Clean photodecomposition of 1-methyl-4-phenyl-1H-tetrazole-5(4H)-thiones to carbodiimides proceeds via a biradical.

The photochemistry of 1-methyl-4-phenyl-1H-tetrazole-5(4H)-thione (1a) and 1-(3-methoxyphenyl)-4-methyl-1H-tetrazole-5(4H)-thione (1b) was studied in acetonitrile at 254 and 300 nm, which involves expulsion of dinitrogen and sulfur to form the respective carbodiimides 5a,b as sole photoproducts. Photolysis of the title compounds in the presence of 1,4-cyclohexadiene trap led to the formation of respective thioureas, providing strong evidence for the intermediacy of a 1,3-biradical formed by the loss of dinitrogen. In contrast, a trapping experiment with cyclohexene provided no evidence to support an alternative pathway of photodecomposition involving initial desulfurization followed by loss of dinitrogen via the intermediacy of a carbene. Triplet sensitization and triplet quenching studies argue against the involvement of a triplet excited state. While the quantum yields for the formation of the carbodiimides 5a,b were modest and showed little change on going from a C(6)H(5) (1a) to mOMeC(6)H(4) (1b) substituent on the tetrazolethione ring, the highly clean photodecomposition of these compounds to a photostable end product makes them promising lead structures for industrial, agricultural, and medicinal applications.