Ultrafast Intramolecular Relaxation and Wave-Packet Motion in a Ruthenium-Based Supramolecular Photocatalyst

The hydrogen-evolving photocatalyst [(tbbpy)2Ru(tpphz)Pd(Cl)2]2+ shows excitation-wavelength dependent catalytic activity, which was correlated to the localization of the initial excitation within the coordination sphere: applying short excitation wavelengths a mixture of Ru→tbbpy and Ru→tpphz 1MLCT transitions are excited, while in the red wing of the absorption band, where the highest catalytic efficiency is observed, Ru→tpphz 1MLCT transitions dominate excitation. In this contribution the excitation-wavelength dependence of the early excited-state relaxation and the occurrence of vibrational coherences are investigated by sub-20-fs transient absorption spectroscopy and DFT/TDDFT calculations. The comparison with the mononuclear precursor [(tbbpy)2Ru(tpphz)]2+ highlights the influence of the catalytic center on these ultrafast processes. For both systems, upon short-wavelength excitation interligand electron transfer contributes significantly to the population of bridge localized excited states. Remarkably, only in the presence of the second metal center the direct long-wavelength excitation of a 1MLCT state localized on the central part of the tpphz bridge leads to coherent wave-packet motion in the excited state, which is preserved during intersystem crossing and reveals a unique strong coupling between the corresponding singlet and triplet states.