Preparation and photoluminescence of some europium (III) ternary complexes with β-diketone and nitrogen heterocyclic ligands

Article abstract of DOI:10.1016/j.jallcom.2013.04.049

Preparation and photoluminescence behavior of four new europium (III) ternary complexes with b-diketones (1-(6-methoxy-naphthalen-2-yl)-3-phenyl- propane-1,3-dione (MNPPD) and 1-(4-tertbutyl- phenyl)-3-(6-methoxy-naphthalen-2- yl)-propane-1,3-dione (BPMPD)) and 2,2-dipyridine (Bipy) or 1,10-phenanthroline (Phen) were reported, in the solid state. Complexes Eu(MPPD)₃Bipy, Eu(BMPD)₃Bipy, Eu(MPPD)₃Phen and Eu(BMPD)₃Phen were characterized by elemental analysis, FT-IR, 1H NMR, UV-vis absorption. The emission spectra show narrow emission bands that arise from the ⁵D₀→ ⁷F_J (J = 0-4) transitions of the europium ion. Based on the emission spectra and luminescence decay curves in solid state, the intensity parameters (Ω τ ), lifetime (η ) and emission quantum efficiency (η ) were determined. The Ω ₂ values indicate that the Eu(III) ion in these complexes is in a highly polarizable chemical environment. Complexes Eu(MPPD)₃Bipy and Eu(MPPD)₃Phen showed a longer lifetime (s) and a higher luminescence quantum efficiency (g), which indicated that the energy transfer to the europium ion from MNPPD ligand is more efficient than that from BPMPD ligand.

Full text of DOI:10.1016/j.jallcom.2013.04.049

Journal of Alloys and Compounds 574 (2013) 54–58
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Preparation and photoluminescence of some europium (III) ternary
complexes with b-diketone and nitrogen heterocyclic ligands
⇑
Dunjia Wang , Yan Pi, Chunyang Zheng, Ling Fan, Yanjun Hu, Xianhong Wei
Hubei Key Laboratory of Pollutant Analysis and Reuse Technology, College of Chemistry and Environmental Engineering, Hubei Normal University, Huangshi 435002, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Preparation and photoluminescence behavior of four new europium (III) ternary complexes with
b-diketones (1-(6-methoxy-naphthalen-2-yl)-3-phenyl-propane-1,3-dione (MNPPD) and 1-(4-tert-
butyl-phenyl)-3-(6-methoxy-naphthalen-2-yl)-propane-1,3-dione (BPMPD)) and 2,2-dipyridine (Bipy)
or 1,10-phenanthroline (Phen) were reported, in the solid state. Complexes Eu(MPPD)3ꢀBipy,
Eu(BMPD)3ꢀBipy, Eu(MPPD)3ꢀPhen and Eu(BMPD)3ꢀPhen were characterized by elemental analysis,
FT-IR, 1H NMR, UV–vis absorption. The emission spectra show narrow emission bands that arise from
Received 14 February 2013
Received in revised form 6 April 2013
Accepted 9 April 2013
Available online 17 April 2013
Keywords:
the 5D0
decay curves in solid state, the intensity parameters (Xt), lifetime (
were determined. The X2 values indicate that the Eu(III) ion in these complexes is in a highly polarizable
chemical environment. Complexes Eu(MPPD)3ꢀBipy and Eu(MPPD)3ꢀPhen showed a longer lifetime (
and a higher luminescence quantum efficiency ( ), which indicated that the energy transfer to the euro-
pium ion from MNPPD ligand is more efficient than that from BPMPD ligand.
?
7FJ (J = 0–4) transitions of the europium ion. Based on the emission spectra and luminescence
Europium (III) ternary complex
Photoluminescence behavior
Luminescent lifetime
Quantum efficiency
Judd–Ofelt intensity parameters
s) and emission quantum efficiency (g)
s
)
g
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
naphthyl nucleus for studying the luminescent properties of their
europium complexes. In this paper, the b-diketone ligands, 1-(6-
methoxy-naphthalen-2-yl)-3-phenyl-propane-1,3-dione (MNPPD)
and 1-(4-tert-butyl-phenyl)-3-(6-methoxy-naphthalen-2-yl)-pro-
pane-1,3-dione (BPMPD), were synthesized. With these two
b-diketones as the first ligand and 2,2-dipyridine (Bipy) or 1,10-
phenanthroline (Phen) as the second ligand, four new europium
(III) ternary complexes, Eu(MPPD)3ꢀBipy, Eu(BMPD)3ꢀBipy,
Eu(MPPD)3ꢀPhen and Eu(BMPD)3ꢀPhen were prepared, character-
ized, and their fluorescent properties were studied as well. In addi-
tion, according to their fluorescence spectra and luminescence
decay curves in the solid state, the Judd–Ofelt intensity parameters
Since Kido reported the europium (III) complex-based electrolu-
minescence devices in 1991 [1], many Europium complexes with
various b-diketones as ligands have received great interest due to
their characteristic luminescence properties and extremely sharp,
intense emission in the visible region [2–4]. This kind of complexes
has promising applications in material science such as sensory
materials [5], electroluminescent materials [6], laser materials [7]
and luminescent probe [8]. However, the absorption and emission
cross section of europium complexes is small because of the for-
biddance for its 4f–4f electronic transitions. To improve their lumi-
nescence intensity, many europium complexes have been reported
in recent years [9–11]. In most cases, the luminescent europium
complexes contain a europium ion and its organic ligand as a pho-
tosensitizer. The sensitization process is that a central metal ion is
excited through the excitation of the ligand and then the excited
energy of the ligand is transferred to the central ions through the
ligand’s triplet-energy level [12,13].
(Xk), radiative (Arad), nonradiative (Anrad), and luminescent quan-
tum yield ( ) were calculated and analyzed.
g
2. Experimental
2.1. Materials and instrumentation
Further investigations into the relationship between the struc-
tures of organic ligands and the energy levels of europium ion
can obtain some evidences for designing high luminescent euro-
pium (III) complexes. Therefore, we designed and synthesized
All chemicals were reagent grade and used without further purification. Ele-
mental analysis (C, H, N) was performed using a Perkin–Elmer 2400 elemental ana-
lyzer. The percentage of europium (III) was determined by the complexometric
titration with EDTA. FT-IR spectra were recorded at 1 cmꢁ1 resolution on a Nicolet
FTIR 5700 spectrophotometer with KBr pellets. 1H NMR spectra were measured on
an Avance III™ 300 MHz NB Digital NMR spectrometer in CDCl3 solution with TMS
as internal standard. Electrospray ionization mass spectra (ESI–MS) were performed
with a Finnigan LCQ Advantage Max spectrometer. The UV–vis spectra were ob-
tained with Hitachi U-3010 spectrometer. Emission spectra and fluorescence
two larger
p-conjugated b-diketones ligands containing the
⇑
Corresponding author. Tel.: +86 714 6515602; fax: +86 714 6573832.
E-mail address: dunjiawang@163.com (D. Wang).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.04.049

D. Wang et al. / Journal of Alloys and Compounds 574 (2013) 54–58
55
lifetimes were measured on a Varian Cary Eclipse fluorescence spectrometer. Melt-
ing points were determined using X-4 digital melting-point apparatus and
uncorrected.
3. Results and discussion
3.1. Synthesis
The synthetic routes for ligands and europium (III) ternary com-
plexes are outlined in Scheme 1. The b-diketones (MNPPD and
BPMPD) were synthesized by Claisen condensation between 6-
methoxy-2-acetylnaphthalene and ethyl benzoate or ethyl 4-tert-
butylbenzoate using sodium amide as the condensing agent. The
europium (III) complexes C1–C4 were obtained by reacting the
b-diketone ligands with EuCl3ꢀ6H2O and 2,2-dipyridine or 1,10-
phenanthroline in ethanol solution according to the method of
Ref. [14].
2.2. Synthesis
1-(6-Methoxy-naphthalen-2-yl)-3-phenyl-propane-1,3-dione (MNPPD): The mix-
ture of 6-methoxy-2-acetylnaphthalene (8.01 g, 0.04 mol), ethyl benzoate (6.0 g,
0.04 mol), NaNH2 (1.95 g, 0.05 mol) and toluene (50 mL) was stirred at 80 °C for
6 h. The reaction mixture was cooled to room temperature, acidified with dilute
hydrochloric acid and then stirred until all solids dissolved. The toluene layer was
separated, washed with a saturated NaHCO3 solution and dried over anhydrous
MgSO4. After solvent removal, the solid residue was recrystallized from ethanol
and MNPPD was obtained as a pale yellow crystal in 52% yield. Mp 143–144 °C;
IR (KBr):
m 3490 (w), 3063 (w), 2975 (w), 2852 (w), 1628 (s), 1599 (m), 1541 (s),
1480 (m), 1391 (s), 1267 (s), 1234 (s), 1197 (s), 1022 (s), 857 (s), 770 (s) cmꢁ1
;
1H NMR (400 MHz, CDCl3): d 3.96 (s, 3H, OCH3), 4.70 (s, 0.14H, keto CH2), 6.99 (s,
1H, enol CH), 7.17–7.18 (m, 2H), 7.51–7.53 (m, 3H), 7.81 (d, 1H, ArAH, J = 8.4 Hz),
7.88 (d, 1H, ArAH, J = 9.6 Hz), 7.99–8.04 (m, 3H), 8.48 (s, 1H), 17.05 (br s, 1H, enol
OH) ppm; ESI–MS: m/z 305.03 [M + 1]+; Anal. Calcd. for C20H16O3: C, 78.93; H, 5.30;
Found C, 79.18; H, 5.16.
3.2. IR spectra
The IR spectra of europium (III) ternary complexes exhibited
noticeable changes as compared with those of the b-diketones.
The characteristic absorption bands of the b-diketones and their
europium (III) complexes are listed in Table 1. The IR spectra of
the free ligands MNPPD and BPMPD showed a weak absorption
at 3490 and 3473 cmꢁ1, respectively, which can be assigned as
their enolic OAH stretching vibration. But in the complexes, the
enolic OꢁA1H absorption is not existent. In the range of 1650–
1500 cm , the strong bands at 1628–1629 cmꢁ1 and 1541–
1545 cmꢁ1 due to the C@O and enolic C@C stretching vibrations
[15,16]. However, the C@O and C@C stretching frequencies of the
europium (III) complexes were red-shifted 19–38 cmꢁ1 with re-
spect to those of the corresponding b-diketone ligands, which cor-
1-(4-tert-butyl-phenyl)-3-(6-methoxy-naphthalen-2-yl)-propane-1,3-dione
(BPMPD): The synthesis was performed as for the previous compound MNPPD and
the pale yellow crystal BPMPD is obtained in 48% yield. Mp 146–147 °C; IR (KBr):
m
3473 (w), 3071 (w), 2950 (s), 2865 (w), 1629 (s), 1608 (m), 1545 (s), 1526 (m), 1481
(m), 1459 (m), 1362 (m), 1300 (m), 1269 (s), 1199 (s), 1111 (s), 1028 (s), 913 (s), 855
(s), 808 (s), 786 (s) cmꢁ1 1H NMR (400 MHz, CDCl3): d 1.37 (s, 9H, C(CH3)3), 3.96 (s,
;
3H, OCH3), 4.70 (s, 0.14H, keto CH2), 6.97 (s, 1H, enol CH), 7.17–7.23 (m, 2H), 7.53
(d, 2H, ArAH, J = 8.0 Hz), 7.82 (d, 1H, ArAH, J = 8.4 Hz), 7.88 (d, 1H, ArAH, J = 9.2 Hz),
7.96–8.01 (m, 3H), 8.47 (s, 1H), 17.10 (br s, 1H, enol OH) ppm; ESI–MS: m/z 361.06
[M + 1]+; Anal. Calcd. for C24H24O3: C, 79.97; H, 6.71; Found C, 80.45; H, 6.62.
2.3. Complexes
responded to the enlarged
p-conjugated system of complexes. In
The general procedure for preparation of europium (III) ternary complexes is
below described: To a solution of b-diketone ligand (3 mmol) and nitrogen hetero-
cyclic ligand (1 mmol) in ethanol (30 ml), which had been neutralized with 3 mmol
NaOH aqueous solution, an ethanol solution containing 1 mmol EuCl3 was added
dropwise and stirred under heating. Then, the mixtures were stirred at 60 °C for
5 h and yellow precipitate was formed. The precipitate was purified by washing
for several times with deionized water and ethanol to remove the free ligands
and salt to get europium (III) ternary complexes.
the complexes, the bands of strong intensity at 1545–1552 cmꢁ1
were attributed to C@N stretching vibrations of Bipy or Phen.
The absorption bands assigned to the coordinated EuAN and EuAO
were observed at 508–517 cmꢁ1 and 474–475 cmꢁ1 for complexes
[17,18], respectively. All these evidences indicated that the euro-
pium (III) ion coordinated to the ligand via the nitrogen atoms of
Bipy or Phen and the carbonyl oxygen atoms of the b-diketones.
Eu(MNPPD)3ꢀBipy (C1): Yellow power, yield 84%; IR
(w), 2838 (w), 1590 (s), 1552 (s), 1516 (s), 1501 (s), 1484 (s), 1452 (s), 1423 (s),
1401 (s), 1262 (s), 1198 (s), 1027 (s), 767 (s), 703 (s), 517 (m), 474 (m) cm–1 1H
m (KBr): 3057 (m), 2934
;
3.3. 1H NMR spectra
NMR (300 MHz, CDCl3): d 2.87 (s, 3H, C@CH), 3.87 (s, 9H, OCH3), 6.24 (d, 6H, ArAH,
J = 8.4 Hz), 6.55–6.59 (m, 9H, ArAH), 6.83 (s, 3H, ArAH), 6.87 (d, 3H,
ArAH, J = 8.7 Hz), 7.00–7.08 (m, 9H, ArAH), 7.31 (s, 3H, ArAH), 8.33 (d, 2H, Bipy–
H, J = 7.2 Hz), 9.66 (br, 2H, Bipy–H), 10.35 (d, 2H, Bipy–H, J = 7.5 Hz), 12.32 (d, 2H,
Bipy–H, J = 8.1 Hz) ppm; Anal. Calcd. for EuC70H53N2O9: C, 69.02; H, 4.39; N, 2.30;
Eu, 12.47; Found C, 68.89; H, 4.35; N, 2.27; Eu, 12.56.
The 1H NMR spectra of europium (III) ternary complexes show
two most obvious changes in comparison with those of the b-dike-
tone ligands. One is that the proton signals at 17.05–17.10 ppm in
the 1H NMR spectra of b-diketones, which due to the protons of
OAH in the keto-enol tautomerism of b-diketones, were not ob-
served in those of their europium complexes. The other is that
the proton signals at d = 6.97–6.99 ppm, which corresponded to
the methine protons in the keto-enol tautomerism of b-diketones,
were shifted to high-field (d = 2.87–2.93 ppm) in their europium
complexes [4,19]. In addition, the chemical shifts for aryl protons
also have some shifts toward high-field in the europium (III) com-
plexes, but those of methoxyl and tert-butyl groups have few
changes.
Eu(BPMPD)3ꢀBipy (C2): Yellow power, yield 81%; IR
2856 (m), 1591 (s), 1545 (s), 1522 (s), 1496 (s), 1438 (s), 1389 (s), 1266 (s), 1197 (s),
1030 (m), 853 (m), 784 (s), 759 (s), 508 (m), 475 (m) cm–1 1H NMR (300 MHz,
m (KBr): 3058 (m), 2961 (s),
;
CDCl3): d 1.33 (s, 27H, C(CH3)3), 2.90 (s, 3H, C@CH), 3.89 (s, 9H, OCH3), 5.86 (d,
6H, ArAH, J = 7.8 Hz), 6.07 (d, 6H, ArAH, J = 7.5 Hz), 6.28 (s, 3H, ArAH), 6.78 (d,
3H, ArAH, J = 7.8 Hz), 6.93–7.01 (m, 9H, ArAH), 8.45 (s, 3H, ArAH), 8.39 (br, 2H,
Bipy–H), 9.60 (br, 2H, Bipy–H), 10.28 (d, 2H, Bipy–H, J = 7.2 Hz), 12.05 (d, 2H,
Bipy–H, J = 8.1 Hz) ppm; Anal. Calcd. for EuC82H77N2O9: C, 71.04; H, 5.60; N, 2.02;
Eu, 10.96; Found C, 70.75; H, 5.76; N, 1.95; Eu, 11.08.
Eu(MNPPD)3ꢀPhen (C3): Yellow power, yield 83%; IR
(w), 2855 (w), 1590 (s), 1551 (s), 1516 (s), 1484 (s), 1453 (s), 1423 (s), 1402 (s),
1262 (m), 1198 (s), 1027 (m), 768 (s), 703 (m), 517 (m), 474 (m) cm–1 1H NMR
m (KBr): 3058 (m), 2941
;
(300 MHz, CDCl3): d 2.90 (s, 3H, C@CH), 3.88 (s, 9H, OCH3), 5.97 (d, 6H, ArAH,
J = 8.1 Hz), 6.18–6.25 (m, 9H, ArAH), 6.55 (s, 3H, ArAH), 6.76 (d, 3H, ArAH,
J = 8.4 Hz), 6.95–7.03 (m, 9H, ArAH), 8.08 (s, 3H, ArAH), 9.06 (br, 2H, Phen–H),
9.92 (br, 2H, Phen–H), 10.56 (br, 2H, Phen–H), 11.02 (br, 2H, Phen–H) ppm; Anal.
Calcd. for EuC72H53N2O9: C, 69.62; H, 4.30; N, 2.26; Eu, 12.23; Found C, 69.55; H,
4.32; N, 2.23; Eu, 12.33.
3.4. UV–visible spectra
The UV–vis absorption spectra for the b-diketone ligands and
their europium complexes in chloroform solution (1 ꢂ 10ꢁ5 mol/
L) was presented in Fig. 1. The UV–vis absorption spectra of com-
plexes C1 and C2, C3 and C4, were analogical because the absorp-
tion spectra of europium complexes were attributed to their
organic ligands and the absorption of europium ion was so weak
that can be ignored. In complexes C1–C4, there were a strong broad
absorption bands at 381, 380, 382 and 380 nm respectively, which
Eu(BPMPD)3ꢀPhen (C4): Yellow power, yield 80%; IR
2855 (m), 1591 (s), 1545 (s), 1521 (s), 1497 (s), 1434 (s), 1390 (s), 1266 (s), 1198 (s),
1031 (m), 843 (m), 783 (m), 730 (m), 509 (m), 474 (m) cm–1 1H NMR (300 MHz,
m (KBr): 3057 (m), 2962 (s),
;
CDCl3): d 1.31 (s, 27H, C(CH3)3), 2.93 (s, 3H, C@CH), 3.85 (s, 9H, OCH3), 5.80 (d,
6H, ArAH, J = 8.1 Hz), 6.02 (d, 6H, ArAH, J = 7.5 Hz), 6.32 (s, 3H, ArAH), 6.81 (d,
3H, ArAH, J = 7.5 Hz), 6.90–6.99 (m, 9H, ArAH), 8.47 (s, 3H, ArAH), 8.98 (br, 2H,
Phen–H), 10.48 (br, 2H, Phen–H), 10.91 (br, 2H, Phen–H), 10.93 (br, 2H, Phen–H)
ppm; Anal. Calcd. for EuC84H77N2O9: C, 71.53; H, 5.50; N, 1.99; Eu, 10.77; Found
C, 71.27; H, 5.56; N, 1.95; Eu, 10.83.
due to the
p–
pꢃ transitions in the conjugated ring system. In

56
D. Wang et al. / Journal of Alloys and Compounds 574 (2013) 54–58
O
O
COOC2H5
COCH3
1) NaNH2, 2) H+
+
H3CO
R
H3CO
Toluene, 80 o
C
R
MNPPD: R=H, BPMPD: R=C(CH3)3
N
N
O
O
O
Eu
NaOH
Ethanol
O
+ EuCl3 + Bipy
H3CO
R
H3CO
R
3
C1 : R = H , C2 : R = C(CH3)3
O
O
N
N
O
Eu
NaOH
Ethanol
+ EuCl3 + Phen
O
H3CO
R
H3CO
R
3
C3 : R = H , C4 : R = C(CH3)3
Scheme 1. The synthetic routes of b-diketones and their complexes.
Table 1
The characteristic IR bands (cmꢁ1) of the free ligands and the complexes.
Compounds
m
(OAH)
m
(C@O)
m
(C@N)
m
(C@C)
m
(EuAN)
m
(EuAO)
MNPPD
BPMPD
C1
C2
C3
3490 (w)
3473 (w)
–
–
–
–
1628 (s)
1629 (s)
1590 (s)
1591 (s)
1590 (s)
1591 (s)
–
–
1541 (s)
1545 (s)
1516 (s)
1522 (s)
1516 (s)
1521 (s)
–
–
–
–
1552 (s)
1545 (s)
1551 (s)
1545 (s)
517 (m)
508 (m)
517 (m)
509 (m)
474 (m)
475 (m)
474 (m)
474 (m)
C4
comparison with their corresponding b-diketone ligands, the main
absorption bands of the europium complexes shifted to longer
wavelengths, which indicated that the larger conjugated chelate
rings were formed in complexes C1–C4.
their emission spectra, all europium complexes exhibited charac-
teristic emission lines for Eu3+ 580 nm (5D0 ? 7F0), 591 nm
:
(5D0 ? 7F1), 612 nm (5D0 ? 7F2), 653 nm (5D0 ? 7F3), and 703 nm
(5D0 ? 7F4) [20]. The emissions bands at about 580 nm and
653 nm in these spectra were very weak, which assigned to the for-
bidden transition for 5D0 ? 7F0 and 5D0 ? 7F3 both in magnetic and
electric dipole schemes. As well-known, the 5D0 ? 7F1 transition is
a parity-allowed magnetic dipole transition and is nonsensitive to
the local structure environment, while the 5D0 ? 7F2 transition is
3.5. Fluorescence emission spectra
The room temperature emission spectra of europium (III) ter-
nary complexes C1–C4 in the solid state were shown in Fig. 2. From
3
MNPPD
250
200
BPMPD
C1
C2
C3
150
2
1
0
C4
5D0 7F2
100
50
0
C1
C2
5D0 7F1
5D0 7F4
5D0 7F3
C3
5D0 7F0
C4
560 580 600 620 640 660 680 700 720
250
300
350
400
450
Wavelength / nm
Wavelength / nm
Fig. 1. UV–vis absorption spectra of ligands and europium (III) ternary complexes
in chloroform solution (1 ꢂ 10ꢁ5 mol/L).
Fig. 2. Room-temperature emission spectra for complexes C1–C4 in the solid state
excited around 400, 402, 399 and 403 nm, respectively.

D. Wang et al. / Journal of Alloys and Compounds 574 (2013) 54–58
57
I0J m01
I01 m0J
700
600
500
400
300
200
100
0
A0J ¼ A01
ð2Þ
C1
C2
C3
C4
where I01 and I0J are the integrated intensities of the 5D0 ? 7F1 and
5D0 ? 7FJ transitions (J = 2 and 4) with
m01 and m0J being the respec-
tive energy barycenters of these transitions. The 5D0 ? 7F1 transi-
tion 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 esti-
mated to be ꢄ50 sꢁ1 [26].
The non-radiative decay rates Anrad can be obtained from the
calculated Arad rates and the experimental decay rates by the fol-
lowing equation [25]:
1
s
¼ Arad þ Anrad
ð3Þ
The obtained radiative decay rates Arad, the non-radiative decay
0.0
0.2
0.4
0.6
0.8
1.0
1.2
rates Anrad and the luminescence quantum efficiency are also pre-
sented in Table 2. From Table 2, complexes C1 and C3 exhibited a
longer lifetime and a higher luminescence quantum efficiency
g
Time / ms
Fig. 3. Luminescence decay curves for complexes C1–C4 in the powder, at room
temperature, monitored at 613, 612, 612 and 612 nm, respectively.
s
g,
which indicated that the energy transfer to the europium ion from
MNPPD ligand is more efficient than that from BPMPD ligand. In
order to further confirm the efficiency of the energy transfer, we
investigated the luminescence decay curves of the free ligands
and the excitation spectra of ligands and complexes in the solid
state (Figs. 4 and 5). From Fig. 4, the free ligands lifetime values
were measured to be 0.012 ms for MNPPD and 0.011 ms for BPMPD
by fitting with mono-exponential. Fig. 5 shows that the excitation
intensities of MNPPD ligand and its complexes C1, C3 were stron-
ger than those of MNPPD ligand and its complexes C2, C4, respec-
tively. Consequently, the excitation energy can be more efficiently
transferred to the europium ion in the Eu-MNPPD complexes com-
pared to the Eu-BPMPD complexes. According to the molecular
structures of ligands [27,28], the main reason might be that the
tert-butyl moiety in complexes C2 and C4 is bad for the formation
of their conjugated system. It is obviously that the increase of the
conjugation length in the b-diketone ligand can improve the radi-
ative properties of the europium (III) complex. This is because the
triplet energy level of ligand gradually decreased with increasing of
the conjugate system, the energy level of MNPPD ligand perfectly
matches the 5D0 energy level of the europium ion and its com-
plexes (C1 and C3) display the longer lifetime and the higher lumi-
nescence quantum efficiency.
an induced electric dipole transition and is a very sensitive to the
condition environment [21]. A prominent feature that may be ob-
served in these spectra is the very high intensity of 5D0 ? 7F2 tran-
sition, which indicated that
a highly polarizable chemical
environment around the europium ions of these complexes was
responsible for the brilliant red emission. As a result, the strong
coordination interactions took place between the organic groups
and the Eu3+ ion in complexes C1–C4. In addition, the very efficient
energy transfer also occurred from the ligands to the central euro-
pium ion in these Eu(III) ternary complexes, which due to the coor-
dination of the ‘‘antenna’’ ligands to the europium ions.
3.6. Luminescence decay time and emission quantum efficiency
In order to further study the photoluminescence properties of
these Eu(III) ternary complexes in solid state, the room tempera-
ture luminescence decay curves of the 5D0 excited state were mea-
sured by monitoring the most intense emission lines (5D0 ? 7F2) at
about 612 nm and the decay curves of complexes C1–C4 were gi-
ven in Fig. 3. The results showed that the luminescence decay pro-
files displayed mono-exponential behavior, indicating the presence
of only one site symmetry from the europium ion [22]. The lifetime
3.7. Judd–Ofelt intensity parameters
values (s) for the emission of complexes C1–C4 are listed in Table 2,
which were obtained from the decay curves for these complexes by
In the standard theory of 4f–4f intensities, the Xt intensity
parameters contain the contributions from the forced electric di-
pole and dynamic coupling mechanisms [29]. In order to investi-
gate the possible structural changes around the emitting center
europium ion among these europium (III) ternary complexes, the
Judd–Ofelt intensity parameters Xt (t = 2, 4) can be estimated from
the emission spectra (Fig. 2) based on the 5D0 ? 7F2 and 5D0 ? 7F4
electronic transitions of the Eu3+ ion and they are calculated
according to the following equation [30]:
fitting with a single exponential.
The luminescence quantum efficiency (g
) for the emitting 5D0
level in complexes C1–C4 at room temperature was obtained
according to their emission spectra and emission decay curves,
which were determined by the following equation [21,23]:
Arad
Arad þ Anrad
g
¼
ð1Þ
where Arad and Anrad are the radiative and non-radiative transition
rates, respectively. The radiative rate Arad can be calculated by sum-
ming over the radiative rates A0J for each 5D0 ? 7FJ transition (RJ
A0J). And A0J is obtained according to the following relation [24,25]:
3ꢀhc3A0J
Xt
¼
ð4Þ
2
ðtÞ
4e2x3
vh D0jU j FJi
5
7
Table 2
The photoluminescence data of complexes in solid state.
Complexes
s
(ms)
Atot (sꢁ1
)
Arad (sꢁ1
)
Anrad (sꢁ1
)
X2 (10ꢁ20 cm2)
X4 (10ꢁ20 cm2)
g (%)
C1
C2
C3
C4
0.365
0.306
0.373
0.314
2740
3268
2681
3185
747
692
693
603
1993
2576
1988
2582
18.3
17.1
16.8
14.2
2.8
2.0
2.4
2.2
27.3
21.2
25.8
18.9

58
D. Wang et al. / Journal of Alloys and Compounds 574 (2013) 54–58
300
250
200
150
100
50
emission lines for europium (III) ion: 580 nm (5D0 ? 7F0), 591 nm
(5D0 ? 7F1), 612 nm (5D0 ? 7F2), 653 nm (5D0 ? 7F3), and 703 nm
(5D0 ? 7F4). The results of lifetime decay curves and the emission
spectra indicated that all complexes had relative long luminescent
lifetime and high quantum efficiency. However, complexes
Eu(MPPD)3ꢀBipy and Eu(MPPD)3ꢀPhen showed a longer lifetime
(s) and a higher luminescence quantum efficiency (g), which
indicated that the energy transfer to the europium ion from
MNPPD ligand is more efficient than that from BPMPD ligand.
The Judd–Ofelt intensity parameters, X2 and X4 were obtained
from the emission spectra in the solid state. The higher value of
the X2 parameters for the Eu(III) complex indicated the europium
ion was located in a more polarizable chemical environment and
suggested a metal–ligand bond with higher covalent character.
0
MNPPD
BPMPD
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Time / ms
Acknowledgment
Fig. 4. The emission decay curves for ligands MNPPD and BPMPD in the powder, at
room temperature, monitored at 436 and 432 nm, respectively.
Financial support for this work from National Natural Science
Foundation of China (No. 21273065).
450
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400
BPMPD
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In this work, four new europium (III) ternary complexes,
Eu(MPPD)3ꢀBipy,
Eu(BMPD)3ꢀBipy,
Eu(MPPD)3ꢀPhen
and
Eu(BMPD)3ꢀPhen were prepared and characterized by elemental
analysis, FT-IR, 1H NMR, UV–vis absorption and photoluminescent
spectroscopy. All europium complexes exhibited the characteristic