INTRAMOLECULAR PYRENE EXCIMER IN PROBING THE SOL-GEL PROCESS

The sol-gel-xerogel process in a system consisting of tetraethyl ortho silicate (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 pro be labeled at one end enabled us to conclude that at these concentrati ons 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 emissi on. 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 t he other end, or by a static process, in which a ground state dimer re sulting from the cyclization of the chain is excited and rearranges to the excimer. The ratios of excimer to monomer fluorescence intensitie s at two excitation wavelengths (345 and 360 mm) are different even at the beginning of the hydrolysis reactions, showing that some pyrene d imers are preformed in the initial solution. From the evolution of thi s 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 poin t and probably of the critical point of drying of the gel. A type I ni trogen adsorption isotherm was determined for the xerogel. Being chara cteristic 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 pol ymers are constrained in one pore, enhancing the intramolecular pyrene aggregation.