Boao Liu

Novel iridium(iii) complexes bearing dimesitylboron groups with nearly 100% phosphorescent quantum yields for highly efficient organic light-emitting diodes

A series of cyclometalated iridium(III) complexes, Ir-B-1, Ir-B-2, Ir-B-3, and Ir-B-4, were synthesized with the 2-phenylpyridine (ppy) type main ligand possessing a dimesitylboron group and the auxiliary ligand containing other main-group element units showing unique charge injection/transporting features. Their thermal stability, photophysical and electrochemical properties, and electroluminescent (EL) performances have been characterized. Time-dependent density functional theory (TD-DFT) calculations were carried out to study the photophysical properties of these complexes. All these complexes can emit intense orange phosphorescence with extremely high quantum yields of nearly 100% measured in neat films at room temperature. Moreover, the solution-processed organic light-emitting devices (OLEDs) using these complexes as orange emitters can show very high EL efficiencies with the maximum luminance efficiency (ηL) of 85.1 cd A−1, corresponding to a power efficiency (ηP) of 69.7 lm W−1 and an external quantum efficiency (ηext) of 28.1%.

Platinum(II) polymetallayne-based phosphorescent polymers with enhanced triplet energy-transfer: synthesis, photophysical, electrochemistry, and electrophosphorescent investigation

Two series of new phosphorescent copolymers with bicarbazole-based platinum(II) polymetallayne backbones have been successfully prepared through Sonogashira cross-coupling with different IrIII ppy-type (ppy = 2-phenylpyridine anion) complexes as phosphorescent centers. The photophysical investigations not only indicate a highly efficient triplet energy-transfer process from the polymetallayne segments to the phosphorescent units in the polymer solution, but also figure out the structure–property relationship between the triplet energy-transfer process and the energy-levels of different excited states. In addition, the phosphorescent copolymers can produce yellow-emitting phosphorescent OLEDs (PHOLEDs) with high EL efficiencies and a current efficiency (ηL) of 11.49 cd A−1, an external quantum efficiency (ηext) of 4.38%, a power efficiency (ηP) of 3.78 lm W−1, and red-emitting PHOLEDs with a ηL of 5.86 cd A−1, ηext of 10.1%, and a ηP of 2.29 lm W−1, representing very decent electroluminescent performances achieved by the phosphorescent copolymers. Herein, this work not only furnishes very important clues for further polishing of this category novel phosphorescent polymer, but also provides a new approach to the design and synthesis of highly efficient phosphorescent copolymers.

Enhancing the electroluminescence performances of novel platinum(II) polymetallayne-based phosphorescent polymers through employing functionalized IrIII phosphorescent units and facilitating triplet energy transfer

Novel orange phosphorescent polymers with platinum(II) polymetallayne-based backbones have been successfully developed through Sonogashira cross-coupling among bicarbazole moieties, functionalized IrIII phosphorescent blocks with electron injection/transporting (EI/ET) features, and trans-[PtCl2(PBu3)2]. Importantly, the very efficient energy-transfer process is observed from the triplet states of the polymetallayne-based backbone to the triplet metal-to-ligand charge transfer states (3MLCT) of the phosphorescent units in the polymer backbone, which will guarantee the high phosphorescent ability of these polymers. Benefiting from the weak conjugation-extending ability of the platinum(II) ions, the polymetallayne-based backbones show high triplet energy-level to effectively block the undesired reverse energy-transfer process. Furthermore, the EI/ET features of the functionalized IrIII phosphorescent units should balance the hole injection/transporting (HI/HT) of the bicarbazole moieties to improve the EL performances of these phosphorescent polymers. Benefiting from these merits, the phosphorescent polymers can furnish solution-processed phosphorescent OLEDs (PHOLEDs) with high EL efficiencies with current efficiency (ηL) of 9.17 cd A−1, external quantum efficiency (ηext) of 4.50% and power efficiency (ηP) of 4.04 lm W−1, representing very decent electroluminescent performances achieved by the orange phosphorescent polymers. This work herein might not only show the great potential of platinum(II) polymetallayne as the host segments in phosphorescent polymers, but also provide a new outlet to design and synthesise highly efficient phosphorescent copolymers.

Homoleptic thiazole-based IrIII phosphorescent complexes for achieving both high EL efficiencies and an optimized trade-off among the key parameters of solution-processed WOLEDs

Two homoleptic thiazole-based IrIII phosphorescent emitters, Ir-3Tz1F and Ir-3Tz2F, with fluorinated 2-phenylthiazole-type ligands were designed and prepared. Their thermal stability, and photophysical and electrochemical properties, as well as electroluminescent (EL) performances in both monochromic OLEDs and solution-processed WOLEDs were investigated. When doped in monochromic OLEDs made by vacuum deposition, Ir-3Tz1F gave the maximum EL efficiencies with ηL of 56.2 cd A−1, ηext of 15.8% and ηp of 50.2 lm W−1. Critically, solution-processed WOLEDs based on Ir-3Tz1F with three primary colors could achieve an excellent trade-off among the stable balanced white EL spectra, a high EL efficiency and a high color rendering index (CRI). The optimized solution-processed WOLED exhibited very attractive EL efficiencies of 33.4 cd A−1, 16.5% and 30.6 lm W−1, while maintaining both a high CRI of ca. 80 and very stable Commission Internationale de L’Eclairage (CIE) coordinates in a wide driving voltage range from 4 V to 11 V.

Optimized trade-offs between triplet emission and transparency in Pt(II) acetylides through phenylsulfonyl units for achieving good optical power limiting performance

Three Pt(II) acetylides have been prepared by coupling trans-[PtCl2(PBu3)2] to ethynyl aromatic ligands with electron-withdrawing phenylsulfonyl units in high yields (>85%). The investigation of their photophysical behavior has shown that the unique conjugation-breaking configuration of the –SO2– linker in the phenylsulfonyl units can afford a very short cut-off wavelength (λcut-off) of <390 nm to the Pt(II) acetylides, furnishing excellent transparency of these compounds. Critically, the triplet quantum yields (ΦP) of the prepared Pt(II) acetylides can be effectively enhanced from 0.52% to 15.92% through increasing the number of fluorine substituents on the phenylsulfonyl units in the organic ligands. Benefiting from their enhanced ΦP, the phenylsulfonyl-based Pt(II) acetylides can exhibit comparable or even better optical power limiting (OPL) performance against 532 nm lasers than the state-of-the-art OPL material C60, indicating their great potential in the field of laser protection. All of these results have provided a new strategy to achieve consistency between high OPL ability and good transparency for OPL materials, representing a valuable attempt for coping with key problems in the field of nonlinear optics.

Novel AuI polyynes and their high optical power limiting performances both in solution and in prototype devices

Three novel AuI polyynes have been prepared in high yield by copolymerization between an AuI complex precursor and different ethynyl aromatic ligands. The investigation of their photophysical behavior has indicated that forming polyynes through polymerization not only maintains the high transparency of the corresponding AuI polyynes similar to those of their corresponding small molecular AuI acetylides, but also effectively enhances their triplet (T1) emission ability. Critically, owing to their enhanced T1 emission ability, all the AuI polyynes exhibit a stronger optical power limiting (OPL) ability against a 532 nm laser than the corresponding small molecular AuI acetylides. The AuI polyynes based on fluorene and triphenylamine ligands show even better OPL performance than the state-of-the-art OPL material C60, indicating their great potential in the field of laser protection. More importantly, in a prototype OPL device made by doping the fluorene-based AuI polyyne into a polystyrene (PS) solid matrix, substantially improved OPL activity has been observed compared with that in the solution, demonstrating its great potential for practical application. All these results have provided a new strategy to achieve a balance between high OPL activity and good transparency for OPL materials, representing a valuable attempt towards developing new OPL materials with high performance to cope with the key problems in the field of nonlinear optics.

Coordination polymers based on bis-ZnII salphen complexes and functional ditopic ligands for efficient polymer light-emitting diodes (PLEDs)

Encouraged by the high photoluminescence (PL) of the bis-ZnII salphen complexes, a series of coordination polymers have been successfully developed through metal–ligand interactions between the bis-ZnII salphen complexes and functional ditopic ligands. These bis-ZnII salphen coordination polymers can exhibit high thermal stability up to 433 °C. Their PL spectra show an emission maximum peaking at ca. 570 nm in solution and ca. 580 nm in neat films. The PL investigation of the neat films for these bis-ZnII salphen coordination polymers has indicated that the functional ditopic ligands can restrain the molecular packing among the bis-ZnII salphen units for enhancing their PL intensity. Hence, the functional ditopic ligands should exhibit great potential in avoiding exciton quenching in the emission layer formed by these bis-ZnII salphen coordination polymers in polymer light-emitting diodes (PLEDs). Benefiting from such a molecular design, these bis-ZnII salphen coordination polymers can exhibit a peak luminance (Lmax) of 8658 cd m−2, a maximum external quantum efficiency (ηext) of 3.18%, a maximum current efficiency (ηL) of 6.2 cd A−1 and a maximum power efficiency (ηP) of 4.4 lm W−1. These attractive results represent the state-of-the-art EL performances ever achieved by Schiff base ZnII coordination polymers, which would provide significant clues for developing high performance PLEDs based on Schiff base ZnII coordination polymers.

Diarylboron-Based Asymmetric Red-Emitting Ir(III) Complex for Solution-Processed Phosphorescent Organic Light-Emitting Diode with External Quantum Efficiency above 28%

Abstract Organic light‐emitting diodes (OLEDs) are one of the most promising technologies for future displays and lighting. Compared with the blue and green OLEDs that have achieved very high efficiencies by using phosphorescent Ir(III) complexes, the red OLEDs still show relatively low efficiencies because of the lack of high‐performance red‐emitting Ir(III) complexes. Here, three highly efficient asymmetric red‐emitting Ir(III) complexes with two different cyclometalating ligands made by incorporating only one electron‐deficient triarylboron group into the nitrogen heterocyclic ring are reported. These complexes show enhanced photoluminescence quantum yields up to 0.96 and improved electron transporting capacity. In addition, the asymmetric structure can help to improve the solubility of Ir(III) complexes, which is crucial for fabricating OLEDs using the solution method. The photoluminescent and oxidation–reduction properties of these Ir(III) complexes are investigated both experimentally and theoretically. Most importantly, a solution‐processed red OLED achieves extremely high external quantum efficiency, current efficiency, and power efficiency with values of 28.5%, 54.4 cd A−1, and 50.1 lm W−1, respectively, with very low efficiency roll‐off. Additionally, the related device has a significantly extended operating lifetime compared with the reference device. These results demonstrate that the asymmetric diarylboron‐based Ir(III) complexes have great potential for fabricating high‐performance red OLEDs.

Novel heterobimetallic Au(I)-Pt(II) polyynes achieving good trade-off between transparency and optical power limiting performance

Two series of new heterobimetallic Au(I)–Pt(II) polyynes have been easily synthesized by cross-coupling under mild conditions. The absorption profiles of these two series of Au(I)–Pt(II) polyynes are quite similar. However, the Au(I)–Pt(II) polyynes with a 1,4-bis(diphenylphosphino)benzene ligand show stronger triplet (T1) emission and superior optical power limiting (OPL) performance than the corresponding Au(I)–Pt(II) polyynes with a 1,3-bis(diphenylphosphino)propane ligand. Hence, the 1,4-bis(diphenylphosphino)benzene ligand is more effective than the 1,3-bis(diphenylphosphino)propane ligand for optimizing the transparency and OPL ability of OPL materials. When compared with the corresponding homometallic Pt(II) polyynes, these heterobimetallic Au(I)–Pt(II) polyynes display a blue shift in their absorption spectra, showing better transparency in the visible-light region. Besides, these heterobimetallic Au(I)–Pt(II) polyynes show stronger OPL ability than their corresponding homometallic Pt(II) polyynes as well as the state-of-the-art OPL material C60, demonstrating their enormous application potential in the nonlinear optics field. In brief, the introduction of Au(I) precursors with tetrahedral diphosphine ligands into the backbone of Pt(II) polyynes can simultaneously achieve enhanced transparency and high OPL ability for OPL materials, providing a new strategy to optimize OPL materials.

Asymmetric Heteroleptic Ir(III) Phosphorescent Complexes with Aromatic Selenide and Selenophene Groups: Synthesis and Photophysical, Electrochemical, and Electrophosphorescent Behaviors

With the aim of evaluating the potential of selenium-containing groups in developing electroluminescent (EL) materials, a series of asymmetric heteroleptic Ir(III) phosphorescent complexes (Ir-Se0F, Ir-Se1F, Ir-Se2F, and Ir-Se3F) have been synthesized by using 2-selenophenylpyridine and one ppy-type (ppy = 2-phenylpyridine) ligand with a fluorinated selenide group. To the best of our knowledge, these complexes represent unprecedented examples of asymmetric heteroleptic Ir(III) phosphorescent emitters bearing selenium-containing groups. Natural transition orbital (NTO) analysis based on optimized geometries of the first triplet state (T1) have shown that the phosphorescent emissions of these Ir(III) complexes dominantly show 3π-π* features of the 2-selenophenylpyridine ligand with slight metal to ligand charge transfer (MLCT) contribution. In comparison with their symmetric parent complex Ir-Se with two 2-selenophenylpyridine ligands, these asymmetric heteroleptic Ir(III) phosphorescent complexes can show much higher phosphorescent quantum yields (ΦP) of ca. 0.90. Both the hole- and electron-trapping ability of these Ir(III) phosphorescent complexes can be enhanced by selenophene and fluorinated selenide groups to improve their EL efficiencies. The EL abilities of these asymmetric heteroleptic Ir(III) phosphorescent emitters fall in the order Ir-Se3F > Ir-Se2F > Ir-Se1F > Ir-Se0F. The highest EL efficiencies have been achieved by Ir-Se3F in the solution-processed OLEDs with external quantum efficiency (ηext), current efficiency (ηL), and power efficiency (ηP) of 19.9%, 65.6 cd A-1, and 57.3 lm W-1, respectively. These encouraging EL results clearly indicate the great potential of selenium-containing groups in developing high-performance Ir(III) phosphorescent emitters.