Abstract: The polythiophenes with the benzenoid-like structure, the equibondlength-like structure, and the quinoid-like structure have been calculated by means of LCAO- CO/EHMO. In order to relate the structure of polymer to its unit structure, the thiophene and its dimer and their fragments which are the structural units of poly- mer have also been calculated. The calculation models and the structural parameters are shown in Fig.1. The polythiophene possesses the symmetries of two-fold helix and glide mirror, so that according to Eq.1 and Eq.2 the calculation can be simplified. The NEWBAND-1 computer program depending on LCAO-CO/EHMO is used in our calculation. The ionization potentials and the orbital exponents of valence electrons which we use are lited in Table 1. Five structure units are considered in calculation of the interaction between structure units.
The charge distributions of atoms are shown in Table 2. In our calculation, the 3d orbitals of S atom have been involved and the result is that S atom is negatively charged, but in the paper [10], the 3d orbitals are neglected and the S atom is postively charged. The contribution of atomic orbitals to HOPO (highest occupied polymer orbital) of the polymer is shown in Table 3. It can be seen that there are rather large contributions from the 3p_z and 3d_(xz) orbital of S atom, which is different from the results of VEH etc~[8,10]. So according to our calculation results the electrons in the polythiophene with p-dopant do not only move along C_α—c_β—C_β—C_α skeleton, which is different from the case in the polypyrrole~[14].
The energy bands of three sorts of polythiophenes are shown in Fig.2. Both VBW and CBW shown in Table 4 are quite large, so that whether the dopant is donor or acdeptor, the doped polythiophene will have nice conductivity. From Fig.2 and Table 4 it has been shown that VBW and CBW from type I to III are becoming larger, while the energy gaps are becoming smaller, which is helpful to conduction of doped polythiohene. However, even for the quinoid-like polythiophene its energy gap is still muc larger than kT of therm-motion at the room temperature, so that it is impossible for it to become conductor due to the electron thermexcitation. Th density of valence state of the benzenoid-like polythiophene is shown in Fig.3.
The relationship between the frontier orbitals of polythiophene (I) and its structural units is shown in Fig.4, from which we can find that there is a correlation between them as in the molecular crystal~[15].
According to our calculation, the energy difference ΔE between the structural units of benzenoid-like polythiophene and the quinoid-like polythiophene is 0.124 eV, which is larger than kT of the therm-motion at the room temperature. Thereby, it is still difficult to create polaron or bipolaron in erms of the thermmotion, that is, it is dopant that brings on the polaron or bipolaron.