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A ππ* State Enables Photoaccumulation of Charges on a π-Extended Dipyridophenazine Ligand in a Ru(II) Polypyridine Complex C

Schindler, Julian, Zhang, Ying, Traber, Philipp, Lefebvre, Jean-François, Kupfer, Stephan, Demeunynck, Martine, Gräfe, Stefanie, Chavarot-Kerlidou, Murielle, Dietzek, Benjamin
Journal of physical chemistry 2018 v.122 no.1 pp. 83-95
Raman spectroscopy, density functional theory, electrons, ligands, moieties, photochemistry, physics
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. 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 ³MLCT 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 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.