Sensitized Photolysis of Organic Azides. A Possible Case of Nonclassical Energy Transfer

Quantum yields for nitrogen evolution were determined in sensitized photolyses of several alkyl azides, phenyl azide, and ethyl azidoformate. Determination of the quantum yield as a function of azide concentration, along with data from the literature on the sensitizers, permitted the calculation of rates of triplet energy transfer from sensitizers to azide. These rates were closely similar for any given sensitizer with n-hexyl, cyclohexyl, and isobutyl azides, but were larger for phenyl azide and smaller for ethyl azidoformate. The energy transfer to the alkyl azides approaches the diffusion-controlled rate only as the sensitizer triplet energy approaches 75-80 kcal/mol, suggesting a triplet energy in this range for the azides. Sensitizers with lower triplet energies are considerably more effective than expected for a classical endothermic energy transfer. It is suggested that the energy transfer is ac-companied by bending of the N-N-N bond angle from its normal 180°, resulting in a “nonspectroscopic” triplet. Extended Hückel theory calculations on methyl azide support the hypothesis that the molecule is more stable with a bent than a linear configuration of the azide group in the first excited state.