Radical pair and triplet state dynamics of a photosynthetic reaction-center model embedded in isotropic media and liquid crystals

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.