Phototriggered Complex Motion by Programmable Construction of Light-Driven Molecular Motors in Liquid Crystal Networks

Fine Tuning of the Rotational Speed of Light-Driven, Second-Generation Molecular Motors by Fluorine Substitution

The elementary steps in the rotation of several second-generation molecular motors are analyzed by finding the minimum energy path between the metastable and stable states and evaluating the transition rate within harmonic transition state theory based on energetics obtained from density functional theory. Comparison with published experimental data shows remarkably good agreement and demonstrates the predictive capability of this approach. While previous measurements by Feringa and co-workers have shown that a replacement of the hydrogen atom at the stereogenic center by a fluorine atom can slow down the rate-limiting thermal helix inversion (THI) step by raising the energy of the transition state, even to the extent that the backreaction in the ground state becomes preferred in some cases, we find that a replacement of a CH3 group by CF3 at the same site accelerates the THI by elevating the energy of the metastable state without affecting the transition state significantly. Since these two fluorine substitutions have an opposite effect on the rate of the THI, the combination of both can provide ways to fine-tune the rotational speed of molecular motors.

Photoactuating artificial muscle from supramolecular assembly of an overcrowded alkene-derived molecular switch

The amplification of molecular motion along length scales for macroscopic muscle-like functions, based on supramolecular polymers, provides attractive opportunities ranging from soft actuators to responsive biomedical materials. Taking the challenge to reveal dynamic assembly parameters governing muscle functions, we present the design of a photoswitch amphiphile based on an overcrowded alkene-derived core, and developed supramolecular artificial muscles. Going from molecular motor amphiphile (MA) to switch amphiphile (SA), taking advantage of high thermal stability of the switch core, a self-recovering of bent SA artificial muscle is observed in post-photoactuation without external intervention. Eliminating molecular motions in SA artificial muscle during the post-photoactuation and aging process enables us to identify correlations between dynamic assembly transformations and macroscopic actuating functions. These findings provide insights into photoactuation and subsequent self-recovery mechanisms from the aspect of dynamic assembly process, which offers new opportunities for developing amphiphile-based supramolecular artificial muscles.

DNA photofluids show life-like motion

As active matter, cells exhibit non-equilibrium structures and behaviours such as reconfiguration, motility and division. These capabilities arise from the collective action of biomolecular machines continuously converting photoenergy or chemical energy into mechanical energy. Constructing similar dynamic processes in vitro presents opportunities for developing life-like intelligent soft materials. Here we report an active fluid formed from the liquid-liquid phase separation of photoresponsive DNA nanomachines. The photofluids can orchestrate and amplify nanoscale mechanical movements by orders of magnitude to produce macroscopic cell-like behaviours including elongation, division and rotation. We identify two dissipative processes in the DNA droplets, photoalignment and photofibrillation, which are crucial for harnessing stochastic molecular motions cooperatively. Our results demonstrate an active liquid molecular system that consumes photoenergy to create ordered out-of-equilibrium structures and behaviours. This system may help elucidate the physical principles underlying cooperative motion in active matter and pave the way for developing programmable interactive materials.

Photoresponsive Coatings by Light-Driven Molecular Motors in Cholesteric Liquid Crystal Microcapsules

Photoresponsive coatings that can change their color in response to light at ambient temperature have large potential applications. Cholesteric liquid crystals (CLCs) are promising photochromic materials, as they are known to reflect light selectively and their optical properties can be modulated with a wide range. However, it remains a major challenge to fabricate photoresponsive coatings that combine fast and good responsivity, fabrication feasibility, and mechanical strength and, more importantly, that can be applied at a large area with excellent stability. In this study, Pickering emulsions containing CLC microdroplets doped with light-driven molecular motors as photoresponsive chiral dopants were prepared via cellulose nanocrystals (CNCs) which serve as both Pickering emulsifiers and alignment agents of CLCs. A melamine-formaldehyde (MF) resin hybrid shell was fabricated via in situ polymerization to form thermally stable CLC microcapsules. These microcapsules were mixed with curable binders, resulting in photoresponsive coatings. The photochromic material which features highly selective addressability of the reflective light wavelength in the visible light region, good reversibility, and viewing angle independence was painted in a large area on both hard and soft substrates, providing a versatile platform for enhanced encryption and smart coatings.

General Synthetic Methodologies for Building Blocks to Construct Molecular Motors

Here we present a collection of reliable, high yielding, and efficient synthetic methods for the preparation of commonly used building blocks to construct molecular rotary motors based on overcrowded alkenes. The easy access and robust synthesis procedures facilitate new endeavors to employ molecular motors in recently emerging fields toward dynamic chemical systems and responsive materials.

General strategy for boosting the performance of speed-tunable rotary molecular motors with visible light

Light-driven molecular rotary motors perform chirality-controlled unidirectional rotations fueled by light and heat. This unique function renders them appealing for the construction of dynamic molecular systems, actuating materials, and molecular machines. Achieving a combination of high photoefficiency, visible-light responsiveness, synthetic accessibility, and easy tuning of dynamic properties within a single scaffold is critical for these applications but remains a longstanding challenge. Herein, a series of highly photoefficient visible-light-responsive molecular motors (MMs), featuring various rotary speeds, was obtained by a convenient one-step formylation of their parent motors. This strategy greatly improves all aspects of the performance of MMs-red-shifted wavelengths of excitation, high photoisomerization quantum yields, and high photostationary state distributions of isomers-beyond the state-of-the-art light-responsive MM systems. The development of this late-stage functionalization strategy of MMs opens avenues for the construction of high-performance molecular machines and devices for applications in materials science and biological systems, representing a major advance in the synthetic toolbox of molecular machines.

Bidirectional Molecular Motors by Controlling Threading and Dethreading Pathways of a Linked Rotaxane

Artificial molecular motors have been presented as models for biological molecular motors. In contrast to the conventional artificial molecular motors that rely on covalent bond rotation, molecular motors with mechanically interlocked molecules (MIMs) have attracted considerable attention owing to their ability to generate significant rotational motion by dynamically shuttling macrocyclic components. The topology of MIM-type rotational molecular motors is currently limited to catenane structures, which require intricate synthetic procedures that typically produce a low synthetic yield. In this study, we develop a novel class of MIM-type molecular motors with a rotaxane-type topology. The switching of the threading/dethreading pathways of the linked rotaxane by protecting/deprotecting the bulky stopper group and changing the solvent polarity enables a net unidirectional rotation of the molecular motor. The threading/dethreading reaction rates were quantitatively evaluated through detailed spectroscopic investigations. Repeated net unidirectional rotation and switching of the direction of rotation were also achieved. Our findings demonstrate that linked rotaxanes can serve as MIM-type molecular motors with reversible rotational direction controlled by threading/dethreading reactions. These motors hold potential as components of molecular machinery.

A Multiresponsive Ferrocene-Based Chiral Overcrowded Alkene Twisting Liquid Crystals

The reversible modulation of chirality has gained significant attention not only for fundamental stereochemical studies but also for numerous applications ranging from liquid crystals (LCs) to molecular motors and machines. This requires the construction of switchable molecules with (multiple) chiral elements in a highly enantioselective manner, which is often a significant synthetic challenge. Here, we show that the dimerization of an easily accessible enantiopure planar chiral ferrocene-indanone building block affords a multi-stimuli-responsive dimer (FcD) with pre-determined double bond geometry, helical chirality, and relative orientation of the two ferrocene motifs in high yield. This intrinsically planar chiral switch can not only undergo thermal or photochemical E/Z isomerization but can also be reversibly and quantitatively oxidized to both a monocationic and a dicationic state which is associated with significant changes in its (chir)optical properties. Specifically, FcD acts as a chiral dopant for cholesteric LCs with a helical twisting power (HTP) of 13 μm-1 which, upon oxidation, drops to near zero, resulting in an unprecedently large redox-tuning of the LC reflection color by up to 84 nm. Due to the straightforward stereoselective synthesis, FcD, and related chiral switches, are envisioned to be powerful building blocks for multi-stimuli-responsive molecular machines and in LC-based materials.

Reconfigurable Visible Light-Driven Liquid Crystalline Network Showing Off-Equilibrium Motions Enabled by Mesogen-Grafted Donor-Acceptor Stenhouse Adducts

Liquid crystalline network (LCN)-based soft actuators have opened up great opportunities to fabricate emerging and intriguing smart materials, serving as potential building blocks for intelligent soft robotics. Endowing LCN actuators with complex responsive behaviors to enhance their intelligence is both challenging and highly demanded. Herein, Donor-Acceptor Stenhouse Adducts (DASAs) molecules with rod-like mesogen and the polymerizable group are judiciously designed and synthesized, which is strong-colored at linear form and de-coloration at cyclic form after visible light. In the colored state, the DASA presents a striking photothermal effect that is capable of driving the motions of LCN film. Upon visible light irradiation, the DASA becomes colorless, making the diminishing photothermal effect. The light-gated switching of the photothermal effect renders the LCN films to be reconfigurable and perform off-equilibrium motions. The varying glass transition temperature of LCN matrix endowing tunable isomerization rates of DASAs and the equilibrium balance of photo- and thermal-isomerization at different temperatures in LCN-P-DASA film mainly guiding the off-equilibrium or stable motions, providing high adjustability of the novel visible light-driven LCN actuators. The multiply modulated LCN-P-DASA film holds great potential in constructing complex visible light-driven soft actuators based on the synergetic effect and interactions of photochemical and photothermal effects.

Closed-Loop Recyclable and Totally Renewable Liquid Crystal Networks with Room-Temperature Programmability and Reconfigurable Functionalities

Dynamic covalent liquid crystal networks (DCv-LCNs) with straightforward (re)programmability, reprocessability, and recyclability facilitates the manufacture of sophisticated LCN actuators and intelligent robots. However, the DCv-LCNs are still limited to heat-assisted programming and polymer-to-polymer reprocessing/recycling, which inevitably lead to deterioration of the LCN structures and the actuation performances after repeated programming/processing treatments, owing to the thermal degradation of the polymer network and/or external agent interference. Here, a totally renewable azobenzene-based DCv-LCN with room-temperature programmability and polymer-to-monomers chemical recyclability is reported, which was synthesized by crosslinking the azobenzene-containing dibenzaldehyde monomer and the triamine monomer via the dynamic and dissociable imine bonds. Thanks to the water-activated dynamics of the imine bonds, the resultant DCv-LCN can be simply programmed, upon water-soaking at room temperature, to yield a UV/Vis light-driven actuator. Importantly, the reported DCv-LCN undergoes depolymerization in an acid-solvent medium at room temperature because of the acid-catalyzed hydrolysis of the imine bonds, giving rise to easy separation and recovery of both monomers in high purity, even with tolerance to additives. The recovered pure monomers can be used to regenerate totally new DCv-LCNs and actuators, and their functionalities can be reconfigured by removing old and introducing new additives, by implementing the closed-loop polymer-monomers-polymer recycling.

Molecular Dynamics, Dielectric Properties, and Textures of Protonated and Selectively Deuterated 4'-Pentyl-4-biphenylcarbonitrile Liquid Crystal

Using liquid crystals in near-infrared applications suffers from effects related to processes like parasitic absorption and high sensitivity to UV-light exposure. One way of managing these disadvantages is to use deuterated systems. The combined 1H and 2H nuclear magnetic resonance relaxometry method (FFC NMR), dielectric spectroscopy (DS), optical microscopy (POM), and differential scanning calorimetry (DSC) approach was applied to investigate the influence of selective deuteration on the molecular dynamics, thermal properties, self-organization, and electric-field responsiveness to a 4'-pentyl-4-biphenylcarbonitrile (5CB) liquid crystal. The NMR relaxation dispersion (NMRD) profiles were analyzed using theoretical models for the description of dynamics processes in different mesophases. Obtained optical textures of selectively deuterated 5CB showed the occurrence of the domain structure close to the I/N phase transition. The dielectric measurements showed a substantial difference in switching fields between fully protonated/deuterated 5CB and selectively deuterated molecules. The DSC thermograms showed a more complex phase transition sequence for partially deuterated 5CB with respect to fully protonated/deuterated molecules.

Development of artificial photosystems based on designed proteins for mechanistic insights into photosynthesis

This review aims to provide an overview of the progress in protein-based artificial photosystem design and their potential to uncover the underlying principles governing light-harvesting in photosynthesis. While significant advances have been made in this area, a gap persists in reviewing these advances. This review provides a perspective of the field, pinpointing knowledge gaps and unresolved challenges that warrant further inquiry. In particular, it delves into the key considerations when designing photosystems based on the chromophore and protein scaffold characteristics, presents the established strategies for artificial photosystems engineering with their advantages and disadvantages, and underscores the recent breakthroughs in understanding the molecular mechanisms governing light-harvesting, charge separation, and the role of the protein motions in the chromophore's excited state relaxation. By disseminating this knowledge, this article provides a foundational resource for defining the field of bio-hybrid photosystems and aims to inspire the continued exploration of artificial photosystems using protein design.

Photo-controllable microcleaner: photo-induced crawling motion and particle transport of azobenzene crystals on a liquid-like surface

Organic crystals of 3,3'-dimethylazobenzene (DMAB) exhibit photo-induced crawling motion on solid surfaces when they are simultaneously irradiated with ultraviolet and visible light from opposite directions. DMAB crystals are candidates for light-driven cargo transporters, having simple chemical compositions and material structures. However, fast crawling motion without significant shape deformation has not yet been achieved. In this study, compared with hydrophilic glass and conventional hydrophobic surfaces with alkyl chains, siloxane-based hybrid surfaces, which are "liquid-like surfaces," result in the fastest crawling motion (4.2 μm min-1) while the droplet-like shape of DMAB crystals is maintained. Additionally, we successfully demonstrate that the DMAB crystals are capable of capturing and carrying silica particles on the hybrid surface. The transport direction is changed on demand without releasing the particles by simply changing the irradiation direction. The particles can be left on the substrate by removing the DMAB crystals via sublimation at room temperature. This result showcases a new concept of "photo-controllable microcleaner" that can operate a series of cargo capture-carry-release tasks. We expect this transporter to contribute to the development of crystal actuators, microfluidics, and microscale molecular flasks/reactors.

Tuning the Properties of Hydrazone/Isosorbide-Based Switchable Chiral Dopants

The long-range supramolecular interactions in liquid crystals (LCs) can be used to amplify and subsequently propagate microscopic structural changes into macroscopic events. Here, we report on a systematic structure-property analysis using 16 chiral photoswitchable dopants composed of bistable hydrazones and chiral isosorbide moieties. Our findings showcase the relationship between the dopant's structure and its helical twisting power (β), and hence, the photophysical properties of the host LC. We show that an increase in the hydrazone CNNH dihedral angle results in an increase in the β value, while alkoxy chains do not lead to such an increase. These results contradict established rules of thumb, stating that structural rigidity and long alky chains are needed for high β values. We also found that the position of the substitution, whether at the 2' or 5' positions of the isosorbide unit, or the attachment of the chiral unit to the rotor or stator phenyl units can have negative or positive additive effects that can either increase or decrease the β values. These results made us hypothesize that unsymmetrically functionalized dopants should result in large Δβ values, which we corroborated experimentally. Moreover, a fluorine-functionalized dopant resulted in higher overall β values, most likely because of π-π interactions. Finally, the dopants were used in modulating and locking in the reflective properties of LC films, yielding multicolor LC canvases that can reflect light from the ultraviolet to the infrared range (i.e., a manipulation of up to ca. 1500 nm of reflected light).

Scalable Approach to Molecular Motor-Polymer Conjugates for Light-Driven Artificial Muscles

The integration of molecular machines and motors into materials represents a promising avenue for creating dynamic and functional molecular systems, with potential applications in soft robotics or reconfigurable biomaterials. However, the development of truly scalable and controllable approaches for incorporating molecular motors into polymeric matrices has remained a challenge. Here, it is shown that light-driven molecular motors with sensitive photo-isomerizable double bonds can be converted into initiators for Cu-mediated controlled/living radical polymerization enabling the synthesis of star-shaped motor-polymer conjugates. This approach enables scalability, precise control over the molecular structure, block copolymer structures, and high-end group fidelity. Moreover, it is demonstrated that these materials can be crosslinked to form gels with quasi-ideal network topology, exhibiting light-triggered contraction. The influence of arm length and polymer structure is investigated, and the first molecular dynamics simulation framework to gain deeper insights into the contraction processes is developed. Leveraging this scalable methodology, the creation of bilayer soft robotic devices and cargo-lifting artificial muscles is showcased, highlighting the versatility and potential applications of this advanced polymer chemistry approach. It is anticipated that the integrated experimental and simulation framework will accelerate scalable approaches for active polymer materials based on molecular machines, opening up new horizons in materials science and bioscience.

Orthogonal Photoswitching in a Porous Organic Framework

The development of photoresponsive systems with non-invasive orthogonal control by distinct wavelengths of light is still in its infancy. In particular, the design of photochemically triggered-orthogonal systems integrated into solid materials that enable multiple dynamic control over their properties remains a longstanding challenge. Here, we report the orthogonal and reversible control of two types of photoswitches in an integrated solid porous framework, that is, visible-light responsive o-fluoroazobenzene and nitro-spiropyran motifs. The properties of the constructed material can be selectively controlled by different wavelengths of light thus generating four distinct states providing a basis for dynamic multifunctional materials. Solid-state NMR spectroscopy demonstrated the selective transformation of the azobenzene switch in the bulk, which in turn modulates N2 and CO2 adsorption.

Photoresponsive Biomimetic Functions by Light-Driven Molecular Motors in Three Dimensionally Printed Liquid Crystal Elastomers

Despite the fascinating developments in design and synthesis of artificial molecular machines operating at the nanoscales, translating molecular motion along multiple length scales and inducing mechanical motion of a three-dimensional macroscopic entity remains an important challenge. The key to addressing this amplification of motion relies on the effective organization of molecular machines in a well-defined environment. By taking advantage of long-range orientational order and hierarchical structures of liquid crystals and unidirectional rotation of light-driven molecular motors, we report here photoresponsive biomimetic functions of liquid crystal elastomers (LCEs) by the repetitive unidirectional rotation of molecular motors using 3D printing. Molecular motors were built in the main chain of liquid crystals oligomers to serve as photoactuators. The oligomers were then used as the ink, and liquid crystal elastomers with different morphologies were printed. The obtained LCEs are able to conduct multiple types of motions including bending, helical coiling, closing of petals, and flipping of wings of a butterfly upon UV illumination, which paves the way for future design of responsive materials with enhanced complex actuating functions.

Molecular Motors' Magic Methyl and Its Pivotal Influence on Rotation

Molecular motors have found a wide range of applications, powering a transition from molecules to dynamic molecular systems for which their motion must be precisely tuned. To achieve this adjustment, strategies involving laborious changes in their design are often used. Herein, we show that control over a single methyl group allows a drastic change in rotational properties. In this regard, we present the straightforward asymmetric synthesis of β-methylated first-generation overcrowded-alkene-based molecular motors. Both enantiomers of the new motors were prepared in good yields and high enantiopurities, and these motors were thoroughly studied by variable-temperature nuclear magnetic resonance (VT-NMR), ultraviolet-visible (UV-vis), and circular dichroism (CD) spectroscopy, showing a crucial influence of the methylation pattern on the rotational behavior of the motors. Starting from a common chiral precursor, we demonstrate that subsequent methylation can drastically reduce the speed of the motor and reverse the direction of the rotation. We show for the first time that complete unidirectionality can be achieved even when the energy difference between the stable and metastable states is small, resulting in the coexistence of both states under ambient conditions without hampering the energy ratcheting process. This discovery opens the way for the design of more advanced first-generation motors.

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

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

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.

Confined Space Nanoarchitectonics for Dynamic Functions and Molecular Machines

Nanotechnology has advanced the techniques for elucidating phenomena at the atomic, molecular, and nano-level. As a post nanotechnology concept, nanoarchitectonics has emerged to create functional materials from unit structures. Consider the material function when nanoarchitectonics enables the design of materials whose internal structure is controlled at the nanometer level. Material function is determined by two elements. These are the functional unit that forms the core of the function and the environment (matrix) that surrounds it. This review paper discusses the nanoarchitectonics of confined space, which is a field for controlling functional materials and molecular machines. The first few sections introduce some of the various dynamic functions in confined spaces, considering molecular space, materials space, and biospace. In the latter two sections, examples of research on the behavior of molecular machines, such as molecular motors, in confined spaces are discussed. In particular, surface space and internal nanospace are taken up as typical examples of confined space. What these examples show is that not only the central functional unit, but also the surrounding spatial configuration is necessary for higher functional expression. Nanoarchitectonics will play important roles in the architecture of such a total system.

Theoretical trends in the dynamics simulations of molecular machines across multiple scales

Over the past few decades, molecular machines have been extensively studied, since they are composed of single molecules for functional materials capable of responding to external stimuli, enabling motion at scales ranging from the microscopic to the macroscopic level within molecular aggregates. This advancement holds the potential to efficiently transform external resources into mechanical movement, achieved through precise control of conformational changes in stimuli-responsive materials. However, the underlying mechanism that links microscopic and macroscopic motions remains unclear, demanding computational development associated with simulating the construction of molecular machines from single molecules. This bottleneck has impeded the design of more efficient functional materials. Advancements in theoretical simulations have successfully been developed in various computational models to unveil the operational mechanisms of stimulus-responsive molecular machines, which could help us reduce the costs in experimental trial-and-error procedures. It opens doors to the computer-aided design of innovative functional materials. In this perspective, we have reviewed theoretical approaches employed in simulating dynamic processes involving conformational changes in molecular machines, spanning different scales and environmental conditions. In addition, we have highlighted current challenges and anticipated future trends in the collective control of aggregates within molecular machines. Our goal is to provide a comprehensive overview of recent theoretical advancements in the field of molecular machines, offering valuable insights for the design of novel smart materials.

Shape-Programmable Liquid-Crystalline Polyurethane-Based Multimode Actuators Triggered by Light-Driven Molecular Motors

In recent years, light-driven soft actuators have been rapidly developed as enablers in the fabrication of artificial robots and biomimetic devices. However, it remains challenging to amplify molecular isomerization to multiple modes of macroscopic actuation with large amplitude and complex motions. Here, a strategy is reported to build a light-responsive liquid-crystalline polyurethane elastomer by phototriggered overcrowded alkene-based molecular motors. A trifunctional molecular motor modified with an ethylene glycol spacer on the rotor and stator functions as a crosslinker and unidirectional stirrer that amplifies molecular motion into macroscopic movement. The shape-programmable polymeric film presents superior mechanical properties and characteristic shape-memory effect. Furthermore, diverse modes of motions including bending, unwinding, and contracting with tunable actuation speed over a wide range are achieved. Such research is hoped to pave a new way for the design of advanced light-responsive soft actuators and robots.

Enlightening dynamic functions in molecular systems by intrinsically chiral light-driven molecular motors

Chirality is a fundamental property which plays a major role in chemistry, physics, biological systems and materials science. Chiroptical artificial molecular motors (AMMs) are a class of molecules which can convert light energy input into mechanical work, and they hold great potential in the transformation from simple molecules to dynamic systems and responsive materials. Taking distinct advantages of the intrinsic chirality in these structures and the unique opportunity to modulate the chirality on demand, chiral AMMs have been designed for the development of light-responsive dynamic processes including switchable asymmetric catalysis, chiral self-assembly, stereoselective recognition, transmission of chirality, control of spin selectivity and biosystems as well as integration of unidirectional motion with specific mechanical functions. This review focuses on the recently developed strategies for chirality-led applications by the class of intrinsically chiral AMMs. Finally, some limitations in current design and challenges associated with recent systems are discussed and perspectives towards promising candidates for responsive and smart molecular systems and future applications are presented.

Accounts of applied molecular rotors and rotary motors: recent advances

Molecular machines are nanoscale devices capable of performing mechanical works at molecular level. These systems could be a single molecule or a collection of component molecules that interrelate with one another to produce nanomechanical movements and resulting performances. The design of the components of molecular machine with bioinspired traits results in various nanomechanical motions. Some known molecular machines are rotors, motors, nanocars, gears, elevators, and so on based on their nanomechanical motion. The conversion of these individual nanomechanical motions to collective motions via integration into suitable platforms yields impressive macroscopic output at varied sizes. Instead of limited experimental acquaintances, the researchers demonstrated several applications of molecular machines in chemical transformation, energy conversion, gas/liquid separation, biomedical use, and soft material fabrication. As a result, the development of new molecular machines and their applications has accelerated over the previous two decades. This review highlights the design principles and application scopes of several rotors and rotary motor systems because these machines are used in real applications. This review also offers a systematic and thorough overview of current advancements in rotary motors, providing in-depth knowledge and predicting future problems and goals in this area.

Light-induced bi-directional switching of thermal conductivity in azobenzene-doped liquid crystal mesophases

The development of systems that can be switched between states with different thermal conductivities is one of the current challenges in materials science. Despite their enormous diversity and chemical richness, molecular materials have been only scarcely explored in this regard. Here, we report a reversible, light-triggered thermal conductivity switching of ≈30-40% in mesophases of pure 4,4'-dialkyloxy-3-methylazobenzene. By doping a liquid crystal matrix with the azobenzene molecules, reversible and bidirectional switching of the thermal conductivity can be achieved by UV/Vis-light irradiation. Given the enormous variety of photoactive molecules and chemically compatible liquid crystal mesophases, this approach opens unforeseen possibilities for developing effective thermal switches based on molecular materials.

Light-Powered Liquid Crystal Polymer Network Actuator Using TiO2 Nanoparticles as an Inorganic Ultraviolet-Light Absorber

Recently, the design and fabrication of light-powered actuators have attracted immense attention because of the manufacturing of intelligent soft robots and innovative self-regulating devices. Accordingly, a liquid crystal polymer network (LCN) provides a promising platform due to its reversible and multistimulus-responsive shape-changing behaviors. In particular, doping nanoparticles with exclusive properties into the LCN can produce interesting results. In this work, we investigated a TiO₂ nanoparticle-based LCN polymer light-powered actuator. TiO₂ nanoparticles as an inorganic ultraviolet (UV)-light absorber can substantially affect the LCN polymer's oscillatory behavior. Our results demonstrate that the oscillation characteristics are directly influenced by the presence of nanoparticles, and we studied the influencing factors. The effectiveness of the elastic modulus, thermomechanical force, and curvature was investigated using different weight percentages of TiO₂ nanoparticles. Our results show that, in the presence of TiO₂ nanoparticles, the polymer chain order and inter-chain interactions in the polymer matrix as well as the structural deformation of relevant polymer surfaces are changed.

Bending Actuation of Hydrogels through Rotation of Light-Driven Molecular Motors

The unidirectional rotation of chemically crosslinked light-driven molecular motors is shown to progressively shift the swelling equilibrium of hydrogels. The concentration of molecular motors and the initial strand density of the polymer network are key parameters to modulate the macroscopic contraction of the material, and both parameters can be tuned using polymer chains of different molecular weights. These findings led to the design of optimized hydrogels revealing a half-time contraction of approximately 5 min. Furthermore, under inhomogeneous stimulation, the local contraction event was exploited to design useful bending actuators with an energy output 400 times higher than for previously reported self-assembled systems involving rotary motors. In the present configuration, we measure that a single molecular motor can lift up loads of 200 times its own molecular weight.

Light-triggered Modulation of Supramolecular Chirality

Dynamically controlling the supramolecular chirality is of great significance in development of functional chiral materials, which is thus essential for the specific function implementation. As an external energy input, light is remote and accurate for modulating chiral assemblies. In non-polarized light control, some photochemically reactive units (e. g., azobenzene, ɑ-cyanostilbene, spiropyran, anthracene) or photo-induced directionally rotating molecular motors were designed to drive chiral transfer or amplification. Besides, photoexcitation induced assembly based physical approach was also explored recently to regulate supramolecular chirality beyond photochemical reactions. In addition, circularly polarized light was applied to induce asymmetric arrangement of organic molecules and asymmetric photochemical synthesis of inorganic metallic nanostructures, in which both wavelength and handedness of circularly polarized light have effects on the induced supramolecular chirality. Although light-triggered chiral assemblies have been widely applied in photoelectric materials, biomedical fields, soft actuator, chiral catalysis and chiral sensing, there is a lack of systematic review on this topic. In this review, we summarized the recent studies and perspectives in the constructions and applications of light-responsive chiral assembled systems, aiming to provide better knowledge for the development of multifunctional chiral nanomaterials.