Host–Guest Interaction of <i>p</i> -Sulfonatothiacalix[4]arene with Zn(II) and Its Role in Structuring Self-Doped Polyaniline Adsorbents

This study presents a comprehensive host–guest investigation of the p-sulfonatothiacalix[4]arene macrocycle and its application as a dopant in the fabrication of a self-doped polyaniline adsorbent for Zn(II) removal. Proton nuclear magnetic resonance (1H NMR) titration of p-sulfonatothiacalix[4]arene with Zn(II) ions revealed a strong binding affinity, quantified by an association constant (Ka) exceeding 104 M–1, indicative of robust encapsulation at the macrocyclic cavity. Due to the limited solubility of the polymer, solid-state NMR spectroscopy was employed to elucidate the complexation behavior of Zn(II) within the self-doped polyaniline, revealing marked heterogeneity and localized variations in charge density while preserving the intrinsic conductivity characteristics of the polymer matrix. Complementary density functional theory (DFT) calculations identified the sulfonate (−SO3–) rim of the thiacalix as the primary binding site for Zn(II), whereas in the self-doped polyaniline, the strongest adsorption resulted from cooperative interactions between the sulfonate groups and the polyaniline backbone. Quantum Theory of Atoms in Molecules (QTAIM) analysis further characterized the interactions, showing predominantly ionic binding with minor covalent contributions in thiacalix–Zn complexes and ionic/dative bonding with weak covalent character in the polymer-Zn system. The integrated NMR and computational insights provide a detailed understanding of the adsorption mechanisms and chelation sites, highlighting the efficacy of p-sulfonatothiacalix[4]arene as a functional dopant for advanced conductive polymer adsorbents targeting heavy metal ions.