PI-DIMERS OF PROTOTYPE HIGH-SPIN POLARONIC OLIGOMERS

Novel well-defined oligomers consisting of two dopable pi-conjugated s egments, 2,2'-bipyrrole or 2,2'-bithiophene, linked via 1,3-phenylene and end-capped with phenyl groups have been synthesized using palladiu m-catalyzed cross-coupling reactions. The molecules are considered as prototypical examples for polaronic ferromagnetic chains based on pyrr ole and thiophene units, which have been proposed as candidates for or ganic magnetic materials. The oligomers are designed to investigate wh ether high-spin (i.e. triplet-state) oligocations can be obtained afte r oxidative doping. We find that the oligomers can be oxidized to the corresponding di(cation radical)s, in which each heterocyclic segment is singly oxidized and carries an unpaired electron, as required for a high-spin state. While these di(cation radical)s are stable at ambien t temperature, UV/visible/near-HR and ESR spectroscopy reveals that th e singly charged cation radical segments reversibly form pi-dimers in solution, especially at low temperatures. This pi-dimerization involve s the intermolecular antiferromagnetic pairing of electron spins and i s detrimental for the formation of high-spin oligomers or polymers via the polaronic concept with oxidized oligopyrrole or oligothiophene se gments as spin-carrying units.