Resonance Raman study of solvent effects on a series of triarylmethane dyes

Abstract Resonance Raman spectroscopy was used to address questions concerning the influence of the solvent on the structure and aggregation state of a series of triarylmethane dyes. These molecules are of interest owing to the dramatic solvent effects exhibited in their optical absorption spectra, which have recently been interpreted in terms of intermolecular interactions which perturb the D 3 symmetry of the cationic dyes. Raman spectra of parafuchsin, malachite green, crystal violet and ethyl violet were compared in various solvents, in order to explore changes in the vibrational modes brought about by aggregation (in water), ion‐pair formation (in non‐polar solvents) and ion–dipole interactions (in polar solution). In triply amino‐substituted dyes, the data point to the possibility that a resonance structure exists having a positive charge on one of the three nitrogens. It is discussed how the resulting reduction in the expected D 3 symmetry of the cation affects the resonance Raman spectrum.