On-surface activation of benzylic C-H bonds for the synthesis of pentagon-fused graphene nanoribbons

Graphene nanoribbons (GNRs) have potential for applications in electronic devices. A key issue, thereby, is the fine-tuning of their electronic characteristics, which can be achieved through subtle structural modifications. These are not limited to the conventional armchair, zigzag, and cove edges, but also possible through incorporation of non-hexagonal rings. On-surface synthesis enables the fabrication and visualization of GNRs with atomically precise chemical structures, but strategies for the incorporation of non-hexagonal rings have been underexplored. Herein, we describe the on-surface synthesis of armchair-edged GNRs with incorporated five-membered rings through the C-H activation and cyclization of benzylic methyl groups. Ortho-Tolyl-substituted dibromobianthryl was employed as the precursor monomer, and visualization of the resulting structures after annealing at 300 °C on a gold surface by high-resolution noncontact atomic force microscopy clearly revealed the formation of methylene-bridged pentagons at the GNR edges. These persisted after annealing at 340 °C, along with a few fully conjugated pentagons having singly-hydrogenated apexes. The benzylic methyl groups could also migrate or cleave-off, resulting in defects lacking the five-membered rings. Moreover, unexpected and unique structural rearrangements, including the formation of embedded heptagons, were observed. Despite the coexistence of different reaction pathways that hamper selective synthesis of a uniform structure, our results provide novel insights into on-surface reactions en route to functional, non-benzenoid carbon nanomaterials.

We have recently achieved on-surface syntheses of dihydroindenofluorene and indenofluorene polymers 3 and 4 ( Fig. 1), respectively, through C-H activation of benzylic methyl groups installed on polyphenylenes, leading to the formation of five-membered rings with methylene-and methine-bridges [35][36][37]. We thus considered the structural extension of AGNR edges with five-membered rings by employing the same concept. To this end, we designed 10,10'-dibromo-2,2'-di-ortho-tolyl-9,9'-bianthracene (12) as a precursor monomer for naphthofused 7-AGNRs with additional five-membered rings. In this work, we report a synthesis of monomer 12 in solution, followed by on-surface fabrication of GNRs using 12 on a Au(111) surface under UHV conditions (Scheme 1). Upon thermal treatment at 300 °C, high-resolution noncontact atomic force microscopy (nc-AFM) revealed formation of pentagon-fused GNR 5 with methylene bridges, although a few fully conjugated five-membered rings with methine bridges as well as some defects due to migration and cleavage of the methyl groups were Scheme 1 Synthetic route to pentagon-fused GNR 5. Reagents and conditions: (i) ( also observed. Interestingly, azulene-incorporated structures were also detected, pointing to unexpected skeletal rearrangements probably induced by C-H activation of the benzylic methyl groups. Our results furnish unprecedented insights into on-surface reactivity of benzylic methyl groups, which are known to play an indispensable role in solution chemistry [38,39]. The purpose of the present paper is to demonstrate their wide potential for achieving novel carbon nanostructures on surfaces.
For the on-surface synthesis of the pentagon-fused AGNRs, monomer 12 was first sublimed onto a Au(111) substrate held at room temperature under UHV conditions. Annealing to 300 °C induced polymerization of 12 via homolytic cleavage of the C-Br bonds to generate diradical intermediates, followed by oxidative cyclodehydrogenation to provide planarized GNRs (apparent height of 2.0 Å with Vb = −0.1 V), as revealed by scanning tunneling microscopy (STM) ( Fig. 2(a) and Fig. S1(a) in the ESM). Some randomly distributed bright protrusions (apparent height of 3.2 Å) were also observed, which we attribute to the presence of unreacted tolyl groups that induce a certain degree of non-planarity due to steric repulsion with the proton within the same bay region. To unambiguously identify the chemical structure of the planar segments of the observed GNRs, we recorded constant-height frequency-shift nc-AFM images with a CO-functionalized tip [40] (Figs. 2(b), 2(c), 2(e), and 2(f)). These images clearly reveal the presence of five-membered rings, confirming the on-surface C-H activation of the benzylic methyl groups to form C-C bonds with the sp 2 carbons of the GNR edges in the same bay regions. Many of the apexes of these five-membered rings appear brighter in nc-AFM imaging mode, which can be attributed to methylene bridges (-CH2-), as previously reported [20,36], corresponding to GNR 5, while very few five-membered rings with singly hydrogenated methine bridges have also been observed (Figs. 2(b)-2(d)).
The nc-AFM images also display structural features that deviate from the ones expected for GNR 5 (Fig. 2). We could clearly identify the bare and methyl-substituted naphtho-fused edges (A and B in Fig. 2(g)) resulting from the removal and migration of methyl groups, respectively. We note that methyl groups are imaged as bright protrusions at 2.1 ± 0.1 Å away from the closest carbon atom of the GNR backbone, in agreement with the literature [41]. On the other hand, CO molecules are sometimes observed, located at variable distances from the GNR (from 3.8 ± 0.1 to 4.4 ± 0.1 Å). Moreover, occasional skeletal rearrangements are revealed, affording different arrays of five-seven-five-membered rings with azulene substructures (C and D in Fig. 2(g)). In particular, the structural feature C demonstrates the formation of a seven-membered ring fully embedded inside the GNR structure, which has rarely been achieved via on-surface synthesis [34,42].
Further annealing to 320 and 340 °C (Fig. S1 in the ESM) resulted in a gradual disappearance of the bright protrusions with apparent height of 3.2 Å, which we tentatively assigned to unreacted tolyl groups. These observations suggest planarization of the unreacted tolyl groups at higher temperatures, in line with similar reports in the literature [33]. After the heating step at 340 °C, almost completely planar GNRs could be obtained (apparent height of 2.0 Å in STM imaging, Fig. 3(a)). While standard STM imaging was not capable of elucidating the precise chemical structure of these GNRs, nc-AFM investigation of some segments of the obtained GNRs revealed similar structural features (Figs. 3(b) and 3(d)) as those observed after the 300 °C annealing (Fig. 2). The presence of five-membered rings with methylene bridges is evidenced by brighter protrusions at their apexes compared with a few fully conjugated pentagons (Figs. 3(b) and 3(d)), indicating that the dehydrogenation of the methylene groups did not efficiently proceed. The slight asymmetry of the five-membered rings in Fig. 3 is attributed to tip asymmetries [43]. Two five-membered rings were sometimes observed on the same side of a repeating unit along the GNR backbone, as a result of methyl migration (Fig. 3(c), central unit). A proper statistical analysis of the occurrence of pentagons with methylene or methine bridges and of the other different structural features observed could not be conducted due to the limited number of GNR segments imaged by nc-AFM, while standard STM images do not allow to unambiguously identify the relevant structural details. However, we could clearly identify the frequent cleavage of benzylic methyl groups from STM images taken after annealing at 300 °C as well as 340 °C,  suggesting the necessity of lowering the C-H activation energy to achieve more selective synthesis of pentagon-fused GNRs.
To characterize the electronic properties of GNR 5, we performed density functional theory (DFT) calculations in gas phase (Fig. 4). These calculations reveal dispersive bands in the band structure of GNR 5, with a relatively small band gap of 0.74 eV. The five-membered rings with methylene bridges do not contribute to the conjugation (Figs. 4(a)-4(c)). The electronic properties of GNR 5 are thus expected to be very similar to those reported for GNR 2 (whose gas-phase structure is reported to have a DFT band gap of 0.74 eV) [33]. On the other hand, dehydrogenation of GNR 5 to GNR 13 (Fig. 4(f)) would drastically change the electronic properties of the GNR, because the carbon atoms at the apexes of five-membered rings would participate in π-conjugation. Such modification introduces two new bands in the calculated band structure of GNR 13 (Figs. 4(f)-4(j)), which reduces the band gap of the freestanding GNR to 0.41 eV.

Conclusion
In conclusion, we synthesized 10,10'-dibromo-2,2'-di-orthotolyl-9,9'-bianthracene (12) as a precursor of pentagon-fused GNRs and demonstrated the on-surface activation of the benzylic C-H bonds coupling against the GNR edges towards formation of GNR 5 incorporating methylene-bridged fivemembered rings. We found GNR 5 to coexist with structural variations involving fully conjugated pentagons with methine bridges as well as products resulting from migration and cleavage of methyl groups. Careful investigation of the chemical structures by nc-AFM with CO-functionalized tips further revealed azulene-incorporating non-benzenoid substructures with five-and seven-membered rings, including an intriguing structural feature with a seven-membered ring completely embedded inside the GNR backbone. Although the formation of GNR 5 lacked selectivity, the detailed information obtained on the on-surface activation of benzylic C-H bonds and the resulting GNR structures are instructive to expand the scope of the on-surface synthesis approach. An overarching goal is the further optimization of monomer design in order to enhance the structural precision of GNRs which is, indeed, seen in other cases.

Monomer synthesis and characterizations
All the experimental details for the synthesis of monomer 12 and characterizations of new compounds are reported in the ESM.

STM and nc-AFM experiments
The on-surface synthesis experiments were performed under UHV conditions with base pressure below 2 × 10 −10 mbar. Au(111) substrates (MaTeck GmbH) were cleaned by repeated cycles of Ar+ sputtering (1 keV) and annealing (470 °C). The precursor molecules were thermally sublimated onto the clean Au(111) surface from quartz crucibles heated at 250 °C, which resulted in a deposition rate of ~ 0.5 Å·min −1 . STM images were acquired with a low-temperature scanning tunneling microscope (Scienta Omicron) operated at 4.7 K in constant-current mode using an etched tungsten tip. Bias voltages are given with respect to the sample. nc-AFM measurements were performed at 4.7 K with a tungsten tip placed on a qPlus tuning fork sensor [44]. The tip was functionalized with a single CO molecule at the tip apex picked up from the previously CO-dosed surface [45]. The sensor was driven at its resonance frequency (24,700 Hz) with a constant amplitude of 70 pm. The frequency shift from 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 (negative) when the tip-surface distance is increased (decreased) with respect to the STM set point at which the feedback loop is opened.

Computational methods
For the gas phase calculations of the infinite ribbons we used the Quantum Espresso package [46] and PAW pseudopotentials from the SSSP precision library [47]. A cutoff of 50 Ry and 400 Ry was used for the plane wave expansion of Kohn Sham orbitals and charge density, respectively. We used the PBE parameterization for the generalized gradient approximation of the exchange correlation functional. All calculations were performed within the AiiDA lab framework [48] based on AiiDA [49]. The full dataset of the calculations is available on the Materials Cloud archive [50]. the European Union's Horizon 2020 research and innovation programme under grant agreement number 785219 (Graphene Flagship Core 2), the Office of Naval Research (No. N00014-18-1-2708), and the Okinawa Institute of Science and Technology Graduate University (OIST). The Swiss National Supercomputing Centre (CSCS) under project ID s904 is acknowledged for computational resources. Skillful technical assistance by Lukas Rotach is gratefully acknowledged. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made.
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