Three N-stabilized rhodamine-based fluorescent probes for Al3+ via Al3+-promoted hydrolysis of Schiff bases

Article abstract of DOI:10.1039/c4nj01357c

Three probes for Al³⁺ detection by both fluorescence and the naked eye in acetonitrile solution were developed. The mechanism of fluorescence was based on the aluminum complexation with rhodamine and subsequent Al³⁺-promoted hydrolysis of the Schiff base. Theoretical calculations indicated that the aluminum complexes tend to hydrolyze to compound L₆. A simple paper test strip system for the rapid monitoring of Al³⁺ was developed, indicating its convenient use in environmental samples.

Full text of DOI:10.1039/c4nj01357c

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DOI: 10.1039/C4NJ01357C
influence in their recognition toward Al3+. Own to more stable
structure, L1 shown the best sensing property, so we selected L1
60 as a representation example to expatiate in the following
discussion. To the best of our knowledge, none of rhodamineꢀ
based probes which fluorescent mechanism is based on the Al3+ꢀ
promoted hydrolysis of the Schiff base has been reported for
Three N-stablization rhodamine-
based fluorescent probes for Al3+
via Al3+-promoted hydrolysis of
Schiff base
detection of Al3+ up to now
.
65
5
Peigang Dinga, Jinhui Wanga, Junye Chenga,
N
O
N
N
O
N
N
Yufen Zhaoa-c and Yong Ye a,c*
R
CHO
EtOH, reflux
N
N
NH2
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted
Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
O
O
R
L4
L
: R=NO , H,CH
1-3 2 3
Scheme 1. Synthetic route of target compounds.
10 New fluorescent probes L1ꢀL3 displayed high sensitivity toward
Al3+
Experimental
70 Apparatus
Fluorescence spectra measurements were performed on a
HITACHI Fꢀ4500 fluorescence spectrophotometer, and the
excitation and emission wavelength band passes were both set at
5.0 nm. Absorption spectra were measured on a UVꢀ2102 doubleꢀ
75 beam UV/VIS spectrometer, Perkin Elmer precisely. NMR
spectra were recorded on a Bruker DTXꢀ400 spectrometer in
CDCl3, using TMS as internal standard. Mass spectral
determination was carried on a HPLC QꢀT of HRꢀMS.
Materials
Introduction
As is known to all, aluminum is the third most abundant
element in the earth’s crust. It widely exists in the environment
15 due to acidic rain and is considered to be toxic in biological
activities.1ꢀ3 Regarding toxicological effects of aluminum, its
primary targets are different from those of heavy metals.4 The
widespread use of aluminum in water treatment, as a food
additive, and in many industrial activities including the
20 manufacturing of cars and computers often exposes people to this
metal.5,6 Excessive exposure of the human body to Al3+ leads to
the malfunction of central nervous system such as Alzheimer’s
disease and Parkinson’s disease.7,8 The WHO recommended the
average daily human intake of Al3+ of around 3−10 mg and
25 weekly tolerable dietary intake as 7 mg kg−1 body weight.
Therefore, trace level determination of Al3+ is highly important.
In the last decade, many methods such as graphite furnace
atomic absorption spectrometry, atomic emission spectrometry,
inductively coupled plasma atomic emission spectrometry and
30 electrochemical methods have been used for aluminum detection.
All of these techniques are generally timeꢀconsuming and
expensive as well. In contrast, optical detection via fluorescence
is an operationally easy technique besides being highly sensitivity.
Due to great changes in the absorption and/or fluorescent spectra
35 of many organic molecules after coordinating with ions,
colorimetric and fluorescence analytical methods are very good
and effective ways to detect metal ions.9ꢀ15
80
All the materials for synthesis were purchased from
commercial suppliers and used without further purification. The
solutions of metal ions were prepared from their nitrate salts,
except for FeCl3, FeCl2, CrCl3 and MnCl2. The metal ions were
prepared as 10.00 mM in water solution.
85 Syntheses
As shown in Scheme 1, the compounds L1-3 were prepared
by reacting L4 with aromatic aldehyde. L4 was synthesized
according to literature.27
L1
90
L4 (0.5 mmoL, 0.24 g) was dissolved in 25 mL ethanol, and
then pꢀnitrobenzaldehyde (1 mmoL, 0.15 g) was slowly added.
The mixture was stirred and refluxed for 12 h at 80 . After
distillation in vacuum, the residue was recrystallized with
methanol and water to give the final product L1 in yield of 78.4%.
95 1H NMR (400 MHz, CDCl3, δ ppm): 8.17 (d, J = 8.0 Hz, 2H),
8.06 (s, 1H), 7.91ꢀ7.90 (m, 1H), 7.72 (d, J = 8.4 Hz, 2H), 7.43 (s,
1H), 7.03 (d, J = 4.0 Hz, 1H), 6.44 (d, J = 8.4 Hz, 1H), 6.28 (s,
2H), 6.14ꢀ6,12 (m, 1H), 3.52ꢀ3.49 (m, 1H), 3.25 (s, 8H), 1.99 (s,
8H); 13C NMR (100 MHz, CDCl3, δ ppm): 168.5, 159.8, 153.9,
100 153.1, 148.8, 148.7, 141.7, 132.6, 130.9, 128.9, 128.8, 128.1,
123.7, 123.6, 122.8, 108.4, 105.7, 98.0, 65.2, 59.3, 47.6, 41.1,
25.5. HRꢀMS: C37H35N5O4 [M+H]+, calcd for 614.2762. Found:
614.2767. (Supporting Information, Figs. S1–S3);
The rhodamine moiety has been used widely in the field of
chemosensors, especially as
a chemodosimeter, given its
40 fluorescence off–on behavior that results from its particular
structural properties. Upon metal binding, its structure can
undergo a change from the spirolactam to an open ring amide,
resulting in a magentaꢀcolored, highly fluorescent compound16.
While some rhodamineꢀbased chemosensors for Al3+ ions
45 have been reported to date17ꢀ26, dual colorimetric and fluorescent
chemosensors for Al3+ were still rare and some of them were not
efficient enough to be selective toward Al3+ or sensed it in
organic solvents. Therefore, developing sensors which are able to
detect Al3+ by both fluorescence and naked eye in aqueous
50 solution are very valuable. In this paper, we report three new Nꢀ
stablization rhodamineꢀbased fluorescent probes which exhibited
sensitive detection toward Al3+ via significant fluorescence
enhancement in solution, and, at the same time, showed a
significant color change from colorless to red. Similar structures
55 had been designed to compounds L1, L2 and L3. By comparing
with each other, we could know that the different electronic
distribution among the chemodosimeters’ structures had great
L2
105
Compound L2 was prepared using a general procedure
which is essentially similar to that used for L1. Yield of L2: 69.3%.
1H NMR (400 MHz, CDCl3, δ ppm): 8.01 (s, 1H), 7.92ꢀ7.90 (m,
1H), 7.58ꢀ7.56 (m, 2H), 7.44ꢀ7.39 (m, 2H), 7.36ꢀ7.31 (m, 3H),
7.05ꢀ7.03 (m, 1H), 6.46 (d, J = 8.8 Hz, 2H), 6.29 (s, 2H), 6.16ꢀ
110 6.14 (m, 2H), 3.44 (s, 4H), 3.25 (t, J = 3.2 Hz, 8H), 2.01ꢀ1.98 (m,
8H); 13C NMR (100 MHz, CDCl3, δ ppm): 168.4, 162.4, 154.0,
153.1, 148.7, 132.4, 130.4, 128.9, 128.4, 128.1, 128.0, 123.7,
122.8, 108.4, 105.8, 98.0, 65.1, 59.0, 47.6, 41.3, 25.5. HRꢀMS:
C37H36N4O2 [M+H]+, calcd for 569.2911. Found: 569.2910.
115 (Supporting Information, Figs. S4–S6).

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DOI: 10.1039/C4NJ01357C
L3
enhancement of absorbance at about 562 nm simultaneously the
color changed into red. Under the identical condition, no obvious
response could be observed upon the addition of other ions
The following compound was prepared using a general
procedure which is essentially similar to that used for L1. Yield of
L3: 62.7%. H NMR (400 MHz, CDCl3, δ ppm): 8.03 (s, 1H), 55 including Zn2+, Mg2+, Ca2+, Cd2+, Cu2+, Pb2+, Hg2+, Ba2+, Ni2+,
1
5
7.94ꢀ7.93 (m, 1H), 7.49 (d, J = 7.6 Hz, 2H), 7.45ꢀ7.43 (m, 2H),
7.15 (d, J = 7.6 Hz, 2H), 7.07ꢀ7.05 (m, 1H), 6.49 (d, J = 8.8 Hz,
2H), 6.32 (s, 2H), 6.17 (d, J = 8.4 Hz, 2H), 3.77ꢀ3.71 (m, 4H),
3.46ꢀ3.29 (m, 8H), 2.36 (s, 3H), 2.02 (s, 8H); 13C NMR (100
MHz, CDCl3, δ ppm): 168.3, 162.3, 154.1, 153.1, 148.7, 140.6,
Fe2+, Mn2+, K+, Li+, Ag+, Co2+ and Na+ except for Fe3+ and Cr3+,
which caused a mild effect compared to Al3+. This interesting
feature demonstrated that compound L1 can serve as a selective
“nakedꢀeye” chemosensor for Al3+.
10 133.7, 132.4, 131.1, 129.1, 128.9, 128.1, 127.9, 123.7, 122.8,
108.4, 105.8, 98.0, 65.1, 59.0, 47.6, 41.3, 25.5, 21.5. HRꢀMS:
C38H38N4O2 [M+H]+, calcd for 583.3068. Found: 583.3074.
(Supporting Information, Figs. S7–S9).
15 Results and discussion
The structure of title compounds L1, L2 and L3 were
1
characterized by H NMR, 13C NMR and HRꢀMS. The results
were in good agreement with the structure showed in Scheme 1.
Fluorescence and UV–vis studies were performed using a 10 ꢁM
20 solution of L1, L2 and L3 in a CH3CN/H2O (95:5, v/v) solution
with appropriate amounts of metal ions. Solutions were shaken
for 120 min before measuring the absorption and fluorescence in
order to make the metal ions chelate with the sensors sufficiently.
Steady-state optical properties
60
65
Fig.2. Absorbance spectra of L1 (10 ꢁM) in CH3CN/H2O (95:5,
v/v) solution with the presence of 10 equiv. of various species.
Inset: the photos of L1 with different metal ions in CH3CN/H2O
(95:5, v/v) solution.
25
Compared with L2+Al3+ and L3+Al3+ L1+Al3+ had higher
,
fluorescence quantum yield (Fig. S10). This means that the strong
electron withdrawing group on benzene benefiting to the chelate
of Al3+ with sensors. The effect of the reaction media for the
binding of L1 with Al3+ was studied, and the results were shown
30 in Fig S11. It was found that solvent have great effect on the
coordination reaction. When the coordination reaction was
To further investigate the interaction of Al3+ and L1, an
ultraviolet titration experiment was carried out (Fig. 3). The Al3+
binding stoichiometry of L1 can be determined from titration and
the Job plot28. A plot of [Al3+]/{[ Al3+] + [L1]} versus the molar
performed in acetonitrile–water solution, high F/F0 and
△
A
70 fraction of Al3+ was provided in Fig. 4. The absorbance reached a
maximum when the ratio was 0.5, indicating a 1:1 stoichiometry
of the Al3+ to L1 in the complex.
values were obtained, indicating that acetonitrileꢀwater media is
favorable for fluorescent measurement. Therefore, acetonitrile–
35 water solution was selected for fluorescent assay and the
colorimetric assay, respectively. As shown in Fig. 1, L1 exhibited
a 190ꢀfold enhancement of fluorescence intensity at peak
wavelength λmax=582 nm in the presence of 10 equiv. Al3+, so we
selected L1 as the representation when expatiating the characters
40 of the three compounds in the following discussion.
75 Fig. 3. Absorbance spectra of L1 (10 ꢁM) in CH3CN/H2O (95:5,
v/v) upon addition of different amounts of Al3+ ions.
Fig. 1. F: Fluorescence intensity (at 582 nm) of L1, L2 and L3
(10ꢁM) in CH3 CN/H 2O (95/5, v/v) with the presence of Al3+
45 (100 ꢁM) (λex = 520 nm); F0: Fluorescence intensity (at 582 nm)
of L1, L2 and L3 (10 ꢁM) only in CH3 CN/H 2O (95/5, v/v) (λex =
520 nm).
UV–vis spectral responses of L1
As shown in Fig. 2, UV–vis spectrum of compound L1
50 (10ꢁM) exhibited only very weak bands over 400 nm. Addition of
10 equiv. Al3+ into solution immediately resulted in a significant

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DOI: 10.1039/C4NJ01357C
30
Fig. 4. Job’s plots of the complexation between L1 and Al3+. Total
concentration of L1+Al3+ was kept constant at 100 ꢁM.
Fig. 6. Fluorescence spectra of L2 (10 ꢁM) in CH3CN/H2O (95:5,
v/v) with the presence of 10 equiv. of various metal ions (λex =
520 nm, slit = 5 nm).
Fluorescence spectral responses of L1
5
As shown in Fig. 5, L1 exhibited a very weak fluorescence in the
absence of metal ions. When 10 equiv. Al3+ was introduced to a
10 uM solution of L1 in CH3CN/H2O (95:5, v/v), a remarkably
enhancement of fluorescence spectra was observed. The
fluorescence enhancement of Al3+ to compound L1 was as high as
10 190ꢀfold. Under the same condition,
a mild fluorescence
enhancement factors was also detected for Fe3+and Cr3+, but Zn2+,
Mg2+, Ca2+, Cd2+, Cu2+, Pb2+, Hg2+, Ba2+, Ni2+, Fe2+, Mn2+, K+,
Li+, Ag+, Co2+ and Na+ showed no obvious changes on
fluorescence intensity and color. From fluorescence spectra of L2
15 and L3, we could know Fe3+ and Cr3+ had more interference (Fig.
6-7). Probe L1 had better fluorescence property. It maybe benefit
from the strong electron withdrawing group. Moreover, we also
confirmed the competitive experiments that the background metal
ions showed very low interference with the detection of in Al3+
20 water solution (Fig. 8). Generally, the detection limit of metal
ions is needed for fluorescence sensor. Under optical conditions,
the linear response for the fluorescence intensity response of
compound L1 was between 8 and 18 ꢁM (Fig. 9), and the
detection limit of Al3+ was measured to be 3.98 ꢁM.
35
Fig. 7. Fluorescence spectra of L3 (10 ꢁM) in CH3CN/H2O (95:5,
v/v) with the presence of 10 equiv. of various metal ions (λex =
520 nm, slit = 5 nm).
25
40
Fig.8. Fluorescence intensity (at 582 nm) of L1 upon the addition
of 100 ꢁM Al3+ in the presence of 100ꢁM background metal ions
in CH3CN/H2O (95/5, v/v) (λex = 520 nm, slit = 5 nm).
Fig. 5. Fluorescence spectra of L1 (10 ꢁM) in CH3CN/H2O (95:5,
v/v) with the presence of 10 equiv. of various metal ions (λex =
520 nm, slit = 5 nm).

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which corresponded to [L1 + H]+ (Fig.S3). The form of L6ꢀAl3+
complex was also confirmed by ESIꢀMS analysis. As shown in
Fig.S14, the [M+H]+ of L6 appeared at m/z 481.4. When 1 equiv.
40 of Al3+ was added to L1 in H2O/CH3CN, the ESIꢀMS spectra
showed the peak of L6ꢀAl3+ complex. The signal at m/z 629.4
(calculated value, 629.4) correspond to [L6+Al3++2HCOOꢀ
+CH3OH]+. Both UVꢀvis and fluorescence data lead to a
significant OFFꢀON signal. From the molecular structure and
45 spectral results of L1, an irreversible fluorescent
chemodosimeters for Al3+ was constructed as shown in Scheme 2.
Firstly, the addition of the Al3+ ion induced a ring opening of the
spirolactam of rhodamine took place. Then, L1-Al was
hydrolyzed into pꢀnitrobenzaldehyde. And it was certified by
50 theoretical calculation (the data was supplied in Supporting
Information). The DFT calculations were performed using the
Gaussian 09 program[29]. The structures of L5, H2O, L6 and pꢀ
nitrobenzaldehyde were optimized at the B3LYP[30ꢀ32]/6ꢀ31G(d)
level in acetonitrile solvent, using the integral equation formalism
55 polarizable continuum model (IEFꢀPCM)[33ꢀ34]. The calculated
results demonstrate that the free energies of L6 and pꢀ
nitrobenzaldehyde is 14.7 kcal/mol lower than those of L5 and
H2O, indicating that this hydrolysis step is an exothermic process.
So in the reaction, L5 tends to hydrolyze to L6 (Fig.12.). From
60 Fig.5, we know that both Cr3+ and Fe3+ can induce the moderate
fluorescence enhancements to L1. The mechanism of L1 with
Fe3+ and Cr3+ were thought to be similar to that of L1 with Al3+.
1H NMR spectra of the complex of L1 + Fe3+ and L1 + Cr3+ was
operated (Fig. S16 and Fig. S18). The same new single peak was
65 observed at 10.179ppm and 10.138ppm, respectively. The ESI
mass spectra of L1+Fe3+ and L1+Cr3+ in CH3CN/H2O (95/5, v/v)
was conducted (Fig. S17 and Fig. S19) and similar hydrolysis
product was found. A kind of Fe3+ꢀinduced Schiff base hydrolysis
mechanism has been reported by Kim and coworkers35.
Fig. 9. The fluorescence intensity (at 582 nm) of compound L1
(10 ꢁM) as a function of the Al3+ concentration in CH3CN/H2O
(95/5, v/v) solution (λex = 520 nm, slit = 5 nm).
5
Fig. 10. Fluorescence intensity (at 582 nm) of L1 (10 ꢁM) to Al3+
in CH3CN/H2O (95:5,v/v) solutions (1) Baseline: 10 ꢁM L1 only;
(2) red line: 10 ꢁM L1 with 10 equiv. Al3+; (3) green line: 10 ꢁM
L1 with 10 equiv. Al3+ and then addition of 30 equiv. Fꢀ; (4) blue
10 line: 10 ꢁM L1with 10 equiv. Al3+ and 30 equiv. Fꢀ then addition
of 10 equiv. Al3+ (λex = 520 nm, slit = 5 nm).
a
Mechanism
To investigate the Al3+ enhancement mechanism, IR spectra
15 of L1 and L1 + Al3+ were taken in KBr disks (Fig.S15.). The
peak at 1694 cm−1, which corresponds to the amide carbonyl
absorption disappeared upon with the addition of Al3+. This
supported the notion that the carbonyl group of L1 is involved in
the coordination of metal ions.
b
20
In addition, the Fꢀꢀadding experiments were conducted to
examine the reversibility of this reaction and the result was
shown in Fig. 10. When Fꢀ (3 equiv. of Al3+) was added to the L1
+ Al3+ CH3CN/H2O solution, the fluorescence intensity at 582 nm
was decreased (green line) and further addition of 10 equiv. Al3+
70
Fig. 11. 1H NMR spectra of L1 + Al3+ (a) and L1 (b) in CDCl3.
In order to investigate the influence of the different acid
concentration on the spectra of L1 and find a suitable pH
75 span in which L1 can selectively detect Al3+ efficiently, the
acid titration experiments were performed. The addition of Al3+
led to the fluorescence enhancement over a wide pH range (1.0–
8.0), which is attributed to opening of the rhodamine spirolactam
structure (Fig.S20). Moreover, a time course of the fluorescence
80 response of L1 upon addition of Al3+ was shown in Fig.S21.
The kinetics of fluorescence enhancement at 593 nm by the
newly developed fluorescent probe was recorded. It indicated that
this reaction between L1 and Al3+ was slow. And, the recognizing
event of L1 with Fe3+ could complete in 80 minutes (Fig.S22).
85 The chemodosimeters L2 and L3 show similar properties as L1
(see supporting information Figs. S23–S34). The detection
25 could not recover the fluorescence (blue line). To investigate the
1
fluorescence quenching mechanism, the H NMR spectra of the
complex of L1 + Al3+ was operated. As it was seen from the Fig.
11, a new single peak was observed at 10.2 ppm when Al3+ was
added to L1. Comparing the 1H NMR spectra of pꢀ
1
30 nitrobenzaldehyde with H NMR spectra of the complex of L1 +
Al3+, it can be confirmed that the new single peak is belong to the
aldehyde proton (Ha) of pꢀnitrobenzaldehyde (Fig. S12). The HRꢀ
MS of L1 + Al3+ in CH3CN/H2O (95/5, v/v) was also conducted
(Fig. S13). An unique peak at m/z 481.2602 corresponding to [L6
35 + H]+ was clearly observed when 1 equiv. of Al3+ was added to
L1 , whereas L1 without Al3+ exhibited peaks only at m/z 614.2

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limits of L2 and L3 were measured to be 32 ꢁM (Fig. S27) and
49 ꢁM (Fig. S34). The three chemodosimeters exhibit irreversible,
selective and sensitive recognition toward Al3+ over other metal
ions. The colorimetric and fluorometric responses between the
sensors L1-L3 and Al3+ can also be conveniently detected by the
naked eye. The fluorescence enhancement of Al3+ to L1, L2 and
L3 is as high as 190, 120 and 60ꢀfold, respectively. These indicate
the differences among the three chemodosimeters’ structures have
great influence in their recognition toward Al3+. Due to the strong
5
10 electron withdrawing group on benzene ring that result in Schiff
base having a good affinity toward Al3+.
40
N
O
N
N
O
N
N
Al3+
N
Fig. 13 Photographs of the test kits with L1 for detecting Al3+ in
(CH3CN/H2O = 95:5, v/v) solution with different concentrations:
(1) 0; (2) 1.0 × 10ꢀ4 M; (3) 1.0 × 10ꢀ3 M; (4) 1.0 × 10ꢀ2 M.
N
N
NO2
Al3+
L5
O
O
NO2
L
1
N
O
N
H2O
NO2
OHC
N
O
45 Conclusions
In summary, we synthesized three fluorescent chemodosiꢀ
NH2
Al3+
L6-Al
Scheme 2. Possible sensing mechanism of L1 with Al3+.
meters L1, L2 and L3. The colorimetric and fluorescent response
to Al3+ can be conveniently detected even by the naked eye,
which provides a facile method for visual detection of Al3+.
50 Theoretical calculation indicated that the electronic distribution
of the N atom of C=N is ꢀ0.436, ꢀ0.46 and ꢀ0.464, respectively
(log file). Own to the strong electron withdrawing group on
benzene, the Schiff base L1 changed stable, which was favor for
high sensitivity for the detection of Al3+. As we know, higher
55 electronegativity of N atom of C=N of Schiff base is easier to
hydrolyze. Furthermore, the mechanism of fluorescence was
found to be the aluminum complexation with rhodamine and
subsequent Al3+ꢀpromoted hydrolysis of the Schiff base. Also,
Iron and chromium complexation had the same hydrolysis
60 mechanism. The simple and convenient test paper may provide an
easy way to detect Al3+ in our daily life.
15 Fig.12. Calculated energyꢀminimized structure of L5, L6 and pꢀ
nitrobenzaldehyde (gray: C atoms; blue: N atoms; red: O atoms;
green: Clꢀ; brown: Al3+).
Many fluorescent sensors for Al3+ detection could only be
20 performed in solution, which would limit their applications under
special circumstances such as onꢀsite detection in situ. To
demonstrate the practical application of our sensor, we prepared
the test papers of sensor L1. It was easily prepared by immersing
a filter paper into the solution of L1 in CH2Cl2 (1 mM) and then
25 drying in air. Next, to different Al3+ concentration solutions（0,
1.0 × 10ꢀ4 M, 1.0 × 10ꢀ3 M, 1.0 × 10ꢀ2 M）, these strips were
immersed for 5 s and taken out of the solution. As depicted in Fig.
13, the color of the test paper changed from colorless to purple
and deepened gradually with the increasing of Al3+ concentration.
30 These paperꢀmade test kits may be used as a simple tool for
detecting Al3+ in environmental samples. To validate its
practicality in real environmental samples, we employed probe L1
in a standard addition method36ꢀ38 to determine Al3+ concenꢀ
trations in water samples from Jinshui River (in Zhengzhou,
35 Henan province, China), no fluorescence enhancement was
observed. When the water samples were spiked with different
Al3+ concentrations (25 ꢁM, 50 ꢁM, and 100 ꢁM) and measured
with the current methods, Al3+ recoveries were about 20% (Table
S1).
Acknowledgments
This work was financially supported by the National Science
65 Foundation of China (No. 21375113) and Program for New
Century Excellent Talents in University (NCETꢀ11ꢀ0950).
Notes and references
a Phosphorus Chemical Engineering Research Center of Henan Province,
the College of Chemistry and Molecular Engineering, Zhengzhou
70 University, Zhengzhou 450052, China. Fax:86 371 67767051; Tel: 86
371 67767050; E-mail: yeyong03@tsinghua.org.cn
b
Department of Chemistry and Key Laboratory for Chemical
biology of Fujian Province, Xiamen University, Xiamen 361005,
China
c
75 Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical
Biology(Ministry of Education), Department of Chemistry, Tsinghua
University, Beijing 100084, China
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
80 DOI: 10.1039/b000000x/
‡
1
2
3
4
* Corresponding author.
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