Functional organogel based on a salicylideneaniline derivative with enhanced fluorescence emission and photochromism

Article abstract of DOI:10.1002/chem.200700321

A new organogelator based on a salicylideneaniline derivative with cholesterol moieties was synthesized, and it was proposed that it could gelate various organic solvents, such as 1-butanol, 1-octanol, butyl acetate, tetrachloromethane, benzene, toluene through combination with a gelation test. From the results of analysis by UV/Vis absorption, circular dichroism (CD), X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) studies and semiempirical (AMI) calculations, we believed that the gelator molecules could self-assemble into left-handed helical nanofibers through unimolecular layer packing, which further twisted into the thicker fibers and constructed 3D networks in the gel phase. Interestingly, the organogel exhibited strong fluorescence enhancement relative to a solution of the same concentration because of the formation of J aggregations. Meanwhile, photochromism of the organogel could take place under UV-light irradiation. Both strong fluorescence emission and photochromism properties were concurrent in one system based on a salicylideneaniline derivative. It was suggested that the self-assembly of the functional organogelator could lead to unique photophysical properties.

Full text of DOI:10.1002/chem.200700321

FULL PAPER
DOI: 10.1002/chem.200700321
Functional Organogel Based on a Salicylideneaniline Derivative with
Enhanced Fluorescence Emission and Photochromism
Pengchong Xue,[a, b] Ran Lu,*[a] Guojun Chen,[a] Yuan Zhang,[c] Hiroyuki Nomoto,[b]
Makoto Takafuji,[b] and Hirotaka Ihara*[b]
Abstract: A new organogelator based
on a salicylideneaniline derivative with
cholesterol moieties was synthesized,
and it was proposed that it could gelate
various organic solvents, such as 1-bu-
tanol, 1-octanol, butyl acetate, tetra-
(TEM) studies and semiempirical
(AM1) calculations, we believed that
the gelator molecules could self-assem-
ble into left-handed helical nanofibers
through unimolecular layer packing,
which further twisted into the thicker
fibers and constructed 3D networks in
the gel phase. Interestingly, the organo-
gel exhibited strong fluorescence en-
hancement relative to a solution of the
same concentration because of the for-
mation of J aggregations. Meanwhile,
photochromism of the organogel could
take place under UV-light irradiation.
Both strong fluorescence emission and
photochromism properties were con-
current in one system based on a sali-
cylideneaniline derivative. It was sug-
gested that the self-assembly of the
functional organogelator could lead to
unique photophysical properties.
chloromethane,
benzene,
toluene
through combination with a gelation
test. From the results of analysis by
UV/Vis absorption, circular dichroism
(CD), X-ray diffraction (XRD), scan-
ning electron microscopy (SEM), and
Keywords: fluorescence
structures photochromism
self-assembly
·
helical
·
·
transmission
electron
microscopy
Introduction
such as fibers, rods, and ribbons, through weak noncovalent
interactions (i.e., hydrogen bonding, p–p, van der Waals, co-
ordination, and charge-transfer interactions). So far many
functional modules have been successfully integrated into
LMMGs, which have specific applications in light harvest-
ing;[2] energy and charge transfer;[3] photo, enzyme, and
other switches;[4] and chiral or molecular-shape selection.[5]
It is well known that salicylideneaniline derivatives could
exhibit unusual functionalities, such as tunable fluores-
cence,[6] photochromism,[7] thermochromism,[8] solvato-
chromism,[9] liquid crystals,[10] and nonlinear optics.[11] To
date, no example of an organogelator that consists of a sali-
cylideneaniline moiety has been reported.
Recently, low-molecular-mass gelators (LMMGs), especially
with functional and chiral moieties, have attracted substan-
tial attention in supramolecular chemistry and materials sci-
ence[1] because some of their physical properties can be
modulated during the formation of the gels, in which the ge-
lator molecules self-assemble into nanoscale superstructures,
[a]Dr. P. Xue, Prof. R. Lu, G. Chen
Key Laboratory for Supramolecular Structure and Materials
Ministry of Education, College of Chemistry
Jilin University, Changchun, 130012 (China)
Fax : (+86)431-8892-3907
As suggested by van Esch and Feringa,[1b] if an organic
compound is expected to be a gelator for an organic solvent
three important guidelines must be followed: 1) strong self-
complementary and unidirectional intermolecular interac-
tions to induce one-dimensional (1D) packing of the mole-
cules need to be present; 2) control of the interfacial energy
between the aggregation and the solvent to adjust the solu-
bility of the gelator and to prevent crystallization is re-
quired; and 3) some factors that favor the cross-linking of
1D fibers to form 3D networks should be considered. Re-
cently, Birkedal and Pattison reported the crystal structure
of bis(4-[salicylideneamino]phenyl)methane (1’),[12] which
E-mail: luran@mail.jlu.edu.cn
[b]Dr. P. Xue, H. Nomoto, M. Takafuji, Prof. H. Ihara
Department of Applied Chemistry and Biochemistry
Kumamoto University, 2-39-1 Kurokami
Kumamoto 860–8555 (Japan)
Fax : (+81)092-342-3662
E-mail: ihara@kumamoto-u.ac.jp
[c]Dr. Y. Zhang
State Key Laboratory of Theoretical and Computational Chemistry
Institute of Theoretical Chemistry
Jilin University, Changchun, 130012 (China)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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has a V-shaped conformation and packs into 1D J aggrega-
tions along the b axis through strong p–p interactions
among the aromatic rings (Figure 1). Such close stacking
Scheme 1. The synthetic route to organogelator 1. DCC=dicyclohexyl
carbodiimide, DMAP=N,N-dimethyl-1,3-propanediamine.
the functional organogelators may result in unusual optical
properties as a result of adjusting the weak intermolecular
interactions, thus leading to supramolecular self-assemblies
with special superstructures.
Figure 1. One-dimensional packing of 1’ along the b axis in the crystal
state.
(the distance between the two adjacent parallel aromatic
rings is 3.6 Š) of the salicylideneaniline groups leads to en-
hanced photoluminescence and non-photochromism under
UV-light irradiation.[7e,13] Therefore, if such Schiff base
dimer is integrated into the designed molecule, it would pos-
sibly self-assemble and be arranged into 1D aggregations
through unidirectional p–p interactions between the aromat-
ic rings. On the other hand, the chiral group in the gelator is
usually considered to be an important factor to prevent crys-
tallization from the solvent;[1c] therefore, many gelators with
cholesterol units have been reported.[14]
Herein, we present the synthesis and characterization of
1, composed of cholesterol and salicylideaniline moieties
(Scheme 1), which is a good gelator in various organic sol-
vents. Investigation of the self-assembly properties of the ge-
lator revealed that 1 could form left-handed helical nanofib-
ers through unimolecular layer packing, which further con-
structed 3D networks in the organogel. Interestingly, the
fluorescence emission intensity of the organogel was more
than 1000 times higher than that of the solution at the same
concentration as a result of the formation of J aggregation in
the gel state. Meanwhile, under UV-light irradiation, the
photochromism of the organogel could be observed. The ap-
pearance of both strong fluorescence emission and photo-
chromism in the salicylideneaniline derivative has rarely
been reported because the strong photoluminescence is con-
sistent with the dense stacking of molecules, which strongly
suppresses photoinduced isomerization. This behavior sug-
gests that the introduction of certain aliphatic moieties into
Results and Discussion
Gelation behavior: A series of organic solvents was em-
ployed to reveal the gelation behavior of 1 (Table 1). Com-
pound 1 readily dissolves in chloroform, dichloromethane,
and THF at room temperature, but was insoluble in lower
alcohols and esters, such as ethanol and ethyl acetate, and
aliphatic hydrocarbon solvents, such as cyclohexane, n-
hexane, and n-decane, even with heating. Compound 1 can
also gelatinize some higher alcohols and esters and some
nonpolar aromatic solvents, including 1-butanol, 1-octanol,
butyl acetate, tetrachloromethane, benzene, and toluene.
However, the minimum gelation concentration (MGC) nec-
essary for gel formation in benzene and toluene is relatively
large (MGC>0.5 wtvol%À1) compared to that in 1-butanol,
Table 1. Gelation properties of 1 in organic solvents.[a]
Solvent
1[b]
Solvent
1[b]
CHCl3
CH2Cl2
THF
ethanol
1-butanol
1-octanol
ethyl acetate
butyl acetate
S
S
benzene[c]
G
G
I
I
I
G
G
G
toluene[c]
S
cyclohexane
I
n-hexane
G
G
I
n-decane
benzene/cyclohexane[d]
benzene/n-hexane[d]
tetrachloromethane
G
[a]G: stable gel formed at room temperature; S: soluble; I: insoluble.
[b]Gelator =1.0 wtvol%À1. [c]MGC is above 0.5 wt/vol%. [d]v/v =1:4;
MGC<0.1 wt/vol%.
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Molecular Aggregation of a Salicylideneaniline–Cholesterol Moiety
FULL PAPER
1-octanol, butyl acetate, and tetrachloromethane (MGC
<0.2 wtvol%À1). The addition of cyclohexane or n-hexane,
in which 1 is poorly soluble, into the benzene or toluene sys-
tems could decrease the MGC. For example, the system
may maintain the gel state below 0.1 wt/vol% in a mixture
of benzene and cyclohexane (v/v 1:4). In this case, the sol-
vents with poor solubility for the organogelator are deemed
to be the trapped solvent to decrease the MGC.[15] As a
result, 1 is an effective gelator for various organic solvents.
UV/Vis absorption and circular dichroism (CD) spectra:
Variable-temperature absorption spectra of 1 in benzene/cy-
clohexane and 1-butanol systems were investigated to moni-
tor the interactions between the chromophores, the salicyli-
deneaniline moieties, during the formation of the gels
(Figure 2). The absorption intensities of gels at lower tem-
peratures are weaker than those at higher temperatures,
thus promoting the interactions between the chromo-
phores.[16] Moreover, relative to the solutions, a red-shift of
Figure 2. Temperature-dependence of the UV/Vis (a and b) and CD spectra (c and d) of the benzene/cyclohexane and 1-butanol organogels, respectively.
The solution of organogel in benzene at 0.1 wtvol%À1 does not give a CD response (see the curve of the center one labeled as “sol 208C” in (c)). e) The
plot of the absorbance and intensity of CD at 343 and 354 nm, respectively, for the benzene/cyclohexane gel as a function of the temperature. f) The plot
of the absorbance and intensity of CD at 343 and 370 nm, respectively, for the 1-butanol gel. The concentrations in all cases are 0.1 wt/vol%.
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H. Ihara, R. Lu et al.
2 nm can be detected for the two gels, thus indicating the
formation of J aggregations of salicylideneaniline in the gel
state.[17] However, the maximum absorption peak of 1’ at
346 nm in dilute solution shifts to 360 nm in the crystal
state, and such a large red-shift (Dl=14 nm; see the Sup-
porting Information) shows one strong exciton coupling at-
tributed to the close distance between the chromophores in
the crystal. The small red-shift phenomenon in the gel phase
suggests that the exciton coupling of the chromophores is
weaker and that the distance between the chromophores is
larger than that in 1’ in the crystal state.[18] It is believed that
the large cholesterol group actually inhibits the closeness of
the chromophores (as will be discussed further later on).
The “melting” temperatures were calculated from Fig-
A
ularly dissolved species, not from gel to sol.[19] There is a
clear dependence of the aggregation melting transition on
the solvent; for example, the melting transition increases
from 558C for the benzene/cyclohexane gel to 778C for the
1-butanol gel at the same concentration.
Figure 3. Temperature-dependent
1H NMR
spectra
of
the
[D6]benzene/[D12]cyclohexane organogel 1 at 0.2 wt/vol%.
AHCTREUNG
CD spectroscopy is considered a powerful tool in the
study of the assembly processes that lead to the helical su-
perstructures. In the CD spectra, both gels give similar sig-
nals and non-bisignate exciton couplets with the opposite
signals, thus indicating no central helical overlay between
the dipole moments of the two gelators in the gels[20] (Fig-
at high temperature;[24] this result can be further confirmed
À
by the FT-IR spectrum, as no N H vibration band at
around 3310 cmÀ1 was found and a strong C=N band at
1622 cmÀ1 appeared.
A
and À164144 degcm2 dmolÀ1 for benzene/cyclohexane and
1-butanol, respectively) reveals a similar intensity of exciton
coupling for both gels. Combined with the results of the
UV/Vis absorption spectra, it is rational that 1 is packed in
an anticlockwise helical direction in both gel phases.[21] In
addition, the melting transitions obtained from temperature-
dependent CD spectra were the same as those from the ob-
servation by UV/Vis spectroscopy (Figure 2e,f).
Small-angle X-ray diffraction investigations (SAXD): The
measurement of the SAXD of the gel can provide informa-
tion about the long-range regulation of the molecular self-
assembly, thereby the packing model of molecules in the gel
phase can be supposed. Figure 4 shows the SAXD spectrum
of the xerogel obtained from the benzene/cyclohexane gel.
The good diffraction pattern was characterized by three re-
flection peaks of 5.81, 2.91 and 1.93 nm, which were exactly
in the ratio of 1:1= :1= , thus indicating a lamellar organization
2
3
Variable-temperature 1H NMR spectra: The NMR spectra
can reveal the interactions not only between the chromo-
phores but also between the cholesterol moieties. The tem-
perature-dependent 1H NMR spectral changes in 1 in a
within the aggregates of gel 1 with an interlayer distance of
5.81 nm.[25]
[D6]benzene/[D12]cyclohexane gel and the assignment of the
N
key peaks for the aromatic and aliphatic (cholesterol and
succinic ester) subunits is shown in Figure 3. The peaks as-
cribed to the aliphatic moieties are consistently visible at
30–908C and become gradually sharper and stronger with
increasing temperature.[22] On the other hand, weak and
broad NMR resonance peaks for the aromatic units can
only be found over 508C. It is well known that the broaden-
ing and decrease in the intensity of the NMR resonance
peaks can be interpreted as an indication of restricted free-
dom of motion, such as the behavior in the solid state.
Therefore, the aromatic moieties behave “solid-like” and ex-
hibit a relatively rigid structure compared to the aliphatic
units. This behavior also suggests that the salicylideneaniline
moiety has a poorer solubility than the cholesterol groups in
the gel phase.[23] Moreover, the peak at d=12.9 ppm illus-
trates that 1 exists as a trans-enol form (E-OH form) even
Figure 4. Small-angle XRD diagram of the xerogel 1 obtained from the
benzene/cyclohexane gel.
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Molecular Aggregation of a Salicylideneaniline–Cholesterol Moiety
FULL PAPER
Transmission electron microscopy (TEM) and scanning elec-
tron microscopy (SEM) investigations: SEM and TEM
images of the 1-butanol (polar solvent) and benzene/cyclo-
hexane (nonpolar solvent) gels were obtained to study the
microstructures of the organogels and the molecular self-as-
sembly properties. A SEM image of the benzene/cyclohex-
ane gel shows that many nanofibers 30–70 nm in width and
tens of micrometers in length form the basis of the 3D net-
works so that the flow of the solvent can be prevented to
induce the formation of a solid-like gel (see Figure 5a).
Moreover, the TEM image of the benzene/cyclohexane gel
shows lots of helical nanofibers. On careful examination, we
found that these fibers are all left-handed helices and the di-
ameter of the thinnest fibers is about 5.8 nm, which is in
agreement with the periodicity of the structure (5.81 nm)
suggested by the XRD studies (Figure 5b). Furthermore, we
also found that two or more fibers 5.8 nm in diameter twist
around each other to form larger fibers with diameters of
11.6, 17.4, and 24.8 nm, thus further confirming the evidence
of the layer structure. Moreover, the helical pitch of the
twisted fiber is independent of the diameter of the fiber,
and all the fibers have similar helical pitches of 45 nm. Simi-
lar to the benzene/cyclohexane gel, some left-handed helical
fibers with diameters of 5.8 nm and helical pitches of ap-
proximately 50 nm were observed in the 1-butanol gel (Fig-
ure 5c). It was thought that the microstructures of the mo-
lecular self-assemblies in both gels are similar.
of 1’ (3.6 Š) in the crystal state because of the effect of the
spatial hindrance of the cholesterol group (Figure 6b). In
addition, the CD spectra and TEM observations suggest
that the molecules are packed in an anticlockwise direction.
Therefore, the molecular packing model in the gel phase
Molecular packing model: Semiempirical (AM1) calcula-
tions were performed to optimize the ground-state geometry
of 1 to reveal the molecular packing model. The results
show that 1 has a V-shaped conformation (Figure 6a), which
coincides with that of 1’. The molecular length for the ex-
tended form is estimated to be 5.78 nm, which is in accord-
ance with the long period of the lamellar structure in the gel
phase. As the results of the UV/Vis spectra indicated that
the aromatic moieties formed J aggregations with weak exci-
ton coupling, it can be confirmed that the gelator molecules
are arranged in parallel along the growth direction of the
fiber and are further extended to form a 1D fibrous struc-
ture. In this model, the distance between two parallel salicy-
lideneaniline moieties reaches up to 5.0 Š, longer than that
Figure 6. a) The optimized structure of 1 through the AM1 quantum me-
chanical calculations. b) Schematic illustration of the monomolecular
layer structure with a period distance of 5.8 nm in the gel state. c) Bimo-
lecular helical packing along the direction of the long axis.
Figure 5. SEM image of the benzene/cyclohexane gel (a) and the TEM images of the benzene/cyclohexane (b) and 1-butanol (c) gels.
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H. Ihara, R. Lu et al.
can be illustrated as shown in Figure 6c. Considering the
helical pitch (namely, 450 and 500 Š for the benzene/cyclo-
hexane and 1-butanol gels, respectively), it can be deduced
that the fiber within a helical pitch includes approximately
90 molecules in the benzene/cyclohexane gel and 100 mole-
cules in the 1-butanol gel, and that the torsion angles of ad-
jacent two molecules in both gels are 4 and 3.68, respective-
ly.
Enhanced fluorescence emission from sol to gel: In general,
after the E-OH form of salicylideneaniline in the ground
state absorbs one photon, the excited-state intramolecular
rapid proton-transfer process from the oxygen to the nitro-
gen atom occurs and yields the excited-state Z-NH form
owing to the lower potential energy.[26] Moreover, the Z-NH
form can undergo a torsion and transform into a photochro-
mic product (i.e., the E-NH form) if there is enough space
to permit the molecule to rotate, as reported by Kawato and
co-workers,[7d] or it can undergo a transition from the excit-
ed state to the ground state accompanied by a longer emis-
sion wavelength[6a] and then return to the E-OH ground
state rapidly. Therefore, the behavior of the Z-NH form can
lead to a large Stokes shift relative to the absorption peak
in the E-OH form. The detailed photoinduced isomerization
and photoluminescence processes are shown in Scheme 2.
Figure 7. Temperature-dependence of the fluorescence emission of the
benzene/cyclohexane (a) and 1-butanol (b) gels. Inset: photograph of the
benzene/cyclohexane gel and benzene sol at 0.1 wt/vol irradiated by 365-
nm light.
Scheme 2. Photoinduced isomerization and photoluminescence processes
of a typical salicylideneaniline moiety.
hanced fluorescence emission of the gel is caused by the for-
mation of molecular aggregations.
In recent years, a few samples with enhanced emission in-
duced by molecular aggregation have been reported; for ex-
ample, Tang and co-workers reported a pentaphenylsilole
derivative with enhanced emission after aggregation[27] and
suggested that the enhanced fluorescence can be explained
by the planar conformation in the solid state, which can acti-
vate the radiation process.[28] Herein, in respect to the ab-
sorption band of the gel with only a small red-shift of 2 nm
relative to that in solution, the contribution of the molecular
planarization to the enhanced emission is not taken into ac-
count. It is well known that an increase in the rigidity of a
molecule can decrease molecular vibrations, probably sup-
press the internal conversion of an excited molecule, and
may increase the fluorescence quantum yield.[29] In addition,
the fluorescence intensity of the p-conjugated chromophores
is correlated with the p aggregation, such as H and J aggre-
gations.[30] H aggregations, in which the molecules are
aligned in parallel to each other in a head-to-head form,
tend to increase the internal conversion from a higher elec-
tronic state into a lower one so that the fluorescence emis-
Under UV-light irradiation, the benzene/cyclohexane and
1-butanol gels emit strong green fluorescence, which gradu-
ally decreases as the temperature increases (Figure 7). The
maximum emission peaks of both gels are located at
520 nm, and a large Stokes shift (177 nm) is observed rela-
tive to the absorption of the gels. Interestingly, at the same
concentration as the benzene/cyclohexane gel, a solution of
1 in benzene, in which the molecules are confirmed to exist
in the unimolecular isolated state because no CD signal is
detected (Figure 2c), gives very weak fluorescence emission
(Ff =2.31”10À5); furthermore, the fluorescence intensity in-
creases gradually when cyclohexane is continually added
into the solution of 1 in benzene to induce the formation of
the self-assembly of 1. When the volume ratio of cyclohex-
ane to benzene reaches 4:1, the fluorescence quantum yield
of the system reaches 0.0241, which is the same as that of
the gel and more than 1000 times greater than that of the so-
lution of 1 in benzene. These results suggest that the en-
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Molecular Aggregation of a Salicylideneaniline–Cholesterol Moiety
FULL PAPER
sion is effectively quenched. In contrast, the molecules are
arranged into a head-to-tail stack as in J aggregation, in
which the transition from the lower couple excited state of
the molecule to the ground state is allowed; as a result, the
absorption peak will red-shift and the fluorescence emission
will be stronger than that of the monomer.[31] In our case,
the gelator molecules form J aggregations and cross-link
into a solid-like 3D network in the gel phase, so the en-
hanced emission of the gel is attributed to the synergetic
effect of the restricted molecular motion and the formation
of J aggregations.
Time-resolved fluorescence spectra were measured to
obtain further information about the excited state of 1. Both
the monomer solution and gel gave double-exponential
decay with average lifetimes of 6.98 and 5.78 ns, respectively
(Figure 8). According to the equations tÀ1 =kr +knr and kr =
based on salicylideneaniline exhibit two mutually exclusive
properties: either thermochromism or photochromism.[7e,32]
In the thermochromic crystal, the molecules are planar and
close-packed through p–p interactions so that strong fluo-
rescence emission can be detected, such as in 1’. On the con-
trary, in the photochromic crystals there is large dihedral
angle between the planar salicylaldimine group and the ani-
line ring, thus inducing an open crystal structure with incom-
pact molecular packing.
From the above discussion, 1 can undergo regular aggre-
gation and emit strong fluorescence in the gel phase, whose
properties are similar to those of the thermochromic crystal.
However, the UV/Vis absorption spectra and molecular
packing model illustrated that the packing of the Schiff base
group was not very tight, and the temperature-dependent
1H NMR spectra suggested that the cholesterol groups pos-
sessed significant freedom of motion. Therefore, it was
worth investigating whether the photochromism of the orga-
nogel could take place under UV irradiation. Figure 9 shows
Figure 8. Fluorescence decay of the benzene/cyclohexane gel and the
benzene sol at 208C and lem =520 nm.
Figure 9. Changes in the UV/Vis absorption spectra of the benzene/cyclo-
hexane gel with irradiation time under 365-nm light from a high-pressure
mercury lamp. Inset: the plot of the absorbance at 343 nm as a function
of the irradiation time.
Ff/t, the radiative rate constant kr and total nonradiative
rate constant knr of 1 in the monomer and J aggregation
state were calculated (Table 2). Although knr increases
the absorption spectral change of the benzene/cyclohexane
gel with the irradiation time. After irradiation with 365-nm
light, the band at 343 nm decreased and a new peak ap-
peared at 450 nm, which could intensify when the irradiation
time was prolonged.[33] On the other hand, the FT-IR spec-
trum of the sample after irradiation revealed two new peaks
at 3326 and 1660 cmÀ1, ascribed to NH and a,b-unsaturated
ketone vibration absorptions, respectively (see the Support-
ing Information). This spectrum suggests that 1 shows signif-
icant photochromic properties in the gel phase, although it
also exhibits emission enhancement as a thermochromic
crystal, which is quite different from the traditional photo-
chromic salicylideneaniline with non-fluorescence. More-
over, after being irradiated for 110 min, the gel state trans-
formed into a solution, although not all molecules existed in
the keto form.
Table 2. The fluorescence quantum yields and the radiative and total
nonradiative rates of the benzene sol and the benzene/cyclohexane gel.
Ff
t1
t2
<t> kr
[106 sÀ1
knr
[108 sÀ1
]
A
]
[ns] [ns]
G
]
N
benzene sol
0.0023 3.43 8.65 6.98
0.0033
4.15
1.43
1.69
benzene/cyclohexane gel 2.41 1.92 9.44 5.78
slightly as a result of the formation of the gel, thus suppress-
ing the fluorescence emission, efficient radiative transition
increased more than 1000 times. Therefore, the aggregation-
induced fluorescence enhancement is mainly ascribed to the
increase in the value of kr.
Photochromism and photoinduced transition from gel to
sol: As deduced from the 3D X-ray structures, the crystals
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H. Ihara, R. Lu et al.
Shimadzu-1601 spectrophotometer. The cell was maintained at 208C
during the irradiation.
Conclusion
Synthesis of the organogelator
In summary, a new organogelator based on a salicylidene-
3-Cholesteryloxycarbonylpropanoic acid (2): Compound 2 was synthe-
sized according to a previously reported method.[35] A solution of choles-
terol (3.87 g, 10 mmol), succinic anhydride (1.00 g, 10 mmol), and tri-
aniline derivative with cholesterol units was synthesized,
which could gel various organic solvents. SEM and TEM
images revealed that the gelator molecules self-assembled
into 1D helical fibers with diameters of approximately
5.8 nm in an anticlockwise direction, which further twisted
into thicker fibers and constructed 3D networks. Through
combining the results of the UV/Vis spectra, CD spectra,
and semiempirical (AM1) calculations, the molecular pack-
ing model in the gel phase was illustrated. Interestingly, the
fluorescence emission intensity of the organogel was more
than 1000 times higher than that of the solution at the same
concentration as a result of the formation of J aggregations
and the restricted molecular motion in the gel state. Mean-
while, the UV irradiation could induce the isomerization of
salicylideneaniline in the organogel because of the large
cholesterol group, thus resulting in a loose packing of the
molecules and permitting the molecule to rotate. Strong
fluorescence emission and photochromism could be involved
in such a gel system based on a salicylideneaniline deriva-
tive, thus indicating that the expected optical properties may
be realized through tuning the self-assembly process of the
functional gelator.
A
reflux for 24 h. The acetone was evaporated and the resulting precipitate
was recrystallized twice from glacial acetic acid. Yield: 65%; m.p. 177.0–
179.08C (177.0–179.08C in reference [34]); IR (KBr): n˜ =1728 (ester car-
bonyl), 1709 (acid carbonyl), 3440 cmÀ1 (hydroxy); elemental analysis
(%) calcd for C31H50O4: C 76.50, H 10.27; found: C 76.01, H 10.10.
2-Hydroxy-4-(3-cholesteryloxycarbonylpropionyloxy)benzaldehyde
(3):
Compound 2 (2.50 g, 5.1 mmol) and 2,4-dihydroxylbenzaldehyde (0.9 g,
6.5 mmol) were dissolved in dry CH2Cl2 (30 mL) containing pyridine
(1.8 mL). The solution was cooled to 08C and a small amount of DMAP
and DCC (2.50 g, 10 mmol) was added. The mixture was stirred for 4 h at
0–58C and left for 24 h at room temperature. A white precipitate was re-
moved by filtration. After the solvent was evaporated under reduced
pressure, the resultant residue was purified by column chromatography
on silica gel (cyclohexane/ethyl acetate 5:1). Yield: 38%; m.p 99.0–
100.08C; IR (KBr): n˜ =3327 (hydroxy), 1770 and 1737 (ester carbonyl),
1661 cmÀ1 (aldehyde carbonyl); elemental analysis (%) calcd for
C38H54O6: C 75.21, H 8.97; found: C 75.10, H 8.86; 1H NMR (300 MHz,
TMS, CDCl3): d=10.24 (s, 1H), 7.60 (d, J=2.2 Hz, 1H), 6.81 (d, J=
2.2 Hz, 1H), 6.74 (s, 1H), 5.37 (d, J=4.2 Hz, 1H), 4.68 (m, 1H), 2.87 (t,
J=7.2 Hz, 2H), 2.72 (t, J=7.2 Hz, 2H), 2.33 (t, J=7.2 Hz, 2H), 2.20–
1.5 ppm (m, 41H).
4,4’-Di(2-hydroxy-4-[3-cholesteryloxycarbonylpropionyloxy]benzylidenea-
mino)diphenylmethane (1): A solution of 3 (0.606 g, 1.0 mmol) and 4,4’-
diaminodiphenylmethane (0.099 g, 0.5 mmol) in ethanol (10 mL) was
heated to reflux for 2 h and cooled to room temperature. The precipitate
formed was collected by filtration and washed by cyclohexane repeatedly.
Yield: 35%; m.p. 217.0–219.08C; IR (KBr): n˜ =3318 (OH), 1763 and
1732 (ester carbonyl), 1622 cmÀ1 (C=N); elemental analysis (%) calcd for
C89H118N2O10: C 77.69, H 8.64, N 2.04; found: C 77.07, H 8.50, N 1.93;
1H NMR (300 MHz, TMS, CDCl3): d=8.60 (s, 2H), 7.36 (s, 2H), 7.23 (d,
J=2.4 Hz, 4H), 6.77 (d, J=2.4 Hz, 4H), 6.72 (d, J=2.1 Hz, 2H), 6.70 (d,
J=2.1 Hz, 2H), 5.38 (d, J=4.2 Hz, 2H), 4.68 (m, 2H), 4.04 (s, 2H), 2.87
(t, J=7.2 Hz, 4H), 2.73 (t, J=7.2 Hz, 4H), 2.33 (t, J=7.2 Hz, 4H), 2.20–
1.5 ppm (m, 82H).
Experimental Section
Instruments: The IR spectra were measured on a Nicolet-360 FT-IR
spectrometer by incorporating the samples in KBr disks. The 1H NMR
spectra were recorded on a Varian-300 EX spectrometer. The tempera-
ture-dependent 1H NMR spectra were measured on JEOL FX-400 spec-
trometer, and the NMR tube containing 1 in solvent was sealed before
measurement. The SEM images were obtained using a JEOL JEM-
5310 LV spectrometer. The CD spectra were recorded on JACSO J-725
spectropolarimeter. The UV/Vis spectra were determined on a Shimad-
zu-1601 spectrophotometer. Microanalysis was performed on a Perkin–
Elmer 240C elemental analyzer. The photoluminescence measurements
were taken on a JACSO PL-5600 spectrofluorometer.[34]
Acknowledgements
This work was financially supported by the National Nature Science
Foundation of China (NNSFC, 20574027) and the Program for New Cen-
tury Excellent Talents in University (NCET).
TEM investigation: A piece of the gel 1 was placed on a carbon-coated
copper grid (400 mesh) followed by naturally evaporating the solvent.
The TEM specimens were examined with a Hitachi mode H600 A-2 ap-
paratus with an accelerating voltage of 100 kV.
Gelation test of organic fluids: The solution of gelator 1 (pre-weighed) in
organic solvent was heated in a sealed test tube (diameter=1 cm) in an
oil bath until the solid was dissolved. The solution was allowed to stand
at room temperature for 6 h, and the state of the mixture was evaluated
by the “stable to inversion of a test tube” method.
[1]a) P. Terech, R. G. Weiss, Chem. Rev. 1997, 97, 3133–3159; b) J. H.
van Esch, B. L. Feringa, Angew. Chem. 2000, 112 2351–2354;
Angew. Chem. Int. Ed. 2000, 39, 2263–2266; c) N. M. Sangeetha, U.
Maitra, Chem. Soc. Rev. 2005, 34, 821–836; d) A. Brizard, R. Oda,
I. Huc, Top. Curr. Chem. 2005, 256, 167–218; e) D. J. Abdallah,
R. G. Weiss, Adv. Mater. 2000, 12, 1237–1247; f) R. J. H. Hafkamp,
M. C. Feiters, R. J. M. Nolte, J. Org. Chem. 1999, 64, 412–426.
[2]a) K. Sugiyasu, N. Fujita, S. Shinkai, Angew. Chem. 2004, 116, 1249–
1253; Angew. Chem. Int. Ed. 2004, 43, 1229–1233; b) T. Nakashima,
N. Kimizuka, Adv. Mater. 2004, 14, 1113–1116.
[3]a) V. K. Praveen, S. J. George, R. Varghese, C. Vijayakumar, A.
Ajayaghosh, J. Am. Chem. Soc. 2006, 128, 7542–7550; b) A. Del
Guerzo, A. G. L. Olive, J. Reichwagen, H. Hopf, J.-P. Desvergne, J.
Am. Chem. Soc. 2005, 127, 17984–17985; c) M. Montalti, L. S.
Dolci, L. Prodi, N. Zaccheroni, M. C. A. Stuart, K. J. C. van Bom-
mel, A. Friggeri, Langmuir 2006, 22, 2299–2303; d) D. Beljonne, E.
Hennebicq, C. Daniel, L. M. Herz, C. Silva, G. D. Scholes, F. J. M.
XRD studies: Diffraction patterns were carried out on a Rigaku D/max-
rA X-ray diffractometer with graphite-monochromated CuKR radiation
(l=1.5418 Š). The accelerating voltage was set at 50 kV, with 100-mA
fluxes at a scanning rate of 0.02 degsÀ1 in the 2q range of 1–108. The
XRD patterns of xerogel were obtained by casting wet gel samples in a
glass flask followed by evaporation of the solvent naturally.
Photochromism: Preweighed 1 was added into a quartz cell and a mix-
ture of benzene and cyclohexane (v/v 1:4) was injected into the cell.
After the solid was dissolved by heating, the system was cooled to room
temperature and the gel formed. A high-pressure mercury lamp was used
as the irradiation-light source with a filter plate (lmax =365 nm). After ir-
radiation for the expected time, the absorption peak was recorded on a
8238
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Chem. Eur. J. 2007, 13, 8231 – 8239

Molecular Aggregation of a Salicylideneaniline–Cholesterol Moiety
FULL PAPER
Hoeben, P. Jonkheijm, A. P. H. J. Schenning, S. C. J. Meskers, R. T.
Phillips, R. H. Friend, E. W. Meijer, J. Phys. Chem. B 2005, 109,
10594–10604; e) J. H. Jung, S. J. Lee, J. A. Rim, H. Lee, T. Bae, S. S.
Lee, S. Shinkai, Chem. Mater. 2005, 17, 459–462; f) T. Sagawa, S. Fu-
kugawa, T. Yamada, H. Ihara, Langmuir 2002, 18, 7223–7228; g) H.
Ihara, T. Yamada, M. Nishihara, T. Sakurai, M. Takafuji, H. Hachi-
sako, T. Sagawa, J. Mol. Liq. 2004, 111, 73–76.
[15]a) R. Wang, C. Geiger, L. H. Chen, B. Swanson, D. G. Whitten, J.
Am. Chem. Soc. 2000, 122, 2399–2400; b) P. C. Xue, R. Lu, D. Li,
M. Jin, C. H. Tan, C. Y. Bao, Z. M. Wang, Y. Y. Zhao, Langmuir
2004, 20, 11234–11239.
[16]a) P. Babu, N. M. Sangeetha, P. Vijaykumar, U. Maitra, K. Rissanen,
A. R. Raju, Chem. Eur. J. 2003, 9, 1922–1932; b) C. Y. Bao, R. Lu,
M. Jin, P. C. Xue, C. H. Tan, G. F. Liu, Y. Y. Zhao, Chem. Eur. J.
2006, 12, 3287–3294.
[17]a) M. Simonyi, Z. Bikµdi, F. Zsila, J. Deli, Chirality 2003, 15, 680–
698; b) S. Yagai, M. Higashi, T. Karatsu, A. Kitamura, Chem. Mater.
2005, 17, 4392–4398.
[18]a) M. Kasha, H. R. Rawls, M. Ashraf El-Bayoumi, Pure Appl.
Chem. 1965, 11, 371–392; b) F. D. Lewis, X. Liu, Y. Wu, X. Zuo, J.
Am. Chem. Soc. 2003, 125, 12729–12731; c) P. C. Xue, R. Lu, Y.
Huang, M. Jin, C. H. Tan, C. Y. Bao, Z. M. Wang, Y. Y. Zhao, Lang-
muir 2004, 20, 6470–6475; d) F. D. Lewis, L. Zhang, X. Liu, X. Zuo,
D. M. Tiede, H. Long, G. C. Schatz, J. Am. Chem. Soc. 2005, 127,
14445–14453.
[19]F. Würthner, Z. Chen, F. J. M. Hoeben, P. Osswald, C. You, P. Jonk-
heijm, J. v. Herrikhuyzen, A. P. H. J. Schenning, P. P. A. M. van der
Schoot, E. W. Meijer, E. H. A. Beckers, S. C. J. Meskers, R. A. J.
Janssen, J. Am. Chem. Soc. 2004, 126, 10611–10618.
[20]C. Thalacker, F. Würthner, Adv. Funct. Mater. 2002, 12, 209–218.
[21]A. Ajayaghosh, C. Vijayakumar, R. Varghese, S. J. George, Angew.
Chem. 2006, 118, 470–474; Angew. Chem. Int. Ed. 2006, 45, 456–
460.
[4]a) T. Suzuki, S. Shinkai, K. Sada, Adv. Mater. 2006, 18, 1043–1046;
b) S. Yagai, T. Nakajima, K. Kishikawa, S. Kohmoto, T. Karatsu, A.
Kitamura, J. Am. Chem. Soc. 2005, 127, 11134–11139; c) Z. Yang,
H. Gu, D. Fu, P. Gao, J. K. Lam, B. Xu, Adv. Mater. 2004, 16, 1440–
1444; d) A. Shumburo, M. C. Biewer, Chem. Mater. 2002, 14, 3745–
3750; e) S. R. Haines, R. G. Harrison, Chem. Commun. 2002, 2846–
2847; f) S. Kiyonaka, K. Sugiyasu, S. Shinkai, I. Hamachi, J. Am.
Chem. Soc. 2002, 124, 10954–10955; g) W. Weng, J. B. Beck, A. M.
Jamieson, S. J. Rowan, J. Am. Chem. Soc. 2006, 128, 11663–11672;
h) S. Kawano, N. Fujita, S. Shinkai, J. Am. Chem. Soc. 2004, 126,
8592–8593; i) K. Tsuchiya, Y. Orihara, Y. Kondo, N. Yoshino, T.
Ohkubo, H. Sakai, M. Abe, J. Am. Chem. Soc. 2004, 126, 12282–
12283.
[5]a) J. J. D. de Jong, T. D. Tiemersma-Wegman, J. H. van Esch, B. L.
Feringa, J. Am. Chem. Soc. 2005, 127, 13804–13805; b) M. de Loos,
J. H. van Esch, R. M. Kellogg, B. L. Feringa, Angew. Chem. 2001,
113, 633–636; Angew. Chem. Int. Ed. 2001, 40, 613–616; c) S.
Kawano, N. Fujita, S. Shinkai, Chem. Commun. 2003, 1352–1353;
d) P. Mukhopadhyay, Y. Iwashita, M, Shirakawa, S. Kawano, N.
Fujita, S. Shinkai, Angew. Chem. 2006, 118, 1622–1625; Angew.
Chem. Int. Ed. 2006, 45, 1592–1595.
[6]a) S. Li, L. He, F. Xiong, Y. Li, G. Yang, J. Phys. Chem. B 2004, 108,
10887–10892; b) A. Ohshima, A. Momotake, R. Nagahata, T. Arai,
J. Phys. Chem. A 2006, 109, 9731–9736.
[7]a) E. Hadjoudis, M. Vittorakis, I. Moustakali-Mavridis, Tetrahedron
1987, 43, 1345–1360; b) E. Hadjoudis, Mol. Eng. 1995, 5, 301–337;
c) J. Zhao, B. Zhao, J. Liu, A. Ren, J. Feng, Chem. Lett. 2000, 268–
269; d) M. Taneda, K. Amimoto, H. Koyama, T. Kawato, Org.
Biomol. Chem. 2004, 2, 499–504; e) E. Hadjoudis, I. M. Mavridis,
Chem. Soc. Rev. 2004, 33, 579–588; f) K. Ogawa, J. Harada, J. Mol.
Struct. 2003, 647, 211–216.
[22]a) D. C. Duncan, D. G. Whitten, Langmuir 2000, 16, 6445–6452;
ˇ
´
´
´
´
´
b) J. Makarevic, M. Jokic, Z. Raza, Z. Stefanic, B. Kojic-Prodic, M.
ˇ
´
Zinc, Chem. Eur. J. 2003, 9, 5567–5580; c) M. Ikeda, T. Nobori,
L. M. J. Schmutz, Chem. Eur. J. 2005, 11, 662–668; d) S. J. George,
A. Ajayaghosh, Chem. Eur. J. 2005, 11, 3217–3227.
[23]M. Tata, V. T. John, Y. Y. Waguespack, G. L. McPherson, J. Am.
Chem. Soc. 1994, 116, 9464–9470.
[24]A. G. J. Ligtenbarg, R. Hage, A. Meetsmaa, B. L. Feringa, J. Chem.
Soc. Perkin Trans. 2 1999, 807–812.
[25]a) M. George, R. G. Weiss, Chem. Mater. 2003, 15, 2879–2888; b) Y.
Zhou, T. Yi, T. Li, Z. Zhou, F. Li, W. Huang, C. Huang, Chem.
Mater. 2006, 18, 2974–2981.
[8]a) T. Inabe, S. Gautier-Luneau, N. Hoshino, K. Okaniwa, H. Oka-
moto, T. Mitani, U. Nagashima, Y. Maruyama, Bull. Chem. Soc. Jpn.
1991, 64, 801–810; b) T. Inabe, I. Luneau, T. Mitani, Y. Maruyama,
S. Takeda, Bull. Chem. Soc. Jpn. 1994, 67, 612–621; c) K. Wozniak,
H. He, J. Klinowski, W. Jones, T. Dziembowska, E. Grech, J. Chem.
Soc. Faraday Trans. 1995, 91, 77–85; d) A. R. Katritzky, I. Ghiviriga,
P. Leeming, F. Soti, Magn. Reson. Chem. 1996, 34, 518–526; e) S. H.
Alarcón, A. C. Olivieri, A. Nordon, R. K. Harris, J. Chem. Soc.
Perkin Trans. 2 1996, 2293–2296; f) T. Sekikawa, T. Kobayashi, T.
[26]T. Sekikawa, T. Kobayashi, T. Inabe, J. Phys. Chem. A 1997, 101,
644–649.
[27]J. Luo, Z. Xie, J. W. Y. Lam, L. Cheng, H. Chen, C. Qiu, H. S.
Kwok, X. Zhan, Y. Liu, D. Zhu, B. Tang, Chem. Commun. 2001,
1740–1741.
[28]S. Kim, Q. Zheng, G. S. He, D. J. Bharali, H. E. Pudavar, A. Baev,
P. N. Prasad, Adv. Funct. Mater. 2006, 16, 2317–2323.
[29]C. Y. Bao, R. Lu, M. Jin, P. C. Xue, C. H. Tan, G. F. Liu, Y. Y. Zhao,
Org. Biomol. Chem. 2005, 3, 2508–2512.
Inabe, J. Phys. Chem.
Harada, T. Fujiwara, S. Yoshida, J. Phys. Chem. A 2001, 105, 3425–
3427.
A 1997, 101, 644–649; g) K. Ogawa, J.
[30]B. An, S. Kwon, S. Jung, S. Y. Park, J. Am. Chem. Soc. 2002, 124,
14410–14415.
[31]A. Chowdhury, S. Wachsmann-Hogiu, P. R. Bangal, I. Raheem,
L. A. Peteanu, J. Phys. Chem. B 2001, 105, 12196–12201.
[32]a) K. Ogawa, Y. Kasahara, Y. Ohtani, J. Harada, J. Am. Chem. Soc.
1998, 120, 7107–7108; b) J. Harada, H. Uekusa, Y. Ohashi, J. Am.
Chem. Soc. 1999, 121, 5809–5810.
[33]K. Amimoto, H. Kanatomi, A. Nagakari, H. Fukuda, H. Koyama, T.
Kawato, Chem. Commun. 2003, 870–871.
[34]The supporting information for this article contains the UV/Vis
spectra of 1’ in dilute solution and the crystal state, the FT-IR spec-
tra of the xerogel before and after irradiation, and the TEM image
of the benzene/cyclohexane gel.
[9]a) A. Ohshima, A. Momotake, T. Arai, J. Photochem. Photobiol. A
2004, 162, 473–479; b) A. Ohshima, A. Momotake, T. Arai, Bull.
Chem. Soc. Jpn. 2006, 79, 305–311.
[10]a) C. V. Yelamaggad, S. A. Nagamani, U. S. Hiremath, D. S. S. Rao,
S. K. Prasad, Liq. Cryst. 2002, 29, 1401–1408; b) R. Achten, A. Kou-
dijs, Z. Karczmarzyk, A. T. M. Marcelis, E. J. R. Sudhçlter, Liq.
ˇ
Cryst. 2004, 31, 215–227; c) M. Sepelj, A. Lesac, U. Baumeister, S.
Diele, D. W. Bruce, Z. Hamersak, Chem. Mater. 2006, 18, 2050–
2058.
ˇ
[11]K. Nakatani, J. A. Delaire, Chem. Mater. 1997, 9, 2682–2684.
[12]H. Birkedal, P. Pattison, Cryst. Struct. Commun. 2006, c62, o139-
o141.
[35]Y. Q. Tian, X. H. Xu, Y. Y. Zhao, X. Y. Tang, T. J. Li, Liq. Cryst.
1997, 22, 87–96.
[13]H. Fukuda, K. Amimoto, H. Koyama, T. Kawato, Org. Biomol.
Chem. 2003, 1, 1578–1583.
[14]a) D. J. Abdallah, R. G. Weiss, Adv. Mater. 2000, 12, 1237–1247;
b) M. Zinic, F. Vçgtle, F. Fages, Top. Curr. Chem. 2005, 256, 39–76.
Received: February 27, 2007
Published online: July 23, 2007
Chem. Eur. J. 2007, 13, 8231 – 8239
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