CHARGE EFFECTS IN PHOTOINDUCED ELECTRON-TRANSFER REACTIONS BETWEEN [RU(BPY)(3)](2+) AND VIOLOGEN DERIVATIVES

Photoexcited [Ru(bpy)(3)](2+) is oxidatively quenched by methylviolog en (MV2+) and 1,1'-bis(2-carboxyethyl)-4,4'-bipyridinium (BCEBP2+) wit h a similar rate constant at pH 2.2; k(q)(obs) = 1.48 x 10(9) and 1.58 x 10(9) mol(-1) dm(3) s(-1) (30 degrees C) for MV2+ and BCEBP2+, resp ectively. 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)(obs) = 1.01 x 10(9) and 1.74 x 10(9) mol(- 1) dm(3) 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 x 10(9) mol(-1) dm(3) s(-1)) proceeds slig htly more rapidly than the reaction between [Ru(bpy)(3)](3+) and . MV (r(kev)(obs) = 2.80 x 10(9) mol(-1) dm(3) s(-1)) at pH 2.2 (30 degree s C), while the former reaction (k(rev)(obs) = 5.85 x 10(9) mol(-1) dm (3) s(-1)) proceeds 2-times as rapidly as the latter reaction (k(rev)( obs) = 2.74 x 10(9) mol(-1) dm(3) s(-1)) at pH 5.0. These differences at pH 5.0 between MV2+ and BCEBP0 are interpreted in terms of charge e ffects on the diffusion and diffusional dissociation of an exciplex an d an encounter complex. The electron-transfer reactions in the exciple x and the encounter complex have been analyzed according to Marcus the ory. The difference in an electronic coupling matrix element (H-rp) be tween MV2+ and BCEBP2+ (or 0) is discussed in terms of the charge effe cts and steric effects.