REDUCTION POTENTIALS OF VESICLE-BOUND VIOLOGENS

Thermodynamic reduction potentials have been determined by using spectroelectrochemical and cyclic voltammetric methods for a homologous series of amphiphilic viologens (N-methyl-N'-alkyl-4,4'-bipyridinium ions, C(n)MV2+) in a variety of media, including dihexadecyl phosphate (DHP), dioctadecyldimethylammonium, and phosphatidylcholine small unilamellar vesicles. In general, potentials for both one-electron steps, i.e., C(n)MV2+ + e- --> C(n)MV+ and C(n)MV+ + e- --> C(n)MV0, were insensitive to the alkyl chain length, which was varied over the range n = 6-20. The single exception was a large decrease (approximately 100 mV) in the first reduction potential for DHP-bound viologens when the chain length was increased from n = 10 to n = 12; this effect was attributed to a change in binding topography. The magnitudes of the reduction potentials were highly dependent upon the vesicle charge; the pattern observed indicated that interfacial electrostatic interactions between the surfactant headgroups and bipyridinium rings were the dominant factors determining the potentials. As discussed in the text, the data allow resolution of several heretofore puzzling observations concerning viologen reactivities in microphase suspensions.