On-surface synthesis of antiaromatic and open-shell indeno[2,1-b]fluorene polymers and their lateral fusion into porous ribbons

Akimitsu Narita; Aliaksandr V. Yakutovich; Carlo A. Pignedoli; Jos\'e I. Urgel; Klaus M\"ullen; Kristjan Eimre; Marco Di Giovannantonio; Pascal Ruffieux; Qiang Chen; Roman Fasel

REVIEW 2 major objections 5 minor 9 references

On-surface synthesis of antiaromatic and open-shell indeno[2,1-b]fluorene polymers and their lateral fusion into porous ribbons

T0 review · 2 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read On a gold surface, unprotected indenofluorene units assemble into antiaromatic open-shell polymer chains with a 0.4 eV band gap.

desk verdict Solid on-surface synthesis and structure work; the antiaromatic open-shell headline rests on gas-phase DFT rather than on-surface evidence, but the paper still deserves a serious referee. read the letter →

arxiv 1908.11158 v1 pith:OAM3YKPL submitted 2019-08-29 cond-mat.mtrl-sci

Marco Di Giovannantonio , Kristjan Eimre , Aliaksandr V. Yakutovich , Qiang Chen , Shantanu Mishra , José I. Urgel , Carlo A. Pignedoli , Pascal Ruffieux

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Klaus Müllen Akimitsu Narita Roman Fasel

This is my paper · ORCID

classification cond-mat.mtrl-sci

keywords on-surfacesynthesisindeno[21-b]fluoreneantiaromaticpolymeropen-shellbiradicaloidscanningtunnelingmicroscopynon-contactatomicforceporousribbonlowbandgap

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper reports a way to make conjugated polymers out of indeno[2,1-b]fluorene units on a gold surface under vacuum, without the bulky protecting groups that solution synthesis requires. The units are 20-π-electron hydrocarbons that Hückel's rule marks antiaromatic and that theory predicts to be open-shell biradicaloids, so the polymer should carry unpaired spins along its backbone. Using scanning tunneling microscopy, spectroscopy, and atomic force microscopy, the authors follow each reaction step and identify the final polymer and a porous ribbon formed by lateral fusion. The polymer adsorbed on Au(111) shows a measured band gap of 0.4 eV, and gas-phase DFT calculations indicate that the antiaromaticity and radical character of the monomer survive polymerization. If right, this opens a route to stable polyradical carbon materials for electronics and spintronics.

What carries the argument

The central object is the indeno[2,1-b]fluorene (IF) unit, a fused 6-5-6-5-6 ring hydrocarbon whose 20-π-electron quinoidal core is antiaromatic by Hückel's rule and has high biradical character. The reaction machinery is a two-stage on-surface sequence on Au(111): Ullmann-type dehalogenative aryl-aryl coupling of 4,4''-dibromo-4',6'-dimethyl-1,1':3',1''-terphenyl forms zigzag chains, and oxidative cyclization of the methyl groups followed by stepwise dehydrogenation builds the five-membered rings and converts CH2 apices to CH. Characterization is carried by CO-functionalized tip nc-AFM imaging, which distinguishes flat from tilted units, and by dI/dV spectroscopy whose frontier-orbital maps are matched to DFT LDOS. The theoretical argument is carried by unrestricted DFT with natural orbital occupation analysis and nucleus-independent chemical shift (NICS) scans on gas-phase oligomers, which quantify the unpaired-electron count and the antiaromaticity retained in longer chains.

What would settle it

Measure spin-resolved tunneling or a Kondo resonance on a flat poly-IF unit on Au(111): if no spin-polarized signal or zero-bias Kondo feature appears at the five-membered-ring apices, the open-shell character is quenched by the metal. A complementary check would be to grow the same polymer on an insulating or weakly interacting surface and look for a magnetic signature such as an ESR signal.

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Extended reading notes

Core claim

The central discovery is that unprotected indeno[2,1-b]fluorene units can be polymerized on Au(111) through a sequence of dehalogenative aryl-aryl coupling and oxidative cyclodehydrogenation, yielding a low-band-gap polymer whose isolated building block is antiaromatic and open-shell. The authors assign the reaction products stage by stage: zigzag chains of the dihydro intermediate after coupling, porous ribbons of tetraindenopyrene motifs upon lateral fusion, and finally flat poly-IF chains after CH2 to CH dehydrogenation. dI/dV spectra on the flat polymer give valence and conduction band edges at about -0.2 V and +0.2 V, i.e. a band gap of 0.4 eV on Au(111), while the porous ribbon shows a 2.2 eV gap. Spin-polarized DFT on gas-phase oligomers predicts an open-shell singlet ground state, with unpaired spin density concentrated at the apices of the five-membered rings, and natural-orbital and NICS analyses show roughly 1.36 odd electrons per repeat unit and persistent antiaromaticity of the five-membered rings. The authors conclude that polymerization preserves the monomer's radical and antiaromatic character, making these unprotected polyradical chains accessible for study.

Load-bearing premise

The paper relies on calculations of isolated molecules to conclude that the polymer on gold is a magnetic antiaromatic chain, but no experiment directly measures the spins on the surface.

Editorial extensions

If this is right

The poly-IF chains on Au(111) have a measured band gap of about 0.4 eV, far below the 3.7 eV of the dihydro intermediate and the 2.2 eV of the porous ribbon, placing them in a regime of interest for narrow-gap organic wires.
Theoretical unpaired-electron counts of about 1.36 per repeat unit and a computed open-shell singlet ground state imply that poly-IF is a polyradical hydrocarbon whose unpaired spins sit at five-membered-ring apices.
NICS calculations show the five-membered rings stay antiaromatic in trimers and pentamers, so the electronic character of the monomer is not washed out by polymerization.
Temperature controls the outcome: methyl cyclization around 200-250 °C, lateral fusion to porous ribbons from 310 °C, and full dehydrogenation to poly-IF at 360-410 °C, giving a synthetic handle on which product forms.
The porous ribbon, formed by cross-dehydrogenative coupling of adjacent chains, is a non-benzenoid porous carbon nanostructure with its own 2.2 eV gap.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

If the spin state survives adsorption, poly-IF could serve as a one-dimensional testbed for magnetism in organic wires; a direct spin probe such as spin-polarized STS or Kondo spectroscopy would settle that question.
The 16-carbon pores of the porous ribbon are chemically accessible sites that might bind metal atoms or molecules, turning the ribbon into a template for periodic magnetic or catalytic arrays.
The same precursor design, with methyl groups as built-in cyclization handles, could be extended to other indenofluorene isomers or substituted derivatives to tune the gap and radical density.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

The core finding that survives scrutiny is the synthesis itself: poly-IF on Au(111) from a simple terphenyl precursor, with careful step-by-step nc-AFM characterization and sensible DFT barriers for the dehydrogenation and lateral coupling. The porous ribbon product is genuinely unexpected and the structural assignment is convincing. This is a real advance over the authors' earlier indenofluorene polymers because it gives the pristine, unprotected [2,1-b] isomer in a polymer, which solution chemistry cannot reach. The nc-AFM simulations match the experiments well, and the reaction sequence is well supported by the temperature-dependent imaging. I would trust the structures. The soft spot is exactly where the reader put it. The open-shell singlet, radical character, and antiaromaticity are computed exclusively for gas-phase oligomers, and the paper says plainly that magnetic structure on surfaces is hard to access with SPM. On a metal like Au(111), hybridization and charge transfer can quench or renormalize radical character, and the paper's own DFT shows chemisorption effects for tilted units. The 0.4 eV STS gap on a flat unit does not by itself prove an open-shell state; a small-gap closed-shell system would give the same signature, so it is consistent with but not evidence for the radical claim. The gas-phase pentamer also twists every other unit by about 35 degrees, while the adsorbed flat units are planar, so the electronic structure used for the headline claim may not be the one on the surface. I do not think this is a fatal flaw, because the synthesis and structural characterization stand independently, but the title and abstract overstate what is demonstrated. One minor note: the paper does not misrepresent the situation; it explicitly flags the experimental challenge and presents the DFT as ab initio prediction. So this is a matter of framing, not fraud. The citation pattern is fine; self-citation is to prior work that is directly relevant. Who is this for? On-surface synthesis people, PAH chemists, and anyone interested in indenofluorene isomers. The structural results deserve to be in the literature. I would send this to peer review, and I would suggest the authors temper the abstract and conclusions to make clear that the open-shell antiaromatic character is a theoretical prediction for the isolated polymer, and add a sentence in the conclusions about the substrate possibly modifying it. If a spin-sensitive measurement is possible, that would settle it, but it is not a prerequisite for publishing the synthesis and the porous ribbon.

Referee Report

2 major / 5 minor

Summary. The manuscript reports the on-surface synthesis of conjugated polymers composed of indeno[2,1-b]fluorene (IF) units on Au(111), starting from a dibromo-dimethyl terphenyl precursor. Using STM, nc-AFM, and STS complemented by DFT, the authors identify a stepwise reaction sequence: dehalogenative aryl-aryl coupling, oxidative cyclization of methyl groups to form 2H-IF units, and further dehydrogenation to IF. They also observe lateral fusion of chains into porous ribbons. STS on a single flat IF unit yields a transport gap of about 0.4 eV. Gas-phase spin-polarized DFT calculations (LC-UBLYP) predict an open-shell singlet ground state with ~1.36 unpaired electrons per IF unit and antiaromatic five-membered rings by NICS analysis. The paper concludes that poly-IF is an antiaromatic open-shell biradicaloid polymer with a very low band gap on Au(111), of interest for spintronics and organic electronics.

Significance. If the electronic-structure claims hold, this is a significant advance: it demonstrates a surface route to unsubstituted indenofluorene polymers and porous ribbons that are not accessible in solution, with rigorous structural characterization. The nc-AFM image matching, the DFT-derived reaction barriers for the stepwise transformations, and the independent prediction of the biradical character (y = 0.645) consistent with prior literature are clear strengths. However, the open-shell and antiaromatic character of the synthesized polymer is inferred solely from gas-phase calculations on free-standing oligomers; no experimental or slab-DFT evidence confirms that these properties survive adsorption on Au(111). Since the title and abstract foreground these properties, the paper's central claim is currently supported only by theory for the surface system. The experimental gap of 0.4 eV is suggestive but not a specific probe of radical character.

major comments (2)

[Theoretical characterization of the electronic structure of poly-IF] The open-shell singlet ground state, radical character, and antiaromaticity of the synthesized poly-IF are established exclusively by gas-phase LC-UBLYP calculations on free-standing oligomers (Table 1, Figure 4). The authors explicitly acknowledge that 'accessing the magnetic structure of open-shell systems with scanning probe techniques represents a challenge' (Section 'Theoretical characterization of the electronic structure of poly-IF'), but no experimental spin probe (spin-polarized STS, Kondo resonance, spin-excitation IETS) and no spin-polarized DFT calculation of the polymer adsorbed on Au(111) are provided. This is a load-bearing gap because the title, abstract, and conclusions present 'open-shell' and 'antiaromatic' as properties of the on-surface polymer. The paper itself shows strong molecule–substrate coupling in the tilted adsorption geometry (Figure 3d,e) and reports that tilted units—the majority—could not be characterized spectroscopically (Section 'Formation of poly-IF'). The measured 0.4 eV gap on a flat unit is consistent with a small-gap closed-shell system and therefore does not independently confirm radical character on the surface. To support the central electronic-structure claim, the authors should either provide spin-polarized DFT results for the adsorbed system or explicitly recast the open-shell/antiaromatic statements as predictions for the isolated polymer that remain to be verified experimentally on Au(111).
[Formation of poly-IF] The reported 0.4 eV transport gap is derived from dI/dV spectra acquired on one flat IF unit (Figure 3l), while the authors state that no clear spectroscopic signals could be obtained on tilted units, which are the easier configuration to find. Because the tilted geometry involves chemisorption and partial sp3 rehybridization of the five-membered-ring apex carbons (Figure 3d,e), the electronic structure of poly-IF is expected to vary between adsorption configurations. The paper does not discuss whether the flat-unit spectrum is representative of the polymer or provide a statistical distribution of measured gaps. The claim that poly-IF on Au(111) is an extremely narrow-gap material should therefore be qualified to the flat adsorption conformer, or supported by measurements on several flat units and a statement about the fraction of units that are spectroscopically accessible.

minor comments (5)

[Abstract] The phrase 'da-ta storage' contains an erroneous hyphen; please ensure the final typeset version does not break the word 'data' across lines.
[Scheme 1 caption] The functional abbreviation 'LC-UBYLP/6-311+G**' is misspelled and should read 'LC-UBLYP/6-311+G**'.
[Computational details] The notation 'U(R)B3LYP/6-311G**' is ambiguous; please specify which calculations used unrestricted and which used restricted B3LYP, since both closed- and open-shell species are discussed.
[Table 1] In Table 1, the natural orbital occupations for the trimer and pentamer are listed without labeling the orbitals beyond 'LUNO' for the monomer; please state explicitly that these are the five lowest-unoccupied natural orbitals (or the defined subset used) for each system.
[Theoretical characterization of the electronic structure of poly-IF] The sentence 'we can safely assume poly-IF to maintain the antiaromaticity of an isolated IF' is too definitive for a calculation on model oligomers; suggest 'the calculations suggest that poly-IF maintains' or 'is predicted to maintain'.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the open-shell and antiaromatic character are independently computed gas-phase predictions that are not fitted to the experimental data.

full rationale

The derivation chain is self-contained and non-circular. The key electronic-structure claims (open-shell singlet ground state, radical character, and antiaromaticity) come from unrestricted DFT calculations on isolated gas-phase oligomers at the LC-UBLYP/6-311+G** level, whose inputs are molecular structure and exchange-correlation functional, not the experimental STM, STS, or nc-AFM data. The biradical character y=0.645 for the monomer is taken from and agrees with the independent calculation of Fukuda et al. (Ref 26), and is recomputed in Table 1 rather than fitted to any measured quantity. The on-surface band gap of 0.4 eV is measured directly from dI/dV peak positions and assigned with LDOS maps; the DFT gap trend is compared afterward, not used to construct the measurement. Self-citations, notably Ref 24, provide context and an established reaction scheme, but the paper re-derives the structural assignments through its own imaging and DFT simulations, so no load-bearing step reduces to its own input. The gas-phase-to-surface extrapolation of the spin state is a scientific limitation that the authors explicitly acknowledge ('accessing the magnetic structure of open-shell systems with scanning probe techniques represents a challenge'), but a limitation in external validity is not a circularity.

Assumptions & free parameters 0 free parameters · 5 assumptions · 0 invented entities

The paper introduces no fitted free parameters: the DFT functionals and basis sets are standard, and the empirical AFM model uses literature parameters. The load-bearing background assumptions are domain assumptions about the reliability of DFT-based image simulation and about the transfer of gas-phase electronic structure calculations to the adsorbed polymer.

assumptions (5)

domain assumption DFT-PBE with van der Waals corrections accurately describes adsorption geometries and STM/nc-AFM images of these molecules on Au(111).
Used to simulate all nc-AFM images and LDOS maps used to assign structures (Figures 1-3), including the two-probe-particle CO-tip model.
domain assumption LC-UBLYP/6-311+G** natural orbital occupations provide reliable biradical character and number of unpaired electrons.
Central to the open-shell singlet claim and to Table 1; the same functional form is used as in Ref. 26.
domain assumption The gas-phase pentamer electronic structure is representative of the polymer on Au(111).
Spin density and NICS are computed only for isolated oligomers; no experimental verification of spin state on the surface. This is the weakest assumption.
domain assumption NICSzz is a valid local aromaticity probe for non-benzenoid systems.
Used to conclude five-membered rings are antiaromatic and six-membered rings aromatic (Figure 4b).
domain assumption Heating steps induce the proposed chemical transformations (dehalogenation, cyclization, dehydrogenation) without alternative side reactions.
The product identifications rely on the assumption that the observed structures correspond to the expected intermediates and products; alternative pathways (e.g., carbene formation) are explicitly excluded by calculated barriers.

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Cite this review

Pith. "Pith review of On-surface synthesis of antiaromatic and open-shell indeno[2,1-b]fluorene polymers and their lateral fusion into porous ribbons." pith.science (2026). https://pith.science/paper/OAM3YKPL

@misc{pith2026190811158,
  author       = {Pith},
  title        = {Pith review of: On-surface synthesis of antiaromatic and open-shell indeno[2,1-b]fluorene polymers and their lateral fusion into porous ribbons},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/OAM3YKPL}},
  note         = {Machine review of arXiv:1908.11158}
}

read the original abstract

Polycyclic hydrocarbons have received great attention due to their potential role in organic electronics and, for open-shell systems with unpaired electron densities, in spintronics and da-ta storage. However, the intrinsic instability of polyradical hydrocarbons severely limits de-tailed investigations of their electronic structure. Here, we report the on-surface synthesis of conjugated polymers consisting of indeno[2,1-b]fluorene units, which are antiaromatic and open-shell biradicaloids. The observed reaction products, which also include a non-benzenoid porous ribbon arising from lateral fusion of unprotected indeno[2,1-b]fluorene chains, have been characterized via low temperature scanning tunneling microscopy/spectroscopy and non-contact atomic force microscopy, complemented by density-functional theory calculations. These polymers present a low band gap when adsorbed on Au(111). Moreover, their pro-nounced antiaromaticity and radical character, elucidated by ab initio calculations, make them promising candidates for applications in electronics and spintronics. Further, they provide a rich playground to explore magnetism in low-dimensional organic nanomaterials.

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Reference graph

Works this paper leans on

9 extracted references · 9 canonical work pages

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Pith tools

Reviewed August 14, 2026 · model on record in the stance chip above.

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(1) Rieger, R.; Müllen, K. Forever Young: Polycyclic Aromatic Hydrocarbons as Model Cases for Structural and Optical Studies. J. Phys. Org. Chem. 2010, 23 (4), 315–325. (2) Tobe, Y. Non-Alternant Non-Benzenoid Aromatic Compounds: Past, Present, and Future. Chem. Rec. 2015, 15 (1), 86–96. (3) Sun, Z.; Ye, Q.; Chi, C.; Wu, J. Low Band Gap Polycyclic Hydroca...

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(18) Shimizu, A.; Tobe, Y. Indeno[2,1-a]Fluorene: An Air-Stable Ortho-Quinodimethane Derivative. Angew. Chem. Int. Ed. 2011, 50 (30), 6906–6910. 19 (19) Shimizu, A.; Kishi, R.; Nakano, M.; Shiomi, D.; Sato, K.; Takui, T.; Hisaki, I.; Miyata, M.; Tobe, Y. I n- deno[2,1-b]Fluorene: A 20-π-Electron Hydrocarbon with Very Low-Energy Light Absorption. Angew. Ch...

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On-Surface Reactions

(9) Lindner, R.; Kühnle, A. On-Surface Reactions. ChemPhysChem 2015, 16 (8), 1582–1592. (10) Bartels, L. Tailoring Molecular Layers at Metal Surfaces. Nat. Chem. 2010, 2 (2), 87–95. (11) Di Giovannantonio, M.; Contini, G. Reversibility and Intermediate Steps as Key Tools for the Growth of Extended Ordered Polymers via On-Surface Synthesis. J. Phys. Conden...

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Comment on ‘Insight into Orga n- ometallic Intermediate and Its Evolution to Covalent Bonding in Surface-Confined Ullmann Polymeriza- tion.’

(28) Hafner, K.; Stowasser, B.; Krimmer, H.-P.; Fischer, S.; Böhm, M. C.; Lindner, H. J. Synthesis and Prop- erties of 1,3,5,7-Tetra-Tert-Butyl-s-Indacene. Angew. Chem. Int. Ed. Engl. 1986, 25 (7), 630–632. (29) Thomas, S.; Kim, K. S. Linear and Nonlinear Optical Properties of Indeno[2,1- b]Fluorene and Its Struc- tural Isomers. Phys. Chem. Chem. Phys. 20...

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