Synthesis of hybrid azobenzene-chalcone dyes and studies on their photophysical properties: Molecular recognition of carbonate and phosphate ions using a novel turn-off fluorescent probe

The work presented here describes the synthesis and comparative physiochemical studies of hybrid compounds C-1 to 5 of azobenzene A-1 and chalcone B-1, which exhibited the characteristic of both. Further, electron-withdrawing and donating substituents analogues have also been synthesized. The hybrid compound is unique in nature because it contains the phenolic -OH, -N = N-, as well as α,β-unsaturated enone functional groups. The photophysical properties of hybrid C-1 behaved more or less like the azobenzene moieties and showed the same pattern of UV–vis absorption as with the change in the polarity of the solvent and the change in the pH of the medium. Thus, in the non-polar solvent or in the water medium, the phenol form dominates with its yellow colour and in polar solvents shows an intense red colour of keto form. They show a dramatic quenching of fluorescence emission in the polar solvent. Furthermore, after screening, C-1 was found to show selective sensing of the aqueous CO32− ion as well as minimally with PO43- and show a characteristic colour change in an organic solvent such as CH3CN. A few ions also changed colour in DMF, but that was due to the only shifting of the tautomerization equilibrium from keto to enol form. The UV–vis as well as fluorescent spectral studies, determination of the detection limits, and binding constants illustrated significant sensing of CO32− ions. A systematic study was performed to understand the binding interactions of the chemosensor by using Job's plot and time-dependent 1H NMR studies that supported the involvement of phenoloc -OH and β carbon of α,β-unsaturated system. The characteristic pattern of UV–vis and interaction mode was confirmed by DFT studies. The applicability of chemosensor C-1 was demonstrated by the development of a paper strip model to check the presence of CO32− ions in different kinds of water in society.