Synthesis of near-infrared absorbing pyrylium-squaraine dye for selective detection of Hg2+

A chemosensor based on symmetrical squaraine dye (SQ) absorbing in the near-infrared region (690–870 nm), is synthesized with quantitative yields by a reaction between squaric acid and 4-methyl 2,6-di-t-butyl pyrylium trifluoromethanesulfonate, and its sensing behavior toward various metal ions has been investigated using UV–visible spectroscopy and fluorescence spectroscopy. SQ shows the highly selective detection behavior toward Hg2+ in comparison with various other metal ions such as Co2+, Mg2+, Pb2+, Zn2+, Cs+, Ag+, Ni2+, Li+, K+, and Na+ due to the soft acid nature and the size of mercuric ion. The selective detection of Hg2+ with the symmetrical-squaraine unit gave rise to a significant bathochromic shift toward the NIR region (λmax 697–840 nm in CH3CN). The absorption studies also indicate a high affinity of Hg2+ toward the formation of 2:1 complexes, which is in well understanding with the observational consequences. Fluorescence studies experimentally proved that only 640 nm absorption peak is responsible for the emission peak at 683 nm. A recognition mechanism based on the binding mode is proposed by means of absorption changes, 1H NMR titration experiments, FT-IR study and theoretical calculations.