Solvent and concentration effects on the visible spectra of tri-para-dialkylamino-substituted triarylmethane dyes in liquid solutions

We have characterized the spectroscopy properties of crystal violet (CV+) and ethyl violet (EV+) in liquid solutions as a function of the solvent type and dye concentration. The analysis of how solvent properties and dye concentration affects the electronic spectra of these tri-para-dialkylamino substituted tryarylmethane (TAM(+)) dyes was performed on the basis of two spectroscopic parameters, namely the difference in wavenumber (Deltanu) between the maximum and the shoulder that appears in the short-wavelength side of the respective maximum visible band (Deltanu = 1/lambda(shoulder) - 1/lambda(max), cm(-1)), and the wavelength of the maximum absorption (lambda(max)). The solvent and the concentration effects on lambda(max) and Deltanu have indicated that both solute/solute (ion-pairing and dye aggregation) and solute/solvent (H-bonding type) interactions modulate the shape of the visible electronic spectra of these dyes in solution. In solvent with small dieletric constant (epsilon < similar to 10), the formation of ion-pairs represents a major contribution to the shaping of these spectra. Upon increasing dye concentration the formation of ion-pairs was characterized by an increase in Deltanu observed concomitantly with a red shift lambda(max). In chloroform and chlorobenzene the ion-pair association constant of CV+ and EV+ with Cl- ions were found to be in the order of 10(6) and 10(5) M-1, respectively. In trichloroethylene the association constant for the CV+Cl- pair was 10(8) M-1. In water, dye aggregation instead of ion-pairing represents a major contribution to the shaping of the visible spectra of CV+ and EV+. Dye aggregation was indicated by an increase in Deltanu observed concomitantly with a blue shift in lambda(max) upon increasing dye concentration. The distinct behavior of lambda(max) for dye aggregation and ion-pairing as a function of dye concentration can therefore assist in the characterization of these two distinct phenomena. The solute/solvent interactions were studied in a series of polar solvents in which solute/solute interactions do not occur in any detectable extent. The dependence found for Deltanu as a function of the Kamlet-Tafts solvatochromic parameters (alpha, beta and pi*) is in keeping with previous inferences indicating that the splitting in the overlapped absorption band of CV+ and EV+ in hydroxilated solvents arises from a perturbation in the molecular symmetry induced by hydrogen bonding (donor-acceptor) type interactions with solvent molecules. A distinction between the effects of solute/solute and solute/solvent interactions on the visible spectra of these dyes is provided. (C) 2002 Elsevier Science B.V. All rights reserved.