Theoretical Studies on Electronic Structure, Spectra and Conductivity of Pristine and Alkali Doped Polyacetylene
The present thesis is a theoretical investigation of the electronic properties of linear polyenes to get an insight into conductive phenomena in pristine and doped polyacetylene. Conductivity in polyacetylene is a complex phenomena and cannot be treated within simple band models. Highly doped polyacetylene has many of the attributes of metals, although its conductivity does not have the dependence on temperature seen in metals.
Emphasis is put on electron transfer (ET) models owing to the fact that electrons (or holes) tend to be localized. Relevant parameters for ET are calculated, such as internal reorganization energy .lambda.i and electronic coupling .DELTA. between localities. The nature of the trapping sites is investigated as well as the influence of alkali atoms in ET. The semiconductor--metal transition that appears at high doping levels is examined.
One or two polyene chains are taken as model compounds for polyacetylene. In some calculations alkali metal atomswere added. Geometries are optimized keeping restrictions according to crystallographic data.
Quantum chemistry methods at different levels of approximation were used. Simple tight-binding and CNDO/S semiempirical methods were used to study large systems. For smaller systems ab initio HF-SCF and correlation corrected methods were used: second-order perturbation theory (MP2), multiconfigurational-SCF (CASSCF), multiconfigurational second order perturbation (CASPT2), and state interaction based on CASSCF wave functions (CASSI) methods.
It is shown that ET theory can be applied to describe the role of hopping in the electronic conductivity of polyacetylene. It is found that in CnHn+2 with even n the reorganization energy .lambda.i is much larger than in systems with odd n. Odd-n polyenes are more ready acceptors of an additional electron. The alkali atoms induces rearrangements in PA structure, and leads to a significant increase of .lambda.i the reorganization energy and the activation energy, and to a moderate increase of the coupling between localities.
The tight-binding method used proves to reproduce to a high degree of accuracy the ab initio results, and allows the elucidation of the nature of solitons and polarons and the possible mechanisms for its creation.
The absorption spectra band below 1 eV is related to the appearance of metallic conductivity, and consists of interchain as well as intrachain transitions.