Researchers at Nagoya University have synthesized a helical graphene nanoribbon (GNR) whose handedness can be switched on demand.
Helical GNRs have drawn interest as a route to chiral carbon materials for optical and electronic devices, but achieving a controllable, switchable twist has been an unsolved problem.
The team took the opposite approach: rather than avoiding this instability, they built a long ladder-polymer chain entirely out of [4]helicene units, fusing them together through quantitative intramolecular multifold cyclization of a nonhelical precursor polymer.
To bias the ribbon toward one handedness, the researchers dissolved it in an optically active solvent.
"For a long time, people saw [4]helicene as unsuitable for chiral materials, because it could not be controlled.
Researchers at Nagoya University have synthesized a helical graphene nanoribbon (GNR) whose handedness can be switched on demand. The team built what they call a poly[4]helicene nanoribbon, a ladder-type polymer made entirely of [4]helicene subunits, and showed that dissolving it in a chiral solvent locks the ribbon into a single, uniform spiral direction, right- or left-handed depending on which mirror-image form of the solvent is used.
Helical GNRs have drawn interest as a route to chiral carbon materials for optical and electronic devices, but achieving a controllable, switchable twist has been an unsolved problem. [4]Helicene, a four-ring helicene unit, was considered too configurationally unstable for this purpose, since it flips its own handedness too quickly to hold a fixed twist on its own. The team took the opposite approach: rather than avoiding this instability, they built a long ladder-polymer chain entirely out of [4]helicene units, fusing them together through quantitative intramolecular multifold cyclization of a nonhelical precursor polymer. Because each subunit is mechanically coupled to its neighbors along the rigid, constrained framework, the helicity inversions of adjacent units became correlated, producing long, interconverting sequences of right- (P) and left- (M) handed [4]helicene subunits rather than random, uncorrelated flipping.
To bias the ribbon toward one handedness, the researchers dissolved it in an optically active solvent. After testing six candidates, they found that beta-pinene, a chiral compound found in pine trees and citrus peel, induced a highly cooperative, nearly complete alignment of the helicene subunits into a single-handed (P)- or (M)-helical form, depending on which enantiomer of beta-pinene was used. This handedness switch was directly observable through circularly polarized luminescence (CPL): the fully (P)- or (M)-helical ribbon emitted right- or left-handed circularly polarized light, respectively, marking the first graphene nanoribbon shown to switch its CPL output this way. The effect was strongest at -90°C and weakened as the temperature approached room temperature.
"We used the same fusion approach already used to make predictable helical ladder polymers. But instead of a stable building block, such as [6]helicene, we used [4]helicene, a 4-ring unit written off as too unstable to be useful," said Nagoya University's Tomoyuki Ikai. "For a long time, people saw [4]helicene as unsuitable for chiral materials, because it could not be controlled. In this paper, we show that once you link enough of these units together, the instability itself becomes the useful part."
The team points to several potential applications: the switchable CPL output could serve optical devices, the ribbon's sensitivity to beta-pinene suggests a possible chiral sensing function, and its graphene backbone offers good electrical conductivity for electronic sensors. The researchers also note that helical structures of this kind are known, in principle, to favor the passage of electrons of one spin over the other, pointing toward possible spintronic applications, though this was not measured in the current work. A current limitation is that the induced twist persists only while the ribbon remains dissolved in the chiral solvent, reverting to a mixed, unstable state once the solvent is removed; the team's next goal is a form of "chiral memory" that would let the ribbon retain its twist after the solvent is gone, building on memory techniques the group has previously explored in other helical systems.
