Electronic properties of heterocyclic aromatic hydroxyl rigid-rod polymers

Using first-principles calculations, we find that the electronic structures of PBT, OH-PBT, OH-PBI, and OH-PBI(N) rigid-rod polymers are very different from those of conventional semiconductors. The close agreement between calculated excitation energies and the observed features in the ultraviolet–visible (UV–Vis) absorption spectra enables us to identify that the major features in the UV–Vis absorption spectra are contributed by N 2s to 2p and C 2s to 2p transitions. For OH-PBI(N), the O atom and the substituted N atom also contribute significantly to the threshold of the UV–Vis absorption spectra. The calculated energy bands show significant inter-molecular pz-orbital coupling perpendicular to the molecular plane.