Hybrid and nonhybrid silica sol-gel systems doped with 1,12-bis(1-pyrenyl)dodecane

A comparison is made between pure inorganic and hybrid organic/inorganic si lica sol-gel systems, at different stages of evolution, using the informati on provided by the fluorescent probe 1,12-bis( 1-pyrenyl)-dodecane. The ini tial sols were prepared using tetraethylorthosilicate (TEOS) as precursor, with acid catalysis, at initial pH 1.2 and 2.5. The hybrid systems were pre pared by incorporating a low molecular weight polymer in the initial mixtur es (polytetrahydrofuran, M-w = 2000), in 1:100 molar ratio to TEOS. The evo lution of the systems was followed by fluorescence spectra of the probe, in corporated in a very low concentration (similar to 10(-7) M). It has been s hown that while in the pure silica systems the probe molecules are encapsul ated in the primary silica particles as they form, in the hybrid systems mo st probes remain in the vicinity of the polymer, which has a strong hydroph obic character. The fluorescence decay curves of aged gels confirm that the initial pH mainly influences the dimensions of the primary silica particle s that are much smaller at pH 1.2 than at pH 2.5. The role of oxygen as a f luorescence quencher has been analyzed, by comparing oxygen-free and air-eq uilibrated samples. The probe is accessible to oxygen throughout the sol-ge l process for both systems at initial pH 2.5, while at pH 1.2 this is only true for the hybrid system.