Polaronic charge states

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... 

EL), conjugated polymers are also of interest as materials for optically or electrically pumped stimulated emission. For effects of this type, the ratio of stimulated emission to photoinduced absorption (PA) is of particular interest for conjugated polymers. In this context, the orign of the PA is controversial the PA can be a result of the formation of either charge-separated polaron pair -states or excimers. Initial experiments support the conjecture that LPPP 26 is significantly superior [49], as the stimulated emission of LPPP 26 is markedly more intense than that of PPV under comparable conditions. 

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. 

The result of the calculation is shown in Fig. 14, it is clear seen that the tunneling from the state 0,0) to the charged state and from the state 1,0) to the neutral state is exponentially suppressed in comparison with the bare tunneling rate F at large values of the electron-vibron interaction constant A. This polaron memory effect can be used to create nano-memory and nanoswitches. At finite voltage the switching between two states is easy accessible through the excited vibron states. It can be used to switch between memory states [112]. 

Polaron Pair State. There are a number of experimental observations which can be interpreted neither by invoking charged excitations injected or photo-generated in the polymer, nor by excitons. However, it may happen that the singlet exciton is broken, as described above, and a pair of charges, negative P and positive P+ polarons, are separated onto adjacent chains, but still bound by the Coulomb attraction. These pairs will be referred to as polaron pairs. Polaron pairs are intermediate states between electronic molecular excitations and free charge carriers. They are formed by excitation of the photo-conductivity in polymers and other molecular solids, as well as... 

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. 

From picosecond transient photoconductivity measurements on PPP films,22 we know that mobile charged states decay within 110 ps. In conventional routes to PPPs, defects like branched chains and large torsion angles of neighboring rings are known to occur. These defects act as shallow or deep traps for positive and negative polarons,38,39 which limit the mobility of charge carriers.40 The synthetic route toward the PPP-type ladder-polymers prevents the described defects and leads to a trap concentration of less than 1 trap per 1000 monomer units,28 whereas substi-... 

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