Ultrafast deactivation mechanism of the excited singlet in the light-induced spin crossover of [Fe(2,2-bipyridine)3]2+

10.1002/chem.201302992
The mechanism of the light-induced spin crossover of the [Fe(bpy)3] 2+ complex (bpy = 2,2’-bipyridine) is studied by combining accurate electronic structure calculations and time-dependent approaches to calculate intersystem crossing rates. We investigate how the initially excited metal-to-ligand charge transfer (MLCT) singlet state deactivates to the final metastable high-spin state. Although ultrafast x-ray free electron spectroscopy has established that the total time scale of this process is on the order of a few tenths of a picosecond, the details of the mechanisms still remain unclear. We determine all the intermediate electronic states along the pathway from low-spin to high-spin and give estimates for the deactivation times of the different stages. The calculations result in a total deactivation time on the same order of magnitude as the experimentally determined rate and indicate that the complex can reach the final high-spin state via different deactivation channels. The optically populated excited singlet state rapidly decays to a triplet state with an Fe-d 6 (t 5 2ge 1 g) configuration either directly or via a triplet MLCT state. This triplet ligand field state could in principle decay directly to the final quintet state, but a much faster channel is provided by internal conversion to a lower-lying triplet state and subsequent intersystem crossing to the high-spin state. The deactivation rate to the low-spin ground state is much smaller, in line with the large quantum yield reported for the process.