Modern Polymer Spect..

4.7 Pol)~(p-pheiz~~lene) 219
ENERGY i 1 0 ~ ~ ~ - '
ENERGY I oZcm
Figure 4-8. Absorption spectra ofa THF solution ofp-terphenyl at (a) early and ib) later stages of
reduction. In (a), the 35600 cni-' band decreases and the 20800 and 10900 cm-' bands increase in
intensity. In (b), the 20800 and 10900 cm-' bands decrease and the 15400 cm-' band increases in
intensity.
and dianions (PPn-) of p-oligophenyls, which are models of negative polarons and
bipolarons, respectively.
4.7.1.2 Radical Anions and Dianions of p-Oligophenyls
The electronic absorption spectra of PPn- and PPH- have been systeinatically
studied [69, 701. When a tetrahydrofuran (THF) solution of PP3 is exposed to a
sodium mirror, its absorption spectrum gives new bands (Figure 4-8). At an early
stage, the band observed at 35700 cm-' (280 nm) arising from neutral PP3
decreases and the bands observed at 20800 and 10900 cm-' (481 and 916 nm)
increase in intensity. These new bands are attributed to the radical anion of PP3
(PP3'-). In the course of this spectral change, an isosbestic point is observed at
about 31 500 cm-', indicating that PP3 is quantitatively converted into PP3'-.
When the solution is further exposed to the sodium mirror, the 20 800 and 10 900
cm-' bands decrease and the band observed at 15400 cm-' (650 nm) increases in
intensity. The 1.5400 cm-' band is ascribed to the dianion of PP3 IPP3'-). During
this reaction, three isosbestic points are observed at about 25 800, 20400, and
13400 cm-'. The THF solutions of PPiz (n = 4,.5, and 6) show similar reduction
reactions which may be described as
PPrz + Na 4 PPi7'- + Na'
PPi7'- + Na i PPn2- + Na'
Since PPn- is stable for a long time, we can conclude that the following disproportionation
reaction does not occur.
2PPn'- 4 PPiZ + PPd