Two-dimensionally extended aromatic polyamines for optimization of charge-transporting properties by partial oxidation

This article describes the synthesis of the two-dimensionally extended aromatic polyamines by polycondensation between the tri-sec-amine monomer and pphenylenediamine-based dibromide, the polyradical generation by chemical and electrochemical oxidations, and the hole-transporting properties. The molecular weights (M_n) of the polyamines, 3a and 3b, were 7700 and 5000, respectively, and both polymers were very soluble in the typical organic solvents, ensuring a good film formation capability by a spin-coating technique. The thermal stability of the aromatic polyamines, elucidated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), was quite high. More important, the cross-conjugated structure is essential for the controlled radical generation and the maintenance of the radical stability. Cyclic voltammograms of the neutral polyamines in the presence of 1 vol % of trifluoroacetic acid and the intervalence bands of the partially oxidized polyamines with NOPF₆ revealed that the generated radicals delocalize over the p-phenylenediamine moieties. The half-life of the polyradicals was about 12 h even under ambient conditions, which allowed us to fabricate and measure the hole-only devices. The charge-transporting properties of the polyamines were dramatically changed by the partial oxidation (20 mol %/aminium unit) with NOPF₆. The neutral polyamines showed the conventional injection-limit behaviors, whereas the hole-transporting behaviors of the polyradicals are bulk-limit and highly dependent on the chemical structure. The efficient hole-transport of the all-conjugated poly(aminium cationic radicaDs was for the first time realized for the polyradical of 3b.