The three kingdoms - Photoinduced electron transfer cascades controlled by electronic couplings

Abstract

Excited states are the key species in photocatalysis, while the critical parameters that govern their applications are: i) excitation energy, ii) accessibility, and iii) lifetime. However, in molecular transition metal-based photosensitizers there is a design tension between the creation of long-lived excited (triplet), e.g., metal-to-ligand charge transfer (3MLCT) states and the population of such states. Long-lived triplet states have low spin-orbit coupling (SOC) and hence their population is low. Thus, a long-lived triplet state can be populated but inefficiently. If the SOC is increased, the triplet state population efficiency is improved - coming at the cost of decreasing the lifetime. A promising strategy to isolate the triplet excited state away from the metal after intersystem crossing (ISC) involves the combination of the transition metal complex and an organic donor/acceptor group. Here we elucidate the excited-state branching processes in a series of Ru(II)-terpyridyl push-pull triads by means of quantum chemical simulations. Scalar-relativistic time-dependent density theory simulations reveal that efficient ISC takes place along 1/3MLCT-gateway states. Subsequently, competitive electron transfer pathways involving the organic chromophore, i.e., 10-methylphenothiazinyl and the terpyridyl ligands are available. The kinetics of the underlying electron transfer processes were investigated within the semi-classical Marcus picture. The electron transfer kinetics were described along efficient internal reaction coordinates that connect the respective photoredox intermediates. The key parameter that governs the population transfer away from the metal either towards the organic chromophore either by means of ligand-to-ligand (3LLCT; weakly coupled) or intra-ligand charge transfer (3ILCT; strongly coupled) states was determined to be the magnitude of the involved electronic coupling.