A ππ* State Enables Photoaccumulation of Charges on a π‑ExtendedDipyridophenazine Ligand in a Ru(II) Polypyridine Complex

International audienceThe pi-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. 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 (MLCT)-M-3 state in which the excess electronic density is localized on the pyridoquinolinone moiety of the extended ligand (t = 105 ps) and (ii) the population of a more long-lived (3)pi pi* state (iota = 9 ns). Notably, the long-lived (3)pi pi* 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 experiment