In the Barbour group we exploit the principles of Crystal Engineering to design new functional materials. In particular, we are interested in the encapsulation of small compounds by noncovalent molecular assemblies. This includes sorption of gases into lattice interstices as well as the capture of substrates in molecular capsules or tubules assembled by means of hydrogen bonding or amphiphilic interactions.
Molecular Encapsulation: We are interested in the controlled assembly of large molecular capsules. Although we derive much of our inspiration from biological systems, the formation of capsules matching the size and complexity encountered in living organisms is currently beyond the synthetic grasp of the chemist. However, we note that viral capsids consist of relatively simple geometrical arrangements of molecules (i.e. polyhedra, helices or a combination of these two morphologies). Since polyhedral arrangements of molecules are often encountered in crystals, we have based our initial approach on the study of supramolecular assemblies in the solid state. We believe that many of the principles governing the formation and stability of self-assembled containers can be unraveled by such studies. By exploiting interactions such as hydrogen bonding and amphiphilic contacts, we have assembled complex, multicomponent capsules with internal volumes of up to 1300 Å3.
Gas Sorption: Crystals composed of purely organic compounds have largely been ignored as gas sorption substrates since the molecules are usually efficiently packed. Packing efficiencies of such materials generally lie in the narrow range of 60 to 67%, while void spaces larger than 25Å3 are seldom encountered. The host lattices of inclusion compounds such as solvates are often described as possessing zero−, one−, two− or three-dimensional solvent-accessible voids if the solvent molecules are located in isolated cavities, channels, layers or networks of channels, respectively. It is attractive to envision that the solvent molecules can be removed from these materials to yield highly porous host lattices analogous to those of zeolites.
In reality, the process of desolvation is almost always accompanied by reassembly of the host molecules in the solid state to form one or more so-called apohost phases, where the pure compound is once again efficiently packed. However, a few exceptions to this phenomenon are known to exist. For example, the apohost phase of calixarene, grown by sublimation at 300ºC under vacuum, forms the same host framework as a series of solvated phases of the compound (Science 2002, 296, 2367). The solvent-accessible volume of the zero-dimensional lattice void is ~153Å3, and it is possible to entrap and stabilize volatile substances such as freons, halons and methane in these interstices at temperatures well above their normal boiling points.
A low-density apohost phase of sublimed p-tert-butylcalixarene possesses zero-dimensional lattice voids of ~235Å3 (Science 2002, 298, 1000). Despite an apparent lack of porosity (i.e. channels that access the voids), these crystals readily and reversibly absorb volatile gases such as N2, O2, CO2 and CH4 (Angew. Chem. Int. Ed. 2004, 43, 2948; Chem. Commun. 2005, 51) at room temperature and relatively low pressures. Since no uptake of hydrogen gas is observed under these conditions, this material can be utilized to separate H2 and CO2 from a mixture of these gases. We aim to further these studies by exploring new systems of organic compounds that are unable to pack efficiently. Specifically, we target molecules that have poor self-complementarity of shape, thus ensuring the presence of lattice voids as potential sites for gas storage.
Metal-Organic Frameworks: The long-standing challenge of designing and constructing new crystalline solid-state materials from molecular building blocks is just beginning to be addressed with success. A conceptual approach that requires the use of secondary building units to direct the assembly of ordered frameworks epitomizes this process: we call this approach reticular synthesis. This chemistry has yielded materials designed to have predetermined structures, compositions and properties. In particular, highly porous frameworks held together by strong metal–oxygen–carbon bonds and with exceptionally large surface area and capacity for gas storage are being prepared and their pore metrics systematically varied and functionalized.
- A crystalline organic substrate absorbs methane under STP conditions. J. L. Atwood, L. J. Barbour, P. K. Thallapally and T. B. Wirsig, Chem. Commun. 2005, 51.
- Towards mimicking viral geometry with metal-organic systems. J. L. Atwood, L. J. Barbour, S. J. Dalgarno, M. J. Hardie, C. L. Raston and H. R. Webb, J. Am. Chem. Soc. 2004, 126, 13170.
- A new type of material for the recovery of hydrogen from gas mixtures. J. L. Atwood, L. J. Barbour and A. Jerga, Angew. Chem. Int. Ed. 2004, 43, 2948.
Featured in: News of the Week, C&E News 2004, 82, 7; Process 2004, Issue 6, p8; Science News, 2004, 165, 380.
- Polymorphism of pure p-tert-butylcalixarene: subtle thermally-induced modifications. J. L. Atwood, L. J. Barbour, G. O. Lloyd and P. K. Thallapally, Chem. Commun. 2004, 922.
- Polymorphism of pure p-tert-butylcalixarene: conclusive identification of the phase obtained by desolvation. J. L. Atwood, L. J. Barbour and A. Jerga, Chem. Commun. 2002, 2952.
- Guest transport in a non-porous organic solid via dynamic van der Waals cooperativity. J. L. Atwood, L. J. Barbour, A. Jerga and B. L. Schottel, Science 2002, 298, 1000.
Featured in: News of the Week, C&E News 2002, 80(44), 8; J. W. Steed, Perspectives in Science 2002, 298, 976; Chemistry Highlights 2002, C&E News 2002, 80(50), 44.
- Storage of methane and freon by interstitial van der Waals confinement. J. L. Atwood, L. J. Barbour and A. Jerga, Science 2002, 296, 2367.
Featured in: Science & Technology Concentrates, C&E News 2002, 80(27), 27; B.C. Gibb, Highlights in Angew. Chem. Int. Ed. 2003, 42, 1686; Chemistry Highlights 2002, C&E News 2002, 80(50), 44.
- Supramolecular stabilization of N2H7+. J. L. Atwood, L. J. Barbour and A. Jerga, J. Am. Chem. Soc. 2002, 124, 2122.