Electron-transfer reactions of quinones, hydroquinones and methyl viologen, photosensitized by tris(2,2′-bipyridine)-ruthenium(II)

Abstract

The photosensitized reactions of tris(2,2′-bipyridine)-ruthenium(II)[Ru(bpy)₃²⁺] with quinones and hydroquinones have been studied by conventional and nanosecond flash photolysis. Quenching of the excited state [*Ru(bpy)₃²⁺] by quinone occurs rapidly (k_q > 2 × 10⁹ dm³ mol^–1 s^–1) but the yield of electron-transfer products observed by nanosecond absorption spectroscopy is small (<8%). Quenching by hydroquinones occurs more slowly (k_q < 5 × 10⁷ dm³ mol^–1 s^–1) but the yield of electron-transfer products is considerably greater. These processes generate the semiquinone radical anion Q^{[graphic omitted]}, Ru(bpy)₃³⁺ and Ru(bpy)₃⁺. Semiquinone disproportionates to give Q and QH₂(hydroquinine). Ru(bpy)₃³⁺ and Ru(bpy)₃⁺ react rapidly (k > 2 × 10⁹ dm³ mol^–1 s^–1) with QH₂ and Q, respectively.

Ru(bpy)₃³⁺ is also formed when Ru(bpy)₃²⁺ is quenched by methyl viologen (MV²⁺) and QH₂ can act as electron donor for the Ru(bpy)₃²⁺/MV²⁺ photoredox couple. No permanent storage of energy is achieved, however, since MV[graphic omitted] reacts rapidly (k > 2 × 10⁹ dm³ mol^–1 s^–1) with quinones which are produced by disproportionation of Q^{[graphic omitted]} or are present in the original hydroquinone samples.