【作者】 刘彬；

【导师】 封继康；

【作者基本信息】 吉林大学 ， 物理化学， 2006， 硕士

【摘要】 本论文利用密度泛函理论对对两个系列的低聚物和高聚物的发光材料进行了一系列的研究。从分子理性设计角度,通过化学计算给出了聚合物的平衡基态几何结构、前线轨道、电离能、电子亲合势、能带带隙、吸收光谱和最低激发态的几何构型及发射光谱等详细信息,为发光材料的实验研究提供了有价值的理论依据,为进一步探索优质高效的发光材料作出了重要贡献。我们研究了在芴(fluorene)的2,8位引入硫芴(dibenzothiophen)对聚芴的电子和光电性质的影响。我们利用低聚物性质随重复单元数的倒数(1/n)呈线性变化的规律,将低聚物的性质外推得到高聚物的相应性质。外推所得结果与实验值符合很好。计算结果表明:增加共轭体系能够使体系能隙变窄,吸收和发射光谱红移。增加聚合物中基团硫芴的含量,能隙变宽,吸收和发射光谱蓝移。芴和硫芴共聚形成的两系列聚合物(PF30T)n、(PF50T)n,其结构相对聚芴发生较大的扭曲,致使原有共轭性变弱,可以提高HOMO-LUMO能隙,吸收和发射光谱的最大吸收和发射波长变短,光谱发生蓝移。电离能和电子亲和势升高,能带带隙变宽。与芴的低聚物相比,较难形成空穴,较易接受电子,本文研究了芴和硫芴的共聚物的电发光性质。结果显示共聚物的吸收光谱、能隙及最低激发能与实验结果符合得很好,是很好的蓝光材料。更多还原

【Abstract】 Since electroluminescent phenomenon of Poly-phenylenevinylene(PPV) are reported by Cambrige University in 1990. Because the dissolublepolymers have many strongpoints such as easy to be deal with, flexile,membranous and their energy gaps can be adjusted by chemicaltechnologies, people come to realize and interest in them. During fourteenyears, many kinds of conjugated polymers have been studied for theelectroluminescence, such as PPV, poly thiophene (PTh), polyperinaphthalene (PPP), polyfluorene and their ramifications and so on. Theirspectra can spread all over the visible spectrum by chemical structuremodification. There are two different approaches to modify the band gaps ofthe conjugation polymers. One is adding groups at the phenyl or methyleneof the backbone, the other is restricting the movement of electrons in themain chain. Additionally, the proper methods to enlarge the band gaps ofpolymers and decrease the effective conjugation length are insertingunconjugated groups in the polymer chain or block the conjugation forconjugated backbone between the luminescent cells, such as insertingpolyester and polyurethane. Therefore, we can select the proper luminescentstaple to decide the absorption and emitting wavelength.What more, there are two different theoretical approaches to evaluatethe band gaps of polymers. One is the polymer approach in which theperiodic structures are assumed for infinite polymers. Another one, theoligomer extrapolation technique, has acquired the increasing popularity inthis field, however. In this approach, a sequence of increasing longeroligomers is calculated, and extrapolation to infinite chain length isfollowed. A distinct advantage of this approach is that it can provide theconvergence behavior of the structural, electronic and spectral properties ofpolymers. In practice, both the oligomer extrapolation and the polymerapproaches are generally considered to be complementary to each other inunderstanding of the properties of polymers. However, we can’t observe orcalculate precise polymer band gap directly. So the oligomer extrapolationis the main way to get the polymer band gap, which is the topic of thepresent work. Here we studied two fluorene-based copolymers polyfluorene,(PF30T)n and (PF50T)n using density functional methods and semiclassicalmodels.The ground-state geometries of oligomers were fully optimized usingthe density functional theory (DFT), B3LYP/6-31G, as implemented inGaussian 03. The results of the optimized structures for the oligomericmolecules of the (PF30T)n (n=1~4) and (PF30T)n (n=1~4) show that thestructural changes softly with increasing chain length in the series of(PF30T)n, as well as (PF50T)n. And it suggests that we can describe thebasic structures of the polymers as their oligomers. The character ofstructure in the (PF50T)n is dramatically twisted in compared with(PF30T)n.ZINDO and TD-DFT/B3LYP calculations of the lowestexcitation energies and the maximal absorption wavelengths (λabs) were thenperformed at the optimized geometries of the ground states. The lowestexcitation energies and the maximal absorption wavelengths show excellentlinearity in our plots. Band gaps of the corresponding polymers wereobtained by extrapolating HOMO-LUMO gaps and the lowest excitationenergies to infinite chain length, as well as the maximal absorptionwavelengths of the polymers. The extrapolation results of Egs and λabs are ingood agreement with the experimental data. The results of each methodindicate the same conclusion that the decreasing of the conjugation in thebackbone of (PF50T)n broadens its band gap. The broader band gap of(PF30T)n causes its shorter maximal absorption wavelengths, comparedwith (PF50T)n.The excited geometries were optimized by ab initioCIS/3-21G and the emission spectra were computed based on the excitedgeometries. The changes of the structures during the excitation can beprefigured from the characters of front orbitals. The structure will be tightwhen the antibonding changes into bonding, otherwise, the bond length willincrease when the bonding changes into antibonding. All of IPs and EAsinvolved in this paper are the energies’ difference between the ions andmolecules. We employed the linear extrapolation technique in this research.The linearity between the calculated IPs, EAs, energy gap, maximalabsorption wavelengths of the oligomers and the reciprocal chain length isexcellent for both homologous series of oligomers. Thus, these values of thepolymers can be obtained by extrapolating the resultant linear relationshipto infinite chain length. In all cases, the energy required to create a hole inthe polymer is ~6eV, while the extraction of an electron from the anionrequires ~1.3eV. The ionization potentials of (PF50T)n are higher then thatof (PF30T)n and electron affinities of it are lower than that of (PF30T)n.This suggests that the (PF50T)n appears to trap and give the electron moreefficiently, compared with (PF30T)n. On all accounts, the rigid twist in thestructure of (PF50T)n result in the conjugation decreasing and it is easy toadd an electron and difficult to ionize compared with (PF30T)n. To theimportance, it results in a broader band gap and shorter maximal absorptionand emission wavelengths in the spectra for (PF50T)n than (PF30T)n.Theenergy band gap broaded, and the spectra blue-shifted.The excited geomertries,the emission spectra are investigated.The theoretic-cal study shows that by modification of chemical structures could greatlymodulate and improve the electronic and optical properties of light-emittingand contribute to orientate the sysnthesis efforts and help understand thestructure-properties relation of these conjugated material.更多还原