Dansylated aminopropyl controlled pore glass: a model for silica-liquid solvation

We have prepared a series of aminopropyl controlled pore glass (CPG) particles that have been labeled with a solvatochromic fluorescent probe molecule (dansyl). We report on the behavior of the attached dansyl reporter as a function of dansyl-to-amine molar ratio (i.e., dansyl loading), solvent dipolarity, and surface-residue end capping. In these experiments, we systematically adjust the dansyl loading by 10(5); a range much larger than ever explored. The dansylated CPG particles were also end capped with trimethylchlorosilane to derivatize most of the residual silanol and/or aminopropyl groups. The attached dansyl molecules can be surrounded by other dansyl molecules; they can be distributed within an ensemble of sites with differing physicochemical properties, and/or they can be distributed in sites that are restrictive to dansyl motion and/or solvent inaccessible. At high dansyl loadings, the majority of the dansyl groups are solvated by other dansyl moieties and solvent does not significantly alter the local microenvironment surrounding the average dansyl molecule (i.e., the cybotactic region) to any significant level. At intermediate dansyl loadings, the average distance between the dansyl groups increases and solvent is able to access/solvate/wet the dansyl groups and alter their cybotactic region to a greater extent. At the lowest dansyl loadings studied, the results suggest that these dansyl moieties are localized within solvent inaccessible/restrictive SiO2 sites (e.g., small pores).