Understanding kinetically controlled spin transitions in bistable spin crossover materials

Abstract

Spin crossover (SCO) materials can be kinetically trapped in a photo-excited metastable state in the so-called LIESST and reverse-LIESST processes. Under these conditions, SCO molecules are excellent light-responsive bistable molecular switches. However, above a certain temperature (T_LIESST and T_r-LIESST, respectively), the relaxation to the ground state becomes favorable and their bistability is suppressed. Understanding the mechanism of these processes, and being able to predict their kinetics, is key to designing SCO switches that are able to operate at room temperature. Herein, we reveal the mechanism of thermally induced spin transitions of the [Fe^II(1-bpp)₂]²⁺ SCO complex, and we predict its T_LIESST (as well as its T_1/2) with unprecedented accuracy. This is possible here thanks to the efficient reconstruction of the low-spin (LS, S = 0), high-spin (HS, S = 2) and intermediate (IS, S = 1) state Free energy surfaces (FESs) with ab initio and machine-learning methods, and the characterization of the minimum energy crossing points (MECPs) connecting those FESs. This approach paves the way for the systematic investigation of molecular features determining the mechanism of kinetically controlled transitions in SCO materials, as well as their temperature-dependent rate constants.