ESR CHARACTERIZATION OF OLIGOMERIC THIOPHENE MATERIALS

The ESR of electrooxidation products of orthogonally bridged dimers of alpha,alpha-coupled thiophene oligomers and spiro-fused phenylene-thi ophene mixed oligomers was studied as a function of temperature. ESR m easurements on the one-electron oxidation products indicate that the c harge remains on a single oligomer unit and does not hop between the t wo bridged oligomer chains for temperatures up to 300 K. This is evide nt from the presence of hyperfine structure in ESR spectra for the dim ers of short-chain oligomers and the small temperature-dependent line broadening for longer chain spiro dimers, characteristic of a low-ener gy process such as the paramagnetic site moving along the oligomer cha in length. The activation energy for a paramagnetic center moving alon g the oligomer chain length has been estimated at <100 cm-1. The incor poration of phenylene into the oligomer unit shifts oxidation potentia ls anodic and also inhibits the mobility of the polaron along the pihc onjugated oligomer segment. Evidence for formation of pi-pi dimers in solutions of 7-mT has also been obtained.