Force-controlled release of small molecules with a rotaxane actuator.

Force-controlled release of small molecules offers great promise for the delivery of drugs and the release of healing or reporting agents in a medical or materials context1-3. In polymer mechanochemistry, polymers are used as actuators to stretch mechanosensitive molecules (mechanophores)4. This technique has enabled the release of molecular cargo by rearrangement, as a direct5,6 or indirect7-10 consequence of bond scission in a mechanophore, or by dissociation of cage11, supramolecular12 or metal complexes13,14, and even by 'flex activation'15,16. However, the systems described so far are limited in the diversity and/or quantity of the molecules released per stretching event1,2. This is due to the difficulty in iteratively activating scissile mechanophores, as the actuating polymers will dissociate after the first activation. Physical encapsulation strategies can be used to deliver a larger cargo load, but these are often subject to non-specific (that is, non-mechanical) release3. Here we show that a rotaxane (an interlocked molecule in which a macrocycle is trapped on a stoppered axle) acts as an efficient actuator to trigger the release of cargo molecules appended to its axle. The release of up to five cargo molecules per rotaxane actuator was demonstrated in solution, by ultrasonication, and in bulk, by compression, achieving a release efficiency of up to 71% and 30%, respectively, which places this rotaxane device among the most efficient release systems achieved so far1. We also demonstrate the release of three representative functional molecules (a drug, a fluorescent tag and an organocatalyst), and we anticipate that a large variety of cargo molecules could be released with this device. This rotaxane actuator provides a versatile platform for various force-controlled release applications.

Pushing vs Pulling: The Unique Geometry of Mechanophore Activation in a Rotaxane Force Actuator.

Mechanophores (mechanosensitive molecules) are usually activated by pulling them with covalently attached polymers. A rotaxane actuator offers a new geometry of activation as the macrocycle pushes against a stoppering mechanophore. Here we compare both pulling and pushing activations and show that pushing is more efficient and selective than pulling. We found that the pulling activation of a bulky furan/maleimide adduct occurs via two competing dissociation pathways: retrocycloaddition and heterolytic cleavage (generating a trityl cation in the process), while the same adduct only cleaves by retrocycloaddition during pushing activation. These results further demonstrate the efficacy and versatility of rotaxane actuators.

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

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

Cooperative and Geometry-Dependent Mechanochromic Reactivity through Aromatic Fusion of Two Rhodamines in Polymers.

The unique topological features of Piezo proteins underlie the lever-like cellular mechanotransduction mechanism. This knowledge inspires us to seek topological/geometric control of mechanochromophores with unprecedentedly amplified, synergistic changes in polymers to serve as ideal stress probes. Here, by judicious placement of two spirolactam rings into aminobenzopyranoxanthene, a series of stereo- and regio-isomeric rhodamine-like mechanophores are developed. With two labile bonds closely coupled into one rigidified scaffold, these π-fused bis-mechanophores enable mechanochromic polymers, featuring cooperative bond scission, low rupture force (lower than rhodamine), and geometry-controlled ring-opening reactivity. Sonication, single-molecule force spectroscopy experiments, and density functional theory calculations provide insight into the force-color relationship and rationalize how the difference in reactivity of the four isomeric mechanophores is affected by their molecular geometry and thermodynamic equilibrium. Our strategy based on the aromatic fusion of bis-mechanophore promises a modular approach to isomeric mechanophores for cooperative bond scission. Also, important insights into internal and external factors governing tandem mechanochemical reactions are gained.

Isotope Effect in the Activation of a Mechanophore.

In mechanochemistry, molecules under tension can react in unexpected ways. The reactivity of mechanophores (mechanosensitive molecules) can be controlled using various geometric or electronic factors. Often these factors affect the rate of mechanical activation but sometimes give rise to alternative reaction pathways. Here we show that a simple isotope substitution (H to D) leads to a reversal of selectivity in the activation of a mechanophore. Remarkably this isotope effect is not kinetic in nature but emerges from dynamic effects in which deuteration reduces the ability of the reactant to follow a post-transition-state concerted trajectory on the bifurcated force-modified potential energy surface. These results give a new insight into the reactivity of molecules under tension.

A Mechanochromic Hydrogen-Bonded Rotaxane.

Tensile forces influence a variety of important biological processes and force sensors are required to study these processes in vivo. Current force sensors are often tailor-made for a specific application, or activate at much higher forces than those observed at the cellular or tissue level. A versatile force sensor, with tunable mechanical and optical properties, activated at low pN forces will be ideal. In this communication, a new mechanoresponsive fluorescent hydrogen-bonded rotaxane, built around a maleimide dye, is reported. Its force-sensing properties are demonstrated in a polyacrylamide gel, a synthetic model of living tissue.

A Catenane as a Mechanical Protecting Group.

Mechanophores (mechanoresponsive molecules) offer great promises for the development of smart force-responsive materials. The activity of a mechanophore can be tuned by altering its structure or the composition of the actuating polymer. Here we show that a [2]catenane can act as a mechanical protecting group by diverting tensional forces away from a mechanically active functional group embedded in one of its rings. This property emerges from the mobility of the two rings of the catenane, which are able to rotate along each other until the tension equalizes over the entirety of the catenated framework. This approach provides a new way to control the mechanical activity of a mechanophore.

Expedient Synthesis of Heterobifunctional Triarylmethane Stoppers for Macromolecular Rotaxanes.

Increasingly complex rotaxane-based molecular devices are interfaced with polymers and surfaces, but suitable bifunctional stoppering groups are lacking. Here, we report a two-step, high-yielding synthesis toward a new class of heterobifunctional triarylmethane stoppers. They possess hydroxyl and ester groups for further functionalization as well as halogen substituents conferring a diagnostic spectroscopic signature. Their utility was demonstrated with the synthesis of a chain-centered macromolecular rotaxane. This new stopper architecture should prove useful to connect rotaxanes with polymers and surfaces for applications in polymer mechanochemistry, single-molecule force spectroscopy, smart materials, and molecular machines.

Mechanical Susceptibility of a Rotaxane.

We have investigated the mechanical dissociation of an ammonium/crown ether rotaxane using experimental (sonication) and computational (CoGEF) methods and found that it breaks faster than its noninterlocked or uncoupled interlocked (i.e., pulled from both sides of the axle) counterparts. This was confirmed by the analysis of the fragments, which are the results of a selective unstoppering reaction. Interestingly, the initial dissociation also triggered the elimination of the axle segment separating the stopper from the ammonium binding station. CoGEF calculations have shown that the constriction of the axle by the macrocycle during the elongation of the rotaxane provokes the accumulation of tensile and torsional stress that ultimately leads to the rupture of a covalent bond in the constricted section of the axle. Overall, these results suggest that the rotaxane architecture acts as a lever that accelerates the dissociation of interlocked covalent bonds. This phenomenon could impact the mechanical properties of slide-ring materials at high strain.

Impact of a Mechanical Bond on the Activation of a Mechanophore.

Mechanical bonds are known to efficiently absorb mechanical energy at low forces, but their behavior at high forces is unknown. Here we investigate the impact of a mechanical bond on the rate of activation of a Diels-Alder mechanophore. Using a combination of experimental and computational techniques, we found that the rate of a retro-Diels-Alder reaction under tension is decreased when the mechanophore is embedded in the axle of a rotaxane due to the presence of a competing high-stress region at the junction between the macrocycle and the axle.

Controlling Reactivity by Geometry in Retro-Diels-Alder Reactions under Tension.

Mechanical force, with its ability to distort, bend, and stretch chemical bonds, is unique in the way it activates chemical reactions. In polymer mechanochemistry, the force is transduced in a directional fashion, and the efficiency of activation depends on how well the force is transduced from the polymer to the scissile bond in the mechanophore (i.e., mechanochemical coupling). We have investigated the effects of regio- and stereochemistry on the rate of force-accelerated retro-Diels-Alder reactions of furan/maleimide adducts. Four adducts, presenting an endo or exo configuration and proximal or distal geometry, were activated in solution by ultrasound-generated elongational forces. A combination of structural (1H NMR) and computational (CoGEF) analyses allowed us to interrogate the mechanochemical activation of these adducts. We found that, unlike its thermal counterpart where the reactivity is dictated by the stereochemistry, the mechanical reactivity is mainly dependent on the regiochemistry. Remarkably, the thermally active distal-exo adduct becomes inert under tension due to poor mechanochemical coupling.

[2]Rotaxane Formation by Transition State Stabilization.

We report on the synthesis of [2]rotaxanes driven by stabilization of the axle-forming transition state. A bifunctional macrocycle, with hydrogen bond donors at one end and acceptors at the other, is used to stabilize the charges that develop during the addition of a primary amine to a cyclic sulfate.

Sequence-Specific ß-Peptide Synthesis by a Rotaxane-Based Molecular Machine.

We report on the synthesis and operation of a three-barrier, rotaxane-based, artificial molecular machine capable of sequence-specific ß-homo (ß3) peptide synthesis. The machine utilizes nonproteinogenic ß3-amino acids, a class of amino acids not generally accepted by the ribosome, particularly consecutively. Successful operation of the machine via native chemical ligation (NCL) demonstrates that even challenging 15- and 19-membered ligation transition states are suitable for information translation using this artificial molecular machine. The peptide-bond-forming catalyst region can be removed from the transcribed peptide by peptidases, artificial and biomachines working in concert to generate a product that cannot be made by either machine alone.

Efficient assembly of threaded molecular machines for sequence-specific synthesis.

We report on an improved strategy for the preparation of artificial molecular machines that can pick up and assemble reactive groups in sequence by traveling along a track. In the new approach a preformed rotaxane synthon is attached to the end of an otherwise fully formed strand of building blocks. This "rotaxane-capping" protocol is significantly more efficient than the "final-step-threading" method employed previously and enables the synthesis of threaded molecular machines that operate on extended oligomer, and potentially polymer, tracks. The methodology is exemplified through the preparation of a machine that adds four amino acid building blocks from a strand in sequence, featuring up to 20-membered ring native chemical ligation transition states.

Mechanical scission of a knotted polymer.

Molecular knots and entanglements form randomly and spontaneously in both biological and synthetic polymer chains. It is known that macroscopic materials, such as ropes, are substantially weakened by the presence of knots, but until now it has been unclear whether similar behaviour occurs on a molecular level. Here we show that the presence of a well-defined overhand knot in a polymer chain substantially increases the rate of scission of the polymer under tension (≥2.6× faster) in solution, because deformation of the polymer backbone induced by the tightening knot activates otherwise unreactive covalent bonds. The fragments formed upon severing of the knotted chain differ from those that arise from cleavage of a similar, but unknotted, polymer. Our solution studies provide experimental evidence that knotting can contribute to higher mechanical scission rates of polymers. It also demonstrates that entanglement design can be used to generate mechanophores that are among the most reactive described to date, providing opportunities to increase the reactivity of otherwise inert functional groups.

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

[This corrects the article DOI: 10.1039/D2SC05051J.].

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

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

Three concomitant C-C dissociation pathways during the mechanical activation of an N-heterocyclic carbene precursor.

Chemical reactions usually proceed through a radical, concerted or ionic mechanism; transformations in which all three mechanisms occur are rare. In polymer mechanochemistry, a mechanical force, transduced along polymer chains, is used to activate covalent bonds in mechanosensitive molecules (mechanophores). Cleavage of a C-C bond often follows a homolytic pathway, but some mechanophores have also been designed that react in a concerted or, more rarely, a heterolytic manner. Here, using 1H- and 19F-nuclear magnetic resonance spectroscopy in combination with deuterium labelling, we show that the dissociation of a mechanophore built around an N-heterocyclic carbene precursor proceeds with the rupture of a C-C bond through concomitant heterolytic, concerted and homolytic pathways. The distribution of products probably arises from a post-transition-state bifurcation in the reaction pathway, and their relative proportion is dictated by the polarization of the scissile C-C bond.