Differentiation of cis- and trans-isomers of the novel napthalene-aza receptor by naked-eye colorimetric anion sensing

Novel chromofluorogenic receptors 1 (cis-isomer) and 2 (trans-isomer) were developed using an aza-nitro-phenyl group as a chromophore, urea moiety as a binding unit, and naphthalene as a fluorophore. The position of nitro-phenyl moiety in the chromophores influences the naked-eye colorimetric anion sensing which differentiates the geometrical isomers. Receptor 2 showed sensitivity toward F− than HP2O73- and its fluorescence emission (λmax = 370 nm) was significantly 'switched-off' at an excitation wavelength of 260 nm in CH3CN:DMSO (90:10, v/v) solution at 25 °C. The fluorescence titration experimental results revealed that the receptor 2 binds strongly with F− than HP2O73- ions in 1:1 stoichiometry. The quantum mechanical calculations through time dependant density functional theory (TD-DFT) using basis set B3LYP/6-31G(d) supported our experimental findings agreeably.