Syntheses and some properties of infrared-absorbing croconium and related dyes

The correlations between the structure and some properties of croconium and related cyanine dyes are discussed. It is shown by PPP MO calculations that the croconium dyes have the same chromophoric system as cyanines. The λmax of the croconium dyes undergoes a bathochromic shift of about 120 nm compared with the corresponding cyanine dyes. This shift can be calculated by the PPP MO method. Negative solvatochromism is observed for croconium dyes. Croconium dyes generally have better solubility than squarylium dyes in organic solvents and have good lightfastness in solvents, greatly improved by adding nickel complex as a stabilizer.