Electronic Properties of Two Series of Blue-Emitting Conjugated Oligomers and Polymers Based on Cyanostilbene:A Quantum Chemistry Study

• 来源期刊: 中国化学(英文版)
• 年，卷（期）: 2007,（10）
• 页码范围: 1491-1498
• 页数: 8页
• 语种: 英文

摘要

One of the drawbacks of the electroluminescence (EL) polymers is that they are usually much better at accepting and transporting holes than electrons due to their inherent richness of π-electrons. One approach improving electron injection and transport in conjugatel polymers is to incorporate moieties with high electron affinities. In this theoretical work, to gain an insight into the chemical structure-property relationships was aimed by controllable modification of the main chain structures. Two cyanovinylene derivatives with 2,7-fluorenylene and p-phenylene moieties in the main chains, namely, poly{(2,5-dimethoxy-p-phenylene-1,4-ylene)-alt-[1,2-bis(p-phenylene)-1-cyanovinylene]} (PPhCN) and poly{[9,9-dimethyl-2,7-fluorenylene]-alt-[1,2-bis(p-phenylene)-1-cyanovinylene]}(PFCN), were studied employing density functional theory (DFT) and time dependent density functional theory (TD-DFT) with B3LYP functional. The electronic properties of the neutral molecules, extrapolated ionization potentials (IP) and electron affinities (EA), and energy gaps were investigated in comparison with pristine poly(2,7-fluorenylene). From comparison with poly(2,7-fluorenylene) (PF), the 1,2-bis(p-phenylene)-1-cyanovi- nylene unit was found to be a good electron-withdrawing moiety for electronic materials and the incorporation of 1,2-bis(pphenylene)-1-cyanovinylene resulted in a narrow band gap and ared shift of both the absorption and photoluminescence emission peaks. Most importantly, the LUMO energies of PFCN are around 1 eV lower than those of PF,which results in the decrement of EA about 0.9 eV, indicating that the 1,2-bis(p-phenylene)-1-cyanovinylene unit has significantly improved the electron-accepting properties of the copolymer PFCN. Substitution of 2,5-dimeth-oxy-p-phenylene for 9,9-dimethyl-2,7-fluorenylene induced larger band gaps and thus a blue-shift in absorption and emission peaks, which can be attributed to the better conjugated backbone in PFCN.