Photosensitized electron-transfer reactions and hydrogen evolution in organized microheterogeneous environments: separation of ground-state xanthene-bipyridinium complexes by silica colloids

Abstract

Rose bengal, Rb{sup 2{minus}}, forms a ground-state complex with N,N{prime}-dimethyl-4,4{prime}-bipyridinium, MV{sup 2+}, with an association constant of K{sub a} = 11,000 {plus minus} 1,100 M{sup {minus}1}. Static electron-transfer quenching of excited Rb{sup 2{minus}} occurs in the complex structure, but charge separation is eliminated due to rapid back electron transfer in the encounter cage complex of photoproducts. In the presence of added SiO{sub 2} colloid particles the (Rb{sup 2{minus}}{hor ellipsis}MV{sup 2+}) complex is separated through the selective association of MV{sup 2+} to the negatively charged colloid interface. Upon illumination of a solution that includes Rb{sup 2{minus}}, MV{sup 2+}, and the sacrificial electron donor triethanolamine (TEOA) in the presence of SiO{sub 2} colloid, the photosensitized formation of MV{sup {sm bullet}+} proceeds effectively, {phi} = 0.1. Mechanistic studies show that the SiO{sub 2} colloid controls the photoinduced electron-transfer process, and stabilization of the intermediate photoproducts against the back-electron-transfer process is achieved.