Synthesis of bisquaraine dyes. Novel homologues of; 1,2-squaraines bearingsymmetrical and unsymmetrical structures

In order to investigate analogous structures of 1,2-squaraine dyes, bisquar aines 1 and 2 are newly prepared as their novel homologues. Symmetrical bis quaraine 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 heteroc yclic compounds 5. The structure of one of the symmetrical dyes is confirme d by X-ray structural analysis, showing a largely extended pi -conjugation system over the whole molecule. The synthesis of unsymmetrical dyes 2, in w hich different aromatic components are introduced, is established by the re actions of 5 with 3'-monosubstituted bisquarates 6. Each bisquaraine dye ex hibits two absorption maxima separated by 56-75 nm, the lower-energy absorp tion of which is observed at 653-757 nm.