Atomic-Resolution Transmission Electron Microscopic
Movies for Study of Organic Molecules, Assemblies, and Reactions:
The First 10 Years of Development
posted on 2017-05-08, 14:51authored byEiichi Nakamura
ConspectusA molecule
is a quantum mechanical entity. “Watching motions
and reactions of a molecule with our eyes” has therefore been
a dream of chemists for a century. This dream has come true with the
aid of the movies of atomic-resolution transmission electron microscopic
(AR-TEM) molecular images through real-time observation of dynamic
motions of single organic molecules (denoted hereafter as single-molecule
atomic-resolution real-time (SMART) TEM imaging). Since 2007, we have
reported movies of a variety of single organic molecules, organometallic
molecules, and their assemblies, which are rotating, stretching, and
reacting. Like movies in the theater, the atomic-resolution molecular
movies provide us information on the 3-D structures of the molecules
and also their time evolution. The success of the SMART-TEM imaging
crucially depends on the development of “chemical fishhooks”
with which fish (organic molecules) in solution can be captured on
a single-walled carbon nanotube (CNT, serving as a “fishing
rod”). The captured molecules are connected to a slowly vibrating
CNT, and their motions are displayed on a monitor in real time. A
“fishing line” connecting the fish and the rod may be
a σ-bond, a van der Waals force, or other weak connections.
Here, the molecule/CNT system behaves as a coupled oscillator, where
the low-frequency anisotropic vibration of the CNT is transmitted
to the molecules via the weak chemical connections that act as an
energy filter. Interpretation of the observed motions of the molecules
at atomic resolution needs us to consider the quantum mechanical nature
of electrons as well as bond rotation, letting us deviate from the
conventional statistical world of chemistry.What new horizons
can we explore? We have so far carried out conformational
studies of individual molecules, assigning anti or gauche conformations
to each C–C bond in conformers that we saw. We can also determine
the structures of van der Waals assemblies of organic molecules, thereby
providing mechanistic insights into crystal formationphenomena
of general significance in science, engineering, and our daily life.
Whereas many of the single organic molecules in a vacuum seen by SMART-TEM
are sufficiently long-lived for detailed studies, molecules with low
ionization potentials (<6 eV) were found to undergo chemical reactions,
for example, [60]fullerene and organometallic compounds possibly via
a hole catalysis mechanism, where a radical cation of CNT generated
under electron irradiation catalyzes the transformation via an electron
transfer mechanism. Common organic molecules whose ionization potentials
are much higher (>8 eV) than that of CNT (5 eV) remain stable for
a time long enough for observation at 60–120 kV acceleration
voltage, as they are not oxidized by the CNT radical cation. Alternatively,
the reaction may have taken place via an excited state of a molecule
produced by energy transfer from CNT possessing excess energy provided
by the electron beam. SMART-TEM imaging is a simple approach to the
study of the structures and reactions of molecules and their assemblies
and will serve as a gateway to the research and education of the science
connecting the quantum mechanical world and the real world.