Theoretical Investigation On Relationship Between Molecular Structures And Photoelectric Properties Of Light-Emitting Organic Metal Complexes

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.