ET(33) dye as a tool for polarity determinations: Application to porous hybrid silica thin-films

ET(33) (2,6-dichloro-4-(2,4,6-triphenyl-N-pyridino)-phenolate) was proven to be an effective tool to determine the polarity of the pore environment in sol–gel matrices prepared using acid catalysis. The probe molecules have been trapped during the formation of the organically modified silica matrices, occupying both, the inner porosity and the outer porosity of the matrix.The effect of the chemical modification of the silica film on the polarity of the pore environment was studied as a function of the organic modifying groups and their relative amount in the silica matrix. The usage of the ET(33) polarity probe allowed the identification of different pore environments (sites) within a matrix, attending to the relative amount of functional groups in the pores. A progressive decrease of the polarity of the samples prepared was observed as the amount of the organic modifying groups was increased. Hybrid matrices could be prepared via sol–gel processing, having polarities between 40 and 60 kcal mol−1 in the Reichardt's ET(30) scale, which can be controlled by the amount and nature of the organic modifying groups. A particular case was observed in samples modified with pentafluorophenyl groups (–C6F5), which showed very high pore polarities, close to that of unmodified matrices, in which pore environments rich in OH groups and rich in –C6F5 groups could not be distinguished, due to their similar polarity.The ET(33) polarity dye was found to provide a four times higher sensitivity than the known probe Nile Red for the determination of the polarity of the pore environment, as reflected by the much larger shift of the absorption peak of the dye due to changes in polarity. Due to its stability in acidic media, the ET(33) is suitable to be used in hybrid matrices prepared using acid catalysis, as are most of those prepared by the sol–gel method.