Differential Recognition of Anions with Selectivity towards F− by a Calix[6]arene–Thiourea Conjugate Investigated by Spectroscopy, Microscopy, and Computational Modeling by DFT

Abstract

Anion recognition studies were performed with triazole-appended thiourea conjugates of calix[6]arene (i.e., compound ⁶L) by absorption and ¹H NMR spectroscopy by using nineteen different anions. The composition of the species of recognition was derived from ESI mass spectrometry. The absorption spectra of compound ⁶L showed a new band at λ=455 nm in the presence of F⁻ due to a charge transfer from the anion to the thiourea moiety and the absorbance increases almost linearly in the concentration range 5 to 200 μm. This is associated with a strong visual color change of the solution. Other anions, such as H₂PO₄⁻ and HSO₄⁻, exhibit a redshift of the λ=345 nm band and the spectral changes are associated with the formation of an isosbestic point at λ=343 nm. ¹H NMR studies further confirm the binding of F⁻ efficiently to the thiourea group among the halides by shifting the thiourea proton signals downfield followed by their disappearance after the addition of more than one equivalent of F⁻. The other anions also showed interactions with compound ⁶L, however, their binding strength follows the order F⁻>CO₃²⁻>H₂PO₄⁻≈CH₃COO⁻>HSO₄⁻. The NMR spectral changes clearly revealed the anion-binding region of the arms in case of all these anions. The anion binding to compound ⁶L indeed stabilizes a flattened-cone conformation as deduced based on the calix-aromatic proton signals and was further confirmed by VT ¹H NMR experiments. The stabilization of the flattened-cone conformation was further augmented by the interaction of the butyl moiety of the nBu₄N⁺ counterion. The structural features of the anion-bound species were demonstrated by DFT computations and the resultant structures carried the features that were predicted based on the ¹H NMR spectroscopic measurements. In addition, SEM images showed a marigold flower-type morphology for compound ⁶L and this has been transformed into a chain-like structure of connected spherical particles in the presence of F⁻. The anion-induced microstructural features are reflective of the binding strength, size, and shape of the anions. The binding strengths of the anions by compound ⁶L were further compared with that of compound ⁴L, a calix[4]arene analogue of compound ⁶L, in order to address the role of the number of arms built on the calixarene platform based on absorption spectroscopy, ¹H NMR spectroscopy, and DFT computations and it was found that compound ⁶L is a better receptor for F⁻, which extends its interactions from all the three arms.