Nanoporous coordination polymers
Extended solids comprised of inorganic and organic components are attracting considerable attention because of their exceptional properties, chemical tailorability, and broad range of potential applications. Recently, a new class of nanoporous coordination polymers known as metal organic frameworks (MOFs) was created that have immense potential for understanding and exploiting molecular interactions in pores. MOFs are crystalline materials with tunable, monolithic pore sizes and cavity properties. Their properties exceed those of virtually all other porous materials, including the lowest density and highest surface area for a crystalline material, tunable photoluminescence, and high capacity for molecular adsorption. These exciting properties are achieved by coupling inorganic clusters with tunable organic ligands that serve as struts, allowing facile manipulation of pore size and surface area through ligand selection. The so-called isoreticular MOFs (for example, see IRMOF-1, Figs. 1, 2) are particularly attractive because of their cubic structure and interchangeable organic linkers.
Fig. 1. The prototypical MOF structure
(IRMOF-1). ZnO4 tetrahedra (blue) are
joined by organic linkers (O, red, C,
black), giving an extended 3D cubic
framework with inter-connected pores
of 11.2 Â aperture width and 18.5Â pore (yellow sphere) diameter [1].
Fig. 2 Space-filling model showing the cubic
nature of IRMOF pores [2].
The development of MOFs for real-world applications is a nascent, but growing area. We are currently working in several areas to realize the exciting potential of these materials:
Fig. 3. Microcantilever design with built-in
piezoresistive stress sensor designed and
fabricated at Georgia Tech by the research
group of Prof. Peter Hesketh. Integration with MEMS devices
The growth of MOF layers on substrates is essential to gaining full use of their properties. At present we are growing MOFs on self assembled monolayers (SAMs) and on oxide materials deposited by atomic layer deposition. We are now able to grow the copper-carboxylate MOF HKUST-1 on microcantilever devices (Fig. 3), as seen in Fig. 4, and have demonstrated that these can be used as chemical sensors.
Luminescent MOFs for radiation detection
The detection and identification of subatomic particles is another important scientific problem with implications for medical devices, radiography, biochemical analysis, particle physics, nuclear nonproliferation, and homeland security. We synthesized new MOFs containing the organic fluorophore stilbene dicarboxylate and find that they emit visible light on nanosecond timescales when irradiated with high-energy protons, alpha particles, and electrons (Fig. 5). A completely new class of scintillation materials is created by this development, with the potential to rationally tailor properties for specific radiation detection applications.
Figure 4. Scanning electron micrograph of a MOF HKUST-1 ([Cu3(TMA)2(H2O)3]n, where TMA = benzene-l,3,5-tricarboxylate) grown on a microcantilever.
Fig. 5.Schematic of scintillation induced by high-energy protons in a MOF.
MOF-adsorbate interactions
Understanding the interactions of solvent molecules with the MOF framework is essential to developing a rational design approach. We are using molecular dynamics to simulate these interactions, employing a non-bonded forcefield we developed for IRMOFs. These calculations predict the limits of stability of IRMOF-1 in the presence of water (Fig. 6), which disrupts the tetrahedral coordination of the Zn(II) ions (Fig. 7).
Figure 6. Simulated lattice parameter as a function of water content. The dashed line indicates the trend toward a much smaller lattice parameter (≈20 Â) at higher water
Fig. 7. Molecular dynamics prediction of the disruption of the IRMOF-1 structure at 2.3 % water. The color scheme is Zn (purple), O (red), C (gray), and H (white), with ZnO4 tetrahedra represented as polygons. The picture shows the formation of hydrogen-bonded chains of water molecules, which disrupt the normal tetrahedral coordination around the Zn(II) ions in the framework.
Publications
- F. P. Doty, C. A. Bauer, A. J. Skulan, P. G. Grant, M. D. Allendorf “Scintillating Metal Organic Frameworks: A New Class of Radiation Detection Materials,” accepted for publication, Adv. Mater. Aug. 2008.
- Greathouse, JA; Allendorf, MD “Force field validation for molecular dynamics simulations of IRMOF-1 and other isoreticular zinc carboxylate coordination polymers,” J. Phys. Chem. C. 112 (2008), 5795-5802.
- D. F. Bahr, J. A. Reid, W. M. Mook, C. A. Bauer, R. Stumpf, A. J. Skulan, N. R. Moody, B. A. Simmons, M. M. Shindel, M. D. Allendorf “Mechanical properties of IRMOF-1 metal-organic framework crystals,” Phys. Rev. B, 76 (2007), 184106.
- C. Bauer, T. Settersten, B. Patterson, T. Timofeeva, V. Liu, B. Simmons, M. D. Allendorf “Influence of connectivity and porosity on ligand-based luminescense in zinc MOFs,” J. Amer. Chem. Soc. 129 (2007), 7136.
- C. Bauer, T. Settersten, B. Patterson, T. Timofeeva, V. Liu, B. Simmons, M. D. Allendorf "Influence of connectivity and porosity on ligand-based luminescense in zinc MOFs," accepted J. Amer. Chem. Soc., March 2007.
- Jeffery A. Greathouse and Mark D. Allendorf "Reactivity of metal-organic framework-5 with water studied by molecular dynamics simulations," JACS, 128 (2006), 10678.
References
- Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O'Keefe, M.; Yaghi, O. M."Systematic design of pore size and functionality in isoreticular MOFs and their application in methane storage," Science 2002, 295, 469.
- Eddaoudi, M.; Moler, D. B.; Li, H.; Chen, B.; Reineke, T. M.; O'Keefe, M.; Yaghi, O. M."Modular chemistry: secondary building units as a basis for the design of highly porous and robust metal-organic carboxylate frameworks," Acc. Chem. Res. 2001, 34, 319.
- Greathouse, J. A.; Allendorf, M. D."The interaction of water with MOF-5 simulated by molecular dynamics," J. Am. Chem. Soc. 2006, 106, 10678.
