Synthesis, characterization and luminescence property of ternary rare earth complexes with azatriphenylenes as highly efficient sensitizers

Article abstract of DOI:10.1016/j.jphotochem.2011.02.026

A series of novel ternary rare earth complexes showing attractive luminescence properties were prepared with both neutral and anion ligands. Three azatriphenylene neutral ligands, 1,10-phenanthroline (phen), dipyrido [3,2-a: 2′,3′-c] quinoxaline (dpq) and dipyrido [3,2-a: 2′,3′-c] phenazine (dppz) were investigated systematically as sensitizers in the ternary rare earth complexes. Benzoate ions (BA^-) and phenoxyacetate ions (POA^-) were chosen as anion ligands. The compositions of these complexes were characterized by elemental analysis, rare earth coordination titration, molar conductivity measurement, IR spectroscopy, UV-vis absorption spectroscopy, ¹H NMR spectroscopy and TGA-DTA. The luminescence spectra, luminescence decay time and quantum efficiency of the complexes were also studied. The very strong luminescence intensities and rather long luminescence lifetimes (typically > 1.0 ms) were achieved for both Eu ³⁺ and Tb³⁺ ternary complexes with phen and dpq as neutral ligands. Moreover, high quantum efficiencies (40-60%) were also obtained for Eu³⁺ ternary complexes with phen and dpq as neutral ligands.

Full text of DOI:10.1016/j.jphotochem.2011.02.026

Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 243–249
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Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, characterization and luminescence property of ternary rare earth
complexes with azatriphenylenes as highly efficient sensitizers
Hui-Juan Suna, Xiao-Tao Fua, Hai-Bin Chua,b,∗, Yan Dua, Xue-Mei Lina, Xin Lia, Yong-Liang Zhaoa,∗∗
a College of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China
b SINOPEC Beijing Research Institute of Chemical Industry, Beijing 100013, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of novel ternary rare earth complexes showing attractive luminescence properties were pre-
pared with both neutral and anion ligands. Three azatriphenylene neutral ligands, 1,10-phenanthroline
(phen), dipyrido [3,2-a: 2ꢀ,3ꢀ-c] quinoxaline (dpq) and dipyrido [3,2-a: 2ꢀ,3ꢀ-c] phenazine (dppz) were
investigated systematically as sensitizers in the ternary rare earth complexes. Benzoate ions (BA−) and
phenoxyacetate ions (POA−) were chosen as anion ligands. The compositions of these complexes were
characterized by elemental analysis, rare earth coordination titration, molar conductivity measurement,
IR spectroscopy, UV–vis absorption spectroscopy, 1H NMR spectroscopy and TGA–DTA. The luminescence
spectra, luminescence decay time and quantum efficiency of the complexes were also studied. The very
strong luminescence intensities and rather long luminescence lifetimes (typically > 1.0 ms) were achieved
for both Eu3+ and Tb3+ ternary complexes with phen and dpq as neutral ligands. Moreover, high quan-
tum efficiencies (40–60%) were also obtained for Eu3+ ternary complexes with phen and dpq as neutral
ligands.
Received 20 October 2010
Received in revised form 18 February 2011
Accepted 25 February 2011
Available online 3 March 2011
Keywords:
Azatriphenylene
Carboxylate
Rare earth complex
Luminescence intensity
Quantum efficiency
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
solvent molecules are typically evolved in the complexes, resulting
in low luminescent efficiencies of rare earth ions. The introduc-
Rare earth compounds have attracted much attention because
of their superior luminescent properties, such as long lumines-
cence lifetimes, large Stokes shift and sharp emission profile, and
their applications in the fields of catalysis, information technique,
display, lighting, nanoscience, agriculture and military [1–4]. How-
ever, the luminescence intensities of rare earth ions are relatively
weak because f–f electronic transitions of these ions are forbidden
by the parity rule.
Direct coordination of the antenna ligands with rare earth ions
can effectively sensitize the central ions by ligand-to-metal energy
transfer. Both anion ligands and neutral ligands have been recog-
nized widely as effective antenna ligands towards rare earth ions.
First, anion ligands, such as ␤-diketones and aromatic carboxylic
acids, not only can form stable complexes with rare earth ions, but
also are able to efficiently receive light and transfer energy to rare
earth ions [5]. Nonetheless, because of the electrostatic repulsion
between anion ligands, anion ligands alone could not meet the high
coordination number requirement of rare earth ions. Consequently,
tion of certain neutral ligands, such as terphenyl-based compounds,
azatriphenylenes and pyridine derivatives, can increase the lumi-
nous efficiency of the rare earth complexes. The reason is that
the neutral ligands can replace the solvent molecules to coordi-
nate with the central ions as well as eliminate energy loss caused
by thermal vibration of solvent molecules [5,6]. Also, they can
improve the rigidity and stability of the complex structures, and
increase the absorption of light energy and energy transfer effi-
ciency so as to enhance the luminous intensity of the complexes
[7,8]. However, rare earth complexes with neutral ligands alone
typically exhibit poor luminescence quenched by anions such as
hydroxyl ions. Besides, the coordination environment as well as
crystal structure of the complexes changes significantly when both
anion and neutral ligands are involved. In addition, intramolecular
energy transfer process between the two ligands would improve
the luminescence efficiency of rare earth complexes. Hence, the
design and synthesis of new ternary complexes with both anion
and neutral ligands have theoretical significance and application
value for excellent luminescent rare earth materials.
The complexes of rare earth ions with aromatic carboxyl
acids have optimal luminescence. Phenanthroline (phen) and 2,2ꢀ-
bipyridine (bipy) are widely used as neutral ligands in ternary
complexes [9–12]. In order to get rare earth complexes with strong
luminescence, good thermodynamic stability and high quantum
efficiency, two neutral azatriphenylene ligands, dipyrido [3,2-a:
∗
Corresponding author at: College of Chemistry and Chemical Engineering, Inner
Mongolia University, Hohhot 010021, China. Tel.: +86 13624849905.
∗∗
Corresponding author. Tel.: +86 13624849905; fax: +86 471 4992984.
E-mail addresses: binghai99@gmail.com (H.-B. Chu), hxzhaoyl@163.com
(Y.-L. Zhao).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.02.026

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H.-J. Sun et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 243–249
2ꢀ,3ꢀ-c] quinoxaline (dpq) and dipyrido [3,2-a: 2ꢀ,3ꢀ-c] phenazine
(dppz), were synthesized and a series of novel rare earth ternary
complexes were prepared with benzoate ions (BA−) or phenoxy-
acetate ions (POA−) as anion ligands and azatriphenylenes as
neutral ligands. Importantly, rather strong luminescence intensi-
ties and long lifetimes have been obtained for both Eu3+ and Tb3+
ternary complexes with phen and dpq as neutral ligands. The high
quantum efficiencies have also been achieved for Eu3+ ternary com-
plexes with phen and dpq as neutral ligands. Moreover, all these
complexes were all non-electrolytes and stable at the room tem-
perature.
250–251 ◦C (literature [13] value: 250 ◦C). 1H NMR (400M-CDCl3),
ppm 9.650–9.625 (q, 2H), 9.278–9.263 (q, 2H), 8.363–8.338 (q, 2H),
7.937–7.912 (q, 2H), 7.812–7.781 (q, 2H); Anal. Found for C18H10N4:
C, 76.23; H, 3.71; N, 19.59; Calcd. C, 76.58; H, 3.57; N, 19.85.
2.3. Preparation of rare earth chloride solution
The solution of rare earth chloride was prepared according to
literature [18]. Rare earth chloride was obtained by the reaction
of rare earth oxide with dilute solution of HCl, heating dissolved,
evaporated, and then dissolved in anhydrous ethanol.
2.4. Synthesis of RE(C6H5COO)3L (RE3+ = Eu3+, Tb3+; L = phen, dpq,
dppz)
2. Materials and methods
2.1. Materials and general methods
0.50 mmol neutral ligand phen was dissolved in absolute
ethanol, and then aqueous C6H5COONa (0.2162 g, 1.5 mmol) was
added. A few minutes later, 0.5 mmol EuCl3 was dropwise added
into this mixture with continuous stirring under constant temper-
ature of 70 ◦C. The pH was readjusted to 6.8 with aqua ammonia.
The resulting mixture was stirred at 70 ◦C for 3 h. The mixture was
filtered and the solid was washed twice with absolute ethanol
and ether, respectively. After dried at 50 ◦C for several hours,
the complex Eu(C6H5COO)3phen was obtained. In the similar
way, Eu(C6H5COO)3dpq, Eu(C6H5COO)3dppz, Tb(C6H5COO)3phen,
Tb(C6H5COO)3dpq and Tb(C6H5COO)3dppz were prepared.
Purities of rare earth oxides are 99.99%. Other reactants and
solvents are all of analytical grade if not mentioned. Elemental
analysis was performed with a PE-2400 elemental analyzer. The
molar concentration of rare earth was determined by EDTA titra-
tion method with xylenol orange as indicator. A Nicolet Nexus
670 FT-IR spectrometer using KBr pellet was used to obtain the
IR spectra in the range of 4000–400 cm−1. Molar conductivity was
measured by DDSJ-308A conductivity meter at room temperature,
with N,N-dimethylformamide (DMF) as solvent and the concentra-
tion of complexes was 1.0 × 10−3 mol L−1. The thermo gravimetric
analysis (TGA) and differential thermal analysis (DTA) were per-
formed by a DTG-50 instrument with a heating rate of 10 ◦C min−1
under a N2 atmosphere. Absorption spectra were recorded on a Tu-
1901 spectrophotometer, using absolute ethanol as solvent and the
concentration of the complexes was 1.0 × 10−5 mol L−1. A Bruker
400 MHz NMR spectrometer was used for the acquisition of 1H
NMR spectra in CDCl3 solution. The excitation, emission spectra
and luminescence decay time of the obtained complexes were
measured using an Edinburgh Instruments FLS920 spectrometer
at room temperature. The excitation slit and emission slit widths
were all 0.1 nm.
2.5. Synthesis of Eu0.5Ln0.5(C6H5COO)3dpq (Ln3+ = La3+, Gd3+
)
The synthetic procedure for Eu0.5La0.5(C6H5COO)3(dpq) was
the same as above mentioned, except that an equimolar
amount of EuCl3 and LaCl3 were added together. Similar to
Eu0.5La0.5(C6H5COO)3(dpq), Eu0.5Gd0.5(C6H5COO)3(dpq) was syn-
thesized.
2.6. Synthesis of RE(POA)3L·1/2H2O (RE3+ = Eu3+, Tb3+; L = phen,
dpq, dppz)
HPOA (0.2267 g, 1.5 mmol) was dissolved in 5 ml absolute
ethanol and aqueous NaOH (1.5 mmol) was added to read-
just the pH to 7. Then 0.5 mmol absolute ethanol solution of
the second ligand phen was added to this mixture with stir-
ring. A few minutes later, 0.5 mmol EuCl3 was added dropwise
into the mixture with continuous stirring under constant tem-
perature of 70 ◦C. Then Eu(POA)3phen·1/2H2O was obtained
similar to the above-mentioned method. Using the same meth-
ods, complexes Eu(POA)3dpq·1/2H2O, Eu(POA)3dppz·1/2H2O,
2.2. Synthesis of dpq and dppz
Dpq and dppz were synthesized according to literatures
[13–17]. Synthesis routes of the ligands are as follows:
O
O
H2SO4, HNO3, NaBr
reflux,1h
N
N
N
N
1,10-phenanthroline-5,6-dione
Tb(POA)3phen·1/2H2O,
Tb(POA)3dpq·1/2H2O
and
Tb(POA)3dppz·1/2H2O were got.
O
O
N
N
N
N
N2
3. Results and discussion
H2NCH2CH2NH2
+
70° , reflux
3.1. Composition analysis and molar conductivity
N
O
N
dpq
The theoretical data and the measured data of complexes
are shown in Table 1. The results of elemental analyses (C,
H, N) and rare earth EDTA titration indicate that the compo-
O
NH2
N
N
N
N
CH3CN, N2
70° , reflux
+
sition of the complexes are RE(C6H5COO)3L (RE3+ = Eu3+, Tb3+
L = phen, dpq, dppz), Eu0.5Ln0.5(C6H5COO)3dpq (Ln3+ = Gd3+, La3+
;
)
NH2
N
N
and RE(POA)3Lꢀ·1/2H2O (RE3+ = Eu3+, Tb3+; Lꢀ = phen, dpq, dppz).
Colors of complexes are white and all the complexes are stable in
the air. The complexes are easily dissolved in dimethylformamide
(DMF) and dimethyl sulfoxide (DMSO) but difficult to dissolve in
water, ethanol and acetone.
dppz
The relevant values of dpq product were consistent with litera-
ture values, mp: 299 ◦C (literature [16] value: 297–299 ◦C). 1H NMR
(400M-CDCl3), ppm 9.506–9.481 (q, 2H), 9.303–9.288 (q, 2H), 8.989
(s, 2H), 7.822–7.791 (q, 2H); Anal. Found for C14H8N4: C, 72.07; H,
3.87; N, 24.02; Calcd. C, 72.40; H, 3.47; N, 24.13. The relevant val-
ues of dppz product were consistent with literature values, mp:
The small molar conductance values of rare earth complexes
in DMF (Table 1) indicate that only a small fraction of rare earth

H.-J. Sun et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 243–249
245
Fig. 1. IR spectra of some ligands and complexes: a, dpq; b, dppz; c, NaBA; d, HPOA; e, Tb(BA)3dppz; f, Eu0.5Ga0.5(BA)3dpq; g, Eu(POA)3dpq1/2H2O; h, Tb(POA)3dppz1/2H2O.
Table 1
Composition analysis (%) and molar conductivities (S cm2 mol−1) (25 ◦C) of the rare earth complexes.
Complex
C%
H%
N%
RE%
ꢀm (S cm2 mol−1
)
EuL3phen
TbL3phen
EuL3dpq
TbL3dpq
EuL3dppz
TbL3dppz
Eu0.5Gd0.5L3dpq
Eu0.5La0.5L3dpq
EuLꢀ3phen·1/2H2O
TbLꢀ3phen·1/2H2O
EuLꢀ3dpq·1/2H2O
TbLꢀ3dpq·1/2H2O
EuLꢀ3dppz·1/2H2O
TbLꢀ3dppz·1/2H2O
56.77(56.98)
56.19(56.42)
55.87(56.23)
55.37(55.71)
58.58(58.72)
58.05(58.22)
55.69(56.03)
56.52(56.72)
54.17(54.41)
53.79(53.94)
53.53(53.90)
53.29(53.47)
55.97(56.25)
55.47(55.82)
3.528(3.333)
3.292(3.300)
2.806(3.101)
3.358(3.073)
3.463(3.159)
3.336(3.132)
3.357(3.090)
3.396(3.129)
4.225(3.806)
3.325(3.773)
3.501(3.572)
3.627(3.543)
3.775(3.597)
3.612(3.570)
3.831(4.028)
3.792(3.989)
7.256(7.496)
7.331(7.428)
7.135(7.026)
7.321(6.965)
7.312(7.470)
7.472(7.562)
3.376(3.526)
3.317(3.496)
6.411(6.619)
6.143(6.566)
6.549(6.250)
6.188(6.202)
21.92(21.85)
22.65(22.62)
20.54(20.33)
20.93(21.06)
18.98(19.06)
19.84(19.75)
20.73(20.61)
19.45(19.63)
19.02(19.13)
19.67(19.82)
18.06(17.95)
18.47(18.62)
17.04(16.95)
17.46(17.59)
18.5
20.7
18.2
17.6
16.2
15.4
16.8
15.8
20.3
18.4
19.2
16.5
18.4
16.5
Remark: the values in brackets are theoretical values; L = BA−, Lꢀ = POA−
.
1411–1418 cm−1 in the complexes, respectively. New bands occur-
ring in the region 411–424 cm−1 could be attributed to the RE-O
absorption peak, which indicated that the carboxyl group of the
benzoate coordinated with RE(III) [21]. For ligand dpq, the ꢁ(C N) at
1391 cm−1 shifted to the vicinity of 1408 cm−1 and overlapped with
the ꢁs bands of carboxyl in the complexes, indicating that the rare
complexes ionize in DMF solution and these complexes are non-
electrolytes [19,20].
3.2. IR Spectra
The IR spectra of ligands phen, dpq, dppz, HPOA, NaBA and
all the obtained complexes were determined in the range of
4000–400 cm−1, some of which are given in Fig. 1.
earth ions coordinated with dpq [22,23]. The ꢁ(C
at 1616 cm−1
N)
of ligand dppz shifted to 1606 cm−1 in the complexes, which
indicated that the rare earth ions coordinated with dppz [24,25].
Spectra analysis of phenoxyacetate series of rare earth com-
plexes: The bands at 1737 cm−1 and 1094 cm−1 could be attributed
Spectra analysis of benzoate series rare earth complexes:
IR spectra of ligand NaBA showed ꢁas at 1552 cm−1 and
ꢁs at 1412 cm−1
,
which shifted to 1530–1542 cm−1 and
to the ꢁ(C
vibration and ı(OH) rocking vibration of ligand HPOA,
O)
which disappeared after the formation of the complexes. Also, new
Fig. 2. UV–vis spectra of the ligands and complex: a, dpq; b, HPOA; c,
Eu(POA)3dpq1/2H2O. The concentrations are 1.0 × 10−5 mol L−1, 3.0 × 10−5 mol L−1
,
1.0 × 10−5mol L−1, respectively.
Fig. 3. TG–DTA curves of Tb(POA)3dppz·1/2H2O in N2 atmosphere.

246
H.-J. Sun et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 243–249
peaks emerged at 1426 cm−1 and 1632–1635 cm−1, which could be
assigned to the symmetric and antisymmetric stretching vibrations
of the carboxylate group, respectively. These changes indicated that
the carboxyl coordinated with the rare earth ions in the complexes.
After introducing the neutral ligands, the ꢁ(C
at 1391 cm−1 of
N)
dpq shifted to the vicinity of 1398 cm−1 in the complexes, indicat-
ing that the rare earth ions coordinated with dpq [22,23]. The ꢁ(C
N)
of ligand dppz at 1616 cm−1 shifted to 1592 cm−1 in the complexes
which revealed the rare earth ions coordinated with dppz [24,25].
3.3. UV–vis absorption spectra
The UV–vis absorption spectra of ligands phen, dpq, dppz, HPOA,
NaBA as well as all complexes were determined in comparison with
absolute ethanol as reference. As shown in Fig. 2, the absorption
peak of phenoxyacetate at 219 nm shifted to 221 nm in the com-
plex Eu(POA)3dpq·1/2H2O, which indicated that the rare earth ions
coordinated with phenoxyacetate. The absorption peak of dpq at
251 nm shifted to 252 nm in the complexes, which indicated that
dpq was involved in the coordination. The data further confirmed
the formation of ternary rare earth complexes.
Fig. 4. Photoluminescence spectra of complexes: a, Eu(BA)3phen (ꢀex = 348 nm); b,
Eu(BA)3dpq (ꢀex = 359 nm); c, Eu(BA)3dppz (ꢀex = 378 nm).
3.4. TGA–DTA
As shown in Fig. 3, the weight loss of the complex
Tb(POA)3dppz·1/2H2O around the endothermic peak at 288.5 ◦C is
0.95%, which could be assigned to the theoretical content (0.99%) of
the complexes containing 0.5 water molecules. Moreover, it should
be coordinated water because of the relative high temperature. The
complex begins to decompose at about 300 ◦C with obvious weight
loss. The weight loss of 40.31% is consistent with the theoretical
value (40.97%) of the complex that loses one dppz and two carboxyl
groups. The aromatic carboxylate ligand was relatively thermally
stable [26], thus the weight loss of complex does not stop at 900 ◦C,
indicating that decomposition of the complex has not yet finished.
3.5. Chemical structure
Fig. 5. Photoluminescence spectra of complexes: d, Tb(BA)3phen (ꢀex = 347 nm);
e, Tb(BA)3dpq (ꢀex = 355 nm); f, Tb(POA)3phen·1/2H2O (ꢀex = 356 nm); g, Tb(POA)3
dpq·1/2H2O (ꢀex = 346 nm).
Most of the rare earth complexes with phen and carboxy-
late have a dimeric structure [RE2(phen)2(carboxylate)6] [27–33],
while some of the related complexes having substituted ben-
given by Carnall et al. [35]. The excitation spectra of the complexes
are essentially the same. They all contain a broadband absorption in
250–400 nm, but the strongest absorption peaks are different (see
Table 2). The emission spectra of all complexes show a characteris-
tic transitions of RE3+ [36,37]. The representative emission spectra
of Eu3+, Tb3+ complexes are presented in Figs. 4 and 5.
zoate ligand have
a monomeric structure. The four ternary
complexes Eu(BA)3phen, Tb(BA)3phen, Eu(POA)3phen·1/2H2O and
Tb(POA)3phen·1/2H2O have a dimeric structure as identified by X-
ray analysis [9,11,12,34]. However, chemical structures of other
complexes with dpq or dppq are still under investigation.
All europium complexes emit the characteristic luminescence of
7
3.6. Luminescence spectra
Eu3+, of which the 5D0 → F1 belongs to magnetic dipole transition,
appearing at nearly 591 nm. 5D0 → F2 belongs to the electric dipole
7
Table 2 gives the data of the excitation and emission spectra of
the complexes in the solid state at room temperature. The spectral
peak identification is based on the data of the free-ion energy levels
transition, appearing in the vicinity of 613 nm. It can be seen from
Table 2 that the intensities of the electric dipole transitions are
stronger than those of the magnetic dipole transitions.
Table 2
Luminescent spectra data of the complexes.
Complex
ꢀex (nm)
5D0
→
7F1
5D0
→
7F2
Complex
ꢀex (nm)
5D4
→
7F6
5D4
→
7F5
ꢀem (nm)
I0 (a. u)
ꢀem (nm)
I0 (a. u)
ꢀem (nm)
I0 (a. u)
1465
6943
68
2205
4962
101
ꢀem (nm)
I0 (a. u)
EuL3phen
EuL3dpq
EuL3dppz
Eu0.5Gd0.5L3dpq
Eu0.5La0.5L3dpq
EuLꢀ3phen 1/2H2O
EuLꢀ3dpq 1/2H2O
EuLꢀ3dppz 1/2H2O
348
359
378
359
354
348
350
386
591
591
589
591
591
592
593
593
3879
2858
91
2772
473
4375
2278
170
614
613
612
613
613
617
614
615
23,400
15,570
287
15,760
2437
9889
8124
555
TbL3phen
TbL3dpq
TbL3dppz
347
355
334
356
346
348
488
489
489
489
488
482
545
545
545
544
545
545
3232
16,340
145
5535
11,200
87
TbLꢀ3phen·1/2H2O
TbLꢀ3dpq·1/2H2O
TbLꢀ3dppz 1/2H2O
Remark: L = BA−, Lꢀ = POA−
.

H.-J. Sun et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 243–249
247
Table 3
The luminescence emission spectra of the terbium complexes
Luminescent life-time and ꢂ2 of the complexes.
are similar and all can be issued as characteristic luminescence of
7
7
7
5D4 → F6 (around 488 nm), 5D4 → F5 (around 545 nm), 5D4 → F4
Complexes
Luminescence lifetime (␮s)
ꢂ2
(around 585 nm) and 5D4 → F3 (around 620 nm) transitions.
7
Tb(C6H5COO)3dpq
1235.89
1649.75
1255.38
188.66
1125.08
1194.14
1675.41
1443.04
110.74
1.172
1.344
1.402
1.469
1.590
1.649
1.655
1.407
1.364
Among them, the intensity of 5D4 → F5 transition is the strongest.
7
Tb(POA)3dpq·1/2H2O
Eu(C6H5COO)3phen
As can be seen from the luminescence data of the complexes,
the luminescence intensities of benzoate series of Eu3+ complexes
are stronger than phenoxyacetate series of Eu3+ complexes. For
Eu3+ complexes with the same acidic ligand but different neu-
tral ligands, the luminescence intensities of phen complexes and
dpq complexes are stronger than those of dppz complexes. The
sequence of energy transmission capacity of the neutral ligands
Eu(C6H5COO)3dppz
Eu0.5Gd0.5(C6H5COO)3dpq
Eu0.5La0.5(C6H5COO)3dpq
Eu(POA)3phen·1/2H2O
Eu(POA)3dpq·1/2H2O
Eu(POA)3dppz·1/2H2O
is phen > dpq > dppz. For Eu0.5Ln0.5(C6H5COO)3dpq (Ln3+ = Gd3+
,
La3+), Gd3+ was found to enhance the luminescence intensity of
Eu3+. However, La3+ adding did not enhance the luminescence
intensity of Eu3+. The difference between Gd(III) and La(III) ions
may result from the different radius of Gd(III) and La(III) ions. The
radius of Gd(III) ion (93.8 pm) is similar with that of Eu(III) ion
(95 pm). However, the radius of La(III) ion (105 pm) is much larger
than that of Eu(III) ion. The luminescence efficiency of the complex
may be lowered down because of the structure distortion in the
complex Eu0.5La0.5(C6H5COO)3dpq [12]. Therefore, only Gd(III) ion
enhanced the emission of Eu(III) ion and La(III) did not. The effect of
neutral ligands on luminescence intensity of Tb3+ complexes is dif-
ferent from that of Eu3+ complexes. For the same acidic ligand but
different neutral ligands, the sequence of luminescence intensity
of the complex is dpq > phen > dppz.
The effect of neutral ligands on the luminescence intensity of
central rare earth ions is investigated by energy difference between
the triplet states of the ligands and the resonance energy level
of the Eu3+ and Tb3+ ions. According to the energy transfer and
intramoleclar energy mechanism [10,38], one of the most impor-
tant factors influencing the luminescence properties of rare earth
complexes is the intramolecular energy transfer efficiency. The
energy transfer rate constants are dependent on the energy dif-
ference, and there should exist an optimal energy difference. If the
energy difference is too large, the energy-transfer rate constant will
decrease due to the diminution of the overlap between the energy
donor (ligands) and acceptor (rare earth ions). On the contrary,
if the energy difference is too small, the energy can backtransfer
from the rare earth ions to the triplet state energy of the ligands. In
the luminescence theory of rare earth complexes, the requirement
for efficient intramolecular energy transfer is that the energy dif-
ference lies within the range 2000–5000 cm−1 [39,40]. The triplet
Fig. 6. Luminescence decay curve and fit curve of Tb(C6H5COO)3dpq.
complexes are longer than dppz complexes. Also, the lifetimes
(typically > 1.0 ms) are much longer than the reported values in
the literatures [8,44–46]. The luminescence lifetime of Tb(BA)3dpq
and Tb(POA)3dpq·1/2H2O were also measured, and corresponding
luminescence decay curves and fit curves are shown in Figs. 6 and 7.
state energies of neutral ligands phen, dpq, dppz are 21,480 cm−1
,
23,800 cm−1 and 18,500 cm−1 [41,42], respectively. And the low-
est excited state energy of Eu3+ and Tb3+ are 17,500 cm−1 and
20,400 cm−1 [43], respectively. The triplet state energy of phen
matches well with the lowest excited state energy of Eu3+ ions (the
energy difference is 3980 cm−1), thus phen is able to transfer energy
to Eu3+ ion effectively. Consequently, the luminescence intensity
of Eu3+ ions could be effectively enhanced. Similarly, the triplet
state energy of dpq matches well with the lowest excited state
energy of Tb3+ ions (the energy difference is 3400 cm−1). There-
fore, the luminescence intensity of the complexes of Tb3+ with dpq
is strong. However, the triplet state energy of dppz is close to the
lowest excited state energy of Eu3+ ions and is lower than that of
Tb3+ ions. Thus the energy easily backtransfers from central ions to
dppz, which results in weak luminescence emission of the central
rare earth ions.
3.7. Luminescence lifetime
The luminescence lifetime of several complexes were deter-
mined and their data were shown in Table 3. From the data
of Eu3+ complexes, the luminescence lifetime of phen or dpq
Fig. 7. Luminescence decay curve and fit curve of Tb(POA)3dpq·1/2H2O.

248
H.-J. Sun et al. / Journal of Photochemistry and Photobiology A: Chemistry 219 (2011) 243–249
Table 4
Luminescence data of the complexes.
EuL3phen
EuL3dppz
Eu0.5Gd0.5 L3dpq
Eu0.5La0.5 L3dpq
EuLꢀ3phen·1/2H2O
EuLꢀ3dpq·1/2H2O
EuLꢀ3dppz 1/2H2O
ꢁ00 (cm−1
ꢁ01 (cm−1
ꢁ02 (cm−1
ꢁ03 (cm−1
I01
)
)
)
)
17,241
16,920
16,287
16,129
3879
17,271
16,978
16,340
16,207
91
17,241
16,920
16,313
16,129
2772
17,241
16,920
16,313
16,129
473
17,241
16,892
16,207
16,129
4375
17,241
16,863
16,287
16,129
2278
8124
3.57
1.443
0.693
322
17,241
16,863
16,260
16,129
170
I02
23,400
6.03
287
3.15
15,760
5.69
2437
5.15
9889
2.26
555
3.26
I02/I01
ꢃ (ms)
1/ꢃ (ms−1
Ar
Anr
ꢄ (%)
1.255
0.797
501.4
295.6
62.91
0.189
5.291
369.4
4921.6
7.00
1.125
0.889
459.1
429.9
51.64
1.194
0.838
469.5
368.5
56.03
1.675
0.597
264.7
332.3
44.33
0.111
9.009
311.8
8697.2
3.46
)
371
46.46
Remark: L = BA−, Lꢀ = POA−
.
complexes and Tb3+ complexes exhibit good characteristic lumi-
nescence emission spectra. For Eu3+ or Tb3+ complexes of the same
acidic ligand but different neutral ligands, the luminescence inten-
sities of phen complexes and dpq complexes are much stronger
than those of dppz complexes. The efficient sensitization of phen
and dpq ligands may result from the energy match between the
triplet state energy of the ligands and the lowest excited state
energy of the rare earth ions. Importantly, the high quantum effi-
ciencies and long lifetimes of the Eu3+ and Tb3+ ternary complexes
with phen and dpq as neutral ligands are advantages over many of
the ternary rare earth complexes investigated so far.
3.8. Luminescence decay times (ꢃ) and emission quantum
efficiency (ꢄ)
The typical decay curves of the Eu3+ complexes were measured.
According to the emission spectrum and lifetime of the Eu3+ first
excited level (ꢃ, 5D0), ꢄ values of the 5D0 Eu3+ excited states were
determined. The ꢄ can be defined as follows [47]:
Ar
ꢄ =
(1)
(Ar + Anr
)
Where Ar and Anr are radiative and nonradiative transition rates,
respectively. Ar can also be obtained by:
ꢀ
Acknowledgements
Ar =
A0J = A00 + A01 + A02 + A03 + A04
(2)
The research work is supported by the National Natural Science
Foundation of China (20461002), Natural Science Foundation of
Inner Mongolia (200711020203), the Opening Foundation for Sig-
nificant Fundamental Research of Inner Mongolia (2010KF03) and
a fund from Inner Mongolia University.
In the above Eq. (2), A0J represents the radiative rates for each
5D0 → FJ (J = 0–4) transitions of Eu3+. A0J can be calculated from
7
the following equation:
ꢁ
ꢂꢁ
ꢂ
I0J
ꢁ01
ꢁ0J
A0J = A01
(3)
I01
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