Syntheses and structures of [7]helicene and double helicene based on dithieno[2,3-b:2′,3′-d]thiophene

Xinming Liuab, Huiliang Suna, Wan Xua, Shisheng Wana, Jianwu Shia, Chunli Li*a and Hua Wang*a
aEngineering Research Center for Nanomaterials, Henan University, Kaifeng, 475004, P. R. China. E-mail:;
bCollege of Chemistry and Chemical Engineering, Shangqiu Normal University, Shangqiu, 476000, P. R. China

Received 23rd November 2017 , Accepted 5th February 2018

First published on 6th February 2018

Based on dithieno[2,3-b:2′,3′-d]thiophene, three novel helicenes including helicene (rac-1), double helicene (rac-2), and benzohexathia[7]helicene (rac-3), and one bull's horn-shaped benzohexathienoacene (4) have been synthesized and reported in this paper. In the helicenes, the sulfur atoms of terminal thiophenes are inside the helical structures. X-ray crystallographic analysis exhibited the helical configuration of rac-1 and the bull's horn-shaped configuration of 4. Multiple types of intermolecular interactions, including S⋯S, C⋯C and π⋯π interactions, were observed between the adjacent molecules in the crystals of rac-1 and 4. In addition, a remarkable bathochromic shift was found in the absorption behaviors for rac-3 and 4 compared to rac-1; the integrated absorbance in rac-2 is approximately twice that of rac-1.


Fused oligothiophenes (thienoacenes) emerge as a more promising new class of π-conjugated organic functional materials, which combine with extended conjugation and rigid planarity.1 Thiophene helicenes possess an aesthetically pleasing helical structure, and may be widely applied in the fields of asymmetric catalysis, circularly polarized luminescence, and molecular recognition.2 Bull's horn-shaped thienoacene possesses an extraordinary compressed sandwich–herringbone arrangement and shows strong intermolecular S⋯C and S⋯S interactions, which might exhibit good electronic properties.3

Fused oligothiophenes (thienoacenes) based on two isomers of dithienothiophene, dithieno[2,3-b:3′,2′-d]thiophene (5) and dithieno[3,2-b:2′,3′-d]thiophene (6), as building blocks have excited studies in a variety of areas ranging from helicene chemistry to materials science (Fig. 1). Among them, the syntheses and structures of aesthetically pleasing carbon–sulfur helicenes and double helicenes based on 5 have been reported by Rajca4 and Wang.5 Rajca has reported a series of studies in preparing carbon–sulfur helicenes, such as [5]-, [7]-, [9]-, and [11]-helicenes.4 With an increase of n, helical carbon–sulfur (C2S)n oligomers possess moderate curvature characteristics of helicenes.6 Wang has prepared a series of carbon–sulfur double helicenes, which bear more solubility-supporting groups.5 A series of organic semiconductor materials based on 5 have also been reported by Hu7 and Wang.8 The synthesis and properties of thienoacenes based on 6 have also been widely studied.8c,9

image file: c7qo01049d-f1.tif
Fig. 1 [7]Helicene molecular structures from 5 and 7.

Dithienothiophenes 5 and 6 are symmetric structural molecules. Their isomer dithieno[2,3-b:2′,3′-d]thiophene (7) has an asymmetric structure, which is also another very important building block. Compound 7 has two α positions, which possess different chemical activities. On the one hand, deriving from the αa position, carbon–sulfur helicenes can be constructed; on the other hand, deriving from the αb position, bull's horn-shaped thienoacene or analogues can be designed.3 However, 7 has not obtained wide attention, mainly because of the lack of efficient synthetic methods.10 In the reported method, an unstable intermediate, 3-(3-bromothiophen-2-yl)thiophene-2-thiol, was employed in preparing 7 under the oxidation of Cu2O for the key step of ring cyclization.10a In our previous research work, three routes were developed for the preparation of 7 and its TMS-protected derivative (TMS)2-7.3 And 6-bromo-2-trimethylsilanyl-dithieno[2,3-b:2′,3′-d]thiophene (11) is the key intermediate for the synthesis of helicene from 7; however, it cannot be obtained from 7 or (TMS)2-7,3,11 and this challenge stimulates our research interest.

In this paper, an efficient synthetic method for the key intermediate 11 was developed, with 2,3-dibromothiophene as a starting material, in which the αb position was protected by the TMS group. Moreover, starting from 11 and 2-bromo-6-trimethylsilanyl-dithieno[2,3-b:2′,3′-d]thiophene (14), four novel thienoacenes including mono-helicene (rac-1) and double helicene (rac-2), benzohexathia[7]helicene (rac-3) derivative and bull's horn-shaped benzohexathienoacene (4) have been synthesized. In addition, their molecular structures and absorption behaviors are also described (Fig. 2 and 4).

image file: c7qo01049d-f2.tif
Fig. 2 Top view and side view of molecular structures for 11, 14, rac-1 and 4. The bromine atoms are connected to the 6- and 2-positions of dithieno[2,3-b:2′,3′-d]thiophenes in 11 and 14, respectively. Carbon, sulfur, and silicon atoms are depicted with thermal ellipsoids set at the 30% probability level, and all hydrogen atoms are omitted for clarity.

Results and discussion

Synthesis of rac-1, rac-2, rac-3 and 4 (Schemes 1 and 2)

Compound 11 is the key intermediate for preparing carbon–sulfur helicene. However, it is very difficult to obtain 11 directly from 7 or (TMS)2-7 due to their high selectivity for bromination at the αb position.3,11 In this work, through TMS-protection to the αb position of 7, we developed an efficient synthesis method for 11. Starting from 2,3-dibromothiophene, via three steps including TMS-protection in the presence of lithium diisopropylamide (LDA) and TMS-Cl, Suzuki coupling, selectivity of Br/Li exchange and deprotonation and cyclization with (PhSO2)2S, 2-trimethylsilanyl-dithieno[2,3-b:2′,3′-d]-thiophene (10) was efficiently prepared in a total yield of 30%. Following the deprotonation and bromination of 10 in the presence of LDA and 1,2-dibromotetrachloroethane, 11 was synthesized in a yield of 95%. Compound 12 was obtained through the highly efficient bromine dance reaction of 11 by using LDA in THF at 0 °C. Following the Br/Li exchange on 12, the resultant aryllithium species were oxidized with CuCl2 to afford 2,2′-di(trimethylsilanyl)-7,7′-bis-dithieno[2,3-b:2′,3′-d]thiophene (13). LDA was introduced to remove the protons on α positions in 13 to afford dilithiated 13. Dilithiated 13 is an amazing intermediate. On the one hand, the reaction of dilithiated 13 with (PhSO2)2S affords the annelated product, [7]helicene rac-1. On the other hand, dilithiated 13 could be oxidized with CuCl2 to develop rac-2. Starting from 11, the total yields of rac-1 and rac-2 are 42% and 23%, respectively. This work provides novel members in the family of thiophene helicene. The molecular configurations of rac-1 and rac-2 are different from these reported thiophene helicene and double helicene, in which all sulfur atoms are outside the helical structure.4,5 In the molecules of rac-1 and rac-2, the sulfur atoms of terminal thiophenes are inside the helical structures.
image file: c7qo01049d-s1.tif
Scheme 1 Synthetic route to rac-1 and rac-2. Reagents and conditions: (a) LDA (1.05 equiv.)/TMSCl (2.0 equiv.), Et2O, −78 °C to r.t.; (b) thiophene-3-boronic acid (1.1 equiv.)/Pd(PPh3)4 (0.03 equiv.)/K2CO3 (2.5 equiv.), THF, 100 °C, 48 h. (c) (i) n-BuLi (2.05 equiv.), Et2O, −78 °C/2 h; (ii) (PhSO2)2S (1.0 equiv.), −78 °C/2 h; (d) (i) LDA (1.1 equiv.), Et2O, −50 °C/2 h; (ii) C2Br2Cl4 (1.0 equiv.), −50 °C to r.t.; (e) LDA (1.5 equiv.), THF, 0 °C/10 h; (f) (i) t-BuLi (2.1 equiv.), Et2O, −78 °C/2 h; (ii) CuCl2 (3.0 equiv.), −78 °C to r.t.; (g) (i) LDA (2.2 equiv.), Et2O, 0 °C/2 h; (ii) (PhSO2)2S (1.0 equiv.), 0 °C/2 h; (h) (i) LDA (4.05 equiv.), Et2O, 0 °C/2 h; (ii) CuCl2 (5.0 equiv.), 60 °C/6 h.

image file: c7qo01049d-s2.tif
Scheme 2 Synthetic route to rac-3 and 4. Reagents and conditions: (a) (i) n-BuLi (1.05 equiv.), THF, −78 °C; (ii) DMF (2.0 equiv.), −78 °C; (b) TiCl4 (5.0 equiv.), Zn (10.0 equiv.), pyridine (5.0 equiv.); (c) hv, iodine (0.5 equiv.), toluene; (d) trifluoroacetic acid, r.t.; (e) 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)-5-trimethylsilyl-thiophene (1.05 equiv.), K2CO3 (2.5 equiv.), Pd(PPh3)4 (0.06 equiv.), water (0.85 mL, 2 M), THF; (f) LDA (1.0 equiv.), THF, −78 °C; (ii) DMF (2.0 equiv.), −78 °C.

In the synthesis of benzohexathia[7]helicene rac-3 and bull's horn-shaped benzohexathienoacene 4, the oxidative photocyclization of 1,2-diarylethylenes in the presence of I2 under an oxygen atmosphere is the crucial step. Following the Li/Br exchange on 14[thin space (1/6-em)]3 in the presence of n-BuLi, N,N-dimethylformamide (DMF) was added into the reaction mixture to produce 6-trimethylsilanyl-dithieno[2,3-b:2′,3′-d] thiophene-2-carbaldehyde (15). After the intermolecular McMurry reaction of 15 using TiCl4/Zn/pyridine, di(6-trimethylsilanyldithieno[2,3-b:2′,3′-d]thiophen-2-yl)ethene (16) was obtained. The oxidative photocyclization of 16 could occur in the presence of iodine in dry toluene via the irradiation of a 450 W Hg medium pressure lamp; a bull's horn-shaped thienoacene 4 was generated, in a total yield of 56%, starting from 14. Compound 17 was obtained through the treatment of 14 with TFA in a yield of 97%. Starting from 17, 2-[(5-trimethylsilyl)-2-thienyl]dithieno[2,3-b:2′,3′-d]thiophene (18) was obtained through Suzuki coupling with 2-(4,4,5,5-tetramethyl-1,3,2-dioxaborolane)-5-trimethylsilyl-thiophene. After formylation and the McMurry reaction, 1,2-bis(2-(5-(trimethylsilyl)thiophen-2-yl)dithieno[2,3-b:2′,3′-d]thiophen-6-yl)-ethene (20) with very poor solubility in organic solvents was obtained. With the precursor 20, benzohexathia[7]helicene rac-3 was synthesized via oxidative photocyclization. Starting from 17, a total yield of 29% was achieved in preparing rac-3.

Crystallographic analysis for rac-1 and 4

The molecular structures of 11, 14, rac-1 and 4 are confirmed through single-crystal X-ray analysis (Fig. 2). Compound 11 belongs to the monoclinic P2(1)/c system. In 11, all the thiophene rings are almost completely coplanar (Fig. 2A and B) and the C2–C3–C5–S3 torsion angle is 0.6°. Compound 14 belongs to the orthorhombic Pnma system. In 14, all the thiophene rings are completely coplanar (Fig. 2a and b) and the torsion angle of S1–C4–C5–C7 is 0°. The crystal of rac-1 belongs to the monoclinic P2(1)/c system. In rac-1, with the formation of the middle thiophene ring from 13 to rac-1, the molecule of rac-1 is compressed with a helical structure (Fig. 2c). The distance between S1⋯S7 is 3.24 Å and the two S atoms point away from each other. The repulsion of the facing terminal thiophene rings causes an interplanar angle between the terminal thiophene rings, 32.3°, which is smaller than 54.1° of Br2-(TMS)2-[7]helicene4a and 43.0° of (TMS)2-[7]helicene.5a The angles between the least-squares planes of the neighboring thiophene rings are between 4.2° and 8.5°. With the middle thiophene ring as a reference, the inner (S1, C4, C5, C7, C9, C11, C13, and S7) helix climb is 1.99 Å and turns in-plane by 268°, and for Br2-(TMS)2-[7]helicene and (TMS)2-[7]helicene are 2.92 Å and 2.18 Å, and 260° and 266°, respectively.4a,5a The crystal packing of rac-1 reveals the existence of multiple types of interactions between the adjacent molecules (Fig. 3A), with distances of 3.59, 3.46 and 3.31 Å for S3⋯S6, S4⋯S6 and S4⋯S4, respectively. The multiple intermolecular interactions stabilize crystal packing and are beneficial for their applications in materials science.7,8a,12
image file: c7qo01049d-f3.tif
Fig. 3 (A) Multiple types of interactions between the adjacent molecules in the packing of rac-1. (B) Molecular packing of the sandwich–herringbone arrangement of 4.

The crystal of 4 belongs to the monoclinic P2(1)/c system. In 4, all seven rings are fused together to form a novel approximately coplanar bull's horn-shaped molecule (Fig. 2d). Its spatial configuration is different from that of helical and linear fused aromatic compounds, but is the same as bull's horn-shaped [7]thienoacene.3 The torsion angles of C5–C6–C9–S3, S2–C8–C10–C15, C8–C10–C15–C16, C10–C15–C16–S5 and S4–C17–C18–C20 are 0.7°, 3.2°, 5.3°, 3.7° and 0.7°, respectively. There are five rings (from ring-B to ring-F) in the middle of 4, forming a [5]helicene-like structure with a little twisting. The dihedral angle between ring-B and ring-F is 10.5°, which is bigger than that of bull's horn-shaped [7]thienoacene3 due to the bigger unit size and possible steric hindrance of benzene instead of thiophene. The angles between the least-squares planes of the neighboring rings are between 2.2° and 3.3° from ring-B to ring-F. Compound 4 possesses a little distorted molecular geometry, lending to a packing of a sandwich–herringbone arrangement in its crystal (Fig. 2B). There are multiple short interactions between the adjacent molecules, such as S2⋯S3 (3.54 Å), C12⋯C18 (3.40 Å), C12⋯C12 (3.40 Å) and π⋯π (3.48 Å) interactions, which are important for thiophene-based organic semiconductors in the field of high field-effect mobility.12 Approximately coplanar bull's horn-shaped compound 4 may show electronic properties similar to those of annulated pentathienoacene and heptathienoacene.

UV-vis spectra were obtained in both solutions (1 × 10−5 M in dichloromethane) and solid thin films, which are shown in Fig. 4 and Fig. S47. In solution, rac-1 and rac-2 have absorption behaviors similar to [7]helicene and double helicene based on dithieno[2,3-b:3′,2′-d]thiophene.5a Moreover, there are significant red shifts, because of the better conjugation of dithieno[2,3-b:2′,3′-d]thiophene than that of dithieno[2,3-b:3′,2′-d]thiophene. Rac-1 has π-electron delocalization including both helical distortion and possible conjugation through sulfur atoms with a maximum absorption peak at 292 nm (ε = 4.5 × 104 M−1 cm−1). Rac-2 has four approximately planar dithieno[2,3-b:2′,3′-d]thiophenes, which are conjugated together with two absorption peaks at 290 nm (ε = 8.4 × 104 M−1 cm−1) and 362 nm (ε = 1.5 × 104 M−1 cm−1). The molar extinction coefficient at the maximum absorption wavelength is almost twice that of rac-1. In thin films, rac-1 and rac-2 presented absorption behaviors similar to their solutions. The bathochromic shift is rather remarkable for rac-3 and 4 than for rac-1, maybe because of the replacement of thiophene with benzene. The short conjugation of dithieno[2,3-b:2′,3′-d]thiophene yields absorption peaks at 278 (ε = 3.5 × 104 M−1 cm−1) and 276 nm (ε = 5.3 × 104 M−1 cm−1) for rac-3 and 4, respectively, and π-electron delocalization including both helical distortion and possible conjugation through the sulfur atoms yields the maximum absorption peaks at 358 nm (ε = 6.0 × 104 M−1 cm−1) and 344 nm (ε = 4.3 × 104 M−1 cm−1) for rac-3 and 4, respectively. In thin films, the spectra of rac-3 and 4 present fine structures and faint red shifts (2 nm) of absorption peaks to their solutions. Furthermore, 4 shows an obvious redshift trailing due to intermolecular π–π stacking in the film (Fig. S47).

image file: c7qo01049d-f4.tif
Fig. 4 UV/vis absorption spectra of rac-1, rac-2, rac-3 and 4 in dichloromethane at room temperature ([C] = 1 × 10−5 M).


With the synthetic strategy of TMS-protection to the αa position and αb position of dithieno[2,3-b:2′,3′-d]thiophene, two key intermediates, 11 and 14, are employed for the construction of thiophene-based helicenes. Four novel thienoacenes including [7]helicene (rac-1) and double helicene (rac-2), benzohexathia[7]helicene (rac-3) and bull's horn-shaped benzohexathienoacene (4) are synthesized. Rac-1 has two inside sulfur atoms in its two terminal thiophene units, which is different from the reported carbon–sulfur helicenes,4,5 in which all sulfur atoms are on the outside of molecular frameworks. Rac-1 and rac-2 are the first isomers of carbon–sulfur [7]helicene and double helicene, respectively. Besides the challenging synthetic work, some of their crystal structures are obtained, which show not only the aesthetically pleasing molecular structures, but also C⋯C, S⋯S and π⋯π intermolecular interactions. Such properties are helpful to benefit their application in asymmetric catalysis2 and organic functional materials, such as OFET.3,7,8

Conflicts of interest

There are no conflicts to declare.


We gratefully acknowledge Mr Pengtao Ma for his assistance in crystal measurements and discussion. This research was financially supported by the National Natural Science Foundation of China (No. 21672054, 51503056), the Innovation Scientists and Technicians Troop Construction Projects of Henan Province (C20150011) and the Foundation for Distinguished Young Scientist of Henan University (No. YQPY20140056).

Notes and references

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Electronic supplementary information (ESI) available. CCDC 1587217–1587220. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c7qo01049d
These authors contributed equally to this work.

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