Realizing Record-High Electroluminescence Efficiency of 31.5 % for Red Thermally Activated Delayed Fluorescence Molecules

Aggregation-induced emissive nanoarchitectures for luminescent solar concentrators

Aggregation-induced emission (AIE), the phenomenon by which selected luminophores undergo the enhancement of emission intensity upon aggregation, has demonstrated potential in materials and biomaterials science, and in particular in those branches for which spectral management in the solid state is of fundamental importance. Its development in the area of luminescent spectral conversion devices like luminescent solar concentrators (LSCs) is instead still in its infancy. This account aims at summarizing relevant contributions made in this field so far, with a special emphasis on the design of molecular and macromolecular architectures capable of extending their spectral breadth to the deep-red (DR) and the near-infrared (NIR) wavelengths. Because of the many prospective advantages characterizing these spectral regions in terms of photon flux density and human-eye perception, it is anticipated that further development in the design, synthesis and engineering of advanced molecular and macromolecular DR/NIR-active AIE luminophores will enable faster and easier integration of LSCs into the built environment as highly transparent, active elements for unobtrusive light-to-electricity conversion.

Unraveling non-radiative decay channels of exciplexes to construct efficient red emitters for organic light-emitting diodes

Exciplex emitters naturally have thermally activated delayed fluorescence characteristics due to their spatially separated molecular orbitals. However, the intermolecular charge transfer potentially induces diverse non-radiative decay channels, severely hindering the construction of efficient red exciplexes. Thus, a thorough comprehension of this energy loss is of paramount importance. Herein, different factors, including molecular rigidity, donor-acceptor interactions and donor-donor/acceptor-acceptor interactions, that impact the non-radiative decay were systematically investigated using contrasting exciplex emitters. The exciplex with rigid components and intermolecular hydrogen bonds showed a photoluminescence quantum yield of 84.1% and a singlet non-radiative decay rate of 1.98 × 106 s-1 at an optimized mixing ratio, respectively, achieving a 3.3-fold increase and a 70% decrease compared to the comparison group. In the electroluminescent device, a maximum external quantum efficiency of 23.8% was achieved with an emission peak of 608 nm, which represents the state-of-the-art organic light-emitting diodes using exciplex emitters. Accordingly, a new strategy is finally proposed, exploiting system rigidification to construct efficient red exciplex emitters that suppress non-radiative decay.

Enhancing Light-Emitting Efficiency of Blue Through-Space Charge Transfer Emitters via Fixing Configuration Induced by Intramolecular Hydrogen Bonding

Closely aligned configuration of the donor (D) and acceptor (A) is crucial for the light-emitting efficiency of thermally activated delayed fluorescence (TADF) materials with through-space charge transfer (TSCT) characteristics. However, precisely controlling the D-A distance of blue TSCT-TADF emitters is still challenging. Herein, an extra donor (D*) located on the side of the primary donor (D) is introduced to construct the hydrogen bonding with A and thus modulate the distance of D and A units to prepare high-efficiency blue TSCT emitters. The obtained "V"-shaped TSCT emitter presents a minimal D-A distance of 2.890 Å with a highly parallel D-A configuration. As a result, a high rate of radiative decay (>107 s-1) and photoluminescence quantum yield (nearly 90%) are achieved. The corresponding blue organic light-emitting diodes show maximum external quantum efficiencies (EQEmax) of 27.9% with a Commission Internationale de L'Eclairage (CIE) coordinate of (0.16, 0.21), which is the highest device efficiency of fluorene-based blue TSCT-TADF emitters. In addition, the TSCT-TADF emitter-sensitized OLEDs also achieve a high EQEmax of 29.3% with a CIE coordinate of (0.12, 0.16) and a narrow emission.

Efficient near-infrared emission benefits from slowing down the internal conversion process

Organic deep-red (DR) and near-infrared (NIR) emitters with high photoluminescence quantum yield (PLQY) are rare due to the strong non-radiative (knr) decay. Here, we report two DR/NIR emitters with high PLQY, TPANZPyPI and TPANZ3PI. Interestingly, the TPANZPyPI film exhibits 46.5% PLQY at 699 nm. Theoretical calculations indicate that TPANZPyPI can achieve this high PLQY in the near-infrared emission region due to its small S1 to S0 internal conversion (IC) rate. Meanwhile, research has found that, compared to TPANZ3PI, TPANZPyPI with a more rigid structure can effectively suppress the T2 to T1 IC process, which is conducive to higher exciton utilization efficiency (EUE). TPANZPyPI's non-doped OLED shows NIR emission with 4.6% @ 684 nm maximum external quantum efficiency (EQEmax). Its doped OLEDs radiate DR with an EQEmax of 6.9% @ 666 nm. These EQEs are among the highest values for hybridized local charge transfer state materials emitting more than 640 nm. This work demonstrates for the first time, based on a combination of theory and experiment, that increasing the molecular rigidity can inhibit the excited state IC process in addition to the S1 to S0 IC, realizing efficient electroluminescence.

Bridging of Cove Regions: A Strategy for Realizing Persistently Chiral Double Heterohelicenes with Attractive Luminescent Properties

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational-stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove-regions bridging strategy for achieving persistently chiral multi-helicenes (incapable of racemization), based on the synthesized proof-of-concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi-helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove-regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi-helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi-helicenes for various applications.

Creating Efficient Red Thermally Activated Delayed Fluorescence Materials with Cyano-Substituted 11,12-Diphenyldipyrido[3,2-a:2',3'-c]phenazine Acceptors

Red luminescent materials are essential components for full color display and white lightening based on organic light-emitting diode (OLED) technology, but the extension of emission color towards red or deep red region generally leads to decreased photoluminescence and electroluminescence efficiencies. Herein, we wish to report two new luminescent molecules (2CNDPBPPr-TPA and 4CNDPBPPr-TPA) consisting of cyano-substituted 11,12-diphenyldipyrido[3,2-a:2',3'-c]phenazine acceptors and triphenylamine donors. As the increase of cyano substituents, the emission wavelength is greatly red-shifted and the reverse intersystem crossing process is promoted, resulting in strong red delayed fluorescence. Meanwhile, due to the formation of intramolecular hydrogen bonds, the molecular structures become rigidified and planarized, which brings about large horizontal dipole ratios. As a result, 2CNDPBPPr-TPA and 4CNDPBPPr-TPA can perform as emitters efficiently in OLEDs, furnishing excellent external quantum efficiencies of 28.8 % at 616 nm and 20.2 % at 648 nm, which are significantly improved in comparison with that of the control molecule without cyano substituents. The findings in this work demonstrate that the introduction of cyano substituents to the acceptors of delayed fluorescence molecules could be a facile and effective approach to explore high-efficiency red or deep red delayed fluorescence materials.

An A-D-A-Type Thermally Activated Delayed Fluorescence Emitter with Intrinsic Yellow Emission Realizing Record-High Red/NIR OLEDs upon Modulating Intermolecular Aggregations

Realizing efficient red/near-infrared (NIR) electroluminescence (EL) by precisely modulating molecular aggregations of thermally activated delayed fluorescence (TADF) emitters is an attractive pathway, yet the molecular designs are elusive. Here, a new approach is proposed to manage molecular aggregation via a mild-twist acceptor-donor-acceptor (A-D-A)-type molecular design. A proof-of-concept TADF molecule, QCN-PhSAC-QCN, is developed that furnishes a fast radiative rate and obvious aggregation-induced emission feature. Its emission bands can be facilely shifted from intrinsic yellow to the red/NIR region via fine-tuning doping levels and molecular aggregates while maintaining elegant photoluminescence quantum yields benefiting from suppressed exciton annihilation processes. As a result, a QCN-PhSAC-QCN-based organic light-emitting diode (OLED) exhibits a record-setting external quantum efficiency (EQE) of 39.1% at a doping ratio of 10 wt.%, peaking at 620 nm. Moreover, its nondoped NIR OLED affords a champion EQE of 14.3% at 711 nm and retains outstanding EQEs of 5.40% and 2.35% at current densities of 10 and 100 mA cm-2 , respectively, which are the highest values among all NIR-TADF OLEDs at similar density levels. This work validates the feasibility of such mild-twist A-D-A-type molecular design for precisely controlling molecular aggregation while maintaining high efficiency, thus providing a promising pathway for high-performance red/NIR TADF OLEDs.

Carbonyl- and Nitrogen-Embedded Multi-Resonance Emitter with Ultra-Pure Green Emission and High Electroluminescence Efficiencies

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with narrow emission spectra have garnered significant attention in future organic light-emitting diode (OLED) displays. However, current C=O/N-embedded MR-TADF systems still lack satisfactory performance in terms of electroluminescence bandwidths and external quantum efficiencies (EQEs). In this study, a C=O/N-embedded green MR-TADF emitter, featuring two acridone units incorporated in a sterically protected 11-ring fused core skeleton, is successfully synthesized through finely controlling the reaction selectivity. The superior combination of multiple intramolecular fusion and steric wrapping strategies in the design of the emitter not only imparts an extremely narrow emission spectrum and a high fluorescence quantum yield to the emitter but also mitigates aggregation-induced spectral broadening and fluorescence quenching. Therefore, the emitter exhibits leading green OLED performance among C=O/N-based MR-TADF systems, achieving an EQE of up to 37.2 %, a full width at half maximum of merely 0.11 eV (24 nm), and a Commission Internationale de l'Éclairage coordinate of (0.20, 0.73). This study marks a significant advance in the realization of ideal C=O/N-based MR-TADF emitters and holds profound implications for the design and synthesis of other MR-TADF systems.

Recent advances in the design of afterglow materials: mechanisms, structural regulation strategies and applications

Afterglow materials are attracting widespread attention owing to their distinctive and long-lived optical emission properties which create exciting opportunities in various fields. Recent research has led to the discovery of many new afterglow materials featuring high photoluminescence quantum yields (PLQY) and lifetimes of up to several hours under ambient conditions. Afterglow materials are typically categorized according to their luminescence mechanism, such as long-persistent luminescence (LPL), room temperature phosphorescence (RTP), or thermally activated delayed fluorescence (TADF). Through rational design and novel synthetic strategies to modulate spin-orbit coupling (SOC) and populate triplet exciton states (T1), luminophores with long lifetimes and bright afterglow characteristics can be realized. Initial research towards afterglow materials focused mainly on pure inorganic materials, many of which possessed inherent disadvantages such as metal toxicity or low energy emissions. In recent years, organic-inorganic hybrid afterglow materials (OIHAMs) have been developed with high PLQY and long lifetimes. These hybrid materials exploit the tunable structure and easy processing of organic molecules, as well as enhanced SOC and intersystem crossing (ISC) processes involving heavy atom dopants, to achieve excellent afterglow performance. In this review, we begin by briefly discussing the structure and composition of inorganic and organic-inorganic hybrid afterglow materials, including strategies for regulating their lifetime, PLQY and luminescence wavelength. The specific advantages of organic-inorganic hybrid afterglow materials, including low manufacturing costs, diverse molecular/electronic structures, tunable structures and optical properties, and compatibility with a variety of substrates, are emphasized. Subsequently, we discuss in detail the fundamental mechanisms used by afterglow materials, their classification, design principles, and end applications (including sensing, anticounterfeiting, and photoelectric devices, among others). Finally, existing challenges and promising future directions are discussed, laying a platform for the design of afterglow materials for specific applications.

Highly Efficient Room-Temperature Light-Induced Synthesis of Polymer Dots: A Programming Control Paradigm of Polymer Nanostructurization from Single-Component Precursor

Polymer dots (PDs) have raised considerable research interest due to their advantages of designable nanostructures, high biocompatibility, versatile photoluminescent properties, and recyclability as nanophase. However, there remains a lack of in situ, real-time, and noncontact methods for synthesizing PDs. Here we report a rational strategy to synthesize PDs through a well-designed single-component precursor (an asymmetrical donor-acceptor-donor' molecular structure) by photoirradiation at ambient temperature. In contrast to thermal processes that normally lack atomic economy, our method is mild and successive, based on an aggregation-promoted sulfonimidization triggered by photoinduced delocalized intrinsic radical cations for polymerization, followed by photooxidation for termination with structural shaping to form PDs. This synthetic approach excludes any external additives, rendering a conversion rate of the precursor exceeding 99%. The prepared PDs, as a single entity, can realize the integration of nanocore luminescence and precursor-transferred luminescence, showing 41.5% of the total absolute luminescence quantum efficiency, which is higher than most reported PD cases. Based on these photoluminescent properties, together with the superior biocompatibility, a unique membrane microenvironmental biodetection could be exemplified. This strategy with programming control of the single precursor can serve as a significant step toward polymer nanomanufacturing with remote control, high-efficiency, precision, and real-time operability.

Highly Efficient Luminescence from a Red Thermally Activated Delayed Fluorescence Emitter with Flexible Conformation of Ancillary Groups

Robust scaffolds were typically applied in thermally activated delayed fluorescence (TADF) molecules to suppress the non-radiative decay, trigger the fast spin-flipping, and enhance the light out-coupling efficiency. Herein, we disclosed for the first time the positive effect of flexible conformation of ancillary groups on the photophysical properties of TADF emitter. The red TADF emitter Ph-TPA with flexible conformation demonstrated small excited-state structural distortion and low reorganization energy compared to the counterpart Mc-TPA with a rigid macrocycle. Consequently, Ph-TPA showed an excellent photoluminescent quantum yield (PLQY) of 92 % and a state-of-the-art external quantum efficiency (EQE) of 30.6 % at 630 nm. This work could deepen our understanding of structure-property relationships of organic luminophores and help us to rationalize the design of efficient TADF materials.

RGB Thermally Activated Delayed Fluorescence Emitters for Organic Light-Emitting Diodes toward Realizing the BT.2020 Standard

With the surging demand for ultra-high-resolution displays, the International Telecommunication Union (ITU) announce the next-generation color gamut standard, named ITU-R Recommendation BT.2020, which not only sets a seductive but challenging milestone for display technologies but also urges researchers to recognize the importance of color coordinates. Organic light-emitting diodes (OLEDs) are an important display technology in current daily life, but they face challenges in approaching the BT.2020 standard. Thermally activated delayed fluorescence (TADF) emitters have bright prospects in OLEDs because they possess 100% theoretical exciton utilization. Thus, the development of TADF emitters emitting primary red (R), green (R), and blue (B) emission is of great significance. Here, a comprehensive overview of the latest advancements in TADF emitters that exhibit Commission Internationale de l'Éclairage (CIE) coordinates surpassing the National Television System Committee (NTSC) and approaching BT.2020 standards is presented. Rational strategies for molecular designs, as well as the resulting photophysical properties and OLED performances, are discussed to elucidate the underlying mechanisms for shifting the CIE coordinates of both donor-acceptor and multiple resonance (MR) typed TADF emitters toward the BT.2020 standard. Finally, the challenges in realization of the wide-color-gamut BT.2020 standard and the prospects for this research area are provided.

Rational Design of Highly Efficient Orange-Red/Red Thermally Activated Delayed Fluorescence Emitters with Submicrosecond Emission Lifetimes

The development of orange-red/red thermally activated delayed fluorescence (TADF) materials with both high emission efficiencies and short lifetimes is highly desirable for electroluminescence (EL) applications, but remains a formidable challenge owing to the strict molecular design principles. Herein, two new orange-red/red TADF emitters, namely AC-PCNCF3 and TAC-PCNCF3, composed of pyridine-3,5-dicarbonitrile-derived electron-acceptor (PCNCF3) and acridine electron-donors (AC/TAC) are developed. These emitters in doped films exhibit excellent photophysical properties, including high photoluminescence quantum yields of up to 0.91, tiny singlet-triplet energy gaps of 0.01 eV, and ultrashort TADF lifetimes of less than 1 µs. The TADF-organic light-emitting diodes employing the AC-PCNCF3 as emitter achieve orange-red and red EL with high external quantum efficiencies of up to 25.0% and nearly 20% at doping concentrations of 5 and 40 wt%, respectively, both accompanied by well-suppressed efficiency roll-offs. This work provides an efficient molecular design strategy for developing high-performance red TADF materials.

A dual-locked triarylamine donor enables high-performance deep-red/NIR thermally activated delayed fluorescence organic light-emitting diodes

Deep-red/near-infrared (DR/NIR) organic light-emitting diodes (OLEDs) have attracted a great deal of attention due to their widespread application fields, such as night-vision devices, optical communication, and information-secured displays. However, most DR/NIR OLEDs show low electroluminescence efficiencies, hampering their applications. Herein, we constructed a high-performance DR/NIR thermally activated delayed fluorescence (TADF) emitter based on an advanced dual-locked triarylamine donor (D) unit. Promisingly, such a novel D segment brings numerous advantages: a larger stereoscopic architecture, an enhanced electron-donating ability, and a stiffer molecular structure. In view of these features, the newly developed emitter DCN-DSP shows redshifted emission, a narrowed ΔEST, an enhanced PLQY value and aggregation-induced emission (AIE) properties, which allows for effectively alleviating concentration quenching compared to the control compound using a conventional triarylamine derivative as D units. The DCN-DSP-based OLEDs with modulated doping concentrations exhibit champion EQEs of 36.2% at 660 nm, 26.1% at 676 nm and 21.3% at 716 nm, which are record-high efficiencies among all TADF OLEDs in the similar emission ranges. This work realizes the efficiency breakthrough of DR/NIR TADF OLEDs, and such a promising molecular design approach may inspire even better DR/NIR TADF emitters in the future.

A Feasible Strategy for a Highly Efficient Thermally Activated Delayed Fluorescence Emitter Over 900 nm Based on Phenalenone Derivatives

Near-infrared (NIR) organic light-emitting diodes (OLEDs) suffer from the low external electroluminescence (EL) quantum efficiency (EQE), which is a critical obstacle for potential applications. Herein, 1-oxo-1-phenalene-2,3-dicarbonitrile (OPDC) is employed as an electron-withdrawing aromatic ring, and by incorporating with triphenylamine (TPA) and biphenylphenylamine (BBPA) donors, two novel NIR emitters with thermally activated delayed fluorescence (TADF) characteristics, namely OPDC-DTPA and OPDC-DBBPA, are first developed and compared in parallel. Intense NIR emission peaks at 962 and 1003 nm are observed in their pure films, respectively. Contributed by the local excited (LE) characteristics in the triplet (T1 ) state in synergy with the charge transfer (CT) characteristics for the singlet (S1 ) state to activate TADF emission, the solution processable doped NIR OLEDs based on OPDC-DTPA and OPDC-DBBPA yield EL peaks at 834 and 906 nm, accompanied with maximum EQEs of 0.457 and 0.103 %, respectively, representing the state-of-the-art EL performances in the TADF emitter-based NIR-OLEDs in the similar EL emission regions so far. This work manifests a simple and effective strategy for the development of NIR TADF emitters with long wavelength and efficiency synchronously.

Realizing High-Efficiency Orange-Red Thermally Activated Delayed Fluorescence Materials through the Construction of Intramolecular Noncovalent Interactions

The development of highly efficient orange and red thermally activated delayed fluorescence (TADF) materials for constructing full-color and white organic light-emitting diodes (OLEDs) remains insufficient because of the formidable challenges in molecular design, such as the severe radiationless decay and the intrinsic trade-off between the efficiencies of radiative decay and reverse intersystem crossing (RISC). Herein, we design two high-efficiency orange and orange-red TADF molecules by constructing intermolecular noncovalent interactions. This strategy could not only ensure high emission efficiency via suppression of the nonradiative relaxation and enhancement of the radiative transition but also create intermediate triplet excited states to ensure the RISC process. Both emitters exhibit typical TADF characteristics, with a fast radiative rate and a low nonradiative rate. Photoluminescence quantum yields (PLQYs) of the orange (TPA-PT) and orange-red (DMAC-PT) materials reach up to 94 and 87%, respectively. Benefiting from the excellent photophysical properties and stability, OLEDs based on these TADF emitters realize orange to orange-red electroluminescence with high external quantum efficiencies reaching 26.2%. The current study demonstrates that the introduction of intermolecular noncovalent interactions is a feasible strategy for designing highly efficient orange to red TADF materials.

A Low-Cost Test Method for Accurate Detection of Different Excited-State Species with a Lifetime Span over 5 Orders of Magnitude in One Time Window

Accurate quantification on the quantum yields (φ) of both the prompt fluorescence (PF) and the delayed fluorescence (DF) species is quite essential for the clarification of molecular design rationales for thermally activated delayed fluorescence (TADF) luminogens. Currently, most φ_PF and φ_DF data of TADF fluorophores were acquired through time-correlated single-photon counting (TCSPC) lifetime measurement systems. However, because of their equal-time-channel working manner, so far all the commercially available TCSPC systems cannot render accurate measurement on φ_PF of TADF materials due to the lack of enough valid data points in the faster decay region of the corresponding photoluminescence (PL) decay curves. Although an intensified charge coupled device (ICCD) system equipped with a streak camera or an optical parametric oscillation laser has been proven to be a powerful tool for accurate determination of φ_PF and φ_DF of TADF fluorophores, the ultrahigh cost of these ICCD systems makes them inaccessible to most users. Herein, by replacing the timing module of a commercial TCSPC system with a low-cost and versatile time-to-digital converter (TDC) module, we developed a modified TCSPC system that can work in an unequal-time-channel manner. The resultant TDC-TCSPC system can not only concurrently determine the accurate lifetime of PF and DF species whose lifetime span even exceeds 5 orders of magnitude in just one time window but also render accurate measurements on φ_PF and φ_DF of TADF fluorophores. The reliability of the TDC-TCSPC method was verified through TCSPC- and ICCD-based comparative experiments on ACMPS, a known TADF fluorophore. Our results not only can provide a low-cost and convenient test method for accurate determination of key experimental data of TADF materials but also will facilitate deeper understanding of the molecular design principles for high-performance TADF materials.

Using azaacene as an acceptor unit to construct an ultraefficient red fluorophore with an EQE over 40

Azaacenes, which have been known for a long time, are of scientific and practical importance in organic electronics. Azaacenes once shone as the luminophore in organic light-emitting diodes (OLEDs). However, due to the low exciton utilization efficiency and/or the aggregation induced quenching (ACQ) effect, N-heteroacene based OLEDs generally showed inferior device performance. In this work, azaacene has been revisited and applied as an acceptor for a red fluorophore (AZA-TPA), where the judicious connection pattern between donor and acceptor maximized the harvest of singlet and triplet excitons, resulting in a high photoluminescence efficiency of 94.6% in doped films (3 wt%). In addition, the linearly-fused polycyclic structure contributed to a high horizontal emitting dipole ratio (Θ_‖ = 90%). As a result, an AZA-TPA-based OLED achieved an unprecedented external quantum efficiency of 41.30% at 610 nm. This work will pave a new path for the development of efficient N-heteroacene-based fluorophores.

Intramolecular charge transfer effect for highly efficient deep red and near infrared thermally activated delayed fluorescence

Thermally activated delayed fluorescence (TADF) materials with emission in the deep red and near infrared (DR/NIR) region are underresearched due to the limited choice of strong donor/acceptor units. The current mainstream strategy for the design of DR/NIR TADFs is to increase the acceptor strength via the introduction of multiple sub-acceptor units, thereby narrowing the bandgap. In this work, the intramolecular charge transfer (ICT) effect was applied for the development of acceptor units to achieve efficient DR/NIR TADFs. The ICT effect within the acceptor unit enhanced the π-electron delocalization, lowered the LUMO and redshifted the emission wavelength. In addition, the fusion of the donor unit into the planar acceptor skeleton rigidified the molecular structure and reduced the non-radiative decay. This proof-of-concept study demonstrated that ICT is an undoubtedly effective strategy for the rational design of efficient DR/NIR TADFs.

Thermally Activated Delayed Fluorescence and Room-Temperature Phosphorescence in Materials with Imidazo-pyrazine-5,6-dicarbonitrile Acceptors

Three donor-acceptor compounds based on the imidazo-pyrazine-5,6-dicarbonitrile (IPDC) acceptor were synthesized. The IPDC emitters exhibit blue to near-infrared (NIR) emission with up to 54 % photoluminescent quantum yield. 9,9-Dimethyl-9,10-dihydroacridine (ACR), phenoxazine (POX), and phenothiazine (PTZ) served as electron donors. IPDC-POX displayed NIR emission in toluene solution, while showing room-temperature phosphorescence in the solid state. IPDC-ACR exhibited yellow thermally activated delayed fluorescence. Interestingly, dual-emissive behavior as well as excitation-dependent thermally activated delayed fluorescence (TADF) was found for IPDC-PTZ, arising from the two conformers of phenothiazine derivatives. Overall, this work describes a novel strong electron acceptor from the fusion of imidazole, pyrazine, and nitrile functional groups into one conjugated heterocycle for materials exhibiting NIR emission, TADF, and/or room-temperature phosphorescence (RTP).

Isomeric thermally activated delayed fluorescence emitters for highly efficient organic light-emitting diodes

The isomeric strategy is an important design concept in molecular design that has a non-negligible influence on molecular properties. Herein, two isomeric thermally activated delayed fluorescence (TADF) emitters (NTPZ and TNPZ) are constructed with the same skeleton consisting of an electron donor and electron acceptor but different connection sites. Systematic investigations show that NTPZ exhibits a small energy gap, large up-conversion efficiency, low non-radiative decay, and high photoluminescence quantum yield. Further theoretical simulations reveal that the excited molecular vibrations play a key role in regulating the non-radiative decays of the isomers. Therefore, an NTPZ based OLED achieves better electroluminescence performances, such as a higher external quantum efficiency of 27.5% compared to a TNPZ based OLED (18.3%). This isomeric strategy not only provides an opportunity to deeply understand the relationship between substituent locations and molecular properties, but also affords a simple and effective strategy to enrich TADF materials.

Poly-L-aspartic acid based nonconventional luminescent biomacromolecules with efficient emission in dilute solutions for Al3+ detection

Nonconventional luminescent macromolecules exhibiting bright fluorescence or phosphorescence emission at high concentrations and solid-state have attracted significant attention due to their promising application in different fields. However, most reported nonconventional luminescent macromolecules show weak or no emission in dilute solutions, limiting their large-scale applications. Herein, nonconventional luminescent biomacromolecules with hydrophobic rigid chains, hydrophilic flexibility and inter- or intra-molecular hydrogen bonding interactions were proposed to achieve effective luminescence in dilute solutions. Poly-L-aspartic acid (PASA) with a fluorescence quantum yield of 4.6 % in a dilute solution (0.8 mg/mL) was synthesized to validate this design strategy. The fluorescence intensity of PASA solution increased with the increase of the concentration, demonstrating a clustering-triggered emission (CTE) effect. Furthermore, the fluorescence intensity significantly enhanced when adding Al³⁺ into PASA aqueous solution via the Al³⁺ recognition effect. The detection limits for Al³⁺ (1.71 × 10^-6 mol/L) meet the World Health Organization (WHO) requirements for food detection. At last, PASA solid-state samples exhibit room temperature phosphorescence emission.

Hyperbranched Conjugated Polymer with Multiple Charge Transfer Enables High-Efficiency Nondoped Red Electroluminescence with Low Driving Voltage

Conjugated polymers featuring thermally activated delayed fluorescence (TADF) attract tremendous attention in both academic and industry communities due to their easy solution processing for fabricating large-area and low-cost high-performance polymer light-emitting diodes (PLEDs). However, current nondoped solution-processed PLEDs frequently encounter significant efficiency roll-offs and unreasonably high operating voltages at high brightness, especially for red-emitting polymers. Herein, we design hyperbranched conjugated polymers (HCPs) with D-A-D type TADF characteristics for high-performance red-emitting PLEDs. Multiple intramolecular charge transfer (ICT) channels induced by quasi-equivalent donors of the TADF core strongly boost the reverse intersystem crossing (RISC) process and singlet excitons radiative transition. Coupling with the efficient energy transfer process generated by structure advantages of HCPs, the strongly electron-withdrawing oxygen atoms located on the TADF cores further accelerate hole transportation from the host chains to the TADF cores. Under a rational regulation of the TADF core ratio, the related nondoped red-emitting device performs an outstanding performance with an EQE_max of 8.39% and exhibits no roll-off while the luminance is less than 100 cd/m² and only 3.3% decrease at 500 cd/m². Simultaneously, the EQE can maintain 7.4% under 1000 cd/m². Furthermore, the corresponding nondoped device exhibits a low turn-on voltage of around 2.5 V and achieves a luminance of 500 cd/m² at 3.5 V and even 1000 cd/m² at 3.9 V. To our knowledge, this is the best performance among all nondoped red PLEDs with high brightness obtained at low operating voltage.

Progresses and Perspectives of Near-Infrared Emission Materials with "Heavy Metal-Free" Organic Compounds for Electroluminescence

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the "heavy metal-free" NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.

The Role of Balancing Carrier Transport in Realizing an Efficient Orange-Red Thermally Activated Delayed-Fluorescence Organic Light-Emitting Diode

Simultaneously realizing improved carrier mobility and good photoluminescence (PL) efficiency in red thermally activated delayed-fluorescence (TADF) emitters remains challenging but important. Herein, two isomeric orange-red TADF emitters, oPDM and pPDM, with the same basic donor-acceptor backbone but a pyrimidine (Pm) attachment at different positions are designed and synthesized. The two emitters show similarly good PL properties, including narrow singlet-triplet energy offsets (0.11 and 0.15 eV) and high photoluminescence quantum yields (ca. 100 and 88%) in doped films. An orange-red organic light-emitting diode (OLED) employing oPDM as an emitter achieves an almost twice as high maximum external quantum efficiency (28.2%) compared with that of a pPDM-based OLED. More balanced carrier-transporting properties are responsible for their contrasting device performances, and the position effect of the Pm substituent leads to significantly distinct molecular packing behaviors in the aggregate states and different carrier mobilities.

A novel orange-red thermally activated delayed fluorescence emitter with high molecular rigidity and planarity realizing 32.5% external quantum efficiency in organic light-emitting diodes

Simultaneous optimization of photoluminescence quantum yield (Φ_PL) and horizontally oriented dipoles (Θ_‖) is considerably challenging for orange and red thermally activated delayed fluorescence (TADF) emitters, due to the conflicts between enhancing molecular rigidity and improving molecular planarity. Herein, a novel orange-red TADF emitter 10-(dipyrido[3,2-a:2',3'-c]phenazin-11-yl)-10H-spiro[acridine-9,9'-fluorene] (SAF-2NP) was constructed with a donor-acceptor structure. The highly rigid donor and acceptor segments ensure the overall rigidity of the emitter. More importantly, the quasi-coplanar structure between the acceptor and the fluorene moiety in the donor unit enlarges the molecular plane without weakening rigidity. Consequently, SAF-2NP exhibited extremely high Φ_PL and Θ_‖ of 99% and 85%, respectively. The optimal organic light-emitting diode using SAF-2NP as the emitter and 4,4'-di(9H-carbazol-9-yl)-1,1'-biphenyl (CBP) as the host demonstrated an unparalleled external quantum efficiency of 32.5% and a power efficiency of 85.2 lm W^-1 without any extra light extraction structure. This work provides a feasible strategy to establish efficient orange and red TADF emitters with both high rigidity and planarity.

Chiral cyclic architectonics with tetraphenylethylenes: conformation immobilization, optical resolution and circularly polarized luminescence

Chiral cyclic tetraphenylethylenes (TPEs) were synthesized by cyclization with chiral binaphthols. The helical conformation can be immobilized and multi-isomers can be separated resulting from (Z/E)-isomerism and helical chirality of TPE. The chiral cyclic TPEs can emit deep blue circularly polarized luminescence (CPL) in both solution and film states.

Pure Hydrocarbon Materials as Highly Efficient Host for White Phosphorescent Organic Light‐Emitting Diodes: A New Molecular Design Approach

To date, all efficient host materials reported for phosphorescent OLEDs (PhOLEDs) are constructed with heteroatoms, which have a crucial role in the device performance. However, it has been shown in recent years that the heteroatoms not only increase the design complexity but can also be involved in the instability of the PhOLED, which is nowadays the most important obstacle to overcome. Herein, we design pure aromatic hydrocarbon materials (PHC) as very efficient hosts in high‐performance white and blue PhOLEDs. With EQE of 27.7 %, the PHC‐based white PhOLEDs display similar efficiency as the best reported with heteroatom‐based hosts. Incorporated as a host in a blue PhOLED, which are still the weakest links of the technology, a very high EQE of 25.6 % is reached, surpassing, for the first time, the barrier of 25 % for a PHC and FIrpic blue emitter. This performance shows that the PHC strategy represents an effective alternative for the future development of the OLED industry.

Distinctive Excited State Symmetry Breaking Dynamics in Typical Donor-Acceptor-Donor Fluorophore: Strong Photoluminescence and Ultrafast Charge Separation from a Partial Charge Transfer State

Understanding the structure-property relationships in organic semiconductors is crucial for controlling their photophysical properties and developing new optoelectronic materials. Quadrupolar molecules, donor-acceptor-donor (DAD), have attracted extensive attention in various optoelectronic applications. However, the systematic studies on the differences on photophysical properties between DAD and simple donor-acceptor (DA) chromophores are rarely reported. Herein we present a comparative study on the excited state dynamics of DA and DAD fluorescence systems using theoretical calculation and transient absorption spectroscopy. Results show that DA and DAD molecules exhibit similar excited state dynamics, which are attributed to the distinctive excited-state symmetry breaking (ESSB) phenomenon observed in a DAD system. The strong photoluminescence (PL) and ultrafast charge separation (CS) from an ESSB-induced partial charge transfer (CT) state were clearly detected in different solvent environments. These results not only offer insight into the excited state dynamics of the DAD fluorescence system but also provide some basic guidelines for designing new optoelectronic materials.

NIR TADF emitters and OLEDs: challenges, progress, and perspectives

Near-infrared (NIR) light-emitting materials show excellent potential applications in the fields of military technology, bioimaging, optical communication, organic light-emitting diodes (OLEDs), etc. Recently, thermally activated delayed fluorescence (TADF) emitters have made historic developments in the field of OLEDs. These metal-free materials are more attractive because of efficient reverse intersystem crossing processes which result in promising high efficiencies in OLEDs. However, the development of NIR TADF emitters has progressed at a relatively slower pace which could be ascribed to the difficult promotion of external quantum efficiencies. Thus, increasing attention has been paid to NIR TADF emitters. In this review, the recent progress of NIR TADF emitters has been summarized along with their molecular design strategies and photophysical properties, as well as electroluminescence performance data of their OLEDs, respectively.

Asymmetric Thermally Activated Delayed Fluorescence Emitter for Highly Efficient Red/Near-Infrared Organic Light-Emitting Diodes

Developing highly efficient red/near-infrared thermally activated delayed fluorescence (TADF) materials is of great importance for organic light-emitting diodes (OLEDs). Here, we reported an asymmetric TADF emitter (TCPQ), which exhibits a high reverse intersystem crossing rate as well as a low non-radiative rate due to molecular symmetry breaking through multiple donor substitution. The coexistence of multiple donors endows TCPQ with not only near-infrared emission but also excellent device performances. As for the TCPQ-based OLEDs, the 10 and 20 wt % doped devices exhibit outstanding external quantum efficiencies (EQEs) of 21.9 and 19.2% with red emission peaks at 612 and 642 nm, respectively. Meanwhile, the non-doped device achieves an EQE of 5.4% with an emission peak at 718 nm, showing near-infrared emission. These device efficiencies are among the best performances of red/near-infrared TADF-OLEDs, demonstrating that the asymmetry design is a potential strategy for constructing long wavelength TADF materials with high efficiency.

Topological Design of Highly Anisotropic Aligned Hole Transporting Molecular Bottlebrushes for Solution-Processed OLEDs

Polyvinyl polymers bearing pendant hole transport functionalities have been extensively explored for solution-processed hole transport layer (HTL) technologies, yet there are only rare examples of high anisotropic packing of the HT moieties of these polymers into substrate-parallel orientations within HTL films. For small molecules, substrate-parallel alignment of HT moieties is a well-established approach to improve overall device performance. To address the longstanding challenge of extension from vapor-deposited small molecules to solution-processable polymer systems, a fundamental chemistry tactic is reported here, involving the positioning of HT side chains within macromolecular frameworks by the construction of HT polymers having bottlebrush topologies. Applying state-of-the-art polymer synthetic techniques, various functional subunits, including triphenylamine (TPA) for hole transport and adhesion to the substrate, and perfluoro alkyl-substituted benzyloxy styrene for migration to the air interface, were organized with exquisite control over the composition and placement throughout the bottlebrush topology. Upon assembling the HT bottlebrush (HTB) polymers into monolayered HTL films on various substrates through spin-casting and thermal annealing, the backbones of HTBs were vertically aligned while the grafts with pendant TPAs were extended parallel to the substrate. The overall design realized high TPA π-stacking along the out-of-plane direction of the substrate in the HTLs, which doubled the efficiency of organic light-emitting diodes compared with linear poly(vinyl triphenylamine)s.

Deep-Red and Near-Infrared Iridium Complexes with Fine-Tuned Emission Colors by Adjusting Trifluoromethyl Substitution on Cyclometalated Ligands Combined with Matched Ancillary Ligands for Highly Efficient Phosphorescent Organic Light-Emitting Diodes

Six novel Ir(C^N)₂(L^X)-type heteroleptic iridium complexes with deep-red and near-infrared region (NIR)-emitting coverage were constructed through the cross matching of various cyclometalating (C^N) and ancillary (LX) ligands. Here, three novel C^N ligands were designed by introducing the electron-withdrawing group CF₃ on the ortho (o-), meta (m-), and para (p-) positions of the phenyl ring in the 1-phenylisoquinoline (piq) group, which were combined with two electron-rich LX ligands (dipba and dipg), respectively, leading to subsequent iridium complexes with gradually changing emission colors from deep red (≈660 nm) to NIR (≈700 nm). Moreover, a series of phosphorescent organic light-emitting diodes (PhOLEDs) were fabricated by employing these phosphors as dopant emitters with two doping concentrations, 5% and 10%, respectively. They exhibited efficient electroluminescence (EL) with significantly high EQE values: >15.0% for deep red light0 (λ_max = 664 nm) and >4.0% for NIR cases (λ_max = 704 nm) at a high luminance level of 100 cd m^-2. This work not only provides a promising approach for finely tuning the emission color of red phosphors via the easily accessible molecular design strategy, but also enables the establishment of an effective method for enriching phosphorescent-emitting molecules for practical applications, especially in the deep-red and near-infrared region (NIR).

Efficient yellow and red thermally activated delayed fluorescence materials based on a quinoxaline-derived electron-acceptor

Highly efficient yellow and red organic light emitting-diodes are realized by employing thermally activated delayed fluorescence emitters based on a new quinoxaline-derived electron-acceptor.