Surface‐enhanced resonance Raman scattering study on the hemicyanine dye 4‐ [2‐(4‐dimethylaminophenyl)ethenyl]‐1‐methylpyridinium iodide adsorbed on a silver electrode surface

Surface‐enhanced resonance Raman scattering (SERRS) spectra were measured at O V (vs Ag/AgCl) for a hemicyanine dye and its protonated form (HC and HCH+) adsorbed on a silver electrode surface. The surface spectrum of HC shows a characteristic feature due to a trans conformation, a band mainly due to a stretching frequency (νcc) at 1580 cm−1, which is appreciably lower than the corresponding frequency for HC in water (1592 cm−1); this result indicates that, on adsorption on the electrode surface, the resonance balance of HC between the quinoid and benzenoid forms shifts to the former. An adsorbate on a silver electrode surface from HCH+ solution at pH 1.0 takes either a protonated form, HCH+, or a deprotonated form, HC, depending on the anodization conditions of granular structures assumes the deprotonated form, whereas the adsorbate on a surface containing smooth parts retains the protonated form; hence the surface morphology is related to the chemical properties of the silver electrode surfaces. It was also found that the adsorbate in the protonated state gives a prominent SERRS band at 885 cm−1 assignable to an out‐of plane bending vibration of the central olefinic group, indicating that the adsorbate assumes a flat orientation on the silver electrode surface.