A new twist on matter? Strange 'Half-Mӧbius' molecule has rare properties chemists have never seen before

Researchers have created an unusually twisted molecule with a never-before-seen electronic structure.

The new molecular architecture, dubbed half-Mӧbius topology, "is another knob that we can turn in order to make and manipulate matter," and expands our fundamental understanding of physics and chemistry, co-lead author Igor Rončević, a lecturer in computational and theoretical chemistry at the University of Manchester in the U.K., told Live Science.

A Mӧbius strip, which is created by twisting a ribbon 180 degrees and then joining the ends, is a mathematically interesting shape that results in a single continuous surface. This weird inverted geometry also has interesting implications for chemists, particularly when they're considering the electronic and spatial properties of molecular structures.

Electrons in revolt

Usually, electrons are localized around a specific atom or bond, but a subset of cyclic compounds, known as conjugated rings, allow the electrons to travel freely throughout the entire loop, above and below the atoms. This delocalization makes conjugated rings more stable than expected, and also influences other properties, including color, optics and reactivity.

However, in a Mӧbius molecule, the electronic orbitals holding the electrons are twisted 180 degrees relative to each other at the junction where the ends meet. The electrons can still move across the whole molecule, but at this junction, some of their properties effectively cancel out, resulting in completely contrasting characteristics and behavior for the overall molecule.

"Chemistry thought that these are the only two options," Rončević said. "But our discovery shows that there's another option, a third option, where we can also rotate by just 90 degrees."

To achieve this, the team, co-led by Leo Gross, principal research scientist at IBM Zurich, created two conjugated systems within a single ring of 13 carbon atoms. The ring contained two chlorine atoms bonded at positions 1 and 7 which isolated these conjugated systems and unevenly separated the electrons on each side. One side of the ring held 13 electrons, while the other held only 11.

"The problem is, electrons like to pair up," Rončević said. "So what they will do in order to pair up is, they will twist the molecule."

The ring, therefore, spontaneously twists itself by 90 degrees ‪—‬ pushing one chlorine atom up and the other down ‪—‬ to align these two separated conjugated systems. This then enables mixing between the two systems, allowing them to share their electrons across the whole molecule.

"At this point, we don't have two separate systems any longer; we have one 24-electron system," Rončević said. The resulting molecule therefore exhibits its own characteristic electronic and magnetic properties, distinct from both standard and Mӧbius structures.

One last twist

The half-Mӧbius molecule's restricted twist angle also results in two possible versions of itself, known as enantiomers.

Because the ring can twist either left or right, the resulting molecules are mirror images of each other ‪—‬ much like left and right hands. This property, technically called chirality, is hugely important throughout chemistry, affecting everything from the synthesis of drug molecules to the production of OLEDs. Intriguingly, by applying a small external voltage the team could freely interconvert a single molecule between the two enantiomers — something that is immensely difficult to achieve using conventional chemistry.

The team supported these experimental findings with detailed computations; the mind-bending complexity of the half-Mӧbius electronic structure necessitated state-of-the-art quantum computers. They published their findings March 5 in the journal Science.

Looking forward, the team intends to focus on exploring the fundamental theory and potential of these molecular architectures.

"We really made a molecule that has a completely new electronic structure, and we want to see what else is possible," Gross said. "We could expand this and explore, for example, several half-Mӧbius twists or even braided ones."