Abstract

Silica gels doped with methylviologen (MV), which is a photoredox and photochromic compound, have been prepared under both acidic and basic conditions. The oxidation process of the reduced species, MV⁺, formed by UV irradiation of the gels, was investigated on the basis of UV absorption measurements. The magnitude of the absorption maximum of MV⁺ in the gels gradually decreased with time after irradiation, as is observed for MV⁺ in solution, indicating that the photoredox behavior of MV in the gel matrix is very similar to that in solution. The lifetime of MV⁺ increased with increasing microviscosity around MV after gelation in the gels prepared under acidic conditions. The lifetime of MV⁺, however, reached a maximum and then gradually decreased as the gel was aged, suggesting that the oxidation of MV⁺ to more stable MV²⁺ readily occurred due to the restriction of the diffusion of MV⁺ and oxidized solvents. Moreover, the lifetime of MV⁺ in the gels prepared under basic conditions reached a maximum at an earlier stage of gel aging than for the gels prepared under acidic conditions because the microviscosity around the MV molecules at the gelation point under basic conditions was greater than that under acidic conditions.