Poly[bis(pyrrol-2-yl)arylenes]: Conducting polymers from low oxidation potential monomeps based on pyrrole via electropolymerization

A series of derivatized bis(pyrrol-2-yl) arylenes, specifically 1,4-bis(pyrrol-2-yl)benzene (1e, BPB), 1,4-bis(pyrrol-2-yl)-2,5-dimethoxybenzene (2e, BPB(OCH3)(2)), 1,4-bis(pyrrol-2-yl)-2,5-diethoxybenzene (3e, BPB(OC2H5)(2)), 1,4-bis(pyrrol-2-yl)-2,5-didodecyloxybene (4e, BPB(OC12H25)(2)), 2,6-bis(pyrrol-2-yl)naphthalene (5e, BPN), and 4,4'-bis(pyrrol-2-yl:,biphenyl (6e, BPBP) have been synthesized. Cyclic voltammetry shows these monomers to oxidize at relatively low potentials to form cation radicals. The lowest peak monomer oxidation potential of +0.15 V vs Ag/Ag+ is observed for 2e, lower than that reported for any other pyrrole-based monomer. Electrochemical polymerization from a 0.01 M monomer, 0.1 M tetrabutylammonium perchlorate (TBAP), and acetonitrile solution was carried out using multiple scanning cyclic voltammetry to yield polymers having E(1/2) values ranging from -0.1 to +0.1 V. These low oxidation potentials cause the polymers to be quite stable in the electrically conducting form. The electronic band gaps for these polymers (measured as the onset of the pi-pi* transition) occurs between 2.3 and 2.4 eV. Upon electrochemical doping, the intensity of the pi-pi* transition decreases while bipolaron bands appear at lower energy. The intensity of these bands increases upon increasing the anodic potential. The redox doping process was found to be reversible upon electrochemical reduction of the films. The unsubstituted polymer poly[1,4-bis(pyrrol-2-yl)benzene], doped with perchlorate, was found to exhibit an average conductivity of 1 S/cm. When the dopant was the anion of bis [(trifluoromethyl)sulfonyl]imide, the conductivity increased to 30 S/cm. These polymers are stable to thousands of double-potential steps before 50% loss of electroactivity occurs.