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

Article abstract of DOI:10.1021/acs.cgd.5b01023

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.

Full text of DOI:10.1021/acs.cgd.5b01023

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Article
Mesophase stabilization in ionic liquid crystals through
pairing equally shaped mesogenic cations and anions
Kathrin Stappert, Gregor Lipinski, Gabriel Kopiec, Eike T. Spielberg, and Anja-Verena Mudring
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01023 • Publication Date (Web): 23 Jul 2015
Downloaded from http://pubs.acs.org on August 6, 2015
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Crystal Growth & Design
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Mesophase stabilization in ionic liquid crystals through
pairing equally shaped mesogenic cations and anions
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Kathrin Stapperta, Gregor Lipinskia, Gabriel Kopieca,b,c, Eike T. Spielberga,d, Anja-Verena
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Mudringa,b,c
*
a former affiliation: Anorganische Chemie III – Materials Engineering and Characterization, Fakultät für Chemie
and Biochemie,
RuhrꢀUniversität Bochum, 44780, Bochum, Germany. Eꢀmail: anja.mudring@ruhrꢀuniꢀbochum.de; Tel: +49ꢀ
234ꢀ32ꢀ27408
bCritical Materials Institute, Ames Laboratory, 333 Spedding Hall, Ames, IA, 50011
cDepartment of Materials Science and Engineering, Iowa State University, 2220 Hoover Hall, Ames, IA, 50011,
mudring@iastate.edu, Tel:+1 515 5095616
dUniversitätsbibliothek, Universität Duisburg Essen, Universitätsstr. 9, 45141 Essen, Germany
Abstract
The synthesis and properties of a set of novel ionic liquid crystals with congruently shaped
cations and anions is 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 DSC (differential scanning calorimetry)
and POM (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.
Keywords. Ionic Liquids. Liquid Crystals. Ionic Liquid Crystals.
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1. Introduction
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Ionic liquids (ILs), salts with a melting point below 100 °C, offer unique chemical and physical
properties and property combinations.0F1 Properties such as negligible vapour pressure, wide
liquid range, good thermal stability, considerable electric conductivity, and large
electrochemical window are advantageous for a number of applications.1F2 In ionic liquid
crystals (ILCs) the properties of ILs (such as mobility) can be combined with those of liquid
crystals (LC). It is thereby possible to endow a useful IL property such as ionic conductivity
with anisotropy. Whilst lyotropic ionic liquid crystals rely on admixture of a second
component, most commonly a classical molecular solvent3 to adopt mesophases, in
thermotropic liquid it is temperature that triggers mesophase formation and in a certain
temperature range a liquid crystal is formed.
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The thermal behavior of ionic liquid crystals is influenced by the shape of the cation
as well as by the shape of the anion as well as by subtle ion-ion interactions. Generally the
thermotropic mesomorphic behavior has been introduced by a rod-shaped cation as the
mesogenic unit. In that respect, the most investigated cation type is the imidazolium cation.
Mesomorphic behavior has been reported for 1-alkyl-3-methylimidazolium salts bearing long
alkyl chains5F4,6F5 and rod-shaped bicationic imidazolium salts.6,7,8
3F 4F Correlating the melting and
2
clearing point with the alkylchain length of the cation has been the focus of numerous
studies.9,10,11
Similar studies, albeit less systematically and comprehensively, have been
carried out for several other types of cations, like pyrrolidinium12,13
piperazinium14, morpholinium14 and triazolium13F15
7F
8F
9F
10F
1F , piperidinium12F14
,
.
As counter anions spherical anions such as simple halides, as well as other high
symmetry anions like tetrafluoroborate, hexafluorophopsphate or perchlorate were
chosen16,17, 18
ions, possibly
14F
15F
16Fthe . Only few attempts have been made to synthesize ILCs with low symmetry
because combinations of organic cations with the
bis(trifluoromethanesulfonyl)amide anion - a weakly coordinating, non-spherical anion
commonly used in ionic liquids - did not lead to the desired formation of mesophases. For
that reason, it is believed that comparatively large and low symmetry anions rather prevent
the alkyl chains of the cations to interact with each other which is a prerequisite for
mesophase
formation.
Only
recently
the
mesophase
formation
of
a
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bis(trifluoromethanesulfonyl)amide could be confirmed19. It was also observed that the
anion shape can substantially impact the cation conformation and the mesomorphic
behavior of the salt. This has been shown for long-chain dialkyl-imidazolium16 and dialkyl-
triazolium15 cations. The simple iodides feature the cation in an U-shaped conformation.
However, for the triiodide a linear cation conformation is found. It appears that the rod-
shape of the triiodide anion forces the dialkyl-cation into a linear conformation. This results
in a much lower interdigitation, hence, interaction of the alkyl-chains on the cation and
ultimately prevents the formation of a mesophase 15.
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Beside these inorganic anions, organic anions could well be used as counter anions
and potentially open up new avenues for stabilizing a mesophase. Sulfonate anions, as
commonly encountered in soaps featuring typically the sodium salt20, are well known to give
lyotropic phases. The formation of thermotropic mesophases of alkylsulfonate or
alkylbenzenesulfonate was observed for different organic cations21,22,23
and rod-shaped
18F
19F
sulfonates like 1-3-dialkylimidazolium cations are mesogenic units. It was proposed that
congruently rod-shaped anions and cations would lead to mesophase stabilization due to a
more efficient packing of the ions. The viability of the concept was demonstrated for
guanidinium sulfonates.24 Since both 1-alkyl-3-methyltriazolium cations and p-alkyloxy-
benzenesulfonate anions with long chains are mesogenic units, the question arises, whether
their combination would lead to an improvement of the ILC properties, i.e. enhance the
mesophase window and lower the melting point. To this avail, a new series of salts with 1-
alkyl-3-methyl-triazolium cations and p-alkyloxy-benzenesulfonate anions with alkyl groups
of different lengths were synthesized (Figure 1) and their thermal behavior studied. To
unravel the effect of rod-shaped, mesogenic anions the corresponding triazolium iodides
where studied as well. A list of the investigated compounds is given in Table 1.
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Figure 1. Structures of cations and anions which were investigated.
Table 1. Investigated compounds and compound numbering scheme.
C14A10 C14A12 C14A14
C10I C12I C14I C10A10 C10A12 C10A14 C12A10 C12A12 C12A14
14
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m
10 12 14
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Anion I- I- I- n= 10 n= 12 n= 14 n= 10 n= 12 n= 14
n= 10 n= 12 n= 14
2. Experimental Section
Synthesis
The alkyl azides were synthesized according to a common literature method21F25. All other
starting materials and solvents were purchased from standard commercial sources and were
used without further purification. No protective atmosphere was used during synthesis.
General procedure for the preparation of 1-alkyl-1,2,3-triazole (1a – 3a)
5g Alkyl azide (23.7 mmol) and 10 ml vinyl acetate were irradiated with microwaves (CEM
Discover) in a closed glass tube for 5 hours at 120 °C. After addition of 200 ml n-hexane, the
suspension was filtered and the solution cooled to -40 °C for 24 hours. The precipitated
product was filtered off and dried overnight in vacuum at room temperature.
1-Decyl-1,2,3-triazole (4a).
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Yield: 70 %, 1H NMR (200 MHz, CDCl3) δ: 7.69 (s, 1H), 7.52 (s, 1H), 4.37 (t,
J
J
J
= 7.2 Hz, 2H), 2.00
= 7.2 Hz, 2H), 1.92
= 7.2 Hz, 2H), 1.99
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– 1.78 (m, 2H), 1.25 (s, 14H), 0.87 (t, J = 6.3 Hz, 3H).
1-Dodecyl-1,2,3-triazole (4b).
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Yield: 71 %, 1H NMR (200 MHz, CDCl3) δ: 7.69 (s, 1H), 7.52 (s, 1H), 4.38 (t,
(dd, = 12.4, 5.7 Hz, 2H), 1.25 (s, 18H), 0.88 (t, = 6.3 Hz, 4H).
J
J
1-Tetradecyl-1,2,3-triazole (4c).
Yield: 75 %, 1H NMR (200 MHz, CDCl3) δ 7.69 (s, 1H), 7.52 (s, 1H), 4.38 (t,
– 1.83 (m, 2H), 1.25 (s, 22H), 0.88 (t, = 6.4 Hz, 3H).
J
1-Alkyl-3-methyl-1,2,3-triazolium iodides (CmI)
A solution of the 1-alkyl-1,2,3-triazole (4a – c) and two equivalents of methyl iodide was
heated under reflux in acetonitrile for three days. After cooling to room temperature the
solution was poured into cold ethyl acetate and kept at -40 °C for 5 hours. The white
precipitate was filtered off and dried under vacuum at room temperature to give the
products in good yields and purity (80 – 85 %).
1-Decyl-3-methyl-1,2,3-triazolium iodide (C10I)
Yield: 85 %, 1H NMR (200 MHz, CDCl3) δ: 9.48 (d,
J
= 1.1 Hz, 1H), 9.38 (d,
J = 1.2 Hz, 1H), 4.74
(t,
J
= 7.4 Hz, 2H), 4.52 (s, 3H), 2.03 (dd,
J
= 8.2, 6.1 Hz, 2H), 1.36 – 1.19 (m, 14H), 0.85 (t, J =
13
6.4 Hz, 3H). C NMR (50 MHz, CDCl3) δ: 132.51, 131.52, 54.53, 41.19, 31.91, 29.92 – 28.73,
26.20, 22.74, 14.20. IR (ATR): ꢀꢁ [cm-1] = 3124, 3072, 3039, 2953, 2918, 2847, 1528, 1463,
+
1353, 1309, 1195, 1129, 1088, 814, 723, 427. MS (FAB): m/z = 224 (100%, C10 ), 84 (12 %,
C10 - C10H21), 350 (10 %, C10+ - CH3 + C10H21). Elemental analysis: calcd % for C13H26IN3 (M =
+
351,2 g/mol): C 44.45, H 7.46, N 11.96, found: C 44.6, H 7.63, N 11.97.
1-Decyl-3-methyl-1,2,3-triazolium iodide (C12I)
Yield: 81 %, 1H NMR (200 MHz, CDCl3) δ: 9.50 (d,
– 4.66 (m, 2H), 4.53 (s, 3H), 2.12 – 1.99 (m, 2H), 1.25 (s, 18H), 0.88 (t,
J
= 1.3 Hz, 1H), 9.34 (d,
J
= 1.4 Hz, 1H), 4.80
13
J
= 6.5 Hz, 3H). C
NMR (50 MHz, CDCl3) δ: 132.55, 131.55, 54.56, 41.20, 31.98, 29.58, 28.94 (s), 26.22 (s),
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22.76, 14.19. IR (ATR): ꢀꢁ [cm-1] = 3124, 3073, 2953, 2916, 1528, 1462, 1353, 1311, 1205,
1193, 1129, 1088, 814, 723. MS (FAB+):
m/z
= 84 (11 %, K+-C12H24), 252 (100 %, K+), 631
6
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(6 %, 2*K+ + I-). Elemental analysis (%) calcd for (C15H30N3I): C 47.5, H 7.97, N 11.08; found: C
8
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47.61, H 7.38, N 11.16.
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1-Decyl-3-methyl-1,2,3-triazolium iodide (C14I)
Yield: 80 %, 1H NMR (200 MHz, CDCl3) δ: 9.52 (s, 1H), 9.36 (d,
Hz, 2H), 4.53 (s, 3H), 2.05 (dd, = 9.1, 6.8 Hz, 2H), 1.29 (d, = 17.8 Hz, 22H), 0.87 (t,
J
= 1.1 Hz, 1H), 4.73 (t,
J
J
= 7.4
= 6.4
J
J
Hz, 3H). 13C NMR (50 MHz, CDCl3) δ: 132.69, 131.55, 54.60, 41.11, 32.05, 29.57, 28.97, 26.26,
22.82, 14.26. IR (ATR): ꢀꢁ [cm-1] = 3125, 3073, 2916, 2846, 1528, 1463, 1202, 1188, 1129,
+
+
1088, 814, 723, 428. MS (FAB): m/z = 280 (100%, C12 ), 84 (12 %, C12 - C12H25), 462 (47 %,
C12+ - CH3 + C12H25). Elemental analysis: calcd % for C13H26IN3 (M = 351,2 g/mol): C 50.12, H
8.44, N 10.31, found: C 50.92, H 8.53, N 10.07.
General procedure for the preparation of 4-(alkyloxy)benzene-1-sulfonyl chlorides (2a-c)
Sodium 4-hydroxybenzenesulfonate dihydrate (1.5 g, 6.45 mmol) and well powdered
potassium hydroxide (1.1 g, 9.7 mmol) were stirred in dimethyl sulfoxide (150 mL) at room
temperature until completely dissolved. The corresponding alkyl bromide (9.7 mmol) was
added and the suspension was stirred at room temperature overnight. The mixture was then
poured into water. The precipitating white solid was filtered off and dried under vacuum.
The resulting alkyloxybenzene sulfonates were used without further purification. In a second
reaction, dimethylformamide (5.5 ml, 71 mmol) was cooled to 0 °C and 5.9 ml (64.5 mmol)
phosphoryl chloride was added dropwise and the solution stirred for further 10 minutes.
Alkyloxybenzene sulfonate (1a-c), dissolved in dichloromethane, was added and the mixture
was allowed to reach room temperature. After stirring for another hour the suspension was
poured over ice and extracted with dichloromethane. The combined organic phases were
dried over sodium sulfate, filtered and the solvent was removed under reduced pressure.
4-(Decyloxy)benzene-1-sulfonylchloride (2a)
1
Yield: 87 %, H NMR (200 MHz, CDCl3) δ: 7.98 – 7.78 (m, 2H), 7.11 – 6.85 (m, 2H), 4.04 (t,
J
=
6.5 Hz, 2H), 1.94 – 1.60 (m, 2H), 1.58 – 1.05 (m, 14H), 0.86 (t, J = 6.4 Hz, 3H).
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4-(Dodecyloxy)benzene-1-sulfonyl chloride (2b)
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1
Yield: 88 %, H-NMR (200 MHz, CDCl3)
δ: 7.96 (d,
J
= 9.1 Hz, 2H), 7.02 (d,
J = 9.1 Hz, 2H), 4.06
7
8
(t, = 6.5 Hz, 2H), 1.89-1.75 (m, 2H), 1.49-1.14 (m, 18H), 0.88 (t,
J
J
= 6.4 Hz, 3H).
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4-(Tetradecyloxy)benzene-1-sulfonyl chloride (2c)
Yield: 89 %, 1H NMR (200 MHz, CDCl3) δ 7.96 – 7.70 (m, 2H), 7.07 – 6.79 (m, 2H), 3.94 (dt, J
= 35.0, 11.9 Hz, 2H), 1.82 – 1.71 (m, 2H), 1.20 (s, 22H), 0.80 (d, J = 6.9 Hz, 3H)
General procedure for the synthesis of methyl 4-(alkyloxy)benzenesulfonates 3a-c
Sodium hydride (0.16 g, 6,47 mmol) was dissolved in 10 ml dry methanol and added
dropwise at 0 °C to a solution of the corresponding sulfonyl chloride (2a-c) (4.31 mmol) in
dry methanol. The reaction mixture was stirred for further 15 minutes and then quenched
with water (0.15 ml, 8.26 mmol). The solvent was removed in vacuum and the residue
dissolved in dichloromethane, dried over sodium sulfate and filtered. The dichloromethane
was evaporated and the residue dried under vacuum to give the a white product.
Methyl 4-(decyloxy)benzenesulfonate (3a)
Yield: 72 %, 1H NMR (200 MHz, CDCl3) δ 7.89 – 7.63 (m, 2H), 7.07 – 6.82 (m, 2H), 4.11 – 3.91
(m, 2H,), 3.71 (s, 3H), 1.89 – 1.58 (m, 2H), 1.53 – 1.00 (m, 14H), 0.87 (t, J = 6.4 Hz, 3H).
Methyl 4-(dodecyloxy)benzenesulfonate (3b)
1
Yield: 70 %, H-NMR (200 MHz, CDCl3):
δ= 7.82 (d,
J
= 8.9 Hz, 2H), 6.99 (d,
J
= 8.9 Hz, 2H),
4.02 (t, J = 6.6 Hz, 2H), 3.72 (s, 3H), 1.88-1.74 (m, 2H), 1.48-1.19 (m, 18H), 0.87 (s, 3H).
Methyl 4-(tetradecyloxy)benzenesulfonate (3c)
Yield: 66 % 1H NMR (200 MHz, CDCl3) δ 7.91 – 7.73 (m, 2H), 7.03 – 6.94 (m, 2H), 4.02 (t, J =
6.5 Hz, 2H), 3.72 (s, 3H), 1.79 (d, J = 8.3 Hz, 2H), 1.26 (s, 22H), 0.86 (d, J = 4.9 Hz, 3H).
General procedure for the preparation of 1- alkyl-3-methyl-triazolium 4-
alkoxybenzenesulfonates (CmAn)
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A solution of the 1-alkyl-1,2,3-triazole (4a-c) and one equivalent of the respective methyl 4-
(alkyloxy)benzenesulfonate was heated under reflux in acetonitrile for three days. After
cooling to room temperature the solution was poured into cold ethyl acetate and kept at -40
°C for 16 hours. The white precipitate was filtered off and dried under vacuum at room
temperature to give the products in good yield and purity (80 – 85 %).
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1- Decyl-3-methyl-triazolium 4-decyloxybenzenesulfonate (C10A10)
1
Yield: 40 %, H NMR (200 MHz, CDCl3) δ: 9.40 (s, 1H), 9.24 (s, 1H), 7.81 (d, J = 8.6 Hz, 2H),
6.84 (d, J = 8.5 Hz, 2H), 4.59 (t, J = 7.4 Hz, 2H), 4.40 (s, 3H), 3.94 (t, J = 6.6 Hz, 2H), 1.93 (s,
13
2H), 1.74 (d, J = 6.8 Hz, 4H), 1.25 (d, J = 6.1 Hz, 26H), 0.87 (t, J = 6.4 Hz, 6H). C NMR (50
MHz, CDCl3) δ: 160.08, 138.61, 136.90, 131.80, 127.50, 113.83, 68,15, 65.33, 40.20, 31.91,
29.45, 26.03, 22.69, 14.13. IR (ATR): ꢀꢁ [cm-1] = 2916s, 2851m, 1596w, 1493w, 1468m,
1245s, 1211s, 1193s, 1120s, 1103m, 1030s, 1003m, 833s, 819m, 799m, 719m, 697m, 609s,
+
+
-
564s, 551m. MS (pos. ESI, MeOH): m/z = 224 (26%, C10 ), 761 (100%, 2*C10 + 1*A10 ), 1298
-
-
-
(72%, 3*C10+ + 2*A10 ), 1836 (12%, 4*C10+ + 3*A10 ). MS (neg. ESI, MeOH): m/z = 172 (8%, A10
-
-
+
+
- C10H21), 313 (100%, A10 ), 850 (8%, 2*A10 + 1*C10 ), 1387 (9%, 3*A10- + 2*C10 ), 1925 (5%,
-
+
4*A10 + 3*C10 ). Elemental analysis: calcd % for C29H51N3O4S (M = 537,8 g/mol): C 64.77, H
9.56, N 7.81, S 5.96, found: C 64.15, H 9.8, N 7.41, S 5.82. Water content: 1.49 %.
1- Decyl-3-methyl-triazolium 4-dodecyloxybenzenesulfonate (C10A12)
Yield: 45 %, 1H NMR (200 MHz, CDCl3) δ: 9.39 (s, 1H), 9.26 (s, 1H), 7.80 (d,
J = 8.7 Hz, 2H),
6.84 (d,
J
= 8.6 Hz, 2H), 4.59 (t,
J
= 7.4 Hz, 2H), 4.39 (s, 3H), 3.94 (t,
= 5.7 Hz, 30H), 0.87 (t,
J
= 6.6 Hz, 2H), 1.98 (d,
13
4H), 1.75 (dd,
J
= 14.1, 6.7 Hz, 2H), 1.25 (d,
J
J
= 6.3 Hz, 6H). C NMR
(50 MHz, CDCl3) δ: 160.27, 138.43, 132.83, 131.92, 127.64, 113.96, 68.26, 54.05, 40.30,
32.02, 29.69, 26.14, 22.80, 14.25. IR (ATR): ꢀꢁ [cm-1] = 2915s, 2850m, 1597w, 1493w, 1468m,
1193s, 1120s, 1030s, 1003m, 832s, 719m, 698m, 605s, 564s. MS (pos. ESI, MeOH): m/z = 224
+
+
-
+
-
+
(27%, C10 ), 789 (100%, 2*C10 + 1*A12 ), 1354 (30%, 3*C10 + 2*A12 ), 1920 (5%, 4*C10
+
-
-
-
3*A12 ). MS (neg. ESI, MeOH): m/z = 172 (5%, A12- - C12H25), 341 (100%, A12 ), 906 (7%, 2*A12
+
+
+ 1*C10 ), 1471 (5%, 3*A12- + 2*C10 ). Elemental analysis: calcd % for C31H55N3O4S (M = 565,85
g/mol): C 65.8, H 9.8, N 7.43, S 5.67, found: C 65.41, H 10.01, N 7.09, S 5.32. Water content:
2.75 %.
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Crystal Growth & Design
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1- Decyl-3-methyl-triazolium 4-tetradecyloxybenzenesulfonate (C10A14)
3
4
5
6
Yield: 53 %, 1H NMR (200 MHz, CDCl3) δ: 9.34 (s, 1H), 9.25 (s, 1H), 7.79 (d,
J
= 8.7 Hz, 2H),
7
8
9
6.83 (d,
J
= 8.7 Hz, 2H), 4.57 (t,
J
= 7.4 Hz, 2H), 4.45 – 4.28 (m, 3H), 3.93 (t,
= 5.6 Hz, 36H), 0.87 (t, J
= 6.4 Hz, 6H). 13C
J = 6.6 Hz, 2H),
1.90 (s, 2H), 1.74 (dd,
J
= 14.1, 6.8 Hz, 2H), 1.24 (d,
J
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NMR (50 MHz, CDCl3) δ: 160.27, 138.39, 132.83, 131.91, 127.65, 113.95, 68.26, 54.04, 40.29,
32.05, 29.81, 26.15, 22.82, 14.26. IR (ATR): ꢀꢁ [cm-1] = 2915s, 2850m, 1597w, 1494w, 1469m,
1193s, 1121s, 1031s, 1003m, 832s, 719m, 697m, 609s, 564s. MS (pos. ESI, MeOH): m/z = 224
+
-
+
(100%, C10 ), 406 (53%, 1*C10+ - CH3 + C14H29), 817 (97%, 2*C10+ + 1*A14 ), 999 (17%, 2*C10
+
-
1*A14 - CH3 + C14H29), 1410 (27%, 3*C10+ + 2*A14 ). MS (neg. ESI, MeOH): m/z = 172 (5%, A14
-
-
-
-
-
+
-
+
C14H29), 369 (100%, A14 ), 962 (6%, 2*A14 + 1*C10 ), 1555 (3%, 3*A14 + 2*C10 ). Elemental
analysis: calcd % for C33H59N3O4S (M = 593,9 g/mol): C 66.74, H 10.01, N 7.08, S 5.4, found: C
66.9, H 10.44, N 6.44, S 4.89. Water content: 1.05 %.
1- Dodecyl-3-methyl-triazolium 4-decyloxybenzenesulfonate (C12A10)
Yield: 74 %, 1H NMR (200 MHz, CDCl3) δ: 9.44 (s, 1H) 9.23 (s, 1H), 7.85 (d, J = 8.9 Hz, 2H), 6.87
(d, J = 8.9 Hz, 2H), 4.63 (t, J = 7.4 Hz, 2H), 4.44 (s, 3H), 3.98 (t, J = 6.6 Hz, 2H), 1.97 (s, 2H),
1.78 (d, J = 7.3 Hz, 2H), 1.64 (s, 4H), 1.28 (d, J = 4.9 Hz, 28H), 0.91 (t, J = 6.4 Hz, 6H). 13C NMR
(50 MHz, CDCl3) δ: 160.16, 138.87, 132.89, 131.94, 127.60, 113.95, 68.27, 54.08, 40.31,
32.04, 29.60, 28.99, 26.16, 22.82, 14.26. IR (ATR): ꢀꢁ [cm-1] = 2917s, 2849m, 1596w, 1468m,
1298w, 1186s, 1120s, 1030s, 1004m, 831m, 819m, 700m, 605s, 562s. MS (pos. ESI, MeOH):
+
-
-
m/z = 252 (21%, C12 ), 817 (100%, 2*C12+ + 1*A10 ), 1382 (49%, 3*C12+ + 2*A10 ). MS (neg. ESI,
-
+
MeOH): m/z = 172 (6%, A10- - C10H21), 313 (100%, A10 ), 878 (10%, 2*A10- + 1*C12 ), 1443 (11%,
-
3*A10 + 2*C12). Elemental analysis: calcd % for C31H55N3O4S (M = 565,85 g/mol): C 65.8, H
9.8, N 7.43, S 5.67, found: C 65.19, H 9.97, N 7.15, S 5.62. Water content: 2.19 %.
1- Dodecyl-3-methyl-triazolium 4-dodecyloxybenzenesulfonate (C12A12)
Yield: 50 %, 1H NMR (200 MHz, CDCl3) δ: 9.41 (s, 1H) 9.23 (s, 1H), 7.81 (d, J = 8.6 Hz, 2H), 6.84
(d, J = 8.6 Hz, 2H), 4.60 (t, J = 7.4 Hz, 2H), 4.41 (s, 3H), 3.94 (t, J = 6.6 Hz, 2H), 1.94 (s, 2H),
13
1.74 (d, J = 6.2 Hz, 4H), 1.25 (d, J = 3.9 Hz, 34H), 0.88 (t, J = 6.4 Hz, 6H). C NMR (50 MHz,
CDCl3) δ: 160.18, 138.71, 132.81, 131.93, 127.59, 113.93, 68.25, 54.08, 40.33, 32.04, 29.74,
26.23, 22.81, 14.25. IR (ATR): ꢀꢂ [cm-1] = 2915s, 2850m, 1597w, 1493w, 1469m, 1194s, 1121s,
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3
+
1030s, 1003m, 833s, 719m, 698m, 610s, 565s. MS (pos. ESI, MeOH): m/z = 252 (38%, C12 ),
4
5
-
-
845 (100%, 2*C12+ + 1*A12 ), 1438 (35%, 3*C12+ + 2*A12 ). MS (neg. ESI, MeOH): m/z = 172
6
7
8
9
-
-
-
+
-
+
(5%, A12 - C12H25), 341 (100%, A12 ), 934 (8%, 2*A12 + 1*C12 ), 1527 (7%, 3*A12 + 2*C12 ).
Elemental analysis: calcd % for C33H59N3O4S (M = 593,9 g/mol): C 66.74, H 10.01, N 7.08, S
5.4, found: C 66.36, H 10.38, N 6.79, S 5.16. Water content: 1.22 %.
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1- Dodecyl-3-methyl-triazolium 4-tetradecyloxybenzenesulfonate (C12A14)
Yield: 49 %, 1H NMR (200 MHz, CDCl3) δ: 9.13 (s, 1H), 8.97 (s, 1H), 7.77 (d,
J
= 8.7 Hz, 2H),
6.83 (d,
J
= 8.7 Hz, 2H), 4.56 – 4.47 (m, 2H), 4.34 (s, 3H), 3.93 (t, J = 6.5 Hz, 2H), 2.04 (s, 8H),
13
1.25 (s, 36H), 0.87 (t, J = 6.2 Hz, 6H). C NMR (50 MHz, CDCl3) δ: 160.22, 138.47, 132.75,
131.88, 127.60, 113.92, 68.24, 53.98, 40.25, 32.03, 29.79, 26.14, 22.80, 14.23. IR (ATR): ꢀꢂ
[cm-1] = 2915s, 2850m, 1597w, 1494w, 1469m, 1194s, 1121s, 1030s, 1003m, 832s, 719m,
+
+
698m, 609s, 564s. MS (pos. ESI, MeOH): m/z = 252 (82%, C12 ), 434 (36%, 1*C12 - CH3 +
-
C14H29), 873 (100%, 2*C12+ + 1*A14 ), 1055 (11%, 2*C12+ + 1*A14- - CH3 + C14H29), 1495 (21%,
-
-
-
3*C12+ + 2*A14 ). MS (neg. ESI, MeOH): m/z = 172 (6%, A14 - C14H29), 369 (100%, A14 ), 990
+
+
(7%, 2*A14- + 1*C12 ), 1611 (3%, 3*A14- + 2*C12 ). Elemental analysis: calcd % for C35H63N3O4S
(M = 621,96 g/mol): C 67.59, H 10.21, N 6.76, S 5.16, found: C 66.87, H 10.61, N 6.08, S 4.75.
Water content: 1.75 %.
1- Tetradecyl-3-methyl-triazolium 4-decyloxybenzenesulfonate (C14A10)
1H NMR (250 MHz, CDCl3) δ: 9.42 (s, 1H), 9.32 (s, 1H), 7.80 (d,
J = 8.8 Hz, 2H), 6.84 (d, J = 8.8
Hz, 2H), 4.60 (t,
J
= 7.4 Hz, 2H), 4.40 (s, 3H), 3.94 (t,
J
= 6.6 Hz, 2H), 1.93 (s, 2H), 1.83 – 1.69
13
(m, 2H), 1.43 – 1.18 (m, 36H), 0.88 (t,
J
= 6.3 Hz, 6H). C NMR (63 MHz, CDCl3) δ: 160.25,
132.83, 131.96, 127.64, 113.98, 68.30, 54.06, 40.30, 32.04, 30.02 – 29.16 (m), 29.00, 26.20,
22.81, 14.23. IR (ATR): ꢀꢂ [cm-1] = 3137, 3124, 2954, 2913, 2871, 2848, 1598, 1580, 1496,
1471, 1456, 1296, 1239, 1198, 1173, 1121, 1101, 1053, 1031, 1005, 824, 717, 699, 606, 561.
+
+
MS (pos. ESI, MeOH): m/z = 280 (28%, C14 ), 462 (6%, C14 -CH3 + C14H25), 873 (100%, 2*C14
+
-
+
-
-
1*A14 ), 1466 (46%, 3*C14 + 2*A10 ). MS (neg. ESI, MeOH): m/z = 172 (8%, A10 - C10H21), 313
-
-
+
-
+
(100%, A10 ), 906 (16%, 2*A10 + 1*C14 ), 1499 (23%, 3*A10 + 2*C14 ). Elemental analysis:
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Crystal Growth & Design
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calcd % for C33H59N3O4S (M = 593.9 g/mol): C 66.74, H 10.01, N 7.08, S 5.40, found: C 66.96,
H 9.62, N 6.97, S 5.56. Water content: 0.93 %.
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7
8
1- Tetradecyl-3-methyl-triazolium 4-dodecyloxybenzenesulfonate (C14A12)
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1H NMR (200 MHz, CDCl3) δ: 9.36 (s, 1H), 9.28 (d,
(d, = 8.8 Hz, 2H), 4.58 (t, = 7.4 Hz, 2H), 4.38 (s, 3H), 3.93 (t,
1.83 – 1.68 (m, 2H), 1.25 (d, = 2.6 Hz, 40H), 0.87 (t,
= 6.4 Hz, 6H). 13C NMR (50 MHz, CDCl3)
J
= 1.3 Hz, 1H), 7.78 (d,
J = 8.8 Hz, 2H), 6.83
J
J
J
= 6.6 Hz, 2H), 1.90 (s, 2H),
J
J
δ: 160.29, 138.35, 132.82, 131.90, 127.65, 113.96, 68.27, 54.04, 40.29, 32.05, 30.17 – 28.80,
26.20, 22.82, 14.25. IR (ATR): ꢀꢂ [cm-1] = 2955, 2916, 2849, 1597, 1579, 1496, 1469, 1301,
1245, 1216, 1186, 1122, 1102, 1030, 1004, 830, 821, 720, 701, 607, 562. MS (pos. ESI,
+
+
-
+
-
MeOH): m/z = 280 (91%, C14 ), 901 (100%, 2*C14 + 1*A12 ), 1522 (8%, 3*C14 + 2*A12 ). MS
-
-
-
+
(neg. ESI, MeOH): m/z = 172 (5%, A12 - C12H29), 341 (100%, A12 ), 962 (8%, 2*A12 + 1*C14 ),
+
1583 (8%, 3*A12- + 2*C14 ). Elemental analysis: calcd % for C35H63N3O4S (M = 621.9 g/mol): C
67.59, H 10.21, N 6.76, S 5.16, found: C 67.44, H 10.76, N 6.47, S 4.82. Water content: 1.72
%.
1- Tetradecyl-3-methyl-triazolium 4-tetradecyloxybenzenesulfonate (C14A14)
1H NMR (200 MHz, CDCl3) δ: 9.38 (s, 1H), 9.14 (s, 1H), 7.80 (d,
Hz, 2H), 4.60 (t, = 7.5 Hz, 2H), 4.42 (s, 3H), 3.94 (t, = 6.6 Hz, 2H), 2.03 – 1.88 (m, 2H), 1.82 –
1.69 (m, 2H), 1.26 (s, 44H), 0.87 (t,
= 6.4 Hz, 6H). 13C NMR (63 MHz, CDCl3) δ: 138.34,
J = 8.7 Hz, 2H), 6.84 (d, J = 8.8
J
J
J
132.78, 131.90, 127.65, 113.97, 68.29, 54.02, 40.27, 32.03, 29.99 – 29.15 (m), 28.99, 26.19,
22.80, 14.21. IR (ATR): ꢀꢂ [cm-1] = 2915, 2850, 1597, 1578.1, 1494, 1469, 1297, 1194, 1121,
+
1031, 1004, 832, 699, 610, 564. MS (pos. ESI, MeOH): m/z = 280 (100%, C14 ), 462 (31%, C14 -
+
-
+
-
CH3 + C14H25), 929 (69%, 2*C14 + 1*A14 ), 1578 (8%, 3*C14 + 2*A14 ). MS (neg. ESI, MeOH):
-
+
-
m/z = 172 (4%, A14- - C14H29), 369 (100%, A14 ), 1018 (6%, 2*A14- + 1*C14 ), 1667 (5%, 3*A14
+
+
2*C14 ). Elemental analysis: calcd % for C37H67N3O4S (M = 650 g/mol): C 68.37, H 10.39, N
6.46, S 4.93, found: C 68.48, H 11.29, N 6.16, S 4.41. Water content: 2.16 %.
Instrumentation
Differential scanning calorimetry (DSC) was performed with a computer-controlled
Phoenix DSC 204 F1 thermal analyzer (Netzsch, Selb, Germany). Measurements were carried
out at a heating rate of 5 °C/min in a cold-sealed aluminum crucible with an argon flow rate
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of 20 mL/min. Given temperatures correspond to the onset of the respective thermal
process.
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Temperature dependent polarized optical microscopy (POM) analyses were carried
out with an Axio Imager A1 microscope (Carl Zeiss MicroImaging GmbH,Göttingen, D)
equipped with a hot stage, THMS600 (Linkam Scientific Instruments Ltd, Surrey, UK), and
Linkam TMS 94 temperature controller (Linkam Scientific Instruments Ltd, Surrey, UK).
Images were recorded at a magnification of 100× as a video with a digital camera during
heating and cooling the sample which was placed between two cover slips. Heating and
cooling rates were 5 °C/min-1.
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Thermogravimetric analysis (TG) were performed with a TG 449 F3 Jupiter (Netzsch,
Selb, Germany). Measurements were carried out in aluminum oxide crucibles with a heating
rate of 5 °C/min and nitrogen as purge gas.
Infrared spectroscopy (IR) was conducted with a Bruker Alpha-P ATR-spectrometer
(Karlsruhe, Germany) in attenuated total reflection configuration. The data evaluation was
carried out with the program OPUS.
13
Nuclear magnetic resonance spectra (1H-NMR and C-NMR) were recorded with a
Bruker Advance DPX 200 spectrometer (Karlsruhe, Germany) at frequencies of 200 MHz and
50 MHz, respectively. Deuterated chloroform (Deutero GmbH) was used as solvent. The
chemical shift δ is given in ppm. The peak multiplicities are abbreviated as follows: s =
singlet, d = doublet, t = triplet, q = quartet, m = multiplet.
Mass spectrometric analysis was carried out on a VG Autospec mass spectrometer
with EBEE geometry (VG instruments, Kent, USA). Fast atom bombardment (FAB) was used
to ionize the samples.
Elemental analysis was conducted with a Vario EL C, H, N, S analyzer (Elmentar,
Hanau, Germany).
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Crystal Growth & Design
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3. Results and Discussion
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Synthesis
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To obtain the triazolium salts, the respective alkylazide was reacted with an excess of vinyl
acetate to give the 1-alky-1,2,3-triazole (Scheme 1). These (4a-4c) were alkylated at the N-3
position with methyl iodide to obtain the desired triazolium iodides (C10I, C12I, C14I). The
methyl sulfonates (3a-c) were obtained from alkylated sodium 4-hydroxybenzenesulfonate
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(
1a-c) via chlorination with phosphoryl chloride in dimethylformamide (DMF) and
dichloromethane (DCM). In the last step the methyl sulfonates (3a-c) were used as
methylation reagents to generate the triazolium cations and the benzenesulfonate anions
(
AnCm). After recrystallization from ethyl acetate all compounds were obtained as white
solids.
Scheme 1. Synthesis of triazolium iodides (CmI) and triazolium sulfonate ion pairs (AnCm). i) CnH(2n+1)Br, KOH ,
DMSO, RT, 24 h; ii) POCl3, DMF, DCM, 1 h; iii) NaOMe, MeOH, RT, 15 min; iv) mw-irradiation, 120 °C, 5 h; v)
acetonitrile, reflux, 3d; vi) acetonitrile, reflux, 3d.
Crystal Structural Analysis
Although it was not possible to grow anhydrous single crystals of sufficient quality for
structure analysis from solution, colorless single crystals of partially hydrated C12A14 could be
obtained from acetonitrile solution. 1-Dodecyl-3-methyl-triazolium 4-tetradecyloxybenzene
ꢄ
sulfonate hydrate (C12A14·0.35 H2O) crystallizes with the triclinic space group ꢃ1 (No. 2) with
two formula units in the unit cell. The elongated ions form a bilayer structure (Figure 2). The
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two crystallographically distinct triazolium cations deviate in the conformation of the alkyl
chain. In one triazolium cation the bond from the second to the third carbon atoms of the
chain is gauche and in the other triazolium cation the bond between the seventh and the
eighth carbon atom (counted from the triazolium head group) is gauche. All other bonds in
the alkyl chains are in trans conformation. The conformation of the alkyl chains of the two
distinct sulfonate anions is all trans, except the bond between the first and the second
carbon after the oxygen atom which is gauche. Alternating cations and anions are arranged
parallel to each other forming layers (Figure 3, left). Their orientation reverses between two
layers. In consequence, a clear segregation of charged, polar domains formed by triazolium
and sulfonate head groups and apolar, hydrophobic alkyl tails of cation and anion are seen in
the structure. The solvent water molecule is situated close to the polar head groups of the
ions. The crystal structure of 1-methyl-3-dodecyl triazolium iodide has been published
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before.26 C12I crystalizes in the triclinic space group ꢃ1 (No. 2) with two formula units in the
ꢄ
unit cell. The rod-shaped cations form an interdigitated bilayer-structure with the triazolium
cations not facing each other directly but alternating with the iodide anions (Figure 3, right).
Figure 2. Crystal packing of 1-dodecyl-3-methyl-triazolium 4-tetradecyloxybenzenesulfonate
(C12A14·x0.35H2O).
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Crystal Growth & Design
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Figure 3. Crystal structure of 1-dodecyl-3-methyl-triazolium 4-tetradecyloxybenzenesulfonate (left) and 1-
dodecyl-3-methyl-triazolium iodide (right).
In comparison with the respective triazolium iodide (Figure 3, right) the alkyl chains are less
interdigitated in the sulfonate and the distance between the polar domains is much larger
(35 Å between the sulfonate vs. 27 Å between the iodide anions). In C12I all alkyl chains of
the cations are parallel to each other and the triazolium head groups alternate with the
iodides. In contrast to that, in C12A14·x0.35H2O the alkyl chains of the cations are not parallel,
but are tilted at different positions (Figure 2). Only the second halves of the alkyl chains run
parallel together with the alkyl chains of the anions. The charged head groups to not
alternate regularly like in the iodide compound. The two triazolium rings and the two
benzene rings of the sulfonates in the unit cell are perpendicular to each other, due to
several hydrogen bonds (Figure 4).
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Figure 4. Hydrogen bonding interactions in C12A14·0.35H2O.
Table 2. H-bond details for C12A14·0.35H2O.
C12A14
H···A (Å)
2.319(2)
2.374(3)
2.496(3)
1.493(3)
1.937(5)
2.807(4)
2.825(3)
H···A (deg)
170.46(1)
154.44(5)
156.10(1)
159.24(2)
136.86(4)
169.91(1)
167.12(2)
C12ꢀH12B···O11B
C50BꢀH50K···O13B
C67ꢀH67···O99
C46BꢀH46C···O99
C48ꢀH48B···O4
C14ꢀH14···O13B
C15ꢀH15D···O12B
Several hydrogen bonds can be found in the structure (Table 2). Within the layers C-H•••O
hydrogen bonds are formed from the single methyl group of the triazolium cation, from the
first carbon of the long alkyl chain and from one CH-group of the triazolium ring to the
oxygen atoms of the sulfonate anions (Figure 4, left). The second triazolium cation features a
C-H•••O hydrogen bond from the first carbon of the long alkyl chain to a water molecule.
This is the reason for the different conformations of the triazolium cations. Beside these
interactions within the layers, there are also hydrogen bonds that interconnect the layers: C-
H•••O hydrogen bonds are formed between the oxygen atoms of the sulfonate group (the
one that is connected to two other cations and the water molecule) and one of the two CH-
groups of the triazolium ring (which has the gauche conformation at the end of the alkyl tail)
and to the single methyl group of this cation (Figure 4, right).
Table 3. Crystallographic and refinement details for C12I and C12A14·0.35H2O.
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C12I
C12A14·0.35H2O
C35H63N3O4S
621.94
Empirical formula
Formular weight (g/mol)
Temperature (K)
Crystal system
C15H30N3I
379.38
170
triclinic
ꢄ
ꢃ1
170
triclinic
ꢄ
ꢃ1
Space group
a = 5.6056(6) Å
b = 8.9070(10) Å
c = 19.523(3) Å
α = 79.80(2) °
β = 83.50(2) °
γ = 81.78(2) °
945.6(2)
a = 8.5428(1) Å
b = 10.3685(2) Å
c = 41.7163(6) Å
α = 95.789(1) °
β = 93.071(1) °
γ = 98.116(1) °
3630.76(7)
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Unit cell dimensions
Volume
Z
Calculated density
(g/cm3)
Absorption coefficient
(mm-1)
2
2
1.17
1.15
2.168
1.098
Θ-range for data
collection
2.34 to 25.00 °
3.2 to 68.0 °
Reflections
8211 / 3342
65716 / 13196
collected/ unique
Refinement method
Data / parameters
Goodness-of-fit on F2
Full-matrix least-squares on F2
3342 / 173
0.989
13196 / 840
1.054
Final R indices
(I>2sigma(I))
R1 = 0.045
wR2 = 0.071
R1 = 0.037
wR2 = 0.098
R1 = 0.077
wR2 = 0.082
R1 = 0.041
wR2 = 0.101
R indices (all data)
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Thermal Investigations
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The thermal properties of all compounds were examined by polarizing optical microscopy
(POM) and differential scanning calorimetry (DSC). The transition temperatures, enthalpies
and phase transition assignments are listed in Table 4 together with the thermal
decomposition temperatures which were determined via thermogravimetric analysis. The
benzenesulfonates decompose between 272 and 292 °C depending on the chain length,
whereas the iodides decompose around 200 °C (Table 4). Thus, the introduction of the rod-
like benzenesulfonate anion leads to increased thermal stability. All investigated
compounds, with exception of C10I, which melts directly to the isotropic liquid, are
thermotropic liquid crystals with quite similar LC behavior. The representative DSC traces of
C12A12 are shown in Figure 5. (The detailed DSC traces of the other compounds can be seen
in supporting information. The phase transition temperatures will be discussed below in
combination with the results from polarizing optical microscopy, which allows for phase
assignment.) C12A12 shows two endothermal events upon heating. Temperature dependent
POM identifies the first transition to be the flow point (SꢀLC) while the second belongs to
the clearing point (LCꢀLISO). Both transitions are reversible and reappear upon cooling,
albeit at slightly lower temperatures, as is commonly observed in ILs and ILCs27.
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Figure 5. DSC traces of C12A12.
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Table 4. Phase transition and thermal decomposition temperatures.
c
Ta/°C (ꢀ H /kJ molꢁ1)
Td [°C]
4
5
6
7
8
b
b
No.
Cr→Cr’ b
Cr‘ ꢀ Crb
Cr→SmAb
-
SmA →LISO
Cr →LISO
C10
C12
C14
I
I
I
-
-
-
74.6 (31.87)
213.7
198.3
214.9
280.4
286.7
280.4
281
-
-
83.1 (98.82)
89.44 (39.12)
40.5 (31.33)
48.8 (43.04)
49.9 (45.95)
54.8 (10.27)
55.6 (48.14)
53.9 (29.66)
73.9 (15.37)
70.6 (33.96)
64.1 (18.18)
132.3 (2.54)
158.1 (1.01)
146 (1.79)
141.9 (1.78)
113.8 (0.88)
157.7 (1.7)
147.9 (1.87)
121.5 (0.81)
151.4 (1.22)
148.9 (0.66)
126 (1.79)
-
-
-
-
-
-
-
-
-
-
-
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-
-
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C10A10
C10A12
C10A14
C12A10
C12A12
C12A14
C14A10
C14A12
C14A14
-
-
-
-
-
-
-
39.0 (13.7)
-
-
280.7
292.6
278.1
272.3
283.6
-
35.7 (22.07)
64.0 (3.78)
-
-
-
47.1 (25.24)
54.8 (11.77)
a Phase transition temperatures (°C) and enthalpies (kJ/mol, in parenthesis) are measured during heating and cooling at 5 K/min. bThe
phase transitions were identified by POM and SAXS measurements. cThermal decomposition temperature
a)
b)
c)
Figure 6. Representative textures as seen between crossed polarizers: oily streak texture characteristic of a
smectic A phase for (a) C12I at 97.4 °C; (b) C10A10 at 70.2 °C; (c) C12A10 at 83.5 °C.
For all compounds except 1-methyl-3-decyl-triazolium iodide (C10I), an oily streak texture
was observed under polarized light. This indicates the formation of a smectic A phase. Three
representative POM textures are shown in Figure 6. The iodides C12I and C14I exhibit a
smectic A phase at temperatures slightly above 80 °C, whereas the triazolium iodide with the
shortest chain length (C10I) melts directly to an isotropic liquid at 75 °C. Also for 1-alkyl-3-
methylimidazolium halides it has been found that a chain length of twelve carbon atoms is
mandatory for the formation of a mesophase5,10. Reducing the alkyl chain length from 14 to
12 carbon atoms for the triazolium iodides lowers both the flow and the clearing point
temperature. However, the flow point is less affected leading to a larger mesophasic window
for C14I compared to C12I. In comparison with the 1-methyl-3-dodecyl-triazolium bromide,
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the iodide C12I has lower flow and clearing point temperatures and similar mesophasic
temperature windows. For the triazolium sulfonates the phase transitions to the smectic A
phase occur at lower temperatures between 40 and 65 °C depeding on the size both of the
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anion and the cation. (Note, that some compounds (C12A10, C12A14, C14A10, C14A14) show an
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additional thermal event at lower temperatures, which will not be discussed in detail but can
be assigned to a solid-solid phase transition.)
Influence of the Anion
The dependence of the phase transition temperatures on the anion is shown in Figure 7 for
each cation. In case of the decyltriazolium compounds the mesomorphic behavior is
obviously induced by the sulfonate counter anion (the decyltriazolium iodide melts directly
to the liquid phase without formation of a LC). With increasing anion length the flow point
increases. The opposite trend can be observed for the clearing points. The compound with
the longest anion chain length (C10A14) and highest size mismatch between cation and anion
has thus the narrowest mesophase temperature window. The effect of the anion alkyl chain
length on the flow point is similar for the other two cations, C12 and C14, with increasing
anion length it decreases. However, for the C12 salts the flow points seem to be largely
unaffected by the anion size, whereas for the C14 cation larger anions lead to lower flow
points. Generally for the same cation, a larger anion does only weakly affect the flow point
but more clearly lowers the clearing point temperature. The mesophase stabilization by
pairing the rod-shaped cation with a rod-shaped anion becomes obvious, when comparing
to the iodides: for C10I no mesophase is observed, for C12 and C14 the flow points for the
sulfonates are observed at lower temperatures than for the iodide. In addition, the
mesophase window is generally wider.
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Figure 7. Phase transition temperatures of the different cations (top, left: C10, top, right: C12, bottom: C14) in
dependence of the anion.
Influence of the cation
To visualize the effect of the cation chain length on the mesophase formation, a comparison
of all relevant transition temperatures is displayed in Figure 8. As mentioned above the
iodide with the shortest chain, C10I, does not form a liquid crystalline phase. In the other two
iodides the temperatures of flow point and clearing point increase with increasing chain
length. For all three sulfonate anions the flow point temperature increases with increasing
cation chain length, ranging from 40.5 °C (C10A10) to 73.9 °C (C14A10). For A12 and A14 a similar
effect on the clearing point is observed: The larger the cation, the higher the clearing point.
For C10 an irregularity is observed. C10A12 has a higher clearing point than C10A10 and C10A14
.
This might be due to the interplay of van der Waals interactions between the alkyl chains
and hydrogen bonding and ionic interactions in the polar regions of the structure. In the
crystal structure of C12A14·0.35H2O the alkyl chains of the cations are not part of the
interdigitation, but are the “counterpart” for the alkyl chains of the sulfonate anion (Figure
2). If the anion is kept at the same length and the cation length is extended, there is less
interdigitation of the alkyl chains of the anions and thus the crystalline phase is stabilized
(Figure 8).
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In summary, the dependence of clearing point temperatures on the cation chain length is
seen to follow the inverse trend compared to the anion chain length (Figure 8 and Figure 7),
i.e. increasing with increasing chain length (Fig. 8). This trend was also observed for
guanidinium sulfonate ion pairs24. The lowest clearing point is measured for the compound
with the shortest cation chain length and the longest anion chain length C10A14 (113.8 °C).
Here the difference in lengths between the cation and anion is the largest (Table 5). The
highest overall clearing point of 157.7 °C was achieved with C12A10 where the cation and the
anion have a very small size difference (Table 5). The liquid crystalline temperature window
is smallest for the ion pair with the longest alkyl chains, C14A14 (62 °C) and largest for the
compound with the shortest alkyl chains C10A10 (105.5 °C). In general the mesophase
temperature window is larger for ions with shorter alkyl chains.
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Figure 8. Phase transition temperatures of the different anions in dependence of the cation chain length.
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Table 5. Length of cation and anion in Å (the length of C12 and A14 were measured in the crystal structure and
the others were estimated by adding or subtracting 2.5 Å for each -CH2CH2- group).
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C10A10 C10A12 C10A14 C12A10 C12A12 C12A14 C14A10 C14A12 C14A14
cation
anion
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20.5
6.5
20.5
1.5
difference 4
1.5
6.5
4. Conclusions
A series of nine triazolium sulfonate ionic liquid crystals with rod-like cations and anions was
synthesized and characterized. The corresponding triazolium iodides where synthesized as
reference materials featuring spherical counter anions. 1-Decyl-3-methyl-triazolium iodide
was the only compound which does not exhibit a liquid crystalline phase, but melts at 75 °C
to an isotropic liquid. Therefore, it qualifies as an ionic liquid. All other compounds, the
iodides as well as the sulfonates, exhibit a smectic A phase, as established by optical
microscopy. In any case, pairing the rod-shaped triazolium cations with long chain sulfonate
anion stabilizes the mesophase. A mesophase temperature window of up to 105 °C for
C10A10 could be achieved. Generally, the melting and clearing temperatures increase with
increasing chain length of the cation. In contrast, with increasing anion alkyl chain length the
clearing point temperatures decrease. In summary, the mesophase stability is increased
through pairing congruently shaped mesogenic ions, whereby the melting and the clearing
point highly depend on the relative alkyl chain length of the cation and anion.
ACKNOWLEDGEMENT
This work was supported in part by the German Science Foundation DFG, Iowa State
University and the Critical Materials Institute, an Energy Innovation Hub funded by the U.S.
Department of Energy, Office of Energy Efficiency and Renewable Energy, Advanced
Manufacturing Office.
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1 Wasserscheid, P.; Welton T. (Eds.), Ionic Liquids in Synthesis, Wiley-VCH, Weinheim, Germany, 2003
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Mingos, D.M. P. (Series Ed.); Kato, T. (Vol. Ed.), Liquid Crystalline Functional Assemblies and Their
Supramolecular Structures, Springer, Berlin Heidelberg, Germany, 2008.
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11 Getsis, A.; Mudring, A.- V. Cryst. Res. Technol. 2008, 43, 1187
12 Getsis, A.; Mudring, A.-V. Z. Anorg. Allg. Chem. 2009, 635, 2214-2221.
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19 Yang, M.; Stappert, K.; Mudring, A.-V. J. Mater. Chem. C, 2014,2, 458-473.
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Stappert, K.; Ünal, D.; Spielberg, E.; Mudring, A.-V. Crystal Growth & Design. Title: "The influence of the
counter anion on the ability of 1-dodecyl-3-methyltriazolium ionic liquids to form mesophases" Manuscript ID:
cg-2014-01564j.
27Mudring, A.-V. Aus. J. Chem. 2010, 63(4) 544–564.
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In ionic liquid crystals the mesophase stability can be increased through pairing congruently shaped
mesogenic ions.
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