Polaronic relaxation by three-electron bond formation in graphitic carbon nitrides

We apply density functional and ionization potential equation of motion coupled cluster theories to investigate hole transport in graphitic carbon nitride (g-C₃N₄), an organic photocatalyst which drives water splitting and oxidative organic reactions. Calculations on small cationic model clusters suggest that the formation of two-center, three-electron bonds involving lone pair electrons on the nitrogen atoms of adjacent monomer units in g-C₃N₄ results in the localization of positive charge; reorganization energies for polaron hopping range from 1.3 to 2.1 eV depending on whether the material is fully condensed into a two-dimensional sheet or linearly polymerized. Similarly, the chemical character of the valence band maximum (VBM) is determined by the strength of the antibonding interaction between lone pair electrons on neighboring monomers; the fully condensed material has a VBM composed predominantly of nitrogen lone pair electrons, whereas the polymer exhibits a VBM of π character.