N-ALKYLCYANOPYRIDINIUM-MEDIATED PHOTOINDUCED CHARGE SEPARATION ACROSSDIHEXADECYL PHOSPHATE VESICLE MEMBRANES

The triplet-photoexcited lfonatophenyl)porphinato]zinc(II)((ZnTPPS4-)- Zn-3) ion underwent rapid one-electron oxidation by a series of N-alky l-4-cyanopyridinium (CnCP+) ions. In aqueous solution, the quenching r ate 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). Charg e 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 phos phate vesicles were included in the reaction medium, both k(q) and k(r ) decreased progressively with increasing chain length. The chain-leng th dependence of k(q) was in quantitative accord with a dyamical model vv herein the relative rate constants were determined by CnCP+ partit ioning between the aqueous phase and the aqueous-organic membrane inte rface, with only the aqueous-phase component being reactive toward (Zn TPPS)-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 oxi dants 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 CnCP+; the characteristic time for this s tep, determined by both transient kinetic spectroscopy and quantum-yie ld, measurements under continuous photolysis, was 2 x 10(-2) s, consis tent with CnCP0 functioning as mobile transmembrane electron carriers. The overall quantum yield for photosensitized C1CP0-mediated transmem brane Co(bpy)(3)(3+) reduction by dithiothreitol was phi approximate t o 0.7. For long-chain analogues. phi was less. an effect that could be quantitatively accounted for by the corresponding decline in k(q) val ues.