Methines, Hemicyanines and Cyanines

In recent years metliines, hemicyanines and cyanines have been the subject of much research as they couple high molar absorption and emission coefficients with a relative ease of molecular manipulation to change the wavelength of absorption, making them an obvious target for chemical designers. 

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. 

The indocyanines and analogues, especially Indocyanine Green, absorbing and 

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