Photoresponsive Organic Cages – Computationally Driven Discovery of Azobenzene-Derived Organic Cages

The incorporation of photoresponsive groups into porous materials is attractive as it offers potential advantages in controlling the pore size and selectivity to guest molecules. A combination of computational modelling and experiment resulted in the synthesis of two azobenzene-derived organic cages based on building blocks identified in a computational screen. Both cages incorporate three azobenzene moieties, and are therefore capable of three-fold isomerization, using either ditopic or tetratopic aldehydes containing diazene functionality. The ditopic aldehyde forms a Tri2Di3 cage via a 6-fold imine condensation and the tritopic aldehyde forms a Tet3Di6 cage via a 12-fold imine condensation. The relative energies and corresponding intrinsic cavities of each isomeric state were computed, and the photoswitching behaviour of both cages was studied by UV–Vis and 1H NMR spectroscopy, including a detailed kinetic analysis of the thermal isomerization for each of the EEZ, EZZ and ZZZ metastable isomers of the Tet3Di6 cage. Both cages underwent photoisomerization, where a photostationary state of up to 77% of the cis-isomer and overall thermal half-life of 110 hours was identified for the Tet3Di6 species. Overall, this work demonstrates the potential of computational modelling to inform the design of photoresponsive materials and highlights the contrasting effects on the photoswitching properties of the azobenzene moieties on incorporation into the different cage species.