Vibrational Solvatochromism at Plasmonic InterfacesProbed by Surface-Enhanced Raman Scattering (SERS)

The vibrational solvatochromism of the nitrile stretching vibration mode of 4-mercaptobenzonitrile (4-MBN) adsorbed on gold nanoparticles under different solvent environments was investigated using surface-enhanced Raman scattering (SERS). Representative spectral data were collected using multipoint mapping measurements for nanoparticles in air and nine solvents with different dielectric properties. The results indicate that the nitrile vibrational frequency exhibits a maximum shift of ∼10.0 cm–1 across different solvents, with the shift behavior following two distinct mechanisms. In aprotic solvents, the frequency redshift qualitatively agrees with a modified Onsager reaction field model. This pure electrostatic model combined with the SERS surface selection rules inferred that 4-MBN molecules adopt a more vertical orientation in DMF and DMSO (tilt angle relative to the surface normal of approximately 30°), while the tilt angle significantly increases in acetone (approximately 60°). The orientation differences originate from variations in solvent polarity that affects the solvation capability of the nitrile terminal group. In protic solvents, solvatochromic blueshift is dominated by the exchange–repulsion effect triggered by hydrogen bonding. In the case of acetonitrile, an aprotic solvent, intermolecular hydrogen-bond-like interactions partially offset the electric field-induced redshift, resulting in only a minimal net shift of approximately 0.5 cm–1. DMF–water mixed solvent experiments reveal that the nitrile frequency changes quasi-linearly with the hydrogen bonding capabilities of the solvent system, confirming that vibrational solvatochromism can serve as a highly sensitive spatial probe to quantify hydrogen donor strength.