Dipolar origin of tilting of rod-like molecules in the smectic C phase

Abstract

We review various proposed molecular mechanisms for tilting of rod-like molecules in the smectic C phase. We analyse some experimental results, viz. (1) unusual odd–even effects in smectic A to nematic (T_AN) and smectic A to smectic C (T_AC) transition temperatures and/or maxima in T_AC in some homologous series of compounds, (2) occurrence of re-entrant SmA and/or hexatic SmB as lower temperature phases, (3) SmC replaced by SmA when a halogen atom is substituted at the end of one of the chains, (4) suppression of SmC phase at high pressures, and in binary mixtures with dissimilar aromatic cores or chain lengths, (5) induction of SmC phase in mixtures of some structural isomers and (6) some recent all-atom simulations. The results can be understood on the basis of our molecular theory in which the tilting occurs due to the repulsive coulomb interaction between neighbouring molecules having off-axis dipoles with transverse components and freely rotating about their long axes in smectic layers. The model also takes into account the entropic cost of tilting which depends on the length to width ratio of the molecules. We have included some simple calculations of the induced dipoles in the chains to account for the odd–even effects.

Keywords:

• smectic C liquid crystals
• molecular tilting in layers
• molecular theories
• off-axis dipoles
• odd–even effects
• re-entrant smectic A phase