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Femtosecond excited-state dynamics and optical bistability in three forms of polyaniline

Photoexcitation and relaxation dynamics of photoinduced defects in the emeraldine base (EB) form of polyaniline, poly (m-aniline) (PMAn) and the emeraldine salt (ES) form of polyaniline embedded in polymethylmathacrylate (PMMA) has been measured using femtosecond time-resolved absorption saturation technology. The relaxation of photoexcitation, observed in EB, consists of two components: an initial ultrafast relaxation (approximately 100 fs) and a following slow relaxation (approximately 30 ps). A slower photoinduced bleaching compared with that in EB and 5-ps relaxation process were observed in PMAn. The ES/PMMA film exhibits the fastest ground recovery (approximately 0.5 ps) of photoexcitation, which is about two orders of magnitude faster than that of EB and an order of magnitude faster than that of PMAn. This ultrafast polaron relaxation in ES/PMMA was observed for the first time. Optical bistability and multistability were obtained by the simulation of the excitation and relaxation processes observed in the three forms of polyaniline. The switching time is strongly affected by the relaxation dynamics of the photoexcited defects.