www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 492 (2000) 137 – 144 Structural effects on the electrochemical and spectroelectrochemical properties of asymmetric viologens on the Au electrode surface S. Abraham John, Fusao Kitamura, Koichi Tokuda, Takeo Ohsaka * Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori -ku, Yokohama 226 -8502, Japan Received 15 May 2000; received in revised form 26 July 2000; accepted 26 July 2000 Abstract Cyclic voltammetry and FTIR spectroscopy were used to study the structural effects on the electrochemical and spectral properties of the asymmetric alkyl viologen, N-ethyl-N-octadecyl viologen (1) on bare and n -alkanethiol (CH 3 (CH 2 ) n SH)-coated Au electrodes. The self-assembled monolayers (SAMs) of 1 on the Au electrode showed multiple redox peak for the first reduction of 1 in aqueous 0.1 M NH 4 PF 6 solution. When the assembly of 1 was inserted into monolayers of n -alkanethiols of n 11, no redox response was observed in 0.1 M NH 4 PF 6 . On the other hand, when the alkyl chain length of thiol decreased from n =11 to 5, the reduction peak potential of 1 shifted to less negative potential. Moreover, the redox reaction of 1 on n -alkanethiol monolayers of n 11 was found to be very slow in the presence of PF 6 - ion, but to be fast in the presence of other supporting electrolytes (typically Cl - , SO 4 2 - and ClO 4 - ions). The  ox / red ratio increases when the alkyl chain length (m) of viologen decreased from m =18 to 16, 14 and 12. The monomer-dimer FTIR spectral features were observed for the SAM of 1 on the bare Au electrode in the presence of PF 6 - ion, whereas only monomer spectral features were observed on the n -hexanethiol (HT)-coated electrode. It is suggested that the redox moiety of 1 was less compact on a bare Au electrode and the entry of more water molecules into the monolayer strongly favors the dimerization. On the other hand, on the HT-coated electrode, the redox moiety of 1 was more compact than on the bare Au electrode and showed monomer spectral features. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Asymmetric viologens; n -Alkanethiols; Hexafluorophosphate anion; Electrochemistry; Spectroelectrochemistry 1. Introduction During the last decade, extensive research work has been directed to the preparation and characterization of well-defined monolayers on gold electrode surfaces us- ing sulfur containing species, such as thiol and disulfides [1–5]. It has been shown that exposure of a clean gold surface to a dilute solution of the thiol or disulfide molecules typically results in a chemisorbed monolayer that excludes both water and ions from the underlying gold surface. Recently, several groups have studied the redox chemistry of monolayers containing immobilized redox species within a well-defined envi- ronment [6 – 16]. In addition, several groups have also prepared mixed monolayers on the electrode surface, in which one of the components of the monolayer is electroactive [17 – 20]. It has been reported that the apparent formal potential for electroactive groups in such monolayers can vary depending on the surface coverage, supporting electrolyte and other factors. Creager and co-workers [17,19] extensively studied the mixed self-assembled monolayers (SAMs) of thiol derivatives of ferrocene coadsorbed onto gold elec- trodes with n -alkanethiol derivatives of various chain length and terminal functionality. They found that the formal potential of ferrocene shifts to more positive potentials as the chain length of the alkanethiol coad- sorbate increases. This shift has been attributed to creation of a less polar, more alkane-like interfacial microenvironment which is favorable to the reduced form of the immobilized ferrocene and is unfavorable to its oxidized form [17,19]. * Corresponding author. Tel.: +81-45-924-5404; fax: +81-45-924- 5489. E-mail address: ohsaka@echem.titech.ac.jp (T. Ohsaka). 0022-0728/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII:S0022-0728(00)00292-8