Light-Induced Translation of Motorized Molecules on a Surface.

Molecular machines are a key component in the vision of molecular nanotechnology and have the potential to transport molecular species and cargo on surfaces. The motion of such machines should be triggered remotely, ultimately allowing a large number of molecules to be propelled by a single source, with light being an attractive stimulus. Here, we report upon the photoinduced translation of molecular machines across a surface by characterizing single molecules before and after illumination. Illumination of molecules containing a motor unit results in an enhancement in the diffusion of the molecules. The effect vanishes if an incompatible photon energy is used or if the motor unit is removed from the molecule, revealing that the enhanced motion is due to the presence of the wavelength-sensitive motor in each molecule.

Unimolecular Submersible Nanomachines. Synthesis, Actuation, and Monitoring.

Unimolecular submersible nanomachines (USNs) bearing light-driven motors and fluorophores are synthesized. NMR experiments demonstrate that the rotation of the motor is not quenched by the fluorophore and that the motor behaves in the same manner as the corresponding motor without attached fluorophores. No photo or thermal decomposition is observed. Through careful design of control molecules with no motor and with a slow motor, we found using single molecule fluorescence correlation spectroscopy that only the molecules with fast rotating speed (MHz range) show an enhancement in diffusion by 26% when the motor is fully activated by UV light. This suggests that the USN molecules give ∼9 nm steps upon each motor actuation. A non-unidirectional rotating motor also results in a smaller, 10%, increase in diffusion. This study gives new insight into the light actuation of motorized molecules in solution.

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 (1)H 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.

Direct observation of mixed-valence and radical cation dimer states of tetrathiafulvalene in solution at room temperature: association and dissociation of molecular clip dimers under oxidative control.

We have observed the mixed-valence and radical cation dimer states of a glycoluril-based molecular clip with tetrathiafulvalene (TTF) sidewalls at low concentration (1 mM) at room temperature. This molecular clip has four consecutive anodic steps in its cyclic voltammogram, which suggests a sequential oxidation of these TTF sidewalls to generate species existing in several distinct charge states: neutral monomers, mixed-valence dimers, radical cation dimers, and fully oxidized tetracationic monomers. The observation of characteristic NIR spectroscopic absorption bands at approximately 1650 and 830 nm in spectroelectrochemistry experiments supports the presence of intermediary mixed-valence and radical cation dimers, respectively, during the oxidation process. The stacking of four TTF radical cations in the dimer led to the appearance of a charge-transfer band at approximately 946 nm. Nanoelectrospray ionization mass spectrometry was used to verify the tricationic state and confirm the existence of other different charged dimers during the oxidation of the molecular clip.

Reading the operation of an acid/base-controllable molecular switch by naked eye.

We report a pH-controllable molecular switch whose switching can be monitored by the naked eye; this system involves the formation of a complex between a [2]rotaxane--featuring dibenzylammonium and 4,4'-bipyridinium stations--and a TTF-side-walled molecular clip.

A macrocycle/molecular-clip complex that functions as a quadruply controllable molecular switch.

Herein, we report the synthesis of a molecular clip with TTF side-walls and its binding behavior towards electron-deficient guests, namely the formation of macrocycle/molecular-clip supramolecular complexes in solution. Four different sets of external stimuli--the K(+)/[2.2.2]cryptand, NH(4) (+)/Et(3)N and (p-BrPh)(3)NSbCl(6)/Zn pairs, and heating/cooling cycles-control the movement of this molecular switch between its threaded and unthreaded states and provide color changes that are observable by the naked eye. This macrocycle/molecular-clip complex system can be considered not only as a quadruple-use molecular switch, but can also be operated by three of these stimuli as a three-input molecular NOR-functioning logic gate that may be monitored by UV-visible spectroscopy.

A switchable macrocycle-clip complex that functions as a NOR logic gate.

We have synthesized a new molecular switch-based on a macrocycle-clip complex-whose switching behavior not only can be controlled through the use of either K+-[2,2,2]cryptand or NH4+-Et3N systems but also provides color changes that are visible to the naked eye; consequently, this system operates as a two-input NOR functioning molecular logic gate.