molecules occur in two versions related to each
other like mirror images; this property is called
chirality. For example, helical polymers are chiral
- they can be either left- or right-handed helices.
The left and right versions differ in their optical
properties, such as their optical activity (they
twist the plane of polarized light in opposite directions).
Molecules whose optical properties can be precisely
controlled - and switched - are highly sought after,
as they present interesting possibilities for new
data storage devices, optical components, or liquid-crystal
displays. American researchers have now developed
a helical polymer with side groups that can be flipped
back and forth synchronously, like Venetian blinds.
The research team headed by Bruce M. Novak from North
Carolina State University and Prasad L. Polavarpu from Vanderbilt
University produced a helical polymer from an achiral building block.
The use of a chiral catalyst made it possible to link the monomers exclusively
into helices twisted in the same direction. Raising the temperature or changing
the solvent causes a sudden - and reversible - change in some of the polymer's
optical properties (optical activity and electronic circular dichroism); contrary
to expectations, one other property (vibrational circular dichroism) remains
unchanged. What is happening with this molecule? Does the direction of the
helix change? The researchers have now been able to prove that isn't the case.
The backbone of the polymer remains the same. The only explanation for these
initially contradictory seeming observations is the following: the polymer
has side chains that stick out from the backbone at an angle, like little flat
wings. All of these “wings” twist around the bond that attaches them to the
backbone. In the end, they point in the opposite direction, relative to the
helix, from where they started. This occurs synchronously, like a Venetian
blind being flipped.
Why does raising the temperature or changing the solvent cause this flip? The
two wing positions are not equivalent. Depending on the polarity of the solvent,
one or the other form of the molecule is stabilized. A higher temperature stabilizes
the less energetically favorable form of the molecule, a lower temperature
stabilizes the more energetically favorable form.
“The coordinated, blind-like flipping of the many side groups as the result of
an external stimulus,” says Novak, “ could also indicate a very interesting potential
for the construction of molecular motors and nanomachines.”
Author: Bruce M. Novak, North Carolina State University Raleigh (USA),
Title: A Thermal and Solvocontrollable Cylindrical Nanoshutter Based on a Single
Screw-Sense Helical Polyguanidine
Angewandte Chemie International Edition , doi: 10.1002/anie.200501977
or David Greenberg (US)
or Julia Lampam (UK)
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