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

Abstract

The hydrogen-evolving photocatalyst [(tbbpy)₂Ru(tpphz)Pd(Cl)₂]²⁺ (tbbpy=4,4′-di-tert-butyl-2,2′-bipyridine, tpphz=tetrapyrido[3,2-a:2′,3′-c:3′′,2′′-h:2′′′,3′′′-j]phenazine) shows excitation-wavelength-dependent catalytic activity, which has been correlated to the localization of the initial excitation within the coordination sphere. 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. Only in the presence of the second metal center, does the excitation of a ¹MLCT state localized on the central part of the tpphz bridge lead to coherent wave-packet motion in the excited state.