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 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 - demonstrate a very different alignment of the azobenzene linkage, which is involved in -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 retains the ability to undergo a reversible ⟷ isomerization upon irradiation with UVA (368 nm) and blue (410 nm) light. Moreover, thermal → back-isomerization (ΔG = 106.5 kJ∙mol, = 141 h) is markedly slowed down as compared to the non-macrocyclic analog. H NMR titration experiments in DMSO-/0.5% water solution reveal that - exhibits a strong preference for dihydrogenphosphate (HPO) over other anions (Cl, MeCO, and PhCO), whereas the photogenerated metastable - shows lower affinity for the HPO anion.