Pore Size Effect on the Dynamics of Excimer Formation for Chemically Attached Pyrene on Various Silica Surfaces

Abstract

Excimer formation by pyrene is a well-known process in solution and on solid surfaces. In solution, excimer formation is highly dependent on the concentration of pyrene. When adsorbed on solid surfaces (i.e., silica surfaces), pyrene has been shown to form ground-state pairs which lead to static excimer emission even at very low surface coverages as a result of a solvent pooling effect induced during solvent removal from the surface. Ground-state pairing on silica surfaces results from a π−π interaction between two adjacent pyrene molecules and can not be avoided even by slow evaporation of the solvent from the surface as the molecules diffuse toward one another during the process. One possible method to alleviate the pairing of pyrene molecules and hence the formation of excimer is to chemically attach pyrene molecules to the silica surface. Chemical attachment, however, does not allow effective control over the spacing between the pyrene molecules to avoid ground-state pairing. To circumvent this, spacer molecules can be incorporated onto the surface by chemical attachment to control the spacing between two adjacent pyrene molecules. Furthermore, by using surfaces that provide various pore sizes, it is possible to control the number of pyrene molecules that can be grafted onto and confined to the pore surface, as well as the steric environment in which the molecules can rotate. Cabosil (fumed silica with no pores) and mesoporous silica surfaces with various pore diameters (i.e., MCM-41) are ideal candidates to examine the feasibility of controlling the spacing between pyrene molecules on a flat surface and confined inside the pores using a cografted spacer molecule (i.e., biphenyl). We have now used such an approach to examine the extent of excimer formation as the ratio of spacer/pyrene molecules is varied on nonporous silica surfaces as well as mesoporous silica surfaces with various pore diameters. Our results show that a decrease in the ratio of spacer/pyrene is accompanied by an increase in the excimer emission on both nonporous and porous silicas. Furthermore, as the pore diameter is increased, an increase in the excimer emission is observed at similar spacer:pyrene grafting ratio. This suggests that a decrease in the separation of pyrene molecules on the surface, as the concentration of biphenyl spacer is decreased relative to pyrene, results in closer proximity of the neighboring pyrene molecules leading to the excimer emission. It is also observed that the alkyl side chain length (i.e., four carbon chain) bearing the pyrene fluorescent probe provides a more facile path for excimer formation by providing the flexibility needed for two pyrenes to reach and interact. When the alkyl side chain is removed and pyrene is directly attached to the surface, the contribution of excimer emission is diminished.