Surface and adsorbates effects on the photochemistry and photophysics of adsorbed perylene on unactivated silica gel and alumina

The effect of the nature of the surface and solvent, the presence of coadsorbed species (O2, Ar and H2O) and the surface loading on some photophysical and photochemical properties of perylene (Per) are reported. Although the absorption and emission spectra of perylene adsorbed on alumina and silica gel of different average pore diameters are very similar to those observed in solution, adsorbed perylene presented spectra with broader bands, loss of vibrational structure, and a small red shift and extension to longer wavelengths. These are consequence of aggregate formation, π-stacking, and surface interactions. The nature of the environment affects also the lifetime of the first excited state, presenting a single-exponential decay when in solution and, at least, a double-exponential decay when adsorbed suggesting the adsorption on at least two major surface regions. As the average pore size increases, the emission spectra resembled more to that in solution and the photodegradation rate also increased. Sample loading causes a red shift of the bands on the emission spectrum and a change in the relative intensities of the first and second bands. Oxygen quenched significantly the fluorescence and accelerated the photodegradation. The participation of 1O2 in the photodegradation process was confirmed as well as the participation of a perylene radical cation, identified by laser diffuse reflectance transient and EPR spectroscopy. Coadsorbed water induced the formation of a new species, probably a perylene–H2O complex on the surface.