Molecular Probing for the Microenvironment of Photonics Materials Prepared by the Sol–Gel Process

The absorption and fluorescence spectra of the solvatochromic dye 9-diethylamino-5H-benzo[a]phenoxazin-5-one (Nile Red) were studied in modified SiO2 gels prepared from tetramethoxysilane (TMOS), tetraethoxysilane (TEOS), triethoxysilane (HTES), and triethoxymethylsilane (MTES). Nile Red showed a remarkable spectral shift during the sol–gel process, the direction and extent of which depended on the gel properties. ENT, empirical parameters of the solvent polarity, were determined based on the spectral shifts. As the precursors for the xerogels changed, ENT of the xerogels decreased in the order TEOS ≈ TMOS > MTES > HTES. ENT of the xerogels were 0.88—0.99 for TMOS and TEOS; this result showed the polarity of the xerogels form TMOS and TEOS to be almost the same as that of water (1.000). On the contrary, the polarity of the xerogels from HTES and MTES were as hydrophobic as haloalkane (0.1—0.3).