Anionic structures organic radical ions

Occasionally such intermolecular effects are specific and have sufficient lifetime to contribute individually to the spectrum. This is the case, for example, when organic radical-anions are studied in solvents of low ionizing power such as tetrahydrofuran. Ion-pairing then becomes important and, when alkali-metal cations are involved, the effect on the spectrum is to split each hyperfine component into four lines, each having one-quarter of the original intensity. This may convert a complicated spectrum into one that is quite uninterpretable, and can be avoided by using a better solvent or non-magnetic cation. However, it also provides evidence that contact ion-pairs are important in such solutions, and yields structural details unobtainable by other techniques. 

In a number of cases quantum-chemical calculations have confirmed the experimentally determined distribution of unpaired electron density in a radical ion [94]. In addition the hyperfine structure of the spectrum can serve as an experimental basis for verification and confirmation of the validity of complex quantum-chemical calculations applied to the structure of organic molecules and radicals. The hyperfine structure can also serve to identify a given type of radical, and to offer information on the coplanarity and isomerism of radical anions it can also be used to determine the rates of transi-... 

The concept of molecular orbitals (MOs) helps to explain the electron structure of ion-radicals. When one electron abandons the highest occupied molecular orbital (HOMO), a cation radical is formed. HOMO is a bonding orbital. If one electron is introduced externally, it takes the lowest unoccupied molecular orbital (LUMO), and the molecule becomes an anion-radical. LUMO is an antibonding orbital. Depending on the HOMO or LUMO involved in the redox reaction, organic donors appear as n, a, or n species, whereas organic acceptors can be tt or a species. Sometimes, a combination of these functions takes place. 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion-radicals. These devices are based on the sandwich-type electrode structures containing organic layers as the electron/hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion-radicals lowers the barrier height for the electron or hole injection. Anion-radicals (e.g., anion-radicals from 4,7-diphenyl-l,10-phenanthroline—Kido and Matsumoto 1998 from tetra (arylethynyl) cyclooctatetraenes—Lu et al. 2000 from bis (1-octylamino) perylene-3,4 9,10-bis (dicarboximide)s— Ahrens et al. 2006) or cation-radicals (e.g., cation-radicals from a-sexithienyl—Kurata et al. 1998 l,l-diphenyl-2-[phenyl-4-A/,A- /i(4 -methylphenyl)] ethylene— Umeda et al. 1990, 2000), all of them are electron or hole carriers. 

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