Yasuhiro Shirai

Recent progress on nanovehicles

Nanovehicles are a new class of molecular machines consisting of a molecular scale chassis, axles, and wheels, that can roll across solid surfaces with structurally defined directions making them of interest to synthetic chemists, surface scientists, chemical engineers, and the general car enthusiast. In this tutorial review, following a brief introduction to the development of nanomachines, our recent progress on the nanovehicle project is presented including the design, synthesis, and testing of a series of nanocars, nanotrucks, and motorized nanocars.

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.

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 (approximately 1.5 eV), determined by the photoelectron spectroscopy, indicates the unique nature of the fullerene-terminated OPEs in which the C(60) moiety can be connected to the Au surface through the conjugated OPE backbone.

Conformation Manipulation and Motion of a Double Paddle Molecule on an Au(111) Surface

The molecular conformation of a bisbinaphthyldurene (BBD) molecule is manipulated using a low-temperature ultrahigh-vacuum scanning tunneling microscope (LT-UHV STM) on an Au(111) surface. BBD has two binaphthyl groups at both ends connected to a central durene leading to anti/syn/flat conformers. In solution, dynamic nuclear magnetic resonance indicated the fast interexchange between the anti and syn conformers as confirmed by density functional theory calculations. After deposition in a submonolayer on an Au(111) surface, only the syn conformers were observed forming small islands of self-assembled syn dimers. The syn dimers can be separated into syn monomers by STM molecular manipulations. A flat conformer can also be prepared by using a peculiar mechanical unfolding of a syn monomer by STM manipulations. The experimental STM dI/dV and theoretical elastic scattering quantum chemistry maps of the low-lying tunneling resonances confirmed the flat conformer BBD molecule STM production. The key BBD electronic states for a step-by-step STM inelastic excitation lateral motion on the Au(111) are presented requiring no mechanical interactions between the STM tip apex and the BBD. On the BBD molecular board, selected STM tip apex positions for this inelastic tunneling excitation enable the flat BBD to move controllably on Au(111) by a step of 0.29 nm per bias voltage ramp.

Investigating the motion of molecular machines on surfaces by STM: The nanocar and beyond

To build up true molecular machines and understand the mechanics of nanoscale motion and manipulation in molecular system, we have created and investigated a family of molecules based around the concept of the nanocar, which has the rolling wheels made of spherical fullerene or carborane molecules. Assisted by scanning tunneling microscopy (STM), we have successfully characterized and manipulated these molecules. In addition, we have observed the behavior of these systems when thermal energy is applied. These initial studies open a new realm of nano-sized mechanical, chemical, and electrical devices.