Activation of a Copper Biscarbene Mechano-Catalyst Using Single-Molecule Force Spectroscopy Supported by Quantum Chemical Calculations

Single-molecule force spectroscopy allows investigation of the effect of mechanical force on individual bonds. By determining the forces necessary to sufficiently activate bonds to trigger dissociation, it is possible to predict the behavior of mechanophores. The force necessary to activate a copper biscarbene mechano-catalyst intended for self-healing materials was measured. By using a safety line bypassing the mechanophore, it was possible to pinpoint the dissociation of the investigated bond and determine rupture forces to range from 1.6 to 2.6 nN at room temperature in dimethyl sulfoxide. The average length-increase upon rupture of the Cu−C bond, due to the stretching of the safety line, agrees with quantum chemical calculations, but the values exhibit an unusual scattering. This scattering was assigned to the conformational flexibility of the mechanophore, which includes formation of a threaded structure and recoiling of the safety line.     

Molecular engineering of mechanophore activity for stress-responsive polymeric materials

Force reactive functional groups, or mechanophores, have emerged as the basis of a potential strategy for sensing and countering stress-induced material failure. The general utility of this strategy is limited, however, because the levels of mechanophore activation in the bulk are typically low and observed only under large, typically irreversible strains. Strategies that enhance activation are therefore quite useful. Molecular-level design principles by which to engineer enhanced mechanophore activity are reviewed, with an emphasis on quantitative structure–activity studies determined for a family of gem-dihalocyclopropane mechanophores.     

Comparison of the reactivity of isomeric 2H- and 3H-naphthopyran mechanophores

Naphthopyrans are molecular switches that produce highly colored merocyanine dyes upon photochemical or mechanochemical activation in polymers. The mechanochromic behavior of these molecular force probes enables the straightforward visualization of stress and/or strain in materials. To date, research on the mechanochemistry of naphthopyran has largely focused on the 3H-naphtho[2,1-b]pyran (3H) scaffold, whereas isomeric 2H-naphtho[1,2-b]pyrans (2H) exhibit complementary properties as suggested from their photochemical reactivity. Here we directly compare the reactivity of two isomeric 2H- and 3H-naphthopyran mechanophores in solution-phase ultrasonication experiments and in crosslinked polydimethylsiloxane elastomers subjected to uniaxial tensile deformation. The prototypical 3H-naphthopyran mechanophore produces a yellow merocyanine dye that reverts quickly, while the 2H-naphthopyran mechanophore generates a red merocyanine dye that reverts significantly slower. The trends in absorption and reversion measured in solution are also reflected in solid polymeric materials activated in tension. Building on recent research into substituent effects, this study identifies naphthopyran isomerism as a simple lever for modulating the mechanochromic properties of the naphthopyran mechanophore used in the development of force-responsive polymers.     

Mechanochemical Release of Fluorophores from a “Flex-activated” Mechanophore

Optical force probes that can release force-dependent and visualized signals with minimal changes in the polymer main chains under mechanical load are highly sought after but currently limited. In this study, we introduce a flex-activated mechanophore (FA) based on the Diels–Alder adduct of anthracene and dimethyl acetylenedicarboxylatea that exhibits turn-on mechanofluorescence. We demonstrate that when FA is incorporated into polymer networks or in its crystalline state, it can release fluorescent anthracenes through a retro-Diels–Alder mechanochemical reaction under compression or hydrostatic high pressure, respectively. The flex-activated mechanism of FA is successfully confirmed. Furthermore, we systematically modulate the force delivered to the mechanophore by varying the crosslinking density of the networks and the applied macroscopic pressures. This modulation leads to incremental increases in mechanophore activation, successive release of anthracenes, and quantitative enhancement of fluorescence intensity. The exceptional potential of FA as a sensitive force probe in different bulk states is highlighted, benefiting from its unique flex-activated mode with highly emissive fluorophore releasing. Overall, this report enriches our understanding of the structures and functions of flex-activated mechanophores and polymeric materials.     

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.

Structure–activity relationships for strategic substitution of the naphthopyran mechanophore scaffold enable polymeric materials with tunable mechanochromic behavior.     

A modular approach to mechanically gated photoswitching with color-tunable molecular force probes

Molecular force probes conveniently report on mechanical stress and/or strain in polymers through straightforward visual cues. Unlike conventional mechanochromic mechanophores, the mechanically gated photoswitching strategy decouples mechanochemical activation from the ultimate chromogenic response, enabling the mechanical history of a material to be recorded and read on-demand using light. Here we report a completely redesigned, highly modular mechanophore platform for mechanically gated photoswitching that offers a robust, accessible synthesis and late stage diversification through Pd-catalyzed cross-coupling reactions to precisely tune the photophysical properties of the masked diarylethene (DAE) photoswitch. Using solution-phase ultrasonication, the reactivity of a small library of functionally diverse mechanophores is demonstrated to be exceptionally selective, producing a chromogenic response under UV irradiation only after mechanochemical activation, revealing colored DAE isomers with absorption spectra that span the visible region of the electromagnetic spectrum. Notably, mechanically gated photoswitching is successfully translated to solid polymeric materials for the first time, demonstrating the potential of the masked diarylethene mechanophore for a variety of applications in force-responsive polymeric materials.

A highly modular and synthetically accessible mechanophore platform enables mechanically gated photoswitching in solution and in solid polymeric materials.     

Mechanophores with a Reversible Radical System and Freezing‐Induced Mechanochemistry in Polymer Solutions and Gels

Visualization and quantitative evaluation of covalent bond scission in polymeric materials are highly important for understanding failure, fatigue, and deterioration mechanisms and improving the lifetime, durability, toughness, and reliability of the materials. The diarylbibenzofuranone‐based mechanophore radical system enabled, through electron paramagnetic resonance spectroscopy, in situ quantitative evaluation of scission of the mechanophores and estimation of mechanical energy induced along polymer chains by external forces. The coagulation of polymer solutions by freezing probably generated force but did not cleave the mechanophores. On the other hand, cross‐linking led to efficient propagation of the force of more than 80 kJ mol^?1 to some mechanophores, resulting their cleavage and generation of colored stable radicals. This mechanoprobe concept has the potential to elucidate other debated issues in the polymer field as well.     

Mechanical Force Induces Ylide‐Free Cycloaddition of Nonscissible Aziridines

The application of aziridines as nonvulnerable mechanophores is reported. Upon exposure to a mechanical force, stereochemically pure nonactivated aziridines incorporated into the backbone of a macromolecule do not undergo cis–trans isomerization, thus suggesting retention of the ring structure under force. Nonetheless, aziridines react with a dipolarophile and seem not to obey conventional reaction pathways that involve C?C or C?N bond cleavage prior to the cycloaddition. Our work demonstrates that a nonvulnerable chemical structure can be a mechanophore.     

Mechanical Force Induces Ylide-Free Cycloaddition of Nonscissible Aziridines

The application of aziridines as nonvulnerable mechanophores is reported. Upon exposure to a mechanical force, stereochemically pure nonactivated aziridines incorporated into the backbone of a macromolecule do not undergo cis–trans isomerization, thus suggesting retention of the ring structure under force. Nonetheless, aziridines react with a dipolarophile and seem not to obey conventional reaction pathways that involve C−C or C−N bond cleavage prior to the cycloaddition. Our work demonstrates that a nonvulnerable chemical structure can be a mechanophore.     

Force-induced redistribution of a chemical equilibrium

Spiropyran (SP) mechanophores (mechanochemically reactive units) can impart the unique functionality of visual stress detection to polymers and have potential for use in smart materials with self-sensing capabilities. These color-generating mechanophores were incorporated into polyurethane via step growth polymerization. Polyurethane, which is inherently a versatile engineering polymer, possesses an optimized balance of mechanical toughness and elasticity to allow for investigation of the kinetics of the mechanochemical response of the SP mechanophore in the bulk polymer via fluorescence and absorbance measurements. The stress-induced 6-π electrocyclic ring-opening to the colored merocyanine (MC) form of the mechanophore was quantified by measuring the change in absorbance of the polymer, while it was held at constant strain. The closing kinetics of the mechanophore was also studied by fluorescence imaging. Finally, the effects of mechanical strain on the equilibrium between the SP and MC forms are reported and discussed.     

How accurately do mechanophores report on bond scission in soft polymer materials?

Mechanophores are molecules that are incorporated into a host material and react to the local mechanical condition—the state of stress or strain—of that host material. Among their many purposes is that of a reporter: Mechanophores whose optical activity changes in response to mechanical cues can reveal bulk material processes that are ordinarily hidden, such as fatigue and fracture. Moreover, they may do so well before a material is fully fractured. To extract quantitative information from the optical signals from embedded mechanophores it is important that the mechanophores, which are generally a minority component of the material, report proportionally and unambiguously on the mechanical condition of the bulk. This is particularly important for early reporting of damage and wear, for which the optical signal from the mechanophore should accurately reflect bulk bond scission. In this article, we develop and analyze a general theory for the quality of optical mechanoreporting by mechanophores in soft materials, based on the Bell-Evans theory of bond breaking. We find, that at the typical low fractions in which mechanophores are incorporated the overall change in strength is limited, but that the proportionality of the reporting can be off by significant amounts, particularly at short times after loading but, for non-scissile bonds, at long times as well.     

Pinpointing Mechanochemical Bond Rupture by Embedding the Mechanophore into a Macrocycle

Mechanophores contain a mechanically labile bond that can be broken by an external mechanical force. Quantitative measurement and control of the applied force is possible through atomic force microscopy (AFM). A macrocycle was synthesized that contains both the mechanophore and an aliphatic chain that acts as a “safety line” upon bond breaking. This ring‐opening mechanophore unit is linked to poly(ethylene glycol) spacers, which allow investigation by single molecule force spectroscopy. The length increase upon rupture of the mechanophore was measured and compared with quantum chemical calculations.     

Off-center Mechanophore Activation in Block Copolymers

Block copolymers (BCPs) are used in numerous applications in modern materials science. Yet, like homopolymers, BCPs can undergo covalent bond scission when mechanically stressed (mechanochemistry), which could lead to unexpected consequences in such applications. BCPs’ heterogeneity may affect force transduction, perhaps changing force distribution and localization. To verify this, a gem-dichlorocyclopropane (gDCC) embedded linear chain is prepared and extended with a poly(methyl methacrylate) block. When stressed in solution, the mechanochemical ring-opening of gDCC is accelerated compared to homopolymers, even though the mechanophores are at the chain ends. Moreover, a higher mechanophore activation selectivity is obtained. These results indicate that mechanochemical response outside, and even far from the chain center is quite prominent in BCPs, and that forces along the polymer chain can efficiently activate multi-mechanophores regions, even when far from the polymer midchain.     

Probing Force with Mechanobase‐Induced Chemiluminescence

Mechanophores capable of releasing N‐heterocyclic carbene (NHC), a strong base, are combined with triggerable chemiluminescent substrates to give a novel system for mechanically induced chemiluminescence. The mechanophores are palladium bis‐NHC complexes, centrally incorporated in poly(tetrahydrofuran) (pTHF). Chemiluminescence is induced from two substrates, adamantyl phenol dioxetane (APD) and a coumaranone derivative, upon sonication of dilute solutions of the polymer complex and either APD or the coumaranone. Control experiments with a low molecular weight Pd complex showed no significant activation and the molecular weight dependence of the coumaranone emission supports the mechanical origin of the activation. The development of this system is a first step towards mechanoluminescence at lower force thresholds and catalytic mechanoluminescence.     

Mechanochemically Triggered Topology Changes in Expanded Porphyrins

A hitherto unexplored class of molecules for molecular force probe applications are expanded porphyrins. This work proves that mechanical force is an effective stimulus to trigger the interconversion between Hückel and Möbius topologies in [28]hexaphyrin, making these expanded porphyrins suitable to act as conformational mechanophores operating at mild (sub-1 nN ) force conditions. A straightforward approach based on distance matrices is proposed for the selection of pulling scenarios that promote either the planar Hückel topology or the three lowest lying Möbius topologies. This approach is supported by quantum mechanochemical calculations. Force distribution analyses reveal that [28]hexaphyrin selectively allocates the external mechanical energy to molecular regions that trigger Hückel–Möbius interconversions, explaining why certain pulling scenarios favor the Hückel two-sided topology and others favor Möbius single-sided topologies. The meso-substitution pattern on [28]hexaphyrin determines whether the energy difference between the different topologies can be overcome by mechanical activation.     

Preparation of enediyne-crosslinked networks and their reactivity under thermal and mechanical conditions

The COGEF technique (COnstrained Geometries simulating External Force) was used to investigate the effects of macroscopic forces on cyclic enediynes, which can undergo Bergman Cyclization (BC). Because the forces needed to activate BC were found to be less than the forces needed for chain scission in polymer backbones, the calculations suggest that enediynes are potentially useful mechanophores. Three enediynes studied computationally were synthesized. The thermal BC reactions for these compounds were studied by DSC and found to be consistent with the predicted thermal sensitivity based on known substituent effects. However, upon incorporation of the enediynes into a polymer matrix as crosslinks, no definitive mechanical activation was observed, and conclusions about the stress-sensitivity of enediynes were unable to be drawn. Model studies suggest that insufficient force was applied to the crosslinks for mechanical activation to be observable by DSC.     

Porous Molecular Capsules as Non-Polymeric Transducers of Mechanical Forces to Mechanophores

Mechanical grinding/milling can be regarded as historically the first technology for changing the properties of matter. Mechanically activated molecular units (mechanophores) can be present in various structures: polymers, macromolecules, or small molecules. However, only polymers have been reported to effectively transduce energy to mechanophores, which induces breakage of covalent bonds. In this paper, a second possibility is presented—molecular capsules as stress-sensitive units. Mechanochemical encapsulation of fullerenes in cystine-based covalent capsules indicates that complexation takes place in the solid state, despite the fact that the capsules do not possess large enough entrance portals. By using a set of solvent-free MALDI (sf-MALDI) and solid-state NMR (ss-NMR) experiments, it has been proven that encapsulation proceeds during milling and in this process hydrazones and disulfides get activated for breakage, exchange, and re-forming. The capsules are porous and therefore prone to collapse under solvent-free conditions and their conformational rigidity promotes the collapse by the breaking of covalent bonds.     

Mechanical Reversibility of Strain‐Promoted Azide–Alkyne Cycloaddition Reactions

Mechanophores, that is, molecules that show a defined response to force, are crucial building blocks of mechanoresponsive materials. The possibility of mechanically induced cycloreversion for a series of triazoles formed via strain‐promoted azide–alkyne cycloaddition reactions was investigated by density functional theory calculations, and these triazoles were compared to the 1,4‐ and 1,5‐regioisomers formed in the reaction of an azide with a terminal alkyne. We show that cycloreversion is in principal possible and that the pulling geometry is the most important parameter that determines the probability of cycloreversion. We further compared triazole stability to the mechanical stability of polymers that are frequently used as force transducers in mechanochemical experiments and identified DIBAC (azadibenzylcyclooctyne) as a promising mechanophore for future applications.     

Rhodamine Mechanophore Functionalized Mechanochromic Double Network Hydrogels with High Sensitivity to Stress

Mechanochromic hydrogels, a new class of stimuli-responsive soft materials, have potential applications in a number of fields such as damage reporting and stress/strain sensing. We prepared a novel mechanochromic hydrogel using a strategy that has been developed to prepare dual-network(DN) hydrogels. A hydrophobic rhodamine derivative(Rh mechanophore) was covalently incorporated into a first network as a cross-linker. This first network embedded with Rh mechanophore within the DN hydrogel was pre-stretched. This guaranteed that the stress could be transferred extensively to the Rh-crosslinked first network once the hydrogel was under an applied force. Interestingly, we found that the threshold stress required to activate the mechanochromism of the hydrogel was less than 200 kPa, and much less than those in previous reports. Moreover, because of the excellent sensitivity of the hydrogel to stress, the DN hydrogel exhibited reversible freezing-induced mechanochromism. Benefiting from the sensitivity of Rh mechanophore to both p H and force, the DN hydrogel showed p H-regulated mechanochromic behavior. Our experimental results indicate that the preparation strategy we used introduces sensitive mechanochromism into the hydrogel and preserves the advantageous mechanical properties of the DN hydrogel. These results will be beneficial to the design and preparation of mechanochromic hydrogels with high stress sensitivity, and foster their practical applications in a number of fields such as damage reporting and stress/strain sensing.     

Derivation of continuous molecular weight distributions from the generating function

A method described in the statistical literature for the numerical inversion of Laplace Transforms of real functions has been adapted for the derivation of molecular weight distributions from calculations of the generating function for a polymeric system. It has been shown that a third-order refinement is sufficiently accurate for molecular weight distributors broader than 4.0. This allows the calculations to be carried out with a precision of 16 decimal digits which is commonly used in Fortran. Where higher precision is available, the treatment is applicable to narrower MWDs.