Dynamic Visualization of Stress/Strain Distribution and Fatigue Crack Propagation by an Organic Mechanoresponsive AIE Luminogen

Highly sensitive and specific uranyl ion detection by a fluorescent sensor containing uranyl-specific recognition sites

Uranium pollution has become a serious threat to human health and environmental safety, making the detection of environmental uranium contamination of great importance. The sensitive and specific detection of uranyl ions, which are the dominant form of uranium in the environment, depends on the specific recognition of uranyl ions by chemical groups. In this study, a novel fluorescent sensor containing a highly specific uranyl ion recognition group is synthesized via the reaction of 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) and 1,1,2,2-tetra(4-carboxylphenyl)ethylene (TPE-(COOH)4). Owing to the effects of aggregation-induced emission (AIE) and intramolecular charge transfer (ICT), the fluorescent sensor, named TPE-EDC, exhibits significant fluorescent properties in aqueous environments. The binding of uranyl ions by specific recognition groups in TPE-EDC leads to a decrease in the ICT effect, thus causing a significant reduction in the emission intensity of TPE-EDC. The attenuation of the fluorescence intensity of TPE-EDC shows an excellent linear relationship with an increase in uranyl ion concentration. TPE-EDC exhibits ultra-sensitive and ultra-selective detection ability for uranyl ions with an ultra-low detection limit of 69 pmol/L and an ultrashort response time of 30 s. These high detection performances render the fluorescent sensor TPE-EDC a promising candidate for early warning of uranium pollution.

Microscopic strain mapping in polymers equipped with non-covalent mechanochromic motifs

The mechanical failure of polymers remains challenging to understand and predict, as it often involves highly localised phenomena that cannot be probed with bulk characterisation techniques. Here, we present a generalisable protocol based on optical microscopy, tensile testing, and image processing that permits the spatially resolved interrogation of mechanical deformation at the molecular level around defects in mechanophore-containing polymers. The approach can be applied to a broad range of polymeric materials, mechanophores, and deformation scenarios.

Mechano-luminescence Behavior of Lanthanide-Doped Fluoride Nanocrystals for Three-Dimensional Stress Imaging

Pervasive mechanical force in nature and human activities is closely related to intriguing physics and widespread applications. However, describing stress distribution timely and precisely in three dimensions to avoid "groping in the dark" is still a formidable challenge, especially for nonplanar structures. Herein, we realize three-dimensional (3D) stress imaging for sharp arbitrary targets via advanced 3D printing, owing to the use of fluoride nanocrystal(NC)-based ink. Notably, a fascinating mechano-luminescence (ML) is observed for the homogeneously dispersed NaLuF₄:Tb³⁺ NCs (∼25 nm) with rationally designed deep traps (at 0.88 and 1.02 eV) via incorporating Cs⁺ ions and using X-ray irradiation. Carriers captured in the corresponding traps are steadily released under mechanical stimulations, which enables a ratio metric luminescence intensity based on the applied force. As a result, a significant mechano-optical conversion and superior optical waveguide of the corresponding transparent printed targets demonstrate stress in 3D with a high spatial and temporal resolution based on stereovision. These results highlight the optical function of the 3D-printed fluoride NCs, which cast light into the black boxes of stress described in space, benefiting us in understanding the ubiquitous force relevant to most natural and engineering processes.

Characterization Techniques of Polymer Aging: From Beginning to End

Polymers have been widely applied in various fields in the daily routines and the manufacturing. Despite the awareness of the aggressive and inevitable aging for the polymers, it still remains a challenge to choose an appropriate characterization strategy for evaluating the aging behaviors. The difficulties lie in the fact that the polymer features from the different aging stages require different characterization methods. In this review, we present an overview of the characterization strategies preferable for the initial, accelerated, and late stages during polymer aging. The optimum strategies have been discussed to characterize the generation of radicals, variation of functional groups, substantial chain scission, formation of low-molecular products, and deterioration in the polymers' macro-performances. In view of the advantages and the limitations of these characterization techniques, their utilization in a strategic approach is considered. In addition, we highlight the structure-property relationship for the aged polymers and provide available guidance for lifetime prediction. This review could allow the readers to be knowledgeable of the features for the polymers in the different aging stages and provide access to choose the optimum characterization techniques. We believe that this review will attract the communities dedicated to materials science and chemistry.

Modulus watch: In situ determination of the gel modulus by timing the fluorescence color change

The gel modulus, a key parameter for gel materials, is traditionally determined by cumbersome rheometer. Recently, probe technologies occur to meet the requirements of in situ determination. Till now, in situ and quantitatively testing of gel materials with unabridged structure informations still remains a challenge. Here, we provide a facile, in situ approach to determine the gel modulus, by timing the aggregation of a doped fluorescence probe. The probe shows green emission during aggregation and shifts to blue once it forms aggregates. The higher modulus of the gel, the longer probe's aggregation time. Furthermore, a quantitative correlation of gel modulus with the aggregation time is established. The in situ method not only facilitates the scientific researches in the field of gels, but also provides a new approach for spatiotemporal materials.

Structure and emission properties of dinuclear copper(i) complexes with pyridyltriazole

A new series of five highly emissive binuclear heteroleptic pyridyltriazole-Cu(i)-phosphine complexes 1-5 was synthesized and examined by different experimental (IR, elemental and thermogravimetric analysis, single crystal X-ray diffraction technique, UV-vis and fluorescence spectroscopy) and quantum chemical aproaches. Complexes 1-5 exhibited excellent stimuli-responsive photoluminescent performance in the solid state at room temperature (quantum yield (QY) = 27.5-52.0%; lifetime (τ) = 8.3-10.7 μs) and when the temperature was lowered to 77 K (QY = 38.3-88.2; τ = 17.8-134.7 μs). The highest QY was examined for complex 3 (52%) that can be explained by the small structural changes between the ground S₀ and exited S₁ and T₁ states leading to the small S₁-T₁ triplet gap and efficient thermally-activated delayed fluorescence. Moreover, complex 4 demonstrates reversible mechanochromic and excitation dependent luminescence.

Pursuing Phase Transitions of a Concentrated Polymer Solution by In Situ Fluorescence Measurements Based On Aggregation-Induced Emission

Concentration-dependent phase transitions in concentrated solutions have remained speculation due to the serious impediment of macromolecule dynamics by intensive topological entanglement or intermolecular interaction as well as the absence of powerful tool for detecting changes in chain or segment movement. Herein, taking a general polymer, namely, poly(vinyl alcohol) (PVA), as an example, a water-soluble fluorescent molecule with aggregation-induced emission (AIE) is introduced into the PVA solutions as a chain dynamics indicator to investigate phase transitions at high concentrations through in situ monitoring of the solvent evaporation process. Two turning points of fluorescent intensity are observed for the first time at mean concentrations of ∼25% and ∼45%, corresponding to the gelation and amorphous-to-crystalline transitions, respectively. Our work offers a fundamental insight into the physical nature of concentrate-dependent nonequilibrium transitions and develops a reliable and sensitive approach based on the AIE phenomenon for following high-concentration-triggered property changes of a polymer solution.

Improving the Sensitivity of the Mechanoluminescence Composite through Functionalization for Structural Health Monitoring

Over the past few years, considerable effort has been directed toward the development and improvement of mechanoluminescence (ML)-based stress sensing as an efficient nondestructive inspection technique. One of the challenges in ML stress sensing is the limited luminescent intensity and sensitivity of the ML-epoxy composite film to the local stress field. Herein, we present a novel approach for increasing the sensitivity of ML composites made of an epoxy resin matrix and SrAl₂O₄:Eu²⁺, Dy³⁺ particles functionalized with (3-aminopropyl)triethoxysilane. We performed a tensile test on an epoxy/ML composite specimen to investigate the effect of surface modification of ML particles on the luminescent sensitivity. A series of characterization analyses were performed on the modified surface to investigate the interfacial bonding. In addition, we applied the modified ML/epoxy paint to one side of the tensile specimen with an artificial invisible notch on the other side to visualize the stress field via light intensity (LI) distribution and then compared the results through a finite-element analysis (FEA). Surface modification of ML particles increased the sensitivity and introduced new chemical bonds, corresponding to a larger stress transfer through interfacial bonding rather than mere mechanical locking. In addition, the applied ML film on the notched specimen could visualize the specific pattern of LI reflecting the presence of a crack, which was confirmed by the FEA simulation. This implies that the proposed method of enhancing the ML film is promising for nondestructively predicting the presence, shape, and residual life of a crack in a specimen.

Polymer Mechanochromism from Force-Tuned Excited-State Intramolecular Proton Transfer

Real-time monitoring of strain/stress in polymers is a big challenge to date. Herein, we for the first time report an ESIPT (excited-state intramolecular proton transfer)-based mechanochromic mechanophore (MM). The synthesis of target MM PhMz-4OH [(2-hydroxyphenyl)benzimidazole with four aliphatic hydroxyls] is quite facile. PhMz-4OH possesses characteristic dual emissions, and its ESIPT activity is greatly affected by steric hindrance. Then, PhMz-4OH was covalently linked into polyurethane chains (PhMz-4OH@PU). Upon stretching, the PhMz-4OH@PU films showed fluorescence color change and spectral variation with the increase in enol emission and blueshift of keto emission due to the force-induced torsion of the dihedral angle between the proton donor and the proton acceptor. The PhMz-4OH@PU films with high mechanophore concentrations (>0.36 mol %) might undergo a two-stage force-responsive process, including torsion of the dihedral angle via force-induced disaggregation and direct chain-transduced force-induced torsion of the dihedral angle. The intensity ratio of enol emission to keto emission (I_E/I_K) shows a quantitative correlation with elongation, and real-time strain sensing is achieved. PhMz-4OH is a successful type II MM (without covalent bond scission) and displays high sensitivity and excellent reversibility to stress. Two control structures PhMz-NH₂ and PhMz-2OH were also embedded into PU but no spectral or color changes were detected, further confirming that mechanochromism of PhMz-4OH@PU films arises from the chain-transduced force. Density function theory (DFT) calculation was performed to study the force-tuned ESIPT process theoretically and rationalize the experimental results. This study might lay the foundation for real-time stress/strain sensing in practical applications.

Fluorescent In Situ 3D Visualization of Dynamic Corrosion Processes of Magnesium Alloys

Magnesium (Mg) alloys as implant materials with excellent biodegradation ability have promising clinical applications for tissue repair and restoration. Although the corrosion processes of Mg alloys in biophysiological media are closely related with their biodegradation ability, only limited methods have been developed for characterization of their corrosion processes, including electrochemical analysis, weight loss measurement, and hydrogen evolution analysis. Moreover, these methods suffer from drawbacks of poor spatiotemporal resolution, static observation, and tedious operation. To tackle these challenges, we herein developed a fluorescent probe PSPA for in situ 3D monitoring of the dynamic corrosion processes of Mg alloys on the basis of its selective turn-on detection ability toward magnesium hydroxide [Mg(OH)₂], which is the main corrosion product of Mg alloys in biophysiological media. As far as we know, this is the first example of a fluorescent probe for the monitoring of corrosion processes of Mg alloys in biophysiological media. We believe this fluorescence analysis method with easy operation and high spatiotemporal resolution advantages will contribute greatly to the clinical applications of Mg alloy implants.

Digital Image Correlation Compatible Mechanoluminescent Skin for Structural Health Monitoring

Monitoring structural health using mechanoluminescent (ML) effects is widely considered as a potential full-field and direct visualizing optical method with high spatial and temporal resolution and simple setup in a noncontact manner. The challenges and uncertainties in the mapping of ML field to effective strain field, however, tend to limit significant commercial ML applications for structural health monitoring systems. Here, however, quantification problems are resolved using the digital image correlation (DIC) method. Specifically, an image containing mechanically induced photon information is processed using a DIC algorithm to measure the strain field components, which enables the establishment of a calibration curve when the ML field is mapped onto the effective strain field using pixel level information. The results show a linear relationship between effective strain and ML intensity despite the plastic flow in ML skin. Furthermore, the calibration curve allows for easy conversion of ML field to effective-strain field at the crack-tip plastic zone of the alloy structure, retaining its spatial resolution. The compatibility of ML skin with the DIC algorithm not only enables the quantification of the ML effects of several organic/inorganic ML materials, but may also be useful in elucidating the fundamentals of the trap-controlled mechanism.

A double-spiro ring-structured mechanophore with dual-signal mechanochromism and multistate mechanochemical behavior: non-sequential ring-opening and multimodal analysis

We have developed a novel aminobenzopyranoxanthene-based mechanophore with a dual-signal response and two mechanogenerated ring-opened isomers, of which the relative distribution is modulated by external force based on the heat–force equilibrium.

Mechanochromic Luminescence of Bionanocomposite Hydrogel

Bionanocomposite hydrogels were prepared from gelatin hydrogel and subsequent ionic crosslinking in the presence of the smectite with adsorbed cyanine dye (pseudoisocyanine). In addition to the improved mechanical properties achieved...

Phototriggered color modulation of perovskite nanoparticles for high density optical data storage

The perovskite nanocrystals-dichloromethane (PNCs-DCM) with tunable fluorescent color under UV light are a new kind of photoresponsive luminescent materials (PLMs), which are qualified to apply in optical data storage.

[1]Benzothieno[3,2-b][1]benzothiophene-Based Dyes: Effect of Ancillary Moiety on Mechanochromism and Aggregation-Induced Emission

It is an established fact that [1]benzothieno[3,2-b][1]benzothiophene (BTBT) is a champion molecule for high mobility OFETs devices. Recently it is also utilized in dye-sensitized solar cells (DSSCs) and organic photovoltaics...

Machine-Learning-Assisted Accurate Prediction of Molecular Optical Properties upon Aggregation

For practical applications, molecules often exist in an aggregate state. Therefore, it is of great value if one can predict the performance of molecules when forming aggregates, for example, aggregation-induced emission (AIE) or aggregation-caused quenching (ACQ). Herein, a database containing AIE/ACQ molecules reported in the literature is first established. Through training, these machine learning (ML) models can build up the structure-property relationship and thus implement fast prediction of AIE/ACQ properties. To this end, a multi-modal approach is proposed, multiple prediction methods are compared and designed, and thus an ensemble strategy is developed. First, multiple molecular descriptors are considered at the same time, major features are extracted by dimensionality reduction, and multi-modal features are synthesized. Then, several state-of-the-art methods are designed and compared to analyze the advantages of the different methods. Finally, the ensemble strategy combines the advantages of the multiple methods to obtain the final prediction result. The reliability of this approach in an unknown molecular space is further verified by three newly designed molecules. Reasonable consistency between model predictions and experimental outcomes is obtained. The result indicates that ML can be a powerful tool to predict molecular properties in the aggregated state, thus accelerating the development of solid-state optical materials.

Fluorescent and stimuli-responsive performance of polymer composites filled with tetraphenylethene derivatives

In this study, a series of tetraphenylethene (TPE) derivatives with 3-butenloxy moieties were synthesized. The developed TPE with different number of substituent groups showed controlled aggregation-induced emission performance and variable...

Fluorescent Supramolecular Organic Frameworks Constructed by Amidinium-Carboxylate Salt Bridges

We report here a set of fluorescent supramolecular organic frameworks (SOFs) that incorporate aggregation-induced emission (AIE) units within their frameworks. The fluorescent SOFs of this study were constructed by linking the tetraphenylethylene (TPE)-based tetra(amidinium) cation TPE⁴⁺ and aromatic dicarboxylate anions through amidinium-carboxylate salt bridges. The resulting self-assembled structures are characterized by fluorescence quantum yields in the range of 4.6∼14 %. This emissive behavior is ascribed to a combination of electrostatic interactions and hydrogen bonds that operate in concert to impede motions that would otherwise lead to excited state energy dissipation. Single-crystal X-ray diffraction analyses revealed that the length of the dicarboxylate anion bridges has a considerable impact on the structural features of the resulting frameworks. Nevertheless, all SOFs prepared in the context of the present study were found to display emissive features characteristic of TPE-based AIE luminogens with only a modest dependence on the structural specifics being seen. The SOFs reported here could be reversibly "broken up" and "reformed" in response to acid/base stimuli. This reversible structural behavior is consistent with their SOF nature.

Molecular copper iodide clusters: a distinguishing family of mechanochromic luminescent compounds

Mechanochromic luminescent materials displaying switchable luminescence properties in response to external mechanical force are currently attracting wide interest because of their multiple potential applications. In the growing number of mechanochromic luminescent compounds, mechanochromic complexes based on copper present appealing features with a large variety of mechanochromic properties and economical advantages over other metals. Among Cu-based compounds, molecular copper iodide clusters of cubane geometry with formula [Cu₄I₄L₄] (L = organic ligand) stand out. Indeed, they can exhibit multiple luminescent stimuli-responsive properties, being particularly suitable for the development of multifunctional photoactive systems. This perspective describes the survey of these mechanochromic luminescent cubane copper iodide clusters. Based on our investigations, their mechanochromic luminescence properties are presented along with the study of the underlying mechanism. Establishment of structure-property relationships supported by various characterization techniques and associated with theoretical investigations permits gaining insights into the mechanism at play. Studies of other researcher groups are also described and illustrate the interest shown by these mechanochromic compounds. Mechanically responsive films are reported, demonstrating their potential use in a range of applications of such copper-based stimuli-responsive materials. Current challenges faced by the development of technological applications are finally outlined.

Amphiphilic polymers for aggregation-induced emission at air/liquid interfaces

Polymersomes and related self-assembled nanostructures displaying Aggregation-Induced Emission (AIE) are highly relevant for plenty of applications in imaging, biology and functional devices. Experimentally simple, scalable and universal strategies for on-demand self-assembly of polymers rendering well-defined nanostructures are highly desirable. A purposefully designed combination of amphiphilic block copolymers including tunable lengths of hydrophilic polyethylene glycol (PEG_m) and hydrophobic AIE polymer poly(tetraphenylethylene-trimethylenecarbonate) (P(TPE-TMC)_n) has been studied at the air/liquid interface. The unique 2D assembly properties have been analyzed by thermodynamic measurements, UV-vis reflection spectroscopy and photoluminescence in combination with molecular dynamics simulations. The (PEG)_m-b-P(TPE-TMC)_n monolayers formed tunable 2D nanostructures self-assembled on demand by adjusting the available surface area. Tuning of the PEG length allows to modification of the area per polymer molecule at the air/liquid interface. Molecular detail on the arrangement of the polymer molecules and relevant molecular interactions has been convincingly described. AIE fluorescence at the air/liquid interface has been successfully achieved by the (PEG)_m-b-P(TPE-TMC)_n nanostructures. An experimentally simple 2D to 3D transition allowed to obtain 3D polymersomes in solution. This work suggests that engineered amphiphilic polymers for AIE may be suitable for selective 2D and 3D self-assembly for imaging and technological applications.

Stimuli-Responsive AIEgens

The unique advantages and the exciting application prospects of AIEgens have triggered booming developments in this area in recent years. Among them, stimuli-responsive AIEgens have received particular attention and impressive progress, and they have been demonstrated to show tremendous potential in many fields from physical chemistry to materials science and to biology and medicine. Here, the recent achievements of stimuli-responsive AIEgens in terms of seven most representative types of stimuli including force, light, polarity, temperature, electricity, ion, and pH, are summarized. Based on typical examples, it is illustrated how each type of systems realize the desired stimuli-responsive performance for various applications. The key work principles behind them are ultimately deciphered and figured out to offer new insights and guidelines for the design and engineering of the next-generation stimuli-responsive luminescent materials for more broad applications.

Stiff and tough PDMS-MMT layered nanocomposites visualized by AIE luminogens

Polydimethylsiloxane (PDMS) is a widely used soft material that exhibits excellent stability and transparency. But the difficulty of fine-tuning its Young's modulus and its low toughness significantly hinder its application in fields such as tissue engineering and flexible devices. Inspired by nacre, here we report on the development of PDMS-montmorillonite layered (PDMS-MMT-L) nanocomposites via the ice-templating technique, resulting in 23 and 12 times improvement in Young's modulus and toughness as compared with pure PDMS. Confocal fluorescence microscopy assisted by aggregation-induced emission (AIE) luminogens reveals three-dimensional reconstruction and in situ crack tracing of the nacre-inspired PDMS-MMT-L nanocomposite. The PDMS-MMT-L nanocomposite is toughened with mechanisms such as crack deflection and bridging. The AIE-assisted visualization of the crack propagation for nacre-inspired layered nanocomposites provides an advanced and universal characterization technique for organic-inorganic nanocomposites.

Machine Learning Enables Highly Accurate Predictions of Photophysical Properties of Organic Fluorescent Materials: Emission Wavelengths and Quantum Yields

The development of functional organic fluorescent materials calls for fast and accurate predictions of photophysical parameters for processes such as high-throughput virtual screening, while the task is challenged by the limitations of quantum mechanical calculations. We establish a database covering >4300 solvated organic fluorescent dyes with 3000 distinct compounds and develop a new machine learning approach aimed at efficient and accurate predictions of emission wavelength and photoluminescence quantum yield (PLQY). Our feature engineering has given rise to a functionalized structure descriptor (FSD) and a comprehensive general solvent descriptor (CGSD), whereby a highly black-box computational framework is realized with consistently good accuracy across different dye families, ability of describing substitution effects and solvent effects, efficiency for large-scale predictions, and workability with on-the-fly learning. Evaluations with unseen molecules suggest a remarkable mean absolute error of 0.13 for PLQY and 0.080 eV for emission energy, the latter comparable to time-dependent density functional theory (TD-DFT) calculations. An online prediction platform was constructed based on the ensemble model to make predictions in various solvents. Our statistical learning methodology will complement quantum mechanical calculations as an efficient alternative approach for the prediction of these parameters.

Stimulus Responsive Zeolitic Imidazolate Framework to Achieve Corrosion Sensing and Active Protecting in Polymeric Coatings

Metal substrates beneath polymeric coatings are susceptible to localized corrosion, which could result in lifetime reduction and catastrophic failure without timely repair treatment. In situ detection of corrosion and repair coating defects are in high demand yet challenging to fulfill so far. Herein, we report a smart polymeric coating by integrating nanosensors into the coating matrix, which is capable of efficient corrosion sensing and active anticorrosion protecting. The nanosensors were constructed by zeolitic imidazolate framework encapsulated with the polyethylene glycol-tannic acid complex. The morphology, chemical constitution, and stimulus responsiveness of nanosensors were systematically analyzed. The generation of local corrosion beneath coating can be promptly sensed and reported by a conspicuous purple color derived from tannic-iron ion coordinates. Meanwhile, local electrochemical impedance spectroscopy results proved that the metal degradation process at the defected interface can be largely inhibited, exhibiting active anticorrosion property. Furthermore, the constructed smart coating possessed superior impermeability and long-term protective performance under simulated seawater and harsh salts spray conditions. This feasible and effective strategy based on simple nanosensors to engineer smart coatings paves a new way to develop high environmental adaptability protective materials with protecting, corrosion sensing, and self-healing functions.

Turning On Solid-State Luminescence by Phototriggered Subtle Molecular Conformation Variations

The development of solid-state intelligent materials, in particular those showing photoresponsive luminescence (PRL), is highly desirable for their cutting-edge applications in sensors, displays, data-storage, and anti-counterfeiting, but is challenging. Few PRL materials are constructed by tethering the classic photochromic systems with newly-emerged solid-state emitters. Selective solid-state photoreactions are demanded to precisely manipulate the luminescent behavior of these emitters, which require dramatic structural change and enough free space, thus limiting the scope of the PRL family. Here, a new PRL material, TPE-4N, that features sensitive and reversible fluorescence switching is reported. The interesting on-off luminescent property of TPE-4N can be facilely tuned through fast phototriggering and thermal annealing. Experimental and theoretical investigations reveal that subtle molecular conformation variation induces the corresponding PRL behavior. The crystalline and amorphous state endows an efficient and weak ISC process, respectively, to turn on and off the emission. The readily fabricated thin-film of TPE-4N exhibits non-destructive PRL behavior with high contrast (>10² ), good light transmittance (>72.3%), and great durability and reversibility under room light for months. Remarkably, a uniform thin-film with such fascinating PRL properties allows high-tech applications in invisible anti-counterfeiting and dynamic optical data storage with micro-resolution.

Color-Tunable and ESIPT-Inspired Solid Fluorophores Based on Benzothiazole Derivatives: Aggregation-Induced Emission, Strong Solvatochromic Effect, and White Light Emission

Organic solid materials with color-tunable emissions have been extensively applied in various fields. However, a rational design and facile synthesis of an ideal fluorophore are still challenging due to the undesirable aggregation-caused quenching effect in concentrated solution and solid form. Herein, we have developed a series of 2-(2'-hydroxyphenyl)benzothiazole (HBT)-derived color-tunable solid emitters by switching functional groups at the ortho-position of a hydroxyl group via formylation and an aldol condensation reaction. By tuning the electron-withdrawing ability and the π-conjugated framework introduced by the functional groups, fluorophores emit light covering the full-color range from blue to near-infrared regions with high quantum yields in their solid form and show a significant solvatochromic effect in polar solvents. The aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE) and excited-state intramolecular proton transfer (ESIPT) involving fluorescence mechanism, along with their inter/intramolecular interactions in crystals, are elucidated to depict the key factors for tunable emissions and high emitting efficiency. Furthermore, high-quality white-light-emitting materials are obtained in various solvents and polydimethylsiloxane (PDMS) films with combined fluorophores. Overall, these studies report a promising strategy for the construction of organic solid materials with color-tunable emission and shed light on methods for obtaining desirable emission efficiency.

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.

Combining Theory and Experiment to Get Insight into the Amorphous Phase of Luminescent Mechanochromic Copper Iodide Clusters

In the field of stimuli-responsive luminescent materials, mechanochromic compounds exhibiting reversible emission color changes activated by mechanical stimulation present appealing perspectives in sensor applications. The mechanochromic luminescence properties of the molecular cubane copper iodide cluster [Cu₄I₄[PPh₂(C₆H₄-CH₂OH)]₄] (1) are reported in this study. This compound can form upon melting an amorphous phase, giving an unprecedented opportunity to investigate the mechanochromism phenomenon. Because the mechanically induced crystalline-to-amorphous transition is only partial, the completely amorphous phase represents the ultimate state of the mechanically altered phase. Furthermore, the studied compound could form two different crystalline polymorphs, namely, [Cu₄I₄[PPh₂(C₆H₄-CH₂OH)]₄]·C₂H₃N (1·CH₃CN) and [Cu₄I₄[PPh₂(C₆H₄-CH₂OH)]₄]·3C₄H₈O (1·THF), allowing the establishment of straightforward structure-property relationships. Photophysical and structural characterizations of 1 in different states were performed, and the experimental data were supported by theoretical investigations. Solid-state NMR analysis permitted quantification of the amorphous part in the mechanically altered phase. IR and Raman analysis enabled identification of the spectroscopic signatures of each state. Density functional theory calculations led to assignment of both the NMR characteristics and the vibrational bands. Rationalization of the photoluminescence properties was also conducted, with simulation of the phosphorescence spectra allowing an accurate interpretation of the thermochromic luminescence properties of this family of compounds. The combined study of crystalline polymorphism and the amorphous state allowed us to get deeper into the mechanochromism mechanism that implies changes of the [Cu₄I₄] cluster core geometry. Through the combination of multistimuli-responsive properties, copper iodide clusters constitute an appealing class of compounds toward original functional materials.

Aggregate Science: From Structures to Properties

Molecular science entails the study of structures and properties of materials at the level of single molecules or small interacting complexes of molecules. Moving beyond single molecules and well-defined complexes, aggregates (i.e., irregular clusters of many molecules) serve as a particularly useful form of materials that often display modified or wholly new properties compared to their molecular components. Some unique structures and phenomena such as polymorphic aggregates, aggregation-induced symmetry breaking, and cluster excitons are only identified in aggregates, as a few examples of their exotic features. Here, by virtue of the flourishing research on aggregation-induced emission, the concept of "aggregate science" is put forward to fill the gaps between molecules and aggregates. Structures and properties on the aggregate scale are also systematically summarized. The structure-property relationships established for aggregates are expected to contribute to new materials and technological development. Ultimately, aggregate science may become an interdisciplinary research field and serves as a general platform for academic research.

Negatively Curved Nanographene with Heptagonal and [5]Helicene Units

Negatively curved nanographene (NG) 4, having two heptagons and a [5]helicene, was unexpectedly obtained by aryl rearrangement and stepwise cyclodehydrogenations. X-ray crystallography confirmed the saddle-shaped structures of intermediate 3 and NG 4. The favorability of rearrangement over helicene formation following radical cation or arenium cation mechanisms is supported by theoretical calculations. NG 4 demonstrates a reversible mechanochromic color change and solid-state emission, presumably benefiting from its loose crystal packing. After resolution by chiral high-performance liquid chromatography, the circular dichroism spectra of enantiomers 4-(P) and 4-(M) were measured and showed moderate Cotton effects at 350 nm (|Δε| = 148 M^–1 cm^–1).

Mechanochromic Fluorescent Polymers Enabled by AIE Processes

Polymeric materials are susceptible to the chain re-conformation, reorientation, slippage, and bond cleavage upon mechanical stimuli, which are likely to further grow into macro-damages and eventually lead to the compromise or loss of materials performance. Therefore, it is of great academic importance and practical significance to sensitively detect the local mechanical states in polymers and monitor the dynamic variations in polymer structures and properties under external forces. Mechanochromic fluorescent polymers (MFP) are a class of smart materials by utilizing sensitive fluorescent motifs to detect polymer chain events upon mechanical stimuli. Taking advantage of the unique aggregation-induced emission (AIE) effect, a variety of MFP systems that can self-report their mechanical states and mechano-induced structural and property changes through fluorescence signals have been developed. In this feature article, an overview of the recent progress on MFP systems enabled by AIE process is presented. The main design principles, including physically doping dispersed or microencapsulated AIE luminogens (AIEgens) into polymer matrix, chemically linking AIEgens in polymer backbones, and utilizing the clusterization-triggered emission of polymers containing nonconventional luminogens, are discussed with representative examples. Perspectives on the existing challenges and problems in this field are also discussed to guide future development.

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.

Dynamic Visible Monitoring of Heterogeneous Local Strain Response through an Organic Mechanoresponsive AIE Luminogen

Local strain concentration is critically important for damage formation of structural components. Therefore, it is of particular interest in developing the structural health monitoring (SHM) method for large-scale, full-field, and on-site monitoring of local strain response in complicated structural components in service. The present work investigated a SHM method based on a pure organic mechanoresponsive luminogen (MRL), 1,1,2,2-tetrakis(4-nitrophenyl)ethane, for heterogeneous local strain concentration. Invisible heterogeneous local strain response in complicated weld joints is transformed into visible fluorescence under monotonic tension and cyclic stress loading. The local strain (<15%) calculated by fluorescence intensity has a good agreement with the results obtained by the conventional digital image correlation method, indicating good measurement accuracy of the calibrated organic MRL method. The heterogeneity of local strain in complicated weld joints increases along with elongation and number of stress cycles. Moreover, the higher mean stress and stress amplitude can induce significantly higher accumulated local strain in the relatively soft fusion zone region. Compared with conventional strain measurement methods, the present organic MRL method opens up new possibilities for large-scale, full-field, and on-site monitoring of local strain concentration and damage in complicated structural components.

A flexible topo-optical sensing technology with ultra-high contrast

Elastic folding, a phenomenon widely existing in nature, has attracted great interests to understand the math and physical science behind the topological transition on surface, thus can be used to create frontier engineering solutions. Here, we propose a topo-optical sensing strategy with ultra-high contrast by programming surface folds on targeted area with a thin optical indicator layer. A robust and precise signal generation can be achieved under mechanical compressive strains (>0.4). This approach bridges the gap in current mechano-responsive luminescence mechanism, by utilizing the unwanted oxygen quenching effect of Iridium-III (Ir-III) fluorophores to enable an ultra-high contrast signal. Moreover, this technology hosts a rich set of attractive features such as high strain sensing, encoded logic function, direct visualisation and good adaptivity to the local curvature, from which we hope it will enable new opportunities for designing next generation flexible/wearable devices.

Emissive Platinum(II) Cages with Reverse Fluorescence Resonance Energy Transfer for Multiple Sensing

It is quite challenging to realize fluorescence resonance energy transfer (FRET) between two chromophores with specific positions and directions. Herein, through the self-assembly of two carefully selected fluorescent ligands via metal-coordination interactions, we prepared two tetragonal prismatic platinum(II) cages with a reverse FRET process between their faces and pillars. Bearing different responses to external stimuli, these two emissive ligands are able to tune the FRET process, thus making the cages sensitive to solvents, pressure, and temperature. First, these cages could distinguish structurally similar alcohols such as n-butanol, t-butanol, and i-butanol. Furthermore, they showed decreased emission with bathochromic shifts under high pressure. Finally, they exhibited a remarkable ratiometric response to temperature over a wide range (223-353 K) with high sensitivity. For example, by plotting the ratio of the maximum emission (I₆₀₀/I₄₈₀) of metallacage 4b against the temperature, the slope reaches 0.072, which is among the highest values for ratiometric fluorescent thermometers reported so far. This work not only offers a strategy to manipulate the FRET efficiency in emissive supramolecular coordination complexes but also paves the way for the future design and preparation of smart emissive materials with external stimuli responsiveness.

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.

Mechanochromic Fluorescent Polymers with Aggregation-Induced Emission Features

Mechanochromic polymers are defined as materials that are able to detect a mechanical stress through an optical output. This feature has evoked a growing interest in the last decades, thanks to the progress of chromogenic molecules whose optical characteristics and chemical functionalities allow their effective insertion in many thermoplastic and thermoset matrices. Among the different types of fluorogenic probes able to detect mechanical solicitations, those with aggregation-induced emission (i.e., AIEgens) have attracted tremendous interest since their discovery in 2001. In the present review, the main principles behind the AIEgens working behavior are introduced along with the current state of knowledge concerning the design and preparation of the derived mechanochromic fluorescent polymers. Examples are provided concerning the most ingenious solution for the preparation of chromogenic materials, starting from different types of commodity plastics or synthetic polymers and combined with the latest AIE technology to provide the most sensitive response to mechanical stress.

Solvation-dependent switching of solid-state luminescence of a fluorinated aromatic tetrapyrazole

Creating stimuli-responsive materials with switchable solid-state luminescence remains a challenge. We report that the solvation of a novel organic fluorophore can be utilized to prepare such a material, which emits in the blue (442-446 nm) region when wet and in the green (497-503 nm) region when dry.

Aggregation-Induced Emission for Visualization in Materials Science

Fluorescent imaging techniques have attracted much attention as a powerful tool to realize the visualization of structural and morphological evolution of various materials. However, the traditional fluorescent dyes usually suffered from aggregation-caused quenching, which severely limits the visualization results. In contrast, aggregation-induced emission (AIE) molecules with high quantum yields in the condensed state showed great opportunities for imaging techniques. In this feature article, recent progresses in visualization with AIE molecules are discussed. Assembly processes including crystallization, gelation process, and dissipative assembly have been observed. To better study information obtained regarding the processes, visualization during reactions, phase transitions, and molecular motions are successfully presented. Based on these successes, AIE molecules were further applied for phase recognition, macro-dispersion evaluation, and damage detection. Finally, we also present the outlook and perspectives, in our opinion, for the development of visualization by AIE molecules.

Luminescence mechanochromism of copper iodide clusters: a rational investigation

A photoluminescent copper iodide cluster has been characterized, and its luminescence mechanochromic properties have been anticipated.