Spectroscopy and defect states in polyaniline

We report the far-infrared through visible photoinduced absorption (PA) spectra of polyaniline in the emeraldine-base and leucoemeraldine-base forms. The direct-absorption spectrum of emeraldine base (EB) has a broad absorption centered at 2 eV (the ``exciton'' band) and an absorption band at \ensuremath{\sim}3.6 eV (the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band gap). The PA spectra of EB for pumping into the ``exciton'' band and across the band gap are nearly identical, indicating the same types of charged defect states are created upon photoexcitation. The direct absorption spectrum of leucoemeraldine base (LB) shows only the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band-gap absorption at \ensuremath{\sim}3.6 eV. For pumping into this absorption band, the PA spectrum of LB is very similar to that of EB, although important differences result from the lack of quinoid structures in this material. Based on our results, we propose a model for the photocreation of defect states in leucoemeraldine base and emeraldine base. The central roles of phenyl-ring rotations and of massive polarons are discussed.