Methine dyes comprise an odd number of methine groups. The end groups can also be part of a heterocycle or the double bonds part of an aromatic system: The methine dye subclasses are based on the these structural differences. Methine dyes can futhermore be classified as cationic, anionic or neutral. If one or more methine groups are replaced by a heteroatom - usually nitrogen - one speaks of heteroanalogous methine dyes.
General properties and classification
Methine dyes can be characterized as polyene dyes with terminalelectron donating and accepting groups. In the majority of cases, these terminal groups contain nitrogen or oxygen atoms. However, while the naturally occurring polyene dyes are limited to yellow to yellow-red tones, almost all colours of the spectrum can be achieved with the predominantly synthetic methine dyes.
Cyanine dyes
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
Hemicyanine dyes
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
Streptocyanine dyes
If both terminal amino and imino groups of the chromophore are open chain, one speaks of streptocyanine dyes.
Members of the oxonol dyes, whose terminal groups contain oxygen as heteroatom, play a role in analogue colour photography.
general structure of oxonol dyes
Styryl dyes
By condensing an active methylene compound 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 by condensation of ethyl cyanoacetate with a p-aminobenzaldehyde derivative.
Synthesis of C.I. Disperse Yellow 31
Di- and triarylmethine dyes
Also amino- and hydroxy-substituted di- and triarylmethines comprise the structural element of a methine dye:
Triarylmethine dyes, resonance structure
Triarylmethine dyes are derived from triphenylmethane, in which at least two of the aromatic rings have electron-donating substituents. 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. 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, as disinfectants, anthelmintics and antifungals. Examples:
Aza-analog methine dyes
If a methine group in a methine dye is replaced by a nitrogen atom, a azamethine dye is obtained. Accordingly, diazamethine dyes are obtained by replacing two methine groups with nitrogen atoms. Diazahemicyanine dyes in particular are an important class of methine dyes and are used as cationic dyes in the dyeing of polyacrylonitrile fibers. Examples:
The quinonimine dyes are aza-analogous members of the diarylmethine dyes:
General structure of quinonimine dyes where X = OH, NH2 or NR2 Y = O or NH
