Helical Threads: Enantiomerically Pure Carbo[6]Helicene Oligomers

We report the synthesis of enantiomerically pure carbo[6]helicene oligomers with buta-1,3-diyne-1,4-diyl bridges between the helicene nuclei. The synthesis of monomeric (±)-2,15-bis[(triisopropylsilyl)ethynyl]carbo[6]helicene was achieved in 25 % yield over six steps. Pure (+)-(P)- and (-)-(M)-enantiomers were obtained by HPLC on a chiral stationary phase. The dimeric (+)-(P)₂ - and (-)-(M)₂ -configured and the tetrameric (+)-(P)₄ - and (-)-(M)₄ -configured oligomers were obtained by sequential oxidative acetylenic coupling. The ECD spectra of the tetrameric oligomers displayed large Cotton effect intensities of Δϵ=-851 m^-1  cm^-1 at λ=370 nm ((M)₄ -enantiomer). We transformed the buta-1,3-diyne-1,4-diyl bridge in the dimeric (P)₂ and (M)₂ oligomer by heteroaromatization into a thiene-2,5-diyl linker. Although the resulting chromophore showed reduced ECD intensities, it exhibited a remarkably strong fluorescence emission at 450-500 nm, with an absolute quantum yield of 25 %.