Time-resolved EPR studies of covalently linked porphyrin-crown ether-quinones dissolved in liquid crystals

Time-resolved EPR spectra are reported for photoexcited porphyrin-quinones with butylene and cis- or trans-cyclohexylene spacers between porphyrin donors and crown ether-quinone accepters, In the nematic phases of liquid crystals, polarized triplet EPR spectra of the porphyrin moiety could be observed at early delay times (0.1-0.6 mu s) after laser flash irradiation, At later delay times (1-2 mu s) the triplet EPR spectra of the charge-separated radical pair state, created by intramolecular electron transfer from the porphyrin donor to the crown ether quinone acceptor, could be detected. The similarity of the zero-field splitting parameters D of the cyclohexylene- and butylene-linked compounds shows that the radical pair of the flexible butylene-linked compound is in a stretched conformation, However, longer laser irradiation of the species with the flexible bridge results in the formation of a novel triplet radical pair species with a shorter porphyrin-quinone separation indicating a photoinduced backfolding of the quinone over the porphyrin plane, Comparison of the cis- and trans-cyclohexylene-linked compounds reveals a distortion of the cyclohexylene spacer in the cis compound.