Ion complexation-controlled columnar mesophase of calix[4]arene-cholesterol derivatives with Schiff-base bridges

Article abstract of DOI:10.1016/j.tetlet.2016.01.047

Two novel calix[4]arene-cholesterol derivatives 7a and 7b with Schiff-base bridges were synthesized in yields of 70-80%. Their structural and conformational characterization had been achieved by NMR, MS, and elemental analysis. Their mesomorphic behaviors were studied by polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. They possess mesomorphic properties with the molecular arrangement of the calixarene bowlic column and Schiff-based cholesterol unit as ancillary lateral column. The complexes of 7a and 7b with AgClO₄ showed no mesophase. These results suggested that the mesophase of compounds 7a and 7b could be tuned by the ion-complexation behavior.

Full text of DOI:10.1016/j.tetlet.2016.01.047

Tetrahedron Letters 57 (2016) 905–909
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Ion complexation-controlled columnar mesophase
of calix[4]arene–cholesterol derivatives with Schiff-base bridges
a
Xiaoyi Zhang a, Hongyu Guo a, Fafu Yang a,b, , Jin Yuan
⇑
a College of Chemistry and Chemical Engineering, Fujian Normal University, Fuzhou 350007, PR China
b Fujian Key Laboratory of Polymer Materials, Fuzhou 350007, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel calix[4]arene–cholesterol derivatives 7a and 7b with Schiff-base bridges were synthesized in
yields of 70–80%. Their structural and conformational characterization had been achieved by NMR, MS,
and elemental analysis. Their mesomorphic behaviors were studied by polarizing optical microscopy, dif-
ferential scanning calorimetry, and X-ray diffraction. They possess mesomorphic properties with the
molecular arrangement of the calixarene bowlic column and Schiff-based cholesterol unit as ancillary lat-
eral column. The complexes of 7a and 7b with AgClO4 showed no mesophase. These results suggested
that the mesophase of compounds 7a and 7b could be tuned by the ion-complexation behavior.
Ó 2016 Elsevier Ltd. All rights reserved.
Received 18 November 2015
Revised 9 January 2016
Accepted 13 January 2016
Available online 14 January 2016
Keywords:
Calixarene
Cholesterol
Synthesis
Mesophase
Complexation
Liquid crystal (LC) materials have gained broad researchers’
interests due to the diverse physical properties for both basic
research and applied sciences, such as organic photovoltaic cells,
organic light-emitting diodes, organic field-effect transistors, gas
sensors.1–4 Recently, the macrocycle-based liquid crystals have
attracted much attention because the mesomorphic property could
be tuned by the complexation behavior of macrocycle unit. For
examples, Cammidge and his co-workers reported the triph-
enylene dimers containing crown ether macrocycle with interest-
ing mesomorphic properties.5,6 Laschat’s group and Peng’s group
synthesized a series of crown ether–triphenylene liquid crystals
and studied the relationship of mesomorphic properties and com-
plexation abilities, respectively.7–13 A cyclodextrin–triphenylene
derivative was also prepared lately and it showed interesting
mesomorphic properties.14
It was well known that, after crown ether and cyclodextrin, cal-
ixarenes were seen as the important supramolecular platforms to
construct all kinds of calixarene derivatives with unique proper-
ties.15–18 Two kinds of calixarene liquid crystals were also reported
up to now.18 One type of calixarene liquid crystal with bowl meso-
phase was synthesized by introducing long alkyl chains on cal-
ixarene skeleton.19–25 Another type of calixarene liquid crystal
was obtained by grafting the triphenylene units onto calixarene
skeleton,26–29 and they were columnar liquid crystals in most
cases. These researches suggested that the complexation behavior
of calixarene liquid crystals influenced greatly on the mesomorphic
properties, such as the transformation or the disappearance of
mesomorphic behavior after complexation. Lately, we synthesized
the novel cholesterol–calixarene liquid crystal and found that their
mesomorphic properties were influenced by the number of sub-
stituents and the structure of bridging chains, but no complexation
behavior was concerned.30 In the present study, two novel calix[4]
arene–cholesterol derivatives with Schiff-base bridges were
designed and synthesized. Moreover, the influences of complexa-
tion behaviors on the mesomorphic properties were investigated
for the first time. The results implied that the liquid crystalline
behaviors could be tuned by the ion complexation.
As the Schiff-base bond possesses good complexation abilities
for soft metal cations, the calix[4]arene–cholesterol derivatives
with Schiff-base bridges were designed as target molecules. The
synthetic routes were illustrated in Scheme 1. According to the lit-
erature method,31 compound 1 was synthesized by the etherifica-
tion of paranitrophenol with chlorohydrins and following
esterification with TsCl. Further, calix[4]arene-1,3-bis-nitro deriva-
tive 2 was prepared by treating compound 1 with calix[4]arene.
Subsequently, compound 2 was deoxidized by hydrazine hydrate
with Pd/C as catalyst and calix[4]arene-1,3-bis-amino derivative
3 was obtained in a yield of 72% after purification with column
chromatography. On the other hand, by reacting cholesterol with
2-chloroacetic acid or 4-chlorobutanoic acid in CH2Cl2 using N,N0-
dicyclohexylcarbodiimide (DCC) and 4-dimethylaminopyridine
⇑
Corresponding author. Tel./fax: +86 591 83465225.
E-mail address: yangfafu@fjnu.edu.cn (F. Yang).
http://dx.doi.org/10.1016/j.tetlet.2016.01.047
0040-4039/Ó 2016 Elsevier Ltd. All rights reserved.

906
X. Zhang et al. / Tetrahedron Letters 57 (2016) 905–909
NO2
NO2
corresponding signals for compounds 7a and 7b. For example, the
NO2
OH
peaks at 160–170 ppm and 60–77 ppm were assigned for C@O,
C@N, OCH, and OCH2, respectively. These data also supported the
structures and conformations of compounds 7a and 7b again.
The liquid crystalline properties of compounds 7a and 7b were
firstly studied by differential scanning calorimetry (DSC). The
results were listed in Figure 1 and Table 1. Both compounds 7a
and 7b exhibited two thermic peaks on second heating and cool-
ing. For compound 7a, two endothermic peaks at 77.4 °C and
117.7 °C were observed on second heating. Also, two exothermic
peaks at 76.2 °C and 105.7 °C appeared on cooling. As for com-
pound 7b, the corresponding endothermic (or exothermic) peaks
were at 75.6 (or 71.7) °C and 118.3 (or 102.9) °C upon heating (or
cooling), respectively. Judging from the heats of fusion between
corresponding crystallization and melting peaks, two phase transi-
tions of solid state–mesophase and mesophase–isotropic phase
could be deduced for the processes of cooling and heating of com-
pounds 7a and 7b. These deductions were also further confirmed
by polarizing optical microscopy (POM). The hysteresis behaviors
observed for compounds 7a and 7b could be attributed to super-
cooling, which was a common phenomenon for such viscous mate-
rials. Comparing with the previous calix[4]arene–cholesterol liquid
crystals with soft alkyl bridging chains,30 compounds 7a and 7b
showed higher phase transfer temperatures and smaller tempera-
ture scopes of mesophase. These phenomena could be ascribed to
the influences of the rigid Schiff-base bridging chains on mesomor-
phic properties.
Furthermore, POM was used to investigate the mesophase tex-
tures of compounds 7a and 7b based on the DSC results. The two
phase transitions of solid state–mesophase and mesophase–isotro-
pic phases were clearly observed on heating and cooling. The phase
transition temperatures were approximately in agreement with
the temperatures of thermic peaks in DSC curves. The mesomor-
phic textures of compounds 7a and 7b at 90 °C are exhibited in
Figure 2. It could be seen that the clear special textures were
observedupon cooling for compounds 7a and 7b, respectively. These
textureswere the typical focal-conic columnar liquid crystals, which
were further confirmed by X-ray diffraction (XRD) analysis.
TsCl
OH
KOH, H2O
Cl
O
O
HO
OH
OH
OH
OH
1
OTs
K2CO3, CH3CN
.
H2O,
Pd-C
N2H4
O
O
O
OH
OH
O
O
O
O
OH
OH
EtOH
O
NO2
2
NO2
3
NH2
NH2
H3C
H3C
H
H
H3C
H
O
OH
O
OH
HO
4
O
O
O
Cl
DCC, DMAP, CH2Cl2
OH
n
N
H3C
H3C
N
HC
CH
H
H3C
O
H
H
Cl
O
n O
O
n
n
5a (n = 1 )
5b (n = 3 )
O
O
O
O
CH3
K2CO3, MeCN
CHO
OHC
OH
H
CH3
H
H
H
H
H
H3C
CH3
CH3
H3C
CH3
CH3
3
MeOH/CHCl3
H
H3C
Figure 3 showed the XRD traces of the mesomorphic states of
compounds 7a and 7b at 90 °C. They exhibited similar peaks with
a long range orientation (2h = 2.7–3.0°, sharp), and two short disor-
dered orientations (2h = 13–20°, broad and 2h = 23°, sharp). These
XRD traces indicated the bowlic columnar liquid crystalline behav-
iors. The peaks at 2h = 2.83° and 2.93° suggested the distances of
31.19 Å and 30.13 Å, respectively. They were in accordance with
7a (n = 1 )
7b (n = 3 )
O
n
H
H
O
O
6a (n = 1 )
6b (n = 3 )
Scheme 1. synthetic routes of compounds 7a and 7b.
(DMAP) as catalysts, the cholesterol chlorinated derivatives 5a and
5b were synthesized in yields of 90% and 88%, respectively. By
treating compounds 5a or 5b with p-hydroxybenzaldehyde in
K2CO3/MeCN system, cholesterol derivatives 6a and 6b were pre-
pared after recrystallization in CHCl3/MeOH in yields of 80% and
75%, respectively. Finally, target compounds 7a and 7b were pre-
pared by refluxing compound 3 and compound 6a or 6b in
MeOH/CHCl3 in ‘1 + 2’ Schiff-base condensation mode. The yields
of compounds 7a and 7b were as high as 74% and 77% after flash
column chromatography, respectively.
The structures of all new compounds were confirmed by ele-
ment analyses, ESI-MS spectra and NMR spectra. In the ESI-MS
spectra, corresponding molecular ion peaks at 1980.6 and 2036.3
were observed for 7a and 7b, indicating the accomplishment of
Schiff-base condensation. In their 1H NMR spectra, two singlets
(1:1) for the tert-butyl groups, one pair of doublets (1:1) for the
methylene bridges and two singlets (1:1) for ArH suggested the
cone conformation for calix[4]arene units. Only one peak for CH@N
groups implied that they might be trans-structures based on the
stabilities of molecules. Moreover, their 13C NMR spectra exhibited
cooling
2.0
7a
Iso
LC
Cr
1.5
1.0
7b
Iso
LC
0.5
Cr
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
Cr
Cr
7b
7a
LC
Iso
Iso
LC
second heating
50
100
150
200
Temperature (oC)
Figure 1. The DSC traces of compounds 7a and 7b on second heating and cooling
(scan rate 8 °C minꢀ1).

X. Zhang et al. / Tetrahedron Letters 57 (2016) 905–909
907
Table 1
Transition temperatures (°C) and enthalpies (kJ molꢀ1) of compounds 7a and 7b
4000
Compd
Phase
T(
D
H)
T(DH)
cooling scan
transitiona
heating scan
3000
2000
1000
0
7a
7b
Cr–Col(Col–Cr)
Col–Iso(Iso–Col)
Cr–Col(Col–Cr)
Col–Iso(Iso–Col)
77.4(12.23)
117.7(13.89)
75.6(11.45)
118.3(14.07)
76.2(13.98)
105.7(13.14)
71.7(12.87)
102.9(13.66)
a
Cr = crystalline, Col = columnar phase, Iso = isotropic.
7a
7b
the diameters of compounds 7a and 7b approximately, implying
the lattice constant arising from the columnar structures. The
broad halos at 13–20° (4.4–6.8 Å) agreed to the average distances
of columnar structures of the molten cholesterol units. The peaks
at 23.4° (3.8 Å) and 24.3° (3.7 Å) were assigned to the intracolum-
nar order of calixarene units and Schiff-base structures, respec-
tively. These XRD traces and analyses were in accordance with
the previous calix[4]arene bowlic columnar liquid crystals with ali-
phatic chains.26–30,32–36 Thus, the proposed columnar layered
molecular arrangements were illustrated in Figure 4 (from a to
b). However, the specific columnar structure (such as hexagonal
or tetragonal columnar) was difficult to be deduced due to the
absence of other distinct XRD peaks, which might suggest the
mixed columnar phases with a low order among the liquid crystal
columns.
0
20
40
60
Scattering angle 2 (degree)
Figure 3. XRD traces of compounds 7a and 7b measured at 90 °C.
3.8Å
3.7Å
In order to investigate the influence of the complexation behav-
iors on the mesomorphic properties, the ionic complexes of 7a and
7b were prepared and their mesomorphic properties were studied.
Considering the effective binding abilities of Schiff-base groups for
soft metallic cations and the hydrogen-bonding action of OH
groups for oxoanions, the complexes of 7a and 7b with AgClO4
were prepared according to the procedure reported by Pedersen
and modified by Laschat.37,38 Typically, the AgClO4 (2 equiv) in
MeCN was added to the solution of compound 7a or 7b (1 equiv)
in CH2Cl2. The resulting slurry was vigorously stirred overnight.
Then the solvent was evaporated. The residue was taken up in boil-
ing CH2Cl2 and the solution was filtered. The filtrate was concen-
trated and dried under vacuum to afford the respective complex
in almost quantitative yields. The elemental analysis of complexes
confirmed the 1:1 molar ratios of hosts/guests. The partial 1H NMR
spectrum of complex of 7a with AgClO4 was illustrated in Figure 5.
The obvious shift was observed for OH, ArH, and CH@N groups,
indicating that the weak interactions between Ag+ and C@N
groups, and the hydrogen bond between ClOꢀ4 and OH groups were
responsible for the formation of the complex. The possible complex
mode was proposed in Figure 5. After the two complexes were pre-
pared, the mesomorphic properties of complexes were investi-
4.4~6.8Å
30~32Å
(b)
ion
(a)
ion
(c)
= calix[4]arene skeleton
= cholesterol unit
= Schiff-base unit
Figure 4. Schematic representation of the columnar layered molecular arrange-
ments and the complexation models.
gated by DSC and POM. The DSC results upon the second heating
of the two complexes are exhibited in Figure 6. Only one peak of
phase transition was observed in DSC curves. Due to the influence
of the bulky organic groups, different orientation of cholesterol
units (two cholesterol units in complexes were closer than that
in neat compounds), and the weak interactions between hosts
Figure 2. Mesomorphic textures of compounds 7a and 7b obtained under POM on cooling at 90 °C (ꢁ400).

908
X. Zhang et al. / Tetrahedron Letters 57 (2016) 905–909
Figure 5. 1H NMR spectra of triads 7a and its 1:1 complex with AgClO4.
0.4
0.2
inorganic ions. We had tried to investigate more complexes with
the other metal ions (K+ and Hg2+) or anion (NOꢀ3 ), but no 1:1 com-
plex was obtained and the structure of complex was difficult to be
elucidated.
7a with AgClO4
7b with AgClO4
0.0
In conclusion, two calix[4]arene–cholesterol derivatives 7a and
7b with Schiff-base bridges were synthesized in yields of 70–80%.
Their structures and conformation were characterized by NMR, MS,
and elemental analysis. Their mesomorphic behaviors were stud-
ied by DSC, POM, and XRD. They possess mesomorphic properties
with the molecular arrangement of the calixarene bowlic column
and Schiff-based cholesterol units as ancillary lateral columns.
The complexes of 7a and 7b with AgClO4 were prepared and exhi-
bit no mesophase. These results implied that the mesomorphic
properties of compounds 7a and 7b could be tuned by the com-
plexation behaviors of compounds 7a and 7b. The studies on the
complexation properties for other organic guests and their influ-
ences on mesomorphic properties will be investigated in the fol-
lowing work.
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
0
50
100
150
200
Temperature (oC)
Figure 6. DSC traces of complexes 7a(7b) with AgClO4 on second heating (scan rate
10 °C minꢀ1).
Acknowledgments
Financial support from the National Natural Science Foundation
of China (No: 21406036), Fujian Natural Science Foundation of
China (No. 2014J01034), Fujian Province Science and Technology
Key Project (2014N0025) and the Program for Innovative Research
Team in Science and Technology in Fujian Province University were
greatly acknowledged.
and guests (the small shifts in 1H NMR suggest the weak interac-
tion), the melting points of complexes were only slight higher than
that of neat compounds 7a and 7b. Moreover, under the POM
observation, no mesophase was seen upon heating and cooling,
although solid phase–isotropic phase transition could be observed.
These results indicated that these complexes had no mesophase.
The disappearance of the mesomorphic property after complexa-
tion might be explained by the fact that the binding of AgClO4
not only enhanced the rigid structures based on the action
between hosts and guests, but also produced the obstacle for
columnar stacking of Schiff-base groups and cholesterol units as
shown in Figure 4c. On the other hand, after the inorganic ions in
complex 7a were removed adequately by extraction with water,
the neat compound 7a could be obtained again. The 1H NMR spec-
trum and phase transition temperatures in DSC curves were the
same as that of initial compound 7a. These results suggested the
liquid crystal behavior could be reinstated after the removal of
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2016.01.
047.
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