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Citation: Butorlin, O.S.; Petrova, A.S.;
Toikka, Y.N.; Kolesnikov, I.E.; Orlov,
S.N.; Ryazantsev, M.N.; Bogachev,
N.A.; Skripkin, M.Y.; Mereshchenko,
A.S. The Structure and Optical
Properties of Luminescent Europium
Terephthalate Antenna Metal–Organic
Frameworks Doped by Yttrium,
Gadolinium, and Lanthanum Ions.
Molecules 2024,29, 3558. https://
doi.org/10.3390/molecules29153558
Academic Editor: Pierre D. Harvey
Received: 3 July 2024
Revised: 23 July 2024
Accepted: 26 July 2024
Published: 28 July 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
molecules
Article
The Structure and Optical Properties of Luminescent Europium
Terephthalate Antenna Metal–Organic Frameworks Doped by
Yttrium, Gadolinium, and Lanthanum Ions
Oleg S. Butorlin 1, Anna S. Petrova 1, Yulia N. Toikka 1, Ilya E. Kolesnikov 1, Sergey N. Orlov 1,2,
Mikhail N. Ryazantsev 1,3 , Nikita A. Bogachev 1, Mikhail Yu. Skripkin 1and Andrey S. Mereshchenko 1,*
1Saint Petersburg State University, 7/9 Universitetskaya emb., Saint Petersburg 199034, Russia;
olbuse@mail.ru (O.S.B.); an.petra04.floreo@gmail.com (A.S.P.); helmi24@mail.ru (Y.N.T.);
ilya.kolesnikov@spbu.ru (I.E.K.); orlov.s.n.1989@yandex.ru (S.N.O.);
mikhail.n.ryazantsev@gmail.com (M.N.R.); n.bogachev@spbu.ru (N.A.B.); skripkin1965@yandex.ru (M.Y.S.)
2Institute of Nuclear Industry, Peter the Great St. Petersburg Polytechnic University (SPbSU), 29,
Polytechnicheskaya Street, Saint Petersburg 195251, Russia
3Nanotechnology Research and Education Centre RAS, Saint Petersburg Academic University,
ul. Khlopina 8/3, Saint Petersburg 194021, Russia
*Correspondence: a.mereshchenko@spbu.ru; Tel.: +7-951-677-5465
Abstract: New heterometallic antenna terephthalate MOFs, namely, (Eu
x
M
1−x
)
2
bdc
3·
4H
2
O (M = Y,
La, Gd) (x = 0.001–1), were synthesized by a one-step method from aqueous solutions. The resulting
compounds are isomorphic to each other; the crystalline phase corresponds to Ln
2
bdc
3·
4H
2
O. Upon
300 nm excitation to the singlet excited state of terephthalate ions, all compounds exhibit a bright
red emission corresponding to the of
5
D
0
–
7
F
J
(J = 0–4) f-f transitions of Eu
3+
ions. The Eu(III)
concentration dependence of the photophysical properties was carefully studied. We revealed that
Gd-doping results in photoluminescence enhancement due to the heavy atom effect. To quantitatively
compare the antenna effect among different compounds, we proposed the new approach, where the
quantum yield of the
5
D
0
formation is used to characterize the efficiency of energy transfer from the
ligand antenna to the Eu3+ emitter.
Keywords: metal–organic framework; luminescence; rare earth; europium; yttrium; gadolinium;
lanthanum; antenna effect
1. Introduction
Metal–organic frameworks (MOFs) represent a large class of crystalline materials de-
fined as porous networks, consisting of metallic ions or clusters linked together by organic
multidentate ligands. Due to their well-defined crystallinity, porosity, high stability, and
wide diversity of structures and topologies, these materials have attracted considerable
attention in the past two decades. Rare-earth element (REE) metal–organic frameworks
(MOFs) are of particular interest due to their unique luminescence properties significantly
determined by the type of lanthanide ion. Thus, REE-MOFs have been revealed as promis-
ing candidates for light-emitting materials, sensors, multimodal image contrast agents,
catalysts, and analytes to reveal the hazardous substances in food and enviroment [
1
–
7
].
The f-f transitions are forbidden by selection rules, which results in inefficiency of direct
excitation of lanthanide ions. This problem can be overcome by using the energy transfer
from the excited ligand to the lanthanide ion, which is called the “antenna effect” [
8
,
9
].
The organic ligands, which are used as “antenna” compounds, have high UV absorption
coefficients, easily coordinate with REE ions, and efficiently transfer energy to REE ions.
The mechanism of the antenna effect can be explained as follows. Upon photon absorption,
the ligand in the ground singlet state (S
0
) is promoted to the singlet excited state (S
1
, S
2
, etc.),
followed by fast internal conversion to a lower excited energy level (S
1
). The excited singlet
Molecules 2024,29, 3558. https://doi.org/10.3390/molecules29153558 https://www.mdpi.com/journal/molecules
Molecules 2024,29, 3558 2 of 14
state can then either (i) return to the ground state (S
1→
S
0
) by internal conversion and
fluorescence or (ii) undergo an intersystem crossing to the triplet state. Through internal
conversion, the ligand reaches the lowest triplet level electronic state, T
1
, followed by the
energy transfer to the REE ion [
10
]. In Eu-based antenna complexes, the quantum yield of
luminescence from the europium ion depends on the relative energy level of the ligand
triplet state and the atomic level of the Eu
3+
ion. Energy transfer has been found to occur
from the lowest energy triplet state of the ligand T
1
to the
5
D
J
(J = 0–3) level of the Eu
3+
ion followed by internal conversion to
5
D
0
. The radiative transitions
5
D
0
–
7
F
J
(J = 0–4) to
the term of ground state
7
F of the Eu
3+
ion correspond to the resulting photoluminescence
of such antenna complexes. For the most efficient energy transfer, the difference in en-
ergy between the ligand triplet state and the
5
D
J
level of Eu
3+
should be approximately
2500–4000 cm
−1
[
11
]. However, it has also been proposed that energy transfer can occur
from a singlet excited state as well [
12
,
13
]. For example, in a study by Shinji Miyazaki
et al., the possibility of two energy transfer pathways was revealed from both the triplet
and singlet levels for the Eu(hfa)
3
(DPPTO)
2
complex (hfa—hexafluoroacetylacetonate,
DPPTO—2-diphenylphosphoryltriphenylene) [14].
The simultaneous presence of both luminescent and nonluminescent REE ions, such
as Sc
3+
, Y
3+
, La
3+
, Gd
3+
, Lu
3+
, can significantly affect the structural and photophysical
properties of these compounds. The structural properties of heterometallic REE-MOFs
have been investigated in several studies. It has been found that at low concentrations
of the luminescent lanthanide ions, substitution occurs isomorphically, without chang-
ing the crystalline structure. However, it has been observed that in some compounds,
the structure changes as concentration increases. For example, Jarley Nascimento et al.
showed that compounds Gd
1−x
Eu
x
(1,4-bdc)
3
(dmf)
2
(H
2
O)
n
(bdc—benzenedecarboxylate,
dmf—dimethylformamide; x = 0.01, 0.03, 0.05, 0.07, 0.09) are isostructural with [Eu
2
(1,4-
bdc)
3
(dmf)
2
(H
2
O)] at the Eu
3+
content between 1 and 7 at. % (at. %—the relative atom
content of a certain lanthanide to all lanthanide atoms). However, at the Eu
3+
concentration
of 9 at. %, the compound was isostructural with [Eu
2
(1,4-bdc)
3
(dmf)
2
(H
2
O)
2
] [
15
]. In
our previous research, we found that the compound (Eu
x
Lu
1−x
)
2
(1,4-bdc)
3·
nH
2
O forms
different crystal structures depending on the concentration of the Eu
3+
ions. At the Eu
3+
concentration range of 6–100 at. %, the samples were found to be isostructural with Ln
2
(1,4-
bdc)
3·
4H
2
O. However at the Eu
3+
concentration of 0–2 at. %, the samples were isostructural
with the Ln
2
bdc
3
. At the Eu
3+
concentration between 3 and 5 at. %, both the Ln
2
bdc
3·
4H
2
O
and the Ln
2
(1,4-bdc)
3
phases were observed in the samples [
16
]. The changes in the crystal
structure can significantly affect the optical properties of compounds, such as the fine
structure of emission spectra, quantum yields, and lifetime values.
Meanwhile, the optical properties of the heterometallic MOFs also depend on the concen-
tration of the luminescent ion in the case of isomorphic substitution of the REE ion by the lumi-
nescent ion in the whole concertation range where the single crystalline phase is formed. How-
ever, very few works have studied such a concentration dependence. Thus, Utochnikova et al.
studied the optical properties of heterometallic solid solutions of (Tb
x
Y
1−x
)
2
(1,4-bdc)
3
(H
2
O)
4
and Eu
x
Gd
1−x
(dbm)
3
(phen) (dbm—dibenzoylmethanate, phen—o-phenantroline) MOFs.
The steep quantum yield rise was observed at the low concentrations of Eu
3+
or Tb
3+
ions.
Then, in the range of 20–100 at. % of Tb
3+
for (Tb
x
Y
1−x
)
2
(1,4-bdc)
3
(H
2
O)
4
and 10–100 at. % of
Eu
3+
for Eu
x
Gd
1−x
(dbm)
3
(phen), the quantum yield does not depend on the concentration
of the luminescent ions. It has also been observed that lifetimes decrease with increasing
concentration of the luminescent ion [17].
We note that the majority of studies focus on compounds with a single or a few concen-
trations of the luminescent lanthanide ion [
17
–
19
] or a narrow range of concentrations [
15
].
The properties of the synthesized compounds have been studied incompletely, and, there-
fore, at the moment, we have limited information about the mechanism of the dopant
concentration effect on photophysical properties of heterometallic REE-MOFs.
In this article, the photophysical properties (photoluminescence decay time constants,
radiative, nonradiative, and total decay rates, quantum efficiencies, and formation quantum
Molecules 2024,29, 3558 3 of 14
yields of the
5
D
0
level; photoluminescence quantum yields, asymmetric ratios) of REE-
MOFs of solid solutions of heterometallic (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O MOFs (M = Y, La,
Gd) were studied in detail in a wide concentration range of the Eu3+ ion (0.1–100 at. %).
2. Results
2.1. Structure and Morphology
The phase composition of (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
nH
2
O (M = Y, Gd, La) with a Eu
3+
con-
centration from 0 to 100 at. % was studied using powder X-ray diffraction (PXRD). The ex-
perimental PXRD patterns at. %of the synthesized materials are presented in
Figure 1a
and
Figure S1 (Supplementary Materials). The positions of the diffraction maxima in the PXRD
patterns indicate that all the synthesized compounds correspond to the Ln
2
(1,4-bdc)
3·
4H
2
O
crystalline phase (Ln = Ce
−
Yb) [
20
], and no additional peaks were observed. In the
Ln
2
(1,4-bdc)
3·
4H
2
O structure (Figure 1b), the octacoordinated lanthanide ions are bound
to the two water molecules and six different terephthalate ions through the
oxygen atoms.
Molecules 2024, 29, x FOR PEER REVIEW 3 of 15
In this article, the photophysical properties (photoluminescence decay time con-
stants, radiative, nonradiative, and total decay rates, quantum efficiencies, and formation
quantum yields of the 5D0 level; photoluminescence quantum yields, asymmetric ratios)
of REE-MOFs of solid solutions of heterometallic (EuxM1−x)2(1,4-bdc)3·4H2O MOFs (M = Y,
La, Gd) were studied in detail in a wide concentration range of the Eu3+ ion (0.1–100 at.
%).
2. Results
2.1. Structure and Morphology
The phase composition of (EuxM1−x)2(1,4-bdc)3‧nH2O (M = Y, Gd, La) with a Eu3+ con-
centration from 0 to 100 at. % was studied using powder X-ray diffraction (PXRD). The
experimental PXRD paerns at. %of the synthesized materials are presented in Figure 1a
and Figure S1 (Supplementary Materials). The positions of the diffraction maxima in the
PXRD paerns indicate that all the synthesized compounds correspond to the Ln2(1,4-
bdc)3·4H2O crystalline phase (Ln = Ce − Yb) [20], and no additional peaks were observed.
In the Ln2(1,4-bdc)3·4H2O structure (Figure 1b), the octacoordinated lanthanide ions are
bound to the two water molecules and six different terephthalate ions through the oxygen
atoms.
The refinement of the unit cell parameters and the calculation of the unit cell volumes
were performed for the selected samples over the Eu3+ concentration range between 0 and
100 at. %. Unit cell parameters (Table S1, Supplementary Materials) were refined using
UnitCell software [21]. This program can retrieve unit cell parameters from diffraction
data using a least-squares method from the positions of the indexed diffraction maxima
of the PXRD paerns (Pawley method [22]). The Eu3+ concentration effect on the unit cell
volumes is shown in Figure 2. For the (EuxLa1−x)2(1,4-bdc)3‧4H2O compounds, the increase
in La3+ content leads to increased unit cell volumes due to a higher ionic radius of La3+ ions
(1.160 Å, the coordination number is eight) than the ionic radius of Eu3+ ions (1.066 Å) [23].
10 15 20 25
Eu
/
Eu
0.5
Y
0.5
Y
/
Eu
0.5
La
0.5
La
/
Eu
0.5
Gd
0.5
Gd
/
Eu
2
(1,4-bdc)
3
·4H
2
O
2θ, deg.
(a) (b)
Figure 1. The XRD paerns of selected (EuxM1−x)2bdc3‧4H2O (M = Gd, La, Y; x = 0, 0.5, 1) and the
simulated XRD paern of Eu2(1,4-bdc)3‧4H2O single-crystal structure were taken from ref. [20] (a).
The crystal structure of Eu2(1,4-bdc)3‧4H2O (b).
The ionic radius of the Gd3+ ion (1.053 Å) is close to that of Eu3+. Therefore, the unit
cell parameters do not change significantly in the (EuxGd1−x)2(1,4-bdc)3‧4H2O series. The
Y3+ ion (0.977 Å) is less than Eu3+; therefore, substitution of Eu3+ by the Y3+ ion results in a
decrease in the unit cell volumes in the (EuxM1−x)2(1,4-bdc)3‧4H2O series. In general, the
Figure 1. The XRD patterns of selected (Eu
x
M
1−x
)
2
bdc
3·
4H
2
O (M = Gd, La, Y; x = 0, 0.5, 1) and the
simulated XRD pattern of Eu
2
(1,4-bdc)
3·
4H
2
O single-crystal structure were taken from ref. [
20
] (a).
The crystal structure of Eu2(1,4-bdc)3·4H2O (b).
The refinement of the unit cell parameters and the calculation of the unit cell volumes
were performed for the selected samples over the Eu
3+
concentration range between 0 and
100 at. %. Unit cell parameters (Table S1, Supplementary Materials) were refined using
UnitCell software [
21
]. This program can retrieve unit cell parameters from diffraction
data using a least-squares method from the positions of the indexed diffraction maxima
of the PXRD patterns (Pawley method [
22
]). The Eu
3+
concentration effect on the unit
cell volumes is shown in Figure 2. For the (Eu
x
La
1−x
)
2
(1,4-bdc)
3·
4H
2
O compounds, the
increase in La
3+
content leads to increased unit cell volumes due to a higher ionic radius
of La
3+
ions (1.160 Å, the coordination number is eight) than the ionic radius of Eu
3+
ions
(1.066 Å) [23].
The ionic radius of the Gd
3+
ion (1.053 Å) is close to that of Eu
3+
. Therefore, the unit
cell parameters do not change significantly in the (Eu
x
Gd
1−x
)
2
(1,4-bdc)
3·
4H
2
O series. The
Y
3+
ion (0.977 Å) is less than Eu
3+
; therefore, substitution of Eu
3+
by the Y
3+
ion results in
a decrease in the unit cell volumes in the (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O series. In general,
the dependence of unit cell volume obeys Vegard’s law [
24
], with a slight deviation from
linearity that falls within the error limits.
Scanning electron microscopy (SEM) was used to reveal the particle morphology and
the porosity of the selected synthesized materials, namely, (Eu
0.5
M
0.5
)
2
(1,4-bdc)
3·
4H
2
O
(M = Y, La, Gd). The resulting compounds had a distinct porous structure, as can be seen
from the SEM images in Figure 3. On average, the particles are between 5 and 20
µ
m in
size, and the pore diameter ranges from 20 to 150 nanometers. This observation confirms
Molecules 2024,29, 3558 4 of 14
that the synthesized compounds are MOFs according to the definition of IUPAC [
25
].
Heterometallic europium terephthalates doped with yttrium and gadolinium formed
spindle-shaped particles, while those with lanthanum formed flake-like particles.
Molecules 2024, 29, x FOR PEER REVIEW 4 of 15
dependence of unit cell volume obeys Vegards law [24], with a slight deviation from lin-
earity that falls within the error limits.
0 102030405060708090100
595
600
605
610
615
620
625
630
635
V
uc
, Å
3
χ(Eu
3+
), at.%
Eu-Y
Eu-Gd
Eu-La
Figure 2. Eu3+ concentration dependence of unit cell volume (Vuc) refined for (EuxM1−x)2(1,4-
bdc)3‧4H2O (M = Gd, La, Y).
Scanning electron microscopy (SEM) was used to reveal the particle morphology and
the porosity of the selected synthesized materials, namely, (Eu0.5M0.5)2(1,4-bdc)3·4H2O (M
= Y, La, Gd). The resulting compounds had a distinct porous structure, as can be seen from
the SEM images in Figure 3. On average, the particles are between 5 and 20 µm in size,
and the pore diameter ranges from 20 to 150 nanometers. This observation confirms that
the synthesized compounds are MOFs according to the definition of IUPAC [25]. Hetero-
metallic europium terephthalates doped with yrium and gadolinium formed spindle-
shaped particles, while those with lanthanum formed flake-like particles.
Figure 2. Eu
3+
concentration dependence of unit cell volume (V
uc
) refined for (Eu
x
M
1−x
)
2
(1,4-
bdc)3·4H2O (M = Gd, La, Y).
Molecules 2024, 29, x FOR PEER REVIEW 5 of 15
Figure 3. SEM images of (Eu0.5M0.5)2(1,4-bdc)3·4H2O (M = Gd, La, Y).
2.2. IR Spectroscopy
To reveal the doping effect on the vibrational structure of the ligands, we measured
the IR spectra of the selected samples of heterometallic (Eu0.5M0.5)2(1,4-bdc)3‧4H2O (M = Y,
Gd, La) and homometallic M2(1,4-bdc)3‧4H2O (M = Y, Gd, La, Eu) terephthalates (Figure
4). The broad band with the maximum at about the 3500 cm−1 region corresponds to the
O-H stretching vibrations of the water molecules coordinated to the metal. The multiple
narrow bands in the 1270–1470 and 1470–1800 cm−1 regions correspond to the symmetric
and asymmetric stretching vibrations of the carboxylic -COO group of the terephthalate
ion, respectively. The data obtained are consistent with the data in the literature obtained
for REE terephthalates [26,27]. The spectral shape, including the fine structure of the ab-
sorption bands and the position of the absorption maxima, is almost identical for the stud-
ied compounds, which indicates the similar structure of these terephthalates. This conclu-
sion is in agreement with PXRD data, demonstrating that all the synthesized compounds
have the same crystalline phase, namely, Ln2(1,4-bdc)3·4H2O.
Figure 3. SEM images of (Eu0.5M0.5)2(1,4-bdc)3·4H2O (M = Gd, La, Y).
Molecules 2024,29, 3558 5 of 14
2.2. IR Spectroscopy
To reveal the doping effect on the vibrational structure of the ligands, we measured
the IR spectra of the selected samples of heterometallic (Eu
0.5
M
0.5
)
2
(1,4-bdc)
3·
4H
2
O (M = Y,
Gd, La) and homometallic M
2
(1,4-bdc)
3·
4H
2
O (M = Y, Gd, La, Eu) terephthalates (Figure 4).
The broad band with the maximum at about the 3500 cm
−1
region corresponds to the O-H
stretching vibrations of the water molecules coordinated to the metal. The multiple narrow
bands in the 1270–1470 and 1470–1800 cm
−1
regions correspond to the symmetric and
asymmetric stretching vibrations of the carboxylic -COO group of the terephthalate ion,
respectively. The data obtained are consistent with the data in the literature obtained for
REE terephthalates [
26
,
27
]. The spectral shape, including the fine structure of the absorption
bands and the position of the absorption maxima, is almost identical for the studied
compounds, which indicates the similar structure of these terephthalates. This conclusion
is in agreement with PXRD data, demonstrating that all the synthesized compounds have
the same crystalline phase, namely, Ln2(1,4-bdc)3·4H2O.
Molecules 2024, 29, x FOR PEER REVIEW 6 of 15
500 1000 1500 2000 2500 3000 3500
0.0
0.2
0.4
0.6
0.8
1.0
ν
as
(C=O
-coo
)
ν(O-H
H2O
)
Abs (Normalized)
ν, cm
−1
Eu
Eu
0.5
Y
0.5
Eu
0.5
Gd
0.5
Eu
0.5
La
0.5
Y
Gd
La
~
ν
s
(C=O
-coo
)
Figure 4. FTIR spectra of (Eu0.5M0.5)2(1,4-bdc)3‧4H2O (M = Y, Gd, La) and M2(1,4-bdc)3‧4H2O (M = Y,
Gd, La, Eu).
2.3. Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was performed for selected heterometallic
(Eu0.5M0.5)2(1,4-bdc)3‧4H2O (M = Y, Gd, La) and homometallic M2(1,4-bdc)3‧4H2O (M = Y,
Gd, La, Eu) terephthalates in the temperature range of 35–200 °C (Figure 5). Weight loss
between 8.5 and 9.3% was observed at a temperature of 120–180 °C for all measured sam-
ples. As previously reported [28], weight loss in this temperature range may be associated
with the dehydration of compounds, leading to the formation of anhydrous terephthalates
with a general formula of M2(1,4-bdc)3. The weight loss of 8.5–9.3% corresponds to the
elimination of 3.8–4.1 water molecules from the initial terephthalates, which is in agree-
ment with the PXRD data, showing that all studied materials are formed in the Ln2(1,4-
bdc)3·4H2O crystalline phase.
50 100 150 200
88
90
92
94
96
98
100
Weigth loss, %
Temperature,
o
C
Eu
Eu
0.5
Y
0.5
Eu
0.5
Gd
0.5
Eu
0.5
La
0.5
Y
Gd
La
Figure 5. TGA curves of selected heterometallic (Eu0.5M0.5)2(1,4-bdc)3‧4H2O (M = Y, Gd, La) and ho-
mometallic M2(1,4-bdc)3‧4H2O (M = Y, Gd, La, Eu) terephthalates measured in the temperature range
of 35–200 °C.
Figure 4. FTIR spectra of (Eu
0.5
M
0.5
)
2
(1,4-bdc)
3·
4H
2
O (M = Y, Gd, La) and M
2
(1,4-bdc)
3·
4H
2
O (
M=Y
,
Gd, La, Eu).
2.3. Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was performed for selected heterometallic
(Eu
0.5
M
0.5
)
2
(1,4-bdc)
3·
4H
2
O (M = Y, Gd, La) and homometallic M
2
(1,4-bdc)
3·
4H
2
O (
M=Y
,
Gd, La, Eu) terephthalates in the temperature range of 35–200
◦
C (Figure 5). Weight loss be-
tween 8.5 and 9.3% was observed at a temperature of 120–180
◦
C for all measured samples.
As previously reported [
28
], weight loss in this temperature range may be associated with
the dehydration of compounds, leading to the formation of anhydrous terephthalates with
a general formula of M
2
(1,4-bdc)
3
. The weight loss of 8.5–9.3% corresponds to the elimina-
tion of 3.8–4.1 water molecules from the initial terephthalates, which is in agreement with
the PXRD data, showing that all studied materials are formed in the Ln
2
(1,4-bdc)
3·
4H
2
O
crystalline phase.
2.4. Luminescent Properties
For all synthesized compounds, emission spectra were measured upon 300 nm excita-
tion into the S
n
singlet state of the terephthalate ion. Figure 6shows the normalized emis-
sion spectra of selected samples with different Eu
3+
content ((Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O
(M = Y, La, Gd) (x = 0.001, 0.01, 0.1, 0.5, 1)). The emission spectra of all studied materials
are given in Figure S2 (Supplementary Materials). All emission spectra contain the same
Molecules 2024,29, 3558 6 of 14
narrow bands, which correspond to the
5
D
0
–
7
F
J
(J = 1, 2, 4) transitions of Eu
3+
:
5
D
0
–
7
F
1
(587.9 and 591.6 nm), 5D0–7F2(614 nm), and 5D0–7F4(696 nm).
Molecules 2024, 29, x FOR PEER REVIEW 6 of 15
500 1000 1500 2000 2500 3000 3500
0.0
0.2
0.4
0.6
0.8
1.0
ν
as
(C=O
-coo
)
ν(O-H
H2O
)
Abs (Normalized)
ν, cm
−1
Eu
Eu
0.5
Y
0.5
Eu
0.5
Gd
0.5
Eu
0.5
La
0.5
Y
Gd
La
~
ν
s
(C=O
-coo
)
Figure 4. FTIR spectra of (Eu0.5M0.5)2(1,4-bdc)3‧4H2O (M = Y, Gd, La) and M2(1,4-bdc)3‧4H2O (M = Y,
Gd, La, Eu).
2.3. Thermogravimetric Analysis
Thermogravimetric analysis (TGA) was performed for selected heterometallic
(Eu0.5M0.5)2(1,4-bdc)3‧4H2O (M = Y, Gd, La) and homometallic M2(1,4-bdc)3‧4H2O (M = Y,
Gd, La, Eu) terephthalates in the temperature range of 35–200 °C (Figure 5). Weight loss
between 8.5 and 9.3% was observed at a temperature of 120–180 °C for all measured sam-
ples. As previously reported [28], weight loss in this temperature range may be associated
with the dehydration of compounds, leading to the formation of anhydrous terephthalates
with a general formula of M2(1,4-bdc)3. The weight loss of 8.5–9.3% corresponds to the
elimination of 3.8–4.1 water molecules from the initial terephthalates, which is in agree-
ment with the PXRD data, showing that all studied materials are formed in the Ln2(1,4-
bdc)3·4H2O crystalline phase.
50 100 150 200
88
90
92
94
96
98
100
Weigth loss, %
Temperature,
o
C
Eu
Eu
0.5
Y
0.5
Eu
0.5
Gd
0.5
Eu
0.5
La
0.5
Y
Gd
La
Figure 5. TGA curves of selected heterometallic (Eu0.5M0.5)2(1,4-bdc)3‧4H2O (M = Y, Gd, La) and ho-
mometallic M2(1,4-bdc)3‧4H2O (M = Y, Gd, La, Eu) terephthalates measured in the temperature range
of 35–200 °C.
Figure 5. TGA curves of selected heterometallic (Eu
0.5
M
0.5
)
2
(1,4-bdc)
3·
4H
2
O (M = Y, Gd, La) and
homometallic M
2
(1,4-bdc)
3·
4H
2
O (M = Y, Gd, La, Eu) terephthalates measured in the temperature
range of 35–200 ◦C.
Molecules 2024, 29, x FOR PEER REVIEW 7 of 15
2.4. Luminescent Properties
For all synthesized compounds, emission spectra were measured upon 300 nm exci-
tation into the Sn singlet state of the terephthalate ion. Figure 6 shows the normalized
emission spectra of selected samples with different Eu3+ content ((EuxM1−x)2(1,4-bdc)3·4H2O
(M = Y, La, Gd) (x = 0.001, 0.01, 0.1, 0.5, 1)). The emission spectra of all studied materials
are given in Figure S2 (Supplementary Materials). All emission spectra contain the same
narrow bands, which correspond to the 5D0–7FJ (J = 1, 2, 4) transitions of Eu3+: 5D0–7F1 (587.9
and 591.6 nm), 5D0–7F2 (614 nm), and 5D0–7F4 (696 nm).
550 600 650 700
I
em
, a.u.
λ
em
, nm
Eu
.
Eu
0.5
Gd
0.5
Eu
0.1
Gd
0.9
Eu
0.01
Gd
0.99
Eu
0.001
Gd
0.999
Eu
0.5
La
0.5
Eu
0.1
La
0.9
Eu
0.01
La
0.99
Eu
0.001
La
0.999
Eu
0.5
Y
0.5
Eu
0.1
Y
0.9
Eu
0.01
Y
0.99
Eu
0.001
Y
0.999
5
D
0
-
7
F
1
5
D
0
-
7
F
2
5
D
0
-
7
F
4
Figure 6. The normalized emission spectra of (EuxM1−x)2(1,4-bdc)3·4H2O (M = Gd, La, Y) at selected
Eu3+ concentrations (given in legend) upon 300 nm excitation.
5D0–7F3 transitions are also present in emission spectra, but they were not observed
due to their weak intensity. The presence of Eu3+ f-f bands in the emission spectra upon
excitation to the Sn singlet state of the terephthalate ion clearly reveals an antenna effect.
Thus, the terephthalate ion absorbs UV radiation followed by efficient energy transfer to
the luminescent lanthanide ion. Upon the excitation, the terephthalate ion is promoted
into the Sn state, followed by the fast internal conversion to the S1 state. Due to the heavy
atom effect caused by the lanthanide atom, the S1 state efficiently undergoes intersystem
crossing to the T1 triplet electronic excited state. The T1 state of the terephthalate ion is
close in energy to the 5D1 energy level of the Eu3+ ion [11,17]. Therefore, an efficient energy
transfer occurs from the sensitizer to the luminescent lanthanide ion. The 5D1 level of Eu3+
then undergoes an internal conversion to the 5D0 state, followed by the emission to the 7FJ
(J = 1, 2, 4) lower-lying energy levels.
The shape of the emission spectra for the Y-, Gd-, and La-doped compounds is iden-
tical to that of the pure europium terephthalate emission spectrum (Figure S3, Supple-
mentary Materials), implying the same coordination environment of Eu3+ in the solid so-
lutions studied. This observation agrees with the PXRD data, which show the presence of
the same crystalline structure, (EuxM1−x)2(1,4-bdc)3‧4H2O (M = Y, Gd, La), among the stud-
ied series. However, the peak intensity of emission spectra depends on the concentrations
of europium ions in the studied solid solutions due to different photoluminescence quan-
tum yields, which will be discussed further.
Figure 6. The normalized emission spectra of (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O (M = Gd, La, Y) at selected
Eu3+ concentrations (given in legend) upon 300 nm excitation.
5
D
0
–
7
F
3
transitions are also present in emission spectra, but they were not observed
due to their weak intensity. The presence of Eu
3+
f-f bands in the emission spectra upon
excitation to the S
n
singlet state of the terephthalate ion clearly reveals an antenna effect.
Thus, the terephthalate ion absorbs UV radiation followed by efficient energy transfer to
the luminescent lanthanide ion. Upon the excitation, the terephthalate ion is promoted
into the S
n
state, followed by the fast internal conversion to the S
1
state. Due to the heavy
atom effect caused by the lanthanide atom, the S
1
state efficiently undergoes intersystem
Molecules 2024,29, 3558 7 of 14
crossing to the T
1
triplet electronic excited state. The T
1
state of the terephthalate ion is
close in energy to the
5
D
1
energy level of the Eu
3+
ion [
11
,
17
]. Therefore, an efficient energy
transfer occurs from the sensitizer to the luminescent lanthanide ion. The
5
D
1
level of Eu
3+
then undergoes an internal conversion to the
5
D
0
state, followed by the emission to the
7
F
J
(J = 1, 2, 4) lower-lying energy levels.
The shape of the emission spectra for the Y-, Gd-, and La-doped compounds is identical
to that of the pure europium terephthalate emission spectrum (Figure S3, Supplementary
Materials), implying the same coordination environment of Eu
3+
in the solid solutions
studied. This observation agrees with the PXRD data, which show the presence of the
same crystalline structure, (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O (M = Y, Gd, La), among the studied
series. However, the peak intensity of emission spectra depends on the concentrations of
europium ions in the studied solid solutions due to different photoluminescence quantum
yields, which will be discussed further.
The photoluminescence decay curves of the (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O phosphors
monitored at 615 nm (
5
D
0
–
7
F
2
transition) are presented in Figure 7(
λex.
= 300 nm). The
decay curves were fitted by a single exponential function:
I=I0·e−t
τ(1)
where τis the observed 5D0lifetime (Table 1).
Molecules 2024, 29, x FOR PEER REVIEW 8 of 15
The photoluminescence decay curves of the (EuxM1−x)2(1,4-bdc)3‧4H2O phosphors
monitored at 615 nm (5D0–7F2 transition) are presented in Figure 7 (λex. = 300 nm). The
decay curves were fied by a single exponential function:
I = I0∙e
-
t
τ (1)
where τ is the observed 5D0 lifetime (Table 1).
0.0 0.5 1.0 1.5
0.1
1
I
em
(615 nm), a.u.
Time, ms
Eu
0.01
Y
0.99
Eu
0.1
Y
0.9
Eu
0.01
La
0.99
Eu
0.1
La
0.9
Eu
0.01
Gd
0.99
Eu
0.1
Gd
0.9
Eu
Figure 7. The 615 nm photoluminescence decay curves of (EuxM1−x)2(1,4-bdc)3‧4H2O (M = Y, La, Gd;
x = 0.01, 0.1).
Table 1. The observed 5D0 lifetime of (EuxM1−x)2(1,4-bdc)3‧4H2O (M = Gd, La, Y; x = 0.01, 0.1).
EuxM1−x τ(5D0), ms
Eu0.01Gd0.99 0.48 ± 0.01
Eu0.1Gd0.9 0.46 ± 0.01
Eu0.01La0.99 0.44 ± 0.01
Eu0.1La0.9 0.43 ± 0.01
Eu0.01Y0.99 0.47 ± 0.01
Eu0.1Y0.9 0.45 ± 0.01
Eu 0.44 ± 0.01
Figure 7 and Table 1 present the photoluminescence decay curves and lifetime data
for the selected concentrations of Eu3+ ions in (EuxM1−x)2(1,4-bdc)3‧4H2O (M = Gd, La, Y) (x
= 0.01, 0.1, and 1) among the whole concentration range. As can be seen from Table 1, all
the samples have similar lifetimes. This allows us to assume that the 5D0 lifetime of the
Eu3+ ion almost does not depend on the Eu3+ content in the 1–100 at. % concentration range
of Eu3+.
The measured photoluminescence quantum yields (PLQYs) of the (EuxM1−x)2(1,4-
bdc)3‧4H2O are shown in Figure 8a. It should be noted that the 5D0 lifetimes do not depend
significantly on the sample composition and are equal to about 0.45 ms. Meanwhile, the
photoluminescence luminescence quantum yields increase with an increase in the Eu3+
concentration from 1 to 10 at. % due to the number of luminescence sites increasing. In
particular, for the Eu–Y series, there is a rapid increase in the quantum yield up to 10%,
after which the PLQY remains within small deviations of values from 10 at. % and reaches
a plateau, up to Eu 100%. For the Eu–La series, there is also a rapid increase in the PLQY
at low concentrations of Eu up to ~12% and a slight decrease in quantum yield to 10% in
the range of Eu concentrations from 20 to 100 at. %. The Eu–Gd series is different from the
Figure 7. The 615 nm photoluminescence decay curves of (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O (M = Y, La,
Gd; x = 0.01, 0.1).
Table 1. The observed 5D0lifetime of (EuxM1−x)2(1,4-bdc)3·4H2O (M = Gd, La, Y; x = 0.01, 0.1).
EuxM1−xτ(5D0), ms
Eu0.01Gd0.99 0.48 ±0.01
Eu0.1Gd0.9 0.46 ±0.01
Eu0.01La0.99 0.44 ±0.01
Eu0.1La0.9 0.43 ±0.01
Eu0.01Y0.99 0.47 ±0.01
Eu0.1Y0.9 0.45 ±0.01
Eu 0.44 ±0.01
Figure 7and Table 1present the photoluminescence decay curves and lifetime data
for the selected concentrations of Eu
3+
ions in (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O (M = Gd, La, Y)
(
x = 0.01
, 0.1, and 1) among the whole concentration range. As can be seen from Table 1,
all the samples have similar lifetimes. This allows us to assume that the
5
D
0
lifetime of
Molecules 2024,29, 3558 8 of 14
the Eu3+ ion almost does not depend on the Eu3+ content in the 1–100 at. % concentration
range of Eu3+.
The measured photoluminescence quantum yields (PLQYs) of the (Eu
x
M
1−x
)
2
(1,4-
bdc)
3·
4H
2
O are shown in Figure 8a. It should be noted that the
5
D
0
lifetimes do not depend
significantly on the sample composition and are equal to about 0.45 ms. Meanwhile, the
photoluminescence luminescence quantum yields increase with an increase in the Eu
3+
concentration from 1 to 10 at. % due to the number of luminescence sites increasing. In
particular, for the Eu–Y series, there is a rapid increase in the quantum yield up to 10%,
after which the PLQY remains within small deviations of values from 10 at. % and reaches
a plateau, up to Eu 100%. For the Eu–La series, there is also a rapid increase in the PLQY
at low concentrations of Eu up to ~12% and a slight decrease in quantum yield to 10% in
the range of Eu concentrations from 20 to 100 at. %. The Eu–Gd series is different from
the Eu–Y and Eu–La ones: the PLQY has a maximum at the Eu
3+
content of 10 at. %.
The PLQY of (Eu
0.1
Gd
0.9
)
2
(1,4-bdc)
3·
4H
2
O is equal to 15%, which is 1.5 times greater than
that of (Eu
0.1
Y
0.9
)
2
(1,4-bdc)
3·
4H
2
O and (Eu
0.1
La
0.9
)
2
(1,4-bdc)
3·
4H
2
O. Upon further Eu
3+
content increase, the PLQY of (Eu
x
Gd
1−x
)
2
(1,4-bdc)
3·
4H
2
O decreases, whereas the PLQY of
(EuxY1−x)2(1,4-bdc)3·4H2O and (EuxLa1−x)2(1,4-bdc)3·4H2O stays about the same (10%).
Molecules 2024, 29, x FOR PEER REVIEW 9 of 15
Eu–Y and Eu–La ones: the PLQY has a maximum at the Eu3+ content of 10 at. %. The PLQY
of (Eu0.1Gd0.9)2(1,4-bdc)3‧4H2O is equal to 15%, which is 1.5 times greater than that of
(Eu0.1Y0.9)2(1,4-bdc)3‧4H2O and (Eu0.1La0.9)2(1,4-bdc)3‧4H2O. Upon further Eu3+ content in-
crease, the PLQY of (EuxGd1−x)2(1,4-bdc)3‧4H2O decreases, whereas the PLQY of
(EuxY1−x)2(1,4-bdc)3‧4H2O and (EuxLa1−x)2(1,4-bdc)3‧4H2O stays about the same (10%).
0 20406080100
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
PLQY
χ(Eu
3+
), at. %
Eu-Y
Eu-Gd
Eu-La
0 20406080100
0.135
0.140
0.145
0.150
0.155
0.160
η
,
5
D
0
Eu-Y
Eu-Gd
Eu-La
χ(Eu3+), at.%
0 20 40 60 80 100
0.0
0.2
0.4
0.6
0.8
1.0
Φ
form.
5
D
0
χ(Eu3+), at.%
Eu-Y
Eu-Gd
Eu-La
(a) (b) (c)
Figure 8. Photoluminescence quantum yield (PLQY) (a), quantum efficiency (η) 5D0 of Eu3+ (b), and
quantum yield of 5D0 formation (c) of (EuxM1−x)2(1,4-bdc)3‧4H2O (M = Gd, La, Y).
Luminescence decay is determined by both radiative and nonradiative processes. Ra-
diative decay rate is determined by dipole transition strength and local-field correction.
Nonradiative processes include multiphonon relaxation, quenching on impurities, and
cooperative processes such as cross-relaxation and energy migration, the influence of
which increases with increasing concentration of europium ions in the solid solution. Ra-
diative and nonradiative decay rates of Eu3+-doped phosphors can be calculated from the
emission spectrum using the 4f–4f intensity theory [29]. The magnetic dipole 5D0–7F1 tran-
sition probability A0–1 = AMD0∙n0
3 = 49 s1. AMD0 is the spontaneous emission probability
of the magnetic dipole 5D0–7F1, 14.65 s−1, and n0 is the refractive index, 1.5 [30]. Radiative
decay rates A0-J (J = 2, 4) of the 5D0–7FJ emission transition can be obtained as follows:
A0–J = A0–1 ∙ν0–1 ∙I0–J
ν0–J∙I0–1 (2)
where ν0–J and I0–J are, respectively, the frequency and intensity of the corresponding tran-
sition 5D0–7FJ in the emission spectrum. The radiative decay rate is the sum of all the A0-J
values ARad = A0–1 + A0–2 + A0–4. The nonradiative decay rate can be determined using the
observed lifetime and the obtained radiative decay rate: Atotal = ARad + ANonrad = 1
τf ; the
quantum efficiency of the 5D0 is
ηD
,
50 = ARad
Atotal
= n(5D0-7FJ)em.
n(5D0) (3)
Knowledge of both the PLQY (Figure 8a) and the quantum efficiency of 5D0 (Figure
8b) allows us to calculate the quantum yield of the 5D0 formation. The studied tereph-
thalate solid solutions are antenna complexes, where 5D0 is populated as a result of the
energy transfer from the initial excited terephthalate ion
(Sn(bdc2−)→S1(bdc2−)→T1(bdc2−)→5D0(Eu3+)). Therefore, the PLQY can be calculated as the
direct product of the quantum efficiency of the 5D0 (ηD
,
50) and quantum yield of the 5D0
formation (Φform.5D0):
PLQY = η,5D0∙Φform.5D0 (4)
Figure 8. Photoluminescence quantum yield (PLQY) (a), quantum efficiency (
η
)
5
D
0
of Eu
3+
(b), and
quantum yield of 5D0formation (c) of (EuxM1−x)2(1,4-bdc)3·4H2O (M = Gd, La, Y).
Luminescence decay is determined by both radiative and nonradiative processes.
Radiative decay rate is determined by dipole transition strength and local-field correction.
Nonradiative processes include multiphonon relaxation, quenching on impurities, and
cooperative processes such as cross-relaxation and energy migration, the influence of which
increases with increasing concentration of europium ions in the solid solution. Radiative
and nonradiative decay rates of Eu
3+
-doped phosphors can be calculated from the emission
spectrum using the 4f–4f intensity theory [
29
]. The magnetic dipole
5
D
0
–
7
F
1
transition
probability
A0–1 =AMD0·n3
0=49 s−1
.
AMD0
is the spontaneous emission probability of
the magnetic dipole
5
D
0
–
7
F
1
, 14.65 s
−1
, and
n0
is the refractive index, 1.5 [
30
]. Radiative
decay rates A0−J(J = 2, 4) of the 5D0–7FJemission transition can be obtained as follows:
A0–J=A0–1 ·ν0–1·I0–J
ν0–J·I0–1 (2)
where
ν0–J
and I
0–J
are, respectively, the frequency and intensity of the corresponding transition
5
D
0
–
7
F
J
in the emission spectrum. The radiative decay rate is the sum of all the
A0−J
values
ARad =A0–1 +A0–2 +A0–4
. The nonradiative decay rate can be determined using the
observed lifetime and the obtained radiative decay rate:
Atotal =ARad +ANonrad =1
τf
; the
quantum efficiency of the 5D0is
η5
,D0=ARad
Atotal
=n(5D0–7FJem.
n(5D0)(3)
Molecules 2024,29, 3558 9 of 14
Knowledge of both the PLQY (Figure 8a) and the quantum efficiency of
5
D
0
(Figure 8b)
allows us to calculate the quantum yield of the
5
D
0
formation. The studied terephthalate
solid solutions are antenna complexes, where
5
D
0
is populated as a result of the energy trans-
fer from the initial excited terephthalate ion (S
n
(bdc
2−
)
→
S
1
(bdc
2−
)
→
T
1
(bdc
2−
)
→5
D
0
(Eu
3+
)).
Therefore, the PLQY can be calculated as the direct product of the quantum efficiency of
the 5D0(η5
,D0) and quantum yield of the 5D0formation (Φform.5D0):
PLQY =η,5D0·Φform.5D0(4)
Therefore, the quantum yield of the
5
D
0
formation (Figure 8c) was calculated as
follows:
Φform.5D0=PLQY
η,5D0
(5)
At Eu
3+
concentrations up to 10 at. %, the quantum yield of the
5
D
0
formation increases
as a result of the number of luminescence sites increasing. For the Eu–Y series, it remains
unchanged up to 100 at. % of Eu
3+
, whereas for the Eu–La and Eu–Gd series, it reaches a
maximum at 10 at. % of Eu
3+
and then decays. The maximum values of quantum yield
of the
5
D
0
formation decrease in a dopant row of Gd–La–Y. The QY of
5
D
0
formation is
close to 100%, which indicates a very-high-efficiency energy transfer from the terephthalate
antenna to the light-emitting Eu3+ ion. We propose that this observation can be explained
by the heavy atom effect, which is more pronounced for the Gd
3+
ion and less pronounced
for the Y3+ ion.
The emission spectrum of the Eu
3+
ion includes a very sensitive forced electric dipole
transition
5
D
0
–
7
F
2
. At the same time, the
5
D
0
–
7
F
1
magnetic dipole transition is not sensitive
to environmental changes. Changes in the environment can be judged by the asymmetry
ratio, which is determined by the ratio between the
5
D
0
–
7
F
2
and
5
D
0
–
7
F
1
transition inten-
sities. The higher the asymmetry coefficient, the further away the luminescence center is
located from the centrosymmetric geometry [
31
]. The effect of Eu
3+
ion concentration on
local symmetry in samples (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O MOFs (M = Y, La, Gd) is shown in
Figure 9. It is seen that an increase in the concentration of the Eu
3+
ion leads to the growth
of the asymmetry ratio due to distortion of the crystal structure near the luminescent atom.
Starting from 10 at. % of Eu
3+
, the asymmetry ratio reaches a plateau and does not change
within the margin of error. Thus, the data once again confirm the fact that three-charged
REE ions replace each other isomorphically in crystalline substances.
Molecules 2024, 29, x FOR PEER REVIEW 10 of 15
Therefore, the quantum yield of the 5D0 formation (Figure 8c) was calculated as fol-
lows:
Φform.5D0=PLQY
η,5D0
(5)
At E u3+ concentrations up to 10 at. %, the quantum yield of the 5D0 formation increases
as a result of the number of luminescence sites increasing. For the Eu–Y series, it remains
unchanged up to 100 at. % of Eu3+, whereas for the Eu–La and Eu–Gd series, it reaches a
maximum at 10 at. % of Eu3+ and then decays. The maximum values of quantum yield of
the 5D0 formation decrease in a dopant row of Gd–La–Y. The QY of 5D0 formation is close
to 100%, which indicates a very-high-efficiency energy transfer from the terephthalate an-
tenna to the light-emiing Eu3+ ion. We propose that this observation can be explained by
the heavy atom effect, which is more pronounced for the Gd3+ ion and less pronounced
for the Y3+ ion.
The emission spectrum of the Eu3+ ion includes a very sensitive forced electric dipole
transition 5D0–7F2. At the same time, the 5D0–7F1 magnetic dipole transition is not sensitive
to environmental changes. Changes in the environment can be judged by the asymmetry
ratio, which is determined by the ratio between the 5D0–7F2 and 5D0–7F1 transition intensi-
ties. The higher the asymmetry coefficient, the further away the luminescence center is
located from the centrosymmetric geometry [31]. The effect of Eu3+ ion concentration on
local symmetry in samples (EuxM1−x)2(1,4-bdc)3·4H2O MOFs (M = Y, La, Gd) is shown in
Figure 9. It is seen that an increase in the concentration of the Eu3+ ion leads to the growth
of the asymmetry ratio due to distortion of the crystal structure near the luminescent atom.
Starting from 10 at. % of Eu3+, the asymmetry ratio reaches a plateau and does not change
within the margin of error. Thus, the data once again confirm the fact that three-charged
REE ions replace each other isomorphically in crystalline substances.
0 20406080100
3.4
3.6
3.8
4.0
4.2
4.4
4.6
Asymmetry ratio
χ(Eu3+), at.%
Eu-Y
Eu-Gd
Eu-La
Figure 9. Asymmetric ratios of (EuxM1−x)2(1,4-bdc)3‧4H2O (M = Gd, La, Y).
The abovementioned results clearly show that the presence of a gadolinium ion leads
to the luminescence enhancement in Eu–Gd terephthalates, whereas this effect is almost
not pronounced in the Eu–La and Eu–Y series. Thus, (Eu0.1Gd0.9)2(1,4-bdc)3·4H2O demon-
strates the maximum PLQY among the Eu–Gd series, 15%. The PLQY value is 4–5% higher
than that in the Eu–La and Eu–Y series at the same Eu3+ ion content (10 at. %). At the higher
Eu3+ concentrations, the values of PLQY decrease smoothly for the Eu–Gd series, reaching
10% for Eu2(1,4-bdc)3·4H2O. For the Eu–La and Eu–Y series, the PLQY stays about the
same (9–11%) up to Eu3+ concentration of 100 at. %. Therefore, we do not observe Eu3+
Figure 9. Asymmetric ratios of (EuxM1−x)2(1,4-bdc)3·4H2O (M = Gd, La, Y).
The abovementioned results clearly show that the presence of a gadolinium ion
leads to the luminescence enhancement in Eu–Gd terephthalates, whereas this effect is
Molecules 2024,29, 3558 10 of 14
almost not pronounced in the Eu–La and Eu–Y series. Thus, (Eu
0.1
Gd
0.9
)
2
(1,4-bdc)
3·
4H
2
O
demonstrates the maximum PLQY among the Eu–Gd series, 15%. The PLQY value is 4–5%
higher than that in the Eu–La and Eu–Y series at the same Eu
3+
ion content (10 at. %). At
the higher Eu
3+
concentrations, the values of PLQY decrease smoothly for the Eu–Gd series,
reaching 10% for Eu
2
(1,4-bdc)
3·
4H
2
O. For the Eu–La and Eu–Y series, the PLQY stays about
the same (9–11%) up to Eu
3+
concentration of 100 at. %. Therefore, we do not observe
Eu
3+
concentration quenching. The Gd-doping effect is also observed for the calculated
values of
5
D
0
formation quantum yield: for (Eu
0.1
Gd
0.9
)
2
(1,4-bdc)
3·
4H
2
O, the QY of
5
D
0
formation values become close to 100%, which indicates a very-high-efficiency energy
transfer from the terephthalate antenna to the light-emitting Eu
3+
ion. The Gd
3+
ion has an
f
7
configuration and, therefore, increases the probability (rate) of S
1
-T
1
intersystem crossing
in the terephthalate ion, which is associated with the heavy atom effect manifested by
paramagnetic Gd
3+
ions. To confirm the proposed mechanism, we measured the emission
spectra of gadolinium and yttrium terephthalates upon 300 nm excitation into the
1ππ
band
of the terephthalate ion. The prominent increase in the phosphorescence band (510 nm)
was observed for Gd
2
(1,4-bdc)
3·
4H
2
O compared with Y
2
(1,4-bdc)
3·
4H
2
O, which confirms
the increase in the intersystem crossing quantum yield resulting from the increase in the
S1-T1nonradiative transition rate (Figure 10) as a result of the presence of the Gd3+ ion.
Molecules 2024, 29, x FOR PEER REVIEW 11 of 15
concentration quenching. The Gd-doping effect is also observed for the calculated values
of 5D0 formation quantum yield: for (Eu0.1Gd0.9)2(1,4-bdc)3·4H2O, the QY of 5D0 formation
values become close to 100%, which indicates a very-high-efficiency energy transfer from
the terephthalate antenna to the light-emiing Eu3+ ion. The Gd3+ ion has an f7 configura-
tion and, therefore, increases the probability (rate) of S1-T1 intersystem crossing in the ter-
ephthalate ion, which is associated with the heavy atom effect manifested by paramag-
netic Gd3+ ions. To confirm the proposed mechanism, we measured the emission spectra
of gadolinium and yrium terephthalates upon 300 nm excitation into the 1ππ band of the
terephthalate ion. The prominent increase in the phosphorescence band (510 nm) was ob-
served for Gd2(1,4-bdc)3·4H2O compared with Y2(1,4-bdc)3·4H2O, which confirms the in-
crease in the intersystem crossing quantum yield resulting from the increase in the S1-T1
nonradiative transition rate (Figure 10) as a result of the presence of the Gd3+ ion.
400 500 600 700
0.0
0.5
1.0
1.5
2.0
2.5
3.0
I
em.
λ
em.
, nm
Y
2
(1,4-bdc)
3
·4H
2
O
Gd
2
(1,4-bdc)
3
·4H
2
O
Figure 10. The emission spectra of Gd2(1,4-bdc)3·4H2O and Y2(1,4-bdc)3·4H2O upon 300 nm excita-
tion.
3. Materials and Methods
Europium (III) chloride hexahydrate, yrium (III) chloride hexahydrate, gadolinium
(III) chloride hexahydrate, and lanthanum (III) chloride hexahydrate were purchased
from Chemcraft (Kaliningrad, Russia). Benzene-1,4-dicarboxylic (terephtalic, H2(1,4-bdc))
acid (>98%), sodium hydroxide (>99%), nickel(II) chloride hexahydrate (>99%), EDTA
disodium salt (0.1 M aqueous solution), and murexide were purchased from Sigma-Al-
drich Chemie GmbH (Tauirchen, Germany) and used without additional purification.
The 0.2 M solutions of EuCl3, YCl3, GdCl3, and LaCl3 were prepared and standardized
using back complexometric titration with EDTA and nickel chloride solution in the pres-
ence of an ammonia buffer (pH ≈ 9). A total of 0.6 mole of sodium hydroxide and 0.3 mole
of terephthalic acid were dissolved in distilled water to obtain a 1 L solution of a 0.3 M
solution of the disodium terephthalate (Na2(1,4-bdc)).
Heterometallic terephthalates with a general formula (EuxM1−x)2(1,4-bdc)3·4H2O (М =
Y, Gd, La) were obtained by mixing 0.2 M EuCl3, 0.2 M MCl3 (M = Y, La, Gd) with 2 mL of
0.3 M Na2bdc water solution. EuCl3 and MCl3 were taken in stoichiometric ratios. The total
volume of 0.2 M EuCl3 and 0.2 M MCl3 solutions was equal to 1 mL. White precipitates of
the resulting terephthalates were separated from the reaction mixture using centrifuga-
tion (4000 g) and washed using deionized water 3 times. The samples of compounds were
then dried at 60 °C.
Figure 10. The emission spectra of Gd
2
(1,4-bdc)
3·
4H
2
O and Y
2
(1,4-bdc)
3·
4H
2
O upon 300 nm
excitation.
3. Materials and Methods
Europium (III) chloride hexahydrate, yttrium (III) chloride hexahydrate, gadolinium
(III) chloride hexahydrate, and lanthanum (III) chloride hexahydrate were purchased from
Chemcraft (Kaliningrad, Russia). Benzene-1,4-dicarboxylic (terephtalic, H
2
(1,4-bdc)) acid
(>98%), sodium hydroxide (>99%), nickel(II) chloride hexahydrate (>99%), EDTA disodium
salt (0.1 M aqueous solution), and murexide were purchased from Sigma-Aldrich Chemie
GmbH (Taufkirchen, Germany) and used without additional purification.
The 0.2 M solutions of EuCl
3
, YCl
3
, GdCl
3
, and LaCl
3
were prepared and standardized
using back complexometric titration with EDTA and nickel chloride solution in the presence
of an ammonia buffer (pH
≈
9). A total of 0.6 mole of sodium hydroxide and 0.3 mole
of terephthalic acid were dissolved in distilled water to obtain a 1 L solution of a 0.3 M
solution of the disodium terephthalate (Na2(1,4-bdc)).
Heterometallic terephthalates with a general formula (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O
(
M=Y
, Gd, La) were obtained by mixing 0.2 M EuCl
3
, 0.2 M MCl
3
(M = Y, La, Gd) with
2 mL
of 0.3 M Na
2
bdc water solution. EuCl
3
and MCl
3
were taken in stoichiometric ratios.
Molecules 2024,29, 3558 11 of 14
The total volume of 0.2 M EuCl
3
and 0.2 M MCl
3
solutions was equal to 1 mL. White
precipitates of the resulting terephthalates were separated from the reaction mixture using
centrifugation (4000
×
g) and washed using deionized water 3 times. The samples of
compounds were then dried at 60 ◦C.
The Eu
3+
/M
3+
(M = Y, Gd, La) ratios in the heterometallic terephthalates were con-
firmed using energy-dispersive X-ray spectroscopy (EDX) (EDX spectrometer EDX-800P,
Shimadzu, Kyoto, Japan). The Eu/M (M = Y, Gd, La) ratios obtained from EDX were consis-
tent with the expected ratios of Eu3+/M3+ (M = Y, Gd, La) taken for the synthesis for Eu3+
content within 1 at. % accuracy for the most of the samples. The EDX data are provided in
the Supplementary Materials in Table S2. X-ray powder diffraction (PXRD) measurements
were performed with a D2 Phaser (Bruker, Billerica, MA, USA) X-ray diffractometer using
Cu K
α
radiation (
λ
= 1.54056 Å). Thermogravimetry curves were obtained using a TG 209
F1 Libra thermo-microbalance (Netzsch, Selb, Germany). The measurement of FTIR spectra
was carried out using the IRAffinity-1 spectrometer (Shimadzu, Kyoto, Japan). To carry
out photoluminescence studies, the synthesized samples (20 mg) and potassium bromide
(300 mg) were pressed into pellets (diameter 13 mm). The photoluminescence data were
obtained with a Fluoromax-4 fluorescence spectrometer (Horiba Jobin Yvon, Kyoto, Japan).
Lifetime measurements were performed with the same spectrometer using a pulsed Xe
lamp (pulse duration: 3
µ
s). The absolute values of the photoluminescence quantum yields
were recorded using a Fluorolog 3 Quanta-phi device (Horiba Jobin Yvon, Kyoto, Japan).
All measurements were performed at 25 ◦C.
4. Conclusions
In this study, we explored the structure and the optical properties of brightly lumi-
nescent heterometallic terephthalate antenna MOFs, namely, (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O
(M = Y, La, Gd) (x = 0.001–1), which were obtained by precipitation from the aqueous
solutions. The crystalline phase of all synthesized compounds corresponds to the Ln2(1,4-
bdc)
3·
4H
2
O (Ln = Ce
−
Yb) [
20
]. Unit cell parameters of the obtained compounds were
refined by Pawley method. The replacement of Eu
3+
ions by the larger La
3+
ions results in
an increase in unit cell parameters in (Eu
x
La
1−x
)
2
(1,4-bdc)
3·
4H
2
O compounds, whereas
the substitution of Eu
3+
by smaller Y
3+
ion results in a decrease in the unit cell volumes in
the (Eu
x
M
1−x
)
2
(1,4-bdc)
3·
4H
2
O series. Unit cell parameters almost do not change in the
(Eu
x
Gd
1−x
)
2
(1,4-bdc)
3·
4H
2
O series due to close values of ionic radii of Eu
3+
and Gd
3+
. The
dependence of unit cell volume obeys Vegard’s law [
24
], which allows one to consider the
studied systems as solid solutions. All compounds demonstrate a pronounced antenna
effect. Upon 300 nm excitation into S
n
singlet state of the terephthalate ion, obtained
compounds demonstrate emission corresponding to the
5
D
0
–
7
F
J
(J = 1, 2, 4) transitions of
Eu
3+
:
5
D
0
–
7
F
1
(587.9 and 591.6 nm),
5
D
0
–
7
F
2
(614 nm, maximal intensity), and
5
D
0
–
7
F
4
(696 nm). The shape of the emission spectra for the Y-, Gd-, and La-doped compounds
is identical to that of pure europium terephthalate emission spectrum due to same local
symmetry of Eu
3+
in the studied solid solutions as a result of crystalline phase isomorphism.
The
5
D
0
excited state lifetime does not depend on the sample composition and is equal to
0.45 ±0.03 ms.
Meanwhile, the peak intensity of emission spectra depends on the concen-
trations of europium ions in the studied solid solutions due to different photoluminescence
quantum yields. Thus, the peak emission intensities and photoluminescence quantum
yields increase with an increase in the Eu
3+
concentration from 1 to 10 at. % due to the
number of luminescence sites increasing. At larger Eu
3+
concentrations, for the Eu–Y and
Eu–La series, PLQY and the peak emission intensities remain the same (about 9–11%),
whereas in the Eu–Gd series, they reach a maximum of 15% at the Eu
3+
content of 10 at. %
and then slightly decrease, reaching 10% in pure europium(III) terephthalate. The quantum
efficiency of the
5
D
0
state was calculated using the relative intensities of
5
D
0
–
7
F
J
(J = 1, 2, 4)
transitions and
5
D
0
excited state lifetimes. To quantitatively compare the antenna effect
among different compounds, we propose to use the new approach. Based on the values of
PLQYs and quantum efficiency of the
5
D
0