The orientation of 2,2′-bipyridine adsorbed at a SERS-active Au(1 1 1) electrode surface

Surface-enhanced Raman scattering (SERS) spectra from 2,2′-bipyridine (22BPY) adsorbed on a SERS-active Au(1 1 1) electrode at several applied potentials were obtained. The SERS-active Au(1 1 1) surface was prepared following an electrochemical cleaning procedure. This procedure involves the application of continuous oxidation–reduction cycles (orcs) within a potential region where no significant surface structural changes are expected to occur. The adsorbed 22BPY may assume several conformations, including the cis- and trans-configurations. Normal Raman spectra of aqueous 22BPY at several different acidities are presented. These spectra are compared to the Raman features of both the ‘free’ 22BPY and its Zn complex in the solid state. The normal Raman experiments showed that unique spectral characteristics were distinguishable for both the cis- and trans-configurations. Based on these results, the potential-dependent orientation (conformation) of 22BPY adsorbed on a SERS-active Au(1 1 1) has been established. The SERS results suggested that, at a positively-charged surface, 22BPY adsorbs end-on, using both nitrogens (cis-configuration). Although no strong spectroscopic evidence suggesting potential-induced reorientation was found, the pyridine rings may no longer be coplanar at the negatively-charged surface. However, the molecules keep the upright position with both nitrogens pointing towards the surface even in these negative limits.