Polyene and Polymethine Dyes

Polyene and polymethine dyes are two structurally related groups of dyes which contain as their essential structural feature one or more methine (-CH=) groups. Polyene dyes contain a series of conjugated double bonds, usually terminating in aliphatic or alicyclic groups. They owe their colour therefore simply to the presence of the conjugated system. In polymethine dyes, electron-donor and electron-acceptor groups terminate either end of the polymethine chain, so that they may be considered as typical donor-acceptor dyes. 

The best-known group of polyene dyes is the carotenoids, which are widely encountered natural colorants. /1-Carotene (104) is given as an 

There has been some interest in extending the absorption range of cyanine dyes to longer wavelengths into the near-infrared region of the spectrum. Consideration of the spectral data for thiazole derivatives 118-120 is of some interest in this respect. Cyanine dye 118 shows the characteristic visible absorption spectrum for a dye of this type, giving a 

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Of particular interest are the intramolecularly ionic meropolymethine dyes (especially the merocyanines), whose electronic structure lies somewhere between that of polyenes and that of polymethines depending on the nature of X and X as well as on solvent polarity [20], These are systems in which an electron-donating group, D, is linked by a conjugated system, R, to an electron-accepting group, A. Their intermediate r-electronic structure can be described in terms of two mesomeric structures, D—R—A D —R—A , as, for example, this special vinylogous merocyanine dye ( = 0,1,2,...) ... 

Nonpolar solute in a nonpolar solvent. In this case, only dispersion forces contribute to the solvation of the solute. Dispersion forces, operative in any solution, invariably cause a small bathochromic shift, the magnitude of which is a function of the solvent refractive index n, the transition intensity, and the size of the solute molecule. The function (n — l)/(2n - -1) has been proposed to account for this general red shift [69, 70]. Corresponding linear correlations between this function of n and Av have been observed for aromatic compounds e.g. benzene [22], phenanthrene [71]), polyenes e.g. lycopene [23], y9-carotene [464]), and symmetrical polymethine dyes e.g. cyanines [26, 27, 292, 293]). 

Fig. 36.9. Sketch of the UV transition energies in polymethine dyes and in polyenes.

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