Nitrogen-embedded buckybowl and its assembly with C60

Curved π-conjugated molecules have attracted considerable interest because of the unique properties originating from their curved π surface. However, the synthesis of such distorted molecules requires harsh conditions, which hamper easy access to heteroatom-containing curved π systems. Here we report the synthesis of a π-extended azacorannulene with nitrogen in its centre. The oxidation of 9-aminophenanthrene provides tetrabenzocarbazole, which is converted to the azabuckybowl through palladium-catalysed intramolecular coupling. The electron-donating nature and curved π surface of the azabuckybowl enable its tight association with C60 in solution and solid states. High charge mobility is observed for the azabuckybowl/C60 assembly. This compound may be of interest in the fields of curved π systems as fullerene hosts, anisotropic π donors and precursors to nitrogen-containing nanocarbon materials.

C urved p-conjugated molecules have captivated numerous scientists [1][2][3][4] . Curving a p system induces a large displacement from a plane to construct threedimensional structures [5][6][7][8][9][10] . Besides their figurative beauty, the curved p surface generates unique functions such as chiroptical properties, anisotropic electron transitions and dynamic motion in solution and solid states [11][12][13] . To enhance these characteristics, the introduction of heteroatoms is an effective strategy. However, the synthesis of heteroatom-containing curved p systems remains a challenge 14,15 . The preparation of distorted p systems requires harsh reaction conditions that do not tolerate heterocyclic molecules.
Buckybowls, that is, bowl-shaped molecules such as corannulenes 16 and sumanenes 17 represent important curved p-conjugated molecules, which can be precursors for the bottom-up synthesis of fullerenes and nanotubes. In 1966, Barth and Lawton 16 reported the first chemical synthesis of corannulene. Since Scott and Siegel's groups [18][19][20][21] developed their straightforward synthesis of corannulene, numerous bowl-shaped hydrocarbons have been synthesized. On the other hand, the nitrogen-embedded bowl-shaped molecules have been sought as model compounds for azafullerenes and nitrogen-doped carbon nanotubes [22][23][24] . Furthermore, dramatic changes in electronic structures of buckybowls by nitrogen are expected. However, the synthesis of buckybowls with internal nitrogen atoms has been still challenging 25 . Oxidative fusion approaches are not compatible with electron-rich nitrogen because of its less tolerant nature for oxidation.
Recently, our group reported the oxidative dimerization of aminoarenes to distorted p-conjugated molecules in a onestep operation 26,27 . We here disclose that the oxidation of 9-aminophenanthrenes affords tetrabenzocarbazoles in good yields. Furthermore, consecutive fusion reactions of tetrabenzocarbazole 2 through palladium-catalysed C-H/C-Cl and C-H/C-Br coupling achieve the synthesis of nitrogenembedded buckybowl 5, that is, 'azabuckybowl' 28 under mild conditions. Owing to the electron-donating nature of the nitrogen atom, azabuckybowl 5 strongly interacts with C 60 to furnish an inclusion complex, which exhibits a substantially high chargecarrier mobility in the solid state.

Results
Synthesis of nitrogen-embedded buckybowl. The synthesis of nitrogen-embedded buckybowl 5 started with the oxidative dimerization of 1 ( Fig. 1) 26 . 9-Aminophenanthrene 1 was oxidized to tetrabenzocarbazole 2 in 94% yield. Reaction of 2 with Pd(OAc) 2 /tricyclohexylphosphine provided singly fused product 3 in 63% yield 29 . The twisted conformation of 3 was unambiguously elucidated by X-ray diffraction analysis ( Supplementary Fig. 18). The bromination of 3 with bromine afforded tribrominated product 4 in 56% yield. Finally, the palladium-catalysed double C-H/C-Br coupling furnished nitrogen-embedded buckybowl 5 in 46% yield. The proton nuclear magnetic resonance ( 1 H NMR) spectrum of 5 exhibited six proton signals in the aromatic region, indicating the formation of a fused and symmetrical molecule.
Structural elucidation and characteristics of azabuckybowl. The bowl-shaped structure of 5 was unambiguously elucidated by X-ray diffraction (Fig. 2). In the crystal, one asymmetric unit contained two independent molecules of 5. The bowl depth, which is defined as the distance between the mean plane that consisted of five carbons at the edge and the centroid of the pyrrole ring, was 1.65 and 1.70 Å. The bowl depth of the central azacorannulene core was 0.90 and 0.92 Å, which is slightly greater than that of corannulene (0.86 Å). The curvature of 5 was further evaluated by Haddon's p-orbital axis vector (POAV) angles 30 . As shown in Fig. 2a, the POAV angles around the central pyrrole ring are in the range of 7.2°-9.3°. These values are comparable with that of corannulene (9.1°). It is noteworthy that the molecules constructed a one-dimensional chain stacking structure in the crystal (Fig. 2c). Distances between the two closest molecules were 3.25 and 3.41 Å, indicating the existence of a p-p interaction.
Bowl-to-bowl inversion of the azabuckybowl was investigated. Azabuckybowl 5 was further functionalized by iridium-catalysed C-H borylation to provide 6 in 80% yield 31,32 . The Suzuki-Miyaura cross-coupling reaction of 6 with 2-bromo-1,3,5triisopropylbenzene furnished the corresponding coupling product 7 in 55% yield. The 1 H NMR spectrum of 7 in 1,2-dichlorobenzene-d 4 at room temperature exhibited three doublet peaks for methyl protons of isopropyl groups at 1.43, 1.39 and 1.13 p.p.m. This non-symmetric feature indicates that 7 shows no bowl-to-bowl inversion at room temperature. As the temperature was raised, two proton signals at 1.43 and 1.13 p.p.m. were gradually broadened (Fig. 2d). Even at 170°C, these signals were not coalesced. Accordingly, the bowl-to-bowl inversion energy (DG z ) was measured by two-dimensional exchange spectroscopy (2D EXSY) experiments. At 393 K, DG z was determined to be 23.3 kcal mol À 1 in 1,2-dichlorobenzene-d 4 ( Supplementary Figs 32 and 33). This value is higher than that of the parent sumanene (DG z ¼ 19.7-20.4 kcal mol À 1 ) 33 . The high bowl-inversion energy of 7 was also supported by theoretical calculations. The inversion barrier of 7 was calculated to be 19.9 kcal mol À 1 by density functional theory (DFT) calculations at the B3LYP/cc-pVDZ level, which is higher than those of sumanene (18.2 kcal mol À 1 ) and corannulene (9.1 kcal mol À 1 ) calculated at the same level of theory 34 . Optical and electrochemical properties of azabuckybowl. Figure 3 shows ultraviolet-visible absorption and emission spectra of 3 and 5 in CH 2 Cl 2 . The lowest energy bands shifted to the low-energy region as the degree of fusion increased. All compounds exhibited fluorescence in the visible region (Fig. 3b). The emission quantum yield of 5 was 17%, which is the highest among buckybowls 35 . The Stokes shifts of 3 (2,800 cm À 1 ) and 5 (1,500 cm À 1 ) were relatively larger than that of a planar molecule. This reflects their excited state dynamics, owing to their distorted characteristics. The electronic structures of 3 and 5 were further investigated by an electrochemical analysis (Supplementary Fig. 22 and Supplementary Table 2). Reversible oxidation waves were observed for all compounds. The first oxidation potentials were lowered in the order of 24345, indicating effective electron donation from the nitrogen atom to the entire p system.
We then examined the protonation behaviour of 5, because 5 was expected to have higher basicity than planar amines. The addition of trifluoroacetic acid (TFA) to a dichloromethane solution of 5 induced a dramatic change in its absorption spectrum (Fig. 3c). Interestingly, the same change was observed on the addition of a one-electron oxidant, tris(4-bromophenyl) aminium hexachloroantimonate (BAHA) (Fig. 3d). We also monitored the electro-oxidative absorption spectrum of 5 in CH 2 Cl 2 , which exhibited essentially the same change (Fig. 3e). These facts strongly indicate that the addition of TFA resulted in the generation of the radical cation species rather than simple protonation. The formation of the radical cation was confirmed by electron spin resonance (ESR) measurements ( Supplementary   Fig. 26). The solution of 5 in the presence of TFA exhibited a distinct signal at g ¼ 2.002, as was the case of the oxidation of 5 with BAHA. The conversion of 5 to the radical cation was almost quantitative under air atmosphere but was substantially lower under argon ( Supplementary Fig. 27). The radical cation generation is likely due to electron transfer between 5 and protonated 5 involving air oxidation 36 . The facile generation of the radical cation from 5 would allow the investigation of the effect of oxidative doping on solid-state properties. This phenomenon also implies that nitrogen-doped electron-rich nanocarbons may undergo a similar radical cation generation by protonation.
Association behaviour of 5 with C 60 . The effect of nitrogen also appeared in the association behaviour of 5 with C 60 . For hydrocarbon buckybowls, their association constants with C 60 were very low to be measured [37][38][39][40] . The incorporation of electron-rich nitrogen in buckybowls should enable tighter binding with electron-deficient fullerenes. The electrochemical analysis revealed its much lower oxidation potential (0.20 V) when compared with corannulene (1.57 V) (Fig. 3f) 41 . The addition of C 60 into an 1,2-dichlorobenzene solution of 5 induced a change in the ultraviolet-visible absorption and emission spectra (Fig. 4b,c). In particular, the appearance of broad absorption bands in the near-infrared region suggests intermolecular charge-transfer interactions between 5 and C 60 . The association behaviour was also monitored by 1 H NMR analysis. On the addition of C 60 into a toluene-d 8   ARTICLE shifted (Fig. 4a). This indicates that 5 and C 60 interacted in a convex-concave manner. This was revealed by the X-ray crystallographic analysis and showed that C 60 was located above the centre of 5 (Fig. 4d,e). The penetration depth of C 60 into 5 measured from the centroid of the pyrrole ring to the centroid of C 60 is 6.82 Å, and that measured from the shortest distance from the concave surface of 5 to a C 60 surface is 3.29 Å, whereas the depths of C 60 into the corannulene/C 60 complex are 6.94 and 3.75 Å. The short distance between 5 and C 60 indicates the presence of attractive interactions between them. Judging from the relatively long distance (43.74 Å), the CH-p interaction between tert-butyl groups and C 60 was not essential. The binding constant was determined to be 3,800 M À 1 by titration with absorption and fluorescence spectra ( Supplementary Figs 23-25).
This value is approximately three times larger than that of perthiolated corannulene. The existence of intermolecular chargetransfer interactions between 5 and C 60 was indicated by the quenching behaviour of the emission on the addition of C 60 to a 1,2-dichlorobenzene solution of 5 (Fig. 4c). The DFT optimization of 5* C 60 afforded nearly the same structure as the crystal structure. The highest occupied molecular orbital was spread over the entire surface of 5, whereas the lowest unoccupied molecular orbital was delocalized on C 60 ( Supplementary Fig. 29).
In addition, oscillator strengths of the absorption of 5*C 60 were simulated by the time-dependent DFT method ( Supplementary  Fig. 30). The broad lowest-energy band in the near-infrared region was assigned as the highest occupied molecular orbitallowest unoccupied molecular orbital transition. These results + 1800 equiv. + 2700 equiv. + 3600 equiv. + 4500 equiv. + 5300 equiv. + 6300 equiv. + 7200 equiv. + 8100 equiv. +9000 equiv. +9900 equiv. supported the conclusion that an intermolecular charge-transfer interaction exists between 5 and C 60 . Finally, we investigated the effect of the association with C 60 of 5 on the charge-carrier mobility of 5 by flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements 42 . The maximum transient conductivity (fSm) of 5 was measured to be 1.5 Â 10 À 5 cm 2 V À 1 s À 1 ( Supplementary  Fig. 28). For the co-crystal of 5* C 60 , the mobility was enhanced to 2.4 Â 10 À 4 cm 2 V À 1 s À 1 (Fig. 5a,b). The chargecarrier generation efficiency (f) was determined to be 4.4 Â 10 À 3 by the transient absorption spectroscopy measurement. Accordingly, the local charge mobility of 5*C 60 was 0.17 cm 2 V À 1 s À 1 . Such a large mobility of 5*C 60 should originate from effective charge separation caused by an electronic interaction between 5 and C 60 . Furthermore, the alignment of 5 and C 60 in the co-crystal may contribute to mobility enhancement. Both C 60 and 5 construct one-dimensional chain alignments in the co-crystal (Fig. 5c).

Discussion
In summary, we have achieved the synthesis of a nitrogenembedded buckybowl under mild conditions. The total yield of azabuckybowl 5 was 11% from 9-bromophenanthrene. We also found that the protonation of 5 resulted in the efficient generation of radical cation species. The nitrogen-embedded buckybowl was sufficiently electron-rich to assemble tightly with C 60 in solution and solid states. The molecular assembly of 5 with C 60 exhibited a significantly high charge mobility (0.17 cm 2 V À 1 s À 1 ). The nitrogen-embedded buckybowl can be a novel molecular entity in the field of curved p systems as fullerene hosts, anisotropic p donors and precursors to nitrogen-containing nanocarbon materials.

Method
Materials and characterization. 1 H NMR (500 MHz) and 13  Ultraviolet-visible-near infrared absorption spectra were recorded using a Shimadzu UV-2550 or JASCO V670 spectrometer. Emission spectra were recorded using a JASCO FP-6500 spectrometer and absolute fluorescence quantum yields were measured by the photoncounting method using an integration sphere. Mass spectra were recorded using a Bruker microTOF by electrospray ionization (ESI) methods. Unless otherwise noted, materials obtained from commercial suppliers were used without further purification.
Synthesis of 3,6-Di-tert-butyl-9-bromophenanthrene. 3,6-Di-tert-butylphenanthrene (0.540 g, 1.86 mmol) was dissolved in CCl 4 (11 ml) in a two-necked flask equipped with a dropping funnel. Br 2 (0.10 ml, 1.95 mmol) and CCl 4 (11 ml) were added into the dropping funnel. The solution was heated to 50°C and then the bromine solution was added slowly over 1 h. After the addition was completed, the mixture was stirred for additional 30 min. The reaction mixture was cooled to room temperature and then quenched with aqueous Na 2 S 2 O 3 . The resulting mixture was extracted with CH 2 Cl 2 and the organic layer was washed with aqueous Na 2 S 2 O 3 , dried over Na 2 SO 4 and concentrated in vacuo. Purification by silica-gel column chromatography (cyclohexane as eluent) afforded the title compound (0.645 g, 1.75 mmol) in 94% yield as a white solid. 1 Synthesis of compound 4. Compound 3 (40.1 mg, 60.3 mmol) was dissolved in CCl 4 (6.0 ml) in a two-necked flask equipped with a dropping funnel. A solution of Br 2 (0.10 ml, 2.0 mmol) in CCl 4 (3.0 ml) was added to the dropping funnel. The mixture was heated to 70°C and then the bromine solution was added slowly over 15 min. After the addition was complete, the mixture was stirred for an additional 12.5 h. The reaction mixture was cooled to room temperature and then quenched with aqueous Na 2 S 2 O 3 . The resulting mixture was extracted with CH 2 Cl 2 and the organic layer was washed with aqueous Na 2 S 2 O 3 , dried over Na 2 SO 4 and concentrated in vacuo. X-ray diffraction analysis. X-ray data were obtained using a Bruker D8 QUEST X-ray diffractometer with an ImS microfocus X-ray source and a large-area (10 cm Â 10 cm) CMOS detector (Photon 100) for 3 and 4, and using a Rigaku  Data 1-4). A fine crystal of 5 for the X-ray diffraction analysis was obtained by the vapour diffusion of methanol into its o-xylene solution. For the X-ray crystal structure of 5 *C 60 , a fine crystal for the X-ray diffraction analysis was obtained by the vapour diffusion of methanol into a toluene solution with a 1:1 mixture of 5 and C 60 . The molecule C 60 was significantly disordered and refined as two disordered rigid bodies by restraining with DFIX, DANG, DELU and SIMU commands as generally used for the refinement of C 60 . The toluene solvent molecules were assigned as two disordered units by using minus part number, because it was located at the special position ( Supplementary Fig. 21). The resolution and data were sufficiently suitable to determine the binding manner in the crystal (R int ¼ 0.0252, 22,169 total reflections and 11,448 unique reflections were observed). The detailed crystallographic data for all compounds are listed in Supplementary Table 1.
Electrochemical analysis. The cyclic voltammogram and differential-pulse voltammogram of 5 were recorded using an ALS electrochemical analyser 612C. Measurements were performed in freshly distilled dichloromethane with tetrabutylammonium hexafluorophosphate as the electrolyte. A three-electrode system was used. The system consisted of a platinum working electrode, a platinum wire and Ag/AgClO 4 as the reference electrode. The scan rate was 100 mVs À 1 . The measurement was performed under nitrogen atmosphere. All potentials are referenced to the potential of ferrocene/ferrocenium cation couple. The data are listed in Supplementary Table 2. The electro-oxidative absorption of 5 was recorded under argon atmosphere with a BAS SEC-F spectroelectrochemical flow cell kit equipped with a DH-2000-BAL as the ultraviolet-visible-near infrared light source and an HR4000CG-ultraviolet-near infrared spectrometer.
Determination of binding constant. The binding constant (K a ) of C 60 with compound 5 was determined by ultraviolet-visible absorption and emission spectral analysis on the titration of C 60 into the 1,2-dichlorobenzene solution of 5.
The fitting was performed with the correlation between the change of absorbance or fluorescence intensity (DX) at 700 and 508 nm, and the initial concentration of the guest ([G] 0 ) using the equation as follows: where De is the gap of molar coefficients between guest and complex, and [H] 0 is the initial concentration of the host ( Supplementary Figs 23 and 24). The estimated K a values by ultraviolet-visible spectral analysis were 3.9 Â 10 3 M À 1 for the first attempt and 3.7 Â 10 3 M À 1 for the second attempt. The K a was also estimated by the emission spectral analysis to be 3.8 Â 10 3 M À 1 . The average K a is 3.8 Â 10 3 M À 1 .
ESR measurement. ESR spectra were recorded at room temperature using a Bruker E500 spectrometer with 2.6f quartz sample tubes. A sample solution of 5 was prepared under air and the ESR tube was sealed. Other samples were prepared by the addition of the degassed solution of TFA and BAHA in CH 2 Cl 2 to the solution of 5.
Time-resolved microwave conductivity measurement. Transient photoconductivity was measured by FP-TRMC 43 . A resonant cavity was used to obtain a high degree of sensitivity in the conductivity measurement. The resonant frequency and microwave power were set at B9.1 GHz and 3 mW, respectively, such that the electric field of the microwave was sufficiently small not to disturb the motion of charge carriers. The conductivity value is converted to the product of the quantum yield f and the sum of charge-carrier mobilities Sm by fSm ¼ Ds (eI 0 F light ) À 1 , where e, I 0 , F light and Ds are the unit charge of a single electron, incident photon density of excitation laser (photons per m 2 ), a correction (or filling) factor (m À 1 ) and a transient photoconductivity, respectively. The sample was set at the highest NATURE COMMUNICATIONS | DOI: 10.1038/ncomms9215 ARTICLE electric field in a resonant cavity. FP-TRMC experiments were performed at room temperature. The measurements of 5 and 5* C 60 were performed for crystalline samples covered with a polyvinyl alcohol film on a quartz substrate.
Theoretical calculations. All calculations were performed using the Gaussian 09 programme 44 . The geometry of 5 þ , in which all tert-butyl groups were replaced with hydrogen, was optimized by the DFT method using the B3LYP 45,46 functional and the 6-31G(d) basis set. The geometry of 5*C 60 was optimized by Zhao's M06-2X functional 47 and the 6-31G(d) basis set. The oscillator strengths of 5 þ and 5* C 60 were calculated by the time-dependent DFT method at the B3LYP/6-31G(d) level ( Supplementary Figs 30 and 31). For calculations of the bowl-to-bowl inversion energy, the ground and transition state geometries of 7 were optimized at the B3LYP/cc-pVDZ level. Zero-point energy and thermal energy corrections were conducted for the optimized structures. The calculation results are summarized in Supplementary Tables 3-7.
Determination of bowl-to-bowl inversion energy by 2D EXSY measurement.
The bowl-to-bowl inversion barrier of 7 was measured by 2D EXSY using the signals for methine protons of isopropyl groups at approximately d ¼ 3.3 and 2.8 p.p.m. (Supplementary Fig. 32) 48 . 2D EXSY measurements were performed in 1,2-dichlorobenzene-d 4 at 393 K with a phase-sensitive nuclear Overhauser effect spectroscopy pulse sequence. The mixing time was increased from 50 to 300 ms. The rate constant (k) was determined using equation as follows: where t m is the mixing time and r is defined by the equation as follows: where I AB and I BA are the intensities of the cross-peaks between two exchangeable signals A and B, and I AA and I BB are the intensities of the diagonal signals ( Supplementary Fig. 33). The free energy (DG z ) of the bowl-to-bowl inversion was finally obtained using the Eyring equation.