Electronic structure of π-conjugated oligomers and polymers: A quantum-chemical approach to transport properties

Nearly 25 years after the discovery that an organic conjugated polymer can be "doped" (that is, in chemical terminology, oxidized or reduced) to metallic-like electrical conductivity, the field of π-conjugated oligomers and polymers has enjoyed a tremendous development, most remarkably underlined by the 2000 Nobel Prize in Chemistry awarded to Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa. Since, in this specific class of polymers, small variations in chemical structure play an essential role and the properties of interest directly depend on the electronic structure, quantum-chemical approaches, that take the chemical nature fully into account, have provided a most valuable input to the field. The results have helped to forge a fundamental understanding of the electronic and optical properties of π-conjugated materials and in guiding the experimental efforts toward novel compounds with enhanced characteristics. It is our main purpose in this contribution to illustrate the impact of quantum-chemical methods, more specifically in relation to transport processes. We do not dwell at all on the theoretical methodologies that have been designed, but rather on the concepts. We first recall the basic electronic-structure aspects of π-conjugated systems, that make their chemistry and physics so rich and fascinating. We then discuss the key role of interchain interactions and their impact on transport.