Polymers and Organic Materials, Electronic States of

In this article, the results of quantum-chemical calculations will be used to provide an introductory overview on the electronic structure of organic semiconductors. The focus is on phenylene-based materials since these form a highly attractive class of p-conjugated systems. These materials can be exploited in organic photodetectors, solar cells, transistors, and electronic circuits; their main application relates to organic light emitting devices and displays, where they serve as blue emitters. Apart from their practical relevance, polyphenylenes also serve as ideal model systems to discuss the electronic properties of organic semiconductors and conjugated polymers and allow the description of general concepts that can then be applied to more complex systems. Here, a start is made from the simple benzene molecule to show how its molecular orbitals are related to the conduction and valence bands of the corresponding polymer, poly(p-phenylene) (PPP) (see Figure 1 for the chemical structure of the corresponding ten-ring oligomer 10PP). (Figure presented) In this way, a number of general properties of the excited states in phenylene-based materials can be derived. To bridge the gap between a molecular description usually found in quantum chemistry and the band structure picture prevalent in condensed matter physics, it will be shown how the molecular orbitals of phenylene chains of increasing length adopt a Bloch-type structure. A similar band-like behavior is found when considering the exciton states of extended p-conjugated molecules. To discuss the influence of molecular conformation, the electronic properties of ‘‘regular’’ poly(fluorene) – a partly bridged derivative of PPP – will be compared....