Engineering visible-to-ultraviolet emission switching of benzothiazole-based mechanoresponsive materials

Abstract

Introducing bulky substituents or twisted structures into luminescent molecules can effectively induce stimuli-responsive emission colour shifts. However, achieving stimuli responsiveness often requires large molecular frameworks to form many intermolecular interactions, which typically result in luminescence colour switching within the visible light region. This study used 2-(4-tert-butylphenylethynyl)benzothiazole (BPEB) to synthesise two polymorphs, BPEB-B and BPEB-N, which exhibit blue and ultraviolet emissions, respectively. Mechanical grinding of BPEB-B induced a visible-to-ultraviolet emission shift. These photophysical changes correlated with the molecular packing in the aggregated state. The visible emission of BPEB-B is attributed to dimer formation, whereas the ultraviolet emission of BPEB-N is due to staircase structures stabilised by C–H/π interactions. Analyses show that mechanical grinding disrupted the aggregates, thereby shifting the emission towards that of the monomer solutions. This result suggests that benzothiazole can serve as a versatile scaffold for visible-to-ultraviolet light-switching materials.

---

Introduction

Organic luminescent materials are widely applied in various fields, including organic light-emitting diodes (OLEDs) and fluorescence bio-imaging.^1,2 Their solid-state luminescent properties depend on their aggregated structures, such as powders and crystals.^3,4 Recently, numerous stimuli-responsive organic luminescent materials have been reported,^5–8 wherein emission colour switching was achieved by changing the aggregated structures through external stimuli such as grinding or heating.^9–13 Reversible luminescence switching by controlling molecular aggregation without changing the chemical structure has potential applications in mechanical or thermal sensors^14,15 and memory devices.^16,17 Luminescence switching mechanisms often rely on the interplay of noncovalent interactions, such as hydrogen bonding mediated by nitrogen and oxygen atoms and π-stacking or C–H/π interactions involving aromatic rings.^18–24 Anthracene and pyrene, which easily facilitate π-stacking interactions, are common structural motifs in luminescent switching materials, where nitrogen or oxygen atoms are introduced as hydrogen-bonding sites (Scheme 1a).^25–32 Furthermore, incorporating sterically bulky substituents or twisted structures into the molecular framework can induce luminescence switching through crystal phase transitions due to increased excluded volume; this is an important molecular design strategy for switching materials.^33–35 Many of these molecules exhibit emission colour switching in the visible light region (380–650 nm),^36–42 and the emission wavelength shifting from the visible to ultraviolet region are highly rare.⁴³ This is because many switching materials possess large molecular structures with extended π-conjugation, which shifts the emission to the visible light region (Scheme 1b).^44,45 The ability to reversibly switch luminescence between visible and undetectable ultraviolet light provides a high level of confidentiality, making it a promising feature for fluorescent anti-counterfeiting devices.⁴⁶ To enable switching in the ultraviolet region, designing molecules with relatively small molecular frameworks is necessary.

Scheme 1 (a) Anthracene-based (b) pyrene-based mechanoresponsive materials.

Benzothiazole possesses the potential to be a compact material for ultraviolet-shift switching, as it features the above-mentioned necessary characteristics and has nitrogen and sulphur atoms. Although there have been few reports on its application as a switching material,^47–49 further development is warranted. This study demonstrates that 2-(4-tert-butylphenylethynyl)benzothiazole (BPEB), a molecule with a relatively simple structure, exhibits two distinct crystal polymorphs: block-shaped crystals that emit blue light and needle-shaped crystals that emit ultraviolet light (Scheme 2). Furthermore, the block-shaped crystals exhibit luminescence switching to the ultraviolet region upon mechanical grinding. Detailed single-crystal X-ray structural analysis reveals that this shift is attributed to the collapse of the aggregate structure, leading to ultraviolet emission from the compact benzothiazole skeleton. This result indicates that benzothiazole is a promising stimuli-responsive scaffold for switching materials with ultraviolet emissions.

Scheme 2 Chemical structure of BPEB synthesized in this study.

Experimental

Materials and methods

This study obtained ¹H-NMR (400 MHz) and ¹³C-NMR (100 MHz) spectra using a Bruker AVANCE III 400 NMR spectrometer in a chloroform-d (CDCl₃) solution, and chemical shifts were reported in parts per million (ppm) using the residual protons in the NMR solvent. All chemicals were of reagent grade and used as purchased.

Synthesis of 2-(4-tert-butyl-phenylethynyl)-benzothiazole (BPEB)

The Sonogashira cross-coupling reaction of 4-tert-butyl-1-ethynylbenzene (0.96 g, 6.1 mmol) and 2-iodobenzothiazole (1.2 g, 5.1 mmol) was performed with Cl₂Pd(PPh₃)₂ (0.18 g, 0.25 mmol), PPh₃ (0.066 g, 0.25 mmol), CuI (0.096 g, 0.51 mmol), and Et₃N (25 mL) at 60 °C in an argon atmosphere. After 3 h, the precipitate formed during the reaction was separated via atmospheric filtration, and the filtrate was poured into a saturated aqueous NH₄Cl solution. The crude product was extracted three times using AcOEt. The collected organic layer was dried over anhydrous Na₂SO₄ and separated via filtration. The filtrate was evaporated in vacuo and subjected to silica-gel column chromatography (eluent: hexane/CH₂Cl₂ = 1/1) to obtain BPEB (0.83 g, 2.8 mmol) in 47% yield as a white powder. ¹H-NMR (400 MHz, CDCl₃) δ 8.12 (d, J = 8.1 Hz, 1H), 7.87 (d, J = 8.0 Hz, 1H), 7.58 (dt, J = 8.6 Hz, 2H), 7.52 (td, J = 8.4 Hz, 1H), 7.47–7.41 (m, 3H), and 1.34 (s, 9H) ppm. ¹³C NMR (100 MHz, CDCl₃) δ 153.6, 153.1, 149.0, 135.5, 132.1, 126.8, 126.2, 125.7, 123.7, 121.4, 118.1, 96.5, 82.4, 77.5, 77.2, 76.8, and 35.1 ppm. IR: ν 2954, 2864, 2213, 1508, 1478, 1432, 1359, 1314, 1263, 1192, 1114, 1091, 1056, 1013, and 936 cm⁻¹.

Crystal growth

Single crystals of BPEB-B and BPEB-N were obtained via recrystallisation from a mixed solvent (CH₂Cl₂/MeOH = 1/1). Approximately 800 mg of BPEB was dissolved in 30 mL of the solvent and allowed to evaporate slowly for five days in a loosely capped screw vial.

Single crystal X-ray diffraction

A suitable crystal was selected and measured using a Rigaku XtaLAB AFC10 (RCD3) quarter-chi single diffractometer. The crystal was maintained at 173 K during data collection. Using Olex2,⁵⁰ the structure was solved with the SHELXT⁵¹ structure solution program using intrinsic phasing and refined with the SHELXL⁵² refinement package using least-squares minimisation.

Powder X-ray diffraction

Measurement was performed using a Rigaku SmartLab-SP/IUA with Cu Kα radiation at 293 K.

Photophysical properties

Ultraviolet-visible (UV-vis) absorption spectra were recorded using a JASCO V-630 spectrometer. Photoluminescence (PL) spectra were measured using a JASCO FP-6300 spectrometer. The PL quantum yields were measured using a Hamamatsu Photonics Quantaurus-QY measurement system C11347-01. The PL lifetime was measured using a Hamamatsu Photonics Quantaurus-Tau lifetime spectrometer C11367-34.

Quantum chemical calculation

Using the Gaussian 16 software package (revision C.01),⁵³ geometry optimisations were performed with the hybrid meta-GGA functional (M06-2X)⁵⁴ and 6-31+G(d) basis set.⁵⁵ The vertical excitation energies of the optimised structures were calculated using a time-dependent self-consistent field approximation at the same level of theory.

Results and discussion

Crystal polymorphism and powder X-ray diffraction

The synthesised BPEB powder was recrystallised using dichloromethane/methanol to yield two types of crystals: block-shaped BPEB-B and needle-shaped BPEB-N. These crystals emitted different luminescent colours when irradiated with an ultraviolet lamp; therefore, they were carefully separated, and their crystal structures were analysed. (Fig. 1 and 2). BPEB-B was assigned to the triclinic P1̄ space group with two molecules per unit cell (Fig. 1a). The benzothiazole aromatic ring and the aromatic ring substituted with a tert-butyl group were twisted by 58.9° using the alkyne (Fig. 1b).

Fig. 1 Crystal structure of 2-(4-tert-butyl-phenylethynyl)-benzothiazole (BPEB)-B. (a) Unit cell, (b) dihedral angle of two aromatic rings, (c and d) dimer structures, and (e) intermolecular CH/S interaction.

Fig. 2 Crystal structure of BPEB-N. (a) Unit cell, (b) dihedral angle of two aromatic rings, (c and d) staircase structures, and (e) intermolecular CH/N interaction.

The molecules formed antiparallel edge-to-face dimers through C–H/π interactions with a C⋯H distance of 291 pm (Fig. 1c and d). Additionally, each dimer was connected via C–H/S interactions (Fig. 1e; H⋯S distance = 296 pm). BPEB-N was assigned to a monoclinic P2₁/c space group with 28 molecules in the unit cell (Fig. 2a). Similar to BPEB-B, the two aromatic rings have twisted structures (Fig. 2b, twist angles = 57.0°, 55.5°, and 50.2°). The molecules were stacked in parallel with a step-like alignment due to C–H/π interactions, with the C⋯H distances of 282 and 285 pm (Fig. 2c and d). Furthermore, these stepped-aligned units were linked via CH/N interactions (Fig. 2e).

To understand the relationship between the two crystal polymorphs better, powder X-ray diffraction (XRD) analysis was performed using mechanically ground BPEB-B and BPEB-N powders (hereinafter called BPEB-Bp and BPEB-Np, respectively). The simulated powder XRD results for BPEB-B and BPEB-N showed distinct sharp peaks (Fig. 3). BPEB-Bp and BPEB-Np showed similar characteristic diffraction patterns (2θ = 11.4°, 19.3°, 20.1°, 21.6°, and 29.8°). This result suggests that the ground powder has a different aggregate structure from the BPEB-B and BPEB-N structures, indicating that the BPEB molecule can adopt three aggregated structures, which have potential as light-emitting switching materials.

Fig. 3 Powder X-ray diffraction patterns of BPEB-Bp and BPEB-Np and simulated powder diffraction patterns of BPEB-B and BPEB-N.

Photophysical properties in solution

The synthesised BPEB powder exhibited good solubility in hexane, THF, CH₂Cl₂, and DMF. UV-vis absorption and PL spectra were measured using the solutions prepared at 10 μM (Fig. 4, Table 1). The absorption maxima (λ_abs) in each solvent were at 315–320 nm. They showed similar absorption spectral shapes, independent of the solvent. These absorptions are attributed to ππ* transitions from HOMO to LUMO through quantum chemical calculations (Fig. 5). The frontier orbitals suggest that local excitation (LE) occurred without intramolecular charge transfer (ICT) characteristics. The emission maxima (λ_em) were approximately 365 nm with multiple vibrational structures in the ultraviolet region. The absorption and emission spectra showed mirror structures; this further suggests that they did not exhibit ICT character but showed LE character. The absolute PL quantum efficiency (PLQY, Φ_em) was 0.01 or 0.02. Free rotation of the alkyne moiety in solution probably led to an extremely low PLQY.

Fig. 4 UV-vis absorption (dotted line) and photoluminescence (solid line) spectra of BPEB in various solvents (10 μM, excited at maximim absorption wavelength).

Table 1 Photophysical properties of BPEB in various solvents (10 μM, excited at maximim absorption wavelength)

Solvents	λ abs [nm] (ε [10^3 L mol^−1 cm^−1])	λ em [nm]	Φ em
Hexane	315 (33.2), 326 (30.3), 337 (25.3)	346, 360	0.01
THF	318 (32.1), 328 (29.9), 340 (25.3)	351, 365	0.01
CH2Cl2	320 (32.6), 331 (29.9), 341 (25.8)	354, 368	0.01
DMF	319 (33.9), 331 (30.0), 341 (25.3)	354, 368	0.02

---

Fig. 5 Calculated frontier orbitals of BPEB.

Photophysical properties of BPEB crystals and powders

The photophysical properties were investigated using BPEB-B and BPEB-N crystals and BPEB-Bp and BPEB-Np powders (Fig. 6, Table 2). BPEB-B exhibited blue emission with a peak at 444 nm and a PLQY of 0.20, whereas BPEB-N exhibited ultraviolet emission with a peak at 378 nm. Comparing the two crystals, BPEB-N showed a significant short-wavelength shift and substantial decrease in the PLQY.

Fig. 6 Photoluminescence spectra and photographs under UV-lamp (365 nm) of (a) BPEB-B, BPEB-Bp (b) BPEB-N, BPEB-Np.

Table 2 Photophysical properties of BPEB crystals and powders

	λ em [nm]	Φ em
BPEB-B	444	0.20
BPEB-Bp	378	0.01
BPEB-N	381	0.01
BPEB-Np	378	0.01

---

Single-crystal XRD (SXRD) analysis revealed that BPEB-B formed a dimer unit with two slightly twisted molecules, whereas BPEB-N had molecules stacked in parallel. Consequently, the red-shifted emission observed for BPEB-B can be attributed to strong stabilisation through electrostatic interactions facilitated by dimer formation. The interaction strength correlated with the high PLQY achieved by suppressing non-radiative deactivation pathways. BPEB-Bp demonstrated a significant short-wavelength shift in its emission maximum, resulting in an ultraviolet emission with a peak wavelength of 381 nm. This is likely due to the mechanical disruption of the dimer, leading to emission characteristics resembling those of the monomer in solution. Moreover, PLQY decreased significantly after the mechanical grinding of BPEB-B, and the value is consistent with the solution's PLQY, which closely resembles that of the monomer. However, the photophysical properties of BPEB-N and BPEB-Np exhibited minimal differences, with only a slight blue shift in the emission wavelength observed after grinding. This minor blue shift can be attributed to the disruption of C–H/π interactions in BPEB-N due to the mechanical grinding, which breaks down the step structure and shifts the emission closer to that of the monomer in solution. Although the ground powder was exposed to solvent vapor to investigate the reversibility of its molecular arrangement, no reversibility was observed (Fig. S5†).

Conclusions

This study demonstrated the synthesis of two distinct crystal polymorphs, BPEB-B and BPEB-N, exhibiting blue and ultraviolet emissions, respectively, from the simple molecule BPEB. Results showed that mechanical grinding of the crystals induced emission colour switching to the ultraviolet region in both polymorphs. These changes in the photophysical properties are closely associated with the molecular packing arrangements in the aggregated state. Specifically, the blue emission of BPEB-B is attributed to the dimer formation, whereas the ultraviolet emission of BPEB-N is attributed to a staircase structure stabilised by C–H⋯π interactions. Mechanically stimulated powders disrupted these aggregates, leading to emissions approaching those of the monomer in solution for both polymorphs. These results suggest that benzothiazoles can serve as versatile molecular scaffolds for materials exhibiting visible-to-ultraviolet light switching.

Data availability

The data supporting this article are included in the ESI.† The crystallographic data for BPEB-B and BPEB-N have been deposited at the CCDC under 2440642, 2440643 and can be obtained from https://www.ccdc.cam.ac.uk/structures/.

Author contributions

R. Y.: data curation, investigation, validation, visualization, writing (original draft preparation), and writing (review and editing); K. K., P. Y., H. F., M. Y., S. Y., T. K.: data curation, investigation, and writing (review and editing); M. M.: conceptualization, data curation, investigation, validation, visualization, funding acquisition, writing (original draft preparation), writing (review and editing), and project administration.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

M. M. acknowledges Fuji Seal Foundation and Shin-Sozai Joho Zaidan Grant. Some measurements were performed at Open Facility Center for Research, Ibaraki University.

References

1. C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913–915.
2. M. M. Swamy, Y. Murai, K. Monde, S. Tsuboi and T. Jin, Bioconjugate Chem., 2021, 32, 1541–1547.
3. Y. Dong, B. Xu, J. Zhang, X. Tan, L. Wang, J. Chen, H. Lv, S. Wen, B. Li, L. Ye, B. Zou and W. Tian, Angew. Chem., Int. Ed., 2012, 51, 10782–10785.
4. C. Shi, Z. Guo, Y. Yan, S. Zhu, Y. Xie, Y. S. Zhao, W. Zhu and H. Tian, ACS Appl. Mater. Interfaces, 2013, 5, 192–198.
5. S. Li and D. Yan, Adv. Opt. Mater., 2018, 6, 1800445.
6. R. Kubota, S. Takahashi, S. Nagai and S. Ito, Chem. – Asian J., 2023, 18, e202300124.
7. Z. Chen, X. Chen, D. Ma, Z. Mao, J. Zhao and Z. Chi, J. Am. Chem. Soc., 2023, 145, 16748–16759.
8. X.-T. Lv, D. Zheng, F.-S. Wan, Y.-L. Liu, Z.-H. Guo, D.-K. Cao and X.-L. Yang, Chem. – Eur. J., 2025, e202403886.
9. S. Varughese, J. Mater. Chem. C, 2014, 2, 3499–3516.
10. Y. Wang, G. Zhang, W. Zhang, X. Wang, Y. Wu, T. Liang, X. Hao, H. Fu, Y. Zhao and D. Zhang, Small, 2016, 12, 6554–6561.
11. X. Zhang, Z. Chi, B. Xu, L. Jiang, X. Zhou, Y. Zhang, S. Liu and J. Xu, Chem. Commun., 2012, 48, 10895–10897.
12. K. Wada, K. Hashimoto, J. Ochi, K. Tanaka and Y. Chujo, Aggregate, 2021, 2, e93.
13. W. Liu, X. Wang, R. Li, S. Sun, Z. Li, J. Hao, B. Lin, H. Jiang and L. Xie, ChemistrySelect, 2022, 7, e202104111.
14. A. Pucci, F. Di Cuia, F. Signori and G. Ruggeri, J. Mater. Chem., 2007, 17, 783–790.
15. M. Kinami, B. R. Crenshaw and C. Weder, Chem. Mater., 2006, 18, 946–955.
16. S.-J. Lim, B.-K. An, S. D. Jung, M.-A. Chung and S. Y. Park, Angew. Chem., Int. Ed., 2004, 43, 6346–6350.
17. M. Irie, T. Fukaminato, T. Sasaki, N. Tamai and T. Kawai, Nature, 2002, 420, 759–760.
18. S.-J. Yoon, J. W. Chung, J. Gierschner, K. S. Kim, M.-G. Choi, D. Kim and S. Y. Park, J. Am. Chem. Soc., 2010, 132, 13675–13683.
19. X. Du, F. Xu, M.-S. Yuan, P. Xue, L. Zhao, D.-E. Wang, W. Wang, Q. Tu, S.-W. Chen and J. Wang, J. Mater. Chem. C, 2016, 4, 8724–8730.
20. B. Xu, J. He, Y. Mu, Q. Zhu, S. Wu, Y. Wang, Y. Zhang, C. Jin, C. Lo, Z. Chi, A. Lien, S. Liu and J. Xu, Chem. Sci., 2015, 6, 3236–3241.
21. S.-J. Yoon and S. Y. Park, J. Mater. Chem., 2011, 21, 8338–8346.
22. H. Y. Zhang, Z. L. Zhang, K. Q. Ye, J. Y. Zhang and Y. Wang, Adv. Mater., 2006, 18, 2369–2372.
23. Y. Liu, X. Tao, F. Wang, X. Dang, D. Zou, Y. Ren and M. Jiang, J. Phys. Chem. C, 2008, 112, 3975–3981.
24. T. Seki and T. Okada, Chemistry, 2024, 30, e202402622.
25. U. Deori, E. Ravindran, G. P. Nanda, S. G. Ramanath, U. Acharya, P. Gandeepan and P. Rajamalli, Chem. – Asian J., 2023, 18, e202300352.
26. T. Seki, N. Tokodai, S. Omagari, T. Nakanishi, Y. Hasegawa, T. Iwasa, T. Taketsugu and H. Ito, J. Am. Chem. Soc., 2017, 139, 6514–6517.
27. Q. Dai, J. Zhang, R. Tan, S. Wang, Y. Li, Q. Li and S. Xiao, Mater. Lett., 2016, 164, 239–242.
28. K. Duraimurugan, M. Harikrishnan, J. Madhavan, A. Siva, S. J. Lee, J. Theerthagiri and M. Y. Choi, Environ. Res., 2021, 194, 110741.
29. H. Yu, M. Li, X. Chen, N. Han, Z. Xu and M. Wang, Eur. J. Org. Chem., 2024, 27, e202400876.
30. T. Wang, N. Zhang, Y. Ge, C. Wang, Z. Hang and Z. Zhang, Macromol. Chem. Phys., 2020, 221, 1900463.
31. K. R. J. Thomas and A. Maurya, ChemPhotoChem, 2024, 8, e202400119.
32. W. Zhao, Z. Ding, Z. Yang, T. Lu, B. Yang and S. Jiang, Chemistry, 2024, 30, e202303202.
33. H. Chung, S. Chen, B. Patel, G. Garbay, Y. H. Geerts and Y. Diao, Cryst. Growth Des., 2020, 20, 1646–1654.
34. X. Zhang, Z. Chi, J. Zhang, H. Li, B. Xu, X. Li, S. Liu, Y. Zhang and J. Xu, J. Phys. Chem. B, 2011, 115, 7606–7611.
35. W. Luo, Y. Tang, X. Zhang, Z. Wu and G. Wang, Adv. Opt. Mater., 2023, 11, 2202259.
36. Z. Ma, M. Teng, Z. Wang, S. Yang and X. Jia, Angew. Chem., Int. Ed., 2013, 52, 12268–12272.
37. H. Mori, K. Nishino, K. Wada, Y. Morisaki, K. Tanaka and Y. Chujo, Mater. Chem. Front., 2018, 2, 573–579.
38. A. Deák, C. Jobbágy, G. Marsi, M. Molnár, Z. Szakács and P. Baranyai, Chemistry, 2015, 21, 11495–11508.
39. X. Zeng, T. Zhou, J. Liu, K. Wu, S. Li, X. Xiao, Y. Zhang, S. Gong, G. Xie and C. Yang, Adv. Opt. Mater., 2018, 6, 1801071.
40. S. K. Suresh, P. D. Raju, A. Krishnan, L. M. Ramachandran and C. V. Suneesh, Chemistry, 2023, 29, e202204030.
41. H. Liu, Y. Shen, Y. Yan, C. Zhou, S. Zhang, B. Li, L. Ye and B. Yang, Adv. Funct. Mater., 2019, 29, 1901895.
42. S. Nakamura, K. Okubo, Y. Nishii, K. Hirano, N. Tohnai and M. Miura, Chemistry, 2023, 29, e202302605.
43. J. Guan, F. Xu, C. Tian, L. Pu, M.-S. Yuan and J. Wang, Chem. – Asian J., 2019, 14, 216–222.
44. S. Yamaguchi, I. Yoshikawa, T. Mutai and K. Araki, J. Mater. Chem., 2012, 22, 20065–20070.
45. A. D. Yan, H. Yang, O. Meng, H. Lin and M. Wei, Adv. Funct. Mater., 2014, 24, 587–594.
46. R. Zou, J. Zhang, S. Hu, F. Hu, H. Zhang and Z. Fu, CrystEngComm, 2017, 19, 6259–6262.
47. B. Fang, L. Lai, M. Chu, Y. Shi and M. Yin, ChemPhotoChem, 2021, 5, 270–274.
48. M. Barłóg, S. K. Podiyanachari, H. S. Bazzi and M. Al-Hashimi, Macromol. Rapid Commun., 2025, e2400914.
49. M. A. Potopnyk, D. Volyniuk, R. Luboradzki, A. Lazauskas and J. V. Grazulevicius, Eur. J. Org. Chem., 2021, 2772–2781.
50. O. V. Dolomanov, L. J. Bourhis, R. J. Gildea, J. A. K. Howard and H. Puschmann, J. Appl. Crystallogr., 2009, 42, 339–341.
51. G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Adv., 2015, 71, 3–8.
52. G. M. Sheldrick, Acta Crystallogr., Sect. C:Struct. Chem., 2015, 71, 3–8.
53. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman and D. J. Fox, Gaussian 16, revision, C. 01, Gaussian Inc., Wallingford, CT, 2016.
54. Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215–241.
55. W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, 56, 2257–2261.

---

Footnote

† Electronic supplementary information (ESI) available. CCDC 2440642 and 2440643. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5ce00379b

---