Introduction

Heterofullerene is a class of fullerenes in which one or more of its carbon atoms are substituted by heteroatoms such as nitrogen, boron, and phosphorous1,2,3,4. Since the substitution of carbon atoms within fullerene frameworks by heteroatoms is a feasible way to adjust its electronic and chemical properties, heterofullerenes are expected to find numerous potential applications in superconductors, optoelectronics, and organic semiconductors5,6,7. As such, heterofullerenes have been an important synthetic target for organic chemists. In contrast to the established methods of synthesizing fullerenes8,9, the synthetic process of heterofullerenes has long been a challenge10. The only successfully synthesized and isolated heterofullerene is an azafullerene, which contains nitrogen atoms within its framework. In 1995, Wudl et al. reported the first synthesis of azafullerene C59N in its dimeric form11. However, thus far, no multiazafullerene has been successfully synthesized and isolated on a macroscopic scale12, presumably due to its “isomeric problem”13,14,15. For instance, attempts to synthesize diazafullerene C58N216,17 leads to the generation of 23 possible isomers18,19,20. Hence, currently the synthesis and isolation of a single isomer is still an open challenge.

Encouraged by the success for the “bottom-up” synthesis of C60 from well-designed aromatic precursors21, researchers have sparked off immense interest amongst the “bottom-up” synthesis of multiazafullerenes from azafullerene precursors. This synthetic approach will allow for a controlled and selective introduction of nitrogen atoms into fullerenes22,23,24,25. However, appropriate synthetic protocols to synthesize nitrogen-embedded polycyclic aromatic molecules as large azafullerene fragments are still lacking26,27,28,29. As far as we are aware, only a handful of multiazafullerene fragments have been reported30,31,32,33,34. As shown in Fig. 1a, triazasumanene 130 and “hydrazinobuckybowl” 232 are considered to be partial fragments of C60-xNx. Meanwhile, the molecular fragments of higher multiazafullerenes are scarce. To our knowledge, the examples include chrysaorole 331 and a corannulene molecule fused with two π-extended pyrroles35. It is worth noting that a pyrrolo[3,2-b]pyrrole-cored nanographene36,37 may be a good precursor for the synthesis of “isomeric multiazafullerenes”, which contain heptagon as well as pentagon and hexagon. In this regard, multiazafullerene fragment molecules are an attractive target, given the fact that the synthesis of multiazafullerene C80-xNx (x ≥ 2) has not been achieved. During our continuous investigations on the synthesis of azafullerene fragment molecules38,39,40,41, we succeeded in achieving the bottom-up synthesis of diaza[80]fullerene fragment t-Bu4C72H24N2 (4a in Fig. 1b). This polycyclic aromatic molecule provides the largest π-surface of a [80]fullerene fragment bearing multiple heteroatoms.

Fig. 1: Representative multiazafullerene fragment molecules.
figure 1

a Molecules reported in literature. b Molecules reported in this manuscript.

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Results and discussion

Synthesis and characterization

Our synthetic strategy shown in Fig. 2 started with the bromination of 2,7-diaminopyrene 542,43 to afford 1,3,6,8-tetrabromopyrene-2,7-diamine (6) in 96% yield. Subsequently, a palladium-catalyzed Suzuki-Miyaura cross-coupling reaction of 6 with an arylboronic acid 7 afforded the corresponding tetraarylated compound 8 in 40% yield. Afterward, an intramolecular cyclization of 8 by treatment with hydrogen chloride followed by air oxidation generates bifunctional iminium salt 9 in 55% yield as a mixture of regioisomers. Following the successful synthesis of iminium salt 9, 1,3-dipolar cycloaddition with 2,2’,6-trichlorodiphenylethyne followed by oxidation with DDQ under ambient air was performed to generate fused pyrrole 10 in 20% yield. Finally, an intramolecular cyclization of 10 was carried out in the presence of Pd(OAc)2, (t-Bu)2MeP ∙ HBF4, and DBU39 to obtain 4a in 37% yield. It is worth noting that 4a should be stored under inert atmosphere due to its sensitivity to oxygen in a solution state, which is comparable to the corannulene/azacorannulene hybrid molecule in our previous report38. The structure of 4a was confirmed by spectroscopic analysis. The 1H NMR spectrum exhibited three singlets, two doublets, and one triplet in aromatic region as well as one singlet in the aliphatic region, which are consistent with the C2v symmetric structure of 4a. In HRMS spectrum, an m/z value of 1145.4867, corresponding to an ion mass of C88H61N2 (m/z = 1145.4835), was observed as a major signal.

Fig. 2
figure 2

Synthetic route to azacorannulene dimer 4a.

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The structure of 4a was further confirmed by X-ray diffraction analysis of its single crystals, which were obtained by slow evaporation from its benzene/diethyl ether solution under argon atmosphere (Fig. 3). The ORTEP structure shows a boat-shaped structure with a fusion of two bowl-shaped azapentabenzocorannulene (APBC) moieties linked by a naphthalene unit. The two terminal azacorannulene bowls are bent in a syn-conformation. The central pyrene unit bends to give quadruple [4]helicene structures, in which two helicenes have a screw sense of P while the other two have M (Fig. 3a). The average interplanar angle between two terminal benzene rings (shown red and blue) of four [4]helicene units was determined to be 33.4°. This angle is larger than a substituted [4]helicene (25.1°)44 but is smaller than a hexabenzocoronene (42.5°)45. Due to the helicene structure, the central pyrene moiety is no longer planar. The dihedral angle between planes formed by C7-C69-C70-C67 and C9-C71-C72-C65 was determined to be 20.5°. The bowl depths, defined as the average perpendicular distance from the mean planes of the hub pyrrole rings (N1-C1-C2-C3-C4 and N2-C35-C36-C37-C38) to each summit atoms of C14, C32, C42 and C60, was determined to be 1.92 Å (Fig. 3b). The bowl depth is deeper than that of APBC (1.38–1.73 Å)26, which is attributed to the steric repulsion between hydrogen atoms in the [4]helicene structure. In the packing structure, two molecules of 4a are packed as a dimeric form with a convex-to-convex π-π interaction (Fig. 3c). The shortest atomic distance between the two molecules is 3.30 Å, which is similar to that of a pentagon- and heptagon-embedded azabuckybowl (3.27 Å)37 and that of a pentagon- and heptagon-embedded nanographene (3.28 Å)46. These results indicate the presence of a strong intermolecular interaction.

Fig. 3: X-ray crystallographic analyses of azacorannulene dimer 4a.
figure 3

a ORTEP structure of 4a with thermal ellipsoids at 50% probability. Hydrogen atoms and t-butyl group are omitted. b Bowl depths of 4a. Hydrogen atoms and t-butyl group are omitted. c Packing structure of 4a in a unit cell. Hydrogen atoms are omitted.

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Conformational analysis

The conformation of molecule 4b, in which t-butyl groups of 4a are replaced by hydro groups, was analyzed by density functional theory (DFT) calculations at the B3LYP/6-31G(d) level of theory. Overall, there are 10 possible conformational isomers which are formed by combinations of the direction of two bowls (syn/anti) and the helicity of four [4]helicene units (P/M), whilst disregarding all enantiomers. Optimization starting from all the ten possible conformers resulted in eight local minimums. The Gibbs free energy values (kcal mol−1) relative to the most stable conformer are summarized in Supplementary Fig. 26. The most stable conformer was found to be syn-III (Fig. 4), which is in agreement with that observed in the X-ray diffraction analysis (Fig. 3) and is opposite to that of a similar bisdibenzocorannulene47. The relative energies of the other 7 conformers range within 5.6 kcal/mol. The transition states of some interconversions were also calculated. The conversion of syn-III into syn-II, which corresponds to the helicene flipping of one [4]helicene unit, has an activation energy of 5.4 kcal/mol. The energy of a bowl inversion (syn-III to anti-II) was calculated to be 15.1 kcal/mol. Considering the reasonably low activation barriers for these interconversions, all the 8 conformers which gave local minimums can equilibrate at room temperature in solution state.

Fig. 4: Interconversion pathways of 4b calculated at the B3LYP/6-31G(d) level of theory.
figure 4

Blue highlights indicate a transition state that involves a helicene flipping of the [4]helicene moiety, while pink highlights indicate a transition state that involves a bowl inversion of the azacorannulene unit.

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Molecular properties

The optical properties of 4a were assessed by absorption and emission spectroscopy (Fig. 5a). The green solution of 4a in hexane, dichloromethane, and dimethyl sulfoxide exhibit comparable absorption bands at 300–720 nm. In dichloromethane, for instance, two major absorption peaks were observed at 447 and 650 nm, which are much larger than those of the parent APBC26 due to its extended π-conjugation system. A time-dependent DFT (TD-DFT) computation at the B3LYP/6-31G(d) level (Supplementary Table 4) indicates that the strong absorption band at around 650 nm corresponds to a HOMO-LUMO transition. To determine the HOMO-LUMO gap experimentally, the electrochemical properties of 4a were investigated by cyclic voltammetry (CV) measurements (Supplementary Fig. 25). Compound 4a showed two overlapped quasi-reversible oxidation peaks at E = ca. +0.16 and +0.25 V (vs. Fc/Fc+), while it showed one reversible reduction peak at E = − 1.77 V. The experimental HOMO-LUMO gap of 4a was determined to be 1.94 eV, which is highly consistent with the absorption at 650 nm. Moreover, 4a exhibits red fluorescence and indicates a positive solvatofluorochromic effect with maximum emission wavelengths (λem) at 665 nm (hexane), 692 nm (dichloromethane), and 706 nm (dimethyl sulfoxide) with quantum yields of ΦF = 0.31, 0.22, and 0.15, respectively. Since APBC shows a comparable fluorescence quantum yield of ΦF = 0.24 in dichloromethane26, the extended π-conjugation of 4a does not significantly affect its fluorescence quantum yield. The observed solvatofluorochromic phenomenon would be induced by the presence of intramolecular charge transfer due to donor-acceptor-donor nature of the molecule. Based on the fact that the HOMO is distributed all over the molecule including the two pyrrole moieties and that LUMO is mainly delocalized at the central pyrene moieties (Fig. 5b), the system involves the APBC cores as a donor moiety and the pyrene unit as an acceptor moiety. The lower fluorescence quantum yields in more polar solvents (Supplementary Table 2) as well as the redox properties in cyclic voltammetry also support our rationale48.

Fig. 5: Optical Properties of azacorannulene dimer 4.
figure 5

a UV/Vis absorption spectra (1.0 × 10−6 M, solid lines) and emission spectra (1.0 × 10−5 M, dashed lines) of 4a in hexane (blue), dichloromethane (green), and dimethyl sulfoxide (red). b HOMO and LUMO of 4b.

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The aromaticity of 4b was characterized by nucleus independent chemical shift (NICS) analysis using DFT calculation at the B3LYP/6-31G(d) level of theory (Fig. 6). The large negative NICS values for the inner pyrrole core (−18.7 ppm) and its four outer benzene rings (−10.1 to −9.7 ppm) show that they are aromatic (Fig. 6a), which is in accordance with those of the reported APBC (Fig. 6b; −18.7 and −10.1 to −9.8 ppm)26. In addition, the central pyrene fragment in 4b has comparable NICS values (−9.8 and −4.6 ppm) with pyrene (Fig. 6c; −12.7 and −5.1 ppm). The anisotropy of the induced current density (ACID) plot of 4b in Fig. 6d shows that the central pyrene moiety has typical ring currents, in which two 6π benzene rings are connected by two carbon–carbon double bonds. In the azacorannulene moiety, clockwise (diamagnetic) 26π ring currents flowing along the core pyrrole moiety and the four outer benzene rings were observed, which substantiates the aromaticity by NICS calculation. These results show that the fusion of two APBC moieties does not significantly change their aromaticity.

Fig. 6: Aromatic properties of azacorannulene dimer 4b.
figure 6

a NICS(0) values of 4b calculated at the B3LYP/6-31G(d) level of theory. b NICS(0) values of APBC. c NICS(0) values of pyrene. d ACID plot of 4b. e ACID plot of APBC.

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Host-guest chemistry

During the investigation on the application of 4a, we discovered its interesting shape-recognition behavior in host-guest chemistry. Inspired by the previous reports of azabuckybowls being utilized as buckycatchers49,50, association behavior of 4a with C60 and a dumbbell-shaped C60 dimer (C120)51 was examined by fluorescence titration. As shown in Fig. 7a, the addition of C120 into a diluted solution of 4a in 1,2-dichlorobenzene resulted in the gradual decrease of its fluorescence intensity. Based on the Benesi–Hildebrand equation, the association constants of 4a toward C60 and C120 were determined to be Ka(C60) = 4.5 × 102 M−1 and Ka(C120) = 2.9 × 103 M−1 respectively (Fig. 7b), which indicate that 4a favors C120 over C60 by one order of magnitude. The Job’s plot for the emission intensity indicates the formation of 1:1 supramolecular assembly of 4a and C120 (Supplementary Fig. 21). It is worth noting that comparable association constants of APBC with C60 and C120 were observed (Fig. 7b), thus showing no distinct selectivity. These results strongly indicate that the boat-shaped structure of 4a recognizes the dumbbell-shape of C120 during the association process in solution, leading to the higher association constant for C120 (Fig. 7c).

Fig. 7: Host–guest chemistry between 4a and C120.
figure 7

a Fluorescence spectra of 4a upon titration with C120. b Association constants of host molecules (4a and APBC) and guest molecules (C60 and C120) determined by fluorescence titration. c One of the possible association modes of 4a and C120 determined by DFT calculation at the B3LYP/6-31G(d) level with Grimme’s D3 dispersion correction.

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In summary, we have demonstrated the bottom-up synthesis of a diaza[80]fullerene fragment molecule 4a. The large nitrogen-containing polycyclic aromatic molecule has a boat-shaped structure which can be viewed as the fusion of two bowl-shaped APBC moieties linked by a fused naphthalene unit. Conformational studies showed that a butterfly-butterfly conformer where the two azabuckybowls bend in the same direction is the most stable, which is consistent with that observed in X-ray diffraction analysis. The unique molecular shape leads to preferable association with a dumbbell-shaped C60 dimer (C120) over C60 through shape recognition. Theoretical analysis revealed the presence of a narrow HOMO-LUMO band gap, resulting in a strong absorption band at around 650 nm. The optical measurement exhibits a red fluorescence and solvatofluorochromic behavior. Importantly, the utilization of fully conjugated bifunctional polycyclic aromatic azomethine ylide 9 in a bottom-up synthetic approach provides a practical method for the selective synthesis of large multiazafullerene fragments.

Methods

Experimental procedure

The synthesis of 4a is as follows: to a mixture of 10 (10 mg, 7.3 μmol), palladium diacetate (4.9 mg, 22 μmol) and di-t-butyl(methyl)phosphonium tetrafluoroborate (16 mg, 66 μmol) were added 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU; 0.5 ml) and N,N-dimethylacetamide (DMA; 2.0 ml). The mixture was stirred for 19 h at 160 °C. After cooling to room temperature and dilution with toluene (5 ml), the mixture was washed with water (3 × 5 ml), dried over sodium sulfate, filtered, and concentrated in vacuo. The resulting mixture was purified by silica gel column chromatography with hexane/dichloromethane (4/1) to obtain 4a as a dark green solid (3.1 mg, 2.7 μmol, 37%). Full experiment details can be found in the Supplementary Information.

Theoretical calculations

All calculations were performed by using Gaussian 16 (revision A.03) program52 by the B3LYP method53,54 with the 6-31G(d) basis set55,56 for structure optimization, vibrational frequency, time-dependent density functional theory, NICS, and ACID calculations. Grimme’s D3 dispersion correction57 was used to investigate the association of 4b with C120. Molecular geometries and transition state (TS) structures were optimized without any symmetry assumptions. Intrinsic reaction coordinate calculations were also performed for all TSs to ensure their true nature. All thermodynamics were obtained by utilizing the standard conditions at 298 K and 1 atm. Energies are presented as ΔG in kcal/mol.