English

• Synthetic nanographene (NG) has made remarkable progress since the pioneering work of Müllen in the middle of 1990s [1-10]. In the last two decades, numerous NGs with various sizes, shapes, properties, and functions have been documented, attracting broad research interests from people working in fields such as synthetic chemistry, photophysics, and materials science. Nonetheless, despite these extraordinary achievements, atomically precise synthesis of large NGs is still highly desirable, providing opportunities for exploring nanoscale molecular entities at a new level of sophistications [11-52].

  Several years ago, we initiated a project on the design and synthesis of nonplanar chiral NGs [53-59]. This allows us to obtain a series of giant chiral NGs, such as hexapole [7]helicene, nitrogen-doped [7]helicene, hexapole [9]helicene, superhelicene, and supertwistacene. Moreover, an X-shaped double π-extended undecabenzo[7]helicene (DPEUB7H) has been documented very recently [59]. It shows a strong panchromatic light absorption capability, in addition to bright near-infrared (NIR) emission and distinguished electronic circular dichroism (ECD) signals. During the course of this study, we wonder what will happen if we break the D₆ symmetry of NG propeller? Therefore, we remove 1 hexabenzocoronene (HBC) subunit of superhelicene [56], which leads to a fan-shaped chiral NG 1 (Fig. 1), composed of 6 HBC subunits and 216 conjugated carbon atoms.

  Figure 1

  

  Figure 1.  Structure of the fan-shaped NG 1 optimized using the Spartan software, MM force field. Note, representative substituents were indicated with stars.

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  The synthesis of NG 1 is presented in Fig. 2A. Briefly, Diels-Alder addition between the cyclopentadienone derivative 2 and the diphenylethyne derivative 3 provides the central hexaphenylbenzene (HPB) platform in 4 (yield, 97%) with triisopropylsilyl protected alkyne groups at its periphery. Then, deprotection of the alkyne, enabled by TBAF, was followed by Sonogashira coupling with 1-(tert‑butyl)–4-iodobenzene, to give the key intermediate 6 (yield, 84%, 2 steps). A second 5-fold Diels-Alder reaction, in the presence of cyclopentadienone derivative 7, yielded the polyphenylene precursor 8 (yield, 14%). Lastly, Scholl oxidation of 8 was carried out in the presence of 2,3-dichloro-5,6-dicyano-1,4-benzo-quinone (DDQ) and trifluo-romethanesulfonic acid (TfOH) in dichloromethane (DCM) at 0 ℃, giving the desired NG 1 as a browned solid in a modest yield of 6%. Detailed synthetic procedures are provided in Supporting information.

  Figure 2

  

  Figure 2.  Synthesis (A) and characterization (B) of 1. Conditions: (a) Ph₂O, 260 ℃, N₂, 48 h; (b) TBAF (tetrabutylammonium fluoride), THF, r.t., 2 h; (c) CuI, Pd(PPh₃)₄, THF, Et₃N, reflux, N₂, 48 h; (d) Ph₂O, 260 ℃, N₂, 72 h; (e) DDQ, TfOH, DCM, 0 ℃, N₂, 4 h. The ¹H NMR (CDCl₃) and mass spectra of 1 are shown at the bottom.

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  It is noticeable that in the final dehydrocyclization reaction, 38 C—C bonds are formed in one step, stitching together 36 phenyl rings of the polyphenylene precursor in a stereoselective manner. In addition, only trace amount of product could be observed, via thin layer chromatography, if the reaction was prolonged up to 12 h, suggesting that 1 is unstable under the reaction environment. According to our experience, NGs with "fully benzenoid" Clar formulas are more amenable to dehydrocyclization. And properly substituted tert‑butyl groups are also beneficial to this kind of reaction.

  The ¹H NMR signals of the aromatic protons (H^a-H^y) in 1 manifest in a broad spectral range over 2.90 ppm (Fig. 2B). Specifically, protons H^a-H^f at the "guard" side of the NG fan, appear at low field (> 9.49 ppm), with H^a observed at 10.85 ppm due to a collective deshielding effect from the three HBC planes nearby. Other protons attached to the edge of its HBC leaves are found at high field up to 7.94 ppm. These ¹H NMR patterns are consistent with those of superhelicene [56] and DPEUB7H [59] with comparable structural features. All resonance signals could be convincingly assigned using 2D NMR techniques (Fig. S2 in Supporting information). Matrix-assisted laser desorption-ionization time-of-flight mass spectrometry of 1 (Fig. 2B, left) shows the desired molecular weight, and the observed isotope pattern is consistent with the simulated one (1, calcd. for C₃₂₂H₂₇₄, 4143.15, found, 4143.16 [M]⁺).

  NG 1 is soluble in common organic solvents such as DCM, chloroform, toluene, and tetrahydrofuran. The solubility of 1 in DCM was determined to be ca. 8 µmol/L, by following the Beer–Lambert law (Fig. S3 in Supporting information). A wine colored solution of 1 in DCM shows strong panchromatic absorption from the ultraviolet to the NIR (Fig. 3A). Three major absorption bands were found at 364, 564 and 675 nm, with a molar absorption coefficient (ɛ) of 332,500, 209,000 and 72,500 L mol⁻¹ cm⁻¹, respectively. Light absorption capability of 1 is comparable to superhelicene, but much weaker than that of the DPEUB7H (ɛ = 844,000 L mol⁻¹ cm⁻¹ at 573 nm). From the onset of the absorption at ca. 800 nm, an optical gap of 1.55 eV is suggested. Upon excitation at 564 nm, 1 emits NIR fluorescence centered at 820 nm, with a quantum yield of 5.5% (τ = 7.0 ns). The shoulder at ca. 900 nm could be attributed to vibronic progression. It is noted that NG with NIR emission may find utilizations for fluorescent molecular sensing and bioimaging [60, 61].

  Figure 3

  

  Figure 3.  (A) Absorption and (B) ECD spectra of 1 in DCM. A visualized image, fluorescence spectrum (λ_ex = 564 nm), and chiral HPLC trace are shown as the inset. A semipreparative COSMOSIL cholester column was used for chiral separation (DCM/isopropanol 50/50, 1.0 mL/min, detected by absorption at 565 nm). (C) Molecular orbitals of 1 calculated by DFT at the B3LYP/6–31 G(d) level. The TD-DFT results are presented also. To simplify the calculation, methyl groups are used as the substituents. The helicity of 1 was assigned based on TD-DFT results.

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  Enantiomers of 1 could be separated using chiral HPLC equipped with a semipreparative COSMOSIL cholester column Perfect mirror-image ECD signals were observed from 300 nm to 800 nm (Fig. 3B). The strongest ECD signal of 1 is found at 405 nm, whose intensity (|Δε| = 704 L mol⁻¹ cm⁻¹) is half that of DPEUB7H (|Δε| = 1375 L mol⁻¹ cm⁻¹ at 430 nm), and is lower than superhelicene also (|Δε| = 1090 L mol⁻¹ cm⁻¹ at 405 nm). The experimental ECD pattern is reproduced by TD-DFT calculations (Fig. S6 in Supporting information), which allows us to assign the helicity convincingly.

  Density functional theory (DFT) calculations were carried out to investigate the optical and electronic properties of 1. As shown in Fig. 3C, the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) are similarly distributed on the upper half of the NG fan, while the HOMO-1 and LUMO+1 are more evenly distributed across the whole molecule except the top HBC plane. According to time-dependent DFT calculations, the major absorption band at the longer wavelength at 675 nm can be attributed to the S₀-S₄ transition with an oscillator strength (f) of 0.391. The theoretical HOMO-to-LUMO transition at 786 nm is insignificant, which has a modest f-value of 0.019).

  In summary, we report the synthesis and characterization of a large fan-shaped chiral NG with 6 HBC subunits and 216 conjugated carbon atoms. Its (chir)optical and electronic properties were comprehensively studied with the assistance of DFT calculations. Results presented herein could be helpful for the development of high-performance functional NGs in the future.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgment

  This work was supported by the National Natural Science Foundation of China (Nos. 21871298, 91956118).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.05.034.

  
  
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• 

  Figure 1  Structure of the fan-shaped NG 1 optimized using the Spartan software, MM force field. Note, representative substituents were indicated with stars.

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  Figure 2  Synthesis (A) and characterization (B) of 1. Conditions: (a) Ph₂O, 260 ℃, N₂, 48 h; (b) TBAF (tetrabutylammonium fluoride), THF, r.t., 2 h; (c) CuI, Pd(PPh₃)₄, THF, Et₃N, reflux, N₂, 48 h; (d) Ph₂O, 260 ℃, N₂, 72 h; (e) DDQ, TfOH, DCM, 0 ℃, N₂, 4 h. The ¹H NMR (CDCl₃) and mass spectra of 1 are shown at the bottom.

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  Figure 3  (A) Absorption and (B) ECD spectra of 1 in DCM. A visualized image, fluorescence spectrum (λ_ex = 564 nm), and chiral HPLC trace are shown as the inset. A semipreparative COSMOSIL cholester column was used for chiral separation (DCM/isopropanol 50/50, 1.0 mL/min, detected by absorption at 565 nm). (C) Molecular orbitals of 1 calculated by DFT at the B3LYP/6–31 G(d) level. The TD-DFT results are presented also. To simplify the calculation, methyl groups are used as the substituents. The helicity of 1 was assigned based on TD-DFT results.

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