Mechanochromic Polymers That Turn Green Upon the Dissociation of Diarylbibenzothiophenonyl: The Missing Piece toward Rainbow Mechanochromism

A Simply Synthesized Shaking-induced Small Molecule System with Repeatable and Instantaneous Discoloration Response

Force-related discoloration materials are highly valuable because of their characteristics of visualization, easy operation, and environment friendliness. Most force-related discoloration materials focus on polymers and depend on bond scission, which leads to insensitivity and unrecoverable. Small-molecule systems based on well-defined molecular structures and simple composition with high sensitivity would exhibit considerable mechanochromic potential. However, to date, researches about force-related discoloration materials based on small molecule solution remain limited and are rarely reported. In this study, we developed a repeatable and instantaneous discoloration small molecule solution system by simple one-pot synthesis method. It exhibited an instantaneous chromic change from yellowish to dark green under shaking and reverting back to yellow within 1 minute after removal of the shaking. Experimental results confirmed that the discoloration mechanism is attributed to the oscillation accelerating the production of unstable ortho-OH phenoxyl radical. The newly developed shaking-induced discoloration small molecule system (SDSMS) promises in field of mechanical force sensing and optical encryption.

Tandem SN2 Nucleophilic Substitution/Phospho-Dieckmann Reaction: One-Step Synthesis of 2-Phosphonyl-3-hydroxybenzo[b]thiophenes

A novel and efficient tandem SN2 nucleophilic substitution/Dieckmann condensation reaction of α-iodomethyl phosphine oxide with methyl thiosalicylate derivatives has been developed by using NaOH as a base, which enables the expeditious synthesis of 2-phosphonyl-3-hydroxybenzo[b]thiophene derivatives in moderate to high yields under simple conditions. This research provides not only a convenient method for the functionalization of benzo[b]thiophenes at the 2-position and 3-position but also new organophosphorus molecules. Furthermore, several new phosphonyl-substituted benzo[b]thiophenes were obtained from the resultant 2-phosphonyl-3-hydroxybenzo[b]thiophenes.

Naphthopyran molecular switches and their emergent mechanochemical reactivity

Naphthopyran molecular switches undergo a ring-opening reaction upon external stimulation to generate intensely colored merocyanine dyes. Their unique modularity and synthetic accessibility afford exceptional control over their properties and stimuli-responsive behavior. Commercial applications of naphthopyrans as photoswitches in photochromic ophthalmic lenses have spurred an extensive body of work exploring naphthopyran-merocyanine structure-property relationships. The recently discovered mechanochromic behavior of naphthopyrans has led to their emergent application in the field of polymer mechanochemistry, enabling advances in the design of force-responsive materials as well as fundamental insights into mechanochemical reactivity. The structure-property relationships established in the photochemical literature serve as a convenient blueprint for the design of naphthopyran molecular force probes with precisely tuned properties. On the other hand, the mechanochemical reactivity of naphthopyran diverges in many cases from the conventional photochemical pathways, resulting in unexpected properties and opportunities for deeper understanding and innovation in polymer mechanochemistry. Here, we highlight the features of the naphthopyran scaffold that render it a powerful platform for the design of mechanochromic materials and review recent advances in naphthopyran mechanochemistry.

Mechanochemically assisted morphing of shape shifting polymers

Morphing in creatures has inspired various synthetic polymer materials that are capable of shape shifting. The morphing of polymers generally relies on stimuli-active (typically heat and light active) units that fix the shape after a mechanical load-based shape programming. Herein, we report a strategy that uses a mechanochemically active 2,2'-bis(2-phenylindan-1,3-dione) (BPID) mechanophore as a switching unit for mechanochemical morphing. The mechanical load on the polymer triggers the dissociation of the BPID moiety into stable 2-phenylindan-1,3-dione (PID) radicals, whose subsequent spontaneous dimerization regenerates BPID and fixes the temporary shapes that can be effectively recovered to the permanent shapes by heating. A greater extent of BPID activation, through a higher BPID content or mechanical load, leads to higher mechanochemical shape fixity. By contrast, a relatively mechanochemically less active hexaarylbiimidazole (HABI) mechanophore shows a lower fixing efficiency when subjected to the same programing conditions. Another control system without a mechanophore shows a low fixing efficiency comparable to the HABI system. Additionally, the introduction of the BPID moiety also manifests remarkable mechanochromic behavior during the shape programing process, offering a visualizable indicator for the pre-evaluation of morphing efficiency. Unlike conventional mechanical mechanisms that simultaneously induce morphing, such as strain-induced plastic deformation or crystallization, our mechanochemical method allows for shape programming after the mechanical treatment. Our concept has potential for the design of mechanochemically programmable and mechanoresponsive shape shifting polymers.

A rational design strategy of radical-type mechanophores with thermal tolerance

Radical-type mechanophores (RMs) are attractive molecules that undergo homolytic scission of their central C-C bond to afford radical species upon exposure to heat or mechanical stimuli. However, the lack of a rational design concept limits the development of RMs with pre-determined properties. Herein, we report a rational design strategy of RMs with high thermal tolerance while maintaining mechanoresponsiveness. A combined experimental and theoretical analysis revealed that the high thermal tolerance of these RMs is related to the radical-stabilization energy (RSE) as well as the Hammett and modified Swain-Lupton constants at the para-position (σp). The trend of the RSE values is in good agreement with the experimentally evaluated thermal tolerance of a series of mechanoresponsive RMs based on the bisarylcyanoacetate motif. Furthermore, the singly occupied molecular orbital (SOMO) levels clearly exhibit a negative correlation with σp within a series of RMs that are based on the same skeleton, paving the way toward the development of RMs that can be handled under ambient conditions without peroxidation.

Multicolor Mechanochromic Epoxy Thermosets That Recognize the Intensity, Type, and Duration of Mechanical Stimulation

Mechanochromic polymers that exhibit multiple color changes under external mechanical stimulation show great potential for sensor applications. Herein, an epoxy thermoset that can reveal the intensity, type, and duration of mechanical stimulation via a combination of disulfide (DS) and rhodamine (Rh) mechanochromophores is reported. A unique multicolor transition occurs upon ball mill or manual grinding because of the different activation energies of DS and Rh. The epoxy changes color depending on the ball mill grinding duration. Simultaneous activation occurs with a mechanochromic time lag between DS and Rh, and the collision energy strongly affects the relative intensity. A more dramatic multicolor response is observed using a mortar and pestle, as sequential activation occurs upon gentle and strong grinding. Various types of mechanical stimulation can cause different aggregates of the activated Rh moiety and vary the relative mechanosensitivities of Rh and DS, which lead to a different color response.

Dipyrene-Terminated Oligosilanes Enable Ratiometric Fluorescence Response in Polymers toward Mechano- and Thermo-Stimuli

Developing fluorescent motifs capable of displaying mechano- and thermo-stimuli reversibly and ratiometrically is appealing for monitoring the deformation or temperature to which polymers are subjected. Here, a series of excimer-type chromophores Si_n-Py (n = 1-3) consisting of two pyrenes linked with oligosilanes of one to three silicon atoms is developed as the fluorescent motif incorporated in a polymer. The fluorescence of Si_n-Py is steered with the linker length where Si₂-Py and Si₃-Py with disilane and trisilane linkers display prominent excimer emission accompanied by pyrene monomer emission. Covalent incorporation of Si₂-Py and Si₃-Py in polyurethane gives fluorescent polymers PU-Si₂-Py and PU-Si₃-Py, respectively, where intramolecular pyrene excimers and corresponding combined emission of excimer and monomer are obtained. Polymer films of PU-Si₂-Py and PU-Si₃-Py display instant and reversible ratiometric fluorescence change during the uniaxial tensile test. The mechanochromic response arises from the reversible suppression of excimer formation during the mechanically induced separation of the pyrene moieties and relaxation. Furthermore, PU-Si₂-Py and PU-Si₃-Py show thermochromic response toward temperature, and the inflection point from the ratiometric emission as a function of temperature gives an indication of the glass transition temperature (T_g) of the polymers. The design of the excimer-based mechanophore with oligosilane provides a generally implementable way to develop mechano- and thermo-dual-responsive polymers.

Theoretical and Experimental Investigations of Stable Arylfluorene-Based Radical-Type Mechanophores

Radical-type mechanophores (RMs) can undergo homolytic cleavage of their central C-C bonds upon exposure to mechanical forces, which affords radical species. Understanding the characteristics of these radical species allows bespoke mechanoresponsive materials to be designed and developed. The thermal stability of the central C-C bonds and the oxygen tolerance of the generated radical species are crucial characteristics that determine the functions and applicability of such RM-containing mechanoresponsive materials. In this paper, we report the synthesis and characterization of two series of arylfluorene-based RM derivatives, that is, 9,9'-bis(5-methyl-2-pyridyl)-9,9'-bifluorene (BPyF) and 9,9'-bis(4,6-diphenyl-2-triazyl)-9,9'-bifluorene (BTAF). BPyF and BTAF derivatives were synthesized without generating any peroxides initially, albeit that BPyF slowly converted to the corresponding peroxide in solution. DFT calculations revealed the importance of the thermodynamic stability and the values of the α-SOMO levels of the corresponding radical species for their thermal stability and oxygen tolerance. Furthermore, the mechanochromism of BTAF was demonstrated by ball-milling a BTAF-centered polymer, which was synthesized by atom-transfer radical polymerization (ATRP).

Polymeric Emissive Materials Based on Dynamic Covalent Bonds

Dynamic covalent polymers, composed of dynamic covalent bonds (DCBs), have received increasing attention in the last decade due to their adaptive and reversible nature compared with common covalent linked polymers. Incorporating the DCBs into the polymeric material endows it with advanced performance including self-healing, shape memory property, and so forth. However, the emissive ability of such dynamic covalent polymeric materials has been rarely reviewed. Herein, this review has summarized DCBs-based emissive polymeric materials which are classified according to the different types of DCBs, including imine bond, acylhydrazone bond, boronic ester bond, dynamic C-C bond, as well as the reversible bonds based on Diels–Alder reaction and transesterification. The mechanism of chemical reactions and various stimuli-responsive behaviors of DCBs are introduced, followed by typical emissive polymers resulting from these DCBs. By taking advantage of the reversible nature of DCBs under chemical/physical stimuli, the constructed emissive polymeric materials show controllable and switchable emission. Finally, challenges and future trends in this field are briefly discussed in this review.

Determining Entanglement Molar Mass of Glassy Polyphenylenes Using Mechanochromic Molecular Springs

Molecular force transduction in tough and glassy poly(meta,meta,para-phenylene) (PmmpP) was investigated as a function of M_n using covalently incorporated mechanochromic donor-acceptor torsional springs based on an ortho-substituted diphenyldiketopyrrolopyrrole (oDPP). Blending oDPP-PmmpP probe chains with long PmmpP matrix chains allowed us to investigate molar-mass-dependent mechanochromic properties for a series of specimens having mechanically identical properties. In the strain-hardening regime, the mechanochromic response (Δλ_max,em) was found to be a linear function of the acting stress and fully reversible, making oDPP-PmmpP a real-time and quantitative stress sensor. For entangled and nonentangled probe chains, distinctly different values of Δλ_max,em were observed, yielding a critical molar mass of M_c ≈ 11 kg mol^-1 for PmmpP. Once physical cross-linking of oDPP in the network of PmmpP was ensured, Δλ_max,em was found to be independent of M_n. The resulting value of M_c is in very good agreement with results from rheology.

Mechanochromic elastomers with different thermo- and mechano-responsive radical-type mechanophores

To design tough soft materials, the introduction of sacrificial bonds into their skeleton is a useful method. The introduction of radical-type mechanophores (RMs), which generate coloured radicals in response to mechanical stimuli, as sacrificial bonds into the cross-linking points of elastomers is expected to be a powerful tool to elucidate the fracture mechanisms as well as the toughening of materials, given that the radicals generated from the RMs are coloured and can be quantitatively evaluated using electron paramagnetic resonance (EPR) measurements. In this study, to investigate the effect of the dynamic nature, i.e., the reactivity, of RMs introduced at the cross-linking points of polymer networks on their macroscopic mechanical properties, polymer networks cross-linked by two different RMs, a symmetric radical-type mechanophore (DFSN) and a non-symmetric radical-type mechanophore (CF/ABF), were synthesized and characterized. Compared to the polymer network cross-linked by DFSN, the network with CF/ABF exhibited higher thermal and mechanical responses, in other words much more sensitive to heat and mechanical force, resulting in better stress relaxation and energy-dissipation properties. These results demonstrate that the reactivity of the radical mechanophore at the cross-linking point is an important factor for designing polymer networks.

Mechanochromophore-linked Polymeric Materials with Visible Color Changes

Mechanical force as a type of stimuli for smart materials has obtained much attention in the past decade. Color-changing materials in response to mechanical stimuli have shown great potential in the applications such as sensors and displays. Mechanochromophore-linked polymeric materials, which are a growing sub-class of these materials, are discussed in detail in this review. Two main types of mechanochromophores which exhibit visible color change, summarized herein, involve either isomerization or radical generation mechanisms. This review focuses on their synthesis and incorporation into polymer matrices, the type of mechanical force used, factors affecting the mechanochromic properties, and their applications. This article is protected by copyright. All rights reserved.

Dynamic covalent polymer networks with mechanical and mechanoresponsive properties reinforced by strong hydrogen bonding

Dynamic polymer materials with superior mechanical properties and mechanochromism are of great importance to a vast variety of applications including stress sensing, damage detecting, soft robot. Herein, mechanoresponsive dynamic covalent...

Thermo- and Mechanochromic Camouflage and Self-Healing in Biomimetic Soft Actuators Based on Liquid Crystal Elastomers

In nature, many mysterious creatures capable of deformation camouflage, color camouflage, and self-healing have inspired scientists to develop various biomimetic soft robots. However, the systematic integration of all the above functionalities into a single soft actuator system still remains a challenge. Here we chemically introduce a multi-stimuli-responsive tetraarylsuccinonitrile (TASN) chromophore into a liquid crystal elastomer (LCE) network through a facile thiol-ene photoaddition method. The obtained TASN-LCE soft actuators not only exhibit reversible shape-morphing and reversible color-changing behavior in response to heat and mechanical compression, but also show excellent self-healing, reprogramming and recycling characteristics. We hope that such a TASN-LCE actuator system endowed with dynamic distortion, thermo- and mechano-chromic camouflage, and self-healing functionalities would pave the way for further development of multifunctional biomimetic soft robotic devices.

A mechanochromic donor-acceptor torsional spring

Mechanochromic polymers are intriguing materials that allow to sense force of specimens under load. Most mechanochromic systems rely on covalent bond scission and hence are two-state systems with optically distinct "on" and "off" states where correlating force with wavelength is usually not possible. Translating force of different magnitude with gradually different wavelength of absorption or emission would open up new possibilities to map and understand force distributions in polymeric materials. Here, we present a mechanochromic donor-acceptor (DA) torsional spring that undergoes force-induced planarization during uniaxial elongation leading to red-shifted absorption and emission spectra. The DA spring is based on ortho-substituted diketopyrrolopyrrole (o-DPP). Covalent incorporation of o-DPP into a rigid yet ductile polyphenylene matrix allows to transduce sufficiently large stress to the DA spring. The mechanically induced deflection from equilibrium geometry of the DA spring is theoretically predicted, in agreement with experiments, and is fully reversible upon stress release.

Visualization of the Necking Initiation and Propagation Processes during Uniaxial Tensile Deformation of Crystalline Polymer Films via the Generation of Fluorescent Radicals

To visualize and simultaneously quantify the necking behavior of crystalline polymer films during uniaxial stretching, tetraarylsuccinonitrile (TASN) moieties were introduced into polymers at the center of the main chain. TASN can produce a relatively stable radical that emits yellow fluorescence in response to mechanical stress. During the uniaxial elongation test of the TASN-centered crystalline polymers, the yellow fluorescence derived from the dissociated TASN radicals was used for microscale observations that showed the orientation of the polymer chains in the stretching direction. Furthermore, by comparing the radical generation in linear and star-shaped TASN-centered crystalline polymers during their tensile deformation, we found that the TASN dissociation ratio is higher in the star-shaped polymer, which has more chains connected to the lamellar crystal. Thus, the microforces generated in the amorphous region during uniaxial stretching were probed via the use of TASN, which allowed a direct visualization of the necking initiation and propagation processes as well as a quantification via electron paramagnetic resonance spectroscopy.

Non-symmetric mechanophores prepared from radical-type symmetric mechanophores: bespoke mechanofunctional polymers

A non-symmetric radical-type mechanophore (CF/ABF) was synthesized by molecular crossing between two radical-type mechanophores. The thermal stability and mechanoresponsiveness of CF/ABF were found to be tunable by altering the properties of the parent RMs. The CF/ABF-centred polymers showed mixed mechanochromism derived from the simultaneous generation of two radical species.

Mechanochemical tools for polymer materials

This review aims to provide a field guide for the implementation of mechanochemistry in synthetic polymers by summarizing the molecules, materials, and methods that have been developed in this field.

A Diarylacetonitrile as a Molecular Probe for the Detection of Polymeric Mechanoradicals in the Bulk State through a Radical Chain-Transfer Mechanism

Since the beginning of polymer science, understanding the influence of mechanical stress on polymer chains has been a fundamental and important research topic. The detection of mechanoradicals generated by homolytic cleavage of the polymer chains in solution has been studied in many cases. However, the detection of mechanoradicals in the bulk is still limited owing to their high reactivity. Herein, we propose an innovative strategy to detect mechanoradicals visually and quantitatively using a chain-transfer agent that generates relatively stable fluorescent radicals as a molecular probe. Mechanoradicals generated by ball milling of polystyrene samples were successfully detected by using a diarylacetonitrile compound as a fluorescent molecular probe through this radical chain-transfer mechanism. This probe enables the visualization and quantitative evaluation of mechanoradicals generated by polymer-chain scission.

Mechanochromic Polymers That Recognize the Duration of the Mechanical Stimulation via Multiple Mechanochromism

Mechanochromic polymers can be used as stress- and damage-detecting sensors in polymeric materials, given that mechanical stimuli can be visualized by color changes. Although many types of mechanochromic polymers have been reported so far, there are only few examples on their further functionalization based on multiple color changes (multicolor mechanochromism). Herein, preliminary results are reported on the use of multicolor mechanochromism to detect the duration of the mechanical stimulation by simply mixing white powders of two mechanochromic polystyrene samples that contain a different radical-type mechanochromophore at the midpoint of each polymer chain and thus exhibit different colors in response to mechanical stimuli. The mechanosensitivity can be tuned via the polymer length and shape, and a combination of these two types of mechanochromic polymers allows detecting the duration with multicolor mechanochromism, i.e., a color change from white to blue upon short exposure to grinding and a color change from white to gray upon longer exposure to grinding. Electron paramagnetic resonance and solid-state UV-vis measurements support the mechanism proposed for this multiple mechanochromism.

Visualization and Quantitative Evaluation of Toughening Polymer Networks by a Sacrificial Dynamic Cross-Linker with Mechanochromic Properties

A difluorenylsuccinonitrile-(DFSN)-based linker, whose central C-C bond is readily cleaved under mechanical stress to generate a relatively stable pink radical species, was introduced into polymer networks. DFSN-based cross-linked polymers exhibit improved mechanical properties as compared to those of the corresponding covalently cross-linked polymers owing to the energy dissipation induced by cleavage of the central DFSN bond. The toughening mechanism of DFSN-based elastomers is qualitatively visualized by the intensity of the pink color and can be quantitatively characterized by electron paramagnetic resonance. These results demonstrate that the extent of DFSN cleavage is the main factor improving the mechanical properties of the polymer networks.

Designing naphthopyran mechanophores with tunable mechanochromic behavior

Mechanochromic molecular force probes conveniently report on stress and strain in polymeric materials through straightforward visual cues. We capitalize on the versatility of the naphthopyran framework to design a series of mechanochromic mechanophores that exhibit highly tunable color and fading kinetics after mechanochemical activation. Structurally diverse naphthopyran crosslinkers are synthesized and covalently incorporated into silicone elastomers, where the mechanochemical ring-opening reactions are achieved under tension to generate the merocyanine dyes. Strategic structural modifications to the naphthopyran mechanophore scaffold produce dramatic differences in the color and thermal electrocyclization behavior of the corresponding merocyanine dyes. The color of the merocyanines varies from orange-yellow to purple upon the introduction of an electron donating pyrrolidine substituent, while the rate of thermal electrocyclization is controlled through electronic and steric factors, enabling access to derivatives that display both fast-fading and persistent coloration after mechanical activation and subsequent stress relaxation. In addition to identifying key structure-property relationships for tuning the behavior of the naphthopyran mechanophore, the modularity of the naphthopyran platform is demonstrated by leveraging blends of structurally distinct mechanophores to create materials with desirable multicolor mechanochromic and complex stimuli-responsive behavior, expanding the scope and accessibility of force-responsive materials for applications such as multimodal sensing.

Fluorescent supramolecular mechanophores based on charge-transfer interactions

Supramolecular mechanofluorophores based on charge-transfer interactions between fluorescent electron-rich pyrene and electron-deficient naphthalene diimide(s) are newly developed and show turn-on fluorescence upon application of mechanical forces.

Dinuclear metal complexes: multifunctional properties and applications

Dinuclear metal complexes have enabled breakthroughs in OLEDs, photocatalytic water splitting and CO₂reduction, DSPEC, chemosensors, biosensors, PDT and smart materials.

Synthesis of well-defined mechanochromic polymers based on a radical-type mechanochromophore by RAFT polymerization: living radical polymerization from a polymerization inhibitor

Synthesis of mechanochromic polymers based on a radical-type mechanochromophore by RAFT polymerization: living radical polymerization from a polymerization inhibitor.

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.

A reversibly mechanochromic conjugated polymer

A reversibly mechanochromic conjugated polymer has been developed. The polymer, polydiacetylene (PDA), obtained by thermal polymerization of a diphenyldisulfide-containing bisdiacetylene undergoes a blue-to-green color change upon grinding. The original blue color of the PDA is regenerated via a metastable red-color state by utilizing a thermochromic (heating-cooling) process.

Puncture-Resistant Hydrogel: Placing Molecular Complexes Along Phase Boundaries

Trendy advances in electric cars and wearable electronics triggered growing awareness in device lethality/survivability from accidents. A divergent design in protection calls for high stress resistance, large ductility, as well as efficient energy dissipation, all from the device itself, while keeping the weight-specific device performance to its premium. Unfortunately, the polymer electrolyte or the ductile elastomer lacks a mechanistic design to resist puncture or tear at a high stress level. Here, we designed molecular complexes along phase boundaries to mitigate the damages by placing these mechanically strong complexes along the phase boundaries or between two immiscible polymers. This puncture-resistant gel, dubbed as gel-nacre, is able to survive a few challenging incidents, including a 400 MPa puncture from a sharp nail, a 1 cm steel ball traveling at 540 km/h, and attempted rupture on stitched samples.

Multicolor Mechanochromism of a Polymer/Silica Composite with Dual Distinct Mechanophores

The development of a multicolor mechanochromic polymer/silica composite is achieved by using two distinct types of mechanochromophores. The multicolor mechanochromism of the composite containing diarylbibenzofuranone in silica-rich domains and naphthopyran in the polymer-rich domain is observed. The obtained composite shows blue, green, and orange colors according to the intensity of applied mechanical stimuli, solvent addition, and lapse of time. This unique multicolor mechanochromic behavior is evaluated by solid-state UV-vis absorption spectroscopy, ab initio steered molecular dynamics simulations, and computed minimum energy paths on force-modified potential energy surfaces. The unique mechanochromism is attributed to the difference in properties, activated colors, and domain locations between the two mechanochromophores.

Mechanochromism of a dumbbell D–π–A–π–D phenothiazine derivative

A dumbbell D–π–A–π–D phenothiazine derivative changes its fluorescence color from orange to red under force stimuli due to π-stacking conversion between H- and J-aggregates.

Introducing static cross-linking points into dynamic covalent polymer gels that display freezing-induced mechanofluorescence: enhanced force transmission efficiency and stability

Freezing polymer gels that are cross-linked by tetraarylsuccinonitrile (TASN) moieties, which can generate pink and fluorescent yellow radicals in response to mechanical stress, induces mechanofluorescence from the dynamic dissociation of the TASN groups.