Strongly fluorescent organogel system comprising fibrillar self-assembly of a trifluoromethyl-based cyanostilbene derivative

Article abstract of DOI:10.1021/ja046215g

The entangled fibrillar structure and its strong fluorescence emission have been observed in a novel class of organogel systems consisting of a CF₃-based π-conjugated organogelator. Copyright

Full text of DOI:10.1021/ja046215g

Published on Web 07/28/2004
Strongly Fluorescent Organogel System Comprising Fibrillar Self-Assembly
of a Trifluoromethyl-Based Cyanostilbene Derivative
†
†
†
†
Byeong-Kwan An, Deug-Sang Lee, Jong-Soon Lee, Yil-Sung Park, Hyung-Su Song, and
Soo Young Park*
School of Materials Science and Engineering, ENG445, Seoul National UniVersity,
San 56-1, Shillim-dong, Kwanak-ku, Seoul 151-744, Korea, and Central R&D center, Dongwoo FineChem Co., Ltd.,
1
177, Pyungtaek-Si, Kyunggi-Do, 451-764, Korea
Received June 26, 2004; E-mail: parksy@plaza.snu.ac.kr
Recently, low-molecular-mass organic gelators (LMOGs) with
π-conjugated molecular structure have gained much interest due
to advantages of the specific nature of organogel networks for
various optoelectronic applications such as enhanced charge
1
transport, fluorescence, and sensing abilities. Relevant to these
applications, the incorporation of the long alkyl chains or steroidal
groups into the gelator structure has been considered very necessary
2
for the effective gelation process, although these structural elements
are normally inactive and undesirable for the optoelectronic
properties. In this communication, we report a new class of LMOGs
with simple trifluoromethyl (CF ) substituents instead of the long
3
alkyl chains or steroidal parts, endowing the gel system with better
potential for optoelectronic applications. Specifically, we demon-
strate in this work a strongly enhanced fluorescence emission gated
by the gelation process.
Figure 1. (a) Photo of CN-TFMBE (0.8 wt/vol %) dissolved in 1,2-
dichloroethane at 60 °C (left vial) and the corresponding organogel at 20
°C (right vial). (b) Fluorescence emission of CN-TFMBE solution (left vial)
and CN-TFMBE gel (right vial) under the UV light (365 nm), the same
vials as in a. (c) SEM images of a dried gel of CN-TFMBE in
1,2-dichloroethane. (d) Fluorescence microscopy images of a CN-TFMBE
organogel. The arrows in the inset photo indicate the node where the fibrous
structures of CN-TFMBE are largely bundled and knotted.
In the course of the systematic syntheses of highly fluorescent
organic chromophores in the solid state, we came to find that one
of our newly synthesized fluorescent molecules, 1-cyano-trans-1,2-
bis-(3′,5′-bis-trifluoromethyl-biphenyl)ethylene (CN-TFMBE), pos-
sessed a strong gelation power at room temperature.
Scheme 1
intensity is enhanced over 170 times in the gel state (see Figure
1b). Therefore, the entangled fibrillar structure of the CN-TFMBE
could be directly observed in the original nondried gel state by
fluorescence optical microscopy (Figure 1d). The inset photo of
Figure 1d shows that the fibrous structures of CN-TFMBE are
largely bundled and knotted at the same node. The presence of
this kind of nodes suggests that the CN-TFMBE gel constitutes a
The powder of the CN-TFMBE was well dissolved in 1,2-
dichloroethane (0.8 wt/vol %) by gentle heating (Figure 1a, left
vial), which, after cooling to room temperature, exhibited no
gravitational flow, as it could be turned upside down safely (Figure
1
a, right vial). It was also found that this gelation process was
2
robust gel system with permanent solidlike networks.
As mentioned earlier, the presence of long alkyl chains or
completely thermoreversible with the gel melting temperature of
about 51 °C as determined by the “dropping ball method”. The
gelation ability of CN-TFMBE was evaluated in various organic
solvents under the same experimental condition as in 1,2-dichlo-
roethane. Gelation of CN-TFMBE was similarly effected in tolune,
chloroform, and tert-butyl alcohol. However, it was partially gelated
in octane and showed no gelation in tetrahydrofuran.
To gain insight into the aggregation morphology of CN-TFMBE
in the organogel, a dried gel sample was transferred onto a glass
slide and subjected to the scanning electron microscopy (SEM)
observation. The SEM images in Figure 1c suggest that CN-TFMBE
gelator creates entangled three-dimensional networks consisting of
the bundles of fibrous aggregates, which must be responsible for
the observed gelation. Figure 1b shows a peculiar example of
steroidal groups has been considered to be essential for the stable
LMOG assemblies.1 It is noted, however, that the simple CF
,2
3
groups are equally effective as demonstrated for the CN-TFMBE
organogel in this work. The origin of this unique gelation capability
of CN-TFMBE is best explained in terms of the two structural
features. First, the strong π-π stacking interactions exerted by rigid
rodlike aromatic segments in CN-TFMBE must play an integral
role in self-assembling of the gelator molecules. The second factor
3
to be considered is the presence of CF components in CN-TFMBE,
which might induce and stabilize molecular assemblies with their
strong secondary bonding forces.3 To evaluate how much these
two factors affect the gel formation, we synthesized and investigated
a series of CN-TFMBE analogues (1-4) shown in Scheme 1.
Compared to CN-TFMBE, the length of the rigid aromatic ring
segments was intentionally reduced in 1 and 2 while retaining four
,4
“
gelation-induced fluorescence emission”: CN-TFMBE gel is
strongly fluorescent, while CN-TFMBE itself is totally nonfluo-
rescent in 1,2-dichloroethane solution. Surprisingly, the fluorescence
CF moieties to investigate the influence of π-π stacking interac-
3
†
Dongwoo FineChem Co., Ltd.
tions on the gel formation. The gelation tests revealed that neither
10232
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J. AM. CHEM. SOC. 2004, 126, 10232-10233
10.1021/ja046215g CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
1
nor 2 showed gelation, probably due to the insufficient inter-
molecular interaction forces. It is noteworthy that the melting points
increase with the number of aromatic phenyl rings in this series of
molecules: 1 (101 °C) < 2 (204 °C) < CN-TFMBE (264 °C).5
This result implies that the elongated aromatic segments lead to
the increased intermolecular interactions, which, in turn, reinforces
the self-assembling process via strong π-π interactions. We further
3
explored the issue of CF contributions by examining the properties
of 3 and 4. These two molecules are identical in chemical
composition and structure to CN-TFMBE with the exception of
-
5
Figure 2. (a) UV/vis absorption spectra of CN-TFMBE (2 × 10 mol
-1
L
) in THF and its nanoparticle suspension (THF/water (1:4) mixture).
terminal methyl groups, i.e., 3 is fully methylated (four -CH
3
),
-5
-1
(
b) PL spectra of CN-TFMBE (2 × 10 mol L ) in THF and its
and 4 is partially methylated (two -CF and two -CH ) analogues.
3
3
nanoparticle suspension. The PL intensities were normalized by the
corresponding UV absorbance. Inset photograph shows the fluorescence
Different from the strong gel formation of CN-TFMBE, no gelation
was again observed for 3 and 4. It is again noted that the melting
-
5
-1
emission of CN-TFMBE (2 × 10 mol L ) in different solvents and in
the solid state (B, benzene; C, chloroform; D, 1,2-dichloroethane; T, THF;
A, acetonitrile; N, nanoparticle suspension; P, powder) under 365 nm UV
light illumination.
points increase with the numbers of CF
3
units: 3 (132 °C) < 4
5
(189 °C) < CN-TFMBE (264 °C), indicating the powerful role of
3
CF groups in reinforcing intermolecular interaction. Due to the
insufficient intermolecular interactions, none of the CN-TFMBE
phase is more dramatically evidenced by the PL studies (Figure
2b). The emission maximum peak of CN-TFMBE is again red-
shifted by about 48 nm with the impressive increase in PL intensity
from the dilute solution to the nanoparticles (over 460 times
enhancement). The inset photograph of Figure 2b shows that
aggregated CN-TFMBE (nanoparticle suspension (N) and powder
(P), respectively) is strongly fluorescent, while isolated CN-TFMBE
in any kind of solvent (benzene (B), chloroform (C), 1,2-
dichloroethane (D), THF (T), and acetonitrile (A)) is virtually
nonfluorescent. This result suggests that the strong fluorescence
emission of CN-TFMBE in the nanoparticle and gel state is not
caused by the special solvatochromic effect but by the unique AIEE
phenomena.
derivatives 1-4 showed fibrous assembly structures that are
6
generally in favor of the gel formation. Therefore, it is properly
3
considered that the four CF units in the CN-TFMBE gelator play
the role of alternative forces to improve intermolecular interaction
much like the long alkyl chain or steroidal group, given that the
strong π-π stacking interactions between the rigid rodlike aromatic
segments provide the essential driving force for gelation in
conjugated LMOGs.
The remarkable fluorescence enhancement from CN-TFMBE
gels can be unambiguously explained within the context of our
previous report on the aggregation-induced enhanced emission
,8
(
AIEE) phenomenon.7 Isolated CN-TFMBE molecules in dilute
solution are considered to be significantly twisted by the steric
interactions in biphenyl units as well as the bulky cyano groups
attached into vinylene moiety, which generally suppresses the
radiative decay channel. On the other hand, the more planar and
conjugated conformation of CN-TFMBE is induced in the solid
state due to the strong intermolecular forces, which tend to optimize
close packing between molecules. This aggregation-induced pla-
narization extends the effective π-conjugation length in the CN-
TFMBE molecule. Furthermore, in the solid state, the bulky and
polar cyano groups in CN-TFMBE play an important role of
favoring J-type aggregation, which restricts the formation of the
In conclusion, we have demonstrated the first example of a simple
conjugated LMOG (CN-TFMBE) without long alkyl chain or
steroidal substituents. The unique gelation capability of CN-TFMBE
is attributed to the cooperative effect of the strong π-π stacking
interactions of rigid rodlike aromatic segments and supplementary
intermolecular interactions induced by four CF units. The remark-
3
able fluorescence increase in the CN-TFMBE gel is attributed to
the AIEE phenomenon.
Acknowledgment. This work has been supported in parts by
Dongwoo FineChem Co., Ltd., and CRM-KOSEF.
Supporting Information Available: Synthetic and experimental
details (PDF). This material is available free of charge via the Internet
at http://pubs.acs.org.
9
excimer complex. Consequently, CN-TFMBE molecules in the
aggregated gel state are likely to show drastically enhanced
fluorescence emission compared with those of isolated state due
to the synergetic effect of intramolecular planarization and restricted
excimer formation. This postulated AIEE phenomenon of CN-
TFMBE is experimentally evidenced by the UV/vis absorption and
photoluminescence (PL) studies of dilute solution (isolated state)
and nanoparticle suspension (aggregated state) of CN-TFMBE
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(
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(
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