Fast Photoinduced Electron Transfer from Polyalkyl- to Polyfluoro-Metalloporphyrins in Lipid Bilayer Membranes
Direct electrochemical evidence shows that photoinduced electron transfer from excited Mg?octaethylporphyrin (MgOEP) to a Mg (or Zn) complex of tetrakis(pentafluorophenyl)porphyrin (TFPP) in lipid bilayers occurs with a second-order rate constant ≥108 M-1 s-1. This reaction is ?100 fold faster than the reported intermolecular rate between porphyrins, and occurs even under aerobic conditions. Electron transfer from the ground state of MgOEP to excited ZnTFPP is observable under anaerobic conditions, but the rate is ?10-fold slower. Time-resolved photoconductivity of the lipid bilayer with the mixed metalloporphyrins suggests that the charge recombination times (τ) of the geminate ion pairs in the MgTFPP?MgOEP and ZnTFPP?MgOEP systems are 20 and 38 ?s, respectively. The MgTFPP- anion reduces O2 with a second-order rate constant of ?107 M-1 s-1, but the oxidation of ZnTFPP- anions by O2 is very slow. The differences between the two systems may arise from different redox potentials of ZnTFPP and MgTFPP. These data prove that, even containing Mg, the least electronegative element which can be stably chelated, a metalloporphyrin with poly electron-withdrawing groups is a good electron acceptor. Our results suggest that such electronegative porphyrins are useful molecular parts for assembling of porphyrin-based biomimetic energy conversion devices.