Photochemical reactions of azo compounds. VI. Determination of quantum yields and some aspects of the mechanism of photochemical cyclodehydrogenation

A satisfactory method has been developed for the determination of quantum yields for the photochemical cyclization reactions of aromatic azo compounds in strongly acidic solutions. With the three azobenzenes selected for study there was no direct correlation between the quantum yields of the cyclization reaction and Hammett substituent constants. A general decrease in quantum yield and increase in yield fraction of cyclized product were observed as the acidity of the reaction medium was progressively raised. At 436 mμ and 25º ø(azobenzene) = 0.016-0.014 (14-18N H2SO4), ø(4-chloroazobenzene) = 0.006-0.005 (14-20N H2SO4), and ø(4-methylazobenzene) = 0.005-0.004 (12-18N H2SO4). Variations in quantum yield with change in temperature to 15º, and with change in wavelength of the exciting radiation to 405 mμ, were investigated with azobenzene and were found to be quite small. A mechanism for the cyclization reaction is now proposed in which a short-lived photoexcited state of the cis -conjugate acid of the azo compound is envisaged as the species involved in the ring-closure step. The variations in quantum yields with changes of substituents and acidity are discussed in relation to this mechanism, and extensions of the mechanism to accommodate the observed ejection and migration of substituent groups from positions of cyclization are considered.