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) ↑(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 ×109 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× 108 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 kr with increasing chain length was attributed to capture of the neutral lipophilic C↓(n)CP↑(o) 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, CnCP↑(+), 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↑(o) functioning as mobile transmembrane electron carriers. The overall quantum yield for photosensitized C↓(1)CP↑(o)-mediated transmembrane Co(bpy)↓(3)↑(3+) reduction by dithiothreitol wasφ≈ 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.