Zero band-gap polymers:: Quantum-chemical study of electronic structures of degenerate π-conjugated systems

In search of very small band-gap polymers, we have quantum-chemically investigated the electronic structures of various polypentafulvalenes (PFVs) fused with six-membered rings. Geometrical parameters of the polymers were optimized through semiempirical Hartree-Fock band calculations at the Austin Model 1 (AM1) level. Electronic structures of the polymers were obtained through modified extended Huckel band calculations by adopting AM1-optimized geometries. It is predicted that poly(di-vinylenedioxy-pentafulvalene) and poly(di-vinylenedithia-pentafulvalene) would possess nearly zero band gaps (corresponding to lambda(max)) despite their large bond-length alternation of similar to 0.1 Angstrom. However, these polymers are expected to be very susceptible to oxidation. Poly(di-2-butenylene-pentafulvalene) and poly(dipyrazinopentafulvalene) are estimated to possess band gaps comparable with that (1.13 eV) of PFV. The calculated band gap of poly(di-ethylenedioxy-pentafulvalene) is 0.77 eV, smaller than that of PFV by 0.36 eV. We explained the evolution of the band gaps of the polymers through molecular orbital arguments.