Reactions at Graphenic Surfaces

We have considerable interest in carbon nanomaterials, including carbon nanotubes and graphenes.  Conducting chemistry at these surfaces is complicated by the fact that the full range of analytical methods that we rely on for small molecule chemistry, especially 1H NMR spectroscopy and single crystal X-ray structure determinations, are not possible on these systems.  We have taken a long-term approach to this area conducting detailed analysis on our materials to confirm the chemical processes that are taken for granted by many in this area.  We typically make use of AFM, SEM, TEM, XPS, Raman, TGA, UV-Vis-NIR, and other methods that are not common for synthetic chemists.  Our synthetic methods at the π-surfaces have enabled some powerful new applications of these materials ranging from sensors to surfactants to structural materials.

The graphene surface allows for dynamic reactions to occur.  We have a number of ongoing efforts in this area, but we were the first to demonstrate concerted rearrangements on the surfaces (Collins, W. R.; Lewandowski, W.; Schmois, E.; Walish, J., Swager, T. M. “Claisen Rearrangement of Graphite Oxide:  A Route to Covalently Functionalized Graphenes” Angew. Chem. Int. Ed. Engl.  2011, 50, 8848-8852).

The first example of a pericyclic reaction at a graphene surface.

We have also been interested in the use of the formal addition of nitrenes to surfaces, and these systems are proposed to have minimal perturbations on the carrier transport.  Below is an example of the coupling of a receptor to the surface.  These structures and their ability to undergo the reversible complexation of methyl-ammonium ions was confirmed on the CNTs by 31P solid state NMR and resistivity measurements (Dionisio, M.; Schnorr, J. M.; Michaelis, V. K.; Griffin, R. G.; Swager, T. M. Dalcanale, R. “Cavitand-Functionalized SWCNTs for N-methylammonium Detection” J. Am. Chem. Soc.  2012, 134, 6540-6543).

Connection of sophisticated receptors creates sensors selective for methyl ammonium ions.

We often conduct measurements on fullerenes, which allow for precise characterization.  We have used this to create metal complexes and materials having electronic interactions through space that modify the electron affinities and use designer interfacial assembling polymers to create superior solar cells (Lobez, J. M.; Andrew, T. L.; Bulovic, V.; Swager, T. M. “Side-Chain Functionalized Polythiophene Additives for Improved Efficiency in Bulk Heterojunction Solar Cells” ACS Nano, 2012, 6, 3044-3056).  The epoxidation of the alkene was shown to modify the electron affinity by removing the through-space interaction between the fullerene and the cyclobutene (Han, G. D.; Collins, W. R.; Andrew, T. L.; Bulović, B.; Swager, T. M. “Cyclobutene–C60 Adducts: N-Type Materials for Organic Photovoltaic Cells with High VOCAdv. Funct. Mater. 2013, 23, 3061-3069).

Designer fullerenes modify the electron affinity of for improved solar cell performance.

We have also developed precision methods for the attachment of aromatic groups through C-C bonds using iodonium reagents.  These methods have been used to create anchoring sites to produce sensors, such as the bio-inspired CO sensor based on the immobilization of iron porphyrins to CNTs.  In addition, the latter method was used to demonstrate what we refer to as voltage modulated chemiresistors.  These sensors are similar to field effect transistors, and the system undergoes internal redox reactions, wherein areas of the sensor construction can be made to be rich in Fe(II) for CO binding to enhance the sensor responses (Savagatrup, S.; Schroeder, V.; He, X.; Lin, S.; He, M.; Yassine, O.; Salama, K. N.; Zhang, X.; Swager, T. M.  “Bio-Inspired Carbon Monoxide Sensors with Voltage-Activated Sensitivity” Angew. Chem. 2017, 56, 14066-14070.)

Biology inspired CO sensors that make use of a field effect transistor configuration.

Graphite is the most stable allotrope of carbon and hence its activation is challenging.  Most methods involve harsh reactions that introduce many defects and even holes in the structures.  We have invested considerable effort into the use of electrochemical methods for graphite activation to form what we call hyper-stage graphites, wherein there are multiple layers of molecules intercalated between individual sheets (Jeon, I., Yoon, B.; He, M.; Swager, T. M. “Hyperstage Graphite: Electrochemical Synthesis and Spontaneous Reactive Exfoliation” Adv. Mater. 2017,  1704538).  Materials that have disordered molecular layers between the graphene sheets are very activated and can be easily dispersed to give defect free graphene or reacted to give graphenes with record levels of covalent functionalization (1 group for every 12 graphene carbons!).  This functionalization is enabling a number of new applications.

X-ray diffraction of electrochemically prepared hyperstage graphene.  The final material spontaneously dissolves with record levels of functionalization.

The dramatic expansion of the graphene can be seen in the video below.  Note a small flexible filament of highly ordered pyrolytic graphite (HOPG) grows to many times in width.