Ultrafast dephasing of photoexcited polarons in primary doped polyaniline

Ultrashort pulse pump-probe and transient anisotropy methods are used to study polaron dynamics of primary doped conductive polyaniline in solution. The degenerate frequency pulses are resonant with the polaron absorption of the emeraldine salt form of polyaniline. Pronounced oscillatory components with frequencies of 160 and 210 cm^-1 are observed, in good agreement with recent Raman and infrared spectral studies, and are likely to represent ground states modes. However, the close agreement with the excited state lifetime suggests that they may have partial excited state character. Photoexcited carrier dynamics have been identified on two distinct time scales. The very fast component, with a 34 fs exponential decay and 90% of the total amplitude, is attributed to the relaxation of the initially delocalized excitations within the polymer chain. This is supported by the transient anisotropy signal exhibiting a 30 fs Gaussian decay. In analogy with exciton dynamics in photosynthetic light harvesting assemblies, the extremely rapid anisotropy decay is attributed to dephasing of the initially delocalized polaron excitation due to diagonal disorder (i.e., variations in the monomer energies) and vibronic coupling. The longer time scale dynamics, occurring with two or more relaxations on >500 fs to >100 ps time scales, reflect the decay from the initially excited Franck-Condon region and the slow recovery of the ground state from a structurally altered, "twisted" intermediate. We propose this occurs by way of an electronic state with charge-separated character as in the isomerization of ethylenic species including retinal and PYP. Kinetic simulations are performed to establish a mechanism for the relaxation dynamics. Explicit inclusion of heterogeneity is necessary to adequately model the data.