Geometrical and electronic structures of new π-conjugated five-membered ring polymers were theoretically investigated. These polymers are analogous to heterocyclic polymers, but adopt as bridging groups ≳CH2, ≳CF2, ≳SiH2, ≳SiF2, ≳C=CH2, ≳C=O and ≳C=S moieties instead of heteroatoms. The ground-state geometries of the polymers were predicted to be quinoid from semiempirical band calculations with AM1 Hamiltonian. The electronic properties of these systems were obtained using the modified extended Hückel method. The calculated band gaps (Eg) were analyzed in terms of geometrical relaxations and electronic effect of the bridging groups using the equation of Eg=ΔEδr+ΔE1–4+ΔEel. The effect of bond-length alternation (ΔEδr) amounts to 1.1–1.4 eV for the aromatic forms and 1.8–1.9 eV for the quinoid forms of the polymers. The interactions (ΔE1–4) between C1 and C4 atoms of the cis-PA type backbone tend to decrease the band gaps of the aromatic forms and to increase the gaps of the quinoid forms as much as 0.2–0.5 eV, depending on the size of a bridging atom. It is found that the electronic effect (ΔEel) of these bridging groups is quite small compared to that found in heterocyclic polymers such as polythiophene, polypyrrole, and polyfuran. ΔEel of ≳CF2, ≳SiH2, and ≳SiF2 bridging groups are negligible and that of the other groups amounts to 0.3–1.0 eV. Therefore, the band gaps of these systems almost correspond to the ΔEδr values which arise from the bond-length alternations, except the case of the polymers with ≳C=O and ≳C=S bridging groups whose π* orbitals strongly interact with the π system of the polymeric backbone.

Theoretical work was performed to investigate the geometrical and electronic structures of four new hypothetical thiophene copolymers: poly(thienylene cyclopentadienylene) (PThPD), poly(thienylene silolylene) (PThS), poly(thienylene oxocyclopentadienylene) (PThOPD) and poly(thienylene thiocyclopentadienylene) (PThTPD). AM1 band calculations showed that the ground-state geometries of PThPD and PThS copolymers are of the aromatic forms whereas those of PThOPD and PThTPD are quinoid. Each intraring structure of the copolymers is predicted to be nearly identical to that of their corresponding parent homopolymers and the bond-length alternation (δr) of each copolymer is equal to the average of δr values for the two corresponding homopolymers. Modified extended Hückel band calculations produced that the band gaps (which correspond to the absorption peaks of π–π* band transition) of the copolymers in their ground states are 1.7 for PThPD, 1.55 for PThS, 1.9 for PThOPD, and 2.09 eV for PThTPD. These values, except for PThPD, are quite smaller than the band gaps calculated for the corresponding homopolymers in the ground states. Decomposition of the band gaps reveals that the gaps of the aromatic forms of PThPD and PThS are dominated by the δr contribution and the electronic effect of the bridging groups and that the gaps of the quinoid types of PThOPD and PThTPD correspond primarily to the δr contribution.

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