Azobenzene-Based Organic Salts with Ionic Liquid and Liquid Crystalline Properties

Two sets of new azobenzene-based
bromide salts are synthesized,
and their thermal photochromic properties are studied. Both sets are
based on the imidazolium cation. The first set (1) features
a symmetric biscation where two imidazolium head groups (Im) with
different alkyl chains (Cn) are connected to a central azobenzene
unit (Azo): [Azo­(C1-Im-Cn)2]; n = 6, 8,
10, 12, 14. The other one contains an n-alkyl-imidazolium
cation (Cn-Im) bearing a terminal azobenzene unit (C1-Azo) substituted
with an alkoxy chain (O-Cm) of either two (2) or six (3) carbon atoms: [C1-Azo-O-Cm-Im-Cn]; m = 2, n =
8, 10, 12 and m = 6, n = 8, 10,
12, 14, 16. For both cation classes, the influence of alkyl chains
of varying length on the thermal phase behavior was investigated by
differential scanning calorimetry (DSC) and polarizing optical microscopy
(POM). For five compounds (Azo­(-C1-Im-C12)2 (1d), Azo­(-C1-Im-C12)2 (1e), C1-Azo-O-C2-Im-C10
(2b), C1-Azo-O-C2-Im-C12 (2c), and C1-Azo-O-C6-Im-C16
(3e)), the formation of a liquid crystalline phase was
observed. The biscationic salts (1) are all comparatively
high melting organic salts (180–240 °C), and only the
two representatives with long alkylchains (C12 and C14) exhibit liquid
crystallinity. The monocationic salts with an O–C2 bridge (2) melt between 140 and 170 °C depending on the alkyl
chain length, but from an alkyl chain of 10 and more carbon atoms
on they form a smectic A liquid crystalline phase. The representatives
of the third set with a O–C6 bridge qualify as ionic liquids
with melting points less than 100 °C. However, only the representative
with a hexadecyl chain forms a liquid crystalline phase. Representative
single crystals for all sets of cations could be grown that allowed
for single crystal structure analysis. Together with small-angle X-ray
scattering experiments they allow for a more detailed understanding
of the thermal properties. Through irradiation with UV-light (320–366
nm) all compounds undergo trans–cis isomerization,
which reverses under visible light (440 nm).