Fluorocarbon and hydrocarbon N-heterocyclic (C5-C7) imidazole-based liquid crystals

Article abstract of DOI:10.1002/asia.201402576

By using three synthetic protocols, a series of fluorocarbon and hydrocarbon N-heterocyclic imidazolebased liquid crystals (LCs) and related imidazolium-based ionic liquid crystals (ILCs) have been prepared. The ring size of the N-heterocycle and the length of the N-terminal chain (on the imidazolium unit in the ILCs) were modified, and the influence of these structural parameters on liquid-crystal phases was investigated by means of polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). These new ILCs exhibit a disordered smectic phase (SmA), good thermal stabilities, a broad smectic phase range, a high dipole moment, relatively low melting points, but high clearing points and strong emission fluorescence relative to imidazole-based LCs. These encouraging results have led us to believe these fluorocarbon and hydrocarbon N-heterocyclic imidazole-based LCs and related imidazolium-based ILCs could be used as new liquid-crystalline materials.

Full text of DOI:10.1002/asia.201402576

FULL PAPER
DOI: 10.1002/asia.201402576
Fluorocarbon and Hydrocarbon N-Heterocyclic (C5–C7) Imidazole-Based
Liquid Crystals
Hongren Chen,[a] Fengying Hong,[a] Guang Shao,[c] Deyu Hang,[d] Lei Zhao,[d] and
Zhuo Zeng*[a, b]
Abstract: By using three synthetic pro-
tocols, a series of fluorocarbon and hy-
drocarbon N-heterocyclic imidazole-
based liquid crystals (LCs) and related
imidazolium-based ionic liquid crystals
(ILCs) have been prepared. The ring
size of the N-heterocycle and the
length of the N-terminal chain (on the
imidazolium unit in the ILCs) were
modified, and the influence of these
structural parameters on liquid-crystal
phases was investigated by means of
polarizing optical microscopy (POM),
stabilities, a broad smectic phase range,
a high dipole moment, relatively low
melting points, but high clearing points
and strong emission fluorescence rela-
tive to imidazole-based LCs. These en-
couraging results have led us to believe
these fluorocarbon and hydrocarbon
N-heterocyclic imidazole-based LCs
and related imidazolium-based ILCs
could be used as new liquid-crystalline
materials.
differential
scanning
calorimetry
(DSC), and X-ray diffraction (XRD).
These new ILCs exhibit a disordered
smectic phase (SmA), good thermal
Keywords: fluorine · heterocycles ·
ionic liquids · mesophases · phase
transitions
Introduction
ILCs with different cations have been described in the lit-
erature, such as ammonium, phosphonium, pyridinium, and
so forth.[4] Imidazolium salts have been intensively investi-
gated as ionic liquids and also introduced into ILC sys-
tems.[5–10] This is because the imidazolium unit is an excel-
lent platform that can be designed to promote liquid-crystal-
line phases and is easily doped by a large diversity of
anions.[11] Imidazolium-based ILCs including symmetrical
and unsymmetrical molecular frameworks have been gener-
ated and most of them show smectic phases.[12] Recently,
mesomorphic imidazolium salts were also used as new vec-
tors for small interfering RNA (siRNA) transfection.[13]
Most of the organic compounds known for optoelectronic
applications still have some major limitations such as low
electron conductivity or luminescence efficiency. An alterna-
tive method to bypass these limitations is to introduce an
electron-acceptor group such as oxadiazole,[14] quinoline,[15]
or other heterocycle moieties.[16] The introduction of a polar
ring into the liquid-crystalline core is a promising approach
to the design of novel liquid crystals for applications in ad-
vanced functional materials. Structures such as N-heterocy-
cles should prove to be highly valuable for further investiga-
tion because of their large dielectric constant, great elec-
tron-transport abilities, and strong electron-withdrawing
properties.[17] The coupling of N-heterocycles and imidazole
in an ionic compound would, in principle, enhance both its
electron transport and conductivity to bypass those limita-
tions. Introducing a fluorine atom or fluorinated group into
liquid-crystal systems will remarkably modify their melting
points, mesophase morphologies, transition temperatures,
and many other very important physical properties, such as
dielectric anisotropy, optical anisotropy, and viscoelastic
properties. Many fluorinated LCs display excellent modified
Liquid crystals (LCs) as soft materials have attracted consid-
erable attention owing to their fascinating properties[1] and
their successful applications in commerce.[2] Ionic liquid
crystals are a class of liquid-crystalline compounds that con-
tain salts of cations and their counterpart anions. They can
be considered materials that possess the properties of both
liquid crystals and ionic liquids. They are significantly differ-
ent from conventional neutral LCs but still display meso-
phases. Their useful properties include ion conductivity, non-
volatility, low viscosity, low melting points, and tunable po-
larity.[3]
[a] H. Chen, F. Hong, Prof. Z. Zeng
College of Chemistry and Environment
South China Normal University
Guangzhou, 510006 (China)
Fax : (+0086)2039310287
E-mail: zhuoz@scnu.edu.cn
[b] Prof. Z. Zeng
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
Shanghai, 200032 (China)
[c] Dr. G. Shao
School of Chemistry and Chemical Engineering
Sun Yat-sen University
Guangzhou, 510275 (China)
[d] Dr. D. Hang, L. Zhao
Beijing Bayi Space LCD Materials Technology Co., Ltd.
Beijing 102205 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201402576.
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properties, and investigations on fluorinated ILCs have re-
ported prospective applications owing to the existence of
fluoro substituents.[18] There is interest in the development
of some fluorinated imidazole-based LCs and related imida-
zolium-based ILCs derivatives in an effort to establish their
structure–property relationships, and to target potential
liquid-crystal display applications.[13]
In this work, we report the design and syntheses of fluoro-
carbon and hydrocarbon imidazole-based liquid crystals
(LCs) and their related imidazolium-based ionic liquid crys-
tals (ILCs). The impact on their properties of N-heterocy-
cles in varying sizes, alkyl chain lengths, and anions has
been investigated. The coupling of a fluorinated heterocycle
and imidazole moiety in these new compounds should ex-
hibit some interesting electron-transport properties because
of their ability to impart lateral and/or longitudinal dipoles
in addition to the accompanying changes in their molecular
geometry.[19] Their mesomorphic properties, some of their
physicochemical properties such as UV and fluorescence,
and their thermal stability properties were investigated.
Scheme 1. Synthesis of fluorocarbon and hydrocarbon N-heterocyclic
(C5–C7) iodobenzene.
nation of I5F with an excess amount of tBuONa in DMSO
at 908C for 12 h generated the 3,4-difluoro-1-(4-iodophe-
nyl)pyrrole (I2F) in high yields. When I6F was treated with
an excess amount of tBuONa in DMSO under a range of
temperatures (50–1508C), a mixture of compounds was ob-
served by TLC that could not be separated by normal
column chromatography owing to their similar polarities
(Scheme 2). b) Transition-metal-catalyzed Ullmann-type
Results and Discussion
Synthesis
Three synthetic protocols for the preparation of N-heterocy-
clic imidazole-based compounds (M2F, M5F–M7F, M5H–
M7H) have been employed in this article. The first synthetic
routine was as follows: a) As we described in our previous
experiments,[20] a series of trifluoromethanesulfonic acid pol-
yfluoroalkyldiyl 1 f and 2 f and 4-toluene sulfonyl alkyldiyl
(1h–3h) esters were treated with 4-iodobenzenamine in the
presence of Et3N and CH3CH2OH to give fluorocarbon and
hydrocarbon N-heterocyclic (C5–C7) iodobenzene com-
pounds I5H–I7H, I5F, and I6F (Scheme 1). Dehydrofluori-
Scheme 2. Dehydrofluorination of compound I5F and I6F.
coupling is one of the most important methods in the forma-
tion of imidazole compounds.[21] Among the transition-metal
catalysts tested, the copper-catalyzed reaction was clearly
the best choice. Two different bases (NaOH,[22] K3PO4
)
[23]
Abstract in Chinese:
were chosen to compare their yields (Scheme 3). We found
that using K3PO4 as base gave better results in terms of
yield in dimethylformamide (DMF) relative to NaOH in
DMSO under ligand-free conditions (Table 1). However,
fluorinated N-heterocyclic compound I5F was dehydro-
fluorinated to yield M2F, and I6F gave an inseparable mix-
ture under these alkaline Ullmann-type coupling conditions.
Using the second synthetic routine, 4-(1H-imidazol-1-yl)a-
niline was prepared first, then further treated with trifluoro-
methanesulfonic acid polyfluoroalkyldiyl and 4-toluene sul-
fonyl alkyldiyl esters, 1 f and 1h, to produce N-heterocyclic
Scheme 3. Ullmann-type coupling reaction in different conditions.
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Zhuo Zeng et al.
Table 1. Prepared N-heterocyclic imidazoles by Ullmann-type coupling reac-
tion.
Therefore, the third synthetic routine was adopted. Fluori-
nated N-heterocyclic imidazoles M5F–M7F were prepared
by Debus–Radziszewski imidazole synthesis[26] in a reasona-
bly good yield. It is worth noting that these protocols are
generally operated in volatile compounds with a base. Fluo-
rocarbon N-heterocyclic anilines (N5F–N7F) were obtained
by reactions of trifluoromethanesulfonic acid polyfluoroal-
kyldiyl 1 f–3 f with two equivalents of p-phenylenediamine
in the presence of Et3N in high yields. N-Heterocyclic imida-
zoles were obtained in a three-step procedure as described
in Debus–Radziszewski imidazole synthesis. The N-hetero-
cyclic anilines were treated with glyoxal through the nucleo-
philic addition to form imine compounds, then they were
treated with formaldehyde and ammonium chloride to yield
quaternary ammonium salt. The quaternary ammonium salt
was further treated with phosphoric acid in methanol to give
the target compounds, fluorocarbon N-heterocyclic imida-
zoles (M5F–M7F), in high yield (Scheme 5). These N-heter-
ocyclic imidazole compounds (M2F, M5F–M7F, M5H–M7H)
were quaternized with the appropriate 1-bromotetradecane
or 1-bromohexadecane bromides in acetonitrile to yield the
N-heterocyclic imidazolium-based ILCs in high yields (2F-
14/18, (5–7)F-14/18, (5–7)H-14/18). The bromine ions of rep-
resentative compounds 5F-14 and 5F-18 were exchanged
with NaBF4 in acetone to give 5F-14B and 5F-18B
(Scheme 6).
R
Products
Yield 1 [%]
55.9
Yield 2 [%]
90.2
M5H
M6H
56.5
55.8
91.3
89.3
M7H
42.3
26.3
M2F
M6F (inseparable mixture)
0
0
58.4
57.1
92.5
89.5
M2F
NH2
MNH2
imidazolium compounds. However, their yields were very
poor (M5H 5.2%, M5F 1.9%) (Scheme 4). It is possible
that it is easy to treat imidazole with 1 f[24] and 1h[25] to give
the corresponding quaternary imidazolium compounds.
Syntheses of fluorinated N-heterocyclic imidazole-based
LCs by using an alkaline Ullmann-type coupling reaction
and cyclization of 4-(1H-imidazol-1-yl)aniline were not suc-
cessful, since fluorine was easily eliminated in the course of
the Ullmann-type coupling reaction and 4-(1H-imidazol-1-
yl)aniline was quaternized with 1 f and 1h under the cycliza-
tion reaction.
Liquid-Crystalline Properties
The phase transitions and thermodynamic data of the non-
ionic imidazoles (M5F–M7F/H, M2F) and the corresponding
imidazolium salts ((5–7)H-14/18, (5–7)F-14/18, 2F-14/18, 5F-
14/18B) were investigated by means of differential scanning
calorimetry (DSC), polarizing optical microscopy (POM),
thermogravimetric analysis (TGA), and for selected com-
pounds, variable-temperature X-ray diffraction (VTXRD).
The transition temperatures of the nonionic imidazoles
M5F/H–M7F/H and M2F are collected in Table 2. All hy-
drocarbon N-heterocyclic nonionic imidazole compounds
exhibit a thermotropic mesomorphic phase. A mosaic tex-
ture of crystal smectic E (SmE) phase was observed by
means of POM for pyrrolidine-, piperidine-, and azepane-
based imidazole compounds M5H–M7H (Figure 1a–c). They
exhibited a narrow phase (e.g., M5H (Cr 178.98C, SmE
Scheme 4. Cyclization of 4-(1H-imidazol-1-yl)aniline.
Scheme 5. Synthesis of N-heterocyclic imidazole LCs by means of the Debus–Radziszewski procedure.
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In the series of the imidazoli-
um salts (5–7)H-14/18, (5–7)F-
14/18, 2F-14/18, and 5F-14/18B,
all compounds show a thermo-
tropic liquid-crystalline phase,
with the exception of the termi-
nal hydrocarbon seven-mem-
bered ring N-heterocyclic com-
pounds 7H-14 (Table 3).
Imidazolium bromides with
terminal fluorocarbon and hy-
drocarbon N-heterocycles, (5–
7)F-14/18 and 2F-14/18, have
a broader range of mesomor-
phic phases than the series of
nonionic imidazoles M5–7F/H
and M2F. These compounds
displayed disordered SmA mes-
ophases (e.g., Figure 2). Some
of these compounds have
a very wide smectic range; for
example, 2F-18 has a smectic
phase range of 113.68C. As can
Scheme 6. Synthesis of N-heterocyclic imidazolium-based ILCs.
Table 2. Thermal behavior of the N-heterocyclic imidazole-based LCs.
Compound Transition temperatures [8C][a] Enthalpies of transition
[JgÀ1
]
M2F
M5F
M6F
M7F
M5H
M6H
M7H
Cr 179.9 [2.56]
Cr 175.4 [3.94]
Cr 82.1 [8.30]
Cr 115.7 [4.73]
Cr 178.9 [3.03]
Cr 104.2 [13.98]
Cr 68.5 [6.47]
SmE 185.5[b]
SmE 178.0[b]
iso
iso
iso
iso
iso
iso
iso
SmE 187.9[b]
SmE 118.7[b]
SmE 73.8[b]
[a] Transition temperature and enthalpy change (in square brackets)
were determined by DSC (peak temperature, first heating scan,
5 KminÀ1À1) and confirmed by POM. Cr=crystalline solid, SmE=crystal
smectic E, iso=isotropic liquid state. [b] Transition temperatures were
determined by POM.
187.98C; Cr=crystalline solid), M6H (Cr 104.28C, SmE
118.78C), M7H (Cr 68.58C, SmE 73.88C); Table 2). Howev-
er, for fluorocarbon N-heterocyclic imidazole compounds,
only 3,3,4,4-tetrafluoropyrrolidine- and 3,4-difluoropyrrole-
based imidazole compounds M2F and M5F show a thermo-
tropic mesomorphic phase. Compound M5F with 3,3,4,4-tet-
rafluoropyrrolidine as the terminal group also displayed
a mosaic texture of the crystal smectic E phase (Figure 1d),
whereas M6F and M7F melted into an isotropic liquid at
82.1 and 115.78C directly with no liquid-crystal texture
being observed. Upon increasing the terminal N-heterocycle
size from pyrrolidine to azepane, their melting points de-
crease dramatically, (e.g., M5H: m.p. 178.98C M6H: m.p.
104.28C M7H: m.p. 68.58C). When comparing fluorocarbon
N-heterocyclic imidazole with hydrocarbon N-heterocyclic
imidazole, M6F with 3,3,4,4,5,5-hexafluoropiperidine as its
terminal group shows the lowest melting point, 82.18C.
Figure 1. a) Mosaic texture of SmE upon cooling to 1798C for M5H.
b) Mosaic texture of SmE upon cooling to 1058C for M6H. c) Mosaic
texture of SmE upon cooling to 708C for M7H. d) Mosaic texture of
SmE upon cooling to 1728C for M5F.
be seen from Table 3, as the terminal N-heterocycle size in-
creases from a five- to a seven-membered ring, the range of
hydrocarbon N-heterocyclic compounds narrows rapidly
(e.g., 5H-14 (Cr 100.88C SmA 123.98C), 6H-14 (Cr 64.18C
SmA 69.68C); see Table 3), and 7H-14 is an isotropic liquid
at 36.88C directly with no liquid-crystal texture being ob-
served, whereas the range of fluorocarbon N-heterocyclic
compounds first decreases and then increases upon increas-
ing the terminal N-heterocycle size (e.g., 5F-14 (Cr 89.18C
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Table 3. Thermal behavior of the N-heterocyclic imidazolium-based ILCs.
Compound
Transition temperatures [8C][a]
Enthalpies of transition [JgÀ1
]
Td [8C][b]
5F-14
5H-14
2F-14
6F-14
6H-14
7F-14
7H-14
5F-18
5H-18
2F-18
6F-18
6H-18
7F-18
5F-14B
5F-18B
Cr 89.1 [2.29]
Cr 100.8 [4.78]
Cr 135.7 [1.10]
Cr 106.0 [12.21]
Cr 64.1 [19.16]
Cr 145.7 [2.04] Cr’ 148.7 [2.95]
Cr 36.8 [10.20]
SmA 157.2 [4.28]
SmA 123.9 [4.20]
SmA 206.9 [0.89]
SmA 112.3[c]
iso
iso
iso
iso
iso
iso
iso
iso
iso
iso
iso
iso
iso
iso
iso
259
257
255
267
259
264
261
247
263
250
263
266
262
273
268
SmA 69.6[c]
SmA 180.4 [8.76]
Cr 108.3 [2.94]
SmA 165.3 [0.23]
SmA 121.5 [1.21]
SmA 248.9 [1.91]
SmA 116.2[c]
Cr 106.0 [3.67] Cr’ 111.7 [1.44]
Cr 122.8 [7.28] Cr’ 135.3 [3.68]
Cr 107.5 [9.06]
Cr 61.2 [19.82]
Cr 80.2 [8.07]
SmA 71.8[c]
SmA 182.3 [18.32]
Cr 84.1 [14.42]
Cr 63.7 [8.26] Cr’ 83.6 [4.48]
SmA 251.5[c]
SmA 254.1ACTHNGUTERNNU[G 2.56]
[a] Transition temperature and enthalpy change (in square brackets) were determined by DSC (peak temperature, first heating scan, 5 KminÀ1À1) and
confirmed by POM. Cr=crystalline solid, N=nematic phase, SmA=Smectic A, iso=isotropic liquid state. [b] Transition temperatures were determined
by POM. [c] Decomposition temperature.
In general, the introduction of terminal nonfluorinated
pyrrolidine, piperidine, and azepane causes a decrease in the
melting temperature relative to the introduction of 3,4- di-
fluoropyrrole, 3,3,4,4-tetrafluoropyrrolidine, 3,3,4,4,5,5-hexa-
fluoropiperidine, and 3,3,4,4,5,5,6,6-octafluoroazepane com-
pounds, for example, 5F-18 (m.p. 108.38C), 5H-18 (m.p.
106.08C), 6F-18 (m.p. 107.58C), 6H-18 (m.p. 61.28C), where-
as compound 5F-14 has a lower melting point (89.18C) than
compound 5H-14. In addition, upon increasing the terminal
Figure 2. a) Texture of the SmA upon cooling to 1668C for 2F-14. b) Tex-
N-heterocycle size from pyrrolidine to azepane, their melt-
ing points decrease dramatically (e.g., 5H-14 (m.p. 100.88C),
6H-14 (m.p. 64.18C), 7H-14 (m.p. 36.88C)). However, the
melting points of the terminal fluorocarbon N-heterocycle
are different when the substituent is nC14H29, such as in (5–
7)F-14, for which the melting points decrease with expan-
sion of the terminal N-heterocycle size from 3,3,4,4-tetra-
fluoropyrrolidine to 3,3,4,4,5,5,6,6-octafluoroazepan. The
melting points increase upon increasing the terminal fluoro-
carbon N-heterocycle size when the substituent is nC18H37 as
in (5–7)F-18. It shows that the length of the N-terminal
chain (on the imidazolium unit in the ILCs) influences the
melting points of terminal fluorocarbon N-heterocyclic imi-
dazolium-based ILCs.
ture of the SmA upon cooling to 1778C for 7F-14.
SmA 157.28C), 6F-14 (Cr 106.08C SmA 112.38C), 7F-14,
(Cr’ 148.7, SmA 180.48C); see Figure 3). In contrast to when
the N-heterocycle is the same size as the terminal group, the
smectic A phase of terminal fluorocarbon N-heterocyclic
compounds is broader than that of terminal hydrocarbon N-
heterocyclic compounds (e.g., 5F-18 (Cr 108.38C SmA
165.38C), 5H-18 (Cr’ 111.7, SmA 121.5), 6F-18 (Cr 107.58C
SmA 116.28C), 6H-18 (Cr 61.2, SmA 71.8)).
The 3,4-difluoropyrrole-based ionic liquid crystals with
BrÀ anion, 2F-14/18, have the highest clearing point relative
to the other terminal fluorocarbon and hydrocarbon N-het-
erocyclic compounds (e.g., 2F-14 and 2F-18 at c.p. 206.9 and
248.98C respectively, and 5F-14, 5H-14, and 7F-14 at c.p.
157.2, 123.9, and 180.48C).
The anion also plays a crucial role in determining the
À
melting point and smectic phase range. The salts with BF4
anion have lower melting points lower and a much broader
smectic phase range than the corresponding salts with BrÀ
anion (e.g., 5F-18 (Cr 108.38C SmA 165.38C), 5F-18B (Cr’
84.18C SmA 254.18C)).
Figure 3. DSC heating curves of compounds 5F-14, 6F-14, and 7F-14.
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Zhuo Zeng et al.
X-ray Diffraction
Smectic phases were observed for these compounds by
means of POM. To obtain further information on the molec-
ular arrangements in the mesophase, variable-temperature
X-ray diffraction (VT-XRD) experiments were performed
on M5H and 2F-14 (for 5F-14, 6F-14, 7F-14, and 5F-18B;
see the Supporting Information). The diffraction patterns
obtained for compounds M5H and 2F-14 are shown in Fig-
ures 4 and 5, respectively.
Figure 5. X-ray diffraction pattern of compound 2F-14 at a) 200 and
b) 508C (before heating). The diffraction peak at 2q=29.78 is from the
glass sample holder.
this temperature range. Both results are consistent with
a smectic A structure. The calculated molecular length of
a molecule of 2F-14 is 28.7 ꢁ, and the layer thickness of 2F-
14 is larger than the molecular length (d/l=1.07; see
Table 4) but significantly smaller than twice the length, thus
indicating bilayer structures. The diffraction plot measured
at 508C (Figure 5c) demonstrates that the smectic A phase
has been quenched at this temperature.
Figure 4. X-ray diffraction pattern of compound M5H at a) 184 and
b) 508C (before heating).
Figure 4 shows the diffraction diagram of the phase transi-
tions of M5H taken at 184 and 508C. When the measuring
temperature was cooled to 1848C only one sharp reflection
at 7.528 was seen with corresponding d spacings of 11.9 ꢁ in
the small-angle region. Three sharp reflections were seen
with d spacings of 5.0, 4.5, and 3.9 ꢁ in the wide-angle
region (Figure 4a). The liquid-crystal state for SmE, SmG,
and SmI shows several sharp outer diffraction peaks as pre-
viously reported.[27] The optical polarizing micrograph (Fig-
ure 1a) reveals a mosaic texture of SmE for M5H in this
temperature range. Both results are consistent with a crystal
smectic E structure, and this is consistent with the fact that
a 1-methylimidazolium (MIm) group attached to rod-shaped
LCs has commonly yielded SmA and SmE phases.[28] When
the measuring temperature was further cooled to 508C
a crystalline substance was obtained.
Figure 5 presents the temperature-dependent X-ray dif-
fraction diagrams obtained from sample of 2F-14 at 200 and
508C. At 2008C (Figure 5a), two sharp scatterings at 3.08
and 6.098 are observed that correspond to a d spacing of
28.7 and 14.4 ꢁ. These reflections are in a 1:1/2 ratio, which
indicates a lamellar ordering in the mesophase. Only a dif-
fuse scattering around 208 is obtained. These results indicate
the formation of a smectic phase at 2008C; the liquid-crystal
state for smectic A, smectic C and smectic F phases show
a diffuse outer ring accompanied by a strong inner diffrac-
tion as reported previously.[27] The polarizing optical micros-
copy (Figure 2a) revealed a lamellar smectic A for 2F-14 in
Table 4. X-ray diffraction analysis data of the compounds.
Compound T [8C] d1 [ꢁ] d2 [ꢁ] d3 [ꢁ] l [ꢁ][a] Ratio d1/l Phase
5F-14
150
110
170
200
26.1
27.8 18.0
24.4 8.2
28.7 14.4
8.7
6.5
6.7
–
28.2
27.4
26.7
26.7
0.93
1.01
0.91
1.07
SmA
SmA
SmA
SmA
6F-14[b]
7F-14[b]
2F-14[b]
–
[a] Molecular length (l) was calculated for the fully extended conforma-
tion (estimated with Gaussian 03). [b] See the Supporting Information.
Thermal Stability
Thermal stabilities, which range from 245 to 2738C were af-
fected by the terminal N-heterocycle, and the substituent
group on the imidazole investigated by thermogravimetric
analysis, which measures decomposition by weight loss.
Data shown in Table 3 indicate that the decomposition tem-
peratures for these compounds, (5–7)H-14/18, (5–7)F-14/18,
2F-14/18, 5F-14/18B, were higher than the clearing points.
The decomposition temperatures were correlated with the
structures of the N-heterocyclic (C5–C7) imidazolium salts
(Figure 6). In general, the new compounds with fluorinated
N-heterocyclic imidazolium salts and a carbon number of 14
were thermally more stable than those constructed from
compounds with a carbon number of 18. However, the new
compounds with nonfluorinated N-heterocyclic imidazolium
and a carbon number of 18 were thermally more stable than
those constructed from a carbon number of 14.
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Table 5. UV-visible and fluorescence spectra analysis data of the com-
pounds.
Compound
lmax [nm]
lexc [nm]
lem [nm]
2F-18
5F-18
6F-18
7F-18
278
281
270
269
282
286
277
276
389
429
411
402
terminal fluorocarbon N-heterocycle of the 3,4-difluoropyr-
role is altered to 3,3,4,4-tetrafluoropyrrolidine.
The emission fluorescence spectra of compounds 2F-18
and (5–7)F-18 are given in Figure 8. All spectra were record-
ed from
a
dilute solution in dichloromethane (1ꢂ
10À6 molLÀ1) at lexc = 282 nm for 2F-18, 286 nm for 5F-18,
277 nm for 6F-18, and 276 nm for 7F-18 (all values of lexc
were obtained from their excitation spectra). All compounds
exhibit strong emission fluorescence from 389 to 429 nm. As
the terminal fluorocarbon N-heterocycle size of the com-
pounds increases from a five- to a seven-membered ring,
their lem decreases from 429 to 402 nm. This arises from the
fact that with the increase in the number of the electron-
withdrawing fluroalkyl groups, the conjugated system is
weakened, which contributes to a blueshift in fluorescence.
Figure 6. Correlation between thermal stability and the structure of N-
heterocyclic (C5–C7) imidazole-based compounds.
The new compounds with a carbon number of 14 and
a fluorocarbon N-heterocycle (3,3,4,4-tetrafluoropyrrolidine,
3,3,4,4,5,5-hexafluoropiperidine, 3,3,4,4,5,5,6,6-octafluoroa-
zepane) as terminal group were more stable than those with
a hydrocarbon N-heterocycle (pyrrolidine, piperidine, aze-
pane) as terminal group, whereas when the compounds were
linked to a carbon number of 18, the stability of these com-
pounds was converse. This suggests that the rigidity of the
molecule and the terminal bromoalkane also influences its
thermal stability, for example, changing the 3,3,4,4-tetra-
fluoropyrrolidine to 3,3,4,4,5,5-hexafluoropiperidine leads to
a decrease in thermal stability.
UV/Vis Absorption and Photoluminescence Spectroscopy
The UV absorption spectra for solutions of compounds 2F-
18 and (5–7)F-18 in dichloromethane are presented in
Figure 7. Their maximum absorption peaks are collected in
Table 5. As the terminal fluorocarbon N-heterocycle size of
the compounds increases from a five- to a seven-membered
ring, their lmax values show a hypsochromic shift of 12 nm,
that is, from 281 nm for 5F-18 to 269 nm for the 7F-18, but
compound 2F-18 shows a bathochromic shift of 3 nm as the
Figure 8. Fluorescence spectra of compounds 2F-18 (dashed line), 5F-18
(dotted line), 6F-18 (dashed-dotted line), and 7F-14 (solid line).
Theoretical Study
Theoretical calculations are of great importance in predict-
ing the structure of ionic liquids.[29] In particular, density
functional theory (DFT) calculations have been found to be
very useful in predicting the molecular structure as well as
interactions present in the molecule.[30–32] DFT computations
were performed with Beckeꢃs three-parameter hybrid func-
tional and the nonlocal correlation of the Lee–Yang–Parr
(B3LYP) method in the gaseous phase.[33] The 6-31+G basis
set was used to obtain the geometry optimization of the
structures of imidazole-based compounds M5F–M7F and
their related imidazolium-based ILCs, (5–7)F-14, with help
of the Gaussian 03 (Revision D.01) suite of programs.[34] The
computed structures were visualized by using the Gauss-
View program.[35] Based on the theoretical calculations to
measure b (hydrogen bonds) and m (dipole) for ILCs, it is
possible to determine the influence of both cation and anion
Figure 7. UV/Vis spectra of compounds 2F-18 (dashed line), 5F-18
(dotted line), 6F-18 (dashed-dotted line), and 7F-14 (solid line).
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structure to obtain a solid background for the following dis-
cussion.[36] The conformation expression is mainly a thermo-
dynamic problem, so the total energy of each different con-
formation can reflect the stability of the structure. For the
six-membered-ring N-heterocycle, both M6F and 6F-14 have
two different conformations, chair and boat forms, with
their total energies being À1303.05774883, À4425.52048797
and À1303.05088990, À4425.510124206 a.u. The total molec-
ular energy of the boat form was larger than that of the
chair form by approximately 18.01 and 27.21 kJmolÀ1
(1 a.u.=2625.51 kJmolÀ1), respectively, thus indicating that
the chair conformation of the terminal 3,3,4,4,5,5-hexafluor-
opiperidine compound is more stable than the boat confor-
mation from a thermodynamic point of view. Thus the ter-
minal 3,3,4,4,5,5-hexafluoropiperidine compound should
exist in the chair conformation. In comparison with
3,3,4,4,5,5-hexafluoropiperidine (C6) and 3,3,4,4-tetrafluoro-
pyrrolidine (C5), the 3,3,4,4,5,5,6,6-octafluoroazepane (C7)
adopts the twisted-boat conformation, which decreases the
molecular ordered arrangement from a five- to a seven-
membered ring. On the basis of their appearance, the re-
ported imidazolium salts can be classified into three differ-
ent types: rod, V, and U shapes.[36] To estimate the degree of
deviation from linearity, the angle (see Table 6, aa, ab,
ac) is defined by the center imidazolium ring and the con-
nected two arms. The V shape of the cation was observed
for ILCs (5–7)F-14. The angles in the V-shaped systems
span a range from 159 to 1618: 5F-14 (aa=159.0778), 6F-14
(ab=160.5778), and 7F-14 (ac=160.4498). This shows that
the mesomorphic behavior is dominated by the size of the
N-heterocycle ring and the angles in the V-shaped systems
Imidazole-based compounds M6F and M7F melted into
an isotropic liquid at 82.1 and 115.78C directly with no ob-
served liquid-crystal texture. The range of mesomorphic
phases for their related imidazolium-based ILCs 5F-14, 6F-
14, and 7F-14 first decreases and then increases as the ter-
minal N-heterocycle size increases. This can be explained by
the geometrical parameters (Figure 9) in which the coplanar
structure of the molecule is destroyed upon expansion of
the terminal N-heterocycle, which is manifested in their di-
hedral angles and bond angles (Table 6) and comes about as
a result of the structures of these molecules, thereby leading
to a change in the number of intermolecular interactions. By
comparing the geometry optimization of the structures of
imidazole-based compounds, M5–7F, and their related imi-
dazolium-based ILCs, (5–7)F-14, it shows three factors that
affect the dipole moment including: 1) the size of the N-
heterocycle ring, 2) the fluoroalkyl substituent on the het-
erocycle, and 3) the different alkyl chains on the cation. The
existence of a high dipole moment in the liquid-crystal mol-
ecule favorably boosts the inter- and intramolecular interac-
tion and influences mesomorphic behavior. Consequently,
these factors result in the diversity of the liquid-crystalline
phase range and clearing point among these compounds.
Evaluation of the Influence of the Nonionic Imidazoles and
Ionic Imidazolium
In contrast to the series of noncharged imidazoles, fluorocar-
bon and hydrocarbon N-heterocyclic imidazolium ionic
liquid crystals have a broader range of mesomorphic phases,
some of which exhibit different phase types (e.g., M5H (Cr
Table 6. Geometrical parameters of M5–7F and (5–7)F-14 calculated with Gaussian 03.
Comp
Dihedral angles [8]
Bond angles [8]
Dipole
moment
[Debye]
Molecular
length
Hydrogen
bonds H···Br
[10À1 nm]
[10À1 nm]
C1-C6-N26-C19
M5F C6-N26-C19-C23
C4-C3-N11-C13
C1-C6-N28-C19
M6F C6-N28-C19-C21
C4-C3-N11-C13
C1-C6-N28-C11
M7F C6-N28-C11-C13
C4-C3-N21-C22
C2-C1-N7-C11
C1-N7-C11-C10
5F-14 C3-C4-N24-C26
C28-N40-C31-C32
À5.274
C6-N26-C19
C3-N11-C12
123.193
126.651
À168.988
À42.940
À46.029
À107.665
À40.198
4.230
2.147
2.903
5.921
12.181
12.083
12.091
C6-N28-C19
C3-N11-C12
121.815
126.607
C6-N28-C11
C3-N21-C2
122.700
126.743
93.900
À41.008
À5.215
C1-N7-C11
123.269
123.966
127.930
159.077
(aa)
À169.261
42.105
C4-N24-C25
C28-N40-C31
C4-N24-N40
10.960
10.677
10.407
28.195
27.425
26.658
2.298
2.218
2.213
À105.015
C6-C1-N72-C64
C1-N72-C64-C68
6F-14 C5-C4-N11-C12
C12-N27-C18-C19
43.349
C1-N72-C64
C4-N11-C12
C12-N27-C18
C4-N11-N27
121.956
124.766
123.574
160.577
(ab)
122.274
126.263
126.702
160.449
(ac)
108.577
À28.085
70.130
C6-C1-N73-C63
C1-N73-C63-C65
7F-14 C5-C4-N11-C13
C15-N27-C18-C19
À15.041
98.315
C1-N73-C63
C4-N11-C13
C15-N27-C18
C4-N11-N27
À33.341
À103.061
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Vis spectra, and fluorescence spectra of the liquid crystals
was investigated. Compared with nonfluorinated N-hetero-
cyclic compounds, fluorinated N-heterocyclic compounds
differ in terms of transition temperatures and mesophase
morphology. For example, fluorinated N-heterocyclic com-
pounds have a broader mesophase range (e.g., 5F-18 (Cr
108.38C SmA 165.38C), 5H-18 (Cr’ 111.7, SmA 121.58C),
6F-18 (Cr 107.58C SmA 116.28C), 6H-18 (Cr 61.28C SmA
71.88C)). However, among nonionic imidazoles, hydrocar-
bon N-heterocyclic nonionic imidazoles have a broader mes-
ophase range than fluorocarbon N-heterocyclic imidazole
(e.g., M5H (Cr 178.98C SmE 187.98C), M5F (Cr 175.48C
SmE 178.0)); M6F and M7F melted directly into an isotropic
liquid at 82.1 and 115.78C with no liquid-crystal texture ob-
served. The low polarizability of fluorine shows the weak in-
termolecular forces of perfluoro systems, which results in
very low surface tensions of liquids. The strong inductive
À
À
effect of the C F bond on the N-heterocycle-polarized C H
bond enables the hydrogen to be involved in hydrogen
bonding with the fluorine of a neighboring molecule.[38]
Any unit that protrudes from the side of the mesogenic
core can result in a disruption in the intermolecular forces
of attraction and molecular packing. The larger size of the
fluoro substituent clearly affects the smectic phase stability,
therefore hydrocarbon N-heterocyclic nonionic imidazoles
have a broader mesophase range. However, for the imidazo-
lium in V-shaped systems, in general, the terminal N-hetero-
cycle relative to the molecule is small, and only the fluoro
substituent is small enough to preserve reasonable liquid
crystallinity. Therefore, although a fluoro substituent clearly
causes a steric effect, the size influence is not too drastic,
which enables it to be usefully incorporated into parent mol-
ecules for beneficial modification of imidazolium properties.
As the terminal N-heterocycle size increases from a five- to
a seven-membered ring, the range of hydrocarbon N-hetero-
cyclic compounds narrows rapidly (e.g., 5H-14 (Cr 100.88C
SmA 123.98C), 6H-14 (Cr 64.18C SmA 69.68C)); 7H-14 is
an isotropic liquid at 36.88C with no liquid-crystal texture
observed, whereas the range of fluorocarbon N-heterocyclic
compounds first decreases and then increases as the termi-
nal N-heterocycle size increases (e.g., 5F-14 (Cr 89.18C
SmA 157.28C), 6F-14 (Cr 106.08C SmA 112.38C), 7F-14
(Cr’ 148.7, SmA 180.48C)). This might be because with the
increase in the terminal N-heterocycle, the coplanar struc-
ture of the molecule is destroyed. However, the increase
also induces a change in shape types and the width of the
molecule, thereby increasing the distance between molecules
and resulting in a decrease in the number of intermolecular
interactions. These factors lead to a change in both the
liquid-crystalline phase ranges and the clearing point
(Scheme 7).
Figure 9. The optimized geometry of M5F–M7F and corresponding ionic
liquid crystals (5–7)F-14.
178.98C SmE 187.98C), 5H-14 (Cr 100.88C SmA 123.98C)).
The nonionic imidazoles M6F and M7F melted directly into
an isotropic liquid at 82.1 and 115.78C with no liquid-crystal
texture observed. But the corresponding ionic imidazolium
compounds, 6H-14/18 and 7F-14/18, have a broad smectic A.
The additional attractive electrostatic interactions provided
by the charged polar groups and polar–apolar interface
force effectively increases the polarizability anisotropy and
rigidity of the molecule, hence influencing the mesophase
behavior, melting point, and the clearing point. For example,
the m (dipole) parameters of noncharged imidazoles M5F in-
crease from 2.147 to 10.960 Debye after quaternization with
appropriate 1-bromotetradecane. The large m (dipole) of
ionic liquids is attributed to two cumulative effects: the ion-
pairing strength and the individual polarizabilities of both
the cation and anion. This is shown by the influence of
anion and cation structures (alkyl chain length of R) on the
dipolarity/polarizability.[37] Thus, the additional charged
polar groups are beneficial to the formation of a lamellar
liquid-crystal phase.
Conclusion
Influence of the Terminal N-Heterocycle
The effect of the terminal N-heterocycle on the melting
point, transition temperatures, mesophase morphology, UV/
A series of novel fluorocarbon and hydrocarbon N-hetero-
cyclic imidazole-based liquid crystals (LCs) and related imi-
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Experimental Section
General Considerations
All the reagents were of analytical grade, purchased from commercial
sources, and used as received. 1H and 19F NMR spectra were recorded
with a 400 MHz spectrometer operating at 400 and 376 MHz, respective-
ly. Chemical shifts are reported relative to Me4Si for 1H, and CCl3F for
19F; the solvent was CDCl3 unless otherwise specified. TGA measure-
ments were performed at a heating rate of 108CminÀ1 with a Netzsch
STA409PC (Germany) instrument. DSC plots were recorded at a scan
rate of 58CminÀ1 with a Netzsch DSC200PC apparatus. Optical micro-
graphs were observed with
a polarizing optical microscope (Nikon
Scheme 7. The structure–property relationship of N-heterocyclic LCs and
ILCs.
LINKAM-THMSE600) equipped with a heating plate (HCS601). Varia-
ble-temperature X-ray diffraction experiments were performed with
a Bruker D8 Avance X-ray diffractometer (using CuKa1 radiation at
a wavelength of 1.54 ꢁ) equipped with a temperature controller. UV/Vis
spectra were recorded with a JASCO UV-530 instrument. Fluorescence
spectra were recorded with a Hitachi LTD spectrophotometer. Elemental
analyses were performed with an EXETER CE-440 instrument.
dazolium-based ionic liquid crystals (ILCs) have been pre-
pared in good yield through Ullmann-type coupling and
Debus–Radziszewski imidazole synthesis. The structure–
property relationship of these new liquid-crystal compounds
is shown in Scheme 7. Their melting points, decomposition
temperatures, clearing points, mesomorphism types, and
fluorescence were determined. Their mesomorphic behavior
and physicochemical properties are adjusted by altering the
polarity of the terminal N-heterocycle. The nonionic imida-
zole M5F with the terminal 3,3,4,4-tetrafluoropyrrolidine
ring adopted a distorted N1-envelope conformation that
shows a mosaic texture of a crystal smectic E phase, and the
hydrocarbon nonionic imidazoles also exhibited the mosaic
texture of the crystal smectic E phase, whereas fluorocarbon
and hydrocarbon N-heterocyclic imidazolium ionic salts dis-
played disordered SmA mesophases. They have a broader
smectic phase range than nonionic imidazole compounds.
Some of these compounds have relatively low melting
points but high clearing temperatures (e.g., 7F-18 (Cr
80.28C SmA 182.38C)). Fluorocarbon N-heterocyclic imida-
zolium ionic compounds exhibit strong emission fluores-
cence (e.g., 5F-18 (lem =429 nm)). These fluorocarbon and
hydrocarbon N-heterocyclic imidazole-based liquid crystals
exhibit good thermal stabilities, broad smectic phase ranges,
high dipole moments, relatively low melting points, but high
clearing points and strong emission fluorescence. Therefore
the imidazole unit is an excellent platform that can be de-
signed to promote liquid-crystalline phases. When compar-
ing the mesomorphic behavior and physicochemical proper-
ties between nonionic (neutral) liquid crystals and ionic
liquid crystals in this work, the N-heterocyclic imidazolium-
based ionic liquid crystals display the highest dipole
moment (5F-14, m=10.96 Debye) and the broadest smectic
phase range (5F-14B, Cr 84.18C SmA 251.58C), which indi-
cates that these N-heterocyclic imidazolium-based ionic
liquid crystals can be used as components to ensure the sta-
bility of certain smectic liquid-crystal states that could be
used as new liquid-crystal materials.
General Procedure for the Preparation of Iodobenzene N-Heterocycle
Compounds (I5–7H, I5–6F)
Trifluoromethanesulfonic acid polyfluoroalkyldiyl (1 f–2 f) and 4-toluene
sulfonyl alkyldiyl (1h–3h) (1 mmol), 4-iodobenzenamine (1 mmol,
219.02 mg), triethylamine (2 mL), and ethanol (2 mL) were placed in
a Pyrex glass tube, sealed, heated at 908C for 24 h, and then allowed to
cool to room temperature. The crude product was added to dichlorome-
thane (50 mL), then washed with water (3ꢂ30 mL) and dried over anhy-
drous sodium sulfate. The solvent was removed under vacuum, and the
residue was purified by chromatography on silica gel to give the target
product. Rf =0.3 (petroleum).
3,3,4,4-Hetrafluoro-1-(4-iodophenyl)pyrrolidine (I5F), yield: 78%.
1H NMR (400 MHz, CDCl3): d=7.55 (d, J=8.8 Hz, 2H), 6.31 (d, J=
8.8 Hz, 2H), 3.78 ppm (m, 4H); 19F NMR (376 MHz, CDCl3): d=
À110.83–À129.53 ppm (m, 1H); MS (ESI): m/z calcd for C10H8F4IN:
344.96 [M+]; found: 344.83; elemental analysis calcd (%) for C10H8F4IN:
C 34.81, H 2.34, N 4.06; found: C 34.79, H 2.33, N 4.05.
3,3,4,4,5,5-Hexafluoro-1-(4-iodophenyl)piperidine (I6F), yield: 75%.
1H NMR (400 MHz, CDCl3): d=7.59 (d, J=8.3 Hz, 2H), 6.72 (d, J=
8.4 Hz, 2H), 3.72 ppm (t, J=8.1 Hz, 4H); 19F NMR (376 MHz, CDCl3):
d=À123.54 (s, 4F), À139.30 ppm (s, 2F); MS (ESI): m/z: 582.2
[M+NH4+]+; elemental analysis calcd (%) for C11H8F6IN: C 33.44, H
2.04, N 3.55; found: C 33.46, H 2.16, N 3.50.
First Synthetic Route: General Procedure for the Preparation of N-
Heterocyclic Imidazolium (M5H–M7H, MNH2, M2F)
1) Aryl iodine (1.0 mmol), imidazole (0.081 g, 1.2 mmol), CuI (9.5 mg,
0.05 mmol), (nBu)4NBr (16 mg, 0.05 mmol), NaOH (80 mg, 2 mmol), and
DMSO (5 mL) were placed in a Pyrex glass tube and sealed, then stirred
at 1208C for 40 h. Then the reaction mixture was diluted with CH2Cl2
(30–40 mL), washed with water (3ꢂ30 mL), then dried over anhydrous
sodium sulfate. The solvent was removed under vacuum, and the residue
was purified by chromatography on silica gel to give the target product.
Rf =0.4 (PE/EtOAc=1:2).
2) CuI (38.4 mg, 0.2 mmol), K3PO4 (0.422 g, 2.0 mmol), imidazole
(0.095 g, 1.4 mmol), aryl iodine (1.0 mmol), and DMF (2 mL) were
placed in a Pyrex glass tube and sealed, then stirred at 1208C for 40 h.
Then the reaction mixture was filtered through Celite (2.5 g), the solution
was diluted with CH2Cl2 (30–40 mL), washed with water (3ꢂ30 mL), and
dried over anhydrous sodium sulfate. The solvent was removed under
vacuum, and the residue was purified by chromatography on silica gel to
give the target product. Rf =0.4 (PE/EtOAc=1:2).
1-[4-(Pyrrolidin-1-yl)phenyl]-1H-imidazole (M5H), yield 1: 56%, yield 2:
90%. 1H NMR (400 MHz, CDCl3): d=7.72 (s, 1H), 7.22 (d, J=8.9 Hz,
2H), 7.17 (d, J=6.7 Hz, 2H), 6.59 (d, J=8.9 Hz, 2H), 3.31 (t, J=6.6 Hz,
4H), 2.11–1.98 ppm (m, 4H); MS (EI) m/z: 213.2 (100); elemental analy-
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Zhuo Zeng et al.
sis calcd (%) for C13H15N3: C 73.21, H 7.09, N 19.70; found: C 73.46, H
7.16, N 19.50.
General Procedure for the Preparation of N-Heterocyclic Imidazolium
(M5F–M7F)
1-[4-(Piperidin-1-yl)phenyl]-1H-imidazole (M6H), yield 1: 57%, yield 2:
91%. 1H NMR (400 MHz, CDCl3): d=8.01 (s, 1H), 7.27 (d, J=7.4 Hz,
3H), 6.98 (d, J=7.8 Hz, 3H), 3.24–3.16 (m, 4H), 2.96 (s, 3H), 2.88 ppm
(s, 3H); MS (EI): m/z: 227.2 (100); elemental analysis calcd (%) for
C14H17N3: C 73.98, H 7.54, N 18.49; found: C 73.90, H 7.47, N 17.96.
N-Heterocyclic aniline (0.01 mol), MeOH (5 mL), and 40% aqueous
glyoxal (1.74 g, 0.012 mol) were placed in a three-necked round-bot-
tomed flask and stirred at room temperature for 16 h to form a yellow
mixture. NH4Cl (1.07 g, 0.02 mol) was added followed by 37% aqueous
formaldehyde (1.62 g, 0.02 mol). The mixture was diluted with MeOH
(40 mL) and heated to reflux for 1 h. H3PO4 (1.4 mL, 85%) was added
slowly over 10 min. The resulting mixture was then stirred at reflux for
another 8 h. After removal of the solvent under vacuum, the dark residue
was poured onto ice (30 g) and neutralized with 40% aqueous KOH so-
lution until the solution was at pH 9. The resulting mixture was extracted
with dichloromethane (3ꢂ30 mL). The organic phases were washed with
water (3ꢂ30 mL) and dried over anhydrous sodium sulfate. The solvent
was removed under vacuum, and the residue was purified by chromatog-
raphy on silica gel to give the target product. Rf =0.4 (PE/EtOAc=1:2).
1-[4-(Azepan-1-yl)phenyl]-1H-imidazole (M7H), yield 1: 56%, yield 2:
89%. 1H NMR (400 MHz, CDCl3): d=7.85 (s, 1H), 7.31–7.14 (m, 4H),
6.73 (m, 2H), 3.49 (m, 4H), 1.81 (s, 4H), 1.57 ppm (d, J=1.4 Hz, 4H);
MS (EI): m/z: 241.3 (100); elemental analysis calcd (%) for C15H19N3: C
74.65, H 7.94, N 17.41; found: C 74.58, H 7.95, N 17.42.
4-(1H-Imidazol-1-yl)aniline (MNH2), yield 1: 57%, yield 2: 90%.
1H NMR (400 MHz, DMSO): d=7.97 (s, 1H), 7.50 (s, 1H), 7.23 (d, J=
7.9 Hz, 2H), 7.03 (s, 1H), 6.65 (d, J=7.9 Hz, 2H), 5.30 ppm (s, 2H).
1-[4-(3,4-Difluoro-1H-pyrrol-1-yl)phenyl]-1H-imidazole (M2F), yield 1:
58%, yield 2: 93%. 1H NMR (400 MHz, CDCl3): d=7.85 (s, 1H), 7.46
(d, J=7.1 Hz, 2H), 7.38 (d, J=8.8 Hz, 2H), 7.27 (d, J=6.2 Hz, 2H), 7.23
(s, 2H), 6.76 ppm (d, J=1.0 Hz, 2H); 19F NMR (376 MHz, CDCl3): d=
À176.33 ppm (s, 2F); MS (EI): m/z: 245.2 (100); elemental analysis calcd
(%) for C13H9F2N3: C 63.67, H 3.70, N 17.14; found: C 63.79, H 3.80, N
16.78.
1-[4-(3,3,4,4-Tetrafluoropyrrolidin-1-yl)phenyl]-1H-imidazole
(M5F),
yield: 85%. 1H NMR (400 MHz, CDCl3): d=8.67 (s, 1H), 7.45 (d, J=
8.7 Hz, 2H), 7.38 (s, 1H), 7.31 (s, 1H), 6.65 (d, J=8.7 Hz, 2H), 3.89 ppm
(m, 4H); 19F NMR (376 MHz, CDCl3): d=À122.91 ppm (m, 4F); MS
(EI): m/z: 285.2 (100); elemental analysis calcd (%) for C13H11F4N3: C
54.74, H 3.89, N 14.73; found: C 54.69, H 3.91, N 14.52.
1-[4-(3,3,4,4,5,5-Hexafluoropiperidin-1-yl)phenyl]-1H-imidazole (M6F),
yield: 84%. 1H NMR (400 MHz, CDCl3): d=7.78 (s, 1H), 7.33 (d, J=
8.9 Hz, 2H), 7.21 (d, J=9.3 Hz, 2H), 7.05 (d, J=8.9 Hz, 2H), 3.92–
3.72 ppm (m, 4H); 19F NMR (376 MHz, CDCl3): d=À123.52 (s, 4F),
À139.27 ppm (s, 2F); MS (EI): m/z: 149.1 (100); elemental analysis calcd
(%) for C14H11F6N3: C 50.16, H 3.31, N 12.53; found: C 50.34, H 3.36, N
12.38.
Second Synthetic Route
MNH2 (0.159 g, 1 mmol), 1 f and 1h (1 mmol), triethylamine (2 mL), and
ethanol (2 mL) were placed in a Pyrex glass tube, sealed, heated at 908C
for 24 h, and then allowed to cool to room temperature. The crude prod-
uct was added to dichloromethane (50 mL), then washed with water (3ꢂ
30 mL) and dried over anhydrous sodium sulfate. The solvent was re-
moved under vacuum, and the residue was purified by chromatography
on silica gel to give the target products M5H, yield: 5%, and M5F, yield:
2%. Rf =0.3 (PE/EtOAc=1:2).
1-[4-(3,3,4,4,5,5,6,6-Octafluoroazepan-1-yl)phenyl]-1H-imidazole (M7F),
yield: 80%. 1H NMR (400 MHz, CDCl3): d=7.77 (s, 1H), 7.34 (d, J=
2.1 Hz, 2H), 7.20 (d, J=7.7 Hz, 2H), 7.03 (d, J=9.1 Hz, 2H), 4.12 ppm
(t, J=12.2 Hz, 4H); 19F NMR (376 MHz, CDCl3): d=À112.38–À112.63
(m, 4F), À128.50 ppm (s, 4F); MS (EI): m/z: 385.2 (100); elemental anal-
ysis calcd (%) for C15H11F8N3: C 46.76, H 2.88, N 10.91; found: C 47.07,
H 3.00, N 10.64.
Third Synthetic Route: General Procedure for the Preparation of
Fluorocarbon N-Heterocyclic Anilines (N5F–N7F)
Trifluoromethanesulfonic acid 2,2,3,3-tretrafluoro-1,4-butanediyl ester
(1 f), trifluoromethanesulfonic acid 2,2,3,3,4,4-hexafluoro-1,5-pentanediyl
ester (2 f), or trifluoromethanesulfonic acid 2,2,3,3,4,4,5,5-octofluoro-1,6-
hexanediyl ester (3 f) (1 mmol), p-phenylenediamine (2 mmol), Et3N
(5 mmol), and ethanol (25 mL) were placed in a round-bottomed flask
fitted with a reflux condenser and heated at reflux for 30 h. After cooling,
the solvent was removed under vacuum. The residue was added to di-
chloromethane (50 mL), then washed with water (3ꢂ20 mL) and dried
over anhydrous sodium sulfate. The solvent was removed under vacuum,
and the residue was purified by chromatography on silica gel to give the
target product. Rf =0.3–0.4 (PE/CH2Cl2 =1:1).
General Procedure for the Preparation of N-Heterocyclic Imidazolium-
Based ILCs ((5–7)H-14, (5–6)H-18, 2F-14/18, (5–7)F-14/18)
N-Heterocyclic imidazole (1 mmol) and 1-bromotetradecane (1.386 g,
5 mmol) or 1- bromohexadecane bromide (1.661 g, 5 mmol) and acetoni-
trile (8 mL) were placed in a Pyrex glass tube and sealed, then stirred at
1208C for 18 h. The solvent was removed under vacuum, and the residue
was purified by chromatography on silica gel to give the target product.
Rf =0.2–0.4 (EtOAc/MeOH=10:1).
1-Tetradecyl-3-[4-(pyrrolidin-1-yl)phenyl]-1H-imidazolium bromide (5H-
14), yield: 82%. 1H NMR (400 MHz, CDCl3): d=10.76 (s, 1H), 7.53 (d,
J=9.0 Hz, 2H), 7.45 (d, J=1.6 Hz, 1H), 7.39 (s, 1H), 6.60 (d, J=9.0 Hz,
2H), 4.56 (t, J=7.4 Hz, 2H), 3.39–3.24 (m, 4H), 2.11–2.00 (m, 4H), 2.02–
1.90 (m, 2H), 1.45–1.15 (m, 22H), 0.88 ppm (t, J=6.9 Hz, 3H); MS
(ESI): m/z: [MÀBrÀ]+; elemental analysis calcd (%) for
C27H44BrN3·0.5H2O: C 64.91, H 9.08, N 8.41; found: C 65.19, H 8.94, N
8.35.
4-(3,3,4,4-Tetrafluoropyrrolidin-1-yl) aniline (N5F), yield: 93%. 1H NMR
(400 MHz, CDCl3): d= 6.73–6.66 (m, 2H), 6.47–6.36 (m, 2H), 3.73 (m,
4H), 3.59–2.51 ppm (br, 2H); 19F NMR (376 MHz, CDCl3): d=À123.04–
À123.39 ppm (m, 4F); MS (ESI): m/z: 235.53 [M+H]+; elemental analy-
sis calcd (%) for C10H10F4N2: C 51.29, H 4.30, N 11.96; found: C,50.77, H
4.48, N 11.79.
4-(3,3,4,4,5,5-Hexafluoropiperidin-1-yl)aniline (N6F), yield: 89%.
1H NMR (400 MHz, CDCl3): d=6.87–6.78 (m, 2H), 6.70–6.61 (m, 2H),
3.65–3.42 ppm (m, 4H); 19F NMR (376 MHz, CDCl3): d=À123.32 (s,
4F), À138.88–À140.68 ppm (m, 2F); MS (EI): m/z: 284.2 (100); elemen-
tal analysis calcd (%) for C11H10F6N2: C 46.49, H 3.55, N 9.86; found: C
46.84, H 3.55, N 9.85.
1-Octadecyl-3-[4-(pyrrolidin-1-yl)phenyl]-1H-imidazolium bromide (5H-
18), yield: 80%. 1H NMR (400 MHz, CDCl3): d=10.80 (s, 1H), 7.52 (d,
J=9.0 Hz, 2H), 7.41 (s, 1H), 7.33 (s, 1H), 6.61 (d, J=7.2 Hz, 2H), 4.57
(t, J=7.4 Hz, 2H), 3.31 (t, J=6.6 Hz, 4H), 2.22–1.84 (m, 6H), 1.45–1.20
(m, 32H), 0.88 ppm (t, J=6.8 Hz, 3H); MS (ESI): m/z: 466.2 [MÀBrÀ]+;
elemental analysis calcd (%) for C31H52BrN3·H2O: C 65.94, H 9.64, N
7.44; found: C 66.39, H 9.35, N 7.42.
4-(3,3,4,4,5,5,6,6-Octafluoroazepan-1-yl)aniline (N7F), yield: 86%.
1H NMR (400 MHz, CDCl3): d=6.82 (d, J=8.5 Hz, 2H), 6.65 (d, J=
8.6 Hz, 2H), 3.89 (t, J=12.7 Hz, 4H), 2.63 ppm (br, 2H); 19F NMR
(376 MHz, CDCl3): d=À113.30 (s, 4F), À128.40 ppm (s, 4F); MS (ESI):
m/z: 335.58 [M+H]+; elemental analysis calcd (%) for C12H10F8N2: C
43.13, H 3.02, N 8.38; found: C 42.98, H 3.38, N 8.07.
1-Tetradecyl-3-[4-(piperidin-1-yl)phenyl]-1H-imidazolium bromide (6H-
14), yield: 78%. 1H NMR (400 MHz, CDCl3): d=10.75 (s, 1H), 7.60 (d,
J=9.1 Hz, 3H), 7.50 (d, J=6.7 Hz, 1H), 7.01 (d, J=8.8 Hz, 2H), 4.55 (t,
J=7.2 Hz, 2H), 3.35–3.14 (m, 4H), 2.05–1.87 (m, 2H), 1.67 (dd, J=26.2,
4.6 Hz, 6H), 1.43–1.17 (m, 22H), 0.88 ppm (t, J=6.8 Hz, 3H); MS (ESI):
m/z: 424.2 [MÀBrÀ]+; elemental analysis calcd (%) for C28H46BrN3·H2O:
C 64.35, H 9.26, N 8.04; found: C 64.81, H 8.92, N 7.89.
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1-Octadecyl-3-[4-(piperidin-1-yl)phenyl]-1H-imidazolium bromide (6H-
18), yield: 77%. 1H NMR (400 MHz, CDCl3): d=10.91 (s, 1H), 7.64 (d,
J=14.0 Hz, 2H), 7.50 (s, 1H), 7.35 (d, J=2.0 Hz, 1H), 7.13 (s, 2H), 4.57
(t, J=8.6 Hz, 2H), 3.40 (m, 4H), 3.33–3.24 (m, 4H), 2.03–1.93 (m, 3H),
1.76 (s, 5H), 1.39 (m, 20H), 0.88 ppm (t, J=6.7 Hz, 9H); MS (ESI): m/z:
480.2 [MÀBrÀ]+; elemental analysis calcd (%) for C32H54BrN3·H2O: C
64.41, H 9.75, N 7.26; found: C 64.09, H 9.38, N 7.32.
11.13 (s, 1H), 7.75 (d, J=9.1 Hz, 2H), 7.53 (s, 1H), 7.36 (s, 1H), 7.10 (d,
J=9.2 Hz, 2H), 4.54 (t, J=7.4 Hz, 2H), 4.16 (t, J=12.0 Hz, 4H), 2.03–
1.92 (m, 2H), 1.42–1.22 (m, 22H), 0.88 ppm (t, J=6.8 Hz, 3H); 19F NMR
(376 MHz, CDCl3): d=À112.24 (m, 4F), À128.48 ppm (s, 4F); MS (ESI):
m/z: 582.2 [MÀBrÀ]+; elemental analysis calcd (%) for
C29H40BrF8N3·H2O: C 51.18, H 6.22, N 6.17; found: C 50.99, H 6.07, N
5.91.
3-Octadecyl-1-[4-(3,3,4,4,5,5,6,6-octafluoroazepan-1-yl)phenyl]-1H-imida-
zolium bromide (7F-18), yield: 78%. 1H NMR (400 MHz, CDCl3): d=
11.04 (s, 1H), 7.75 (d, J=9.0 Hz, 2H), 7.57 (s, 1H), 7.39 (s, 1H), 7.09 (d,
J=9.0 Hz, 2H), 4.53 (t, J=7.4 Hz, 2H), 4.23 (d, J=8.7 Hz, 4H), 2.03–
1.92 (m, 3H), 1.39–1.22 (m, 29H), 0.91–0.87 ppm (m, 3H); 19F NMR
(376 MHz, CDCl3): d=À112.26 (t, J=13.2 Hz, 4F), À128.48 ppm (s, 4F);
MS (ESI): m/z: 638.2 [MÀBrÀ]+; elemental analysis calcd (%) for
C33H48BrF8N3·2H2O: C 52.52, H 6.95, N 5.57; found: C 52.82, H 6.83, N
5.91.
1-Tetradecyl-3-[4-(azepan-1-yl)phenyl]-1H-imidazolium bromide (7H-14),
yield: 74%. 1H NMR (400 MHz, CDCl3): d=10.76 (s, 1H), 7.55 (d, J=
8.3 Hz, 2H), 7.48 (s, 1H), 7.38 (s, 1H), 6.82 (d, J=8.7 Hz, 2H), 4.56 (t,
J=7.8 Hz, 2H), 3.71–3.26 (m, 4H), 2.02–1.92 (m, 3H), 1.80 (s, 6H), 1.41–
1.23 (m, 23H), 0.88 ppm (t, J=6.4 Hz, 3H); 13C NMR (101 MHz,
CDCl3): d=151.31, 134.78, 122.82, 122.37, 120.43, 111.85, 105.10, 50.28,
49.50, 31.88, 30.37, 29.89–28.72 (multicarbon in alkyl chain), 27.14, 26.81,
26.22, 22.62, 14.06 ppm; MS (ESI): m/z: 438.2 [MÀBrÀ]+; HRMS (EI):
m/z calcd for C29H48N3: 438.3853; found: 438.3843.
1-Tetradecyl-3-[4-(3,4-difluoro-1H-pyrrol-1-yl)phenyl]-1H-imidazolium
bromide (2F-14), yield: 83%. 1H NMR (400 MHz, CDCl3): d=11.33 (s,
1H), 7.95 (d, J=8.8 Hz, 2H), 7.60 (s, 1H), 7.48 (d, J=8.8 Hz, 2H), 7.40
(s, 1H), 6.79 (s, 2H), 4.56 (t, J=9.0 Hz, 2H), 2.07–1.95 (m, 2H), 1.45–
1.21 (m, 22H), 0.88 ppm (t, J=6.7 Hz, 3H); 19F NMR (376 MHz, CDCl3):
d=À175.07 ppm (s, 1F); MS (ESI): m/z: 442.1 [MÀBrÀ]+; elemental
analysis calcd (%) for C27H38BrF2N3·2H2O: C 58.06, H 7.58, N 7.52;
found: C 58.28, H 7.19, N 7.19.
General Procedure for the Preparation of N-Heterocyclic Imidazolium
ILCs (5F-14/18)
A solution of 1-tetradecyl-3-[4-(3,3,4,4-tetrafluoropyrrolidin-1-yl)phenyl]-
1H-imidazolium bromide (562.52 mg, 1 mmol) or 1-octadecyl-3-[4-
(3,3,4,4-tetrafluoropyrrolidin-1-yl)phenyl]-1H-imidazolium
bromide
(618.63 mg, 1 mmol) and acetone (8 mL) was placed in a Pyrex glass tube
and sealed, then heated to 608C. NaBF4 (219.58 mg, 2 mmol) was added
and the reaction mixture was heated to reflux for 3 h. Inorganic salts
were filtered off and washed with acetone. The solvent was removed
under vacuum, and the residue was purified by chromatography on silica
gel to give the target product. Rf =0.2–0.4 (EtOAc/MeOH=10:1).
1-Octadecyl-3-[4-(3,4-difluoro-1H-pyrrol-1-yl)phenyl]-1H-imidazolium
bromide (2F-18), yield: 84%. 1H NMR (400 MHz, CDCl3): d=11.17 (s,
1H), 7.95 (d, J=8.9 Hz, 2H), 7.76–7.54 (m, 1H), 7.46 (d, J=4.9 Hz, 3H),
6.79 (s, 2H), 4.54 (d, J=5.6 Hz, 2H), 2.20–1.87 (m, 2H), 1.57–1.04 (m,
30H), 0.88 ppm (t, J=6.7 Hz, 3H); 19F NMR (376 MHz, CDCl3): d=
À175.12 ppm (d, J=10.8 Hz, 2F); MS (ESI): m/z: 498.1 [MÀBrÀ]+; ele-
mental analysis calcd (%) for C31H46BrF2N3·2H2O: C 60.58, H 8.20, N
6.84; found: C 60.34, H 7.86, N 6.50.
1-Tetradecyl-3-[4-(3,3,4,4-tetrafluoropyrrolidin-1-yl)phenyl]-1H-imidazoli-
um fluoroborate (5F-14B), yield: 82%. 1H NMR (400 MHz, CDCl3): d=
9.13 (s, 1H), 7.53 (d, J=8.9 Hz, 3H), 7.43 (s, 1H), 6.64 (d, J=5.3 Hz,
2H), 4.32 (t, J=6.3 Hz, 2H), 3.84 (t, J=8.8 Hz, 4H), 2.00–1.84 (m, 2H),
1.40–1.12 (m, 22H), 0.88 ppm (m, 3H); 19F NMR (376 MHz, CDCl3): d=
À119.53–À126.19 (m, 4F), À151.19 ppm (d, J=19.6 Hz, 4F); MS (ESI):
m/z: 482.6 [MÀBrÀ]+; elemental analysis calcd (%) for C27H40BF8N3: C
56.95, H 7.08, N 7.38; found: C 56.52, H 7.53, N 7.54.
1-Tetradecyl-3-[4-(3,3,4,4-tetrafluoropyrrolidin-1-yl)phenyl]-1H-imidazoli-
um bromide (5F-14), yield: 86%. 1H NMR (400 MHz, CDCl3): d=11.09
(s, 1H), 7.74 (d, J=8.9 Hz, 2H), 7.49 (s, 1H), 7.36 (s, 1H), 6.67 (d, J=
8.8 Hz, 2H), 4.55 (t, J=7.4 Hz, 2H), 3.92–3.82 (m, 4H), 1.99 (m, 2H),
1.43–1.23 (m, 22H), 0.88 ppm (t, J=6.8 Hz, 3H); 19F NMR (376 MHz,
CDCl3): d=À123.16 ppm (m, 4F); MS (ESI): m/z: 482.1 [MÀBrÀ]+; ele-
mental analysis calcd (%) for C27H40BrF4N3·H2O: C 55.86, H 7.29, N
7.24; found: C 56.17, H 6.98, N 7.12.
1-Octadecyl-3-[4-(3,3,4,4-tetrafluoropyrrolidin-1-yl)phenyl]-1H-imidazoli-
um fluoroborate (5F-18B), yield: 85%. 1H NMR (400 MHz, CDCl3): d=
9.18 (s, 1H), 7.55 (m, 4H), 6.67 (m, 2H), 4.35 (m, 2H), 3.88 (t, J=14.4,
4H), 1.95 (d, J=20.0 Hz, 3H), 1.51–1.13 (m, 33H), 0.97–0.80 ppm (m,
3H); 19F NMR (376 MHz, CDCl3): d=À120.64–À125.87 (m, 4F),
À151.01 ppm (t, J=18.8 Hz, 4F); MS (ESI): m/z: 538.8 [MÀBrÀ]+; ele-
mental analysis calcd (%) for C31H48BF8N3: C 59.52, H 7.73, N 6.72;
found: C 59.82, H 7.52, N 6.54.
1-Octadecyl-3-[4-(3,3,4,4-tetrafluoropyrrolidin-1-yl)phenyl]-1H-imidazoli-
um bromide (5F-18), yield: 84%. 1H NMR (400 MHz, CDCl3): d=11.10
(s, 1H), 7.74 (d, J=8.9 Hz, 2H), 7.49 (s, 1H), 7.36 (s, 1H), 6.67 (d, J=
8.8 Hz, 2H), 4.55 (t, J=7.4 Hz, 2H), 3.88 (m, 4H), 2.07–1.90 (m, 2H),
1.44–1.19 (m, 30H), 0.88 ppm (t, J=6.8 Hz, 3H); 19F NMR (376 MHz,
CDCl3): d=À123.16 ppm (m, 4F); MS (ESI): m/z: 538.3 [MÀBrÀ]+; ele-
mental analysis calcd (%) for C31H48BrF4N3: C 60.19, H 7.82, N 6.79;
found: C 59.82, H 7.71, N 6.55.
Acknowledgements
1-Tetradecyl-3-[4-(3,3,4,4,5,5-hexafluoropiperidin-1-yl)phenyl]-1H-imida-
zolium bromide (6F-14), yield: 84%. 1H NMR (400 MHz, CDCl3): d=
10.98 (s, 1H), 7.78 (d, J=9.0 Hz, 2H), 7.65 (s, 1H), 7.46 (s, 1H), 7.10 (d,
J=9.0 Hz, 2H), 4.52 (t, J=7.4 Hz, 2H), 3.88 (t, J=7.5 Hz, 4H), 2.03–1.92
(m, 2H), 1.30 (m, 22H), 0.88 ppm (t, J=6.8 Hz, 3H); 19F NMR
(376 MHz, CDCl3) d=À123.63 (s, 4F), À139.11 ppm (s, 2F); MS (ESI):
m/z: 532.2 [MÀBrÀ]+; elemental analysis calcd (%) for
C28H40BrF6N3·H2O: C 53.33, H 6.71, N 6.66; found: C 53.87, H 6.51, N
6.61.
The authors gratefully acknowledge the support of the National Natural
Science Foundation of China (21272080), the Department of Science and
Technology, Guangdong Province (2010A020507001-76, 5300410, FIPL-
05-003), and the Scientific Research Foundation for the Returned Over-
seas Chinese Scholars, State Education Ministry.
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zolium bromide (7F-14), yield: 80%. 1H NMR (400 MHz, CDCl3): d=
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Received: May 28, 2014
Published online: September 26, 2014
Chem. Asian J. 2014, 9, 3418 – 3430
3430
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