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Abstract
The synthesis of circumazulene, a nonalternant isomer of circumnaphthalene, and its π-expanded derivatives poses a considerable challenge due to the lack of a suitable synthetic strategy. In this work, we present our efforts toward achieving tetrabenzo-fused circumazulene (1) through both solution and on-surface syntheses. In the case of in-solution synthesis, we obtained a product (P) with the desired target mass, but the structural verification proved to be challenging owing to the presence of various structural isomers. In the on-surface synthesis approach, a series of unexpected azulene-embedded nanographenes were obtained, including a molecule with an additional pentagonal ring (U1) based on the backbone of 1. Furthermore, theoretical calculations were conducted to shed light on these unexpected structures and to investigate their aromaticity. This work opens a new avenue for the design and synthesis of novel nonalternant graphene nanostructures incorporating circumarene.
Highlights
Circumarenes, a unique class of nanographenes (NGs) characterized by a distinctive zig-zag edge, feature a central aromatic core surrounded by one outer layer of fused benzenoid rings.[1–3] Among these, circumbenzene[4–8] and circumnaphthalene[9] (Figure 1a) stand as the smallest circumarenes, synthesized by Scholl and Clar, respectively, in the 20th century
We report our endeavors towards the synthesis of tetrabenzo-fused circumazulene (1) (Figure 1b), where the benzannulation strategy is employed to expand the circumazulene with defined curvature and to further tune its chemical and electronic properties.[27–32]
For on-surface synthesis, a series of unexpected azuleneembedded NGs were found after the cyclodehydrogenation reaction
Summary
Circumarenes, a unique class of nanographenes (NGs) characterized by a distinctive zig-zag edge, feature a central aromatic core surrounded by one outer layer of fused benzenoid rings.[1–3] Among these, circumbenzene (coronene)[4–8] and circumnaphthalene (ovalene)[9] (Figure 1a) stand as the smallest circumarenes, synthesized by Scholl and Clar, respectively, in the 20th century. Circumbenzene (coronene)[4–8] and circumnaphthalene (ovalene)[9] (Figure 1a) stand as the smallest circumarenes, synthesized by Scholl and Clar, respectively, in the 20th century. Their soluble derivatives were later employed in supramolecular self-assembly and electronic device applications.[10–12]. The replacement of hexagons in circumarenes with pentagon, heptagon, or joined non-hexagonal rings (i.e., azulene or heptalene) leads to local changes in strain, dipole, and molecular symmetry, which has a significant impact on their geometries and chemical, electronic, mechanical, and magnetic properties.[22,23]. [7, 7]circulene, containing a heptalene unit (two jointed heptagons), was synthesized by Yamamoto in 1991, which has a distinctive 8-shaped geometry.23d In contrast, [5, 7]circulene (namely, circumazulene), another important member among non-hexagonal ring-embedded circumar-
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