Strong fluorescence emission induced by supramolecular assembly and gelation: Luminescent organogel from nonemissive oxadiazole-based benzene-1,3,5-tricarboxamide gelator

Article abstract of DOI:10.1039/b311648d

Supramolecular aggregation of a novel nonfluorescent gelator yields highly luminescent organogels in aprotic organic solvents through intermolecular hydrogen bonding, which is a key motif for both self-assembly and photophysical process control.

Full text of DOI:10.1039/b311648d

Strong fluorescence emission induced by supramolecular assembly and
gelation: luminescent organogel from nonemissive oxadiazole-based
benzene-1,3,5-tricarboxamide gelator†
Seung Yeul Ryu,a Sehoon Kim,a Jangwon Seo,a Young-Woon Kim,a Oh-Hoon Kwon,b Du-Jeon Jangb
and Soo Young Park*a
a School of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea.
E-mail: parksy@plaza.snu.ac.kr; Fax: +82 2 886 8331; Tel: +82 2 880 8327
b School of Chemistry, Seoul National University, Seoul National University, Seoul 151-747, Korea
Received (in Cambridge, UK) 23rd September 2003, Accepted 24th October 2003
First published as an Advance Article on the web 11th November 2003
Supramolecular aggregation of a novel nonfluorescent gelator
yields highly luminescent organogels in aprotic organic solvents
through intermolecular hydrogen bonding, which is a key motif
for both self-assembly and photophysical process control.
completely dissolved TOBA gels into clear solutions, suggesting
that the intermolecular H bonding between amide groups is a key
motif for the gel formation.
The X-ray diffractogram (XRD) of a TOBA xerogel obtained by
drying a 0.1 wt% chloroform gel showed a distinct small-angle
peak and a wide-angle broad halo.† Only one small-angle peak at
2q ≈ 2.2° suggests that the gel contains one-dimensional columnar
structures with a diameter of ~ 4.0 nm. Moreover, the absence of
higher order diffractions in the small-angle region means that this
one-dimensional structure does not possess intercolumnar struc-
tural coherence. On considering the simulated size of the fully
extended planar geometry of TOBA ( ~ 6 nm, MM+, HyperChem
5.0), it can be mentioned that disk-shaped TOBA molecules stack
in the face-to-face manner through axial H bonding, similar to the
stacking of other benzene-1,3,5-tricarboxamides.6 The difference
between column diameter and calculated molecular size is most
probably due to the presence of long flexible dodecyl groups and
the possible geometry change of the core scaffold by axial H
bonding of amide groups.6b It is also noted from the wide-angle
halo that rigid order is not promoted between TOBA molecules
within a column and the average interdisk distance is 3.5–4.4 Å
which is compatible with the values for H-bonded benzene-
1,3,5-tricarboxamide strands.6b,c Actually, the xerogel sample
showed no birefringence between the crossed polarizers. A
scanning electron microscopy (SEM) image (Fig. 1a) shows the
morphology of the TOBA xerogel that consists of intertwining
bundles of fibrils, which further intertwine to form a large rope
elongated over hundreds of micrometers. A transmission electron
microscopy (TEM) image of a 0.005 wt% sample in chloroform
(containing a small part of fibrillar precipitate) offers a closer look
at the self-assembled structures of TOBA. As shown in Fig. 1b,
highly elongated fibrillar aggregates with various diameters are
occasionally fused and split to form a network with junction zones.
The smallest aggregate with a diameter of ~ 5 nm can be assigned
as a single strand fibril, based on good accordance with the column
diameter and the absence of intercolumnar structural coherence
from the XRD result.
Organogels are a new class of materials in supramolecular
chemistry, composed of a self-assembled superstructure of low
molecular weight organogelators through specific interactions and
a large volume of organic liquid immobilized therein.1 In addition
to the academic importance, optical and electronic applications of
organogels are being actively investigated particularly with p-
electron conjugated organogelators2,3 or anisotropic gel medium
like liquid crystals.4 As an attempt to obtain a new functional
organogelator with potental photonics applications, we have
designed and synthesized a benzene-1,3,5-tricarboxamide deriva-
tive (TOBA) with three 2,5-diphenyl[1,3,4]oxadiazole arms. Inter-
estingly, TOBA showed an unusual optical property related to the
supramolecular assembly and gelation: the monomer state is totally
nonfluorescent, whereas hydrogen (H)-bonded ones are highly
luminescent. This phenomenon is a novel supramolecular version
of the aggregation-induced enhanced emission that is of growing
interest in organic nanoscience.5 Here we report the supramolecular
assembly and gelation of TOBA in some organic solvents
accompanied by a striking fluorescence change.
To study optical properties arising from the partly conjugated
oxadiazole arms, the gel-state absorption spectrum of a xerogel cast
sample was measured and compared with that of the monomer state
in a H-bond-disturbing protic cosolvent (20 mM in CHCl3/methanol
= 9/1 by volume).† Regardless of the thickness, xerogel samples
gave a markedly broadened band relative to that of the monomer
state, with almost the same peak position at 324 nm (Fig. 1c). This
absorption broadening was also observed in a homogeneous
chloroform solution (20 mM),† suggesting that TOBA molecules
readily self-assemble, even in extremely dilute aprotic conditions,
to form a H-bonded aggregate.3 Sources for spectral broadening are
quite complex in the supramolecular system, possibly including
intra- and interfibrillar chromophore interactions, spectral shifts in
H-bonded complexes, and a geometrical distribution of monomer
units within each self-assembled fibril.
TOBA† is insoluble in nonpolar hydrocarbon solvents, alcohols
and also in poor solvents for macromolecules such as diethyl ether,
ethyl acetate, acetone, and acetonitrile. However, it showed robust
gelating capabilities for THF, 1,4-dioxane, and most chlorinated
solvents including chloroform, dichloroethane, and o-dichlor-
obenzene. In chloroform, for example, TOBA is still insoluble at
room temp. but turns into a clear solution by heating vigorously to
the boiling point in a capped vial. Upon cooling to room temp.,
immobile gels are readily formed within several minutes at TOBA
contents as low as 0.05 wt%. Addition of a small drop of methanol
† Electronic Supplementary Information (ESI) available: Synthetic and
experimental details, X-ray diffractograms, H-bonded aggregate-state
absorption and emission spectra, and original data for Fig. 1c and 2. See
http://www.rsc.org/suppdata/cc/b3/b311648d/
More profound changes by self-assembly of TOBA were found
in the photoluminescence (PL) spectra (Fig. 1c). When excited at
330 nm, the monomer state was almost nonfluorescent (the middle
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T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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sample in Fig. 1d), while the chloroform gel of the same
concentration emitted 100 times stronger fluorescence (on the right
side of Fig. 1d). To the best of our knowledge, this PL enhancement
of TOBA is unique and also the amplitude of modulation is the
largest among the reported organogels showing PL changes usually
arising from excimer formation.2e,f,3 Judging from the similar blue
luminescence observed from the H-bonded aggregate sample (on
the left side of Fig. 1d), it is concluded that the luminescence of
TOBA is turned on by supramolecular aggregation into H-bonded
fibrils.†
To investigate the origin of the PL enhancement by H-bonded
supramolecular aggregation, TOBA samples in different states
were examined by a picosecond time-resolved fluorescence study.†
As shown in Fig. 2,† a monomer sample (20 mM in CHCl3/
methanol = 9/1 by volume) without H bonds gave a very short-lived
component of 90 ps,‡ indicating that its lowest-excited singlet state
(S1) undergoes a facile nonradiative relaxation process such as
intersystem crossing (ISC) or twisted intramolecular charge
transfer, both of which are often observed in benzanilide deriva-
tives.7 In contrast, a H-bonded aggregate sample (20 mM in CHCl3)
gave mainly a long-lived component of 1.1 ns,‡ inferring that such
a nonradiative process is suppressed to elongate the lifetime of the
S1 state. At this stage, the most plausible mechanism of non-
radiative relaxation in the monomer state is suggested to be ISC on
the basis of the observation of very long-lived transients (in
microseconds) instead of any Stokes-shifted fluorescing ones. We
also found from experiments and semiempirical quantum chemical
calculations that the H bonding between TOBA molecules plays an
important role in aggregation-induced luminescence generation by
providing significant singlet–triplet splitting to reduce the rate of
ISC. This is a totally different mechanism from those of other
reported aggregation-induced emitting systems.5 The semiempir-
ical quantum chemical calculation and the full results studied by
time-resolved fluorescence and transient-absorption spectroscopy
will be reported elsewhere in detail.
In conclusion, a novel chromophoric but nonfluorescent ben-
zene-1,3,5-tricarboxamide (TOBA) has been proven to be a
powerful organogelator for some aprotic organic solvents. It has
been demonstrated that the face-to-face intermolecular H bonding
is a key motif for inducing strong fluorescence as well as
supramolecular aggregation. It was inferred that the nonradiative
relaxation process is inactivated in the H-bonded supramolecular
assembly and gel states leading to the enhanced fluorescence
emission.
This work was supported in part by CRM-KOSEF. DJJ thanks
the Strategic National R&D Program.
Notes and references
‡ Actually, the fluorescence decay of the monomer state has an additional
long-lived component, speculated to come from a small contribution of a H-
bonded state. In turn, the short-lived component (90 ps), presumably arising
from a monomer state, also contributes to the fluorescence decay of the H-
bonded aggregate state.
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Fig. 1 SEM (a) and TEM (b) images of TOBA xerogel formed from 0.1 wt%
chloroform gel (a) and precipitate sample from 0.005 wt% chloroform
solution (b). (c) Absorption and PL spectra of TOBA : xerogel film (solid
absorption), 20 mM solution in CHCl3/methanol = 9/1 (dotted absorption),
0.1 wt% gel in CHCl3 (solid PL), and 0.1 wt% solution in CHCl3/methanol
= 9/1 (dotted PL). (d) Fluorescence image of TOBA taken under 365 nm
illumination: (from left) 20 mM solution in CHCl3, 0.1 wt% solution in
CHCl3/methanol = 9/1, and 0.1 wt% gel in CHCl3.
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Fig. 2 Fluorescence kinetic profiles for monomer (2) and H-bonded
aggregate (8) states of TOBA. Samples were excited at 386 nm and
monitored above 420 nm. Solid lines are convoluted fits for the profiles.
C h e m . C o m m u n . , 2 0 0 4 , 7 0 – 7 1
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