Charge effects in photoinduced electron-transfer reactions between [Ru(bpy)3]2+ and viologen derivatives

Article abstract of DOI:10.1246/bcsj.71.2281

Photoexcited *[Ru(bpy)₃]²⁺ is oxidatively quenched by methylviologen (MV²⁺) and 1,1′-bis(2-carboxyethyl)-4,4′-bipyridinium (BCEBP²⁺) with a similar rate constant at pH 2.2; k_q^bs = 1.48×10⁹ and 1.58×10⁹ mol^-1 dm³ s^-1 (30 °C) for MV²⁺ and BCEBP²⁺, respectively. However, *[Ru(bpy)₃]²⁺ is much more slowly quenched by MV²⁺ than by BCEBP⁰ at pH 5.0, where the superscript "0" represents that BCEBP is neutral; k_q^bs = 1.01 × 10⁹ and 1.74×10⁹ mol^-1 dm³ s^-1 for MV²⁺ and BCEBP⁰, respectively. The reverse electron-transfer reaction between [Ru(bpy)₃]³⁺ and one-electron-reduced ·BCEBP⁺ (k_rev^obs = 3.33×10⁹ mol^-1 dm³ s^-1) proceeds slightly more rapidly than the reaction between [Ru(bpy)₃]³⁺ and ·MV+ (k_rev^obs = 2.80×10⁹ mol^-1 dm³ s^-1) at pH 2.2 (30 °C), while the former reaction (k_rev^obs = 5.85×10⁹ mol^-1 dm³ s^-1) proceeds 2-times as rapidly as the latter reaction (k_rev^obs = 2.74×10⁹ mol^-1 dm³ s^-1) at pH 5.0. These differences at pH 5.0 between MV²⁺ and BCEBP⁰ are interpreted in terms of charge effects on the diffusion and diffusional dissociation of an exciplex and an encounter complex. The electron-transfer reactions in the exciplex and the encounter complex have been analyzed according to Marcus theory. The difference in an electronic coupling matrix element (H_rp) between MV²⁺ and BCEBP^{2+ or 0} is discussed in terms of the charge effects and steric effects.