  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.
