Four-Site Cooperative Spin Crossover in a Mononuclear Fe~(II) Complex

摘要

Spin crossover (SCO), one of the most impressive examples of molecular bistability, is of interest for a number of applications in molecular electronics, including molecular memory, switches, and sensors. SCO-active Fe~(II) compounds switch between high-spin (HS, ~5T2) and low-spin (LS, ~1A1) electronic states in response to an external stimulus such as temperature, pressure, light irradiation, or magnetic/electric fields. The HS<->LS transition, which is accompanied by a visible color change, can be abrupt, gradual, and/or hysteretic. Dinuclear and polynuclear Fe~(II) compounds showing two-step SCO have been synthesized, based on the hypothesis that SCO sites are most likely to communicate if they are covalently linked. In two-step SCO, the HS<->LS transition proceeds via a thermodynamically stable intermediate electronic state, typically comprising a specific stoichiometric combination of HS and LS Fe~(II) atoms. Synthesis of Fe~(II) compounds that can exhibit more than three addressable magnetic states has proven to be more difficult.