Dual‐Controlled Macroscopic Motions in a Supramolecular Hierarchical Assembly of Motor Amphiphiles

Supramolecular assemblies of amphiphilic donor-acceptor Stenhouse adducts as macroscopic soft scaffolds

In the design of photoharvesting and photoresponsive supramolecular systems in aqueous medium, the fabrication of amphiphilic photoswitches enables a noninvasive functional response through photoirradiation. Although most aqueous supramolecular assemblies are driven by high-energy and biodamaging UV light, we have previously reported a design of amphiphilic donor-acceptor Stenhouse adducts (DASAs) controlled by white light. Herein, we present a series of DASA amphiphiles (DAs) with minor structural modifications on the alkyl linker chain length connecting the DASA motif with the hydrophilic moiety. The excellent photoswitchability in organic medium and the photoresponsiveness in aqueous medium, driven by visible light, were investigated by UV-vis absorption spectroscopy. The assembled supramolecular nanostructures were confirmed by electron microscopy, while the supramolecular packing was revealed by X-ray diffraction analysis. Upon visible-light irradiation, significant transformations of the DA geometry enabled transformations of the supramolecular assemblies on a microscopic scale, subsequently disassembling macroscopic soft scaffolds of DAs. The current work shows promising use for the fabrication of visible-light-controlled macroscopic scaffolds, offering the next generation of biomedical materials with visible-light-controlled microenvironments and future soft-robotic systems.

Aqueous Supramolecular Transformations of Motor Bola-Amphiphiles at Multiple Length-Scale

Molecular motor amphiphiles have already been widely attempted for dynamic nanosystems across multiple length-scale for developments of small functional materials, including controlling macroscopic foam properties, amplifying motion as artificial molecular muscles, and serving as extracellular matrix mimicking cell scaffolds. However, limiting examples of bola-type molecular motor amphiphiles are considered for constructing macroscopic biomaterials. Herein, this work presents the designed two second generation molecular motor amphiphiles, motor bola-amphiphiles (MBAs). Aside from the photoinduced motor rotation of MBAs achieved in both organic and aqueous media, the rate of recovering thermal helix inversion step can be controlled by the rotor part with different steric hindrances. Dynamic assembled structures of MBAs are observed under (cryo)-transmission electron microscopy (TEM). This dynamicity assists MBAs in further assembling as macroscopic soft scaffolds by applying a shear-flow method. Upon photoirradiation, the phototropic bending function of MBA scaffolds is observed, demonstrating the amplification of molecular motion into macroscopic phototropic bending functions at the macroscopic length-scale. Since MBAs are confirmed with low cytotoxicity, human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can grow on the surface of MBA scaffolds. These results clearly demonstrate the concept of designing MBAs for developing photoresponsive dynamic functional materials to create new-generation soft robotic systems and cell-material interfaces.

Red-light-controlled supramolecular assemblies of indigo amphiphiles at multiple length scales

Amphiphilic molecules functionalized with photoresponsive motifs have attractive prospects for applications in smart functional bio-material ranging from cell-material interfaces to drug delivery systems owing to the precisely controllable functionality of self-assembled hierarchical supramolecular structures in aqueous media by a non-invasive light stimulation with high temporal- and spatial-resolution. However, most of reported photoresponsive amphiphiles are triggered by bio-damaging UV-light, which greatly limits the potential in bio-related applications. Herein, we present newly designed red-light controlled N,N'-diaryl-substituted indigo amphiphiles (IA), exhibiting excellent photoswitchablity and photostability with dual red-/green-light in organic media. Meanwhile, aqueous solutions of IA assembled into supramolecular structures in both microscopic and macroscopic length-scale, though the photoresponsiveness of IA is slightly compromised in aqueous media. At macroscopic length-scale, morphological changes of IA macroscopic scaffold prepared by a shear-flow method can be fine adjusted upon red-light irradiation. Moreover, the preferential attachment of live h-MSCs to IA macroscopic scaffold surface also indicates a good biocompatibility of IA macroscopic scaffold. These results provide the potential for developing the next generation of red-light controlled soft functional materials with good biocompatibility.

Metal Cross-Linked Supramolecular Gel Noodles: Structural Insights and Antibacterial Assessment

Achieving precise control over gelator alignment and morphology is crucial for crafting tailored materials and supramolecular structures with distinct properties. We successfully aligned the self-assembled micelles formed by a functionalized dipeptide 2NapFF into long 1-D "gel noodles" by cross-linking with divalent metal chlorides. We identify the most effective cross-linker for alignment, enhancing mechanical stability, and imparting functional properties. Our study shows that Group 2 metal ions are particularly suited for creating mechanically robust yet flexible gel noodles because of their ionic and nondirectional bonding with carboxylate groups. In contrast, the covalent nature and high directional bonds of d-block metal ions with carboxylates tend to disrupt the self-assembly of 2NapFF. Furthermore, the 2NapFF-Cu noodles demonstrated selective antibacterial activity, indicating that the potent antibacterial property of the copper(II) ion is preserved within the cross-linked system. By merging insights into molecular alignment, gel extrusion processing, and integrating specific functionalities, we illustrate how the versatility of dipeptide-based gels can be utilized in creating next-generation soft materials.

Light-Triggered Disassembly of Molecular Motor-based Supramolecular Polymers Revealed by High-Speed AFM

Photoresponsive supramolecular polymers have a major potential for applications in responsive materials that are externally triggered by light with spatio-temporal control of their polymerisation state. While changes in macroscopic properties revealed the adaptive nature of these materials, it remains challenging to capture the dynamic depolymerisation process at the molecular level, which requires fast observation techniques combined with in situ irradiation. By implementing in situ UV illumination into a High-Speed Atomic Force Microscope (HS-AFM) setup, we have been able to capture the disassembly of a light-driven molecular motor-based supramolecular polymer. The real-time visualisation of the light-triggered disassembly process not only reveals cooperative depolymerisation, it also shows that this process continues after illumination is halted. Combining the data with cryo-electron microscopy and spectroscopy approaches, we obtain a molecular-level description of the motor-based polymer dynamics reminiscent of actin chain-end depolymerisation. Our detailed understanding of supramolecular depolymerisation will drive the development of future responsive polymer systems.

Photo-responsive functional materials based on light-driven molecular motors

In the past two decades, the research and development of light-triggered molecular machines have mainly focused on developing molecular devices at the nanoscale. A key scientific issue in the field is how to amplify the controlled motion of molecules at the nanoscale along multiple length scales, such as the mesoscopic or the macroscopic scale, or in a more practical perspective, how to convert molecular motion into changes of properties of a macroscopic material. Light-driven molecular motors are able to perform repetitive unidirectional rotation upon irradiation, which offers unique opportunities for responsive macroscopic systems. With several reviews that focus on the design, synthesis and operation of the motors at the nanoscale, photo-responsive macroscopic materials based on light-driven molecular motors have not been comprehensively summarized. In the present review, we first discuss the strategy of confining absolute molecular rotation into relative rotation by grafting motors on surfaces. Secondly, examples of self-assemble motors in supramolecular polymers with high internal order are illustrated. Moreover, we will focus on building of motors in a covalently linked system such as polymeric gels and polymeric liquid crystals to generate complex responsive functions. Finally, a perspective toward future developments and opportunities is given. This review helps us getting a more and more clear picture and understanding on how complex movement can be programmed in light-responsive systems and how man-made adaptive materials can be invented, which can serve as an important guideline for further design of complex and advanced responsive materials.

Tailoring Multicontrolled Supramolecular Assemblies of Stiff-Stilbene Amphiphiles into Macroscopic Soft Scaffolds as Cell-Material Interfaces

Biocompatible synthetic supramolecular systems have shed light on biomedical and tissue-regenerative material applications. The intrinsic functional applicability, tunability, and stimuli-responsiveness of synthetic supramolecular systems allow one to develop various multicontrolled supramolecular assemblies in aqueous media. However, it remains highly challenging to use state-of-the-art supramolecular assemblies of photoresponsive amphiphiles controlled by multiple stimulations in fabricating macroscopic materials. Herein, we demonstrate a stiff-stilbene amphiphile (SA) multicontrolled supramolecular assembling system that comprises two different charged end groups. The excellent photoswitchabilities of SA in both organic and aqueous media are demonstrated. Furthermore, multiple stimuli, i.e., light, pH, and counterions, are applied to control the supramolecular assembling behaviors, which are monitored by circular dichroism spectroscopy and electron microscopies. This multicontrolled supramolecular system can be systematically assembled into macroscopic soft functional scaffolds, whose structural parameters are investigated by electron microscopies and X-ray diffraction techniques, suggesting the large aspect ratio of SA nanostructures assembled into macroscopic soft scaffolds. The fabricated soft functional scaffold is highly biocompatible for photocontrolled biotarget encapsulation/release selectively, as well as a cell-material interface for diverse cells' attachment. This new synthetic multicontrolled soft functional material provides a new strategy toward the development of next-generation controllable and biocompatible soft functional materials.

Supramolecular Gel-to-Gel Transition Induced by Nanoscale Structural Perturbation via the Rotary Motion of Feringa's Motor

Supramolecular rather than covalent molecular engineering on Feringa motors can provide an alternative toolkit for tuning the properties of motorized materials through appropriate supramolecular structural perturbations, which are underexplored. Herein, a multicomponent supramolecular gel system is successfully prepared by employing an ultra-low molecular weight gelator and a modulator-Feringa motor. The electron microscopic, spectroscopic, and rheological data revealed that the morphology and mechanical properties of the gel can be tuned via a crystallographic mismatch branching (CMB) mechanism simply by adding varied amounts of motor modulators. Notably, the rotary motion of the motor is preserved in such a multicomponent gel system, and the morphology and rheology of the gel can be further altered by the motor's rotary motion that promotes the structural perturbation, resulting in seldomly seen gel-to-gel transition events. The work shown here offers prospects to utilize a supramolecular perturbation strategy to deliver responsiveness from molecular motors to the corresponding bulk 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.

Photoresponsive Supramolecular Polymers: From Light-Controlled Small Molecules to Smart Materials

Photoresponsive supramolecular polymers are well-organized assemblies based on highly oriented and reversible noncovalent interactions containing photosensitive molecules as (co-)monomers. They have attracted increasing interest in smart materials and dynamic systems with precisely controllable functions, such as light-driven soft actuators, photoresponsive fluorescent anticounterfeiting and light-triggered electronic devices. The present review discusses light-activated molecules used in photoresponsive supramolecular polymers with their main photo-induced changes, e.g., geometry, dipole moment, and chirality. Based on these distinct changes, supramolecular polymers formed by light-activated molecules exhibit photoresponsive disassembly and reassembly. As a consequence, photo-induced supramolecular polymerization, "depolymerization," and regulation of the lengths and topologies are observed. Moreover, the light-controlled functions of supramolecular polymers, such as actuation, emission, and chirality transfer along length scales, are highlighted. Furthermore, a perspective on challenges and future opportunities is presented. Besides the challenge of moving from harmful UV light to visible/near IR light avoiding fatigue, and enabling biomedical applications, future opportunities include light-controlled supramolecular actuators with helical motion, light-modulated information transmission, optically recyclable materials, and multi-stimuli-responsive supramolecular systems.

Sterically Hindered Stiff-Stilbene Photoswitch Offers Large Motions, 90% Two-Way Photoisomerization, and High Thermal Stability

Molecular photoswitches have been widely used as molecular machines in various fields due to the small structures and simple motions generated in reversible isomerization. However, common photoswitches, as represented by azobenzene (AB), cannot combine both large motions and high thermal stability, which are critically important for some practical applications in addition to high photoisomerization yields. Here, we focus on a promising photoswitch, stiff stilbene (SS), and its derivative, sterically hindered SS (HSS). The detailed investigation of their performance with a comparison to AB demonstrated that HSS is an outstanding photoswitch offering larger motions than AB and SS, ca. 90% photoisomerization in both E-to-Z and Z-to-E directions, and significantly high thermal stability with a half-life of ca. 1000 years at room temperature. The superior performance of HSS promises its use in various applications, even where previous photoswitches have troubles and are unavailable.

Visible-light controlled supramolecular transformations of donor-acceptor Stenhouse adducts amphiphiles at multiple length-scale

Designing responsive, adaptive, and dynamic supramolecular systems in water, the incorporation of photoresponsive units in amphiphilic molecular structures enables functional responses in a non-invasive way by using light. However, in aqueous media, vast majority of reported synthetic photoresponsive molecular amphiphiles are commonly driven by high energy and bio-damaging UV-light for supramolecular transformation at multiple length-scale. Herein, we present newly designed visible-light controlled supramolecular assembly of donor-acceptor Stenhouse adducts amphiphiles (DA) with excellent stability and solubility in aqueous media. The excellent photoswitchability in organic media and photoresponsiveness in aqueous media driven by visible-light are found, as confirmed with UV-vis absorption and NMR spectroscopies. Supramolecular assembly at multiple length-scale of DAs is investigated with electron microscopies and X-ray diffraction to show large aspect-ratio of nanostructures assembled into macroscopic soft scaffolds. Upon visible-light irradiation, the large geometrical transformation of DAs enables supramolecular transformations, and subsequently destabilizes the macroscopic soft scaffold to release fluorophores from the scaffolds. These results provide the feasibility in developing the next generation of visible-light controlled macroscopic soft functional scaffold from supramolecular assembly across multiple length-scale without and offer ample opportunity to design future soft robotic materials and functional biomaterials.

Aqueous Supramolecular Co-Assembly of Anionic and Cationic Photoresponsive Stiff-Stilbene Amphiphiles

Fabrication of macroscopic soft functional materials, such as macroscopic photoresponsive soft materials and artificial muscles, can be commonly prepared by charge screening of supramolecular assemblies with inorganic salt solutions using a shear-flow method. However, some of the charged end-groups of photoresponsive molecular amphiphiles cannot be stabilized with inorganic salt solutions to fabricate macroscopic soft materials. We design and synthesize stiff stilbene amphiphiles (SAs) functionalized with anionic phosphite and cationic quaternary ammonium end groups and determine their photochemical and supramolecular assembly properties. Supramolecular co-assembly of anionic and cationic nanotubes of SAs allows to transform into nanoribbons, confirmed by TEM, CAC, and Zeta potential measurements. Nanoribbons of anionic and cationic SAs can be prepared into macroscopic soft materials with inorganic salt solutions and surprisingly also with deionized water. The macroscopic soft material of anionic and cationic SAs can be stabilized at low concentration ∼5 mM. Meanwhile, the photoresponsiveness of the macroscopic soft materials is retained to provide macroscopic morphological change upon photoirradiation. These results exhibit the feasibility in fabrication of macroscopic functional soft materials from supramolecular assembly across multiple length-scale without help of inorganic salts and offer ample opportunity in developing future soft supramolecular robotic systems. This article is protected by copyright. All rights reserved.

Controlling the shape morphology of origami-inspired photoresponsive hydrogels

The concept of origami has influenced the development of responsive materials that can mimic complex functions performed by living organisms. An ultimate goal is to discover and design soft materials that can be remotely actuated into diverse structures. To achieve this goal, we design and synthesize here a light-responsive spiropyran hydrogel system that can display dynamic shape changes upon irradiation with local light. We use a continuum polymer model to analyze the behavior of the constructed photoactive hydrogel, which is in good agreement with the experimental results. We explore different buckling modalities and patterns in a different range of parameters. The synthesis and fabrication of these materials demonstrate that the theoretical model can be used to drive the development of responsive photoactive systems.

Photoactuating Artificial Muscles of Motor Amphiphiles as an Extracellular Matrix Mimetic Scaffold for Mesenchymal Stem Cells

Mimicking the native extracellular matrix (ECM) as a cell culture scaffold has long attracted scientists from the perspective of supramolecular chemistry for potential application in regenerative medicine. However, the development of the next-generation synthetic materials that mimic key aspects of ECM, with hierarchically oriented supramolecular structures, which are simultaneously highly dynamic and responsive to external stimuli, remains a major challenge. Herein, we present supramolecular assemblies formed by motor amphiphiles (MAs), which mimic the structural features of the hydrogel nature of the ECM and additionally show intrinsic dynamic behavior that allow amplifying molecular motions to macroscopic muscle-like actuating functions induced by light. The supramolecular assembly (named artificial muscle) provides an attractive approach for developing responsive ECM mimetic scaffolds for human bone marrow-derived mesenchymal stem cells (hBM-MSCs). Detailed investigations on the photoisomerization by nuclear magnetic resonance and UV–vis absorption spectroscopy, assembled structures by electron microscopy, the photoactuation process, structural order by X-ray diffraction, and cytotoxicity are presented. Artificial muscles of MAs provide fast photoactuation in water based on the hierarchically anisotropic supramolecular structures and show no cytotoxicity. Particularly important, artificial muscles of MAs with adhered hBM-MSCs still can be actuated by external light stimulation, showing their ability to convert light energy into mechanical signals in biocompatible systems. As a proof-of-concept demonstration, these results provide the potential for building photoactuating ECM mimetic scaffolds by artificial muscle-like supramolecular assemblies based on MAs and offer opportunities for signal transduction in future biohybrid systems of cells and MAs.

Coordination-directed self-assembly of molecular motors: towards a two-wheel drive nanocar

Designing and constructing hierarchical and stimuli-responsive motorized nanocar systems to perform useful tasks on-demand is highly imperative towards molecular nanotechnology. In this work, a most simplified two-wheel nanocar was successfully prepared through a facile strategy of coordination-directed self-assembly. The nanocar meso-AgL2 features a central pseudo square-planar Ag(I) which was bridged by two enantiomeric motors as the wheels that ensure the car moves in the same direction when observed externally. Thanks to the electronic push-pull characteristic of L and ³ILCT triplet sensitization, this nanocar can be driven by visible light up to 500 nm. Furthermore, it could be disassembled into individual motor elements through the addition of pyridine, thus allowing dynamic regulation over the function of the nanocar. Importantly, our STM imaging results showed very organized tilted layered structures for meso-AgL2 on highly oriented pyrolytic graphite (HOPG) that are quite similar to its crystalline ones, paving the way for future single molecule manipulations. The nanocar reported here represents the first example of integrating individual motors into a hierarchical motorized nanocar system via the facile coordination-directed self-assembly method and may offer a good starting point to realize its robotic functions, e.g., metal transportation and release.

Controlling the formation and alignment of low molecular weight gel 'noodles'

We show how to control the formation and alignment of gel ‘noodles’. Nanostructure alignment can be achieved reproducibly by extensional deformation as the filaments form. Using a spinning technique, very long and highly aligned filaments can be made. The Young's moduli of the gel noodles are similar to that of a bulk gel. By using two syringe pumps in a concentric flow setup, we show that a filament-in-filament morphology can be created.

Extensional deformations induce nanostructure alignment in low molecular weight gel noodles during injection.

Multistate Switching of Spin Selectivity in Electron Transport through Light-Driven Molecular Motors

It is established that electron transmission through chiral molecules depends on the electron's spin. This phenomenon, termed the chiral-induced spin selectivity (CISS), effect has been observed in chiral molecules, supramolecular structures, polymers, and metal-organic films. Which spin is preferred in the transmission depends on the handedness of the system and the tunneling direction of the electrons. Molecular motors based on overcrowded alkenes show multiple inversions of helical chirality under light irradiation and thermal relaxation. The authors found here multistate switching of spin selectivity in electron transfer through first generation molecular motors based on the four accessible distinct helical configurations, measured by magnetic-conductive atomic force microscopy. It is shown that the helical state dictates the molecular organization on the surface. The efficient spin polarization observed in the photostationary state of the right-handed motor coupled with the modulation of spin selectivity through the controlled sequence of helical states, opens opportunities to tune spin selectivity on-demand with high spatio-temporal precision. An energetic analysis correlates the spin injection barrier with the extent of spin polarization.

Self-Assembly of Photoresponsive Molecular Amphiphiles in Aqueous Media

Amphiphilic molecules, comprising hydrophobic and hydrophilic moieties and the intrinsic propensity to self-assemble in aqueous environment, sustain a fascinating spectrum of structures and functions ranging from biological membranes to ordinary soap. Facing the challenge to design responsive, adaptive, and out-of-equilibrium systems in water, the incorporation of photoresponsive motifs in amphiphilic molecular structures offers ample opportunity to design supramolecular systems that enables functional responses in water in a non-invasive way using light. Here, we discuss the design of photoresponsive molecular amphiphiles, their self-assembled structures in aqueous media and at air-water interfaces, and various approaches to arrive at adaptive and dynamic functions in isotropic and anisotropic systems, including motion at the air-water interface, foam formation, reversible nanoscale assembly, and artificial muscle function. Controlling the delicate interplay of structural design, self-assembling conditions and external stimuli, these responsive amphiphiles open several avenues towards application such as soft adaptive materials, controlled delivery or soft actuators, bridging a gap between artificial and natural dynamic systems.

Tunable Photocontrolled Motions of Anil-Poly(ethylene terephthalate) Systems through Excited-State Intramolecular Proton Transfer and Trans-Cis Isomerization

With the combination of excited-state intramolecular proton transfer and trans-cis isomerization as microscopic molecular motions under light stimulus, multiple photodeformable processes are achieved in anil-poly(ethylene terephthalate) systems, including simple bending, dancing butterflies, and switches. The doping films can realize light-driven contraction as large as 70% and bending angle of about 141°, upon a simple stretching process. The internal mechanism is confirmed by transient absorption spectra, and the relationship between molecular structure and photocontrolled motion is investigated by theoretical calculations and crystal analysis. This work provides a convenient approach by utilizing anils to fabricate reversible actuations with desirable geometries, greatly contributing to the applications and manufacturing of soft robots and related research.

Stepwise Adsorption of Alkoxy-Pyrene Derivatives onto a Lamellar, Non-Porous Naphthalenediimide-Template on HOPG

The development of new strategies for the preparation of multicomponent supramolecular assemblies is a major challenge on the road to complex functional molecular systems. Here we present the use of a non-porous self-assembled monolayer from uC₃₃ -NDI-uC₃₃ , a naphthalenediimide symmetrically functionalized with unsaturated 33 carbon-atom-chains, to prepare bicomponent supramolecular surface systems with a series of alkoxy-pyrene (PyrOR) derivatives at the liquid/HOPG interface. While previous attempts at directly depositing many of these PyrOR units at the liquid/HOPG interface failed, the multicomponent approach through the uC₃₃ -NDI-uC₃₃ template enabled control over molecular interactions and facilitated adsorption. The PyrOR deposition restructured the initial uC₃₃ -NDI-uC₃₃ monolayer, causing an expansion in two dimensions to accommodate the guests. As far as we know, this represents the first example of a non-porous or non-metal complex-bearing monolayer that allows the stepwise formation of multicomponent supramolecular architectures on surfaces.

Multi-modal control over the assembly of a molecular motor bola-amphiphile in water

We report multi-modal-control over the assembly behaviour of a first-generation molecular motor bola-amphiphile in water by light, pH and the choice of counter-ions. These findings open up opportunities for the development of materials that reconfigurate enabling complex functions in response to different stimuli.

Dynamic Assemblies of Molecular Motor Amphiphiles Control Macroscopic Foam Properties

Stimuli-responsive supramolecular assemblies controlling macroscopic transformations with high structural fluidity, i.e., foam properties, have attractive prospects for applications in soft materials ranging from biomedical systems to industrial processes, e.g., textile coloring. However, identifying the key processes for the amplification of molecular motion to a macroscopic level response is of fundamental importance for exerting the full potential of macroscopic structural transformations by external stimuli. Herein, we demonstrate the control of dynamic supramolecular assemblies in aqueous media and as a consequence their macroscopic foam properties, e.g., foamability and foam stability, by large geometrical transformations of dual light/heat stimuli-responsive molecular motor amphiphiles. Detailed insight into the reversible photoisomerization and thermal helix inversion at the molecular level, supramolecular assembly transformations at the microscopic level, and the stimuli-responsive foam properties at the macroscopic level, as determined by UV-vis absorption and NMR spectroscopies, electron microscopy, and foamability and in situ surface tension measurements, is presented. By selective use of external stimuli, e.g., light or heat, multiple states and properties of macroscopic foams can be controlled with very dilute aqueous solutions of the motor amphiphiles (0.2 weight%), demonstrating the potential of multiple stimuli-responsive supramolecular systems based on an identical molecular amphiphile and providing opportunities for future soft materials.