Infrared spectroscopic study of polaron formation in electrochemically synthesised poly(3-alkylpyrroles)

Abstract

Ex-situ infrared absorption spectroscopy was performed on electrochemically synthesised poly(3-alkylpyrroles) as a function of the alkyl chain length (poly(3-hexylpyrrole) or poly(3-decylpyrrole)) and the counterion (tosylate, perchlorate, tetrafluoroborate, or hexafluorophosphate). More accurate vibrational assignments were made on the basis of the band shift on deuteration of 3-decylpyrrole. In-situ FT-IR spectroscopy was also performed varying the oxidation state of electrochemically polymerised poly(3-alkylpyrroles) from the “undoped” neutral state to the “doped” conductive one. The enhancement and frequency shift of infrared bands as a function of the potential was studied from −0.4 V (vs. Ag/AgCl (sat.)) to 1.0 V for poly(3-decylpyrrole)perchlorate (P3DP(ClO₄)) and from −0.5 V to 0.7 V for poly(3-decylpyrrole)tosylate (P3DP(TsO)). The charge passed during the entire oxidative process is associated with the generation of charge carriers. Infrared absorption bands at approx. 1500 cm⁻¹ and 1530 cm⁻¹, ascribed to cooperative skeletal stretching of pyrrole moieties, relate to the formation of polarons and bipolarons, respectively. Both types of charge carriers coexist throughout the potential values where oxidation of the polymer takes place. Bipolarons prevail over polarons at higher potential values (higher than 0.4 V and 0.0 V for P3DP(ClO₄) and P3DP(TsO) respectively). The effective shielding of the positive charge by tosylate, due to its effective coordination to the hydrogen of the pyrrole moiety, enhances the formation of the bipolaron over the polaron at lower potentials (0.0 V) with respect to the case when perchlorate is employed (0.4 V). The shielding of the tosylate causes the bipolarons to extend over fewer pyrrole moieties than in the case of perchlorate with a consequent increase of the stiffness of the oxidised polymer. This gives rise to a lower electrical conductivity in the case of P3DP(TsO) (10⁻³ S cm⁻¹) with respect to P3DP(ClO₄) (10⁻¹ S cm⁻¹).