Mesophase Stabilization in Ionic Liquid Crystals through Pairing Equally Shaped Mesogenic Cations and Anions

The
synthesis and properties of a set of novel ionic liquid crystals
with congruently shaped cations and anions are reported to check whether
pairing mesogenic cations with mesogenic anions leads to a stabilization
of a liquid crystalline phase. To that avail 1-alkyl-3-methyl-triazolium
cations with an alkyl chain length of 10, 12, and 14 carbon atoms
have been combined with p-alkyloxy-benzenesulfonate
anions with different alkyl chain lengths (n = 10,
12, and 14). The corresponding triazolium iodides have been synthesized
as reference compounds where the cation and anion have strong size
and shape mismatch. The mesomorphic behavior of all compounds is studied
by differential scanning calorimetry and polarizing optical microscopy.
All compounds except 1-methyl-3-decyltriazolium iodide, which qualifies
as an ionic liquid, are thermotropic ionic liquid crystals. All other
compounds adopt smectic A phases. A comparison of the thermal phase
behavior of the 1-methyl-3-decyltriazolium bromides to the corresponding p-alkoxy-benzensulfonates reveals that definitely the mesophase
is stabilized by pairing the rod-shaped 1-alkyl-3-methyltriazolium
cation with a rod-like anion of similar size.