Design And Theorectical Investigation On The Organic Metal Complexes Light-emitting Materials

Organic light-emitting diodes (OLEDs) and other electroluminescent devices based on iridium and platinum cyclometalated complexes have received much attention because of their enormous potential in the light emitting and the flat panel display. Recently experimental chemistries have done a number of researches to tune the emission color from blue to red and improve the external quantum efficiency. And the main requirement for OLEDs of the next generation is to design efficient phosphorescent dopants to emit sharp colors and improve emitting lifetime with very high phosphorescence quantum yields. At present, the scientific research workers' work has mainly been concentrated on the synthesis of high performance compounds and the researches of luminescence nature. In recent years, the researches on the phosphorescent material and apparatus based on the heavy metal complexes, especially, the iridium complexes have become the hot spot of researches in the organic electroluminescence domain. With the development of the past several decades, the application of metal complexes based on the (C^N)2Ir(LX) structure as phosphorescent material has become more and more mature. Their short phosphorescent lifetime reduces the T-T quenching and the excited state saturated effect, enhances the apparatus brightness greatly. Their luminescence color cover the entire visible wave band, resulting in that preparing the entire color apparatus becomes possible (red, green, blue for three basics true colors). Compared with metal iridium complexes in which iridium has the d6 electronic configuration, the Pt(II) complexes in which Pt(II) has the d8 electronic configuration usually present a plane molecular configuration, therefore, there are very strong intermolecular interaction in both the liquid state and the solid state, one kind of typical interaction isπ-πstacking effect; another important effect is the interaction between the metal - metal - ligand. In recent years, in order to achieve an in-depth understanding on the electroluminescence essence of the Pt(II) complexes material, the scientific research workers have investigated the metal complexes both experimentally and theoretically. But they need the sufficient support of theory which results in the empirical understanding of the mechanism of luminescence and there is no exact direction for experimental work. In this work, density functional theory was used to investigate the organic metal phosphorescent complexes. The ground-state and excited-state geometries were optimized by means of B3LYP and CIS methods, respectively. The absorption and emission spectra were calculated with the aid of TD-DFT method based on the optimized ground-state and excited-state geometries, respectively. In addition, the compositions of molecular orbitals, energy gaps, the distribution of charge, ionization potentials, electronic affinities, and reorganization energy, etc. have also been calculated and analyzed. The theoretical results show that we can design superior photoluminescent materials by modification of chemical structure. The following is our main work.1 The properties of electronic structure and the optical properties of a series of Ir(ppy)2(ptz) complexes have been studied by the theoretical calculation and design. The computed results show that the true reason of spectrum shift of complex 5a2 is not in agreement with the experiment conclusion. It is not because of the increases of the HOMO-LUMO gaps by the introduction of the electron-donating substituting group. In fact, this kind of energies is reduced. The true reason is that the introduction of substituting group changes the nature of transition character, more frontier molecular orbitals are concerned with the transition process. Moreover, by theory design, complex 5ax will be an extremely good blue light material because it has a good luminescence nature and charge transfer performance.2 The electronic structure and photoelectricity nature of a series of Ir(ppy)2(pic) complexes have been studied. The computed results indicate that the nature of the substituent at the 4-position of the pyridyl moiety of the auxiliary pic ligand can influence both the distributions of HOMO and LUMO and their energies. When an electron-donating group is introduced at this position, the distribution of LUMO on the auxiliary ligand will nearly completely shift to other phenylpyridine ligand, while an electron-withdrawing substituent is introduced at this position, the distribution of LUMO nearly have no obvious change. In addition, both the electron-donating and electron-withdrawing substituents at this position don't change the distribution of HOMO, but they can influence the energies of the frontier molecular orbitals. At the same time, the nature of substituting group also has the very tremendous influence on the distribution of charge on the auxiliary ligand, especially when an electron-withdrawing substituent is introduced at this position, this effect is remarkable. In addition, this kind of substituting groups influencing the distribution of charge only influences the ligand which is connected to the substituent remarkably, especially, the distribution of charge of skeleton atoms at pic ligand which directly connect the substituent. And the substituents nearly have no influence on the other ligands. The above factors are the reason of complex N984 having a highly phosphorescent green emitter with high electroluminescence efficiency, which meets the need for display and illumination application. In addition, introducing the substituting group simultaneously changed the molecular charge transfer rate and balance. Especially when an electron-withdrawing group is introduced, the transfer rate of electrons will be changed remarkably.3 The electronic structure and illumination mechanism of a series of Ir(ppy)2(LX) complexes FIrpic, FIrmpic, FIrpca, and FIrprza have been studied. We can obtain the following main conclusion. (1) The energies and electron distribution of frontier molecular orbitals can be adjusted availably by applying some subsituent at auxiliary ligand or changing the auxiliary ligand. The energies of frontier molecular orbitals have an important influence on transition characteristics in absorption and emission processes because they determine the transition probability of electrons in frontier molecular orbitals in absorption and emission processes. Then they can impact the spectrum color and purity remarkably. (2) The charge transfer properties are also affected by the energies of frontier molecular orbitals. Therefore we can design the better performance electroluminescence material by adjusting the structure of the auxiliary ligand and the subsituent at the auxiliary ligand.4 Based on the most classical Pt(II) metal complex FPt, the electronic structure and the spectrum nature of a series of Pt(II) metal complexes have been studied and designed via the computation by the DFT method. Moreover, we introduced the UB3LYP method optimizing the excited-state geometry structure, and compared its accuracy with that of CIS method. The computed results indicate that the computed spectral data based on the excited-state geometry which were computed by UB3LYP method are not in agreement with the experiment values. From the computation results we can obtain the following conclusions. (1) Both the electron-withdrawing substituents at the 3- or 5-position of the phenyl moiety of ppy ligand and electron-donating group at 4-position of the pyridyl part of ppy liangd nearly unchange the distribution of HOMOs and LUMOs, but change the energies of HOMOs and LUMOs, and this kind of transition processes involves the frontier molecular orbitals HOMO-1, HOMO, LUMO and LUMO+1. Therefore introducing above substituting group will influence the molecular illumination color remarkably. (2) When auxiliary ligand acac is changed to a strong electron-withdrawing ligand pyridyltetrazole, the molecular structure does not present the plane configuration, but presents a very obvious spatial structure. This structure can reduce intermolecular interaction which affects the luminous efficiency. More importantly, this complex has extremely balanced electronic and hole transfer capacity. In summary, we may forecast NFPt could be one kind of very good phosphorescent material because it has no obvious shift compared with complex FPt but has an obvious spatial structure and a good charge transfer nature.These researches may provide some theory clue for the experimental synthesis of new highly efficient electroluminescence material.