Polymethine dyes

Example: C.I. Basic Red 12[4]

The most well-known group of methine dyes are the cationic cyanine dyes. Both terminal groups of the chromophore contain nitrogen atoms, whereas the amino or imino group is part of a heterocycle.

general structure of cyanine dyes

Example: C.I. Basic Yellow 11[5]

In the case of hemicyanine dyes, one terminal amino group is part of a heterocycle, while the second terminal group is open-chain.

general structure of hemicyanine dyes

If both terminal amino and imino groups of the chromophore are open chain, one speaks of streptocyanine dyes.

general structure of streptocyanine dyes

Merocyanine dyes have an amino and a carbonyl group as end groups of the polyene structural element. Both a neutral and a zwitterionic resonance structure can be described for this compound type.[6]

general structure of merocyanine dyes

Example: Oxonol Blau[7]

Members of the oxonol dyes, whose terminal groups contain oxygen as heteroatom, play a role in analogue colour photography[8] and the following of enzymatic reactions via the use of barbituric and thiobarbituric terminal groups.[9][10]

general structure of oxonol dyes

By condensing an active methylene compound (e.g. malonic acid dinitrile) with a benzaldehyde derivative, styryl dyes are obtained. By integrating a benzene ring into the polyene part, these compounds have a styrene partial structure. Styryl dyes are used as disperse dyes in the dyeing of polyester fibers.

Example: Preparation of C.I. Disperse Yellow 31 (3) by condensation of ethyl cyanoacetate (1) with a p-aminobenzaldehyde derivative.[3]^{: S. 58}

Synthesis of C.I. Disperse Yellow 31 (3)

Also amino- and hydroxy-substituted di- and triarylmethines comprise the structural element of a methine dye:

Triarylmethine dyes, resonance structure (R=H, alkyl)

Triarylmethine dyes are derived from triphenylmethane, in which at least two of the aromatic rings have electron-donating substituents (e.g. amino groups, secondary and tertiary alkylamino groups or hydroxy groups). They are therefore also referred to in older literature as triarylmethane dyes. For the dyes, resonance structure can be described as a carbenium ion or with an iminocyclohexadiene or cyclohexadienone partial structure. The central carbon atom in the dyes is therefore not sp³- but sp²-hybridized.[1]^{: S. 58}

Triarylmethine dyes usually show intensive, luminous colourings with different light fastness. In addition to dyeing and printing textiles, leather and paper, they are also used for printing inks, inks, ballpoint pen pastes and as microscopic dyes. As functional dyes, some representatives of this dye class are used as indicators (phenolphthalein), as disinfectants, anthelmintics and antifungals (e.g. malachite green, crystal violet).[11]

Examples:

• 
  Michlers Hydrol
• 
  C.I. Solvent Yellow 34
• 
  Phenolphthalein (pH 8,2-12)
• 
  C.I. Basic Violet 14 (Fuchsin)
• 
  C.I. Basic Green 4 (malachite green)
• 
  C.I. Basic Violet 3 (crystal violet)