Substituent-tuned structure and luminescence sensitizing towards Al3+ based on phenoxy bridged dinuclear EuIII complexes

Article abstract of DOI:10.1039/c7ra02386c

To develop a Ln^III complex-supported chemsensor, two new phenoxy bridged dinuclear Eu^III complexes, [Eu₂(H₂L)₃(NO₃)₃]·3CH₃CN (EuL) and [Eu₂(H₂L′)₂(NO₃)₄]·3CH₃CN (EuL′), constructed by two new structurally related salicylamide salen-like ligands, 1-(2-hydroxy-benzamido)-2-(2-hydroxy-5-nitro-benzylideneamino)-ethane (H₃L) and 1-(2-hydroxy-benzamido)-2-(2-hydroxy-4-diethylamino-benzylideneamino)-ethane (H₃L′), have been synthesized and structural analysis shows that the different substitution groups on 2-(iminomethyl)phenol moiety have significant effects on their structures. Upon excitation of the ligand-centered absorption band at 375 nm, emissions both originating from ligands and Eu^III ions were observed in the two Eu^III compounds with the Eu^III-centered emission intensity more than three times higher than that of ligand-centered emission. The capability of EuL and EuL′ for selective detection Al³⁺ ions were evaluated and the results indicate EuL exhibits a turn-on luminescent enhancement as high as 5.7 fold with K_d = 1.53 × 10^-4 in CH₃CN, but comparable compound EuL′ could not detect Al³⁺ among various cations. The considerably 'turn-on' luminescence response of EuL concomitantly led to the apparent color change from reddish to brilliant red, which could also be identified by naked eyes easily under UV lamp. This luminescence enhanced response can be explained in terms of the decrease of non-radiative transitions in EuL in addition to excited-state intra-molecular proton transfer (ESIPT) and photo-induced electron transfer suppression upon Al³⁺ coordination which is also rationalized by a theoretical calculation.

Full text of DOI:10.1039/c7ra02386c

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Substituent-tuned structure and luminescence
3
+
sensitizing towards Al based on phenoxy bridged
dinuclear Eu complexes†
Cite this: RSC Adv., 2017, 7, 25549
III
Jing-Jing Zhou, Xue-Qin Song, * Yuan-Ang liu and Xiao-Long Wang
III
III
To develop a Ln complex-supported chemsensor, two new phenoxy bridged dinuclear Eu
0
0
complexes, [Eu
two new structurally related salicylamide salen-like ligands, 1-(2-hydroxy-benzamido)-2-(2-
2 2 3 3 3 3 2 2 2 3 4 3
(H L) (NO ) ]$3CH CN (EuL) and [Eu (H L ) (NO ) ]$3CH CN (EuL ), constructed by
3
hydroxy-5-nitro-benzylideneamino)-ethane (H L) and 1-(2-hydroxy-benzamido)-2-(2-hydroxy-4-
0
diethylamino-benzylideneamino)-ethane (H
3
L ), have been synthesized and structural analysis shows
that the different substitution groups on 2-(iminomethyl)phenol moiety have significant effects on
their structures. Upon excitation of the ligand-centered absorption band at 375 nm, emissions both
III
III
III
originating from ligands and Eu ions were observed in the two Eu compounds with the Eu -
centered emission intensity more than three times higher than that of ligand-centered emission.
0
3+
The capability of EuL and EuL for selective detection Al
ions were evaluated and the results
ꢁ4
indicate EuL exhibits a turn-on luminescent enhancement as high as 5.7 fold with K
d
¼ 1.53 ꢀ 10
among various cations. The
0
3
in CH CN, but comparable compound EuL could not detect Al
3+
considerably ‘turn-on’ luminescence response of EuL concomitantly led to the apparent color
change from reddish to brilliant red, which could also be identified by naked eyes easily under UV
lamp. This luminescence enhanced response can be explained in terms of the decrease of non-
radiative transitions in EuL in addition to excited-state intra-molecular proton transfer (ESIPT) and
Received 26th February 2017
Accepted 29th April 2017
DOI: 10.1039/c7ra02386c
3+
photo-induced electron transfer suppression upon Al coordination which is also rationalized by
rsc.li/rsc-advances
a theoretical calculation.
III
a Ln -centred excited state has been reached via the antenna
effect, the rules for radiative de-excitation are the same and the
1
Introduction
Luminescent lanthanide complexes display unrivalled spectro- probability is weak. As a consequence, the excited-state life-
III
scopic properties, which place them in a special category in the times of Ln can be extremely long, up to a few milliseconds for
III
III
luminescent toolbox. The attractive luminescence properties, visible Eu and Tb emitters.
especially, their characteristic line-like sharp emission bands
Aluminum, a group 3 metal, is the third most abundant
which are unaffected by the Ln coordination environment metal element in the crust of the earth, accounting for
make Ln luminescence appealing for a broad range of prac- approximately 8% of its mass. It is well-known that aluminum is
III
III
1
tical applications such as bioanalysis and biological imaging,
extensively used in our daily life and is also one important metal
3
+
2
3
light conversion devices, display materials, and luminescent element applied in industry production. However, Al is toxic
4
probes. Since most of these f / f transitions are forbidden by and environmentally harmful. Due to the frequent use of
3
+
the Laporte rule, the direct excitation of luminescent lantha- aluminum vessels and foil, the risk of Al ions absorption by
nide(III) cations is inefficient. However, this limitation can be the human body is increasing. For example, the iron-binding
3
+
overcomed with the help of a highly absorbing “antenna” protein can carry Al ions to the brain, which can further
III
5
3+
located in a sufficiently close proximity to the Ln . Once hurt the central nervous system. Al has been implicated as the
fatal factor in Alzheimer's disease, and it also prevents plant
growth on acid grounds. The World Health Organization (WHO)
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou,
China. E-mail: songxq@mail.lzjtu.cn; Fax: +86-0931-4938755; Tel: +86-0931- prescribed the average human intake of aluminium is about 3–
4
938755
10 mg per day with a weekly dietary intake of 7 mg per day
†
Electronic supplementary information (ESI) available: A brief statement in CIF
6
bodyweight. Thus, it is urgent for development of selective and
0

les, coordination polyhedron of EuL and EuL (Fig. S1 and S2), Job's plot
3+
sensitive Al chemsensors. Up to now, some uorescent che-
3+
between EuL and Al (Fig. S3), ESI-MS spectra of EuL$Al (Fig. S4) and LOD
curves (Fig. S5). CCDC 1534744 and 1534745. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c7ra02386c
3+
7
mosensors have been designed for detection of Al . However,
the uorescence lifetimes of typical organic compounds,
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including common aluminum-selective uorescent sensor
molecules, are in the nanosecond region. On the other hand,
among numerous sensors, luminescent lanthanide complexes,
2
Experimental
2.1 Materials and physical measurements
III
III
in particular Eu and Tb complexes receiving enormous Carbon, nitrogen and hydrogen analyses were performed using
interest due to their excellent optical properties in aid of an an EL elemental analyzer. Melting point were determined on
1
“
antenna effect”, such as large Stokes shis and high color a Koer apparatus. H NMR spectra of the ligands were recor-
purity. An efficient strategy to prepare lanthanide complexes for ded in d -DMSO solution at room temperature on a Bruker 400
6
detecting metal ion is to use lanthanide compounds with instrument operating at a frequency of 400 MHz and referenced
nonbonded functional ligand sites, where the lanthanide to tetramethylsilane (0.00 ppm) as an internal standard.
compounds provides luminescence, and the nonbonded func- Chemical shi multiplicities are reported as s ¼ singlet, d ¼
tional sites interact with the metal ions. However, immobiliza- doublet, t ¼ triplet and m ¼ multiplet. Infrared spectra were
tion of functional site within the lanthanide compounds has obtained with KBr discs on a Nicolet FT-170SX instrument in
ꢁ1
been challenging due to their high reactivity during the the wavenumber range of 4000–400 cm with an average of 128
III
ꢁ1
synthesis. Up to now, only a few investigations on Ln complex scans and 4 cm of spectral resolution. ESI-MS were deter-
based chemsensors with luminescence enhancement have been mined on a HRMS spectrometer (model: QTOF Micro YA263).
2
+
8
+
9
+
10
11
carried out to detect Zn , Ag , K , as well as some anions.
UV-vis absorption spectra were determined with a Varian UV-
These chemosensors were designed proting from an analyte- Cary100 spectrophotometer. Luminescence spectra were recor-
induced alteration of antenna, shortening distance between ded on a F-7000 uorescence spectrophotometer (Japan Hitachi
III
the antenna and Ln or ligand exchange in solution. To our company) at room temperature. The lifetime and quantum yield
III
best knowledge, chemosensors based on Ln complexes are were measured at room temperature on FLS920 Steady State
3
+
unprecedented for Al because of the poor coordination ability &Time-resolved
uorescence
Spectrometer
(Edinburgh
3
+
and strong hydration aptitude of Al . Inspired by these cases Instrument).
together with a continuation of our interest in the design of
III
12
luminescent Ln complexes, we dedicate to develop a chem-
2.2 Synthesis of the ligand
III
sensor based on Ln complexes of salicylamide salen-like
The synthesis of 1-(2-hydroxy-benzamido)-2-(2-hydroxy-5-nitro-
ligands by ingenious ligand design. Considering nitryl and
diethamino group possess weaker binding abilities toward
metal atoms than imine and amide group, when it situated
within a highly electron-rich conjugate system, the lone pair
electrons of N atoms can be spread and thereby realize the
effective transfer of electrons and further inuence the optical
properties through environmental perturbation. Therefore,
introduction of nitryl or diethylamino into salen-like salicyla-
mide ligands may be a highly promising platform for
construction of luminescent lanthanide chemsensor. We expect
benzylideneamino)-ethane (H L) and 1-(2-hydroxy-benzamido)-
3
0
2
-(2-hydroxy-4-diethylamino-benzylideneamino)-ethane (H L )
3
13
are according to literature with minor modication and
using 2-hydroxy-5-nitrobenzaldehyde and 2-hydroxy-4-
diethylaminobenzaldehyde instead (Scheme 1).
ꢂ
H
3
L. Yellow solid. Mp 212–213 C. Analytical data, calc. for
16 15 3 5
C H N O : C, 58.36; H, 4.59; N, 12.76; found: C, 58.81, H, 5.57,
ꢁ
1
N, 12.79; IR (KBr, y, cm ): 3381 (w), 1658 (s), 1636 (s), 1517 (s),
452 (m), 1373 (m), 1306 (s), 1236 (m), 1092 (m), 821 (m), 748
1
1
(m), 495 (m). H NMR (d -DMSO, 400 MHz): d: 3.66 (m, 2H,
III
6
that metal ion binding to unoccupied coordination site of a Ln
CH ), 3.87 (m, 2H, CH ), 6.67 (d, 1H, ArH), 6.89 (m, 2H, ArH),
2
2
complex constructed by the salicylamide salen-like ligand
would be conducive to a large luminescent response through
the modulation of the sensitization process. Therefore, in this
7
1
.40 (m, 1H, ArH), 7.81 (m, 1H, ArH), 8.05 (m, 1H, ArH), 8.41 (s,
H, CH]N), 9.00 (t, 1H, NH, J ¼ 4 Hz), 12.33 (s, 1H, OH), 14.19
(s, 1H, OH).
III
contribution, two new phenoxy bridged dinuclear Eu
0
ꢂ
H
3
L . Pale solid. Mp 145–146 C. Analytical data, calc. for
compounds, [Eu
2
(H
2
L)
3
(NO
3
)
3
]$3CH
3
CN (EuL) and [Eu
2
(H
2
-
C
20
H
25
N
3
O
3
: C, 67.58; H, 7.09; N, 11.82; found: C, 67.57, H, 7.07,
0
0
L ) (NO ) ]$2CH CN (EuL ) based on two structurally related
2
3 4
3
ꢁ1
N, 11.84; IR (KBr, y, cm ): 3292 (w), 3081 (w), 2974 (m), 2924
salicylamide salen-like ligands were prepared by solvent evap-
oration method. Both the structural and luminescent investi-
gations revealed that the different push–pull electronic
substitution group on 2-(iminomethyl)phenol moiety of the
salicylamide salen-like ligands have signicant effect on their
(
(
(
3
1
m), 1656 (s), 1517 (s), 1354 (s), 1267 (s), 1215 (s), 1143 (s), 1010
s), 954 (w), 864 (w), 761 (m), 648 (m), 603 (m), 594 (m). H NMR
1
d
6
-DMSO, 400 MHz): d: 1.04 (t, 6H, CH
.51 (m, 2H, CH ), 3.62 (t, 2H, CH ), 5.90 (d, 1H, ArH), 6.13 (dd,
H, ArH), 6.84 (m, 2H, ArH), 7.04 (d, 1H, ArH), 7.33 (m, 1H,
3 2
), 3.29 (m, 4H, CH ),
2
2
3
+
structures as well as luminescence responses toward Al . We
found that EuL exhibits a turn-on luminescent enhancement as
ArH), 7.79 (dd, 1H, ArH), 8.18 (s, 1H, CH]N), 8.89 (t, 1H, NH).
ꢁ
4
high as 5.7 fold with K ¼ 1.53 ꢀ 10 in CH CN, but compa-
d
3
0
0
3+
2.3 Synthesis of EuL and EuL
rable compound EuL could not detect Al under the same
experimental conditions. To the best of our knowledge, EuL is
General procedure. To a acetonitrile solution of 0.1 mmol
III
3+
0
the rst luminescence-enhanced Eu complex-supported Al
H L/H L , 27 uL (0.2 mmol) triethylamine was added to make
3
3
sensor. The sensing mechanism is presumably due to the a clear solution. Then 0.1 mmol Eu(NO ) $6H O (0.446 mg) was
3
3
2
formation of EuL$Al which result in effective suppression of added as a solid, and the solution was stirred for another 4 h to
non-radiative transitions, excited-state intra-molecular proton obtain a clear solution which was ltered into a sealed 10–20
transfer (ESIPT) and photo induced electron transfer.
mL glass vial for crystallization at room temperature. Aer
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3 3
Scheme 1 The synthesis route of the ligand H L and H L .
15
2
about three weeks, pale yellow single crystals suitable for crystal structure was solved by direct methods and rened on F by
analysis were obtained and collected by ltration, washed with a full-matrix least-squares procedure based on all data mini-
2
2 2
2 2 1/2
cold acetonitrile, and dried in the air.
mizing w ¼ [S[w(F ꢁ F ) ]/S(F ) ] , R ¼ S||F | ꢁ |F ||/S|F |
R
o
c
o
o
c
o
2
o
2 2
c
1/2
[Eu
2
(H
2
3
L)
3
(NO
3
)
3
]$3CH
3
CN (EuL): (yield: 36.7 mg, 52% based and S ¼ [S[w(F
ꢁ F
) ]/(n ꢁ p)] . SHELXL-2014 was used for
16
on Eu(NO
)
3
$6H
2
O). Analytical data (%), calcd: C, 41.47; H, 2.90; both structure solutions and renements. All non-hydrogen
ꢁ
1
N, 11.64; found: C, 41.22; H, 2.88; N, 11.68; IR (KBr, y, cm ): atoms were rened anisotropically. The positions of hydrogen
3
1
456 (w), 1606 (s), 1511 (s), 1470 (s), 1446 (s), 1383 (m), 1217 (s), atoms were calculated and isotropically xed in the nal
˚
168 (s), 1130 (m), 1074 (m), 734 (m), 594 (m).
renement [d(C–H) ¼ 0.95 A, with the isotropic thermal
0
0
[
Eu (H L ) (NO ) ]$2CH CN (EuL ): (yield: 37.6 mg, 56% parameter of Uiso(H) ¼ 1.2 Uiso(C)]. The SMART and SAINT
2
2
2
3 4
3
17
based on Eu(NO ) $6H O). Analytical data (%), calcd: C, 39.35; soware packages were used for data collection and reduction
3
3
2
H, 4.05; N, 12.52; found: C, 39.48; H, 4.08; N, 12.50; IR (KBr, y, respectively. Crystallographic diagrams were drawn using the
ꢁ
1
18
cm ): 3394 (w), 2974 (w), 1602 (s), 1510 (s), 1474 (s), 1308 (m), DIAMOND soware package. The structures were examined
1
242 (s), 1145 (m), 1012 (m), 816 (m), 741 (m), 584 (m).
using the Add sym sub routine of PLATON to ensure that no
additional symmetry could be applied to the models. Also
0
severely disordered acetonitrile molecules in EuL and EuL ,
19
2.4 X-ray crystallographic study
were removed by SQUEEZE during the structural renements.
3
For details about the squeezed material, see cif data in ESI.†
Therefore, three and one crystalline acetonitrile molecules
which were determined on the basis of elemental microanalysis
and the data treated with the SQUEEZE routine within PLATON
Suitable yellow block crystals of EuL (0.21 ꢀ 0.16 ꢀ 0.12 mm )
0
3
and EuL (0.16 ꢀ 0.12 ꢀ 0. 06 mm ) were coated with per-

uoropolyether oil before mounting. Intensity data were recor-
ded with a Bruker Smart Apex II CCD diffractometer with
0
were added to the molecular formula of EuL and EuL respec-
˚
agraphite-monochromated Mo-Ka radiation (l ¼ 0.071073 A)
tively. A summary of the relevant crystallographic data and the
using the u–2q scan mode. No crystal decay were observed

nal renement details are given in Table 1, important bond
during the data collections. Absorption corrections based on
14
lengths are listed in Table 2.
multi-scan using the SADABS soware were applied. The
0
Table 1 Crystal data and structure refinement parameters for EuL and EuL
Empirical formula
C
48
H
42Eu
2
N
12
O
24
C
42
2 11 18
H51Eu N O
Crystal system, space group
Unit cell dimensions
Trigonal, R3ꢀ
Monoclinic, C2/c
˚
ꢂ
ꢂ
˚
ꢂ
a ¼ 17.4511(7) A, a ¼ 90
a ¼ 28.3358(12) A, a ¼ 90
˚
˚
ꢂ
b ¼ 17.4511(7) A, b ¼ 90
b ¼ 20.1143(7) A, b ¼ 118.325(4)
˚
ꢂ
˚
ꢂ
c ¼ 40.5949(16) A, g ¼ 120
c ¼ 20.98177(7) A, g ¼ 120
3
˚
ꢁ
3
˚
Volume
10 706.5(10) A
10 526.8(8) A
3
ꢁ3
Z, calculated density
Absorption coefficient
F(000)
6, 1.732 kg m
8, 1.643 kg m
ꢁ1
ꢁ1
1.795 mm
4392
2.441 mm
4530
Crystal size
Theta range for data collect
Limiting indices
Reections collected/unique
Completeness to theta ¼ 25.01
Data/restraints/parameters
0.21 ꢀ 0.16 ꢀ 0.12 mm
0.16 ꢀ 0.12 ꢀ 0.06 mm
ꢂ
ꢂ
3.60 to 25.50
3.46 to 26.00
ꢁ21 # h # 11, ꢁ16 # k # 16, ꢁ49 # l # 30
ꢁ34 # h # 33, ꢁ24 # k # 18, ꢁ25 # l # 24
7665/4442 [R(int) ¼ 0.0358]
21 725/10 272 [R(int) ¼ 0.0452]
99.7%
99.2%
4442/45/269
1.036
10 272/79/672
1.053
2
Goodness-of-t on F
Final R indices [I > 2 sigma(I)]
R indices (all data)
Largest diff. peak and hole
R
R
1
¼ 0.0539, wR
2
2
¼ 0.1675
¼ 0.1820
R
R
1
¼ 0.0514, wR
2
2
¼ 0.1221
¼ 0.1388
1
¼ 0.0772, wR
1
¼ 0.0780, wR
ꢁ
3
ꢁ3
˚
˚
1.861 and ꢁ1.410 e A
2.622 and ꢁ1.354 e A
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0
Table 2 Selected bond lengths ( A˚ ) for EuL and EuL
[
2 2 3 3 3 3
Eu (H L) (NO ) ]$3CH CN (EuL)
Eu1–O1 2.311(5) Mn1–O1 1.846(2)
Eu2–O2 2.510(5)
Eu1–O7 2.498(6)
Eu1–O6 2.558(5)
Eu2–O3 2.401(5)
Eu2–O1 2.480(4)
0 0
2 2 2 3 4 3
Eu (H L ) (NO ) ]$2CH CN (EuL )
[
Eu1–O3 2.169(6)
Eu1–O8 2.406(5)
Eu2–O1 2.270(4)
Eu1–O1 2.267(5)
Eu1–O11 2.423(5)
Eu2–O4 2.304(5)
Eu1–O4 2.286(4)
Eu1–O10 2.492(5)
Eu2–O14 2.387(5)
Eu1–O2 2.302(5)
Eu2–O6 2.210(5)
Eu2–O13 2.396(6)
Eu1–O7 2.400(5)
Eu2–O5 2.260(5)
Eu2–O17 2.423(6)
Eu2–O16 2.437(5)
˚
ꢂ
2
.5 Sensing experiment
of 3.576 (2) A and a Eu1–O1–Eu2 angle of 96.46(6) . Upon
further investigation, we nd there are hydrogen bonds between
imine and phenolic hydroxyl groups with imine groups being
hydrogen bonding acceptor and phenolic hydroxyl groups being
hydrogen bonding donor. The existence of these intramolecular
hydrogen bonds could prevent imine nitrogen atom from
coordination. What deserves special note is the preorganized
cavity existed in EuL, where the three phenolic hydroxyl groups
on 2-(iminomethyl)phenol moiety are arranged in a radial
manner anchored with Lewis base sites which can be very useful
for binding suitable metal ions.
Luminescence spectra were recorded on a Hitachi F-7000
spectrophotometer. Spectroscopic measurements were recor-
ded in acetonitrile solution. Stock solutions (0.01 M) of the
+
+
2+
2+
2+
2+
2+
3+
2+
nitrate salts of Li , Na , Mg , Ca , Ba , Mn , Fe , Fe , Co ,
2
+
2+
2+
2+
2+
3+
Ni , Cu , Cd , Hg , Zn and Al in distilled water were
prepared. Stock solutions of EuL (0.1 mM) were prepared in
acetonitrile solution. Test solutions were prepared by placing 2
mL of the probe stock solution into a test tube and adding an
appropriate aliquot of each ions stock. The mixture solution
was stirred for 30 seconds, and waited about one minute.
Complex formation was monitored following the absorption
spectral changes. Both the excitation and emission slit widths
were 5.0 and 5.0 nm, respectively.
To conrm the role of different substitution groups in the
0
self-assembly process, H
3
L was used instead of H
3
L to perform
the reaction under the same experimental conditions. As ex-
pected, compound EuL crystallizes in the monoclinic C2/c
0
space group with a 1 : 1 ligand metal stoichiometric ratio which
is quite different from that of EuL due to the substitution of
nitryl with diethylamino group. A view of the molecular struc-
3
Result and discussion
3.1 Crystal structure of EuL
ture with partly numbering scheme is depicted in Fig. 2a. Two
III
Signal-crystal X-ray diffraction analysis revealed that EuL is crystallographically independent Eu ions (Eu1 and Eu2), are
a dinuclear complex with a ligand metal stoichiometric ratio of doubly bridged by two phenoxy of salicylamide with the
3
: 2. As shown in Table 1, EuL crystallizes in the trigonal system
ꢀ
space group R3, which allows a 3-fold rotational axis passes
through Eu ions and thus to give a C molecular symmetry. In
3
III
the asymmetry unit of EuL, there are two crystallographically
III
independent Eu ions (Eu1 and Eu2) with one third occupancy,
ꢁ
one partly deprotoned H2L and one nitrate anions (Fig. 1a).
Eu1 is coordinated by three phenoxy atoms (O1) of salicylamide
ꢁ
group from three crystallgraphically equivalent H
2
L
ligands
and six oxygen atoms (O6 and O7) from three crystallgraphically
equivalent nitrate anions. Meanwhile, Eu2 is coordinated by
three amide oxygen atoms (O2), three phenoxy oxygen atoms
(
O1) and three phenolic hydroxyl atoms (O3) from three crys-
ꢁ
tallgraphically equivalent H
2
L ligands. Exact geometry analysis
by SHAPE 2.1 soware shows that the inner coordination sphere
III
of the two nine-coordinated Eu ions are residing in two dis-
torted tricapped trigonal prism sharing one bottom with
a deviation of 13.096 for Eu1 and 12.686 for Eu2 from the ideal
D3h symmetry (Fig. S1 and Table S1†). The Eu–O bond distances
˚
vary from 2.311(5) to 2.558(5) A and the O–Eu–O bond angles
ꢂ
fall in the range of 51.69(5)–158.72(9) , which are exactly
III
20
comparable to those reported in other Eu compounds. As
ꢁ
shown in Fig. 1b, the H2L ligands in EuL adopt a chelating-
bridging tetradentate m -h :h :h coordination mode, and as
a result, Eu1 and Eu2 ions are linked by three phenoxy atoms
4
1
1
2
Fig. 1 a) Asymmetric unit of EuL with thermal ellipsoids at 50%
probability; (b) molecular structure of EuL showing intramolecular
(
O1) engendering a [Eu (m-O)
2
3
] core with a Eu1/Eu2 separation hydrogen bonds between imine and phenolic hydroxyl groups.
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Fig. 2 (a) A view of the molecular structure of EuL with partly numbering scheme; (b) molecular structure of EuL showing intramolecular
hydrogen bonds between imine and phenolic hydroxyl group.
remaining coordinating sites fullled by carbonyl oxygen atom, obvious changes can be observed with continuously titrated of
0
ꢁ
III
phenolic hydroxy group from the same ligand H
oxygen atoms from two bidentate nitrate anions. Contrast to Eu is 3 : 2. Meanwhile, the change observed in Fig. 3c shows
Eu compound of H
2
L , and four Eu , suggesting that the stoichiometry between the ligand and
III
III
III
3
L, two Eu ions are eight-coordinated and a 1 : 1 ligand metal stoichiometric ratio. All these results are
their coordination geometries are distorted square antiprisms consistent with the structural analysis discussed above. Further
3
+
3
sharing one side with a deviation of 5.258 for Eu1 and 7.034 for incremental addition of Al to the EuL in CH CN, obvious
Eu2 from the ideal D4d symmetry (Fig. S2 and Table S1†) based changes in the absorption bands are also observed (Fig. 3b). The
on exact geometry analysis by SHAPE 2.1 soware. Similar to absorbance at 232 nm is almost unchanged with concomitant
0
ꢁ
that in EuL, H2L also adopts tetradentate coordination modes hypsochromic shi in the absorbance at 349 nm and simulta-
with one phenolic hydroxy group and one amide oxygen atom neous development of a new peak at 260 nm. Aer addition of 1
3
+
chelating to one europium ion as well as one phenoxy atom equiv. of Al , saturation point is reached. However, no such
bridging another europium ion. As a result, imine nitrogen signicant change in the absorption spectrum of EuL was
atoms were also deprived of coordination due to the existence of observed with other tested metal cations. All these may indicate
3
+
molecular hydrogen bonds between imine and phenolic a formation of a complex between EuL and Al . By compar-
3
+
0
hydroxy groups (Fig. 2b). Notably, the distance of two phenolic ision, addition of Al to the resulted EuL solution bring no
0
hydroxyl groups on 2-(iminomethyl)phenol moiety in EuL is spectra change (Fig. 3d), indicating there are no electronic
˚
0
3
+
0
3
.847 A. Clearly, the structural difference between EuL and EuL
interaction between Al and EuL .
are owing to the electronic property of nitryl and diethylamino
group in spite of their poor coordination abilities. Such differ-
ences in structure must have an impact on their luminescence
sensing ability to metal ions.
3
.3 Luminescence and sensing properties
As shown in Fig. 4, excitation of the absorption bands at 326 nm
or 349 nm produce blue uorescence positioned at 406 and
4
4
71 nm for H
3
L + NEt
3
, and broad emission band at lmax
¼
3
.2. UV-vis absorption studies
0
III
06 nm for H
3
L + NEt
3
. Upon coordination to Eu ions, pink
III
3+
0
The UV-vis titration of Eu and Al were conducted using emission of EuL and EuL were observed by naked eyes
0
a 10.0 mM solution of H
shown in Fig. 3a and c, the absorption spectrum of H
thylamine and H
3
L/H
3
L + triethylamine in CH
3
CN. As following excitation with a standard UV lamp (lex ¼ 365 nm).
3
L + trie- Excitation of the ligand-centered absorption band at 375 nm,
0
III
3
L + triethylamine exhibit different absorption emissions both originating from the ligand and Eu ions were
properties owing to the different electronic properties of nitryl observed with the Eu -centered emission (619 nm) intensity
III
and diethylamino group: the former shows three strong more than three times higher than that of ligand-centered
0
absorption bands centered at 208, 238 and 252 nm with several emissions (432 nm) for EuL for EuL , conrming that the
III
relative weak broad bands ranging from 314 to 404 nm which energy transfers from ligands' chromophore to Eu ions occur
III
tail out to 450 nm, while the latter exhibits three relatively weak (Fig. 4). The Eu -centered emissions shows ve characteristic
5
7
absorption bands centered at 215, 224 and 335. Upon addition bands originated from the D
0
state to the F
J
(J ¼ 0, 1, 2, 3, 4) at
III
5
0
of Eu , there were clearly ve isosbestic points at 237, 249, 276, about 581, 594, 619, 653 and 698 nm. The rst transition ( D /
F ) observed at the wavelength 581 nm is symmetric, indicating
0
points at 275 and 348 nm for H L + triethylamine, indicating only one type of europium were present in all complexes
7
3
21 and 403 nm for H
3
L + triethylamine, and two isosbestic
0
3
21
5
7
the formation of the corresponding compound. Obviously, the studied. The second transition ( D
/
F1) with magnetic
0
absorbance of H
the stoichiometric ratio of Eu : L reaches 2 : 3, and no longer intensive maxima correspond to the D
3
L + triethylamine shows a linear increase until dipole nature observed at the wavelength 594 nm. The most
III
5
7
0
2
/ F transition for
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III
Fig. 3 (a) UV-vis titration of H
EuL (10.0 mM) upon the addition of increasing concentration of Al in CH
3
L + triethylamine (10.0 mM) with the addition of increasing concentration of Eu in CH
3
CN; (b) UV-vis titration of
3+
0
3
CN; (c) UV-vis titration of H
3
L + triethylamine (10.0 mM) upon the
CN; (d) UV-vis titration of EuL (10.0 mM) with the addition of increasing concentration of Al in
III
0
3+
addition of increasing concentration of Eu in CH
CH CN.
3
3
0
0
Fig. 4 Emission spectra of H
3
L/H
3
L (black solid line) and EuL/EuL (red solid line) in CH
3
CN.
0
EuL and EuL are found at wavelength 619 nm. The ratios of the lanthanide rst coordination sphere as ascertained by
5
7
5
7
0
intensities corresponding to D / F and D / F transi- structural analysis, the luminescence of EuL and EuL are weak.
0
2
0
1
0
0
tions are 5.8 and 2.4 for EuL and EuL respectively, showing
a strong deviation from central symmetry of Eu coordination metal ions was examined by addition of 1.0 quiv. aqueous
geometry in EuL compared to that in EuL . The
transition observed at 653 nm is very weak because it is Mg , Al , Co , Fe , Ni , Fe , Mn , Cu Zn , Cd , and
forbidden. It is well-known that the presence of OH oscillators Hg to the solution of EuL and EuL in CH
in the lanthanide rst coordination sphere provides an efficient the same conditions. As depicted in Fig. 5, upon Al addition,
non-radiative path. Therefore, owing to the combined effects 5.7 times signicant luminescence enhancement was observed
of excited state intramolecular proton transfer (ESIPT) from for EuL. While for EuL , a 6.2 times enhancement was caused by
The selectivity of EuL and EuL towards various common
III
0
5
7
+
+
2+
D
0
/
F
3
solution containing different metal ions such as Li , Na , Ca ,
2
+
3+
2+
2+
2+
3+
2+
2+
2+
2+
2
+
0
3
CN and tested under
3
+
22
0
+
phenolic –OH to imine-N and the presence of OH oscillators in Li addition. We can also see from Fig. 5, that transition metal
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3
Fig. 5 Comparison of the luminescence intensity of EuL and EuL (0.1 mM) upon addition of various metal ions in CH CN.
cations with unpaired d-electrons have a little varying degrees acetonitrile solution. As shown in Fig. 8, the sequential
0
quenching effect on the luminescence intensity of EuL and EuL
enhancing of emission intensity at 619 nm of EuL was recorded
1
0
3+
as compared to other transition metal with d electron as a result of gradual increased concentration of Al aqueous
conguration, alkaline metal ions and alkaline earth metal solution. The emission intensity at 619 nm increases gradually
3
+
ions. Furthermore, the considerably ‘turn-on’ luminescence with the increase in concentration of Al up to a mole ratio
3
+
response of EuL led to the apparent color change from pink to EuL : Al ¼ 1 : 1 and then becomes saturated. A luminescence
brilliant red which could also be identied by naked eyes easily enhancement of 5.7-fold at 619 nm with the quantum yield
under UV lamp (Fig. 6).
increasing from F ¼ 4.5% to F ¼ 16.9% can be realized upon
3+
0
To explore the effective applications of EuL and EuL , the addition of 1.0 equiv. Al . The lifetimes of EuL and its complex
3
+
0
3+
luminescence response of EuL towards Al and EuL towards with Al in acetonitrile were also measured. The lifetime of EuL
+
3+
Li in presence of typical competing ions were studied and the is quite short (0.16 ms). Upon addition of 1.0 equiv. Al , the
results were shown in Fig. 7. Data in Fig. 7 shows that there is lifetime of EuL increased to 0.94 ms. Unfortunatly, no crystal-
3
+
+
3+
little interference of EuL for detection of Al in presence of Li , line product was obtained upon Al addition.
+
2+
2+
3+
2+
2+
2+
2+
2+
2+
Na , Mg , Mn , Fe , Ni , Fe , Cu , Zn , Cd , Co as
Job's method was further employed to determine the
2
+
follows. In the case of Cu , quenching of the uorescence composition of the complex, the luminescence emission
3
+
III
3+
signal was observed. The response of EuL for Al detection in intensity of Eu for complexation between EuL and Al has
2
+
2+
2+
2+
2+
2+
the presence of Mn , Ni , Co , Zn , Cd , and Hg is rela- a maximum at a mole fraction of 0.5, thus indicative of the
23
tively low but clearly detectable. Thus, EuL can be used as formation of a 1 : 1 complex. (Fig. S3†). As shown in Fig. 9,
3
+
3+
a selective luminescent chemosensor for Al detection in Benesi–Hiderband plot of [Imax ꢁ I )/(I ꢁ I )] vs. 1/[Al ] gives
0
0
ꢁ
4
2
presence of most competing metal ions. Comparably, very straight line with slope Kd ¼ 1.53 ꢀ 10 M with R ¼ 0.99,
0
+
signicant interference exists in EuL for detecting Li in pres- indicating a 1 : 1 moderately strong binding of EuL towards
2
+
2+
3+
2+
2+
2+
2+
2+
2+
2+
3+
ence of Mg , Mn , Fe , Ni , Fe , Cu , Zn , Cd , Co , Ca
Al . This conclusion has also been certied by electrospray
and Hg . These results clearly indicated that EuL rather than ionization mass spectrum (ESI-MS). The positive ion mass
2
+
0
3+
EuL could be used as a potential metal-selective luminescent spectrum of EuL upon addition of 1.0 equiv. of Al exhibited
sensor even in presence of a large excess of other competitive intense peaks at m/z ¼ 1473.76 and m/z ¼ 1642.09, which can be
+
+
metal ions in acetonitrile.
assigned to the ion of [EuL ꢁ H] , [EuL ꢁ 2H + Al + 3H O]
2
3
+
Further determination of sensitivity was carried out by respectively (Fig. S4†). The LOD of Al calculated by using 3s
3
+
gradually adding Al
ion aqueous solution to EuL in method was found to be 0.33 mM. (Fig. S5†) which is lower than
Fig. 6 The visual fluorescence response of EuL (0.1 mM) upon addition of metal ions (0.5 mM) in CH
3
CN under UV lamp at 365 nm.
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3
+
0
+
Fig. 7 Bar chart illustrating luminescence response of EuL + Al and EuL + Li at 619 nm towards different cations in CH
3
CN (lex ¼ 375 nm).
0
3+
n+
n+
+
+
2+
2+
3+
2+
2+
2+
2+
2+
Conditions: EuL ¼ 0.1 mM, EuL ¼ 0.1 mM, Al ¼ 0.1 mM, M ¼ 0.01 M; where, M ¼ Li , Na , Mg , Mn , Fe , Ni , Fe , Zn , Cd , Co
,
2+
2+
2+
Cu , Ca and Hg
.
possibility of the formation of EuL$Al is expected to reduce. For
this purpose, 20 mL of 2 M HCl solution was added sequentially
to the acetonitrile solution of EuL aer addition of 1.0 equiv.
3
+
Al ions. As shown in Fig. 10, the luminescence intensity of
III
Eu decreased a lot upon gradual addition of HCl, which may
be due to the dissociation of EuL$Al as the result of the
protonation of phenolic hydroxy group. This strongly supports
3
+
the state that the interaction between Al and EuL is through
the formation of EuL$Al at the expense of destroying intra-
molecular hydrogen bonding between imine and phenolic
hydroxy groups. Therefore, the enhancement of the lumines-
3
+
cent intensity of EuL upon Al incorporation is presumably due
to metal binding to the cavity formed by three phenolic hydroxyl
group in EuL followed by excited-state intra-molecular proton
transfer (ESIPT) and photo induced electron transfer
suppression.
Fig. 8 Luminescence titration spectra of EuL (0.1 mM) upon gradual
3+
3
addition of Al (0.1 mM) in CH CN (lex ¼ 375 nm).
According to the analysis of the structural features, we found
that in the molecular structure of EuL, three phenolic hydroxyl
groups provide a cavity with hard Lewis base chelating sites
together with the geometric uniqueness of the cavity leads to
3+
Fig. 9 Benesi–Hiderband plot of EuL toward Al
.
3
+
the maximum tolerable limit of Al . Such a low detection limit
as well as the selective emission enhancement reach to
a conclusion that EuL is an effective sensor for selective detec-
3
+
tion of Al in acetonitrile.
To explore the mechanism of the luminescence turn-on Fig. 10 Concentration-dependent luminescence quenching of
3
+
compound EuL$Al (0.1 mM) after adding different amounts of 2 M HCl
at room temperature.
behavior for Al ions, we performed HCl titrations where the
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a high affinity for Al (Scheme 2). Among common metal ions
RSC Advances
3
+
To further understand the relationship between the struc-
3
+
3+
examined herein, charge-to-size ratio of Al ions is so high that ture of EuL and the respective optical response to Al , we
it is expected to destroy the triple intramolecular N/H–O carried out density function theory (DFT) calculations with the
hydrogen bonds in EuL to form EuL$Al, where the three B3LYP/6-31 G(d) basis set using the Gaussian 09 program. As
phenolic hydroxy groups of EuL act as simple electron donor to displayed in Fig. 11, the HOMO of EuL is distributed on sali-
3
+
ꢁ
Al ions through Lewis acid-base type interactions. Based on cylamide moiety of three H
2
L
ligands, meanwhile, the LUMO
3
+
the analysis above, Al binds to EuL through three hydroxy of EuL is localized on 2-(iminomethyl)phenol moiety, indicating
3
+
groups and the hexacoordination of Al can be satised by the presence of PET process from salicylamide group of the
three water molecules. Obviously, nitryl groups with strong molecule to the 2-(iminomethyl)-5-nitrophenol moiety. The
3
+
electron withdrawing effect in EuL plays a key role in con- optimized structure of EuL$Al complex shows that the Al ion
structing this distinctive architecture and the consequent binds to EuL at phenolic oxygen atoms of 2-(iminomethyl)-5-
3
+
detection of Al ion.
nitrophenol moiety, with three water molecules to satisfy the
need of saturated coordination. In the EuL$Al complex, the
electron density of HOMO is also localized on salicylamide
moieties, while the electron density of LUMO is localized on the
2-(iminomethyl)-5-nitrophenol moiety. For the electron density
as well as energy gap of EuL$Al compared with that of EuL only
changed slightly, which excludes the possibility that the
III
increasing of Eu luminescence is due to a change of the nature
of the antenna triplet state. Therefore we can suggest that the
observed increase in luminescence intensity, lifetime and
5
III
quantum yield of the D
0
level of Eu is mainly related to the
decrease in non-radiative transitions due to the absent of OH
III
3+
oscillators in the Eu rst coordination shell upon Al coor-
dination followed by excited-state intra-molecular proton
transfer (ESIPT) and photo induced electron transfer
suppression.
3+
Scheme 2 Sensing mechanism of EuL towards Al
.
Fig. 11 HOMO–LUMO energy levels and the molecular orbital plots of EuL and EuL$Al.
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6
(a) J. Barcelo and C. Poschenrieder, Environ. Exp. Bot., 2002,
4
Conclusions
48, 75; (b) B. Valeur and I. Leray, Chem. Rev., 2000, 205, 3; (c)
III
In summary, a novel luminescence-enhanced Eu complex-
supported chemosensor EuL was developed by tuning the
substitution group on 2-(iminomethyl)phenol moiety of salicy-
lamide salen-like ligands. The strong electron withdrawing
capability of nitryl group led to the formation of the phenoxy
bridged dinuclear Eu complex anchored with Lewis base sites
which is distinctive for binding Al ions. Its sensing ability for
a wide range of metal ions (Li , Na , Mg , Ca , Mn , Fe , Fe ,
Co , Ni , Cu , Zn , Cd , Hg and Al ) was tested and the
results indicated that EuL exhibited high selectivity and sensi-
Z. Krejpcio and R. W. P. J. Wojciak, Environ. Stud., 2002, 11,
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III
3
+
+
+
2+
2+
2+
2+
3+
2
+
2+
2+
2+
2+
2+
3+
3
+
tivity for Al in the presence of various metal cations. Job's plot
and mole–ratio curves revealed a 1 : 1 stoichiometry between
EuL and Al . To the best of our knowledge, this is the rst
3
+
III
example of Eu based luminescent sensor for the detection of
3
+
Al that demonstrated signicant luminescence enhancement.
The detection limit was sufficiently low to determine the
3
+
micromolar levels of Al and EuL could be served as an excellent
lanthanide chemsensor for highly toxical aluminum ion. The
sensing mechanism can be explained in terms of the decrease
of non-radiative transitions in EuL in addition to excited-state
intra-molecular proton transfer (ESIPT) and photo-induced
8 K. Hanaoka, K. Kikuchi, H. Kojima, Y. Urano and T. Nagano,
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9 W. S. Liu, T. Q. Jiao, Y. Z. Li, Q. Z. Liu, M. Y. Tan, H. Wang
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3
+
electron transfer suppression upon Al coordination which is 10 A. Thibon and V. C. Pierre, J. Am. Chem. Soc., 2009, 131, 434.
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not ideal luminescent sensors for it can't be used in pure
aqueous solution.
J. L. Yuan, Chem. Commun., 2008, 9, 1127; (b) J. W. Wang,
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(
1
c) C. G. Gulgas and T. M. Reineke, Inorg. Chem., 2008, 47,
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grant 21661019).
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