Excited State Branching Processes in a Ru(II)-Based Donor-Acceptor-Donor System

Excited state properties such as excitation energy, accessibility of the respective excited state either by direct or indirect population transfer, and its lifetime govern the application of these excited states in light-driven reactions, for example, photocatalysis using transition metal complexes. Compared with triplet metal-to-ligand charge transfer (³MLCT) states, charge-separated (³CS) excited states involving organic moieties, such as triplet intra-ligand or ligand-to-ligand charge transfer (³ILCT and ³LLCT) states, tend to possess longer-lived excited states due to the weak spin-orbit coupling with the closed-shell ground state. Thus, the combination of inorganic and organic chromophores enables isolating the triplet states onto the organic chromophore. In this study, we aim to elucidate the excited-state relaxation processes in a Ru(II)-terpyridyl donor-acceptor-donor system (RuCl) in a joint spectroscopic-theoretical approach combining steady-state and time-resolved spectroscopy as well as quantum chemical simulations and dissipative quantum dynamics. The electron transfer (ET) processes involving the low-lying ³MLCT, ³ILCT, and ³LLCT excited states were investigated experimentally and computationally within a semiclassical Marcus picture. Finally, dissipative quantum dynamical simulations-capable of describing incomplete ET processes involving all three states-enabled us to unravel the competitive relaxation channels at short and long timescales among the strongly coupled ³MLCT-³ILCT states and weakly coupled ³MLCT-³LLCT and ³ILCT-³LLCT states.