The synthesis and optical properties of bis-squarylium dyes bearing arene and thiophene spacers

A series of arene- and thiophene-bridged bis-squarylium dyes were synthesized using the corresponding bis-squaric acids as precursors and their electronic absorption and fluorescence emission properties were investigated. The arene-bridged, bis-squarylium dyes exhibited electronic absorption in the visible to near-infrared regions. The combination of the pyrene spacer with the benzindoline heterocycles resulted in significantly red-shifted absorption at 770 nm. The arene-bridged dyes were fluorescence emissive to some extent and some compounds exhibited fluorescence emission in the near-infrared region. Thiophene-bridged dyes, in which thiophene-, bithiophene-, and terthiophene spacers were employed, displayed electronic absorption in the near-infrared region from 717 to 807 nm; an increase in the number of thiophene rings in the spacer led to a blue shift of the absorption maxima due to disruption of π-electron conjugation. The thiophene-bridged dyes exhibited fluorescence emission in the near-infrared region, although the fluorescence intensities were much smaller than those of the fluorescent squarylium dye.