Ultrafast dynamics of polarons in conductive polyaniline: comparison of primary and secondary doped forms

Time- and wavelength-resolved pump-probe measurements are performed on the conductive, primary and secondary doped, forms of polyaniline in solution to investigate the relaxation dynamics of photoexcited polarons. Contrasting dynamics observed in the two forms allow investigation of electronic (and structural) relaxation of this material. Pump pulse at 800 nm photoexcites an electron from the valence to polaron band, and subsequent relaxation dynamics are probed by a white light continuum pulse. Three distinct features are observed in the primary doped sample: (1) absorption near time zero in the 900-1025 nm probe wavelength region; (2) delayed absorption from 850 to 1025 nm; (3) pronounced oscillations with frequencies of 165 and 210 cm(-1). The first two features are associated with intraband absorptions to higher-lying states in the polaron band from the initial excited and conformationally changed intermediate states. The oscillations reflect torsional motions associated with photoexcitation and relaxation. In the secondary doped material, only bleaching and stimulated emission are observed throughout the whole spectral region; neither transient absorption signals nor oscillatory dynamics are observed. Kinetic modeling is performed to establish the mechanism of relaxation. We propose that the excited polaron relaxes nonradiatively to the ground state through an intermediate state(s) with a twisted geometry. Our measurements and analysis allow us to describe the structure of the polaron bands for primary and secondary doped polyaniline; they are in the small and large polaron limits, respectively, and are consistent with a bandgap for the primary doped form and without a bandgap (i.e., metallic) for the secondary doped material.