Dyes polymethine

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

A great number of different heterocycHc residues have been used as the terminal groups of PMDs. Examples appear throughout this article. PMDs containing residues with quaternary nitrogen atoms are traditionally called cyanine dyes (qv). 

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 

According to one classification (15,16), symmetrical dinuclear PMDs can be divided into two classes, A and B, with respect to the symmetry of the frontier molecular orbital (MO). Thus, the lowest unoccupied MO (LUMO) of class-A dyes is antisymmetrical and the highest occupied MO (HOMO) is symmetrical, and the TT-system contains an odd number of TT-electron pairs. On the other hand, the frontier MO symmetry of class-B dyes is the opposite, and the molecule has an even number of TT-electron pairs. 

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

The hterature on polymethine dyes has been reviewed (3,4,7,9—11,21). Reference 3 is the best among recent (1995) sources on the chemistry and spectral properties of this dye class it also contains a large bibhography. 

Dye Aggregation. Another important kind of chromophore interaction is the dye aggregation. A number of reviews exist that treat the stmcture and appHcation of polymethine dye aggregates (3,48—51) (see also Cyaninedyes). 

The phenomenon of dye aggregation was discovered in the 1930s (52,53). Polymethine dyes were found to form molecular complexes in solution or on certain crystal surfaces. Molecules within aggregates are bound together by nonvalence bonds, ie, resonance interactions exist between them. 

As conjugated systems with alternating TT-charges, the polymethine dyes are comparatively highly reactive compounds (3). Substitution rather than addition occurs to the equalized TT-bond. If the nucleophilic and electrophilic reactions are charge-controHed, reactants can attack regiospeciftcaHy. 

The piC values of polymethine dyes depend on terminal group basicity (64) thus the protonation abHity diminishes if the basic properties of the residues decrease, passing from benzimidazole, quinoline, benzothiazole, to indolenine. On the other hand, the piC of higher homologues increases with chain lengthening. The rate constant of protonation is sensitive to other features, for example, substituents and rings in the chain and steric hindrance for short-chain dyes. 

The dianilinopolycarhocyanines, CgH5HN=CH-(-CH=CH NHCg Hg, are useful synthetic iatermediates for polymethine dyes, offering needed spectral and other properties (3,4,73,74). The asymmetrical polymethines appear to be ambivalent systems, and the number of possible reaction paths iacreases considerably as a result (75,76). 

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