An INDO/S-CI study of the photoinduced absorption spectrum of polyemeraldine

The optical properties of polyemeraldine base (PEB) are studied using the semi-empirical INDO/S-CI method. The theoretically calculated optical absorption spectrum is in very good agreement with experimental results showing absorption peaks at ∼2 and ∼4 eV. We find the lower absorption to be due to a localized HOMO to LUMO type of excitation and the high energy absorption to be a mixture of transitions from low lying orbitals to the LUMO and π-π^* transitions. Upon optical pumping, new absorptions are experimentally observed at 0.9, 1.4, and 3.0 eV. We show that absorptions associated with positive and negative polarons as well as positive bipolarons account very well for the two higher peaks. No explanation for the 0.9 eV peak in terms of polarons or bipolarons could be found.