Effect of Alkyl Chain Length on the Structure of Viologen Adsorbed on a Cu(100) Electrode

The redox behavior and the potential dependent adsorption structure of alkylated viologen on a Cu(100) electrode were investigated by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM). Heptyl viologen (1,1'-diheptyl-4,4'-bipyridinium dichloride) (DHV) and ethyl viologen (1,1'-diethyl-4, 4'-bipyridinium dibromide) (DEV) showed different current waves during CV measurements in a KCl-containing electrolyte solution. The dicationic heptyl viologen molecules showed a highly ordered two dimension (2D)'dot-array' structure on the c(2 x 2)-Cl modified Cu(100) electrode surface in the STM images while the dicationic ethyl viologen molecules did not show any structures. With a decrease in the electrode potential, a one-electron reduction of the dication heptyl viologen resulted in the appearance of a stripe pattern that was formed by radical heptyl viologen as shown in the STM images. A more compact stripe pattern was visible in the radical ethyl viologen phase. The adsorption structure of heptyl viologen strongly depends on the electrode potential but the structure of the ethyl viologen adsorbed on the Cu(100) electrode was less response to the electrode potential.