Charge Effects in Photoinduced Electron-Transfer Reactions Between [Ru(bpy)_3]~(2+) and Viologen Derivatives

摘要

Photoexcited ~*[Ru (bpy)_3]~(2+) is oxidatively quenched by methylviologen (MV~(2+)) and 1,1'-bis (2-carboxyethy)-4,4'-bipyridinium (BCEBP~(2+)) with a similar rate constant at pH 2.2; k_q~(obs) = 1.48 * 10~9 and 1.58 * 10~9 mol~(-1) s~(-1) (30 ℃) for MV~(2+) an BCEBP~(2+), respectively. However, ~*[Ru (bpy)_3]~(2+) is much more slowly quenched by MV~(2+) than by BCEBP~0 at pH 5.0, where the superscript "0" represents that BCEBP is neutral; k_q~(obs) = 1.01 * 10~9 and 1.74 * 10~9 mol~(-1) s~(-1) for MV~(2+) and BCEBP~0, respectively. The reverse electron-transfer reaction between [Ru (bpy)_3]~(2+) and one-electron-reduced ·BCEBP~+ (k_(rev)~(obs) = 3.33 * 10~9 mol~(-1) dm~3 s~(-1)) proceeds slightly more rapidly than the reaction between *[Ru (bpy)_3]~(3+) and ·MV~+ (k_(rev)~(obs) = 2.80 * 10~9 mol~(-1) dm~3 s~(-1)) at pH 2.2 (30 ℃), while the former reaction (k_(rev)~(obs) = 5.85 * 10~9 mol~(-1) dm~3 s~(-1)) proceeds 2-times as rapidly as the latter reaction (k_(rev)~(obs) = 2.74 * 10~9 mol~(-1) dm~3 s~(-1)) at pH 5.0. These differences at pH 5.0 between MV~(2+) and BCEBP~0 are interpreted in terms of charge effects on the diffusion and diffusional dissociation of an exciplex and an encounter complex. The electron-transfer reaction 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 MV~_(2+) and BCEBP~(2+ or 0) is discussed in terms of the charge effects and steric effects.