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

Abstract

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 (Δ ν) between the maximum and the shoulder that appears in the short-wavelength side of the respective maximum visible band (Δ ν=1/ λ shoulder−1/ λ max, cm −1), and the wavelength of the maximum absorption ( λ max). The solvent and the concentration effects on λ max and Δ ν 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 ( ε<∼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 Δ ν observed concomitantly with a red shift in λ 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 Δ ν observed concomitantly with a blue shift in λ max upon increasing dye concentration. The distinct behavior of λ 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 Δ ν as a function of the Kamlet–Tafts solvatochromic parameters ( α, β and π*) 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.