Photosensitized electron-transfer reactions and H₂ evolution in organized microheterogeneous environments: Separation of ground-state xanthene-bipyridinium complexes by SiO₂ colloids

Rose bengal, Rb^2-, forms a ground-state complex with N,N′-dimethyl-4,4′-bipyridinium, MV²⁺, with an association constant of K_a = 11000 ± 1100 M^-1. Static electron-transfer quenching of excited Rb^2- 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 SiO₂ colloid particles the [Rb^2-⋯MV²⁺] complex is separated through the selective association of MV²⁺ to the negatively charged colloid interface. Upon illumination of a solution that includes Rb^2-, MV²⁺, and the sacrificial electron donor triethanolamine (TEOA) in the presence of SiO₂ colloid, the photosensitized formation of MV^•+ proceeds effectively, φ = 0.1. Mechanistic studies reveal that TEOA reduces excited Rb^2- in the primary electron-transfer process. The intermediate photoproducts, TEOA^•+ and Rb^•3-, are stabilized against back-electron-transfer reactions by means of electrostatic interactions with the SiO₂ interface, leading to the electrical repulsion of Rb^•3- from the colloid interface. The control of the recombination process of the intermediate photoproducts leads to the subsequent effective reduction of MV²⁺. A xanthene dye-bipyridinium complex is also formed between eosin, Eo^2-, and N,N′-dibenzyl-3,3′-dimethyl-4,4′-bipyridinium, BMV²⁺, K_a = 17000 ± 3400 M^-1. The complex is separated by a SiO₂ colloid that is immobilized with Pd metal catalyst sites. Separation of the complex allows charge separation and subsequent H₂ evolution (or hydrogenation of ethylene) upon illumination of the microheterogeneous assembly in the presence of TEOA. Mechanistic studies show that the SiO₂ colloid controls the photoinduced electron-transfer process, and stabilization of the intermediate photoproducts against the back-electron-transfer process is achieved.