Density Functional Theory Study of the Light-Induced Excited Spin-State Trapping Effect in Spin Crossover Materials

Spin crossover dynamics in phosphorene complexes [Fe(dppen)₂X₂] (X = Br and Cl) is investigated using DFT calculations. A parameter-free computational scheme is devised to model all-atom free energy surfaces and kinetic barriers associated with the low and high spin states interconversions. The influence of halogens (X) and solvent environments (vacuum, CHCl₃ and CH₂Cl₂) on the spin crossover properties of the compounds is explored. We present full ab initio calculations on the interplay between electronic states, spin-orbit interaction, and molecular vibrations in determining the thermal and kinetic spin crossover properties. Theoretical calculations reproduce the experimentally observed trend on the influence of halogens in these compounds - Br in the coordination sphere slows kinetic relaxations of photo-induced trapped HS state and reduces thermal equilibrium temperature when compared to Cl. This results from a smaller energy gap between LS and HS states for Br. Moreover, we find that the solvents strongly affect the spin crossover properties but the changes do not follow a simple relationship with the energy gap.