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arxiv: 2603.02040 · v2 · submitted 2026-03-02 · ❄️ cond-mat.mes-hall

On-surface synthesis and aromaticity of large cyclocarbons

Lisanne Sellies , Marco Vitek , Yueze Gao , Fabian Paschke , Florian Albrecht , Jakob Eckrich , Beren Dempsey , Stefano Barison
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Leonard-Alexander Lieske Samuele Piccinelli Alberto Baiardi Ivano Tavernelli Harry L. Anderson Igor Ron\v{c}evi\'c Leo Gross
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Pith reviewed 2026-05-15 16:59 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall
keywords cyclocarbonsaromaticityon-surface synthesisscanning tunnelling spectroscopytransport gapcarbon ringsannulenestip-induced chemistry
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The pith

Transport gaps in tip-synthesized carbon rings show aromatic 4n+2 species retain a larger gap than anti-aromatic 4n ones up to at least N=42, with the difference shrinking as rings grow larger.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The work creates monocyclic carbon rings containing up to 88 atoms on a salt surface by successive tip-induced bond rearrangements and fusions. Scanning-tunnelling spectroscopy records the transport gap for each ring, revealing a clear oscillation: rings whose atom count is a multiple of 4 (anti-aromatic) have smaller gaps than those with 4n+2 atoms (aromatic). This oscillation weakens with increasing size and is predicted to disappear, signalling the loss of distinct aromatic character. At N=42 the gap difference is still measurable and accompanying calculations find ring currents whose magnitude matches that of benzene. The results supply a direct experimental test of how aromaticity evolves in the pure-carbon limit of annulenes.

Core claim

Tip-induced chemistry on NaCl produces C20, C22, C42, C44, C46, C66 and C88; their measured transport gaps are smaller for N=4n than for N=4n+2 at small sizes, the amplitude of this alternation decreases with N, and large-active-space calculations predict ring currents at N=42 that remain comparable in strength to those in benzene.

What carries the argument

Transport gap extracted from scanning-tunnelling spectra, which reports the HOMO-LUMO separation modulated by aromatic stabilisation or destabilisation in the monocyclic carbon framework.

If this is right

  • Aromaticity remains detectable at N=42 with ring currents of benzene-like magnitude.
  • The 4n versus 4n+2 gap difference shrinks steadily and is expected to vanish at large N.
  • Larger cyclocarbons can serve as model systems for conductance and quantum-interference studies in atomic carbon wires.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the oscillation truly disappears, cyclocarbons beyond a few dozen atoms will behave electronically like ordinary conjugated chains rather than aromatic or anti-aromatic rings.
  • The same manipulation sequence could be used to insert heteroatoms or defects into the rings to test how aromaticity is locally quenched.
  • Persistent ring currents at N=42 imply that carbon rings of this size may still support delocalised currents useful for designing molecular-scale current loops.

Load-bearing premise

The gap recorded on the surface equals the intrinsic gap of the isolated cyclocarbon without substantial distortion from the substrate, charging, or tip-induced changes.

What would settle it

Finding identical transport gaps for C20 and C22, or for C42 and C44, on the same surface would falsify the reported aromaticity oscillation.

Figures

Figures reproduced from arXiv: 2603.02040 by Alberto Baiardi, Beren Dempsey, Fabian Paschke, Florian Albrecht, Harry L. Anderson, Igor Ron\v{c}evi\'c, Ivano Tavernelli, Jakob Eckrich, Leo Gross, Leonard-Alexander Lieske, Lisanne Sellies, Marco Vitek, Samuele Piccinelli, Stefano Barison, Yueze Gao.

Figure 1
Figure 1. Figure 1: On-surface synthesis of C22 and C44. Laplace-filtered AFM data measured with CO￾funcƟonalized Ɵps and corresponding chemical structures. a, Precursor 1. b, Cyclo[22]carbon, C22. c, Two C22 molecules with a centre-to-centre-distance of 5 nm. d–i, GeneraƟon of C44 from two C22, along with an energy diagram (d) calculated using OX-BLYP30, including energies of transiƟon structures (red plateaus, for details s… view at source ↗
Figure 2
Figure 2. Figure 2: On-surface synthesis of C66 and C88. Sequence of Laplace-filtered AFM data (upper panels) and respecƟve chemical structures (boƩom panels). a, C22 adjacent to C44 L(24,22) . b, Lemniscate C66 L(24,24,22) formed by fusion (pulse parameters: Vp = 4.5 V, Δz = 8.5 Å, IP = 7 pA). c, Lemniscate C66 L(46,22) formed by ring-opening (VP = 4.5 V, Δz = 8 Å, IP = 9 pA). d, C66 formed by ring opening (VP = 4.7 V, Δz = … view at source ↗
Figure 3
Figure 3. Figure 3: Transport gaps of cyclocarbons. a, STS measured gaps Δexp , i.e., energy differences between the PIR and the NIR for the studied cyclocarbons on monolayer NaCl on Au(111) (for the STS spectra and estimation of errors, see Supplementary Fig. S34). The dashed line, which only serves as guidance, was obtained by fitting Δexp = ae –bN + c, with a, b, and c as parameters. b, Gas￾phase calculated IPEA gaps Δcalc… view at source ↗
Figure 4
Figure 4. Figure 4: Theory results for even cyclo[N]carbons up to N = 100. a, Ring current. b, Aromatic stabilization energy (ASE). c, Automerization barrier. Calculations were performed with OX￾BLYP30/def2-TZVP, further details in SI sections 10–15. d, Transport gaps Δcalc of C20 and C22. Ionization potentials (IP), electron affinities (EA), and Δcalc (IP – EA) were computed using Hartree￾Fock (HF), heat bath CI (HCI), SqDRI… view at source ↗
read the original abstract

Molecular rings of N carbon atoms, that is, cyclo[N]carbons, or C$_N$, can be formed by tip-induced chemistry [1-7]. Because of their monocyclic geometry, cyclocarbons are fundamentally important for testing theoretical models of aromaticity [8-11]. Here, we synthesized large cyclo[N]carbons, with N up to 88, by tip-induced chemistry on a NaCl surface and studied their aromaticity by measuring their transport gaps by scanning tunnelling spectroscopy. We first generated C$_{20}$ and C$_{22}$, and then fused multiple cyclocarbons [5-7] by means of atom manipulation, obtaining C$_{42}$, C$_{44}$, C$_{46}$, C$_{66}$ and C$_{88}$. In agreement with predictions obtained using a finely tuned density functional [12-15] and large active space approximate configuration interaction calculations executed on quantum hardware [16, 17], we observe a substantially smaller transport gap for C$_{20}$ (N = 4n) compared to C$_{22}$ (4n+2), and for C$_{44}$ (4n) relative to C$_{42}$ (4n+2). In larger cyclocarbons, the oscillation of the transport gap between anti-aromatic N = 4n and aromatic N = 4n+2 cyclocarbons becomes smaller, and is expected to eventually vanish with increasing N, indicating non-aromaticity. Our experimental results show that aromaticity persists at N = 42, and theory predicts ring currents comparable in magnitude to that of benzene in cyclocarbons of this size. In the future, such large cyclocarbons could be used to study conductance, quantum interference, and the effects of aromaticity in single atomic carbon wires and circuits.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 2 minor

Summary. The manuscript reports tip-induced on-surface synthesis of cyclocarbons up to C88 on NaCl, including fusion of smaller rings to form C42, C44, C46, C66 and C88. Scanning tunneling spectroscopy measurements reveal smaller transport gaps for N=4n species (C20, C44) than for N=4n+2 (C22, C42), with the oscillation amplitude decreasing at larger N; the authors interpret this as persistence of aromaticity up to N=42, consistent with tuned DFT and quantum-hardware CI calculations that predict ring currents comparable to benzene.

Significance. If the measured gaps faithfully report gas-phase aromatic character, the work provides the first experimental extension of Hückel aromaticity/anti-aromaticity to cyclocarbons beyond C22 and demonstrates that the oscillation survives to N=42 before expected damping. The combination of atomically precise synthesis, STS gap data, and comparison to large-scale quantum-hardware calculations constitutes a notable advance for testing aromaticity models in monocyclic carbon systems and for future single-molecule carbon-wire studies.

major comments (1)
  1. [Results and Discussion sections on STS gap measurements and interpretation] The central claim equates the observed STS transport-gap oscillation on NaCl directly to intrinsic free-molecule aromaticity (smaller gap for N=4n anti-aromatic vs. 4n+2 aromatic, persisting to N=42). No experimental controls are described to bound differential surface effects such as image-charge stabilization, substrate-induced bond-length alternation, or tip-induced charging that could vary systematically with ring size or parity. While agreement with gas-phase tuned DFT and quantum-hardware results is reported, this does not substitute for a control measurement (different decoupling layer or bias-dependent charging check) that would be required to make the interpretation load-bearing.
minor comments (2)
  1. [Abstract and Methods] The abstract and main text refer to 'finely tuned density functional' without specifying the tuning procedure or parameters in the main manuscript; a brief methods paragraph or SI reference would improve reproducibility.
  2. [Figure captions] Figure captions for the STS spectra should explicitly state the number of independent measurements and the fitting procedure used to extract transport gaps.

Simulated Author's Rebuttal

1 responses · 1 unresolved

We thank the referee for the positive assessment of the significance of our work and for the constructive major comment. We address the concern regarding the interpretation of the STS transport gaps below.

read point-by-point responses

  1. Referee: [Results and Discussion sections on STS gap measurements and interpretation] The central claim equates the observed STS transport-gap oscillation on NaCl directly to intrinsic free-molecule aromaticity (smaller gap for N=4n anti-aromatic vs. 4n+2 aromatic, persisting to N=42). No experimental controls are described to bound differential surface effects such as image-charge stabilization, substrate-induced bond-length alternation, or tip-induced charging that could vary systematically with ring size or parity. While agreement with gas-phase tuned DFT and quantum-hardware results is reported, this does not substitute for a control measurement (different decoupling layer or bias-dependent charging check) that would be required to make the interpretation load-bearing.

    Authors: We agree that surface-induced effects represent a valid concern for any STS measurement on a decoupling layer and that explicit experimental controls would strengthen the claim. The observed gap oscillation matches the parity dependence and damping trend predicted by our gas-phase tuned-DFT and quantum-hardware CI calculations; this quantitative agreement across multiple ring sizes makes a purely substrate-driven origin unlikely, as image-charge stabilization or bond-length alternation would need to reproduce the same 4n vs. 4n+2 alternation and its systematic reduction with N. In the revised manuscript we will add a dedicated paragraph in the Discussion that (i) estimates the magnitude of image-charge shifts using a simple electrostatic model, (ii) notes the absence of bias-dependent charging signatures in our dI/dV spectra, and (iii) discusses why substrate-induced bond-length alternation is expected to be small on NaCl for these symmetric rings. We cannot, however, perform new control experiments on an alternative decoupling layer within the scope of this revision, as the on-surface synthesis protocol and molecular stability are specific to NaCl. revision: partial

standing simulated objections not resolved
  • New experimental controls on a different decoupling layer or explicit bias-dependent charging checks cannot be supplied in the revised manuscript.

Circularity Check

0 steps flagged

No circularity; experimental synthesis and STS gaps are independent of cited theory

full rationale

The paper's core chain is tip-induced on-surface synthesis of C_N rings on NaCl followed by direct STS transport-gap measurements. These experimental steps produce the reported gap oscillations (smaller for N=4n than 4n+2, persisting to N=42) without reference to or equivalence with the DFT or quantum-hardware CI predictions, which are invoked only for post-hoc comparison and aromaticity interpretation. No equation or claim reduces a measured quantity to a fitted parameter or self-citation by construction; the cited theory papers supply independent benchmarks rather than load-bearing premises. The work is therefore self-contained as an experimental report.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard chemical assumptions about Hückel aromaticity and the interpretation of transport gaps as aromaticity indicators. The theory component invokes a finely tuned density functional whose specific parameters are referenced but not re-derived here.

free parameters (1)
  • tuned DFT parameters
    The density functional is described as finely tuned; exact values or fitting procedure are not restated in the abstract but are required for the theoretical predictions.
axioms (2)
  • domain assumption Hückel's 4n+2 / 4n rule determines aromatic vs anti-aromatic character in monocyclic systems
    Invoked to assign aromaticity labels and to interpret smaller transport gaps as anti-aromatic signatures.
  • domain assumption Transport gap measured by STS on NaCl faithfully reflects the gas-phase aromaticity of the cyclocarbon
    Central link between experiment and aromaticity classification; surface and tip effects are assumed negligible.

pith-pipeline@v0.9.0 · 5699 in / 1487 out tokens · 65712 ms · 2026-05-15T16:59:30.839939+00:00 · methodology

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Reference graph

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    Synthetic protocols: C22-ketal: To a soluƟon of compound S1 (ref. 1) (80 mg, 0.177 mmol) in THF/H2O (6 mL/0.06 mL) was added tetrabutylammonium fluoride (TBAF , 1.0 M in THF , 390 µL, 0.39 mmol) at 0 °C. The soluƟon was sƟrred for 1 h at 0 °C, then water (10 mL) and petroleum ether (20 mL) were added, the layers were separated, and the aqueous phase was ex...

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