Two-dimensionally extended organic high-spin poly(aminium cationic radical)s and their magnetic force microscopic images

Abstract

Purely organic high-spin poly(aminium cationic radical)s of two-dimensionally extended soluble aromatic polyamines P1 and P2 were prepared by the palladium-catalyzed polycondensation of two multifunctional monomers, followed by chemical or electrochemical oxidations. The chemical structures of the precursor polyamines were fully characterized by 1H- and 13C nuclear magnetic resonance (NMR), infrared and MALDI-TOF mass spectra (MS). The aminium radical sites of N,N,N′,N′-tetraaryl-p-phenylenediamines were connected through the m-phenylene ferromagnetic coupler, which satisfies the non-Kekulé-type molecular design. Oxidation of radical sites was facilitated by electron-donating methoxy substituents. The first oxidation potential (Eo′(1)) of P2 with methoxy substituents, evaluated from the cyclic voltammograms in CH2Cl2, was 0.69 V (vs Ag/Ag+), whereas the Eo′(1) of P1 without electron-donating substituents was 0.78 V. This result was further verified by spectroelectrochemistry measurements. The electrochemically oxidized P2 displayed a more intense low-energy band at ca. 800 nm ascribed to aminium cationic radicals and bications. Chemical oxidation of P1 and P2 with NOPF6 solubilized with 18-crown-6 in CH2Cl2 gave the corresponding poly(aminium cationic radical)s. The high-spin ground states were confirmed by the temperature dependence of forbidden electron spin resonance (ESR) transition peak intensities, and the average spin quantum number (S) determined by the magnetic curves reached 8.4/2 at low temperatures. Remarkably, when a small amount of trifluoroacetic acid was added, the half-life of polyradicals reached ca. 1 week at room temperature (RT). This result prompted us to observe the single polymer-based molecular and magnetic images by atomic force microscopy and magnetic force microscopy, respectively. The precursor polyamine P1 showed globular-shaped polymer particles with an average vertical distance of 1.43 nm. After chemical oxidation, the vertical distance of P1 definitely decreased, reflecting the improved p-type character of the nitrogen atoms. Moreover, the detection of weak MFM images supported the presence of magnetic spins in the single polymers.