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

摘要

The triplet-photoexcited [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II) (~3ZnTPPS~(4-)) ion underwent rapid one-electron oxidation by a series of N-alkyl-4-cyanopyridinium (C_nCP~+) ions. In aqueous solution, the quenching rate constants (k_q approx= 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 approx= 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 dynamical model wherein the relative rate constants were determined by C_nCP~+ partitioning between the aqueous phase and the aqueous-organic membrane interface, with only the aqueous-phase component being reactive toward ~3ZnTPPS.~(4-) The decrease in k_r with increasing chain length was attributed to capture of the neutral lipophilic C_nCP~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_nCP~+, 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_nCP~+; 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_nCP~0 functioning as mobile transmembrane electron carriers. The overall quantum yield for photosensitized C_1CP~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.