Poly polarons

Eig. 2. Lattice distortions associated with the neutral, polaron, and bipolaron states in poly(p-phenylene). 

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...

The electronic band structure of a neutral polyacetylene is characterized by an empty band gap, like in other intrinsic semiconductors. Defect sites (solitons, polarons, bipolarons) can be regarded as electronic states within the band gap. The conduction in low-doped poly acetylene is attributed mainly to the transport of solitons within and between chains, as described by the intersoliton-hopping model (IHM) . Polarons and bipolarons are important charge carriers at higher doping levels and with polymers other than polyacetylene. 

On the basis of their results the authors concluded that the crossover from the neutral to oxidised form of poly pyrrole, and vice versa, requires the number of spins to pass through a maximum, i.e. the creation and anihilation of bipolarons involves the passage through the polaron state. They expressed this as a two-step redox reaction ... 

Diverse, chemically stable pedant polarons can be introduced into a polymer chain. The resultant poly(cation-radical)s are really stable and easily handled. They are described as monthly stable at room temperature in air (Murata et al. 2004, 2005). These properties open a way to advanced applications of magnetic organic molecules in the future. 

Importantly, deep oxidation of polyaniline leads to a material that becomes insulating and spinless. This phenomenon was demonstrated in case of poly(fV-methylaniline) by monitoring ESR signal and electric conductivity of the sample (Wei et al. 2007). Deep oxidation results in the formation of the so-called polaron pairs that are evidenced by optical spectra. Because the hopping probability of two polarons on a single chain is too small, polaron pairs do not contribute to electric conductivity and ESR signal. 

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). 

Fig. 2 Schematic representation of the doping of poly(p-phenylene ethynylene) (PPE) under formation of polarons (radical cations, radical anions) and bipolarons (dications, dianions). Note that multiple carriers can coexist on each macromolecule...

Although measurements continue to be made, it seems quite clear that native DNA is certainly not the band-type conductor first suggested by Eley and Spivey [35]. Disorder effects, polaron coupling, and the narrow band nature of the transport all suggest that A-DNA, or any native DNA, should be an insulator at long length scales. Quoted measurements indeed support this conductivity of less than 10 S/cm [76], or l/g>10 2 for lengths 40 nm [65], -4-10 ° Q for poly(GC) [77], or >10 2 for 1.8-/im DNA [52]. 

A number of excimers, notably adjacent C-C and A-A, and exciplexes consisting of adjacent A-C and A-T, have been identified at 78 K [42]. Only A-T emits at room temperature [43], however, the emission being seen from poly(A T)-poly(A T). Significantly, the lifetime of this fluorescence at room temperature is 7 ns [44]. Of course, this is the radiative Hfetime, thus a lower limit on the possible lifetime of the excitation. Thus, despite the vigorous thermal motion, this excitation, which involves a distortion quite similar to that of a polaron, lasts at least nanoseconds. 

It is perhaps useful to mention that the use of electronically conducting polymers, such as poly(acetylene), (CH) poly(pyrrole), (C4H5N)X, and poly(aniline), (CeHgNf ), has been proposed for positives for lithium batteries. The electrochemical process of these lithium-polymer positives is somewhat similar to an intercalation reaction. On charging, the polymer (P) is oxidized by acquiring a positive charge to form a polaron, and this is... 

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. 

At the molecular level, studies of the doping mechanism in the poly(heterocycles) reveal that very short lived radical cations ("polarons") immediately decay (combine) to form dications ("bi-... 

Since we had shown earlier18 that the species generated upon (dark) doping were delocalized cations (polarons), one can conclude from the above experiments that in poly(thiophene) irradiation generates both cations and cation radicals in sharp contrast to what is observed in (CH)x-... 

The polaronic strategy has also been applied to polymers, incorporating m-phenylene units as coupling links in n-conjugated polymer chains of poly(thienylene ketone) (Colle Dal et al. 1999) as well as of polyarylamine, polyacetylene, polythiophene (Kaisaki et al. 1991 Murray et al. 1994 Bushby et al. 1997a). Scheme 1-66 presents these polaronic polymers. 

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