EFFECTS OF ENVIRONMENTAL-FACTORS ON THE ELECTROOPTICAL PROPERTIES OF CONJUGATED POLYMERS CONTAINING OLIGO(OXYETHYLENE) SUBSTITUENTS

The electrochemical and spectroelectrochemical properties of poly[3-(3,6-dioxaheptyl)thiophene] (PDHT) films deposited on transparent ITO electrodes have been analyzed in various electrolytic media (0.1 and 1 M Bu4NClO4 and LiClO4 in CH3CN and 0.1 and 3.5 M LiClO4 in H2O) by cyclic voltammetry (CV), UV-visible absorption spectroscopy, and cyclic voltabsorptometry (CVA). The voltammograms recorded in these various media involve successive wave systems whose characteristics are strongly dependent on the solvent and on the nature and concentration of the electrolyte cation, Bu4N+ leads to a positive shift of the peak's potential and to a drop of electroactivity, whereas, in contrast, Li+ improves the resolution of the voltammetric waves and produces a negative shift of the peak's potential and an increase of the doping level. PDHT remains highly electroactive in aqueous media in which the increase of the concentration of lithium cations produces also an enhancement of the voltammetric waves and an increase of the doping level. In situ optical spectra of films undoped in these various media show that, in CH3CN, the increase of [Li+] leads to a slight bathochromic shift of the bands' maxima and to an enhancement of the resolution of the vibronic fine structure, whereas the reverse effects are observed with Bu4N+ or aqueous media. The CVAs recorded at the absorption maximum of the neutral polymer show successive slope inflexions tightly correlated to the CV waves and providing evidence for distinct oxidation stages and for structural relaxation during the redox processes. Li+ increases the magnitude of the first step and decreases the corresponding potential, whereas Bu4N+ has the opposite effect and the CVA recorded in aqueous media involve only one step for both the oxidation and the reduction processes. These results are discussed in terms of feedback effects of the interactions between the oligo(oxyethylene) substituents and the chemical environment of the polymer on the processes of charge and mass transport in the polymer and on the geometry and rigidity of the conjugated poly(thiophene) backbone.