DFT, SERS, and Single-Molecule SERS of Crystal Violet

Applying the method of density functional theory calculations, we examine the Raman and surface-enhanced Raman spectra (SERS) of crystal violet. The resulting optimized structure is of point symmetry D3, and the calculated Raman spectrum provides an excellent match with the observed normal Raman spectrum. This provides a reliable assignment of the symmetry and normal modes of the observed spectrum, which consists of bands assigned to modes of either a1 or e symmetry. The e modes are not split, showing that D3 symmetry remains, even on the surface. The SERS spectra, both normal and single-molecule, are dominated by the nontotally symmetric e vibrations, which are preferentially enhanced in accord with the Herzberg−Teller-surface selection rules. The mechanism involves intensity borrowing through vibronic coupling between a charge-transfer state and the lowest-lying π → π* transition. A quantitative measure of the degree of charge transfer is obtained by analyzing the potential dependence of SERS intensities. This indicates a considerable contribution of charge-transfer intensity to the overall SERS enhancement.