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 tra nsfer in a photosynthetic model system, which consists of a linear str ucture 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-napththalenediimide. This supramolecular elect ron donor-acceptor array was isotropically oriented in toluene, and an isotropically oriented in liquid crystal matrices, and studied by time -resolved electron paramagnetic resonance spectroscopy. Photoexcitatio n of B results in a two-step electron transfer to yield the radical io n pair, A(.+)-B-C-.-. Charge recombination within A(.+)-B-C-.- produce s a molecular triplet state, A-B-C-3, which exhibits the unique spin- polarized electron paramagnetic resonance signal that has been observe d only in photosynthetic reaction-center proteins.