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A roughly 8-nanometre ring built from 18 porphyrin units shows weak evidence of global aromatic and antiaromatic ring currents in different oxidation states. The 2025 study frames it as an apparent upper size limit for butadiyne-linked porphyrin nanorings—not a maximum size for every kind of aromatic molecule.
What the 18-porphyrin study found
The molecule, called c-P18, is a π-conjugated nanoring assembled from 18 butadiyne-linked porphyrin units. Its diameter is about 8 nm. In “An 18-Porphyrin Nanoring at the Size Limit for Global Aromaticity,” published online August 28, 2025, and in Journal of the American Chemical Society 147(36) on September 10, the authors asked how large a macrocyclic π-system can become while still sustaining a ring current around its circumference. The JACS paper describes the synthesis and measurements.
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The team used template-directed synthesis, including Vernier templating, to make the ring. Scanning tunneling microscopy confirmed its size and shape. To probe aromaticity, researchers bound the nanoring to a fluorinated template and measured it during oxidation titrations using fluorine-19 nuclear magnetic resonance (19F NMR).
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The evidence was not that c-P18 is uniformly aromatic. Instead, shoulder signals in the NMR measurements were interpreted as weak global ring-current signals in two distinct oxidation states:
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| Oxidation state | Interpretation | π-electron circuit |
|---|---|---|
| 10+ | Weak global aromatic ring-current evidence | 242 π-electrons |
| 12+ | Weak global antiaromatic ring-current evidence | 240 π-electrons |
Those electron counts describe the circuits in the respective charged states; they are not a single, oxidation-state-independent count for the molecule. A split-ring control supported the interpretation that the NMR signals arose from currents circulating around the whole nanoring rather than local effects alone. Calculations using nucleus-independent chemical shifts (NICS) qualitatively predicted the aromatic and antiaromatic behavior in the relevant states, supporting—but not replacing—the experimental interpretation.
Why the authors call this a size limit
The authors reported that ring currents in c-P18 are at least two times weaker than those in the related 12-porphyrin nanoring. Their explanation is that larger rings are more conformationally disordered: bending and twisting can disrupt the coherence needed for electrons to circulate around the full circumference. They infer that only a fraction of the 18-ring molecules adopt a geometry suitable for a global current.
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That makes “size limit” a carefully bounded conclusion. It concerns global aromaticity in the studied family of butadiyne-linked porphyrin nanorings, under the conditions and oxidation states examined. It does not establish that larger aromatic macrocycles are impossible. More rigid, more strongly coupled structures—such as edge-fused porphyrin nanobelts—could potentially sustain global currents at larger sizes.
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A 2019 study had reported evidence for global aromaticity in porphyrin nanorings with circuits reaching 162 π-electrons. Its findings also highlighted that aromatic behavior depends on oxidation state, molecular constitution and conformation. The 18-unit result pushes the scale further while showing that size alone does not determine whether a clear global current persists. The 2019 Nature Chemistry paper provides that earlier comparison.
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What the result does—and does not—say
- It is evidence for both behaviors: the 10+ state showed weak aromatic-current evidence, while the 12+ state showed weak antiaromatic-current evidence.
- It is not a claim of strong aromatic stabilization: the measured signals were weak and the currents were at least two times weaker than in the 12-porphyrin homologue.
- It points to a structural constraint: conformational flexibility may make global electronic coherence harder to sustain as these particular rings grow.
- It is not a universal molecular cutoff: other macrocycles with greater rigidity or stronger coupling may behave differently.
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