Dimethylviologen dichloride (MV(2+)(Cl-)(2)) was introduced into silica gel pores by impregnation or addition to the soi during sol-gel synthesis. The evacuated silica gel samples containing MV(2+) were reduced by 320-nm irradiation to form a stable dimethylviologen cation radical MV(+), which was detected by electron spin resonance and diffuse reflectance spectroscopies. The counteranion Cl- is shown to be the electron donor. The photoyield and stability of the photoproduced MV(+) radical depend on the pore size of the silica gel which ranged from 1.7 to 14 nm. In a large pore silica gel the MV(+) photoyield is larger and the MV(+) cation radical is more stable than in a small pore silica gel. Replacing the two methyl groups of dimethylviologen with buty and heptyl alkyl chains did not affect the photoyield or stability of the photoproduced viologen cation radical in the silica gels. It is suggested that stability of the photoproduced MV(+) is controlled by its distance from Cl trapped within the pores. The trend of increasing MV(+) stability and photoyield with increasing pore size is due to a larger separation distance between the MV(+) and the Cl trapping site in the larger pores. It is found that the photoyields are about 5 times greater in impregnated vs sol-gel synthesized samples. Thus, impregnation seems to be the optimum method for preparation of the most efficient photoactive assemblies.