Induction of cholesteric mesophases by simple cyclic derivatives of p,p '-disubstituted 1,2-diphenylethane-1,2-diols: Importance of shape and polarizability effects

A systematic study of the cholesteric induction in nematic solvents (MBBA a nd E7) by some cyclic derivatives of unsubstituted and p,p'-disubstituted-1 ,2-diphenylethane-1, shows that the values of the twisting power are signif icantly dependent on the nature of the link connecting the two oxygen atoms and on the nature of the p,p'-substituents. This result has been interpret ed considering that the nature of the bridge affects the overall molecular shape and the p,p'-substituents affect both the molecular polarizability an d shape. This investigation points out that the polarizability of the solut e and the solvent is the main parameter in determining the value of the twi sting power while electrostatic arene-arene interactions contribute to a le ss extent. It has been also observed that solutes having the same structure and the same absolute configuration can induce cholesteric helix of opposi te sign depending on the substituent on the aromatic ring. This finding ind icates that configurational assignments by cholesteric induction are reliab le only if high values of twisting power are measured.