The Excited Triplet State of Azoalkanes: Electron Spin Polarization and Magnetic Field Effects During Triplet-Sensitized Photolysis of trans-Azocumene in Solution

The triplet-sensitized photodecomposition of azocumene into nitrogen and cumyl radicals is investigated by time-resolved electron paramagnetic resonance and absorption spectroscopy. The radicals are found to be created spin polarized with a yield depending on the strength of the applied magnetic field. The phenomenon arises because in triplet azocumene, the decay into radicals competes with a fast triplet-sublevel selective intersystem crossing back to the azocumene ground state. The size of the initial spin polarization of the radicals and the magnetic field effect on their yield are determined in solvents of different viscosities. Data analysis yields rate constants for the intersystem crossing and the cleavage reaction of triplet azocumene as well as its zero-field splitting D ZFS. At room temperature in nonpolar solvents, the most probable values are: k x = k y = 1.2 × 1011 s−1 and k z = 1.9 × 1010 s−1 for the intersystem crossing from the energetically lower and upper triplet substates, respectively, k p = 1.6 × 109 s−1 for the cleavage reaction and for the zero-field splitting D ZFS = −3.4 × 1010 s−1 (0.18 cm−1).