title: Thiophene-Based Double Helices: Syntheses, X-ray Structures, and Chiroptical Properties.
abstract: We demonstrate facile and efficient construction of conjugated double helical ladder oligomers from the saddle-shaped cyclooctatetrathiophene (COTh) building blocks. The key step involves deprotonation of tetra[3,4]thienylene (β,β-COTh) with n-BuLi which displays remarkably high ipsilateral selectivity. Three racemic double helical ladder oligomers, rac-DH-1, rac-DH-2, and rac-DH-3, containing two, three, and five COTh annelated moieties are efficiently synthesized by diastereoselective coupling of the racemic precursors. The X-ray crystallographic studies of rac-DH-1, rac-DH-2 and rac-DH-3 unambiguously revealed that each double helical scaffold has two single helices intertwined with each other via the C-C single bonds. Following removal of TMS groups, double helical ladder oligomer rac-DH-1-D had sufficient solubility to be resolved via chiral HPLC, thus enabling determination of its chirooptical properties such as CD spectra and optical rotation. (+)-DH-1-D has a large barrier for racemization, with lower limit of ΔG(‡) > 48 kcal mol(-1), which may be compared to DFT-computed barrier of 51 kcal mol(-1). The enantiomers of DH-1-D show 1 order of magnitude stronger chirooptical properties than the carbon-sulfur [7]helicene, as determined by the anisotropy factor g = Δε/ε = -0.039, based on Δεmax = -11 and ε = 2.8 × 10(2) L mol(-1) cm(-1) in cyclohexane at 327 nm.
authors.affiliation: Engineering Research Center for Nanomaterials, Henan University , Kaifeng 475004, China.
authors.forename: Sheng
authors.initials: S
authors.lastname: Zhang
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588, United States.
authors.forename: Xinming
authors.initials: X
authors.lastname: Liu
authors.affiliation: Engineering Research Center for Nanomaterials, Henan University , Kaifeng 475004, China.
authors.forename: Chunli
authors.initials: C
authors.lastname: Li
authors.affiliation: Engineering Research Center for Nanomaterials, Henan University , Kaifeng 475004, China.
authors.forename: Lu
authors.initials: L
authors.lastname: Li
authors.affiliation: Engineering Research Center for Nanomaterials, Henan University , Kaifeng 475004, China.
authors.forename: Jinsheng
authors.initials: J
authors.lastname: Song
authors.affiliation: Engineering Research Center for Nanomaterials, Henan University , Kaifeng 475004, China.
authors.forename: Jianwu
authors.initials: J
authors.lastname: Shi
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588, United States.
authors.forename: Martha
authors.initials: M
authors.lastname: Morton
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588, United States.
authors.forename: Suchada
authors.initials: S
authors.lastname: Rajca
authors.affiliation: Department of Chemistry, University of Nebraska , Lincoln, Nebraska 68588, United States.
authors.forename: Andrzej
authors.initials: A
authors.lastname: Rajca
authors.affiliation: Engineering Research Center for Nanomaterials, Henan University , Kaifeng 475004, China.|Collaborative Innovation Center of Nano Functional Materials and Applications of Henan Province , Kaifeng 475004, China.
authors.forename: Hua
authors.initials: H
authors.lastname: Wang