Synthesis, Structures and Properties of Heptabenzo[7]circulene and

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Synthesis, Structures and Properties of Heptabenzo[7]circulene and Octabenzo[8]circulene Sai Ho Pun, Yujing Wang, Ming Chu, Chi Kit Chan, Yuke Li, Zhifeng Liu, and Qian Miao J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b03910 • Publication Date (Web): 27 May 2019 Downloaded from http://pubs.acs.org on May 27, 2019

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Journal of the American Chemical Society

Synthesis, Structures and Properties of Heptabenzo[7]circulene and Octabenzo[8]circulene Sai Ho Pun, Yujing Wang, Ming Chu, Chi Kit Chan, Yuke Li, Zhifeng Liu, Qian Miao* Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China

ABSTRACT: This study puts forth two new members of fully ortho-benzannulated [n]circulenes, heptabenzo[7]circulene and octabenzo[8]circulene, which are new negatively curved nanographenes and also represent unprecedented structures of septuple [4]helicene and octuple [4]helicene, respectively. The successful synthesis of them through Scholl reaction in good to excellent yields takes advantage of the reactivity of naphthalene. Quantum chemistry calculations reveal that heptabenzo[7]circulene and octabenzo[8]circulene are both flexible π-molecules and adopt saddle-shaped geometry of C2 and D2d symmetry, respectively, at the global energy minimum in agreement with the single crystal structures. A serendipitous discovery from this study is that tetra(tbutyl) octabenzo[8]circulene in the single crystals self-assemble into a supramolecular nanosheet with an unprecedented motif of ππ stacking. Such a new molecular packing mode, in combination with the demonstrated semiconducting property of octabenzo[8]circulene, suggests a new supramolecular two-dimensional (2D) material.

INTRODUCTION Curved polycyclic aromatics have attracted growing interest not only owing to their unique structures and properties but also because they play an important role in bottom-up approaches to carbon allotropes of defined curvature.4,5 One common way to induce curvature in an otherwise flat polycyclic aromatic hydrocarbon is to embed a non-hexagonal ring 6 as demonstrated by [n]circulenes, which consist of a central n-membered ring surrounded with n fused benzenoid rings. [n]Circulenes differ from cycloarenes, 7, 8 1, 2, 3

such as kekulene,9 septulene 10 and octulene,11 which comprise annelated benzene rings forming a macrocycle with inward-pointing C-H bonds. [n]Circulene is positively curved like a bowl when n = 3 to 5, or negatively curved like a saddle when n = 7 to 16 as revealed by quantum chemistry calculations. 12 In agreement with this, [7]circulene and [8]circulene are basic structural units of theoretical threedimensional carbon allotropes of negative curvature, 5 which are known as Mackay crystals or carbon Schwarzites.13 So far, only [5]circulene (or corannulene),14 [6]circulene (or coronene), 15 and [7]circulene 16 have been synthesized. In contrast, [8]circulene is predicted to be highly unstable and still remains elusive,17 although its peri-substituted derivatives were synthesized in 2013.18 Expansion of [n]circulenes through ortho- and peri-benzannulation leads to nanographenes of defined curvature. So far, only three fully ortho-benzannulated [n]circulenes, namely, 6 tetrakis(trimethylsilyl)tetrabenzo[4]circulene (1), 19 20 pentabenzo[5]circulene (2), and hexabenzo[6]circulene (3), as shown in Figure 1a have been synthesized. Hexa-perihexabenzocoronene (4 in Figure 1b), 21 whose derivatives have been widely studied, 22 is the only known member of fully

peri-benzannulated [n]circulenes. Penta-peripentabenzo[5]circulene and hepta-peri-heptabenzo[7]circulene (5 in Figure 1b) remain elusive although N-hetero analogues of penta-peri-pentabenzo[5]circulene 23and attempted synthesis of 5 were reported recently.24

Figure 1 (a) Fully ortho-benzannulated [n]circulenes synthesized earlier; (b) documented fully peri-benzannulated [n]circulenes. Herein, we report two new members of fully orthobenzannulated [n]circulenes, heptabenzo[7]circulene (6) and

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octabenzo[8]circulene (7a–b) as shown in Figure 2. These molecules are new negatively curved nanographenes, 25, 26 which not only contain important structural information of negatively curved carbon allotropes, but also can, in principle, be used as templates or monomer units for synthesizing carbon nanostructures of negative curvature and graphenes with defined defects. Detailed below are the synthesis, structural analysis, stereochemistry and electronic properties of 6 and 7a–b. A serendipitous discovery from this study is the molecular packing of 7b in the single crystals, which presents an unprecedented motif of π-π stacking and promises a new supramolecular two-dimensional (2D) material.

and 7b in a yield of 92% and 83%, respectively. In contrast, the Scholl reaction of 1,4,9,12-tetraphenyltetraphenylene Scheme 1. Synthesis of 6.

Figure 2 Structure of heptabenzo[7]circulene (6) and octabenzo[8]circulenes (7a–b).

RESULTS AND DISCUSSION Synthesis. Heptabenzo[7]circulene (6) and octabenzo[8]circulenes (7a–b) were synthesized through a general route including three key steps: formation of the central seven- or eight-membered ring, Diels–Alder reaction of strained alkyne, and Scholl reaction.27, 28, 29 Scheme 1 shows the synthesis of 6 starting from pentacenone 8, which was also used by Nuckolls and coworkers in the synthesis of hexabenzo[6]circulene (3).20 The Büchner–Curtius–

Scheme 2 Synthesis of 7a–b.

Schlotterbeck reaction 30 of 8 expanded its cyclohexanone moiety to a seven-membered ring.26 Treatment of 9 with lithium diisopropylamide followed by quenching with triflic anhydride led to alkenyl triflate 10 in a yield of 42%. Subsequent elimination reaction of 10 with tBuOK generated a strained alkyne in situ, which, as a dienophile, reacted with 1,3-diphenylisobenzofuran (11a) in the Diels–Alder cycloaddition to afford 12 in a yield of 42%. The reaction of 12 with iodotrimethylsilane and sodium iodide enabled cleavage of oxygen bridge and aromatization to give 13 in a very good yield. The final Scholl reaction of 13 with DDQ and triflic acid at room temperature 31 afforded 6 in an excellent yield (90%), which corresponds to a yield of 97.4% for each C–C bond. As shown in Scheme 2, the synthesis of 7a–b started with nickel(0)-catalyzed [2+2+2+2] cycloaddition of diyne 14 following the reported methods 32 to form the eight-membered ring. The resulting dinaphthocyclooctatetraene (15) 33 was converted into diyne 17 by bromination and the subsequent elimination of HBr. Diyne 17 was not purified or characterized due to its poor solubility. Instead, the formation of 17 was confirmed by the two-fold Diels-Alder reactions of 17 with 1,3-diarylisobenzofurans (11a and 11b), which gave adducts 18a and 18b, respectively, in very good yields. Similar to 12, 18a–b reacted with iodotrimethylsilane and sodium iodide affording 19a–b with the oxygen bridges cleaved. The Scholl reactions of 19a and 19b, similar to that of 13, resulted in 7a


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tetrabenzo[8]circulene in a yield of 47% as reported by Whalley and coworkers.34 Therefore, the high yields of 7a–b as well as 6 from the Scholl reactions can be attributed to the naphthalene subunits in the reactants, which are electronricher than benzene subunits and can facilitate formation of CC bonds at their α-positions.35 DFT Calculations. The structures of heptabenzo[7]circulene and octabenzo[8]circulene were studied with density functional theory (DFT) calculations and X-ray crystallography. Molecular geometry optimization of 6 at the B3LYP/6-31G* level of DFT revealed a pair of saddle-shaped enantiomers of C2 symmetry (C2-6) at the global minima. Figure 3a shows the two enantiomers of C2-6 labeling the helicity of each [4]helicene moiety and the C2 axis. The two enantiomers can be interconverted through a transition state of Cs symmetry with an energy barrier of 3.8 kcal/mol, which corresponded to a rate constant of 1.0 × 1010 s−1 at 25 °C, as estimated using the Eyring equation k = κ(kBT/h)exp(−ΔG‡/RT) and assuming a value of unity for the transmission coefficient (κ). 36 It is worth noting that the enantiomerization of 6 in fact inverts the helicity of only one of the seven [4]helicene moieties (highlighted in yellow in Figure 3b). In comparison to 6, tetrabenzo[7]circulene was reported earlier to adopt a similar saddle-shaped geometry of C2 symmetry at the global minima but have a Cs symmetry at the local minimum. 37 The calculated enantiomerization pathway of C2-tetrabenzo[7]circulene, where the Cs conformer is an intermediate, has a higher energy barrier (12.2 kcal/mol) than 6 since both the [4]helicene moieties in tetrabenzo[7]circulene invert the helicity during the enantiomerization.

symmetric planes (σ). D2d-7a is achiral and have four P[4]helicene and four M-[4]helicene moieties, while D2-7a exists as a pair of enantiomers, which are less stable the D2d conformer by 7.9 kcal/mol. Changing helicity in two [4]helicene moieties in D2d-7a leads to D2-7a through the transition state TS-1 with an energy barrier of 9.2 kcal/mol as shown in Figure 4b. In accompany with inversion of helicity in another two [4]helicene moieties, D2-7a converts back to D2d-7a with the same energy barrier (9.2 kcal/mol), which corresponds to a rate constant of 1.1 × 106 s−1 at 25 °C. The calculated large rate constants for the conformational change of 6 and 7a indicate both molecules are flexible at room temperature in agreement with the fact that the 1H NMR spectra of 6 and 7a both show only two well-resolved proton signals in aromatic region (Supporting Information). The slower conformational change of 7a may be attributed to the pseudoration process of its central [8]circulene moiety, where the wave-like movement 2 of eight successive benzenoid rings requires more energy than that of seven successive benzenoid rings in [7]circulene.38, 39

Figure 4. (a) Energy-minimized models of D2d-7a and D2-7a, and (b) conformational change of 7a with relative Gibbs free energy (kcal/mol) as calculated at the B3LYP/6-31G* level of DFT.

Figure 3. (a) Molecular models of C2-6; (b) calculated pathway for the enantiomerization of 6 with relative Gibbs free energy as calculated at the B3LYP/6-31G* level of DFT. As revealed by the DFT calculations at the same level, 7a is shaped like a saddle of D2d symmetry at the global minimum and a twisted-saddle of D2 symmetry at the local minima. Figure 4a shows the D2d and D2 symmetric conformers of 7a (D2d-7a and D2-7a, respectively) labeling the C2 axes and

Crystal Structures. Heptabenzo[7]circulene (6) is soluble in common organic solvents such as CHCl3, CH2Cl2, THF and toluene. Single crystals of 6 were obtained by slow diffusion of methanol vapor into its solution in CHCl3. X-ray crystallography revealed a triclinic unit cell containing two molecules of 6 and five molecules of CHCl3 with disorder. In this crystal, 6 exists as a pair of enantiomers shaped like a twisted saddle with approximate C2 symmetry, as depicted in Figure 5a, in agreement with the DFT calculated model. The [7]circulene moiety in 6 has a depth of saddle of 1.73 Å, while



[7]circulene 16 is less curved with a depth of saddle of 1.10 Å as shown in Figure S1 in the Supporting Information. The greater curvature of 6 is presumably due to its [4]helicene moieties, which introduce additional crowdedness. Figure 5b shows the molecular packing of 6 in the crystalline state, where the two enantiomers are shown in blue and yellow, as viewed along the b axis of the unit cell. The neighboring enantiomers interact with each other having one benzenoid ring overlapped in a face-to-face manner with a π-to-π distance of 3.77 Å.

but unanswered question whether octabenzo[8]circulene with different substituents or without substitutents can form similar π-stacked nanosheets in the solid state.

Figure 5. (a) Structure of 6 in the single crystal with the C2 axis; (b) molecular packing of 6 in the crystal as viewed along the b axis of the unit cell. (Carbon atoms are shown as ellipsoids at 50% probability level. In (b) two enantiomers of 6 are shown in yellow and blue, and all hydrogen atoms and solvent molecules are removed for clarity.) Octabenzo[8]circulenes (7a–b) have poorer solubility than 6 in common organic solvents. Single crystals of 7b suitable for X-ray crystallography were obtained by very slow diffusion of iPrOH vapor into its solution in CS during three weeks, while 2 our attempts to grow single crystals of 7a were unsuccessful. As shown in Figure 6a, the saddle-shaped polycyclic backbone of 7b in the crystalline state is nearly the same as the DFTcalculated structure of 7a. However, 7b has a D2 symmetry rather than a D2d symmetry due to loss of the two σ planes as a result of attaching four t-butyl groups to octabenzo[8]circulene. Figure 6a shows the three C2 symmetry axes, C2-x, C2-y and C2-z, which are vertical to each other. As a result of the D2 symmetry, 7b is chiral and exists in the crystalline state as a pair of enantiomers, which are shown in blue and yellow in Figure 6b–d. Interestingly, molecules of 7b in the single crystals self-assemble into one-molecule-thick supramolecular nanosheets, which are separated by co-crystallized solvent (CS2) molecules as shown in Figure 6b. In each nanosheet, every molecule of 7b interlocks with four adjacent enantiomers in the ab plane as shown in (Figure 6c). A closer look along the b axis of the unit cell reveals that such an interlock involves insertion of a benzenoid ring into the space above or below the saddle of an adjacent enantiomer as shown in Figure 6d. This leads to face-to-face (not exactly parallel) and edge-to-face π-π interactions with C-to-C contacts of 3.54 to 3.65 Å. Each molecule of 7b donates two benzenoid rings for insertion, and simultaneously accepts two benzenoid rings into its spaces above and below the saddle. As a result, the crystal structure of 7b presents a unique type of twodimensional (2D) π-stacking, which is neither available to planar π-molecules nor known for negatively curved nanographenes 4 or multiple helicenes.40 This unprecedented packing motif may be related to the saddle-shaped π-backbone of high symmetry and tert-butyl groups, which fill the empty space inside the π-stacked nanosheet. It remains an interesting

Figure 6. (a) Structure of 7b in the single crystal with the C2 axes (the tert-butyl groups are removed for clarity, and the carbons that are bonded to tert-butyl groups are shown in magenta); (b) molecular packing of 7b as viewed along the b axis of the unit cell (two enantiomers of 7b are shown with yellow and blue stick models and CS2 is shown with the spacefilling model); (c) molecular packing of 7b as viewed along the c axis of the unit cell (two enantiomers of 7b are shown with yellow and blue space-filling models); (d) π-π interactions between two enantiomers of 7b in the crystals as viewed along the a axis of the unit cell (carbon atoms are shown as ellipsoids at 50% probability level). Electronic Properties. The optical and electrochemical properties of 6 and 7a were studied with UV-vis absorption spectroscopy and cyclic voltammetry. As shown in Figure 7, the absorption spectrum of 6 exhibits a broad absorption with the longest-wavelength absorption maximum at 483 nm while that of 7a exhibits a wide shoulder until about 560 nm. In comparison to 6 and 7a, hexabenzo[6]circulene (3) exhibits blue-shifted absorption, in agreement with the π-extended backbones of 6 and 7a. As estimated from the absorption edge, the HOMO-LUMO gaps of 6 is 2.56 eV. In contrast, the


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HOMO→LUMO transition of 7a is strongly forbidden as found from the TD-DFT calculations (Supporting Information), and the absorption band of 7a in the range of 440 nm to 560 nm can be attributed to the HOMO−1→LUMO and HOMO−2→LUMO transitions. The yellow solution of 6 in CH2Cl2 exhibits weak green luminescence and the orange solution of 7a in CH2Cl2 exhibits weak orange luminescence upon irradiation with UV light. The weak luminescence of 6 and 7a in solution can be attributed to their highly flexible polycyclic framework, whose conformational change consumes the energy of excited state resulting in radiationless internal conversion. The cyclic voltammograms of 6 and 7a in CH2Cl2 both exhibit one reversible oxidation wave with essentially the same half-wave oxidation potential of 0.64 V versus ferrocenium/ferrocene, based on which their HOMO energy levels are estimated as −5.74 eV.41 As found from DFT calculations at the B3LYP/6-31G* level, the HOMOs of 6 and 7a are distributed similarly on the naphthalene moieties. This may account for the same half-wave oxidation potential for 6 and 7a, which is a result of oxidation of the naphthalene moieties.

supramolecular nanosheet structure of 7b suggest a new supramolecular 2D material, which may offer two-dimension charge transport within the π-stacked nanosheet if crystalline monolayers 44 of 7b can be fabricated through proper methods.

CONCLUSIONS In summary, two new members of fully ortho-benzannulated [n]circulenes, heptabenzo[7]circulene (6) and octabenzo[8]circulene (7a–b), were successfully synthesized through a general route, which included three key steps: formation of the central seven- or eight-membered ring, Diels– Alder reaction of strained alkyne, and Scholl reaction. In particular, the success of the Scholl reaction relied on the reactivity of naphthalene moieties. As revealed by the DFT calculations, 6 and 7a are both flexible π-molecules and adopt saddle-shaped geometry of C2 and D2d symmetry, respectively, at the global energy minimum in agreement with the single crystal structures of 6 and 7b. A serendipitous discovery from this study is that 7b in the single crystals self-assemble into a supramolecular nanosheet with an unprecedented motif of π-π stacking. In view that 6 and 7a–b all function as p-type semiconductors in vacuum-deposited thin film transistors, the π-stacked nanosheet of 7b suggests a new supramolecular 2D material if a crystalline monolayer of 7b could be obtained. As found from our most recent preliminary experiments, the Scholl reactions for synthesis of 6 and 7a–b reported here can tolerate halogen subsitutents on the precursors leading to halogenated heptabenzo[7]circulenes and octabenzo[8]circulenes. Research on polymerization of these compounds is in progress in our laboratory to approach novel three-dimensional carbon-rich networks of negative curvature.

ASSOCIATED CONTENT

Figure 7. Absorption spectra of 3, 6 and 7a in CH2Cl2 (1×10−5 mol/L). The HOMO energy levels and π-π interactions of 6 and 7a–b suggest they can function as p-type organic semiconductors in the solid state. To test their semiconductor properties, thin films of 6 and 7a–b were deposited by thermal evaporation under high vacuum onto a silicon substrate, which had successive layers of silica, alumina, and 12cyclohexyldodecylphosphonic acid (CDPA) as a composite dielectric material. 42 Here CDPA formed a self-assembled monolayer (SAM) on alumina to provide an ordered dielectric surface with a relatively high surface energy. 43 As measured in ambient air, 6, 7a and 7b all behaved as p-type semiconductors with average field effect mobilities of (3.2 ± 1.0) × 10−5 cm2/(V s), (4.8 ± 3.2) × 10−6 cm2/(V s) and 1.4 ± 0.7) × 10−3 cm2/(V s), respectively. These low field effect mobilities can be attributed to the amorphous nature of the films, which was found from the absence of diffraction peaks when the film was investigated with X-ray diffraction (XRD). The fact that 7b has much higher mobility than 6 and 7a is presumably due to its unique 2D π-π stacking although the film lacks long range ordering. On the other hand, our preliminary efforts to deposit crystalline films of 7b using drop-casting and dip-coating techniques only resulted in isolated small crystals, which were not suitable for fabrication of thin film transistors. The semiconductor property and

Details of synthesis and characterization, fabrication, and characterization of organic thin film transistors, NMR spectra, the crystallographic information files (CIF) for 6 and 7b. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author Email: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Ms. Hoi Shan Chan (the Chinese University of Hong Kong) for the single crystal crystallography. This work was supported by the Research Grants Council of Hong Kong (GRF14300218) and the Croucher Senior Research Fellowship.

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