Triplet harvesting with 100% efficiency by way of thermally activated delayed fluorescence in charge transfer OLED emitters

TADF Dye-Loaded Nanoparticles for Fluorescence Live-Cell Imaging

Thermally activated delayed fluorescence (TADF) molecules offer nowadays a powerful tool in the development of novel organic light emitting diodes due to their capability of harvesting energy from non-emissive triplet states without using heavy-metal complexes. TADF emitters have very small energy difference between the singlet and triplet excited states, which makes thermally activated reverse intersystem crossing from the triplet states back to the singlet manifold viable. This mechanism generates a long-lived delayed fluorescence component which can be explored in the sensing of oxygen concentration, local temperature, or used in time-gated optical cell-imaging, to suppress interference from autofluorescence and scattering. Despite this strong potential, until recently the application of TADF outside lighting devices has been hindered due to the low biocompatibility, low aqueous solubility and poor performance in polar media shown by the vast majority of TADF emitters. To achieve TADF luminescence in biological media, careful selection or design of emitters is required. Unfortunately, most TADF molecules are not emissive in polar media, thus complexation with biomolecules or the formation of emissive aggregate states is required, in order to retain the delayed fluorescence that is characteristic of these compounds. Herein, we demonstrate a facile method with great generalization potential that maintains the photophysical properties of solvated dyes by combining luminescent molecules with polymeric nanoparticles. Using an established swelling procedure, two known TADF emitters are loaded onto polystyrene nanoparticles to prepare TADF emitting nanomaterials able to be used in live-cell imaging. The obtained particles were characterized by optical spectroscopy and exhibited the desired TADF emission in aqueous media, due to the polymeric matrix shielding the dye from solvent polarity effects. The prepared nanoparticles were incubated with live human cancer cells and showed very low cytotoxicity and good cellular uptake, thus making fluorescence microscopy imaging possible at low dye concentrations.

Excited State Dynamics of Thermally Activated Delayed Fluorescence from an Excited State Intramolecular Proton Transfer System

We describe the photophysical processes that give rise to thermally activated delayed fluorescence in the excited state intramolecular proton transfer (ESIPT) molecule, triquinolonobenzene (TQB). Using transient absorption and time-resolved photoluminescence spectroscopy, we fully characterize prompt and delayed emission, phosphorescence, and oxygen quenching to reveal the reverse intersystem crossing mechanism (rISC). After photoexcitation and rapid ESIPT to the TQB-TB tautomer, emission from S1 is found to compete with thermally activated ISC to an upper triplet state, T2, very close in energy to S1 and limiting photoluminescence quantum yield. T2 slowly decays to the lowest triplet state, T1, via internal conversion. In the presence of oxygen, T2 is quenched to the ground state of the double proton transferred TQB-TC tautomer. Our measurements demonstrate that rISC in TQB occurs from T2 to S1 driven by thermally activated reverse internal conversion from T1 to T2 and support recent calculations by Cao et al. (Cao, Y.; Eng, J.; Penfold, T. J. Excited State Intramolecular Proton Transfer Dynamics for Triplet Harvesting in Organic Molecules. J. Phys. Chem. A 2019, 123, 2640-2649).

Rational Molecular Design of Highly Efficient Yellow-Red Thermally Activated Delayed Fluorescent Emitters: A Combined Effect of Auxiliary Fluorine and Rigidified Acceptor Unit

Molecular design strategies are crucial to develop highly efficient and long-wavelength thermally activated delayed fluorescent (TADF) emitters because the inherent limitation of the energy gap law degrades the efficiency of the red or orange TADF emitters. To resolve the low efficiency issue, we designed and synthesized two TADF emitters, 4,4'-(6-(9,9-dimethylacridin-10(9H)-yl)-7-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCNAc) and 11-(9,9-dimethylacridin-10(9H)-yl)-12-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCNAc), by utilizing fluorine and peripheral cyano-substituted quinoxaline and phenazine acceptors of 4,4'-(6-fluoroquinoxaline-2,3-diyl)dibenzonitrile (FDQCN) and 11-fluorodibenzo[a,c]phenazine-3,6-dicarbonitrile (FBPCN), respectively. A fluorine atom at the ortho position of the acridine donor acts as an auxiliary acceptor to minimize the singlet-triplet energy gap (ΔE_ST) below 0.1 eV and promotes the reverse intersystem crossing (RISC) process. Organic light-emitting diodes (OLEDs) fabricated with FDQCNAc and FBPCNAc emitters demonstrated high external quantum efficiencies (EQEs) of 27.6 and 23.8% in the yellow-red TADF OLEDs, respectively. In particular, the combination of the F auxiliary acceptor unit and the rigidified FBPCN acceptor unit enabled red-shifted emission by about 58 nm without much sacrifice of the EQE in the red region.

Understanding and Designing Thermally Activated Delayed Fluorescence Emitters: Beyond the Energy Gap Approximation

In this article recent progress in the development of molecules exhibiting Thermally Activated Delayed Fluorescence (TADF) is discussed with a particular focus upon their application as emitters in highly efficient organic light emitting diodes (OLEDs). The key aspects controlling the desirable functional properties, e. g. fast intersystem crossing, high radiative rate and unity quantum yield, are introduced with a particular focus upon the competition between the key requirements needed to achieve high performance OLEDs. The design rules required for organic and metal organic materials are discussed, and the correlation between them outlined. Recent progress towards understanding the influence of the interaction between a molecule and its environment are explained as is the role of the mechanism for excited state formation in OLEDs. Finally, all of these aspects are combined to discuss the ability to implement high level design rules for achieving higher quality materials for commercial applications. This article highlights the significant progress that has been made in recent years, but also outlines the significant challenges which persist to achieve a full understanding of the TADF mechanism and improve the stability and performance of these materials.

Exciton Interactions, Excimer Formation, and [2π+2π] Photodimerization in Nonconjugated Curcuminoid-BF2 Dimers

We describe the synthesis of a series of covalently linked dimers of quadrupolar curcuminoid-BF₂ dyes and the detailed investigation of their solvent-dependent spectroscopic and photophysical properties. In solvents of low polarity, intramolecular folding induces the formation of aggregated chromophores, the UV/Vis absorption spectra of which display the optical signature characteristic of weakly-coupled H-aggregates. The extent of folding and, in turn, of ground-state aggregation is strongly dependent on the nature of the flexible linker. Steady-state and time-resolved fluorescence emission spectroscopies show that the Frenkel exciton relaxes into a fluorescent symmetrical excimer state with a long lifetime. Furthermore, our in-depth studies show that a weakly emitting excimer lies on the pathway toward a photocyclomer. Two-dimensional ¹ H NMR spectroscopy and density functional theory (DFT) allowed the structure of the photoproduct to be established. To our knowledge, this represents the first example of a [2π+2π] photodimerization of the curcuminoid chromophore.

Dual-Phase Thermally Activated Delayed Fluorescence Luminogens: A Material for Time-Resolved Imaging Independent of Probe Pretreatment and Probe Concentration

Developing luminescent probes with long lifetime and high emission efficiency is essential for time-resolved imaging. However, the practical applications usually suffer from emission quenching of traditional luminogens in aggregated states, or from weak emission of aggregation-induced emission type luminogens in monomeric states. Herein, we overcome this dilemma by a rigid-and-flexible alternation design in donor-acceptor-donor skeletons, to achieve a thermally activated delayed fluorescence luminogen with high emission efficiency both in the monomeric state (quantum yield up to 35.3 %) and in the aggregated state (quantum yield up to 30.8 %). Such a dual-phase strong and long-lived emission allows a time-resolved luminescence imaging, with an efficiency independent of probe pretreatment and probe concentration. The findings open opportunities for developing luminescent probes with a usage in larger temporal and spatial scales.

Investigation of Thermally Activated Delayed Fluorescence from a Donor-Acceptor Compound with Time-Resolved Fluorescence and Density Functional Theory Applying an Optimally Tuned Range-Separated Hybrid Functional

Emitters showing thermally activated delayed fluorescence (TADF) in electroluminescent devices rely on efficient reverse intersystem crossing (rISC) arising from small thermal activation barriers between the lowest excited triplet and singlet manifolds. A small donor-acceptor compound consisting of a demethylacridine donor and a methylbenzoate acceptor group is used as a model TADF emitter. The spectroscopic signatures of this system are characterized using a combination of photoluminescence and photoluminescence excitation, and the photoluminescence decay dynamics are recorded between delays of 2 ns and 20 ms. Above T = 200 K, our data provide convincing evidence for TADF at intermediate delays in the microsecond range, whereas triplet-triplet annihilation and slow triplet decay at later times can be observed over the entire temperature range from T = 80 K to room temperature. Moreover, close to room temperature, we find a second and faster up-conversion mechanism, tentatively assigned to reverse internal conversion between different triplet configurations. An interpretation of these experimental findings requires a calculation of the deformation patterns and potential minima of several electronic configurations. This task is performed with a range-separated hybrid functional, outperforming standard density functionals or global hybrids. In particular, the systematic underestimation of the energy of charge transfer (CT) states with respect to local excitations within the constituting chromophores is replaced by more reliable transition energies for both kinds of excitations. Hence, several absorption and emission features can be assigned unambiguously, and the observed activation barriers for rISC and reverse internal conversion correspond to calculated energy differences between the potential surfaces in different electronic configurations.

Thermally activated triplet exciton release for highly efficient tri-mode organic afterglow

Developing high-efficient afterglow from metal-free organic molecules remains a formidable challenge due to the intrinsically spin-forbidden phosphorescence emission nature of organic afterglow, and only a few examples exhibit afterglow efficiency over 10%. Here, we demonstrate that the organic afterglow can be enhanced dramatically by thermally activated processes to release the excitons on the stabilized triplet state (T₁^*) to the lowest triplet state (T₁) and to the singlet excited state (S₁) for spin-allowed emission. Designed in a twisted donor–acceptor architecture with small singlet-triplet splitting energy and shallow exciton trapping depth, the thermally activated organic afterglow shows an efficiency up to 45%. This afterglow is an extraordinary tri-mode emission at room temperature from the radiative decays of S₁, T₁, and T₁^*. With the highest afterglow efficiency reported so far, the tri-mode afterglow represents an important concept advance in designing high-efficient organic afterglow materials through facilitating thermally activated release of stabilized triplet excitons.

The development of organic afterglow materials that do not contain heavy metals is of interest for biological applications. Here, the authors report on the development of a thermally activated organic molecule with tri-mode afterglow and an afterglow efficiency of up to 45%.

Mixed-Host Systems with a Simple Device Structure for Efficient Solution-Processed Organic Light-Emitting Diodes of a Red-Orange TADF Emitter

Charge balance, concentration quenching, and exciton confinement are the most important factors for realizing the use of thermally activated delayed fluorescence (TADF) emitters for organic light-emitting diodes. Red-orange organic light-emitting diodes of a TADF emitter 2-[4 (diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one (TXO-TPA) have been reported by doping in a mixed p-type host system of poly(N-vinylcarbazole) (PVK) and 1,3-bis(N-carbazolyl)benzene (mCP) via solution-processed. We have demonstrated the peak external quantum efficiency of 9.75%, maximum current efficiency of 19.36 cd/A, and power efficiency of 12.17 lm/W along with a CIE coordinate of (0.45, 0.51). The devices were compared with different doping concentrations of TXO-TPA, and a comparative investigation on the effect of the thickness electron transport layer was studied. The results clearly indicated that this solution-processed TXO-TPA device structure is a promising strategy to develop highly efficient but simple OLED structures.

An effective strategy for simply varying relative position of two carbazole groups in the thermally activated delayed fluorescence emitters to achieve deep-blue emission

The development of efficient deep-blue thermally activated delayed fluorescence (TADF) materials is especially important for organic light-emitting devices as displays and lighting sources. However, finding suitable deep-blue TADF emitters is still challenging. Based on an experimentally reported blue-light TADF emitter DCZ-TTR, two new molecules (DCZ1-TTR and DCZ2-TTR) have been designed to investigate the impact of the change of relative position in two carbazole groups on their TADF properties. Density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations coupled with the Marcus rate theory have been performed. It is found that the absorption and emission spectra simulated using the BMK functional can reproduce the available experimental data very well. The fluorescence emissions of DCZ1-TTR and DCZ2-TTR are predicted to show clear blue-shifting in cyclohexane with respect to their analogue DCZ-TTR. Especially, the emission wavelength of DCZ2-TTR is calculated to be 435nm, in the deep-blue light range. According to the Marcus rate theory, the rates of reverse intersystem crossing of DCZ1-TTR and DCZ2-TTR are estimated to be two orders of magnitude larger than that of DCZ-TTR, which is more favorable for the occurrence of delayed fluorescence. This strongly suggests that our newly designed two molecules DCZ1-TTR and DCZ2-TTR can be also expected to be potential blue-light or even deep-blue-light TADF emitters. This may be an effective strategy for realizing deep-blue emission by simply varying relative position of two carbazole groups in the TADF molecules. To our best knowledge, this is a novel finding, which may be useful in preparing highly efficient deep-blue TADF-OLED materials.

Observation of Nonradiative Deactivation Behavior from Singlet and Triplet States of Thermally Activated Delayed Fluorescence Emitters in Solution

Excited states of emissive organic molecules undergo various kinds of quenching phenomena such as vibration-coupled quenching depending on their environmental conditions. Because bright singlet excitons in purely organic molecules exhibiting thermally activated delayed fluorescence (TADF) can access dark triplet excited states, photogenerated singlet excitons can decay nonradiatively through both singlet and triplet excited states. Here, we investigated nonradiative decay behavior, including internal and external exciton quenching processes, of various types of TADF materials in solution. Under air-saturated conditions, both the lowest singlet and triplet excited states of almost all of the TADF materials showed oxygen quenching. We considered the effect of oxygen quenching for both spin states to develop a method for determination of the triplet contribution to the total photoluminescence quantum yield from the transient photoluminescence profiles. Furthermore, we observed a clear energy gap law for the internal nonradiative processes.

CN-Modified Imidazopyridine as a New Electron Accepting Unit of Thermally Activated Delayed Fluorescent Emitters

Two efficient thermally activated delayed fluorescent (TADF) emitters were developed by utilizing CN-modified imidazopyridine as an acceptor unit. The CN-modified imidazopyridine acceptor was combined with either an acridine donor or a phenoxazine donor through a phenyl linker to produce two TADF emitters, Ac-CNImPy and PXZ-CNImPy. The acridine-based Ac-CNImPy emitter exhibited sky-blue emission with a CIE coordinate of (0.18, 0.38), whereas the phenoxazine-donor-based PXZ-CNImPy showed greenish-yellow emission with a CIE coordinate of (0.32, 0.58). A high photoluminescence quantum yield of 80 % was observed for the PXZ-CNImPy emitter compared with 40 % for the Ac-CNImPy emitter. Organic light-emitting diodes based on the PXZ-CNImPy emitter demonstrated high external quantum efficiency of 17.0 %. Hence, the CN-modified imidazopyridine unit can be considered as a useful electron acceptor for the future design of highly efficient TADF emitters.

Towards Highly Efficient TADF Yellow-Red OLEDs Fabricated by Solution Deposition Methods: Critical Influence of the Active Layer Morphology

Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence emitters (TADF) in simple device structures fabricated by solution processing are strongly dependent on a suitable host molecular conformation and morphology. Herein, we report the fabrication of highly efficient yellow-red TADF-based OLEDs via solution processing in a simple, two-organic-layer device structure. The devices were fabricated at different weight concentrations of 5%, 8%, and 10% emitter in an n-/p-type mixed host matrix, and their characteristics were studied. The device performance was compared with different thickness parameters for both the emitting layer (EML) and the electron transport layer (ETL) in various solvents, including chlorobenzene, dichlorobenzene, and chloroform. By optimizing the mixed ratio of EML, yellow-red OLEDs of 2-[4 (diphenylamino)phenyl]-10,10-dioxide-9H-thioxanthen-9-one (TXO-TPA) emitter in an n-/p-type host matrix of poly(N-vinylcarbazole):1,3-Bis[2-(4-tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (PVK:OXD-7) as a blend for the active layer were fabricated. In the best results, the device exhibited a lower turn-on voltage at around 6 V, with an external quantum efficiency (EQE) of 18.44%, current efficiency of 36.71 cd/A, and power efficiency of 14.74 Lm/W for the 8% emitter concentration. The importance of solvent for improving the electrical properties, together with organic layer thickness and host effect for the charge carrier’s transport and device characteristics are also discussed.

The design, synthesis and performance of thermally activated delayed fluorescence macromolecules

The design, synthesis and performance of thermally activated delayed fluorescence macromolecules are summarized, and the typical solution-processed polymeric and dendritic emitters are also organized herein as a function of EL emission color.

Photoluminescent properties and molecular structures of dinuclear gold(i) complexes with bridged diphosphine ligands: near-unity phosphorescence from 3XMMCT/3MC

Dinuclear gold(i) complexes with bridged diphosphine ligands display near-unity phosphorescence in the crystalline state at room-temperature.

Enhancing the thermally activated delayed fluorescence of nido-carborane-appended triarylboranes by steric modification of the phenylene linker

The introduction of a methyl group into the 4-position of the phenylene linker of nido-carborane–triarylborane D–A dyads, i.e., at the ortho position to the nido-carborane cage, largely enhances their thermally activated delayed fluorescence.

Molecular Design Strategy of Thermally Activated Delayed Fluorescent Emitters Using CN-Substituted Imidazopyrazine as a New Electron-Accepting Unit

Thermally activated delayed fluorescence (TADF)-based organic light-emitting diodes (OLEDs) have attracted enormous attention recently due to their capability to replace conventional phosphorescent organic light-emitting diodes for practical applications. In this work, a newly designed CN-substituted imidazopyrazine moiety was utilized as an electron-accepting unit in a TADF emitter. Two TADF emitters, 8-(3-cyano-4-(9,9-dimethylacridin-10(9H)-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (Ac-CNImPyr) and 8-(3-cyano-4-(10H-phenoxazin-10-yl)phenyl)-2-phenylimidazo[1,2-a]pyrazine-3-carbonitrile (PXZ-CNImPyr), were developed based on the CN-substituted imidazopyrazine acceptor combined with acridine and phenoxazine donor, respectively. A CN-substituted phenyl spacer was introduced between the donor and acceptor for a sufficiently small singlet-triplet energy gap (ΔE_ST ) and molecular orbital management. Small ΔE_ST of 0.07 eV was achieved for the phenoxazine donor-based PXZ-CNImPyr emitter. As a result, an organic light-emitting diode based on the PXZ-CNImPyr emitter exhibited a high external quantum efficiency of up to 12.7 %, which surpassed the EQE limit of common fluorescent emitters. Hence, the CN-modified imidazopyrazine unit can be introduced as a new acceptor for further modifications to develop efficient TADF-based OLEDs.

Organic Light-Emitting Diodes Based on Conjugation-Induced Thermally Activated Delayed Fluorescence Polymers: Interplay Between Intra- and Intermolecular Charge Transfer States

In this work, interactions between different host materials and a blue TADF polymer named P1 are systematically investigated. In photoluminescence, the host can have substantial impact on the photoluminescence quantum yield (PLQY) and the intensity of delayed fluorescence (Φ_DF), where more than three orders of magnitude difference of Φ_DF in various hosts is observed, resulting from a polarity effect of the host material and energy transfer. Additionally, an intermolecular charge-transfer (CT) emission with pronounced TADF characteristics is observed between P1 and 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), with a singlet-triplet splitting of 7 meV. It is noted that the contribution of harvested triplets in monochrome organic light-emitting diodes (OLEDs) correlates with Φ_DF. For devices based on intermolecular CT-emission, the harvested triplets contribute ~90% to the internal quantum efficiency. The results demonstrate the vital importance of host materials on improving the PLQY and sensitizing Φ_DF of TADF polymers for efficient devices. Solution-processed polychrome OLEDs with a color close to a white emission are presented, with the emission of intramolecular (P1) and intermolecular TADF (PO-T2T:P1).

Strategy for tuning the up-conversion intersystem crossing rates in a series of organic light-emitting diodes emitters relevant for thermally activated delayed fluorescence

Accurate prediction on the up-conversion intersystem crossing rate (k_UISC) is a critical issue for the molecular design of an efficient thermally activated delayed fluorescence (TADF) emitter, and the k_UISC rate is considered to be mainly determined by the spin-orbit coupling matrix element (SOCME) and the singlet-triplet energy difference (∆E_ST). In the present contribution, we strategically designed a series of organic molecules, bearing an isoindole-dione core as the electron acceptor (A) unit and dinitrocarbazolyl, carbazolyl, diphenylcarbazolyl, dicarbazolyl and tercarbazolyl groups as the electron donor (D) units, respectively. Their SOCME and ∆E_ST values between the S₁ and T₁ states were calculated by the DFT and TD-DFT methodes, and the k_UISC rates were estimated by using the semiclassical Marcus theory. The present studies indicate that as the π-conjugation in the D unit enhances, the ∆E_ST value gradually decreases, and the k_UISC rate gradually increases. The molecule using tercarbazolyl as the D moiety is found to exhibit the largest k_UISC in the present computations, as high as 1.22 × 10⁶ s^-1, which is of the same order of magnitude as an experimentally observed highly-efficient TADF emitter using a 4-benzoylpyridine as the A unit and the same tercarbazolyl group as the D moiety. The present results sufficiently prove the necessity of introducing strong electron-rich substituent groups when designing highly efficient TADF emitters.

Understanding and Manipulating the Interplay of Wide-Energy-Gap Host and TADF Sensitizer in High-Performance Fluorescence OLEDs

Comprising an emitting layer (EML) constituting a wide-energy-gap host, a thermally activated delayed fluorescence (TADF) sensitizer and a conventional fluorescent dopant, TADF-sensitizing-fluorescence organic light-emitting diodes (TSF-OLEDs) highly depend on component interplay to maximize their performance, which, however, is still under-researched. Taking the host type (TADF or non-TADF) and the recombination position (on the host or on the TADF sensitizer) into consideration, the interplay of host and TADF sensitizer is comprehensively studied and manipulated. A wide-energy-gap host with TADF and recombination of charges on it are both required to maximize device performances by triggering multiple sensitizing processes to eliminate exciton losses. Based on those findings, a maximum external quantum efficiency (EQE)/power efficiency (PE) of 23.2%/76.9 lm W^-1 is realized with a newly developed TADF host, significantly outperforming the reference devices. Further device optimization leads to unprecedently high EQE/PE of 24.2%/89.5 lm W^-1 and a half-lifetime of over 400 h at an initial luminance of 2000 cd m^-2 , with the peak PE being the highest value among the reported TSF-OLEDs. This work reveals the importance of manipulating the component interplay in EMLs, opening a new avenue toward highly efficient TSF-OLEDs.

Molecular Design Strategies for Color Tuning of Blue TADF Emitters

New thermally activated delayed fluorescence (TADF) blue emitter molecules based on the known donor-acceptor-donor (D-A-D)-type TADF molecule, 2,7-bis(9,9-dimethylacridin-10-yl)-9,9-dimethylthioxanthene-S,S-dioxide (DDMA-TXO2), are reported. The motivation for the present investigation is via the use of rational molecular design, based on DDMA-TXO2, to elevate the organic light emitting diode (OLED) performance and obtain deeper blue color coordinates. To achieve this goal, the strength of the donor (D) unit and acceptor (A) units have been tuned with methyl substituents. The methyl functionality on the acceptor was also expected to modulate the D-A torsion angle in order to obtain a blue shift in the electroluminescence. The effect of regioisomeric structures has also been investigated. Herein, we report the photophysical, electrochemical, and single-crystal X-ray crystallography data to assist with the successful OLED design. The methyl substituents on the DDMA-TXO2 framework have profound effects on the photophysics and color coordinates of the emitters. The weak electron-donating methyl groups alter the redox properties of the D and A units and consequently affect the singlet and triplet levels but not the energy gap (ΔE_ST). By systematically manipulating all of the aforementioned factors, devices have been obtained with acceptor-substituted III with a maximum external quantum efficiency of 22.6% and Commission Internationale de l'Éclairage coordinates of (0.15, 0.18) at 1000 cd m^-2.

Solution-Processed Highly Efficient Bluish-Green Thermally Activated Delayed Fluorescence Emitter Bearing an Asymmetric Oxadiazole-Difluoroboron Double Acceptor

Difluoroboron (BF₂)-containing dyes have attracted great interest owing to their exceptionally high luminescence efficiency and good electron-withdrawing properties. However, only a few reports on difluoroboron-based thermally activated delayed fluorescence (TADF) have been addressed. In this contribution, a novel BF₂-containing TADF molecule of BFOXD, which contains two acceptor fragments of oxadiazole (OXD) and BF₂ and one donor unit of 9,9-dimethylacridine, was synthesized and characterized. For comparison, the precursor of OHOXD bearing one acceptor unit was also investigated. Both molecules clearly show TADF characteristics with sky-blue emission in solution and film state. Additionally, OHOXD undergoes excited-state intramolecular proton transfer-coupled intramolecular charge transfer processes. Using 9-(4-tert-butylphenyl)-3,6-bis(triphenylsilyl)-9H-carbazole (CzSi) as the host, the organic light-emitting diodes fabricated via a solution process show maximum external quantum efficiency (EQE) of 2.98 and 13.8% for OHOXD- and BFOXD-based devices, respectively. While the bipolar TADF host of 10-(4-((4-(9H-carbazol-9-yl)phenyl)sulfonyl)phenyl)-9,9-dimethyl-9,10-dihydroacridine (CzAcSF) is utilized instead of CzSi, the OHOXD- and BFOXD-based devices exhibit better performances with the maximum EQEs of 12.1 and 20.1%, respectively, which render the most efficient and the bluest emission ever reported for the BF₂-based TADF molecules. This research demonstrates that introduction of one more acceptor unit into the TADF molecule could have a positive effect on emission efficiency, which opens a new way to design high-efficiency TADF molecules.

Design of Thermally Activated Delayed Fluorescent Assistant Dopants to Suppress the Nonradiative Component in Red Fluorescent Organic Light-Emitting Diodes

Organic light-emitting diodes are currently under research to achieve high efficiency and long life by using thermally activated delayed fluorescence (TADF) materials. In particular, many studies have focused on ensuring high efficiency in fluorescent devices by introducing TADF materials. Herein, four kinds of orange-colored TADF materials were synthesized and introduced into 5,10,15,20-tetraphenylbisbenz[5,6]indeno[1,2,3-cd:1',2',3'-lm]perylene (DBP) red fluorescent devices as assistant dopants. These TADF materials assisted in achieving high efficiency in DBP devices by reducing nonradiative process by Dexter energy transfer and harvesting singlet excitons by a Förster resonance energy transfer process. Among the four TADF materials, 2-(3,5-di-tert-butylphenyl)-6-(9,9-diphenylacridin-10(9H)-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (DtBIQAP) showed a higher reverse intersystem crossing rate and a smaller nonradiative rate constant than the other two materials, which can reduce the exciton loss process. As a result, the DtBIQAP-assisted DBP device showed a high maximum external quantum efficiency of 18.2 % and color coordinates of (0.63, 0.37) in red fluorescent organic light-emitting diodes. This study provided a strategy of developing assistant dopants for high external quantum efficiency in TADF-assisted fluorescent devices.

Modulation of Thermally Activated Delayed Fluorescence in Waterborne Polyurethanes via Charge-Transfer Effect

Here, we designed several waterborne polyurethanes (WPUs) with efficient thermally activated delayed fluorescence (TADF) via serving charge-transfer (CT) states as a mediate bridge between singlet and triplet states to boost reverse intersystem crossing (RISC). By tuning substituents of diphenyl sulfone (DS), we found that O,O'- and S,S'-substituted DS covalently incorporated in WPUs solely show typical fluorescence emission with lifetimes in the nanosecond range. Interestingly, TADF appears by replacing the substituent with the nitrogen atom, of which lifetimes are up to ≈10 microseconds and ≈1 millisecond in air and vacuum, respectively, even though the energy gap between singlet and triplet states (ΔE_ST ) is still large for generating TADF. To explain this phenomenon, an energy level mode based on CT states and an ³ (n-π*) receiver state was proposed. By the rational modulation of CT states, it is possible to tune the ΔE_ST to render TADF-based materials suitable for versatile applications.

Electroabsorption Spectroscopy as a Tool for Probing Charge Transfer and State Mixing in Thermally Activated Delayed Fluorescence Emitters

Solid-state electroabsorption is demonstrated as a powerful tool for probing the charge transfer (CT) character and state mixing in the low-energy optical transitions of two structurally similar thermally activated delayed fluorescent (TADF) materials with divergent photophysical and device performances. The Liptay model is used to fit differentials of the low-energy absorption bands to the measured electroabsorption spectra, with both emitters showing CT characteristics and large changes in dipole moments upon excitation despite the associated absorption bands appearing to be structured. High electric fields then reveal transfer of oscillator strength to a state close to the CT in the better performing molecule. With supporting TDDFT-TDA and DFT/MRCI calculations, this state showed ππ* characteristics of a local acceptor triplet that strongly mixes with the σπ* of the CT. The emitter with poor TADF performance showed no evidence of such mixing.

Strategic-tuning of radiative excitons for efficient and stable fluorescent white organic light-emitting diodes

The emerging thermally activated delayed fluorescence materials have great potential for efficiencies in organic light-emitting diodes by optimizing molecular structures of the emitter system. However, it is still challenging in the device structural design to achieve high efficiency and stable device operation in white organic light-emitting diodes. Here we propose a universal design strategy for thermally activated delayed fluorescence emitter-based fluorescent white organic light-emitting diodes, establishing an advanced system of “orange thermally activated delayed fluorescence emitter sensitized by blue thermally activated delayed fluorescence host” combined with an effective exciton-confined emissive layer. Compared to reference single-layer and double-layer emissive devices, the external quantum efficiency improves by 31 and 45%, respectively, and device operational stability also shows nearly fivefold increase. Additionally, a detailed optical simulation for the present structure is made, indicating the validity of the design strategy in the fluorescent white organic light-emitting diodes.

Demonstrating white organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters with high performance and operational stability remains a challenge. Here, the authors show efficient and stable white OLEDs based on a double-dopant TADF system.

Four-Coordinate Boron Emitters with Tridentate Chelating Ligand for Efficient and Stable Thermally Activated Delayed Fluorescence Organic Light-Emitting Devices

A new class of four-coordinate donor-acceptor fluoroboron-containing thermally activated delayed fluorescence (TADF) compounds bearing a tridentate 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligand has been successfully designed and synthesized. Upon varying the donor moieties from carbazole to 10H-spiro[acridine-9,9'-fluorene] to 9,9-dimethyl-9,10-dihydroacridine, these boron derivatives exhibit a wide range of emission colors spanning from blue to yellow with a large spectral shift of 2746 cm^-1 , with high PLQYs of up to 96 % in the doped thin film. Notably, vacuum-deposited organic light-emitting devices (OLEDs) made with these boron compounds demonstrate high performances with the best current efficiencies of 55.7 cd A^-1 , power efficiencies of 58.4 lm W^-1 and external quantum efficiencies of 18.0 %. More importantly, long operational stabilities of the green-emitting OLEDs based on 2 with half-lifetimes of up to 12 733 hours at an initial luminance of 100 cd m^-2 have been realized. This work represents for the first time the design and synthesis of tridentate dppy-chelating four-coordinate boron TADF compounds for long operational stabilities, suggesting great promises for the development of stable boron-containing TADF emitters.

A Highly Efficient Red Metal-free Organic Phosphor for Time-Resolved Luminescence Imaging and Photodynamic Therapy

Developing highly efficient red metal-free organic phosphors for biological applications is a formidable challenge. Here, we report a novel molecular design principle to obtain red metal-free organic phosphors with long emission lifetime (504.6 μs) and high phosphorescence efficiency (14.6%) from the isolated molecules in the crystal. Furthermore, the well-dispersed phosphorescent nanodots (PNDs) with the particle size around 5 nm are prepared through polymer-encapsulation in an aqueous solution, which show good biocompatibility and low cytotoxicity. The metal-free PNDs are successfully applied to time-resolved luminescence imaging to eliminate background fluorescence interference both in vitro and vivo as well as effective photodynamic anticancer therapy for the first time. This work will not only pave a pathway to develop highly efficient metal-free RTP materials but also expand the scope of their applications to biomedical fields.