Bipolaronic charge states

Polaronic and bipolaronic charge states are well known in electroactive polymers, and can be observed in model oligomers with overall delocalization lengths as small as 16 atoms. It has been suggested that localized charge states may be involved in oligomers and polymers having enhanced proper-... 

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

There is no doubt that polaronic and bipolaronic charge states can be supported in stable form in small oligomers in solution, or incorporated as part of a copolymer sequence. Enhanced x properties can derive from either N, P or BP states as a function of increased delocalization length, and we anticipate several families of copolymers based on the modeling studies discussed in this paper to become available in the near future to test these proposals. 

Figure 9.1. Formation of polaronic and bipolaronic charge states during oxidative doping of PTV.

Figure 9.3. Possible spectroscopic transitions for doped -conjugation sequences exhibiting either polaronic or bipolaronic charge state formation.

Spangler, C. W. and Havelka, K. O., Design of new nonlinear optic-active polymers. Use of delocalized polaronic or bipolaronic charge states, in Materials for Nonlinear Optics Chemical Perspectives (ACS Symp. Ser., 455), Marder, S. R., Sohn, J. E., and Stucky, G. D., Eds., American Chemical Society, Washington, D.C., 1991, 661. 

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...

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... 

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. 

In PPV a PI A peak at 1.4 eV has been assigned to a triplet-triplet transition from the triplet exciton [148,150], as in PDA (it is not known if the near equality in the energies is accidental). But, in addition, two other induced absorptions are observed near 0.6 and 1.6 eV, and since they are associated with the characteristic IR bands (such as the 0.45-eV band in PA), they should be due to a charged state. The absence of ODMR signal suggests that they have no spin and would then be bipolarons. In improved PPV (see Fig. 15), the PIA spectrum contains only the triplet peak [151], suggesting that the presence of the other features is a consequence of strong localization in a defective polymer. Similar results are found in other CPs, but up to now evidence for PIA due to polarons is elusive. 

Figure 5.3. The reduction (doping) of trans-polyacetylene. In the ideal case, the mobility of the charged states allows the bipolaron to collapse into a pair...

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. 

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