Chemists Thread Eight Molecular Helices Into Single Nanographenes

Researchers have developed a modular stereoselective synthetic strategy to precisely control the handedness of helicene units within large polycyclic aromatic hydrocarbons. Published in Nature Synthesis, the study details the preparation of hexabenzocoronenes and graphene nanoribbons with up to eight helicene motifs, each with defined stereochemistry. This advancement paves the way for a new class of chiral nanocarbons with tunable optical and electronic properties, a key objective in molecular materials research.

Helicenes are ortho-fused polycyclic aromatic compounds with a non-planar, screw-like geometry. The two possible screw senses, P for right-handed and M for left-handed, are mirror images, making helicenes inherently chiral. The arrangement of helicene units in a nanographene scaffold significantly affects the molecule's chiroptical activity. Uniform helicene alignment enhances chiroptical responses, whereas mixed arrangements can diminish them. Achieving complete stereochemical control in multi-helical architectures has been challenging due to the difficulty in separating stereoisomers formed during synthesis.

The new strategy employs a modular design where stereodefined building blocks are assembled around a central carbon-rich core. This allows chemists to control the P or M configuration of each helicene during formation, transforming a previously uncontrolled process into a rational synthesis. The approach is versatile enough to produce hexabenzocoronene derivatives and graphene nanoribbons with up to eight helicene units aligned in the same direction.

Uniform helix alignment in chiral nanographenes enhances interactions with circularly polarized light, resulting in significant circular dichroism signals and, in some cases, circularly polarized luminescence. These properties are valuable for next-generation display technologies, optical data storage, and chiral photonics. The chiroptical effects scale with the degree of chiral order, making nanographenes with eight aligned helices particularly effective.

The materials also exhibit near-infrared emission, useful for biological imaging, telecommunications, and optical sensing. The rigid, planarized nanocarbon frameworks reduce the energy gap between ground and excited states, shifting emission towards longer wavelengths. The helicenes contribute to both the red shift and chiral character of the emitted light. The modular synthetic strategy allows for precise tuning of photophysical and chiroptical properties by adjusting helicene units.

The synthesis relies on established carbon-carbon bond-forming reactions, including oxidative cyclodehydrogenation, to fuse aromatic rings into an extended graphene-like lattice. Diastereoselective steps lock in helix handedness, while enantioselective steps allow for the selection of globally left- or right-handed products. This combination of stereocontrol enables the construction of nanographenes with up to eight aligned helices.

Multi-helical nanographenes are characterized by their diastereomeric relationships, with molecules having the same helix configurations but different arrangements being diastereomers. Enantiomers are fully right- or left-handed molecules. Sophisticated analytical tools, such as nuclear magnetic resonance spectroscopy and circular dichroism spectroscopy, are used to distinguish and characterize these species.

This research contributes to the field of chiral nanocarbons, which includes helically twisted graphene nanoribbons and chiral carbon nanotube models. These structures bridge molecular chemistry and materials science, offering the electronic behavior of graphene with added chiral functionality. Stereoselective synthesis of single enantiomers or defined diastereomers in a modular fashion represents progress toward practical applications and systematic structure-property studies.

The modular strategy suggests potential extensions, such as varying helicene substituents to modulate solubility and assembly, or incorporating heteroatoms into the nanographene core for tailored electronic structures. Longer graphene nanoribbons with helicene arrays could combine anisotropic charge transport with strong circular dichroism, relevant to chiral spintronics. The near-infrared emission also opens possibilities for biomedical applications if materials can be made biocompatible. The immediate achievement is the deliberate installation of multiple helical elements in a nanographene, transforming complex mixtures into rationally designed materials for advanced optoelectronics.