| Abstract
| - One serious problem associated with polyfluorene and derivatives (PFs) as blue luminescent polymers is thesignificant energy barrier for hole or electron injections; thus they usually face charge injection and transportdifficulties with the currently available cathode and anode materials. The incorporation of an electron-donatingor -accepting unit is expected to improve the recombination of the charge carriers. In this paper, we applyquantum-chemical techniques to investigate three fluorene-based copolymers, copoly(2,5-ethylenedioxythiophene-alt-9,9‘-dimethylfluorene) (PEF), copoly(2,5-pyridine-alt-9,9‘-dimethylfluorene) (PPyF), and poly[(fluorene-2,7-diyl)-alt-(1,3,4-oxadiazole-2,5-diyl)] (PFO), in which ΔH-L [the energy difference between thehighest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), when n= ∞], the lowest excitation energies (Eg), ionization potentials (IP), electron affinities (EA), and λabs and λemare fine-tuned by the regular insertion of electron-donating unit 3,4-ethylenedioxythiophene (EDOT) or electron-withdrawing units pyridine and 1,3,4-oxadiazole. The results show that the alternate incorporation of electron-donating moiety EDOT increases the HOMO energy and thus reduces the IPs, and consequently the holeinjection was greatly improved. On the other hand, even though both kinds of charge carriers will improvethe electron-accepting ability, the results show that electron-withdrawing moieties greatly facilitate the electron-transporting. Especially in PFO, the highly planar structural character resulted from the strong push−pulleffect between the fluorene ring and the 1,3,4-oxadiazole ring and a weak interaction between the nitrogenand oxygen atoms in 1,3,4-oxadiazole ring and the hydrogen atom of the fluorene ring, significantly loweringthe LUMO energy levels and thus improve the electron-accepting and transporting properties by the lowLUMO energy levels.
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