A new water-soluble near-infrared croconium dye

A new water-soluble near-infrared (NIR) absorbing croconium dye, 2,5-bis[(4-carboxylic-piperidylamino)thiophenyl]-croconium, was synthesised by condensing croconic acid with 2-(4-carboxylic-piperidylamino)thiophene and characterised by 1H NMR, high-resolution MS and UV–vis spectra. The dye showed a sharp and intense absorption band peaking near 800 nm and little absorption in visible region in several water-soluble polymers. It also exhibited excellent photostability in water, methanol and in the plastic films. Due to its good water solubility, a water-based coating process was developed to make NIR blocking plastic filters for use as attenuators of stray light in NIR laser systems.