Doping-induced bands

Meanwhile, the IR study on the photoexcited trans-(CH)x, in which the excited electron-hole pair is expected to change rapidly (a time of order 10 13 sec) into a charged soliton pair (Su and Schrieffer, 1980), has revealed the complete absence of the band around 900 (888) cm-1, the other two bands (1260 and 1370 cm-1) remaining (Blanchet et al., 1983 Vardeny et al., 1983). Hence the doping-induced band at 888 cm-1 is currently considered to be due to a pinned mode of dopant-induced carriers in the (CH), chain. 

In Figure 4.10 a typical uv-vis-nir spectrum of a partially doped conjugated polymer is presented. The coexistence of the n—n transition band and two doping induced bands can be observed. 

Fig. 6. V vs. for the normal modes of trans-polyacetylene. The experimental values are ( ) Raman shifts for different excitation wavelengths (a) infrared doping-induced bands and ( ) infrared bands induced by photoexcitation.

It has been shown that, upon doping, three new bands appear in the infrared, with such a very large absorption coefficient that, even for weak doping, they dominate the whole IR spectrum [34,35]. The doping-induced bands show the following characteristics in the frequencies Vj and the intensities... 

On this basis, the plot of v vs. R (Fig. 6) keeps its validity also for the IR spectra of doped PA, and the doping-induced bands can be accounted for, in frequencies and intensities, as the modes which contain, to various extents, the. Si mode, just as in the case of the Raman spectra of pristine polyacetylene. 

Based on the fact, that doping-induced bands come from collective phonons in the doped Id-lattice, phonon dispersion curves have been calculated for different values of I [39]. For long, but finite, doped chains, k K) phonons become the so-called "translational modes", k phonons may form a progression of bands, with rapidly decreasing infrared intensity, just like the case of long, but finite, n-alkanes. These modes are generally called in the literature "shape modes". 

Figure 8.17. IR spectra of a,a -dimethylquaterthiophene (DMQtT) doped with BF4 ions (DMQ1T-BF4) and iodine (DMQtT-I) in comparison with the neutral species (DMQtT ). The asterisks indicate the doping-induced bands occurring at 1400 and 1340 cm . Reprinted with permission from Reference 22. Copyright 1991 The Royal Society of Chemisty.

Figure 8.43. The near to mid-IR feature of DMSxT doped with iodine. Two different spectra taken with different apparatus were jointed at an energy of 0.6 eV (see text). The two subgap bands are observed in the near-IR region (peaking at 0.71 and 1.45 eV) and doping-induced bands in the mid-IR can be noted below 0.2 eV Reprinted with permission from Reference 174. Copyright 1993 American Chemical Society.

The doping- and photoinduced absorption spectra of polythiophene, substituted polythiophenes, and their composites have been reported [145-166]. Let us discuss the subgap absorptions due to charged excitations in unsubstituted polythiophene. The optical absorption spectrum of BF -doped polythiophene [42,46] and the photoinduced absorption spectrum of polythiophene [145] are shown in Figure 20. In the spectrum of BFJ -doped polythiophene (Fig. 20a), doping-induced bands are observed at 0.73 and 1.68 eV, which are located below the gap edge, 1.94 eV. The 0.73-... 

Fig. 28.12 Infrared spectrum of doped PA (at very small doping, 0.0017 h). The arrows indicate the doping-induced bands. (From Ref. 41.)...

---