Effect of doping systems, heat and time on the electrical conductivity of poly-p-phenylenes

Poly-p-phenylenes (PPPs) were synthesized by the Kovacic procedure using CuCl2 as the oxidant and AlCl3 as the catalyst. The alternating electrical conductivity of PPPs was investigated considering the doping system (dopant-solvent), heat treatment, and time. Doped poly-p-phenylenes remained semiconductors even 1 year after doping with a lower conductivity compared to the initial one. The most effective doping systems for PPPs were FeCl3-ACN (ACN: acetonitrile) and FeCl3-N (N: nitrobenzene), while SnCl4-ACN yielded material with a lower but comparable conductivity. FTIR spectra and X-ray diffractograms of doped materials having a lower conductivity (i.e. PPP doped with SnCl4-ACN) approached that of the undoped PPP. The conductivity depends on the stability of the polymer-dopant complex, which is affected by the electron donor-acceptor (EDA) interactions between the dopant and the solvent. Annealing after doping resulted in a decrease in conductivity, due to the thermal deactivation of the polymer-dopant complex. The ratio of sigma'(tau)/sigma'(0) (where sigma'(tau) and sigma'(0) are the real part of the electrical conductivity at log f = 7 and log f = 1, respectively) ranges from 10(1) for the doped polymers at a time of t = 0 to 10(1)-10(2) at t = 12 months, and to 10(4) for the heat-treated doped polymers. The conductivity measurements suggested an indirect method to characterize the stability of the complexes formed.