The crystal chemistry of spin crossover (SCO) behavior in coordination compounds can potentially be in association with smart materials—promising materials for applications as components of memory devices, displays, sensors and mechanical devices and, especially, actuators, such as artificial muscles. This Special Issue is devoted to various aspects of SCO and related research, comprising 18 interesting original papers on valuable and important SCO topics. Significant and fundamental scientific attention has been focused on the SCO phenomena in a wide research range of fields of fundamental chemical and physical and related sciences, containing the interdisciplinary regions of chemical and physical sciences related to the SCO phenomena. Coordination materials with bistable systems between the LS and the HS states are usually triggered by external stimuli, such as temperature, light, pressure, guest molecule inclusion, soft X-ray, and nuclear decay. Since the first Hofmann-like spin crossover (SCO) behavior in {Fe(py)2[Ni(CN)4]}n (py = pyridine) was demonstrated, this crystal chemistry motif has been frequently used to design Fe(II) SCO materials to enable determination of the correlations between structural features and magnetic properties.

The crystal chemistry of spin crossover (SCO) behavior in coordination compounds can potentially be in association with smart materials-promising materials for applications as components of memory devices, displays, sensors and mechanical devices and, especially, actuators, such as artificial muscles. This Special Issue is devoted to various aspects of SCO and related research, comprising 18 interesting original papers on valuable and important SCO topics. Significant and fundamental scientific attention has been focused on the SCO phenomena in a wide research range of fields of fundamental chemical and physical and related sciences, containing the interdisciplinary regions of chemical and physical sciences related to the SCO phenomena. Coordination materials with bistable systems between the LS and the HS states are usually triggered by external stimuli, such as temperature, light, pressure, guest molecule inclusion, soft X-ray, and nuclear decay. Since the first Hofmann-like spin crossover (SCO) behavior in {Fe(py)2[Ni(CN)4]}n (py = pyridine) was demonstrated, this crystal chemistry motif has been frequently used to design Fe(II) SCO materials to enable determination of the correlations between structural features and magnetic properties.

The phenomenon of spin-crossover has a large impact on the physical properties of a solid material, including its colour, magnetic moment, and electrical resistance. Some materials also show a structural phase change during the transition. Several practical applications of spin-crossover materials have been demonstrated including display and memory devices, electrical and electroluminescent devices, and MRI contrast agents. Switchable liquid crystals, nanoparticles, and thin films of spin-crossover materials have also been achieved. Spin-Crossover Materials: Properties and Applications presents a comprehensivesurvey of recent developments in spin-crossover research, highlighting the multidisciplinary nature of this rapidly expanding field. Following an introductory chapter which describes the spin-crossover phenomenon and historical development of the field, the book goes on to cover a wide range of topics including Spin-crossover in mononuclear, polynuclear and polymeric complexes Structure: function relationships in molecular spin-crossover materials Charge-transfer-induced spin-transitions Reversible spin-pairing in crystalline organic radicals Spin-state switching in solution Spin-crossover compounds in multifunctional switchable materials and nanotechnology Physical and theoretical methods for studying spin-crossover materials Spin-Crossover Materials: Properties and Applications is a valuable resource for academic researchers working in the field of spin-crossover materials and topics related to crystal engineering, solid state chemistry and physics, and molecular materials. Postgraduate students will also find this book useful as a comprehensive introduction to the field.

C. Brady, J.J. McGarvey, J.K. McCusker, H. Toftlund, D.N. Hendrickson: Time-Resolved Relaxation Studies of Spin Crossover Systems in Solution.- V. Ksenofontov, P. Gütlich et al.: Spin Crossover under Pressure.- A. Bousseksou, F. Varret, M. Goiran, K. Boukheddaden, J.P. Tuchagues: The Spin Crossover Phenomenon under High Magnetic Field.- J.-P. Tuchagues, A. Bousseksou, G. Molnár, J.J. McGarvey, F. Varret: The Role of Molecular Vibrations in the Spin Crossover Phenomenon.- W. Linert, M. Grunert, A.B. Koudriavtsev: Isokinetic and Isoequilibrium Relationships in Spin Crossover Systems.- H. Winkler, A.I: Chumakov, A.X. Trautwein: Nuclear Resonant Forward and Nuclear Inelastic Scattering Using Synchrotron Radiation for Spin Crossover Systems.- M. Sorai: Heat Capacity Studies of Spin Crossover Systems.- H. Spiering et al.: Cooperative Elastic Interactions in Spin Crossover Systems.- H. Paulsen, A.X. Trautwein: Density Functional Theory Calculations for Spin Crossover Complexes.- J.-F. Létard, P. Guionneau, L. Goux-Capes: Towards Spin Crossover Applications.

The results obtained in this thesis demonstrate the importance of supramolecular chemistry for the design of potential new SCO clusters. Starting from synthesizing the suitable ligands with suitable functional groups, novel host-guest systems could be obtained where the guest play a major role in tuning the physical properties of the guest. Two bis-pyrazolylpyridine lignads, H2L4 and H2L6 were designed and prepared to achieve the assembly of transition metal ions in a triple-stranded helicate fashion where the central cavity can encapsulate different counterions depending on the size of this cavity. The N-H groups found in these ligands which usually directed toward the internal cavity help in the encapsulation of hydrogen acceptor anions. 13 iron-based compounds have been crystallized and studied adding significantly to the helical SCO compounds in the literature. The crystal structure for all the compounds were resolved, which allowed for an extensive study of supramolecular interactions and the influence of these interactions on the magnetic properties of the compounds. The first part of the thesis deals with spin-crossover dinuclear triple-stranded helicates compounds with encapsulated halide ions using H2L4. Six of such helicates with different encapsulated halide or counter ions were synthesized: Cl2"Fe2(H2L4)3]Cl(PF6)2·5.7CH3OH (1) Br2"Fe2(H2L4)3]Br(PF6)2·4CH3OH (2) Cl2"Fe2(H2L4)3]Cl(PF6)2·3CH3OH·1H2O (1a) Br2"Fe2(H2L4)3]Br(PF6)2·1CH3OH·1H2O (2a) Cl2"Fe2(H2L4)3](I3)3·3(Et2O) (3) Br2"Fe2(H2L4)3](I3)3·3(Et2O) (4) These isostructural compounds consist of triple-stranded helicates that encapsulate halide ion inside their cavity. The main difference is the kind of outer counterions and lattice solvents which affect dramatically the magnetic properties of these compounds as a result of changing the supramolecular interactions. Changing the halide ion from chloride to bromide in 1 and 2, respectively, shift the spin transition by 30 K. The SCO occur here from [HS-LS] to [HS-HS] upon heating. Compounds 1a and 2a are the water solvate helicates that produced from single-crystal to single-crystal exchange by exposing the crystals to the ambient water. This exchange leads to important changes; the {X2"[Fe2(H2L4)3]}3+ helicate are now symmetric and the two Fe centers are crystallographically identical. This change in the solvent affects dramatically the SCO behavior of the helicates. Two-step SCO from [LS-LS] à [HS- LS]à [HS-HS] states was observed in the bulk magnetic studies. Compounds 3 and 4 consists of {X2"[Fe2(H2L4)3]}3+ triple-stranded helicates similar to the one seen in previous helicates where the counter ions are now three triodide (I3- ) linear ions occupying the outer space formed between the helical strands and the solvent is ether. In these complexes the iron centers remain in the HS state through all the temperatures. The change of the solvents used in the reaction yielded different supramolecular compounds using the same ligands H2L4. Dimerized mononuclear helicates {X2"Fe(H2L4)3]2}3+ was prepared where a halide ion is encapsulated inside the cavity formed by the intercalating dimers. Five dimerized triple-stranded helicates are presented in this thesis: Cl2"Fe(H2L4)3]2(OH)(PF6)2·H2O (5). Cl2"Fe(H2L4)3]2(FeCl4)3·2C3H6O·4C7H8 (6). Br2"Fe(H2L4)3]2(OH)(PF6)2·H2O (7). I2"Fe(H2L4)3]2(PF6)2.23(I)0.21(I3)0.56·2CH3OH (8) I2"Fe(H2L4)3]2(I)2(I3)0.6(OH)0.4·0.6H2O·2CH3OH·2C3H6O (9) In every mononuclear helicate, one pyrazolyl-pyridine side of each ligand is not coordinated to any metal ion. The SCO behavior of the dimer is also affected by the nature of the halide ions which make hydrogen bonding with the N-H groups of the pyrazole rings. Using the ligand H2L6, the encapsulation of [M(III)(ox)3]3- (M = Fe and Cr; ox = oxalate) metal complexes inside the helical cavity of [Fe2(H2L6)3]4+ was achieved. Two of such triple-stranded helicates are presented in this thesis: Fe(C2O4)32"Fe2(H2L6)3](BF4)·4CH3OH·3.7H2O (10). Cr(C2O4)32"Fe2(H2L6)3](BF4)·1.4CH3OH·6H2O (11). The Fe(II) ions of the helicate exhibit SCO behavior and LIESST effect in the case of the encapsulated chromium oxalate complex. Interestingly, the guest [Cr(III)(ox)3]3- exhibits SIM-like behavior at low temperatures. This is the first example where a host- guest system exhibits both LIESST effect and SIM behavior.

Nowadays, the idea that molecule can be used as an active element in an electronic device stimulates scientific activity of chemistry and physics laboratories worldwide. The information storage capacity from technological demands is growing exponentially, which relies much on the development of nanosciences. The objective is to store data as quickly as possible in a device as small as possible. One of the most promising strategies is based on the concept of molecular bistability, the switching between two electronic states of a molecule in the same way that a binary switch. It is thus possible to pass in a reversible and detectable manner from one state (OFF = 0) to another state (ON = 1) under the influence of a controlled external stimulus. The spin transition (ST) phenomenon that switches the system between high spin (HS) and low spin (LS) states is a typical example of molecular bistability. The two states can be distinguished with different magnetic, optical and structural properties and can be induced by an external perturbation like the temperature, the light, the pressure, a magnetic field or the inclusion of a guest molecule. When the structural changes associated with the spin transition are transmitted in a cooperative manner across the network molecules, the transitions will occur with steepness and possibly accompanied by hysteresis loop (the first order transition). So, spin transition molecular materials should offer many opportunities in terms of applications in the field of electronics, information storage, digital display, photonics and photo-magnetism. Among the different families of compounds, coordination polymers arouse much interest due to their bistability near room temperature. The judicious choice of ligands and counter-anions make possible to modulate the final properties of these compounds and even in some cases to synergistically combine different physical properties. The work developed in this thesis attempt to address the different issues related to the challenge of coordination polymers based nanoscale materials with spin transition. The synthesis of inorganic bistable materials, their development in micro- and nanoparticles, thin layers, their organization and their physical properties are shown. The materials in the microscopic scale have mostly the same physical properties as those measured at the macroscopic scale. However, at the nanoscale, materials can exhibit physical properties that are far from those of bulk compounds. It is therefore imperative to understand more about the phenomena related to material size decrease to develop nanotechnology. The fundamental study of these nanomaterials is necessary and represents a major challenge today, which is of prime importance for the development of future applications. The development of nanoscale materials through the control of certain systematic models permits to improve our understanding of specific effects at the nanoscale. For example, in the case of spin crossover complex, the most important question is: how downsizing effect influences the transition temperature, the cooperativity and the width of hysteresis loop? In this context, this thesis is devoted to the design and the synthesis of various size spin crossover nano and micro-materials with different morphologies. To accomplish this, we developed the reverse-micelle technique and adopted innovative matrix-free synthetic approaches.