Push–pull type quinoidal perylene showing solvent polarity dependent diradical character and negative solvatochromism

Abstract

Synthesis and physical properties of a new push–pull type quinoidal perylene (DA-Per) are reported. Three resonance forms, a closed-shell quinoid, an open-shell diradicaloid and a closed-shell zwitterion, all contribute to the ground-state electronic structure. The molecule shows solvent polarity dependent diradical character and negative solvatochromism, which can be explained by the subtle balance among the three resonance forms in different solvent environments.

---

Recent studies demonstrated that π-conjugated molecules with intermediate π-bonding could show open-shell diradical character in the ground state.^1–6 For example, quinoidal oligo-para-phenylenes,^7–12 quinoidal oligothiophenes,^13,14 and polycyclic hydrocarbons embedded with a quinodimethane unit,^15–20 have an intrinsic tendency to recover aromaticity and become open-shell diradicals. This type of molecule usually possesses a small energy gap and shows amphoteric redox behaviour. They show paramagnetic properties at increased temperatures due to the thermal population from the singlet ground state to the triplet excited state. In addition, they exhibit enhanced third-order non-linear optical (NLO) properties (e.g., two-photon absorption) at an intermediate diradicaloid state.^21–25 However, so far, most of the studies have been conducted for symmetric molecules, while quinoidal molecules with asymmetric push–pull type structure are rarely investigated.^26,27 There are very limited examples of molecules showing both open-shell diradical character and dipolar zwitterionic character because they are counterparts to each other.^28,29 It was also demonstrated that π-conjugated molecules with a zwitterionic character usually show interesting solvatochromism and second-order NLO properties.^30–32 In particular, T. Marks et al. showed that highly twisted π-zwitterions were good second-order NLO chromophores with ultrahigh hyperpolarizability.^33–38 Herein, we report a new push–pull type quinoidal perylene molecule DA-Per, which contains a dicyanomethanide unit as an electron donor and a pyridinium moiety as an electron acceptor (Fig. 1a). The molecule can be drawn in three major resonance forms: a closed-shell quinoidal form, a closed-shell zwitterionic form, and an open-shell diradical form (Fig. 1a). The latter two forms can be stabilized by aromatic stabilization energy of the aromatic perylene unit. Our detailed studies revealed interesting solvent polarity dependent open-shell diradical character and unusual negative solvatochromism for DA-Per in different environments.

Fig. 1 (a) Chemical structure and three representative resonance forms of DA-Per. (b) Synthesis of DA-Per. Reagents and conditions: (a) 4-pyridinylboronic acid, Pd₂(dba)₃, Sphos, K₃PO₄, toluene, 110 °C, 12 h; (b) (i) malononitrile, NaH, Pd₂(dba)₃, dppf, THF; (ii) MeOTf, DCM, r.t., 20 min; (iii) NaHCO₃ (aq).

Compound DA-Per was synthesized according to the scheme shown in Fig. 1b. One-fold Suzuki coupling reaction between the 3,10-dibromo-1,1-bis(4-tetradecylphenyl)-1H-cyclopenta[ghi]perylene 1³⁹ and 4-pyridinylboronic acid (1 equivalent) gave the compound 2 in 70% yield. The Pd-catalyzed Takahashi coupling reaction between 2 and malononitrile in the presence of sodium hydride in anhydrous THF provided the aryl malononitrile intermediate, which was treated with methyl trifluoromethanesulfonate (MeOTf) followed by deprotonation with aqueous NaHCO₃ solution to afford the final target compound DA-Per in an overall 75% yield from 2. Compound DA-Per is stable and can be purified by silica gel column chromatography on trimethylamine deactivated silica gel. It is soluble in normal organic solvents because the 4-tetradecylphenyl groups attached to the sp³-carbon at the perylene bay region can help to suppress aggregation.

The electronic absorption spectra of DA-Per in different solvents show an unusual hyperchromic shift upon increase of the solvent polarity, that is, negative solvatochromism (Fig. 2a). The absorption maximum (λ_max) in chloroform, tetrahydrofuran (THF), dichloromethane (DCM), acetone, DMSO and methanol appears at 875, 822, 809, 702, 654 and 634 nm, respectively (Table 1). The peak at 716 nm observed in chloroform is similar to that of an H-aggregated perylene dimer.²⁷ However, concentration dependent UV-vis measurements indicate that there is almost no aggregation at low concentration (5 × 10⁻⁶–5 × 10⁻⁵ M) (Fig. S1 in the ESI†). This band may originate from some solvent polarity dependent higher-energy electronic transitions. The peak at about 480 nm observed in methanol could be correlated to the absorption of the aromatic perylene bridge due to the large stabilization of the zwitterionic form in the very polar solvent. The observed negative solvatochromism indicates that the molecule has larger dipole moment (μ) in the singlet ground state (S₀) than that in the first singlet excited state (S₁). The ground-state and the first singlet excited-state structures of DA-Per are optimized at the B3LYP/6-31G(d,p) level of theory (the long aliphatic chains are replaced by methyl groups during the calculations). Environment effects on S₀ and S₁ geometries are included by using the polarizable continuum model (PCM). Under gas phase, a large ground-state dipole moment (μ_g = 23.49 Debye) is calculated, and the negative/positive charges are mainly distributed at the dicyanomethide/pyridinium moiety, respectively, according to the calculated electrostatic potential map (Fig. 2b). The μ_g becomes larger with the increase of the solvent dielectric constant (ε_r) (Table 1). The calculated dipole moments at the first singlet excited state (μ_e) are slightly larger than that μ_g when the environment is gas phase and chloroform. However, when the solvent becomes more polar (in DCM, THF, acetone, DMSO and methanol), the μ_e is smaller than μ_g (Table 1), which explains the observed negative solvatochromism. Time-dependent (TD) DFT calculations (B3LYP/6-31G(d,p) combined with PCM) indicate that the longest wavelength absorption band is originated from the HOMO → LUMO electronic transition. The HOMO and LUMO are partially segregated, but have large spatial overlap, leading to a large oscillator strength (f = 1.068 under gas phase) and intense absorption (Fig. S8 and Table S1, ESI†). TD DFT calculations in different solvents also properly predict that the absorption maximum will show hyperchromic shift with increase of the solvent polarity (Table 1), and the trend is in agreement with the experimental observation, although there is still quite large deviation from the absolute wavelengths. We should also note that the observed solvent-dependent spectral change cannot be simply correlated with a single solvent polarity parameter (neither E_T(30) nor ε_r). Hydrogen bonding between the methanol molecules with the CN group, or between THF molecules with the acidic protons in the pyridinium unit, may explain the abnormality in methanol and THF.

Fig. 2 (a) Electronic absorption spectra of DA-Per in different solvents. (b) Optimized geometry with dipole moment (left) and the calculated electrostatic potential map (right) of DA-Per (gas phase).

Table 1 Calculated (calcd) dipole moment (μ) in the ground state (μ_g) and the first singlet excited state (μ_e), diradical character (y₀), singlet–triplet energy gap (ΔE_S–T), absorption maximum (λ_max), and oscillator strength (f), together with the experimentally observed result (Obs.) λ_max and molar absorption coefficient (ε) of DA-Per in different solvents with different solvent polarity parameters E_T(30), and dielectric constants (ε_r)

Solvents	Dipole moment (μ) (D)			λ max (nm)		ε (M^−1 cm^−1)/f		y 0	ΔES–T (kcal mol^−1)	Solvent polarity
	μ g (S0)	μ e (S1)	Δμeg	Obs.	Calcd	Obs. (ε)	Calcd (f)			E T(30)	ε r
Gas	23.49	27.47	+3.98	—	627.0	—	1.0913	0.4261	−7.17	—	—
Chloroform	39.70	39.78	+0.08	875	717.0	15 490	1.2723	0.3545	−9.38	39.1	4.7
DCM	43.85	42.34	−1.51	822	712.8	25 860	1.1695	0.3388	−9.91	40.7	8.9
THF	42.85	41.70	−1.15	809	711.9	22 720	1.1858	0.3426	−9.78	37.4	7.4
Acetone	46.84	44.41	−2.43	703	700.4	18 730	1.0677	0.3286	−10.26	42.2	20.7
DMSO	48.67	45.47	−3.20	654	702	15 030	1.0446	0.3243	−10.40	45.1	48.9
Methanol	47.75	45.10	−2.65	634	694.8	7630	1.0312	0.3262	−10.35	55.4	32.6

---

The ¹H NMR spectrum of DA-Per in CD₂Cl₂ shows that the resonances for the protons on the π-conjugated backbone are significantly broadened and cooling the solution down to −80 °C leads to sharpening of the peaks (Fig. 3a). Such observation indicates that DA-Per in CD₂Cl₂ may display open-shell diradical character and the NMR broadening is due to the existence of thermally populated paramagnetic triplet diradical species, a typical phenomenon for many open-shell singlet diradicaloids.^1–6 A broad ESR signal (g_e = 2.0029) was determined (Fig. 4a). The ESR intensity (I) decreased with decreasing temperature (T) for the solid sample (Fig. S2 in the ESI†) and fitting of the IT–T data by Bleaney–Bows equation⁴⁰ gave a singlet–triplet energy gap (ΔE_S–T) of −2.26 kcal mol⁻¹ (Fig. 4b). The spin densities of the triplet diradical are mainly distributed along the quinoidal conjugated backbone (Fig. 4b). Similar NMR broadening was observed in CDCl₃ and THF-d₈ at room temperature (Fig. S3 and S4 in the ESI†). On the other hand, the ¹H NMR spectra of DA-Per in polar DMSO-d₆, acetone-d₆, and methanol-d₄ all exhibit sharp peaks (Fig. 3b and Fig. S5 and S6 in the ESI†), indicating a closed-shell nature. The resonances in DMSO-d₆ are clearly assigned by using a 2D COSY technique (Fig. 3b). In agreement with the NMR spectra, ESR measurements of the DA-Per in different solvents with the same concentration (c = 1.66 × 10⁻⁴ M) show that the ESR intensity decreases in the order of CHCl₃, DCM, THF, acetone, methanol and DMSO. There are only very weak signals in the latter three polar solvents (Fig. 4a). These observations suggest that the diradical character is also dependent on the polarity of the solvent, that is, in the more polar solvents, the zwitterionic form is more stabilized, leading to smaller diradical character.

Fig. 3 (a) VT ¹H NMR spectra of DA-Per (500 MHz, aromatic region) in CD₂Cl₂. (b) 2D-COSY NMR spectrum of DA-Per (500 MHz, rt) in DMSO-d₆.

Fig. 4 (a) ESR spectra of DA-Per in different solvents at the same concentration (c = 1.66 × 10⁻⁴ M). (b) Fitted IT–T curve for DA-Per based on the VT ESR data measured in powder; I: integrated ESR intensity; T: temperature. Inset is the calculated spin density distribution map of the triplet state. (c) Calculated (RAS-SF/6-31G*) natural orbital profiles (isovalue: 0.04) and occupation numbers (in the parentheses).

To deeply understand this trend, we conducted multi-reference restricted active space spin flip (RAS-SF)^41,42 calculations based on the optimized triplet state geometries in different solvents. The diradical character index (y₀) is defined as the occupation number of the lowest unoccupied natural orbital (LUNO). According to the calculations, the diradical character decreases with the increase of solvent polarity in the order of CHCl₃, DCM, THF, acetone, methanol and DMSO (Table 1), which is the same as that of the ESR experiment. The unpaired electron densities are distributed along the π-conjugated backbone and the diradical character is calculated to be 0.426 in the gas phase (Fig. 4c). Similarly, the calculated vertical singlet–triplet excitation energy (ΔE_S–T) increases with increasing solvent polarity in the same order (Table 1). It should be noted that the calculated data only provide a reference to understand the trend, and it is not meaningful to compare the absolute values between the experiment and calculation (e.g., ΔE_S–T).

Compound DA-Per showed different electrochemical behavior in DCM and DMSO (Fig. 5). In DCM, two oxidation waves with half-wave potential E^ox_1/2 at 0.47 and 0.86 V (vs. Fc⁺/Fc) and two overlapped reduction waves with half-wave potential E^red_1/2 at −1.36 and −1.46 V (vs. Fc⁺/Fc). In DMSO, one oxidative wave with E^ox_1/2 at 0.87 V and one reduction wave with E^red_1/2 at −1.50 V were observed. Therefore, the electrochemical energy gap in DCM (1.83 eV) is smaller than that in DMSO (2.37 eV), in agreement with the optical energy gaps, which can be explained by the large contribution of open-shell electronic structure in DCM and the large zwitterionic contribution in DMSO.

Fig. 5 Cyclic voltammograms of DA-Per in DMSO and DCM with n-Bu₄N·PF₆ as a supporting electrolyte, and scan rate at 50 mV s⁻¹.

In conclusion, a new push–pull type quinoidal perylene based diradicaloid DA-Per was synthesized, which shows a negative solvatochromic effect due to a larger dipole moment at the first singlet excited state compared to that in the ground state. Notably, the open-shell diradical character also exhibits solvent dependence, and it displays closed-shell nature in the very polar solvents (DMSO, MeOH, acetone), while open-shell singlet diradical character in the less polar solvents (CHCl₃, DCM, THF). Therefore, a subtle balance between the three resonance forms is tunable by solvent polarity. This work represents a rare report on diradicaloids showing solvent polarity dependent radical character. Investigation on the solvent dependent NLO properties is in our plan.

Experimental

Synthesis of compound 2

A mixture of compound 1 (0.48 g, 0.5 mmol), pyridine-4-boronic acid (61.5 mg, 0.5 mmol, 1.0 equiv.), Pd₂(dba)₃ (22.9 mg, 5 mol%), SPhos (40.1 mg, 20 mol%), and K₃PO₄ (425 mg, 2.5 equiv.) in toluene (100 mL) was degassed and heated at 110 °C for 12 h. After cooling to room temperature, water was added, and the reaction mixture was extracted with chloroform (50 mL). The organic layer was dried over sodium sulfate and the solvent was removed under vacuum. The residue was subjected to column chromatography (silica gel, hexane/DCM = 1/1) to afford the title product 2 (0.33 g) in 70% yield. ¹H NMR (CDCl₃, 500 MHz): δ ppm 8.77 (br, 2H), 8.27 (m, 2H), 8.09 (d, J = 8.50 Hz, 1H), 7.99 (br, 2H), 7.92 (s, 1H), 7.77–7.66 (m, 3H), 7.61 (s, 1H), 7.19 (d, J = 8.10 Hz, 4H), 7.00 (d, J = 8.10 Hz, 4H), 2.42 (m, 2H), 1.58–1.25 (m, 8H), 1.25–1.18 (m, 36H), 0.88–0.78 (m, 12H); ¹³C NMR (CDCl₃, 125 MHz): δ ppm 149.90, 149.05, 145.99, 145.76, 139.73, 136.09, 135.79, 132.71, 131.10, 128.31, 127.86, 127.43, 126.17, 125.93, 124.63, 121.34, 45.67, 36.78, 31.86, 29.74, 29.52, 27.66, 27.30, 22.66, 14.11. HR-MS (APCI): m/z = 964.5403, calcd for C₆₆H₇₉BrN (M + 1): m/z = 964.5390, error = −1.4 ppm.

Synthesis of compound DA-Per

To an ice-cooled suspension of NaH (46 mg, 1.15 mmol, 4.8 equiv.) in anhydrous THF (10 mL) was added dropwise via syringe a solution of malononitrile (49.5 g, 0.57 mmol, 2.4 equiv.) in anhydrous THF (2.0 mL) under argon. The mixture was stirred for 1 h at room temperature and then compound 2 (231 mg, 0.24 mmol), Pd₂(dba)₃ (11.0 mg, 5 mol%), dppf (20.0 mg, 15 mol%), and THF (20 mL) were added subsequently under argon. The mixture was purged with argon for 0.5 h and was then heated at 85 °C for 12 h. The solvent was removed under reduced pressure, water was added and the reaction mixture was extracted with DCM (50 mL). The organic layer was dried over sodium sulfate and the solvent was removed under vacuum. The residue was dissolved in dry DCM (20 mL) and then a solution of methyl trifluoromethanesulfonate (39.8 mg, 0.24 mmol) in DCM (2 mL) was added. The mixture was stirred at room temperature for 20 min and then poured into ice water and neutralized with saturated aqueous NaHCO₃ solution (2 M, 50 mL). The organic layer was separated, washed with water, and dried over anhydrous Na₂SO₄. The solvent was removed under vacuum and the residue was subjected to column chromatography (deactivate silica gel by Et₃N, acetone/DCM = 1/1) to afford the title product DA-Per (0.17 g) in an overall 75% yield from 2. ¹H NMR (DMSO-d₆, 500 MHz): δ ppm 9.05 (d, J = 6.65 Hz, 2H), 8.95 (d, J = 8.65 Hz, 1H), 8.48 (d, J = 7.50 Hz, 1H), 8.42 (d, J = 7.50 Hz, 1H), 8.35 (d, J = 6.65 Hz, 2H), 8.12 (s, 1H), 7.94 (s, 1H), 7.83 (d, J = 8.50 Hz, 1H), 7.74 (m, 1H), 7.67 (m, 1H), 7.26 (d, J = 8.25 Hz, 4H), 7.06 (d, J = 8.25 Hz, 4H), 4.40 (s, 3H), 2.42 (m, 2H), 1.52–1.43 (br, 8H), 1.24–0.99 (m, 36H), 0.78–0.74 (m, 12H). HR-MS (APCI): m/z = 963.6567, calcd for C₇₀H₈₁N₃: m/z = 963.6530, error = −3.8 ppm.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by MOE Tier 3 programme (MOE2014-T3-1-004), NRF Investigatorship (NRF-NRFI05-2019-0005), MOE Tier 2 grant (MOE2018-T2-2-094) and NSFC grant (21801074).

Notes and references

1. M. Abe, Chem. Rev., 2013, 113, 7011.
2. Z. Sun, Z. Zeng and J. Wu, Acc. Chem. Res., 2014, 47, 2582.
3. Z. Zeng, X. Shi, C. Chi, J. T. L. Navarrete, J. Casado and J. Wu, Chem. Soc. Rev., 2015, 44, 6578.
4. T. Kubo, Chem. Lett., 2015, 44, 111.
5. T. Y. Gopalakrishna, W. Zeng, X. Lu and J. Wu, Chem. Commun., 2018, 54, 2186.
6. C. Liu, Y. Ni, X. Lu, G. Li and J. Wu, Acc. Chem. Res., 2019, 52, 2309.
7. A. E. Tschitschibabin, Chem. Ber., 1907, 40, 1810.
8. L. K. Montgomery, J. C. Huffman, E. A. Jurczak and M. P. Grendze, J. Am. Chem. Soc., 1986, 108, 6004.
9. K. Dimroth, W. Umbach and K. H. Blocher, Angew. Chem., Int. Ed. Engl., 1963, 2, 620.
10. R. West, J. A. Jorgenson, K. L. Stearly and J. C. Calabrese, J. Chem. Soc., Chem. Commun., 1991, 2, 1234.
11. G. Li, H. Phan, T. S. Herng, T. Y. Gopalakrishna, C. Liu, W. Zeng, J. Ding and J. Wu, Angew. Chem., Int. Ed., 2017, 56, 5012.
12. G. Li, Y. Han, Y. Zou, J. J. C. Lee, Y. Ni and J. Wu, Angew. Chem., Int. Ed., 2019, 58, 14319–14326.
13. T. Takahashi, K. I. Matsuoka, K. Takimiya, T. Otsubo and Y. Aso, J. Am. Chem. Soc., 2005, 127, 8928.
14. R. P. Ortiz, J. Casado, V. Hernandez, J. T. López Navarrete, P. M. Viruela, E. Orti, K. Takimiya and T. Otsubo, Angew. Chem., Int. Ed., 2007, 46, 9057.
15. K. Ohashi, T. Kubo, T. Masui, K. Yamamoto, K. Nakasuji, T. Takui, Y. Kai and I. Murata, J. Am. Chem. Soc., 1998, 120, 2018.
16. A. Shimizu, T. Kubo, M. Uruichi, K. Yakushi, M. Nakano, D. Shiomi, K. Sato, T. Takui, Y. Hirao, K. Matsumoto, H. Kurata, Y. Morita and K. Nakasuji, J. Am. Chem. Soc., 2010, 132, 14421.
17. A. Shimizu, Y. Hirao, K. Matsumoto, H. Kurata, T. Kubo, M. Uruichi and K. Yakushi, Chem. Commun., 2012, 48, 5629.
18. Z. Sun, K.-W. Huang and J. Wu, J. Am. Chem. Soc., 2011, 133, 11896.
19. Z. Sun, S. Lee, K. Park, X. Zhu, W. Zhang, B. Zheng, P. Hu, Z. Zeng, S. Das, Y. Li, C. Chi, R. Li, K. Huang, J. Ding, D. Kim and J. Wu, J. Am. Chem. Soc., 2013, 135, 18229.
20. R. Huang, H. Phan, T. S. Herng, P. Hu, W. Zeng, S. Dong, S. Das, Y. Shen, J. Ding, D. Casanova and J. Wu, J. Am. Chem. Soc., 2016, 138, 10323.
21. M. Nakano, H. Nagao and K. Yamaguchi, Phys. Rev. A: At., Mol., Opt. Phys., 1997, 55, 1503.
22. M. Nakano, R. Kishi, T. Nitta, T. Kubo, K. Nakasuji, K. Kamada, K. Ohta, B. Champagne, E. Botek and K. Yamaguchi, J. Phys. Chem. A, 2005, 109, 885.
23. M. Nakano, R. Kishi, S. Ohta, H. Takahashi, T. Kubo, K. Kamada, K. Ohta, E. Botek and B. Champagne, Phys. Rev. Lett., 2007, 99, 033001.
24. S. Ohta, M. Nakano, K. Kubo, K. Kamada, K. Ohta, R. Kishi, N. Nakagawa, B. Champagne, E. Botek, A. Takebe, S. Umezaki, M. Nate, H. Takahashi, S. Furukawa, Y. Morita, K. Nakasuji and K. Yamaguchi, J. Phys. Chem. A, 2007, 111, 3633.
25. K. Kamada, K. Ohta, T. Kubo, A. Shimizu, Y. Morita, K. Nakasuji, R. Kishi, S. Ohta, S. I. Furukawa, H. Takahashi and M. Nakano, Angew. Chem., Int. Ed., 2007, 46, 3544.
26. S. Inoue, Y. Aso and T. Otsubo, Chem. Commun., 1997, 1105.
27. Z. Zeng, S. Lee, M. Son, K. Fukuda, P. M. Burrezo, X. Zhu, Q. Qi, R.-W. Li, J. T. López Navarrete, J. Ding, J. Casado, M. Nakano, D. Kim and J. Wu, J. Am. Chem. Soc., 2015, 137, 8572.
28. Y. Hirao, N. Nagamachi, K. Hosoi and T. Kubo, Chem. – Asian J., 2018, 13, 510.
29. Y. Hirao, Y. Hamamoto, N. Nagamachi and T. Kubo, Phys. Chem. Chem. Phys., 2019, 21, 12209.
30. C. Reichardt, Chem. Rev., 1994, 94, 2319.
31. L. Beverina and G. A. Pagani, Acc. Chem. Res., 2014, 47, 319.
32. A. J.-T. Lou and T. J. Marks, Acc. Chem. Res., 2019, 52, 1428.
33. I. D. L. Albert, T. J. Marks and M. A. Ratner, J. Am. Chem. Soc., 1998, 120, 11174.
34. H. Kang, A. Facchetti, P. Zhu, H. Jiang, Y. Yang, E. Cariati, S. Righetto, R. Ugo, C. Zuccaccia, A. Macchioni, C. L. Stern, Z. Liu, S.-T. Ho and T. J. Marks, Angew. Chem., Int. Ed., 2005, 44, 7922.
35. H. Kang, A. Facchetti, H. Jiang, E. Cariati, S. Righetto, R. Ugo, C. Zuccaccia, A. Macchioni, C. L. Stern, Z. Liu, S.-T. Ho, E. C. Brown, M. A. Ratner and T. J. Marks, J. Am. Chem. Soc., 2007, 129, 3267.
36. G. S. He, J. Zhu, A. Baev, M. Samoc, D. L. Frattarelli, N. Watanabe, A. Facchetti, H. Ågren, T. J. Marks and P. N. Prasad, J. Am. Chem. Soc., 2011, 133, 6675.
37. Y. Shi, D. Frattarelli, N. Watanabe, A. Facchetti, E. Cariati, S. Righetto, E. Tordin, C. Zuccaccia, A. Macchioni, S. L. Wegener, C. L. Stern, M. A. Ratner and T. J. Marks, J. Am. Chem. Soc., 2015, 137, 12521.
38. A. J.-T. Lou, S. Righetto, C. Barger, C. Zuccaccia, E. Cariati, A. Macchioni and T. J. Marks, J. Am. Chem. Soc., 2018, 140, 8746.
39. W. Zeng, H. Phan, T. S. Herng, T. Y. Gopalakrishna, N. Aratani, Y. Ni, Z. Zeng, H. Yamada, J. Ding and J. Wu, Chem, 2017, 2, 81.
40. B. Bleaney and K. D. Bowers, Proc. R. Soc. London, Ser. A, 1952, 214, 451.
41. M. Head-Gordon, Chem. Phys. Lett., 2003, 372, 508.
42. D. Casanova and M. Head-Gordon, Phys. Chem. Chem. Phys., 2009, 11, 9779.

---

Footnote

† Electronic supplementary information (ESI) available: Additional spectra and characterization data, and theoretical calculation details. See DOI: 10.1039/c9qm00566h

---