THEORETICAL AB-INITIO INVESTIGATION OF THE DIPOLE POLARIZABILITIES OFOLIGOTHIOPHENES AND POLYTHIOPHENE

Ab initio coupled and uncoupled Hartree-Fock polarizabilities are comp uted for increasingly large oligothiophenes using the 3-21G, 6-31G*, 6-311G*, and the Sadlej medium-size polarized atomic basis sets. The comparison with experimental values highlights the large sensitivity o f the measured polarizabilities with respect to working conditions. By extrapolating to the infinite chain limit the polarizability values o f increasingly large oligomers, we compute the asymptotic polarizabili ties per thiophene ring. The transversal, perpendicular, and longitudi nal asymptotic coupled Hartree-Fock polarizabilities per thiophene rin g are, using the 3-21G (631G*) atomic basis set, 49.7 (52.5), 15.2 (2 0.6), and 184.1 (190.0) a.u., respectively. Polythiophene is more pola rizable than polydiacetylene, polyyne, and polysilane but less polariz able than polyacetylene which, in contrast to polythiophene, is much m ore difficult to process. Using a procedure which relates the polariza bility and the topology of the one-particle states, the major contribu tion to the polarizability is attributed to pi-symmetry crystalline or bitals located at the top of the valence bands, which look like an alt ernation of segments of the highest occupied crystalline orbitals of t he all-trans and trans-cisoid polyacetylene. However, a sizeable a con tribution domes from sigma-conjugated segments formed by the sulfur at oms and their two adjacent carbon atoms.