Spectroscopic investigation of thermal treatment of doped polypyrrole

The results of a UV/VIS spectroscopic investigation of thin films of oxidiz ed polypyrrole doped with p-toluenesulfonate, dodecylsulfate or copper phth alocyanine tetrasulfonate anions immediately after electrochemical synthesi s and during heat treatment are presented. The absorption bands in the resu lting spectra were identified with the electronic transitions of the polaro ns, bipolarons and transverse bipolarons as follows: the absorption bands b elow ca. 1.4 and at 1.8 eV are both attributed to the sum of the polarons a nd transverse bipolarons, the band at 2.44-2.5 eV to the sum of polarons, b ipolarons and transverse bipolarons and the band around 3.2-3.6 eV to the b and gap transition of polypyrrole. The identity of the absorption band abov e 3.8 eV remains uncertain. The absorbance changes of bands in the UV/VIS s pectra collected during sample aging and heat treatment were explained as a result of the rearrangement of the polypyrrole structure. The main process es 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 hea ting at 80 degrees C seem to be caused mainly by an interchain dissociation of the transverse bipolarons to polarons and by their slower intrachain re combination into bipolarons and the polypyrrole neutral form. The structura l rearrangement induced by the sample heating is a very slow process becaus e the UV/VIS spectra were not stable even after 35 h. The changes taking pl ace in the polypyrrole structure during sample cooling are the opposite of those during the heating but they are only partially reversible in magnitud e. The polypyrrole samples doped with p-toluenesulfonate and copper phthalo cyanine tetrasulfonate exhibited very similar behavior whereas dodecylsulfa te doped samples exhibited an extraordinary behavior owing their liquid-lik e solid-state micellar structure. The magnitudes of the polypyrrole spectra changes showed that only a relatively small proportion of the species pres ent within the polypyrrole were influenced by these processes.