| Summary: | We report a fully organic pyridine-tetrapyrrolic U-shaped acyclic receptor <b>10</b>, which prefers a supramolecular pseudo-macrocyclic dimeric structure (<b>10</b>)<sub>2</sub> in a less polar, non-coordinating solvent (e.g., CHCl<sub>3</sub>). Conversely, when it is crystalized from a polar, coordinating solvent (e.g., <i>N</i>,<i>N</i>-dimethylformamide, DMF), it exhibited an infinite supramolecular one-dimensional (1D) “zig-zag” polymeric chain, as inferred from the single-crystal X-ray structures. This supramolecular system acts as a potential receptor for strong acids, e.g., <i>p</i>-toluenesulfonic acid (PTSA), methane sulfonic acid (MSA), H<sub>2</sub>SO<sub>4</sub>, HNO<sub>3</sub>, and HCl, with a prominent colorimetric response from pale yellow to deep red. The receptor can easily be recovered from the organic solution of the host–guest complex by simple aqueous washing. It was observed that relatively stronger acids with p<i>K</i><sub>a</sub> < −1.92 in water were able to interact with the receptor, as inferred from <sup>1</sup>H NMR titration in tetrahydrofuran-<i>d<sub>8</sub></i> (THF-<i>d</i><sub>8</sub>) and ultraviolet–visible (UV–vis) spectroscopic titrations in anhydrous THF at 298 K. Therefore, this new dynamic supramolecular receptor system may have potentiality in materials science research.
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