Multiple reduction states with different conductivities of polybenzimidazobenzophenanthroline (BBL) studied with infrared spectroelectrochemistry

Polybenzimidazobenzophenanthroline (BBL), a conjugated ladder polymer, has interesting features such as n-type conductivity, good photoconductivity, large nonlinear optical properties and the highest electron affinity among known n-type semiconducting polymers, which makes the material an interesting candidate for optoelectronical applications. BBL can be reduced (n-doped) electrochemically and shows various reduction states. We report results on FTIR spectroelectrochemical studies of the reduction reactions using ATR-FTIR spectroscopy. The spectra were recorded in a three electrode spectroelectrochemical cell in situ during continuous potential cycling of a BBL coated germanium reflection element in contact with the electrolyte solution under applied potential. In contrast to most of the other conjugated polymers, the polymer shows four reversible redox reactions during n-doping, corresponding to five different forms of BBL. From the spectral response, a reduction scheme insulator to conductor to insulator to conductor to insulator was found.