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 slowly to produce both syn (2) and anti (3) dimers. Under aerated conditions photolysis of 1 at surface coverages <1% of a monolayer gives mainly tetracene-5,12-endoperoxide (4) and very little dimer. The endoperoxide slowly decomposes to form 5,12-tetracenequinone (5), 5,12-dihydroxytetracene (6) and 5,12-dihydro-5,12-dihydroxytetracene (7), respectively. As the surface coverage is increased above 1% of a monolayer, dimerization becomes more pronounced as a result of pairing and/or aggregation. The photooxidation process is mediated by the addition of singlet molecular oxygen to 1. Tetracene also undergoes a dark oxidation reaction on the silica surface to give 5 (major), 6 and 7 (miners) as products. The first-order rate constant for this Lewis acid-catalyzed oxidation was determined to be 1 x 10(-5) s(-1).