Bipolaron signature

Optical Signatures of Solitons, Polarons, and Bipolarons The formation of these excitations generates energy levels corresponding to optical transitions below the fundamental absorption that between the valence and the conduction band in one-electron models (but see Section... 

When conjugated Tt-sequences are chemically oxidized by I2, AsFs, NOPFft, SbCls or a host of other reagents (oxidatively doped) to the conductive state, new optical transitions are observed which correspond to the formation of new energy levels within the gap. Transitions involving either polaron or bipolaron states have their own unique optical signatures, and can be schematically represented as shown in Figure 9.3. 

These model compounds were oxidatively doped in solution (SbCl5/CH2Cl2) and compared to the oxidative behavior of the corresponding dithienylpolyenes. The spectra were essentially identical, showing clearly that both dithienylpolyene segments were oxidized to bipolarons, and that their signature spectra were indistinguishable from the model monomer (Table 9.6). 

The broad absorptions of CPs characteristic of charge carriers such as bipolarons tail off in the IR, near ca. 6 /im, and subsequently, broad band variations in electro-chromic signature are then absent in most cases for conventional dopants. This can be seen fi om Fig. 3-15. showing Specular Reflectance for a poly(aromatic amine)/perchlorate here the "tails" of the bipolaron bands from the far-Visible and near-IR and the substantial dynamic range therein are clearly visible to ca. 6 fim. Beyond 6 / m, however, there is a nearly featureless absorption, and very little broad band variation in dynamic range. This tailing of the bipolaron absorptions well in to the IR is a characteristic feature of most CPs. 

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