Triphenylmethine dyes

Carbinol bases are obtained by treating cationic triphenylmethine dyes with alkali.77 However, not all carbinol bases are stable. For example, 52 readily eliminates water to form the neutral quinonimide45 53 (Eq. 7). 

The valence-bond (resonance) description of the triphenylmethine dye Malachite Green (125) is illustrated in Figure 6.5. Comparison with Figure 6.4 reveals their structural similarity compared with cyanine dyes. Formally, the dye contains a carbonium ion centre, as a result of a contribution from resonance form II. The molecule is stabilised by resonance that involves delocalisation of the positive charge on to the p-amino... 

The triarylmethane dyes were also very important leather dyes in the past. The central carbon is sp2-hybridized (triarylmethine) and can add a hydroxyl group to form the respective carbinol base with spP-hybridized carbon. The dyes react like Lewis acids and some species act as pH indicators (e g., phenolphthaleine). The term cationic dyes is therefore recommended for this dye class instead of basic dyes. Nowadays only the sulfonated triphenylmethine dyes are used for leather. 

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