Time-resolved EPR studies of photoinduced electron transfer reactions in photosynthetic model porphyrin quinone triads

Abstract

Time-resolved electron paramagnetic resonance (EPR) investigations, performed at microwave frequencies of 9.5 GHz (X band) and 95 GHz (W band), on porphyrin quinone quinone and porphyrin porphyrin quinone triads are presented. Time-resolved EPR at different microwave frequencies makes it possible to elucidate the electron transfer (ET) reactions after pulsed laser excitation and to determine their intermediates, the charge-separated radical pair (RP) states. After photoexcitation in the nematic and soft glass phase of a liquid crystal (LC), spin-polarized EPR spectra are observed for the RPs and for the triplet states of the porphyrins created by spin-selective intersystem crossing (ISC) from the excited porphyrin singlet state. The EPR polarization patterns of the RPs are discussed in terms of the favoured decay channels of the photoexcited singlet state of the porphyrin donor. The decay pathway can be singlet ET to the quinone(s) followed by singlet/triplet mixing to yield RPs with triplet character, or triplet ET after ISC from the porphyrin singlet to the triplet state, or a superposition of both pathways. It is demonstrated that the nature of the linking bridge between donor and acceptor, i.e., aliphatic cyclohexylene or aromatic phenylene groups, significantly influences the polarization patterns of the RP spectra. Saturation (inversion) recovery measurements reveal that the polarization pattern of the radical pairs is governed mainly by a combination of spin relaxation and charge recombination of the radical pairs. In addition, information about the orientation of the guest molecules in the LC matrix with respect to the long axes of the LC molecules can be obtained. The EPR spectrum of the excited triplet state of free-base porphyrin quinone systems, in which no ET occurs, is used as an additional probe for investigating the orientation distributions. In the porphyrin porphyrin quinone triads the energy and ET processes depend strongly on the type of metallation of the porphyrins, specifically, whether the distal, the vicinal, or both porphyrins bear a zinc atom in their macrocycles.