Structural investigation and photoluminescent properties of gadolinium(III), europium(III) and terbium(III) 3-mercaptopropionate complexes

Article abstract of DOI:10.1007/s10895-013-1288-5

This work reports on the synthesis, crystallographic determination and spectroscopic characterization of gadolinium(III), terbium(III) and europium(III) 3-mercaptopropionate complexes, aqua-tris(3-mercaptopropionate) lanthanide(III) - [Ln(mpa)₃(H₂O)]. The Judd-Ofelt intensity parameters were experimentally determined from emission spectrum of the [Eu(mpa)₃(H₂O)]complex and they were also calculated from crystallographic data. The complexes are coordination polymers, where the units of each complex are linked together by carboxylate groups leading to an unidimensional and parallel chains that by chemical interactions form a tridimensional framework. The emission spectrum profile of the [Eu(mpa) ₃(H₂O)] complex is discussed based on point symmetry of the europium(III) ion, that explains the bands splitting observed in its emission spectrum. Photoluminescent analysis of the [Gd(mpa)₃(H ₂O)] complex show no efficient ligand excitation but an intense charge transfer band. The excitation spectra of the [Eu(mpa)₃(H ₂O)] and [Tb(mpa)₃(H₂O)] complexes do not show evidence of energy transfer from the ligand to the excited levels of these trivalent ions. Therefore the emission bands are originated only by direct f-f intraconfigurational excitation of the lantanide(III) ions.

Full text of DOI:10.1007/s10895-013-1288-5

J Fluoresc (2014) 24:203–211
DOI 10.1007/s10895-013-1288-5
ORIGINAL PAPER
Structural Investigation and Photoluminescent Properties
of Gadolinium(III), Europium(III) and Terbium(III)
3-Mercaptopropionate Complexes
E. R. Souza & I. O. Mazali & F. A. Sigoli
Received: 10 July 2013 /Accepted: 9 August 2013 /Published online: 18 August 2013
#
Springer Science+Business Media New York 2013
.
.
Abstract This work reports on the synthesis, crystallo-
graphic determination and spectroscopic characterization of
gadolinium(III), terbium(III) and europium(III) 3-mercapto
propionate complexes, aqua-tris(3-mercaptopropionate)
lanthanide(III) - [Ln(mpa)3(H2O)]. The Judd-Ofelt intensity
parameters were experimentally determined from emission
spectrum of the [Eu(mpa)3(H2O)]complex and they were also
calculated from crystallographic data. The complexes are
coordination polymers, where the units of each complex are
linked together by carboxylate groups leading to an unidimen-
sional and parallel chains that by chemical interactions form a
tridimensional framework. The emission spectrum profile of
the [Eu(mpa)3(H2O)] complex is discussed based on point
symmetry of the europium(III) ion, that explains the bands
splitting observed in its emission spectrum. Photoluminescent
analysis of the [Gd(mpa)3(H2O)] complex show no effi-
cient ligand excitation but an intense charge transfer band.
The excitation spectra of the [Eu(mpa)3(H2O)] and
[Tb(mpa)3(H2O)] complexes do not show evidence of
energy transfer from the ligand to the excited levels of
these trivalent ions. Therefore the emission bands are
originated only by direct f-f intraconfigurational excitation
of the lantanide(III) ions.
Keywords Luminescence Lanthanide
3-mercaptopropionate
Introduction
Coordination polymers consist on spatial structures formed by
metal ions and ligands where the unities of the complex are
bonded forming chains in one, two or three dimensions [1, 2].
This class of compounds has a wide possibility of spatial
organization [3, 4]. Some coordination polymers form porous
structures allowing them to be applicable in many areas, such
as gas purification and separation [5–7], drugs storage [8],
heterogeneous catalysis [9] etc.
Metal ions in coordination polymers have been studied and
developed due to their wide applications such as gas storage,
insertion of other molecules in the structure lattices [10]. Their
spatial structures can be extended in one (1D), two (2D) or three
dimensions(3D). To build these kinds of structures it is com-
mon to use multidentate ligands, for example carboxylates. The
carboxylate group may present many coordination modes, from
mono to tridentate [11], and that is one of the reasons because
the carboxylate ligands are widely used in building inorganic
frameworks. The ligand 3-mercaptopropionic acid (Hmpa), is
often used as surface stabilizer of metal nanoparticles [12, 13]
and quantum dots [14, 15]. As nucleophile, the sulfur atom of
the Hmpa may interfere in photochemistry of other complexes
such as iron(II) complexes [16] or act as reducing agent [17,
18]. For instance, Su et al. [19] used Hmpa combined to DNA-
Cu/Ag nanoclusters to detect Cu(II) ions by fluorescence
quenching. Also in the biological field, Prasad et al. [20]
observed that the Schiff base oxovanadium(IV) complexes of
phenanthroline bases does not show DNA photocleavage ac-
tivity anymore in presence of 3-mercaptopropionic acid
(added as reducing agent), due to the poor redox activity
oxovanadium(IV) complexes showing quasi-reversible
CCDC 883721 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-013-1288-5) contains supplementary material,
which is available to authorized users.
:
:
E. R. Souza I. O. Mazali F. A. Sigoli (*)
Laboratory of Functional Materials - Institute of Chemistry,
University of Campinas - UNICAMP, Campinas, SP 13083-970,
Brazil
e-mail: fsigoli@iqm.unicamp.br

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J Fluoresc (2014) 24:203–211
100
90
80
70
60
50
40
V(IV)–V(III) redox processes near -0.6 V. Most of the reports
about the use of 3-mercaptopropionic acid on coordination
compounds are related to transition metal ions.
515 °C
TG
14
12
10
8
DTA
3.58%
Specifically about lanthanaide(III) complexes with
mercaptopropionate ligand, Bear et. al. [21] determined the
stability constants in aqueous solution for many lanthanide 2-
and 3-mercaptopropionate complexes, concluding that the
ligands act as monodentate. Later Choppin and Martinez-
Perez [22] corrected the chemical stability values obtained
previously and measured the complexation enthalpy and en-
tropy energies (ΔH, ΔS) of the 2- and 3-mercaptopropionate
ligand with the trivalent lanthanide ions. It is important to state
that these measurements concerns on the coordination of only
one ligand to an aqueous lanthanide trivalent ion.
6
4
42.16%
800
2
0
100
200
300
400
500
600
700
900
Temperature (°C)
However, considering the best of our knowledgement there
is no description about the crystal structure and
photoluminescent properties of the lantanthanides(III)-based
3-mercaptopropionate complexes. Therefore, this work de-
scribes the crystalline structure of three isomorphic one tridi-
mensional lanthanide coordination polymers using 3-
mercaptopropionate acid as ligand (Fig. 1). The structural
and photoluminescent properties of gadolinium(III),
terbium(III) and europium(III) complexes will be shown and
discussed.
Fig. 2 TG/DTA of the [Eu(mpa)3(H2O)] complex. The data were
obtained between 27 °C and 900 °C under synthetic air dynamic
atmosphere
pellets. The photoluminescence data were obtained in a
Fluorolog-3 spectrofluorometer (Horiba FL3-22-iHR320),
Table 1 Crystal data and structures for [Eu(mpa)3(H2O)] complex
C9H14EuO7S3
Z =8
Mr =482.34
F(000)=1880
Dx=1.950 Mg m−3
Orthorhombic, Pbca
Experimental Details
Hall symbol: -P 2 ac2ab Mo Kα radiation, λ=0.71073 Å
a =12.2866 (5) Å
b =7.7334 (3) Å
c =34.5819 (15) Å
V =3285.9 (2) Å3
μ=4.22 mm−1
The lanthanides oxides were purchased from Aldrich and used
without further purification. The lanthanides(III) chloride
(LnCl3) were prepared by dissolving Eu2O3, Gd2O3 and
Tb4O7 in HCl. The excess of acid was eliminated by evapo-
ration and the LnCl3 were dried under vacuum into a desic-
cator. Then 3 · 10−3 mol of Hmpa were added to 5 mL of water
containing 3 · 10−3 mol of NaOH in order to neutralize de acid.
After solubilization 1 · 10−3 mol of RECl3 in 5 mL of water
was added under stirring. The solutions were maintained
under stirring for 1 hour and then left to evaporate. After
1 week the crystals of the complexes were filtered off, washed
using cold water and kept under vacuum into a desiccator. The
thermogravimetric analyses were obtained using a TA SDT
Q600 from 27 °C up to 900 °C with 10 °C/min as a heating
rate under a synthetic air flow of 100 mL/min. The Fourier
transform infrared spectra (FT-IR) were carried out with a
Bomem MB – Series model B 100 FTIR spectrometer, with
a resolution equal to 4 cm−1 and 32 scans per spectrum, in KBr
T =296 K
colourless
0.95×0.16×0.14 mm
Radiation source: fine-
focus sealed tube
Graphite
R
int=0.034
θmax=27.9°, θmin=1.2°
47527 measured
reflections
3912 independent
reflections
h =−15→16
k =−9→10
3463 reflections with I > l =−16→45
2σ(I)
Refinement on F2
Primary atom site location: structure-invariant
direct methods
Least-squares matrix: full Secondary atom site location: difference
Fourier map
R[F2>2σ(F2)]=0.038
wR(F2)=0.081
S =1.24
Hydrogen site location: inferred from
neighbouring sites
H atoms treated by a mixture of independent
and constrained refinement
w =1/[σ2(Fo2)+(0.P)2+25.5935P] where
Fig. 1 Molecular structure of the
HO
SH
P=(Fo2+2Fc2)/3
Hmpa ligand
3912 reflections
189 parameters
0 restraints
(Δ/σ)max=0.008
Δ〉max=1.14 e Å −3
Δ〉min=−2.31 e Å −3
O

J Fluoresc (2014) 24:203–211
205
Table 2 Distances be-
tween europium(III) ion
and surrounding oxygen
atoms in the
[Eu(mpa)3(H2O)]
complex
Bond
Distance (Å)
Eu1—O6
Eu1—O1i
Eu1—O7
Eu1—O2ii
Eu1—O3
Eu1—O4
Eu1—O1
Eu1—O5
Eu1—O2
2.365 (4)
2.393 (4)
2.444 (4)
2.463 (4)
2.468 (4)
2.470 (4)
2.488 (3)
2.509 (4)
2.513 (4)
Fig. 3 ORTEP diagram of the [Eu(mpa)3(H2O)] crystal
Table 3 Angles be-
tween Eu3+ ion and sur-
rounding oxygen atoms
in the [Eu(mpa)3(H2O)]
complex
with double-gratings (1,200 gr/mm, 330 nm blaze) in the
excitation monochromator and double-gratings (1,200 gr/
mm, 500 nm blaze) in the emission monochromator. An
ozone-free Xenon lamp of 450 W (Ushio) was used as a
radiation source. The excitation spectra were obtained be-
tween 200 and 600 nm and they were corrected in real time
according to the lamp intensity and the optical system of the
excitation monochromator using a silicon diode as a reference.
The emission spectra were carried out between 350 and
720 nm using the front face mode at 22.5°. All of them were
corrected according to the optical system of the emission
monochromator and the photomultiplier response (Hamama-
tsu R928P). The time resolved emission spectra were mea-
sured using a phosphorimeter system using time delay in a
range from 0.015 ms to 0.13 ms, in order to get only the
emissions from triplet states of the ligands. The emission
decay curves were obtained with a pulsed 150 W Xenon lamp
using a phosphorimeter. The absolute quantum yields were
measured using a Quanta-φ (Horiba F-309) integrating sphere
equipped with an optical-fibers bundle (NA=0.22 - Horiba-
FL-3000/FM4-3000).X-Ray data were collected in a Bruker
Apex with CCD detector and using Mo Kα radiation. The
crystal structure was solved and refined using the software
SHELXL97[23]. All estimated standard derivations (esds),
except the esd in the dihedral angle between two l.s. planes,
Atoms
Angle (°)
O6—Eu1—O1i
O6—Eu1—O7
O1i—Eu1—O7
O6—Eu1—O2ii
O1i—Eu1—O2ii
O7—Eu1—O2ii
O6—Eu1—O3
O1i—Eu1—O3
O7—Eu1—O3
O2ii—Eu1—O3
O6—Eu1—O4
O1i—Eu1—O4
O7—Eu1—O4
O2ii—Eu1—O4
O3—Eu1—O4
O6—Eu1—O1
O1i—Eu1—O1
O7—Eu1—O1
O2ii—Eu1—O1
O3—Eu1—O1
O4—Eu1—O1
O6—Eu1—O5
O1i—Eu1—O5
O7—Eu1—O5
O2ii—Eu1—O5
O3—Eu1—O5
O4—Eu1—O5
O1—Eu1—O5
O6—Eu1—O2
O1i—Eu1—O2
O7—Eu1—O2
O2ii—Eu1—O2
O3—Eu1—O2
O4—Eu1—O2
O1—Eu1—O2
O5—Eu1—O2
73.81 (13)
142.20 (13)
71.83 (13)
139.32 (14)
144.18 (14)
72.40 (14)
76.82 (14)
76.29 (13)
109.30 (15)
117.73 (12)
77.54 (14)
143.17 (14)
140.23 (14)
71.17 (15)
75.01 (15)
125.42 (13)
106.49 (12)
79.82 (12)
68.42 (11)
52.05 (12)
72.46 (14)
73.47 (14)
117.38 (12)
109.43 (14)
73.70 (13)
141.25 (15)
74.75 (14)
135.96 (12)
75.10 (14)
69.08 (11)
78.34 (13)
101.99 (11)
140.13 (12)
124.57 (14)
157.98 (13)
51.62 (12)
Fig. 4 Coordination polyhedron around europium(III) ion showing the
tricapped trigonal prism geometry

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J Fluoresc (2014) 24:203–211
Fig. 5 Viewing of one chain of the [Eu(mpa)3(H2O)] complex along c axis (a) and along b axis (b). Each one dimensional chain is surrounded by other
six parallel one dimensional structures (c)
are estimated using the full covariance matrix. The cell esds
are taken into account individually in the estimation of esds in
distances, angles and torsion angles; correlations between esds
in cell parameters are only used when they are defined by
crystal symmetry. An approximate (isotropic) treatment of cell
esds is used for estimating esds involving l.s. planes. Refine-
ment of F2 against all reflections. The weighted R-factor wR
and goodness of fit S are based on F2, conventional R-factors
R are based on F, with F set to zero for negative F2. The
threshold expression of F2 > 2sigma(F2) is used only for
calculating R-factors(gt) etc. and is not relevant to the choice
of reflections for refinement. R-factors based on F2 are statis-
tically about twice as large as those based on F, and R- factors
based on all data will be even larger.
(a)
(b)
(c)
(d)
(e)
Results and Discussion
The yields of the chemical reactions were: [Eu(mpa)3(H2O)]:
46 %; [Gd(mpa)3(H2O)]: 58 %; [Tb(mpa)3(H2O)]: 65 %.
Elemental analysis of the [Eu(mpa)3(H2O)] complex (%):
calcd/obs for C9H17O7S3Eu: C, 22.28/21.91; H, 3.53/
2.15; Eu, 31.32/32.33 %. Elemental analysis of the
[Gd(mpa)3(H2O)] complex (%): calcd/obs for C9H17O7S3Gd:
C, 22.04/22.31; H, 3.49/2.07; Tb, 32.06/31.71 %. Elemental
analysis of the [Tb(mpa)3(H2O)] complex (%): calcd/obs for
2600
2500
4000
3000
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 6 FT-IR spectra of the Hmpa ligand (a), its ionic sodium salt (b) and
the complexes: [Eu(mpa)3(H2O)] (c), [Gd(mpa)3(H2O)] (d) and
[Tb(mpa)3(H2O)] (e)

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207
Table 4 FT-IR bands (cm −1) of the Hmpa, its ionic salt, Nampa, and [Ln(mpa)3(H2O)] complexes
Samples
Hmpa
Nampa
[Eu(mpa)3(H2O)]
[Gd(mpa)3(H2O)]
[Tb(mpa)3(H2O)]
νO-H
3080
3360
3470, 3160
3470, 3160
3470, 3190
νC-H
2947
2921, 2912
2942,2929
2942,2929
2942,2929
Acid dimer
νS-H
2664, 2570
–
–
–
–
–
–
2565, 2552
2565, 2552
2565, 2552
νC=O
1713
1415
1250
930
–
–
–
–
–
δ
C-O-H (in plane)
νC-OH
C-O-H (out of plane)
νas(COO
–
–
–
–
–
–
–
–
δ
–
–
–
–
−
1547
1417
130
1576, 1552
1431
1576, 1550
1430
1577, 1550
1431
)
νs(COO
−
)
–
ΔνCOO
−
–
145, 121
1309, 1300
146, 120
1309, 1300
146, 121
1309, 1300
νas(C-S)
–
1314, 1301
C9H17O7S3Tb: C, 21.96/22.00; H, 3.48/1.86; Tb, 32.29/
31.61 %.
complex is shown in Fig. 3 with labeled atoms. The structure
was determined as an orthorrombic system and space group
Pbca. Powder X-Ray diffractions of the obtained complexes
(ESI – Figure S3) present the similar diffraction pattern for all
complexes, indicating that they are isomorphic. Therefore
only the crystallographic data of the [Eu(mpa)3(H2O)] com-
plex are shown and it will be used for the explanation of the
photoluminescent data.
From the ORTEP diagram is possible to observe that the
europium(III) ions are coordinated by nine oxygen ions where
one of them belongs to the water molecule. The geometry of
the first coordination sphere around the metallic ion forms a
slightly distorted tricapped trigonal prism (Fig. 4), where the
europium(III) ions are located in a point symmetry close to
D3h.
The thermogravimetric curves of the [Ln(mpa)3(H2O)]
complexes are quite similar, the curve of the [Eu(mpa)3(H2O)]
complex is shown in Fig. 2 and the TG/DTA curves of
[Tb(mpa)3(H2O)] and [Gd(mpa)3(H2O)] ones are shown on
electronic supplementary material (ESI – Figure S1). In gen-
eral, just one endothermic event is observed at 78 °C and it
may be attributed to the loss of a coordinated water molecule,
corresponding to a mass loss varying from 3.4 up to 3.6 % for
all complexes. The decomposition of the ligand occurs from
200 up to 700 °C. The final residual masses are between 42
and 45 % of the initial ones and the [Eu(mpa)3(H2O)] residue
was identified as europium oxysulfates, Ln2O2SO4 by powder
X-Ray diffraction of the residues (ESI – Figure S2).
Details of the crystal structure of the [Eu(mpa)3(H2O)]
complex are shown in Table 1. Bond distances and angles
around the europium(III) ion are shown in Tables 2 and 3,
respectively. The ORTEP diagram of the [Eu(mpa)3(H2O)]
Each europium(III) ion is coordinated by six carboxylate
groups. Two of them are bridging mode bonded, two as chelate
mode and two carboxylate primarily bonded as a chelate also
bond as bridging mode to a neighbor europium(III) ion. The
77 K: λem = 380 nm
300 K: λem = 385 nm
77 K: λex = 250 nm
300 K: λem = 250 nm
(a)
(b)
77 K: λem = 380 nm
S0→S1
S0→S1
120x
275
300
325
350
250
275
300
325
350
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 7 Excitation (a) and emission (b) spectra of the [Gd(mpa)3(H2O)] complex obtained at 77 and at 300 K. The inset shows a magnification of the
excitation spectrum obtained at 77 K

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J Fluoresc (2014) 24:203–211
baricentro: 17257 cm-1
FWHM= 9,7 cm-1
λex, 77 K = 396 nm
λ
em, 77 K = 618 nm
5L6
S1→S0
5G3
5D4
7F0→
T1→S0
5D2
578
579
580
5D0→
7F2
0.015
0.041
0.068
0.095
0.12
7F1
7F4
7F0
7F3
ex, 300 K = 396 nm
300
350
400
450
Wavelength (nm)
λ
λem, 300 K = 618 nm
Fig. 8 Time resolved emission spectra of the [Gd(mpa)3(H2O)] complex
obtained at 77 K using time delay between 0.015 ms and 0.13 ms
250
300
350
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 10 Excitation and emission spectra of the [Eu(mpa)3(H2O)] com-
plex obtained at 77 and 300 K. The inset shows the 5D1→5D1 transition
obtained at 77 K
carboxylate groups are bond to the central ion in two different
modes (Fig. 4). The coordination centers are unidimensionally
bonded along b axis. The angle formed by consecutive
europium(III) ions is 145.21°, forming a zigzag chain, where
the europium(III) positions are alternated along with b axis.
Each sequence of europium(III) chain is surrounded by six
other parallel one dimensional structures forming a 3D frame-
work (Fig. 5c).
The FT-IR spectra of the Hmpa ligand, its ionic sodium salt
and the [Ln(mpa)3(H2O)] complexes are shown in Fig. 6. The
bands attributed to the COOH group of the Hmpa (Fig. 6a),
such as C=O stretching (1,713 cm−1), symmetric and asym-
metric acid dimmer stretching (2,664 and 2,570 cm−1), C-OH
sctretching (1,250 cm−1) and wagging (1,415 and 930 cm−1)
does not appear in the spectra of the complexes (Fig. 6c-d) .
Instead, symmetric and asymmetric stretching modes of the
carboxylate group are observed in the complexes FT-IR spec-
tra (Fig. 6c-d). In order to evaluate the energy of the vibra-
tional modes of the purely ionic carboxylate group, its sodium
mercaptopropionate salt, Na(mpa), was prepared. The sym-
metric stretching mode of the carboxylate group of the sodium
salt is observed at 1,417 cm−1 and the asymmetric one appears
around 1,547 cm−1. In the case of complexes, the difference
between the wavenumbers of asymmetric and symmetric
modes of carboxylate group (Δν) is indicative of its coordi-
nation modes [24]. When a carboxylate ligand is coordinated
by a monodentate mode the value of Δν of complex is higher
than the value of Δν of ligand salt; for bidentate chelate mode
the value of Δν of complex is smaller than the value of Δν of
ligand salt and for bidentate bridge mode the value of Δν of
complex is a similar to the value of Δν of ligand salt24. In the
case of [Ln(mpa)3(H2O)] complexes, the symmetric stretching
mode is around 1,430 cm−1 and the asymmetric mode of the
carboxylate group splits in two bands (1,552 and 1,576 cm−1).
The asymmetric stretching mode at 1,576 cm−1 leads Δν
equal to 146 cm−1 indicating a chelate bridging mode and
the other asymmetric stretching mode at 1,552 cm−1 leads Δν
equal to 121 cm−1 indicating a bidentate chelate mode, as
determined by the ORTEP diagram (Fig. 3).
In the FT-IR spectra of the [Ln(mpa)3(H2O)] complexes,
the S-H stretching mode also splits in two bands the first one at
2,565 and the second at 2,552 cm−1 (Fig. 6-inset). This result
indicates the presence of hydrogen interactions between S-H
groups. The crystalline structure determined by X-Ray dif-
fraction indicates that the smallest distance between two sulfur
atoms is equal to 3.735 Å that makes possible the existence of
hydrogen interaction. Therefore, the splitting of the S-H
stretching band may be attributed to the symmetric and asym-
metric stretching of the two interacting S-H groups. The main
vibrational bands of the ligand and the complexes are shown
in Table 4.
5L10
5G6
λ
em, 77 K = 542 nm
5G4
λ
ex, 77 K = 352 nm
5D4
→
7F5
7F6
→
5L6
5H7
5H5
7F6
7F4
7F3
5F3, 5I7
7F1
7F2
7F0
λ
em, 300 K = 545 nm
λ
ex, 300 K = 369 nm
250 300 350
400
450
500
550
600 650
Wavelength (nm)
The excitation and emission spectra of the [Gd(mpa)3(H2O)]
complex are shown in Fig. 7. The excitation spectra obtained at
Fig. 9 Excitation and emission spectra of the [Tb(mpa)3(H2O)] complex
obtained at 77 and 300 K

J Fluoresc (2014) 24:203–211
209
Table 5 Judd-Ofelt intensity parameters (Ω), polarizability of bonding
atoms (α), charge factor (g), emission lifetime (τ) of 5D0 level, emission
coefficients (A), ratio of the integrated areas of the 5D0→7F0 and 5D0→7F2
transitions (R02), quantum efficiency (η) and absolute quantum yield (Φ) of
[Eu(mpa)3(H2O)] complex
Ω2
Ω4
α(COO−) α(H2O) g(COO-) g(H2O)
τ
Arad Anrad Atot
R02
η
Φ
(10−20 cm2) (10−20 cm2)
(ms) (s−1 (s−1 (s−1
)
)
)
(10−3
)
(%) (%)
Spectral values 6.05
Crystal values 6.06
5.43
1.34
–
–
–
–
0.80 312
938
1250 0.55
2.19
25
16
3.60
0.50
0.90
0.30
–
–
–
–
–
–
300 and at 77 K show just one intense band with maximum at
energies higher than 250 nm. According to its energy and
relative intensity this band may be attributed to a charge transfer
band (CT). At 275 and 320 nm it is possible to observe two
other bands much less intense than CT one (Fig. 7a-inset). The
latter bands may be attributed to the excitation of the ligand
from fundamental state S0 to the excited states S2 and S1,
respectively. Under excitation at 250 nm the [Gd(mpa)3(H2O)]
complex has an emission band with maximum at 380 nm
(Fig. 7b).
Time resolved emission spectra (Fig. 8) of the
[Gd(mpa)3(H2O)] was performed in order to attribute the
broad emission band shown in Fig. 7b. These spectra were
obtained at 77 K under excitation at 250 nm. From the time
resolved emission spectra (Fig. 8) it is possible to distinguish
two emission bands. At short delay time (0.015 ms), the first
band, between 270 and 290 nm has a higher intensity than the
second one observed between 350 and 450 nm. As a function
of the delay time their intensities are inverted due to the
difference between the emission lifetimes. Therefore, the data
indicates that the bands may be attributed to the spin-allowed
S1→S0 transition (fluorescence) and to the spin-forbidden
S1→T1 transition (phosphorescence), respectively.
similar. The excitation spectra do not show any charge transfer
band. That means that there is no energy transfer from the
ligand to the excited level of the terbium(III) ion once the
5
emission spectrum is monitored at the D4→7F5 transition.
Both excitation and emission spectra of the [Tb(mpa)3(H2O)]
just show transitions attributed to f-f intraconfigurational of
terbium(III) ion due to the high energies of the ligand excited
states. Terbium(III) complexes present low back-transfer rate
when the energy of the triplet state of the ligand is higher than
22,300 cm−1 [25, 26], that is about 1,850 cm−1 higher than the
5D4 emitting level of the terbium(III) ion. The absolute quan-
tum yield of the [Tb(mpa)3(H2O)] complex, excited at 369 nm
(7F6→5L10 transition) was determined as 19 %.
The excitation spectra of the [Eu(mpa)3(H2O)] complex
(Fig. 10) were obtained at 77 and 300 K exhibiting only
7
narrow transitions that arises from F0,1 levels of the
europium(III) ion to its excited levels (Fig. 10). The emission
spectra obtained at obtained at 77 and 300 K are quite similar
5
showing no significant emission from ligand or from D1
level. The number of Stark splitting of the europium(III)
5
transitions that arises from D0 emitting level is higher than
expected for a D3h symmetry. That happens because of the
distortion of the symmetry site around the europium(III) ion
5
The excitation and emission spectra (Fig. 9) of the
[Tb(mpa)3(H2O)] obtained at 300 and at 77 K are quite
the complex. The D0→7F0 transition (Fig. 10-inset), that
should be absent for europium(III) ions in a D3h symmetry is
[Eu(mpa)3(H2O)]
[Tb(mpa)3(H2O)]
[Eu(mpa)3(H2O)]
[Tb(mpa)3(H2O)]
1
2
3
4
1
2
3
4
Time (ms)
Time (ms)
Fig. 11 Emission decay curves of the [Eu(mpa)3(H25O)] and
[Tb(mpa)3(H2O)] complexes obtained at 300 K, monitoring the D0→7F2
transition of the europium(III) ion and the transition 5D4→7F5 of the
terbium(III) ion. The excitations were done at 396 nm and 352 nm for the
[Eu(mpa)3(H2O)] and [Tb(mpa)3(H2O)] complexes, respectively

210
J Fluoresc (2014) 24:203–211
detected indicating the distortion around the D3h symmetry.
The 5D0→7F0 transition is narrow (FWHM=9.7 cm−1,
Fig. 10-inset) indicating a well distributed point symmetry
around europium(III) ions through the sample.
The sum of all spontaneous emission coefficient gives the
radiative rate Arad. The sum of radiative Arad and non-
radiative Anrad rates is called total rate Atot, that is equal to
the inverse of the lifetime τ of emitting state. So that the
quantum efficiency is calculated as:
The luminescent parameters of the [Eu(mpa)3(H2O)] were
measured from the spectral data and are shown on Table 5.
Figure 11 exhibit the emission decay curves of the emitting
levels, 5D0 and 7F5 of the europium(III) and terbium(III) ions,
respectively. The curves were adjusted with a first order
exponential decay function indicating that in both complexes
the lanthanide ions are located in a similar chemical environ-
ment as identified by monocrystal X-ray diffraction (Fig. 3).
Despite the water molecule in its coordination sphere, the
europium(III) ion has a relatively long emission lifetime,
0.80 ms if compared with other europium(III) carboxylate
coordination polymers26 and even to europium(III) β-
diketonate complexes. It happens probably due to the poly-
meric structure of the europium(III) complex that results in
less vibronic coupling decreasing the non-radioactive rates
Arad
Arad
Atot
η ¼
¼
¼ τArad
Arad þ Anrad
Judd-Ofelt intensity parameters as well as polarizability
and charge factor of the bonding atoms [29] of the
[Eu(mpa)3H2O] complex were calculated from radiative rate
obtained from the emission spectrum at 300 K and using the
Judd-Ofelt theory (Table 5), respectively.
From Table 5 it is possible to notice the low values of decay
rates (A) when compared to other complexes [30, 31], even
hydrated carboxylates.26 Despite the low Arad, value, the
quantum efficiency of the [Eu(mpa)3(H2O)] complex is not
so low due to the relatively low value of the Atot leading to a
considerable value of the emission lifetime [32]. From the data
(Table 5) one may observe the higher values of polarizability
(α) than the charge factor (g) of the carboxylate group indi-
cating the important contribution of the dynamic coupling
mechanism on the intensity emission of the europium(III)
ion in the complex.
5
from the D0 emitting level. Due to the higher energy of the
5D4 terbium(III) emitting level compared to the 5D0
europium(III) emitting level, usually terbium(III) complexes
have higher lifetime values, 1.42 ms for [Tb(mpa)3(H2O)]
complex.
5
To each observable D0→7FJ transition in the emission
spectrum it is associated a spontaneous emission coefficient
5
A
0→J. D0→7F1 transition is strictly allowed by magnetic
dipole mechanism, its value is almost insensitive to changes
in the chemical environment around europium(III) ion and it is
assumed to be A0→1=50 s−1. This transition is an internal
reference and is used to calculate A0→2 and A0→4, as follows:
Conclusion
A set of three lanthanide(III) 3-mercaptopropionate complexes
were synthesized and characterized. The complexes are coor-
dination polymers, where the units of each complex unit is
linked together by carboxylate groups leading to an unidimen-
sional and parallel chains that by chemical interactions form a
tridimensional (3D) framework. Photoluminescent data of
[Gd(mpa)3(H2O)] complex suggest a high energy of the T1
state (28,000 cm−1) that do not contribute to the energy transfer
to europium(III) or terbium(III) on their respective complexes.
However the structural rigidity of the complexes contributes to
decrease non-radiactive processes from emitting levels 5D0 and
5D4 of the europium(III) and terbium(III), respectively, as
shown by the values of emission lifetime, quantum efficiency
and absolute quantum yield.
ꢀ
ꢁꢀ
ꢁ
σ0→1
σ0→J
I0→J
I0→1
A0→J ¼ A0→1
where σ and I are, respectively, the barycenter and the inte-
grated area under the transition at emission spectrum. Using
experimental values of A0→J, the experimental intensity pa-
rameters Ωλ [27, 28] are calculated as
3hc3A0→J
Ωλ ¼
ꢂ
ꢂ
ꢂ
2
5D0〉
λ
4e2ω3χ〈7 F j
U
ꢂ
Acknowledgment The authors acknowledge to Capes, CNPq and
FAPESP for financial support. The authors also would like to thank Prof.
C. H. Collins (IQ-UNICAMP, Campinas, Brazil) for English revision and
the Laboratory of Advanced Optical Spectroscopy (LMEOA/IQ-
UNICAMP/FAPESP grant # 2009/54066-7) for use of its equipment.
This work is a contribution of the National Institute of Science and
Technology in Complex Functional Materials (CNPq-MCT/Fapesp).
where χ =n(n +2)2/9 is the Lorentz local field correction
assuming refractive index n =1.5, and 〈7Fj‖Uλ‖5D0〉 are the
square reduced matrix elements, and are equal to 0.0032 for
J =2 and 0.0023 for J =4.

J Fluoresc (2014) 24:203–211
211
16. Chmura A, Szaciłowski K, Waksmundzka-Góra A, Stasicka Z (2006)
Nitric Oxide 14:247
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