Synthesis and spectroscopic behavior of highly luminescent trinuclear europium complexes with tris-β-diketone ligand

Article abstract of DOI:10.1016/j.jallcom.2014.05.222

A new tris-β-diketone ligand, 2-[4,6-bis-(1-benzoyl-2-oxo-2-phenyl- ethyl)-[1,3,5]triazin-2-yl]-1,3-diphenyl-propane-1,3-dione (H₃L), and its trinuclear europium complexes, Eu₃(DBM)₆L (C1), Eu₃(DBM)₆(Bipy)₃L (C2) and Eu ₃(DBM)₆(Phen)₃L (C3) were synthesized and their spectroscopic behaviors were studied by FT-IR, ¹H NMR, UV-vis and photoluminescence spectroscopic techniques. These europium complexes exhibited the characteristic emission bands that arise from the ⁵D₀ → ⁷F_J (J = 0-4) transitions of the europium ion in solid state. The Ω₂ and Ω₄ intensity parameters, lifetime (τ) and luminescence quantum yield (η) were calculated according to the emission spectra and luminescence decay curves in solid state. The results indicated that these trinuclear europium complexes displayed a longer lifetime (τ) and higher luminescence quantum efficiency (η), especially complexes C2 (τ = 0.820 ms, η = 46.5%) and C3 (τ = 0.804 ms, η = 47.4%), which due to the effect of two additional europium ion lumophors and the introduction of the third ligands, Bipy or Phen in trinuclear complexes. Their Ω₂ values demonstrated that the europium ion in these complexes is in a highly polarizable chemical environment.

Full text of DOI:10.1016/j.jallcom.2014.05.222

Journal of Alloys and Compounds 613 (2014) 13–17
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Synthesis and spectroscopic behavior of highly luminescent trinuclear
europium complexes with tris-b-diketone ligand
⇑
Dunjia Wang , Yan Pi, Hua Liu, Xianhong Wei, Yanjun Hu, Jing Zheng
Hubei Collaborative Innovation Center for Rare Metal Chemistry, Hubei Key Laboratory of Pollutant Analysis and Reuse Technology, Hubei Normal University, Huangshi 435002, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
new tris-b-diketone ligand, 2-[4,6-bis-(1-benzoyl-2-oxo-2-phenyl-ethyl)-[1,3,5]triazin-2-yl]-1,3-
Received 23 February 2014
Received in revised form 24 May 2014
Accepted 31 May 2014
diphenyl-propane-1,3-dione (H3L), and its trinuclear europium complexes, Eu3(DBM)6L (C1),
Eu3(DBM)6(Bipy)3L (C2) and Eu3(DBM)6(Phen)3L (C3) were synthesized and their spectroscopic behaviors
were studied by FT-IR, 1H NMR, UV–vis and photoluminescence spectroscopic techniques. These euro-
Available online 12 June 2014
pium complexes exhibited the characteristic emission bands that arise from the 5D0
?
7FJ (J = 0–4) tran-
) and
) were calculated according to the emission spectra and luminescence
decay curves in solid state. The results indicated that these trinuclear europium complexes displayed a
longer lifetime and higher luminescence quantum efficiency ), especially complexes C2
= 0.820 ms, = 46.5%) and C3 ( = 0.804 ms, = 47.4%), which due to the effect of two additional euro-
sitions of the europium ion in solid state. The X2 and X4 intensity parameters, lifetime (
luminescence quantum yield (
s
Keywords:
Trinuclear europium complex
Tris-b-diketone
Spectroscopic behavior
Luminescent lifetime
Quantum efficiency
Judd–Ofelt intensity parameters
g
(s
)
(g
(s
g
s
g
pium ion lumophors and the introduction of the third ligands, Bipy or Phen in trinuclear complexes. Their
X2 values demonstrated that the europium ion in these complexes is in a highly polarizable chemical
environment.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
Generally, one efficient method of improving the luminescent
properties is the introduction of organic ligands with multiple
Since Tang and Van Slyke reported Organic electroluminescent
diodes in 1987 [1], Organic light emitting diodes have been carry-
ing out considerable research over the past three decades [2,3]. In
particular, europium complexes have been attracted attention
owing to their highly monochromatic red light and extremely
sharp emission [4]. However, the quantum efficiency of most euro-
pium complexes is unfortunately still low because the Laporte-for-
bidden 4f–4f transition prevents direct excitation of europium
luminescence [5]. Chemistry and materials scientists have realized
that it is crucial to design and synthesize organic ligands, which
have better energy transfer property to the europium ion.
To overcome the problem, organic ligands must have a high
absorption coefficient, which can sensitize the europium ion bet-
ter. Consequently, many europium complexes with the high
absorption coefficient ligands, especially b-diketone ligands, have
been designed and synthesized [6–8]. Recently, we also reported
some new europium complexes with b-diketone ligands and inves-
tigated their photoluminescence properties [9,10].
binding sites in europium complexes [11]. Therefore, to further
develop the luminescent properties of europium complexes, we
designed and synthesized a new tris-b-diketone ligand, 2-[4,
6-bis-(1-benzoyl-2-oxo-2-phenyl–ethyl)-[1,3,5]triazin-2-yl]-1,3-
diphenyl- propane-1,3-dione (H3L). With this tris-b-diketone as
the first ligand, dibenzoylmethane (DBM) as the second ligand
and 2,2-dipyridine (Bipy) or 1,10-phenanthroline (Phen) as the
third ligand, three new trinuclear europium complexes, Eu3(-
DBM)6L, Eu3(DBM)6(Bipy)3L, Eu3(DBM)6(Phen)3L were synthesized
and characterized. Their photoluminescence behaviors have also
been studied by fluorescence spectroscopy. Furthermore, based
on their photoluminescence spectra and luminescence decay
curves, the Judd–Ofelt intensity parameters (Xk), radiative (Arad),
nonradiative (Anrad), and photoluminescence quantum yield (g)
were calculated and analyzed.
2. Experimental
2.1. Materials and instrumentation
Dibenzoylmethane, 2,4,6-trichloro-1,3,5-triazine, potassium, europium oxide,
2,2-dipyridine and 1,10-phenanthroline were purchased from Shanghai Chemical
Reagent Company Ltd. (Shanghai, China). Other reagents were used as received or
were purified using standard procedures.
⇑
Corresponding author. Tel.: +86 714 6515602; fax: +86 714 6573832.
E-mail address: dunjiawang@163.com (D. Wang).
http://dx.doi.org/10.1016/j.jallcom.2014.05.222
0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

14
D. Wang et al. / Journal of Alloys and Compounds 613 (2014) 13–17
Melting points were determined on X-4 digital melting-point apparatus and
dibenzoylmethane and 2,4,6-trichloro-1,3,5-triazine by refluxing
in benzene in the presence of potassium tert-butanolate. The trinu-
clear europium complexes C1–C3 were prepared by enolization of
the tris-b-diketone and dibenzoylmethane ligands in the presence
of sodium ethoxide at first, and then complexation using Bipy or
Phen as the third ligand with europium chloride in ethanol solu-
tion according to the method of Ref. [11].
uncorrected. Infrared spectra were recorded on a Nicolet FTIR 5700 spectrophotom-
eter in KBr pellets. 1H NMR spectra were performed on an Avance III™ 300 MHz NB
Digital NMR spectrometer using TMS as internal standard and CDCl3 and DMSO-d6
as the solvent. The UV–Vis spectra were measured on a Hitachi U-3010 spectrom-
eter. Electrospray ionization mass spectroscopy (ESI-MS) was performed using a
Finnigan LCQ Advantage Max spectrometer. Fluorescence spectra and lifetimes
were recorded on a Varian Cary Eclipse fluorescence spectrometer. Elemental anal-
ysis (C, H, N) was measured using a Perkin–Elmer 2400 elemental analyzer. The per-
centage of europium (III) was determined by the complexometric titration with
EDTA.
3.2. Spectroscopic characterization
In the IR spectra, trinuclear europium complexes exhibited
noticeable changes in comparison with those of the tris-b-diketone
ligand. Their characteristic absorption peaks were summarized in
Table 1. Firstly, the IR spectra of the tris-b-diketone showed a weak
peak at 2893 cmꢀ1 due to the saturated CAH stretching vibration,
but which is nonexistent in the complexes. Secondly, In complexes,
the strong C@O stretching vibrations in the regions of 1601–
1597 cmꢀ1 were red-shifted 37–41 cmꢀ1 with respect to those of
the tris-b-diketone, and new absorption peaks were observed at
the region of 1523–1520 cmꢀ1, which ascribed to the enolic C@C
stretching vibrations of trinuclear europium complexes [12,13].
Thirdly, the medium absorptions of complexes in the region of
510–512 cmꢀ1 and 468–472 cmꢀ1, which attributed to the Eu–N
and Eu–O stretching vibrations [14,15], respectively. These results
indicated that the formation of trinuclear europium complexes
C1–C3.
2.2. Synthesis of tris-b-diketone (H3L)
2-[4,6-bis-(1-benzoyl-2-oxo-2-phenyl–ethyl)-[1,3,5]triazin-2-yl]-1,3-diphenyl-
propane-1,3-dione (H3L): The freshly cut metal potassium (1.95 g, 50 mmol) was
added in part to 40 mL of tert-butanol with stirring. The suspension was heated
to 80 °C until the potassium was completely exhausted. The excess tert-butanol
was evaporated under reduced pressure and the white residue was dissolved in
20 mL of benzene. To this mixture,
a solution of dibenzoylmethane (11.20 g,
50 mmol) in benzene (50 mL) was added and stirred at 60 °C for 4 h. Then 2,4,6-tri-
chloro-1,3,5-triazine (1.85 g, 10 mmol) was added to the reaction mixture and kept
to reflux for 8 h. The reaction was monitored by TLC using ethyl acetate/petroleum
ether (1:3) as eluent. After cooling to the room temperature, the mixture was
poured into 50 mL ice-water; then acidified with dilute hydrochloric acid and
extracted with chloroform. The organic layer was washed with a saturated NaHCO3
solution, dried over anhydrous MgSO4. After solvent removal, the product was
recrystallized from ethanol to obtain the tris-b-diketone (H3L). Yellow crystals,
yield 38%, Mp 110–112 °C; IR (KBr):
m 3058 (w), 2893 (w), 1638 (s), 1565 (s),
1502 (s), 1315 (s), 1246 (s), 1178 (m), 1091 (m), 965 (m), 878 (m), 748(s), 695(s)
cmꢀ1 1H NMR (300 MHz, CDCl3): d 1.69 (s, 3H, CH), 6.58–7.86 (m, 30H) ppm;
;
ESI–MS: m/z 748.43 [M+1]+; Anal. Calcd. for C48H33N3O6: C, 77.10; H, 4.45; N,
5.62; Found C, 77.42; H, 4.43; N, 5.65.
In the 1H NMR spectra, the keto-CH proton of the tris-b-dike-
tone exhibited a singlet of at d 1.69 ppm and the enolic proton sig-
nal was not observed, which indicated that the tris-b-diketone
ligand existed only one single tautomer as the keto form. But in
complexes C1–C3, the 1H NMR spectra did not show the presence
of the keto-CH proton signals, which attributed to the enolization
of the tris-b-diketone in its complexes. The results also confirmed
that the formation of these trinuclear europium complexes. In
addition, the proton signals of chelated Bipy or Phen were found
to shift notably to lower fields in comparison with those of the free
Bipy or Phen, which due to the electron-withdrawing inductive
effect of complexation [16].
Fig. 1 shows the UV–vis absorption spectra for the b-diketone
ligands and europium complexes in chloroform solution
(1 ꢁ 10ꢀ5 mol/L). The kmax of the europium complex C1 is
363 nm, which is red-shifted 21 and 45 nm, respectively, compared
with those of the DBM ligand (kmax = 342 nm) and tris-b-diketone
ligand (kmax = 318 nm). However, the kmax values for europium
complexes C2 and C3 are 374 and 377 nm, respectively, which
caused a large red shift in comparison with those of their b-dike-
tone ligands. This is because the introduction of the third ligand
Bipy or Phen resulted in the bigger conjugated system in com-
plexes C2 and C3. The results further indicated that the formation
of trinuclear europium complexes C1–C3.
2.3. Synthesis of trinuclear europium complexes
Eu3(DBM)6L (C1): The tris-b-diketone (H3L) (0.75 g, 1 mmol), DBM (1.35 g,
6 mmol) and sodium ethoxide (0.62 g, 9 mmol) were dissolved in 40 ml anhydrous
ethanol and stirred at 60 °C for 30 min. To this solution, an ethanol solution con-
taining 3 mmol EuCl3, which was obtained by reaction of Eu2O3 and concentrated
HCl, was added dropwise and reacted at this temperature for 12 h. The reaction
solution was cooled to the room temperature and a yellow solid was precipitated.
The precipitate was separated by suction filtration, purified by washing for several
times with ethanol and deionized water to give the trinuclear europium complex
C1. Yield: 68%. IR
1289(s), 1239(s), 1219(s), 1184(m), 1032(m), 865(m), 752(s), 696(s), 468(m)
cm–1 1H NMR (300 MHz, DMSO-d6): d 4.58 (s, 6H, C@CH), 6.21–7.93 (br, m, 90H)
m (KBr): 3073(m), 1601(s), 1554(s), 1520(s), 1499(s), 1309(s),
;
ppm; Anal. Calcd. for Eu3C138H96N3O18: C, 65.25; H, 3.81; N, 1.65; Eu, 17.95; Found
C, 65.61; H, 3.75; N, 1.68; Eu, 17.88.
Eu3(DBM)6(Bipy)3L (C2): Same procedure as for Eu3(DBM)6L (C1), but the reac-
tion mixture with H3L (0.75 g, 1 mmol), DBM (1.35 g, 6 mmol), sodium ethoxide
(0.62 g, 9 mmol), Bipy (0.47 g, 3 mmol) and 3 mmol EuCl3. The trinuclear europium
complex C2 was obtained as a yellow power, yield 63%. IR
1598(s), 1552(s), 1522(s), 1496(s), 1309(s), 1296(s), 1243(s), 1218(s), 1190(m),
1050(m), 882(m), 748(s), 699(s), 512(m), 471(m) cm–1 1H NMR (300 MHz,
DMSO-d6): d 4.66 (s, 6H, C@CH), 6.35–8.23 (br, m, 90H), 8.35 (br, 6H, Bipy–H),
9.63 (br, 6H, Bipy–H), 10.48 (d, 6H, Bipy–H, J = 7.2 Hz), 12.06 (d, 6H, Bipy–H,
J = 7.8 Hz) ppm; Anal. Calcd. for Eu3C168H120N9O18: C, 67.07; H, 4.02; N, 4.19; Eu,
15.15; Found C, 67.36; H, 3.99; N, 4.23; Eu, 15.25.
Eu3(DBM)6(Phen)3L (C3): Same procedure as for Eu3(DBM)6L (C1), but the reac-
tion mixture with H3L (0.75 g, 1 mmol), DBM (1.35 g, 6 mmol), sodium ethoxide
(0.62 g, 9 mmol), Phen (0.55 g, 3 mmol) and 1 mmol EuCl3. The trinuclear europium
complex C3 was obtained as a yellow power, yield 66%. IR
1597(s), 1549(s), 1523(s), 1497(s), 1316(s), 1293(s), 1245(s), 1227(s), 1209(s),
1195(m), 1045(m), 875(s), 798(m), 757(s), 701(s), 510(m), 472(m) cmꢀ1 1H NMR
(300 MHz, DMSO-d6): d 4.73 (s, 6H, C@CH), 6.41–8.42 (br, m, 90H), 9.05 (br, 6H,
Phen–H), 10.33 (br, 6H, Phen–H), 10.75 (br, 6H, Phen–H), 11.02 (br, 6H, Phen–H)
ppm; Anal. Calcd. for Eu3C174H120N9O18: C, 67.84; H, 3.93; N,4.09; Eu, 14.80; Found
C, 67.58; H, 3.98; N, 4.13; Eu, 14.77
m (KBr): 3067(m),
;
3.3. Photoluminescence properties
m (KBr): 3071(m),
The photoluminescence spectra of trinuclear europium com-
plexes C1–C3 in the solid state at room temperature are shown
in Fig. 2. The principal features appearing in the emission spectra
were the presence of the narrow bands in the 560–720 nm region
that were attributed to the 5D0 ? 7FJ transitions (J = 0–4) of the
europium (III) ion. The five expected Stark components were well
resolved and the hypersensitive 5D0 ? 7F2 transition was very
strong. The results indicated a highly polarizable chemical environ-
ment around the europium (III) ion, and were responsible for the
brilliant-red emission color of these complexes. The presence of
only one peak corresponding to 5D0 ? 7F0 transition around
580 nm suggested the existence of one local site of symmetry for
the europium complexes [17]. Additionally, the emission intensity
;
3. Results and discussion
3.1. Synthesis
The synthetic procedures of the tris-b-diketone (H3L) and trinu-
clear europium complexes are summarized in Scheme 1. The tris-
b-diketone was synthesized by nucleophilic substitution between

D. Wang et al. / Journal of Alloys and Compounds 613 (2014) 13–17
15
O
O
N
Cl
N
O
O
Ph
Ph
Ph
1) t-BuOK 2) H+
Benzene, 80 oC
N
N
N
Ph
O
+
O
Cl
Cl
N
O
O
Ph Ph
H3L
Ph
Ph
O
O
Ph
O
Ph
O
O
Eu
O
O
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
Ph
O
NaOC2H5 , DBM
Ph
+ EuCl3
N
N
Ph
O
O
Ethanol
O
N
O
O
N
Ph
Ph
O
O
O
Ph Ph
Eu
Eu
Ph
O
O
Ph Ph
O
O
O
Ph
Ph
C1
Ph
Ph
Ph
N
O
N
O
O
Eu
O
N
O
O
O
O
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
N
N
Ph
NaOC2H5
, DBM
Ph
+ EuCl3 + Bipy
Ph
O
O
Ethanol
N
N
Ph
O
Ph
O
O
Ph
O
N
O
O
Ph Ph
O
N
O
N
Eu
Eu
O
Ph Ph
Ph
O
O
O
N
N
Ph
O
O
C2
Ph
Ph
Ph
Ph
N
N
O
O
O
O
O
N
O
Eu
O
O
N
Ph
Ph
Ph
Ph
Ph
O
Ph
Ph
N
Ph
O
NaOC2H5
, DBM
Ph
Ph
O
+ EuCl3 + Phen
O
Ph
Ethanol
N
Ph
N
O
Ph
O
Ph
O
N
N
O
O
Ph Ph
N
O
N
Eu
Eu
O
Ph Ph
Ph
O
O
O
N
Ph
O
O
Ph
C3
Ph
Scheme 1. The synthetic routes of tris-b-diketone and europium complexes.
of complexes C2 and C3 are much stronger than that of complex C1
due to the coordination of the ‘‘antenna’’ ligands to the europium
ion, which brings about an efficient energy transfer from the
ligands to central the Eu3+ ion [18]. This is because that the third
ligands, Bipy and Phen, have high affinity toward various lantha-
nide ions, especially toward Eu3+ ion. The asymmetry of the com-

16
D. Wang et al. / Journal of Alloys and Compounds 613 (2014) 13–17
Table 1
Comparison of the characteristic IR data (cmꢀ1) of the tris-b-diketone ligand and
trinuclear europium complexes.
1.0
0.8
0.6
0.4
0.2
0.0
Compound
m(CAH)
m(C@O)
m(C@C)
m
(EuAN)
m(EuAO)
H3L
C1
C2
2893 (w)
1638 (s)
1601 (s)
1598 (s)
1597 (s)
–
–
–
–
–
–
–
1520 (s)
1522 (s)
1523 (s)
468 (m)
471 (m)
472 (m)
C2
C3
512 (m)
510 (m)
C3
3.0
2.5
2.0
1.5
1.0
0.5
DBM
H L
C31
C2
C3
C1
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Time / ms
Fig. 3. Luminescence decay curves for complexes C1–C3 in the powder, at room
temperature, monitored at 612 nm.
The lifetime, non-radiative (Anrad) and radiative (Arad) rates are
related through the equation Atot = 1/s = Arad + Anrad. The radiative
(Arad) rates were obtained by summing over the radiative rates
P
0.0
A0J for each 5D0 ? 7FJ transition (
A0J) of the europium ion. And
J
275
300
325
350
375
400
425
450
A0J is obtained according to the following equation [20]:
Wavelength / nm
I0J m01
A0J ¼ A01
ð1Þ
Fig. 1. UV–vis spectra of ligands and trinuclear europium complexes in chloroform
solution (1 ꢁ 10ꢀ5 mol/L).
I01 m0J
where I0J is the area under the emission curve related to the
5D0 ? 7FJ transitions, m0J is energy barycenter of transition. The
5D0 ? 7F1 transition does not depend on the local ligand field seen
by europium ion and, thus, may be used as a reference, the value of
A01 is estimated to be ꢂ50 sꢀ1 [5].
5D0 7F2
C1
1.0
C2
C3
0.8
According to the emission spectra and lifetimes of complexes
C1–C3, the luminescent quantum efficiency (g) for the emitting
5D0 level can be obtained from the equation [21]:
0.6
0.4
Arad
g
¼
ð2Þ
Arad þ Anrad
The calculated radiative decay rates Arad, the non-radiative
decay rates Anrad and the quantum efficiency are shown in Table
0.2
5D0 7F1
g
5D0 7F4
5D0 7F0
5D0 7F3
640 660
2. The results indicated these trinuclear europium complexes pre-
sented the higher luminescence quantum efficiency, especially
complexes C2 (g = 46.5%) and C3 (g = 47.4%), which was in agree-
0.0
560
580
600
620
680
700
720
ment with the results from their luminescence emission intensity.
From these results, the trinuclear europium complexes C2 and
C3 have much longer lifetime values and much higher quantum
efficiency as compared to their corresponding dibenzoylmethane
(DBM) mononuclear europium complexes, such as Eu(DBM)3ꢃPhen
Wavelength / nm
Fig. 2. Room-temperature emission spectra for complexes C1, C2 and C3 in solid
state, excited at 397, 401 and 402 nm, respectively.
(
s
= 0.503 ms,
0.25 ms,
= 25.2%) [10], Eu(alkoxy-DBM)3ꢃBipy
= 20–23%) [23], which due to the effect of two additional euro-
g
= 31.3%) [22], Eu(4-methoxy-DBM)3ꢃBipy
(s =
plexes C2 and C3 is increased and prevented the 4f–4f transition
ascribed to the ligands, Bipy and Phen, participating in the coordi-
nation. This is much helpful to the emission intensity of europium
complexes.
g
(s
= 0.24 ms,
g
pium (III) ion lumophors in trinuclear complexes. Therefore, the
introduction of ligands with multiple binding sites in europium
complexes could effectively improve the luminescent properties
of europium complexes.
Fig. 3 presents the luminescence decay curves of 5D0 level for
complexes C1–C3 by monitoring the emission at 612 nm at room
temperature. The lifetime values (s) (Table 2) for the emission of
these complexes were obtained from fittings of the decay curves.
It was found that the decay curves for these europium complexes
exhibited a mono-exponential behavior, which was in agreement
with one chemical environment around where the Eu3+ ion existed
[19]. The much longer lifetime values had been observed for com-
3.4. Judd–Ofelt analysis
It is very important to obtain the Judd–Ofelt intensity parame-
ters because it is possible to gain information about the chemical
environment around the europium ion. The Judd–Ofelt intensity
parameters Xt (t = 2, 4) can be calculated from the emission spec-
tra (Fig. 2) using the 5D0 ? 7F2 and 5D0 ? 7F4 electronic transitions
of the europium ion according to the following equation [4]:
plexes C2 (
s
= 0.820 ms) and C3 (s = 0.804 ms) as compared to the
complex C1 (
s
= 0.279 ms). This may be due to the introduction of
the third ligands, Bipy or Phen, resulting in the photoluminescence
stability of the trinuclear europium complex.

D. Wang et al. / Journal of Alloys and Compounds 613 (2014) 13–17
17
Table 2
The photoluminescence data of europium complexes in solid state.
Complex
s
(ms)
Atot (sꢀ1
)
Arad (sꢀ1
)
Anrad (sꢀ1
)
X2 (10ꢀ20 cm2)
X4 (10ꢀ20 cm2)
g (%)
C1
C2
C3
0.279
0.820
0.804
3584
1220
1244
536
567
590
3048
653
654
13.5
13.7
14.5
1.1
2.1
2.0
15.0
46.5
47.4
3
ꢀ
3hc A0J
tively improve the luminescence properties of europium
complexes.
Xt
¼
ð3Þ
2
ðtÞ
4e2x3
v
h D0kU k FJi
5
7
Acknowledgment
where J = 2 and 4,
v is a Lorentz local field correction that is given
2
by
v
¼ n20ðn20 þ 2Þ =9, with
a refraction index, n0 of 1.5, and
2
ð2Þ
5
7
Financial support for this work from National Natural Science
Foundation of China (No. 21273065).
h D0kU k F2i are the squared reduced matrix elements whose val-
ues are 0.0032 and 0.0023 for J = 2 and 4, respectively.
The X2 and X4 intensity parameters of trinuclear europium
complexes C1–C3 are listed in Table 2. It was found that these com-
plexes had high values of the X2 intensity parameter, which
reflected the hypersensitive character of the 5D0 ? 7F2 transition
and indicated that the europium ion is in a highly polarizable
chemical environment. On the other hand, complexes C2 and C3
exhibited the higher values for the X4 parameter as compared to
the complex C1, which suggested that steric effect prevent the
third ligands, Bipy or Phen, from getting closer to the europium
ion.
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In summary, we designed and synthesized three new trinuclear
europium complexes, Eu3(DBM)6L (C1), Eu3(DBM)6(Bipy)3L (C2)
and Eu3(DBM)6(Phen)3L (C3). Their spectroscopic behaviors were
investigated by FT-IR, 1H NMR, UV–vis and photoluminescence
spectroscopic techniques. These europium complexes exhibited
the characteristic emission lines for europium ion in solid state.
The only one peak corresponding to 5D0 ? 7F0 transition and the
mono-exponential behavior for their decay curves indicated the
presence of only one europium (III) site in the chemical environ-
ment. In addition, the X2 and X4 intensity parameters, the radia-
tive decay rate Arad, the nonradiative rates Anrad, the lifetime (
and the quantum yield ( ) were calculated according to the emis-
sion spectra and the lifetime of the 5D0 emitting level in solid state.
The trinuclear europium complexes displayed a longer lifetime (
and higher luminescence quantum efficiency ( ), especially com-
plexes C2 = 0.820 ms, = 46.5%) and C3 = 0.804 ms,
= 47.4%). These results indicated that the introduction of ligands
with multiple binding sites in europium complexes could effec-
s)
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(s
[22] H. Liang, F. Xie, Spectrochim. Acta, Part A 75 (2010) 1191.
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Galyametdinov, Russ. J. Phys. Chem. A 87 (2013) 2018.
g