Daisy chain architectures: from discrete molecular entities to polymer materials

Daisy chain architectures, made by the self-complementary threading of an axle covalently linked to a macrocycle, represent a particularly intriguing family of supramolecular and mechanically interlocked (macro)molecules. In this review, we discuss their recent history, their modular chemical structures, and the various synthetic strategies to access them. We also detail how their internal sliding motions can be controlled and how their integration within polymers can amplify that motions up to the macroscopic scale. This overview of the literature demonstrates that the peculiar structure and dynamics of daisy chains have already strongly influenced the research on artificial molecular machines, with the potential to be implemented from nanometric switchable devices to mechanically active soft-matter materials.

Exploration of molecular machines in supramolecular soft robotic systems

Soft robotic system, a new era of material science, is rapidly developing with advanced processing technology in soft matters, featured with biomimetic nature. An important bottom-up approach is through the implementation of molecular machines into polymeric materials, however, the synchronized molecular motions, acumination of strain across multiple length-scales, and amplification into macroscopic actuations remained highly challenging. This review presents the significances, key design strategies, and outlook of the hierarchical supramolecular systems of molecular machines to develop novel types of supramolecular-based soft robotic systems.

Mechanically interlocked polymers based on rotaxanes

The nature of mechanically interlocked molecules (MIMs) has continued to encourage researchers to design and construct a variety of high-performance materials. Introducing mechanically interlocked structures into polymers has led to novel polymeric materials, called mechanically interlocked polymers (MIPs). Rotaxane-based MIPs are an important class, where the mechanically interlocked characteristic retains a high degree of structural freedom and mobility of their components, such as the rotation and sliding motions of rotaxane units. Therefore, these MIP materials are known to possess a unique set of properties, including mechanical robustness, adaptability and responsiveness, which endow them with potential applications in many emerging fields, such as protective materials, intelligent actuators, and mechanisorption. In this review, we outline the synthetic strategies, structure-property relationships, and application explorations of various polyrotaxanes, including linear polyrotaxanes, polyrotaxane networks, and rotaxane dendrimers.

Post-Synthetic Macrocyclization of Rotaxane Building Blocks

Although not often encountered, cyclic interlocked molecules are appealing molecular targets because of their restrained tridimensional structure which is related to both the cyclic and interlocked shapes. Interlocked molecules such as rotaxane building blocks may be good candidates for post-synthetic intramolecular cyclization if the preservation of the mechanical bond ensures the interlocked architecture throughout the reaction. This is obviously the case if the modification does not involve the cleavage of either the macrocycle's main chain or the encircled part of the axle. However, among the post-synthetic reactions, the chemical linkage between two reactive sites belonging to embedded elements of rotaxanes still consists of an underexploited route to interlocked cyclic molecules. This Review lists the rare examples of macrocyclization through chemical connection between reactive sites belonging to a surrounding macrocycle and/or an encircled axle of interlocked rotaxanes.

An Asymmetric Mechanism in a Symmetric Molecular Machine

The design of molecular architectures exhibiting functional motions is a promising area for disruptive technological development. Toward this goal, rotaxanes and catenanes, which undergo relative motions of their subunits in response to external stimuli, are prime candidates. Here, we report on the computational analysis of the contraction/extension of a bistable [c2]daisy chain rotaxane. Using free-energy calculations and transition path optimizations, we explore the free-energy landscape governing the functional motions of a prototypical molecular machine with atomic resolution. The calculations reveal a sequential mechanism in which the asynchronous gliding of each ring is preferred over the concerted movement. Analysis of the underlying free-energy surface indicates that the formation of partially rearranged intermediates entails crossing of much smaller barriers. Our findings illustrate an important design principle for molecular machines, namely that efficient exploitation of thermal fluctuations may be realized by breaking down the large-scale functional motions into smaller steps.

Design of Collective Motions from Synthetic Molecular Switches, Rotors, and Motors

Precise control over molecular movement is of fundamental and practical importance in physics, biology, and chemistry. At nanoscale, the peculiar functioning principles and the synthesis of individual molecular actuators and machines has been the subject of intense investigations and debates over the past 60 years. In this review, we focus on the design of collective motions that are achieved by integrating, in space and time, several or many of these individual mechanical units together. In particular, we provide an in-depth look at the intermolecular couplings used to physically connect a number of artificial mechanically active molecular units such as photochromic molecular switches, nanomachines based on mechanical bonds, molecular rotors, and light-powered rotary motors. We highlight the various functioning principles that can lead to their collective motion at various length scales. We also emphasize how their synchronized, or desynchronized, mechanical behavior can lead to emerging functional properties and to their implementation into new active devices and materials.

[c2]Daisy Chain Rotaxanes as Molecular Muscles

Bistable [ c2]daisy chain rotaxanes represent a particularly intriguing class of interlocked molecules that can produce internal sliding movements with a net contraction or extension at the single-molecule level. These nanometric motions show some analogies with the sliding motions of actin and myosin filaments in sarcomeres, and this is why [ c2]daisy chain rotaxanes have been also named as “molecular muscles,” as their first synthesis in 2000. In this minireview, the authors discuss the recent history of these molecules, their modular chemical structures, and the various synthetic pathways described in the literature to access them. The authors also detail how their internal motions can be controlled and characterized by a number of chemical and physical tools. The authors finally show that their integration within polymers and materials can give access to synchronized motions and amplifications up to the macroscopic scale. Overall, the numerous examples that have been described in the literature to date demonstrate that this family of molecules has already strongly influenced the entire field of research on artificial molecular machines, and has the potential to be implemented as actuators working at all scales, from nanometric-switchable devices to mechanically active soft matter materials.

Fragment motion in motor molecules: basic concepts and application to intra-molecular rotations

Torque directions T_ref along the CC backbone of stilbene used in fragment motion analysis, ground (GS) and excited state (ES).

Integration of molecular machines into supramolecular materials: actuation between equilibrium polymers and crystal-like gels

In this article, the dynamic structure of complex supramolecular polymers composed of bistable [c2]daisy chain rotaxanes as molecular machines that are linked by ureidopyrimidinones (Upy) as recognition moieties was studied. pH actuation of the integrated mechanically active rotaxanes controls the contraction/extension of the polymer chains as well as their physical reticulation. Small-angle neutron and X-ray scattering were used to study in-depth the nanostructure of the contracted and extended polymer aggregates in toluene solution. The supramolecular polymers comprising contracted nanomachines were found to be equilibrium polymers with a mass that is concentration dependent in dilute and semidilute regimes. Surprisingly, the extended polymers form a gel network with a crystal-like internal structure that is independent of concentration and reminiscent of a pearl-necklace network.

Controlled Sol-Gel Transitions by Actuating Molecular Machine Based Supramolecular Polymers

The implementation of artificial molecular machines in polymer science is an important objective that challenges chemists and physicists in order to access an entirely new class of smart materials. To design such systems, the amplification of a mechanical actuation from the nanoscale up to a macroscopic response in the bulk material is a central issue. In this article we show that bistable [c2]daisy chain rotaxanes (i.e., molecular muscles) can be linked into main-chain Upy-based supramolecular polymers. We then reveal by an in depth quantitative study that the pH actuation of the mechanically active rotaxane at the nanoscale influences the physical reticulation of the polymer chains by changing the supramolecular behavior of the Upy units. This nanoactuation within the local structure of the main chain polymer results in a mechanically controlled sol-gel transition at the macroscopic level.

Successive Translocation of the Rings in a [3]Rotaxane

A [2]rotaxane, a [3]rotaxane and the corresponding thread containing two succinamide (succ) binding stations and a central redox-active pyromellitimide (pmi) station were studied. Infrared spectroelectrochemical experiments revealed the translocation of the macrocycle between the succinamide station and the electrochemically reduced pmi station (radical anion and dianion). Remarkably, in the [3]rotaxane, the rings can be selectively translocated. One-electron reduction leads to the translocation of one of the two macrocycles from the succinamide to the pyromellitimide station, whereas activation of the shuttle through two-electron reduction results in the translocation of both macrocycles: the dianion, due to its higher electron density and hence greater hydrogen-bond accepting affinity, is hydrogen bonded to both macrocycles. Systems with such an on-command contraction are known as molecular muscles. The relative strengths of the binding between the macrocycle and the imide anions could be estimated from the hydrogen-bond-induced shifts in the C=O stretching frequencies of hydrogen-bond accepting amide groups of the macrocycle.

Computational prediction of optimal metal ions to induce coordinated polymerization of muscle-like [c2]daisy chains

Schematic representations of the polymerized [c2]daisy chain linked with Fe²⁺ in contracted vs. elongated forms by computational simulation.

Hierarchical Self-Assembly of Supramolecular Muscle-Like Fibers

An acid-base switchable [c2]daisy chain rotaxane terminated with two 2,6-diacetylamino pyridine units has been self-assembled with a bis(uracil) linker. The complementary hydrogen-bond recognition patterns, together with lateral van der Waals aggregations, result in the hierarchical formation of unidimensional supramolecular polymers associated in bundles of muscle-like fibers. Microscopic and scattering techniques reveal that the mesoscopic structure of these bundles depends on the extended or contracted states that the rotaxanes show within individual polymer chains. The observed local dynamics span over several length scales because of a combination of supramolecular and mechanical bonds. This work illustrates the possibility to modify the hierarchical mesoscopic structuring of large polymeric systems by the integrated actuation of individual molecular machines.