Unusual isomerization behavior of organic solutes at the aqueous-silica interface

Abstract

Experiments described in this thesis address the question of how strong association between water molecules and the silica surface alter the solvation and isomerization behavior of adsorbed organic molecules from bulk solution limits. The work was motivated by the hypothesis that the ice-like structure induced by strong hydrogen bonding with the surface silanol groups would restrict solute isomerization. This hypothesis was tested using 2 surface-specific spectroscopic techniques: second harmonic generation (SHG) and time-correlated single photon counting in a total internal reflection geometry (TIR-TCSPC). This work examined two different 7-aminocoumarin dyes (Coumarin 151 and Coumarin 152) and dimethylaminobenzonitrile (DMABN). Coumarin 152 and DMABN both isomerize to form a twisted intramolecular charge transfer (TICT) state upon photoexcitation, whereas Coumarin 151 forms a simple (planar) intramolecular charge transfer state. SHG studies characterized the local solvation environment surrounding adsorbed molecules by providing electronic excitation energies that were compared to bulk excitation energies in different representative solvents. TIR-TCSPC measured the time-resolved emission of adsorbed molecules and quantified a solute's tendency to form TICT (or ICT) isomers at the aqueous-silica interface. Together, SHG and TIR-TCSPC provide a cohesive description of the local polarity across an aqueous-silica interface and how restricted solvent dynamics change a solute's photophysical chemistry. TIR-TCSPC studies reported that both C152 and DMABN are unable to isomerize to TICT states at the aqueous-silica interface, acting as if they were solvated in a nonpolar solvent or in a confined geometry. SHG studies confirm that the aqueous-silica interface is, in fact, more polar than the bulk aqueous limit, strongly implying that the observed effects are dynamic in origin rather than polarity driven. In contrast, studies of C151 show that this solute is largely insensitive to anisotropic, restrictive surface effects. Together results from these three molecules lead us to conclude that adsorption to the strongly associating aqueous-silica interface restricts large amplitude isomerization in organic molecules. Adsorption to less strongly associating interfaces does not cause this restriction. In the event that photo-induced isomerization does not require large amplitude motion, interfacial solvation has little effect on adsorbed solute behavior.