A π π* state enables photoaccumulation of charges on a π-extended dipyridophenazine ligand in a Ru(II) polypyridine complex

The π-extended dipyrido-[3,2-a:2′,3′-c]phenazine (dppz) ligand of the Ru(II)complex [Ru(bpy)2(oxo-dppqp)](PF6)2 (oxo-dppqp = dipyrido[3,2-a:2',3'-c]pyrido[2",3"-4,5,6]quinolino[2,3-h]phenazin-15-one, bpy = 2,2′-bipyridine) enables the mononuclear complex for visible light-driven accumulation of two electrons on a single ligand structure. While this has been shown before, the excited-state physics underlying this promising feature in terms of photocatalysis are exploited in this manuscript. The photophysics of the complex was investigated by excitation wavelength-dependent resonance Raman and transient absorption spectroscopy in combination with time-dependent density functional theory. The results show that excitation with visible light leads to the population of the two excited-state branches: (i) the population of a short living 3MLCT state in which the additional charge is localized on the pyridoquinolinone moiety of the extended ligand (τ = 105 ps) and(ii) the population of a more long-lived 3ππ* state (τ = 9 ns). Notably, the long-lived 3ππ*state rather than a 3MLCT state is prone to reductive quenching by the sacrificial electron donor and hence presents the critical excited-state intermediate in the photochemical charge accumulation experiments.