The effect of two additional Eu3+ lumophors in two novel trinuclear europium complexes on their photoluminescent properties

Article abstract of DOI:10.1039/c2pp25284h

Two novel trinuclear europium complexes based on trisphen(1,3,5-tris{4-((1, 10-phenanthroline-[5,6-d]imidazol-2yl)phenoxy)methyl}-2,4,6-trimethyl-benzene) as a second ligand were designed, synthesized, and characterized by FT-IR, ¹H NMR, UV-visible, photoluminescence (PL) spectroscopy, elemental analysis (EA) and ESI-MS. The geometries of these two trinuclear europium complexes were predicted using the Sparkle/PM3 model and suggested a chemical environment of very low symmetry around the lanthanide ions (C₁), which is in agreement with the luminescent spectra. CV analysis demonstrated that the trinuclear complexes possessed excellent electro-injection abilities. The effects of two additional Eu³⁺ lumophors in these trinuclear europium complexes on their photoluminescent properties were investigated in detail. The results indicated that these trinuclear europium complexes exhibited highly luminescent quantum efficiencies and experimental intensity parameters in the solid state. Especially, due to the contribution of the two additional Eu³⁺ lumophors in the trinuclear europium complexes, the quantum efficiency of the trinuclear complex Eu₃(TTA)₉trisphen was higher (ca. 34%) than the mononuclear europium complex Eu(TTA) ₃imidazophen.

Full text of DOI:10.1039/c2pp25284h

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The effect of two additional Eu3+ lumophors in two
novel trinuclear europium complexes on their
photoluminescent properties†
Cite this: Photochem. Photobiol. Sci., 2013,
12, 330
Chaolong Yang,a,b Jing Xu,a,b Jianying Ma,a,b Dongyu Zhu,c Yunfei Zhang,a,b
Liyan Lianga and Mangeng Lu*a
Two novel trinuclear europium complexes based on trisphen(1,3,5-tris{4-((1,10-phenanthroline-[5,6-d]imi-
dazol-2yl)phenoxy)methyl}-2,4,6-trimethyl-benzene) as a second ligand were designed, synthesized, and
characterized by FT-IR, 1H NMR, UV-visible, photoluminescence (PL) spectroscopy, elemental analysis (EA)
and ESI-MS. The geometries of these two trinuclear europium complexes were predicted using the
Sparkle/PM3 model and suggested a chemical environment of very low symmetry around the lanthanide
ions (C1), which is in agreement with the luminescent spectra. CV analysis demonstrated that the trinuc-
lear complexes possessed excellent electro-injection abilities. The effects of two additional Eu3+ lumo-
phors in these trinuclear europium complexes on their photoluminescent properties were investigated in
detail. The results indicated that these trinuclear europium complexes exhibited highly luminescent
quantum efficiencies and experimental intensity parameters in the solid state. Especially, due to the con-
tribution of the two additional Eu3+ lumophors in the trinuclear europium complexes, the quantum
efficiency of the trinuclear complex Eu3(TTA)9trisphen was higher (ca. 34%) than the mononuclear euro-
pium complex Eu(TTA)3imidazophen.
Received 12th July 2012,
Accepted 22nd September 2012
DOI: 10.1039/c2pp25284h
www.rsc.org/pps
Unfortunately, the absorption coefficient of an europium ion
is very low. To overcome the problem, organic ligands, which
Introduction
As one of the most promising next generation low-cost full- have much larger absorption coefficients, are usually coordi-
colour flat-panel displays, OLEDs have attracted much research nated to sensitize the europium complexes. So, in order to
interest since Tang and Van Slyke began the pioneering work improve the luminescent properties of the europium com-
in 1987.1 For commercial applications, only three primary plexes, the design and synthesis of organic ligands which have
colors of red, green, and blue are required. Until now, the pure larger absorption coefficients is very important.
red luminescent material is still a challenge, although many
One efficient approach to increase the luminescent proper-
metal complexes showing red emission have been reported.2–5 ties is to introduce ligands with multiple binding sites.6,7 So far,
Among those red light-emitting materials, europium com- organic europium complexes have been studied for twenty
plexes are particularly attractive, because they can emit highly years. However, the majority of europium complexes studied are
monochromatic red light at around 612 nm with a 5–10 nm mononuclear and binuclear complexes.8–10 There are few
half peak bandwidth due to the electronic transitions of the reports of the synthesis and properties for trinuclear europium
central europium ion. In addition, they can offer 100% emis- complexes.11 Some work6,7,12,13 has shown that the quantum
sion intrinsic quantum efficiency, theoretically, since this kind efficiencies of bi- and trinuclear europium complexes are higher
of transition is not restricted by the spin inhibition rule. (about 10%) than mononuclear ones, and the lifetimes of these
europium complexes were longer than the mononuclear ones.
1,10-Phenanthroline (phen), a classic chelating bidentate
ligand for transition metal ions, has played an important role
in the development of coordination chemistry and still con-
tinues to be of considerable interest as a versatile starting
material for organic, inorganic and supramolecular chem-
istry.14 Phen is a rigid planar, hydrophobic, electron-accepting
heteroaromatic system, whose nitrogen atoms are beautifully
placed to act cooperatively in cation binding. These structural
aKey Laboratory of Polymer Materials for Electronics, Guangzhou institute of
Chemistry, Chinese Academy of Sciences, Guangzhou 510650, PR China.
E-mail: mglu@gic.ac.cn; Fax: +86 20 85232978
bGraduate School of the Chinese Academy of Sciences, Beijing 100039, PR China
cSchool of Chemistry and Engineering, Sun Yat-sen University, Guangzhou 510275,
PR China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c2pp25284h
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features determine its coordination ability toward metal ions. prepared in high yield (80%) from 1,3,5-tris[(4-formylphenoxy)-
As is well-known, many studies have confirmed that 1,10-phe- methyl]-2,4,6-trimethylbenzene and 1,10-phenanthroline-5,6-
nanthroline is an excellent second ligand for europium com- dione. The trinuclear europium complexes are commonly syn-
plexes, it can reduce the non-radiative decay of the excited thesized by dissolving ligands in warm ethanol, neutralized
states of the europium ion, improve the stability of the euro- pH = 7 with dilute sodium hydroxide solution, followed by the
pium complexes, and increase the energy transfer efficiency addition of the europium chloride solution.
from the ligands to the europium ion.
We tried our best to obtain single crystals of the trinuclear
In this work, to further improve the luminescent properties europium complexes in our experiment, unfortunately, we
of europium complexes, two novel trinuclear europium com- failed. To deeply understand the complex structure, we opti-
plexes, with trisphen as a second ligand, were designed and mized the molecular structures of the trinuclear europium
synthesized. The structures of the ligand and trinuclear euro- complexes using the Sparkle/PM3 model,16 and the optimized
1
pium complexes were characterized by FT-IR, UV-vis, H NMR, results are displayed in Fig. S1 and S2,† respectively. Every tri-
13C NMR, ESI-MS and elemental analysis. The geometries were nuclear Eu-complex consists of three parts with a small unit of
predicted using the Sparkle/PM3 model. The electrochemical europium complexes, where every unit was eight-coordinate
properties were investigated by circle voltammetry (CV). The and the coordination polyhedron can be approximately
effects of two additional Eu3+ lumophors in these trinuclear described as a distorted bicapped trigonal prism. The two tri-
europium complexes on their photoluminescent properties nuclear europium complexes belong to the C1 point-group.
were investigated in detail by photoluminescent (PL) spec- The spherical coordinates on the bonded oxygen and nitrogen
troscopy in solid state and in THF solution. We expect that atoms with respect to a coordinate system centred on the Eu3+
these two novel trinuclear complexes can exhibit excellent ion are given in Tables S1 and S2.† These structural data were
luminescent properties.
consistent with a chemical environment of very low symmetry
around the Eu3+ ions. An interesting point, predicted by the
present structure optimization, is that the ligand DBM oxygen
atoms were considerably closer to the Eu3+ ion than the TTA
oxygen atoms in these trinuclear europium complexes.
Further, the Eu → N bonds with the trisphen ligand were
longer than the Eu → O bonds with the TTA (DBM) ligands,
this result was in good agreement with the recently reported
results in ref. 17.
The FT-IR spectra of trisphen and the trinuclear europium
complexes are shown in Fig. 1. For complex Eu3(DBM)9tris-
phen, the characteristic stretching vibration absorption peaks
of –CvO in the DBM ligand shifted from ca. 1697 cm−1 to
1629 cm−1, and a new absorption peak was observed at
1550 cm−1, which is attributed to the –CvC stretching
vibration of DBM coordinated to the europium ion. These
results suggest that coordination bonds were formed between
the DBM ligand and europium ion. The phenanthroline ring
Results and discussion
With the purpose of improving the properties of europium
complexes, the trisphen ligand, with three binding sites, has
been designed and synthesized, one trisphen molecule can
efficiently coordinate with three europium ions and nine TTA
(DBM) molecules. Then, two novel trinuclear europium com-
plexes with trisphen as a second ligand were prepared by a
simple method. We expect that these two novel europium com-
plexes can exhibit excellent luminescent properties. The syn-
thetic routes for trisphen and the trinuclear europium
complexes are outlined in Scheme 1. The trisphen was readily
Scheme 1 Synthetic procedures of the trisphen ligand and trinuclear euro-
pium complexes.
Fig. 1 FT-IR spectra of trisphen and the trinuclear europium complexes.
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vibration of trisphen was observed at 1520 cm−1, although the
intensity was relatively weaker. Meanwhile, two new absorption
peaks were observed at ca. 516 cm−1 and 424 cm−1, and were
attributed to the stretching of O → Eu and N → Eu, respect-
ively. For complex Eu3(TTA)9trisphen, the typical asymmetric
vibrations of the carbonyl group of the TTA ligand were
observed at about 1600 cm−1 from the FT-IR spectra of the cor-
responding complexes, while the bond at about 579 cm−1
revealed the presence of O → Eu, which could not be observed
in the ligands. Meanwhile, the peaks of phen at 740 cm−1, cor-
responding to the stretching vibration of C–H bonds, shifted
to 720 cm−1. The FT-IR results further confirmed the confor-
mation of the trinuclear europium complexes.
The UV-vis absorption spectra of the ligands and trinuclear
europium complexes in THF solution (1 × 10−5 mol L−1) are
shown in Fig. 2. Two main absorption bands of the trinuclear
Eu-complex were easily observed, at 284.2 nm and 340.2 nm
for complex Eu3(TTA)9trisphen, and at 283.2 nm and 351.6 nm
for complex Eu3(DBM)9trisphen, respectively, which were
Fig. 3 1H NMR spectra of trisphen and trinuclear europium complexes in
attributed to the singlet–singlet π → π enol absorption. Due to DMSO-d6: (a) trisphen, (b) Eu3(TTA)9trisphen, (c) Eu3(DBM)9trisphen.
the formation of larger conjugated chelate rings in the trinuc-
lear Eu-complexes, those main absorption bands shifted to
peak at 7.81–7.92 ppm was attributed to the protons at the
5-position of phenanthroline ring. The lanthanide-based com-
plexes were well-known as shift reagents due to the paramag-
netism of the lanthanide ion. Therefore, the peaks of trisphen
in the trinuclear europium complexes were very different with
free tris-phenantroline. The obvious difference was the line
broadening of certain peaks close to the paramagnetic euro-
pium ion. For complex Eu3(TTA)9trisphen, compared with free
tris-penanthroline, the proton of the –NH group is shifted
from 13.6 to 13.72 ppm, and the peak was broad. The peaks
for the protons at the 2-, 3-, 5-positions shifted from 9.04 to
10.79 ppm, 8.91 to 10.33 ppm, and 7.85 to 9.58 ppm, respect-
ively. The single peak at 3.62 ppm was attributed to the
protons at the α-position (COCHCO) of the nine TTA ligands.
For complex Eu3(DBM)9trisphen, the obvious change in the 1H
NMR spectrum for trisphen was the peaks for the 2-, 3-, 5-posi-
tions shifted from 9.04 to 11.66 ppm, 8.91 to 9.55 ppm, and
7.85 to 9.03 ppm. The single peak for the proton of the –NH
group shifted from 13.6 to 13.97 ppm. The single peak at
3.62 ppm was attributed to the protons at the α-position
(COCHCO) of the nine DBM ligands. In addition, the signal
for the protons at the 4-positions shifted from 8.26 to
8.39 ppm, where the doublet peak changed to a broad singlet
peak.
longer wavelengths compared with corresponding ligands.
The 1H NMR spectra of trisphen and trinuclear europium
complexes obtained at 400 MHz in d-DMSO solutions were
1
shown in Fig. 3. The H NMR spectrum of trisphen consisted
of three singlet peaks at 13.6, 5.23 and 2.42 ppm, three
doublet peaks at 8.90–8.92, 8.27–8.29, and 7.25–7.35 ppm, one
triplicate peak at 9.02–9.03 ppm, and one multiplicate peak at
7.81–7.92 ppm. The singlet peaks at 13.6, 5.23 and 2.42 ppm
were attributed to the proton of the –NH group of the imida-
zole ring, three –CH2– groups, and three –CH3 groups, respect-
ively. The double peaks at 8.90–8.92, 8.27–8.29 and 7.34–7.35
were attributed to the protons at the 3-, 4- and 6-positions of
the phenanthroline ring and phenyl group with J = 8, 8.3 and
4 Hz. The peak at 9.02–9.03 ppm was attributed to the protons
at the 2-position of the phenanthroline ring. The multiplicate
The luminescent properties of the trinuclear europium
complexes, in the solid state and in THF solution, were investi-
gated in detail at room temperature. The excitation spectra of
all complexes in the solid state and in THF solution were
5
7
obtained by monitoring the D0 → F2 transition of the euro-
pium ion, which are shown in Fig. 4 and 5, respectively. For
the solution excitation spectra, there was one intense broad
band at 245–425 nm, while in the solid state excitation spectra,
it can be seen clearly that intense broad bands between
200 nm and 475 nm dominate a large portion of the excitation
Fig. 2 UV-vis spectra of ligands and trinuclear europium complexes in THF
solution (1 × 10−5 mol L−1).
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The mechanism of the energy transfer from ligands to
metal ions has been widely discussed to interpret the lumines-
cence of lanthanide compounds.18 From the results discussed
above, we can presume that the energy gaps of the europium
ions were comparable with the triplet state energy of the
ligands trisphen, TTA or DBM, and that efficient energy trans-
fer can take place from the ligands to the europium ions, we
will discuss in detail the energy transfer progress below. These
complexes exhibited the characteristic red emission of euro-
pium ions, by 365 nm excitation, which suggested that these
complexes could be potential red fluorescent materials.
The emission spectra of these complexes in the solid state
are shown in Fig. 4, and the characteristic europium ion emis-
sion was not observed. The lines were distributed mainly in
the 550–710 nm range, which were associated with the 4f → 4f
5
7
transitions of the D0 excited state to the low-lying FJ(J = 0, 1,
2, 3 and 4) levels of Eu3+ ions. No emission peaks from the
ligands were observed under this excitation, further confirm-
ing that the energy transfer from the ligands to the europium
ion center was quite efficient in all trinuclear europium com-
plexes. As is well-known, the emissions of europium ions were
usually employed as a sensitive probe to investigate the coordi-
nation and local environment around cations. The europium
ion transitions that are observed in the emission spectrum
occur via three main mechanisms: forced electric dipole (ED),
magnetic dipole (MD), and dynamic coupling. From the emis-
sion spectra, we can see the emission bands at about 580 and
653 nm were very weak, since their corresponding transitions
5D0 → 7F0,3 were forbidden both in the MD and ED schemes. A
prominent feature, that may be noted in these spectra, was the
Fig. 4 The PL spectra of trinuclear europium complexes in the solid state. (Left)
Excitation spectra (λem = 614 nm for all complexes). (Right) Emission spectra
[λex = 367 nm for Eu3(TTA)9trisphen, 372 nm for Eu3(DBM)9trisphen]. (Inset)
Luminescent image from powder of trinuclear europium complexes excited
at 365 nm (★: false peak).
very high intensity of the 5D0
→
7F2 transition at 614 nm. It
was well-known to us that the 5D0 → 7F1 transition was a parity
allowed MD and is non-sensitive to the local structure environ-
ment, while the 5D0
→
7F2 transition was a typical ED tran-
sition and was sensitive to the coordination environment to
the europium ion. When the interaction of the rare earth
complex with its local environment was stronger, the complex
became more non-symmetrical, and the intensity of the elec-
tric-dipolar transitions became more intense. As a result, the
Fig. 5 The PL spectra of trinuclear europium complexes in THF solution (1 ×
10−5 L−1). (Left): Excitation spectra (λem = 614 nm for all complexes). (Right)
Emission spectra [λex = 359 nm for Eu3(TTA)9trisphen, 381 nm for Eu3(DBM)9tris-
phen]. (Inset) Luminescent image from THF solution of trinuclear europium com-
plexes excited at 365 nm.
integration intensity ratio of the D0 → 7F2 transition to D0 →
7F1 transition (I2/I1) had been widely used as an indicator of
europium ion site symmetry. The calculated results were
shown in Table 1.
5
5
Whether in the solid state or in THF solution, the intensity
ratios of these trinuclear complexes were very high. In the
solid state, the intensity rations (I2/I1) of the complexes
Eu3(TTA)9trisphen and Eu3(DBM)9trisphen were 16.9 and 15.1,
respectively, while in THF solution they were 15.2 and 14.6,
respectively. This ratio was only possible when the europium
ion did not occupy a site with inversion symmetry. It was clear
that strong coordination interactions occur between the
organic ligands and europium ion. Further, the emission
spectra for all europium complexes. From comparison of the
UV-vis spectra in Fig. 2, these bands were attributed to the
π → π* transitions of the ligands (trisphen, DBM or TTA). In
comparison with the excitation broad bands of the ligands,
the direct excitation peaks of the europium ion was much
weaker. This suggested that the emission of europium by the
ligands were much more efficient than direct excitation of the
trinuclear europium complexes. Meanwhile, the strong bright-
red emission of the trisnuclear europium complexes in the
solid state and in THF solution, upon illumination with a
365 nm excitation light provided by a 12 W ultraviolet lamp,
can be easily observed by the naked eye (inset of Fig. 4 and 5).
5
spectra of the complexes showed only one line for the D0 →
7F0 transition, indicating the presence of a single chemical
environment around the europium ions.19 In addition, no
emission from the ligands was observed in Fig. 4, which
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Table 1 Solid state and solution luminescent data of trinuclear europium complexes
In the solid state
In THF solution
Complexes
τb (ms)
Arad (s−1
)
I2/I1
η (%)
Ω2 (10−20 cm2)
Ω4 (10−20 cm2)
I2/I1
τb (ms)
Φ (%)
Eu3(TTA)9trisphen
0.78
0.67
0.37
0.29
827.99
730.62
837.2
16.9
15.1
12.31
18.56
64.58
48.95
30.67
23.13
21.5
13.8
—
1.3
1.1
—
15.2
14.6
11.47
15.53
0.63
0.39
0.24
0.13
22.8
3.55
10.83
1.69
Eu3(DBM)9trisphen
Eu(TTA)3imidazophena
Eu(DBM)3imidazophena
797.6
—
—
a These two compounds were synthesized according to ref. 24. b The integrated intensity ratio of the 5D0 → 7F2 transition to 5D0 → 7F2.
indicated that a very efficient energy transfer occurred from
the ligands to the central europium ion.
In addition to the emission spectra of solid complexes, the
room temperature emission spectra of all the trinuclear euro-
pium complexes in THF solution were investigated (Fig. 5).
Compared with the solid state, no great differences in the
emission spectra for all complexes in THF solution were
observed, except the emission intensity.
As is well-known for europium ions, the sensitive ability of
the ligand TTA was better than the ligand DBM, so the relative
emission intensity of the corresponding trinuclear europium
complex Eu3(TTA)9trisphen was stronger than complex
Eu3(DBM)9trisphen under the same conditions. In particular,
5
7
the emission intensity for D0 → F2 of Eu3(TTA)9trisphen was
13-fold stronger than the complex Eu3(DBM)9trisphen in THF
solution.
Fig. 6 The decay curves of the trinuclear europium complexes in the solid state
and in THF solution (λem = 614 nm, λex = 360 nm).
For Eu3(TTA)9trisphen and Eu3(DBM)9trisphen, the Com-
mission International de Éclairage (CIE) chromaticity coordi-
nates from the emission spectra (λex = 370 nm for solid and
λex = 362 nm in THF) were (0.66, 0.33) and (0.66, 0.33) in the
solid state, (0.64, 0.32) and (0.56, 0.27) in THF solution, respect-
ively (Fig. S3†), and indicated that they belong to the pure red
emission. This result is very important because it indicated an
advantage of rare-earth complexes for preparing OLEDs.
To better understand the luminescent properties of the tri-
nuclear europium complexes in the solid state and THF sol-
ution, the room temperature (RT) luminescence decay curves
of the 5D0 excited state were measured by monitoring the most
complexes). Eqn (1) is a means to determine the η values from
experimental spectroscopic data.20
Arad
Arad þ Anrad
η ¼
ð1Þ
where, Arad and Anrad are radiative and non-radiative transition
rates, respectively. The denominator in eqn (1) is calculated
from the lifetime of the emitting level (1/τ = Arad + Anrad). In
the case of europium luminescence the value of Arad can be
estimated by spectral analysis with the help of eqn (2).21
7
intense emission lines (5D0 → F2) of the europium ion center
4
at 614 nm, and under excitation of a 360 nm xenon lamp. As
shown in Fig. 6, whether in the solid state or in THF solution,
the decay curves of the complexes exhibited monoexponential
behavior, indicative of the presence of a single chemical
environment around the europium ion in these complexes,
which was in agreement with the results of only one 5D0 → 7F0
line in the emission spectra and the calculated results of
Sparkle/PM3. The luminescent lifetime values (τ) of Eu3(TTA)9-
trisphen and Eu3(DBM)9trisphen were 0.78 ms and 0.67 ms in
the solid state, and 0.63 ms and 0.39 ms in THF solution,
respectively.
X
A0ꢀ1hω0ꢀ1
S0ꢀJ
Arad
¼
ð2Þ
S0ꢀ1
J¼0 hω0ꢀJ
7
where, J represents the final F0–6 levels, S is the integrated
intensity of the particular emission lines and hω stands for the
corresponding transition energies. A0–1 is the Einstein coeffi-
cient of spontaneous emission between the 5D0 and the 7F1
Stark levels. The branching ratios for the 5D0
→
7F5,6 tran-
sitions must be neglected as they are too weak to be observed
experimentally. Therefore, their influence can be ignored in
5
5
7
the depopulation of the D0 excited state. The D0 → F1 tran-
sition does not depend on the local ligand field seen by euro-
pium ions and, thus, may be used as a reference for the whole
5
The intrinsic luminescent quantum efficiency (η) of the D0
emission level in these trinuclear europium complexes at room
temperature was obtained based on the luminescence data
(emission spectra and emission decay curves of these
spectrum, in vacuo A0–1 = 14.65 s−1 22
.
An average refractive
index equal to 1.5 was considered, leading to A0–1 ≈ 50 s−1
.
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According to the above discussion, the intrinsic lumines- here, ν is the average transition energy in cm−1, h is the Planck
cence quantum efficiency of these trinuclear europium com- constant, 2J + 1 is the degeneracy of the initial state. S(ED) and
plexes in the solid state can be determined, as shown in S(MD) are the ED and MD strengths, respectively. Among all
Table 1.
these transitions, the 5D0 7F0,3,5 transitions are forbidden,
→
The solution luminescent yields of the trinuclear europium both in MD and ED schemes (S(ED) and S(MD) are zero). In
complexes were determined using quinine sulfate (dissolved in addition, the 5D0 → 7F1 transition is an isolated MD transition,
0.5 M H2SO4 with a concentration of 10−6 M, assuming ΦPL of and has no ED contribution, which is practically independent
0.55) as a standard.23 The quantum yield is calculated accord- of the lanthanide ions chemical environment and can be used
ing the following formula:
as a reference. The experimental parameters Ω2 and Ω4 can be
calculated according to the equation:26
Ar S n2
A Sr nr
Φ ¼ Φr
ð3Þ
3hc3A0ꢀλ
2
ꢁ
ꢁ
ꢁ
ꢁ
Ωλ ¼
ð5Þ
2
kλl
4e2w3χk5D0
U
7FJl
where Φ is the fluorescence quantum yield, S represents the
area of the corrected emission fluorescence spectrum, A is the
absorbance of the solution at the exciting wavelength, and n is
the refractive index of the solvent used. The subscript r
denotes the reference substance, of which the fluorescence
quantum yield is already known. The calculated results were
shown in Table 1.
2
where, e is the electronic charge. χ = n0 (n0 + 2)2/9 is a Lorenz
local field correction. The square reduced matrix elements are
<5D0kU(2)kF2>2 = 0.0032 and <5D0kU(4)kF4>2 = 0.0023, and an
average index of refraction equal to 1.5 was used. The Ω2 and
Ω4 intensity parameters for all complexes are shown in
Table 1. A point to be noted in these results was the relatively
high Ω2 parameter for these two trinuclear europium com-
plexes. This might be interpreted as being a consequence of
Whether in the solid state or THF solution, these trinuclear
europium complexes exhibited high luminescence quantum
efficiencies or yields, indicating that the energy transfer from
the ligands to the center europium ions was very efficient.
From Table 1, we found that the trinuclear europium com-
plexes based on TTA as a β-diketone ligand exhibited higher
luminescence quantum efficiencies (yield) than the complexes
based on DBM as the β-diketone ligand. This finding was in
good agreement with the results from luminescent emission
intensity. Especially in the solid state, due to the contribution
of two additional Eu3+ lumophors, the luminescent quantum
efficiency of the trinuclear europium complexes Eu3(TTA)9tri-
sphen and Eu3(DBM)9trisphen (64.58% and 48.95%, respect-
ively) were higher than that of the mononuclear europium
complexes Eu(TTA)3imidazophen (30.67%) and Eu(DBM)3imi-
dazophen (η = 23.13%). In addition, the quantum yield of the
trinuclear europium complexes in THF solution were higher
(about 2-fold) than the corresponding mononuclear europium
complexes. These results indicated that the introduction of
ligands with multiple binding sites to the europium complex
can effectively increase the quantum efficiency of the euro-
pium complexes.
5
7
the hypersensitive behavior of the D0 → F2 transition, indi-
cating that the europium ion was in a highly polarizable
chemical environment in these trinuclear complexes.
It was known that in lanthanide complexes, neutral ligands
often play a role in absorbing and transporting energy to the
other ligands or the center metal ions. According to absorp-
tion spectra of trisphen and TTA (DBM) (Fig. 7), it was found
that there was a large area of overlap between the absorption
peaks of trisphen and TTA (DBM), from 297 to 400 nm. This
meant that nearly all the radiation from trisphen can be
absorbed by TTA (DBM). This wide overlap showed the possi-
bility of the highly efficient energy transfer from trisphen to
TTA (DBM).
To demonstrate the energy transfer progress, the triplet
energy of the ligand trisphen was calculated using the con-
figuration interaction single (CIS) based on the intermediate
To investigate the possible structural changes around the
emitting center europium ion among these trinuclear com-
plexes, the experimental intensity parameters Ω2 and Ω4 (the
5
7
Ω6 parameter was not determined because the D0 → F6 tran-
sition could not be experimentally detected) can be calculated
from the emission spectra as described in ref. 25. In particular,
Ω2 was more sensitive to the symmetry and sequence of ligand
fields. To produce faster europium radiation rates, antisymme-
trical europium complexes with large Ω2 parameters needed to
be designed. The spontaneous emission probability A0λ (λ = 2,
4) of the transitions were related to its dipole strength accord-
ing to the equation:
Fig. 7 Schematic energy level diagram and energy transfer processes for the
trisnuclear europium complexes. S1 represents the first excited singlet state and
T1 represents the first excited triplet state.
ꢀ
ꢀ
ꢀ
ꢀ
4
3
2
A0λ ¼ ð64π ν Þ=j3hð2J þ 1Þjk nðn þ 2Þ=9 SðEDÞ þ S
l
ð4Þ
ðMDÞ
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neglect of differential overlap/spectroscopic (INDO/S)27
method implemented in the ZINDO software.28 The values of
the triplet energy for TTA and DBM were obtained from ref. 29
and 30. The triplet energy levels of trisphen, TTA and DBM
were 2.23, 2.35, and 2.53 eV, respectively. The singlet state
energy levels of trisphen, TTA and DBM were estimated by
referencing their absorbance edges, which were 3.45 eV, 3.12
eV and 3.25 eV, respectively. For comparison, the singlet and
triplet state energy levels are illustrated in Fig. 7.
Table 2 Electrochemical data of the europium complexes
a
b
c
d
Eox
Ered
EHOMO
ELUMO
(eV)
Eg
(eV)
Complexes
(V)
(V)
(eV)
Eu3(TTA)9trisphen
1.76
−1.57
−1.42
−1.5
−6.14
−6.09
−6.3
−2.81
−2.96
−3.27
−2.42
3.33
3.13
3.03
3.21
Eu3(DBM)9trisphen 1.71
Eu(TTA)3phene
Eu(DBM)3phenf
1.53
1.25
−1.95
−5.63
a Estimated according to the reduction potential and the UV-vis
absorption spectra. b Measured in DMF solution at 298 K. EHOMO
=
c
d
−(Eox + 4.38) eV. ELUMO = −(Ered + 4.38) eV.e These values for Eu-
(TTA)3phen were reported in ref. 32. f These values for Eu(DBM)3phen
were reported in ref. 33.
In Fig. 7, it can be seen that the S1 energy level of trisphen
matches that of TTA (DBM) and the 5D0 energy level of Eu3+
ion. So the energy absorbed by trisphen could be transferred
to TTA (DBM) or the Eu3+ ion directly. However, it is known
that the gap between the two energy levels should be inter-
mediate, where being too big or small would decrease the
efficiency of the energy transfer. The energy gap of the S1
energy levels between trisphen and TTA or DBM were 0.33 eV
and 0.20 eV, respectively. So it was appropriate to improve the
energy transfer from the S1 of trisphen to that of TTA or DBM.
At the same time, due to the S1 energy levels of TTA (DBM)
being higher than the T1 energy levels of trisphen, the energy
transfer from the S1 of TTA (DBM) to the T1 of trisphen was
fairly efficient. Meanwhile, as the T1 state of trisphen was
higher than the energy level of the 5D0 of Eu3+ ion, so the
which the Eg was estimated based on the UV-vis absorption
edge. Thus, the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO) energy
levels (EHOMO and ELUMO) of the trinuclear europium com-
plexes were calculated according to the empirical relationship
formula: EHOMO = −e (Eox + 4.38) and ELUMO = −e (Ered + 4.38)
eV, proposed by the Brédas group.31 The electrochemical data
of the trinuclear europium complexes are listed in Table 2.
The HOMO and LUMO energy levels of the trinuclear com-
plexes Eu3(TTA)9trisphen and Eu3(DBM)9trisphen are −6.14
and −2.81 eV, and −6.3 and −2.96 eV, respectively. Compared
to the corresponding mononuclear europium complexes Eu-
(TTA)3phen and Eu(DBM)3phen, these two trinuclear europium
complexes exhibited a decrease in the HOMO and LUMO
energy levels. As is well-known, the decreased LUMO energy
levels were available to facilitate electro injection and transpor-
tation from the cathode to the emitters of the europium com-
plexes. Therefore, two additional Eu3+ ions in the trinuclear
europium complexes may significantly tune the electrochemi-
cal and transporting property of the europium complexes.
5
efficient energy transfer from the T1 of trisphen to the D0 of
the Eu3+ ion can occur, which can improve the ability of
energy transfer from the ligands to the emitter Eu3+ ions.
The electrochemical properties of the trinuclear europium
complexes were investigated by cyclic voltammetry (CV) at
298 K in acetonitrile solutions versus the saturated calomel
electrode (SCE) (Fig. 8), and the reduction peaks of these tri-
nuclear complexes were easily observed, the values of the
reduction potential (Ered) were −1.57 eV for Eu3(TTA)9trisphen,
and −1.42 eV for Eu3(DBM)9trisphen, respectively, but their
oxidation peaks disappeared. The oxidation potential (Eox) had
to be estimated based on the energy gap (Eg) and Ered, in
Conclusions
In this work, two new trinuclear europium complexes were
designed and synthesized, and their luminescent properties
were carefully studied. Their ground state geometries were
obtained from the Sparkle/PM3 model and pointed to a chemi-
cal environment of very low symmetry around the Eu3+ ions.
Whether in the solid state or in THF solution, all trinuclear
complexes exhibited strong luminescent intensities and long
luminescent lifetimes. The results of the decay curves and
emission spectra of the trinuclear complexes indicated that
there was only one luminescence centre, and the europium ion
was located in a polarized chemical environment. Most impor-
tantly, due to the contribution of the two additional Eu3+
lumophors in the trinuclear europium complexes, these two
complexes exhibited much longer lifetimes and higher intrin-
sic quantum efficiencies than the corresponding mononuclear
europium complex. These results indicated that the introduc-
tion of ligands with multiple binding sites to europium com-
plexes could effectively increase the luminescent properties of
the europium complexes. Further investigation indicated that
Fig. 8 Cyclic voltammogram of the trinuclear europium complexes measured
in acetonitrile solution, containing 0.1
50 mV s−1
M Bu4NClO4 at 298 K. Scans rate:
.
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Paper
trisphen has a mezzo first triplet excited energy level (T1) C 75.83, H 4.61, N 15.39; Calcd for C69H48N12O3: C 75.81, H
5
between the first T1 of TTA (DBM) and the D0 of the Eu3+ ion, 4.43, N 15.38. FT-IR/cm−1, 3122 (NH), 1609, 1480, 1243, 1179,
which may support one more additional energy transfer 986. UV-vis (nm), 283, 326.
5
routine from the T1 energy level of trisphen to the D0 of the
Eu3+ ion, and consequently results in the improvement of
energy transfer in these trinuclear europium complexes.
General procedure of synthesizes of europium complexes
Trisphen (1 mmol), DBM or TTA (9 mmol), and sodium
hydroxide (9 mmol) were dissolved in 60 mL THF under stir-
ring. Then the solution was heated to 70 °C, and 40 mL anhy-
drous ethanol with 3 mmol EuCl3·6H2O was added dropwise
Experimental section
to the solution, and kept at this temperature for 5 h. The
Starting materials were of reagent grade and used without
mixed solution was cooled to room temperature and the sol-
further purification, unless otherwise stated. All solvents were
vents removed to give the crude product. The complexes were
purified with conventional methods before use. Mesitylene,
purified by reprecipitation from cyclohexane and washed three
dibenzoylmethane (DBM) and 2-thenoyltrifluoroacetone (TTA)
times with 50 mL ethanol every time to give the desired trinuc-
lear europium complexes.
were purchased from Aldrich Chemical Company. Eu2O3
(99.99%) was purchased from a Chinese company, Beijing
Founder. EuCl3·6H2O was obtained by dissolving Eu2O3 in con-
centrated chlorohydric acid.
FT-IR spectra were carried out using a Tensor 27 (Bruker)
Fourier Transform Infrared Spectrometer. Elemental analysis
data were obtained from a Vario EL elemental analyzer. NMR
spectra were taken on a DRX-400 MHz (Bruker) superconduct-
ing-magnet NMR spectrometer with TMS as an internal stan-
Eu3(TTA)9trisphen was synthesized as
a yellow-white
powder in 75% yield. Eu3C141H84N12O21S9 found (calc.): C
47.59 (47.84), H 2.41 (2.39), and N 4.63 (4.75). ESI-MS: 3677.9.
1H NMR (400 MHz, DMSO-d6): δ 13.65 (s, –NH, 3H), 10.78 (s,
phenanthroline ring, 6H), 10.32 (s, phenanthroline ring, 6H),
9.64 (s, phenanthroline ring, 6H), 8.29 (s, phenyl, 6H), 7.83 (d,
J = 7.5 Hz, Th-H, 9H), 7.34 (d, J = 7.5 Hz, phenyl, 6H), 6.47 (br,
s, Th-H, 9H), 6.26 (br, s, Th-H, 9H), 5.22 (s, –CH2–, 6H), 3.61
dard. UV-vis absorption spectra were determined on
a
(br, s, COCHCO, 9H), 2.43 (s, –CH3, 9H) ppm. FT-IR/cm−1
,
Shimadz spectrophotometer (UV 2550). CV measurements
were performed on a computer-controlled CHI600D electroche-
mical analyzer with a Pt working electrode, a Pt plate counter
electrode, and an SCE reference electrode immersed in 0.1 M
Bu4NClO4 in dry acetonitrile purged with dried argon. The
scanning rate was 50 mV s−1, and all electrochemical poten-
tials were calibrated with the ferrocene/ferrocenium (Fc/Fc+)
standard. The photoluminescence (PL) measurements in the
solid state and in the THF solutions were conducted in a
Hitachi F-4600 florescence spectrophotometer. Luminescence
lifetimes were obtained with a FLS920 steady state spec-
trometer with a pulsed xenon lamp.
1603, 1539, 738, 578, 464. UV-vis (nm), 283, 344.
Eu3(DBM)9trisphen was synthesized as yellow-white powder
in 67% yield. Eu3C204H129N6O18, found (calc.): C 68.9 (68.86),
H 3.92 (4.16), and N 4.83 (4.72). ESI-MS: 3702.5. 1H NMR
(400 MHz, DMSO-d6): δ 13.98 (s, –NH, 3H), 11.64 (br, s, phe-
nanthroline ring, 6H), 9.56 (br, s, phenanthroline ring, 6H),
9.04 (s, phenanthroline ring, 6H), 8.37 (br, s, phenyl, 6H), 7.85
(s, phenyl, 6H), 7.34 (s, phenyl, 9H), 6.79–6.87 (d, J = 5.8 Hz,
phenyl, 18H), 6.63 (s, phenyl, 18H), 6.43 (s, phenyl, 9H), 6.19
(s, phenyl, 9H), 5.61 (s, phenyl, 9H), 5.28 (s, –CH2–, 6H), 4.02
(br, s, COCHCO, 9H), 2.47 (s, –CH3, 9H) ppm. FT-IR/cm−1
,
1603, 1550, 1519, 738, 516, 424. UV-vis (nm), 283, 352.
Synthesis of 1,3,5-tris{4-((1,10-phenanthroline-[5,6-d]imidazol-
2-yl)phenoxy)methyl}-2,4,6-trimethylbenzene (trisphen)
Trisphen was prepared according to the procedure described
in the literature with some modification.15 A mixture of 1,3,5-
tris[(4-formylphenoxy)methyl]-2,4,6-tri-methylbenzene (0.76 g,
1.5 mmol), 1,10-phenanthroline-5,6-dione (0.94 mg,
4.5 mmol), and NH4Ac (8.79 g, 114 mmol) in 90 mL glacial
acetic acid was heated to 125 °C for 6 h. After cooling to room
temperature, the solution was diluted with 1 L ice water. Neu-
tralization of the solution with aqueous ammonia solution
gave a yellow precipitate. The precipitate was then filtered and
subsequently washed with ethanol and ethyl ether, respect-
ively, affording the desired product as a yellow solid. Yield:
Acknowledgements
The authors appreciate the Guangdong-Hongkong Technology
Cooperation Finding (project no. 2009A091300012) and the
Natural Science Foundation of Guangdong Province of China
(project no.: 8251065004000001 and E06200692).
Notes and references
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1.31 g (80%). H NMR (400 MHz, DMSO-d6): δ 2.42 (s, –CH3,
9H), 5.23 (s, –CH2–, 6H), 7.33 (d, J = 7.3 Hz, phenyl, 6H), 7.85
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