Molecular Damage Detection in an Elastomer Nanocomposite with a Coumarin Dimer Mechanophore

A Dual-Pathway Responsive Mechanophore for Intelligent Luminescent Polymer Materials

Mechanoluminescent units, when integrated into polymer matrices, undergo structural transformations in response to mechanical force, resulting in changes in fluorescence. This phenomenon holds considerable promise for the development of stress-sensing materials. Despite the high demand for robust, tunable mechanoluminescent mechanophores for force assessment and smart force-responsive materials, strategies for their design and synthesis remain underdeveloped. In an attempt to address this challenge, we have introduced a novel dual-pathway responsive mechanophore, bis(2-(2-(tert-butyldimethylsilanyloxy)benzylidene)amino)aryl disulfides (SSTBS), which, when incorporated into polymer chains, exhibits fluorescence upon the combined application of force and chemical stimulus, irrespective of their sequence. This property is facilitated by the disulfide bond's sensitivity to mechanical force and the fluoride anion-induced desilylation and deprotonation. Notably, the force-responsive threshold of the SSTBS mechanophore can be finely tuned by TBAF treatment, as supported by both experimental and computational studies, providing a simple, yet effective means, to regulate polymer force responsiveness on demand. We believe that the strategy developed in this investigation will shed light on the design of mechanophores for the fabrication of intelligent luminescent polymer materials and advance the development of smart force-reporting systems.

Stereochemical Impacts on Acyclic Mechanochemical Carbon-Carbon Bond Activation

The influence of stereochemistry on the mechanochemistry rate is studied using a new mechanophore based on a benzopinacol (BP) skeleton. Two sets of BP diastereomers, the meso R,S and the R,R/S,S were isolated, incorporated into the center of a poly(methyl acrylate), and their mechanical activation rate was measured in solution. Under mechanical stress, the central C-C bond in BP is cleaved, providing two independent benzophenone molecules with higher UV-absorption coefficient at 254 nm. Monitoring the reaction rate spectroscopically indicates that the chiral R,R/S,S enantiomers react ~1.4 fold faster compared to the meso R,S diastereomer. In-silico modeling indicates that a hydrogen bond between the syn hydroxyls in the R,R diastereomer becomes shorter with stress, reducing the maximal force required for C-C bond scission, while this bond is inexistent in the meso diastereomer, as the hydroxyl are anti to each other. Our results indicate that in polymer where free rotation around bonds is possible, non-covalent interactions between backbone substituents, which are affected by relative stereochemistry, can play a fundamental role in the mechanochemical stability of the polymer.

Constructing Strong and Tough Polymer Elastomers via Photoreversible Coumarin Dimer Mechanophores

Advanced elastomers with outstanding strength, toughness, and reusability hold significant potential for diverse applications. Using photochemistry and mechanochemistry to develop such materials has become a very effective strategy. Here, we report that photoreversible coumarin-based mechanophores that can make force-/light-triggered cycloreversion are chemically incorporated into polyurethane elastomers to simultaneously enhance their strength and toughness. Coumarin dimer mechanophore cross-linkers are formed in the polyurethane elastomer after exposure to 365 nm irradiation, leading to networks with dramatically enhanced mechanical properties. The tensile strength of the HNA-PU2000 elastomer with coumarin dimer mechanophore cross-linkers could increase from 13.9 to 26.9 MPa, elongation at break from 726 to 1053%, and toughness from 25.6 to 119.7 MJ·m-3, in contrast to its counterpart HNA-PU2000 elastomer without coumarin dimer mechanophores. Additionally, the polyurethane elastomer exhibits a decent reusable capability via force-/light-triggered cycloreversion of coumarin dimer mechanophores under mechanical stress or 254 nm irradiation. This mechano-/photochemical strategy to improve strength, toughness, and reusable capability of elastomers provides a facile approach for the development of high-performance elastomer materials.

Polymer Mechanochemistry in Confined Spaces

With the development of mechanophores, polymer mechanochemistry has emerged as a powerful tool for creating force-responsive materials with a variety of desired functions, ranging from color change to molecular release. However, it remains challenging to improve the efficiency of mechanochemical activation, especially for mechanophores embedded within polymer networks, which has profound implications for translating mechanochemical responses into materials-centered applications. The physical and chemical conditions under spatial confinement differ significantly from those in the surrounding bulk environment, offering opportunities to facilitate mechanochemical activation. In this Minireview, we discuss and summarize recent progress in polymer mechanochemistry within confined spaces including surfaces/interfaces, polymer assemblies, and other nanostructures, specifically focusing on the effects of spatial confinement on the enhancement of mechanophore activation. We envision that combining confinement effects with advances in molecular and materials engineering will further improve the activation efficiency, capitalizing more fully on the potential of mechanophores toward practical applications.

A New Class of Mechano-Responsive Polyurethane Via Anthracene -TAD Diels-Alder (DA) Click Chemistry

Smart or stimuli-responsive polymers have garnered significant interest in the scientific community due to their response to different stimuli like pH, temperature, light, mechanical force, etc. Mechanophoric polymer is an intriguing class of smart polymers that respond to external mechanical force by producing fluorescent moieties and can be utilized for damage detection and stress-sensing assessment. In recent reports on mechanophoric polymers, different mechanophoric motifs such as spiropyran, rhodamine, coumarin, etc. are explored. This investigation reports a new kind of mechanophoric polyurethane (PU) adduct based on Diels-Alder (DA) click chemistry. Here, an anthracene(An)-end capped tri-armed urethane system is synthesized, followed by a DA reaction using bis-(1,2,4-triazoline-3,5-dione) (bis-TAD) derivative. The incorporation of bis-TAD in the urethane system renders the anthracene inactive ("turn-off") by dismantling its conjugation as a result of a successful DA reaction. The soft PU translated into a harder material through bis-TAD linkages between polymer chains as evident from nanoindentation (NINT) analysis. The resulting material reverts back to its fluorescent "turned-on" mode owing to a force-accelerated retro-Diels-Alder (r-DA) reaction. Besides the mechanophoric attributes, the material demonstrates self-healing behavior examined by microscopic investigations. This innovative approach can be a potential route to design responsive polymers with dynamic functionalities for advanced material applications.

Isocyanide-based multicomponent reactions for the synthesis of benzopyran derivatives with biological scaffolds

Benzopyrans (BZPs) are among the most privileged and influential small O-heterocycles that form the core of many natural compounds, commercial drugs, biological compositions, agrochemicals, and functional materials. BZPs are divided into six general categories including coumarins, chromans, 2H-chromenes, 4H-chromenes, chromones, and 4-chromanones, each of which is abundant in many plants and foods. These oxygenated heterocyclic compounds are fascinating motifs and have extensive applications in biology and materials science. Hence, numerous efforts have been made to develop innovative approaches for their extraction and synthesis. However, most of them are step-by-step or multi-step strategies that suffer from waste material generation and a tedious extraction process. Isocyanide-based multicomponent reactions (I-MCRs) offer a highly efficient method for overcoming these problems. The I-MCR is a simple and environmentally friendly one-pot domino procedure that does not require intermediate isolation or workup and is generally more efficient in material usage. This review covers all research articles related to I-MCRs for synthesizing BZP derivatives from the beginning to the middle of the year 2023. This strategy will be useful for organic and pharmaceutical chemists to design new drugs and optimize the synthesis steps of biological compounds and commercial drugs with benzopyran cores.

Coumarin Dimer Is an Effective Photomechanochemical AND Gate for Small-Molecule Release

Stimulus-responsive gating of chemical reactions is of considerable practical and conceptual interest. For example, photocleavable protective groups and gating mechanophores allow the kinetics of purely thermally activated reactions to be controlled optically or by mechanical load by inducing the release of small-molecule reactants. Such release only in response to a sequential application of both stimuli (photomechanochemical gating) has not been demonstrated despite its unique expected benefits. Here, we describe computational and experimental evidence that coumarin dimers are highly promising moieties for realizing photomechanochemical control of small-molecule release. Such dimers are transparent and photochemically inert at wavelengths >300 nm but can be made to dissociate rapidly under tensile force. The resulting coumarins are mechanochemically and thermally stable, but rapidly release their payload upon irradiation. Our DFT calculations reveal that both strain-free and mechanochemical kinetics of dimer dissociation are highly tunable over an unusually broad range of rates by simple substitution. In head-to-head dimers, the phenyl groups act as molecular levers to allow systematic and predictable variation in the force sensitivity of the dissociation barriers by choice of the pulling axis. As a proof-of-concept, we synthesized and characterized the reactivity of one such dimer for photomechanochemically controlled release of aniline and its application for controlling bulk gelation.

Mechanochromic Polymers Based on Mechanophores

In recent years, diverse stimuli-responsive allochroic materials have rapidly been developed, and smart materials with mechanochromic properties in particular have received increasing attention. This is because force fields have the advantage of being large and controllable compared to other stimulation modalities. Mechanochromic polymers mainly convert mechanical force signals into optical signals, which makes them suitable for applications in bionic actuators, encryption, and signal sensing. In this review, we summarize recent research progress in the design and development of mechanochromic polymers that are classified into two categories. The first category comprises those based on mechanophores that are physically dispersed in polymer matrices in the form of supramolecular aggregates. The second category comprises those based on mechanophores that are covalently linked to polymer networks. We focus on the working mechanisms of the mechanophores and their potential applications, which include damage monitoring and signal sensing.

Effect of Polymer Composition and Morphology on Mechanochemical Activation in Nanostructured Triblock Copolymers

The effect of composition and morphology on mechanochemical activation in nanostructured block copolymers was investigated in a series of poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate) (PMMA-b-PnBA-b-PMMA) triblock copolymers containing a force-responsive spiropyran unit in the center of the rubbery PnBA midblock. Triblock copolymers with identical PnBA midblocks and varying lengths of PMMA end-blocks were synthesized from a spiropyran-containing macroinitiatior via atom transfer radical polymerization, yielding polymers with volume fractions of PMMA ranging from 0.21 to 0.50. Characterization by transmission electron microscopy revealed that the polymers self-assembled into spherical and cylindrical nanostructures. Simultaneous tensile tests and optical measurements revealed that mechanochemical activation is strongly correlated to the chemical composition and morphologies of the triblock copolymers. As the glassy (PMMA) block content is increased, the overall activation increases, and the onset of activation occurs at lower strain but higher stress, which agrees with predictions from our previous computational work. These results suggest that the self-assembly of nanostructured morphologies can play an important role in controlling mechanochemical activation in polymeric materials and provide insights into how polymer composition and morphology impact molecular-scale force distributions.

Phosphorescent extensophores expose elastic nonuniformity in polymer networks

Networks and gels are soft elastic solids of tremendous technological importance that consist of cross-linked polymers whose structure and connectivity at the molecular level are fundamentally nonuniform. Pre-failure local mechanical responses are not understood at the level of individual crosslinks, despite the enormous attention given to their macroscopic mechanical responses and to developing optical probes to detect their loci of mechanical failure. Here, introducing the extensophore concept to measure nondestructive forces using an optical probe with continuous force readout proportional to deformation, we show that the crosslinks in an elastic polymer network extend, fluctuate, and deform with a wide range of molecular individuality. Requiring little specialized equipment, this foundational single-molecule phosphorescence approach, applied here to polymer science and engineering, can be useful to a broad science and engineering community.

Ultrasound triggered organic mechanoluminescence materials

Ultrasound induced organic mechanoluminescence materials have become one of the focal topics in wireless light sources since they exhibit high spatiotemporal resolution, biocompatibility and excellent tissue penetration depth. These properties promote great potential in ultrahigh sensitive bioimaging with no background noise and noninvasive nanodevices. Recent advances in chemistry, nanotechnology and biomedical research are revolutionizing ultrasound induced organic mechanoluminescence. Herein, we try to summarize some recent researches in ultrasound induced mechanoluminescence that use various materials design strategies based on the molecular conformational changes and cycloreversion reaction. Practical applications, like noninvasive bioimaging and noninvasive optogenetics, are also presented and prospected.

Concurrent and Mechanochemical Activation of Two Distinct and Latent Fluorophores via Retro-Diels-Alder Reaction of an Anthracene-Aminomaleimide Adduct

Generally, a typical mechanochromophore produces color change through chemical transformation into one or two identical new chromophores/fluorophores under applied mechanical force. Herein, we introduce a novel mechanophore based on an anthracene-aminomaleimide Diels-Alder (DA) adduct featuring two distinct and latent fluorophores. This nonfluorescent mechanophore undergoes retro-DA reaction upon mechanochemical activation in solution and the solid state, generating the respective anthracene and aminomaleimide fragments simultaneously, both of which are highly emissive with different fluorescent colors. In addition, the aminomaleimide fluorophore exhibits sensitive fluorescence on-off response to protic solvents or polar solvents, which enables dual-color mechanochromism from this single mechanophore.

Ultrasound controlled mechanophore activation in hydrogels for cancer therapy

Mechanophores are molecular motifs that respond to mechanical perturbance with targeted chemical reactions toward desirable changes in material properties. A large variety of mechanophores have been investigated, with applications focusing on functional materials, such as strain/stress sensors, nanolithography, and self-healing polymers, among others. The responses of engineered mechanophores, such as light emittance, change in fluorescence, and generation of free radicals (FRs), have potential for bioimaging and therapy. However, the biomedical applications of mechanophores are not well explored. Herein, we report an in vitro demonstration of an FR-generating mechanophore embedded in biocompatible hydrogels for noninvasive cancer therapy. Controlled by high-intensity focused ultrasound (HIFU), a clinically proven therapeutic technique, mechanophores were activated with spatiotemporal precision to generate FRs that converted to reactive oxygen species (ROS) to effectively kill tumor cells. The mechanophore hydrogels exhibited no cytotoxicity under physiological conditions. Upon activation with HIFU sonication, the therapeutic efficacies in killing in vitro murine melanoma and breast cancer tumor cells were comparable with lethal doses of H2O2 This process demonstrated the potential for mechanophore-integrated HIFU combination as a noninvasive cancer treatment platform, named "mechanochemical dynamic therapy" (MDT). MDT has two distinct advantages over other noninvasive cancer treatments, such as photodynamic therapy (PDT) and sonodynamic therapy (SDT). 1) MDT is ultrasound based, with larger penetration depth than PDT. 2) MDT does not rely on sonosensitizers or the acoustic cavitation effect, both of which are necessary for SDT. Taking advantage of the strengths of mechanophores and HIFU, MDT can provide noninvasive treatments for diverse cancer types.

A Simple Mechanochromic Mechanophore Based on Aminothiomaleimide

Mechanochromic mechanophores have promising applications in stress sensing and damage detection. Here we report a simple mechanofluorochromic mechanophore based on aminothiomaleimide (ATM). Poly(methyl acrylate) containing this mechanophore (ATM-PMA) was synthesized by atom transfer radical polymerization (ATRP) using an ATM-derived difunctional initiator. To investigate its mechanofluorochromism, the solution of ATM-PMA was subjected to ultrasonication, and size exclusion chromatography (SEC) and fluorescence spectroscopy were employed to monitor the changes in molecular weight and fluorescence emission. The results showed that the molecular weight of ATM-PMA decreased upon ultrasonication, accompanied by a shift of fluorescence emission from bright yellow to light blue. This mechanophore of a simple functional group of ATM has great potential to be used in mechanochromic polymer materials.

Self-Strengthening of Cross-Linked Elastomers via the Use of Dynamic Covalent Macrocyclic Mechanophores

The creation of polymeric materials that self-strengthen in response to a mechanical force is an important objective in the field of polymer chemistry. Here, the mechanochemical strengthening of cross-linked elastomers using macrocyclic mechanophores that contain a dynamic covalent disulfide bond is reported. Cross-linked poly(hexyl methacrylate) (CPHMA) polymers with macrocyclic mechanophores inserted at the cross-linking points were synthesized via free radical polymerization. Tensile and swelling tests showed that the addition of the macrocyclic mechanophores to the CPHMA polymers successfully impart them with self-strengthening functionality following compression, without the need for any additives such as monomers and modifiers, or any other stimuli.

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.