Acceleration of Diels-Alder reactions by mechanical distortion

A scalable photo-mechanochemical platform for sustainable photoredox catalysis by resonant acoustic mixing

Photocatalysis has greatly advanced in organic synthesis but still confronts challenges, including light attenuation in reaction media and excessive solvent utilization. These issues lead to inefficiencies, particularly in heterogeneous cloudy mixtures and in scaling-up applications. Integrating photocatalysis with mechanochemistry offers a nascent but promising solution to these challenges. Herein, we present a scalable photo-mechanochemical platform that combines visible-light photocatalysis with Resonant Acoustic Mixing (RAM), enabling efficient cross-coupling reactions under solvent-minimised conditions. This approach demonstrates broad substrate tolerance, accommodating a variety of aryl (hetero) halides and N-, O-, P-, S-nucleophiles. The protocol supports scaling up to 300 mmol, representing a 1500-fold increase, while maintaining exceptionally low catalyst loading and achieving up to 9800 turnover numbers (TON). The generality of this platform is further validated by its applicability to other synthetic transformations.

Biphasic mechanochemistry of single-chain polymerization

Mechanical forces can induce chemical reactions, produce chemical signals, and alter reaction kinetics. Here, using magnetic tweezers-based single-molecule force spectroscopy, we study the force effects on the ring-opening metathesis polymerization (ROMP) of single-polymer chains, during which nonequilibrium conformational entanglements can form and unravel stochastically. We find a surprising, biphasic force dependence of polymerization kinetics: The single-chain polymerization rate initially slows down with increasing stretching forces, reaching a minimum, and then accelerates at higher forces. Analysis of real-time single-chain growth trajectories allows for dissecting the polymerization process into two distinct regimes, one with and the other without entanglement formation, unveiling the biphasic force dependence in both regimes. Two different mechanisms likely operate for the biphasic dependence: a force-induced entanglement tightening and then splitting and a force-induced catalyst structural distortion that switches the reaction pathway between reactant states of different stability and reactivity. These findings and insights point to opportunities of using force to manipulate polymerization reactions and tune the physiochemical properties of synthetic polymers.

Flexocatalysis: Regulating peroxymonosulfate activation by flexoelectricity

Challenges related to energy shortages and environmental pollution are driving extensive research in catalysis. Flexocatalysis, which extends flexoelectricity to mechanocatalysis, is a promising mechanism for catalytic processes. Owing to its size-dependent effects, flexoelectricity has become particularly significant and plays a dominant regulatory role in chemical reactions within nanocatalysis. In this study, we integrated flexocatalysis with peroxymonosulfate (PMS) activation for efficient water purification. The simulation results show that flexoelectric polarization can induce a strong flexoelectric field in δ-MnO2 nanosheets. This built-in electric field subsequently drives the migration of electrons and holes to the reaction interface, thereby activating PMS and promoting rapid generation of reactive species for the degradation of organic pollutants. At low concentrations of flexoelectric catalysts, we achieved excellent degradation efficiency that was 6.6 times greater than that obtained through the thermal activation of PMS. This study demonstrated that flexoelectricity can function as a switch for PMS activation and offers a promising approach for sustainable water remediation.

Chemifriction and Superlubricity: Friends or Foes?

The mechanisms underlying chemifriction (the contribution of interfacial bonding to friction) in defected twisted graphene interfaces are revealed using fully atomistic molecular dynamics simulations based on machine-learning potentials. This involves stochastic events of consecutive bond formation and rupture between single vacancy defects that may enhance friction. A unique shear-induced interlayer atomic transfer healing mechanism is discovered that can be harnessed to design a run-in procedure to restore superlubric sliding. This mechanism should be manifested as negative differential friction coefficients that are expected to emerge under moderate normal loads. A physically motivated phenomenological model is developed to predict the chemifriction effects in experimentally relevant sliding velocity regimes. This allows us to identify a distinct transition between logarithmic increase and logarithmic decrease of the friction force with increasing sliding velocity. While demonstrated for homogeneous graphitic contacts, a similar mechanism is expected to occur in other homogeneous or heterogeneous defected two-dimensional material interfaces.

Regulation of Reaction Pathways in Coordinated Chains by Directional Mechanical Force

Mechanochemistry refers to chemical reactions induced by mechanical forces. Due to different reaction mechanisms, products obtained through mechanochemistry can be distinct from those produced by thermochemistry and photochemistry. Scanning probe microscopy is a powerful tool for studying single-molecule mechanochemical processes. Mechanical force is a vector that has both magnitude and direction. Previous studies have focused on triggering reactions by forces and measuring their magnitude. In this work, we use the direction of the force to regulate the reaction pathway in a spin-crossover coordinated chain. The chains are prepared via the dehydrogenated coordination reaction between tetrahydroxybenzene molecules and Ni atoms on Au(111). The Ni atoms in the chain alternate between a high-spin state and a low-spin state. By altering Ni-O bond lengths and O-Ni-O angles through the directional mechanical force, a chemical process occurs, and the spin state of Ni undergoes a transition. With the attraction from a Au tip, the Ni atom is pulled from high-spin to low-spin state. With the repulsion from a C60-functionalized tip, the low-spin Ni atom is pushed to the high-spin state. The force to induce the reaction is measured by qPlus atomic force microscopy. This study provides an approach for regulating chemical pathways.

Using Near-Infrared Irradiation for Heating Mechanochemical Reactions in Organic-Dye-Doped Epoxy Milling Jars

Albeit mechanochemistry is a novel promising technology that gives access to reactivity under solvent-free conditions, heating such reactions is sometimes compulsory to obtain satisfactory results in terms of conversion, selectivity and/or yield. In this work, we developed a novel approach using a dye that absorbs NIR photons and release the energy as heat. Hence, de novo milling jars in epoxy resin doped with the dye were thus produced to obtain reactors that would produce heat upon irradiation at 850 nm. Temperature profiles were recorded, depending on the irradiance, dye charge in the resin, and milling frequency, showing an excellent control of the temperature. The usefulness of the heating jar was then demonstrated in mechanochemical reactions that are known to require heat to yield the desired product, namely Diels-Alder reactions with high activation energies and the newly developed rearrangement of a sydnone into corresponding 1,3,4-oxadiazolin-2-one.

On the Valorisation of Chitin-Derived Furans by Milling

Chitin-derived furans offer a sustainable alternative feedstock for nitrogen appended aromatic compounds. Herein, we address the challenge of using chitin-derived furans, 3-acetamido-5-acetylfuran (3A5AF) and 3-acetamido-5-furfural aldehyde (3A5F), to favour the formation of exo Diels-Alder adducts and 4-acetylaminophthalimides respectively, using a mechanochemical ball-milling technique. Mechanochemical activation is explored through the synthesis of 7-oxa-norbornene backbones with novel substitution pattern from 3A5AF in yields up to 77 % and improved exo:endo selectivity compared to solution-phase reactions. The synthesis of 4-acetylaminophthalimides from 3A5F in yields up to 79 % is also showcased from hydrazone derivatives.

An efficient and flexible approach for local distortion: distortion distribution analysis enabled by fragmentation

Distortion can play crucial roles in influencing structures and properties, as well as enhancing reactivity or selectivity in many chemical and biological systems. The distortion/interaction or activation-strain model is a popular and powerful method for deciphering the origins of activation energies, in which distortion and interaction energies dictate an activation energy. However, decomposition of local distortion energy at the atomic scale remains less clear and straightforward. Knowing such information should deepen our understanding of reaction processes and improve reaction design. Herein, an efficient, general and flexible fragmentation-based approach was proposed to evaluate local distortion energies for various chemical and biological molecules, which can be obtained computationally and/or experimentally. Moreover, our distortion analysis is readily applicable to multiple structures from molecular dynamics (or the minimum energy path) as well as can be evaluated by different computational chemistry methods. Our systematic analysis shows that our approach not only aids computational and experimental chemists in visualizing (relative) distortion distributions within molecules (distortion map) and identifies the key distorted pieces, but also offers deeper understanding and insights into structures, reaction mechanisms and dynamics in various chemical and biological systems. Furthermore, our analysis offers indices of local distortion energy, which can potentially serve as a new descriptor for multi-linear regression (MLR) or machine learning (ML) modelling.

Stereoelectronic Effect of Protecting Groups on the Stability of Galactosyl Donor Intermediates

Using methods of DFT, we investigated the effect of electron withdrawing and electron donating groups on the relative stability of tentative glycosyl donor reaction intermediates. The calculation shows that by changing the stereoelectronic properties of the protecting group, we can influence the stability of the dioxolenium type of intermediates by up to 10 kcal mol-1, and that by increasing nucleophillicity of the 4-O-Bz group, the dioxolenium intermediate becomes more stable than a triflate-donor pair. We exploited this mechanism to design galactosyl donors with custom protecting groups on O2 and O4, and investigated the outcome of the reaction with cyclohexanol. The reaction showed no change in the product distribution, which suggests that the neighboring group participation takes precedence over remote group participation due to kinetic barriers.

Mechanochemical Radical Transformations in Organic Synthesis

Organic synthesis has historically relied on solution-phase, polar transformations to forge new bonds. However, this paradigm is evolving, propelled by the rapid evolution of radical chemistry. Additionally, organic synthesis is witnessing a simultaneous resurgence in mechanochemistry, the formation of new bonds in the solid-state, further contributing to this shift in the status quo. The aforementioned advances in radical chemistry have predominantly occurred in the solution phase, while the majority of mechanochemical synthesis advances feature polar transformations. Herein, we discuss a rapidly advancing area of organic synthesis: mechanochemical radical reactions. Solid-state radical reactions offer improved green chemistry metrics, better reaction outcomes, and access to intermediates and products that are difficult or impossible to reach in solution. This review explores these reactions in the context of small molecule synthesis, from early findings to the current state-of-the-art, underscoring the pivotal role solid-state radical reactions are likely to play in advancing sustainable chemical synthesis.

Boosting Reactive Oxygen Species Generation via Contact-Electro-Catalysis with FeIII-Initiated Self-cycled Fenton System

Contact Electro-Catalysis (CEC) using commercial dielectric materials in contact-separation cycles with water can trigger interfacial electron transfer and induce the generation of reactive oxygen species (ROS). However, the inherent hydrophobicity of commercial dielectric materials limits the effective reaction sites, and the generated ROS inevitably undergo self-combination to form hydrogen peroxide (H2O2). In typical CEC systems, H2O2 does not further decompose into ROS, leading to suboptimal reaction rates. Addressing the generation and activation of H2O2 is therefore crucial for advancing CEC. Here, we synthesized a catalyst by loading the dielectric material polytetrafluoroethylene (PTFE) onto ZSM-5 (PTFE/ZSM-5, PZ for short), achieving uniform dispersion of the catalyst in water for the first time. The introduction of an FeIII-initiated self-cycling Fenton system (SF-CEC), with the synergistic effects of O2 activation and FeIII-activated H2O2, further enhanced ROS generation. In the FeIII-initiated SF-CEC system, the synergistic effects of ROS and protonated azo dyes enabled nearly 99 % degradation of azo dyes within 10 minutes, a sixfold improvement compared to the CEC system. This represents the fastest degradation rate of methyl orange dye induced by ultrasound to date. Without extra oxidants, this system enabled stable dissolution of precious metals in weakly acidic solutions at room temperature, achieving 80 % gold dissolution within 2 hours, 2.5 times faster than similar CEC systems. This study also corrects the unfavorable perception of CEC applications under acidic conditions, providing new insights for the fields of dye degradation and precious metal recovery.

On the physical processes of mechanochemically induced transformations in molecular solids

Initiating or sustaining physical and chemical transformations with mechanical force - mechanochemistry - provides an opportunity for more sustainable chemical processes, and access to new chemical reactivity. These transformations, however, do not always adhere to 'conventional' chemical wisdom, making them difficult to design and rationalise. This challenge is exacerbated by the fact that not all mechanochemical transformations are equal, with mechanical force playing a different role in different types of processes. In this review we discuss some of the different roles mechanical force can play in mechanochemical transformations, set primarily against the author's own research. We classify mechanochemical reactions broadly as those (1) where mechanical energy is for mixing, (2) where mechanical energy alters the stability of the reagent, and (3) where mechanical energy directly excites the solid. Finally, we demonstrate how - while useful - these classifications have fuzzy boundaries and concepts from across them are needed to understand many mechanochemical reactions.

Applications of Diels-Alder Chemistry in Biomaterials and Drug Delivery

Recent studies, leveraging click chemistry reactions, have significantly advanced the fields of biomaterials and drug delivery. Of these click reactions, the Diels-Alder cycloaddition is exceptionally valuable for synthetic organic chemistry and biomaterial design, as it occurs under mild reaction conditions and can undergo a retrograde reaction, under physiologically relevant conditions, to yield the initial reactants. In this review, potential applications of the Diels-Alder reaction are explored within the nexus of biomaterials and drug delivery. This includes an emphasis on key platforms such as polymers, nanoparticles, and hydrogels which utilize Diels-Alder for drug delivery, functionalized surfaces, bioconjugation, and other diverse applications. Specifically, this review will focus on the use of Diels-Alder biomaterials in applications of tissue engineering and cancer therapies, while providing a discussion of the advantages, platforms, and applications of Diels-Alder click chemistry.

Navigating Ball Mill Specifications for Theory-to-Practice Reproducibility in Mechanochemistry

The rising prospects of mechanochemically assisted syntheses hold promise for both academia and industry, yet they face challenges in understanding and, therefore, anticipating respective reaction kinetics. Particularly, dependencies based on variations in milling equipment remain little understood and globally overlooked. This study aims to address this issue by identifying critical parameters through kinematic models, facilitating the reproducibility of mechanochemical reactions across the most prominent mills in laboratory settings, namely planetary and mixer mills. Through a series of selected experiments replicating major classes of organic, organometallic, transition metal-catalyzed, and inorganic reactions from literature, we rationalize the independence of kinematic parameters on reaction kinetics when the accumulated energy criterion is met. As a step forward and to facilitate the practicability of our findings, we provide a freely accessible online tool[†] that allows the calculation of respective energy parameters for different planetary and mixer mills. Our work advances the current understanding of mechanochemistry and lays the foundation for future rational exploration in this rapidly evolving field.

Nonaqueous Contact-Electro-Chemistry via Triboelectric Charge

Mechanochemistry revolutionizes traditional reactions through mechanical stimulation, but its reaction efficiency is limited. Recent advancements in utilizing triboelectric charge from liquid-solid contact electrification (CE) have demonstrated significant potential in improving the reaction efficiency. However, its efficacy remains constrained by interfacial electrical double-layer screening in aqueous solutions. This study pioneered chemistry in nonaqueous systems via CE for catalysis and luminescence. Density functional theory simulations and experiments revealed varying electron transfer capabilities and chemoselectivity of CE across different solvents. Phenol degradation via CE in dimethyl sulfoxide (DMSO) exhibited a rate over 40 times faster than that of traditional mechano-driven chemistry. A more intuitive comparison revealed that CE degradation of phenol in DMSO exhibits a 30-fold rate improvement compared to deionized water, where the degradation remains incomplete. Luminol oxidation by radicals generated solely via CE in DMSO eliminates the dependence on traditional catalysts and side reactions, establishing a pure and simple system for investigating the reaction mechanisms. A high and stable luminescence characteristic was maintained for 3 months, enhancing the imaging accuracy and stability exponentially. This study underscores the impact of triboelectric charge on reaction efficiency and chemoselectivity, establishing a new paradigm in nonmetal catalysis, mechanoluminescence, and providing profound insights into reaction kinetics.

Molecular Recognition in Mechanochemistry: Insights from Solid-State NMR Spectroscopy

Molecular-recognition events are highly relevant in biology and chemistry. In the present study, we investigated such processes in the solid state under mechanochemical conditions using the formation of racemic phases upon reacting enantiopure entities as example. As test systems, α-(trifluoromethyl)lactic acid (TFLA) and the amino acids serine and alanine were used. The effects of ball-milling and resonant acoustic mixing (RAM) on the formation of racemic phases were probed by using solid-state Nuclear Magnetic Resonance (NMR) spectroscopy. In a mixer mill, a highly efficient and fast racemic phase formation occurred for both TFLA and the two amino acids. RAM led to the racemic phase for TFLA also, and this process was facilitated upon employing pre-milled enantiopure entities. In contrast, under comparable conditions RAM did not result in the formation of racemic phases for serine and alanine.

Simulation of a Diels-Alder reaction on a quantum computer

The simulation of chemical reactions is an anticipated application of quantum computers. Using a Diels-Alder reaction as a test case, in this study we explore the potential applications of quantum algorithms and hardware in investigating chemical reactions. Our specific goal is to calculate the activation barrier of a reaction between ethylene and cyclopentadiene forming a transition state. To achieve this goal, we use quantum algorithms for near-term quantum hardware (entanglement forging and quantum subspace expansion) and classical post-processing (many-body perturbation theory) in concert. We conduct simulations on IBM quantum hardware using up to 8 qubits, and compute accurate activation barrier in the reaction between cyclopentadiene and ethylene by accounting for both static and dynamic electronic correlation. This work illustrates a hybrid quantum-classical computational workflow to study chemical reactions on near-term quantum devices, showcasing the potential for performing quantum chemistry simulations on quantum hardware to predict activation barriers in agreement with those predicted by CASCI.

Mechanochemical Controlled Radical Polymerization: From Harsh to Mild

Mechanochemistry constitutes a burgeoning field that investigates the chemical and physicochemical alterations of substances under mechanical force. It enables the synthesis of materials which is challenging to obtain via thermal, optical or electrical activation methods. In addition, it diminishes reliance on organic solvents and provides a novel route for green chemistry. Today, as a distinct branch alongside electrochemistry, photochemistry, and thermochemistry, mechanochemistry has emerged as a frontier research domain within chemistry and material science. In recent years, the intersection of mechanochemistry with controlled radical polymerization has witnessed rapid advancements, providing new routes to polymer science. Significantly, we have experienced breakthroughs in methods relying on sonochemistry, piezoelectricity and contact electrification. These methodologies not only facilitate the synthesis of polymers with high molecular weight but also enable precise control over polymer chain length and structure. Transitioning from harsh to mild conditions in mechanochemical routes has facilitated a significant improvement in the controllability of mechanochemical polymerization. From this perspective, we introduce the progress of mechanochemistry in controlled radical polymerization in recent years, aim to clarify the historcial development of this topic.

Room-Temperature Optically Detected Coherent Control of Molecular Spins

Optically interfaced molecular spins are a promising platform for quantum sensing and imaging. Key for such applications is optically detecting coherent spin manipulation at room temperature. Here, using the photoexcited triplet state of organic chromophores (pentacene doped in p-terphenyl), we optically detect coherent spin manipulation with photoluminescence contrasts exceeding 15% at room temperature, both in a molecular crystal and thin film. We further demonstrate how multifrequency spin control could enhance such systems. These results open opportunities for room-temperature quantum sensors that capitalize on the versatility of synthetic chemistry.

Decomposition of Forces in Protein: Methodology and General Properties

In contrast to the central role played by the structure of biomolecules, the complementary force-based view has received little attention in past studies. Here, we proposed a simple method for the force decomposition of multibody interactions and provided some techniques to analyze and visualize the general behavior of forces in proteins. It was shown that atomic forces fluctuate at a magnitude of about 3000 pN, which is huge in the context of cell biology. Remarkably, the average scalar product between atomic force and displacement universally approximates -3kBT. This is smaller by an order of magnitude than the simple product of their fluctuation magnitudes due to the unexpectedly weak correlation between the directions of force and displacement. The pairwise forces are highly anisotropic, with elongated fluctuation ellipsoids. Residue-residue forces can be attractive or repulsive (despite being more likely to be attractive), forming some kind of tensegrity structure stabilized by a complicated network of forces. Being able to understand and predict the interaction network provides a basis for rational drug design and uncovering molecular recognition mechanisms.

Dynamic Covalent Bond-Based Polymer Chains Operating Reversibly with Temperature Changes

Dynamic bonds can facilitate reversible formation and dissociation of connections in response to external stimuli, endowing materials with shape memory and self-healing capabilities. Temperature is an external stimulus that can be easily controlled through heat. Dynamic covalent bonds in response to temperature can reversibly connect, exchange, and convert chains in the polymer. In this review, we introduce dynamic covalent bonds that operate without catalysts in various temperature ranges. The basic bonding mechanism and the kinetics are examined to understand dynamic covalent chemistry reversibly performed by equilibrium control. Furthermore, a recent synthesis method that implements dynamic covalent coupling based on various polymers is introduced. Dynamic covalent bonds that operate depending on temperature can be applied and expand the use of polymers, providing predictions for the development of future smart materials.

Charge Redistribution in Mechanochemical Reactions for Solid Interfaces

Mechanochemical strategies are widely used in various fields, ranging from friction and wear to mechanosynthesis, yet how the mechanical stress activates the chemical reactions at the electronic level is still open. We used first-principles density functional theory to study the rule of the stress-modified electronic states in transmitting mechanical energy to trigger chemical responses for different mechanochemical systems. The electron density redistribution among initial, transition, and final configurations is defined to correlate the energy evolution during reactions. We found that stress-induced changes in electron density redistribution are linearly related to activation energy and reaction energy, indicating the transition from mechanical work to chemical reactivity. The correlation coefficient is defined as the term "interface reactivity coefficient" to evaluate the susceptibility of chemical reactivity to mechanical action for material interfaces. The study may shed light on the electronic mechanism of the mechanochemical reactions behind the fundamental model as well as the mechanochemical phenomena.

Mechanochemical C-H Arylation and Alkylation of Indoles Using 3 d Transition Metal and Zero-Valent Magnesium

Although the 3 d transition-metal catalyzed C-H functionalization have been extensively employed to promote the formation of valuable carbon-carbon bonds, the persistent problems, including the use of sensitive Grignard reagents and the rigorous operations (solvent-drying, inert gas protection, metal pre-activation and RMgX addition rate control), still leave great room for further development of sustainable methodologies. Herein, we report a mechanochemical technology toward in-situ preparation of highly sensitive organomagnesium reagents, and thus building two general 3 d transition-metal catalytic platforms that enables regioselective arylation and alkylation of indoles with a wide variety of halides (including those containing post transformable functionalities and heteroaromatic rings). This mechanochemical strategy also brings unique reactivity and high step-economy in producing functionalized N-free indole products.

Reversible strain-promoted DNA polymerization

Molecular strain can be introduced to influence the outcome of chemical reactions. Once a thermodynamic product is formed, however, reversing the course of a strain-promoted reaction is challenging. Here, a reversible, strain-promoted polymerization in cyclic DNA is reported. The use of nonhybridizing, single-stranded spacers as short as a single nucleotide in length can promote DNA cyclization. Molecular strain is generated by duplexing the spacers, leading to ring opening and subsequent polymerization. Then, removal of the strain-generating duplexers triggers depolymerization and cyclic dimer recovery via enthalpy-driven cyclization and entropy-mediated ring contraction. This reversibility is retained even when a protein is conjugated to the DNA strands, and the architecture of the protein assemblies can be modulated between bivalent and polyvalent states. This work underscores the utility of using DNA not only as a programmable ligand for assembly but also as a route to access restorable bonds, thus providing a molecular basis for DNA-based materials with shape-memory, self-healing, and stimuli-responsive properties.

Intermediates in Mechanochemical Reactions

Mechanochemical reactions offer methodological and environmental advantages for chemical synthesis, constantly attracting attention within the scientific community. Besides unmistakable sustainability advantages, the conditions under which mechanochemical reactions occur, namely solventless conditions, sometimes facilitate the isolation of otherwise labile or inaccessible products. Despite these advantages, limited knowledge exists regarding the mechanisms of these reactions and the types of intermediates involved. Nevertheless, in an expanding number of cases, ex situ and in situ monitoring techniques have allowed for the observation, characterization, and isolation of reaction intermediates in mechanochemical transformations. In this Minireview, we present a series of examples in which reactive intermediates have been detected in mechanochemical reactions spanning organic, organometallic, inorganic, and materials chemistry. Many of these intermediates were stabilized by non-covalent interactions, which played a pivotal role in guiding the chemical transformations. We believe that by uncovering and understanding such instances, the growing mechanochemistry community could find novel opportunities in catalysis and discover new mechanochemical reactions while achieving simplification in chemical reaction design.

Exploring mechanochemistry of pharmaceutical cocrystals: effect of incident angle on molecular mixing during simulated indentations of two organic solids

The solid-state reaction of the active pharmaceutical ingredient theophylline with citric acid is a well-established example of a mechanochemical reaction, leading to a model pharmaceutical cocrystal. Here, classical force field molecular dynamics was employed to investigate the molecular mixing and structural distortion that take place on the mechanically driven indentation of a citric acid nanoparticle on a slab of crystalline theophylline. Through non-equilibrium molecular dynamics simulations, a 6 nm diameter nanoparticle of citric acid was introduced onto an open (001) surface of a theophylline crystal, varying both the angle of incidence of the nanoparticle between 15° and 90° and the indentation speed between 1 m s-1 and 16 m s-1. This theoretical study enabled the evaluation of how these two parameters promote molecular mixing and overall structural deformation upon the mechanical contraction of theophylline and citric acid, both of which are important parameters underlying mechanochemical cocrystallisation. The results show that the angle of incidence plays a key role in the molecular transfer ability between the two species and in the structural disruption of the initially spherical nanoparticles. Changing the indentation speed, however, did not lead to a discernible trend in molecular mixing, highlighting the importance of the incident angle in mechanochemical events in the context of supramolecular chemistry, such as the disruption of the crystal structure and molecular transfer between molecular crystals.

Shear-activation of mechanochemical reactions through molecular deformation

Mechanical stress can directly activate chemical reactions by reducing the reaction energy barrier. A possible mechanism of such mechanochemical activation is structural deformation of the reactant species. However, the effect of deformation on the reaction energetics is unclear, especially, for shear stress-driven reactions. Here, we investigated shear stress-driven oligomerization reactions of cyclohexene on silica using a combination of reactive molecular dynamics simulations and ball-on-flat tribometer experiments. Both simulations and experiments captured an exponential increase in reaction yield with shear stress. Elemental analysis of ball-on-flat reaction products revealed the presence of oxygen in the polymers, a trend corroborated by the simulations, highlighting the critical role of surface oxygen atoms in oligomerization reactions. Structural analysis of the reacting molecules in simulations indicated the reactants were deformed just before a reaction occurred. Quantitative evidence of shear-induced deformation was established by comparing bond lengths in cyclohexene molecules in equilibrium and prior to reactions. Nudged elastic band calculations showed that the deformation had a small effect on the transition state energy but notably increased the reactant state energy, ultimately leading to a reduction in the energy barrier. Finally, a quantitative relationship was developed between molecular deformation and energy barrier reduction by mechanical stress.

Reactive Simulations of Silica Functionalization with Aromatic Hydrocarbons

Reactive molecular dynamics simulations are used to model the covalent functionalization of amorphous silica with aromatic hydrocarbons. Simulations show that the surface density of silanol-terminated phenyl, naphthyl, and anthracenyl molecules is lower than the maximum value calculated based on molecule geometry, and the simulation densities decrease faster with the number of aromatic rings than the geometric densities. The trends are analyzed in terms of the surface-silanol bonding configurations, tilt angles, local conformational ordering, and aggregation of surface-bound molecules under steady-state conditions. Results show that the surface density is affected by both the size and symmetry of the aromatic hydrocarbons. The correlations among bonding, orientation, and surface density identified here may guide the selection or design of molecules for functionalized surfaces.

In Situ Nanomechanics: Opportunities Based on Superplastic Nanomolding

In situ nanomechanics, referring to the real-time monitoring of nanomechanical deformation during quantitative mechanical testing, is a key technology for understanding the physical and mechanical properties of nanoscale materials. This perspective reviews the progress of in situ nanomechanics from the aspects of preparation and testing of nanosamples, with a major focus on one-dimensional (1D) nanostructures and discussions of their challenges. We highlight the opportunities provided by in situ nanomechanics combined with the superplastic nanomolding technique, especially in the aspects of regulating physical and chemical properties which are highly exploitable for mechanoelectronics, mechanoluminescence, piezoelectronics, piezomagnetism, piezothermography, and mechanochemistry.

Chemical Patterning on Nanocarbons: Functionality Typewriting

Nanocarbon materials have become extraordinarily compelling for their significant potential in the cutting-edge science and technology. These materials exhibit exceptional physicochemical properties due to their distinctive low-dimensional structures and tailored surface characteristics. An attractive direction at the forefront of this field involves the spatially resolved chemical functionalization of a diverse range of nanocarbons, encompassing carbon nanotubes, graphene, and a myriad of derivative structures. In tandem with the technological leaps in lithography, these endeavors have fostered the creation of a novel class of nanocarbon materials with finely tunable physical and chemical attributes, and programmable multi-functionalities, paving the way for new applications in fields such as nanoelectronics, sensing, photonics, and quantum technologies. Our review examines the swift and dynamic advancements in nanocarbon chemical patterning. Key breakthroughs and future opportunities are highlighted. This review not only provides an in-depth understanding of this fast-paced field but also helps to catalyze the rational design of advanced next-generation nanocarbon-based materials and devices.

Mechanochemical Iron-Catalyzed Nitrene Transfer Reactions: Direct Synthesis of N-Acyl Sulfonimidamides from Sulfinamides and Dioxazolones

A mechanochemical synthesis of sulfonimidamides by iron(II)-catalyzed exogenous ligand-free N-acyl nitrene transfer to sulfinamides is reported. The one-step method tolerates a wide range of sulfinamides with various substituents under solvent-free ambient conditions. Compared to its solution-phase counterpart, this mechanochemical approach shows better conversion and chemoselectivity. Mechanistic investigations by ESI-MS revealed the generation of crucial nitrene iron intermediates.

Two-Legged Molecular Walker and Curvature: Mechanochemical Ring Migration on Graphene

Attaining controllable molecular motion at the nanoscale can be beneficial for multiple reasons, spanning from optoelectronics to catalysis. Here we study the movement of a two-legged molecular walker by modeling the migration of a phenyl aziridine ring on curved graphene. We find that directional ring migration can be attained on graphene in the cases of both 1D (wrinkled/rippled) and 2D (bubble-shaped) curvature. Using a descriptor approach based on graphene's frontier orbital orientation, we can understand the changes in binding energy of the ring as it translates across different sites with variable curvature and the kinetic barriers associated with ring migration. Additionally, we show that the extent of covalent bonding between graphene and the molecule at different sites directly controls the binding energy gradient, propelling molecular migration. Importantly, one can envision such walkers as carriers of charge and disruptors of local bonding. This study enables a new way to tune the electronic structure of two-dimensional materials for a range of applications.

Mechanistic model for quantifying the effect of impact force on mechanochemical reactivity

Conventional mechanochemical synthetic tools, such as ball mills, offer no methodology to quantitatively link macroscale reaction parameters, such as shaking frequency or milling ball radius, to fundamental drivers of reactivity, namely the force vectors applied to the reactive molecules. As a result, although mechanochemistry has proven to be a valuable method to make a wide variety of products, the results are seldom reproduceable between reactors, difficult to rationally optimize, and hard to ascribe to a specific reaction pathway. Here we have developed a controlled force reactor, which is a mechanochemical ball mill reactor with integrated force measurement and control during each impact. We relate two macroscale reactor parameters-impact force and impact time-to thermodynamic and kinetic transition state theories of mechanochemistry utilizing continuum contact mechanics principles. We demonstrate force controlled particle fracture of NaCl to characterize particle size evolution during reactions, and force controlled reaction between anhydrous copper(II) chloride and (1, 10) phenanthroline. During the fracture of NaCl, we monitor the evolution of particle size as a function of impact force and find that particles quickly reach a particle size of ∼100 μm largely independent of impact force, and reach steady state 10-100× faster than reaction kinetics of typical mechanochemical reactions. We monitor the copper(II) chloride reactivity by measuring color change during reaction. Applying our transition state theory developed here to the reaction curves of copper(II) chloride and (1, 10) phenanthroline at multiple impact forces results in an activation energy barrier of 0.61 ± 0.07 eV, distinctly higher than barriers for hydrated metal salts and organic ligands and distinctly lower than the direct cleavage of the CuCl bond, indicating that the reaction may be mediated by the higher affinity of Fe in the stainless steel vessel to Cl. We further show that the results in the controlled force reactor match rudimentary estimations of impact force within a commercial ball mill reactor Retsch MM400. These results demonstrate the ability to quantitatively link macroscale reactor parameters to reaction properties, motivating further work to make mechanochemical synthesis quantitative, predictable, and fundamentally insightful.

Strain-Driven Formal [1,3]-Aryl Shift within Molecular Bows

Delving into the influence of strain on organic reactions in small molecules at the molecular level can unveil valuable insight into developing innovative synthetic strategies and structuring molecules with superior properties. Herein, we present a molecular-strain engineering approach to facilitate the consecutive [1,2]-aryl shift (formal [1,3]-aryl shift) in molecular bows (MBs) that integrate 1,4-dimethoxy-2,5-cyclohexadiene moieties. By introducing ring strain into MBs through tethering the bow limb, we can harness the intrinsic mechanical forces to drive multistep aryl shifts from the para- to the meta- to the ortho-position. Through the use of precise intramolecular strain, the seemingly impractical [1,3]-aryl shift was realized, resulting in the formation of ortho-disubstituted products. The solvent and temperature play a crucial role in the occurrence of the [1,3]-aryl shift. The free energy calculations with inclusion of solvation support a feasible mechanism, which entails multistep carbocation rearrangements, for the formal [1,3]-aryl shift. By exploring the application of molecular strain in synthetic chemistry, this research offers a promising direction for developing new tools and strategies towards precision organic synthesis.