Hydrophilic oligo(oxyethylene)-derivatized poly(3,4-ethylenedioxythiophenes): Cation-responsive optoelectroelectrochemical properties and solid-state chromism

Electroactive pi-conjugated polymers have been synthesized by electrochemical polymerization of precursors consisting of two (3,4-ethylenedioxythiophene) (EDOT) groups linked by oligo(oxyethylene) chains of variable length. The analysis of the cyclic voltammetric behavior in the presence of various cations (Bu4N+, L+, Na+, Ca2+, Sr2+, and Ba2+) shows that the nature and charge of the electrolyte cation exert a considerable influence on the potential of the anodic peak associated with the oxidation of the polymer, with positive shifts of several hundreds of millivolts in the presence of doubly charged cations. Spectroelectro-chemistry shows that complexation of metal cations by the polyether network induces conformational changes in the pi-conjugated backbone, which significantly contributes to the observed increase of the oxidation potential and the band gap. The polymers exhibit a high hydrophilicity, and experiments performed in the presence of Ba2+ show that replacement of acetonitrile by water produces a ca. 500 mV negative shift of the oxidation peak accompanied with a 0.30 eV decrease of the band gap, which reaches a minimal value under these conditions. These phenomena indicative of an enhancement of the effective conjugation in the polymer backbone are attributed to the removal of Ba2+ cations with concomitant swelling of the polymer by a specific solvation of the oligo(oxyethylene) network by water molecules. As a consequence of this high structural flexibility and environmental stability, the polymers exhibit fast, intense, and reversible ionochromic effects in the solid state.