Love triumphs over hate to make exotic new compound
Two identical, highly charged rings (in their stick representation) are interlocked and inseparable, a constitution which represents a homo[2]catenane. |
Northwestern
University graduate student Jonathan Barnes had a hunch for creating an exotic
new chemical compound, and his idea that the force of love is stronger than
hate proved correct. He and his colleagues are the first to permanently
interlock two identical tetracationic rings that normally are repelled by each
other. Many experts had said it couldn't be done. On the surface, the rings
hate each other because each carries four positive charges (making them
tetracationic). But Barnes discovered by introducing radicals (unpaired
electrons) onto the scene, the researchers could create a love-hate
relationship in which love triumphs.
Unpaired
electrons want to pair up and be stable, and it turns out the attraction of one
ring's single electrons to the other ring's single electrons is stronger than
the repelling forces.
The
process links the rings not by a chemical bond but by a mechanical bond, which,
once in place, cannot easily be torn asunder.
The study detailing this new class of stable organic radicals
will be published Jan. 25 by the journal Science.
"It's
not that people have tried and failed to put these two rings together -- they
just didn't think it was possible," said Sir Fraser Stoddart, a senior
author of the paper. "Now this molecule has been made. I cannot
overemphasize Jonathan's achievement -- it is really outside the box. Now we
are excited to see where this new chemistry leads us."
Sir
Fraser is the Board of Trustees Professor of Chemistry in the Weinberg College
of Arts and Sciences at Northwestern. In the late 1980s, he was one of the
early pioneers to introduce an additional type of bond, the mechanical bond,
into chemical compounds.
The new
Northwestern compound has attractive electronic characteristics and can be made
quickly and inexpensively. Down the road, it may be possible to expand this
first linked pair into a longer chain-like polymer where this methodology could
be useful in new technologies for batteries, semiconductors and electronic
memory devices.
Driven
by curiosity, Barnes only began to look at the radical chemistry of the ring
cyclobis (paraquat-p-phenylene) two years ago, nearly 25 years after the ring
was first made.
"I
wondered what would happen if we took it all the way to the max," said
Barnes, the paper's first author and a member of Stoddart's group. "Can we
take two of these rings, each with four positive charges, and make them live
together?"
The
rings repel each other like the positive poles of two magnets. Barnes saw an
opportunity where he thought he could tweak the chemistry by using radicals to
overcome the hate between the two rings.
"We
made these rings communicate and love each other under certain conditions, and
once they were mechanically interlocked, the bond could not be broken,"
Barnes said.
Barnes'
first strategy -- adding electrons to temporarily reduce the charge and bring
the two rings together -- worked the first time he tried it. He, Stoddart and
their colleagues started with a full ring and a half ring that they then closed
up around the first ring (using some simple chemistry), creating the mechanical
bond.
When
the compound is oxidized and electrons lost, the strong positive forces come
roaring back -- "It's hate on all the time," Barnes said -- but then
it is too late for the rings to be parted. "That's the beauty of this
system," he added.
Most
organic radicals possess short lifetimes, but this unusual radical compound is
stable in air and water. The compound tucks the electrons away inside the
structure so they can't react with anything in the environment. The tight
mechanical bond endures despite the unfavorable electrostatic interactions.
The two
interlocked rings house an immense amount of charge in a mere cubic nanometer
of space. The compound, a homo[2]catenane, can adopt one of six oxidation
states and can accept up to eight electrons in total.
"Anything
that accepts this many electrons has possibilities for batteries," Barnes
said.
"Applications
beckon," Stoddart agreed. "Now we need to spend more time with
materials scientists and people who make devices to see how this amazing
compound can be used."
Source: Northwestern University
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Posted by Unknown
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