Infrared photoexcitation and doping studies of poly(3-methylthienylene)

Abstract
We report detailed photoinduced and doping-induced absorption spectra of poly(3-methylthienylene) (P3MT) in the infrared-frequency range. The four infrared-active translational (T) modes of bipolarons and the weak-ring (R) modes associated with the ring deformation were observed, as in polythiophene (PT). Detailed analysis based on the amplitude-mode formalism shows that the nondegenerate ground state leads to an ‘‘extrinsic’’ contribution to the energy gap of 0.26 eV in P3MT, and that the pinning is weaker than in PT.