Reversed triplet mechanism CIDEP in triplet azoalkanes

The CIDEP of transient alkyl radicals, generated by triplet sensitization of symmetrical azoalkanes, is studied by time-resolved EPR (TREPR) spectroscopy. The polarization is found to consist of two net components, one reflecting the polarization of the triplet sensitizer, and the other being attributable to a reversed triplet mechanism taking place in the triplet sensitized azo compound. The latter amounts to −8.4±1.5 times the Boltzmann polarization for triplet sensitized AIBN at room temperature in benzene. For the benzophenone/AIBN system it is found that spin relaxation in triplet benzophenone and quenching by AIBN occurs with 3T1=(6.4±1.3) ns and kqT=(9.3±0.2)×108 M−1 s−1 under these conditions.