Synthesis, self-assembly, metal binding properties of triazole azobenzene based polycatenar dyes through click chemistry

Article abstract of DOI:10.1016/j.dyepig.2017.10.025

Two series of novel polycatenar dyes consisting of a triazole azobenzene core were synthesized efficiently by copper-catalyzed azide-alkyne cycloaddition (CuAAC) reaction. These compounds can self-assemble into SmC and Col_hex/p6mm liquid-crystalline phases, and form multistimuli responsive organogels. Bicontinuous cubic phase (Cub_V) could be induced in the binary system of a bicatenar compound with SmC phase and a hexacatenar compound with Col_hex/p6mm phase. Photophysical investigation indicates that these compounds have reversible photoresponsive properties in solution, liquid-crystalline and gel states. They can also act as chemosensors of Fe³⁺ and sodium dithionite with high selectivity.

Full text of DOI:10.1016/j.dyepig.2017.10.025

Accepted Manuscript
Synthesis, self-assembly, metal binding properties of triazole azobenzene based
polycatenar dyes through click chemistry
Huiru Chen, Ruilin Zhang, Hongfei Gao, Huifang Cheng, Haipeng Fang, Xiaohong
Cheng
PII:
S0143-7208(17)31731-X
10.1016/j.dyepig.2017.10.025
DYPI 6323
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 11 August 2017
Revised Date: 25 September 2017
Accepted Date: 15 October 2017
Please cite this article as: Chen H, Zhang R, Gao H, Cheng H, Fang H, Cheng X, Synthesis, self-
assembly, metal binding properties of triazole azobenzene based polycatenar dyes through click
chemistry, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.10.025.
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ACCEPTED MANUSCRIPT
Graphical abstract
Synthesis, self-assembly, metal binding properties of triazole
azobenzene based polycatenar dyes through click chemistry
Huiru Chen,a
‡
Ruilin Zhang,a,b
‡
Hongfei Gao,a Huifang Cheng,a Haipeng Fang,a
Xiaohong Cheng*a
[a] Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry School of
Chemical Science and Technology, 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, Yunnan 650500, P. R. China
‡Both authors contributed equally to this work

ACCEPTED MANUSCRIPT
Synthesis, self-assembly, metal binding properties of triazole
azobenzene based polycatenar dyes through click chemistry
Huiru Chen,a
‡
Ruilin Zhang,a,b
‡
Hongfei Gao,a Huifang Cheng,a Haipeng Fang,a
Xiaohong Cheng*a
[a]
Key Laboratory of Medicinal Chemistry for Natural Resources, Chemistry School of
Chemical Science and Technology, 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, Yunnan 650500, P. R. China
‡Both authors contributed equally to this work
Abstract: Two series of novel polycatenar dyes consisting of a triazole azobenzene core were
synthesized efficiently by copper-catalyzed azide-alkyne click (CuAAC) reaction. These
compounds can self-assemble into SmC and Colhex/p6mm liquid-crystalline phases, and form
multistimuli responsive organogels. Bicontinuous cubic phase (CubV) could be induced in the
binary system of a bicatenar compound with SmC phase and a hexacatenar compound with
Colhex/p6mm phase. Photophysical investigation indicates that these compounds have reversible
photoresponsive properties in solution, liquid-crystalline and gel states. They can also act as
chemosensors of Fe3+ and sodium dithionite with high selectivity.
Key words: Azobenzene; Liquid crystal; Photoresponsible behavior; Fluorescent chemosensor
1. Introduction
Liquid crystals (LCs) combining mobility and order on the molecular level have found wide
application not only in electrooptical devices but also in molecular engineering nanotechnologies
[1,2]. Novel mesogens by combination of different conjugated units such as azobenzene [3],
heterocycles [4,5,6,7,8] into the skeletons of the classical liquid crystals have been reported in
order to create advanced functional nanomaterials. Among them, azobenzene-based LCs have
caused significant attention because the photoisomerization of the azobenzene provides a general
way for the photocontrol of molecular structure and function [3]. Furthermore the azobenzene
based LCs have found wide potentials as optoelectronic sensing devices [9] and organic
light-driven actuators etc [10,11,12,13,14,15,16,17,18,19,20].
Polycatenar LCs consisting of a long aromatic rod-like core and multiple terminal flexible
chains represent as a kind of nonconventional LCs [21,22,23,24,25,26,27]. Due to the
nanosegregation of the aromatic rigid core and flexible chains, different LC phases like nematic,
smectic (lamellar), bicontinuous cubic (CubV), columnar and even micellar cubic (CubI) phases
depending on the number of the peripheral tails have been observed in such compounds. CubV
phases, which represent intermediate states between lamellar and columnar organization, are
interwoven 3D networks of channels, while CubI phases are formed by the regular organization of
closed spheroidic aggregates, which have been reported in azobenzene-based [ 28 ],
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oligothiophene-based [29], and bent-shaped polycatenar LCs [30].
Click chemistry is an easy and fast access to create a diversity of structures applicable for
pharmaceutical and material targets. The most representative “click chemistry” is
copper-catalyzed azide-alkyne click (CuAAC) reaction [31,32,33], which can combine organic
units into more complex architectures. The 1,2,3-triazole ring generated by the click chemistry has
highly chemical and thermal stabilities, a good hydrogen-bond-accepting ability [34,35] as well as
a nitrogen (N)-donor ability [36,37,38]. 1,2,3-triazole can enhance the molecular anisotropy when
linear connected to other aromatic groups, meanwhile it can lead to an increase in the molecular
dipole and dielectric anisotropy. Despite these advantages, the CuAAC reaction has only been
used for synthesis of calamitic [39,40,41] or discotic liquid-crystalline materials [42,43], seldom
for polycatenar LCs [30,44], and so far as we know there is no report on polycatenar liquid crystal
combining azobenzene core with triazole wings.
We have recently reported novel amphotropic azobenzene derivatives [ 45 ],
cholesterol-azobenzene dimesogens [46] which can display interesting properties such as
photoinduced liquid crystalline phase and photoresponsive chiral amplification effect. Considering
the useful properties and wide applications, photoswitchable azobenzene based polycatenars
[28,47,48] have received little attention to date. Furthermore the mostly reported azobenzene
based LCs contain the azo linkage either at the periphery [49,50,51] or only in a few cases at the
central of the molecules [28,52,53].
Therefore herein to continue our interest in study the novel azobenzene mesogens, we report the
facile synthesis and properties of series of novel polycatenar LCs ACm/n and AEm/n with an
azobenzene core and 1,2,3-triazole units at both sides via the CuAAC click reaction. Alkoxy
substituted phenyl units are attached at both terminals via acetate linkage (X = COO, series ACm/n)
or ether linkage (X = CH2O, series AEm/n). In the notation the letter m gives the number of alkoxy
chains and the superscript n identifies the alkoxy chain length.
The effect of various linkages X as well as the number and length of the terminal alkyl chains
on the self-assembly behavior of these compounds was investigated. Our study shows that these
compounds can self-assemble into liquid crystalline phases in the bulk states and form
multistimuli responsive organogels in organic solvents. They have reversible photoresponsive
properties, and can act as chemosensor towards Fe3+ and sodium dithionite with high selectivity.
2. Experimental
2.1. Materials
The structures of the compounds are shown in Scheme 1. Reactions requiring an inert gas
atmosphere were conducted under argon and the glassware was oven-dried (120 °C).
Commercially available chemicals were used as received. The solvents were dried and distilled by
1
conventional methods. H-NMR and 13C-NMR spectra were recorded on a Bruker-DRX-400
spectrometer with tetramethylsilane (TMS) as the internal standard. Elemental analysis was
performed using an Elemental VARIO EL elemental analyzer. Thin-layer chromatography was
performed on aluminum plates precoated with 5735 silica gel 60 PF254 (Merck). Column
chromatography was performed on Merck silica gel 60 (230-400 mesh).
2.2. Methods and characterization
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A mettler heating stage (FP 82 HT) was used for polarizing optical microscopy (POM, Optiphot
2, Nikon) and DSC were recorded with a DSC 200 F3 Maia calorimeter (NETZSCH). Their
optical properties (UV-vis and fluorescent emission) were done on a UNIC UV2600A and
HITACHI F-7000 spectrophotometers. SEM experiments were carried out on a QUNT200
scanning electron microscopy (SEM, USA). All pictures were taken digitally. For the sample
preparation, the gel was placed on an aluminum foil paper for some time until the gel became dry
gel, then the sample was gold plated, finally the sample was put into the scanning electron
microscopy for observation.
Small-angle powder diffraction (SAXS) experiments were performed in transmission mode
with synchrotron radiation at the 1W2A SAXS beamline at Beijing Accelerator Laboratory and the
beamline BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF). A modified Linkam hot
stage with a thermal stability within 0.2 °C was used, with a hole for the capillary drilled through
the silver heating block and mica windows attached to it on each side. Samples were held in the
poly(imide) (Kapton) film. A MarCCD 165 detector was used. q calibration and linearization were
verified using several orders of layer reflections from silver behemate.
For electron density reconstruction, fourier reconstruction of the electron density was carried
out using the general formula for 2D periodic systems:
ꢅ
ꢀꢁꢂ = ∑
⋅ |ꢉꢃꢄ| ⋅ cos(ꢊꢃꢄꢋ) ⋅ (cos(2ꢌ ⋅ (ℎꢍ + ꢎꢏ)))
(1)
ꢆꢇꢈ
ꢃꢄ
here m being the multiplicity, F the structure factor, which is proportional to the intensity and φ
the phase of the reflex.
2.3. Synthesis
Full synthesis scheme is shown in Scheme 1. 4-Azidophenol 1 [54], alkoxybenzoic acid 2m/n
[55,56], alkoxybenzyl chlorides 3m/n [57,58], aryl azides 4m/n [59,60] and 5m/n [30],
4,4'-dihydroxyazobenzene 6 [61] and 4,4'-dipropargyloxyazobenzene 7 [62] were prepared
according to literature procedures. A CuAAC click reaction between 4,4'-dipropargyl
oxyazobenzene 7 and aryl azides 4m/n and 5m/n respectively to yield the target products ACm/n
and AEm/n respectively. All products were purified using column chromatography. Experimental
procedures and analytical data are collated in the Supporting Information (SI).
Scheme 1. Synthesis of the ACbm/n, ACm/n and AEm/n, Reagents and conditions: i) a) NaNO2, HCl, H2O, 0 °C, 1
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h; b) NaN3, 0-5 °C, 12 h; ii) EtOH, KOH, 90 °C, 6 h; iii) a) LiAlH4, THF, 0 °C, 2 h; b) SOCl2, THF, RT, 2 h; iv)
DCC, DMAP, DCM, RT, 12 h; v) NaH, THF, 70 °C, 20 h; vi) a) NaNO2, HCl, H2O, 0 °C, 1 h; b) phenol, NaOH,
0-5 °C, 12 h; vii) propargyl bromide, K2CO3, acetone, 50 °C, 12 h; viii) CuSO4·5H2O, sodium ascorbate, TBA :
H2O = 1 : 1, THF, 40 °C, 12 h.
3. Results and discussion
3.1. Mesomorphic properties
Table 1 Transition temperatures, associated enthalpy values (in brackets) and X-ray data of compounds ACm/n,
AEm/n and ACb3/12a
Comp.
AC1/14
AC3/10
R1, R2, R3
T/°C [∆H/ kJ mol−1]
a/nm (T/°C)
n
-
R1 = R3 = H, R2 = OC14H29
R1 = R2 = R3 = OC10H21
Cr 160 [103.62] SmC 186 [2.11] Iso
Cr 144 [68.29] Colhex 177 [6.53] Iso
-
5.30(180)
3.9
AC3/14
AE3/10
AE3/14
ACb3/12
-
R1 = R2 = R3 = OC14H29
R1 = R2 = R3 = OC10H21
R1 = R2 = R3 = OC14H29
R1=R2=R3=OCH2CH(C12H25)2
Cr 142 [22.39] Colh 165 [1.23] Iso
Cr 116 [26.38] Colhex 162 [2.83] Iso
Cr 116 [197.41] Colhex 146 [4.40] Iso
Amorphous
-
5.25(125)
5.55(140)
-
3.9
3.6
-
a Transition temperatures were determined by DSC (peak temperature from the first heating scan, at a rate of 5 K min−1 for AC3/10,
AC3/14, AE3/10, AE3/14; 1 K min−1 for compounds AC1/14). Abbreviations: Cr = crystal; SmC = smectic C phase; Colhex = hexagonal
columnar phase; Colh = hexagonal columnar phase (assignment based only on optical investigations, no XRD performed; Iso = isotropic
liquid. Abbreviations: a = lattice parameter determined by XRD; n = number of molecules in the cross section of a column in the Colhex
phases (with assumed height of 0.45 nm). For Colhex phase: n = (a2/2) h(NA/M)ρ, NA = Avogadro constant, M = molecular mass,
3
assuming a density of ρ = 1 g/cm3.
The LC self-assembly of compounds ACm/n and AEm/n were investigated by polarized optical
microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). All
target compounds except ACb3/12 exhibit enantiotropic (thermodynamically stable) LC phases.
The phase transition temperatures and enthalpies of the azobenzene derivatives are listed in Table
1.
3.1.1 Compounds (AC1 /14
columnar phases
,
AC3 /n and AE3 /n) and their formation of smectic and
The rod-like dicatenar compound AC1/14 with one tetradecoxy chain at each end displays a
schlieren texture as typical for smectic C phase (Fig. 1a) [63]. The small-angle X-ray scattering
(SAXS) of this smectic C phase (Fig. 1b) shows that two reflection peaks are in the small angle
region with a ratio of d001/d002 ≈ 2, which confirms the smectic organization [1]. The layer spacing
was calculated to be d = 4.7 nm at 170 °C (Table S1), while the molecular length Lmax = 8.2 nm
(calculated by the MM2 method [64] and assuming a molecule in the most stretched conformation
with all-trans conformation for the alkyl chains [65]). The ratio of the layer spacing (d) and the
molecule length (Lmax) is 0.57, indicating a monolayer structure in the SmC phase where the fully
extended molecules take the orientation with a tilt angle θ ≈ 55° (calculated by the relationship
cosθ = d/Lmax) from the layer normal (the inset of Fig. 1b). The value of the tilt angle is consistent
with those of the reported SmC phases [66,67].
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(b)
(a)
Fig. 1. SmC phase of compound AC1/14: (a) texture as seen between crossed polarizers at T = 180 °C; (b) SAXS
diffractogram at T = 170 °C; the inset shows the model of the molecular organization in monolayer of the SmC
phase.
With increasing the number of the alkyl chains, all the other hexacatenar compounds AC3/n (n
= 10, 14) and AE3/n (n = 10, 14) with 6 alkyl chains attached equally at each terminal display
columnar mesophases. The columnar phases were identified by the typical spherulitic fan-like
textures under POM. The investigations with an additional λ-plate under POM indicate that the
columnar phases are optically negative (Fig. 2a and Fig. S1a-c). This means that the major
intramolecular π-conjugation pathway, which is along the long axis of the aromatic cores, is
perpendicular to the column long axis. The columnar phases were also investigated by SAXS. For
compounds AC3/10, AE3/10 and AE3/14, there are three small angle reflections with a ratio of
their reciprocal spacing 1: 31/2 : 2, which can be indexed to the 10, 11, 20 reflections of hexagonal
lattice with p6mm symmetry (Fig. 2b and Fig. S3-S4 and Table S2-S4). The number of molecules
organized in a slice of the columns with a height of h = 0.45 nm (a typical value for the maximum
of the diffuse wide angle scattering) ncell could be calculated as shown in Table S5. Therefore it
can be assumed that for these compounds approximately 4 molecules self-assemble into an overall
disk-like stratum and they successively stack one another to form supramolecular columns, and
these columns in turn self-organize to form a 2D parallelogram lattice (Fig. 2c), which was similar
with the reported polycatenar mesogens [22]. The self-assembly model of Colhex/p6mm phase is in
complete agreement with the reconstructed electron density map (Fig. 2d and Fig. S3b, S4b),
where the high electron density areas (blue, purple) involving the azobenzene cores, which are
surrounded by a medium electron density continuum (green) involving the benzylether groups,
triazoles and some of the alkyl chains. The majority of the alkyl chains form the low electron
density areas (red). It should be noted that the intermolecular π-π* interactions between the
rod-like cores as well as nanosegregation of the aromatic center from the surrounding alkyl chain
moieties would promote the assembly of these molecules into such columnar structures.
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(a)
(b)
(c)
(d)
Fig. 2. Colhex phase of compound AE3/14: (a) texture as seen between crossed polarizers at T = 130 °C, the inset
shows the same region with λ-plate; (b) SAXS diffractogram at T = 140 °C; (c) the model showing the
organization of compound AE3/14 in the Colhex phase; (d) electron density map as reconstructed from the
diffraction pattern in (b), for the color code, see bar on the right.
Though the LC phase structure of compound AC3/14 was not characterized by SAXS, the
investigations of the binary mixtures provided evidence that the columnar phase of AC3/14 is a
hexagonal columnar phase too, just like that of AE3/14 (Fig. 3a). This was indicated by the
continuous growth of the spherulitic texture of compound AC3/14 into the region of the
Colhex/p6mm phase of AE3/14 without any intermediate minimum, maximum, or visible
miscibility gap [68].
(b)
(a)
Col
CubV
SmC
AC3/14
AE3/14
AC3/14
AC1/14
Fig. 3 (a) Contact region between the triangular honeycomb type LC phases of AE3/14 and AC3/14 at T = 142 °C;
the dark areas represent the homeotropically aligned regions of the Colhex/p6mm phases; the development of the
contact region is shown in Fig. S5; (b) Contact region between AC3/14 and AC1/14 at T = 163 °C; The red arrows
indicate the approximate position of the boundary between the two compounds.
Attempts to change the mesophase structure to the micellar cubic structure by the branching of
the peripheral chains were unsuccessful. Compound ACb3/12 is completely isotropic and
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relatively viscous, but no cubic lattice could be found by XRD. There is only one diffuse
scattering peak in the small angle range at about d = 4.19 nm (Fig. S6). Hence, the LC phase was
completely lost, and additionally, no crystallization was observed. It appears that the chain
branching leads to peripheral overcrowding, which distorts the core packing of the aromatics and
destabilizes the ordered aggregates.
3.1.2 Binary mixture of different molecules
According to the thermotropic liquid-crystalline phase sequence formed by the polycatenar
molecules [69], CubV phases could occur as intermediate phases between layer-like and columnar
organization of these molecules with the increase of the interface curvature. In order to predict the
possibility that the target molecules can self-assemble into CubV liquid-crystalline phase, we
investigated the contact region between AC3/14 and AC1/14. During the cooling process, an
isotropic texture was observed in the contact region, which separated the spherulitic texture of
AC3/14 and the schlieren texture of AC1/14 (Fig. 3b). The equal molar ratio binary mixture of
AC3/14 and AC1/14 showed an isotropic mesophase in the temperature range of 125 °C-173 °C,
which is consistent with the DSC trace of this binary mixture (Fig. S2f). The typical feature of the
induced optically isotropic mesophase (viscoelasticty, supercool ability) is identified with those of
the cubic mesophases of the pure compounds, confirmed its cubic phase nature. The self-assemble
structure of this cubic phase needs to be further investigated by the SAXS experiment.
It is interesting to compare these triazole-azobenzene polycatenars with our previously reported
polycatenar mesogen IE3/14 [30] (Fig. 4). It clearly shows that replacement of bisphenylmethane
bent central core in IE3/14 with azobenzene leads to both higher melting and clearing
temperatures, and prevention the formation of the cubic phase for herein reported
triazole-azobenzene polycatenars AE3/14 and AC3/14. The reduced flexibility of the azobenzene
core in compounds AE3/14 and AC3/14 compared with the bisphenylmethane core in the reported
compound IE3/14 should be responsible for these observations.
Fig. 4. Bar graph summarizing the thermal behavior of compounds ACm/n, AEm/n under investigation and the
reported compound IE3/14.
Therefore the structure-LC property relationship of these triazole-azobenzene polycatenars
focusing on the effects of the number, length and type of terminal alkyl chains as well as the type
of linkage on their LC self-assembly behavior was systematically studied. In all cases, shearing
the samples led to flow which removed the textures. This confirms the LC states of these materials.
With increasing of the number of alkyl chains, the interface curvature between the nanosegregated
regions of the triazole-azobenzene cores and the flexible chains should be increased. This
structural variation results in the transition from the lamellar (SmC) phase for bicatenar compound
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to hexagonal columnar (Colhex) phases for all hexacatenar compounds, which is similar to the
mesomorphism of amphiphiles and block molecules [70], chain branching leads to none
mesogenic material. It seems that it is difficult to achieve the CubI phase in these compounds,
because of their long rigid cores.
All ether compounds AEm/n display broader range of mesophases with both reduced melting
and isotropic temperature compared with the corresponding ester analogues ACm/n (Table 1). This
should be attributed to less rigid and less polar ether group (X = CH2O) contained in AEm/n
compared with the carboxyl group (X = COO) contained in ACm/n.
3.2. Photoisomerization behavior in liquid crystalline and solution
All the liquid-crystalline phases can respond to the UV irradiation immediately by change their
textures to isotropic state and recovery to their LC textures after removing the UV light (Fig. 5 and
Fig. S7). For example, compound AC1/14 was cooled from the isotropic liquid state to 170 °C in
the range of the SmC phase (Fig. 5a). It was found that a phase transition from the SmC phase to
the isotropic phase was realized in just 10 s under UV irradiation (Fig. 5b). Then the UV light was
turned off after 15 s (Fig. 5c), schlieren textures of a typical SmC appeared again. It was believed
that azo isomerization trans-cis disrupted the schlieren structure of the SmC phase, thus an
order-decreasing phase transition was realized. It seems that the trans-cis photoisomerization
effect on the liquid-crystalline property of the compounds reported here is very strong.
(a)
(b)
(c)
Fig. 5. Textural changes as observed by POM at the photoinduced SmC-Iso transition and the relaxation Iso-SmC
as observed for compound AC1/14 at 170 °C: (a) before UV irradiation; (b) after UV irradiation for 10 s; (c) after
visible light irradiation for 15 s.
The possible photochemical switching process of SmC-Iso reversible transition of AC1/14 is
schematically represented in Fig. 6. Similarly, the compound AE3/14 showed photoisomerization
reversible transition of Col-Iso (Fig. S7). Such photoisomerizations could be useful for light
sensitive displays or data storage applications [45,46].
Additionally, such compounds exhibit the expected reversible trans-cis photoresponsive
behavior in solution (Fig. S8-S12). The trans-cis transition under UV light irradiation led to a
decrease of the π-π* band at 355 nm and an increase of the n-π* band at 450 nm, while the reverse
trans-cis transition under visible light irradiation led to an increase of the absorption at around 355
nm and a decrease at around 450 nm. For these compounds, under the irradiation of 365 nm UV
light of its CH2Cl2 solution (1×10-5 mol L-1), the trans-cis transition was completely achieved
about 120 s. Under visible light, the reverse trans-cis transition process was achieved within 240s.
8

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(c)
(b)
(a)
UV-light
Vis-light
UV-light
Vis-light
Cis
Trans
Fig. 6. Schematic illustration of the possible photochemical switching process in AC1/14: (a) SmC phase; (b)
SmC-Iso transition; (c) isotropic phase.
3.3. UV sensor property towards Na2S2O4
Sodium dithionite (Na2S2O4) is an important reducing agent in the textile and pulp industry. The
azobenzene could exhibit reactivity toward Na2S2O4 by cleaving the -N=N- bond to the
phenylamine group [71,72]. As shown in Fig. 7a, AC3/14 had a yellow color in THF/H2O solution
(3:1, v/v at room temperature), and turned to pale and finally colorless after addition of the
Na2S2O4 solution, indicating the cleavage of azobenzene moiety. The chemical reaction was
monitored by UV absorption spectra (Fig. 7b), which shows an intense absorption peak at 357 nm
owing to the π-π* absorption of trans-azobenzene. Time-dependent UV absorption spectra shows
that, with the addition of Na2S2O4 solution, the UV absorption peak at 356 nm slowly disappeared
and the cleavage reaction almost finished in 10 min (Fig. 7b). We also studied the reactivity of
AC3/14 with other sulfur-carrying ions such as Na2SO4, Na2SO3, Na2S and NaHS, which are the
major interferents for sodium dithionite detection. The UV absorption spectra of AC3/14 solutions
did not show obvious change, revealing the good selectivity of this reaction towards Na2S2O4 (Fig.
7c). Therefore AC3/14 shows high reactivity toward Na2S2O4, which providing a potential sodium
dithionite sensor.
(a)
(b)
(c)
Na2S2O4
Fig. 7. (a) Schematic demonstration of compound AC3/14 reaction with Na2S2O4 and the corresponding photos; (b)
Time-dependent UV absorption spectra of AC3/14 solution(1×10-5 mol L-1) in the presence of Na2S2O4 (1×10-3 mol
L-1); (c) UV absorption spectra of AC3/14 solution (1×10-5 mol L-1) in the presence of Na2SO4, Na2SO3, Na2S,
NaHS and Na2S2O4 (1×10-3 mol L-1).
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3.4. Absorption and emission properties
The UV-vis absorption and fluorescence spectroscopic data of the chosen representative
compounds AC1/14, AC3/14, AE3/14 in THF solution are shown in Fig. 8. These compounds
show the maximum absorption peak at about 355 nm and the maximum emission peak around 402
nm in THF solution, which may be attributed to the π-π* transition. The absorption and
fluorescence spectra of all these compounds in solutions show similar profiles, indicating that the
molecular transmittance is mainly due to the azobenzene conjugate bridge structures in
chromophores [73].
Fig. 8. Absorptions and fluorescence spectra of compounds AC1/14, AC3/14 and AE3/14 in THF (1×10-6 mol L-1).
For further investigation of the influence of solvent polarity on the optical properties of the
compound of AC3/14, the UV-vis and fluorescence spectra were measured in various solvents (Fig.
9). The main peaks of their absorption spectra locate at about 355 nm, which have very little
changes in different organic solvents (Fig. 9a). With the increasing of the polarity of the solvents,
the emission peaks showed only slightly red-shifted from about 392 nm (hexane) to about 414 nm
(CHCl3) (Fig. 9b). The Stokes shifts found to be 40, 44, 51 and 55 nm for cyclohexane, DCM,
THF and CHCl3 respectively (Table S6). Little influence of the solvent polarity on the optical
properties of these compounds means that weak intramolecular charge transfer (ICT) exists in
these molecules. The conformation and electron distributions of ACm/n and AEm/n, calculated
based on density functional theory (DFT) with the Gaussian 03W program package at
B3LYP/(6-31G, d) level with the model compound AC3/14, having methoxy groups instead of the
OC14H29 chains indicate that the electron distributions of the HOMO and LUMO of AC3/14 are all
located on the central phenyl moiety and azo groups (Fig. S13). This corresponds with weak ICT
effect existing in these compounds deduced from the weak solvent effect.
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(a)
(b)
Fig. 9. (a) UV-vis spectra of AC3/14 (1×10-6 mol L-1) at 20 °C in cyclohexane, DCM, THF, trichloromethane; (b)
fluorescence spectra of AC3/14 (1×10-6 mol L-1) in cyclohexane, DCM, THF, trichloromethane excited at 350 nm,
355 nm, 355 nm, 360 nm respectively.
3.5. Gelation properties
The gelation ability of these compounds was tested in various organic solvents at a
concentration of 3.0 mg mL-1. Among the synthesized polycatenars, only hexacatenar compounds
AC3/14 and AE3/14 can form yellow gels in 1,4-dioxane and cyclohexane, and the results are
summarized in Table 2. The bicatenar AC1/14 and other hexacatenars with short or branching
alkyl chains have no gelation properties. These results indicate that the number or the length of
terminal alkyl chains influences the gelation properties. This is probably due to the presence of
alkyl chains maintaining the subtle balance between solubility and precipitation of the gelator
molecules in a given solvent and also facilitating van del Waals interaction of gelator molecules
[74]. Yellow gels could be obtained by cooling the heated solution of AC3/14 or AE3/14 quickly
below 20 ºC or slowly down to room temperature.
The scanning electron microscopy (SEM) images of the xerogels formed by AC3/14 and
AE3/14 are significantly different. For ester compound AC3/14, its xerogel from polar solvent of
1,4-dioxane (Fig. 10 left) shows the formation of three-dimensional networks composed of twisted
nanobelts. The approximate width of the nanobelt is 0.56-0.60 µm, and the length is more than 3.0
µm. While xerogel of AC3/14 from less polar solvent of cyclohexane shows the formation of the
less ordered plate aggregates (Fig. S14a). In contrary, xerogel of ether compound AE3/14 from
polar solvent of 1,4-dioxane shows the porous three-dimensional networks composed of entangled
fibrous aggregates (Fig. S14b), while the unordered lump aggregates are observed in the xerogel
of AE3/14 from less polar solvent of cyclohexane (Fig. S14c). Therefore gels from the more ploar
solvent of 1,4-dioxane solvent displayed more ordered three-dimensional networks than gels form
less polar solvent of cyclohexane, which displayed only less ordered plate or lump aggregates. The
reason is that in polar solvents, the dipolar-dipolar interaction between polar solvents and triazole
or azo units, as well as other non-covalent interactions such as π-π interaction, hydrogen bonding
between the triazole or azobenzene units etc, all could drive the gel formation. Additionally, the
more polar ester linkages in AC3/14 lead to stronger π-π interaction, therefore the formation of
more ordered gel structure is observed for ester compound AC3/14 as compared with the ether
compound AE3/14.
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Table 2 Gelation properties of AC3/14 and AE3/14a
AC3/14
AE3/14
AC3/14
AE3/14
Solvent
THF
Solvent
CH2Cl2
S
S
P
P
P
G
S
S
P
P
S
G
S
G
P
P
S
P
S
G
P
P
S
P
Ethyl acetate
Hexane
Cyclohexane
Ethanol
Methanol
Acetone
n-Butanol
Toluene
1,4-Dioxane
CH3CN
a S = solution, P = precipitation, G = gelation, gels formed at room temperature (20 ºC).
The organogels exhibit multiple stimuli-responsive behavior namely gel-sol reversible trans
process upon exposure to a number of environmental stimuli including light, temperature, and
shear etc (Fig. 10 right). Irradiation with UV light, application of heat or shear resulted in a sol
state through disruption of the non-covalent interactions between the molecules, and the gel state
can be recovered by the removal of such stimuli. Such multiple stimuli-responsive behaviors could
be useful for delivery vehicles of small drug molecules [75], energy transfer [76], hardeners of
solvents and sensors etc [77].
Mechano-Responsive
UV-light
Light-Responsive
Vis-light
Thermo-Responsive
Fig. 10. SEM images of xerogel formed by AC3/14 in 1,4-dioxane and the scale bar is 2 µm (left); multistimuli
responsive organogels formed by compound AC3/14 in 1,4-dioxane (3.0 mg mL-1) (right).
3.6. Chemosensor behavior
Recently considerable efforts have been devoted to the development of chemosensors for the
selective detection of heavy metal ions of environmental and biological importance. Among the
heavy and transition metal ions, Fe3+ as a physiologically important metal ion, plays a catalytic
role in chemical and biological processes such as oxygen metabolism and electron transfer, and
both its deficiency and excess in the human body can induce a variety of diseases [78,79]. The
1,2,3-triazole motif introduced by click chemistry has its potential binding ability to transition
metalions [80,81]. Hence, We have investigated the binding properties of ACm/n for metal ions in
THF : H2O = 1 : 1, such as Ag+, Cd2+, Mg2+, Fe3+, Hg2+, Li+, Cu2+, Pb2+, Zn2+, Ni+, Co2+, K+. The
result of the representative compound AC3/14 is shown in Fig. 11. During the addition of Fe3+, the
fluorescence intensity was dramatically decreased, which was the most significant change in
fluorescence signals compared with the other metal ions.
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(a)
(b)
Fig. 11. Fluorescence spectra of compound AC3/14 (1×10-6 mol L-1) in the presence of different metal ions (1 ×
-
10-5 mol L-1) at room temperature, all anions are ClO4 , at pH = 7.0. (a, b) THF: H2O = 1 : 1 (λex = 355 nm);
(F0-F)/F0×100 depicts the cation selective fluorescence quenching efficiency of compound AC3/14 namely the
fluorescence responses of different metal ions; abbreviation: F0 = the fluorescence emission maximum of blank
sample; F = the fluorescence emission maximum of samples with addition of different metal ions.
We also performed the selectivity study on Fe3+. In fluorescence spectra, an emission band
centered at 415 nm appears upon addition of Fe3+ (from 0 to 25 equiv), and the emission intensity
decreased linearly with [Fe3+]total. Nearly 10 equiv of metal ions being required for the saturation
of the fluorescence intensity under the titration conditions, which was the most significant change
in fluorescence signals compared with the other metal ions (Fig. 12) [82]. To further determine the
sensitivity level, the detection limits [83,84] was ascertained to be 7.88 × 10-7 mol L-1 based on a
3σ/slope analysis under these experimental conditions (Fig. S15). By comparison, the detection
limits of these compounds are lower than those of the reported compounds with two triazole units
[85]. Therefore, compound AC3/14 could be used as a highly sensitive fluorescent chemosensor
for the detection of Fe3+ in THF aqueous solution.
(a)
(b)
Fig. 12. (a, b) Fluorescence spectra of compound AC3/14 (1×10-6 mol L-1) at 20 °C upon addition of Fe3+ (from 0
to 25.0 equiv) in THF: H2O = 1 : 1 (λex = 355 nm).
4. Conclusion
Therefore, it is possible to construct of a multifunctional molecule by using small different
functional tectons with different functions. The novel triazole-azobenzene polycatenars reported
here can self-assemble into LC phases. The increase of the interface curvature by increasing the
number of alkyl chain leads to a mesophase transition from SmC to hexagonal columnar phases.
13

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In the contact region between the SmC phase of compound AC1/14 and hexagonal columnar phase
of AC3/14, a reversed bicontinuous cubic phase was induced. The structure-property relationship
of these triazole-azobenzene polycatenars focusing on the effects of the number, length and type
of terminal alkyl chains as well as the linking groups on the self-assembly into lamellar and
columnar phases is systematically studied.
Additionally, these compounds can self-assemble into gels with different morphologies in
different organic solvents. The azobenzene as photosensitive unit endowing these compounds with
the reversible photoresponsive behavior in liquid crystalline, solution and gel states is well
demonstrated. The high reactivity of azobenzene toward Na2S2O4 makes these compounds to be a
potential sodium dithionite sensor. Metal recognizing properties of triazole units lead to these
compounds acting as Fe3+ fluorescence sensor among a series of metal cations in THF-H2O
solution. These multi functional polycatenar dyes should have great potentials in optical electronic,
biological as well as medical areas.
Acknowledgement
This work was supported by National Natural Science Foundation of China (Nos. 21664015,
21364017, 21274119 and 21602195) and the Yunnan Provincial Department of Education
Foundation (Nos. ZD2015001 and 2016FD008). We thank beamline 1W2A at Beijing Accelerator
Laboratory and beamline BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF), China. The
calculations were performed with the support of the Yunnan University Supercomputer Center.
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Polycatenar dyes consisting of a triazole azobenzene core have been successfully synthesized via
click chemistry.
These polycatenar dyes can self-assemble into SmC and Colhex/p6mm mesophases and organogels
with different morphologies.
Bicontinuous cubic phase (CubV) could be induced in the binary system of SmC phase and
Colhex/p6mm phase.
These polycatenar dyes exhibit reversible photoresponsive behavior in solution, liquid-crystalline
states and gel states.
These polycatenar dyes can act as chemosensors of Fe3+ and Na2S2O4 with high selectivity.