Fusing of Seven HBCs toward a Green Nanographene Propeller

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Fusing of Seven HBCs toward a Green Nanographene Propeller Yanpeng Zhu, Xiaoyu Guo, Yang Li, and Jiaobing Wang J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 12 Mar 2019 Downloaded from http://pubs.acs.org on March 12, 2019

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Journal of the American Chemical Society

Fusing of Seven HBCs toward a Green Nanographene Propeller Yanpeng Zhu†, Xiaoyu Guo†, Yang Li‡, Jiaobing Wang*† E-mail: [email protected]

Abstract This work presents a green chiral nanographene propeller (NP), which is built by fusing seven hexabenzocoronenes in a helical arrangement. It contains 258 conjugated carbon atoms, and represents the largest three-dimensional conjugated polycyclic aromatic hydrocarbons ever prepared using scalable solution chemistry. Despite of an unusual molecular size, single crystal X-ray structural analysis (resolution, 0.9 Å) and baseline chiral resolution are achieved. NP is soluble in various organic solvents, and can be fully characterized by common spectroscopic and voltammetric techniques. It has a strong panchromatic absorption from ultraviolet to the near infrared (λmax = 659 nm, ε = 179000 M-1cm-1). For instance, more than half of the spectral range between 300 and 800 nm witnesses an extinction coefficient larger than 100000 M-1cm-1. Moreover, a record-high Cotton effect in the visible spectrum is observed for enantiopure NP, with a |∆ε| value of 1182 and 1090 M-1 cm-1 at 374 and 405 nm, respectively. These photophysical properties evolve significantly compared with that of the propeller-shaped hexapole [7]helicene.

†School

of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China

‡Instrumental

Analysis and Research Center, Sun Yat-Sen University, Guangzhou 510275, China

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Introduction Chemists of different research areas are trying to construct and investigate various nanoscale molecular entities to push the limits of our understanding of the molecular world. For instance, gold clusters1, selfassembled molecular cages2, porphyrin nano-tapes3 and -rings4 of various sizes and shapes, have been prepared for studying their structures and intrinsic properties, testifying new synthetic methods, or identifying novel functional materials. In this context, synthetic nanographene sets a prominent example, which is initiated and established by Müllen and coworkers since the middle of 1990s5. In the last two decades, various nanographene compounds have been prepared, displaying attractive photophysical properties, which may be useful as the next-generation optical and electronic materials6-8. Nonetheless, when the molecular dimension and level of structural complexity of the target nanographene increase further, both its precise synthesis and full characterization encounter formidable challenges, and even reach the limit in many cases9,10, due to incomplete chemical transformation, side reactions, solubility issue, and difficult purification. Currently, except for graphene nanoribbon11, the largest synthetic nanographene contains 222 conjugated carbon atoms (C222)9. But its planar structure and large π-surface bring about strong molecular stacking, and lead to extremely poor solubility. As a result, mass spectroscopy is the only technique available for its structural elucidation, and no any solution-phase spectroscopic data of C222 can be obtained. This dilemma has been addressed later by edge chlorination, which yields the nonplanar C222Cl42 with enhanced solubility12. Thereby, its absorption spectrum in toluene could be recorded. In addition, attaching of multiple branched alkyl chains at the periphery of planar nanographene (e.g. C168) can also enhance solution processability13. But, with increasing molecular size, the π-π attraction will overtake the solubilization force, making this method less effective, because the former scales with the area, while the later correlates with the perimeter14. Thus, it is evident from this background that large atomically precise


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synthetic nanographenes, with e.g. > 200 conjugated carbon atoms and clear structure-property correlation, are quite rare12,15,16, and overcoming this limitation is of paramount importance for the further

Figure 1. Molecular model of NP (top) optimized with the Spartan software, MM force field. The central/peripheral HBC unit is indicated with shadow, or in purple for clarity. Stars represent the substituent groups on the representative HBC, or DBP blade. H7H, with HBC and DBP (dibenzo[e,l]pyrene) subunits, is shown at the bottom.

development of synthetic nanographene. Here, we report a chiral nanographene propeller (NP), which is constructed by fusing seven hexabenzocoronene (HBC) subunits in a helical arrangement (Figure 1)17. One HBC stays at the center, to which six others are attached at the periphery. It is a higher homologue of our recently documented multiple helicenes18, i.e. hexapole [7]helicene (H7H), and can also be regarded as a new member of superhelicenes19 derived from superbenzene20. NP contains 258 conjugated carbon atoms. To our knowledge, it is the largest atomically precise three-dimensional conjugated polycyclic aromatic hydrocarbons (PAHs) ever prepared using scalable solution chemistry. Despite of an exceptional



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nanoscale of 3.5 nm in width, the structure of NP is unambiguously verified by single crystal X-ray diffraction. Furthermore, in stark contrast to the planar nanographene compounds, which tend to form aggregates due to strong π-π stacking, NP is soluble in various organic solvents. Therefore, it can be fully characterized by common spectroscopic and voltammetric measurements, which enable its intrinsic optical and electrochemical properties to be disclosed in a comprehensive manner. For instance, we observed a strong panchromatic absorption from ultraviolet to the near infrared and a record high Cotton effect in the visible spectral range. Additionally, by comparing of NP with the 150-C H7H, it allows us to probe the evolution of the photophysical properties of synthetic nanographene, at the edge of its size domain currently reached in this field.

Figure 2. Synthesis and characterization of 3, 4, and NP. (A) Reaction scheme, (B) Mass and 1H-NMR data. Models of 3, 4, and NP are optimized using the Spartan software, MM force field. One of the six newly formed phenyl rings in 3, and the new


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bonds in 4 and NP are marked with distinct colors. 1 is prepared using the Sonogashira coupling from hexaiodo-HPB. Molecular model of 3 is close to the crystal structure of its unsubstituted analogue21.

Result and Discussion The synthesis of NP is outlined in Figure 2. Briefly, the polyphenylene precursor 3 is prepared following a modified literature method21. A six-fold Diels-Alder addition between an alkyne derivative 1 and a cyclopentadienone compound 2 gives 3 in 50% yield. With this step, all 43 phenyl rings of the final NP are in place, providing the hexaphenylbenzene (HPB) based polyphenylene skeleton for further transformation. Two consecutive oxidative dehydrocyclization reactions convert 3 into NP via a hexaHBC substituted benzene 4 as the intermediate. Altogether 42 new C-C bonds are formed with a two-step yield of 60% (an average yield of ca. 99% per new C-C bond). When 3 is mixed with DDQ (2,3-dichloro-5,6-dicyano-1,4-benzoquinone) at 0 ̊C in the presence of TFSA (trifluoromethanesulfonic acid) for 1.5 h, the peripheral HPBs are converted into HBCs cleanly. The product 4, as the charged species in a higher oxidation state22, precipitates out from the reaction mixture, preventing further transformation. But when a milder condition is applied using MSA (methanesulfonic acid) as the catalytic acid, 4 retains in the solution, whose HBC units are stitched together, by forming six new C-C bonds, to generate the central HBC core of the final product. The reaction takes about 12 h to finish at 25 ̊C. In addition, we found that direct subjection of 3 to the DDQ-MSA condition, at either 0 or 25 ̊C, yielded some unidentified byproducts, without NP observed. If FeCl3 was utilized as the oxidant, NP could also be formed from 3 via 4 as the intermediate. But the reaction was more sluggish, after 7 days, both 4 and NP (~1/1, mole ratio) were present in the reaction mixture, as judged from the 1H-NMR data.

It is reasonable that the dehydrocyclizing process occurs in a stepwise manner starting from the peripheral HPB, because the electron-donating t-butyl groups will decrease its oxidation potential, facilitating the



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Scholl reaction. Interestingly, we note that if one phenyl ring is removed from each of the periphery HPB of 3 (altogether six, one of them is circled, see Figure 2), a 37-ring polyphenylene precursor (without the t-butyl groups) will be formed, whose dehydrocyclization reaction will generate the planar C222, which is mentioned above9. Mass and NMR results (Figure 2B) are fully consistent with the reaction scheme. The mass spectra inform the detaching of 72 and 12 hydrogen atoms, from 3 and 4, respectively, to give the desired NP with expected isotope patterns (Figure S1). Using a combination of different 2D-NMR techniques, all resonance signals are readily assigned. Changes over multiplicity and chemical shifts are quite revealing. For instance, aromatization of the HPB units in 3 removes protons HG-J, all other protons left turn from doublet to singlet, and manifest a significant shift to the low field. Further transformation from 4 to NP is supported by the loss of protons Hf’. In addition, Hd-e appear in the relatively high field because of the shielding effect from the neighboring HBC blade. As can be easily recognized from the molecular structure, a group of three singlet resonance signals (1.88, 1.57, and -0.23 ppm, Figure S1) is observed for the periphery t-butyl groups. And the one between Hd and He is shielded from the nearby π-surface, appearing in the high-field end of the NMR spectrum at -0.23 ppm. Single crystals suitable for X-ray diffraction study were grown by vapor diffusion of hexane into a mixture of NP and ferrocene in CS2/CH2Cl2 (10/1, v/v) at 0 ̊C. NP crystallizes in the C2 chiral space group. Spontaneous resolution, as happened for some helicene compounds23, lead to the packing of only one form (M or P) of enantiomer in the crystal (Figure 3B). Consistent with the simulated molecular model (Figure 1), six peripheral HBCs are fused to the central one via rings-d, and they tilt unidirectionally around the central helical axis, resulting in a propeller-shaped chiral geometry. An average dihedral angle of 33.2̊ is observed between the mean planes of the outside and inner HBCs, which is in accordance with computational result of 33.8̊. But in the crystal, the exact value varies between 32.6̊ and 33.8̊ to adapt for a close-packed organization. Rings (c, i, d, j) of the embedded [7]helicene (Figure 3A, cyan) experience


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a significant distortion, as revealed by the average nonplanarity values (ring-c, 0.034 Å; ring-i, 0.070 Å; ring-d, 0.107 Å; ring-j, 0.058 Å). While other rings (a, b, e, g, h) are more flat by comparison. According to the crystal structure, bonds along the "bay" area are elongated significantly. For instance, the average bond lengths of C1-C2 (1.46 Å), C2-C3 (1.42 Å), and C3-C4 (1.44 Å) are much longer than the bond in benzene (1.40 Å). Based on the NICS (nucleus-independent chemical shift) calculations24, rings a-f are aromatic, and g-j are antiaromatic. This result reflects the “fully benzenoid”25 Clar formulas of the HBC subunit. In addition, despite of being a Clar π-sextet, ring-d displays the weakest aromaticity because of severe distortion. The local aromaticity determined from NICS is in agreement with that evaluated by the harmonic oscillator model of aromaticity (Table S4).

Figure 3. (A) Crystal structure of NP (CCDC no. 1895152), and the packing mode (B, side view; C, top view). For clarity, the t-butyl groups are not shown, and representative peripheral HBC (purple) and embedded [7]helicene (cyan) are marked with color. The absolute chirality could not be determined from the X-ray data, and the (P)-isomer is presented tentatively. NICS(0) values are indicated on the corresponding rings.



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In the crystal, NPs are arranged in a layered organization (Figure 3B) without intermolecular π-π stacking. The interlayer distance between the mean planes of NP’s central HBC from adjacent layers was found to be 1.14 nm. In each layer (Figure 3C), they are positioned in a 2D hexagonal lattice in agreement with the molecular symmetry. To maximize the van der Waals contact, slip-stacked NP orients differently relative to the neighbors above and below, by rotating around its central helical axis. Single crystal X-ray analysis of the 258-C NP is remarkable, considering that X-ray structures of synthetic nanographenes containing conjugated carbon atoms close to 100 are uncommon already12,26,27. Moreover, NP’s crystal data has a resolution of 0.9 Å, which is impressive compared with the X-ray structures of other nanoscale molecular entities such as self-assembled molecular cages and small proteins, for which high resolution crystal data are more difficult to achieve28. A rigid, robust, and compact molecular skeleton of NP is expected to contribute strongly to the successful X-ray structural analysis.

Figure 4. (A) Side view of the molecular model of NP covered with t-butyl groups. (B) Visualized image of NP (0.5 µM) in different solvents, 1% CS2, v/v. The maximum absorption of NP, in these solvents, shifts modestly between 644 (in hexane) and 666 nm (in CS2), with essentially constant absorption coefficients, see SI.

Although NP has an extraordinary size, which reaches ca. 3.50 nm in width and 1.25 nm in height (Figure 4), the propeller-shaped structure and the t-butyl groups at the edge, prevent it from stacking by π-π interaction, as indicated from the crystal data. Thus, NP is soluble in common organic solvents such as


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hexane, tetrahydrofuran, dichloromethane, acetone, ethyl acetate, dimethylformamide, and different alcohols except methanol. Particularly, in solvents such as toluene, chloroform, and CS2, the solubility reaches to the 10-4 M range or even higher, as is measured by following the Beer–Lambert law at varying concentrations. Outstanding solubility of NP allows us to systematically study its optical and electrochemical properties by using different solution-phase spectroscopic and voltammetric measurements. In addition, it is also essential for performing chiral resolution by using high performance

Figure 5. (A) Absorption and (B) CD spectra of NP (1.0 µM) in toluene at 25 ̊C. Inset of (A), NIR emission of NP at 77 K, λex = 405 nm. Inset of (B), chiral HPLC trace of NP using a semipreparative COSMOSIL Cholester column (hexane/isopropanol,



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25/75, 3.0 mL/min, detected by absorption at 450 nm). Helicity of NP is assigned based on a comparison of the CD pattern with that H7H, which has known chirality, see SI.

liquid chromatography (HPLC).

NP shows an intense panchromatic absorption, spanning from ultraviolet to the near infrared (Figure 5A). Four major bands manifest in toluene at 367, 405, 471, and 659 nm, respectively, with several shoulders evident. And the residual absorption tails until 900 nm (Figure S4). Solution of NP is green (Figure 4B), due to a prominent absorption around 659 nm. From 300 to 800 nm, more than half of the spectral range witnesses an extinction coefficient larger than 100000 M-1cm-1. The ε–value at the maximum absorption of 659 nm (179000 M-1cm-1) is much more intensive relative to that of the 150-C H7H at 618 nm (102000 M-1cm-1)18a. It demonstrates NP’s remarkable capability to capture light, which is highly desirable for various optical materials and devices29. Moreover, absorption of NP red-shifts evidently, ca. 41 nm, compared with H7H due to a significant expansion of the conjugated π-system. This difference could be even larger if an all-carbon H7H could be synthesized and used for comparison, considering that the electron-donating methoxyl groups, in the previously synthesized H7H (Figure 1), are expected to narrow the transition gap due to an elevation of its occupied frontier orbiters. Furthermore, we note that the absorption maximum of the propeller-shaped 258-C NP is shorter than, but approaching that of a fully annulated C222Cl47 nanographene (686 nm)12. Both of them have a conjugated π-system of ca. 3.0 nm in width.

NP has a weak NIR fluorescence at room temperature (Figure S4), with two maxima centered at 836 and 934 nm (λex = 405 nm, Φ = 0.016). While at low temperature of 77 K, both fluorescence (λmax = 863 nm, τ = 6 ns) and phosphorescence (λmax = 955 nm, τ = 4.7 µs) are observed (Figure 5A, inset). The mixed singlet-triplet character of the emission at 77 K was confirmed by lifetime measurement (Figure S5). A


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small singlet-triplet splitting (150 meV) is expected to enhance the spin-orbit interaction and vibronic coupling, resulting in an efficient intersystem crossing14,30.

Electrochemistry of NP is examined by cyclic voltammetry and differential pulse voltammetry measurements in CH2Cl2 with n-Bu4PF6 as the supporting electrolyte, which disclose an amphoteric multistage redox behavior. A group of six reversible single-electron oxidation waves (E1-6) is observed at 0.52, 0.75, 1.01, 1.14, 1.22, and 1.36 V (E, vs redox couple of decamethylferrocene), respectively. The first two of them (E1-2) is much lower than the oxidation potential of HBC (E = 1.02 V), while the last three are raised evidently by comparison. This redox behavior indicates a strong electronic coupling of the peripheral HBC blades in the large conjugated π-system31. In addition, two reversible reduction waves manifest at -1.44 and -1.73 V, respectively (Figure S7).

To rationalize the optical and electrochemical properties of NP, we carried out computational studies using the density functional theory (DFT) at the level of B3LYP/6-31G(d). The non-degenerate HOMO (-4.32 eV) and the doubly degenerate HOMO-1 (-4.40 eV) are closer in energy and well-separated from the HOMO-2 (-4.88 eV). This energy-level distribution is in agreement with the six-fold oxidizing process discussed above32. According to the time-dependent DFT calculation, the HOMO-1→LUMO+1 transition (energy gap, 1.80 eV, 687 nm), with a dominant oscillator strength of 0.87, can be correlated to the maximum absorption at 659 nm. While other theoretically weak and low-lying transitions, due to destructive combinations of the frontier orbitals, appear as shoulders (735, 800 nm) in the longer wavelength. Finally, we address the propeller chirality of NP by using chiral HPLC and circular dichroism (CD) spectroscopy (Figure 5B). Baseline separation of the (M)- and (P)-form enantiomers of NP is readily achieved with a commercially available COSMOSIL Cholester column. Optically pure NP obtained thereby shows a series of strong mirror-image Cotton effects across the whole UV-NIR spectral window.



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Specifically, the |∆ε| values at 374 (1182 M-1 cm-1) and 405 nm (1090 M-1 cm-1) are extremely large, which, to our knowledge, set a record for the strongest Cotton effect in the visible spectral range known for any synthetic molecules33,34. Considering that NP has an extremely intensive absorption (ε = 397000 M-1cm-1) around 367 nm, which coincides with the absorption profile of hexa-tert-butyl HBC (λmax = 361 nm, Figure S12), we assume that the strong Cotton effects of NP at this spectral range (350-420 nm) originate from the local transitions of its helically-arranged HBC subunits. Moreover, we compared the distribution and intensity of the CD signals of NP with those of H7H. Their CD patterns resemble each other closely (Figure S10), in consistence with their similar propeller-shaped configuration. But, in a stark contrast, CD intensity of the former is generally much stronger by comparison. For instance, |∆ε| at the maximum absorption of NP at 659 nm (308 M-1 cm-1, |∆ε|/ε = 0.0017) is almost five times more intensive than the corresponding value of H7H at 618 nm (66 M-1 cm-1, |∆ε|/ε = 0.0007).

Conclusion This work discloses a chiral nanographene propeller NP, which is constructed by fusing seven HBCs in a helical arrangement. Its structure is confirmed by single crystal X-ray diffraction. NP contains 258 conjugated carbon atoms, and represents the largest three-dimensional conjugated PAHs ever prepared using scalable solution chemistry. It is fully characterized by MS, NMR, and various optical and electrochemical studies, which reveal a series of unusual properties such as strong panchromatic absorption, and a record-high Cotton effect in the visible spectral range. These photophysical properties evolve significantly compared with its 150-C lower homologue, H7H. Precise synthesis and comprehensive characterization of NP hint that large synthetic nanographene has more potential to release.


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Experimental Section 1. Hexa(4-iodophenyl)benzene (300 mg, 0.233 mmol), 1-(tert-butyl)-4-ethynylbenzene (446 mg, 2.82 mmol), Pd(PPh3)2Cl2 (24 mg, 0.034 mmol), and CuI (7 mg, 0.037mmol) were suspended in toluene/Et3N (20 mL/5 mL) and stirred for 24 h at 50 oC under a nitrogen atmosphere. After cooling to room temperature (RT, 25 ± 2 oC), the reaction mixture was washed with saturated NH4Cl (aq). The organic layer was dried over anhydrous Na2SO4 and concentrated in vacuo. The residue was recrystallized from CH2Cl2/CH3OH (1:2, v/v) to give 1 as a yellowish solid (336 mg, 98 %). Mp > 300 oC. 1H NMR (300 MHz, CDCl3) δ 7.29 (d, J = 8.5 Hz, 12H), 7.21 (d, J = 8.5 Hz, 12H), 7.00 (d, J = 8.0 Hz, 12H), 6.70 (d, J = 8.2 Hz, 12H), 1.19 (s, 54H). 13C NMR (75 MHz, CDCl3) δ 151.47, 140.17, 140.03, 131.40, 131.38 (overlap), 130.55, 125.40, 120.89, 120.39, 89.72, 89.06, 34.90, 31.31. MALDI-TOF MS calculated for C114H102 1471.80, found: 1471.71 [M]+.

3. In a 10 mL Schlenk flask, compound 1 (90 mg, 0.060 mmol) and cyclopentadienone 2 (340 mg, 0.56 mmol) were dissolved in 1.5 mL diphenyl ether. The reaction mixture was stirred at 300 oC under a nitrogen atmosphere for 48 h. After cooling to RT, the reaction mixture was poured into methanol (50 mL), and the precipitate formed was collected. The crude product was purified via silica gel column chromatography using PE/CH2Cl2 (10:1, v/v, PE: petroleum ether) as the eluent to give 3 as a white solid (150 mg, 50%). Mp > 300 oC. 1H NMR (300 MHz, CDCl3) δ 6.91 – 6.72 (m, 101H), 6.68 (d, J = 8.1 Hz, 12H), 6.56 (d, J = 8.1 Hz, 12H), 6.47 (d, J = 8.0 Hz, 12H), 5.83 (d, J = 8.0 Hz, 12H), 1.13 (s, 170H), 1.02 (s, 111H). 13C NMR (75 MHz, CDCl3) δ 147.51, 147.45, 141.22, 141.09, 140.51, 140.17, 138.74, 138.52, 138.37, 138.14, 137.24, 136.94, 132.61, 131.60, 131.38, 131.18, 130.89, 130.76, 123.62, 123.23, 122.97, 34.24, 34.22, 31.88, 31.39. MALDI-TOF MS calculated for C378H414 4957.25, found: 4957.26 [M]+.



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4. To a solution of 3 (100 mg, 0.01 mmol) and DDQ (107 mg, 0.47 mmol) in anhydrous CH2Cl2 (80 mL), was added TfOH (4 mL) dropwise under a nitrogen atmosphere. The solution was stirred at 0 oC for 1.5 h. After quenched with Et3N, the reaction mixture was washed with water (50 mL × 3). The organic layer was dried over anhydrous Na2SO4 and concentrated in vacuo. The residue was purified by silica gel column chromatography using PE/CH2Cl2 (v/v, 8:1) as the eluent to give 4 as a yellow solid (92 mg, 92 %). Mp > 300 oC. 1H NMR (300 MHz, CDCl3) δ 9.80 (s, 12H), 9.04 (s, 13H), 8.96 (s, 12H), 8.69 (s, 12H), 8.64 (s, 12H), 8.50 (s, 12H), 1.69 (s, 45H), 1.53 (s, 90H), 0.86 (s, 90H). 13C NMR (101 MHz, CDCl3) δ 148.62, 148.49, 148.26, 144.65, 138.68, 130.57, 130.39, 130.29, 130.06, 129.80, 127.18, 124.13, 123.79, 123.66, 123.62, 120.60, 120.21, 119.99, 119.88, 118.84, 118.57, 118.42, 118.30, 118.10, 35.70, 35.52, 34.90, 32.08, 31.94, 31.24. MALDI-TOF MS calculated for C378H342 4884.69, found: 4884.75 [M]+.

NP. To a solution of compound 4 (71 mg, 0.014 mmol) and DDQ (59 mg, 0.26 mmol) in anhydrous CH2Cl2 (100 mL), was added MSA (5 mL) dropwise under a nitrogen atmosphere. The solution was stirred at RT for 12 h. After quenched with Et3N, the solution was washed with water (100 mL× 3). The CH2Cl2 layer was dried over anhydrous Na2SO4, and concentrated in vacuo. The residue, dissolved in CS2, was purified by silica gel column chromatography using PE/CH2Cl2 (v/v, 1:1) as the eluent. The crude product was further purified by recrystallization from ethanol (30 mL) to offer NP as a dark green solid (47 mg, 65 %). Mp > 300 oC. 1H NMR (400 MHz, CDCl3) δ 9.31 (s, 12H), 9.17 (s, 12H), 8.68 (s, 12H), 8.49 (s, 12H), 7.97 (s, 12H), 1.89 (s, 45H), 1.59 (s, 90H), -0.22 (s, 90H).

13C

NMR (75 MHz,

CDCl3) δ 148.73, 148.36, 147.72, 131.68, 130.91, 130.46, 130.18, 129.36, 128.05, 126.96, 125.50, 124.59, 124.11, 123.84, 123.72, 122.83, 121.43, 120.60, 120.33, 119.63, 118.90, 118.67, 118.36, 118.12, 35.74, 35.42, 33.51, 32.14, 31.87, 29.62. MALDI-TOF MS calculated for C378H330 4872.59, found: 4872.61 [M]+.


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Supporting Information MS and NMR spectra, photophysical and electrochemical data, X-ray structures, and computational results (PDF). Single crystal X-ray diffraction data (CIF).

Notes The authors declare no competing financial interests.

Acknowledgement This research was supported by the National Natural Science Foundation of China (21871298) and the Sun Yat-Sun University.

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