Probing the ‘elephant’: On the essential difference between graphenes and polycyclic aromatic hydrocarbons

Abstract

There is a crucial difference between the structure of a polycyclic aromatic hydrocarbon (PAH) molecule and that of a single-layer sp2-hybridized carbon cluster; its misunderstanding leads to unresolvable inconsistencies when attempting to rationalize the behavior of graphenes across their impressively wide spectrum of existing or potential applications. The PAHs are not ‘nanographenes’, as is too often assumed, because not all the edges in graphenes are H-saturated. This increasingly obvious fact has profound implications for both ‘physical’ (e.g., magnetism, luminescence, thermoelectric power) and ‘chemical’ properties (e.g., functionalization, adsorption, reactivity, electrocatalysis) of graphene-based carbon materials. As revealed by computational quantum chemistry, the triplet ground state of graphenes, containing carbene-like active sites at the zigzag edge or in basal-plane vacancies, offers a consistent explanation for their wide-ranging physicochemical properties and behavior. The most salient consequences of such σ-π coupling of valence electrons, as well as of tailorable singlet-triplet and HOMO-LUMO gaps, account not only for the creation of holes in the conduction band, and thus positive thermoelectric power, but also for the pH-sensitive basicity of oxygen-free graphene-based materials, as well as the ubiquitous (electro)chemical interactions with molecular oxygen including the transition from a peroxy to dioxiranyl intermediate.