Luminescence and Photolysis in Organic- Inorganic Hybrid Coordination Polymers

Yan, Jingjing

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Luminescence and Photolysis in Organic- Inorganic Hybrid Coordination Polymers

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Azobenzene has become a ubiquitous component of functional molecules and polymeric materials because of the light-induced trans to cis isomerization of the diazene group. In contrast, there are very few applications utilizing azobenzene luminescence since excitation energy typically dissipates via non-radiative pathways. Inspired by our earlier studies with 2,2'-bis[N,N'-(2-pyridyl)methyl]diaminoazobenzene (AzoAMoP) and related compounds, we investigated a series of five aminoazobenzene derivatives and their corresponding silver complexes. Four of the aminoazobenzene ligands, which exhibit no emission under ambient conditions, form silver coordination polymers that are luminescent at room temperature. AzoAEpP (2,2'-bis[N,N'-(4- pyridyl)ethyl]diaminoazobenzene) assembles into a three- dimensional coordination polymer (AgAAEpP) that undergoes a reversible loss of emission upon the addition of metal coordinating analytes like pyridine. The switching behavior is consistent with the disassembly-reassembly of the coordination polymer driven by displacement of the aminoazobenzene ligands by coordinating analytes. Luminescent metal-organic frameworks (MOFs) have been explored extensively as potential probes for nitroaromatic molecules, which are common constituents of explosive devices. Guest encapsulation within MOF pores is often cited as the prerequisite for emission changes, but the evidence for this signal transduction mechanism is often inadequate. Using the unique bipyridyl ligand AzoAEpP (2,2'-bis[N,N'-(4-pyridyl)ethyl]diaminoazobenzene), we constructed two luminescent pillared paddle-wheel Zn2+ MOFs using aryl dicarboxylate ligands benzene 1,4-dicarboxylic acid (ABMOF-1) and 1,4-aphthalenedicarboxylic acid (ABMOF-2). While both MOFs exhibit luminescence, 2,4-dinitrophenol only extinguishes ABMOF-1 emission. Since the size of the pores in ABMOF-1 preclude guest inclusion, we used X-ray photoelectron spectroscopy (XPS) to confirm the surface interaction and obtain insight into the nature of the quenching process. XPS experiments utilized a fluorinated nitroaromatic molecule, 4-trifluoromethyl-2,6-dinitrophenol, that extinguishes ABMOF-1 emission, and verified surface adsorption through a series of angle-resolved (ARXPS) and argon-ion sputter depth profile experiments. By further developing these techniques, we hope to develop a general instrumental approach for distinguishing between the various intermolecular interactions between MOFs and analytes that lead to changes in luminescence. Photoswitchable components can modulate the properties of metal organic frameworks (MOFs); however, photolabile building blocks remain underexplored. A new strut NPDAC (2-nitro-1,4-phenylenediacetic acid) that undergoes photodecarboxylation has been prepared and incorporated into a MOF using post-synthetic linker exchange (PSLE) from the structural analogue containing PDAC (p- phenylenediacetic acid). Irradiation of NPDAC-MOF leads to MOF decomposition and concomitant formation of amorphous material. In addition to complete linker exchange, MOFs containing a mixture of PDAC and NPDAC can be obtained through partial linker exchange. In NPDAC30-MOF which contains approximately 30% NPDAC, the MOF retains crystallinity after irradiation, but the MOF contains defect sites consistent with loss of decarboxylated NPDAC linkers. The defect sites can be repaired by exposure to additional PDAC or NPDAC linkers at a much faster rate than the initial exchange process. The photoremoval and replacement process may lead to a more general approach to customizable MOF structures. The combination of high surface area and large void spaces make metal organic frameworks (MOFs) attractive for guest encapsulation; however, guest release is almost exclusively dictated by intermolecular forces, so engineering the desired rate of delivery can require on an overly empirical approach. For applications such as drug delivery, complete control over the temporal and spatial facets of guest release are desirable to minimize both waste and adverse side effects. Recently, we demonstrated that triphenylacetic acid could be used to seal dye molecules within MOF-5, but guest release required digestion of the framework by treatment with acid. Inspired by our photodecarboxylation strategy to control the release of metal ions, we synthesized the sterically bulky photocapping group [bis-(3-nitro-benzyl)-amino]-(3-nitro-phenyl)-acetic acid (PC1) to explore light-initiated guest release. A comparative study using PC1 and the smaller analog 3-nitrophenylacetic acid (PC2) demonstrated that crystal violet (CV) could be encapsulated in MOF-5 and released by photodecarboxylation of the capping group.

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