Photochemical cyclodehydrogenation of Lewis acid-conjugates of azobenzenes

Irradiation of the conjugate acids of azobenzene (1a) with Lewis acids like anhydrous AlCl₃, anhydrous SnCl₄ and anhydrous FeCl₃ in 1,2-dichloroethane results in cyclodehydrogenation to benzo[c]cinnoline (2a). The formation of benzidine (3a) along with 2a suggests that the reaction is a photochemical disproportionation. The absorption spectra of the conjugate acids in 1,2-dichloroethane reveal that inversion of the n → π* and π → π* singlet state energies occurs on complexation of the azo nitrogen with the Lewis acid. Irradiation of the Lewis acid-conjugates of 2-methylazobenzene (1b), 2,2′-dimethylazobenzene (1c), 4,4′-dimethylazobenzene (1d) and 4-chloroazobenzene (1e) also results in cyclodehydrogenation.