Color-saturated green-emitting molecules with high Commission Internationale de L’Eclairage (CIE) y values have great potential applications for displays and imaging. Here, we linked the outer phenyl groups in multiple-resonance (MR)-type blue-emitting B (boron)-N (nitrogen) molecules through bonding and spiro-carbon bridges, resulting in rigid green emitters with thermally activated delayed fluorescence. The MR effect and multiple interlocking strategy greatly suppressed the high-frequency vibrations in the molecules, which emit green light with a full-width at half-maximum of 14 nm and a CIE y value of 0.77 in cyclohexane. These were the purest green molecules with quantum efficiency and color purity that were comparable with current best quantum dots. Doping these emitters into a traditional green-emitting phosphorescence organic light-emitting diode (OLED) endowed the device with a Broadcast Service Television 2020 color-gamut, 50% improved external quantum efficiency, and an extremely high luminescence of 5.1 × 105 cd/m2, making it the greenest and brightest OLED ever reported.

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 new series of tetracoordinated boron-enabled thermally activated delayed fluorescence (TADF) materials with a donor–acceptor BF2-type framework were designed and conveniently synthesized. Difluoroboron plays a critical role and acts as a key to coordinate with the latent acceptor of the 2-(4-phenylpyridin-2-yl)­phenol (PPyPOH) moiety to realize TADF. TADF materials are air-stable and have a high photoluminescence quantum yield of up to 99%. NOBF2-Cz- and NOBF2-DPCz-doped blue OLEDs demonstrated EQEs of 11.0% with CIE coordinates of (0.14, 0.16) and 15.8% with (0.14, 0.28) and high brightness of 6761 and 19383 cd/m2 could be achieved, respectively. Moreover, the blue OLED doped with NOBF2-DPCz and the green OLED doped with NOBF2-DMAC achieved operational lifetimes at 50% of initial luminance (L 0 = 500 cd/m2), LT50, of 54 and 920 h, respectively. This work indicates that these tetracoordinated difluoroboron molecules can act as efficient and stable TADF materials for OLED applications.