as-Indaceno[3,2,1,8,7,6-ghijklm]terrylene as a near-infrared absorbing C70-fragment

Carbon and hydrogen are fundamental components of organic molecules and a fascinating plethora of functions can be generated using these two elements. Yet, realizing attractive electronic structures only by using carbon and hydrogen remains challenging. Herein, we report the synthesis and properties of the C70 fragment as-indaceno[3,2,1,8,7,6-ghijklm]terrylene, which exhibits near-infrared (NIR) absorption (up to ca. 1300 nm), even though this molecule consists of only 34 carbon and 14 hydrogen atoms. A remarkably small highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) gap is confirmed by electrochemical measurement and theoretical calculations. Furthermore, as-indacenoterrylene is stable despite the absence of peripheral substituents, which contrasts with the cases of other NIR-absorbing hydrocarbons such as diradicaloids and antiaromatic molecules. The results of this study thus offer fundamental insights into the design of hydrocarbons with a small band gap.


Results
Synthesis of as-indacenoterrylene 3. The synthesis of as-indacenoterrylene 3 is shown in Fig. 2a. The synthetic approach is based on the incorporation of sp 3 -hybridized carbon atoms into the fused π-system to generate the curved structure. The bromine-lithium exchange reaction between 1-bromonaphthalene and butyllithium, followed by the addition of 1,4,5,8-tetrachloroanthraquinone (4), afforded the corresponding dinaphthylated diols. The crude mixture was methylated with iodomethane in the presence of sodium hydride, which provided a mixture of cis-adduct 5 and trans-adduct 6 in 22% and 64% yield (over two steps), respectively. These isomers exhibit different solubility in CH 2 Cl 2 and were separated by filtration. The cis-isomer 5 engaged in a Pd-catalysed intramolecular C-H/C-Cl coupling 17 to provide methoxy-substituted as-indacenoterrylene 7 in 21% yield, wherein the two five-membered rings are located on the same side. Another isomer, in which the two five-membered rings are located on the opposite side, was not detected. The methoxy group in 7 was transformed to hydrogen using triethylsilane (HSiEt 3 ) and trifluoromethanesulfonic acid (TfOH) 46 , which afforded 8 in 90% yield. The 1 H nuclear magnetic resonance (NMR) signal of the internal hydrogens was observed at 5.91 ppm ( Supplementary  Fig. 5), which is comparable to that of hydrofullerene C 60 H 2 (5.93 ppm) 47 . Finally, abstraction of hydrogen from 8 using 1,4dichloro-2,6-dicyano-p-benzoquinone (DDQ) furnished as-indacenoterrylene 3 in 73% yield. Although 3 exhibits very low solubility in most common organic solvents, it slightly dissolved in CH 2 Cl 2 /CS 2 , o-dichlorobenzene and N-methylpyrrolidone. The attempted reductive aromatization of 7 with SnCl 2 and HCl resulted in decomposition without the formation of any isolable products. In addition, no fully fused product was detected in the reaction mixture when trans-adduct 6 was subjected to the intramolecular C-H/C-Cl coupling. This result could be due to the large structural distortion of the expected product having oppositely directed sp 3 carbons.
The structures of methoxy-substituted as-indacenoterrylene 7 and hydrogenated as-indacenoterrylene 8 were unequivocally determined by single-crystal X-ray diffraction analysis (Fig. 2b). Both 7 and 8 adopt a bowl-shaped structure with two sp 3hybridized carbon atoms in the central six-membered ring. Notably, 7 and 8 can be regarded as internally functionalized bowl-shaped aromatics, which have been rarely explored [48][49][50] . The methoxy groups of 7 are aligned perpendicular to the πsurface, which shows a bowl-depth of 2.10 Å. Unfortunately, the crystal data of 8 are insufficient for a detailed structural analysis due to severe disorder. The 1 H NMR spectrum of as-indacenoterrylene 3 in CDCl 3 / CS 2 exhibited seven signals in the range of 7.21-6.78 ppm, supporting a C s -symmetric structure ( Supplementary Fig. 7). These signals are upfield-shifted by ca. 1 ppm compared to those of other C 70 fragments, except for antiaromatic dicyclopentaperylene 2 (refs. [31][32][33][34][35][36][37][38][39][40][41] ). The upfield-shifted 1 H NMR signals are characteristic for π-conjugated systems with narrow HOMO-LUMO gaps as theoretical studies suggest that, in such systems, the quantum-chemical perturbation of excited states to the ground state results in the increase of paratropic ring current 51,52 . The high-resolution atmospheric pressure chemical ionization time-of-flight (APCI-TOF) mass spectrum of 3 showed a parent ion peak at m/z = 423.1184 (calcd for C 34 H 14 , m/z = 423.1168 [M + H] + ), which is in accordance with the expected molecular composition.
Bowl-to-bowl inversion. Variable-temperature 1 H NMR spectra of 9 were recorded in CDCl 3 ( Supplementary Fig. 22). At 0°C, the signals that arise from the ortho-methyl groups of the mesityl substituent were observed as a pair of singlets. At 35°C, these singlets coalesced to a single singlet at 2.08 ppm. A plot of the   Fig. 24). This value is smaller than the calculated rotation barrier of the peripheral mesityl group (26.1 kcal mol -1 ) ( Supplementary Fig. 25). Hence, the observed temperature dependence of NMR spectra can be interpreted in terms of an inversion of the bowl-shaped structure.
Electronic structures. The ultraviolet (UV)/vis/NIR absorption spectra of 3, 7 and fullerene C 70 are shown in Fig. 3a. Methoxysubstituted 7 exhibits weak absorption bands in the range of 400-520 nm, and the presence of vibrational bands implies structural rigidity. The absorption of 7 is blueshifted compared to that of fullerene C 70 due to the disrupted conjugation on account of the central sp 3 -hybridized carbon atoms. In sharp contrast, asindacenoterrylene 3 displays a significantly redshifted absorption with a peak top at 952 nm and a tail to ca. 1300 nm. The optical HOMO-LUMO gap is 1.30 eV. The extinction coefficient of the broad absorption band (4.1 × 10 3 cm -1 M -1 ) is considerable in comparison to those of the forbidden absorption of indene-fused corannulene derivative 1 (ref. 21 ) and dicyclopentaperylene 2 (ref. 41 ). As the forbidden S 0 -S 1 transition is a diagnostic feature of antiaromatic molecules [42][43][44][45] , the allowed nature of the lowenergy absorption of 3 cannot be explained in terms of a potential contribution of antiaromaticity. Interestingly, the absorption tail of 3 is even comparable to that of quarteranthene 55 , which is a zigzag-edge nanocarbon that consists of 56 carbon atoms. The NIR absorption band of 3 obeys the Lambert-Beer's law and was matched with that of mesityl-substituted derivative 9 (Supplementary Fig. 26). These results clearly suggest a negligible effect of aggregation on the NIR absorption. The electrochemical properties of 2-mesityl-as-indacenoterrylene 9 were studied by cyclic voltammetry (Fig. 3b), using the ferrocene/ferrocenium couple as an external reference, which revealed one reversible oxidation wave at 0.17 V as well as two reversible reduction waves at -1.16 and -1.45 V. The electrochemical HOMO-LUMO gap (ΔE = 1.33 eV) is in good agreement with the optical HOMO-LUMO gap. Indene-fused corannulene derivative 1 exhibited redox potentials E ox1 and E red1 at 0.33 and -0.81 V, respectively 21 . Dicyclopentaperylene 2 displayed redox potentials E ox1 and E red1 at 0.33 and -0.96 V, respectively 41 . The ΔE value of 9 is slightly wider than that of 1 and comparable to that of 2. The redox potentials of 9 are by ca. 0.2-0.3 V positively shifted relative to those of 1 and 2, which is indicative of a higher electron-donating ability.
To evaluate the electronic structure of 3, DFT calculations were conducted at the CAM-B3LYP/6-311+G(d,p) level of theory ( Supplementary Figs. 27 and 29). Time-dependent (TD)-DFT calculations predicted the presence of the HOMO-LUMO transition at 820 nm with an oscillator strength of 0.1226, which is in good agreement with the experimental absorption spectrum ( Supplementary Fig. 28, Supplementary Table 1). The calculated HOMOs and LUMOs of 3 and its fragments, as-indacenopyrene and terrylene, are shown in Fig. 4a and Supplementary Fig. 31. The HOMO of 3 is delocalized along the longitudinal axis, whose distribution and energy level are almost identical to those of the HOMO of terrylene rather than those of HOMO of asindacenopyrene. In contrast, the LUMO of 3 is mainly located at the centre of the molecule, which resembles the LUMO of asindacenopyrene and energetically much lower than the LUMO of terrylene by 0.7 eV. The high electron-deficiency of as-indacenopyrene can be attributed to the incorporated non-alternant hydrocarbon segments 56 .
Nucleus-independent chemical shift (NICS) 57-59 calculations and anisotropy of the induced current density (ACID) 60 calculations were conducted for as-indacenoterrylene 3 at the B3LYP/6-31G(d) level (Fig. 4c, d). The NICS(0) iso values, except for that at the five-membered ring, fall in the range -3.9 to 6.4 ppm. An essentially similar trend was also observed for the NICS (1) zz values (Fig. 4c, Supplementary Table 2) 59 . The ACID plot indicates a negligible contribution of the macrocyclic ring current. Besides, the ACID calculations predict counter-clockwise ring current at the five-membered rings, which accord with the positively large NICS values. Although the exact reason remains unclear, this observation can be accounted for by the local antiaromaticity at the five-membered rings, which could be due to the cyclopentadienyl cation-like electronic contribution associated with the electron-deficient character of as-indacenopyrene segment. These results suggest that the overall electronic structure of 3 cannot be rationalized satisfactorily in term of antiaromaticity.
Calculations of 3 at the CASSCF(2,2)/6-31G level of theory furnished a diradical index (y = 0.05) that is comparable to that of bisanthene (y = 0.07) and much smaller than that of teranthene (y = 0.54) 55 . The increase of the calculation level (CASSCF(4,6)/6-31G(d)) again provided a relatively small diradical index of y = 0.07. In addition, the closed-shell (CS) singlet, open-shell (OS) singlet and triplet states of 3 were optimized at the (U)CAM-B3LYP/6-311+G(d,p) level (Supplementary Tables 3-5). The OS singlet state is structurally almost identical with the CS singlet state, being slightly more stable than the CS singlet state by 0.9 kcal mol -1 with the small diradical index of y = 0.03. The triplet state is energetically unfavourable with the energy gap ΔE ST of 6.7 kcal mol -1 . These results suggest the non-dominant contribution of an OS diradical state, which will be due to a small energetical gain given by the formation of one Clar's sextet in its resonance structure ( Supplementary  Fig. 36) 61 . This assumption has been supported by the experimental fact that heating an o-dichlorobenzene solution of 3 at 100°C for 12 h in air resulted in no distinct spectral change of the absorption, indicating that 3 is remarkably stable despite the absence of peripheral substituents (Supplementary Fig. 35).
The above-mentioned studies indicate that the electronic structure of as-indacenoterrylene 3 cannot be satisfactorily explained in terms of its antiaromaticity and diradical character. One plausible description could be the coexistence of asindacenopyrene-and terrylene units, which are moderately merged together without diminishing the individual electronic character. TD-DFT calculations predict that, while the S 0 -S 1 transition of terrylene is allowed (oscillator strength = 0.8363) with a large gap of 2.52 eV, that of as-indacenopyrene is forbidden (oscillator strength = 0.0118) with a small gap of 0.97 eV ( Supplementary Figs. 33 and 34). The oscillator strength and energy gap of 3 (0.1226 and 1.51 eV) lie between those of terrylene and as-indacenopyrene. Importantly, the transition dipoles of 3, as-indacenopyrene and terrylene are directed in the same molecular axis, which could be the origin of the moderate mixing of the transitions. Because of the coexistence of electronaccepting and electron-donating segments 62 , the S 0 -S 1 transition of 3 could include some contribution of the charge-transfer (CT) character. Indeed, the electrostatic potential maps suggest that the S 0 -S 1 transition associates with the change of orbital-and chargedistributions ( Supplementary Figs. 30 and 32). The ill-defined absorption feature of 3 and 9 with the absence of vibrational bands in the NIR region is reminiscent of CT character.

Discussion
We have synthesized the C 70 fragment as-indaceno[3,2,1,8,7,6ghijklm]terrylene 3. The synthetic concept is based on the generation of curvature by incorporation of sp 3 -hybridized carbon atoms in the fused π-system. The structure was assigned based on NMR spectroscopy and mass spectrometry. The physical properties were probed by UV/vis/NIR absorption spectroscopy, cyclic voltammetry, temperature-dependent NMR analysis, and theoretical calculations. Even though 3 contains only 34 carbon and 14 hydrogen atoms, it absorbs in the NIR region up to ca. 1300 nm. The absorption tail of 3 is comparable to organic diradicaloid 1 and antiaromatic hydrocarbon 2. However, as-indacenoterrylene 3 exhibits outstanding stability despite the absence of peripheral substituents, which stands in sharp contrast to the behaviour of other NIR-absorbing hydrocarbons such as diradicaloids and antiaromatic molecules. The current study thus offers effective design guidelines for aromatic hydrocarbons with narrow HOMO-LUMO gaps.

Methods
Materials and characterization. 1 H NMR (500 MHz) and 13 C NMR (126 MHz) spectra were recorded on a Bruker AVANCE III HD spectrometer. Chemical shifts were reported as the delta scale in ppm relative to CDCl 3 (δ = 7.26) for 1 H NMR and CDCl 3 (δ = 77.16 ppm) for 13 C NMR. The 1 H and 13 C NMR spectra of structurally novel chemical compounds are shown in Supplementary Figs. 1-17. Mass spectra were recorded using a Bruker microTOF by APCI methods. UV/vis/ NIR absorption spectra were recorded on a Shimadzu UV-2550 or JASCO V670 spectrometer. X-ray data were obtained using a Rigaku CCD diffractometer   (7). A Schlenk tube containing 5 (100 mg, 0.154 mmol), Pd(OPiv) 2 (36.8 mg, 0.119 mmol), PCy 3 •HBF 4 (87.6 mg, 0.238 mmol) and K 2 CO 3 (220 mg, 1.59 mmol) was evacuated and then refilled with N 2 . To the tube, dry and degassed N,Ndimethylacetamide (DMA) (6.0 ml) was added. The mixture was stirred at 130°C for 2 h and then poured into CH 2 Cl 2 . The mixture was passed through Celite. After removing the solvent, the purification by silica-gel column chromatography (CH 2 Cl 2 as eluent) afforded 7 in 21% yield (16.5 (15 mL) and CS 2 (15 mL) were added. After stirring, a solution of DDQ in CH 2 Cl 2 (6.21 mM, 8.30 mL) was added. The mixture was stirred at room temperature for 30 min. Saturated NaHCO 3 aq. was added to the mixture. The reaction mixture was extracted with CH 2 Cl 2 /CS 2 (1/1). The combined organic layers were dried over Na 2 SO 4 and passed through short silica-gel (CH 2 Cl 2 as an eluent). After removing the solvent, the purification by washing with hexane, MeOH and CHCl 3 afforded 3 in 73% yield (8.50 mg, 20.1 μmol) as a dark red solid.
Theoretical calculations. All calculations were carried out using the Gaussian 09 program 63 . Initial geometry of 3 was obtained from the X-ray structure of 7. The gas phase S 0 -and S 1 -geometries of 3 were optimized with the DFT method at the CAM-B3LYP/6-311+G(d,p) level 64 . Molecular orbitals were calculated at the CAM-B3LYP/6-311+G(d,p) level. The oscillator strengths of 3 were computed by the TD-DFT formalism at the CAM-B3LYP/6-311+G(d,p) level. The NICS and ACID calculations were conducted at the B3LYP/6-31G(d) level.
For the calculations of the possible diradical contribution, the geometries of 3 with the CS singlet, OS singlet and triplet states were optimized at the (U)CAM-B3LYP/6-311+G(d,p) level. The stability analysis was conducted to check the stability of the OS singlet solution. The diradical index y was obtained on the basis of the natural orbital occupation number (NOON) calculation. We also conducted CASSCF calculations at the CS optimized structure to determine y at the CASSCF (2,2)/6-31G and CASSCF(4,6)/6-31G(d) levels.
The TS geometry and intrinsic reaction coordinate were calculated at the B3LYP/6-31G(d) level by using the GRRM17 program 65 . The energy and freq calculations of the TS and ground state geometries were conducted at the CAM-B3LYP/6-311+G(d,p) level. Received: 17 April 2020; Accepted: 13 July 2020;