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.

A series of tetradentate Pt­(II) emitters containing fused 5/6/6 metallocycles have been designed and synthesized. Molecular geometries play a critical role in determining the photophysical properties. Their emission spectra are significantly affected by the geometries of the molecular core skeletons, the substituents, even hydrogen atoms, and their positions, which are further supported by X-ray crystallographic analyses and theoretical calculations. The generation of excimer emissions is observed in the tetradentate 5/6/6 Pt­(II) emitters for the first time and found to be concentration-dependent both in the solution and solid states. All of the Pt­(II) emitters have high photoluminescent quantum efficiency of up to 100% and luminescent lifetime as short as 1.4 μs at room temperature, achieving a radiative rate of 7.14 × 105 s–1. Their emission color can be easily tuned to cover the whole visible region (λmax = 464–632 nm) through selective synthetic modification of the heteroaromatic rings of the ligands. Pt­(1-ptz)-based sky blue organic light-emitting diode (OLED) demonstrates a maximum external quantum efficiency (EQE) of 14.5%, yet maintains an EQE of 12.7% at a high brightness of 1000 cd/m2. This work demonstrates that these tetradentate Pt­(II) complexes can act as efficient phosphorescent emitters for OLED applications.

The synthesis and photophysical characterization of a series of tetradentate cyclometalated M­(tzpPh-O-CzPy-R) complexes and their analogues are reported, where M is palladium or platinum and a tetradentate cyclometalating ligand contains tzpPh (3-phenyl-[1,2,4]­triazolo­[4,3-a]­pyridine) and CzPy (carbazolylpyridine) moieties linked with an oxygen atom. Variations of the σ-electron-donating group R on the ligand significantly affect the photophysical properties of the complexes. By using the strong electron-withdrawing tzp portion as an acceptor and the carbazole portion as a donor, a series of Pd­(II)-based metal-assisted delayed fluorescence (MADF) materials was developed. Electrochemical analysis demonstrates the irreversible reduction process occurs on the tzp ring and the irreversible oxidation process mainly occurs on the metal-phenyl moiety. This is in agreement with the HOMO and LUMO distributions by the DFT calculations, which also shows that the Pt­(II) complex has more metal orbital character than those of the Pd­(II) complexes. Most of the Pd­(II) complexes reported here are highly emissive at 77 K in 2-MeTHF with luminescent lifetimes in the millisecond range (τ = 1.96–2.36 ms) and λmax = 488–499 nm; however, the luminescent lifetimes are shortened to the microsecond range (τ = 26.7–152.9 μs in solution and 57.0–109.9 μs in thin film respectively) at room temperature. The quantum efficiency of the Pd­(II) complexes can be increased by more than 8-fold through structure modification with σ-donating groups on the ligand. Especially, the Pd­(tzp-3) has a small ΔE ST of 0.228 eV and exhibits strong typical MADF in PMMA film. The Pt­(II) complex Pt­(tzp-2) exhibits high thermal stability (ΔT 0.5% = 440 °C) and high quantum efficiency (Φ = 50.1%) in dichloromethane solution with τ of 15.8 μs. The Pt­(tzp-2) based bright green OLED achieved a peak EQE of 8.7% and a maximum brightness of 28280 cd/m2 using an unoptimized device structure.

A series of phenylpyridine (ppy)-based 6/5/5 N*C^N^O and biphenyl (bp)-based 6/5/6 N*C^C*N Pt­(II) complexes employing tetradentate ligands with nitrogen or oxygen atoms as bridging groups have been developed. Ligand structural modifications have great influences on the electrochemical, photophysical, and excited-state properties, as well as photostabilities of the Pt­(II) complexes, which were systematically studied by experimental and theoretical investigations. The time-dependent density functional theory calculations and natural transition orbital analyses reveal that Pt­(bp-6), Pt­(bp-7), and Pt­(bp-8) have dominant ligand-centered (3LC) mixed with small metal-to-ligand charge-transfer (3MLCT) characters in T1 states, resulting in relatively low quantum efficiencies (ΦPL) of 5–33% and 12–32% in dichloromethane solution and PMMA film, respectively. By contrast, Pt­(ppy-1) possesses much more 3MLCT character in the T1 state, enabling a high ΦPL of 95% in dichloromethane and 90% in DPEPO film, and large radiative decay rates. The strength of the Pt–N1 coordination bond plays a critical role in the photostability. Pt­(ppy-1)- and Pt­(bp-6)-doped polystyrene films demonstrate long photostability lifetimes of 150 min for LT97 and LT98.5, respectively. A Pt­(ppy-1)-based green OLED using 26mCPy as host realized a peak EQE of 18.5%, which still maintained an EQE of 10.4% at 1000 cd/m2, and an L max of over 40 000 cd/m2 was achieved. This study should provide a valuable reference for the further development of efficient and stable phosphorescent Pt­(II) complexes.

A novel host material of “M”-type carbazole/fluorene-based mDCzPF with a high triplet energy by utilizing meta-substituted phenyl groups as linkers was developed. It was demonstrated that the position of the substituents significantly affected the molecular configuration and dipole moment, which played a critical role in the device performances. Red phosphorescent OLED utilizing the “M”-type mDCzPF as the host represented a 10-fold operational lifetime improvement over the OLED using a “V”-type pDCzPF linked by para-substituted phenyl groups as the host because of the good charge transport ability of the mDCzPF. Additionally, the “M”-type mDCzPF host was also compatible with a blue emitting phosphorescent emitter PtNON. The PtNON-doped OLED using mDCzPF as the host exhibited a peak EQE of 18.3% with a small roll off, yet maintained an EQE of 13.3% at a high brightness of 5000 cd/m2. Thus, the novel “M”-type mDCzPF could be employed as stable host material for efficient OLED emitting across the whole visible spectrum. This study should provide a viable method for designing new host materials for the development of stable and efficient phosphorescent OLEDs.