The photochemistry of a number of azocyclopropanes has been studied: cis-1-(methyl-trans-azo)-2-phenylcyclopropane, trans-10(methyl-trans-azo)-2-phenylcyclopropane, trans-azocyclopropane and cis-azocyclopropane. Direct irradiation into the lowest n $\rightarrow\pi$* band led to nearly exclusive cis-trans isomerization of the azo group. Minor processes for the phenylcyclopropanes were cyclopropyl ring isomerization, rig expansion of 1-methyl-5-phenyl-2-pyrazoline and formation of styrene. These processes suggest formation of a biradical intermediate which is apparently not formed in the case of trans- and cis-azocyclopropane.
Singlet sensitization of cis-1-(methyl-trans-azo)-2-phenylcyclopropane with benzene increased the quantum yields of the processes competing with azo group isomerization. The increased efficiencies indicate that the second excited state of the azoalkane contributed to the observed photochemistry. On the other hand, the second excited state of trans- and cis-azocyclopropane is not accessible to benzene singlet sensitization and so led to exclusive interconversion of the two azo isomers.
Direct irradiation into the short wavelength UV band of trans- and cis-azocyclopropane in pentane produced more decomposition products than n $\rightarrow\pi$* excitation. The efficiency of trans $\rightarrow$ cis azo group isomerization decreased at the expense of the C-N and cyclopropyl C-C homolyses. These two cleavages accounted for a large number of products including nitrogen, cyclopropane, ethylene and 1-cyclopropyl-2-pyrazoline. Cyclopropyl ring cleavage to a biradical intermediate followed by reclosure to regenerate the starting azoalkane was postulated as one of the major processes to rationalize the low quantum yield of products. Formation of products propene, allylcyclopropane and 1,5-hexadience upon irradiation at 193 nm suggested production of vibrationally excited cyclopropyl radicals which rearrange to allyl. Short wavelength irradiation of trans-azocyclopropane in the vapor phase led to a large number of decomposition products.
Triplet-sensitization of cis-1-(methyl-trans-azo)-2-phenylcyclopropane produced both azo and cyclopropyl group isomerization products, but not styrene. The differing product distribution relative to direct irradiation indicates that the triplet state is not exclusively involved in the singlet photochemistry. The unusually high quantum yield found for triplet-sensitized isomerization of trans-azocyclopropane to the cis isomer was initially rationalized in terms of cyclopropyl ring cleavage to form a biradical intermediate. However, a stereospecifically labelled deuterium analog of trans-azocyclopropane, prepared to test the biradical hypothesis, showed that a such an intermediate is probably not formed upon triplet sensitization.