Polarons Correlation with Optical Transition

The states in the gap and the associated optical transitions for P" are shown in Figure 22.3a. The polaron energy states in the gap are SOMO and LUMO, respectively, separated by 2wo(P) [52]. Then three optical transitions Pf, P2, and P3 are possible [52-54]. In oligomers, the parity of the HOMO, SOMO, LUMO, and LUMO + 1 levels alternates they are g, u, g, and u, respectively. Therefore, the transition vanishes in the dipole approximation, and the polaron excitation is then characterized by the appearance of two correlated optical transitions below Eg. Even for long chains in the Hiickel approximation, transition P3 is extremely weak, and therefore the existence of two optical transitions upon doping or photogeneration indicates that polarons were created [51,54]. Unfortunately, polaron transitions have not been calculated for an infinite correlated chain. This is a possible disorder-induced relaxation of the optical selection rules that may cause ambiguity as to the number of optical transitions associated with polarons in real polymer films. 

FIGURE 22.26 (a) Transient PM spectrum of PDPA-nB solution at t = 0 that shows an SE and two correlated PA bands of excitons the onset of a third PA band is assigned. The inset shows the decay kinetics of the SE (full line) and two PA bands (dashed lines), (b) The steady state PM spectrum at 80 K that shows two other correlated PA bands and photoinduced IRAVs of polaron excitations. The inset shows the energy levels and optical transitions associated with a positively charged polaron excitation. (From Korovyanko, O.J., et al., Phys. Rev. B, 67, 035114, 2003. With permission.)... 

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