INTRAMOLECULAR PYRENE EXCIMER IN PROBING THE SOL-GEL PROCESS

The sol-gel-xerogel process in a system consisting of tetraethyl orthosilicate (TEOS)/water/ethanol with acid catalysis was monitored, using as probes polystyrene chains labeled either at one or both ends with a l-pyrene derivative, in very low concentrations (<10(-6) M). The probe labeled at one end enabled us to conclude that at these concentrations pyrene emits only in the monomeric form. For the chain labeled at both ends, besides the monomeric emission, a new band centered at 470 nm was observed, which was attributed to intramolecular excimer emission. The excimer forms either by a dynamic process, in which an excited pyrene at one end of the chain encounters a ground state monomer at the other end, or by a static process, in which a ground state dimer resulting from the cyclization of the chain is excited and rearranges to the excimer. The ratios of excimer to monomer fluorescence intensities at two excitation wavelengths (345 and 360 mm) are different even at the beginning of the hydrolysis reactions, showing that some pyrene dimers are preformed in the initial solution. From the evolution of this ratio at 345 nm with time (which mainly monitors the dynamic excimer formation process), it has been possible to detect an increase in the local medium viscosity with the progress of reaction. The same ratio at 360 nm (which mainly monitors the excimer formation via the dimer) increases over time, initially slowly and afterwards very steeply, and finally attains a plateau, allowing the determination of the gel point and probably of the critical point of drying of the gel. A type I nitrogen adsorption isotherm was determined for the xerogel. Being characteristic of a microporous solid, it indicates that the average radius of the pores is around 10 Angstrom, which is lower than the radius of gyration of the polymer chain, supporting that in the xerogel the polymers are constrained in one pore, enhancing the intramolecular pyrene aggregation.