Imidazole-bearing tetraphenylethylene: Fluorescent probe for metal ions based on AIE feature

Article abstract of DOI:10.1039/c1nj20122k

With an aggregation-induced emission feature, a suspension of imidazole-bearing tetraphenylethylene in a tetrahydrofuran-water mixture with a high water fraction is highly emissive, and can be completely quenched by Fe³⁺ ions due to the coordin

Full text of DOI:10.1039/c1nj20122k

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PAPER
Imidazole-bearing tetraphenylethylene: fluorescent probe for metal ions
based on AIE featurew
Ning Bian,ab Qi Chen,b Xi-Long Qiu,ab Ai-Di Qi*a and Bao-Hang Han*b
Received (in Montpellier, France) 15th February 2011, Accepted 18th April 2011
DOI: 10.1039/c1nj20122k
With an aggregation-induced emission feature, a suspension of imidazole-bearing
tetraphenylethylene in a tetrahydrofuran–water mixture with a high water fraction is highly
emissive, and can be completely quenched by Fe3+ ions due to the coordination between
imidazole moieties and the Fe3+ ion.
The development of artificial fluorescent receptors for the
sensing and recognition of environmentally and biologically
important ionic species, especially transition-metal ions, is
currently of great interest, because their fluorescence-based
sensing and recognition are highly sensitive, convenient, cost-
effective, and easy to scale-up to a high-throughput screening
format. The toxicity of certain metal ions has been a constant
cause of environmental concern. Thirteen transition-metal
ions, such as chromium, manganese, cobalt, copper, zinc,
molybdenum, silver, mercury, cadmium, lead, iron, and nickel,
are listed as ‘‘priority pollutants’’ by the Environmental
Protection Agency.1
hereditary hemochromatosis.7 Therefore, much attention has
been drawn to the design of probes for the detection of iron
ion based on conjugated polymers or organic dyes.8 However,
the detection sensitivity is strongly dependent on the organic
solvent content in the detecting solution system. Most of them
work well in a pure organic solvent medium or an aqueous
solution containing at least 50% organic co-solvent. Furthermore,
aggregation-caused fluorescence quenching of traditional dyes
often takes place when dispersed in aqueous media or interacted
with biomacromolecules, resulting in a drastically negative effect
on efficiencies and sensitivities of biosensors or bioprobes.9
These limitations, including organic co-solvent dependence
and aggregation-caused quenching, to some extent, lower the
sensitivity and restrict the application of sensors in biological
systems and environmental determinations.
Recently, molecules with aggregation-induced emission
(AIE) characteristics have been developed and have drawn
much attention due to their enhanced emission in aggregate
form or solid-state.2 The AIE effect can significantly improve
the fluorescence quantum yields of the molecules by up to
three orders of magnitude, enhancing the photoluminescence
intensity from faint luminophores into strong emitters.3 Tetra-
phenylethylene (TPE), as one of most studied AIE luminophors,
has already been demonstrated to show practical applications
in OLEDs,4 chemosensors,5 and bioprobes,6 owing to their
facile preparation and easy functionalization.
Imidazole has been widely employed in coordination
chemistry and supramolecular chemistry, and is a good ligand
for transition-metal ions.10 In this work, an imidazole-bearing
tetraphenylethylene derivative, Im-TPE, was prepared as a
fluorescent probe for an iron ion assay based on the aggregation-
induced emission effect.
The synthetic route to Im-TPE is shown in Scheme 1.
Dihydroxy-tetraphenylethylene (2) was prepared from
methoxybenzophenone through two steps that were a McMurry
and demethylated reaction, respectively, according to a reported
method.6a After being treated with 1,2-dibromoethane under
basic conditions, dihydroxy-tetraphenylethylene was converted
to TPE-based bromide (3), which was reacted with imidazole
to afford the desired product Im-TPE efficiently.
Iron(III) is an important trace element for biological
systems, which plays an essential role in oxygen uptake of
heme, oxygen metabolism, and a cofactor in many enzymatic
reactions. But the concentration of iron(III) must be moderate
in mammals, both its insufficiency and excess can lead to a
variety of diseases such as anemia, hepatic cirrhosis, and
Im-TPE is soluble in common organic solvents such as
acetonitrile, chloroform, and tetrahydrofuran (THF), but
insoluble in water. In dilute THF solution, Im-TPE, as
expected, shows almost no luminescence. However, its suspension
in a THF–water mixture with a high water fraction is highly
emissive. As shown in Fig. 1, from the molecular solution in
THF to the aggregate suspension in a THF–water mixture
(1 : 99, v/v), the photoluminescence emission of Im-TPE at
471 nm significantly increases. Furthermore, Im-TPE shows a
a Tianjin Key Laboratory of Modern Chinese Medicine and College of
Traditional Chinese Medicine, Tianjin University of Traditional
Chinese Medicine, Tianjin 300193, China.
E-mail: qiaidi@tjutcm.edu.cn
b National Center for Nanoscience and Technology, Beijing 100190,
China. E-mail: hanbh@nanoctr.cn; Fax: +86-10-82545576
w Electronic supplementary information (ESI) available: Details of
related fluorescence spectra and Hill plot of Im-TPE, mass spectrum,
1H NMR, and 13C NMR spectra of Im-TPE and TPE derivative 3. See
DOI: 10.1039/c1nj20122k
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The concentration of Im-TPE in a THF–water mixture
with 99% water fraction was fixed at 20 mM, and the final
concentration of metal ions was fixed at 50 mM. It can be seen
that Cu2+, Fe2+, and Fe3+ lead to an efficient fluorescence
quenching of Im-TPE. Especially, when treated with Fe3+
nearly complete fluorescence quenching of Im-TPE was
observed. The other metal ions such as Zn2+, Cd2+, Mn2+
,
,
Ni2+, and Co2+ cause a slight fluorescence quenching of
Im-TPE. As a control study, dimethoxy-tetraphenylethylene
(DMO-TPE) was also exposed to the same amount of Fe3+
under the same condition, and its fluorescence spectrum in the
solution shows almost no change (see supporting information,
Fig. S1w). Therefore, the corresponding changes of Im-TPE
fluorescence spectra in the presence of different metal ions can
be ascribed to the existence of imidazole groups. The distinct
responsive behaviors of Im-TPE to various metal ions reveal
the different coordination ability of transition-metal ions with
imidazole receptors. It should be noted that the fluorescence
intensity of Im-TPE has been enhanced ca. 10% by the Ag+
ion compared with the intensity of the blank system.
Such fluorescence enhancement observed for Im-TPE in the
presence of Ag+ is attributed to the coordination of imidazole
moieties of Im-TPE with Ag+ ions leading to formation of
coordination complexes that may further aggregate due to
the low solubility. A similar result was also reported for
Ag+-induced aggregation of adenine-functionalized TPE.5
The photograph given in Fig. 2b clearly displays the emissive
nature of Im-TPE affected by various metal ions in a
THF–water mixture with a 99% water fraction under UV
light illumination (365 nm).
Scheme 1 Synthetic route to Im-TPE.
Fig. 1 Fluorescent spectra of Im-TPE (20 mM) in a THF–water
mixture (excitation wavelength was 320 nm).
strong blue emission in solid-state. These data clearly indicate
that Im-TPE exhibits a significant feature of aggregation-
induced emission.
The competition experiment was also carried out by adding
Fe3+ to the aqueous solutions of Im-TPE in the presence of
another metal ion, shown in Fig. 3. The results indicate that
the sensing of Fe3+ by Im-TPE is hardly affected by these
commonly coexistent ions, which shows the fluorescence
response of Im-TPE is selective towards Fe3+ rather than other
Fig. 2a shows the fluorescent spectra of Im-TPE in the
presence of various metal ions (Ag+, Cd2+, Co2+, Cu2+, Mn2+
,
Ni2+, Zn2+, Fe2+, and Fe3+) at the same concentration.
Fig. 3 Fluorescent response of Im-TPE (20 mM) with various other
metal ions. Violet bars represent the addition of various metal ions
(50 mM) to Im-TPE solution (20 mM), black bars represent subsequent
addition of Fe3+ (50 mM) to the former solution. For all measure-
ments, the excitation wavelength was set at 320 nm and the emission
was monitored at 471 nm.
Fig. 2 (a) Fluorescent spectra of Im-TPE (20 mM) in the presence of
various metal ions (50 mM) in THF–water (1 : 99, v/v); (b) the
photograph displays the emissive nature of Im-TPE affected by
various metal ions under UV light illumination (365 nm).
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transition-metal ions investigated. In addition, the anions such
as ClÀ, CH3COOÀ, NO3À, and SO4 show a slight effect on
inset in Fig. 4a, the nonlinear curve of the Stern–Volmer plot
indicates that the mechanism of the quenching process involves
complexation rather than collisional deactivation in this case.11
The possible quenching mechanism is proposed through
experimental phenomenon. At first, no luminescence is
observed when Im-TPE is dissolved in THF solution. Thanks
to the AIE characteristics, strong emission is detected as
Im-TPE is dispersed in a THF–water mixture with 99% water
fraction owing to aggregate formation. The efficient fluorescence
quenching is caused by successive addition of aliquots of Fe3+
ions to the diluted Im-TPE suspension in a THF–water
mixture. Imidazole moiety is an excellent acceptor to Fe3+
and the suspension converted gradually into more water-
soluble Im-TPE–Fe3+ complex. Fig. S4 displays the turbid
Im-TPE suspension in a THF–water mixture with 99% water
fraction, which turns clear when treated with Fe3+.w
Therefore, the aggregated Im-TPE is gradually disassembled,
and the corresponding fluorescence intensity reduces signifi-
cantly. We observed that the molar ratio of Fe3+ ions to the
Im-TPE is about 1.0 when the fluorescence intensity
approaches the minimum (see supporting information,
Fig. S5w), where the aggregate is completely broken.12
The reduction in fluorescent intensity is used to obtain the
constant in a Hill plot through log[(I À Imin)/(Imax À Imin)]
versus log[Fe3+] of the emission data and yield a log Ka of
12.67 (see supporting information, Fig. S3w).13 The result
indicates that after being attached to the TPE, the imidazole
moieties might efficiently chelate iron ions to form a more
water-soluble nonfluorescent complex from the highly emissive
Im-TPE aggregate form. Thus, the strong luminescence of
TPE is quenched. As for a d5 ion such as Fe3+, the five
electrons may present as two orbitals occupied by two pairs of
electrons and one orbital having single occupancy. So it could
form an inner-orbital complex for Fe3+, which is more stable
than the other imidazole–metal complexes.14 Therefore, when
treated with Fe3+, nearly complete fluorescence quenching of
Im-TPE is observed in aqueous solution, due to the disassembly
of the aggregate suspension to the more water-soluble
Im-TPE–Fe3+ complex. So Im-TPE can be utilized as a
selective chemosensor to iron ion at low concentration.
2À
the emission of Im-TPE (see supporting information, Fig. S2w),
which can exclude disturbance from anions. In light of the
data aforementioned, it can be confirmed that Im-TPE is a
useful fluorogenic sensing material for selective detection of
Fe3+ in the presence of other ions.
To study the quenching behavior of Fe3+ ion in details, we
investigated the decrease in fluorescence intensity upon
successive addition of aliquots of aqueous solution of Fe3+
ion to the diluted Im-TPE solution in a THF–water mixture
with 99% water fraction. As shown in Fig. 4a, quenching of
photoluminescence is observed at a very low level of Fe3+, and
the fluorescent intensity decreases rapidly upon the increase in
the concentration of Fe3+. At the very beginning, the Fe3+
ions of only 5 mM, which is only 25% of the Im-TPE
concentration, cause the fluorescence emission of Im-TPE to
quench to 70% of its original emission intensity. When the
concentration of Fe3+ increases up to 30 mM, almost no
luminescence is observed. The emission quenching is observed
at a concentration of Fe3+ as low as 1.0 mM, which is not the
lowest detecting concentration compared to the reported
literature.8 However, it is a promising example of the selective
detection of Fe3+ with efficiently prepared Im-TPE in aqueous
solution based on the AIE characteristics. As shown by the
In order to confirm the above mechanism, an additional
experiment was conducted employing EDTA as a much
stronger chelating agent. Fig. 4b shows that the fluorescent
intensity of Im-TPE–Fe3+ system can be recovered to the
original value of Im-TPE in a THF–water mixture when
treated with EDTA. This indicates that the fluorescence
quenching of Im-TPE induced by the chelating iron ion is
due to the formation of coordination complexes between
Im-TPE and iron ions. Excess EDTA can snatch the Fe3+
ions from the imidazole moieties and interrupt the interaction
between the Fe3+ ions and the imidazole moieties to form new
more stable complexes with a much higher stability constant
(log Ka = 25.1). Due to the much stronger complex binding
between EDTA and Fe3+ ions, the more water-soluble
Im-TPE–Fe3+ complex is destroyed, and the fluorescence of
Im-TPE switches on in the aggregate form. The absorbance
spectrum also confirms that the new chelated complex was
formed by Im-TPE and Fe3+ ions. The broad and flat
absorbance peaks of Im-TPE at 325 and 253 nm indicate
Fig. 4 (a) Fluorescence spectra and Stern–Volmer plot (inset) of
Im-TPE (20 mM) in the absence and presence of Fe3+ ions at different
concentrations in a THF–water mixture (1 : 99, v/v); (b) fluorescence
spectra of Im-TPE (20 mM), Im-TPE–Fe3+ mixture, and Im-TPE–Fe3+
mixture (Fe3+, 50 mM) with addition of EDTA (70 mM) in a THF–water
mixture (1 : 99, v/v) at room temperature.
c
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Experimental section
Synthesis of TPE derivative 3
To a mixture of TPE 2 (2.3 g, 6.3 mmol) and potassium
carbonate (7.09 g, 51.3 mmol) in acetone (20 mL) was added
1,2-dibromoethane (4.5 mL, 51.9 mmol). The mixture was
refluxed under stirring overnight. The reaction mixture was
cooled to room temperature and filtered. After evaporation of
the filtrate, the crude product was purified by a silica gel
column using petroleum ether and ethyl acetate (30 : 1) as
eluent to give compound 3 as a white solid in 68% yield
(2.4 g). 1H NMR (400 MHz, CDCl3): d 7.12–7.07 (m, 6H),
7.04–6.99 (m, 4H), 6.93 (dd, J = 8.8, 12 Hz, 4H), 6.64 (dd,
J = 8.8, 13.2 Hz, 4H), 4.21 (m, 4H), 3.60 (dd, J = 6.3,
13.8 Hz, 4H). 13C NMR (100 MHz, CDCl3): d 156.6, 144.1,
139.9, 137.2, 131.5, 127.8, 114.0, 67.8, 58.6, 29.3.
Fig. 5 Absorbance spectra of Im-TPE (20 mM) in the absence and
presence of Fe3+ in a THF–water (1 : 99, v/v) mixture.
the aggregate state in the aqueous solution. After adding Fe3+
into Im-TPE suspension, the absorbance peaks get sharper
and narrower, and are blue-shifted to 304 and 245 nm,
respectively (Fig. 5). It can be inferred that the aggregated
Im-TPE is disassembled due to formation of the more water-
soluble Im-TPE–Fe3+ complex, which is also an evidence for
the proposed quenching mechanism.
Synthesis of Im-TPE
A solution of imidazole (26 mg, 0.382 mmol) in anhydrous
THF was added to a stirred suspension of sodium hydride
(16 mg, 0.667 mmol) at room temperature. The mixture was
stirred for 1 h and then a solution of 3 (90 mg, 0.156 mmol)
and tetrabutylammonium iodide (56 mg) in THF was added.
The mixture was stirred overnight at room temperature. The
reaction mixture was diluted with water (15 mL) and extracted
twice by ethyl acetate (50 mL). The combined organic layer
was dried with anhydrous sodium sulphate and then concen-
trated. Silica gel chromatography (petroleum ether–ethyl
acetate, 20 : 1) afforded the product as a light yellow solid
with 65% yield (16.2 mg). Mp.: 145–147 1C. 1H NMR
(400 MHz, CDCl3): d 7.66 (s, 2H), 7.10 (m, 8H), 7.01 (m,
6H), 6.98 (d, J = 8.72 Hz, 4H), 6.58 (d, J = 8.72 Hz, 4H), 4.30
(t, J = 5.12 Hz, 4H), 4.14 (t, J = 5.2 Hz, 4H). 13C NMR
(100 MHz, CDCl3): d 156.4, 144.1, 139.8, 137.6, 137.3, 132.7,
131.4, 129.1, 127.9, 126.5, 119.7, 113.7, 67.1, 46.7. FTIR (KBr)
In summary, imidazole-bearing tetraphenylethylene, Im-TPE,
is efficiently prepared and is highly emissive in a THF–water
mixture with a high water fraction (99%) due to the
aggregation-induced emission feature of tetraphenylethylene
moieties. When exposed to Fe3+ ions (50 mM), nearly complete
quenching of the Im-TPE fluorescence is observed. Further
treatment of Im-TPE–Fe3+ mixture with EDTA can make the
fluorescence emission of resulting solution recover almost to
the original value, which proves that the fluorescence quenching
of Im-TPE induced by the formation of more water-soluble
coordinated complexes between Im-TPE and Fe3+ ions.
As a fluorescent probe for iron ion assay in 99% water
solution, owing to its sensitivity and selectivity, Im-TPE will
have potential applications in biosensing and environmental
analysis.
3035, 2932, 2875, 1607, 1509, 1464, 1244, 817, 759, 699 cmÀ1
.
ESI(+)-MS: calcd for C36H32N4O2: 552.6 [M]; found 553.4
[M+H]+.
This work was supported by the Ministry of Science and
Technology of China (National Basic Research Program,
Grant 2007CB808000) and the National Science Foundation
of China (Grants 20972035 and 21002017).
Interference experiment of Im-TPE with different metal ions
To a solution of Im-TPE in a THF–water mixture with 99%
water fraction was added the solution of various metal ions in
the same condition. The final concentration of Im-TPE was
fixed at 20 mM, and the final concentration for the metal ions
was fixed at 50 mM. After addition of the metal ion, the sample
was allowed to incubate at room temperature for 30 min prior
to recording a spectrum. Then, a Fe3+ ion solution (50 mM)
was added to the previous solution and incubated at room
temperature for 30 min to record another spectrum. The
excitation wavelength was fixed at 320 nm and the emission
was monitored at 471 nm.
Materials and instrumentations
All chemical reagents were commercially available and used as
received unless otherwise stated. Deionized water was purified
by a Millipore purification system (Milli-Q water). THF was
dried over sodium and distilled under an atmosphere of dry
nitrogen.
The 1H and 13C NMR spectroscopy was performed by a
Bruker DMX400 NMR spectrometer using deuterated chloro-
form as solvent. UV-visible spectra were obtained using a
Perkin-Elmer Lamda 950 UV-vis-NIR spectrophotometer and
quartz cells with 1 cm path length. The fluorescence spectra
were recorded by Perkin-Elmer LS55 luminescence spectro-
meter in a conventional cell with 1 cm path length. Mass
spectrum was obtained using Fennigan LCQ Deca XP MAX
liquid chromatography-mass spectrometry (LC-MS).
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