Solvent effects on the spectroscopic properties of triarylmethane dyes

The spectroscopic characteristics of two highly symmetric triarylmethane dyes, Crystal Violet and Ethyl Violet, and two triarylmethanes displaying lower symmetry, Victoria Blue R and Victoria Pure Blue BO, were investigated in a variety of polar solvents. The solvent effects observed on the electronic spectra of the highly symmetric dyes add momentum to previous objections to the rotamers model, a model traditionally used to explain the appearance of a shoulder in the short-wavelength side of the maximum absorption band of Crystal Violet in ethanol. We have found that, in polar solvents the magnitude of the splitting between the overlapped absorption bands of Crystal Violet is dependent on the solvatochromic properties of the media. The nature of this dependence suggested the existence of specific interactions between Crystal Violet and the surrounding solvent molecules. We hypothesize that these solute-solvent interactions represent the symmetry-breaking event that lowers the symmetry of Crystal Violet in solution, and gives rise to the appearance of the spectroscopic shoulder at the short-wavelength side of its maximum absorption band. Analogous results were obtained for Ethyl Violet. The solvent effects observed on the absorption spectra of the triarylmethanes displaying lower symmetry were different from those observed for the highly symmetrical compounds. This difference was rationalized in terms of the distinct nature of the overlapping absorption bands characteristic of these two distinct sets of dyes.