Ultrafast spin crossover photochemical mechanism in [Fe(2,2'−bipyridine)3]2+ revealed by quantum dynamics

The photoinduced spin crossover reaction of [FeII(2,2'-bipyridine)3]2+ is a light-induced transformation of the initial singlet low-spin configuration (1A1) in a quintet high-spin state (5T2) in the sub-picosecond timescale. The photochemical mechanism is still under debate, especially concerning the role of triplet intermediate excited states. Using wavepacket dynamics, we show that, upon excitation to a metal-ligand charge transfer (MLCT) state, the metal-centered (MC) triplet manifold (3T1) is responsible for the ultrafast transfer to the 5T2 state, leading to a mechanism of the type 1MLCT rightwards arrow 3MLCT rightwards arrow 3T1 rightwards arrow 5T2. This photochemical pathway is possible thanks to the ligand vibronic effects on increasing the effective triplet/quintet metal-centered couplings, facilitating the relaxation in the MLCT band and modulating the relative position of the MC states to allow an efficient transfer population between MLCT and MC.