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A ring about 8 nanometres across, built from 18 porphyrin units, has produced evidence of weak global aromatic and antiaromatic currents—but only in particular oxidation states. The 2025 result points to an apparent upper size limit for global aromaticity in butadiyne-linked porphyrin nanorings, not a universal maximum for aromatic molecules.
What did the researchers make?
In a 2025 study, Holmes and colleagues synthesized c-P18, a π-conjugated nanoring containing 18 porphyrin units joined by butadiyne linkers. The ring is approximately 8 nm in diameter. The team used classical template-directed synthesis with Vernier templating, and scanning tunneling microscopy confirmed the ring’s size and shape. The study appeared in the Journal of the American Chemical Society online on August 28, 2025, and in volume 147, issue 36, on September 10, 2025.
What does “aromaticity” mean for a ring this large?
Aromaticity is often introduced through small molecules such as benzene, but it can also describe a shared electronic circuit around a larger conjugated macrocycle. In this study, the question was whether electrons could sustain a global ring current around the entire circumference of c-P18—not whether every part of the molecule was aromatic in every condition.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThe answer depends on oxidation state. By changing the ring’s charge, the researchers tested states with different electron circuits. Their findings indicate weak global aromatic behavior in the 10+ state and weak global antiaromatic behavior in the 12+ state. “Fully aromatic” therefore would be misleading as a general description of c-P18.
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How did the team detect a global ring current?
The researchers attached c-P18 to a fluorinated template and used oxidation titrations monitored by 19F nuclear magnetic resonance (NMR). The ring current affects the magnetic environment of nearby fluorine atoms; changes in the NMR signals can therefore provide evidence about circulation of electrons around the macrocycle.
Shoulder signals in the 10+ and 12+ states were interpreted as evidence of weak global currents. A split-ring complex served as a control: its results supported the interpretation that the signals in the intact nanoring came from currents circulating around the whole ring. NICS calculations—computational estimates of magnetic effects associated with aromaticity—were qualitatively consistent with aromaticity and antiaromaticity in the corresponding states. The evidence is thus a combination of experimental signals, controls and calculations, rather than a direct image of electrons moving around the ring.
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What are the circuit sizes, and how strong are the signals?
| c-P18 oxidation state | Interpreted behavior | Hückel circuit size |
|---|---|---|
| 10+ | Weak global aromatic ring-current evidence | 242 π-electrons |
| 12+ | Weak global antiaromatic ring-current evidence | 240 π-electrons |
These electron counts describe the circuits for the specified oxidation states; they are not a single, oxidation-state-independent count for the molecule. The authors report that the currents in c-P18 are at least two times weaker than those in its homologous 12-porphyrin ring. The finding is evidence of a global response, but not of a strong current comparable to the smaller ring.
Why might a larger ring have weaker global currents?
The authors point to conformational disorder. A flexible nanoring can adopt many shapes, and distortions may make it harder for the electronic wave function to remain coherent around the entire circumference. In their interpretation, only a fraction of c-P18 molecules adopt a conformation suited to a global current. Increasing the ring’s size does not automatically strengthen aromatic behavior: the extended circuit also has to maintain the structural and electronic coherence needed for a current to travel around it.
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Is this the largest aromatic molecule possible?
No. The authors’ proposed limit applies to global aromaticity in the specific family of butadiyne-linked porphyrin nanorings, and to the evidence available for that family. Their conclusion is that c-P18 defines its apparent upper size limit for a global ring current—not that no larger aromatic macrocycle can exist.
The result extends a 2019 report of global aromaticity in porphyrin nanorings with circuits up to 162 π-electrons. That earlier study also underscored that aromatic behavior depends on oxidation state, molecular constitution and conformation. More rigid and strongly coupled structures, including edge-fused porphyrin nanobelts, could potentially sustain global currents at larger sizes; c-P18 does not settle what those different architectures can achieve. The 2019 study was published in Nature Chemistry.
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