Polymethine and Related Chromophores

Methine dyes are characterized by a chain of conjugated double bonds with an odd number of carbon atoms between the two terminal groups X and Y, which may be considered as being capable of charge exchange. 

One of the substituents X and Y is an electron acceptor and the other as an electron donor. The dyes can be cationic, anionic, or neutral. 

Methine dyes have an extended system of conjugated double bonds. A large number of resonance structures may therefore be formulated, of which (1) and (2) are the most important, i.e., the electron density is lowest on the nitrogen atoms. 

In all the following formulas only one resonance structure is shown. To indicate that a jr-electron system with many possible resonance structures is involved, the total charge of the molecule is given outside a square bracket. 

Cyanine dyes (3) are the best known polymethine dyes. Nowadays, their commercial use is limited to sensitizing dyes for silver halide photography and as infrared absorbers for optical data storage and other (bio)imaging applications. However, derivatives of cyanine dyes provide important dyes for polyacrylonitrile. 

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