Anion-responsive luminescent Eu3+ complexes with ring-like rigid quinoline-amide ligands

Article abstract of DOI:10.1016/j.inoche.2010.04.020

Two novel ring-like rigid quinolineamide ligands, 2-[2-(1,3-dioxolan-2-yl) quinolin-8-yloxy]-N-benzylacetamide (L^I) and 3-[2-(1,3-dioxolan-2-yl) quinolin-8-yloxy]-1,1-diphenylpropan-2-one (L^II), were designed to assemble the anion-responsive luminescent europium complexes. Crystallographic studies of the EuL^I(NO₃)₃ and EuL ^II(NO₃)₃ revealed that the tetradentate ligands L^I and L^II cooperatively coordinated with Eu³⁺ ion and some available sites around central europium ions for guest anions were reserved in the complexes. The luminescent properties of the Eu(III) nitrate complexes in acetonitrile solutions were investigated. And the lowest triplet state energy levels of the ligands are well placed to allow energy transfer to the resonance level of Eu(III). At the same time, the luminescence titration experiments and the Job's plot analysis demonstrated the formation of 1:1 complexes in the solutions. The luminescence intensities at 616 nm were enhanced by addition of NO₃^- or Cl^- anion to the mixed acetonitrile solutions of Eu(CF₃SO₃)₃ and the ligands.

Full text of DOI:10.1016/j.inoche.2010.04.020

Inorganic Chemistry Communications 13 (2010) 882–886
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Anion-responsive luminescent Eu3+ complexes with ring-like rigid
quinoline–amide ligands
⁎
Hai-Ping Wang, Hai-Gang Li, Guang-Nong Lu, Ning Tang, Wei-Sheng Liu, Yu Tang
Key Laboratory of Nonferrous Metal Chemistry and Resources Utilization of Gansu Province, State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel ring-like rigid quinoline–amide ligands, 2-[2-(1,3-dioxolan-2-yl)quinolin-8-yloxy]-N-benzylace-
tamide (LI) and 3-[2-(1,3-dioxolan-2-yl)quinolin-8-yloxy]-1,1-diphenylpropan-2-one (LII), were designed to
assemble the anion-responsive luminescent europium complexes. Crystallographic studies of the EuLI(NO3)3
and EuLII(NO3)3 revealed that the tetradentate ligands LI and LII cooperatively coordinated with Eu3+ ion and
some available sites around central europium ions for guest anions were reserved in the complexes. The
luminescent properties of the Eu(III) nitrate complexes in acetonitrile solutions were investigated. And the
lowest triplet state energy levels of the ligands are well placed to allow energy transfer to the resonance level
of Eu(III). At the same time, the luminescence titration experiments and the Job's plot analysis demonstrated
the formation of 1:1 complexes in the solutions. The luminescence intensities at 616 nm were enhanced by
addition of NO−3 or Cl− anion to the mixed acetonitrile solutions of Eu(CF3SO3)3 and the ligands.
© 2010 Elsevier B.V. All rights reserved.
Received 21 March 2010
Accepted 21 April 2010
Available online 29 April 2010
Keywords:
Anion-responsive
Luminescence
Europium complex
Quinoline–amide ligand
Introduction
type ligands would coordinate chelated with the central lanthanide
ions in equatorial plane and several available coordination sites in
A lot of luminescent lanthanide complexes has been applied in
biochemical sensors, naked-eye detection of inorganic anions and
other chemical systems, because the organic ligand-sensitized
complexes combine the outstanding optical properties of the
lanthanide ions (microsecond to millisecond lifetimes, characteristic
and narrow emission bands, and large Stokes shifts), effective
intramolecular energy transfer to the central lanthanide ions (the
so-called “antenna effect”) of designed organic ligands and the
regulated structures of complexes [1–4]. Because trivalent lanthanide
ions have high coordination flexibility and lack of preferential
geometries, the researchers' attention has been focused on the
structure-property relationships of lanthanide complexes in the past
decades [5–7]. According to the dynamic and unsaturated coordina-
tion features, various designed organic ligands and their luminescent
lanthanide complexes which exhibited anion-responsive lumines-
cence profiles had been synthesized [8–10].
In order to study the anion-responsive luminescence behavior of
the europium complexes, two tetradentate ring-like rigid quinoline–
amide ligands, 2-[2-(1,3-dioxolan-2-yl)quinolin-8-yloxy]-N-benzyl-
acetamide (LI) and 3-[2-(1,3-dioxolan-2-yl)quinolin-8-yloxy]-1,1-
diphenylpropan-2-one (LII) (Scheme 1), in which the quinoline
skeleton was the effective photoantenna [8,11] for excitation of the
europium ions were designed. It is expected that the quinoline–amide
axial to interact with guest anions were reserved in the complex, thus
the novel lanthanide complexes might exhibit anion-responsive
luminescence profiles. As a part of our systematic studies, herein we
report the assembly and anion-responsive luminescent properties of
the Eu3+ complexes with the rigid quinoline–amide ligands LI and LII.
The synthetic route for the ligands 2-[2-(1,3-dioxolan-2-yl)
quinolin-8-yloxy]-N-benzylacetamide (LI) and 3-[2-(1,3-dioxolan-2-
yl)quinolin-8-yloxy]-1,1-diphenylpropan-2-one (LII) is shown in
Scheme 1. The 2-(1,3-dioxolan-2-yl)quinolin-8-ol (b) [12] was
prepared through the acetalation reaction between 2-carbaldehyde-
8-hydroxyquinoline (a) [13] and glycol. The ligands LI and LII [14]
were synthesized via that the amide groups were introduced by the
etherification reaction between the chloroacetamide type compounds
and 2-(1,3-dioxolan-2-yl)quinolin-8-ol (b). The introduced oxygen
atoms are expected to function as “hard” donors for lanthanide
cations and form steady five-membered chelate rings. The europium
complexes EuLI(NO3)3 and EuLII(NO3)3 were readily prepared by
mixing stoichiometric amounts of ligands and Eu(NO3)3·6H2O in
mixed solvents of chloroform and ethyl acetate (v:v=1:1). The data
of the elemental analysis indicate that the complexes conform to a 1:1
metal-to-ligand stoichiometry and the conductivity measurements
for the complexes in acetonitrile solutions indicate that the complexes
are non-electrolyte [15].
Slow diffusion of diethyl ether into the acetonitrile solutions of the
europium complexes afforded colorless rhombic crystals. The single-
crystal X-ray analysis of the complexes EuLI(NO3)3 and EuLII(NO3)3
reveal that they are isomorphous with the central Eu atom
⁎
Corresponding author. Tel.: +86 9318912552; fax: +86 9318912582.
E-mail address: tangyu@lzu.edu.cn (Y. Tang).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.04.020

H.-P. Wang et al. / Inorganic Chemistry Communications 13 (2010) 882–886
883
Scheme 1. The synthetic route for the ligands LI and LII.
coordinated with 10 donor atoms [16]. Six of them belong to three
bidentate nitrate groups and the remaining four are from the
tetradentate ligands (Figs. 1a and 2a). The coordination polyhedra
around Eu3+ are both distorted bicapped square antiprisms (Figs. 1b
and 2b). The ligands wrap around the metal ion with its oxygen atoms
and nitrogen atoms and form a ring-like coordination structure
together with the Eu atom. The four donor atoms of the ligands are not
quite coplanar, their deviation from the mean plane being in the range
0.080–0.146 Å and 0.021–0.037 Å. The mean absolute deviations are
0.113 Å and 0.040 Å from the mean plane. The europium atoms lie out
of the planes by 0.279 Å and 0.368 Å.
The excitation (left) and emission (right) spectra of the Eu(NO3)3
complexes with ligands LI and LII in acetonitrile solutions at room
temperature are shown in Fig. 3). The excitation spectra, which are
obtained by monitoring within the 5D0 →7F2 transition of Eu3+, both
exhibit a broad excitation band (BEB) between 320 and 400 nm which
is ascribed to the π–π* electron transition of the ligands. Upon
excitation at the maximum excited wavelength of the ligands'
absorption band, the emission spectra of all samples reveal the
characteristic emission bands of the Eu3+ ion at ∼580 nm, ∼590 nm
and ∼616 nm, indicating that the energy transfer from the ligands to
central ion Eu3+ (antenna effect) seems comparatively efficient. The
relative intensities ratio value η(5D0 →7F2/5D0 →7F1) of the EuLI
(NO3)3 and EuLII(NO3)3 are 3.1 and 3.6, respectively, indicating that
the Eu3+ ions do not lie in centro-symmetric coordination sites, in
agreement with the crystal structure analysis. The lifetimes of EuLI
(NO3)3 and EuLII(NO3)3 in CH3CN solutions are 1.40 ms and 1.44 ms,
respectively, fitted from the luminescence decay curves by single
exponential process.
In order to acquire the lowest triplet excited state T1 of the ligands
LI and LII, the phosphorescence spectra of the Gd3+ complexes
were measured at 77 K in mixed solvents of methanol and ethanol
Fig. 1. The coordination sphere (a) and coordination polyhedron (b) of EuLI(NO3)3 (purple: Eu ion; blue: N atom; red: oxygen atom). The key bond distances and angles (Å and °): Eu
(1)–O(1) 2.352(3), Eu(1)–O(2) 2.639(3), Eu(1)–O(3) 2.469(3), Eu(1)–N(2) 2.598(3), Eu(1)–O(5) 2.459(3), Eu(1)–O(6) 2.462(3), Eu(1)–O(8) 2.481(3), Eu(1)–O(9) 2.553(3), Eu
(1)–O(11) 2.475(4), Eu(1)–O(12) 2.452(4), and O(1)–Eu(1)–O(2) 61.27(9), N(2)–Eu(1)–O(2) 60.49(9), O(3)–Eu(1)–N(2) 62.15(10), O(5)–Eu(1)–O(6) 50.35(12), O(8)–Eu(1)–O
(9) 49.54(10), O(12)–Eu(1)–O(11) 50.10(14).

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H.-P. Wang et al. / Inorganic Chemistry Communications 13 (2010) 882–886
Fig. 2. The coordination sphere (a) and coordination polyhedron (b) of EuLII(NO3)3 (purple: Eu ion; blue: N atom; red: oxygen atom). The key bond distances and angles (Å and °): Eu
(1)–O(1) 2.378(2), Eu(1)–O(2) 2.6073(19), Eu(1)–O(3) 2.460(2), Eu(1)–N(2) 2.567(2), Eu(1)–O(5) 2.466(2), Eu(1)–O(7) 2.486(2), Eu(1)–O(8) 2.505(2), Eu(1)–O(10) 2.532(2),
Eu(1)–O(11) 2.521(2), Eu(1)–O(13) 2.491(2), and O(1)–Eu(1)–O(2) 61.52(6), N(2)–Eu(1)–O(2) 61.70(7), O(3)–Eu(1)–N(2) 63.09(7), O(5)–Eu(1)–O(7) 51.39(8), O(8)–Eu(1)–O
(10) 49.73(8), O(13)–Eu(1)–O(11) 50.36(8).
(v:v=1:1). The triplet state energy levels T1 of the ligands LI and LII,
which were calculated from the shortest wavelength phosphores-
cence band, are 19,084 and 19,417 respectively. These energy
levels are above the lowest excited resonance level 5D0 of Eu3+
(17,286 cm−1), however, below the lowest excited resonance level
5D4 of Tb3+ (20,545 cm− 1). The results of the triplet state energy
levels T1 of the ligands LI and LII indicate that these ligands are suitable
sensitizers for the europium ion.
To study the stoichiometries between the ligand LI or LII and Eu3+
,
the luminescence titration experiments were also carried out using
10−4 mol L−1 ligand LI or LII in acetonitrile solution (Fig. 4). The
emission peaks at ∼592 and ∼617 nm increased upon gradual
addition of Eu(NO3)3 and remained constant after 1.0 equiv Eu
(NO3)3 was added. A Job's plot analysis exhibited a maximum at
0.5 mol fraction of Eu(NO3)3 (inset of Fig. 4), indicating the formation
of 1:1 complexes in the solutions.
Lanthanide luminescence behaviors are influenced by the change
of the coordination environment around the lanthanide centre [17].
Considering the coordinatively unsaturated characters of the quino-
line–amide type ligands, europium complexes with ligands LI and LII
might exhibit anion-responsive luminescence profiles for particular
anions, because the stoichiometry, geometry, and structure of the
ternary “ligand–lanthanide–anion” complex are often alerted by the
additional coordination from external anion [18].
When ligand LI or LII was mixed with an equimolar quantity of
Eu(CF3SO3)3 in CH3CN solutions (0.5×10−4 mol L−1), characteristic
Eu3+ luminescence spectra were observed. The anion responsiveness
studies of the lanthanide complexes are investigated and the relative
luminescence intensities changes at 616 nm for LI–Eu(CF3SO3)3 and
LII–Eu(CF3SO3)3 complexes upon addition of three equivalents
tetrabutylammonium salt containing the F−, Cl−, Br−, I−, ClO−4
,
NO−3 , CH3CO−2 , HSO4− or H2PO−4 anions are summarized in Fig. 5. The
Eu3+ complexes with ligands LI or LII exhibited the similar anion-
response luminescence behaviors which the luminescence increased
upon addition of Cl− or NO3−. Considering the descriptions of the
single-crystal structures and complexation in CH3CN solution, the
ligands and Eu3+ ion might prefer the formation of 1:1 complexes, in
which the ligands would occupy the coordination sites of the central
Eu3+ ion in equatorial plane and the potential exchangeable
coordination sites of the central Eu3+ ion in axial would be kept
back. We deduce that based on the appropriate coordination capacity
and radius of anions, Cl− and NO−3 might coordinate into the inner
coordination sphere of central Eu3+ and prevent luminescence
quenching by the water or solvent molecules.
Fig. 3. Room temperature excitation (monitored within the 5D0 →7F2 transition) and
emission (monitored at the maximum excited wavelength) spectra for EuLI(NO3)3 (a)
and EuLII(NO3)3 (b) in CH3CN solution ([complex]=1×10−3 mol L−1).

H.-P. Wang et al. / Inorganic Chemistry Communications 13 (2010) 882–886
885
Fig. 5. Anion-responsive profiles of luminescent quinoline–amide ligands–Eu3+
complexes [Eu(CF3SO3)3]=0.5×10− 4 mol L− 1 [ligand]=0.5×10− 4 mol L− 1
a:
excitation at 317 nm, slit widths 10/10 nm; b: excitation at 320 nm, slit widths 10/
10 nm; [nBu4NX]=1.5×10−4 mol L−1
;
;
.
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[5] D. Parker, Coord. Chem. Rev. 205 (2000) 109.
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[9] L.J. Charbonnière, R. Schurhammer, S. Mameri, G. Wipff, R.E. Ziessel, Inorg. Chem.
44 (2005) 7151.
Fig. 4. Luminescence spectra of LI and LII (1.0×10−4 mol L−1) in CH3CN upon addition of
different concentrations of Eu(NO3)3 (0, 2.0×10−5, 4.0×10−5, 6.0×10−5, 8.0×10−5
1.0×10−4, and 1.2×10−4 mol L−1, respectively). The Job's plots for LI and LII vs. Eu3+
,
.
The results of this work represent novel ligands design based on
quinoline acted as effective photoantennae for excitation of the
europium ion and pre-controlled structures of the complexes affected
by the polydentate rigid ligands. Sensitized by the ring-like rigid
quinoline–amide ligands, the Eu3+ complexes have shown the
[10] T. Terai, K. Kikuchi, S. Iwasawa, T. Kawabe, Y. Hirata, Y. Urano, T. Nagano, J. Am.
Chem. Soc. 128 (2006) 6938.
[11] W.N. Wu, N. Tang, L. Yan, J. Fluoresc. 18 (2007) 101.
[12] A.R. Katritzky, W.Q. Fan, Q.L. Li, Tetrahedron Letters 28 (1987) 1195 2-(1,3-
dioxolan-2-yl)quinolin-8-ol (b): white solid, yield 90%.1H NMR (400 MHz, CDCl3,
ppm): 8.209–8.188(d, 1H, J =8.4 Hz), 8.164(s, 1H), 7.673–7.652(d, 1H,
J=8.4 Hz), 7.482–7.442(t, 1H, J=8.4 Hz), 7.341–7.318(dd, 1H, J=1.2 Hz,
J=8.4 Hz), 7.202–7.181(dd, 1H, J=0.8 Hz, J=7.6 Hz), 5.990(s, 1H), 4.254–
4.194(m, 2H), 4.170–4.110(m, 2H).
characteristic europium emission and selectivity for Cl− and NO3−
.
[13] E.G. Petkova, R.D. Lampeka, M.V. Gorichko, K.V. Domasevitch, Polyhedron 20
(2001) 747.
Acknowledgements
[14] Ligand LI: white solid, yield 76%,1H NMR (CDCl3, 400 MHz):8.868(s, 1H), 8.225–
8.204(d, 1H, J=8.4 Hz), 7.673–7.652 (d, 1H, J=8.4 Hz), 7.539–7.506(t, 2H,
J=6.6 Hz), 7.295–7.219(m, 6H), 5.697(s, 1H), 4.854(s, 1H), 4.562–4.547(d, 2H,
J=6.0 Hz), 4.159–4.099(m, 2H), 4.053–3.993(m, 2H). IR (KBr pellet, cm-1) ν:
3076 (m, Ar–H), 2924 (m, C–H), 1670 (s, C O), 1555 (m), 1475 (m), 1431 (m),
1379 (m), 1318 (m), 1256 (m), 1106 (s), 838 (m), 757 (m). Ligand LII: white solid,
yield 80%,1H NMR (CDCl3, 400 MHz):8.178–8.156(d, 1H, J=8.8 Hz), 7.668–7.647
(d, 1H, J=8.4 Hz), 7.429–7.324(m, 10H), 7.058–7.046(d, 1H, J=4.8 Hz), 6.001(s,
1H), 4.935(s, 1H), 4.245–4.185(m, 2H), 4.147–4.087(m, 2H). IR (KBr pellet, cm-1)
ν: 3057 (m, Ar–H), 2914 (m, C–H), 1685 (s, C O), 1597 (m), 1487 (m), 1428 (m),
1377 (m), 1319 (m), 1254 (m), 1108 (s), 840 (m), 764 (m), 701 (m).
[15] Element analysis (%) for EuLI(NO3)3 Calcd.: C 35.91, H 2.87, N 9.97; found: C 36.13,
H 2.78, N 9.78. IR (KBr pellet, cm-1) ν: 1651(s, C O), NO3-(C2v): 1479(s, ν1),
1312(s. ν4), 1034 (m, ν2), 817(m, ν5). Element analysis (%) for EuLII(NO3)3
Calcd.: C 40.85, H 2.90, N 9.16; found: C 41.31, H 2.83, N 8.66. IR (KBr pellet, cm-1)
ν: 1634 (s, C O), NO3-(C2v): 1490 (s, ν1), 1306 (s. ν4), 1030 (m, ν2), 816(m, ν5).
[16] Data for single crystal of the complexes was collected on a Bruker SMART CCD
detector diffractometer using graphite-monochromatic Mo Kα radiation
(λ=0.71073 Å) at 298 2 K. The structures were solved by direct methods and
refined by full-matrix least squares techniques with anisotropic thermal factors
for all nonhydrogen atoms. The hydrogen atoms were added geometrically and
not refined. All calculations were performed using the program package SHELXTL
This work was supported by the National Natural Science
Foundation of China (Project No. 20931003) and the program for
New Century Excellent Talents in University (NCET-06-0902).
Appendix A. Supplementary material
CCDC 770381 and CCDC 770382 contain the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via http://
www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associat-
ed with this article can be found, in the online version, at doi:10.1016/
j.inoche.2010.04.020.
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