Programmed Assembly of Porous Organic Frameworks

Dr. Michael D. Ward
Department of Chemical Engineering and Materials Science
University of Minnesota

Date: Thursday, January 27, 2005
Time: 4:00 p.m.
Place: Engineering II, Room 3361

ABSTRACT

Organic materials promise useful solid-state properties through proper design and synthesis of their molecular components. This potential often is thwarted, however, by the inability to guide the assembly of molecules into the specific solid-state structures required for selected properties. Even the most innocent structural modification to a molecular component can lead to completely unanticipated solid-state structures, reflecting the delicate and non-covalent nature of the intermolecular interactions that govern molecular assembly. Consequently, crystal structure prediction, even with the emergence of advanced computational methods, remains one of the foremost challenges in organic solid-state chemistry. In a rare case of true “crystal engineering,” an extensive family of molecular crystals with predictable and controllable lattice architectures has been prepared by using a structurally robust two-dimensional hydrogen-bonding network consisting of complementary guanidinium ions and sulfonate moieties. The persistence of this network permits the synthesis of lamellar host frameworks in which organodisulfonates serve as “pillars” to create well defined and adjustable pore sizes, shapes and physicochemical characteristics. Guest molecules serve as templates that direct the assembly of many compositionally identical framework isomers, which are distinguished by the connectivity topology of the pillars. The dependability of these lamellar architectures simplifies crystal design and enables strategies for retrosynthetic de novo design of polar crystals for second harmonic generation and separations of isomeric organic compounds through selective inclusion. Organo mono sulfonate inclusion compounds, which are not constrained to lamellar architectures, can be coerced to form hexagonal cylinder phases in which the compliant guanidinium-sulfonate network achieves a positive curvature. The lamellar and cylindrical architectures can be organized into structural “phase diagrams,” which is reminiscent of “soft matter” microstructures and provokes questions about structural compliance, curvature and length scale. The hexagonal cylinder phases also serve as a platform for networks of cylinders connected by tritopic linkers, illustrating an unusual example of “topological transference” with rigorous preservation of crystal symmetry. Collectively, these studies illustrate that control of crystal structure can be achieved with persistent supramolecular networks, particularly if these networks are sufficiently compliant so that optimized packing of organic components can be achieved.