Synthesis of bisquaraine dyes. Novel homologues of 1,2-squaraines bearing symmetrical and unsymmetrical structures

Abstract

In order to investigate analogous structures of 1,2-squaraine dyes, bisquaraines 1 and 2 are newly prepared as their novel homologues. Symmetrical bisquaraine dye 1, in which the same heterocyclic components are introduced at the 3 and 3′ positions of the bisquaryl skeleton, is obtained by reaction of 4,4′-bi(3-isopropoxycyclobut-3-ene-1,2-dione) 3 with a series of heterocyclic compounds 5. The structure of one of the symmetrical dyes is confirmed by X-ray structural analysis, showing a largely extended π-conjugation system over the whole molecule. The synthesis of unsymmetrical dyes 2, in which different aromatic components are introduced, is established by the reactions of 5 with 3′-monosubstituted bisquarates 6. Each bisquaraine dye exhibits two absorption maxima separated by 56–75 nm, the lower-energy absorption of which is observed at 653–757 nm.