Metal organic frameworks (MOFs) are coordination polymers containing metal ions and organic ligands that form porous 3D structures. Guest molecules can be encapsulated within the pores of the MOF crystals, and subsequently released by diffusion. Recently we have shown that treating MOF-5 containing guest molecules with bulky carboxylate ligands blocks MOF pores and prevents guest diffusion; however, the basis for this guest trapping phenomenon remains unclear. Three guests of differing sizes: crystal violet, azobenzene, and 4-nitrophenol were encapsulated in MOF-5 and the number of guest molecules within the MOF was quantified. On average, each MOF-5 unit cell contains 1-3 molecules of crystal violet, 3-5 molecules of azobenzene, or 1-2 molecules of 4-nitrophenol. Identical MOF-5 samples containing each of the three guests were capped with triphenylacetic acid (TPAA), and the leakage of the guests subsequently was monitored by UV-vis spectroscopy. Capped MOFs showed dramatic reductions of guest release compared to untreated samples. The location of the capping coordination was also interrogated by X-ray photoelectric spectroscopy (XPS), which can identify surface elements of materials. A fluorinated TPAA derivative was synthesized and used as a capping group since fluorine exhibits a distinct peak in XPS. Preliminary XPS measurements were inconclusive, so future efforts will seek to improve the experimental protocols so that the location and nature of MOF capping can be elucidated. Details about the MOF sealing will allow suitable capping groups to be chosen for various combinations of guests and MOFs.

Trapping guest molecules in MOF-5 with bulky capping groups

2019 Spring National Meeting

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