Structural effects on the electrochemical and spectroelectrochemical properties of asymmetric viologens on the Au electrode surface

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 (CH3(CH2)nSH)-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 NH4PF6 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 NH4PF6. 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 PF6− ion, but to be fast in the presence of other supporting electrolytes (typically Cl−, SO42− and ClO4− ions). The Γox/Γred 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 PF6− 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.