Photoacoustic spectroscopy study on the co-fluorescence effect of Eu3+-La3+-Hba solid complexes

Article abstract of DOI:10.1016/S0022-3697(03)00157-4

The photoacoustic (PA) spectra of Eu³⁺-La³⁺-Hba complexes are reported (Hba: benzoic acid). It is found that the PA intensity of the ligand bears a relation to the energy transfer processes. For Eu³⁺-La³⁺-Hba coprecipitates, with increasing the concentration of La³⁺, the PA intensity in the region of the ligand absorption decreases firstly, then increases. It is indicated that the addition of the second rare earth ion La³⁺-changes the relaxation processes of the complexes. The changes of fluorescence spectra turn out to be complementary to the PA spectra. The intramolecular energy transfer and intermolecular energy transfer processes in the coprecipitates are discussed.

Full text of DOI:10.1016/S0022-3697(03)00157-4

Journal of Physics and Chemistry of Solids 64 (2003) 1333–1337
www.elsevier.com/locate/jpcs
Photoacoustic spectroscopy study on the co-fluorescence
effect of Eu3þ–La3þ–Hba solid complexes
*
Yue-tao Yang , Shu-yi Zhang
State Key Laboratory of Modern Acoustics and Institute of Acoustics, Nanjing University, Nanjing 210093,
People’s Republic of China
Received 29 July 2002; accepted 4 March 2003
Abstract
The photoacoustic (PA) spectra of Eu3þ–La3þ–Hba complexes are reported (Hba: benzoic acid). It is found that the PA
intensity of the ligand bears a relation to the energy transfer processes. For Eu3þ–La3þ–Hba coprecipitates, with increasing the
concentration of La3þ, the PA intensity in the region of the ligand absorption decreases firstly, then increases. It is indicated that
the addition of the second rare earth ion La3þ–changes the relaxation processes of the complexes. The changes of fluorescence
spectra turn out to be complementary to the PA spectra. The intramolecular energy transfer and intermolecular energy transfer
processes in the coprecipitates are discussed.
q 2003 Elsevier Science Ltd. All rights reserved.
1. Introduction
short enough to take intermolecular energy transfer. The
fluorescence enhancement and quenching phenomena
have been found for the coprecipitates of rare earth b-
diketone complexes [5]. Similarly to the b-diketone
complexes, rare earth complexes with aromatic car-
boxylic acids exhibit excellent luminescence properties.
It is necessary to study the co-fluorescence effect of
those compounds in solid states. Such study can be
helpful in better understanding regarding the co-
fluorescence mechanism.
Rare earth complexes have received great interest
because of their important roles in the study of
luminescent materials, NMR shift reagents and biologi-
cal systems. Increasingly more attention has been paid
on rare earth complexes in solid states [1–2]. The
addition of Y3þ or certain lanthanide ions can
considerably enhance the fluorescence intensities of the
chelates of Eu3þ, Sm3þ and Tb3þ in solution. This type
of fluorescence actually is an intrinsic fluorescence
phenomenon called ‘co-fluorescence’ effect [3]. The co-
fluorescence can be found in chelate suspensions and in
a micellar environment. While in an actual solution,
there is no co-fluorescence enhancement because the
long distance between the chelates makes intermolecular
energy transfer impossible [4]. In solids, especially in
coprecipitates, the distance between chelates could be
The photoacoustic (PA) technique has been widely
used to investigate the chemical and physical properties
of many samples [6]. The PA technique enables to obtain
spectra on any type of solids whether it be crystalline,
powder or gel. It can be a direct monitor of the energy
gap and nonradiative relaxation channels, and the
complement of absorption and photoluminescence spec-
troscopic techniques [7]. In this paper, Eu3þ–La3þ–Hba
(Hba: benzoic acid) complexes are prepared. Using PA
and fluorescence spectroscopy, the luminescence proper-
ties of Eu3þ influenced by La3þ, and the relaxation
processes in Eu3þ–La3þ–Hba coprecipitates are studied
in depth from two aspects: nonradiative process and
radiative process.
*
Corresponding author. Tel.: þ86-25-3594066; fax: þ86-25-
3313690.
E-mail address: yyang@nju.edu.cn (Y. Yang).
0022-3697/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0022-3697(03)00157-4

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Y.-t. Yang, S.-y. Zhang / Journal of Physics and Chemistry of Solids 64 (2003) 1333–1337
2. Experimental
3. Results and discussion
2.1. Preparation of Eu12xLax(Hba)3 complexes
3.1. Photoacoustic spectra
Eu12xLax(Hba)3 (x ¼ 0.0,0.15,0.35,0.65,0.75,0.85,0.95,
1.0) complexes were prepared by mixing the solution of
ammonium benzoate, europium chloride and lanthanum
chloride in proportion (pH 5–6). Each mixture was
digested for 4 h in a water bath, cooled to room
temperature, then filtered. The solids were washed free of
Cl21 and NHþ4 and sucked dry. The anhydrous compounds
were obtained after drying at 85 8C. Eu12xNdx(Hba)3
complexes were prepared according to the procedures
described above. Their elemental and thermal analyses
were performed and were consistent with the expected
formulae.
The PA signal is obtained by detecting the heat generated
through the nonradiative transitions by the sample after
absorbing a periodically varying incident light [11]. The PA
spectrum of Eu0.85La0.15(Hba)3 is shown in Fig. 1. The
broad PA band around 304 nm is assigned to the p 2 p*
transition of the complex [12]. For the central Eu3þ ion of
the complex, only f–f transitions are observed from 300 to
700 nm in Fig. 1 [13–14]. Since the p 2 p* transition of the
ligand is far more stronger than the parity forbidden f–f
transitions [15], the f–f transitions of Eu3þ in the region of
300 , 400 nm are lost in the strong p 2 p* transition.
It is known that after excitation there are two kinds of
relaxation processes: radiative and nonradiative processes.
As the PA spectrum only corresponds to the nonradiative
relaxation process, the PA signals of energy levels of Eu3þ
,
which have fluorescence properties, are quite weak or
2.2. Spectroscopic measurements
5
vanished [16]. For Eu3þ, the relaxation of D0 can not be
The PA spectra containing both amplitude and phase
information were measured on
5
monitored by PA spectroscopy, and the PA signals of D1,
a single-beam spec-
5D2 at 469 and 540 nm are also quite weak. It can be
interpreted by the relaxation model according to the
previous paper [17].
trometer constructed in our laboratory [8]. Excitation
source was a 500 W xenon lamp. The optical system
was a monochromator and a variable speed mechanical
chopper at a frequency of 30 Hz, which was used to
modulate the light source intensity. The acoustic signal
was monitored with the sample placed in an indigenous
photoacoustic cell fitted with an electret microphone.
The output signal from the microphone was amplified
by a preamplifier and then fed to a lock-in-amplifier
with a reference signal imputed from the chopper. The
final signal was normalized for the changes in lamp
intensity using a reference by carbon black. The PA
spectra of all the complexes were recorded in the region
of 300 , 700 nm.
The infrared spectra were measured with pellets using
KBr on Magna IR-750 spectrometer. The infrared spectra
of Eu12xLax(Hba)3 were identical with that of Eu(Hba)3,
which indicated the nature of metal-ligand bonding was
similar for the two types of complexes. For the
complexes, the absorption bands of characteristic asym-
metric ðnasÞ and symmetric ðnsÞ stretching vibrations of
carboxyl group shifted to lower frequencies compared
with those of sodium benzoate. Both of the absorption
bands ðnas and nsÞ showed splitting. The difference ðDnÞ
between nas and ns ranged from 118 to 123 cm21 for the
complexes, which was less than that of sodium benzoate
ðDn ¼ 137 cm21Þ: According to the IR spectral results, it
was suggested that the carboxyl groups coordinated with
Ln3þ in both chelating and bridging ways [9–10], and
Eu12xLax(Hba)3 had the structure of a polynuclear
form.
With the change of La3þ (mole fraction) in the
complexes, the PA band location of the ligand hardly
changes, and the PA intensity changes correspondently.
These changes are recorded in Fig. 2. With increasing
the concentration of La3þ, the PA absorbency of Eu12x-
Lax(Hba)3 decreases. When seventy five percent of Eu3þ in
Eu(Hba)3 are substituted by La3þ, the PA intensity becomes
the weakest (at 304 nm). Then the PA intensity increases
rapidly. When all Eu3þ in Eu(Hba)3 are replaced by La3þ
,
the PA intensity becomes the strongest. As we know, PA
intensity of the ligand is the sum of the nonradiative
relaxation of the ligand, the energy transfer and the
following nonradiative relaxations of the central Ln3þ ion
[17,18]. For Eu12xLax(Hba)3, the PA signal of the ligand
changes with the addition of La3þ, which indicates
The fluorescence spectra were taken with a Hitachi 850
fluorescence spectrophotometer.
Fig. 1. PA spectrum of Eu0.85La0.15(Hba)3.

Y.-t. Yang, S.-y. Zhang / Journal of Physics and Chemistry of Solids 64 (2003) 1333–1337
1335
Fig. 2. Variation of the PA intensity as a function of the mole
fraction of La3þ
Fig. 4. Variation of the emission intensity as a function of the mole
fraction of La3þ
.
.
3.3. Energy transfer processes
the relaxation processes of the complexes change corre-
spondingly. The changes can be reflected in their fluor-
escence spectra.
3.3.1. Intramolecular energy transfer
The PA signal is selectively sensitive only to the
nonradiative relaxation process, according to Adams et al.
[19] the PA signal ðPÞ can be written as,
3.2. Fluorescence spectra
P ¼ KAabs
g
ð1Þ
The shapes of fluorescence spectra of Eu12xLax(Hba)3
are almost the same. The fluorescence spectra of Eu0.85
-
where Aabs is the absorbency of the sample, g is the
probability of nonradiative transitions after excitation, and
K is a coefficient which is determined by the thermal
properties of the sample and by the spectrometer.
The fluorescence signal ðEÞ; which is proportional to the
probability of radiative transitions after excitation, can be
expressed similarly as [20],
La0.15(Hba)3 are shown in Fig. 3. In the emission spectrum,
the four characteristic emissions of Eu3þ at 582 nm
7
7
7
(5D0 ! F0), 595 nm (5D0 ! F1), 619 nm (5D0 ! F2),
703 nm (5D0 ! F4) have been clearly observed. Among
7
them, the emission at 619 nm is the strongest.
The emission intensities of Eu12xLax(Hba)3 versus the
concentration of La3þ are shown in Fig. 4. With the increase
of La3þ mole fraction, the fluorescence intensity increases
remarkably. When seventy five percent of Eu3þ in Eu(Hba)3
are replaced by La3þ, the fluorescence intensity becomes the
strongest. Then the fluorescence intensity decreases quickly.
Figs 2 and 4 are mutually complementary. As the
probability of radiative transition increases, the PA intensity
exhibits a corresponding decrease [16].
E ¼ K0Aabsð1 2 gÞ
So Eq. (3) is obtained after combining Eqs. (1) and (2),
ð2Þ
ꢀ
ꢁ
1
1
¼ c
2 1
ð3Þ
R
g
where R ¼ P=E; and c ¼ K0=K: For the thermal properties of
the title complexes are almost the same and the experimen-
tal condition does not change, we can take c as a constant. It
can be inferred from Eq. (3) that as the probability of
nonradiative transition g increases, R exhibits a correspond-
ing increase. The PA and emission intensities are listed in
Table 1 with the values of R for comparison.
Table 1
PA ðl ¼ 304 nmÞ and emission intensities ðlex ¼ 304 nmÞ of the
title compounds
PA relative
Emission relative R ¼ P=E
intensity ðPÞ intensity ðEÞ
Eu(Hba)3
1.00
1.00
1.31
1.70
0.71
1.00
0.62
0.33
1.62
Eu0.85La0.15(Hba)3 0.81
Eu0.25La0.75(Hba)3 0.56
Eu0.05La0.95(Hba)3 1.15
Fig. 3. Fluorescence spectra of Eu0.85La0.15(Hba)3. (a) Excitation
spectrum ðlem ¼ 619 nmÞ; (b) emission spectrum ðlex ¼ 304 nmÞ:

1336
Y.-t. Yang, S.-y. Zhang / Journal of Physics and Chemistry of Solids 64 (2003) 1333–1337
It can be seen from Fig. 3(a) that the excitation spectrum
is a broad band due to the absorption of the ligand Hba,
which means that there is an efficient energy transfer
process between Hba and Eu3þ. This is also confirmed by
the emission spectra of Fig. 3(b). Since the excitation
wavelength is fixed in the region of the ligand absorption,
the existence of characteristic emissions of Eu3þ demon-
strates the energy transfer process from the ligand to Eu3þ
.
According to the Dexter’s theory [21], the energy
transfer probability ðTÞ is proportional to the overlap
integral:
!
2pZ2
ð
T ¼
FsðEÞ1aðEÞdE
ð4Þ
h
where FsðEÞ represents the observed shape of the emission
band of the energy donor and the 1aðEÞ is the shape of the
absorption band of acceptor.
Fig. 5. Energy transfer and relaxation processes of Eu12xLax(Hba)3.
the bridging carboxyl groups in Eu12xLax(Hba)3 coprecipi-
tates, according to the IR spectral results. Thus the
fluorescence intensity of Eu3þ is enhanced considerably
by the addition of La3þ through the intermolecular energy
transfer processes.
It has been proposed that there should be a matching
energy difference between the triplet state of the ligand and
the resonance energy level of the central ion for rare earth
complexes that exhibit efficient intramolecular energy
transfer [22]. If the energy gap is too large, the energy
transfer rate will decrease due to the diminution in the
overlap between the donor and acceptor. If the energy gap is
too small, the efficiency of energy transfer will decrease
because of the thermal deexcitation process [23]. The
resonance energy level (5D0) of Eu3þ is at 17300 cm21, and
the triplet state (T1) of Hba is at 24800 cm21. The energy
gap between Hba (T1) and Eu3þ (5D0) is suitable. So the
excited energy on T1 is transferred to the fluorescent ion
Eu3þ, which results the characteristic line-type emissions of
The energy transfer processes of Eu12xLax(Hba)3 are
illustrated in Fig. 5.
After seventy five percent of Eu3þ in Eu(Hba)3 are
substituted by La3þ, however, the fluorescence intensity of
Eu3þ begins to decrease quickly because of over replacing
the fluorescent ion Eu3þ by La3þ, which results in the
excessive decrease of the acceptor.
As can be seen in Table 1, for Eu12xLax(Hba)3 (x: 0.15
or 0.75) complex, the addition of La3þ enhances the
fluorescence intensity of Eu3þ. As the probability of
radiative transition increases, the value of R exhibits a
conrespondent decrease. While for Eu0.05La0.95(Hba)3, the
addition of La3þ weakens the fluorescence intensity of Eu3þ
due to the excessive decrease of the acceptor in the complex.
The value of R for Eu0.05La0.95(Hba)3 increases remarkably,
because the probability of nonradiative transition increases.
For Eu3þ–Nd3þ–Hba systems, the result is different.
The fluorescence intensity of Eu3þ is quenched remarkably
with the addition of the second rare earth ion Nd3þ at any
concentration. This is because that the numerous closely
packed excited states of Nd3þ provide paths for efficient
quenching of the excited states of the complex, and Nd3þ
ion acts as a competitive energy acceptor with Eu3þ ion. A
large part of the excited energy of Eu(Hba)3 molecules is
transferred to Nd3þ ion through an intermolecular energy
transfer process, dominantly following by nonradiative
transitions to the ground state which results in weakening
Eu3þ
.
3.3.2. Intermolecular energy transfer
For Eu12xLax(Hba)3 complexes, the intramolecular
energy transfer can explain the characteristic emissions of
Eu3þ, but it cannot explain the changes of fluorescence
intensity of Eu3þ with the addition of La3þ. Considering the
Foster [24] and Dexter’s theories, energy can be transferred
to molecules in short distance by an intermolecular energy
transfer. The efficiency of intermolecular energy transfer is
dependent on close approach or contact between the donor
and acceptor. For the system studied, Eu12xLax(Hba)3 are
prepared by coprecipitation. The short distance between
molecules in the coprecipitates makes the intermolecular
energy transfer possible.
La3þ has no low-lying energy levels, the energy
absorbed by La(Hba)3 molecule cannot be dissipated
through the central La3þ ion. The excited energy on the
ligand triplet state would be gathered. So La(Hba)3
molecules can be energy donors and become the enhancing
chelates. The stabilized triplet state of La(Hba)3 transfers
the energy to the nearby molecules Eu(Hba)3 (acceptor) in
the aggregated particles. The excited energy of La(Hba)3
may also be transferred to the neighbor Eu(Hba)3 through
the luminescence intensity of Eu3þ
.
Experimental results show that the fluorescence
enhancement and quenching phenomena cannot be found
in the mixtures of Eu(Hba)3 and Ln(Hba)3 (Ln3þ: La3þ or
Nd3þ) in any proportion. As the mixture is obtained in a
mechanical way, the distance between the molecules is too
far to incur an intermolecular energy transfer.

Y.-t. Yang, S.-y. Zhang / Journal of Physics and Chemistry of Solids 64 (2003) 1333–1337
1337
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4. Conclusions
The PA and fluorescence spectra of Eu3þ–La3þ–Hba
complexes are measured and interpreted. With the increase
of La3þ mole fraction, the fluorescence intensity of Eu3þ
increases correspondently, and reaches it maximum for
Eu0.25La0.75(Hba)3, then decreases quickly. The ratio ðRÞ of
PA to fluorescence signals is used to interpret the co-
fluorescence effect of the complexes. For Eu12xLax(Hba)3
(x: 0.15, 0.75) complexes, the additions of La3þ enhance the
fluorescence intensity of Eu3þ considerably. The values of R
decrease as the probability of radiative transition increases
for the complexes. While for Eu0.05La0.95(Hba)3, the
addition of La3þ weakens the fluorescence intensity of
Eu3þ. The value of R for Eu0.05La0.95(Hba)3 increases, for
the probability of nonradiative transition increases. The
energy transfer processes of the coprecipitates are inter-
preted in depth from two aspects: nonradiative process and
radiative process.
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