Toward Negatively Curved Carbons - Accounts of Chemical Research

Article Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Toward Negatively Curved Carbons Sai Ho Pun and Qian Miao*

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Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China CONSPECTUS: Negatively curved carbons are theoretical carbon allotropes as proposed by embedding heptagons or octagons in a graphitic lattice. Unlike five-membered rings in fullerenes, which induce positive curvature, the seven- or eight-membered rings induce negative curvature, giving rise to a variety of esthetic carbon nanostructures known as Mackay crystals or carbon schwarzites. In addition, hypothetical toroidal carbon nanotubes consisting of five-, six-, and seven-membered rings present positive curvature on the outside and negative curvature on the inside of the torus. These carbon allotropes with negative curvature are predicted to have interesting properties and potential applications on the basis of computational studies but are yet to be synthesized. A promising bottom-up approach to these intriguing but still imaginary carbon structures is organic synthesis of negatively curved polycyclic arenes, which are also known as negatively curved nanographenes. They not only are segments of negatively curved carbon allotropes containing important structural information but also can in principle be used as templates or monomer units for the synthesis of carbon schwarzites and toroidal carbon nanotubes. This Account describes research on the design, synthesis, structure, stereochemical dynamics, and properties of negatively curved nanographenes, with emphasis on our efforts in this field. In our designs of negatively curved nanographenes, a few heptagon- or octagon-embedded π systems were employed as basic structural units, including [7]circulene, heptagon-embedded hexa-peri-benzocoronene, tetrabenzodipleiadiene, and [8]circulene. They present a saddle-shaped geometry and consist of a relatively small number of sp2 carbon atoms. By expanding or connecting these structural units, we designed and synthesized larger negatively curved nanographenes consisting of up to 96 sp2 carbon atoms. A method of key importance in the synthesis of negatively curved nanographenes is the Scholl reaction, which enables the formation of multiple carbon−carbon bonds in a single step by intramolecular oxidative cyclodehydrogenation. The unique structures of negatively curved nanographenes were studied by experimental and computational methods. In particular, X-ray crystallography of single crystals revealed remarkably curved π faces accompanied by severe out-of-plane deformation of benzenoid rings, which sheds light on the limit of π bonds and the aromaticity of polycycles. As found mainly from calculations, the flexible polycyclic frameworks of negatively curved nanographenes are associated with stereochemical dynamics that is not available for planar polycyclic aromatics. In addition, some negatively curved nanographenes have been found to function as organic semiconductors in the solid state. We envision that the study of negatively curved nanographenes will serve as an important initial step toward the eventual synthesis of new carbon allotropes with negative curvature and new frontiers of nanocarbon materials.

1. INTRODUCTION Carbon allotropes comprising exclusively sp2-hybridized carbon atoms present a variety of esthetic nanostructures with delocalized π electrons, such as fullerenes, carbon nanotubes, and graphene, giving rise to interesting electrical, optical, magnetic, thermal, and chemical properties.1 A layer of covalently bonded sp2 carbon atoms forms a surface, which may lie flat like a carpet or curve like a bowl or a saddle depending on the patterns of carbon atoms. The overall feature of this surface is reflected by a geometric quantity called the polygonal curvature of the surface,2 which is discussed in section 2. Consisting of six-membered rings exclusively, graphene has zero curvature. Five-membered rings induce positive curvature, as displayed in fullerenes, while seven- or eight-membered rings induce negative curvature, as displayed in theoretical carbon allotropes of negative curvature, which are also known as Mackay crystals or carbon schwarzites. © XXXX American Chemical Society

Mackay crystals are named after A. L. Mackay, who, together with H. Terrones, first proposed negatively curved graphite foams3 by embedding octagons in the graphitic lattice, as demonstrated by the example shown in Figure 1a. Carbon schwarzites are named after H. A. Schwarz, who was the first to describe periodic minimal surfaces, the topological model for Mackay crystals.4 More members of negatively curved carbon allotropes have been proposed by introducing sevenmembered carbon rings into frameworks of sp2 carbon atoms.5−7 In addition, hypothetical toroidal carbon nanotubes8 comprising five-, six-, and seven-membered rings (Figure 1b) present positive curvature on the outside and negative curvature on the inside of the torus. These negatively curved carbon allotropes are predicted to have various interesting Received: March 28, 2018

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DOI: 10.1021/acs.accounts.8b00140 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research

shown in Figure 2a. Fullerene C60 consists of 20 regular hexagons and 12 regular pentagons, where each vertex is

Figure 1. (a) A Mackay crystal with octagons highlighted in orange. (b) A toroidal carbon nanotube with pentagons and heptagons highlighted in blue and red, respectively.

Figure 2. Schematic drawings of the polygonal surfaces of (a) graphene and (b) C60 with zero and positive polygonal curvature, respectively, at a vertex.

properties and potential applications on the basis of computational studies.9 For example, Tománek and co-workers studied a negatively curved carbon nanotube junction with spin density functional calculations and found that it carried a net magnetic moment in the ground state.10 First-principles calculations also suggested the potential application of carbon schwarzites in lithium ion batteries as the anode materials, where the presence of pores in schwarzites can lead to three-dimensional lithium ion diffusion paths with relatively low energy barriers.11 Despite their esthetic structures and interesting properties, the negatively curved carbon allotropes are yet to be synthesized. We envision that organic synthesis of negatively curved polycyclic arenes containing seven- or eight-membered rings is a promising bottom-up approach12 to the imaginary carbon allotropes with negative curvature. These polycyclic arenes, also known as negatively curved nanographenes, are segments of negatively curved carbon allotropes containing important structural information. Moreover, the negatively curved carbon allotropes could in principle be synthesized from a negatively curved polycyclic arene, which may be used as a template or a monomer unit in a controlled growth process. With these ideas in mind, we initiated our studies of negatively curved polycyclic arenes in 2010 and have reported a series of negatively curved nanographenes since 2012. Almost in parallel to our work, Itami, Scott, and co-workers reported a negatively curved nanographene containing five heptagons in 2013,13 and the groups of Wu14 and Suzuki15 reported octagon-embedded nanographenes in the same year. This Account aims to provide an overview of the design, synthesis, structure, stereochemical dynamics, and properties of negatively curved nanographenes, with emphasis on our own work.

surrounded by two hexagons and one pentagon. The polygonal curvature at the vertex (Figure 2b) is positive because the internal angle of the regular pentagon (3π/5) is smaller than 2π/3. A seven-membered ring of sp2 carbon atoms in a polycyclic arene has a greater tendency to curve than a six-membered ring for two reasons. First, the carbon atoms in a non-aromatic seven-membered ring do not need to be in the same plane to maximize the aromaticity. Second, if the seven-membered ring stays flat, its average internal angle (128.6°) is larger than the typical bond angle for sp2 carbons (120°). For example, in [7]circulene (1 in Figure 3a), the seven-membered ring is

2. BRIEF CONSIDERATION OF GEOMETRY Carbon atoms in a polycyclic arene can be considered as spatially located on a polygonal surface that is made of convex polygons.16 If all of the rings in a polycyclic arene are planar, each ring is a polygonal region, and each carbon atom is a vertex of this polygonal surface S. As a result, the polygonal curvature is completely determined by all of the interior angles of the polygons at each vertex of S. The polygonal curvature of S at a vertex (v) is defined as k(v) = 2π − s(v), where s(v) is the sum of the radian measures of these interior angles.2 Therefore, the surface may lie flat or curve in some way, depending on the sum of the interior angles of the polygonal regions meeting at a vertex. Graphene has a hexagonal lattice, where each vertex is surrounded by three regular hexagons. Because the internal angle of a regular hexagon is 2π/3, the polygonal curvature at each vertex of graphene is zero, as

Figure 3. (a) [7]Circulene and its seven-membered ring in the crystal structure, with the centroid shown as a red ball. (b) Representative structures of flat polycyclic arenes containing seven-membered rings.

curved while the seven benzenoid rings are essentially flat, as found from the crystal structure.17 Eight-membered rings of sp2 carbon atoms are usually not planar because cyclooctatetraene is tub-shaped to avoid the flat antiaromatic structure and to keep the typical bond angle (120°) for sp2 carbons. The interior of a nonplanar ring contributes to the curvature. Therefore, in order to calculate the polygonal curvature of a polycyclic arene containing nonplanar rings, the interior of each nonplanar ring (not necessarily a seven or eight-membered ring) needs to be subdivided into triangles, B


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Accounts of Chemical Research which are always planar. These triangles meet at a new vertex,18 at which the curvature is negative if the sum of the interior angles of these triangles is larger than 360° (or 2π).2 As shown in Figure 3a, the seven-membered ring of [7]circulene in the crystal structure can be divided into seven triangles at its geometric center (centroid), at which the sum of the interior angles of these triangles is 366°. It is worth noting that not all polycyclic arenes containing sevenmembered rings are negatively curved. When the sevenmembered ring is not heavily fused with benzenoid rings, it can still keep flat to maximize conjugation. Figure 3b shows two examples of this kind of polycyclic arene (2 and 3), which exhibit essentially flat π backbones in their crystal structures.19,20

Scheme 1. Syntheses of (a) 1 and (b) 12

3. DESIGN AND SYNTHESIS Negatively Curved Nanographenes Containing Seven-Membered Rings

To design negatively curved nanographenes, we considered a few heptagon-embedded π systems as basic structural units, which present a saddle-shaped geometry and consist of a relatively small number of sp2 carbon atoms. Larger negatively curved nanographenes can be constructured by expanding or connecting them. These basic structural units include 1 (Figure 3a) and 4−7 (Figure 4). [7]Circulene (1), having

Figure 4. Basic structural units for negatively curved nanographenes containing seven-memerbed rings.

one seven-membered ring surrounded by seven successive benzenoid rings, is the first member of the family of negatively curved polycyclic arenes containing seven-memerbed rings.17 Molecules 4−6 have one or two heptagons embedded in the framework of hexa-peri-benzocoronene (HBC). Tetrabenzodipleiadiene (7), unlike dipleiadiene (3), is curved like a shallow saddle, as found from its energy-minimized model. The synthesis of 1 was first reported by Yamamoto and coworkers in a brief communication in 198317 and detailed in a full article in 1988.21 Scheme 1a shows this synthesis starting from sulfur-containing macrocycle 8, which was prepared from 5,5′-dimethyl-2,2′-dinitrobiphenyl in five steps. The Stevens rearrangement converted sulfonium salt 9 to sulfide 10. Oxidation of 10 followed by pyrolysis enabled elimination of sulfur, yielding macrocyclic diene 11. Photocyclization of 11 produced a seven-membered ring at the center. The subsequent formylation and intramolecular McMurry reaction afforded 1. Using a similar strategy, the same research group also synthesized [7.7]circulene 12 from tetraene 13, as shown in Scheme 1b.22 On the basis of [7]circulene, we designed 1423 and 15,24 which formally result from expansion of [7]circulene by annulation of benzenoid rings ortho or peri to [7]circulene, respectively, as shown in Figure 5. Scheme 2 shows our

Figure 5. Expansion of [7]circulene by benzannulation.

synthesis of tetrabenzo[7]circulene (14) from dione 16, which contains a seven-membered ring preformed by an intramolecular Friedel−Crafts acylation. Ramirez olefination of dione 16 yielded compound 17 but, to our surprise, was not able to enforce reaction of the second carbonyl group even with a large excess of CBr4 and PPh3, likely because the carbonyl group in 17 is blocked by the bromine atoms on the convex face, as found from the crystal structure. Suzuki coupling of 17 with 2-bromophenylboronic acid and subsequent Pd-catalyzed cyclization afforded ketone 18, which differed from 17 by being reactive in the Ramirez olefination, likely because of the reduced steric hindrance. Repeating the Suzuki coupling and Pd-catalyzed cyclization finally resulted in 14. Hepta-peri-heptabenzo[7]circulene (15) has an odd number of π electrons and thus, in its neutral form, is an open-shell molecule with the radical delocalized in the π backbone. We considered ketone 19 (Scheme 3a) to be a synthetic precursor to 15 since it has essentially the same polycyclic framework as C


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or para to activating substituents,25 we introduced four methoxy groups into 21a to obtain 21b. The Scholl reaction of 21b afforded 22 with the formation of four C−C bonds. To complete the polycyclic framework of 19, two more C−C bonds are yet to be formed. Our attempts to synthesize 4−6 without extra substituting groups were unsuccessful. As a result, alkoxy groups were attached to 4−6 to facilitate the synthesis. As shown in Scheme 4, the synthesis of heptagon-embedded HBC 23 was adapted

Scheme 2. Synthesis of Tetrabenzo[7]circulene (14)

Scheme 4. Synthesis of Heptagon-Embedded HBCs 23 and 27a/b

Scheme 3. Attempted Synthesis of Hepta-periheptabenzo[7]circulene

from Müllen’s well-known synthesis of substituted HBCs that includes the Diels−Alder reaction and the Scholl reaction as two key steps.26 The seven-membered ring in 23 was introduced from 24, which generated a strained alkyne in situ as the dienophile for the Diels−Alder reaction with cyclopentadienone 25. In order to increase the solubility of the final product, the methyl groups in 26a were changed to hexyl groups in 26b. The Scholl reaction of 26b with FeCl3 as both the Lewis acid and oxidant provided 23 in 62% yield. The position of the alkoxy groups in 26b was found to play a key role in this reaction. It is necessary to equip the heptagoncontaining hexaphenylbenzene with alkoxy groups ortho or para to the reaction sites since the Scholl reaction is known to form C−C bonds preferably ortho or para to activating substituents.25 A mixture of inseparable products was obtained

15. Scheme 3a shows our attempts to synthesize 19, which unfortunately stopped at 21a. Subjection of 21a or 20 to the Scholl reaction for oxidative cyclodehydrogenation under different conditions failed to afford 19. As inspired by a report that the Scholl reaction formed C−C bonds preferably ortho D


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Accounts of Chemical Research when the analogous precursors had alkoxy groups meta to the reaction sites. As shown in Scheme 4b, diketone 27a was synthesized in a similar way from cyclopentadienone 28, which already contained a seven-membered ring.27 The second sevenmembered ring in 27a was formed by an intramolecular Friedel−Crafts acylation mediated by methanesulfonic acid after the Scholl reaction of 29. After our synthesis of 23 and 27a, Campaña and co-workers also reported the synthesis of methoxylated and tert-butylated derivatives of 5 in 2017 through a cobalt-catalyzed cyclotrimerization of polyfunctionalized alkynes followed by the Scholl reaction.28 One advantage of 27a over 23 is the two carbonyl groups, which can allow convenient extension of the π backbone. To improve solubility of the π-extended products, the propyl groups in 27a were changed to hexyl groups in 27b (Scheme 4). Then the π backbone of 27b was extended in two different ways by reactions of the ketones, as shown in Scheme 5.

Scheme 6. Synthesis of 38

Scheme 5. Synthesis of 30 and 31

expansion reaction under common conditions, four tert-butyl groups were introduced to bisanthenequinone to increase the solubility. Our synthesis started with the preparation of tetratert-butylbisanthenequinone (32) from di-tert-butylanthrone (33) by oxidative coupling and subsequent photocyclization. The ring expansion of 32 using TMSCHN2 resulted in diketone 34 as one regioisomer, where the positions of the carbonyl groups, however, were not determined. The enolate of 34 reacted with triflic anhydride to give 35, a ditriflate derivative of 7. To expand the polycyclic framework of 35, it was treated with tBuOK, resulting in a strained alkyne in situ, which reacted with cyclopentadienone 36 in the Diels−Alder cycloaddition to afford 37. The Scholl reaction of 37 enabled the formation of 10 carbon−carbon bonds, affording 38, a saddle-shaped nanographene consisting of 86 sp2 carbon atoms.29 A common strategy used by us in the above syntheses was to form seven-membered rings at an early stage of the synthesis and then to expand the polycyclic framework from heptagoncontaining building blocks. Using a different approach, Itami, Scott, and co-workers synthesized C80H30 (39a), a grossly warped nanographene, and its deca-tert-butyl derivative (39b) from corannulene through the formation of five seven-

Nucleophilic addition of fluorene anion to the carbonyl groups followed by dehydration and cyclodehydrogenation under the Scholl reaction condition resulted in 30. Ramirez olefination followed by Suzuki coupling and the Scholl reaction resulted in 31. To synthesize the structural unit 7 (Figure 4), ring expansion of the cyclohexanone moieties in bisanthenequinone was used as a key strategy for the formation of sevenmembered rings, as shown in Scheme 6. Since unsubstituted bisanthenequinone was too insoluble to allow a successful ring E


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Accounts of Chemical Research Scheme 7. Synthesis of Grossly Warped Nanographenes 39a and 39b

membered rings in the last step.13 As shown in Scheme 7, direct C−H bond arylation of corannulene with tris(obiphenylyl)boroxin catalyzed by Pd(OAc)2/o-chloranil resulted in 40 with a lack of regioselectivity. The same catalysts also enabled the reaction of corannulene with tris(p-tertbutylphenyl)boroxin to afford 41 with 10-fold direct C−H bond arylation. Alternatively, a self-correcting, exhaustive C−H borylation of corannulene with an iridium catalyst yielded 42, which reacted in the Suzuki−Miyaura coupling to give isomerically pure pentakis(biphenylyl)corannulene 43. The Scholl reactions of 40 and 43 with DDQ and triflic acid and of 41 with FeCl3 afforded 39a and 39b, respectively, with the formation of 10 carbon−carbon bonds in one step. Negatively Curved Nanographenes Containing Eight-Membered Rings

[8]Circulene (44 in Figure 6) is the only known structural unit for negatively curved nanographene containing eight-membered rings. However, unsubstituted [8]circulene is predicted to be highly unstable and still remains elusive.30 As found from density functional theory (DFT) calculations,15 the ground state of [8]circulene has the shape of a tub with D2d symmetry, having three C2 axes and two symmetry planes (σ), as shown in Figure 6 (one C2 axis at the intersection of the two symmetry planes is not shown). Tetrabenzo[8]circulene (46) (Figure 6) can be considered as a result of expanding 44 by benzannulation along two C2 axes, and twisted nanographenes 47a and 47b (Figure 6) can be considered as a result of fusing [8]circulene with two units of dibenzo[cd,pq]bisanthene along a C2 axis of [8]circulene. Scheme 8a shows the synthesis of peri-substituted [8]circulenes 45a and 45b through a fourfold Pd-catalyzed

Figure 6. [8]Circulene (44), tetrabenzo[8]circulene (46), and nanographenes 47a and 47b containing an [8]circulene subunit as shown in red.

annulation of tetraiodotetraphenylene with diarylethynes.14 Scheme 8b shows the synthesis of tetrabenzo[8]circulene in two different ways: an inward approach as demonstrated by Sakamoto and Suzuki in 201315 and an outward approach as demonstrated by Whalley and co-workers in 2014.31 In the inward approach, the eight-membered ring formed through the F


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Accounts of Chemical Research Scheme 8. (a) Synthesis of Peri-Substituted [8]Circulenes 45a and 45b; (b) Syntheses of Tetrabenzo[8]circulene (46)

Scheme 9. (a) Synthesis of Nanographenes 47a and 47b; (b) Structures of the Incompletely Cyclized Products of the Scholl Reaction of 57a

Scholl reaction of macrocycle 49, which was prepared by Suzuki coupling of o-terphenylboronic ester 48 and odibromobenzene. The outward approach employed the double Diels−Alder reaction of sulfoxide 50a with Sondheimer− Wong diyne 51 to form tetraphenylene 52a and subsequent intramolecular Pd-catalyzed arylation to obtain the framework of 46. Two years later, Whalley and co-workers modified the outward approach by synthesizing 46 through the Scholl reaction of 52b in a higher yield.32 In 2017 we synthesized 47a and 47b from macrocyclic precursors 57a and 57b, respectively,33 using an inward strategy similar to Susuki’s approach to 46, as shown in Scheme 9. To synthesize 57a, macrocyclic diene 54 was prepared by Suzuki coupling of boronic acid ester 53 with 1,2dibromo-4,5-dimethoxybenzene. Bromination of 54 followed by elimination of HBr resulted in macrocyclic diyne 55, whose strained carbon−carbon triple bonds reacted as dienophiles in the twofold Diels−Alder reaction with cyclopentadienone 56 to afford 57a. To tune the solubility of the final product, 57a was converted to 57b by replacing the methyl groups with

propyl groups. The Scholl reactions of 57a and 57b under strongly acidic conditions (CF3SO3H) with DDQ as the oxidant at 0 °C gave 47a and 47b, respectively, with the formation of 14 carbon−carbon bonds. The low yield of 47a (18%) can be attributed to the formation of two incompletely cyclized products 58 and 59, which were isolated and unambiguously identified. Treatment of 58 or 59 with DDQ and CF3SO3H essentially did not afford 47a. This suggests that the eight-membered ring in 47a is formed at an early stage in the sequence of forming 14 carbon−carbon bonds. G


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4. STRUCTURES, STEREOCHEMICAL DYNAMICS, AND PROPERTIES Negatively curved nanographenes are attractive structures not only because the structural information contained in these molecules sheds light on imaginary carbon schwarzites but also because they usually have flexible polycyclic frameworks associated with stereochemical dynamics that is not available for planar polycyclic aromatics. Moreover, as a result of embedding heptagons or octagons in a graphitic framework, negatively curved nanographenes often exhibit severe out-ofplane deformation of benzenoid rings, which provides new insight into the limit of π bonds and the aromaticity of polycycles. Most of the structural details of negatively curved nanographenes to date have been disclosed by single-crystal Xray crystallography, while the study of stereochemical dynamics has largely relied on calculations. [7]Circulene (1) is found to exist as a pair of enantiomers in crystals and has a crystallographic twofold symmetry with a C2 axis of symmetry,17 as shown in Figure 7a. In agreement with

Figure 8. Calculated enantiomerization pathway of C2-14.

with an energy barrier of 12.2 kcal/mol, which indicates fast interconversion (7.1 × 103 s−1) at room temperature. The polycyclic framework of 23 in crystals is shaped like a saddle with approximate Cs symmetry,26 as shown in Figure 9a.

Figure 7. Crystal structures of (a) 1 and (b) 14 with the C2 axes.

the crystal structure, the most stable conformer of 1 is calculated to have a C2-symmetric geometry. The stereochemical pathway for enantiomerization of C2-1 has not been conclusively established yet.1 Calculations with the selfconsistent field method showed that the enantiomerization of C2-1 occurs via a planar D7h-symmetric transition state with an energy barrier of 8.5 kcal/mol.34 In contrast, a more recent theoretical study indicated that the enantiomerization of C2-1 occurs through a Cs-symmetric structure that is slightly higher in Gibbs free energy than the C2 conformer by 0.11 kcal/mol, as calculated at the B3LYP/6-31G(d,p) level of DFT.35 This enantiomerization process of 1 can be viewed as a part of pseudorotation, which involves a continuous wavelike motion of the seven hexagons around the heptagon.1 Similar to 1, tetrabenzo[7]circulene (14) in crystals exists as a pair of enantiomers that are shaped like a twisted saddle with approximate C2 symmetry, as depicted in Figure 7b.23 The [7]circulene moiety in 14 is more curved than 1 itself, presumably as a result of the additional crowdedness introduced by its two [4]helicene moieties. In agreement with the crystal structure, DFT calculations at the B3LYP/631G(d,p) level revealed a chiral C2-symmetric structure at the global minimum and an achiral Cs-symmetric structure at a local minimum that is higher in Gibbs free energy than C2-14 by 7.6 kcal/mol. Figure 8 shows the calculated enantiomerization pathway of C2-14, which involves Cs-14 as an intermediate and two transition states, (P)- and (M)-TS, a pair of enantiomers. This pathway does not involve planarization of the [7]circulene moiety but happens through a continuous wave of the seven hexagons progressing around the heptagon

Figure 9. Structures of (a) 23, (b) 27b, (c) 30, and (d) 31 with the hexyl groups removed for clarity.

With two heptagons embedded in the HBC skeleton, the π backbone of 27b in crystals has a saddle-shaped geometry with approximate C2v symmetry.27 This saddle is 10.6 Å wide and 3.5 Å deep in the upper part, as shown in Figure 9b, and is 7.9 Å wide and 3.0 Å deep in the lower part. Similarly, the π backbone of 30 in crystals is shaped like a saddle with approximate C2v symmetry27 but is wider and deeper than that in 27b in the upper part as a result of π extension, as shown in Figure 9c. The geometry of 30 is dominated by the two heptagons, which induce negative curvature, while each pentagon induces a very shallow bowl-shaped geometry around itself. As found from the crystal structure, 31 has an even more curved π face (Figure 9d) than 30 because the two [4]helicene moieties in 31 induce extra contortion in its polycyclic framework.27 Computational studies on the stereochemical dynamics of 23 and 30 were not described in the original reports26,27 but are disclosed here for the first time. To reduce the computational cost, simplified molecules 23′ and 30′ with four methyl groups replacing the n-hexyl groups in 23 and 30, respectively, were calculated at the B3LYP/6-31G(d) level of DFT. The global minimum structure of 23′ has a saddleshaped polycyclic framework, which is essentially the same as that of 23 in the crystals. As found from the calculations, the H


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The π backbone of 38 in crystals is shaped like a saddle with approximate C2v symmetry that is 14.8 Å wide and 6.0 Å deep,29 as shown in Figure 11a. One striking structural feature

polycyclic framework of 23′ inverts itself through a twisted transition state (TS-23′ in Figure 10a) with an energy barrier

Figure 10. Calculated pathways for the saddle-to-saddle inversions of (a) 23′ and (b) 30′.

of 22.4 kcal/mol, which corresponds to a rate constant of 2.4 × 10−4 s−1 at 25 °C as estimated using the Eyring equation. The saddle-to-saddle inversion of 23 in solution was conveniently monitored with variable-temperature 1H NMR spectroscopy since the seven-membered ring in 23 has two chemically different benzylic protons, which interconvert during the inversion process. When 23 is heated from room temperature to 150 °C, these benzylic protons exhibited a pair of wellresolved doublet signals without movement and broadening of the signals. This suggests that the saddle-to-saddle inversion of 23 is a slow process on the NMR time scale with an activation free energy higher than 20.1 kcal/mol, in agreement with the DFT-calculated result. The relatively rigid polycyclic framework of 23 may be attributed to the sp3 carbon in the sevenmembered ring, which interrupts the continuous wavelike motion of the benzenoid rings around the seven-membered ring. Similarly, the inversion of [1,2]dihydro[7]circulene is calculated to have a higher energy barrier than that of [7]circulene.1 Nanographene 30′ is calculated to have a much more flexible π backbone, which inverts itself through a twisted transition state (TS-30′) with a significantly lower energy barrier (13.3 kcal/mol), as shown in Figure 10b. In agreement with this, 30 exhibits stronger fluorescence in the solid state than in solution. This aggregation-enhanced emission phenomenon can be attributed to the fact that conformational change of the flexible π backbone in solution consumes the energy of the excited state, resulting in radiationless internal conversion, whereas the restriction of intramolecular motions in the solid state enhances the photoluminescence.

Figure 11. (a) Crystal structure of 38 with tert-butyl groups removed. (b) The central naphthalene moiety of 38 in crystals. (c) Calculated pathway from saddle-38 to twisted-38.

of 38 is the central naphthalene moiety (displayed in red in Figure 11a,b) exhibiting a large out-of-plane deformation, which is measured as 77.2° from the corresponding dihedral angles, as shown in Figure 11b. This severe deformation, to the best of our knowledge, is the greatest bending of naphthalene moieties in conjugated polycyclic aromatics. Molecular geometry optimization at the B3LYP/6-31G level of DFT revealed three stable conformers, saddle-38 at the global energy minimum and twisted-38 and twisted saddle-38 at local energy minima, as shown in Figure 11c. Saddle-38 with C2v symmetry is the dominant conformer and has essentially the same structure as 38 in the crystals with minor differences in the width and depth of the saddle. As shown in Figure 11c, the calculated pathway from saddle-38 to twisted-38 involves movement of tert-butyl groups. Saddle-38 first converts into twisted saddle-38 through transition state TS1-38, accompanied by upward movement of one tert-butyl group (shown in green) attached to the central tetrabenzodipleiadiene moiety. Then twisted saddle-38 converts into twisted-38 through transition state TS2-38, accompanied by upward movement of another tert-butyl group (shown in green). To complete the I


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Accounts of Chemical Research saddle-to-saddle inversion, twisted-38 can further convert into saddle-38 through two transition states that are enantiomers of TS1-38 and TS2-38, accompanied by upward movement of another two tert-butyl groups. The highest point on the pathway of saddle inversion has a relative Gibbs free energy of 16.0 kcal/mol, which corresponds to a rate constant of 11.6 s−1 at room temperature. This indicates that the saddle inversion of 38 is slower than that of 30. Further calculations on nanographene C86H32, the parent molecule of 38 without tertbutyl substituents, indicated that attachment of tert-butyl groups to the central tetrabenzodipleiadiene moiety of C86H32 stabilizes the saddle conformation and makes this nanographene less flexible. This finding has received support from experiments on debutylation of 38, and the effect of the tertbutyl groups is attributable to the atom crowding around them, which is less severe in the saddle conformation. As found from the crystal structure of 39a (Figure 12),13 its geometry is largely determined by the five heptagons, which

Figure 13. Structures of (a) 45a and (b) 46 in crystals.

molecule 45a′ having eight phenyl groups replacing the tolyl groups in 45a was calculated at the ωB97X-D/6-31G** level of DFT.36 The saddle-to-saddle inversion of 45a′ was found to occur through pseudorotation that resembles the motion of a running wave around the molecular periphery with a calculated energy barrier of 16.3 kcal/mol. The 1H NMR signals of the xylyl groups in 45b were well-resolved at 303 K but merged into two broad bands at 433 K. On the basis of a coalescence temperature slightly higher than 443 K, the energy barrier for the saddle-to-saddle inversion was estimated to be 20.7 kcal/ mol, which is higher than the value calculated for 45a′ since the methyl groups in the m-xylyl substituents in 45b provide additional steric influence. Single crystals of tetrabenzo[8]circulene (46) contain two conformers in a 1:1 ratio. The conformers both are shaped like a deep saddle with S4 symmetry but differ in the torsion angle (α) between the C1−C2 and C5−C6 bonds in the central cyclooctatetraene moiety.15 Figure 13b shows one conformer of 46 with α = 31.9°. Although the S4 symmetry should give six signals in the 1H NMR spectrum, only three signals (in agreement with a D2d symmetry were observed in solution, suggesting that 46 is a rather flexible molecule. As found from DFT calculations at the B3LYP/6-31(d) level, 46 has two D2dsymmetric saddle-shaped conformers, LM-46 and GM-46, at the local energy minimum and the global energy minimum, respectively. The S4-symmetric structures found in the crystals are close to the structure of LM-46, with similar bond lengths and angles in the eight-membered ring, although LM-46 is less stable than GM-46 by 5.1 kcal/mol. The saddle-to-saddle inversion of GM-46 occurs via a calculated pathway that involves LM-46 as the intermediate and a pair of S4-symmetric transition states with an activation energy of 7.3 kcal/mol. This pathway can be viewed as part of a pseudorotation. GM-46 is the structure in solution but not in the solid state as a result of the crystal-packing force and low-energy pseudorotation. The crystal of 47b is found to contain a pair of enantiomers having a twisted ribbonlike π backbone with approximate D2 symmetry,33 as shown in Figure 14a. The polycyclic framework of 47b is 2.03 nm long along the C2-x axis and twists along two directions. The end-to-end twists are 142.4° and 140.2° along the C2-x axis and the C2-y axis, respectively, as measured from the torsion angles defined by the corresponding terminal carbon atoms. The central [8]circulene moiety in 47b in crystals is shaped like a twisted saddle presenting negative curvature. To study the dynamic behaviors of 47a and 47b at lower computational cost, a simplified molecule 47a′ with eight methyl groups replacing the tert-butyl groups in 47a was calculated at the B3LYP/6-31G(d) level of DFT. The calculations revealed twisted-47a′ as the global energy

Figure 12. Structure of the PMPMP conformer of 39a in crystals.

induce negative curvature in a unique double-concave structure; the pentagon induces small local positive curvature in the central corannulene moiety, which adopts a shallow bowl-shaped geometry with a bowl depth of 0.37 Å. Each seven-membered ring in 39a is surrounded by six benzenoid rings in a hexa[7]circulene moiety. With the five hexa[7]circulene moieties having either P or M chirality, 39a exists as a pair of enantiomers with PMPMP and MPMPM configurations in the crystals. However, 39a is conformationally flexible and racemizes rapidly in solution, as indicated by 1H NMR spectroscopy. DFT calculations at the B3LYP/6-31G(d) level indicated that 39a processes two different dynamic behaviors, a bowl-to-bowl inversion and enantiomerization. The energy barrier calculated for the bowl-to-bowl inversion of the central corannulene is as low as 1.7 kcal/mol because of the shallowness of the bowl in 39a. The calculated energy barrier for enantiomerization between the PMPMP and MPMPM conformers is 18.9 kcal/mol. As evidence for the calculated flexible framework, rapid equalization of the 10 benzo groups on the perimeter has been observed in the 1H NMR spectrum of 39a. The [8]circulene framework of 45a is shaped like a saddle in crystals with approximate D2d symmetry,14 as shown in Figure 13a. The central eight-membered ring in 45a has a tub-shaped structure in which all of the bond lengths (approximately 1.45 Å) and bond angles (around 125°) are almost identical. To study the dynamic behaviors of 45a and 45b, a simplified J


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effect mobility (10−5 to 10−4 cm2 V−1 s−1), which is attributable to the amorphous nature of the films.23,27

5. CONCLUSION AND OUTLOOK This Account has summarized the recent progress in the chemistry of negatively curved nanographenes with emphasis placed on our own efforts. Just as the emergence of aromatic bowls led to a rational, controlled synthesis of C60 and opened the field of geodesic polycyclic aromatics, the preliminary success in the studies of negatively curved nanographenes can be regarded as an important initial step toward the programmable synthesis of negatively curved carbon allotropes. We envision that one of the next steps toward negatively curved carbons will be the synthesis of negatively curved carbon nanobelts and nanocages containing heptagons or octagons. We have made preliminary efforts in this direction by synthesizing the sp2 carbon nanoring C50H28, which contains two heptagons and one octagon, as shown in Figure 15.37

Figure 15. Crystal structure of C50H28, a carbon nanoring containing two heptagons (red) and one octagon (orange). Figure 14. (a) Crystal structure of 47b with substituent groups removed. (b) Calculated pathway from twisted-47a′ to saddle-47a′.

High-quality graphene has been grown by reactions of flat polycyclic arenes on hot copper foil,38 and carbon nanotubes have been synthesized using cycloparaphenylenes as templates and ethanol as the carbon source.39 In view of these successful bottom-up syntheses, negatively curved carbon allotropes might be synthesized in a similar manner using the negatively curved nanographenes, carbon nanobelts, and carbon nanocages as monomer units or templates. Success in this direction will greatly expand the frontiers of carbon nanoscience.

minimum and saddle-47a′ as the local energy minimum, as shown in Figure 14b. The polycyclic framework of twisted47a′ with D2 symmetry is essentially the same as that of 47b in crystals. On the other hand, saddle-47a′ is shaped like a saddle with C2v symmetry. Twisted-47a′ is more stable than saddle47a′ by 4.1 kcal/mol. The interconversion of twisted-47a′ and saddle-47a′ occurs via a continuous wavelike motion of the eight hexagons around the central octagon, and the transition state for this process is TS-47a′, which is shaped as a twisted saddle with C2 symmetry, as shown in Figure 14b. The energy barrier from twisted-47a′ to TS-47a′ is as small as 6.2 kcal/ mol, which is slightly lower than that of 46 and indicates a highly flexible polycyclic framework. In agreement with the calculated results, 47a exhibits aggregation-induced emission due to restriction of intramolecular motions in the aggregated states. As a result of their flexible polycyclic frameworks, negatively curved nanographenes usually exhibit higher solubility than planar polycycyclic aromatics with similar size. The dissolution of negatively curved nanographenes is an entropy-favored process since these flexible molecules in solution can adopt many conformations that are frozen in the solid state. Sufficient solubility has allowed negatively curved nanographenes to be purified by crystallization from solution and characterized by X-ray crystallography on solution-grown single crystals. Moreover, 14 and 30 have been found to function as p-type organic semiconductors in thin-film transistors with low field-

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Qian Miao: 0000-0001-9933-6548 Notes

The authors declare no competing financial interest. Biographies Sai Ho Pun was born in Hong Kong in 1993. He received his Bachelor’s degree in chemistry from the Chinese University of Hong Kong in 2012. In the same year, he joined the group of Prof. Qian Miao at the same university for Ph.D. studies. His research interest is the synthesis and characterization of negatively curved polycyclic aromatics. K


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(17) Yamamoto, K.; Harada, T.; Nakazaki, M.; Naka, T.; Kai, Y.; Harada, S.; Kasai, N. Synthesis and Characterization of [7]Circulene. J. Am. Chem. Soc. 1983, 105, 7171−7172. (18) The value of the curvature depends on the selected vertex that divides the polygon into triangles. For better estimation of the curvature, the vertex should be chosen so that the sum of the areas of the triangles is minimized, as reported by us earlier (see the Supporting Information for ref 26). Here the geometric center is chosen as the vertex for convenience and does not change the sign of the curvature. (19) Yang, X.; Liu, D.; Miao, Q. Heptagon-Embedded Pentacene: Synthesis, Structures and Thin Film Transistors of Dibenzo[d,d′]benzo[1,2-a:4,5-a′]dicycloheptenes. Angew. Chem., Int. Ed. 2014, 53, 6786−6790. (20) Vogel, E.; Neumann, B.; Klug, W.; Schmickler, H.; Lex, J. Synthesis of Dipleiadiene by “Serendipity”. Angew. Chem., Int. Ed. Engl. 1985, 24, 1046−1048. (21) Yamamoto, K.; Harada, T.; Okamoto, Y.; Chikamatsu, H.; Nakazaki, M.; Kai, Y.; Nakao, T.; Tanaka, M.; Harada, S.; Kasai, N. Synthesis and Molecular Structure of [7]Circulene. J. Am. Chem. Soc. 1988, 110, 3578−3584. (22) Yamamoto, K.; Saitho, Y.; Iwaki, D.; Ooka, T. [7.7]Circulene, a Molecule Shaped like a Figure of Eight. Angew. Chem., Int. Ed. Engl. 1991, 30, 1173−1174. (23) Gu, X.; Li, H.; Shan, B.; Liu, Z.; Miao, Q. Synthesis, Structure, and Properties of Tetrabenzo[7]circulene. Org. Lett. 2017, 19, 2246− 2249. (24) Yang, X.; Miao, Q. Studies toward the Synthesis of Hepta-periheptabenzo[7]Circulene. Synlett 2016, 27, 2091−2094. (25) King, B. T.; Kroulik, J.; Robertson, C. R.; Rempala, P.; Hilton, C. L.; Korinek, J. D.; Gortari, L. M. J. Org. Chem. 2007, 72, 2279. (26) Luo, J.; Xu, X.; Mao, R.; Miao, Q. Curved Polycyclic Aromatic Molecules That Are π-Isoelectronic to Hexabenzocoronene. J. Am. Chem. Soc. 2012, 134, 13796−13803. (27) Cheung, K. Y.; Xu, X.; Miao, Q. Aromatic Saddles Containing Two Heptagons. J. Am. Chem. Soc. 2015, 137, 3910−3914. (28) Márquez, I. R.; Fuentes, N.; Cruz, C. M.; Puente-Muñoz, V.; Sotorrios, L.; Marcos, M. L.; Choquesillo-Lazarte, D.; Biel, B.; Crovetto, L.; Gómez-Bengoa, E.; González, T.; Martin, R.; Cuerva, J. M.; Campañ a, A. G. Versatile Synthesis and Enlargement of Functionalized Distorted Heptagon-Containing Nanographenes. Chem. Sci. 2017, 8, 1068−1074. (29) Pun, S. H.; Chan, C. K.; Luo, J.; Liu, Z.; Miao, Q. A Dipleiadiene-Embedded Aromatic Saddle Consisting of 86 Carbon Atoms. Angew. Chem., Int. Ed. 2018, 57, 1581−1586. (30) Salcedo, R.; Sansores, L. E.; Picazo, A.; Sansón, L. [8]Circulene. Theoretical Approach. J. Mol. Struct.: THEOCHEM 2004, 678, 211− 215. (31) Miller, R. W.; Duncan, A. K.; Schneebeli, S. T.; Gray, D. L.; Whalley, A. C. Synthesis and Structural Data of Tetrabenzo[8]Circulene. Chem. - Eur. J. 2014, 20, 3705−3711. (32) Miller, R. W.; Averill, S. E.; Van Wyck, S. J.; Whalley, A. C. General Method for the Synthesis of Functionalized Tetrabenzo[8]Circulenes. J. Org. Chem. 2016, 81, 12001−12005. (33) Cheung, K. Y.; Chan, C. K.; Liu, Z.; Miao, Q. A Twisted Nanographene Consisting of 96 Carbon Atoms. Angew. Chem., Int. Ed. 2017, 56, 9003−9007. (34) Shen, M.; Ignatyev, I. S.; Xie, Y.; Schaefer, H. F., III. [7]Circulene: A Remarkably Floppy Polycyclic Aromatic C28H14 Isomer. J. Phys. Chem. 1993, 97, 3212−3216. (35) Hatanaka, M. Puckering Energetics and Optical Activities of [7]Circulene Conformers. J. Phys. Chem. A 2016, 120, 1074−1083. (36) Feng, C.-N.; Hsu, W.-C.; Li, J.-Y.; Kuo, M.-Y.; Wu, Y.-T. PerSubstituted [8]Circulene and Its Non-Planar Fragments: Synthesis, Structural Analysis, and Properties. Chem. - Eur. J. 2016, 22, 9198− 9208. (37) Cheung, K. Y.; Yang, S.; Miao, Q. From Tetrabenzoheptafulvalene to sp2 Carbon Nano-rings. Org. Chem. Front. 2017, 4, 699− 703.

Qian Miao was born in 1977 in Chengdu, China, and graduated from the University of Science and Technology of China with a B.Sc. degree in 2000. He received his Ph.D. degree from Columbia University in 2005 under the direction of Prof. Colin Nuckolls and was a postdoctoral scholar with Prof. Fred Wudl at the University of California, Los Angeles. He joined the Chinese University of Hong Kong as an Assistant Professor in 2006 and was promoted to Associate Professor in 2012 and then Professor in 2016. His research interests include the design and synthesis of novel polycyclic aromatic molecules with interesting structures and useful applications and the development of high-performance organic semiconductor materials and devices.

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ACKNOWLEDGMENTS We thank all of the co-workers for their contributions to the chemistry described in this Account. We also thank Chi Kit Chan and Prof. Zhifeng Liu for the DFT calculations on 23′ and 30′. This work was supported by the Research Grants Council of Hong Kong (CRF C4030-14G).

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Toward Negatively Curved Carbons - Accounts of Chemical Research

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