Rong-Huei Yi

Publication date: 15 February 2023

Source: Chemical Engineering Journal, Volume 454, Part 1

Author(s): Lisi Zhan, Yang Tang, Weimin Ning, Guohua Xie, Cheng Zhong, Shaolong Gong, Chuluo Yang

Pyridine-based Ir(III) complexes have been rationally designed to improve their emission brightness. The blue phosphorescent organic light-emitting diodes based on complex 3 dopant (functionalized with new −CF₃ ligand) exhibit excellent performance with a maximum external quantum efficiency of 22.0% and extremely low efficiency roll-off. Upon increasing the current density to 250 mA cm⁻², high brightness value of 53 400 cd m⁻² is achieved.

Abstract

Herein, a family of six [3+2+1] coordinated 2-(2′,4′-difluorophenyl) pyridine-based Iridium(III) complexes with intermolecular interactions have been designed to improve their emission properties. These molecular interactions have been regarded as an effective way to suppress non-radiative decay and enhance the photoluminescence quantum yield (PLQY) of the light-emitting materials. Specifically, complex 3 functionalized with new −CF₃ ligand exhibits PLQY of up to 100%, and the emission peak at ≈485 nm with short excited-state lifetime down to 1 µs. Therefore, the blue phosphorescent organic light-emitting diode (OLED) based on complex 3 dopant exhibits excellent performance with a maximum external quantum efficiency of 22.0% and low efficiency roll-off. Upon increasing the current density to 250 mA cm⁻², high brightness value of 53 400 cd m⁻² is achieved, which is not attainable with the similar molecular structure that we have reported recently, indicating the importance of the presence of the (CF···H/CN···H) interactions. Given the well-overlapping of the emission spectra of these Iridium(III) complexes and absorption spectrum of v-DABNA emitter, complex 3 has been successfully applied as a sensitizer for v-DABNA-based OLED, and a maximum current efficiency of 27.13 cd A⁻¹ with high brightness level of up to 10 000 cd m⁻² have been achieved.

Recent advances in long-wavelength light-emitting electrochemical cells (LECs) are reviewed from the perspectives of materials, including ionic transition metal complexes, small molecules, conjugated polymers, and perovskites, as well as device engineering. Device engineering techniques, including spectral modification by adjusting the microcavity effect, and light outcoupling enhancement, are also discussed. This review aims to pave the way for researchers to design materials and device engineering techniques for long-wavelength LECs in the applications of displays, bio-imaging, telecommunication, and night-vision displays.

Abstract

Long-wavelength light-emitting electrochemical cells (LECs) are potential deep-red and near infrared light sources with solution-processable simple device architecture, low-voltage operation, and compatibility with inert metal electrodes. Many scientific efforts have been made to material design and device engineering of the long-wavelength LECs over the past two decades. The materials designed the for long-wavelength LECs cover ionic transition metal complexes, small molecules, conjugated polymers, and perovskites. On the other hand, device engineering techniques, including spectral modification by adjusting microcavity effect, light outcoupling enhancement, energy down-conversion from color conversion layers, and adjusting intermolecular interactions, are also helpful in improving the device performance of long-wavelength LECs. In this review, recent advances in the long-wavelength LECs are reviewed from the viewpoints of materials and device engineering. Finally, discussions on conclusion and outlook indicate possible directions for future developments of the long-wavelength LECs. This review would like to pave the way for the researchers to design materials and device engineering techniques for the long-wavelength LECs in the applications of displays, bio-imaging, telecommunication, and night-vision displays.

A synthetic methodology is implemented for functionalizing brominated BN-containing multiple-resonance framework with multifarious electron groups, such as donors, acceptors, and moieties without obvious push–pull electron properties. The optimized m-DPAcP–BNCz-based organic light-emitting diode (OLED) exhibits green emission with a full-width at half-maximum (FWHM) of 28 nm and a maximum external quantum efficiency (EQE) of 40.6%.

Abstract

It is of important strategic significance to develop high-efficiency narrowband organic electroluminescent materials that can be employed to fabricate ultrahigh-definition displays with wide color gamut. This topic implies a great challenge to molecular design and synthesis, especially for the development of universality, diversity, scalability, and robustness of molecular architectonics. In this work, a synthetic methodology is demonstrated for functionalizing brominated BN-containing multiple-resonance (MR) frameworks with multifarious functional groups, such as donors, acceptors, and moieties without obvious push–pull electron properties. The m-DPAcP–BNCz-based organic light-emitting diode (OLED) exhibits green emission with a full-width at half-maximum (FWHM) of 28 nm and a maximum external quantum efficiency (EQE) of 40.6%. The outstanding performance of m-DPAcP–BNCz is attributed to the perfect integration of the inherent advantages of the MR framework and the donor–acceptor configuration, which can not only achieve bathochromic shift and narrowband emission, but also obtain high photoluminescence (PL) quantum yield (Φ_PL) and horizontal emitting dipole orientation ratio (Θ_//). This straightforward and efficient approach provides insightful guidance for the construction and enrichment of more high-efficiency narrowband emitters.

Color-tunable and time-dependent organic afterglow materials are achieved via efficient phosphorescence resonance energy transfer within three-component doping PVA systems, in which indolocarbazole isomers with ultralong blue RTP emissions act as energy donors while commercially available fluorescent dyes with emission colors ranging from green to red act as energy acceptors.

Abstract

Organic ultralong room-temperature phosphorescence (RTP) materials have promising applications in anti-counterfeiting. To improve the encryption level, the exploration of organic materials with tunable solid-state long persistent luminescence is in urgent need. Herein, a series of organic ultralong RTP polymeric systems are prepared by doping versatile indolocarbazole isomers into the poly(vinyl alcohol) (PVA) matrix. Notably, the doping film 11,12-ICz@PVA exhibits excellent RTP property with an ultralong lifetime of 2.04 s and a high phosphorescence quantum yield of 44.1%. Theoretical calculations reveal that this excellent RTP property can be attributed to the strong electrostatic attraction resulting from the synergistic double hydrogen-bond between the isomer 11,12-ICz and PVA matrix. More impressively, color-tunable and time-dependent long persistent luminescence is successfully achieved through efficient phosphorescence energy transfer between the indolocarbazole isomers with ultralong blue RTP emissions and commercially available fluorescent dyes with emission colors ranging from green to red doped into the PVA matrix. Besides, diversified encryption patterns are fabricated to demonstrate the promising applications of these water-soluble doping PVA systems with tunable solid-state persistent luminescence in advanced anti-counterfeiting technology.

This is a review on the design of purely organic small molecules to overcome the energy gap law and achieve highly efficient near-infrared light emission. Representative deep-red to near-infrared molecules reported over the past 5 years are introduced and analyzed. Promising molecular design strategies to achieve higher quantum efficiencies are also reviewed.

Abstract

Organic light-emitting materials in the near-infrared (NIR) region are important to realize next-generation lightweight and wearable applications in bioimaging, photodynamic therapy, and telecommunications. Inorganic and organometallic light-emitting materials are expensive and toxic; thus, the development of purely organic light-emitting materials is essential. However, the development of highly efficient NIR light-emitting materials made of organic materials is still in its infancy. Therefore, this review outlines molecular design strategies for developing organic small-molecule NIR light-emitting materials with high emission efficiency that can overcome the energy-gap law to be applied to next-generation wearable devices. After briefly reviewing the basic knowledge required for the NIR emission of organic molecules, representative high-efficiency molecules reported over the past 5 years are classified according to their core moieties, and their molecular design, physical properties, and luminescence characteristics are analyzed. Further, the perspective and outlook regarding the development of next-generation high-efficiency NIR organic light-emitting materials are provided.

A rational molecular design strategy for developing efficient thermally-activated delayed fluorescence materials is presented through the additional introduction of donor (D) or acceptor (A) units in D-A framework, enabling the corresponding organic light-emitting diodes with external quantum efficiency enhanced from 8.5% to 11.6% and up to 27.5%, respectively, due to improved photophysical and horizontal dipole ratio properties.

Abstract

Thermally-activated delayed fluorescence (TADF) emitters are usually constructed with twisted donor-acceptor (D-A) frameworks, while studies on the relationship between diverse D-A structures are still in high demand to achieve high-performance emitters. Herein, by adopting triphenylamine as electron donor and dibenzo[a,c]phenazine as electron acceptor, three TADF molecules are reported with different frameworks of D-A (TPZ), D-A-D (DPZ) and D-A-A (APZ). Theoretical and experimental results demonstrate that different D-A frameworks play significant effects on photophysical, horizontal dipole ratio, and electroluminescence properties of the TADF molecules. In comparison, the APZ-OLED device achieves the best performance with a maximum external quantum efficiency of 27.5%, resulting from its low energy gap between the singlet and triplet, effective reverse intersystem crossing, high photoluminescence quantum yield, and horizontal dipole ratio. This work provides an insight into the relationship between efficient TADF emitters and rational molecular design engineering.

Publication date: 15 February 2023

Source: Chemical Engineering Journal, Volume 454, Part 1

Author(s): Zhaoran Hao, Nengquan Li, Jingsheng Miao, Zhongyan Huang, Xialei Lv, Xiaosong Cao

A pair of organic enantiomers with aggregation-induced emission and circularly polarized thermally activated delayed fluorescence properties have been developed successfully. Furthermore, it is found that their crystals can emit strong blue thermally activated delayed fluorescence and produce bright sky-blue mechanoluminescence and remarkable mechanofluorochromism under the stimuli of force.

Abstract

The development of circularly polarized thermally activated delayed fluorescence (CP-TADF) luminogens with stimuli-response characteristics remains challenging. Herein, a pair of organic enantiomers, S-CzTA and R-CzTA, with aggregation-induced emission properties, have been successfully developed by introducing chiral 1,2,3,4-tetrahydronaphthalene and carbazole to phthalimide. They present CP-TADF properties in toluene solutions, giving dissymmetric factors of 0.84×10⁻³ and −1.03×10⁻³, respectively. In the crystalline state, both S-CzTA and R-CzTA can emit intense blue TADF and produce very bright sky-blue mechanoluminescence (ML) and remarkable mechanofluorochromism (MFC) under the stimuli of mechanical force. Single-crystal analysis and theoretical calculation results suggest that their ML activities are probably associated with their chiral and polar molecular structures and unique non-centrosymmetric molecular packing modes. Furthermore, the MFC properties of the enantiomers likely originate from the destruction of crystal structure, leading to the planarization of molecular conformation. This work may provide helpful guidance for developing new CP-TADF materials with force-stimuli-responsive properties.

Crystal City in a chiral world: The Crystal City is illuminated by chiral suns shining left-circularly polarized light (L-CPL) or right-circularly polarized light (R-CPL). The light of the two suns interacts with the skyscraper crystals differently revealing differences in their inner crystal structure. More information can be found in the Research Article by V. Andrushchenko and co-workers. (DOI: 10.1002/chem.202201922). Image designed by Tomáš Belloň at IOCB Prague.

Blue thermally activated delayed fluorescence (TADF) molecules are systematically designed and synthesized by introducing various functional substituent groups to explore the structure–property correlation of TADF mechanism. By both theoretical and experimental methods, the investigations reveal that for the design of efficient TADF emitters, the role of direct intersystem crossing between the same charge transfer states is important.

Abstract

A molecular structural approach is applied by introducing substituent groups (X) to explore the structure–property correlation of thermally activated delayed fluorescence (TADF) mechanism and develop blue TADF materials. D–A–X emitters show blue emissions from 446 to 487 nm and exhibit high rate constants of reverse intersystem crossing (k _rISC) from 0.76 × 10⁶ to 2.13 × 10⁶ s⁻¹. Organic light emitting diodes (OLEDs) based on D–A–X emitters exhibit efficient external quantum efficiency from 17.2% to 23.9%. Furthermore, the theoretical analysis of spin–flip transitions between states of various nature reveals that the highest rISC rates can be achieved by the increase of charge-transfer (CT) strength and enhancement of direct transition between triplet (³CT) and singlet (¹CT) charge transfer states. Rotational tolerance of dihedral angle, low energy gap, and low reorganization energy between the ³CT and ¹CT states provides fast rISC even when triplet states of different (LE) nature have much higher energy not to enable the three-level interaction. By both experimental and theoretical methods, the investigations reveal that for the design of efficient TADF-OLED emitters, the enhancement of the ³CT–¹CT transition is as much important as that of ³LE–¹CT.

A deep-blue multiple-resonance-type thermally activated delayed fluorescence (MR-TADF) compound (3tPAB) is selected as sensitizer for both blue traditional fluorescence and MR-TADF organic light-emitting diodes. TADF-sensitized fluorescent (TSF) and TADF-sensitized TADF (TST) device using 3tPAB as sensitizer presents maximum external quantum efficiency (EQE_max) of 14.4% and 33.9%, respectively. Compared with the device without sensitizer, the efficiency is increased ≈2.4 and 1.25 times, respectively.

Abstract

Due to the limitation of donor and acceptor group selection, the efficient thermally activated delayed fluorescence (TADF) type sensitizer used for blue organic light-emitting diodes (OLEDs) is rare. Multiple resonance (MR) type TADF emitters can easily achieve efficient blue emission. And the compounds exhibit small Stokes shift and lower absorption energy under the same emission color compared with traditional TADF, mitigating the damage of high-energy absorption of sensitizer on material stability. However, their characteristics as sensitizers have not been explored. In this work, a deep-blue MR-TADF compound (3tPAB) is selected as a sensitizer for both blue traditional fluorescence and MR-TADF OLED. Given the improved photoluminescence quantum yield and the utilization of triplet excitons, the TADF-sensitized fluorescent device using 3tPAB as sensitizer presents a maximum external quantum efficiency (EQE_max) of 14.4%, showing ≈2.4 times increase compared with the device without sensitizer. More impressively, TADF-sensitized TADF (TST) device using 3tPAB as sensitizer and MR-TADF compound PhDMAC-BN as emitter exhibits EQE_max of 33.9%. And in TST device, efficient Förster energy transfer is demonstrated, thus, the device can maintain high color purity. The work first demonstrates the feasibility of MR-TADF as a sensitizer and provides a new strategy for developing high-performance blue OLED with high color purity.

Color-switchable optical resonator is realized from an epoxy resin liquid microdroplet, doped with fluorescent dye cyano-substituted oligo(phenylenevinylene) (COPV) and photoisomerizable azobenzene derivative. The liquid-state cis-isomer of the azobenzene works as a precipitant for COPV that leads to aggregation/disaggregation of COPV by optical control, accompanying a large aggregation-induced emission shift in the droplet resonator.

Abstract

Emission-switchable fluorophores often include stimuli-responsive units in their molecular structures. This strategy works well, but the applicable compounds are limited to the derivatives of several kinds of photochromic molecules such as diarylethene, azobenzene, and spiropyran. Here, this work presents a simple methodology based on a photoresponsive precipitant for achieving color-switchable photoluminescence. A luminescent dye, cyano-substituted oligo(phenylenevinylene) (COPV), features both twisted intramolecular charge transfer and aggregation-induced emission shift properties, leading to the change in the luminescence color from green to red upon precipitation. The COPV, together with photoisomerizable precipitant azobenzene (C6), is doped into spherical droplets of epoxy resin (ER) in a liquid state. The photoisomerization of C6 induced by UV irradiation and heating alternatively precipitates out and dissolves COPV in ER and changes the photoluminescence color, while maintaining the optical microresonator properties. This study will open a promising way for assembling/disassembling novel emission color-switchable systems.

High-power-conservation white organic light-emitting diodes based on all thermally activated delayed fluorescence system are realized through using a host 248DBTTPO with triple P≐O orientations. The record-high power efficiency of 108.6 lm W⁻¹ is the combined result of ultralow-driving voltages (2.6 V for turn on) and the state-of-the-art external quantum efficiency (27.2% for maximum), owing to wide-range carrier/exciton migration.

Abstract

Thermally activated delayed fluorescence (TADF) white organic light-emitting diodes (WOLED) hold great potential for daily lightings, but should overcome the current limitation in power conservation. Despite amount of efforts on developing efficient TADF emitters, it is demonstrated here that the host matrix actually controls exciton formation and allocation, therefore strongly determines power efficiency. Through multiplying P≐O orientations, 248DBTTPO matrix establishes “diffusion predominance” mode to support multidirectional carrier/exciton migration, which combines the advantages of “recombination predominance” and “transport predominance” modes in alleviating dopant–dopant interactions and rationalizing carrier and energy transfer. 248DBTTPO matrix effectively balances carrier flux, singlet radiation, and triplet quenching suppression of TADF emitters, and exciton allocation in its complementary white TADF systems. As consequence, 248DBTTPO endows its trilayer single-emissive-layer WOLEDs with the state-of-the-art external quantum efficiency up to 27%, ultralow driving voltages, and consequently the record-high power efficiency of 108.6 lm W⁻¹, which breaks through key bottleneck of TADF lighting.

The progress of ketone-based TADF emitters with narrow-band properties is summarized. The molecular design involved in maintaining excellent device efficiency and extraordinary color purity is highlighted.

Abstract

Carbonyl-containing derivatives show enduring vitality in the field of thermally activated delayed fluorescence (TADF) materials; they can realize high device efficiency by using both singlet and triplet excitons for electroluminescence. Recently, a system based on fused ketone/amine exhibited huge potential for constructing multi-resonance TADF (MR-TADF) emitters, which exhibit higher narrow-band emission than conventional TADF emitters with twisted donor-acceptor (D-A) structure. Herein, we summarize current research progress in both traditional and MR-type ketone derivatives with TADF characteristics for introducing the molecular design strategy of maintaining high device efficiency while keeping narrow-band emission profile. We hope this review can inspire the emergence of more high-performance narrow-band materials.

The synthesis and late-stage diversification of BN-embedded dibenzocorannulenes (B₂N₂-DBCs) based on one-shot halogenative borylation and successive cross-coupling reaction is reported. An organic light-emitting diode employing one of the derivatives as an emitter exhibited a high external quantum efficiency of 6.6 % and long operational lifetime of 907 h at an initial luminance of 1000 cd m⁻².

Abstract

We report the synthesis and late-stage diversification of a new class of hetero-buckybowl, BN-embedded dibenzocorannulenes (B₂N₂-DBCs). The synthesis is achieved via one-shot halogenative borylation, comprising the nitrogen-directed haloboration of alkyne and an intramolecular bora-Friedel-Crafts reaction, which provides BN-embedded dibenzocorannulene possessing two bromo substituents (B₂N₂-DBC-Br). B₂N₂-DBC-Br undergoes diversification via coupling reactions to provide a variety of arylated derivatives (B₂N₂-DBC-R), exhibiting strong blue fluorescence. An organic light-emitting diode (OLED) employing one of the derivatives as an emitter exhibited a high external quantum efficiency of 6.6 % and long operational lifetime of 907 h at an initial luminance of 1000 cd m⁻², indicating the significant potential for the development of efficient and stable hetero-buckybowl-based OLED materials.