Clockwise helical stacking of triphenylbenzene-based organogelator induced by peripheral chirality

Article abstract of DOI:10.1016/j.tet.2007.03.125

We report on the synthesis and self-assembly of a new discotic organogelator based on π-conjugated triphenylbenzene with three peripheral chiral substituents. It is found that the introduction of chiral groups to the C₃-symmetric molecule led t

Full text of DOI:10.1016/j.tet.2007.03.125

Tetrahedron 63 (2007) 7443–7448
Clockwise helical stacking of triphenylbenzene-based
organogelator induced by peripheral chirality
*
Chunyan Bao, Ming Jin, Ran Lu, Zhiguang Song, Xinchun Yang, Dongpo Song,
Tinghua Xu, Guofa Liu and Yingying Zhao
Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, College of Chemistry,
Jilin University, Changchun 130012, PR China
Received 30 October 2006; revised 12 March 2007; accepted 16 March 2007
Available online 24 March 2007
Abstract—We report on the synthesis and self-assembly of a new discotic organogelator based on p-conjugated triphenylbenzene with three
peripheral chiral substituents. It is found that the introduction of chiral groups to the C3-symmetric molecule led to a hexagonal columnar
chiral stacking of the molecules in a clockwise direction in the organogel fibers. And the fluorescent emission of the gel decreased significantly
compared to the monomer state at the same concentration, as a result of the aggregation of the nonplanar triphenylbenzene core in the gel
phase.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
stacking in gel systems due to the introduction of chirality
in the discotics.15,16,26,27 However, reports on the chiral
self-assembly of discotic molecules based on a p-conju-
gated core are limited. Such system could find potential pho-
tonics applications due to their unique optical properties. In
our previous work,28 it was found that triphenylbenzene-
based organogelators with achiral side chains can self-
assemble into right-handed and left-handed helical fibers
simultaneously with equal probability. These fibers showed
unusual aggregation-induced emission enhancement. The
formation of helical fibers was due to the helical inducement
of the core cooperating with the packing and arrangement of
the alkoxy chains. The morphologies of the obtained helixes
can be adjusted by the peripherally substituted alkoxy
chains.
Helical architectures are very common in nature, such as the
DNA double helix,1 the collagen triple helix,2 and the a-he-
lical motifs in proteins,3 and they have received considerable
interests of scientists. Recently, the control of helicity in the
supramolecular chemistry is an attractive goal because of its
possible applications in material science, chemical sensing,
enantioselective catalysis and so on.4–9 The remarkable con-
trol of helicity in supramolecular self-assemblies usually
depends on cooperative hydrogen bonding10,11 and p–p
stacking.12–14 In particular, the preparation of helical fibers
or ribbons in low molecular weight organogel systems has
attracted broad attention, with the introduction of chiral
groups, such as chiral alkyl chains,15–17 amino acids,18,19
glucose,20–22 and cholesterol,23–25 into the organogel mole-
cules playing an important role in the formation of helical
microstructures.
In this paper, we intended to bring chiral groups into the pe-
riphery of this triphenylbenzene-based system to prepare a fi-
brous organogel with a single direction of helical stacking.
Therefore, a C3-symmetrical discotic molecule 5 with chiral
side chains ((R)-(ꢀ)-1-cyclohexyl-ethylamine) connected to
the triphenylbenzene core through amide groups (as shown
in Scheme 1) was synthesized, and its self-assembly and op-
tical properties were studied. Compound 5 could form stable
fibrous organogels in aromatic solvents and CCl4, and the
introduction of the chiral groups into the periphery of the
molecules induced the hexagonal columnar stacking of
the molecules in a clockwise direction. It was also found
that the emission could be adjusted by the sol–gel transition,
which may find some useful applications in sensors and
other photoelectric devices.
To increase insight into helical stacking phenomena in orga-
nogels, the exact nature and relative importance of the sec-
ondary interactions were studied. For this purpose, discotic
molecules were selected as highly suitable building blocks
for the formation of the helical structures because of their
predisposition for stacking. Over the past few years, several
examples have been presented to show perfect helical
Keywords: Organogel; Self-assembly; Helical stacking; Triphenylbenzene;
Fluorescence.
* Corresponding author. Tel.: +86 431 8499179; fax: +86 431 8923907;
e-mail: luran@mail.jlu.edu.cn
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.03.125

7444
C. Bao et al. / Tetrahedron 63 (2007) 7443–7448
NH2
NO2
O
SiCl4
Pb/C, 50% NH2NH2•H2O
C2H5OH
O2N
C2H5OH
NH2
H2N
O2N
NO2
1
yield: 80%
2
O
HO
O
O
O
OCH3
TEA
THF
Br
+
O
Cl
N
H
K2CO3, DMF, 50 °C
H2N
H3CO
yield: 65%
3
(R)-(-)-1-Cyclohexyl-ethylamine
O
O
1M NaOH
O
3
N
H
THF, 60 °C 24 h
HO
yield: 95%
4
R
O
HN
DCC, DMAP
O
+
4
2
R =
Dry THF, 0-5 °C
O
N
H
O
N
H
NH
R
O
R
yield: 60%
5
Scheme 1. The synthetic route for compound 5.
2. Results and discussion
2.1. Synthesis
compound 5 could form stable fibrous gel in benzene, tolu-
ene, xylene, and CCl4 at a concentration of 1.0 wt %. The
obtained gels are thermoreversible in the organic solvents,
for example, the gelator is insoluble at room temperature,
turns into clear solutions by heating to reflux, and upon cool-
ing to room temperature, the immobile gel appears. This pro-
cess can be repeated many times. The microstructures of the
gels were observed by SEM and TEM. Figure 1a shows the
SEM image of the gel obtained from toluene, which exhibits
well-defined fibers with diameter of ca. 100 nm. The fibers
have a tendency to be parallel to each other and form fibrous
bundles, and then three-dimensional networks. For the gel in
CCl4, the SEM investigation did not image clear morpho-
logies, while the TEM image (see Fig. 1b) shows short inter-
twined fibers with diameter of 30–50 nm, without a network
structure. The gel in CCl4 broke into solution and precipi-
tated after one week, while the gel in toluene remained for
several months. These phenomena suggest that the chiral
compound 5 could form more stable gel in toluene than in
CCl4, and the solvents play an important role in process of
gel formation.
The synthetic route for the triphenylbenzene derivative 5 is
shown in Scheme 1. It was prepared through acylation of
1,3,5-tris(4-aminophenyl)benzene with the corresponding
chiral benzoyl chloride. Compounds 3 and 4 were synthe-
sized according to the methods described in Refs. 29 and
30. All the compounds were characterized with Fourier trans-
1
form infrared (FTIR) spectroscopy, H NMR, C, H, and N
elemental analyses and matrix assisted laser desorption ion-
ization/time-of-flight mass spectrometry (MALDI-TOFMS).
2.2. Gelation ability of compound 5
The gelation properties of the discotic compound 5 were in-
vestigated in various solvents. As summarized in Table 1,
Table 1. Gelation test of compound 5 in different solvents at 1.0 (wt/vol)%
(5.0 mg compound 5 in 5.0 mL solvent)
Entry Solvent
Gelation Entry Solvent
Gelation
2.3. Self-assembling properties of compound 5
1
2
3
4
THF
Toluene TG
Benzene TG
S
5
6
7
8
CCl4
Ethanol
Chloroform
Ethanol/water (1:2)
TG
S
S
FTIR and UV–vis absorption spectra were used to elucidate
the driving forces of the gelation process for compound
5. The FTIR spectra (not shown here) for the xerogel from
toluene showed that the N–H stretching band appeared at
Xylene
TG
G
G, turbid gel; TG, stable transparent gel; S, soluble.

C. Bao et al. / Tetrahedron 63 (2007) 7443–7448
7445
ethanol solution at 10-5 M
CCl4 gel at 1.0 wt%
200
250
300
350
400
450
500
wavelength (nm)
Figure 2. UV–vis absorption spectra (the absorptions around 230–300 nm
for the triphenylbenzene were normalized) of compound 5 in CCl4 gel at
1.0 wt % (the dash line), and in ethanol solution at 10ꢀ5 M (the solid line).
compound 5 suggests that the triphenylbenzene cores of the
molecules have formed coplanar stagger or co-facial stack-
ing. The optimized Chemdraw 3D CPK model of the mole-
cule 5 showed that the triphenylbenzene core was not planar
and the dihedral angle between the benzene rings was about
35ꢂ in the monomeric state. Therefore, the triphenylbenzene
core must form co-facial stacking in the self-assembly of
compound 5. By combining these facts with the results of
FTIR spectra, it is concluded that compound 5 in the gel
should form helical stacking by the cooperation of p–p in-
teractions and H-bonds, as reported in our previous work.28
Figure 1. (a) SEM image of gel obtained from toluene and (b) TEM image
of gel obtained from CCl4, the concentration in both is 1.0 wt %.
3321 cmꢀ1 and two amide bands were located at 1663 and
1641 cmꢀ1, respectively, with nearly equal intensities. As
known, peaks of 3321 and 1641 cmꢀ1 can be attributed
to the characteristic peaks of intermolecular H-bonded
amides,31,32 which indicates that hydrogen bonding has par-
ticipated in the formation of the gel. However, the peak located
at 1663 cmꢀ1 can be ascribed to free amide units, in other
words, there are some non-H-bonded amide groups in the
gel phase. As shown in Scheme 1, there are six amide groups
in compound 5, three of them are connected directly to the
triphenylbenzene core while the others are further away
from the benzene rings and are connected to chiral cyclo-
hexyl groups at periphery. Based on the molecular conforma-
tion, it is reasonable to deduce that the amide groups close to
the triphenylbenzene can more easily form H-bonds due to
the p–p stacking of the cores than the peripheral ones,
which existed as free amides.
CD spectroscopy is a very good tool to test the chiral struc-
tures of the aggregates,21,22 so the CD spectra of the gel and
of the solution were compared to investigate the effect of
a chiral periphery on the stacking of the discotic molecules.
As shown in Figure 3, the ethanol solution of compound 5
does not show any CD signal, while the gel in CCl4 shows
a strong positive Cotton effect, and the crossover lq¼0 value
is very close to the lmax values (280 nm) in the UV–vis ab-
sorption spectra, which suggests that CD bands are attributed
6
3
0
Figure 2 shows the UV–vis spectra of the CCl4 gel at
1.0 wt % (7.2ꢁ10ꢀ3 M, tested using the 0.1 cm thin quartz
cell) and of the ethanol solution of compound 5 at a concen-
tration of 1ꢁ10ꢀ5 M (in monomer state since the ethanol can
break hydrogen bonds). The absorption band red-shifts to
281 nm in the gel phase compared with that at 256 nm in
the ethanol solution, which indicates that J-aggregates have
formed in the molecules in gel phase as a result of the p–p
interaction between triphenylbenzene cores. This actually
plays a key role in the formation of the gel. Generally, the
discotic molecules prefer to form H-aggregates by the stack-
ing of the coplanar cores, so the formation of J-aggregates in
-3
ethanol solution at 10-4
gel from CCl4
M
-6
250
300
350
400
wavelength (nm)
Figure 3. CD spectra for compound 5 in ethanol solution at 10ꢀ4 M (the
solid line) and in gel from CCl4 (the dash line).

7446
C. Bao et al. / Tetrahedron 63 (2007) 7443–7448
(a)
(b)
Figure 4. The XRD pattern for xerogel of compound 5 from CCl4.
to the exciton coupling and the dipole moments are oriented
in a clockwise direction in the aggregate of the organogela-
tors. As reported in our previous work,28 the co-facial stack-
ing of the triphenylbenzene core induced the same rotational
direction of benzene rings due to the steric hindrance and the
probability for left- and right-handed rotations was equal.
Here, the difference of the compound is due to the introduc-
tion of the chiral groups ((R)-(ꢀ)-1-cyclohexyl ethyl) in the
side chains of compound. Therefore, the appearance of a pos-
itive Cotton effect in the CD spectra for the gel indicates that
the introduction of the chiral groups in discotic molecules
induces the triphenylbenzene cores to rotate in a clockwise
direction to form the chiral stack.
XRD was then used to investigate how the clockwise-
stacked molecules further stacked into fibers. Figure 4 shows
the small-angle XRD pattern of the self-assembling xerogel
of compound 5 from CCl4. It gives three weak and broad
peaks at the d-spacing of 1.90 nm, 1.16 nm, and 0.96 nm, re-
spectively, with the proportion of 1 : 1= 3 : 1= 4, indicat-
ing the lattice to be indexed to a hexagonal arrangement of
columns. The low intensity and the broad peaks suggest
that the columns are not well correlated with each other and
the molecular stacking is not very close. This is perhaps the
reason why the helical structure could not be observed easily
in the SEM and TEM images. Based on the above results, a
tentative stacking model can be proposed as the inset of
Figure 4 and Scheme 2, the clockwise-stacked molecules
further aggregate into hexagonal column, which finally
self-assemble into fibers and networks.
Scheme 2. The possible stacking model of compound 5 in the gel phase: (a)
face-on view and (b) edge-on view.
pffiffiffi
pffiffi
of the PL intensity. As reported, J-aggregates as well as the
coplanarity of the molecules in the aggregates favor an
enhancement of the emission.35,36 In this gel system, the
Fluorescent spectroscopy can also provide important
information about the molecular organization of fluoro-
phores,33,34 such as the triphenylbenzene cores in the present
system. Figure 5 gives the fluorescent emission spectra of 5
in the toluene gel and in a solution of toluene/methanol
(v/v¼9:1) at the same concentration of 1.0 wt %, in which
the molecules are presented as aggregates and monomers,
respectively, because the addition of trace amount of meth-
anol could avoid the molecular aggregation. It was found
that the emission of the toluene gel is much weaker than
that in the solution, when excited at 300 nm. This suggested
that the aggregation of the molecules results in the decrease
Figure 5. PL spectra of compound 5 in (a) toluene/methanol (9:1) solution
and (b) in toluene gel at 1.0 wt % (excited at 300 nm).

C. Bao et al. / Tetrahedron 63 (2007) 7443–7448
7447
molecules aggregated into J-aggregates for compound 5, but
the triphenylbenzene cores of the molecule are not stacked
in a coplanar manner but rather twisted into clockwise col-
umns. We argue that the two competitive factors may induce
the emission of gel to decrease greatly to 1/50 of that in the
solution. This emission change between the solution and gel
state may find some potential applications in optical mate-
rials.
aqueous solution (100 mL) was added and stirred for 10 min.
The obtained yellow precipitate was filtered and dried, and
directly added in 100 mL ethanol containing Pd/C (10%,
0.8 g). The mixture was heated to reflux, then 80%
NH2NH2$H2O (20 mL) was added dropwise to the hot
solution. After refluxing for another 10 h, the precipitate
was collected by filtration, the solution was cooled to room
temperature, and the white precipitate was collected. The
solid was recrystallized in ethanol and dried at vacuum, and
5.6 g white solid was obtained. Yield: 80%. Mp>200 ꢂC;
FTIR (KBr, cmꢀ1): 3420, 3344, 1618, 1592, 1515. Elemen-
tal analysis calculated for C24H21N3: C, 82.02; H, 6.02;
3. Conclusions
1
In summary, we synthesized a new discotic organogelator
consisting of a p-conjugated triphenylbenzene core and
chiral peripheral groups. The FTIR and UV–vis absorption
spectra indicated that hydrogen bonds and p–p interactions
provided by the amides and p-conjugated triphenylbenzene
favored the formation of helical organogel fibers, and the
chiral groups in the side chains induced the stacking in a sin-
gle clockwise direction. At the same time, the aggregation of
the molecules resulted in the decrease of the emission in the
gel phase. This chirality-induced single direction helix may
find some useful applications in designing new supramolec-
ular systems.
N, 11.96. Found: C, 82.04; H, 6.01; N, 11.94%; H NMR
(CDCl3, 300 MHz): 7.59 (s, 3H, ArH), 7.50–7.45 (m, 6H,
ArH), 6.78–6.48 (m, 6H, ArH), 3.73 (s, 6H, NH2).
4.1.2. Methyl-4-(5-(1-cyclohexylethylcarbamoyl)pentyl-
oxy)benzoate (3). A dry THF solution (10 mL) containing
6-bromo-hexanoyl chloride (0.011 mol, 1.68 mL) was
slowly added to a dry THF solution (20 mL) with (R)-(ꢀ)-
1-cyclohexyl-ethylamine (0.01 mol, 1.47 mL) and NEt3
(0.025 mol, 3.4 mL) at 0 ꢂC with stirring, and then the reac-
tion mixture was stirred at room temperature for 12 h. The
resulting solution was filtered and the filtrate was evaporated
to dryness. The obtained liquid (1-cyclohexyl-ethyl)-amide-
6-bromo-hexanoic acid was directly added to DMF (70 mL)
solution containing 4-hydroxy-benzoic acid methyl ester
(0.005 mol, 0.77 g), tetrabutyl ammonium bromide (0.17 g),
and K2CO3 (0.05 mol, 6.9 g), which was heated to 50 ꢂC for
96 h. Then the mixture was poured into water (150 mL) and
acidified with HCl aq (10%) to pH¼2. The obtained white
solid was recrystallized from ethyl acetate. Yield: 65%.
Mp 124.0–125.0 ꢂC; [a]D20 +17.9 (c 1, THF); FTIR (KBr,
cmꢀ1): 3294, 2941, 2854, 1724, 1640, 1607, 1550, 1510,
1435. Elemental analysis calculated for C22H33NO4: C,
70.37; H, 8.86; N, 3.73. Found: C, 70.35; H, 8.84; N,
4. Experimental section
Dry ethanol was obtained by distillation from Na and dry
tetrahydrofuran (THF) by distillation from Na/K/benzo-
phenone. (R)-(ꢀ)-1-Cyclohexyl-ethylamine (Aldrich, 98%,
ee: 95%, [a]2D0 ꢀ4, neat) was used as received.
4.1. Characterization
1H NMR spectra were determined with Varian-300 EX and
JEOL JNM-500 EX. UV–vis absorption spectra were
recorded with a Shimadzu UV-2201 UV–vis spectrophoto-
meter. Fourier transform infrared (FTIR) spectra for the
samples of xerogels were measured at room temperature
on a Nicolet Impact 410 FTIR spectrometer. C, H, and N
elemental analyses were performed on a Perkin–Elmer
240C elemental analyzer. Optical rotations were measured
on a WZZ-1S polarimeter at l¼589 nm. X-ray diffraction
(XRD) patterns were obtained on a Japan Rigaku D/max-
gA. XRD is equipped with graphite monochromatized Cu
1
3.75%; H NMR (CDCl3, 500 MHz): 7.97 (d, J¼8.5, 2H,
ArH), 6.88 (d, J¼8.5, 2H, ArH), 4.00 (t, J¼6.0, 2H, OCH2),
3.88 (m, 4H, OCH3, CH), 2.19 (t, J¼8.5, 2H, COCH2),
1.83 (m, 2H, CH2), 1.72 (m, 5H, CH2, CH), 1.65 (m, 2H,
CH2), 1.52 (m, 2H, CH2), 1.33–1.22 (m, 4H, CH2), 1.07
(d, J¼6.5, 3H, CH3), 0.98 (m, 2H, CH2).
4.1.3. 4-(5-(1-Cyclohexylethylcarbamoyl)pentyloxy)
benzoic acid (4). Compound 3 (0.005 mol, 1.88 g) was
dissolved in THF (150 mL), and NaOH aq (1 M, 25 mL) was
added. After the reaction mixture was heated at 60 ꢂC for
24 h, the resulting solution was evaporated to dryness. The
obtained solid was dissolved in water (300 mL) and the
aqueous solution was acidified by concentrated HCl to
pH¼1. The obtained white precipitate was filtered, washed
with water, and then dried. The product was obtained by re-
crystallization from ethanol. Yield: 95%. Mp>200 ꢂC; [a]D20
+25.0 (c 1, THF); FTIR (KBr, cmꢀ1): 3296, 2936, 2853,
1693, 1640, 1607, 1549, 1429. Elemental analysis calculated
for C21H31NO4: C, 69.87; H, 8.64; N, 3.87. Found: C,
˚
Ka radiation (l¼1.5418 A) and employs a scanning rate of
0.02 ꢂ/s in the 2q range from 0.7ꢂ to 10ꢂ. The measured sam-
ple was prepared by casting the gels on glass slide and dried
at room temperature. Transmission electron microscopy
(TEM) images were taken with a Hitachi mode H600A-2 ap-
paratus. The samples for TEM measurement were prepared
by wiping a small amount of gel onto a 200-mesh copper
grid followed by natural evaporation of the solvent. Fluores-
cence spectra were measured in a Japan Hitachi 850 fluores-
cence spectrophotometer at room temperature.
1
69.90; H, 8.60; N, 3.90%; H NMR (CDCl3, 300 MHz):
4.1.1. 1,3,5-Tris(4-aminophenyl)benzene (2).28 SiCl4
(20 mL, 0.18 mol) was added dropwise to a dry ethanol
solution (60 mL) containing 10 g (0.06 mol) p-nitroace-
tophenone at 0 ꢂC. Ayellow precipitate was formed immedi-
ately and then the mixture was refluxed for 10 h. After the
mixture was cooled to room temperature, saturated NH4Cl
8.02 (d, J¼8.7, 2H, ArH), 6.91 (d, J¼9.0, 2H, ArH),
4.02 (t, J¼6.7, 2H, OCH2), 3.74 (m, 1H, CH), 2.20 (t,
J¼6.5, 2H, COCH2), 1.85 (m, 2H, CH2), 1.73 (m, 5H,
CH2, CH), 1.62 (m, 2H, CH2), 1.45 (m, 2H, CH2), 1.30–
1.20 (m, 4H, CH2), 1.06 (d, J¼6.0, 3H, CH3), 0.98 (m, 2H,
CH2).

7448
C. Bao et al. / Tetrahedron 63 (2007) 7443–7448
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tion (5 mL) of 1,3,5-tri(4-aminophenyl) benzene (2) (0.12 g,
0.34 mmol) was added dropwise to a THF solution (15 mL)
containing 4 (0.6 g, 1.65 mmol), DCC (dicyclohexyl-carbo-
diimide) (0.8 g, 3.9 mmol), and DMAP (4-dimethylamino
pyridine) (0.05 g) at 0 ꢂC. After stirring overnight at room
temperature the solution was filtered and the remaining solu-
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precipitate was filtered and recrystallized in little THF twice,
followed by drying in a vacuum oven. Yield: 60%.
Mp>200 ꢂC, [a]2D0 +53.3 (c 1, THF); FTIR (KBr, cmꢀ1):
3321, 2931, 2853, 1665, 1640, 1609, 1527. Elemental anal-
ysis calculated for C87H108N6O9: C, 75.62; H, 7.88; N, 6.08.
1
Found: C, 75.97; H, 7.82; N, 6.02%; H NMR (CDCl3,
500 MHz) (present as aggregates): 7.56 (m, 9H, ArH),
6.89 (m, 12H, ArH), 6.09 (d, J¼9.0, 6H, ArH), 3.96 (t, J¼
6.0, 6H, OCH2), 3.60 (m, 3H, CH), 2.05 (m, 6H, COCH2),
1.74 (m, 9H, CH2, CH), 1.67–1.15 (m, 36H, CH2), 1.12 (d,
J¼6.5, 9H, CH3), 1.08–0.84 (m, 6H, CH2); MALDI-TOF
MS: m/z, calcd for 1,3,5-tris{4-[5-(1-cyclohexylethylcarb-
amoyl)pentyloxy]-benzoylamino}phenyl benzene: 1381.8;
found: 1382.4.
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Gelator 5 (5 mg) in different solvents (0.5 mL) was heated in
sealed test tubes in an oil bath until the solid dissolved. After
the solutions were allowed to stand at room temperature for
5 h, the state of the mixture was evaluated by the ‘stable to
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Acknowledgements
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The work was financially supported by the National Natural
Science Foundation of China (NNSFC, No. 20574027) and
the Program for New Century Excellent Talents in Univer-
sity (NCET).
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