Photoisomerization and Mesophase Formation in Azo-Ionic Liquids

Article abstract of DOI:10.1021/acs.cgd.9b01018

Ionic liquids present a versatile, highly tunable class of soft functional materials. Aside from being low melting salts, they can be endowed with additional functionalities. In N-alkylimidazolium halides, which are a prominent class of ionic liquids (ILs), the imidazolium cation was linked via an ether-bridge to an azobenzene unit in order to obtain photoresponsive materials through photoinduced trans-cis isomerization. The azobenzene unit, in turn, was modified with electron-donating or -withdrawing groups such as methyl-, tert-butyl-, methoxy-, N,N-dimethylamino, and nitro groups to study their influence on the photoisomerization and phase behavior. Endowing the imidazolium additionally with a long alkyl chain allows the materials to potentially form liquid crystalline (LC) mesophases before melting into the isotropic liquid. All studied compounds qualify as ionic liquids, and all, except for the nitro-compound, show the formation of smectic mesophases melting to the isotropic liquid. The compounds with the bulkiest aliphatic substituent, the tert-butyl, shows the lowest melting point, the largest mesophase window, and an efficient photochemical trans-cis conversion (>90%). In summary, by tuning sterically and electronically the cationic part of ILs, a photoswitchable room temperature liquid crystal could be developed and design guidelines for photoresponsive ionic liquids could be obtained.

Full text of DOI:10.1021/acs.cgd.9b01018

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Article
Photoisomerization and mesophase formation in azo-ionic liquids
Guillaume Bousrez, Kathrin Stappert, Magdalena Wilk-Kozubek, Brando Adranno, Hanwen
Pei, Eike T. Spielberg, Volodymyr Smetana, Anja-Verena Mudring, and Olivier Renier
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.9b01018 • Publication Date (Web): 07 Nov 2019
Downloaded from pubs.acs.org on November 8, 2019
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Photoisomerization and mesophase formation in azo-
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Olivier Renier , Guillaume Bousrez , Kathrin Stappert , Magdalena Wilk-Kozubek , Brando
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Adranno , Hanwen Pei , Eike T. Spielberg , Volodymyr Smetana and Anja-Verena Mudring*
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Department of Materials and Environmental Chemistry, Stockholm University, Svante
Arrhenius väg 16C, 10691 Stockholm, Sweden. Email: anja-verena.mudring@mmk.su.se
b
former address: AnorganischeChemie III – Materials Engineering and Characterization,
Fakultät für Chemie and Biochemie, Ruhr-Universität Bochum, 44780, Bochum, Germany.
Abstract
Ionic liquids present a versatile, highly tunable class of soft functional materials. Aside from being
low melting salts, they can be endowed with additional functionalities. In N-alkylimidazolium
halides, which are a prominent class of ionic liquids (ILs), the imidazolium cation was linked via
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an ether-bridge to an azobenzene unit in order to obtain photoresponsive materials through
photoinduced trans-cis isomeration. The azobenzene unit, in turn, was modified with electron
donating or withdrawing groups such as methyl-, tert-butyl-, methoxy-, N,N-dimethylamino and
nitro groups to study their influence on the photoisomerization and phase behavior. Endowing the
imidazolium additionally with a long alkyl chain allows the materials to potentially form liquid
crystalline (LC) mesophases before melting into the isotropic liquid. All, studied compounds
qualify as ionic liquids; and all, except for the nitro-compound, show the formation of smectic
mesophases melting to the isotropic liquid. The compounds with the bulkiest aliphatic substituent,
the tert-butyl, shows the lowest melting point and exhibits largest mesophase window, and shows
an efficient photochemical trans-cis conversion (>90%). In summary, by tuning sterically and
electronically the cationic part of ILs, photoswitchable room temperature liquid crystal could be
developed and design guidelines for photoresponsive ionic liquids could be obtained.
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1. Introduction
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Ionic liquids (ILs) have received considerable attention in recent years. Their physical and
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chemical properties are of great interest to many research topics. In fact, they exhibit a melting
point below 100°C, are good electrical conductors and can be easily tailored to fit ones needs.3
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ILs can be used as solvents that exhibit extremely low, virtually non-existent vapor pressure, as
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templates for the formation of zeolites, or even as reagents. Among their applications, one can
find catalysis, extraction, and electrochemistry.13–15 Typically, ILs have an organic cation (most
commonly ammonium, sulphonium, phosphonium, imidazolium, pyridinium, picolinium,
pyrrolidinium, thiazolium, oxazolium or pyrazolium) and an anionic part which is typically inorganic
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and weakly basic (Cl , Br , PF , etc). Due to their cationic part being organic, ILs can be easily
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tailored by well-known substitution reactions (i.e. Suzuki, Williamson, etc…). Another interesting
feature of ILs is their ability to adopt mesophases and form ionic liquid crystals (ILCs). 17 They
usually are observed in an IL containing one or two anisotropically shaped ions (rod-like or disk
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like). They exhibit order along one or more crystallographic orientations but will simultaneously
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demonstrate a random arrangement of cations/anions in the other directions. This results most
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ILCs can also be induced by the combination of rigid (i.e. aromatic) and flexible groups (i.e.
aliphatic). The self-assembly and alignment of such structural features is of great influence on the
formation of liquid crystalline phases. Thermotropic liquid crystals form mesophases in a certain
temperature range. The thermal behavior of ILCs is influenced by the shape of the cation as well
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as by the shape of the anion and their interaction. A change in anion can result in completely
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different phase transition (cold crystallization for BF vs only solid-solid transition with CF SO
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Azobenzene based liquid crystals (Azo-ILs) have attracted considerable attention due to the
possibility of using light instead of electricity to manipulate their properties.23–25 By photoirradiation
of the appropriate wavelength, it is possible to switch between the two possible isomeric states:
typically, the trans conformation is the thermodynamically favored form which can be converted
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to the cis state by UV-light irradiation. The reverse reaction (cis to trans) can be induced either
thermally (Figure 1) or by photoirradiation in the visible range. However, this optical switching is
only possible if the absorption spectra of the cis and trans form are sufficiently different. There
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are different mechanisms discussed for the isomerization process, inversion or rotation.27,28 The
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reaction mechanism depends on different factors, such as solvent, pressure and substituents.29,30
Recently, Azo-ILs have been a splendid platform to study different applications. In fact, it has
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been demonstrated that they could be used as light-controlled ion transporters. It has even been
shown that azobenzene-imidazolium ILs can enhance the release time of drug delivery systems.
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The photoswitchable abilities of these ILs have also shown great promise in controlling the
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conductivity of IL in aqueous solutions. In addition, when introduced into block copolymer, azo-
ILs can induce reversible changes in viscoelasticity.33,34
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This reversible cis-trans photoisomerization process offers a wide range of applications such
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as optical data storage, optical switching, polarization holography as well as the possibility of
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using azo-group bearing molecules as photolabile surfactants. For this reason, azobenzene
based polymers and liquid crystalline polymers have been studied intensively over the last
decade.34,40,41 The study of azobenzene based ILCs has shown that the ionic character has a
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great influence on the mesomorphic behavior. Therefore, ILs with photochromic functionalities
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have been developed, and the photoresponsive conductivity of azobenzene based ILs has been
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investigated together with other structural aspects.
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Figure 1. Energy profile for the trans-cis isomerization of azobenzenes.
The aim of this study is to determine the influence of different substituents at the azobenzene
moiety on the efficiency of the photoinduced trans-cis isomerization as well as on the thermal
phase behavior in order to provide improved design guidelines for the deliberate development of
this interesting class of materials. To this avail, we have chosen to study imidazolium bromides,
where the cation is functionalized with a dodecyl-side chain that can act as a mesogenic group
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and an azobenzene moiety linked via an ether-bridge to the cationic head group to allow for
additional flexibility and varied the –R group by exploring methyl-, tert-butyl-, methoxy-, N,N-
dimethylamino and nitro groups (Figure 2).
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Figure 2. General structure of the studied compounds (R = Me (1), t-Bu (2), OMe (3), NMe (4),
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NO (5)).
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2. Experimental Section
2.1.
Materials
All chemicals were purchased from Sigma-Aldrich or VWR and used as received.
2.2.
Synthesis
Synthesis of phenyldiazenylphenol derivatives 1b-5b
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Aniline derivative (1a-5a) (10 mmol, 1 eq.) and concentrated hydrochloric acid (37%) (2.5 mL) were
dissolved in water (10 mL). NaNO (690 mg, 10 mmol, 1 eq.) in 1.5 mL water was slowly added at 0°C. The
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solution was stirred during 4h. Phenol (940 mg, 10 mmol, 1 eq.), Na CO (1,092 mg, 10.3 mmol, 1.03 eq.)
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and NaOH (400 mg, 10 mmol, 1 eq.) were dissolved in water (10 mL) and then added dropwise to the
solution. The mixture was stirred for another 4h at room temperature. After adjusting the pH to 7 with
hydrochloric acid the precipitated product was filtered off, washed with water (3 x 20 mL) and dried under
vacuum at room temperature for 24h.
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4-Methylphenyldiazenylphenol (1b)
1
Orange solid. H NMR (400 MHz, CDCl , /ppm): 2.42 (s, 3H), 6.93 (d, J = 8.4 Hz, 2H), 7.29 (d, JH-
3
H-H
-1
=
8.0 Hz, 2H), 7.79 (d, JH-H = 7.6 Hz, 2H), 7.86 (d, JH-H = 8.0 Hz, 2H). max (cm ): 3025, 2917, 2792,
H
1
7
601, 1579, 1501, 1459, 1404, 1369, 1275, 1220, 1184, 1156, 1139, 1101, 1035, 1020, 938, 825, 755, 721,
05, 645, 614, 596, 545, 532, 503, 465, 423, 414.
4-Tert-butylphenyldiazenylphenol (2b)
1
Orange solid. H NMR (400 MHz, CDCl , /ppm): 1.37 (s, 9H), 6.93 (d, J = 8.4 Hz, 2H), 7.52 (d, JH-
3
H-H
-1
= 8.4 Hz, 2H), 7.82 (d, JH-H = 8.8 Hz, 2H), 7.85 (d, JH-H = 8.4 Hz, 2H). max (cm ): 3356, 3056, 2958,
902, 2866, 1901, 1784, 1589, 1500, 1460, 1431, 1393, 1362, 1265, 1226, 1140, 1103, 1022, 1010, 947,
37, 789, 756, 718, 699, 632, 565, 532, 492, 450, 421.
H
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4-Methoxyphenyldiazenylphenol (3b)
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Orange solid. H NMR (400 MHz, CDCl , /ppm): 3.88 (s, 3H), 6.93 (d, J = 8.4 Hz, 2H), 7.00 (d, JH-
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H-H
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=
9.2 Hz, 2H), 7.83 (d, JH-H = 8.4 Hz, 2H), 7.88 (d, JH-H = 8.4 Hz, 2H). max (cm ): 3410, 3025, 2958,
2838, 2052, 1981, 1596, 1582, 1494, 1461, 1438, 1352, 1317, 1304, 1233, 1176, 1149, 1103, 1028, 1014,
59, 945, 841, 750, 731, 645, 604, 555, 531, 504, 429, 409.
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-Dimethylaminophenyldiazenylphenol (4b)
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Brown solid. H NMR (400 MHz, CDCl , /ppm): 3.06 (s, 6H), 6.74 (d, J = 8.8 Hz, 2H), 6.92 (d, JH-H
3
H-H
-1
= 8.8 Hz, 2H), 7.76 (d, JH-H = 8.8 Hz, 2H), 7.82 (d, JH-H = 9.2 Hz, 2H). max (cm ): 3062, 3024, 2910, 2802,
2
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672, 1598, 1582, 1558, 1521, 1499, 1478, 1442, 1393, 1371, 1314, 1269, 1230, 1146, 1102, 1062, 843,
33, 818, 796, 754, 718, 646, 589, 551, 537,511, 497, 434, 413, 402.
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-Nitrophenyldiazenylphenol (5b)
1
Brown solid. H NMR (400 MHz, CDCl , /ppm): 6.97 (d, J = 8.8 Hz, 2H), 7.93 (d, JH-H = 8.8 Hz,
3
H-H
-1
2
1
H), 7.97 (d, JH-H = 8.8 Hz, 2H), 8.36 (d, JH-H = 8.8 Hz, 2H). max (cm ): 3383, 3110, 1604, 1584, 1503,
457, 1429, 1402, 1379, 1332, 1281, 1178, 1136, 1101, 1006, 859, 845, 755, 722, 685, 640, 603, 536, 510,
422, 400.
Synthesis of 1-(4-((2-bromoethyl)oxy)phenyl)-2-(phenyl)diazene derivatives 1c-5c
A mixture of 1b-5b (15 mmol, 1 eq.), 1,2-dibromoethane (45 mmol, 3 eq.) and K CO (55.5 mmol, 3.7
2
3
eq.) in acetonitrile (10 mL) was heated under refluxed during 20 h. After cooling down to room temperature
the suspension was poured into water and extracted with dichloromethane. The combined organic phases
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were dried with magnesium sulfate, filtered and the solvent was removed under vacuum. The product was
dried under vacuum at room temperature for 20 hours to give an orange powder.
1-(4-((2-Bromoethyl)oxy)phenyl)-2-(4-methylphenyl)diazene (1c)
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Orange solid. H NMR (400 MHz, CDCl , /ppm): 2.43 (s, 3H), 3.68 (t, J = 6.0 Hz, 2H), 4.37 (t, JH-H
3
H-H
= 6.0 Hz, 2H), 7.02 (d, JH-H = 8.8 Hz, 2H), 7.30 (d, JH-H = 7.6 Hz, 2H), 7.80 (d, JH-H = 8.4 Hz, 2H), 7.91 (d,
-1
JH-H = 8.4 Hz, 2H). max (cm ): 3025, 2921, 2861, 2736, 1598, 1579, 1492, 1453, 1416, 1381, 1315, 1295,
1
275, 1235, 1142, 1114, 1073, 1013, 962, 880, 849, 820, 784, 745, 724, 705, 641, 633, 618, 571, 551, 528,
513, 422, 400.
1-(4-((2-Bromoethyl)oxy)phenyl)-2-(4-tert-butylphenyl)diazene (2c)
1
Orange solid. H NMR (400 MHz, CDCl , /ppm): 1.37 (s, 9H), 3.68 (t, J = 6.4 Hz, 2H), 4.38 (t, JH-H
3
H-H
=
6.0 Hz, 2H), 7.02 (d, JH-H = 8.8 Hz, 2H), 7.52 (d, JH-H = 8.4 Hz, 2H), 7.83 (d, JH-H = 8.4 Hz, 2H), 7.92 (d,
-1
JH-H = 8.8 Hz, 2H). max (cm ): 3054, 2958, 2866, 1599, 1584, 1497, 1456, 1416, 1389, 1363, 1297, 1281,
1
5
264, 1246, 1225, 1178, 1161, 1143, 1105, 1077, 1010, 969, 943, 878, 836, 768, 720, 698, 631, 603, 568,
46, 525, 511, 468, 453, 423.
1-(4-((2-Bromoethyl)oxy)phenyl)-2-(4-methoxyphenyl)diazene (3c)
1
Orange solid. H NMR (400 MHz, CDCl , /ppm): 3.67 (t, J = 6.4 Hz, 2H), 3.89 (s, 3H), 4.37 (t, JH-H
=
3
H-H
6
.4 Hz, 2H), 7.01 (d, JH-H = 8.8 Hz, 2H), 7.01 (d, JH-H = 9.2 Hz, 2H), 7.89 (d, JH-H = 8.8 Hz, 2H), 7.89 (d,
-1
JH-H = 8.8 Hz, 2H). max (cm ): 3061, 2998, 2969, 2910, 2859, 2838, 2728, 2617, 2556, 2036, 1999, 1913,
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1592, 1495, 1454, 1440, 1418, 1386, 1304, 1282, 1243, 1177, 1142, 1102, 1071, 1010, 957, 877, 843, 812,
7
83, 744, 731, 640, 580, 548, 506, 419.
1-(4-((2-Bromoethyl)oxy)phenyl)-2-(4-dimethylaminophenyl)diazene (4c)
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Brown solid. H NMR (400 MHz, CDCl , /ppm): 3.08 (s, 6H), 3.67 (t, J = 6.0 Hz, 2H), 4.36 (t, JH-H
3
H-H
-1
= 6.0 Hz, 2H), 6.76 (d, JH-H = 8.8 Hz, 2H), 6.99 (d, JH-H = 8.8 Hz, 2H), 7.82-7.86 (m, 4H). max (cm ): 3068,
3
8
029, 2854, 2795, 1676, 1594, 1513, 1495, 1442, 1403, 1356, 1308, 1223, 1168, 1146, 1059, 1011, 941,
16, 751, 730, 691, 641, 613, 575, 547, 535, 522, 417, 406.
1-(4-((2-Bromoethyl)oxy)phenyl)-2-(4-nitrophenyl)diazene (5c)
1
Brown solid. H NMR (400 MHz, CDCl , /ppm): 3.69 (t, J = 6.0 Hz, 2H), 4.41 (t, JH-H = 6.0 Hz, 2H),
3
H-H
-1
7
.06 (d, JH-H = 8.4 Hz, 2H), 7.97-8.00 (m, 4H), 8.37 (d, JH-H = 8.8 Hz, 2H). max (cm ): 3105, 3073, 2969,
2923, 2851, 1783, 1688, 1600, 1580, 1521, 1493, 1456, 1419, 1403, 1385, 1341, 1296, 1279, 1243, 1181,
1
4
135, 1106, 1068, 1012, 966, 920, 881, 857, 841, 816, 754, 723, 683, 636, 578, 546, 537, 510, 495, 458,
18.
Synthesis of ionic liquids 1-5
A mixture of 1c-5c (10 mmol, 1 eq.) and dodecylimidazole (10 mmol, 1 eq.) was heated under reflux in
acetonitrile (5 mL) for three days. After cooling down the product was washed with ethyl acetate (3 x 20
mL). To obtain the precipitation of the final product the Schlenk tube is put at -80°C during one hour. The
precipitate was filtered off and dried under vacuum at room temperature overnight.
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45
We have reported on 1 previously. Compound 1 in the current study corresponds to 2c in ref
4
5. We have re-synthesized the material and measured additional chemical and physical
properties. We have re-evaluated its crystal structure (c.f. CCDC1036110 to 1902398, no
changes to the data and refinement results) to compare its structural similarities to compound 3
reported here.
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1
-Dodecyl-3-(2-(4-((4-methylphenyl)diazenyl)phenoxy)ethyl)-imidazolium bromide (1)
1
Orange solid. H NMR (400 MHz, CDCl , /ppm): 0.82 (t, J = 6.4Hz, 3H), 1.17 (s, 14H), 1.25 (s, 4H),
3
H-H
1
6
.84-1.85 (m, 2H), 2.37 (s, 3H), 4.25 (t, JH-H = 7.2 Hz, 2H), 4.47 (t, JH-H = 4.4 Hz, 2H), 4.93-4.94 (m, 2H),
.98 (d, JH-H = 8.8 Hz, 2H), 7.23 (d, JH-H = 8.0 Hz, 2H), 7.37 (s, 1H), 7.72 (d, JH-H = 8.0 Hz, 2H), 7.79-7.72
1
3
(m, 3H), 10.39 (s, 1H). C NMR (100 MHz, CDCl , /ppm): 14.1, 21.5, 22.7, 26.3, 29.0, 29.3, 29.4, 29.5,
3
2
1
1
1
5
9.6 (2C), 30.2, 31.9, 49.4, 50.3, 66.6, 114.8 (2C), 121.6, 122.6 (2C), 123.5, 124.6 (2C), 129.7 (2C), 137.2,
-1
41.2, 147.6, 150.6, 159.6. max (cm ): 3168, 3168, 3133, 3089, 3063, 2951, 2923, 2867, 2848, 1599, 1583,
570, 1498, 1474, 1462, 1411, 1377, 1359, 1312, 1297, 1241, 1222, 1190, 1152, 1113, 1102, 1055, 1042,
017, 1006, 974, 945, 919, 919, 873, 841, 821, 805, 790, 770, 751, 726, 705, 694, 645, 634, 618, 594, 548,
34, 466, 423. ESI TOF m/z (positive mode) 475.4086 (calculated m/z = 475.3431).
1-Dodecyl-3-(2-(4-((4-tert-butylphenyl)diazenyl)phenoxy)ethyl)-imidazolium bromide (2)
1
Orange solid. H NMR (400 MHz, CDCl , /ppm): 0.82 (t, J = 6.4 Hz, 3H), 1.17 (s, 14H), 1.25 (s,
3
H-H
4
H), 1.31 (s, 9H), 1.83-1.85 (m, 2H), 2.44 (s, 1H), 4.25 (t, JH-H = 7.2 Hz, 2H), 4.48 (t, JH-H = 4.4 Hz, 2H),
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4.93-4.94 (m, 2H), 6.99 (d, JH-H = 8.0 Hz, 2H), 7.36 (d, JH-H = 1.2 Hz, 1H), 7.46 (d, JH-H = 8.4 Hz, 2H),
13
7
2
.74-7.82 (m, 5H), 10.31 (s, 1H). C NMR (100 MHz, CDCl , /ppm): 14.1, 22.6, 26.3, 29.0, 29.3, 29.4,
3
9.5, 29.6 (2C), 30.2, 31.3 (3C), 31.9, 34.9, 49.4, 50.3, 66.6, 114.8 (2C), 121.6, 122.4 (2C), 123.5, 124.6
-1
(2C), 126.0 (2C), 137.2, 147.7, 150.5, 154.2, 159.6. max (cm ): 3358, 3149, 3126, 3086, 3059, 2958, 2920,
0
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850, 1649, 1602, 1586, 1553, 1500, 1467, 1420, 1397, 1363, 1322, 1300, 1258, 1240, 1181, 1156, 1143,
107, 1054, 1013, 951, 918, 863, 844, 807, 777, 722, 693, 659, 630, 600, 566, 535, 479, 429, 401. ESI TOF
m/z (positive mode) 517.4017 (calculated m/z = 517.3901).
1
-Dodecyl-3-(2-(4-((4-methoxyphenyl)diazenyl)phenoxy)ethyl)-imidazolium bromide (3)
1
Yellow solid. H NMR (400 MHz, CDCl , /ppm): 0.80 (t, J = 6.8 Hz, 3H), 1.16 (s, 14H), 1.25 (s,
3
H-H
4
H), 1.84-1.87 (m, 2H), 3.82 (s, 3H), 4.21 (t, JH-H = 7.6 Hz, 2H), 4.45 (t, JH-H = 8.4 Hz, 2H), 4.95 (t, JH-H
=
8.4 Hz, 2H), 6.93 (d, JH-H = 7.6 Hz, 4H), 7.20 (s, 1H), 7.60 (s, 1H), 7.79 (d, JH-H = 8.8 Hz, 2H), 7.81 (d, JH-H
1
3
=
8.8 Hz, 2H), 10.60 (s, 1H). C NMR (100 MHz, CDCl , /ppm): 14.2, 22.8, 26.4, 29.1, 29.4, 29.5, 29.6,
3
29.7 (2C), 30.3, 32.0, 49.6, 50.5, 55.7, 66.9, 114.3 (2C), 114.8 (2C), 121.3, 123.5, 124.6 (2C), 124.6 (2C),
-1
1
1
37.8, 147.0, 147.8, 159.3, 161.9. max (cm ): 3169, 3134, 3087, 3061, 2947, 2923, 2864, 2849, 1599, 1583,
570, 1500, 1474, 1463, 1422, 1413, 1382, 1371, 1359, 1315, 1298, 1238, 1223, 1178, 1151, 1108, 1057,
1040, 974, 940, 919, 873, 844, 820, 807, 770, 751, 728, 694, 643, 619, 595, 548, 528, 489, 453, 436, 411.
ESI TOF m/z (positive mode) 491.3576 (calculated m/z = 491.3381).
1
-Dodecyl-3-(2-(4-((4-dimethylaminophenyl)diazenyl)phenoxy)ethyl)-imidazolium bromide (4)
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Brown solid. H NMR (400 MHz, CDCl , /ppm): 0.82 (t, J = 6.4 Hz, 3H), 1.17 (s, 14H), 1.23 (s, 4H),
3
H-H
1
6
.81-1.82 (m, 2H), 3.01 (s, 6H), 4.21 (t, JH-H = 7.2 Hz, 2H), 4.42 (t, JH-H = 4.4 Hz, 2H), 4.87-4.88 (m, 2H),
1
3
.67 (d, JH-H = 8.4 Hz, 2H), 6.93 (d, JH-H = 8.4 Hz, 2H), 7.33 (s, 1H), 7.72-7.78 (m, 5H), 10.28 (s, 1H). C
NMR (100 MHz, CDCl , /ppm): 14.1, 22.6, 26.2, 29.0, 29.3, 29.4, 29.5, 29.6 (2C), 31.9 (2C), 40.3 (2C),
3
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9.4, 50.2, 66.5, 111.5 (2C), 114.8 (2C), 121.6, 123.5, 123.8 (2C), 124.6 (2C), 137.1, 143.4, 148.0, 152.2,
-1
158.5. max (cm ): 3357, 3133, 3035, 2922, 2851, 1898, 1671, 1597, 1584, 1561, 1511, 1497, 1466, 1449,
1
6
449, 1405, 1353, 1322, 1293, 1228, 1164, 1150, 1101, 1054, 1005, 944, 913, 890, 833, 823, 778, 724,
86, 655, 642, 595, 548, 535, 439, 430, 405. ESI TOF m/z (positive mode) 504.4475 (calculated m/z =
504.3697).
1-Dodecyl-3-(2-(4-((4-nitrophenyl)diazenyl)phenoxy)ethyl)-imidazolium bromide (5)
1
Brown solid. H NMR (400 MHz, CDCl , /ppm): 0.87 (t, J = 7.2 Hz, 3H), 1.23 (s, 14H), 1.34 (s, 4H),
3
H-H
1
.61 (s, 2H), 1.94 (s, 2H), 4.26 (t, JH-H = 7.2 Hz, 2H), 4.59 (s, 2H), 5.04 (s, 2H), 7.05 (d, JH-H = 8.8 Hz, 2H),
7.18 (s, 1H), 7.53 (s, 1H), 7.97 (d, JH-H = 8.4 Hz, 2H), 7.99 (d, JH-H = 8.4 Hz, 2H), 8.37 (d, JH-H = 8.4 Hz,
13
2
3
1
1
5
H), 10.80 (s, 1H). C NMR (100 MHz, CDCl , /ppm): 14.2, 22.8, 26.4, 29.1, 29.4, 29.5, 29.6, 29.7 (2C),
3
0.3, 32.0, 49.6, 50.6, 67.0, 115.1 (2C), 121.4, 123.3 (2C), 123.6, 124.8 (2C), 125.7 (2C), 137.6, 147.5,
-1
48.4, 155.8, 161.0. max (cm ): 3382, 3056, 2921, 2850, 2446, 2090, 1926, 1597, 1544, 1519, 1503, 1462,
403, 1340, 1300, 1251, 1163, 1137, 1106, 1051, 1030, 1005, 914, 859, 837, 777, 754, 736, 721, 684, 635,
85, 542, 511, 491, 442. ESI TOF m/z (positive mode) 506.3727 (calculated m/z = 506.3126).
Synthesis of dodecylimidazole (6)
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Potassium hydroxide (100 mmol, 2 eq) was added to a solution of imidazole (50 mmol, 1 eq) in DMSO
(
20 mL) and the mixture was stirred for 30 min at room temperature. Dodecyliodide (50 mmol, 1 eq) was
added portion-wise under vigorous stirring in a water bath and the stirring was continued overnight. The
mixture was then quenched with water (200 mL) and extracted with diethyl ether (3 x 25 mL). The
combined extracts were washed with water, dried over anhydrous magnesium sulphate and the solvent was
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evaporated off under reduced pressure.
1
Orange oil. H NMR (400 MHz, CDCl , /ppm): 0.82 (t, J = 7.2 Hz, 3H), 1.19 (s, 18H), 1.67-1.72 (m,
3
H-H
-1
2
1
H), 3.84 (t, JH-H = 7.2 Hz, 2H), 6.83 (s, 1H), 6.96 (s, 1H), 7.37 (s, 1H). max (cm ): 3106, 2922, 2852,
678, 1568, 1506, 1465, 1376, 1282, 1228, 1163, 1108, 1076, 1031, 907, 807, 725, 663, 624.
2.3.
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 sealed aluminum crucible with an argon flow rate of 40 mL/min. The
samples were placed in aluminum pans which were cold-sealed. Given temperatures correspond
to the maximum of the respective thermal process.
Thermogravimetric Analysis (TG) was performed with a TG 449 F3 Jupiter (Netzsch, Selb,
Germany). Measurements were carried out in aluminum oxide crucibles with a heating rate of
1
0°C/min and nitrogen as purge gas.
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The 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.
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H and C NMR spectra were recorded at room temperature in CDCl or DMSO on a Bruker
3
400 MHz spectrometer equipped with a BBO probe. Chemical shifts are reported in delta (δ) units,
expressed in parts per million (ppm). The following abbreviations used for the observed
1
multiplicities: s (singlet), d (doublet), t (triplet), q (quartet), m (multiplet for unresolved lines). H
13
NMR chemical shifts were referenced to the residual solvent signal for CDCl (7.26 ppm) and C
3
NMR chemical shifts were referenced to the solvent signal of CDCl (77.16 ppm).
3
The ESI mass spectrometry was conducted with A SYNAPT G2-S HDMS Q-ToF Mass
Spectrometer (Waters, Manchester, UK) in positive and negative ion mode.
Optical analyses were made by heated-stage polarized optical microscopy (POM) 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
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magnification of 100× as a video with a digital camera during heating and cooling the sample
which was places between two cover slips. Heating and cooling rates were 5°C/min.
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UV-Vis absorption spectra of methanolic solutions of 1–5 were recorded on an Agilent
Technologies Cary 50 UV-Vis spectrophotometer using 1 cm light-path quartz cuvettes. UV-Vis
absorption spectra of neat samples, which were molten before the measurement, were recorded
on an Agilent Technologies Cary 5000 UV-Vis-NIR spectrophotometer equipped with a Praying
Mantis diffuse reflectance accessory. The irradiation of the samples was conducted in a box
equipped with an open UV lamp installed in the upper part of the hood (CAMAG). For irradiation,
the samples were placed in the box and exposed to UV light of 366 nm for 10 min. UV-Vis
measurements and UV irradiation were performed at room temperature.
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Crystals of compounds 1–3 were obtained from recrystallization of the corresponding samples
45
in dichloromethane/acetonitrile solution. Compound had been studied by us previously. We did
the reinvestigation of the crystal structure as compound 3 is isostructural with compound 1 and
we discuss their similarities and differences." Data sets were collected using various single-crystal
X-Ray diffractometer a Stoe IPDS-I (Mo K , at 100 K), a Bruker D8 Venture (Mo K , at 293 K) and
α
α
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
an Agilent SuperNova (Cu K , at 107 K). Crystal structure solution by direct methods using SIR
α
4
6
47
48
9
2 or SHELXT yielded the heavy atom positions. Refinement with SHELXL-97 allowed for
the localization of the remaining atom positions. Hydrogen atoms were added and treated with
the riding atom mode. Data reduction was performed with the program package X-Red49 or
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
50
51
52
SAINT and absorption corrections were carried out with the programs X-Shape or SADABS.
5
3
To illustrate the crystal structures, the program Diamond was used.
Theoretical calculations based on the DFT (density functional theory) method are utilized to
investigate the mechanism of photoisomerization of compounds 1-5. All of the calculations are
performed by the Gaussian09 program package.54 Ground state geometry optimization of
azobenzene compounds and the calculation of HOMO-LUMO gap are done using B3PW91
5
5
functional and 6-311++G(d,p) basis set in gas phase. Note that, polarization functions improve
the description of molecular geometries. The diffuse function are represented by “+” sign, which
considers the effect of electrons far away from the nucleus. The polarizable continuum model
(
PCM) is utilized to evaluate the bulk solvent effects on the absorption spectra.56
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2
3. Results and Discussion
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
.1.
Synthesis
In a first step an azo-coupling between a substituted aniline and phenol (Scheme 1) was carried
out, followed by a Williamson ether synthesis with 1,2-dibromoethane. Finally, conversion to the
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
bromide salt was achieved by alkylation of dodecylimidazole.
OH
NH2
OH
(
i)
N
+
N
R
R
R
1a-5a
1b-5b
OH
O
Br
(ii)
N
Br
N
N
+
N
Br
R
1b-5b
1c-5c
O
O
N
N
C H
12 25
Br
(iii)
N
N
N
C H
2 25
N
1
+
N
N
R
R
1
c-5c
6
1-5
Scheme 1. Synthesis of compounds 1–5. i) NaNO , NaOH, Na CO , HCl, H O, r.t., 4h; ii) K CO ,
2
2
3
2
2
3
CH CN, reflux, 20h; iii) CH CN, reflux, 72h.
3
3
3.2.
Structure Analysis
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Single crystals of compounds 1 and 3 could be grown from a dichloromethane/acetonitrile
solution. The hemihydrate of 2 could be obtained by isothermal evaporation of acetonitrile solution
at ambient conditions. Despite intense trying various solvents and solvent combinations, crystals
of 4 and 5 with sufficient quality for X-ray structure analysis could not be obtained.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Compounds 1 and 3 crystallize in the triclinic space group P1 and are isostructural. The alkyl
chain and the azobenzene unit planes are oriented virtually perpendicular (65–75°) to the
imidazolium ring but parallel to each other, resembling the letter “U” from a certain perspective
(Figure 3 top). Nevertheless, these chains (the arms of the molecule) are entirely orthogonal in
the projection on the ac plane, resembling the letter “V” (Figure 3 bottom). Expectedly, the
azobenzene unit adopts a trans conformation in all crystal structures. The organic cations form
bilayers with head-to-head arrangement. The latter connects through offset head-to-head ···
interactions (dCg···Cg = 3.91 Å, slippage = 1.68 Å) and weak and strongly delocalized C−H···Br
−
interactions (dH···Br above 2.9 Å). Each Br anion is hypothetically bound to six large organic
cations, mostly through aromatic ring H atoms. Different arms of the neighboring intralayer cations
connect through rather strong C−H··· interactions (dH···Cg = 2.93 Å for 1 and dH···Cg = 2.84 Å for 3)
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2
forming a zigzag. The interlayer contacts are limited to the weak C−H··· (dH···Cg = 4.08 Å) and
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
weak electrostatic interactions between the terminal -CH groups of the alkyl chains and the -CH
3
3
groups of the R substituents. It is interesting that in the case of -OCH substituent the distances
3
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
between the closest -CH groups slightly increase but due to the presence of the oxygen atom
3
one -CH group moves closer to the arm of the neighboring organic cation adding one more
3
comparable interaction.
The crystal structure of 2·0.5 H O (Figure 4) exhibits similar bilayers though with different
2
internal ordering due to different conformation of the cations and enhanced bonding between two
parts of the bilayer perhaps due to formation of the solvate. In contrast to 1 and 3, the alkyl chain
and the azobenzene unit planes form a smaller angle (45−50°) with the imidazolium ring and,
instead of parallel stacking, build zigzag chains along the a axis. Within one layer cations alternate
approximately along [011] being orthogonal to each other. Head-to-head ··· interactions
between the imidazolium rings are shorter (dCg···Cg = 3.52 Å, slippage = 0.79 Å), while no C−H···
interactions are observed between the arms of different molecules. Instead, strong C−H···
interactions (dH···Cg = 3.56 Å) appeared between the arms of the same molecule connecting the
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
second benzene ring with the terminal -C H group and stabilizing the “U” shape. The interlayer
2
5
contacts are now represented by a more extended network of -CH groups from tert-butyl groups
3
and long carbon chains. Each tert-butyl group is involved in seven weak interlayer electrostatic
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
interactions with three different cations (solely -CH groups), two intralayer and two intramolecular
3
interactions (mostly -CH groups).
2
The hydrogen bond network is significantly extended not at least due to the presence of water.
–
The latter form strong associates with the two neighboring Br anions (d
= 2.43−2.47 Å) and
H···Br
accept H-bonds from four neighboring cations, exclusively imidazolium and  methylene
hydrogens from the both parts of the bilayer. The architecture of C−H···Br interactions is different
–
also compared to 1 and 3. Each Br anion is connected to four cations with strong preference to
one part of the bilayer (~90% in average). These bonds are not restricted to imidazolium and 
methylene hydrogens and may include even hydrogens from the first benzene ring and both arms
of the same cation. In average C−H···Br contacts in 2 are comparable but the most significant
fraction is located around 2.9 Å. Though the water molecule significantly affected the polar part
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2
of the structure it is hard to estimate its role in total structural changes in 2, especially mutual
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
reorientation of the long carbon chains and azobenzene units.
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 1. Crystallographic and refinement details for compounds 1, 2·0.5 H O and 3.
2
Compound
Substituent
Empirical
1
Me
2·0.5 H O
3
2
t-Bu
OMe
C H N OBr
C
33H N O Br
C H N O Br
3
0
1
43
4
50
4
1.5
30 43
4
2
902398
1902400
1902399
CCDC
Formula weight
Space group, Z
SG No.
555.59
P1, 2
2
606.68
Cc, 8
571.59
P1, 2
2
9
a, Å
6.854(5)
8.234(6)
27.248(2)
90.87(9)
90.87(9)
109.43(7)
1449.7(18)
170
52.972(6)
8.6963(8)
15.054(1)
90
6.8342(1)
8.1601(1)
27.6004(4)
88.892(2)
89.877(2)
69.752(1)
1443.81(4)
170
b, Å
c, Å
α, °
β, °
98.138(3)
90
γ, °
Volume
6865.1(9)
293
Temperature [K]
Calculated
Absorption
F(000)
1.27
1.174
1.31
1.446
1.228
2.192
588
2584
604
Θ-range for data
2.6 to 25.00 °
2.3 to 26.8 °
-66 <h< 66
-11 <k< 11
-17 <l< 19
13651/ 686
0.988
3.2 to 68.0 °
-6 <h< 8
-9 <k< 9
-33 <l< 33
5260/ 336
1.129
-
7 <h< 8
Index ranges
-9 <k< 9
-
32 <l< 28
Data/
4366/ 325
1.069
Goodness-of-fit
R ; ωR [I >2σ
0.047; 0.063
0.116; 0.129
−
0.049; 0.110
0.118; 0.135
0.019(12)
0.035; 0.038
0.095; 0.096
−
1
2
0
R1; ωR2 (all
Flack parameter
Largest diff. peak
0.34/ -0.52
0.30/ -0.28
0.75/ -0.51
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2
3
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5
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7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 3. Projections of the crystal structure of compound 1 on the bc (top) and the ac (bottom)
plane.
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2
Figure 4. Projections of the crystal structure of compound 2·0.5 H O on the ac plane.
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
3
.3.
Thermal behavior
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
The thermal phase behavior 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 2. The thermal decomposition temperatures and
water content were determined via thermogravimetric analysis (TGA). All compounds decompose
between 250 and 270°C. TGA confirms compounds 1, 3, 4 and 5 to be anhydrous and 2 to be a
hemihydrate. The second heating and cooling traces for all compounds are displayed in Figure
5
. All following heating and cooling cycle show the same thermal events.
Table 2. Thermal properties of compounds 1–5.
heatinga
coolinga
No
Substituent Transitionb
c
T [°C]
ΔH [kJ/mol] T [°C] ΔH [kJ/mol] T [°C]
d
.
SmACr
Me
63.6
-16.41
1
Cr SmA
132.9
18.34
263.7
2
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6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
SmALISO
SmA Cr
Cr SmA
SmA LISO
169.6
2.66
-52.60
38.72
3.95
166.9
-2.87
-4.18
66.8
99.5
2
3
t-Bu
270.3
258.4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
193.5
101.4
190.6
Cr  LISO
21.18
OMe
LISO SmA
107.4
57.1
30.4
88.5
77.9
-2.69
-16.32
-12.82
-1.52
SmA  Cr
Cr SmA
SmA LISO
Cr LISO
31.3
89.7
12.16
1.27
4
5
NMe2
NO2
258.3
256.4
101.7
13.29
-14.05
a
Phase-transition temperatures (°C) and enthalpies are measured during heating and cooling
b
c
at 5°C/min;
the phase transitions were identified by POM measurements;
thermal
decomposition temperature.
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2
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5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5. DSC thermogramms of compounds 1–5. For each compound the upper (lower) line
nd
corresponds to the 2 heating (cooling) curve.
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1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Compounds 1 and 2 with methyl and tert-butyl (t-Bu) substituents, respectively, show one
exothermic and two endothermic events during heating (Figure 5). The exothermic event
corresponds to a cold crystallization (63.6°C (1) and 66.8°C (2)). Temperature dependent POM
identifies the first endothermic transition (132.9°C/99.5°C) to be the flow point (SLC) and the
second transition (169.6°C/193.5°C) as the clearing point (LCLISO). The liquid crystalline phase,
which could be identified as a smectic A phase by polarizing microscopy, reappears at 166.9°C
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
(1) and 190.6°C (2) upon cooling the isotropic liquid. No crystallization occurs during further
cooling. Compound 3 melts into an isotropic liquid at 101.4°C without showing other phase
transitions. During cooling for 3, a smectic A phase is formed at 107.4°C. Crystallization occurs
upon further cooling at 57.1°C. Compound 4 which features a N,N-dimethylamino substituent
shows two phase transition upon heating as well as upon cooling. It exhibits a smectic A phase
around room temperature and clears at 89.7°C. Both transitions are reversible and reappear
during cooling, albeit at slightly lower temperatures (Table 2). Compound 5, where the cation
contains a nitro group in para position of the azobenzene ring, does not exhibit a mesophase
(Figure 5). Upon heating it melts directly into the isotropic liquid at 101.7°C. It recrystallizes during
cooling at 77.9°C. Compound 2 (t-Bu) has the largest liquid crystalline temperature window:
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almost 100°C, while compound 4 (NMe ) has the lowest flow point (31.3°C) and the lowest
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
clearing point (89.7°C).
Although compounds 1 and 3 have a similar crystal structure (Figure 3), their thermal behavior
is entirely different. Compound 3 does not exhibit a mesophase upon heating whereas compound
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1, which shows a cold crystallization upon (re-)heating, forms a liquid crystalline phase over a
wide temperature range. It appears, that the additional oxygen atom of the -OMe group is able to
efficiently stabilize the crystalline structure and hinders the mesophase formation, probably due
to an immobilization of the azogroup. Compound 2 shows a similar thermal behavior to compound
1, but has a substantially larger mesophase temperature window, in fact the largest amongst all
investigated compounds. In contrast, compound 4 has a small mesophase range of 58.4°C. No
liquid crystalline phase is formed by compound 5. The crystal structure of 2 shows that between
the layers the substituents are facing each other (Figure 4). Thus, if there are polar groups like in
compounds 4 and 5, repulsive forces are operating that prevent recrystallization. Whereas with
aliphatic substituents van der Waals interactions can hold the structure together.
2
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