Preparation and EPR spectroscopic investigation of conjugated oligomers containing semiquinone repeat units

Three protected catechol-containing conjugated oligomers have been prepared using Pd-mediated coupling reactions. Two of the oligomers are poly[o-phenylenevinylene] type while the other is poly[o-phenyleneethynylene] type. One of the poly[o-phenylenevinylene]-type oligomers is regioregular while the other is regiorandom. The repeat units can be deprotected and oxidized to yield orthoquinone groups, which were subsequently reduced electrochemically to yield paramagnetic semiquinone repeat units. At spin concentrations greater than similar to 0.25 per repeat unit, precipitation occurs preventing preparation of oligomers with greater spin, concentrations using electrochemical methods. The frozen solution EPR spectra for all of the soluble semiquinone-containing oligomers exhibit fine structure, indicating the existence of high-spin (S > 1/2) states that arise from exchange coupling (J) of semiquinone groups through the oligomer backbone. Consistent with the x-connectivity of two of the oligomers, the intensities of their EPR signals vary linearly with inverse absolute temperature suggesting J greater than or equal to 0. Unexpectedly, the same result was obtained for the regiorandom oligomer for which antiferromagnetic coupling was expected. The latter result is explained by the law-spin concentration.