Ground-state structural dynamics in doped and undoped polyaniline films probed by two-dimensional infrared vibrational echo spectroscopy.

Two-dimensional infrared vibrational echo spectroscopy (2D-IR VES) provides information about the structural dynamics occurring on the ultrafast time scale, a temporal regime that is comparable to that of charge-hopping events in conducting polymer films. In this study, 2D-IR VES is used to study polyaniline (PANI) thin films in three states of varying conductivity: emeraldine base (PANI-EB, semiconducting), emeraldine salt (PANI-ES) doped with dinonylnaphthalene sulfonic acid (conductive), and PANI-ES doped with camphor sulfonic acid (highly conductive). UV-visible and FTIR spectroscopies were used to characterize the static electronic and structural differences between these materials, and then these results were compared to the dynamical results from 2D-IR VES. The electronic ground state ultrafast dynamics for the PANI-EB reveal very fast motions that are not present in either of the PANI-ES samples. Despite differences in conductivity, no significant dynamical differences are observed for the films prepared with the two dopants. We interpret these results in light of previous work on the structural ordering induced by doping with sulfonic acids and the possible correlations between charge carrier mobilities and low frequency structural dynamics.