Radical ions: Where organic chemistry meets materials sciences

Abstract

Radical ions generated by electron transfer reactions are known as important intermediates in organic chemistry. On the other hand, their formation, recombination and transport in organic materials is responsible for a series of attractive physical properties. Radical ion formation is often accompanied by structural changes being well understood in small organic molecules, which also constitute repeating units of intensively studied macromolecules. Therefore, an approach is described herein to compare and combine the structural and energetic description of monomeric and oligomeric radical ions with that of partially oxidized or reduced polymeric materials.Many optical and electrical properties of high-molecular-weight conjugated polymers closely correspond to those of oligomers containing only a few repeating units. These oligomers can be synthesized as monodisperse species, facilitating the spectroscopic description and enabling systematic studies of physical properties as a function of chain length (Sect. 2).The mode of charge and spin distribution on conjugated chains is a central question for conducting polymers which are electrical insulators and semiconductors in the neutral, pristine state (Sect. 3). Both, intra- and interchain charge transport have to be considered in describing the overall conductivity. Electroactive polymers are applied as change storage materials, e.g. in rechargeable batteries, where the detailed charging mechanisms and minimization of Coulomic repulsion in highly charged states are crucial (Sect. 4). Electron transfer can also induce chemical reactions under formation or cleavage of σ-bonds (Sect. 5). While this is an unwanted side effect in the doping of conjugated polymers electrooxidation of suitable π-systems is a common method of producing electroactive and conducting polymers. Conductivity does not necessarily require polymeric materials, but is also obtained in radical ion salts and charge transfer complexes which are crystalline one dimensional conductors (Sect. 6). Their electrical conductivity can adequately be described as an electron hopping process between neighboring molecular layers. While the mobility of charge is important in processes like electrical conductivity or photo- and electroluminescence, localized radical states with as many unpaired electrons as possible are needed in magnetic materials (Sect. 7). Finally, the control of electron transfer processes in radical ion states can be used in molecular electronics (Sect. 8).