Covalent Doping of g-C3N4 with Carbazole and Benzochalcogenadiazoles: Simulation of Electronic Properties in Alternative Approximations

The paper considers the results of theoretical analysis of the electronic properties of compounds based on carbon nitride fragments consisting of three heptazine rings (melon) covalently bound to heterocyclic substitutes (electron-withdrawing 2,1,3-benzochalcogenadiazoles and electron-donor carbazole). The simulation was performed at two alternative levels: molecular gas-phase model and the model of one-dimensional polymer with periodic boundary conditions. These levels made it possible to compare the energy gap between the frontier orbitals in the molecular model and the band gap of the chain polymer for the same compound. It was found that, on the one hand, selenium-containing heterocycles reduce the energy difference between the frontier orbitals to a much larger extent than other covalently bonded dopants of this series and, on the other hand, doping of melon with only carbazole minimizes the band gap. Upon effective doping that minimizes the energy difference between the frontier orbitals, the highest occupied molecular orbital is localized in the donor part of the molecule, while the lowest unoccupied molecular orbital is localized in the acceptor part of the molecule. It was also shown that doped melons form complexes with benzyl alcohol via non-covalent bonding. The lower strength of such bonds in melon complexes substituted with electron-withdrawing selenium-containing groups implies more efficient oxidation of the alcohol in these systems. The simulation results adequately describe the available experimental data on the band gap of melon doped with electron-withdrawing molecules. The fast gas-phase calculations are suitable for a qualitative estimation of the dependences of the frontier orbital energy difference on the dopant type and "dopant-melon" ratio, while more laborious calculations of polymer structures provide a correct estimation of the band gaps of g-C3N4 polymers upon the addition of various dopants.