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.
Spectral changes caused by the effect of the additional ring in transversely conjugated naphthalene analogues of Crystal Violet, Malachite Green, and Michler's Hydrol Blue are described and discussed. Although peri-hindrance of the terminal dimethylamino-groups leads to incomplete conversion of dye base into dye in acid solution, and to ready protonation of the crowded groups, the univalent cations are shown to be electronically symmetrical and pronounced bathochromic shifts of the first bands are observed. The carbonyl group of bis-(4-dimethylamino-lnaphthyl) ketone is unusually resistant to reduction.
Spectral studies show that terminal nitrogen atoms attached to longitudinally conjugated naphthalene groups in analogues of Crystal Violet, Malachite Green, and Michler's Hydrol Blue have a greater tendency to conjugate with the central carbon atom than those in the corresponding 9.9-dimethylfluorene analogues. Conjugation is most effective in the Crystal Violet analogue (I1 ; R = NMe,) in which a high resonance-energy counterbalances the high energy of the quinonoid naphthalene group. 6
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