Electrochemical and EQCM studies on the assembly of asymmetric viologen onbare and alkanethiol-coated Au electrodes

Cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQ CM) techniques have been used to study the influence of anions and the alky l chain length of thiol on the redox behavior of asymmetric viologen, N-eth yl-N'-octadecyl viologen (C18EV2+) on bare and alkanethiol (CH3(CH2)(n)SH, n = 3, 5, 7, 11, 17)-coated electrodes. The grazing angle reflectance FTIR spectral and cyclic voltammetric blocking experiments suggested that the al kyl chains of the n-octadecanethiol (n-ODT) monolayers were disordered grea tly in the C18EV2+ inserted n-ODT monolayers when compared to pure n-ODT mo nolayers. On the other hand, the assembly of C18EV2+ was more highly ordere d on n-ODT monolayers than on a bare Au electrode. The assembly of C18EV2On bare and alkanethiol-coated Au electrodes showed a single redox wave for the first reduction of C18EV2+ in 0.1 M of KCl and Na2SO4. A sharp redox p eak was observed for the assembly of C18EV2+ on bare and short alkyl chain thiol (n = 3, 5, 7)-coated electrodes whereas a broad redox wave was observ ed on long alkyl chain thiol (n = 11, 17)-coated electrodes in 0.1 M NaClO4 . On the other hand, a multiple redox peak was observed for the first reduc tion of C18EV2+ on a bare Au electrode in 0.1 M NH4PF6. In the case of alka nethiol-coated electrodes, no redox response was observed for the assembly of C18EV2+ on long chain alkanethiol-coated electrodes (n = 11, 17) while a single redox peak was observed on short chain alkanethiol (n = 3, 5, 7)-co ated electrodes in 0.1 M NH4PF6. The observed lack of a redox response of C 18EV2+ on the long chain alkanethiol-coated electrodes has been explained b y the non-entry of water molecules in the presence of very weakly hydrated PF, anions. In these electrodes, the PF6- ions precipitate the assembly of C18EV2+ in the absence of water molecules and it is inactive electrochemica lly. When the same electrode was transferred into the supporting electrolyt es of KCl, Na2SO4 or NaClO4, the PF6-ions were exchanged readily with these anions and the assembly became electroactive. EQCM data indicate that the ingress of water molecules into the assembly of C18EV2+ increases as its as sembly is bound to the electrodes coated with alkanethiols of shorter alkyl chain length. The ingress of water molecules also depends on the anions pr esent in the solution. The ingress of water molecules into the assembly of C18EV2+ on bare and alkanethiol-coated Au electrodes increases in the order of PF6- < ClO4- < Cl- < SO42-. (C) 1999 Elsevier Science S.A. All rights r eserved.