Light-powered autonomous and directional molecular motion of a dissipative self-assembling system

Photo- and Redox-Driven Artificial Molecular Motors

Directed motion at the nanoscale is a central attribute of life, and chemically driven motor proteins are nature's choice to accomplish it. Motivated and inspired by such bionanodevices, in the past few decades chemists have developed artificial prototypes of molecular motors, namely, multicomponent synthetic species that exhibit directionally controlled, stimuli-induced movements of their parts. In this context, photonic and redox stimuli represent highly appealing modes of activation, particularly from a technological viewpoint. Here we describe the evolution of the field of photo- and redox-driven artificial molecular motors, and we provide a comprehensive review of the work published in the past 5 years. After an analysis of the general principles that govern controlled and directed movement at the molecular scale, we describe the fundamental photochemical and redox processes that can enable its realization. The main classes of light- and redox-driven molecular motors are illustrated, with a particular focus on recent designs, and a thorough description of the functions performed by these kinds of devices according to literature reports is presented. Limitations, challenges, and future perspectives of the field are critically discussed.

Dissipative Catalysis with a Molecular Machine

We report on catalysis by a fuel-induced transient state of a synthetic molecular machine. A [2]rotaxane molecular shuttle containing secondary ammonium/amine and thiourea stations is converted between catalytically inactive and active states by pulses of a chemical fuel (trichloroacetic acid), which is itself decomposed by the machine and/or the presence of additional base. The ON-state of the rotaxane catalyzes the reduction of a nitrostyrene by transfer hydrogenation. By varying the amount of fuel added, the lifetime of the rotaxane ON-state can be regulated and temporal control of catalysis achieved. The system can be pulsed with chemical fuel several times in succession, with each pulse activating catalysis for a time period determined by the amount of fuel added. Dissipative catalysis by synthetic molecular machines has implications for the future design of networks that feature communication and signaling between the components.

Photoswitchable Dissipative Two-Dimensional Colloidal Crystals

Control over particle interactions and organization at fluid interfaces is of great importance both for fundamental studies and practical applications. Rendering these systems stimulus-responsive is thus a desired challenge both for investigating dynamic phenomena and realizing reconfigurable materials. Here, we describe the first reversible photocontrol of two-dimensional colloidal crystallization at the air/water interface, where millimeter-sized assemblies of microparticles can be actuated through the dynamic adsorption/desorption behavior of a photosensitive surfactant added to the suspension. This allows us to dynamically switch the particle organization between a highly crystalline (under light) and a disordered (in the dark) phase with a fast response time (crystallization in ≈10 s, disassembly in ≈1 min). These results evidence a new kind of dissipative system where the crystalline state can be maintained only upon energy supply.

Threading-gated photochromism in [2]pseudorotaxanes

Rigid, Y-shaped imidazole compounds containing the bis(thienyl)ethene moiety were designed and synthesized. The 4,5-bis(benzothienyl)-2-phenylimidazolium cations were then used as axles for [2]pseudorotaxane formation with 24-membered crown ether wheels. It was demonstrated using 1H NMR spectroscopy, UV-Vis absorption and emission spectroscopies that this host-guest interaction results in significant changes in the photochromic properties of the imidazolium axles. This is a rare example of gated photochromism, which exploits the recognition event of an interpenetrated molecular system to tune the photochromic properties in one of the components.

Pumping a Ring-Sliding Molecular Motion by a Light-Powered Molecular Motor

Designing artificial molecular machines to execute complex mechanical tasks, like coupling rotation and translation to accomplish transmission of motion, continues to provide important challenges. Herein, we demonstrated a novel molecular machine comprising a second-generation light-driven molecular motor and a bistable [1]rotaxane unit. The molecular motor can rotate successfully even in an interlocked [1]rotaxane system through a photoinduced cis-to -trans isomerization and a thermal helix inversion, resulting in concomitant transitional motion of the [1]rotaxane. The transmission process was elucidated via ¹H NMR, ¹H-¹H COSY, HMQC, HMBC, and 2D ROESY NMR spectroscopies, UV-visible absorption spectrum, and density functional theory calculations. This is the first demonstration of a molecular motor to rotate against the appreciably noncovalent interactions between dibenzo-24-crown-8 and N-methyltriazolium moieties comprising the rotaxane unit, showing operational capabilities of molecular motors to perform more complex tasks.

Photochromic rotaxanes and pseudorotaxanes

Among stimulus-responsive molecular ring-on-thread rotaxanes and pseudorotaxanes, variants incorporating photochromic sub-units are attracting considerable attention as their properties and structure can be remotely and precisely controlled, additionally without producing chemical waste. The focus herein is on photoswitching-driven assembly/disassembly and modulation of properties resulting from light-activated isomerization or changes in electronic properties.

Mechanical behaviour of contractile gels based on light-driven molecular motors

The networking of individual artificial molecular motors into collective actuation systems is a promising approach for the design of active materials working out of thermodynamic equilibrium. Here, we report the first mechanical studies on active polymer gels built by integrating light-driven rotary molecular motors as reticulation units in polymer networks. We correlate the volume ratio before and after light irradiation with the change of the elastic modulus, and we reveal the universal maximum mechanical efficiency of such gels related to their critical overlap concentration before chemical reticulation. We also show the major importance of heterogeneities in the macroscopic contraction process and we confirm that these materials can increase their internal energy by the motorized winding of their polymer chains.

Uncertainty quantification of a DNA origami mechanism using a coarse-grained model and kinematic variance analysis

Significant advances have been made towards the design, fabrication, and actuation of dynamic DNA nanorobots including the development of DNA origami mechanisms. These DNA origami mechanisms integrate relatively stiff links made of bundles of double-stranded DNA and relatively flexible joints made of single-stranded DNA to mimic the design of macroscopic machines and robots. Despite reproducing the complex configurations of macroscopic machines, these DNA origami mechanisms exhibit significant deviations from their intended motion behavior since nanoscale mechanisms are subject to significant thermal fluctuations that lead to variations in the geometry of the underlying DNA origami components. Understanding these fluctuations is critical to assess and improve the performance of DNA origami mechanisms and to enable precise nanoscale robotic functions. Here, we report a hybrid computational framework combining coarse-grained modeling with kinematic variance analysis to predict uncertainties in the motion pathway of a multi-component DNA origami mechanism. Coarse-grained modeling was used to evaluate the variation in geometry of individual components due to thermal fluctuations. This variation was incorporated in kinematic analyses to predict the motion pathway uncertainty of the entire mechanism, which agreed well with experimental characterization of motion. We further demonstrated the ability to predict the probability density of DNA origami mechanism conformations based on analysis of mechanical properties of individual joints. This integration of computational analysis, modeling tools, and experimental methods establish the foundation to predict and manage motion uncertainties of general DNA origami mechanisms to guide the design of DNA-based nanoscale machines and robots.

Light-driven control of the composition of a supramolecular network

The composition of a supramolecular network, constituted by several cucurbituril receptors and guests, can be controlled by the reversible and all-photonic switching of a dithienylethene guest.

Design of photo-activated molecular machines: highlights from the past ten years

Researchers continue to generate ingenious (supra)molecular structures in which light can trigger controlled and directed movements of the components.

Controlling interfacial interactions of supramolecular assemblies by light-responsive overcrowded alkenes

A light-responsive supramolecular polymer was constructed by an AB-type monomer containing a light-responsive overcrowded alkene. The primary assemblies of the supramolecular polymer can further undertake secondary self-assembly by interfacial host-guest connections, which can be manipulated by light stimuli to convert into discrete primary assemblies.

An Artificial Molecular Shuttle Operates in Lipid Bilayers for Ion Transport

Inspired by natural biomolecular machines, synthetic molecular-level machines have been proven to perform well-defined mechanical tasks and measurable work. To mimic the function of channel proteins, we herein report the development of a synthetic molecular shuttle, [2]rotaxane 3, as a unimolecular vehicle that can be inserted into lipid bilayers to perform passive ion transport through its stochastic shuttling motion. The [2]rotaxane molecular shuttle is composed of an amphiphilic molecular thread with three binding stations, which is interlocked in a macrocycle wheel component that tethers a K⁺ carrier. The structural characteristics enable the rotaxane to transport ions across the lipid bilayers, similar to a cable car, transporting K⁺ with an EC₅₀ value of 1.0 μM (3.0 mol % relative to lipid). We expect that this simple molecular machine will provide new opportunities for developing more effective and selective ion transporters.

Photoactive Molecular-Based Devices, Machines and Materials: Recent Advances

Molecular and supramolecular-based systems and materials that can perform predetermined functions in response to light stimulation have been extensively studied in the past three decades. Their investigation continues to be a highly stimulating topic of chemical research, not only because of the inherent scientific value related to a bottom-up approach to functional nanostructures, but also for the prospective applications in diverse fields of technology and medicine. Light is an important tool in this context, as it can be conveniently used both for supplying energy to the system and for probing its states and transformations. In this microreview we recall some basic aspects of light-induced processes in (supra)molecular assemblies, and discuss their exploitation to implement novel functionalities with nanostructured devices, machines and materials. To this aim we illustrate a few examples from our own recent work, which are meant to illustrate the trends of current research in the field.

Reversible Photocontrol of DNA Melting by Visible-Light-Responsive F4-Coordinated Azobenzene Compounds

Spatiotemporal control over the regulation of intra- and intermolecular motions in naturally occurring systems is systematically studied to expand the toolbox of mechanical operations in multicomponent nanoarchitectures. DNA is ideally suited for programming light-powered processes that are based on a minimalist molecular design. Here, the noncovalent incorporation of bistable photoswitches into B-like DNA moieties is shown to trigger the thermal transition midpoint of the duplexes by converting visible light into directed mechanical work by orchestrating the collective actions of the photoresponsive chromophores and the host DNA nanostructures. Besides its practical applications, the resulting hybrid nanosystem bears unique features of modulability, biocompatibility, reversibility, and addressability, which are key components for developing molecular photon-controlled programmed materials.

Energy consumption in chemical fuel-driven self-assembly

Nature extensively exploits high-energy transient self-assembly structures that are able to perform work through a dissipative process. Often, self-assembly relies on the use of molecules as fuel that is consumed to drive thermodynamically unfavourable reactions away from equilibrium. Implementing this kind of non-equilibrium self-assembly process in synthetic systems is bound to profoundly impact the fields of chemistry, materials science and synthetic biology, leading to innovative dissipative structures able to convert and store chemical energy. Yet, despite increasing efforts, the basic principles underlying chemical fuel-driven dissipative self-assembly are often overlooked, generating confusion around the meaning and definition of scientific terms, which does not favour progress in the field. The scope of this Perspective is to bring closer together current experimental approaches and conceptual frameworks. From our analysis it also emerges that chemically fuelled dissipative processes may have played a crucial role in evolutionary processes.

A pyridinium/anilinium [2]catenane that operates as an acid-base driven optical switch

A two-station [2]catenane containing a large macrocycle with two different recognition sites, one bis(pyridinium)ethane and one benzylanilinium, as well as a smaller DB24C8 ring was synthesized and characterized. ¹H NMR spectroscopy showed that the DB24C8 ring can shuttle between the two recognition sites depending on the protonation state of the larger macrocycle. When the aniline group is neutral, the DB24C8 ring resides solely at the bis(pyridinium)ethane site, while addition of acid forms a charged benzylanilinium site. The DB24C8 then shuttles between the two charged recognition sites with occupancy favoring the bis(pyridinium)ethane site by a ratio of 4:1. The unprotonated [2]catenane has a deep yellow/orange color when the DB24C8 ring resides solely at the bis(pyridinium)ethane site and changes to colorless when the crown ether is shuttling (i.e., circumrotating) back and forth between the two recognition sites thus optically signalling the onset of the shuttling dynamics.

ATP-Driven Temporal Control over Structure Switching of Polymeric Micelles

An adenosine triphosphate (ATP)-fueled micellar system in the out-of-equilibrium state was constructed based on 4,5-diamino-1,3,5-triazine (DAT)-containing block copolymer. The block copolymer self-assembled into spherical micelles in equilibrium steady state at pH higher than its p K_a. The pendant DAT residues in protonated form acted as ATP catchers via hydrogen bonding and electrostatic interactions. Activated by ATP fuel, the polymeric micelles spontaneously disrupted into small aggregates of ATP/polymer hybrid complexes. The consumption of ATP energy via the enzymatic hydrolysis led to dissociation of the complexes and reversible formation of polymeric micelles. A transient self-assembly cycle, in which the assembly underwent autonomous division-fusion motion, was created using ATP fuel and enzyme; the switching of assembly structure was sustained by continuous supply of ATP fuel. This DAT-containing block copolymer have good biocompatibility, and drug-loaded micelles display ATP-responsive release behavior. It is expected that this ATP-fueled supramolecular assembly system will provide a functional platform for biomimic chemistry and therapeutic applications.

Highly ordered molecular rotor matrix on a nanopatterned template: titanyl phthalocyanine molecules on FeO/Pt(111)

Molecular rotors, motors and gears play important roles in artificial molecular machines, in which rotor and motor matrices are highly desirable for large-scale bottom-up fabrication of molecular machines. Here we demonstrate the fabrication of a highly ordered molecular rotor matrix by depositing nonplanar dipolar titanyl phthalocyanine (TiOPc, C₃₂H₁₆N₈OTi) molecules on a Moiré patterned dipolar FeO/Pt(111) substrate. TiOPc molecules with O atoms pointing outwards from the substrate (upward) or towards the substrate (downward) are alternatively adsorbed on the fcc sites by strong lateral confinement. The adsorbed molecules, i.e. two kinds of molecular rotors, show different scanning tunneling microscopy images, thermal stabilities and rotational characteristics. Density functional theory calculations clarify that TiOPc molecules anchoring upwards with high adsorption energies correspond to low-rotational-rate rotors, while those anchoring downwards with low adsorption energies correspond to high-rotational-rate rotors. A robust rotor matrix fully occupied by low-rate rotors is fabricated by depositing molecules on the substrate at elevated temperature. Such a paradigm opens up a promising route to fabricate functional molecular rotor matrices, driven motor matrices and even gear groups on solid substrates.

Optical vs. chemical driving for molecular machines

Molecular machines use external energy to drive transport, to do mechanical, osmotic, or electrical work on the environment, and to form structure. In this paper the fundamental difference between the design principles necessary for a molecular machine to use light or external modulation of thermodynamic parameters as an energy source vs. the design principle for using an exergonic chemical reaction as a fuel will be explored. The key difference is that for catalytically-driven motors microscopic reversibility must hold arbitrarily far from equilibrium. Applying the constraints of microscopic reversibility assures that a coarse grained model is consistent with an underlying model for motion on a single time-independent potential energy surface. In contrast, light-driven processes, and processes driven by external modulation of the thermodynamic parameters of a system cannot in general be described in terms of motion on a single time-independent potential energy surface, and the rate constants are not constrained by microscopic reversibility. The results presented here call into question the value of the so-called power stroke model as an explanation of the function of autonomous chemically-driven molecular machines such as are commonly found in biology.

Unveiling the mechanism of the promising two-dimensional photoswitch - Hemithioindigo

The control of internal molecular motions by outside stimuli is a decisive task in the construction of functional molecules and molecular machines. Light-induced intramolecular rotations of photoswitches have attracted increasing research interests because of the high stability and high reversibility of photoswitches. Recently, Henry et al. reported an unprecedented two-dimensional controlled photoswitch, the hemithioindigo (HTI) derivative Z1, whose single bond rotation in dimethyl sulphoxide (DMSO) solvent and double bond rotation in cyclohexane solvent can be induced by visible light (J. Am. Chem. Soc. 2016, 138, 12,219). Here we investigate the intramolecular rotations of the HTI and Z1 in different polar solvents by time-dependent density functional theory (TDDFT) and Nonadiabatic dynamic simulations. Due to the steric hindrance between methyl and thioindigo fragment, the rotations of Z1 in the excited state are obstructed. Interestingly, the HTI exhibits two distinct rotation paths in DMSO and cyclohexane solvents at about 50fs. The intermolecular hydrogen bonds between HTI and DMSO play an important role in the rotation of HTI in DMSO solvent. Therefore, the HTI is a more promising two-dimensional photoswitch compared with the Z1. Our finding is thus of fundamental importance to understand the mechanisms of this class of photoswitches and design complex molecular behavior.

Self-Propelled Micro/Nanomotors for Sensing and Environmental Remediation

Self-propelled micro/nanomotors have gained attention for successful application in cargo delivery, therapeutic treatments, sensing, and environmental remediation. Unique characteristics such as high speed, motion control, selectivity, and functionability promote the application of micro/nanomotors in analytical sciences. Here, the recent advancements and main challenges regarding the application of self-propelled micro/nanomotors in sensing and environmental remediation are discussed. The current state of micro/nanomotors is reviewed, emphasizing the period of the last five years, then their developments into the future applications for enhanced sensing and efficient purification of water resources are extrapolated.

Reversible photoswitching of encapsulated azobenzenes in water

Efficient molecular switching in confined spaces is critical for the successful development of artificial molecular machines. However, molecular switching events often entail large structural changes and therefore require conformational freedom, which is typically limited under confinement conditions. Here, we investigated the behavior of azobenzene-the key building block of light-controlled molecular machines-in a confined environment that is flexible and can adapt its shape to that of the bound guest. To this end, we encapsulated several structurally diverse azobenzenes within the cavity of a flexible, water-soluble coordination cage, and investigated their light-responsive behavior. Using UV/Vis absorption spectroscopy and a combination of NMR methods, we showed that each of the encapsulated azobenzenes exhibited distinct switching properties. An azobenzene forming a 1:1 host-guest inclusion complex could be efficiently photoisomerized in a reversible fashion. In contrast, successful switching in inclusion complexes incorporating two azobenzene guests was dependent on the availability of free cages in the system, and it involved reversible trafficking of azobenzene between the cages. In the absence of extra cages, photoswitching was either suppressed or it involved expulsion of azobenzene from the cage and consequently its precipitation from the solution. This finding was utilized to develop an information storage medium in which messages could be written and erased in a reversible fashion using light.

Ring-through-ring molecular shuttling in a saturated [3]rotaxane

Mechanically interlocked molecules such as rotaxanes and catenanes comprise two or more components whose motion relative to each other can be controlled. A [2]rotaxane molecular shuttle, for example, consists of an axle bearing two recognition sites and a single macrocyclic wheel that can undergo a to-and-fro motion along the axle-shuttling between the recognition sites. The ability of mechanically interlocked molecules to undergo this type of large-amplitude change is the core mechanism behind almost every interlocked molecular switch or machine, including sophisticated mechanical systems such as a molecular elevator and a peptide synthesizer. Here, as a way to expand the scope of dynamics possible at the molecular level, we have developed a molecular shuttling mechanism involving the exchange of rings between two recognition sites in a saturated [3]rotaxane (one with no empty recognition sites). This was accomplished by passing a smaller ring through a larger one, thus achieving ring-through-ring molecular shuttling.

Molecular rotary motors: Unidirectional motion around double bonds

The field of synthetic molecular machines has quickly evolved in recent years, growing from a fundamental curiosity to a highly active field of chemistry. Many different applications are being explored in areas such as catalysis, self-assembled and nanostructured materials, and molecular electronics. Rotary molecular motors hold great promise for achieving dynamic control of molecular functions as well as for powering nanoscale devices. However, for these motors to reach their full potential, many challenges still need to be addressed. In this paper we focus on the design principles of rotary motors featuring a double-bond axle and discuss the major challenges that are ahead of us. Although great progress has been made, further design improvements, for example in terms of efficiency, energy input, and environmental adaptability, will be crucial to fully exploit the opportunities that these rotary motors offer.

Cyclodextrin Rotaxane with Switchable Pirouetting

A [1]rotaxane with two threaded α-cyclodextrin (α-CD) wheels was synthesized in 92% yield using a one-pot process at room temperature that employed spontaneous α-CD threading onto a 12-carbon alkyl chain in water followed by an oxime condensation reaction that attached two boronic acid-containing stopper groups. Rapid pirouetting of the threaded α-CD wheels around the encapsulated dumbbell was switched "ON" or "OFF" by the presence of chemical additives that controlled boronate ester bond formation between the interlocked components.

Light-driven micro/nanomotors: from fundamentals to applications

Light, as an external stimulus, is capable of driving the motion of micro/nanomotors (MNMs) with the advantages of reversible, wireless and remote manoeuvre on demand with excellent spatial and temporal resolution. This review focuses on the state-of-the-art light-driven MNMs, which are able to move in liquids or on a substrate surface by converting light energy into mechanical work. The general design strategies for constructing asymmetric fields around light-driven MNMs to propel themselves are introduced as well as the photoactive materials for light-driven MNMs, including photocatalytic materials, photothermal materials and photochromic materials. Then, the propulsion mechanisms and motion behaviors of the so far developed light-driven MNMs are illustrated in detail involving light-induced phoretic propulsion, bubble recoil and interfacial tension gradient, followed by recent progress in the light-driven movement of liquid crystalline elastomers based on light-induced deformation. An outlook is further presented on the future development of light-driven MNMs towards overcoming key challenges after summarizing the potential applications in biomedical, environmental and micro/nanoengineering fields.