Raman spectra of polypyrrole and its 2,5-¹³C-substituted and C-deuterated analogues in doped and undoped states

Raman and optical absorption spectra were studied on electrochemically polymerized polypyrrole in as-polymerized doped (highly conductive), partially undoped and substantially undoped 'neutral' (insulating) states. Vibrational assignments were made on the basis of the data of 2,5-¹³C-substituted and C-deuterated derivatives. The Raman result of the neutral polymer is consistent with the α,α′-linked all-s-trans coplanar and conjugated structure. In the as-polymerized polymer there exist both radical cation and dication in which the bond orders of C₃C₄ and interring CC are increased as compared to those in the neutral polymer. The structural distortion in the dication is greater than that in the radical cation. Changes in optical absorption and Raman spectra during reduction (undoping) were explained by the conversions of the dication to the radical cation and the radical cation to the neutral species.