Rational Design of Azo-Azomethine Receptors for Sensing of Inorganic Fluoride: Construction of Molecular Logic Gates and DFT Study

Abstract

New azo-azomethine receptors, HLn (n = 1–3), have been synthesised via condensation reaction of 5-(4-X-phenyl)-azo-salicylaldehyde (X = NO2, Cl and CH3) with (4-nitrobenzylidene)hydrazine. The receptor with a p-NO2 substituent on the aromatic ring of the azo moiety (HL1) has excellent sensitivity and selectivity towards basic anions with proper discrimination between F− and AcO− or H2PO4− in DMSO–water (4 : 1). A Job’s plot displays a 1 : 1 stoichiometry between HL1 and F− alone with a detection limit of 0.737 μM for fluoride ions. The solvatochromic behaviour of HL1 was probed by studying its UV-vis spectra in four pure organic solvents of different polarities and a meaningful correlation was observed. Furthermore, HL1 was used for detection of inorganic fluoride in toothpaste. The systematic density functional theory (DFT) and time dependent-DFT calculations have been carried out to investigate the mechanism of colourimetric sensing of fluoride ion by HL1 in the gas phase and in solution. Moreover, by using F− and H+ as chemical inputs, and the absorbance as output, a INHIBIT logic gate was constructed, which exhibits ‘Write–Read–Erase–Read’ ability without obvious degradation in its optical output.