Croconium dyes

The squarylium (4.6) and croconium (4.7) dyes are closely related structurally to cyanines but are in fact donor-acceptor molecules and consequently the design principles of near-IR absorbers based on these chromophores are different. The synthesis of these chromophores is achieved easily by reacting either squaric acid, or preferably an alkylated derivative, e.g. di-n-butyl squarate, or croconic acid with electron-donor molecnles. The croconium dyes absorb at significantly longer wavelengths than the sqnarylinms as shown in Figure 4.2. 

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