Toward intelligent molecular machines: directed motions of biological and artificial molecules and assemblies

A Multiresponsive Ferrocene-Based Chiral Overcrowded Alkene Twisting Liquid Crystals

The reversible modulation of chirality has gained significant attention not only for fundamental stereochemical studies but also for numerous applications ranging from liquid crystals (LCs) to molecular motors and machines. This requires the construction of switchable molecules with (multiple) chiral elements in a highly enantioselective manner, which is often a significant synthetic challenge. Here, we show that the dimerization of an easily accessible enantiopure planar chiral ferrocene-indanone building block affords a multi-stimuli-responsive dimer (FcD) with pre-determined double bond geometry, helical chirality, and relative orientation of the two ferrocene motifs in high yield. This intrinsically planar chiral switch can not only undergo thermal or photochemical E/Z isomerization but can also be reversibly and quantitatively oxidized to both a monocationic and a dicationic state which is associated with significant changes in its (chir)optical properties. Specifically, FcD acts as a chiral dopant for cholesteric LCs with a helical twisting power (HTP) of 13 μm-1 which, upon oxidation, drops to near zero, resulting in an unprecedently large redox-tuning of the LC reflection color by up to 84 nm. Due to the straightforward stereoselective synthesis, FcD, and related chiral switches, are envisioned to be powerful building blocks for multi-stimuli-responsive molecular machines and in LC-based materials.

Macrocycle Unidirectional Transport Along a Linear Molecule by a Two-Step Chemical Reaction Sequence

Chemical systems displaying directional motions are relevant to the operation of artificial molecular machines. Herein we present the functioning of a molecule capable of transporting a cyclic species in a preferential direction. Our system is based on a linear, non-symmetric, positively charged molecule. This cation integrates into its structure two different reactive regions. On one side features a bulky ester group that can be exchanged by a smaller substituent; the other extreme contains an acid/base responsive moiety that plays a dual role, as part of the recognition motif and as a terminal group. In the acidic state, a dibenzo-24-crown-8 ether slides into the linear component attracted by the positively charged recognition site. It does this selectively through the extreme that contains the azepanium group, since the other side is sterically hindered. After base addition, intermolecular interactions are lost; however, the macrocycle is unable to escape from the linear component since the energy barrier to slide over the neutral azepane is too large. Therefore, a metastable mechanically interlocked molecule is formed. A second reaction, now on the ester functionality, exchanges the bulky mesityl for a methyl group, small enough to allow macrocycle dissociation, completing the directional transit of the ring along the track.

Conformational Switching of a Nano-Size Urea-Bridged Zn(II)Porphyrin Dimer by External Stimuli

For the first time, explicit stabilization of all the three conformers, viz. (cis,cis), (cis,trans) and (trans,trans), of a 'nano-sized' highly-flexible urea-bridged Zn(II)porphyrin dimer have been achieved via careful manipulations of external stimuli such as solvent dielectrics, temperature, anionic interactions, axial ligation and surface-induced stabilization. The conformers differ widely in their structures, chemical and photophysical properties and thus have vast potential applicability. X-ray structural characterizations have been reported for the (cis,cis) and (cis,trans)-conformers. While (cis,cis) conformer stabilized exclusively in dichloromethane, more polar solvents resulted in the stabilization of (cis,trans) and (trans,trans)-conformers. Low temperature promotes the stabilization of (cis,trans)-conformer while rise in temperature facilitates flipping to the (cis,cis) one. Significantly, exclusive stabilization of the (trans,trans)-isomer has been illustrated using acetate anion which facilitates H-bonding with the two amide linkages of the urea spacer. Remarkably, HOPG surface facilitates stabilization of the energetically challenging (trans,trans)-conformer via CH⋅⋅⋅π and π⋅⋅⋅π interactions with the solid surface to the porphyrinic cores. DFT calculations demonstrate that the relative stability of the conformers can be modulated upon slight external perturbations as also observed in the experiment. Several factors contributing towards the conformational landscape for the highly flexible urea-bridged porphyrin dimers have been mapped.

Blue Light Controlled Supramolecular Soft Robotics of Phenylazothiazole Amphiphiles for Rapid Macroscopic Actuations

Nature preprograms sophisticated processes in operating molecular machines at the nanoscale, amplifying the molecular motion across multiple length-scales, and controlling movements in living organisms. Supramolecular soft robotics serve as a new alternative to hard robotics, are able to transform and amplify collective motions of the supramolecularly assembled molecular machines in attaining macroscopic motions, upon photoirradiation. By taking advantage of oriented supramolecular macroscopic soft scaffold, here the first rapid macroscopic movements of supramolecular robotic materials driven by visible light are presented. Head-tail amphiphilic structure is designed with the phenylazothiazole motif as the photoswitching core. Unidirectionally aligned nanostructures of the amphiphilic phenylazothiazoles are controlled by non-invasive blue light irradiation and bends toward the light source, demonstrating a fast macroscopic actuation of supramolecular robotic systems (up to 17° s-1) in aqueous media. Through meticulous X-ray diffraction and electron microscopy analyzes, macroscopic actuation mechanism is illustrated in a tight relation to molecular geometric transformations upon photoisomerization. By elucidating the key macroscopic actuation parameters, this paves the way for the next generation design of supramolecular soft robotic systems with enhanced biomimetic actuating functions.

Pushing a bistable [2]rotaxane out of equilibrium and isolation of the metastable-state co-conformation

Incorporating a steric barrier between the two stations in a bistable [2]rotaxane based on monopyrrolotetrathiafulvalene and cyclobis(paraquat-p-phenylene) allows the high-energy metastable-state co-conformation to be physically isolated following a single redox cycle, thus making it possible to store energy (4.4 J L-1) and to follow its interconversion back to the ground-state co-conformation.

Insight from Electrochemical Analysis in the Radical Cation State of a Monopyrrolotetrathiafulvalene-Based [2]Rotaxane

Mechanically interlocked molecules are a class of compounds used for controlling directional movement when barriers can be raised and lowered using external stimuli. Applied voltages can turn on redox states to alter electrostatic barriers but their use for directing motion requires knowledge of their impact on the kinetics. Herein, we make the first measurements on the movement of cyclobis(paraquat-p-phenylene) (CBPQT4+) across the radical-cation state of monopyrrolotetrathiafulvalene (MPTTF) in a [2]rotaxane using variable scan-rate electrochemistry. The [2]rotaxane is designed in a way that directs CBPQT4+ to a high-energy co-conformation upon oxidation of MPTTF to either the radical cation (MPTTF⋅+) or the dication (MPTTF2+). 1H NMR spectroscopic investigations carried out in acetonitrile at 298 K showed direct interconversion to the thermodynamically more stable ground-state co-conformation with CBPQT4+ moving across the oxidized MPTTF2+ electrostatic barrier. The electrochemical studies revealed that interconversion takes place by movement of CBPQT4+ across both the MPTTF•+ (19.3 kcal mol-1) and MPTTF2+ (18.7 kcal mol-1) barriers. The outcome of our studies shows that MPTTF has three accessible redox states that can be used to kinetically control the movement of the ring component in mechanically interlocked molecules.

Unraveling the Origin of Multichannel Circularly Polarized Luminescence in a Pyrene-Functionalized Topologically Chiral [2]Catenane

Mechanically interlocked molecules (MIMs) are promising platforms for developing functionalized artificial molecular machines. The construction of chiral MIMs with appealing circularly polarized luminescence (CPL) properties has boosted their potential application in biomedicine and the optical industry. However, there is currently little knowledge about the CPL emission mechanism or the emission dynamics of these related MIMs. Herein, we demonstrate that time-resolved circularly polarized luminescence (TRCPL) spectroscopy combined with transient absorption (TA) spectroscopy offers a feasible approach to elucidate the origins of CPL emission in pyrene-functionalized topologically chiral [2]catenane as well as in a series of pyrene-functionalized chiral molecules. For the first time, direct evidence differentiating the chiroptical signals originating from either topological (local state emission) or Euclidean chirality (excimer state emission) in these pyrene-functionalized chiral molecules has been discovered. Our work not only establishes a novel and ideal approach to study CPL mechanism, but also provides a theoretical foundation for the rational design of novel chiral materials in the future.

Aqueous Supramolecular Transformations of Motor Bola-Amphiphiles at Multiple Length-Scale

Molecular motor amphiphiles have already been widely attempted for dynamic nanosystems across multiple length-scale for developments of small functional materials, including controlling macroscopic foam properties, amplifying motion as artificial molecular muscles, and serving as extracellular matrix mimicking cell scaffolds. However, limiting examples of bola-type molecular motor amphiphiles are considered for constructing macroscopic biomaterials. Herein, this work presents the designed two second generation molecular motor amphiphiles, motor bola-amphiphiles (MBAs). Aside from the photoinduced motor rotation of MBAs achieved in both organic and aqueous media, the rate of recovering thermal helix inversion step can be controlled by the rotor part with different steric hindrances. Dynamic assembled structures of MBAs are observed under (cryo)-transmission electron microscopy (TEM). This dynamicity assists MBAs in further assembling as macroscopic soft scaffolds by applying a shear-flow method. Upon photoirradiation, the phototropic bending function of MBA scaffolds is observed, demonstrating the amplification of molecular motion into macroscopic phototropic bending functions at the macroscopic length-scale. Since MBAs are confirmed with low cytotoxicity, human bone marrow-derived mesenchymal stem cells (hBM-MSCs) can grow on the surface of MBA scaffolds. These results clearly demonstrate the concept of designing MBAs for developing photoresponsive dynamic functional materials to create new-generation soft robotic systems and cell-material interfaces.

Formylation boosts the performance of light-driven overcrowded alkene-derived rotary molecular motors

Artificial molecular motors and machines constitute a critical element in the transition from individual molecular motion to the creation of collective dynamic molecular systems and responsive materials. The design of artificial light-driven molecular motors operating with high efficiency and selectivity constitutes an ongoing fundamental challenge. Here we present a highly versatile synthetic approach based on Rieche formylation that boosts the quantum yield of the forward photoisomerization reaction while reaching near-perfect selectivity in the steps involved in the unidirectional rotary cycle and drastically reducing competing photoreactions. This motor is readily accessible in its enantiopure form and operates with nearly quantitative photoconversions. It can easily be functionalized further and outperforms its direct predecessor as a reconfigurable chiral dopant in cholesteric liquid crystal materials.

All-visible-light-driven salicylidene schiff-base-functionalized artificial molecular motors

Light-driven rotary molecular motors are among the most promising classes of responsive molecular machines and take advantage of their intrinsic chirality which governs unidirectional rotation. As a consequence of their dynamic function, they receive considerable interest in the areas of supramolecular chemistry, asymmetric catalysis and responsive materials. Among the emerging classes of responsive photochromic molecules, multistate first-generation molecular motors driven by benign visible light remain unexplored, which limits the exploitation of the full potential of these mechanical light-powered systems. Herein, we describe a series of all-visible-light-driven first-generation molecular motors based on the salicylidene Schiff base functionality. Remarkable redshifts up to 100 nm in absorption are achieved compared to conventional first-generation motor structures. Taking advantage of all-visible-light-driven multistate motor scaffolds, adaptive behaviour is found as well, and potential application in multistate photoluminescence is demonstrated. These functional visible-light-responsive motors will likely stimulate the design and synthesis of more sophisticated nanomachinery with a myriad of future applications in powering dynamic systems.

Supramolecular assemblies of amphiphilic donor-acceptor Stenhouse adducts as macroscopic soft scaffolds

In the design of photoharvesting and photoresponsive supramolecular systems in aqueous medium, the fabrication of amphiphilic photoswitches enables a noninvasive functional response through photoirradiation. Although most aqueous supramolecular assemblies are driven by high-energy and biodamaging UV light, we have previously reported a design of amphiphilic donor-acceptor Stenhouse adducts (DASAs) controlled by white light. Herein, we present a series of DASA amphiphiles (DAs) with minor structural modifications on the alkyl linker chain length connecting the DASA motif with the hydrophilic moiety. The excellent photoswitchability in organic medium and the photoresponsiveness in aqueous medium, driven by visible light, were investigated by UV-vis absorption spectroscopy. The assembled supramolecular nanostructures were confirmed by electron microscopy, while the supramolecular packing was revealed by X-ray diffraction analysis. Upon visible-light irradiation, significant transformations of the DA geometry enabled transformations of the supramolecular assemblies on a microscopic scale, subsequently disassembling macroscopic soft scaffolds of DAs. The current work shows promising use for the fabrication of visible-light-controlled macroscopic scaffolds, offering the next generation of biomedical materials with visible-light-controlled microenvironments and future soft-robotic systems.

Photochromic Ln-MOFs: A Platform for Metal-Photoswitch Cooperativity

Optoelectronic devices based on lanthanide-containing materials are an emergent area of research due to imminent interest in a new generation of diode materials, optical and magnetic sensors, and ratiometric thermometers. Tailoring material properties through the employment of photo- or thermochromic moieties is a powerful approach that requires a deep fundamental understanding of possible cooperativity between lanthanide-based metal centers and integrated switchable units. In this work, we probe this concept through the synthesis, structural analysis, and spectroscopic characterization of novel photochromic lanthanide-based metal-organic materials containing noncoordinatively integrated photoresponsive 4,4'-azopyridine between lanthanide-based metal centers. As a result, a photophysical material response tailored on demand through the incorporation of photochromic compounds within a rigid matrix was investigated. The comprehensive analysis of photoresponsive metal-organic materials includes single-crystal X-ray diffraction and diffuse reflectance spectroscopic studies that provide guiding principles necessary for understanding photochromic unit-lanthanide-based metal-organic framework (MOF) cooperativity. Furthermore, steady-state and time-resolved diffuse reflectance spectroscopic studies revealed a rapid rate of photoresponsive moiety attenuation upon its integration within the rigid matrix of lanthanide-based MOFs in comparison with that in solution, highlighting a unique role and synergy that occurred between stimuli-responsive moieties and the lanthanide-based MOF platform, allowing for tunability and control of material photoisomerization kinetics.

High-order layered self-assembled multicavity metal--organic capsules and anti-cooperative host-multi-guest chemistry

The construction and application of metal-organic cages with accessible internal cavities have witnessed rapid development, however, the precise synthesis of complex metal-organic capsules with multiple cavities and achievement of multi-guest encapsulation, and further in-depth comprehension of host-multi-guest recognition remain a great challenge. Just like building LEGO blocks, herein, we have constructed a series of high-order layered metal-organic architectures of generation n (n = 1/2/3/4 is also the number of cavities) by multi-component coordination-driven self-assembly using porphyrin-containing tetrapodal ligands (like plates), multiple parallel-podal ligands (like clamps) and metal ions (like nodes). Importantly, these high-order assembled structures possessed different numbers of rigid and separate cavities formed by overlapped porphyrin planes with specific gaps. The host-guest experiments and convincing characterization proved that these capsules G2-G4 could serve as host structures to achieve multi-guest recognition and unprecedentedly encapsulate up to four C60 molecules. More interestingly, these capsules revealed negative cooperation behavior in the process of multi-guest recognition, which provides a new platform to further study complicated host-multi-guest interaction in the field of supramolecular chemistry.

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.

Out-of-Equilibrium Mechanical Disruption of β-Amyloid-Like Fibers using Light-Driven Molecular Motors

Artificial molecular motors have the potential to generate mechanical work on their environment by producing autonomous unidirectional motions when supplied with a source of energy. However, the harnessing of this mechanical work to subsequently activate various endoenergetic processes that can be useful in materials science remains elusive. Here, it is shown that by integrating a light-driven rotary motor through hydrogen bonds in a β-amyloid-like structure forming supramolecular hydrogels, the mechanical work generated during the constant rotation of the molecular machine under UV irradiation is sufficient to disrupt the β-amyloid fibers and to trigger a gel-to-sol transition at macroscopic scale. This melting of the gel under UV irradiation occurs 25 °C below the temperature needed to melt it by solely using thermal activation. In the dark, a reversible sol-gel transition is observed as the system fully recovers its original microstructure, thus illustrating the possible access to new kinds of motorized materials that can be controlled by advanced out-of-equilibrium thermodynamics.

Fabrication strategies for chiral self-assembly surface

Chiral self-assembly is the spontaneous organization of individual building blocks from chiral (bio)molecules to macroscopic objects into ordered superstructures. Chiral self-assembly is ubiquitous in nature, such as DNA and proteins, which formed the foundation of biological structures. In addition to chiral (bio) molecules, chiral ordered superstructures constructed by self-assembly have also attracted much attention. Chiral self-assembly usually refers to the process of forming chiral aggregates in an ordered arrangement under various non-covalent bonding such as H-bond, π-π interactions, van der Waals forces (dipole-dipole, electrostatic effects, etc.), and hydrophobic interactions. Chiral assembly involves the spontaneous process, which followed the minimum energy rule. It is essentially an intermolecular interaction force. Self-assembled chiral materials based on chiral recognition in electrochemistry, chiral catalysis, optical sensing, chiral separation, etc. have a broad application potential with the research development of chiral materials in recent years.

Photo-responsive functional materials based on light-driven molecular motors

In the past two decades, the research and development of light-triggered molecular machines have mainly focused on developing molecular devices at the nanoscale. A key scientific issue in the field is how to amplify the controlled motion of molecules at the nanoscale along multiple length scales, such as the mesoscopic or the macroscopic scale, or in a more practical perspective, how to convert molecular motion into changes of properties of a macroscopic material. Light-driven molecular motors are able to perform repetitive unidirectional rotation upon irradiation, which offers unique opportunities for responsive macroscopic systems. With several reviews that focus on the design, synthesis and operation of the motors at the nanoscale, photo-responsive macroscopic materials based on light-driven molecular motors have not been comprehensively summarized. In the present review, we first discuss the strategy of confining absolute molecular rotation into relative rotation by grafting motors on surfaces. Secondly, examples of self-assemble motors in supramolecular polymers with high internal order are illustrated. Moreover, we will focus on building of motors in a covalently linked system such as polymeric gels and polymeric liquid crystals to generate complex responsive functions. Finally, a perspective toward future developments and opportunities is given. This review helps us getting a more and more clear picture and understanding on how complex movement can be programmed in light-responsive systems and how man-made adaptive materials can be invented, which can serve as an important guideline for further design of complex and advanced responsive materials.

Releasing a bound molecular spring with light: a visible light-triggered photosalient effect tied to polymorphism

Here we present a study on the solid state properties of trans tetra-ortho-bromo azobenzene (4Br-Azo). Two distinct crystal polymorphs were identified: the α-phase and β-phase. Notably, only the β-phase exhibited an extraordinary photosalient effect (jumping/breaking) upon exposure to a wide range of visible light. Powder X-ray diffraction and Raman spectroscopy revealed that the β-phase is metastable and can transition to the α-phase when subjected to specific stimuli like heat and light. Furthermore, single crystal X-ray diffraction and density functional theory calculations highlighted the significance of a highly strained conformer in the β-phase, showing that the metastability of the phase potentially arises from relieving this strain. This metastability leads to a light induced phase transition, which appears to be the cause of the photosalient effect in these crystals. Interestingly the polymorphism at the core of 4Br-Azo's dynamic behavior is based on different arrangements of halogen based intermolecular interactions. It is possible that continued study on combining visible light capturing chromophores with halogen interaction-based polymorphism will lead to the discovery of even more visible light controlled dynamic crystal materials.

Highly Ordered Co-Assembly of Bisurea Functionalized Molecular Switches at the Solid-Liquid Interface

Immobilization of stimulus-responsive systems on solid surfaces is beneficial for controlled signal transmission and adaptive behavior while allowing the characterization of the functional interface with high sensitivity and high spatial resolution. Positioning of the stimuli-responsive units with nanometer-scale precision across the adaptive surface remains one of the bottlenecks in the extraction of cooperative function. Nanoscale organization, cooperativity, and amplification remain key challenges in bridging the molecular and the macroscopic worlds. Here we report on the design, synthesis, and scanning tunneling microscopy (STM) characterization of overcrowded alkene photoswitches merged in self-assembled networks physisorbed at the solid-liquid interface. A detailed anchoring strategy that ensures appropriate orientation of the switches with respect to the solid surface through the use of bis-urea groups is presented. We implement a co-assembly strategy that enables the merging of the photoswitches within physisorbed monolayers of structurally similar 'spacer' molecules. The self-assembly of the individual components and the co-assemblies was examined in detail using (sub)molecular resolution STM which confirms the robust immobilization and controlled orientation of the photoswitches within the spacer monolayers. The experimental STM data is supported by detailed molecular mechanics (MM) simulations. Different designs of the switches and the spacers were investigated which allowed us to formulate guidelines that enable the precise organization of the photoswitches in crystalline physisorbed self-assembled molecular networks.

Tailoring Multicontrolled Supramolecular Assemblies of Stiff-Stilbene Amphiphiles into Macroscopic Soft Scaffolds as Cell-Material Interfaces

Biocompatible synthetic supramolecular systems have shed light on biomedical and tissue-regenerative material applications. The intrinsic functional applicability, tunability, and stimuli-responsiveness of synthetic supramolecular systems allow one to develop various multicontrolled supramolecular assemblies in aqueous media. However, it remains highly challenging to use state-of-the-art supramolecular assemblies of photoresponsive amphiphiles controlled by multiple stimulations in fabricating macroscopic materials. Herein, we demonstrate a stiff-stilbene amphiphile (SA) multicontrolled supramolecular assembling system that comprises two different charged end groups. The excellent photoswitchabilities of SA in both organic and aqueous media are demonstrated. Furthermore, multiple stimuli, i.e., light, pH, and counterions, are applied to control the supramolecular assembling behaviors, which are monitored by circular dichroism spectroscopy and electron microscopies. This multicontrolled supramolecular system can be systematically assembled into macroscopic soft functional scaffolds, whose structural parameters are investigated by electron microscopies and X-ray diffraction techniques, suggesting the large aspect ratio of SA nanostructures assembled into macroscopic soft scaffolds. The fabricated soft functional scaffold is highly biocompatible for photocontrolled biotarget encapsulation/release selectively, as well as a cell-material interface for diverse cells' attachment. This new synthetic multicontrolled soft functional material provides a new strategy toward the development of next-generation controllable and biocompatible soft functional materials.

Coupled Rocking Motion in a Light-Driven Rotary Molecular Motor

Coupled motion is ubiquitous in Nature as it forms the base for the direction, amplification, propagation, and synchronization of movement. Herein, we present experimental proof for the coupling of the rocking motion of a dihydroanthracene stator moiety with the light-induced rotational movement of an overcrowded alkene-based molecular motor. The motor was desymmetrized, introducing two different alkyl substituents to the stator part of the molecular scaffold, resulting in the formation of two diastereomers with opposite axial chirality. The structure of the two isomers is determined with nuclear Overhauser effect spectroscopy NMR and single-crystal X-ray analysis. The desymmetrization enables the study of the coupled motion, that is, rotation and oscillation, by 1H NMR, findings that are further supported by density functional theory calculations. A new handle to regulate the rotational speed of the motor through functionalization in the bottom half was also introduced, as the thermal barrier for thermal helix inversion is found to be largely dependent on the alkyl substituents and its orientation toward the upper half of the motor scaffold. In addition to the commonly observed successive photochemical and thermal steps driving the rotation of the motor, we find that the motor undergoes photochemically driven rotation in three of the four steps of the rotation cycle. Hence, this result extends the scope of molecular motors capable of photon-only rotary behavior.

Giant Crystalline Molecular Rotors that Operate in the Solid State

Molecular motion in the solid state is typically precluded by the highly dense environment, and only molecules with a limited range of sizes show such dynamics. Here, we demonstrate the solid-state rotational motion of two giant molecules, i.e., triptycene and pentiptycene, by encapsulating a bulky N-heterocyclic carbene (NHC) Au(I) complex in the crystalline media. To date, triptycene is the largest molecule (surface area: 245 Å2 ; volume: 219 Å3 ) for which rotation has been reported in the solid state, with the largest rotational diameter among reported solid-state molecular rotors (9.5 Å). However, the pentiptycene rotator that is the subject of this study (surface area: 392 Å2 ; volume: 361 Å3 ; rotational diameter: 13.0 Å) surpasses this record. Single-crystal X-ray diffraction analyses of both the developed rotors revealed that these possess sufficient free volume around the rotator. The molecular motion in the solid state was confirmed using variable-temperature solid-state 2 H spin-echo NMR studies. The triptycene rotor exhibited three-fold rotation, while temperature-dependent changes of the rotational angle were observed for the pentiptycene rotor.

Recent Advances in Nanomechanical Measurements and Their Application for Pharmaceutical Crystals

Mechanical behavior of pharmaceutical crystals directly impacts the formulation development and manufacturing of drug products. The understanding of crystal structure-mechanical behavior of pharmaceutical and molecular crystals has recently gained substantial attention among pharmaceutical and materials scientists with the advent of advanced nanomechanical testing instruments like nanoindentation. For the past few decades, instrumented nanoindentation was a popular technique for measuring the mechanical properties of thin films and small-length scale materials. More recently it is being implemented to investigate the mechanical properties of pharmaceutical crystals. Integration of correlative microscopy techniques and environmental control opened the door for advanced structure-property correlation under processing conditions. Preventing the degradation of active pharmaceutical ingredients from external factors such as humidity, temperature, or pressure is important during processing. This review deals with the recent developments in the synchronized nanomechanical measurements of pharmaceutical crystals toward the fast and effective development of high-quality pharmaceutical drug products. This review also summarizes some recent reports to intensify how one can design and control the nanomechanical properties of pharmaceutical solids. Measurement challenges and the scope for studying nanomechanical properties of pharmaceutical crystals using nanoindentation as a function of crystal structure and in turn to develop fundamental knowledge in the structure-property relationship with the implications for drug manufacturing and development are discussed in this review. This review further highlights recently developed capabilities in nanoindentation, for example, variable temperature nanoindentation testing, in situ imaging of the indented volume, and nanoindentation coupled Raman spectroscopy that can offer new quantitative details on nanomechanical behavior of crystals and will play a decisive role in the development of coherent theories for nanomechanical study of pharmaceutical crystal.

Modulation of supramolecular chirality by stepwise axial coordination in a nano-size trizinc(ii)porphyrin trimer

Herein, we report a chiral guest's triggered spring-like contraction and extension motions coupled with unidirectional twisting in a novel flexible and 'nano-size' achiral trizinc(ii)porphyrin trimer host upon step-wise formation of 1 : 1, 1 : 2, and 1 : 4 host-guest supramolecular complexes based on the stoichiometry of the diamine guests for the first time. During these processes, porphyrin CD responses have been induced, inverted, and amplified, and reduced, respectively, in a single molecular framework due to the change in the interporphyrin interactions and helicity. Also, the sign of the CD couplets is just the opposite between R and S substrates which suggests that the chirality is dictated solely by the stereographic projection of the chiral center. Interestingly, the long-range electronic communications between the three porphyrin rings generate trisignate CD signals that provide further information about molecular structures.

Directing and Understanding the Translation of a Single Molecule Dipole

Understanding the directed motion of a single molecule on surfaces is not only important in the well-established field of heterogeneous catalysis but also for the design of artificial nanoarchitectures and molecular machines. Here, we report how the tip of a scanning tunneling microscope (STM) can be used to control the translation direction of a single polar molecule. Through the interaction of the molecular dipole with the electric field of the STM junction, it was found that both translations and rotations of the molecule occur. By considering the location of the tip with respect to the axis of the dipole moment, we can deduce the order in which rotation and translation take place. While the molecule-tip interaction dominates, computational results suggest that the translation is influenced by the surface direction along which the motion takes place.

Benchmark Dynamics of Dipolar Molecular Rotors in Fluorinated Metal-Organic Frameworks

Fluorinated Metal-Organic Frameworks (MOFs), comprising a wheel-shaped ligand with geminal rotating fluorine atoms, produced benchmark mobility of correlated dipolar rotors at 2 K, with practically null activation energy (E_a =17 cal mol^-1 ). ¹ H T₁ NMR revealed multiple relaxation phenomena due to the exchange among correlated dipole-rotor configurations. Synchrotron radiation X-ray diffraction at 4 K, Density Functional Theory, Molecular Dynamics and phonon calculations showed the fluid landscape and pointed out a cascade mechanism converting dipole configurations into each other. Gas accessibility, shown by hyperpolarized-Xe NMR, allowed for chemical stimuli intervention: CO₂ triggered dipole reorientation, reducing their collective dynamics and stimulating a dipole configuration change in the crystal. Dynamic materials under limited thermal noise and high responsiveness enable the fabrication of molecular machines with low energy dissipation and controllable dynamics.

Target Recognition in Tandem WW Domains: Complex Structures for Parallel and Antiparallel Ligand Orientation in h-FBP21 Tandem WW

Protein-protein interactions often rely on specialized recognition domains, such as WW domains, which bind to specific proline-rich sequences. The specificity of these protein-protein interactions can be increased by tandem repeats, i.e., two WW domains connected by a linker. With a flexible linker, the WW domains can move freely with respect to each other. Additionally, the tandem WW domains can bind in two different orientations to their target sequences. This makes the elucidation of complex structures of tandem WW domains extremely challenging. Here, we identify and characterize two complex structures of the tandem WW domain of human formin-binding protein 21 and a peptide sequence from its natural binding partner, the core-splicing protein SmB/B'. The two structures differ in the ligand orientation and, consequently, also in the relative orientation of the two WW domains. We analyze and probe the interactions in the complexes by molecular simulations and NMR experiments. The workflow to identify the complex structures uses molecular simulations, density-based clustering, and peptide docking. It is designed to systematically generate possible complex structures for repeats of recognition domains. These structures will help us to understand the synergistic and multivalency effects that generate the astonishing versatility and specificity of protein-protein interactions.

Amplification of integrated microscopic motions of high-density [2]rotaxanes in mechanically interlocked networks

Integrating individual microscopic motion to perform tasks in macroscopic sale is common in living organisms. However, developing artificial materials in which molecular-level motions could be amplified to behave macroscopically is still challenging. Herein, we present a class of mechanically interlocked networks (MINs) carrying densely rotaxanated backbones as a model system to understand macroscopic mechanical properties stemmed from the integration and amplification of intramolecular motion of the embedded [2]rotaxane motifs. On the one hand, the motion of mechanical bonds introduces the original dangling chains into the network, and the synergy of numerous such microscopic motions leads to an expansion of entire network, imparting good stretchability and puncture resistance to the MINs. On the other hand, the dissociation of host-guest recognition and subsequent sliding motion represent a peculiar energy dissipation pathway, whose integration and amplification result in the bulk materials with favorable toughness and damping capacity. Thereinto, we develop a continuous stress-relaxation method to elucidate the microscopic motion of [2]rotaxane units, which contributes to the understanding of the relationship between cumulative microscopic motions and amplified macroscopic mechanical performance.

Investigation of the motion of fullerene-wheeled nano-machines on thermally activated curved gold substrates

The current study presents one of the first investigations in which the simultaneous effect of the curved gold substrates and temperature changes on C₆₀ and C₆₀-wheeled nano-machines’ migration was evaluated. For this aim, the cylindrical and concave substrates with different radii were chosen to attain the size of the most appropriate substrate for nano-machines. Results indicated that the chassis' flexibility substantially affected the nanocar's mobility. Nano-machines' deviation from their desired direction was adequately restricted due to selected substrate geometries (The cylindrical and concave). Besides, for the first time, the effect of the substrate radius changes on nano-machine's motion has been investigated. Our findings revealed that adjusting the value of radius results in a long-range movement for nano-machines as well as a sufficient amount of diffusion coefficient even at low temperatures (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$75\; \text{K}$$\end{document}75K or \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$150\; \text{K}$$\end{document}150K). As a result, the aforementioned substrates could be utilized as the optimized geometries for C₆₀ and nanocar at all temperatures. At the same time, the nanotruck displayed an appropriate performance merely on the small cylindrical substrate (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$radius=17.5 \; \text{\AA} $$\end{document}radius=17.5Å) at high temperatures (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$500\; \text{K}$$\end{document}500K and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$600\; \text{K}$$\end{document}600K).

Hierarchically supramolecular polymerization of anthraquinone dye to chiral aggregates via 2D-monolayered nanosheets: the unanticipated role of pathway complexity

An anthraquinone dye underwent supramolecular polymerization, affording 2D-monolayered nanosheets in a kinetically controlled state. The nanosheets then transformed into hierarchically chiral aggregates in a thermodynamically controlled step. The unanticipated role played by pathway complexity was clearly unravelled in this work, highlighting the diversified pathways in the supramolecular polymerization of various building blocks.

Chemically Driven Rotatory Molecular Machines

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

Recent Advances in Fuel-Driven Molecular Switches and Machines

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

Investigation of fullerene motion on thermally activated gold substrates with different shapes

In the current study, the regime of motion of fullerene molecules on substrates with different shapes at a range of specific temperatures has been investigated. To do so, the potential energy of fullerene molecules was analyzed using the classical molecular dynamics method. C₂₀, C₃₆, C₅₀, C₆₀, C₇₂, C₇₆, C₈₀, and C₉₀ fullerene molecules were selected due to their spherical shapes with different sizes. In addition, to completely analyze the behavior of these molecules, different gold substrates, including flat, concave, the top side of the step (upward step), and the downside of the step (downward step) substrates, were considered. Specifying the regime of the motion at different temperatures is one of the main goals of this study. For this purpose, we have studied the translational and rotational motions of fullerene molecules independently. In the first step of the investigation, Lennard-Jones potential energy of fullerene molecules was calculated. Subsequently, the regime of motion of different fullerenes has been classified, based on their displacement and sliding velocity. Our findings indicated that C₆₀ is appropriate in less than [Formula: see text] of the conditions. However, C₂₀, C₇₆ and C₈₀ molecules were found to be appropriate candidates in most cases in different conditions while they were incompetent only in seven situations. As far as a straight-line movement is considered, the concave geometry demonstrated a better performance compared to the other substrates. In addition, C₇₂ indicated less favorable performance concerning the range of movement and diffusion coefficients. All in all, our investigation helps to understand the performance of different fullerene molecules on gold substrates and find their probable application, especially as a wheel in nano-machine structures.

Flexible organic crystals. Understanding the tractable co-existence of elastic and plastic bending

As an emerging class of flexible materials, mechanically bendable molecular crystals are broadly classified as elastic or plastic. Nevertheless, flexible organic crystals with mutually exclusive elastic and plastic traits, with contrasting structural requirements, co-existing under different stress settings are exceptional; hence, it is imperative to establish the concurring factors that beget this rare occurrence. We report a series of halogen-substituted benzil crystals showing elastic bending (within ∼2.45% strain), followed by elastoplastic deformation under ambient conditions. Under higher stress settings, they display exceptional plastic flexibility that one could bend, twist, or even coil around a capillary tube. X-ray diffraction, microscopy, and computational data reveal the microscopic and macroscopic basis for the exciting co-existence of elastic, elastoplastic, and plastic properties in the crystals. The layered molecular arrangement and the weak dispersive interactions sustaining the interlayer region provide considerable tolerance towards breaking and making upon engaging or releasing the external stress; it enables restoring the original state within the elastic strain. Comparative studies with oxalate compounds, wherein the twisted diketo moiety in benzil was replaced with a rigid and coplanar central oxalate moiety, enabled us to understand the effect of the anisotropy factor on the crystal packing induced by the C[double bond, length as m-dash]O⋯C tetral interactions. The enhanced anisotropy depreciated the elastic domain, making the oxalate crystals more prone to plastic deformation. Three-point bending experiments and the determined Young's moduli further corroborate the co-existence of the elastic and plastic realm and highlight the critical role of the underlying structural elements that determine the elastic to plastic transformation. The work highlights the possible co-existence of orthogonal mechanical characteristics in molecular crystals and further construed the concurrent role of microscopic and macroscopic elements in attaining this exceptional mechanical trait.

Stimuli-Responsive Adaptive Nanotoxin to Directly Penetrate the Cellular Membrane by Molecular Folding and Unfolding

Biological nanomachines, including proteins and nucleic acids whose function is activated by conformational changes, are involved in every biological process, in which their dynamic and responsive behaviors are controlled by supramolecular recognition. The development of artificial nanomachines that mimic the biological functions for potential application as therapeutics is emerging; however, it is still limited to the lower hierarchical level of the molecular components. In this work, we report a synthetic machinery nanostructure in which actuatable molecular components are integrated into a hierarchical nanomaterial in response to external stimuli to regulate biological functions. Two nanometers core-sized gold nanoparticles are covered with ligand layers as actuatable components, whose folding/unfolding motional response to the cellular environment enables the direct penetration of the nanoparticles across the cellular membrane to disrupt intracellular organelles. Furthermore, the pH-responsive conformational movements of the molecular components can induce the apoptosis of cancer cells. This strategy based on the mechanical motion of molecular components on a hierarchical nanocluster would be useful to design biomimetic nanotoxins.

Disulfide-Mediated Reversible Polymerization toward Intrinsically Dynamic Smart Materials

The development of a dynamic chemistry toolbox to endow materials dynamic behavior has been key to the rational design of future smart materials. The rise of supramolecular and dynamic covalent chemistry offers many approaches to the construction of dynamic polymers and materials that can adapt, respond, repair, and recycle. Within this toolbox, the building blocks based on 1,2-dithiolanes have become an important scaffold, featuring their reversible polymerization mediated by dynamic covalent disulfide bonds, which enables a unique class of dynamic materials at the intersection of supramolecular polymers and adaptable covalent networks. This Perspective aims to explore the dynamic chemistry of 1,2-dithiolanes as a versatile structural unit for the design of smart materials by summarizing the state of the art as well as providing an overview of the fundamental challenges involved in this research area and its potential future directions.

Transport of a self-propelled tracer through a hairy cylindrical channel: interplay of stickiness and activity

Active transport of biomolecules assisted by motor proteins is imperative for the proper functioning of cellular activities. Inspired by the diffusion of active agents in crowded cellular channels, we computationally investigate the transport of an active tracer through a polymer grafted cylindrical channel by varying the activity of the tracer and stickiness of the tracer to the polymers. Our results reveal that the passive tracer exhibits profound subdiffusion with increasing stickiness by exploring deep into the grafted polymeric zone, while purely repulsive one prefers to diffuse through the pore-like space created along the cylindrical axis of the channel. In contrast, the active tracer shows faster dynamics and intermediate superdiffusion even though the tracer preferentially stays close to the dense polymeric region. This observation is further supported by the sharp peaks in the density profile of the probability of radial displacement of the tracer. We discover that the activity plays an important role in deciding the pathway that the tracer takes through the narrow channel. Interestingly, increasing the activity washes out the effect of stickiness. Adding to this, van-Hove functions manifest that the active tracer dynamics deviates from Gaussianity, and the degree of deviation grows with the activity. Our work has direct implications on how effective transportation and delivery of cargo can be achieved through a confined medium where activity, interactions, and crowding are interplaying. Looking ahead, these factors will be crucial for understanding the mechanism of artificial self-powered machines navigating through the cellular channels and performing in vivo challenging tasks.

Controlling rotary motion of molecular motors based on oxindole

Molecular motors are essential components of artificial molecular machines, which can be used to manipulate and amplify mechanical motion at the nanoscale to create machine-like function. Since the discovery of light-driven rotary molecular motors, the field has been widely developed, including the introduction of molecular motors based on oxindole by our group in 2019. The rotational properties of molecular motors, e.g. absorption wavelength, quantum yield and rotation speed, often critically depend on substituent effects. Up to now, the substituent effects of oxindole-based molecular motors have not yet been investigated. Herein, we present a family of oxindole-based molecular motors functionalised at three different positions on the motor core, with either CN or OMe groups. The motors prepared in this work retain the favourable features of oxindole-based motors, i.e. simple synthesis and visible light addressability. We find that functionalisation has substantial effects on the absorption wavelength of the motors, meanwhile the rotation speed is unaffected. Furthermore, we found that functionalisation of the oxindole molecular motors increases their quantum efficiency considerably in comparison to previous motors of their class.

We present a new family of oxindole-based functionalised at three positions on the upper and lower halves, with methoxy or cyano groups. We find that this allows the absorption wavelength and quantum yields of these motors to be tuned.

Catalytic synthesis of azoarenes via metal-mediated nitrene coupling

Various valuable properties of azoarenes (“azo dyes”), including their vivid colors and their facile cis-trans photoisomerization, lead to their wide use in the chemical industry. As a result, ~700,000 metric...

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.

Electron ratcheting in self-assembled soft matter

Ratcheted multi-step hopping electron transfer systems can plausibly produce directional charge transport over very large distances without requiring a source-drain voltage bias. We examine molecular strategies to realize ratcheted charge transport based on multi-step charge hopping, and we illustrate two ratcheting mechanisms with examples based on DNA structures. The charge transport times and currents that may be generated in these assemblies are also estimated using kinetic simulations. The first ratcheting mechanism described for nanoscale systems requires local electric fields on the 10⁹ V/m scale to realize nearly 100% population transport. The second ratcheting mechanism for even larger systems, based on electrochemical gating, is estimated to generate currents as large as 0.1 pA for DNA structures that are a few μm in length with a gate voltage of about 5 V, a magnitude comparable to currents measured in DNA wires at the nanoscale when a source-drain voltage bias of similar magnitude is applied, suggesting an approach to considerably extend the distance range over which DNA charge transport devices may operate.