Influence of the Counteranion on the Ability of 1‑Dodecyl-3-methyltriazolium Ionic Liquids to Form Mesophases

The influence of the counteranion
on the ability of the mesogenic
cation 1-methyl-3-dodecyl-triazolium to form mesophases is explored.
To that avail, salts of the cation with anions of different size,
shape, and hydrogen bonding capability such as Cl–, Br–, I–, I3–, PF6–, and Tf2N– [bis­(trifluorosulfonyl)­amide] were synthesized
and characterized. The crystal structures of the bromide, the iodide,
and the triiodide reveal that the cations form bilayers with cations
oriented in opposite directions featuring interdigitated alkyl tails.
Within the layers, the cations are separated by anions. The rod-shaped
triiodide anion forces the triazolium cation to align with it in this
crystal structure but due to its space requirement reduces the alkyl
chain interdigitation which prevents the formation of a mesophase.
Rather the compound transforms directly from a crystalline solid to
an (ionic) liquid like the analogous bis­(trifluorosulfonyl)­amide.
In contrast, the simple halides and the hexafluorophosphate form liquid
crystalline phases. Their clearing points shift with increasing anion
radius to lower temperatures.