Polymorph‐Dependent Thermally Activated Delayed Fluorescence Emitters: Understanding TADF from a Perspective of Aggregation State

Current research on thermally activated fluorescence (TADF) emitters is mainly based on the molecular levels, while the aggregation states of TADF emitters are to be explored deeply. Now two multifunctional emitters are reported with simultaneous TADF, aggregation induced emission (AIE), and multicolor mechanochromic luminescence (MCL) features. Both emitters also show polymorph‐dependent TADF emission. Crystal structure analysis reveals that the polymorphism is ascribed to the mutable conformations in different aggregation states. This work brings new insight to TADF emitters from a perspective of aggregation states.     

Recent Advancements in and the Future of Organic Emitters: TADF‐ and RTP‐Active Multifunctional Organic Materials

Organic emitting compounds that are based on π‐conjugated skeletons have emerged as promising next‐generation materials for application in optoelectronic devices. In this Minireview, recent advances in the development of organic emitters that irradiate room‐temperature phosphorescence and/or thermally activated delayed fluorescence with extraordinary luminescence properties, such as aggregation‐induced emission, mechanochromic luminescence, and circularly polarized luminescence, are discussed.     

Multiple Anti‐Counterfeiting Guarantees from a Simple Tetraphenylethylene Derivative – High‐Contrasted and Multi‐State Mechanochromism and Photochromism

Herein the novel tetraphenylethylene (TPE) derivative 1 was designed with an integration of aggregation‐induced emission (AIE), multi‐state mechanochromism and self‐recovery photochromism. The molecule was susceptible to grinding, heating and vapor fuming and showed corresponding transition of its emission colors. The heated powder or single crystal of 1 exhibited reversible photochromism. After a short period of UV irradiation, it showed a bright red color, but recovered to its original white appearance within 1 min. The photochromism is due to the formation of photocyclization intermediates upon UV irradiation, while the eversible mechanochromism is attributed to the weak molecular interactions derived from head‐to‐tail stacking of the molecules. This reversible multi‐state, high‐contrasted and rapid responsive mechanochromic and photochromic property cooperatively provide double enhancement of a multimode guarantee in advanced anti‐counterfeiting.     

Aggregation‐Induced Emission Properties of Copper Iodide Clusters

The aggregation‐induced emission (AIE) properties of two different copper iodide clusters have been studied. These two [Cu₄I₄L₄] clusters differ by their coordinated phosphine ligand and the luminescent mechanochromic properties are only displayed by one of them. The two clusters are AIE‐active luminophors that exhibit an intense emission in the visible region upon aggregation. The formed particles present luminescent thermochromism comparable to that of the bulk compounds. The observed AIE properties can be attributed to suppression of nonradiative relaxation of the excited states in a more rigid state, in relation to the large structural relaxation of the excited triplet state. The differences observed in the AIE properties of the two clusters can be related to the different ligands. A correlation between the luminescence mechanochromic properties and the AIE effect is not straightforward, but the formation of “soft” molecular solids is a common characteristic that can explain the photoactive properties of these compounds.     

A 1,3‐Indandione‐Functionalized Tetraphenylethene: Aggregation‐Induced Emission,Solvatochromism, Mechanochromism,and Potential Application as a Multiresponsive Fluorescent Probe

A tetraphenylethene (TPE) derivative substituted with the electron‐acceptor 1,3‐indandione (IND) group was designed and prepared. The targeted IND‐TPE reserves the intrinsic aggregation‐induced emission (AIE) property of the TPE moiety. Meanwhile, owing to the decorated IND moiety, IND‐TPE demonstrates intramolecular charge‐transfer process and pronounced solvatochromic behavior. When the solvent is changed from apolar toluene to highly polar acetonitrile, the emission peak redshifts from 543 to 597 nm. IND‐TPE solid samples show an evident mechanochromic process. Grinding of the as‐prepared powder sample induces a redshift of emission from green (peak at 515 nm) to orange (peak at 570 nm). The mechanochromic process is reversible in multiple grinding–thermal annealing and grinding–solvent‐fuming cycles, and the emission of the solid sample switches between orange (ground) and yellow (thermal/solvent‐fuming‐treated) colors. The mechanochromism is ascribed to the phase transition between amorphous and crystalline states. IND‐TPE undergoes a hydrolysis reaction in basic aqueous solution, thus the red‐orange emission can be quenched by OH^? or other species that can induce the generation of sufficient OH^?. Accordingly, IND‐TPE has been used to discriminatively detect arginine and lysine from other amino acids, due to their basic nature. The experimental data are satisfactory. Moreover, the hydrolyzation product of IND‐TPE is weakly emissive in the resultant mixture but becomes highly blue‐emissive after the illumination for a period by UV light. Thus IND‐TPE can be used as a dual‐responsive fluorescent probe, which may extend the application of TPE‐based molecular probes in chemical and biological categories.     

Aggregation‐Induced Emission‐Active 1,4‐Dihydropyridine‐Based Dual‐Phase Fluorescent Sensor with Multiple Functions

A N‐2‐phenylethyl‐substituted 1,4‐dihydropyridine derivative (NDHP) containing 5,5‐dimethylcyclohexane‐1,3‐dione and naphthylethylene was designed and synthesized. NDHP acts as a multifunctional fluorescent sensor in dual phases. The crystal structure analysis confirms that the NDHP molecules have highly twisted conformations. The twisted conformation results in aggregation‐induced emission properties and solid‐state emission, by restricting the intramolecular free rotation in the aggregated or solid state. In the solid state, NDHP exhibits reversible mechanochromic properties as a result of the transition between the amorphous and crystalline states. NDHP also exhibits a rare phenomenon of acid‐fumed solid‐state emission enhancement owing to the change in packing mode from a zigzag arrangement to J‐aggregation. The solid‐state stimuli‐responsive fluorescence switching is applied to realize a rewritable optical recording media and a multiple output combinational logic system. In solution, NDHP shows a selective fluorescence response for environmentally harmful Hg²⁺, with a limit of detection of 2.7 nm . This results from the “turn‐on” responsive behavior owing to the Hg²⁺‐triggered aggregation of the NDHP molecules. NDHP is also used in the imaging of intracellular Hg²⁺ in HeLa cells. These findings provide a feasible and attractive route for developing multifunctional fluorescent sensors for use in dual phases.     

N‐Boc‐Indolylbenzothiadiazole Derivatives: Efficient Full‐Color Solid‐State Fluorescence and Self‐Recovering Mechanochromic Luminescence

Herein, the solid‐state emission with good fluorescence quantum yields of N‐Boc‐indolylbenzothiadiazoles as a new class of fluorophores is described. Their solid‐state emission covers the wide range of the visible spectrum and the emission color can be tuned easily by changing the substituents on the two heteroaromatic rings. Among these, 3‐methylindolyl derivatives exhibit moreover autonomously self‐recovering mechanochromic luminescence, whereby the original solid‐state emission could be recovered spontaneously at room temperature after exposure to a mechanical stimulus. The emission color, as well as the recovery time for the color change could be tuned via the introduction of different substituents on the benzothiadiazole ring. We propose that the mechanism of the autonomously self‐recovering mechanochromic luminescence of 3‐methylindolylbenzothiadiazoles is based on a partial amorphization of the crystals upon exposure to the mechanical stimulus, followed by autonomous recovering in the form of recrystallization.     

Bismuth‐Based Coordination Polymers with Efficient Aggregation‐Induced Phosphorescence and Reversible Mechanochromic Luminescence

Two bismuth coordination polymers (CPs), (TBA)[BiBr₄(bp4mo)] (TBA=tetrabutylammonium) and [BiBr₃(bp4mo)₂], which are based on the rarely used simple ditopic ligand N‐oxide‐4,4′‐bipyridine (bp4mo), show mechanochromic luminescence (MCL). High solid‐state phosphorescence quantum yields of up to 85 % were determined for (TBA)[BiBr₄(bp4mo)] (λ_em=540 nm). Thorough investigations of the luminescence properties combined with DFT and TDDFT calculations revealed that the emission is due to aggregation‐induced phosphorescence (AIP). Upon grinding, both samples became amorphous, and their luminescence changed from yellow to orange and red, respectively. Heating or exposure to water vapor led to the recovery of the initial luminescence. These materials are the first examples of mechanochromic phosphors based on bismuth(III).     

Ring Size Effects on Multi‐Stimuli Responsive Luminescent Properties of Cyclic Amine Substituted β‐Diketones and Difluoroboron Complexes

Emissive β‐diketones (bdks) and difluoroboron complexes (BF₂bdks) show multi‐stimuli responsive luminescence in both solution and the solid state. A series of bdk ligands and boron coordinated dyes were synthesized with different cyclic amine substituents in the 4‐position to explore ring size effects on various luminescent properties, including solvatochromism, viscochromism, aggregation‐induced emission (AIE), mechanochromic luminescence (ML) and halochromism. Red‐shifted absorption and emission were observed in CH₂Cl₂ for both bdk ligands and boron dyes with increasing substituent ring size. The compounds displayed bathochromic emission in more polar solvents, and higher fluorescence intensity in more viscous media. The AIE compounds exhibited enhanced emission when aggregated. For solid‐state properties, a large emission wavelength shift was shown for the piperidine substituted bdk after melt quenching on weighing paper. Large blue‐shifted emissions were observed in all the boron dye spin cast films after trifluoroacetic acid vapor annealing, and the original emissions were partially recovered after triethylamine vapor treatment.     

Controlling the Emergence and Shift Direction of Mechanochromic Luminescence Color of a Pyridine‐Terminated Compound

In this study, mechanochromic luminescence was induced in a complex of mechano‐inactive compounds. Dye/acid complexes containing the same π‐conjugated backbones were prepared. While the luminophore showed blue and red shifts in photoluminescence spectra when combined with different acids by grinding, it exhibited slight mechanoresponsiveness itself. Also, compounds with similar molecular backbones to the dye/acid complex were synthesized to clarify the color change mechanism. The compounds showed both blue and red shifts in photoluminescence and diffuse reflectance spectra upon grinding, indicating that mechanochromic luminescence in the hydrogen‐bonded complex is like its monomeric analogue and that aggregation structure plays an important role in mechanoresponsive behavior rather than the π‐conjugated structure. It was shown that a color change can be mechanically induced by imitating the solid‐state aggregation structure of other mechanoresponsive compounds without synthetic modification.     

Aggregation‐Induced Emission and Aggregation‐Promoted Photo‐oxidation in Thiophene‐Substituted Tetraphenylethylene Derivative

Aggregation‐induced emission combined with aggregation‐promoted photo‐oxidation has been reported only in two works quite recently. In fact, this phenomenon is not commonly observed for AIE‐active molecules. In this work, a new tetraphenylethylene derivative (TPE‐4T) with aggregation‐induced emission (AIE) and aggregation‐promoted photo‐oxidation was synthesized and investigated. The pristine TPE‐4T film exhibits strong bluish‐green emission, which turns to quite weak yellow emission after UV irradiation. Interestingly, after solvent treatment, the weakly fluorescent intermediate will become bright‐yellow emitting. Moreover, the morphology of the TPE‐4T film could be regulated by UV irradiation. The wettability of the TPE‐4T microcrystalline surface is drastically changed from hydrophobic to hydrophilic. This work contributes a new member to the aggregation induced photo‐oxidation family and enriches the photo‐oxidation study of tetraphenylethylene derivatives.     

Manipulation of Molecular Aggregation States to Realize Polymorphism,AIE, MCL,and TADF in a Single Molecule

Multifunctional emitting materials are scarce and need to be further explored. Now, a newly anthraquinone derivative, 2‐(phenothiazine‐10‐yl)‐anthraquinone (PTZ‐AQ) was designed and synthesized and found to demonstrate polymorphism, multi‐color emission, aggregation‐induced emission (AIE), mechanochromic luminescence (MCL), and thermally activated delayed fluorescence (TADF) in its different solid forms. It is shown for the first time that TADF properties of a compound can be systematically tuned via its aggregation state. The optimized PTZ‐AQ crystal shows a small singlet–triplet energy splitting of 0.01 eV and exhibits red TADF with a photoluminescence quantum yield as high as 0.848. This study shows that the unique multiple functions can be integrated into one single compound through controlling the aggregation states, which provides a new strategy for the investigation and application of multifunctional organic materials.     

Manipulation of Molecular Aggregation States to Realize Polymorphism,AIE, MCL,and TADF in a Single Molecule

Multifunctional emitting materials are scarce and need to be further explored. Now, a newly anthraquinone derivative, 2‐(phenothiazine‐10‐yl)‐anthraquinone (PTZ‐AQ) was designed and synthesized and found to demonstrate polymorphism, multi‐color emission, aggregation‐induced emission (AIE), mechanochromic luminescence (MCL), and thermally activated delayed fluorescence (TADF) in its different solid forms. It is shown for the first time that TADF properties of a compound can be systematically tuned via its aggregation state. The optimized PTZ‐AQ crystal shows a small singlet–triplet energy splitting of 0.01 eV and exhibits red TADF with a photoluminescence quantum yield as high as 0.848. This study shows that the unique multiple functions can be integrated into one single compound through controlling the aggregation states, which provides a new strategy for the investigation and application of multifunctional organic materials.     

A Mechanochromic Luminescent Dye Exhibiting On/Off Switching by Crystalline–Amorphous Transitions

A mechanochromic luminescent dye based on a simple aminomaleimide skeleton was readily synthesized in a one‐pot process. It exhibited an on/off mechanochromic luminescent switching property dependent on external stimuli, unlike a traditional mechanochromic color change. The green emission was turned on by grinding in a mortar and turned off by heating or treatment with dichloromethane. In the crystalline state, two molecules were stacked by cofacial π–π interactions, which caused concentration self‐quenching. The crystalline‐to‐amorphous transition induced by grinding removed cofacial π–π stacking, which led to intensive emission. Crystallizing processes recovered the cofacial π–π stacking, resulting in elimination of the emission. Theoretical calculations and X‐ray diffraction analyses revealed that the dye molecule was distorted in the crystalline state; thus even a mechanical stimulus caused the crystalline‐to‐amorphous transition.     

Anion‐, Solvent‐, Temperature‐, and Mechano‐Responsive Photoluminescence in Gold(I) Diphosphine‐Based Dimers

A series of [Au₂(nixantphos)₂](X)₂ (nixantphos=4,6‐bis(diphenylphosphino)‐phenoxazine; X=NO₃, 1 ; CF₃COO, 2 ; CF₃SO₃, 3 ; [Au(CN)₂], 4 ; and BF₄, 5 ) complexes that exhibit intriguing anion‐switchable and stimuli‐responsive luminescent photophysical properties have been synthesized and characterized. Depending on their anions, these complexes display yellow ( 3 ), orange ( 4 and 5 ), and red ( 1 and 2 ) emission colors. They exhibit reversible thermo‐, mechano‐, and vapochromic luminescence changes readily perceivable by the naked eye. Single‐crystal X‐ray studies show that the [Au₂(nixantphos)₂]²⁺ cations with short intramolecular Au ??? Au interactions are involved as donors in an infinite N?H ??? X (X=O and N) hydrogen‐bonded chain formation with CF₃COO^? ( 2 C ) and aurophilically linked [Au(CN)₂]^? counterions ( 4 C ). Both crystals show thermochromic luminescence; their room temperature red ( 2 C ) and orange ( 4 C ) emission turns into yellow upon cooling to 77 K. They also exhibit reversible mechanochromic luminescence by changing their emission color from red to dark ( 2 C ), and orange to red ( 4 C ). Compounds 1 – 5 also display reversible mechanochromic luminescence, altering their emission colors between orange ( 1 ) or red ( 2 ) to dark, as well as between yellow ( 3 ) or orange ( 4 and 5 ) to red. Detailed photophysical investigations and correlation with solid‐state structural data established the significant role of N?H ??? X interactions in the stimuli‐responsive luminescent behavior.     

Mechanochromic Luminescence from Crystals Consisting of Intermolecular Hydrogen‐Bonded Sheets

Introduction of functional groups that can form intermolecular hydrogen bonds into highly‐emissive luminophores is a promising way to induce mechanochromic luminescence. Herein, we report that a 9,10‐bis(phenylethynyl)anthracene derivative featuring two amide groups forms green‐emissive crystals based on two‐dimensional hydrogen‐bonded molecular sheets. Mechanical grinding changed the emission from green to yellow, owing to a transition from a crystalline to an amorphous phase. Infrared spectroscopy revealed that mechanical stimuli disrupted the linear hydrogen‐bonding formation. A thermal treatment recovered the original green photoluminescence.     

Stimulus‐Triggered Formation of an Anion–Cation Exciplex in Copper(I) Complexes as a Mechanism for Mechanochromic Phosphorescence

The investigation of the mechanisms of mechanochromic luminescence is of fundamental importance for the development of materials for photonic sensors, data storage, and luminescence switches. The structural origin of this phenomenon in phosphorescent molecular systems is rarely known and thus the formulation of structure–property relationships remains challenging. Changes in the M–M interactions have been proposed as the main mechanism with d¹⁰ coinage metal compounds. Herein, we describe a new mechanism—a mechanically induced reversible formation of a cation–anion exciplex based on Cu–F interactions—that leads to highly efficient mechanochromic phosphorescence and unusual large emission shifts from UV‐blue to yellow for Cu^I complexes. The low‐energy luminescence is thermo‐ and vaporesponsive, thus allowing the generation of white light as well as for recovering the original UV‐blue emission.     

Towards meso‐Ester BODIPYs with Aggregation‐Induced Emission Properties: The Effect of Substitution Positions

Three meso‐ester boron dipyrromethene (BODIPY) dyes have been synthesized and functionalized with aggregation‐induced emission (AIE)‐active tetraphenylethene or triphenylethene moieties. It was found that functionalizing at the different positions of the BODIPY core resulted in the final dye having different emission properties in response to aggregation: from aggregation‐induced quenching (ACQ) to being AIE active. X‐ray crystallographic analysis was thus performed to provide an explanation for these differences.     

Unusual Salt‐Induced Color Modulation through Aggregation‐Induced Emission Switching of a Bis‐cationic Phenylenedivinylene‐Based π Hydrogelator

The synthesis, hydrogelation, and aggregation‐induced emission switching of the phenylenedivinylene bis‐N‐octyl pyridinium salt is described. Hydrogelation occurs as a consequence of π‐stacking, van der Waals, and electrostatic interactions that lead to a high gel melting temperature and significant mechanical properties at a very low weight percentage of the gelator. A morphology transition from fiber‐to‐coil‐to‐tube was observed depending on the concentration of the gelator. Variation in the added salt type, salt concentrations, or temperature profoundly influenced the order of aggregation of the gelator molecules in aqueous solution. Formation of a novel chromophore assembly in this way leads to an aggregation‐induced switch of the emission colors. The emission color switches from sky blue to white to orange depending upon the extent of aggregation through mere addition of external inorganic salts. Remarkably, the salt effect on the assembly of such cationic phenylenedivinylenes in water follow the behavior predicted from the well‐known Hofmeister effects. Mechanistic insights for these aggregation processes were obtained through the counterion exchange studies. The aggregation‐induced emission switching that leads to a room‐temperature white‐light emission from a single chromophore in a single solvent (water) is highly promising for optoelectronic applications.     

High‐Performance Quinoline‐Malononitrile Core as a Building Block for the Diversity‐Oriented Synthesis of AIEgens

In vivo fluorescent monitoring of physiological processes with high‐fidelity is essential in disease diagnosis and biological research, but faces extreme challenges due to aggregation‐caused quenching (ACQ) and short‐wavelength fluorescence. The development of high‐performance and long‐wavelength aggregation‐induced emission (AIE) fluorophores is in high demand for precise optical bioimaging. The chromophore quinoline‐malononitrile (QM) has recently emerged as a new class of AIE building block that possesses several notable features, such as red to near‐infrared (NIR) emission, high brightness, marked photostability, and good biocompatibility. In this minireview, we summarize some recent advances of our established AIE building block of QM, focusing on the AIE mechanism, regulation of emission wavelength and morphology, the facile scale‐up and fast preparation for AIE nanoparticles, as well as potential biomedical imaging applications.