Highly fluorescent supramolecular gels with chirality transcription through hydrogen bonding

Article abstract of DOI:10.1039/b802096e

A highly fluorescent organogel with transparency was formed through a hydrogen (H)-bonding interaction between a non-fluorescent and achiral 2-(3′,5′-bis-trifluoromethyl-biphenyl-4-yl)-3-(4-pyridin-4-yl- phenyl)-acrylonitrile (CN-TFMBPPE) monomer and chiral sergeant l-tartaric acid (TA) (or d-TA), with gel formation being accompanied by a drastic fluorescence enhancement as well as chirality induction. The Royal Society of Chemistry.

Full text of DOI:10.1039/b802096e

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Highly fluorescent supramolecular gels with chirality transcription
through hydrogen bondingw
Jangwon Seo, Jong Won Chung, Eun-Hye Jo and Soo Young Park*
Received (in Cambridge, UK) 6th February 2008, Accepted 11th March 2008
First published as an Advance Article on the web 17th April 2008
DOI: 10.1039/b802096e
A highly fluorescent organogel with transparency was formed
through a hydrogen (H)-bonding interaction between a non-
fluorescent and achiral 2-(30,50-bis-trifluoromethyl-biphenyl-
4-yl)-3-(4-pyridin-4-yl-phenyl)-acrylonitrile (CN-TFMBPPE)
monomer and chiral sergeant L-tartaric acid (TA) (or D-TA),
with gel formation being accompanied by a drastic fluorescence
enhancement as well as chirality induction.
In the present study, we sought to add specific functional
properties to CN-TFMBE, particularly the hydrogen (H)-bond-
ing, optical chirality, and gel transparency. To achieve this, we
modified the chemical structure of CN-TFMBE to the pyridine-
containing trifluoromethyl-based cyanostilbene (CN-TFMBPPE,
see the structure in Fig. 1), which was unlikely to have sufficient
self-assembling capability (with only two CF3 units) and so
needed prior complex formation with 3,5-bistrifluoromethyl
benzoic acid to attain the proper self-assembling ability for the
formation of supramolecular fibers and gels. Moreover, we
sought to prepare a transparent organogel from CN-TFMBPPE
via H-bonding with L-tartaric acid (TA)2a,3a,d as a chiral
component, in order to demonstrate chiral transcription in a
supramolecular system with concomitant fluorescence turn-on.
CN-TFMBPPE, 2-(30,50-bis-trifluoromethyl-biphenyl-4-yl)-
3-(4-pyridin-4-yl-phenyl)-acrylonitrile, was synthesized in
three steps as shown in Scheme S1 (see ESIw).z As expected,
CN-TFMBPPE was soluble in common organic solvents such
as 1,2-dichloroethane (DCE) and did not form a gel. To
examine the effect of complementary H-bonding, an equimo-
lar amount of 3,5-bistrifluoromethyl benzoic acid was added
to a 1 wt% solution of CN-TFMBPPE in DCE; initially, the
CN-TFMBPPE was soluble in DCE and showed no visible
color or fluorescence. As the H-bonded complex was gener-
ated and thus self-assembly proceeded, a translucent partial
gel with a characteristic absorption color and AIEE fluores-
cence slowly formed over several hours (see Fig. 1). In the FT-
IR spectrum of the dried gel (see ESIw), the intermolecular
H-bonding of the complex was evidenced by the appearance of
Recently, chiral supramolecular assemblies have attracted con-
siderable attention as candidate materials for use in biological
and electro-optical applications since the recognition and tran-
scription of chirality in the self-assembly process has a key role in
mimicking the development of biological helical structures with
specific function as well as in controlling unique optoelectronic
device applications.1 The first chiral helical supramolecular
polymer was prepared by Lehn and co-workers via molecular-
recognition-directed self-assembly from complementary chiral
components.2 Subsequently, this design strategy has been used
to construct chiral supramolecules from achiral monomers and
has been applied to a variety of self-assembled systems including
thin films, oligomers, liquid crystals and organogels.3
broad peaks at ca. 2420 cmꢀ1 and 1920 cmꢀ1 1g,3e,5
In addi-
.
Previously, we reported a special class of aromatic organo-
gelator (CN-TFMBE) that formed highly fluorescent organo-
gels with aggregation-induced enhanced emission (AIEE).4 A
unique characteristic of this class of AIEE dye is that it is
virtually non-fluorescent in the molecular state in solution but
becomes highly fluorescent upon self-assembly into supramo-
lecular fibers. This means that the self-assembly process of
CN-TFMBE can be monitored in real time by looking for
‘fluorescence turn-on’ with the naked eye. The strong self-
assembly capability of CN-TFMBE was attributed to co-
operative intermolecular interactions induced by the four
CF3 units in addition to the strong p–p interaction of the
planarized aromatic segments, which are twisted in isolated
molecules in solution.4b
tion, a SEM image of the dried gel revealed a typical network
structure of entangled and fibrillar aggregates. Similar to
CN-TFMBE, the gel forming capability of the
Fig. 1 (a) Schematic representation of the hydrogen (H)-bonded
complex between CN-TFMBPPE and 3,5-bistrifluoromethyl benzoic
acid (BA). (b) SEM image of a dried gel (partial) formed from the
H-bonded complex between CN-TFMBPPE (1 wt%, 1 equiv.) and BA
(1 equiv.) in DCE. Inset displays photo images of the corresponding
CN-TFMBPPE solution (left) and H-bonded complex gel (right) state.
(under daylight).
Organic Nano-Photonics Laboratory, School of Materials Science &
Engineering, Seoul National University, Seoul 151-744, Korea.
E-mail: parksy@snu.ac.kr; Fax: +82 2-886-8331;
Tel: +82 2-880-8327
w Electronic supplementary information (ESI) available: Synthetic
details and FT-IR, XRD, optical data. See DOI: 10.1039/b802096e
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Scheme 1 H-bonded complex between CN-TFMBPPE and L-TA,
and photo image of its transparent gel (L-TA complex gel (1 wt%))
from CN-TFMBPPE (1 wt%, 1 equiv.) and L-TA (0.5 equiv.) in
DCE–THF (10 : 1).
Fig. 2 (a) SEM Image of a dried L-TA complex gel (1.0 wt%)
(prepared in Scheme 1) and (b) Its magnified image.
CN-TFMBPPE–3,5-bistrifluoromethyl benzoic acid complex
is attributed to the presence of CF3 units at the four peripheral
positions. The present results thus support the hypothesis that
the self-assembly of these entities relies on cooperative inter-
molecular interactions induced by four peripheral CF3 units in
addition to the strong p–p interaction of the planarized
aromatic segment.
aggregation (vide infra). In the wide angle, the peak at 4.6 A is
regarded to represent the intercomplex distance.
The AIEE effect has been shown to originate from the
combined planarization and J-type aggregation of CN-TFMBE
molecules during supramolecular assembly.4a Absorption and
photoluminescence excitation (PLE) spectra of a CN-TFMBPPE
(2 equiv., 0.25 wt%)–L-TA (1 equiv.) complex gel as well as the
dried gel are shown in Fig. 3a. It is clearly shown that the
absorption spectra of the gel consists of two main bands centered
at 342 nm and 431 nm (see the deconvolution result in the inset),
which are attributed to the monomer and J-aggregate absorption
bands, respectively. It must be noted that the 431 nm J-aggregate
band is observed in the spectrum of the complex gel and becomes
more pronounced in the dried gel, whereas the 342 nm monomer
band exactly matches the absorption band of the monomer
(CN-TFMBPPE, see ESIw). In Fig. 3a, the PLE spectrum of
CN-TFMBPPE–L-TA complex is shown to be comprised of a
J-aggregation band with only a weak monomer band, indicating
that the strong AIEE fluorescence in the gel state is caused mainly
by J-type aggregation. In the dried gel state, monomer band
made practically no contribution to the PLE spectrum, indicating
the importance of J-stacking to the enhanced fluorescence in the
gel state.8 Before gel formation, the CN-TFMBPPE solution was
very weakly fluorescent with an emission maximum of 430 nm.
Upon gelation, however, strong green emission at 514 nm with an
intensity of 240 times that of the solution prior to gelation,
appeared immediately, as shown in Fig. 3b. This strong and
red-shifted AIEE fluorescence in the gel state is attributed to the
H-bonding mediated transformation from the twisted conforma-
tion in the sol state (which leads to non-radiative decay) to a CN-
TFMBPPE–L-TA complex assembly with a planar conformation
and J-type aggregation.4,8 As a result of this AIEE effect, a very
high solid-state fluorescence quantum yield of 57% (ꢂ3%) was
obtained from a dried gel of the CN-TFMBPPE–L-TA complex.
To investigate the possible chirality transcription in the self-
assembling AIEE system, we recorded circular dichroism (CD)
spectra of the CN-TFMBPPE complexes with L-TA or D-TA.
(see Fig. 3c). In the region of the J-aggregate band, the CD
spectra exhibited strong signals with opposite signs (positive for
L-TA complex gel, and negative for D-TA complex gel), which is
presumably due to the formation of chiral aggregates when the
achiral molecule CN-TFMBPPE interacts with the chiral TA.3f
In summary, a highly fluorescent chiral organogel with trans-
parency was prepared via the H-bonding mediated supramole-
cular assembly of non-fluorescent and achiral CN-TFMBPPE
and chiral L-TA (or D-TA). Due to the AIEE effect, the supra-
molecular assembly process could be monitored in the real time
by fluorescence turn on.
To improve the gel stability as well as to induce chiral
organization in this system, L-TA was employed as a chiral H
bond donor to CN-TFMBPPE. Surprisingly, a transparent and
highly fluorescent organogel promptly formed when
2 equiv. of CN-TFMBPPE (1 wt%, in 500 ml of DCE) was
mixed with 1 equiv. of L-TA (in 50 ml of tetrahydrofuran), as
shown in Scheme 1. The formation of this gelation was accom-
panied by the characteristic appearance of a bright yellow color
with AIEE fluorescence turn-on. As for the CN-TFMBPPE–3,5-
bistrifluoromethyl benzoic acid interaction, broad peaks indica-
tive of H-bonding between the pyridine of CN-TFMBPPE and
the carboxylic acid group of L-TA were observed at ca. 2500
cmꢀ1 and 1920 cmꢀ1 in the FT-IR spectrum (see ESIw).1g,3e,5 In
addition, the presence of strong peaks at 3430 cmꢀ1 and 1730
cmꢀ1 indicative of H-bonded OH and CQO stretching, respec-
tively, suggested that intermolecular H-bonding6 between adja-
cent molecules of the CN-TFMBE–L-TA trimer complex was
additionally contributing to the supramolecular self-assembly.
This was indirectly supported by the observation that gelation
did not occur when succinic acid which lacks an OH unit, was
used instead of L-TA. Furthermore, the intercomplex H-bonding
between the neighbouring TA’s is so specific as to bring about
supramolecular self-assembly and resultant gel formation purely
between homo chiral TA units. Neither the gelation nor the
remarkable fluorescence enhancement happened in the case of
racemic TA mixtures. The CN-TFMBPPE–L-TA complex gel
showed a sponge-like morphology (see Fig. 2a) most likely as a
result of the release of entrapped solvent during drying.7 The
different morphology of L-TA–CN-TFMBPPE complex com-
pared to the CN-TFMBPPE–3,5-bistrifluoromethyl benzoic acid
gel can be attributed to the additional H-bonding feature of the
former system. Closer inspection of the L-TA–CN-TFMBPPE
comlex gel reveals that the sponge-like film consists of stacked
nano-fibrillar aggregates with rather obscure boundaries (see
Fig. 2b), giving only a single broad peak in the small angle region
of XRD (see ESIw). The excellent optical transparency of the gel
must arise from this specific morphological feature. Considering
that the molecular length of the trimer complex is considered to
be B50 A, the peak at 16.8 A in the XRD pattern is most likely
to originate from J-type stacking by inclination of trimer com-
plex, of whose optical property (J-aggregate absorption and
enhanced fluorescence) is consistent with that typical of J-type
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diffraction patterns were carried out on Bruker GADDS instrument.
The QY of the solid film was measured using IESP-150B (Sumitomo
Heavy Industries Advanced Machinery Co. Ltd.) including a 15 cm
integrating sphere equipped with a Xe-lamp (CW500W) and CCD
attached to a monochromator. The details of the analytical procedure
used to obtain the QY for the solid films are described elsewhere.9
Synthesis of 2-(30,50-bis-trifluoromethyl-biphenyl-4-yl)-3-(4-pyridin-
4-yl-phenyl)-acrylonitrile (CN-TFMBPPE): To the mixture of 4-(pyr-
idin-4-yl)benzaldehyde (0.30 g, 1.64 mmol) and 2-(30,50-bis(trifluoro-
methyl)biphenyl-4-yl)acetonitrile (0.54 g, 1.64 mmol) in tert-butyl
alcohol (30 mL) and THF (2 drops), tetrabutylammonium hydroxide
(TBAH) (1M solution in methanol) (0.16 mL) was slowly dropped.
After stirring at 50 1C for 2 h, the reaction mixture was poured into
water. The precipitate was collected by filtration and was dried. The
crude product was purified by column chromatography (dichloro-
methane) and precipitated in methanol to afford a pale-yellow solid
(0.50 g, yield = 62%); mp = 280 1C; 1H NMR (300 MHz, CDCl3)
d[ppm]: 8.72 (d, 2H, pyridine-H), 8.06 (m, 4H, Ar–H), 7.90 (s, 1H,
Ar–H), 7.86 (d, 2H, Ar–H), 7.78 (d, 2H, Ar–H), 7.72 (d, 2H, Ar–H),
7.67 (s, 1H, vinyl), 7.56 (d, 2H, Ar–H); MS (FAB+) (m/z): calcd for
C28H16F6N2: 494, found: 495; anal. calcd for C28H16F6N2: C,
68.02; H, 3.26; N, 5.67, found: C, 67.89; H, 3.00; N, 5.46.
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Fig. 3 (a) UV-vis. Absorption (solid) and PLE (dash–dot) spectra of
an L-TA complex gel (0.25 wt%) and the corresponding dried gel. PLE
was probed at 530 nm. Inset displays two Gaussian-fit peaks for
absorption spectrum of gel. (b) PL spectra of CN-TFMBPPE
(0.25 wt%, 1 equiv.) in DCE before (black, sol) and after (red, gel)
adding L-TA (0.5 equiv.) in THF. Inset shows a fluorescence photo image
of the gel. (c) CD spectra of L-TA and D-TA complex gels (0.25 wt%).
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This work was supported by the Korea Science and
Engineering Foundation (KOSEF) through the National
Research Lab. Program funded by the Ministry of Science
and Technology (No. 2006-03246).
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Notes and references
z General experimentals: UV-vis absorption, circular dichroism (CD)
and fluorescence spectra were recorded on a HP 8452-A, Jasco J-715
and Shimadzu RF-500 spectrofluorophotometer, respectively. FE-
SEM images were acquired on a JSM-6330F (JEOL). FT-IR spectra
were measured on MIDAC corp. FT-IR PRS spectrometer. X-Ray
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This journal is The Royal Society of Chemistry 2008
2796 | Chem. Commun., 2008, 2794–2796