Calculations of helical twisting power
When a nematic liquid crystal phase is doped with a small amount of a chiral molecule, the whole phase becomes chiral, and the liquid crystal director (which describes the preferred direction of alignment for the molecules) is twisted through space to form a helix. Some molecules are better than others at inducing this twist, and the effectiveness of an individual molecule is measured by its Helical Twisting Power (HTP). There is a lot of industrial interest in making new molecules with very large HTPs, to use as chiral dopants for chiral polymer films. However, it is difficult to relate chemical structure to HTP.
In this part of the project, we developed new methods for predicting HTP prior to synthesis.
a) A Monte Carlo program was developed that allowed a chiral molecule, represented by atomistic potentials, to be simulated in a Gay-Berne (GB) solvent. The GB solvent formed a twisted nematic phase, in which the director was forced to twist through 90 degrees over the simulation box through employment of twisted periodic boundary conditions. In a series of simulations the chiral dopant was gradually mutated into its enantiomer, and the free energy change for this process was measured. The measured free energy change leads directly to a quantitative measure of the helical twisting power.
b) In a second project several single molecule techniques, to see if any of these can yield accurate HTP values. This would allow very rapid screening of molecules for industrials prior to them embarking on an expensive and time-consuming synthetic programme. The most successful approach to date has been pursued in collaboration with Maureen Neal’s group in Coventry, using a scaled chirality tensor. This has predicted good HTP values for a large number of molecules with a rigid chiral framework. Further work into this and other methods is currently underway to extend these to the study of flexible chiral dopants, using the Monte Carlo program developed in the first part of this project.
Finally, our work suggests that each separate molecular conformation has a different HTP, and constant NVT Monte Carlo simulations show how the overall HTP value changes with increases in temperature (as higher energy conformations with different HTPs start to become populated). This mechanism explains the temperature-induced reversal in HTP that occurs with some materials. It also explains the fact that some molecules can have different HTPs in different solvents. The latter is caused by some conformations being preferentially selected in certain solvents.