Optimizing the Stability of Trapped Metastable Spin States

Abstract

In the domain of transition metal complexes, there are several possibilities for generating light-induced switches accompanied by drastic modifications of magnetic, volume and/or optical properties.1–4 Among the best known and extensively studied systems, we can cite metal-centered thermal spin-transition (observed in iron(II) SCO materials), valence tautomerism (catecholate complexes of Co(II)), metal-to-metal charge transfer (Prussian Blue analogs), metal-to-ligand charge transfer (nitroprusside complexes), and finally, all ligand modifications that induce a change of spin state, such as isomerization (stilbenoid complexes) and open/close (diarylethene type ligands) processes. The objective of this chapter is to focus on the Light-Induced Excited Spin-State Trapping (abbreviated as LIESST) encountered in iron(II) spin-crossover complexes,5 which is of major interest for the design of optical switches,6 and to reflect on ten years of work since we introduced the T(LIESST) procedure.7 This systematically measures the limiting temperature above which a photomagnetic effect in a material is erased, by warming the sample from 10 K at a rate of 0.3 K min−1. Until now, any applications based on the LIESST effect have been prohibited because the lifetimes of the photomagnetic states are sufficiently long only at low temperatures. Nevertheless, it seems that certain factors can affect the stability of the photoinduced HS state and the T(LIESST) limit has been increased progressively. This chapter is organized into four subsections. Section 19.2 briefly recalls some background considerations on the Light-Induced Excited Spin-State Trapping (LIESST) effect and on the various strategies used to compare the photomagnetic properties of iron(II) SCO materials, including the ‘inverse energy gap’ law introduced by Hauser which represents the first guideline allowing some prediction of the lifetime of a photoinduced high spin state. This will lead us to introduce the T(LIESST) procedure used to compare