Photolysis of aromatic azides. Part 3.—Quantum yield and mechanism

Abstract

The quantum yield of photolysis in a group of aromatic azides was measured in solution at 25°C and in hydrocarbon matrices at 77°K. A more detailed kinetic study was carried out on 1-azido-naphthalene. From the results, together with the spectral assignments made in part 1 and 2, and chemical evidence, it is concluded that bond dissociation occurs via a hot ground state, by vibrational excitation of a critical stretching mode in the azido group. Vibrational energy from higher excited states is carried over into the ground state and is responsible for the observed wavelength dependence of the quantum yield. The efficiency of bond fission in response to irradiation is determined by the strength of the dissociating bond, by the excess vibrational energy available and by the efficiency of transmission of skeletal vibrations to the side chain. This last factor is thought to be sensitive to the geometry of the molecule and to the symmetry of the electronic transition involved.