Spin bistability

The present chapter is organized in three sections the first section is devoted to basic theoretical backgroimd concerning the spin-crossover phenomenon, viz ligand field theory, thermodynamics and cooperativity the second section reports on some examples that we feel are particularly relevant to illustrate the main directions in which research on synthetic aspects of cooperativity is being directed finally, the third section describes three approaches, up to now reported, concerning spin bistability in supramolecular and molecular systems and memory effect. 

Several other light-induced phenomena associated with spin transition systems have recently been reported. These include light induced thermal hysteresis (LITH), which is another example of light induced bistability, discovered for the SCO compound [Fe(PMBiA)2(NCS)2] which undergoes a very abrupt thermal ST around 170 K with hysteresis [174]. Irradiation of the sample at 10 Kwith green light resulted in the population of the LIESST state. When the temperature was raised to 100 K and lowered back to 10 K under continuous irradiation a wide thermal hysteresis loop resulted. The... 

Hauser, Jeftic, Romstedt, Hinek and Spiering (1999) Cooperative phenomena and light-induced bistability in iron(II) spin-crossover compounds [242],... 

Real (1999) Bistability in iron(II) spin crossover systems a supramolecu-lar function [243]. 

Keywords Spin transition Coordination polymers Bistability Hofmann-like clathrates Supramolecular networks... 

Switching also implies molecular and supramolecular bistability since it resides in the reversible interconversion of a molecular species or supramolecular system between two thermally stable states by sweeping a given external stimulus or field. Bistability in isolated molecules or supermolecules is, for instance, found in optical systems such as photochromic [8.229] or thermochromic substances or devices, in electron transfer or magnetic processes [8.239], in the internal transfer of a bound substrate between the two binding sites of a ditopic receptor (see Section 4.1 see also Fig. 33) [6.77]. Bistability of polymolecular systems is of a supramolecular nature as in a phase transition or a spin transition, both of which involve an assembly of interacting species. 

Real, Jose Antonio, Bistability in Iron (II) Spin-Crossover Systems A Supramolecular Function, 5, 53. 

The preparation of chiral magnetic materials is not limited to those that show paramagnetism and magnetic ordering. Another important phenomenon is that of spin crossover, as it can lead to bistable systems, that in addition can be switched by light [213,214]. A homochiral spin crossover... 

Only a few multinuclear cyanide complexes are known to exhibit spin crossover behavior. In all the cases described in Section IV.D, a gradual spin transition is observed, which can be explained by relatively weak intermolecular interactions in the crystal strucmres of these compounds. It is generally known that stronger interactions between individual SCO centers lead to cooperative behavior, abmpt spin transitions, and thermal hysteresis or bistability over a finite temperature range (276). The latter property renders SCO compounds attractive materials for the development of magnetic sensors and memory devices. It is expected that extended... 

Rather, magnetic bistability originates purely from intramolecular magnetic interactions, as firmly established by dilution experiments [48-50]. In essence, SMM behavior arises from the association of a high-spin ground... 

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. 

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