Theoretical study of geometrical and electronic structures of new π-conjugated poly (arylene vinylene) analogs

Abstract

Theoretical work was performed to investigate the geometrical and electronic structures of new vinylene copolymers: poly(cyclopentadienylene vinylene) (PPDV), poly(silolylene vinylene) (PSV) and poly(oxocyclopentadienylene vinylene) (POPDV). Semi-empirical AM1 band calculations demonstrate that these vinylene copolymers in the ground states have quinoid ring structures as do their parent homopolymers. The copolymers keep the neighboring rings anti-coplanar and the rings are connected to the vinylene linkage in the trans-form. Through modified extended Hückel band calculations, the bandgaps (which correspond to the absorption peaks of the π—π* band transition) of the polymers in the ground states are predicted to be 1.7 eV for PPDV, 1.9 eV for PSV and 2.5 eV for POPDV. The calculated bandgaps are analyzed in terms of geometrical relaxations and the electronic effect of the bridging groups using the equation Eg = ΔEδr + ΔE1–4 + ΔEel. It is concluded that incorporation of the vinylene linkage slightly weakens the interaction of the bridging groups with the conjugated carbon backbone. As a result of the weak interaction, the bond-length alternations (δr) slightly increase in the aromatic forms and decrease in the quinoid forms by about 0.01 Å. The ionization potentials and electron affinities of the copolymers slightly change compared to those of the parent cyclic polymers. However, the vinylene copolymerization is found not to invert the relative stabilities of the aromatic and quinoid forms of the parent cyclic polymers. Also, the bandgaps are not influenced much when the electronic effects of the bridging groups are small. The bandgap analysis reveals that the gaps of the vinylene copolymers mainly correspond to the contribution from the bond-length alternations. However, in POPDV, the contributions from the C1–C4 interactions and the electronic effect of the >C=O group are not negligible.