Ab initio study of charged polyenes as charge-carrier models in conducting polymers

Ab initio self-consistent-field calculations are presented which deal with the structure of charged polyenes, as model systems for charge carriers in polyacetylene. We used a double-zeta-quality basis set, and determined the structure of clusters up to C22H24 2+, C21H23+, and C21H23 -. Fully optimized geometries and electronic properties are reported. The vibrational spectra of the clusters up to C12H14 2+, C11H13 +, and C11H13 - were obtained. All of these data are analyzed and compared with experimental data available on doped polyacetylene and with previous theoretical studies. Differences from ab initio data for neutral clusters are discussed. The results suggest that both positively and negatively charged solitons have a defect width of 15 carbon atoms. The C11H13 + cation and the corresponding anion display two equally strong infrared bands both assigned to strongly mixed C-C stretches and C-H bends in agreement with the ir spectrum of doped polyacetylene. The deuterated analogs have one strong band and another one intensity increasing with cluster size, a trend in accord with experimental data. The spectrum of C12H14 2+ has several strong bands (C-C stretches and C-H bends), while the deuterated analog shows two closely spaced bands, one of which is of low intensity. We comment on the mutual exclusion of ir and Raman bands and on the centrosymmetrical character of defects in doped polyacetylene. Our study suggests that charged solitons and not bipolarons may be a better representation of the defects.