Steric effects in di- and tri-arylmethane dyes. Part 13. Electronic absorption spectra of derivatives of Crystal Violet, Malachite Green, and Michler's Hydrol Blue exhibiting simultaneous central and terminal steric distortion

Abstract

Spectral changes caused by the simultaneous crowding effects of o,m-dimethyl groups in Crystal Violet, Malachite Green, and Michler's Hydrol Blue are described and discussed in terms of molecular orbital theory. 2,3-Dimethyl derivatives of these dyes generally absorb at shorter wavelengths than their 2,5-dimethyl isomers owing to the increased spatial demands of the buttressed pair of substituents. Electronic symmetry is retained in derivatives of Crystal Violet but structurally unsymmetrical derivatives of Michler's Hydrol Blue and Malachite Green show red and blue deviations respectively. Diminished conjugation of terminal dimethylamino-groups with the central carbon atom leads to ready protonation of the crowded groups and, in certain cases, to incomplete conversion of dye base into dye in acid solution.