N-Alkylcyanopyridinium-Mediated Photoinduced Charge Separation across Dihexadecyl Phosphate Vesicle Membranes

The triplet-photoexcited [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II) ( 3 ZnTPPS 4- ) ion underwent rapid one-electron oxidation by a series of N -alkyl-4-cyanopyridinium (C n CP + ) ions. In aqueous solution, the quenching rate constants ( k q ≅ 4 × 10 9 M -1 s -1 ) were insensitive to the N -alkyl chain length over the range examined ( n = 1−16). Charge recombination was also rapid ( k r ≅ 1.3 × 10 8 M -1 s -1 ) and independent of the chain length. However, when dihexadecyl phosphate vesicles were included in the reaction medium, both k q and k r decreased progressively with increasing chain length. The chain-length dependence of k q was in quantitative accord with a dyamical model wherein the relative rate constants were determined by C n CP + partitioning between the aqueous phase and the aqueous−organic membrane interface, with only the aqueous-phase component being reactive toward 3 ZnTPPS. 4- The decrease in k r with increasing chain length was attributed to capture of the neutral lipophilic C n CP 0 radicals within the hydrocarbon phase of the membrane. Net photoinduced transmembrane reduction was observed in vectorially organized systems containing oxidants within the inner aqueous phase of the vesicles and sensitizer, C n CP +, and a sacrificial electron donor in the external aqueous phase. Transmembrane reduction of occluded Co(bpy) 3 3+ was accompanied by 1:1 stoichiometric uptake of C n CP +; the characteristic time for this step, determined by both transient kinetic spectroscopy and quantum-yield measurements under continuous photolysis, was 2 × 10 -2 s, consistent with C n CP 0 functioning as mobile transmembrane electron carriers. The overall quantum yield for photosensitized C 1 CP 0 -mediated transmembrane Co(bpy) 3 3+ reduction by dithiothreitol was φ ≈ 0.7. For long-chain analogues, φ was less, an effect that could be quantitatively accounted for by the corresponding decline in k q values.