A ππ* 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)](PF 6 ) 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. Although this has been shown before, the excited-state physics underlying this promising feature are exploited in this work. 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-lived 3 MLCT state in which the excess electronic density is localized on the pyridoquinolinone moiety of the extended ligand (τ = 105 ps) and (ii) the population of a more long-lived 3 ππ* state (τ = 9 ns). Notably, the long-lived 3 ππ* state rather than a 3 MLCT 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.