Toward tailored xerogel composites: Local dipolarity and nanosecond dynamics within binary composites derived from tetraethylorthosilane and ORMOSILs, oligomers or surfactants

We explore the potential of xerogel composites to tailor the behavior of ac tive dopants that are sequestered within the xerogel. Toward this end, we r eport on the local dipolarity and dynamics of two fluorescent probes (pyren e and rhodamine 6G, R6G) each co-doped at low concentration directly into a series of binary xerogel composites. The composites that we have investiga ted are composed of tetraethylorthosilicate (Si(OCH2CH3)(4),TEOS) plus one of several organically-modified silanes (ORMOSILs), organic oligomers, or a common surfactant. For convenience we divide these xerogel composites into two classes: (1) xerogels wherein the organic character arises from the ad dition of an ORMOSIL co-monomer, possessing a non-hydrolyzable organic func tional group, that becomes covalently incorporated with in the xerogel and (2) xerogels wherein the organic content is adjusted by adding organic olig omers or a surfactant. Six organically-modified silylalkoxides of the form R' Si-n(OR)(4-n) were investigated as ORMOSILs. Poly(ethylene glycol), Nafi on, and Ionene 6,2 were tested as oligomers. Triton X-100 was used as the s urfactant. To estimate the local dipolarity within these composites we used the static fluorescence from pyrene molecules that were sequestered within the composites. These experiments showed that the local dipolarity surroun ding the average pyrene molecule can be tuned significantly, but this depen ds on the actual organic species that one uses to prepare the xerogel compo site. Time-resolved fluorescence anisotropy measurements were used to quant ify the R6G mobility within the same composites. These results demonstrate that certain organic additive scan be used to adjust the R6G mobility withi n the xerogel composite.