Ladder Polymers as New Polymeric Conductors

Electrical conductivity of heteroaromatic ladder polymers of the benzimidazobenzophenanthrolin type, BBL and BBB, and the phenothiazine type, PTL, was investigated. Doping studies of initially insulating BBL and BBB polymer films were carried out using various electron donors and acceptors. PTL polymer showed unexpectedly high conductivity (σ =10–5 ohm-1 cm-1); doping with ASF, brought about a minor increase in conductivity. With BBL and BBB polymers, a negligible doping effect was observed with halogens, a significant effect with AsF5 and a marked effect with SO3 (σ =1 × 10–2 ohm-1 can-1) and H2SO4 (σ =2 ohm-1 cm-1). A large increase of the thickness and the weight in the doped film is probably due to the planar layered structure of the polymers. The room temperature conductivity of potassium metal-doped (at 300–500°C) BBL film was 2 × 10–1 ohm-1 cm-1, while the potassium naphthalide-doped sample had a conductivity of 1 ohm-1 cm-1. In the acceptor-dopd samples, band broadening was observed in the 1700 (C=O) and 1580 (C=N) cm-1 regions, resulting from their frequency shifts (25 30 cm-1) toward higher wave numbers. This can be explained by an increase of force constants in C=O and C=N bonds by π-polarization induced by the complexation with the acceptors on the iminonitrogen which is the primary complexation site. The exact same band broadening was observed around these frequencies in protonic acid-doped samples. Therefore, acid doping is interpreted as a Lewis acid-type charge-transfer complexation, not merely a protonation.