PHOTOCHEMISTRY OF TETRACENE ADSORBED ON DRY SILICA - PRODUCTS AND MECHANISM

The photochemistry of tetracene (1) has been studied on dry silica at very low surface coverages (less than 3% of a monolayer). Irradiation of degassed samples of 1 adsorbed on silica surface proceeds very slow ly to produce both syn (2) and anti (3) dimers. Under aerated conditio ns photolysis of 1 at surface coverages <1% of a monolayer gives mainl y tetracene-5,12-endoperoxide (4) and very little dimer. The endoperox ide slowly decomposes to form 5,12-tetracenequinone (5), 5,12-dihydrox ytetracene (6) and 5,12-dihydro-5,12-dihydroxytetracene (7), respectiv ely. As the surface coverage is increased above 1% of a monolayer, dim erization becomes more pronounced as a result of pairing and/or aggreg ation. The photooxidation process is mediated by the addition of singl et molecular oxygen to 1. Tetracene also undergoes a dark oxidation re action on the silica surface to give 5 (major), 6 and 7 (miners) as pr oducts. The first-order rate constant for this Lewis acid-catalyzed ox idation was determined to be 1 x 10(-5) s(-1).