Synthesis, structure, thermal and luminescent behaviors of lanthanide-Pyridine-3,5-dicarboxylate frameworks series

Article abstract of DOI:10.1016/j.tca.2010.06.010

The isostructural series of lanthanide pyridine-3,5-dicarboxylates of the formula [Ln₂pdc₃(dmf)₂]·(dmf) _x(H₂O)_y where Ln are lanthanides from La(III) to Lu(III); pdc^2--C₅

Full text of DOI:10.1016/j.tca.2010.06.010

Thermochimica Acta 509 (2010) 120–127
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Thermochimica Acta
journal homepage: www.elsevier.com/locate/tca
Synthesis, structure, thermal and luminescent behaviors of
lanthanide—Pyridine-3,5-dicarboxylate frameworks series
Renata Łyszczek∗
Department of General and Coordination Chemistry, Faculty of Chemistry, University of Maria Curie-Skłodowska, M.C. Skłodowskiej Sq. 2, 20-031 Lublin, Poland
a r t i c l e i n f o
a b s t r a c t
Article history:
The isostructural series of lanthanide pyridine-3,5-dicarboxylates of the formula [Ln2pdc3
(dmf)2]·(dmf)x(H2O)y where Ln are lanthanides from La(III) to Lu(III); pdc2− – C5H3N(COO)2; dmf
– N,Nꢀ-dimethylformamide have been prepared under solvothermal conditions. The X-ray analyses
show that the complexes are three-dimensional frameworks possessing one-dimensional rhomboid
channels occupied by disordered solvent molecules. Thermal behavior of the complexes has been
investigated using the TG/DSC and TG/FTIR measurements. The luminescence properties of the Eu(III)
and Tb(III) complexes have been studied. They show very strong characteristic emissions in the red and
green regions, respectively.
Received 25 March 2010
Received in revised form 10 June 2010
Accepted 12 June 2010
Available online 18 June 2010
Keywords:
MOFs
Lanthanides
TG-DSC
© 2010 Elsevier B.V. All rights reserved.
TG-FTIR
Luminescence
1. Introduction
detector systems, light-emitting diodes, solid state lasers [26–29].
The lanthanide ions are characterized by unique optical properties
Metal–organic frameworks (MOFs) have attracted huge atten-
tion as a new class of hybrid porous solids. Large surface area, low
density and controllable channel size as well as acceptable thermal
stability make them promising materials in a wide variety of fields
like: gas storage (hydrogen, carbon dioxide, methane, hydrogen,
oxygen) and gas separation (notably greenhouse gases), organic
synthesis, and heterogeneous catalysis [1–13]. Metal–organic
frameworks are well characterized by crystalline architectures,
which can be self-assembled by the coordination of metal ions with
bridging linkers (O or/and N donors). Construction of such com-
pounds is governed not only by the coordination geometry of the
metal ions but also by their affinity for donor atoms. As reported
in the literature, the majority of the investigated metal–organic
frameworks are based on the first raw of d-block transition metal
centers [14–20], whereas the lanthanide coordination polymers
are less documented. Variable and high coordination numbers
as well as a wide variety of coordination environments of lan-
thanide cations may cause difficulty in the design and synthesis
of lanthanide coordination polymers of desirable topology. On the
other hand, coordination polymers containing lanthanide ions form
fascinating structures along with the special magnetic and lumines-
cence properties [21–25], that make them useful in material science
including light conversion molecular devices i.e. light sources,
due to their spectrally narrow f-f electronic transitions. Unfortu-
nately, lanthanide transitions are forbidden by Laporte selection
rules that cause to weak absorbance and low quantum yields
[28,30]. Complexation of the Ln(III) ions with strongly absorbing
ligands leads to the formation of luminescent MOF devices by the
energy transfer from the linker excited state to the appropriate
metal level. This coupling leads to the increase in luminescence
and is widely known as the “antenna” effect.
Since, the lanthanide ions have high affinity for lig-
ands containing oxygen atoms, policarboxylate linkers
such as terephthalic [31–34], trimesic [35–38] and 2,6-
naphthalenedicarboxylic [39–41] acids are widely used in the
synthesis of lanthanide–organic frameworks. On the other hand,
pyridine-3,5-dicarboxylic acid is exploited in the synthesis of
coordination polymers due to its rigid multidentate character as
well as “antenne” character [42,43].
As was mentioned above, metal–organic frameworks are widely
explored due to their potential applications. One of the most impor-
tant requirements imposed to the MOFs apart from rigid and porous
structure is their thermal stability under air conditions [41]. Usage
of the coordination polymers of transition metals at high temper-
ature is limited due to their weak thermal stability (up to about
300 ◦C) [15,17]. On the contrary lanthanide complexes form more
thermally stable compounds that provide opportunities for their
applications at higher temperature [24,39,37].
The methods of thermal analysis not only allow to deter-
mine the thermal stability of the coordination polymers but
∗
Tel.: +48 81 537 57 43.
E-mail address: renata.lyszczek@poczta.umcs.lublin.pl.
0040-6031/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.tca.2010.06.010

R. Łyszczek / Thermochimica Acta 509 (2010) 120–127
121
are also indispensable tools for the estimation of solvent con-
tents in channels. The characteristic structural feature of the
microporous metal–organic frameworks is formation of three-
dimensional skeletons with the channels occupied by solvent
molecules. They are highly disordered and it is impossible to locate
them by the X-ray diffraction measurements. The next very impor-
tant application of thermal analysis (especially coupled techniques
TG-FTIR) in investigations of microporous coordination polymers
is recognization of type of solvent molecules distributed within the
channels. The syntheses of MOFs are very often carried out by the
solvothermal method using a mixture of solvents. Due to different
affinity of solvent molecules for the metal–organic framework, dif-
ferent molecules can be included into the pores. For such a reason, a
number of papers dealing with thermal characteristics of coordina-
tion polymers still increases [44–46]. There were applied thermal
analysis methods also in course of kinetic investigations of desol-
vation process in coordination polymers as well as determination
of thermodynamic function of these compounds [47–51].
The TG-FTIR coupled measurements have been carried out using
a Netzsch TG apparatus coupled with a Bruker FTIR IFS66 spec-
trophotometer. The samples of about 7–12 mg were heated up to
1000 ◦C at a heating rate of 10 ◦C min−1 in flowing argon atmo-
sphere.
The luminescence spectra were recorded in the solid state at
room temperature on the FP-6300 Jasco fluorescence spectrometer.
Single-crystal diffraction data were measured at room tem-
perature in the ω/2ꢀ mode on the Oxford Diffraction Xcalibur
diffractometer using the graphite-monochromated Mo K␣ radi-
ation (ꢁ = 0.71073 Å). Crystal structures were solved by direct
methods using SHELXS97 [58] and refined by the full-matrix least-
squares on F2 using the SHELXL97 program [59].
3. Results and discussion
The reaction of lanthanide(III) salts with pyridine-3,5-
dicarboxylic acid in the N,Nꢀ-dimethylformamide solution
leads to the formation of complexes of the general formula:
[Ln2pdc3(dmf)2]·(dmf)x(H2O)y, where Ln = lanthanides from La(III)
to Lu(III); pdc – C5H3N(COO)22−, dmf – N,Nꢀ-dimethylformamide
molecules, x = 0.5–1, y = 1 or 0. The detailed formulas of complexes
as well as their elemental analyses are given in Table 1.
The obtained complexes are hardly soluble in water as well
as in the common organic solvents such as methanol, ethanol,
N,Nꢀ-dimethylformamide, tetrahydrofuran, dimethyl sulfoxide and
benzene.
So far, no systematic investigations across the lanthanide
pyridine-3,5-dicarboxylate series obtained by the solvothermal
method have been carried out. Hitherto, investigations on pyri-
dine 3,5-dicarboxylates only with selected lanthanide ions such as
erbium(III), ytterbium(III) [52] and europium(III) [53] obtained by
means of solvothermal method have been described.
Following my ongoing research attempts toward the synthesis
of lanthanide polycarboxylates coordination polymers [54–56], in
my previous work, I investigated the complexes of light lanthanides
(from La to Gd) with dinicotinic acid obtained by the classical
method [57]. Herein, I have reported a series of lanthanide(III)
complexes with pyridine-3,5-dicarboxylic acid obtained by the
solvothermal method from N,Nꢀ-dimethylformamide solution.
The intention of the presented paper was to determine the
thermal properties of the investigated lanthanide dinicotinates
by TG-DSC and TG-FTIR methods in correlation with their crystal
structures. The results from the thermogravimetric and differen-
tial thermal calorimetry allowed to obtain information concerning
with thermal decomposition of complexes in air and argon atmo-
sphere. The TG-FTIR method was applied as an excellent technique
for qualitative determination of molecules included in the channels
which was impossible from the X-ray single-crystal analysis. The
next scope of this paper was to found out, the differences among
series of isostructural lanthanide dinicotinates based on thermal
investigations. Additionally, the luminescence properties of Eu(III)
and Tb(III) complexes have been investigated.
3.1. Structural description
Under solvothermal conditions, the investigated compounds
have been grown in the in the form of like-needle transparent
crystals. Unfortunately, they were not suitable for complete X-
ray single-crystal measurements because of their fragility. For
some monocrystals, it was possible to determine the framework of
the crystal structure whereas large disorder of solvent molecules
included in the channels caused incapability of precise determi-
nation of uncoordinated solvent molecules. These molecules were
determined by thermogravimetric and elemental measurements.
The unit cell parameters from the selected compounds given in
Table 2 reveal that the complexes are isostructural, which is fur-
ther confirmed by the powder X-ray diffraction method (Fig. 1)
and IR measurements. The lanthanide dinicotinates crystallize in
the orthorhombic system and Pnma space group. The parameters
of cell units are very similar and the only differences observed are
a result of lanthanide contraction. The whole series of the investi-
gated compounds is isotructural with pyridine-3,5-dicarboxylates
of Eu [53], Er and Y [52] previously reported.
The complexes obtained from dmf solution form three-
dimensional polymeric structures while compounds synthesized
under hydrothermal conditions constitute the 1D two stranded
polymeric chains [42]. The frameworks of reported complexes
are built up from pyridine-3,5-dicarboxylate linkers and Ln(III)
ions. Each Ln(III) is coordinated by eight oxygen atoms derived
from six pyridine-3,5-dicarboxylate molecules. Carboxylate groups
are of threedentate bridging-chelating and bidentate bridging
character. Additionally, the central atom is coordinated by the
carbonyl oxygen atom of the monodentate dmf molecule. The
structures possess 1D channels occupied by totally disordered N,Nꢀ-
dimethylformamide and water molecules in the crystal lattice. As
the affinity of oxygen atoms for lanthanide atoms is greater than for
nitrogen atoms, coordination of the dinicotinate ligand by N atom
does not take place. In the coordination polymers of transition met-
als with dinicotinate liagnds, nitrogen atoms always participate in
metal bonding [60–62].
2. Experimental
The studied compounds were prepared by reacting of 1.5mmol
of pyridine-3,5-dicarboxylic acid (dinicotinic acid) (250 mg) with
lanthanide(III) chloride hydrates (obtained by dissolving of
0.5mmol of lanthanide oxide in hydrochloric acid) in 10 mL N,Nꢀ-
dimethylformamide by a solvothermal technique, in Teflon-lined
autoclaves which were heated under an autogenous pressure to
140 ◦C for 5 days and then cooled to room temperature for 24 h.
The infrared spectra of the complexes and pyridine-3,5-
dicarboxylic acid were recorded in KBr discs on a SPECORD M80
spectrophotometer over the range 4000–400 cm−1. The X-ray pow-
der diffractions of the studied complexes were recorded on a HZG
4 diffractometer, using Ni filtered CuK␣ radiations. Measurements
were taken over the range of 2ꢀ = 5–70◦.
Thermal analyses of the prepared complexes were carried out
by the TG-DSC method using the SETSYS 16/18 analyser (Setaram).
The samples (about 5–9 mg) were heated in ceramic crucibles up to
850 ◦C at a heating rate of 10 ◦C min−1 in dynamic air atmosphere
(v= 0.75 dm3 h−1).

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R. Łyszczek / Thermochimica Acta 509 (2010) 120–127
Table 1
Results of elemental analyses of lanthanide complexes with dinicotinic acid and dmf.
Complex
%C
%H
%N
Calc.
Found
Calc.
Found
Calc.
Found
[La2pdc3(dmf)2]·dmf0.5(H2O)
[Ce2pdc3(dmf)2]·dmf0.5(H2O)
[Pr2pdc3(dmf)2]·dmf0.5(H2O)
[Nd2pdc3(dmf)2]·dmf0.5(H2O)
[Sm2pdc3(dmf)2]·dmf(H2O)
[Eu2pdc3(dmf)2]·dmf0.5(H2O)
[Gd2pdc3(dmf)2]· dmf0.5(H2O)
[Tb2pdc3(dmf)2]·dmf
35.13
35.04
35.16
36.29
34.84
35.74
33.84
34.88
34.64
34.48
34.33
34.22
33.99
34.22
34.82
39.21
34.79
35.65
34.62
35.61
33.56
34.76
34.42
34.01
33.89
33.67
33.52
33.67
2.93
2.92
2.93
3.02
3.29
2.98
2.82
3.10
3.08
3.06
3.05
3.04
3.02
3.04
2.75
2.65
2.76
2.85
3.02
2.76
2.56
2.87
2.76
2.81
2.86
2.76
2.84
2.68
7.86
7.88
7.92
8.17
8.13
8.05
7.62
8.14
8.08
8.04
8.01
7.98
7.93
7.98
7.56
7.63
7.51
8.10
7.87
7.78
7.54
7.38
7.86
7.71
7.92
7.66
7.63
7.53
[Dy2pdc3(dmf)2]·dmf
[Ho2pdc3(dmf)2]·dmf
[Er2pdc3(dmf)2]·dmf
[Tm2pdc3(dmf)2]·dmf
[Yb2pdc3(dmf)2]·dmf
[Lu2pdc3(dmf)2]·dmf
Table 2
The unit cell parameters of lanthanide complexes with pyridine-3,5-dicarboxylic
acid and dmf.
Orthorhombic system, Pnma space group
a (Å)
b (Å)
c (Å)
Z (Å3)
[La2pdc3(dmf)2]·dmf0.5(H2O) 8.224(2) 30.284(6) 15.71893) 3914.7(4)
[Ce2pdc3(dmf)2]·dmf0.5(H2O) 8.201(4) 30.190(5) 15.686(6) 3883(4)
[Tb2pdc3(dmf)2]·dmf
[Ho2pdc3(dmf)2]·dmf
[Yb2pdc3(dmf)2]·dmf
8.557(1) 28.511(7) 15.939(1) 3888.58
8.517(1) 28.458(4) 15.806(1) 3830.92
8.438(1) 28.43(4)
15.693(1) 3762(9)
The spectra of free acid as well as the described complexes were
recorded in the infrared range (Fig. 2). All complexes show a broad
absorbance band in the range 3400–3200 cm−1 characteristic of
the stretching vibrations of OH bonds from water molecules as
well as hydrogen bonds. The bands at 3060 and 2924 cm−1 were
ascribed to the stretching vibrations of the CH bonds in the CH3
groups of N,Nꢀ-dimethylformamide molecules. The bands derived
from stretching the C O group from dmf appear in the range
1690–1650 cm−1. The strong band at 1656 cm−1 can be assigned
to the coordinated carbonyl groups whereas the remaining weak
bands are derived from physically bonded dmf molecules [63].
Deprotonation of pyridine-3,5-dicarboxylic acid was confirmed by
the absence of the stretching vibrations of C O at 1720 cm−1 [64].
Fig.
2. IR
spectra
of
(a)
pyridine-3,5-dicarboxylic
acid;
(b)
[Nd2pdc3(dmf)2]·dmf0.5(H2O) and (c) [Er2pdc3(dmf)2]·dmf.
Thus the spectra of the lanthanide complexes show the bands origi-
nating from carboxylate groups bands in the range 1600–1560 and
1400–1380 cm−1 from the stretching asymmetric and symmetric
vibrations, respectively [65–66]. These data are in good agreement
with the crystal structures determined by X-ray analysis.
The bands from the stretching asymmetric vibrations are
split that is typical of unequivalency of carboxylate groups. The
infrared spectra confirm that carboxylate groups are of three-
dentate bridging-chelating and bidentate-bridging character. The
strong band at 1552 cm−1 was assigned to the coupled vibration of
deformation and rocking vibration of CH3 group as well as stretch-
ing vibrations of CN bond from the dmf molecules.
3.2. Thermal analysis
To study thermal decomposition of these complexes, the
thermogravimetric analysis (TG) and the differential scanning
calorimetry (DSC) were performed for crystalline samples. The
investigated complexes decompose in the identical way during
heating in the temperature range 30–400 ◦C in air as well as in
argon atmosphere (Figs. 3 and 4). In such temperature, only sol-
vent molecules are evolved, and changing of atmosphere does not
influence on the way of desolvatation. Obviously, slight differences
in the temperature of processes can be observed due to presence of
Fig.
1. XRD
patterns
of
(a)
[Nd2pdc3(dmf)2]·dmf0.5(H2O)
and
(b)
[Tm2pdc3(dmf)2]·dmf.

R. Łyszczek / Thermochimica Acta 509 (2010) 120–127
123
The as made complexes show a gradual mass loss in a wide
temperature range which depending on the nature of the lan-
thanide ions. An initial mass loss in the range about 30–150 ◦C
is attributed to the removal of weakly bonded water and dmf
molecules included in the channels of the frameworks. The release
of physically bound solvent molecules is accompanied by a very
weak endothermic effect on the DSC curve at about 80 ◦C. Next,
on the TG curve there is observed weight loss due to the removal
of N,Nꢀ-dimethylformamide molecules coordinated to metal ions
(Fig. 3). For the complexes of light lanthanides from La(III) to Sm(III)
this process occurs in the two hardly distinguishable steps. The first
one takes place in the temperature range about 150–260 ◦C and is
associated with a weak endothermic effect at about 250 ◦C. The next
stage occurs in the temperature range 250–330 ◦C (except for the
cerium complex which is less stable). Among light lanthanide com-
plexes, only lanthanum complex exhibits two very well-separated
endothermic effects assigned to the dmf molecules release at 236 ◦C
and 312 ◦C, respectively. Next, after solvent molecules evolution,
slow mass loss is observed due to further decomposition of the
unstable network (Table 3).
Fig. 3. TG-DSC curves of [Nd2pdc3(dmf)2]·dmf0.5(H2O) recorded in air (—) and argon
(- - -).
The proposed way of solvent molecules evolution from the
structure of complexes has been confirmed by the TG-FTIR anal-
ysis (Fig. 5). The FTIR spectra of gaseous products evolved during
heating in the temperature range 30–200 ◦C show absorption
bands only in the wave number 4000–3500 and 1800–1300 cm−1
attributed to stretching and deformation vibrations of water
molecules [56,57]. These bands are observed during heating of
only light lanthanide complexes (from La to Eu). Above this
temperature characteristic bands derived from gaseous N,Nꢀ-
dimethylformamide molecules appear. The bands at 2939 and
2849 cm−1 were assigned to the stretching vibrations of the CH
bond from the CH3 groups. The band located at 1724 cm−1 can be
attributed to the stretching vibrations of the carbonyl CO group. The
band at 1493 cm−1 was assigned to stretching vibrations of the CN
bond. Methyl groups of dmf molecules give characteristic bands
derived from asymmetric and symmetric stretching vibrations at
1482 and 1382 cm−1, respectively [63]. Further heating causes col-
lapse of frameworks accompanied by evolution of carbon dioxide
molecules giving characteristic doublet bands at 2359, 2343 cm−1
and those in the range 750–600 cm−1 derived from the valence and
deformation vibrations, respectively. At little higher temperature,
the FTIR spectra of evolved products show bands at 3075, 3017 and
1217, 1152 cm−1 assigned to the stretching and deformation vibra-
tions of C–C and C–N groups from the free pyridine ring [54,57]. At
the same time, carbon dioxide and carbon monoxide are released.
Carbon monoxide gives an easily detectable double-band with the
Fig. 4. TG-DSC curves of [Er2pdc3(dmf)2]·dmf recorded in air (—) and argon (- - -).
disordered solvent molecules within the channels. The complexes
decomposein twowell-separated mass loss stages. Theresultsfrom
thermal decomposition of the investigated compounds in air were
given in Table 3. The first weight loss is consistent with departure
of uncoordinated as well as covalent bonded solvent molecules
whereas the second one is attributed to the decomposition and
burning (air) of the desolvated form of complexes.
Table 3
Thermogravimetric data obtained from TG-DSC curves during heating of [Ln2pdc3(dmf)2]·(dmf)x(H2O)y complexes in air.
Complex
T1
Removal products
Mass loss (%)
Calc.
T2
ꢂT3
Mass loss (%)
Calc.
Found
Found
[La2pdc3(dmf)2]·dmf0.5(H2O)
[Ce2pdc3(dmf)2]·dmf0.5(H2O)
[Pr2pdc3(dmf)2]·dmf0.5(H2O)
[Nd2pdc3(dmf)2]·dmf0.5(H2O)
[Sm2pdc3(dmf)2]·dmf0.5(H2O)
[Eu2pdc3(dmf)2]·dmf0.5(H2O)
[Gd2pdc3(dmf)2]· dmf0.5(H2O)
[Tb2pdc3(dmf)2]·dmf
30–310
30–250
30–320
30–360
30–330
30–290
30–300
30–260
30–260
30–260
30–250
30–260
30–260
30–260
2.5dmf, H2O
2.5dmf, H2O
2.5dmf, H2O
2.5dmf, H2O
2.5dmf, H2O
2.5dmf, H2O
2.5dmf, H2O
3dmf
3dmf
3dmf
3dmf
3dmf
20.59
20.54
24.36
21.27
22.90
20.90
19.84
20.93
21.07
20.97
20.89
20.81
20.67
20.58
22.50
20.30
23.70
20.70
22.00
20.30
19.00
22.42
20.42
22.00
20.64
24.18
21.17
21.00
–
–
–
–
–
310–750
250–460
320–570
360–620
330–620
420–630
420–720
460–600
460–600
450–700
490–700
460–700
480–700
460–700
66.53
64.99
65.00
64.29
66.25
63.23
64.12
63.77
64.70
63.81
63.52
63.40
62.79
62.59
66.30
64.00
65.27
63.54
64.62
63.00
63.00
60.50
64.11
62.30
62.67
62.50
61.36
61.00
290–420
300–420
260–460
260–460
260–450
250–490
260–460
260–480
260–460
[Ho2pdc3(dmf)2]·dmf
[Dy2pdc3(dmf)2]·dmf
[Er2pdc3(dmf)2]·dmf
[Tm2pdc3(dmf)2]·dmf
[Yb2pdc3(dmf)2]·dmf
3dmf
3dmf
[Lu2pdc3(dmf)2]·dmf
pdc – C7H3NO42−; dmf – C3H7NO; T1 – temperature range of removal solvent molecules from crystal structures of the complexes; T2 – temperature range of thermal stability
of Ln2pdc3; T3 – temperature range of degradation of Ln2pdc3 to suitable lanthanide oxides.

124
R. Łyszczek / Thermochimica Acta 509 (2010) 120–127
Fig. 5. Stacked plot of FTIR spectra of the evolved gases for [Nd2pdc3(dmf)2]·dmf0.5(H2O).
maxima at 2177, 2113 cm−1, respectively. Additionally, some bands
at 3071, 1605, 1484 and 1448 cm−1 are observed. These bands have
been assigned to the stretching and deformation vibrations of some
hydrocarbons [65]. As the final solid products of thermal decom-
position of the complexes in air atmosphere, suitable oxides are
formed (Ln2O3 for La, Nd, Sm and CeO2, Pr6O11).
Thermal decomposition of the complexes from Eu(III) to Lu(III),
occurs in a slightly different manner (Fig. 4). During heating in air,
similarly to the complexes described earlier, they start to decom-
pose at 30 ◦C. At first, they lose only physically bonded solvent
molecules, included in the channels of the frameworks. Next, on
the TG curve, a significant weight loss is observed according to the
release of dimethylformamide molecules coordinated with central
atoms [34]. This process occurs in one step compared to the light
lanthanide complexes and is accompanied by a strong endothermic
effect on the DSC curve at about 250 ◦C. This temperature is diag-
nostic for the removal of covalently bonded dmf molecules [34].
The values of enthalpy reaction of dmf molecules release are in the
range 60–111 kJ/mol. The TG-FTIR spectra of the evolved gaseous
products in temperature range 30–260 ◦C evidently confirm the
suggested way of solvent molecules release.
As the product of thermal desolvation, the compounds of the
formula Ln2(pdc)3 are formed. These compounds are stable in a
wide temperature range (about 260–460 ◦C) as can be seen from
Table 3. In such temperature range, no weight losses were observed
on the TG curve as well as the FTIR spectra do not show any bands
that are indicative to formation of stable compounds (Fig. 6). Above
460 ◦C, further heating results in the decomposition process that
is accompanied by evolution of carbon dioxide, carbon monoxide,
pyridine and hydrocarbons in the inert atmosphere.
Fig. 7. The excitation spectrum of [Eu2pdc3(dmf)2]·dmf0.5(H2O) (monitored at
615 nm).
As the final solid products of decomposition of heavier lan-
thanide dinicotinates in the air atmosphere suitable oxides of the
formula Ln2O3 (Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu) and Tb4O7 are formed.
3.3. Luminescence properties
The [Eu2pdc3(dmf)2]·dmf0.5(H2O) and [Tb2pdc3(dmf)2]·dmf
complexes exhibit strong visible luminescence in the red and green
regions, respectively. The excitation spectrum of the europium
complex was obtained by monitoring the emission of Eu(III) ions
7
at 615 nm (50D → F2). As can be seen from Fig. 7, excitation
Fig. 6. Stacked plot of FTIR spectra of the evolved gases for [Er2pdc3(dmf)2]·dmf.

R. Łyszczek / Thermochimica Acta 509 (2010) 120–127
125
Fig. 8. Comparison of the emission spectra of [Eu2pdc3(dmf)2]·dmf0.5(H2O) excited at different wavelength (nm).
weak lines characteristic of the Eu3+ energy levels. The lines at
362 nm, 376 nm, 395 nm, 465 nm and 535 nm were assigned to
the 7F0 → D4, 7F0 → G2,4–6
,
7F0 → L6, 7F1 → D3 and 7F0 → D2
5
5
5
5
5
intrinsic transitions from Eu3+, respectively. These bands were
selected as excitation wavelength for the Eu(III) ions as the most
intense excitation peaks. The emission spectra of the europium
complex, obtained with the different excitation wavelengths are
shown in Fig. 8. These spectra consists of several Eu(III) emission
7
peaks due to the 5D0 → Fj transitions whereas those emission
from the ligand are not observed. The bands at 579, 591, 619,
5
7
5
7
651, and 697 nm were assigned to the 0D → F0, 0D → F1,
5
7
5
7
5
7
0D → F2, 0D → F3, and 0D → F4 f-f transitions of Eu(III) ions,
respectively. The strongest and most useful emissions for struc-
5
7
5
7
tural determination are observed in the 0D → F1 and 0D → F2
7
transition regions. The 50D → F1 transition corresponds to a mag-
netic dipole transition and is relatively unaffected by the local
Fig. 9. The excitation spectrum of [Tb2pdc3(dmf)2]·dmf (monitored at 544 nm).
5
7
environment. The 0D → F2 transitions of predominantly electric
dipole character are co-called hypersensitive transition because
of their sensitivity to the coordination environment [27,29,30].
spectrum exhibits a progressive increase of the excitation abil-
ity toward shorter wavelengths in the range 220–400 nm which
can be assigned to the excited levels of the pdc2− ligands. Addi-
tionally, this spectrum shows a series of sharp and relatively
7
7
The transitions at 50D → F3 and 50D → F4 are nearly invisible for
[Eu2pdc3(dmf)2]·(dmf)0.5(H2O) complex. As can be seen in Fig. 8,
the emission intensities are related to the intrinsic excitation lines
Fig. 10. Comparison of the emission spectra of [Tb2pdc3(dmf)2]·dmf excited at different wavelength (nm).

126
R. Łyszczek / Thermochimica Acta 509 (2010) 120–127
intensity of europium(III). In this complex, pdc2− ligand shows
much weaker intensity than in the [Tb2pdc3(dmf)2]·dmf complex.
The stronger emission intensity was obtained rather by the direct
an excitation of the lanthanide ions. The similar effect was observed
in heterbinuclear complex of Eu(III) and Zn(II) ions [67]. This may
be an explanation of such influence of the excitation wavelength
on the intensity of emissions.
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The excitation spectrum of terbium(III) complex was detected
at 544 nm (Fig. 9). The spectrum shows a broad band in the range
220–350 nm with the maximum absorption peak at 324 nm, which
can be assigned to the emission of the ligand-to-metal charge trans-
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and 487 nm are observed. The lines at 352 nm, 369 nm and 378 nm
were assigned to the 7F6 → G6–4
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5L10,9
,
5D2 and 7F6 → D3 transi-
5
5
tions, respectively. The intensities of transitions from Tb3+ states
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The emission spectrum shown in Fig. 10 under the excita-
tion at 352 nm displays the five emission peaks at 489, 543,
582, 619 and 703 nm, respectively attributed to the intra 4f8
7
7
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7
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5D4 → F6, 5D4 → F5, 5D4 → F4, 5D4 → F3 and 5D4 → F2 tran-
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the region 540–555 nm due to 5D4 → F4 transition [23,24,34]. The
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excitation energy from ligand to metal. The influence of excita-
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for terbium complex as it can be seen from Fig. 10. When the
[Tb2pdc3(dmf)2]·dmf complex was excited with the absorption
wavelength of the ligand at 324 nm, the strongest emission of ter-
bium takes place. This excitation energy is absorbed by this ligand
and then occurs an efficient energy transfer from ligand triplet state
to the europium(III) ions. This excitation process is more efficient
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populate its emitting levels by direct excitation [26,68,69]. These
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4. Conclusions
Summing up, I have synthesized a series of the isostructural
lanthanide complexes with pyridine-3,5-dicarboxylic acid under
solvothermal conditions. The polymeric complexes have three-
dimensional structures with 1D channels occupied by disordered
solvent molecules. The complexes are stable at room tempera-
ture but heating causes release of free as and coordinated solvent
molecules. The stable intermediate compounds Ln2pdc3 are formed
during heating complexes of heavier lanthanides (from Eu to Lu).
The proposed manner of decomposition has been confirmed by the
TG-FTIR spectra of gas-phase products of heating. The complexes
[Eu2pdc3(dmf)2]·dmf0.5(H2O) and [Tb2pdc3(dmf)2]·dmf show very
intense luminescence emissions which can be applied in potential
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