Matrix effects and mechanisms of the spectral shifts of coumarin 440 doped in sol–gel-derived gel glass

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Abstract
The spectral and temporal changes of the emission from four types of gel glass doped with coumarin 440 prepared by using tetraethoxysilane (TEOS), 3-glycidoxypropyltrimethoxysilane (GPTMS), methyltriethoxysilane (MTES), and vinyltriethoxysilane (VTES) as sol–gel precursors, respectively, were studied. The observed significant spectral shifts of coumarin 440 in the TEOS– and TEOS/GPTMS–gel glass are due to two types of matrix effects on different time scales. The formation of hydrogen bonding between the carbonyl and amine groups of coumarin 440 and the residual Si–OH and H 2 O in the TEOS– and TEOS/GPTMS–gel glass shifts the emission spectra to longer wavelengths on the subnanosecond time scale with respect to that in the MTES– and VTES–gel glass. The temperature and time dependent spectral shifts on the nanosecond time scale are attributed to the reorientation of the surrounding matrix molecules during the lifetime of the excited state of coumarin 440. No residual Si–OH groups H 2 O molecules are found in the MTES– and VTES–gel glass prepared by acid-catalyzed hydrolysis and basic-catalyzed condensation, and the emission spectra of coumarin 440 in these matrices show no dependence on time and temperature.