Chemistry at the Dirac Point: Diels–Alder Reactivity of Graphene

Abstract

Most of the interesting physics of graphene results from the singular electronic band structure at the so-called Dirac point, where the conduction and valence bands cross in momentum space. Although graphene is very stable thermodynamically, the electronic structure at the Dirac point facilitates basal plane chemistry including pericyclic reactions such as the Diels-Alder reaction. We have discovered a series of facile Diels-Alder reactions in which graphene can function either as a diene when paired with tetracyanoethylene and maleic anhydride or as a dienophile when paired with 2,3-dimethoxybutadiene and 9-methylanthracene. In this Account, we seek to rationalize these findings using simple arguments based on considerations of orbital symmetry and the frontier molecular orbital theory. The graphene conduction and valence bands (HOMO and LUMO) cross at the Dirac point, which defines the work function (W = 4.6 eV). Thus, the HOMO and LUMO form a degenerate pair of orbitals at this point in momentum space with the same ionization potential (IP) and electron affinity (EA). Based on the importance of the energies of the HOMO (-IP) and LUMO (-EA) in frontier molecular orbital (FMO) theory, graphene should be a reactive partner in Diels-Alder reactions due to the very high-lying HOMO and low-lying LUMO (energies of -4.6 eV). Inspection of the orbital symmetries of the degenerate pair of half-occupied band orbitals at the Dirac point confirms that with the appropriate orbital occupancies, both diene and dienophile reaction partners should undergo concerted Diels-Alder reactions with graphene that are allowed based on the Woodward-Hoffmann principles of orbital symmetry.

Graphene-Based Polymer Nanocomposites: Chemistry and Applications

“…Graphene is known as a Diels-Alder substrate due to its ability to function as either diene or dienophile (Fig. 5) [113]. The covalent functionalization of graphene via Diels-Alder reactions is an efficient approach to reverse engineering the conductivity of graphene for electronic and optical applications [112].…”

Organometallic Chemistry of Carbon Nanotubes and Graphene

“…This type of chemistry has been widely used as a means for bandgap engineering in graphene [29,30], and examples include hydrogenation [31,32], halogenation [29,33], oxidation (ozonolysis) [34], carboxylation [35], hydroxylation and epoxidation [36][37][38][39], radical additions [2,4,16,30,[40][41][42][43][44][45][46][47], carbene addition [48][49][50], nitrene addition [51,52], Diels-Alder reactions [17,53] including benzyne addition [54,55] and 1,3-dipolar cycloaddition [56] reactions. The graphene derivatives isolated to date, when incorporated in electronic field effect devices, show low conductivity and significantly reduced carrier mobility.…”

“…All of the extended periodic π-electron graphitic structures are narrow or zero bandgap materials, and thus the electrondonor and electron-acceptor interactions between the HOMOs and LUMOs of the π-systems, and the d-orbitals of the transition metals will be enhanced by the high-lying HOMO and low-lying LUMO of the graphitic surfaces [9,17].…”

“…Fullerenes [10][11][12][13] and CNTs [14] are curved carbon materials with demonstrated ability to serve as primary ligands, whereas graphene represents a new class of periodic, planar two-dimensional π-ligands which has been recently reported to have a rich organometallic chemistry [6], in analogy with the coordination chemistry of polyaromatic hydrocarbons (PAHs) [15]. The flat two-dimensional π-surface of graphene exhibits a unique chemical reactivity as a result of the electronic structure at the Dirac point and this provides the opportunity to perform a wide range of chemical reactions [6,9,16,17]. Functionalization of CNTs with transition metal complexes alters their electronic and chemical properties and single-walled carbon nanotubes (SWCNTs) have been already used as transition metal supports in heterogeneous [18][19][20] and homogeneous catalysis [5].…”

Multi-type quantum dots photo-induced doping enhanced graphene/semiconductor solar cell

“…33 The electron affinity of Gr (the D-value of vaccum level and Dirac point) is about 4.6 eV (ref. 34) and its work function is varied depending on the doping concentration. Therefore, a depletion region is formed in the interface of heterostructure by a build-in barrier under van der Waals force due to a clear difference of work function between Gr and GaAs.…”