Self-assembly of luminescent 12-metal Zn-Ln planar nanoclusters with sensing properties towards nitro explosives

Article abstract of DOI:10.1039/c8tc02426j

Two types of Zn-Ln nanoclusters [Nd₂Zn₂L₂(L′)₂(OAc)₂(NO₃)₂(H₂O)₂] (1) and [Ln₆Zn₆L₂(L′′)₂O₂(OAc)₁₈] (Ln = Nd (2) and Sm (3)) were prepared using one long Schiff base ligand (H₂L) with a naphthyl backbone and two short polydentate ligands (H₂L′ and H₂L′′). 1 and 2-3 show linear and planar structures with sizes of approximately 10 × 10 × 30 ? and 10 × 15 × 22 ?, respectively. The chromogenic Zn/ligand components can act as efficient sensitizers for lanthanide luminescence, and thus 1-3 show visible and NIR emissions. In these clusters, the Schiff base ligands display a “U-shaped” configuration, which is suitable to form ππ interactions and hydrogen bonds with nitro aromatic compounds. Interestingly, Zn-Sm cluster 3 shows visible and NIR luminescence sensing of nitro explosives and exhibits high sensitivity to 1,4-DNB and TNT at the ppb level.

Full text of DOI:10.1039/c8tc02426j

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Nguyên T. K. Thanh, Xiaodi Su et al.
Fine-tuning of gold nanorod dimensions and plasmonic properties using
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DOI: 10.1039/C8TC02426J
Journal of Materials Chemistry C
ARTICLE
Self-assembly of Luminescent 12-metal Zn-Ln Planar Nanoclusters
with Sensing Properties towards Nitro Explosives
Received 00th January 20xx,
Accepted 00th January 20xx
a
*,a
b
a
a
a
a
Dongmei Jiang, Xiaoping Yang, Xiaohui Zheng, Le Bo, Ting Zhu, Hongfen Chen, Lijie Zhang
and Shaoming Huang*
,a
DOI: 10.1039/x0xx00000x
Two types of Zn-Ln nanoclusters [Nd
were prepared using one long Schiff base ligand (H
and H L''). 1 and 2-3 show linear and planar structures with sizes of approximately 10 × 10 × 30 Å and 10 × 15 × 22 Å,
2
Zn
2
L
2
(L')
2
(OAc)
2
(NO
3
)
2
(H
2
O)
2
] (1) and [Ln
6
Zn
6
L
2
(L'')
2
O
2
(OAc)18] (Ln = Nd (2) and Sm (3))
www.rsc.org/
2
L) with a naphthyl backbone and two short polydentate ligands (H
2
L'
2
respectively. The chromogenic Zn/Ligand components can act as efficient sensitizers for lanthanide luminescence, thus 1-3
show visible and NIR emissions. In these clusters, the Schiff base ligands display an "U-shaped" configuration, which is
suitable to form ··· interactions and hydrogen bonds with nitro aromatic compounds. Interestingly, the Zn-Sm cluster 3
shows visible and NIR luminescent sensing of nitro explosives and exhibits high sensitivity to 1,4-DNB and TNT at ppb level.
Ln,17 Mn-Ln18 and Ni-Ln19 clusters have been synthesized for
the studies of magnetic properties (i. e. single-molecule
Introduction
2+
10
magnets (SMM)). Zn ion has saturated d electronic
configuration, which prevents the quenching of lanthanide
luminescence through d-d transitions (i.e. f→d energy
transfer). The light-absorbing Zn(II) chromophores have been
used as sensitizers for lanthanide emission in Zn-Ln complexes
Selective and sensitive detection of explosives such as nitro-
aromatics has been received extensive attention due to its
potential application in national security and environmental
1-3
protection. For example, explosives such as nitrobenzene
NB), 1,4-dinitrobenzene (1,4-DNB), 2,4,6-trinitrotoluene (TNT)
(
20
(
"antenna effect"). Recently, Q.-F. Sun et al. reported the
and cyclotrimethylene trinitramine (RDX) are common
ingredients of industrial explosives (Scheme 1a). Recently,
attention has focused on the fluorescence-based detection
methods due to their high sensitivities, portability and short
response times. A variety of fluorescent materials including
conjugated polymers, nanoparticles and metal-organic
frameworks (MOFs) have been extensively used for the
sensing of explosives.
Luminescent lanthanide-based complexes have been
employed for the detection of various cations, anions
and small molecules,
properties such as large Stokes' shifts, high color purity and
relatively long luminescence lifetimes. For example, many
Eu(III)- and Tb(III)-based metal-organic frameworks with visible
luminescence have been designed for this purpose.
recent years, high-nuclearity heterometallic d-f nanoclusters
have attracted considerable attention due to their unique
physical and chemical properties.
luminescent sensing properties toward nitro-aromatic
21
4
explosives of a Eu cluster in solution. To the best of our
knowledge, there are no reports on the luminescent sensing of
explosives based on high-nuclearity d-f nanoclusters.
An effective strategy towards the construction of
luminescent polynuclear lanthanide complexes is the use of
two kinds of chromogenic ligands with different sizes and
coordination modes to meet the requirement of high
4
-6
3
+
22
coordination numbers of Ln ions. Salen-type Schiff base
ligands have been widely used in the synthesis of d-f
7
,8
9,10
1
1,12
due to their specific luminescence
2
3-28
complexes.
As part of our continuing studies focused on
the design of functional lanthanide-based materials,2
9,30
we
13
report here two types of luminescent Zn-Ln nanoclusters with
specially designed Schiff base ligand N,N'-bis(3-
methoxysalicylidene)-1,5-naphthalenediamine (H L), which has
an electron-rich naphthyl backbone (Scheme 1b). The new
clusters are [Nd Zn (L') (OAc) (NO (H O) and
Ln Zn (L'') (OAc)18] (Ln = Nd ( ) and Sm ( )). Besides the
long Schiff base ligand, aromatic bidentate ligand 1,3-
diphenylpropane-1,3-dione (H L') and tridentate ligand 3-
a
7
-12
In
2
2
L
2
2
2
3
)
2
2
2
]
(1)
14-16
2
Many high-nuclearity Cu-
[
6
6
L
2
2
O
2
2
3
a.College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou
2
3
25035, China.; Zhejiang Key Laboratory of Carbon Materials, Wenzhou 325035,
methoxy-benzene-1,2-diol (H
ligands for the formation of
approximately 20 Å, which is advantageous for the formation
of large metal clusters. and show linear and planar
2
L'') are selected as secondary
China.
b.
1
-
3
. The length of H L is
2
Chemical Biology Research Center, School of Pharmaceutical Science, Wenzhou
Medical University, Wenzhou 325035, China.
*Email - xpyang@wzu.edu.cn
1
2-3
Electronic Supplementary Information (ESI) available: Experimental and
characterization details, additional figures and tables. CCDC 1842949-1842951 for structures with sizes of approximately 10 × 10 × 30 Å and 10 ×
1-3. For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/x0xx00000x.
This journal is © The Royal Society of Chemistry 20xx
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Page 2 of 10
ARTICLE
Journal Name
5 × 22 Å, respectively. It is noticeable that, in these clusters, In the NdZnLL' moiety, Nd and Zn2+ ions are linked by one
3
+
1
View Article Online
DOI: 10.1039/C8TC02426J
-
the Schiff base ligands exhibit an "U-shaped" configuration,
OAc anion and phenolic oxygen atoms of two L ligands with a
O
O
O
N+
N+
N+
-
O
-O
-O
nitrobenzene (NB) 2-nitrotoluene (2-NT) 4-nitrotoluene (4-NT)
O-
N+
O-
N+
O
O
N+
30 Å
O
-O
O
3+
2+
Figure 1. A view of the crystal structure of
1 (Nd : blue; Zn :
acetophenone (4-NBAP)
1,4-dinitrobenzene (1,4-DNB)
green)
O-
N+
-
O
N+
separation of 3.487 Å. Nd3+ ion is nine-coordinate, surrounded
O
N+
O
N+
O
N+
O
N+
O
-
and OAc-
O-
O-
N
O
by nine oxygen atoms from L and L' ligands, and NO
anions. Zn ion shows an octahedral geometry, coordinated
3
-O
-O
N
O
N
N
2+
N
N+
N
N
with four oxygen and two nitrogen atoms from two L ligands,
O
N+
N+
+ O
N
O-
-
-
O
O
O-
one OAc anion and one H
2
O molecule. The Schiff base ligand is
-
O
bound to all metal ions through its N and phenoxide and
methoxy O atoms, while the L' ligand is only bound to one Nd3+
2
,4,6-trinitrotoluene (TNT) cyclotrimethylene
cyclotetramethylene
trinitramine (RDX) tetranitramine (HMX)
(a)
ion through its two O atoms. The metric dimensions of 1
measure approximately 10 × 10 × 30 Å. The Zn-O, Zn-N and
Nd-O bond lengths range from 1.922 Å to 2.400 Å, 2.130 Å to
Ln
OCH3
Zn
O
N
2
.196 Å, and 2.285 Å to 2.536 Å, respectively.
N
O
Zn
H3CO
The construction of the Zn-Ln nanoclusters appears to be
Ln
the secondary ligand dependant. Reactions of H
with Zn(OAc) ·2H O and Ln(NO ·6H O (Ln = Nd and Sm)
under the same experimental conditions as above produced
Scheme 1. (a) The structures of nitro explosives; (b) The yellow solutions from which isomorphous and were
coordination mode of Schiff base ligand H L in isolated as yellow crystalline solids. As shown in Figure 2, the
X-ray structure of reveals a centrosymmetric core with two
which may help to "capture" guest aromatic compounds such equivalent Nd Zn LL''O(OAc) moieties linked by two Schiff
as nitro-aromatic explosives through π-π and electrostatic base ligands. In each Nd Zn LL''O(OAc)
moiety, the Nd3+ ions
interactions. Among the Ln(III) ions, Sm(III) may show emission are 8-coordinate, surrounded by eight oxygen atoms from L
2 2
L and H L''
20 Å
2
2
3
)
3
2
(b)
2
3
2
1-3.
2
3
3
9
3
3
9
4
6
4
6
-
2-
bands both in the visible ( G5/2→ H
J
) and in the NIR ( G5/2→ F
J
)
and L'' ligands, and OAc and O anions. They are linked
3
1
2-
-
ranges. Interestingly, the 12-metal Zn-Sm cluster shows together by the L'' ligand, one O and four OAc anions with an
visible and NIR luminescent sensing of nitro explosives, and average separation of 3.632 Å. Three Zn ions have
exhibits high sensitivity to nitro-aromatic 1,4-DNB and TNT at tetrahedral, square pyramidal and triangular bipyramidal
ppb level. To the best of our knowledge,
report of high-nuclearity d-f nanocluster for both visible and ions are linked by L and L'' ligands, and OAc and O anions.
3
2+
3
+
2+
3 represents the first geometries, respectively. Each Nd ion and its closest two Zn
-
2-
NIR luminescent sensing of nitro-aromatic explosives.
The Zn(II)···Nd(III) distances range from 3.385 Å to 3.909 Å in 2.
2+
3+
The Schiff base ligand is bound to two Zn and two Nd ions
2
+
through its N and O atoms. The L'' ligand is bound to two Zn
Results and discussion
3+
and three Nd ions through its three O atoms. The metric
dimensions of
Zn-O, Zn-N and Nd-O bond lengths range from 1.934 Å to
.387 Å, 2.079 Å to 2.114 Å, and 2.216 Å to 2.603 Å
respectively.
In the solid state, the structures of
networks (Figure 3a), with guest molecules such as Et
MeOH and H O accommodated within the channels. For
2 measure approximately 10 × 15 × 22 Å. The
Synthesis and structures
The Schiff base ligand H
-hydroxy-3-methoxybenzaldehyde
naphthalenediamine in refluxing ethanol with a yield of 98%.
2
L was synthesized from the reaction of
2
2
with 1,5-
32
1
2
and
3
have open 3D
O,
2
The H NMR spectrum of H L shows a signal for imino proton (-
2
CH=N-) at 8.76 ppm (Figure S1 in the ESI), while the signal for
the aldehyde proton (Ar-CHO) of the reactant at 10.0 ppm
2
-
-
example, in
3
intermolecular Ar-H···O(OAc ) and (OAc )C-
)C-
between each
disappears. In the presence of Et
with Zn(OAc) ·2H O and Nd(NO
produced a yellow solution from which
yellow crystalline solid. A view of the crystal structure of
shown in Figure 1. The X-ray structure of reveals a
centrosymmetric core with two equivalent NdZnLL' moieties.
3
N, reaction of H
·6H O in refluxing methanol
was isolated as pale
is
2 2
L and H L'
-
H···O(OAc ) hydrogen bonds (2.472 Å–2.669 Å) and (-OCH
3
2
2
3
)
3
2
33,34
H···
interactions (2.932 Å–4.001 Å)
1
molecule occur with eight of its closest neighbours, resulting in
a network architecture with extended channels running along
the ac-axes measuring approximately 7 × 7 Å (Figure 3a). In
1
1
2
| J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry C
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ARTICLE
Figure 3b, a panoramic scanning electron microscopy (SEM)
image shows the crystalline nature of . Energy dispersive X-
The photophysical properties of
1
-
3
were studied in DMF
View Article Online
DOI: 10.1039/C8TC02426J
3
solution and the solid state. The free ligand H
absorption bands at 220, 274, 356 nm, which are red-shifted
upon co-ordination to metal ions in (Figure 4). For the free
ligand H L, excitation at the absorption wavelength produces
broad emission bands at 468, 482 and 499 nm (Figure S5).
Upon excitation of the ligand-centered absorption bands,
2
L exhibits
1-3
2
1
exhibit typical NIR luminescence of Nd ( F3/2→4Ij/2
shows visible
and NIR emission spectra for Sm ( G5/2 → Hj/2 transitions, j =
3+
4
and
2
transitions, j = 9, 11 and 13) (Figure 5), while
3
3+
4
6
4
6
5
, 7, 9 and 11; G5/2 → Fj/2 transitions, j = 1, 3, 5, 7 and 9)
(
Figure 6). show two excitation bands from 277 nm to 411
1-3
nm, in agreement with their absorption spectra, confirming
that the energy transfers from the chromogenic ligands to Ln3+
ions occur. For each Zn-Ln cluster, the luminescence spectrum
in the solid state is similar to that in the solution. The
excitation and emission wavelengths (λex and λem), the
absorption of excitation wavelengths (ε), quantum yields (
emission lifetimes (τ) and energy transfer efficiencies (ηsens) of
in solution are given in Table 1.
As shown in Figure 5, both
bands of Nd3+ at about 873, 1066 and 1335 nm. The emission
lifetimes (τ) of and in DMF are 6.08 μs and 6.10 μs,
respectively (Figure S6). The intrinsic quantum yields (
em),
1-3
1
and 2 display NIR emission
1
2
2
2Å

Ln) of
3
+
Figure 2. Two views of the planar structure of
the bc-axis (top) and a-axis (lower). (Nd : blue; Zn : green)
2
: viewed along Nd emission in
1
and
2
are calculated as 2.43% and 2.44%,
3+
2+
35
respectively, using
Ln = τ/τ
0
(τ
0
= 250 μs, the natural lifetime
3+
of Nd ). The emission quantum yields (
em) of
1
and
2
are
0
.75% and 0.95%, respectively. Thus, the efficiencies (ηsens) of
(a)
(b)
3+
the energy transfer from Zn/Ligand-center to Nd
1
and
2
are
calculated as 30.86% and 38.93%, respectively, using ηsens
=
36

em
/

Ln
.
This indicates that the chromogenic ligands in
2
has
higher energy transfer efficiency than in
1
. It is found that
3+
there is one coordinated H
2
O molecule near the Nd ion in 1,
(
c)
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
H L
2
1
2
3
H L:×2
2
Figure 3. Space filling view along the ac-axis (a), SEM image (b),
and EDX spectrum (c) of
3
.
ray spectroscopy (EDX) analysis of
3 indicates that the molar
2
00
250
300
350
400
450
500
ratio of Zn/Sm is 0.97 (Figure 3c), in agreement with the crystal
structure.
nm
2
Figure 4. UV-Vis spectra of the free H L and clusters 1-3 in DMF
Powder XRD patterns of
patterns generated from single crystal X-ray data (Figure S3).
Thermogravimetric analyses show that on heating and
undergo weight losses of about 2% and 5% before 100 C
Figure S4), respectively, assigned to the loss of uncoordinated
solvent molecules such as Et O, MeOH and H O. Melting point
measurements show that 1-3 are thermodynamically stable,
1-3 are similar to their simulated at 298 K.
1
2-3
o
1
2
(
2
2
o
o
starting to discompose from 172.4 C to 175.7 C (ESI).
Luminescence properties
400
600
800
/nm
1000 1200 1400

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ARTICLE
Journal Name
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DOI: 10.1039/C8TC02426J
Figure 5. Excitation and emission spectra of
1
and
2
in DMF.
Figure 7. The visible emission lifetime of 3 in DMF.
Table 1. The excitation and emission wavelengths (λex and λem), the absorption of excitation wavelengths (ε), quantum yields (em),
a
emission lifetimes (τ) and energy transfer efficiencies (ηsens) of 1-3 in solution. ( The addition of 30 µM 1,4-DNB)
5
-1
-1
Clusters
λ
ex (nm) / ε (× 10 M cm )
λ
em (nm)
τ (µs)
Φ
em (%)
ηsens (%)
1
2
3
311/0.07, 411/0.03
872, 1067, 1334
6.08
0.75
30.86
38.93
38.55
—
300/0.17, 404/0.09
874, 1065, 1335
6.10
0.95
Vis. 277/0.33, 364/0.29
NIR 279/0.34, 360/0.28
562, 598, 645, 706
912, 953, 1030, 1178
24.75 (21.01 a)
23.15
0.32 (0.09 a)
0.29
which may efficiently quench the luminescence of lanthanide Luminescent sensing of explosives
ions through non-radiative exchange of electronic energy of In the planar structure of and
2
3
, two Schiff base ligands
3+
37
Ln to the high vibrational modes of OH
-
groups.
3
exhibit an outward "U-shaped" configuration with two
, the G5/2 → 6H7/2 benzene rings turning to the same side of H
L and almost
9/2 (645 nm) transitions are perpendicular to the middle naphthalene ring (Figures 1, 2 and
hypersensitive transitions and responsible for the most intense S7a). The sensing ability of for nitro explosives such as
lines of Sm(III) emission (Figure 6). The visible emission lifetime nitrobenzene (NB), 2-nitrotoluene (2-NT), 4-nitrotoluene (4-
τ) of is 24.75 μs in DMF (Figure 7), which is comparable to NT), 4-nitrobenzene acetophenone (4-NBAP), 2,4,6-
values reported for other Sm(III) complexes.
4
For the visible emission spectrum of
2
4
6
(598 nm) and
G
5/2
→
H
3
(
3
3
8,39
The intrinsic trinitrotoluene
(TNT),
1,4-dinitrobenzene
(1,4-DNB),
3
+
quantum yield (
.83%, using
Ln) of Sm emission in 3 is calculated as cyclotrimethylene trinitramine (RDX) and cyclotetramethylene
0

Ln = τ/τ
0
(τ
0
= 3.0 ms,40 the natural lifetime of tetranitramine (HMX) has been investigated in DMF (Scheme
Sm ). The visible emission quantum yield (
3+
4
em) of
3
is 0.32%. 1b). The intensities of the visible emission at 645 nm ( G5/2
→
6
4
6
The efficiency (ηsens) of the energy transfer from the Zn/Ligand-
H9/2) and NIR emission at 953 nm ( G5/2 → F5/2) of
3
were
3+
36
center to Sm is calculated as 38.55%, using ηsens
=

em
/

Ln
.
recorded as the explosives were added with different
For
3, the main line in the NIR area is at 953 nm due to the concentrations. The luminescence intensities at 645 nm of the
4
6
3+
G
5/2 → F5/2 transition, and a long NIR emission lifetime (τ), up Sm ions are gradually decreased as the concentrations of
to 23.15 μs was recorded in DMF (Figure S6). The NIR emission explosives increase (Figures 8 and S8). Interestingly, the visible
quantum yield (em) of
3
is 0.29%.
lanthanide luminescence was found to be quenched rapidly
upon the addition of 1,4-DNB and TNT. For example, the visible
emission intensities are decreased about 50% when the
concentrations of added 1,4-DNB and TNT solutions are 7.5
and 21 µM, respectively. However, the concentrations of other
nitro explosives added should be more than 1000 µM to
3
-Vis
3-NIR
reduce the visible emission intensities of
As shown in Figure 9, the luminescence quenching percentages
of towards 1,4-DNB and TNT are 70% and 51%, respectively.
While, all of the other nitro explosives have minor effects on
the luminescence intensity of at low content (< 30 µM), with
the quenching efficiencies less than 10%. This indicates that
3 by half (Figure S8).
3
3
3
400
600
800
/nm
1000
1200
has high sensitivity towards 1,4-DNB and TNT over other nitro
explosives.

Figure 6. Excitation and emission spectra of 3 in DMF.
3
2
2
2
1
0μM
8μM
4μM
1μM
8μM
4
4
3
3
2
2
1
1
0
.5
.0
.5
.0
.5
.0
.5
.0
15μM
11μM
3
3
-Vis
-Vis
9
μM
7.5μM
μM
4.5μM
.5μM
2.5μM
6
3
1.5μM
1.0μM
0.5μM
0μM
550
600
650
700
750
.5
0
/nm
.00
0.05
0.10
0.15
0.20
Time (ms)
4
| J. Name., 2012, 00, 1-3
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Journal Name
Figure 8. Decrease in visible luminescence intensity of
ARTICLE
solution resulted in significant luminescence quVeienwcAhritnicgle O(ntlhinee
3
(
λ
ex
=
3
64 nm) in DMF (15 µM) upon the addition of different increase in concentration of 1,4-DNB wasDaOlIs:o105.10µ3M9/Ce8aTcCh0t2i4m26eJ).
concentrations of 1,4-DNB.
The further addition of another explosive still showed little
Figure 9. The visible luminescence quenching efficiencies of
ex = 364 nm) (15 µM) towards explosives (30 µM).
3
(λ
Figure 10. The visible luminescence quenching constants (KSV
)
of 3 towards explosives.
The luminescence quenching constants (KSV) can be
calculated by the Stern-Volmer (SV) equation,41
18.5μM
6.5μM
14.5μM
0
I /I = KSV[A] + 1,
1
0
where I and I are the emission intensities before and after the
addition of the explosive, respectively, and [A] is the molar
concentration of the explosive. As shown in Figure 10, the KSV
11.0μM
7.5μM
6.0μM
4.5μM
values for the response of
3
to 1,4-DNB and TNT are calculated
3.0μM
2.0μM
1.5μM
1.0μM
0.5μM
as 75350 M- and 40910 M , respectively, which are among
1
-1
the highest values of known fluorescent complexes for the
detection of nitro explosives.
other explosives are only from 179 M to 1010 M . The
detection limit can be calculated by the 3σ/Ksv equation, where
σ is the standard deviation. The visible luminescence detection
5,6,42,43
While the KSV values to
0μM
-
1
-1
800
900
1000 1100 1200 1300

/nm
limits of
respectively, demonstrating the extremely high sensitivity of
towards the detection of these two explosives at the ppb level. concentrations of 1,4-DNB.
For the NIR luminescence of , the additional of
3
to 1,4-DNB and TNT are 0.08 µM and 0.15 µM, Figure 11. Decrease in NIR luminescence intensity of
3
(
λ
ex
=
3
360 nm) in DMF (15 µM) upon the addition of different
3
explosives leads to varying degrees of weakening of the
emission intensities (Figures 11, S8 and S9). For example, when
the concentration of added 1,4-DNB solution is 11 µM, the
emission intensity is decreased about 50%. As shown in Figure
1
2, the KSV values for the response of 3 to nitro-aromatic
explosives are higher than those to nitro-aliphatic explosives
such as RDX and HMX. It is noticeable that, the KSV value to
3
-1
1
1
,4-DNB (104.97 × 10 M ) is much greater than others (3.47 ×
3
-1
3
-1
0 M - 28.17 × 10 M ), indicating that
3 exhibits higher
sensibility to 1,4-DNB. The NIR luminescence detection limit of
to 1,4-DNB is calculated as 2.86 µM. The reusability of was
investigated using the powder XRD. The powder XRD pattern
of the recycled sample of after treated with 1,4-DNB is still Figure 12. The NIR luminescence quenching constants (KSV) of
3
3
3
similar to its simulated pattern generated from single crystal X-
ray data (Figure S3d), indicating that the cluster is reusable in
the sensing of nitro explosives.
3
towards explosives.
effect on luminescence intensity, but the further addition of
The selectivity of
other explosives was investigated. The initial luminescence stepwise decrease in luminescence intensities demonstrates
spectrum of (15 µM) in DMF was recorded. As shown in the high selectivity of for 1,4-DNB even in the presence of
Figure 13, at first very few luminescence quenching was other nitro explosives.
observed when another explosive was added twice (the Photoinduced electron transfer (PET) and resonance
3 towards 1,4-DNB in the presence of 1,4-DNB gave significant luminescence quenching again. The
3
3
increase in concentration of explosive was 5 µM each time). energy transfer (RET) mechanisms have been widely used to
However, the addition of equal amount of 1,4-DNB to this explain the high sensitivity of fluorescence-quenching-based
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Journal of Materials Chemistry C
Page 6 of 10
ARTICLE
Journal Name
sensors towards nitroaromatic explosives.44 For the PET sensing of TNT. While almost no overlap was oVibewseArrtviceledOnflioner
process, usually, the conduction band (CB) of the chemosensor other explosives. In addition, it is noticDeOaIb:l1e0.1t0h3a9t/,C8bToCt0h2412,64J-
lies higher than the LUMO energies of the nitro analytes
DNB and TNT show higher absorption at the excitation
100
-
-
2
3
LUMO
80
60
40
20
-
4
-
5
6
1
1
1
1
1
1
,4-DNB-4-NB
,4-DNB-2-NT
,4-DNB-4-NT
,4-DNB-4-NBAP
,4-DNB-RDX
,4-DNB-HMX
-
-
7
HOMO
NB
-8
-9
0
10
20
30
40
50
60
2-NT
TNT
1,4-DNB
RDX
Explosives (μM)
Figure 13. The decrease in luminescence intensities of
3 upon Scheme 2. HOMO and LUMO energies for the selected
the addition of various nitro explosives followed by 1,4-DNB in explosives.
DMF.
NB
2-NT
Scheme 2),45,46 and the electrons can be transferred from the
electron rich chemosensor (i.e. H L) to electron-deficient nitro
analytes, resulting in fluorescence quenching (Scheme S1). The
luminescence of results from the antenna effect in which the
Zn/L moieties absorb the energy and transfer it to Sm ions.
Thus, in when the excitation energy absorbed by Zn/L
0.8
.6
0.4
(
4
-NBAP
2
TNT
0
1
,4-DNB
3
RDX
HMX
Em-3
3+
3
moieties is consumed through the PET process, the lanthanide
luminescence intensity is decreased. Since charge transfer is
generally considered to be a short range interaction, the
0
.2
0.0
configuration of the Schiff base ligands in
significant role in the sensing selectivity to nitro explosives. As
shown in Figure S7a, the "U-shaped" configuration of the Schiff Figure 14. Spectral overlap between normalized absorbance
3
may play a
300
400
500
nm
600
700
base ligands is found to be more suitable to form
interactions and -CH···O(NO -Ar) hydrogen bonds with 1,4-DNB
and TNT, which have more than one nitro groups, than other
explosives, resulting in the high sensitivity to these two wavelength
explosives. For 1,4-DNB and TNT, the intensities of their M cm and 0.20 × 10 M cm for 1,4-DNB and TNT,
interactions with the Schiff base ligand may be different. As respectively). This means 1,4-DNB and TNT can compete with

···

spectra of explosives and the normalized emission spectra of
(λex = 364 nm) in DMF.
3
2
5
(
364 nm) than other explosives (i.e. ε = 0.10 × 10
-1
-1
5
-1
-1
shown in Figure S7b, when -CH···O(NO
are formed between and explosives, 1,4-DNB has more the probability of energy transfer from Zn/Ligand moieties to
overlap with the naphthyl ring of L than TNT, resulting in a Sm
stronger interaction between 1,4-DNB and L. Thus, shows a luminescence. Compared with 1,4-DNB, TNT has higher
higher quenching constant (KSV) towards 1,4-DNB. The absorption at the excitation wavelength and its absorption
formation of a ground-state charge-transfer (CT) complex spectrum has a larger overlap with the emission spectrum of
between the -electron-rich Schiff base ligands of and the Thus, for PET and RET mechanisms, the greater KSV value of
electron-poor 1,4-DNB was investigated by UV-vis titration and towards 1,4-DNB indicates that the PET mechanism is
2
-Ar) hydrogen bonds the Zn/Ligand moieties for the light energy, thus decreasing
3
3+
and subsequently quenching the lanthanide
4
3
3
3
.

3
3
21
lifetime measurements. The addition of 1,4-DNB caused a dominant in the luminescence quenching process of
significant blue-shift and increase in the absorbance intensity
3.
at 288 nm (Figure S10). Meanwhile, the lifetime of
obviously from 24.75 μs to 21.01 μs when the concentration of
added 1,4-DNB is 30 μM (Table 1).
3 decreases
Experimental
Materials and methods
Energy transfer is considered to be a long range process.
When the emission band of the fluorophore has an effective
overlap with the absorption band of the analyte, the
resonance energy transfer (RET) can occur, resulting in the
quenching of fluorescence. As shown in Figure 14, the
absorption spectrum of TNT has a large overlap with the
2
The Schiff-base ligand H L was prepared according to well-
29
established procedures.
Physical measurements: NMR:
1
AVANCE III AV500. 500 spectrometer ( H, 500 MHz) at 298 K;
IR: Nicolet IS10 spectrometer; Elemental analyses (C, H, N) of
compounds were carried out on a EURO EA3000 elemental
o
analysis after dried in an oven at 100 C for 2 hours. Melting
emission spectrum of
3, resulting in the highly selective
points were obtained in sealed glass capillaries under
6
| J. Name., 2012, 00, 1-3
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Page 7 of 10
Journal of Materials Chemistry C
Journal Name
ARTICLE
-
1
dinitrogen and are uncorrected. The thermogravimetric cm ): 1610 (s), 1550 (s), 1460 (s), 1390 (s), 1210 (m), 974 (w),
View Article Online
DOI: 10.1039/C8TC02426J
analyses were carried out on a TA Instruments Q600. The UV- 786 (w), 747 (w), 689 (w).
visible absorption spectra were recorded at RT using an UV-
3
600 spectrophotometer. Field emission scanning electron Photophysical Studies
microscopy (FESEM) images were recorded on a Nova Visible and NIR luminescence spectra were recorded on a FLS
NanoSEM 200 scanning electron microscope.
980 fluorimeter. The light source for the spectra was a 450 W
xenon arc lamp with continuous spectral distribution from 190
to 2600 nm. Liquid nitrogen cooled Ge PIN diode detector was
Synthetic procedures
H
2
L.
Hydroxy-3-methoxybenzaldehyde (20.0 mmol, used to detect the NIR emissions from 800 nm to 1700 nm.
.0430 g) was dissolved in 15 mL EtOH, and a solution of 1,5- The temporal decay curves of the fluorescence signals were
naphthalenediamine (10.0 mmol, 1.5808 g) in 20 mL EtOH was stored by using the attached storage digital oscilloscope. The
then added drop by drop. The resulting solution was stirred quantum yields ( em) were obtained by using an integrating
and heated under reflux for 2.5 hours. It was allowed to cool sphere, according to eqn
3

em = Nem / Nabs, where Nem and Nabs
and was then filtered. The solid was washed with EtOH (3 × 5 are the numbers of emitted and absorbed photons,
ml) and then dried in the air at room temperature to give respectively. Systematic errors have been deducted through
yellow product. Yield (based on 1,5-naphthalenediamine): the standard instrument corrections. All the measurements
4
6
.1793 g (98%). Elemental analysis: Found: C, 73.44; H, 5.66; N, were carried out at room temperature.
.55%; Calc. for C26 : C, 73.23; H, 5.20; N, 6.57%. IR
22 2 4
H N O
-
1
(CH
3
CN, cm ): 1600 (s), 1507 (m), 1460 (s), 1410 (m), 1250 (s), Crystallography
1
1
080 (m), 965 (s), 781 (s), 685 (m). H NMR δ H (500 MHz,
Data were collected on a Smart APEX CCD diffractometer
3
CDCl ) 8.76 (2H, s), 8.26 (2H, d), 7.56 (2H, m), 7.29 (2H, d), 7.26 with graphite monochromated Mo-Kα radiation (λ = 0.71073
(
4H, s), 7.11 (2H, d), 7.06 (2H, d), 6.95 (2H, t), 3.99 (6H, s).
Nd Zn (L) (L (OAc) (NO (H O) ). Zn(OAc) ·2H
0.40 mmol, 0.0876 g), Nd(NO ·6H
L (0.30 mmol, 0.1333 g) and H L' (0.35 mmol, 0.0785 g) were were solved by direct methods and refined anisotropically
Å) at 190 K. The data set was corrected for absorption based
[
2
2
2
'
)
2
2
3
)
2
2
2
]
(1
2
2
O
on multiple scans and reduced using standard methods. Data
47
(
H
3
)
3
2
O (0.30 mmol, 0.1315 g), reduction was performed using DENZO-SMN. The structures
2
2
dissolved in MeOH (50 mL) at room temperature, and a using full-matrix least-squares methods with the SHELX 97
48
solution of triethylamine in EtOH (1.00 mol/L, 1 ml) was then program package. Coordinates of the non-hydrogen atoms
added. The resulting solution was stirred and heated under were refined anisotropically, while hydrogen atoms were
reflux for 30 mins. It was allowed to cool and was then filtered. included in the calculation isotropically but not refined.
Diethyl ether was allowed to diffuse slowly into the filtrate at Neutral atom scattering factors were taken from Cromer and
49
room temperature and pale yellow crystals were obtained Waber. Crystallographic data for 1-3 have been deposited
after one month. The crystals were filtered off, washed with with the Cambridge Crystallographic Data (CCDC reference
MeOH (5 ml) and dried in the air for one week. Yield (based on numbers 1842949-1842951). These data can be obtained free
o
Nd(NO
3
)
3
·6H
2
O): 0.1400 g (47 %). m. p. > 172.4 C (dec.). of charge via www.ccdc.cam.ac.uk/data_request/cif. Selected
Elemental analysis: Found: C, 51.85; H, 3.77; N, 4.15%. Calc. bond lengths of
Nd : C, 51.81; H, 3.69; N, 4.22%. IR (CH CN, Information).
cm ): 1610 (s), 1460 (s), 1390 (s), 1210 (s), 975 (m), 786 (s),
41 (s), 684 (m).
Nd Zn (L''
same as that for
Nd(NO ·6H O (0.35 mmol, 0.1534 g), H
g) and H L'' (0.40 mmol, 0.0561 g). Pale yellow single crystals
of
Nd(NO
Elemental analysis: Found: C, 35.76; H, 3.80; N, 1.49%. Calc. for
Zn : C, 36.34; H, 4.13; N, 1.44%. IR (CH CN, 0.0953, wR
cm ): 1610 (s), 1550 (s), 1390 (s), 1210 (s), 975 (w), 792 (m), goodness-of-fit of 1.083.
43 (m), 686 (w).
Sm Zn (L''
same as that for
Sm(NO ·6H O (0.30 mmol, 0.1356 g), H
g) and H L'' (0.40 mmol, 0.0565 g). Pale yellow single crystals wR
of
Sm(NO
1-3 are given in Tables S1-S3 (Supporting
For C86
H
72
N
6
2
O
24Zn
2
3
-
1
1
: C86
H
72
N
6
O
24Zn
2
Nd
2
, orthorhombic, space group Pbca, a
7
= 15.923(3), b = 20.416(4), c = 24.500(5) Å, α = 90°, β = 90°, γ =
3
-3
[
6
6
L
2
)
2
O
2
(OAc)18
] (2). The procedure was the 90°, V = 7965(3) Å , Z = 4, Dc = 1.662 g cm , μ(Mo-Kα) = 1.961
-
1
1
using Zn(OAc)
2
·2H
2
O (0.40 mmol, 0.0876 g), mm , F(000) = 4008, T = 190 K. R
L (0.40 mmol, 0.1705 6779 independent reflections with a goodness-of-fit of 1.086.
Nd , monoclinic, space group P2(1)/n,
1 2
= 0.0781, wR = 0.2313 for
3
)
3
2
2
2
2
: C118
H
158
N
4
O63Zn
6
6
2
were formed after one month. Yield (based on a = 18.0116(4), b = 22.1288(5), c = 19.8728(3) Å, α = 90°, β =
o
3
-
3 3 2
) ·6H O): 0.0882g (39%). m. p. > 174.6 C (dec.). 114.9620(10)°, γ = 90°, V = 7180.9(3) Å , Z = 2, Dc = 1.803 g cm
3
-1
, μ(Mo-Kα) = 3.201 mm , F(000) = 3876, T = 190 K. R
1
=
C
118
H
158
N
4
O
63Nd
6
6
3
2
= 0.2705 for 7678 independent reflections with a
-
1
7
3
: C103
H
110
N
4
O
58Zn
6 6
Sm , monoclinic, space group P2(1)/n,
[
6
6
L
2
)
2
O
2
(OAc)18
] (3). The procedure was the a = 18.465(7), b = 22.557(8), c = 20.143(7) Å, α = 90°, β =
3
-3
1
using Zn(OAc) ·2H O (0.45 mmol, 0.0988 g), 115.879(6)°, γ = 90°, V = 7549(5) Å , Z = 2, Dc = 1.592 g cm ,
2 2
-1
3
)
3
2
2
L (0.30 mmol, 0.1333 μ(Mo-Kα) = 3.306 mm , F(000) = 3536, T = 190 K. R
1
= 0.0994,
2
2
= 0.2647 for 12887 independent reflections with a
were formed after one month. Yield (based on goodness-of-fit of 1.021.
2
o
3
)
3
·6H
2
O): 0.0850 g (47%). m. p. > 175.7 C (dec.).
Elemental analysis: Found: C, 35.41; H, 3.77; N, 1.47%. Calc. for
Zn : C, 33.94; H, 3.54; N, 1.54%. IR (CH CN,
Conclusions
C
103
110
H N
4
O
58Sm
6
6
3
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ARTICLE
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1
3 F. J. Steemers, W. Verhoeven, D. N. Reinhoudt,VEie.wBA.rtVicalenOndliener
DOI: 10.1039/C8TC02426J
4 A. Müller, Nature 2007, 447, 1034–1035.
In conclusion we have successfully demonstrated the
application of a "two-ligand" approach to the construction of
Tol, J. Am. Chem. Soc., 1995, 117, 9408–9414.
1
1
two types of Zn-Ln nanoclusters
ligand (H L) featuring a electron-rich naphthyl backbone and
two short polydentate ligands (H L' and H L''). The 4-metal Zn-
Nd cluster and 12-metal Zn-Nd/Sm clusters show linear
and planar structures with sizes of approximately 10 × 10 × 30
Å and 10 × 15 × 22 Å, respectively. In the solid state, both
and structures have open 3D networks with extended
channels (i.e. 7 × 7 Å for ). All
of lanthanide ions. For the Zn-Nd clusters,
luminescence properties than . In these clusters, the Schiff
base ligands exhibit an "U-shaped" configuration. The Zn-Sm
cluster shows visible and NIR luminescent sensing of nitro
explosives and exhibits high selectivity and sensitivity to 1,4-
DNB and TNT. To the best of our knowledge, represents the
1-3 with one long Schiff base
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Conflicts of interest
There are no conflicts to declare.
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Journal of Materials Chemistry C
Journal Name
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Journal of Materials Chemistry C
Page 10 of 10
View Article Online
DOI: 10.1039/C8TC02426J
Table of Contents
Selfꢀassembly of Luminescent 12ꢀmetal ZnꢀLn Planar Nanoclusters with Sensing Properties
towards Nitro Explosives
Dongmei Jiang, Xiaoping Yang, Xiaohui Zheng, Le Bo, Ting Zhu, Hongfen Chen, Lijie Zhang and
Shaoming Huang
Two types of 4- and 12-metal Zn-Ln nanoclusters (Ln = Nd and Sm) were constructed using one long Schiff base
ligand with a naphthyl backbone and two short polydentate ligands, and the 12-metal Zn-Sm cluster shows
interesting visible and NIR luminescent sensing of nitro explosives with high sensitivity to 1,4-DNB and TNT at
ppb level.
Hydrogen bond
30µM
8µM
24µM
21µM
18µM
UV-vis
2
1
,4-DNB
π
e
π
1
1
9
7
6
4
3
2
1
1
0
0
5µM
1µM
µM
.5µM
µM
.5µM
.5µM
.5µM
.5µM
.0µM
.5µM
µM
π
π
e
550
600
650
/nm
700
750
Vis/NIR
Hydrogen bond
λ