Boron difluoride hemicurcuminoid as an efficient far red to near-infrared emitter: toward OLEDs and laser dyes

Progress and Perspective toward Continuous-Wave Organic Solid-State Lasers

A continuous-wave (CW) organic solid-state laser is highly desirable for spectroscopy, sensing, and communications, but is a significant challenge in optoelectronics. The accumulation of long-lived triplet excitons and relevant excited-state absorptions, as well as singlet-triplet annihilation, are the main obstacles to CW lasing. Here, progress in singlet- and triplet-state utilizations in organic gain media is reviewed to reveal the issues in working with triplets. Then, exciton behaviors that inhibit light oscillations during long excitation pulses are discussed. Further, recent advances in increasing organic lasing pulse widths from microseconds toward the indication of CW operation are summarized with respect to molecular designs, advanced resonator architectures, triplet scavenging, and potential triplet contribution strategies. Finally, future directions and perspectives are proposed for achieving stable CW organic lasers with significant triplet contribution.

Neoteric Advances in Oxygen Bridged Triaryl Boron-based Delayed Fluorescent Materials for Organic Light Emitting Diodes

Since their first demonstration, thermally activated delayed fluorescence (TADF) materials have been emerged as the most promising emitters because of their promising applications in optoelectronics, typified by organic light-emitting diodes (OLEDs). In which, the rigid oxygen bridged boron acceptor-featured (DOBNA) emitters have gained tremendous impetus for OLEDs, which is ascribed to their excellent external quantum efficiency (EQE). However, these materials often displayed severe efficiency roll-off and poor operational stability. Therefore, there needs to be a comprehensive understanding of the aspect of the molecular design and structure-property relationship. To the best of our knowledge, there is no detailed review on the structure-function outlook of DOBNA-based emitters emphasizing the effect of the nature of donor units, their number density, and substitution pattern on the physicochemical properties, excited state dynamics and OLED performance were reported. To fill this gap, herein we presented the recent advancements in DOBNA-based acceptor featured TADF materials by classifying them into several subgroups based on the molecular design i. e. donor-acceptor (D-A), D-A-D, A-D-A, and multi-resonant TADF (MR-TADF) emitters. The detailed design concepts, along with their respective physicochemical and OLED performances were summarized. Finally, the prospective of this class of materials in forthcoming OLED displays is also discussed.

Tailoring D-π-A architectures with hybridized local and charge transfer fluorophores exhibiting high electroluminescence exciton utilization and low threshold amplified spontaneous emission

Novel amorphous compounds which could simultaneously use 25% singlet excitons and 75% triplet excitons as the energy source for light amplification enable the reduction of the threshold current density for electrically pumped organic semiconductor laser diodes (OSLDs); however, there is always a trade-off between the high radiative decay rate of the local excited (LE) state that is required for amplified spontaneous emission (ASE) and high exciton utilization benefiting from the charge-transfer (CT) state during electroluminescence (EL). Herein, we have explored a delicate balance to achieve both low ASE threshold and high EL exciton utilization by adopting a carefully tailored hybridized local and charge-transfer (HLCT) molecular design. A series of donor-π-acceptor (D-π-A) molecules (SBz-1, SBz-2 and SBz-3) are synthesized, and the structural change mainly refers to the spatial distance between D and A which could regulate the excited-state character via adjusting the CT length. Notably, the ASE phenomenon with a low threshold (2.97 μJ cm-2) and a high exciton utilization of 57.6% are achieved at the same time for SBz-2 with an appropriate CT length. The results provide guidance for molecular design toward harvesting triplet excitons in organic laser materials.

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.

Progresses and Perspectives of Near-Infrared Emission Materials with "Heavy Metal-Free" Organic Compounds for Electroluminescence

Organic/polymer light-emitting diodes (OLEDs/PLEDs) have attracted a rising number of investigations due to their promising applications for high-resolution fullcolor displays and energy-saving solid-state lightings. Near-infrared (NIR) emitting dyes have gained increasing attention for their potential applications in electroluminescence and optical imaging in optical tele-communication platforms, sensing and medical diagnosis in recent decades. And a growing number of people focus on the "heavy metal-free" NIR electroluminescent materials to gain more design freedom with cost advantage. This review presents recent progresses in conjugated polymers and organic molecules for OLEDs/PLEDs according to their different luminous mechanism and constructing systems. The relationships between the organic fluorophores structures and electroluminescence properties are the main focus of this review. Finally, the approaches to enhance the performance of NIR OLEDs/PLEDs are described briefly. We hope that this review could provide a new perspective for NIR materials and inspire breakthroughs in fundamental research and applications.

Dielectric behavior of curcuminoid polymorphs on different substrates by direct soft vacuum deposition

Our work examines the structural-electronic correlation of a new curcuminoid, AlkCCMoid, as a dielectric material on different substrates. For this purpose, we show a homemade sublimation method that allows the direct deposition of molecules on any type of matrix. The electronic properties of AlkCCMoid have been evaluated by measurements on single crystals, microcrystalline powder, and sublimated samples, respectively. GIWAXS studies on surfaces and XRD studies on powder have revealed the existence of polymorphs and the effect that substrates have on curcuminoid organization. We describe the dielectric nature of our system and identify how different polymorphs can affect electronic parameters such as permittivity, all corroborated by DFT calculations.

Cibalackrot Dendrimers for Hyperfluorescent Organic Light-Emitting Diodes

Hyperfluorescent organic light-emitting diodes (HF-OLEDs) enable a cascading Förster resonance energy transfer (FRET) from a suitable thermally activated delayed fluorescent (TADF) assistant host to a fluorescent end-emitter to give efficient OLEDs with relatively narrowed electroluminescence compared to TADF-OLEDs. Efficient HF-OLEDs require optimal FRET with minimum triplet diffusion via Dexter-type energy transfer (DET) from the TADF assistant host to the fluorescent end-emitter. To hinder DET, steric protection of the end-emitters has been proposed to disrupt triplet energy transfer. In this work, the first HF-OLEDs based on structurally well-defined macromolecules, dendrimers is reported. The dendrimers contain new highly twisted dendrons attached to a Cibalackrot core, resulting in high solubility in organic solvents. HF-OLEDs based on dendrimer blend films are fabricated to show external quantum efficiencies of >10% at 100 cd m^-2 . Importantly, dendronization with the bulky dendrons is found to have no negative impact to the FRET efficiency, indicating the excellent potential of the dendritic macromolecular motifs for HF-OLEDs. To fully prevent the undesired triplet diffusion, Cibalackrot dendrimers HF-OLEDs are expected to be further improved by adding additional dendrons to the Cibalackrot core and/or increasing dendrimer generations.

Amplified Spontaneous Emission from Zwitterionic Excited-State Intramolecular Proton Transfer

The unique four-level photocycle characteristics of excited-state intramolecular proton transfer (ESIPT) materials enable population inversion and large spectral separation between absorption and emission through their respective enol and keto forms. This leads to minimal or no self-absorption losses, a favorable feature in acting as an optical gain medium. While conventional ESIPT materials with an enol-keto tautomerism process are widely known, zwitterionic ESIPT materials, particularly those with high photoluminescence, are scarce. Facilitated by the synthesis and characterization of a new family of 2-hydroxyphenyl benzothiazole (HBT) with fluorene substituents, HBT-Fl1 and HBT-Fl2, we herein report the first efficient zwitterionic ESIPT lasing material (HBT-Fl2). The zwitterionic ESIPT HBT-Fl2 not only shows a remarkably low solid-state amplified spontaneous emission (ASE) threshold of 5.3 μJ/cm² with an ASE peak at 609 nm but also exhibits high ASE photostability. Coupled with its substantially large Stokes shift (≈236 nm ≈10,390 cm^-1) and an extremely small overlap of excited-state absorption with ASE emission, comprehensive density functional theory (DFT) and time-dependent DFT studies reveal the zwitterionic characteristics of HBT-Fl2. In opposition to conventional ESIPT with π-delocalized tautomerism as observed in analogue HBT-Fl1 and parent HBT, HBT-Fl2 instead shows charge redistribution in the proton transfer through the fluorene conjugation. This structural motif provides a design tactic in the innovation of new zwitterionic ESIPT materials for efficient light amplification in red and longer-wavelength emission.

Synthesis and n-Type Semiconducting Properties of Bis(dioxaborin) Compounds Containing a π-Extended 2,2'-Bithiophene Structure

Bis(dioxaborin) compounds containing π-conjugated systems have been studied as n-type semiconductors for organic field-effect transistors (OFETs). In this study, with the aim of investigating the effect of the extension of the π-conjugation on the n-type semiconducting properties and stability of bis(dioxaborin) compounds, we synthesized new compounds containing 2,2'-bithiophene derivatives extended with an olefin or an acetylene spacer. The absorption maxima of the compounds containing olefin spacers were greatly red-shifted compared with those of the original compound without a π-spacer. The newly synthesized compounds exhibited high electron affinity, and the olefin spacers effectively reduced the on-site Coulomb repulsion in the two-electron reduction of the compounds. An OFET fabricated using one of these compounds having a layer-by-layer crystal structure exhibited n-type semiconductor behavior with a low threshold voltage, most likely due to the small on-site Coulomb repulsion. The electron-transporting properties were investigated by theoretical calculations based on the Marcus theory.

Recent advances in dioxaborine-based fluorescent materials for bioimaging applications

Fluorescent materials are continuously contributing to important advances in the field of bioimaging. Among these materials, dioxaborine-based fluorescent materials (DBFM) are arousing growing interest. Due to their rigid structures conferred by a cyclic boron complex, DBFM possess appealing photophysical properties including high extinction coefficients and quantum yields as well as emission in the near infrared, enhanced photostability and high two-photon absorption. We herein discuss the recent advances of DBFM that found use in bioimaging applications. This review covers the development of fluorescent molecular probes for biomolecules (DNA, proteins), small molecules (cysteine, H₂O₂, oxygen), ions and the environment (polarity, viscosity) as well as polymers and nanomaterials used in bioimaging. This review aims at providing a comprehensive and critical insight on DBFM by highlighting the assets of these promising materials in bioimaging but also by pointing out their limitations that would require further developments.

Aggregation-induced emissions in the mesogenic BF2 complexes of aroylhydrazines

PL peak intensity of 1a-BF2 (n = 12) versus f_w of THF/H₂O in pure THF (f_w = 0) and THF/water with a specific f_w. The concentration is 10 μM and PL emission was excited at 376 nm. Photos were taken under UV illumination.

Organic solid-state lasers: a materials view and future development

Lasing applications have spread over various aspects of human life. To meet the developing trends of the laser industry towards being miniature, portable, and highly integrated, new laser technologies are in urgent demand. Organic semiconductors are promising gain medium candidates for novel laser devices, due to their convenient processing techniques, ease of spectral and chemical tuning, low refractive indexes, mechanical flexibilities, and low thresholds, etc. organic solid-state lasers (OSSLs) open up a new horizon of simple, low-cost, time-saving, versatile and environmental-friendly manufacturing technologies for new and desirable laser structures (micro-, asymmetric, flexible, etc.) to unleash the full potential of semiconductor lasers for future electronics. Besides the development of optical feedback structures, the design and synthesis of robust organic gain media is critical as a vigorous aspect of OSSLs. Herein, we provide a comprehensive review of recent advances in organic gain materials, mainly focused on organic semiconductors for OSSLs. The significant breakthroughs toward electrical pumping of OSSLs are emphasized. Opportunities, challenges and future research directions for the design of organic gain media are also discussed.

High-Performance Solution-Processed Red Thermally Activated Delayed Fluorescence OLEDs Employing Aggregation-Induced Emission-Active Triazatruxene-Based Emitters

Two novel red thermally activated delayed fluorescence (TADF) emitters [triazatruxene (TAT)-dibenzo[a,c]phenazine (DBPZ) and TAT-fluorine-substituted dibenzo[a,c]phenazine (FDBPZ)] were developed by incorporating TAT as the electron donor (D) and DBPZ or FDBPZ as the electron acceptor (A). Both compounds showed aggregation-induced emission behaviors and bright red emission in neat films. Benefited from the rigid and large planar conjugated structure of TAT and DBPZ, TAT-DBPZ and TAT-FDBPZ realized high photoluminescence quantum yields in solid states. Meanwhile, the large steric hindrance between TAT and DBPZ segments produced small singlet-triplet energy splitting (ΔE_ST), leading to short delayed fluorescence lifetimes and high reverse intersystem crossing (RISC) rate (>10⁶ s^-1) for both compounds. The solution-processable doped organic light-emitting diodes (OLEDs) based on TAT-DBPZ achieved a high external quantum efficiency (EQE) of 15.4% with a red emission peak at 604 nm, which was one of the highly efficient solution-processable red TADF OLEDs. TAT-FDBPZ-based doped devices also showed a red emission peak at 611 nm with a maximum EQE of 9.2% and low-efficiency roll-off ratios of 1.0% at 100 cd m^-2 and 19% at 1000 cd m^-2. Furthermore, their solution-processable nondoped devices displayed EQEs of 5.6 and 2.9% with the red-shifted emission peaks at 626 and 641 nm, respectively. These results indicate the huge potential of utilization of TAT as the donor unit to achieve highly efficient and low-efficiency roll-off solution-processable red TADF OLEDs.

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.

Four-Coordinate Organoboron Platforms for Efficient Red Phosphorescent Organic Light-Emitting Diodes

So far both three- and four-coordinate organoboron compounds have been widely applied in organic light-emitting diode (OLED) materials. However, the use of four-coordinate organoboron compounds as host materials is rarely reported. In this work, two new four-coordinate organoboron compounds, namely 8-(4-(9H-carbazol-9-yl)phenyl)-6,6-difluoro-6H-6λ⁴ ,7λ⁴ -benzo[4',5']imidazo[1',2':3,4][1,3,2]diazaborolo[1,5-a]pyridine (B1PCz) and 8-(3-(9H-carbazol-9-yl)phenyl)-6,6-difluoro-6H-6λ⁴ ,7λ⁴ -benzo[4',5']imidazo[1',2':3,4][1,3,2]diazaborolo[1,5-a]pyridine (B1MCz), were successfully designed, synthesized, and fully characterized. The red OLEDs using B1PCz and B1MCz as host materials achieved relatively high device performance with a maximum external quantum efficiency of 14.8 % and 11.8 %, respectively. These results will expand the scope of organoboron compounds for OLED materials and reveal the great potential of four-coordinate organoboron materials.

Simple construction of deep-red hexaazatrinaphthylene-based thermally activated delayed fluorescence emitters for efficient solution-processed OLEDs with a peak at 692 nm

Two solution-processable deep-red thermally activated delayed fluorescence emitters based on a 5,6,11,12,17,18-hexaazatrinaphthylene acceptor core were developed through a simple and catalyst-free nucleophilic substitution reaction. The solution-processed OLEDs using these emitters achieved a peak external quantum efficiency of 4.8% with the emission peak at 692 nm.

Effect of substitution on the excited state photophysical and spectral properties of boron difluoride curcumin complex dye and their derivatives: A time dependent-DFT study

The optical, charge transport and electronic properties of boron difluoride curcumin (BFC) complex have been explored using the DFT (Density Functional Theory) method and B3LYP functional with the combination of 6-31 + G(d,p) as a basis set. The influence of substitution with various electron releasing and withdrawing groups on the above properties is analyzed and discussed in this work. The results reveal that the BFC complex on additional electron releasing substitution experiences redshifts in the optical transitions, and this is correlated with the dipole moment, NBO charges, HOMO-LUMO energy gap. Further, the absorption (λ_abs) and emission (λ_ems) spectra of substituted and unsubstituted BFCs are calculated using Time-Dependent Density Functional Theory (TD-DFT). The results show that the electron releasing groups strongly influence the absorption and emission spectra of BFC. Electron releasing groups in BFC derivatives generate the wavelength shift (Bathochromic), but the electron-withdrawing groups in BFC don't affect the λ_abs and λ_ems when compare to its original (parent) compound. The output of the research work strongly recommends that the amino, phenyl and N, N'-dimethylamino derivatives are potential candidates to act as fluorescent materials due to enhance the emission behavior of BFC and also can be used as an electron/charge transport material for organic light-emitting diodes (OLEDs).

Efficient Orange-Red Thermally Activated Delayed Fluorescence Emitters Feasible for Both Thermal Evaporation and Solution Process

Development of red thermally activated delayed fluorescence (TADF) emitters has been lagging behind when compared with those of blue and green fluorophores, especially for solution-processable ones. In this work, two novel orange-red TADF emitters 3,6-di(10H-phenoxazin-10-yl)dibenzo[a,c]phenazine (DBPZ-DPXZ) and 10,10'-(11,12-bis(3,5-di-tert-butylphenyl)dibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (tDBBPZ-DPXZ) are developed. A high-performance orange-red TADF emitter, DBPZ-DPXZ, is first prepared by connecting a rigid acceptor and two rigid donor segments. While this design strategy endows DBPZ-DPXZ with an excellent TADF performance leading to a vacuum-processed organic light-emitting diode (OLED) with a high external quantum efficiency (EQE) of 17.8%, the rigid segments limit its solubility and applications in solution-processed devices. Based on this prototype, tDBBPZ-DPXZ is designed with the addition of 3,5-di-tert-butylphenyl groups to boost its solubility with barely an influence on the photophysical properties. In particular, tDBBPZ-DPXZ maintains nearly an identical photoluminescence quantum yield of 83% and singlet-triplet energy splitting of 0.03 eV with EQE of 17.0% in a vacuum-processed orange-red OLED. Furthermore, it can be applied on the orange-red solution-processed OLED realizing an EQE as high as 10.1%, representing one of the state-of-the-art results of the reported orange-red solution-processed TADF-OLEDs. This work provides an effective strategy to address the conflicting requirements between high efficiency and good solubility and develop efficient soluble orange-red TADF emitters.

Realizing 22.5% External Quantum Efficiency for Solution-Processed Thermally Activated Delayed-Fluorescence OLEDs with Red Emission at 622 nm via a Synergistic Strategy of Molecular Engineering and Host Selection

Developing high-efficiency solution-processable thermally activated delayed-fluorescence (TADF) emitters, especially in longer wavelength regions, is a formidable challenge. Three red TADF emitters, namely NAI_R1, NAI_R2, and NAI_R3, are developed by phenyl encapsulation and tert-butyl substitution on a prototypical 1,8-naphthalimide-acridine hybrid. This design strategy not only grants these molecules high solubility, excellent thermal stability, and good film-forming ability, but also pulls down their charge-transfer (CT) energy levels excited states. Furthermore, dispersing these emitters into two different host materials of mCP and mCPCN finely tailors their CT-state energy levels. More importantly, a synergistic combination of molecular engineering and host selection can effectively manipulate the competition between the radiative and nonradiative decay rates of the CT singlet states of these emitters and the reverse intersystem crossing from their triplet to singlet states. Consequently, the optimal combination of NAI_R3 emitter and mCP host successfully results in a state-of-the-art external quantum efficiency (EQE) of 22.5% for solution-processed red TADF organic light-emitting diodes (OLEDs) with an emission peak above 620 nm. This finding demonstrates that a synergistic strategy of molecular engineering and host selection with TADF emitters could provide a new pathway for developing efficient solution-processable TADF systems.

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.

High-Efficiency Red and Near-Infrared Organic Light-Emitting Diodes Enabled by Pure Organic Fluorescent Emitters and an Exciplex-Forming Cohost

Three D-A-D-configured molecules DTPBT, DTPNT, and DTPNBT with high quantum yield of orange red (628 nm), red (659 nm), and deep-red/NIR (710 nm) fluorescence, respectively, were developed as emitting dopants in an exciplex-forming cohost (TCTA:3P-T2T) for high-efficiency fluorescence-based organic light-emitting diodes (OLEDs). The obtained physical properties together with theoretical calculations analyzed from these new molecules establish a clear structure-property relationship, in which the feature of central acceptor 2,1,3-benzothiadiazole (BT), naphtho[1,2-c:5,6-c']bis[1,2,5]thiadiazole (NT), and 2,1,3-naphthothiadiazole (NBT) plays the crucial role for governing the physical characteristics. The optimized device configured as ITO/HAT-CN/TAPC/TCTA/TCTA:3P-T2T:5% emitter/3P-T2T/LiF/Al gave a record-high efficiency of orange red (591 nm, 15%), red (647 nm, 10%), and deep-red/NIR (689 nm, 9%) electroluminescent devices. The effective harvest of triplet excitons with an exciplex-forming system in conjunction with efficient energy transfer between the exciplex and the dopant is beneficial for such high device efficiencies. More importantly, the stable exciplex-forming cohost and fast radiative decay rate of DTPNT render this particular device exhibiting high device stability as indicated by the low efficiency roll-off under high current densities (EQE (external quantum efficiency) values of 8.1% at 1000 cd m^-2 and 6.8% at 10,000 cd m^-2). These results reveal the potential of employing an exciplex-forming system as cohost for fluorescent dopants to furnish high-efficiency OLEDs with an emission wavelength extending to the red or even the NIR range.

Phenothiazinen-Dimesitylarylborane-Based Thermally Activated Delayed Fluorescence: High-Performance Non-doped OLEDs With Reduced Efficiency Roll-Off at High Luminescence

We report a phenothiazinen-dimesitylarylborane thermally activated delayed fluorescence (TADF) molecule that exhibits high external quantum efficiency (EQE) in non-doped organic light-emitting diodes (OLEDs) at high luminescence. The non-doped device shows green electroluminescence with an emission peak of 540 nm and a maximum EQE of 19.66% obtained at a luminescence of ~170 cd m-2. The EQE is still as high as 17.31% at a high luminescence of 1,500 cd m-2 with small efficiency roll-off.

Difluoroboron Chelation to Quinacridonequinone: A Synthetic Method for Air-Sensitive 6,13-Dihydroxyquinacridone via Boron Complexes

This study aims to perform the chelation of difluoroboron (BF₂ ) to quinacridonequinone (QQ). The resulting dark green solid was determined to be QA-BF₂ , which is a BF₂ complex of 6,13-dihydroxyquinacridone (QA-OH), and not QQ-BF₂ , which is a BF₂ complex of QQ. This result indicated that QQ-BF₂ was first generated as an O,O-bidentate chelate, which immediately underwent a two-electron reduction to produce QA-BF₂ . This compound was converted to air-sensitive QA-OH by undergoing hydrolysis in argon. Since QA-OH has a strong electron-donating property, it easily produced QQ via air oxidation in the solution. QA-OH also acts as a reducing reagent for quinones. The crystal packing of QA-OH is a herringbone type with short π⋅⋅⋅π contacts, and a good hole mobility has been suggested by theoretical calculations. Herein, a new synthetic method from QQ to QA-OH using BF₂ chelation and hydrolysis was proposed. QA-BF₂ and QA-OH are useful organic functional pigments and reducing reagents.

Fluorescent BF2 complexes of pyridyl-isoindoline-1-ones: synthesis, characterization and their distinct response to mechanical force

Three boron-pyridyl-isoindoline-1-one based dyes (B1, B2, and B3) with varied side groups were synthesized and their mechanochromic fluorescence properties were studied.

Control of the dual emission from a thermally activated delayed fluorescence emitter containing phenothiazine units in organic light-emitting diodes

A novel TADF emitter showing dual emission is synthesized and applied to OLEDs with an external quantum efficiency of 11.5%.

The effect of conformational isomerism on the optical properties of bis(8-oxyquinolato) diboron complexes with a 2,2'-biphenyl backbone

A fluorescent bis(8-oxyquinolato) diborinic complex with a central 2,2'-biphenyl backbone 1 and its octafluoro analogue 2 were synthesized to study the optical-structural relationship of sterically encumbered molecules featuring close intramolecular π-stacking interactions involving chromophore units. The crystal structure of 1 revealed a unique π-stacked arrangement of two pendant phenyl groups and two 8-oxyquinolato ligands (Q) located in the inner part of the complex. Unlike 1, the closely related complex 2 features conformational isomerism, and two major forms, namely 2-syn and 2-anti, are observed in solution to a varying extent depending on the solvent polarity. Form 2-syn, a geometrical analogue of 1, is preferable in polar solutions, whereas its rotational isomer 2-anti featuring π-stacking interactions between the terminal phenyl group and Q ligand dominates in benzene and chloromethane solutions. The observed conformational equilibria strongly affect the optical properties of the system, specifically leading to a significant increase of the quantum yield of emission (from 22% in MeCN to 38% in benzene) accompanied by a bathochromic shift (Δλ = 10 nm) of absorption and hypsochromic shifts (Δλ = -8 nm) of emission spectra with decreasing solvent polarity. This effect was ascribed to the variation in frontier orbital distributions.

Blue-Shifting Intramolecular Charge Transfer Emission by Nonlocal Effect of Hyperbolic Metamaterials

Metallic nanostructures permit controlling various photophysical processes by coupling photons with plasmonic oscillation of electrons confined in the tailored nanostructures. One example is hyperbolic metamaterial (HMM) leading to an enhanced spontaneous emission rate of emitters located nearby. Noting that emission in organic molecules is from either π-π* or intramolecular charge-transfer (ICT) states, we address here how HMM modifies ICT emission spectral features by comparing them with a spectral shift dependent on the local polarity of the medium. The 7.0 nm blue shift is observed in ICT emission from 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dispersed into a polymer matrix prepared on HMM multilayered structure, while no spectral shift is observed in π-π* emission from perylene diimide. In the frame of the Lippert-Mataga formalism, the blue shift is explained by the HMM nonlocal effects resulting from 8% decrease in refractive index and 18% reduction in dielectric permittivity. This phenomenon was also shown in a hemicurcuminoid borondifluoride dye yielding 15.0 nm blue shift. Such a capability of spectral shift control in films by HMM structure opens new prospects for engineering organic light-emitting devices.

Solvatochromism, acidochromism and aggregation-induced emission of propeller-shaped spiroborates

Propeller-shaped pyridyl-enolato-catecholate/-salicyl spiroborates (Sborepy1–6) were synthesized.