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The Influence of Electron-Withdrawing Groups at the 2- and 2′-Positions of Dibenzothienylethenes on Molecular Switching

Abstract

Three dibenzothienylethenes that carry a methyl, a trifluoromethyl, or a fluoro substituent in the 2- and 2′-position were synthesised in short multistep syntheses, respectively. The trifluoromethyl and fluoromethyl substituents significantly improve the absorption properties of the corresponding open and closed isomers (both in the UV-A and in the visible range). The increasing electron deficiency (i) enlarges the portion of the closed and coloured diarylethene isomer in the corresponding photostationary states and slows down the closing and opening isomerisation, (ii) the switching reactions, both closing and opening, are slowed down, and (iii) the switching quantum yields are lowered.

Key words

molecular switches - photoswitches - diarylethenes - quantum yields - kinetics - photochromism

• References and Notes

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• 16 Under inert conditions benzo[b]thiophene (7.00 g, 51.1 mmol, 1.00 equiv) was dissolved in dry THF and stirred for 30 min at –78 °C. n-BuLi (26.6 mL, 2.5 M in hexane, 66.5 mmol, 1.30 equiv) was added slowly, and the mixture was stirred for 30 min. MeI (4.90 mL, 11.2 g, 78.9 mmol, 1.54 equiv) was added over 15 min. The reaction was stirred for 20 min at –78 °C, for 3 h at r.t., quenched with water (100 mL), and extracted with EtOAc (4 × 50 mL). The organic layer was dried over Na₂SO₄, and the solvents were removed in vacuo. After purification by flash chromatography (silica, hexane), 5 could be obtained as a colourless solid in 97% yield (7.26 g, 49.0 mmol). TLC (SiO₂, hexane): R_f = 0.54. ¹H NMR (400 MHz, DMSO-d ₆): δ = 7.85 (dp, J = 7.8, 0.7 Hz, 1 H), 7.74–7.64 (m, 1 H), 7.35–7.21 (m, 2 H), 7.12 (p, J = 1.2 Hz, 1 H), 2.56 (d, J = 1.2 Hz, 3 H) ppm. ¹³C NMR (101 MHz, DMSO-d ₆): δ = 140.57, 140.09, 138.88, 124.19, 123.46, 122.57, 122.05, 121.78, 15.74 ppm. MS (EI): m/z [M]⁺ calcd for C₉H₈S: 148.0342; found: 148.0341.
• 17 A solution of Br₂ (2.10 mL, 6.55 g, 41.5 mmol, 1.02 equiv) in CHCl₃ (60 mL) was slowly added to a solution of 5 (6.00 g, 40.8 mmol, 1.00 equiv) in CHCl₃ (300 mL) over 5 h at r.t. The mixture was stirred for 15 h at r. t., quenched with sat. aq Na₂SO₃ (50 mL), and extracted with CH₂Cl₂ (4 × 100 mL). The combined organic layers were washed with sat. aq NaCl (2 × 100 mL), dried over Na₂SO₄, and the solvents were removed in vacuo. Purification by flash chromatography (silica, hexane) gave 6 as a colourless solid in 96% yield (8.68 g, 38.2 mmol). TLC (SiO₂, hexane): R_f = 0.25. ¹H NMR (400 MHz, DMSO-d ₆): δ = 8.00–7.94 (m, 1 H), 7.68–7.63 (m, 1 H), 7.51–7.38 (m, 2 H), 2.54 (s, 3 H) ppm. ¹³C NMR (101 MHz, DMSO-d ₆): δ = 137.62, 136.57, 135.65, 125.38, 125.14, 122.75, 121.92, 105.78, 15.17 ppm. MS (EI): m/z [M] ⁺ calcd for C₉H₇BrS: 225.9448; found: 225.9446.
• 18 Under inert conditions, a solution of 6 (1.33 g, 5.86 mmol, 1.00 equiv) in dry THF (8 mL) was stirred at –78 °C for 20 min. n-BuLi (2.80 mL, 2.5 M in hexane, 7.00 mmol, 1.20 equiv) was added. The mixture was stirred for 1 h. Octafluorocyclopentene (1.32 mL, 2.0 M in THF, 2.20 mmol, 0.38 equiv) was added. The reaction was warmed up to r.t. overnight. Water (2 mL) was added, and the reaction was extracted with Et₂O (4 × 20 mL). The organic layer was washed with sat. aq NaCl (2 × 30 mL), dried over Na₂SO₄, and the solvents were removed in vacuo. Flash chromatography (silica, hexane) gave 1 as a colourless solid in 73% yield (1.00 g, 2.15 mmol, isomeric mixture). ¹H NMR (400 MHz, DMSO-d ₆): δ = 7.92 (d, J = 8.0 Hz, 2 H), 7.88–7.79 (m, 2 H), 7.62 (d, J = 8.7 Hz, 4 H), 7.46 (t, J = 7.6 Hz, 2 H), 7.41–7.35 (m, 2 H), 7.26 (d, J = 9.2 Hz, 4 H), 2.27 (s, 6 H) ppm. ¹³C NMR (126 MHz, CDCl₃-d ₁): δ = 141.64, 141.13, 137.29, 137.16, 137.03, 133.18, 133.04, 131.60, 131.45, 128.11, 126.16, 126.07, 123.58, 123.42, 123.33, 123.04, 121.18, 121.13, 121.06, 120.77, 120.62, 118.37, 115.15, 110.45, 97.33, 14.12 ppm. MS–FAB: m/z [M]⁺ calcd for C₂₃H₁₄F₆S₂: 468.0438; found: 468.0436.
• 19 Ye Y. Künzi SA. Sanford MA. Org. Lett. 2012; 14: 4979
• 20 To a solution of benzo[b]thien-2-yl-boronic acid (1.00 g, 5.62 mmol, 1.00 equiv), CuCl (0.56 g, 5.62 mmol, 1.00 equiv), and NaSO₂CF₃ (2.63 g, 16.9 mmol, 3.00 equiv) in a mixture of MeOH–CH₂Cl₂–H₂O (11.2 mL/11.2 mL/8.99 mL), tert-butyl hydroperoxide solution (3.82 mL, 70% in H₂O, 28.1 mmol, 5.00 equiv) were added at 0 °C and stirred overnight. Et₂O (90 mL) was added, and the organic layer was washed with sat. aq NaHCO₃ (70 mL), sat. aq Na₂SO₃ (25 mL), and dried over Na₂SO₄. The solvents were removed in vacuo. Purification by flash chromatography (silica, hexane) gave 8 as a colourless solid in 77% yield (769 mg, 4.33 mmol). TLC (SiO₂, hexane): R_f = 0.64. ¹H NMR (400 MHz, DMSO-d ₆): δ = 8.19–8.12 (m, 2 H), 8.06–8.01 (m, 1 H), 7.61–7.50 (m, 2 H) ppm. ¹³C NMR (101 MHz, DMSO-d ₆): δ = 139.43, 137.56, 129.01, 127.35, 127.19, 125.66, 123.79, 123.16, 121.28 ppm. MS (EI): m/z [M]⁺ calcd for C₉H₅F₃S: 202.0059; found: 202.0057.
• 21 Under inert conditions 8 (250 mg, 1.24 mmol, 1.00 equiv) were dissolved in dry THF (5 mL) and stirred for 20 min at –78 °C. n-BuLi (544 μL 2.0 M in hexane; 1.36 mmol, 1.10 equiv) were added slowly, and the mixture was stirred for 1 h. Octafluorocyclopentene (279 μL, 2.0 M in THF, 0.560 mmol, 0.450 equiv) were added, and the mixture was warmed to r.t. overnight. The reaction was quenched with sat. aq NH₄Cl (4 mL) and extracted with Et₂O (3 × 30 mL). The organic layer was dried over Na₂SO₄, and the solvents were removed in vacuo. Purification by flash chromatography (silica, hexane) gave 2 as a colourless solid in 75% yield (267 mg, 0.463 mmol). TLC (SiO₂, hexane): R_f = 0.21. ¹H NMR (400 MHz, DMSO-d ₆): δ = 8.24 (dd, J = 6.4, 2.7 Hz, 1 H), 8.14 (d, J = 7.9 Hz, 2 H), 8.06 (d, J = 8.1 Hz, 2 H), 7.78–7.65 (m, 3 H), 7.59–7.46 (m, 4 H) ppm. ¹³C NMR (126 MHz, CDCl₃-d ₁): δ = 141.23, 141.00, 140.80, 140.53, 138.65, 138.44, 137.57, 136.80, 132.08, 131.78, 131.49, 131.16, 127.68, 127.46, 125.83, 125.54, 124.38, 124.36, 122.77, 122.34, 121.72, 120.56, 120.45, 118.40, 117.67, 117.47, 115.58, 115.41, 115.19, 113.50, 113.30, 113.13, 111.33, 111.13, 110.93 ppm. MS–FAB: m/z [M]⁺ calcd for C₂₃H₈F₁₂S₂: 575.9870; found: 575.9869.
• 22 Under inert conditions, benzo[b]thiophene (2.30 g, 17.2 mmol, 1.00 equiv) was dissolved in dry THF (140 mL) and stirred for 20 min at –78 °C. n-BuLi (12.8 mL, 2.5 M in hexane, 32.0 mmol, 1.86 equiv) was added dropwise, and the resulting mixture was stirred for 1 h at –78 °C. N-Fluorobenzenesulfonimide (12.5 g, 39.7 mmol, 3.21 equiv) was added in two portions at an interval of 15 min. The reaction mixture was stirred for 2 h at –78 °C, 16 h at r.t., stopped by addition of water (30 mL), and extracted with EtOAc (4 × 50 mL). The organic layer was dried over Na₂SO₄, and the solvents were removed in vacuo. After flash chromatography (silica, hexane) 9 could be obtained as a colourless oil in 59 % yield (1.53 g, 10.1 mmol). TLC (SiO₂, hexane): R_f = 0.71. ¹H NMR (300 MHz, DMSO-d ₆): δ = 7.90 (d, J = 7.7 Hz, 1H), 7.73 (d, J = 6.3 Hz, 1 H), 7.44–7.32 (m, 2 H), 7.11 (d, J = 2.7 Hz, 1 H) ppm. ¹³C NMR (75 MHz, DMSO-d ₆): δ = 161.88, 135.63, 131.20, 125.28, 124.51, 124.45, 123.60, 123.51, 122.81, 104.09, 103.94 ppm. MS (EI): m/z [M]⁺ calcd for C₈H₅FS: 152.0091; found: 152.0092.
• 23 Under inert conditions, 9 (1.18 g, 7.72 mmol, 1.00 equiv) was dissolved in dry CHCl₃ (8 mL). NaOAc (1.17 g, 14.2 mmol, 1.85 equiv) was added, and the mixture was stirred for 15 min at r.t. A solution of Br₂ (394 μL 1.23 g, 7.72 mmol, 1.00 equiv) in dry CHCl₃ (2 mL) was added dropwise over 30 min. The solution was stirred for 2 h at r.t., quenched with sat. aq Na₂SO₃, and extracted with hexane (4 × 25 mL). The organic layer was dried over Na₂SO₄, and the solvent was removed under reduced pressure. Purification of the crude product was done by flash chromatography (silica, hexane). The product was dried in vacuo overnight to remove last traces of the volatile starting material 9. The pure product 10 was isolated as pale yellow oil in 76% yield (1.35 g, 5.56 mmol). TLC (SiO₂, hexane): R_f = 0.76. ¹H NMR (300 MHz, DMSO-d ₆): δ = 8.02 (d, J = 8.0 Hz, 1 H), 7.68 (d, J = 7.8 Hz, 1 H), 7.55 (dd, J = 7.6 Hz, 1 H), 7.48 (dd, J = 7.7 Hz, 1 H) ppm. ¹³C NMR (75 MHz, DMSO-d ₆): δ = 160.02, 157.15, 133.58, 129.34, 126.35, 126.08, 126.03, 123.69, 122.19, 122.13, 88.45, 88.32 ppm. MS (EI): m/z calcd for C₈H₄SFBr: 229.2; found: 230.1.
• 24 Under inert conditions, n-BuLi (1.05 mL, 2.0 M in hexane, 2.10 mmol, 1.01 equiv) was dissolved in dry THF (1 mL) at –78 °C. A solution of 10 (480 mg, 2.08 mmol, 1.00 equiv) in dry THF (700 μL) was added dropwise over 15 min and stirred for 45 min at –78 °C. Octafluorocyclopentene (261 μL, 3.6 M in THF, 0.936 mmol, 0.45 equiv) was added, and the reaction mixture was stirred for 24 h at r.t. The reaction was quenched with water (4 mL) and extracted with Et₂O (4 × 20 mL). The organic layer was dried over Na₂SO₄, and the solvents were removed in vacuo. The product was purified by flash chromatography (silica, hexane) and 3 was isolated as a colourless solid in 46% yield (204 mg, 0.428 mmol, isomeric mixture). TLC (SiO₂, hexane): R_f = 0.35. ¹H NMR (500 MHz, CDCl₃-d ₁): δ = 7.61–7.55 (m, 1 H), 7.50 (dd, J = 6.5, 2.5 Hz, 1 H), 7.35–7.27 (m, 2 H) ppm. ¹³C NMR (126 MHz, CDCl₃-d ₁): δ = 165.63, 163.24, 141.69, 138.37, 137.49, 137.28, 137.06, 133.59, 130.58, 127.23, 126.00, 125.50, 125.47, 122.72, 122.16, 117.98, 117.97, 117.60, 115.92, 115.73, 115.54, 113.87, 113.68, 113.48, 113.28, 111.52, 111.31, 111.11, 109.35, 109.15, 108.95, 104.00, 103.94, 77.41, 77.16, 76.91 ppm. MS–FAB: m/z calcd for C₂₃H₈F₈S₂: 475.9934; found: 475.9935.
• 25 Megerle U. Lechner R. König B. Riedle E. Photochem. Photobiol. Sci. 2010; 9: 1400
• 26 Sumi T. Takagi Y. Yagi A. Morimoto M. Irie M. Chem. Commun. 2014; 50: 3928