A mechanical actuator driven electrochemically by artificial molecular muscles

Photo-responsive [2]catenanes: synthesis and properties

A series of novel dithienylethene-based macrocycles containing ammonium moieties has been synthesized.

Dithienylethene-based rotaxanes: synthesis, characterization and properties

The photochromic materials have been widely applied in many fields.

Reversible actuation of polyelectrolyte films: expansion-induced mechanical force enables cis-trans isomerization of azobenzenes

Fabrication of light-driven actuators that can prolong their deformation without constant irradiation poses a challenge. This study shows the preparation of polymeric actuators that are capable of reversible bending/unbending movements and prolonging their bending deformation without UV irradiation by releasing thermally cross-linked azobenzene-containing polyelectrolyte films with a limited free volume from substrates. Layer-by-layer assembly of poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl sodium salt} (PAZO)-poly(acrylic acid) (PAA) complexes (noted as PAZO-PAA) with poly(allylamine hydrochloride) (PAH) produces azobenzene-containing PAZO-PAA/PAH films. UV irradiation induces trans-cis isomerization of azobenzenes and allows large-scale bending deformation of the actuators. The actuators prolong the bending deformation even under visible light irradiation because the cis-trans back isomerization of azobenzenes is inhibited by the limited free volume in the actuators. Unbending of actuators is attained by exposing the actuators to a humid environment at room temperature. Film expansion in a humid environment produces a mechanical force that is sufficiently strong to enable the cis-trans back isomerization of azobenzenes and restore the bent actuators to their original configuration. The capability of the force produced by film expansion for cis-trans azobenzene isomerization can be helpful for designing novel polymeric actuators.

Interfacial nanoarchitectonics: lateral and vertical, static and dynamic

The exploration of nanostructures and nanomaterials is essential to the development of advanced functions. For such innovations, nanoarchitectonics has been proposed as a novel paradigm of nanotechnology aimed at assembling nanoscale structural units into predesigned configurations or arrangements. In this Feature Article, we provide an overview of several recent research works from the viewpoint of interfacial nanoarchitectonics with features developed in lateral directions or grown in vertical directions with construction on solid, static, or flexible dynamic surfaces. Lateral nanoarchitectonics at a static interface provides molecular organization by bottom-up nanoarchitectonics and can also be used to realize device integration by top-down nanoarchitectonics. In particular, in the latter case, the fabrication of novel devices, so-called atomic switches, are introduced as a demonstration of atomic-level electronics. Lateral nanoarchitectonics at dynamic interfaces is exemplified by 2D molecular patterning and molecular machine operation induced by macroscopic motion. The dynamic nature of interfaces enables us to operate molecular-sized machines by macroscopic mechanical stimuli such as our hand motion, which we refer to as hand-operated nanotechnology. Vertical nanoarchitectonics is mainly discussed in relation to layer-by-layer (LbL) assembly. By using this technique, we can assemble a variety of functional materials in ultrathin film structures of defined thickness and layer sequence. The organization of biomolecules (or even living cells) within thin films and their integration with device structures is exemplified. Finally, the anticipated research directions of interfacial nanoarchitectonics are described.

From the bottom up: dimensional control and characterization in molecular monolayers

Self-assembled monolayers are a unique class of nanostructured materials, with properties determined by their molecular lattice structures, as well as the interfaces with their substrates and environments. As with other nanostructured materials, defects and dimensionality play important roles in the physical, chemical, and biological properties of the monolayers. In this review, we discuss monolayer structures ranging from surfaces (two-dimensional) down to single molecules (zero-dimensional), with a focus on applications of each type of structure, and on techniques that enable characterization of monolayer physical properties down to the single-molecule scale.

Photoreaction of matrix-isolated dihydroazulene-functionalized molecules on Au{111}

Dihydroazulenes are photochromic molecules that reversibly switch between two distinct geometric and conductivity states. Molecular design, surface attachment, and precise control over the assembly of such molecular machines are critical in order to understand molecular function and motion at the nanoscale. Here, we use surface-enhanced Raman spectroscopy on special atomically flat, plasmonically enhanced substrates to measure the photoreaction kinetics of isolated dihydroazulene-functionalized molecules assembled on Au{111}, which undergo a ring-opening reaction upon illumination with UV light and switch back to the initial isomer via thermal relaxation. Photokinetic analyses reveal the high efficiency of the dihydroazulene photoreaction on solid substrates compared to other photoswitches. An order of magnitude decrease in the photoreaction cross section of surface-bound dihydroazulenes was observed when compared with the cross sections of these molecules in solution.

Sequence-specific peptide synthesis by an artificial small-molecule machine

The ribosome builds proteins by joining together amino acids in an order determined by messenger RNA. Here, we report on the design, synthesis, and operation of an artificial small-molecule machine that travels along a molecular strand, picking up amino acids that block its path, to synthesize a peptide in a sequence-specific manner. The chemical structure is based on a rotaxane, a molecular ring threaded onto a molecular axle. The ring carries a thiolate group that iteratively removes amino acids in order from the strand and transfers them to a peptide-elongation site through native chemical ligation. The synthesis is demonstrated with ~10(18) molecular machines acting in parallel; this process generates milligram quantities of a peptide with a single sequence confirmed by tandem mass spectrometry.

Organic switches for surfaces and devices

The pursuit to achieve miniaturization has tantalized researchers across the fields of chemistry, physics, biology, materials science and engineering for over half a century because of its many alluring potential applications. As alternatives to traditional "top-down" manufacturing, "bottom-up" approaches, originating from the (supra)molecular level, have enabled researchers to develop switches which can be manipulated on surfaces at nanoscale dimensions with deft precision using simple external triggers. Once on surfaces, these organic switches have been shown to modulate both the physical and chemical surface properties. In this Progress Report, we shed light on recent advances made in our laboratories towards integrated systems using all-organic switches on a variety of substrates. Design concepts are revealed, as well as the overall impact of all-organic switches on the properties of their substrates, while emphasizing the considerable promise and formidable challenges these advanced composite materials pose when it comes to conferring function on them.

Photoresponsive molecules in well-defined nanoscale environments

Stimuli-responsive molecules are key building blocks of functional molecular materials and devices. These molecules can operate in a range of environments. A molecule's local environment will dictate its conformation, reactivity, and function; by controlling the local environment we can ultimately develop interfaces of individual molecules with the macroscopic environment. By isolating molecules in well-defined environments, we are able to obtain both accurate measurements and precise control. We exploit defect sites in self-assembled monolayers (SAMs) to direct the functional molecules into precise locations, providing a basis for the measurements and engineering of functional molecular systems. The structure and functional moieties of the SAM can be tuned to control not only the intermolecular interactions but also molecule-substrate interactions, resulting in extraction or control of desired molecular functions. Herein, we report our progress toward the assembly and measurements of photoresponsive molecules and their precise assemblies in SAM matrices.

Facile route to an all-organic, triply threaded, interlocked structure by templated dynamic clipping

Encaged! Three-terminal interlocked molecular species were obtained by dynamic (2+3) assembly of a cagelike macro-bicycle around a trifurcated trispyridinium π guest. The complex is stabilized by π-π interactions and multiple [C-H⋅⋅⋅O] and [C-H⋅⋅⋅N] interactions. Uncomplexed guest molecules cocrystallize alongside the threaded complexes in the solid state, thus giving extended π-stacked columns.

Muscle-like supramolecular polymers: integrated motion from thousands of molecular machines

Pumping iron: Double-threaded rotaxanes can be linked to coordination units and polymerized in the presence of iron or zinc ions. pH modulation triggers cooperative contractions (or extensions) of the individual rotaxanes, thus resulting in an amplified motion of the muscle-like supramolecular chains with changes of their contour lengths of several micrometers (see picture).

Thermally-induced phase transition of pseudorotaxane crystals: changes in conformation and interaction of the molecules and optical properties of the crystals

This paper presents a pseudorotaxane that acts as a thermally driven molecular switch in the single-crystal state. Crystals of the cationic pseudorotaxane consisting of dibenzo[24]crown-8 (DB24C8) and N-(xylylammonium)-methylferrocene as the cyclic and axle component molecules, respectively, undergo crystalline-phase transition at 128 °C with heating and 116 °C with cooling, according to differential-scanning-calorimetry measurements. X-ray crystallographic analyses revealed that the phase transition was accompanied by rotation of the 4-methylphenyl group of the axle component molecule and a simultaneous shift in the position of the PF(6)(-) counteranion. Crystalline phase transition changes the conformation and position of the DB24C8 molecule relative to the ammonium cation partially; the interaction between the cyclic component and the PF(6)(-) anion in the crystal changes to a greater extent. Moreover, there are changes in the vibration angle (θ) and birefringence (Δn) on the (001) face of the crystal transitionally; θ is rotated by +12°, and Δn is decreased from 0.070 to 0.059 upon heating across the phase transition temperature. The phase transition and accompanying change in the optical properties of the crystal occur reversibly and repeatedly upon heating and cooling processes. The switching rotation of the aromatic plane of the molecule induces a change in the optical anisotropy of the crystal, which is regarded as a demonstration of a new type of optical crystal. Partial replacement of the PF(6)(-) anion with the bulkier AsF(6)(-) anion forms crystals with similar crystallographic parameters. An increase in the AsF(6)(-) content decreases the reversible-phase-transition temperature gradually down to 99 °C (T(end)) and 68 °C (T(exo)) ([AsF(6)(-)]:[PF(6)(-)] = 0.4:0.6).

Surface-enhanced Raman spectroscopy to probe photoreaction pathways and kinetics of isolated reactants on surfaces: flat versus curved substrates

We identify and control the photoreaction paths of self-assembled monolayers (SAMs) of thiolate-linked anthracene phenylethynyl molecules on Au substrate surfaces, and study the effects of nanoscale morphology of substrates on regioselective photoreactions. Two types of morphologies, atomically flat and curved, are produced on Au surfaces by controlling substrate structure and metal deposition. We employ surface-enhanced Raman spectroscopy (SERS), combined with Raman mode analyses using density functional theory, to identify the different photoreaction paths and to track the photoreaction kinetics and efficiencies of molecules in monolayers. The SAMs on curved surfaces exhibit dramatically lower regioselective photoreaction kinetics and efficiencies than those on atomically flat surfaces. This result is attributed to the increased intermolecular distances and variable orientations on the curved surfaces. Better understanding of the morphological effects of substrates will enable control of nanoparticle functionalization in ligand exchange in targeted delivery of therapeutics and theranostics and in catalysis.

Mechanostereochemistry and the mechanical bond

The chemistry of mechanically interlocked molecules (MIMs), in which two or more covalently linked components are held together by mechanical bonds, has led to the coining of the term mechanostereochemistry to describe a new field of chemistry that embraces many aspects of MIMs, including their syntheses, properties, topologies where relevant and functions where operative. During the rapid development and emergence of the field, the synthesis of MIMs has witnessed the forsaking of the early and grossly inefficient statistical approaches for template-directed protocols, aided and abetted by molecular recognition processes and the tenets of self-assembly. The resounding success of these synthetic protocols, based on templation, has facilitated the design and construction of artificial molecular switches and machines, resulting more and more in the creation of integrated functional systems. This review highlights (i) the range of template-directed synthetic methods being used currently in the preparation of MIMs; (ii) the syntheses of topologically complex knots and links in the form of stable molecular compounds; and (iii) the incorporation of bistable MIMs into many different device settings associated with surfaces, nanoparticles and solid-state materials in response to the needs of particular applications that are perceived to be fair game for mechanostereochemistry.

Scanning electrochemical microscopy monitoring in microcantilever platforms

The deflection of cantilever systems may be performed by an indirect electrochemical method that consists of measuring the local cantilever activity and deflection in a feedback generation-collection configuration of the SECM. This is illustrated during the electrochemically assisted adsorption of Br onto a gold-coated cantilever, either in its pristine state or previously coated with a thin organic barrier. It is further extended to the adsorption of an antibody in a heterogeneous immunoassay at an allergen-coated microcantilever platform. In both reactions, the cantilever deflection is qualitatively detected from the SECM tip current measurement and a quantitative estimate is obtained through modeling. This electroanalytical strategy provides an alternative approach to standard optical detection. It can overcome some limitations of the optical method by allowing electrochemical characterization of nonconductive cantilevers and appropriate use for closed systems.

Molecular recognition: from solution science to nano/materials technology

In the 25 years since its Nobel Prize in chemistry, supramolecular chemistry based on molecular recognition has been paid much attention in scientific and technological fields. Nanotechnology and the related areas seek breakthrough methods of nanofabrication based on rational organization through assembly of constituent molecules. Advanced biochemistry, medical applications, and environmental and energy technologies also depend on the importance of specific interactions between molecules. In those current fields, molecular recognition is now being re-evaluated. In this review, we re-examine current trends in molecular recognition from the viewpoint of the surrounding media, that is (i) the solution phase for development of basic science and molecular design advances; (ii) at nano/materials interfaces for emerging technologies and applications. The first section of this review includes molecular recognition frontiers, receptor design based on combinatorial approaches, organic capsule receptors, metallo-capsule receptors, helical receptors, dendrimer receptors, and the future design of receptor architectures. The following section summarizes topics related to molecular recognition at interfaces including fundamentals of molecular recognition, sensing and detection, structure formation, molecular machines, molecular recognition involving polymers and related materials, and molecular recognition processes in nanostructured materials.

The importance of the rotor in hydrazone-based molecular switches

The pH-activated E/Z isomerization of a series of hydrazone-based systems having different functional groups as part of the rotor (R = COMe, CN, Me, H), was studied. The switching efficiency of these systems was compared to that of a hydrazone-based molecular switch (R = COOEt) whose E/Z isomerization is fully reversible. It was found that the nature of the R group is critical for efficient switching to occur; the R group should be a moderate H-bond acceptor in order to (i) provide enough driving force for the rotor to move upon protonation, and (ii) stabilize the obtained Z configuration, to achieve full conversion.

Mechanical control of nanomaterials and nanosystems

In situations of power outage or shortage, such as periods just following a seismic disaster, the only reliable power source available is the most fundamental of forces i.e., manual mechanical stimuli. Although there are many macroscopic mechanical tools, mechanical control of nanomaterials and nanosystems has not been an easy subject to develop even by using advanced nanotechnological concepts. However, this challenge has now become a hot topic and many new ideas and strategies have been proposed recently. This report summarizes recent research examples of mechanical control of nanomaterials and nanosystems. Creation of macroscopic mechanical outputs by efficient accumulation of molecular-level phenomena is first briefly introduced. We will then introduce the main subject: control of molecular systems by macroscopic mechanical stimuli. The research described is categorized according to the respective areas of mechanical control of molecular structure, molecular orientation, molecular interaction including cleavage and healing, and biological and micron-level phenomena. Finally, we will introduce two more advanced approaches, namely, mechanical strategies for microdevice fabrication and mechanical control of molecular machines. As mechanical forces are much more reliable and widely applicable than other stimuli, we believe that development of mechanically responsive nanomaterials and nanosystems will make a significant contribution to fundamental improvements in our lifestyles and help to maintain and stabilize our society.

Great expectations: can artificial molecular machines deliver on their promise?

The development and fabrication of mechanical devices powered by artificial molecular machines is one of the contemporary goals of nanoscience. Before this goal can be realized, however, we must learn how to control the coupling/uncoupling to the environment of individual switchable molecules, and also how to integrate these bistable molecules into organized, hierarchical assemblies that can perform significant work on their immediate environment at nano-, micro- and macroscopic levels. In this tutorial review, we seek to draw an all-important distinction between artificial molecular switches which are now ten a penny-or a dime a dozen-in the chemical literature and artificial molecular machines which are few and far between despite the ubiquitous presence of their naturally occurring counterparts in living systems. At the single molecule level, a prevailing perspective as to how machine-like characteristics may be achieved focuses on harnessing, rather than competing with, the ineluctable effects of thermal noise. At the macroscopic level, one of the major challenges inherent to the construction of machine-like assemblies lies in our ability to control the spatial ordering of switchable molecules-e.g., into linear chains and then into muscle-like bundles-and to influence the cross-talk between their switching kinetics. In this regard, situations where all the bistable molecules switch synchronously appear desirable for maximizing mechanical power generated. On the other hand, when the bistable molecules switch "out of phase," the assemblies could develop intricate spatial or spatiotemporal patterns. Assembling and controlling synergistically artificial molecular machines housed in highly interactive and robust architectural domains heralds a game-changer for chemical synthesis and a defining moment for nanofabrication.

Redox-responsive self-healing materials formed from host-guest polymers

Expanding the useful lifespan of materials is becoming highly desirable, and self-healing and self-repairing materials may become valuable commodities. The formation of supramolecular materials through host-guest interactions is a powerful method to create non-conventional materials. Here we report the formation of supramolecular hydrogels and their redox-responsive and self-healing properties due to host-guest interactions. We employ cyclodextrin (CD) as a host molecule because it is environmentally benign and has diverse applications. A transparent supramolecular hydrogel quickly forms upon mixing poly(acrylic acid) (pAA) possessing β-CD as a host polymer with pAA possessing ferrocene as a guest polymer. Redox stimuli induce a sol-gel phase transition in the supramolecular hydrogel and can control self-healing properties such as re-adhesion between cut surfaces.

Experimental demonstration of a single-molecule electric motor

For molecules to be used as components in molecular machines, methods that couple individual molecules to external energy sources and that selectively excite motion in a given direction are required. Significant progress has been made in the construction of molecular motors powered by light and by chemical reactions, but electrically driven motors have not yet been built, despite several theoretical proposals for such motors. Here we report that a butyl methyl sulphide molecule adsorbed on a copper surface can be operated as a single-molecule electric motor. Electrons from a scanning tunnelling microscope are used to drive the directional motion of the molecule in a two-terminal setup. Moreover, the temperature and electron flux can be adjusted to allow each rotational event to be monitored at the molecular scale in real time. The direction and rate of the rotation are related to the chiralities of both the molecule and the tip of the microscope (which serves as the electrode), illustrating the importance of the symmetry of the metal contacts in atomic-scale electrical devices.