PRIMARY PHOTOCHEMICAL PROCESSES OF ANTHRACENE ADSORBED ON SILICA-GEL

Time resolved and steady state absorption and emission characteristics of anthracene adsorbed at surface coverages of less than 5% of a monolayer on silica gel have been investigated as a function of sample loading. At low loadings, steady state absorption and emission spectra show considerable similarities with those from dilute methanolic solutions. As loading is increased, analysis of the spectra using the Kubelka-Munk theory reveals an apparent decrease in molar absorption coefficient which is attributed to the formation of aggregates. Studies of steady state emission spectra as a function of sample loading reveal a decrease in emission intensity at higher loadings, indicative of nonemitting aggregates. Transient absorption due to the triplet state of anthracene, the rate of decay of which increases with increased loading, has been studied using diffuse reflectance laser flash photolysis. At low loadings, the decay of delayed fluorescence due to triplet-triplet annihilation can be described using a fractal dimensional rate constant, but with increasing loading the kinetics conform more closely to a two-dimensional model. Transient absorption studies show that, in addition to the triplet state, the anthracene cation radical is formed by a two-photon or multiphoton process with a yield which depends on the concentration of monomeric anthracene present on the surface, as does the triplet yield. No transient absorption attributable to excited states of aggregated species has been observed. Time resolved emission studies reveal, in addition to the prompt fluorescence and delayed fluorescence originating from triplet-triplet annihilation, two further emissions; one centered at 450 nm, assigned as arising from the anthracene excimer, and one at 530 nm, assigned as arising from a reaction product present on the surface prior to irradiation.