The electron spin dynamics associated with intramolecular electron transfer in a photosynthetic model system, which consists of a linear structure of the type A-B-C, is described. In this structure, donor A is a p-methoxyaniline, chromophore B is a 4-amino-1,8-naphthalimide, and acceptor C is a 1,4:5,8-naphthalenediimide. This supramolecular electron donor−acceptor array was isotropically oriented in toluene, and anisotropically oriented in liquid crystal matrices, and studied by time-resolved electron paramagnetic resonance spectroscopy. Photoexcitation of B results in a two-step electron transfer to yield the radical ion pair, A•+-B-C•-. Charge recombination within A•+-B-C•- produces a molecular triplet state, A-B-3*C, which exhibits the unique spin-polarized electron paramagnetic resonance signal that has been observed only in photosynthetic reaction-center proteins.

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Intramolecular electron transfer (ET) in cis and trans isomers of a covalently linked porphyrin–cyclohexylene–quinone, oriented in liquid crystals (LCs), is monitored by time-domain electron paramagnetic resonance (EPR) spectroscopy over an extended range of temperature, i.e., 210<T<320 K. The spectra of both isomers exhibit different line shape behavior and temperature dependence, as compared to those found in isotropic solutions (ethanol). Whereas in ethanol the range of detection is quite narrow, i.e., 130<T<160 K, the one in the LC environment includes also the nematic fluid phase, thus allowing one to monitor the ET products in this phase. The difference in spectra between the two environments is analyzed in terms of the magnetization projection on the LCs director, L. The different spectra of the two isomers are interpreted in terms of their different molecular geometry. In the case of the trans isomer, both triplet- and probably singlet-initiated ET routes can concurrently be detected, and the free energy of the charge-separated state is estimated from the spectral dynamics.