Some 3dn (4 ≤ n ≤ 7) transition metal compounds exhibit a cooperative transition between a low-spin (LS) and a high-spin (HS) state. This transition is abrupt and occurs with a thermal hysteresis, which confers a memory effect on the system. The intersite interactions and thus the cooperativity are magnified in polymeric compounds such as [Fe(Rtrz)3]A2·nH2O in which the Fe2+ ions are triply bridged by 4-R-substituted-1,2,4-triazole molecules. Moreover, in these compounds, the spin transition is accompanied by a well-pronounced change of color between violet in the LS state and white in the HS state. The transition temperatures of these materials can be fine tuned, using an approach based on the concept of a molecular alloy. In particular, it is possible to design a compound for which room temperature falls in the middle of the thermal hysteresis loop. These materials have many potential applications, for example, as temperature sensors, as active elements of various types of displays, and in information storage and retrieval.

https://science.sciencemag.org/content/sci/279/5347/44.full.pdf

Spin-Transition Polymers: From Molecular Materials Toward Memory Devices

The magnetic properties of many magnetic materials can be controlled by external stimuli. The principal focus here is on the thermal, photochemical, electrochemical, and chemical control of phase transitions that involve changes in magnetization. The molecular compounds described herein range from metal complexes, through pure organic compounds to composite materials. Most of the Review is devoted to the properties of valence-tautomeric compounds, molecular magnets, and spin-crossover complexes, which could find future application in memory devices or optical switches.

Control of magnetic properties through external stimuli.

The behaviour of spin crossover compounds is among the most 
striking and fascinating shown by relatively simple molecular species. This 
review aims to draw attention to the various ways in which spin crossover 
phenomena are manifested in iron(II) complexes, to offer some 
rationalisation for these, and to highlight their possible applications. 
Typical examples have been selected along with more recent ones in order to 
give an overall view of the scope and development of the area. The article 
is structured to provide the basic material for those who wish to enter the 
field of spin crossover.

Spin crossover phenomena in Fe(II) complexes

Triazoles and tetrazoles: Prime ligands to generate remarkable coordination materials

Zero-field splitting in metal complexes

Structural changes associated with the spin transition in bis(isothiocyanato)bis(1,10-phenanthroline)iron: a single-crystal x-ray investigation

This paper is concerned with the first two-step spin crossover presented by a polynuclear molecular compound, viz., the dinuclear iron(II) complex [Fe(bt)(NCS) 2 ] 2 bpym, where bt stands for 2,2'-bi-2-thiazoline and bpym for the bridging ligand 2,2'-bipyrimidine. The synthesis of the compound is described. Variable-temperature magnetic susceptibility and 57 Fe Mossbauer spectrometry data provide evidence for an overall S=2 (HS)↔S=0 (LS) spin-crossover behavior. They show that the transition takes place in two steps

Two-step spin crossover in the new dinuclear compound [Fe(bt)(NCS)2]2bpym, with bt = 2,2'-Bi-2-thiazoline and bpym = 2,2'-bipyrimidine : experimental investigation and theoretical approach

High-spin α low-spin transition in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O). X-ray crystal structure and magnetic, mössbauer and EPR properties

A method to obtain ligand-driven light-induced spin changes (LD-LISC) in transition-metal molecular compounds is described. It is based on the utilization of ligands capable of being photochemically modified, such as cis-trans photoisomerizable ligands. A prerequisite for the spin conversion to be observed upon ligand photoisomerization is that one at least of the two complexes formed with ligand isomers presents a thermally-induced spin crossover. Two complexes of the type FeL 4 (NCS) 2 with L=trans-stpy (complex 1) and L=cys-stpy (complex 2), stpy standing for 4-styrylpyridine, are presented

Toward Ligand-Driven Light-Induced Spin Changing. Influence of the Configuration of 4 Styrylpyridine (stpy) on the Magnetic Properties of FeII(stpy)4(NCS)2 Complexes. Crystal Structures of the Spin-Crossover Species Fe(trans-stpy)4(NCS)2 and of the High-Spin Species Fe(cis-stpy)4(NCS)2

Magnetic interaction and spin transition in iron(II) dinuclear compounds. Crystal structure of (.mu.-2,2'-bipyrimidine)bis[(2,2'-bipyrimidine)bis(thiocyanato)iron(II)]

Jacqueline Zarembowitch

Papers

Structural changes associated with the spin transition in bis(isothiocyanato)bis(1,10-phenanthroline)iron: a single-crystal x-ray investigation

Two-step spin crossover in the new dinuclear compound [Fe(bt)(NCS)2]2bpym, with bt = 2,2'-Bi-2-thiazoline and bpym = 2,2'-bipyrimidine : experimental investigation and theoretical approach

High-spin α low-spin transition in [Fe(NCS)2(4,4′-bis-1,2,4-triazole)2](H2O). X-ray crystal structure and magnetic, mössbauer and EPR properties

Toward Ligand-Driven Light-Induced Spin Changing. Influence of the Configuration of 4 Styrylpyridine (stpy) on the Magnetic Properties of FeII(stpy)4(NCS)2 Complexes. Crystal Structures of the Spin-Crossover Species Fe(trans-stpy)4(NCS)2 and of the High-Spin Species Fe(cis-stpy)4(NCS)2

Magnetic interaction and spin transition in iron(II) dinuclear compounds. Crystal structure of (.mu.-2,2'-bipyrimidine)bis[(2,2'-bipyrimidine)bis(thiocyanato)iron(II)]