Defunctionalization of Ester-Substituted Electrochromic Dioxythiophene Polymers

Abstract

This work demonstrates a simple process to form insoluble poly(3,4-propylenedioxythiophene) (Poly[ProDOT]) films by defunctionalization of a soluble form, which is useful in the fabrication of multilayer polymer devices. Three new ester disubstituted Poly[ProDOTs], soluble in common organic solvents, including chloroform, methylene chloride, toluene, tetrahydrofuran, and ethyl acetate, have been synthesized by oxidative polymerization using iron(III) chloride. These Poly[ProDOT-diesters] have the expected repeat unit stuctures along with GPC determined number-average molecular weights ranging from 9000 to 12 000 g mol-1. Dilute polymer solutions in toluene exhibited red fluorescence with quantum efficiencies from 0.24 to 0.32. Homogeneous thin films were formed by spray casting polymer solutions onto ITO coated glass slides and compared to films prepared by electropolymerization. These Poly[ProDOT-diesters] are electroactive, switching from a dark blue-purple to a transmissive sky blue when potentials are applied between −0.9 and +0.3 V vs Fc/Fc.+ A thin film saponification method was developed and rendered the spray-cast films insoluble by submersion into 0.1 M KOH in hot methanol for 1 h to remove the solubilizing ester group. In the case of a bis(heptanoate) Poly[ProDOT-diester] (1), spectroelectrochemistry showed minimal change in its electronic spectra after methanolysis and the resulting alcohol-substituted polymer (Poly[ProDOT-diol] (4)) could be repeatably switched between neutral and oxidized states in subsecond times. In contrast, when a more highly functionalized Poly[ProDOT-tetraester] (3) was used, a small change in the electronic spectrum which is associated with a distinct color change from burgundy to blue was observed upon methanolysis to Poly[ProDOT-tetraol] (6). The insoluble alcohol-substituted polymer films were found to be efficient hole transport layers in polymer light-emitting diodes.