Spectroscopic and spectroelectrochemical properties of a poly(alkylthiophene)-oligoaniline hybrid polymer

By copolymerization of 3-octylthiophene with thiophene containing aniline tetramer in the 3-position we have prepared a hybrid copolymer, poly(3-octyl-2,5-thienylene-co-3-oligoaniline-2,5-thienylene), exhibiting very interesting spectroscopic and spectroelectrochemical properties. The UV-vis-NIR spectrum of this new hybrid copolymer, in addition to the band ascribed to the pi-pi* transition in the poly(2,5-thienylene) chain, shows two bands at 330 nm and ca. 580 nm which can be attributed to the transitions in the pendant oligoaniline groups, namely to the pi-pi* transition in the benzoid ring and to the excitonic-type transition between the HOMO orbital of the benzoid ring and the LUMO orbital of the quinoid ring. Electrochemical activity of poly(3-octyl-2,5-thienylene-co-3-oligoaniline-2,5-thienylene) was tested in nonaqueous electrolytes combining cyclic voltammetry, LTV-vis-NIR spectroelectrochemistry, and Raman spectroelectrochemistry. All techniques unequivocally show that both the oligoaniline side chains and the poly(2,5-thienylene) main chain can be electrochemically doped. The doping starts by the oxidation of aniline tetramer substituents and is followed by the oxidation of the poly(2,5-thienylene) main chain. Because of strong resonance effect Raman spectroelectrochemistry turned out to be a very selective probe of the polymer doping. The blue excitation line probes selectively the vibrations originating from undoped segments of the poly(2,5-thienylene) main chain whereas the red one probes only the undoped and doped oligoaniline substituents. The infrared excitation line enhances resonantly signals due to vibration of the doped parts of the polymer and reveals the sequence of doping. Protonation of pending oligoaniline groups with diphenyl phosphate lowers the potential of the onset of the doping as revealed by cyclic voltammetry and LTV-vis-NIR spectroelectrochemistry.