Very bright mechanoluminescence and remarkable mechanochromism using a tetraphenylethene derivative with aggregation-induced emission

Achieving Ultrasound-Excited Emission with Organic Mechanoluminescent Materials

Unlike traditional photoluminescence (PL), mechanoluminescence (ML) achieved under mechanical excitation demonstrates unique characteristics such as high penetrability, spatial resolution, and signal-to-background ratio (SBR) for bioimaging applications. However, bioimaging with organic mechanoluminescent materials remains challenging because of the shallow penetration depth of ML with short emission wavelengths and the absence of a suitable mechanical force to generate ML in vivo. To resolve these issues, the present paper reports the achievement of ultrasound (US)-excited fluorescence and phosphorescence from purely organic luminogens for the first time with emission wavelengths extending to the red/NIR region, with the penetrability of the US-excited emission being considerably higher than that of PL. Consequently, US-excited subcutaneous phosphorescence imaging can be achieved using a mechanoluminescent-luminogen-based capsule device with a quantified intensity of 9.15 ± 1.32 × 104 p s-1 cm-2 sr-1 and an SBR of 24. Moreover, the US-excited emission can be adequately tuned using the packing modes of the conjugated skeletons, dipole orientation of mechanoluminescent luminogens, and strength and direction of intermolecular interactions. Overall, this study innovatively expands the kind of excitation sources and the emission wavelengths of organic mechanoluminescent materials, paving the way for practical biological applications based on US-excited emission.

Dual-channel mechano-phosphorescence: a combined locking effect with twisted molecular structures and robust interactions

Organic mechanoluminescence materials, featuring dual emission and ultralong phosphorescence characteristics, exhibit significant potential for applications in real-time stress sensing, pressure-sensitive lighting, advanced security marking techniques, and material breakage monitoring. However, due to immature molecular design strategies and unclear luminescence mechanisms, these materials remain rarely reported. In this study, we propose a valuable molecular design strategy to achieve dual-channel mechano-phosphorescence. By introducing the arylphosphine oxide group into a highly twisted molecular framework, enhanced intra- and intermolecular interactions could be achieved within rigid structures, leading to dual-channel mechanoluminescence with greatly promoted ultralong phosphorescence. Further investigations reveal the substantial boosting effect of intra- and intermolecular interactions on mechanoluminescence and ultralong phosphorescence properties by locking the highly twisted molecular skeleton. This work provides a concise and guiding route to develop novel smart responsive luminescence materials for widespread applications in material science.

Spontaneous Symmetry Breaking of Achiral Molecules Leading to the Formation of Homochiral Superstructures that Exhibit Mechanoluminescence

Chirality, with its intrinsic symmetry-breaking feature, is frequently utilized in the creation of acentric crystalline functional materials that exhibit intriguing optoelectronic properties. On the other hand, the development of chiral crystals from achiral molecules offers a solution that bypasses the need for enantiopure motifs, presenting a promising alternative and thereby expanding the possibilities of the self-assembly toolkit. Nevertheless, the rational design of achiral molecules that prefer spontaneous symmetry breaking during crystallization has so far been obscure. In this study, we present a series of six achiral molecules, demonstrating that when these conformationally flexible molecules adopt a cis-conformation and engage in multiple non-covalent interactions along a helical path, they collectively self-assemble into chiral superstructures consisting of single-handed supramolecular columns. When these homochiral supramolecular columns align in parallel, they form polar crystals that exhibit intense luminescence upon grinding or scraping. We therefore demonstrate our molecular design strategy could significantly increase the likelihood of symmetry breaking in achiral molecular synthons during self-assembly, offering a facile access to novel chiral crystalline materials with unique optoelectronic properties.

Stimuli-fluorochromic smart organic materials

Smart materials based on stimuli-fluorochromic π-conjugated solids (SFCSs) have aroused significant interest due to their versatile and exciting properties, leading to advanced applications. In this review, we highlight the recent developments in SFCS-based smart materials, expanding beyond organometallic compounds and light-responsive organic luminescent materials, with a discussion on the design strategies, exciting properties and stimuli-fluorochromic mechanisms along with their potential applications in the exciting fields of encryption, sensors, data storage, display, green printing, etc. The review comprehensively covers single-component and multi-component SFCSs as well as their stimuli-fluorochromic behaviors under external stimuli. We also provide insights into current achievements, limitations, and major challenges as well as future opportunities, aiming to inspire further investigation in this field in the near future. We expect this review to inspire more innovative research on SFCSs and their advanced applications so as to promote further development of smart materials and devices.

Precise peripheral design enables propeller-like squaraine dye with highly sensitive and wide-range piezochromism

Piezochromic fluorescent (PCF) materials that feature high sensitivity and wide-range switching are attractive in intelligent optoelectronic applications but their fabrication remains a significant challenge. Here we present a propeller-like squaraine dye SQ-NMe₂ decorated with four peripheral dimethylamines acting as electron donors and spatial obstacles. This precise peripheral design is expected to loosen the molecular packing pattern and facilitate more substantial intramolecular charge transfer (ICT) switching caused by conformational planarization under mechanical stimuli. As such, the pristine SQ-NMe₂ microcrystal exhibits significant fluorescence changes from yellow (λ_em = 554 nm) to orange (λ_em = 590 nm) upon slight mechanical grinding and further to deep red (λ_em = 648 nm) upon heavy mechanical grinding. Single-crystal X-ray diffraction structural analysis of two SQ-NMe₂ polymorphs provides direct evidence to illustrate the design concept of such a piezochromic molecule. The piezochromic behavior of SQ-NMe₂ microcrystals is sensitive, high-contrast, and easily reversible, enabling cryptographic applications.

Realizing Photoswitchable Mechanoluminescence in Organic Crystals Based on Photochromism

Organic mechanoluminescent (ML) materials possessing photophysical properties that are sensitive to multiple external stimuli have shown great potential in many fields, including optic and sensing. Particularly, the photoswitchable ML property for these materials is fundamental to their applications but remains a formidable challenge. Herein, photoswitchable ML is successfully realized by endowing reversible photochromic properties to an ML molecule, namely 2-(1,2,2-triphenylvinyl) fluoropyridine (o-TPF). o-TPF shows both high-contrast photochromism with a distinct color change from white to purplish red, as well as bright blue ML (λ_ML  = 453 nm). The ML property can be repeatedly switched between ON and OFF states under alternate UV and visible light irradiation. Impressively, the photoswitchable ML is of high stability and repeatability. The ML can be reversibly switched on and off by conducting alternate UV and visible light irradiation in cycles under ambient conditions. Experimental results and theoretical calculations reveal that the change of dipole moment of o-TPF during the photochromic process is responsible for the photoswitchable ML. These results outline a fundamental strategy to achieve for the control of organic ML and pave the way to the development of expanded smart luminescent materials and their applications.

Mechanoluminescence Affected by Trap Types and Excitation State Positions in Mg3Ca3(PO4)4:Eu2+/Mn2+/Ce3+ for Multimode Anticounterfeiting

Mechanoluminescence (ML) materials with tunable emissions can serve in many practical applications; however, their underlying mechanism still needs further clarification. Herein, we developed Eu²⁺-/Mn²⁺-/Ce³⁺-activated Mg₃Ca₃(PO₄)₄ (MCP) phosphors and studied their luminescence properties by device fabrication. The intense blue ML is obtained by fabricating MCP:Eu²⁺ into the polydimethylsiloxane elastomer matrix. The red ML of relatively weak intensity is received in Mn²⁺ activator, but the ML for the Ce³⁺ dopant is nearly quenched in the same host. The possible reason is proposed from the analysis of the relative positions between the excitation state and conduction band, together with the trap types. The appropriate location of the excited energy levels in the band gap allows for a larger probability of efficient ML when shallow traps near the excitation states are created synchronously as an effective energy transfer (ET) channel. The concentration-dependent ML for the MCP:Eu²⁺,Mn²⁺-based devices indicates that the emitting light color can be tailored, where several ET processes among oxygen vacancies, Eu²⁺, Ce³⁺, and Mn²⁺, occur. The luminescence manipulation with dopants and excitation sources demonstrates the potential applications in visualized multimode anticounterfeiting. These findings open up many possibilities for constructing new ML materials by introducing appropriate traps into the band structures.

Bright Organic Mechanoluminescence and Remarkable Mechanofluorochromism from Circularly Polarized TADF Enantiomers with Aggregation-Induced Emission Properties

The development of circularly polarized thermally activated delayed fluorescence (CP-TADF) luminogens with stimuli-response characteristics remains challenging. Herein, a pair of organic enantiomers, S-CzTA and R-CzTA, with aggregation-induced emission properties, have been successfully developed by introducing chiral 1,2,3,4-tetrahydronaphthalene and carbazole to phthalimide. They present CP-TADF properties in toluene solutions, giving dissymmetric factors of 0.84×10^-3 and -1.03×10^-3 , respectively. In the crystalline state, both S-CzTA and R-CzTA can emit intense blue TADF and produce very bright sky-blue mechanoluminescence (ML) and remarkable mechanofluorochromism (MFC) under the stimuli of mechanical force. Single-crystal analysis and theoretical calculation results suggest that their ML activities are probably associated with their chiral and polar molecular structures and unique non-centrosymmetric molecular packing modes. Furthermore, the MFC properties of the enantiomers likely originate from the destruction of crystal structure, leading to the planarization of molecular conformation. This work may provide helpful guidance for developing new CP-TADF materials with force-stimuli-responsive properties.

Multistimuli-responsive materials based on a zinc(II) complex with high-contrast and multicolor switching

The development of stimulus-responsive luminescent materials, especially those based on a single compound exhibiting multicolor and high-contrast (Δλ_em ≥ 100 nm) chromic properties, is a critical challenge. In this work, we synthesized and characterized a zinc(II) complex (1). As expected, 1 displays aggregation-induced emission enhancement (AIEE) in THF/H₂O mixtures, and remarkable multicolor switching under external stimuli in the solid state. Complex 1 shows reversible mechanochromic luminescence behavior with a large wavelength shift (Δλ_em = 100 nm) during the grinding-fuming cycles, due to the phase transformation between the crystalline and amorphous states. More impressively, 1 exhibits obvious acidochromic properties (Δλ_em = 130 nm) which originate from the adsorption of vapor and a gas-solid reaction on the crystal surface. Furthermore, 1 exhibits electrochemical oxidation behavior accompanied by quenching of yellow-green emission due to the overlap of an emission band and an absorption band. The above-mentioned color changes under ambient light can also be observed by the naked eye during the mechanical, acid-base vapor and electrical stimulation. Based on the high-contrast and multicolor switching, complex 1 was successfully developed into test papers and films in the field of rapid detection of mechanical stimuli and HCl/NH₃ vapors.

Advances in Pure Organic Mechanoluminescence Materials

Mechanoluminescence (ML) is the production of light from materials in response to the external stimulus of mechanical action. For organic compounds, the production of ML is tightly associated with fracture and plastic deformation of materials with piezoelectric effect in crystal lattice, and the ML property is highly dependent on the molecular packing mode, spatial conformation, and intermolecular interaction in the solid state. In the past few years, our group focused on the molecular design of pure organic ML compounds, with an attempt to discover different features of ML in pursuing the inherent emission mechanism and potential practical applications. We successfully found polymorph-dependent ML, ML with a phosphorescent property, conformation-dependent ML, ML with odd-even effect, wearable ML devices applied in heartbeat and pressure detection, etc. In this Perspective, we aim to deepen the understanding of ML and provide some guidance for the molecular design of organic light-emitting materials through the combination of our contributions.

Multi-stimuli distinct responsive D–A based fluorogen oligomeric tool and efficient detection of TNT vapor

P1 shows distinct emission responses with multi-stimuli, i.e., quenching for TNT sensing, red shifting for acid and base vapors, blue shifting against MFC behavior, and solvent polarity-dependent emission.

Luminescent polymorphic crystals: mechanoresponsive and multicolor-emissive properties

Polymorphic organic crystals that can switch their photophysical properties in response to mechanical stimuli are highlighted.

Influence of additives on the polymorphic manipulation of organic fluorescent crystals and its mechanism

The influence and mechanism behind using additives to isolate metastable fluorescent polymorphs were carefully investigated.

A triphenylamine derivative and its Cd(ii) complex with high-contrast mechanochromic luminescence and vapochromism

A triphenylamine derivative and its Cd(ii) complex exhibited predominant mechanochromism and vapochromism with high-contrast color and emission changes.

Mechanoluminescence Rebrightening the Prospects of Stress Sensing: A Review

The emergence of new applications, such as in artificial intelligence, the internet of things, and biotechnology, has driven the evolution of stress sensing technology. For these emerging applications, stretchability, remoteness, stress distribution, a multimodal nature, and biocompatibility are important performance characteristics of stress sensors. Mechanoluminescence (ML)-based stress sensing has attracted widespread attention because of its characteristics of remoteness and having a distributed response to mechanical stimuli as well as its great potential for stretchability, biocompatibility, and self-powering. In the past few decades, great progress has been made in the discovery of ML materials, analysis of mechanisms, design of devices, and exploration of applications. One can find that with this progress, the focus of ML research has shifted from the phenomenon in the earliest stage to materials and recently toward devices. At the present stage, while showing great prospects for advanced stress sensing applications, ML-based sensing still faces major challenges in material optimization, device design, and system integration.

Quantifiable stretching-induced fluorescence shifts of an elastically bendable and plastically twistable organic crystal

Organic crystals with mechanical stimulus-response properties are being developed increasingly nowadays. However, the studies involving tensile-responsive crystals are still lacking due to the strict requirement of crystals with good flexibility. In this work, an organic crystal with the ability of elastic bending and plastic twisting upon loading stress and shearing force, respectively, is reported. The deformability in different directions enables the crystal to be a model for tensile-responsive study. Indeed, blue shifts of fluorescence were observed when the tensile forces loaded upon the needle-shaped crystal were stretched to a certain degree. The mathematical correlation between emission wavelength changes and stretching strain was obtained for the first time, which proves that the crystal has a potential application for tension sensors. In addition, a low detection limit and high sensitivity enabled the crystal to have the ability to detect tension variations in precision instruments. Theoretical calculations and X-ray crystal structure analyses revealed the mechanism of emission wavelength shifts caused by molecular movement during the stretching process. The presented crystal successfully overcame the limitations of traditional mechanochromic organic crystals, which have difficulty in responding to tensile forces.

Study of a phosphorescent cationic iridium(III) complex displaying a blue-shift in crystals

We report the synthesis and the characterization of a new cationic iridium(III) complex featuring two 1-(p-methoxyphenyl)-5-methoxybenzimidazole cyclometallating ligands and a dimethylbipyridine ancillary ligand. The complex has been fully characterized by 1D and 2D NMR (¹H, ¹³C, ¹⁹F and ³¹P), elemental analysis and high-resolution mass spectrometry (HRMS). The photoluminescence studies performed in a solution, on amorphous powder and on crystals revealed an unexpected behavior. Indeed, the emission spectra observed in both solution (CH₂Cl₂) and amorphous powder samples are centered at around 580 nm, whereas in crystals the emission displays a large hypsochromic shift of ∼800 cm^-1 (λ_em = 558 nm). X-ray diffraction experiments, photophysical studies and DFT calculations allow for rationalizing the hypsochromic shift.

Restriction of Twisted Intramolecular Charge Transfer Enables the Aggregation-Induced Emission of 1-(N,N-Dialkylamino)-naphthalene Derivatives

Understanding the mechanisms of aggregation-induced emission (AIE) is essential for the rational design and deployment of AIEgens toward various applications. Such a deep mechanistic understanding demands a thorough investigation of the excited-state behaviors of AIEgens. However, because of considerable complexity and rapid decay, these behaviors are often not experimentally accessible and the mechanistic comprehension of many AIEgens is lacking. Herein, utilizing detailed quantum chemical calculations, we provide insights toward the AIE mechanism of 1-(N,N-dialkylamino)-naphthalene (DAN) derivatives. Our theoretical analysis, corroborated by experimental observations, leads to the discovery that modulating the formation of the twisted intramolecular charge transfer (TICT) state (caused by the rotation of the amino groups) and managing the steric hindrance to minimize solid-state intermolecular interactions provides a plausible explanation for the AIE characteristics of DAN derivatives. These results will inspire the deployment of the TICT mechanism as a useful design strategy toward AIEgen development.

Multistimuli Responsive Solid-State Emission of a Zinc(II) Complex with Multicolour Switching

The development of smart luminescent materials, especially those stimulus-responsive fluorescent materials that can switch between different colors repeatedly under external stimulation based on a single molecule, is of great significance but a challenge. In this work, a novel zinc(II)-Schiff base complex (ZnL₂) was obtained and characterized. Upon exposure to the HCl and NH₃ vapors, it displayed remarkable tricolor acidochromic behavior with high contrast and rapid response under the ambient light as well as UV light (365 nm). The XPS analyses of ZnL₂ crystals before and after HCl/NH₃ fuming show that the acidochromism originates principally from the adsorption of vapor and the gas-solid reaction equilibrium on the crystal surface. The reddish-brown color of the HCl-fumigated ZnL₂ crystals could be attributed to the generation of HL at the surface of ZnL₂, and red-shifted emission could be ascribed to the self-absorption effect. The single crystal X-ray diffraction data indicate that these processes cause slight changes in the molecular conformation and crystal packing. ZnL₂ shows reversible mechanochromic luminescence behavior between yellow and orange emission during the grinding-fuming/heating cycles due to the modulation between amorphous and crystalline states. Moreover, ZnL₂ was successfully made into test paper for the rapid detection of HCl/NH₃ vapors and mechanical stimuli.

Mechanoluminescence and aggregation-enhanced emission (AEE) of an In-MOF based on a 9,9'-diphenyl-9H-fluorene tetraacid linker

A water-stable In-MOF, constructed based on a conformationally-flexible tetraacid linker, i.e., 2,7-bis(3,5-dicarboxyphenyl)-9,9'-diphenyl-9H-fluorene, i.e., H4DPF, is shown to exhibit a significantly enhanced solid-state fluorescence quantum yield (φf) of 23% in comparison with that of the linker (φfca. 4%) as a consequence of rigidification of the latter by metalation. Application of external stimulus in the form of grinding of the In-MOF leads to a drastic enhancement by 29%, φf from 23 to 52%. Solid-state absorption and emission spectra show that the absorption in the region of 368-550 nm gets diminished with a concomitant change in the emission maximum with a blue shift upon grinding. Fluorescence enhancement with grinding is correlated with a gradual reduction in the size of the particles, as established by SEM analysis. MOF particle aggregation has been invoked to account for the observed fluorescence enhancement in addition to a subtle conformational change in the structure of the linker upon grinding. Intriguingly, the ground MOF particles exhibit aggregation behaviour in the DMF-water solvent system with the emission further increasing up to 75% for the increase in the water fraction (fw) from 0 to 60%; hydrophobic aggregation of particles evidently leads to a change in the conformation of the linker and particle aggregation-enhanced emission (AEE). De-aggregation of particles ensues for fw = 70-90%, as reflected by a gradual decrease in the emission intensity. It is shown that the suspension of ground In-MOF particles in water permits sensing of metal ions, in particular Al3+ ions, by fluorescence quenching with detection at a sub-ppb level. The observed results comprise first demonstration of both mechanoluminescence and AEE of MOF particles.

Mechanically induced single-molecule white-light emission of excited-state intramolecular proton transfer (ESIPT) materials

Described herein is the first example of mechanically induced single-molecule white-light emission based on excited-state intramolecular proton transfer (ESIPT) materials. The mechanism of mechanochromism is clearly disclosed by powder and single crystal X-ray diffraction (XRD) data, infrared spectroscopy, and fluorescence up-conversion measurement, etc. 2-(2'-Hydroxyphenyl)oxazole (6b) with a herringbone packing motif exhibits a predominant keto-form emission, giving off yellowish-green fluorescence. Mechanical grinding transforms the herringbone packing motif into a brickwork packing motif, decreases the intermolecular distances, which results in an enhanced intermolecular charge-transfer interaction, and therefore changes the ESIPT dynamics, leading to an enhanced enol-form emission and white fluorescence. Herringbone-packing 6b is thermodynamically more stable than brickwork-packing 6b. Thus, the latter can convert to the former by solvent fuming or thermal annealing.

Light-emitting analogues based on triphenylamine modified quinoxaline and pyridine[2,3-b]pyrazine exhibiting different mechanochromic luminescence

A series of light-emitting analogues show different mechanochromic luminescence properties with emission color changes of 42 and 10 nm.

From Molecules to Polymers-Harnessing Inter- and Intramolecular Interactions to Create Mechanochromic Materials

The development of mechanophores as building blocks that serve as predefined weak linkages has enabled the creation of mechanoresponsive and mechanochromic polymer materials, which are interesting for a range of applications including the study of biological specimens or advanced security features. In typical mechanophores, covalent bonds are broken when polymers that contain these chemical motifs are exposed to mechanical forces, and changes of the optical properties upon bond scission can be harnessed as a signal that enables the detection of applied mechanical stresses and strains. Similar chromic effects upon mechanical deformation of polymers can also be achieved without relying on the scission of covalent bonds. The dissociation of motifs that feature directional noncovalent interactions, the disruption of aggregated molecules, and conformational changes in molecules or polymers constitute an attractive element for the design of mechanoresponsive and mechanochromic materials. In this article, it is reviewed how such alterations of molecules and polymers can be exploited for the development of mechanochromic materials that signal deformation without breaking covalent bonds. Recent illustrative examples are highlighted that showcase how the use of such mechanoresponsive motifs enables the visual mapping of stresses and damage in a reversible and highly sensitive manner.

Restriction of Conformation Transformation in Excited State: An Aggregation-Induced Emission Building Block Based on Stable Exocyclic C=N Group

The development of aggregation-induced emission (AIE) building block and deciphering its luminescence mechanism are of great significance. Here a feasible strategy for the construction of AIE unit based on E-Z isomerization (EZI) of exocyclic C=N double bond is proposed. Taking [1,2,4]thiadiazole[4,3-a]pyridine (TZP) derivative as an example, its aryl-substituted derivative (TZPP) shows obvious AIE character. The analysis of spectral data and theoretical calculations indicates that fast structural relaxation of TZPP in the emissive state plays a key role in a low fluorescence quantum yield in dilute solution, which should be caused by the small energy gap between locally excited (LE) state and twisted intramolecular charge transfer state. When in solid state, the bright emission with LE state characteristic reappears due to the large shift barrier of geometry transformation. As a potential building block for AIEgens with special heterocyclic structure, these findings would open up opportunities for developing various functional materials.

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A new aggregation-induced emission building block

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A novel AIE mechanism with spectral measurements and theoretical calculations

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Available starting materials resulting in convenient synthesis and modification

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A stable exocyclic C=N double bond in heterocycles

A mechanochromic cyclemetalated cationic Ir(iii) complex with AIE activity by strategic modification of ligands

Two new aggregation-induced emission (AIE)-active cyclemetalated cationic Ir(iii) complexes have been rationally designed and synthesized by introducing O-H substituents into Schiff base ligands. π-Hydrogen bonding is successfully exploited for the first time to realize the mechanochromic luminescence (MCL) property by the synergistic effect of O-H and F substituents in complex 1. An X-ray crystal structure analysis of the two complexes suggests that the intramolecular hydrogen bonding effectively restricted the molecular motions, thus causing typical AIE characteristics. More importantly, a loosely packed structure constructed from intermolecular hydrogen bonding interactions (O-Hπ and C-HF) is obtained, and it is susceptible to mechanical stimulation. Powder X-ray diffraction (PXRD) studies also prove that the MCL behavior of complex 1 is caused by the reversible phase transition from crystalline to amorphous state under grinding and solvent recrystallization, leading to a change in emission colors. A re-writable phosphorescence data recording device was fabricated using complex 1 as the active material. Our molecular design strategies provide a new avenue for achieving efficient phosphorescence materials with AIE and MCL properties.

Equilibrium and thermodynamic studies of chromic overcrowded fluorenylidene-acridanes with modified fluorene moieties

Chromic materials, an important class of stimuli-responsive materials, have aroused extensive attention in recent years. Normally, their color is based on changes in morphology. Few examples of chromic material based on conformational isomerization, such as in overcrowded alkenes, have been reported previously. Furthemore, experimental thermodynamic studies of overcrowded bistricyclic aromatic enes have not been carried out to our knowledge. Here, we show that N-phenyl-substituted fluorenylidene-acridanes, with a properly modified fluorene moiety, performs chromisms originating from conformational changes. Thermodynamic studies determine equilibrium constants, changes in enthalpy, entropy, and free energy in solution, enabling in-depth understanding of the equilibrium behavior of overcrowded alkenes and providing useful information for designing functional chromic compounds. Single-crystal X-ray diffraction analysis of fluorenylidene-acridanes in this work clearly shows well-tuned charge transfer from the acridane to the fluorene moiety. Various chromic behaviors such as mechanochromism, thermochromism, solvatochromism, vapochromism, and proton-induced chromism also support understanding of conformational isomerism.

Aggregation-Induced Emission: New Vistas at the Aggregate Level

Aggregation-induced emission (AIE) describes a photophysical phenomenon in which molecular aggregates exhibit stronger emission than the single molecules. Over the course of the last 20 years, AIE research has made great strides in material development, mechanistic study and high-tech applications. The achievements of AIE research demonstrate that molecular aggregates show many properties and functions that are absent in molecular species. In this review, we summarize the advances in the field of AIE and its related areas. We specifically focus on the new properties of materials attained by molecular aggregates beyond the microscopic molecular level. We hope this review will inspire more research into molecular ensembles at and beyond the meso level and lead to the significant progress in material and biological science.

Amphiphilic gemini-iridium (III) complex for rapid and selective detection of picric acid in water and intracellular

Inspired by the structure and properties of gemini surfactant, a novel amphiphilic gemini-iridium complex (GIC-Ir) has been developed, which can spontaneously form vesicles by self-assembly and exhibit excellent dispersibility and high emission intensity in water. The emission of GIC-Ir can be rapidly and selectively quenched by picric acid (PA) due to the aromatic groups and two long-chain quaternary ammonium (QA) groups with positive charge, which endow GIC-Ir vesicles outstanding capability to capture negatively charged PA, and greatly promote the interaction between GIC-Ir and PA. Theoretical calculations and spectral studied indicated that the photoinduced electron transfer and resonance energy transfer may be responsible to the emission quenching. Furthermore, the real water samples and in vitro studies further prove that GIC-Ir can be used as a promising chemosensor for the detection of PA both in water and intracellular.

Precise Molecular Design for High-Performance Luminogens with Aggregation-Induced Emission

Precise design of fluorescent molecules with desired properties has enabled the rapid development of many research fields. Among the different types of optically active materials, luminogens with aggregation-induced emission (AIEgens) have attracted significant interest over the past two decades. The negligible luminescence of AIEgens as a molecular species and high brightness in aggregate states distinguish them from conventional fluorescent dyes, which has galvanized efforts to bring AIEgens to a wide array of multidisciplinary applications. Herein, the useful principles and emerging structure-property relationships for precise molecular design toward AIEgens with desirable properties using concrete examples are revealed. The cutting-edge applications of AIEgens and their excellent performance in enabling new research directions in biomedical theranostics, optoelectronic devices, stimuli-responsive smart materials, and visualization of physical processes are also highlighted.

AIPE-active Pt(ii) complexes with a tunable triplet excited state: design, mechanochromism and application in anti-counterfeiting

A series of AIPE-active Pt(ii) complexes exhibit tunable triplet excited state properties, mechanochromic behavior and potential application in anti-counterfeiting.

Spontaneous chiral self-assembly of achiral AIEgens into AIEgen-silica hybrid nanotubes

Antipodal chiral AIEgen-silica hybrid mesostructures were fabricated by spontaneous chiral self-assembly of the achiral amphiphilic AIEgen 2-[4-(1,2,2-triphenylethenyl)phenoxy]-acetic acid via a co-structure directing route in the absence of any symmetry-breaking agent. The produced AIEgen-silica hybrid mesostructured nanotubes showed chiral features with optical activity originating from J-aggregation with an excess of one stereochemical hand of the helical conformation of the AIEgen.

Spiro-Structure: A Good Approach to Achieve Mechanoluminescence Property

Following the development of organic mechanoluminescence (ML) materials, molecular packing was proved as the key point to the emission process under an external force. In this text, with the introduction of spiro-(fluorene-9-9'-xanthene) (SFX) unit as the building block, the molecular packing of the resultant compound (BSFXA) was optimized with the interlaced mode, directly leading to the efficient ML effect. The key role of SFX with a spiro-structure can be further confirmed by the ML inactivity of reference compound BFA with the replacement of SFX unit by dimethyl fluorene (MeF), which provided a novel strategy to construct organic ML luminogens.