The Design, Synthesis And Properties Study Of New Squaraine-based Fluorescent Probes For Cations

As a class of novel organic functional dye, squaraine and their derivatives aregaining significant interest in the field of chemistry. Due to the unique D-A-Dstructure and the strong absorption and emission characters in the visible and nearinfrared region, squaraine has been used as the chemosensor in the ion recognition.In this paper, we have synthesized six new squaraine dyes, which have not beenreported so far. These structures have been characterized by IR,1H NMR,13C NMR,MS and EA. Furthermore, the recognition properties and the response mechanismhave been studied by the absorption and emission spectra.(1) Mercury is a deadly toxin to humans in the environment. Thus, preparationof a highly specific, facile, and cost-effective mercury chemosensor, which isapplicable to the typical biological milieux, is an important goal. The fact thatdithiocarbamate (DTC) is an effective ligand for heavy metal ions prompted us toselect (phenylazanediyl)-bis-(ethane-2,1-diyl)-bis-diethylcarbamodithioate as abinding arm to synthesize two new squaraine dyes40&41which can specificallydetect the Hg2+cation in the complex aqueous samples containing common metalions, even in the presence of thiophillic cation Ag+and Pb2+, by means ofcolorimetric and fluorescent monitors. We also carried out the1H NMR titrationexperiment, ESI-MS analysis, single-crystal X-ray diffraction structural analysis andtheoretical calculation to investigate the recognition mechanism. The experimentalrusults exhibit that the detection limit of40reaches7.1×10-9M (3σ/k), which isbelow the limit of Hg2+in drinking water (2ppb) set by the U.S. EnvironmentalProtection Agency (EPA). The Job’s plot showed a maximum value at the0.66molfraction of Hg2+ions, which is the signature of a1:2binding ratio between the dye40and Hg2+. The squaraine dye41is more solutable and anti-aggregating due to theasymmetric modification. The detection limit of41to Hg2+is as low as4.5×10-8M, and Job’s plot and Benesi-Hildebrand Plot analysis suggest a1:1binding ratiobetween the dye and Hg2+along with an association constant of1.36×105M-1.(2) Squaraine dyes are notorious with their low solubility and processability.Herein, the thio-ether was introduced to the aniline side arms to improve thesolubility, and four new squaraine dyes42,43,44&45were synthesized. In theEtOH/H2O (50:50, v/v) solution, the fluorescence of42was quenched by Hg2+orAg+. The detection limits of42to Hg2+and Ag+are1.3×10-7M and1.8×10-7M,respectively. Interestingly, the chelating agent EDTA and42have a competitivecoordination to Hg2+. In the solution, EDTA extract Hg2+and affranchise the dye,leading the fluorescence restore. While the Ag+can not be extracted by EDTA fromthe complex, so that42could tell Hg2+from Ag+by this approach.(3) Zincum is an important trace element, so that the design and preparation ofhighly effective Zn2+chemosensors is a significant goal. It was reported that thecompounds modified by pyridine, such as DPA, can be designed as a promisingfluorescence probe for Zn2+. With this design principle, we synthesized squarainedyes46&47for Zn2+detection. However, the pyridinyl substituted squaraine dyeswere found inferior in chemical stability, and the further study is still under process.