Construction of Chiral triple-Decker Nd(III) Nanocluster with High NIR Luminescence Sensitivity toward Co(II)

Article abstract of DOI:10.1021/acs.inorgchem.0c01146

One Nd(III) complex [Nd3L3(OAc)3] (1) was synthesized from a conjugate Schiff base ligand H2L. It shows a chiral triple-decker structure (1.1 × 1.2 × 1.8 nm) with Nd(III) ions sandwiched between the Schiff base ligands. 1 exhibits NIR Nd(III) luminescence, and the LMET efficiency is calculated to be 13.8percent. It displays high luminescence sensitivity and selectivity to Co(II). The KSV value and LOD of 1 to Co(II) are 9.96 × 104 M-1 and 0.97 μM, respectively.

Full text of DOI:10.1021/acs.inorgchem.0c01146

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Construction of Chiral “Triple-Decker” Nd(III) Nanocluster with High
NIR Luminescence Sensitivity toward Co(II)
Xia Liu, Yanan Ma, Xiaoping Yang,* Dongliang Shi, Desmond Schipper, and Richard A. Jones
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ABSTRACT: One Nd(III) complex [Nd3L3(OAc)3] (1) was synthesized from a conjugate Schiff base ligand H2L. It shows a chiral
“triple-decker” structure (1.1 × 1.2 × 1.8 nm) with Nd(III) ions sandwiched between the Schiff base ligands. 1 exhibits NIR Nd(III)
luminescence, and the LMET efficiency is calculated to be 13.8%. It displays high luminescence sensitivity and selectivity to Co(II).
The KSV value and LOD of 1 to Co(II) are 9.96 × 104 M−1 and 0.97 μM, respectively.
ecently much attention has focused on the fluorescent
response for selective detection of various analytes due to
N,N′-bis(3-methoxysalicylidene)(4-methylphenylene)-1,2-dia-
mine (H2L, Scheme S1) that has one methyl group on the
central benzene ring. 1 shows a chiral “triple-decker” structure
(1.1 × 1.2 × 1.8 nm) with the Nd(III) ions sandwiched
between the alternating layers of Schiff base ligands.
Interestingly, luminescent sensing studies show that 1 displays
high sensitivity and selectivity to Co(II). To the best of our
knowledge, this is the first report on the NIR luminescent
detection of Co(II) based on chiral lanthanide complex.
The Schiff-base ligand was synthesized from the reaction of
2-hydroxy-3-methoxybenzaldehyde and 4-methylbenzene-1,2-
diamine (yield, 82%).34 The signal of an imino proton
R
its potential application in environment, biology, and
medicine.1−6 Cobalt is one of the biological essential trace
elements and plays a critical role in the metabolism of ions and
formation of hemoglobin. However, an excessive level of cobalt
in the body may cause adverse health effects such as blindness,
deafness, and hypothyroidism.7,8 Cobalt has been classified by
the International Agency for Research on Cancer (IARC) as a
possible group (2B) human carcinogen. The toxic dose of
cobalt is 500 mg, and the maximum tolerable limit in the diet is
10 mg/L.9 Owing to the versatile applications of cobalt in the
industry, it has been found as a contaminant in the
environment. Much effort has been devoted to the develop-
ment of fluorescent sensors for the Co(II) detection, such as
fluorescent organics,10−12 metal nanoparticles (NPs),13−15 and
metal−organic frameworks (MOFs).16−18 In early research in
this field, Govindaraju et al. developed some interesting
fluorescent Schiff bases and chromogenic resorufin dyes for the
detection of Co(II) in organisms.19−21 So far, many visible
luminescent lanthanide complexes bearing Eu3+ and Tb3+ ions
have been used as probes to detect various analytes;22−24
however, there are very few reports on the NIR luminescent
lanthanide complexes in this field.25,26 NIR fluorescent probes
may offer the advantage of lower levels of absorption and
autofluorescence of biomolecules in this wavelength range.
For the past few years, the construction of chiral metal
complexes has attracted much attention of researchers due to
their potential application in pharmaceuticals, catalysis, and
functional materials.27−29 We have previously reported the
synthesis of one chiral complex [Yb3(L′)3] with Schiff base
ligand N,N′-bis(5-bromo-3-methoxysalicylidene)phenylene-
1,2-diamine (H2L′, Scheme S1). In the “triple-decker”
structure of this complex, the back H2L′ ligand deflects in a
clockwise (or anticlockwise) fashion to form a Δ (or Λ)
designation.30 It is found that some factors such as the types of
metal salts and ligand structures may affect the construction of
“multi-decker” lanthanide complexes.31−33 With this consid-
eration, one lanthanide complex [Nd3L3(OAc)3] (1) was
synthesized by the use of a new conjugate Schiff base ligand
1
(−CHN−) in the H NMR spectrum of H2L is found at
8.61 ppm (Figure S1). The reaction of H2L with Nd(OAc)3·
6H2O in refluxing MeOH gave the complex, and the slow
diffusion of diethyl ether into the reaction solution led to the
formation of the crystalline product of 1. In the chiral “triple-
decker” structure of 1 (Figure 1a), the backmost Schiff base
ligand deflects counter clockwise, resulting in the “Λ”
designation as found in [Yb3(L′)3].28 The coordination
number of all Nd(III) is eight. Nd(1) is surrounded by eight
oxygen atoms from the front Schiff base ligand and three OAc−
anions. Nd(2) is bound to the N2O2 and O2O2 donor sets of
two back L2− ligands, while Nd(3) is coordinated with two
N2O2 sets of the L2− ligands. The coordination geometry about
the Nd(III) ions is a slightly distorted dodecahedron. The
Nd(III) ions in 1 are linked together by two L2− ligands and
one OAc− anion, and the Nd(III)···Nd(III) distances are from
3.730 to 3.853 Å.
In the solid state, 1 reveals an open 3D channel structure
(Figure 1b). Intermolecular (OAc−)H···O(OAc−) hydrogen
bonding (i.e., 2.461 Å−2.666 Å), π···π (i.e., 3.625−3.990 Å),
Received: April 19, 2020
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(Figure S7b). Thus, the intrinsic quantum yields (ΦLn) of
Nd(III) and LMET efficiency (ηsens) in 1 are calculated to be
1.6% and 13.8%, respectively, using equations ΦLn = τ/τ037 (τ0
= 250 μs, the natural lifetime of Nd(III)) and ηsens = Φem/
ΦLn.38
In the structure of the complex, there are 18 electronegative
O atoms (from L2− ligands and OAc− anions), which may help
to attract the metal cations in solution. Since the solubility of 1
in water is poor, the luminescent sensing of 1 to metal cations
was investigated in CH3CN. Interestingly, the addition of
Co(II), Cu(II), Pd(II), Mg(II), Cr(III), Ag(I), and Mn(II)
decreases the luminescence intensity of 1, while the addition of
Cd(II), Al(III), and Hg(II) slightly increases the luminescence
(Figures 2 and S8). It is noticeable that the addition of Co(II)
Figure 1. (a) Chiral “tri-decker” structure of 1. (The aromatic methyl
groups are drawn in black to show the anticlockwise deflection of the
back Schiff base ligand.) (b) Open 3D channel structure along the a
axis. (c) SEM image. (d) EDX spectroscopy.
Figure 2. Lanthanide luminescent response of 1 (10 μM) to the
addition of Co(II) in CH3CN (λex = 296 nm).
and (OAc−)−H···π interactions (i.e., 2.811 Å−3.102 Å)35,36
are formed among neighbor molecules in the crystal packing
structure of 1. The empty volume calculated by the PLATON
program is 16.9% of the total crystal volume. The scanning
electron microscopy (SEM) image of 1 is displayed in Figure
1c. The energy dispersive X-ray (EDX) spectroscopy exhibits
that the weight content of Nd(III) in 1 is about 23% (Figure
1d), which is also confirmed by inductively coupled plasma-
atomic emission spectrometry (ICP-AES, Figure S3A). These
results as well as the elemental analysis of 1 (see ESI) indicate
that the crystalline product of the complex is pure. Due to the
escaping of solvent molecules in the 3D channel structure, the
crystal packing structure of 1 collapses rapidly after the
crystalline product is separated from the mother solution. A
large background is found in the powder XRD spectrum of 1,
indicating the amorphous nature of the complex in the solid
state (Figure S4). Both thermogravimetric analysis (Figure S5)
and melting point measurement (see ESI) exhibit that 1 is
stable in the solid state until heated to 206 °C. The stability of
1 in solution is confirmed by a molar conductivity study. The
complex is neutral in CH3CN, in agreement with its crystal
structure.
leads to a rapid decrease of the luminescence. For example, the
emission intensity at 1066 nm is decreased more than 50%
when the concentration of added Co(II) is 25 μM. However,
for other metal cations, the concentrations needed to decrease
the intensity by half are much higher than 25 μM (Figures 2
and S7). The linear relationship between luminescence
intensity decreases and the concentrations of added metal
cations can be fitted into the Stern−Volmer (S−V) equation,39
which gives the quenching constants (KSV) of 1 to metal
cations. The KSV value of 1 to Co(II) is 9.96 × 104 M−1, which
is among the highest values reported so far for the fluorescent
sensing of Co(II).10−18 As shown in Figure 3, this value is
much larger than those of 1 to other metal cations (<0.4 × 104
M−1). The limit of detection (LOD) of 1 to Co(II) can be
calculated by 3σ/KSV, where σ is the standard deviation
The free Schiff base ligand and the complex show a broad
absorption band at about 300 nm (Figure S6). The
photoluminescence spectrum of free H2L exhibits an emission
band with λmax = 482 nm (Figure S7a). The emission spectrum
of 1 in either solution or the solid state displays the NIR
luminescence of Nd(III) (Figure S7b). The excitation
spectrum of 1 exhibits the strongest peak at about 300 nm,
which is similar to the absorption of free H2L. This indicates
that the lanthanide emission is sensitized by the Schiff base
ligand through the ligand-to-metal energy transfer (LMET)
process. The NIR emission lifetime (τ) and quantum yield
(Φem) of 1 are measured to be 4.0 μs and 0.22%, respectively
Figure 3. Lanthanide luminescence quenching constants (KSV) of 1 to
metal cations.
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calculated from 21 blank measurements of 1.40 The LOD of 1
to Co(II) is 0.97 μM, which is smaller than the maximum
permitted level of 1.7 μM of groundwater quality criterion
defined by the Department of Environmental Protection of
New Jersey.7,8 In addition, 1 is recyclable for the detection of
Co(II). The NIR luminescent response behavior of the
recycled sample of 1 after treatment with Co(II) is similar to
that before treatment with Co(II) (Figure S9). ICP-AES
analysis shows that the weight content of Nd(III) in the
recycled sample of 1 has not changed (Figure S3B). These
results indicate that 1 meets the basic requirements of
application for Co(II) chemosensors.16,41
value and LOD of 1 to Co(II) are 9.96 × 104 M−1 and 0.97
μM, respectively. To the best of our knowledge, 1 represents
the first report of a chiral lanthanide complex for NIR
luminescent detection of Co(II). Further studies focused on
the improvement of the solubility of 1 in H2O by the
introduction of some water-soluble groups such as carboxylic
and sulfonic groups into the Schiff base ligand are in progress.
ASSOCIATED CONTENT
■
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* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c01146.
Chelation enhancement of the quenching emission (CHEQ)
and chelation-enhanced fluorescence (CHEF) effects have
been used to explain the influence of added metal cations on
the lanthanide luminescence.42 For example, Co(II), Cu(II),
Pd(II), Cr(III), and Mn(II) have d−d transitions, which may
quench the luminescence of 1 by energy transfers from Nd(III)
to these d-block metal cations.43,44 In addition, the added
metal cations may affect the LMET process in 1, resulting in
the luminescence intensity changes. It is found that the LMET
efficiency (ηsens) in 1 is decreased to 9.8% after the addition of
25 μM Co(II) (in this case, the luminescence lifetime (τ) and
quantum yield (Φem) of 1 are measured to be 2.3 μs and
0.09%, respectively (Figure S10)).37,38 Meanwhile, the
luminescence intensity is also dependent on the ability of the
complex to absorb light energy at the excitation wavelength
(λex). UV−vis titration of 1 indicates that the addition of
Co(II) (e.g., 67.9 μM) decreases the absorption of the
complex at λex = 296 nm (Figure S11),45 which is unfavorable
to the sensitization effect of the Schiff base ligand to the
lanthanide luminescence.46
1
Detailed synthesis of ligand H2L and complex 1, H
NMR, IR, TGA, PXRD and UV−vis spectra, lumines-
cence properties, and X-ray crystallography (PDF)
Accession Codes
CCDC 1988628 contains 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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: + 44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Xiaoping Yang − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325035, China;
orcid.org/0000-0002-4370-517X; Email: xpyang@
wzu.edu.cn
Authors
Xia Liu − College of Chemistry and Materials Engineering,
Wenzhou University, Wenzhou 325035, China
Yanan Ma − College of Chemistry and Materials Engineering,
Wenzhou University, Wenzhou 325035, China
Dongliang Shi − College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou 325035, China
Desmond Schipper − The University of Texas at Austin,
Department of Chemistry and Biochemistry, Austin, Texas
78712, United States
The selectivity of 1 to Co(II) in the presence of other
competitive metal cations was investigated. As shown in Figure
4, the existence of another metal cation with the same
concentration does not affect the high quenching percentage of
1 to Co(II).
Richard A. Jones − The University of Texas at Austin,
Department of Chemistry and Biochemistry, Austin, Texas
78712, United States; orcid.org/0000-0003-4174-6530
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.0c01146
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the National Natural Science
Foundation of China (No. 21771141), and the Welch
Foundation of the United States (No. F-816).
Figure 4. Luminescence quenching percentages of 1 (10 μM) before
and after the addition of Co(II) (50 μM) in the presence of another
competitive metal cation (50 μM) in CH3CN.
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