Anion Recognition by a Pincer-Type Host Constructed from Two Polyamide Macrocyclic Frameworks Jointed by a Photo-Addressable Azobenzene Switch

A sterically crowded light-responsive host 1 was synthetized with a 93% yield by applying a post-functionalization protocol utilizing the double amidation of 4,4′-azodibenzoyl dichloride with a readily available 26-membered macrocyclic amine. X-ray structures of two hydrates of trans-1 demonstrate a very different alignment of the azobenzene linkage, which is involved in T-shape or parallel-displaced π⋯π stacking interactions with the pyridine-2,6-dicarboxamide moieties from the macrocyclic backbone. Despite the rigidity of the macrocyclic framework, which generates a large steric hindrance around the azobenzene chromophore, the host 1 retains the ability to undergo a reversible cis⟷trans isomerization upon irradiation with UVA (368 nm) and blue (410 nm) light. Moreover, thermal cis→trans back-isomerization (ΔG(0) = 106.5 kJ∙mol(−1), t(½) = 141 h) is markedly slowed down as compared to the non-macrocyclic analog. (1)H NMR titration experiments in DMSO-d(6)/0.5% water solution reveal that trans-1 exhibits a strong preference for dihydrogenphosphate (H(2)PO(4)(−)) over other anions (Cl(−), MeCO(2)(−), and PhCO(2)(−)), whereas the photogenerated metastable cis-1 shows lower affinity for the H(2)PO(4)(−) anion.