Synthesis and bright luminescence of lanthanide (Eu(III), Tb(III)) complexes sensitized with a novel organic ligand

Article abstract of DOI:10.1016/j.cplett.2003.12.093

A novel organic ligand, 6-[(benzylamino) carbonyl]-2-pyridine carboxylic acid (HBAP), and the corresponding lanthanide complexes, tris(6-[(benzylamino) carbonyl]- 2-pyridine carboxylato) lanthanide(III) (Ln-BAP, Ln=Eu, Tb, Gd), have been designed and synthesized. The lanthanide (Eu(III), Tb(III)) complexes were efficiently sensitized by BAP ligand. The fluorescence quantum yields were investigated by comparison with a luminescence standard, and the yields were 15±3%, 34±3% for the solid europium and terbium complexes respectively. The lowest triplet level of HBAP ligand was calculated from the phosphorescence spectrum of Gd-BAP complex, and the energy transfer mechanisms in the lanthanide complexes were discussed.

Full text of DOI:10.1016/j.cplett.2003.12.093

Chemical Physics Letters 385 (2004) 345–350
www.elsevier.com/locate/cplett
Synthesis and bright luminescence of lanthanide (Eu(III), Tb(III))
complexes sensitized with a novel organic ligand
Bao-Li An a,b,*, Meng-Lian Gong , Kok-Wai Cheah , Ji-Ming Zhang , King-Fai Li
b,
c
a
c
*
a
Department of Chemistry, College of Science, Shanghai University, Shanghai 200436, PR China
b
State Key Laboratory of Optoelectronic Materials and Technologies, Sun Yat-sen University, Guangzhou 510275, PR China
c
Department of Physics, Hong Kong Baptist University, Hong Kong, PR China
Received 6 June 2003; in final form 15 December 2003
Published online: 25 January 2004
Abstract
A novel organic ligand, 6-[(benzylamino) carbonyl]-2-pyridine carboxylic acid (HBAP), and the corresponding lanthanide
complexes, tris(6-[(benzylamino) carbonyl]- 2-pyridine carboxylato) lanthanide(III) (Ln-BAP, Ln ¼ Eu, Tb, Gd), have been designed
and synthesized. The lanthanide (Eu(III), Tb(III)) complexes were efficiently sensitized by BAP ligand. The fluorescence quantum
yields were investigated by comparison with a luminescence standard, and the yields were 15 ꢀ 3%, 34 ꢀ 3% for the solid europium
and terbium complexes respectively. The lowest triplet level of HBAP ligand was calculated from the phosphorescence spectrum of
Gd-BAP complex, and the energy transfer mechanisms in the lanthanide complexes were discussed.
Ó 2004 Elsevier B.V. All rights reserved.
1
. Introduction
dicarboxylate) Ln(III) (Ln ¼ Eu, Tb) complexes have
good photoluminescent properties [9] and high thermal
stability [10,11], but has not been applied to OEL device
because of the following reasons: first, the ionic com-
pound could not be prepared as a thin film using vac-
uum vapor deposition; second, the compound could not
be dissolved in non-polar solvents, and could not be
made uniform thin film by spin coating method. In
principle, electroneutral metal complexes may form
uniform thin film in vacuum vapor deposition and are
reasonably stable to heat, which is required for fabri-
cation of OLEDs [12]. Investigating the relationship
between the structures of organic ligands and the energy
levels of lanthanide ions will provide evidences for de-
signing high luminescent lanthanide organic complexes.
In our earlier work two new organic ligands, that effi-
ciently sensitize europium(III) luminescence, but do not
sensitize terbium(III) luminescence at room temperature
[13,14], were synthesized. In this paper, a novel organic
ligand, HBAP, which sensitized europium(III) lumines-
cence as well as terbium(III) luminescence, was designed
and synthesized,. The corresponding lanthanide com-
plexes, Eu-BAP, Tb-BAP and Gd-BAP, were synthe-
sized, and the luminescence properties for the lanthanide
complexes were investigated.
Since a brightly photoluminescent europium complex
was first reported by Weissman in 1942 [1], great
achievement has been gained in research of lanthanide
organic complexes. Complexes of lanthanide ions with
organic ligands have been doped in polymers for optical
amplification [2,3], and luminescent lanthanide com-
plexes have been used in medical diagnosis in reality,
where they are used to detect small amounts of bio-
molecules that can tell about the physical state of a
patient [4,5]. Presently organic electroluminescence
(
OEL) has been studied extensively for its opportunities
of low drive voltage, suitability for integrated circuit and
potential application for large flat panel display [6,7].
Several ternary lanthanide complexes have been applied
to fabricate OEL devices (OLEDs), and great achieve-
ments have been gained in improving the brightness
and electroluminescence efficiencies of OELDs with
europium(III) and terbium(III) organic complexes
as luminescent centers [8]. Sodium tris(2,6-pyridine
*
Corresponding authors. Fax: +86-21-661-327-97.
E-mail addresses: anbaolii@263.sina.com, blan@mail.shu.edu.cn
B.-L. An), cesgml@zsu.edu.cn (M.-L. Gong).
(
0
009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.12.093

346
B.-L. An et al. / Chemical Physics Letters 385 (2004) 345–350
2
. Experimental
solution containing 0.4 mmol EuCl3, which was ob-
tained by a reaction of Eu2O3 (99.95%, Zhu-Jiang
smeltery Co.) and HCl (6.0 mol/l). The pH value of the
mixture was adjusted to 6 by adding an aqueous solu-
tion of NaOH. The mixture was stirred 24 h at 40 °C.
After deposited 24 h, a white precipitate deposited from
the solution, then was filtered out and washed with de-
ionized water and 95% ethanol. The precipitate was
purified with mixed solution of 95% ethanol and ace-
tone, and dried in vacuum at room temperature for 24 h.
Eu(BAP)36H2O complex was obtained with high yield
of 73%.
2
.1. Preparation of 6-[(benzylamino) carbonyl]-2-pyri-
dine carboxylic acid
2,6-Dichlorocarbonyl pyridine (DCP) was prepared
by a reaction of 2,6-pyridine dicarboxylic acid (H DPA)
2
and thionyl chloride [13] with a high yield of 95%. 14.7
mmol of benzylamine dissolved in dried benzene was
slowly added to 14.7 mmol of DCP dissolved in dried
benzene, and stirring was continued for 6 h at 50 °C
until the precipitate vanished. The benzene solution was
concentrated in vacuum and then hydrolyzed at 40 °C
for 2 h. The pH value of the solution was adjusted to
Tb(BAP)3 ꢁ 4H2O and Gd(BAP)3 ꢁ 5H2O were pre-
pared as the synthesis method of Eu(BAP) ꢁ 6H O with
3
2
1
0.0 using 2.0 mol/l aqueous NaOH. The mixture was
filtered and the pH value of the filtrate was adjusted to
.0 using 2.0 mol/l HCl, giving a white precipitate. The
the same yield of 71%.
The complexes were verified by elemental analysis
and IR spectroscopy. The elemental analysis data for
Eu-BAP were: found (cal for EuC42H45N6O15) C 49.30
(49.18), H 4.25 (4.42), N 8.10 (8.19) and Eu 14.98
(14.81). IR (KBr) mmax: 3424.0 (m, mHO–H), 3224.4 (w,
3
white precipitate was recrystallized in 95 % ethanol:ac-
etone:water (2:1:2, volume ratio), giving 1.64 g HBAP
with a yield 43.6 % and mp 181–184 °C. The elemental
analysis data for C14H12N2O3 (HBAP) were: found (cal)
C 65.80 (65.62), H 4.93 (4.72), and N 11.15 (10.93). IR
mN–H), 3083.9 (m, mC–H), 2930.0 (w, mas (CH2)), 1635.9 (s,
ꢂ
mas (COO )), 1585.5 (m, m
, band I), 1563.0 (m, dN–H,
C@O
ꢂ
(
KBr) mmax: 3600–2500 (s, mCOO–H), 3372.1 (s, mN–H),
band II), 1366.5 (s, m (COO )), 1276.8 (m, mC–N), 754.3,
s
3
2
116.0, 3069.6, 3034.0 (w, mC–H), 2672.0, 2611.0, 2501.0,
441.0 (w, Fermi resonance), 1718.9 (s, mC@O), 1657.8
725.4, 695.0 (m, d
). The elemental analysis
C–H ðin planeÞ
data for Tb-BAP were: found (cal for TbC42H41N6O13)
C 50.45 (50.61), H 4.72 (4.15), N 8.22 (8.43), Tb 15.62
(15.94). IR (KBr) mmax: 3423.7 (m, mHO–H), 3224.3 (w,
mN–H), 3083.6 (w, mC–H), 2924.0 (w, mas (CH2)), 1636.6 (s,
(
(
s, mC@O, band I), 1538.2 (s, dN–H, band II), 1457.0
s, dCH(CH2)), 1222.9 (s, mC–N, band III), 756.0, 727.3 (s,
1
d
). H-NMR in CD3SOCD3: 13.026 (1 H, s),
3
C–H ðin planeÞ
3
ꢂ
9
.698 (1 H, t, J ¼ 6.5 Hz), 8.303 (1 H, d, d, J ¼ 7.5 Hz,
mas (COO )), 1590.0 (s, mC@O, band I), 1568.4 (s, dN–H,
4
3
4
ꢂ
J ¼ 2.0 Hz), 8.259 (1 H, d, d, J ¼ 8.0 Hz, J ¼ 2.0 Hz),
band II), 1367.7 (s, m (COO )), 1277.9 (m, mC–N), 754.7,
725.7, 695.0 (m, d
data for Gd-BAP were: found (cal for GdC H N O )
42 43 6 14
s
3
3
8
7
.229 (1 H, t, J ¼ 7.5 Hz), 7.336 (4 H, h, J ¼ 3 Hz),
). The elemental analysis
C–H ðin planeÞ
3
4
.260 (1 H, h, J ¼ 3 Hz), 4.586 (2H, d, J ¼ 6.5 Hz).
þ
FAB-MS: m=z 257 ([M + H] ).
The synthesis route is expressed as follows:
C 49.69 (49.79), H 4.02 (4.28), N 8.20 (8.30), Gd 15.39
(15.52). IR (KBr) mmax: 3396.8 (m, mHO–H), 3260.8 (w,
mN–H), 3138.9, 3080.7 (w, mC–H), 2892.8 (w, m(CH2)),
ꢂ
1
632.7 (s, mas (COO )), 1580.6 (s, mC@O), 1558.2 (s,
O
ꢂ
C
N
C
Cl
Cl
dN–H), 1373.5 (s, ms (COO )), 1277.9 (m, mC–N), 759.2,
COOH
7
28.2, 688.4 (m, d
). The crystal structure of a
C–H ðin planeÞ
SOCl2
N
similar europium complex, Eu(DPAP) ꢁ12H O (DPAP
3
2
is 6-(diphenylamino carbonyl)-2-pyridine carboxylic
COOH
acid), has been obtained [15]. The central Eu3 is
wrapped by three molecules of the ligand, two oxygen
and one nitrogen atoms from each DPAP ligand coor-
þ
O
H DPA
DCP
2
O
O
dinated to Eu3 , and 12 molecules of H2O locate around
the outside of Eu(DPAP)3 complex. The structures of
the lanthanide complexes are proposed as follows:
þ
C
OH
1
)
CH2 NH2
N
C
2
) H2O
N CH
2
H
O
C
O
HBAP
N
Ln
2
and Gd(BAP)
.2. Preparation of Eu(BAP)
ꢁ 5H
3
ꢁ 6H
2
O, Tb(BAP)
3
ꢁ 4H
2
O
C
O
3
HN
3
2
O
CH2
Eu(BAP)3 ꢁ 6H2O complex was prepared by adding
1
.2 mmol of HBAP dissolved in ethanol to an aqueous

B.-L. An et al. / Chemical Physics Letters 385 (2004) 345–350
347
2
.3. Measurements and apparatus
1
1
0
0
.5
.0
.5
.0
Elemental analysis for the synthesized ligand and the
complexes were carried out with Elementar vario EL
elemental analyzer. IR spectra in the region 4000–400
cm were recorded on a Bruker infrared spectropho-
tometer using conventional KBr method. FAB-MS
2
74
282
ꢂ1
327
(c)
2
72
(b)
(a)
spectra were performed on a VG ZAB-HS spectrometer.
1H-NMR spectra were recorded on a USA Varian
UNITYINOVA-500 spectrometer (500 MHz). The ex-
citation and emission spectra were measured using a
Hitachi F4500 fluorescence spectrophotometer with ex-
citation and emission slits of 2.5 nm and with PMT at
200
250
300
350
400
Wavelength (nm)
ꢂ5
Fig. 1. UV absorption for (a) 2.5 ꢃ 10 mol/l HBAP ethanol solution,
ꢂ5
ꢂ5
(
l Eu(BAP)3 ethanol solution.
b) 3.3 ꢃ 10 mol/l Tb(BAP)3 ethanol solution and (c) 2.5 ꢃ 10 mol/
7
00 V at room temperature. Phosphorescence spectra
were recorded on SPEX 1934D phosphorescence spec-
trometer at 77 K with both excitation and emission slits
at 4.0 mm and with the light pulse width of 1.0 ms and
pulse interval of 0.001 ms. Fluorescence lifetimes were
measured using an EMG 201 MSC quasi-molecular
of Eu3 ions. Different from Tb , an Eu ion has
þ
3þ
3þ
tendency to get one electron to reach half-filled 4f shell
7
3þ
(4f ), and some Eu complexes have been observed to
have electron transfer absorption bands [17,18].
5
7
3
08 nm laser by monitoring D ! F emission line of
0
2
3þ 5 7
3þ
Eu or by monitoring D4! F5 emission line of Tb
at room temperature. The fluorescence yields were
þ
3.2. Excitation and emission spectra
measured with He–Cd 325 nm laser with Y2O3:3%Eu3
as luminescence standard. To evaluate the accuracy of
the method, the efficiency of sodium salicylate (60%) as
a standard was determined, and the results showed that
the method is accurate with a relative error of ꢀ10%
The excitation spectrum monitored at 614 nm for the
dilute ethanol solution of Eu(BAP)3 showed two exci-
tation bands centered at 275 and 330 nm, respectively
(Fig. 2), and the longer wavelength excitation band was
assigned to an electron transfer absorption band. A wide
excitation band in region of 220–300 nm and some
narrow excitation peaks located at 362, 376 and 395 nm
were observed for the solid europium complex, and the
narrow peaks were ascribed to various f–f transitions of
[
16,17]. Electronic absorption spectra were recorded on
a Varlan UV–Vis spectrophotometer.
3
. Results and discussion
Eu3 ions.
The emission spectra for Eu-BAP either in solid state
or in ethanol solution consist of two main lines at 593.7/
þ
3
.1. UV absorption spectra
5
7
5
7
UV absorption spectra for HBAP, Eu(BAP)3 and
592.2 nm ( D0 ! F1) and 614.3 / 614.6 nm ( D0 ! F2),
Tb(BAP)3 dilute ethanol solutions are shown in Fig. 1,
and the maximum absorption wavelengths and molec-
ular absorption coefficients are given in Table 1. The
wide absorption band at 327 nm for Eu(BAP)3 ethanol
solution was assigned to an electron transfer from the
highest filled molecular orbital, mainly localized on the
coordinating oxygen anions, to the partly filled 4f shell
and the emission at 614 nm is the strongest. Different
from Eu3 -b-dikitonate complexes [19], the 614 nm peak
for Eu(BAP)3 complex was sharper, showing higher
color purity for the latter. It is interesting that the
þ
5
7
3þ
D0 ! F0 transition of the Eu complex was observed
to be very weak in ethanol solution, but was not
observed in solid state. This suggests that the actual
Table 1
The photophysical properties of Eu(BAP)3 and Tb(BAP)3 complexes
Sample
UV absorption
Luminescence
ex (nm)
Lifetimes
kmax (nm)
ꢀ ꢃ 10ꢂ4a
1.01
1.54
1.46
k
k
em (nm)
I (a.u.)
s (ms)
ꢂ5
ꢂ5
ꢂ5
2
2
3
.5 ꢃ 10 mol/l HBAP
272
274
274
b
.5 ꢃ 10 mol/l Eu(BAP)
3
3
275
283
271
300
614.5
544.0
614.3
542.8
1328
0.41 ꢀ 0.01
1.55 ꢀ 0.02
1.67 ꢀ 0.01
1.31 ꢀ 0.01
c
.3 ꢃ 10 mol/l Tb(BAP)
2602
8979
8335
Eu(BAP)
Tb(BAP)
3
ꢁ 6 H
2
O
O
3
ꢁ 4 H
2
a
b
c
ꢂ1
3
ꢂ1
.
The unit of ꢀ is mol dm cm
The excitation and emission slits are 5.0 nm.
2
.5 ꢃ 10ꢂ5 mol/l Tb(BAP)3 ethanol solution was determined.

348
B.-L. An et al. / Chemical Physics Letters 385 (2004) 345–350
5
7
5
7
D → F
0 2
1
1
500
000
275
620.1 nm ( D4 ! F3), and the magnetic dipole transi-
(
a)
Em.
Ex.
30
5 7
tion, D4 ! F5, is the strongest.
5
7
3
D → F
0 1
5
00
0
3
.3. Lifetimes and yields
(
b) 256
614.3
Typical decay curves for Eu-BAP complex are
showed in Fig. 4. The lifetime values were calculated
through the first order exponential decay method [22],
2
93
8
4
000
000
0
Ex.
Em.
5
93.7
5
and the results were given in Table 1. The lifetime of D0
3þ
excited state of Eu ions in solid state was evidently
2
00 250 300 350 400 550 600 650 700
Wavelength (nm)
longer than that in dilute ethanol solution. It has been
observed that the decay rate of Ln3 excited states is
directly proportional to the number of water molecules
in the first coordination sphere of the lanthanide ions
þ
ꢂ5
Fig. 2. Excitation and emission spectra for: (a) 2.5 ꢃ 10
Eu(BAP)3 ethanol solution and (b) solid Eu(BAP)3 ꢁ 6H2O.
mol/l
[
coordination sphere can be calculated based on the
following equation [23]:
23,24]. The number of water molecules (q) in the first
symmetry of Ln(BAP)3 deviates slightly from the higher
3
ꢂ
symmetry of Ln(dpa) complexes (D ) [20] where the
3
3
5
7
3þ
D ! F transition is forbidden. The symmetry of Eu
q ¼ AðsH O ꢂ sD O ꢂ aÞ;
0
0
2
2
complex in ethanol solution may be little different from
that in solid state, which is ascribed to one molecule of
water coordinated to Eu3 in the first coordination
sphere in ethanol solution. The frequency of the
where the constant A was determined to be 1.11 wa-
ters ꢁ ms, and the value for the parameter a was 0.30
þ
ꢂ1
ms representing the effect of second and subsequent
coordination sphere molecules. The s represents the
lifetime (ms) in the solvent indicated. The lifetime values
5
7
3þ
D0! F0 transition can be used to diagnose Eu ion
environment in the first coordination sphere based on
the nephelauxetic parameters of the coordinating atoms
for Eu(BAP)
ordinated to Eu in solid state, while one molecule of
3
indicated that no molecule of water co-
3þ
5
7
[
(
21]. The calculated value of the D ! F transition
3þ
0
0
water coordinated to Eu in ethanol solution. Water
3þ
ꢂ1
17 236 cm ) for the 10 coordination europium com-
molecules in the first coordination sphere of Eu ion
plex with one molecule of water in the first coordination
sphere is proximate to the observed value of the tran-
provide a ready non-radiative de-excitation pathway for
the excited state. This is attributed to a weak coupling
between the O–H vibrational ladder and the excited
state [24].
ꢂ1
sition (17 234 cm ).
The excitation spectrum for the dilute ethanol solu-
tion of Tb(BAP)3 complex showed an excitation band at
5
3þ
The lifetime values of D4 excited state of Tb ions
for Tb(BAP)3 are given in Table 1. However, the lifetime
2
83 nm, and a wide excitation band in region of 260–360
nm was observed for the solid terbium complex (Fig. 3).
The emission spectra for Tb-BAP complex either in
solid state or in ethanol solution consist of four main
5
3þ
of D excited state of Tb ions in solid state is shorter
4
than that in dilute ethanol solution.
The fluorescence quantum yields for original
Eu(BAP)3 ꢁ 6H2O and Tb(BAP)3 ꢁ 4H2O were measured
5
7
lines at 491.7/491.0 nm ( D4 ! F6), 542.8/544.2 nm
5
7
5
7
(
D4 ! F5), 582.0/582.1 nm ( D4 ! F4) and 620.0/
0
.25
(
a)
3
2
1
000
000
000
0
5
7
0.20
0.15
0.10
2
83
D4 F5
→
(
a)
5
7
D → F
4
6
Em.
Ex.
0
0.00
.05
5
D
7
→
F
4
4
5
7
D → F
4
3
0.000
0.001
0.002
0.003
0
.15
(
b)
542.8
(
b)
2
98
331 Ex.
Em.
8
4
000
491.7
500
0.10
0
0
.05
.00
000
0
5
82
620
0.000 0.002 0.004 0.006 0.008 0.010
Time (s)
2
00
300
400
600
Wavelength (nm)
ꢂ5
Fig. 4. The decay curves of the emission at 614 nm for (a) 2.5 ꢃ 10
ꢂ5
Fig. 3. Excitation and emission spectra for: (a) 3.3 ꢃ 10
Tb(BAP)3 ethanol solution and (b) solid Tb(BAP)3 ꢁ 4H2O.
mol/l
mol/l Eu(BAP)3 ethanol solution and (b) original Eu(BAP)3 ꢁ 6H2O.
The line curves are the first-order exponential decay fit curves.

B.-L. An et al. / Chemical Physics Letters 385 (2004) 345–350
349
by comparison with a luminescence standard [16] using
the following formula:
S1(L)
6
P7/2
ꢀ
ꢁꢀ
ꢁ
1
1
ꢂ r
DUx
30
st
q
x
¼
ST
q ;
ꢂ rx
DUST
5
Dj, j =
5
where rST and r are the amount of exciting radiation
2
1
0
T (L)
T (L)
D4
x
1
1
2
0
reflected by the standard and by the sample, respec-
tively, and qST and q
x
are the quantum yields of the
standard and the sample, respectively. The terms of DU
x
1
0
7
and DUST are the integrated photon flux for the sample
and for the standard, respectively. The yield of
Fj, j =
7
3
4
5
3þ
Fj, j =
2
1
Y O :3%Eu is 99 %, used as a standard [16]. The yield
2
3
8
S0(L)
S7/2
0
values are 15 ꢀ 3%, 34 ꢀ 3% for the original europium
6
3+
3+
Eu
Gd
BAP ligand
3+
Tb
and terbium complexes, respectively.
Fig. 5. Energy transfer scheme.
3
.4. Triplet state of the ligand and energy transfer
The lowest triplet state of HBAP ligand, T1(L), is
about 23500 cm , calculated from the maximum
ꢂ1
It is easy to understand why the fluorescence quan-
tum yield for Tb-BAP is much higher than that for Eu-
BAP. The excitation energy was partially lost in the
emission wavelength of the phosphorescence spectrum
for Gd(BAP) complex in dimethyl sulfoxide (DMSO)
3
5
5
solution determined at 77 K. The T (L) is higher by 3000
2 0
internal relaxation process from D to D with non-
1
ꢂ1
5
3þ
cm than the D4 state of Tb , and matches with the
þ
radiation relaxation via ligand vibration, so the yield for
the europium complex was much lowered.
lowest excited state of Tb3 according to the energy gap
study on the Eu3 and Tb
þ
3þ
b-diketonate chelates
25,26]. The terbium complex emitted very strong green
[
characteristic emission of Tb3 ions either in solid state
þ
4. Conclusions
or in ethanol solution.
The excitation spectra for Eu(BAP) complex showed
that most of the excitation energy was mainly absorbed
by the ligand, and then the energy was transferred to the
3
A novel organic ligand, HBAP, and the correspond-
ing lanthanide (III) complexes, Ln(BAP) (Ln ¼ Eu, Tb,
3
Gd), have been synthesized. The lanthanide complexes
excited state levels of the central Eu3 ions, D2 (21480
þ
5
are electroneutral molecules, in which the central lan-
thanide ions are wrapped by three BAP ligands with
some water molecules located around the outer shell of
the complexes. The europium complex emits strong red
ꢂ1
5
ꢂ1
cm ) and D1 (18700 cm ), through the T1(L) of BAP
ligand. The overlap area between the T1(L) and D2 was
calculated to be seven times more than that between the
5
5
5
3þ
T (L) and D , hence, the D was the main acceptor of
characteristic emission of Eu ions either in solid state
1
1
2
the transferred energy. The energy could not be directly
ꢂ1
or in dilute ethanol solution at room temperature, and
the luminescence yield is 15 ꢀ 3% in solid state. The
terbium complex emits very strong green characteristic
emission of Tb3 ions either in solid state or in dilute
ethanol solution at room temperature, and the lumi-
nescence yield is 34 3% in solid state. BAP ligand sen-
sitizes europium(III) emission as well as terbium(III)
emission. Both Eu(BAP)3 and Tb(BAP)3 are good
fluorescence materials.
5
transferred to D (17300 cm ) state through the T (L)
0
1
due to the diminution in overlap between donor and
acceptor according to DexterÕ theory [27]. The electrons
in the D2 states relaxed to the lowest excited states,
þ
5
5
D0, through internal relaxation process. Characteristic
emission of europium(III) ions was observed when
5
electrons transferred from D to the ground state levels
0
(7
mechanism for Eu(BAP) complex is given in Fig. 5.
F , j ¼ 0, 1, 2, 3, 4). The dominant luminescence
j
3
The energy transfer mechanism for Tb(BAP)3 com-
plex is similar to that for Eu(BAP)3, and the dominant
luminescence mechanism is given as energy transferred
from the T1(L) of the ligand to the central terbium ions.
Acknowledgements
This work was financially supported by a grant from
the scientific and technical project of Guangdong
province (B10502), a grant from the State Key Labo-
ratory of Optoelectronic Materials and Technologies
(2000), and also supported by a grant from adolescent
fund of Shanghai Municipal Education Committee
(03AQ95).
The lowest excited state level of Gd3 ions ( P7=2) is
þ
6
much higher than the T (L), and the energy can not be
1
transferred to the central gadolinium(III) from BAP li-
gand (Fig. 5). So phosphorescence emission from BAP
ligand was observed for Gd(BAP)3 complex, while no
Gd3 ion luminescence was found.
þ

3
50
B.-L. An et al. / Chemical Physics Letters 385 (2004) 345–350
[14] B.L. An, K.W. Cheah, W.K. Wong, J.X. Shi, N.S. Xu, Y.S. Yang,
M.L. Gong, J. Alloys Comp. 352 (2003) 143.
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