An AIE active Y-shaped diimidazolylbenzene: Aggregation and disaggregation for Cd2+ and Fe3+ sensing in aqueous solution

Article abstract of DOI:10.1039/c4ob01635a

A simple Y-shaped dimb with AIE properties was designed and synthesized. It showed selective fluorescence turn-on toward Cd²⁺ ion in MeCN-water (2:8, v/v) through aggregation, and also selective fluorescence turn-off toward Fe³⁺ ion in MeCN-water (1:99, v/v) through disaggregation.

Full text of DOI:10.1039/c4ob01635a

Organic &
Biomolecular Chemistry
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An AIE active Y-shaped diimidazolylbenzene:
aggregation and disaggregation for Cd2+ and Fe3+
sensing in aqueous solution†
Cite this: Org. Biomol. Chem., 2014,
12, 9524
Received 31st July 2014,
Accepted 10th October 2014
Chengming Li, Chao Gao, Jingbo Lan, Jingsong You and Ge Gao*
DOI: 10.1039/c4ob01635a
www.rsc.org/obc
A simple Y-shaped dimb with AIE properties was designed and syn-
thesized. It showed selective fluorescence turn-on toward Cd2+
ion in MeCN–water (2 : 8, v/v) through aggregation, and also selec-
tive fluorescence turn-off toward Fe3+ ion in MeCN–water (1 : 99,
v/v) through disaggregation.
Fluorescence sensing for metal ions is an attractive and impor-
tant subject in supramolecular, environmental and biological
chemistry.1 Classical fluorescent sensor design is the combi-
nation of a signaling fluorophore and a metal binding unit.
The selective and efficient binding between a certain metal ion
and the binding unit induces the fluorescence variation (on or
off) of the signaling fluorophore through photo-induced elec-
tron transfer (PET), intramolecular charge transfer (ICT),
metal–ligand charge transfer (MLCT), fluorescence resonance
energy transfer (FRET), etc.,2 to realize sensing. Based on these
conventional signaling mechanisms, a myriad of fluorescent
sensors have been synthesized. However, selectivity and sensi-
tivity still need to be improved. Therefore, novel sensor design
utilizing a new signaling mechanism is of great importance
and significance.
Scheme 1 Four channels in aggregation/disaggregation triggered
sensing.
Hg–thymine complex.3 More favorable and attractive is the use
of the reverse process, disaggregation, to design fluorescent
turn-on sensors (Scheme 1, channel II). A squaraine-based
sensor reported by Fu et al. formed a H-aggregate in AcOH–
H2O (4 : 6, v/v) and its fluorescence was quenched. The coordi-
nation with Hg2+ resulted in the dissociation of the H-aggre-
gate and the recovery of fluorescence.4
On the other hand, a phenomenon opposite to the ACQ was
disclosed by Tang et al. that silole and tetraphenylethylene
molecules were not or are weakly fluorescent in solutions but
emitted a strong fluorescence in the solid state, which they
referred to as aggregation induced emission (AIE).5 This abnor-
mal fluorescence behavior quickly attracted much attention for
the sensor design and a few fluorescent turn-on sensors for
metal ions have been developed accordingly through channel
III.6 Recently, the sensor design strategy using the above men-
tioned three channels was conceptually reviewed by Wang
et al.7 and Chang et al.8 However, channel IV, the fluorescence
turn-off by disaggregation via AIE, has not been reported in
any sensing process yet.
Due to our continuous efforts on the development of imida-
zole-based fluorogenic skeletons,9 we recently became inter-
ested in the design and synthesis of AIE active imidazole
fluorophores.10 The V-shaped 1,3-diimidazolylbenzene (V-dimb)
is structurally characteristic of two open coordination sites of
the imidazolyl N atoms, which can serve as a linker to form
coordination polymers.11 Previously, the You group reported
that V-dimb selectively coordinated with Ag+ ion to form
a coordination polymer and to gelate solvents.12 V-dimb
Aggregation of organic fluorophores is
a ubiquitous
phenomenon. Disaggregation is its reverse process. These two
processes open up four signal channels for the sensor design
(Scheme 1). Aggregation usually results in quenching of the
fluorescence, which is known as aggregation caused quench-
ing (ACQ). Fluorescent turn-off sensors can be obtained when
an analyte is capable of inducing the aggregation of a specific
fluorophore (Scheme 1, channel I). For example, Shangguan
et al. reported that the fluorescence of tetra-(thymin-1-yl-acet-
amido)-phthalocyanine Zn(II) (4T-ZnPc) was selectively
quenched by the addition of Hg2+ due to the aggregation
induced by the intermolecular cross-linking of the thymine–
Key Laboratory of Green Chemistry and Technology of Ministry of Education, College
of Chemistry, Sichuan University, 29 Wangjiang Road, Chengdu 610064, PR China.
E-mail: gg2b@scu.edu.cn; Fax: (+86) 28-85412203
†Electronic supplementary information (ESI) available: Detailed experimental
procedures, analytical data, supplementary figures and NMR spectra. See DOI:
10.1039/c4ob01635a
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Scheme 2 The structures of differently shaped dimbs.
was also used to construct double helixes in a cobalt vanadate
system.13 These results indicated that the aggregation state of
V-dimb could be selectively tuned by different metal ions. We
therefore introduced two electron-donating and π-conjugating
dimethylaminophenylethynyl groups (DMAPE) para to the imi-
dazole moieties on V-dimb and synthesized an X-shaped dimb
(X-dimb),10a expecting to endow the V-dimb AIE properties
and capability of selective metal ion sensing via the AIE mecha-
nism. The X-dimb actually exhibited AIE characteristics, but
was not able to recognize any metal ion, probably because the
two DMAPE groups hindered the coordinating polymerization.
We then proposed that only one DMAPE group at the meta posi-
tion of the imidazole units could reduce the steric hindrance,
facilitate the coordinating polymerization and eventually realize
metal ion sensing via AIE. Herein, we report the synthesis of
this Y-shaped dimb (Y-dimb) and its AIE properties. The Y-
dimb exhibited dual selective sensing of Cd2+ and Fe3+ in
aqueous solution via channel III and IV, respectively
(Scheme 2).
Y-dimb was easily obtained by a three-step convergent syn-
thesis as depicted in Scheme 3. The Sonogashira coupling of
4-bromo-N,N-dimethylaniline with ethynyltrimethylsilane gave
the intermediate 1 in 89% yield. The bromide substituted
dimb 2 was obtained in 55% yield by a 2 : 1 stoichiometric
N-arylation of imidazole with 1,3,5-tribromobenzene. The desily-
lation of 1 followed by the coupling with 2 in a one-pot
manner afforded the desired Y-dimb as a yellow-colored solid
in 68% yield. The structure was confirmed by 1H and 13C
NMR, and HRMS (ESI†).
Fig. 1 Plot of the emission intensity of Y-dimb (20 μM) versus the water
fraction in the MeCN–water mixture. Inset: the photo of Y-dimb in
MeCN–water mixtures under UV light.
tration of 20 μM emitted blue fluorescence with a maximum
emission at 471 nm under UV irradiation. Upon increasing
addition of water, the fluorescence weakened gradually with a
red shift of the maximum emission wavelength to 501 nm
until the water fraction reached 85% (Fig. 1). The red shift of
the fluorescence suggested a narrowed band gap, probably
because the enlarged solvent polarity elevated the HOMO
energy level.14 Meanwhile, the higher environmental polarity
was beneficial to the stabilization of the twisted intramolecular
charge transfer (TICT) state of Y-dimb, which quenched the
fluorescence through various nonradiative decay pathways. As
the amount of water was added to reach 90%, the blue fluo-
rescence reappeared with an intensity similar to the pure
MeCN solution. When the water fraction was increased to
92%, the fluorescence enhancement was about 28-fold higher
compared with the lowest fluorescence at the water fraction of
85%. The addition of more water resulted in slight fluo-
rescence quenching, probably due to the change in the aggre-
gation state. The Tyndall phenomenon revealed the formation
of the nano-sized aggregates of Y-dimb (Fig. S2, ESI†).15 There-
fore, Y-dimb is an AIE active molecule as we designed. In
addition, the dynamic light scattering (DLS) measurement
showed average particle sizes of 255, 342 and 825 nm in
MeCN–water solutions of 2 : 8, 15 : 85 and 1 : 92 (v/v), respect-
ively (Fig. S6†). It suggested that the aggregation size signifi-
cantly influenced the fluorescence of Y-dimb, probably
because the larger aggregates were more compact restricting
the free rotation.
Y-dimb could be dissolved in common organic solvents
such as chloroform, dichloromethane, tetrahydrofuran, ethyl
acetate, acetone, acetonitrile (MeCN), methanol, etc., but was
insoluble in water. The MeCN solution of Y-dimb at a concen-
The metal ion sensing experiment was then conducted
using the 20 μM solution of Y-dimb in a MeCN–water mixture
of 2 : 8 (v/v), in which the fluorescence background was very
low. A variety of ions (50 equiv.), Cd2+, Ag+, Zn2+, Cu2+, Co2+,
Ca2+, Ni2+, Mg2+, Mn2+, Fe2+, Fe3+, Cr3+, K+ and Hg2+, were
added and well dispersed. Under UV light, a green-colored
fluorescence was clearly seen by the naked eyes in the cell with
Cd2+ while no visible change could be observed in the other
Scheme 3 The synthetic route of Y-dimb. Conditions: (a) Pd(dppf)Cl2,
PPh3, CuI, Et3N, 80 °C; (b) CuI, K2CO3, DMF, 150 °C; (c) Pd(PPh3)4, CuI,
KOH (50% aq), Et3N, THF, 80 °C.
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Fig. 3 SEM images of (a) nanofibres and (b) nanoparticles observed in a
MeCN–water mixture (2/8, v/v), respectively.
in Fig. 3a, nano fibres were formed. Similarly, the SEM photo
of Y-dimb with FeCl3 exhibited only small nanoparticles
(Fig. 3b). The nano fibres observed in the presence of Cd2+
indicated that the Cd2+ ion strongly tended to form coordi-
nation polymers with Y-dimb in solution, which restrained the
intramolecular rotation of Y-dimb and shut down the non-
radiative decay pathway, resulting in the fluorescence enhance-
ment. However, the Fe3+ ion hydrolyzed in the aqueous
solution and formed Fe(OH)3 particles, so that the fluo-
rescence of Y-dimb was not turned on. The DLS experiments
showed that average particle sizes in the 2 : 8 MeCN–water
solutions of FeCl3 with and without Y-dimb were 1281 and
1106 nm, respectively (Fig. S8†), suggesting the nanoparticles
observed by SEM in the presence of FeCl3 was Fe(OH)3.
Fig. 2 The histogram of the fluorescence variations of Y-dimb (20 μM)
in the presence of Cd2+, Ag+, Zn2+, Cu2+, Co2+, Ca2+, Ni2+, Mg2+, Mn2+
,
Fe2+, Fe3+, Cr3+, K+ and Hg2+ cations (50 equiv.) in MeCN–H2O (2 : 8,
v/v), excited at 357 nm. Insets: (a) fluorescence spectra of Y-dimb
accordingly; (b) photo taken under UV light accordingly.
cells (Fig. 2, inset b). The fluorescence measurement on the
fluorimeter also showed the selective fluorescence turn-on by
Cd2+ (Fig. 2, inset a).16 The histogram showed about 20-fold
enhancement caused by the addition of Cd2+, a small increase
by Ag+, tiny quenching by Cu2+, Fe2+, Fe3+ and Hg2+, and a neg-
ligible variation by the other ions tested (Fig. 2). It is note-
worthy that Y-dimb showed an excellent selectivity between
Cd2+ and Zn2+, which only a few sensors could achieve because
they are neighbouring elements in the IIB group with similar
physical and chemical properties.17 The Tyndall experiment
confirmed the formation of the nano-sized aggregates caused
by the addition of Cd2+ ion, suggesting that the sensing
process was based on the AIE mechanism (Fig. S3, ESI†). The
DLS measurement showed an average particle size of 1990 nm
(Fig. S7†). It needs to be pointed out that the Tyndall pheno-
menon was also observed in the cell with Fe3+, which otherwise
exhibited a small fluorescence quenching.
Although the disaggregation of Y-dimb-Cd2+ aggregates
occurred upon addition of both Fe3+ and Cu2+ in MeCN–water
(2 : 8, v/v), the disaggregation could only be selectively trig-
gered by Fe3+ in MeCN–water (1 : 99, v/v), which made Y-dimb
a potential fluorescent turn-off sensor18 through channel IV.
In these mixed solvents, Y-dimb was dispersed as nanoparti-
cles and emitted a strong green fluorescence. Upon addition of
Fe3+, the fluorescence gradually diminished. When 10 equiv.
of Fe3+ was added, the fluorescence vanished completely
(Fig. 4). It should be noted that the disaggregation was caused
by the protonation of Y-dimb due to the strong hydrolysis of
Fe3+, which led to quenching of the fluorescence. Other metal
The competitive experiment was then conducted (Fig. S4,
ESI†). The addition of 50 equiv. of Cd2+ into the cells of
Y-dimb with 50 equiv. of Na+, K+, Ca2+, Mg2+, Co2+ and Fe2+,
respectively, showed similar fluorescence enhancement to that
without any disturbing ion. However, the presence of Mn2+
and Ni2+ decreased one third of the fluorescence increase,
while Cr3+, Zn2+ and Hg2+ almost cut the fluorescence
enhancement in half. The most pronounced influence was
from Cu2+ and Fe3+ ions, which resulted in a very small fluo-
rescence variation, indicating that the coordination polymers
between Y-dimb and Cd2+ could not be formed in these
environments.
To shed light on the structure of the nano aggregates, a
25 μL aqueous solution of CdCl2 (30 mM) was added into 3 mL
of MeCN–water solution of Y-dimb (20 μM) and mixed
thoroughly. An aliquot was then dropped on a clean glass
slide. After the solvent naturally evaporated, the slide was
Fig. 4 Fluorescence titration of Y-dimb (20 μM) with Fe3+ in MeCN–
water solution (1 : 99. v/v). Insets: photos of solutions of Y-dimb and in
viewed under the scanning electron microscope (SEM). As seen the presence of 10 equiv. of Fe3+ under UV light.
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ions tested only induced small fluorescence variations even at
a much higher concentration (Fig. S5, ESI†).
8 D. Zhai, W. Xu, L. Zhang and Y.-T. Chang, Chem. Soc. Rev.,
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In conclusion, we designed and synthesized an AIE active
Y-dimb. Based on the AIE mechanism, Y-dimb could not only
selectively sense Cd2+ through aggregation with a fluorescence
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