A squaraine-based chemosensor for Hg2+ and Pb2+

A novel unsymmetrical cationic squaraine dye (), absorbing in the near-infrared region (600–850 nm), was synthesized by reacting a semi-squaric acid derivative 3-[4-(N,N-dioctylamino)phenyl]-4-hydroxycyclobutene-1,2-dione with the squarylium dye, bis(3-methylbenzothiazol-2-ylidene)squaraine. Addition of micromolar amounts of Hg2+ and Pb2+ to solutions of in dichloromethane brings about significant changes in its absorption spectrum, resulting in a change in colour of the solution from green to pink. The absorption spectrum of the dye is relatively insensitive to other environmentally relevant metal ions, making it possible to visually detect Hg2+ and Pb2+ in the presence of other metal ions. Molecular modeling of the complexes at the density functional theory (DFT) and semiempirical PM3 levels strongly suggests that the oxygen atoms are the most likely choice for cation binding. The calculations also indicate a high affinity of Hg2+ and Pb2+ toward the formation of 2:1 complexes, which is in good agreement with the experimental results.