Molecule with a Split Personality

July 31, 2007

If you take a strip of paper, twist one end by 180° and then stick the two ends together to form a ring, the result is called a Möbius strip, a geometric shape with only one surface and one edge.

You can prove this by making a line along the strip with a pencil: Without lifting the pencil you get back to your starting point in the end—with the whole strip marked: at each point, both sides of the strip are marked by the line. Ring-shaped aromatic molecules can also have a topology like that of the Möbius strip, with only one side.

Polish researchers have now made a porphyrin-like ring that can do something its paper analogue can’t. As the team led by Lechosław Latos-Grażyński reports in the journal Angewandte Chemie, the new molecule can switch back and forth between the one-sided Möbius topology and a “normal” two-sided state (Hückel topology)—without breaking the ring.

An aromatic compound is a nearly planar ring (or ring system) with bonding, yet freely mobile, electron pairs from double bonds. These so called π electrons reside in a kind of “electron cloud” with a part above and a part below the plane of the ring. This is the classic Hückel topology. Even rings that are twisted into a figure eight can have this topology.

If the ring system is twisted by 180°, the result is a Möbius topology; there is no longer a difference between the upper and lower “electron cloud”. Like the pencil line on the Möbius strip made of paper, the two clouds merge together to form a single continuous surface.

The Polish researchers have now synthesized a large molecular ring, which can be classified as an expanded porphyrin analogue, which can do what the glued-together paper strip cannot: Without having to break a single bond in the figure-eight-shaped ring system, it can switch back and forth between the Hückel and Möbius topologies.

Even the figure-eight shape is retained. The trick to this molecule with a “split personality” is two aromatic six-membered rings that lie directly opposite each other in the large ring system and form the crossing point of the figure eight. These two freely spinning rings can either lie flat on top of each other, with their planes lying in parallel, or one of the rings can be twisted by 90° so that their planes lie perpendicular to each other. In the parallel arrangement, there is a clear distinction of the upper and lower parts of the π-electron cloud, in accordance with a Hückel topology—a solution of the compound in this configuration is green in color.

The perpendicular arrangement is the Möbius arrangement and is blue in color. The rotated six-membered ring provides the twist required to unite the upper and lower electron clouds. Which topology the molecule prefers depends on the type of solvent and the temperature.

Citation: Lechosław Latos-Grażyński, Expanded Porphyrin with a Split Personality: A Hückel–Möbius Aromaticity Switch, Angewandte Chemie International Edition, doi: 10.1002/anie.200700555

Source: Angewandte Chemie

Explore further: DNA art imitates life: Construction of a nanoscale Mobius strip

Related Stories

Recommended for you

In colliding galaxies, a pipsqueak shines bright

February 20, 2019

In the nearby Whirlpool galaxy and its companion galaxy, M51b, two supermassive black holes heat up and devour surrounding material. These two monsters should be the most luminous X-ray sources in sight, but a new study using ...

Research reveals why the zebra got its stripes

February 20, 2019

Why do zebras have stripes? A study published in PLOS ONE today takes us another step closer to answering this puzzling question and to understanding how stripes actually work.

When does one of the central ideas in economics work?

February 20, 2019

The concept of equilibrium is one of the most central ideas in economics. It is one of the core assumptions in the vast majority of economic models, including models used by policymakers on issues ranging from monetary policy ...

Correlated nucleons may solve 35-year-old mystery

February 20, 2019

A careful re-analysis of data taken at the Department of Energy's Thomas Jefferson National Accelerator Facility has revealed a possible link between correlated protons and neutrons in the nucleus and a 35-year-old mystery. ...

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.