 Press Release
Angewandte Chemie International Edition 2008, 47, 6077–6079
doi: 10.1002/anie.200801431 Nr. 26/2008 Molecular Hula HoopSpinning motion of a molecular rotor detectedContact: Akira Harada, Osaka University (Japan)
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Single-Molecule Imaging of Rotaxanes Immobilized on Glass Substrates: Observation of Rotary Movement
Humans have long been trying to make the dream of nanoscopic robots come true.
The dream is, in fact, taking on some aspects of reality. Nanoscience
has produced components for molecular-scale machines. One such device is
a rotor, a movable component that rotates around an axis. Trying to
observe such rotational motion on the molecular scale is an extremely
difficult undertaking. Japanese researchers at the Universities of Osaka
and Kyoto have now met this challenge. As Akira Harada and his team
report in the journal Angewandte Chemie, they were able to get
“snapshots” of individual molecular rotors caught in motion.
© Wiley-VCH
As the subject of their study the researchers chose a rotaxane. This is a
two-part molecular system: A rod-shaped molecule is threaded by a
second, ring-shaped molecule like a cuff while a stopper at the end of
the rod prevents the ring from coming off. The researchers attached one
end of the rod to a glass support. To observe the rotational motions of
the cuff around the sleeve, the scientists attached a fluorescing side
chain to the cuff as a probe.
To observe the rotation of the ring around the rod, the researchers used a
microscopic technique called defocused wide-field total internal
reflection fluorescence microscopy. This gave snapshots of individual
rotaxane molecules in the form of emission patterns. In simplified
terms, if the cuff is motionless, the patterns make it possible to
calculate the direction in which the probe emits its fluorescent light.
This makes it possible to calculate the orientation of the cuff, which
remains constant for every snapshot. However, if the cuff is rotating,
the emission pattern does not reveal the spatial orientation of the
probe.
The researchers showed that the cuff of the rotaxane does not rotate if the
sample is dry. However, when it is wet they can see very rapid
rotational and vibrational motion. The cuff rotates faster than the time
required to snap a picture: the rotational speed is thus over 360° in
300 milliseconds.
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