Surfaces change the properties of solvents from bulk solution limits, often in ways that are difficult to predict.  This talk will explore how surface induced anisotropy creates markedly different solvating properties with two polar solvents, acetonitrile and methanol. Specifically, resonance enhanced second harmonic generation (SHG) is used to measure the surface activity and electronic structure of two adsorbed 7-amino-coumarin solutes, Coumarin 151 (C151) and Coumarin 152 (C152) at silica/liquid interfaces, and time resolved fluorescence measurements performed in a total internal reflection geometry (TR-TIRF) measure the emission lifetimes of these two solutes in the near-surface region.  Across the silica/liquid interface, acetonitrile forms a local solvation environment that is more polar than bulk solution, whereas the silica/methanol interface is significantly less polar than bulk solution.  These differences are assigned to differences in solvent structure imposed on the interfacial region by the substrate.  TR-TIRF data from C151 and C152 adsorbed to silica/methanol interfaces support the SHG measurements, but similar experiments performedTR-TIRF data from silica/acetonitrile interfaces appear to contradict SHG results with the fluorescence lifetime showing little evidence of the polar solvation implied by the excitation wavelengths.  This discrepancy may reflect differences in the properties measured by the two types of experiments:  SHG samples adiabatic excitation energies and is sensitive to the solvent organization around solutes in their ground excited states, whereas TR-TIRF measures the response of excited state solvent molecules with solvation structure equilibrated to the solute's excited state.

Linear and nonlinear optical studies of structure, organization, and reactivity across solid/liquid interfaces

247th National Meeting (2014)

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