The electronic absorption spectra of di- and tri-arylmethane dyes containing fluorine

Abstract

A comprehensive series of derivatives of Malaohite Green, Crystal Violet and Michler's Hydrol Blue has been synthesised containing meta-trifluoromethyl groups. Such dyes have not been previously prepared because of earlier difficulties encountered in synthesising N.N-dimethyl-2-tri -fluoromethylaniline. A synthetic route to this compound has now been established. The preparation of the required intermediate, 4-dimethylamino-3-trifluoromethylphenyl-lithium, afforded a
route to a number of novel compounds containing trifluoromethyl groups ortho to a dimethylamino- group.
The observed visible spectra of the dyes have been correlated with structural features. Apparent inconsistencies in the response of Crystal Violet to the introduction of substituents have been interpreted in terms of a possible loss of degeneracy of the NBMO pair in the derivatives of this compound.
Novel analogues of Malachite Green, Crystal Violet and Michler's Hydrol Blue have been prepared containing terminal piperidino- and pyrrolidino- groups. Differences in the electronic absorption spectra of the parent systems have been correlated with the varying basicities of the amino- groups and to differing abilities of the terminal groups
to stabilise resonance forms. Comprehensive series of derivatives of these dyes have been prepared containing orthotrifluoromethyl groups. The effect of substitution on their visible spectra has been shown to follow the trends established for their known dimethylamino- analogues.
The readiness of Piperidine Violet and its 2'-trifluoromethyl derivative to protonate when in strongly acidic media was not shared by their pyrrolidine analogues. This behaviour was illustrated by the production of Piperidine Green type spectra for these dyes when determined in solutions with a comparatively low concentration of acetic acid.
The A max for the x bands of 3'-trifluoromethyl Malachite Green, 2'-trifluoromethyl Piperidine Green and 2'-trifluoromethyl Pyrrolidine Green exhibit deviations to shorter wavelength consistent with steric hindrance in systems possessing electronic asymmetry. In all three cases di-substitution leads to é re-establishment of symmetry.
An apparatus is described which has been developed to enable simultaneous steam distillation and solvent extraction.