Diazocines\nas Guests of Cucurbituril Macrocycles:\nLight-Responsive Binding and Supramolecular Catalysis of Thermal Isomerization

The\nphotoswitching of supramolecular host–guest complexes\nis the basis of numerous molecularly controlled macroscopic functions,\nsuch as sol–gel transition, photopharmacology, the active transport\nof ions or molecules, light-powered molecular machines, and much more.\nThe most commonly used systems employ photoactive azobenzene guests\nand synthetic host molecules, which bind as the stable <i>E</i> isomers and dissociate as the <i>Z</i> forms after exposure\nto UV light. We present a new, extraordinarily efficient cucurbit[7]uril\n(CB7)/diazocine host/guest complex with inverted stability that self-assembles\nunder UV irradiation and dissociates in the dark. The association\nconstants of the <i>Z</i> and <i>E</i> isomers\nin water differ by more than 10<sup>4</sup>-fold. We also show that\nthe thermally activated <i>E</i> → <i>Z</i> isomerization is significantly accelerated by CB7, which is a rare\ncase of enzyme-like catalysis by transition state stabilization without\nproduct inhibition. In contrast to CB7, cucurbit[8]uril (CB8) binds\nboth isomers with high affinity, showing good selectivity (∼1000-fold)\ntoward the <i>Z</i> isomer. Notably, this isomer preferentially\nbinds CB8 relative to CB7 by a factor greater than 1 × 10<sup>6</sup>. We also use the system to introduce a supramolecular photoacid\nthat builds on the increased basicity of a guest bound to CB7 and\non the extremely high affinity of the <i>E</i> isomer, which\nis utilized to displace the acid from CB7, thereby switching the pH\nof the solution.