Spectroscopic investigation of thermal treatment of doped polypyrrole

Abstract

The results of a UV/VIS spectroscopic investigation of thin films of oxidized polypyrrole doped with p-toluenesulfonate, dodecylsulfate or copper phthalocyanine tetrasulfonate anions immediately after electrochemical synthesis and during heat treatment are presented. The absorption bands in the resulting spectra were identified with the electronic transitions of the polarons, bipolarons and transverse bipolarons as follows: the absorption bands below ca. 1.4 and at 1.8 eV are both attributed to the sum of the polarons and transverse bipolarons, the band at 2.44–2.5 eV to the sum of polarons, bipolarons and transverse bipolarons and the band around 3.2–3.6 eV to the band gap transition of polypyrrole. The identity of the absorption band above 3.8 eV remains uncertain. The absorbance changes of bands in the UV/VIS spectra collected during sample aging and heat treatment were explained as a result of the rearrangement of the polypyrrole structure. The main processes acting during the sample aging are the interchain conversion of polarons into transverse bipolarons and the interaction of polarons with transverse bipolarons yielding bipolarons. The spectral changes induced by sample heating at 80 °C seem to be caused mainly by an interchain dissociation of the transverse bipolarons to polarons and by their slower intrachain recombination into bipolarons and the polypyrrole neutral form. The structural rearrangement induced by the sample heating is a very slow process because the UV/VIS spectra were not stable even after 35 h. The changes taking place in the polypyrrole structure during sample cooling are the opposite of those during the heating but they are only partially reversible in magnitude. The polypyrrole samples doped with p-toluenesulfonate and copper phthalocyanine tetrasulfonate exhibited very similar behavior whereas dodecylsulfate doped samples exhibited an extraordinary behavior owing their liquid-like solid-state micellar structure. The magnitudes of the polypyrrole spectra changes showed that only a relatively small proportion of the species present within the polypyrrole were influenced by these processes.