Self-assembly of supramolecualr metallogelator containing 2-(2'-Hydroxyphenyl) benzoxazole/Zn(II) chelate

Article abstract of DOI:10.1166/jnn.2010.2972

A low molar mass organogelator 1 containing 2-(2'-hydroxyphenyl)benzoxazole (HPB) unit with long alkyl chain was synthesized by the reaction with HPB and octyl isocyanate in THF at room temperature. A new chelate-based organogelator 1-Zn(ll) was prepared

Full text of DOI:10.1166/jnn.2010.2972

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Journal of
Nanoscience and Nanotechnology
Vol. 10, 6929–6933, 2010
Self-Assembly of Supramolecualr
Metallogelator Containing 2-(2ꢀ-Hydroxyphenyl)
benzoxazole/Zn(II) Chelate
Tae Hyeon Kim1ꢀ2, Na Young Kwon1, Ji Hye Son1, Changduk Yang3,
Minjung Lee4, and Taek Seung Lee1ꢀ∗
1Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and Textile System Engineering,
Chungnam National University, Daejeon 305-764, Korea
2Film Research Institute, Kolon Central Research Park, Kolon Industries, Inc., 212 Gongdan-dong,
Gumi, Gyeongbuk 730-030, Korea
3School of Energy Engineering, Ulsan National Institute of Science and Technology, Ulsan 689-805, Korea
4Department of Oriental Medicine and Food Biotechnology, Joongbu University, 101 Daehack-ro, Chubu-myon,
Kumsan-gun, Chungnam 312-702, Korea
A low molar mass organogelator 1 containing 2-(2ꢀ-hydroxyphenyl)benzoxazole (HPB) unit with long
alkyl chain was synthesized by the reaction with HPB and octyl isocyanate in THF at room temper-
ature. A new chelate-based organogelator 1-Zn(II) was prepared with the reaction of 1 and zinc(II)
acetate in methanol and dichloromethane at mild condition. The gelation ability of organogelator
was tested by heating–cooling method in various organic solvents, and the opaque gel was formed
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from DMF. Well-deveIlPop:e2d0s5e.2lf-1a7s.s2e2m5b.2le6d Ostnru:cWtureedo,f1o6rgManaorg2e0l 1w6as1c1o:4nfi6r:m52ed with field emission-
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scanning electron microscope (FE-SEM) and transmission electron microscope (TEM), and the
optical properties of organogel upon aggregation were monitored by UV-Vis and fluorescence spec-
troscopy. 1-Zn(II) was self-assembled to the sheet-like structure with the thickness of fully extended
length of molecules.
Keywords: Organogel, Metallogel, Fluorescence, Metal-Chelate, Self-Assembly.
1. INTRODUCTION
various electronic states arising from inorganic metal
components but also limitless possibilities of molecular
design resulting from organic ligands.
Supramolecular structures composed of low molecular
mass molecules have attracted much interest due to their
uniquecharacteristics and a wide range of potential appli-
cations such as templates of nano-scale inorganic mate-
rials,1–5 organic soft materials,6ꢀ7 and optical sensors.8–11
The gelation is an intriguing phenomenon demonstrated by
small molecules in aqueous or organic solvents resulting
from weak secondary interactions leading to the formation
of three dimensional supramolecular structures of nano-
meter to micrometer dimensions.1–14
Among various supramolecular structures, low molec-
ular mass organogels with metal-chelate moieties, called
metallogels, have been widely investigated to apply to
advanced soft and nanomaterials.15 The metal chelate moi-
eties are useful materials because they have not only
The self-assembled gelation for supramolecular struc-
ture is organized by intermolecular physical interactions
facilitated by hydrogen bond and other subsequent weak
interactions such as ꢁ–ꢁ interaction of heterocyclic rings,
van der Waals force of long alkyl chains, and electrostatic
forces.16–22 Along with a number of reports on the gelation
from small molecules, many efforts are devoted to devel-
opment of gels with optical absorption or fluorescence.23ꢀ24
In this study, we report on the gel formation of a new
chelate-based organogelator composed with HPB/zinc(II)-
chelate derivative at the central part of the gelator molecule
and long alkyl chains at lateral both sides, which linked
together by urea linkages. We expected that their chelate
structure would be of great advantage to gel formation
because of their strong ꢁ–ꢁ interactions, which were one
of the main driving forces for the gelation.
∗Author to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2010, Vol. 10, No. 10
1533-4880/2010/10/6929/005
doi:10.1166/jnn.2010.2972
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Self-Assembly of Supramolecualr Metallogelator Containing 2-(2ꢀ-Hydroxyphenyl)benzoxazole/Zn(II) Chelate
Kim et al.
2. EXPERIMENTAL DETAILS
2.3.2. 5-Octylureyl-2-(2ꢀ-hydroxy-4ꢀ-methyl
phenyl)benzoxazole (1)
2.1. Chemicals
1.52 g (6.30 mmol) of 2 in 50 ml of THF was added
into a mixture of 3.43 g (22.1 mmol) of octyl isocyanate
and 50 ml of THF with stirring. The reaction mixture was
stirred at room temperature for 2 days. The product was
filtered and washed with acetone for several times. Without
further purification, a product was dried in vacuo (yield:
All starting materials were obtained from commercial
supplies and used as received. 2,4-Diaminophenol dihy-
drochloride and octyl isocyanate were purchased from
Aldrich. 4-Methylsalicylic acid and polyphosphoric acid
(PPA) were purchased from TCI and Acros, respec-
tively. Other chemicals were purchased from Duksan and
Samchen.
1
1.46 g, 63%); H NMR (300 MHz, CDCl3): ꢂ = 11.30
(s, OH, 1H), 7.89 (d, J = 8.1 Hz, Ar–H, 1H), 7.66 (d,
J = 1.50 Hz, Ar–H, 1H), 7.53 (d, J = 8.70 Hz, Ar–H,
1H), 7.31 (d, J = 2.10 Hz, Ar–H, 1H), 6.95 (s, Ar–H, 1H),
6.84 (d, J = 9.00 Hz, Ar–H, 1H), 6.2 (s, NH, 1H), 4.60 (s,
NH, 1H), 3.29 (q, J = 25.2 Hz, CH2, 2H), 2.41 (s, CH3,
3H), 1.56–1.27 (m, 15H). Anal. Calcd: C, 69.85; H, 7.39;
N, 10.62; O, 12.14; Found: C, 70.01; H, 7.23; N, 10.19;
O, 12.57.
2.2. Instrumentation
1H NMR spectra were recorded on Bruker Biospin
DRX300 (Korea Basic Science Institute). UV-Vis spectra
were recorded on Lambda 35 (PerkinElmer) and fluores-
cent spectra were measured on Cary Eclipse (Varian). FE-
SEM and energy dispersive spectroscopy (EDS) images of
the xerogels were obtained using a JEOL JSM-7000F with
an accelerating voltage of 5.0 kV.
2.3.3. 1-Zn(II)
0.12 g (0.64 mmol) of zinc acetate in 10 ml methanol was
slowly added into 0.51 g (1.30 mmol) of 1 in THF with
stirring. The reaction mixture was stirred at room tem-
perature overnight and poured into 350 ml methanol. The
product was filtered and washed with methanol for sev-
2.3. Synthesis (Scheme 1)
2.3.1. 5-Amino-2-(2ꢀ-hydroxy-4ꢀ-methyl
phenyl)benzoxazole (2)
6.43 g (32.7 mmol) of 2,4-diaminophenol dihydrochlo-
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eral times. The solid was recrystallized with DMF. With-
ride and 4.97 g (32.7 mmol) of 4-methylsalicylic acid
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out further purification, the product was dried in vacuo
were mixed with 150 ml of PPA. The reaction mixture
Copyright: American Scientific Publishers
1
ꢁ
(yield: 0.363 g, 59%); H NMR (300 MHz, CDCl3): 7.88
was heated to 150 C and stirred for 12 h. The resulting
(d, J = 8.1 Hz, Ar–H, 2H), 7.67 (d, J = 1.50 Hz, Ar–H,
2H), 7.54 (d, J = 8.70 Hz, Ar–H, 2H), 7.33 (d, J = 2.10
Hz, Ar–H, 2H), 6.97 (s, Ar–H, 2H), 6.85 (d, J = 9.00 Hz,
Ar–H, 2H), 6.21 (s, NH, 2H), 4.59 (s, NH, 2H), 3.30
(q, J = 25.2 Hz, CH2, 4H), 2.42 (s, CH3, 6H), 1.54–1.28
(m, 30H). Anal. Calcd: C, 64.67; H, 6.61; N, 9.84; O,
11.24; Zn, 7.65; Found: C, 63.15; H, 6.76; N, 9.62; O,
12.14; Zn, 8.33.
mixture was cooled to room temperature and poured into
ice-water with vigorous stirring. The solution was neutral-
ized by addition of sodium bicarbonate. The crude product
was collected by filtration followed by recrystallization in
ethanol. Further purification was carried out with column
chromatography with ethyl acetate-toluene mixture. After
evaporation, the product was dried in vacuo (yield: 2.03 g,
26%); 1H NMR (300 MHz, DMSO-d6): ꢂ = 11.31 (s, OH,
1H), 8.79 (s, Ar–H, 1H), 8.13 (d, J = 2.40 Hz, Ar–H, 1H),
7.90 (d, J = 2.10 Hz, Ar–H, 1H), 7.67 (d, Ar–H, 2H), 7.45
(m, Ar–H, 1H), 6.97 (s, Ar–H, 1H), 6.94 (s, NH, 2H), 0.86
(t, 3H).
2.4. Gelation Test
A mixture of a weighed 1-Zn(II) and DMF in a screw-
capped vial was heated around boiling point until a clear
solution appeared. After clear solution was obtained, mix-
ture was cooled to room temperature under ambient condi-
tion. Yellowish opaque gel was confirmed by the inversion
of a test vial method.
OH
COOH
OH
HO
NH2
N
O
PPA
150 o
C
H2N
NH2
OH
2
H
N
H
N
N
O
OCN C8H17
THF, r.t.
O
O
1
2.5. Optical Measurement
O
O
N
N
H
N
H
Zn(II)(CH3COO)2
O
For optical spectroscopic measurements, aggregates of
1-Zn(II) dispersed in DMF was prepared by vigorous
shaking of corresponding gel at room temperature, on the
while a clear solution was prepared by heating the aggre-
gates followed by cooling to room temperature.
Zn
H
N
H
N
CH2Cl2, CH3OH, r.t.
N
O
O
1-Zn(II)
Scheme 1. Synthetic route of 1-Zn(II).
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Kim et al.
Self-Assembly of Supramolecualr Metallogelator Containing 2-(2ꢀ-Hydroxyphenyl)benzoxazole/Zn(II) Chelate
3. RESULTS AND DISCUSSION
(A)
(B)
1-Zn(II) was gelled at a concentration of 5% (w/v) in
DMF by heating–cooling method (Fig. 1). 1-Zn(II) showed
good solubility in DMF at high temperature in spite of low
solubility at room temperature. When the temperature of
1-Zn(II) solution was cooled down from boiling point to
room temperature, transparent and weakly fluorescent solu-
tion was transferred to opaque and bluish fluorescent gel.
Furthermore, opaque gel turned to the transparent solution
by re-heating to boiling point. It was allowed that the cycles
of gel-to-sol transition upon heating and sol-to-gel transi-
tion upon cooling were repeated reversibly.
(C)
(D)
The self-assembled structure was confirmed with FE-
SEM and TEM microscope. The FE-SEM image reveals
that 1-Zn(II) in DMF (5% (w/v)) self-assembled to
wrinkled paper-like structure composed of thin sheet-
like aggregates as shown in Figures 2(A and B). The
3-dimensional gel structure was formed by entanglement
of wrinkled paper-like structure. Interestingly, sheet struc-
ture with a few nm in thickness was observed, which
related to the thickness of the fully extended length of
1-Zn(II) molecules. The fully extended length of 1-Zn(II)
was calculated approximately 3.7 nm by Materials Stu-
dio 4.3. With these results, we presumed that the long
axis of 1-Zn(II) molecules was located perpendicularly
to the thin sheet plane. The evidence of this assumption
Fig. 2. FE-SEM (A, B) and TEM (C, D) photographs of freeze-dried
gel of 1-Zn(II) from DMF-gel (5% (w/v)), Scale bar; (A) 1 ꢃm;
(B) 100 nm; (C) 200 nm; (D) 10 nm.
DMF was shifted to the long wavelength from 337 nm to
369 nm and increased approximately two times than that of
1 solution in DMF at the same concentration (Fig. 4). The
large shift of absorption maximum from 338 nm to 369 nm
was caused by metal-to-ligand charge transfer (MLCT)
from zinc atom to HPB moieties.25 These results means
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was also supported by TEM image (Figs. 2(C and D)).
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TEM sample was prepared by the method that 1-Zn(II)-
DMF gel was located on carbon-coated copper grid and
freeze-dried. The zinc atom were located homogeneously
on all area of the thin sheet without any aggregation, which
means that the thin sheet was composed with one layer of
1-Zn(II) molecules. Furthermore, any directional pattern
was not observed, which means that 1-Zn(II) was ran-
domly packed each other. Further evidence was confirmed
by EDS mapping image (Fig. 3). The EDS images showed
also that zinc atom existed uniformly on all field of the
thin sheet without any aggregates and any pattern.
Copyright: American Scientific Publishers
that 1-Zn(II) was composed with one zinc atom and two
1 molecules. In fluorescence spectra, 1 solution in DMF
showed dual emission peak at 420 nm and 470 nm due to
the enol-keto tautomer of HPB moieties (Fig. 5).26 How-
ever, 1-Zn(II) solution in DMF showed only one emission
maximum at 437 nm, which means that one kind of energy
transfer was existed in the case of chelate. The emission
maximum of 1-Zn(II) shifted to longer wavelength than
(A)
(B)
Optical changes upon chelation of 1-Zn(II) were con-
firmed with UV-Vis and fluorescence spectroscopy. In UV-
Vis spectra, absorption maximum of 1-Zn(II) solution in
(C)
(D)
Fig. 1. Sol–gel process of 1-Zn(II) in DMF (5% (w/v)) by heating–
Fig. 3. EDS mapping measurement of freeze-dried gel from DMF-gel
cooling method.
((A) FE-SEM photograph, (B) carbon, (C) oxygen and (D) zinc).
J. Nanosci. Nanotechnol. 10, 6929–6933, 2010
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Self-Assembly of Supramolecualr Metallogelator Containing 2-(2ꢀ-Hydroxyphenyl)benzoxazole/Zn(II) Chelate
Kim et al.
1.0
were located perpendicularly to the sheet plane. By further
aggregation, thin sheets formed the thick sheets by pack-
ing and entangled to wrinkled paper-like structure, which
was fixed the solvents to form a gel.
0.8
0.6
0.4
0.2
0.0
4. CONCLUSION
We demonstrate the gel formation of a new chelate-
based organogelator composed with HPB/zinc(II)-chelates
derivative at central part of gelator molecule and long
alkyl chain at lateral both sides, which linked together
by urea linkages. These molecules self-assembled to wrin-
kled paper-like structure, which composed with thin sheet-
like aggregates. It is presume that 1-Zn(II) aggregated to
ꢄ-sheet structure with aggregation, and the long axis of
1-Zn(II) molecules were located perpendicularly to the
sheet plane.
300
400
500
600
Wavelength
Fig. 4. UV-Vis spectra of 1 (solid, 1.0 × 10−5 M), 1-Zn(II) solution
(dashed, 1.0 × 10−5 M), 1-Zn(II) aggregates (dotted, 1.0 × 10−5 M) in
DMF.
that of enol form of HPB moieties from 426 nm to 437 nm.
These were also caused by MLCT from zinc atom to HPB
moieties.
Optical changes of sol–gel transition of 1-Zn(II) were
also confirmed with UV-Vis and fluorescence spectroscopy
(Figs. 4 and 5). Upon aggregation, the absorption max-
imum of 1-Zn(II) aggregates shifted to longer wave-
length and became broader than that of 1-Zn(II) solution.
Besides, fluorescence emission of 1-Zn(II) aggregates was
also shifted to longer wavelength and broadened more than
Acknowledgment: This work was supported by Korea
Research Foundation Grant funded by Korean Government
(KRF-2008-313-D01272).
References and Notes
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Kim et al.
Self-Assembly of Supramolecualr Metallogelator Containing 2-(2ꢀ-Hydroxyphenyl)benzoxazole/Zn(II) Chelate
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Received: 31 December 2008. Accepted: 10 July 2009.
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J. Nanosci. Nanotechnol. 10, 6929–6933, 2010
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