Area-Controllable Nanoplatelets from Rapid Photocontrolled Living Crystallization-Driven Self-Assembly of an Alternating Copolymer

Photocontrolled living self-assembly has attracted considerable interest due to its noninvasive, remote control, and real-time features; however, it has remained much less explored compared to other stimuli-responsive self-assembly systems. Here, a novel photocontrolled living crystallization-driven self-assembly (P-CDSA) system was constructed by employing an alternating copolymer, poly((hexylthienyl stiff-stilbene)-alt-poly(ethylene glycol)) containing photosensitive stiff-stilbene derivative, as the precursor. The photoinduced trans-to-cis isomerization of the stiff-stilbene derivative segments could occur quickly upon 365 nm light irradiation, leading to a rapid P-CDSA process producing size-controllable nanoplatelets within 2 min at room temperature. Taking advantage of the repetitive characteristic of alternating copolymers, the nanoplatelet morphology was independent of the molecular weight (MW) and its distribution (Đ) of the copolymer. The areas of the nanoplatelets were precisely controlled by adjusting the unimer-to-seed mass ratio, following a linear relationship. Additionally, the lengths of the major and minor axes followed a sublinear growth trend, enabling tailored nanoplatelet dimensions. The area could also be programmed by sequential light on/off switching, showing a linear dependence on light irradiation time. This study demonstrates the first example of photocontrolled two-dimensional CDSA and opens a new avenue for controlling over the area of 2D architectures.

A Photoswitchable Chloride-Binding [2]Rotaxane

Control over the binding properties of anion receptors by external stimuli can be advantageous in various applications such as extraction and transport processes. Toward a light-responsive anion receptor with high binding affinity and selectivity, a stiff-stilbene photoswitch is incorporated into the macrocycle of a mechanically interlocked, chloride-binding [2]rotaxane structure. UV-Vis and 1H NMR studies show reversible transformation between Z/E-isomers upon light irradiation, causing changes in motional dynamics and binding affinity. Photoswitching also takes place in the presence of chloride, as monitored by 1H NMR spectroscopy, which results in its concomitant uptake and release. Our results show the suitability of rotaxanes as light-responsive ion receptors, which could serve as prototypes for supramolecular pumps.

A photoswitchable [2]catenane receptor

A [2]catenane-based receptor functionalized with stiff-stilbene can be reversibly switched with 340/385 nm light between its Z- and E-isomers, which leads to a considerable change in chloride binding affinity. Photoisomerization in the presence of chloride allows for in situ on demand guest uptake and release.

Supramolecular Control of the Photoisomerization of a Coumarin-Based Photoswitch

The complex formation of the cationic stilbene-type photoswitch CP with the anionic macrocycles carboxylato-pillar[5]arene (WP5) and carboxylato-pillar[6]arene (WP6) has been investigated in aqueous solution by optical spectroscopic, NMR and isothermal calorimetric experiments and theoretical calculations. Subsequently, the photoisomerization reactions of the supramolecular complexes formed have been studied. CP consists of a 7-diethylamino-coumarin fluorophore and an N-methylpyridinium unit, which are connected via an ethene bridge. The trans isomer of CP is fluorescent, and its cis isomer is dark. The binding constants of the WP6 complexes of the two photoisomers of CP are larger by 2 orders of magnitude than those of the respective complexes with WP5, and trans-CP forms more stable complexes with the individual pillararenes than the cis isomer. As shown by NMR spectroscopic measurements and theoretical calculations, the two isomers of CP form external complexes with WP5 and inclusion complexes with WP6. On complexation with WP6, the quantum yields of both the trans-to-cis and cis-to-trans photoisomerization reactions of CP increase significantly, and the fluorescence quantum yield of trans-CP is also enhanced. These changes are due to the suppression of the TICT deactivation process, which is characteristic of 7-dialkylamino-coumarin derivatives.

Cascading Macrocycle and Helix Motions in a Foldarotaxane Molecular Shuttle

The design of a dynamically assembled foldarotaxane was envisioned with the aim of operating as a two cascading trigger-based molecular shuttle. Under acidic conditions, both the macrocycle and helix were localized around their respective best molecular stations because they are far enough from each other not to alter the stability of complexes. The pH-dependent localization of the macrocycle along the encircled axle allowed us to modulate the association between the helical foldamer and its sites of interaction on the axle. Under kinetic control-at low concentration and room temperature-when the foldarotaxane supramolecular architecture is kinetically stable, the pH-responsive translation of the macrocycle along the thread triggered the gliding of the helix away from its initial best station. At higher concentration-when helix assembly/disassembly process is accelerated-the system reached the equilibrium state. A new foldarotaxane isomer then appeared through the change of the relative position of the helix and macrocycle along the thread. In this isomer, the helix segregated the macrocycle away from its best station. The fine control of the kinetic and thermodynamic processes, combined with the control of pH, allowed the reciprocal segregation of the helix or the ring away from their respective best sites of interaction.

A Photoswitchable Macrocycle Controls Anion-Templated Pseudorotaxane Formation and Axle Relocalization

Important biological processes, such as signaling and transport, are regulated by dynamic binding events. The development of artificial supramolecular systems in which binding between different components is controlled could help emulate such processes. Herein, we describe stiff-stilbene-containing macrocycles that can be switched between (Z)- and (E)-isomers by light, as demonstrated by UV/Vis and 1 H NMR spectroscopy. The (Z)-isomers can be effectively threaded by pyridinium halide axles to give pseudorotaxane complexes, as confirmed by 1 H NMR titration studies and single-crystal X-ray crystallography. The overall stability of these complexes can be tuned by varying the templating counteranion. However, upon light-induced isomerization to the (E)-isomer, the threading capability is drastically reduced. The axle component, in addition, can form a heterodimeric complex with a secondary isophthalamide host. Therefore, when all components are combined, light irradiation triggers axle exchange between the macrocycle and this secondary host, which has been monitored by 1 H NMR spectroscopy and simulated computationally.

Allosteric control of olefin isomerization kinetics via remote metal binding and its mechanochemical analysis

Allosteric control of reaction thermodynamics is well understood, but the mechanisms by which changes in local geometries of receptor sites lower activation reaction barriers in electronically uncoupled, remote reaction moieties remain relatively unexplored. Here we report a molecular scaffold in which the rate of thermal E-to-Z isomerization of an alkene increases by a factor of as much as 104 in response to fast binding of a metal ion to a remote receptor site. A mechanochemical model of the olefin coupled to a compressive harmonic spring reproduces the observed acceleration quantitatively, adding the studied isomerization to the very few reactions demonstrated to be sensitive to extrinsic compressive force. The work validates experimentally the generalization of mechanochemical kinetics to compressive loads and demonstrates that the formalism of force-coupled reactivity offers a productive framework for the quantitative analysis of the molecular basis of allosteric control of reaction kinetics. Important differences in the effects of compressive vs. tensile force on the kinetic stabilities of molecules are discussed.

Coupled Rotary and Oscillatory Motion in a Second-Generation Molecular Motor Pd Complex

Molecular machines offer many opportunities for the development of responsive materials and introduce autonomous motion in molecular systems. While basic molecular switches and motors carry out one type of motion upon being exposed to an external stimulus, the development of molecular systems capable of performing coupled motions is essential for the development of more advanced molecular machinery. Overcrowded alkene-based rotary molecular motors are an ideal basis for the design of such systems as they undergo a controlled rotation initiated by light allowing for excellent spatio-temporal precision. Here, we present an example of a Pd complex of a second-generation rotary motor whose Pd center undergoes a coupled oscillatory motion relative to the motor core upon rotation of the motor. We have studied this phenomenon by UV-vis, NMR, and density functional theory calculations to support our conclusions. With this demonstration of a coupled rotation-oscillation motion powered by a light-driven molecular motor, we provide a solid basis for the development of more advanced molecular machines integrating different types of motion in their operation.

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.

Noncovalently bound and mechanically interlocked systems using pillar[n]arenes

Pillar[n]arenes are pillar-shaped macrocyclic compounds owing to the methylene bridges linking the para-positions of the units. Owing to their unique pillar-shaped structures, these compounds exhibit various excellent properties compared with other cyclic host molecules, such as versatile functionality using various organic synthesis techniques, substituent-dependent solubility, cavity-size-dependent host-guest properties in organic media, and unit rotation along with planar chiral inversion. These advantages have enabled the high-yield synthesis and rational design of pillar[n]arene-based mechanically interlocked molecules (MIMs). In particular, new types of pillar[n]arene-based MIMs that can dynamically convert between interlocked and unlocked states through unit rotation have been produced. The highly symmetrical pillar-shaped structures of pillar[n]arenes result in simple NMR spectra, which are useful for studying the motion of pillar[n]arene wheels in MIMs and creating sophisticated MIMs with higher-order structures. The creation and application of polymeric MIMs based on pillar[n]arenes is also discussed.

Dynamics of Excitons in Conjugated Molecules and Organic Semiconductor Systems

The exciton, an excited electron-hole pair bound by Coulomb attraction, plays a key role in photophysics of organic molecules and drives practically important phenomena such as photoinduced mechanical motions of a molecule, photochemical conversions, energy transfer, generation of free charge carriers, etc. Its behavior in extended π-conjugated molecules and disordered organic films is very different and very rich compared with exciton behavior in inorganic semiconductor crystals. Due to the high degree of variability of organic systems themselves, the exciton not only exerts changes on molecules that carry it but undergoes its own changes during all phases of its lifetime, that is, birth, conversion and transport, and decay. The goal of this review is to give a systematic and comprehensive view on exciton behavior in π-conjugated molecules and molecular assemblies at all phases of exciton evolution with emphasis on rates typical for this dynamic picture and various consequences of the above dynamics. To uncover the rich variety of exciton behavior, details of exciton formation, exciton transport, exciton energy conversion, direct and reverse intersystem crossing, and radiative and nonradiative decay are considered in different systems, where these processes lead to or are influenced by static and dynamic disorder, charge distribution symmetry breaking, photoinduced reactions, electron and proton transfer, structural rearrangements, exciton coupling with vibrations and intermediate particles, and exciton dissociation and annihilation as well.

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.

Stabilization of Antitumor Agent Busulfan through the Encapsulation within a Water-Soluble Pillar[5]arene

The complexation of antitumor agent busulfan by negatively charged carboxylatopillar[5]arenein water is reported. The encapsulation within carboxylatopillar[5]arenein reduces the hydrolytic degradation of busulfan from 90.7 % to 25.2 % after 24 days and accordingly enhances its stability by providing a hydrophobic shelter for busulfan in water. Moreover, the complexation result in 12 times improvement of water solubility for busulfan. Our result provides a supramolecular approach for stabilizing the anticancer agent busulfan.

Photomodulation of Transmembrane Transport and Potential by Stiff-Stilbene Based Bis(thio)ureas

Membrane transport proteins fulfill important regulatory functions in biology with a common trait being their ability to respond to stimuli in the environment. Various small-molecule receptors, capable of mediating transmembrane transport, have been successfully developed. However, to confer stimuli-responsiveness on them poses a fundamental challenge. Here we demonstrate photocontrol of transmembrane transport and electric potential using bis(thio)ureas derived from stiff-stilbene. UV-vis and 1H NMR spectroscopy are used to monitor E-Z photoisomerization of these bis(thio)ureas and 1H NMR titrations reveal stronger binding of chloride to the (Z)-form than to the (E)-form. Additional insight into the binding properties is provided by single crystal X-ray crystallographic analysis and DFT geometry optimization. Importantly, the (Z)-isomers are much more active in transmembrane transport than the respective (E)-isomers as shown through various assays. As a result, both membrane transport and depolarization can be modulated upon irradiation, opening up new prospects toward light-based therapeutics as well as physiological and optopharmacological tools for studying anion transport-associated diseases and to stimulate neuronal activity, respectively.

Push-Pull Stiff-Stilbene: Proton-Gated Visible-Light Photoswitching and Acid-Catalyzed Isomerization

Donor-acceptor substituted stiff-stilbene is shown to undergo isomerization induced by visible light avoiding the need for harmful UV light. This visible-light photoswitching is inhibited by protonation of the dimethylamino-donor unit, disrupting the push-pull character and thus, gating of the photochromic properties is allowed by acid/base addition. Remarkably, the addition of a mild acid also triggers fast thermal back-isomerization, which is unprecedented for stiff-stilbene photoswitches usually having a very high energy barrier for this process. These combined features offer unique orthogonal control over switching behavior by light and protonation, which is investigated in detail by 1 H NMR and UV/Vis spectroscopy. In addition, TD-DFT calculations are used to gain further insight into the absorption properties. Our results will help elevating the level of control over dynamic behavior in stiff-stilbene applications.

The Contributions of Model Studies for Fundamental Understanding of Polymer Mechanochemistry

The exciting field of polymer mechanochemistry has made great empirical progress in discovering reactions in which a stretching force accelerates scission of strained bonds using single molecule force spectroscopy and ultrasonication experiments. Understanding why these reactions happen, i.e., the fundamental physical processes that govern coupling of macroscopic motion to chemical reactions, as well as discovering other patterns of mechanochemical reactivity require complementary techniques, which permit a much more detailed characterization of reaction mechanisms and the distribution of force in reacting molecules than are achievable in SMFS or ultrasonication. A molecular force probe allows the specific pattern of molecular strain that is responsible for localized reactions in stretched polymers to be reproduced accurately in non-polymeric substrates using molecular design rather than atomistically intractable collective motions of millions of atoms comprising macroscopic motion. In this review, we highlight the necessary features of a useful molecular force probe and describe their realization in stiff stilbene macrocycles. We describe how studying these macrocycles using classical tools of physical organic chemistry has allowed detailed characterizations of mechanochemical reactivity, explain some of the most unexpected insights enabled by these probes, and speculate how they may guide the next stage of mechanochemistry.

Molecular photoswitches in aqueous environments

Molecular photoswitches enable dynamic control of processes with high spatiotemporal precision, using light as external stimulus, and hence are ideal tools for different research areas spanning from chemical biology to smart materials. Photoswitches are typically organic molecules that feature extended aromatic systems to make them responsive to (visible) light. However, this renders them inherently lipophilic, while water-solubility is of crucial importance to apply photoswitchable organic molecules in biological systems, like in the rapidly emerging field of photopharmacology. Several strategies for solubilizing organic molecules in water are known, but there are not yet clear rules for applying them to photoswitchable molecules. Importantly, rendering photoswitches water-soluble has a serious impact on both their photophysical and biological properties, which must be taken into consideration when designing new systems. Altogether, these aspects pose considerable challenges for successfully applying molecular photoswitches in aqueous systems, and in particular in biologically relevant media. In this review, we focus on fully water-soluble photoswitches, such as those used in biological environments, in both in vitro and in vivo studies. We discuss the design principles and prospects for water-soluble photoswitches to inspire and enable their future applications.

Pillar[5]arene-Based 3D Hybrid Supramolecular Polymer for Green Catalysis in Water

Pillar[n]arene-based supramolecular polymers have attracted great interest because of their tunable morphologies and external stimuli responsiveness. However, most of the investigations of supramolecular polymers previously reported were focused on their formation and transformation, and investigations on their applications are rare. Herein, we designed and prepared hybrid polymeric materials by incorporating Pd nanoparticles into a supramolecular polymer, constructed from a pillar[5]arene dimer and a three-arm guest. The obtained hybrid polymer was fully characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, scanning electron microscopy-energy-dispersive X-ray mapping, and X-ray diffraction technologies. Importantly, the hybrid supramolecular polymeric materials exhibited desirable catalytic activity for reductions of toxic nitroaromatics and C-C bond-forming Suzuki-Miyaura reaction in aqueous solution.

Enhanced E/Z-photoisomerization and luminescence of stilbene derivative co-coordinated in di-β-diketonate lanthanide complexes

A new tetradentate chelating ligand appending a stilbene derivative, E-N',N'-bis(pyridin-2-ylmethyl)-4-styrylbenzohydrazide (HL) was synthesized, together with two β-diketonates (4,4,4-trifluoro-1-phenylbutane-1,3-dionate, tfd), with or without the trifluoroacetate anion present as a ligand for coordination with lanthanide(iii) ions to form [Ln(tfd)2(HL)(CF3CO2)] (LnC49H36F9N4O7, Ln = La (1), Nd (2), Eu (3), Gd (4)) and [Yb(tfd)2(L)] (YbC47H35F6N4O5 (5), L = deprotonated HL). All five complexes were structurally characterized, and five crystals were obtained and analyzed by single-crystal X-ray diffraction. The quantum yield of trans-to-cis photoisomerization of the stilbene group in gadolinium complex 4 was enhanced about five-fold compared with that of HL itself. Other complexes showed slightly enhanced or depressed photoisomerization. The total luminescence quantum yield/sensitization efficiency of europium complex 3 in the solid state and acetonitrile solution were 22.1%/96.7% and 19.3%/97.9%, respectively. The transfer of ligand energy to the Eu3+ ion was highly efficient. This enhanced photoisomerization and luminescence of the stilbene group within complexes was found to be related to the energy level of lanthanide ions and whether a ligand-to-metal center or ligand-to-ligand charge transfer process was present. The interpretation of experimental results is rationally supported by time-dependent density-functional theory calculations. In complex 4, except for the intramolecular absorption transition of HL ligand itself (IL, πHL-π*HL), the presence of the ligand-to-ligand charge transfer transition from tfd to HL (LLCT, πtfd-π*HL) and the triplet state energy of HL being unable to transfer to the higher 6P7/2 excited energy level of the Gd3+ ion would facilitate HL photoisomerization. For complex 3, this was due to reversed ligand-to-ligand charge transfer transition from HL to tfd (LLCT, πHL-π*tfd) and its energy transfer to the metal center. Although the observed radiative lifetimes of NIR luminescent complexes 2 and 5 were around 10 μs, these systems contained only two diketone ligands, indicating that HL still had a certain promoting effect compared with tris(diketonate) lanthanide complexes. These results offer an important route for the design of new lanthanide-based molecular switching materials.

Photoinduced Strain-Assisted Synthesis of a Stiff-Stilbene Polymer by Ring-Opening Metathesis Polymerization

Developing a novel strategy to synthesize photoresponsive polymers is of significance owing to their potential applications. We report a photoinduced strain-assisted synthesis of main-chain stiff-stilbene polymers by using ring-opening metathesis polymerization (ROMP), activating a macrocyclic π-bond connected to a stiff-stilbene photoswitch through a linker. Since the linker acts as an external constraint, the photoisomerization to the E-form leads to the stiff-stilbene being strained and thus reactive to ROMP. The photoisomerization of Z-form to E-form was investigated using time-dependent NMR studies and UV/Vis spectroscopy. The DFT calculation showed that the E-form was less stable due to a lack of planarity. By the internal strain developed due to the linker constraint through photoisomerization, the E-form underwent ROMP by a second generation Grubbs catalyst. In contrast, Z-form did not undergo polymerization under similar conditions. The MALDI-TOF spectrum of E-form after polymerization showed the presence of oligomers of >5.2 kDa.

Helix Inversion Controlled by Molecular Motors in Multistate Liquid Crystals

Unravelling the rules of molecular motion is a contemporary challenge that promises to support the development of responsive materials and is likely to enhance the understanding of functional motion. Advances in integrating light-driven molecular motors in soft matter have led to the design and realization of chiral nematic (cholesteric) liquid crystals that can respond to light with modification of their helical pitch, and also with helix inversion. Under illumination, these chiral liquid crystals convert from one helical geometry to another. Here, a series of light-driven molecular motors that feature a rich configurational landscape is presented, specifically which involves three stable chiral states. The succession of chiral structures involved in the motor cycle is transmitted at higher structural levels, as the cholesteric liquid crystals that are formed can interconvert between helices of opposite handedness, reversibly. In these materials, the dynamic features of the motors are thus expressed at the near-macroscopic, functional level, into addressable colors that can be used in advanced materials for tunable optics and photonics.

Stiff-Stilbene Photoswitches: From Fundamental Studies to Emergent Applications

Stiff-stilbene, a sterically restricted fused ring analogue of stilbene, has been regularly used as a model compound in theoretical studies of stilbene photoisomerization. Lately, owing to its excellent photoswitching properties, it is increasingly being applied to reversibly control the properties and function of chemical as well as biological systems. Stiff-stilbene photoswitches possess a number of advantageous properties including a high quantum yield for photoisomerization and a high thermal stability. Furthermore, they undergo a large geometrical change upon isomerization and their synthesis is straightforward. Herein, we provide an overview of the basic properties of stiff-stilbene and of recent applications in supramolecular chemistry, catalysis, and biological systems.

Double Rotors with Fluxional Axles: Domino Rotation and Azide-Alkyne Huisgen Cycloaddition Catalysis

The simple preparation of the multicomponent devices [Cu4 (A)2 ]4+ and [Cu2 (A)(B)]2+ , both rotors with fluxional axles undergoing domino rotation, highlights the potential of self-sorting. The concept of domino rotation requires the interconversion of axle and rotator, allowing the spatiotemporal decoupling of two degenerate exchange processes in [Cu4 (A)2 ]4+ occurring at 142 kHz. Addition of two equiv of B to rotor [Cu4 (A)2 ]4+ afforded the heteromeric two-axle rotor [Cu2 (A)(B)]2+ with two distinct exchange processes (64.0 kHz and 0.55 Hz). The motion requiring a pyridine→zinc porphyrin bond cleavage is 1.2×105 times faster than that operating via a terpyridine→[Cu(phenAr2 )]+ rupture. Finally, both rotors are catalysts due to their copper(I) content. The fast domino rotor (142 kHz) was shown to suppress product inhibition in the catalysis of the azide-alkyne Huisgen cycloaddition.

Suppression of Undesirable Isomerization and Intermolecular Reactions of Double Bonds by a Linked Rotaxane Structure

For luminescent materials, the isomerization and intermolecular reactions of their double bonds are often undesirable because they cause a reduction in the luminescence properties of the π-system. Herein, we report a new methodology to simultaneously prevent isomerization and intermolecular reactions by utilizing the steric effect of a linked rotaxane structure. The ring units are covalently linked in order to prevent any undesired shuttling effect from occurring during isomerization. In addition, the insulated structure provides robust optical properties by prevention of intermolecular reactions. Bulky linked rotaxane structures on both sides of the N=N and C=C double bonds suppress E/Z isomerization; photoluminescence quantum yield (PLQY) measurements reveal that this results in suppression of PLQY reduction caused by isomerization. Moreover, an improvement in the stability under light irradiation and air atmosphere is demonstrated.

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.

Synthesis of a new solvent-responsive pillar[5]arene-based [1]rotaxane molecular machine

In this work, we designed a new pillar[5]arene-based molecular machine responsive to the polarity of different solvents, which can exist in an interlocked structure in CDCl₃ and CD₃OD, and can exist in an extended form in DMSO and was studied by ¹H and 2D NMR spectroscopy, HR(MS) and fluorescence spectroscopy.

Effect of ring rize on photoisomerization properties of stiff stilbene macrocycles

A series of stiff stilbene macrocycles have been studied to investigate the possible impact of the macrocycle ring size on their photodynamic properties. The results show that reducing the ring size counteracts the photoisomerization ability of the macrocycles. However, even the smallest macrocycle studied (stiff stilbene subunits linked by a six carbon chain) showed some degree of isomerization when irradiated. DFT calculations of the energy differences between the E- and Z-isomers show the same trend as the experimental results. Interestingly the DFT study highlights that the energy difference between the E- and Z-isomers of even the largest macrocycle (linked by a twelve carbon chain) is significantly higher than that of the stiff stilbene unit itself. In general, it is indicated that addition of even a flexible chain to the stiff stilbene unit may significantly affect its photochemical properties and increase the photostability of the resulting macrocycle.

"Seeing" and Controlling Photoisomerization by (Z)-/(E)-Isomers with Aggregation-Induced Emission Characteristics

Efficient photoisomerization of chromophores is important in living systems, and structural constraints of protein pocket on chromophores are the probable reason for moving their dynamic reaction equilibrium forward. On the other hand, photochemical reaction to switch a molecule from one isomer to the other with different geometry and property in a high yield will continue to play a vital role in the synthetic chemistry and material science. Because of the important role of efficient photoisomerization, a biomimetic approach for "seeing" and controlling the photoisomerization is developed by using the technology of aggregation-induced emission (AIE) with supramolecular chemistry. It is revealed that a (Z)-isomer of a 2-ureido-4[1H]-pyrimidinone-containing tetraphenylethene (TPE-UPy) can be photoisomerized into supramolecular polymer form of its (E)-counterpart in chloroform in a high reaction yield of 68.1%. The yield is further enhanced to 100% in THF as aggregates of supramolecular polymers of (E)-TPE-UPy are formed, which completely inhibits the reverse photoreaction to form (Z)-TPE-UPy. In chloroform with organic acid, a mixture of equal amounts of (E)- and (Z)-isomers was obtained due to the disruption of the formation of intermolecular hydrogen bonds. The AIE characteristics of the isomers allow us to directly "see" the "turn-on" photoisomerization process by distinct fluorescence color changes, and the photoisomerization observed here may enable the development of a promising generation of optical power limiting 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.

Regulation of Axial Chirality through Dynamic Covalent Bond Constrained Biaryls

A strategy of dynamic covalent chemistry within constrained biaryls was developed for the modulation of axial chirality. The ring fusion partners of amide and aldehyde allowed the manipulation of ring/chain equilibrium and chirality transfer within cyclic diastereomeric hemiaminal. Dynamic covalent reactions (DCRs) with alcohols, thiols, and secondary amines further enabled the reversal of chirality relay and thereby regulation of axial chirality. Moreover, a combination of NMR, X-ray, and density functional theory results shed light on the structural basis of chirality transfer, exhibiting modest to excellent diastereoselectivity under thermodynamic control. The critical role of the amide unit in the modulation of axial chirality was also corroborated. Finally, the chiroptical signal was controlled through changing solvents, DCRs, and stimuli-responsive switching of DCRs.

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

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

Recent advances in the development of rotaxanes and pseudorotaxanes based on pillar[n]arenes: from construction to application

This article summarizes recent advances in the development of rotaxanes and pseudorotaxanes based on pillar[n]arenes: from construction to application.