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), <i>N</i>-acetyltyrosine (<i>N</i>-Ac-Tyr), and hydroquinone (H₂Q) 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-state ¹H 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 <i>N</i>-Ac-Tyr and H₂Q. Such a photoinduced electron transfer does not occur with protonated GMP, however. ¹H 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