Synthesis, characterization, and physical properties of oligo(1-(N,N-dimethylamino)pyrrole)s and their doped forms, precursors of candidates for molecular flat-band ferromagnets

Abstract

Fully oxidized poly(1-(N,N-dimethylamino)pyrrole)s (PDMAPyⁿ⁺), which are synthesized through doping with anions with the same number of polymer units as the original pyrrole, have been proposed as candidates for molecular flat-band magnets from the theoretical calculations of Aoki and coworkers (Phys. Rev. B: Condens. Matter Mater. Phys., 2003, 68, 140403; Phys. Rev. B: Condens. Matter Mater. Phys., 2010, 82, 235127). Here we describe the synthesis of neutral forms of oligo(1-(N,N-dimethylamino)pyrrole)s (ODMAPys) from the dimer (2) to the dodecamer (12). The crystal structures of 2–6 (i.e., up to the hexamer) were determined by single crystal X-ray analysis. The chemical and electrochemical formation of their oxidized forms is also reported. Electrochemical measurements of the ODMAPys indicated that oligomers 4, 6, and 8 underwent 2e⁻ oxidation, whereas 10 and 12 showed quasi-reversible 4e⁻ oxidation. The oxidized forms maintained considerable chemical stability; 4²⁺, 6²⁺, 8²⁺, 10⁴⁺, and 12⁴⁺ were isolated as SbCl₆⁻ salts. The neutral forms were colorless (λ_max = 240–270 nm), whereas the oxidized forms exhibited strong blue color with λ_max = 670–950 nm. EPR spectroscopy indicated that the dications 4²⁺, 6²⁺, and 8²⁺ in the solid salts were intrinsically diamagnetic, but the tetracation of 10⁴⁺ was paramagnetic at 4.4 K, which was supported by magnetic measurements. These physical properties of the ODMAPys are different from those common to polypyrroles, but consistent with their predicted unique electronic structures.