Prediction of pitch in twisted nematics: Puzzling cases

The correlation between structure of chiral molecules and macroscopic properties of cholesteric phases is not obvious, but often the experimental behaviour can be rationalised by resorting to simple rules. However, there are cases in which solvent / temperature changes or introduction of substituents in the chiral molecules can produce dramatic and unexpected effects, including helix inversion. Some significant examples will be presented and discussed on the basis of theoretical predictions derived by the ‘surface tensor’ mean field model for molecular structures optimised with quantum-mechanical methods.