Synthetic Routes toward Carborane-Wheeled Nanocars

A new set of aryleneethynylene derivatives bearing three, four, and six p-carboranes as potential wheels attached to a semirigid chassis have been designed and synthesized. These molecules are expected to move in predetermined patterns on atomically smooth surfaces, depending on their specific configuration

Synthesis and Photoisomerization of Fullerene− and Oligo(phenylene ethynylene)−Azobenzene Derivatives

The presence of fullerenes and oligo(phenylene ethynylene)s (OPEs) in azobenzene derivatives have a large effect on the photoisomerization behavior of the molecules. Fullerenes reduce the photoisomerization yield for cis isomers, and the OPEs, when directly attached to the azobenzenes, have a similar yet smaller effect when compared with the fullerenes. While these effects have not been previously considered for fullerene− and OPE−azobenzene derivatives, they were clearly detected in our work using NMR and UV–vis spectroscopy methods. The intramolecular electronic energy transfer between the fullerene and azobenzene moiety was examined in two cases in which separation of the two functional groups was small, as in 1, or large, as in 2. Almost no photoisomerization was observed for 1, while significant photoisomerization was observed for 2, apparently due to the effective isolation and blocking of electronic communication between the two functional groups

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

Toward a Light-Driven Motorized Nanocar: Synthesis and Initial Imaging of Single Molecules

A second generation motorized nanocar was designed, synthesized, and imaged. To verify structural integrity, NMR-based COSY, NOESY, DEPT, HSQC, and HMBC experiments were conducted on the intermediate motor. All signals in 1H NMR were unambiguously assigned, and the results were consistent with the helical structure of the motor. The nanocar was deposited on a Cu(111) surface, and single intact molecules were imaged by scanning tunneling microscopy (STM) at 5.7 K, thereby paving the way for future single-molecule studies of this motorized nanocar atop planar substrates

Fullerene/Thiol-Terminated Molecules

A series of fullerene-terminated oligo(phenylene ethynylene) (OPEs) have been synthesized for potential use in electronic or optoelectronic device monolayers. Electronic properties such as the energy levels and the distribution of HOMOs and LUMOs of fullerene-terminated OPEs have been calculated using the ab initio method at the B3LYP/6-31G(d) level. The calculations have revealed the concentration of frontier orbitals on the fullerene cage and a narrow distribution of HOMO−LUMO energy gaps. Ultraviolet photoelectron spectroscopy and inverse photoemission spectroscopy studies have been performed to further examine the electronic properties of the fullerene-terminated OPEs on gold surfaces. The obtained broad photoelectron spectra suggest that there are strong intermolecular interactions in the fullerene self-assembled monolayers, and the small bandgap (∼1.5 eV), determined by the photoelectron spectroscopy, indicates the unique nature of the fullerene-terminated OPEs in which the C60 moiety can be connected to the Au surface through the conjugated OPE backbone

Surface-Rolling Molecules

Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely, nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic components that include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use of spherical wheels based on C60 and freely rotating axles based on alkynes permits directed nanoscale rolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles the same motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A new synthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, and the latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provided definitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. The studies here underscore the ability to control directionality of motion in molecular-sized nanostructures through precise molecular design and synthesis

Surface-Rolling Molecules

Design, syntheses, and testing of new, fullerene-wheeled single molecular nanomachines, namely, nanocars and nanotrucks, are presented. These nanovehicles are composed of three basic components that include spherical fullerene wheels, freely rotating alkynyl axles, and a molecular chassis. The use of spherical wheels based on C60 and freely rotating axles based on alkynes permits directed nanoscale rolling of the molecular structure on gold surfaces. The rolling motion observed by STM resembles the same motion performed by macroscopic entities in which rolling occurs perpendicular to the axles. A new synthesis methodology, in situ ethynylation of fullerenes, was developed for the realization of the fullerene-wheeled molecular machines. Four generations of the fullerene-wheeled structures were developed, and the latest fourth generation nanocar, 3b, along with three-wheeled triangular compounds, 4a and 4b, provided definitive evidence for fullerene-based wheel-like rolling motion, not stick-slip or sliding translation. The studies here underscore the ability to control directionality of motion in molecular-sized nanostructures through precise molecular design and synthesis