Synthesis of macrocyclised dimetric compounds and their liquid crystal transition behaviours

Abstract

Cyclic C-n dimers were synthesised by the ring-closing metathesis of linear L-n dimers with allyl tails under high dilution in the presence of the first Grubbs catalyst. The C-n dimers form a smectic phase similarly to the precursor L-n dimers, but the smectic structure and phase transition behaviour are remarkably different from those in L-n in the following respects. First, the C-n dimers invariably form a smectic A phase, although the L-n dimers as well as the conventional dimers form smectic A and smectic CA phases depending on whether the number of carbons on the spacer is even or odd. Second, the isotropisation temperature of the C-n dimer is significantly higher than that of the L-n dimer so that the smectic temperature span is expanded to 100°C from around 30°C for L-n. Third, the layer order in the smectic phase of the C-n dimer is remarkably higher than that of the L-n dimer, especially when n is small. These differences are explained as an effect of macrocyclisation, which forces the spacer to fold and thereby causes the two mesogens within a molecule to face each other closely.

Keywords:

• liquid crystalline dimer
• macrocyclisation
• ring-closing metathesis
• cyclic dimer
• smectic phase

Acknowledgements

This research was supported by the Grant-in-Aid for Creative Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan.