FAST PHOTOINDUCED ELECTRON-TRANSFER FROM POLYALKYL-METALLOPORPHYRINS TO POLYFLUORO-METALLOPORPHYRINS IN LIPID BILAYER-MEMBRANES

Direct electrochemical evidence shows that photoinduced electron trans fer from excited Mg-octaethylporphyrin (MgOEP) to a Mg (or Zn) complex of tetrakis(pentafluorophenyl)porphyrin (TFPP) in lipid bilayers occu rs with a second-order rate constant greater than or equal to 10(8) M- 1 s(-1). This reaction is similar to 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 excit ed ZnTFPP is observable under anaerobic conditions, but the rate is si milar to 10-fold slower. Time-resolved photoconductivity of the lipid bilayer with the mixed metalloporphyrins suggests that the charge reco mbination times (tau) of the geminate ion pairs in the MgTFPP-MgOEP an d ZnTFPP-MgOEP systems are 20 and 38 mu s, respectively. The MgTFPP- a nion reduces O-2 with a second-order rate constant of similar to 10(7) M-1 s(-1), but the oxidation of ZnTFPP- anions by O-2 is very slow. T he differences between the two systems may arise from different redox potentials of ZnTFPP and MgTFPP. These data prove that, even containin g Mg, the least electronegative element which can be stably chelated, a metalloporphyrin with poly electron-withdrawing groups is a good ele ctron acceptor. Our results suggest that such electronegative porphyri ns are useful molecular parts for assembling of porphyrin-based biomim etic energy conversion devices.