Light-Operated Machines Based on Threaded Molecular Structures

Directing Coupled Motion with Light: A Key Step Toward Machine-Like Function

Molecular photoactuators can control shape and chemical or physical properties of the responsive system they are embedded in. These effects are usually mediated by supramolecular interactions and can be amplified to perform work at the micro- and macroscopic scale, for instance, in materials and biomimetic systems. While many studies focus on the observable outcome of these events, photoresponsive structures can also translate their conformational change to molecular components and perform work against random Brownian motion. Stereochemical cascades can amplify light-generated motion to a distant moiety of the same molecule or molecular assembly, via conformationally restricted stereogenic elements. Being able to control the conformation or motion of molecular systems remotely provides prospects for the design of the smallest machines imaginable. This Focus Review emphasizes the emergence of directed, coupled motion of remote functionalities triggered by light-powered switches and motors as a tool to control molecular topology and function.

A Light-Operated Molecular Cable Car for Gated Ion Transport

Inspired by the nontrivial and controlled movements of molecular machines, we report an azobenzene-based molecular shuttle PR2, which can perform light-gated ion transport across lipid membranes. The amphiphilicity and membrane-spanning molecular length enable PR2 to insert into the bilayer membrane and efficiently transport K⁺ (EC₅₀ =4.1 μm) through the thermally driven stochastic shuttle motion of the crown ether ring along the axle. The significant difference in shuttling rate between trans-PR2 and cis-PR2 induced by molecular isomerization enables a light-gated ion transport, i.e., ON/OFF in situ regulation of transport activity and single-channel current. This work represents an example of using a photoswitchable molecular machine to realize gated ion transport, which demonstrates the value of molecular machines functioning in biomembranes.

ON/OFF Control of the Flipping Motion of Diuranyl Bis(Salophen) Macrocycle by Extremely Strong Binding with Fluoride Ion

Diuranyl bis(salophen) complex 1 features a relatively slow conformational motion, induced by an intramolecular O═U═O···UO₂ binding motif, which interconverts the two nonsymmetric halves of the ligand. This flipping motion, which constitutes one of the fundamental molecular motions, can be completely halted by addition of fluoride anion, which is bound to 1, reaching one of the highest affinities reported to date. This system represents a model to study flipping dynamics in light of the possibility of developing novel types of molecular machines based on it.

Threading-gated photochromism in [2]pseudorotaxanes

Rigid, Y-shaped imidazole compounds containing the bis(thienyl)ethene moiety were designed and synthesized. The 4,5-bis(benzothienyl)-2-phenylimidazolium cations were then used as axles for [2]pseudorotaxane formation with 24-membered crown ether wheels. It was demonstrated using 1H NMR spectroscopy, UV-Vis absorption and emission spectroscopies that this host-guest interaction results in significant changes in the photochromic properties of the imidazolium axles. This is a rare example of gated photochromism, which exploits the recognition event of an interpenetrated molecular system to tune the photochromic properties in one of the components.

Dynamics of individual molecular shuttles under mechanical force

Molecular shuttles are the basis of some of the most advanced synthetic molecular machines. In these devices a macrocycle threaded onto a linear component shuttles between different portions of the thread in response to external stimuli. Here, we use optical tweezers to measure the mechanics and dynamics of individual molecular shuttles in aqueous conditions. Using DNA as a handle and as a single molecule reporter, we measure thousands of individual shuttling events and determine the force-dependent kinetic rates of the macrocycle motion and the main parameters governing the energy landscape of the system. Our findings could open avenues for the real-time characterization of synthetic devices at the single molecule level, and provide crucial information for designing molecular machinery able to operate under physiological conditions.

Molecular shuttles are bi-stable and stimuli-responsive systems that are considered potential elements for molecular machinery. Here, the authors use optical tweezers to measure the force dependent real-time kinetics of individual molecular shuttles under aqueous conditions.

Photochromism into nanosystems: towards lighting up the future nanoworld

The ability to manipulate the structure and function of promising nanosystems via energy input and external stimuli is emerging as an attractive paradigm for developing reconfigurable and programmable nanomaterials and multifunctional devices. Light stimulus manifestly represents a preferred external physical and chemical tool for in situ remote command of the functional attributes of nanomaterials and nanosystems due to its unique advantages of high spatial and temporal resolution and digital controllability. Photochromic moieties are known to undergo reversible photochemical transformations between different states with distinct properties, which have been extensively introduced into various functional nanosystems such as nanomachines, nanoparticles, nanoelectronics, supramolecular nanoassemblies, and biological nanosystems. The integration of photochromism into these nanosystems has endowed the resultant nanostructures or advanced materials with intriguing photoresponsive behaviors and more sophisticated functions. In this Review, we provide an account of the recent advancements in reversible photocontrol of the structures and functions of photochromic nanosystems and their applications. The important design concepts of such truly advanced materials are discussed, their fabrication methods are emphasized, and their applications are highlighted. The Review is concluded by briefly outlining the challenges that need to be addressed and the opportunities that can be tapped into. We hope that the review of the flourishing and vibrant topic with myriad possibilities would shine light on exploring the future nanoworld by encouraging and opening the windows to meaningful multidisciplinary cooperation of engineers from different backgrounds and scientists from the fields such as chemistry, physics, engineering, biology, nanotechnology and materials science.

A light-driven molecular machine based on stiff stilbene

We report a new molecular design for optically triggered nm-scale translation of a submolecular component relative to another.

Excitation Energy Delocalization and Transfer to Guests within MII4L6 Cage Frameworks

We have prepared a series of M^II₄L₆ tetrahedral cages containing one or the other of two distinct BODIPY moieties, as well as mixed cages that contain both BODIPY chromophores. The photophysical properties of these cages and their fullerene-encapsulated adducts were studied in depth. Upon cage formation, the charge-transfer character exhibited by the bis(aminophenyl)-BODIPY subcomponents disappeared. Strong excitonic interactions were instead observed between at least two BODIPY chromophores along the edges of the cages, arising from the electronic delocalization through the metal centers. This excited-state delocalization contrasts with previously reported cages. All cages exhibited the same progression from an initial bright singlet state (species I) to a delocalized dark state (species II), driven by interactions between the transition dipoles of the ligands, and subsequently into geometrically relaxed species III. In the case of cages loaded with C₆₀ or C₇₀ fullerenes, ultrafast host-to-guest electron transfer was observed to compete with the excitonic interactions, short-circuiting the I → II → III sequence.

High-Yielding Syntheses of Crown Ether-Based Pyridyl Cryptands

Pyridinium bis(trifluoromethylsulfonyl)imide (PyTFSI)-templated syntheses of 2,6-pyridyl cryptands of cis(4,4')-dibenzo-30-crown-10 (3a), the p-bromobenzyloxy derivative 3b, bis(m-phenylene)-32-crown-10 (5), cis(4,4')-dibenzo-27S-crown-9 (7), cis(4,4')-dibenzo-27L-crown-9 (9), and cis(4,4')-dibenzo-24-crown-8 (11) are reported. Here we provide a fast (12 h), high-yielding (89%, 74%, 80%, and 62% for 3a, 3b, 5, and 9, respectively) templation method without the use of a syringe pump. The yields for 7 (19%) and 11 (26%) were lower than with the previous pseudo-high-dilution method, indicating ineffective templation in these cases. Coupled with our previously developed templated syntheses of dibenzo crown ethers, this protocol makes powerful cryptand hosts readily available in gram quantities in good yields from methyl 4(or 3)-hydroxy-3(or 4)-benzyloxybenzoate.

Four-component zinc-porphyrin/zinc-salphen nanorotor

An off-axis supramolecular rotor was composed of four components: a zinc-porphyrin based stator with four phenanthroline stations and a zinc-salphen based rotator were self-assembled with DABCO and four copper(i) ions to furnish the rotor ROT-2 in quantitative yield.

Five-State Rotary Nanoswitch

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

Bidirectional Photomodulation of Surface Tension in Langmuir Films

Switching systems operating in a cooperative manner capable of converting light energy into mechanical motion are of great interest for optical devices, data storage, nanoscale energy converters and molecular sensing. Herein, photoswitchable monolayers were formed at the air-water interface from either a pure bis(thiaxanthylidene)-based photoswitchable amphiphile or from a mixture of the photoswitchable amphiphile with a conventional lipid dipalmitoylphosphatidylcholine (DPPC). Efficient photoisomerization of the anti-folded to syn-folded geometry of the amphiphile's central core induces changes in the surface pressure in either direction, depending on the initial molecular density. Additionally, the switching behavior can be regulated in the presence of DPPC, which influences the packing of the molecules, thereby controlling the transformation upon irradiation. Bis(thiaxanthylidene)-based photoswitchable monolayers provide a promising system to explore cooperativity and amplification of motion.

Allosteric Regulation of the Rotational Speed in a Light-Driven Molecular Motor

The rotational speed of an overcrowded alkene-based molecular rotary motor, having an integrated 4,5-diazafluorenyl coordination motif, can be regulated allosterically via the binding of metal ions. DFT calculations have been used to predict the relative speed of rotation of three different (i.e., zinc, palladium, and platinum) metal dichloride complexes. The photochemical and thermal isomerization behavior of these complexes has been studied in detail using UV–vis and ¹H NMR spectroscopy. Our results confirm that metal coordination induces a contraction of the diazafluorenyl lower half, resulting in a reduction of the steric hindrance in the “fjord” region of the molecule, which causes an increase of the rotational speed. Importantly, metal complexation can be accomplished in situ and is found to be reversible upon the addition of a competing ligand. Consequently, the rotational behavior of these molecular motors can be dynamically controlled with chemical additives.

Synthesis and Photostability of Unimolecular Submersible Nanomachines: Toward Single-Molecule Tracking in Solution

The synthesis and photophysical properties of a series of photostable unimolecular submersible nanomachines (USNs) are reported as a first step toward the analysis of their trajectories in solution. The USNs have a light-driven rotatory motor for propulsion in solution and photostable cy5-COT fluorophores for their tracking. These cy5-COT fluorophores are found to provide an almost 2-fold increase in photostability compared to the previous USN versions and do not affect the rotation of the motor.

The eternal youth of azobenzene: new photoactive molecular and supramolecular devices

The development of multicomponent chemical systems that can perform predetermined functions under external control – i.e., molecular devices – is a challenging task in chemistry and a fascinating objective in the frame of a bottom-up approach to nanostructures. Photochromic units undergo profound changes in their chemical and/or electronic structure upon light excitation, and are highly interesting for the construction of photocontrollable molecular devices, machines and materials. The E–Z photoisomerization of azobenzene – owing to its high efficiency, excellent reversibility and significant physico-chemical differences between the two forms – is a highly useful reaction in this regard. Azobenzene photoisomerization has been known for almost 80 years and has been exploited to implement light-induced functionalities with a large variety of compounds, biomolecules, nanosystems and materials. Here we present some of our recent investigations highlighting how this outstanding photochrome can be utilized to develop (supra)molecular systems with valuable light-induced functionalities.

Redox-dependent self-sorting toggles a rotary nanoswitch

The pyridine–pyrimidine (py–pym) arm as the moving part of the two-state nanomechanical rotary switch [Cu(1)]⁺ is toggled reversibly between two stations using one-electron oxidation/reduction.

Coupled molecular motions driven by light or chemical inputs: spiropyran to merocyanine isomerisation followed by pseudorotaxane formation

A dual stimuli-responsive guest molecule containing a spiropyran moiety can be isomerised to a merocyanine species promoting pseudorotaxane formation.

From self-sorted coordination libraries to networking nanoswitches for catalysis

This feature article sketches our long way from the development of dynamic heteroleptic coordination motifs to the self-sorting of multi-component libraries and finally the design of a new family of triangular nanomechanical switches, which are useful for ON–OFF control of catalysis and in bidirectional communication.