Influence of ring-torsion dimerization on the band gap of aromatic conjugated polymers

We report quantum-chemical calculations on the geometric structures, ring-torsion potential curves, and electronic band structures in leucoemeraldine base, poly(paraphenylene sulfide), and pernigraniline base. Our aim is to define the origin of the band gap in the framework of an electron-lattice- coupled problem. The results turn out to be particularly interesting in the sense that we are able to fully discriminate between the relative contributions to the gap coming from bond-length dimerization and ring-torsion dimerization. We show that in the case of the leucoemeraldine-base form, which in the ground state has no Peierls contribution to the gap, the influence of ring-torsion dimerization is negligible. A similar result is found for poly(paraphenylene sulfide). On the contrary, in the case of pernigraniline base, which in the ground state possesses a Peierls gap, ring-torsion dimerization contributes to as much as about 40% of the full band-gap value. Very interestingly, it appears that in this polymer the effects of bond-length and ring-torsion dimerizations are additive.