Surface enhanced raman scattering and photodimerization of pyridyl-substituted ethylenes at a silver electrode surface

Abstract

Pyridyl-substituted ethylenes have been observed to photodimerize while adsorbed at a polycrystalline silver electrode surface. Surface Raman spectra reveal that vinylpyridine and bis-pyridyl ethylene dimerize to products in which ethylenic bonds have been reduced. The potential dependence of the photochemical threshold wavelength implicates the role of adsorbateto-metal charge transfer excitation in the dimerization mechanism.Adsorbate-to-metal charge transfer leaves a cation radical which initiates dimerization with nearest neighbor adsorbate molecules. Photochemistry occurs only under conditions of heavy surface coverage. A prerequisite to the topochemical dimerization of ethylenic compounds in the solid state requires that the separation between neighboring ethylene groups lie between 3.7 and 4.1 Å. The silver lattice parameter of 4.08 Å and the absence of photochemistry under light surface coverages appear to correlate well to the solid state topochemical prerequisite. Observed isotopic shifts in the surface Raman spectra of the photoproducts prepared in H 2O/D 2O electrolytes indicates that solvent proton transfer occurs during dimerization. Adsorbate surface orientation was determined and the structures of photoproducts are identified.