Entangled excited state branching processes in a Ru(II)-based push-pull triad

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 (GS). Thus, the combination of both inorganic and organic chromophores enables the isolation of triplet excited states onto the organic chromophore. Herein, we aim to elucidate the entangled excited-state relaxation processes in a Ru(II)-terpyridyl push-pull triad (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 kinetics of the underlying electron transfer (ET) processes involving the low-lying 3MLCT, 3ILCT and 3LLCT excited states were investigated experimentally and computationally within a semi-classical Marcus picture, which allowed us to evaluate the ET processes between along the 3MLCT-3ILCT and the 3MLCT-3LLCT channels. Finally, dissipative quantum dynamical simulations – capable of describing incomplete ET processes involving all three states of interest – enabled us to unravel the competitive excited state relaxation channels at the short timescale vs. at the long timescale among the strongly coupled 3MLCT-3ILCT states as well as the weakly coupled 3MLCT/3ILCT-3LLCT states.