Modern Polymer Spect..

2 12 4 T4hvritionnl Spectvoscopy of Intact nnd Doped Conjugrited Poljmiers
Table 4-2. Self-localized excitations and chemical terminologies.
Self-localized excitation Chemical term Charge Spin
positive polaron
negative polaron
positive bipolaron
negative bipolaron
neutral soliton
positive soliton
negative soliton
radical cation
radical anion
dication
dianion
neutral radical
cation
anion
+e
--E
+ 2e
- 2e
0
+e
-e
Figure 4-3. Schematic structures of self-localized excitations in poly( p-phenylenej and tucms-polyacetylene.
(a) Positive polaron; (b) negative polaron; (c) positive bipolaron; id) negative bipolaron;
(e) positive polaron; (fj negative polaron: (g) neutral soliton: (11) positive soliton; (i) negative soliton;.
D, donor; A, acceptor; +, positive charge: -, negative charge; 0, unpaired electron.
In trans-polyacetylene having a degenerate ground-state structure, soliton excitations
can be formed between the A and B phases (see Figure 4-2a). Solitons are
classified into neutral, positive, and negative types according to their charge. A
neutral soliton has no charge and spin 1/2 (Figure 4-3g). A positive (or a negative)
soliton has charge +e (or -e) and no spin (Figure 4-3h, i). A neutral soliton, a
positive soliton, and a negative soliton correspond, respectively, to a neutral radical,
a cation, and an anion of a linear tvans-oligoene with an odd number of carbon
atoms, CnHn+2 (where n is an odd number).
Bond-alternation defects or misfits in polyenes were studied using the Huckel MO
method in the early 1960s [36, 45, 461, though these are nothing but solitons proposed
by Su, SchrieHer, and Heeger [lS, 191 about two decades after. These workers
have studied solitons in tvnns-polyacetylene by using a simple tight-binding Hamil-