The efficient synthesis of dibenzo[d,d′]benzo[1,2-b:4,3-b′]dithiophene and cyclopenta[1,2-b:4,3-b′]bis(benzo[d]thiophen)-6-one

With 3,3′-bi[benzo[b]thiophenyl] as starting material, dibenzo[d,d′]benzo[1,2-b:4,3-b′]dithiophene, a [5]heterohelicene, was synthesized efficiently in 60% yield via formylation and McMurry reaction. Cyclopenta[1,2-b:4,3-b′]bis(benzo[d]thiophen)-6-one, another interesting helical ketone, was also prepared in 79% yield via deprotonation and ketonization of 3,3′-bi[benzo[b]thiophenyl]. In addition, the single-crystal structure of dibenzo[d,d′]benzo[1,2-b:4,3-b′]dithiophene and UV–vis spectra of both title compounds are described.


X-ray structural analysis
A single crystal of 1 was obtained by the slow evaporation of a solution of 1 in CHCl 3 /CH 3 OH (5/1, v/v). The crystal structure of 1 was confirmed by single-crystal X-ray analysis ( Figure 2 and Figure 3). 1 has non-planar π-extended frameworks, and its molecule is compressed and dominated by a helical structure ( Figure 2). The distance between the two H atoms H(5)A…H(18)A is 2.062 Å and these H atoms point away from each other. The repulsion of the facing terminal benzene rings causes an interplanar angle of 31.7° between the terminal benzene rings. The angles between the least-squares planes of neighboring rings are between 7.5° and 9.5°. Using the middle benzene ring as a reference, the inner (C(5), C(6), C(7), C(12), C(13), and C(18)) helix increases by 1.46 Å and turns in-plane by 237.0° [22,23].  The crystal packing structure of 1 ( Figure 3) is based on π-stacking along the a-axis in which the plane-to-plane distance is ca. 3.857 Å. The lack of contact between the π-stacks indicates that the crystal has one-dimensional (1D) electronic structure [13].

UV-vis spectra of 1 and 2
The UV-vis spectra of 1 and 2 in chloroform are shown in Figure 4. 1 shows the main peaks at 323 nm, 335 nm, and 372 nm, and 2 gives two broad peaks at 339 nm and 380 nm. Both 1 and 2 possess molecular connectivities of cross-conjugated π-system that shows π-electron delocalization with helical distortion.

Conclusion
Derivatives of benzothiophene, thienothiophene, and thienodithiophene with good characteristics for OFETs were recently reported [5][6][7]11,24,25]. Because of higher π-electron delocalization, compounds 1 and 2 could be used in OFET and/or con-ducting polymers. From 3 as starting material, 1 and 2 have been efficiently obtained in a total yield of 60% and 79%, respectively, in our work. The efficient synthesis of 1 and 2 will facilitate the synthetic approaches to various organic functional materials, by using 1 and 2 as building blocks, and/or versatile intermediates. The measurement of the hole/electron mobilities of compounds 1 and 2 is in progress.

Synthesis of dibenzo[d,d′]benzo[1,2-b:4,3-b′]dithiophene (1)
TiCl 4 (0.16 mL, 1.5 mmol, 3.0 equiv) was carefully added to dry THF (20 mL) at 0 °C, after keeping at 0 °C for 10 min, zinc dust (0.20 g, 3.0 mmol, 6.0 equiv) was added, and the mixture refluxed for 2 h. Pyridine (0.12 mL, 1.5 mmol, 3.0 equiv) was added and the mixture was heated under reflux for further 1 h. After cooling to ambient temperature, a solution of 4 (0.1607 g, 0.5 mmol) in dry THF (10 mL) was added and the reaction mixture was heated under reflux overnight. The reaction mixture was quenched with 18% HCl (20 mL) at 0 °C, extracted with CHCl 3 (4 × 30 mL), washed with H 2 O (3 × 30 mL), and dried over MgSO 4 . After the removal of the solvent in vacuo, the crude product was purified by column chromatography on silica gel with PE (60-90 °C)/ethyl acetate ( 3 . A colorless crystal with a size of 0.29 mm × 0.15 mm × 0.10 mm was used for measurement at 296(2) K in ω scan mode (Bruker Smart APEX X-ray diffractometer, CCD detector, Mo K α radiation (λ = 0.71073 Å)). The data were corrected for Lorentz and polarization effects and absorption corrections were performed using the SADABS [26] program. The crystal structures were solved using the SHELXTL [27] program and refined using full matrix leastsquares. The positions of hydrogen atoms were calculated theoretically and included in the final cycles of refinement in a riding model along with attached carbons. The final cycle of full matrix least-squares refinement was based on 7988 independent reflections [I > 2σ(I)] and 181 variable parameters with R1 = 0.0335, wR2 = 0.0846.

Supporting Information
Supporting information features experimental procedures and spectroscopic analysis for compounds 1, 2, and 4.