A Molecule's Topological Switch

August 16, 2007

A newly synthesized, ring-shaped molecule can switch back and forth between a one-sided, Möbius-strip topology and a "normal" two-sided state — without breaking the ring.

An aromatic compound is a nearly planar ring (or ring system) in which alternating double and single bonds link carbon (or other) atoms. The freely mobile electron pairs from double bonds reside in a kind of electron cloud, with a part of the cloud above and a part below the plane of the ring. This configuration is known as the classic Hückel (two-sided) topology. If the ring is twisted by 180 degrees, the two electron clouds merge to form a single, continuous surface, described as the Möbius topology.

Now, Polish researchers led by Lechoslaw Latos-Grazynski of the University of Wroclaw have synthesized a large molecular ring that can do what a paper Möbius 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 ring's basic figure-eight shape is retained. The findings appear in the article "Expanded Porphyrin with a Split Personality: A Hückel–Möbius Aromaticity Switch," which was published in the journal Angewandte Chemie (International Edition).

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 atop each other, with their planes resting in parallel, or one of the rings can be twisted by 90 degrees so that their planes lie perpendicular to each other.

In the parallel setup, there is a clear distinction between the upper and lower parts of the electron cloud, in accordance with a Hückel topology. A solution of the compound in this configuration is green in color. The perpendicular configuration is the Möbius arrangement, and, in solution, it is blue in color. The rotated six-membered ring provides the twist required to unite the upper and lower electron clouds.

The topology the molecule takes depends on the type of solvent and temperature.

Source: Angewandte Chemie (International Edition), July 2, 2007; EurekAlert, Aug. 2, 2007.