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 0 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 p(z)-orbital coupling perpendicular to the molecular plane. (C) 2003 Elsevier Ltd. All rights reserved.