Polymethine bridge

The longer the polymethine bridge in a cyanine dye, the higher the absorbance and emission wavelengths become. In general, for each incremental increase of n, the absorbance and... 

Ghromophore Interaction. A great number of systems containing several polymethine chromophores exist. As the simplest objects of this sort, dyes with coupled chromophores, or bis-dyes, can be considered (3,10) as follows (15), where M is either a conjugated or a nonconjugated bridge. 

The synthesis of polymethines with dimethylene bridges starts with aUcychc ketones (31), ketals (32), and enamines (X = NR2), or enol ethers (X = OR) (33). They possess two activated centers, methine or methylene groups, which react with Vilsmeief s reagent to produce the corresponding dyes. 

Polymethines with trimethylene bridges have been obtained by iateraction of the corresponding salts with 2,6-dimethoxy-l,4-dihydtobenzene (34) (74), for example, as follows, where Y = O, S, or Se. 

AT1 cm ). and therefore, these dyes are excitable not only with red (635 or 670-nm) but also with blue (380, 405, and 470 nm) diode lasers or LEDs (Fig. 1). Carbonyl containing substituents such as 1,3-indanedione, cyanoacetic ester, barbituric, and thiobarbituric acid form intramolecular H-bond with the polymethine hydrogens of the squaraine bridge. As a result, the molar absorptivities and quantum yields of these dyes are substantially decreased. 

---