ELECTRONIC-STRUCTURES OF HETEROCYCLIC LADDER POLYMERS - POLYPHENOTHIAZINE, POLYPHENOXAZINE, AND POLYPHENOQUINOXALINE

Geometrical and electronic structures of three heterocyclic ladder polymers, polyphenothiazine (PTL), polyphenoxazine (POL), and polyphenoquinoxaline (PQL), were investigated with the extended Huckel crystal orbital method. Fully optimized geometries were obtained by total energy calculations using the solid-state version of the modified neglect of diatomic overlap (MNDO) method. Two types of bandgaps were found: one at the Brillouin zone edge arising from a Peierls distortion and the other at the center of the Brillouin zone (k = 0) due to heteroatomic perturbations. The bandgap at the Fermi level, E(F), is due to Peierls distortion and is small, 0.3-0.5 eV, which reflects the experimentally observed substantial conductivity (10(-6) 10(-5) OMEGA-1 cm-1) of pristine PTL. At high doping levels (charge transfer of around +/- 0.8e/unit cell), the Peierls gaps are predicted to disappear, but the gaps due to heteroatoms are essentially unchanged. Therefore, the heteroatoms are responsible for certain experimental observations: short electron delocalization lengths in ladder polymers as determined by N-14 electron nuclear double resonance measurements, and only a small (2-4 orders of magnitude) increase in conductivity and spin concentrations of doped PTL. From the assignments of band symmetries, it is found that a transition between highest occupied band and lowest unoccupied band is IR active in a high-frequency region (> ca. 3000 cm-1) and that two direct optical transitions (B(g) <-- A(u)) from the Fermi level to the higher conduction bands can occur in the visible range as observed experimentally from the quinoid form of triphenodithiazine.