Tethered Helical Ladder-Type Aromatic Lactams.

Tethered nonplanar aromatics (TNAs) make up an important class of nonplanar aromatic compounds showing unique features. However, the knowledge on the synthesis, structures, and properties of TNAs remains insufficient. In this work, a new type of TNAs, the tethered aromatic lactams, is synthesized via Pd-catalyzed consecutive intramolecular direct arylations. These molecules possess a helical ladder-type conjugated system of up to 13 fused rings. The overall yields ranged from 3.4 to 4.3%. The largest of the tethered aromatic lactams, 6L-Bu-C14, demonstrates a guest-adaptive hosting capability of TNAs for the first time. When binding fullerene guests, the cavity of 6L-Bu-C14 became more circular to better accommodate spherical fullerene molecules. The host-guest interaction is thoroughly studied by X-ray crystallography, theoretical calculations, fluorescence titration, and nuclear magnetic resonance (NMR) titration experiments. 6L-Bu-C14 shows stronger binding with C70 than with C60 due to the better convex-concave π-π interaction. P and M enantiomers of all tethered aromatic lactams show distinct and persistent chiroptical properties and demonstrate the potential of chiral TNAs as circularly polarized luminescence (CPL) emitters.

Two Birds with One Stone: Polymerized Thermally Activated Delayed Fluorescence Small Molecules.

Thermally activated delayed fluorescence (TADF) polymers show a great potential in low-cost, large-area and flexible full-color flat-panel displays. One of the most promising design rules is based on TADF+Linker, where a small molecular TADF unit is bonded to each other by a simple linker. Unlike the expensive vacuum deposition for small molecules, these polymerized TADF small molecules (Poly-TADF-SMs) are capable of cost-effective solution processing. Meanwhile, the good luminescent property of small molecular TADF emitters can be well inherited by Poly-TADF-SMs so as to bridge the efficiency gap between small molecules and polymers. Herein, we will highlight the recent progress of Poly-TADF-SMs, together with emphasis on their molecular design, photophysical and electroluminescence properties.

D-O-A based organic phosphors for both aggregation-induced electrophosphorescence and host-free sensitization.

Pure organic phosphors capable of room-temperature phosphorescence show a great potential in organic light-emitting diodes, while it is limited by the big challenge to realize efficient electroluminescence under electric excitation. Herein, we develop a class of organic phosphors based on acridine as the electron donor, triazine as the electron acceptor and oxygen as the bridge between them. Benefitting from the characteristic donor-oxygen-acceptor geometry, these compounds are found to behave an exciting aggregation-induced organic room-temperature electrophosphorescence, and achieve a record-high external quantum efficiency of 15.8% for non-doped devices. Furthermore, they can sensitize multi-resonant emitters in the absence of any additional wide bandgap host, leading to an effective narrowband emission with a peak external quantum efficiency of 26.4% and a small full-width at half maximum of 26 nm. The results clearly indicate that donor-oxygen-acceptor geometry is a promising strategy to design organic phosphors suitable for organic light-emitting diodes.

Polymerized Thermally Activated Delayed-Fluorescence Small Molecules: Long-Axis Polymerization Leads to a Nearly Concentration-Independent Luminescence.

Nowadays numerous thermally activated delayed fluorescence (TADF) polymers have been developed for PLEDs to realize high device performance and tunable emission colors. However, they often possess a strong concentration dependence on their luminescence including aggregation-caused quenching (ACQ) and aggregation-induced emission (AIE). Herein, we first report a nearly concentration-independent TADF polymer based on the strategy of polymerized TADF small molecules. It is found that when a donor-acceptor-donor (D-A-D) type TADF small molecule is polymerized through its long-axis direction, the triplet state is distributed along the polymeric backbone to effectively suppress the unwanted concentration quenching. Unlike the short-axis one with an ACQ effect, the photoluminescent quantum yield (PLQY) of the resultant long-axis polymer remains almost unchanged with the increasing doping concentration. Accordingly, a promising external quantum efficiency (EQE) up to 20 % is successfully achieved in a whole doping control window of 5-100 wt. %.

In situ interfacial passivation with an arylphosphine oxide and phosphonate electron transporting layer for efficient all-solution-processed PeQLEDs.

Perovskite quantum dot light-emitting diodes (PeQLEDs) have emerged as a promising candidate for high-quality lightings and displays, where an electron transporting layer (ETL) is required to achieve balanced charge transport and thus high performance. However, the ETL is often thermally-deposited under vacuum, since the low-cost solution process would damage the underlying perovskite quantum dots (PeQDs). Here, we demonstrate efficient all-solution-processed PeQLEDs based on arylphosphine oxide (SPPO13) and phosphonate (TPPO) as the ETL. Benefitting from the coordination between PO and exposed Pb atoms, in situ interfacial passivation occurs during the solution deposition of SPPO13 or TPPO on PeQDs. As a result, bilayer films (PeQDs/ETL) exhibit improved photoluminescence quantum yields and prolonged lifetimes compared with single layer PeQDs. Correspondingly, all-solution-processed PeQLEDs are fabricated successfully via an orthogonal solvent strategy, revealing bright green emission with a promising current efficiency of 24.1 cd A-1 (12.1 lm W-1, 6.47%) and CIE coordinates of (0.12, 0.79).

Sterically-Locked Donor-Acceptor Conjugated Polymers Showing Efficient Thermally Activated Delayed Fluorescence.

Donor-acceptor (D-A) conjugated polymers often possess a significant frontier molecular orbital overlap because of the conjugation elongation, leading to no thermally activated delayed fluorescence (TADF) caused by a large singlet-triplet energy splitting (▵EST ). Herein a novel steric locking strategy is proposed by incorporating methyl groups into D-A conjugated polymers. Benefitting from the methyl hindrance, the torsion between the donor and acceptor can be well tuned to form a sterically-locked conformation, so that the unwanted relaxation toward planarity and thus conjugation elongation is prevented to boost hole-electron separation. The resultant D-A conjugated polymer achieves an extremely low ΔEST of 0.09 eV to enable efficient TADF. The corresponding doped and non-doped devices are fabricated via a solution process, revealing a record-high external quantum efficiency (EQE) of 24.0 % (79.4 cd A-1 , 75.0 lm W-1 ) and 15.3 % (50.9 cd A-1 , 47.3 lm W-1 ).

An Electroactive Pure Organic Room-Temperature Phosphorescence Polymer Based on a Donor-Oxygen-Acceptor Geometry.

An electroactive room-temperature phosphorescence (RTP) polymer has been demonstrated based on a characteristic donor-oxygen-acceptor geometry. Compared with the donor-acceptor reference, the inserted oxygen atom between donor and acceptor can not only decrease hole-electron orbital overlap to suppress the charge transfer fluorescence, but also strengthen spin-orbital coupling effect to facilitate the intersystem crossing and subsequent phosphorescence channels. As a result, a significant RTP is observed in solid states under photo excitation. Most noticeably, the corresponding polymer light-emitting diodes (PLEDs) reveal a dominant electrophosphorescence with a record-high external quantum efficiency of 9.7 %. The performance goes well beyond the 5 % theoretical limit for typical fluors, opening a new door to the development of pure organic RTP polymers towards efficient PLEDs.

Meta Junction Promoting Efficient Thermally Activated Delayed Fluorescence in Donor-Acceptor Conjugated Polymers.

The meta junction is proposed to realize efficient thermally activated delayed fluorescence (TADF) in donor-acceptor (D-A) conjugated polymers. Based on triphenylamine as D and dicyanobenzene as A, as a proof of concept, a series of D-A conjugated polymers has been developed by changing their connection sites. When the junction between D and A is tuned from para to meta, the singlet-triplet energy splitting (ΔEST ) is found to be significantly decreased from 0.44 to 0.10 eV because of the increasing hole-electron separation. Unlike the para-linked analogue with no TADF, consequently, the meta-linked polymer shows a strong delayed fluorescence. Its corresponding solution-processed organic light-emitting diodes (OLEDs) achieve a promising external quantum efficiency (EQE) of 15.4 % (51.9 cd A-1 , 50.9 lm W-1 ) and CIE coordinates of (0.34, 0.57). The results highlight the bright future of D-A conjugated polymers used for TADF OLEDs.

Unravelling Electroplex Emission from Long-Range Charge Transfer Based on a Phosphorescent Dendrimer as the Electron Donor.

We report the exceptionally long-range charge-transfer-induced electroplex between a neat dendrimer emitter and the adjacent electron-transporting layer (ETL). Interestingly, the electroplex exists even in the dilute emitter with a sufficiently low concentration (0.5 wt %) in an inert host. The iridium dendrimer with the carbazole-based dendritic ligands exhibits bright emission, peaking at 536 nm, with a full width at half-maximum (fwhm) of 77 nm in the devices without any ETLs. Unexpectedly, once the ETLs are inserted, a significantly broadened emission (fwhm = 115 nm) is detectable under electroluminescence. Taking advantage of the broad interfacial electroplex emission, a hybrid warm-white device was demonstrated by combining a sky-blue thermally activated delayed fluorescence emitter, exhibiting a maximum external quantum efficiency of 13.7%, which is an order of magnitude higher than that of any other reported works based on the electroplex white organic light-emitting diodes.

Trap-Controlled White Electroluminescence From a Single Red-Emitting Thermally Activated Delayed Fluorescence Polymer.

Single white-emitting polymers have been reported by incorporating the second-generation carbazole dendron into the side chain of a red-emitting thermally activated delayed fluorescence (TADF) polymer. Due to the prevented hole trap effect, in this case, excitons can be generated simultaneously on the polymeric host and the red TADF dopant to give a dual emission. Consequently, a bright white electroluminescence is achieved even at a dopant loading as high as 5 mol.%, revealing a maximum luminous efficiency of 16.1 cd/A (12.0 lm/W, 8.2%) and Commission Internationale de l'Eclairage (CIE) coordinates of (0.42, 0.32). The results clearly indicate that the delicate tuning of charge trap is a promising strategy to develop efficient single white-emitting polymers, whose low-band-gap chromophore content can be up to a centesimal level.

In Situ Light-Initiated Ligands Cross-Linking Enables Efficient All-Solution-Processed Perovskite Light-Emitting Diodes.

Solution process has been considered to be an effective method to fabricate an emitting layer (EML) in light-emitting diodes (LEDs). However, the fabrication of the charge transport layer (CTL) above the perovskite EML by solution processing is challenging. Herein, we incorporated polymerizable molecules, conjugated linoleic acid (CLA), as surface ligands to passivate perovskite QDs. The polymerized CLA can create a cross-linked QD film, which allows the solution deposition of subsequent CTLs. The theoretical calculations reveal that the binding energy of polymerized CLA with QDs increased, and the strong ligands' binding state can better passivate the surface and improve the stability of QDs. As a result, all-solution-processed multilayer perovskite LEDs were fabricated with performance of a max luminance of 2470 cd/m2 for CsPbBr3-based devices and a peak EQE of 2.67% for CsPbI3-based devices. These results demonstrate that the in situ light-initiated ligands cross-linking can be an effective strategy in all-solution-processed optoelectronic devices.

Bridging Small Molecules to Conjugated Polymers: Efficient Thermally Activated Delayed Fluorescence with a Methyl-Substituted Phenylene Linker.

Based on a "TADF + Linker" strategy (TADF=thermally activated delayed fluorescence), demonstrated here is the successful construction of conjugated polymers that allow highly efficient delayed fluorescence. Small molecular TADF blocks are linked together using a methyl-substituted phenylene linker to form polymers. With the growing number of methyl groups on the phenylene, the energy level of the local excited triplet state (3 LEb ) from the delocalized polymer backbone gradually increases, and finally surpasses the charge-transfer triplet state (3 CT). As a result, the diminished delayed fluorescence can be recovered for the tetramethyl phenylene containing polymer, revealing a record-high external quantum efficiency (EQE) of 23.5 % (68.8 cd A-1 , 60.0 lm W-1 ) and Commission Internationale de l'Eclairage (CIE) coordinates of (0.25, 0.52). Combined with an orange-red TADF emitter, a bright white electroluminescence is also obtained with a peak EQE of 20.9 % (61.1 cd A-1 , 56.4 lm W-1 ) and CIE coordinates of (0.36, 0.51).

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.

Achieving Deep-Blue Thermally Activated Delayed Fluorescence in Nondoped Organic Light-Emitting Diodes through a Spiro-Blocking Strategy.

A deep-blue thermally activated delayed fluorescence (TADF) emitter TXADO-spiro-DMACF has been reported for nondoped organic light-emitting diodes (OLEDs) by integrating an appropriate blocking unit with the donor (D)-acceptor (A)-donor (D)-type TADF emitter via a spiro linkage. Benefiting from the characteristic perpendicular arrangement, the intermolecular interactions are expected to be weakened to some degree. As a result, TXADO-spiro-DMACF shows a very small bathochromic shift of 8 nm associated with a narrowed full width at half maximum of 54 nm on going from solution to the film. The corresponding nondoped device successfully achieves a bright deep-blue emission, revealing Commission Internationale de l'Eclairage coordinates of (0.16, 0.09) and a peak external quantum efficiency of 5.3% (5.3 cd/A, 5.9 lm/W). The results clearly indicate that spiro-blocking is a promising strategy to develop deep-blue TADF emitters capable of nondoped OLEDs.

Efficient Red Phosphorescent Polymers with Trap-Assisted Charge Balance: Molecular Design, Synthesis, and Electroluminescent Properties.

Three classes of red phosphorescent polymers (PF-H- x, PF-DPO- x, and PF-DPA- x, where x denotes the mole content of Ir complex) have been designed and synthesized, where the C∧N ligand of the tethered dopant bis(2,4-diphenylquinolyl)iridium(acetylacetonate) is substituted by hydrogen (H), diphenylphosphine oxide (DPO), and diphenylamine (DPA), respectively. It is found that the electron-withdrawing DPO group can lower the lowest unoccupied molecular orbital (LUMO) level of the phosphor, whereas the electron-donating DPA group leads to an upshifted highest occupied molecular orbital (HOMO) level of the phosphor. Following a sequence of PF-DPA- x, PF-H- x, and PF-DPO- x, the electron trap depth between dopant and host is gradually up from 0.43 to 1.01 eV, and the hole trap depth is correspondingly down from 0.74 to 0.46 eV. As a result, PF-DPO- x achieves the most balanced charge transport in the emitting layer among these polymers, revealing a record-high luminous efficiency (LE) of 10.3 cd/A and Commission Internationale de L'Eclairage (CIE) coordinates of (0.62, 0.33) on the basis of the simple single-layer device structure. Compared with PF-H- x (3.8 cd/A) and PF-DPA- x (1.2 cd/A) containing the same Ir content, the significantly improved performance indicates that trap-assisted charge balance is a promising strategy to optimize the device efficiency of red phosphorescent polymers.

Teaching an Old Poly(arylene ether) New Tricks: Efficient Blue Thermally Activated Delayed Fluorescence.

Polymer light-emitting diodes are attractive for optoelectronic applications owing to their brightness and ease of processing. However, often metals have to be inserted to increase the luminescence efficiency, and producing blue emitters is a challenge. Here we present a strategy to make blue thermally activated delayed fluorescence (TADF) polymers by directly embedding a small molecular blue TADF emitter into a poly(aryl ether) (PAE) backbone. Thanks to the oxygen-induced negligible electronic communication between neighboring TADF fragments, its corresponding blue delayed fluorescence can be inherited by the developed polymers. These polymers are free from metal catalyst contamination and show improved thermal stability. Through device optimization, a current efficiency of 29.7 cd/A (21.2 lm/W, 13.2%) is realized together with Commission Internationale de L'Eclairage coordinates of (0.18, 0.32). The value is competitive with blue phosphorescent polymers, highlighting the importance of the PAE backbone in achieving high-performance blue delayed fluorescence at a macromolecular level.

High-Energy-Level Blue Phosphor for Solution-Processed White Organic Light-Emitting Diodes with Efficiency Comparable to Fluorescent Tubes.

A high-energy-level blue phosphor FIr-p-OC8 has been developed for solution-processed white organic light-emitting diodes (WOLEDs) with comparable fluorescent tube efficiency. Benefiting from the electron-donating nature of the introduced alkoxy, FIr-p-OC8 shows not only efficient blue light but also elevated highest occupied molecular orbital/lowest unoccupied molecular orbital levels to well match the dendritic host H2. Consequently, the hole scattering between FIr-p-OC8 and H2 can be prevented to favor the direct exciton formation on the blue phosphor, leading to reduced driving voltage and thus improved power efficiency. By exploiting this approach, a maximum power efficiency of 68.5 lm W-1 is achieved for FIr-p-OC8-based white devices, slightly declining to 47.0 lm W-1 at a practical luminance of 1,000 cd m-2. This efficiency can be further raised to 96.3 lm W-1 @ 1,000 cd m-2 when a half-sphere is applied to increase light out-coupling. We believe that our results can compete with commercial fluorescent tubes, representing an important progress in solution-processed WOLEDs.

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.

Fused Isoindigo Ribbons with Absorption Bands Reaching Near-Infrared.

Through fusing isoindigo (IID) units at 6,7;6',7'-positions, a series of new near-infrared (NIR) absorbing and stable ribbon-like conjugated molecules, namely nIIDs in which n represents the number of IID units, have been synthesized. The optical band gaps of the molecules are lowered from 2.03 eV of 1IID to 1.12 eV of 6IID with the increase of the conjugation length. 3IID, 4IID, and 6IID have strong absorption in the NIR region and exhibit photothermal conversion efficiencies of greater than 50 % under laser irradiation at λ=808 nm.

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.