Photoinduced localized charged excitations in polyaniline

Abstract

We report the infrared photoinduced absorption spectra of polyaniline in the ‘emeraldine’ base form while pumping into the two optical transitions at 2.0 eV and 4.0 eV, respectively. For optical pumping at energies near 2.0 eV, we found photoinduced infrared-active vibrational (IRAV) modes that are essentially identical to those observed subsequent to protonation. We propose that the optical absorption at 2.0 eV in the emeraldine form is due to an $\text{n}\text{−π}{}^{\mathrm{\ast }}$ transition from the non-bonding nitrogen lone-pair to the conduction ($\text{π}{}^{\mathrm{\ast }}$ band. With 4.0 eV optical pumping, the IRAV modes are significantly different, demonstrating that a different species is produced when pumping into the $\text{π−π}{}^{\mathrm{\ast }}$ transition. Our analysis, based on the amplitude mode formalism, indicates that the charged photoexcitations and the polarons that are generated through protonation are localized and not highly mobile.