TIME DYNAMICS OF GROWTH AND DECAY OF PHOTOINDUCED DEFECTS IN POLYANILINES

Polyanilines differ from other electronic polymers in the presence of very long-lived photoinduced absorptions, attributed to pinning of ring-rotational polarons of large effective mass. We report the growth and decay kinetics of long-lived photoinduced absorption in polyanilines. The intensity of the photoinduced 1130 cm-1 peak is monitored for periods of hours both with exciting laser light on and after it has been turned off. Photoinduced signal growth exhibits a stretched-exponential behavior in all oxidation states of polyaniline at low temperatures and does not saturate even after hours of irradiation. Photoinduced signal decay (after the pump light is turned off) is very slow, with half-life of the order of days. The evolution of the electronic photoinduced peak at 1.4 eV scales with that of vibrational peaks, indicating that the mass of photoinduced defects does not change with time. Several models that give rise to stretch-exponential kinetics are reviewed. Comparison with light-induced electron spin resonance studies suggests that serial growth and decay are the origins of the very large range of time dynamics in this system.