Conformational Transition of Ion-Radicals

This problem has attracted considerable attention, and intriguing results have been obtained. It would be important, however, to develop some applications in organic photodiodes, transistors, sensors, and charge-storage devices. 

For instance, poly-p-phenylenes in their doped states manifest high electric conductivity (Shacklette et al. 1980). Banerjee et al. (2007) isolated the hexachloroantimonate of 4 -di(tert-butyl)-p-quaterphenyl cation-radical and studied its x-ray crystal structure. In this cation-radical, 0.8 part of spin density falls to the share of the two central phenyl rings, whereas the two terminal phenyl rings bear only 0.2 part of spin density. Consequently, there is some quinoidal stabilization of the cationic charge or polaron, which is responsible for the high conductivity. As it follows from the theoretical consideration by Bredas et al. (1982), the electronic structure of a lithium-doped quaterphenyl anion-radical also differs in a similar quinoidal distortion. With respect to conformational transition, this means less freedom for rotation of the rings in the ion-radicals of quaterphenyl. This effect was also observed for poly-p-phenylene cation-radical (Sun et al. 2007) and anion-radical of quaterphenyl p-quinone whose C—O bonds were screened by o,o-tert-hutyl groups (Nelsen et al. 2007). 

During transformation from the neutral molecules to the corresponding anion-radicals, the rate of the fragment rotation, relative to one another, decreases. This also results in the nonequivalence of the meta and ortho protons. Thus, 3-acetylpyridine gives two different anion-radicals as conformational isomers (Cottrell and Rieger 1967). In the case of 3-benzoylpyridine anion-radical, the phenyl group rotates freely about the carbonyl center, whereas the rotation of the pyridyl group is limited. The ESR spectrum shows that the spin density in the phenyl ortho positions is half of that 

Benzene p-dialdehyde (terephthalic aldehyde) produces the anion-radical at the potential of the limiting current of its first reduction wave in DMSO with tetrapropylammonium salt as a supporting electrolyte. The anion-radicals demonstrate two ESR spectra. One of them is superimposed 

The oxygen-bridged triphenylamine (2,2 6, 2 6 ,6-trioxatriphenylamine) has a shallow-bowl structure. The corresponding cation-radical acquires a practically planar structure. This cation-radical is very stable, neither dimerization tendency in the crystalline form nor oxygenation 

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