Polymethine

According to general usage, the term cyanine designates any cationic dye in which two nuclei of different or same nature are linked by a mono or polymethine chain. When these groups (-CH= are replaced partially or totally by one or several nitrogen atoms, the cationic dye is called azacyanine. 

These dyes possess two independent chromophoric chains of even methine (neutro) and uneven methine (cyanine) fixed on a central ketometbylene nucleus. The methylene reactive group is first used for the neutrocyanine synthesis in position 5. the, quaternization of which can ensure a subsequent polymethine synthesis in position 2 of a cationic dye by ordinary means (Scheme 58). As indicated, this quaternized neutrocyanine (37) may as well give another neutrocyanine. 

In WORM disks, pentamethine and heptamethine are the materials of choice (see Cyanine dyes Polymethine dyes). 

The simplest PMDs include the polymethines (2), streptocyanines (3), oxonols (4), and merocyanines (5). 

Polymethines with branched polymethine chains also exist. Among these, PMD with symmetrically branched chains are the best known they are referred to as trinuclear polymethine dyes (TPMD) (6). 

PMDs demonstrate pronounced absorption and contain fluorescence bands that are relatively narrow and highly intense, which arise from electron transitions occurring within the polymethine chromophore... 

For convenience, unsymmetrical PMDs should be considered as the derivatives of the corresponding symmetrical polymethines, commonly called... 

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