A chemically powered unidirectional rotary molecular motor based on a palladium redox cycle

Directionality Reversal and Shift of Rotational Axis in a Hemithioindigo Macrocyclic Molecular Motor

Molecular motors are central driving units for nanomachinery, and control of their directional motions is of fundamental importance for their functions. Light-driven variants use easy to provide, easy to dose, and waste-free fuel with high energy content, making them particularly interesting for applications. Typically, light-driven molecular motors work via rotations around dedicated chemical bonds where the directionality of the rotation is dictated by the steric effects of asymmetry in close vicinity to the rotation axis. In this work, we show how unidirectional rotation around a virtual axis can be realized by reprogramming a molecular motor. To this end, a classical light-driven motor is restricted by macrocyclization, and its intrinsic directional rotation is transformed into a directional rotation of the macrocyclic chain in the opposite direction. Further, solvent polarity changes allow to toggle the function of this molecular machine between a directional motor and a nondirectional photoswitch. In this way, a new concept for the design of molecular motors is delivered together with elaborate control over their motions and functions by simple solvent changes. The possibility of sensing the environmental polarity and correspondingly adjusting the directionality of motions opens up a next level of control and responsiveness to light-driven nanoscopic motors.

Enantioselective Synthesis of Heteroatom-Linked Non-Biaryl Atropisomers

Atropisomers hold significant fascination, not only for their prevalence in natural compounds but also for their biological importance and wide-ranging applications as chiral materials, ligands, and organocatalysts. While biaryl and heterobiaryl atropisomers are commonly studied, the enantioselective synthesis of less abundant heteroatom-linked non-biaryl atropisomers presents a formidable challenge in modern organic synthesis. Unlike classical atropisomers, these molecules allow rotation around two bonds, resulting in low barriers to enantiomerization through concerted bond rotations. In recent years the discovery of new configurationally stable rare non-biaryl scaffolds such as aryl amines, aryl ethers and aryl sulfones as well as innovative methodologies to control their configuration have been disclosed in the literature and constitute the topic of this minireview.

Atroposelective catalysis

Atropisomeric compounds-stereoisomers that arise from the restricted rotation about a single bond-have attracted widespread attention in recent years due to their immense potential for applications in a variety of fields, including medicinal chemistry, catalysis and molecular nanoscience. This increased interest led to the invention of new molecular motors, the incorporation of atropisomers into drug discovery programmes and a wide range of novel atroposelective reactions, including those that simultaneously control multiple stereogenic axes. A diverse set of synthetic methodologies, which can be grouped into desymmetrizations, (dynamic) kinetic resolutions, cross-coupling reactions and de novo ring formations, is available for the catalyst-controlled stereoselective synthesis of various atropisomer classes. In this Review, we generalize the concepts for the catalyst-controlled stereoselective synthesis of atropisomers within these categories with an emphasis on recent advancements and underdeveloped atropisomeric scaffolds beyond stereogenic C(sp2)-C(sp2) axes. We also discuss more complex systems with multiple stereogenic axes or higher-order stereogenicity.

Chiral dinitrogen ligand enabled asymmetric Pd/norbornene cooperative catalysis toward the assembly of C-N axially chiral scaffolds

C - N axially chiral compounds have recently attracted significant interest among synthetic chemistry community due to their widespread application in pharmaceuticals, advanced materials and organic synthesis. Although the emerging asymmetric Catellani reaction offers great opportunity for their modular and efficient preparation, the only operative chiral NBE strategy to date requires using half stoichiometric amount of chiral NBE and 2,6-disubstituted bromoarenes as electrophiles. We herein report an efficient assembly of C-N axially chiral scaffolds through a distinct chiral ligand strategy. The crucial chiral source, a biimidazoline (BiIM) chiral dinitrogen ligand, is used in relatively low loading and permits the use of less bulky bromoarenes. The method also features the use of feedstock plain NBE, high reactivity, good enantioselectivity, ease of operation and scale-up. Applications in the preparation of chiral optoelectronic material candidates featuring two C-N chiral axes and a chiral ligand for asymmetric C-H activation have also been demonstrated.

Molecular Ratchets and Kinetic Asymmetry: Giving Chemistry Direction

Over the last two decades ratchet mechanisms have transformed the understanding and design of stochastic molecular systems-biological, chemical and physical-in a move away from the mechanical macroscopic analogies that dominated thinking regarding molecular dynamics in the 1990s and early 2000s (e.g. pistons, springs, etc), to the more scale-relevant concepts that underpin out-of-equilibrium research in the molecular sciences today. Ratcheting has established molecular nanotechnology as a research frontier for energy transduction and metabolism, and has enabled the reverse engineering of biomolecular machinery, delivering insights into how molecules 'walk' and track-based synthesisers operate, how the acceleration of chemical reactions enables energy to be transduced by catalysts (both motor proteins and synthetic catalysts), and how dynamic systems can be driven away from equilibrium through catalysis. The recognition of molecular ratchet mechanisms in biology, and their invention in synthetic systems, is proving significant in areas as diverse as supramolecular chemistry, systems chemistry, dynamic covalent chemistry, DNA nanotechnology, polymer and materials science, molecular biology, heterogeneous catalysis, endergonic synthesis, the origin of life, and many other branches of chemical science. Put simply, ratchet mechanisms give chemistry direction. Kinetic asymmetry, the key feature of ratcheting, is the dynamic counterpart of structural asymmetry (i.e. chirality). Given the ubiquity of ratchet mechanisms in endergonic chemical processes in biology, and their significance for behaviour and function from systems to synthesis, it is surely just as fundamentally important. This Review charts the recognition, invention and development of molecular ratchets, focussing particularly on the role for which they were originally envisaged in chemistry, as design elements for molecular machinery. Different kinetically asymmetric systems are compared, and the consequences of their dynamic behaviour discussed. These archetypal examples demonstrate how chemical systems can be driven inexorably away from equilibrium, rather than relax towards it.

Stepwise Operation of a Molecular Rotary Motor Driven by an Appel Reaction

To date, only a small number of chemistries and chemical fueling strategies have been successfully used to operate artificial molecular motors. Here, we report the 360° directionally biased rotation of phenyl groups about a C-C bond, driven by a stepwise Appel reaction sequence. The motor molecule consists of a biaryl-embedded phosphine oxide and phenol, in which full rotation around the biaryl bond is blocked by the P-O oxygen atom on the rotor being too bulky to pass the oxygen atom on the stator. Treatment with SOCl2 forms a cyclic oxyphosphonium salt (removing the oxygen atom of the phosphine oxide), temporarily linking the rotor with the stator. Conformational exchange via ring flipping then allows the rotor and stator to twist back and forth past the previous limit of rotation. Subsequently, the ring opening of the tethered intermediate with a chiral alcohol occurs preferentially through a nucleophilic attack on one face. Thus, the original phosphine oxide is reformed with net directional rotation about the biaryl bond over the course of the two-step reaction sequence. Each repetition of SOCl2-chiral alcohol additions generates another directionally biased rotation. Using the same reaction sequence on a derivative of the motor molecule that forms atropisomers rather than fully rotating 360° results in enantioenrichment, suggesting that, on average, the motor molecule rotates in the "wrong" direction once every three fueling cycles. The interconversion of phosphine oxides and cyclic oxyphosphonium groups to form temporary tethers that enable a rotational barrier to be overcome directionally adds to the strategies available for generating chemically fueled kinetic asymmetry in molecular systems.

Studies on the Configurational Stability of Tropolone-Ketone-, Ester-, and Aldehyde-Based Chiral Axes

Recent studies have revealed that tropolone-amide aryl C-C(O) rotational barriers are dramatically higher than those of analogous benzamide-based systems, and as a result, they have an increased likelihood of displaying high configurational stability. Studies on other tropolone-based chiral axes are important to assess the generality of this phenomenon. Herein, we describe a series of studies on the rotational barriers of tropolone-ketone, tropolone-ester, and tropolone-aldehyde chiral axes. These studies are complemented with computational modeling of the dynamics of these and analogous benzenoid variants to illuminate the impact that tropolone may have on aryl-C(O) configurational stability.

Switched "On" Transient Fluorescence Output from a Pulsed-Fuel Molecular Ratchet

We report the synthesis and operation of a molecular energy ratchet that transports a crown ether from solution onto a thread, along the axle, over a fluorophore, and off the other end of the thread back into bulk solution, all in response to a single pulse of a chemical fuel (CCl3CO2H). The fluorophore is a pyrene residue whose fluorescence is normally prevented by photoinduced electron transfer (PET) to a nearby N-methyltriazolium group. However, crown ether binding to the N-methyltriazolium site inhibits the PET, switching on pyrene fluorescence under UV irradiation. Each pulse of fuel results in a single ratchet cycle of transient fluorescence (encompassing threading, transport to the N-methyltriazolium site, and then dethreading), with the onset of the fluorescent time period determined by the amount of fuel in each pulse and the end-point determined by the concentration of the reagents for the disulfide exchange reaction. The system provides a potential alternative signaling approach for artificial molecular machines that read symbols from sequence-encoded molecular tapes.

The Effect of Intramolecular Hydrogen Bonds on the Rotational Barriers of the Biaryl C-C Axis

Axially chiral compounds are attracting more attention recently. Although hydrogen bonds are reported as a vital weak force that influences the properties of compounds, the effect of intramolecular hydrogen bonds on the atropisomerization of the Caryl -Caryl single bonds has not yet been well quantitatively investigated. Here, a series of axially chiral biaryl compounds were synthesized to study the effect of hydrogen bonds on the rotational barriers of the biaryl C-C axis. Experimental studies demonstrated that the rotational barrier of hydrogen bonding biaryl 9 was significantly lower (46.7 kJ mol-1 ) than biaryl 10 without hydrogen bonds. Furthermore, theoretical studies revealed that the intramolecular hydrogen bond stabilized the transition state (TS) of tri-ortho-substituted biaryl 9, relieving the steric repulsion in the TS. We believe that this study will provide chemists with a deeper understanding of the atropisomerization process of axially chiral biaryl compounds.

Chiral Phosphoric Acid-Catalyzed Enantioselective Synthesis of Axially Chiral Compounds Involving Indole Derivatives

Indoles are one of the most ubiquitous subclass of N-heterocycles and are increasingly incorporated to design new axially chiral scaffolds. The rich profile of reactivity and N-H functionality allow chemical derivatization for enhanced medicinal, material and catalytic properties. Although asymmetric C-C coupling of two arenes gives the most direct access of axially chiral biaryl scaffolds, this chemistry has been the remit of metal catalysis and works efficiently on limited substrates. Our group has devoted special interest in devising novel organocatalytic arylation reactions to fabricate biaryl atropisomers. In this realm, indoles and derivatives have been reliably used as the arylation partners in combination with azoarenes, nitrosonapthalenes and quinone derivatives. Their efficient interaction with chiral phosphoric acid catalyst as well as the tunability of electronics and sterics have enabled excellent control of stereo-, chemo- and regioselectivity to furnish diverse scaffolds. In addition, indoles could act as nucleophiles in desymmetrization of 1,2,4-triazole-3,5-diones. This account provides a succinct illustration of these developments.

Chiral Transient Ligand Enabled Enantioselective Synthesis of Atropisomers Decorated with Unactivated Olefins via a Palladium-Catalyzed C-H Olefination

Herein, atroposelective synthesis of axially chiral biaryls with unactivated olefins by a palladium-catalyzed C-H olefination using a chiral transient directing group strategy has been disclosed. This protocol is well compatible with a variety of biaryl-2-aldehydes as well as various olefins such as allyl sulfonamides and allyl sulfones to provide the atroposelective olefinated products in synthetically useful yields with excellent enantioselectivities up to >99% ee. In addition, a wide number of axially chiral biaryl alcohols were synthesized by the simple diversification of the products in excellent enantioselectivity.

Stereodynamic Strategies to Induce and Enrich Chirality of Atropisomers at a Late Stage

Enantiomers, where chirality arises from restricted rotation around a single bond, are atropisomers. Due to the unique nature of the origins of their chirality, synthetic strategies to access these compounds in an enantioselective manner differ from those used to prepare enantioenriched compounds containing point chirality arising from an unsymmetrically substituted carbon center. In particular stereodynamic transformations, such as dynamic kinetic resolutions, thermodynamic dynamic resolutions, and deracemizations, which rely on the ability to racemize or interconvert enantiomers, are a promising set of transformations to prepare optically pure compounds in the late stage of a synthetic sequence. Translation of these synthetic approaches from compounds with point chirality to atropisomers requires an expanded toolbox for epimerization/racemization and provides an opportunity to develop a new conceptual framework for the enantioselective synthesis of these compounds.

A [3]-catenane non-autonomous molecular motor model: Geometric phase, no-pumping theorem, and energy transduction

We study a model of a synthetic molecular motor-a [3]-catenane consisting of two small macrocycles mechanically interlocked with a bigger one-subjected to time-dependent driving using stochastic thermodynamics. The model presents nontrivial features due to the two interacting small macrocycles but is simple enough to be treated analytically in limiting regimes. Among the results obtained, we find a mapping into an equivalent [2]-catenane that reveals the implications of the no-pumping theorem stating that to generate net motion of the small macrocycles, both energies and barriers need to change. In the adiabatic limit (slow driving), we fully characterize the motor's dynamics and show that the net motion of the small macrocycles is expressed as a surface integral in parameter space, which corrects previous erroneous results. We also analyze the performance of the motor subjected to step-wise driving protocols in the absence and presence of an applied load. Optimization strategies for generating large currents and maximizing free energy transduction are proposed. This simple model provides interesting clues into the working principles of non-autonomous molecular motors and their optimization.

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.

Organocatalytic Atroposelective Cross-Coupling of 1-Azonaphthalenes and 2-Naphthols

Atroposelective cross-coupling is one of the most appealing routes to construct axially chiral binaphthyl molecules due to the modular and succinct nature. Although transition-metal-catalyzed cross-couplings offer reliable synthetic means, alternative reaction modes that could be applied to broader substrate range without their pre-functionalization is highly desirable. Herein we show that the application of chiral Brønsted acid catalyst as organocatalyst could accomplish cross-coupling of 1-azonaphthalenes and 2-naphthols with high efficiency, exclusive C4-selectivity as well as excellent enantioselectivity and functional group compatibility. The identification of acylimidazolinone auxiliary for azo activating group, effective remote catalyst control and arene resonance effect synergistically play key roles in the development of this method. The utility is further demonstrated by transformations of the products into other binaphthyl compounds with perfectly retained axial chirality.

Driving a Third Generation Molecular Motor with Electrons Across a Surface

Excitation of single molecules with electrons tunneling between a sharp metallic tip of a scanning tunneling microscope and a metal surface is one way to study and control dynamics of molecules on surfaces. Electron tunneling induced dynamics may lead to hopping, rotation, molecular switching, or chemical reactions. Molecular motors that convert rotation of subgroups into lateral movement on a surface can in principle also be driven by tunneling electrons. For such surface-bound motor molecules the efficiency of motor action with respect to electron dose is still not known. Here, the response of a molecular motor containing two rotor units in the form of overcrowded alkene groups to inelastic electron tunneling has been examined on a Cu(111) surface in ultrahigh vacuum at 5 K. Upon vibrational excitation, switching between different molecular conformations is observed, including conversion of enantiomeric states of chiral conformations. Tunneling at energies in the range of electronic excitations causes activation of motor action and movement across the surface. The expected unidirectional rotation of the two rotor units causes forward movements but with a low degree of translational directionality.

Sulfoxide-Directed or 3d-Metal Catalyzed C-H Activation and Hypervalent Iodines as Tools for Atroposelective Synthesis

ConspectusThe expanding applications of atropisomeric compounds combined with the growing diversity of such chiral molecules translate into an urgent need for innovative synthetic strategies allowing their rapid, efficient, and sustainable synthesis. Recently, the C-H activation approach has provided new opportunities for synthesizing axially chiral compounds. The two complementary approaches allowing implementation of the C-H activation methodology toward the synthesis of the chiral molecules imply either ortho-functionalization of the preexisting prochiral or atropo-unstable biaryl substrates or direct C-H arylation of sterically encumbered aromatics. The first approach required the preinstallation of a directing group on a biaryl precursor, which drastically limits the diversity of thus generated products. To tackle this important synthetic limitation, we have envisioned using a chiral sulfoxide as both directing group and chiral auxiliary. Indeed, in addition to efficiently coordinating the Pd-catalyst thus allowing chiral induction, the sulfoxide moiety can be easily removed, via the sulfoxide/lithium exchange, after the C-H activation step, thus guaranteeing an almost unlimited postdiversification of the atropisomeric products. The efficiency and generality of this concept could be illustrated by developing atropo-diastereoselective oxidative Heck reaction, direct acetoxylation, and iodination, as well as direct arylation. Besides, the synthetic utility of this methodology was demonstrated by designing an expedient synthesis of a direct steganone precursor. This unique transformation also allowed us to build up unprecedented triaryl scaffolds with two perfectly controlled chiral axes, original chiral skeletons for new ligand design. While considering the atroposelective direct arylations, the clear antagonism between the harsh reaction conditions frequently required for the coupling of two sterically hindered compounds and the atropo-stability of the new product, resulted in the scarcity of such transformations. To solve this fundamental challenge, we have focused on the application of a low-valent cobalt catalyst, prompted to catalyze C-H activation of indoles at the C2 position under extremely mild reaction conditions (room temperature). Accordingly, atroposelective C2-arylation of indoles could be achieved using an original carbene ligand and delivering the uncommon atropoisomerically pure indoles in excellent yields and enantioselectivities. Detailed combined experimental and theoretical mechanistic studies shed light on the mechanism of this transformation, providing strong evidence regarding the origin of the enantioselectivity. Finally, the antagonism between steric hindrance required to guarantee the atropo-stability of a molecule and harsh reaction conditions required to couple two partners is a strong limitation not only for the development of atroposelective C-H arylation reaction but also for the development of direct synthesis of the C-N axially chiral compounds. Despite the long history and incredible advances achieved in Ullmann-Goldberg and Buchwald-Hartwig couplings, atroposelective versions of such transformations have remained unprecedented until recently. Our idea to tackle this challenging issue consisted in using hypervalent iodines as highly reactive coupling partners, thus allowing the desired N-arylations to occur at room temperature. This hypothesis could be validated by reporting first atropo-diastereoselective Cu-catalyzed N-arylation, using sulfoxide λ³-iodanes as the coupling partners. Subsequently, the enantioselective version of this atroposelective N-arylation was successfully established by using a chiral Cu-complex bearing a BOX ligand. In conclusion, we report herein designing tailored-made solutions to provide new synthetic strategies to construct the atropisomeric molecules, including biaryls and C-N axially chiral molecules.

Cation controlled rotation in anionic pillar[5]arenes and its application for fluorescence switch

Controlling molecular motion is one of hot topics in the field of chemistry. Molecular rotors have wide applications in building nanomachines and functional materials, due to their controllable rotations. Hence, the development of novel rotor systems, controlled by external stimuli, is desirable. Pillar[n]arenes, a class of macrocycles, have a unique planar chirality, in which two stable conformational isomers pR and pS would interconvert by oxygen-through-the-annulus rotations of their hydroquinone rings. We observe the differential kinetic traits of planar chirality transformation in sodium carboxylate pillar[5]arene (WP5-Na) and ammonium carboxylate pillar[5]arene (WP5-NH₄), which inspire us to construct a promising rotary platform in anionic pillar[5]arenes (WP5) skeletons. Herein, we demonstrate the non-negligible effect of counter cations on rotational barriers of hydroquinone rings in WP5, which enables a cation grease/brake rotor system. Applications of this tunable rotor system as fluorescence switch and anti-counterfeiting ink are further explored.

Design, Synthesis, and Physicochemical Studies of Configurationally Stable β-Carboline Atropisomers

Axially chiral atropisomers have energetic barriers to rotation, ΔG_rot, that prevent racemization of the respective enantiomers. We used computational modeling to develop a suite of 10 bio-inspired 1-aryl-β-carbolines with varying ΔG_rot, from which a strong structure-activity relationship was observed for 2-substituted-1-naphthyl substituents. We then synthesized two of these atropisomers, 1d and 1f, by a four-step racemic synthesis and resolved the enantiomers via chiral chromatography. Racemization studies revealed experimental ΔG_rot values of 39.5 and 33.0 kcal/mol for 1d and 1f, respectively, which were consistent with our computational results. These atropisomers exhibited long half-lives, which allowed for their physicochemical characterization and stereochemical assignment via UV-vis spectroscopy, fluorescence spectroscopy, electronic circular dichroism, and vibrational circular dichroism.

Chemically Driven Rotatory Molecular Machines

Molecular machines are at the frontier of biology and chemistry. The ability to control molecular motion and emulating the movement of biological systems are major steps towards the development of responsive and adaptive materials. Amazing progress has been seen for the design of molecular machines including light-induced unidirectional rotation of overcrowded alkenes. However, the feasibility of inducing unidirectional rotation about a single bond as a result of chemical conversion has been a challenging task. In this Review, an overview of approaches towards the design, synthesis, and dynamic properties of different classes of atropisomers which can undergo controlled switching or rotation under the influence of a chemical stimulus is presented. They are categorized as molecular switches, rotors, motors, and autonomous motors according to their type of response. Furthermore, we provide a future perspective and challenges focusing on building sophisticated molecular machines.

Atropisomers with Axial and Point Chirality: Synthesis and Applications

Enantiopure atropisomers have become increasingly important in asymmetric synthesis and catalysis, pharmaceutical science, and material science since the discovery of inherent features of axial chirality originating from rotational restriction. Despite the advances made in this field to date, it remains highly desirable to construct structurally diverse atropisomers with potentially useful functions. We propose superposition to match axial and point chirality as a potentially useful strategy to access structurally complex and diverse building blocks for organic synthesis and pharmaceutical science because merging atropisomeric backbones with one or more extra chiral elements can topologically broaden three-dimensional environments to create complex scaffolds with multiple tunable parameters. Over the past decade, we have successfully implemented a strategic design for the superposition of axial and point chirality to develop a series of enantiopure atropisomers and have utilized the synergistic functions of these molecules to enhance chirality transfer in various catalytic asymmetric transformations.In this Account, we present several novel atropisomers with superposed axial and point chirality developed in our laboratory. In our studies, this superposition strategy was used to design and synthesize both biaryl and non-biaryl atropisomers from commercially available chiral sources. Consequently, these atropisomers were used to demonstrate the importance of the synergetic functions of axial and point chirality in specific enantioselective reactions. For example, aromatic amide-derived atropisomers, simplified as Xing-Phos arrays, were broadly employed in Ag-catalyzed [3 + 2] cycloaddition by a series of reactions of aldiminoesters with activated alkenes and imines, as well as being used as chiral solvating agents for the discrimination of optically active mandelic acid derivatives. Considering the powerful potential of non-biaryl atropisomers for asymmetric catalysis, we also explored the transition-metal-catalyzed enantioselective construction of a novel backbone of non-biaryl atropisomers (Ar-alkene, Ar-N axis) bearing both axial and point chirality for the design and synthesis of chiral ligands and functional molecules.The studies presented herein are expected to stimulate further research efforts on the development of functional atropisomers by superposition of matching axial and point chirality. In addition to tunable electron and stereohindrance effects, the synergy between matching chiral elements of axial/point chirality and functional groups is proven to be a special function that cannot be ignored for promoting reactivity and chirality-transfer efficiency in enantioselective synthesis. Consequently, our novel types of scaffolds with superposed axial and point chirality that are capable of versatile coordination with various metal catalysts in asymmetric catalysis highlight the power of the superposition of matching axial and point chirality for the construction of synthetically useful atropisomers.

Acid-base-induced fac → mer isomerization of luminescent iridium(iii) complexes

Luminescent Ir(C^N)3 complexes (C^N = cyclometalated arylpyridine ligand) exist in the form of two stable isomers with distinct photophysical and electrochemical properties: fac and mer. Herein, we show that fac-Ir(C^N)3 complexes can be converted into the thermodynamically less stable mer forms by a consecutive reaction with first acid and then base. The chemically induced isomerization is fast, quantitative, and stereoselective, and it can be inversed by light. The new isomerization process opens the possibility to use highly luminescent Ir(C^N)3 complexes as molecular switches.

Intrinsically unidirectional chemically fuelled rotary molecular motors

Biological systems mainly utilize chemical energy to fuel autonomous molecular motors, enabling the system to be driven out of equilibrium¹. Taking inspiration from rotary motors such as the bacterial flagellar motor² and adenosine triphosphate synthase³, and building on the success of light-powered unidirectional rotary molecular motors^4-6, scientists have pursued the design of synthetic molecular motors solely driven by chemical energy^7-13. However, designing artificial rotary molecular motors operating autonomously using a chemical fuel and simultaneously featuring the intrinsic structural design elements to allow full 360° unidirectional rotary motion like adenosine triphosphate synthase remains challenging. Here we show that a homochiral biaryl Motor-3, with three distinct stereochemical elements, is a rotary motor that undergoes repetitive and unidirectional 360° rotation of the two aryl groups around a single-bond axle driven by a chemical fuel. It undergoes sequential ester cyclization, helix inversion and ring opening, and up to 99% unidirectionality is realized over the autonomous rotary cycle. The molecular rotary motor can be operated in two modes: synchronized motion with pulses of a chemical fuel and acid-base oscillations; and autonomous motion in the presence of a chemical fuel under slightly basic aqueous conditions. This rotary motor design with intrinsic control over the direction of rotation, simple chemical fuelling for autonomous motion and near-perfect unidirectionality illustrates the potential for future generations of multicomponent machines to perform mechanical functions.

Recent Advances in Fuel-Driven Molecular Switches and Machines

The molecular switches and machines arena has entered a new phase in which molecular machines operate under out-of-equilibrium conditions using appropriate fuel. Unlike the equilibrium version, the dissipative off-equilibrium machines necessitate only one stimulus input to complete each cycle and decrease chemical waste. Such a modus operandi would set significant steps towards mimicking the natural machines and may offer a platform for advancing new applications by providing temporal control. This review summarises the recent progress and blueprint of autonomous fuel-driven off-equilibrium molecular switches and machines.

Active Mechanical Threading by a Molecular Motor

Molecular motors transform external energy input into directional motions and offer exquisite precision for nano‐scale manipulations. To make full use of molecular motor capacities, their directional motions need to be transmitted and used for powering downstream molecular events. Here we present a macrocyclic molecular motor structure able to perform repetitive molecular threading of a flexible tetraethylene glycol chain through the macrocycle. This mechanical threading event is actively powered by the motor and leads to a direct translation of the unidirectional motor rotation into unidirectional translation motion (chain versus ring). The mechanism of the active mechanical threading is elucidated and the actual threading step is identified as a combined helix inversion and threading event. The established molecular machine function resembles the crucial step of macroscopic weaving or sewing processes and therefore offers a first entry point to a “molecular knitting” counterpart.

Construction and physical properties of low-dimensional structures for nanoscale electronic devices

Over the past decades, construction of nanoscale electronic devices with novel functionalities based on low-dimensional structures, such as single molecules and two-dimensional (2D) materials, has been rapidly developed. To investigate their intrinsic properties for versatile functionalities of nanoscale electronic devices, it is crucial to precisely control the structures and understand the physical properties of low-dimensional structures at the single atomic level. In this review, we provide a comprehensive overview of the construction of nanoelectronic devices based on single molecules and 2D materials and the investigation of their physical properties. For single molecules, we focus on the construction of single-molecule devices, such as molecular motors and molecular switches, by precisely controlling their self-assembled structures on metal substrates and charge transport properties. For 2D materials, we emphasize their spin-related electrical transport properties for spintronic device applications and the role that interfaces among 2D semiconductors, contact electrodes, and dielectric substrates play in the electrical performance of electronic, optoelectronic, and memory devices. Finally, we discuss the future research direction in this field, where we can expect a scientific breakthrough.

Controlling forward and backward rotary molecular motion on demand

Synthetic molecular machines hold tremendous potential to revolutionize chemical and materials sciences. Their autonomous motion controlled by external stimuli allows to develop smart materials whose properties can be adapted on command. For the realisation of more complex molecular machines, it is crucial to design building blocks whose properties can be controlled by multiple orthogonal stimuli. A major challenge is to reversibly switch from forward to backward and again forward light-driven rotary motion using external stimuli. Here we report a push-pull substituted photo-responsive overcrowded alkene whose function can be toggled between that of a unidirectional 2^nd generation rotary motor and a molecular switch depending on its protonation and the polarity of its environment. With its simplicity in design, easy preparation, outstanding stability and orthogonal control of distinct forward and backward motions, we believe that the present concept paves the way for creating more advanced molecular machines.

Being able to control motion at the molecular level is vital for many future developments in the molecular sciences. Here, the authors report the controlled forward and backward rotation of a molecular motor guided by external stimuli.

Autonomous fuelled directional rotation about a covalent single bond

Biology operates through autonomous chemically fuelled molecular machinery¹, including rotary motors such as adenosine triphosphate synthase² and the bacterial flagellar motor³. Chemists have long sought to create analogous molecular structures with chemically powered, directionally rotating, components^4-17. However, synthetic motor molecules capable of autonomous 360° directional rotation about a single bond have proved elusive, with previous designs lacking either autonomous fuelling^7,10,12 or directionality⁶. Here we show that 1-phenylpyrrole 2,2'-dicarboxylic acid^18,19 (1a) is a catalysis-driven^20,21 motor that can continuously transduce energy from a chemical fuel^9,20-27 to induce repetitive 360° directional rotation of the two aromatic rings around the covalent N-C bond that connects them. On treatment of 1a with a carbodiimide^21,25-27, intramolecular anhydride formation between the rings and the anhydride's hydrolysis both occur incessantly. Both reactions are kinetically gated^28-30 causing directional bias. Accordingly, catalysis of carbodiimide hydration by the motor molecule continuously drives net directional rotation around the N-C bond. The directionality is determined by the handedness of both an additive that accelerates anhydride hydrolysis and that of the fuel, and is easily reversed additive³¹. More than 97% of fuel molecules are consumed through the chemical engine cycle²⁴ with a directional bias of up to 71:29 with a chirality-matched fuel and additive. In other words, the motor makes a 'mistake' in direction every three to four turns. The 26-atom motor molecule's simplicity augurs well for its structural optimization and the development of derivatives that can be interfaced with other components for the performance of work and tasks^32-36.

Insights from an information thermodynamics analysis of a synthetic molecular motor

Information is physical, a realization that has transformed the physics of measurement and communication. However, the flow between information, energy and mechanics in chemical systems remains largely unexplored. Here we analyse a minimalist autonomous chemically driven molecular motor in terms of information thermodynamics, a framework that quantitatively relates information to other thermodynamic parameters. The treatment reveals how directional motion is generated by free energy transfer from chemical to mechanical (conformational and/or co-conformational) processes by 'energy flow' and 'information flow'. It provides a thermodynamic level of understanding of molecular motors that is general, complements previous analyses based on kinetics and has practical implications for machine design. In line with kinetic analysis, we find that power strokes do not affect the directionality of chemically driven machines. However, we find that power strokes can modulate motor velocity, the efficiency of free energy transfer and the number of fuel molecules consumed per cycle. This may help explain the role of such (co-)conformational changes in biomachines and illustrates the interplay between energy and information in chemical systems.

Off-Equilibrium Speed Control of a Multistage Molecular Rotor: 2-Fold Chemical Fueling by Acid or Silver(I)

Driving conformational motion in defined off-equilibrium oscillations can be achieved using chemical fuels. When the ultrafast turnstile 1 (k₂₉₈> 10¹² Hz) was fueled with 2-cyano-2-phenylpropanoic acid (Fuel 1), the diprotonated rotor [H₂(1)]²⁺ (k₂₉₈ = 84.0 kHz) formed as a transient regaining the dynamics of the initial turnstile after consumption of the fuel (135 min). Upon addition of silver(I) (Fuel 2) to turnstile 1, the metastable rotor [Ag₂(1)]²⁺ (k₂₉₈ = 1.57 Hz) was initially furnished, but due to a consequentially triggered S_N2 reaction, the Ag⁺ ions were consumed as insoluble AgBr along with regeneration of 1 (within 3 h). The off-equilibrium fast ⇆ slow rotor conversions fueled by acid and silver(I) were directly monitored by fluorescence and ¹H NMR. In addition, metal ion exchange was fueled enabling off-equilibrium oscillations between rotors [Li₂(1)]²⁺ ⇆ [Ag₂(1)]²⁺. In the end, both sustainability and efficiency of the process were increased in unison by using the interfering proton waste in the formation of a [2]pseudorotaxane.

Chemically Fueled Self-Assembly in Biology and Chemistry

Life is a non-equilibrium state of matter maintained at the expense of energy. Nature uses predominantly chemical energy stored in thermodynamically activated, but kinetically stable, molecules. These high-energy molecules are exploited for the synthesis of other biomolecules, for the activation of biological machinery such as pumps and motors, and for the maintenance of structural order. Knowledge of how chemical energy is transferred to biochemical processes is essential for the development of artificial systems with life-like processes. Here, we discuss how chemical energy can be used to control the structural organization of organic molecules. Four different strategies have been identified according to a distinguishable physical-organic basis. For each class, one example from biology and one from chemistry are discussed in detail to illustrate the practical implementation of each concept and the distinct opportunities they offer. Specific attention is paid to the discussion of chemically fueled non-equilibrium self-assembly. We discuss the meaning of non-equilibrium self-assembly, its kinetic origin, and strategies to develop synthetic non-equilibrium systems.

Acylhydrazone Subunits as a Proton Cargo Delivery System in 7-Hydroxyquinoline

The reimagined concept of long-range tautomeric proton transfer using crane subunits is shown by designing and synthesising two new acylhydrazones containing a 7-hydroxyquinoline (7-OHQ) platform. The acylhydrazone subunits attached to the 7-OHQ at the 8th position act as crane arms for delivering proton cargo to the quinoline nitrogen. Light-induced tautomerization to their keto forms leads to Z/E isomerization of the C=C axle bond, followed by proton delivery to the quinoline nitrogen by the formation of covalent or hydrogen bonds. The axle's being either an imine or ketimine bond is the structural difference between the studied compounds. The -CH₃ group in the latter provides steric strain, resulting in different proton transport pathways. Both compounds show long thermal stability in the switched state, which creates a tuneable action of bidirectional proton cargo transport by using different wavelengths of irradiation. Upon the addition of acid, the quinoline nitrogen is protonated; this results in E/Z configuration switching of the acylhydrazone subunits. This was proven by single-crystal X-ray structure analysis and NMR spectroscopy.

Long-term switching of single photochromic triads based on dithienylcyclopentene and fluorophores at cryogenic temperatures

Photochromic molecules can be reversibly converted between two bistable forms by light. These systems have been intensively studied for applications as molecular memories, sensing devices, or super-resolution optical microscopy. Here, we study the long-term switching behavior of single photochromic triads under oxygen-free conditions at 10 K. The triads consist of a photochromic unit that is covalently linked to two strong fluorophores that were employed for monitoring the light-induced conversions of the switch via changes in the fluorescence intensity from the fluorophores. As dyes we use either perylene bisimide or boron-dipyrromethen, and as photochromic switch we use dithienylcyclopentene (DCP). Both types of triads showed high fatigue resistance allowing for up to 6000 switching cycles of a single triad corresponding to time durations in the order of 80 min without deterioration. Long-term analysis of the switching cycles reveals that the probability that an intensity change in the emission from the dyes can be assigned to an externally stimulated conversion of the DCP (rather than to stochastic blinking of the dye molecules) amounts to 0.7 ± 0.1 for both types of triads. This number is far too low for optical data storage using single triads and implications concerning the miniaturization of optical memories based on such systems will be discussed. Yet, together with the high fatigue resistance, this number is encouraging for applications in super-resolution optical microscopy on frozen biological samples.

Ion shuttling between emulsion droplets by crown ether modified gold nanoparticles

Selective unidirectional transport of barium ions between droplets in a water-in-chloroform emulsion is demonstrated. Gold nanoparticles (GNPs) modified with a thiolated crown ether act as barium ion complexing shuttles that carry the ions from one population of droplets (source) to another (target). This process is driven by a steep barium ion concentration gradient between source and target droplets. The concentration of barium ions in the target droplets is kept low at all times by the precipitation of insoluble barium sulfate. A potential role of electrostatically coupled secondary processes that maintain the electroneutrality of the emulsion droplets is discussed. Charging of the GNP metal cores by electron transfer in the presence of the Fe(ii)/Fe(iii) redox couple appears to affect the partitioning of the GNPs between the water droplets and the chloroform phase. Processes have been monitored and studied by optical microscopy, Raman spectroscopy, cryogenic scanning electron microscopy (cryo-SEM) and zeta potential. The shuttle action of the GNPs has further been demonstrated electrochemically in a model system.

Recent Advances in Catalytic Asymmetric Construction of Atropisomers

Atropisomerism is a stereochemical behavior portrayed by three-dimensional molecules that bear rotationally restricted σ bond. Akin to the well-represented point-chiral molecules, atropisomerically chiral compounds are finding increasing utilities in many disciplines where molecular asymmetry is influential. This provides steady demand on atroposelective synthesis, where numerous synthetic pursuits have been rewarded with conceptually novel and streamlined methods while expanding the structural diversity of atropisomers. This review summarizes key achievements in stereoselective preparation of biaryl, heterobiaryl, and nonbiaryl atropisomers documented between 2015 and 2020. Emphasis is placed on the synthetic strategies for each structural class, while examples are cited to illustrate the potential applications of the accessed atropochiral targets.

Photo-responsive Helical Motion by Light-Driven Molecular Motors in a Liquid-Crystal Network

Controlling sophisticated motion by molecular motors is a major goal on the road to future actuators and soft robotics. Taking inspiration from biological motility and mechanical functions common to artificial machines, responsive small molecules have been used to achieve macroscopic effects, however, translating molecular movement along length scales to precisely defined linear, twisting and rotary motions remain particularly challenging. Here, we present the design, synthesis and functioning of liquid‐crystal network (LCN) materials with intrinsic rotary motors that allow the conversion of light energy into reversible helical motion. In this responsive system the photochemical‐driven molecular motor has a dual function operating both as chiral dopant and unidirectional rotor amplifying molecular motion into a controlled and reversible left‐ or right‐handed macroscopic twisting movement. By exploiting the dynamic chirality, directionality of motion and shape change of a single motor embedded in an LC‐network, complex mechanical motions including bending, walking and helical motion, in soft polymer materials are achieved which offers fascinating opportunities toward inherently photo‐responsive materials.

The Transient Covalent Bond in Abiotic Nonequilibrium Systems

Biochemical systems accomplish many critical functions with by operating out-of-equilibrium using the energy of chemical fuels. The formation of a transient covalent bond is a simple but very effective tool in designing analogous reaction networks. This Minireview focuses on the fuel chemistries that have been used to generate transient bonds in recent demonstrations of abiotic nonequilibrium systems (i.e., systems that do not make use of biological components). Fuel reactions are divided into two fundamental classifications depending on whether the fuel contributes structural elements to the activated state, a distinction that dictates how they can be used. Reported systems are further categorized by overall fuel reaction (e.g., hydrolysis of alkylating agents, carbodiimide hydration) and illustrate how similar chemistry can be used to effect a wide range of nonequilibrium behavior, ranging from self-assembly to the operation of molecular machines.

Atroposelective transformation of axially chiral (hetero)biaryls. From desymmetrization to modern resolution strategies

Atroposelective transformations of (hetero)biaryls are classified into desymmetrization, kinetic resolution, dynamic kinetic resolution, and dynamic kinetic asymmetric transformation depending on the nature and behavior of the starting material.

Effects of n → π* Orbital Interactions on Molecular Rotors: The Control and Switching of Rotational Pathway and Speed

The role of n → π* orbital interactions in the rotational pathway and barrier of biaryl-based molecular rotors was elucidated through a combined experimental and computational study. The n → π* interaction in the transition state can lead to the acceleration of rotors. The competition between the n → π* interaction and hydrogen bonding further enabled the reversal of the pathway and greasing/braking the rotor in response to acid/base stimuli, thereby creating a switchable molecular rotor.

Reversible Soft Mechanochemical Control of Biaryl Conformations through Crosslinking in a 3D Macromolecular Network

Tuning the dihedral angle (DA) of axially chiral compounds can impact biological activity, catalyst efficiency, molecular motor performance, or chiroptical properties. Herein, we report gradual, controlled, and reversible changes in molecular conformation of a covalently linked binaphthyl moiety within a 3D polymeric network by application of a macroscopic stretching force. We managed direct observation of DA changes by measuring the circular dichroism signal of an optically pure BINOL-crosslinked elastomer network. Stretching the elastomer resulted in a widening of the DA between naphthyl rings when the BINOL was doubly grafted to the elastomer network; no effect was observed when a single naphthyl ring of the BINOL was grafted to the elastomer network. We have determined that ca. 170 % extension of the elastomers led to the transfer of a mechanical force to the BINOL moiety of 2.5 kcal mol^-1  Å^-1 (ca. 175 pN) in magnitude and results in the opening of the DA of BINOL up to 130°.