Abstract
Asymmetric pyrylium–squarylium dyes ( 5a– b) were prepared by direct condensation of squaric acid with two different kinds of heterocycles. Symmetrical dyes 5d–f were isolated as by-products. Asymmetric dye 5c was synthesized via the traditional way using monosubstituted squaric acid as the key intermediate. Dyes 5a–f were characterized with the aid of 1H-NMR, IR, mass spectroscopy and elemental analysis. The dyes exhibit high melting points and absorption maxima in the range of 720–760 nm with high extinction coefficients and higher solubility in organic solvents than other squarylium dyes.
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