Cyanine Dyes: Self Aggregation and Behaviour in Surfactants

Abstract

The cyanine dyes came to the limelight in 1856 [1] for their application to impart light sensitivity to silver halide emulsions in a region of the spectrum to which the silver halide is normally not sensitive. Numerous applications in various areas are being published every year on this subject. Now it has gained importance for their use in photodynamic therapy and in various biological activities. The generic cyanine dyes consist of two nitrogen centers, one of which are positively charged and are linked by a conjugated chain of an odd number of carbon atoms to the other nitrogen ( 1 ). This has been studied as ?Push-pull? alkenes and forms the basis of the polymethine dyes, which contain the streptopolymethine unit as the chromophore. These dyes are classified as cationic streptopolymethines and hemicyanines ( 1 ), anionic streptopolymethine oxonols ( 2 ), neutral streptopolymethine merocyanines ( 3 ) and zwitterionic squaraine-based cyanines ( 4 ) depending upon the charge of the streptomethine unit. Generally these dyes have an all-trans geometry in their stable form. These dyes undergo photo-isomerization, which can be studied by using various techniques such as flash photolysis, transient absorption, picosecond time-resolved spectroscopy, etc. [2-5]. The cyanines, merocyanines and squaraine dyes are substances with a variety of colors, but are not widely used for dyeing purposes, as they are decolorized by light and acid.