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Reports: ND353977-ND3: Photo-Functional Switches and Metal Organic Frameworks Containing Azobenzene Groups

Shawn Burdette, PhD, Worcester Polytechnic Institute

Overview

The following specific aims were proposed originally:

1.     Identify a modification strategy for converting an inactive AB into a one that undergoes isomerization

2.     Design a chelator where coordination traps AB in the cis form

3.     Design AB ligands that serve as the core for MOFs

4.     Determine the extent to which functionalized ABs can assemble MOFs from dinuclear paddle-wheel cores

Research activities in 2014 were primarily focused exclusively on aim 3 and 4 owing to promising results growing metal organic frameworks (MOFs).

3. Design AB ligands that serve as the core for MOFs.

In the original proposal, we described a series of 6 azobenzened-based ligands that we envisioned as components of MOFs, and in preliminary results described the synthesis of 5 ligands and the corresponding silver complexes (Figure 1). We spent so time trying to prepare the sixth member of the group, AzoAMmP, which contains a m-pyridine nitrogen donor and an ethyl spacer. Unfortunately, the necessary precursors, either the m-bromoethyl- or m-acetaldehyde-pyridine appears to be highly unstable. The compounds decompose readily, so obtaining reagents for further reactions has proven impossible.

With the remaining compounds, we have continued working with and characterizing the silver complexes. Notably, all the ligands form 2-dimensional polymers except AzoAEpP, which makes a 3-dimensional material owing to 4-coordinate silver sites as shown by x-ray crystallography (Figure 2). This material has more interesting optical properties and exhibits luminescence in the solid state and when dispersed in solution. We have also attempted to look at its photoactivity. We observe the material is unstable to prolonged exposure to ~350 nm light from a 3 W LED. We suspect photoreduction of silver, however, we are still studying the process and photoproducts as the ligand also decomposes under intense irradiation. We anticipate finishing this investigation by the end of the calendar year.

4. Determine the extent to which functionalized ABs can assemble MOFs from dinuclear paddle-wheel cores.

In the original proposal, we proposed using the bipyridylazobenzenes (Figure 1) in conjunction with different dicarboxylates to assemble dinuclear zinc paddle wheel MOFs. We have successfully assembled MOFs with all the bipyridylazobenzenes except AzoAMP-1 and AzoAEoP. The lack of MOF formation was expected with AzoAMP-1 since we have previously shown that it does not interact with zinc. Likewise, the AzoAEoP ligand did not form a polymer with silver, so the coordination requirements of the ligand seem to disfavor spanning different metal sites. As a representative example, the x-ray crystal structure of a AzoAMmP MOF is shown in Figure 3. These MOFs exhibit differing degrees of luminescence in the solid state and when dispersed in solution. The study of the photoreactivity of these materials is ongoing.

While preparing different MOFs, we made the serendipitous discovery that AzoAEpP assembles into a MOF with benzene-1,4-dicarboxylic acid that contains the azo group in the cis isomer (Figure 4). All the other silver and zinc MOF structures contain trans-azobenzenes. This was unexpected because the energy barrier between the trans and cis isomers in unmodified azobenzene is 12 kcal/mole, and the trans isomers in our ligands are further stabilized by an intramolecular hydrogen bond between the azo nitrogen atom and the anilino hydrogen atoms. Thermal isomerization of the AzoAEpP ligand apparently triggers trapping of the cis-isomer in the MOF.

While we have yet to see any photoactivity in the cis-MOF, we can remove the AzoAEpP ligand chemically. By adding the MOF crystals to pyridine, we can observe the loss of AzoAEpP by UV-vis spectroscopy. The initially clear pyridine solution turns orange-red. Simultaneously, the orange-red crystals are replaced by a white microcrystalline material. We hypothesized that this new material was a two dimensional MOF containing dinuclear zinc paddle wheel complexes capped by pyridine groups. We prepared crystals of this material independently, and obtained the x-ray structure; in addition, we were able to match the powder diffraction pattern of the product of the pyridine displacement assay.

Owing to the unique nature of this MOF, we are focusing our attention on finishing this project. We also anticipate finishing this project within the next 3-6 months.