Helicene Monomers and Dimers: Chiral Chromophores Featuring Strong Circularly Polarized Luminescence.

The synthesis and chiroptical properties of a series of enantiomerically pure, C-2-symmetrical carbo[6]helicene dimers are reported. Two helicene cores are connected through a buta-1,3-diyne-1,4-diyl linker or a heteroaromatic bridge and bear arylethynyl substituents at their 15-positions. This ensures the possibility of extended electronic communication throughout the whole molecule. The new chromophores exhibit intense ECD spectra with strong bands in the UV/Vis region well above 400 nm. The anisotropy factor g(abs) (defined as Delta epsilon/epsilon) reaches values up to 0.047, which are unusually large for single organic molecules. They also display blue fluorescence, with good quantum yields (phi(f)similar to 0.25). The emitted light is circularly polarized to an outstanding extent: in some cases, the luminescence dissymmetry factor g(lum)=2(I-L-I-R)/(I-L+I-R) attains values of |0.025|. To the best of our knowledge, such values are among the highest ever reported for non-aggregated organic fluorophores.