How nanographene transforms through oxidation

Graphene and its molecular fragments, known as nanographenes, are key materials for next-generation organic electronics due to their tunable π-electron frameworks. The physical and electronic properties of nanographenes are highly sensitive to molecular size, shape, and especially edge topology. Among the various edge types, “gulf edges”—deep concave sites on the molecular boundary—have remained largely unexplored despite their potential to induce strong curvature and unique electronic behavior.

The collaborative research group led by Ehime University designed and synthesized a nitrogen-containing nanographene derivative, fused octapyrrolylanthracene (fOPA), by introducing eight pyrrole rings onto an anthracene skeleton.

This new molecule was obtained in only two synthetic steps. X-ray crystallographic analysis revealed that steric repulsion between hydrogen atoms at the gulf-edge regions induces a natural bending of the molecule, leading to a ladder-like conformation. Quantum chemical calculations further confirmed that this nonplanar structure is more stable than the twisted alternative.

Electrochemical studies showed that fOPA undergoes up to four reversible oxidation processes. Upon stepwise oxidation, the molecule’s electronic character changes dramatically. The dicationic species (fOPA²⁺) adopts a singlet diradical configuration with two spatially separated spins localized at the gulf-edge regions, while the tetracationic species (fOPA⁴⁺) exhibits a closed-shell aromatic structure accompanied by global diatropic ring currents. These features were verified through ESR and NMR spectroscopy, along with ACID and NICS computational analyses.

The discovery of such oxidation-dependent switching between open-shell and closed-shell electronic states within a single curved π-framework demonstrates a new concept in molecular electronics—where structural flexibility and redox activity are inherently coupled. This could open pathways to novel organic conductors, molecular switches, and responsive optoelectronic materials.

The findings are published in the journal Organic Letters.