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

Abstract

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.

Synthesis and Characterization of peri‐Heptacene on a Metallic Surface

“…Frequently, the on‐surface synthesis of targeted nanostructures via oxidative ring‐closure of precursors equipped with methyl groups presents several reaction pathways that hamper their selectivity [53–56] . Therefore, in spite of the successful formation of 7‐PA , we have further conducted a careful investigation of the byproducts obtained after annealing of the sample at 300 °C.…”

“…[41] Frequently, the on-surface synthesis of targeted nanostructures via oxidative ring-closure of precursors equipped with methyl groups presents several reaction pathways that hamper their selectivity. [53][54][55][56] Therefore, in spite of the successful formation of 7-PA, we have further conducted a careful investigation of the byproducts obtained after annealing of the sample at 300 °C. are formed due to the cleavage of one or two methyl groups from 1 during the annealing of the sample together with the rotation of naphthalenyl units and subsequent ring rearrangement reactions as deduced from the constant-height STM images, as shown in Figure 2c-g 2d, e, g), can be potentially transformed into fully conjugated NGs via STM tip-induced hydrogen removal, in agreement with previous reports.…”

Synthesis and Characterization of peri‐Heptacene on a Metallic Surface

“…[41] Frequently, the on-surface synthesis of targeted nanostructures via oxidative ring-closure of precursors equipped with methyl groups presents several reaction pathways that hamper their selectivity. [53][54][55][56] Therefore, in spite of the successful formation of 7-PA, we have further conducted a careful investigation of the byproducts obtained after annealing of the sample at 300 °C. 2d, e, g), can be potentially transformed into fully conjugated NGs via STM tip-induced hydrogen removal, in agreement with previous reports.…”

Defect-Induced π-Magnetism into Non-Benzenoid Nanographenes

“…The formation of such NGs is attributed to the oxidative ring closure of the majority of the methyl groups, together with the dissociation of several of them per NG prior to cyclization. Such removal of methyl moieties was previously reported in the synthesis of several NGs [30,37,[42][43][44][45] and GNRs [22,46] and is concomitant to the annealing step at temperatures where the oxidative ring closure is expected to occur on Au(111), therefore being not possible to achieve the synthesis of E.…”

“…For instance, the nature of the electronic ground state of long pursued members of the acene [14][15][16][17][18][19] and triangulene [20,21] families, widely discussed in theoretical studies, have only recently been untangled. Contemporarily, a successful strategy toward the on-surface formation of open-shell NGs, one-dimensional polymers and graphene nanoribbons (GNRs) is the surface-assisted oxidative ring closure between a methyl group and the neighboring aryl moiety of a properly predesigned molecular precursor, which occurs after thermal activation [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. Such synthetic approach, though often successful, presents some limitations related to the cleavage of methyl groups prior to cyclization, which may lead to the formation of topological defects inducing an open-shell ground state to NGs [30,37].…”