Charge Transfer Excited State Research Articles

Hybrid Local and Charge Transfer Emitters Utilizing Hyperconjugation Effect Towards Solution‐Processed Ultra‐Deep‐Blue OLEDs with External Quantum Efficiency Approaching 12 %

AbstractHybrid local and charge transfer (HLCT) excited state materials, which possess weak donor‐acceptor (D–A) pure organic structures, deserve one of the most promising efficient and stable blue emitters. Through high‐lying reverse intersystem crossing (hRISC) process, 75 % triplet excitons generated by electrical excitation could be harvested and utilized in organic light‐emitting diodes (OLEDs). However, there are still significant challenges to achieve high‐efficiency ultra‐deep‐blue HLCT emitters with low Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinate y values. Here, a series of novel blue HLCT emitters based on spiro[1,8‐diazafluorene‐9,2′‐imidazole] structure were designed and synthesized by fine‐tuning the spiro[fluorene‐9,2′‐imidazole] core structure in our previous work through heteroatom substitution and hyperconjugation effect. The target emitters were endowed with excellent photophysical and electrochemical merits, thermal stability and solution processibility. The solution‐processed OLED based on 4′,5′‐bis(4‐(9H‐carbazol‐9‐yl)phenyl)spiro[1,8‐diazafluorene‐9,2′‐imidazole] (NFIP‐CZ) achieved efficient ultra‐deep‐blue emission (CIEx,y=0.1581, 0.0422) with the maximum external quantum efficiency (EQEmax), maximum current efficiency (CEmax) and maximum power efficiency (PEmax) of 11.94 %, 4.07 cd ⋅ A−1 and 2.56 lm ⋅ W−1. The record EQE is a breakthrough in both solution‐processed and vacuum vapor deposition ultra‐deep‐blue HLCT OLEDs currently.

Modulating Charge-Transfer Excited States of Multiple Resonance Emitters via Intramolecular Covalent Bond Locking.

Advanced multiple resonance thermally activated delayed fluorescence (MR-TADF) emitters with high efficiency and color purity have emerged as a research focus in the development of ultra-high-definition displays. Herein, we disclose an approach to modulate the charge-transfer excited states of MR emitters via intramolecular covalent bond locking. This strategy can promote the evolution of strong intramolecular charge-transfer (ICT) states into weak ICT states, ultimately narrowing the full-width at half-maximum (FWHM) of emitters. To modulate the ICT intensity, two octagonal rings are introduced to yield molecule m-DCzDAz-BNCz. Compounds m-CzDAz-BNCz and m-DCzDAz-BNCz exhibit bright light-green and green fluorescence in toluene, with emission maxima of 504 and 513 nm, and FWHMs of 28 and 34 nm, respectively. Sensitized organic light-emitting diodes (OLEDs) employing emitters m-CzDAz-BNCz and m-DCzDAz-BNCz exhibit green emission with peaks of 508 and 520 nm, Commission Internationale de L'Eclairage (CIE) coordinates of (0.12, 0.65) and (0.19, 0.69), and maximum external quantum efficiencies (EQEs) of 30.2 % and 32.6 %, respectively.

Structural Rigidification Strategy Based on Self-Assembly Enabled Reversible Excited-State Conversion of Iridium(III) Complexes for Multiple-Stimulus-Responsive Data Encryption.

Stimulus-responsive chromic materials exhibit color-switching properties under specific external stimuli and have been widely used in various fields. Transition-metal complexes show great potential applications as promising candidates for stimulus-responsive chromic materials, as their excited states not only depend on the chemical composition but are also affected by the intermolecular stacking modes. Owing to the intrinsic difficulty in the ordered stacking of the octahedral configuration, changing the stacking modes of iridium(III) complexes for multiple-stimulus responsiveness remains a significant challenge. In this work, we propose a structural rigidification strategy based on self-assembly to reversibly regulate the excited states of iridium(III) complexes, therefore achieving color switch under different stimulus conditions. We prepare cationic iridium(III) complexes by using tetrakis(perfluorophenyl)-borate ([B(PhF5)4]-) as the counterion, whose matching tetrahedral configuration and electron-deficient aromaticity enables polar-π interaction with the octahedral iridium(III) cations, inducing self-assembly to form structural rigidification. The structural rigidity restricts the large conformational changes of the metal-to-ligand charge transfer (3MLCT) excited state, and facilitates the conversion from the 3MLCT to the ligand-center (3LC) excited state in aggregated states. The excited-state conversion results in a 54 nm blue shift (from yellow to sky blue) in the photoluminescence spectra. As a result, we report a series of cationic iridium(III) complexes with different responses to low temperature, vapor fuming, and mechanical force, therefore achieving multiple-stimulus-responsive data encryption. Our work provides a novel strategy to achieve ordered stacking of octahedral complexes, shows a deeper understanding of the photophysical processes of transition-metal complexes, and offers a new perspective to develop multiple-stimulus-responsive chromic materials.

Rapid Quantification of Oxytetracycline Based on Fluorescence Enhancement Influenced by pH.

Accurate quantification of antibiotics in environmental samples is typically challenging due to the low antibiotic concentrations and the complexity of environmental matrices. This paper presents a fluorescence spectrometry method for determining oxytetracycline under alkaline conditions. The ionic distribution of the oxytetracycline solution was analyzed based on its dissociation constant. The dimethylamino group plays a crucial role in this method, as it promotes intramolecular charge transfer in the electronic excited state through its electron-donating capability with a lone electron pair. The presented method is straightforward, cost-effective, and holds potential for analyzing oxytetracycline in water sample after further investigation.

Hybrid Local and Charge Transfer Emitters Utilizing Hyperconjugation Effect Towards Solution-Processed Ultra-Deep-Blue OLEDs with External Quantum Efficiency Approaching 12.

Hybrid local and charge transfer (HLCT) excited state materials, which possess weak donor-acceptor (D-A) pure organic structures, deserve one of the most promising efficient and stable blue emitters. Through high-lying reverse intersystem crossing (hRISC) process, 75% triplet excitons generated by electrical excitation could be harvested and utilized in organic light-emitting diodes (OLEDs). However, there are still significant challenges to achieve high-efficiency ultra-deep-blue HLCT emitters with low Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinate y values. Here, a series of novel blue HLCT emitters based on spiro[1,8-diazafluorene-9,2'-imidazole] structure were designed and synthesized by fine-tuning the spiro[fluorene-9,2'-imidazole] core structure in our previous work through heteroatom substitution and hyperconjugation effect. The target emitters were endowed with excellent photophysical and electrochemical merits, thermal stability and solution processibility. The solution-processed OLED device based on 4',5'-bis(4-(9H-carbazol-9-yl)phenyl)spiro[1,8-diazafluorene-9,2'-imidazole] (NFIP-CZ) achieved efficient ultra-deep-blue emission (CIEx,y = 0.1581, 0.0422) with the maximum external quantum efficiency (EQEmax), maximum current efficiency (CEmax) and maximum power efficiency (PEmax) of 11.94%, 4.07 cd·A-1 and 2.56 lm·W-1. The record EQE is a breakthrough in both solution-processed and vacuum vapor deposition ultra-deep-blue HLCT-OLEDs currently.

Photoinduced charge transfer assisted through external electric field and ternary hydrogen bonding strategies

Understanding the mechanisms governing interfacial charge transfer in photoactive layer is crucial for optimizing photogenerated charge separation efficiency. In this study, the interfacial charge transfer process is regulated by applying external electric field (Fext) and ternary hydrogen bonding strategies. We observe significant changes in the excited state properties and charge transfer parameters at the interface under Fext conditions. This facilitates an in-depth exploration of the effects of Fext on the microscopic details of the interface at the molecular level. Implementing the ternary hydrogen bonding strategy significantly enhances the electron-attracting ability of fullerene. Comparative analysis of the PTB7-Th:PC71BM and PTB7-Th:C7:PC71BM systems under Fext = 0, reveals that the ternary hydrogen bonding strategy increases the charge transfer pathway and significantly improves the intermolecular charge separation rate (KCS). Additionally, we evaluated the relationship between hydrogen bond strength and Fext, demonstrating that Fext significantly enhances the hydrogen bond strength between C7:PC71BM. These studies provide deeper insights into the role of Fext and ternary hydrogen bonding strategies in charge transfer process, providing a valuable theoretical framework for designing more efficient organic solar cells (OSCs).

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.

Deep-blue electroluminescence with orthogonal donor-acceptor structure: The role of charge-transfer excited state component in hybrid local and charge-transfer (HLCT) excited state

In this work, three orthogonally bonded phenanthroimidazole based donor-acceptor (D-A) structured blue-emissive materials N-SBPMCN, N-ABPMCN and N-TBPMCN with different donor moieties are designed synthesized. Theoretical combined experimental researches proved that the involving of weak donor (spiro-fluorene and anthracene) and strong donor triphenylamine can form different locally-emissive (LE) dominated hybrid local and charge-transfer (HLCT) excited state, which can contribute to their high photoluminescent quantum yield (PLQY), satisfied electroluminescence performance and blue purity. Specifically, N-TBPMCN with strong donor triphenylamine that possesses more CT excited state components demonstrates the best electroluminescence performance and blue purity among the three materials, revealing that the molecular design of orthogonal D-A structure is of great potential in blue-emissive functional materials. Additionally, we also discovered that intermolecular CT state can also contribute to high exciton utilization.

Mechanochromic luminescent material with high quantum yield for multi-mode anti-counterfeiting applications

The multi-mode anti-counterfeiting materials, constructed by mechanochromic luminescent materials with high quantum yield, can effectively prevent illegal counterfeiting, but their design and construction still present significant challenges. Herein, a twisting D-A-D conjugated molecule, FBtF, is obtained with a special hybridized local and charge transfer excited state, and shows approximately 100 % photoluminescence quantum yield in the coating film. The self-assembled FBtF microplates exhibit bathochromic-shifted emission from green to orange, accompanied by the photoluminescence quantum yield increase from 47.8 % to 89.0 % upon grinding, which is the highest in reported MCL materials, to the best of our knowledge. In contrast, a D-π-D type molecule (FBenF) and a D-D’-D molecule (FAnF) maintain the blue-emission after grinding. Through mechanistic investigation, the force-induced molecular conformational planarization and the breakage of the intermolecular charge transfer collaboratively lead to the emission-color change and photoluminescence quantum yield increase. The FBtF microplates are designed as ink for anti-counterfeiting applications based on multi-mode output signals, including the UV light-excited image and the grinding-induced emission color change together with the photoluminescence quantum yield increase. The written tags exhibit high stability in various harsh environments, and demonstrate superior anti-counterfeiting performance in practical applications, such as medicine anti-counterfeiting.

(Invited) Luminescent Coordination Complexes as a Molecular Probe in Supramolecular Organic-Inorganic Assemblies

Guest molecules entrapped in a confined space can show unusual chemical and physical properties because their behaviors are manipulated by multiple noncovalent interactions from the interiors of the space. Thus, it is important to understand the local environment in the confined space and noncovalent interactions between the guest molecule and the interiors for establishing a new molecular system based on the unusual chemical and physical phenomena from the trapped molecules. Photophysical properties of coordination complexes are easily adjusted by variation of metal ions and organic ligands, leading to a fine-tuning of their excited states related to photoluminescence. In particular, a charge-transfer excited state in the coordination complexes is an environment-sensitive feature and thus they will act as a molecular probe for elucidating a local environment around the complexes. Herein we confirmed the photophysical properties of IrIII complexes within a hydrogen-bonded hexameric capsule, which are derived from their mixed states of 3MLCT/3LLCT excited states. The photophysical properties of IrIII complexes were altered by the encapsulation within the hydrogen-bonded capsule, owing to the effective modulation of a local environment around the complex by noncovalent interaction. This result also provides us a better understanding of the internal space within the hydrogen-bonded capsule. Figure 1

Design of Charge Transfer Donor-Acceptor Co-Crystals with Long Lived Charge Carriers

Organic donor-acceptor co-crystals with long exciton lifetimes have emerged as a promising class of semiconductors for their unique photophysical properties. These are crystalline, single-phase materials composed of two or more different compounds in a stoichiometric ratio, providing a platform for unveiling the structure–property relationships at a molecular level. Importantly, co-crystals that are packed through π stacking interactions have the ability to form charge transfer (CT) excitons within donor-acceptor pairs.1,2,3 We hypothesize that co-crystals with a high degree of CT will have longer exciton lifetimes. Therefore, we design and synthesized of new co-crystals using derivatives of N,N-bis(3-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (PDI, acceptor) with a series of donor molecules, including peri-xanthenoxanthene (PXX), perylene, and triphenylene. High quality single crystals were isolated by slow evaporation at room temperature from suitable solvent mixtures. Their crystal structures were successfully refined by single-crystal X-ray diffraction and the phase purity was validated by powder diffraction. Crystal structures revealed that the driving force for forming these co-crystals is π...π stacking. We measured diffuse reflectance UV/visible spectra of each of these co-crystals and compared with computed spectra to assign the observed electronic transitions. DFT calculations provide additional insights into the nature of the excited state CT interactions which can be derived from ground state electronic structure calculations. Transient absorption measurements were conducted to understand their long exciton lifetime for each of as synthesized co-crystals. We conclude that this study supported our previous hypothesis and such materials are potentially useful in applications ranging from photovoltaics and opto-electronics to photocatalysis. References 1) A. Abou Taka, J. E. Reynolds Iii, N. C. Cole-Filipiak, M. Shivanna, C. J. Yu, P. L. Feng, et al. Phys. Chem. Chem. Phys. 2023, 25, 27065.2) L. Sun, Y. Wang, F. Yang, X. Zhang and W. Hu. Adv. Mater. 2019, 31, 1902328.3) I. Schlesinger, N. E. Powers-Riggs, J. L. Logsdon, Y. Qi, S. A. Miller, R. Tempelaar, et al. Chem. Sci., 2020, 11, 9532–9541.

(Invited) Computational Analysis of the Excited States in the Film States of Non-Fullerene Acceptors

Non-fullerene acceptors are currently widely used as the acceptors of organic thin film solar cells due to their high efficiency. However, the molecular mechanism is still unclear. We have studied the excited states in the film states of non-fullerene acceptors together with experimental collaborators by using theoretical methods. In this presentation, we will present recent progress of our research. For example, we will talk about the excited states in the film states of ITIC. ITIC is a popular acceptor-donor-acceptor-type non-fullerene acceptor. Recently, our collaborator, Prof. Yamakata at Okayama University, Japan, found that the charge separation can take place in ITIC acceptor film even without donor molecules. We investigated this phenomenon by using MD simulations and quantum chemical calculations. MD simulations of the ITIC film showed that the acceptor parts of ITIC are stacked with each other. The dominant angle between two stacked ITIC was found to be ~180 degrees, making J-type dimer. On the other hand, it was also found that there are quite a few V-type dimers with the angles less than 90 degrees. Quantum chemical calculations revealed that the dipole moment in the excited state of J-type dimer is almost zero whereas that of V-type dimer is quite large, indicating strong charge-transfer excited state. Therefore, it is considered that the charge separation of ITIC film can take place in the V-type dimers. We also investigated the excited states in the film states of ITIC analogs. Recently, our collaborator, Prof. Ie at Osaka University, Japan, synthesized several ITIC analogs with different sidechains and found the different efficiency of solar cells. We theoretically investigated the excited states in the film sates of ITIC analogs as in the case of ITIC. It is found that the modification of side chains can change the distributions of angles between two stacked molecules and excited-state properties. These findings highlight the importance of controlling stacking structures in the film state for the efficiency of organic thin film solar cells.

Deep Blue Emitter with a Combination of Hybridized Local and Charge Transfer Excited State and Aggregation-Induced Emission Features for Efficient Non-Doped OLED.

Herein, a deep blue emitter (PI-TPB-CN) with a synergistic effect of hybridized local and charge transfer excited state (HLCT) and aggregation-induced emission (AIE) properties is successfully designed and synthesized to improve the performance of deep blue organic light-emitting diodes (OLEDs). It is constructed using a 1,2,4,5-tetraphenylbenzene (TPB) as an π-conjugated AIE core being asymmetrically functionalized with a phenanthro[9,10-d]imidazole (PI) as a weak donor (D) and a benzonitrile (CN) as an acceptor (A), thereby formulating D-π-A type fluorophore. Its HLCT and AIE properties verified by theoretical calculations, solvatochromic effects, and transient photoluminescence decay experiments, bring about a strong blue emission (452 nm) with a high photoluminescence quantum yield of 74 % in the thin film. PI-TPB-CN is successfully employed as a blue emitter in OLEDs. Non-doped OLED with the structure of ITO/HAT-CN (6 nm)/NPB (30 nm)/TCTA (10 nm)/PI-TPB-CN (30 nm)/TPBi (40 nm)/LiF (1 nm)/Al (100 nm) demonstrates excellent electroluminescence (EL) performance with blue emission (451 nm) and maximum external quantum efficiency (EQEmax) of 7.38 %. The device with a thinner layer of PI-TPB-CN (20 nm) and TPBi (30 nm) exhibits a deeper blue emission (444 nm) with CIE coordinates of (0.156, 0.096), a low turn-on voltage of 3.0 V, and EQEmax of 6.45 %.

Sandwich-type thermally activated delayed fluorescence molecules with through-space charge transfer excited state for red OLEDs

Exploring through-space charge transfer (TSCT) excited state for the design of thermally activated delayed fluorescence (TADF) emitters has been receiving increasing interest for organic light-emitting diodes (OLEDs). In this work, we developed two TSCT-TADF molecules, namely 2DPXZ-QX and 2DPXZ-DFQX, which have sandwich-type donor-acceptor-donor (D-A-D) structures supported by two carbazole bridges. Dibenzo[a,c]phenazine (QX) and its fluorinated derivative (DFQX) were used as the acceptors and O-bridged triphenylamine (DPXZ) was used as the donor. The two donors and acceptor in each molecule are aligned in a face-to-face orientation and thus result in the presence of intramolecular π–π interactions, as revealed by their single crystal X-ray structures. The emission maxima (λPL) of 2DPXZ-QX and 2DPXZ-DFQX in doped 1,3-bis(N-carbazolyl)benzene films are 595 and 600 nm with photoluminescence quantum yields of 67 % and 54 %, respectively. The delayed fluorescence lifetimes are 8.8 and 6.7 μs. OLEDs based on the new sandwich-type emitters show maximum external quantum efficiencies of up to 19.1 %.

A new parameterization of the DFT/CIS method with applications to core-level spectroscopy.

Time-dependent density functional theory (TD-DFT) within a restricted excitation space is an efficient means to compute core-level excitation energies using only a small subset of the occupied orbitals. However, core-to-valence excitation energies are significantly underestimated when standard exchange-correlation functionals are used, which is partly traceable to systemic issues with TD-DFT's description of Rydberg and charge-transfer excited states. To mitigate this, we have implemented an empirically modified combination of configuration interaction with single substitutions (CIS) based on Kohn-Sham orbitals, which is known as "DFT/CIS." This semi-empirical approach is well-suited for simulating x-ray near-edge spectra, as it contains sufficient exact exchange to model charge-transfer excitations yet retains DFT's low-cost description of dynamical electron correlation. Empirical corrections to the matrix elements enable semi-quantitative simulation of near-edge x-ray spectra without the need for significant a posteriori shifts; this should be useful in complex molecules and materials with multiple overlapping x-ray edges. Parameter optimization for use with a specific range-separated hybrid functional makes this a black-box method intended for both core and valence spectroscopy. Results herein demonstrate that realistic K-edge absorption and emission spectra can be obtained for second- and third-row elements and 3d transition metals, with promising results for L-edge spectra as well. DFT/CIS calculations require absolute shifts that are considerably smaller than what is typical in TD-DFT.

First-Principles Computational Screening of Two-Dimensional Polar Materials for Photocatalytic Water Splitting.

The band gap constraint of the photocatalyst for overall water splitting limits the utilization of solar energy. A strategy to broaden the range of light absorption is employing a two-dimensional (2D) polar material as photocatalyst, benefiting from the deflection of the energy level due to their intrinsic internal electric field. Here, by using first-principles computational screening, we search for 2D polar semiconductors for photocatalytic water splitting from both ground- and excited-state perspectives. Applying a unique electronic structure model of polar materials, there are 13 photocatalyst candidates for the hydrogen evolution reaction (HER) and 8 candidates for the oxygen evolution reaction (OER) without barrier energies from the perspective of the ground-state free energy variation calculation. In particular, Cu2As4Cl2S3 and Cu2As4Br2S3 can catalyze HER and OER simultaneously, becoming promising photocatalysts for overall water splitting. Furthermore, by combining ground-state band structure calculations with excited-state charge distribution and transfer calculated by linear-response time-dependent density functional theory (LR-TDDFT) and time-dependent ab initio nonadiabatic molecular dynamics (NAMD), respectively, the rationality of the 2D polar material model has been manifested. The intrinsic built-in electric field promotes the separation of charge carriers while suppressing their recombination. Therefore, our computational work provides a high-throughput method to design high-performance photocatalysts for water splitting.

Benchmarking Charge-Transfer Excited States in TADF Emitters: ΔDFT Outperforms TD-DFT for Emission Energies.

Charge-transfer (CT) excited states are crucial to organic light-emitting diodes (OLEDs), particularly to those based on thermally activated delayed fluorescence (TADF). However, accurately modeling CT states remains challenging, even with modern implementations of (time-dependent) density functional theory [(TD-)DFT], especially in a dielectric environment. To identify shortcomings and improve the methodology, we previously established the STGABS27 benchmark set with highly accurate experimental references for the adiabatic energy gap between the lowest singlet and triplet excited states (ΔEST). Here, we diversify this set to the STGABS27-EMS benchmark by including experimental emission energies (Eem) and use this new set to (re)-evaluate various DFT-based approaches. Surprisingly, these tests demonstrate that a state-specific (un)restricted open-shell Kohn-Sham (U/ROKS) DFT coupled with a polarizable continuum model for perturbative state-specific nonequilibrium solvation (ptSS-PCM) provides exceptional accuracy for predicting Eem over a wide range of density functionals. In contrast, the main workhorse of the field, Tamm-Dancoff-approximated TD-DFT (TDA-DFT) paired with the same ptSS-PCM, is distinctly less accurate and strongly functional-dependent. More importantly, while TDA-DFT requires the choice of two very different density functionals for good performance on either ΔEST or Eem, the time-independent U/ROKS/PCM approaches deliver excellent accuracy for both quantities with a wide variety of functionals.

Nitrogen-Embedding Strategy for Short-Range Charge Transfer Excited States and Efficient Narrowband Deep-Blue Organic Light Emitting Diodes.

The development of deep-blue organic light-emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh-definition displays with wide color gamut. Herein, based on the nitrogen-embedding strategy for modifying the short range charge transfer excited state energies of multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron-embedded 1,3-bis(carbazol-9-yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic-based MR-TADF molecules, Py-BN and Pm-BN, exhibit deep-blue emissions with high photoluminescence quantum yields of 93 % and 94 %, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep-blue hyperfluorescent OLED based on Py-BN exhibited a maximum external quantum efficiency of 27.7 % and desired color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR-TADF emitters.

Nitrogen‐Embedding Strategy for Short‐Range Charge Transfer Excited States and Efficient Narrowband Deep‐Blue Organic Light Emitting Diodes

AbstractThe development of deep‐blue organic light‐emitting diodes (OLEDs) featuring high efficiency and narrowband emission is of great importance for ultrahigh‐definition displays with wide color gamut. Herein, based on the nitrogen‐embedding strategy for modifying the short range charge transfer excited state energies of multi‐resonance (MR) thermally activated delayed fluorescence (TADF) emitters, we introduce one or two nitrogen atoms into the central benzene ring of a versatile boron‐embedded 1,3‐bis(carbazol‐9‐yl)benzene skeleton. This approach resulted in the stabilization of the highest occupied molecular orbital energy levels and the formation of intramolecular hydrogen bonds, and thus systematic hypsochromic shifts and narrowing spectra. In toluene solution, two heterocyclic‐based MR‐TADF molecules, Py‐BN and Pm‐BN, exhibit deep‐blue emissions with high photoluminescence quantum yields of 93 % and 94 %, and narrow full width at half maximum of 14 and 13 nm, respectively. A deep‐blue hyperfluorescent OLED based on Py‐BN exhibited a maximum external quantum efficiency of 27.7 % and desired color purity with Commission Internationale de L'Eclairage (CIE) coordinates of (0.150, 0.052). These results demonstrate the significant potential for the development of deep blue narrowband MR‐TADF emitters.

Regulating Planarized Intramolecular Charge Transfer for Efficient Single-Phase White-Light Emission in Undoped Metal-Organic Framework Nanocrystals.

The technology of combining multiple emission centers to exploit white-light-emitting (WLE) materials by taking advantage of porous metal-organic frameworks (MOFs) is mature, but preparing undoped WLE MOFs remains a challenge. Herein, a pressure-treated strategy is reported to achieve efficient white photoluminescence (PL) in undoped [Zn(Tdc)(py)]n nanocrystals (NCs) at ambient conditions, where the Commission International del'Eclairage coordinates and color temperature reach (0.31, 0.37) and 6560 K, respectively. The initial [Zn(Tdc)(py)]n NCs exhibit weak-blue PL consisting of localized excited (LE) and planarized intramolecular charge transfer (PLICT) states. After pressure treatment, the emission contributions of LE and PLICT states are balanced by increasing the planarization of subunits, thereby producing white PL. Meanwhile, the reduction of nonradiative decay triggered by the planarized structure results in 5-fold PL enhancement. Phosphor-converted light-emitting diodes based on pressure-treated samples show favorable white-light characteristics. The finding provides a new platform for the development of undoped WLE MOFs.