A photoswitchable tetra-azo macrocycle enabling light-controlled host-in-host binding and release of cucurbit[5]uril

Abstract

Photoresponsive host macrocycles are attractive due to their wide scope of applications in controlled molecular recognition, information storage, switchable catalysis, and smart materials. One of the challenges along this line lies in the construction of large-sized hosts with multiple photoactive groups, e.g., azobenzene, for light-controlled guest capture and release. Herein, we report a giant hydrogen-bonded aramide macrocycle incorporating four azobenzene units that enables light-controlled binding and release of cucurbit[5]uril (CB[5]). Single-crystal X-ray diffraction reveals that the tetra-azo-macrocycle adopts a relatively rigid figure of eight conformation constrained by intramolecular hydrogen bonding within the macrocyclic framework. Photochemical studies demonstrate efficient photoisomerization from the thermally stable E,E,E,E-isomer to other isomeric states, reaching a maximum conversion of 95.7%. The amide oxygen-decorated cavity of the macrocycle selectively accommodates the smaller host CB[5] through multi-point hydrogen-bonding interactions, giving rise to the formation of a host-in-host complex. Most notably, light irradiation triggers the release of CB[5] in the presence of sodium cations, in sharp contrast to previously reported host-in-host systems, in which guest release is typically difficult and relies solely on subtle host shape changes. This work demonstrates a rare example of exploiting a H-bonded aramide macrocycle with multiple azobenzene units for manoeuvring the uptake and release process in response to an external photo-stimulus and also implicates new possibilities for designing multi-state light-responsive materials.