Synthesis and self-assembly of bent core polycatenar mesogens with binding selectivity to Hg2+

Article abstract of DOI:10.1039/c7nj00225d

Bent core polycatenar mesogens, consisting of a methylene central core and two 1,2,3-triazole dendronic wings, have been synthesized via a copper-catalyzed azide-alkyne click reaction and investigated by polarizing microscopy, DSC, XRD scattering, SEM and photoluminescence measurements. All these compounds can self-assemble into thermotropic micellar liquid crystalline phases in the bulk state and form organogels in various organic solvents. They also show binding selectivity to Hg²⁺ among a series of cations in CH₃CN-CH₂Cl₂ solution.

Full text of DOI:10.1039/c7nj00225d

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Synthesis and selfꢀassembly of bent core polycatenar mesogens with
binding selectivity to Hg2+
Huifang Cheng,‡a Ruilin Zhang,‡ab Taihao Li,a Xiongwei Peng,a Meng Xia,a Yulong
Xiao,a Xiaohong Cheng*a
[a] Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry
Department, Yunnan University
Kunming,
Yunnan 650091,
P. R. China
Fax: (+86) 871 65032905
Eꢀmail: xhcheng@ynu.edu.cn
[b] Forensic Medicine of Kunming Medical University, Kunming 650500, P. R.
China
‡Both authors contributed equally to this work
Abstract: Bent core polycatenar mesogens, consisting of a methylene central core,
and two 1,2,3ꢀtriazole dendronic wings, have been synthesized via copperꢀcatalyzed
azide–alkyne click reaction and investigated by polarizing microscopy, DSC, XRD
scattering, SEM and photoluminescence measurements. All these compounds can
selfꢀassemble into thermotropic micellar liquid crystalline phases in bulk state and
form organogels in various organic solvents. They also show binding selectivity to
Hg2+ among a series of cations in CH3CNꢀCH2Cl2 solution.
1. Introduction
Bent shaped molecules and polycatenar molecules are two kinds of nonconventional
liquid crystals (LCs) which have been intensively studied recently because of their
ability to form new mesogenic nanostructures1 that are known as novel functional
materials.2,3 The achiral bent shaped molecules with a bent angle of 105ꢀ140° can
form chiral supramolecular structures which could be potentially used as ferroelectric
materials.4,5 Polycatenar molecules6 consisting of a long aromatic rod core and more
than two flexible terminal chains can form nematic, smectic, bicontinuous cubic,
columnar mesophases7,8,9,10 and in few cases, micellar cubic phases.11,12,13

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Thermotropic micellar cubic phases based on the selfꢀassembly of spherical
supramolecular aggregates have received considerable attention, because of their
complex three dimensional structures,14 and of their potential applications in different
fields of nanotechnology.15,16 The first formation of thermotropic micellar cubic
phases was reported in the case of conical amphiphiles 17 and their dendritic
analogues.18 Although these mesophases have been later found in several different
types of taperꢀshaped and dendritic amphiphilic, starꢀshaped, bananaꢀshaped, rodꢀlike,
discꢀlike molecules based on the nanosegregation,19,20 the design and controlled
formation of these micellar cubic phases is still a challenge.
Click chemistry, i.e., Cu(I)ꢀcatalyzed azide–alkyne cycloaddition (CuAAC)21,22
represents one of the most efficient approaches for interconnecting smaller units to
larger entities. The resulting 1,2,3ꢀtriazole can be used as the aromatic component of
the extended aromatic blocks, enhancing nanoscale assembly of the resulting
compounds into ordered structures such as LC phases or gel phases, etc. Additionally,
the 1,2,3ꢀtriazole can act as nitrogen (N)ꢀdonor allowing chelation with heavy
transition metal ions.23 The organogels have great potential for many applications in
different fields24 such as stimuliꢀresponsive materials,25 electronics,26 catalysis,27
templated synthesis,28 pollution control.29 Furthermore, the recognition of toxic
heavyꢀ and transitionꢀmetal ions (for example, Hg2+)30 via the chelating effect has
attracted considerable attention because of its environmental and biological
importance.
Bent core polycatenar molecules obtained by combining the bent core with multi
periphery chains have been designed and synthesized to form polar columnar
phases.31 However, to the best of our knowledge, a bent core polycatenar molecule
based on 1,2,3ꢀtriazole with a CH2 central core has never been reported. Considering
the facile synthesis procedure of click reaction and the special properties of the
1,2,3ꢀtriazole unit, herein we firstly designed and synthesized bent core polycatenar
molecules consisting of a methylene central unit and two wings of 1,2,3ꢀtriazole
dendrons via click reactions (Fig. 1).
The influence of the dendronic generation number as well as the length of the

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terminal alkyl chain on the molecular selfꢀassembly in bulk state and in solution is
studied together with the binding ability of metal ions to these compounds.
Figure 1. Bent core polycatenar mesogens IE3/14 and IIEm/n.
2. Synthesis and characterization
2.1. Synthesis
All of the target compounds IEm/n and IIEm/n were synthesized by click reaction
between the bisacetylene 10 and the appropriate substituted aromatic azides (IENm/n)
or
IIENm/n
(Scheme
1).
Reduction
with
of
commercially
available
yielded
32
4,4'ꢀdihydroxybenzophenone
8
LiAlH4/AlCl3
4,4'ꢀdihydroxydiphenylmethane 9, which was etherified with propargyl bromide to
yield bisacetylene 10. 4ꢀAzidophenol 7 was synthesized from commercially available
4ꢀaminophenol 6 by formation of the diazonium salt and subsequently substituted
with sodium azide. 7 was etherified with the appropriate 3,4,5ꢀtrialkoxybenzyl
chlorides 2m/n 33 to give the aromatic azides IENm/n. Commercially available
3,4ꢀdihydroxybenzoate 4 was etherified with propargyl bromide to give the
3,4ꢀdipropargyloxylbenzoate 5, which was used for the CuAAC reaction with the aryl
azides 3m/n, yielding the dendritic aromatic azides IIENm/n. Finally click reaction
between bisacetylene 10 and aromatic azides IENm/n or IIENm/n produced the target
bent core polycatenars IEm/n and IIEm/n. All of the target compounds were purified
by column chromatography. For more details, see SI.

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Scheme 1. Synthesis of compounds IEm/n and IIEm/n: Reagents and conditions: i) CnH2n+1Br,
DMF, K2CO3, 90 °C; ii) LiAlH4, THF, 25 °C, 2 h; iii) SOCl2, THF, 25 °C, 1 h; iv) KI, NaN3,
DMF, 45 °C, 12 h; v) K2CO3, propargyl bromide, acetone, reflux; vi) a) tertꢀbutanol, THF, H2O,
sodium ascorbate, CuSO4·5H2O, 25 °C, 20 h; b) LiAlH4, THF, 25 °C, 2 h; c) SOCl2, THF, 25 °C,
1 h; d) KI, NaN3, DMF, 45 °C, 12 h; vii) a) NaNO2/HCl, 0 °C, 1 h; b) NaN3, 0~5 °C, 5 h; viii)
THF, NaH, 70 °C, 30 h; ix) LiAlH4, AlCl3, THF, 65 °C, 48 h; x) KOH, propargyl bromide, acetone,
reflux.
2.2. Mesomorphic properties
The liquid crystalline properties of the triazole bent core polycatenars IE3/14 and
IIE6/n (n = 10, 12, 14) were studied by polarizing optical microscopy (POM,
Optiphot 2, Nikon, in conjunction with a FP 82 HT heating stage, Mettler),
differential scanning calorimetry (DSC, NETZSH 200F3) and Xꢀray diffraction (XRD)
techniques. The phase transition temperatures of compounds IE3/14 and IIE6/n (n =
10, 12, 14) are collected in Table 1.
Table 1. The phase transition temperatures with corresponding enthalpy values of compounds
IE3/14 and IIEm/n.[a]
Comp
m
3
6
6
6
n
T/°C [∆H/kJ molꢀ1]
IE3/14
14
10
12
14
Cr 68.1[41.24] Cub /
93[b] Iso
Pm3n
I
IIE6/10
IIE6/12
IIE6/14
Cr 91.4[29.6] Cr 107.7[15.3] Cub /
127.3[49.2] Iso
Pm3n
1
2
I
Cr 82.4[44.3] Cr 91.6[25.4] Cr 106.7[13.53] Cub /
112.5[3.4] Iso
Pm3n
1
2
3
I
Cr 84.4[30.38] Cub /
108.1[2.49] Iso
Pm3n
I
[a]
Transition temperatures were determined by DSC (2 K minꢀ1 for IE3/14 and IIEm/n) and confirmed by POM,
peak temperatures of the first heating scans are given; values in brackets are transition temperatures upon cooling;
abbreviations: Cr = crystalline solid, Iso = isotropic liquid, Cub / = micellar cubic mesophases with space
Pm3n
I

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groups
[b] Transition temperature determined by POM.
Pm3n
.
Compound IE3/14 with the first generation dendrons is a hexacatenar compound as
both of the wings have three tetradecoxy chains at each end, while compounds with
the second generation dendronic wings (IIE6/n) are dodecylcatenar compounds
having six alkyl chains at each end. All these polycatenar compounds are mesogens
forming micellar cubic phase with Pm3n space group. The increase of dendronic
generation number (from IE3/14 to IIE6/14) causes increases in both of the melting
and clearing temperatures of the corresponding compounds. The elongation of the
terminal alkyl chains gives a slight decreases in both the melting and clearing
temperatures of compounds IIE6/n (n = 10, 12, 14).
a)
b)
Figure 2. (a) DSC heating and cooling curves of IIE6/14 (2 K·minꢀ1); the inset shows the DSC
peaks corresponding to the CubI–Iso and Iso–CubI transitions; (b) SAXS diffractogram of the
Cub/
phase of compound IIE6/14 at T = 100 °C.
Pm3n
Results of POM show that all of these phases are optically isotropic mesophases
and soft solids with viscoelastic response on shear forces, as typical for cubic
mesophases (Fig. S1).34 By increasing the temperature, sharp transitions to highly
fluid isotropic liquids occurs at defined temperatures, which can also be detected as
distinct peaks in the DSC traces (Fig. 2a, DSC curves of IIE6/14, Fig. S2). The
formation of the optically isotropic mesophases can be supercooled, in the case of the
formation of mesophases with 3D periodicity (Fig. 2a). These POM and DSC
observations are indicative of cubic mesophases. The optically isotropic mesophases
of these compounds were investigated by XRD (Fig. 2b, Table 2, Figs. S3ꢀ5, Tables

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S2ꢀ6). The positions of the sharp spotꢀlike reflections in the smallꢀangle region are in
line with a cubic lattice with Cub/
space group35 (see Fig. 2b and Figs. S3ꢀ5).
Pm3n
For example in the SAXS pattern of the optically isotropic mesophase of compound
IIE6/14, there are reflections (110), (200), (210), (211), (220), (310) consistent with a
cubic lattice with Pm3n space group (Fig. 2b). The calculated lattice parameters are
acub = 11.04 nm, 9.05 nm, 9.46 nm and 9.32 nm for IE3/14, IIE6/10, IIE6/12 and
IIE6/14 respectively (Table 2, Table S1). In Cub/ Pm3n phases, the molecules are
organized in closed soft spheroidic aggregates and there are 8 of these aggregates in
each unit cell.36 The number of molecules per aggregate decreases as the number of
the dendronic wings increases (µ = 49.10, 17.60, 17.83 and 15.34 for IE3/14, IIE6/10,
IIE6/12 and IIE6/14 respectively) (Table 2).
Table 2. Lattice parameters and other data[a] derived from the XRD results of compounds IEm/n
and IIEm/n.
Comp.
m
n
Phase plane/
space group
Unit cell parameters
Micelle size D/nm
µ
α[o]
a
cub /nm (T /°C)
11.04 (75)
9.05 (95)
IE3/14
IIE6/10
IIE6/12
IIE6/14
3
6
6
6
14
10
12
14
5.52
4.53
4.73
4.66
49.10
17.60
17.83
15.34
14.7
40.9
40.4
46.9
CubI/
Pm3n
CubI/
Pm3n
CubI/
9.46 (90)
Pm3n
CubI/
9.32 (100)
Pm3n
[a]
acub = lattice parameter; µ = number of molecules in each aggregate of the Cub /
lattice (ncell/8).37 D =
Pm3n
I
diameter of the spherical supramolecular aggregates, calculated according to D = 0.5×acub, based on the optimized
packing of sphere with minimized interfacial area by using WeaireꢀPhelan foam;38 α = the molecular solid angle,
calculated according to α = 4π/µ.37
The formation of a micellar type cubic phase for such bent core polycatenar
compounds might be attributed to the conformational flexibility of the central
methylene group and side ether linkages among the aromatic units, which increases
the alkyl chain flexibility. This favours a strong interface curvature, and leads to the
formation of the micellar cubic phase. The central bent angle of a single molecule is
109° due to the sp3ꢀhybridized methylene group, which is much larger than the
molecular solid angle, calculated based on the lattice parameter (Table 2). This
indicates that the bent angle is significantly reduced during the selfꢀorganization

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process and the molecule takes a conical configuration, which can selfꢀassemble into
closed isolated spherical objects and in turn pack in a periodic cubic lattice39 (Fig. 3).
Increase in the dendronic generation number simultaneously increases the number of
the terminal alkyl chains which leads to the increase of solid angle (α) of the conical
molecule. Due to larger solid angles, a lower number of molecules is organized in the
aggregates and the diameter of the sphere is smaller. For example, there are about 49
molecules per sphere for hexacatenar compound IE3/14 (α = 14.7°, D = 5.52 nm),
whereas the sphere aggregates of the dodecylcatenar compounds IIE6/14 (α = 46.9°,
D = 4.66 nm) are much smaller (15 molecules per sphere). The effect of the
chainꢀlength on the aggregate size is in line with the slight increase of the solid angle
on chain elongation (by comparison among IIE6/10, IIE6/12 and IIE6/14 in Table 2).
IE3/14
IIE6/14
Figure 3. Selfꢀassembly of compound IE3/14 (a) and compound IIE6/14 (b) into coneꢀlike cubic
phase with
lattice.
Pm3n
2.3. Gel properties
The gelation property of the representative compounds IE3/14 and IIE6/12 was tested
in various organic solvents at a concentration of 5.0 mg/mL and the results are
summarized in Table 3 and Table 4, respectively. The compound in an organic solvent
(1 mL) was heated in a septumꢀcapped test tube until the solid was dissolved. The
solution was then left to cool at room temperature in air. The phase transformations
were confirmed by visual observation (Figs. 4a, 4d, Fig. S6). Gel formation was
observed either while cooling or immediately after the cooling process. Hexacatenar

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compound IE3/14 can form gels in different solvents such as acetone, DMF, ethyl
acetate, 1,4ꢀdioxane and nꢀhexane whereas the dodecylcatenar compound IIE6/12 can
form gels in acetone and nꢀhexane only. The shielding effect of the terminal nonpolar
alkyl chains on triazole units could reduce the interaction between the solvent and the
triazole unit leading to the reduced gelation ability of IIE6/12 in which the number of
alkyl chains is two times higher than in IE3/14. To obtain a deeper insight into the
morphology of the molecular aggregates, the gels were investigated by scanning
electron microscopy (SEM). The SEM images of xerogels formed by IE3/14 and
IIE6/12 in polar solvents (Figs. S7cꢀe and Figs. 4bꢀc respectively) show the formation
of threeꢀdimensional networks composed of entangled fibrous aggregates. For
example, in the case of the xerogel formed by IIE6/12 in acetone, the approximate
diameter of the fibers is 100ꢀ400 nm and the length is more than 20 ꢁm. On the other
hand, the SEM images of xerogels formed in nꢀhexane (Fig. S7a and Figs. 4eꢀf) and in
ethyl acetate (Fig. S7b) shows the formation of plate aggregates. The strength of the
interaction between the solvent and the triazole unit, πꢀπ interactions, as well as van
der Waals forces, may play an important role in the type of aggregation, of the
compounds in solvents as seen in many triazole based gelators.23d,40 In the case of
polar solvent, stronger dipolarꢀdipolar interactions are expected between the solvent
and the triazole unit, which driving the formation of a more ordered threeꢀdimensional
network composed of entangled fibrous aggregates with respect to gels formed in
nonpolar or less polar solvents.
The organogels obtained exhibit multiple stimuliꢀresponsive behavior, namely
gelꢀsol reversible trans process upon exposure to environmental stimuli, such as
temperature and shear (Fig. 5). Application of heat or shear results in the formation of
a sol state through disruption of the nonꢀcovalent interactions between the molecules.
The gel state can be recovered by removal of such stimuli. Such stimuliꢀresponsive
behaviours could be useful for drug controlled release,41 hardeners of solvents and
sensors.42
Table 3. Gelation test results of compound IE3/14.[a]

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Solvent
IE3/14
Solvent
IE3/14
Ethyl acetate
Acetone
DMF
G
G
G
S
Chloroform
nꢀHexane
S
G
S
Cyclohexane
1,4ꢀDioxane
Toluene
G
Acetonitrile
P
[a] G = stable gel at room temperature; S = soluble; P = precipitation.
Table 4. Gelation test results of compound IIE6/12.[a]
Solvent
IIE6/12
Solvent
IIE6/12
Ethyl acetate
Acetone
DMF
P
G
P
S
S
Chloroform
nꢀHexane
S
G
S
Cyclohexane
1,4ꢀDioxane
Toluene
S
Acetonitrile
[a] G = stable gel at room temperature; S = soluble; P = precipitation.
a)
c)
b)
d)
e)
f)
Figure 4. Gels of IIE6/12 (a) in acetone; (d) in nꢀhexane; Selected SEM images of the dry gel
made from IIE6/12 in acetone with different magnification: (b) scale bar is 5 ꢁm; (c) scale bar is 2
ꢁm; the dry gel made from IIE6/12 in nꢀhexane with different magnification: (e) scale bar is 20 ꢁm;
(f) scale bar is 10 ꢁm.

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Figure 5. Multistimuli responsive organogels of compound IIE6/12 in acetone (5.0 mg/mL).
2.4. Chemosensor behaviour
The selectivity of compounds IEm/n and IIEm/n for metal ions was examined by
monitoring the change of fluorescence intensity upon addition of 10 equiv. solutions
of various metal ions, including Ag+, Cd2+, Co2+, Cu2+, Fe3+, Hg2+, K+, Li+, Mg2+, Ni2+,
Pb2+, Zn2+. The results of the representative compounds IE3/14 and IIE6/12 are shown
in Fig. 6. It should be noted that there are few reports on fluorescent sensing and
detection of metal ions by asymmetric and symmetric compounds with central groups
like polymethylene, 43 polyoxyethylene, 44 calix[4]arene, 45 , 46 , 47 anthracene, 48
naphthyl,49 deoxycholic acid50 and BODIPY51 etc, coupled at one side or both sides
with 1,2,3ꢀtriazole linked conjugates. In most cases, the symmetric compounds with
two triazole units can recognize more different metal ions than the asymmetric
analogues containing only one triazole unit or than analogues containing no triazole
unit.43,45,47 Similar phenomena are also observed in compounds IE3/14 and IIE6/12
.
By comparison, the fluorescence intensity of the hexacatenar compound IE3/14,
featuring two triazole binding sites, was quenched only by the addition of Hg2+, while
the fluorescence intensity of dodecylcatenar compound IIE6/12, featuring six triazole
units, was quenched by the addition of more than one kind of metal ions such as Hg2+,
Cu2+, Pb2+ and Co2+.43,45,47,52 These results show that the increased number of triazole
units increases the number of binding sites for different ions. For the selective
response of IE3/14 and IIE6/12 to Hg2+, spectral titration was performed in solution of
CH2Cl2 : CH3CN = 1 : 1 (v/v). With the addition of Hg2+, the emission peaks
gradually decreased (Figs. S8aꢀb, Figs. S9aꢀb) and finally reached its fluorescence
quenching plateau at ~ 7 equiv for IE3/14 and at ~ 16 equiv for IIE6/12. To further

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determine the sensitivity level, the detection limits53 for IE3/14 and IIE6/12 with
Hg2+ were ascertained to 1.08 × 10ꢀ7 mol/L and 4.45 × 10ꢀ7 mol/L based on a 3σ/slope
analysis under these experimental conditions, respectively (Fig. 7). By comparison,
the detection limits of these compounds are lower than those of the reported
compounds with two triazole units,44 but higher than the reported cyclophoshpazene
derivative54 containing six triazole units without terminal alkyl chains, indicating
again the shielding effect of the terminal alkyl chains on triazole units, which could
reduce the binding ability of triazole units towards metal ions.
2000
a)
100
b)
blank
Ag+
Cd2+
1600
1200
800
400
0
80
60
40
20
0
Co2+
Cu2+
Fe3+
Hg2+
K+
Li+
Mg2+
Ni2+
Pb2+
Zn2+
275
300
325
350
375
400
425
450
λ/nm
3000
2500
2000
1500
1000
500
c)
d) 100
blank
Ag+
Cd2+
Co2+
Cu2+
Fe3+
Hg2+
K+
80
60
40
20
0
Li+
Ni2+
Mg2+
Pb2+
Zn2+
-20
0
250
300
350
400
450
λ
/nm
Figure 6. Fluorescence spectra at room temperature at pH = 7.0. in CH2Cl2 : CH3CN = 1 :1
mixture (λex = 240 nm) of IE3/14 (6.4 × 10ꢀ6 M) (a), IIE6/12 (5.0 × 10ꢀ6 M) (c); (F0ꢀF)/F0×100
depicts the cation selective fluorescence quenching efficiency namely the fluorescence responses
of different metal ions of IE3/14 (b), IIE6/12 (d); abbreviation: F0 = the fluorescence emission
maximum of the blank sample; F = the fluorescence emission maximum of samples after addition
of different metal ions.

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a)
b)
Figure 7. Plots of the detection limits for Hg2+ evaluated from the fluorescence emission of (a)
IE3/14; (b) IIE6/12.
3. Conclusions
We have developed a new type of bent core polycatenar compounds with a methylene
center via click chemistry. These compounds can selfꢀassemble into a liquid
crystalline micellar cubic phase with Pm3n symmetry in their pure states due to the
strong interface curvature. The increase in the dendronic generation number, with a
consequent increases in the number of the terminal alkyl chains, as well as the
elongation of the terminal alkyl chains could lead to the increase of the solid angle.
This, in turn, leads to a lower number of molecules organized in the micellar
aggregates and therefore a smaller diameter of the micelle. These compounds can
form organogels in different organic solvents. The morphologies of the xerogels
prepared from these gelators strongly depend on the polarity of the gelling solvent.
Due to the shielding effect of the terminal alkyl chains on triazole units,
dodecylcatenar compounds with second generation dendrons show less gelation
ability than the hexacatenar compound with the first generation dendrons.
Additionally, the hexacatenar compound shows high selectivity exclusively for Hg2+
with a relative low detection limit, while the dodecylcatenar compounds exhibit
slightly lower selectivity being capable of binding several metal ions such as Hg2+,
Cu2+, Pb2+ and Co2+. The higher the number of the triazoles in the compounds, the
higher the number of different types of metal ions that can be bonded. The shielding
effect of the terminal alkyl chains also leads to a slightly reduced sensitivity to Hg2+

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ion of the dodecylcatenar compounds with the second generation dendrons, compared
to hexacatenar compounds with the first generation dendrons or to compounds
without terminal alkyl chains. As far as we know, there are few reports on materials
with LC, gel and chemosensor properties simultaneously.55 These compounds can be
considered as promising new generation multifunctional materials which can be
potentially applied as different advanced functional materials.
Acknowledgements
Financial support of this research by NSFC (No. 21664015, 21364017 and 21274119)
and the YEDF(ZD2015001) is greatly acknowledged. We thank beamline 1W2A at
Beijing Accelerator Laboratory and beamline BL16B1 at Shanghai Synchrotron
Radiation Facility (SSRF), China.
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DOI: 10.1039/C7NJ00225D
Graphical Abstract
Synthesis and self-assembly of bent core polycatenar mesogens with
binding selectivity to Hg2+
Huifang Cheng,‡a Ruilin Zhang,‡ab Taihao Li,a Xiongwei Peng,a Meng Xia,a Yulong Xiao, a Xiaohong
Cheng*a
[a] Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry
Department, Yunnan University
Kunming,
Yunnan 650091,
P. R. China
Fax: (+86) 871 65032905
E-mail: xhcheng@ynu.edu.cn
[b] Forensic Medicine of Kunming Medical University, Kunming 650500, P. R.
China
‡Both authors contributed equally to this work
Target compounds can self-assemble into Cub /
LC phases and organogels, simultaneously
Pm3n
I
these compounds have binding selectivity to Hg2+.