Polymethine Hemicyanines

Dyes, polymethine used for dyes having at least one electron donor and one electron acceptor group linked by methine groups or aza analogues aUopolar cyanine, dye bases, complex cyanine, hemicyanine, merocyanine, oxonol, streptocyanine, and styryl. Supersensitization has been reported for these types—18 cites for cyanines, 3 for merocyanine, and 6 for all other polymethine types. 

The strategies used in the synthesis of polymethine dyes are illustrated for a series of indoline derivatives in Scheme 6.1. There is an even wider range of synthetic routes to polymethine dyes than is described here, but they are based for the most part on a similar set of principles. The starting material for the synthesis of this group of polymethine dyes is invariably 2-methylene-1,3,3-trimethylindolenine (121), known universally as Fischer s base. As illustrated in the scheme, compound 121 may be converted by formylation using phosphoryl chloride and dimethylformamide into compound 122, referred to as Fischer s aldehyde, which is also a useful starting material for this series of polymethine dyes. When compound 121 (2 mol) is heated with triethylorthoformate (1 mol) in the presence of a base such as pyridine, the symmetrical cyanine dye, C. I. Basic Red 12 109 is formed. The synthesis of some hemicyanines may be achieved by... 

Hemicyanines are polymethines with an ammonium salt at one end of the chain. Typically these can be pyridinium (3.30), indolenium (3.31a, X = C(CHj)2) and ben-zothiazolium (3.31b, X = S) derivatives. The pyridinium derivatives fluoresce in the long wavelength red region, whilst the others are in the near-infrared, which is a very useful property for certain laser dye outlets and in fluorescent imaging in medical diagnosis and biological probes. 

Crowns with Polymethines Dyes. The ready synthesis, see Figure 3.18, of crown ethers attached to polymethines or hemicyanines has made these compounds a fertile area of research and application in metal ion sensing. AU of these polymethine derivatives are cation responsive e.g. (3.84b) forms complexes with Na, Mg, Ca, Sr and Ba with an increase in fluorescence intensity. 

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