Reduction versus Rearrangement of 6,13-Dihydro-6,13-diarylpentacene-6,13-diols Affording 6,13- and 13,13′-Substituted Pentacene Derivatives

Abstract

Pentacene has excellent semi-conducting properties but its practical use in organic thin film transistors (OTFTs) gives rise to a lot of problems caused by its sensitivity to oxygen and its very low solubility. In order to solve the problems involved in the use of pentacene, different aryl substituents were introduced on the 6- and 13-positions. Formation of the pentacene ring system may be accompanied by the rearrangement of the starting dihydropentacenediol to a 13,13′-disubstituted pentacen-6-one for electron-rich aryl substituents. 6-Monosubstituted pentacenes were also prepared.

substituted pentacenes - OTFTs - thiophene substituents - rearrangement - pentacenones

References

1 Katz HE. Bao Z. Gilat SL. Acc. Chem. Res.  2001,  34:  359 

2 Lin Y.-Y. Gundlach DJ. Nelson SF. Jackson TN. IEEE Transactions on Electron Devices  1997,  44:  1325 

3a Anthony JE. Eaton DL. Parkin SR. Org. Lett.  2002,  4:  15 

3b Vogel KM, Vogel DE, and Terrance SP. inventors; US  0105365. 

3c Meng H. Bendikov M. Mitchell G. Helgeson R. Wudl F. Boa Z. Siegrist T. Kloc C. Chen CH. Adv. Mat.  2003,  13:  1090 

3d Eaton DL, Parkin S, and Anthony JE. inventors; US  6690029. 

4a Maulding DR. Roberts BGJ. J. Org. Chem.  1969,  34:  1734 

4b Wolak MA. Jang BB. Palilis LC. Kafafi ZH. J. Phys. Chem. B  2004,  18:  5492 

5 Bruckner V. Tomasz J. Acta Chim. Hung.  1961,  28:  405 

6

Synthesis of 6,13-Dihydro-6,13-dithien-2-ylpentacene-6,13-diol(2c):
n-BuLi (4.0 mL, 9.8 mmol) was added to a solution of thiophene (0.78 mL, 9.8 mmol) in THF (75 mL) at -78 °C under argon atmosphere. After 10 min pentacenequinone (1.0 g, 3.4 mmol) was added and the mixture was allowed to warm up to r.t. and stirred overnight. The reaction was worked up with 1 M HCl (50 mL), extracted with CH₂Cl₂ (150 mL) and washed with H₂O (3 × 50 mL). After drying over MgSO₄ and evaporation in vacuo, the crude product was purified by column chomatography over silica gel using petroleum ether-CH₂Cl₂ (15:85). The product was isolated in 58% yield as a mixture of the cis- and trans-isomers (4:6); mp 134 °C. ¹H NMR (300 MHz, CDCl₃): δ trans-isomer = 3.3 (s, 2 H), 6.8 (d, 2 H), 7.0 (t, 2 H), 7.3 (d, 2 H), 7.5 (m, 4 H), 7.9 (m, 4 H), 8.0 (m, 4 H), 8.2 (s, 4 H), 8.6 (s, 4 H); δ cis-isomer = 3.1 (s, 2 H), 5.9 (d, 2 H), 6.3 (t, 2 H), 6.9 (d, 2 H), 7.6 (m 4 H). ¹³C NMR (75 MHz, CDCl₃): δ = 72.6, 124.2, 125.5, 126.2, 126.4, 127.2, 128.3, 132.8, 138.4, 149.0. MS (CI) [MH⁺]: m/z = 477 (and 459: rearranged product).
Synthesis of 6,13-Dihydro-6,13-bis(4-methoxy-phenyl)pentacene-6,13-diol (2a):
The procedure is the same as for 2c except purification was done by washing the crude product with MeOH (30 mL). The pure product was collected by filtration. Yield 34%, mp 232 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.1 (s, 2 H), 3.5 (s, 6 H), 6.1 (d, 4 H), 6.5 (d, 4 H), 7.5 (m, 4 H), 7.9 (m, 4 H), 8.4 (s, 4 H). ¹³C NMR (75 MHz, CDCl₃): δ = 55.0, 76.4, 112.6, 124.8, 126.3, 128.1, 129.0, 132.7, 134.7, 139.6, 158.2. MS (CI) [MH⁺]: m/z = 525.
Synthesis of 6,13-Dihydroxy-6,13-(4-acetyl)phenyl-pentacene-6,13-diol (2b):
The procedure is the same as for 2c, the bromoacetophenone was protected with 1,2-ethanediol, except column chromatography over silica gel was carried out with EtOAc-petroleum ether-CH₂Cl₂ (2:4:4). Yield 11%; mp 229-232 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.4 (s, 6 H), 2.5 (s, 2 H), 6.9 (d, 4 H), 7.3 (d, 4 H), 7.6 (m, 4 H), 8.0 (m, 4 H), 8.4 (s, 4 H). ¹³C NMR (75 MHz, CDCl₃): δ = 26.6, 76.4, 125.7, 127.1, 127.4, 128.3, 132.9, 135.7, 139.0, 147.4, 197.8. MS (CI) [MH⁺]: m/z = 549 (and 531: rearranged product).
Synthesis of 6,13-Dihydro-6′,13′-bis(benzothien-2-yl)pentacene-6,13-diol (2d):
The procedure is the same as for 2c except column chromatography over silica gel was carried out with petroleum ether-CH₂Cl₂ (1:9). Yield 87%; mp trans: 258-260 °C, cis: 296-300 °C. ¹H NMR (300 MHz, CDCl₃): δ trans-isomer = 3.0 (s, 2 H), 7.1 (s, 2 H), 7.3 (m, 4 H), 7.5 (m, 4 H), 7.6-7.7 (m, 2 H), 7.7-7.8 (m, 2 H), 7.9 (m, 4 H), 8.3 (s, 4 H); δ cis-isomer = 3.2 (s, 2 H), 5.9 (s, 1 H), 6.6 (d, 2 H), 6.7 (t, 2 H), 6.9 (t, 2 H), 7.3 (d, 2 H), 7.6 (m, 4 H), 8.0-8.1 (m, 4 H), 8.6 (s, 4 H). ¹³C NMR (75 MHz, CDCl₃): δ trans-isomer = 75.4, 122.9, 124.5, 125.0, 125.1, 127.5, 127.6, 128.8, 133.7, 138.9, 139.8, 141.1, 153.0; δ cis-isomer = 53.7, 121.2, 123.6, 124.5, 126.9, 128.4, 133.1, 138.5, 139.9. MS (CI) [MH⁺]: m/z 559 (rearranged product).
Synthesis of 6,13-Dihydro-13,13′-bis(5-methoxythien-2-yl)pentacene-6-one (4):
The procedure is the same as for product 2c except column chromatography over silica gel was carried out with petroleum ether-CH₂Cl₂ (1:9). Yield 55%; the yield rose to 77% when the work up was done with NH₄Cl. Mp 117-120 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.8 (s, 6 H), 5.9 (d, 2 H), 6.2 (d, 2 H), 7.5-7.6 (m, 4 H), 7.9 (d + s, 4 H), 8.0-8.1 (d, 2 H), 8.9 (s, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 53.2, 60.1, 102.4, 126.7, 127.2, 128.5, 128.8, 129.8, 130.2, 132.3, 135.3, 136.8, 143.0, 167.2. MS (CI) [MH⁺]: m/z = 519.
Synthesis of 6,13-Dihydro-6′,13′-bis-(5-methoxythien-2-yl)pentacene-6,13-diol (2e):
The procedure is the same as for product 2c except work up with NH₄Cl and column chromatography over silica gel was carried out with petroleum ether-CH₂Cl₂ (1:9) and 0.5% Et₃N. Yield 80%; mp 85-95 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.9 (s, 2 H), 6.0 (d, 2 H), 6.4 (d, 2 H), 7.5 (m, 4 H), 7.9 (m, 4 H), 8.2 (s, 4 H). ¹³C NMR (75 MHz, CDCl₃): δ = 60.1, 74.5, 103.1, 125.0, 126.4, 126.7, 128.2, 133.0, 136.7, 139.0, 167.3. MS (CI) [MH⁺]: m/z = 519.2 (rearranged product), 559.2 (Na adduct).

7

Synthesis of 6,13-Dithien-2-ylpentacene (3c):
A suspension of diol 2c (2.0 g, 4.2 mmol), NaI (4.3 g, 29 mmol) and NaH₂PO₂ (4.8 g, 40 mmol) in HOAc (100 mL) was refluxed for 1 h. The product 3c was isolated by filtration and washed with H₂O (3 × 50 mL) and MeOH (2 × 25 mL). After drying in vacuo, 6,13-dithien-2-ylpentacene was obtained in 87% yield; mp 300-305 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.3 (m, 4 H), 7.4 (d, 2 H), 7.5 (t, 2 H), 7.8 (m, 6 H), 8.5 (s, 4 H). ¹³C NMR (75 MHz, CDCl₃): δ = 125.4, 125.5, 127.1, 127.4, 128.6, 130.0, 131.4, 139.5. MS (CI) [MH⁺]: m/z = 443.
Synthesis of 6,13-Bis(4-methoxyphenyl)pentacene (3a):
The procedure is the same as for 3c. Yield 91%; mp 303 °C. ¹H NMR (300 MHz, CDCl₃): δ = 4.1 (s, 6 H), 7.2 (m, 4 H), 7.3 (d, 4 H), 7.6 (d, 4 H), 7.8 (m, 4 H), 8.4 (s, 4 H). MS (MH⁺): m/z = 491.
Synthesis of 6,13-Bis(4-acetylphenyl)pentacene (3b):
The procedure is the same as for 3c. Yield 58%; mp >300 °C. ¹H NMR (300 MHz, CDCl₃): δ = 2.8 (s, 6 H), 7.3 (m, 4 H), 7.7 (m, 4 H), 7.8 (d, 4 H), 8.2 (s, 4 H), 8.3 (d, 4 H). MS [MH⁺]: m/z = 515.
Synthesis of 6,13-Bis(benzothien-2-yl)pentacene (3d):
The procedure is the same as for 3c. Yield 59%; mp >300 °C. ¹H NMR (300 MHz, CDCl₃): δ = 7.5 (m, 2 H), 7.6 (m, 6 H), 7.7 (m, 2 H), 7.9 (m, 4 H), 8.0 (m, 4 H), 8.6 (m, 4 H). MS [MH⁺]: m/z = 542.

8 Smet M. Van Dijk J. Dehaen W. Synlett  1999,  495 

9

Synthesis of 6,13-Dihydro-6,6′-bis(5-methoxythien-2-yl)-13,13′-dihydropentacene (5):
A solution of diol 2e (0.50 g, 0.90 mmol), ZnI₂ (0.86 g, 2.7 mmol) and NaCNBH₃ (0.57 g, 9.0 mmol) in 1,2-dichloroethane (50 mL) was refluxed during the night. After filtration of the salts, the filtrate was acidified, extracted with CH₂Cl₂ (50 mL) and washed with H₂O (2 × 30 mL). After drying over MgSO₄ and evaporation in vacuo. The crude product was purified by two successive column chromatographies over silica gel using CH₂Cl₂ and CH₂Cl₂-petroleum ether (5:5), respectively. The product was characterized to be 6,13-dihydro-6,6′-bis(5-methoxythien-2-yl)-13,13′-dihydropentacene; mp >300 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.8 (s, 6 H), 4.1 (s, 2 H), 6.0 (d, 2 H), 6.2 (d, 2 H), 7.4-7.5 (m, 6 H), 7.8 (m, 6 H). ¹³C NMR (75 MHz, CDCl₃): δ = 36.9, 55.2, 60.1, 102.5, 125.4, 125.5, 126.3, 126.6, 127.0, 127.4, 128.5, 132.1, 132.8, 135.2, 135.3, 142.1, 166.6. MS (CI) [MH⁺]: m/z = 505.

10

Synthesis of 13,13′-dithien-2-ylpentacene-6-one (6):
A solution of diol 2c (0.26 g, 0.55 mmol) and BF₃·OEt (0.10 mL, 8.2 mmol) in CH₂Cl₂ (20 mL) was stirred at r.t. during the night. After adding NaOH (1 M, 20 mL), the reaction mixture was extracted with CH₂Cl₂. The organic layer was washed with H₂O (3 × 100 mL), dried with MgSO₄ and evaporated in vacuo. The product 6 could be obtained after column chromatography over silica gel using CH₂Cl₂, with a yield of 44.5%; mp >300 °C. ¹H NMR (300 MHz, CDCl₃): δ = 6.6 (d, 2 H), 6.9 (t, 2 H), 7.3 (d, 2 H), 7.6 (m, 4 H), 7.8 (s + d, 4 H), 8.1 (d, 2 H), 8.9 (s, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 52.8, 125.2, 126.2, 126.4, 127.1, 128.3, 128.4, 128.8, 128.9, 129.7, 130.0, 132.2, 135.2, 143.6, 152.0, 184.8. MS (CI) [MH⁺]: m/z = 459.
Synthesis of 13,13′-Bis(4-methoxyphenyl)pentacene-6-one (7):
The procedure is the same as for 6 except column chromatography over silica gel was carried out with using CH₂Cl₂-petroleum ether (5:5); yield 1%. ¹H NMR (300 MHz, CDCl₃): δ = 3.7 (s, 6 H), 6.7, 6.8 (d, d, 8 H), 7.5 (s + m, 6 H), 7.7 (d, 2 H), 8.0 (d, 2 H), 8.8 (s, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 55.2, 113.2, 126.7, 128.1, 128.2, 128.5, 129.0, 129.1, 129.3, 129.5, 131.5, 131.6, 131.7, 134.9, 138.6, 144.9, 158.1, 186.0. MS (CI) [MH⁺]: m/z = 507.
Synthesis of 6,13-Dihydro-13,13′-bis(benzothien-2-yl)pentacene-6-one (8):
The procedure is the same as for 6 except column chromatography over silica gel was carried out with CH₂Cl₂-petroleum ether (5:5). Yield 18%, mp >300 °C. ¹H NMR (300 MHz, CDCl₃): δ = 6.9 (s, 2 H), 7.3 (m, 4 H), 7.6 (m, 6 H), 7.7 (d, 2 H), 7.8 (d, 2 H), 8.0 (s, 2 H), 8.1 (d, 2 H), 8.9 (s, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 56.9, 116.9, 118.4, 118.8, 119.1, 120.5, 121.4, 122.3, 122.8, 123.5, 123.6, 125.8, 128.3, 131.4, 133.2, 134.3, 142.7, 171.7. MS (CI) [MH⁺]: m/z = 559.

11 Smet M. Van Dijk J. Dehaen W. Tetrahedron  1999,  55:  7859 

12

Synthesis of 13-Hydroxy-13′-thien-2-ylpentacen-6-one (9a):
n-BuLi (2.0 mL, 4.9 mmol) was added to a solution of thiophene (0.39 mL, 4.9 mmol) in THF (20 mL) under argon atmosphere at -78 °C. After 10 min, this solution was added dropwise to a suspension of pentacenequinone (2 g, 6.5 mmol) in THF (120 mL), which was cooled to -78 °C. The mixture was allowed to warm up to r.t. and stirred during the night. The reaction mixture was worked up with HCl (1 M, 25 mL). The precipitate was filtered, washed with H₂O (3 × 30 mL), MeOH (2 × 20 mL), and dried in vacuo. The filtrate was extracted with CH₂Cl₂ (75 mL), washed with H₂O (3 × 50 mL), dried over MgSO₄ and evaporated in vacuo. After column chromatography over silica gel using petroleum ether-CH₂Cl₂ (3:7) on both fractions separately, the total yield was 39%; mp 248 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.3 (s, 1 H), 6.3-6.4 (d, 1 H), 6.7 (t, 1 H), 7.1 (d, 1 H), 7.5-7.6 (m, 4 H), 7.9 (d, 2 H), 8.0 (d, 2 H), 8.5 (s, 2 H), 8.8 (s, 2 H). MS [MH⁺]: m/z = 393.
Synthesis of 6,13-Dihydro-6-thien-2-yl-13-(4-methoxy-phenyl)pentacene-6,13-diol (10):
n-BuLi (0.60 mL, 1.4 mmol) was added to a solution of 4-bromoanisole (0.27 g, 1.5 mmol) in THF (30 mL) under argon atmosphere at -78 °C. After 10 min, ketone 9a (0.38 g, 0.97 mmol) was added. The mixture was allowed to warm up to r.t. and stirred during the night. The reaction mixture was worked up with HCl (1 M, 10 mL), was extracted with CH₂Cl₂ (50 mL), washed with H₂O (3 × 50 mL), dried over MgSO₄ and evaporated in vacuo. After purification by column chromatography on silica gel using petroleum ether-CH₂Cl₂ (2:8), diol 10 was used in the next reaction. Yield after column chromatography was 58%. MS [MH⁺]: m/z = 501.
Synthesis of 6-Thien-2-yl-13-(4-methoxy-phenyl)penta-cene (11):
A suspension of diol 10 (0.09 g, 0.2 mmol), NaI (0.38 g, 2.5 mmol) and NaH₂PO₂ (0.38 g, 4.3 mmol) was refluxed in HOAc (20 mL) during 1 h. The mixture was allowed to cool down and the product was filtered. The residue was washed with H₂O (3 × 20 mL) and MeOH (2 × 10 mL). After drying in vacuo, 6-thien-2-yl-13-(4-methoxyphenyl)pentacene was obtained with a yield of 36%; mp 272-274 °C. ¹H NMR (300 MHz, CDCl₃): δ = 4.0 (s, 3 H), 7.2, 7.3 (m, 6 H), 7.4 (d, 1 H), 7.5 (t, 1 H), 7.55 (d, 2 H), 7.7, 7.8 (m, 5 H), 8.3 (s, 2 H), 8.5 (s, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 55.5, 114.1, 125.2, 125.4, 125.8, 127.0, 127.4, 128.2, 128.5, 128.9, 130.0, 130.2, 132.7, 138.3, 159.3. MS [MH⁺]: m/z = 467.

13

Smet, M. unpublished results

14

Synthesis of 6,13-Dihydro-6′-thien-2-yl-13,13′-dihydro-pentacene (12a):
A suspension of ketone 9a (0.1 g, 0.25 mmol), ZnI₂ (0.24 g, 0.75 mmol) and NaBH₃CN (0.16 g, 2.5 mmol) in 1,2-dichloroethane (40 mL) was refluxed overnight. The salts were filtered off and the filtrate was acidified with HCl (3 M, 20 mL). After extraction with CH₂Cl₂ (20 mL) and washing with H₂O (3 × 50 mL), the organic layer was dried over MgSO₄ and evaporated in vacuo. The product was purified by column chromatography over silica gel using CH₂Cl₂-petroleum ether (5:5). The title compound was obtained as a white solid in 92% yield; mp 228-230 °C. ¹H NMR (300 MHz, CDCl₃): δ = 4.2 (s, 2 H), 5.8 (s, 1 H), 6.5 (d, 1 H), 6.8 (t, 1 H), 7.1 (d, 1 H), 7.5 (m, 4 H), 7.8-7.9 (m, 4 H), 8.0 (s, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 48.4, 63.1, 124.8, 125.1, 125.5, 125.9, 126.0, 126.2, 126.6, 127.3, 127.6, 129.5, 132.8, 135.5, 137.8. MS (CI) [MH⁺]: m/z = 363. The same reaction in toluene gives a lower yield (44%) and 6-hydroxy-13-thien-2-ylpentacene (12%) as a side-product.
Synthesis of 6,13-Dihydro-6′-(4-methoxyphenyl)-13,13′-dihydropentacene (12b):
The procedure is the same as for 12a. Yield 51%; mp 155 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.7 (s, 3 H), 4.1 (s, 2 H), 5.5 (s, 1 H), 6.7 (d, 2 H), 7.0 (d, 2 H), 7.4 (m, 4 H), 7.8 (m, 8 H). ¹³C NMR (75 MHz, CDCl₃): δ = 51.9, 55.6, 114.1, 125.9, 126.1, 127.0, 127.6, 129.7, 132.9, 133.0, 134.1, 136.3, 139.1, 158.5. MS (CI) [MH⁺]: m/z = 387.
Synthesis of 6,13-Dihydro-6′-(4-chlorophenyl)-13,13′-dihydropentacene (12b):
The procedure is the same as for 12a. Yield 50%; mp 174 °C. ¹H NMR (300 MHz, CDCl₃): δ = 4.0 (s, 1 H), 4.1 (s, 1 H), 5.5 (s, 1 H), 7.0 (d, 2 H), 7.1 (d, 2 H), 7.4 (m, 4 H), 7.8 (m, 8 H). ¹³C NMR (75 MHz, CDCl₃): δ = 36.8, 51.7, 125.2, 125.5, 125.7, 126.0, 126.8, 127.4, 127.7, 128.5, 129.7, 132.5, 132.8, 135.8, 136.0, 138.0, 140.4. MS (CI) [MH⁺]: m/z = 391.

15

Synthesis of 6,13-Dihydro-13-hydroxy-13-(4-methoxy)phenylpentacene-6-on (9b):
Bromoanisole (2.8 mL, 22 mmol) in dry THF (10 mL) was added dropwise over a period of 30 min to a suspension of Mg (0.56 g, 23 mmol) in dry THF (10 mL) in the presence of a catalytic amount of iodine and bromoanisole. This mixture was than added dropwise to a suspension of pentacenequinone (6.0 g, 20 mmol) in THF (120 mL). The reaction mixture was stirred during the night and acidified with HCl (1 M). The mixture was extracted with CH₂Cl₂, washed with H₂O and dried over MgSO₄. After evaporation in vacuo, the crude product was purified by column chromatography on silica gel using petroleum ether-CH₂Cl₂ (4:6). The desired pentacene derivative 9b was obtained in 8% yield; mp 289-291 °C. ¹H NMR (300 MHz, CDCl₃): δ = 3.0 (s, 1 H), 3.7 (s, 3 H), 6.7 (d, 2 H), 7.2 (d, 2 H), 7.5-7.6 (m, 4 H), 7.9 (d, 2 H), 8.0 (d, 2 H), 8.3 (s, 2 H), 8.9 (s, 2 H). ¹³C NMR (75 MHz, CDCl₃): δ = 55.2, 113.7, 126.5, 126.9, 128.1, 128.8, 129.0, 129.2, 129.8, 132.3, 136.0, 151.8, 155.6. MS (CI) [MH⁺]: m/z = 417.
6,13-Dihydro-13-hydroxy-13-(4-chloro)phenyl-pentacene-6-one (9c):
The procedure is the same as for 9b except column chromatography over silica gel using petroleum ether-CH₂Cl₂ (2:8); yield 8%. ¹H NMR (300 MHz, CDCl₃): δ = 3.2 (s, 1 H), 7.2 (s, 3 H), 7.3 (s, 3 H), 7.5 (d, 5 H), 7.8 (d, 2 H), 8.0 (m, 2 H), 8.2 (2 H), 8.9 (m, 2 H). MS (CI) [MH⁺]: m/z = 421. Melting point and ¹³C NMR data were not obtained, due to unremovable contaminating pentacenequinone.

16

Synthesis of 6-Thien-2-ylpentacene (13a):
Dihydropentacene 12a (90 mg, 0.25 mmol) was refluxed for 3 h in the presence of DDQ (64 mg, 0.28 mmol) in dioxane (20 mL). After cooling and evaporating in vacuo, the product was purified by column chromatography over silica gel using CH₂Cl₂. The yield of the impure product was 31%. ¹H NMR (300 MHz, CDCl₃): δ = 6.3 (m, 1 H), 6.8 (m, 1 H), 7.3 (m, 1 H), 7.5 (m, 4 H), 7.9 (d + s, 2 H + 2 H), 8.0 (d, 2 H), 8.2 (1 H), 8.4 (2 H). MS (CI) [MH⁺]: m/z = 361.

17

Spectral data: ¹H NMR (300 MHz, CDCl₃): δ = 6.0 (s, 1 H), 6.7 (d, 1 H), 6.8 (t, 1 H), 7.0 (d, 1 H), 7.4-7.5 (m, 4 H), 7.8 (d, 2 H), 7.9 (s, 2 H), 8.0 (d, 2 H), 8.8 (s, 2 H). MS [MH⁺]: m/z = 377.