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 sixsteps. 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 Delta epsilon = -851 M-1 cm(-1) at lambda = 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%.