| Abstract: | Quantum-mechanical methods were employed to calculate electronic band structures for the polybenzobisoxazole (PBO) and polybenzobisthiazole (PBT) chains originally synthesized and much studied because of their utility as high-performance fibers and films. For cis-PBO, trans-PBO, and trans-PBT chains in their coplanar conformations, the band gaps in the axial direction were found to be 1.72, 1.62, and 1.73 eV, respectively. Since trans-PBT is nonplanar, calculations on it were also carried out as a function of the rotation angle ? about the C—C bond joining the two +ing systems in the repeat unit. The band gap was found to increase markedly with increase in nonpla-narity, as would be expected from the decrease in charge delocalization. The calculations suggest the most likely value of ? to be ca. 30°, in good agreement with the experimental value 23° obtained by x-ray analysis of a crystalline trans-PBT model compound. At this value of ?, the calculated value of the band gap is.1.98 eV. All of these values are very close to the corresponding values of 1.4-1.9 eV reported for trans-polyacetylene, which should encourage further theoretical and experimental investigations of the electronic properties of these polymers. |
