Light-induced triplet electron transfer in cyclohexene-bridged porphyrin–quinones detected by time-resolved electron paramagnetic resonance spectroscopy

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Abstract
Light-induced triplet electron transfer (ET) is observed in two isomeric cyclohexene-bridged porphyrin–quinones (P–Q) in highly viscous ethanol at temperatures between 130 and 150 K. Both the primary porphyrin triplet, TP–Q, and the triplet radical pair, T(P·+–Q·–), identified by their dipolar splittings and polarization patterns, are detected by time-resolved continuous-wave electron paramagnetic resonance (cw-EPR). A theoretical model is presented by which the kinetic traces at the turning points of the powder-type EPR spectrum of T(P·+–Q·–) are simulated. From the fits to the experimental data, values for the ET rate, kET, and the exchange integral, J, are estimated. At 150 K, for example, kET= 0.26 × 106 s–1(±0.02 × 106 s–1) and J=–1370 G (±60 G). The rates obtained for the charge recombination from the singlet radical pair to the ground state indicate that a distribution of J values has to be considered, even for P–Qs with a relatively rigid bridge.

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