Pyrylocyanines. 14. Unsymmetrical pyrylocyanines based on 2,3-polymethylene-4,6-diphenylpyrylium salts

Abstract

Unsymmetrical 4,6-diphenylpyrylo-2-carbo- and dicarbocyanines, as well as the analogous pyridocyanines, containing 3-ethylbenzothiazolium and 1,3,3-trimethyl-3H-indolium residues and an ethylene or trimethylene bridging grouping between the α position of the polymethine chain and the β position of the pyrylium or pyridinium residue were synthesized. The introduction of bridging groups is generally accompanied by a bathochromic effect and intensification (in the pyrylocyanine series) or weakening (in the pyridocyanine series) of the fine structure of the absorption bands. On the basis of the deviations calculated from the centers of the absorption bands and the dependences of the vinylene shifts and the absorption band widths on the structure of the dyes it was concluded that the trimethylene bridging groups in the examined cases increase the electron-donor character of the heterocyclic residues in the dyes more markedly than the ethylene bridging groups.