A molecular tuner: Substituent controlled π/π···hole induced selective colorimetric intramolecular push-pull fluoride sensor

Abstract

Controlling the anion selectivity of synthetic anion receptors towards a particular anion is challenging task for researcher. In the case of bipodal receptors this become trickier since anion can approach the recognition elements of receptors from any direction. In order to overcome this problem, we adopted the new strategy in which the receptor molecules are functionalized with electron donating and/or withdrawing group. In the way of hunting such a receptors, herein Thio-semicarbazide based chemosensors 1-(2,4-dinitrophenyl)-4-(4-nitrophenyl) thiosemicarbazide L1, and 1-(2,4-dimethylphenyl)-4-(4-nitrophenyl) thiosemicarbazide L2 with electron withdrawing (EW) and donating (ED) groups have been synthesized for selective colorimetric anion sensing. Substituent effects on the strength of those receptors L1 and L2 have been studied by naked-eye colorimetric experiments, Ultraviolet-Visible (UV-Vis) spectrometric titration studies, Fourier-Transform Infrared experiments (FT-IR), 1H-NMR, 19F-NMR spectroscopic studies and Molecular Electrostatic Potential (MEP) surface analysis. When we have nitro substitution on both end of the thio-semicarbazide receptor L1, there were no selectivity was found with anions and receptor L1 shows visible color change with the all the tested anions. On the other hand, instead of nitro, when we have methyl substitution on one end and nitro substitution at an another end of the thio-semicarbazide receptor L2, then the receptor L2 show excellent selectivity towards fluoride ion with high association constant. Additionally, for azide considerable bathochromic shift of 70 nm also observed with reference to absorption values of cyanide and fluoride anion which will clearly distinguish the presence of azide ion from cyanide and fluoride. The observed bathochromic shift is mainly attributed due to the intramolecular charge transfer (ICT). In this work, we have successfully tuned the selectivity of synthesized receptors via generating a push-pull effect originated by electron withdrawing and donation substitutions on both end of the receptor L2. The enhanced selectivity of L2 is achieved by altering the acidity of -NHs of thio-semicarbazide derivative L2 by substituting EW and ED groups. In this present work, we have tuned the selectivity of receptor is via intramolecular push-pull effect by electron withdrawing and donating substituent.

Keywords

• Bipodal receptors
• π-hole interaction, fluoride Sensor
• Anion-π interactions
• Colorimetric sensor, and M. E.P Surface