Bipolaron state

Eig. 2. Lattice distortions associated with the neutral, polaron, and bipolaron states in poly(p-phenylene). 

So far, electrochemical measurements have not provided any direct proof for the formation of a bipolaron state in oligbmers or polymers which is significantly more stable than the polaron state. In general, in terms of energy the redox potentials E° for bipolaron formation should be much lower than the potentials Ej for polaron formation (/E / < /E /). However, more recent electrochemical and ESR spectroscopic studies by Nechtschein et al. indicate that the bipolaron state is not much more stable than the polaron state... 

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. 

The electronic absorption can be attributed to the lowest polaron and/or bipolaron states in the gap (see Figure 3.72). It is clear from the above discussion that some form of transition in the carrier behaviour occurs near c. —0.2 V this is reinforced by a consideration of the IRAV absorptions in Figure 3.83(b). The IRAV bands are reasonably sharp up until —0.2 V after which they broaden and appear less well-defined. 

The occurrence of bipolaronic states in the polymer chains promotes optical absorption prior to the n-n gap transitions. In fact, referring to the example (9.30) of the band structure of doped heterocyclic polymers, transitions may occur from the valence band to the bipolaronic levels. These intergap transitions are revealed by changes in the optical absorptions, as shown by Fig. 9.8 which illustrates the typical case of the spectral evolution of polydithienothiophene upon electrochemical doping (Danieli et al., 1985). 

During the past three years we have been studying the chemical (SbCl5) oxidation of well-characterized oligomers of polyacetylene, poly[p-phenylene vinylene] (PPV) and poly[2,5-thienylene vinylene] (PTV) in order to determine how polaron and bipolaron states can be preferentially formed and stabilized. 

In all cases, the oxidations proceed via two consecutive one-electron transfers forming polaron and bipolarons states consecutively, with the bipolaron being the more stable state in all cases. A typical bipolaron formed from a bis-(p-methoxy phenyl) polyene is shown below (n = 4). In fact, both as the conjugation... 

In all cases, formation of a bipolaron state coincides with a complete bleaching of the original tt-tt polyene transition, which in effect moves the optical absorption "window" several hundred nm to the red. More recently we have discovered that polaronic state formation (EDG = MeO) can be preferentially controlled if the quantity of oxidizing agent is carefully monitored (25). 

Fig. 4.3 The bipolaron state in poly(p -phenylenevinylene) is shown schematically.

We should mention that another possible explanation of the Pauli-like behavior has recently been proposed in terms of contributions of the triplet-excited bipolaronic states to the spin susceptibility [94]. As shown by Bussac and Zuppiroli, the polaron-bipolaron energy difference U is essentially determined by the interdopant distance Ld. The authors also consider the bipolaron triplet state, which reduces to two separate polarons for large Ld. Due to disorder, the Ld are distributed, giving rise to a distribution for the energy of the magnetic states. Summing over the distribution yields a Curie-like contribution, plus a smoothy temperature-dependent term, which resembles a Pauli contribution. 

Optical measurements upon doping show the characteristic features of polarons and bipolarons i.e. the two sub-gap electronic absorption bands predicted by SSH theory [13]. Voss et al. [110] measured difference spectra obtained upon doping for PPV and some soluble derivatives and found broad electronic features in the mid and near infrared, corresponding to the two subgap transitions associated with the bipolaron state. Doping-induced spectra taken by Voss et al. for a series of PPV derivatives are shown in Fig. lVD-5. Similar results are seen in the doping case for PPy [111], as well as for PT [112]. Vardeny et al. 

Microporous hosts related to zeolites, i.e., Cu(II)-exchanged alumina- and chromia-pillared layered a-Sn- and Zr-phosphates, have been studied for the polymerization of pyrrole introduced from the vapor phase. XPS data and optical spectra suggested that more than one type of polymer was formed in the pillared hosts, with mixed neutral/bipolaron states on low-oxidation level polymer. The absence of significant conductance (less than 10 S/cm) was associated with short pol5uneric units. 

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