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_gs) and the maximal absorption wavelength lambda_(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 lambda_(abs) of the polymers were also obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1 = 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.
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机译：通过具有B3LYP泛函的密度泛函理论研究了低聚物的结构，电离能（IPs），电子亲和力（EAs）和HOMO-LUMO间隙（DELTA_（H-L））。聚芴（PF）和聚（2,7-芴-alt-co-5,7-二氢二苯并[c，e] oxepin）（PFDBO）的低聚物的最低激发能（E_gs）和最大吸收波长lambda_（abs） ）是采用时变密度泛函理论（TD-DFT）和ZINDO进行研究的。通过将HOMO-LUMO间隙和最低激发能外推到无限长链，可以得到相应聚合物的带隙和有效共轭长度。聚合物的IP，EA和λ（abs）也可以通过将低聚物的IP，EA和lambda_（abs）外推至等于零（1 / n = 0）的逆链长度来获得。对于PFDBO，根据外推法，IP和EA较高，并且带隙大于PF。结果表明，七元环中PFDBO的剧烈扭曲结构导致链中共轭的减少。与PF相比，这些导致PFDBO蓝移的最大吸收和发射波长。
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