Structurally Tunable Donor-Bridge-Fluorophore Architecture Enables Highly Efficient and Concentration-Independent Narrowband Electroluminescence.

Luminescent materials with narrowband emission have extraordinary significance for developing ultrahigh-definition display. B-N-containing multiple resonance thermally activated delayed fluorescence (MR-TADF) materials are strong contenders. However, their device performances pervasively encounter detrimental aggregation-caused quenching effect that is highly vulnerable to doping concentration, complicating device fabrication. Therefore, constructing highly efficient and concentration-independent MR-TADF emitters is of pragmatic importance for improving device controllability and reproducibility, simplifying manufacturing procedures, and conserving production costs. Here, by systematic arrangement of donor triphenylamine and fluorophore BNCz on distinct bridges, a spatial confinement strategy has been developed with a donor-bridge-fluorophore architecture. Structurally fine modulation and progressive evolution to construct molecular entities with congested steric hindrance effect that can suppress intermolecular interactions without substantially affecting the luminescence tone of fluorophore BNCz, resulting in highly efficient and concentration-independent narrowband emitters; through isomer engineering, two isomers BN-PCz-TPA and TPA-PCz-BN with different crystal stacking patterns are synthesized by altering the connection mode between triphenylamine and BNCz. As a result, BN-PCz-TPA-based device showcases maximum external quantum efficiency (EQE) of 36.3% with narrow full-width at half-maximum of 27 nm at 10 wt% doping concentration. Even at 20 wt% doping concentration, the maximum EQE remains at 32.5% and the emission spectrum is almost unchanged.

Strategies to Diversification of the Mechanical Properties of Organic Crystals.

Structurally ordered soft materials that respond to complementary stimuli are susceptible to control over their spatial and temporal morphostructural configurations by intersectional or combined effects such as gating, feedback, shape-memory, or programming. In the absence of general and robust design and prediction strategies for their mechanical properties, at present, combined chemical and crystal engineering approaches could provide useful guidelines to identify effectors that determine both the magnitude and time of their response. Here, we capitalize on the purported ability of soft intermolecular interactions to instigate mechanical compliance by using halogenation to elicit both mechanical and photochemical activity of organic crystals. Starting from (E)-1,4-diphenylbut-2-ene-1,4-dione, whose crystals are brittle and photoinert, we use double and quadruple halogenation to introduce halogen-bonded planes that become interfaces for molecular gliding, rendering the material mechanically and photochemically plastic. Fluorination diversifies the mechanical effects further, and crystals of the tetrafluoro derivative are not only elastic but also motile, displaying the rare photosalient effect.

Bridging of Cove Regions: A Strategy for Realizing Persistently Chiral Double Heterohelicenes with Attractive Luminescent Properties.

The racemization of chiral organic compounds is a common chemical phenomenon. However, it often poses configurational-stability issues to the application of this class of compounds. Achieving chiral organic compounds without the risk of racemization is fascinating, but it is challenging due to a lack of strategies. Here, we reveal the cove-regions bridging strategy for achieving persistently chiral multi-helicenes (incapable of racemization), based on the synthesized proof-of-concept double hetero[4]helicenes featuring macrocycle structures with a small 3D cavity. Additionally, we demonstrate that the strategy is also effective in tuning the electronic structures of multi-helicenes, resulting in a conversion from luminescence silence into thermally activated delayed fluorescence (TADF) for the present system. Furthermore, red circularly polarized TADF based on small double [4]helicene systems is achieved for the first time using this strategy. The disclosed cove-regions bridging strategy provides an opportunity to modulate the electronic structures and luminescent properties of multi-helicenes without concern for racemization, thus significantly enhancing the structural and property diversity of multi-helicenes for various applications.

Elastic Organic Crystals Exhibiting Amplified Spontaneous Emission Waveguides with Standard Red Chromaticity of the Rec.2020 Gamut.

The use of mechanically flexible molecular crystals as optical transuding media is demonstrated for a plethora of applications; however, the spectral peaks of optical outputs located mainly in the range of 400-600 nm are insufficient for practical telecommunication and full-color display applications. Herein, two elastically bendable organic crystals are reported that show red emission of the rec.709 gamut under 365 nm UV light irradiation yet generate rec.2020 gamut red optical waveguides and amplified spontaneous emissions when irradiated by a 355 nm laser. Capitalizing on the extended π-conjugation and donor-acceptor character, as well as mechanical elasticity, these organic crystals exhibit flexible optical waveguides with Commission Internationale de L'Eclairage (CIE) coordinates of (0.70, 0.29), nearly identical to the red chromaticity of the rec.2020 gamut required for ultrahigh-definition (UHD) displays. Notably, one of the elastic crystals functions as a soft resonance cavity, resulting in amplified spontaneous emission waveguides with CIE coordinates of (0.71, 0.29) and the standard red chromaticity of the rec.2020 gamut, both in straight and bent states. This study presents a new avenue for the development of high-purity red-emissive crystalline materials to create all-organic, lightweight, and mechanically compliant optical telecommunication and UHD display devices.

Carbonyl- and Nitrogen-Embedded Multi-Resonance Emitter with Ultra-Pure Green Emission and High Electroluminescence Efficiencies.

Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters with narrow emission spectra have garnered significant attention in future organic light-emitting diode (OLED) displays. However, current C=O/N-embedded MR-TADF systems still lack satisfactory performance in terms of electroluminescence bandwidths and external quantum efficiencies (EQEs). In this study, a C=O/N-embedded green MR-TADF emitter, featuring two acridone units incorporated in a sterically protected 11-ring fused core skeleton, is successfully synthesized through finely controlling the reaction selectivity. The superior combination of multiple intramolecular fusion and steric wrapping strategies in the design of the emitter not only imparts an extremely narrow emission spectrum and a high fluorescence quantum yield to the emitter but also mitigates aggregation-induced spectral broadening and fluorescence quenching. Therefore, the emitter exhibits leading green OLED performance among C=O/N-based MR-TADF systems, achieving an EQE of up to 37.2 %, a full width at half maximum of merely 0.11 eV (24 nm), and a Commission Internationale de l'Éclairage coordinate of (0.20, 0.73). This study marks a significant advance in the realization of ideal C=O/N-based MR-TADF emitters and holds profound implications for the design and synthesis of other MR-TADF systems.

The Factor beyond Schmidt's Criteria Impacting the Photo-Induced [2+2] Cycloaddition Reactivity and Photoactuation of Molecular Crystals Based on Cyclic Chalcone Analogues.

Generally, the potential reactive "olefin pairs" in the molecular crystals satisfying Schmidt's criteria could undergo topological [2+2] cycloaddition. In this study, another factor that affects the photodimerization reactivity of chalcone analogues was found. The cyclic chalcone analogues of (E)-2-(2,4-dichlorobenzylidene)-2,3-dihydro-1H-inden-1-one (BIO), (E)-2-(naphthalen-2-ylmethylene)-2,3-dihydro-1H-inden-1-one (NIO), (Z)-2-(2,4-dichlorobenzylidene)benzofuran-3(2H)-one (BFO), and (Z)-2-(2,4-dichlorobenzylidene)benzo[b]thiophen-3(2H)-one (BTO) have been synthesized. While the geometrical parameters for the molecular packing of the above four compounds did not exceed Schmidt's criteria, [2+2] cycloaddition did not occur in the crystals of BIO and BTO. The single crystal structures and Hirshfeld surface analyses revealed that interactions of C=O⋅⋅⋅H (CH2 ) existed between adjacent molecules in the crystal of BIO. Therefore, the carbonyl and methylene groups linked with one carbon atom in carbon-carbon double bond were tightly confined in the lattice, acting as a tweezer to inhibit free movement of the double bond and suppressing [2+2] cycloaddition. In the crystal of BTO, similar interactions of Cl⋅⋅⋅S and C=O⋅⋅⋅H (C6 H4 ) prevented free movement of the double bond. In contrast, the intermolecular interaction of C=O⋅⋅⋅H only exists around the carbonyl group in the crystals of BFO and NIO, leaving the C=C double bonds to move freely and allowing the occurrence of [2+2] cycloaddition. Driven by photodimerization, the needle-like crystals of BFO and NIO displayed evident photo-induced bending behavior. This work demonstrates that the intermolecular interactions around carbon-carbon double bond affect the [2+2] cycloaddition reactivity except for Schmidt's criteria. These findings provide valuable insights into the design of photomechanical molecular crystalline materials.

Toward Narrowband and Efficient Blue Fluophosphors by Locking the Stretching Vibration of Indolocarbazole Skeletons.

Developing efficient and color-saturated deep-blue emitting molecules with small Commission Internationale de L'Eclairage (CIE) y values is challenging and has great potential for wide-color gamut displays. Herein, we introduce an intramolecular locking strategy to restrain molecular stretching vibrations of the emission spectral broadening. By cyclizing rigid fluorenes and connecting electron-donating groups to the indolo[3,2,1-jk]-indolo[1',2',3':1,7]indolo[2,3-b]carbazole (DIDCz) framework, the in-plane swing of peripheral bonds and stretching vibrations of the indolocarbazole skeleton are restricted due to an increased steric hindrance from cyclized groups and diphenylamine auxochromophores. As a result, reorganization energies at the high-frequency region (1300-1800 cm-1) are reduced, realizing pure blue emission with a small full-width-at-half-maximum (FWHM) of 30 nm by suppressing shoulder peaks of polycyclic aromatic hydrocarbon (PAH) frameworks. The fabricated bottom-emitting organic light-emitting diode (OLED) exhibits an efficient external quantum efficiency (EQE) of 7.34% and deep-blue coordinates of (0.140, 0.105) at a high brightness of 1000 cd/m2. The FWHM of the electroluminescent spectrum is only 32 nm, which is one of the narrowest electroluminescent emissions among the reported intramolecular charge transfer fluophosphors. Our current findings provide a new molecular design strategy to conceive efficient and narrowband emitters with small reorganization energies.

Distorted B/O-containing nanographenes with tunable optical properties.

We report the synthesis of two B/O-containing nanographenes, which feature the fusion of three or six planar B/O-heterocycles onto one hexa-peri-hexabenzocoronene π-framework. Incorporation of the B/O-heterocycles not only leads to distorted geometries, but also modulates the electronic structures and results in gradually red-shifted absorptions and fluorescence.

Molecular Twisting Affects the Solid-State Photochemical Reactions of Unsaturated Ketones and the Photomechanical Effects of Molecular Crystals.

Three groups of chalcone derivatives and their analogues involving halogen atoms (X=F, Cl, Br) have been synthesized. Firstly, the nearly planar acyclic chalcone derivatives were inclined to undergo photo-induced stereospecific [2+2] cycloaddition, which triggered the crystals to exhibit macroscopic motions of bending or cracking. In particular, the single-crystal-to-single-crystal transformation happened upon UV irradiation of the crystals, which was helpful for the understanding photomechanical effects. Cyclic 3,4-dihydronaphthalene-based chalcone analogues possess a more twisted conformation, and they tend to undergo trans-cis isomerization. No photomechanical effect was observed for the crystals of the cyclic chalcone analogues due to the lower isomerization rate. The twist degree of chroman-based molecules was in between of the first two, [2+2] cycloaddition and trans-cis isomerization simultaneously took place in crystals. Photo-induced bending and twisting were observed for the crystals of chroman-based chalcone analogues. Therefore, the differences in molecular dihedral angles in α,ß-unsaturated ketones were responsible for their photochemical characters and in turn to tune the photomechanical effects. In this work, a bridge between the molecular structures and solid-state photochemical reactions triggered photomechanical crystals is built.

Crystalline Porous Organic Salt for Ultrarapid Adsorption/Desorption-Based Atmospheric Water Harvesting by Dual Hydrogen Bond System.

In recent years, the porous sorbent-assisted atmospheric water harvesting (AWH) method has emerged as an effective approach for solving water crises without geographical restrictions. However, there is a limited array of porous adsorbent materials that can be used for AWH, which are inadequate to meet the needs under different climatic conditions. In light of this, herein, we synthesize a new crystalline porous organic salt (CPOS; denoted as CPOS-6) possessing a dual hydrogen bond system and verify its applicability toward AWH for the first time. Unlike other reported CPOSs, CPOS-6 displays an S-shaped water sorption isotherm owing to the presence of the dual hydrogen bond system. Under simulated drought conditions in Xinjiang Uygur Autonomous Region, CPOS-6 exhibits long-term water adsorption-desorption cycling stability, low water desorption temperature, and ultrarapid adsorption-desorption kinetics. The results confirm that CPOS-6 is an effective sorbent material for AWH.

Ribbon-Type Boron-Doped Polycyclic Aromatic Hydrocarbons: Conformations, Dynamic Complexation and Electronic Properties.

Molecular ribbons (MRs), one-dimentional topological polycyclic aromatic hydrocarbons (PAHs), are of importance for synthetic chemistry and material sciences. Herein, we disclose an effective strategy to develop boron-doped MRs, i.e. photochemical cyclization on conjugated organoboranes for rapid π-extension. A series of ribbon-type boron-doped PAHs that own multiple cove edges were synthesized using Mallory photoreaction in solution. Two of them feature isomeric C68 B2 π-skeletons with 2.2 nm in length, thus representing a new kind of boron-doped MRs. The boron atoms endow them with sufficient Lewis acidity, and notably, the formed Lewis acid-base adducts based on boron-doped MRs display the photo-induced dual-dissociation behavior in the excited state and thus photochromism property. Moreover, despite of the highly contorted topological conformations, their potential utility as organic semiconductor was demonstrated by fabrication of organic field-effect transistors.

Integrating Low-Temperature-Resistant Two-Dimensional Elastic-Bending and Reconfigurable Plastic-Twisting Deformations into an Organic Crystal.

Integrating plasticity and elasticity in anisotropic molecular crystals is theoretically possible and is beneficial in enabling rich and complex deformations; however, it is much harder to implement in practice. Herein, we report a centimeter-long organic crystal that is two-dimensionally (2D) elastically bendable not only at room temperature but also at ultralow temperatures (-196 °C). The straight crystal can also be manually twisted and reconfigured to form arbitrary right-handed or left-handed helical structures. The integration of low-temperature-resistant (LTR) 2D elastic-bending and reconfigurable plastic-twisting deformations into one organic crystal expands the perspectives of the emerging crystal flexibility. Taking advantage of the unique multiple flexibility characteristics, spatial controllability of optical transmission for cryogenic applications and reusability of light-polarization rotations have been implemented simultaneously in an organic crystal.

PO-containing dibenzopentaarenes: facile synthesis, structures and optoelectronic properties.

Incorporation of heteroatoms into polyarenes has been developed as an effective approach to alter their intrinsic structures and properties. Herein, we designed and synthesized two PO-containing dibenzopentaarene isomers (5a and 5b) and studied their structures and properties, along with those of dibenzopentaarenes containing six-membered Si- and B-heterocycles (3 and 4). These heterocyclic polyarenes have similar frameworks to well-known heptazethrene, and thus can be regarded as members of the heteroatom-doped zethrene system. The heterocycles greatly affect not only the molecular and packing structures but also the electronic structures and properties. Notably, while compounds 3 and 4 adopt almost planar geometries, 5a possesses a clearly curved conformation, leading to its brick-type slipped and dense π-π stacking mode. Moreover, the electron-withdrawing PO groups endow 5a and 5b with simultaneously lowered lowest unoccupied molecular orbital (LUMO)/highest occupied molecular orbital (HOMO) levels, whereas the p-π conjugation of the B atoms in 4 leads to its smaller energy gap and thus remarkably red-shifted absorption and fluorescence bands by over 80 nm, though all of these molecules possess similar closed-shell structures. This study thus deepens the understanding of heteroatom-doping effects, which may be expanded to develop other heteroatom-doped zethrene materials.

A Low-Temperature-Resistant Flexible Organic Crystal with Circularly Polarized Luminescence.

Flexible organic crystals with unique mechanical properties and excellent optical properties are of paramount significance for their wide applications in various research fields such as adaptive optics and soft robotics. However, low-temperature-resistant flexible organic crystal with circularly polarized luminescence (CPL) has never been reported. Herein, chiral organic crystals with CPL activity and low-temperature flexibility (77 K) are fabricated by the solvent diffusion method from chiral Schiff base, S(R)-4-bromo-2-(((1-phenylethyl)imino)methyl)phenol (S(R)-BPEMP). The corresponding chirooptical properties for the two enantiomeric crystals were thoroughly investigated, including the measurements of circular dichroism (CD) and CPL. To the best of our knowledge, this is the first report on low-molecular-weight flexible organic crystals with CPL activity, and we believe that the results will give a new impetus to the research of organic crystals.

Thermo-Induced Single-Crystal-to-Single-Crystal Transformations and Photo-Induced [2+2] Cycloaddition Reactions in Polymorphs of Chalcone-Based Molecular Crystals: Multi-Stimuli Responsive Actuators.

The polymorphs of 2ClChMe-4 in Form I (ribbon-like crystal) and Form II (block-like crystal) were prepared, and they exhibited curling/flipping and expansion upon heating on account of single-crystal-to-single-crystal transformations. The irreversible phase transformations occurred separately at 53.2 °C and 57.8 °C for the crystals in Form I and Form II, during which the molecular conformation of 2ClChMe-4 changed and the molecules slipped along the (100) plane. Movement at the molecular level resulted in changes of cell parameters, which in turn led to macroscopic motions of the crystals upon heating. Additionally, the ribbon-like crystals of 2ClChMe-4 showed photo-induced bending driven by [2+2] cycloaddition. Accordingly, an actuator showing reversible bending behavior was fabricated triggered by light and heat successively. Like biomimetic self-actuators, such multi-stimuli mechanical responsive molecular crystals might have potential applications in soft robots, artificial muscles and microfluidic systems.

A TADF Emitter Featuring Linearly Arranged Spiro-Donor and Spiro-Acceptor Groups: Efficient Nondoped and Doped Deep-Blue OLEDs with CIEy <0.1.

Reported herein is a molecular design strategy of deep-blue emitters for resolving the lack of highly efficient deep-blue organic light-emitting diodes (OLEDs) featuring CIEy (Commission Internationale de l'Eclairage) color coordinates matching the display requirements (<0.1). The strategy is to combine weak spiro-donor and spiro-acceptor groups into a linear donor-π-acceptor type of thermally-activated delayed fluorescence molecule through a sterically bulky π-spacer. The strategy endows an emitter with deep-blue emission, a narrower emission bandwidth (51 nm in toluene), a high photoluminescence quantum yield (0.95 in toluene), weak concentration quenching, and efficient triplet-exciton utilization, which are all attractive characteristics for emitters of deep-blue OLEDs with lower CIEy coordinates. Owing to the rational design, the emitter has realized not only highly efficient doped deep-blue OLEDs with external quantum efficiencies (EQEs) up to 25.4 % and CIEy less than 0.1 but also so far the most efficient nondoped deep-blue OLED (EQE up to 22.5 %) with CIEy less than 0.1.

Self-Waveguide Single-Benzene Organic Crystal with Ultralow-Temperature Elasticity as a Potential Flexible Material.

With the increasing popularity and burgeoning progress of space technology, the development of ultralow-temperature flexible functional materials is a great challenge. Herein, we report a highly emissive organic crystal combining ultralow-temperature elasticity and self-waveguide properties (when a crystal is excited, it emits light from itself, which travels through the crystal to the other end) based on a simple single-benzene emitter. This crystal displayed excellent elastic bending ability in liquid nitrogen (LN). Preliminary experiments on optical waveguiding in the bent crystal demonstrated that the light generated by the crystal itself could be confined and propagated within the crystal body between 170 and -196 °C. These results not only suggest a guideline for designing functional organic crystals with ultralow-temperature elasticity but also expand the application region of flexible materials to extreme environments, such as space technology.

Engineering Mechanical Compliance of an Organic Compound toward Flexible Crystal Lasing Media.

Recently, organic crystals with mechanical flexibility have been emerging as a hot research topic due to their great potentials in flexible optoelectronics. However, organic crystals exhibiting elastic bending or plastic bending are relatively rare. In this study, we proposed a strategy to improve the probability of crystal flexibility as well as to regulate the mechanical properties by controlling polymorphism. Three different emissive organic polymorphs Cry-G, Cry-Y, and Cry-O with elastic, plastic, and brittle natures, respectively, were obtained by fine-tuning crystallization conditions of a diaryl ß-diketone compound. Cry-G was found to transduce light and amplify the self-waveguided emission efficiently along the crystal body in the elastically bent state, demonstrating its multifunctional applications in flexible optical devices. This study is of great scientific significance not only to engineer mechanical compliance of organic crystals but also to highlight the utility of "crystal flexibility".

Achieving High-Performance Pure-Red Electrophosphorescent Iridium(III) Complexes Based on Optimizing Ancillary Ligands.

Two new iridium(III) complexes were synthesized by introducing two trifluoromethyl groups into an ancillary ligand to develop pure-red emitters for organic light-emitting diodes (OLEDs). The electron-donating ability of the ancillary ligands is suppressed, owing to the electron-withdrawing nature of trifluoromethyl groups, which can reduce the HOMO energy levels compared with those of compounds without trifluoromethyl groups. However, the introduction of trifluoromethyl groups into the ancillary ligand has little impact on the LUMO energy levels. Therefore, a well-tuned, pure-red, excited-state energy was achieved by regulating the relative energy level between the HOMO and LUMO. OLEDs with these complexes as emitters showed high external quantum efficiencies (EQEs) of 26 % and realized high EQEs of about 25 % and fairly low driving voltages of 3.3-3.6 V for practical luminance of 1000 cd m-2 , as well as excellent Commission Internationale de L'Eclairage (CIE) coordinates of (0.66, 0.33) and (0.67, 0.33); thus, this demonstrates the successful molecular design strategy by modifying the electron-donating ability of ancillary ligand.

Optical Waveguiding Organic Single Crystals Exhibiting Physical and Chemical Bending Features.

Bendable (elastic and plastic) organic single crystals have been widely studied as emerging flexible materials with highly ordered packing structures. However, even though manifold bendable organic crystals have been recently reported, most of them bend in response to only one stimulus. Herein, we report an organic single crystal of (Z)-4-(1-cyano-2-(4-(dimethylamino)phenyl)vinyl)benzonitrile, which bends under external stress (physical process) and also hydrochloric acid atmosphere (chemical process). This observation indicates that a single organic crystal, whose structure has been optimized simultaneously at both the molecular and supramolecular levels, may display multiple crystal-bending modes. Furthermore, the crystals exhibit bright orange-yellow emission and can serve as an active low-loss optical waveguide in both the straight and the bent state, which indicates a potential optical application.