High-Triplet-Energy Dendrons: Enhancing the Luminescence of Deep Blue Phosphorescent Iridium(III) Complexes

Twisted Carbazole Dendrons for Solution-Processable Green Emissive Phosphorescent Dendrimers

A family of first-generation dendrimers containing 3,5-bis(carbazolyl)phenyl dendrons attached to a green emissive fac-tris(2-phenylpyridyl)iridium(III) core were prepared. The solubility of the dendrimers was imparted by the attachment of tert-butyl surface groups to the carbazole moieties. The dendrimers differed in the number of dendrons attached to each ligand (one or two dendrons) as well as the degree of rotational restriction within the dendrons. The densities of the films containing the doubly dendronized materials were higher than those of their mono-dendronized counterparts, with the dendrimer containing two rotationally constrained dendrons per ligand having the highest density at 1.12 ± 0.04 g cm^-3. The dendrimers were found to have high photoluminescence quantum yields (PLQYs) in solution of between 80 and 90%, with the doubly dendronized materials having the lower values and a red-shifted emission. The neat film PLQY values of the dendrimers were less than those measured in solution although the relative decrease was smaller for the doubly dendronized materials. The dendrimers were incorporated into solution-processed bilayer organic light-emitting diodes (OLEDs) composed of neat or blend emissive layers and an electron transport layer. The best-performing devices had the dendrimers blended with a host material and external quantum efficiencies as high as 14.0%, which is higher than previously reported results for carbazole-incorporating emissive dendrimers. A feature of the devices containing blends of the doubly dendronized materials was that the maximum efficiency was relatively insensitive to the concentration in the host between 1 and 7 mol %.

Cibalackrot Dendrimers for Hyperfluorescent Organic Light-Emitting Diodes

Hyperfluorescent organic light-emitting diodes (HF-OLEDs) enable a cascading Förster resonance energy transfer (FRET) from a suitable thermally activated delayed fluorescent (TADF) assistant host to a fluorescent end-emitter to give efficient OLEDs with relatively narrowed electroluminescence compared to TADF-OLEDs. Efficient HF-OLEDs require optimal FRET with minimum triplet diffusion via Dexter-type energy transfer (DET) from the TADF assistant host to the fluorescent end-emitter. To hinder DET, steric protection of the end-emitters has been proposed to disrupt triplet energy transfer. In this work, the first HF-OLEDs based on structurally well-defined macromolecules, dendrimers is reported. The dendrimers contain new highly twisted dendrons attached to a Cibalackrot core, resulting in high solubility in organic solvents. HF-OLEDs based on dendrimer blend films are fabricated to show external quantum efficiencies of >10% at 100 cd m^-2 . Importantly, dendronization with the bulky dendrons is found to have no negative impact to the FRET efficiency, indicating the excellent potential of the dendritic macromolecular motifs for HF-OLEDs. To fully prevent the undesired triplet diffusion, Cibalackrot dendrimers HF-OLEDs are expected to be further improved by adding additional dendrons to the Cibalackrot core and/or increasing dendrimer generations.

Synthesis of Insulated Heteroaromatic Platinum-Acetylide Complexes with Color-Tunable Phosphorescence in Solution and Solid States

Phosphorescence colors of cyclodextrin-based insulated Pt-acetylide complexes were tuned by the molecular engineering of the chromophores. A series of Pt complexes bearing various acetylide ligands, including heteroaromatics, were prepared via self-inclusion of the linked macrocycles with the complexes. The decline in the inclusion efficiency derived from the heteroaromatics was overcome by the late-stage insulation via intramolecular slippage after the construction of the Pt-acetylide complexes. The cyclic protection of the thus-formed complexes prevented phosphorescence quenching via molecular interactions, even in the solid state. Accordingly, the tuned emission colors in a dilute system were replicated in the solid state.

Cyclometalated Ir(iii) complexes towards blue-emissive dopant for organic light-emitting diodes: fundamentals of photophysics and designing strategies

The fundamental photophysics of cyclometalated Ir(iii) complexes and surveys design strategies for efficient blue phosphorescent Ir(iii) complexes are summarised.

Efficient blue, green and red iridium(iii) complexes with noncovalently-linked pyrazole/pyrazolide rings for organic light-emitting diodes

Blue, green and red iridium(iii) complexes using a new pyrazole/pyrazolide ancillary ligand system, and their application in OLEDs with EQEs of up to 21.0% and low efficiency roll-off.

Solution-Processible Blue Fluorescent Dendrimers with Carbazole/Diphenylamine Hybrid Dendrons for Power-Efficient Organic Light-Emitting Diodes

Two blue fluorescent dendrimers named PVAC2 and PVACA have been newly synthesized and investigated, where the carbazole/diphenylamine hybrid dendron is adopted instead of oligocarbazole. Compared with the reference dendrimer PVCt3, the emission maxima of PVAC2 and PVACA are found to be red-shifted accompanied by a slight reduction of the photoluminescence quantum yield in films. Most importantly, the highest occupied molecular orbital level is elevated from -5.35 eV of PVCt3 to -5.20 eV of PVAC2 and -4.95 eV of PVACA. Because of the favored hole injection, the turn-on voltage is accordingly decreased from 3.6 to 3.2 and 2.6 V. The value of PVACA is even lower than the theoretical limit of 2.78 V. In addition, PVAC2 exhibited the best nondoped device performance, showing a nearly doubled power efficiency of 4.80 lm/W relative to PVCt3 (2.37 lm/W). The results clearly indicate that dendron engineering is also a promising strategy to develop solution-processible blue fluorescent dendrimers capable of being used for power-efficient organic light-emitting diodes.

What accounts for the color purity of tetradentate Pt complexes? A computational analysis

In order to improve the texture of human visual perception and broaden the range of certain optical applications, many phosphorescent complexes exhibiting narrow emission spectra have been prepared through reasonable molecular design. For example, by adding a particular group such as tert-butyl (tbu) to a suitable position of PtON1 and PtON7, the peak width of a relevant vibronic band caused by the specific vibrational normal modes could be dramatically restrained in the emission spectra at room temperature. For the purpose of finding an effective approach to replace the trial-and-error manner, the microscopic mechanism of such high color purity was elucidated by computational investigation. In this study, we aim to identify the reason that causes sharp emission associated with the relevant vibrational normal modes. Here, these modes can be labeled to the emission peak by the vibrationally resolved emission spectra. Based on the displacement vectors of relevant normal modes and the vibrationally resolved spectra, the most possible reason for the higher color purity is that tbu in a specific location can restrain the structural deformation between the first triplet excited state (T₁) and the ground state (S₀). That is to say, the relevant Huang-Rhys factor (S_k) of specific vibrational modes would be decreased. For these compounds, the total bandwidth and the height of the intermediate and high-frequency regions which are in direct proportion to S_k would be decreased to obtain the higher color purity by tbu in a particular position. What is more, the best position for tbu in order to suppress the structural deformation was also considered. In the meantime, radiative (k_r) and nonradiative (k_nr) decay rates of T₁ were investigated to seek the effective phosphorescent complexes.

N-Benzoimidazole/Oxadiazole Hybrid Universal Electron Acceptors for Highly Efficient Exciplex-Type Thermally Activated Delayed Fluorescence OLEDs

Recently, donor/acceptor type exciplex have attracted considerable interests due to the low driving voltages and small singlet-triplet bandgaps for efficient reverse intersystem crossing to achieve 100% excitons for high efficiency thermally activated delayed fluorescence (TADF) OLEDs. Herein, two N-linked benzoimidazole/oxadiazole hybrid electron acceptors were designed and synthesized through simple catalyst-free C-N coupling reaction. 24iPBIOXD and iTPBIOXD exhibited deep-blue emission with peak at 421 and 459 nm in solution, 397 and 419 nm at film state, respectively. The HOMO/LUMO energy levels were −6.14/−2.80 for 24iPBIOXD and −6.17/−2.95 eV for iTPBIOXD. Both compounds could form exciplex with conventional electron donors such as TAPC, TCTA, and mCP. It is found that the electroluminescent performance for exciplex-type OLEDs as well as the delayed lifetime was dependent with the driving force of both HOMO and LUMO energy offsets on exciplex formation. The delayed lifetime from 579 to 2,045 ns was achieved at driving forces close to or larger than 1 eV. Two TAPC based devices possessing large HOMO/LUMO offsets of 1.09–1.34 eV exhibited the best EL performance, with maximum external quantum efficiency (EQE) of 9.3% for 24iPBIOXD and 7.0% for iTPBIOXD acceptor. The TCTA containing exciplex demonstrated moderate energy offsets (0.88–1.03 eV) and EL efficiency (~4%), while mCP systems showed the poorest EL performance (EQE <1%) and shortest delayed lifetime of <100 ns due to inadequate driving force of 0.47–0.75 eV for efficient exciplex formation.

Green-emitting dendritic alkynylgold(iii) complexes with excellent film morphologies for applications in solution-processable organic light-emitting devices

Bipolar dendritic alkynylgold(iii) complexes that exhibit excellent film morphologies in solid-state thin films have been designed and synthesized. Solution-processable OLEDs with EQEs >14.5% have been achieved.

Multinuclear Iridium Complex Encapsulated by Oligocarbazole Dendrons for Enhanced Nondoped Device Efficiency

A dendritic multinuclear Ir complex, namely Cz-3IrB-IrG, has been designed and synthesized by introducing the second-generation oligocarbazole dendrons into its periphery. Because of the characteristic encapsulation, the intermolecular interactions could be effectively alleviated to prevent the unwanted triplet-triplet annihilation stemmed from the outer blue Ir complexes. Compared with 3IrB-IrG in the absence of dendrons, the film photoluminescence quantum yield of Cz-3IrB-IrG is greatly increased from 0.46 to 0.82 together with a small blue-shifted emission from 524 to 520 nm. On the basis of Cz-3IrB-IrG as the emitting layer alone, the nondoped device realizes a promising luminous efficiency of 40.9 cd/A (12.0%), much higher than that of 3IrB-IrG (32.6 cd/A, 9.7%). The obtained improvement clearly indicates that further dendronization toward multinuclear Ir complex will provide an alternative strategy to construct highly efficient phosphors used for nondoped phosphorescent organic light-emitting diodes.

Tailoring the Emission of Fluorinated Bipyridine-Chelated Iridium Complexes

New functionalized tris(2',6'-difluoro-2,3'-bipyridinato-N,C4')iridium(III) ((dfpypy)₃Irs) complexes, including small molecules and their dendrimer embedded analogoues, were synthesized and characterized. It is demonstrated that both the fac-(dfpypy)₃Ir-based polyphenylene dendrimers and (triisopropylsilyl)ethynyl (TIPSE)-substituted (dfpypy)₃Ir complexes induce large bathochromic shifts (∼50 nm) of emission bands compared with fac-(dfpypy)₃Ir. This is due to the pronounced ³π-π* character of emissive excited states and the extended conjugation. A further remarkable feature is the small bathochromic shift of the emissions of fac-tris(2-phenylpyridine)iridium (fac-(ppy)₃Ir)-based polyphenylene dendrimers when compared to those of the iridium (Ir) complex core. Obviously, the triplet metal-to-ligand charge transfer makes emission less sensitive to extended conjugation than the ³π-π* transition. This finding suggests new concepts for designing blue phosphorescent dendrimer emitters. Both the dendrimers and the TIPSE-substituted (dfpypy)₃Ir complexes represent new green and the trimethylsilyl-functionalized (dfpypy)₃Ir new blue phosphorescent emitters. Incorporation of TIPSE moieties into the ligands of iridium complex gives rise to enhanced phosphorescence.

Tetranuclear Iridium Complex with a Self-Host Feature for High-Efficiency Nondoped Phosphorescent Organic Light-Emitting Diodes

A tetranuclear Ir complex 3IrB-IrG was newly designed and synthesized with one green-emitting complex as the core, whose periphery is encapsulated by three blue-emitting complexes via a nonconjugated linkage. In the case of such a multinuclear system, a self-host feature can be formed, showing negligible electron communication, efficient outside-in energy transfer, and unique shielding effect. When using 3IrB-IrG as the emitting layer in the absence of host, the corresponding nondoped device reveals a state-of-art luminous efficiency of 32.6 cd/A (34.2 lm/W, 9.7%) as well as CIE coordinates of (0.38, 0.58). The performance significantly outperforms that of the bare mononuclear complex IrG (8.4 cd/A, 9.2 lm/W, 2.5%), highlighting the potential of self-host multinuclear complexes to realize high-efficiency nondoped PhOLEDs for the first time.

Density functional theory investigation on iridium(iii) complexes for efficient blue electrophosphorescence

The geometrical structures, electronic structures, optoelectronic properties and phosphorescence efficiencies of blue-emitting phosphors [Ir(fpmi)2(pyim)], [Ir(pyim)2(fpmi)], [Ir(fpmi)2(fptz)], [Ir(fpmi)2(pypz)] and [Ir(tfmppz)2(pyim)]), were investigated by DFT and TDDFT methods.

High efficiency green OLEDs based on homoleptic iridium complexes with steric phenylpyridazine ligands

The doped and non-doped OLED devices based on 3 exhibit very high EQE of 25.2% and 15.2%, respectively.

A spectroscopic study on the satellite vibronic band in phosphorescent Pt-complexes with high colour purity

To understand the relationship between the narrowing of an emission band and structural changes, we synthesised tetradentate Pt-complexes. Pt-1 has two directly connected carbazole (Cz) moieties, Pt-2 has two additional methyl groups to Pt-1, and Pt-3 has one Cz moiety. The absorption and emission spectra of Pt-2 were identical to those of Pt-1. Pt-3's emission was observed at a shorter wavelength compared to the others. We achieved phosphorescence with high colour purity by introducing a tetradentate ligand. All the Pt-complexes showed a vibronic structure in the emission spectra measured at 77 and 300 K. The 0-0 vibronic band of the Pt-complexes is quite intense compared to the 0-1 vibronic band, which may be due to less structural change of the fused tetradentate ligand in the excited state relative to the ground state. The spacing of the 0-0 and 1-0 vibronic bands is 1487 and 1323 cm^-1, respectively. To understand the origin of the satellite vibronic bands, we carried out vibrational spectroscopic (IR and Raman) measurements and theoretical calculations to analyse the infrared spectrum. In addition, we carried out a transient Raman experiment to obtain the vibronic information of an excited Pt-1. The vibronic spacing in the emission was caused by the displacement of the potential energy curve in the excited state. The highest occupied molecular orbital is populated with a Cz moiety and the lowest unoccupied molecular orbital is localized at the terminal pyridine moiety. For the triplet state, however, the highest singly occupied molecular orbital is delocalized on the pyrazole or imidazole moiety, as well as the pyridine moiety. These groups are located at the terminal site of the ligand, and are less rigidified and more flexible. Therefore, the major origin of the satellite vibration band in emission spectra is the stretching of the terminal groups.

Donor-σ-Acceptor Motifs: Thermally Activated Delayed Fluorescence Emitters with Dual Upconversion

A family of organic emitters with a donor-σ-acceptor (D-σ-A) motif is presented. Owing to the weakly coupled D-σ-A intramolecular charge-transfer state, a transition from the localized excited triplet state (³ LE) and charge-transfer triplet state (³ CT) to the charge-transfer singlet state (¹ CT) occurred with a small activation energy and high photoluminescence quantum efficiency. Two thermally activated delayed fluorescence (TADF) components were identified, one of which has a very short lifetime of 200-400 ns and the other a longer TADF lifetime of the order of microseconds. In particular, the two D-σ-A materials presented strong blue emission with TADF properties in toluene. These results will shed light on the molecular design of new TADF emitters with short delayed lifetimes.

Dendron engineering in self-host blue iridium dendrimers towards low-voltage-driving and power-efficient nondoped electrophosphorescent devices

Low-voltage-driving and power-efficient nondoped electrophosphorescent devices have been realized by increasing the dendron's HOMO energy level to favor effective hole injection and promote exciton formation.

Sky-blue phosphorescence from bis- and tris-cyclometalated iridium(iii) complexes bearing carbazole-based dendrons: fabrication of non-doped multilayer organic light-emitting diodes by solution processing

Non-doped multilayer OLEDs employing liposoluble and alcohol-insoluble dendrimers were fabricated by solution processing, exhibiting excellent charge balance factors of 0.9.

Self-Host Blue-Emitting Iridium Dendrimer Containing Bipolar Dendrons for Nondoped Electrophosphorescent Devices with Superior High-Brightness Performance

A novel self-host blue-emitting iridium dendrimer, namely, B-CzPO, has been designed and synthesized via a postdendronization route, where a bipolar carbazole/triphenylphosphine oxide hybrid is selected as the peripheral dendron instead of the p-type oligocarbazole used in unipolar analogue B-CzG2. This structural modification can render B-CzPO with more balanced charge transportation relative to that of B-CzG2. As a result of the significantly reduced efficiency roll-off, the nondoped phosphorescent organic light-emitting diodes (PhOLEDs) of B-CzPO show a superior high-brightness performance, revealing a luminous efficiency of 21.2, 16.1, and 10.5 cd/A at 1000, 5000, and 10 000 cd/m², respectively. Compared with that of B-CzG2 (i.e., 7.8 cd/A @5000 cd/m²), more than doubled high-brightness performance is achieved for B-CzPO. The results indicate that the design of self-host phosphorescent dendrimers with a bipolar feature will be a promising strategy to develop efficient nondoped PhOLEDs suitable for high-brightness applications including general illumination and micro displays.

Single molecular tuning of the charge balance in blue-emitting iridium dendrimers for efficient nondoped solution-processed phosphorescent OLEDs

Charge balance modulation at a single dendritic molecule is demonstrated to be far superior to physical blending due to the elimination of phase segregation, leading to improved EQEs and negligible efficiency roll-off.

Spotlight on the ligand: luminescent cyclometalated Pt(iv) complexes containing a fluorenyl moiety

A family of Pt(iv) complexes with 2-(9,9-dimethylfluoren-2-yl)pyridine is reported, including bis- and tris-cyclometalated derivatives, which exhibit intense ³LC phosphorescent emissions in fluid solution.

Functionalization of phosphorescent emitters and their host materials by main-group elements for phosphorescent organic light-emitting devices

Phosphorescent organic light-emitting devices (OLEDs) have attracted increased attention from both academic and industrial communities due to their potential practical application in high-resolution full-color displays and energy-saving solid-state lightings. The performance of phosphorescent OLEDs is mainly limited by the phosphorescent transition metal complexes (such as iridium(III), platinum(II), gold(III), ruthenium(II), copper(I) and osmium(II) complexes, etc.) which can play a crucial role in furnishing efficient energy transfer, balanced charge injection/transporting character and high quantum efficiency in the devices. It has been shown that functionalized main-group element (such as boron, silicon, nitrogen, phosphorus, oxygen, sulfur and fluorine, etc.) moieties can be incorporated into phosphorescent emitters and their host materials to tune their triplet energies, frontier molecular orbital energies, charge injection/transporting behavior, photophysical properties and thermal stability and hence bring about highly efficient phosphorescent OLEDs. So, in this review, the recent advances in the phosphorescent emitters and their host materials functionalized with various main-group moieties will be introduced from the point of view of their structure-property relationship. The main emphasis lies on the important role played by the main-group element groups in addressing the key issues of both phosphorescent emitters and their host materials to fulfill high-performance phosphorescent OLEDs.

Efficient Red-Emitting Platinum Complex with Long Operational Stability

A tetradentate cyclometalated Pt(II) complex, PtN3N-ptb, was developed as an emissive dopant for stable and efficient red phosphorescent OLEDs. Devices employing PtN3N-ptb in electrochemically stable device architectures achieved long operational lifetimes with estimated LT97, of over 600 h at luminances of 1000 cd/m(2). Such long operational lifetimes were achieved utilizing only literature reported host, transporting and blocking materials with known molecular structures. Additionally, a thorough study of the effects of various host and transport materials on the efficiency, turn on voltage, and stability of the devices was carried out. Ultimately, maximum forward viewing EQEs as high as 21.5% were achieved, demonstrating that Pt(II) complexes can act as stable and efficient dopants with operational lifetimes comparable or superior to those of the best literature-reported Ir(III) complexes.

Yellow/orange emissive heavy-metal complexes as phosphors in monochromatic and white organic light-emitting devices

Owing to the electron spin-orbit coupling (SOC) and fast intersystem crossing (ISC), heavy-metal complexes (such as iridium(III), platinum(II) and osmium(II) complexes, etc.) are phosphorescent emitters at room temperature. Since 1998, heavy-metal complexes as phosphors have received considerable academic and industrial attention in the field of organic light-emitting diodes (OLEDs), because they can harvest both the singlet (25%) and triplet (75%) excitons for emission during the electro-generated processes. Among all the visible colors (blue, green, yellow, orange and red), the yellow/orange heavy-metal complexes play an important role for realizing full-color OLEDs as well as high-efficiency white OLEDs, and thus the development of highly efficient yellow/orange heavy-metal complexes is a pressing concern. In this article, we will review the progress on yellow/orange heavy-metal complexes as phosphors in OLEDs. The general principles and useful tactics for designing the yellow/orange heavy-metal complexes will be systematically summarized. The structure-property relationship and electrophosphorescence performance of the yellow/orange heavy-metal complexes in monochromatic phosphorescent OLEDs (PhOLEDs) and white OLEDs (WOLEDs) will be comprehensively surveyed and discussed.

Harvesting all electrogenerated excitons through metal assisted delayed fluorescent materials

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) .

Solution-processed oxadiazole-based electron-transporting layer for white organic light-emitting diodes

Alcohol-soluble oxadiazole-based compound as electron-transporting layer in solution-processed OLEDs shows much improved efficiency compared to control devices.

Synthesis and properties of greenish-blue-emitting iridium dendrimers with N-phenylcarbazole-based polyether dendrons by a post-dendronization route

Solution processible greenish-blue-emitting Ir dendrimers with N-phenylcarbazole based polyether dendrons are developed via a convenient post-dendronization route.

Efficient bifunctional materials based on pyrene- and triphenylamine-functionalized dendrimers for electroluminescent devices

These materials show a promising bifunctional property as blue emissive layer and hole-transporting layer for high efficiency solution processed blue and Alq3-based green OLEDs with much simpler device architecture.

Facile synthesis of self-host functional iridium dendrimers up to the fourth generation with N-phenylcarbazole-based polyether dendrons for non-doped phosphorescent organic light-emitting diodes

Self-host functional iridium dendrimers with N-phenylcarbazole-based polyether dendrons.

Theoretical research on the effect of regulated π-conjugation on the photophysical properties of Ir(iii) complexes

Larger π-skeleton-conjugation in host ligands of Ir(iii) carbene complexes can result in remarkably blue-shifted emission and can effectively improve the quantum efficiency.