PHOTODECOMPOSITION OF ANTHRACENE ON DRY SURFACES - PRODUCTS AND MECHANISM

The photochemistry of anthracene (1) was studied on dry surfaces of si lica, Cab-O-Sil (fumed silica) and alumina (neutral) at low coverages (less than 11% of a monolayer). The adsorption of 1 onto these surface s from cyclohexane obeys a Freundlich adsorption isotherm, demonstrati ng a distribution of adsorption sites for interactions between 1 and t he surface. Photolysis of 1 (lambda(ex) = 350 nm) adsorbed on silica, Cab-O-Sil or alumina, under deaerated conditions, proceeds slowly to g ive the anthracene-9,10-photodimer (2) as the only product. Diffuse re flectance and fluorescence spectroscopy show that a novel process, inv olving the formation of a stable ground state pair between two molecul es of I takes place on all three surfaces at coverages of more than 1% of a monolayer. We propose a dimerization mechanism involving a singl et excimer formed from the stable ground state pairs. Photolysis at th e solid-air interface, on the other hand, is considerably faster and p roceeds to give 2 and photo-oxidation products of 1. The primary oxida tion product is anthracene-9,10-endoperoxide (3) which undergoes therm al decomposition on the surface to give 9,10-anthraquinone (4), 9,10-d ihydro-9,10-dihydroxyanthracene (5), bianthronyl (6), 9-bydroxyanthron e (7) and 0'-dihydroxy-9,9',10,10'-tetrahydro-9,9'-bianthryl (8) as th e secondary products. Photo-oxidation is mediated by the addition of s inglet molecular oxygen to ground state 1.