Fluorescence enhancement upon gelation and thermally-driven fluorescence switches based on tetraphenylsilole-based organic gelators

Article abstract of DOI:10.1016/j.cplett.2009.05.029

Two new organic gelators 1 and 2 based on the silole (silacyclopentadiene) framework were designed with the end to develop switchable fluorescent organogels, by making use of the aggregation-induced emission (AIE) feature of silole derivatives. As for oth

Full text of DOI:10.1016/j.cplett.2009.05.029

Chemical Physics Letters 475 (2009) 64–67
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Chemical Physics Letters
journal homepage: www.elsevier.com/locate/cplett
Fluorescence enhancement upon gelation and thermally-driven fluorescence
switches based on tetraphenylsilole-based organic gelators
a
a
Ming Wang a,b, Deqing Zhang a, , Guanxin Zhang , Daoben Zhu
*
a Beijing National Laboratory for Molecular Sciences, Organic Solids Laboratory, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
b Graduate School of Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two new organic gelators 1 and 2 based on the silole (silacyclopentadiene) framework were designed
with the end to develop switchable fluorescent organogels, by making use of the aggregation-induced
emission (AIE) feature of silole derivatives. As for other silole derivatives, compounds 1 and 2 exhibited
AIE behavior as indicated by the significant fluorescence enhancement by introducing water to the THF
solutions. Compounds 1 and 2 can gel hexane, methylcyclohexane and heptane. Large fluorescence
enhancement was observed for compounds 1 and 2 after gelation. Moreover, their fluorescence intensi-
ties can be changed reversibly accompanying the gel–solution transition through alternating cooling and
heating. Therefore, thermally-driven fluorescence switches can be achieved with organogels based on 1
and 2.
Received 18 March 2009
In final form 13 May 2009
Available online 18 May 2009
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
observed for gelation with a gelator containing anthracene and
urea moieties as reported by us recently [26].
A number of organic gelators with various structures have been
described recently [1–5] and various derivatives of urea [6–10],
cholesterol [11,12], and amine acid [13] have been found to be
efficient gelators. Among them chromophore-based gelators have
received more attentions recently, since interesting spectral varia-
tion is observed in accompanying the corresponding gel–solution
phase transition for these gelators. For instance, Ajayaghosh et al.
reported the absorption spectral shifts and the tuning of the
excitation energy transfer for oligo(p-phenylenevinylene) derived
gelators after gelation [14,15] and gels with turnable emission
[16–18]. By making use of the viscosity difference between gel
and solution phases, we and others successfully achieved the
thermal modulation of the pyrene monomer/excimer fluorescence
and supramolecular exciplex formation after gelation [8,19]. In a
few cases, gelation-induced-fluorescence-enhancement was ob-
served. Such fluorescent organogels are especially interesting since
organogels are thermally responsive and accordingly thermal fluo-
rescence modulation can be realized. As a result, these fluorescent
organogels may find potential applications in areas such as optical
devices and sensors [20–26]. Park and co-workers reported the
fluorescence increase after gelation with 1-cyano-trans-1,2-bis-
(3050-bis-trifluoromethyl-biphenyl)ethylene(CN-TFMBE) [20,21].
Similar phenomena were described for gelators with oxadiazole
[22], benzoxazole [23], naphthalene [24] and salicylideneaniline
In this report, we report new examples of gelation-induced-
fluorescence-enhancement with silole (silacyclopentadiene)-based
gelators 1 and 2 (Scheme 1). The molecular design for the gelators
is based on the Aggregation-Induced Emission (AIE) feature of
silole [27,28], which was reported by Tang, Zhu and their cowork-
ers. In fact, silole derivatives with AIE feature have attracted much
more efforts and even interesting chemo-/biosensors have been
developed based on silole molecules [29–31]. Besides, urea and
cholesterol moieties are connected to the silole core in gelators 1
and 2, respectively, to enhance the intermolecular interactions
and thus facilitate the molecular assembly and gel formation as
reported early [6–12]. The results show that (1) compounds 1
and 2 exhibit AIE behaviors; (2) compounds 1 and 2 can gel a
few organic solvents; (3) gelation with compounds 1 and 2 is
accompanied with fluorescence enhancement and as a result
thermally-driven fluorescence switch is achieved.
2. Result and discussion
As shown in Fig. 1, both compounds 1 and 2 exhibited rather
weak fluorescence in THF solution (1.0 ꢀ 10ꢁ5 M). But, their fluo-
rescence intensities were significantly enhanced after addition of
water. For instance, the emission intensity at 459 nm of compound
1 (1.0 ꢀ 10ꢁ5 M in THF) increased by nearly 66 times when water
was added to the THF solution and the water content reached
95% (see Fig. 1 A); moreover, the emission maximum was shifted
from 459 nm to 465 nm. Similarly, for compound 2 (1.0 ꢀ 10ꢁ5 M
in THF) the fluorescence intensity at 480 nm was enhanced by
[25] moieties.
A significant fluorescence enhancement was
* Corresponding author.
E-mail address: dqzhang@iccas.ac.cn (D. Zhang).
0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2009.05.029

M. Wang et al. / Chemical Physics Letters 475 (2009) 64–67
65
Ph
Ph
Ph
Ph
Si
H
H
N
a
N
Ph
Ph
Ph
Ph
O
1
Si
NH2
b
Ph
Ph
Ph
3
Ph
Si
H
O
N
O
2
Scheme 1. Chemical structure of silole-based gelators and synthetic approach. (a) C12H25NCO, CH2Cl2, 40%. (b) Cholesteryl chloroformate/TEA, CH2Cl2, 46%.
190 times and the emission maximum was red-shifted by 3 nm
when the volume ratio of THF vs. water reached 5:95 (see Fig. 1
B). As for other silole derivatives [27,28], the fluorescence quantum
yields of compounds 1 and 2 in THF were rather low, being
5.85 ꢀ 10ꢁ4 and 2.3 ꢀ 10ꢁ3, respectively, by reference to 9, 10-
diphenylanthrance. After aggregation induced by addition of water
to the THF solutions of 1 and 2, their fluorescence quantum yields
increased. For instance, the fluorescence quantum yields of 1 and 2
were measured to be 0.011 and 0.023, respectively, in the mixed
solvent of THF/H2O (v/v, 5/95). The fluorescence life-times of 1
and 2 were measured in THF and the mixed solvent of THF/H2O.
For the THF solution of compound 1 (1.0 ꢀ 10ꢁ5 M), the fluores-
cence life-time was estimated to be 161 20 ps. But, when water
was added to the THF solution (THF/H2O, v/v, 5/95), the fluores-
cence exhibited a double-exponential decay, corresponding to
s
1 = 2.1 0.1 ns and s2 = 5.8 0.07 ns (A1/A2 = 24:76). For the THF
solution of 2 (1.0 ꢀ 10ꢁ5 M), a double-exponential fluorescence
decay was observed, corresponding s1 = 94 14 ps and s2 = 3.0
0.2 ns (A1/A2 = 94:6). After addition of water to the THF solution
(THF/H2O, v/v, 5/95), the fluorescence life times were prolonged
to be 2.9 0.2 ns (s1) and 6.7 0.1 ns (s2) (A1/A2 = 21:79), respec-
tively. The increase of the fluorescence life-times and fluorescence
enhancement after addition of water were due to the aggregation
of molecules of 1 and 2 as reported early [28,32].
After introducing water to the THF solutions of 1 and 2, red-
shifts were detected for their absorption spectra of 1 and 2 (see
Fig. S1 in ESI). These results indicate that the enhanced-emission
observed for compounds 1 and 2 can be ascribed to the formation
of J-type aggregation of 1 and 2 after addition of water according to
previous studies [28].
A
c
1200
800
400
0
The gelation experiments were performed in a number of sol-
vents for compounds 1 and 2. The results indicated that the gela-
tion abilities of compounds
1 and 2 were generally weak.
Compound 1 can gel in hexane and methylcyclohexane. When
the corresponding hot solutions of 1 in hexane (23 mg/mL) and
in methylcyclohexane (25 mg/mL) were cooled down, opaque
white gels were formed respectively (as shown in Fig. 2). As antic-
ipated, a network of long fibers with widths about 100 nm was
formed (see Fig. 3A for the SEM image of xerogel of 1). Molecules
of 1 in the gel phase were orderly arranged as indicated by XRD
a
b
375
450
525
λ (nm)
600
675
B
2000
1500
1000
500
0
c
a
b
375
450
525
λ (nm)
600
675
750
Fig. 1. The emission spectra of compounds 1 (10
THF–water mixture with different volume percentages of water: (a) 0% H2O; (b) 50%
H2O; (c) 95% H2O.
l
M) (A) and 2 (10
l
M) (B) in the
Fig. 2. Illustration of the gel formation with compounds 1 (23 mg/mL in hexane)
and 2 (150 mg/mL in n-heptane), and the photos of the formed gels under the UV
light (365 nm) illumination.

66
M. Wang et al. / Chemical Physics Letters 475 (2009) 64–67
Fig. 3. SEM image of xerogel formed with 1 in hexane and that with 2 in n-heptane.
3.14° and 6.99° were detected in the XRD spectrum of the xerogel,
corresponding to d-spacing of 4.22 nm, 2.12 nm and 1.4 nm,
respectively. They may correspond to the intermolecular separa-
tions in different directions. Interestingly, they are exactly in the
ratio of 1:1/2:1/3, which implies a lamellar organization for mole-
cules of 2 within the aggregates of the xerogel according to previ-
ous studies [25,33].
a) Sol
b) Gel
b
Gel
Gel
2000
1000
0
2000
1000
0
The 1H NMR spectral results of 1 measured at different temper-
atures and concentrations (see Fig. S3 in ESI) indicated that the
intermolecular H-bonds were formed between the urea moieties
of neighboring molecules of 1 and these intermolecular H-bonds
may be responsible for the gel formation of 1 [7,10]. The fact that
gelation cannot occur for the methylcyclohexane solution of 1 con-
taining tetrabutylammonium fluoride provides support for the
above assumption (see Fig. S4 in ESI). It is probable that intermo-
lecular H-bonds between the urea moieties would be disrupted
Sol
Sol
a
375
450
525
600
675
in the presence of fluoride anion [34]. But, the corresponding
stacking and van der Waals interactions may also contribute to
the gel formation with 1.
p–
λ (nm)
p
Fig. 4. The fluorescence spectra of compound 1 in n-hexane (23 mg/mL) before and
after gel formation; inset shows the reversible variation of the emission intensity at
453 nm accompanying the solution–gel phase transition.
As anticipated, the solution of compound 1 exhibited rather
weak fluorescence (see Fig. 4). However, its fluorescence intensity
increased largely after gel formation. For instance, the fluorescence
intensity of 1 (23 mg/mL in hexane) at 470 nm was enhanced by 50
times and simultaneously the emission maximum was blue-
shifted by 17 nm after gelation as shown in Fig. 4 [35]. Indeed,
the fluorescence intensity enhancement after gelation can be easily
distinguished by naked-eye (see photos under UV light illumina-
tion in Fig. 2). The xerogel formed with compound 1 exhibited
strong emission as indicated by the fluorescence confocal micro-
scopic image (see Fig. 5A). Such fluorescence intensity enhance-
ment was likely due to the formation of self-assembly aggregates
(see SEM images in Fig. 3) of 1 during the gelation process accord-
ing to previous investigations [27,28]. Interestingly, the fluores-
cence intensity was reduced again after the transformation of
the gel into the solution by heating. In this way, the fluorescence
analysis. Three diffraction signals at 3.46°, 5.57° and 7.10° were
observed as shown in Fig. S2A, corresponding to d-spacing of
2.55 nm, 1.57 nm and 1.25 nm, respectively. The molecular length
of 1 was estimated to be 2.68 nm. Thus, the first diffraction peak
may relate to the molecular length of 1, and the other two diffrac-
tion signals may correspond to the intermolecular separations in
different directions. Among the solvents tested, compound 2 can
only lead to the gelation of n-heptane. As indicated in Fig. 2, a
pale-yellow gel was formed by cooling the hot solution of 2
(150 mg/mL) in n-heptane. A worm-like structure was observed
in the SEM image of xerogel of 2 as shown in Fig. 3B. As shown
in Fig. S2B, one sharp and two broad diffraction signals at 2.05°,
Fig. 5. Fluorescence confocal microscopic images of the xerogels based on 1 and 2.

M. Wang et al. / Chemical Physics Letters 475 (2009) 64–67
67
Acknowledgments
Gel
4000
2000
0
4000
2000
b
a) Sol
b) Gel
Gel
The present research was financially supported by NSFC, the
State Basic Research Program and Chinese Academy of Sciences.
This work was also partially supported by the NSFC-DFG joint pro-
ject (TRR61).
Sol
Sol
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.cplett.2009.05.029.
a
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solution transition through alternating cooling and heating (see in-
set of Fig. 3). Similar emission variation was observed for the
ensemble of 1 in methylcyclohexane. Accordingly, a thermally-dri-
ven fluorescence switch can be established with organic gels based
on compound 1.
The gelation-induced-fluorescence-enhancement was also ob-
served for the ensemble of 2 in heptane as shown in Fig. 6. The
fluorescence intensity at 480 nm increased by more than six times
after gel formation, and simultaneously the emission maximum
was shifted from 480 nm to 483 nm after gelation. Also, the photo
of the gel under UV light illumination (see Fig. 2) and fluorescence
confocal microscopic measurement (see Fig. 5B) indicated the fluo-
rescence enhancement after gelation with compound 2. Moreover,
the fluorescence intensity of 2 can be reversibly tuned by alternat-
ing heating and cooling as shown in the inset of Fig. 4.
In summary, we describe new examples of gelation-induced
fluorescence enhancement with gelators 1 and 2 by making use
of the aggregation-induced emission (AIE) feature of silole deriva-
tives. As for other silole derivatives, compounds 1 and 2 exhibit AIE
behavior as indicated by the significant fluorescence enhancement
by introducing water to the THF solutions. Although the gelation
abilities of 1 and 2 are weak, compounds 1 and 2 do gel a few
solvents including hexane, methylcyclohexane and heptane. Of
particular interest is the observation of large fluorescence
enhancement for compounds 1 and 2 after gelation. Moreover,
their fluorescence intensities can be changed reversibly accompa-
nying the gel–solution transition through alternating cooling and
heating. Therefore, thermally-driven fluorescence switches can be
achieved with organogels based on 1 and 2. These switchable fluo-
rescent organogels may found potential applications in informa-
tion storage and optical devices.
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