Synthesis of Single-Molecule Nanocars

The drive to miniaturize devices has led to a variety of molecular machines inspired by macroscopic counterparts such as molecular motors, switches, shuttles, turnstiles, barrows, elevators, and nanovehicles. Such nanomachines are designed for controlled mechanical motion and the transport of nanocargo. As researchers miniaturize devices, they can consider two complementary approaches: (1) the “top-down” approach, which reduces the size of macroscopic objects to reach an equivalent microscopic entity using photolithography and related techniques and (2) the “bottom-up” approach, which builds functional microscopic or nanoscopic entities from molecular building blocks. The top-down approach, extensively used by the semiconductor industry, is nearing its scaling limits. On the other hand, the bottom-up approach takes advantage of the self-assembly of smaller molecules into larger networks by exploiting typically weak molecular interactions. But self-assembly alone will not permit complex assembly. Using nanomachines, we hope to eventually consider complex, enzyme-like directed assembly. With that ultimate goal, we are currently exploring the control of nanomachines that would provide a basis for the future bottom-up construction of complex systems.This Account describes the synthesis of a class of molecular machines that resemble macroscopic vehicles. We designed these so-called nanocars for study at the single-molecule level by scanning probe microscopy (SPM). The vehicles have a chassis connected to wheel-terminated axles and convert energy inputs such as heat, electric fields, or light into controlled motion on a surface, ultimately leading to transport of nanocargo. At first, we used C60 fullerenes as wheels, which allowed the demonstration of a directional rolling mechanism of a nanocar on a gold surface by STM. However, because of the low solubility of the fullerene nanocars and the incompatibility of fullerenes with photochemical processes, we developed new p-carborane- and ruthenium-based wheels with greater solubility in organic solvents. Although fullerene wheels must be attached in the final synthetic step, p-carborane- and ruthenium-based wheels do not inhibit organometallic coupling reactions, which allows a more convergent synthesis of molecular machines. We also prepared functional nanotrucks for the transport of atoms and molecules, as well as self-assembling nanocars and nanotrains.Although engineering challenges such as movement over long distance and non-atomically flat surfaces remain, the greatest current research challenge is imaging. The detailed study of nanocars requires complementary single molecule imaging techniques such as STM, AFM, TEM, or single-molecule fluorescence microscopy. Further developments in engineering and synthesis could lead to enzyme-like manipulation and assembly of atoms and small molecules in nonbiological environments

Synthesis of Single-Molecule Nanocars

The drive to miniaturize devices has led to a variety of molecular machines inspired by macroscopic counterparts such as molecular motors, switches, shuttles, turnstiles, barrows, elevators, and nanovehicles. Such nanomachines are designed for controlled mechanical motion and the transport of nanocargo. As researchers miniaturize devices, they can consider two complementary approaches: (1) the “top-down” approach, which reduces the size of macroscopic objects to reach an equivalent microscopic entity using photolithography and related techniques and (2) the “bottom-up” approach, which builds functional microscopic or nanoscopic entities from molecular building blocks. The top-down approach, extensively used by the semiconductor industry, is nearing its scaling limits. On the other hand, the bottom-up approach takes advantage of the self-assembly of smaller molecules into larger networks by exploiting typically weak molecular interactions. But self-assembly alone will not permit complex assembly. Using nanomachines, we hope to eventually consider complex, enzyme-like directed assembly. With that ultimate goal, we are currently exploring the control of nanomachines that would provide a basis for the future bottom-up construction of complex systems.This Account describes the synthesis of a class of molecular machines that resemble macroscopic vehicles. We designed these so-called nanocars for study at the single-molecule level by scanning probe microscopy (SPM). The vehicles have a chassis connected to wheel-terminated axles and convert energy inputs such as heat, electric fields, or light into controlled motion on a surface, ultimately leading to transport of nanocargo. At first, we used C60 fullerenes as wheels, which allowed the demonstration of a directional rolling mechanism of a nanocar on a gold surface by STM. However, because of the low solubility of the fullerene nanocars and the incompatibility of fullerenes with photochemical processes, we developed new p-carborane- and ruthenium-based wheels with greater solubility in organic solvents. Although fullerene wheels must be attached in the final synthetic step, p-carborane- and ruthenium-based wheels do not inhibit organometallic coupling reactions, which allows a more convergent synthesis of molecular machines. We also prepared functional nanotrucks for the transport of atoms and molecules, as well as self-assembling nanocars and nanotrains.Although engineering challenges such as movement over long distance and non-atomically flat surfaces remain, the greatest current research challenge is imaging. The detailed study of nanocars requires complementary single molecule imaging techniques such as STM, AFM, TEM, or single-molecule fluorescence microscopy. Further developments in engineering and synthesis could lead to enzyme-like manipulation and assembly of atoms and small molecules in nonbiological environments

Toward Chemical Propulsion: Synthesis of ROMP-Propelled Nanocars

The synthesis and ring-opening metathesis polymerization (ROMP) activity of two nanocars functionalized with an olefin metathesis catalyst is reported. The nanocars were attached to a Hoveyda−Grubbs first- or second-generation metathesis catalyst via a benzylidene moiety. The catalytic activity of these nanocars toward ROMP of 1,5-cyclooctadiene was similar to that of their parent catalysts. The activity of the Hoveyda−Grubbs first-generation catalyst-functionalized nanocar was further tested with polymerization of norbornene. Hence, the prospect is heightened for a ROMP process to propel nanocars across a surface by providing the translational force

Synthesis of Highly Fluorescent BODIPY-Based Nanocars

The convergent synthesis of inherently highly fluorescent nanocars incorporating 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY)-containing axles and p-carborane wheels is reported. These nanocars are expected to exhibit rolling motion with predetermined patterns over smooth surfaces, depending on their chassis. Their quantum yields of fluorescence (ΦF > 0.7) make them excellent candidates for imaging and tracking by single-molecule fluorescence microscopy. An analogue as a stationary control with tert-butyl groups instead of p-carborane wheels was also synthesized

Molecular Machinery: Synthesis of a “Nanodragster”

The synthesis and imaging by scanning tunneling microscopy of a mixed wheeled nanovehicle composed of a p-carborane small-wheeled short front axle and a C60 large-wheeled long rear axle that has been termed a nanodragster due to the structural relation to a dragster are reported. This nanodragster is expected to exhibit motion at a lower temperature than pure C60-wheeled nanocars and should allow the investigation of the role played by p-carborane wheels in directional motion

Precise Rate Control of Pseudorotaxane Dethreading by pH-Responsive Selectively Functionalized Cyclodextrins

A family of cyclodextrins functionalized with zero, one, two, or six amines was shown to control the rate of their threading and dethreading on a molecular axle depending on the pH and their substitution pattern. The originality of this system lies in the rate control of the switch by operating the stimulus directly on the macrocycle

Intramolecular versus Intermolecular Hydrogen Bonding of Coordinated Acetate to Organic Acids: A Neutron, X-ray, and Database Study

The effect of strong intra- and intermolecular hydrogen bonding on molecular geometry in carboxylate coordination complexes is examined by means of a search of the Cambridge Crystallographic Database. Accurate hydrogen-bonding parameters for two representative examples, [Cu2(μ-O2CCH3)4(CH3CO2H)2] (1; intramolecular) and [Cu2(μ-O2CCH3)4(H2O)2]·2CH3CO2H (2; intermolecular), are determined by single-crystal neutron diffraction. Electronic effects are probed by the synthesis and characterization of a further mixed carboxylate derivative, [Cu2(μ-O2CCH3)2(μ-O2CCF3)2 (H2O)2]·2CH3CO2H (5)

Micrometer-Scale Translation and Monitoring of Individual Nanocars on Glass

Nanomachines designed to exhibit controlled mechanical motions on the molecular scale present new possibilities of building novel functional materials. Single molecule fluorescence imaging of dye-labeled nanocars on a glass surface at room temperature showed a coupled translational and rotational motion of these nanoscale machines with an activation energy of 42 ± 5 kJ/mol. The 3 nm-long dye-labeled carborane-wheeled nanocars moved by as much as 2.5 μm with an average speed of 4.1 nm/s. Translation of the nanocars due a wheel-like rolling mechanism is proposed and this is consistent with the absence of movement for a three-wheeled nanocar analogue and the stationary behavior of unbound dye molecules. These findings are an important first step toward the rational design and ultimate control of surface-operational molecular machines

Synthesis of Fluorescent Dye-Tagged Nanomachines for Single-Molecule Fluorescence Spectroscopy

In an effort to elucidate the mechanism of movement of nanovehicles on nonconducting surfaces, the synthesis and optical properties of five fluorescently tagged nanocars are reported. The nanocars were specifically designed for studies by single-molecule fluorescence spectroscopy and bear a tetramethylrhodamine isothiocyanate fluorescent tag for excitation at 532 nm. The molecules were designed such that the arrangement of their molecular axles and p-carborane wheels relative to the chassis would be conducive to the control of directionality in the motion of these nanovehicles

Mechanostereoselective One-Pot Synthesis of Functionalized Head-to-Head Cyclodextrin [3]Rotaxanes and Their Application as Magnetic Resonance Imaging Contrast Agents

A versatile, five-component, one-pot synthesis of cyclodextrin (CD) [3]­rotaxanes using copper-catalyzed azide–alkyne cycloaddition has been developed. Head-to-head [3]­rotaxanes of α-CD selectively functionalized by one or two gadolinium 1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid monoamide complexes were obtained mechanostereoselectively. The magnetic resonance imaging efficiency, expressed by the longitudinal proton relaxivity of the rotaxanes, was significantly improved as compared to the functionalized CD. In vitro and in vivo preclinical studies showed a higher contrast and retention in the kidney than gadolinium 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid complex, demonstrating the potential of these rotaxanes as MRI contrast agent