Functionalization of Graphene Flake Surfaces With p-Substituted Phenyl Radicals: Estimation of Activation Energies From Hammett Substituent Constants

Chemical modification of flat graphene surfaces composed of pure sp² carbons is extremely challenging. In this study, we used density functional theory to investigate the addition of phenyl radical derivatives to the surface of graphene flakes and clarified the effect of the nature of the substituents on the activation energy using Hammett plots. Circumcoronene was used as a model for the graphene flakes, and 13 para-substituted phenyl radicals were used as radical species. The activation energy consists of the distortion energies of the graphene flakes and radical species and the interaction energy between the graphene flakes and radical species. The interaction energy is 30%–40% of the activation energy. The interaction energy between the nitrophenyl radical and graphene flake is 3.1 kcal/mol, 2.3 kcal/mol lesser than that between the N,N-dimethylamino-phenyl radical and graphene flake. As the electron-withdrawing properties of the substituents increase, the interaction energy decreases, and consequently, the activation energy decreases. Furthermore, natural energy decomposition analysis shows that the intermolecular interactions are stabilized by charge transfer and electrostatic interactions, the magnitudes of which increase when increasing the Hammett constants of the substituents.