On-Surface Synthesis of Anthracene-Fused Zigzag Graphene Nanoribbons from 2,7-Dibromo-9,9′-bianthryl Reveals Unexpected Ring Rearrangements

On-surface synthesis has emerged as a powerful strategy to fabricate unprecedented forms of atomically precise graphene nanoribbons (GNRs). However, the on-surface synthesis of zigzag GNRs (ZGNR) has met with only limited success. Herein, we report the synthesis and on-surface reactions of 2,7-dibromo-9,9′-bianthryl as the precursor toward π-extended ZGNRs. Characterization by scanning tunneling microscopy and high-resolution noncontact atomic force microscopy clearly demonstrated the formation of anthracene-fused ZGNRs. Unique skeletal rearrangements were also observed, which could be explained by intramolecular Diels–Alder cycloaddition. Theoretical calculations of the electronic properties of the anthracene-fused ZGNRs revealed spin-polarized edge-states and a narrow bandgap of 0.20 eV.


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General Methods

Synthesis and characterizations
All reactions working with air-or moisture-sensitive compounds were carried out under an argon atmosphere using standard Schlenk line techniques.Unless otherwise noted, all starting materials and other chemicals were purchased from commercial sources and used without further purification.Thin layer chromatography (TLC) was done on silica gel coated aluminum sheets with F254 indicator and column chromatography separation was performed with silica gel (particle size 0.063-0.200mm).

Sample preparation
Ultrahigh vacuum (UHV) experiments were performed on single crystal Au(111) substrates (MaTeck GmbH).The substrates were cleaned by repeated cycles of sputtering with Ar + -ions and subsequent annealing to 400-450 °C.A custom built evaporator was used to deposit the precursors onto the clean substrate by sublimation.The sublimation temperature was adjusted to obtain a deposition rate of ~0.04 monolayer /min.The substrates were annealed to the specified temperature for 15-60 min to trigger the on-surface reactions.Sample temperatures were measured with a pyrometer (Optris).All sample preparations were performed in a preparation chamber (base pressure: 2 ×10 −10 mbar) and transferred to the attached STM chamber (base-pressure: 7 ×10 −11 mbar) for analysis.

Scanning tunneling microscopy (STM) and non-contact atomic force microscopy (nc-AFM) measurements
The STM and nc-AFM experiments were performed using a low-temperature scanning tunneling microscope (Scienta Omicron) operated at 4.7 K with a tungsten tip placed on a qPlus® tuning fork sensor. 1 The tip was functionalized with a single CO molecule at the tip apex picked up from the previously CO-dosed surface. 2 The CO molecule was picked up from the Au(111) surface by scanning the surface at -20 mV and 120-200 pA.The sensor was driven at its resonance frequency (22255 Hz, quality factor 30.8k) with a constant amplitude < 100 pm.The nc-AFM images shown in the main text have been Laplace filtered to enhance the contrast.The raw data is shown in the supporting information.
The frequency shift from the resonance of the tuning fork was recorded in constant-height mode using Omicron Matrix electronics and HF2Li PLL by Zurich Instruments.The Δz is positive when the tip-

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surface distance is increased with respect to the STM set point and negative when the tip-surface distance is decreased.Δz is set to zero when the feedback loop is switched off.

Computational details
The density functional theory (DFT) calculations were executed using the AiiDAlab platform 3 based on AiiDA 4 and the CP2K code. 5To emulate the surface-adsorbate interaction, a repeated slab method was adopted.The simulation cell consisted of 4 atomic layers of Au along the [111] direction.One side of the slab was passivated by a layer of hydrogen atoms to suppress the Au(111) surface state.To decouple the simulation cell from its periodic replicas 40 Å of vacuum was included in the simulation cell in the direction perpendicular to the surface.Electronic states were expanded using a TZV2P Gaussian basis set 6 for C and H species and a DZVP basis set for Au species.For the plane-wave basis set, a cutoff of 600 Ry was utilized.The core electrons of the atoms were represented using norm-conserving Goedecker-Teter-Hutter pseudopotentials. 7The exchange-correlation functional's generalized gradient approximation 8 was approached via the PBE parameterization, while van der Waals interactions were treated using Grimme's D3 scheme. 9The gold surface was modeled using a supercell of size 29.48 × 30.64 Å 2 (equivalent to 600 Au atoms).Equilibrium geometries were ascertained by fixing the bottom two slab layers to their ideal bulk positions and relaxing the remaining atoms until forces dropped below 0.005 eV/Å.For nc-AFM simulations in AiiDAlab, the equilibrium geometries and CP2K-derived electrostatic potential were used with Hapala's probe particle code. 10The GNR's DFT band structure calculations were performed with the Quantum Espresso software package 11 and employed the PBE exchange-correlation functional.A plane wave basis with an energy cutoff of 400 Ry for the charge density was used together with PAW pseudopotentials (SSSP) 12 and a Monkhorst k-mesh of 13 × 1 × 1.
The cell and atomic geometries were relaxed until forces were smaller than 0.001 a.u.
The residue was purified by silica gel column chromatography to give the title compound (41.0 mg, 52% yield) as light yellow solid. 1

Figure S1 .Figure S2 .
Figure S1.Constant-current STM images of polymer 10 before and after the tip manipulation are shown in a and b (V = -1.5 V, I = 50 pA, colorbar indicate the heights).After the image in a was acquired, the tip was approached closer to the surface with the tunneling parameters V = 10 mV and I = 1 nA.The tip was then made to move in a straight line upwards along the polymer.The STM image shown in b was acquired after the tip manipulation, revealing the intact but different adsorption geometry of the polymer, evidencing covalent linking between the individual monomers.

Figure S4 .
Figure S4.(a) STM and (b) nc-AFM images of a GNR segment.Panel c displays the chemical structure deduced from the nc-AFM image, with rings derived from a single precursor molecule distinctly colored to emphasize observed ring rearrangements.The initial finding, marked as 'i' in panel c, reveals the absence of a phenylene ring, also indicated by a black arrow in panel b, which can be explained by the extrusion of benzyne as discussed in the main text.Furthermore, a bright protrusion in the nc-AFM image is attributed to a cove-edge, which is twisted out of plane due to steric repulsion of C-H bonds (labelled 'ii').Lastly, an additional ring, situated atop the segment can be seen (marked 'iii'), indicating ring migration.The tunneling parameters for the STM image are V = 20 mV, I = 100 pA (colorbars in a indicate height).The feedback loop was switched off (Δz = 0) on top of the GNR segments at V = 20 mV, I = 100 pA (colorbars in b indicate the frequency shift).

Figure S5 .
Figure S5.Possible intermediate structures after the partial cyclodehydrogenation of 11b.