Voltammetric and electroreflectance study of thiol-functionalized viologenmonolayers on polycrystalline gold: Effect of anion finding to a viologen moiety

The monolayers of thiol-functionalized viologens, i.e., dihexafluorophospha te salts of N-butyl-N'-(4-mercaptobutyl)-4, 4'-bipyridinium (C4V2+C4SH) and N-pentyl-N'-(5-mercaptopentyl)-4,4'-bipyridinium (C5V2+C5SH), on a polycry stalline gold electrode were characterized in several aqueous electrolyte s olutions using cyclic voltammetry and electroreflectance (ER) spectroscopy. The voltammetric response for the viologen dication/radical cation couple (V2+/V.+) in Na2SO4, K2SO4, KPF6, or KF solution was found to be electrolyt e anion dependent. However, considerable differences due to the electrolyte anion or the alkyl chain length in the fractional content of dimer forms i n the V.+ state and the average orientation of the V.+ moiety, both estimat ed from the results of ER spectral measurements, were not observed. A large voltammteric peak width was observed in KF solution. Because the voltammet ric measurements suggested a strong and weak binding to the viologen moiety respectively for PF6- and F- ions, the dependence of voltammetric response on the concentration of PF6- was investigated quantitatively for a monolay er of C4V2+C4SH in the mixed solutions of KPF6 and KF at a constant ionic s trength. It was found that one PF6- ion additionally binds to a viologen mo iety upon the oxidation of V.+ to V2+. Th,ratio of the binding constant to V2+ and V.+ was estimated to be approximately 1.2 x 10(4) M-1 at an ionic s trength of electrolyte of 0.4 M. The absence of considerable dependence of the PF6- binding on the ionic strength was suggested by voltammetric measur ements in the solutions containing solely KPF6. The electron-transfer rate constants for the monolayers were estimated to be no less in the order of m agnitude than 1 x 10(4) s(-1).