pH Dependence of Photoinduced Electron Transfer with [Ru(TAP) 3 ] 2 .

The quenching of the excited state of [Ru(TAP)3 ]2+ (TAP = 1,4,5,8-tetraazaphenanthrene) by guanosine-5'-monophosphate (GMP), N-acetyltyrosine (N-Ac-Tyr), and hydroquinone (H2 Q) has been studied in aqueous solution over a wide range of pH values including, for the first time, strongly acidic media. This quenching by electron transfer was examined by steady-state1 H photochemically induced dynamic nuclear polarization (photo-CIDNP) as well as by more conventional techniques, among which are pulsed laser-induced transient absorption and emission experiments. A deeper knowledge of the photochemical behavior of [Ru(TAP)3 ]2+ has been gained thanks to the combined use of these two approaches, photo-CIDNP and electronic spectroscopies, highlighting their complementarity. In contrast to what was believed, it is found that the protonated excited state of [Ru(TAP)3 ]2+ may give rise to an electron transfer with N-Ac-Tyr and H2 Q. Such a photoinduced electron transfer does not occur with protonated GMP, however.1 H photo-CIDNP experiments are expected to be particularly promising for characterization of the reductive quenching of excited-state ruthenium(II) polypyridyl complexes comprising several nonequivalent protonation sites.