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.
Highlights
Graphene nanoribbons (GNRs) are a new class of next-generation carbon nanomaterials and have attracted widespread attention due to their unique electronic and magnetic properties
Since the initial demonstration of 7-atomwide armchair-type GNRs (7-AGNRs) on a Au(111) surface using 10,10'-dibromo-9,9'-bianthryl (DBBA) as a precursor [11], a range of GNRs with different structures have been prepared via on-surface synthesis, such as armchair GNRs (N-AGNRs) with different widths N (N = 5, 6, 8, 9, 10, 13, 15 and 17) [12,13,14,15,16,17,18,19], zigzag GNRs (ZGNRs) [20], chevron-type GNRs [11, 21], chiral (3,1)-GNRs containing zigzag and armchair edges [22], and heteroatom-doped GNRs with boron, nitrogen, oxygen, or sulfur [23,24,25,26,27,28,29,30]. π-Extension of 7-AGNRs through aryl-substitution of DBBA turned out to be of special value
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
Summary
Graphene nanoribbons (GNRs) are a new class of next-generation carbon nanomaterials and have attracted widespread attention due to their unique electronic and magnetic properties. Their electronic band structures are critically depending upon structural features such as width, length and edge-types [1,2,3,4]. Precision synthesis of GNRs appears as a major challenge. Crommie and co-workers demonstrated a synthesis of 13-AGNRs 1 (Fig. 1) using bis(biphenylyl)-substituted DBBA as the monomer [16]. GNRs comprising carbon hexagons, and sequences of 5-6-7 carbon rings and 4-5-7 rings were
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