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

Abstract

Silica gels doped with bis(2,2′-bipyridine)[2-(2-pyridyl)benzimidazole]ruthenium(II), [Ru(bpy)₂(pbimH)]²⁺, which dissociates a proton from the imino 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 conditions 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 than that under acidic conditions, indicating that the mobility of the Ru complex in silica gels was more strongly restricted under basic conditions than under acidic conditions.