Understanding the Mechanochemistry of Ladder-Type Cyclobutane Mechanophores by Single Molecule Force Spectroscopy

Revealing Mechanochemical Force Distributions with Polymechanophore Block Copolymers

Polymer mechanochemistry is most studied under ultrasonication conditions, where the force distribution along a polymer chain is typically modeled as a parabola centered at the chain midpoint. How far the forces required for mechanoactivation extend toward the chain ends remains a question of interest. Investigating the mechanochemical reactivity of mechanophores at defined locations off chain center and toward termini would provide valuable information regarding this, but preparing such polymers to study the effects of mechanophore location and chain length on mechanoactivation has been synthetically cumbersome. Using an operationally simple procedure, we synthesized a series of block copolymers containing a block of mechanophores by living ring opening metathesis polymerization of a ladderene-type mechanophore and a norbornene, with good control over block positions and lengths. We found that for polymers of initial degrees of polymerization (DP) ≈ 1000, terminal mechanophore blocks activated less than more centrally located ones. However, simply by extending the length of the mechanochemically inert block, terminal mechanophore blocks of DP ≈ 200 achieved surprisingly high degrees of mechanoactivation comparable to those of centrally located ones or mechanophore homopolymers, after an induction period. These findings revealed the broad range of high force coverage along a polymer chain under sonication conditions and the possibility to still achieve high degrees of productive mechanochemistry far away from chain centers.

The Design, Synthesis, and Characterization of Photochromic and Mechanochromic Functional Fibers

Mechanically responsive polymer materials have garnered significant interest due to their unique ability to respond to external forces, leading to groundbreaking applications in visual stress mapping and damage detection. However, their use in fibers remains relatively unexplored. In this study, a mechanoresponsive polymer is synthesized by incorporating a spiropyran (SP) mechanophore into a polyurethane backbone. Based on this mechanoresponsive polyurethane, mechanochromic fibers are fabricated via solution spinning. The fibers exhibits a clear color change from colorless to blue when subjected to external tensile stress or ultraviolet irradiation, triggered by the activation of the SP mechanophore. Furthermore, the mechanical sensitivity of the fibers can be tailored by adjusting the orientation of the polymer chains through hot stretching. Fibers subjected to higher pre-stretching during post-processing displayed a mechanochromic response at lower strain levels. Various pre-stretched fibers are then made into a barcode, which exhibits distinct dynamic information via color change during stretching. The innovative application of function fibers, capable of storing information through mechanochromic response, offers a novel approach to the development of anti-counterfeit fibers.

Advancing the Mechanosensitivity of Atropisomeric Diarylethene Mechanophores through a Lever-Arm Effect

Understanding structure-mechanical activity relationships (SMARs) in polymer mechanochemistry is essential for the rational design of mechanophores with desired properties, yet SMARs in noncovalent mechanical transformations remain relatively underexplored. In this study, we designed a subset of diarylethene mechanophores based on a lever-arm hypothesis and systematically investigated their mechanical activity toward a noncovalent-yet-chemical conversion of atropisomer stereochemistry. Results from Density functional theory (DFT) calculations, single-molecule force spectroscopy (SMFS) measurements, and ultrasonication experiments collectively support the lever-arm hypothesis and confirm the exceptional sensitivity of chemo-mechanical coupling in these atropisomers. Notably, the transition force for the diarylethene M3 featuring extended 5-phenylbenzo[b]thiophene aryl groups is determined to be 131 pN ± 4 pN by SMFS. This value is lower than those typically recorded for other mechanically induced chemical processes, highlighting its exceptional sensitivity to low-magnitude forces. This work contributes a fundamental understanding of chemo-mechanical coupling in atropisomeric configurational mechanophores and paves the way for designing highly sensitive mechanochemical processes that could facilitate the study of nanoscale mechanical behaviors across scientific disciplines.

Outstanding enrichment of ladderane lipids in anammox bacteria: Overlooked effect of pH

Ladderane lipids synthesised by anammox bacteria hold significant potential for applications in jet fuel, drug delivery, and optoelectronics. Despite the widespread use of anammox bacteria in nitrogen removal from wastewater, the optimal conditions for maximising ladderane production remain unclear, limiting their broader application. To address this, we operated a fed-batch bioreactor with anammox bacteria, gradually adjusting the pH from 6.5 to 7.5 while regularly sampling for microbial community composition (Illumina sequencing), proteins, and ladderane lipids (UHPLC-HRMS). Our findings reveal that ladderane production positively correlates with rising pH increasing nearly fivefold as pH rose from 6.5 to 7.5, with a notable shift towards lipids containing two ladderane alkyl chains at higher pH. However, the conditions at an alkaline pH range also induced mild stress in anammox bacteria, as evidenced by our proteomic and microbial community data. Therefore, we propose maintaining a pH above 7.5 to enrich ladderane-rich anammox biomass but emphasise the need for gradual adaptation. This approach could optimise anammox installations for producing high-value ladderane lipids from wastewater.

Force-Assisted Orbital Crossing in Mechanochemical Oxirane Ring Opening

Polymer mechanochemistry induces chemical reactivity by applying a directed force, which can lead to unexpected reaction mechanisms. Strained cyclic molecules are often used in force-sensitive motifs because of the strong force coupling of ring-opening reactions. In this computational study, the force dependence of the ring-opening reactions of oxirane will be investigated. Density functional theory and multireference methods were used to investigate the electronic character of both symmetry-allowed and symmetry-forbidden reactions. In the latter case, an orbital crossing occurs during the reaction course, forcing the Woodward-Hoffmann-forbidden reaction to proceed via a diradical pathway. The performance of broken-symmetry density functional theory is evaluated and compares well to high-accuracy CASPT2, MRCI, and ic-MRCC computations. Due to the high ring strain, the barrier heights of both ring-opening reactions are steeply reduced by the application of an external force. Furthermore, the use of unsaturated linkers was shown to yield a significant reduction of the barrier heights, explaining previous experimental findings. Finally, we show through analysis of the PES topology how the external force transforms characteristic points such as saddle points and bifurcations, providing insights into force-dependent mechanism changes.

Fluorination Affects the Force Sensitivity and Nonequilibrium Dynamics of the Mechanochemical Unzipping of Ladderanes

When multiple reaction steps occur before thermal equilibration, kinetic energy from one reaction step can influence overall product distributions in ways that are not well predicted by transition state theory. An understanding of how the structural features of mechanophores, such as substitutions, affect reactivity, product distribution, and the extent of dynamic effects in the mechanochemical manifolds is necessary for designing chemical reactions and responsive materials. We synthesized two tetrafluorinated [4]-ladderanes with fluorination on different rungs and found that the fluorination pattern influenced the force sensitivity and stereochemical distribution of products in the mechanochemistry of these fluorinated ladderanes. The threshold forces for mechanochemical unzipping of ladderane were decreased by α-fluorination and increased by γ-fluorination; these changes correlated to the different stabilizing or destabilizing effects of fluorination patterns on the first transition state. Using ab initio steered molecular dynamics (AISMD), we compared the product distributions of synthesized and hypothetical ladderanes with different substitution patterns. These calculations suggest that fluorination on the first two bonds of ladderane gives rise to a larger fraction of dynamic trajectories and a larger fraction of E-alkene product through a mechanism resulting from larger momentum because of the greater atomic mass of fluorine. Fluorination on the third and fourth rungs instead gives a larger fraction of E-alkene product primarily due to electronic effects. These combined experimental and computational studies of the mechanochemical unzipping of fluorinated ladderanes provide an example of how relatively simple substituents can affect the extent of nonstatistical dynamics and, thus, mechanochemical outcomes.

The Restoring Force Triangle: A Mnemonic Device for Polymer Mechanochemistry

In polymer mechanochemistry, mechanophores are specific molecular units within the macromolecular backbone that are particularly sensitive to tension. To facilitate understanding of this selective responsiveness, we introduce the restoring force triangle (RFT). The RFT is a mnemonic device intended to provide intuitive insight into how external tensile forces (i.e., stretching) can selectively activate scissile bonds, thereby initiating mechanically driven chemical reactions. The RFT utilizes two easily computable parameters: the effective bond stiffness constant, which measures a bond's resistance to elongation, and the bond dissociation energy, which is the energy required to break a bond. These parameters help categorize reactivity into thermal and mechanical domains, providing a useful framework for developing new mechanophores that are responsive to force but thermally stable. The RFT helps chemists intuitively understand how tensile force contributes to the activation of a putative mechanophore, facilitating the development of mechanochemical reactions and mechano-responsive materials.

Strain learning in protein-based mechanical metamaterials

Mechanical deformation of polymer networks causes molecular-level motion and bond scission that ultimately lead to material failure. Mitigating this strain-induced loss in mechanical integrity is a significant challenge, especially in the development of active and shape-memory materials. We report the additive manufacturing of mechanical metamaterials made with a protein-based polymer that undergo a unique stiffening and strengthening behavior after shape recovery cycles. We utilize a bovine serum albumin-based polymer and show that cyclic tension and recovery experiments on the neat resin lead to a ~60% increase in the strength and stiffness of the material. This is attributed to the release of stored length in the protein mechanophores during plastic deformation that is preserved after the recovery cycle, thereby leading to a "strain learning" behavior. We perform compression experiments on three-dimensionally printed lattice metamaterials made from this protein-based polymer and find that, in certain lattices, the strain learning effect is not only preserved but amplified, causing up to a 2.5× increase in the stiffness of the recovered metamaterial. These protein-polymer strain learning metamaterials offer a unique platform for materials that can autonomously remodel after being deformed, mimicking the remodeling processes that occur in natural materials.

Force Transduction Through Distant Force-Bearing Regioisomeric Linkages Affects the Mechanochemical Reactivity of Cyclobutane

Fundamental understanding of mechanochemical reactivity is important for designing new mechanophores. Besides the core structure of mechanophores, substituents on a mechanophore can affect its mechanochemical reactivity through electronic stabilization of the intermediate or effectiveness of force transduction from the polymer backbone to the mechanophore. The latter factor represents a unique mechanical effect in considering polymer mechanochemistry. Here, we show that regioisomeric linkage that is not directly adjacent to the first cleaving bond in cyclobutane can still significantly affect the mechanochemical reactivity of the mechanophore. We synthesized three non-scissile 1,2-diphenyl cyclobutanes, varying their linkage to the polymer backbone via the o, m, or p-position of the diphenyl substituents. Even though the regioisomers share the same substituted cyclobutane core structure and similar electronic stabilization of the diradical intermediate from cleaving the first C-C bond, the p isomer exhibited significantly higher mechanochemical reactivity than the o and m isomers. The observed difference in reactivity can be rationalized as the much more effective force transduction to the scissile bond through the p-position than the other two substitution positions. These findings point to the importance of considering force-bearing linkages that are more distant from the bond to be cleaved when incorporating mechanophores into polymer backbones.

Understanding the Mechanochemistry of Mechano-Radicals in Self-Growth Materials by Single-Molecule Force Spectroscopy

Recent research on mechano-radicals has provided valuable insights into self-growth and adaptive responsive materials. Typically, mechanophores must remain inert in the absence of force but respond quickly to external tension before other linkages within the polymer network. Azo compounds exhibit promising combinations of mechanical stability and force-triggered reactivity, making them widely used as mechano-radicals in force-responsive materials. However, the activation conditions and behavior of azo compounds have yet to be quantitatively explored. In this study, we investigated the mechanical strength of three azo compounds using single-molecule force spectroscopy. Our results revealed that these compounds exhibit rupture forces ranging from ~500 to 1000 pN, at a loading rate of 3×104 pN s-1. Importantly, these mechanophores demonstrate distinct kinetic properties. Their unique mechanical attributes enable azo bond scission and free radical generation before causing major polymer backbone damage of entire material during polymer network deformation. This fundamental understanding of mechanophores holds significant promise for the development of self-growth materials and their related applications.

Synthesis and Reactivity of an X-Shaped Molecule: Reversible Formation and Cleavage of a Four-Membered Ring in Response to External Stimuli

We herein report the synthesis and reactivity of an X-shaped molecule featuring three four-membered rings (4MRs) arranged in a ladder configuration. This molecule exhibits a reversible opening and closure of the central 4MR upon exposure to light irradiation and thermal treatment. The central 4MR of this molecule is also cleaved via electrochemical and chemical reductions. The stimuli-responsiveness of the X-shaped molecule is attributed to the small energy gap difference between its open and closed states, stemming from the antiaromatic character of its precursor.

A Thermally Stable SO2-Releasing Mechanophore: Facile Activation, Single-Event Spectroscopy, and Molecular Dynamic Simulations

Polymers that release small molecules in response to mechanical force are promising candidates as next-generation on-demand delivery systems. Despite advancements in the development of mechanophores for releasing diverse payloads through careful molecular design, the availability of scaffolds capable of discharging biomedically significant cargos in substantial quantities remains scarce. In this report, we detail a nonscissile mechanophore built from an 8-thiabicyclo[3.2.1]octane 8,8-dioxide (TBO) motif that releases one equivalent of sulfur dioxide (SO2) from each repeat unit. The TBO mechanophore exhibits high thermal stability but is activated mechanochemically using solution ultrasonication in either organic solvent or aqueous media with up to 63% efficiency, equating to 206 molecules of SO2 released per 143.3 kDa chain. We quantified the mechanochemical reactivity of TBO by single-molecule force spectroscopy and resolved its single-event activation. The force-coupled rate constant for TBO opening reaches ∼9.0 s-1 at ∼1520 pN, and each reaction of a single TBO domain releases a stored length of ∼0.68 nm. We investigated the mechanism of TBO activation using ab initio steered molecular dynamic simulations and rationalized the observed stereoselectivity. These comprehensive studies of the TBO mechanophore provide a mechanically coupled mechanism of multi-SO2 release from one polymer chain, facilitating the translation of polymer mechanochemistry to potential biomedical applications.

Azobenzene as a photoswitchable mechanophore

Azobenzene has been widely explored as a photoresponsive element in materials science. Although some studies have investigated the force-induced isomerization of azobenzene, the effect of force on the rupture of azobenzene has not been explored. Here we show that the light-induced structural change of azobenzene can also alter its rupture forces, making it an ideal light-responsive mechanophore. Using single-molecule force spectroscopy and ultrasonication, we found that cis and trans para-azobenzene isomers possess contrasting mechanical properties. Dynamic force spectroscopy experiments and quantum-chemical calculations in which azobenzene regioisomers were pulled from different directions revealed that the distinct rupture forces of the two isomers are due to the pulling direction rather than the energetic difference between the two isomers. These mechanical features of azobenzene can be used to rationally control the macroscopic fracture behaviours of polymer networks by photoillumination. The use of light-induced conformational changes to alter the mechanical response of mechanophores provides an attractive way to engineer polymer networks of light-regulatable mechanical properties.

Mechanochemistry of Pterodactylane

Pterodactylane is a [4]-ladderane with substituents on the central rung. Comparing the mechanochemistry of the [4]-ladderane structure when pulled from the central rung versus the end rung revealed a striking difference in the threshold force of mechanoactivation: the threshold force is dramatically lowered from 1.9 nN when pulled on the end rung to 0.7 nN when pulled on the central rung. We investigated the bicyclic products formed from the mechanochemical activation of pterodactylane experimentally and computationally, which are distinct from the mechanochemical products of ladderanes being activated from the end rung. We compared the products of pterodactylane's mechanochemical and thermal activation to reveal differences and similarities in the mechanochemical and thermal pathways of pterodactylane transformation. Interestingly, we also discovered the presence of elementary steps that are accelerated or suppressed by force within the same mechanochemical reaction of pterodactylane, suggesting rich mechanochemical manifolds of multicyclic structures. We rationalized the greatly enhanced mechanochemical reactivity of the central rung of pterodactylane and discovered force-free ground state bond length to be a good low-cost predictor of the threshold force for cyclobutane-based mechanophores. These findings advance our understanding of mechanochemical reactivities and pathways, and they will guide future designs of mechanophores with low threshold forces to facilitate their applications in force-responsive materials.

Dual Ratiometric Fluorescence Monitoring of Mechanical Polymer Chain Stretching and Subsequent Strain-Induced Crystallization

Tracking the behavior of mechanochromic molecules provides valuable insights into force transmission and associated microstructural changes in soft materials under load. Herein, we report a dual ratiometric fluorescence (FL) analysis for monitoring both mechanical polymer chain stretching and strain-induced crystallization (SIC) of polymers. SIC has recently attracted renewed attention as an effective mechanism for improving the mechanical properties of polymers. A polyurethane (PU) film incorporating a trace of a dual-emissive flapping force probe (N-FLAP, 0.008 wt %) exhibited a blue-to-green FL spectral change in a low-stress region (<20 MPa), resulting from conformational planarization of the probe in mechanically stretched polymer chains. More importantly, at higher probe concentrations (∼0.65 wt %), the PU film showed a second spectral change from green to yellow during the SIC growth (20-65 MPa) due to self-absorption of scattered FL in a short wavelength region. The reversibility of these spectral changes was demonstrated by load-unload cycles. With these results in hand, the degrees of the polymer chain stretching and the SIC were quantitatively mapped and monitored by dual ratiometric imaging based on different FL ratios (I525/I470 and I525/I600). Simultaneous analysis of these two mappings revealed a spatiotemporal gap in the distribution of the polymer chain stretching and the SIC. The combinational use of the dual-emissive force probe and the ratiometric FL imaging is a universal approach for the development of soft matter physics.

Furan Release via Force-Promoted Retro-[4+2][3+2] Cycloaddition

Mechanophores (mechanosensitive molecules) have been instrumental in the development of various force-controlled release systems. However, the release of functional organic molecules is often the consequence of a secondary (nonmechanical) process triggered by an initial bond scission. Here we present a new mechanophore, built around an oxanorbornane-triazoline core, that is able to release a furan molecule following a force-promoted double retro-[4+2][3+2] cycloaddition. We explored this unprecedented transformation experimentally (sonication) and computationally (DFT, CoGEF) and found that the observed reactivity is controlled by the geometry of the adduct, as this reaction pathway is only accessible to the endo-exo-cis isomer. These results further demonstrate the unique reactivity of molecules under tension and offer a new mechanism for the force-controlled release of small molecules.

An Algorithm Predicting the Optimal Mechanical Response of Electronic Energy Difference

The use of mechanical forces at the molecular level has been shown to be an interesting tool for modulating different chemical and physical molecular properties. The so-called covalent mechanochemistry deals with the application of precise mechanical forces that induce specific changes in the structure, stability, reactivity, and other physical properties. The use of this kind of force to modulate photophysical properties and photochemical reactivity has also been studied. Nevertheless, the general problem of mechanical modulation of the energy gap between two electronic states has been addressed only with the development of simple theoretical models. Here, we develop and implement an algorithm providing the Largest energy Gap variation with Minimal mechanical Force (LGMF) that allows the determination of the optimal mechanical forces tuning the electronic energy gap, as well as to identify the maximum mechanical response of a molecular system to the application of any mechanical stimulus. The algorithm has been implemented for diverse molecular systems showing different degrees of flexibility. The phyton code of the algorithm is available in a public repository.

Acceleration of Diels-Alder reactions by mechanical distortion

Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics.

Swelling-Activated, Soft Mechanochemistry in Polymer Materials

Swelling in polymer materials is a ubiquitous phenomenon. At a molecular level, swelling is dictated by solvent-polymer interactions, and has been thoroughly studied both theoretically and experimentally. Favorable solvent-polymer interactions result in the solvation of polymer chains. For polymers in confined geometries, such as those that are tethered to surfaces, or for polymer networks, solvation can lead to swelling-induced tensions. These tensions act on polymer chains and can lead to stretching, bending, or deformation of the material both at the micro- and macroscopic scale. This Invited Feature Article sheds light on such swelling-induced mechanochemical phenomena in polymer materials across dimensions, and discusses approaches to visualize and characterize these effects.

Bond breaking of furan-maleimide adducts via a diradical sequential mechanism under an external mechanical force

Substituted furan-maleimide Diels-Alder adducts are bound by dynamic covalent bonds that make them particularly attractive mechanophores. Thermally activated [4 + 2] retro-Diels-Alder (DA) reactions predominantly proceed via a concerted mechanism in the ground electronic state. We show that an asymmetric mechanical force along the anchoring bonds in both the endo and exo isomers of proximal dimethyl furan-maleimide adducts favors a sequential pathway. The switching from a concerted to a sequential mechanism occurs at external forces of ≈1 nN. The first bond rupture occurs for a projection of the pulling force on the scissile bond at ≈4.3 nN for the exo adduct and ≈3.8 nN for the endo one. The reaction is inhibited for external forces up to ≈3.4 nN for the endo adduct and 3.6 nN for the exo one after which it is activated. In the activated region, at 4 nN, the rupture rate of the first bond for the endo adduct is computed to be ≈3 orders of magnitude larger than for the exo one in qualitative agreement with recent sonication experiments [Z. Wang and S. L. Craig, Chem. Commun., 2019, 55, 12263-12266]. In the intermediate region of the path between the rupture of the first and the second bond, the lowest singlet state exhibits a diradical character for both adducts and is close in energy to a diradical triplet state. The computed values of spin-orbit coupling along the path are too small for inducing intersystem crossings. These findings open the way for the rational design of DA mechanophores for polymer science and photochemistry.

Covalent Mechanochemistry and Contemporary Polymer Network Chemistry: A Marriage in the Making

Over the past 20 years, the field of polymer mechanochemistry has amassed a toolbox of mechanophores that translate mechanical energy into a variety of functional responses ranging from color change to small-molecule release. These productive chemical changes typically occur at the length scale of a few covalent bonds (Å) but require large energy inputs and strains on the micro-to-macro scale in order to achieve even low levels of mechanophore activation. The minimal activation hinders the translation of the available chemical responses into materials and device applications. The mechanophore activation challenge inspires core questions at yet another length scale of chemical control, namely: What are the molecular-scale features of a polymeric material that determine the extent of mechanophore activation? Further, how do we marry advances in the chemistry of polymer networks with the chemistry of mechanophores to create stress-responsive materials that are well suited for an intended application? In this Perspective, we speculate as to the potential match between covalent polymer mechanochemistry and recent advances in polymer network chemistry, specifically, topologically controlled networks and the hierarchical material responses enabled by multi-network architectures and mechanically interlocked polymers. Both fundamental and applied opportunities unique to the union of these two fields are discussed.

Computational Study of Mechanochemical Activation in Nanostructured Triblock Copolymers

Force-driven chemical reactions have emerged as an attractive platform for diverse applications in polymeric materials. However, the microscopic chain conformations and topologies necessary for efficiently transducing macroscopic forces to the molecular scale are not well-understood. In this work, we use a coarse-grained model to investigate the impact of network-like topologies on mechanochemical activation in self-assembled triblock copolymers. We find that mechanochemical activation during tensile deformation depends strongly on both the polymer composition and chain conformation in these materials. Activation primarily occurs in the tie chains connecting different glassy domains and in loop chains that are hooked onto each other by physical entanglements. Activation also requires a higher stress in materials having a higher glassy block content. Overall, the lamellar samples show the highest percent activation at high stress. In contrast, at low stress, the spherical morphology, which has the lowest glassy fraction, shows the highest activation. Additionally, we observe a spatial pattern of activation, which appears to be tied to distortion of the self-assembled morphology. Higher activation is observed in the tips of the chevrons formed during deformation of lamellar samples as well as in the centers between the cylinders in the cylindrical morphology. Our work shows that changes in the network-like topology in different morphologies significantly impact mechanochemical activation efficiencies in these materials, suggesting that this area will be a fruitful avenue for further experimental research.

Excited State Charge-Transfer Complexes Enable Fluorescence Color Changes in a Supramolecular Cyclophane Mechanophore

Mechanochromic mechanophores are reporter molecules that indicate mechanical events through changes of their photophysical properties. Supramolecular mechanophores in which the activation is based on the rearrangement of luminophores and/or quenchers without any covalent bond scission, remain less well investigated. Here, we report a cyclophane-based supramolecular mechanophore that contains a 1,6-bis(phenylethynyl)pyrene luminophore and a pyromellitic diimide quencher. In solution, the blue monomer emission of the luminophore is largely quenched and a faint reddish-orange emission originating from a charge-transfer (CT) complex is observed. A polyurethane elastomer containing the mechanophore displays orange emission in the absence of force, which is dominated by the CT-emission. Mechanical deformation causes a decrease of the CT-emission and an increase of blue monomer emission, due to the spatial separation between the luminophore and quencher. The ratio of the two emission intensities correlates with the applied stress.

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.

The Mechanochemical Synthesis and Activation of Carbon-Rich π-Conjugated Materials

Mechanochemistry uses mechanical force to break, form, and manipulate chemical bonds to achieve functional transformations and syntheses. Over the last years, many innovative applications of mechanochemistry have been developed. Specifically for the synthesis and activation of carbon-rich π-conjugated materials, mechanochemistry offers reaction pathways that either are inaccessible with other stimuli, such as light and heat, or improve reaction yields, energy consumption, and substrate scope. Therefore, this review summarizes the recent advances in this research field combining the viewpoints of polymer and trituration mechanochemistry. The highlighted mechanochemical transformations include π-conjugated materials as optical force probes, the force-induced release of small dye molecules, and the mechanochemical synthesis of polyacetylene, carbon allotropes, and other π-conjugated materials.

Mechano-Nanoarchitectonics: Design and Function

Mechanical stimuli have rather ambiguous and less-specific features among various physical stimuli, but most materials exhibit a certain level of responses upon mechanical inputs. Unexplored sciences remain in mechanical responding systems as one of the frontiers of materials science. Nanoarchitectonics approaches for mechanically responding materials are discussed as mechano-nanoarchitectonics in this review article. Recent approaches on molecular and materials systems with mechanical response capabilities are first exemplified with two viewpoints: i) mechanical control of supramolecular assemblies and materials and ii) mechanical control and evaluation of atom/molecular level structures. In the following sections, special attentions on interfacial environments for mechano-nanoarchitectonics are emphasized. The section entitled iii) Mechanical Control of Molecular System at Dynamic Interface describes coupling of macroscopic mechanical forces and molecular-level phenomena. Delicate mechanical forces can be applied to functional molecules embedded at the air-water interface where operation of molecular machines and tuning of molecular receptors upon macroscopic mechanical actions are discussed. Finally, the important role of the interfacial media are further extended to the control of living cells as described in the section entitled iv) Mechanical Control of Biosystems. Pioneering approaches on cell fate regulations at liquid-liquid interfaces are discussed in addition to well-known mechanobiology.

Single-Molecule Force Spectroscopy of a Tetraaryl Succinonitrile Mechanophore

Fluorescent damage reporters that use mechanochemical activation of a covalent bond to elicit an optical signal are emerging tools in material mechanics as a means to access the nanoscale distribution of forces inside materials under stress. A promising class of damage reporters are tetraaryl succinonitriles (TASN), whose mechanical activation results in stable fluorescent radical species. However, in-depth insights into the molecular mechanics of TASN activation are absent, precluding their use as quantitative mechanoprobes. Here we perform single-molecule force spectroscopy experiments to provide these insights. We use a bridged version of the TASN unit, embedded in multi-mechanophore polymer, to enable multiplexed mechanochemical measurements at the single-molecule level. Our experiments reveal that TASN activates at surprisingly low forces and short time scales compared to other covalent mechanophores. These results establish TASN as a promising candidate for reporting the lower end of relevant forces in material mechanics.

Bridging pico-to-nanonewtons with a ratiometric force probe for monitoring nanoscale polymer physics before damage

Understanding the transmission of nanoscale forces in the pico-to-nanonewton range is important in polymer physics. While physical approaches have limitations in analyzing the local force distribution in condensed environments, chemical analysis using force probes is promising. However, there are stringent requirements for probing the local forces generated before structural damage. The magnitude of those forces corresponds to the range below covalent bond scission (from 200 pN to several nN) and above thermal fluctuation (several pN). Here, we report a conformationally flexible dual-fluorescence force probe with a theoretically estimated threshold of approximately 100 pN. This probe enables ratiometric analysis of the distribution of local forces in a stretched polymer chain network. Without changing the intrinsic properties of the polymer, the force distribution was reversibly monitored in real time. Chemical control of the probe location demonstrated that the local stress concentration is twice as biased at crosslinkers than at main chains, particularly in a strain-hardening region. Due to the high sensitivity, the percentage of the stressed force probes was estimated to be more than 1000 times higher than the activation rate of a conventional mechanophore.

The Contributions of Model Studies for Fundamental Understanding of Polymer Mechanochemistry

The exciting field of polymer mechanochemistry has made great empirical progress in discovering reactions in which a stretching force accelerates scission of strained bonds using single molecule force spectroscopy and ultrasonication experiments. Understanding why these reactions happen, i.e., the fundamental physical processes that govern coupling of macroscopic motion to chemical reactions, as well as discovering other patterns of mechanochemical reactivity require complementary techniques, which permit a much more detailed characterization of reaction mechanisms and the distribution of force in reacting molecules than are achievable in SMFS or ultrasonication. A molecular force probe allows the specific pattern of molecular strain that is responsible for localized reactions in stretched polymers to be reproduced accurately in non-polymeric substrates using molecular design rather than atomistically intractable collective motions of millions of atoms comprising macroscopic motion. In this review, we highlight the necessary features of a useful molecular force probe and describe their realization in stiff stilbene macrocycles. We describe how studying these macrocycles using classical tools of physical organic chemistry has allowed detailed characterizations of mechanochemical reactivity, explain some of the most unexpected insights enabled by these probes, and speculate how they may guide the next stage of mechanochemistry.