Conformational and configurational analysis of 4,4'-biphenanthryl derivatives and related helicenes by circular dichroism spectroscopy and cholesteric induction in nematic mesophases

Coupling the analysis of circular dichroism (CD) spectra with the study of cholesteric mesophases induced in nematic liquid crystals (LC) allows one to deduce some stereochemical features of 3,3'disubstituted-4,4'-biphenanthryl derivatives. The exciton coupling model can explain the CD spectra of the open-chain derivatives, thus confirming their absolute configuration; the LC technique indicates an s-trans conformation in agreement with molecular mechanics computations. Compounds with a bridge between the 3 and 3' positions are conformationally rigid and similar in shape, and their absolute configuration can be predicted by the LC technique. On the other hand, their CD spectra are different and not easy to interpret as a consequence of the different chromophores present.