1. Introduction

The polycyclic aromatic hydrocarbons, named as PAHs, are generated by linear fusion and two-dimensional growth of benzene as shown in Scheme 1. Last decades have witnessed that PAHs show excellent optical and electrical properties, therefore PAHs have been widely applied to fabricate various organic electronics such as OLED, OFET, and so on ^[1]. Concerning effective intermolecular π-orbital overlaps, PAHs with planar shape and large p-conjugated system are required. However, the extension of p-conjugation of PAHs may decrease the chemical stability, which would limit the fabrication of organic electronics under the ambient conditions. For example, hexacene ^[2] and heptacene ^[3] show diradical feature; therefore, they can react with oxygen to lose the continuous conjugation. In this regard, the modulation of electronic states of PAHs is highly desirable. An effective way toward this purpose is proper lowering the aromaticity of PAHs, which can be achieved by two strategies ^[4]: (i) introducing heteroatoms into the π-conjugated system; (ii) imbedding nonhexagonal rings into hexagonal sheets. The latter may result in PAHs with non-planar π-surface.

While most of attentions have been paid to PAHs having planar p-conjugated system, the non-planar PAHs seem amazing due to their unique physical, chemical, and assembling features ^[5]. The non-planar PAHs have been created with the geometries of bracelet ^[6], saddle ^[7], bowl ^[8], MÖbius band ^[9], helicenes ^[10], and so on. To date, most of the curved PAHs are generated by embedding non-hexagonal rings into the hexagonal sheets. Fullerenes, discovered in 1985 ^[11], are typical molecules with curved conjugation and they possess the electron-deficient convex surface. Fullerenes have been widely employed as electron acceptor in OPV ^[12] and conducting component in organic superconductor ^[13]. The extraordinary properties of fullerenes stimulate intensive studies on PAHs with curved π-electron system. These studies are not only limited to the “closed” system, but also directed to the “open” system. In comparison with the “closed” one, the “open” system shows various structures and properties that can be modulated by chemical functionalization.

The bowl-shaped PAHs, also named as bucky bowls or π-bowls, are representative PAHs with open curved π-surface. Bucky bowls are mainly derived from two representative fragments of C₆₀, corannulene (1) and sumanene (2), as shown in Scheme 2. Bucky bowls have concave and convex surfaces with different distribution of π-electron cloud ^[14]. Consequently, they show distinct properties and assembly features as compared with fullerenes. Recently, bucky bowls are of growing interest owing to their multidiscipline applications. They have been employed as synthetic intermediates for C₆₀ ^[15], end-cap of carbon nanotube ^[16], warped nanographene ^[17], and so on. They can coordinate with various metal ions ^[18] and encapsulate fullerenes ^[19]. Recently, bucky bowls have been applied to fabricate electronic devices, e.g., the ambipolar OFET ^[20]. While bucky bowls are rich in chemistry and material science, the key issue in this field lies on the development of practical synthetic strategies, because the synthesis of bucky bowls are challenging due to the high ring strain energy. In this short account, we focus on the synthesis of the representative bowl-shaped conjugated polycycles. As for the structure and property of compounds herein and their analogues, one may find in the literatures ^[8].

2. Corannulene and the representative derivatives

Corannulene (1) is a representative fragment of C₆₀ with C5v symmetry. Chemically speaking, corannulene is constructed by fusion of five benzene rings onto the edges of cyclopentane, therefore shows large ring strain energy. Corannulene was firstly synthesized by Lawton and Barth in 1966 ^[21], two decades earlier than the discovery of C₆₀. However, this synthesis suffers from the tedious route (17 linear steps) and low yields (overall yield ＜1%). Consequently, the further research on 1 is limited since then. In 1991, Scott’s group disclosed a much practical synthetic route as shown in Scheme 3. This synthesis is inspired by the early reports of Brown, i.e., the terminal alkynes isomerize reversibly under flash vacuum pyrolysis (FVP) condition by 1, 2-shifts of hydrogen atom and the resulting vinylidenes can be trapped by insertion into nearby C-H bonds ^[22]. By employing FVP as the key technique, Scott and co-workers successfully made the ring closure of compound 3 at 1000 ℃ to form 1 in 10% yield ^[23]. However, compound 3 shows strong tendency to polymerization rather than isomerization during heating at high temperature, which leads to the low production yield of 1. To solve this issue, Scott’s group selected various precursors of 1, and they found the best one was 4, a “masked” version of 3. Compound 4 sublimes cleanly without polymerization and loses 2 mol of HCl in the hot zone to generate 3 in situ, which then cyclizes twice to afford 1 in 35%-40% yield ^[24]. This breakthrough sparks many groups to join in this field.

While Scott’s FVP strategy has been successfully employed to create a variety of PAHs with bowl-shaped conjugation ^[8c], the FVP process cannot be easily handled. To avoid the bimolecular reaction in gas phase, the slow sublimation of precursor in reasonable period (usually 10-30 min) is highly required; thus the reaction is mainly run in small scale. Moreover, the precursor should be sufficiently robust during sublimation into gas phase, say, the FVP strategy shows poor tolerance of substrate. In this regard, the synthesis of 1 under mild synthetic conditions (e.g., in solution phase) became an important goal. In 1996, Siegel and coworkers reported the first solution phase synthesis of corannulene derivative ^[25]. Inspired by this progress, Siegel ^[26] and Rabideau ^[27] groups independently disclosed the non-pyrolytic synthesis of the pristine corannulene as shown in Scheme 4. Compound 5 is tetrabrominated with NBS under forcing condition to produce 6, which undergoes ring closure in the presence of the reduction agents (Condition A) to afford 1 in 70%-80% yield. This synthesis is further refined by Sygula and Rabideau (Condition B), and leads to the large-scale production of 1 ^[28]. Recently, Siegel’s group has reported the kilogram-scale synthesis of 1 ^[29], which significantly promotes the application of this peculiar molecule in multiple terminals. To further modulate the electronic structure and explore novel PAHs with curved π-surface, the functionalization of 1 has been performed ^[8a-c,8f]. In the following, we will introduce several representative examples in this area.

The fusion of 1, 4-benzodithiin rings onto the flanking benzene of 1 is achieved in two steps as shown in Scheme 5. Firstly, corannulene is transformed into decachlorocorannulene (7) in 60% yield ^[30]. In the second step, a nucleophilic aromatic reaction of compound 7 with sodium benzene-1, 2-dithiolate results in the 1, 4-benzodithiin-fused corannulene (8) in 50% yield ^[31]. X-ray crystal structure analysis reveals that 8 adopts ‘fly trap’ type conformation. Compound 8 is electron rich (π-donor) owing to the decoration of ten sulfur atoms on the flanking benzenes. On the other hand, the convex surface of C₆₀ is electron deficient (π-acceptor). Thus, compound 8 has the priority, in both electronic states and shape complementary, to encapsulate fullerene molecules. In fact, Scott’s group successfully prepared the donor-acceptor type supramolecular complexes between 8 and C₆₀/C₇₀, and the donor: acceptor ratio was 1: 1 as testified by the ¹ HNMR titration ^[32].

To gain corannulene-based organic semiconductors, Pei’s group synthesized the imide-fused corannulene derivative (13) in four linear steps with an overall yield of 50% (Scheme 6) ^[20].his modification leads to two substantial effects.irstly, the molecular dipole moment, both the magnitude and direction, is distinctly changed. Consequently, compound 13 forms ordered columnar stack with the smallest π-π stack distance (3.39Å) among corannulene derivatives. Secondly, the HOMO energy level of 13 remains almost the same to that of 1, whereas the LUMO energy level is clearly lowered owing to the decoration of imide group.herefore, compound 13 has a HOMO-LUMO energy gap narrower than that of 1.urther study indicates that 13 shows ambipolar transport property with the hole and electron mobilities of 0.05 cm² V^-1 s^-1 and 0.02 cm² V^-1 s^-1, respectively ^[20]. Very recently, Pei and co-workers have synthesized corannulene derivatives 14-18. Compound 14 is a p-type organic semiconductor ^[33], 16 is an n-type organic semiconductor ^[34], and 17 and 18 can serve as the non-fullerene organic acceptors to fabricate OPV ^[35].

Despite of peripheral functionalization, doping of heteroatoms into backbone of corannulene is promising as the heteroatoms would bring distinct influence on packing motifs and band structures. However, the heteroatom doping of corannulene is very challenging.to, okimaru, and Nozaki reported the first heteracorannulnene by replacing the sp² carbon on the fivemembered ring with nitrogen atom ^[36]. However, the simple replacement of any sp² carbon on the pristine corannulene with nitrogen atom would reduce the chemical stability, i.e., formation of cationic species by oxygen.n this regard, the replacement of sp² carbon with nitrogen as well as the fusion of benzene rings on the rim of corannulnene is simultaneously carried out as shown in Scheme 7. Employing compound 19 as starting material, the 1, 3- dipolar cycloaddition with diarylethyne (20) results in compound1 21, which then undergoes the palladium-catalyzed intramolecular cyclization to afford compound 22, a nitrogen-doped corannulene with the extended conjugation.his modification brings significant influence on both the structural and electronic features.or example, compound 22 is a much deeper π-bowl and displays distinctly red-shifted and much strong fluorescence, as compared with the pristine corannulene.

3. Sumanene and the representative derivatives

In comparison with corannulene (1), sumanene (2) is a more strained bucky bowl due to embedding three pentagonal rings into the hexagonal sheets. Hence, the synthesis of 2 is highly challenging because of the large strain energy. Mehta and coworkers tried the synthesis of 2 from tris(bromomethyl)triphenylene through FVP, whereas only the mono-cyclized and dicyclized products were obtained ^[37].he first successful synthesis of was accomplished by Hirao and co-workers in 2003 ^[38]. As shown in Scheme 8, trimerization of norbornadiene (23) afforded the mixture of 25 (syn-form) and 26 (trans-from) in 7% yield (Condition A). On the other hand, the stepwise transmetallation via an organotin 24 increases the yield of 25/26 mixture to 47% (Conditions B and C). Alkene-bridge exchange of the syn-form 25 affords 27 in 30% yield via Ru-catalyzed tandem metathesis reaction under an ethylene atmosphere.n contrast, the reaction of trans-form6 under the same condition does not give the desired compound 27.inally, the dehydrogenation of 27 afforded in 70% yield.his progress inspires the further studies on sumanene including peripheral functionalization, coordination with metal ions, and application in electronic materials; one may find these topics in a recent personal account of Hirao’s group ^[8g].

Further modification on sumanene is directed to the introduction of heteroatoms into its backbone, which results in the heterasumanenes ^[39].he heterasumanenes can be achieved by two strategies: (i) replacing sp² carbons on the flanking benzenes with heteroatoms and (ii) replacing the benzylic carbons with heteroatoms.In 2013, Sakurai group reported the synthesis of triazasumanene (32) with three flanking benzenes replaced by pyridines as shown in Scheme 9 ^[40].he Pd-catalyzed cyclotrimerization of enantiopure 28 exclusively afforded the syn-form 29, which was hydrolysed, followed by condensation, to afford bowl-shaped lactam 30. Because the direct transformation of 30 into 32 was unsuccessful, 30 was then converted to thioimidate 31 in three steps.n the final step, 31 was dehydrogenated by employing Ph₃CBF₄ as oxidant and a nonnucleophilic baseTBMP as acid scavenger.he overall yield of this six-step synthesis for 32 is 16%. Compound 32 has a bowl-tobowl inversion energy (42.2 kcal mol^-1)much higher than that of 2(20.3 kcal mol^-1), therefore this compound shows very stable chirality.

The heterasumanenes derived from 2 by replacing benzylic carbons with heteroatoms are promising, because the electronic structures of the five-membered rings differ from each other in varying heteroatoms. A rational approach toward such heterasumanenes is to build three heteroatom bridges at the bay regions of triphenylene. However, theoretical investigations suggest it would be challenging to synthesize curved heterasumanenes from planar triphenylene due to large strain energy for ring cyclization ^[41]. On the other hand, heterasumanenes with flat backbone can be accessed through this strategy. Employing a triphenylene analogue HBT (33) as starting material, Kawashima and co-workers synthesized trisilasumanene (38), which is of planar conjugation ^[42]. As shown in Scheme 10, the ring cyclization was achieved step-wisely, taking the advantage of sila-Friedel-Crafts reaction on the benzene ring developed by the same group. However, this synthesis suffers from the low efficiency of sila-Friedel-Crafts reaction, i.e., the total yield for this five-step synthesis is about 0.7%. Compound 38 shows strong blue emission in both solution and solid state, plausibly ascribable to the unique photophysical nature of the silole rings ^[43].

Saito’s group disclosed the synthesis of group 14 elements (Si, Ge, Sn) doped sumanenes without substituents on the flanking benzenes ^[39b]. Scheme 11 shows the synthesis of trigermasumanene (42) from triphenylene (39), as a representative example for this series. Unlike HBT (33) which can be fully lithiated on the bay regions, the selective and full lithiation on bay positions of 39 is impossible, and the rim positions are partially lithiated simultaneously.herefore, the construction of three germanium bridges at the bay regions of 39 is achieved one by one. While the reagents are same for each step, one should be careful about the amount of n- BuLi in the lithiation step and the reaction temperature for the insertion of germanium. Employing the similar strategy, Saito and co-workers prepared heterasumanenes (43-45) containing multiple heteroatoms ^[44], whereas an attempt to create tristannosumanene (Sn-doped) failed to afford the desired one. Recently, Saito’s group succeeded the synthesis of compound 46, a phosphorus doped sumanene derivative ^[45].

Among heterasumanenes, the sulfur-doped sumanene seems amazing, because sulfur-containing conjugated molecules, such as thiophene and tetrathiafulvalene (TTF), show excellent optical and electrical properties. Moreover, sulfur atom can form various van der Waals contacts in solid state and has different valence states that can be chemically modulated.rithiasumanene can be derived from sumanene by replacing benzylic carbons with sulfur. Compared with sumanene, five-membered rings in trithiasumanene are of aromaticity, and the molecule is changed from electron-deficient to electron-rich by replacing Cp ring with thiopehene.n chemcial sense, trithiasumanene can also be derived from coronene by replacing three flanking benzene rings with thiophene rings. Although aromaticity remains, the conjugated system changes from planar to bowl-shaped.rithiasumanene consists of two subunits, benzotrithiophene (47) and triphenylene (39).he retrosynthetic analysis thus provides two plausible synthetic routes: Routes A and B start from compounds 47 and 39, respectively.heoretical study shows that route A is much better than route B, because the latter has large ring strain energy ^[41].

In 1999, Otsubo’s group successfully synthesized trithiasumanene (54) through route A ^[46], which was even earlier than the theoretical study. Employing 47 as starting material, Otusbo and co-workers synthesized 54 in four linear steps as shown in Scheme 12.he key step for this synthesis is the ring closure of compounds 52 and 53 under FVP condition (1000 ℃, 0.005 Torr, N₂ flow). Consequently, the synthesis suffers from low efficiency (overall yield, 6%) and small scale production (tens of milligrams). Meanwhile, compound 47 is much more expensive than 39.hus, the key issue in trithiasumanene chemistry lies in (i) development of more practical route and (ii) large scale production. Moreover, the synthetic strategy should be applied to synthesize other types of trichalcogenasumanenes.

In comparison with route A, the route B shows advantage in starting material.he derivatives of 39 are cheaper than those of 47, and can be easily prepared in hundred-gram scale. However, the challenge is that it is thermodynamically unfavorable to construct three sulfur bridges into triphenylene ^[41]. Only the mono- and di-bridged species are formed in trace amount at around 500 ℃ as reported by Klemm ^[47]. While difficult to construct three sulfur bridges at bay positions of triphenylene in one time, it is possible to build up multiple less strained 1, 2-dithiin rings in single step. Aiming at this purpose, Shao’s group disclosed a non-pyrolytic two-step synthesis of trithiasumanene (57) from HBT (33) ^[48].his strategy lies in (i) ring cyclization, fusion of multiple thiophene and 1, 2-dithiin rings at the bay regions of 33, and (ii) ring contraction, desulfurization of 1, 2-dithiin to form the desired 57. As shown in Scheme 13, in the first step, compound 33 was hexalithiated with n-BuLi at 60 ℃, followed by insertion of sulfur.his one-pot reaction almost quantitatively constructs two 1, 2-dithiin rings and one thiophene ring, i.e., compound 55.n the second step, compound 55 was desulfurized by heating with copper nano powder at 200 ℃ to afford 57 in 30% yield, along with 5% of 56 which can be transformed into 57.his protocol shows advantages in cheaper starting material, mild condition, and multigram scale production.nspired by this progress, Shao and coworkers synthesized triselenasumanene (59).n fact, this strategy is more practical to synthesize 59.n the ring cyclization step, compound 58 was produced in yield of 70%, and was then converted to 59 quantitatively.

Theoretical investigation reveals that the chalcogen atoms (O, S, Se) show important contribution on the frontier orbitals of compounds 57 and 58, particularly on the HOMO orbital.herefore, compounds 57 and 58 have two structural features: (i) electron-rich arises from butoxy groups and chacogenole rings; (2) ring strain from the bowl-shaped structure. An oxidation of 57 and 59 by Oxone resulted in the cleavage of one of the flanking benzene rings (Scheme 14), as reported by Shao’s group ^[49]. On the other hand, HBT (33) is electron-rich without ring strain, and sumanene (2) is a more strained but electron-deficient.he oxidation of 33 and 2 by Oxone did not afford ring-opening products.herefore, this peculiar reaction is driven by the cooperative effect of the electron-rich nature and ring strain. On the basis of the ring-opening products (61 and 62), Shao and coworkers prepared a series of ^[5-6]-fused system (compounds 64-66) through facile transformations.he central ^[5-6]-fused frameworks of 64-66 are planar due to ring expansion. Compounds 64-66 form the head-to-tail type columnar stacks with the π-π stack distance of 3.50Å. Compared with 57 and 59, compounds 64-66 display significantly red-shifted absorbance and emission, and the fluorescence quantum yield up to 40%.

4. Concaved conjugated polycycles other than bucky bowls

Apart from the derivation of fullerene fragments (corannulene and sumanene), the bowl-shaped conjugated molecules can also be created by replacing the sp² carbon on benzene-based twodimensional sheet with multiple heteroatoms.his is because the bond lengths of C=C and C-X (X = N, O, P, S) are different. Additionally, heteroatoms have lone pair electrons on the p-orbitals, therefore they tend to take tetrahedron geometry rather than the flat one even if the p-orbital electrons can involve in the conjugated framework. A typical example is triphenylamine, which adopts a Pyramid-like geometry.n the following, we will introduce several typical examples for this series.

In 1997, Krebs and co-workers reported a concaved heterocycle (TOTPP, 72) based on the triphenyphosphine skeleton ^[50]. As shown in Scheme 15, they synthesized 72 in four linear steps with the final step being an intramolecular nucleophilic aromatic reaction of compound 71.he phosphorus atom on 72 was further oxidized by H₂O₂ to form theOTPPO (73) in 63% yield. Crystallographic study shows that 72 takes a bowl-shaped conformation and forms the polarized stacks in solid state.heoretical investigation reveals that compound 72 has a dipole moment of 3.3 ± 0.2 with a direction along the concave face. convex face. Consequently, the crystals of 72 exhibit pyroelectric property with pyroelectric coefficient of -3 µC m^-2 K^-1 -1 at room temperature.

Inspired by Kreb’s work, Yamamura and co-workers have prepared a series of concaved heterocycles based on triphenyphosphine ^[51]. As a typical example, the synthesis of a sulfur analogue (TSTPP, 76) of 72 is shown in Scheme 16. Employing the same precursor (compound 71) for the synthesis of 72, they synthesized 76 in four linear steps with overall yield of 35% ^[51a].he key step for this synthesis is the Newman-Kwart rearrangement of compound 75. All these compounds so far synthesized by Yamamura and co-workers adopt the bowl-shaped conformation, whereas the bowl depth can be finely tuned by varying the chalcogen atoms and the convalence state of the central phosphorus atom as well ^[51a]. Yamamura and co-workers also demonstrated that these compound show strong propensity to encapsulate fullerene molecules ^[51b-d].

Okada’s group has developed a different analogue of compound 72 by replacing phosphorus atom with nitrogen atom, ., a triphenylamine based concaved heterocyleOTPA ^[52].OTPA (85) was synthesized in five linear steps with two-fold intramolecular nucleophilic aromatic reaction to construct the desired structure (Scheme 17).OTPA is an excellent electron donor and can form stable cation radical species upon electrochemical oxidation and/or reaction with electron acceptors.heoretical calculation reveals that the spin density of the cation radical is mainly on the central nitrogen atom.OTPA adopts bowl shape in the neutral state, whereas the conformation is changed to flat in its cation radical state.

Finally, we would like to introduce the bowl-shaped molecules with the bottom opened, for example, chrysaoroles and chrysaorenes developed by Stepien´ group ^[53]. Chrysaorole differs from typical bucky bowls in that its hub consists of a uniquely large, 18- membered ring ^[53a]. However, the presence of six hydrogen atoms in the hub makes the remaining opening quite small.he depth of the bowl is 1.96Å, much larger than that of corannulene (0.87Å). Stepien´ group has revealed that geometries of the chrysaorenes depend on the number of the fluorenophane subunit [53b]. When three fluorenophanes fused in sequence to form a large macrocycle, the resulting ^[3]Chrysaorene is of bowl shape. On the other hand, whereas four fluorenophanes fused one, ^[4]chrysaorene, is planar.he chrysaorenes show geometry-dependent magnetic properties and are strongly fluorescent.

5. Conclusion

In this short review, we highlight the recent progress on the chemistry of bowl-shaped conjugated polycycles.he bowlshaped molecules are mainly constructed on the basis of two subunits of fullerene C₆₀, the C_5v-symmetric corannulnene and the C_3v-symmetric sumanene.he breakthrough for the synthesis of these two basic backbones inspires intensive research on the bowlshaped polycycles including peripheral functionalization and heteroatom doping of corannulene and sumanene, as wells as replacing the sp² carbon on the conjugated two-dimensional sheet with multiple heteroatoms.he achievement of the synthesis polycycles leads to the multiple applications of bowl-shaped conjugated in synthetic chemistry, coordination chemistry, supramolecular chemistry, and materials sciences.

Acknowledgments We are grateful to the National Natural Science Foundation of China (Nos.1522203,1372111, and1190034) and State Key Laboratory of Applied Organic Chemistry for the financial support.