Synthesis and properties of two cationic narrow band gap conjugated polyelectrolytes

We report the design, synthesis, and optical and electronic properties of two novel narrow band gap conjugated polyelectrolytes (NBGCPEs) based on a poly[2,6-(4,4-bis-alkyl-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] donor/acceptor backbone. Comparison with the properties of the neutral precursor material shows that the ionic component in these cationic NBGCPEs leads to a red-shift in the absorption spectra and to a modification of the polymer electronic energy levels. Both the HOMO and the LUMO are lowered in energy, with the net effect being dependent on the choice of counterion, i.e. bromide vs tetrakis(1-imidazolyl)borate. Moreover, we unexpectedly find n-type transport in thin-film transistors, as opposed to the widely studied p-type transport in neutral systems with isoelectronic backbones. From these observations we conclude that introduction of ionic functionalities adjacent to semiconducting polymers that exhibit charge-transfer excitations offers unique opportunities for materials design.