Static and dynamic structural memory in polyaniline thin films

Steady-state UV-visible and FTIR spectroscopies were used to characterize the electronic and structural changes that occur in polyaniline (PANI) thin films over the course of a single deprotonation and reprotonation cycle. The dedoping from the emeraldine salt (PANI-ES) to the emeraldine base (PANI-EB) form was achieved by treatment with a weak base (ammonia gas), and the PANI-ES was recovered by exposure to humid air and then dry air. The spectroscopic changes were classified into two general categories: those in which the recovered sample features were intermediate to the initial PANI-ES and the deprotonated PANI-EB and those in which the recovered sample features changed monotonically from the starting PANI-ES toward a unique observable. Two-dimensional IR vibrational echo spectroscopy (2D-IR VES) was then used to demonstrate that ultrafast structural dynamics on the time scales of hundreds of femtoseconds to a few picoseconds could also be organized into these two categories. In contrast, it was found that the slower dynamics on the tens of ps time scale appear unperturbed by the dramatic structural changes of the dedoping-redoping cycle. We discuss the relevance of these dynamics to charge mobilities in the initial and final PANI-ES states and compare their behavior to the film electrical resistances over the course of the protonation cycle. We show that specific structural dynamics are correlated with changes in the film conductivities and that PANI films have a memory of not only the static molecular structures of the as-cast materials but also some of the dynamics that are inherent to those morphologies.