QM/MM nonadiabatic dynamics simulation on ultrafast excited-state relaxation in osmium(II) compounds in solution

Photoinduced excited-state relaxation dynamics of organometallic compounds, due to their potential applications in solar cells, organic light-emitting diodes, photocatalysis, etc., have been recently explored by a wealth of ultrafast experimental techniques. However, the corresponding nonadiabatic dynamics simulations are rarely reported. In this work, the early-time excited-state relaxation dynamics of two Os(11) compounds i.e. Os(bpy)(3) and Os(bpy)(2)(dpp) (Os1 and Os2) have been investigated by our recently implemented TD-DFT based generalized surface-hopping dynamics simulation method in combination with the quantum mechanics/molecular mechanics (QM/MM) approach. The metal-to-ligand charge transfer (MLCT) excited singlet states from the Os atom to the bpy and dpp ligands are first populated in the Franck-Condon region. These initially populated excited singlet states will be converted through a series of ultrafast internal conversion and intersystem crossing processes to the lowest triplet state as a result of significant nonadiabatic and spin-orbit couplings among singlet and triplet electronic excited states. The intersystem crossing rates are estimated to be 72 and 53 fs for Os1 and Os2, respectively, which are consistent with experimental measured 100 fs and less than 50 fs. The time-dependent transition density analysis reveals that the MLCT excited states are dominant in the entire relaxation dynamics for either Os1 or Os2; however, the MLCT character changes in these processes. In Os1, the initially populated MLCT state is mainly from the Os atom to one bpy ligand, which is changed to the other two MLCT states. In Os2, the MLCT state that is mainly from the Os atom to the two bpy ligands is converted to the MLCT state from the Os to the dpp ligand. Finally, we have found that inter-ligand electron transfer plays a major role in the excited-state relaxation dynamics of Os1 and Os2; while, the hole transfer is minor. Our results also show that the ligand properties have significant influence on the excited-state relaxation dynamics of organometallic compounds. The insights gained in the present study provide atomistic insights for early-time excited-state relaxation dynamics of Osmium(II) compounds.