The ESR Characterization of Oligomeric Thiophene Materials

The ESR of electro-oxidation products of orthogonally bridged dimers of alpha,alpha-coupled thiophene oligomers and spiro-fused phenylene-thiophene mixed oligomers was studied as a function of temperature. ESR measurements on the one electron oxidation products indicate that the charge remains on a single oligomer unit and does not hop between the two bridged oligomer chains for temperatures up to 300K. This is evident from the presence of hyperfine structure in ESR spectra for the dimers of short chain oligomers and the small temperature dependent line broadening for longer chain spiro dimers, characteristic of a low energy process such as the paramagnetic site moving along the oligomer chain length. The activation energy for a paramagnetic center moving along the oligomer chain length has been estimated at 100/cm. The incorporation of phenylene into the oligomer unit shifts oxidation potentials anodic and also inhibits the mobility of the polaron along the pi-conjugated oligomer segment. Evidence for formation of pi-pi dimers in solutions of 7-mT has also been obtained. Prepared in cooperation with IBM Almaden Research Ctr., San Jose, CA.