Self-Calibrating Mechanochromic Fluorescent Polymers Based on Encapsulated Excimer-Forming Dyes

Microstructure induction of quaternary ammonium chitosan microcapsules based on magnetic field and study of their aroma release

Traditional pressure-sensitive microcapsules used in textiles face challenges of insufficient environmental friendliness in the production process and uncontrollable fragrance release. To address this issue, this study utilized quaternary ammonium chitosan and silica as wall materials to develop a magnetic aromatic microcapsule. The microstructure of the microcapsules was controlled by magnetic field induction, and its evolution pattern was investigated. After magnetic field induction, the microcapsules exhibited a trend of evolving from spherical to asymmetrical shapes, accompanied by significant changes in mechanical properties. Asymmetrical microcapsules showed higher adhesion and lower stiffness. When applied to cotton textiles, the cotton textiles treated with asymmetrical microcapsules released 63.40 % of lavender essential oil after 200 friction cycles, representing an 11.3 % improvement in release efficiency compared to regular microcapsules, indicating better mechanical stimulus responsiveness. Additionally, in antibacterial tests, aromatic cotton exhibited a 96.52 % inhibition ratio against Escherichia coli. In summary, this study explores methods to adjust the mechanical properties of microcapsules and the relationship between mechanical properties and microstructure, providing a new approach for functional textiles.

Exploring structural and optical properties of a new series of soft salts based on cyclometalated platinum complexes

A series of nine new soft salts based on two platinum(II) complexes, namely ([Pt(C^N)(CN)2]-[Pt(C^N)(en)]+) (en = ethane-1,2-diamine), has been developed and synthesized. Their photophysical properties in both solution and the solid state were described. All soft salt complexes exhibit phosphorescence emission with PLQY in the solid state up to 0.36. Most of these materials displayed aggregation-induced emission (AIE) or aggregation-induced emission enhancement (AIEE) in water/DMSO solutions as the water ratio increased. Structure-property relationships were analyzed in relation to emission properties. The presence of the free nitrogen atoms in soft salt complexes with a C^N pyrimidine-based ligand allowed for reversible sensitivity to acidic vapors, resulting in the quenching of phosphorescence emission. Additionally, for selected soft salts, we described reversible vapochromism behaviour, making these new materials interesting for multi-detection purposes in anti-counterfeiting applications.

CO2-Sourced Polymer Dyes for Dual Information Encryption

Large amounts of small molecule dyes leak into the ecosystems annually in harmful and unsustainable ways. Polymer dyes have attracted much attention because of their high migration resistance, excellent stability, and minimized leakage. However, the complex synthesis process, high cost, and poor degradability hinder their widespread application. Herein, green and sustainable polymer dyes are prepared using natural dye quercetin (Qc) and CO2 through a one-step process. The CO2-sourced polymer dyes show strong migration resistance, high stability, and can be degraded on demand. Additionally, the CO2-sourced polymer dyes showed unique responses to Zn2+, leading to significantly enhanced fluorescence, highlighting their potential for information encryption/decryption. The CO2-sourced polymer dyes can solve the environmental hazards caused by small molecule dye leakage and promote the carbon cycle process. Meanwhile, the one-step synthesis process is expected to achieve sustainable and widespread utilization of CO2-sourced polymer dyes.

Photo-modulated activation of organic bases enabling microencapsulation and on-demand reactivity

A method is developed for facile encapsulation of reactive organic bases with potential application for autonomous damage detection and self-healing polymers. Highly reactive chemicals such as bases and acids are challenging to encapsulate by traditional oil-water emulsion techniques due to unfavorable physical and chemical interactions. In this work, reactivity of the bases is temporarily masked with photo-removable protecting groups, and the resulting inactive payloads are encapsulated via an in situ emulsion-templated interfacial polymerization method. The encapsulated payloads are then activated to restore the organic bases via photo irradiation, either before or after being released from the core-shell carriers. The efficacy of the photo-activated capsules is demonstrated by a damage-triggered, pH-induced color change in polymeric coatings and by recovery of adhesive strength of a damaged interface. Given the wide range of potential photo-deprotection chemistries, this encapsulation scheme provides a simple but powerful method for storage and targeted delivery of a broad variety of reactive chemicals, promoting design of diverse autonomous functionalities in polymeric materials.

High-contrast reversible multiple color-tunable solid luminescent ionic polymers for dynamic multilevel anti-counterfeiting

Dynamic color-tunable luminescent materials, which possess huge potential applications in advanced multilevel luminescence anti-counterfeiting, are of considerable interest. However, it remains challenging to develop simple high-contrast reversible multiple (triple or more than triple) color-tunable high-efficiency solid luminescent materials with low cost, facile synthesis, and good processability. Herein, by simply grafting charged multi-color AIEgen-based chromophores into polymers, a series of high-efficiency multiple color-tunable luminescent single ionic polymers are constructed through tuning feed ratios, counter anions and reaction solvents. Remarkably, some ionic polymers can not only achieve rare high-contrast reversible multiple color-tunable emission in solid states in response to different solvent stimuli, but also could realize excitation-dependent color-tunable emission. To the best of our knowledge, such charming multiple (triple or more than triple) color-tunable solid polymers responding to multiple external stimuli are still rare. Based on comparative studies of emission spectra, excitation spectra and fluorescence lifetimes before and after swelling, it could be inferred that solvent stimuli could induce microstructure changes of these ionic polymers and then change the aggregated-states of their corresponding AIE-active emission centers. Moreover, the different solvent stimuli could induce to produce different degrees of microstructure changes, resulting in their unique multiple color-tunable emission. More significantly, these smart color-tunable ionic polymers show great promise for applications in dynamic multilevel (three-level or even more than three-level) anti-counterfeiting.

A dinoflagellate-inspired mechanochromic film for fast and reversible information encryption and display

Inspired by dinoflagellates, we developed a flexible film consisting of spiropyran-based soft polyacrylate and Zn(OTf)₂. The open-ring form of spiropyran coordinated with Zn(OTf)₂ under stretching to produce a visible fluorescent color change from colorless to yellow. The potential of this film was demonstrated for fast and reversible information encryption and decryption.

FAP-α-Instructed Coumarin Excimer Formation for High Contrast Fluorescence Imaging of Tumor

Emissive excimers, which are formed by planar polycyclic aromatic fluorophores (e.g., coumarin), enable high contrast tumor imaging. However, it is still challenging to "turn on" excimer fluorescence in physiological dilute solutions. The biocompatible CBT-Cys click condensation reaction enables both intra- and intermolecular aggregations of the as-loaded fluorophores on the probe molecules, which may promote the generation of emissive excimers in a synergistic manner. As a proof-of-concept, we herein design a fluorescence probe Cbz-Gly-Pro-Cys(StBu)-Lys(coumarin)-CBT (Cbz-GPC(StBu)K(Cou)-CBT), which can be activated by FAP-α under tumor-inherent reduction conditions, undergo a CBT-Cys click reaction, and self-assemble into coumarin nanoparticle Cou-CBT-NP to "turn on" the excimer fluorescence. In vitro and in vivo studies validate that this "smart" probe realizes efficient excimer fluorescence imaging of FAP-α-overexpressed tumor cells with high contrast and enhanced accumulation, respectively. We anticipate that this probe can be applied for diagnosis of FAP-α-related diseases in the clinic in near future.

Efficient chirality transfer from chiral amines to oligo(p-phenylenevinylene)s to fabricate chiroptical materials

Oligo(p-phenylenevinylene) (OPV)-based luminophores show versatile luminescence properties based on their structural and packing arrangements, and however have seldomly been utilized in fabricating chiroptical materials. Here, we report a co-assembly strategy to introduce hydrophilic chiral amines into OPV-appended carboxylic acids via salt-bridge type hydrogen bonds. The coassembly in aqueous media allows for efficient chirality transfer with chiroptical activities. The participation of chiral amines altered the aromatic stacking from H- to J-type, which greatly enhanced the luminescence of the OPV compounds and facilitated the emergence of Cotton effects as well as circularly polarized luminescence. This work demonstrates the successful coassembly of OPV luminophores into chiral assemblies with tunable optical activity, showing potential for chiroptical application in optical chirality sensing.

Supramolecular Rings as Building Blocks for Stimuli-Responsive Materials

Stimuli-responsive polymers are of great interest due to their ability to translate changing environmental conditions into responses in defined materials. One possibility to impart such behavior is the incorporation of optically active molecules into a polymer host. Here, we describe how sensor molecules that consist of a π-extended benzothiadiazole emitter and a naphthalene diimide quencher can be exploited in this context. The two optically active entities were connected via different spacers and, thanks to attractive intramolecular interactions between them, the new sensor molecules assembled into cyclic structures in which the fluorescence was quenched by up to 43% when compared to solutions of the individual dyes. Detailed spectroscopic investigations of the sensor molecules in solution show that the extent of donor/acceptor interactions is influenced by various factors, including solvent polarity and ion concentration. The new sensor molecule was covalently incorporated into a polyurethane; the investigation of the optical characteristics in both the solid and solvent-swollen states indicates that a stimulus-induced formation of associated dye pairs is possible in polymeric materials. Indeed, a solvatochromic quenching effect similar to the behavior in solution was observed for solvent-swollen polymer samples, leading to an effective change of the green emission color of the dye to a yellow color.

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.

Mechanofluorochromic Difluoroboron β-diketonates based Polymer Composites: Towards Multi-Stimuli Responsive Mechanical Stress Probes

Developing mechano-responsive fluorescent polymers that exhibit distinct responses to distinct mechanical stresses requires a careful design of the fluorophore in order to tune its interactions with the polymer. A series of mechanofluorochromic (MFC) polymer composites has been prepared by dispersing difluoroboron diketonates complexes with various alkyl side chain lengths (DFB-alkyl) in linear low-density polyethylene (LLDPE). Observation of the resulting polymer composites under microscope reveals different aggregate sizes of the three DFB-alkyl, thus confirming the functionalization by alkyl side chains as a powerful approach to control the aggregation process in a polymer. Besides, the three polymer composite samples are shown to be sensitive to both stretching and scratching, thereby consisting in the first reported example of MFC polymer responding to these two distinct mechanical stimuli. To establish a structure-property relationship, our strategy consisted in applying controlled tensile or friction forces while simultaneously monitoring fluorescence changes. Interestingly, the intensity of the MFC response to both stretching and scratching depends on the alkyl chain length and thus on the aggregation properties of the fluorophore. According to a time-resolved fluorescence study, emission was found to originate from different species following the type of applied stress (tensile or friction force). This article is protected by copyright. All rights reserved.

Autonomous Visualization of Damage in Polymers by Metal-Free Polymerizations of Microencapsulated Activated Alkynes

The development of autonomous materials with desired performance and built-in visualizable sensing units is of great academic and industrial significance. Although a wide range of damage indication methods have been reported, the "turn-on" sensing mechanism by damaging events based on microcapsule systems, especially those relying on chemical reactions to elicit a chromogenic response, are still very limited. Herein, a facile and metal-free polymerization route with an interesting reaction-induced coloration effect is demonstrated. Under the catalysis of 1,4-diazabicyclo[2.2.2]octane (DABCO), the polymerizations of difunctional or trifunctional activated alkynes proceed very quickly at 0 °C in air. A series of polymers composed of stereoregular enyne structure (major unit) and divinyl ether structure (minor unit) are obtained. Both the catalyst and monomers are colorless while the polymerized products are deep-colored. This process can be applied for the damage visualization of polymers using the microencapsulation technique. Microcapsules containing the reactive alkyne monomer are prepared and mixed in a DABCO-dispersed polymer film. Both the external and internal damage regions of this composite film can be readily visualized once the reaction is initiated from the ruptured microcapsules. Moreover, the newly formed polymer automatically seals the cracks with an additional protection function.

Activation of Mechanophores in a Thermoset Matrix by Instrumented Scratch

Scratches in polymer coatings and barrier layers negatively impact optical properties (haze, light transmission, etc.), initiate routes of degradation or corrosion (moisture permeability), and nucleate delamination of the coating. Detecting scratches in coatings on advanced materials systems is an important component of structural health monitoring but can be difficult if the defects are too small to be detected by the naked eye. The primary focus of the present work is to investigate scratch damage using fluorescence lifetime imaging microscopy (FLIM) and mechanical activation of a mechanophore (MP)-containing transparent epoxy coating. The approach utilizes a Berkovich tip to scratch MP-epoxy coatings under a linearly increasing normal load. The goal is to utilize the fluorescent behavior of activated MPs to enable the detection of microscale scratches and molecular scale changes in polymeric systems. Taking advantage of the amine functionality present in a polyetheramine/bisphenol A epoxy network, a modified rhodamine dye is covalently bonded into a transparent, thermoset polymer network. Following instrumented scratch application, subsequent fluorescence imaging of the scratched MP-epoxy reveals the extent of fluorescence activation induced by the mechanical deformation. In this work, the rhodamine-based mechanophore is used to identify both ductile and fracture-dominated processes during the scratch application. The fluorescence intensity increases linearly with the applied normal load and is sensitive to fracture dominated processes. Fluorescence lifetime and hyperspectral imaging of damage zones provide additional insight into the local (nanoscopic) environment and molecular structure of the MP around the fracture process zone, respectively. The mechanophore/scratch deformation approach allows a fluorescence microscope to probe local yielding and fracture events in a powerful way that enhances the optical characterization of damage zones formed by standard scratch test methods and leads to novel defect detection strategies.

Force-mediated molecule release from double network hydrogels

The incorporation of mechanosensitive linkages into polymers has led to materials with dynamic force responsivity. Here we report oxanorbornadiene cross-linked double network hydrogels that release molecules through a force-mediated retro Diels-Alder reaction. The molecular design and tough double network of polyacrylamide and alginate promote significantly higher activation at substantially less force than pure polymer systems. Activation at physiologically relevant forces provides scope for instilling dynamic mechanochemical behavior in soft biological materials.

Rhodamine-Installed Polynorbornenes: Molecular Design, Structure, and Stimuli-Responsive Properties

The synthesis of a number of tailored architectures of rhodamine dye-norbornene conjugate monomers and corresponding homopolymers derived from them is described. The impact of the monomer architecture on the mechanochromic, photochromic, and thermochromic properties of rhodamine-modified polynorbornenes is reported. Color changes were caused by the reversible interconversion between the "open" and "closed" spirolactam form of the covalently attached dye. Monomers were synthesized in two principle architectures that varied on: (1) the number of polymerizable norbornene groups tethered to a bifunctional rhodamine dye; (2) the presence of flexible methylene spacers between the dye and the polymerizable norbornene groups. Introduction of norbornene groups on each of the two hydroxy groups of a bifunctional rhodamine resulted in a cross-linked polymer that exhibited better mechanochromic, photochromic, and thermochromic properties compared to the corresponding polymer without cross-links, derived from the derivatization of bifunctional rhodamine with only one norbornene. The introduction of flexible methylene spacers between the two polymerizable norbornenes and the dye molecule resulted in a polymeric framework with rapidly reversible color-changing properties upon mechanical or photostimulation. The ideal monomer molecular structure, whereby (1) attaching norbornene on both sides of the rhodamine dye and (2) methylene spacers between the dye and norbornenes on both sides afforded the nonpareil polymer structure that was capable of thermoreversible mechanochromic and photochromic features, and irreversible thermochromic features. These new materials may find utility as multi-stimuli-responsive soft materials.

Multicolor Mechanofluorophores for the Quantitative Detection of Covalent Bond Scission in Polymers

The fracture of polymer materials is a multiscale process starting with the scission of a single molecular bond advancing to a site of failure within the bulk. Quantifying the bonds broken during this process remains a big challenge yet would help to understand the distribution and dissipation of macroscopic mechanical energy. We here show the design and synthesis of fluorogenic molecular optical force probes (mechanofluorophores) covering the entire visible spectrum in both absorption and emission. Their dual fluorescent character allows to track non-broken and broken bonds in dissolved and bulk polymers by fluorescence spectroscopy and microscopy. Importantly, we develop an approach to determine the absolute number and relative fraction of intact and cleaved bonds with high local resolution. We anticipate that our mechanofluorophores in combination with our quantification methodology will allow to quantitatively describe fracture processes in materials ranging from soft hydrogels to high-performance polymers.

Phosphorescent Soft Salt Based on Platinum(II) Complexes: Photophysics, Self-Assembly, Thermochromism, and Anti-counterfeiting Application

A new platinum(II) complex-based soft salt S1, ([Pt(tpp)(ed)]⁺[Pt(pba) (CN)₂]^-) (tpp = 2-(4-(trifluoromethyl)phenyl)pyridine, ed = ethane-1,2-diamine, pba = 4-(2-pyridyl)benzaldehyde), was designed and synthesized. UV-visible absorption and photoluminescence (PL) spectra were studied to elucidate the nature of ground and excited states. The soft salt complex was found to show self-assembly properties with the assistance of electrostatic, π-π stacking, and Pt···Pt interactions, resulting in the remarkable emergence of low-energy absorption and PL bands. Morphological transformation of S1 from undefined nanosized aggregates to nanofibers with different solvent compositions has been demonstrated. Interestingly, a luminescent polymer film was prepared by doping S1 into a polyethylene glycol matrix. The film displayed distinctive emission color change from yellow to red upon heating. Eventually, a high-level anti-counterfeiting application was accomplished using a time-resolved imaging technique based on the thermochromic luminescence property and long emission decay time displayed by S1. It is anticipated that this work can provide deep insights into the control of intermolecular interactions between cationic and anionic complexes of soft salt upon exposure to different external stimuli, resulting in the development of smart luminescent materials for various applications.

Continuous color tuning of single-fluorophore emission via polymerization-mediated through-space charge transfer

Tuning emission color of molecular fluorophores is of fundamental interest as it directly reflects the manipulation of excited states at the quantum mechanical level. Despite recent progress in molecular design and engineering on single fluorophores, a systematic methodology to obtain multicolor emission in aggregated or solid states, which gives rise to practical implications, remains scarce. In this study, we present a general strategy to continuously tune the emission color of a single-fluorophore aggregate by polymerization-mediated through-space charge transfer (TSCT). Using a library of well-defined styrenic donor (D) polymers grown from an acceptor (A) fluorophore by controlled radical polymerization, we found that the solid-state emission color can be fine-tuned by varying three molecular parameters: (i) the monomer substituent, (ii) the end groups of the polymer, and (iii) the polymer chain length. Experimental and theoretical investigations reveal that the color tunability originates from the structurally dependent TSCT process that regulates charge transfer energy.

Ultrahigh-Sensitive Compression-Stress Sensor Using Integrated Stimuli-Responsive Materials

Measurement of mechanical stresses, such as compression, shear, and tensile stresses, contributes toward achieving a safer and healthier life. In particular, the detection of weak compression stresses is required for healthcare monitoring and biomedical applications. Compression stresses in the order of 10⁶ -10¹⁰  Pa have been visualized and/or quantified using mechano-responsive materials in previous works. However, in general, it is not easy to detect compression stresses weaker than 10³  Pa using conventional mechano-responsive materials because the dynamic motion of the rigid mechano-responsive molecules is not induced by such a weak stress. In the present work, weak compression stresses in the order of 10⁰ -10³  Pa are visualized and measured via the integration of stimuli-responsive materials, such as layered polydiacetylene (PDA) and dry liquid (DL), through response cascades. DLs consisting of liquid droplets covered by solid particles release the interior liquid and collapse with application of a weak compression stress. The color of the layered PDA is changed by the spilled liquid as a chemical stress. A variety of weak compression stresses, such as expiratory pressure, are visualized and colorimetrically measured using the paper-based device of the integrated stimuli-responsive materials. Diverse mechano-sensing devices can be designed via the integration of stimuli-responsive materials.

Multicolor mechanochromism of a multinetwork elastomer that can distinguish between low and high stress

We report our findings on a multicolor mechanochromic elastomer that is able to discriminate between low and high stress. The key point of our design depends on the introduction of two UV-inert mechanophores into different polymer networks.

Polymer-Based Mechanochromic Composite Material Using Encapsulated Systems

The development of mechanochromic or self-reporting polymers that can indicate damage or fatigue of materials with an optical signal has become of paramount interest to ensure the reliability of the materials and prevent catastrophic failure. This technology can potentially find usefulness for various applications, including in situ monitoring of mechanical events and structural health monitoring systems. An emerging and versatile approach to achieve mechanochromic properties relies on the encapsulation of dye solutions that can be released and activated (chemically or physically) when the walls of the capsules are mechanically damaged. While the mechanochromic effect can be achieved with different types of dyes and operating principles, this framework can also be designed with encapsulating-containers of different shapes and shell materials, such as microcapsules, hollow glass fibers, vascular networks, and micelles, making this concept applicable to a broad range of polymer matrices. An overview of the different encapsulation approaches that have been employed to prepare mechanochromic polymers is given, with a focus on the containers used for this purpose. A brief description of the containers' preparation is provided, and their associated chromic operating principles and progress in their designs are reviewed.

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.

The Mechanism of Flex-Activation in Mechanophores Revealed By Quantum Chemistry

Flex-activated mechanophores can be used for small-molecule release in polymers under tension by rupture of covalent bonds that are orthogonal to the polymer main chain. Using static and dynamic quantum chemical methods, we here juxtapose three different mechanical deformation modes in flex-activated mechanophores (end-to-end stretching, direct pulling of the scissile bonds, bond angle bendings) with the aim of proposing ways to optimize the efficiency of flex-activation in experiments. It is found that end-to-end stretching, which is a traditional approach to activate mechanophores in polymers, does not trigger flex-activation, whereas direct pulling of the scissile bonds or displacement of adjacent bond angles are efficient methods to achieve this goal. Based on the structural, energetic and electronic effects responsible for these observations, we propose ways of weakening the scissile bonds experimentally to increase the efficiency of flex-activation.

Mechanochromic LLDPE Films Doped with NIR Reflective Paliogen Black

The perylene bisimide derivative Paliogen Black (P-black) is proposed as a new chromogenic probe that shows visible (vis) and near-infrared (NIR) responses after mechanical solicitations of host linear low-density polyethylene (LLDPE) films. P-black is reported to display strong absorption in the vis spectrum and unusual reflective and cooling features in the NIR region. Uniaxial deformation of the 2.5, 5, and 10 wt% P-black/LLDPE films yields a dichroic absorption under polarized light with color variations attributed by the computational analysis to the distinct anisotropic behavior of the transition dipole moments of P-black chromophores. When LLDPE films are deformed, P-black aggregates reduce their size from ≈30-40 µm to ≈5-10 µm that, in turn, causes reflectivity losses of about 30-40% at the maximum elongation. This gives rise to warming of 5-6 °C of the locally oriented film placed in contact with a black substrate under the illumination with an IR lamp for 5 s. These features combined with the high sensitivity of the vis-NIR response toward mechanical solicitations render P-black as a new solution to detect uniaxial deformations of plastic films through both optical and thermal outputs.

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.

Fabrication of microcapsule-type composites with the capability of underwater self-healing and damage visualization

Inspired by biology, underwater self-healing polymer composites with damage-healing visible agents were successfully designed and prepared. The healing agents, same as epoxy resin matrices, were encapsulated and embedded into a matrix that contained fluorescent latent curing agents. The results of investigation on healing properties revealed that the fluorescent latent curing agents and the microcapsules in the matrix play two roles. First, the matrix could be self-healed via a crosslinking reaction between the amine group and epoxy resin, in which the amine group could be released from the fluorescent latent curing agents (FLCAs) after exposure to water. Second, the fluorescent dyes released under water could indicate the scratches and healing area visually. Embedding 15 mass% microcapsules and 6 mass% FLCAs in self-healing materials yielded a healing efficiency of 85.6% and the most efficient fluorescence detection. Self-healing materials can be repaired underwater and they show the location of damage, which is of great significance in applications such as water conservation engineering, environmental treatment engineering, ship engineering and ocean engineering.

Supramolecular Oligourethane Gel with Multicolor Luminescence Controlled by Mechanically Sensitive Hydrogen-Bonding

We report a new type of mechanically sensitive multicolor luminescent oligourethane gel (OUA-gel). The conformation of the oligomeric chains can be controlled by changing the strength of hydrogen bonds. The optical properties of the oligomers are highly dependent on the conformations which vary in response to mechanical stresses and phase transitions. The design relies on the introduction of a single mechanical chromophore, aurintricarboxylic acid, with propeller-like, spatially crowded, and highly twisted conformations, and the presence of three carboxyl groups, which provide multidirectional hydrogen-bonding opportunities. Introducing dimethylsulfoxide (DMSO) as an additional H-bond acceptor molecule leads to a viscous OUA-gel which exhibits multiemission colors because of changes in the chain conformation within the matrix, which are induced by different strengths of H bonds. The conformation can be adjusted by mechanical force or temperature, both of which influence the H-bonding. The multifunctional and multicolored mechanochromism of the OUA-gel has great promise in sensing applications. The results represent a substantial step toward understanding the mechanism of polychromism in soft materials and the molecular design of advanced smart materials.

Establishing charge-transfer excitons in 2D perovskite heterostructures

Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA₂PbI₄:PEA₂SnI₄, functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 μs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA₂PbI₄/PEA₂SnI₄ heterostructure when shearing-away PEA₂SnI₄ film onto the surface of PEA₂PbI₄ film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA₂PbI₄:PEA₂SnI₄ interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites.

Forming charge transfer excitons (CTEs) exclusively within perovskite structures remains as an unexplored issue. Here, the authors report the establishment of CTEs for demonstrating broad light emission within quasi-2D perovskite heterostructures, presenting “intermolecular-type” excited states.

Mechanochromic Polymers Based on Microencapsulated Solvatochromic Dyes

The development of polymers with built-in sensors that provide readily perceptible optical warning signs of mechanical events has received considerable interest. A simple and versatile concept to bestow polymers with mechanochromic behavior is the incorporation of dye-filled microcapsules. Such capsules release their cargo when their shell is damaged, and the dye is subsequently activated through a chemical or physical change that causes a chromogenic response. Here, we report the preparation of fluorescent poly(urea-formaldehyde) microcapsules containing solutions of a solvatochromic cyanostilbene dye and their integration in different polymers. When objects made from such composites are damaged, the dye solution is released from the containers, diffuses into the matrix, and the solvent evaporates. As a result, the polarity around the dye molecules changes, and this leads to a change of the fluorescence color. Alternatively, the dye is blended into the polymer matrix, microcapsules are loaded with a solvent, and the release of the latter triggers the color change. Both mechanisms afford ratiometric signals because the capsules that remain intact or dye molecules that are not exposed to the solvent can be used as a built-in reference; therefore, a quantitative assessment of the damage inflicted on the material is a priori possible.

Stimuli-Responsive Benzothiadiazole Derivative as a Dopant for Rewritable Polymer Blends

A new rod-shaped benzothiadiazole fluorophore, namely, 4,7-di-(4-nonylphenyl)benzo[c][1,2,5]thiadiazole, which strongly emits fluorescence both in solution and in solid state has been synthesized, and its photophysical properties were rationalized with the help of density functional theory calculations. This molecule crystallizes in two distinct light-emitting crystalline phases, which can be interconverted in response to pressure, temperature, and solvent vapors. Powder X-ray diffraction indicates that in both polymorph, molecules adopt a lamellar packing, the different interlayer spacing being the main difference between the two structures. Single-crystal analysis of one of the polymorphs allows us to identify weak interaction planes, which presumably facilitates the polymorphic transformation through mechanically or thermally induced sliding processes. The polymorphic transformation and the origin of the switchable fluorescence have been rationalized through a spectroscopic and theoretical study. This study suggests that the different colors observed are due to different intermolecular aromatic interactions owing to the displacement of the molecules with respect to the layer normal. Interestingly, blending this molecule with a biodegradable polymer such as poly(vinyl alcohol) gives rise to a thermally activated reversible switchable fluorescent system, which entitles this material as an attractive candidate for technological applications, such as thermal sensors, security inks, or rewritable paper.

A Facile Strategy To Prepare Smart Coatings with Autonomous Self-Healing and Self-Reporting Functions

Herein, we report a smart coating with autonomous self-healing and self-reporting functions by simple integration of one-component microcapsules into the matrix without external intervention. The microcapsules containing hexamethylene diisocyanate (HDI) solution of aggregation-induced emission luminogens (AIEgens) were synthesized, and their properties, such as their composition, thermal stability, morphology, and damage-indicating ability, were investigated systematically. The AIEgen/HDI microcapsule-embedded coatings display adaptive self-repair of scratches and simultaneous high-contrast indication of the healed damage. Two commercialized AIEgens, tetraphenylethylene (TPE) and its derivative with dimethoxyl and benzylidene-methyloxazolone moieties (DM-TPE-BMO), were utilized as examples to demonstrate the feasibility of this concept in diverse polymer matrixes (including blue autofluorescent matrixes). It was found that the content of AIEgens can even be lowered to 0.05 wt %. This facile, economical, and feasible strategy toward the dual functions of self-repairing and self-sensing provides a new route for enhancing the longevity and reliability of polymer coatings, which is appealing and of great importance in practical applications.

Color-Based Optical Detection of Glass Transitions on Microsecond Timescales Enabled by Exciplex Dynamics

Every measurement technique operates on a given timescale and measurements using emissive small molecule sensors are no exception. A family of luminescent sensors providing first optical characterization of dynamic phenomena in polymers at a timescale of several microseconds is described. This performance originates from the dynamics manifested in the excited state of the sensor molecules where diffusioncontrolled events select the emission color while radiative phenomena define the global operation timescale. Since the mechanism responsible for signal generation is confined to the short lived excited state of emissive probe, it is possible observe an unprecedented link between the timescale of sensory action and that of photoluminescence. An application of this new methodology is demonstrated by performing general, short timescale detection of glass transitions in a temperature ranges precluding the informative range of conventional techniques by tens of degrees.

Mechano-chromic protein-polymer hybrid hydrogel to visualize mechanical strain

In a proof-of-concept study, a mechano-chromic hydrogel was synthesized here, via chemoenzymatic click conjugation of fluorophore-labeled fibronectin into a synthetic hydrogel co-polymers (i.e., poly-N-isopropylacrylamide/polyethylene glycol). The optical FRET response could be tuned by macroscopic stretch.

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.

Anti-Stokes Stress Sensing: Mechanochemical Activation of Triplet-Triplet Annihilation Photon Upconversion

The development of methods to detect damage in macromolecular materials is of paramount importance to understand their mechanical failure and the structure-property relationships of polymers. Mechanofluorophores are useful and sensitive molecular motifs for this purpose. However, to date, tailoring of their optical properties remains challenging and correlating emission intensity to force induced material damage and the respective events on the molecular level is complicated by intrinsic limitations of fluorescence and its detection techniques. Now, this is tackled by developing the first stress-sensing motif that relies on photon upconversion. By combining the Diels-Alder adduct of a π-extended anthracene with the porphyrin-based triplet sensitizer PtOEP in polymers, triplet-triplet annihilation photon upconversion of green to blue light is mechanochemically activated in solution as well as in the solid state.

Multicolor Organometallic Mechanophores for Polymer Imaging Driven by Exciplex Level Interactions

Photoluminescent compounds can undergo various structural changes upon interaction with light. When these changes manifest themselves in the excited state, the resulting emitters can obtain a sensory function. In this work, we designed coordination compounds that can vary their emission color in response to thermal and mechanical stimuli. When embedded in a polymer matrix, Cu-NHC sensors act as mechanophores, and their color-based response can readily describe mechanical stress and phase transition phenomena. A strong practical advantage of new mechanophores over previous generations of organometallic stress sensors stems from their reliance on emission color variations that are easy to detect. In a broad context, our work implies that emission color variations that we often view as thermally governed can also be triggered mechanically and used to generate sensory information.

Mechano- and Photoresponsive Behavior of a Bis(cyanostyryl)benzene Fluorophore

The mechanoresponsive behavior and photochemical response of a new bis(cyanostyryl)benzene fluorophore (CSB-5) were investigated. Green fluorescence with λ_em,max of 507 nm was found for CSB-5 in chloroform solution, mirroring the behavior of a previously reported similar dye (CSB-6). Alternatively, crystalline samples of CSB-5 exhibited orange fluorescence with λ_em,max of 620 nm, attributable to excimer emission. Although the emission color change was not clearly noticeable by naked eye, CSB-5 exhibited mechanochromic luminescence, due to transformation into the amorphous state upon grinding the crystalline powder. Interestingly, rubbed films of CSB-5 prepared on glass substrates exhibited a pronounced emission color change from orange to green when exposed to UV light. This response is the result of a photochemical reaction that occurs in the amorphous state and which causes a decrease of the excimer emission sites so that the emission color changes from excimer to monomer. The crystalline material did not display such a photoinduced emission color change and the difference in photochemical reactivity between crystalline and amorphous states was exploited to pattern the emission color of rubbed films.

Bio-Inspired, Self-Toughening Polymers Enabled by Plasticizer-Releasing Microcapsules

A new concept for the design of self-toughening thermoplastic polymers is presented. The approach involves the incorporation of plasticizer-filled microcapsules (MCs) in an intrinsically rigid and brittle matrix polymer. The intriguing adaptability that this simple tactic enables is demonstrated with composites composed of a poly(lactic acid) (PLA) matrix and 5-20% w/w poly(urea-formaldehyde) (PUF) MCs that contained hexyl acetate as plasticizer. At low strain (<1.5%), the glassy PLA/MC composites remain rigid, although the intact MCs reduce the Young's modulus and tensile strength by up to 50%. While the neat PLA shows brittle failure at a strain of around 2.5%, the composites yield in this regime, because the MCs rupture and release their plasticizing cargo. This effect leads up to 25-fold increase of the elongation at break and 20-fold increase of the toughness vis-à-vis the neat PLA, while the impact on modulus and ultimate stress is much smaller. Ballistic impact tests show that the self-toughening mechanism also works at much higher strain rates than applied in tensile tests and the operating mechanism is corroborated through systematic thermomechanical studies that involved dynamic mechanical testing and thermal analysis.

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.

Tracking Local Mechanical Impact in Heterogeneous Polymers with Direct Optical Imaging

Structural heterogeneity defines the properties of many functional polymers and it is often crucial for their performance and ability to withstand mechanical impact. Such heterogeneity, however, poses a tremendous challenge for characterization of these materials and limits our ability to design them rationally. Herein we present a practical methodology capable of resolving the complex mechanical behavior and tracking mechanical impact in discrete phases of segmented polyurethane-a typical example of a structurally complex polymer. Using direct optical imaging of photoluminescence produced by a small-molecule organometallic mechano-responsive sensor we observe in real time how polymer phases dissipate energy, restructure, and breakdown upon mechanical impact. Owing to its simplicity and robustness, this method has potential in describing the evolution of complex soft-matter systems for which global characterization techniques fall short of providing molecular-level insight.

Autonomous Damage Detection in Multilayered Coatings via Integrated Aggregation-Induced Emission Luminogens

Detection and assessment of small-scale damage at early stages are essential for polymeric materials to extend lifetime, avoid catastrophic structural failure, and improve cost-efficiency. Previous self-reporting coatings provide visual indication of surface damage but have been limited to a single layer without information on the depth of crack penetration. Here, we present a novel strategy for autonomous indication of damage in multilayered polymeric materials using aggregation-induced emission luminogens (AIEgens). Three different AIEgens are encapsulated and layered into polymeric coatings. When scratches of varying depths penetrate the coating layers, different combinations of AIEgens are activated to visually detect the depth of damage based on the corresponding fluorescent colors. The AIEgen-based detection mechanism makes this system a powerful tool for damage indication in a variety of polymeric coatings.