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Light-responsive molecular systems, capable of interconverting between isomers using light, can be integrated into biological membranes to mimic and control their dynamic behaviour. This Review discusses the key design principles and experimental challenges while discussing and highlighting recent advances.

Covalent polymers with helical conformations offer an adaptive scaffold for smart materials, but polymer-to-monomer deconstruction is inhibited by the covalent backbone. Now it has been shown that poly(disulfide)s can be folded into helices driven by side-chain hydrogen-bonding self-assembly, resulting in a synthetic helical polymer that can be fully recycled.

The light-driven power stroke of a unidirectional molecular motor is studied using ultrafast fluorescence spectroscopy. The evolution on the excited-state energy surface is observed on the 100 fs timescale and is accompanied by damped coherent molecular motion. The implications of these observations for the operation of the molecular motors are discussed.

Using supramolecular chemistry principles, thermodynamically metastable, yet kinetically stable, poly(disulfide)s with tunable mechanical properties can be recycled into crystalline monomers with quantitative yields and monomer purity >90%.

The amplification of molecular motion along length scales for macroscopic muscle-like functions provides attractive opportunities ranging from soft actuators to responsive biomedical materials. Here, the authors design a photoswitch amphiphile based on an overcrowded alkene-derived core and develop supramolecular polymer-based artificial muscles.

Liquid-liquid phase separation (LLPS) is a crucial process, but this behaviour in supramolecular assemblies is not well understood. Here, the authors report the development of a molecular-motor driven LLPS system, with the separation process driven by nanoscale rotary motion.

Light and redox switches are of interest for responsive materials, actuators and robotics. Here the authors integrated a redox- and photoresponsive switch into a polymer hydrogel enabling reversible redox-switching with high spatio-temporal precision.

Overcrowded alkene-derived molecular motors convert light and heat into chirality-directed unidirectional rotary motion, but the efficiency of their photochemical isomerization remains limited. Now formylation of the motor core has been shown to boost all aspects of motor photochemistry by improving photochemical efficiency, diminishing competing processes and redshifting absorption.

Photoclick reactions have found applications ranging from surface functionalization and polymer crosslinking to protein labelling and bioimaging, but they typically require near-UV and mid-UV light to proceed. This Review presents and discusses strategies and recent advances for long wavelength-driven photoclick reactions.

Nature frequently shows exquisite control over reactions both of water and in water. Here, the enantioselective conjugate addition of water to an enone — a reaction that has no equivalent in conventional homogeneous catalysis — is catalysed by a copper complex of an achiral ligand that is non-covalently bound to DNA.

Avoiding equal probability for clockwise and anticlockwise rotation is essential for the function of molecular motors, and both biological and synthetic systems take advantage of chirality to control the rotary direction. Now it has been shown, by integrating two rotor moieties in a symmetric meso motor design, that light-driven unidirectional rotary motion can be achieved in an achiral system.

Typically in asymmetric catalysis each product enantiomer is produced using a different enantiomer of catalyst. Here, the authors show a photoswitchable bisphosphine ligand, capable of altering the stereoselectivity of a palladium catalysed process and producing either enantiomer of product.

Avoiding the development of microbial antibiotic resistance is a major challenge. Now the incorporation of a photoswitchable group into quinolones has been used to create ‘smart’ antibiotics that can be activated with light. The subsequent loss of activity (within hours) should prevent the build-up of active antibiotics in the environment.

Cross-coupling reactions typically are carried out under dilute conditions of ethereal solvents. Here, the authors report a method for palladium catalysed cross-coupling of organolithium reagents that avoids the use of any added solvent with reaction times of only minutes.

Direct alkylation of amines with alcohols is an atom-efficient and environmentally benign process. Here, the authors show that catalysts based on iron—an inexpensive and readily available metal—can carry out this process with high conversions and good substrate scope.

Unidirectional rotation in a synthetic molecular motor is typically driven by intrinsic asymmetry or sequences of chemical transformations. Here, the authors control the direction of a molecule’s rotation through supramolecular binding of a chiral guest and subsequent transfer of its chiral information.

The Sonogashira cross-coupling is a key strategy in modern synthesis for C–C bond formation and introduction of the versatile alkyne functionality into organic molecules. Now, a complementary method is reported based on the palladium-catalysed cross-coupling of lithium acetylides with aryl bromides.

A centimetre-long string formed by the hierarchical self-assembly of a photoresponsive amphiphilic molecular motor — composed of 95 wt% of water — undergoes muscle-like contraction. Under irradiation, rotary motion at the molecular level is amplified through non-covalent interactions to sustain a fast macroscopic mechanical motion of large amplitude.

Single-stranded DNA encoders containing polyadenine domains endow colloidal gold nanoparticles with programmable bond valence, orthogonality and reconfigurability, thus achieving post-synthetic control over colloidal structures.

The inherent instability of poly(sulfur) renders the conversion of elementary sulfur into sulfur-rich polymers challenging which limits the use of this industrial by-product. Here, the authors demonstrate that inorganic sulfur can be utilized by copolymerizing with cyclic disulfides, producing high-performance thermoplastic elastomers with self-healing ability and degradability.

Chiral liquid-crystal materials with optical properties that can be tuned, erased and reversed offer new opportunities in labelling, displays and anti-counterfeiting.

Light-driven artificial molecular switches and motors afford dynamic control of various molecular systems, ranging from catalytic to biological. This Perspective provides insight into the challenges that must be addressed to transition the field from the proof-of-concept stage to realization of its myriad applications.

A molecular motor in a liquid-crystal film uses light to turn items thousands of times larger than itself.

Photoswitching of stereochemistry opens up the possibility of controlling the selectivity of catalytic reactions in a non-invasive manner. Here the authors report a set of phosphoramidite-based molecular switches that allow reversible switching of chirality—and hence tuning of catalytic performance—even as competing pairs of chiral catalysts.

The circadian clock is an internal mechanism that controls various physiological processes, such as the sleep-wake cycle, but its precise regulation is challenging. Here, the authors develop a visible light-responsive inhibitor of casein kinase I which controls the period and phase of cellular and tissue circadian rhythms in a reversible manner.

First-generation light-driven rotary molecular motors received considerable interest in various areas of chemistry, but those driven by benign visible light remain less explored. Here the authors describe all-visible-light-driven first-generation molecular motors which show an adaptive behaviour and explore application in multistate photoluminescence.

Controlling the movements of molecular systems through external stimuli is crucial for the construction of nanoscale mechanical machines. A spring-like compound has now been prepared — a double helicate that retains its handedness under ion-triggered extension and contraction.

Autonomous propulsion of microparticles using catalytic olefin polymerization, and directional rotation of a molecule on a metal surface using electrons from the tip of a scanning tunnelling microscope.

While molecular photoswitches have proven useful in many fields, selective and reversible control over multiple switches is an unsolved challenge. Here, the authors report on a system consisting of two classes of photoswitches that can be addressed orthogonally and demonstrate the applicability in phase-transfer control.

Developing enantiodivergent catalysts capable of preparing both enantiomeric products from one substrate in a controlled fashion is challenging. Now, a manganese(III)-salen complex with a chiral photoswitchable phosphate counterion is reported for the epoxidation of alkenes in a stereoselective manner, where irradiation with light allows access to either enantiomer of the epoxide.

It will take a concerted effort for nanomachines to express themselves on a macroscopic scale. Could self-organizing helical polymers help?

A homochiral rotary molecular motor shows autonomous unidirectional rotation around a single bond driven by a chemical fuel.

Controlling molecular motion at nanoscale is important for the design of nanomachines. Here the authors use ultrafast vibrational and electronic spectroscopy to characterize the mechanism of motion of a light driven molecular motor designed to support translational movement.

General and efficient methodologies for the construction of homochiral phosphorus stereocentres are sought-after. Now, merging Pd-catalysed cross-coupling with phosphorus arylation of phosphoramidites provides access to a broad array of asymmetric P-compounds by axial-to-central chirality transfer from BINOL.

Combining photofunctionalities in a single molecule is challenging due to inherent detrimental interactions. Here, the authors construct a molecular motor that exhibits photoinduced rotary motion together with bright photoluminescence.

The design of photoswitches which operate in the visible light regime, show a large separation of absorption bands and are functional in various solvents is challenging. Here the authors report Iminothioindoxyls as visible-light operated photoswitches with a band separation of 100 nm.

Overcrowded alkene-based motors are made part of the crystalline structure of a metal–organic framework. It is shown that they retain the same rotatory speed and unidirectionality as in solution.

The application of photoswitches as light-responsive triggers for phase transitions of porous materials remains poorly explored. Here, the authors report a light-responsive flexible metal-organic framework which undergoes pore contraction upon combined application of light irradiation and adsorption stress via a buckling process of the framework-embedded azobenzene photoswitch.

Despite numerous potential applications, the development of light-responsive solid materials based on molecular photoswitches is impeded by the low efficiency of photoisomerization in the solid environment. Now a robust, solid porous material made from tetraphenylmethane and a photoswitchable overcrowded alkene exhibits nearly quantitative photoisomerization in the bulk and in photomodulation of gas uptake.

Switching the chirality of helicates non-invasively is challenging, even though helicates were discovered some decades ago. Now, by incorporating a light-driven molecular motor into a Cu(I)-helicate, the chirality of the metal helicate as well as the assembly and disassembly processes can be switched non-invasively by light and heat.

Molecular motors and switches change conformation under the influence of an external stimulus and can be incorporated into functional systems, allowing the construction of adaptive materials and switchable catalysts. Here, the authors present two molecular motor-functionalized porphyrin macrocycles for future photo-switchable catalysis.

Vesicle-capped nanotubes made from amphiphilic molecules with photoswitchable cores undergo controlled disassembly when exposed to ultraviolet light, and this process can be followed in real time with fluorescence microscopy.

Organolithiums are the prototypical nucleophilic reagent, but the direct use of them as nucleophiles in catalytic asymmetric synthesis has been problematic. Long sought after, conditions have now been identified that allow the highly enantioselective catalytic formation of C–C bonds by a direct S_N2′ reaction of organolithiums with allylic halides.

Control of motion at the molecular level is an integral requirement for the development of future nanoscale machinery. Now, governed by the fundamental reactivity principles of organometallic chemistry, a biaryl rotor is shown to exhibit 360° unidirectional rotary motion driven by the conversion of two simple fuels.

Biological rotary motors can alter their mechanical function by changing the direction of rotary motion. Now, researchers have designed a synthetic light-driven rotary motor in which the direction of rotation can be reversed on command by changing the chirality of the molecular motor through base-induced epimerization.