Modern Polymer Spect..

220 4 Vibvofionnl Slwctvoscopy oj Intact and Doped Conjugated Po1ynier.s
band I1
-
‘6 b?, @6 d hg
4 -fs- 4 *-
$4 ‘4
tJ3 tJ3
(a)
(b)
I
I
I
I
I
I
I
I
Figure 4-9. Schematic energy level diagrams. (a) The
radical anion of biphenyl IPP2.-); (b) the dianion of
biphenyl (PP2’-). 0, electron; arrow. electronic transition.
The inolecular orbital levels are taken from 1771.
This reaction is similar to the process of the intramolecular formation of a bipolaron
from two polarons.
The electronic transition energies [69, 701 of the radical anions and dianions of
p-oligophenyls are plotted in Figure 4-7. Two strong bands are observed for the
radical anions and are called bands I and I1 (curves b and c in Figure 4-7, respectively).
On the other hand, one intense band is observed for the dianions, which
is called band I’ (curve d in Figure 4-7). In the electronic absorption spectra of
the radical cations and dications of a-oligothiophenes [71-73], two strong bands
and one strong band are commonly observed for the radical cations and dications,
respectively. In the case of the radical cations of oligophenylenevinylenes, two
bands are observed [74, 751, although different results are also reported [76].
The absorption spectra of conducting polymers including poly( p-phenylene) have
been discussed within the frame work of the Hiickel approxiination [7, 101. In order
to correlate the spectra of the radical anions and dianions of y-oligophenyls with
those of doped poly( p-phenylene), we will discuss the observed absorption spectra
of the charged p-oligophenyls on a similar theoretical basis. The energy diagram of
PP2’- is shown schematically in Figure 4-9a. Molecular orbital levels in this figure
are taken from the results calculated by the Pariser-Parr-Pople-SCF-MO method
for PP2 (021, symmetry) [77]. A Pariser-Parr-Pople-SCF-MO-CI calculation for
PP2‘- [78] shows that band I is assigned to the transition mostly attributable to
d6. These transitions are
the electronic excitation of dl0 t d7 and band I1 to d7 +-
polarized along the long molecular axis. The a, and d9 levels are nonbonding with
respect to the inter-ring CC bond. It is reasonable to consider that these assignments
hold also for PPn- (12 = 3-6). Moreover, band I’ of PP2’- is assigned to the transition
mostly attributable to the electronic excitation bl,,