Highly Efficient Phosphorescent Tetradentate Platinum(II) Complexes Containing Fused 6/5/6 Metallocycles

Abstract

A series of neutral tetradentate Pt(II) complexes with fused 6/5/6 metallocycles and biphenyl (bp)-containing ligands have been designed and synthesized. All bridging atoms adopt nitrogens designed as an acridinyl group (Ac), an aza acridinyl group (AAc), and an aza carbazolyl group (ACz), which can effectively tune their LUMO energy levels. Their HOMO energy levels can be well-controlled through molecular modifications on the bp moieties with electron-donating and electron-withdrawing groups. These molecular modifications also have profound effects on the electrochemical and photophysical properties and photostabilities of the Pt(II) complexes. The ground-states and excited states are systematically studied by density functional theory (DFT), timedependent density functional theory (TD-DFT), and natural transition orbital (NTO) calculations. All the Pt(II) complexes exhibit admixed 3 (LC/MLCT) characters in T 1 states with various proportions, which are strongly structure-dependent. These 6/5/6 Pt(II) complexes demonstrate high quantum efficiencies in dichloromethane solutions (Φ PL = 27−51%) and in doped PMMA films (Φ PL = 36−52%) at room temperature with short luminescence lifetimes of 1.6−9.5 μs and 7.6−9.0 μs, respectively. They emit green light with dominant peaks of 512−529 nm in solutions and 512−524 nm in doped PMMA films, respectively. Importantly, Pt(bp-2) exhibits highly stable emission colors with the same dominant peaks at 512 nm in various matrixes and also demonstrates a long photostability lifetime, LT 80 , at 80% of initial luminance, of 190 min, which is doped in polystyrene films (5 wt %) excited by UV light of 375 nm at 500 W/m 2 . These studies indicate that these 6/5/6 Pt(II) complexes can act as good phosphorescent emitters for OLED applications and should provide a viable route for the development of efficient and stable Pt(II)-based phosphorescent emitters.

N-Heterocyclic Carbene-Based Tetradentate Pd(II) Complexes for Deep-Blue Phosphorescent Materials

“…A similar result was also observed in the previously reported tetradentate Pt(II) complexes. 28 Electrochemical properties of the NHC-based Pd(II) complexes were investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in anhydrous dimethylformamide under a nitrogen atmosphere (Figures 5, S3−S5, Table 1). 30 All the Pd(II) complexes exhibit irreversible redox processes in CV due to nucleophilic attack of solvent molecules to the metal center.…”

“…30 All the Pd(II) complexes exhibit irreversible redox processes in CV due to nucleophilic attack of solvent molecules to the metal center. 28,30,35,55,56 The Pd(II) complexes have oxidation potentials in a small region between 0.55 and 0.61 V, revealing the occurrence of the oxidation process mainly on the electron-rich metal-phenyl-carbazole (M-Ph-Cb) moiety. 55 This result is in good agreement with their highest occupied molecular orbital (HOMO) distribu- 1) with the involvement of reduction processes on the electron-deficient Py and Ph/ NHC moieties, respectively.…”

“…The substituent modifications on the pyridine or phenyl ring have no effect on the absorption energy levels, which are affected by the NHC moieties of the Pd(II) complexes (Figures S7 and 7). Natural transition orbital (NTO) analysis 28 reveals that the π → π* transitions are from Ph → Ph/NHC and Cb → Py for S4). Time-dependent density functional theory (TD-DFT) calculations and analysis 28 reveal that S 0 → S 1 processes of the Pd(II) complexes are mainly from HOMO → LUMO transitions (96−98%), whereas S 0 → T 1 processes are only 68−73% from HOMO → LUMO transitions, and HOMO → LUMO+3 transitions (11− 12%) also greatly contribute to the S 0 → T 1 processes for the PdON5 analogues (Tables S4−S10).…”

“…Natural transition orbital (NTO) analysis 28 reveals that the π → π* transitions are from Ph → Ph/NHC and Cb → Py for S4). Time-dependent density functional theory (TD-DFT) calculations and analysis 28 reveal that S 0 → S 1 processes of the Pd(II) complexes are mainly from HOMO → LUMO transitions (96−98%), whereas S 0 → T 1 processes are only 68−73% from HOMO → LUMO transitions, and HOMO → LUMO+3 transitions (11− 12%) also greatly contribute to the S 0 → T 1 processes for the PdON5 analogues (Tables S4−S10). All the NHC-based Pd(II) tetradentate complexes emit deep-blue light in various matrixes, such as in 2-methyltetrahydrofuran (2-MeTHF) at 77 K, in degassed dichloromethane solution at RT or in doped PMMA film at RT (Figures 8 and S9, Table 2).…”

Phosphorescent Manganese(II) Complexes and Their Emerging Applications

“…[77][78][79][80][81][82] So far, many commercialized emitters are based on these phosphors (such as iridium(III) complex). [83][84][85][86][87][88] However, these noble metals often have some disadvantages (such as high cost and low abundance in nature). The exploration of low-cost, relatively abundant, and less-toxic phosphorescent manganese(II) complexes could be the key to solve this problem.…”