Strain-Driven Open-Shell Emergence in a Carbon Nanoring

Controlling open-shell character in aromatic π-conjugated hydrocarbons is central to the development of magnetic, optoelectronic, and quantum materials. Conventional strategies rely on Clar 's sextet redistribution or steric protection to stabilize singlet diradicals, inherently limiting accessible molecular frameworks. Here we demonstrate a conceptually distinct approach in which geometric strain alone induces open-shell emergence. By embedding anthracene, normally a closed-shell aromatic unit, into carbon nanoring, bending at the 9,10-positions disrupts π-bonding and promotes diradical formation. We synthesized two nanorings, [2.4]-and [2.8]cyclo(9,10)anthrylene paraphenylene (CAPP). Structural analysis shows that [2.8]CAPP adopts a planar, closed-shell configuration, whereas enhanced strain localization in [2.4]CAPP generates bent anthracene units and an open-shell electronic structure equivalent to a cyclic Chichibabin's hydrocarbon dimer. This work establishes strain-driven electronic reorganization as a general design principle for open-shell π-systems beyond traditional stabilization paradigms.