【作者】 房晓红；

【导师】 许并社；

【作者基本信息】 太原理工大学 ， 材料学， 2009， 博士

【摘要】 有机电致发光器件具有驱动电压低、响应速度快、视角广、发光亮度和发光效率高以及易于调制颜色实现全色显示等优点,而且有机材料具有重量轻、柔性强、易于加工等特点,可用于制作超薄大面积平板显示、可折叠的“电子报纸”以及高效率的户外和室内照明器件,这些都是传统的无机电致发光器件和液晶显示器所无法比拟的。上述特点使得有机电致发光成为电致发光领域内一个新的研究热点,受到了化学、光学、材料学等相关学科领域的广泛重视。近年来,发光有机金属配合物因其在电致发光中的潜在应用而成为一个十分活跃的研究领域。人们对有机金属配合物光电性质的实验研究很多,但由于发光、载流子传输等微观过程的复杂性,其微观机制尚未探明,因此有机金属配合物发光、传输等性质的理论研究越来越受到重视。目前,量子化学计算方法已被广泛用于研究物质分子的结构、性能及其结构与性能之间的关系等问题,并获得了一些理想的结果。论文运用密度泛函理论(DFT),对有机电致发光领域中具有代表性的8-羟基喹啉金属配合物、席夫碱金属配合物的几何结构和电子结构进行了研究,分析结构对其性能的影响,进而为设计合成具有性能优良的有机电致发光材料提供理论指导。1、实验研究指出,8-羟基喹啉锂(Liq)可用作电致发光器件的发光层、电子传输层,也可以用作电子注入层。论文从分子设计的角度出发,采用密度泛函理论较为系统地研究了给/吸电子取代基对Liq光电性能的影响,获得了一些有价值的研究结果,为进一步改善Liq的性能提供理论指导。研究结果表明,不同取代基与母体形成不同的共轭,取代基-CN、-OCH3很好地参与了整个π体系共轭,对体系性质影响最大,而-CF3、-CH3CH2CH2、-CH3、-Cl与体系的共轭作用较弱,对体系性质影响相对较小。给电子基取代,加强了N和Li共价性和O与Li的静电作用,吸电子基取代减弱了N和Li共价性和O与Li的静电作用。吸电子基-CF3、-CN、-Cl在5位取代Liq都使其最高占据轨道(HOMO)、最低空轨道(LUMO)能级降低,但吸电子基的强弱对Liq的LUMO、HOMO及带隙的影响不具有规律性,-CF3、-CN使Liq带隙增大,而-Cl使Liq带隙减小,这里还需考虑取代基的共轭效应的协同作用。给电子基-CH3、-CH3CH2CH2、-OCH3在5位取代Liq都使LUMO、HOMO升高,带隙减小,给电子性越强,影响越显著。-CN在5取代,显著增加了Liq的电子亲和势,降低了分子轨道的简并度,使分子轨道能级展宽,电子更易于注入和传输。与Liq及其它衍生物相比,5-CN-Liq是一种更好的电子注入和传输材料。2、8-羟基喹啉铝作为一种重要的电子传输材料和理想的有机电致绿光材料,它的发光性质得到了广泛的研究。目前对Alq3的研究主要集中在如何通过分子剪裁和聚集态结构的调控来改变其发光光谱以及提高器件的效率和寿命。马东阁等设计合成了双核8-羟基喹啉铝(DAlq3),发现其电致发光性能优于同样器件结构下8-羟基喹啉铝(Alq3)的性能,他们认为由于DAlq3具有相对较高的电子迁移率,有利于电子和空穴的传输平衡。论文研究了分子的化学修饰对载流子传输性能的影响,从微观的角度解释了DAlq3比Alq3具有更高电子迁移率的本质原因。基于Marcus电子转移理论,利用DFT方法,对Alq3和DAlq3分子间及分子内的电子转移进行了理论计算,计算Alq3和DAlq3的重组能、电子亲和势(EA)和电离势(IP)。基于跳跃模型构建DAlq3和Alq3的电荷转移路径,计算DAlq3和Alq3的电荷耦合矩阵元。利用重组能及电荷耦合矩阵元计算Alq3和DAlq3的电子迁移率。结果表明,在氧化还原过程中,DAlq3分子几何结构扭曲变形小于Alq3,因此导致DAlq3的电子和空穴重组能均比Alq3的小。DAlq3的电子耦合矩阵元远大于Alq3的电子耦合矩阵元。据Marcus电子转移理论计算得出DAlq3的电子迁移率约为Alq3的2.7倍,这源于DAlq3较小的电子重组能λ(e)和较大的电子耦合矩阵元HAB(e),这就解释了为什么DAlq3比Alq3有更好的电子传输特性,与实验观测是一致的。在进行电荷迁移率计算时,对于DAlq3和Alq3,电荷耦合矩阵元是比重组能更为关键的参数。电子亲和势(EA)和电离势(IP)计算分析也表明,DAlq3比Alq3更有利于电子传输。3、席夫碱金属配合物由于其具有药物、催化、非线性光学、电致发光等性能而被广泛研究。水杨醛类双席夫碱金属配合物具有较大共轭体系的四齿含氮配体,是一类很好的发光化合物。在实验中发现水杨醛缩乙二胺锌Zn(salen)有着有趣的光致发光和电致发光性能。论文借助于密度泛函理论讨论分子结构及其聚集态结构对发光性能影响,对实验现象进行了合理的解释。首先根据实验研究结果,借鉴类似化合物的微观结构,构建了Zn(salen)三种可能的分子构型,即单体、二聚体和螺旋状体。利用DFT方法对三种不同构型Zn(salen)分子的几何和电子结构进行理论研究。三种Zn(salen)的稳定性顺序是螺旋状体>二聚体>单体,螺旋状体为热力学上最稳定的一种构型。三种构型Zn(salen)的前线分子轨道特征表明,相对于单体及二聚体,螺旋状体的LUMO更加离域,而且螺旋状体中平行的亚水杨基分子片段之间具有π-π相互作用,这表明螺旋状体拥有更好的电子传输特性。三种构型Zn(salen)的带隙大小次序是单体>二聚体>螺旋状体,对应的发光光谱依次红移。利用电子光谱对Zn(salen)的电子跃迁性质进行了分析,对三种构型Zn(salen)的紫外-可见吸收光谱的归属进行了详细指认。实验观察到的Zn(salen)可变的光致发光特性源于Zn(salen)不同的分子构型和电子结构,Zn(salen)可变的电致发光性能表明在电压驱动下,Zn(salen)的构型会发生转变,从而从理论上解释了水杨醛缩乙二胺锌可变的光致发光和电致发光性能。更多还原

【Abstract】 Organic electroluminescent devices have many merits, for example, lower drive voltage, quick response, broad visual angle, high luminescence brightness and efficiency, easy modulation of color for whole color display and so on. At the same time, organic materials have characteristics of lightweight, good flexibility and easy processibility. Therefore, organic electroluminescent materials can be applied to ultrathin and big area flat display, foldable "electronics newspaper" and high-efficiency outdoors and indoors illuminating apparatuses. At these aspects, all traditional inorganic electroluminescent devices and liquid crystal display are incomparable. So organic electroluminescence has become a new research hotspot in electroluminescence field and attached great attention of chemistry, optics, and materials science.In recent years, light-emitting organic metal complexes have become a very active research field because of their potential applications in electroluminescence. There are a lot of experimental studies on photoelectric properties of organic metal complexes, but because of the complexity of some microcosmic processes, such as luminescence and charge carrier transporting and so on, the microcosmic mechanism of these photoelectric properties has not been understood distinctly. So theoretical investigation on these properties for light-emitting organic metal complexes has attracted more and more attention.At present, the quantum chemistry calculation method has been widely used to study the material molecular structure, properties and the relationship between structure and properties, and obtained some satisfactory results. In this dissertation, the geometric structure, electronic structure, and the relationship between structure and properties for 8-hydroxyquinoline-metal complexes, Schiff base metal complexes which are representative in organic electroluminescence field were studied by density functional theory (DFT). These will provide theoretical guidance for design and synthesis of organic electroluminescent material with excellent performance.1. Experimental researchs indicated that 8-hydroxyquinoline lithium (Liq) can be applied as light-emitting, electron transport and electron injection layer in organic electroluminescent device (OELD). In this paper, from the point of view of molecular design, the influence of electron donating and withdrawing substituents on photoelectric properties of Liq are systematically studied by density functional theory (DFT), several significant research results were achieved to provide theoretical guidance for further improving the performance of Liq.The research results showed that the different substituents form different conjugate with the parent population. The substituents of -CN, -OCH3 participate in theπ-conjugated system properly, and have a great influence on the properties of the system. While the substituents of -CF3, -CH3, -CH3CH2CH2, -Cl have the weaker conjugated effects with the system, and have relatively a little influence on the properties of the system. The electron donating substituents enhance O-Li electrostatic interaction and N-Li covalency, and the electron withdrawing substituents weaken O-Li electrostatic interaction and N-Li covalency. The electron withdrawing groups of -CF3, -CN, -Cl attached to 5-position of Liq make LUMO and HOMO energy level lower, but the dependence of the LUMO, HOMO and band gap on their inductive effect strength lack regularity. The -CF3, -CN increase band gap of Liq, however the -Cl decreases band gap of Liq. Here the synergistic action of conjugated effects should be included. The electron donating groups of -CH3, -CH3CH2CH2, -OCH3 attached to 5- position of Liq make LUMO and HOMO energy level higher and band gap decrease, the stronger interaction between the substituent and the molecular orbital, the more remarkable the influence will be. The -CN substitution at 5-position of Liq make the electron affinity significant increase, the degeneracy of molecular orbital energy levels decrease, and the molecular orbital energy levels broaden, so that electron injection and transport become more easy. It can be concluded that 5-CN-Liq is a kind of better electron injection and transport materials than Liq and other derivatives of Liq.2. 8-hydroxyquinoline aluminum is an important electron transport and an ideal green light emitting material. Its luminescent properties had been studied extensively. Current research for Alq3 focuses on modulating its light-emitting wavelength and improving the efficiency and lifetime of OELD by tailoring molecule and controlling aggregation structure. A dinuclear aluminum 8-hydroxyquinoline complex (DAlq3) based on 8-hydroxyquinoline and 5, 5′-methylene-bis(8-hydroxyquinoline) ligand had been designed and synthesized by Ma Dong-ge et al. It was found that the electroluminescent performance of DAlq3 is better than that of Alq3 in the same structure devices. They thought that the DAlq3 possesses relatively higher electron mobility compared with Alq3, which balances transport of electron and hole in OELD.In this dissertation, the effects of the molecular chemical modification on the charge transport properties were investigated, it can be explained from the microcosmic aspect that DAlq3 has a higher electron mobility than Alq3. The theoretical calculation on intermolecular and intramolecular charge transfer for DAlq3 and Alq3 were carried out based on the Marcus electron transfer theory using DFT methods. The reorganization energy, electron affinities (EA) and ionization potentials (IP) of DAlq3 and Alq3 were obtained. The charge transfer pathways of DAlq3 and Alq3 were constructed based on intermolecular hopping model, and then charge coupling matrix elements for DAlq3 and Alq3 were calculated. Electron mobilities of DAlq3 and Alq3 were estimated ultimately using reorganization energy and charge coupling matrix element. The results showed, upon oxidation and reduction, the geometry structure distortion of DAlq3 is smaller than that of Alq3, consequently lead to smaller reorganization energy of DAlq3 both for electron and hole than that of Alq3. Electronic coupling matrix element of DAlq3 is much larger than that of Alq3. According to the Marcus electron transfer theory, we estimated that electron mobility of DAlq3 is about 2.7 times of that of Alq3. This is due to the smaller electron reorganization energyλ(e) and larger electronic coupling matrix element HAB(e) of DAlq3 than that of Alq3. This explains why DAlq3 has better electron transporting property and then higher EL efficiency than Alq3, which is agreement with experimental observations. The charge coupling matrix element is a more crucial parameter than the reorganization energy for DAlq3 and Alq3 for charge mobility calculation. Moreover, from the viewpoint of the electron affinity (EA) and ionization potential (IP), DAlq3 is more favorable for electron transport than Alq3.3. Schiff base metal complexes have been extensively studied because of their properties of drugs, catalysis, nonlinear optics, electroluminescent and so on. Salicylaldehyde bis-Schiff base metal complexes have quadridentate nitrogen-ligand with a larger conjugated system, are regarded as a class of very good light-emitting compounds. The experimental results revealed that N,N’-bis(salicylidene)-ethylenediamine)zinc(Zn(salen) exhibited very intersting photoluminescent and electroluminescent properties.In this thesis, the effects of molecular structure and aggregation structure on the light-emitting properties were discussed, and the experimental phenomena were explained reasonably by density functional theory (DFT). First of all, according to experimental results, and profitting from the microstructure of similar compounds, three possible molecular configurations of Zn(salen) were constructed, namely the monomer, the dimer and the helical. The theoretical studies on geometric and electronic structures for three different configurations of Zn(salen) were carried out by means of DFT method. The order of stabilization of three species of Zn(salen) is the helical structure > the dimer > the monomer. This suggest that the helical is the most favored conformation thermodynamically. The frontier molecular orbital characteristics of three species of Zn(salen) show that the LUMO of the helical has more delocalized character than that of the monomer and the dimer. In addition,π-πinteraction of the parallel salicylidene segments for the helical also provides an efficient pathway for electron transport. Therefore the helical has better electron transporting property than the monomer and the dimer. The order of energy band gap of three species of Zn(salen) is the monomer > the dimer > the helical, corresponding the PL and EL spectra exhibit red shift in turn. The properties of electronic transition for Zn(salen) were analyzed using electronic spectra, the attribution of UV-Vis absorption spectra peaks for three configurations of Zn(salen) are designated specially. The variability of photoluminescence properties observed in experiment result from diversity of different molecular structure and electronic structure of Zn(salen), the various electroluminescence properties of Zn(salen) shows that the configurations of Zn(salen) also is changed at driving-voltage. The changes on PL and EL properties of Zn(salen) can be clarified eventually in theory.更多还原