Photosensitized Electron-Transfer Reactions and H2 Evolution in Organized Microheterogeneous Environments: Separation of Ground-State Xanthene-Bipyridinium Complexes by SiO2 Colloids

Article abstract of DOI:10.1021/j100370a064

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 2-....MV²⁺> complex is separated through the selective association of MV²⁺ to the negatively charged colloid interface.Upon illumination of a solutionthat 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 theSiO2 colloid controls the photoinduced electron-transfer process, and stabilization of the intermediate photoproducts against the back-electron-transfer process is achieved.