Diazocines as Guests of Cucurbituril Macrocycles: Light-Responsive Binding and Supramolecular Catalysis of Thermal Isomerization

Abstract

The photoswitching of supramolecular host–guest complexes is the basis of numerous molecularly controlled macroscopic functions, such as sol–gel transition, photopharmacology, the active transport of ions or molecules, light-powered molecular machines, and much more. The most commonly used systems employ photoactive azobenzene guests and synthetic host molecules, which bind as the stable E isomers and dissociate as the Z forms after exposure to UV light. We present a new, extraordinarily efficient cucurbit[7]uril (CB7)/diazocine host/guest complex with inverted stability that self-assembles under UV irradiation and dissociates in the dark. The association constants of the Z and E isomers in water differ by more than 10⁴-fold. We also show that the thermally activated E → Z isomerization is significantly accelerated by CB7, which is a rare case of enzyme-like catalysis by transition state stabilization without product inhibition. In contrast to CB7, cucurbit[8]uril (CB8) binds both isomers with high affinity, showing good selectivity (∼1000-fold) toward the Z isomer. Notably, this isomer preferentially binds CB8 relative to CB7 by a factor greater than 1 × 10⁶. We also use the system to introduce a supramolecular photoacid that builds on the increased basicity of a guest bound to CB7 and on the extremely high affinity of the E isomer, which is utilized to displace the acid from CB7, thereby switching the pH of the solution.