Spin Crossover - Switching the spin state of materials

Spin conversion is a widespread phenomenon found, for example, in geology, biology and magnetism. The vast majority of spin-conversion systems are made up of coordination complexes, the best known of which is found in hemoglobin, involved in the capture, transport and release of oxygen in the blood. These complexes are built on the coordination of metal ions by organic ligands. The composition and arrangement of these ligands around the metal ions determine their electronic configuration. Some ions hesitate between two configurations, and the application of a perturbation such as temperature, pressure or light enables them to switch from one configuration to the other. This switching of electronic configuration, known as spin conversion, is accompanied by numerous changes in physico-chemical properties (color, magnetism, mechanics, electricity, volume, density, etc.). A wide range of applications are envisaged for these systems, from intelligent pigments and molecular electronics to sensors and artificial muscles. The spin conversion phenomenon originates in the metal coordination sphere of these complexes. In the solid state, this phenomenon propagates via a network of intermolecular interactions. The richness of molecular engineering makes it an ideal tool for designing molecules and molecular networks with properties tailored to specific applications. The aim of this article is to present the spin conversion phenomenon and its richness in broad outline, in order to provide engineers with the essential tools for effective technology watch on the subject. This article presents the conditions required to obtain spin-converted molecules and materials, the chemical and physical means of inducing and detecting spin conversion, and the consequences on the properties of the material undergoing the conversion, in order to help meet application specifications.