Squarylium Functional Dyes Having Absorption Bands in a Wide Range of Wavelength and Applicability to Various Fields

Novel bis-squaraine dyes have been synthesized in which two squaraine moieties are conjuga-tively linked by a methine group, thiophene, oligothiophene or arene spacer. New unsymmetrical squarylium dyes have been also prepared by condensation of mono-substituted squaric acid (semi-squaraine acid) with heterocyclic compounds with active methyl group or arylamines. These dyes exhibit large and intense absorption in the near-infrared region, and the X-ray structural analyses of these dyes show a planar structure. Semi-squaraine acids are key precursors for the synthesis of unsymmetrical squarylium dyes, and exhibited their absorption maxima at the shorter wavelength. Applications of squarylium dyes to photoconductive materials, photovoltatic devices, biological and environmental analysis are described. A Ca2+ ion-induced allostreic assem-bling of the squarylium dimmer was confirming by ESI-MS measurement showing the formation of metallo supramolecular complexes, that is the H-aggregate composed of at least three units.