Surface manipulation of a curved polycyclic aromatic hydrocarbon-based nano-vehicle molecule equipped with triptycene wheels

Divergent Synthesis of Molecular Winch Prototypes

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.

Molecular Gears: From Solution to Surfaces

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.

Desymmetrised pentaporphyrinic gears mounted on metallo-organic anchors

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.

Dipolar Nanocars Based on a Porphyrin Backbone

The design and synthesis of a new family of nanocars is reported. To control their motion, we integrated a dipole which can be tuned thanks to strategic donor and acceptor substituents at the 5- and 15-positions of the porphyrin backbone. The two other meso positions are substituted with ethynyltriptycene moieties which are known to act as wheels. Full characterization of nine nanocars is presented as well as the electrochemistry of these push-pull molecules. DFT calculations allowed us to evaluate the magnitude of the dipoles and to understand the electrochemical behavior and how it is affected by the electron donating and accepting groups present. An X-ray crystal structure of one nanocar has also been obtained.

Train of Single Molecule-Gears

Two molecule-gears, 1.2 nm in diameter with six teeth, are mounted each on a single copper adatom separated exactly by 1.9 nm on a lead surface using a low-temperature scanning tunneling microscope (LT-STM). A functioning train of two molecule-gears is constructed complete with a molecule-handle. Not mounted on a Cu adatom axle, this ancillary molecule-gear is mechanically engaged with the first molecule-gear of the train to stabilize its step-by-step rotation. Centered on its Cu adatom axle, the rotation of the first gear of the train step by step rotates the second similar to a train of macroscopic gears. From the handle to the first and to this second molecule-gear, the exact positioning of the two Cu adatom axles on the lead surface ensures that the molecular teeth-to-teeth mechanics is fully reversible.

Tri-Stable Structural Switching in 2D Molecular Assembly at the Liquid/Solid Interface Triggered by External Electric Field

A tri-stable structural switching between different polymorphisms is presented in the 2D molecular assembly of a 5-(benzyloxy)isophthalic acid derivative (BIC-C12) at the liquid/solid interface. The assembled structure of BIC-C12 is sensitive to the applied voltage between the STM tip and the sample surface. A compact lamellar structure is exclusively observed at positive sample bias, while a porous honeycomb structure or a quadrangular structure is preferred at negative sample bias. Selective switching between the lamellar structure and the honeycomb structure or the quadrangular structure is realized by controlling the polarity and magnitude of the sample bias. The transition between the honeycomb structure and the quadrangular structure is, however, absent in the assembly. This tri-stable structural switching is closely related to the molecular concentration in the liquid phase. This result provides insights into the effect of external electric field on molecular assembly and benefits the design and construction of switchable molecular architectures on surfaces.