Photodecomposition of diazanaphthalenes adsorbed on silver colloid surfaces

The surface photoreactivity of four diazanaphthalenes (phthalazine, cinnoline, quinazoline and quinoxaline) adsorbed on aqueous colloidal silver was studied by following the time evolution of the SERS spectra of their photoproducts with laser irradiation time in a flow cell. All of these molecules were found to photodecompose through the cleavage of N=N or N-C bonds of the heterocycle while the benzene ring remained intact. Cinnoline, which adsorbed as a sigma-complex "standing up" on the surface, showed very little reactivity. Quinazoline at high surface concentrations was also found to form a sigma-complex (likely due to the fact that more adsorbate could be packed in that orientation). It, too, showed essentially no surface photoreactivity. To the contrary, quinazoline at low surface concentrations formed a pi-complex, "lying down" on the surface. In this orientation quinazoline photoreacted rapidly. For quinoxaline, two distinct photoprocesses were observed depending on the presence or absence of coadsorbed chloride ion on the surface. The wavelength response of the formation rate of these products in the absence and presence of chloride suggests that the coadsorbed halide modifies the electronic properties (probably locally) of the silver. Multiple photoproducts were observed to be formed in several cases. Although an unequivocal identification of the products was not possible, plausible assignments were suggested on the basis of the observed vibrational spectra.