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

Tunable Emission and Structural Insights of 6-Arylvinyl-2,4-bis(2'-hydroxyphenyl)pyrimidines and Their O∧N∧O-Chelated Boron Complexes

In this study, we present the synthesis and photophysical characteristics of a novel series of 6-arylvinyl-2,4-bis(2'-hydroxyphenyl)pyrimidines. These compounds exhibit nonemissive properties attributed to the potential occurrence of a excited-state intramolecular proton transfer process from the OH groups to the nitrogen atoms of the pyrimidine ring. The introduction of an acid for protonation of the pyrimidine ring results in a significant enhancement of the fluorescence response, easily perceptible to the naked eye. Notably, these molecules serve as intriguing rigid O∧N∧O ligands for boron chelation. The incorporation of boron atoms promotes structural planarity, increases rigidity, and successfully restores fluorescence in both solution and the solid state. Moreover, the photoluminescence was found to be strongly influenced by the nature of the end groups on the arylvinylene fragment, allowing for the modulation of the emission color and covering the optical spectrum from blue to red. Strong emission solvatochromism was observed in various solvents, a finding that supports the formation of intramolecular charge-separated emitting states, particularly when terminal electron-donating groups are present in the structure. X-ray diffraction analysis enables the determination of inter- and intramolecular interactions, as well as molecular packing structures, aiding in the rationalization of distinct luminescent behaviors in the solid state. All experimental findings are elucidated through extensive density functional theory (DFT) and time-dependent DFT calculations.

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

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

Achieving efficient and stable blue thermally activated delayed fluorescence organic light-emitting diodes based on four-coordinate fluoroboron emitters by simple substitution molecular engineering

Achieving both high efficiency and high stability in blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) is challenging for practical displays and lighting. Here, we have successfully developed a series of sky-blue to pure-blue emitting donor-acceptor (D-A) type TADF materials featuring a four-coordinated boron with 2,2'-(pyridine-2,6-diyl)diphenolate (dppy) ligands, i.e.1-8. Synergistic engineering of substituents on the phenyl bridge as well as the electronic properties and the attached positions of heteroatom N-donors not only enables fine-tuning of the emission colors, but also modulates the nature and energies of their triplet excited states that are important for the reverse intersystem crossing (RISC). Particularly for the compound with two methyl substituents on the phenyl bridge (compound 8), RISC is significantly facilitated through the vibronic coupling of the energetically close-lying triplet charge transfer (3CT) and the triplet local excited (3LE) states, when compared to analogue 7. Efficient sky-blue to pure-blue OLEDs with electroluminescence peaks (λ EL) at 460-492 nm have been obtained, in which ca. five-fold higher external quantum efficiencies (EQEs) of 18.9% have been demonstrated by 8 than that by 7. Moreover, ca. thirty times longer device operational half-lifetimes (LT50) of 9113 hours for 8 than that for 7 as well as satisfactory LT50 reaching 26 643 hours for 6 at an initial luminance of 100 cd m-2 have also been demonstrated. To the best of our knowledge, these results represent one of the best high-performance blue OLEDs based on tetracoordinated boron TADF emitters. Moreover, the design strategy presented here has provided an attractive strategy for enhancing the device performance of blue TADF-OLEDs.

O-B(F)←N Functionalized Copolymers with Delayed Fluorescence and P-Type Semiconducting Characteristics

Conjugated polymers with integrating properties of delayed fluorescence and photovoltaic responses simultaneously are scarcely reported due to the generally contradictory requirements for molecular structures to achieve the two properties. Herein, an O-B(F)←N functionalized fused unit (M) with multiple resonance features, small energy gap between lowest singlet excited state (S1) and triplet excited state (T1) (ΔEST = 0.23 eV), and delayed fluorescence (τD = 0.75 µs), is designed. Selecting three benzodithiophene (BDT) derivatives as co-units to copolymerize with M, leading to a series of O-B(F)←N embedded polymers also maintaining delayed fluorescence (τD = 0.4-0.5 µs). Moreover, p-type semiconductor characteristics are tested for these polymers with hole mobilities in the range of 10-6-10-5 cm2/Vs. Devices with obviously photovoltaic responses are prepared using these polymers as donors and Y6 as the acceptor, affording a preliminary efficiency of 5.05%. This work successfully demonstrates an effective strategy to design conjugated polymers with integrating properties of delayed fluorescence and photovoltaic performance simultaneously by introducing O-B(F)←N functional groups to polymer backbones.

The Blue Problem: OLED Stability and Degradation Mechanisms

OLED technology has revolutionized the display industry and is promising for lighting. Despite its maturity, there remain outstanding device and materials challenges to address. Particularly, achieving stable and highly efficient blue OLEDs is still proving to be difficult; the vast array of degradation mechanisms at play, coupled with the precise balance of device parameters needed for blue high-performance OLEDs, creates a unique set of challenges in the quest for a suitably stable yet high-performance device. Here, we discuss recent progress in the understanding of device degradation pathways and provide an overview of possible strategies to increase device lifetimes without a significant efficiency trade-off. Only careful consideration of all variables that go into OLED development, from the choice of materials to a deep understanding of which degradation mechanisms need to be suppressed for the particular structure, can lead to a meaningful positive change toward commercializable blue devices.

High-Performance Ultraviolet Organic Light-Emitting Diodes Enabled by Double Boron-Oxygen-Embedded Benzo[m]tetraphene Emitters

Ultraviolet organic light-emitting diodes (UV OLEDs) have attracted increasing attention because of their promising applications in healthcare, industry, and agriculture; however, their development has been hindered by the shortage of robust UV emitters. Herein, we embedded double boron-oxygen units into nonlinear polycyclic aromatic hydrocarbons (BO-PAHs) to regulate their molecular configurations and excited-state properties, enabling novel bent BO-biphenyl (BO-bPh) and helical BO-naphthyl (BO-Nap) emitters with hybridized local and charge-transfer (HLCT) characteristics. They could be facilely synthesized in gram-scale amounts via a highly efficient two-step route. BO-bPh and BO-Nap showed strong UV and violet-blue photoluminescence in toluene with full width at half-maximum values of 25 and 37 nm, along with quantum efficiencies of 98 and 99%, respectively. A BO-bPh-based OLED showed high color purity UV electroluminescence peaking at 394 nm with Commission Internationale de l'Eclairage (CIE) coordinates of (0.166, 0.021). Moreover, the device demonstrated a record-high maximum external quantum efficiency (EQE) of 11.3%, achieved by successful hot exciton utilization. This work demonstrates the promising potential of double BO-PAHs as robust emitters for future UV OLEDs.

Encoding Hole-Particle Information in the Multi-Channel MolOrbImage for Machine-Learned Excited-State Energies of Large Photofunctional Materials

We present a novel class of one-electron multi-channel molecular orbital images (MolOrbImages) designed for the prediction of excited-state energetics in conjunction with the state-of-the-art VGG-type machine-learning architecture. By representing hole and particle states in the excitation process as channels of MolOrbImages, the revised VGG model achieves excellent prediction accuracy for both low-lying singlet and triplet states, with mean absolute errors (MAEs) of <0.08 and <0.1 eV for QM9 molecules and large photofunctional materials with up to 560 atoms, respectively. Remarkably, the model demonstrates exceptional performance (MAE < 1 kcal/mol) for the T1 state of QM9 molecules, making it a non-system-specific model that approaches chemical accuracy. The general rules attained, for instance, the improved performance with well-defined MO energies and the reduced overfitting concern via the inclusion of physically insightful hole-particle information, provide invaluable guidelines for the further design of orbital-based descriptors targeting molecular excited states.

Double boron-oxygen-fused polycyclic aromatic hydrocarbons: skeletal editing and applications as organic optoelectronic materials

An efficient one-pot strategy for the facile synthesis of double boron-oxygen-fused polycyclic aromatic hydrocarbons (dBO-PAHs) with high regioselectivity and efficient skeletal editing is developed. The boron-oxygen-fused rings exhibit low aromaticity, endowing the polycyclic aromatic hydrocarbons with high chemical and thermal stabilities. The incorporation of the boron-oxygen units enables the polycyclic aromatic hydrocarbons to show single-component, low-temperature ultralong afterglow of up to 20 s. Moreover, the boron-oxygen-fused polycyclic aromatic hydrocarbons can also serve as ideal n-type host materials for high-brightness and high-efficiency deep-blue OLEDs; compared to single host, devices using boron-oxygen-fused polycyclic aromatic hydrocarbons-based co-hosts exhibit dramatically brightness and efficiency enhancements with significantly reduced efficiency roll-offs; device 9 demonstrates a high color-purity (Commission International de l'Eclairage CIEy = 0.104), and also achieves a record-high external quantum efficiency (28.0%) among Pt(II)-based deep-blue OLEDs with Commission International de l'Eclairage CIEy < 0.20; device 10 achieves a maximum brightnessof 27219 cd/m2 with a peak external quantum efficiency of 27.8%, which representes the record-high maximum brightness among Pt(II)-based deep-blue OLEDs. This work demonstrates the great potential of the double boron-oxygen-fused polycyclic aromatic hydrocarbons as ultralong afterglow and n-type host materials in optoelectronic applications.

Highly Twisted Thermally Activated Delayed Fluorescence (TADF) Molecules and Their Applications in Organic Light-Emitting Diodes (OLEDs)

Thermally activated delayed fluorescence (TADF) materials have attracted great potential in the field of organic light-emitting diodes (OLEDs). Among thousands of TADF materials, highly twisted TADF emitters have become a hotspot in recent years. Compared with traditional TADF materials, highly twisted TADF emitters tend to show multi-channel charge-transfer characters and form rigid molecular structures. This is advantageous for TADF materials, as non-radiative decay processes can be suppressed to facilitate efficient exciton utilization. Accordingly, OLEDs with excellent device performances have also been reported. In this Review, we have summarized recent progress in highly twisted TADF materials and related devices, and give an overview of the molecular design strategies, photophysical studies, and the performances of OLED devices. In addition, the challenges and perspectives of highly twisted TADF molecules and the related OLEDs are also discussed.

Spiroborate-Based Host Materials with High Triplet Energies and Ambipolar Charge-Transport Properties

Organic light-emitting diodes (OLEDs) receive considerable attention because of their commercial use in flat panel displays. Herein, highly efficient spiroborate-based host materials are reported for use in blue OLEDs. Our designed spiroborates (SBOX) were simple to synthesize and exhibited high triplet excitation energies, narrow S-T gaps, and balanced charge carrier mobilities. A blue OLED containing one of the designed spiroborates, SBON, as a host exhibited a high external quantum efficiency (27.6 %) and low turn-on voltage (3.7 V) compared to those observed using 3,3'-di(9H-carbazol-9-yl)-1,1'-biphenyl (17.6 % and 4.5 V, respectively), indicating their potential as host materials in OLEDs.

Excited-State Dynamics in Borylated Arylisoquinoline Complexes in Solution and in cellulo

Two four-coordinate organoboron N,C-chelate complexes with different functional terminals on the PEG chains are studied with respect to their photophysical properties within human MCF-7 cells. Their excited-state properties are characterized by time-resolved pump-probe spectroscopy and fluorescence lifetime microscopy. The excited-state relaxation dynamics of the two complexes are similar when studied in DMSO. Aggregation of the complexes with the carboxylate terminal group is observed in water. When studying the light-driven excited-state dynamics of both complexes in cellulo, i. e., after being taken up into human MCF-7 cells, both complexes show different features depending on the nature of the anchoring PEG chains. The lifetime of a characteristic intramolecular charge-transfer state is significantly shorter when studied in cellulo (360±170 ps) as compared to in DMSO (∼960 ps) at 600 nm for the complexes with an amino group. However, the kinetics of the complexes with the carboxylate group are in line with those recorded in DMSO. On the other hand, the lifetimes of the fluorescent state are almost identical for both complexes in cellulo. These findings underline the importance to evaluate the excited-state properties of fluorophores in a complex biological environment in order to fully account for intra- and intermolecular effects governing the light-induced processes in functional dyes.

Thermally activated delayed fluorescence with dual-emission and pressure-induced bidirectional shifting: cooperative effects of intramolecular and intermolecular energy transfer

Different from the conventional piezochromic materials with a mono-redshift of single emission, our well-designed molecule demonstrates a sensitive turn-on and color-tunable piezochromic luminescence in response to the hydrostatic pressure. The molecule PXZ-W-SOF possesses dual-emission and pressure-induced bidirectional shifting characteristics. On the basis of in-depth experimental studies, on one hand, it is confirmed that the origin of the dual-emission behavior is the intramolecular charge transfer, namely thermally activated delayed fluorescence (TADF), and the intermolecular excimer; on the other hand, the emission of the excimer exhibits three-step variations with increasing pressure, which is mainly attributed to the molecular structure and its crystal packing state. The remarkable color change of PXZ-W-SOF from sky-blue to green to deep-blue during the whole process of boosting and releasing pressure is a result of intramolecular and intermolecular energy-transfer interactions. The PXZ-W-SOF molecular model is an extremely rare example of highly sensitive fluorescence tuning driven by TADF and excimer conversion under mechanical stimulation, thus providing a novel mechanism for the field of piezochromism. The unique molecular design also offers a new idea for rare deep-blue and ultraviolet TADF materials.

π-Extended Ligands in Two-Coordinate Coinage Metal Complexes

Two-coordinate carbene-M^I-amide (cMa, M^I = Cu, Ag, Au) complexes have emerged as highly efficient luminescent materials for use in a variety of photonic applications due to their extremely fast radiative rates through thermally activated delayed fluorescence (TADF) from an interligand charge transfer (ICT) process. A series of cMa derivatives was prepared to examine the variables that affect the radiative rate, with the goal of understanding the parameters that control the radiative TADF process in these materials. We find that blue-emissive complexes with high photoluminescence efficiencies (Φ_PL > 0.95) and fast radiative rates (k_r = 4 × 10⁶ s^-1) can be achieved by selectively extending the π-system of the carbene and amide ligands. Of note is the role played by the increased separation between the hole and electron in the ICT excited state. Analysis of temperature-dependent luminescence data and theoretical calculations indicate that the hole-electron separation exerts a primary effect on the energy gap between the lowest-energy singlet and triplet states (ΔE_ST) while keeping the radiative rate for the singlet state relatively unchanged. This interpretation provides guidelines for the design of new cMa derivatives with even faster radiative rates in addition to those with slower radiative rates and thus extended excited state lifetimes.

Four-Coordinate Monoboron Complexes with 8-Hydroxyquinolin-5-Sulfonate: Synthesis, Crystal Structures, Theoretical Studies, and Luminescence Properties

8-Hydroxyquinolin-5-sulfonic acid (8HQSA) was combined with 3-pyridineboronic acid (3PBA) or 4-pyridineboronic acid (4PBA) to give two zwitterionic monoboron complexes in crystalline form. The compounds were characterized by elemental analysis, single-crystal X-ray diffraction studies, and IR, 1H NMR, UV-Visible, and luminescence spectroscopy. The analyses revealed compounds with boron atoms adopting tetrahedral geometry. In the solid state, the molecular components are linked by charge-assisted (B)(O-H···−O(S) and N+-H···O(S) hydrogen bonds aside from C-H···O contacts and π···π interactions, as shown by Hirshfeld surface analyses and 2D fingerprint plots. The luminescence properties were characterized in terms of the emission behavior in solution and the solid state, showing emission in the bluish-green region in solution and large positive solvatofluorochromism, caused by intramolecular charge transfer. According to TD-DFT calculations at the M06-2X/6-31G(d) level of theory simulating an ethanol solvent environment, the emission properties are originated from π-π * and n-π * HOMO-LUMO transitions.

Recent advances in the synthesis of luminescent tetra-coordinated boron compounds

Tetra-coordinated boron compounds offer a plethora of luminescent materials. Different chelation around the boron center (O,O-, N,C-, N,O-, and N,N-) has been explored to tune the electronic and photophysical properties of tetra-coordinated boron compounds. A number of fascinating molecules with interesting properties such as aggregation induced emission, mechanochromism and tunable emission by changing the solvent polarity were realised. Owing to their rich and unique properties, some of the molecules have shown applications in making optoelectronic devices, probes and so on. This perspective provides an overview of the recent developments of tetra-coordinated boron compounds and their potential applications.

Photochromic dithienylethene-containing four-coordinate boron(III) compounds with a spirocyclic scaffold

A new series of four-coordinate boron compounds bearing a photochromic dithienylethene-containing C^C ligand and an ancillary N^C ligand have been successfully designed and synthesised. These compounds exhibit reversible photochromism upon photoexcitation with percentage conversions of 71-96% and readily tuneable photocycloreversion quantum yields by convenient modification of the ancillary ligand to turn on the thermally activated upconversion from the lower-lying unreactive excited state to the higher-lying photoreactive excited state.

Intense Organic Afterglow Enabled by Molecular Engineering in Dopant-Matrix Systems

We report intense dopant-matrix afterglow systems with an afterglow efficiency (Φ_AG) of 47% and an afterglow lifetime (τ_AG) of 1.3 s. Luminescent difluoroboron β-diketonate (BF₂bdk) dopants and their deuterated counterparts are designed with naphthalene and carboxylic acid groups. After doping into benzoic acid (BA) matrices, room-temperature afterglow brightness and afterglow duration of the BF₂bdk-BA materials have unexpectedly been found to reach the levels of those at 77 K, which indicates that hydrogen bonding between BF₂bdk and BA, as well as the deuteration technique, can reduce k_nr + k_q of BF₂bdk triplets to very small values even at room temperature. Detailed studies reveal that the BF₂bdk possesses typical ¹ICT characters in the S₁ state and distinct ³LE composition in the T₁ state, and thus shows a high Φ_ISC and a small k_P to obtain a high Φ_AG and a long τ_AG. Besides, triplet-triplet annihilation has been found in the dopant-matrix system at high doping concentrations to further increase Φ_AG.

Multi-resonant thermally activated delayed fluorescence emitters based on tetracoordinate boron-containing PAHs: colour tuning based on the nature of chelates

Multi-resonant thermally activated delayed fluorescence (MR-TADF) materials have attracted considerable attention recently. The molecular design frequently incorporates cycloboration. However, to the best of our knowledge MR-TADF compounds containing nitrogen chelated to boron are still unknown. Reported herein is a new class of tetracoordinate boron-containing MR-TADF emitters bearing C^N^C- and N^N^N-chelating ligands. We demonstrate that the replacement of the B–C covalent bond in the C^N^C-chelating ligand by the B–N covalent bond affords an isomer, which dramatically influences the optoelectronic properties of the molecule. The resulting N^N^N-chelating compounds show bathochromically shifted absorption and emission spectra relative to C^N^C-chelating compounds. The incorporation of a tert-butylcarbazole group at the 4-position of the pyridine significantly enhances both the thermal stability and the reverse intersystem crossing rate, yet has a negligible effect on emission properties. Consequently, high-performance hyperfluorescent organic light-emitting diodes (HF-OLEDs) that utilize these molecules as green and yellow-green emitters show a maximum external quantum efficiency (η_ext) of 11.5% and 25.1%, and a suppressed efficiency roll-off with an η_ext of 10.2% and 18.7% at a luminance of 1000 cd m⁻², respectively.

A new class of tetra-coordinate boron-containing MR-TADF emitters and their corresponding high-performance hyperfluorescent organic light-emitting diodes have been successfully achieved.

Highly efficient carbazolylgold(iii) dendrimers based on thermally activated delayed fluorescence and their application in solution-processed organic light-emitting devices

A new class of C^C^N ligand-containing carbazolylgold(iii) dendrimers has been designed and synthesized. High photoluminescence quantum yields of up to 82% in solid-state thin films and large radiative decay rate constants in the order of 10⁵ s⁻¹ are observed. These gold(iii) dendrimers are found to exhibit thermally activated delayed fluorescence (TADF), as supported by variable-temperature emission spectroscopy, time-resolved photoluminescence decay and computational studies. Solution-processed organic light-emitting diodes (OLEDs) based on these gold(iii) dendrimers have been fabricated, which exhibit a maximum current efficiency of 52.6 cd A⁻¹, maximum external quantum efficiency of 15.8% and high power efficiency of 41.3 lm W⁻¹. The operational stability of these OLEDs has also been recorded, with the devices based on zero- and second-generation dendrimers showing maximum half-lifetimes of 1305 and 322 h at 100 cd m⁻², respectively, representing the first demonstration of operationally stable solution-processed OLEDs based on gold(iii) dendrimers.

A new class of carbazolylgold(iii) C^C^N dendrimers with thermally activated delayed fluorescence properties has been designed and synthesized for the realzaqtion of operationally stable solution-processed organic light-emitting devices.

Recent advances and future challenges in the synthesis of 2,4,6-triarylpyridines

2,4,6-Triarylpyridines are key building blocks to access functional molecules that are used in the design of advanced materials, metal-organic frameworks, supramolecules, reactive chemical intermediates and drugs. A number of synthetic protocols to construct this heterocyclic scaffold have been developed to date, the most recent of which (2015-present) are included and discussed in the present review. An emphasis has been placed on the utility of each synthetic approach in view of the scope of aryl/hetaryl substituents, limitations and an outlook of each method to be used in applied sciences.

Rational Utilization of Intramolecular Hydrogen Bonds to Achieve Blue TADF with EQEs of Nearly 30% and Single Emissive Layer All-TADF WOLED

A series of highly efficient blue thermally activated delayed fluorescence (TADF) compounds, SON-Cz, SON-tBuCz, and SON-PhCz, were developed. Pyridinyl was introduced as the bridging unit between carbazole donors and sulfone acceptor. Intramolecular hydrogen bonds between the pyridine N atom and carbazole H atoms were detected in single crystals, which suppressed the twisting of carbazole rings and dramatically increased the molecular rigidity. At the same time, tert-butyl or phenyl were incorporated at the 3,6-sites of carbazole ring to tune electron donating ability or enlarge HOMO delocalization. All these hydrogen bonds featured TADF compounds exhibited much improved photoluminescence quantum yields (PLQYs) and excellent efficiencies in their doped blue organic light-emitting diodes. In particular, SON-tBuCz and SON-PhCz exhibited the maximum external quantum efficiencies (EQEs) of 29.59% and 28.22% with CIE coordinates of (0.17, 0.22) and (0.21, 0.36), respectively. The excellent performance benefits from the carbazole structure modification and the intramolecular hydrogen bonds, which bring more rigid structures and eliminate nonradiative transitions. Furthermore, a single emissive layer all-TADF white OLED was fabricated using SON-tBuCz as the blue emitter and 4CzTPN-Ph as the orange emitter to give an EQE of 23.51% with a high CRI of 71, which is among the top efficiencies ever reported for all-TADF WOLEDs so far.

TADF-Type Organic Afterglow

We report a highly efficient dopant-matrix afterglow system enabled by TADF mechanism to realize afterglow quantum yields of 60-70 %, which features a moderate rate constant for reverse intersystem crossing (k_RISC ) to simultaneously improve afterglow quantum yields and maintain afterglow emission lifetime. Difluoroboron β-diketonate (BF₂ bdk) compounds are designed with multiple electron-donating groups to possess moderate k_RISC values and are selected as luminescent dopants. The matrices with carbonyl functional groups such as phenyl benzoate (PhB) have been found to interact with and perturb BF₂ bdk excited states by dipole-dipole interactions and thus enhance the intersystem crossing of BF₂ bdk excited states. Through dopant-matrix collaboration, the efficient TADF-type afterglow materials have been achieved to exhibit excellent processability into desired shapes and large-area films by melt casting, as well as aqueous afterglow dispersions for potential bioimaging applications.

A Luminescent Two-Coordinate AuI Bimetallic Complex with a Tandem-Carbene Structure: A Molecular Design for the Enhancement of TADF Radiative Decay Rate

A luminescent bimetallic Au^I complex comprised of N-heterocyclic carbene (NHC) and carbazole (Cz) ligands, that is, (NHC')Au(NHC)AuCz has been synthesized and studied. Both carbene ligands in the bimetallic complex act as electron acceptors in tandem to increase the energy separation between the ground and excited state, which is higher than those found in either monometallic analogue, (NHC)AuCz and (NHC')AuCz. A coplanar geometry designed into the tandem complex ensures sufficient electronic coupling between the π-orbitals of the ligands to impart a strong oscillator strength to the singlet intra-ligand charge-transfer (¹ ICT) transition. Theoretical modelling indicates that the emissive ICT excited state involves both NHC ligands. The tandem complex gives blue luminescence (λ_max =480 nm) with a high photoluminescent quantum yield (Φ_PL =0.80) with a short decay lifetime (τ=0.52 μs). Temperature-dependent photophysical studies indicate that emission is via thermally assisted delayed fluorescence (TADF) and give a small singlet-triplet energy difference (ΔE_ST =50 meV, 400 cm^-1 ) consistent with the short TADF lifetime.

New luminescent tetracoordinate boron complexes: an in-depth experimental and theoretical characterisation and their application in OLEDs

New luminescent 2-iminopyrrolyl boron complexes with different BX2 moieties are extensively studied via complementary experimental and theoretical methodologies, including application in OLEDs.

Complexation of 2,7-diazapyrene with boron for structural and electronic tuning

Complexation of tetra(hydroxyphenyl)-2,7-diazapyrene with boron provided the boron complexes as anti- and syn-isomers. The structural difference induces self-association behaviour of the syn-isomer and isomerisation of the anti-isomer in the solution and solid states.

Molecular Conformational Effect on Optical Properties and Fluoride Induced Color Changes in Triarylborane-Vinylbithiophene-BODIPY Conjugates

Three new triads [bis(mesityl)boryl (Mes₂B)-vinylbithiophene-BODIPY] bearing zero (1), two (2), and four (3) methyl groups on the BODIPY core are synthesized, and their optical properties are reported. The vinyl linker between the thiophene rings in the spacer moiety improved the electronic communication between the boryl and BODIPY units. It displayed a bathochromic shift in the absorption and emission spectra compared to the Mes₂B-bithiophene-BODIPY triad reported elsewhere. These compounds exhibit intriguing multiple-emission features due to an incomplete energy transfer from donor borane to the acceptor BODIPY unit. These compounds' photoluminescence color can be conveniently fine-tuned by fluoride binding at the coordinatively unsaturated tricoordinate boron center. Triad 3, with a rigid molecular structure, showed a sharp emission band, whereas triads 1 and 2, with flexible molecular structures, displayed broad emission bands with a robust bathochromic shift, ascribed to their excited state structural reorganizations. These triads selectively bind fluoride ions and show colorimetric responses, which can be seen with an unaided eye. DFT computational studies were performed to rationalize the optical signatures of these compounds.

Boron(iii) β-diketonate-based small molecules for functional non-fullerene polymer solar cells and organic resistive memory devices

A class of acceptor-donor-acceptor chromophoric small-molecule non-fullerene acceptors, 1-4, with difluoroboron(iii) β-diketonate (BF2bdk) as the electron-accepting moiety has been developed. Through the variation of the central donor unit and the modification on the peripheral substituents of the terminal BF2bdk acceptor unit, their photophysical and electrochemical properties have been systematically studied. Taking advantage of their low-lying lowest unoccupied molecular orbital energy levels (from -3.65 to -3.72 eV) and relatively high electron mobility (7.49 × 10-4 cm2 V-1 s-1), these BF2bdk-based compounds have been employed as non-fullerene acceptors in organic solar cells with maximum power conversion efficiencies of up to 4.31%. Moreover, bistable resistive memory characteristics with charge-trapping mechanisms have been demonstrated in these BF2bdk-based compounds. This work not only demonstrates for the first time the use of a boron(iii) β-diketonate unit in constructing non-fullerene acceptors, but also provides more insights into designing organic materials with multi-functional properties.

6,12-Dihydro-6,12-diboradibenzo[def,mno]chrysene: A Doubly Boron-Doped Polycyclic Aromatic Hydrocarbon for Organic Light Emitting Diodes by a One-Pot Synthesis

One-pot synthesis of a new doubly boron-doped polycyclic aromatic hydrocarbon of 6,12-dimesityl-6,12-dihydro-6,12-diboradibenzo[def,mno]chrysene (MDBDBC) was reported. MDBDBC features a rigid planar electron-deficient core structure and demonstrates good chemical and thermal stabilities. A low-lying LUMO of -3.53 eV, a low locally excited triplet energy of 1.92 eV, as well as green electroluminescence with maximum EQE of 4.9% were found for MDBDBC, suggesting its potential as an n-type unit for future organic light emitting diode applications.

Iodine-Catalyzed Synthesis of N,N'-Chelate Organoboron Aminoquinolate

We disclose a novel method for the synthesis of fluorescent N,N'-chelate organoboron compounds in high efficiency by treatment of aminoquinolates with NaBAr₄/R'COOH in the presence of an iodine catalyst. These compounds display high air and thermal stability. A possible catalytic mechanism based on the results of control experiments has been proposed. Fluorescence quantum yield of 3b is up to 0.79 in dichloromethane.

Density Functional Studies on Photophysical Properties of Boron-Pyridyl-Imino-Isoindoline Dyes: Effect of the Fusion

In this work, to make out the aryl-fusion effect on the photophysical properties of boron-pyridyl-imino-isoindoline dyes, compounds 1-5 were theoretically studied through analyses of their geometric and electronic structures, optical properties, transport abilities, and radiative (k r) and non-radiative decay rate (k nr) constants. The highest occupied molecular orbitals of aryl-fused compounds 2-5 are higher owing to the extended conjugation. Interestingly, aryl fusion in pyridyl increases the lowest unoccupied molecular orbital (LUMO) level, while isoindoline decreases the LUMO level; thus, 4 and 5 with aryl fusion both in pyridyl and isoindoline exhibit a similar LUMO to 1. Compounds 4 and 5 show relatively low ionization potentials and high electron affinities, suggesting a better ability to inject holes and electrons. Importantly, the aryl fusion is conducive to the decrease of k IC. The designed compound 5 exhibits a red-shifted emission maximum, low λh, and low k IC, which endow it with great potential for applications in organic electronics. Our investigation provides an in-depth understanding of the aryl-fusion effect on boron-pyridyl-imino-isoindoline dyes at molecular levels and demonstrates that it is achievable.

Recent Advancement in Boron-Based Efficient and Pure Blue Thermally Activated Delayed Fluorescence Materials for Organic Light-Emitting Diodes

In the last few years, electron-deficient materials have been actively researched for application in organic light-emitting diode (OLED) as dopant and electron-transporting materials. The boron-containing materials are interesting as they give good emissive properties in solid state with an electron-accepting character. Recently, many boron-containing materials are used as emissive materials for thermally activated delayed fluorescence (TADF) OLED applications. In this review, boron acceptor-based push-pull small molecules used for application in blue TADF OLEDs are reviewed, covering their different types of acceptor, molecular design, structure-property relation, material properties, and device properties. Also, the importance of boron acceptors to address the key issue of blue TADF OLEDs is discussed.

Organolithium-Mediated Postfunctionalization of Thiazolo[3,2-c][1,3,5,2]oxadiazaborinine Fluorescent Dyes

An effective method for transition-metal-free postfunctionalization of thiazolo[3,2-c][1,3,5,2]oxadiazaborinine dyes via direct lithiation of the 1,3-thiazole ring was developed. The reaction allows valuable regioselective C-H modification of these N,O-chelated organoboron chromophores incorporating different groups, including C-, Hal-, Si-, S-, Se-, and Sn-substituents. As a result, a library of novel fluorescent 1,3-thiazole-based organoboron complexes has been synthesized and characterized. The influence of the donor/acceptor strength of the substituent E on the photophysical properties has been established. The compound with a bulky lipophilic substituent (SnBu₃) exhibits a relatively high solid-state photoluminescence quantum yield of 44%.

Responsive ruthenium complex probe for phosphorescence and time-gated luminescence detection of bisulfite

Sensitive and selective quantification of specific analytes is of great significance in analytical and environmental sciences, as well as in the food industry. Herein, we report the design, synthesis, characterization, and application of a responsive ruthenium(ii) complex probe, Ru-azo, for phosphorescence and time-gated luminescence (TGL) detection of bisulfite, an important additive in the food industry. Upon a specific nucleophilic addition reaction between bisulfite and the azo group of Ru-azo, a new ruthenium(ii) complex, Ru-SO3, was obtained, which resulted in a remarkable increase in phosphorescence intensity, allowing the bisulfite detection to be achieved. In addition, long-lived emissions of Ru-azo (τ = 258 ns) and Ru-SO3 (τ = 261 ns) also enabled the TGL detection of bisulfite in autofluorescence-rich food samples. Through theoretical computations, the photoinduced electron transfer (PET) process within the ruthenium(ii) complex was validated, which unveiled the rationality of the luminescence "off-on" response of Ru-azo to bisulfite. The probe showed advantages of good water solubility, and high sensitivity, selectivity and accuracy for responding to bisulfite, facilitating its application in phosphorescence and TGL detection of bisulfite in aqueous and food samples.

Luminescent halogen-substituted 2-(N-arylimino)pyrrolyl boron complexes: the internal heavy-atom effect

New luminescent halogen-substituted 2-iminopyrrolyl boron complexes exhibited an internal-heavy atom effect depending on the position of the halogen atom, and activity in OLEDs.

Combining the qualities of carbazole and tetraphenyl silane in a desirable main chain for thermally activated delayed fluorescence polymers

A series of green TADF polymers with carbazole and a tetraphenyl silane copolymer main chain were developed for use in non-doped solution processed OLEDs.