| Density Functional Theory and Time-dependent Density Functional Theory were employed to shed deep light on the luminescent properties and the photo-deactivation mechanism of a series of phosphorescent cyclometalated Pt(II) and Ir(III) complexes. At the initial stage of the research, we chose five(N^C)Pt(acac) complexes bearing different main-group moieties functionalized 2-phenypyridine(ppy) ligand as the stud y objects. Through explorations on the tuning of emission wavelength and phosphorescence quantum efficiency arising from the addition of substituent groups, the relationship between structure and property for cyclometalated Pt(II) complexes was clearly unveiled. Next, we mainly focus our attention on the effect of tert-butyl unit o n photo-deactivation mechanism of Pt(II) complexes bearing tetradentate coordination ligand, the influence of controlling π-conjugation on the structural rigidity and the relaxation dynamics of cyclometalated(C^C*)Pt(II)(NHC) complexes as well as the nature of the dual emission behavior of cyclometalated Ir(III) complexes. Consequently, as revealed, the phosphorescence emission energy and the luminescence efficiency can be considerably tuned through incorporating strong electron-withdrawing or electron-donating groups into the cyclometalated ligand. Moreover, based on the elucidation of the photo-deactivation pathway of transition-metal complexes, it can be found that the increase in the number of tert-butyl unit added to the ligand of tetradentate coordinated Pt(II) complexes has little influence on the thermal-deactivation process, while the radiative decay process and the temperature-independent non-radiative decay process are affected greatly. In addition, it can also be concluded that enlarging π-conjugation at appropriate position of cyclometalated(C^C*)Pt(II)(NHC) complexes is a judicious strategy to design robust phosphorescent emitters applied in OLEDs, since the structural rigidity is greatly enhanced and the occurrence of thermal-deactivation is efficiently depressed. Therefore, the unveiling and exploration of photo-deactivation mechanism of phosphorescent transition-metal complexes has significant importance in deepening the learning of emissive mechanism and providing theoretical guidance for the accurate design of phosphorescent materials. This work mian includes four aspects as follow:1. Exploration of Phosphorescent Platinum(II) Complexes Functionalized by Distinct Main-Group Units to Search for Highly Efficient Blue Emitters Applied in Organic Light-Emitting Diodes: A Theoretical StudyIn this study, five cyclometalated Pt(II) complexes were chosen as research subjects to investigate the effects of main-group moieties on the electronic structure, photophysical properties and radiative deactivation processes of the phosphorescent metal complexes. Density functional theory(DFT)/time-dependent DFT investigation was conducted to gain a better understanding of the properties of these Pt(II) complexes, including the ground and triplet state geometries, absorption spectra and emission wavelength. Moreover, the self-consistent spin-orbit coupling TDDFT(SOC-TDDFT) was used to calculate zero-field splitting(ZFS), radiative rate and radiative lifetime to unveil the radiative deactivation processes for these complexes. The results reveal that the different main-group moieties added on the 4′-position of the phenyl ring in [Pt(ppy)(acac)] could not only dramatically affect molecular and electronic structure, absorption and luminescence properties, but also radiative deactivation processes. And the emission wavelengths of five complexes are in the range from 434 to 562 nm. Furthermore, among the studied complexes, the designed complex 4 shows great potential to serve as an efficient deep-blue-light emitter in OLED.2. Exploring the Photodeactivation Pathways of Pt[O^N^C^N] Complexes: A Theoretical PerspectiveIn this article, the influence of tert-butyl unit on photo-deactivation pathways of Pt[O^N^C^N](O^N^C^N=2-(4-(3,5-di-tert-butylphenyl)-6-(3-(pyridin-2-l)phenyl)-pyridi n-2-yl)phenolate) was elaborated by DFT/TDDFT calculations. To further explore the factors which determine the radiative processes, the transition dipole moments of the singlet excited states, spin-orbit coupling(SOC) matrix elements and energy gaps between the lowest triplet excited states and singlet excited states were calculated. As demonstrated by the results, compared with Pt-3, Pt-1 and Pt-2 have larger SOC matrix elements between the lowest triplet excited states and singlet excited states, an indicator that they have faster radiative decay processes. In addition, the SOC matrix elements between the lowest triplet excited states and ground states were also computed to elucidate the temperature-independent non-radiative decay processes. Moreover, the temperature-dependent non-radiative decay mechanisms were also explored via the potential energy profiles.3. Theoretical Insights into the Phosphorescence Quantum Yields of Cyclometalated(C^C*) Platinum(II) NHC Complexes: π-Conjugation Controls the Radiative and Non-Radiative Decay ProcessesIn this article, the radiative and non-radiative decay processes of four cyclometalated(C^C*) platinum(II) N-heterocyclic carbene(NHC) complexes were unveiled via density functional theory(DFT) and time-dependent density functional theory(TD-DFT). In order to explore the influence of π-conjugation on quantum yields of(NHC)Pt(acac)(NHC=N-heterocyclic carbene, acac=acetylacetonate) complexes, the factors that determine the radiative process, including singlet-triplet splitting energies, transition dipole moments and spin-orbit coupling(SOC) matrix elements between the lowest triplet states and singlet excited states were calculated. In addition, the SOC matrix elements between the lowest triplet state and the ground state as well as Huang-Rhys factors were also computed to descript the temperature-independent non-radiative decay processes. And the triplet potential energy surfaces were investigated to elucidate the temperature-dependent non-radiative decay processes. The results indicate that complex Pt-1 has higher radiative decay rate than complexes Pt-2-4 due to the larger SOC matrix elements between the lowest triplet states and singlet excited states. However, complexes Pt-2-4 have smaller Huang-Rhys factors, smaller SOC matrix elements between the lowest triplet and the ground states and higher active energy barriers than complex Pt-1, indicating that complexes Pt-2-4 have smaller non-radiative decay rate constants. According to these results, one may discern why complex Pt-2 has higher phosphorescence quantum efficiency than complex Pt-1, meanwhile, it can be inferred that the non-radiative decay process plays an important role in the whole photo-deactivation process. In addition, on the basis of complex Pt-2, Pt-5 was designed to investigate the influence of substitution group on the photo-deactivation process of rigid(NHC)Pt(acac) complex.4. Dual Emission: Theoretical Unveil the Photo-Deactivation Mechanism of a Neutral Heteroleptic Iridium(III) ComplexIn this article, the photo-deactivation mechanism of dual emission of a neutral iridium(III) complex was explored by using the density function theory(DFT) and time-dependent density function theory(TD-DFT). In order to explore the phosphorescence quantum yield of this iridium(III) complex, the radiative decay constant of each emission excited state was computed with the help of the TD-DFT calculation including the spin-orbit coupling(SOC). Simultaneously, the factors, including the transition dipole moments, energy gaps and SOC elements between the emission triplet state and singlet excited stats, are taken into account to elaborate the radiative decay constants. Additionally, the temperature-independent and the temperature-dependent non-radiative processes also were considered to reveal non-radiative decay. The calculated results indicate that the order of the two emission excited states can cause vital effect on the phosphorescence quantum yield, which is enormously significant for whole understanding the properties of photo-deactivation of phosphorescent emitters. |
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