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 %. Bigger, better! An efficient route to enantiopure 2,15‐dialkynylated carbo[6]helicenes and buta‐1,3‐diyne‐1,4‐diyl‐ and thiene‐2,5‐diyl‐linked dimeric and tetrameric oligomers is described. The chiroptical properties are correlated with the extent of π‐electron delocalization. X‐ray analysis provided conformational information consistent with DFT‐calculated structures. The thiene‐2,5‐diyl‐bridged dimers display intense fluorescence emission, which is a rare property in carbohelicene chemistry (see scheme).