Synthesis of 5,7,12,14-Tetraarylpentacenes from Pentacene-5,7,12,14-tetrone and Characterisation of the Tetrol Intermediates

Abstract

Pentacene has excellent semi-conducting properties but its practical use in organic thin-film transistors (OTFTs) has some drawbacks caused by its sensitivity to oxygen and low solubility. In order to overcome these problems one can introduce substituents on the ring system. In this context, we are investigating the possibility of introduction of aryl substituents on the pentacene backbone. 5,7,12,14-Tetraaryl-substituted pentacenes are described in this article. They have been prepared from the 5,7,12,14-aryl-substituted pentacene-5′,7′,12′,14′-tetrol. We isolated all isomers of the tetrol. We also found that the 5,7,12,14-thienylpentacene-5′,7′,12′,14′-tetrol can undergo rearrangement to afford both 5,5′,7,7′-tetrathi­enylpentacene-12,14-one and 5,5′,12,12′-tetrathienylpentacene-7,14-one.

Key words

synthesis - arenes - reduction - pentacene derivatives - pentacene tetrols

References and Notes

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• 4a Anthony JE. Eaton DL. Parkin SR. Org. Lett.  2002,  4:  15 
• 4b Vogel KM, Vogel DE, and Terrance SP. inventors; US  0105365. 
• 4c Meng H. Bendikov M. Mitchell G. Helgeson R. Wudl F. Boa Z. Siegrist T. Kloc C. Chen CH. Adv. Mater. (Weinheim, Ger.).  2003,  13:  1090 
• 4d Eaton DL, Parkin S, and Anthony JE. inventors; US  6690029. 
• 5 Miao Q. Chi X. Xiao SX. Zeis R. Lefenfeld M. Siegrist T. Steigerwald ML. Nuckolls C. J. Am. Chem. Soc.  2006,  128:  1340 
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9

Characterisation of 5,7,12,14-Pentacene-tetra-one.
Mp >300 °C. ¹H NMR (300 MHz, D₂SO₄): δ = 8.1 (m, 4 H), 8.4 (m, 4 H), 9.2 (s, 2 H). ¹³C NMR (300 MHz, D₂SO₄ in double tube with DMSO): δ = 128.4, 129.0, 131.0, 136.3, 137.5, 185.0. MS (CI) [MH⁺]: m/z = 339.

10

General Synthesis of 5,7,12,14-Dihydro-5,7,12,14-tetraarylpentacene-5′,7′,12′,14′-tetrol ( 3).
n-BuLi (6 equiv) was added to a solution of the(bromo)aryl (6 equiv) in dry THF at -78 °C under argon atmosphere. After 10 min pentacene-5,7,12,14-one (1 equiv) 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 and extracted with EtOAc. After drying over MgSO₄ and evaporation in vacuum, the crude product was purified by column chromatography on silica gel. The different isomers were separated.
Example of Characterisation: 5,7,12,14-Dihydro-5,7,12,14-tetrabenzothienylpentacene-5′,7′,12′,14′-tetra-ol ( 3b).
Mp 280 °C [cis/cis(trans)/cis-isomer], 170 °C (cis/trans/trans-isomer), 185 °C [trans/cis(trans)/trans-isomer]. ¹H NMR (400 MHz, DMSO, 50 °C): δ [trans/cis(trans)/trans-isomer] = 6.7 (s, 4 H), 6.8 (s, 4 H), 7.2 (t, 8 H), 7.3 (m, 4 H), 7.4 (d, 8 H), 7.6 (m, 4 H), 8.0 (s, H). ¹H NMR (400 MHz, DMSO): δ (cis/trans/trans-isomer) = 5.7 (s, 1 H), 5.8 (s, 1 H), 6.7 (d, 2 H), 6.8 (s, 1 H), 6.9-7.2 (m, 8 H), 7.3 (s, 1 H), 7.4 (m, 10 H), 7.5-7.6 (m, 6 H), 7.8 (d, 2 H), 7.9-8.0 (m, 4 H), 8.3 (s, 1 H), 8.4 (s, 1 H). ¹H NMR (400 MHz, DMSO): δ [cis/cis(trans)/cis-isomer) = 6.0/6.1 (s, 4 H), 6.7/6.8 (d, 4 H), 6.9 (t, 4 H), 7.0 (t, 4 H), 7.5/7.6 (d, 4 H), 7.6 (m, 4 H), 8.1 (m, 4 H), 8.7/8.8 (s, 2 H). ¹³C NMR (400 MHz, DMSO, 50 °C): δ [trans/cis(trans)/trans-isomer] = 71.9, 121.3, 121.6, 122.7, 123.2, 123.5, 127.4, 127.8, 128.3, 138.7, 138.8, 139.1, 139.9, 155.6. ¹³C NMR (400 MHz, DMSO): δ (cis/trans/trans-isomer) = 71.0, 71.4, 71.9, 72.7, 121.3, 121.4, 121.6, 121.9, 122.3, 122.4, 123.5, 123.6, 123.7, 123.8, 124.2, 125.5, 125.9, 127.6, 127.8, 127.9, 128.7, 137.9, 138.1, 138.8, 138.9, 139.0, 139.2, 139.3, 139.5, 139.6, 140.0, 140.2, 140.8, 141.2, 151.8, 152.0, 156.5, 156.9. ¹³C NMR (400 MHz, DMSO): δ [cis/cis(trans)/cis-isomer] = 71.6/71.5, 121.6/121.5, 122.2/122.1, 122.6, 123.5, 123.7, 124.2/124.1, 126.0, 127.8/127.7, 138.1/138.0, 139.2/139.1, 140.0/139.9, 140.8/140.6, 152.1/151.8. MS (ESI+) [MH⁺]: m/z = 857.4 (1 × rearrangement).

11

General Synthesis of 5,7,12,14-Tetraarylpentacene ( 4).
A suspension of tetra-ol 3 (1 equiv), NaI (14 equiv) and NaH₂PO₂ (same amount as NaI) in AcOH was heated at reflux for 3 h. The product 4 was isolated by filtration and washed with H₂O and MeOH. After drying in vacuum, 5,7,12,14-tetraarylpentacene was obtained.

13

Characterisation of 5,7,12,14-Tetrathienylpentacene ( 4a).
Mp >300 °C. ¹H (400 MHz, CDCl₃, in concentric tube): δ = 6.7 (d, 4 H), 6.9 (t, 4 H), 7.0 (d, 2 H), 7.9 (m, 4 H), 8.0 (m, 4 H), 8.9 (s, 2 H). MS (APCI+) [MH⁺]: m/z = 607.2.
Characterisation of 5,7,12,14-Tetrabenzothienyl-pentacene ( 4b).
Mp >300 °C. ¹H NMR (400 MHz, CDCl₂CDCl₂): δ = 7.3 (m,
8 H), 7.35 (s, 4 H), 7.5 (m, 4 H), 7.6 (m, 4 H), 7.7 (m, 4 H), 8.0 (m, 4 H), 8.8 (s, 2 H). MS (APCI+) [MH⁺]: m/z = 806.3.