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 mono layer on silica gel have been investigated as a function of sample loa ding. At low loadings, steady state absorption and emission spectra sh ow considerable similarities with those from dilute methanolic solutio ns. As loading is increased, analysis of the spectra using the Kubelka -Munk theory reveals an apparent decrease in molar absorption coeffici ent which is attributed to the formation of aggregates. Studies of ste ady state emission spectra as a function of sample loading reveal a de crease in emission intensity at higher loadings, indicative of nonemit ting aggregates. Transient absorption due to the triplet state of anth racene, the rate of decay of which increases with increased loading, h as been studied using diffuse reflectance laser flash photolysis. At l ow loadings, the decay of delayed fluorescence due to triplet-triplet annihilation can be described using a fractal dimensional rate constan t, but with increasing loading the kinetics conform more closely to a two-dimensional model. Transient absorption studies show that, in addi tion 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 sta tes of aggregated species has been observed. Time resolved emission st udies reveal, in addition to the prompt fluorescence and delayed fluor escence originating from triplet-triplet annihilation, two further emi ssions; one centered at 450 nm, assigned as arising from the anthracen e excimer, and one at 530 nm, assigned as arising from a reaction prod uct present on the surface prior to irradiation.