Charge Effects in Photoinduced Electron-Transfer Reactions between [Ru(bpy)3]2+and Viologen Derivatives

Abstract 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; kqobs = 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; kqobs = 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+ (krevobs = 3.33 × 109 mol−1 dm3 s−1) proceeds slightly more rapidly than the reaction between [Ru(bpy)3]3+ and ·MV+ (krevobs = 2.80 × 109 mol−1 dm3 s−1) at pH 2.2 (30 °C), while the former reaction (krevobs = 5.85 × 109 mol−1 dm3 s−1) proceeds 2-times as rapidly as the latter reaction (krevobs = 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 (Hrp) between MV2+ and BCEBP2+ or 0 is discussed in terms of the charge effects and steric effects.