Systematic study of the luminescent europium-based nonanuclear clusters with modified 2-hydroxybenzophenone ligands

Article abstract of DOI:10.1021/ic401191e

The reaction of 2-hydroxybenzophenone derivatives with europium ions has afforded a new family of luminescent nonanuclear Eu(III) clusters. Crystal structure analysis of the clusters reveals that the metal core comprises two vertex-sharing square pyramida

Full text of DOI:10.1021/ic401191e

Article
pubs.acs.org/IC
Systematic Study of the Luminescent Europium-Based Nonanuclear
Clusters with Modified 2‑Hydroxybenzophenone Ligands
Bin Zhang, Ting Xiao, Chunmei Liu, Qian Li, Yanyan Zhu, Mingsheng Tang, Chenxia Du,*
and Maoping Song*
The College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou, 450052, P. R. China
S
* Supporting Information
ABSTRACT: The reaction of 2-hydroxybenzophenone derivatives
with europium ions has afforded a new family of luminescent
nonanuclear Eu(III) clusters. Crystal structure analysis of the clusters
reveals that the metal core comprises two vertex-sharing square
pyramidal units. Most of these complexes show emissions typical of
Eu3+ ion under visible light excitation (400−420 nm) at room
temperature. Photophysical characterization and DFT study reveal a
correlation between luminescent efficiencies of Eu(III) complexes
and the electronic features of the ligands, which can be tuned by the
nature of substituents in the 4-position of the ligands. The ligands
with a fluorine substituent possess more suitable triplet energy levels, resulting in more intensive luminescence.
tags for biological molecules.9 It is well-known that
luminescence from trivalent europium arises from electronic
INTRODUCTION
■
Lanthanide-based high-nuclearity clusters have attracted
transitions between the 4f orbitals, which are forbidden on
considerable attention in recent years, not only for their
symmetry grounds. Consequently, the use of chromophore
aesthetically pleasing structures but also because of their
ligands with strong light harvesting is necessary for efficient
intrinsic properties.1 From the perspective of applications,
sensitizing of the Eu3+ ion through intramolecular energy
envisioned or realized, polynuclear lanthanide complexes have
transfer.10 For highly luminescent Eu(III) complexes, the
shown promising results. For example, unique magnetic
chelating ligands should have suitable triplet energy levels to
properties of certain lanthanide hydroxide clusters have been
match the D1 or D0 level of Eu3+ ion.11 In fact, only a few
kinds of ligands have been successfully used for producing
luminescent europium oxido/hydroxido clusters with precisely
defined structures until now.12 In order to investigate the
influence of supporting ligands on the structural patterns and
luminescent behaviors of the clusters, it is of great importance
to systematically explore new chelating ligands for optimizing
sensitization processes in luminescent europium clusters. The
2-hydroxybenzophenone derivatives have been demonstrated
to be good antennae for Eu3+ ion luminescence.13 Their energy
levels of the triplet states can be fine-tuned by incorporating
substitutions into the ligands, which can result in more efficient
intramolecular energy transfer in terms of ligand (S1) → ligand
(T1) → Ln(III)*. Of particular significance, this kind of ligands
shows great potential to stabilize the oxo and hydroxo
components of the metal core, due to the good affinity in
bridging europium ions through oxo and hydroxo groups and
the proper steric hindrance to prevent the core from further
aggregation. However, europium clusters with 2-hydroxybenzo-
phenone derivatives as supporting ligands are rarely reported
until now, especially for the high-nuclearity clusters.14 With all
of the above in mind, we extended our interests to design and
5
5
demonstrated in the form of single-molecule magnets2 and
long-lived and narrow emission bands ranging from near-
infrared to visible regions of the complexes with lanthanide-
hydroxo core have also been observed in cluster−polymer
hybrid thin films.3 However, the construction of polynuclear
lanthanide complexes remains a great challenge ascribing to
their variable and high coordination numbers as well as
adventitious hydrolysis of the metal ions.4 To date, the most
successful synthetic route to single-sized lanthanide hydroxide
clusters is the ligand-controlled hydrolytic approach which is
exemplified by clusters stabilized by diketonate ligands.5
A
careful analysis of the supporting ligands used for various
lanthanide-based clusters suggests that the ancillary ligand
should present (i) easily accessible chelation regions to bind the
Ln(III) ions and (ii) proper steric hindrance to bridge metal
centers but prevent polymer formation.6 Despite extensive
attempts aimed at producing lanthanide oxido/hydroxido
clusters, only a few kinds of bridging ligands have been
successfully used for this purpose until now.7 Thus, research
activities in pursuing novel lanthanide clusters are of
fundamental interest and practical significance.
Europium based oxido/hydroxido clusters featuring lumines-
cence are of particular significance because of the intrinsic long-
lived, sharp band red emission of Eu3+ ions,8 which are highly
pursued as phosphors in full-color display and as luminescent
Received: May 14, 2013
© XXXX American Chemical Society
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synthesize novel high-nuclearity europium clusters with this
kind of ligands. In this paper, we report six novel diabolo-
shaped Eu9 clusters assembled by two square pyramidal
pentanuclear units via the apical metal center Eu(III) ion.
The complexes with a general core of [Eu(III)9(L)16(μ3-
OH)8(μ4-O)(μ4-OH)] have been confirmed by single-crystal
X-ray diffraction, infrared (IR) spectroscopy, and elemental
analysis (C, H, and N). The photophysical properties of the
ligands and complexes have been thoroughly investigated,
which shows that the substituents on the ligands play an
important role for optimizing the ligand-to-europium energy
transfer process. Quantum chemical calculations were con-
ducted to help better understand the experimental results.
precipitates of lanthanide oxides or hydroxides. Then, a series
of new europium oxido/hydroxido clusters of general core
[Eu(III)9(L)16(μ3-OH)8(μ4-O)(μ4-OH)] were formed. This
synthetic procedure is schematically illustrated in Scheme 3
Scheme 3. Rationales of the Ligand-Controlled Hydrolytic
Approach to Assembly of the Clusters (Eu9-3)
RESULTS AND DISCUSSION
■
Synthesis of the Ligands and Complexes. A series of 2-
hydroxybenzophenone derivatives have been employed to
investigate the effect of substituents on the photophysical
properties of the europium clusters (Scheme 1). Ligands HL1−
Scheme 1. Diagram of α-Hydroxybenzophenone Derivatives
(Eu9-3 as an example). In the following sections, the
europium(III) complexes based on HL1, HL2, HL3, HL4,
HL5, and HL6 will be abbreviated as Eu9-1, Eu9-2, Eu9-3, Eu9-
4, Eu9-5, and Eu9-6, respectively.
Crystal Structure Descriptions. The full data collections
and the refinements of the structures have been done for Eu9-1,
Eu9-3, Eu9-4, and Eu9-6, while only the cell parameters have
been determined to confirm the structure of Eu9-2 due to the
low crystallinity (Table S1 in the Supporting Information). X-
ray diffraction analysis reveals that the four compounds are all
nonanuclear clusters with a similar core structure (Figure S1,
Supporting Information), although they are different in
crystallography. Each core can be viewed as two vertex-sharing
square pyramids arranged in a “staggered” fashion with their
basal planes rotated by 45° with respect to each other (Figure
S2, Supporting Information). The only difference of these
complexes lies in that there are no solvent molecules in the
asymmetric units of Eu9-3, Eu9-4, and Eu9-6, while Eu9-1
contains two water and two methanol molecules with a formula
of [Eu(III)9(L1)16(μ3-OH)8(μ4-O)(μ4-OH)]·2H2O·2CH3OH.
Thus, only the structure of the Eu9-3 cluster is briefly described
as a representative.
HL4 were purchased from Alfa Aesar company without further
purification. The ligands of (4-fluoro-2-hydroxyphenyl)-
phenylmethanone (HL5) and (4-fluoro-2-hydroxyphenyl)-(4-
methlphenyl)methanone (HL6) were synthesized by a similar
procedure according to the literature (Scheme 2).15 Moderate
yields for both ligands were obtained after column chromatog-
raphy, ranging from 79% for HL5 to 72% for HL6, respectively.
Scheme 2. Synthetic Pathway of the Ligands HL5 and HL6
(TEA: Triethylamine)
X-ray crystal diffraction reveals that compound Eu9-3
consists of a nonanuclear [Ln9(μ4-O)(μ4-OH)(μ3-OH)8] core
and 16 deprotonated peripheral HL3 ligands. Selected bond
lengths and angles for the complex Eu9-3 are listed in Table S2
in the Supporting Information. As shown in Figure 1, the
molecular structure of Eu9-3 can be viewed as two [Eu5]
clusters with Eu(1,2,3,4) forming the basal planes and the
central Eu(5) atom as the shared apex. The coordination
environment around the Eu(5) ion at the vertex is an almost
perfect square antiprism by connecting with eight μ3-bridging
hydroxo groups. The bond lengths of Eu−O range from
2.455(2) to 2.472(2) Å, comparable to those in the reported Eu
complex.16 Each basal Eu(III) ion is eight-coordinate by two
μ3-OH, one μ4-(O, OH), and five oxygen atoms from
deprotonated HL3 ligands through chelating/bridging modes,
forming a distorted square antiprism geometry.17 The average
bond length of Eu−(μ4-(O, OH)) (2.597(3) Å) is apparently
longer than that of Eu−(μ3-OH) (2.351(3) Å), while the
average bond length of Eu−O(phenolic group) (2.379(3) Å) is
The procedure employed in the synthesis of the series of
nonanuclear clusters relies on the reaction of 1 equiv of
europium salt, EuCl3·xH2O, with 2 equiv of the HL ligand in
methanol using excess triethylamine as base. It seems that the
relative small steric hindrance of these ligands and their good
affinity for chelating and bridging Eu(III) ions may play an
essential role in forming and stabilizing the nonanuclear metal
clusters. In such cases, the L− anions preoccupy part of the
coordination sphere of the bulky lanthanide ions with μ2:η2
coordination mode, leaving only a limited number of sites
available for aqua coordination. As a result, deprotonation of
the lanthanide-activated aqua ligands upon pH increase will be
limited, and lead eventually to the well-defined polynuclear
lanthanide clusters rather than the intractable and undesirable
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in acetonitrile solutions (2.0 × 10−5 mol/cm3) at room
temperature (for nonanuclear clusters, the concentrations of
the solutions were based on Eu(III) ion). Considering that the
nonanuclear cluster might dissociate into smaller species such
as EuL2(OH)(H2O)2 and EuL3(H2O)2 in the dilute acetonitrile
solution, we measure the absorption spectra for both the
mononuclear europium complexes (EuL3(H2O)2: Eu-1 to Eu-
6) and nonanuclear clusters (Eu9-1 to Eu9-6) in either solid
state or acetonitrile solution (Figures S4 and S5, Supporting
Information). The results show that there is no obvious
difference between the absorption spectra of monomers and
clusters under both conditions, indicating that the coordinated
ligands in monomers and clusters are under identical influence
of the Eu(III) ion. As shown in Figure 2, the absorption bands
Figure 1. Molecular structure of the Eu9-3 cluster. For clarity,
hydrogen atoms have been omitted.
shorter than that of Eu−O(carbonyl group) (2.387(3) Å). In
the basal plane, four Eu(III) ions form a square-planar
geometry with adjacent Eu···Eu distances ranging from
3.6515(3) to 3.6648(3) Å.
It is worth noting that, whatever ligands are used, the final
compounds are all nonanuclear clusters in this system. The
obvious tendency and recurrence of forming nonanuclear
lanthanide−hydroxo cluster cores suggests that such a kind of
ancillary ligands is more prevalent than other species to form
high-nuclearity lanthanide clusters with well-defined structure
through ligand-controlled hydrolysis of the lanthanide ions.12a
IR Characterizations. A comparison of the IR spectra of
the complexes with the ligands provides evidence for the
coordination mode of the ligands (Figure S3, Supporting
Information); the relevant characteristic bands of ligands and
complexes are given in Table 1. The broad absorptions in the
3100−3650 cm−1 region for the complexes can be assigned to
the stretching of OH. The absence of absorption in the
ν(OH) region of the free ligands is considered as evidence
that there exists a strong intramolecular hydrogen bond
between OH and CO groups, which considerably weakens
and broadens the ν(OH) vibration. The relatively strong
ν(CO) vibrations at 1601−1620 cm−1 in these complexes
are appreciably red-shifted by a range of 14−29 cm−1 relative to
that of the free ligands, indicating the coordination of CO
groups with Eu3+ ions. The stretching frequencies of PhO
vibrations of the complexes (1240−1254 cm−1) are also red-
shifted relative to the free ligands. The comparison of the IR
spectra obtained for the six complexes exhibits a similar feature
in the 4000−400 cm−1 region, which indicates the similar
structural features of these complexes.
Figure 2. UV−vis absorption spectra of the ligands and the
corresponding europium clusters in acetonitrile solution (c = 2.0 ×
10−5 mol·cm−3). The concentrations of the clusters were based on
Eu(III) ion. All spectra are normalized to a constant intensity at the
maximum.
located in the region 250−290 nm are primarily attributed to
the π−π* electronic transitions of the ligands. The lower
energy transitions are associated with the shift of electron
density from the phenolate oxygen to the π* orbitals of the
aromatic rings with maxima at 300−350 nm. The molar
absorption coefficient values of the ligands, calculated at the
absorption maxima of the lower energy bands (λmax), illustrate
good light absorption ability of the ligands (Table 2). The
introduction of a fluorine or methoxy group in the 4-position of
the HL2 ligand moderately shifts the lower energy absorption
bands toward the blue region. Furthermore, due to the reduced
π-conjugation, the ligand HL1 displays a more apparent blue
shift with the lower energy absorption band at 290−340 nm
(λmax = 313 nm). Upon complexation with Eu(III) ions, as
expected, the absorption profiles are dominated by the ligands
and the lower energetic transitions are significantly red-shifted
about 2879−3978 cm−1. Interestingly, the lower energy
absorption bands of these complexes extend to the visible
region, rendering the corresponding europium clusters the
Electronic Absorption Spectra. The UV−vis absorption
spectra of the free ligands (HL1−HL6) and the corresponding
nonanuclear europium clusters (Eu9-1 to Eu9-6) were recorded
Table 1. The Relevant Characteristic IR Bands (cm−1) of the Ligands and the Complexes
Eu9-1/HL1
Eu9-2/HL2
Eu9-3/HL3
Eu9-4/HL4
Eu9-5/HL5
Eu9-6/HL6
ν(OH)
ν(CO)
ν(PhO)
3449/-
3356/-
3334/-
3332/-
3420/-
3396/-
1601/1622
1243/1255
1611/1627
1240/1244
1607/1629
1247/1259
1604/1633
1247/1257
1620/1634
1254/1258
1619/1634
1252/1258
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Table 2. The Absorption Maxima of the Lower Energy Bands (λmax) and the Related Molar Absorption Coefficient Values (ε)
for the Ligands and Europium Clusters
HL1
HL2
HL3
HL4
HL5
HL6
ε/104 L mol−1 cm−1 (λmax/nm)
0.64 (313)
Eu9-1
0.35 (338)
Eu9-2
1.14 (324)
Eu9-3
1.26 (324)
Eu9-4
0.53 (321)
Eu9-5
0.63 (320)
Eu9-6
a
ε /104 L mol−1 cm−1 (λmax/nm)
1.09 (344)
2879
0.63 (385)
3612
1.96 (369)
2.23 (368)
3690
0.93 (368)
3978
1.12 (366)
3927
b
ΔE(λmax) /cm−1
3764
b
a
The concentrations of the clusters in acetonitrile were based on Eu(III) ion. The differences in absorption maxima of the lower energy bands in
energy units (cm−1).
potential property of being long wave sensitized luminescent
materials.18
Photoluminescence Properties. The solid-state excita-
tion spectra of complexes (Eu9-1 to Eu9-6) are recorded at
room temperature. As shown in Figure S6 in the Supporting
Information, the excitation spectra of complexes (Eu9-2 to Eu9-
6), obtained by monitoring the 5D0 → 7F2 transition of Eu(III)
ion at 614 nm, are very similar. They all feature strong bands in
the range 350−450 nm with the maximal absorptions around
405 nm. The excitation spectrum of complex Eu9-1 shows a
maximal absorption around 372 nm owing to the reduced
conjugated system of HL1; furthermore, the narrow band at
464 nm corresponding to the 7F0
→
5D2 hypersensitive
transition of Eu(III) ion can also be clearly observed. The
excitation spectra overlap well with the absorption spectra of
the europium clusters, evidencing that the emissions arise from
the antenna effects of the ligands.19
Figure 3. Emission spectra of clusters Eu9-1 to Eu9-6 at 298 K in the
solid state (λex = 372 for Eu9-1 and 405 nm for the others).
The europium clusters exhibit the characteristic Eu(III)
emissions when excited at 372 nm for Eu9-1 and 405 nm for
the others at both ambient and low temperatures, indicating
efficient energy transfer from the ligands to the Eu(III) center.
Due to the similar coordination environment around the
Eu(III) center, crystal field splittings of the 7FJ levels (J = 0−4)
extracted from emission spectra (77 K) are identical. Several
features can be outlined from the analysis of the luminescence
spectrum of the representative cluster Eu9-3 at 77 K (Figure S7,
Supporting Information). The well resolved five sharp peaks at
578, 590, 614, 652, and 700 nm correspond to the transitions
and by the intrinsic quantum yields (ΦEu) (eq 1).22 The
intrinsic quantum yield of the luminescent europium complex
(ΦEu) can be evaluated from eqs 2 and 3,23 where τobs/τrad are
the observed and radiative lifetimes of Eu(5D0), AMD,0 is the
5
7
spontaneous emission probability for the D0 → F1 transition
in vacuo (14.65 s−1), Itot/IMD is the ratio of the total integrated
7
5
5D0 → FJ emissions (J = 0−4) to the magnetic dipole D0 →
7F1 transition, and the refractive index of the medium (n) is
taken to be equal to 1.5 in the solid sample that is commonly
encountered for coordination complexes.
5
7
from the D0 excited state to the FJ (J = 0; 1; 2; 3; 4) ground
state multiplet.20 It is well-known that the 5D0 → 7F1 magnetic
dipole transition at 590 nm is independent of the coordination
sphere, while the electric dipole transition 5D0 → 7F2 at 614 nm
is extremely sensitive to the nature and symmetry of the
coordinating environment, which can be further enhanced by
the distortion of the symmetry around the europium ion.21
Φ
= η × ΦEu
sens
(1)
(2)
tot
ΦEu = τ /τrad
obs
1/τrad = AMD,0 × n3 × (Itot/IMD
)
(3)
Table 3 summarizes the Φtot, τobs, and other photophysical
parameters. The presence of O−H oscillators may be a key
factor causing the small luminescence quantum yields and
Thus, the domination of the electric dipole 5D0
→
7F2
transition at 614 nm indicates the presence of a low-symmetry
Eu(III) site, which is confirmed by the crystal structure. The
Table 3. Radiative Lifetimes (τrad), Luminescence Lifetimes
(τobs), Sensitization Efficiencies (ηsens), Intrinsic Quantum
Yields (ΦEu), and Overall Quantum Yields (Φtot) for the
Clusters Eu9-1 to Eu9-6 at 298 K in Solid State
5
7
singlet of the D0 → F0 transition at about 578 nm indicates
the presence of only single site symmetry of Eu3+ ion
corresponding to the basal Eu(III) in the nonanuclear cluster,
5
7
because the D0 → F0 transition is strictly forbidden (by the
electric and magnetic dipole mechanisms) for the highly
symmetrical coordination around the central Eu(III) ion.12a
Notably, although the emission spectra of the clusters at 298
K have analogical profiles because of their similar structures, the
luminescence efficiencies of the complexes with different
chelating ligands decrease in the sequence of Eu9-6 > Eu9-5
> Eu9-4 > Eu9-3 > Eu9-2 > Eu9-1 (Figure 3). Within a general
paradigm, the overall luminescence quantum yield of the
europium complex upon excitation of the chromophore is
a
complexes
τrad (ms)
τobs (ms)
ηsens (%)
ΦEu (%)
Φtot (%)
Eu9-1
Eu9-2
Eu9-3
Eu9-4
Eu9-5
Eu9-6
2.58
2.59
2.60
2.61
2.62
2.61
0.51
0.30
0.33
0.38
0.46
0.48
19.8
11.6
12.7
14.6
17.6
18.4
<0.5
<0.5
1.5
11.8
26.0
40.9
45.7
3.8
7.2
8.4
a
Φtot was measured using an integrating sphere method with
integrated emission ranging from 550 to 750 nm.
determined by the sensitization efficiency of the ligand (ηsens
)
D
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Table 4. The Lowest Singlet and Triplet Energy Levels of the Ligands in the Gadolinium Complexes
HL1
HL2
HL3
HL4
HL5
HL6
E(S1), cm−1
24694
23094
5794
21834
17761
461
23310
17921
621
23529
18181
881
23786
18416
1116
23809
18498
1198
E(T1), cm−1
ΔE(T1−5D0), cm−1
lifetimes.24 It is worth noting that the radiative lifetimes (τrad)
of the complexes are similar to each other, resulting in
comparable intrinsic quantum yields (ΦEu) within the range
11.6−19.8%. The results imply that the luminescence
efficiencies of the complexes mainly depend on the energy
transfer efficiencies between the ligands and the Eu(III) ions.
Energy Transfer between Ligands and Eu3+. The most
well documented mechanism for the energy transfer from a
chromophore ligand to europium ion involves the intersystem
crossing (ISC) from the first excited singlet state of the ligand
to the lowest triplet state, which then transfers energy to the
europium-emitting resonance level.25 Accordingly, we have
determined the relevant ligand-centered electronic states in the
complexes. On account of the difficulty in observing the singlet
state by fluorescent emissions of the corresponding non-
emissive Ln complex (Gd, La, Lu) at room temperature, the
energy levels of S1 are estimated with a reported method by
referring to the UV−vis absorption edges of the corresponding
mononuclear gadolinium complexes (GdL3(H2O)2 shortened
as Gd-1 to Gd-6) (Figure S8, Supporting Information).26 Since
the lowest-lying excited level of the Gd(III) ion (6P7/2, at 32150
cm−1) is too high to permit any energy transfer from the ligand
to the metal center, the phosphorescence spectra of the Gd3+
complexes can reveal the lowest triplet energy levels of the
ligands.27 The triplet energy levels of the ligands are
determined by the phosphorescence spectra of the gadolinium
complexes (Gd(L)3(H2O)2) at 77 K (Figure S9, Supporting
Information). For comparison, the singlet and triplet state
energy levels are summarized in Table 4. The results show that
the energy differences between singlet and triplet states for
HL3−HL6, ΔE(1ππ*−3ππ*), are larger than 5000 cm−1, while
those of HL1 and HL2 only amount to 1600 and 4073 cm−1,
respectively. According to Reinhoudt’s empirical rule, the
intersystem crossing process becomes effective when
ΔE(1ππ*−3ππ*) is at least 5000 cm−1.28 Thus, the intersystem
crossing processes of ligands (HL3−HL6) are efficient, which
render these ligands the potential ability to transfer energy to
europium ion efficiently.
Eu9-6, following the increasing order of the triplet state energy
levels of the corresponding ligands. This result implies that the
triplet energy levels of this kind of ligands can be fine-tuned by
the substituents on the ligands, leading to a more energetically
compatible, efficient energy transfer process to the 5D0 emitting
level of Eu3+.
Computational Studies. It is known that the optoelec-
tronic properties depend mainly on the frontier molecular
orbitals (FMOs), especially the highest occupied molecular
orbital (HOMO, H) and the lowest unoccupied molecular
orbital (LUMO, L).31 In this subsection, comprehensive
density functional theory (DFT) calculations using the
B3LYP functional have been performed on the free ligands to
ascertain the effects of the substituents involved in tuning
electronic features of the ligands. Moreover, as a simplified
model, the corresponding mononuclear europium complexes
(Eu(L)3(H2O)2) are calculated at the B3LYP/6-311G+
+(d,p)//SDD (Eu) level to analyze the influence of the
Eu(III) ion. The optimized geometrical structures of the ligands
and complexes are depicted in Figure S10 (Supporting
Information). There are two conformations for ligands HL3
and HL4 due to the less coexistent orientations of methoxy
groups in the clusters (named HL3′ and HL4′). The HOMO
and LUMO distribution, energy levels, and energy gaps of the
ligands and mononuclear europium complexes are shown in
Figure 4 and Table S10 (Supporting Information), respectively.
It is well-known that a suitable energy gap between the triplet
energy level of the ligand (T1) and the radiation state of the
Eu3+ ion (5D0) is required for an optimal ligand-to-europium
energy transfer process.29 As shown in Table 4, the energy gap
ΔE(T1 → 5D0) of 5794 cm−1 for HL1 is obviously too high to
allow an effective energy transfer, resulting in a poor
sensitization efficiency of the ligand. As already shown by
Crosby et al., the triplet state of the HL2 ligand (17761 cm−1) is
critically close to the D0 emitting state, which can lead to
the thermally assisted back-energy transfer from the europium
5D0 state to the triplet state of the ligand (BET) and a very low
luminescence efficiency of Eu9-2. Interestingly, the introduc-
tion of substituents on the ligand (HL2) apparently raises the
lowest triplet states of the ligands with the trend HL3 < HL4 <
HL5 < HL6, lying between the 5D1 (19 070 cm−1) and 5D0 (17
300 cm−1) excited states of the europium ion.30 As shown in
Table 3, the ligand sensitization efficiencies (ηsens) increase
obviously with the trend Eu9-2 < Eu9-3 < Eu9-4 < Eu9-6 <
Figure 4. Presentation of the energy levels, energy gaps, and orbital
composition distribution of the HOMO and LUMO for the ligands.
Orbital analyses of the compositions of the HOMOs and
LUMOs for HL1−HL6 show that the HOMOs are mainly
located at phenolate moieties of the ligands while the LUMOs
extend to the whole molecules (Figure 4).32 The HOMO
energy of HL1 is similar to those of HL3 and HL4, due to the
similar HOMOs. The energy of the LUMO, however, is
considerably higher owing to the reduced π-conjugation and
thus resulting in a larger HOMO−LUMO energy gap (HLG)
for HL1 (4.63 eV) compared with that of the other ligands. On
the contrary, the HLG of the parent ligand HL2 is the smallest
(4.24 eV) due to the contribution of the phenyl π-orbital. It is
13e
5
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interesting to note that the HOMO and LUMO energies of the
ligands depend to a large extent on the nature of the
substituents. Similar to the observation for tris(8-hydroxyqui-
nolinato) aluminum (Alq3), a methoxy substitution at the 4-
position of the phenolate ring raises both the HOMO and
LUMO levels due to its electron-donating property, whereas
the fluorine atom mainly exhibits a strong electron-withdrawing
effect to lower the energy levels.33 Since 4-OMe particularly
affects the LUMO level, while 4-fluorine atom influences the
HOMO level significantly, the energy gaps of HL3 and HL5 are
larger than that of parent ligand HL2. Moreover, the
attachment of an extra electron-donating methyl groups in
HL4 and HL6 led to a slight effect on the orbital energies with
the HLGs raised by 0.01 eV compared with HL3 and HL5,
respectively. As a result, the HLGs increase in the sequence of
HL2 < HL3 < HL4 < HL5 < HL6 < HL1, which is in
agreement with the excited state levels of these ligands.
As expected, europium complexes gave smaller HOMO−
LUMO energy gaps due to the combination of three ligands
and one trivalent europium ion (Table S10, Supporting
Information). We found that the HOMOs for the complexes
are mainly based on two phenolate moieties, while the LUMOs
are localized on two entire ligands. Moreover, population
analysis revealed that the contributions of the europium ions to
HOMO and LUMO are negligible (less than 5%) except for
Eu-1, of which the Eu(III) ions account for 22.10% of the
LUMO. The results interpret very weak metal−ligand mixing in
the HOMOs and LUMOs. The calculated data suggests that
the substituents affect the frontier molecular orbitals of these
complexes apparently, which is similar to that observed in the
corresponding ligands. Thus, the HLG increases in the order
Eu-2 < Eu-3 < Eu-4 < Eu-5 < Eu-6 < Eu-1. On the basis of the
discussion, it is reasonable to conclude that the optical
properties of these clusters are dependent on the electronic
features of the ligands, which can be fine-tuned by the
substituents in the ligands.
EXPERIMENTAL SECTION
■
Materials. Diethyl ether and triethylamine (TEA) were dried using
standard procedures. All the other reagents were used as received from
commercial sources, unless otherwise stated.
Physical Measurements. 1H and 13C NMR spectra were obtained
using a Bruker DPX-400 MHz spectrometer in CDCl3. The following
standard abbreviations are used for characterization of NMR signals: s,
singlet; d, doublet; t, triplet; m, multiplet; dd, doublet of doublets.
HRMS were determined on a Waters Q-Tof Micro MS/MS System
ESI spectrometer. Elemental analysis for C, H, and N was carried out
with a Carlo-Erba1106 Elemental Analyzer. IR spectra were performed
on a Perkin-Elmer FTIR GX instrument using KBr pellets in the 400−
4000 cm−1 region. Absorption spectra were measured on a Perkin-
Elmer Lambda 900 spectrometer at room temperature. The excitation
and emission spectra of the Eu(III) complexes in solid state at room
temperature (298 K) were recorded at an angle of 22.5° (front face)
on a HORIBA JOBIN YVON FluoroMax-P spectrophotometer
equipped with both continuous and pulsed xenon lamps, and the
emission spectra at 77 K of the clusters and mononuclear gadolinium
complexes were recorded by cooling the sample with liquid nitrogen in
a Dewar flask. The observed luminescence lifetimes of the europium
clusters were determined under excitation maximum and monitoring
the 5D0−7F2 with a HORIBA JOBIN YVON FluoroMax-P
spectrophotometer. The emission (550−750 nm) quantum yields of
the clusters were obtained with a SPEX Fluorolog spectrofluorimeter
at room temperature using an integrating sphere method.34 Three
measurements were carried out for each sample. The errors in the
quantum yield values associated with this technique were estimated
within 10%.35
X-ray Crystallography. Crystal diffraction data for the complexes
Eu9-1, Eu9-3, Eu9-4, and Eu9-6 were collected on an Oxford
Diffraction Xcalibur CCD diffractometer with graphite-monochrom-
atized Mo Kα radiation (λ = 0.71073 Å) at room temperature using
the ω-scan technique. The program CrysAlisPro was employed for
data collection and data reduction.36 The structures were solved by
direct methods, and the non-hydrogen atoms were refined anisotropi-
cally by a full-matrix least-squares method on F2 using the SHELXTL-
97 program.37 Part of the MeO groups of the HL4 are refined to be
disordered over two possible sites with site occupancy factors of
0.52:0.48. Moreover, in order to keep the global neutrality, we
imposed a disorder on the occupancy of the hydrogen atom supported
by the μ4-O groups (μ4-(O, OH) 50:50). Similar phenomena were also
previously reported.12a The hydrogen atoms were included in the
structure factor calculation at ideal positions using a riding model.
Complete crystal structure results of the complexes as CIF files
including bond lengths, angles, and atomic coordinates are available as
Supporting Information.
Computational Methodology. The ground-state geometries of
the ligands were optimized with density functional theory (DFT)
using Becke’s three-parameter hybrid exchange functional combined
with Lee−Yang−Parr’s correlation functional (B3LYP).38 Stuttgart-
Dresden effective core potential (designed as SDD ECP) basis sets39
were used for the Eu(III) ion, while the 6-311G++(d,p) basis set was
employed for all other atoms. All calculations were done in the gas
phase with the Gaussian 09 package.40 Vibrational frequencies were
calculated at the same theoretical level to confirm that each
configuration was a minimum on the potential energy surface. Graphic
representations for optimized geometrical structures and molecular
orbitals for the ground state were obtained with GaussView.
CONCLUSIONS
■
In summary, six novel diabolo-shaped nonanuclear Eu(III)
clusters with appealing structures and interesting optical
properties have been synthesized with modified 2-hydrox-
ybenzophenone as supporting ligands. X-ray diffraction analysis
reveals that the compounds are all nonanuclear clusters with a
similar core [Eu(III)9(L)16(μ3-OH)8(μ4-O)(μ4-OH)], which
comprises two vertex-sharing square pyramids arranged in a
“staggered” fashion. The photophysical studies show that all the
complexes exhibit characteristic red luminescence of Eu3+ under
long-wavelength excitation with the luminescent efficiencies
decreasing in the sequence Eu9-6 > Eu9-5 > Eu9-4 > Eu9-3 >
Eu9-2 > Eu9-1. The phenomena are rationalized by the studies
on the lowest electronic excited states of these ligands. The
supporting DFT computational results confirm that the excited
energy levels of these ligands can be fine-tuned by
incorporating substitutions into the ligands. The introduction
of fluorine or methoxy substituents on the ligand, especially for
the fluorine, can apparently raise the lowest triplet states of the
ligands to the values facilitating efficient energy transfer process
to the 5D0 emitting level of Eu3+, which result in more efficient
luminescence. Taken collectively, manipulation of the elec-
tronic features of the 2-hydroxybenzophenone by substituents
is a viable strategy for obtaining new luminescent europium
clusters. The specific design of polynuclear complexes with
interesting optical properties is now under way.
Syntheses. The methanol solutions of LnCl3·xH2O (0.1 mol/L)
were prepared from their oxides using a modified version of a
previously published procedure.41 The europium complexes
(EuL3(H2O)2: Eu-1 to Eu-6) are synthesized according to the
literature procedure.13a,b
Synthesis of (4-Fluoro-2-hydroxyphenyl)phenylmethadone
(HL5). Benzoyl chloride (2.81 g, 20.0 mmol) was added slowly to a
diethyl ether solution (50 mL) of 3-fluorophenol (2.00 g, 17.8 mmol)
and triethylamine (2.02 g, 20.0 mmol) under a nitrogen atmosphere.
The mixture was refluxed for about 1 h under stirring. Then, the
F
dx.doi.org/10.1021/ic401191e | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
reaction was quenched with water (∼10 mL) and the pH of the
solution was adjusted to 5 with 10 wt % NaOH solution. The solution
was extracted with ethyl acetate (3 × 50 mL), and the organic phase
was then dried (MgSO4), evaporated, and concentrated to dryness
under reduced pressure. The resulting crude product was mixed with
AlCl3 (2.53 g, 19.0 mmol) under a nitrogen atmosphere, and the
resultant mixture was heated for 30 min at 200 °C. After cooling to
room temperature, the solid was slowly added to ice-cooled 10% HCl
solution under stirring. The final product was then obtained through
C, 53.35; H, 3.91. IR (KBr, cm−1): 3332 (w), 1604 (s), 1516 (s), 1364
(s), 1247 (s), 1113 (m), 1030 (w), 836 (m), 764 (m), 584 (w).
[Eu(III)9(L5)16(μ3-OH)8(μ4-O)(μ4-OH)] (Eu9-5, yield: 32%):
Anal. Calcd for Eu9C208H137F16O42 (%): C, 50.17; H, 2.77. Found
(%): C, 50.10; H, 2.69. IR (KBr, cm−1): 3420 (m), 1621 (s), 1532 (s),
1418 (s), 1254 (s), 990 (m), 853 (m), 758 (m), 579 (m).
[Eu(III)9(L6)16(μ3-OH)8(μ4-O)(μ4-OH)] (Eu9-6, yield: 29%):
Anal. Calcd for Eu9C224H169F16O42 (%): C, 51.70; H, 3.27. Found
(%): C, 51.65; H, 3.21. IR (KBr, cm−1): 3396 (m), 1619 (s), 1532 (s),
1417 (s), 1252 (s), 990 (w), 701 (m).
1
column chromatography with a moderate yield of 79% (3.04 g). H
NMR (400 MHz, CDCl3) δ 12.43 (d, 1H), 7.62 (m, 4H), 7.52 (m,
2H), 6.76 (dd, 1H), 6.60 (m, 1H). 13C NMR (100 MHz, CDCl3) δ
200.6, 167.5 (d), 165.8 (d), 137.8, 136.0 (d), 132.1, 129.0, 128.5, 116.2
(d), 107.0 (d), 105.1 (d). Anal. Calcd for C13H9FO2 (%): C, 72.22; H,
4.20. Found (%): C, 72.20; H, 4.19. HRMS (ESI) calcd for C13H10FO2
[M + H]+: 217.0665, found 217.0663.
ASSOCIATED CONTENT
■
S
* Supporting Information
X-ray crystallographic data of Eu9-1, Eu9-3, Eu9-4, and Eu9-6
in CIF format. Crystal data and structure refinement
parameters for clusters, thermal ellipsoid plot of the clusters,
selected bond lengths and angles for Eu9-3, IR spectra for the
ligands and clusters, absorption spectra of the mononuclear
europium complexes (Eu-1 to Eu-6) and nonanuclear euro-
pium clusters (Eu9-1 to Eu9-6) in solid state and in acetonitrile
at room temperature, excitation spectra of clusters Eu9-1 to
Eu9-6 at 298 K, luminescence spectrum of the cluster Eu9-3 at
77 K in the solid state, UV−vis absorption spectra of the
mononuclear gadolinium complexes, phosphorescence spectra
of the mononuclear gadolinium complexes (Gd(L)3(H2O)2) at
77 K, optimized ground-state structures of the ligands and
mononuclear europium complexes (Eu-1 to Eu-6) together
with Cartesian coordinates, the HOMO and LUMO distribu-
tion, and energy levels and energy gaps of the mononuclear
europium complexes. This material is available free of charge
via the Internet at http://pubs.acs.org.
Synthesis of (4-Fluoro-2-hydroxyphenyl)-(4-methylphenyl)-
methadone (HL6). This compound was prepared by using the
1
method described for the synthesis of HL5. Yield 72% (2.95 g). H
NMR (400 MHz, CDCl3) δ 12.45 (d, 1H), 7.63 (dd, 1H), 7.56 (d,
2H), 7.31 (d, 2H), 6.74 (m, 1H), 6.58 (m, 1H), 2.45 (s, 3H). 13C
NMR (100 MHz, CDCl3) δ 200.3, 167.3 (d), 165.7 (d), 142.9, 135.9
(d), 135.0, 129.3, 129.1, 116.3 (d), 106.8 (d), 105.0 (d), 21.6. Anal.
Calcd for C14H11FO2 (%): C, 73.03; H, 4.82. Found (%): C, 73.00; H,
4.79. HRMS (ESI) calcd for C14H12FO2 [M + H]+: 231.0821, found
231.0826.
Synthesis of Mononuclear Gadolinium Complexes Gd(L)3(H2O)2.
The ligand HL (3 mmol) and GdCl3·xH2O (1.00 mol) were dissolved
in 30 mL of ethanol, and then, the pH of the mixture was adjusted to 7
with 20 wt % NaOH aqueous solution. The resultant solution was
heated and stirred at 60 °C for 3 h, and then cooled to room
temperature. The precipitates were filtered and purified by
recrystallization from an ethanol−water mixed solution, dried in air,
and then dried in a vacuum.
Gd-1 (GdC27H25O11). Yield: 63%. Anal. Calcd (%): C, 47.50;
H, 3.69. Found: C, 47.45; H, 3.63.
Gd-2 (GdC39H25O8). Yield: 82%. Anal. Calcd (%): C, 60.14; H,
3.24. Found: C, 60.18; H, 3.19.
Gd-3 (GdC42H31O11). Yield: 70%. Anal. Calcd (%): C, 58.05;
H, 3.60. Found: C, 57.93; H, 3.54.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: dcx@zzu.edu.cn
*E-mail: mpsong@zzu.edu.cn.
Notes
Gd-4 (GdC45H37O11). Yield: 72%. Anal. Calcd (%): C, 59.33;
H, 4.09. Found: C, 59.38; H, 4.02.
The authors declare no competing financial interest.
Gd-5 (GdC39H22F3O8). Yield: 65%. Anal. Calcd (%): C, 56.24;
H, 2.66. Found: C, 56.22; H, 2.61.
Gd-6 (GdC42H28F3O8). Yield: 69%. Anal. Calcd (%): C, 57.66;
ACKNOWLEDGMENTS
■
This work was supported by the Specialized Research Fund for
the Doctoral Program of Higher Education (20114101130002)
and the National Natural Science Foundation of China
(21071128).
H, 3.23. Found: C, 57.61; H, 3.18.
Synthesis of Clusters: General Procedure. A 1 mmol portion of
EuCl3·xH2O and 2 mmol of HL were dissolved in 10 mL of methanol.
After stirring for 15 min, 4 mmol of triethylamine was added dropwise
to the solution, and then, the mixture was stirred for 3 h at room
temperature. The resultant solution was left unperturbed to allow the
slow evaporation of the solvent. Yellow block single crystals, suitable
for X-ray diffraction analysis, were formed after 5 days. Yields: 29−43%
based on Eu.
[Eu(III)9(L1)16(μ3-OH)8(μ4-O)(μ4-OH)]·2H2O·2CH3OH (Eu9-1,
yield: 36%): Anal. Calcd for Eu9C146H165O62 (%): C, 40.98; H, 3.89.
Found (%): C, 40.86; H, 3.81. IR (KBr, cm−1): 3449 (m), 1601 (s),
1517(s), 1371 (s), 1243 (s), 1138 (m), 1063 (m), 983 (m), 848 (m),
788 (w), 572 (w).
[Eu(III)9(L2)16(μ3-OH)8(μ4-O)(μ4-OH)] (Eu9-2, yield: 31%):
Anal. Calcd for Eu9C208H153O42 (%): C, 53.24; H, 3.29. Found (%):
C, 53.19; H, 3.22. IR (KBr, cm−1): 3356 (w), 3057 (w), 1611(s), 1332
(m), 1240 (s), 1148 (m), 941 (w), 760 (m), 701 (m).
[Eu(III)9(L3)16(μ3-OH)8(μ4-O)(μ4-OH)] (Eu9-3, yield: 43%):
Anal. Calcd for Eu9C224H185O58 (%): C, 52.01; H, 3.61. Found (%):
C, 51.98; H, 3.52. IR (KBr, cm−1): 3334 (w), 1607 (s), 1517(s), 1358
(m), 1247 (s), 1114 (m), 1029 (w), 841 (w), 702 (m), 599 (w).
[Eu(III)9(L4)16(μ3-OH)8(μ4-O)(μ4-OH)] (Eu9-4, yield: 41%):
Anal. Calcd for Eu9C240H217O58 (%): C, 53.41; H, 4.05. Found (%):
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