Insight into the light-induced spin crossover of [Fe(bpy)3]2+ in aqueous solution from molecular dynamics simulation of d–d excited states

Abstract

Molecular dynamics (MD) simulations are performed for d–d excited states of the aqueous [Fe(bpy)₃]²⁺ system using a previously developed model Hamiltonian. Specifically, the characters of d–d excited states and of transitions among these states are explored to gain clues about electronic relaxation during the photo-excited metal-to-ligand charge transfer (MLCT) to the lowest quintet d–d states. By evaluating the spin–orbit couplings in various nuclear configurations through MD simulations, strong mixing among low-lying d–d states with different spin multiplicities is found not to be expected in most of the sampled nuclear configurations except for surface crossing regions. The lifetimes of triplet d–d states are evaluated by Fermi's golden rule using equilibrium MD simulations. The internal conversion from upper-lying triplet to lower-lying triplet states is estimated to occur with a lifetime of order 100 fs accompanied by the distortion of the [Fe(bpy)₃]²⁺ complex structure. This result is consistent with the discussion in another computational study, which evaluated the intersystem crossing rates by Fermi's golden rule using electronic structure calculations. In contrast, the present MD simulations cannot provide a clear picture of intersystem crossings from the lowest triplet d–d state after the above-mentioned internal conversion. Based on this result, possible relaxation mechanisms are discussed.