Perylenebisimide-based fluorescent chemosensors for selective detection of Zn2+ in aqueous solution

Article abstract of DOI:10.2174/157017812802139663

Novel perylenebisimide-based fluorescent chemosensors PBI-5 and PBI-6 were designed and synthesized. Their selective recognition behavior toward metal ions was investigated by visual colorimetry and fluorescence spectroscopy. PBI-5 successfully exhibits a

Full text of DOI:10.2174/157017812802139663

Letters in Organic Chemistry, 2012, 9, 503-508
503
Perylenebisimide-based Fluorescent Chemosensors for Selective Detection
of Zn2+ in Aqueous Solution
Xuefeng Conga , Ningjie Wua, Lei Mab, Mingliang Lia, Rongqi Zhoua, Ge Gaoa, Jingbo Lana,*
aKey Laboratory of Green Chemistry and Technology of Ministry of Education, College of Chemistry, Sichuan Univer-
sity, 29 Wangjiang Road, Chengdu 610064, P. R. China
bCenter for Drug Evaluation, State Food and Drug Administration, Beijing 100038, P. R. China
Received December 13, 2011: Revised July 3, 2012: Accepted July 03, 2012
Abstract: Novel perylenebisimide-based fluorescent chemosensors PBI-5 and PBI-6 were designed and synthesized.
Their selective recognition behavior toward metal ions was investigated by visual colorimetry and fluorescence spectros-
copy. PBI-5 successfully exhibits a fluorescence turn-on response for Zn2+ and switch-off response for Cu2+ in neutral
buffer solution, but the fluorescence turn-on response is moderately perturbed by Cd2+ and Pb2+. PBI-6 can effectively
eliminate or weaken Pb2+ and Cd2+ induced interference and shows a better selectivity for Zn2+. Live cell imaging experi-
ments indicates that both PBI-5 and PBI-6 can successfully enter the cell and apply in cell fluorescence imaging.
Keywords: Cell fluorescence imaging, colorimetry assay, fluorescent chemosensor, perylenebisimide, photoinduced electron
transfer, selective recognition.
INTRODUCTION
analysis [5]. Design and synthesis of organic sensing mole-
cules that either “turn on” or “switch off” their fluorescence
in the presence of a particular metal ion has been drawn in-
creasing attention in the areas of chemical and biological
sciences [6]. Recently, a variety of fluorescence sensors con-
taining quinoline [7], fluorescein [8], porphyrin [9], schiff-
bases [10], calixarene [11], and peptides [12] have been de-
signed and synthesized for the selective detection of free
zinc ion. However, due to fluorescence quenching, the de-
pendence on organic solvents or, more specifically, the inter-
ference from other metal ions, there are still in great demand
to develop new fluorescent sensors with more versatile and
improved performance.
Zinc is an essential constituent of more than two hundred
metalloenzymes found in the human body and the second
most abundant transition metal after iron, which plays key
roles in the synthesis and stabilization of genetic regulators
and is necessary for cell division and the synthesis and deg-
radation of carbohydrates, lipids and proteins [1]. Zinc defi-
ciency may cause unbalanced metabolism, which is impli-
cated in a number of severe effects including poor prenatal
development, growth retardation, mental retardation, im-
paired nerve conduction, reproductive failure, susceptibility
to infections, etc [2]. Whereas Zn2+ in excess of physiologi-
cal need could also cause poisoning and the symptoms of
acute zinc ion toxicity include abdominal pain, nausea, vom-
iting, lethargy, anaemia and dizziness. Meanwhile, some
work has also been undertaken suggesting that high levels of
zinc may perturb calcium, magnesium, copper and iron ab-
sorption [3]. Thus fast and accurate detection of zinc ion
becomes increasingly important. Atomic absorption spectro-
photometry is the most widely used method for the determi-
nation of zinc, whereas this methodology can usually only
measure the total zinc concentration. Moreover, the enrich-
ment step is necessary when zinc content is too low to be
directly determined [4], and thus straightforward and direct
estimation of free zinc ion in environment and biological
samples remains challenging.
Perylenebisimide (PBI) and its derivatives are a class of
privileged organic dyes, which have excellent light fastness,
high fluorescence quantum yields, good chemical and ther-
mal stability, and versatile reactivity, and offer an opportu-
nity for the construction of a series of fluorescent chemosen-
sors [13]. PBI-based chemosensors have been designed and
widely applied for the optical detection of variety of anions,
cations and biological molecules in recent years [14]. Re-
cently, we reported two kinds of fluorescent sensors based
on PBIs utilizing cyclen (1,4,7,10-tetraazacyclododecane)
and DPA (di-2-picolylamine) as receptor (the recognition
site), which can serve as highly selective and sensitive indi-
cators for the detection of Pb2+ and Cu2+ respectively [15].
We wanted to know that the different roles of cyclen and
DPA in the detection of metal ions, and what would happen
if cyclen and DPA were together installed onto the PBI-
chromophore. Therefore, in this report we present the design
and synthesis of cyclen/DPA-bifunctionalized perylene-
bisimide PBI-5 and di-DPA-functionalized perylenebisimide
PBI-6 and the spectroscopic investigations toward different
metal ions.
Organic small molecule fluorescence probe can offer
many advantages such as high sensitivity, simplicity,
tunability, and low cost, especially for real-time and online
* Address correspondence to this author at the College of Chemistry, Si-
chuan University, Chengdu 610064, P. R. China; Tel: 86-28-85412285;
Fax: 86-28-85412285; E-mail: jingbolan@scu.edu.cn
1875-6255/12 $58.00+.00
© 2012 Bentham Science Publishers

504 Letters in Organic Chemistry, 2012, Vol. 9, No. 7
Cong et al.
Cl Cl
O
N
O
O
N
O
R
N
N
N
N
R
N
N
N
R
Boc
N
Boc
Boc
Cl Cl
N
N
4
R = Boc
Cl Cl
TFA, CH2Cl2
RT, 12 h
N
O
O
O
O
O
O
PBI-5.6TFA R = H
Toluene, 110 oC
24 h
H2N
2
+
Cl Cl
O
N
O
O
N
O
N
H2N
N
Cl Cl
N
N
N
N
N
N
N
1
3
Cl Cl
PBI-6
Scheme 1. Synthetic routes of PBI-5 and PBI-6.
Fig. (1). (a) Fluorescence responses of PBI-5 (2 ꢀM) in 10 mM HEPES (pH = 7.2) to various concentrations of Zn2+ (0-100ꢀM). The inset of
(a) shows the relative emission intensity (ꢀmax = 559 nm) vs. the concentration of Zn2+. (ꢀex = 523 nm). (b) Fluorescence responses of PBI-
5 (2 ꢀM) in 10 mM HEPES (pH = 7.2) to 20 ꢀM of different metal ions (gray bars), followed by the addition of 20 ꢀM Zn2+ (black bars): 1,
no metal ion; 2, Cd2+; 3, Pd2+; 4, Na+; 5, Mg2+; 6, Cu2+; 7, Hg2+; 8, K+; 9, Ca2+; 10, Cr3+; 11, Ni2+; 12, Ba2+; 13, Fe2+; 14, Fe3+; 15, Mn2+; 16,
Ag+. (ꢀex = 523 nm).
RESULTS AND DISCUSSION
excitation at 523 nm (see Fig. S1 in Supplementary Mate-
rial).
The new fluorescent host molecule PBI-5 was synthe-
sized as shown in Scheme 1. Reaction of 1,6,7,12-
tetrachloro-3,4,9,10-perylenetetracarboxylic dianhydride (1)
with 10-(2-aminoethyl)-1,4,7,10-tetraazacyclododecane-1,4,
7-tricarboxylate tri-tert-butyl ester (2) and N, N-bis(2-
pyridylmethyl)ethylenediamine (3) in one-pot gave 4 [16],
the Boc group of which was deprotected with TFA to afford
PBI-5, which was isolated as PBI-5·6TFA.
A fluorescence titration of Zn2+ with the fluorescent sen-
sor PBI-5 was performed. As shown in Fig. (1a), the metal-
free form of PBI-5 in 10 mM HEPES buffer exhibited weak
fluorescence emission, which may be due to the photoin-
duced electron transfer (PET) from the donor nitrogen atoms
to the acceptor PBI-chromophore. Upon addition of Zn2+, the
fluorescence intensity of PBI-5 increased gradually with the
retardation of photoinduced electron transfer by chelation of
nitrogen atoms of cyclen and DPA to Zn2+, with no shifting
in the excitation and emission maxima. About 10 equiv of
Zn2+ was required for the saturation of the fluorescence in-
tensity under the titration conditions, which indicated an
equilibrium driven coordination interaction. The Job’s plot
showed that PBI-5 formed a 1:1 complex with Zn2+ (see Fig.
S2 in Supplementary Material). While the ESI-TOF mass
spectrum of a mixture of PBI-5 with Zn(ClO4)2 revealed the
formation of a 1:2 complex through the metal coordination
In neutral buffer solution, PBI-5 exhibits a good solubil-
ity because the partial nitrogen atoms in its cyclen unit are
protonated and form ammonium salts [17]. Spectrum meas-
urements with PBI-5 were performed under simulated
physiological conditions (10 mM HEPES, buffer pH = 7.2),
which would eliminate the disturbance of protonation in-
duced by pH effect. The UV/Vis spectrum of PBI-5 in 10
mM HEPES buffer solution exhibits a maximal absorption
wavelength at 526 nm, and the fluorescence spectrum of
which gives a maximal emission wavelength at 559 nm upon

Perylenebisimide-based Fluorescent Chemosensors
Letters in Organic Chemistry, 2012, Vol. 9, No. 7
505
Fig. (2). (a) Fluorescence responses of PBI-6 (2 ꢁM) in 10 mM HEPES/DMSO (v/v = 90/10, pH = 7.2) to various metal ions (20 ꢁM). The
inset of (a) shows the colorimetry assay of PBI-6 (30 μM) in 10 mM HEPES/DMSO (v/v = 90/10, pH = 7.2) under a UV lamp (365 nm) by
addition of 10 equiv. various metal ions (From left to right: no metal ion, Mg2+, Ag+, Cu2+, Pb2+, Zn2+, Fe3+, Cr3+, Hg2+, Cd2+). (b) Fluores-
cence responses of PBI-6 (2 ꢁM) in 10 mM HEPES/DMSO (v/v = 90/10, pH = 7.2) to various concentrations of Zn2+ (0-200ꢁM). (ꢀex = 500
nm).
also generated a measurable response of PBI-5 in fluores-
cence enhancement, which was consistent with the results of
our previous studies [15a]. Therefore, we assumed that Pb2+-
induced interference arose primarily out of the change of
aggregation tendency of probe molecules due to the chela-
tion of cyclen unit to Pb2+. Moreover, it was noteworthy that
when 10 equiv of Pb2+ and Zn2+ were added together to the
buffer solution of PBI-5, the fluorescence intensity of which
was remarkably enhanced up to 7-fold, which was about the
sum of the fluorescence enhancement induced by Pb2+ and
Zn2+, respectively. This result suggests that Pb2+ and Zn2+
involve in the coordination with cyclen and DPA respec-
tively. The co-ordination of cyclen with Pb2+ depress the
aggregation tendency of host molecules, and the coordina-
tion of DPA with Zn2+ retard the photoinduced electron
transfer from nitrogen atoms in DPA unit to the PBI-
chromophore. Both coordination interactions result in en-
hancement of fluorescence intensity, and thus the fluores-
cence enhancement appears a cumulative effect.
To avoid the interference of Pb2+ and further improve
the selectivity of the sensor, we prepared the symmetrical di-
DPA-functionalized perylenebisimide PBI-6 (Scheme 1).
Reaction of 1,6,7,12-tetrachloro-3,4,9,10-perylenetetra-
carboxylic dianhydride (1) with N, N-bis(2-pyridyl-
methyl)ethylenediamine (3) afforded PBI-6 easily. The wa-
ter-solubility of PBI-6 was far lower than that of PBI-5. This
fact indicates again that the partial nitrogen atoms of cyclen
are protonated in neutral water solution which results in in-
crease of the water-solubility of PBI-5.
interaction to cyclen and DPA respectively, where there was
a major signal at m/z 558.5483 and 1216.0428 assigned to
the species [PBI-5+2Zn+2H2O]2+ and [PBI-5+2Zn+2H2O
+ClO4]+ respectively (see Fig. S3 in Supplementary Mate-
rial). The possible reason is that the fluorescence response of
PBI-5 to Zn2+ should only be attributed to the chelation of
DPA to Zn2+. Although cyclen has been recognized to be a
versatile metal chelator, the chelation of cyclen to Zn2+ has
not changed emission of PBI-chromophore. Our previous
studies had also shown that Zn2+ did not make fluorescence
responses of cyclen-functionalized perylenebisimide deriva-
tives [15a].
The selectivity of the fluorescence response of PBI-5 to a
range of physiological and environmentally relevant metal
ions was then examined and the results were depicted in Fig.
1b (and see Fig. S4 in Supplementary Material). It was ob-
served that PBI-5 (2 μM) showed no significant response
when treated with 10 equiv of different metal ions including
Na+, K+, Mg2+, Ca2+, Ba2+, Fe2+, Fe3+, Ni2+, Mn2+, Cr3+, Cu2+,
Ag+, and Hg2+. However, the addition of Zn2+ resulted in a
remarkable increase in fluorescence intensity. Importantly,
the presence of Na+, K+, Mg2+, Ca2+, Fe2+, Fe3+, and Cu2+,
which were highly prevalent species in biological systems,
did not induce any obvious increase in fluorescence inten-
sity. The addition of Cu2+ resulted in fluorescence quenching
due to its powerful coordination with nitrogen-containing
ligands and the recognized role as a fluorescence quencher
[18]. The competition experiment also revealed Cu2+ could
limit the turn-on Zn2+ response of PBI-5. However, in a
sense, it was suggested that PBI-5 could be employed as a
fluorescent Cu2+ probe by switch-off modes. Owing to the
similar properties of Cd2+ and Zn2+, Cd2+ cause a moderate
fluorescence enhancement when it was placed in HEPES
buffer solution of PBI-5. Cd2+ is not present in significant
amounts in biological systems, which is generally present at
concentrations ꢀ100-fold less than Zn2+ in biological and
environmental systems, thus Cd2+-induced interference
probably would be negligible [17, 18a]. Unfortunately, Pb2+
Subsequently, the selectivity of PBI-6 toward various
metals was then examined. Fig. (2a) displays the fluores-
cence responses of PBI-6 (2 μM) in 10 mM HEPES/DMSO
(v/v = 90/10, pH = 7.2) to a range of physiological and envi-
ronmentally relevant metal ions. Upon addition of 10 equiv
of metal ions, Zn2+ caused an about 40-fold fluorescence
intensity enhancement. As expected, the addition of Pb2+ did
not result in discernible interference. Among a series se-
lected other metal ions, only Cd2+ perturbed the emission

506 Letters in Organic Chemistry, 2012, Vol. 9, No. 7
Cong et al.
PBI-6 (5 μM) in a physiological saline for 40 min at 37 ºC,
washed to remove the remaining dye, and then Zn2+ (50 μM)
was added and incubated for another 30 min. As shown in
Fig. (3), a clear red fluorescent image was observed obvi-
ously from fluorescence microscopy. The experimental re-
sults revealed that PBI-6 would be a potentially useful rea-
gent for fluorescence imaging of Zn2+ in living biological
samples.
In conclusion, we have developed a new type of fluores-
cent chemosensors based on perylenebisimide for the detec-
tion of Zn2+. Both cyclen/DPA-bifunctionalized perylene-
bisimide PBI-5 and di-DPA-functionalized perylenebisimide
PBI-6 exhibit fluorescence turn-on responses for Zn2+ in
neutral HEPES buffer solution. Especially, PBI-6 can effec-
tively eliminate Pb2+-induced interference, weaken Cd2+-
induced interference, and show a better selectivity for Zn2+
compared to a range of physiological and environmentally
relevant other metal ions. We also evaluated their capability
of biological applications and found that both PBI-5 and
PBI-6 could successfully penetrate the cell membrane and
apply in the fluorescence imaging of living cells.
Fig. (3). Brightfield and fluorescent images of Zn2+ in SM-7721
cells for PBI-6 (5 ꢀM). (left) bright-field transmission image and
(right) fluorescence image of SM-7721 cells incubated with PBI-6
for 40 min after the addition of Zn2+ (50 ꢀM) for another 30 min.
weakly, which induced an about 4-fold fluorescence en-
hancement. Compared to the 40-fold fluorescence enhance-
ment observed for Zn2+, it was obvious that PBI-6 was
highly selective and demonstrated substantial fluorescence
turn-on ability for Zn2+ over the other metal ions tested. Vis-
ual colorimetry assay further underscored the selective re-
sponse of PBI-6 to Zn2+. An obvious fluorescence turn-on
response was observed as the addition of Zn2+ into the aque-
ous solution of PBI-6 under a UV lamp (365 nm), while the
equimolar amounts of other metal ions did not lead to sig-
nificant changes (Fig. 2a inset). Under the above experimen-
tal conditions, the competitive coordination ability of PBI-6
to Zn2+ in the presence of other metal ions was also investi-
gated and the results are depicted in Fig. (S6) in Supplemen-
tary Material. Except Cu2+ and Hg2+ resulted in fluorescence
quenching, a series selected metal ions, including Pb2+ and
Cd2+, did not obviously interfere with the fluorescence turn-
on response to Zn2+. The Zn2+ titration experiment of PBI-6
showed a similar trend with PBI-5, and the fluorescent inten-
sity of PBI-6 increased as the Zn2+ concentration increased
(Fig. 2b). The Job’s plot showed that PBI-6 formed a 1:2
complex with Zn2+ (see Fig. S7 in Supplementary Material).
The binding constant (log K = 6.8 ± 0.2) of PBI-6 with Zn2+
was calculated via a nonlinear least-squares analysis (see
Fig. S8 in Supplementary Material) [19]. The 1:2 stoichio-
metry was further confirmed by the electrospray ionization
(ESI) mass spectrum analysis. High solution ESI-TOF mass
spectroscopy showed two peaks at m/z 653.9596 and
1404.8601 assigned to the species [PBI-6+2Zn+2ClO4+
2H]2+ and [PBI-6+2Zn+3ClO4]+ (see Fig. S9 in Supplemen-
tary Material).
Experimental Sections
NMR spectra were obtained on a Bruker AMX-400, Var-
ian INOVA-400 (400 MHz), or a Bruker AMX-600. The 1H
NMR (400 MHz) chemical shifts were measured relative to
CDCl3 or DMSO-d6 as the internal reference (CDCl3: ꢀ =
7.26 ppm; DMSO-d6: ꢀ = 2.50 ppm). The 13C NMR (100
MHz or 150 MHz) chemical shifts were given using CDCl3
and DMSO-d6 as the internal standard (CDCl3: ꢀ = 77.16
ppm; DMSO-d6: ꢀ = 39.52 ppm). High-resolution mass spec-
tra (HR-MS) were obtained with a Waters-Q-TOF-Premier
(ESI). Melting points were determined with XRC-1 and are
uncorrected. Absorption spectra were detected on a HI-
TACHI U-2910 spectrometer. Fluorescent emission spectra
were collected on a Horiba JobinYvon-Edison fluoromax-4
fluorescence spectrometer.
Unless otherwise noted, all reagents were obtained from
commercial suppliers and used without further purification.
10-(2-aminoethyl)-1,4,7,10-tetraazacyclododecane-1,4,7-tri-
carboxylate tri-tert-butyl ester (2) [20], N, N-bis(pyridin-2-
ylmethyl)ethane-1,2-diamine (3) [15b] were prepared ac-
cording to the literature procedures. Solvents were dried over
CaH2 or sodium and freshly distilled prior to use. Unless
otherwise indicated, all syntheses and manipulations were
carried out under N2 atmosphere.
Fluorescence Spectra
The capability of cellular uptake and staining of dyes
were assessed via incubating hepatoma cell line SM-7721
with PBI-5 and PBI-6 (5 μM) in a physiological saline con-
taining 1% and 5% DMSO for 40 min at 37 °C respectively.
It is encouraging that both PBI-5 and PBI-6 could success-
fully penetrate the cell membrane and apply in the fluores-
cence imaging of living cells (see Fig. S10 in Supplementary
Material). Subsequently, a preliminary study on the Zn2+-
selective detection of PBI-6 in living cells was carried out by
fluorescence microscopy. SM-7721 cells were cultured in
DMEM (containing 10% fetal bovine serum (FBS), 100
IU/mL penicillin, 100 μg/mL streptomycin), incubated with
DMSO was either HPLC or spectroscopic grade and wa-
ter was distilled for twice in the optical spectroscopic stud-
ies. Chloride (Zn2+, Cd2+, Hg2+, Cu2+, Pb2+, K+, Ca2+, Mg2+,
Ba2+, Mn2+, Cr3+, Fe3+, Fe2+, Na+, Ni2+), nitrate (Ag+) were
prepared in water as stock solutions for each measurement.
Each time a 3 mL of receptor solution was filled in a quartz
cell of 1 cm of optical path length and the stock solution of
metal ion was dropped into a quartz cell using a microsy-
ringe. The volume of metal ions stock solution added was
less than 100 μL to remain the concentration of receptor con-
stant. The excitation and emission slits of fluorescence spec-
tra were set at 5.0 nm if not specified.

Perylenebisimide-based Fluorescent Chemosensors
Letters in Organic Chemistry, 2012, Vol. 9, No. 7
507
Experimental Procedure for 4: 1,6,7,12-tetrachloro-
3,4,9,10-perylenetetracarboxylic dianhydride 1 (582 mg, 1.1
mmol), 10-(2-aminoethyl)-1,4,7,10-tetraazacyclododecane-
1,4,7-tricarboxylate tri-tert-butyl ester 2 (567 mg, 1.1 mmol)
and N, N-bis(pyridin-2-ylmethyl)ethane-1,2-diamine 3 (267
mg, 1.1 mmol) was suspended in toluene (10 mL) under N2
and stirred for 24 h at 110 ºC. After cooling to room tem-
perature, the solvent were removed under reduced pressure,
then the residue was purified by column chromatography on
silica gel eluting with CH2Cl2/EtOAc (1:3, v/v) to give the
desired product as a red solid (580 mg) at a yield of 40%.
Mp = 232-234 ºC; 1H NMR (400 MHz, CDCl3): ꢀ = 8.70 (s,
2H), 8.59 (s, 2H), 8.37 (d, J = 4.8 Hz, 2H), 7.41-7.33 (m,
4H), 7.01 (t, J = 6.6 Hz, 2H), 4.52-4.30 (m, 4H), 3.98-3.86
(m, 4H), 3.57-3.39 (m, 12H), 3.00-2.89 (m, 8H), 1.46 (s,
27H) ppm; 13C NMR (150 MHz, CDCl3): ꢀ = 162.3, 162.1,
159.5, 149.1, 136.2, 135.6, 135.5, 133.2, 133.0, 131.6, 131.4,
128.9, 128.5, 123.5, 123.4, 123.3, 123.2, 123.1, 122.0, 121.3,
79.7, 79.5, 60.5, 51.3, 49.9, 48.3, 38.7, 28.8, 28.7, 28.6 ppm;
HRMS (ESI): m/z calcd for C63H67Cl4N9O10 [M+H]+
1252.3814, found 1252.3806.
Centre of Testing & Analysis, Sichuan University for NMR
measurements.
CONFLICT OF INTEREST
Declared none.
SUPPLEMENTARY MATERIAL
Supplementary data of 1H NMR, 13C NMR, High-
resolution mass spectra, Optical data and Cell fluorescence
imaging of sensors PBI-5 and PBI-6 are available on the
publishers Web site along with the published article.
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Experimental Procedure for PBI-6: 1,6,7,12-
tetrachloro-3,4,9,10-perylenetetracarboxylic dianhydride 1
(582 mg, 1.1 mmol) and N, N-bis(pyridin-2-
ylmethyl)ethane-1,2-diamine 3 (727 mg, 3.0 mmol) were
suspended in toluene (50 mL) under N2 and stirred for 24 h
at 110 ºC. After cooling to room temperature, the solvent
was removed under reduced pressure, then the residue was
purified by column chromatography on silica gel eluting
with CH2Cl2/MeOH (30:1, v/v). A subsequent recrystalliza-
tion form a mixture of EtOAc and MeOH gave pure product
(511 mg) at a yield of 52%. Mp = 140-142 ºC; 1H NMR (400
MHz, CDCl3): ꢀ = 8.61 (s, 4H), 8.38 (d, J = 4.8 Hz, 4H),
7.39-7.33 (m, 8H), 7.02 (t, J = 6.0 Hz, 4H), 4.54-4.47 (m,
2H), 4.37-4.31 (m, 2H), 3.98-3.85 (m, 8H), 3.05-2.92 (m,
4H) ppm; 13C NMR (100 MHz, CDCl3): ꢀ = 162.0, 159.5,
149.0, 136.2, 135.4, 132.9, 131.5, 128.5, 123.4, 123.3, 123.1,
121.9, 60.4, 51.2, 38.6 ppm; HRMS (ESI): m/z calcd for
C52H36Cl4N8O4 [M+H]+ 979.1662, found 979.1677.
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ACKNOWLEDGEMENTS
This work was supported by grants from the National
NSF of China (Nos 20872101 and 21172155). We thank the
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