Bipolaron models

In principle, such propositions resemble the bipolaron model, which presents the physicist s view of the electronic properties of doped conducting polymers 53-159) The model was originally constructed to characterize defects in solids. In chemical terminology, bipolarons are equivalent to diionic spinfree states of a system (S = 0)... 

Whereas the intermediate existence of polarons has been unequivocally proved by ESR measurements and optical absorption data, up to now, the existent of bipolarons has been only indirectly deduced from the absence of the ESR signal and the disappearance of the visible polaron bands from the optical absorption spectrum On the other hand, spinfree — diionic-charge — states in aromatics, whose optical properties bear a remarkably resemblence to the predictions of the bipolaron model, have long been known Further evidence of bipolarons is the fact that doped... 

The above findings are in approximate agreement with the theoretical calculations for the bipolaron model. These predict that the bipolaron state (=diion) in PPy extends over 4 pyrrole rings, and in PPP over 5 rings. 

Bipolaron — Bipolarons are double-charged, spinless quasiparticles introduced in solid state physics [i]. A bipolaron is formed from two -> polarons (charged defects in the solid). For chemists the double-charged states mean dications or dianions, however, bipolarons are not localized sites, they alter and move together with their environment. By the help of the polaron-bipolaron model the high conductivity of -> conducting polymers can be explained. 

The Ising, Small-Bipolaron Model of Mean-Field Theory... 

An important part of Andre and Bredas s work during the 1980s was to analyze and extend the soliton, polaron and bipolaron models to a number of cases [54] polypyrrole, polyparaphenylene, poly anilines, polythiophenes, etc. More details on the theory of conducting polymers can be found in specialized monographs [55]. 

At present it would appear that the polaron-bipolaron model is fully accepted by the scientific community. The main conductivity features of PPPs can easily be explained on the basis of this band energy scheme, and several experiments based on spectroscopic determinations have confirmed the theory. In particular, electron energy loss spectroscopy (EELS) of PPPs doped by AsFs [219] clearly showed the presence of two peaks near 1 eV and 2 eV, corresponding to transitions from the valence band to two states in the gap [219b], in fair agreement with the predictions of Bredas et al. [224]. 

Transport phenomena were also investigated and correlated with an appropriate state density diagram. In addition to the classical polaron and bipolaron model of Bredas et al, two kinds of localized states... 

---