 281.43
	10 (Q-HS*)	2.185(3) 2.174(3) 0.391	0.409	71.47	88.95 266.90 285.92
	(Q-HS*-LS) at 80 K	0.229	0.216	0.284	0.354	10.17	29.96	96.61 122.78

Table III .

 III

	T / K (spin state)	S1-Br1 / Å	S2-Br2 / Å	S3-Br3 / Å	S4-Br4 / Å	S3-S3 / Å	H45-H33 / Å	H48-H42 / Å	πpy•••πpy / Å	Br2-C13H13 /Å Br2-H13-C13 / °S2-H32 / Å S2-H32-C32 /	°S2-H42 / Å S2-H42-C42 /	°350
	(HS)	3.465 3.685 3.763 4.264 3.441 2.144	2.253	4.048	3.731 / 143.05	2.887 / 146.16	2.911 / 158.15
	300 (HS)	3.444 3.677 3.736 4.221 3.401 2.138	2.212	3.978	3.696 / 143.15	2.848 / 148.83	2.904 / 143.57
	250 (HS)	3.430 3.704 3.720 4.237 3.377 2.153	2.179	3.923	3.685 / 143.70	2.82 / 150.73	2.875/ 145.11
	200 (HS)	3.421 3.724 3.695 4.228 3.344 2.188	2.144	3.840	3.672 / 142.61	2.794 / 153.07	2.853 / 144.39
	180 (HS)	3.418 3.719 3.684 4.217 3.332 2.163	2.143	3.835	3.67 / 140.95	2.779 / 152.22	2.861 / 143.02
	170 (HS)	3.414 3.727 3.673 4.218 3.331 2.204	2.168	3.812	3.652 / 144.73	2.769 / 155.43	2.831 / 146.45
	150 (LS)	3.476 3.496 3.643 3.496 3.637 2.787	2.541	5.281	3.634 / 151.69	2.979 / 133.70	3.108 / 106.41
	120 (LS)	3.460 3.474 3.620 3.477 3.609 2.751	2.559	5.285	3.611 / 150.91	2.976 / 134.50	3.176 / 126.36
	100 (LS)	3.451 3.477 3.612 3.477 3.606 2.784	2.565	5.303	3.641 / 151.85	2.966 / 133.80	3.173 / 127.22
	80 (LS)	3.449 3.462 3.590 3.466 3.590 2.768	2.531	5.310	2.646 / 152.27	2.976 / 134.16	3.154 / 127.63
	80 (Q-HS*)	3.398 3.750 3.622 4.225 3.297 2.181	2.131	3.738	3.652 / 141.56	2.747 / 158.71	2.813 / 144.29
	10 (hν-HS*)	3.390 3.773 3.594 4.266 3.253 2.208	2.092	3.897	3.637 / 140.851	2.738 / 159.88	2.805 / 145.04
	At 80 K, in the LS state (Figure III.25b), the shapes of the fingerprints for Fe1 and Fe2

10. Selected interatomic distances in polymorph I-pBrA at various temperatures.

Table III .

 III 11. The unit-cell parameters, Rp and wRp values extracted from PXRD for polymorph I-pBrA. The data were analyzed by the Le Bail method using Fullprof software.

	Temp	Spin					nit-cell	Rp	wRp
	/K	State	a/Å	b/Å	c/Å	β/°U volume
							/Å 3
	100	LS	15.82111 20.95809	17.7064	109.7222	5526.691	0.163 0.148
	90	h -HS*	16.4653	21.0725	16.5736	108.7868	5447.383	0.151 0.151
	90	LS	15.8291	20.9501	17.7196	109.6995	5532.248
	80	h -HS*	16.4596	21.0767	16.5699	108.7448	5443.438	0.155 0.158
	80	LS	15.8290	20.9391	17.7171	109.6876	5528.617
	70	h -HS*	16.4356	21.0513	16.5562	108.6577	5427.263	0.138 0.139
	70	LS	15.8128	20.9002	17.7088	109.7800	5507.263
	60	h -HS*	16.4353	21.0727	16.5569	108.6927	5431.767	0.163 0.149
	60	LS	15.8232	20.8988	17.7161	109.7606	5513.514
	50	h -HS*	16.4230	21.0768	16.5409	108.7304	5422.310	0.140 0.148
	50	LS	15.8113	20.8784	17.6921	109.7166	5497.989
	40	h -HS*	16.4120	21.0718	16.5312	108.7219	5414.498	0.138 0.153
	40	LS	15.8129	20.8498	17.6966	109.7524	5491.215
	30	h -HS*	16.4093	21.0135	16.5341	108.7423	5398.928	0.157 0.162
	30	LS	15.7872	20.8543	17.6838	109.6638	5482.512
	20	h -HS*	16.3989	21.0028	16.5305	108.7404	5391.653	0.144 0.150
	20	LS	15.7956	20.8100	17.6833	109.6811	5473.028
	15	h -HS*	16.4008	20.9810	16.5225	108.7345	5385.246	0.102 0.101
	15	LS	15.7995	20.8117	17.6858	109.7054	5474.798
	10	Q-HS* 16.5140(4) 20.9864(5) 16.3611(5) 108.785(3)	5368.2(3)	SC-XRD

Table III .

 III 12. Crystal data and structure refinement data for polymorph II-pBrA at selected temperatures (full table with complementary temperatures inAppendix 5)

	Empirical formula	C26H18Br2FeN6S2
	Formula weight / g mol -1	694.25	694.25
	Temperature / K	350	120
	Radiation MoKα / Å	λ = 0.71073	λ = 1.54184
	Crystal size / mm 3	0.18 × 0.12 × 0.06
	Crystal system	monoclinic
	Space group	P21/n
	a / Å	16.688(12)	16.3086(10)
	b / Å	21.087(13)	20.7674(2)
	c / Å	17.393(13)	16.8871(2)
	α / °90
	β / °107.342(19)	107.5110(10)
	γ / °90
	Volume / Å 3	5842(7)	5454.40(9)
	Z-formula	8
	ρcalc / g cm -3	1.579	1.691
	μ / mm -1	3.421	9.523
	F(000)	2752.0	2752.0
	2θ range for data collection / °2.97 -50.05	6.606 -136.492
	Index ranges
	Reflections collected	107438	39776
	Independent reflections	10 308 [Rint = 0.0724,	9 897 [Rint = 0.0285,
		Rsigma = 0.0547]	Rsigma = 0.0196]
	Data/restraints/parameters	10308/0/673	9897/0/667
	Goodness-of-fit on F2	1.008	1.055
	Final R indexes [I>=2σ (I)]	R1 = 0.0554, wR2 = 0.1397	R1 = 0.0351, wR2 = 0.0956
	Final R indexes [all data]	R1 = 0.1440, wR2 = 0.1822	R1 = 0.0371, wR2 = 0.0972
	Largest diff. peak/hole / e Å -3	0.79/-0.60	1.15/-0.71

-19 ≤ h ≤ 19, -25 ≤ k ≤ 21, -20 ≤ l ≤ 20 -19 ≤ h ≤ 18, -25 ≤ k ≤ 24,

-20 ≤ l ≤ 20

Table III .

 III 13. Actually, HS fraction can be also calculated based on the average <Fe-N> distance, labeled as HS fraction2 in Table III.13. Figure III.42 reports the evolution of this calculated HS fraction from the dihedral angle as function of the temperature. One can clearly see that the evolution of the order-disorder of the phenyl ring correlates with the SCO curve in II-pBrA.

Table III .

 III 13. The summary of structural data of the disorder phenyl ring in II-pBrA.

	Temperature / K	120	200	250	300	330	350
	C33a,C32a,C35a,C36a 1	1.0	1.0	0.630	0.810	0.792	0.790
	C33b,C32b,C35b,C36b 1	0.0	0.0	0.370	0.190	0.208	0.210
	dihedral angle / °0	0	22.19	56.6	64.42	68.44
	HS fraction1 / % 2	0	0	0.32	0.82	0.94	1
	<Fe1-N>/<Fe2-N> / Å	1.966 / 1.961	1.966 / 1.961	2.015 / 2.017	2.112 / 2.129	2.153 / 2.156	2.140 / 2.144
	HS fraction2 / % 3	0	0	0.26	0.78	0.95	0.89

Table III .

 III 15 summarizes the experimental and structure refinement datas at selected temperatures.

	Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism:
	Bromine Substitution of PM-zBrA Family

Table III .

 III 15. Crystal data and structure refinement data for polymorph III-pBrA at selected temperatures (full table with complementary temperatures in Appendix 5)

	Empirical formula		C26H18N6S2FeBr2
	Formula weight / g mol -1		694.25
	Temperature / K	300	90	10 (thermally quenched)
	Radiation MoKα / Å		λ = 0.71073
	Crystal size / mm 3	0.26 × 0.22 × 0.14	0.28 × 0.24 × 0.14	0.22 × 0.24 × 0.20
	Crystal system		triclinic
	Space group		P-1
	a / Å	9.9504(3)	10.3686(14)	9.8698(12)
	b / Å	16.4925(5)	15.804(2)	16.0292(14)
	c / Å	18.4995(6)	17.839(2)	18.2045(13)
	α / °102.6780(10)	98.998(3)	102.601(7)
	β / °90.0430(10)	92.149(3)	90.214(8)
	γ / °105.2950(10)	107.243(3)	104.530(9)
	Volume / Å 3	2851.42(15)	2746.5(6)	2715.6(5)
	Z-formula		4
	ρcalc / g cm -3	1.617	1.679	1.698
	μ / mm -1	3.504	3.638	3.680
	F(000)	1376.0	1376.0	1376.0
	2θ range for data collection / °3.838 -53.59	2.74 -50.05	3.916 -50.054
	Index ranges
	Reflections collected	86 509	42 764	20 752
	Independent reflections	12 184 [Rint = 0.0613, Rsigma = 0.0563]	9 188 [Rint = 0.1058, Rsigma = 0.1410]	9 575 [Rint = 0.1279, Rsigma = 0.1929]
	Data/restraints/parameters	12 184/0/667	9 188/0/661	9 575/0/587
	Goodness-of-fit on F 2	0.991	1.028	1.007
	Final R indexes [I>=2σ (I)]	R1 = 0.0455, wR2 = 0.0882	R1 = 0.0582, wR2 = 0.1030	R1 = 0.1025, wR2 = 0.2392
	Final R indexes [all data]	R1 = 0.1147, wR2 = 0.1101	R1 = 0.1495, wR2 = 0.1325	R1 = 0.1682, wR2 = 0.2927
	Largest diff. peak/hole / e Å -3	0.99/-0.67	0.84/-0.72	1.28/-1.74

-12 ≤ h ≤ 12, -20 ≤ k ≤ 20, -23 ≤ l ≤ 23 -11 ≤ h ≤ 12, -18 ≤ k ≤ 18, -21 ≤ l ≤ 21 -11 ≤ h ≤ 11, -19 ≤ k ≤ 16, -21 ≤ l ≤ 21

  .16), in the two Fe 2+ sites. These differences in σ3 values, σ3, between the two ligands of the same molecule, are 15° and 38° for Fe1 and Fe2, respectively. This clearly indicates a stronger dissymmetry of the ligand geometry in Fe2. The average of the bond lengths, <Fe-N>, are 2.176(6) Å and 2.177(6) Å in Fe1 and Fe2 entities at 300 K clearly indicating HS states for both sites.At 90 K, both molecules tend toward similar geometries with a reduction of σ3 to 2° in Fe1 and 9° in Fe2. This causes the decrease of the RMSD value to 0.177 Å. The differences that remain between the molecules are related to motion of NCS -groups as seen in the

	Compound			III-pBrA
	Temperature / K	300	300	90	90	10quench	10quench
	Fe 2+ site	Fe1	Fe2	Fe1	Fe2	Fe1	Fe2
	<Fe-N> / Å	2.176(6) 2.177(6) 1.948(4) 1.943(4) 2.154(5) 2.159(6)
	ζ / Å	0.365	0.409	0.081	0.047	0.312	0.340
	Σ / °70.68	83.77	57.82	53.80	65.40	81.50
	Θ / °258.77	257.88	163.41	159.49	250.62	259.97
	Shape b	1.584	1.628	0.705	0.645	1.446	1.572
	SC-N-Fe / °157.78 /	170.68 /	173.51 /	171.01 /	154.32 /	170.07 /
		174.63	160.26	170.95	169.13	172.44	159.07
	(SC)N-Fe-N(CS) / °94.75	95.83	87.57	87.77	93.55	95.78
	σ1 / °89.92	80.95	99.90	97.84	90.52	81.78
	σ2 / °166.44	165.27	176.65	174.98	166.69	166.95
	σ3 / °51.08 /	68.28 /	55.35 /	70.39 /	49.68 /	63.57 /
		65.57	30.75	53.10	61.71	58.63	29.73

superposition of molecules (Figure III.49e) and in Table III.16. The averages of the bond lengths, <Fe-N>, are 1.948(4) Å and 1.943(4) Å at 90 K, in Fe1 and Fe2, respectively, corresponding to full LS states. Table III.16. The coordination sphere parameters in polymorph III-pBrA.

Table III .

 III 17. Unit-cell parameters at various temperatures after thermal quenching in polymorph III-pBrA.

	Temperature	a / Å	b / Å	c / Å	V / Å 3	Spin state
	quench 10K	9.926(12)	16.134(18)	18.307(10)	2761(5)	Q-HS*
	increase to 70K	10.01(2)	16.22(3)	18.30(3)	2782(9)	Q-HS*
	quench 72K	10.394(16)	15.86(2)	17.80(2)	2760(7)	LS
	relax 10 K	10.410(18)	15.79(4)	17.78(3)	2744(

Table III .

 III 18. Crystal data and structure refinement data for polymorph IV-pBrA.

	Empirical formula	C26H18Br2FeN6S2
	Formula weight / g mol -1	694.25
	Temperature / K	300	85
	Radiation MoKα / Å	λ = 0.71073
	Crystal size / mm 3	0.24 × 0.2 × 0.16
	Crystal system	monoclinic
	Space group	P21/n
	a / Å	9.83160(1)	9.7221(4)
	b / Å	13.4637(2)	13.1922(6)
	c / Å	21.5638(3)	21.4557(9)
	α / °90	90
	β / °100.7442(8)	100.4530(10)
	γ / °90	90
	Volume / Å 3	2804.35(6)	2706.2(2)
	Z-formula	4	4
	ρcalc / g cm -3	1.644	1.704
	μ / mm -1	3.563	3.693
	F(000)	1376.0	1376.0
	2θ range for data collection / °6.052 -54.994	3.642 -55.072
		-12 ≤ h ≤ 12,	-10 ≤ h ≤ 12,
	Index ranges	-17 ≤ k ≤ 17,	-17 ≤ k ≤ 17,
		-28 ≤ l ≤ 28	-27 ≤ l ≤ 27
	Reflections collected	161654	43535
	Independent reflections	6429 [Rint = 0.0363, Rsigma = 0.0139]	6247 [Rint = 0.0255, Rsigma = 0.0141]
	Data/restraints/parameters	6429/0/334	6247/0/334
	Goodness-of-fit on F 2	1.149	1.043

Table III .19. The

 III

	Compound	IV-pBrA
	Temperature / K	300	85
	Fe 2+ site	Fe1	Fe1
	<Fe-N> / Å	2.174(7)	2.168(6)
	ζ / Å	0.346	0.311
	Σ / °88.65	88.61
	Θ / °288.95	291.35
	Shape b	1.979	1.981
	SC-N-Fe / °156.25 / 155.33	154.56 / 153.65
	(SC)N-Fe-N(CS) / °97.09	96.59
	σ1 / °79.73	79.61
	σ2 / °166.89	167.08
	σ3 / °71.10 / 33.10	71.12 / 32.2.

coordination sphere parameters in polymorph IV-pBrA. σ1:N-Fe-N with N from imine; σ2:N-Fe-Nwith N from pyridine; σ3,dihedral angle of phenyl and pyridine in one ligand. a values obtained using the OctaDist program (https://octadist.github.io). b values obtained using the SHAPE program (ref 6).

Table II .20.

 II The shortest contacts between molecules in the HS state of four polymorphs.

	Compound s	S---Br / Å	S---H / Å S-C-H / °Br---H / Å Br-C-H / °N---H / Å N-C-H / °S3---S3 / Å	π---π / Å	H-H / Å
	I-pBrA	3.444 / 3.677 3.736 / 4.221	2.848 / 148.83 2.904 / 143.57	2.909 / 143.15	3.013 /148.8	3.441	3.978	2.212
		3.433 / 3.632	2.499 / 152.28	2.952 / 141.55	2.940 / 152.34		-
	II-pBrA(*)	3.823 / 3.899	2.840 / 137.82	3.023 / 127.09		4.366		1.957
		3.553 / 3.563	2.750 / 158.29	3.102 / 137.05 2.957 / 161.91		-
	III-pBrA	3.631 / 3.724	2.888 / 160.93			3.486		2.534
			3.043 / 139.97		2.857 / 125.06		-
	IV-pBrA	3.730	3.100 / 114.67 3.089/ 132.52	2.931 / 153.33 3.883		2.314

*Data used at 350 K for II-pBrA and 300 K for others

  80

Table III .

 III 21. Summary of relevant parameters for the [Fe(PM-zBrA)2(NCS)2] compounds.

	Compounds	SCO	T1/2 / K	T(LIESST) / T(TIESST) / K	Space Group
	I-oBrA	Gradual	220	50
	I-mBrA	Fully HS	-	-	C2/c ↔ P21/c
	II-mBrA	Gradual	163	no	C2/c
	I-pBrA	Abrupt	170 / 181	107 / 109	P21/n
	II-pBrA	Gradual	260	no	P21/n
	III-pBrA*	Abrupt	198 / 205	-/ 70	P-1
	IV-pBrA	Fully HS	-	-	P21/n

* data for III-pBrA are coming from the SCXRD results.

Table IV . 1 .

 IV1 Summary of relevant parameters for the 10 compounds described in part II and III.

	From this table we can first report the [T1/2, T(LIESST)] couples on the corresponding
		SCO	T1/2	T(LIESST)	Space	ζ	Θ	S	comment
			/ K	/ T(TIESST)	Group (HS and	/ Å	/ Å
				/ K	LS)
	I-oFA	Gradual	215	55 / -		unknown structure
	I-mFA Gradual &	215	32 / -	C2/c	0.017(Fe1) /	-6(Fe1) /	0.067(Fe1) /	Fe1 SCO
		incomplete				0.262(Fe2)	66(Fe2)	0.59(Fe2)	active; Fe2,
									HS;
	I-pFA Gradual 2	100(T11/2)	55/57	R-3	0.181(Fe1) /	57(Fe1) /	0.79(Fe1) /	R-3 and
		steps	/140(T21/2)			0.335(Fe2)	62(Fe2)	0.68(Fe2)	nanoporous
	I-oBrA	Gradual	220	55 / -		unknown structure
	I-mBrA Fully HS	-	-	C2/c ↔	-	-		phase
					P21/c				transition
									without SCO
	II-mBrA Gradual	163	no	C2/c	0.276	40	0.46
	I-pBrA Abrupt &	170(T1/2↓)	107 / 109	P21/n	0.28(Fe1) /	95(Fe1) /	0.91(Fe1) /	rotation of
		hysteresis	/181(T1/2↑)			0.35(Fe2)	122(Fe2)	1.23(Fe2)	phenyl ring
									and reverse
									volume
									expansion
	II-pBrA Gradual	260	no	P21/n	0.28(Fe1) /	107(Fe1) /	0.89(Fe1) /	order-
						0.27(Fe2)	95(Fe2)	1.09(Fe2)	disorder of
									phenyl ring
	III-	Abrupt &	198(T1/2↓)/2	-/ 70	P-1	0.28(Fe1) /	95(Fe1) /	0.92(Fe1) /	rotation of
	pBrA*	hysteresis	05(T1/2↑)			0.36(Fe2)	98(Fe2)	0.96(Fe2)	phenyl ring
	IV-pBrA Fully HS	-	-	P21/n	-	-		-

Table IV . 3 .

 IV3 Summary of the SCO characteristics of new compounds in the family of [Fe(PM-pXA)2(NCE)2].S (E = S and Se, S = Solvent, X = F and Br) additionally obtained in this work.

	SCO	T1/2
		/ K 170(T1/2↓) T(LIESST) / K /181(T1/2↑)
		Shape

Table IV

 IV

.1 and Table IV.3.

Table IV . 4 .

 IV4 Overview of some geometrical characteristics of the coordination sphere and T(LIESST) for selected compounds based on bidentate ligands. The selection is made though out the literature and this work on the efficiency of T(LIESST), see text for details.

	label	Compounds	ζ	Θ	S	T(LIESST)	comment	Ref.
			/ Å	/ °		/ K
		Without symmetry change
	1	I-[Fe(PM-BiA)2(NCS)2]	0.46	110	1.15	78
	11	[FeL 1 3]ClO4	0.163	73	0.8	48		17
	12	trans-[Fe(Cladpat)2(NCS)2]	0.165	65	0.27	30		18
	13	[Fe(abpt)2(NCSe)2]	0.05	78.1	0.54	32		19
	14 C-[Fe(abpt)2(NCS)2]	0.03	92.5	0.61	54		20
	15 D-[Fe(abpt)2(NCS)2]	0.001	83	0.48	36		21
	16	trans-[Fe(Fc-tzpy)2(NCSe)2]•CHCl3 0.051	85.8	0.55	39		22
	17 A-trans-[Fe(tzpy)2(NCS)2]	0.09	95	0.7	54		23
	18	[FeL 2 2(NCSe)2]	0.07	89	0.66	50		24
	19	[Fe(L 3 )2(NCS)2]	0.08	89.6	0.65	40		25
	20	[Fe(DPyDT)2(NCS)2]0.5CH3O	0.142	74.3	0.45	66		26
	21	[Fe(tap)2(NCS)2]CH3CN	0.29	99.8	0.76	52		27
		With symmetry change
	22 II-[Fe(PM-PEA)2(NCS)2]	0.5	72	0.92	65	Phase transition 4
	23	[Fe(pic)3]Cl2.EtOH	0.02	130	1.71	45		28
	24	[Fe(pic)3]Cl2.MeOH	0.009 117.6 1.526	50		29
	25	trans-[Fe(L 4 F)2(NCS)2].2CH3CN	0.24	64	0.31	58		30
	26	[Fe(H2B(pz)2)2phen]	0.006	39.4	0.32	44		16
	27 D'-[Fe(abpt)2(NCS)2]	0.108	49	0.29	108	Photo-	21
							isomerization
	28 I-pFA	0.355	62	0.68	55		This work
	29	S-pFA.C7H8	0.37	179	1.78	82	Twinning	This work
	30	Se-pFA.C7H8	0.28	179	1.71	70	Twinning	This work
	31	Se-pFA.C9H12	0.35	151	1.5	100	Solvent rotation This work
	32	S-pBrA.C9H12	0.34	100	1.12	40	Solvent disorder This work

Table 1 .

 1 Summary of various functions of distance and curvature mapped on Hirshfeld surfaces.

	Function	Symbol and	Mapping range
		definition
	distance from the surface to the nearest	de	red through green to blue
	nucleus external to the surface
	distance from the surface to the nearest	di	red through green to blue
	nucleus internal to the surface
	curvatures	C	-4 to +4
	normalized contact distance	dnorm	red through white to blue
	Shape index, in terms of principle	S	-1 to +1
	curvatures k1 and k2

Table 2 .

 2 Experimental conditions of PXRD measurements.

	Compounds	Sample No.	2θ range / °Time / min
	[Fe(PM-oFA)2(NCS)2]	PGWG130D	8-80	34
	[Fe(PM-mFA)2(NCS)2]	PGWG130G	5-80	90
	[Fe(PM-pFA)2(NCS)2]·0.47CHCl3	PGWG128E	3-80	93
	[Fe(PM-oBrA)2(NCS)2]	PGWG130E	4-38	113
	I-[Fe(PM-mBrA)2(NCS)2]	PGWG32O	8-80	34
	II-[Fe(PM-mBrA)2(NCS)2]	PGWG128I	8-80	34
	I-[Fe(PM-pBrA)2(NCS)2]	PGWG79B	4-38	113
	II-[Fe(PM-pBrA)2(NCS)2]	PGWG51	4-38	113

Table 5 .

 5 Crystal data and structure refinement data for polymorph I-

[Fe(PM-pBrA)2(NCS)2].

	Empirical formula			C26H18Br2FeN6S2
	Formula weight / g mol -1			694.25
	Temperature / K	350	250	180	170	100
	Radiation MoKα / Å			λ = 0.71073
	Crystal size / mm 3			0.2 × 0.1 × 0.1
	Crystal system			monoclinic
	Space group			P21/n
	a / Å	16.8011(4)	16.7009(3)	16.6098(15)	16.6046(15)	15.8230(6)
	b / Å	21.3926(3)	21.2963(3)	21.1736(16)	21.1608(16)	21.0048(7)
	c / Å	16.8227(4)	16.6548(3)	16.5520(13)	16.5325(13)	17.6823(7)
	α / °90	90	90	90	90
	β / °109.54	109.1890(10)	109.065(4)	108.988(4)	109.72
	γ / °90	90	90	90	90
	Volume / Å 3	5698.3(2)	5594.45(17)	5501.9(8)	5492.9(8)	5532.3(4)
	Z-formula	8	8	8	8	8
	ρcalc / g cm -3	1.618	1.649	1.676	1.679	1.667
	μ / mm -1	3.507	3.572	3.632	3.638	3.612
	F(000)	2752.0	2752.0	2752.0	2752.0	2752.0
	2θ range for data collection / °4.596 -50.032 4.616 -50.094	4.234 -50.054	4.232 -50.054	4.746 -50.008
	Index ranges
	Reflections collected	19491	19170	33236	33169	17992
	Independent reflections	10011 [Rint = 0.0279, Rsigma = 0.0397]	9854 [Rint = 0.0232, Rsigma = 0.0328]	9667 [Rint = 0.1729, Rsigma = 0.1654]	9657 [Rint = 0.1676, Rsigma = 0.1585]	9551 [Rint = 0.0416, Rsigma = 0.0528]
	Data/restraints/p arameters	10011/0/661	9854/0/667	9667/0/667	9657/0/667	9551/0/667
	Goodness-of-fit on F 2	1.033	1.031	0.949	0.955	1.057
	Final R indexes	R1 = 0.0547,	R1 = 0.0492,	R1 = 0.0692,	R1 = 0.0677,	R1 = 0.0542,
	[I>=2σ (I)]	wR2 = 0.1441	wR2 = 0.1245	wR2 = 0.1264	wR2 = 0.1225	wR2 = 0.1237
	Final R indexes	R1 = 0.0967,	R1 = 0.0695,	R1 = 0.2017,	R1 = 0.1912,	R1 = 0.0838,
	[all data]	wR2 = 0.1677	wR2 = 0.1377	wR2 = 0.1653	wR2 = 0.1600	wR2 = 0.1419
	Largest diff. peak/hole / e Å -3 0.88/-0.78	1.47/-1.34	0.76/-0.58	0.88/-0.61	1.63/-0.77
		R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2

-19 ≤ h ≤ 19, -25 ≤ k ≤ 25, -20 ≤ l ≤ 20 -19 ≤ h ≤ 19, -25 ≤ k ≤ 25, -19 ≤ l ≤ 19 -16 ≤ h ≤ 19, -25 ≤ k ≤ 25, -19 ≤ l ≤ 19 -16 ≤ h ≤ 19, -25 ≤ k ≤ 25, -19 ≤ l ≤ 19 -18 ≤ h ≤ 18, -24 ≤ k ≤ 24, -21 ≤ l ≤ 21

Table 6 .

 6 Crystal data and structure refinement data for polymorph II-

[Fe(PM- pBrA)2(NCS)2].

  Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2

	Empirical formula		C26H18Br2FeN6S2
	Formula weight / g mol -1		694.25
	Temperature / K	330	300	250	10
	Radiation MoKα / Å		λ = 0.71073
	Crystal size / mm 3		0.18 × 0.12 × 0.06
	Crystal system		monoclinic
	Space group			P21/n
	a / Å	16.688(12) 16.5583(5)	16.4370(7)	16.1982(8)
	b / Å	21.087(13) 20.9993(6)	20.9170(9)	20.6684(10)
	c / Å	17.393(13) 17.2361(6)	17.0599(8)	16.8258(10)
	α / °90	90	90	90
	β / °107.342(19) 107.1025(9) 107.3862(14)	107.492(6)
	γ / °90	90	90	90
	Volume / Å 3	5842(7)	5728.2(3)	5597.4(4)	5372.6(5)
	Z-formula			8
	ρcalc / g cm -3	1.579	1.610	1.648	1.717
	μ / mm -1	3.421	3.489	3.570	3.720
	F(000)	2752.0	2752.0	2752.0	2752.0
	2θ range for data collection / °2.97 -50.05	2.998 -54.354	3.02 -51.362	3.64 -52.744
	Index ranges
					≤ 21
	Reflections collected	107438	72498	80083	37342
	Independent reflections	10308 [Rint = 0.0724, Rsigma = 0.0547]	12711 [Rint = 0.0627, Rsigma = 0.0671]	10628 [Rint = 0.0615, Rsigma = 0.0481]	10883 [Rint = 0.0653, Rsigma = 0.0717]
	Data/restraints/para meters	10308/0/673 12711/0/656 10628/0/656	10883/0/667
	Goodness-of-fit on F 2	1.008	1.005	1.001	1.035
	Final R indexes [I>=2σ (I)]	R1 = 0.0554, wR2 = 0.1397	R1 = 0.0671, wR2 = 0.1447	R1 = 0.0472, wR2 = 0.1050	R1 = 0.0489, wR2 = 0.1109
	Final R indexes [all data]	R1 = 0.1440, wR2 = 0.1822	R1 = 0.1559, wR2 = 0.1798	R1 = 0.1039, wR2 = 0.1291	R1 = 0.0757, wR2 = 0.1265
	Largest diff. peak/hole / e Å -3	0.79/-0.60	1.05/-0.58	1.09/-0.47	2.31/-0.98

R1 = -19 ≤ h ≤ 19, -25 ≤ k ≤ 21, -20 ≤ l ≤ 20 -21 ≤ h ≤ 21, -26 ≤ k ≤ 26, -22 ≤ l ≤ 22 -20 ≤ h ≤ 20, -22 ≤ k ≤ 25, -20 ≤ l ≤ 20 -20 ≤ h ≤ 20, -25 ≤ k ≤ 18, -19 ≤ l

Table 7 .

 7 Crystal data and structure refinement data for polymorph III-

[Fe(PM- pBrA)2(NCS)2].

	Empirical formula		C26H18N6S2FeBr2
	Formula weight / g mol -1		694.25
	Temperature / K	220	210	150
	Radiation MoKα / Å		λ = 0.71073
	Crystal size / mm 3		0.28 × 0.24 × 0.14
	Crystal system		triclinic
	Space group		P-1
	a / Å	9.9422(4)	9.9447(7)	10.4005(6)
	b / Å	16.3778(6)	16.3647(12)	15.8356(9)
	c / Å	18.4452(6)	18.4366(14)	17.8941(10)
	α / °102.7020(10)	102.720(2)	99.009(2)
	β / °90.0740(10)	90.089(2)	92.205(2)
	γ / °105.0720(10)	105.015(2)	107.245(2)
	Volume / Å 3	2823.71(18)	2821.4(4)	2768.6(3)
	Z-formula	4	4	4
	ρcalc / g cm -3	1.633	1.634	1.666
	μ / mm -1	3.539	3.542	3.609
	F(000)	1376.0	1376.0	1376.0
	2θ range for data collection / °2.644 -53.464	2.268 -50.052	2.314 -49.826
	Index ranges
	Reflections collected	56481	46044	53684
	Independent reflections	12010 [Rint = 0.0315, Rsigma = 0.0288]	9830 [Rint = 0.0465, Rsigma = 0.0565]	9552 [Rint = 0.0517, Rsigma = 0.0523]
	Data/restraints/parameters	12010/0/667	9830/0/667	9552/0/667
	Goodness-of-fit on F 2	1.059	1.136	1.056

-12 ≤ h ≤ 12, -20 ≤ k ≤ 20, -21 ≤ l ≤ 23 -11 ≤ h ≤ 11, -19 ≤ k ≤ 19, -21 ≤ l ≤ 21 -12 ≤ h ≤ 12, -18 ≤ k ≤ 18, -16 ≤ l ≤ 21

R1 = Σ||F0|-|FC||/Σ|F0|, wR2 = [Σw(F0 2 -FC 2 ) 2 /Σw(F0 2 ) 2 ] 1/2

occupancy;

calculated from dihedral angle of disordered phenyl ring;

caculated from the <Fe-N> distances.

* Data for III-pBrA coming from the SCXRD results, the other ones from magnetic data

2 II-[Fe(PM-BiA)2(NCS)2]0.33

Acknowledgements

i Acknowledgements Acknowledgements ii patient to explain me the results because I know almost nothing about NMR. As an expert on photomagnetism, I learned many things on Corine's classes. Patrick was also very kind to help and train me for the VSM and CHNF measurements. He can operate almost all the instruments in the lab. I would like to thank many people who also participated on my thesis project. I would like to express many thanks also to Dr. Nathalie DARO and Dr. Elodie TAILLEUR for their kind training on experiments and synthesis during the first year of the PhD. I would like to express many thanks also to Eric LEBRAUD, Dr. Stanislav PECHEV and Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family

The thermally quenched Q-HS* structure was also determined at 10 K. The compound crystallizes in the same space group P-1 with two molecules in the asymmetric units (Figure

III.49f).

The ligand conformation is very similar to that at 300 K with σ3 of 9° and 34° for Fe1 and Fe2, respectively (Table III.16). The average bond lengths <Fe-N>, are 2.154(5) Å

and 2.159(6) Å for Fe1 and Fe2 centers, respectively, which indicates the HS state. The superposition of the molecules at various temperatures are shown in Figure III.50. From the overlay of the Fe1 (Fe2) site in the HS and LS states, one can see the huge modifications of ligand conformation with RMSD of 0.385 Å and 0.906 Å for Fe1 and Fe2, respectively (Figure III.50a). The atomic displacements of NCS -contribute to the large value of RMSD in Fe1. For Fe2, in addition to the motion of NCS -there is also the huge motion of pyridine and phenyl rings, and the rotation of bromophenyl ring (Br3C6) by 67°. Similar differences can be seen from the overlay of the thermally quenched Q-HS* and LS structures (Figure III.50b). The RMSD value for Fe1 and Fe2 are 0.393 Å and 0.949 Å, respectively, which are a little bigger than between HS and LS. In this case, the evolution of the dihedral angle of bromophenyl (Br3C6) rings in Fe2 leads to a rotation of 70°. Finally, the overlay of the HS and Q-HS* structures show their similarity with RMSD value lower than 0.1 Å (Figure III.50c). Part III -Promoting Distortion of the Coordination Polyhedron Using Polymorphism: Bromine Substitution of PM-zBrA Family

General Conclusions and Perspectives

General Conclusions and Perspectives

Table of contents

For the quenched structure at 10 K, the crystal packing is similar to the one at 300 K (Figure

III.56).

The {Fe1Fe2}∞ zig-zag chains are similar to that at 300 K with the same direction.

Within the chains, the Fe1---Fe2 distances are 10.931 Å and 11.012 Å with Fe2-Fe1-Fe2 angle of 140.859°. The layer and ribbon are also similar to that at 300 K. So, the molecular packing is very similar between the thermally quenched at 10 K and 300 K. The single-crystal density at 10 K is larger than 300 K due to the thermal contraction.

Unit-cell parameters as a function of temperature of III-pBrA

In order to determine the SCO temperature more accurately we followed the thermal evolution of the unit-cell parameters with the idea of looking at the unit-cell volume variation to check whether it behaves similarly to polymorph I-pBrA, or not. As seen in In addition, polymorphism on this new compounds allow to evidence that ΔΘ is not the unique parameter to correlate with the T(LIESST) and that Δ has to be taken into account.

However, this aspect has to be confronted with first the other members of the PM-L family then complexes based on bidentate ligands. Coordination sphere scale

IV.2.2. Comparison with the whole PM-L family

The breathing of the coordination sphere is anisotropic, the HS state being bigger and more distorted. This includes both the angular distortion that measures the angular deviation of the coordination sphere to a perfect octahedron, Σ and Θ, and anisotropy of bond lengths, , that measures the deviation of these bond lengths from the average value <Fe-L>. More than a given value, it is their variations upon spin crossover that seem to be relevant as influent parameters on the lifetime of hv-HS*. This is particularly the case for ΔΘ and Δζ as discussed in Part IV.2. divided by the same sum for the crystal (the procrystal). ρi(r) is a spherically-averaged atomic electron density centred on nucleus A, and the promolecule and procrystal are sums over the atoms belonging to the molecule and to the crystal, respectively. The Hirshfeld surface is then defined in a crystal as that region around a molecule where w(r) ≥ 0.5. That is, the region where the promolecule contribution to the procrystal electron density exceeds that from all other molecules in the crystal.

eq.1

The useful property to map onto the surface is the distance from the surface to the nearest nucleus external to the surface, which we call de. This property provides an immediate picture of the nature of intermolecular contacts in the crystal. In a similar fashion, we can define di, the distance from the surface to the nearest nucleus internal to the surface (Table 1). The value of dnorm (eq.2) is a normalized contact distance that informs the shortest contacts (red area on the surface) or the the longest ones (blue area on the surface). Derived from the Hirshfeld surface, the 2D plot of di vs de provides a visual summary of the frequency of each combination of de and di across the surface of a molecule. So they not only indicate which intermolecular interactions are present, but also the relative area of the surface corresponding to each kind of interaction. Quantitative comparisons between contributions to crystal packing from various types of intermolecular contacts can be done based on Hirshfeld surfaces and fingerprints.

eq.2

Appendix 2 -Single-crystal and Powder X-ray Diffraction Experiments.

Single-crystal X-ray diffraction (SCXRD): The variable temperature structural determinations all along the thermal SCO were performed on a Kappa Nonius-Brüker diffractometer (λ = 0.71071 Å) or Bruker Apex II diffractometer (λ = 0.71071 Å) equipped with an Oxford Cryosystem nitrogen cryostat.

Collaborating with Dr. Sébastien Pillet in the University of Lorraine (Nancy, France), the thermally quenched states at 10 K were collected with an Oxford Diffraction SuperNova diffractometer equipped with an Atlas CCD detector, using a helium openflow cryosystem.

Absorption corrections, data reduction and unit cell refinements were performed using the SADABS and SAINT programs. [1][2][3] The structures were solved and refined using weighted fullmatrix least-squares on F 2 in olex2. [1][2][3] The constraint code AFIX / EADP were used when Powder X-ray diffractograms were acquired using a PANalytical X'Pert 3 Powder (CuKα, X'Celerator detector). The crystals were crushed firstly and then transfer to the sample holder carefully, using a blade to make the surface smooth. The conditions of PXRD measurements are shown in Table 2.

Photocrystallography of Powder X-Ray Diffraction was performed using a Panalytical Photomagnetic measurements were performed on the same SQUID as above coupled with a set of laser diodes via an optical fiber to the magnetometer cavity. The powder samples were prepared as thin layers and irradiated at 10 K. The detailed conditions for magnetic and photomagnetic measurements are reported in Table 3. 2.17/-2.47 6.80/-2.70 Le second point important est que la durée de vie de l'état photo-induit est impliquée dans un processus multi-échelle. En effet, les composés présentant des changements structuraux notables (ordre-désordre, brisure de symétrie, macle…) ne suivent pas cette règle des hautes valeurs de ΔΘ et (Figure R3b). Cela implique que la T(LIESST) peut être influencée par toutes les échelles du matériau : la distorsion anisotrope de la sphère de coordination, la variation de la conformation du ligand, la modification des interactions, le réarrangement moléculaire ainsi que la brisure de symétrie. Toutes ces modifications structurales peuvent renforcer ou affaiblir l'influence de la respiration anisotrope de la sphère de coordination sur la durée de vie de l'état métastable. Il y a donc une forte intrication entre l'influence de la respiration de la sphère de coordination sur les échelles supérieures et, en retour, une influence des échelles supérieures sur la sphère de coordination. Là encore, d'un point de la synthèse, les défis sont nombreux pour que le réseau cristallin ne vienne pas perturber négativement une chimie spécifique sur la sphère de coordination.

En conclusion, les vingt-trois nouveaux composés de ce travail offrent un large panel de comportements innovants, comme par exemple des expansions volumiques négatives ou nulles à la transition de spin et l'absence de transition à plusieurs étapes malgré des sites métalliques indépendants à l'intérieur du cristal. De plus des températures de relaxation élevées ont été obtenues et comparées à la littérature permettant de dégager une vision multi-échelle de la T(LIESST). Cela permet d'envisager de nouvelles voies de conception de matériaux commutables vers une température de relaxation T(LIESST) élevée. Ces travaux élargissent considérablement la richesse des perspectives basées sur la conversion de spin.

Résumé