N-alkylcyanopyridinium-mediated photoinduced charge separation across dihexadecyl phosphate vesicle membranes

The triplet-photoexcited [5,10,15,20-tetrakis(4-sulfonatophenyl)porphinato]zinc(II)((ZnTPPS4-)-Zn-3) ion underwent rapid one-electron oxidation by a series of N-alkyl-4-cyanopyridinium (CnCP+) ions. In aqueous solution, the quenching rate constants (k(q) congruent to 4 x 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) congruent to 1.3 x 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 vv herein the relative rate constants were determined by CnCP+ partitioning between the aqueous phase and the aqueous-organic membrane interface, with only the aqueous-phase component being reactive toward (ZnTPPS)-Zn-3.(4-) The decrease in k(r) with increasing chain length was attributed to capture of the neutral lipophilic CnCP0 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 CnCP+; the characteristic time for this step, determined by both transient kinetic spectroscopy and quantum-yield, measurements under continuous photolysis, was 2 x 10(-2) s, consistent with CnCP0 functioning as mobile transmembrane electron carriers. The overall quantum yield for photosensitized C1CP0-mediated transmembrane Co(bpy)(3)(3+) reduction by dithiothreitol was phi approximate to 0.7. For long-chain analogues. phi was less. an effect that could be quantitatively accounted for by the corresponding decline in k(q) values.