Heteroatom-Doped Nanographenes with Structural Precision

Abstract

Nanographenes, which are defined as nanoscale (1-100 nm) graphene cutouts, include quasi-one-dimensional graphene nanoribbons (GNRs) and quasi-zero-dimensional graphene quantum dots (GQDs). Polycyclic aromatic hydrocarbons (PAHs) larger than 1 nm can be viewed as GQDs with atomically precise molecular structures and can thus be termed nanographene molecules. As a result of quantum confinement, nanographenes are promising for next-generation semiconductor applications with finite band gaps, a significant advantage compared with gapless two-dimensional graphene. Similar to the atomic doping strategy in inorganic semiconductors, incorporation of heteroatoms into nanographenes is a viable way to tune their optical, electronic, catalytic, and magnetic properties. Such properties are highly dependent not only on the molecular size and edge structure but also on the heteroatom type, doping position, and concentration. Therefore, reliable synthetic methods are required to precisely control these structural features. In this regard, bottom-up organic synthesis provides an indispensable way to achieve structurally well-defined heteroatom-doped nanographenes. Polycyclic heteroaromatic compounds have attracted great attention of organic chemists for decades. Research in this direction has been further promoted by modern interest in supramolecular chemistry and organic electronics. The rise of graphene in the 21st century has endowed large polycyclic heteroaromatic compounds with a new role as model systems for heteroatom-doped graphene. Heteroatom-doped nanographene molecules are in their own right promising materials for photonic, optoelectronic, and spintronic applications because of the extended π conjugation. Despite the significant advances in polycyclic heteroaromatic compounds, heteroatom-doped nanographene molecules with sizes of over 1 nm and their relevant GNRs are still scarce. In this Account, we describe the synthesis and properties of large heteroatom-doped nanographenes, mainly summarizing relevant advances in our group in the past decade. We first present several examples of heteroatom doping based on the prototypical nanographene molecule, i.e., hexa-peri-hexabenzocoronene (HBC), including nitrogen-doped HBC analogues by formal replacement of benzene with other heterocycles (e.g., aromatic pyrimidine and pyrrole and antiaromatic pyrazine) and sulfur-doped nanographene molecules via thiophene annulation. We then introduce heteroatom-doped zigzag edges and a variety of zigzag-edged nanographene molecules incorporating nitrogen, boron, and oxygen atoms. We finally summarize heteroatom-doped GNRs based on the success in the molecular cases. We hope that this Account will further stimulate the synthesis and applications of heteroatom-doped nanographenes with a combined effort from different disciplines.

Figure 1

Schematic illustration of graphene and its nanoscale subunits (i.e., nanographenes), including nanographene molecules (atomically precise graphene quantum dots) and graphene nanoribbons.

Figure 2

(a) Synthesis of HBCs from the corresponding hexaphenylbenzene precursors via cyclodehydrogenation. (b) Columnar self-assembly of disc-shaped nanographene molecules. Reproduced from ref (24). Copyright 2007 American Chemical Society.

Figure 3

(a) General synthetic route to heteroatom-doped HBC analogues. (b) Synthesis of N-doped HBC 4 incorporating pyrimidine rings. (c) Synthesis of hexapyrrolohexaazacoronenes 6 incorporating pyrrole rings. (d) Several N-doped HBC analogues with a mixture of benzene and pyrrole rings. (e, f) Extensions of the hexapyrrolohexaazacoronene family, as exemplified by compounds 10–14.

Figure 4

(a) Chemical structures of aromatic HBC 1a and antiaromatic pyrazine-embedded HBC 15. (b) STM and (c) nc-AFM images of 15. Scale bars: 2 Å. A molecular model is partially superimposed on the STM image, and a white arrow pointing to the N atoms indicates the molecular symmetry axis. Reproduced with permission from ref (39). Published by Springer Nature. (d) Synthetic route to 15. TBA, tri-n-butylamine; DMSO, dimethyl sulfoxide; DDQ, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.

Figure 5

Syntheses of S-doped nanographene molecules. m-CPBA, m-chloroperoxybenzoic acid.

Figure 6

Synthesis of S-doped nanographene molecules 27 and 28 via postfunctionalization of perchlorinated HBC 26. DMI, 1,3-dimethyl-2-imidazolidinone. The single-crystal structure of perchlorinated HBC 26 was reproduced with permission from ref (43). Copyright 2013 Macmillan Publishers Ltd.

Figure 7

Chemical structures of (a, b) hexa-cata-hexabenzocoronene (29) and dibenzotetrathienocoronenes 30 as well as (c, d) hexathienocoronenes 31 and 32.

Figure 8

Schematic illustration of armchair and zigzag edge structures of nanographene molecules.

Figure 9

Synthesis of N-doped zigzag peripheries 35 and 38.

Figure 10

Synthesis of NBN-doped zigzag peripheries based on the 1,9-diaza-9a-boraphenalene motif. (a) Synthetic route to monomers 40 and further derivatizations. (b) Synthetic route to “dimer” 45. NBS, N-bromosuccinimide; o-DCB, o-dichlorobenzene.

Figure 11

Schematic representations of peri-acenes and heteroatom-doped peri-acene-type nanographene molecules as a segment of full zigzag GNRs.

Figure 12

(a) Synthesis of OBO-doped peri-tetracenes 49. (b) Another method to synthesize the uncyclized precursor 48a and an isomer of 48b with different substituent positions (compound 51). TfOH, trifluoromethanesulfonic acid.

Figure 13

(a) Synthetic route to OBO-doped peri-hexacene 55. (b) STM and (c) nc-AFM images of 55 on a Au(111) surface. (d) Schematic illustration of molecular assembly via metal coordination and O···H hydrogen bonding. Reproduced from ref (69). Copyright 2017 American Chemical Society.

Figure 14

(a) Synthetic route to OBO-fused double [7]helicene 57. (b) Single-crystal structures of the (M,M) and (P,P) isomers. (c) Circular dichroism spectra of the (M,M) and (P,P) isomers. Reproduced from ref (70). Copyright 2016 American Chemical Society.

Figure 15

Synthetic routes to oxaborin-annelated pyrene derivatives 60 and 63 and their solutions in CHCl₃ under UV light (365 nm). Images were reproduced with permission from ref (71). Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 16

(a) Surface-assisted synthesis of chevron-type GNR 65. (b) STM image of GNR 65 on a Au(111) surface. Reproduced with permission from ref (72). Copyright 2010 Macmillan Publishers Ltd. (c) Surface-assisted synthesis of N-doped chevron-type GNRs 67. (d) High-resolution STM image of GNR 67b, displaying a side-by-side alignment due to inter-ribbon N···H interactions. Reproduced with permission from ref (74). Copyright 2014 AIP Publishing LLC. (e) Surface-assisted synthesis of GNR heterojunction 69. (f, g) STM images of GNR heterojuction 69, with N-doped and pristine GNR segments highlighted in blue and light-gray dashed lines, respectively. Reproduced with permission from ref (75). Copyright 2014 Macmillan Publishers Ltd.

Figure 17

(a) Surface-assisted synthesis of S-doped GNR 71. (b) Distinct isomers of 70 due to restriction of thienyl group rotation on Au(111). (c) Schematic illustration of GNR 71 on Au(111). (d) Large-area STM image of GNR 71. (e) High-resolution STM image of GNR 71 with an overlaid chemical structure. Reproduced with permission from ref (76). Copyright 2017 Tsinghua University Press and Springer-Verlag GmbH Germany.

Figure 18

Schematic representation of (a) armchair GNRs, (b) zigzag GNRs, and (c) chiral GNRs. The edge configuration of chiral GNRs in (c) is defined by the translation vector C_h, described as C_h = na₁ + ma₂ = (n, m), where a₁ and a₂ represent the unit vectors of the graphene lattice. Reproduced from ref (78). Copyright 2018 American Chemical Society.

Figure 19

(a) Surface-assisted synthesis of OBO-doped (4,1)-GNR 74. (b) Large-area STM image and (c) high-resolution STM image of GNR 74 on Au(111). (d) nc-AFM image of GNR 74. (e, f) Theoretically calculated models of inter-ribbon interactions for heterochiral and homochiral assemblies of GNR 74, respectively. Reproduced from ref (78). Copyright 2018 American Chemical Society.