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.
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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.Journal of Materials Chemistry A 01/2000; 10(12):2765-2768. DOI:10.1039/b003647l · 7.44 Impact Factor