Steering photoinduced electron transfer in intramolecular photocatalysts by peripheral ligand control

Bridged photosensitizer-catalyst systems are promising models to study photocatalytic hydrogen evolution. However, the systems in literature are structurally diverse and therefore hard to compare. Many systems show highly complex photophysics including several accepting orbitals for the excited state, as a result catalytic activity is hard to predict. Here we present a bimetallic Ru-Pt photocatalyst bearing peripheral spectator ligands at the ruthenium(II) photocenter as a member of the Ru-tpphz-Pt family. Consequently, it features a single acceptor tpphz ligand and socalled unidirectional electron transfer, i.e., electron transfer without co-occurring transfer to peripheral ligands, from the excited state. Thus – and in contrast to recently used peripheral ligands – the new spectator ligands do not disrupt electron transfer towards the catalytic center. By comparison to known systems, this facilitates unprecedented insight into the importance of electron transfer from the bridge to the catalytic center moving towards more rational design of oligonuclear photocatalysts.