The structures, ionization potentials (IPs), electron affinities (EAs), and HOMO-LUMO gaps (Delta(H-L)) of the oligomers are studied by the density functional theory with B3LYP functional. The lowest excitation energies (E(g)s) and the maximal absorption wavelength Delta(abs) of oligomers of polyfluorene (PF) and poly(2,7-fluorene-alt-co-5,7-dihydrodibenz[c,e]oxepin) (PFDBO) are studied employing the time-dependent density functional theory (TD-DFT) and ZINDO. Band gaps and effective conjugation lengths of the corresponding polymers were obtained by extrapolating HOMO-LUMO gaps and the lowest excitation energies to infinite chain length. The IPs, EAs, and Delta(abs) of the polymers were also obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n = 0). For PFDBO, IPs and EAs are higher and the band gap is larger than those of PF's from the extrapolation. The outcome shows that the dramatically twisted structure of PFDBO in the seven-membered ring results in the decreased conjugation in the chain. These cause both the maximal absorption and emission wavelengths of PFDBO blue shift compared with PF.