The Influences of Structural Evolution of Silica Gel Glass on the Photophysical Properties of Benzoic Acid

Benzoic acid was dispersed into silica gel glass at molecular level by sol-gel process. The luminescence properties of benzoic acid in gel glass heat-treated at various temperatures were studied. The changes in gel glass structure and the surrounding environment of benzoic acid resulted in different electronic transitions, and thus the different luminescence spectra. The fluorescence bands centered at 320 nm and 355 nm, respectively induced by the S_1* S₀ transition of benzoic acid and S_1n* S₀ transition of benzoate anions, respectively, were observed when the doped gel glass was heat-treated at the temperatures below 200°C and at the temperature range 300 400°C, respectively. On heating at 500°C, benzoate anions were steadily immobilized in the cages of the Si–O network with the SiO₄ tetrahedra formation and the RTP of benzoate anions is observed. It is proposed that the luminescence spectra of benzoic acid can be used as a structural evolution probe of gel glass.