Improving Fatigue Resistance and Autonomous Switching of pH Responsive Hydrazones by Pulses of a Chemical Fuel

The chemically triggered reversible switching of pH-responsive hydrazones involves rotary motion-induced configurational changes, serving as a prototype for constructing an array of molecular machines. Typically, the configurational isomerization of such switches into two distinct forms (E/Z) occurs through the alteration of the pH the medium, achieved by successive additions of acid and base stimuli. However, this process results in intermittent operation due to the concomitant accumulation of salt after each cycle, limiting switching performance to only a few cycles (5-6). In this context, we introduce a novel strategy for the autonomous E/Z isomerization of hydrazones in acetonitrile using pulses of trichloroacetic acid as a chemical fuel. The use of this transient acid enabled reversible switching of hydrazones even after 50 cycles without causing significant fatigue. To test the broad viability of the fuel, a series of ortho/para-substituted hydrazones were synthesized and their switching performance was investigated. The analysis of kinetic data showed a strong dependency of switching operations including the lifetime of transient state, on the electronic properties of substituents. Finally, a distinct color change from yellow to orange due to reversible switching of the para-methoxy substituted hydrazone was employed for the creation of rewritable messages on commercially available paper.

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.

Cinematographic study of stochastic chemical events at atomic resolution

The advent of single-molecule atomic-resolution time-resolved electron microscopy (SMART-EM) has created a new field of 'cinematic chemistry,' allowing for the cinematographic recording of dynamic behaviors of organic and inorganic molecules and their assembly. However, the limited electron dose per frame of video images presents a major challenge in SMART-EM. Recent advances in direct electron counting cameras and techniques to enhance image quality through the implementation of a denoising algorithm have enabled the tracking of stochastic molecular motions and chemical reactions with sub-millisecond temporal resolution and sub-angstrom localization precision. This review showcases the development of dynamic molecular imaging using the SMART-EM technique, highlighting insights into nanomechanical behavior during molecular shuttle motion, pathways of multistep chemical reactions, and elucidation of crystallization processes at the atomic level.

Guest-Shuttling in a Nanosized Metallobox

An iridium-conjoined long and narrow metallorectangle was obtained by combining a quinoxalinophenanthrophenazine-connected Janus-di-imidazolylidene ligand and pyrazine. The size and shape of this assembly together with the fused polyaromatic nature of its panels provides it with properties that are uncommon for other metallosupramolecular assemblies. For example, this nanosized 'slit-like' metallobox is able show very large binding affinities with planar organic molecules in such a way, that the cavity is asymmetrically occupied by the guest molecule. This unsymmetrical conformation leads to the existence of a large amplitude motion of these guests, which slide between the two sides of the cavity of the host, thus constituting rare examples of molecular shuttles.

Bispidine as a promising scaffold for designing molecular machines

The development of artificial molecular machines is a challenging endeavor. Herein, we have synthesized a series of bispidine diamides D1-D6 that exhibit rotation reminiscent of a motor motion. Dynamic NMR, X-ray diffraction, quantum mechanical calculations, and molecular dynamics simulations provided insights into their rotational dynamics. All the diamides D1-D6 exhibited mutually independent rotation around the two bispidine arms. However, the rate of rotation and the presence or absence of directionality in amide bond rotation were found to depend on the solvent, temperature, and nature of substitution on the amide carbonyl. These engineered systems may aid in the development of biologically relevant synthetic molecular motors. Studies on homochiral and heterochiral bispidine-peptides revealed that the direction of rotation can be controlled by chirality and the nature of the amino acid.

Controlling molecular shuttling in a rotaxane with weak ring recognition sites

We describe a rotaxane molecular shuttle encompassing triazolium and tertiary ammonium units as weak recognition sites for the ring. Such a design, which differs from that of typical controllable rotaxanes, allows the precise tuning of the ring distribution among the two sites - i.e., the coconformational equilibrium - by changing the solvent polarity or the nature of the counteranions. Shuttling of the ring between the two stations can also be toggled by acid-base stimuli. Such an approach is paradigmatic to obtain rotaxanes that can sense environmental changes and transduce them into a coconformational response and opens avenues for novel applications in sensing and stimuli-responsive materials.

The Diversity of Cucurbituril Molecular Switches and Shuttles

Ring translocation switches and shuttles featuring a macrocycle (or a ring molecule) navigating between two or more stations continue to attract attention. While the vast majority of these systems are developed in organic solvents, the cucurbituril (CB) macrocycles are ideally suited to prepare such systems in water. Indeed, their stability and their relatively high affinity for relevant guest molecules are key attributes toward translating the progresses made in organic solvents, into water. This concept article summarizes the findings, key advances and multiple possibilities offered by CBs toward advanced molecular switches and shuttles in water.

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.

[3]Foldarotaxane-mediated synthesis of an improbable [2]rotaxane

The wrapping of an aromatic oligoamide helix around an active ester-containing [2]rotaxane enforced the sliding and the sequestration of the surrounding macrocycle around a part of the axle for which it has no formal affinity. The foldamer-mediated compartmentalization of the [2]rotaxane shuttle was subsequently used to prepare an improbable rotaxane.

Foldaxanes: Rotaxane-like Architectures from Foldamers

Mechanically interlocked molecules such as rotaxanes and catenanes contain free-moving components that cannot dissociate and have enabled the investigation and control of various translational and rotational molecular motions. The architecture of pseudo-rotaxanes and of some kinetically labile rotaxanes is comparable to that of rotaxanes but their components are reversibly associated and not irreversibly interlocked. In other words, pseudo-rotaxanes may fall apart. This Account focuses on a peculiar family of rotaxane-like architectures termed foldaxanes.Foldaxanes consist of a helically folded oligomer wound around a rod-like dumbbell-shaped guest. Winding of the helix around the rod thus entails an unwinding-rewinding process that creates a kinetic barrier. It follows that foldaxanes, albeit reversibly assembled, have significant lifetimes and may not fall apart while defined molecular motions are triggered. Foldaxanes based on helically folded aromatic oligoamide hosts and oligo(alkyl carbamate) guests can be designed rationally through the inclusion of complementary binding motifs on the rod and at the inner rim of the helix so that helix length and rod length match. Single helical foldaxanes (bimolecular species) and double helical foldaxanes (trimolecular species) have thus been produced as well as poly[n]foldaxanes, in which several helices bind to long rods with multiple binding stations. When the binding stations differ and are organized in a certain sequence, a complementary sequence of different stacked helices, each matching with their binding station, can be assembled, thus reproducing in an artificial system a sort of translation process.Foldaxane helix handedness may be controlled by stereogenic centers on the rod-like guest. Handedness can also be transmitted from helix to helix in polyfoldaxanes. Foldaxane formation has drastic consequences for the rod properties, including its stiffening and the restriction of the mobility of a macrocycle already interlocked on the rod. Fast translation (without dissociation) of helices along rod-like guests has been demonstrated. Because of the helical nature of the hosts, translation may be accompanied by rotation in various sorts of screw-like motions. The possibility, on longer time scales, for the helix to dissociate from and reassociate to the rod has allowed for the design of complex, kinetically controlled supramolecular pathways of a helix on a rod. Furthermore, the design of helices with a directionality, that is, with two distinct termini, that bind to nonsymmetrical rod-like guests in a defined orientation makes it possible to also control the orientation of molecular motion. Altogether, foldaxanes constitute a distinct and full-of-potential family of rotaxane-like architectures that possess designer structures and allow orchestration of the time scales of various supramolecular events.

Recent Progress in Light-Driven Molecular Shuttles

Molecular shuttles are typical molecular machines that could be applied in various fields. The motion modes of wheel components in rotaxanes could be strategically modulated by external stimuli, such as pH, ions, solvent, light, and so on. Light is particularly attractive because it is harmless and can be operated in a remote mode and usually no byproducts are formed. Over the past decade, many examples of light-driven molecular shuttles are emerging. Accordingly, this review summarizes the recent research progress of light-driven molecular shuttles. First, the light-driven mechanisms of molecular motions with different functional groups are discussed in detail, which show how to drive photoresponsive or non-photoresponsive molecular shuttles. Subsequently, the practical applications of molecular shuttles in different fields, such as optical information storage, catalysis for organic reactions, drug delivery, and so on, are demonstrated. Finally, the future development of light-driven molecular shuttle is briefly prospected.

Photochemical Energy Conversion with Artificial Molecular Machines

The exploitation of sunlight as a clean, renewable, and distributed energy source is key to facing the energetic demand of modern society in a sustainable and affordable fashion. In the past few decades, chemists have learned to make molecular machines, that is, synthetic chemical systems in which energy inputs cause controlled movements of molecular components that could be used to perform a task. A variety of artificial molecular machines operated by light have been constructed by implementing photochemical processes within appropriately designed (supra)molecular assemblies. These studies could open up new routes for the realization of nanostructured devices and materials capable to harness, convert, and store light energy.

Designing light-driven rotary molecular motors

The ability to induce and amplify motion at the molecular scale has seen tremendous progress ranging from simple molecular rotors to responsive materials. In the two decades since the discovery of light-driven rotary molecular motors, the development of these molecules has been extensive; moving from the realm of molecular chemistry to integration into dynamic molecular systems. They have been identified as actuators holding great potential to precisely control the dynamics of nanoscale devices, but integrating molecular motors effectively into evermore complex artificial molecular machinery is not trivial. Maximising efficiency without compromising function requires conscious and judicious selection of the structures used. In this perspective, we focus on the key aspects of motor design and discuss how to manipulate these properties without impeding motor integrity. Herein, we describe these principles in the context of molecular rotary motors featuring a central double bond axle and emphasise the strengths and weaknesses of each design, providing a comprehensive evaluation of all artificial light-driven rotary motor scaffolds currently present in the literature. Based on this discussion, we will explore the trajectory of research into the field of molecular motors in the coming years, including challenges to be addressed, potential applications, and future prospects.

Radical-pairing-induced molecular assembly and motion

Radical-pairing interactions between conjugated organic π-radicals are relative newcomers to the inventory of molecular recognition motifs explored in supramolecular chemistry. The unique electronic, magnetic, optical and redox-responsive properties of the conjugated π-radicals render molecules designed with radical-pairing interactions useful for applications in various areas of chemistry and materials science. In particular, the ability to control formation of radical cationic or anionic species, by redox stimulation, provides a flexible trigger for directed assembly and controlled molecular motions, as well as a convenient means of inputting energy to fuel non-equilibrium processes. In this Review, we provide an overview of different examples of radical-pairing-based recognition processes and of their emerging use in (1) supramolecular assembly, (2) templation of mechanically interlocked molecules, (3) stimuli-controlled molecular switches and, by incorporation of kinetic asymmetry in the design, (4) the creation of unidirectional molecular transporters based on pumping cassettes powered by fuelled switching of radical-pairing interactions. We conclude the discussion with an outlook on future directions for the field.

Interplay between a Foldamer Helix and a Macrocycle in a Foldarotaxane Architecture

The design and synthesis of a novel rotaxane/foldaxane hybrid architecture is reported. The winding of an aromatic oligoamide helix host around a dumbbell-shaped thread-like guest, or axle, already surrounded by a macrocycle was evidenced by NMR spectroscopy and X-ray crystallography. The process proved to depend on the position of the macrocycle along the axle and the associated steric hindrance. The macrocycle thus behaves as a switchable shield that modulates the affinity of the helix for the axle. Reciprocally, the foldamer helix acts as a supramolecular auxiliary that compartmentalizes the axle. In some cases, the macrocycle is forced to move along the axle to allow the foldamer to reach its best recognition site.

Translational dynamics of a non-degenerate molecular shuttle imbedded in a zirconium metal-organic framework

A new [2]rotaxane molecular shuttle linker based on the binding of a 24-crown-8 ether macrocycle at a benzimidazole recognition site was synthesised. The shuttling dynamics of the linker were studied in solution and the structure confirmed by X-ray crystallography. A multivariate Zr(iv) MOF, UWDM-11, containing the new MIM linker and primary linker tetramethylterphenyldicarboxylate was synthesised and the translational motion of the molecular shuttle studied in the solid state. The use of a 13C enriched MIM linker allowed the dynamics of both activated and mesitylene-solvated UWDM-11 to be elucidated by VT 13C CPMAS SSNMR. The incorporation of mesitylene into the pores of UWDM-11 resulted in a significant increase in the barrier for thermally driven translation of the macrocycle.

Aryl Migration on Graphene

The design and synthesis of molecular systems able to carry out movements resembling macroscopic objects is an exciting and challenging endeavor. Molecules that can walk covalently on a track have been demonstrated, and we now report how aryl groups that can migrate over a graphene surface. Specifically, we describe a system comprised of covalently functionalized aryl groups on graphene that undergo continuous aryl shifts. The dynamic aryl shift allows the aryl groups on graphene to effectively migrate step-by-step wherein each step involves reversible bond breaking and making that is initiated by a combination of an activated arene and p-doping of the graphene surface. Raman spectroscopic mapping of the distribution of the covalent attachment revealed that activated 4-methoxyphenyl groups migrate several microns from regions of high functionalization to regions with no prior functionalization.

Photoactivated Artificial Molecular Machines that Can Perform Tasks

Research on artificial photoactivated molecular machines has moved in recent years from a basic scientific endeavor toward a more applicative effort. Nowadays, the prospect of reproducing the operation of natural nanomachines with artificial counterparts is no longer a dream but a concrete possibility. The progress toward the construction of molecular-machine-based devices and materials in which light irradiation results in the execution of a task as a result of nanoscale movements is illustrated here. After a brief description of a few basic types of photoactivated molecular machines, significant examples of their exploitation to perform predetermined functions are presented. These include switchable catalysts, nanoactuators that interact with cellular membranes, transporters of small molecular cargos, and active joints capable of mechanically coupling molecular-scale movements. Investigations aimed at harnessing the collective operation of a multitude of molecular machines organized in arrays to perform tasks at the microscale and macroscale in hard and soft materials are also reviewed. Surfaces, gels, liquid crystals, polymers, and self-assembled nanostructures are described wherein the nanoscale movement of embedded molecular machines is amplified, allowing the realization of muscle-like actuators, microfluidic devices, and polymeric materials for light energy transduction and storage.

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

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

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.

Step-by-step reaction-powered mechanical motion triggered by a chemical fuel pulse

Natural molecular machines perform important tasks in organisms under a reasonable energy supply strategy: a series of step-by-step biochemical reactions after the intake of a fuel such as glucose. As analogues of biomolecular machines, most artificial molecular machines and shuttles are still powered by two opposite stimuli, such as acid and base or oxidation and reduction. This inconvenient stimulus method for artificial molecular machines and shuttles creates great obstacles for realizing more functions of artificial molecular machines and shuttles owing to low energy utilization efficiency and troublesome operation. In this work, we report a new step-by-step reaction system combining two known reactions: (1) the formation of [bis(trifluoroacetoxy)iodo]benzene from the reaction of iodosylbenzene and trifluoroacetic acid; and (2) the catalytic oxidation of alcohols by [bis(trifluoroacetoxy)iodo]benzene in the presence of TEMPO. Owing to the acid absorption features of the first reaction and the acid releasing characteristics of the second reaction, the new step-by-step reaction obtains a pH oscillation feature. Taking advantage of the pH oscillation feature, the new reaction cycle triggered by iodosylbenzene was coupled with an acid-base switchable helicarene-based molecular shuttle. Benefitting from the reaction rate difference of the two known reactions (the reaction rate of the first reaction is faster than that of the second reaction), the movement of the helicarene on the axle could be controlled automatically by the reaction system through adding iodosylbenzene to a solution of alcohol, TEMPO and protonated rotaxane, and the manual addition of another opposite stimulus could be avoided.

Light triggers molecular shuttling in rotaxanes: control over proximity and charge recombination

The lifetime of a charge separated state is enhanced by the effects of solvent polarity and the coordination controlled shuttling of a dumbbell in a porphyrin/fullerene rotaxane.

We present the synthesis of novel rotaxanes based on mechanically interlocked porphyrins and fullerene and their advanced investigations by means of photophysical measurements. To this end, a fullerene-capped dumbbell-type axle containing a central triazole was threaded through strapped (metallo)porphyrins—either a free-base or a zinc porphyrin. Femtosecond-resolved transient absorption measurements revealed charge-separation between the porphyrin and fullerene upon light excitation. Solvent polarity and solvent coordination effects induced molecular motion of the rotaxanes upon charge separation and enabled, for the first time, subtle control over the charge recombination by enabling and controlling the directionality of shuttling.

Machine Metathesis: Thermal and Catalyzed Exchange of Piston Rods in Multicomponent Nanorotor/Nanoslider Ensemble

Three-component nanorotor R1 ( k₂₉₈ = 80 kHz) and two-component slider-on-deck DS2 ( k₂₉₈ = 440 kHz) were prepared from rotator S1 and stator [Cu₃(1)]³⁺ and from S2 and deck D, respectively. Mixing of R1 with DS2 leads to clean metathesis, furnishing the slower nanodevices R2 ( k₂₉₈ = 29.6 kHz) and DS1 ( k₂₉₈ = 32.2 kHz). Exchange of the piston rods S1 and S2 is completed within 22 min (uncatalyzed) or 3 min (catalyzed) at 298 K.

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.

pH-Gated photoresponsive shuttling in a water-soluble pseudorotaxane

Phototriggered ring shuttling in a water-soluble fluorescent pseudorotaxane can be enabled and disabled at different pH values.

Self-locked dipillar[5]arene-based pseudo[1]rotaxanes and bispseudo[1]rotaxanes with different lengths of bridging chains

The relationships between lengths of bridging chains and self-locked behaviors of dipillar[5]arene-based pseudo[1]rotaxanes and bispseudo[1]rotaxanes were studied in detail.

Fourier transform infrared and Raman spectroscopy in the study of phase transitions in dipyrazolium iodide triiodide: Experimental and theoretical analysis

The paper presents the Infrared and Raman spectra of the powdered [C₃N₂H₅⁺]₂[I^-∙I₃^-] crystal at the temperature intervals of 11-270K, covering two low-temperature phase transitions: discontinuous at 182/188K (cooling/heating) and continuous at 254K. The research shows that the vibrational states of the pyrazolium cations change significantly during discontinuous phase transition (III→II), while the continuous nature of successive structural transformation is more subtle and displays an insignificant change in the temperature coefficient of numerous vibrations during the II→I PT at 254K. The spectacular changes at Raman spectra above 188K confirm a huge rebuilding of inorganic network from [I^-∙I₃^-] to [I₄^2-]. Additionally, a complete geometry optimization was carried out in the solid state in order to obtain minimum structures and bonding properties. The theoretical results correspond well with the experimental data. Moreover, the infrared spectrum in harmonic approximation was calculated, and a comparative vibrational analysis was performed. CRYSTAL09 vibrational results appear to be in a good agreement with the experimental ones.

Artificial molecular motors

Artificial molecular motors take inspiration from motor proteins, nature's solution for achieving directional molecular level motion. An overview is given of the principal designs of artificial molecular motors and their modes of operation. We identify some key challenges remaining in the field.

Light-induced piston nanoengines: ultrafast shuttling of a styryl dye inside cucurbit[7]uril

A proof of principle for an ultrafast molecular shuttle based on the light-operated movement of a styryl dye inside cucurbit[7]uril was described.

Five-State Rotary Nanoswitch

In our quest to develop artificial multistate devices, we synthesized the nanomechanical switch 1 that is characterized by a tetrahedral core equipped with four pending arms. The rotary arm with its azaterpyridine terminal is intramolecularly coordinated to a zinc(II) porphyrin station that is the terminus of another arm in 1. The two other arms carry identical sterically shielded phenanthroline stations. The 2-fold alternate addition of a copper(I) ion and [1,10]-phenanthroline (1 equiv each) results in the formation of five different switching states (State I→ State II→ State III→ State IV→ State V → State I), which force the toggling arm to move back and forth between the zinc(II) porphyrin and phenanthroline stations separated by a distance of 25 Å. All switching states constitute clean single species, except for State III, and thus are fully characterized by spectroscopic methods and elemental analysis. Finally, the initial state of nanoswitch was reset by addition of cyclam for complete removal of the copper(I) ions.

Synthesis and Photoisomerization of Substituted Dibenzofulvene Molecular Rotors

The synthesis, spectral and structural characterization, and photoisomerization of a family of 2-substituted dibenzofulvene molecular actuators based on (2,2,2-triphenylethylidene)fluorene (TEF) are reported. The 2-substituted species investigated are nitro (NTEF), cyano (CTEF), and iodo (ITEF). X-ray structures of these three compounds and three intermediates were determined to assign alkene configuration and investigate the effects of the 2-substituents on steric gearing. The addition-elimination reaction of Z-9 with trityl anion to form Z-10 proceeded with complete retention of configuration. Rates of photoisomerization were measured at irradiation wavelengths between 266-355 nm in acetonitrile/dioxane solutions at room temperature. Photoisomerization quantum yields (φ) were calculated by means of a mathematical model that accounts for a certain degree of photodecomposition in the cases of CTEF and ITEF. Quantum yields vary significantly with substituent, having maximum values of φ=0.26 for NTEF, 0.39 for CTEF, and 0.50 for ITEF. NTEF is photochemically robust and has a large quantum yield for photoisomerization in the near-UV, making it a particularly promising drive rotor moiety for light-powered molecular devices.

An Artificial Molecular Transporter

The transport of substrates is one of the main tasks of biomolecular machines in living organisms. We report a synthetic small-molecule system designed to catch, displace, and release molecular cargo in solution under external control. The system consists of a bistable rotaxane that behaves as an acid-base controlled molecular shuttle, whose ring component bears a tether ending with a nitrile group. The latter can be coordinated to a ruthenium complex that acts as the load, and dissociated upon irradiation with visible light. The cargo loading/unloading and ring transfer/return processes are reversible and can be controlled independently. The robust coordination bond ensures that the cargo remains attached to the device while the transport takes place.

Dethreading of a Photoactive Azobenzene-Containing Molecular Axle from a Crown Ether Ring: A Computational Investigation

Pseudorotaxanes formed by a dibenzo[24]crown-8 ring (R) and a dialkylammonium axle bearing either two E- or two Z-azobenzene units (EE-A or ZZ-A) revealed useful for the construction of light-powered molecular machines and motors, as they provide the opportunity of photocontrolling self-assembly/disassembly processes. The potential energies profiles for the dethreading of these complexes have been investigated by adopting a combination of first-principles molecular dynamics, metadynamics and quantum-chemical geometry optimization approaches. While the dethreading of the EE-A axle is associated with a monotonic energy increase, for that of the ZZ-A axle a transition state and an intermediate structure, in which the components are still threaded together, are found. The rate determining step for the dethreading of the ZZ axle has a higher energy barrier than that of the EE axle, in agreement with the experimental kinetic data. Moreover, the results suggest that the elliptic shape of the ring cavity is important for discriminating between the E and Z terminal azobenzene during dethreading.

Unprecedented transformation of [I−·I3−] to [I42−] polyiodides in the solid state: structures, phase transitions and characterization of dipyrazolium iodide triiodide

Dipyrazolium iodide triiodide, [C₃N₂H₅⁺]₂[I⁻·I₃⁻], has been synthesized and studied by means of X-ray diffraction, differential scanning calorimetry, dielectric measurements, and UV-Vis spectroscopy.

Organic nanofibers embedding stimuli-responsive threaded molecular components

While most of the studies on molecular machines have been performed in solution, interfacing these supramolecular systems with solid-state nanostructures and materials is very important in view of their utilization in sensing components working by chemical and photonic actuation. Host polymeric materials, and particularly polymer nanofibers, enable the manipulation of the functional molecules constituting molecular machines and provide a way to induce and control the supramolecular organization. Here, we present electrospun nanocomposites embedding a self-assembling rotaxane-type system that is responsive to both optical (UV-vis light) and chemical (acid/base) stimuli. The system includes a molecular axle comprised of a dibenzylammonium recognition site and two azobenzene end groups and a dibenzo[24]crown-8 molecular ring. The dethreading and rethreading of the molecular components in nanofibers induced by exposure to base and acid vapors, as well as the photoisomerization of the azobenzene end groups, occur in a similar manner to what observed in solution. Importantly, however, the nanoscale mechanical function following external chemical stimuli induces a measurable variation of the macroscopic mechanical properties of nanofibers aligned in arrays, whose Young's modulus is significantly enhanced upon dethreading of the axles from the rings. These composite nanosystems show therefore great potential for application in chemical sensors, photonic actuators, and environmentally responsive materials.