Highly Efficient Near-Infrared Electrofluorescence from a Thermally Activated Delayed Fluorescence Molecule

Recent Advances in Thermally Activated Delayed Fluorescence-Based Organic Afterglow Materials

Thermally activated delayed fluorescence (TADF)-based materials are attracting widespread attention for different applications owing to their ability of harvesting both singlet and triplet excitons without noble metals in their structures. As compared to the conventional fluorescence and room-temperature phosphorescence pathways, TADF originates from the reverse intersystem crossing process from the excited triplet state (T1) to the singlet state (S1). Therefore, TADF emitters enabling activated and long lifetime T1 excitons are potential candidates for generating long-lived afterglow emission, an effect that can still be observed for a while by the naked eye after the removal of the excitation light source. Recently, TADF-based organic afterglow materials featuring high photoluminescence quantum yields and long lifetimes above 100 ms under ambient conditions, have emerged for advanced information security, high-contrast biological imaging, optoelectronic devices, and intelligent sensors, whereas the related systematic review is still lacking. Herein, the recent progress in TADF-based organic afterglow materials is summarized and an overview of the photophysical mechanism, design strategies, and the performances for relevant applications is given. In addition, the challenge and perspective of this area are given at the end of the review.

Rational Molecular Design for Balanced Locally Excited and Charge- Transfer Nature for Two-Photon Absorption Phenomenon and Highly Efficient TADF-Based OLEDs

The pursuit of highly efficient thermally activated delayed fluorescence (TADF) emitters with two-photon absorption (2PA) character is hampered by the concurrent achievement of a small singlet-triplet energy gap (ΔEST) and high photoluminescence quantum yield (ΦPL). Here, by introducing a terephthalonitrile unit into a sterically crowded donor-π-donor structure, inducing a hybrid electronic excitation character, we designed unique TADF emitters possessing 2PA ability. This rational molecular design was achieved through a main π-conjugated donor-acceptor-donor backbone in line with locally excited feature renders a large oscillator strength and transition dipole moment, maintaining a high 2PA cross-section value. The ancillary N-donor-acceptor-donor with charge transfer character highly balances the TADF phenomenon by minimizing ΔEST. A near-unity ΦPL value with a large radiative decay rate over an order of magnitude higher than the intersystem crossing rate and a high horizontal orientation ratio of 0.95 were simultaneously attained for TPCz2NP. The organic light-emitting diodes fabricated with this material exhibit a high maximum external quantum efficiency of 25.4 % with an elevated 2PA cross-section (σ2) value up to 143 GM at 850 nm. These findings offer a venue for designing high-performance TADF emitters with exceptional performance inclusive of 2PA properties, expanding for future functional material design.

The Golden Age of Thermally Activated Delayed Fluorescence Materials: Design and Exploitation

Since the seminal report by Adachi and co-workers in 2012, there has been a veritable explosion of interest in the design of thermally activated delayed fluorescence (TADF) compounds, particularly as emitters for organic light-emitting diodes (OLEDs). With rapid advancements and innovation in materials design, the efficiencies of TADF OLEDs for each of the primary color points as well as for white devices now rival those of state-of-the-art phosphorescent emitters. Beyond electroluminescent devices, TADF compounds have also found increasing utility and applications in numerous related fields, from photocatalysis, to sensing, to imaging and beyond. Following from our previous review in 2017 ( Adv. Mater. 2017, 1605444), we here comprehensively document subsequent advances made in TADF materials design and their uses from 2017-2022. Correlations highlighted between structure and properties as well as detailed comparisons and analyses should assist future TADF materials development. The necessarily broadened breadth and scope of this review attests to the bustling activity in this field. We note that the rapidly expanding and accelerating research activity in TADF material development is indicative of a field that has reached adolescence, with an exciting maturity still yet to come.

Highly Efficient Near-Infrared Luminescent Radicals with Emission Peaks over 750 nm

Purely organic molecules exhibiting near-infrared (NIR) emission possess considerable potential for applications in both biological and optoelectronic technological domains, owing to their inherent advantages such as cost-effectiveness, biocompatibility, and facile chemical modifiability. However, the repertoire of such molecules with emission peaks exceeding 750 nm and concurrently demonstrating high photoluminescence quantum efficiency (PLQE) remains relatively scarce due to the energy gap law. Herein, we report two open-shell NIR radical emitters, denoted as DMNA-Cz-BTM and DMNA-PyID-BTM, achieved through the strategic integration of a donor group (DMNA) onto the Cz-BTM and PyID-BTM frameworks, respectively. We found that the donor-acceptor molecular structure allows the two designed radical emitters to exhibit a charge-transfer excited state and spatially separated electron and hole levels with non-bonding characteristics. Thus, the high-frequency vibrations are effectively suppressed. Besides, the reduction of low-frequency vibrations is observed. Collectively, the non-radiative decay channel is significantly suppressed, leading to exceptional NIR PLQE values. Specifically, DMNA-Cz-BTM manifests an emission peak at 758 nm alongside a PLQE of 55 %, whereas DMNA-PyID-BTM exhibits an emission peak at 778 nm with a PLQE of 66 %. Notably, these represent the pinnacle of PLQE among metal-free organic NIR emitters with emission peaks surpassing 750 nm.

Boosting the Efficiency of Red Thermally Activated Delayed Fluorescence via Conjugation Enhancement in Push-Pull Naphthalimide Derivatives

In this work, we synthesize a series of push-pull compounds bearing naphthalimide as the electron acceptor and tetraphenylethylene (TPE)/triphenylamine (TPA)/phenothiazine (PTZ) as the electron rich/electron donor units. These moieties are arranged in highly conjugated quadrupolar structures. The structure-property relationships are investigated through a joint experimental time-resolved spectroscopic and computational TD-DFT study. The femtosecond transient absorption and fluorescence up-conversion experiments reveal ultrafast photoinduced intramolecular charge transfer. This is likely the key factor leading to efficient spin-orbit CT-induced intersystem crossing for the TPA- and PTZ-derivatives as well as to small singlet-to-triplet energy gap. Consequently, evidence for a delayed fluorescence component is found together with the main prompt emission in the fluorescence kinetics both in solution and in thin film. The weight of the Thermally Activated Delayed Fluorescence (TADF) is greatly enhanced when these fluorophores are used as guests in solid-state host matrices. TADF is interestingly revealed in the orange-red region of the visible. Such long wavelength emission is here observed with surprisingly large fluorescence quantum yields, thanks to the conjugation enhancement achieved in these newly synthesized structures relative to previous studies. Our findings may be thus promising for the future development of efficient third generation TADF-based OLEDs.

Capturing the Interplay Between TADF and RTP Through Mechanically Flexible Polymorphic Optical Waveguides

Polymorphism plays a pivotal role in generating a range of crystalline materials with diverse photophysical and mechanical attributes, all originating from the same molecule. Here, we showcase two distinct polymorphs: green (GY) emissive and orange (OR) emissive crystals of 5'-(4-(diphenylamino)phenyl)-[2,2'-bithiophene]-5-carbaldehyde (TPA-CHO). These polymorphs display differing optical characteristics, with GY exhibiting thermally activated delayed fluorescence (TADF) and OR showing room temperature phosphorescence (RTP). Additionally, both polymorphic crystals display mechanical flexibility and optical waveguiding capabilities. Leveraging the AFM-tip-based mechanophotonics technique, we position the GY optical waveguide at varying lengths perpendicular to the OR waveguide. This approach facilitates the exploration of the interplay between TADF and RTP phenomena by judiciously controlling the optical path length of crystal waveguides. Essentially, our approach provides a clear pathway for understanding and controlling the photophysical processes in organic molecular crystals, paving the way for advancements in polymorphic crystal-based photonic circuit technologies.

Boosting External Quantum Efficiency to 12.0 % of an Ultraviolet OLED by Engineering the Horizontal Dipole Orientation of a Hot Exciton Emitter

Currently, much research effort has been devoted to improving the exciton utilization efficiency and narrowing the emission spectra of ultraviolet (UV) fluorophores for organic light-emitting diode (OLED) applications, while almost no attention has been paid to optimizing their light out-coupling efficiency. Here, we developed a linear donor-acceptor-donor (D-A-D) triad, namely CDFDB, which possesses high-lying reverse intersystem crossing (hRISC) property. Thanks to its integrated narrowband UV photoluminescence (PL) (λPL: 397 nm; FWHM: 48 nm), moderate PL quantum yield (ϕPL: 72 %, Tol), good triplet hot exciton (HE) conversion capability, and large horizontal dipole ratio (Θ//: 92 %), the OLEDs based on CDFDB not only can emit UV electroluminescence with relatively good color purity (λEL: 398 nm; CIEx,y: 0.161, 0.040), but also show a record maximum external quantum efficiency (EQEmax) of 12.0 %. This study highlights the important role of horizontal dipole orientation engineering in the molecular design of HE UV-OLED fluorophores.

Intermolecular charge transfer and solid-state solvent effect synergistically induce near-infrared thermally activated delay fluorescence in the guest-host system

The host matrix is an important means to tune emission color and improve luminescence efficiency of near-infrared (NIR) thermally activated delay fluorescence (TADF) light-emitting diodes. However, the mechanism of NIR TADF of the guest-host systems is still unclear. Namely, there is a controversy on whether the formation of J-aggregation, solid-state solvent effect, molecular polarization or intermolecular charge transfer (CT) is responsible for the NIR TADF. Here, the morphologies, geometrical and electronic structures, and photophysical properties are explored by combining molecular dynamics simulation, density functional theory and thermal vibration correlation function theory for the guest-host (TPAAP: TPBi) films with different concentrations. It is found that the red TADF is generated largely by the solid-state solvent effect in the low 1 wt% doped film while the NIR TADF is attributed to the synergistic effect of solid-state solvent and guest-guest intermolecular CT in the high 20 wt% film. These findings provide a deeper understanding of the mechanism of NIR-TADF of the guest-host systems.

Palladium(II)-Catalyzed Tandem γ-C(sp2)-H Arylation and Cyclization for the Construction of Natural Product-Like Polycyclic Fused ortho-Quinone Scaffolds

Here, we report a novel strategy for the direct construction of polycyclic fused ortho-quinone scaffolds through palladium(II)-catalyzed tandem γ-C(sp2)-H arylation and cyclization of arylglyoxals with aryl iodides. This transformation features unique tandem transient directing of γ-C(sp2)-H arylation and cyclization reaction mode, broad substrate scope, especially for the aromatic substrates containing oxygen and sulfur atoms, and avoiding the common issue of aromatization due to the construction of the hexatomic ring.

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.

Three-dimensional sensing of surfaces by projection of invisible electroluminescence from organic light-emitting diodes

Organic light-emitting diodes (OLEDs) that efficiently emit near-infrared (NIR) light and consume little power will create valuable applications for OLEDs beyond just displays. Here, we report such a NIR-OLED with high operational stability that can be used as a light source for three-dimensional sensing of object's surfaces. Using a narrow-energy-gap material as a host for producing NIR hyperfluorescence system, we fabricated a NIR-OLED exhibiting intense emission at 930 nm with a high external electroluminescence quantum efficiency of more than 1% at a current density of 100 milliamperes per square meter without any degradation even after more than 300 hours of operation. The NIR-OLEDs were integrated with dense complementary metal-oxide semiconductor circuits to make a micro-NIR-OLED projector (0.21 inch, 230,400 pixels). By actively driving the projector on a pixel by pixel and projecting their emission onto objects, we successfully scanned and sensed the surfaces in three dimensions with invisible NIR.

Azaacene containing iridium(III) phosphors: elaboration of the π-conjugation effect and application in highly efficient solution-processed near-infrared OLEDs

Azaacenes have attracted wide research interest due to their tremendous potential in organic electronics. However, near-infrared (NIR) light-emitting iridium(III) phosphors bearing azaacene derivatives are rarely investigated. In this contribution, two solution-processable heteroleptic iridium(III) complexes, namely DBPzIr and PPzIr, are rationally designed and synthesized, and they contain a rigid phenanthrene- or pyrene-fused diazaacene core and two peripheral groups of 4-tert-butyl-phenyl attached at the 12,13-positions in the core, respectively. The effects of the diazaacene core and appending groups on the optoelectronic properties of both complexes are systematically investigated. A dramatically red-shifted NIR emission peak at 789 nm with a photoluminescence quantum yield (PLQY) of 14% is observed in PPzIr compared with the 746 nm emission with a PLQY of 40% in DBPzIr. Taking advantage of their photophysical properties, the solution-processed device doped with DBPzIr achieves a maximum external quantum efficiency (EQEmax) of 8.00% with a radiance of 54 866 mW Sr-1 m-2 at 716 nm and the device doped with PPzIr exhibits a significantly red-shifted emission at 772 nm with an EQEmax of 3.53%. The achieved device performance is among the best values in the reported NIR-OLEDs based on iridium(III) complexes via a solution process at the same color gamut. Our study indicates that the reasonable collocation of the rigid diazaacene chelating core and flexible peripheral groups in the iridium(III) complex is of great significance in designing highly efficient NIR emitters.

Carbon Dots for Electroluminescent Light-Emitting Diodes: Recent Progress and Future Prospects

Carbon dots (CDs), as emerging carbon nanomaterials, have been regarded as promising alternatives for electroluminescent light-emitting diodes (LEDs) owing to their distinct characteristics, such as low toxicity, tuneable photoluminescence, and good photostability. In the last few years, despite remarkable progress achieved in CD-based LEDs, their device performance is still inferior to that of well-developed organic, heavy-metal-based QDs, and perovskite LEDs. To better exploit LED applications and boost device performance, in this review, a comprehensive overview of currently explored CDs is presented, focusing on their key optical characteristics, which are closely related to the structural design of CDs from their carbon core to surface modifications, and to macroscopic structural engineering, including the embedding of CDs in the matrix or spatial arrangement of CDs. The design of CD-based LEDs for display and lighting applications based on the fluorescence, phosphorescence, and delayed fluorescence emission of CDs is also highlighted. Finally, it is concluded with a discussion regarding the key challenges and plausible prospects in this field. It is hoped that this review inspires more extensive research on CDs from a new perspective and promotes practical applications of CD-based LEDs in multiple directions of current and future research.

Efficient and Bright Organic Radical Light-Emitting Diodes with Low Efficiency Roll-Off

Organic radicals have been of interest due to their potential to replace nonradical-based organic emitters, especially for deep-red/near-infrared (NIR) electroluminescence (EL), based on the spin-allowed doublet fluorescence. However, the performance of the radical-based EL devices is limited by low carrier mobility which causes a large efficiency roll-off at high current densities. Here, highly efficient and bright doublet EL devices are reported by combining a thermally activated delayed fluorescence (TADF) host that supports both electron and hole transport and a tris(2,4,6-trichlorophenyl)methyl-based radical emitter. Steady-state and transient photophysical studies reveal the optical signatures of doublet luminescence mechanisms arising from both host and guest photoexcitation. The host system presented here allows balanced hole and electron currents, and a high maximum external quantum efficiency (EQE) of 17.4% at 707 nm peak emission with substantially improved efficiency roll-off is reported: over 70% of the maximum EQE (12.2%) is recorded at 10 mA cm-2 , and even at 100 mA cm-2 , nearly 50% of the maximum EQE (8.4%) is maintained. This is an important step in the practical application of organic radicals to NIR light-emitting devices.

Narrowband Pure Near-Infrared (NIR) Ir(III) Complexes for Solution-Processed Organic Light-Emitting Diode (OLED) with External Quantum Efficiency Over 16

Highly efficient near-infrared (NIR) emitters have significant applications in medical and optoelectronic fields, but the development stays a great challenge due to the energy gap law. Here, we report two NIR phosphorescent Ir(III) complexes which display emission peaks around 730 nm with a narrow full width at half maximum of only 43 nm. Therefore, pure NIR luminescence can be obtained without having a very long emission wavelength, thus alleviating the restriction of the energy gap law, and obtaining impressively high photoluminescence quantum yield up to 0.70. More importantly, the pure NIR organic light-emitting diode (OLED) fabricated by the solution-processed mothed shows outstanding device performance with the highest external quantum efficiency of 16.43 %, which sets a new record for solution-processed NIR-OLEDs based on different emitters. This work sheds light on the development of Ir(III) complexes with narrowband emissions as highly efficient pure NIR-emitters.

A Simple Molecular Design Strategy for Luminescent Radicals to Achieve Near-Infrared Emission

The spin-allowed doublet emission of luminescent radicals has recently attracted significant attention. However, the spectral range of most reported luminescent radical emitters and their corresponding organic light-emitting diodes (OLEDs) is confined to the red and deep red regions, with only a few extending to the near-infrared region, specifically in the context of an emission peak exceeding 800 nm. Herein, a luminescent radical, 2-(4-(bis(2,4,6-trichlorophenyl)methyl radical)-3,5-dichlorophenyl)-4-phenyl-4H-thieno[3,2-b]indole (TTM-2PTI), with NIR emission peaking at 830 nm in toluene, was obtained through attaching a 4-phenyl-4H-thieno[3,2-b]indole group to the TTM radical core. An organic light-emitting diode (OLED) utilizing TTM-2PTI as the emitter exhibits electroluminescence (EL) emission peaking at 870 nm, which is the longest EL wavelength among the doublet-emissive near-infrared (NIR) OLEDs. This work provides a simple molecular design strategy to achieve NIR emission of radicals by leveraging the lower steric hindrance and electron-donating ability of thiophene.

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.

Survey of Recent Advances in Molecular Fluorophores, Unconjugated Polymers, and Emerging Functional Materials Designed for Electrofluorochromic Use

In this review, recent advances that exploit the intrinsic emission of organic materials for reversibly modulating their intensity with applied potential are surveyed. Key design strategies that have been adopted during the past five years for developing such electrofluorochromic materials are presented, focusing on molecular fluorophores that are coupled with redox-active moieties, intrinsically electroactive molecular fluorophores, and unconjugated emissive organic polymers. The structural effects, main challenges, and strides toward addressing the limitations of emerging fluorescent materials that are electrochemically responsive are surveyed, along with how these can be adapted for their use in electrofluorochromic devices.

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).

Thermally Activated Delayed Near-Infrared Photoluminescence from Functionalized Lead-Free Nanocrystals

As an analogue to thermally activated delayed fluorescence (TADF) of organic molecules, thermally activated delayed photoluminescence (TADPL) observed in molecule-functionalized semiconductor nanocrystals represents an exotic mechanism to harvest energy from dark molecular triplets and to obtain controllable, long-lived PL from nanocrystals. The reported TADPL systems have successfully covered the visible spectrum. However, TADF molecules already emit very efficiently in the visible, diminishing the technological impact of the less-efficient nanocrystal-molecule TADPL. Here we report bright, near-infrared TADPL in lead-free CuInSe₂ nanocrystals functionalized with carboxylated tetracene ligands, which results from efficient triplet energy transfer from photoexcited nanocrystals to ligands, followed with thermally activated reverse energy transfer from ligand triplets back to nanocrystals. This strategy prolonged the nanocrystal exciton lifetime from 100 ns to 60 μs at room temperature.

Uncovering the Mechanism of Thermally Activated Delayed Fluorescence in Coplanar Emitters Using Potential Energy Surface Analysis

Planarized emitters exhibiting thermally activated delayed fluorescence (TADF) have attracted attention due to their narrow emission spectra, improved photostability, and high quantum yields, but with large singlet-triplet energy gaps (ΔE_ST) and no heavy atoms, the origin of their TADF remains a subject of debate. Here we prepare two isomeric, coplanar donor-acceptor compounds, with HMAT-2PYM performing dual TADF and room-temperature phosphorescence but with HMAT-4PYM exhibiting only prompt fluorescence. Although conventional TADF design principles suggest that neither isomer should exhibit TADF, we reveal differences in the excited state potential energy surfaces that enable spin-flip processes in only one isomer. We also find that hydrogen bonding is absent between the planar units of these emitters, despite earlier claims of intramolecular hydrogen bonding in similar compounds. Overall, this work demonstrates that potential energy surface analysis is a practical strategy for designing coplanar TADF materials that might otherwise be overlooked by conventional TADF design metrics.

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.

Tuning the Properties of Donor-Acceptor and Acceptor-Donor-Acceptor Boron Difluoride Hydrazones via Extended π-Conjugation

Molecular materials with π-conjugated donor-acceptor (D-A) and acceptor-donor-acceptor (A-D-A) electronic structures have received significant attention due to their usage in organic photovoltaic materials, in organic light-emitting diodes, and as biological imaging agents. Boron-containing molecular materials have been explored as electron-accepting units in compounds with D-A and A-D-A properties as they often exhibit unique and tunable optoelectronic and redox properties. Here, we utilize Stille cross-coupling chemistry to prepare a series of compounds with boron difluoride hydrazones (BODIHYs) as acceptors and benzene, thiophene, or 9,9-dihexylfluorene as donors. BODIHYs with D-A and A-D-A properties exhibited multiple reversible redox waves, solid-state emission with photoluminescence quantum yields up to 10%, and aggregation-induced emission (AIE). Optical band gaps (or highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) gaps) determined for these compounds (2.02-2.25 eV) agree well with those determined from cyclic voltammetry experiments (2.05-2.42 eV). The optoelectronic properties described herein are rationalized with density functional theory calculations that support the interpretation of the experimental findings. This work provides a foundation of understanding that will allow for the consideration of D-A and A-D-A BODIHYs to be incorporated into applications (e.g., organic electronics) where fine-tuning of band gaps is required.

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.

Introduction of a triphenylamine substituent on pyridyl rings as a springboard for a new appealing brightly luminescent 1,3-di-(2-pyridyl)benzene platinum(II) complex family

The preparation and characterization of three new complexes, namely [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-4,6-difluoro-benzene)Cl] ([PtL1Cl]), [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-5-triphenylamine-benzene)Cl] ([PtL2Cl]), and [Pt(1,3-bis(4-triphenylamine-pyridin-2-yl)-5-mesityl-benzene)Cl] ([PtL3Cl]), is reported. All of them are highly luminescent in dilute deaerated dichloromethane solution (Φ_lum = 0.88-0.90, in the yellow-green region; the λ_max,em in nm for the monomers are: 562, 561 and 549 for [PtL1Cl], [PtL2Cl] and [PtL3Cl], respectively).[PtL1Cl] is the most appealing, being characterized by a very long lifetime (103.9 μs) and displaying intense NIR emission in concentrated deaerated solution (Φ_lum = 0.66) with essentially no "contamination" by visible light < 600 nm. This complex allows the fabrication of both yellow-green and deep red/NIR OLEDs; OLED emissions are in the yellow-green (CIE = 0.38, 0.56) and deep red/NIR (CIE = 0.65, 0,34) regions, for [PtL1Cl] 8 wt% (with 11% ph/e EQE) and pure [PtL1Cl] (with 4.3% ph/e EQE), respectively.

Acceptor-Donor-Acceptor π-Stacking Boosts Intramolecular Through-Space Charge Transfer towards Efficient Red TADF and High-Performance OLEDs

Organic push-pull systems featuring through-space charge transfer (TSCT) excited states have been disclosed to be capable of exhibiting thermally activated delayed fluorescence (TADF), but to realize high-efficiency long-wavelength emission still remains a challenge. Herein, we report a series of strongly emissive orange-red and red TSCT-TADF emitters having (quasi)planar and rigid donor and acceptor segments which are placed in close proximity and orientated in a cofacial manner. Emission maxima (λ_em) of 594−599 nm with photoluminescence quantum yields (PLQYs) of up to 91% and delayed fluorescence lifetimes of down to 4.9 μs have been achieved for new acceptor-donor-acceptor (A-D-A) molecules in doped thin films. The presence of multiple acceptors and the strong intramolecular π-stacking interactions have been unveiled to be crucial for the efficient low-energy TSCT-TADF emissions. Organic light-emitting diodes (OLEDs) based on the new A-D-A emitters demonstrated electroluminescence with maximum external quantum efficiencies (EQEs) of up to 23.2% for the red TSCT-TADF emitters. An EQE of 18.9% at the brightness of 1000 cd m⁻² represents one of the highest values for red TADF OLEDs. This work demonstrates a modular approach for developing high-performance red TADF emitters through engineering through-space interactions, and it may also provide implications to the design of TADF emitter with other colours.

Boron-Based Multi-Resonance TADF Emitter with Suppressed Intermolecular Interaction and Isomer Formation for Efficient Pure Blue OLEDs

Multi-resonance (MR) thermally activated delayed fluorescent (TADF) emitters are highly attractive due to their superior color purity as well as efficient light-harvesting ability from singlets and triplets. However, boron and nitrogen-based MR-TADF emitters suffer from their strong π-π interaction owing to their rigid flat cores. Herein, a boron-based multi-resonance blue TADF emitter with suppressed intermolecular interaction and isomer formation is developed through a simple synthetic process by introducing meta-xylene and meta-phenyphenyl groups to the core. The MR-TADF emitter, mBP-DABNA-Me, shows a narrowband blue emission with a peak at 467 nm, along with full width at half maximum of 28 nm, and photoluminescence quantum yield of 97%. Notably, highly efficient pure blue organic light-emitting diode (OLED) is realized using mBP-DABNA-Me, showing a maximum external quantum efficiency of 24.3% and a stable blue emission with a Commission Internationale de L'Eclairage coordinate of (0.124, 0.140). The color purity of the OLED is maintained at a high doping concentration of over 20%, attributed to the suppressed intermolecular interaction between the MR emitters.

Optimizing Intermolecular Interactions and Energy Level Alignments of Red TADF Emitters for High-Performance Organic Light-Emitting Diodes

Adequately harvesting all excitons in a single molecule and inhibiting exciton losses caused by intermolecular interactions are two important factors for achieving high efficiencies thermally activated delayed fluorescence (TADF). One potential approach for optimizing these is to tune alignment of various excited state energy levels by using different doping concentrations. Unfortunately, emission efficiencies of most TADF emitters decrease rapidly with concentrations which limits the window for energy level tunning. In this work, by introducing a spiro group to increase steric hindrance of a TADF emitter (BPPXZ) with a phenoxazine and a dibenzo[a,c]phenazine, emission efficiency of the resulting molecule (BPSPXZ) is much less affected by concentration increase. This enables exploitation of the concentration effects to tune energy levels of its excited states for obtaining simultaneously small singlet-triplet energy offset and large spin-orbital coupling, leading to high-efficiency reverse intersystem crossing. With these merits, organic light-emitting diodes (OLEDs) using the BPSPXZ emitter from 5 to 60 wt% doping can all deliver EQE of over 20%. More importantly, record-high EQEs of 33.4% and 15.8% are respectively achieved in the optimized and nondoped conditions. This work proposes a strategy for developing red TADF emitters by optimizing the intermolecular interaction and energy level alignments to facilitate exciton utilization over wide doping concentrations.

Donor manipulation for constructing a pH sensing thermally activated delayed fluorescent probe to detect alkaliphiles

pH homeostasis is essential for alkaliphiles, given their widespread use in biotechnological applications. However, quantitative monitoring of alkaline pH in alkaliphiles remains challenging. Here, we synthesized for the first time, a thermally activated delayed fluorescent (TADF) pH probe: NI-D-OH. Our probe exhibits a good linear relationship between fluorescence intensity and pH in the neutral to alkaline range (pH 7.0-8.6), as well as long-lived TADF emission. We further show that NI-D-OH can be used to monitor intracellular pH in living organisms, and evaluate the effect of Na⁺ on the pH homeostasis, demonstrating the potential for alkaline pH monitoring and time-resolved fluorescence imaging.

Donor-Acceptor Molecule Based High-Performance Photothermal Organic Material for Efficient Water Purification and Electricity Generation

In this contribution, a unique donor-acceptor conjugated organic-small-molecule photothermal material, namely GDPA-QCN, is presented. Bulky dendritic triphenylamine (GDPA) was grafted onto quinoxaline-6,7-dicarbonitrile (QCN) with a phenyl ring as a bridge to form an "umbrella" architecture. Benefited from the particular molecular structure, in solid state, GDPA-QCN molecules adopted a loose packing mode due to the steric effect of "umbrella head" dendritic triphenylamine and flexible molecular structure feature, which allows efficient intramolecular motions and consequently elevates energy dissipation by taking the pathway of thermal deactivation within broad absorption range. The GDPA-QCN solid has high solar-thermal conversion efficiency with an absorption range from 300 to 1100 nm, which can promote superior water purification and electricity generation performance.

An extended π-backbone for highly efficient near-infrared thermally activated delayed fluorescence with enhanced horizontal molecular orientation

Near-infrared thermally activated delayed fluorescence (NIR-TADF) materials with emission over 700 nm have been insufficiently investigated mainly due to the limited choice of strong donor/acceptor units for molecular construction and the limited electronic coupling between the donors and acceptors. Herein, a novel D-A1-A2-A3 configuration was developed for the design of a NIR-TADF material (TPA-CN-N4-2PY), in which three types of sub-acceptor units (CN: cyano; N4: dipyrido[3,2-a:2',3'-c]phenazine; PY: pyridine) were incorporated into a molecular skeleton to reinforce the electron-accepting strength. The attachment of two pyridine units on TPA-CN-N4 produced TPA-CN-N4-2PY with an extended π-backbone, which shifted the electroluminescence (EL) emission into the NIR region and enhanced the horizontal ratio of emitting dipole orientation (Θ_//) simultaneously. TPA-CN-N4-2PY-based OLEDs demonstrated a record-high external quantum efficiency (EQE) of 21.9% with an emission peak at 712 nm and Θ_// = 85% at the doping ratio of 9.0 wt%. On the contrary, the parent compound TPA-CN-N4-based OLEDs at the same doping ratio achieved an EQE of 23.4% at 678 nm with Θ_// = 75%. This multiple sub-acceptors approach could enrich the design strategy of the NIR-TADF materials, and the large conjugated system could improve the Θ_// for achieving efficient emitters.

Synthesis, Aromaticity, and Application of peri-Pentacenopentacene: Localized Representation of Benzenoid Aromatic Compounds

We report the synthesis and optoelectronic properties of TIPS-peri-pentacenopentacene (TIPS-PPP), a vertical extension of TIPS-pentacene (TIPS-PEN) and a low-band-gap material with remarkable stability. We found the synthetic conditions to avoid the competition between 1,2- and 1,4-addition of lithium acetylide on the large aromatic dione. The high stability of TIPS-PPP is due to the peri-fusion which increases the aromaticity by generating two localized aromatic sextets that are flanked with 2 diene fragments, similar to two fused-anthracenes. Like TIPS-PEN, TIPS-PPP shows the archetypal 2D brickwall motif in crystals with a larger transfer integral and smaller reorganization energy. The high mobility of up to 1 cm²  V^-1  s^-1 was obtained in an organic field-effect transistor fabricated by a wet process. Also, TIPS-PPP was used as a near-infrared (NIR) emitter for NIR organic-light-emitting-diode devices resulting in a high external quantum efficiency at 800 nm.

Efficient deep red/near-infrared thermally activated delayed fluorescence emitters via molecular reconstruction: theoretical insights

Molecular bonding and molecular fusion can achieve an effective red-shift of intrinsic emission and efficient DR and NIR emission.

Tandem rigidification and π-extension as a key tool for the development of a narrow linewidth yellow hyperfluorescent OLED system

Hyperfluorescence (HF), a relatively new phenomenon utilizing the transfer of excitons between two luminophores, requires careful pairwise tuning of molecular energy levels and is proposed to be the crucial step towards the development of new, highly effective OLED systems. To date, barely few HF yellow emitters with desired narrowband emission but moderate external quantum efficiency (EQE < 20%) have been reported. This is because a systematic strategy embracing both Förster resonance energy transfer (FRET) and triplet to singlet (TTS) transition as complementary mechanisms for effective exciton transfer has not yet been proposed. Herein, we present a rational approach, which allows, through subtle structural modification, a pair of compounds built from the same donor and acceptor subunits, but with varied communication between these ambipolar fragments, to be obtained. The TADF-active dopant is based on a naphthalimide scaffold linked to the nitrogen of a carbazole moiety, which through the introduction of an additional bond leads not only to π-cloud enlargement, but also rigidifies and inhibits the rotation of the donor. This structural change prevents TADF, and guides bandgaps and excited state energies to simultaneously pursue FRET and TTS processes. New OLED devices utilizing the presented emitters show excellent external quantum efficiency (up to 27%) and a narrow full width at half maximum (40 nm), which is a consequence of very good alignment of energy levels. The presented design principles prove that only a minor structural modification is needed to obtain commercially applicable dyes for HF OLED devices.

The rigidification with simultaneous π-extension of TADF-active dye leads to fluorescent dopant with fine-tuned energy levels. These used as hyperfluorescent OLED device shows extraordinary EQE and brightness due to effective FRET and TTS processes.