On the microenvironments surrounding dansyl sequestered within class I and II xerogels

We report on the static and time-resolved fluorescence spectroscopy of dansyl sequestered within two xerogels that were formed from tetraethylorthosilane (TEOS) mixed with ppm levels of dansylamide or dansyltriethoxysilane. These are referred to as class I and II xerogels, respectively. Experiments were performed on these samples as they progressed from liquids to xerogels over a period of 1 year. The results show that the average local microenvironment surrounding the dansyl dopant is different in the class I and II xerogels. Specifically, even though the dipolarity surrounding the dansyl dopant increases as both samples progress from liquids to xerogels, the local dansyl microenvironment within a class II xerogel is always more heterogeneous in comparison to the class I xerogel. In an aged xerogel, the local dipolarity surrounding the dansyl dopant is greater for the class I xerogel than for the class II xerogel. These results are discussed in terms of a model that has the dansyl locating preferentially within Si-OH- and SiO₂-rich regions for the class I and II xerogels, respectively.