The synthesis and structure of an azobenzene functionalized isoreticular metal-organic framework (azo-IRMOF-74-III) [Mg 2 (C 26 H 16 O 6 N 2 )] are described and the ability to controllably release a guest from its pores in response to an external stimulus has been demonstrated. Azo-IRMOF-74-III is an isoreticular expansion of MOF-74 with an etb topology and a 1-D hexagonal pore structure. The structure of azo-IRMOF-74-III is analogous to that of MOF-74, as demonstrated by powder X-ray diffraction, with a surface area of 2410 m 2 g À1 BET. Each organic unit within azo-IRMOF-74-III is decorated with a photoswitchable azobenzene unit, which can be toggled between its cis and trans conformation by excitation at 408 nm. When propidium iodide dye was loaded into the MOF, spectroscopic studies showed that no release of the luminescent dye was observed under ambient conditions. Upon irradiation of the MOF at 408 nm, however, the rapid wagging motion inherent to the repetitive isomerization of the azobenzene functionality triggered the release of the dye from the pores. This light-induced release of cargo can be modulated between an on and an off state by controlling the conformation of the azobenzene with the appropriate wavelength of light. This report highlights the ability to capture and release small molecules and demonstrates the utility of self-contained photoactive switches located inside highly porous MOFs.

As the world transitions from fossil fuels to clean energy sources in the coming decades, many technological challenges will require chemists and material scientists to develop new materials for applications related to energy conversion, storage, and efficiency. Because of their unprecedented adaptability, metal-organic frameworks (MOFs) will factor strongly in this portfolio. By utilizing the broad synthetic toolkit provided by the fields of organic and inorganic chemistry, MOF pores can be customized to suit a particular application. Of particular importance is the ability to tune the strength of the interaction between the MOF pores and guest molecules. By cleverly controlling these MOF-guest interactions, the chemist may impart new function into the Guest@MOF materials otherwise lacking in vacant MOF. Herein, we highlight the concept of the Guest@MOF as it relates to our efforts to develop these materials for energy-related applicatons. Our work in the areas of H2 and noble gas storage, hydrogenolysis of biomass, light-harvesting, and conductive materials will be discussed. Of relevance to light-harvesting applications, we report for the first time a postsynthetic modification strategy for increasing the loading of a light-sensitive electron-donor molecule in the pores of a functionalized MIL-101 structure. Through the demonstrated versatility of these approaches, we show that, by treating guest molecules as integral design elements for new MOF constructs, MOF science can have a significant impact on the advancement of clean energy technologies.

Non-monotonic catalytic behavior in an isoreticular MOF series results from subtle structural differences that alter the substrate binding energy.