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Photodecomposition of anthracene on dry surfaces: products and mechanism

1995; Elsevier BV; Volume: 86; Issue: 1-3 Linguagem: Inglês

10.1016/1010-6030(94)03945-q

1873-2666

Reza Debestani, Keith J. Ellis, Michael E. Sigman,

Analytical Chemistry and Sensors

The photochemistry of anthracene (1) was studied on dry surfaces of silica, CabOSil (fumes silica) and alumina (neutral) at low coverages (less than 11% of a monolayer). The adsorption of 1 onto these surfaces from cyclohexane obeys a Freundlich adsorption isotherm, demonstrating a distribution of adsorption sites for interactions between 1 and the surface. Photolysis of 1 (λex=350 nm) adsorbed on silica, CabOSil or alumina, under deaerated conditions, proceeds slowly to give the anthracene-9,10-photodimer (2) as the only product. Diffuse reflectance and fluorescence spectroscopy show that a novel process, involving the formation of a stable ground state pair between two molecules of 1 takes place on all three surfaces at coverages of more than 1% of a monolayer. We propose a dimerization mechanism involving a singlet excimer formed from the stable ground state pairs. Photolysis at the solid-air interface, on the other hand, is considerably faster and proceeds to give 2 and photo-oxidation products of 1. The primary oxidation product is anthracene-9,10-endoperoxide (3) which undergoes thermal decomposition on the surface to give 9,10-anthraquinone (4), 9,10-dihydro-9,10-dihydroxyanthracene (5), bianthronyl (6), 9-hydroxyanthrone (7) and 10,10′-dihydroxy-9,9′,10,10′-tetrahydro-9,9′-bianthryl (8) as the secondary products. Photo-oxidation is mediated by the addition of singlet molecular oxygen to ground state 1.