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

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, k_ET, and the exchange integral, J, are estimated. At 150 K, for example, k_ET= 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.