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.