A photophysical study of the sol/gel transition in silica: Structural dynamics and oscillations, room-temperature phosphorescence and photochromic gel glasses

We report the use of photophysical probes for studying the sol/gel transition of silicon tetra-alkoxide undergoing hydrolysis and condensation-polymerization, for studying the properties of the final silica gel glass, and for studying the formation of reversed-phase organic silica gel glasses during hydrolysis of phenyl and ethyl trimethoxysilanes. Pyrene fluorescence and excimerization process were used to probe, stage by stage, the dynamics of the structural changes that occur during the sol to wet gel to dry gel transitions and to probe the effects of parameter changes such as pH, water/silane ratio and alkoxy groups. Unlike adsorption on silica surfaces, pyrene trapped in the silica glass shows monomeric emission only. Structural oscillations are observed upon addition of surface active agent. To the best of our knowledge, this is the first far-from-equilibrium dissipative structure, in which a polymeric system oscillates. We also report the uses of the final gel glass as a novel room-temperature trap of organic molecules. The possibility to trap an organic molecule in inorganic glass opens many new opportunities and applications, two of which are reported: (a) Room temperature phosphorescence is obtained for a number of molecules, e.g., pyrene. (b) The first organic-dye (spiropyrane)/inorganic glass (SiO₂) photochromic material is described.