A cationic sulfur-hydrocarbon triradical with an excited quartet state†

Shuxuan Tang , Huapeng Ruan , Zhaobo Hu , Yue Zhao , You Song and Xinping Wang *
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China. E-mail: xpwang@nju.edu.cn

First published on 14th January 2022

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For the importance in fundamental science and electronic areas, main-group element-based radical species are under the spotlight.^1,2 Compared to various kinds of reported main-group monoradicals and diradicals, research on main-group triradicals and multiradicals is quite rare.^3,4 Triradical molecules, featuring three unpaired electrons, can be divided into two categories according to its ground state.^5–7 If all of the unpaired electrons in a triradical are ferromagnetically coupled in pairs, its ground state would be a quartet (S = 3/2, Fig. 1a).⁶ If two of the unpaired electrons are antiferromagnetically coupled, the ground state would be a doublet (S = 1/2, Fig. 1b).⁷ Until now, studies on organic triradical species (i.e., C/N/O-based) have revealed their potential application in magnetic areas.^5–7 For example, a doublet triangle triradical may feature spin frustration, which can be treated as the structural foundation and origin of quantum spin liquid.^7,8 However, structurally characterized main-group element-based triradicals, beyond C/N/O elements, are sparse. Only four examples have been reported up to now (Scheme 1), which are either neutral (A–C) or anionic (D).³ Triradical C features a quartet ground state while the strong intermolecular interaction among triradicals A and B prevent further exploration on their intramolecular magnetic coupling. The metal complex D, constructed by three diborane radical anions fused by a Fe^II ion through metal–ligand coordination bonds, was observed to feature a doublet ground state though zero-field splitting (ZFS) of its thermally populated quartet state has not been observed.

Sulfur-hydrocarbon radicals are attracting a great deal of attention in organic electronic and magnetic areas.⁹ Thianthrene is a typical sulfur-hydrocarbon, and has been proved to feature high stability under chemical oxidation.^9e,10 We speculate that thianthrene radicals could be applied as structural units for building triradicals by using a three-dimensional hydrocarbon, triptycene, as a rigid central framework. In this work, a triptycene-bridged tris(thianthrene) compound 1 was designed and synthesized. 1 was prepared by refluxing the DMF solution of 2,3,6,7,14,15-hexabromotriptycene¹¹ and 1,2-benzenedithiol¹² in the existence of potassium carbonate for 12 h (Scheme S1, ESI†). It was isolated as a white solid in 71% yield after purification (see the ESI†). Cyclic voltammetry of 1 reveals three oxidation peaks (1.38 V, 1.43 V and 1.55 V), consistent to the differential pulse voltammetry (DPV) result (Fig. S3, ESI†),¹³ indicating that 1 could undergo a three-electron oxidation process.

Inspired by the electrochemical oxidation result, chemical oxidation reaction of 1 with three equivalents of NO[Al(OR_F)₄] (OR_F = OC(CF₃)₃) was carried out.¹⁴ The suspension of 1 turned to dark purple immediately after added the oxidant, accompanied by the formation of nitric oxide bubbles. Interestingly, crystallization of the reaction filtrate at −35 °C and room temperature afforded two different kinds of crystals for the triradical trication salts, 2a and 2b, respectively (Scheme 2). Both crystals are thermally stable under anaerobic condition at room temperature, and characterized by single-crystal X-ray diffraction, EPR spectroscopy and SQUID measurements.

For comparison, single-crystal structure of 1 was also obtained (Fig. 2a). Geometries of 2a and 2b are illustrated in Fig. 2 and Fig. S5–S9 (ESI†). Selected bond distances are given in Table S2 (ESI†).¹⁵ The neutral compound 1 crystallizes in the hexagonal space group P6₃/m. Three equivalent thianthrene units are separated through the central triptycene rigid framework and each is bent relative to the S–S axis. One-dimensional pore passages with diameters around 5.84 Å, were found along the c axis in the crystal structure of 1 (Fig. S8, ESI†). Although both are composed of triradical trication salts, 2a and 2b have different crystal packing patterns as the triclinic space group P and the monoclinic space group P2₁/c, respectively. In 2a, the trications are completely separated by counteranions without intermolecular π–π interactions (Fig. S9, ESI†), while weak π–π interactions were found among the triradical trication dimers in 2b with the distances of 3.565 Å between the benzene rings of the neighboring trications (Fig. 2c).

Upon three equivalent oxidations, the triptycene-bridged tris(thianthrene) structures remain while all thianthrene units in 2a and 2b become planar (Fig. 2b and c), suggesting that the oxidations occur on all three thianthrene branches. Correspondingly, the C–S bond distances of the planarized rings decrease (Table S2, ESI†), similar to the reported changes in C–S bond lengths from neutral thianthrene to thianthrene radical cation.^10b The average C–S bond distances become shorter from neutral compound 1 (1.785 Å) to 2a (1.721 Å) and 2b (1.717 Å), while the average C–S–C bond angles turn wider from 1 (98.6°) to 2a (107.4°) and 2b (107.4°).

To further explore the electronic structures of 2a and 2b, continuous-wave EPR (CW-EPR) measurements were carried out. The frozen solution EPR spectrum of 2a in dibutyl phthalate (DBP) at 88 K reveals ZFS with parameters D = 27.0 G and E = 1.2 G, due to the thermally excited quartet state (Fig. 3a). The g factor is anisotropic with g_x = 2.0062, g_y = 2.0072 and g_z = 2.0108 (Fig. S10, ESI†). Half-field signal of the forbidden Δm_s = ±2 transition was clearly observed, while 1/3-field signal of the forbidden Δm_s = ±3 transition was absent, which is likely attributed to the relatively large doublet–quartet energy gap, or the weak intensity of the 1/3-field signal compared to the half-field signal.^3d,6c The powder samples of 2a were also found to exhibit the half-field signals of forbidden transition (Δm_s = ±2) at both room temperature and 88 K (Fig. S11 and S12, ESI†). Quite similar to 2a, the EPR results of 2b exhibit ZFS (D = 27.0 G and E = 1.2 G) in frozen solution EPR spectrum (Fig. 3b and Fig. S13, ESI†), and half-field signals not only in the frozen solution EPR spectrum, but also in the powder EPR spectra at both room temperature and 88 K (Fig. S14 and S15, ESI†).

The electron spin relaxation of 2a was investigated by pulse EPR spectroscopy (Fig. S16 and S17, Tables S3 and S4, ESI†). The electron spin–lattice relaxation time T₁ was measured with the three-pulse inversion recovery sequence, π–T–π/2–τ–π–τ–echo. T₁ was obtained as 11.92 μs at 6 K, and decreased upon temperature rising (Fig. S16, ESI†). The quantum coherence time T₂ was measured by the two-pulse echo decay sequence, π/2–τ–π–τ–echo, and was recorded as 156.4 ns at 6 K. Different from T₁, T₂ was shown to be increased upon temperature rising (Fig. S17, ESI†).

Variable temperature magnetic susceptibility measurements were performed on the powder samples of 2a and 2b (Fig. 3c and d). The χ_MT versus T plot of 2a was 0.978 cm³ mol⁻¹ K at 310 K, and then gradually decreased upon cooling, indicating the antiferromagnetic intramolecular interaction in 2a (Fig. 3c). Besides, the χ_MT value was shown to be 0.244 cm³ mol⁻¹ K at 2 K, lower than that expected for a discrete S = 1/2 state with g = 2 (0.375 cm³ mol⁻¹ K). Several repeat measurements using different batches of samples show the same results, indicating that this unusual behaviour is ascribed to the nature of the compound itself. Considering the slight difference of the C–S bond distances among each S-doped ring (Table S2, ESI†), the triradical trication in 2a should be treated as a scalene triangular three-spin model. The Heisenberg–Dirac–van Vleck (HDVV) Hamiltonian can be described as Ĥ = −2J₁₂Ŝ₁Ŝ₂ − 2J₁₃Ŝ₁Ŝ₃ − 2J₂₃Ŝ₂Ŝ₃. However, for both isosceles and scalene triangle triradical models, the relative energies of its three spin states (one quartet state and two doublet states) depend only on two energy gaps, which means that the three different coupling parameters in scalene models cannot be determined unequivocally.¹⁶ In this sense, we would consider the triradical trication of 2a as an isosceles triangle approximately, in order to work out the coupling parameters. The HDVV Hamiltonian can be described as Ĥ = −2J′(Ŝ₁Ŝ₂ + Ŝ₁Ŝ₃) − 2J₂₃Ŝ₂Ŝ₃, where J′ is the approximate coupling parameter.¹⁶ The experimental χ_MT data were fitted by PHI program,¹⁷ and the isotropic g factor was determined to be 2.00 with a mass factor w = 0.95.¹⁸ The best fit can be obtained with J′ = −51.5 cm⁻¹, J₂₃ = −57.1 cm⁻¹, zj = −1.65 cm⁻¹ and TIP = 6.126 × 10⁻⁴ cm³ mol⁻¹. The doublet–quartet energy gap is determined to be −165.7 cm⁻¹ (−473.8 cal mol⁻¹).^7d The deviation of the fitting result below 90 K is probably attributed to the complicated coupling competition among the three spins in 2a, which features a relatively large difference from a simple Weiss mean field. The molecule of 2a displays as a geometric frustration, and the three antiferromagnetic coupling constants are closed to each other, suggesting that 2a is also spin-frustrated. This implies that the three spins may arrange in nonlinear. The net spins cannot equal to 2 × 1/2 − 1/2 = 1/2, but be less than 1/2 in a canting state. Upon cooling below 90 K, the antiferromagnetic coupling is dominant relative to the thermal disturbance from temperature, which results in the continuous decrease of χ_MT. The experimental data are only the expressions of the spin arrangement, which is no longer following the theoretical spin Hamiltonian equation above. This is the reason for the large deviation between the fitting results and the experimental curve below 90 K.

The χ_MT versus T plot of 2b is similar to that of 2a (Fig. 3d). Fitted with the HDVV Hamiltonian Ĥ = −2J’(Ŝ₁Ŝ₂ + Ŝ₁Ŝ₃) − 2J₂₃Ŝ₂Ŝ₃, the best fitting parameters w = 0.95, g = 2.00, J′ = −50.9 cm⁻¹, J₂₃ = −55.9 cm⁻¹, zj = −1.90 cm⁻¹, TIP = 6.488 × 10⁻⁴ cm³ mol⁻¹ and the doublet–quartet energy gap −162.7 cm⁻¹ (−465.2 cal mol⁻¹) were obtained. The fitting results of 2b reveal that the weak intermolecular π–π interactions have little influence on the intramolecular isotropic exchange coupling. The UV-vis-NIR spectrum of 1 exhibits absorption peaks at 266 nm and 310 nm (Fig. 4). The absorption spectra of 2a and 2b are similar to each other, which both show peaks at around 282 nm, 300 nm, 580 nm, and broad absorptions at around 878 nm (Fig. 4).

Theoretical calculations were carried out to explore the electronic structures of the triradical trication in 2a and 2b, and geometry optimizations were performed at the (U)PBE0/6-311G* level (Table S5–S7, Fig. S20 and S21, ESI†). A doublet ground state of the triradical trication was obtained, with the doublet–quartet energy gap ΔE = −192.4 cm⁻¹ (−550.0 cal mol⁻¹), close to the results obtained from SQUID measurements. The calculated geometries are in good accordance with the experimental data. The C–S bond distances of all three thianthrene rings decrease upon oxidation (Table S6, ESI†), consistent with the crystal structures (Table S2, ESI†). The spin density distributions for the triradical trication are mainly delocalized on all of the S-doped rings (Fig. 5a and b), with significant contribution from every sulfur atom. The spin density distributions of the two S atoms in each thianthrene units are 0.60, 0.60 and −0.60, respectively.

In summary, the triptycene-bridged tris(thianthrene) compound 1 was designed and synthesized. Upon three equivalent oxidations of 1, triradical trication salts 2a and 2b were isolated as thermally stable crystals. The experimental and theoretical calculation results reveal the doublet ground states for 2a and 2b. The triradical trications in 2a and 2b are the first examples of cationic main-group element-based triradicals. The electron spin–lattice relaxation time T₁ of 2a was recorded to be 11.92 μs at 6 K. It is noteworthy that T₁ is close to that of several reported electron spin qubits, suggesting the development potential of main-group radicals at quantum information processing.^19,20 More researches on open-shell sulfur-hydrocarbons with intriguing physical property are ongoing in our laboratory.

We thank the National Key R&D Program of China (Grants 2016YFA0300404 and 2018YFA0306004, X. W.) and the National Natural Science Foundation of China (Grants 21525102 and 21690062, X. W.) for financial support. The calculations were performed at the High-Performance Computing Centre of Nanjing University. Dr Li Zhang is acknowledged for the discussions on the theoretical calculations during revision.

Conflicts of interest

There are no conflicts to declare.

References

1. (a) P. P. Power, Chem. Rev., 2003, 103, 789–809 CrossRef CAS PubMed ; (b) R. G. Hicks, Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds, John Wiley & Sons, Ltd, 2010 CrossRef ; (c) J. Hankache and O. S. Wenger, Chem. Rev., 2011, 111, 5138–5178 CrossRef CAS PubMed ; (d) T. Chivers and J. Konu, Comprehensive Inorganic Chemistry II, ed. K. Poeppelmeier, Elsevier, Amsterdam, 2nd edn, 2013, pp. 349–373 Search PubMed ; (e) C. D. Martin, M. Soleilhavoup and G. Bertrand, Chem. Sci., 2013, 4, 3020–3030 RSC ; (f) K. Chandra Mondal, S. Roy and H. W. Roesky, Chem. Soc. Rev., 2016, 45, 1080–1111 RSC ; (g) Y. Su and R. Kinjo, Coord. Chem. Rev., 2017, 352, 346–378 CrossRef CAS .
2. (a) F. Breher, Coord. Chem. Rev., 2007, 251, 1007–1043 CrossRef CAS ; (b) M. Abe, Chem. Rev., 2013, 113, 7011–7088 CrossRef CAS PubMed .
3. (a) A. W. Cordes, R. C. Haddon, R. G. Hicks, R. T. Oakley, T. T. M. Palstra, L. F. Schneemeyer and J. V. Waszczak, J. Am. Chem. Soc., 1992, 114, 5000–5004 CrossRef CAS ; (b) A. W. Cordes, R. C. Haddon, R. G. Hicks, D. K. Kennepohl, R. T. Oakley, L. F. Schneemeyer and J. V. Waszczak, Inorg. Chem., 1993, 32, 1554–1558 CrossRef CAS ; (c) T. Nozawa, M. Ichinohe and A. Sekiguchi, Chem. Lett., 2015, 44, 56–57 CrossRef ; (d) L. Wang, J. Li, L. Zhang, Y. Fang, C. Chen, Y. Zhao, Y. Song, L. Deng, G. Tan, X. Wang and P. P. Power, J. Am. Chem. Soc., 2017, 139, 17723–17726 CrossRef CAS PubMed .
4. (a) A. Rodriguez, F. S. Tham, W. W. Schoeller and G. Bertrand, Angew. Chem., Int. Ed., 2004, 43, 4876–4880 CrossRef CAS PubMed ; (b) A. Hinz, A. Schulz and A. Villinger, J. Am. Chem. Soc., 2015, 137, 9953–9962 CrossRef CAS PubMed .
5. (a) K. Yoshizawa, A. Chano, A. Ito, K. Tanaka, T. Yamabe, H. Fujita, J. Yamauchi and M. Shiro, J. Am. Chem. Soc., 1992, 114, 5994–5998 CrossRef CAS ; (b) M. Guillemot, L. Toupet and C. Lapinte, Organometallics, 1998, 17, 1928–1930 CrossRef CAS ; (c) A. I. Krylov, J. Phys. Chem. A, 2005, 109, 10638–10645 CrossRef CAS PubMed ; (d) J. K. Mahoney, D. Martin, C. E. Moore, A. L. Rheingold and G. Bertrand, J. Am. Chem. Soc., 2013, 135, 18766–18769 CrossRef CAS PubMed .
6. (a) F. Kanno, K. Inoue, N. Koga and H. Iwamura, J. Phys. Chem., 1993, 97, 13267–13272 CrossRef CAS ; (b) S. Suzuki, T. Wada, R. Tanimoto, M. Kozaki, D. Shiomi, K. Sugisaki, K. Sato, T. Takui, Y. Miyake, Y. Hosokoshi and K. Okada, Angew. Chem., Int. Ed., 2016, 55, 10791–10794 CrossRef CAS PubMed ; (c) D. Shimizu and A. Osuka, Angew. Chem., Int. Ed., 2018, 57, 3733–3736 CrossRef CAS PubMed ; (d) A. Nagata, S. Hiraoka, S. Suzuki, M. Kozaki, D. Shiomi, K. Sato, T. Takui, R. Tanaka and K. Okada, Chem. – Eur. J., 2020, 26, 3166–3172 CrossRef CAS PubMed ; (e) C. Shu, M. Pink, T. Junghoefer, E. Nadler, S. Rajca, M. B. Casu and A. Rajca, J. Am. Chem. Soc., 2021, 143, 5508–5518 CrossRef CAS PubMed ; (f) E. V. Tretyakov, P. V. Petunin, S. I. Zhivetyeva, D. E. Gorbunov, N. P. Gritsan, M. V. Fedin, D. V. Stass, R. I. Samoilova, I. Y. Bagryanskaya, I. K. Shundrina, A. S. Bogomyakov, M. S. Kazantsev, P. S. Postnikov, M. E. Trusova and V. I. Ovcharenko, J. Am. Chem. Soc., 2021, 143, 8164–8176 CrossRef CAS PubMed .
7. (a) J. Fujita, M. Tanaka, H. Suemune, N. Koga, K. Matsuda and H. Iwamura, J. Am. Chem. Soc., 1996, 118, 9347–9351 CrossRef CAS ; (b) O. B. Borobia, P. Guionneau, H. Heise, F. H. Köhler, L. Ducasse, J. Vidal-Gancedo, J. Veciana, S. Golhen, L. Ouahab and J.-P. Sutter, Chem. – Eur. J., 2005, 11, 128–139 CrossRef PubMed ; (c) S. Suzuki, A. Nagata, M. Kuratsu, M. Kozaki, R. Tanaka, D. Shiomi, K. Sugisaki, K. Toyota, K. Sato, T. Takui and K. Okada, Angew. Chem., Int. Ed., 2012, 51, 3193–3197 CrossRef CAS PubMed ; (d) R. Kurata, D. Sakamaki, M. Uebe, M. Kinoshita, T. Iwanaga, T. Matsumoto and A. Ito, Org. Lett., 2017, 19, 4371–4374 CrossRef CAS PubMed ; (e) Y. Wu, M. D. Krzyaniak, J. F. Stoddart and M. R. Wasielewski, J. Am. Chem. Soc., 2017, 139, 2948–2951 CrossRef CAS PubMed .
8. (a) K. Kanoda and R. Kato, Annu. Rev. Condens. Matter Phys., 2011, 2, 167–188 CrossRef CAS ; (b) C. Broholm, R. J. Cava, S. A. Kivelson, D. G. Nocera, M. R. Norman and T. Senthil, Science, 2020, 367(6475), eaay0668 CrossRef CAS PubMed ; (c) J. R. Chamorro, T. M. McQueen and T. T. Tran, Chem. Rev., 2021, 121, 2898–2934 CrossRef CAS PubMed .
9. (a) T. Kubo, M. Sakamoto, M. Akabane, Y. Fujiwara, K. Yamamoto, M. Akita., K. Inoue, T. Takui and K. Nakasuji, Angew. Chem., Int. Ed., 2004, 43, 6474–6479 CrossRef CAS PubMed ; (b) G. Li, T. Y. Gopalakrishna, H. Phan, T. S. Herng, J. Ding and J. Wu, Angew. Chem., Int. Ed., 2018, 57, 7166–7170 CrossRef CAS PubMed ; (c) J. J. Dressler, M. Teraoka, G. L. Espejo, R. Kishi, S. Takamuku, C. J. Gómez-García, L. N. Zakharov, M. Nakano, J. Casado and M. M. Haley, Nat. Chem., 2018, 10, 1134–1140 CrossRef CAS PubMed ; (d) J. E. Barker, J. J. Dressler, A. C. Valdivia, R. Kishi, E. T. Strand, L. N. Zakharov, S. N. MacMillan, C. J. Gómez-García, M. Nakano, J. Casado and M. M. Haley, J. Am. Chem. Soc., 2020, 142, 1548–1555 CrossRef CAS PubMed ; (e) S. Tang, L. Zhang, H. Ruan, Y. Zhao and X. Wang, J. Am. Chem. Soc., 2020, 142, 7340–7344 CrossRef CAS PubMed .
10. (a) O. Hammerich and V. D. Parker, Electrochimi. Acta, 1973, 18, 537–541 CrossRef CAS ; (b) H. Bock, A. Rauschenbach, C. Näther, M. Kleine and Z. Havlas, Chem. Ber., 1994, 127, 2043–2049 CrossRef CAS .
11. T.-Z. Xie, K. Guo, M. Huang, X. Lu, S.-Y. Liao, R. Sarkar, C. N. Moorefield, S. Z. D. Cheng, C. Wesdemiotis and G. R. Newkome, Chem. – Eur. J., 2014, 20, 11291–11294 CrossRef CAS PubMed .
12. D. M. Giolando and K. Kirschbaum, Synthesis, 1992, 451–452 CrossRef CAS .
13. A. Maiti, J. Stubbe, N. I. Neuman, P. Kalita, P. Duari, C. Schulzke, V. Chandrasekhar, B. Sarkar and A. Jana, Angew. Chem., Int. Ed., 2020, 59, 6729–6734 CrossRef CAS PubMed .
14. A. Decken, H. D. B. Jenkins, G. B. Nikiforov and J. Passmore, Dalton Trans., 2004, 2496–2504 RSC .
15. X-ray data for 1, 2a and 2b are listed in Table S1 (ESI†).
16. (a) S. Ferrer, F. Lloret, E. Pardo, J. M. Clemente-Juan, M. Liu-González and S. García-Granda, Inorg. Chem., 2012, 51, 985–1001 CrossRef CAS PubMed ; (b) A. Escuer, G. Vlahopoulou, F. Lloret and F. A. Mautner, Eur. J. Inorg. Chem., 2014, 83–92 CrossRef CAS .
17. N. F. Chilton, R. P. Anderson, L. D. Turner, A. Soncini and K. S. Murray, J. Comput. Chem., 2013, 34, 1164–1175 CrossRef CAS PubMed .
18. For the relatively large molecular weight of 2a and 2b (>3500 g mol⁻¹), the results of magnetic susceptibility measurements would obviously suffer from impurities and external conditions. Moreover, the large molecular weight will result in a slight overcorrection for diamagnetism during Pascal correction. References: (a) G. Spagnol, K. Shiraishi, S. Rajca and A. Rajca, Chem. Commun., 2005, 5047–5049 RSC ; (b) K. Kato and A. Osuka, Angew. Chem., Int. Ed., 2019, 58, 8546–8550 CrossRef CAS PubMed .
19. (a) J. M. Zadrozny and D. E. Freedman, Inorg. Chem., 2015, 54, 12027–12031 CrossRef CAS PubMed ; (b) Z. Hu, B. Dong, Z. Liu, J. Liu, J. Su, C. Yu, J. Xiong, D. Shi, Y. Wang, B. Wang, A. Ardavan, Z. Shi, S. Jiang and S. Gao, J. Am. Chem. Soc., 2018, 140, 1123–1130 CrossRef CAS PubMed ; (c) F. Santanni, A. Albino, M. Atzori, D. Ranieri, E. Salvadori, M. Chiesa, A. Lunghi, A. Bencini, L. Sorace, F. Totti and R. Sessoli, Inorg. Chem., 2021, 60, 140–151 CrossRef CAS PubMed .
20. (a) E. Moreno-Pineda, C. Godfrin, F. Balestro, W. Wernsdorfer and M. Ruben, Chem. Soc. Rev., 2018, 47, 501–513 RSC ; (b) M. Atzori and R. Sessoli, J. Am. Chem. Soc., 2019, 141, 11339–11352 CrossRef CAS PubMed .

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization data and theoretical calculations. CCDC 2098339–2098341. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1cc06904g

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