Polaron stability

Fig. 10. Formation of the bipolaron (= diion) state in poly-p-phenylene upon reduction In the model it is assumed that the ionized states are stabilized by a local geometric distortion from a benzoid-like to a chinoid-Iike structure. Hereby one bipolaron should thermodynamically become more stable than two polarons despite the coulomb repulsion between two similar charges...

In a second possibility, the polaron-like hopping model, a structural distortion of the DNA stabilizes and delocalizes the radical cation over several bases. Migration of the charge occurs by thermal motions of the DNA and its environment when bases are added to or removed from the polaron [23]. 

The stabilization of the radical cation by forming a polaron is a trade-off between its delocalization and the energy required to distort the DNA structure. The former lowers the kinetic energy of the intrinsically quantum mechanical migrating radical cation, and the latter will be determined by factors that are independent of specific base sequence, such as the force constants of bonds in the sugar diphosphate backbone. 

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). 

The disappearance of the sharp Verwey transition was discussed by Mott (1979), who suggested that at low temperatures the material is a Wigner glass , the electrons (Fe2 + ions) being frozen into random sites and the whole system stabilized by the fluorine. Discussion of the thermopower measurements show, according to Mott (1979), that a hopping mechanism is operative at low T. Ihle and Lorenz (1985), however, consider that the electrons in the wrong sites move by a small polaron band mechanism. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

MODELING ELECTRQACTIVE SEGMENTS FORMATION AND STABILIZATION OF POLARONIC AND BIPOLARONIC CHARGE STATES... 

During the past three years we have been studying the chemical (SbCl5) oxidation of well-characterized oligomers of polyacetylene, poly[p-phenylene vinylene] (PPV) and poly[2,5-thienylene vinylene] (PTV) in order to determine how polaron and bipolaron states can be preferentially formed and stabilized. 

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