New fluorescent amide-functionalized phenylethynylthiophene low molecular weight gelator

Article abstract of DOI:10.1021/ol052542x

An amide-functionalized phenylethynylthiophene gelator has been synthesized. Self-assembly of this molecule via cooperative hydrogen bonding and π-stacking induced gelation of a variety of organic solvents. The presence of aggregates was confirmed by conc

Full text of DOI:10.1021/ol052542x

ORGANIC
LETTERS
2006
Vol. 8, No. 3
387-390
New Fluorescent Amide-Functionalized
Phenylethynylthiophene Low Molecular
Weight Gelator
Chia-Chen Tsou†,‡ and Shih-Sheng Sun*,†
Institute of Chemistry, Academia Sinica, 115 Nankang, Taipei, Taiwan, Republic of
China, and Department of Chemistry, National Central UniVersity, 320 Chungli,
Taiwan, Republic of China
sssun@chem.sinica.edu.tw
Received October 20, 2005
ABSTRACT
An amide-functionalized phenylethynylthiophene gelator has been synthesized. Self-assembly of this molecule via cooperative hydrogen bonding
and -stacking induced gelation of a variety of organic solvents. The presence of aggregates was confirmed by concentration-dependent
absorption and fluorescence properties. SEM and TEM studies reveal the formation of fiberlike nanostructure.
π
The unique ability of small organic gelators to immobilize
organic solvents via the cooperative effect of noncovalent
interactions, such as hydrogen bonding, π-π stacking,
donor-acceptor, and van der Waals interactions, has received
considerable current interest for various potential applica-
tions.1 In order for these assemblies to form, it is crucial to
control the intermolecular interactions in such a way that
the molecules are able to self-organize into higher order
structures but without transformation to a crystalline state.
Many structure prototypes of low-molecular-weight organic
gelators have been reported in the recent literature.2
π-conjugated systems.3 Surprisingly, to the best of our know-
ledge, there is no report of low molecular weight organic
gelators based on phenylenethynylene derivatives although
phenylene ethynylene derivatives have played significant
roles in the recent advances of optoelectronic applications.4
Herein, we report the first example of thermoreversible low
molecular weight organic gelator of fluorescent phenylen-
ethynylene derivative via cooperative hydrogen-bond- and
π-stacking-induced supramolecular assembly.
(3) (a) Schoonbeek, F. S.; van Esch, J. H.; Wegewijs, B.; Rep, D. B. A.;
de Haas, M. P.; Klapwijk, T. M.; Kellogg, R. M.; Feringa, B. L. Angew.
Chem., Int. Ed. 1999, 38, 1393-1397. (b) Wang, R.; Geiger, C.; Chen, L.;
Swanson, B.; Whitten, D. G. J. Am. Chem. Soc. 2000, 122, 2399-2400.
(c) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148-
5149. (d) Ajayaghosh, A.; George, S. J.; Praveen, V. K. Angew. Chem.,
Int. Ed. 2003, 42, 332-335. (e) Sugiyasu, K.; Fujita, N.; Shinkai, S. Angew.
Chem., Int. Ed. 2004, 43, 1229-1233. (f) George, S. J.; Ajyaghosh, A.
Chem. Eur. J. 2005, 11, 3217-3227. (g) Wu¨rther, F.; Hanke, B.; Lysetska,
M.; Lambright, G.; Harms, G. S. Org. Lett. 2005, 7, 967-970. (h) Lampkins,
A. J.; Abdul-Rahim O.; Li, H.; Castellano, R. K. Org. Lett. 2005, 7, 4471-
4474.
In contrast, only scant work has been explored on the
structures and properties of organogels based on the rigid
* To whom correspondence should be adressed. Tel: +011-886-2-
27898596. Fax: +011-886-2-27831237.
† Academia Sinica.
‡ National Central University.
(1) Hoeben, F. J. M.; Jonkheijm, P.; Meijer, E. W.; Schenning, A. P. H.
Chem. ReV. 2005, 105, 1491-1546.
(2) (a) Terech, P.; Weiss, R. G. Chem. ReV. 1997, 97, 3133-3160. (b)
Abdallah, D. J.; Weiss, R. G. AdV. Mater. 2000, 12, 1237-1247. (c) van
Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263-2266.
(4) Schenning, A. P. H. J.; Meijer, E. W. Chem. Commun. 2005, 3245-
3258.
10.1021/ol052542x CCC: $33.50
© 2006 American Chemical Society
Published on Web 01/06/2006

Scheme 1. Synthesis of Amide-Functionalized Phenylethynylthiophene Derivatives
The amide-functionalized phenylethynylthiophenes 7 and
can be stored for months without showing sign of decom-
position. It should be noted that compound 8 is not effective
in gelating solvents that strongly compete for hydrogen-bond
formation. Addition of MeOH or DMSO into hot toluene
solution of 8 resulted in no gel formation after cooling the
solution, indicative of the crucial role of self-complementary
hydrogen bonding in the gelation process of 8.
The optical properties of compounds 7 and 8 are sum-
marized in Table 1. Both compounds show very similar
photophysical properties in dilute solution. Figure 1 compares
the absorption and fluorescent spectra of 8 measured in dilute
toluene solution (5 × 10-6 M) at 25 °C and at concentration
slightly higher above critical gelation concentration (2 mM).
The spectra of freeze-dried film are also included for
comparison. In dilute solution 8 displays typical π-π*
transitions in the UV and near UV region. An extra shoulder
appears at ∼440 nm in gelling solution, and it is even more
prominent and tailing into visible region in the absorption
spectrum of freeze-dried film. A weak fluorescent band
appears at 448 nm in dilute toluene solution with quantum
yield of 0.0087. The fluorescent spectra are red-shifted in
8 were prepared in four steps starting from 2,3,4,5-tetrabro-
mothiophene according to Scheme 1. Palladium-catalyzed
cross-coupling procedures were used extensively in the
syntheses.5 All new compounds synthesized were character-
ized by 1H and 13C NMR, mass spectrometry, and elemental
analysis.6 Compound 7 is soluble in virtually all common
organic solvents and forms a clear homogeneous solution at
room temperature. In contrast, compound 8 is only soluble
in certain organic solvents at elevated temperature. After the
solution was cooled, a nonflowing material was formed. The
gelation propensity of 8 was assessed in different solvents.
It exhibited remarkable capability of gelating a variety of
organic solvents including benzene (2 mg/mL), toluene (3
mg/mL), p-xylene (5 mg/mL), chloroform (8 mg/mL),
dichloromethane (8 mg/mL), 1,2-dichloroethane (5 mg/mL),
and 1-heptanol (1 mg/mL).7 In the case of 1-heptanol, one
molecule of 8 is capable of immobilizing ∼12000 molec-
ules of 1-heptanol, and thus, 8 can be considered as a
supergelator.3f,8 Except for dichloromethane, gels formed
from the solvents listed above are highly transparent. The
sol-gel interconversion is fully thermoreversible by several
cycles of heating and cooling. In addition, the gels from 8
Table 1. Photophysical Properties in CH2Cl2 Solution at 293 K
(5) Heck, R. F. Palladium Reagents in Organic Syntheses; Academic
Press: Orlando, 1985.
(6) The characterization and spectral analyses are included in Supporting
Information.
(7) The critical gelation concentration for each solvent is included in
parentheses.
(8) Luboradzki, R.; Gronwald, O.; Ikeda, A.; Shinkai, S. Chem. Lett.
2000, 1148-1149.
absorption
emission
compd
λmax, nm (ꢀ × 10-3, M-1 cm-1
)
λem, nm
Φem
7
8
334 (86.8), 390 (sh, 30.7)
335 (94.3), 388 (sh, 32.8)
458
448
0.0038
0.0087
388
Org. Lett., Vol. 8, No. 3, 2006

1
IR and H NMR measurements were employed to probe
the role of amide functional groups play in the assembly
1
process. Figure 3 shows the variable-temperature H NMR
Figure 1. Normalized absorption and emission spectra of 8 in dilute
solution, gelling solution, and dried film.
1
Figure 3. Variable-temperature H NMR of 8 in the aromatic
region.
gelling solution and film. The fluorescence maxima exhibit
bathochromic shift to 506 and 523 nm in gelling solution
and film, respectively. Variable-temperature absorption and
fluorescence spectra of 8 in toluene (5 × 10-6 M) also
revealed the transition from self-assembly nanostructure to
the molecularly isolated molecule as the temperature raised
from -10 to +70 °C (see Figure 2 for fluorescence spectra).
1
spectra of 8. As expected, the H NMR spectrum of 8 (2
mM) in CDCl3 at 298 K shows broad proton signals
indicating extensive aggregation in solution. Unlike the
chemical shift of “free” N-H peak, the chemical shift of
N-H peak downfield shifts to 9.41 ppm that is evidenced
of the formation of intermolecular hydrogen bonds11 and
confirmed by dilution H NMR experiments.6 The proton
1
signals were getting sharper upon heating the solution.
Concomitantly, the N-H peak upfield shifts to 9.32 ppm
indicating the disruption of intermolecular hydrogen bonds.
In addition, the evidence of reducing aromatic π-π interac-
tions is provided by the downfield shift of â protons of
pyridines from 8.29 ppm (298 K) to 8.39 ppm (338 K). The
IR spectra of 7 and 8 in toluene solution exhibit the same
vibrational bands at 3342 cm-1 that are assigned to the
intramolecular hydrogen-bonded N-H stretching frequency.
On the other hand, the N-H vibrational frequency of 8 in
toluene gel and in dried film shifts to lower values at 3339
and 3327 cm-1, respectively. All these observations point
toward the intermolecular aggregation of 8 in solution with
the participation of both hydrogen-bonding and π-π stacking
interactions.
The morphology of dried gels of 8 was studied by scanning
electron microscopy (SEM) and transmission electron mi-
crograph (TEM). The SEM images of 8 from CHCl3 display
three-dimensional networks of fiber bundles with 200-300
nm in diameter and several micrometers in length (Figure
4a). We believe the gathering of numerous fiber bundles
Figure 2. Variable-temperature fluorescence spectra of 8 in toluene
(1 × 10-5 M).
The prominent difference of spectra in dilute solution
compared to the spectra in gelling solution and freeze-dried
film as well as the spectral transition in the variable
temperature flourescence spectra indicate the possibility of
molecular aggregation through amide hydrogen bonding and/
or π-stacking.9 The fluorescence quantum yield also enhances
to 0.091 in film. This result is in contrast to the common
observation of decreasing fluorescence quantum yield in
π-stacking film compared to dilute solution.10
(9) The formation of excimer in the excited state is ruled out on the
basis of closely resemblance between their absorption and excitation spectra
for both dilute solution and gelling solution.
(10) Turro, N. J. Modern Molecular Photochemistry, 2nd ed.; University
Sciences Books: Sausalito, 1991.
(11) (a) Huang, B.; Parquette, J. R. Org. Lett. 2000, 2, 239-242. (b)
Dado, G. P.; Gellman, S. H. J. Am. Chem. Soc. 1993, 115, 4228-4245. (c)
Stevens, E. S.; Sugawara, N.; Bonora, G. M.; Toniolo, C. J. Am. Chem.
Soc. 1980, 102, 7048-7050.
Org. Lett., Vol. 8, No. 3, 2006
389

wide-angle region. The broad band in the wide-angle region
is most likely due to the interdigitated packing of the alkoxyl
chains or the intermolecular distance within a column.3h,12
In summary, we have synthesized the first example of low
molecular weight organic gelator based on phenylene ethy-
nylene derivative that is capable of immoblizing a variety
of organic solvents to form stable organogels. The general
properties of 8 are similar to many other reported gelators.
However, it exhibits very unique optical property. Even
though with apparently more ordered π-stacking and inter-
molecular interactions in the self-assembled three-dimen-
sional architecture, the fluorescence of these gels is greatly
enhanced compared to their solution states. We are currently
investigating in more detail the role of conjugation and
aliphatic chains in controlling the self-assembly and gelation
behavior of these amide-functionalized phenylenethynylenes
aiming to tune their optical properties and explore the
potential applications in optoelectronics.
Acknowledgment. We are grateful to the National
Science Council in Taiwan (Grant Nos. 93-2113-M-001-028,
93-2113-M-001-029, 94-2113-M-001-014) and Academia
Sinica for support of this research.
Figure 4. SEM image (a), TEM images (b, c), and XRD pattern
(d) of 8. The inset in (d) shows the expansion of small-angle region
of the spectrum.
Supporting Information Available: Synthetic details,
characterization data, and 1H NMR spectra of dilution
experiments. This material is available free of charge via
the Internet at http://pubs.acs.org.
forms the steric intertwined structure that results in the
immobilization of the organic solvents. The TEM images
of 8 from 1-heptanol solution display tape-like morphology
with 50-80 nm in diameter (Figure 4b,c). Powder X-ray
diffraction pattern of toluene film of 8 (Figure 4d) reveals
long-range periodic order where three reflection peaks
appearing at d ) 36.8, 21.5, and 18.0 Å in the small-angle
region, which indicates a macroscopic order of hexagonal
packing.12 A diffuse reflection at d ) 3.9 Å is observed in
OL052542X
(12) (a) Kim, C.; Kim, K. T.; Chang, Y.; Song, H. H.; Cho, T.-Y.; Jeon,
H.-J. J. Am. Chem. Soc. 2001, 123, 5586-5587. (b) Hashimoto, M.; Ujiie,
S.; Mori, A. AdV. Mater. 2003, 15, 797-800. (c) Hsu, H.-F.; Lin, M.-C.;
Lin, W.-C.; Lai, Y.-H.; Lin, S.-Y. Chem. Mater. 2005, 15, 2115-2118.
390
Org. Lett., Vol. 8, No. 3, 2006