Ratiometric Flapping Force Probe That Works in Polymer Gels

A mitochondrial targeted dual ratiometric near-infrared fluorescent probe based on ICT effect for the detection of SO2 derivative and its bioimaging

SO2 derivatives play an important role in many metabolic processes, excessive ingestion of them can lead to serious complications of various diseases. In this work, a novel dual ratiometric NIR fluorescent probe XT-CHO based on ICT effect was synthesized for detecting SO2 derivative. In the design of the probe, the α, β-unsaturated bond formed between benzopyran and coumarin was used as the reaction site for SO2, meanwhile, the extended π-conjugate system promoted maximum emission wavelength of the probe up to 708 nm. Notably, the probe exhibited high selectivity and sensitivity for detecting SO2, the limit of detection reached 2.13 nM and 58.5 nM in fluorescence spectra and UV-Vis absorption spectra, respectively. The reaction mechanism of SO2 and XT-CHO had been verified by 1H NMR, ESI-MS spectra and DFT calculation. Moreover, the probe was successfully applied in detecting endogenous and exogenous SO2 in living cells and proved possessed the mitochondrial targeted ability.

Advancing Dynamic Polymer Mechanochemistry through Synergetic Conformational Gearing

Harnessing mechanical force to modulate material properties and enhance biomechanical functions is essential for advancing smart materials and bioengineering. Polymer mechanochemistry provides an emerging toolkit for exploring unconventional chemical transformations and modulating molecular structures through mechanical force. One of the key challenges is developing innovative force-sensing mechanisms for precise and in situ force detection. This study introduces mDPAC, a dynamic and sensitive mechanophore, demonstrating its mechanochromic properties through synergetic conformational gearing. Its unique mechanoresponsive mechanism is based on the simultaneous conformational synergy between its phenazine and phenyl moieties, facilitated by a worm-gear-like structure. We confirm mDPAC's complex mechanochemical response and elucidate its mechanotransduction mechanism through our experimental data and comprehensive simulations. The compatibility of mDPAC with hydrogels is particularly notable, highlighting its potential for applications in aqueous biological environments as a dynamic force sensor. Moreover, mDPAC's multicolored mechanochromic responses facilitate direct force sensing and visual detection, paving the way for precise and real-time mechanical force sensing in bulk materials.

High-Sensitive Spatiotemporal Distribution Imaging of Compression Stresses Based on Time-Evolutional Responsiveness

Mechanoresponsive materials have been studied to visualize and measure stresses in various fields. However, the high-sensitive and spatiotemporal imaging remain a challenging issue. In particular, the time evolutional responsiveness is not easily integrated in mechanoresponsive materials. In the present study, high-sensitive spatiotemporal imaging of weak compression stresses is achieved by time-evolutional controlled diffusion processes using conjugated polymer, capsule, and sponge. Stimuli-responsive polydiacetylene (PDA) is coated inside a sponge. A mechanoresponsive capsule is set on the top face of the sponge. When compression stresses in the range of 6.67-533 kPa are applied to the device, the blue color of PDA is changed to red by the diffusion of the interior liquid containing a guest polymer flowed out of the disrupted capsule. The applied strength (F/N), time (t/s), and impulse (F·t/N s) are visualized and quantified by the red-color intensity. When a guest metal ion is intercalated in the layered structure of PDA to tune the responsivity, the device visualizes the elapsed time (τ/min) after unloading the stresses. PDA, capsule, and sponge play the important roles to achieve the time evolutional responsiveness for the high-sensitive spatiotemporal distribution imaging through the controlled diffusion processes.

Cyclooctatetraene-Embedded Carbon Nanorings

With the prosperity of the development of carbon nanorings, certain topologically or functionally unique units-embedded carbon nanorings have sprung up in the past decade. Herein, we report the facile and efficient synthesis of three cyclooctatetraene-embedded carbon nanorings (COTCNRs) that contain three (COTCNR1 and COTCNR2) and four (COTCNR3) COT units in a one-pot Yamamoto coupling. These nanorings feature hoop-shaped segments of Gyroid (G-), Diamond (D-), and Primitive (P-) type carbon schwarzites. The conformations of the trimeric nanorings COTCNR1 and COTCNR2 are shape-persistent, whereas the tetrameric COTCNR3 possesses a flexible carbon skeleton which undergoes conformational changes upon forming host-guest complexes with fullerenes (C60 and C70), whose co-crystals may potentially serve as fullerene-based semiconducting supramolecular wires with electrical conductivities on the order of 10-7 S cm-1 (for C60⊂COTCNR3) and 10-8 S cm-1 (for C70⊂COTCNR3) under ambient conditions. This research not only describes highly efficient one-step syntheses of three cyclooctatetraene-embedded carbon nanorings which feature hoop-shaped segments of distinctive topological carbon schwarzites, but also demonstrates the potential application in electronics of the one-dimensional fullerene arrays secured by COTCNR3.

Molecular Probing of the Microscopic Pressure at Contact Interfaces

Obtaining insights into friction at the nanoscopic level and being able to translate these into macroscopic friction behavior in real-world systems is of paramount importance in many contexts, ranging from transportation to high-precision technology and seismology. Since friction is controlled by the local pressure at the contact it is important to be able to detect both the real contact area and the nanoscopic local pressure distribution simultaneously. In this paper, we present a method that uses planarizable molecular probes in combination with fluorescence microscopy to achieve this goal. These probes, inherently twisted in their ground states, undergo planarization under the influence of pressure, leading to bathochromic and hyperchromic shifts of their UV-vis absorption band. This allows us to map the local pressure in mechanical contact from fluorescence by exciting the emission in the long-wavelength region of the absorption band. We demonstrate a linear relationship between fluorescence intensity and (simulated) pressure at the submicron scale. This relationship enables us to experimentally depict the pressure distribution in multiasperity contacts. The method presented here offers a new way of bridging friction studies of the nanoscale model systems and practical situations for which surface roughness plays a crucial role.

Effect of the Activation Force of Mechanophore on Its Activation Selectivity and Efficiency in Polymer Networks

In recent decades, more than 100 different mechanophores with a broad range of activation forces have been developed. For various applications of mechanophores in polymer materials, it is crucial to selectively activate the mechanophores with high efficiency, avoiding nonspecific bond scission of the material. In this study, we embedded cyclobutane-based mechanophore cross-linkers (I and II) with varied activation forces (fa) in the first network of the double network hydrogels and quantitively investigated the activation selectivity and efficiency of these mechanophores. Our findings revealed that cross-linker I, with a lower activation force relative to the bonds in the polymer main chain (fa-I/fa-chain = 0.8 nN/3.4 nN), achieved efficient activation with 100% selectivity. Conversely, an increase of the activation force of mechanophore II (fa-II/fa-chain = 2.5 nN/3.4 nN) led to a significant decrease of its activation efficiency, accompanied by a substantial number of nonspecific bond scission events. Furthermore, with the coexistence of two cross-linkers, significantly different activation forces resulted in the almost complete suppression of the higher-force one (i.e., I and III, fa-I/fa-III = 0.8 nN/3.4 nN), while similar activation forces led to simultaneous activations with moderate efficiencies (i.e., I and IV, fa-I/fa-IV = 0.8 nN/1.6 nN). These findings provide insights into the prevention of nonspecific bond rupture during mechanophore activation and enhance our understanding of the damage mechanism within polymer networks when using mechanophores as detectors. Besides, it establishes a principle for combining different mechanophores to design multiple mechanoresponsive functional materials.

Molecular machines working at interfaces: physics, chemistry, evolution and nanoarchitectonics

As a post-nanotechnology concept, nanoarchitectonics combines nanotechnology with advanced materials science. Molecular machines made by assembling molecular units and their organizational bodies are also products of nanoarchitectonics. They can be regarded as the smallest functional materials. Originally, studies on molecular machines analyzed the average properties of objects dispersed in solution by spectroscopic methods. Researchers' playgrounds partially shifted to solid interfaces, because high-resolution observation of molecular machines is usually done on solid interfaces under high vacuum and cryogenic conditions. Additionally, to ensure the practical applicability of molecular machines, operation under ambient conditions is necessary. The latter conditions are met in dynamic interfacial environments such as the surface of water at room temperature. According to these backgrounds, this review summarizes the trends of molecular machines that continue to evolve under the concept of nanoarchitectonics in interfacial environments. Some recent examples of molecular machines in solution are briefly introduced first, which is followed by an overview of studies of molecular machines and similar supramolecular structures in various interfacial environments. The interfacial environments are classified into (i) solid interfaces, (ii) liquid interfaces, and (iii) various material and biological interfaces. Molecular machines are expanding their activities from the static environment of a solid interface to the more dynamic environment of a liquid interface. Molecular machines change their field of activity while maintaining their basic functions and induce the accumulation of individual molecular machines into macroscopic physical properties molecular machines through macroscopic mechanical motions can be employed to control molecular machines. Moreover, research on molecular machines is not limited to solid and liquid interfaces; interfaces with living organisms are also crucial.

Mechanochemical Approaches to Fundamental Studies in Soft-Matter Physics

Stretching a segment of a polymer beyond its contour length makes its (primarily backbone) bonds more dissociatively labile, which enables polymer mechanochemistry. Integrating some backbone bonds into suitably designed molecular moieties yields mechanistically and kinetically diverse chemistry, which is becoming increasingly exploitable. Examples include, most prominently, attempts to improve mechanical properties of bulk polymers, as well as prospective applications in drug delivery and synthesis. This review aims to highlight an emerging effort to apply the concepts and experimental tools of mechanochemistry to fundamental physical questions in soft matter. A succinct summary of the state-of-the-knowledge of the field, with emphasis on foundational concepts and generalizable observations, is followed by analysis of 3 recent examples of mechanochemistry yielding molecular-level details of elastomer failure, macromolecular chain dynamics in elongational flows and kinetic allostery. We conclude with reasons to assume that the highlighted approaches are generalizable to a broader range of physical problems than considered to date.

Ultrasound-Sensitive Intelligent Nanosystems: A Promising Strategy for the Treatment of Neurological Diseases

Neurological diseases are a major global health challenge, affecting hundreds of millions of people worldwide. Ultrasound therapy plays an irreplaceable role in the treatment of neurological diseases due to its noninvasive, highly focused, and strong tissue penetration capabilities. However, the complexity of brain and nervous system and the safety risks associated with prolonged exposure to ultrasound therapy severely limit the applicability of ultrasound therapy. Ultrasound-sensitive intelligent nanosystems (USINs) are a novel therapeutic strategy for neurological diseases that bring greater spatiotemporal controllability and improve safety to overcome these challenges. This review provides a detailed overview of therapeutic strategies and clinical advances of ultrasound in neurological diseases, focusing on the potential of USINs-based ultrasound in the treatment of neurological diseases. Based on the physical and chemical effects induced by ultrasound, rational design of USINs is a prerequisite for improving the efficacy of ultrasound therapy. Recent developments of ultrasound-sensitive nanocarriers and nanoagents are systemically reviewed. Finally, the challenges and developing prospects of USINs are discussed in depth, with a view to providing useful insights and guidance for efficient ultrasound treatment of neurological diseases.

Suppressive Photochromism and Promotive Mechanochromism of Rhodamine Mechanophore by the Strategy of Poly(methyl acrylate)/Polyurethane Interpenetrating Polymer Network

As molecular design and the structure-property relationships of photochemical molecules established in the literature serve as a convenient reference for mechanophore exploration, many typical mechanophores suffer undesired responses to UV light or even sunlight in bulk polymers. We developed a strategy of a poly(methyl acrylate)/polyurethane (PMA/PU) interpenetrating polymer network (IPN) to suppress the photochromic property of the mechanophore and promote its mechanochromic property. A widely used rhodamine mechanophore (Rh-2OH) was first incorporated into polyurethane (P1). Then P1 was swollen in methyl acrylate and photopolymerized to prepare a PMA2.8/PU IPN (P2). Different from photo/force-responsive P1, P2 selectively responded to force because the low free volume in IPN greatly hinders photoisomerization of the rhodamine spirolactam, suggesting that a simple IPN strategy successfully resolves the giant problem of nonselective response to photo/force for photochromic mechanophores. Moreover, PMA/PU IPN enhanced the mechanical property, resulting in a higher mechanochemical activation ratio than PU, and the prestretching effect of PMA/PU IPN promoted the force sensitivity of rhodamine mechanophores significantly. We believe that the strategy can be applied to other mechanophores, promoting their application in more complicated environments.

Light, Heat, and Force-Responsive Polyolefins

Stimuli-responsive polymers have found applications as shape-memory materials, optical switches, and sensors, but the installation of these responsive properties in non-polar and inert polyolefins is challenging. In this contribution, a series of spiropyran (SP)-based comonomers are synthesized and copolymerized with ethylene or ethylene/cyclic monomers. In addition to great mechanical and surface properties, these functionalized polyolefins responded to light, heat, and force, which induced changes in the polymer structure to transmit color or mechanical signals. These interesting responsive properties are also installed in a series of commercial polyolefin materials through reactive extrusion, making the scalable production of these materials possible.

Polymer mechanochemistry: from single molecule to bulk material

The field of polymer mechanochemistry has experienced a renaissance over the past decades, primarily propelled by the rapid development of force-sensitive molecular units (i.e., mechanophores) and principles governing the reactivity of polymer networks for mechanochemical transduction or material strengthening. In addition to fundamental guidelines for converting mechanical energy input into chemical output, there has also been increasing focus on engineering applications of polymer mechanochemistry for specific functions, mechanically adaptive material systems, and smart devices. These endeavors are made possible by multidisciplinary approaches involving the development of multifunctional mechanophores for mechanoresponsive polymer systems, mechanochemical catalysis and synthesis, three-dimensional (3D) printed mechanochromic materials, reasonable design of polymer network topology, and computational modeling. The aim of this minireview is to provide a summary of recent advancements in covalent polymer mechanochemistry. We specifically focus on productive mechanophores, mechanical remodeling of polymeric materials, and the development of theoretical concepts.

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.

Environment-sensitive fluorescence of COT-fused perylene bisimide based on symmetry-breaking charge separation

Flexible and aromatic photofunctional system (FLAP) is composed of flapping rigid aromatic wings fused with a flexible 8π ring at the center such as cyclooctatetraene (COT). A series of FLAP have been actively studied for the interesting dynamic behaviors. Here, we synthesized a new flapping molecule bearing naphtho-perylenebisimide wings (NPBI-FLAP), in which two perylene units are arranged side by side. As a reference compound, we also prepared COT-fused NPBI (NPBI-COT) that contains only single perylene unit. In both compounds, inherent strong fluorescence of the NPBI moiety is almost quenched and the FL lifetime becomes much shortened in highly polar solvents (acetone and DMF). Through the analyses of environment-sensitive fluorescence, electrochemical reduction/oxidation, and femtosecond transient absorption, the fluorescence quenching behavior was attributed to rapid symmetry-breaking charge separation (SB-CS) for NPBI-FLAP and to intramolecular charge transfer (ICT) for NPBI-COT. Most of the excited species of these compounds decay with the bent geometry, which is in contrast with the excited-state planarization behavior of a previously reported COT-fused peryleneimides with the double-headed arrangement of the perylene moieties. These results indicate that changing the fusion manners between COT and other π skeletons offers new functional molecules with distinct dynamics.

Imaging of Accumulated Mechanical Stresses Using Self-Assembled Layered Conjugated Polymer

When mechanical stresses, such as tensile, compressive, and frictional stresses, are applied to objects by various motions, they are accumulated in materials. Conventional mechanoresponsive materials and sensors detect one-time applied stress. However, the accumulated stresses are not visualized or measured in previous works. The present study demonstrated imaging and sensing of not only one-time but also accumulated tensile, compressive, and frictional stresses. Polyurethane (PU) film was combined with 2D layered polydiacetylene (PDA), a stimuli-responsive color-changing polymer. PDA generally exhibits no color changes with the application of tensile and compression stresses because the molecular motion leading to the color change is not induced by such mechanical stresses. Here the versatile mechanoresponsiveness was achieved using a block copolymer guest partially intercalated in the layered PDA. As the interlayer and outerlayer segments interact with PDA and PU, respectively, the applied stresses to the film are transferred from PU to PDA via the block copolymer guest. The color changes of the film imaged and quantified the accumulated work depending on the number and strength of the applied multiple stresses such as tensile, compressive, and frictional stresses. The design strategy of materials and methodology of sensing can be applied to the development of new sensors for accumulated mechanical stresses in a wide range of length and strength scales.

Thermally Stable Photomechanical Molecular Hinge: Sterically Hindered Stiff-Stilbene Photoswitch Mechanically Isomerizes

Molecular photoswitches are extensively used as molecular machines because of the small structures, simple motions, and advantages of light including high spatiotemporal resolution. Applications of photoswitches depend on the mechanical responses, in other words, whether they can generate motions against mechanical forces as actuators or can be activated and controlled by mechanical forces as mechanophores. Sterically hindered stiff stilbene (HSS) is a promising photoswitch offering large hinge-like motions in the E/Z isomerization, high thermal stability of the Z isomer, which is relatively unstable compared to the E isomer, with a half-life of ca. 1000 years at room temperature, and near-quantitative two-way photoisomerization. However, its mechanical response is entirely unexplored. Here, we elucidate the mechanochemical reactivity of HSS by incorporating one Z or E isomer into the center of polymer chains, ultrasonicating the polymer solutions, and stretching the polymer films to apply elongational forces to the embedded HSS. The present study demonstrated that HSS mechanically isomerizes only in the Z to E direction and reversibly isomerizes in combination with UV light, i.e., works as a photomechanical hinge. The photomechanically inducible but thermally irreversible hinge-like motions render HSS unique and promise unconventional applications differently from existing photoswitches, mechanophores, and hinges.

Microscopic strain mapping in polymers equipped with non-covalent mechanochromic motifs

The mechanical failure of polymers remains challenging to understand and predict, as it often involves highly localised phenomena that cannot be probed with bulk characterisation techniques. Here, we present a generalisable protocol based on optical microscopy, tensile testing, and image processing that permits the spatially resolved interrogation of mechanical deformation at the molecular level around defects in mechanophore-containing polymers. The approach can be applied to a broad range of polymeric materials, mechanophores, and deformation scenarios.

Force-Triggered Atropisomerization of a Parallel Diarylethene to Its Antiparallel Diastereomers

This paper describes a mechanical approach to inducing the atropisomerization of a parallel diarylethene into its antiparallel diastereomers exhibiting distinct chemical reactivity. A congested parallel diarylethene mechanophore in the (R_a,S_a)-configuration with mirror symmetry is atropisomerized to its antiparallel diastereomers with C₂ symmetry under ultrasound-induced force field. The resulting stereochemistry-converted material gains symmetry-allowed reactivity toward conrotatory photocyclization.

A dual-mode ratiometric aptasensor for accurate detection of pathogenic bacteria based on recycling of DNAzyme activation

Pathogenic bacteria have a significant impact on food safety. Herein, an innovative dual-mode ratiometric aptasensor was constructed for ultrasensitive and accurate detection of Staphylococcus aureus (S. aureus) based on recycling of DNAzyme activation on gold nanoparticles-functionalized MXene nanomaterials (MXene@Au NPs). Electrochemiluminescent (ECL) emitter-labeled probe DNA (probe 2-Ru) containing the blocked DNAzyme was partly hybridized with aptamer and then captured by electrochemical (EC) indicator-labeled probe DNA (probe 1-MB) on electrode surface. When S. aureus presented, the conformation vibration of probe 2-Ru activated the blocked DNAzymes, leading to recycling cleavage of probe 1-MB and ECL tag close to electrode surface. Based on the reverse change tendencies of ECL and EC signals, aptasensor achieved S. aureus quantification from 5 to 10⁸ CFU/mL. Moreover, the self-calibration characteristic of the aptasensor with dual-mode ratiometric readout ensured the reliable measurement of S. aureus in real samples. This work showed useful insight into sensing foodborne pathogenic bacteria.

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.

Swelling-induced Mechanochromism in Multinetwork Polymers

We report a novel and versatile approach to achieving swelling-induced mechanochemistry using a multinetwork (MN) strategy that enables polymer networks to repeatedly swell with monomers and solvents. The isotropic expansion of the first network (FN) provides sufficient force to drive the mechanochemical scission of a radical-based mechanophore, difluorenylsuccinonitrile (DFSN). Although prompt recombination generally occurs in such highly mobile environments, the resulting pink radicals are kinetically stabilized in the gels, probably due to limited diffusion in the extended polymer chains. Moreover, the DFSN embedded in the isotropically strained chain exhibits increased thermal reactivity, which can be reasonably explained by an entropic contribution of the FN to the dissociation. The utility of the MN polymers is demonstrated not only in terms of swelling-force-induced network modification, but also in the context of tunable reactivity of the dissociative unit through proper design of the hierarchical network architecture.

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.

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.

Stimuli-Responsive Sponge for Imaging and Measuring Weak Compression Stresses

Imaging and measuring compression stresses secure a safe and healthy life. Compression stresses in kPa range are not easily detected by conventional mechanoresponsive materials because microscopic molecular motion of the chromophores is not induced by such weak stresses. Moreover, imaging of the stress distribution is not achieved so far. The present study shows a sponge device combining two stimuli-responsive materials, a capsule releasing interior liquid and color-changing polymer in responses to compression stress and chemical stimulus, respectively. The stimuli-responsive capsule is dispersed on a melamine sponge comprised of the fibers with coating the layered polydiacetylene (PDA). The application of weak compression stresses induces collapse of the capsules, outflow of the interior liquid, and subsequent irreversible color change of PDA. The cascading response in the sponge device colorimetrically enables imaging of the distribution and measuring the strength of the compression stresses in kPa range. Furthermore, the device demonstrates imaging and measuring unknown weak compression stresses applied by the irregular-shaped objects. A couple of clinical issues in surgical operation of intestine are studied using the stress-imaging sponge device. The device and its design strategy can be applied to stress imaging in a variety of fields.

4D Direct Laser Writing of Submerged Structural Colors at the Microscale

Biomimetic stimuli-responsive structure colors (SCs) can improve the visualization and identification in the micro functional structure field such as information encryption/decryption and smart actuators. However, it is still challenging to develop the ability to 4D print arbitrary submerged colorful patterns with stimuli-responsive materials at the microscale. Herein, a hydrogel photoresist with feature resolution (98 nm) for the fabrication of 4D microscopic SCs by the femtosecond direct laser writing method is developed. The 4D printed woodpile SCs are grouped as pixel palettes with various laser parameters and they spanned almost the entire color space. The coloring mechanism of diffraction gratings is not only investigated by optics microscopy and spectroscopy but also supported by simulation. Moreover, the 4D printed hydrogel-integrated amphichromatic fish constructions and pixelated painting can visually discolor reversibly by regulating the solution pH. This finding promises an ideal coloring method for sensors, anti-counterfeiting labels, and transformable photonic devices.

Molecular Machines and Microrobots: Nanoarchitectonics Developments and On-Water Performances

This review will focus on micromachines and microrobots, which are objects at the micro-level with similar machine functions, as well as nano-level objects such as molecular machines and nanomachines. The paper will initially review recent examples of molecular machines and microrobots that are not limited to interfaces, noting the diversity of their functions. Next, examples of molecular machines and micromachines/micro-robots functioning at the air-water interface will be discussed. The behaviors of molecular machines are influenced significantly by the specific characteristics of the air-water interface. By placing molecular machines at the air-water interface, the scientific horizon and depth of molecular machine research will increase dramatically. On the other hand, for microrobotics, more practical and advanced systems have been reported, such as the development of microrobots and microswimmers for environmental remediations and biomedical applications. The research currently being conducted on the surface of water may provide significant basic knowledge for future practical uses of molecular machines and microrobots.

Conformer Ring Flip Enhances Mechanochromic Performance of ansa-Donor-Acceptor-Donor Mechanochromic Torsional Springs

Mechanochromophores based on conformational changes of donor-acceptor-donor (DAD) springs allow sensing of forces acting on polymer chains by monotonic changes of absorbance or photoluminescence (PL) wavelength. Here, we identify a series of thiophene (D)-flanked quinoxalines (A) as molecular torsional springs for force sensing in bulk polymers at room temperature. The mode of DAD linkage to the polymer matrix and linker rigidity are key parameters that influence the efficacy of force transduction to the DAD spring and thus mechanochromic response, as probed by in situ PL spectroscopy of bulk films during stress-strain experiments. The largest shift of the PL maximum, and thus the highest sensitivity, is obtained from an ansa-DAD spring exhibiting bridged D units and a stiff A linker. Using detailed spectroscopy and density functional theory calculations, we reveal conformer redistribution in the form of a thiophene ring flip as the major part of the overall mechanochromic response. At forces as low as 27 pN at early stages of deformation, the ring flip precedes mechanically induced planarization of the ansa-DAD spring, the latter process producing a PL shift of 21 nm nN^-1. Within the stress-strain diagram, the thiophene ring flip and DAD planarization are thus two separated processes that also cause irreversible and reversible mechanochromic responses, respectively, upon sample failure. As the thiophene ring flip requires much smaller forces than planarization of the DAD spring, such micromechanical motion gives access to sensing of tiny forces and expands both sensitivity and the force range of conformational mechanochromophores.

Dynamic Timing Control of Molecular Photoluminescent Systems

Dynamic control of molecular photoluminescence offers chemical solutions to designing functional emissive materials. Although stimuli-switchable molecular luminescent systems are well established, how to encode these dynamic emissive systems with a "timing" feature, that is, time-dependent luminescent properties, remains challenging. This Concept aims to summarize the design principles of dynamic timing molecular photoluminescent systems by discussing the state-of-the-art of this topic and the shaping of fabrication strategies at both the molecular and supramolecular levels. An outlook and perspectives are given to outline the future opportunities and challenges in the rational design and potential applications of these smart emissive systems.

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.

Photoconductance from the Bent-to-Planar Photocycle between Ground and Excited States in Single-Molecule Junctions

Single-molecule conductance measurements for 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) may offer unique insight into the bent-to-planar photocycle between the ground and excited states. Herein, we employ DPAC derivative DPAC-SMe as the molecular prototype to fabricate single-molecule junctions using the scanning tunneling microscope break junction technique and explore photoconductance dependence on the excited-state structural/electronic changes. We find up to ∼200% conductance enhancement of DPAC-SMe under continuous 340 nm light irradiation than that without irradiation, while photoconductance disappears in the case where structural evolution of the DPAC-SMe is halted through macrocyclization. The in situ conductance modulation as pulsed 340 nm light irradiation is monitored in the DPAC-SMe-based junctions alone, suggesting that the photoconductance of DPAC-SMe stems from photoinduced intramolecular planarization. Theoretical calculations reveal that the photoinduced structural evolution brings about a significant redistribution of the electron cloud density, which leads to the appearance of Fano resonance, resulting in enhanced conductance through the DPAC-SMe-fabricated junctions. This work provides evidence of bent-to-planar photocycle-induced conductance differences at the single-molecule level, offering a tailored approach for tuning the charge transport characteristics of organic photoelectronic devices.

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.

Copper(I) Catalyzed Decarboxylative Synthesis of Diareno[a,e]cyclooctatetraenes

Diareno[a,e]cyclooctatetraenes find widespread applications as building blocks, ligands, and responsive cores in topologically switchable materials. However, current synthetic methods to these structures suffer from low yields or operational disadvantages. Here, we describe a practical three-step approach to diareno[a,e]cyclooctatetraenes using an efficient copper(I) catalyzed double decarboxylation as the key step. The sequence relies on cheap and abundant reagents, is readily performed on scale, and is amenable also to unsymmetrical derivatives that expand the utility of this intriguing class of structures.

Supramolecular Polymerization of a Photo-Fluttering Chiral Monomer: A Temporarily Suspendable Chain Growth by Light

Using a photochemically fluttering thiophene-fused cyclooctatetraene derivative (COT) as a nonplanar chiral monomer, we have succeeded in remotely suspending the supramolecular polymerization in a temporal manner by a completely new strategy. The COT monomer with an 8π electron core adopts a saddle shape in the ground state and flutters 5.8 × 10³ times faster upon photoirradiation than in the dark as a result of the stabilized planar conformation by the excited-state aromaticity (Baird aromaticity). Detailed investigation revealed that without photoirradiation the rate constant of the fluttering motion is 1/560 times smaller than that of the chain elongation, indicating that the fluttering of COT does not affect the chain elongation in the dark. In contrast, under photoirradiation (365 nm), the fluttering of COT is at least 11 times more rapid than the chain elongation, thereby suppressing the elongation event. The rapid fluttering of COT to suspend the chain elongation is not accompanied by a decrease in active monomer concentration, leading to depolymerization.