Yang Li, Feng Ji-Kang, Ren Ai-Min
State Key Laboratory of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry, Jilin University, Changchun 130023, People's Republic of China.
J Comput Chem. 2005 Jul 30;26(10):969-79. doi: 10.1002/jcc.20231.
Poly(fluorene)-type materials are widely used in polymer-based emitting devices. During operation there appears, however, an additional emission peak at around 2.3 eV, leading to both a color instability and reduced efficiency. The incorporation of the carbazole units has been proven to efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate two series of alternating fluorene/carbazole oligomers and copolymers poly[2,7-(N-(2-methyl)-carbazole)-co-alt-2,7-m(9,9-dimethylfluorene)], namely, PFmCz (m = 1,2) and gain a detailed understanding of the influence of carbazole units on the electronic and optical properties of fluorene derivatives. The electronic properties of the neutral molecules, HOMO-LUMO gaps (Delta(H-L)), in addition to the positive and negative ions, are studied using B3LYP functional. The lowest excitation energies (E(g)s) and the maximal absorption wavelength lambda(abs) of PFmCz (m = 1,2) are studied, employing the time-dependent density functional theory (TD-DFT). The properties of the two copolymers, such as Delta(H-L), E(g), IPs, and EAs were obtained by extrapolating those of the oligomers to the inverse chain length equal to zero (1/n = 0). The outcomes showed that the carbazole unit is a good electron-donating moiety for electronic materials, and the incorporation of carbazole into the polyfluorene (PF) backbone resulted in a broadened energy gap and a blue shift of both the absorption and photoluminescence emission peaks. Most importantly, the HOMO energies of PF1Cz and PF2Cz are both a higher average (0.4 eV) than polyfluorene (PF), which directly results in the decreasing of IPs of about 0.2 eV more than PF, indicating that the carbazole units have significantly improved the hole injection properties of the copolymers. In addition, the energy gap tends to broaden and the absorption and emission peaks are gradually blue-shifted to shorter wavelengths with an increase in the carbazole content in the copolymers. This is due to the interruption of the longer conjugation length of the backbone in the (F1Cz)(n) series.
聚芴类材料广泛应用于聚合物发光器件中。然而，在器件运行过程中，会出现一个额外的发射峰，其能量约为2.3 eV，这导致了颜色不稳定和效率降低。已证明引入咔唑单元可有效抑制酮缺陷发射。在本论文中，我们应用量子化学技术研究了两个系列的交替芴/咔唑低聚物和共聚物聚[2,7-(N-(2-甲基)-咔唑)-co-alt-2,7-m(9,9-二甲基芴)]，即PFmCz（m = 1,2），并详细了解咔唑单元对芴衍生物电子和光学性质的影响。使用B3LYP泛函研究了中性分子、HOMO-LUMO能隙（Δ(H-L)）以及正负离子的电子性质。采用含时密度泛函理论（TD-DFT）研究了PFmCz（m = 1,2）的最低激发能（E(g)s）和最大吸收波长λ(abs)。通过将低聚物的性质外推到反链长等于零（1/n = 0），得到了两种共聚物的性质，如Δ(H-L)、E(g)、IPs和EAs。结果表明，咔唑单元是电子材料中良好的给电子基团，将咔唑引入聚芴（PF）主链导致能隙变宽，吸收峰和光致发光发射峰均发生蓝移。最重要的是，PF1Cz和PF2Cz的HOMO能量均比聚芴（PF）的平均能量高0.4 eV，这直接导致其IPs比PF降低约0.2 eV，表明咔唑单元显著改善了共聚物的空穴注入性能。此外，随着共聚物中咔唑含量的增加，能隙趋于变宽，吸收峰和发射峰逐渐蓝移至更短波长。这是由于(F1Cz)(n)系列主链较长共轭长度的中断所致。
