Picosecond photoinduced absorption and long-lived effects in polyaniline

We present results of picosecond photoinduced absorption studies on poly(pernigraniline base) (PNB), the degenerate-ground-state form of polyaniline, and poly(emeraldine base), a nondegenerate form. When pumped at the 2.1 eV absorption edge, both polymers display photoinduced absorption in the 1.3-1.6 eV region due to charged polarons. PNB displays additional absorption at >1.5 eV due to neutral solitons. For each of these polymers, the photogenerated polaron absorption at fixed pump/probe time delay is found to lose oscillator strength with continued exposure to the pump beam, decaying in a stretched-exponential manner over several minutes and saturating at about 10 percent of its initial value. In PNB, the soliton absorption saturates completely and is replaced by a slowly-decaying bleaching signal which is comparable in magnitude. We consider a possible origin of these effects to be buildup of longlived electronic states in the polymers.