Twisted molecules lock down electrons

Electrons have a tendency to gather in the vicinity of ‘twists’ in cyclacene molecules, an important finding in the development of nonlinear optical molecules.

Researchers from China and Japan have studied how the twist in a molecule can affect the way its electrons are positioned; a fundamental factor in understanding how molecules interact with light.

Zhi-Ru Li and colleagues from Jilin University in China, along with Kikuo Harigaya of AIST, Tsukuba and Feng Long Gu of Kyushu University, Fukuoka, Japan created a computer simulation of a molecule shaped like a Möbius strip—a loop with a twist that has only one, continuous face. Chemists have already created various molecular analogues to this mathematical figure that have unusual electronic and optical properties.¹

Fig. 1: The structures of linear strip (up panel), normal cyclacene (middle panel), and Mobius strip (low panel) of nitrogen-substituted polyacelenes.

The scientists calculated how clouds of electrons are spread around these twisted molecules, known as cyclacenes, and compared their results with simple loops and linear strips (Fig.1). Their analysis was based on a strip made of seven interconnected hexagonal carbon-based molecules which each contained a nitrogen atom. The electronic properties of the molecule can be partially inferred from the distance between the nitrogen and neighbouring atoms.

The section of the molecule around the twist was found to have the greatest separation between nitrogen atoms. Furthermore, the greatest variation was in the separation between nitrogen and carbon atoms—an indication of the strain present in the vicinity of this section of the compound. However, in the non-distorted part of the Möbius cyclacene, atomic separations were similar to those of untwisted loops and flat strips.

These results suggest that electrons in Möbius cyclacene are less free to move around the molecule compared with untwisted loops and flat strips. “The twist in the molecule reduces the delocalization of electrons,” says Gu.

Electron delocalization is particularly important in nonlinear optics, a field which relies on materials whose electrons exhibit extreme responses to light. These materials can be useful in a number of different situations, for example, doubling the frequency of infrared lasers to produce visible light emission.

The team hope that understanding the effects of twists in molecules will be useful in their broader goal of designing high performance nonlinear optical molecules with strong electron delocalization.