DETERMINATION OF THE ENERGY-LEVELS OF RADICAL PAIR STATES IN PHOTOSYNTHETIC MODELS ORIENTED IN LIQUID-CRYSTALS WITH TIME-RESOLVED ELECTRON-PARAMAGNETIC-RESONANCE

We report the results of time-resolved electron paramagnetic resonance (TREPR) studies of photoinduced charge separation in a series of biom imetic supramolecular compounds dissolved in oriented liquid crystal s olvents. The molecules contain a chlorophyll-like (zinc 9-desoxomethyl pyropheophorbide a) electron donor, D (ZC), and two electron accepters with different reduction potentials, i.e., pyromellitimide, A(1) (PI) , and 1,8:4,5-naphthalenediimide, A(2) (NI). The compounds investigate d are ZCPI, ZCNI, and ZCPINI, and they have small but well-defined dif ferences of their ion-pair energies. Temperature-dependent TREPR studi es on this series of compounds permit the determination of the radical pair energy levels as the solvent reorganization energy increases fro m the low-temperature crystalline phase, through the soft glass phase, to the nematic phase of the liquid crystal. As the temperature is inc reased, the radical pair with the lowest energy is the first to exhibi t triplet-initiated charge separation as the solvent reorganization en ergy increases in the liquid crystal. The energy levels of the radical pairs and the solvent reorganization energy are determined by using t he known singlet: and triplet excited state energy levels of ZC, the e lectrochemically determined relative energies between the radical ion pairs in polar isotropic solvents, and the TREPR data. All these yield information about the ordering of the radical ion pair energy levels relative to the excited-state energy levels of ZC.