The proposed research is problem oriented: they aim to understand at microscopic level, and thereby gain more control over, the chemical factors which influence the structure and electronic structure of highly doped (well-conducting) and pristine (semiconducting) conjugated polymers. Their low-dimensional nature causes very large electron-proton interactions and nuclear rearrangements, which lie at the core of electron-proton interactions and nuclear rearrangements, which lie at the core of their unusual electrical and optical properties. Since no quantum mechanical method exists now which can treat the total energy and the band structure of these systems simultaneously, a pragmatic approach is chosen. They combine full geometry optimizations (at ab initio level or at semi-empirical self- consistent-field, SCF, level for the large unit cell systems) with subsequent energy band calculations using appropriate non- SCF semi-empirical methods. The energy gaps in these systems are due to the nuclear rearrangements and pertubations caused by chemical substitutions. A broad range of polymers will be studied to find good candidates for very small band gap systems. The Peierls active distortion modes leading to semiconducting gaps in ladder-type polymers have been subject to controversy. Calculations will determine which of these modes leads to the largest energy gain. In the highly doped conducting polymers the location and orientation of dopants will be determined by total energy optimizations. Ab initio calculations will determine the dopant-polymer hybridization and the interchain hopping interaction. The quantitative aspects of bond length equalization upon doping, including the asymetry between donars and acceptors will be determined in conjunction with a new cis-trans isomerization mechanism. The popular qualitative concept of quinoid vs. aromatic forms, which is fundamental in the theory of these polymers, will be put on a quantitative basis through full geometry optimizations. A systematic search for novel groups of compounds with small energy gaps and with two or more energetically close nuclear configurations will be attempted.
