Spin-crossover phenomenon molecules

Spin-crossover phenomenon represents probably one of the best examples of molecular and supramolecular bistability. This phenomenon is observed both in solution as well as in the solid state. In the first case, the process is essentially molecular, owing to the isolation of molecules. In the solid state, the situation is in general quite different and involves cooperative character for the phenomenon. 

Cooperativity is one of the most appealing and elusive facets of the spin-crossover phenomenon. It is a main aspect because discontinuity in the magnetic and optical properties along with thermal hysteresis confer to these systems potential memory effect. Nevertheless, because most of the spin-crossover systems are discrete in nature, cooperativity stems from assemblies of molecules held together by nonco-valent interactions and, consequently, difficult to control. 

The cooperative mechanism of the spin-crossover phenomenon is well imderstood in some aspects. For instance, theoretical models account for the thermal evolution of the system. However, due to their phenomenologic character, the key parameters accounting for intermolecular interactions do not reflect the relevance of the microscopic details which could orientate us in designing molecules that can recognize each other and combine to produce systems with prescribed characteristics. Consequently, very general principles guide the syntiietic chemistry in the search for suitable spin-crossover compounds often trial and error or serendipity are the sole strategies. 

Spin-crossover phenomenon is one of the hot topics in the assembled complexes [5-20]. In the study of spin-crossover complexes, intermolecular interaction (cooperativity) is important. High-spin and low-spin states can coexist at the same temperature (bistability), depending on the cooperativity. The bistability will be stabilized by the cooperativity. In this situation, two states of bistability are interconverted by outer stimuli, bringing a function of molecular switch to the sample. In order to achieve the bistability, assembly of complexes becomes important. In this context, it is important to investigate the assembled complexes, especially from the viewpoint of spin-crossover phenomenon, that is, whether the presence of a guest molecule has a great influence on the construction of assembled structures and the spin state of the metal ions [21,22]. 

OCCURRENCE OF SPIN-CROSSOVER PHENOMENON IN ASSEMBLED COMPLEXES Fe(NCX)2(bpa>2 (X = S, Se, BH3) BY ENCLATHRATING GUEST MOLECULES [25-27]... 

Multi-temperature X-ray diffraction data for a series of spin-crossover complexes differing in cooperativity indicates that the molecule and crystal volume variations upon spin conversion are similar in all the cases irrespective of the cooperative nature of the spin conversion [47]. So, a systematic structural analysis of specifically designed spin-crossover compounds should be of utmost importance to establish correlations between intermolecular interactions and cooperativity. The comparative structural study of [Fe(phen)2(NCS)2] and [Fe(btz)2(NCS)2] where btz = 2,2 -bi-4,5-dihydrothiazine (Figure 10) represents the sole example so far reported oriented in this direction [48,49]. It illustrates the dependence of the nature of the phenomenon on the efficiency of the intermolecular contacts in transmiting the intramolecular reorganization upon spin conversion. 

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