| As the rapid development of technology and society, organic light-emitting diodes(OLEDs) have obtained extensive attention over the recent decades for their widely applications in flat panel display, solid-state lighting and electronic information. Transition metal platinum complex as luminescent materials for OLED have demonstrated the ability to harvest both singlet and triplet excitons due to the spin-orbit coupling(SOC) effect, resulting in a theoretical 100% phosphorescence quantum efficiency; and their emission colour can be altered in the visible region flexibly by judicious modification of the ancillary ligand; moreover, the ligand of platinum complexes can be designed as bidentate, tridentate, and tetradentate due to the square-planar configuration. In recent years, the tetradentate complexes have been widely concerned and studied because of their rigid framework which can enhance the thermal stability and reduce the nonradiative decay pathway.In this paper, we studied a new potential class of synthetic tetradentate Pt(II) complexes with a novel structure of Pt[N^C-O-LL’]. N^C is a cyclometalating ligand such as phenyl-methylimidazole(pmi) or phenyl-pyrazole(ppz), and LL’ is an ancillary chelate ligand such as phenoxyl-pyridine(popy) or carbazolyl-pyridine(cbpy). A large ligand is formed when the N^C ligand and the LL’ ligand are bridged with oxygen, which greatly enhance the rigidity of the complexes. The photophysical properties of the complexes can be tuned by changing the N^C and LL’ moieties in the ligand. Four Pt(II) complexes Pt[ppz-O-popy](1), Pt[pmi-O-popy](2), Pt[pmi-O-cbpy](3), Pt[ppz-O-cbpy](4) of highly efficient blue-emitting and two designed complexes Pt[pmi-O-cbmi](5) and Pt[pmi-O-cbbm](6)(where cbmi=carbazolyl-imidazole, cbbm=carbazolyl-benzimidazole) based on 3-4 were studied in this paper. Complexes 3-6 introduced carbazole group and complexes 5-6 substituted the pyridine of LL’ ligand with imidazole and benzimidazole. To have an in-depth understanding of the essential mechanisms of the luminescence, the geometrical structures, absorption, emission properties, radiative decay rate constant(kr) and reorganization energy(λ) were investigated by density functional theory and time-dependent density functional theory calculations; in addition, the nonradiative decay processes were also investigated in order to obtain a more reliable efficiency comparison for 1-6.The results confirm that the extended conjugation of complexes 3 and 4 can not only improve the electronic structures and photophysical properties but also enhance the capability and balance of charge transfer. Thereupon, the following designed of two derivatives 5 and 6 with extended conjugation based on complexes 3 and 4 with carbazole group was also considered for promising materials exploitation and theoretical understanding. Complexes 3-6 have better rigidity and charge transfer performance than that of 1-2 due to the introduction of carbazole group. Additionally, the kr of 5 and 6 are greatly improved compared with 1-4, which may result in higher phosphorescence quantum efficiency. The charge transfer balance performance and the kr of 5 and 6 are greatly improved compared with 1-4, which may result in higher phosphorescence quantum efficiency and the emission colour has almost no change. Therefore, complexes 5 and 6 through judicious molecular design would be promising candidates as highly efficient blue-emitting phosphorescent materials for OLEDs applications. We hope this work can contribute to the further design and development of highly efficient blue-emitting phosphorescent materials. |
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