LIGHT-INDUCED TRIPLET ELECTRON-TRANSFER IN CYCLOHEXENE-BRIDGED PORPHYRIN QUINONES DETECTED BY TIME-RESOLVED ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPY

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, (T)P-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 x 10(6) s-1 (+/-0.02 x 10(6) 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.