Quantitative time-resolved EPR CIDEP study of the photodecomposition of trans -azocumene in solution

The cumyl radical system, which is created after laser flash irradiation oftrans-azocumene in benzene solution at room temperature, is investigated using time-resolved EPR spectroscopy. From the quantitative analysis of EPR time-profiles at different microwave powers the spin relaxation timesT 1=3.5±0.3 μs andT 2=2.5±0.1 μs are evaluated as well as the magnitude of the chemically induced electron polarization (CIDEP), which is generated by the radical pair mechanism (RPM). The geminate RPM polarization is found to be considerably smaller than the F-pair one, 32±2 and 48±5 in units of the Boltzmann polarization, respectively. This is attributed to an initial radical separation in the geminate pair, caused by the cleavage reaction. Besides cleavage, the photoexcitedtrans-azocumene also decays via isomerization to the thermally unstablecis-isomer, the lifetime of which is found to be 14±3 μs at 293 K in benzene, three times longer than in cyclohexane. The quantum yield of free radicals, escaping from the primary cage, is determined as 0.28±0.06 for the decay of the excitedtrans-azocumene and 0.18±0.04 for the thermal cleavage of thecis-isomer. The self-termination of cumyl radicals proceeds with a rate constant 2k t=7±1)·108 M−1s−1 in benzene at R