Photochemically driven overcrowded-alkene-based molecular motors have become ubiquitous in molecular nanotechnology due to their reliable unidirectional rotation, modular design, and responsiveness to light. For all the designs of such motors reported to this day, the directional bias of the motion is a consequence of the stereochemistry of the molecule, defined by one or multiple (pro-)chiral stereogenic centers. Such designs nevertheless suffer from incompatibility with various potential experimental conditions, involving for instance the use of strong bases or redox-stimuli. To expand the application scope of molecular motors, we envisioned to replace this point-chirality by a stable helical element, directly integrated within the structure of the molecular motor and use it as the sole inherent chiral information used to drive the unidirectional rotation of the overcrowded-alkene-based motor. Herein we report the design, synthesis, and characterization of three overcrowded-alkenes featuring a helical half, accompanied by a detailed study of their rotational properties, showing the importance of a subtle interplay of distinct isomerization processes.

We present the first integration of a platinahelicene within a nanographene (NG), giving access to near-infrared circularly polarized phosphorescence with dissymmetry factors glum reaching 4 × 10-3. For comparison, a novel NG featuring a borahelicene displayed a blue-green circularly polarized fluorescence with glum = 6 × 10-4.

Molecular motors have been operated in a myriad of environments since their inception more than two decades ago. Of particular interest are systems in which a structural frame of reference for the motor's rotary motion is established. Examples include motors adsorbed on surfaces, embedded in framework materials, used as dopants in liquid crystals, or incorporated into polymer matrices. Embedding the molecular motor as a guest in a supramolecular cage assembly, however, remains an unexplored strategy. Such systems are particularly attractive, as they would allow the motor's rotation to drive mechanical motion from within the assembly, provided the motion does not lead to the guest motor's release. Here, the first motor-nanocage host-guest system in which a thermal and light-driven full 360° unidirectional rotational cycle occurs within the confined space of the nanocage is reported. We identified key structural elements that enable the formation of a host-molecular motor complex with unprecedented stability, governed by a noncovalent interaction between the motor's alkyl-COOH moiety and a carbonyl residue on the cage. This strategy allows the formation of stable host-guest complexes without relying on a size-induced fit, as is commonly observed in other inclusion complexes. This enables rotation to occur within the cage cavity despite dramatic geometric changes. We envision this strategy as a valuable tool for developing a new generation of molecular motors operating in confined spaces.

A variety of rotary molecular machine prototypes powered by light, chemical energy, or electrons have been synthesized and their operation in solution, gels, or on surfaces has been demonstrated. However, little data regarding their performances have been disclosed. Here, we report on the synthesis of molecules incorporating a five-arm rotor and the direct measurement of the work required to block the rotation around the central atom. We used single-molecule force spectroscopy (SMFS) to detect the free rotary oscillations and measure the work performed by the molecules against the mechanical load. We show that the chemical nature of the arms influences the energy barrier, causing differences in the work that the molecules can generate. Our results illustrate that SMFS, which is now widely used to probe linear displacements at a few tens of nanometer scale in macromolecules, can detect rotary motions around a single atom in a tiny synthetic molecule.

The modulation of molecular recognition underpins numerous wide-ranging applications and has inspired the development of a myriad of switchable receptors, in particular photo- or redox-responsive hosts. Herein, we report a highly versatile three-state cation receptor family and switch system based on an overcrowded alkene strapped with crown ethers, which can be switched by both redox and light stimuli, thereby combining the advantages of both approaches. Specifically, the neutral switches can be quantitatively converted between anti- and syn-folded receptor geometries by irradiation, leading to the discovery of a significant increase or decrease in cation binding affinity, which was exploited to shuttle the pseudorotaxane-forming dibenzylammonium guest between the switchable crown ethers of slightly different sizes. Alternatively, two-electron oxidation to the orthogonal, dicationic, nonvolatile state completely turns off cation binding to the host, thereby ejecting the guest. Upon reduction, the metastable syn-folded state is first formed, which then thermally relaxes, resulting in a unique, autonomous, and cation-dependent multistate switching cascade.

Stimuli-responsive materials based on molecular switches, introducing life-like properties such as adaptive behavior in an aqueous environment, are fascinating, providing numerous opportunities to control functions and enable future applications like actuators and soft robotics. Light-responsive molecular systems are receiving particular attention, due to the non-invasive stimulus and distinctive spatio-temporal control possible with photoswitches. In contrast, redox-switching is quantitative, non-volatile and associated with significant changes in material properties, but lacks spatio-temporal precision. Herein we address this challenge in the first proof-of-principle demonstration of light-gated redox switching of polymer hydrogel materials, thereby combining the advantages of both strategies. We present a unique approach where irradiation controls the intrinsic redox properties of the system. This is enabled by the reversible and versatile light- and redox-responsive bisthioxanthylidene switch embedded in a polymer hydrogel, whose two-electron oxidation potential is strongly modulated by light. As a result, oxidation of the material, which is associated with large changes in color, fluorescence, swelling and actuation can be carried out in water with high precision in space and time by photo-masking. This light-gated redox-patterning of the material can be exploited for numerous functions including, as demonstrated here, complex motion and reversible surface texturing.

Chirality is intrinsic to nature, and control of molecular chirality remains at the forefront of the chemical sciences. The introduction of fundamentally new principles to break symmetry, leading to the formation of a specific enantiomer, is particularly challenging with important implications in numerous scientific fields. Herein we present a helically chiral dynamic molecular switch whose enantiomers are connected via a single symmetric metastable state that can be populated via light or redox stimuli. Relaxation of the prochiral state in the presence of a bound chiral ammonium guest results in the preferential formation of a specific enantiomer through a conceptually unique supramolecular deracemization process. Importantly, the formed host enantiomer is stable even in the absence of guest, enabling reversible and stable chiral information transfer, while oxidation of the host results in an enantioenriched dication that is insensitive to light, allowing orthogonal chiral memory.

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.

While hexaphenylsilacyclopentadiene (hexaphenylsilole) is viewed as an archetypal Aggregation-Induced Emission (AIE) luminogen, its isostructural hydrocarbon surrogate hexaphenylcyclopentadiene has strikingly never been investigated in this context, most probably due to a lack of synthetic availability. Herein, we report a straightforward synthesis of hexaphenylcyclopentadiene, via the direct perarylation of cyclopentadiene upon copper(I) catalysis under microwave activation, with the formation of six new C-C bonds in a single synthetic operation. Using zirconocene dichloride as a convenient source of cyclopentadiene and a variety of aryl iodides as coupling partners, this copper-catalyzed cross-coupling reaction gave rise to a series of unprecedented hexaarylcyclopentadienes, displaying promising properties as AIE luminogens. In addition, these propeller-shaped perarylcyclopentadienes are direct precursors of π-extended conjugated polycyclic compounds, and their cyclodehydrogenation under Scholl reaction conditions yielded helicenic 17,17-diarylcyclopenta[l,l’]diphenanthrenes. These structurally complex polyannelated fluorene derivatives can now be prepared in only two synthetic steps from cyclopentadiene for applications as organic semiconductors or fluorophores.

We synthesised star-shaped ruthenium(II) complexes containing one or five styryl-BODIPY units arranged around a central trisindazolylrutheniumcyclopentadienyl fragment. A symmetric complex was obtained following a five-fold Suzuki-Miyaura reaction of the pentabrominated key precursor while desymmetrisation was next achieved via a modular synthetic approach based on sequential chemoselective Suzuki-Miyaura reactions on a key building block bearing one iodophenyl and four bromophenyl groups. Star-shaped systems containing five BODIPY, five styryl-BODIPY, or mixed BODIPY units arranged around a central trisindazolylrutheniumcyclopentadienyl fragment were prepared.

Towards complex coupled molecular motions, the remote handedness inversion of a helicene moiety was achieved by a rotary molecular motor. The use of a specifically engineered dynamic helicene stator in a novel overcrowded-alkene second-generation molecular motor based on a fluorinated dibenzofluorene fragment allows for an unprecedented control over helicity inversion. This is achieved by the mechanical coupling of the rotation of the rotor to the helicene inversion of the stator half via a remote chirality transmission process. Thus, the unidirectional rotary motion generated upon irradiation is used to invert the dynamic stereochemistry of a helicene, leading to a 6-step cycle with eight intermediates. In this cycle, both alternation between P and M configurations of the helicene stator and dynamic thermal interconversion (paddling motion) can be achieved. In-depth computational and spectroscopic studies were performed to support the associated mechanism. The control over coupled motion and dynamic helicity offers prospects for the development of complex responsive systems.

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.

A variety of rotary molecular motors powered by light, chemical energy or tunnelling electrons have been synthesized and their operation in solution, in gels or on surfaces has been demonstrated. However, the single-molecule mechanics of such machines remain scarcely studied and very little data regarding their quantified performances have been disclosed. Here, we report on the synthesis of a series of molecules incorporating a five-arm rotor subunit and the direct quantitative measurement of the work generated by its rotation around the central ruthenium atom. We have used single-molecule force spectroscopy (SMFS) to detect the autonomous oscillations. We demonstrate that a mechanical load does not prevent rotation and show that the chemical nature of the arms influences the energy barrier, causing differences in the work that the motor can generate. Our results illustrate that SMFS, which is now widely used to probe processes at the scale of a few tens of nanometers in biomacromolecules, can detect motions around a single atom in a tiny synthetic molecule.

Mastering intermolecular gearing is crucial for the emergence of complex functional nanoscale machineries. However, achieving correlated motion within trains of molecular gears remains highly challenging, due to the multiple degrees of freedom of each cogwheel. In this context, we designed and synthesised a series of star-shaped organometallic molecular gears incorporating a hydrotris(indazolyl)borate anchor to prevent diffusion on the surface, a central ruthenium atom as a fixed rotation axis, and an azimuthal pentaporphyrinic cyclopentadienyl cogwheel specifically labelled to monitor its motion by non-time-resolved Scanning Tunneling Microscopy (STM). Desymmetrisation of the cogwheels was first achieved sterically, i.e. by introducing one tooth longer than the other four. For optimal mechanical interactions, chemical labelling was also investigated as a preferential way to induce local contrast in STM images, and the electronic properties of one single paddle were modulated by varying the porphyrinic scaffold or the nature of the central metal. To reach such a structural diversity, our modular synthetic approach relied on sequential cross-coupling reactions on a penta(p-halogenophenyl)cyclopentadienyl ruthenium(ii) key building block, bearing a single pre-activated p-iodophenyl group. Chemoselective Sonogashira or more challenging Suzuki-Miyaura reactions allowed the controlled introduction of the tagged porphyrinic tooth, and the subsequent four-fold cross-couplings yielded the prototypes of pentaporphyrinic molecular gears for on-surface studies, incorporating desymmetrised cogwheels over 5 nm in diameter.

We report the synthesis of conceptually new prototypes of molecular winches with the ultimate aim to investigate the work performed by a single ruthenium-based molecular motor anchored on a surface by probing its ability to pull a load upon electrically-driven directional rotation. According to a technomimetic design, the motor was embedded in a winch structure, with a long flexible polyethylene glycol chain terminated by an azide hook to connect a variety of molecular loads. The structure of the motor was first derivatized by means of two sequential cross-coupling reactions involving a penta(4-halogenophenyl)cyclopentadienyl hydrotris(indazolyl)borate ruthenium(II) precursor and the resulting benzylamine derivative was next exploited as key intermediate in the divergent synthesis of a family of nanowinch prototypes. A one-pot method involving sequential peptide coupling and Cu-catalyzed azide-alkyne cycloaddition was developed to yield four loaded nanowinches, with load fragments encompassing triptycene, fullerene and porphyrin moieties.

The photophysical properties of heteroleptic rotor-like Ru(ii) complexes containing both a cyclopentadienyl-type ligand and a hydrotris(indazolyl)borate chelating unit with a piano stool structure (Ar5L1-Ru-S1 and L3-Ru-S1) and their corresponding subunits have been investigated. The complexes show peculiar absorption features when compared with their related ligands or fragments. L3-Ru-S1 was found to be non-emissive, while Ar5L1-Ru-S1 showed a weak emission with a quantum yield of 0.27%. With the help of DFT calculations, we demonstrate that the new absorption features can be attributed to ruthenium-based charge transfer transitions which involve the π* orbitals of the phenyl substituents of the cyclopentadienyl ligand.

The synthesis of ruthenium complexes incorporating an overcrowded pentaarylcyclopentadienyl ligand has been investigated, and higher efficiency has been reached using chlorine-functionalised precursors when compared with their brominated counterparts. A new methodology for the preparation of chlorocyclopentadienes has been developed which is well adapted for highly sterically hindered compounds and works with either electron rich or poor systems.

A molecular motor that has been previously shown to rotate when fueled by electrons through a scanning tunneling microscope tip has been functionalized with a terarylene photochrome fragment on its rotating subunit. Photoisomerization has been performed under UV irradiation. Variable-temperature 1H NMR and UV-vis studies demonstrate the rotational motion and its braking action after photoisomerization. The braking action can be reversed by thermal heating. Once the rigid and planar closed form is obtained, the rotation is effectively slowed at lower temperature, making this new rotor a potential motor with an independent response to electrons and light.

This review highlights the major efforts devoted to the development of molecular gears over the past 40 years, from pioneering covalent bis-triptycyl systems undergoing intramolecular correlated rotation in solution, to the most recent examples of gearing systems anchored on a surface, which allow intermolecular transmission of mechanical power. Emphasis is laid on the different strategies devised progressively to control the architectures of molecular bevel and spur gears, as intramolecular systems in solution or intermolecular systems on surfaces, while aiming at increased efficiency, complexity and functionality.