Switching of C-C and C-N Coupling/Cleavage for Hypersensitive Detection of Cu2+by a Catalytically Mediated 2-Aminoimidazolyl-Tailored Six-Membered Rhodamine Probe

Article abstract of DOI:10.1021/acs.orglett.0c02814

A robust six-membered rhodamine spirocyclic probe 1 containing a versatile 2-aminoimidazolyl moiety was elaborately designed and synthesized via an attractive C-C and C-N coupling strategy to improve the performance in the detection of ultralow transition

Full text of DOI:10.1021/acs.orglett.0c02814

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Letter
Switching of C−C and C−N Coupling/Cleavage for Hypersensitive
Detection of Cu2+ by a Catalytically Mediated 2‑Aminoimidazolyl-
Tailored Six-Membered Rhodamine Probe
Lin-Lin Yang,† A-Ling Tang,† Pei-Yi Wang,* and Song Yang*
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ABSTRACT: A robust six-membered rhodamine spirocyclic probe 1 containing a
versatile 2-aminoimidazolyl moiety was elaborately designed and synthesized via an
attractive C−C and C−N coupling strategy to improve the performance in the detection
of ultralow transition metal ions. Probe 1 allowed the highly hypersensitive detection of
Cu2+ with a superior picomolar limit of detection (35 pM) and nanomolar naked-eye
performance (80 nM) via the switching of C−C and C−N cleavage by a catalytic
hydrolysis mode.
ransition metal ions are naturally indispensable ingre-
dients that are involved in diverse essential physiological
established by rationally decorating various fluorophores, such
as rhodamine,10 naphthalimide,11 fluorescein,12 cyanine dyes,13
anthracene,14 and BODIPY.15 Among these sensors, rhod-
amine-based fluorescent sensors [such as 1−4 (Figure 1)]
bearing a classical five-membered spirolactam skeleton have
been strongly highlighted and investigated due to their
prominent photophysical and photostability properties and
can be used to sensitively distinguish Cu2+ ions via the flexible
regulation of C−N bond cleavage along with fluorescence
“turn-on” and/or color transformation.8,16 Despite the out-
standing progress achieved in this field, several challenges and
problems persist. These issues include the frequent nanomolar
and micromolar levels of the corresponding limit of detection
(LOD) and naked-eye recognition for metal ion analytes, and
susceptible detection under harsh circumstances. These
problems limit the realistic applications of probes for
monitoring the dynamic fluctuations of ultralow analytes in
subcellular microenvironments and natural environments. For
example, the dynamic concentration of free Cu2+ ions in
mammalian cells is <10−17 mol L−1,17 and tap water contains
latent nanomolar concentrations of Cu2+. Therefore, develop-
ing innovative and practical fluorescent probes for the real-
T
and metabolic processes in many living organisms. The loss of
their homeostasis is closely associated with diverse dysfunc-
tions and/or hematological manifestations.1 Copper, one of
the critically important transition metals, is responsible for the
normal functioning of various organs and metabolic
processes.1a,2 However, Cu2+ deficiency can lead to growth
failure and neurodegenerative disorders, whereas excessive
amounts of Cu2+ in biological processes can cause serious
syndromes, including Alzheimer’s disease,3 Wilson’s disease,4
and metabolic disorders such as obesity and diabetes.5
Additionally, superfluous metal ions lead to environmental
pollution that seriously threatens the health of plants,
microorganisms, and animals, including humans. Thus,
monitoring the distribution and dynamic fluctuations of
metal ions in subcellular microenvironments and/or ambient
surroundings can promote the in-depth understanding of
diverse metal-mediated physiological and pathological pro-
cesses for disease diagnosis and simultaneously track the
invisible environmental pollutants for ecological protection.6
In the past 50 years, the detection of biologically and/or
environmentally important transition metal ions by fluorescent
probes, especially a probe for the selective and rapid sensing of
Cu2+ ions, has experienced explosive development.7 Since
Czarnik et al.8 reported the pioneering work for the
colorimetric detection of Cu2+ by using a typical rhodamine-
based five-membered spirocyclic hydrazide (1) in 1997, a
considerable amount of sensitive chromogenic chemosensors
for detecting Cu2+ ions,9 in vitro and in cell, have been
Received: August 21, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02814
© XXXX American Chemical Society
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Figure 1. Classical rhodamine-based five-membered spirolactam structures 1−4 and six-membered rhodamine spirocyclic probes for the detection
of Cu2+.
Figure 2. (a) Synthesis of probes 1 and 2. (b) Proposed mechanism for C−C and C−N coupling six-membered cyclization. R = NH2 or CH3.
time, rapid, quantitative, and naked-eye detection of metal ions
at nanomolar and picomolar levels is challenging.
an “unstable” characteristic in acidic solutions, thus introduc-
ing uncertainty and limitations into its application.16 There-
fore, improving the performance of the classical five-membered
spirolactam scaffold and exploring new favorable spirocyclic
frameworks are necessary.
Six robust rhodamine-based probes possessing different six-
membered spirocyclic skeletons have been recently reported to
have perceptible capability for monitoring Cu2+,18 Hg2+,19 and
ClO−.20 Among them, three probes bearing corresponding six-
membered thiosemicarbazide moieties and the spirolactam
Careful investigations have found that traditional rhod-
amine-based five-membered spirolactam structures can hardly
improve the current detection capability for several possible
reasons, such as the stereotypical cleavage of C−N bonds, the
mode of chelation between analytes and probes, the low
efficiency and inadequacy of hydrolysis, and the disturbance of
fluorescence caused by nonplanar hydrolysis products.9,16
Meanwhile, the strained spirolactam structure usually displays
B
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pattern (Figure 1) can selectively distinguish Cu2+ by involving
C−N bond cleavage along with the formation of an
isothiocyanate group and the chelation of metal ions along
with C−C bond cleavage. Notably, these interesting probes
continue to display some limitations, with either a low
fluorescence quantum yield18a,b or a weak sensitivity.18c
Inspired by the intriguing frameworks mentioned above, the
rational fabrication of innovative six-membered spirocycles
may advance the performance and provide highly reliable and
hypersensitive fluorescent chemosensors for the detection of
metal ions at ultralow concentrations. For this purpose, herein,
a robust six-membered rhodamine spirocyclic probe 1
containing a versatile 2-aminoimidazolyl moiety was elabo-
rately designed and synthesized on the basis of an attractive
C−C and C−N coupling structure. This probe is reported for
the first time and confirmed on the basis of its single-crystal
structure, which is distinctly different from classical five-
membered spirolactam structures. Within this molecule, the
versatile 2-amino-carbonyl imidazole motif executes a dual
function: promoting the binding affinity21 for Cu2+ to induce
the rearrangement of the whole molecular electronic property
accompanied by subsequent C−C bond cleavage and
accelerating the Cu2+-mediated catalytic hydrolysis22 of
carbonyl imidazole along with C−N bond cleavage. On the
basis of this proposal, we expect that this probe can achieve
ultralow Cu2+ detection by elaborately exploiting the
compelling C−C and C−N cleavage approach along with the
release of violently fluorescent and colorimetric rhodamine.
Probe 1 displayed highly selective and hypersensitive detection
of Cu2+ with a superior picomolar LOD and nanomolar naked-
eye performance.
A facile synthetic route for the construction of rhodamine-
based C−N- and C−C coupling six-membered fluorescent
probes 1 and 2 is illustrated in Figure 2a. Generally, rhodamine
B was treated with POCl3 to provide a chlorinated
intermediate, which was then reacted with 2-aminoimidazole
or 2-methylimidazole under the mediation of the base
triethylamine to afford final product 1 or 2, respectively. A
possible mechanism for the C−N and C−C coupling six-
membered cyclization was proposed as shown in Figure 2b.
The frameworks of probes 1 and 2 were confirmed on the basis
of NMR and HRMS analyses and their relevant single-crystal
structures (Figures S1−S8 and Tables S1 and S2).
The fluorescence responses of probe 1 before and after the
addition of Cu2+ ions in various organic solvents or water/
organic solvents were investigated to acquire an improved
system for Cu2+ ion detection. Notably, excellent detection
performance was obtained in an acetonitrile solvent with 770-
fold fluorescence enhancement after incubation with an
equivalent amount of Cu2+ for 10 min at 25 °C (Figure S9).
Subsequently, an acetonitrile/water mixed solution [3:7 (v/v)]
was applied in the following tests (Figure S10). Further
spectroscopy investigations were executed to evaluate the
selectivity and sensitivity of probe 1. As depicted in Figure 3a−
c, violet fluorescence at 578 nm (Φ = 0.63) and strong
absorption at 556 nm along with a naked-eye recognition effect
were triggered and generated only after the addition of Cu2+
but not that of other metal ions, indicating that a reliable
colorimetric probe for detecting Cu2+ was established.
Competition experiments revealed the anti-interference
performance of probe 1 for distinguishing Cu2+, leading to
the fluorescence “turn on” only by the latter supplementation
of Cu2+ (Figures S11 and S12). A titration experiment on
Figure 3. (a) Fluorescence and (b) absorption spectra of probe 1 (10
μM) upon addition of 10 μM Cu2+ and other ions [λex = 556 nm, slits
3/3, 3:7 (v/v) CH3CN/H2O]. (c) Probe 1 (10 μM) upon addition of
10 μM metal ions: (1) probe 1, (2) Cu2+, (3) Ca2+, (4) Ni2+, (5) Ba2+,
(6) Cr3+, (7) Co2+, (8) Hg2+, (9) Al3+, (10) Fe3+, (11) Zn2+, (12) Li+,
(13) Pb2+, (14) Cd2+, (15) Mg2+, (16) K+, (17) Na+, (18) Ag+, and
(19) Fe2+.
probe 1 toward various dosages of Cu2+ revealed a
predominant linear detection range of 30−1300 nM [R2 =
0.9924 (Figure S13)], suggesting that this probe could enable
the quantitative analysis of Cu2+ at the nanomolar level. A
superior picomolar LOD (35 pM, 2.22 ppt) and nanomolar
naked-eye performance [80 nM, 5.08 ppb (Figure S14)] were
achieved, further confirming that probe 1 could serve as an
extremely superior tool for selective and hypersensitive
detection of Cu2+, thus demonstrating that the rational design
of the six-membered rhodamine spirocyclic probe could
improve the performance of classical five-membered spirolac-
tam structures in the detection of metal ions. Additionally, this
probe showed a particularly robust performance in detecting
Cu2+ over a harsh pH range of 3.6−11.6 in CH3CN/HEPES
buffer (Figure S15). By contrast, probe 2 showed insignificant
detection effects toward Cu2+ (Figure S16), indicating that the
amino part promoted the binding affinity21 with Cu2+ to
induce the rearrangement of the whole molecular electronic
property accompanied by subsequent C−C bond cleavage.
The time-dependent fluorescence intensity of probe 1
triggered by diverse Cu2+ concentrations was recorded to
investigate the detection mechanism of the probe. At Cu2+
concentrations of 10, 7.5, 5, and 2.5 μM, the final fluorescence
intensity could reach a stable equilibrium at approximately 5, 9,
15, and 45 min, respectively, indicating a catalytic mode for
detecting Cu2+ (Figure 4a). A distinct blue shift from 587 to
578 nm was observed as illustrated in Figure 4b. This shift
might be ascribed to the transformation of the C−C severed
product into C−C and C−N severed products (Figure 5).
After the purification and identification of the final fluorescent
product, the original rhodamine returned (Figures S17−S19),
validating our initial assumption. Meanwhile, a 1H NMR
titration experiment (Figure S20) was performed to reveal the
detection mechanism. Multiple newly generated small peaks
were observed after the introduction of Cu2+, indicating that a
complicated component was probably created by either C−C
cleavage or both C−C and C−N cleavage. High-performance
liquid chromatography (HPLC) of probe 1 reacting with Cu2+
was provided to monitor the process of catalytic hydrolysis. As
shown in Figures S21−S39, Cu2+ detection was definitely
directed by a catalytic hydrolysis mode and affording a final
conversion yield to rhodamine with 95.4%, in agreement with
our original design.
C
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with PBS buffer (0.01 M, pH 7.4) thrice, and subsequently
treated with 5 μM Cu2+ for 1 h at 37 °C. Finally, the cells were
washed with PBS buffer thrice before being imaged. Cells with
strong red fluorescence (Figure 6) were observed only by
Figure 6. Fluorescence imaging of probe 1 responding to Cu2+ in
living A549 cells. (a1) Bright-field and (a2) fluorescence images of
A549 cells incubated with 5 μM probe 1 for 30 min at 37 °C. (a3)
Merged image of panels a1 and a2. (b1) Bright-field and (b2)
fluorescence images of A549 cells incubated with 5 μM probe 1 and
then 5 μM Cu2+ for 1 h at 37 °C. (b3) Merged image of panels b1 and
b2. Red channel, 510−560 nm.
incubating the cells with probe 1 upon addition of Cu2+,
thereby indicating that probe 1 could penetrate the cell
membrane and selectively detect Cu2+ in cell.
In summary, a robust six-membered rhodamine spirocyclic
probe 1 containing a versatile 2-aminoimidazolyl moiety was
rationally designed and constructed via an attractive C−C and
C−N coupling strategy to improve the performance encoun-
tered by classical five-membered spirolactam structures in
ultralow metal ion detection. The results of testing found that
probe 1 displayed highly selective and hypersensitive detection
of Cu2+ with an excellent picomolar LOD (35 pM, 2.22 ppt)
and nanomolar naked-eye performance (80 nM, 5.08 ppb) via
a catalytically mediated C−C and C−N cleavage along with
violent fluorescence “turn-on” (Φ = 0.63, for the new product).
A significant linear detection range of 30−1300 nM was
afforded for the quantification of nanomolar levels of Cu2+.
Further bioimaging studies revealed that probe 1 showed good
cell membrane permeability and practicable capability for
monitoring Cu2+ in living cell lines. We anticipate that this
intriguing C−C and C−N coupling/cleavage approach can
open up a new avenue for developing hypersensitive
fluorescent probes for the desired ultralow analyte detection,
which improves our understanding of diverse metal-mediated
physiological and pathological processes for disease diagnosis.
Figure 4. Time-dependent fluorescence intensity spectra of (a) probe
1 (10 μM) reacted with various concentrations of Cu2+ and (b) probe
1 (10 μM) upon the detection of 2.5 μM Cu2+. λex = 556 nm, slits 3/
3, CH3CN:H2O = 3:7.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02814.
Materials and methods; synthesis, NMR, HRMS, and
crystal structures of probes 1 and 2; calculation of the
relative fluorescence quantum yields and detection limit;
sensitivity of probe 1 for Cu2+ in different solvents;
competition experiments; quantitative determination of
Cu2+; detection of nanomolar level Cu2+ by the naked
eye; influence of pH on the detection of Cu2+; sensitivity
Figure 5. Possible mechanism for Cu2+-induced catalytic hydrolysis
sensing cycle of probe 1 in a CH3CN/H2O solution.
A549 cells were used for fluorescence imaging to explore the
cell detection capability of probe 1 toward Cu2+. Initially,
probe 1 showed low cytotoxicity toward mammalian cell lines
measured via MTT assays (Figure S40). Briefly, A549 cells
were incubated with 5 μM probe 1 for 0.5 h at 37 °C, washed
1
of probe 2 toward Cu2+; H NMR and HRMS spectra
for the newly generated product; HPLC analysis of
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probe 1 reacting with Cu2+; imaging experiments using
(3) (a) Barnham, K. J.; Masters, C. L.; Bush, A. I. Nat. Rev. Drug
Discovery 2004, 3, 205−214. (b) Savelieff, M. G.; Lee, S.; Liu, Y.; Lim,
M. H. ACS Chem. Biol. 2013, 8, 856−865. (c) Multhaup, G.;
Schlicksupp, A.; Hesse, L.; Beher, D.; Ruppert, T.; Masters, C. L.;
Beyreuther, K. Science 1996, 271, 1406−1409. (d) Lee, S.; Barin, G.;
Ackerman, C. M.; Muchenditsi, A.; Xu, J.; Reimer, J. A.; Lutsenko, S.;
Long, J. R.; Chang, C. J. J. Am. Chem. Soc. 2016, 138, 7603−7609.
(4) (a) Lutsenko, S. Biochem. Soc. Trans. 2008, 36, 1233−1238.
(b) Huster, D. Ann. N. Y. Acad. Sci. 2014, 1315, 37−44.
(5) (a) Huster, D.; Purnat, T. D.; Burkhead, J. L.; Ralle, M.; Fiehn,
O.; Stuckert, F.; Olson, N. E.; Teupser, D.; Lutsenko, S. J. Biol. Chem.
2007, 282, 8343−8355. (b) Huster, D.; Lutsenko, S. Mol. BioSyst.
2007, 3, 816−824. (c) Engle, T. E. J. Anim. Sci. 2011, 89, 591−596.
(6) (a) Zhu, H.; Fan, J.; Wang, B.; Peng, X. Chem. Soc. Rev. 2015, 44,
4337−4366. (b) Cao, D.; Liu, Z.; Verwilst, P.; Koo, S.; Jangjili, P.;
Kim, J. S.; Lin, W. Chem. Rev. 2019, 119, 10403−10519.
(7) Wu, D.; Sedgwick, A. C.; Gunnlaugsson, T.; Akkaya, E. U.;
Yoon, J.; James, T. D. Chem. Soc. Rev. 2017, 46, 7105−7123.
(8) For pioneering five-membered works, see: Dujols, V.; Ford, F.;
Czarnik, A. W. J. Am. Chem. Soc. 1997, 119, 7386−7387.
(9) Sivaraman, G.; Iniya, M.; Anand, T.; Kotla, N. G.; Sunnapu, O.;
Singaravadivel, S.; Gulyani, A.; Chellappa, D. Coord. Chem. Rev. 2018,
357, 50−104.
the chelating agent of Cu2+; and a comparison of
detection results of reported five- and six-membered
rhodamine-based probes (PDF)
Accession Codes
CCDC 1961727 and 1991171 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
■
Song Yang − State Key Laboratory Breeding Base of Green
Pesticide and Agricultural Bioengineering, Key Laboratory of
Green Pesticide and Agricultural Bioengineering, Ministry of
Education, Guizhou University, Guiyang 550025, China;
orcid.org/0000-0003-1301-3030; Email: jhzx.msm@
gmail.com
(10) (a) Kumar, M.; Kumar, N.; Bhalla, V.; Sharma, P. R.; Kaur, T.
Org. Lett. 2012, 14, 406−409. (b) Liu, C.; Jiao, X.; Wang, Q.; Huang,
K.; He, S.; Zhao, L.; Zeng, X. Chem. Commun. 2017, 53, 10727−
10730. (c) Huo, F.; Wang, L.; Yin, C.; Yang, Y.; Tong, H.; Chao, J.;
Zhang, Y. Sens. Actuators, B 2013, 188, 735−740. (d) Wang, L.; Du,
W.; Hu, Z.; Uvdal, K.; Li, L.; Huang, W. Angew. Chem., Int. Ed. 2019,
58, 14026−14043. (e) Zhou, L.; Zhang, X.; Wang, Q.; Lv, Y.; Mao,
G.; Luo, A.; Wu, Y.; Wu, Y.; Zhang, J.; Tan, W. J. Am. Chem. Soc.
2014, 136, 9838−9841. (f) Liu, Y.; Su, Q.; Chen, M.; Dong, Y.; Shi,
Y.; Feng, W.; Wu, Z. Y.; Li, F. Adv. Mater. 2016, 28, 6625−6630.
(11) (a) Goswami, S.; Sen, D.; Das, N. K. Org. Lett. 2010, 12, 856−
859. (b) Li, M.; Ge, H.; Arrowsmith, R. L.; Mirabello, V.; Botchway,
S. W.; Zhu, W.; Pascu, S. I.; James, T. D. Chem. Commun. 2014, 50,
11806−11809.
(12) (a) Yang, Y.; Huo, F.; Yin, C.; Chu, Y.; Chao, J.; Zhang, Y.;
Zhang, J.; Li, S.; Lv, H.; Zheng, A.; Liu, D. Sens. Actuators, B 2013,
177, 1189−1197. (b) Choi, M. G.; Cha, S.; Lee, H.; Jeon, H. L.;
Chang, S. K. Chem. Commun. 2009, 7390−7392. (c) Huo, F. J.; Yin,
C. X.; Yang, Y. T.; Su, J.; Chao, J. B.; Liu, D. S. Anal. Chem. 2012, 84,
2219−2223.
Pei-Yi Wang − State Key Laboratory Breeding Base of Green
Pesticide and Agricultural Bioengineering, Key Laboratory of
Green Pesticide and Agricultural Bioengineering, Ministry of
Education, Guizhou University, Guiyang 550025, China;
orcid.org/0000-0002-9904-0664; Email: pywang888@
126.com
Authors
Lin-Lin Yang − State Key Laboratory Breeding Base of Green
Pesticide and Agricultural Bioengineering, Key Laboratory of
Green Pesticide and Agricultural Bioengineering, Ministry of
Education, Guizhou University, Guiyang 550025, China
A-Ling Tang − State Key Laboratory Breeding Base of Green
Pesticide and Agricultural Bioengineering, Key Laboratory of
Green Pesticide and Agricultural Bioengineering, Ministry of
Education, Guizhou University, Guiyang 550025, China
Complete contact information is available at:
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(13) (a) Li, P.; Duan, X.; Chen, Z.; Liu, Y.; Xie, T.; Fang, L.; Li, X.;
Yin, M.; Tang, B. Chem. Commun. 2011, 47, 7755−7757.
(b) Hirayama, T.; Van de Bittner, G. C.; Gray, L. W.; Lutsenko, S.;
Chang, C. J. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 2228−2233.
(14) (a) Xie, Y.; Ding, Y.; Li, X.; Wang, C.; Hill, J. P.; Ariga, K.;
Zhang, W.; Zhu, W. Chem. Commun. 2012, 48, 11513−11515.
(b) Yang, L.; Song, Q.; Damit-Og, K.; Cao, H. Sens. Actuators, B 2013,
176, 181−185.
Author Contributions
†L.-L.Y. and A-L.T. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(15) Li, Q.; Guo, Y.; Shao, S. Sens. Actuators, B 2012, 171, 872−877.
(16) Chen, X.; Pradhan, T.; Wang, F.; Kim, J. S.; Yoon, J. Chem. Rev.
2012, 112, 1910−1956.
■
The authors acknowledge the financial support of the National
Natural Science Foundation of China (21702037, 21662009,
31860516, and 21877021), the Research Project of the
Ministry of Education of China (20135201110005 and
213033A), the Key Technologies R&D Program
(2014BAD23B01), the Guizhou Provincial S&T Program
([2017]5788), and the Program of Introducing Talents of
Discipline to Universities of China (111 Program, D20023).
(17) Rae, T. D.; Schmidt, P. J.; Pufahl, R. A.; Culotta, V. C.;
O’Halloran, T. V. Science 1999, 284, 805−808.
(18) (a) Wu, C.; Bian, Q. N.; Zhang, B. G.; Cai, X.; Zhang, S. D.;
Zheng, H.; Yang, S. Y.; Jiang, Y. B. Org. Lett. 2012, 14, 4198−4201.
(b) Wang, B.; Cui, X.; Zhang, Z.; Chai, X.; Ding, H.; Wu, Q.; Guo, Z.;
Wang, T. Org. Biomol. Chem. 2016, 14, 6720−6728. (c) Majumdar,
A.; Lim, C. S.; Kim, H. M.; Ghosh, K. ACS Omega 2017, 2, 8167−
8176.
REFERENCES
(19) (a) Yang, Z.; Hao, L.; Yin, B.; She, M.; Obst, M.; Kappler, A.;
Li, J. Org. Lett. 2013, 15, 4334−4337. (b) Biswal, B.; Mallick, D.; Bag,
B. Org. Biomol. Chem. 2016, 14, 2241−2248.
(20) Wang, Z.; Zhang, Q.; Liu, J.; Sui, R.; Li, Y.; Li, Y.; Zhang, X.;
Yu, H.; Jing, K.; Zhang, M.; Xiao, Y. Anal. Chim. Acta 2019, 1082,
116−125.
■
(1) (a) Carter, K. P.; Young, A. M.; Palmer, A. E. Chem. Rev. 2014,
114, 4564−4601. (b) Barnham, K. J.; Bush, A. I. Chem. Soc. Rev. 2014,
43, 6727−6749.
(2) Gaggelli, E.; Kozlowski, H.; Valensin, D.; Valensin, G. Chem. Rev.
2006, 106, 1995−2044.
E
https://dx.doi.org/10.1021/acs.orglett.0c02814
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
Letter
(21) Xie, P.; Guo, F.; Li, D.; Liu, X.; Liu, L. J. Lumin. 2011, 131,
104−108.
(22) (a) Yang, Y.; Zhao, Q.; Feng, W.; Li, F. Chem. Rev. 2013, 113,
192−270. (b) Kroll, H. J. Am. Chem. Soc. 1952, 74, 2036−2039.
(c) Bender, M. L.; Turnquest, B. W. J. Am. Chem. Soc. 1957, 79,
1889−1893.
F
https://dx.doi.org/10.1021/acs.orglett.0c02814
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