Excitation‐wavelength‐dependent charge transfer resonance of bipyridines on silver nanoparticles: surface‐enhanced Raman scattering study

Charge transfer (CT) resonance mechanisms of 2,2′‐bipyridine (2,2′‐BiPy), 2,4′‐bipyridine (2,4′‐BiPy), and 4,4′‐bipyridine (4,4′‐BiPy) on silver nanoparticle surfaces have been comparatively investigated by means of surface‐enhanced Raman scattering (SERS) at the excitation wavelengths of 457, 514, 633, and 785 nm. A combination of the electromagnetic (EM) and charge transfer (CT) contributions should affect the SERS intensities for the bipyridine compounds adsorbed on silver nanoparticle surfaces. The CT resonance is assumed to occur in dissimilar ways for the bipyridine compounds, as evidenced from their different excitation‐wavelength‐dependent SERS enhancement factors. Ab initio density functional theory (DFT) calculations at the level of B3LYP/LANL2DZ have been carried out for the bipyridine‐Ag complexes.