Absorption and emission behavior of bis(2,2 '-bipyridine)[2-(2-pyridyl)benzimidazole]ruthenium(II) doped in silica gel matrices

Silica gels doped with bis(2,2'-bipyridine)[2-(2-pyridyl)benzimidazole]ruth enium(II), [Ru(bpy)(2)(pbimH)](2+), which dissociates a proton from the imi no group upon light irradiation, were prepared by the sol-gel method under both acidic and basic conditions. Changes in the absorption spectra of the [Ru(bpy)(2)(pbimH)](2+)-doped silica sols and gels prepared under basic con ditions with the transition from sols to wet gels were similar to those of [Ru(bpy)(2)(pbimH)](2+) in methanol-water solution with a change from basic to acidic conditions. Therefore, the acid-base equilibrium is shifted from the deprotonated form, [Ru(bpy)(2)(pbim)](+), to the protonated form, [Ru( bpy)(2)(pbimH)](2+), during the sol-gel transition. At the gelation point, the wavelength of the emission maximum under basic conditions was shorter t han that under acidic conditions, indicating that the mobility of the Ru co mplex in silica gels was more strongly restricted under basic conditions th an under acidic conditions.