Materials Organic Chemistry:
Research in our lab focuses on the design, synthesis, and analysis of new organic materials utilizing molecular recognition, self-assembly, and dynamic covalent chemistry. By themselves, and especially in combination, molecular recognition, self-assembly, and dynamic covalent reactions have enabled molecules and supramolecules of incredible complexity to be constructed from relatively simple starting materials. The breadth and modularity of these synthetic approaches makes them amenable to address questions and solve problems across multiple chemical, environmental, biological, and materials disciplines. Our interests range from the development of stimuli-responsive mechanically interlocked polymers, to the dynamic synthesis of covalent organic polygons and nanoparticles, to the preparation of artificial receptors for sensing and industrial waste remediation applications. We rely on a variety of tools - NMR and UV-Vis spectroscopies, isothermal titration and differential scanning calorimetries, Atomic Force Microscopy (AFM), dynamic light scattering (DLS), computational modeling, and others - to understand the formation, structure, and functional properties of the new materials developed in our lab. Though our research is interdisciplinary, our interests in noncovalent self-assembly and dynamic processes are found throughout all projects.

Mechanically Interlocked Molecules
Molecules can be connected or associated with each other by a number of different means, with covalent bonds, noncovalent interactions, and metal-ligand coordination being the most common. Mechanical bonds, on the other hand, represent a more "exotic" manner of joining two or more molecules. Catenanes, for example, are molecules composed of two macrocyclic rings looped through each other so that they are mechanically bound and cannot be separated. Developing ways of controlling the positions of two mechanically interlocked molecules relative to each other has allowed interlocked molecules to be developed into a variety of "molecular machines" such as motors, muscles, and switches. Less explored, however, have been mechanically interlocked polymer systems. One route to their synthesis is shown schematically in Scheme 1A. It is expected that mechanically interlocked polymers will have properties that differ from purely covalent polymers. In particular, because monomer units can move relative to each other without breaking the polymer they will likely be more resistant to stress. It is also possible to prepare responsive interlocked polymers whose properties change with external stimuli. The synthesis of mechanically interlocked polymers is very modular, and small variations in their structure may generate many different polymers with widely varying properties. A variety of molecular recognition motifs and self-assembling systems (Scheme 1B) are also being studied en route to the synthesis of mechanically interlocked polymers.

Scheme 1. (A) Graphical representation of the synthesis of a mechanically interlocked polymer from the self-assembly and polymerization of two complementary molecules. (B) The template-directed dynamic self-assembly of a supramolecular host-guest complex.

Covalent Organic Polygons and Nanoparticles

The condensation of boronic acids with organic diols provides an efficient and versatile route to boronate esters. This synthetic protocol has been applied to the dynamic assembly of covalent organic frameworks (COFs) and complex macrocycles and cage compounds. In our lab we prepare fairly simple boronic acids and organic diols, which we can then condense to form structures such as covalent organic polygons (Scheme 2) and synthetic nanoparticles. The judicious design of target molecules allows for a range of organic polygons and nanoparticles of well-defined shapes, sizes, and geometries to be synthesized. These structures are not only interesting themselves but they also have the potential to be self-assembled onto solid substrates. The conformations and stability of their surface monolayers can be investigated using scanning probe microscopy techniques such as AFM and STM. Covalent organic polygons may ultimately be used to pattern and systematically functionalize solid substrates for the purposes of developing hybrid solid-organic materials.

Scheme 2. The condensation of anthracene-9,10-diboronic acid with a triphenylene tetraol derivative to generate a hexagonal covalent organic polygon.

Artificial Receptors
Our group is also interested in the design and synthesis of novel artificial receptor molecules. Specifically, we are interested in developing new derivatives of cucurbiturils and glycolurils (Figure 1). Cucurbiturils - whose name derives from their resemblance to pumpkin cucurbitaceae plants - are known to bind hydrophobic guest molecules strongly within their hollow interior cavity and have found uses in catalysis, chemical separations, and in drug delivery systems. The very poor solubility of cucurbiturils, however, has hindered much of their development. We are synthesizing new, soluble variants of cucurbiturils and investigating their receptor properties. Our lab is also involved in the synthesis of new glycoluril derivatives, especially polymerizable glycoluril molecular "clips." We hope to apply these artificial receptors to environmental remediation given their potential to bind hydrophobic guests such as polychlorinated contaminants, which are commonly formed as waste products during industrial processes that involve chlorine. Other potential applications range from biomolecular sensing to enzyme mimetics to synthetic "molecular Velcro."

Figure 1. Chemical structures of cucurbit[6]uril and a glycoluril-based molecular clip.

• Li, S.-S.; Northrop, B. H.; Yuan, Q.-H.; Wan, L.-J.; Stang, P. J. "Surface Confined Metallosupramolecular Architectures: Formation and STM Characterization." Acc. Chem. Res. 2009, 42, 249-259.
   
• Northrop, B. H.; Yang, H.-B.; Stang, P. J. "Exploring the Limits of Self-Organization in the Self-Assembly of Supramolecular Metallacycles." Inorg. Chem. 2008, 47, 11257-11268.
   
• Northrop, B. H.; Yang, H.-B.; Stang, P. J. "Coordination-Driven Self-Assembly of Functionalized Supramolecular Metallacycles." Chem. Commun. 2008, 5896-5908.
   
• Wang, L.; Chen, Q.; Pan, G.-B.; Wan, L.-J.; Zhang, S.; Northrop, B. H.; Stang, P. J.
  "Nanopatterning of Donor/Acceptor Hybrid Supramolecular Architectures on HOPG: An STM Study." J. Am. Chem. Soc. 2008, 130, 13433-13441.
   
• Northrop, B. H.; Glöckner, A.; Stang, P. J. "Functionalized Hydrophobic and Hydrophilic Self-Assembled Supramolecular Rectangles." J. Org. Chem. 2008, 73, 1787-1794.
   
• Li, S.-S.; Yan, H.-J.; Wan, L.-J.; Yang, H.-B.; Northrop, B. H.; Stang, P. J. "The Control of Supramolecular Rectangle Self-Assembly with a Molecular Template." J. Am. Chem. Soc. 2007, 129, 9268-9269.
   
• Northrop, B. H.; Norton, J. E.; Houk, K. N. "On the Mechanism of Peripentacene Formation from Pentacene: Computational Studies of a Prototype for Graphene Formation from Smaller Acenes." J. Am. Chem. Soc. 2007, 129, 6536-6546.
   
• Williams, A. R.; Northrop, B. H.; Chang, T.; Stoddart, J. F.; White, A. J. P.; Williams, D. J. "Suitanes"  Angew. Chem. Int. Ed. 2006, 45, 6665-6669.
   
• Northrop, B. H.; Aric¢, F.; Tangchiavang, N.; Badjić, J. D.; Stoddart, J. F. "Template-Directed Synthesis of Mechanically Interlocked Molecular Bundles using Dynamic Covalent Chemistry." Org. Lett. 2006, 8, 3899-3902.
   
• Brough, B.; Northrop, B. H.; Schmidt, J. J.; Tseng, H.-R.; Houk, K. N.; Stoddart, J. F.; Ho, C.-M. "Evaluation of Synthetic Linear Motor-Molecule Actuation Energetics." Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 8583-8588.
   
• Northrop, B. H.; O?Malley, D. P.; Zografos, A. L.; Baran, P. S.; Houk, K. N. "The Mechanism of the Vinylcyclobutane Rearrangements of Sceptrin to Ageliferin and Nagelamide E." Angew. Chem. Int. Ed. 2006, 45, 4126-4130.
   
• Braunschweig, A. B.; Northrop, B. H.; Stoddart, J. F. "Structural Control at the Organic-Solid Interface." J. Mater. Chem. 2006, 16, 32-44.
   
• Northrop, B. H.; Houk, K. N. "The Vinylcyclobutane-Cyclohexene Rearrangement: Theoretical Exploration of Mechanism and Relationship to the Diels-Alder Potential Surface.' J. Org. Chem. 2006, 71, 3-13.
   
• Lui, Y.; Flood, A. H.; Bonvallet, P. A.; Vignon, S. A.; Northrop, B. H.; Tseng, H.-R.; Jeppesen, J. O.; Huang, T. J.; Brough, B.; Baller, M.; Magonov, S.; Solares, S. D.; Goddard, W. A.; Ho, C.-M.; Stoddart, J. F. "Linear Artificial Molecular Muscles." J. Am. Chem. Soc. 2005, 127, 9745-9759.