Triarylmethine dyes

Arylmethane leuco dyes are converted into di- or triarylmethane dyes on oxidation. This class of dye precursors sometimes is referred to as leuco di- or triphenylmethane dyes, or di- or triphenylmethane leuco dyes. The use of the term di- or triarylmethane dyes can be misleading as the central carbon atom is a carbonium ion. Instead, the term di- and triarylmethine dye is recommended for this class as it correlates with the well-known polymethine dyes. Nevertheless, it has not been commonly used. 

The R group possesses Jt-electrons or lone pairs of electrons that can interact with the rest of the Jt-electron system. The most important electron donor is the amino group. Triarylmethine dyes are usually divided into mono-, di-, and triami-notriarylmethine dyes. In some di- and triarylmethine dyes, the ring carbon atoms ortho to the central methine carbon atom are bonded via a heteroatom to form a heterocyclic six-membered ring. These include the acridine, xanthene, and thiox-anthene dyes. 

The diphenylmethyl and triphenylmethyl cations can be considered the basic chromophores of di- and triarylmethane dyes 11 —41. However, the electronic state of the donor-substituted di- and triarylmethine dyes can be described better by breaking down the chromophore of these dyes into a straight- or branched-chain polymethine subchromophore, respectively, and two ethylene units (from the formal cleavage of the benzene rings) [2], This model allows the similarities between the di- and triarylmethine dyes and the polymethine s to be recognized. 

In 1862 Nicholson sulfonated basic dyes and thus created the first amphoteric and acid leather dyes. Triaiylmethane (triarylmethine) dyes (Table 5.3) and their different uses are typical examples of this development. 

Cationic dyes form positively charged dye ions by dissociation, and the positive electric charge is delocalized over the entire molecule. They are usually di- and triarylmethine dyes such as C.I. Basic Green 4, 42000 [569-64-2] (31), which are also known as triarylmethane dyes or triaryl carbenium ions [15], oxazine dyes such as C.I. Basic Blue 3, 51005 [2787-91-9] (32), orthiazine dyes. 

Cyclized di- or triarylmethine dyes are also known as acridine, xanthene, or thioxanthene dyes. Rhodamine GG (33), C.I. Basic Red 1, 45160 [989-38-8], is of the most interest for the paper industry. Rhodamine dyes dye paper in strongly fluorescent red shades. 

M Figure 7.9 Malachite green cation, a triarylmethine dye characterized by three phenyl rings. 

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. 

Di- and triarylmethines. These dyes are characterized by systems of two or three linked substituted phenyl rir (aiyl groups). Common functionalities attached to the phenyl rings include dimethylamine, OH, and 0 groups. Some of ftiese molecules can exist as cations. There are also multiple-linked ring dyes in this class, characterized by an aza linkage... 

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