Polarons and Bipolarons

Figure 7-2H. (a) ll-l ADMR and (b) A-PADMR spectra of u-6T film. The peaks of polarons, bipolarons, and triplet cxei-(ons are labeled. 

We investigated the ultrafast dynamics in a Na-NaBr melt at 1073 K by fs pump probe absorption spectroscopy. A simple model was used to simulate the dynamics of polaron-, bipolaron- and Drude-type electrons. The relaxation times for polarons and bipolarons are 210 fs and 3 ps, respectively. The existence of an isosbestic point at 1.35 eV indicates an inter-conversion between bipolarons and Drude-type electrons. 

Poly(phenylene vinylene), PPV, and its soluble derivatives have emerged as the prototypical luminescent semiconducting polymers. Since PPV has a nondegenerate ground state, structural relaxation in the excited state leads to the formation of polarons, bipolarons, and neutral excitons. However, prior to treating the structural relaxation in the excited state, one needs to develop a satisfactory description of the electronic excited states. 

Jahn-Teller Polarons, Bipolarons and Inhomogeneities. A Possible Scenario for Superconductivity in Cuprates... 

Bredas JL, Street GB (1985) Polarons, Bipolarons, and Solitons in Conducting Polymers. Accounts of Chemical Research 18( 10) 309—315... 

A revisal of the equilibrium between polarons and doubly charged states of the polymer chains shows that the previous assumptions on the extension of polaron, bipolarons and polaron pairs were not justified. A more general formulation is presented for the equilibrium concentrations and the kinetics. But the differences are almost negligible up to the maximum charge concentrations that can be achieved in accumulation layers. The resulting rate constants for formation and dissociation of (immobile) bipolarons can be estimated using a rate constant for the bipolaron formation determined recently by Salleo and Street, and indicate that these processes can cause the hysteresis on the time scale of the measurements. 

A powerful technique for probing the underlying physics in a material is the investigation of the behaviour of photo-excited states, which are produced by absorption of light. Electroactive conjugated polymers exhibit a wide variety of photoexcitations, such as solitons, polarons, bipolarons, and excitons. ... 

J. L. Bredas, G. B. Street, Polarons, bipolarons, and solitons in conducting polymers, Accounts of Chemical Research 1985, 18, 309. 

As the brief history of conducting polymers tells, initially physicists played the dominant role in research of these materials. Naturally, they borrowed names for the species and phenomena from the nomenclature of their own field, as we have already seen in the case of doping. The terms polaron, bipolaron, and sometimes soliton are frequently seen in the literature on conducting polymers. These terms have been borrowed from solid-state physics. By polaron (Fig. 2a) in connection with CPs we mean radical ion introduced by oxidizing or reducing a conducting polymer molecule. The simple molecular orbital theory shows that the charge of a... 

Next, we discuss the electronic transitions due to polarons, bipolarons, and soli-tons in a polymer chain on the basis of a theoretical study reported by Fesser et al. [48] on a continuum electron-phonon-coupled model. The electronic energy levels of a neutral infinite polymer and those of polarons, bipolarons, and solitons are shown schematically in Figure 4-5. 

Methodology of Raman Studies of Polarons, Bipolarons, and Soli tons 215... 

Polarons, bipolarons, and solitons are different from each other as described above. It is expected that we can detect these excitations separately by electronic absorption, ESR, and vibrational spectroscopies. In particular, geometric changes induced by doping can be studied by vibrational (Raman and infrared) spectroscopy. 

The stability of polarons, bipolarons, and solitons has been studied theoretically... 

Unsaturated long-chain hydrocarbon molecules with their extended ir-electron system are suitable for bridging the gap between quantum chemistry and solid-state theory. A simple step-potential model of the nearly free 7T electron coupled to the lattice distortions is presented. The geometry of the molecule is obtained by requiring self-consistency between bond length and 7r-electron density in the middle of each bond. This quantum-mechanical model has no free parameter and leads to a satisfactory description of unsaturated hydrocarbon chains and rings and of defect states in rran.y-polyacetyl-ene called solitons, such as kinks, polarons, bipolarons, and excitons. 

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