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).

本研究合成了两套全新的偶氮苯类溴盐（azobenzene-based bromide salts），并对其热性能与光致变色性能展开了研究。两类化合物均以咪唑鎓阳离子（imidazolium cation）为基础骨架。
第一套化合物（标记为1）为对称双阳离子结构：两个带有不同烷基链（Cₙ）的咪唑鎓头部基团（Im）连接于中心偶氮苯单元（Azo）之上，通式为[Azo­(C₁-Im-Cₙ)₂]，其中n的取值为6、8、10、12、14。
另一套化合物则包含带有末端偶氮苯单元（C₁-Azo）的正烷基-咪唑鎓阳离子（Cₙ-Im），该偶氮苯单元还接有碳数分别为2（标记为2）或6（标记为3）的烷氧基链（O-Cₘ），通式为[C₁-Azo-O-Cₘ-Im-Cₙ]；其中当m=2时，n的取值为8、10、12；当m=6时，n的取值为8、10、12、14、16。
针对这两类阳离子结构，研究人员借助差示扫描量热法（differential scanning calorimetry, DSC）与偏光光学显微镜（polarizing optical microscopy, POM），考察了不同长度的烷基链对其热相行为的影响。
在五种化合物中均观察到了液晶相（liquid crystalline phase）的形成，这五种化合物分别为：[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）以及C₁-Azo-O-C₆-Im-C₁₆（3e）。
双阳离子盐（1）均为熔点较高的有机盐，熔点区间为180~240 ℃，仅烷基链长度为12和14的两种衍生物表现出液晶性。
带有O-C₂桥连的单阳离子盐（2）的熔点介于140~170 ℃之间，具体数值随烷基链长度变化而改变；但当烷基链碳数达到10及以上时，该类化合物会形成近晶A相液晶（smectic A liquid crystalline phase）。
第三套带有O-C₆桥连的化合物属于离子液体（ionic liquids），其熔点低于100 ℃；但仅十六烷基链取代的衍生物能够形成液晶相。
所有阳离子类型均成功培养出了代表性单晶，可用于单晶结构分析（single crystal structure analysis）。结合小角X射线散射（small-angle X-ray scattering）实验数据，能够更深入地理解其热性能的内在机制。
所有化合物经紫外光（320~366 nm）辐照后均会发生反式-顺式异构化（trans–cis isomerization），该异构化过程可通过可见光（440 nm）照射实现可逆。