Efficient deep-blue-emitting tetradentate platinum complexes with a narrow spectral bandwidth are presented, which demonstrate CIEx ≈ 0.15 and CIEy < 0.1. Ultimately, an organic light-emitting diode (OLED) with 24.8% peak external quantum efficiency and CIE coordinates of (0.147, 0.079) is fabricated using PtON7-dtb.

Phosphorescent organic light-emitting diodes (OLEDs) are leading candidates for next-generation displays and solid-state lighting technologies. Much of the academic and commercial pursuits in phosphorescent OLEDs have been dominated by Ir(III) complexes. Over the past decade recent developments have enabled square planar Pt(II) and Pd(II) complexes to meet or exceed the performance of Ir complexes in many aspects. In particular, the development of N-heterocyclic carbene-based emitters and tetradentate cyclometalated Pt and Pd complexes have significantly improved the emission efficiency and reduced their radiative lifetimes making them competitive with the best reported Ir complexes. Furthermore, their unique and diverse molecular design possibilities have enabled exciting photophysical attributes including narrower emission spectra, excimer -based white emission, and thermally activated delayed fluorescence. These developments have enabled the fabrication of efficient and "pure" blue OLEDs, single-doped white devices with EQEs of over 25% and high CRI, and device operational lifetimes which show early promise that square planar metal complexes can be stable enough for commercialization. These accomplishments have brought Pt complexes to the forefront of academic research. The molecular design strategies, photophysical characteristics, and device performance resulting from the major advancements in emissive Pt and Pd square planar complexes are discussed.

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In order to develop organic light-emitting diodes with improved optical properties, a series of phosphorescent complexes exhibiting narrow-band emission spectra are prepared and color tuned to emit efficiently across the whole visible spectrum through a judicious molecular design. Devices employing a green narrow-band phosphorescent emitter are fabricated and demonstrate an internal quantum efficiency of close to unity and impressive device operational lifetimes, estimate at over 70,000 hours at a practical luminance of 100 cd/m 2 . Additionally a deep blue narrow-band emitter is incorporated into a device setting which demonstrates a peak external quantum efficiency of 17.6% and CIE coordinates of (0.14, 0.09).

Using a single tetradentate platinum emitter dubbed Pt7O7, efficient and stable white organic light-emitting diodes are developed. The excimer-based white devices achieve an external quantum efficiency (EQE) of 24.5%, coordinates of (0.37, 0.42) based on the Commission internationale de l'éclairage (CIE) system, and a color rendering index (CRI) of 70. Moreover, devices of Pt7O7 in a stable structure demonstrate operational lifetimes (50% initial luminance) of 36 h at an elevated driving current of 20 mA cm2, which corresponds to over 10,000 h at 100 cd m2.

Deep blue emitters are highly desired and intensively investigated for organic light emitting diodes for high quality full color display or solid state lighting applications. Here a new strategy for deep blue phosphorescent emission is demonstrated through the incorporaiton of 6-membered metal chelation rings in the tetradentate metal complex molecular structure. The platinum metal complex PtNON was synthesized and fully characterized. The emission spectra of PtNON shows a deep blue emission peaking at 438 nm at 77 K and a red-shifted, broadened, and structureless emission band centered at 508 nm at room temperature. The photoluminescent quantum yield of PtNON reaches a high value of 83%, and its luminescent lifetime remains as short as 3.76 μs in a 5% doped PMMA thin film. Organic light emitting diodes devices with PtNON as an emissive material were fabricated, achieving a peak device efficiency of 24.4% in a device structure designed for charge confinement. Furthermore, device architectures using known stable components with PtNON demonstrated an operational lifetime to 70% initial luminance estimated at over 30 000 h at 100 cd/m2 while also retaining moderate efficiencies over 10% despite the high triplet energy over 2.8 eV for the emitter.

wileyonlinelibrary.comlayers, or through the combination of fl uorescent and phosphorescent materials. [9][10][11] With these structures, typically employing iridium complexes, WOLEDs have achieved external quantum effi ciencies over 20%, color rendering index (CRI) over 80, and power effi ciencies over 100 lm/W when advanced outcoupling techniques are employing. [ 12 ] However, the strategy of using multiple emissive materials depends on the precise control over various energy transfer processes within the device which can signifi cantly complicate the device fabrication and have significant tradeoffs between device effi ciency and emission color. [ 13 ] Furthermore, the precise color balance of WOLEDs containing multiple emissive materials can be signifi cantly perturbed by variations in the driving conditions, or through different aging processing of the various materials. [ 14 ] Thus, it is strongly desired to achieve an effi cient WOLED containing a single emissive material which is effi cient, stable, and can be fabricated within a single emissive layer.One major approach to achieve single doped white OLEDs is through the exploitation of the excimer emission properties of square planar complexes for a broad white emission. [ 15 ] In excimer based OLEDs, white emission is achieved through the combination of blue emission from an isolated dopant molecule and orange-red emission of two or more closely stacked dopant molecules. Much of the existing reports of excimer based white OLEDs employ either bidentate or tridentate cyclometalating ligands, both of which have typically demonstrated external quantum effi ciency (EQE) less than 20% and often poor CRI or Commission internationale de l'éclairage (CIE) coordinates. [ 16,17 ] One exception is the recent development of platinum(II) bis(methyl-imidazolyl)benzene chloride (Pt-16) resulted in a device with peak EQE of 20.1%, CRI of 80, and CIE of (0.33,0.33). [ 18 ] However, it was demonstrated that the monomer species of Pt-16 was ineffi cient, leading to a lower overall effi ciency and an unavoidable tradeoff between optimal color and highest effi ciency. Furthermore, the N^C^N complexes and other tridentate analogs require Cl − or other monoanionic ligands as the fourth coordinating ligand which may be potentially unstable so a new molecular design motif is needed. [ 19 ] Here, we report the synthesis of a series of tetradentate Pt complexes based on a phenyl methyl-imidazole emissive ligand ( Figure 1 ), which demonstrate effi cient emission from both the excimer and monomer achieving a peak EQEs of 24% ± 2% for all three emitters at concentrations from 2% to 16%. The effect A series of tetradentate platinum complexes that exhibit both effi cient monomer and excimer emission are synthesized. Via small modifi cations to the cyclometalating ligands, both the monomer and excimer emission energy can be separately tuned. Devices employing all of the developed emitters demonstrate impressively high external quantum effi ciencies (EQEs) within the range of 22% to 27%...

Highly efficient and stable palladium complexes that exhibit both phosphorescence and delayed fluorescence are developed. It is demonstrated that the emission from the two processes can be separately tuned through rational ligand modification. External quantum efficiencies over 20% are achieved and stable devices demonstrate an operational lifetime to 90% initial luminance estimated at over 20 000 h at 100 cd m(-2) .