Self-assembly synthesis, structural features, and photophysical properties of dilanthanide complexes derived from a novel amide type ligand: Energy transfer from Tb(III) to Eu(III) in a heterodinuclear derivative

Article abstract of DOI:10.1021/ic402338q

A novel amide type ligand benzyl-N,N-bis[(2′-furfurylaminoformyl) phenoxyl)ethyl]-amine (L) has been designed and applied for the self-assembly generation of homodinuclear lanthanide coordination compounds [Ln ₂(μ₂-L)₂(NO₃)₆(EtOH) ₂] [Ln = Eu (1), Tb (2), and Gd (3)] and a heterodinuclear derivative [EuTb(μ₂-L)₂(NO₃)₆(EtOH) ₂] (4). All the complexes have been characterized by the X-ray single-crystal diffraction analyses. They are isostructural, crystallize in a monoclinic space group P2₁/c, and form [2 + 2] rectangular macrocycle structures. Compound 4 is the first example of a [2 + 2] rectangular macrocycle heterodinuclear EuTb complex assembled from an amide type ligand. In 4, the discrete 0D dimeric [EuTb(μ₂-L)₂(NO₃) ₆(EtOH)₂] units are extended, via the multiple N-H·O hydrogen bonds, into a 2D supramolecular network that has been topologically classified as a uninodal 4-connected underlying net with the sql [Shubnikov tetragonal plane net] topology. The triplet state ( ³ππ*) of L studied by the Gd(III) complex 3 demonstrated that the ligand beautifully populates Tb(III) emission (Φ = 52%), whereas the corresponding Eu(III) derivative 1 shows weak luminescence efficiency (Φ = 0.7%) because the triplet state of L has a poor match with ⁵D ₁ energy level of Eu(III). Furthermore, the photoluminescent properties of heterodinuclear complex 4 have been compared with those of the analogous homodinuclear compounds. The quantum yield and lifetime measurements prove that energy transfer from Tb(III) to Eu(III) is being achieved, namely, that the Tb(III) center is also acting to sensitize the Eu(III) and enhancing Eu(III) emission in 4.

Full text of DOI:10.1021/ic402338q

Article
pubs.acs.org/IC
Self-Assembly Synthesis, Structural Features, and Photophysical
Properties of Dilanthanide Complexes Derived from a Novel
Amide Type Ligand: Energy Transfer from Tb(III) to Eu(III)
in a Heterodinuclear Derivative
Cunji Gao,† Alexander M. Kirillov,‡ Wei Dou,† Xiaoliang Tang,† Liangliang Liu,† Xuhuan Yan,† Yujie Xie,†
Peixian Zang,† Weisheng Liu,† and 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, P. R. China
‡Centro de Quimica Estrutural, Complexo I, Instituto Superior Tecnico, The University of Lisbon, Av. Rovisco Pais,
1049-001 Lisbon, Portugal
S
* Supporting Information
ABSTRACT: A novel amide type ligand benzyl-N,N-bis[(2′-furfurylaminoformyl)phenoxyl)ethyl]-amine (L) has been designed
and applied for the self-assembly generation of homodinuclear lanthanide coordination compounds [Ln2(μ2-L)2-
(NO3)6(EtOH)2] [Ln = Eu (1), Tb (2), and Gd (3)] and a heterodinuclear derivative [EuTb(μ2-L)2(NO3)6(EtOH)2] (4).
All the complexes have been characterized by the X-ray single-crystal diffraction analyses. They are isostructural, crystallize in a
monoclinic space group P21/c, and form [2 + 2] rectangular macrocycle structures. Compound 4 is the first example of a [2 + 2]
rectangular macrocycle heterodinuclear EuTb complex assembled from an amide type ligand. In 4, the discrete 0D dimeric
[EuTb(μ2-L)2(NO3)6(EtOH)2] units are extended, via the multiple N−H···O hydrogen bonds, into a 2D supramolecular
network that has been topologically classified as a uninodal 4-connected underlying net with the sql [Shubnikov tetragonal plane
net] topology. The triplet state (3ππ*) of L studied by the Gd(III) complex 3 demonstrated that the ligand beautifully populates
Tb(III) emission (Φ = 52%), whereas the corresponding Eu(III) derivative 1 shows weak luminescence efficiency (Φ = 0.7%)
because the triplet state of L has a poor match with 5D1 energy level of Eu(III). Furthermore, the photoluminescent properties of
heterodinuclear complex 4 have been compared with those of the analogous homodinuclear compounds. The quantum yield and
lifetime measurements prove that energy transfer from Tb(III) to Eu(III) is being achieved, namely, that the Tb(III) center is
also acting to sensitize the Eu(III) and enhancing Eu(III) emission in 4.
dinuclear lanthanide coordination compounds attract consid-
erable research interest due to their potential biological,3
medical,4 chemical, and technological importance.5 Energy
transfer between two lanthanide ions in heterodinuclear
complexes or mixed lanthanide systems has long been known
in solution6 and in the solid state.7 The use of a second
lanthanide ion as an energy transfer acceptor can make the
resonance energy transfer measurement more sensitive because
INTRODUCTION
■
Recently, the design of novel lanthanide coordination
compounds with conjugated organic ligands has drawn more
and more chemists’ interests, due to their attractive photo-
physical properties (large Stokes shifts, long radiative lifetimes,
high luminescence quantum yields, narrow bandwidths)
suitable for various applications, namely as optical probes,
liquid crystalline materials, or biological labeling reagent.1
Eu(III) and Tb(III) complexes are of special interest because of
the high luminescence quantum efficiencies and the versatility
of their coordination environments.2 Among these studies,
Received: September 16, 2013
Published: January 8, 2014
© 2014 American Chemical Society
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the emissions from a donor and an acceptor are readily
resolvable as sharply spiked peaks. When the Eu(III) ion is used
as an energy receptor from Tb(III), it is common to observe
that the luminescence from Eu(III) is strongly sensitized to the
detriment of Tb(III) emission.8 Recent studies have reported
the importance of energy transfer modulation between the
Tb(III) and Eu(III) ions aiming to obtain solid light-emitting
materials and probes based on luminescent switching.9 Reddy
and co-workers have recently reported that the efficient energy
transfer (η = 86%) occurs from Tb(III) to Eu(III) in a
coordination polymer.10
As a result, single-crystal X-ray structure analyses proved that all
the complexes are isostructural. The 1:1 molar ratio of Eu(III)/
Tb(III) in a heterometallic compound 4 was further proved by
the EDX analysis (Eu/Tb = 0.93; Figure S1 and Table S1 in the
Supporting Information). Elemental analysis and IR spectral
data for the complexes 1−4 are listed in Table 1. In the FTIR
spectra, the peak at 1640 cm−1 which is assigned to the
vibration absorption of a carbonyl group of L is replaced by a
new absorption peak at 1615−1620 cm−1 in complexes 1−4
(Figure S2 in Supporting Information), suggesting that the
oxygen atoms of the carbonyl groups are coordinated to the
central lanthanide ions.
Thermal Stability Studies. To study the thermal stabilities
of the complexes 1−4, thermogravimetric analyses (TGA) and
differential scanning calorimetry (DSC) were performed on the
crystalline samples (Figure S3 in Supporting Information). All
the compounds easily lose the crystallization ethanol molecules,
the complete elimination of which is observed at 203 °C for 1
(calcd, 4.71%; found, 5.06%), 200 °C for 2 (calcd, 4.67%;
found, 4.98%), 227 °C for 3 (calcd, 4.68%; found, 5.03%), and
192 °C for 4 (calcd, 4.69%; found 4.53%). After reaching these
temperatures, the compounds begin to decompose, resulting in
a concomitant elimination of the two L moieties and three
nitrate groups (e.g., calcd, 79.69%; found, 78.17% for 1; calcd,
79.41%; found, 77.95% for 4). The decomposition process is
essentially complete at 600 °C for all the samples.
Inspired by the aforementioned progress, we were prompted
to design a new ligand with two very flexible amide type arms,
namely, benzyl-N,N-bis[(2′-furfurylaminoformyl)phenoxyl)-
ethyl]-amine (L) (Scheme 1), and utilized it to construct a
Scheme 1. Chemical Structure of the Ligand L
Crystal Structure Descriptions. The single-crystal X-ray
analyses of the complexes 1−4 confirm that they are all
isostructural dinuclear derivatives and bear the nine-coordinate
metal centers. Their coordination environments are filled by
two O atoms from carbonyl groups of two μ2-L moieties, six O
atoms from three bidentate nitrate groups, and one O atom
from the terminal coordinated ethanol molecule. The structural
analyses unambiguously prove that the complexes 1−4 form
discrete [2 + 2] rectangular macrocycles with the sizes 7.246 ×
7.557 Å2 (1), 7.522 × 7.256 Å2 (2), 7.226 × 7.546 Å2 (3), and
7.264 × 7.546 Å2 (4), respectively. Compound 4 represents the
first example of a [2 + 2] rectangular macrocycle EuTb
heterodinuclear complex assembled from an amide type ligand.
The molecular structure of 1 shows that two μ2-L ligands
adopt a face-to-face orientation and are linked together by two
Eu(III) ions, generating a discrete zero-dimensional (0D)
dinuclear rectangular macrocycle structure (Figure 1). Each
discrete unit consists of two independent Eu(III) centers. As
shown in Figure S4 (Supporting Information), the μ2-L
moieties reveal the intramolecular π−π interactions of the
two benzene ring planes which are parallel to each other, with
the centroid to centroid distance of 3.749 Å. Besides, two
benzene rings from two arms of each ligand interact through
the C−H···π interactions referred as a T-shape arrangement.
The coordination polyhedron around Eu(III) is a distorted
tricapped trigonal prism. The distance between two Eu(III)
ions is 13.243 Å. In addition, the discrete dinuclear units are
further assembled into a two-dimensional (2D) supramolecular
series of new lanthanide coordination compounds [Ln2(μ2-
L)2(NO3)6(EtOH)2] bearing homodinuclear [Ln = Eu (1); Tb
(2); Gd (3)] and heterodinuclear Eu(III)/Tb(III) [Ln = Eu
and Tb (4)] cations. This semirigid ligand can lead to discrete
−
structures, while the NO3 anion can act as a bidentate ligand
thus satisfying the need for high coordination number of
lanthanide centers. All of the obtained dinuclear complexes
have been structurally characterized by X-ray single-crystal
diffraction, elemental analysis, and IR spectroscopy. The
photoluminescent properties of the dilanthanide complexes
have been studied, revealing that the energy can transfer from
Tb(III) to Eu(III) and assist sensitization of Eu(III). This will
allow modulating the luminescence color by tuning the
excitation band or distance between two lanthanide ions in
order to design a multicolored light emitter.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The homo- and
heterodinuclear lanthanide nitrate coordination compounds
1−4 were obtained by the self-assembly reaction of lanthanide
nitrates with L in ethyl acetate. They were isolated as highly air
stable powders that are soluble in dimethyl formamide,
dimethylsulfoxide, methanol, ethanol, and acetone, slightly
soluble in acetonitrile, and insoluble in chloroform and diethyl
ether. After two weeks of slow diffusion at room temperature of
the diethyl ether to the mixed ethanol and acetonitrile solution
of 1−4, the crystals suitable for X-ray analysis were obtained.
Table 1. Elemental Analysis and IR Spectral Data of Complexes 1−4
found (calcd) (%)
complex
C
H
N
Ln
IR (cm−1) v (CO)
1
2
3
4
45.02 (45.45)
44.83 (45.13)
44.89 (45.21)
44.97 (45.29)
3.83 (4.23)
3.76 (4.20)
3.75 (4.20)
3.76 (4.21)
8.42 (8.60)
8.44 (8.53)
8.24 (8.55)
8.25 (8.46)
15.25 (15.55)
15.88 (16.14)
15.86 (16.00)
Eu 7.24 (7.75)
1615
1620
1615
1618
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Figure 1. Thermal ellipsoid (30% probability level) plot of complex 1.
All hydrogen atoms were omitted for clarity. Color scheme: gray, C;
red, O; blue, N; pink, Eu.
structure through intermolecular hydrogen bonds. Hydrogen
bonding parameters in 1 are given in Table S2 (Supporting
Information). The crystal structures of 2 and 3 are essentially
similar to that of 1 and are thus not discussed.
The structure of the heterodinuclear complex 4 (Figure 2a)
resembles those of homodinuclear analogues 1−3. However, in 4
each lanthanide element site is populated by a half Eu(III) and a
half Tb(III), so each crystal unit contains one Eu(III) and one
Tb(III) atom. Two μ2-L ligands adopt a face-to-face orientation
and are coordinated by two metal centers generating a discrete 0D
heterodimetallic rectangular macrocycle structure.
In 4, each dinuclear [EuTb(μ2-L)2(NO3)6(EtOH)2] block is
linked to four adjacent ones via the intermolecular N3−H3A···
O11 [3.062 Å] hydrogen bonds (Figure 2b, Table S2 in
Supporting Information), thus giving rise to the generation of a
H-bonded 2D layer. To better understand the structure of this
H-bonded network, we have carried out its topological analysis11
following the concept of the simplified underlying net.12 Thus, the
molecular units [EuTb(μ2-L)2(NO3)6(EtOH)2] have been reduced
to the respective centroids and considered as the 4-connected
nodes. The obtained underlying network (Figure 2c) has been
analyzed from the topological viewpoint, revealing a uninodal
4-connected net with the sql [Shubnikov tetragonal plane net]
topology described by the point symbol of (44·62).11,13 Although
various Eu or Tb containing sql nets have been identified,14 to our
knowledge, coordination compound 4 represents the first
heterometallic EuTb network of this topological type.11,15
Figure 2. Structural fragments of 4: (a) thermal ellipsoid (30%
probability level) plot of the discrete dinuclear unit. All hydrogen
atoms were omitted for clarity. Color scheme: gray, C; red, O; blue, N;
pink, Eu; green, Tb. (b) Linkage of the “central” (red) dinuclear unit
with four adjacent ones (gray) via the intermolecular N3−H3A···O11
hydrogen bonds (dotted lines), forming a H-bonded 2D layer. (c)
Topological representation of the underlying uninodal 4-connected
net with the sql topology and the point symbol of (44·62); centroids of
the [EuTb(μ2-L)2(NO3)6(EtOH)2] nodes are depicted in red. (b, c)
Views along the a-axis.
Photoluminescent Properties of the Complexes. The
excitation and emission spectra of the homodinuclear
complexes 1, 2, and heterodinuclear analogue 4 were recorded
on the crystalline samples at room temperature, and the
relevant photophysical data are summarized in Table S3
(Supporting Information). The excitation spectrum of 1
7
7
7
(Figure 3) illustrates that the F0→5D4, F0→5G0−4, F0→5L6,
two splitting peaks of the 5D0→7F4 transition occur at 689 and
7F0→5D2,1 and F1→5D1 excitation bands of Eu(III) ion are
7
5
701 nm. The hypersensitive transition D0 →7F2 locating at
more intense than excitation band of L,16 thus proving that the
luminescence sensitization through direct excitation of the
central Eu(III) ion is much more efficient than L. The emission
spectrum of 1 was detected at 327 nm which is the maximum
excitation wavelength of L. Maximum intensities situated at
579, 591, 617, 649, 689, and 701 nm, respectively, were
observed for the 5D0 →7FJ (J = 0, 1, 2, 3, and 4) transitions, and
617 nm is a strong band, leading to a red luminescence emission
of 1.17 The intensity ratio of I(5D0 → F2)/I(5D0 → F1) about
5.1 indicates that Eu(III) ions in 1 occupy sites with low symmetry
and have no inversion center,1e,18 which is in agreement with the
crystal structure analysis.
7
7
Figure 4 describes the steady-state excitation and emission
spectra of the Tb(III) complex at room temperature. The
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Figure 3. Excitation (λem = 618 nm) and emission (λex = 327 nm)
spectra for complex 1 recorded on the crystalline sample at room
temperature. Inset image refers to a red emission in 1 under the
365 nm UV lamp.
Figure 5. Emission spectra of complexes 1 (red line) and 4 (black
line), λexc = 327 nm recorded on the crystalline sample at room
temperature. Inset image refers to a dull-red emission in 4 under the
365 nm UV lamp.
5
large portion of the energy from the D4 level of the Tb(III)
ion, thus promoting its fast luminescence quenching.
Furthermore, the color in the luminescence image of the
heterodinuclear compound 4 shows a dull-red emission under
the 365 nm UV lamp (Figure 5).
Energy Transfer from Tb(III) to Eu(III) in Hetero-
dinuclear Compound 4. It is well-known that energy transfer
from the triplet state of the ligand to the resonance level of the
Ln(III) ion is regarded as an important factor influencing the
luminescent properties of lanthanide complexes.20 To elucidate
the energy transfer process in 4, the singlet energy level of the
ligand L has been determined by the UV−vis absorption
spectrum of the complex 3, which was measured in methanol
solution at room temperature. The wavelength of the
absorbance edge of complex 3 is 313 nm (Figure S5 in
Supporting Information), indicating that its singlet energy level
(1ππ*) is 31 949 cm−1. Furthermore, we have used the
compound [Gd2(μ2-L)2(NO3)6(EtOH)2] (3) to discuss the
triplet energy level (3ππ*) of the ligand L, aiming at assessing
its efficiency in the energy-transfer process to the central
lanthanide ions. The phosphorescence spectrum of 3 was
measured at 77 K in a methanol−ethanol solution (1:1 v/v)
(Figure S6 in Supporting Information). As a result, the triplet
energy level (3ππ*) of L, corresponding to its lower-wavelength
emission edge, is 24 631 cm−1 (406 nm). It is interesting to
note that the 3ππ* of this newly designed ligand lies well above
the lowest excited resonance level 5D1 of Eu(III) (19 000 cm−1)
Figure 4. Excitation (λem = 544 nm) and emission (λex = 327 nm)
spectra for complex 2 recorded on the crystalline sample at room
temperature. Inset image refers to a green emission in 2 under the
365 nm UV lamp.
excitation spectrum is a broad band from 220 to 350 nm, which
is associated with the electronic absorption of the ligand. This
proves L can effective populate the Tb(III) complex. The
emission spectrum of 2 shows four characteristic peaks
19
5
7
resulting from the D4 → FJ (J = 6, 5, 4, 3) transitions.
5
The most intense one lying at 544 nm agrees with the D4 →
7F5 transition. Moreover, there is an efficient ligand-to-metal
energy-transfer process in this complex proved by the absence
of the emission of L.
5
and D4 of Tb(III) (20 500 cm−1). Thus, the absorbed energy
Figure 5 shows the photoluminescent emission spectrum of
heterodinuclear compound 4 accompanied by the emission
spectrum of 1, which were recorded in the 450−720 nm range
upon excitation at 327 nm at room temperature. Upon ligand
excitation, the EuTb derivative 4 displays both the Eu(III)
of ligand L could be transferred to the central Eu(III) and
Tb(III) ions. Reinhoudt’s empirical rule21 states that ligand-to-
metal energy transfer becomes effective when ΔE(1ππ* −
3ππ*) is at least 5000 cm−1. The energy gap between 1ππ* and
3ππ* for the ligand L is 7318 cm−1, thus pointing out that this
newly designed ligand has a relatively good intersystem crossing
efficiency. On the other hand, it is very important that there is a
7
7
(5D0 → FJ) and Tb(III) (5D4 → FJ) transitions. Complex 4
exhibits enhanced luminescence intensity at 617 nm (2.1-fold)
corresponding to the 5D0 → 7F2 transition of Eu(III) ion
compared to that of compound 1. The presence of Tb(III)
enhances the luminescence intensity of the Eu(III) emission.
On the other hand, the luminescence intensity at 544 nm of the
3
suitable energy gap between the ππ* and the lanthanide ion
emissive states since the lanthanide ion emissive states must be
efficiently populated for the emission to occur. The empirical
rule proposed by Latva22 states that an optimal ligand-to-metal
energy transfer process for a lanthanide ion needs [ΔE =
E(3ππ*) − E(5DJ)] of 2500−4000 cm−1 for Eu(III) and 2500−
4500 cm−1 for Tb(III). The calculation of such parameters
7
5D4 → F5 transition decreases 41.8-fold in comparison with
that of complex 2, which indicates the relatively high efficiency
of Tb(III) → Eu(III) energy-transfer. It is possible that the
presence of Eu(III) in the heterodinuclear complex 4 diverts a
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5
Table 2. Radiative (ARAD) and Nonradiative (ANR) Decay Rates, D0/5D4 Lifetimes (τobs), Radiative Lifetimes (τRAD), Intrinsic
Quantum Yields (ΦLn), Energy Transfer Efficiencies (Φsen), and Overall Quantum Yields (Φoverall) for Complexes 1 and 2
complex
ARAD (s−1
)
ANR (s−1
)
τobs (μs)
τrad (μs)
ΦLn (%)
Φsen (%)
Φoverall (%)
a
1
443
2742
314
858
947
3
5
2
2257
14
5
0.7
b
a
2
1116
1
85
62
52
a
b
Absolute quantum yield. τobs at 77 K.
indicates that ΔE is 4131 cm−1 for Tb(III), which indicates
energy absorbed by L can transfer to Tb(III) efficiently.23 By
contrast, ΔE is 7331 cm−1 for Eu(III), thus indicating that the
ligand L cannot populate Eu(III) efficiently.
in vacuo, and ITOT/IMD shows the ratio of the total integrated
intensity of the corrected Eu(III) emission spectrum to the
integrated intensity of the magnetic dipole 5D0
transition:
→
7F1
The overall luminescence quantum yields, Φ, of 1, 2, and 4 in
the solid state were found to be 0.7%, 52%, and 2.5%,
respectively, using an integrating sphere when excited at
327 nm. Lifetime values of two homodinuclear lanthanide
coordination compounds 1 and 2, and heterodinuclear
lanthanide derivative 4, were measured in order to get further
insight into the light-conversion processes and to determine the
mechanism of the Tb → Eu energy transfer.
ARAD = 1/τRAD = AMD,0 × n3 × (ITOT/IMD
The intrinsic quantum yield for Tb3+ (ΦTb) estimated
through eq 4 is 85% on the basis of the hypothesis that the
decay process at 77 K in a deuterated solvent is purely
radiative.30,20e
)
(3)
ΦTb = τobs(298K)/τ
obs(77K)
(4)
The D0 (Eu3+) and D4 (Tb3+) lifetime values (τobs) were
determined from the luminescence decay profiles for the
complexes 1 and 2 at both 298 and 77 K monitored within the
more intense lines of the 5D0 → 7F2 and 5D4 → 7F5 transitions,
respectively (Figures S7−S9 in Supporting Information), and
the relevant data are listed in Table 2. The shorter 5D0 lifetime
(τobs = 314 3 μs) is detected for complex 1 at 298 K. The
lifetime is two component corresponding to two exponential
functions (⟨τobs⟩ = 858 5 μs) at 77 K indicating the presence
of two emissive Eu3+ centers.24 The temperature dependent
lifetime reflects the thermally activated deactivation processes.25
The observed luminescence decay profiles of complex 2
corresponding to single exponential functions at 298 K
(τobs = 947 2 μs) demonstrate the existence of one emissive
5
5
The calculated intrinsic quantum yields and energy transfer
efficiencies data for complexes 1 and 2 are listed in Table 2.
Poor sensitization efficiency existed in the Eu3+ complex mainly
3
because of the larger energy gap between ππ* and the excited
5D0 energy level.
Hence, the luminescence lifetime value of heterodinuclear deri-
vative 4 attained 0.792 0.004 ms (5D0) and 0.915 0.002 ms
(5D4) from the luminescent decay profiles by both fitting
with monoexponential curve (Figure S10 in Supporting
Information), indicating the single chemical environment
around the emitting lanthanide ions. The lifetime of Eu(III)
in 4 almost doubled because of the deceasing concentration of
Eu3+ ions, and the energy-transfer from Tb3+ to Eu3+ in this
heterodinuclear derivative. The lifetime of the central Tb3+ ions
in 4 decreases moderately, which may be caused by the
following two reasons: (1) The Eu3+ reduces the concentration
quenching effect between the Tb3+ ions, and the decrease of the
concentration of Tb3+ in complex 4 may give rise to the
increased lifetime of Tb3+. (2) The Tb3+ → Eu3+ energy transfer
leads to the decreased lifetime of Tb3+ ions. The observed
change in lifetimes of the Tb(III) and Eu(III) emission in a
EuTb heterodinuclear complex, compared to that of pure
Tb(III) and Eu(III) emission, further proves that the energy
transfer between Tb(III) and Eu(III) indeed exsits. If the main
mechanism of the energy transfer process would be dipole−
dipole, the through-space energy-transfer effiency can be
estimated by the following eq 5:
5
Tb3+ center. The shorter D0 lifetimes detected for the Eu3+
complex 1 may be caused by the dominant nonradiative decay
channels associated with vibronic coupling due to the
coordinated solvent molecules. On the other hand, a longer
5D4 lifetime value at 298 K (τobs= 947 2 μs) has been gained
for the Tb3+ complex even though there is a solvent molecule in
the first coordination sphere since the vibrational deactivators
of the excited states of Ln3+ ions are present.25b,26 The D4
5
lifetime values of Tb(III) complex 2 indicate that it is
essentially temperature independent because of τRAD decreasing
by ∼15%, while temperature goes down from 298 to 77 K.
These observations indicate that the Tb(III) complex lacks
thermally activated deactivation processes.
The emission of complexes 1 and 2 can be analyzed in terms
of eq 1 where Φoverall and ΦLn are the ligand-sensitized and
intrinsic luminescence quantum yields of Ln(III); Φsen is the
efficiency of the energy transfer from ligand to Ln(III), and
τobs/τRAD are the observed and the radiative lifetimes of
Ln(III).27
η = 1 − (τ/τ0) = 1/[1 + (R/R0)6]
(5)
Here, τ is the Tb(5D4) lifetime in the EuTb heterodinuclear
complex, τ0 is the lifetime in the pure Tb(5D4) complex, R is
the Eu···Tb separation, and R0 is the critical distance for 50%
transfer.31 The energy-transfer efficiency is calculated to be 3%
using the above equation when τ = 0.915 0.002 ms and τ0 =
0.947 0.002 ms. The value for R0 generally lies between 8 and
10 Å for Eu(III)−Tb(III) pairs.32 Using a value of 13.2 Å for R
and an average value of 9.0 Å for R0 with the above equation,
we obtained a theoretical energy-transfer efficiency of η = 9.1%.
Also, this efficiency resembles the one calculated by the
experimental lifetime values of complex 4. Hence, we can
conclude that the main energy-transfer path is a dipole−dipole
Φ
= Φ ΦLn
(1)
overall
sen
ΦLn can be obtained by the following eq 2:
ΦLn = τ /τRAD
(2)
obs
For the Eu(III) complex, the radiative lifetime (τRAD) can be
calculated from eq 3,28 where n is the refractive index (1.5 for
solid state metal−organic complexes).29 AMD,0 (14.65 s−1) is the
5
7
spontaneous emission probability for the D0 → F1 transition
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heating rate of 10 °C min−1 under air. Mass spectra were recorded on
a Bruker UHR-TOF maXis 4G mass spectrometer. The FT-IR spectra
were recorded on a Nicolet Nexus 670 instrument in the 4000−
1
400 cm−1 region using KBr pellets. H NMR (400 MHz) and 13C
NMR (100 MHz) spectra were obtained with a Bruker Avance 400
spectrometer in CDCl3 solutions, with tetramethylsilane (Si(CH3)4) as
an internal standard. Absorption spectra were measured on a Varian
UV-Cary100 spectrophotometer. The steady-state luminescence
spectra and the lifetime measurements were measured on an Edin-
burgh Instruments FSL920 fluorescence spectrometer, with 450 W
Xe arc lamp as the steady-state excitation source or Nd-pumped
OPOlette laser as the excitation source for lifetime measurements. The
77 K phosphorescence spectrum was recorded on a Hitachi F-4500
fluorescence spectrophotometer. The quantum yields of the complexes
in the solid state were determined according to an absolute method of
Wrighton34 using an integrating sphere (150 mm diameter, BaSO4
coating) from Edinburgh Instruments FLS920. Three parallel
measurements were carried out for each sample, so that the presented
value corresponds to the arithmetic mean value. The errors in the
quantum yield values associated with this technique were estimated to
be within 10%. The lifetime measurement was obtained on an
Edinburgh Instruments FLS920 fluorescence spectrometer using a Nd
pumped OPOlette laser as the excitation source. EDX analysis was
carried out with a scanning electron microscope, Hitachi S-4800,
Chiyoda-ku, Japan.
Figure 6. Energy level scheme showing energy transfer processes in
compound 4.
mechanism. The energy level scheme depicted in Figure 6
summarizes the intramolecular energy transfer processes taking
place in heterodinuclear coordination compound 4.
Synthesis of the Benzyl-N,N-bis[(2′-furfurylaminoformyl)-
phenoxyl)ethyl]-amine (L). The β,β′-dichlorodiethylamine hydro-
chloride salt (0.174 g, 1 mmol) and potassium carbonate (0.276 g,
2 mmol) were refluxed in acetone (25 cm3) for 30 min, and then the
benzyl bromide (0.171 g, 1 mmol) was added to the solution. The hot
solution was filtered off after refluxing for 12 h. The product was
obtained after solvent was removed under reduced pressure. The
above product was added to a mixture of N-furfurylsalicylamide
(0.492 g, 2 mmol), potassium carbonate (0.552 g, 4 mmol), and
distilled dimethylformamide (DMF) (20 cm3) and then warmed to
90 °C stirring for 12 h. The mixture was poured into water (100 cm3)
after cooling to room temperature. The product was extracted into
dichloromethane, washed with brine, dried over Na2SO4, and
concentrated under reduced pressure. The residue was purified by
silica gel chromatography to obtain L as a white solid. Yield: 76%, mp
81−83 °C. Anal. Calcd for C35H35O6N3: C, 70.81; H, 5.94; N, 7.08.
Found: C, 70.72; H, 6.10; N, 6.89. IR (KBr pellet, cm−1): 3330 m,
2930 w, 1640 s ν(CO), 1600 w, 1529 s, 1489 w, 1450 w, 1307 w,
CONCLUSIONS
■
In this Article, we have reported the self-assembly synthesis,
crystal structures, and photoluminescent properties of four new
homodinuclear and heterodinuclear lanthanide (Ln = Eu, Tb,
Gd, EuTb) coordination compounds derived from a novel
amide ligand, benzyl-N,N-bis[(2′-furfury-laminoformyl)-
phenoxyl)ethyl]-amine (L). The single-crystal X-ray diffraction
analyses have revealed the isostructural character of all the
lanthanide derivatives 1−4, which are composed of zero-
dimensional (0D) rectangular macrocycle blocks. Moreover, an
interesting feature of EuTb heterodinuclear derivative 4
consists of the assembly of discrete dinuclear 0D units into a
2D H-bonded network with the sql topology, which has not yet
been identified in a heterometallic EuTb compound. Under the
excitation, both complexes 1 and 2 can show the characteristic
luminescence emissions of the central Eu(III) and Tb(III) ions.
The relative intensities and overall luminescence quantum
yields show that the ligand L can effectively sensitize the
luminescence of Tb(III) rather than Eu(III). Another note-
worthy feature of the current study concerns a Tb(III)-to-
Eu(III) energy transfer ability of the heterodinuclear compound
4, which has been investigated in detail by the measurement of
the energy levels of the ligand L and the luminescence lifetimes
of the excited-state energy levels of the central Eu(III) and
Tb(III) ions. We believe the obtained results should deserve
further exploration, namely toward the construction of
lanthanide based color displays, luminescence sensors, and
photonic devices.
1
1240 m, 1157 w, 1107 w, 1041 w, 754 w, 732 w, 656 w, 518 w. H
NMR (CDCl3, 400 MHz): δ 8.37 (s, 2H), 8.20 (dd, 2H, J = 8 Hz),
7.38 (t, 2H, J = 6.8 Hz), 7.27 (s, 7H), 7.06 (t, 2H, J = 7.4 Hz), 6.78 (d,
2H, J = 8.0 Hz), 6.27 (dd, 2H, J = 4.8 Hz), 6.22 (d, 2H, J = 2.8 Hz),
4.60 (d, 4H, J = 5.2 Hz), 4.10 (t, 4H, J = 6.0 Hz), 3.70 (s, 2H), 2.95 (t,
4H, J = 5.6 Hz). 13C NMR (CDCl3, 100 MHz): δ 165.1, 156.4, 151.8,
141.9, 137.9, 132.8, 132.4, 128.8, 128.5, 127.5, 121.6, 121.5, 112.4,
107.4, 66.6, 59.1, 53.4. 36.8. MS (ESI+): m/z 594 (M + H)+.
Synthesis of the Lanthanide Complexes 1−4. [Ln2(μ2-
L)2(NO3)6(EtOH)2] (1−3). An ethyl acetate solution (5 cm3) of
Ln(NO3)3·6H2O [Ln = Eu (0.045 g, 0.1 mmol for 1), Tb (0.045 g,
1 mmol for 2), and Gd (0.045 g, 0.1 mmol for 3)] was dropped into a
solution of L (0.059 g, 0.1 mmol in ethyl acetate (5 cm3). The mixture
was filtered off after stirred at r.t. for 4 h. After washing with ethyl
acetate three times, complexes 1−3 were furnished by dried in vacuo
under P4O10 for two days, yield: 62−75%. Elemental analysis details
follow. Found for 1, C74H82Eu2N12O32: C, 45.02; H, 3.83; N, 8.42; Eu,
15.25. Anal. Calcd: C, 45.45; H,4.23; N, 8.60, Eu, 15.55. Found for 2,
C74H82Tb2N12O32: C, 44.83; H, 3.76; N, 8.44; Tb, 15.88. Anal. Calcd:
C, 45.13; H, 4.20; N, 8.53; Tb, 16.14. Found for 3, C74H82Gd2N12O32:
C, 44.89; H, 3.75; N, 8.24; Gd, 15.86. Anal. Calcd: C, 45.21; H, 4.20;
N, 8.55; Gd, 16.00. The three complexes show essentially similar IR
spectra, the characteristic bands of which have similar shifts, suggesting
that they are structurally similar. IR details follow (KBr pellet, cm−1).
For complex 1: 3356 m, 1671 s, 1615 s ν(CO), 1595 m, 1566 m,
1494 s, 1474 s, 1383 w, 1307 s, 1236 w, 1111 w, 1029 m, 760 s. For
complex 2: 3356 m, 1671 s, 1620 s ν(CO), 1595 m, 1566 m, 1494 s,
EXPERIMENTAL SECTION
■
Materials and Instrumentation. Lanthanide nitrates were
prepared by dissolving Eu2O3 (99.99%, Shanghai Yuelong) in nitric
acid (7 mol L−1) followed by successive evaporating to remove excess
acid. N-Furfurylsalicylamide33 was synthesized on the basis of the
literature. Other chemicals were all commercially available and purified
by standard methods. Elemental analyses were conducted using an
Elementar Vario EL. The amounts of lanthanide ions in complexes
1−4 were tested by inductively coupled plasmas (ICP) which were
recorded on IRIS Advantage ER/S spectrophotometer. Thermogravi-
metric analyses (TGA) and differential scanning calorimetry (DSC)
were performed using a STA 449C thermal analyzer up to 900 °C at a
940
dx.doi.org/10.1021/ic402338q | Inorg. Chem. 2014, 53, 935−942

Inorganic Chemistry
Article
Table 3. Crystal Data and Structure Refinements for Compounds 1−4
1
2
3
4
empirical formula
fw
C74H82Eu2N12O32
1955.44
C74H82Tb2N12O32
1969.36
C74H82Gd2N12O32
1966.02
C74H82EuN12O32Tb
1962.40
cryst syst
space group
a (Å)
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
monoclinic
P21/c
16.338(4)
13.490(3)
19.958(5)
90.00
16.338(4)
13.490(3)
19.958(5)
90.00
16.287(8)
13.456(6)
19.899(9)
90.00
16.345(9)
13.493(7)
19.954(11)
90.00
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
V (Å3)
Z
Dcalcd (g cm−3
μ(Mo Kα) (mm−1
104.485(2)
90.00
104.485(2)
90.00
104.544(5)
90.00
104.685(4)
90.00
4258.7(16)
2
4258.7(16)
2
4221(3)
2
4257(4)
2
)
1.525
1.536
1.547
1.531
)
1.549
1.737
1.648
1.643
F(000)
GOF on F2
1984
1992
1988
1988
0.985
1.056
1.078
1.013
final R indices, [I > 2σ(I)]
R1 = 0.0497
wR2 = 0.1230
R1 = 0.0762
wR2 = 0.1453
R1 = 0.0455
wR2 = 0.1147
R1 = 0.0706
wR2 = 0.1347
R1 = 0.0566
wR2 = 0.1360
R1 = 0.1075
wR2 = 0.1766
R1 = 0.0515
wR2 = 0.1243
R1 = 0.0833
wR2 = 0.1450
R indices (all data)
1475 s, 1384 m, 1308 s, 1236 m, 1111 w, 1030 m, 760 s. For complex
3: 3359 m, 1669 w, 1615 s ν(CO), 1596 m, 1566 m, 1493 s, 1476 s,
1384 w, 1308 s, 1240 w, 1111 w, 1029 m, 756 s.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tangyu@lzu.edu.cn. Phone: +86-931-8912552. Fax:
+86-931-8912582.
[EuTb(μ2-L)2(NO3)6(EtOH)2] (4). An ethyl acetate solution (5 cm3)
of Eu(NO3)3·6H2O (0.022 g, 0.05 mmol) and Tb(NO3)3·6H2O
(0.023 g, 0.05 mmol) was dropped into a solution of L (0.059 g,
0.1 mmol) in ethyl acetate (5 cm3). The mixture was filtered off after
stirring at r.t. for 4 h. After washing with ethyl acetate three times, product
4 was obtained by dried in vacuo under P4O10 for two days, yield: 64%.
Anal. Found for C74H82EuN12O32Tb: C, 44.97; H, 3.76; N, 8.25, Eu, 7.24;
Tb, 7.82. Anal. Calcd.: C, 45.29; H, 4.21; N, 8.46; Eu, 7.75; Tb, 8.10. IR
(KBr pellet, cm−1): 3356 m, 3300 m, 1670 s, 1618 s ν(CO)), 1566 s,
1494 s, 1474 s, 1374 m, 1307 s, 1237 m, 1029 m, 757 s.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Natural Science
Foundation of China (Projects 21071068, 20931003) and the
National Science Foundation for Fostering Talents in Basic
Research of the National Natural Science Foundation of China
(Project J1103307) is acknowledged. A.M.K. acknowledges the
Foundation for Science and Technology (FCT), Portugal
(PTDC/QUI-QUI/121526/2010, PEst-OE/QUI/UI0100/2013).
Unfortunately, the reliable mass spectrometry data for complex 4 could
not be obtained because the complex is rather unstable in solutions.
Single-Crystal X-ray Crystallography. Single crystals of the
complexes were obtained using an anhydrous diethyl ether diffused
into a mixed EtOH/MeCN solution of 1−4 for about two weeks at
room temperature. The X-ray single-crystal measurements were
carried out on a Bruker SMART CCD detector diffractometer at
294 K, using graphite-monochromatic Mo Kα radiation (λ = 0.710 73 Å).
Semiempirical absorption corrections were applied using the SADABS
program. The structures were solved by direct methods and refined by full-
matrix least-squares on F2 using the SHELXS-97 and SHELXL-97
programs.35 Details of crystallographic parameters, data collection, and
refinement for these complexes are listed in Table 3. Hydrogen bonds in
crystal packing of 1−4 are listed in Table S2 (Supporting Information).
Representative bond distances [Å] and angles [deg] are given in Table S4
(Supporting Information). CCDC 954976 (1), 954977 (2), 960612 (3),
and 954978 (4) include all supplementary crystallographic data of these
four new lanthanide complexes.
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ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files with crystallographic data, selected bonding
parameters and intramolecular π−π and C−H···π stacking
interactions, photophysical data, EDX analysis results and EDX
spectrum, thermal analysis plots, FTIR spectra, absorption
spectrum and phosphorescence spectrum, and low temperature
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