Photoinduced electron transfer between lipidporphyrinato-zinc(II) and -iron(III) complexes in a phospholipid vesicular membrane

A tetraphenylporphyrin derivative having four alkylphosphocholine groups and a covalently-bound axial imidazole (lipidporphyrin) is easily self-organized in water to form spherical micelles or bilayer membranes with phospholipid molecules. The photoinduced electron transfer reactions between lipidporphyrinato-zinc(II) (1) and -iron(III) (2) complexes in these molecular assemblies were studied by fluorescence spectroscopy and laser flash photolysis. A mixture of 1 and 2 (molar ratio: 1/1) produced non-fluorescent micelles. The red-shifted Soret band absorption, relative to that of the methanolic monomer solution, suggests the formation of the photodeactive complex made of 1 and 2 in the ground state. On the other hand, both chromophores were homogeneously dispersed into the bilayer membrane of 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC), and the reductive electron transfer from the excited singlet state of 1 to 2 was observed. While this resulted in a significant decrease in the yield of intersystem crossing, the gradual quenching of the excited triplet state of 1 by 2 was also measured. In both cases, the corresponding Stern–Volmer plots showed a linear relationship and yielded quenching rate constants of 1.2×1011 and 6.2×104 M−1 s−1 via the excited singlet state and the triplet state, respectively. In the presence of excess triethanolamine as a sacrificial reagent, the intermolecular electron transfer became partly irreversible, giving an intramolecularly imidazole-coordinated Fe(II) complex which is capable of reversibly binding dioxygen like hemoglobin.