Photophysics of an Intramolecular Hydrogen-Evolving Ru-Pd Photocatalyst

Photoinduced electron-transfer processes within a precatalyst for intramolecular hydrogen evolution [(tbbpy)2Ru(tpphz)PdCl2]2+ (RuPd; tbbpy=4,4′-di-tert-butyl-2,2′-bipyridine, tpphz=tetrapyrido[3,2-a:2′,3′c:3′′,2′′,-h:2′′′,3′′′-j]phenazine) have been studied by resonance Raman and ultrafast time-resolved absorption spectroscopy. By comparing the photophysics of the [(tbbpy)2Ru(tpphz)]2+ subunit Ru with that of the supramolecular catalyst RuPd, the individual electron-transfer steps are assigned to kinetic components, and their dependence on solvent is discussed. The resonance Raman data reveal that the initial excitation of the molecular ensemble is spread over the terminal tbbpy and the tpphz ligands. The subsequent excited-state relaxation of both Ru and RuPd on the picosecond timescale involves formation of the phenazine-centered intraligand charge-transfer state, which in RuPd precedes formation of the Pd-reduced state. The photoreaction in the heterodinuclear supramolecular complex is completed on a subnanosecond timescale. Taken together, the data indicate that mechanistic investigations must focus on potential rate-determining steps other than electron transfer between the photoactive center and the Pd unit. Furthermore, structural variations should be directed towards increasing the directionality of electron transfer and the stability of the charge-separated states.