Theoretical Studies On The Electronic Excited-state And Spectroscopic Properties Of Transition Metal Complexes

Transition metal complexes as optoelectronic materials have become a fascinating field in recent years for their diverse potential applications in flat-panel displays, memory and sensors due to their diversity of molecule and electronic structures and various photonic and electronic properties. Especially in the area of phosphorescent materials based on transition metal complexes, platinum and iridium complexes have attracted increasing attention due to their intriguing optoelectronic properties and excellent phosphorescent efficiency. With the development of research, it is more and more necceary to study electronic structures, excited state properties, charge transfer properties and other optoelectronic properties through quantum chemical methods. Theoretical studies become the strong complement for experimental investigation. And it can help us optimize the structure of the complexes and explain the phosphorescent mechanism.In this paper, we have systematically studied he electronic structure, ground-and excited-state properties, absorption and emission properties and charge transfer propeties of several kinds of platinum complexes and iridium complexes. The investigation on the structure–property relationship can provide the basis for the design of new functional materials and be helpful to improve their performance. The following is the main results:1. A series of dinuclear alkynyl platinum (II) terpyridyl complexes have been investigated theoretically using density functional theory (DFT) methods to investigate their electronic structures, excited-state properties, and spectroscopic properties. It is found that the introduction of substituents can enlargeÏ€-conjugation of the molecule and improve the HOMO and LUMO levels, resulting in the red-shift of absorption. In addition, the complexes containing different substituents have different HOMO and LUMO distributions. It can be realized to tune their optical and electronic properties through introducing the various substituents, aiming at gaining excellent functional materials.2. Cyclometalated iridium (III) complexes based on the structure of Ir(C^N)2(acac) have been designed by introducing different substituents on the CN ligands. DFT calculations have been carried out to investigate the influence on the optical and electronic properties caused by the introduction of substituents.From the calculation results, the substituents stabilized the HOMOs and LUMOs of the complexes and brought some changes of their HOMO-LUMO energy gaps. The carbazole group leads to slight blue shift of the absorption, and the oxadiazole group and dimesitylboryl group lead to red shift.Furthermore, the introduction of the hole-transporting group carbazole to complex 2 improves its hole-injecting and -transporting abilities, which is useful for the fabrication of highly efficient optoelectronic devices. And the introduction of electron-transporting group oxadiazole and electron-accepting group dimesitylboryl groups improves the charge transporting abilities of complexes 3 and 4, as well as leading to the good balance between hole- and electron-transfer for them.3. A theoretical analysis based on DFT/TDDFT approach is provided to explain the different quantum effeciency of a series of platinum(II) complexes containing differentβ-diketonate ligands. We mainly focus on the SOC effect and the factors in close connection with radiative decay rate and nonradiative decay rate of the complexes to discuss their quantum efficiency. The results show that we can explain or predict the quantum efficiency of these complexes, and it is helpful for the design and study of this kind of materials.4. We have studied the substituent effect of dimesitylboron groups on the optical and electronic properties of both the closed-ring and open-ring isomers of the diarylethene molecule by DFT/TDDFT calculation and investigated on the modulation of photochromic properties in an organoboron-based diarylethene by fluoride ion. It is found that the introduction of dimestiylboron groups on DTE compund can elongate its conjugated length and change the excited state properties fromÏ€â†'Ï€* transition to charge transfer state. Furthermore, the photochromic properties can be tuned through the binding of F- by boron center and the excited state of dithienylethene compund is changed again from charge transfer state toÏ€â†'Ï€* transition. Hence, a fine control of the photochromic properties is realized.