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.

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. %.

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.

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).

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.

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.

Design, synthesis, and optoelectronic properties of dendrimeric Pt(II) complexes and their ability to inhibit intermolecular interaction.

Dendrimeric Pt(II) complexes [(C(∧)N)Pt(dpm)] and [Pt(C(∧)N)2] (Hdpm = dipivaloylmethane, HC(∧)N = 1,2-diphenylbenzoimidazole and its derivatives containing the carbazole dendrons) have been synthesized and characterized systematically. All of the complexes display green emission in the range of 495-535 nm that originated from the 360-440 nm absorption bands, which are assigned to dπ(Pt)→π*(L) metal-to-ligand charge transfer (MLCT) mixed with intraligand π(L)→π*(L) transition. Solution photoluminescence quantum yield (φp 0.26-0.31) of the heteroleptic complexes [(C(∧)N)Pt(dpm)] obviously increases when compared with that of complex [(C(∧)N)Pt(acac)]. Organic light-emitting diode devices based on these Pt(II) complexes with a multilayer configuration were fabricated and gave desirable electroluminescent (EL) performances, such as non- or less red-shifted EL spectra, in comparison with the photoluminescence spectra and slow efficiency roll-off with increasing brightness or current density. Complex [(t-BuCzCzPBI)Pt(dpm)] (where t-BuCzCzPBI = 1-(4-(3,6-di-(3,6-di-t-butyl-carbazol-9-yl))carbazol-9-yl)phenyl-2-phenylbenzoimidazole) showed the best performance, with a maximum current efficiency of 29.31 cd/A and a maximum external quantum efficiency (EQE) of 9.04% among the fabricated devices. Likewise, for homoleptic [Pt(t-BuCzCzPBI)2] dendrimer, the powder φp (0.14) and maximum EQE (0.74%) improve by 7 and 7.4 times, respectively, as high as they do for nondendrimeric [Pt(1,2-diphenylbenzoimidazole)2] (0.02, 0.10%), although its efficiency is still lower than that of the heteroleptic counterpart due to the severely distorted square-planar geometry of the emitting core. These results reveal that large steric hindrance from ancillary ligand (dpm) or the homoleptic conformation can effectively inhibit intermolecular interaction for these dendrimeric Pt(II) complexes.

Self-host blue-emitting iridium dendrimer with carbazole dendrons: nondoped phosphorescent organic light-emitting diodes.

A blue-emitting iridium dendrimer, namely B-G2, has been successfully designed and synthesized with a secondgeneration oligocarbazole as the dendron, which is covalently attached to the emissive tris[2-(2,4-difluorophenyl)-pyridyl]iridium(III) core through a nonconjugated link to form an efficient self-host system in one dendrimer. Unlike small molecular phosphors and other phosphorescent dendrimers, B-G2 shows a continuous enhancement in the device efficiency with increasing doping concentration. When using neat B-G2 as the emitting layer, the nondoped device is achieved without loss in efficiency, thus giving a state-of-art EQE as high as 15.3% (31.3 cdA1, 28.9 lmW1) along with CIE coordinates of (0.16, 0.29).

Peripheral Substitution Engineering of MR-TADF Emitters Embedded With B‒N Covalent Bond Towards Efficient BT.2020 Blue Electroluminescence.

Compared with the classical boron/nitrogen (B/N) doped ones, multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters embedded with B-N covalent bond behave a significantly blue-shifted narrowband TADF, and thus show a greater potential in ultrapure blue organic light-emitting diodes (OLEDs). As a proof of concept, herein a peripheral substitution engineering is demonstrated based on such a B‒N embedded parent core. The simple approach is found to ensure easy synthesis via a one-pot lithium-free borylation-annulation, manipulate the excited states through different electronic coupling between core and substituent, and introduce the steric hindrance to minimize the unwanted spectral broadening. Impressively, ultrapure blue OLEDs are realized to give a high external quantum efficiency of 20.3% together with Commission Internationale de l'Éclairage coordinates of (0.152, 0.046). The performance is well competent with those of B/N doped MR-TADF emitters, clearly highlighting that the B‒N embedded framework is a novel promising paradigm towards efficient BT.2020 blue standard.

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.

White electroluminescence from all-phosphorescent single polymers on a fluorinated poly(arylene ether phosphine oxide) backbone simultaneously grafted with blue and yellow phosphors.

On the basis of a fluorinated poly(arylene ether phosphine oxide) backbone with both high triplet energy and appropriate HOMO/LUMO levels, highly efficient all-phosphorescent single white-emitting polymers were designed and successfully synthesized via a "two-step addition" strategy. Simultaneous blue and yellow triplet emissions were achieved to generate white electroluminescence with a promising luminous efficiency as high as 18.4 cd/A (8.5 lm/W, 7.1%) and CIE coordinates of (0.31, 0.43).

Highly efficient blue electrophosphorescent polymers with fluorinated poly(arylene ether phosphine oxide) as Backbone.

In view of the tolerance of F atoms in FIrpic to the nucleophilic aromatic substitution polymerization, an activated fluorinated poly(arylene ether phosphine oxide) backbone is used to construct novel blue electrophosphorescent polymers containing FIrpic as the blue emitter, because they can be synthesized under a milder temperature of 120 °C. Compared with the counterparts prepared at high temperature (165 °C), unexpected bathochromic shift is successfully avoided, and a state-of-art luminous efficiency as high as 19.4 cd A(-1) is achieved. The efficiency is comparable to the corresponding physical blend system, which indicates that the fluorinated poly(arylene ether phosphine oxide) has the potential to be used as the platform for the development of high-performance all-phosphorescent white polymer based on single polymer system.

Red-emitting dendritic iridium(III) complexes for solution processable phosphorescent organic light-emitting diodes.

Functionalization of a red phosphorescent iridium(III) complex core surrounded by rigid polyphenylene dendrons with a hole-transporting triphenylamine surface allows to prevent the intermolecular aggregation-induced emission quenching, improves charge recombination, and therefore enhances photo- and electroluminescence efficiencies of dendrimer in solid state. These multifunctional shape-persistent dendrimers provide a new pathway to design highly efficient solution processable materials for phosphorescent organic light-emitting diodes (PhOLEDs).

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).

Phosphonate functionalized oxadiazole derivative as an efficient electron transporting material for solution-processed blue electrophosphorescent devices.

An efficient electron transporting material, P-OXD (1,3-bis[(4-(4-diethylphosphoryl-butyl-phenyl))-1,3,4-oxidiazol-2-yl]phenylene), has been synthesized and thoroughly characterized. Due to its alcohol-soluble nature, P-OXD can be spin-coated atop the light emitting layer to form high quality film without dissolving the underlying layer. As a consequence, the double-layer blue electrophosphorescent device has been successfully fabricated, giving a peak luminous efficiency of 10.5 cd/A, and a maximum brightness of 8200 cd/m² with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.16, 0.33). The promising results indicate that P-OXD has a potential application in solution-processed multilayer polymer 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.

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.

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.