Steric and Electronic Effects in Basic Dyes

Absorption spectra of several derivatives of Malachite Green containing more than one substituent in the phenyl ring have been examined. In most cases where steric effects are unimportant an additive relationship is found between the electronic effects of the substituents and the position of the first-frequency absorption band. The presence of a second unsaturated electron-withdrawing group in the phenyl ring leads to a further reduction in the intensity of the first band. In certain cases, the overall electron-withdrawal is such that the equilibrium between dye-base and univalent dye cation in acid solution is shifted in favour of the former species.