Photoresponsive Organic Cages─Computationally Inspired 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 modeling 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 3-fold isomerization, using either ditopic or tetratopic aldehydes containing diazene functionality. The ditopic aldehyde forms a Tri ²Di ³ cage via a 6-fold imine condensation and the tritopic aldehyde forms a Tet ³Di ⁶ cage via a 12-fold imine condensation. The relative energies and corresponding intrinsic cavities of each isomeric state were computed, and the photoswitching behavior 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 Tet ³Di ⁶ cage. Both cages underwent photoisomerization, where a photostationary state of up to 77% of the cis-isomer and overall thermal half-life of 110 h was identified for the Tet ³Di ⁶ species. Overall, this work demonstrates the potential of computational modeling 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.