Facile synthesis and anion binding studies of fluorescein/benzo-12-crown-4 ether based bis-dipyrromethane (DPM) receptors

Abstract

Two novel fluorescein as well as benzo-12-crown-4 ether functionalized dipyrromethane receptors (DPM3 and DPM4) have successfully been synthesized. The anion (used as their TBA salts) binding studies of thus prepared DPM3 and DPM4 receptors were evaluated by the UV-visible spectrophotometric titrations. Binding affinities as well as the stoichiometry were determined through the UV-visible titrations data with the involvement of the BindFit (v0.5) package available online at https://supramolecular.org. Moreover, binding events were validated by means of the comparison of the partial ¹H-NMR spectrum of the simple host molecule with that of the host–guest complex, and the 1 : 1 stoichiometry were further confirmed by the Job's method of continuous variation. From the results, we observed the binding constant (K_a) values of DPM3/DPM4 with various tested anions in the range of 516.07 M⁻¹ to 63789.81 M⁻¹, depending upon the nature/shape/size of the anions. Moreover, the anion–π interactions were confirmed by the partial ¹H-NMR spectral data, and further supported by the literature reported systems. The authors hope that such types of valued receptors will be benefitted in future for the recognizing/binding of a variety of biologically important anions.