Extended Hückel Calculations on Defect States in the Pi System of Polyazine
William B. Euler, Journal of Physical Chemistry, 1987, 91, 5795 – 5800
Abstract
The extended Hückel method is used to calculate the π electronic structure of polyazine, –(N=CH–CH=N)x–. The band structure of the polymer is deduced from long-chain model compounds using monomer molecular orbitals as the basis set. This gives results in agreement with tight-binding calculations. Polyazine is found to have a valence bandwidth of 1.4 eV, a conduction bandwidth of 1.3 eV, and a band gap of 2.3 eV. The effect of a single defect or a double defect centered on either carbon or nitrogen was considered. For single atom defects, whether centered at carbon or nitrogen, one midgap state is created, the band gap is decreased, and the wavefunction of the new state is delocalized over four to five monomer units. In contrast, when two defects are formed, two midgap states appear, the band gap increases, but the new wave functions are still delocalized over four to five monomer units. The various kinds of defects can be formed from one another by simple one- or two-bond shifts in the π system. The importance of these kinds of shifts to charge transport is discussed.