Crystal violet’s shoulder

Abstract

The symmetry of the crystal violet cation has been the subject of ongoing discussion since G. N. Lewis and co-workers called attention to the unexpected shoulder in its optical absorption spectrum. Strikingly absent from this debate is a description of the X-ray crystal structure of the common chloride salt, that compound to which the vast majority of the spectroscopic analyses pertain, and whose large crystals account for the familiar name. This absence is especially curious since a preliminary structure determination was made as early as 1943. Here, we present single crystal structures of two forms of crystal violet chloride, a monoclinic monohydrate and a trigonal nonahydrate. In principle, the pair is well suited to address whether desymmetrization is responsible for the shoulder since the cation is asymmetric in the monoclinic crystal, while it is axially symmetric in the trigonal crystal. Absorption spectra of the two crystalline forms were identical and dominated by peaks assigned to aggregates. Raman spectra also could not distinguish the hydrates from one another. Mixed crystals of benzene-1,2-dicarboxylic acid (phthalic acid) containing mM concentrations of crystal violet were prepared in order to study the cation fixed in a dissymmetric medium, yet free from aggregate absorption. Through this work, and a reevaluation of the literature, we show that the two prevailing theories for the shoulder, that there are two isomers in equilibrium, and that desymmetrization gives rise to two excited states, are not mutually exclusive.