Absolute Configuration of Hexahelicene

HEXAHELICENE (I), first synthesized and resolved by Newman and Lednicer1,2, is a classical example of an inherently dissymmetric chromophore3 whose dissymmetry extends in a helical fashion through the entire molecule. Two theoretical treatments linking the sign of the long wavelength Cotton effect4 and the chirality of hexahelicene are at variance with each other. One treatment predicted that (+)-hexahelicene would correspond to a right-handed helix5 (Ia), the other to a left-handed helix6 (Ib). (But after complete configuration interaction in a π-electron SCF calculation, a negative long wavelength Cotton effect is predicted for hexahelicene of left-handed chirality7.) Although the configuration of a (−)-thia-helicene has been found to have left-handed helicity by X-ray methods8,9, until this report no experimental data rigorously establishing the absolute configuration of hexahelicene have appeared (Fig. 1).