Synthesis and photoluminescent properties of Eu (III) complexes with fluorinated β-diketone and nitrogen heterocyclic ligands

Article abstract of DOI:10.1016/j.dyepig.2016.05.026

Two fluorinated β-diketones and their six europium (III) complexes using 2,2-dipyridine, 1,10-phenanthroline or 4′-phenyl-terpyridine as the secondary ligand were synthesized and characterized. The photoluminescence behavior for these complexes was investigated in solid state in detail. Based on the emission spectra and luminescence decay curves of these complexes in solid state, the Judd-Ofelt intensity parameters (Ω_t), lifetime (τ) and luminescence quantum efficiency (η) were determined. The Ω₂ values indicate that the europium (III) ion is in a highly polarizable chemical environment in these complexes. Europium (III) complexes with the secondary ligands 2,2-dipyridine or 1,10-phenanthroline exhibited much better photoluminescence properties than complexes with the secondary ligand 4′-phenyl-terpyridine. Especially, europium (III) complexes of 4,4,4-trifluoro-1-(4-fluorophenyl)-butane-1,3-dione containing the secondary ligands 2,2-dipyridine or 1,10-phenanthroline showed a longer lifetime (τ = 0.799 and 0.826 ms, respectively) and a higher luminescence quantum efficiency (η = 56.5 and 56.1, respectively) among these complexes.

Full text of DOI:10.1016/j.dyepig.2016.05.026

Accepted Manuscript
Synthesis and photoluminescent properties of Eu (III) complexes with fluorinated β-
diketone and nitrogen heterocyclic ligands
Dan Wang, Zheng Luo, Zhao Liu, Dunjia Wang, Ling Fan, Guodong Yin
PII:
S0143-7208(16)30216-9
10.1016/j.dyepig.2016.05.026
DYPI 5258
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 14 March 2016
Revised Date: 14 May 2016
Accepted Date: 17 May 2016
Please cite this article as: Wang D, Luo Z, Liu Z, Wang D, Fan L, Yin G, Synthesis and
photoluminescent properties of Eu (III) complexes with fluorinated β-diketone and nitrogen heterocyclic
ligands, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.05.026.
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ACCEPTED MANUSCRIPT
Graphical Abstract
4
4
50
00
C1: Eu(TMPD) Bipy
3
C2: Eu(TFPD) Bipy
350
3
C3: Eu(TMPD) Phen
3
3
2
2
1
1
5
0
00
50
00
50
00
0
C4: Eu(TFPD) Phen
3
C5: Eu(TMPD) Phterpy
3
C6: Eu(TFPD) Phterpy
3
O
O
F
F F
R
(
TMPD : R = -OCH , TFPD : R = -F)
3
C1
C2
C3
C4
C5
C6
5
60
580
600
620
640
660
680
700
720
Wavelength / nm

ACCEPTED MANUSCRIPT
Synthesis and photoluminescent properties of Eu (III) complexes with
fluorinated β-diketone and nitrogen heterocyclic ligands
Dan Wang, Zheng Luo, Zhao Liu, Dunjia Wang*, Ling Fan, Guodong Yin
College of Chemistry and Chemical Engineering, Hubei Collaborative Innovation
Center for Rare Metal Chemistry, Hubei Key Laboratory of Pollutant Analysis and
Reuse Technology, Hubei Normal University, Huangshi 435002, China
Received…
Abstract: Two fluorinated β-diketones and their six europium (III) complexes using
2
,2-dipyridine, 1,10-phenanthroline or 4′-phenyl-terpyridine as the secondary ligand
were synthesized and characterized. The photoluminescence behavior for these
complexes was investigated in solid state in detail. Based on the emission spectra
and luminescence decay curves of these complexes in solid state, the Judd−Ofelt
intensity parameters (Ωt), lifetime (τ) and luminescence quantum efficiency (η)
were determined. The Ω2 values indicate that the europium (III) ion is in a highly
polarizable chemical environment in these complexes. Europium (III) complexes
with the secondary ligands 2,2-dipyridine or 1,10-phenanthroline exhibited much
better photoluminescence properties than complexes with the secondary ligand
4
4
′-phenyl-terpyridine.
Especially,
europium
(III)
complexes
of
,4,4-trifluoro-1-(4-fluorophenyl)-butane-1,3-dione containing the secondary
ligands 2,2-dipyridine or 1,10-phenanthroline showed a longer lifetime (τ = 0.799
1

ACCEPTED MANUSCRIPT
and 0.826 ms, respectively) and a higher luminescence quantum efficiency (
and 56.1, respectively) among these complexes.
η = 56.5
Keywords: Europium (III) complex, fluorinated β-diketone, heterocyclic nitrogen
donors, photoluminescence property, quantum efficiency, Judd−Ofelt
intensity parameters.
_
*
___________________
Corresponding author. Tel.: +86 714 6515602; fax: +86 714 6573832
E-mail address: dunjiawang@163.com (Dunjia Wang)
2

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1
. Introduction
β-Diketones have a high complexing ability due to their O,O-donor ligands,
which are widely used in coordination with transition metal ions [1]. Especially,
their europium (III) complexes have been of great attention to the chemists because
of their high fluorescence emission efficiency, long fluorescence lifetime and
extremely sharp emission bands [2−4]. Many applications have been found in laser
materials, chemical sensor, fluorescent probes, photoluminescent materials,
magnetic molecular materials and organic light-emitting diodes [5−7]. Recently, the
design, synthesis and optical properties of many novel europium (III) complexes
with β-diketones have been reported [8–10]. In addition, some reports demonstrated
that the β-diketones containing high-energy oscillators, such as C−H and O−H
bonds, could quench the metal excited states nonradiatively, thereby resulting in
lower luminescence intensities and shorter excited-state lifetimes [11,12]. Thus, it is
very important to replace C−H bonds with C−F bonds in the synthesis and design of
new β-diketone ligands in order to improve the luminescent properties of europium
(III) complexes.
Therefore, with further investigations into the relationship between the
structure of organic ligands and the luminescent behavior of europium (III)
complexes, we designed and synthesized the fluorinated β-diketones with
4
-methoxyphenyl and 4-fluorophenyl moieties to investigate the spectroscopic and
3

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photoluminescence properties of their europium complexes. Herein, the fluorinated
β-diketone ligands, 4,4,4-trifluoro-1-(4-methoxyphenyl)-butane-1,3-dione (TMPD)
and 4,4,4-trifluoro-1-(4-fluorophenyl)-butane-1,3-dione (TFPD), were synthesized
by the Claisen condensation. With these two fluorinated β-diketones as the first
ligand and 2,2-dipyridine (Bipy), 1,10-phenanthroline (Phen) or 4′-phenyl-
terpyridine (Phterpy) as the second ligand, six new europium (III) ternary complexes,
Eu(TMPD) ⋅Bipy,
Eu(TFPD) ⋅Bipy,
Eu(TMPD) ⋅Phen,
Eu(TFPD) ⋅Phen,
3
3
3
3
Eu(TMPD) ⋅Phterpy and Eu(TFPD) ⋅Phterpy, were prepared, characterized, and
3
3
their photoluminescence behavior were studied as well. Meanwhile, the Judd–Ofelt
intensity parameters (Ωλ), radiative (A ), nonradiative (A ), and luminescent
rad
nrad
quantum yield (
η) were calculated and analyzed according to their
photoluminescence spectra and luminescence decay curves in the solid state.
2
. Experimental
2
.1. Apparatus and Chemicals
−1
FT-IR spectra were recorded in KBr pellets at 1 cm resolution on a Nicolet
FTIR 5700 spectrophotometer. Melting points were measured using X-4 digital
melting-point apparatus and uncorrected. Elemental analysis (C, H, N) was
performed on a Perkin−Elmer 2400 elemental analyzer. The contents of europium
1
(
III) were determined by the complexometric titration with EDTA. H NMR spectra
4

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TM
were measured on an Avance III 300 MHz NB Digital NMR spectrometer in
CDCl or DMSO-d solution with TMS as internal standard. Electrospray ionization
3
6
mass spectra (ESI−MS) were carried out on a Finnigan LCQ Advantage Max
spectrometer. The UV-vis spectra were measured on a Hitachi U-3010 spectrometer.
The fluorescence excitation, emission spectra and fluorescence lifetimes were
obtained on a Varian Cary Eclipse fluorescence spectrometer.
4
-Methoxyacetophenone, 4-fluoroacetophenone, ethyl trifluoroacetate, sodium
methoxide, europium(III) oxide, 2,2-dipyridine and 1,10-phenanthroline were
purchased from Sun Chemical Technology (Shanghai) Co., Ltd. 4′-Phenyl-
terpyridine (Phterpy) was synthesized by our group in the literature [13]. Europium
trichloride was obtained by dissolving Eu O in concentrated hydrochloric acid.
2
3
Other Reagents used were of analytical grade, and were used without further
purification.
2
.2. Synthesis of fluorinated
β-diketone ligands
4
,4,4-Trifluoro-1-(4-methoxyphenyl)-butane-1,3-dione (TMPD): 4-Methoxy-
acetophenone (3.0g, 20 mmol) and sodium methoxide (2.16 g, 40 mmol) were added
into 80 ml dry benzene, and the mixture was stirred for 15min. To this mixture ethyl
trifluoroacetate (5ml, 42 mmol) was added dropwise, and then stirred at 50 °C for 8h.
The resulting mixture was cooled to the room temperature and acidified with dilute
5

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hydrochloric acid. The organic layer was collected, washed with a saturated
NaHCO solution and dried over anhydrous MgSO . The organic solvent was
3
4
removed by rotary evaporation. The residue was recrystallized with ethanol to give
o
the light yellow crystal TMPD in yield 70%. Mp 56−57 C; IR (KBr): ν 3452 (b, m),
3
059 (m), 2972(w), 2850 (m), 1602 (s), 1509 (s), 1450 (m), 1313 (m), 1271 (s), 1256
(
(
s), 1195 (s), 1170 (s), 1140 (s), 1110 (s), 1069 (m), 1020 (m), 844 (s), 794 (s), 698
−1 1
m) cm ; H NMR (300 MHz, CDCl ): δ 3.88 (s, 3H, OCH3), 6.51 (s, 1H, enol CH),
3
6
.99 (d, 2H, Ar−H, J = 8.1 Hz), 7.94 (d, 2H, Ar−H, J = 8.4 Hz), 15.43 (brs, 1H,
+
enol OH); ppm; ESI−MS: m/z 246.30 [M] . Anal. Calcd. for C H O F : C, 53.67; H,
11
9
3 3
3
.68; Found C, 53.85; H, 3.66.
,4,4-Trifluoro-1-(4-fluorophenyl)-butane-1,3-dione (TFPD): The synthesis
4
was performed as for the previous compound TMPD and the colorless crystal
o
TFPD was obtained in 67% yield. Mp 42−43 C; IR (KBr): ν 3442 (b, m), 3077 (m),
1
600 (s), 1510 (s), 1432 (m), 1275 (s), 1159 (s), 1122 (s), 1103 (s), 1063 (s), 1013
−1 1
(
m), 917 (m), 859 (s), 799 (s), 695 (s) cm ; H NMR (300 MHz, CDCl ): δ 6.54 (s,
3
1
1
H, enol CH), 7.21 (t, 2H, Ar−H, J = 8.2 Hz), 7.99 (dd, 2H, Ar−H, J =5.6, 8.0 Hz),
+
5.12 (brs, 1H, enol OH); ppm; ESI−MS: m/z 234.02 [M] ; Anal. Calcd. for
C H O F : C, 51.30; H, 2.58; Found C, 51.53; H, 2.57.
1
0
6
2 4
2
.3. Preparation of europium (III) ternary complexes (C1−C6)
The procedure for preparation of all these europium (III) complexes is below
6

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described: The fluorinated β-diketone ligand (3 mmol), nitrogen heterocyclic ligand
(1 mmol) and NaOH (0.12 g, 3 mmol) were dissolved in 30 ml ethanol and stirred at
5
0 °C for 15min. To this an ethanolic solution containing 1 mmol EuCl was added
3
dropwise and the mixture was stirred at 60 ºC for 5h. The resulting mixture was
cooled to the room temperature and the light yellow solid was precipitated. The
precipitate was purified by washing for several times with deionized water and
ethanol to remove the free ligands and salt to give europium (III) ternary complexes
(
C1−C6).
o
Eu(TMPD) ⋅Bipy (C1): Light yellow power, yield 79%, mp 179−181 C; IR ν
3
(KBr): 3046(w), 2966(w), 2842(w), 1599(s), 1548(s), 1505(s), 1466(s), 1439(s),
1
293(s), 1263(s), 1246(s), 1175(s), 1139(s), 1029 (s), 942 (m), 846 (m), 791 (s), 762
–
1
1
(
m), 690 (m), 576(m), 472(m) cm ; H NMR (300 MHz, CDCl ): δ 3.03 (s, 3H,
3
C=CH), 3.93 (s, 9H, OCH ), 6.82 (d, 6H, Ar−H, J = 7.7Hz), 7.46 (d, 6H, Ar−H, J =
3
7
.8Hz), 8.54 (d, 2H, Bipy−H, J = 6.0Hz), 9.83 (br, 2H, Bipy−H), 10.76 (d, 2H,
Bipy−H, J = 6.4Hz), 12.44 (br, 2H, Bipy−H) ppm. Anal. Calcd. for EuC H N O F :
4
3
32
2
9 9
C, 49.49; H, 3.09; N, 2.68; Eu, 14.56; Found C, 49.23; H, 3.07; N, 2.69; Eu, 14.61.
o
Eu(TFPD) ⋅Bipy (C2): Light yellow power, yield 81%, mp 192−194 C; IR ν
3
(KBr): 3051(m), 1592(s), 1544(s), 1502(s), 1476(m), 1308(s), 1286(s), 1238(s),
1
5
186(s), 1161(s), 1133(s), 1064(m), 1014(m), 851(m), 790(s), 758(m), 693 (m),
–
1 1
76(m), 474(m) cm ; H NMR (300 MHz, CDCl ): δ 3.26 (s, 3H, C=CH), 7.00 (t,
3
7

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6
6
H, Ar−H, J = 8.3Hz), 7.40 (t, 6H, Ar−H, J = 6.7Hz), 8.52 (d, 2H, Bipy−H, J =
.7Hz), 9.77 (t, 2H, Bipy−H, J = 7.4Hz), 10.47 (d, 2H, Bipy−H, J = 7.7Hz), 12.78
(
br, 2H, Bipy−H) ppm. Anal. Calcd. for EuC H N O F : C, 47.68; H, 2.30; N,
40
23
2
6 12
2
.78; Eu, 15.08; Found C, 47.92; H, 2.29; N, 2.75; Eu, 14.96.
Eu(TMPD) ⋅Phen (C3): Light yellow power, yield 80%, mp 204−206 C; IR ν
o
3
(KBr): 3072(w), 2998(w), 2842(w), 1599(s), 1543(s), 1504(s), 1466(m), 1295(s),
1
5
9
2
263(s), 1178(s), 1142(s), 1028(m), 943(m), 844(m), 789(s), 729(m), 692 (m),
–
1 1
77(m), 473(m) cm ; H NMR (300 MHz, CDCl ): δ 3.06 (s, 3H, C=CH), 3.91 (s,
3
H, OCH ), 6.79 (d, 6H, Ar−H, J = 8.0Hz), 7.37 (d, 6H, Ar−H, J = 8.1Hz), 8.53 (d,
3
H, Phen−H, J = 7.6Hz), 8.90 (br, 2H, Phen−H), 10.1 (s, 2H, Phen−H), 10.59 (d, 2H,
Phen−H, J = 7.8Hz) ppm. Anal. Calcd. for EuC H N O F : C, 50.62; H, 3.02; N,
4
5
32
2
9 9
2
.62; Eu, 14.23; Found C, 50.84; H, 3.04; N, 2.65; Eu, 14.17.
Eu(TFPD) ⋅Phen (C4): Light yellow power, yield 82%, mp 190−192 C; IR ν
o
3
(KBr): 3036(w), 1595(s), 1545(s), 1502(s), 1312(s), 1289(s), 1239(m), 1189(m),
–
1
1
160(s), 1143(s), 1036(m), 845(m), 791(s), 726(m), 690 (m), 577(m), 474(m) cm ;
1
H NMR (300 MHz, CDCl ): δ 3.27 (s, 3H, C=CH), 6.96 (br, 6H, Ar−H), 7.28 (br,
3
6
H, Ar−H), 8.49 (br, 2H, Phen−H), 9.10 (br, 2H, Phen−H), 9.92 (br, 2H, Phen−H),
0.49 (br, 2H, Phen−H) ppm. Anal. Calcd. for EuC H N O F : C, 48.90; H, 2.25;
1
42
23
2
6 12
N, 2.72; Eu, 14.73; Found C, 49.15; H, 2.23; N, 2.75; Eu, 14.79.
8

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o
Eu(TMPD) ⋅Phterpy (C5): Light yellow power, yield 73%, mp 202−204 C; IR
3
ν (KBr): 3078(w), 2948 (w), 2841 (w), 1598 (s), 1537 (s), 1496 (s), 1471(s), 1406
(
m), 1296 (s), 1253 (s), 1178 (s), 1134 (s), 1024(s), 943 (m), 843 (m), 789 (s),
–
1 1
6
3
90(m), 574(m), 471(m) cm ; H NMR (300 MHz, CDCl ): δ 3.02 (s, 3H, C=CH),
3
.80 (s, 9H, OCH ), 6.56 (br, 6H, Ar−H), 6.90 (br, 6H, Ar−H), 7.56 (br, 3H,
3
Terpy−H), 7.72 (br, 2H, Terpy−H), 7.88 (br, 2H, Terpy−H), 8.07 (br, 2H, Terpy−H),
.82 (br, 2H, Terpy−H), 10.37 (br, 2H, Terpy−H), 12.09 (br, 2H, Terpy−H) ppm.
Anal. Calcd. for EuC H N O F : C, 54.19; H, 3.28; N, 3.51; Eu, 12.70; Found C,
8
5
4
39
3
9 9
5
4.49; H, 3.24; N, 3.49; Eu, 12.76.
Eu(TFPD) ⋅Phterpy (C6): Light yellow power, yield 72%, mp 214−216 C; IR
o
3
ν (KBr): 3073(m), 1596 (s), 1538 (s), 1492 (s), 1409(m), 1299 (s), 1237 (s), 1146 (s),
–
1
1
1
011(m), 944 (m), 845 (m), 789 (s), 573(m), 468(m) cm ; H NMR (300 MHz,
CDCl ): δ 3.28 (s, 3H, C=CH), 6.73 (br, 6H, Ar−H), 6.91 (br, 6H, Ar−H), 7.53 (br,
3
3
2
H, Terpy−H), 7.69 (br, 4H, Terpy−H), 7.83 (d, 2H, Terpy−H, J = 7.5Hz), 8.67 (d,
H, Terpy−H, J = 7.6Hz), 10.39 (d, 2H, Terpy−H, J = 7.0Hz), 12.03 (br, 2H,
Terpy−H) ppm. Anal. Calcd. for EuC H N O F : C, 52.77; H, 2.61; N, 3.62; Eu,
51
30
3
6 12
1
3
3
3.09; Found C, 52.97; H, 2.56; N, 3.65; Eu, 12.98.
. Results and discussion
.1. Synthesis
9

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The synthetic routes for fluorinated β-diketone ligands and their europium (III)
complexes are outlined in Scheme 1. The fluorinated β-diketones (TMPD and TFPD)
were synthesized by Claisen condensation between 4-Methoxyacetophenone or
4
-fluoroacetophenone and ethyl trifluoroacetate using sodium methoxide as the
condensing agent in benzene. Their europium (III) ternary complexes C1−C6 were
prepared by the reaction of the fluorinated β-diketone ligands with Europium
chloride
’-phenyl-terpyridine (Phterpy) in ethanol solution according to the method of the
literature [14].
and
2,2-dipyridine
(Bipy),
1,10-phenanthroline
(Phen)
or
4
<
Scheme 1>
3
.2. IR spectra
The characteristic infrared absorption bands of the free fluorinated β-diketones
and their europium (III) ternary complexes are listed in Table 1. In the fluorinated
β-diketones, the IR spectra of the uncoordinated ligands TMPD and TFPD showed
−1
a broad absorption at 3452 and 3442 cm , respectively, which can be attributed to
−1
their enolic O−H stretching vibration. The strong bands at 1602−1600 cm and
−1
1
510−1509 cm were due to their C=O and enolic C=C stretching vibrations. But in
the complexes C1−C6, their enolic O−H absorption of the IR spectra was absence.
1
0

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−1
−1
The strong absorption bands appeared at 1599–1592 cm and 1505–1492 cm
assigned to their C=O and enolic C=C stretching vibrations, and red-shifted 3–18
−1
cm with respect to those of the corresponding β-diketone ligands. The results
suggested that the coordination bands were formed between the fluorinated
−1
β-diketones and europium (III) ion. Meanwhile, the strong bands at 1537−1548 cm
in complexes were attributed to C=N stretching vibrations of the nitrogen
heterocyclic ligands (Bipy, Phen or Phterpy). The new absorption bands at 577−573
−1
−1
cm and 468−474 cm were assigned to the stretching vibrations of the coordinated
Eu−N and Eu−O, respectively [15,16]. These evidences further confirmed the
conformation of europium (III) ternary complexes with the fluorinated β-diketones
and nitrogen heterocyclic ligands.
<
Table 1>
1
3
.3. H NMR spectra
1
The H NMR spectra of europium (III) ternary complexes exhibited three most
obvious changes in comparison with those of the β-diketone and nitrogen
heterocyclic ligands. The first one is that the enolic proton signals at δ =
1
5.43−15.12 ppm in the β-diketones were not observed in those of their europium
11

ACCEPTED MANUSCRIPT
complexes. The second is that the methine proton signals at δ = 6.54−6.51 ppm in
the keto-enol tautomerism of β-diketones were shifted to high-field at δ = 3.28−3.02
ppm in their europium complexes, which was due to the closeness for these protons
binding site of the β-diketones and the paramagnetic europium ion [4,17]. The third
is that the aryl proton signals of the nitrogen heterocyclic ligands in complexes
exhibited a larger downfield shift compared with those of the corresponding free
ligands [18,19], which was attributed to the electron-withdrawing effect of
complexation. Meanwhile, the chemical shifts for aryl protons of the β-diketones
had some shifts toward high-field in the europium (III) complexes, but those of
methoxyl group had few changes. Here, we chose the complex C1 as an example to
1
discuss their chemical shifts. The H NMR spectrum of the complex C1 was
presented in Fig. 1. The Fig. 1 showed two broad two-proton singlet peaks at δ 12.44
and 9.83 ppm, which were due to H H and H H , respectively. The two
1
1’
3
3’
two-proton doublet (J = 6.4Hz, 6.0Hz) peaks at δ 10.76 and 8.54 ppm was assigned
to H H and H H , respectively. Proton H H and H H both exhibited a
4
4’
2
2’
6
6’
5
5’
six-proton doublet (J = 7.8Hz, 7.7Hz) peaks at δ 7.46 and 6.82 ppm, respectively.
The nine-proton singlet peak at δ 3.93 ppm was attributable to the H of the
8
methoxyl group and the three-proton singlet peak at δ 3.03 ppm was corresponding
to the methine protons H7.
1
2

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<
Figure 1>
3
.4. UV-visible spectra
The UV−vis absorption spectra for the ligands and their europium
−5
corresponding complexes in chloroform solution (1 × 10 mol/L) was shown in Fig.
2
. For complexes C1−C6, two main absorption bands at 332−340 nm and 266−276
nm were observed, which were due to the characteristic π–π* enol absorption of the
β-diketone ligands and the π–π* absorption of the nitrogen donor heterocyclic
ligands, respectively. Compared with their corresponding β-diketone ligands, the
absorption bands at 332−340 nm in complexes were red-shifted 6−10 nm, which
was attributed to the formation of the large conjugated chelate rings. The absorption
bands at 266−276 nm in complexes were blue shifted 6−14 nm in comparison with
the corresponding nitrogen heterocyclic ligands, caused by the perturbation induced
by the metal coordination. These results suggested that all ligands had coordinated
to europium (III) ion.
<
Figure 2>
3
.5. Photoluminescence properties
The excitation spectra of complexes C1−C6 in solid state at room temperature
1
3

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were shown in Fig. 3, which was obtained by monitoring the emission at the
5
7
hypersensitive D → F transition at 612 nm. These excitation spectra exhibited a
0
2
weak band at 263−272 nm and a strong broad band at 366−378 nm, corresponding
to the absorber of the nitrogen donor heterocyclic ligands and fluorinated
β-diketones, respectively. The results suggested that the nitrogen donor and
β-diketone ligands were both binding to the europium (III) ion, and their presence
together afforded a strong sensitization of the emission. In addition, their excitation
spectra also present narrow lines at 464nm, assigned to the europium (III) ion
7
5
transition for F → D [20].
0
2
<
Figure 3>
The photoluminescence spectra of complexes C1−C6 in the solid state under
excitation at 368, 370, 369, 366, 378 and 370 nm, respectively, were presented in
Fig. 4. From the emission spectra, all europium (III) complexes exhibited
5
7
characteristic sharp peaks in the region of 580–710nm associated with the D → F
0
J
5
(
J = 0–4) transitions of the europium (III) ion. The five expected peaks of the D →
0
7
F0−4 transitions are well resolved, and their emissions have only one sharp peak in
5
7
the region of the D → F transition, suggesting that the europium (III) ion occupies
0
0
1
4

ACCEPTED MANUSCRIPT
only a single site and a single chemical environment exists around it [21]. The
5
7
D → F transition was split into two or three components in their emission spectra,
0
1
indicating that the environment of the center europium (III) ion was asymmetric.
5
7
Compared to other transitions, the very high intensity of D → F transition may be
0
2
observed in these spectra, pointing to a highly polarizable chemical environment
around the europium ions, which was responsible for the brilliant red emission of
these complexes. It was also observed that the emission intensities of complexes
C1−C4 were much stronger than those of complex C5 and C6, which was attributed
to the high affinity of the second ligands, Bipy and Phen, toward various lanthanide
ions, especially toward europium (III) ion.
<
Figure 4>
To further understand the photoluminescence behavior of these europium (III)
5
complexes in solid state, the luminescence decay curves of the D excited state in
0
room temperature were determined by monitoring the most intense emission lines
5
7
5
(
D → F ) at about 612 nm. Fig. 5 presented the luminescence decay curves of D
0
2
0
level for complexes C1–C6 by monitoring the emission of 612, 613, 612, 612, 612
and 611 nm, respectively. The luminescence decay curves of all europium (III)
complexes fitted a first-order exponential decay law, indicating the presence of a
1
5

ACCEPTED MANUSCRIPT
single chemical environment around the europium ion in complexes, which was in
5
7
accord with the results of only one D → F line in their emission spectra. The
0
0
5
lifetime values (
τ) of the emitting D level are summarized in Table 2, which were
0
obtained from the luminescence decay curves of complexes C1−C6.
<
Figure 5>
<
Table 2>
5
In Table 2, the lifetime (
τ) of the emitting state D , nonradiative (A ),
0
nrad
radiative (A ) and tot-radiative rates (A ) can be related through the following
rad
tot
equation [3]:
1
=
Arad + A = Atot
nrad
(1)
τ
The radiative (A ) rates were obtained by summing over the radiative rates A
rad
0J
5
7
for each D → F transition (Σ A ) of the europium (III) ion and A is calculated
0
J
J
0J
0J
according to the following relation [3].
I0J ν 01
A = A
0
J
01
(2)
I01 ν 0J
5
7
where I is the integrated intensities of the D → F transitions (J = 1, 2 and 4) and
0
J
0
J
1
6

ACCEPTED MANUSCRIPT
5
7
ν0J is energy barycenters of these transitions. The magnetic dipole allowed D → F
0
1
transition was taken as the reference for the whole spectrum and the value of A is
0
1
−1
estimated to be ~50 s [22].
5
The luminescent quantum efficiency (η) for the emitting D level in complexes
0
C1−C6 can be calculated by the following equation [23].
Arad
η =
(3)
A + A
rad
nrad
The calculated photoluminescence data of complexes C1−C6 (A , A , A and
rad
nrad
tot
η
) are also presented in Table 2. It is clear that complexes C1−C4 containing the
secondary ligands Bipy and Phen exhibited a longer lifetime ( ) and a higher
luminescence quantum efficiency ( ), which was in agreement with the results from
τ
η
the excitation and emission intensity of complexes C1−C6. This could be due to the
introduction of the ligands Bipy and Phen, which were useful ligands with high
affinity toward europium (III) ion. The asymmetry of complexes C1−C4 was
increased and prevented the 4f–4f transition attributed to the secondary ligands Bipy
or Phen participating in the coordination [24]. This is beneficial to the
photoluminescence properties of complexes C1−C4. In addition, complexes C1, C3
and C5 based on the β-diketone TMPD displayed the shorter lifetime (
τ) and lower
quantum efficiency ( ) in comparison with the corresponding complexes C2, C4
η
1
7

ACCEPTED MANUSCRIPT
and C6 based on the β-diketone TFPD as ligand. This is because the β-diketone
TMPD contains high-energy oscillators C−H bonds of methoxyl group in center
benzene ring [12], which are able to quench the europium (III) ion excited states
nonradiatively, thereby leads to shorter lifetime and lower quantum efficiency of
complexes C1, C3 and C5. Consequently, complexes C2 and C4 with the
β-diketone TFPD and the secondary ligands Bipy or Phen have longer lifetime (
τ
=
=
0
5
3
.799 and 0.826 ms, respectively) and higher luminescence quantum efficiency (
6.5 and 56.1, respectively).
η
.6. Judd−Ofelt intensity parameters
The the Judd−Ofelt intensity parameters, Ω2 and Ω4, are sensitive to the
changes in the chemical environment around the europium (III) ion. To further
investigate the possible structural changes around the emitting center europium ion
in complexes C1−C6, the experimental intensity parameters Ωt (t = 2, 4) can be
5
7
5
7
calculated from the emission spectra based on the D → F and D → F electronic
0
2
0
4
transitions of the europium (III) ion according to the following equation [25]:
3
3
hc A0
J
Ω =
t
2
(4)
2
3
5
(t) 7
4
e
ω χ D U
FJ
0
where
ω is the angular frequency of the transition, A , where J = 2 and 4, are the
0
J
5
7
coefficient of spontaneous emission for the D → F transition,
χ is the Lorentz
0
J
1
8

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2
0
2
0
2
local-field correction term given by
χ = n (n + 2) / 9 , n is the refractive index of the
5
(2)
7
2
medium (in this case n = 1.5), the squared matrix elements are 〈 D U
F2 〉
=
0
5
(4)
7
2
0
.0032 and 〈 D U
F4 〉 = 0.0023.
0
The the Judd−Ofelt intensity parameters Ω2 and Ω4 are given in Table 2. It was
found that complexes C2 and C4 exhibited higher values of the Ω2 parameters,
which indicated a more polarizable chemical environment in their complexes and
suggested a higher covalent character between the europium (III) ion and ligands.
Meanwhile, complexes C5 and C6 with the Phterpy ligand both revealed a higher
value for the Ω4 parameter, which was due to the perturbation on the coordination
effect of the bidentate β-diketones by the steric factors from the Phterpy ligand [26].
4
. Conclusions
The six new europium (III) complexes, Eu(TMPD) ⋅Bipy (C1),
3
Eu(TFPD) ⋅Bipy (C2), Eu(TMPD) ⋅Phen (C3), Eu(TFPD) ⋅Phen (C4),
3
3
3
Eu(TMPD) ⋅Phterpy (C5) and Eu(TFPD) ⋅Phterpy (C6), were synthesized and
3
3
characterized, and their photophysical properties were investigated in detail. Their
characteristics emission lines of europium (III) ion in emission spectra evidently
exhibited that europium (III) ion was presented in a polarizable chemical
environment acting as only one luminescent centre. The Judd−Ofelt intensity
parameters, Ω2 and Ω4 were determined from their emission spectra in solid state.
1
9

ACCEPTED MANUSCRIPT
Complexes C2 and C4 displayed higher values of the Ω2 parameters, suggested a
higher covalent character between the europium (III) ion and ligands. The higher Ω4
values for Complexes C5 and C6 indicated a perturbation on the coordination effect
by the steric factors from the Phterpy ligand. Meanwhile, complexes C1−C4 with
the secondary ligands Bipy or Phen exhibited much better photoluminescence
properties than complexes C5 and C6 with the secondary ligand Phterpy, which was
assigned to the steric effect preventing the ligand Phterpy from getting closer to the
europium (III) ion. In particular, complexes C2 and C4 showed a longer lifetime (
0.799 and 0.826 ms, respectively) and a higher luminescence quantum efficiency
= 56.5 and 56.1, respectively). These results indicated that the europium (III)
τ
=
(
η
complexes with the fluorinated β-diketone could effectively improve their
photoluminescence properties.
Acknowledgment
The authors would like to acknowledge the support from National Natural
Science Foundation of China (No. 21542009) and the Educational Commission of
Hubei Province, PR China (No. B2015132).
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2
5

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Tables:
−1
Table 1 The characteristic IR peaks (cm ) of β-diketones and their complexes
Compound ν (O−H) ν (C=O) ν (C=N) ν (C=C) ν (Eu−N) ν (Eu−O)
TMPD
TFPD
C1
3452 (m) 1602 (s)
3442 (m) 1600 (s)


1509 (s)
1510 (s)










1599 (s) 1548 (s) 1505 (s) 576 (m) 472 (m)
1592 (s) 1544 (s) 1502 (s) 576 (m) 474 (m)
1599 (s) 1543 (s) 1504 (s) 577 (m) 473 (m)
1595 (s) 1545 (s) 1502 (s) 577 (m) 474 (m)
1598 (s) 1537 (s) 1496(s) 574 (m) 471 (m)
1596 (s) 1538 (s) 1492 (s) 573 (m) 468 (m)
C2
C3
C4
C5
C6
Table 2 The photoluminescence data of europium (III) complexes in solid state
−1
−1
−1
−20
−20
Complex
τ
(ms) A (s ) A (s ) A (s ) Ω2(10
Ω4(10
η (%)
tot
rad
nrad
2
2
cm )
cm )
C1
C2
C3
C4
C5
C6
0.702
1424
1252
1350
1211
4065
2890
609
815
14.53
2.06
42.8
56.5
48.1
56.1
13.0
21.9
0.799
0.741
0.826
0.246
0.346
708
650
679
530
634
544
700
17.50
15.84
16.62
12.11
14.94
2.18
1.96
2.02
2.44
2.59
532
3535
2256
2
6

ACCEPTED MANUSCRIPT
Schemes:
N
O
N
Eu
Bipy
O
F
F
F
O
3
R
+
CF COOEt
3
C1 : R = -OCH , C2 : R = -F
3
R
Benzene,
NaOCH3
N
O
N
O
O
Eu
F
NaOH
Phen
O
+
EuCl3
F
F
Ethanol
F
R
F F
3
R
(
TMPD : R = -OCH , TFPD : R = -F)
3
C3 : R = -OCH , C4 : R = -F
3
N
N
N
Eu
Phenylterpy
O
O
F
F F
3
R
C5 : R = -OCH , C6 : R = -F
3
Scheme 1 The synthetic routes of fluorinated β-diketones and their complexes
2
7

ACCEPTED MANUSCRIPT
Figure Captions:
1
Fig. 1. H NMR (300 MHz) spectrum of the complex C1 in CDCl .
3
Fig. 2. UV−vis absorption spectra of ligands TMPD, TFPD, Bipy, Phen, Phterpy
−5
and complexes C1−C6 in chloroform solution (1 × 10 mol/L).
Fig. 3. Excitation spectra of complexes C1−C6 in solid state at room
temperature monitoring at 612 nm.
Fig. 4. Emission spectra of complexes C1−C6 in solid state at room temperature,
under excitation at 368, 370, 369, 366, 378 and 370 nm, respectively.
Fig. 5. Luminescence decay curves for complexes C1−C6 in the powder at room
temperature monitoring at 612, 613, 612, 612, 612 and 611 nm, respectively.