Covalent doping of carbon nitride with carbazole and benzochalcodiazoles: simulation of electronic properties in alternative approximations

The results of theoretical consideration of the electronic properties are presented for compounds based on carbon nitride fragments consisting of three heptazine rings (melon) covalently bonded with heterocyclic substitutes (electron acceptors like 2,1,3-benzochalcodiazoles and electron donors like carbazole). The simulation has been performed at two alternative levels: a molecular gas-phase model and a model of one-dimensional polymer with periodic boundary conditions. These levels have allowed comparing the energy change of frontier orbitals and the band gap of a chain polymer for the same compounds. It has been found that, on the one hand, the selenium-containing heterocycles reduce the energy change of frontier orbitals significantly more than other covalently bonded dopants of the considered series; on the other hand, the doping of melon with mere carbazole minimizes the band gap. At efficient doping, which minimizes the energy change of frontier orbitals the highest occupied orbital is localized on the electron-donor fragment of the molecule, and the lowest unoccupied orbital is localized on the electron-acceptor fragment of the molecule. It has also been shown that the doped melons form complexes with benzyl alcohol due to appearance of non-covalent bonds between them. Wherein, the lower strength of such bonds in complexes with melon, substituted with the electron-acceptor selenium-containing fragments, suggests more efficient oxidation of an alcohol in such systems. The simulation results appropriately correspond to the available experimental data of the band gap of melon doped with electron acceptor molecules. The fast gas-phase calculations are suitable for qualitative estimation of dependences of the energy change of frontier orbitals upon their type and the amount of covalently bonded dopant, whereas the more expensive calculations of polymer structures allow a correct estimation of the band gap values of carbon nitride polymers with various dopants.