Chemical oxidation of a double-twisted nanographene

Abstract

Chemical oxidation of a double-twisted nanographene led to quantitative generation of corresponding dicationic species, which exhibited an intense absorption band tailing to 1500 nm wherein two orthogonally arranged π-skeletons contributed to transitions and a high dissymmetry factor of −0.0209 was recorded at 1443 nm in the NIR-II region.

---

Twisted nanographenes (NGs) have just emerged as a novel class of non-planar NGs, while the role of the twist operation is still under exploration.^1,2 In most cases, installation of multiple [n]helical units is a way to prepare twisted NGs. Therefore, this type of NGs is often regarded as conventional helical NGs with paying less attention to the role of twist operation.^3–10 Nevertheless, twisted NGs possessing a large π-surface were recently revealed.^11,12 However, it is, in general, difficult to prepare the sole conformation during the synthesis of twisted NGs so that the products are likely obtained as a mixture of multiple conformations including twisted, wavy, and even combined geometries.¹³ For instance, in 2020, Wang and co-workers reported the synthesis of C₁₅₀ NGs which were obtained as a conformational mixture containing unidirectionally-twisted (ut), alternatively-twisted (at), and randomly-twisted geometries.¹⁴ At nearly the same time, a research group led by Maçôas and Campaña reported C₁₁₄ NG and its analogue containing a heptagonal ring, the former of which contains ut and at geometries with a ratio of 1.2 : 1, while the latter was conformationally pure.¹⁵

Recently, we reported conformation-selective synthesis of a double-twisted NG featuring two orthogonally arranged π-skeletons centered at a contorted pyrene core. This twisted NG exhibited a robust conformational stability with retaining D₂ symmetry.¹⁶ In that work, the NG was found to undergo two-electron ejection in a redox process with producing an electrochemically stable dicationic species (1²⁺) (Fig. 1). Herein, we report the generation of the dicationic species by chemical oxidation and clarify the influence of twisted geometry on its chiroptoelectronic properties.

Fig. 1 A dicationic double-twisted NG (1²⁺) studied herein.

Chemical oxidation of 1 was performed with 2.2 equiv. of NOSbF₆ in C₂H₄Cl₂ under a N₂ atmosphere (Fig. 2a). After addition of the oxidant at 0 °C, the solution was allowed to warm to room temperature. During 2 h, the compound was mostly precipitated. The resulting solid was collected by centrifugation using C₂H₄Cl₂/n-hexane (1 : 20) to afford 1²⁺[SbF₆⁻]₂ in 83% isolated yield as a brown solid. It should be noted that 1²⁺[SbF₆⁻]₂ was stable in CD₂Cl₂ at room temperature under N₂ for at least 19 days (Fig. S4, ESI†) as confirmed by ¹H NMR showing nine sets of sharp signals (Fig. 2b). Upon comparing the chemical shifts of 1²⁺[SbF₆⁻]₂ with those for neutral 1, significant upfield shifts were found for protons labeled with H(1), H(6), and H(9), while opposite shifts were confirmed for those labeled with H(3), H(4), and H(5), indicating a change in local aromaticity at each hexagon. These protons were fully assigned according to through-space correlation peaks observed by NOESY (Fig. 2c). Owing to the dicationic charge, ¹³C signals were overall shifted down-field, while two characteristic signals showing considerable shifts appeared at δ 148.53 and 144.35 ppm (Fig. 2d). The simulated ¹³C spectrum of 1²⁺ (GIAO-B3LYP/6-311 + G(2d,p)//B3LYP-D3/6-31G(d); Fig. S5, ESI†) reproduces the spectral features with the two signals being assignable to the pyrene carbons. This implies that the cationic charge is strongly distributed on the pyrene moiety in 1²⁺.

Fig. 2 (a) Chemical oxidation of 1 (the arrows represent NOE correlations). (b) ¹H NMR (600 MHz, CD₂Cl₂) spectra of 1 and 1²⁺[SbF₆⁻]₂. (c) NOESY (600 MHz, CD₂Cl₂) spectrum of 1²⁺[SbF₆⁻]₂. (d) ¹³C NMR spectra of 1 (151 MHz, CD₂Cl₂) and 1²⁺[SbF₆⁻]₂ (151 MHz, CD₂Cl₂/CD₃CN = 5 : 1).

To get better insights into the aromatic character of 1²⁺, theoretical calculations were performed for both 1 and 1²⁺. Fig. 3a shows an optimized structure of 1²⁺, showing that H(3) is located 5.634 Å above an edge of ring F and H(4) and H(5) face ring E with distances of 4.350 and 2.361 Å, respectively. Therefore, the chemical shifts of these protons should be affected considerably. The 3D ICSS (iso-chemical shielding surface) plots clearly demonstrate the significant change in the shielding environment at ring C (Fig. 3b). The NICS(0) (nucleus independent chemical shift) value at C changes from −8.2 (1) to +8.6 (1²⁺) upon 2e⁻ oxidation. This change was also verified by ACID (anisotropy of current-induced density) plots where diatropic ring currents at C in 1 are switched to paratropic nature in 1²⁺ (Fig. 3c). The significant change in aromaticity was also confirmed for rings E and G where NICS(0) values varied from −1.4 (1) to +3.0 (1²⁺) and from −9.7 (1) to −7.6 (1²⁺), respectively (Fig. 3b). From these information, the observed up-field shifts for H(1), H(9), and H(6) are ascribed to a weakened deshielding effect caused by decreased aromaticity in G for the former as well as shielding effect from weak antiaromaticity attained by E for the rest. In addition, the down-field shifts for H(3), H(4), and H(5) are due to a weakened shielding effect caused by lowered aromaticity of F for the former as well as a deshielding effect from weak antiaromaticity attained by E for the rest. The electrostatic potential (ESP) map of 1²⁺ exhibits localization of the cationic charge on the pyrene moiety (Fig. 3d), being matched well with the resonance structure (Fig. 3e) expected from ¹³C spectral feature (Fig. 2d).

Next, absorption spectra were recorded in CH₂Cl₂ (Fig. 4a). Neutral 1 exhibits a strong absorption band at 592 nm. By chemical oxidation to 1²⁺[SbF₆⁻]₂, an intense broadband absorption was observed at 1175 nm (ε = 41 583 M⁻¹ cm⁻¹), tailing to 1500 nm in the NIR-II (1000–1700 nm) region. Since 1²⁺[SbF₆⁻]₂ is a chiral nanographene with two-fold twist chirality and four-fold helical chirality, we first performed optical resolution of rac-1 to obtain (MM,MMMM)–1 which shows dissymmetry factors (g_CD) of 6.5 × 10⁻³ at 342 nm and 6.9 × 10⁻⁴ at 592 nm.¹⁶ After chemical oxidation to (MM,MMMM)–1²⁺[SbF₆⁻]₂, the circular dichroism (CD) spectrum was recorded in CH₂Cl₂ (Fig. 4b), being demonstrative of a wide range of chiroptical activities. The highest Cotton effect of (MM,MMMM)–1²⁺[SbF₆⁻]₂ was found at 536 nm where g_CD is obtained to be +1.9 × 10⁻³. The slightly higher g_CD value of −3.5 × 10⁻³ was observed at 680 nm with a negative Cotton effect. Notably, the highest g_CD value with an order of 10⁻² (−2.09%) was detected at 1443 nm in the near-infrared II (NIR-II) region. Such a large anisotropy factor,^17–22 especially in the NIR region,^23–25 is quite rare among known chiral nanocarbon materials.

Fig. 3 (a) Optimized structure of 1²⁺ (B3LYP-D3/6-31G(d)). (b) 3D ICSS plots of 1 and 1²⁺ with NICS(0) (units in ppm) (HF/6-31 + G(d,p)). (c) ACID plots (B3LYP/6-31G(d)). (d) ESP map of 1²⁺ (B3LYP-D3/6-31G(d)). (e) The most probable resonance structure of 1²⁺.

Fig. 4 (a) UV-vis-NIR absorption spectra of 1 and 1²⁺[SbF₆⁻]₂. (b) CD and g_CD spectra (CH₂Cl₂) of (MM,MMMM)–1²⁺[SbF₆⁻]₂.

To verify the origin of high g_CD, we computed the optoelectronic nature of (MM,MMMM)–1²⁺[SbF₆⁻]₂ (Fig. 5). The intense NIR-II bands were then assigned to S₀ → S₂ (λ_calc = 1172 nm) and S₀ → S₃ (1135 nm) with significant contributions from excitations corresponding to HOMO–1 → LUMO (92%) and HOMO → LUMO (74%), respectively (Fig. 5a). These transitions are electronically allowed with moderately large oscillator strengths (f) of 0.1728 and 0.3912, respectively, and mostly occur within the terropyrene moiety which is one of the twisted π-skeletons in 1²⁺. The lowest energy transition (S₀ → S₁) was confirmed at 1380 nm, featuring an excitation, with a 72% contribution, from HOMO-2, which shows significant delocalization of orbital coefficients on the twisted tetrabenzotetracene moiety, to the LUMO with a strong character of the contorted pyrene. The oscillator strength of this transition was suggested to be smaller (f = 0.0156) by a tenth compared with those of S₀ → S₂ (0.1728) and S₀ →S₃ (0.3912). This is primarily owing to the smaller magnitude of electric transition dipole moment, |μ| (S₀ → S₁, 182 × 10⁻²⁰ esu cm; S₀ → S₂, 557 × 10⁻²⁰ esu cm; S₀ → S₃, 825 × 10⁻²⁰ esu cm).

Fig. 5 (a) Optical transitions of (MM,MMMM)–1²⁺[SbF₆⁻]₂ with molecular orbitals. (b) Pictorial representation of transition dipole moments μ and m. The calculations were performed at the TD CAM-B3LYP-D3/SDD & 6-31G(d)//B3LYP-D3/SDD & 6-31G(d) level of theory (SDD was applied for Sb) and transition energies were calibrated with 0.72.²⁶

For the lowest energy transition of (MM,MMMM)–1²⁺[SbF₆⁻]₂, the size of the magnetic transition dipole moment was estimated to be |m| = 2.13 × 10⁻²⁰ erg G⁻¹, while the angle between the two transition dipole moments was given as 123°. The arrangement of the two transition dipole moments was therefore illustrated as shown in Fig. 5b. Accordingly, the g_calc was obtained as −0.0254 which is in accordance with the experimental observation (g_CD = −0.0209). For common organic molecules, |μ| tends to be larger by ca. 300 times than |m| wherein the physical origin is a ratio between the two transition dipole moments as large as ea₀/μ_B ≈ 274 (in CGS units)^27,28 where e is the elementary charge, a₀ is the Bohr radius, and μ_B is the Bohr magneton. Under this circumstance, g_calc is simply described as 4(|m|/|μ|) cos θ which obviously suggests that g_calc is more likely to be perturbed by an electric term for the electromagnetic wave of interest than the magnetic term. The |μ|/|m| ratio of 85.3 for 1²⁺[SbF₆⁻]₂ is, however, one order smaller than those for common chiral organic molecules, consequently leading to a large g_CD value even in the NIR-II region.

In summary, we synthesized a stable dicationic double-twisted nanographene, 1²⁺[SbF₆⁻]₂, wherein the cationic charge is delocalized on the central pyrene core. Notably, owing to the small |μ|/|m| ratio, a high dissymmetry factor of −0.0209 was recorded at 1443 nm in the NIR-II region. The robust stability of the dicationic NG is achieved probably due to steric protection of the central pyrene core by the peripheral π-wings arranged in a propeller manner. This would be of benefit for its usage as an NIR-II chiral chromophore.

Financial support was provided by the National Natural Science Foundation of China (No. 2022161035), the Education Department of Inner Mongolia Autonomous Region (NJYT22089), the Funding Scheme for High-Level Overseas Chinese Students’ Return, International Collaborative Research Program of Institute for Chemical Research (ICR), Kyoto University (2022-39 and 2023-46), the JSPS KAKENHI Grant Number JP22H04538, the Advanced Technology Institute Research Grants 2023, ISHIZUE 2024 of Kyoto University, and the JACI Prize for Encouraging Young Researchers.

Data availability

The data supporting this article are included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Notes and references

1. Y.-F. Wu, L. Zhang, Q. Zhang, S.-Y. Xie and L.-S. Zhang, Org. Chem. Front., 2022, 9, 4726–4743.
2. H. V. Anderson, N. D. Gois and W. A. Chalifoux, Org. Chem. Front., 2023, 10, 4167–4197.
3. R. K. Dubey, M. Melle-Franco and A. Mateo-Alonso, J. Am. Chem. Soc., 2022, 144, 2765–2774.
4. A. Bernhardt, D. Čavlović, M. Mayländer, O. Blacque, C. M. Cruz, S. Richert and M. Juríček, Angew. Chem., Int. Ed., 2024, 63, e202318254.
5. S. E. Penty, G. R. F. Orton, D. J. Black, R. Pal, M. A. Zwijnenburg and T. A. Barendt, J. Am. Chem. Soc., 2024, 146, 5470–5479.
6. R. Eichelmann, P. Jeudy, L. Schneider, J. Zerhoch, P. R. Mayer, J. Ballmann, F. Deschler and L. H. Gade, Org. Lett., 2024, 26, 1172–1177.
7. R. Bam, W. Yang, G. Longhi, S. Abbate, A. Lucotti, M. Tommasini, R. Franzini, C. Villani, V. J. Catalano, M. M. Olmstead and W. A. Chalifoux, Angew. Chem., Int. Ed., 2024, 63, e202404849.
8. Y. Liu, Z. Li, M.-W. Wang, J. Chan, G. Liu, Z. Wang and W. Jiang, J. Am. Chem. Soc., 2024, 146, 5295–5304.
9. A. Swain, K. Radacki, H. Braunschweig and P. Ravat, Chem. Sci., 2024, 15, 11737–11747.
10. X. Liu, Z. Jin, F. Qiu, Y. Guo, Y. Chen, Z. Sun and L. Zhang, Angew. Chem., Int. Ed., 2024, 63, e202407547.
11. C. Wallerius, O. Erdene-Ochir, E. V. Doeselar, R. Alle, A. T. Nguyen, M. F. Schumacher, A. Lützen, K. Meerholz and S. H. Pun, Precis. Chem, 2024, 2, 488–494.
12. B. Borrisov, G. M. Beneventi, Y. Fu, Z. Qiu, H. Komber, Q. Deng, P. M. Greißel, A. Cadranel, D. M. Guldi, J. Ma and X. Feng, J. Am. Chem. Soc., 2024, 146, 27335–27344.
13. Y. Li, Z. Jia, S. Xiao, H. Liu and Y. Li, Nat. Commun., 2016, 7, 11637.
14. S. Ma, J. Gu, C. Lin, Z. Luo, Y. Zhu and J. Wang, J. Am. Chem. Soc., 2020, 142, 16887–16893.
15. S. Castro-Fernández, C. M. Cruz, I. F. A. Mariz, I. R. Márquez, V. G. Jiménez, L. Palomino-Ruiz, J. M. Cuerva, E. Maçôas and A. G. Campaña, Angew. Chem., Int. Ed., 2020, 59, 7139–7145.
16. Y. Dong, Z. Zhang, Y. Hashikawa, H. Meng, F. Bai, K. Itami and C. Chaolumen, Angew. Chem., Int. Ed., 2024, 63, e202406927.
17. S. Sato, A. Yoshii, S. Takahashi, S. Furumi, M. Takeuchi and H. Isobe, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 13097–13101.
18. W. Zheng, T. Ikai, K. Oki and E. Yashima, Nat. Sci, 2022, 2, e20210047.
19. Y. Hashikawa, S. Okamoto, S. Sadai and Y. Murata, J. Am. Chem. Soc., 2022, 144, 18829–18833.
20. M. Toya, T. Omine, F. Ishiwari, A. Saeki, H. Ito and K. Itami, J. Am. Chem. Soc., 2023, 145, 11553–11565.
21. Y. Hashikawa, S. Okamoto and Y. Murata, Nat. Commun., 2024, 15, 514.
22. G.-F. Huo, W.-T. Xu, Y. Han, J. Zhu, X. Hou, W. Fan, Y. Ni, S. Wu, H.-B. Yang and J. Wu, Angew. Chem., Int. Ed., 2024, 63, e202403149.
23. Y. Nakakuki, T. Hirose, H. Sotome, M. Gao, D. Shimizu, R. Li, J. Hasegawa, H. Miyasaka and K. Matsuda, Nat. Commun., 2022, 13, 1475.
24. Y. Hashikawa, S. Sadai, S. Okamoto and Y. Murata, Angew. Chem., Int. Ed., 2023, 62, e202215380.
25. S. T. Bao, S. Louie, H. Jiang, Q. Jiang, S. Sun, M. L. Steigerwald, C. Nuckolls and Z. Jin, J. Am. Chem. Soc., 2024, 146, 51–56.
26. Y. Hashikawa, S. Okamoto and Y. Murata, Commun. Chem., 2020, 3, 90.
27. T. Mori, Chem. Rev., 2021, 121, 2373–2412.
28. H. Kubo, T. Hirose, T. Nakashima, T. Kawai, J. Hasegawa and K. Matsuda, J. Phys. Chem. Lett., 2021, 12, 686–695.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4cc05264a

---