Structure-property correlation of benzoyl thiourea derivatives as organogelators

Article abstract of DOI:10.1016/j.molstruc.2012.07.040

A series of N-benzoyl-N′-aryl thiourea derivatives (1-4) can form stable gels from a variety of organic solvents ranging from protic to aprotic or polar to apolar at concentrations below 5 mg/mL. The gelation properties and structures of the resulting gels were investigated by ¹H-NMR, FTIR, UV-vis, SEM, and XRD. The gels were anion-responsive and the driving forces for its formation were the hydrogen bonding and van der Waals interaction. The SEM images of the xerogels prepared from the organogels formed in acetonitrile, cyclohexane and acetone showed a network of elongated fibers. The results of XRD suggested that the dry gels had a layer structure.

Full text of DOI:10.1016/j.molstruc.2012.07.040

Journal of Molecular Structure 1031 (2013) 43–48
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Structure–property correlation of benzoyl thiourea derivatives as organogelators
Yao-Dong Huang a,b, , Xue-Lin Dong a,b, Li-Li Zhang a,b, Wei Chai a,b, Ji-Young Chang c,d
⇑
a Department of Pharmaceutical Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
b Key Laboratory of Systems Bioengineering, Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China
c Department of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul 151-744, Republic of Korea
d Hyperstructured Organic Materials Research Center, College of Engineering, Seoul National University, Seoul 151-744, Republic of Korea
h i g h l i g h t s
"
"
"
N-benzoyl-N0-aryl thioureas are powerful organogelators.
The driving forces of gelation are hydrogen bonding and van der Waals interaction.
The xerogels have clear fibrous networks and possess lamellar structures.
a r t i c l e i n f o
a b s t r a c t
A series of N-benzoyl-N0-aryl thiourea derivatives (1–4) can form stable gels from a variety of organic sol-
vents ranging from protic to aprotic or polar to apolar at concentrations below 5 mg/mL. The gelation
properties and structures of the resulting gels were investigated by 1H-NMR, FTIR, UV–vis, SEM, and
XRD. The gels were anion-responsive and the driving forces for its formation were the hydrogen bonding
and van der Waals interaction. The SEM images of the xerogels prepared from the organogels formed in
acetonitrile, cyclohexane and acetone showed a network of elongated fibers. The results of XRD suggested
that the dry gels had a layer structure.
Article history:
Received 10 March 2012
Received in revised form 23 July 2012
Accepted 23 July 2012
Available online 1 August 2012
Keywords:
Benzoyl thiourea
Gelator
Ó 2012 Elsevier B.V. All rights reserved.
Self-assembly
Hydrogen bonds
Anion-responsive organogels
1. Introduction
fields, and mechanical stress, has increasingly attracted attention
because of its applications in new functional materials, such as
Low molecular weight gelators (LMWGs) have been investi-
gated extensively in the past two decades, not only because of their
various potential applications, but also because the exact mecha-
nism of gel formation is still poorly understood [1]. Under gelation
conditions, the gelator molecules self-assemble through noncova-
optoelectronic devices, drug delivery, controlled release, and
molecular sensing [6–8]. The discovery and exploration of smart
gel are highly desired and encouraged.
We have been interested in synthesizing heterocyclic com-
pounds for some time. In the course of trying to purify several N-
benzoyl-N0-aryl thiourea derivatives, we found 1–4 could gelate
many organic solvents and turned out to be a family of efficient
organogelators. Furthermore, the gel-to-sol transition of gels could
be tuned by anion Fꢀ. N-benzoyl thiourea derivatives, noticeable
for their carbonyl and thioamide groups which can bind to various
metal ions, are widely used as ligands in the coordination chemis-
try [9] and have also attracted considerable attention owing to
their interesting biological activities [10]. N-benzoyl thioureas
and their metal complexes are potential building blocks for supra-
molecular architectures. Several studies have reported on their li-
quid crystalline behaviors [11]. In this paper, we report the
gelling properties and supramolecular architecture structures of
N-benzoyl-N0-aryl thiourea derivatives 1–4. Although numerous
examples of the organogelators based on urea or thiourea deriva-
lent interactions such as hydrogen bonding,
p–p stacking, van
der Waals interaction, and dipole–dipole interaction, into three-
dimensional networks to immobilize the solvent molecules. In
the gelation of carbohydrate- [2], nucleic acid- [3], amino acid-
[4], and urea-based gelators [5], hydrogen bonding plays a very
important role and is the major driving force of gelation. Of the
LMWG gels reported, smart gel—the gel can be responsive to multi-
ple stimuli such as temperature, pH, ions, light, electric/magnetic
⇑
Corresponding author at: Department of Pharmaceutical Engineering, Ministry
of Education, School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300072, China. Tel.: +86 22 87402150; fax: +86 22 27890166.
E-mail address: huangyaodong@tju.edu.cn (Y.-D. Huang).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.07.040

44
Y.-D. Huang et al. / Journal of Molecular Structure 1031 (2013) 43–48
tives have been reported [5,12], to our best knowledge, they have
2.5. Preparation of xerogels
not included N-benzoyl-N0-aryl thiourea series.
The gels prepared according to the process described in Sec-
tion 2.3 were placed in the atmosphere for 48 h to generate xerogels.
2. Experimental
3. Results and discussion
2.1. Materials and instrumentation
3.1. Gelation properties
All the chemicals were purchased from commercial chemical
suppliers and used without further purification. Acetonitrile was
dried by distillation over CaH2. 1H nuclear magnetic resonance
(1H NMR) spectra were recorded with a Bruker 400 spectrometer
operated at 300 MHz. Fourier transform infrared spectroscopy
(FTIR) measurements were performed on a Bio-Rad FTS 6000 spec-
trometer. Ultraviolet–visible spectroscopy (UV–vis) absorption
spectra were recorded with a Lambda35 spectrometer. X-ray dif-
The gelation test results are listed in Table 1. Compounds 1–4
gelated a wide variety of solvents ranging from apolar to polar
ones, while compound 5, bearing two methoxy groups as side
chains, did not gelate any of the solvents tested. The gelation abil-
ity of 1–4 had a positive trend with the length of the side chains. A
compound with longer side chains gelated a broader range of sol-
vents with a lower MGC than that with shorter side chains. It is
well known that the gelation ability of LMWG is highly related to
a hydrophobic–hydrophilic balance of the molecule, which in-
cludes the balance of polar hydrogen bonding units and non-polar
alkyl chains [13]. Because 1–5 have different lengths of alkyl
chains, it can be deduced that the longest alkyl chains render 4 a
best balance for organogelation while 1 has too short the alkyl
chains and tends to precipitate. Apparently, a suitable length of
side chains is essential for the effective gelation of organic solvents
in the present cases.
fraction (XRD) were checked on a XPERT Philips (Cu K
a radiation
k = 1.54056 Å) at a scan speed of 0.02° sꢀ1 in the 2h range from
0.8° to 15.0°. Scanning electron microscopy (SEM) images were ta-
ken by Hitachi S-4800 microscope. Single crystal data were col-
lected on a CAD4/PC instrument.
2.2. Synthesis
N-benzoyl-N0-aryl thiourea derivatives 1–5 were prepared
according to the literature [11(d)] (Scheme 1) and their structures
were identified by 1H NMR [11(e)].
3.2. Morphology of the gels
(a) RBr, K2CO3, DMF, 70–80 °C, 5 h; (b) C2H5OH, KOH, reflux, 4 h,
pH = 1–2; (c) SOCl2, reflux, 6 h; (d) CH3CN, NH4SCN, reflux, 1.5 h;
(e) RBr, DMF, 70–80 °C, 5 h; (f) NH2NH2ꢁH2O, C2H5OH, graphite, re-
flux, 12 h; (g) CH3CN, reflux, 3 h.
To obtain a macroscopic aspect of the network structures of the
gels, xerogels of compounds 1–4 were examined by SEM. As shown
in Fig. 1, all the xerogels have clear fibrous network structures
formed by the long fibers with the wide range of about 0.2–
5.0 lm. To a certain extent, the morphologies of the xerogels are
2.3. Gelation test
independent of the gelating solvents but apparently different in
fine structures. For example, the xerogel of 4 in 1-butanol has
the thickest fibers (Fig. 1E) while 1 in cyclohexane exhibits a most
twisted network (Fig. 1B); the network formed by 4 in acetonitrile
(Fig. 1D) is much tighter than that by 1 in acetonitrile (Fig. 1A).
A weighted gelator was mixed with a certain amount of the sol-
vent in a test tube, and the mixture was heated until the solid was
dissolved. The resulting solution was then cooled to room temper-
ature, and termed a gel if no fluid solvent ran down when the test
tube was inverted. The addition of certain amount solvent and the
heating–cooling process was repeated until gel cannot be formed,
and the minimum gelation concentration (MGC) can be calculated.
3.3. Single-crystal X-ray analysis
Though the single crystal structure of 5 was previously reported
[14], we performed the X-ray analysis to obtain more information
about the intermolecular interactions. As shown in Fig. 2, aryl-
amine NAH and carbonyl O form an intramolecular hydrogen bond
[Hꢁ ꢁ ꢁO = 1.90(2) Å, \NAHꢁ ꢁ ꢁO = 137.3(13)°] while benzoyl amide
NAH and thiocarbonyl S constitute an intermolecular hydrogen
bond [Hꢁ ꢁ ꢁS = 2.63(16) Å, \NAHꢁ ꢁ ꢁS = 165.5(11)°]. Obviously, the
intramolecular hydrogen bond is more powerful than the intermo-
lecular one. The dihedral angle of the aromatic rings of A and B0 or
B and A0 is 72.4° with the center-to-center distance of 5.17 Å,
which is much larger than the optimum distance of ca. 3.80 Å
2.4. Preparation of single crystal
Single-crystal analysis can provide important information to
elucidate the molecular structure of gelator and the packing mode
in the aggregation. However, all attempts to obtain single crystals
from 1 to 4 suitable for X-ray analysis failed. Fortunately, single
crystal of 5 could be obtained by slowly evaporating a chloro-
form-hexane solution of 5 in the atmosphere, and was used as a
reference.
COOMe
(a)
COOMe COOH
(c)
COCl
CONCS
(d)
(b)
OR
O
S
OH
OR
OR
OR
OR
(g)
N
H
N
H
NO2
NO2
NH2
RO
1:R=-C6H13
2:R=-C8H17
3:R=-C10H21
(e)
(f)
OH
OR
OR
4:R=-C12H25
5:R=-CH3
Scheme 1. Synthetic route for 1–5.

Y.-D. Huang et al. / Journal of Molecular Structure 1031 (2013) 43–48
45
Table 1
Gelation properties of N-benzoyl-N0-aryl thiourea derivatives 1–5.a
Solvent
1
2
3
4
5
Hexane
PG
<16
<16
PG
<50
S
PG
PG
<6
PG
<6
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
Cyclohexane
n-octane
Diethyl ether
Ethyl acetate
Benzene
Toluene
Acetonitrile
Acetone
<16
<16
PG
<20
<25
<50
<5
<12
<25
<30
PG
<10
<8
<10
<16
<16
<50
S
<10
<25
S
<12
PG
<12
<25
<25
<5
<12
<25
<25
PG
<10
<8
<10
<16
<16
<50
S
<10
<16
S
<12
PG
<10
<16
<16
<5
<12
<20
<25
PG
S
<20
<50
<50
<30
PG
<12
<16
<16
<16
S
S
S
<25
<50
S
CCl4
1,2-Dichloroethane
Methanol
Ethanol
2-Propanol
1-Butanol
1-Hexanol
DMFb
<6
<8
<10
<10
<12
<50
<25
<10
<12
S
Dioxane
Fig. 2. Hydrogen bonds and crystal packing mode of 5.
Diethylamine
Triethylamine
Aniline
[15]. The result indicates that there are no p–p interactions among
the molecules of 5.
CH2Cl2
CHCl3
S
S
S
S
S
S
S
S
THFb
3.4. FTIR studies
a
PG = partial gel; S = solution; values denote MGC (mg/mL).
DMF: N,N-dimethylformamide; THF: tetrahydrofuran.
b
Fig. 3 shows the FTIR spectra of 4 at different states. It is obvious
that the spectrum of xerogel of 4 obtained from CCl4 (Fig. 3b) is
very similar to that of the crystal from CHCl3 (Fig. 3a), implying
Fig. 1. SEM photographs of the xerogels from the gels of (A) 1/acetonitrile (20 mg/mL), (B) 1/cyclohexane (20 mg/mL), (C) 1/acetone (50 mg/mL), (D) 4/acetonitrile (20 mg/
mL), (E) 4/1-butanol (20 mg/mL), and (F) 4/acetone (20 mg/mL).

46
Y.-D. Huang et al. / Journal of Molecular Structure 1031 (2013) 43–48
Table 2
that the noncovalent interactions in the gel are almost the same as
those in the crystal.
vas(CH2) and ms(CH2) for xerogels of compounds 1–4.
As shown in Fig. 3b and c, the stretching vibrations of NH, C@O
and C@S shifted from 3425, 1671, 1150 cmꢀ1 in the CHCl3 solution
to 3339, 1669, 1141 cmꢀ1 in the CCl4 xerogel, respectively. The
same observations were obtained from the spectra of 5 measured
in the solution and crystal states (not shown), indicating that com-
pounds 4 and 5 had the same molecular interactions. The slight
red-shift (1–2 cmꢀ1) of the carbonyl absorption could be attributed
to the formation of intra-hydrogen bonding rather than inter-
hydrogen bonding. These results suggested that intermolecular
hydrogen bonding should be one of the driving forces for the for-
mation of the gel networks [16]. As can also be seen from Fig. 3,
the two absorption bands for CH2 appear at 2928 and 2854 cmꢀ1
in CHCl3 solution but shift to 2916 and 2848 cmꢀ1 after gelation.
The lower wavenumber reveals a decrease in the fluidity of alky
chains due to the strong organization of alky groups via van der
Waals interaction [17,18].
Compound
1
2
3
4
vas(CH2)/cmꢀ1
2921
2854
2918
2852
2917
2850
2916
2848
m
s(CH2)/cmꢀ1
It is known that the antisymmetric and symmetric stretching
vibration absorptions of CH2[vas(CH2) and ms(CH2)] are highly re-
lated to the conformation of the hydrocarbon chain [19]. When
the alkyl chains adopt the all-trans conformation, vas(CH2) and
m
s(CH2) are around at 2918 cmꢀ1 and 2848 cmꢀ1, respectively;
while the blue-shift of the bands indicates the increase in confor-
mation disorder, i.e., gauche conformation, in the hydrocarbon
Fig. 4. UV–visible absorption of 4 in the gel state (acetonitrile, 5 mg/mL) (solid line)
and in the solution state (acetonitrile, 1.2 ꢂ 10ꢀ5 M) (dashed line).
chain [20]. As shown in Table 2, frequencies of vas(CH2) and ms(CH2)
for xerogels of 1–4 are in the range of 2921–2916 cmꢀ1 and 2854–
2848 cmꢀ1, respectively, suggesting that the hydrocarbon chains of
1–4 adopt almost all-trans conformation in organogels. From 1 to 4,
the signals of two amide protons shift slightly but steadily to lower
field along with increasing concentration (Fig. 5). Such a shift re-
sults from the restricting freedom of motion, indicating the pres-
ence of hydrogen bonding interaction [22]. The addition of 1
equivalent of tetra-n-butylammonium fluoride (TBAF) results in
the disappearance of two amide proton signals, implying the reac-
tion of Fꢀ with amide protons. Fig. 6 shows the temperature-
dependent 1H NMR spectra of 4 in CDCl3. Both the two amide pro-
tons shift to higher field with increasing temperature from 298 K to
318 K. This phenomenon can be ascribed to the gradual breakdown
of the hydrogen bonding network at elevated temperatures. These
results demonstrated that the hydrogen bonding is a driving force
for the gel formation [23].
the values of vas(CH2) and ms(CH2) decrease slowly with increasing
alky chain length. This decrease means that the van der Waals
interactions of the alky chains are getting more powerful [21].
3.5. UV–vis spectroscopy studies
The UV–visible spectra of 4 in the solution and gel state were
measured (Fig. 4). In an acetonitrile solution (1.2 ꢂ 10ꢀ5 M), the
two absorption peaks assigned to E2 band and B band of 4 appear
at 211 and 288 nm, respectively. Upon gelation in acetonitrile
(5 mg/mL), E2 band is red-shifted to 249 nm while B band remains
at the same position, indicating that no
the formation of the gel network.
p–p interaction involves in
As to the protons in benzene rings, no signal shifting is observed
both in Fig. 5 (concentrations at 10, 30, 40 mg/mL) and Fig. 6. The
3.6. 1H NMR spectroscopy studies
results suggest no p–p interaction is involved in the aggregation of
4. This conclusion is consistent with the observations in the UV–vis
1H NMR spectra can reveal hydrogen bonding interactions. In
the concentration-dependent 1H NMR measurement of 4 in CDCl3,
Fig. 3. FTIR spectra of 4: (a) the crystal from CHCl3, (b) the xerogel from CCl4, and
Fig. 5. The 1H NMR spectra of 4 in CDCl3 at different concentrations and in the
(c) CHCl3 solution.
presence of 1 equivalent TBAF at the concentration of 30 mg/mL.

Y.-D. Huang et al. / Journal of Molecular Structure 1031 (2013) 43–48
47
Fig. 6. Temperature-dependent 1H NMR spectra of 4 in CDCl3 (30 mg/mL).
Fig. 8. XRD pattern of the xerogel from 4 in acetonitrile gel inset with a molecular
model of compound 4 performed by SYBYL 6.9.2.
and could be reasoned from the result of the single crystal X-ray
diffraction analysis of 5.
3.7. Anion response test
Table 3
Bragg reflections observed in xerogels of 1–4.
It has been reported that some organogels can be tuned by an-
ions through the formation/destruction of hydrogen bonding be-
tween anion and reactive hydrogen [24]. To verify the existence
of hydrogen bonding in the gels, the acetonitrile gel of 4 was cho-
sen as a sample for the anion response test. Five anions, Fꢀ, Clꢀ,
Brꢀ, Iꢀ and AcOꢀ were added as their corresponding tetra-n-butyl-
ammonium salts into four acetonitrile gels of 4 respectively. Four
gels remained unchanged (Fig. 7a) but the gel with 1 equivalent
of Fꢀ became a homogeneous solution, which turned into gel again
when a protic solvent such as CH3OH, was added (Fig. 7b). The gel–
sol and sol–gel transitions are a reflection of the destruction/for-
mation of hydrogen bonding network among gelator molecules.
In fact, the result of 1H NMR titration has proved the formation
of hydrogen bonding between Fꢀ and reactive amide protons
(Fig. 5), which will destroy the hydrogen bonding among mole-
cules of 4.
Compound
Solvent
d100 (Å)
d200 (Å)
d300 (Å)
1
2
3
4
Acetone
Acetone
Acetone
Acetonitrile
18.36
22.63
27.97
30.27
9.18
11.45
13.79
15.12
6.06
7.44
8.97
10.06
performed by SYBYL 6.9.2 and the length of the molecule com-
puted by PM3 method was 33.46 Å (inset of Fig. 8), a little longer
than the first peak’s spacing of the xerogel of 4. This result obvi-
ously implies that the molecules of 4 are twisted in the self-assem-
bling process, probably due to the influences of hydrogen bonds
and van der Waals interactions.
4. Conclusion
3.8. Powder X-ray diffraction
The present study demonstrates that N-benzoyl-N0-aryl thio-
urea derivatives 1–4 are efficient organogelators and the formed
gels show thermo- and anion-induced reversible gel-to-sol phase
transition. FTIR, 1H NMR and UV–vis spectroscopy revealed that
both hydrogen bonding and van der Waals interaction were the
X-ray diffraction (XRD) technique was used to investigate the
structures of xerogels. The resulting spectrum of the xerogel ob-
tained from 4 in acetonitrile is shown in Fig. 8. The long spacing
d values of the aggregate are 30.27, 15.12, and 10.06 Å, which are
almost exactly 1:1/2:1/3, indicating a lamellar packing of the gela-
tor [25]. Similar XRD were performed on the xerogels of 1, 2, 3
from acetone and the results are listed in Table 3. The spacing val-
ues of 1, 2, 3 also have a reciprocal ratio of 1:1/2:1/3, which also
suggests that the gelator molecules are aggregated in a lamellar
structure. The interlayer distances of xerogels of 1, 2, 3, 4 are
18.36, 22.63, 27.97, 30.27 Å, respectively, increasing expectedly
with the length increase of the side chains. On the basis of single
driving forces for gelation but no
p–p interaction existed. The
XRD results suggested that the gels from 1 to 4 had the lamellar
structures and the interlayer distance increased with increasing
the length of the side chain.
Acknowledgement
The work was financially supported by the Graduate School of
Tianjin University.
crystal data of 5, molecular modeling of compound
4 was
Fig. 7. State changes of the acetonitrile gel of 4 (5 mg/mL) in anion response test: (a) responses to different anions, and (b) sol–gel transition by adding 1 equivalent of CH3OH
to the solution obtained after addition of Fꢀ.

48
Y.-D. Huang et al. / Journal of Molecular Structure 1031 (2013) 43–48
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