The role of triplet intermediates in the photoinduced spin crossover reaction of [FeII(2,2'-bipyridine)3]2+ is still under debate. Employing quantum dynamics, we show that the metal-centered (MC) triplets are responsible for the transfer to the quintet high-spin state. This photochemical pathway is made possible thanks to bipyridine stretching vibrations, facilitating the transfer between the initial metal-ligand charge transfer (MLCT) states to the MC triplets. These results show the central role of ligands in modulating the excited state spectrum and the photochemical mechanism, opening the route for increased metal-centered lifetime that increases the effciency of iron-based photocatalysts.
A new model has been developed, in which we increase the lifetime of MLCT states to ~13 ps, by introducing a methyl in between the pyridine ([Fe(dipyridin-2-ylmethane)3]2+).