Bipolaron absorption

Table 3.7 Polaron and bipolaron absorptions in polypyrrole (after Christensen and Hamnett, 1991) ...

Table I. Stabilized Polaron and Bipolaron Absorption Spectra...

Figure IVD-5 Doping-induced spectra for PPV and a series of its derivatives. Note vibrational modes in the mid-IR and bipolaron absorption bands in the near-IR. (Taken...

Recently, Hong et al. reported the optical properties of electrochemically synthesized P3MT NWs that were separated from nanoporous AI2O3 templates by treatment with HF [62]. Figure 9a shows the UV-vis absorption spectrum of P3MT NWs that were synthesized at a lower temperature and higher applied current than those shown in Fig. 8. A broad and relatively intense bipolaron absorption band was observed at 780 nm and a relatively weak n-n transition peak was observed at 390 nm, which indicates that the P3MT NWs shown in Fig. 9 were more heavily doped than those shown in Fig. 8. 

The process of formation of a multilayer film on the ITO coated glass from sequential addition of PABA/RNA bilayers was observed with UV-Vis Spectroscopy as shown in Figure 3.34. The film growth observed with the deposition of additional bilayers suggests that the multilayer formation of PABA/RNA is reproducible with sequential deposition. All spectra exhibit an intense and sharp peak attributed to the w-tt and bipolaron band transitions. The bipolaron absorption band at 800 nm, associated with complexation of RNA with PABA, increases linearly with the number of PABA/RNA bilayers (Figure 3.34 inset). The linear relationship between absorbance and the number of deposited bilayers indicates that the deposition was reproducible from layer to layer, i.e., the amount of PABA adsorbed in each bilayer was the same. In addition to these results, multilayer formation was observed with ellipsometric and X-ray photoelectronic spectroscopy. The linear increase in film thickness with number of PABA/RNA bilayers was observed using ellipsometry. The average thickness of the PABA/RNA bilayer built up on a silicon substrate was approximately 10 nm. Additionally, X-ray photoelectron... 

Using the SPEL (spectroelectrochemical) curves shown in the next chapter for at least three different CPs, identify plausible ir - ir and bipolaron absorptions, and draw out the electronic transitions they represent in a band structure diagram for each CP. 

The P(Py) system depicted in Fig. 3-1 represents a particularly well behaved and more "ordered system, where the evolution from the polaron to the bipolaron bands is clearly visible. Fig. 3-2 shows another such SPEL of an experiment P(Py) system, showing more clearly the disappearance of the polaron absorption (ca. 520 nm) with increased doping, to yield the broader bipolaron absorptions (beyond 700 nm). Figs. 3-3. 3-4. and 3 5 show more representative CP systems Fig. 3-5 also shows the alternative, %-Transmission representation. 

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. 

Several other derivative measurement parameters may also of interest in the electrochromism of CPs for specialized applications. The most straightforward of these are differential measurements, e.g. of the differential Absorbance or Transmittance of various doped states of a CP with reference to its pristine, undoped state. Figs. 3-34 and 3-35 show two such differential plots. In the first, (a) shows the standard SPEL, while (b) shows its differential the differential plot simply serves to enhance the trends observed in the SPEL, but also serves to accurately pinpoint the isosbestic point. In the second, the differential absorbance monitoring a bipolaron absorption at 700 nm shows that up to an applied potential of ca. 0.6 V, the formation of bipolarons appears minimal after 0.6 V, however, bipolarons form rapidly. 

Well-defined CVs can also be used to indirectly extract information such as CP bandgaps (E and the relative strength and position of polaron and bipolaron absorptions. While the latter has been carried out effectively only for poly(aromatic amines). Fig. 4-19 below shows an example of the computation of the former from voltammetric data, using the simple formula = E, - Ep ed- Fig. 4-24 summarizes bandgaps for a number of CPs computed primarily from electrochemical data. 

Onoda et al. [37] carried out an interesting electropolymerization of oligothiophenes having a double bond (ethene or vinylene group) between thiophene chains, shown in Fig. 14-3a. In these, the conformational restriction introduced by the rigid vinylene groups is said to have some effect on the bipolaron absorptions. Fig. 14-3b shows a SPEL for one of these polymers a reddish brown to dark blue electro-chromism is observed, with calculated bandgaps ca. 0.2 eV smaller than that of P(T). 

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