pH Dependence of Photoinduced Electron Transfer with
[Ru(TAP)<sub>3</sub>]<sup>2+</sup>
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Abstract
The quenching of
the excited state of [Ru(TAP)<sub>3</sub>]<sup>2+</sup> (TAP = 1,4,5,8-tetraazaphenanthrene)
by guanosine-5′-monophosphate (GMP), <i>N</i>-acetyltyrosine
(<i>N</i>-Ac-Tyr), and hydroquinone (H<sub>2</sub>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-state <sup>1</sup>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)<sub>3</sub>]<sup>2+</sup> 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)<sub>3</sub>]<sup>2+</sup> may give rise to an electron transfer
with <i>N</i>-Ac-Tyr and H<sub>2</sub>Q. Such a photoinduced
electron transfer does not occur with protonated GMP, however. <sup>1</sup>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