小分子铱配合物及其电致发光

由于磷光金属配合物可以同时利用单线态和三线态激子发光,使有机电致发光器件的理论内量子效率达到100%,突破了25%的极限。因而以磷光金属配合物为发光材料制成的器件备受关注。在这些金属配合物中,铱配合物由于具有较强的发光特性、发光波长可调性、较好的热稳定性和电化学稳定性以及能够形成便于蒸镀的中性分子,而成为最有应用潜力的电致磷光材料。本文综述了近几年铱配合物磷光材料在分子设计与合成方法、发光机理及器件构筑等方面的研究进展。特别介绍与讨论了磷光铱配合物的两种发光机理,即基于同配体铱配合物或异配体铱配合物的主配体到中心金属离子的电荷转移三线态(³MLCT)发射和基于异配体铱配合物的辅助配体三线态(³LC)发射。根据反应条件的差异,归纳总结了合成铱配合物常用的4种方法以及合成fac式和mer式的铱配合物的方法。还根据材料的发光颜色及其电致发光的不同,对磷光铱配合物材料进行了分类与讨论。此外,简要介绍了用于器件制作的主体材料。最后,展望了金属有机配合物电致磷光材料的发展前景,并提出了今后磷光材料的发展方向。     

Nearly 100% exciton utilization in highly efficient red OLEDs based on dibenzothioxanthone acceptor

Improving the utilization of excitons has always been an important topic for the development of electroluminescence devices. In this work, we designed and synthesized three red TADF emitters TPA-DBT12, TPA-DBT3 and DTPA-DBT by employing dibenzothioxanthone (DBT) acceptor framework to stabilize the locally excited triplet state to participate in the reverse intersystem crossing (RISC) process. The fast RISC process and singlet radiation decay process gave rise to evidently enhanced exciton utilization. All of the red OLEDs based on these materials showed maximum EQE over 11% and high exciton utilization close to 100%. This work not only extend the acceptor framework for red materials but also provide a new perspective for the design of highly efficient red TADF materials with 100% exciton utilization by managing locally excited triplet state.     

Recent progress in thermally activated delayed fluorescence emitters for nondoped organic light-emitting diodes

Nondoped organic light-emitting diodes (OLEDs) have drawn immense attention due to their merits of process simplicity, reduced fabrication cost, etc. To realize high-performance nondoped OLEDs, all electrogenerated excitons should be fully utilized. The thermally activated delayed fluorescence (TADF) mechanism can theoretically realize 100% internal quantum efficiency (IQE) through an effective upconversion process from nonradiative triplet excitons to radiative singlet ones. Nevertheless, exciton quenching, especially related to triplet excitons, is generally very serious in TADF-based nondoped OLEDs, significantly hindering the pace of development. Enormous efforts have been devoted to alleviating the annoying exciton quenching process, and a number of TADF materials for highly efficient nondoped devices have been reported. In this review, we mainly discuss the mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for nondoped devices. We further classify their molecular structures depending on the functional A groups and offer an outlook on their future prospects. It is anticipated that this review can entice researchers to recognize the importance of TADF-based nondoped OLEDs and provide a possible guide for their future development.

The mechanism, exciton leaking channels, and reported molecular design strategies of TADF emitters for high-performance nondoped OLEDs are summarized. Their molecular structures depending on the functional A groups are further classified.     

Method for accurate experimental determination of singlet and triplet exciton diffusion between thermally activated delayed fluorescence molecules

Understanding triplet exciton diffusion between organic thermally activated delayed fluorescence (TADF) molecules is a challenge due to the unique cycling between singlet and triplet states in these molecules. Although prompt emission quenching allows the singlet exciton diffusion properties to be determined, analogous analysis of the delayed emission quenching does not yield accurate estimations of the triplet diffusion length (because the diffusion of singlet excitons regenerated after reverse-intersystem crossing needs to be accounted for). Herein, we demonstrate how singlet and triplet diffusion lengths can be accurately determined from accessible experimental data, namely the integral prompt and delayed fluorescence. In the benchmark materials 4CzIPN and 4TCzBN, we show that the singlet diffusion lengths are (9.1 ± 0.2) and (12.8 ± 0.3) nm, whereas the triplet diffusion lengths are negligible, and certainly less than 1.0 and 1.2 nm, respectively. Theory confirms that the lack of overlap between the shielded lowest unoccupied molecular orbitals (LUMOs) hinders triplet motion between TADF chromophores in such molecular architectures. Although this cause for the suppression of triplet motion does not occur in molecular architectures that rely on electron resonance effects (e.g. DiKTa), we find that triplet diffusion is still negligible when such molecules are dispersed in a matrix material at a concentration sufficiently low to suppress aggregation. The novel and accurate method of understanding triplet diffusion in TADF molecules will allow accurate physical modeling of OLED emitter layers (especially those based on TADF donors and fluorescent acceptors).

A method for measuring triplet diffusion between TADF molecules is presented, and implications of limited triplet diffusion for OLEDs discussed.     

Ultrafast dynamics of exciton fission in polycrystalline pentacene

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ～80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kasha's Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.     

Molecular Design Tactics for Highly Efficient Thermally Activated Delayed Fluorescence Emitters for Organic Light Emitting Diodes

Recently, pure organic thermally activated delayed fluorescence (TADF) emitters have attracted considerable interest from the scientific community in the field of organic light emitting diodes (OLEDs) as they can theoretically realize 100 % of the internal quantum efficiency by exploiting both the singlet and triplet excitons via the reverse intersystem crossing enabled by small singlet‐triplet energy splitting. Currently, the external quantum efficiency of the TADF emitters is reaching the level of phosphorescent emitters. Therefore, the TADF approach is considered as a potential alternative to the low efficiency conventional fluorescent and expensive phosphorescent emitters. In this account, we summarized our recent development of blue and green TADF molecular designs to improve the device performances of the TADF devices.     

Manipulating exciton dynamics of thermally activated delayed fluorescence materials for tuning two-photon nanotheranostics

Rational manipulation of energy utilization from excited-state radiation of theranostic agents with a donor–acceptor structure is relatively unexplored. Herein, we present an effective strategy to tune the exciton dynamics of radiative excited state decay for augmenting two-photon nanotheranostics. As a proof of concept, two thermally activated delayed fluorescence (TADF) molecules with different electron-donating segments are engineered, which possess donor–acceptor structures and strong emissions in the deep-red region with aggregation-induced emission characteristics. Molecular simulations demonstrate that change of the electron-donating sections could effectively regulate the singlet–triplet energy gap and oscillator strength, which promises efficient energy flow. A two-photon laser with great permeability is used to excite TADF NPs to perform as theranostic agents with singlet oxygen generation and fluorescence imaging. These unique performances enable the proposed TADF emitters to exhibit tailored balances between two-photon singlet oxygen generation and fluorescence emission. This result demonstrates that TADF emitters can be rationally designed as superior candidates for nanotheranostic agents by the custom controlling exciton dynamics.

Exciton dynamics can be manipulated rationally in the design of TADF materials for nanotheranostics. Regulating the ΔE_ST and f promises efficient energy flow for tailoring balances between singlet oxygen generation and fluorescence emission.     

Highly Efficient Nondoped OLEDs with Negligible Efficiency Roll‐Off Fabricated from Aggregation‐Induced Delayed Fluorescence Luminogens

Purely organic emitters that can efficiently utilize triplet excitons are highly desired to cut the cost of organic light‐emitting diodes (OLEDs), but most of them require complicated doping techniques for their fabrication and suffer from severe efficiency roll‐off. Herein, we developed novel luminogens with weak emission and negligible delayed fluorescence in solution but strong emission with prominent delayed components upon aggregate formation, giving rise to aggregation‐induced delayed fluorescence (AIDF). The concentration‐caused emission quenching and exciton annihilation are well‐suppressed, which leads to high emission efficiencies and efficient exciton utilization in neat films. Their nondoped OLEDs provide excellent electroluminescence efficiencies of 59.1 cd A⁻¹, 65.7 lm W⁻¹, and 18.4 %, and a negligible current efficiency roll‐off of 1.2 % at 1000 cd m⁻². Exploring AIDF luminogens for the construction of nondoped OLEDs could be a promising strategy to advance device efficiency and stability.     

Benzimidazobenzothiazole‐Based Bipolar Hosts to Harvest Nearly All of the Excitons from Blue Delayed Fluorescence and Phosphorescent Organic Light‐Emitting Diodes

Much effort has been devoted to developing highly efficient organic light‐emitting diodes (OLEDs) that function through phosphorescence or thermally activated delayed fluorescence (TADF). However, efficient host materials for blue TADF and phosphorescent guest emitters are limited because of their requirement of high triplet energy levels. Herein, we report the rigid acceptor unit benzimidazobenzothiazole (BID‐BT), which is suitable for use in bipolar hosts in blue OLEDs. The designed host materials, based on BID‐BT, possess high triplet energy and bipolar carrier transport ability. Both blue TADF and phosphorescent OLEDs containing BID‐BT‐based derivatives exhibit external quantum efficiencies as high as 20 %, indicating that these hosts allow efficient triplet exciton confinement appropriate for blue TADF and phosphorescent guest emitters.     

Molecular design of efficient white‐light‐emitting fluorene‐based copolymers by mixing singlet and triplet emission

A new strategy to realize efficient white‐light emission from a binary fluorene‐based copolymer (PF‐Phq) with the fluorene segment as a blue emitter and the iridium complex, 9‐iridium(III)bis(2‐(2‐phenyl‐quinoline‐N,C³′)(11,13‐tetradecanedionate))‐3,6‐carbazole (Phq), as a red emitter has been proposed and demonstrated. The photo‐ and electroluminescence properties of the PF‐Phq copolymers were investigated. White‐light emission with two bands of blue and red was achieved from the binary copolymers. The efficiency increased with increasing concentration of iridium complex, which resulted from its efficient phosphorescence emission and the weak phosphorescent quenching due to its lower triplet energy level than that of polyfluorene. In comparison with the binary copolymer, the efficiency and color purity of the ternary copolymers (PF‐Phq‐BT) were improved by introducing fluorescent green benzothiadiazole (BT) unit into polyfluorene backbone. This was ascribed to the exciton confinement of the benzothiadiazole unit, which allowed efficient singlet energy transfer from fluorene segment to BT unit and avoided the triplet quenching resulted from the higher triplet energy levels of phosphorescent green emitters than that of polyfluorene. The phosphorescence quenching is a key factor in the design of white light‐emitting polyfluorene with triplet emitter. It is shown that using singlet green and triplet red emitters is an efficient approach to reduce and even avoid the phosphorescence quenching in the fluorene‐based copolymers. The strategy to incorporate singlet green emitter to polyfluorene backbone and to attach triplet red species to the side chain is promising for white polymer light‐emitting diodes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 453–463, 2008     

Progress in next-generation organic electroluminescent materials: material design beyond exciton statistics

Exciton (or spin) statistics is a physical principle based on the statistics of spin multiplicity. In electroluminescence, injected electrons and holes have randomized spin states, and usually form singlet or triplet excitons in the ratio of 1:3. Exciton statistics determines that the upper limit of internal quantum efficiency is 25% in fluorescent devices, since only singlet exciton can decay radiatively. However, both experimental and theoretical evidence indicate that the actual efficiency can exceed the exciton statistics limit of 25% by utilizing materials with special electronic structure and optimized device structures. These results bring light to break through the exciton statistics limit and develop new-generation fluorescent materials with low cost and high efficiency. Recently, the exciton statistics, which has attracted great attention in the past decade, is being rejuvenated due to the discovery of some fluorescent materials with abnormally high efficiencies. In view of their significance in theoretical research of organic semiconductors and developing new-generation OLED materials, such materials are widely investigated in both academic institutions and industry. Several key issues still require further clarification for this kind of materials, such as the molecular design concepts. Herein, we review the progress of the materials with efficiency exceeding the exciton statistics limit, and the routes to improve exciton utilization efficiency. In the end, we present an innovative pathway to fully harvest the excitons in fluorescent devices, namely, “hot exciton” model and relevant fluorescence material with hybridized local and charge-transfer (HLCT) excited state.     

Hyperfluorescent polymers enabled by through-space charge transfer polystyrene sensitizers for high-efficiency and full-color electroluminescence

Fluorescent polymers are suffering from low electroluminescence efficiency because triplet excitons formed by electrical excitation are wasted through nonradiative pathways. Here we demonstrate the design of hyperfluorescent polymers by employing through-space charge transfer (TSCT) polystyrenes as sensitizers for triplet exciton utilization and classic fluorescent chromophores as emitters for light emission. The TSCT polystyrene sensitizers not only have high reverse intersystem crossing rates for rapid conversion of triplet excitons into singlet ones, but also possess tunable emission bands to overlap the absorption spectra of fluorescent emitters with different bandgaps, allowing efficient energy transfer from the sensitizers to emitters. The resultant hyperfluorescent polymers exhibit full-color electroluminescence with peaks expanding from 466 to 640 nm, and maximum external quantum efficiencies of 10.3–19.2%, much higher than those of control fluorescent polymers (2.0–3.6%). These findings shed light on the potential of hyperfluorescent polymers in developing high-efficiency solution-processed organic light-emitting diodes and provide new insights to overcome the electroluminescence efficiency limitation for fluorescent polymers.

Hyperfluorescent polymers with high efficiency and full-color electroluminescence are developed by using through-space charge transfer polystyrenes as sensitizers for exciton utilization and fluorescent chromophores as emitters for light emission.     

High‐Performance Non‐doped OLEDs with Nearly 100 % Exciton Use and Negligible Efficiency Roll‐Off

Non‐doped organic light‐emitting diodes (OLEDs) possess merits of higher stability and easier fabrication than doped devices. However, luminescent materials with high exciton use are generally unsuitable for non‐doped OLEDs because of severe emission quenching and exciton annihilation in neat films. Herein, we wish to report a novel molecular design of integrating aggregation‐induced delayed fluorescence (AIDF) moiety within host materials to explore efficient luminogens for non‐doped OLEDs. By grafting 4‐(phenoxazin‐10‐yl)benzoyl to common host materials, we develop a series of new luminescent materials with prominent AIDF property. Their neat films fluoresce strongly and can fully harvest both singlet and triplet excitons with suppressed exciton annihilation. Non‐doped OLEDs of these AIDF luminogens exhibit excellent luminance (ca. 100000 cd m^?2), outstanding external quantum efficiencies (21.4–22.6 %), negligible efficiency roll‐off and improved operational stability. To the best of our knowledge, these are the most efficient non‐doped OLEDs reported so far. This convenient and versatile molecular design is of high significance for the advance of non‐doped OLEDs.     

Theoretical investigation on reverse intersystem crossing from upper triplet to lowest singlet: A “hot exciton” path for blue fluorescent OLEDs

Nowadays, blue fluorescent organic light-emitting diodes (FOLEDs) have attracted considerable attention from both academia and industry. According to spin statistics, electrical excitation results in the formation of ∼25% singlet excitons and ∼75% triplet excitons (signifying ~75% energy loss), which triggered wide-ranging efforts to harvest as many triplet excitons as possible. The materials that can convert triplet excitons into singlet excitons from the high-lying excited triplet states (referred as “hot exciton” channel) to realize high efficiency were reported, which can also efficaciously avoid the accumulation of triplet excitons in T₁ state. In this study, by means of density functional theory (DFT) and time-dependent DFT, we have theoretically investigated the electronic and photophysical properties of 16 newly designed molecules with donor-bridge-acceptor framework to search for the blue FOLED materials exploiting the “hot exciton” path. Important properties, such as singlet-triplet energy gaps, absorption and emission parameters, and reverse intersystem crossing rates (k_RISC), of five target molecules were studied. The calculated results demonstrate that thiophene-diphenylamine (k_RISC up to 1.03 × 10⁸ seconds⁻¹) may have promising potential as blue FOLED materials by virtue of the “hot exciton” effect.     

Highly Efficient Organic Blue Electroluminescent Materials and Devices with Mesoscopic Structures

Due to the difficulty in achieving high efficiency and high color purity simultaneously, blue emission is the limiting factor for the performance and stability of OLEDs. Since 2003, we have been working on organic light‐emitting diodes (OLEDs), especially on blue light. After a series of molecular designs, novel strategies have been proposed from different aspects. At first, highly efficient deep blue emission could be achieved through molecular design with highly twisted structure to suppress fluorescence quenching and redshift. Deep blue emitters with high efficiency in solid state, a twisted structure with aggregation induced emission (AIE) characteristics was incorporated to inhibit molecular aggregation, and triplet‐triplet fusion (TTF) and hybridized localized charge transfer (HLCT) were adopted to increase the ratio of triplet exciton used. Secondly, a highly efficient blue OLED could be achieved through improving charge transport. New electron transport materials (ETMs) with wide band gap were developed to control charge transport balance in devices. Thirdly, a highly efficient deep blue emission could be achieved through a mesoscopic structure of out‐coupling layer. A mesoscopic photonic structured organic thin film was fabricated on the top of metal electrode by self‐aggregation in order to improve the light out‐coupling efficiency.     

Singlet/triplet exciton dissociation and charge recombination in donor-acceptor ThQs-C60/PDIxCN2 complexes

The donor-acceptor interface plays a critically important role in determining the power conversion efficiency of organic solar cells via controlling charge separation (CS) and recombination (CR) processes. Here, we combine the electronic structure calculations with electron transfer rate theory to clarify the CS and CR processes in ThQs-C₆₀ /PDIxCN₂ donor-acceptor complexes. The results reveal that in ThQs-PDIxCN₂ the CS comes from both the dissociations of photo-induced singlet exciton and singlet fission-induced triplet exciton with a high efficiency, whereas in ThQs-C₆₀ only the singlet exciton dissociation can take place because the triplet exciton lies below the charge-transfer exciton. However, very high CR rates in ThQs-PDIxCN₂ obliterate the benefit of fast CS, inversely leading to the ThQ-C₆₀ complex with a better cell efficiency. The present results are consistent with experimental observation and may furnish a possible patten to improve the overall conversion efficiency. © 2018 Wiley Periodicals, Inc.     

Cooperative exciton spectra in molecular crystals

It is shown that the singlet exciton spectrum in the crystal is coupled with that of the two-particle excitations (singlet, triplet excitons, magnons, triplet exciton-magnon). Splitting on the excitation spectrum occurs when the composite energy of the two-particle excitations is close to that of the singlet exciton.     

Phosphorescent Pt(II) Emitters for OLEDs: From Triarylboron‐Functionalized Bidentate Complexes to Compounds with Macrocyclic Chelating Ligands

The development of organic light‐emitting diodes (OLEDs) has attracted enormous research efforts from both academia and industry in the past decades and tremendous progress has been made. However, the low operation lifetime of the blue phosphorescent OLEDs remains as one of the greatest bottlenecks limiting further applications of OLEDs. To address this problem, design and synthesis of triplet emitters with high phosphorescence quantum yield (Φ_P) and adequate thermal, chemical, electrical and ultraviolet (UV) stabilities are vital. This review summarizes the progress we made on the development of efficient and robust phosphorescent emitters based on cyclometalated Pt(II) compounds, particularly the ones with blue emission, starting from complexes with triarylboron‐functionalized bidentate ligand to molecules incorporating tetradentate and macrocyclic ligands, with emphasis on their structure‐property relationships.     

Quinoline-based aggregation-induced delayed fluorescence materials for highly efficient non-doped organic light-emitting diodes

Three new emitters,namely 10,10'-(quinoline-2,8-diyl)bis(10 H-phenoxazine)(Fene),10,10'-(quinoline-2,8-diyl)bis(10 H-phenothiazine)(Fens) and 10,10'-(quinoline-2,8-diyl)bis(9,9-dimethyl-9,10-dihydroacridine)(Yad),featuring quinoline as a new electron acceptor have been designed and conveniently synthesized.These emitters possessed small singlet-triplet splitting energy(ΔEst) and twisted structures,which not only endowed them show thermally activated delayed fluorescence(TADF)properties but also afforded a remarkable aggregation-induced emission(AIE) feature.Moreover,they also showed aggregation-induced delayed fluorescence(AIDF) property and good photoluminescence(PL) property,which are the ideal emitters for non-doped organic light-emitting diodes(OLEDs).Furthermore,high-performance non-doped OLEDs based on Fene,Fens and Yad were achieved,and excelle nt maximum external quantum efficiencies(EQE_(max)) of 14,9%,13.1% and 17,4%,respectively,were obtained.It was also found that all devices exhibited relatively low turn-on voltages ranging from 3.0 V to3.2 V probably due to their twisted conformation and the AIDF properties.These results demonstrated the quinoline-based emitters could have a promising application in non-doped OLEDs.     

Dibenzo[a,j]phenazine‐Cored Donor–Acceptor–Donor Compounds as Green‐to‐Red/NIR Thermally Activated Delayed Fluorescence Organic Light Emitters

A new family of thermally activated delayed fluorescence (TADF) emitters based on U‐shaped D‐A‐D architecture with a novel accepting unit has been developed. All investigated compounds have small singlet‐triplet energy splitting (ΔE_ST) ranging from 0.02 to 0.20 eV and showed efficient TADF properties. The lowest triplet state of the acceptor unit plays the key role in the TADF mechanism. OLEDs fabricated with these TADF emitters achieved excellent efficiencies up to 16 % external quantum efficiency (EQE).