Delayed Fluorescence Emitters Research Articles

Thermally Activated Delayed Fluorescence Emitters Based on a Special Tetrahedral Silane Core.

Based on the tetraphenylsilane skeleton, a new class of thermally activated delayed fluorescence (TADF) molecules have been designed and synthesized. Benefiting from the unique tetrahedron architecture of tetraphenylsilane, the intermolecular distance between TADF units can be enlarged and thus weakened the aggregation-induced quenching of triplet excitons. By adjusting the numbers of TADF subunits, the spin-orbit coupling processes can be controlled, leading to efficient up-conversion processes. The related OLEDs are fabricated through the solution processing technology, and pure-blue and green electroluminescence were observed with maximum external quantum efficiencies (EQEmax) of 6.6 and 13.8% as well as Commission Internationale de l'Eclairage coordinates of (0.14, 0.15) and (0.25, 0.45), respectively. This study provides a new idea for designing color-tunable TADF emitters through spatial structure regulation.

A chiral spirofluorene‐embedded multiple‐resonance thermally activated delayed fluorescence emitter for efficient pure‐green circularly polarized electroluminescence

AbstractThe simultaneous achievement of chiral multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters with narrowband and circularly polarized electroluminescence (CPEL) poses a challenge. Herein, a MR‐TADF emitter, Spiro‐BNCz, embedding spirofluorene structure was developed and chiral separated, whose emission peaks at 528 nm in toluene with Commission Internationale de L'Eclairage coordinates of (0.26, 0.69). The conjugation extension caused by embedding sulfur substituted spirofluorene on B/N framework shortens the singlet‐triplet energy gap and increases spin‐orbital coupling matrix element. Therefore, a fast reverse intersystem crossing rate constant of 2.8 × 106 s−1 and a high photoluminescence efficiency of 92% in film were achieved. The organic light‐emitting diode (OLED) exhibits a maximum external quantum efficiency of 32.3%. Particularly, the circularly polarized OLEDs based on Spiro‐BNCz enantiomers show symmetric CPEL with dissymmetry factors (|gEL|) ≈ 10−3.

Enhancing operational stability of OLEDs based on subatomic modified thermally activated delayed fluorescence compounds

The realization of operationally stable blue organic light-emitting diodes is a challenging issue across the field. While device optimization has been a focus to effectively prolong device lifetime, strategies based on molecular engineering of chemical structures, particularly at the subatomic level, remains little. Herein, we explore the effect of targeted deuteration on donor and/or acceptor units of thermally activated delayed fluorescence emitters and investigate the structure-property relationship between intrinsic molecular stability, based on isotopic effect, and device operational stability. We show that the deuteration of the acceptor unit is critical to enhance the photostability of thermally activated delayed fluorescence compounds and hence device lifetime in addition to that of the donor units, which is commonly neglected due to the limited availability and synthetic complexity of deuterated acceptors. Based on these isotopic analogues, we observe a gradual increase in the device operational stability and achieve the long-lifetime time to 90% of the initial luminance of 23.4 h at the luminance of 1000 cd m−2 for thermally activated delayed fluorescence-sensitized organic light-emitting diodes. We anticipate our strategic deuteration approach provides insights and demonstrates the importance on structural modification materials at a subatomic level towards prolonging the device operational stability.

Electron donor–acceptor type delayed fluorescence emitters with inverted singlet and triplet excited states

Emitters having a negative energy difference between their lowest singlet and triplet excited states are promising candidates for developing next-generation organic light-emitting diodes (OLEDs) because their inverted singlet and triplet (IST) excited states enable exothermic reverse intersystem crossing, which results in a fast rate constant for reverse intersystem crossing and facilitates the efficient harvesting of triplet excitons. This study reports the development of organic emitters comprising 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene as an electron acceptor and 10H-spiro[acridine-9,9′-fluorene] as an electron donor that exhibit IST-excited states. We confirmed the presence of IST excited states in our emitters using photoluminescence spectroscopy, time-correlated single-photon counting, and excited-state density functional theory calculations. In addition, by varying the number of electron donors and the dihedral angles between the electron donor and acceptor, we identified the underlying reasons for the inversion of the singlet and triplet states. Furthermore, we achieved high external quantum efficiencies of up to 30 % by utilizing the IST emitters as emitting layers in OLEDs.

Thermally activated delayed fluorescence emitters with bulky four-coordinate boron acceptors for high-performance solution-processed OLEDs

Although considerable progress has been made in vacuum-deposited thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs), the development of inexpensive solution-processed TADF-OLEDs still lags, primarily owing to low luminous efficiency. Herein, we designed and synthesized two TADF emitters by introducing bulky four-coordinate boron into pyrimidine acceptor, producing a simple and versatile approach to dramatically enhance performance. The introduction of bulky four-coordinate boron substantially reduces the singlet–triplet energy gap and improves photoluminescent quantum yield as well as improves solubility by inhibiting intermolecular aggregation or crystallization, resulting in an excellent TADF characteristics and highly efficient solution-processed OLEDs. Finally, the Spiro-PMB-SPAc emitter provides an OLED device with a remarkable maximum external quantum efficiency of 38.21 %, which reaches 32.50 % at a luminance of 500 cd m−2. This state-of-the-art performance validates the superiority of the boron-nitrogen coordination strategy in the reconstruction of pyrimidine-based TADF molecules for highly efficient solution-processed OLEDs.

Marching Toward Long‐Wavelength Narrowband Emissive Multi‐Resonance Delayed Fluorescence Emitters for Organic Light Emitting Diodes

AbstractOver the past decade, thermally activated delayed fluorescence (TADF) emitters have garnered tremendous impetus because of their ability to harvest 100% excitons for the light emission in organic light emitting diodes (OLEDs). However, despite their superior external quantum efficiencies (> 35%), the broad emission spectra with associated full‐width‐at‐half maximum (FWHM > 70 nm) present a limiting factor that must be solved. Recently, multiple‐resonance TADF (MR‐TADF) materials based on the heteroatom doped polyaromatic hydrocarbons have gained astonishing attention owing to their remarkable narrowband emission (FWHM < 30 nm). However, emission of the majority of reported MR‐TADF emitters falls in the blue/green region, which inevitably jeopardizes their application in full‐color OLEDs. Therefore, there is an urgent need to develop the new molecular designs for expanding the color‐gamut of MR‐TADF emitters, i.e., λem > 550 nm without compromising the narrowband emission. To the best of current knowledge, no detailed reviews focusing on the different design strategies for producing long‐wavelength (> 550 nm) MR‐TADF emitters have been reported to date. To this end, a review highlighting the recent design advances for constructing long‐wavelength MR‐TADF emitters is presented, and their photophysics and OLED performance is discussed. Finally, the current status and future prospects of long‐wavelength MR‐TADF materials are discussed.

A Design Strategy for Multiple Resonance‐Induced Pure Violet Thermally Activated Delayed Fluorescence Emitters with a Narrow Emission Band

AbstractThis study proposes a novel approach to develop highly efficient, narrow‐emitting violet materials based on boron and oxygen polycyclic aromatic hydrocarbon multiple resonance structure. Herein, B‐2OCz is developed by fusing indole with a 5,9‐dioxa‐13bboranaphtho[3,2,1‐de]anthracene (DOBNA) core to enhance its thermally activated delayed fluorescence (TADF) properties and molecular rigidity. On the other hand, the B‐2OCz‐Si is decorated with a bulky tetraphenylsilyl substituent. B‐2OCz‐Si exhibits exceptional features such as violet emission at 397 nm, a very small full width at half maximum of 16 nm, and 82% of photoluminescence quantum yield. The B‐2OCz‐Si devices achieve a high external quantum efficiency of over 15%, violet emission with a peak wavelength of 423 nm, and color coordinates of (0.156, 0.037). Furthermore, the B‐2OCz‐Si is used as an electron transport type host material for phosphorescent organic light‐emitting diodes (PhOLEDs), based on its high triplet energy and TADF properties. As compared to the conventional triazine based host materials, these newly developed DOBNA‐based materials display superior device lifetime performance. All these potential aspects corroborate that this new class of DOBNA‐based materials can work as a promising host material for PhOLEDs and violet‐emitting fluorescent devices.

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.

Red thermally activated delayed fluorescence emitters based on phenazine-derived electron donors and solution-processed OLEDs

Despite recent advancements in red thermally activated delayed fluorescence (TADF) emitters, most reported red TADF emitters feature rigid acceptor units, which make them insoluble and unsuitable for solution-processed organic light-emitting diodes (OLEDs). To tackle this issue, two new TADF emitters, 2tBuInPz-BP33 and 2tBulnPz-XO33, have been designed by connecting a novel phenazine-based electron-donor to the derivative of benzophenone acceptor. The highly twisted donor-acceptor structure, along with an appropriate distribution of the highest occupied molecular orbital/lowest unoccupied molecular orbital, has resulted in a very small singlet-triplet energy gap. Additionally, the peripheral tert-butyl-modified multi-ring phenazine donor unit increases the molecular solubility and enhances the electron-donating ability of donors. Finally, solution-processed OLEDs based on 2tBulnPz-XO33 exhibit a red emission peak at 626 nm and low efficiency roll-off, achieving an external quantum efficiency of 6.9%.

Highly Efficient Electrochemiluminescence from Imidazole-Based Thermally Activated Delayed Fluorescence Emitters.

A family of novel Thermally Activated Delayed Fluorescence (TADF) emitters have been synthetized by a straightforward and metal-free synthesis, and structurally characterized. In this work we keep fixed the acceptor moiety, 4-(1H-imidazol-1-yl)benzonitrile, and systemically tested different donors to correlate their photophysical and electrochemical properties with their performance in electrochemiluminescence using both benzoyl peroxide as coreactant and coreactant free (annihilation) conditions. Some compounds exceed the efficiency of the standard [Ru(bpy)3]Cl2 up to 28 times with benzoyl peroxide and 38 times in annihilation. Interestingly, we found that the efficiency is mainly dictated by the electrochemical reversibility of redox processes rather than by the photophysical properties in terms of photoluminescence quantum yield or excited state lifetime. In addition, the annihilation electrochemiluminescence efficiency strongly depend on the pulse sequence. The imidazole moiety can be conveniently alkylated allowing the insertion of functional groups, such a carboxylic acid, enabling practical applications.

(Invited) Structural and Dynamics Analysis of Organic Semiconducting Materials by Solid State NMR

Recently, our group have successfully developed several blue and green thermally activated delayed fluorescence (TADF) emitters showing high external quantum efficiencies in OLEDs through high-throughput screening based on quantum chemical calculations. Although the conformation, aggregated structure and dynamics of TADF emitters in amorphous states is expected to be the origin of reverse intersystem crossing and resulting high device performance, the experimental approach toward these information is still limited because typical diffraction technique cannot be used to amorphous materials due to the lack of long-range order. Thus we have carried out multiscale simulations and solid-state NMR (ssNMR) analysis of organic amorphous thin films. The ssNMR is one of the powerful technique to obtain the structural and dynamics information of amorphous aggregated materials. In this presentation, we demonstrate structural and dynamics analysis of amorphous organic semiconducting materials using ssNMR.Reference(1) “Analysis of Molecular Orientation in Organic Semiconducting Thin Films Using Static Dynamic Nuclear Polarization Enhanced Solid-State NMR Spectroscopy”, Suzuki, K.; Kubo, S.; Aussenac, F.; Engelke, F.; Fukushima, T.; Kaji, H. Angew. Chem. Int. Ed. 2017, 56, 14842.(2) “Efficient blue thermally activated delayed fluorescence emitters showing very fast reverse intersystem crossing”, Ren, Y., Wada, Y., Suzuki, K., Kusakabe, Y., Geldsetzer, J. and Kaji, H. Appl. Phys. Express, 2021, 14, 071003.(3) “Solution-processable thermally activated delayed fluorescence emitters for application in organic light emitting diodes”, Suzuki, K.; Adachi, C.; Kaji, H. J. Soc. Inf. Disp. 2017, 25, 480.(4) “Imidazole Acceptor for Both Vacuum-Processable and Solution-Processable Efficient Blue Thermally Activated Delayed Fluorescence”, Kusakabe, Y.; Wada, Y.; Misono, T.; Suzuki, K.; Shizu, K.; Kaji, H ACS Omega, 7, 16740 - 16745 (2022).(5) “Triarylboron-based Fluorescent Organic Light-emitting Diodes with External Quantum Efficiencies Exceeding 20%”, Suzuki, K.; Kubo, S.; Shizu, K.; Fukushima, T.; Wakamiya, A.; Murata, Y.; Adachi, C.; Kaji, H.; Angew. Chem. Int. Ed. 2015, 54, 15231.(5) “Purely organic electroluminescent material realizing 100% conversion from electricity to light”, Kaji, H.; Suzuki, H.; Fukushima, T.; Shizu, K.; Suzuki, K.; Kubo, S.; Komino, T.; Oiwa, H.; Suzuki, F.; Wakamiya, A.; Murata, Y.; Adachi, C. Nat. Commun. 2015, 6, 8476.

Spiro Boron‐Nitrogen Molecules Based Thermally Activated Delayed Fluorescence Emitter for Highly Efficient Solution‐Processed Organic Light‐Emitting Diodes

AbstractSpiro compounds with unique structural and electronic properties are beneficial for thermally activated delayed fluorescence (TADF) emitters. Herein, a series of spiro boron‐nitrogen (B‐N) compounds are reported with donor–acceptor (D‐A) interaction through homoconjugation. Three phenyl acridine‐containing spiro B‐N compounds exhibit highly efficient photoluminescence (PL) in degassed toluene and solid‐state with obvious TADF character. The separation of HOMO and LUMO for these compounds is confirmed by theoretical calculations, as is the small ΔEST of phenyl acridine‐containing compounds. These three compounds are used as emitting layers in solution‐processed organic light‐emitting diodes (OLEDs), resulting in deep‐blue to green emission colors with different acceptor moieties. Impressively, an excellent external quantum efficiency (EQE) of 22.1% is recorded for the TPA‐s‐FMesBF‐based OLED device, which is comparable with the highest value from solution‐processed OLEDs based on spiro D‐A emitting materials. This work presents a facile synthetic route to novel B, N‐substituted spiro compounds, as well as the structure‐property relationship of spiro D‐A compounds for highly efficient TADF emitters.

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.

Towards red thermally activated delayed fluorescence emitter: Molecular design and property prediction of phenothiazine-triphenyltriazine derivative

A series of thermally activated delayed fluorescence (TADF) molecules with the linkage of the electron-donating group (phenothiazine, PTZ) and electron-withdrawing groups (2,4,6-triphenyl-1,3,5-triazine type, TRZ) have been designed and investigated for efficient red TADF emitters by means of quantum chemical calculations. The geometrical and electronic structures, absorption and emission properties, reverse intersystem crossing (RISC), radiative and nonradiative decay processes of the PTZ-TRZ-based molecules were analyzed. The calculated results indicate that the designed molecules possess the relatively large RISC rates (>105 s−1) and exhibit the red emission, indicating that they have the potential to be an efficient red emitting TADF material. Furthermore, the position of N atom in the TRZ acceptor group have significant effects on RISC process and emission color. This work provides in-depth understanding of the structure–property relationship of PTZ-TRZ-based TADF molecules, and offers the guideline for the screening of the efficient pure red TADF emitters.

Hybridization of short-range and long-range charge transfer excited states in multiple resonance emitter

Multiple resonance (MR) thermally activated delayed fluorescence emitters have been actively studied as pure blue dopants for organic light-emitting diodes (OLEDs) because of excellent color purity and high efficiency. However, the reported MR emitter, 2,5,13,16-tetra-tert-butylindolo[3,2,1-jk]indolo[1′,2′,3′:1,7]indolo[2,3-b]carbazole (tDIDCz) based on bis-fused indolocarbazole framework could not demonstrate efficient triplet-to-singlet spin crossover. In this work, we report two isomeric MR emitters designed to promote triplet exciton harvesting by reconstructing the electronic structure of tDIDCz. To manage excited states, strong electron donors were introduced at the 2,5-/1,6-position of tDIDCz. As a result, 2,5-positions managed tDIDCz shows long-range charge transfer characteristics while preserving the MR nature. Quantum chemical calculation demonstrates direct spin-orbit coupling by long-range charge transfer and spin-vibronic coupling assisted reverse intersystem crossing by short-range charge transfer simultaneously contribute to triplet-to-singlet spin crossover. Consequently, high performance blue OLED recorded a high external quantum efficiency of 30.8% at a color coordinate of (0.13, 0.13).

Design and Synthesis of Red Through-Space Charge Transfer Thermally Activated Delayed Fluorescence Emitters with Donor/Acceptor/Donor Stacking.

Red through-space charge transfer thermally activated delayed fluorescence (TSCT TADF) materials named SAF36DCPP and SAF27DCPP with sandwiched structures were synthesized. Single crystals indicated that the intramolecular C-H···π interactions play a vital role in rigidifying the sandwiched structure, which results in a fluorescence yield of 63% for SAF36DCPP compared to 40% for SAF27DCPP. Organic light-emitting diodes with SAF36DCPP as the emitter realized a maximum external quantum efficiency of 16.12%.

High‐Performance Multi‐Resonance Thermally Activated Delayed Fluorescence Emitters for Narrowband Organic Light‐Emitting Diodes

AbstractMulti‐resonance thermally activated delayed fluorescence (MR‐TADF) emitters have drawn considerable attention because of their remarkable optoelectronic properties of high emission efficiency and narrow emission profile, and represent an active subject of cutting‐edge research in the organic electroluminescence (EL). However, the slow reverse intersystem crossing (RISC) rate of MR‐TADF emitter caused by the large energy gap (ΔEST) and small spin‐orbit coupling (SOC) matrix elements between the singlet and triplet excited states limits their further development in organic EL devices. Currently, innovative molecular design strategies have been developed including heavy atom integration, π‐extended MR framework and metal perturbation, and so on to improve the RISC process of MR‐TADF emitters for high‐performance EL devices. Here, an overview is presented on the recent progress of MR‐TADF emitters with fast RISC rate ( > 10−5 s−1), with particular attention to the molecular design, optoelectronic properties, and device performance of organic light‐emitting diodes (OLEDs), which intends to systematize the knowledge in this subject for the thriving development of highly efficient MR‐TADF emitters. Finally, the challenges and future prospects of MR‐TADF materials are discussed comprehensively.

Brominated B1-Polycyclic Aromatic Hydrocarbons for the Synthesis of Deep-Red to Near-Infrared Delayed Fluorescence Emitters.

Bromo-functionalized B1-polycyclic aromatic hydrocarbons (PAHs) with LUMOs of less than -3.0 eV were synthesized and used in cross-couplings to form donor-acceptor materials. These materials spanned a range of S1 energies, with a number showing thermally activated delayed fluorescence and significant emission in the near-infrared region of the spectrum. These B1-PAHs represent a useful family of acceptors that can be readily synthesized and functionalized.

Torsion Angle Analysis of a Thermally Activated Delayed Fluorescence Emitter in an Amorphous State Using Dynamic Nuclear Polarization Enhanced Solid-State NMR.

The torsion angle between donor and acceptor segments of a thermally activated delayed fluorescence (TADF) molecule is one of the most critical factors in determining the performance of TADF-based organic light-emitting diodes (OLEDs) because the torsion angle affects not only the energy gap between the singlet and triplet but also the oscillator strength and spin-orbit coupling. However, the torsion angle is difficult to analyze, because organic molecules are in an amorphous state in OLEDs. Here, we determined the torsion angle of a highly efficient TADF emitter, DACT-II, in an amorphous state by dynamic nuclear polarization enhanced solid-state NMR measurements. From the experimentally obtained chemical shift principal values of 15N on carbazole, we determined the average torsion angle to be 52°. Such quantification of the torsion angles in TADF molecules in amorphous solids will provide deep insight into the TADF mechanism in amorphous OLEDs.

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.