Theoretical Design of Conjugated Organic Polymers

Predictions of excitation energies of π-conjugated oligomers and band gaps of conducting organic polymers at various theoretical levels are reviewed and compared to experimental data in gas, liquid, and solid phases. High level ab initio calculations reproduce the experimental excitation energies of polyenes in the gas phase very well. The assignment of the peaks to adiabatic and vertical excitations is, however, not certain. Solvent effects lower and disorder increases the excitation energies. In the solid state, bands of molecular crystals split and shift compared to individual molecules. Therefore, theoretical data should not agree completely with experiment, unless all medium effects are accounted for. Band structure calculations usually neglect most of the medium effects but can produce trends, helping with the analysis of substituent effects. Properties of oligomers with increasing size do not necessarily depend linearly on inverse chain length. Even for defect free oligomers in the gas phase a saturation limit seems to exist. Theoretical investigations are particularly useful for studying underlying causes of changes in properties upon chemical modification. Understanding these factors can lead to structure property relationships that are needed for“tailoring” new systems for specific applications.