Synthesis and properties of near-infrared absorbing asymmetric pyrylium-squarylium dyes containing tertiary butyl groups

Asymmetric pyrylium-squarylium dyes (5a-b) were prepared by direct condensa tion of squaric acid with two different kinds of heterocycles. Symmetrical dyes 5d-f were isolated as by-products. Asymmetric dye 5c was synthesized v ia the traditional way using monosubstituted squaric acid as the key interm ediate. Dyes 5a-f were characterized with the aid of H-1-NMR, IR, mass spec troscopy and elemental analysis. The dyes exhibit high melting points and a bsorption maxima in the range of 720-760 nm with high extinction coefficien ts and higher solubility in organic solvents than other squarylium dyes. (C ) 2000 Elsevier Science Ltd. All rights reserved.