Mechanochromic Materials Research Articles

Reversible Mechanochromic Luminescence of Tetranuclear Cuprous Complexes.

Mechanochromic luminescence materials have attracted rapidly growing interest. Nevertheless, the designed synthesis of such materials remains a challenge, and there have been few examples based on weak intramolecular interactions. Herein, we report a new approach for preparing mechanochromic luminescence materials of Cu(I) complexes, i.e., constructing a photoluminescence system that bears a large coplanar multinuclear Cu(I) unit showing weak intramolecular π···π interactions with the planar rings of the coordinated ligands in the molecule. Using it, a series of novel mechanochromic luminescent tetranuclear Cu(I) complexes have been successfully designed and synthesized. As revealed by single-crystal X-ray crystallography, these Cu(I) complexes share an identical {Cu4[μ3-η2(N,N),η1(N),η1(N)-pyridyltetrazole]2}2+ planar fragment whose coplanar pyridyl rings exhibit weak intramolecular π···π interactions with the phenyl rings of the coordinated phosphine ligands in the molecule. All of these Cu(I) complexes exhibit reversible mechanochromic luminescence, which can be attributed to the change in the rigidity of the molecular structure resulting from the disruption and restoration of intramolecular π···π interactions between the pyridyl and phenyl rings triggered by grinding and CH2Cl2 vapor, as supported by powder X-ray diffraction and Fourier transform infrared spectrometry. In addition, the results might provide a new route for developing mechanochromic luminescence materials of Cu(I) complexes for intelligent responsive luminescent devices.

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.

Mechanochromism of Structural‐Colored Materials

AbstractStructural colors widespread in nature have been theoretically in‐depth studied and manufactured through the artificial approaches such as self‐assembly and nanolithography thanks to the rapidly developing nanotechnology. Structural colors based on elastomers and hydrogels can be readily modulated upon mechanical stimuli, via adjusting the structural parameters such as interplanar spacing and diffractive angle. Recent breakthroughs in mechanochromic structural‐colored materials has not only enriched their varieties but also strengthened their competences remarkably. The emerging mechanoresponsive materials have been applied in various fields such as sensors, displays, anti‐counterfeiting, and healthcare. This review focuses on the recent progresses in mechanochromic structural‐colored materials with different architectures including photonic crystals, liquid crystals, and photonic glasses. A discussion on their structures and fabrications are presented, along with their merits and demerits by comparison. Special attention is given to the application potentials of these mechanochromic materials in diverse domains, followed by detailed discussion of their mechanism. Last but not least, the current challenges as well as the trends of structural‐colored mechanochromic materials are proposed.

Mechanical approaches to dynamic, reversible mechanochromism based on thin film interference

Mechanochromic materials have received considerable attentions due to their potential applications in structural health monitoring, finger printing, stretchable electronics, and soft robotics. Herein we present a new strategy for polymeric mechanochromism by synergistic integration of mechanical engineering with thin film interference. This new strategy enables scalable and affordable manufacturing of mechanochromic polymeric systems that exhibit dynamically reversible mechanochromic properties with high sensitivity to smaller strains (0–60%) without the requirement of external ultraviolet light sources. We show that the mechanicanochromic properties can be tailored by using different mechanical approaches. In our kirigami approach via cutting, the coupling of buckling-induced kirigami and viewing angle-dependent interference color leads to unique anisotropic, load direction-dependent mechanochromism. In our tunable reflectivity shield approach via cracking, the polymer-metal bilayer film is used as a mechanically tunable shield to program its interference reflection color intensity and the visibility of the underlying dyed elastomeric substrate layer. The coupling of strain-dependent polymer-metal cracking and tunable reflection and transmission of the interference coloration film yields isotropic, load direction-independent mechanochromism.

Highly Stretchable Tough Elastomers Crosslinked by Spiropyran Mechanophores for Strain‐Induced Colorimetric Sensing

AbstractColorimetric spiropyran as a promising mechanophore has proven to be useful in visualizing stress/strain sensing and damage warning in polymeric materials. However, it remains challenging to prepare spiropyran‐containing mechanochromic materials with excellent mechanical properties. Herein, highly stretchable and tough double‐network elastomers crosslinked by spiropyran mechanophores are reported. The spiropyran‐crosslinked elastomer features both force and UV light‐induced color changes originating from the structural transformation from ring‐close spiropyran form to ring‐open merocyanine form of the mechanophores, and excellent mechanical properties (favorable elasticity of 480 kPa, large stretchability of 3400% strain, and high toughness of 8.4 kJ m−1) benefiting from the energy dissipation by breakage of sacrificial bonds. In particular, it is demonstrated that the multi‐mechanoresponsive and mechanically strong elastomer can be served as a conceptual sensor to quantify mechanical stretchability by a visible color change. This work shows promising potential for mechanochemical sensing applications.

Regulating Color Activation Energy of Mechanophore-Linked Multinetwork Elastomers

To lower color activation stress/strain while maintaining excellent mechanical properties for mechanochromic materials has been a challenge. Here, a structure of multinetwork polyacrylates was adop...

A dihedral-angle-controlled mechanochromic luminescent material: Application for pressure sensing

For most of the mechanochromic luminescent (MCL) materials, their mechanisms are based on the phase transition. When they are fabricated into devices or films, the MCL cannot be fully activated due to uncontrollable phase state of the materials. In this study, a new MCL material, (E)-6-chloro-N-(4-(dimethylamino)benzylidene)benzo[d]thiazol-2-amine (CDBTA), functioning based on dihedral-angle change, was reported. The initial state of CDBTA glows green emission (520 nm), while grinding changes its colour into yellow-orange (551 nm). The switching between two emission states can be stably switched with high fatigue resistance. The underlying MCL mechanism was verified by crystal analyses on two forms of single crystal, namely Green-form (G-form) and Yellow-form (Y-form), respectively. Molecules in G-form crystal adopt the twisted conformation by the rotation of the donor-acceptor (D-A) biplane. The dihedral angle is thus fixed by a variety of intermolecular interactions. Under mechanical stimuli, these interactions are destructed and the intramolecular torsional stress is released. Thus the molecules cannot maintain twisted conformation but form D-A coplane similar to the Y-form. Such dihedral-angle-controlled structural relaxation makes the π-electron highly delocalized to facilitate bathochromic-shift. The MCL stems from the changes of dihedral angle of the D-A biplane rather than that in packing mode or phase state, therefore CDBTA offers the possibility to fully activate the MCL performances when instrumented. Encouraged by this advantage, a CDBTA film sensor for pressure detection was fabricated, which exhibits a gradual red-shift with pressure and low detection limit down to 15.21 MPa.

Gradient Photonic Materials Based on One-Dimensional Polymer Photonic Crystals.

In nature, animals such as chameleons are well-known for the complex color patterns of their skin and the ability to adapt and change the color by manipulating sophisticated photonic crystal systems. Artificial gradient photonic materials are inspired by these color patterns. A concept for the preparation of such materials and their function as tunable mechanochromic materials is presented in this work. The system consists of a 1D polymer photonic crystal on a centimeter scale on top of an elastic poly(dimethylsiloxane) substrate with a gradient in stiffness. In the unstrained state, this system reveals a uniform red reflectance over the entire sample. Upon deformation, a gradient in local strain of the substrate is formed and transferred to the photonic crystal. Depending on the magnitude of this local strain, the thickness of the photonic crystal decreases continuously, resulting in a position-dependent blue shift of the reflectance peak and hence the color in a rainbow-like fashion. Using more sophisticated hard-soft-hard-soft-hard gradient elastomers enables the realization of stripe-like reflectance patterns. Thus, this approach allows for the tunable formation of reflectance gradients and complex reflectance patterns. Envisioned applications are in the field of mechanochromic sensors, telemedicine, smart materials, and metamaterials.

Precise molecular design for BN-modified polycyclic aromatic hydrocarbons toward mechanochromic materials

This paper reports on a novel molecular-level design strategy to achieve a novel MCL material base on boron and nitrogen aromatics. The introduction of boron nitrogen unit is the key to design the MCL smart materials.

Mechanochromic luminescent materials of bimetallic Cu(i) complexes showing thermally activated delayed fluorescence

Reversible mechanochromism is related to the breaking and restoring of hydrogen-bonding and luminescence color is tunableviathe substituent.

Color and shape reversible, recoverable and repeatable mechanochromic shape memory polycaprolactone: a single material with dual functions

A polycaprolactone-based mechanochromic shape memory material exhibits reversible and repeatable shape and color as a result of its crystallinity.

Tricolor fluorescence switching in a single component mechanochromic molecular material

Reversible tricolor switching in a mechanochromic α-dicyanodistyrylbenzene molecule has been developed. The tricolor switching (blue, orange, and green) is implemented by external stimuli of heat, solvent-vapor exposure, and mechanical force.

The abnormal solvatochromism, high-contrast mechanochromism and internal mechanism of two AIEE-active β-diketones

Tradition β-diketones are rarely used as fluorescent emitters and mechanochromic materials. Here, TPED2F and TPED2T with tiny structural differences (Furan unit and thiophene unit) and the same asymmetric β-diketone skeleton were designed and synthesized. Both of them show abnormal solvatochromism, water-sensitive fluorescence emission, marked aggregation-induced enhanced emission (AIEE) activity and high-contrast mechanochromism. Furthermore, crystal TPED2T displays inferior quantum efficiency (0.08) and hypsochromic-shift fluorescence emission (493 nm) in contrast with crystal TPED2F presenting 0.20 quantum efficiency and 516 nm emission maxima. However, ground and cured TPED2T exhibit deeper bathochromic-shift (54 nm and 59 nm in turn) than ground (37 nm) and cured (1 nm) TPED2F. In addition, ground TPED2F show reversible mechanochromism by heating, but mechanochromism of ground TPED2T is irreversible whether heating or solvent fuming. Based on 1H NMR, 13C NMR, UV–vis absorption spectra, emission spectra, XRD, DSC, single crystal analysis and theoretical calculation, the abnormal solvatochromism and water-sensitive fluorescence emission are attributed to enol-keto tautomerism and solvent polarity induced conformation changes of excited states, while heavy atom effect and wide band gap respectively lead to inferior quantum efficiency and hypsochromic-shift emission for crystal TPED2T compared with TPED2F. Moreover, excellent crystallization ability and close melting point before/after grinding endow TPED2F with thermal reversible mechanochromism. The amorphous state provided grinding and melting may contain distinct local energy traps, resulting in different fluorescence emission. Finally, existence forms of TPED2F and TPED2T depend on intramolecular hydrogen bonds, conjugate structure and small relative energy.

Bioinspired Healable Mechanochromic Function from Fluorescent Polyurethane Composite Film.

Camouflage and wound healing are two vital functions for cephalopods to survive from dangerous ocean risks. Inspired by these dual functions, herein, we report a new type of healable mechanochromic (HMC) material. The bifunctional HMC material consists of two tightly bonded layers. One layer is composed of polyvinyl alcohol (PVA) and titanium dioxide (TiO2) for shielding. Another layer contains supramolecular hydrogen bonding polymers and fluorochromes for healing. The as‐synthesized HMC material exhibits a tunable and reversible mechanochromic function due to the strain‐induced surface structure of composite film. The mechanochromic function can be further restored after damage because of the incorporated healable polyurethane. The healing efficiency of the damaged HMC materials can even reach 98 % at 60 °C for 6 h. The bioinspired HMC material is expected to have potential applications in the information encryption and flexible displays.

Elastic Photonic Microbeads as Building Blocks for Mechanochromic Materials

Colloidal assemblies have been used to make mechanochromic materials. As interparticle spacing of colloids in the elastomeric polymer matrix can be changed by mechanical strain, the structural colo...

Pyrene Functionalized Mechanochromic Elastomer with Switchable White Fluorescence

AbstractMechanochromic polymers that change their photophysical properties on activation by mechanical force are now widely studied, which can signal mechanical stress and visualize material damage ahead of structure failure. However, white‐light‐emitting mechanochromic materials are still very rare. Here, a pyrene group is introduced into a mechanochromic elastomer. A blue‐green fluorescence is exhibited in the original elastomer through rational control of the aggregation degree of pyrene groups. During continuous stretching, due to the “closed” form of rhodamine‐based mechanophore changing into “opened” form, a remarkable white fluorescence can be seen in this process (chromaticity coordinate: x = 0.31, y = 0.31). In the processes of stretching and relaxing, an obvious red shift in fluorescence is detected by the naked eye, which is useful to monitor transient stress in polymer materials. The present study provides a new guideline of the molecular structure design for white‐light emitting mechanochromic materials.

Design and Synthesis of a Well-Controlled Mechanoluminescent Polymer System Based on Fluorescence Resonance Energy Transfer with Spiropyran as a Force-Activated Acceptor and Nitrobenzoxadiazole as a Fluorescent Donor

Colorimetric mechanophores, which change color through force-induced covalent bond breakage, have been incorporated into polymer chains to create mechanochromic materials. However, the color change...

A turn-on mechanochromic luminescent material serving as pressure sensor and rewritable optical data storage

Mechanochromic luminescent (ML) materials can be roughly classified into two categories: turn-off type and turn-on type. Clearly the latter is more reliable because it has higher sensitivity and lower false-positive signals or background noises. Herein, a turn-on type ML material, (E)-4-((2-hydroxy-5-methylbenzylidene)amino)-3-methylbenzoic acid (MMA), is presented, which is designed by integrating appropriate electron donor-acceptor (D-A) moieties into a twisted conjugation core. MMA displays both aggregation-induced emission (AIE) and intramolecular charge transfer (ICT) properties. In addition, it glows weakly but exhibits notable emission enhancement under mechanical stimuli, and the two emission states can be repeatedly switched by grinding-fumigation treatment. Unlike the ML materials based on crystalline-amorphous phase transition, the ML of MMA is ascribed to the coupling and decoupling of the D-A units, regardless of the phase state. And this new working mechanism is confirmed by crystallographic data and DFT calculations. A pressure sensor based on MMA was fabricated, affording a low detection limit down to 21.6 Mpa. MMA further facilitates the fabrication of a ML recording medium, which allows for piezo-writing and fumigation-erasing of data, demonstrating a reliable prototype of optical data storage.

Mechanochromic luminescent materials with aggregation-induced emission: Mechanism study and application for pressure measuring and mechanical printing

In this work, we report the synthesis and emission behavior of two analogue compounds, 2-((E)-((9H-fluoren-2-yl)methylene)amino)-3-aminomaleonitrile (C1) and 2-amino-3-((E)-(4-(diphenylamino)benzylidene)amino)maleonitrile (C2). The two compounds exert both aggregation-induced emission (AIE) nature and intramolecular charge transfer (ICT) character. They show mechanochromic luminescence (MCL) in different ways: The C1 solid powder has strong green emission but would be quenched without color change after grinding. While C2 shows strong green emission in powder form but turns orange-red with a 30% loss in the quantum efficiency after grinding. The force-induced emission of both C1 and C2 can be changed back to the starting state by a simple immersion treatment in organic solvents, suggesting high fatigue resistance. Their MCL mechanisms were studied base on spectroscopic analysis combined with quantum chemical calculation. Powder x-ray diffraction (PXRD) diffractograms suggest a decrease in crystallinity rather than a full amorphization, excluding the phase transition mechanism that has been widely accepted for most of the MCL materials in prior reports. The photophysical process has been modeled according to the quantum chemical calculation using DFT-PBE0/6-311g (d,p), two stable isomers of C1 were detected, which exhibit structural relaxation with a conical intersection seam. The conical intersection of C1 can be accessed by mechanical stimuli but is unable to reach in the starting crystalline phase due to conformation rigidity. In contrast, such structural relaxation is less possible with C2 due to high emission in solution state. And the MCL mechanism mainly involves conformation planarizartion, which leads to bathochromic-shift with its emission. The new working mechanism with C1 is superior to other counterparts on account of its sensitivity and practicability. A new type of pressure sensor based on C1 film was developed, showing remarkable quenching effect with the pressure. A linear relation between pressure and emission intensity was obtained, offering a low detection limit down to 22.92 Mpa. This mechanochromic system succeeds in mechanical printing using MCL: Pressure up to 30 Mpa clearly imprints the cartoon patterns in detail with high contrast using the MCL materials, demonstrating a feasible prototype of printing technology.

Enabling Room-Temperature Mechanochromic Activation in a Glassy Polymer: Synthesis and Characterization of Spiropyran Polycarbonate.

Mechanochromic functionality realized through force-responsive molecules (i.e., mechanophores) has great potential for spatially localized damage warning in polymers. However, in structural plastics, for which damage warning is most critical, this approach has had minimal success because brittle failure typically precedes detectable color change. Herein, we report on the room-temperature mechanochromic activation of spiropyran in high Tg bisphenol A polycarbonate. The mechanochromic functionality was introduced by polymerization of dihydroxyspiropyran as a comonomer while retaining the excellent thermomechanical properties of the polycarbonate. The mechanochromic behavior is thoroughly evaluated in response to changes in stress, deformation, and time, providing new insights regarding how loading history controls stress accumulation in polymer chains. In addition, a new method to incorporate mechanochromic functionality in structures without dispersing costly mechanophores in the bulk is demonstrated by using a mechanochromic laminate. The room-temperature mechanochromic activation in a structural polymer combined with the new and efficient preparation and processing methods bring us closer to the application of mechanochromic smart materials.