Luminescent behavior and energy transfer in homometallic Eu and ion associating Eu-Zn complexes with a pentadentate ligand

Article abstract of DOI:10.1016/j.inoche.2012.09.010

A pentadentate ligand L (2-(2-((bis(pyridin-2-ylmethyl)amino)methyl) phenoxy) -N,N-diphenylacetamide) was self-assembled with Eu (NO ₃)₃ or Zn (NO₃)₂/Eu (NO ₃)₃ to give homometallic complex ([EuL(NO ₃)₃], 1) or ion associating complex ([ZnL(NO ₃)]₂·[Eu(NO₃)₅] ·CH₃CN, 2), respectively. The photoluminescent behavior and energy transfer process were studied in the two complexes by excitation in different ways, in which 2 shows Fo?rster energy transfer mechanism.

Full text of DOI:10.1016/j.inoche.2012.09.010

Inorganic Chemistry Communications 25 (2012) 48–50
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Luminescent behavior and energy transfer in homometallic Eu and ion associating
Eu–Zn complexes with a pentadentate ligand
⁎
⁎
Hai-Ping Wang, Shi-Chao Wei, Mei Pan , Kang Li, Zi Wang, Cheng Yan, Cheng-Yong Su
MOE Laboratory of Bioinorganic and Synthetic Chemistry/KLGHEI of Environment and Energy Chemistry, State Key Laboratory of Optoelectronic Materials and Technologies, School of
Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou, 510275, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A pentadentate ligand L (2-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenoxy) -N,N-diphenylacetamide)
was self-assembled with Eu (NO3)3 or Zn (NO3)2/Eu (NO3)3 to give homometallic complex ([EuL(NO3)3],
1) or ion associating complex ([ZnL(NO3)]2·[Eu(NO3)5]·CH3CN, 2), respectively. The photoluminescent
behavior and energy transfer process were studied in the two complexes by excitation in different ways, in
which 2 shows Förster energy transfer mechanism.
Received 30 August 2012
Accepted 5 September 2012
Available online 13 September 2012
Keywords:
Ion associating complex
Luminescence
© 2012 Elsevier B.V. All rights reserved.
Förster energy transfer
Usually, via the strong absorption of organic ligand and then
effective “Dexter” energy transfer to the rare earth metal center coordi-
nated with the ligand directly, the so called “antenna effect” can be
achieved to generate specific luminescence in Lanthanide complexes
[1–6]. As an alternative way, “Förster” mechanism can be accomplished
in some Lanthanide complexes in which the energy is transferred from
organic ligand to non-coordinated metal center through space instead
of via direct bond connections[7–9]. Ion associating complex can be
formed by the electrostatic interaction between positively (or negative-
ly) charged metal–organic coordination groups with oppositely charged
ions or groups to form a neutral complex, which have been widely
reported in some charge-transfer complexes[10]. However, ion associat-
ing complexes containing both d and f metal centers and their lumines-
cent energy transfer behaviors have not been studied extensively. As a
matter of fact, due to the suitable distance between the ion pair linked
by electronic interactions instead of direct coordination bonds in this
kind of complexes, it affords a suitable platform for the luminescent
generation via “Förster” energy transfer mechanism. In this paper, we
report a new 3d–4f bimetallic ion associating complex (2), in which
the pentadentate-ligand-coordinated Zn (II) cations form electronic
interactions with nitrate-bonded Eu (III) anions. Its luminescent proper-
ties show “Förster” type energy transfer character compared with pure
homometallic Eu complex (1)[11–13].
The single-crystal of 1 is crystallized in P21/n space group and the
central Eu (III) ion is ten-coordinated, in which the pentadentate
ligand affords three N and two O atoms, and simultaneously, five O
atoms from two bidentate and one monodentate nitrates help to
satisfy the {N3O7} distorted bicapped square antiprism coordination
atmosphere (Fig. 1a). The tripodal arms from the ligand wrap around
the metal ion and form a pincer-like coordination configuration. Struc-
tural analysis reveals that 2 is crystallized in P21 chiral space group. It is
interesting that the compound consisted of organic ligand-coordinated
zinc cationic centers and inorganic nitrate anion-bonded europium
anionic centers (Fig. 1b). In [ZnL(NO3)]+ organic coordination unit, Zn
(II) center is encapsulated in an {N3O3} 6-coordinating geometry with
three N atoms and two O atoms from the ligand and one O atom from
a nitrate. The three arms of the ligand are also arranged in a pincer-
like configuration. Besides this kind of coordination cationic unit,
there exists another kind of Eu (III) center in complex 2, namely the
inorganic nitrate anion-bonded [Eu (NO3)5]2− anionic unit. The coordi-
nation sphere of Eu (III) is totally satisfied by ten O atoms from five
inorganic nitrate anions, which also forms a distorted bicapped square
antiprism atmosphere. Two ligand-coordinated cations and one inor-
ganic anion-bonded anion coexisted in one unsymmetric unit of the
crystal. In general, the two kinds of oppositely charged metallic centers
are linked by electrostatic interactions to form an ion associating
complex.
The luminescent spectra of homonuclear complex 1 and ion asso-
ciating complex 2 are very similar and are shown in Fig. 2. We can see
that by monitoring the 5D0→ 7 F2 transition of Eu3+ at 618 nm, an
excitation spectrum with the background of a broad excitation band
between 270 and 425 nm is obtained, which can be ascribed to the
⁎
Corresponding authors at: School of Chemistry and Chemical Engineering, Sun Yat-Sen
University, Guangzhou, 510275, China. Tel.: +86 20 8411 5178; fax: +86 20 8411 5178.
E-mail addresses: panm@mail.sysu.edu.cn (M. Pan), cesscy@mail.sysu.edu.cn
(C.-Y. Su).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.09.010

H.-P. Wang et al. / Inorganic Chemistry Communications 25 (2012) 48–50
49
Fig. 1. The coordination unit in complexes 1 (a) and 2 (b).
Fig. 2. Solid state luminescence (a) and lifetime decay curves (b) of the two complexes.
π–π∗ electron transition of the ligands. Meanwhile, some distinct sharp
f–f transition peaks of Eu (III) can be observed at 395 (7 F0→5 L6) and
465 (7 F0→5D2). Upon excitation at 318 (based on the absorption of
the ligand), 395 and 465 nm (based on the hypersensitive transition
absorptions of the Eu (III) center), the emission spectra of the two
complexes both reveal the characteristic emission bands of the Eu3+
ion at 579 (5D0→7 F0), 593 (5D0→7 F1), 612 (5D0→7 F2) and 650 nm
(5D0→7 F3).
or mono-exponential decay curves can be observed (Fig. 2b), respec-
tively [14]. As shown in Table 1, the two complexes both have double
lifetimes upon excitation at the absorption wavelength of the ligand,
0.5 (~20%)/1.1 (~80%)ms for complex 1, and 0.3 (~20%)/0.9 (~80%)ms
Table 1
Luminescent lifetimes of complexes 1 and 2 excited by different wavelength.
Sample
Excitation wavelength/nm
τ1/ms
τ2/ms
Although the luminescent emission peaks are almost identical
with excitation either from the absorption of the ligand or Eu (III)
center, their lifetimes show important differences, which can give
some information about the energy transfer mechanism during differ-
ent excitation approaches. We can see that by monitoring the 613 nm
emission under the excitation of 318 or 395/465 nm, bi-exponential
Complex 1
318
395
465
318
395
465
0.4720 (~20%)
1.1212
1.1308
0.3031 (~20%)
0.9416
0.9257
1.1237 (~80%)
Complex 2
0.8937 (~80%)

50
H.-P. Wang et al. / Inorganic Chemistry Communications 25 (2012) 48–50
Scheme 1. Energy transfer mechanism in complexes 1 (a) and 2 (b).
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for complex 2. However, a single lifetime is obtained for the two com-
plexes upon excitation at 395 or 465 nm (f–f transitions of the Eu (III)
center), with the value of about 1.1 ms for complex 1, and 0.9 ms for com-
plex 2.
As we know, mono-exponential lifetime relates to one kind of
excited state of the luminescent center, while bi-exponential lifetime
indicates the existence of multiple excited states or different kinds of
luminescent centers. As the cases in complexes 1 and 2 excited by
318 nm, the emission of Eu (III) center happens via a complicated
process involving ligand absorption and then energy transfer from
the ligand to the metal center. The energy transfer can happen by
different pathways, for example, either from the singlet or triplet
states of the ligand [15], therefore, multiple excited states in the lumi-
nescent Eu (III) center with different lifetimes can be observed. On
the other hand, the direct absorption of the f–f transition of Eu (III)
center affords only one kind of excited state, and mono-exponential
lifetime can be expected. From the above analysis, we can deduce
that, although the coordination environments of Eu (III) center in
the two complexes are totally different, they can be excited from
similar origins. Firstly, they can both be excited by the absorption of
the ligands, while by different ways. In complex 1, the ligand is coor-
dinated with Eu (III) center directly, therefore, Dexter energy transfer
dominantly occurs. While in complex 2, the energy transition from
the ligand in [ZnL(NO3)]+ organic coordination unit to Eu (III) center
in [Eu(NO3)5]2− anionic unit should happen by Förster mechanism.
The [ZnL(NO3)]+ units serving as donor chromophores, may transfer
energy to the [Eu(NO3)5]2− anionic units as acceptor chromophores
through nonradiative dipole–dipole coupling effect. But unluckily, in
both complexes 1 and 2, insufficient energy transfer from the ligand
to Eu (III) center results in low luminescent quantum yield (~3% for com-
plex 1 and 5% for complex 2). In an alternative way, both complexes can
be excited by direct hypersensitive absorption of the Eu (III) center at
395 and 465 nm, which results in mono-exponential lifetimes. Compar-
ing the luminescent lifetime values of the two complexes, we can see
that those for 1 are slightly longer than those for 2. This is in accordance
with their structural differences, in which the Eu (III) center in 1 is well
shielded by pentadentate organic ligand against outer solvents, while
only NO3− anions surrounding the Eu (III) center in 2 cannot afford
such good protection. The mechanism of the luminescent emission in
the two complexes is represented in Scheme 1.
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phenanthridine appended lanthanide complexes: analysis of triplet mediated
energy transfer processes in terbium, europium and neodymium complexes, J.
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of coordinative and inorganic lanthanide centers showing dual emission via
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108 (2004) 8084–8088.
[11] Preparation of the ligand: 2-(2-((bis(pyridin-2-ylmethyl)amino)methyl)phenoxy) -N,
N-diphenylacetamide (L). A mixture of 2-((bis(pyridin-2-ylmethyl)amino) methyl)
phenol (3.2 g, 10 mmol), 2-chloro-N, N-diphenylacetamide (2.46 g, 10 mmol),
K2CO3 (1.66 g, 12 mmol), and KI (0.5g, 3.3mmol) in DMF (20 mL) was stirred at 80
°C for 12h. After cooling, the mixture was poured into 150mL of ice water and allowed
to stand overnight. The deposited brown solid was extracted with CHCl3 (3 × 40 mL)
and the solvent was removed in vacuo. The dark brown compound was purified by
flash chromatography (silica gel) using ethyl acetate as eluent. 1H NMR (300 MHz,
CDCl3) δ 8.48 (d, J = 5.0 Hz, 2H), 7.61 (d, J = 3.9 Hz, 3H), 7.55 (d, J = 7.5 Hz, 1H),
7.31 (t, J = 8.0 Hz, 4H), 7.24–7.12 (m, 4H), 7.10 (dd, J = 9.0, 4.4 Hz, 2H), 6.95 (t, J =
7.4 Hz, 1H), 6.73 (d, J = 8.2 Hz, 1H), 4.57 (d, J = 14.4 Hz, 2H), 3.82 (s, 3H), 3.64
(s, 2H). Mass (ESI): m/z 467.156 (M+1).
[12] Preparations of complexes 1 and 2: A solution of the ligand L (0.1 mmol, 514 mg)
in acetonitrile (10 mL) was added to a stirred solution of Eu (NO3)3·6H2O
(0.1 mmol, 446 mg) in acetonitrile (10 mL) at room temperature for 1 h. After
filtration, slow diffusion of diethyl ether into the filtrate over 72 hours afforded
pale yellow crystals of 1. Anal. Calcd. (%) for C33H34N7O13Eu (1·2H2O): C, 44.26;
H, 3.55; N, 11.11. Found (%): C, 44.60; H, 3.86; N, 11.03. A solution of the ligand
L (0.1 mmol, 514 mg) in acetonitrile (10 mL) was added to a stirred solution of
Zn (NO3)2·6H2O (0.1 mmol, 297 mg) and Eu (NO3)3·6H2O (0.05 mmol,
223 mg) in acetonitrile (10 mL) at room temperature for 1 h. After filtration,
slow diffusion of diethyl ether into the filtrate over one week afforded colorless
crystals of 2. Anal. Calcd. (%) for C66H62N15O26EuZn2 (2·H2O): C, 44.63; H, 3.57;
N, 11.73. Found (%): C, 44.94; H, 3.54; N, 11.91.
[13] Crystal data for 1: C33H30EuN7O11, Mr = 852.60, monoclinic, space group P21/n, a =
12.2441 (1) Å , b =18.4985 (2) Å , c =14.9315 (2) Å, β =90.1550(10)o, V =
3381.94 (6) Å3, Z = 4, T = 293K, Dcalcd. = 1.675 g.cm-3, μ = 13.902 mm-1
,
R=0.0237, wR2 = 0.0595 and GOF =1.020. Crystal data for 2: C68H60EuN16O25Zn2,
Mr = 1784.02, colorless crystal, monoclinic, space group P21, a = 11.9955(2)Å,
b = 11.8303 (3) Å, c = 26.5310 (6) Å, V = 3757.38 (14) Å3, Z = 2, T =293 K,
Acknowledgments
Dcalcd. = 1.577 g.cm-3 μ = 7.404 mm-1, R = 0.0666, wR2 = 0.1830 and GOF =
,
This work was supported by the National Basic Research Program of
China (973 Program, 2012CB821701), NSF of China (Grants U0934003,
20903120, 21121061, 21173272), the RFDP of Higher Education of
China (No. 20090171120042), the Fundamental Research Funds for
the Central Universities (No. 09lgpy15), and FDYT (No. LYM09004).
1.028. The crystal structures of 1 and 2 were solved by direct methods using
SHELXS-97 program and refined on F2 by full-matrix leastsquares using SHELXL-97
program. All non-H atoms were refined anisotropically while all H atoms were placed
in geometrically idealized positions.
[14] M.E. Hagerman, S.J. Salamone, R.W. Herbst, A.L. Payeur, Tris (2,2′-bipyridine)
ruthenium (II) cations as photoprobes of clay tactoid architecture within hectorite
and laponite Films, Chem. Mater. 15 (2003) 443–450.
[15] C. Yang, L.-M. Fu, Y. Wang, J.-P. Zhang, W.-T. Wong, X.-C. Ai, Y.-F. Qiao, B.-S. Zou, L.-L.
Gui, A highly luminescent europium complex showing visible-light-sensitized red
emission: direct observation of the singlet pathway, Angew. Chem. Int. Ed. 43
(2004) 5010–5013.
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