The nature and mobility of the species present when acridine is adsorbed from high vacuum onto thermally pretreated alumina and silica have been investigated by ground-and excited-state spectroscopic techniques. For alumina treated at high temperatures (>500 °C) -bonded acridine is the main species present, while the acridinium cation AH+ predominates at lower activation temperatures (-100 °C). The triplet-triplet absorption at 480 nm of AH+ has been directly observed without a triplet sensitizer, and the decay is nonexponential. On silica the main adsorbed species is hydrogen-bonded acridine which shows triplet-triplet absorption at 435 nm and which, for samples of low coverage or high pretreatment temperature, has an exponential decay with a lifetime of about 30 ms. For samples of high coverage or low pretreatment temperature, the triplet decay is faster and nonexponential, and delayed fluorescence is observed. This arises from triplet-triplet annihilation, a consequence of the mobility of acridine on the silica surface for which the two-dimensional bimolecular rate constant is 8 x 1013 dm2 mol"1 s"1. No delayed fluorescence occurs when silica has been pre.rented at high temperatures, conditions which favor dehydroxylation of the surface to form siloxane units. These have a high activation barrier to translational motion and thereby impede the lateral mobility of the adsorbate. For alumina, the nature of the surface permits virtually no mobility of the acridine, and hence no significant bimolecular processes are observed on this adsorbent.
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