This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed.

Thermal, pressure and light switchable spin-crossover materials

The article deals with coordination compounds of iron(II) that may exhibit thermally induced spin transition, known as spin crossover, depending on the nature of the coordinating ligand sphere. Spin transition in such compounds also occurs under pressure and irradiation with light. The spin states involved have different magnetic and optical properties suitable for their detection and characterization. Spin crossover compounds, though known for more than eight decades, have become most attractive in recent years and are extensively studied by chemists and physicists. The switching properties make such materials potential candidates for practical applications in thermal and pressure sensors as well as optical devices. The article begins with a brief description of the principle of molecular spin state switching using simple concepts of ligand field theory. Conditions to be fulfilled in order to observe spin crossover will be explained and general remarks regarding the chemical nature that is important for the occurrence of spin crossover will be made. A subsequent section describes the molecular consequences of spin crossover and the variety of physical techniques usually applied for their characterization. The effects of light irradiation (LIESST) and application of pressure are subjects of two separate sections. The major part of this account concentrates on selected spin crossover compounds of iron(II), with particular emphasis on the chemical and physical influences on the spin crossover behavior. The vast variety of compounds exhibiting this fascinating switching phenomenon encompasses mono-, oligoand polynuclear iron(II) complexes and cages, polymeric 1D, 2D and 3D systems, nanomaterials, and polyfunctional materials that combine spin crossover with another physical or chemical property.

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Spin state switching in iron coordination compounds

Communication between iron(II) building blocks in cooperative spin transition phenomena

In this chapter we attempt to review the potential for the application of the spin crossover (SCO) phenomenon in various domains, such as molecular electronics, data storage, display devices. It is evident that SCO properties, such as room-temperature working range, chemical stability, low addressing power, short addressing time, full reversibility, are of promising value in the context of the stringent limits necessary in the future development of information technology, due to the unceasing miniaturization of the components. Of course, many requirements must be fulfilled before any use in a genuine device becomes feasible. Some of these are emphasized and discussed here. Additionally, this review reports recent progress in non-linear optics and photomagnetism of SCO materials.

Towards spin crossover applications

Multifunctionality in spin crossover materials

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)]

Antonio Real

Papers

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)]