π-Dimers of Prototype High-Spin Polaronic Oligomers

Abstract

Novel well-defined oligomers consisting of two dopable π-conjugated segments, 2,2‘-bipyrrole or 2,2‘-bithiophene, linked via 1,3-phenylene and end-capped with phenyl groups have been synthesized using palladium-catalyzed cross-coupling reactions. The molecules are considered as prototypical examples for polaronic ferromagnetic chains based on pyrrole and thiophene units, which have been proposed as candidates for organic magnetic materials. The oligomers are designed to investigate whether high-spin (i.e. triplet-state) oligocations can be obtained after 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 ambient temperature, UV/visible/near-IR and ESR spectroscopy reveals that the singly charged cation radical segments reversibly form π-dimers in solution, especially at low temperatures. ...