Theoretical Studies On The Optical Properties Of Iridium Complexes And Related Molecular Design

The efficiencies of organic light-emitting devices(OLED)have been boosted rapidly in recent years since the first device was reported by Tang from Kodak company in 1987. Compared with liquid crystal display (LCD), OLEDs have the advantages such as fast response, wide visual angle, flexible display, low temperature-resistant and so on. Compared with cathode ray tube (CRT), OLEDs show the advantages in small volume, lightweight, high efficiency, high brightness, low voltage DC drives and large areas of full color display. OLEDs with potential performance in display technology will be the most competitive technology for the third generation flat panel display.Recently, considerable research efforts have been focused on phosphorescent transition metal complexes for using as room temperature dyes. These materials have intriguing photophysical, photochemical and excited state redox properties and high internal quantum efficiency (ηint) with up to almost 100% in principle. They possess the strong spin-orbit couplings which allow fast intersystem crossing among the low-lying electronic states. Phosphorescent d6 metal complexes of Ir (III), Ru (II) and Os (II) have shown high efficiency in OLEDs. Among these complexes, iridium (III) complexes are regarded as the most effective materials in OLEDs, in which they exhibit a high thermal stability, a short life time in excited states and the tunable color in the region of blue to red through modification of the ligand or introducing a variety of electron donors or acceptors in the ligand. To achieve wide-range color emission in use of full color display, many different classes of homoleptic (Ir(C^N)3) and heteroleptic (Ir(C^N)2L^X) iridium (III) complexes have been developed (C^N is a cyclometalating ligand and L^X is an ancillary ligand). However, there are still some problems that need to be solved in such system, for example, the light-emitting mechanism of transition metal phosphoresce complexes, the lack of blue light-emitting materials and host materials for blue emitting materials and the effect of the ancillary ligands on the characters of the complexes.The rapid development of the advanced science and technique greatly promotes the progress of modern computational chemistry. The theoretical study and experimental finding would illuminate each other and they are interrelated and inextricably linked. On one hand, the comparison between calculated results and experiment finding can test the reliability and accuracy of electronic structure theory, which shows the dependence of theory on experiments; On the other hand, the developed electronic structure theory provide impressive support and necessary explanations to the experimental finding; Furthermore, to predict the potential properties of the complexes which has not been synthesized on experiment, theoretical studies have demonstrated the independence and forward-looking characteristic .The macroscopic properties of the molecular is the concrete embodiment of the microscopic properties, so to study the electronic structure of the molecular is the most effective way to learn the macroscopic properties of the molecular and mast the molecular reaction rules. The absorption and emission properties of the transition metal complexes are extremely complicated microcosmic process, including some fundamental problems of quantum theory such as the electronic structure of ground stated and excited state, relativistic effect of the orbital angular momentum and so on. So theoretical study on the light-emitting properties of transition metal complex is not only to explore and design of the new type organic-metal light-emitting complex and give important guiding significance and forward-looking characteristic, but it is an important theoretical subject itself. Through theoretical study, we could abstract and built reasonable theoretical model from the calculate results based on the real molecules, so that we can reveal the nature and features of transition metal complexes and give some hints on the molecular design.In this dissertation, we solve some problems with theoretical method on Ir(III) as phosphorescence light-emitting materials. Specifically, it is divided into four parts as follows:Part I: A series of four phosphorescent Ir (III) heteroleptic complexes with different ancillary ligands Ir(dfppy)2(pic) (FIrpic) and Ir(dfppy)2(acac) (FIracac) and homoleptic complexes Ir(ppy)3 with different geometric isomers as facial (fac) and meridional (mer) structure are investigated theoretically, where ppy = 2-phenylpyridyl, dfppy = 2-(2,4-difluorophenyl)pyridine, ancillary ligands pic = picolinic acid and acac = acetoylacetonate. We present a thorough theoretical analysis of the structure and electronic properties of a series of Ir(III) complex in S0 state and S1, T1 states to explore the mechanism of phosphorescence. For the first time, we reveal the nature of the S1 state that significant changes of the geometrical and electronic structures of Ir (III) complexes take place with the reflection in the abruptly elongated bond length of Ir-N increased about 0.32-0.79 ? and the precipitously falling-off d (eg*) orbital from LUMO+15 (or LUMO+10) in the S0 to LUMO (or LUMO+2), respectively. The following intersystem crossing easily takes place because of the participation of the falling d orbital in the S1 state. The most probable light-emitting mechanism is proposed by the analysis of the excited-state properties of the Ir (III) complexes. Combined with our calculated results, the indirect possible reason to the high phosphorescence quantum efficiency from structure-property view of Ir (III) complexes is discussed. Our calculated results provide important information for understanding the light-emitting process and afford a new way to evaluate the luminous efficiency, which will be helpful for the design of highly efficient phosphorescent Ir (III) complexes.Part II:Based on the characteristics of highly efficient phosphorescent Ir (III) complexes we put forward in previous chapter, we design two series blue-emitting Ir (III) complexes. The design strategy is to change the structure of representative blue-emitting complex FIrpic, and our goal is to design the molecules with high efficiency as FIrpic and pure blue emitting color. Two approaches are adopted: on the one hand, keep the cyclometalating ligand dfppy, and alter the ancillary ligand pic. The calculated results show that this series of complexes have less than 12 nm blue shift compared with FIrpic; On the other hand, keep ancillary ligand and change cyclometalating ligand. The designed complexes show 22 nm～38 nm blue shift. the results reveal that changing cyclometalating ligand is a more effective way in turning the emission color compared with ancillary ligand.Part III : Two series of heteroleptic Ir(dfppy)2L^X, Ir(ppy)2L^X iridium (III) complexes have been studied to explore the ancillary ligand effects on the properties of the complexes. For the series of Ir(dfppy)2L^X, the ancillary participate in the composition of the frontier orbital, and the lower triplet energy which determine the light-emitting color of complexes. This work demonstrates tuning the emission color of metal complexes in a simple way by only changing the structures of ancillary ligands. We show how the nature of substituents with electron-donating and electron-withdrawing groups can significantly influence the photophysical properties of Ir(dfppy)2L^X complexes. Combined with experimental and DFT calcualtions, we have shown the detailed electronic structures and complicated absorption and emission process involved. For the emission, the transition of FIracac is [d(Ir)+π(C^N)→π*(C^N)]; and those of FIrdbm, FIrnatfac are [d(Ir)+π(L^X)→π* (L^X)]. It is shown that all of the low-lying excitations calculated in this study are categorized as MLCT and ILCT characters. The properties of the complexes are both affected by C^N and L^X ligands. For the series of Ir(ppy)2L^X complexes, we adopt the Mucus electron transfer theory to explore the ancillary effect on the carrier mobility. The introduction of substituents to the ancillary ligands will change the hole and electron recombination energy, and the size of substituents adjust the molecular packing mode to change the transfer integral. The calculated results show that Ir(ppy)2acac is an electron transport material and the electron mobility is 1.23E-02 cm2/V s; while Ir(ppy)2dmd,Ir(ppy)2FBDK are hole and electron balance transport materials. Ir(ppy)2dmd show better carrier mobility and the hole and electron mobility are 2.70E-03,1.30E-02 cm2/V s, respectively. Part IV: Based on density functional theory (DFT) calculations, a new series of bipolar host molecules for efficient blue electrophosphorescence devices are designed by linkage of hole-transporting moiety carbazole (CZ) and electron-transporting unit diphenylphosphoryl (ph2P=O) to the core molecules with high triplet energies. The electronic structures in the ground states, cationic and anionic states, and lowest triplet states of the designed molecules have been studied with focusing on triplet energies, spin density distributions, ionization potentials, electron affinities and the influence of molecular topology. Designed bipolar host molecules possess the following features: (1) Higher triplet energy,～3.0 eV, and the triplet energy can be turned by different substitute position of CZ , ph2P=O. (2) HOMO and LUMO separation and localization in the respective hole- and electron-transporting moieties which will be helpful for the control of the carrier transportation propertiex. (3) Thermal stability and morphological of host material are taken into account in the molecular design. (4) Adjust the number of CZ and ph2P=O can achieve the hole and electronic balance transporting.