Novel C3-symmetrical triphenylbenzene-based organogelators with different linkers between phenyl ring and alkyl chain

Article abstract of DOI:10.1016/j.tet.2010.02.094

A new family of 1,3,5-triphenylbenzene-based organogelators with approximately identical side chain length have been synthesized and fully characterized. The molecular aggregation can be promoted by hydrogen-bonding and van der Waals interaction. The magnitudes of hydrogen bonding interactions between the linkers on the triphenylbenzene cores are qualitatively in the order of -CONHNHCO-, -NHCONH->-NHCO->-CONH->-NHCSNH-, consequently resulting in different gelation behaviors. The sol-gel transitions of gels 3 and 4 in toluene are thermodynamically reversible, while the formations of gels 2 and 5 in dioxane are kinetically controlled. The urea-based gel 2 can selectively recognize F^- ion.

Full text of DOI:10.1016/j.tet.2010.02.094

Tetrahedron 66 (2010) 3553–3563
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Tetrahedron
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Novel C3-symmetrical triphenylbenzene-based organogelators with different
linkers between phenyl ring and alkyl chain
*
*
Yabing He, Zheng Bian , Chuanqing Kang, Yanqin Cheng, Lianxun Gao
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of Chinese Academy of Sciences,
Changchun 130022, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new family of 1,3,5-triphenylbenzene-based organogelators with approximately identical side chain
length have been synthesized and fully characterized. The molecular aggregation can be promoted by
hydrogen-bonding and van der Waals interaction. The magnitudes of hydrogen bonding interactions
between the linkers on the triphenylbenzene cores are qualitatively in the order of –CONHNHCO–,
–NHCONH–>–NHCO–>–CONH–>–NHCSNH–, consequently resulting in different gelation behaviors. The
sol-gel transitions of gels 3 and 4 in toluene are thermodynamically reversible, while the formations of
gels 2 and 5 in dioxane are kinetically controlled. The urea-based gel 2 can selectively recognize Fꢀ ion.
Ó 2010 Elsevier Ltd. All rights reserved.
Received 24 September 2009
Received in revised form 22 February 2010
Accepted 26 February 2010
Available online 10 March 2010
1. Introduction
accumulation structures may be tuned by the chain length and the
number of chains.9 Different from the rigid benzene core, a tri-
In the past decade, the introduction of the beautiful C3-sym-
metry into molecular structure has acquired much interest from the
supramolecular chemists.1 Especially, many C3-symmetrical low
molecular weight organo- or hydrogelators (LMWGs) have been
designed and synthesized successfully.2–11 In common, the cis,cis-
1,3,5-cylcohexane triamide or triamidobenzene cores are used as
the gelating scaffolds. The parallel orientation of the hydrogen
bond-based units on the cores provides strong uniaxial in-
termolecular interactions affording 1 D self-assembly perpendicu-
lar to the plane of the molecule. By further extension toward the
outside from the core, a broad scope of functionalities can be in-
troduced.2–8 Furthermore, the research for the tuning of mechan-
ical or thermal properties of the gels, the scope of gelated solvents
and morphology of the gels is elementary in the field of gel
chemistry.3 A better understanding of the structure-property re-
lationship of a gelator is of great interest and importance. Ben L.
Feringa et al.10 and Masamichi Yamanaka et al.11 ever systematically
developed the C3-symmetric organogelator with a broad structural
variation. Interestingly, the intermolecular interactions strongly
depend on the nature of the core units, the type of hydrogen-
bonding units and the side groups, and consequently there are
large differences in aggregation properties, thermal stability, and
morphology between the various compounds.
phenylbenzene molecule possesses the labile conformation, and
a propeller-like one is energetically favorable. In particular, the C2-
symmetrical para-terphenylen-1,400-ylenebis(dodecanamide) can
self-assemble into the nanotube structure, but is almost insoluble
in any solvent at room temperature.12 The
p–p interaction between
rigid terphenylene segments and the intermolecular hydrogen
bonding between amido units have a synergistic effect on the for-
mation of extremely stable aggregation. So the exploration for the
structurally similar C3-symmetrical triphenylbenzene-centered
molecules seems to be very interesting. In this paper, we describe
the aggregation behaviors of a new family of triphenylbenzene-
centered molecules 1–6 with approximately identical side chain
length. Our interest is focused on the effect of different hydrogen
bond-based linkers between phenyl ring and long alkyl chain on
gelation properties (Scheme 1).
2. Results and discussion
2.1. Synthesis
A family of triphenylbenzene-centered molecules 1–6 were
prepared according to the procedures described in Scheme 2. Tet-
rachlorosilane catalyzed condensation of p-nitroacetophenone in
dry ethanol gave 1,3,5-tri(4-nitrophenyl)-benzene. Reduction of the
nitro group proceeded in the presence of hydrazine with Pd/C as
catalyst in THF under reflux and provided triamino 7. Trithiourea 1
and triurea 2 were obtained by the reaction of triamino 7 with
dodecyl thioisocyanate or isocyanate in dry THF, respectively.
Recently, Lu Ran et al. reported a series of 1,3,5-triphe-
nylbenzene derivatives with long alkoxy sidechains whose
* Corresponding authors. Tel.: þ86 431 85262265; fax: þ86 431 85697831;
e-mail addresses: bianzh@ciac.jl.cn (Z. Bian), lxgao@ciac.jl.cn (L. Gao).
0040-4020/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.02.094

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Y. He et al. / Tetrahedron 66 (2010) 3553–3563
2.2. Gel test
R
R
All compounds 1–6 were first studied with regard to their ge-
lation behavior in various solvents using a test tube inversion
method (Fig. 1). The organogelation properties of 1–6 are listed in
Table 1. Trithiourea 1 has poor organogelation ability and only at
ꢀ10 ꢁC forms the gels in aromatic solvents such as toluene, xylene,
mesitylene, possibly due to weak hydrogen bonding interactions
between thiourea groups. However, triurea 2 forms the gels in
chloroform and cyclic ether solvents such as THF and dioxane with
low critical gel concentration (CGC). When the urea groups are
replaced by the amide groups, triamide 3 exhibits a high CGC value
in chloroform, but it shows good gelation ability in aromatic sol-
vents. In contrast, triurea 2 is insoluble in aromatic solvents. When
the linking mode of amide is inverted, the resulting derivative 4 has
1
R
2
3
R
R
R
1: R = NHCSNHC12H25
2: R = NHCONHC12H25
3: R = NHCOC11H23
6: R = CONHC12H25
4: R = CONHC12 25
H
5: R = CONHNHCOC11H23
Scheme 1.
Treating triamino 7 with dodecanoyl chloride in dry THF using
triethylamine as base gave triamide 3. Suzuki cross-coupling of
1,3,5-tribromobenzene and p-methoxycarbonylphenyl boronic acid
in the presence of Pd(PPh3)4 as catalyst and Na2CO3 as base fol-
lowed by basic hydrolysis provided 8. Likewise, 9 was obtained
from 1,3,5-tribromobenzene and m-methoxycarbonylphenyl
boronic acid. Heating
8 in SOCl2 to give 1,3,5-tri(p-chloro-
carbonylphenyl)benzene followed by reaction with dodecan-
1-amine or dodecanehydrazide provided the investigated
derivatives 4 or 5, respectively. Triamide 6 was prepared as de-
scribed for 4, starting from 9 and dodecan-1-amine. The chemical
structures of all compounds were confirmed by 1H NMR, 13C NMR,
FTIR, ESI-MS, and elemental analysis.
Figure 1. Gel photographs. (a) gel 3 in chloroform (20 mg mLꢀ1), (b) gel 3in toluene
(20 mg mLꢀ1), (c) gel 5 in THF (6 mg mLꢀ1), (d) gel 5in dioxane(4 mg mLꢀ1), (e) gel 2 in
chloroform (3 mg mLꢀ1), (f) gel
2 2 in dioxane
in THF (10 mg mLꢀ1), (g) gel
(1.5 mg mLꢀ1), (h) gel 4 in ethyl acetate (5 mg mLꢀ1), (i) gel 4 in toluene (10 mg mLꢀ1).
NH2
O2N
NO2
NO2
C12H25NCO or C12H25NCS
.
Pd/C, NH2NH2 H2O
reflux 10 h
SiCl4, EtOH
RT, 12 h; then reflux 5 h
1, 2, 3
or C11H23COCl, NEt3
RT, 24 h
0 °C, 30 min;
then reflux 10 h
O
7
NH2
H2N
NO2
COOH
1. Pd(PPh3)4, Na2CO3
PhCH3/MeOH/H2O
reflux, 48 h
COOCH3
Br
1. SOCl2, reflux 12 h
+
4, 5
2. C11H23CONHNH2, NEt3
or C12H25NH2, NEt3
RT, 24 h
2. 6 M NaOH, MeOH/H2O
reflux, 8 h
3. conc. HCl
HOOC
Br
Br
Br
B(OH)2
8
COOH
COOH
O
1. Pd(PPh3)4, Na2CO3
PhCH3/MeOH/H2O
Br
OCH3
reflux, 48 h
1. SOCl2, reflux 12 h
+
6
2. C12H25NH2, NEt3
RT, 24 h
COOH
2. 6 M NaOH, MeOH/H2O
reflux, 8 h
Br
B(OH)2
3. conc. HCl
HOOC
9
Scheme 2. The synthetic routes to the compounds 1–6.

Y. He et al. / Tetrahedron 66 (2010) 3553–3563
3555
alkyl chains are observed at 2928 cmꢀ1 and 2856 cmꢀ1 in THF so-
Table 1
Gelation properties of 1–6a
lution, indicating the presence of a gauche-form alkyl chain. But
these IR bands of the xerogel appearing at 2924 cmꢀ1
(
n
asC–H) and
sC–H) correspond to the alkyl chain in the all-trans
form.13 Such low wavenumber shifts of the IR bands of
asC–H and
sC–H demonstrate the decreased mobility of the alkyl chain. Con-
solvent
1
2
3
4
5
6
2852 cmꢀ1
(n
CHCl3
THF
S
S
S
P
P
P
VS
VS
VS
I
G (2.0)
G (3.0)
G (1.0)
I
I
P
I
I
I
G (20)
S
S
P
P
P
G (5.0)
G (5.0)
G (5.0)
I
S
S
S
WG
G (5.0)
G (2.5)
S
S
S
P
P
P
P
P
P
I
n
Dioxane
EtOAc
acetone
Acetonitrile
Toluene
Xylene
Mesitylene
n-Hexane
DMF
n
G (5.0)
P
P
G (10)
G (10)
G (10)
I
I
I
I
I
I
I
I
S
sequently, it is clear that the van der Waals interaction between the
alkyl chains is present in the gel.
Moreover, for all xerogels 4 prepared from different solvents, the
peaks arising from the hydrogen-bonded amide are independent on
solvents and are consistent with those in the solid states, indicating that
the nature of intermolecular hydrogen bonding in the solid and gel is
the same, and does not significantly depend on the gelling solvents.
The peaks of other compounds assigned to hydrogen-bonded
amides and alkyl chains are summarized in Table 2.
I
P
S
S
S
S
a
S¼solution, P¼precipitation, G¼gelation, I¼insoluble, VS¼viscous solution,
WG¼weak gel. The values in the brackets denote the CGC values (mg mLꢀ1) of
gelators at 25 ꢁC.
Table 2
relatively poor gelation ability in aromatic solvents, but it can gelate
ethyl acetate. With further increase of the number of amide bond, 5
loses the gelation ability in aromatic solvents, but can gelate THF
and dioxane. When the linking position of amide is changed from
p- to m-position of the phenyl ring, 6 completely loses gelation
ability in tested solvents. According to Masamichi’s results,11 the m-
substituted phenyl ring region could not be flexible enough, and
consequently, it is difficult for 6 to form the effective 1 D aggregate.
Evidently, triurea 2 is the best organogelator for polar solvents, and
triamide 3 for aromatic solvents.
The gelation time depends on the tested solvents. In general, the
formation of a gel need one minute in aromatic solvents, three
minutes in cyclic ether solvents and ethyl acetate, and at least
15 min in chloroform. The gelation also depends on the cooling
rate. When the hot solutions in dioxane are slowly cooled to room
temperature (1 ꢁC minꢀ1), 2 and 5 do not form gel but precipitate.
The formed gels are stable at room temperature for months. It
should be noted that gels 2 and 3 in chloroform can be damaged by
modest shake with hand.
FT-IR data of xerogels 2–5 from different solvents (cmꢀ1
)
Xerogel
nN–H
nC]O
dN–H
n
asC–H
nsC–H
2 (CHCl3)
2 (THF)
2 (Dioxane)
3 (CHCl3)
3 (Toluene)
4 (EtOAc)
4 (CHCl3/n-hexane)
4 (THF/n-hexane)
4 (Toluene)
5 (THF)
3324
3323
3333
3292
3291
3299
3299
3299
3300
3205
3208
1645
1643
1644
1660
1659
1635
1636
1635
1636
1601
1601
1555
1555
1559
1524
1526
1544
1545
1544
1544
1471
1471
2923
2922
2925
2923
2923
2923
2924
2923
2923
2922
2924
2852
2852
2854
2852
2852
2852
2852
2852
2852
2853
2853
5 (Dioxane)
For xerogel 2 from THF, the N–H and C]O stretching and N–H
bendingvibration modes ofureagroupsarelocatedat3323,1643, and
1555 cmꢀ1, characteristic of hydrogen-bonding mode of stabilization
of aggregates of urea derivatives.14 The xerogels 3 and 5 also have the
similar behaviors. The absorption bands of 3 and 5 arising from N–H
and C]O stretching and N–H bending vibration are observed at 3292,
1660, 1524 cmꢀ1 and 3205, 1601, 1471 cmꢀ1, respectively, indicating
the hydrogen bonding interactions between amide groups.15
The gel test also shows that 2–5 are moderately soluble in many
solvents, suggesting that the intermolecular interaction is weak in
contrast to that of para-terphenylen-1,400-ylenebis(dodecanamide)
and there couldn’t exist a synergistic effect to promote extremely
steady aggregation.12
2.4. NMR investigation
NMR technique is employed to probe the assembly process. Due
to poor solubility of 2 and 5 in CDCl3, only 3 and 4 were charac-
terized by concentration-dependent (CD) and temperature-
2.3. FT-IR study
1
IR spectroscopy is a powerful tool for studying hydrogen
bonding and van der Waals forces. Figure 2 shows FT-IR spectra of 4
in THF solution and xerogel. In a THF solution, the typical absorp-
tion bands, characteristic of the non-hydrogen bonded amide
dependent (TD) NMR methods. In H NMR spectra of 4 at 27 ꢁC,
the peaks assigned to amide shift downfield with increasing con-
centration (Fig. 3a), derived from multimolecular aggregation. This
is also supported by TD-NMR experiments. With increasing tem-
perature, the peaks assigned to amide shift upfield (Fig. 3b).
Moreover, these signal peaks become broader at higher concen-
tration or lower temperature. All these observations point toward
the intermolecular aggregation of 4 in solution by hydrogen
bonding interaction. Compound 3 also exhibits the similar prop-
erties (Fig. 3c–d). However, the proton signals assigned to the tri-
phenylbenzene unit don’t exhibit a significant shift, suggesting that
groups, are observed around 3455 cmꢀ1
and 1527 cmꢀ1
ethyl acetate appear around 3298 cmꢀ1
and 1545 cmꢀ1
dN–H) due to hydrogen bonded amide groups. These
(
nN–H), 1654 cmꢀ1
dN–H). In contrast, the IR peaks of the xerogel from
nN–H), 1636 cmꢀ1
nC]O
(nC]O)
(
(
(
)
(
results indicate that one of the driving forces for organogelation is
intermolecular hydrogen bonding between the amide groups.
In addition, the absorption bands arising from the antisym-
metric (
n
asC–H) and symmetric (
n
sC–H) stretching vibrations of the
the intermolecular
p–
p
interactions are relatively weak.
1505
1545
2856
2852
3298
N-H
1636
1654
C-H
as
1501
N-H
C-H
s
C=O
3298
2928 2924
1527
1700
1650
1600
1550
1500
1450
2950 2925 2900 2875 2850 2825 2800
3500 3450 3400 3350 3300 3250 3200
-1
-1
-1
wavenumber / cm
wavenumber / cm
wavenumber / cm
Figure 2. FT-IR spectra of 4 in THF solution (1.0ꢂ10ꢀ2 M) (dashed line) and xerogel from ethyl acetate (solid line).

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Y. He et al. / Tetrahedron 66 (2010) 3553–3563
a
b
e
d
Figure 3. Concentration and temperature dependence of chemical shifts of 4 (a and b) and 3 (c and d).
According to FT-IR and NMR analyses, the aggregation of the
gelator molecules is promoted mainly by hydrogen bonding and van
de Waals interaction. Furthermore, referred to abovementioned gel
test, it can be seen that the magnitudes of intermolecular hydrogen
bonding inter-actions are qualitatively in the order of –CONHNHCO–,
–NHCONH–>–NHCO–>–CONH–>–NHCSNH–.
2.5. DSC analyses
The thermal properties of the gels were investigated by differ-
ential scanning calorimetry (DSC). Representative sets of thermo-
grams with gels 2 in dioxane, 3 in toluene, 4 in toluene, and 5 in
dioxane are shown in Figure 4. The DSC curves of gels 3 and 4 in
toluene clearly exhibit very broad endothermic transitions upon
a
b
c
d
Figure 4. DSC curves of gels 2 in dioxane (a), 3 in toluene (b), 4 in toluene (c), and 5 in dioxane (d) with a concentration of 1.52ꢂ10ꢀ3 mol Lꢀ1, 2.23ꢂ10ꢀ2 mol Lꢀ1, 1.06ꢂ10ꢀ2 mol Lꢀ1
,
and 3.9ꢂ10ꢀ3 mol Lꢀ1, respectively. Heating and cooling rate, 2 ꢁC minꢀ1
.

Y. He et al. / Tetrahedron 66 (2010) 3553–3563
3557
melting with the maxima at 115.4 ꢁC and 102.1 ꢁC, respectively,
characteristic of the supramolecular aggregates. When the mix-
tures are cooled down, the sharp exothermic transitions are ob-
served with maxima at 98.7 ꢁC and 94.8 ꢁC, respectively. Repeated
cycle shows similar transition behaviors. These phenomena suggest
that the sol-gel transitions of gels 3 and 4 in toluene are thermo-
dynamically reversible.
Particularly, the endothermic transitions disappear in the second
cycle. Combined with above-mentioned experimental results that 2
and 5 form precipitate on cooling slowly, a conclusion can be drawn
that the formations of gels 2 and 5 in dioxane are kinetically
controlled.
2.6. Microscopy
The DSC curves of gels 2 and 5 in dioxane also show broad en-
dothermic transitions on heating, but no evident exothermic
transition on cooling in the first heating and cooling cycle.
It is well known that a supramolecular gel is often formed by
self-assembled nanofibers. The scanning electronic microscopy
Figure 5. SEM images of the xerogels 2 from chloroform (a), dioxane (b), and THF (c), precipitate 2 in dioxane when cooling slowly (d), xerogel 3 from toluene (e), xerogel 4 from
ethyl acetate (f), and xerogel 5 from THF (g) and dioxane (h).

3558
Y. He et al. / Tetrahedron 66 (2010) 3553–3563
Figure 6. TEM images of gels of 2 from dioxane (a) and THF (b).
(SEM) images of xerogels 2 prepared from chloroform, dioxane, and
THF, show a three-dimensional network formed by the entangle-
ment of the nanofibers with a length of several micrometers and
a diameter of 30–40 nm, 70–80 nm, and 60–200 nm, respectively
(Fig. 5a–c), which is consistent with the gel transparency. In
transparent chloroform gel 2, the diameters of fibers are smaller
than those in opaque gels 2 from THF or dioxane. The large di-
ameter of nanofibers may result in the opaqueness of a gel.16 In-
terestingly, the precipitate of 2 formed by cooling slowly in dioxane
is composed of thicker fibers than gel 2 in dioxane (Fig. 5b and 5d).
Slow cooling rate and strong intermolecular hydrogen bonding
interaction can together promote further aggregation of 1 D poly-
meric chains of gelator molecules.
The transmission electronic microscopy (TEM) images of gels 2
in dioxane and THF also demonstrated the existence of a well-
developed network structure composed of fibrous aggregates
(Fig. 6). The aspect ratios and diameters of the observed fibers by
TEM are in accordance with the observations from SEM images. The
gels 3–5 from different solvents also consist of many nanofibers as
confirmed by SEM and TEM. In particular, the nanofibers of gel 5 in
THF are longer than those in dioxane, and many fibers in dioxane
seem broken, consistent with the observation that the gel in di-
oxane is easily damaged.
of water droplets on glass slides coated with xerogels 2–4 from
different solvents. The surface wettabilities are strongly dependent
on the linker units and gelling solvents.17 In comparison, the
xerogel film of 4 from ethyl acetate gives a hydrophobic surface
with a maximum contact angle of 140.7ꢁ (Fig. 7e).
2.7. UV–vis and photoluminescence (PL) spectra
The UV–vis absorption spectra of 2–5 in gel and dilute solution
were measured (Fig. 8). For all compounds in gel, the absorption
peaks assigned to the triphenylbenzene display a broader band
than those in dilute solution, characteristic of the supramolecular
aggregates. It should be noted that for gel 5, an extra shoulder peak
appears at w325 nm in contrast to dilute solution (Fig. 8d), in-
dicating the existence of some aggregated species.
Figure 9 shows the emission spectra of 2–5 in gel and solution at
different concentrations. The changes of the fluorescence in-
tensities of 2–4 in solution from 2ꢂ10ꢀ6 M to 2ꢂ10ꢀ4 M are typical
of aggregation quenching. In particular, the intensity of maximum
emission peak of 5 in solution first decreases and then increases in
the same concentration range, suggesting that 5 possesses strong
self-association ability even in very dilute solution. Moreover, the
fluorescence spectra of gels 2–5 exhibit several new peaks in the
long wavelengths in contrast to those in dilute solution, pre-
sumably due to the aggregated species.18
Specific morphologies of the surfaces of xerogel may possess
quite different surface wettabilities. Figure 7 displays some pictures
Figure 7. Water droplets on glass slides coated with xerogels 2 from chloroform (a), THF (b), and dioxane (c), xerogels 3 from toluene (d), xerogels 4 from ethyl acetate (e), and
toluene (f), and xerogels 5 from THF (g) and dioxane (h).

Y. He et al. / Tetrahedron 66 (2010) 3553–3563
3559
b
a
c
d
Figure 8. Absorption spectra of 2 in chloroform solution and gel (a), 3 in toluene solution and gel (b), 4 in THF solution and in THF/n-hexane gel (c), and 5 in THF solution and gel (d).
a
b
c
d
Figure 9. Fluorescence spectra of 2 in chloroform solution and gel (a), 3 in toluene solution and gel (b), 4 in toluene solution and gel (c) and 5 in THF solution and gel (d).
lex¼290 nm.
Interestingly, the maximum emission peak of 5 in gel is red-
shifted by ~52 nm in comparison with that in dilute solution,
while the maximum emissions of gels 2–4 only exhibit the slight
shifts. The triphenylbenzene core is not planar due to the steric
hindrance and the hydrogen bonding interactions between the
linkers. The phenyl ring might rotate to some degree to cope with
the formation of the hydrogen bonding.9 For gels 2–4, the
hydrogen bonding between the linkers may be formed flexibly so
that a propeller-like conformation of the cores is still favorable.
However, a rigid connecting mode (two adjacent hydrogen
bonds) between the –CONHNHCO– linkers in gel 5 can promote
the cofacial stacking of the cores (Fig. 10). Thus, a better conju-
gation between the cores results in the red-shift of emission
bands.19

3560
Y. He et al. / Tetrahedron 66 (2010) 3553–3563
a
b
800
N
N
H
H
N
N
H
H
H
H
N
N
N
N
N
O
H
O
H
O
H
O
O
O
600
400
200
0
d
c
O
O
O
H
H
H
N
N
N
O
O
O
H
H
H
N
N
N
N
N
N
H
H
H
O
O
O
350
400
450
500
550
wavelength / nm
Figure 10. The hydrogen-bonding interactions between urea (a), amide (b and c), and
–CONHNHCO– (d) linkers.
Figure 11. Fluorescence spectra of 4 in THF solution (dash line) and in toluene gel
(solid line) at the same concentration (1.1ꢂ10ꢀ2 mol Lꢀ1). lex¼327 nm.
Moreover, a remarkable phenomenon is the fluorescence en-
hancement of 4 obtained in the gelation (Fig. 11). When excited at
327 nm, gel 4 in toluene exhibits stronger emission than a THF
solution of 4 at the same concentration. This is considered as the
consequence of aggregation-induced enhanced emission.20 The
gelator molecule in solution is conformationally flexible, which
typically renders molecules experience faster non-radiative decay
from excited state. Upon formation of gel, however, the resulting
ordered aggregates due to intermolecular hydrogen bonding
greatly reduce the conformational flexibility and, therefore, slow
down the non-radiative decay processes with enhanced fluores-
cence quantum yields.
discotic molecules are apt to be arranged into a columnar
architecture.1,3,21 However, Lu et al. previously reported that
when alkoxy sidechains of the discotic molecules based on tri-
phenylbenzene become longer or the number of chains is in-
creased, the columnar architecture can be transformed into the
lamellar one.9 As shown in Figure 12, the WAXD patterns of
xerogels 2–5 reveal a lamellar structure with the interlayer dis-
tance of 2.92 nm, 2.49 nm, 2.72 nm, and 2.39 nm, respectively, in
the low angle region with the reciprocal spacing ratio of 1:1/2:1/3.
These results show that different linker units don’t significantly
affect the secondary accumulation structure. In addition, the dif-
fractions in wide-angle region give a very broad peak around 20ꢁ,
attributable to the packing of the alkyl chains by van der Walls
interaction.22
2.8. Wide angle X-ray diffraction (WAXD)
In order to study the molecular packing structures of the gel-
fibers, WAXD were performed on the xerogels. Generally, the
a
b
c
d
Figure 12. WAXD patterns of xerogels 2 (a), 3 (b), 4 (c), and 5 (d).

Y. He et al. / Tetrahedron 66 (2010) 3553–3563
3561
2.9. The effect of anion on gel
a
Considering that the urea moieties are able to bind anion, the
effect of anion on gel was assessed. A chloroform gel 2 is trans-
formed into a homogenous solution after the addition of 3 equiv of
Fꢀ ions (Fig. 13), while only partial disintegration of the gel is
caused on the addition of 1 equiv of Fꢀ ion. However, addition of
3 equiv of other anions, such as Iꢀ, Brꢀ, Clꢀ, BFꢀ4 , and PF6ꢀ does not
result in the gel-sol phase transition under the same condition.
Therefore, gel 2 may be used to selectively recognize Fꢀ ion.
b
Figure 13. Illustration of the gel-sol phase transition of 2 by the addition of 3 equiv of
(n-C4H9)4NF (TBAF).
The 1H NMR spectrum of 2 in CDCl3 is also recorded in the
presence of 3.0 equiv of Fꢀ ions. The proton signals assigned to urea
groups disappear, due to deprotonation by Fꢀ ion according to
previous reports.23 Thus, the intermolecular H-bonding between
the urea moieties would be disrupted in the presence of Fꢀ ion,
leading to the gel-sol phase transition. This also suggests the im-
portance of hydrogen bonding in the formation of the gel. With
increasing amounts of Fꢀ ion, the proton signals attributable to
aromatic part shift downfield, indicative of the reduction of in-
Figure 15. The UV–vis spectra of gel 2 before (dashed line) and after (solid line)
addition of 3 equiv of Fꢀ ions (a) and normalized fluoresc-ence spectra of gel 2 after
addition of 3–9 equiv of Fꢀ ions (b).
l
¼360 nm, c¼3.0ꢂ10ꢀ3 mol Lꢀ1
.
termolecular p–p interactions (Fig. 14).
–NHCONH–>–NHCO–>–CONH–>–NHCSNH–,
consequently
resulting in different gelation behaviors. The sol-gel transitions of
gels 3 and 4 in toluene are thermodynamically reversible, and the
formations of gels 2 and 5 in dioxane are kinetically controlled.
Slow cooling will lead to the precipitations of 2 and 5. In gels 2–4,
a propeller-like conformation of the triphenylbenzene core is still
favorable, but the triphenylbenzene is approximately coplanar in
gel 5. The urea-based gel 2 can selectively recognize Fꢀ ion.
4. Experimental
4.1. Materials
Figure 14. 1H NMR spectral changes of 2 in CDCl3 at 27 ꢁC upon the addition of 3–
9 equiv of TBAF. c¼1.0ꢂ10ꢀ3 mol Lꢀ1
.
UV–vis and fluorescence spectra are also used to monitor the
process. When the gel is completely turned into clear solution on
addition of 3 equiv of Fꢀ ions, the maximum absorption peak red-
shifts from 277 nm to 292 nm (Fig. 15a), and the fluorescence in-
tensity is weakened. Furthermore, on addition of 3–9 equiv of Fꢀ
ions, the maximum emission peak red-shifts from 403 nm to
434 nm (Fig. 15b). These can be well understood as a result of
deprotonation of urea groups.
All reagents and solvents are obtained from commercial sup-
plies and used directly without further purification unless other-
wise stated. The solvents for spectroscopic studies are purified
according to standard methods.
4.2. Synthesis of the compounds investigated
4.2.1. 1,3,5-Tri(4-(3-dodecylthioureido)phenyl)benzene 1. A solution
of dodecylisothiocyanate (2.14 g, 9.39 mmol) in dry THF (20 mL)
3. Conclusions
was slowly added to a solution of 1, 3, 5-tris(4-amino-
phenyl)benzene (1.00 g, 2.85 mmol) in dry THF (30 mL) under ni-
trogen. The reaction mixture was stirred overnight at room
temperature and then refluxed for 5 h. After cooling to room
temperature, the solvent was removed under vacuum and the
residue was purified by silica column chromatography (dichloro-
methane/THF¼6/1) followed by recrystallization with ethyl acetate
to give the off-white solid (1.42 g, 48%). 1H NMR (CDCl3, 400 MHz)
In this work, we have described the synthesis and aggregation
properties of a family of C3-symmmetrical discotic molecules
tailored with various hydrogen bonding motifs as linkers. The
molecular aggregation of 2–5 can be promoted by hydrogen
bonding interactions between linkers and van der Waals in-
teractions between the hydrophobic alkyl sidechains. However, the
p
–
p
stacking between triphenylbenzene cores is not significant.
d
(ppm): 7.75 (s, 3H), 7.73 (d, J¼8.0 Hz, 6H), 7.70 (s, 3H), 7.33 (d,
The magnitudes of hydrogen bonding interactions between the
triphenylbenzene-centered molecules with approximately identi-
cal side chain length are qualitatively in the order of –CONHNHCO–,
J¼8.0 Hz, 6H), 6.09 (s, 3H), 3.66 (m, 6H), 1.60 (m, 6H), 1.24–1.31 (m,
54H), 0.86 (t, J¼8.0 Hz, 9H); 13C NMR (CDCl3, 100 MHz)
d (ppm):
180.48, 141.34, 139.24, 136.04, 128.83, 125.33, 125.01, 45.65, 31.87,

3562
Y. He et al. / Tetrahedron 66 (2010) 3553–3563
29.60, 29.54, 29.50, 29.30, 29.25, 28.98, 26.90, 22.65, 14.09; FT-IR
(KBr, cmꢀ1): 3241, 2923, 2852, 1539, 1512, 1466, 1446, 1316, 1248,
829, 700, 521; MS (ESI) (m/z) calcd for C63H96N6S3: 1032.7, found:
1033.1. Anal. Calcd for C63H96N6S3: C, 73.20; H, 9.36; N, 8.13. Found:
C, 73.07; H, 9.39; N, 8.09.
127.21, 125.25, 33.31, 31.28, 29.01, 28.80, 28.70, 28.57, 25.08, 22.07,
13.92; FT-IR (KBr, cmꢀ1): 3205, 2955, 2922, 2853, 1601, 1562, 1471,
1070, 956, 843; MS (ESI) (m/z) calcd for C63H90N6O6: 1026.7, found:
1027.2. Anal. Calcd for C63H90N6O6: C, 73.65; H, 8.83; N, 8.18. Found:
C, 73.70; H, 8.79; N, 8.25.
4.2.2. 1,3,5-Tri(4-(3-dodecylureido)phenyl)benzene 2. A solution of
dodecylisocyanate (2.11 g, 9.96 mmol) in dry THF (20 mL) was
slowly added to a solution of 1,3,5-tris(4-aminophenyl)benzene
(1.00 g, 2.85 mmol) in dry THF (30 mL). The reaction mixture was
stirred overnight at room temperature and then refluxed for 5 h.
After cooling to room temperature, the gel-like reaction mixture
was concentrated, filtered, and washed with ethyl acetate to give
a white waxy solid. Finally, the white solid was dried for 6 h at 60 ꢁC
under vacuum (1.03 g, 73%). 1H NMR (DMSO-d6, 400 MHz, 373 K)
4.2.6. 1,3,5-Tri(3-dodecylcarbamoylphenyl)benzene 6. 1,3,5-Tri-(m-
carbonylphenyl)benzene (0.50 g, 1.14 mmol) in SOCl2 (30 mL) was
refluxed overnight and excess SOCl2 was removed in vacuo. The
obtained carbonyl chloride was dissolved in dry THF (40 mL). Then
a solution containing dodecan-1-amine (0.85 g, 4.56 mmol) and
triethylamine (0.95 mL, 6.84 mmol) was added dropwise at 0 ꢁC.
After the reaction mixture was stirred for 24 h at room tempera-
ture, the solvent was evaporated under vacuum and the residue
was purified by silica gel chromatography (CH2Cl2/EtOAc¼10/1) to
give pure white solid (0.95 g, 89%). 1H NMR (CDCl3, 400 MHz)
d
(ppm): 8.20 (s, 3H), 7.70 (s, 3H), 7.65 (d, J¼8.4 Hz, 6H), 7.50 (d,
J¼8.4 Hz, 6H), 5.94 (br, 3H), 3.14 (m, 6H),1.50 (m, 6H),1.33 (m, 54H),
d
(ppm): 8.06 (s, 3H), 7.77 (s, 3H), 7.74 (m, 6H), 7.51 (t, J¼7.6 Hz, 3H),
0.88 (t, J¼6.4 Hz, 9H); 13C NMR (DMSO-d6, 100 MHz, 373 K)
6.37 (br, 3H), 3.48 (m, 6H), 1.66 (m, 6H), 1.25–1.38 (m, 54H), 0.87 (t,
d
(ppm): 154.70, 140.77, 139.78, 132.70,126.43, 121.84, 117.79, 38.75,
J¼6.8 Hz, 9H); 13C NMR (CDCl3, 100 MHz)
d (ppm):167.50, 141.59,
30.53, 29.11, 28.24, 28.04, 27.87, 25.74, 21.21, 12.91; FT-IR (KBr,
cmꢀ1): 3323, 2922, 2852, 1643, 1588, 1555, 1528, 1234, 1069, 833,
653, 526; MS (ESI) (m/z) calcd for C63H96N6O3: 984.8, found: 985.2.
Anal. Calcd for C63H96N6O3: C, 76.78; H, 9.82; N, 8.53. Found: C,
76.67; H, 9.89; N, 8.49.
141.04, 135.44, 130.12, 128.99, 125.96, 125.43, 40.31, 31.89, 29.61,
29.34, 27.07, 22.66, 14.09; FT-IR (KBr, cmꢀ1): 3290, 2956, 2921, 2851,
1631, 1541, 1468, 1327, 869, 811, 698; MS (ESI) (m/z) calcd for
C63H93N3O3: 939.7, found: 940.3. Anal. Calcd for C63H93N3O3: C,
80.46; H, 9.97; N, 4.47. Found: C, 80.43; H, 10.03; N, 4.39.
4.2.3. 1,3,5-Tri(4-dodecanamidophenyl)benzene 3. Dodecanoyl chlo-
ride (1.40 g, 6.40 mmol) was dissolved in dry THF (10 mL), and the
solution was added dropwise to dry THF (15 mL) solution consisting
of 1,3,5-tris(4-aminophenyl)-benzene (0.5 g, 1.42 mmol) and trie-
thylamine (1.20 mL, 8.54 mmol) at 0 ꢁC. After stirring at room
temperature for 24 h, the reaction mixture was purified by silica gel
column chromatography (dichloromethane/THF¼1/1) to give pure
4.2.7. 1,3,5-Tri(4-aminophenyl)benzene 7. Tetrachloride silicon
(20 mL, 0.18 mol) was added dropwise to a solution of p-nitro-
acetophenone (10.00 g, 0.06 mol) in absolute ethanol (60 mL) at
0 ꢁC. A yellow precipitate was formed immediately and then the
reaction mixture was refluxed for 10 h. After cooled to room
temperature, the mixture was poured to ice-water and stirred for
10 min. The resulting yellow precipitate was filtered and dried,
and added to ethanol (200 mL) directly with Pd/C (0.8 g), which
was heated to reflux. Then NH2NH2$H2O (80%, 25 mL) was added
dropwise to the hot solution. After refluxing for 10 h, the pre-
cipitate was taken off by filtration and the solution was cooled to
room temperature, and the white precipitate was collected. The
solid was recrystallized in ethanol and dried under vacuum. 1H
white product (0.87 g, 65%). 1H NMR (CDCl3, 400 MHz)
d (ppm):
7.65 (s, 3H), 7.61 (s, 12H), 7.26 (s, 3H), 2.39 (t, J¼8.0 Hz, 6H), 1.75 (m,
6H), 1.26–1.40 (m, 48H), 0.88 (t, J¼8.0 Hz, 9H); 13C NMR (DMSO-d6,
150 MHz)
d (ppm): 171.28, 141.02, 139.01, 134.57, 127.21, 122.87,
119.27, 36.42, 31.25, 28.98, 28.95, 28.89, 28.75, 28.67, 28.63, 25.06,
22.05, 13.89; FT-IR (KBr, cmꢀ1): 3292, 2923, 2852, 1660, 1599, 1524,
825, 527; MS (ESI) (m/z) calcd for C60H87N3O3: 897.7, found: 898.1.
Anal. Calcd for C60H87N3O3: C, 80.22; H, 9.76; N, 4.68. Found: C,
80.35; H, 9.59; N, 4.62.
NMR (DMSO-d6, 400 MHz)
d
(ppm): 7.48 (s, 3H), 7.47 (d, J¼8.8 Hz,
6H), 6.66 (d, J¼8.8 Hz, 6H), 5.22 (s, 6H); 13C NMR (DMSO-d6,
150 MHz)
d (ppm): 148.31, 141.57, 128.06, 127.42, 120.37, 114.18;
FT-IR (KBr, cmꢀ1): 3434, 3355, 3210, 3028, 1621, 1607, 1515, 1280,
4.2.4. 1,3,5-Tri(4-dodecylcarbamoylphenyl)benzene 4. 1,3,5-Tri-(p-
carbonylphenyl)benzene (0.60 g, 1.14 mmol) in SOCl2 (30 mL) was
refluxed overnight and excess SOCl2 was removed. The resulting
carbonyl chloride was dissolved in dry THF (50 mL). Then, a solu-
tion consisting of dodecan-1-amine (1.02 g, 5.47 mmol) and trie-
thylamine (1.20 mL, 8.21 mmol) in dry THF (20 mL) was added
dropwise. The reaction mixture was stirred overnight. The product
was purified by silica gel column chromatography (CH2Cl2/
1177, 829; MS (ESI) (m/z) calcd for C24H21N3: 351.2, found: 351.5.
4.2.8. 1,3,5-Tri(p-carboxyphenyl)benzene 8. To a mixture of 1,3,5-
tribromobenzene (5.0 g, 15.88 mmol), p-methoxy-carbonylphenyl
boronic acid (11.43 g, 63.53 mmol), Na2CO3 (8.42 g, 79.41 mmol),
and Pd(PPh3)4 (1.38 g, 1.19 mmol) were added degassed toluene–
methanol–water (80/40/40 mL) under an argon atmosphere. The
resulting reaction mixture was stirred for 48 h under reflux. After
removal of the solvent, the residue was extracted with dichloro-
methane (80ꢂ3 mL), washed with brine (80 mL), dried over an-
hydrous MgSO4 and concentrated in vacuo. The residue was
purified by silica gel column chromatography (petroleum ether/
dichloromethane¼1/5) to give 1,3,5-tri(p-methoxycarbonyl-
phenyl)benzene (5.41 g, 71%), which was hydrolyzed with 6 M
NaOH to afford the title compound. 1H NMR (DMSO-d6, 400 MHz)
EtOAc¼1/1) (0.86 g, 67%). 1H NMR (CDCl3, 400 MHz)
d (ppm): 7.81
(d, J¼8.0 Hz, 6H), 7.65 (s, 3H), 7.59 (d, J¼8.0 Hz, 6H), 6.48 (br, 3H),
3.48 (m, 6H),1.65 (m, 6H),1.26–1.40 (m, 54H), 0.88 (t, J¼6.8 Hz, 9H);
13C NMR (CDCl3, 100 MHz)
d (ppm): 167.17, 143.28, 141.38, 134.05,
127.54, 127.20, 125.44, 40.24, 31.89, 29.68, 29.60, 29.33, 27.06,
22.66, 14.08; FT-IR (KBr, cmꢀ1): 3299, 2924, 2852, 1636, 1545, 1505,
1466, 1438, 1314, 845, 767, 694; MS (ESI) (m/z) calcd for
C63H93N3O3: 939.7, found: 940.2. Anal. Calcd for C63H93N3O3: C,
80.46; H, 9.97; N, 4.47. Found: C, 80.53; H, 10.05; N, 4.40.
d
(ppm): 13.04 (s, 3H), 8.10 (s, 3H), 8.06 (s, 12H); 13C NMR (DMSO-
d6, 150 MHz)
d (ppm): 167.11, 143.81, 140.72, 130.02, 129.88, 127.39,
125.54; FT-IR (KBr, cmꢀ1): 3382, 3014, 2660, 2538, 1697, 1608, 1569,
1512, 1447, 1416, 1393, 1317, 1286, 1241, 1181, 1016, 847, 764; MS
(ESI) (m/z) calcd for C27H18O6: 438.1, found: 438.6.
4.2.5. 1,3,5-Tri(4-dodecanamidocarbamoylphenyl)benzene
5. Compound 5 was synthesized as described for 4. 1H NMR
(DMSO-d6, 400 MHz)
d (ppm): 10.38 (s, 3H), 9.85 (s, 3H), 8.11 (s,
3H), 8.09 (d, J¼8.0 Hz, 6H), 8.03 (d, J¼8.0 Hz, 6H), 2.20 (m, 6H), 1.57
4.2.9. 1,3,5-Tri(m-carboxyphenyl)benzene 9. Compound 9 was syn-
(m, 6H), 1.27 (m, 48H), 0.86 (t, J¼6.4 Hz, 9H); 13C NMR (DMSO-d6,
thesized as described for 8. 1H NMR (DMSO-d6, 400 MHz)
d
(ppm):
100 MHz)
d (ppm): 171.58, 165.04, 142.78, 140.67, 131.66, 128.02,
13.12 (s, 3H), 8.36 (s, 3H), 8.17 (d, J¼7.6 Hz, 3H), 8.01 (d, J¼7.6 Hz,

Y. He et al. / Tetrahedron 66 (2010) 3553–3563
3563
3H), 7.99 (s, 3H), 7.66 (t, J¼7.6 Hz, 3H); 13C NMR (DMSO-d6,
Schenning, A. P. H. J.; Meijer, E. W. Chirality 2008, 20, 1016; (c) Ollvler, C.; Solarl,
´
´
E.; Scopelltl, R.; Severln, K. Inorg. Chem. 2008, 47, 4454; (d) Martın-Rapun, R.;
Byelov, D.; Palmans, A. R. A.; de Jeu, W. H.; Meijer, E. W. Langmuir 2009, 25,
8794; (e) van Herrikhuyzen, J.; Jonkheijm, P.; Schenning, A. P. H. J.; Meijer, E. W.
Org. Biomol. Chem. 2006, 4, 1539; (f) Castaldi, M. P.; Gibson, S. E.; Rudd, M.;
White, A. J. P. Chem.dEur. J. 2006, 12, 138; (g) Hennrich, G.; Ortiz, P. D.; Cavero,
E.; Hanes, R. E.; Serrano, J. L. Eur. J. Org. Chem. 2008, 4575; (h) Benincori, T.;
Marchesi, A.; Mussini, P. R.; Pilati, T.; Ponti, A.; Rizzo, S.; Sannicolo`, F. Chem.dEur.
J. 2009, 15, 86; (i) Feng, X.; Pisula, W.; Zhi, L.; Takase, M.; Mu¨llen, K. Angew. Chem.,
Int. Ed. 2008, 47, 1703; (j) Schnopp, M.; Haberhauer, G. Eur. J. Org. Chem. 2009,
4458; (k) Castaldi, M. P.; Gibson, S. E.; Rudd, M.; White, A. J. P. Angew. Chem., Int.
Ed. 2005, 44, 3432; (l) Wang, J. Y.; Yan, J.; Ding, L.; Ma, Y.; Pei, J. Adv. Funct. Mater.
2009, 19, 1746; (m) Angiolini, L.; Benelli, T.; Giorgini, L. Chirality. doi:10.1002/chir.
150 MHz)
d (ppm): 167.24, 141.07, 140.19, 131.77, 131.55, 129.31,
128.64, 127.78, 125.00; FT-IR (KBr, cmꢀ1): 3005, 2889, 2649, 2560,
1702, 1606, 1582, 1486, 1426, 1394, 1302, 1272, 1239, 750, 687, 672;
MS (ESI) (m/z) calcd for C27H18O6: 438.1, found: 438.4.
4.3. Preparation of the gels
The gels were prepared by heating the organogelator and the
solvent in a vial (35ꢂ15 mm) until the complete dissolution of the
solid. After the solution is allowed to stand at room temperature
(25ꢃ5 ꢁC) for 30 min, the state of the mixture is evaluated by the
‘stable to inversion of test tube’ method.
´
´
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20712; (n) Pinter, A.; Haberhauer, G. Chem.dEur. J. 2008, 14, 11061; (o) Garcı-
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Frutos, E. M.; Go´mez-Lor, B.; Monge, A.; Gutie´rrez-Puebla, E.; Alkorta, I.; Elguero,
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Mu¨llen, K.; Samorı`, P. Small 2009, 5, 112; (r) van den Hout, K. P.; Martı´n-Rapu´ n, R.;
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4.4. Measurements
1H NMR and 13C NMR spectra are recorded on Bruker AV400 or
AV600 spectrometer at 300 K unless otherwise stated. Chemical
shifts are reported downfield with TMS (
d
¼0.00 ppm) as the
standard. For 13C NMR spectra, chemical shifts are reported on the
scale relative to deuterated solvent as internal standard (CDCl3,
d
¼77.00 ppm, DMSO-d6, ¼39.50 ppm). The coupling constants are
d
reported in hertz. Mass spectra (ESIMS) are recorded with a Thermo
Finnigan LCQ instrument. IR spectra for the samples of xerogels and
in THF solutions are performed on a Bio-Rad FTS135 infrared
spectrometer at room temperature. Elemental analyses (C, H, N) are
carried out with a VarioEL instrument. For differential scanning
calorimetry measurements, a DSC Q100 TA instrument is used. UV–
vis spectra are recorded on an UV-2500 UV–vis spectrophotometer
at room temperature. The fluorescent spectra are recorded on
Perkin–Elmer LS 50B fluorescence spectrophotometer at room
temperature. TEM is performed on a JEOL JEM-1011 transmission
electron microscope operated at an acceleration voltage of 100 kV.
Samples are prepared by wiping a small amount of gel samples
onto carbon-coated copper grid followed by naturally evaporating
the solvent. SEM pictures are taken using an XL 30 ESEM FEG field
emission scanning electron microscope with 20 kV operating
voltage. PXRD patterns are recorded by PW1710 BASED X-ray dif-
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fractometer with CuKa1 radiation source. CA measurements are
performed using the sessile drop method (Dataphysics, OCA 20).
The water droplets are introduced using a microsyringe, and im-
ages are captured to measure the angle of the liquid-solid interface.
Each sample is recorded at three different points. Chromatographic
purification is conducted using 100–200 mesh silica gel.
15. (a) Suzuki, M.; Nigawara, T.; Yumoto, M.; Kimura, M.; Shirai, H.; Hanabusa, K.
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Tetrahedron Lett. 2003, 44, 6841; (b) Vela´zquez, D. G.; Dı´az, D. D.; Ravelo, A. G.;
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Acknowledgements
The work is supported by the National Natural Science Funda-
tion of China (20502024) and Free Exploration Project of Innovation
III of Changchun Institute of Applied Chemistry (CX07QZJC-02).
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Supplementary data
Supplementary data associated with this article can be found in
the online version, at doi:10.1016/j.tet.2010.02.094.
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