A novel pyrene-containing fluorescent organogel derived from a quinoline-based fluorescent porbe: Synthesis, sensing properties, and its aggregation behavior

Article abstract of DOI:10.1039/c4ra02373k

A new family of pyrene-containing compounds 2-4 derived from aminoquinoline-containing fluorescent probe 1 were successfully synthesized and well characterized. The investigation of the absorption and emission spectra of these compounds revealed that the photophysical properties were significantly affected by the substitution of pyrene. Moreover, these compounds exhibited selective fluorescence behaviour towards Zn²⁺ in aqueous solution. The gelation properties of these compounds were investigated by the "stable to inversion of a test tube" method. Interestingly, 1,8-bis-substituted compound 4 displayed stable gel-formation properties in acetone, dioxane, tetrahydrofuran, ethyl acetate, chloroform, and dichloromethane. The morphologies of the xerogels were investigated by scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). The concentration- and temperature-dependent emission properties, and concentration- and temperature-variable ¹H NMR spectroscopy of compound 4 were investigated, which suggested that both π-π stacking interaction and hydrogen bonding were the driving forces for the process of self-aggregation and the gel formation. In addition, studies on the luminescence properties indicated that 4 had the ability to form a fluorescent organogel with interesting fluorescence behaviour. More importantly, it was found that compound 4 could form a stimuli-responsive gel that had a sensitive gel-to-sol transition response to heating or adding Zn²⁺. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02373k

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PAPER
A novel pyrene-containing fluorescent organogel
derived from a quinoline-based fluorescent porbe:
synthesis, sensing properties, and its aggregation
behavior†
Cite this: RSC Adv., 2014, 4, 19538
a
a
b
a
*
Chang-Bo Huang, Li-Jun Chen, Junhai Huang and Lin Xu
A new family of pyrene-containing compounds 2–4 derived from aminoquinoline-containing fluorescent
probe 1 were successfully synthesized and well characterized. The investigation of the absorption and
emission spectra of these compounds revealed that the photophysical properties were significantly
affected by the substitution of pyrene. Moreover, these compounds exhibited selective fluorescence
behaviour towards Zn2+ in aqueous solution. The gelation properties of these compounds were
investigated by the “stable to inversion of a test tube” method. Interestingly, 1,8-bis-substituted
compound 4 displayed stable gel-formation properties in acetone, dioxane, tetrahydrofuran, ethyl
acetate, chloroform, and dichloromethane. The morphologies of the xerogels were investigated by
scanning electron microscopy (SEM) and laser scanning confocal microscopy (LSCM). The
concentration- and temperature-dependent emission properties, and concentration- and temperature-
variable 1H NMR spectroscopy of compound 4 were investigated, which suggested that both p–p
stacking interaction and hydrogen bonding were the driving forces for the process of self-aggregation
and the gel formation. In addition, studies on the luminescence properties indicated that 4 had the ability
to form a fluorescent organogel with interesting fluorescence behaviour. More importantly, it was found
that compound 4 could form a stimuli-responsive gel that had a sensitive gel-to-sol transition response
Received 20th March 2014
Accepted 15th April 2014
DOI: 10.1039/c4ra02373k
www.rsc.org/advances
to heating or adding Zn2+
.
p–p stacking, hydrogen-bonding, electrostatic interactions,
hydrophobic interactions, and van der Waals interactions.
Among various organogels, uorescent organogels con-
structed from p-conjugated molecules have drawn signicant
interest due to their distinctive advantages such as diversity,
exibility, and promising applications in optoelectronics, eras-
able thermal imaging, and sensing of explosives like trinitro-
toluene.6 Moreover, a remarkable variation in the prole of the
uorescence spectrum of a uorescent organogel may accom-
pany a sol-to-gel or gel-to-sol phase transition process, which
may provide the crucial information at a molecular level about
the structure changes during the phase transition process.
Thus, the incorporation of chromophores makes it easy to
predict the gelation phenomenon or explain the structure–
property relationship although the gelator–gelator and gelator–
solvent interactions are intricate. As a result, a number of
uorescent organogels based on various aromatic building
blocks including porphyrins,7 pyrenyl,8 perylene bisimides,9
oligo-(p-phenylenevinylene),10 and naphthalimides11 have been
investigated during the recent past years.
Introduction
Over the past few decades, supramolecular chemistry1 has wit-
nessed tremendously rapid development in the construction of
delicated functional architectures2,3 including smart so
matter.4 Particularly, organogels have attracted a great deal of
attention in the areas that range from supramolecular chem-
istry to materials science because of their unique supramolec-
ular structures and potential applications in template synthesis,
drug delivery, and biomimetic systems.5 Generally, organogels
are formed through the self-assembly of gelator molecules to
the generation of three-dimensional supramolecular network
structures with a lot of void spaces occupied by solvent mole-
cules under the suitable conditions. The driving forces for
gelator molecules to self-assemble into the entangled nano-
structures are generally weak non-covalent interactions such as
aDepartment of Chemistry, East China Normal University, 3663 N. Zhongshan Road,
Shanghai, 200062, P.R. China. E-mail: lxu@chem.ecnu.edu.cn; Fax: +86 21-6223-
5137
bZhangjiang R&D Center, Shanghai Institute of Pharmaceutical Industry, 1320 W.
On the other hand, the stimuli-responsive organogels are of
much interest due to their benets for the development of
sensor devices or applications like drug delivery, controlled
Beijing Road, Shanghai, 200040, P.R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra02373k
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release, and catalysis, etc.12 Stimuli-responsive organogels can 1,6-bis-substituted, and 1,8-bis-substituted branches, were
be rendered sensitive to the external stimulus, such as designed and synthesized. It was found that 2–4 featured the
temperature,13 light irradiation,14 sound,15 pH,16 oxidative/ interesting spectroscopic properties due to the existence of
reductive reactions,17 and anions,18 by incorporating a receptor pyrene and quinoline moieties. All of them exhibited good yet
unit or a spectroscopically active unit as part of gelator mole- different sensing properties to Zn2+. Interestingly, compound 4
cule. For instance, organogels that response to light irradiation presented a good gel formation property in many solvents such
have been obtained by incorporating azobenzene, spiropyran, as dichloromethane, chloroform, acetone, and tetrahydrofuran,
stilbene, etc. into the corresponding organogels.14a,d,17b Simi- etc. More importantly, it was found that heating or adding Zn2+
larly, if the organogels contain acidic or basic groups, the gel– could induce the gel–sol phase transition. This work provides
sol transition can be reversibly switched by appropriately an interesting and successful example of constructing stimuli-
changing the pH value.19 Notably, even though considerable responsive uorescent organogel, which obviously enriches the
efforts have been devoted to the development of stimuli- library of stimuli-responsive uorescent organogels and might
responsive organogels, the related reports are still limited. be helpful in preparing such type of organogels.
This might be caused by the fact that the preparation of
such organogel needs an effective coupling between the recog-
nition unit and the self-organization unit. Furthermore, it has
Results and discussion
Synthesis
been recognized that the receptor unit should be an active
component participating in the self-assembly process. There-
fore, it is still challenging yet desirable to develop stimuli-
responsive organogels, especially stimuli-responsive uores-
cent organogels.
Compound 1 was synthesized according to the literature
procedure.20 The preparation of pyrene-containing compounds
2–4 was outlined in Scheme 2. Iodination of 8-aminoquinoline
afforded 5, followed by the reaction with chloroacetyl
chloride in room temperature to give compound 6 in high yield
(98.2%). Subsequently, a further nucleophile substitution
reaction of 6 with 2-(2-aminoethoxy)ethanol was carried out to
provide the key intermediate 7. In addition, 1-ethynylpyrene 9,
1,6-diethynylpyrene 14, and 1,8-diethynylpyrene 15 were
prepared according to our previous reports.8e It was worth
noting that bromination of pyrene resulted in two isomers 1,6-
dibromopyrene 10 and 1,8-dibromopyrene 11, which were
difficult to be isolated from each other. However, a further
alkynylation of 10 and 11 was performed to prepare a mixture of
12 and 13, which could be easily puried with column chro-
matography to give pure 1,6-diethynyl and 1,8-diethynylpyrene,
respectively. Finally, sonogashira coupling reaction of 7 with 9,
14, or 15 generated the series of pyrene-containing compounds
2–4 in acceptable yields, respectively. The molecular structures
of compounds 2–4 were characterized by 1H NMR, 13C NMR,
and mass spectroscopy, which were in full agreement with the
Herein, in order to construct a stimuli-responsive uores-
cent organogel, a new family of pyrene-containing compounds
2–4 were synthesized in this work. Compounds 2–4 were derived
from the uorescent probe 1, which was reported to exhibit
characteristic changes in its uorescence prole in response to
Zn2+ through the binding of Zn2+ with three nitrogen atoms
(Scheme 1).20 The following principles have been considered in
the design of 2–4. Firstly, the long chain, which contains amide
and NH groups, not only can act as Zn2+ acceptor but also have
the ability to form multiple hydrogen bonds. It's well known
that multiple hydrogen bonds may provide driving forces to
form organogel in the appropriate solvents.21 Secondly, as a p-
conjugated molecule, pyrene and its derivatives have been
widely explored in the eld of luminescent material.22 With the
introduction of ethynyl subunits into the pyrene skeleton, the
conjugated fusion of pyrene moiety with quinoline moiety may
result in the strong intermolecular p–p stacking, which
provides another powerful driving force to form organogel.
Thirdly, in order to investigate the inuence of the substitution
on photophysical properties, sensing properties, and organogel
formation, a family of compounds 2–4 with monosubstituted,
1
desired structures. It is noteworthy that the H NMR spectrum
of 4 displayed two singlets at 7.76 and 8.42 ppm, which were
assigned to the protons on 4-, 5-, and 9-, 10-substituted location
1
of pyrene, respectively (Fig. S3 in ESI†).21b However, in the H
NMR spectrum of 3, none of the singlet could be found in the
aromatic region. It means that the isomers 3 and 4 could be
distinguished from each other base on the singlets.
Investigation of spectral properties of 2–4
The absorption and emission spectra of compounds 2–4 in
acetone (10ꢀ5 M) were presented in Fig. 1. The obtained results
indicated that the absorption properties were signicantly
affected by the substitution of pyrene. For example, two
absorption bands were observed for 2 at lmax of 391 and 415 nm
along with a shoulder peak at 366 nm. Unlike 2, the bisub-
stituted pyrene derivative 3 had two maximum absorption
bands located at 430 and 448 nm, respectively. Moreover, the
onset of the absorption of 3 was very broad, which was from 350
Scheme 1 Molecular structures of compounds 2–4.
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Scheme 2 Synthesis route of target compounds 2–4.
(absorption band usually appearing at 320 nm) and 1 (absorp-
tion band appearing at 305 nm), compounds 2–4, especially 3
and 4, showed pronounced red-shied absorption due to the
extension of p-conjugation. The additional substituted 8-ami-
noquinoline group in 3 and 4, as compared to 2, induced a ꢁ35
nm red shi in the absorption spectrum. Moreover, the broad
and loss of vibronic structure absorption spectra of 2–4 indi-
cated that there existed charge transfer between pyrene and 8-
aminoquinoline.
The UV-Vis absorption of 2–4 in different solvents including
dichloromethane, ethyl acetate, tetrahydrofuran, dime-
thylbenzene, dioxane, toluene, chloroform, n-propanol, tetra-
hydrofuran–water (1 : 1, v/v), acetone–water (1 : 1, v/v), dioxane–
water (1 : 1, v/v), and n-propanol–water (1 : 1, v/v) were studied.
As shown in Fig. S7–S9a,† the shapes of the absorption spectra
of 2–4 showed almost no change in various solvents. Mean-
while, a slight shi in wavelength and a moderate change of
extinction coefficient of 2–4 in different solvents were observed.
The absorption spectra shis may arise from either the physical
intermolecular solute–solvent interactions (e.g. hydrogen-
bonding interaction) or the chemical processes such as charge
transfer or proton transfer, etc.23 Moreover, a relative higher
molar extinction coefficient in the tested solvents indicated the
more effective p / p* and n / p* transition for the absorp-
tion bands in the p-conjugated molecules. Therefore, the
results disclosed that the solvents featured the moderate effect
Fig. 1 UV-Vis (a) and fluorescence emission (b) spectra of 2–4 (10 mM)
in acetone.
to 430 nm. Compound 4, however, which had the same number on UV absorption spectra of 2–4.
of substitution groups as 3 but the different substitution posi-
Compared to the absorption spectra, the emission spectra of
tion, exhibited two main absorption regions with bands at lmax compounds 2–4 exhibited the slighter dependence on the
of 370 and 432 nm, respectively. Furthermore, there were two substitution of pyrene in acetone (Fig. 1b). For instance,
shoulder peaks located at 410 and 450 nm. It was proposed that compounds 3 and 4 showed the similar emission spectral
the intramolecular charge transfer (ICT) effect induced different shapes and exhibited two main emission regions with bands
UV-Vis absorptions of isomers 3 and 4. Compared to free pyrene about at 465 and 490 nm in acetone. The uorescence emission
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spectra for 2–4 were also measured in various solvents for
certifying the inuence of solvent on their emission (Fig. S7–S9b
in ESI†). The results indicated that uorescence emission of
compound 2 was more sensitive to solvents than that of 3 and 4.
For example, in different tested solvents, the emission spectra
of compounds 3 and 4 showed the similar shapes and a slight
wavelength shi. However, compound 2 displayed totally
different emission spectra in non-protonic solvents and
protonic solvents. In the non-protonic solvents such as dioxane,
ethyl acetate, and tetrahydrofuran, compound 2 showed two
main emission band peaks at 436 and 460 nm, which was the
characteristic feature of monomer of pyrene derivatives. In
comparison, however, compound 2 exhibited the excimer peak
at the longer wavelength regions in the protonic solvents such
as n-propanol, tetrahydrofuran–water, acetone–water, dioxane–
water, and n-propanol–water. This observation might be caused
by the fact that the bisubstituted 3 and 4, as compared with
monosubstituted 2, have the larger steric hindrance, thus pre-
venting the formation of excimer in protonic solvents.
Metal ions sensing properties of 2–4
In order to evaluate the selectivity of 2–4, various metal ions
such as Zn2+, Mn2+, Ba2+, Hg2+, Ni2+, Cu2+, Co2+, Pb2+, Mg2+,
Cd2+, Fe2+, Al3+, Ag+, Li+, and Na+ were employed. As shown in
Fig. 2, upon adding various metal ions to the solution of 2, 3,
and 4, respectively, only Zn2+ induced a remarkable red-shi of
emission spectra by comparison with that of free 2–4 in the
solution. For example, with the addition of increasing amounts
of Zn2+ to 2, a slight decrease in uorescent intensity and a 38
nm red-shi from 480 to 518 nm of uorescence emission were
observed. According to the previous report, it is speculated that
the red-shi of emission of 2 upon adding Zn2+ was attributed
to the deprotonation of amide when binding with Zn2+
.
20
Though a signicant uorescence quenching was detected with
the addition of Cu2+, Co2+, Fe2+, and Pb2+ due to their well-
known paramagnetic effect,24 it should be noted that the uo-
rescence emission wavelength did not change. As shown in
Fig. S10–S12 in ESI†, about 2-fold increase in the uorescence
intensity ratio of 2 at 518 and 480 nm (I518nm/I480nm) was
observed following the addition of 4 equiv. Zn2+. Under the
identical conditions, Cu2+ induced 50% increase in the uo-
rescence intensity ratio at 518 to 480 nm (I518nm/I480nm). More
interestingly, a remarkable red-shi of 81 nm was observed for 3
with the addition of 4 equiv. Zn2+, which induced about 41-fold
increase in the uorescence intensity ratio of 3 at 546 and 465
nm (I546nm/I465nm). Similarly, the addition of Zn2+ to the solu-
tion of 4 resulted in a 57 nm red-shi from 466 to 523 nm of
Fig. 2 Fluorescence emission spectra of 2 (a, 10 mM), 3 (b, 10 mM), and
4 (c, 10 mM) in the presence of 40 mM of Zn2+ in acetone–water (1 : 1,
v/v, 10 mM Tris–HCl, pH 7.2).
and the corresponding complex (4 equiv. Zn2+ was added to the
solution of 2, 3, and 4, respectively), however, all of them dis-
played selective uorescence behaviour towards Zn2+ in
aqueous solution.
Gelation properties
uorescence emission and 14-fold increase in the uorescence The gelation properties of compounds 1–4 were investigated by
intensity ratio value at 523 and 466 nm (I523nm/I466nm). However, the “stable to inversion of a test tube” method in various polar
under the identical conditions, Cu2+ induced a 1.8-fold or nonpolar, aromatic or nonaromatic solvents such as n-
increasing of uorescence intensity ratio at 523 to 466 nm, ca. heptane, acetone, dioxane, tetrahydrofuran, ethyl acetate,
12% compared to Zn2+ (Fig. 3). The other ions produced minor toluene, dimethylbenzene, n-propanol, chloroform, dichloro-
changes in the uorescence intensity ratio value. From the view methane, n-propanol, tetrahydrofuran–water (1 : 1, v/v),
of the application of uorescent probe, 2, 3 and 4 did not exhibit acetone–water (1 : 1, v/v), dioxane–water (1 : 1, v/v), and n-
better behavior than that of 1 due to the large overlap of uo- propanol–water (1 : 1, v/v). The results were summarized in
rescence emission spectra between free compound (2, 3, and 4) Table 1. It was surprised to nd that compound 4 exhibited the
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dioxane, n-propanol, chloroform, and dichloromethane, it
precipitated gradually out of these solvents upon subsequently
cooling to the room temperature. The results revealed that the
structural effects, such as the number of substitution groups
and the substitution position have an observable inuence on
the gel formation.
The critical gelation concentrations (CGC) of 4 in acetone,
chloroform, and dichloromethane was 8.24 mg mLꢀ1, indi-
cating that one molecule of 4 could entrap approximately 1377,
1256, and 1562 solvent molecules, respectively. Moreover, the
CGC of 4 in dioxane, tetrahydrofuran, and ethyl acetate was
determined to be 13.73 mg mLꢀ1. These data demonstrated that
4 formed LMOGs more easily in acetone, chloroform, and
dichloromethane than that in dioxane, tetrahydrofuran, and
ethyl acetate.
Fig. 3 Fluorescence intensity ratio of 4 (10 mM) at 523 and 466 nm
(I523nm/I466nm) in the presence of 4 equiv. various metal ions to free 4
(10 mM) at 523 and 466 nm (I523nm/I466nm) in acetone–water (1 : 1, v/v,
10 mM Tris–HCl, pH 7.2).
To investigate the morphologies of the obtained organogels,
scanning electron microscopy (SEM) was performed with the
corresponding xerogels (air-dried gels). Gel samples were slowly
evaporated to dryness, and then the resultant xerogels were
examined (Fig. 5 and 6 and Fig. S13–S17 in ESI†). As shown in
Fig. 5, the morphologies of xerogels were dependent on the
solvents of gelation. For example, the xerogels of 4 from polar
solvents such as acetone, dioxane, and tetrahydrofuran exhibi-
ted twining agglomerates, while cross-linked massive
morphology was found in the case of ethyl acetate. The solvent
dependent morphology of gel 4 may be due to the dipolar
Table 1 Gel properties of 1–4 in organic solventsa
Solvent
1
2
3
4
n-Heptane
Acetone
Dioxane
Tetrahydrofuran
Ethyl acetate
Toluene
Dimethylbenzene
n-Propanol
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
P
I
I
I
I
P
P
P
I
I
S
S
S
S
S
S
S
S
S
S
S
S
S
G(8.24)b
G(13.73)
G(13.73)
G(13.73)
I
I
P
G(8.24)
G(8.24)
I
I
PG
I
Chloroform
Dichloromethane
Tetrahydrofuran–water
Acetone–water
Dioxane–water
n-Propanol–water
I
I
I
a
G ¼ gel; I ¼ insoluble; PG ¼ partial gel; P ¼ precipitation; S ¼ solution.
b
The values in parentheses are the CGC in mg mLꢀ1
.
stable gel-formation property in acetone, dioxane, tetrahydro-
furan, ethyl acetate, chloroform, and dichloromethane (Fig. 4).
While compounds 1–3 could not gel in the tested solvents. For
instance, compound 2 was insoluble in all of the tested solvents
both at room temperature and at the boiling point of the tested
solvents. Similarly, compound 3 was insoluble in n-heptane,
acetone, tetrahydrofuran, ethyl acetate, toluene, and dime-
thylbenzene both at room temperature and at the boiling point
of the tested solvents. Although compound 3 could dissolve in
Fig. 5 SEM images of the xerogels of 4 in (a) acetone; (b) dioxane; (c)
tetrahydrofuran; (d) ethyl acetate; (e) chloroform; (f) dichloromethane.
Scale bars: 5.0 mm.
Fig. 4 The gel state photographs (a, under ambient; b, irradiated at
365 nm by a handheld UV lamp) of 4 in various solvents: (1) acetone; (2)
dioxane; (3) tetrahydrofuran; (4) ethyl acetate; (5) chloroform; (6) Fig. 6 SEM images of the xerogels of 4 in chloroform. Scale bars: (a)
dichloromethane.
1.0 mm; (b) 2.0 mm; (c) 5.0 mm; (d) 10.0 mm.
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structure of 4 and the competition interactions between gela-
tor–gelator and solvent–gelator. Moreover, for the xerogel from
compound 4 in nonpolar solvents such as chloroform and
dichloromethane, it appeared as a 3D network comprised of
entangled bers with diameter of 200–600 nm.
Study of the aggregation process in solution
With the aim to obtaining the insight into the aggregation mode
during the gel formation, a further spectroscopic investigation
of these gelators was carried out. It should be noted that the 4
showed concentration- and temperature-dependent emission
properties (Fig. 7a and b). For example, upon increasing the
concentration from 1.0 ꢂ 10ꢀ6 to 1.0 ꢂ 10ꢀ3 M at 298 K, a broad
structureless band at 507 nm was observed, accompanied by the
disappearance of the emission bands at 466 and 494 nm, which
indicated the formation of a pyrene excimer. Meanwhile, as
shown in Fig. 7b, upon warming a gelled solution of 4 from 298
to 328 K, the emission spectra of 4 featured a slight blueshi
(ca. 10 nm). Furthermore, when the solution was heated at
328 K, there appeared a new peak at 500 nm. Over the same
temperature range the sample undergoes a gel–sol transition.
Therefore, the thermally induced shi in the emission spectra
was clearly associated with the phase transition.
Fig. 8 Partial 1H NMR spectra (400 MHz, CD2Cl2–CD3OD ¼ 3 : 2,
298 K) of compound 4 at different concentrations.
to probe the driving forces for the self-assembly process (Fig. 8
and 9). The 1H NMR spectra displayed distinctly concentration-
and temperature-dependent features. For instance, as the
concentration increased, the resonance signals corresponding
to the aromatic protons displayed an obvious upeld (e.g., H9,10
and H4,5, upelded from 8.84 and 8.09 ppm to 8.39 and 7.73
ppm, respectively). Similarly, the aromatic protons showed an
obvious upeld shi upon decreasing temperature (e.g., H9,10
and H4,5, upelded from 8.46 and 7.79 ppm to 8.25 and 7.61
ppm, respectively). It is noteworthy that, even in the gelling
solvent (CDCl3), the aromatic protons showed upeld shi
when the sample was cooled from the hot solution state to the
gel state (Fig. 10). It is well known that the upeld shi of the
aromatic proton resonance of a p-conjugated system reects an
increase in the p–p stacking interaction.8g Thus, the extent of
the p–p stacking interaction in 4 increased with the increase of
concentration and the decrease of temperature.
NMR spectra can not only reveal the changes in hydrogen
bonding, but also disclose the interactions between aromatic
moieties.25 Therefore, the concentration- and temperature-
1
dependent H NMR spectroscopic studies of 4 were performed
Moreover, it was found that the NH resonance displayed an
obvious upeld shi upon the increase of temperature, which
was indicative of the existence of hydrogen-bonding interac-
tions during the formation of supramolecular aggregate in
the case of 4. Temperature dependence of NH chemical shi
(Dd/DT) has been widely used as a tool for the study of intra-
molecular and intermolecular H-bonding during the
1
Fig. 7 Emission spectra of 4 in acetone (a) at different concentrations Fig. 9 Partial H NMR spectra (400 MHz, CD2Cl2–CD3OD ¼ 3 : 2) of
and (b) at different temperatures (CGC concentration).
compound 4 at different temperature.
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Moreover, the moderate coefficient (Dd/DT) in ppb/K ¼ (1.37
ppb/T) indicated a moderate intermolecular hydrogen bonding
in self-assembled state responsible for gelation. Therefore,
concentration- and temperature-1H NMR spectroscopic studies
suggested that both p–p stacking interaction and hydrogen
bonding were the driving forces for the process of self-assembly
and gelation of the gelator 4.
Based on all above experimental facts, a possible mechanism
for the formation of such LMOG was proposed (Fig. 12).
Initially, the primary-ordered aggregates were formed from the
simple building block mostly driven by p–p stacking interac-
tion, hydrogen-bonding, and other possible non-covalent
interactions. Subsequently, these primary aggregates evolved to
be the secondary ber-like structures or twining agglomerates,
which ultimately intertwine to form large intense three-
dimensional networks or other morphologies with large bulks.
Although it is still very difficult to demonstrate the denite
process of the formation of such organogel, there is no doubt
that p–p stacking interaction and hydrogen-bonding played an
important role in the formation of the ordered aggregates.
1
Fig. 10 Partial H NMR spectra (400 MHz, CDCl3) of compound 4 at
different temperature.
assembling process for the hydrogen bonded gelator mole-
cules.6f A small (Dd/DT) value for the NH resonance indicated
the participation of NH proton in an internal H-bond or it may
also be interpreted as an unbound NH that was completely
exposed to the solvent. On the other hand, relatively larger
temperature coefficient in solvent showed the involvement of
amide NHs in intermolecular H-bonding. VT-NMR studies
(Fig. 11) conducted with compound 4 over the temperature
range 293–323 K showed that there was a steady upeld
chemical shi of NH protons upon increasing temperature.
Emission investigation of uorescent organogel
As mentioned previously, the uorescent organogels containing
p-conjugated chromophores have received considerable atten-
tion because of their potential applications in optoelectronics,
erasable thermal imaging, and sensing of explosives.6 Inspired
by the existence of a large p-conjugated system in compound 4,
the further spectroscopic investigation on this new uorescent
organogel was carried out. As shown in Fig. 13, compound 4
showed two shoulder peaks located at 468 and 495 nm in
solution, which arised from the non-aggregated of pyrene-con-
taining compound. Under self-assembly conditions, in the gel
state, 4 exhibited a excimer emission peak centered at 546 nm.
This indicated that pyrenyl group of this gelator 4 was dimer-
ized in the gel state, which might be caused by the presence of
strong p–p stacking interaction. The observed signicant red
shi of uorescence emission maxima was obvious due to the
presence of a strong molecular aggregation in the gel state.
Thus, the uorescence emission investigation of 4 provided
further evidence that p–p stacking interaction contributed to
Fig. 11 Chemical shift of NH resonance (ppm) of 4 at different
temperature in CDCl3.
Fig. 12 Schematic illustration for the formation of gels from 4 in tested solvents.
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shown in Fig. 14. Moreover, the investigation of LSCM on
samples of 4 offered similar morphologies as that observed in
the SEM studies. The 3-D networks comprised by bers struc-
tures were found in LSCM images of 4. The results indicated
that 4 had the ability to form uorescent organogel with good
uorescence behaviour.
Heating or Zn(II)-induced gel–sol phase transition
Similar to the reported gels,13 the gel–sol interconversion of 4
was fully thermoreversible by several cycles of heating and
cooling. As shown in Fig. 15, the gel was found to be stable in
the room temperature, however, when the temperature of gel
was heated to the boiling point of the solvent, a clear and yellow
solution was observed. Subsequently, when the hot solution of 4
was cooled down to the room temperature, a stable gel re-
formed. The gel returned to a yellow solution aer re-heating.
The cycles of gel-to-sol transition upon heating and sol-to-gel
transition upon cooling were reversible.
Fig. 13 Nomalized fluorescence emission spectra of compound 4 in
solution (10ꢀ5 M) and in the gel state (CGC concentration) in acetone.
As discussed above, compound 4 was able to bind with Zn2+
through the binding of NH group with Zn2+. Thus, the inter-
molecular H-bonding between the NH groups would be dis-
rupted in the presence of Zn2+. As shown in Fig. 15, the addition
of 0.5 equiv. Zn2+ to the gel of 4 induced an apparent and fast
gel-to-sol transition within 30 seconds. This observation
revealed that the addition of Zn2+ destroyed the H-bonding, one
of the driving forces for the gel formation, thus leading to the
fast gel-to-sol transition. The results proved that compound 4
could form a stimuli-responsive gel that characterized a sensi-
tive gel-to-sol transition response to Zn2+. Moreover, the solu-
tion of 4 containing Zn2+ could not be transformed into the gel
under heating and cooling, which also provided support for the
assumption that the intermolcular H-bonds between the NH
groups should be responsible for the gel formation.
Fig. 14 LCSM images (a and d) and bright-field images (b and e) of
xerogels of compound 4 in chloroform. (c) Was the overlap of (a) and
(b). (f) Was the overlap of (d and e). lex ¼ 405 nm, emission was
collected at 450–550 nm.
the formation of organogel. Moreover, as shown in Fig. 4b, upon
irradiation at 365 nm by a handheld UV lamp, compound 4
exhibited stable and chartreuse uorescence gels in various
solvents such as acetone, dioxane, tetrahydrofuran, ethyl
acetate, chloroform and dichloromethane. With the aim to
obtaining further insight into the morphologies and optical
properties of this new LMOG, the laser scanning confocal
microscopy (LSCM) was performed on the xerogels of 4.
Notably, the green luminescence was emitted from a nanoscale
structure, which was directly observed by LSCM images as
With the aim to obtaining the further insight into the stimuli-
responsive properties of gelator 4, the investigations of the
responsive behavior of organogels 4 to Zn2+ with different
concentrations and in different solutions have been carried out.
As shown in Fig. S18a,† the addition of 0.1 equiv. Zn2+ to the gel
of 4 in acetone induced the partial gel-to-sol transition within 10
minutes. However, the addition of 0.2 equiv. Zn2+ to the gel of 4 in
acetone induced an apparent and fast gel-to-sol transition within
1 minute. The results proved that the gel of 4 exhibited highly
sensitive gel-to-sol transition response to Zn2+ in acetone.
Fig. 15 Photographs demonstrating of the cycles of gel–sol transition of 4 in acetone by heating and cooling and the gel–sol transition of 4 in
acetone by addition of Zn2+
.
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formation property in acetone, dioxane, tetrahydrofuran, ethyl
acetate, chloroform, and dichloromethane. The morphologies
of the xerogels were investigated by scanning electron micros-
copy (SEM) and laser scanning confocal microscopy (LSCM).
The concentration- and temperature-dependent emission
1
properties, and concentration- and temperature-dependent H
NMR spectroscopic of 4 revealed that both p–p stacking inter-
action and hydrogen bonding were the driving forces for the
process of self-aggregation and the gel formation. In addition,
the studies on luminescence properties indicated that 4 had the
ability to form uorescent organogel with good uorescence
behaviour. More importantly, it was found that that compound
4 could form a stimuli-responsive gel that characterized a
Fig. 16 Normalized fluorescence emission spectra of compound 4 in
the gel state (CGC concentration) absence of Zn2+ (red curve) and in
the presence of 0.5 equiv. Zn2+ (black curve) in acetone.
sensitive gel-to-sol transition response to heating or Zn2+
.
In summary, we have designed and synthesized a family of
ethynylpyrene derivatives, which presented interesting lumi-
nescence properties and different gel formation properties.
More importantly, this work provide an interesting and
successful example of constructing stimuli-responsive uores-
cent organogel derived from an aminoquinoline-based uo-
rescent Zn2+ probe with an alkoxyethylamino chain as receptor.
This research obviously enriches the library of stimuli-respon-
sive uorescent organogels and might be helpful in preparing
such type of organogels.
Moreover, the solvent inuence on the stimuli-responsive
property of 4 was discussed. As shown in Fig. S18a,† the addi-
tion of 0.4 equiv. Zn2+ to the gel of 4 in dioxane induced the
partial gel-to-sol transition within 30 minutes. The addition of
Zn2+ up to 0.5 equiv. was able to induce total gel-to-sol transi-
tion within 15 minutes. The results indicated that the gel of 4
was more stable in dioxane than that in acetone, which might
be attributed to the fact that dioxane has a higher viscosity than
acetone.
Experimental section
In order to give a better understand of the application of the
gel, the uorescent change of the gel with the addition of Zn2+
has been carried out. For example, as shown in Fig. 16, the
compound 4 showed an emission band peak at 546 nm in the
gel state (red curve). However, the addition of 0.5 equiv. Zn2+ to
the gel of 4 induced a gel-to-sol transition accompanied with a 7
nm blue-shi from 546 nm to 539 nm (black curve) of the
uorescence emission. The blue-shi of emission provided the
further evidence that a weaker intermolecular aggregation of
compound 4 existed in the solution state than that in the gel
state.
All reagents and solvents were purchased from commercial
sources. THF was distilled from sodium, Et3N and Et2NH were
dried from potassium hydroxide; both solvents were degassed
under N2 for 30 min before use. All reactions were performed in
standard glassware under an inert N2 atmosphere. Compounds
5, 9, 14, and 15 were prepared as the reported procedure. Unless
specically mentioned, 1H NMR and 13C NMR spectra were
recorded on Bruker 400 MHz NMR Spectrometer at 298 K. The
1H and 13C NMR chemical shis were reported relative to
residual solvent signals. Coupling constants (J) are denoted in
Hz and chemical shis (d) in ppm. Multiplicities were denoted
as follows: s ¼ singlet, d ¼ doublet, m ¼ multiplet.
The metal salts used were Fe(ClO4)2, Zn(ClO4)2$6H2O,
Co(ClO4)2$6H2O, Ni(ClO4)2$6H2O, Ba(ClO4)2$3H2O, Pb(ClO4)2$
3H2O, Cd(ClO4)3$6H2O, Cu(ClO4)2$6H2O, Mn(ClO4)2$6H2O,
LiClO4$3H2O, NaClO4$H2O, AgClO4$H2O, Hg(ClO4)2$3H2O,
Al(ClO4)3$9H2O, and Cu(ClO4)2$6H2O.
Conclusions
By utilizing the sonogashira coupling reaction of ethynylpyrene
with 5-I-aminoquinoline derivatives as a key step, a new family
of pyrene-containing compounds 2–4 derived from uorescent
probe 1 were successfully synthesized. The molecular structures
of these new compounds were fully characterized by using
multiple nuclear NMR (1H and 13C) and mass spectrometry. It
was found that compounds 2–4 showed pronounced red-shied
absorption and emission compared to free pyrene and 1.
Moreover, the absorption and emission properties of these
compounds were signicantly affected by the substitution of
pyrene, such as the numbers and the position of substituent.
Furthermore, these compounds exhibited the selective uores-
cence behaviour towards Zn2+ in aqueous solution.
UV-Vis and uorescence measurements
UV/Vis spectra were recorded on a Cary 50Bio UV/Visible spec-
trophotometer. Emission spectra were measured on a Cary
Eclipse uorescence spectrophotometer. Samples for absorp-
tion and emission measurements were contained in 1 cm ꢂ 1
cm quartz cuvette.
Gelation test
It was interesting to nd that compounds 2–4 displayed A known weight of the compound to be tested and a measured
unexpectedly different gelation behaviour, for example, only volume of selected pure solvent were placed into a sealed test
1,8-bis-substituted compound 4 exhibited the stable gel- tube and the system was heated in a water bath until the solid
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was dissolved, then, the solution was cooled to room tempera- 66.4, 60.2, 60.1, 53.2, 48.9. HRMS m/z: (M + H)+ calcd for
ture in air, and nally the test tube was inversed to look at if the
solution inside could still ow. A positive test is obtained if the
ow test is negative. When a gel was formed at this stage, it was
denoted as “G”. For systems, in which only solution was
remained until the end of the tests, they were referred to as “S”
(solution). When the gelator of a system appeared as precipitate,
the system was denoted as “P” (precipitation). The system, in
which the potential gelator could not be dissolved even at the
boiling point of the solvent, was called an insoluble system (I).
C50H45N6O6, 825.3401; found, 825.3422.
Synthesis of compound 4
A 250 mL Schlenk ask was charged with compounds 7 (1.8
mmol, 750 mg), 15 (0.799 mmol, 200.0 mg) and cuprous iodide
(0.06 mmol, 12.2 mg), degassed, and back-lled three times
with N2. Triethylamine (60 mL) and dried THF (60 mL) were
introduced into the reaction ask by syringe. The reaction was
stirred under an inert atmosphere at 75 ꢃC for 12 h. The solvent
was removed by evaporation on a rotary evaporator. The residue
was puried by column chromatography on silica gel
(dichloromethane–methanol, 99 : 1) to afford 4 as red solid
(51.1%). 1H NMR (CD2Cl2–CD3OD ¼ 3 : 2, 400 MHz) d: 8.76 (d, J
¼ 4.0 Hz, 2H), 8.64 (d, J ¼ 8.4 Hz, 2H), 8.53 (d, J ¼ 8.0 Hz, 2H),
8.42 (s, 2H), 7.97 (d, J ¼ 7.6 Hz, 2H), 7.85 (d, J ¼ 8.0 Hz, 2H), 7.76
(s, 2H), 7.68 (d, J ¼ 8.0 Hz, 2H), 7.49 (q, J ¼ 4.4 Hz, 2H), 3.70 (dd,
J ¼ 9.2, 4.8 Hz, 8H), 3.60–3.54 (m, 8H), 2.92 (t, J ¼ 9.6, 4.4 Hz,
4H). 13C NMR (DMSO-d6, 100 MHz) d: 171.2, 149.7, 137.8, 135.3,
134.6, 132.1, 131.1, 130.8, 130.3, 128.2, 127.7, 126.4, 125.8,
123.4, 117.6, 115.0, 113.5, 92.9, 92.7, 72.2, 70.2, 60.2, 53.4, 49.0.
HRMS m/z: (M + H)+ calcd for C50H45N6O6, 825.3401; found,
825.3416.
SEM measurements
SEM images of the xerogels were obtained using an S-4800
(Hitachi Ltd) with an accelerating voltage of 1.0 kV or 10.0 kV.
Samples were prepared by dropping dilute gels onto a silicon
wafer.
Synthesis of compound 2
A 250 mL Schlenk ask was charged with compounds 7 (2
mmol, 830.5 mg), 9 (1 mmol, 226.3 mg) and cuprous iodide
(0.05 mmol, 9.5 mg), degassed, and back-lled three times with
N2. Triethylamine (70 mL) and dried THF (70 mL) were intro-
duced into the reaction ask by syringe. The reaction was stir-
red under an inert atmosphere at 75 ꢃC for 12 h. The solvent was
removed by evaporation on a rotary evaporator. The residue was
puried by column chromatography on silica gel (dichloro-
methane–methanol, 99 : 1) to afford 2 as yellow solid (37.3%).
1H NMR (DMSO-d6, 400 MHz) d: 11.34 (s, 1H), 9.09 (s, 1H), 8.97
(d, J ¼ 8.4 Hz, 1H), 8.79 (d, J ¼ 8.0 Hz, 1H), 8.71 (d, J ¼ 9.2 Hz,
1H), 8.41 (m, 6H), 8.27 (q, J ¼ 8.8 Hz, 2H), 8.16 (d, J ¼ 8.0 Hz,
2H), 7.90 (q, J ¼ 4.0 Hz, 1H), 3.89 (s, 2H), 3.68 (s, 2H), 3.53 (d, J ¼
4.4 Hz, 2H), 3.48 (d, J ¼ 4.4 Hz, 4H), 3.04 (s, 2H). 13C NMR
(DMSO-d6, 100 MHz) d: 149.8, 138.0, 135.1, 134.7, 132.0, 131.1,
130.9, 130.8, 130.5, 129.9, 129.0, 128.5, 127.8, 127.2, 126.8,
126.1, 126.0, 125.0, 124.8, 123.7, 123.5, 123.4, 116.6, 116.1,
114.3, 93.1, 92.1, 72.2, 67.9, 60.1, 51.2, 47.8.
Acknowledgements
The work was nancially supported by the National Natural
Science Foundation of China (no. 21302058), the Research Fund
for the Doctoral Program of Higher Education of China (no.
20130076120006), the Opening Project of Shanghai Key Labo-
ratory of Chemical Biology, and the Fundamental Research
Funds for the Central Universities. We also appreciate valuable
discussion with Prof. Hai-Bo Yang (East China Normal
University).
Notes and references
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Synthesis of compound 3
A 250 mL Schlenk ask was charged with compounds 7 (2.16
mmol, 895.8 mg), 14 (0.719 mmol, 180.0 mg) and cuprous
iodide (0.07 mmol, 13.7 mg), degassed, and back-lled three
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was stirred under an inert atmosphere at 75 C for 12 h. The
solvent was removed by evaporation on a rotary evaporator. The
residue was puried by column chromatography on silica gel
(dichloromethane–methanol, 99 : 1) to afford 3 as red solid
(38.9%). 1H NMR (DMSO-d6, 400 MHz) d: 11.53 (s, 2H), 9.00 (d, J
¼ 2.8 Hz, 2H), 8.85 (d, J ¼ 8.4 Hz, 2H), 8.72 (d, J ¼ 8.0 Hz, 2H),
8.61 (d, J ¼ 8.8 Hz, 2H), 8.37 (d, J ¼ 8.0 Hz, 2H), 8.34–8.26 (m,
4H), 8.01 (d, J ¼ 8.0 Hz, 2H), 7.82 (q, J ¼ 4.0 Hz, 2H), 3.62 (t, J ¼
7.2, 10.4 Hz, 4H), 3.52–3.47 (m, 8H), 3.45 (d, J ¼ 3.2 Hz, 4H), 2.83
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127.7, 125.8, 123.4, 117.7, 115.1, 113.5, 92.7, 72.2, 72.1, 70.0,
ꢃ
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