Polaron-like radical cations Polarons

Beyond numerous studies of soluble reaction intermediates and products (as an example see a study of electroreduction of nitrosobenzene [23] or investigations of chromium aryl complexes [24]) this design has also been employed successfully in studies of polymer films deposited onto these electrodes. Films showing redox activity and, in many cases, intrinsic electronic conductivity [intrinsically conducting polymers (ICPs)] have been studied for an overview see [25, 26]. In a typical set of spectra (Fig. 5.5) obtained with a film of poly aniline, optical absorptions corresponding to the 7T 7T transition (around 330 nm) and to further transitions involving species like radical cations (polarons) and dications (bipolarons) formed in the sequence of electrooxidation of the film are observed. 

Generation of polaron-like radical cations and bipolaron-like dications in diarylpolyenes... 

III. GENERATION OF POLARON-LIKE RADICAL CATIONS AND BIPOLARON-LIKE DICATIONS IN POLYMER OLIGOMERS... 

The Mechanism of Long-Distance Radical Cation Transport in Duplex DNA Ion-Gated Hopping of Polaron-Like Distortions... 

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 hole-resting-site and polaron-like hopping models can be distinguished by the distance and sequence behavior of radical cation migration. Analysis of the hole-resting-site model leads to the prediction that the efficiency of radical cation migration will drop ca. ten-fold for each A/T base pair that separates the G resting sites [33]. 

The phonon-assisted polaron-like hopping model is unique because it is built upon an understanding of the dynamical nature of DNA in solution. The fundamental assumption of this model is that the introduction of a base radical cation into DNA will be accompanied by a consequent structural change that lowers the energy for the system. 

Fig. 10 Two schematic representations of a polaron-like species in DNA. In the top drawing, the base pairs of DNA are represented by the horizontal lines the sugar diphosphate backbone is represented by the vertical lines. The polaronic distortion is enclosed in the box and extends over some number of base pairs. This is shown schematically by drawing the base-pair lines closer together. In the lower figure, a specific potential po-laron is identified, AAGGAA, and the radical cation is presented as being delocalized over this sequence. Movement of the polaron from one AAGGAA sequence to the next requires thermal activation...

Schuster GB, Landman U (2004) The Mechanism of Long-Distance Radical Cation Transport in Duplex DNA Ion-Gated Hopping of Polaron-Like Distortions. 236-. 139-161 Schwarz H, see Schroder D (2003) 225 129-148... 

Liu C-S, Schuster GB (2003) Base sequence effects in radical cation migration in duplex DNA support for the polaron-like hopping model. J Am Chem Soc 125 6098-6102 Llano J, Eriksson LA (2004) First principles electrochemical study of redox events in DNA bases and chemical repair in aqueous solution. Phys Chem Chem Phys 6 2426-2433... 

Schuster GB, Landman U (2004) The mechanism of long-distance radical cation transport in duplex DNA Ion-gated hopping of polaron-like distortions. Top Curr Chem 236 139-161 Schussler H, Jung E (1989) Protein-DNA crosslinks induced by primary and secondary radicals. Int J Radiat Biol 56 423-435... 

FTIR spectroscopy has been used to monitor the conducting states of a conducting polymer as well as to know if a doping experiment is successful [86, 87], The FTIR and UV-Vis spectra of unsubstituted PANI is similar to that of substituted PANI though with slight band shifts. Doped PANI and its derivatives exist in the emeraldine salt forms which are essentially delocalized polysemiquinone radical cations whose stability is maintained by the presence of dopant anions. The degree of electron delocalization in the polysemiquinone forms of the doped PANI manifests itself in the form of an electronic-like band at ca. 1100 cm 1 associated with polarons [86], The structures of emeraldine base and emeraldine salt form of PANI are presented in Figure 6. 

A polaron may be a radical cation (oxidation) or a radical anion (rednction). Like a free carrier, a polaron has a spin of 1/2 and a charge of e. Polaron formation creates new localised electronic states in the band gap (Figure 1.6). Theoretical studies indicate that the polaron states of PPy are symmetrically located about 0.5 eV from the band edges [31]. The lower energy states are occupied by a single unpaired electron. 

Oudard et al. [719, 720] recorded ECESR spectra of PPy in a molten salt electrolyte liquid at room temperature. The ESR signal was observed only in a very narrow range of electrode potentials that closely correlate with the electrochemical oxidation current the line shape was not Dysonian, contrary to the observation reported previously. Evaluation of kinetic data implies slow formation of radical cations (polarons) and fast disappearance, most likely by recombination into spinless dications. 

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