New chelate complexes of trivalent y and lanthanides (Eu, Ho, Yb) with a triazene N-oxide: Synthesis, structural characterization and luminescence properties

Article abstract of DOI:10.1016/j.ica.2010.11.007

Deprotonated 3-(4-nitrophenyl)-1-phenyltriazene N-oxide reacts with YCl₃·6H₂O and LnCl₃·6H ₂O (Ln = Eu, Ho, Yb) to give the monoclinic chelate complexes [Y{O₂N(C₆H₄)NNN(O)Ph}

Full text of DOI:10.1016/j.ica.2010.11.007

Inorganica Chimica Acta 366 (2011) 203–208
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
New chelate complexes of trivalent Y and lanthanides (Eu, Ho, Yb) with a triazene
N-oxide: Synthesis, structural characterization and luminescence properties
a,
a
a
b
Gelson Manzoni de Oliveira a, , Manfredo Hörner , Aline Machado , Davi F. Back , Jorge H.S.K. Monteiro ,
⇑
⇑
Marian R. Davolos b
a Universidade Federal de Santa Maria, UFSM, Departamento de Química, 97105-900 Santa Maria, RS, Brazil
b Universidade Estadual Paulista, UNESP, Instituto de Química, Laboratório de Materiais Luminescentes, Rua Francisco Degni s/n, 14800-900 Araraquara, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2010
Received in revised form 25 October 2010
Accepted 3 November 2010
Available online 12 November 2010
Deprotonated 3-(4-nitrophenyl)-1-phenyltriazene N-oxide reacts with YCl3Á6H2O and LnCl3Á6H2O
(Ln = Eu, Ho, Yb) to give the monoclinic chelate complexes [Y{O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2O (1)
(Ph = C6H5; Et = C2H5) and [LnIII{O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2OÁ{CH3OH } {LnIII = Eu (2), Ho (3),
*
*
Yb (4), in which the metal centers present a square antiprismatic configuration. As already observed
for hydrated ammonium complexes of triazene-oxides ligands with (C6H4)ÀNO2 groups, multiple, effec-
tive OÁÁÁH and NÁÁÁH interactions hold the species in supramolecular 3D assemblies. The optical and the
luminescent properties of the triazene-oxide europium complex 2 are also presented and fully discussed.
Ó 2010 Elsevier B.V. All rights reserved.
Dedicated to the memory of Professor
Herbert Schumann
Keywords:
Lanthanides(III) chelate complexes
Triazene oxides complexes of lanthanides
Luminescence of lanthanides(III) complexes
1. Introduction
triazenes oxides are in general good chelators, with a noteworthy
tendency to achieve stable five-membered rings (A) with transition
Triazenes present a remarkable ability to perform variable coor-
dination modes with transition metals, showing also a noteworthy
trend to achieve intermolecular, secondary metal–ligand and li-
gand–ligand interactions. Such characteristics make these species
also proficient to assemble unique supramolecular aggregates. Re-
cently we have reported some examples of these classes of com-
pounds, with 1D [1–4] and 2D [5–7] polymeric assemblies. The
generation ‘‘in situ’’ of an alkali metal derivative was also investi-
gated, by reaction of the relatively unstable 3-(4-carboxyphenyl)-
1-methyltriazene N-oxide with KOH [8].
The early reported species called triazenes 1-oxides [9] present
a hydroxyl group in the N1-atom of the triazene chain and result
from the equilibrium shown in the Scheme 1.
In solution, triazenes oxides appear in the form I, while the tau-
tomeric form II is preferred in the solid state (powder or crystal-
line). Because of the hard (medium) basic character of the O (N)
atoms, associated with the negative charge of the deprotonated
form, and the proximity of the two coordinative sites (O and N),
metal ions [10,11].
R
'
N
N
R
N
O
M
A
Due to the strong hydration of lanthanide salts, their com-
plexation (chelation) is only achieved with stereochemically
appropriated chelators, like, for example, the triazene oxide 3-(4-
nitrophenyl)-1-phenyltriazene N-oxide. This species presents a
relatively simple architecture, allied with the nearness of the coor-
dinative sites and with the hard basic character of the OÀ ligand, in
this case an indispensable requisite for the displacement of the
strong coordinated water molecules of the lanthanide salts. These
theoretical predictions have been already demonstrated with the
synthesis of the chelate complexes [LnIII{O2N(C6H4)NNN(O)-
Ph}4](Et3NH)ÁH2O (M = La3+
, ; Ph = C6H5; Et = C2H5) [12].
Dy3+
⇑
Corresponding authors. Tel.: +55 55 3220 8757; fax: +55 55 3220 8031.
E-mail addresses: manzonideo@smail.ufsm.br (G. Manzoni de Oliveira), hoerner.
Although known as good chelating agents for some lanthanides,
so far triazenes have never been outstanding by their fluorescent
manfredo@gmail.com (M. Hörner).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.11.007

204
G. Manzoni de Oliveira et al. / Inorganica Chimica Acta 366 (2011) 203–208
nol at 65 °C. Small portions of YCl3Á6H2O (0.0152 g, 0.05 mmol), or
R
R'
N
R
R'
N
LnCl3Á6H2O {0.05 mmol: 0.0183 g (2); 0.01896 g (3); 0.0194 g (4)}
were added under continuous stirring to the yellow solution. After
addition of five drops of triethylamine the color of the solution
turned dark-purple. The mixture was refluxed slowly under stir-
ring for 2 h and then filtered for isolation of a deep-red precipitate.
The slow evaporation of the solvent at room temperature yielded
red crystals of 1–4. Yields: 88% (1); 81% (2); 89% (3); 79% (4).
N
O
N
H
N
N
OH
I
II
Complex 1: Melting point: 174 °C. Anal. Calc. for C54H54YN17O13
C, 52.39; H, 4.40; N, 19.23. Found: C, 48.34; H, 3.91; N, 17.67%. IR
spectra (KBr): 3416 [strong, s(C@C)];
s(NÀH)]; 1659 [medium,
as(NNN)]; 1237 [strong,
:
Scheme 1. Tautomeric equilibrium of triazenes 1-oxides.
activities, still less by their ability to transfer energy to a lanthanide
ion and so excite its luminescence.
m
m
1311 [strong,
ms(NO2)]; 1401 [strong, m
m
s(NÀO)]; 1105 cmÀ1 [medium,
s(NÀN)].
m
The rare-earth elements are characterized for showing the elec-
tronic configuration [Xe] 4fn (n = 0–14). The outer lying 5s25p6
filled subshells of these elements are not luminescent, but the core
electrons 4f show high luminescence activity due to intraconfigu-
rational transitions 4f–4f, which are not allowed by Laporte and
spin rules. Because of the effective shielding of the 4f orbitals
and selection rules restrictions, the lanthanide transitions are little
affected by the crystalline field and are narrow bands. The low
crystalline field influence makes the lanthanides very good struc-
tural probes, especially Eu3+ [13] that shows the characteristic
transitions 5D0 ? 7FJ (J = 0, 1, 2, 3 and 4), all in the red region,
which depend on the number of nonsymmetrical sites
(5D0 ? 7F0), environment (5D0 ? 7F2) and long range effects
(5D0 ? 7F4). The narrow bands are very interesting and useful in
processes requesting high efficiency and monochromaticity. The
most exploited rare-earths are Sm3+, with emission in the orange
region, Eu3+ (red emission) [14], and Tb3+ with green emission.
The theoretical tools provide a good deal of informations about
various and important features, like, for example, coordination
polyhedrons and triplet/singlet states energies.
Complex 2: Melting point: 193 °C. Anal. Calc. for C54H54Eu-
N17O13: C, 49.85; H, 4.18; N, 18.30. Found: C, 49.22; H, 3.73; N,
17.92%. IR spectra (KBr): 3074 [s,
1398 [s, s(NO2)]; 1329 [s, as(NNN)]; 1236 [s,
1174 cmÀ1 [m,
s(N–N)].
m
s(N–H)]; 1585 [m,
m
s(C@C)];
m
m
ms(N–O)];
m
Complex 3: Melting point: 197 °C. Anal. Calc. for C54H54Ho-
N17O13: C, 49.36; H, 4.14; N, 18.12. Found: C, 49.22; H, 3.73; N,
17.92%. IR spectra (KBr): 3190 [s,
1336 [s, s(NO2)]; 1325 [s, as(NNN)]; 1222 [s,
1110 cmÀ1 [m,
s(N–N)].
m
s(N–H)]; 1602 [m,
m
s(C@C)];
m
m
ms(N–O)];
m
Complex 4: Melting point: 196 °C. Anal. Calc. for C55H58Yb-
N17O14: C, 47.75; H, 4.30; N, 17.53. Found: C, 49.22; H, 3.73; N,
17.92%. IR spectra (KBr): 3106 [s,
1331 [s, s(NO2)]; 1311 [s, as(NNN)]; 1237 [s,
1173 cmÀ1 [m,
s(N–N)].
m
s(NÀH)]; 1653 [m,
m
s(C@C)];
m
m
ms(N–O)];
m
2.3. X-ray crystallography
Data were collected with a Bruker APEX II CCD area-detector
diffractometer and graphite-monochromatized Mo K radiation.
We report now some further results on the complex structural
chemistry of triazenes and lanthanides together with luminescence
studies carried out with the new chelates [YIII{O2N(C6H4)NNN(O)-
Ph}4](Et3NH)ÁH2O (1) and [LnIII{O2N(C6H4)NNN(O)Ph}4](Et3NH)Á
a
The structures were solved by direct methods using SHELXS [15].
Subsequent Fourier-difference map analyses yielded the positions
of the non-hydrogen atoms. Refinements were carried out with
the SHELXL package [15]. All refinements were made by full-matrix
least-squares on F2 with anisotropic displacement parameters for
all non-hydrogen atoms. Hydrogen atoms were included in the
refinement in calculated positions.
III
*
H2OÁ{CH3OH*} {Ln = Eu (2), Ho (3), Yb (4)}, obtained by deproto-
nation of 3-(4-nitrophenyl)-1-phenyltriazene N-oxide with trieth-
ylamine and further reaction with YCl3Á6H2O/LnCl3Á6H2O. We
demonstrate also the worth of theoretical methods associated with
the characteristic transitions of the Eu3+ luminescence to help the
crystallographic structural studies and to explain the low lumines-
cence of the parent complexes. Since the optical and structural
properties of the ‘‘semi’’ lanthanide yttrium arouse increasing inter-
est, results regarding its triazene complex have been also included
in this work.
2.4. Optical and luminescence experiments
For optimization of the ligand ground state geometry the RM1
model was used [16], implemented in MOPAC2009 package [17].
The optimization of the complex ground state geometry was car-
ried out with the Sparkle/AM1 model [18] looking for a good result
and low CPU time. Singlet and triplet states energies were calcu-
lated with the INDO/S-CIS [19,20] implemented in zindo package
[21].
2. Experimental
2.1. General
The ligand absorption spectrum in UV–Vis region was per-
formed using dimethylsulfoxide (DMSO) as solvent in the Lambda
14P Perkin–Elmer equipment. The emission spectra of the com-
plexes were recorded at ꢀ77 K in a Spectrofluorimeter Fluorolog
Horiba Jobin Yvon FL3-222 model with a 450 W Xenon lamp.
Elemental analyses for C, H and N were performed at a Shima-
dzu EA 112 microanalysis instrument. IR spectra were recorded on
a Tensor 27 – Bruker spectrometer with KBr pellets in the 4000–
400 cmÀ1 region. Methanol was dried with Mg/I2 and distilled prior
to use. The complex [Gd{O2NPhNNN(O)Ph}4](Et3NH)ÁH2O, em-
ployed for determination of the triplet energy level of the ligand,
was prepared according the standard procedure used in this work
and identified with basis on elemental analysis and IR spectra.
3. Results and discussion
3.1. Crystal structure
2.2. Synthesis of [Y{O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2O (1) and
[Ln{O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2OÁ{CH3OH*} {LnIII = Eu (2), Ho
The X-ray crystal data and the experimental conditions for the
analyses of [YIII{O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2O (1) and
*
(3), Yb (4)}
III
[Ln {O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2OÁ{CH3OH } {LnIII = Eu (2),
*
*
The
ligand
3-(4-nitrophenyl)-1-phenyltriazene
N-oxide
Ho (3), Yb (4)}, are given in Table 1. Table 2 resumes selected bond
(0.0514 g, 0.2 mmol) was dissolved in 10 mL of anhydrous metha-
distances for the title complexes with basis on the templates of

G. Manzoni de Oliveira et al. / Inorganica Chimica Acta 366 (2011) 203–208
205
Table 2
Figs. 1 and 2 (structure of 2), and Table 3 summarizes the most
Selected bond lengths [Å] for 1, 2, 3 and 4.
important bond angles of complex 2. Fig. 1 shows the whole
molecular structure (asymmetric unit) of 2, with exception of the
triethylammonium (counter) ion and the H2O solvate molecule
(the hydrogen atoms were also omitted by clarity). Fig. 2 displays
the core of the anionic component of all complexes, with atom
labeling for complex 2.
1
2
3
4
Y
Eu
Ho
Yb
M–O1
M–O2
M–O3
M–O4
M–N43
M–N23
M–N33
M–N13
2.270(2)
2.281(2)
2.289(2)
2.297(2)
2.516(3)
2.539(3)
2.552(3)
2.572(3)
2.322(4)
2.333(4)
2.335(4)
2.345(4)
2.561(5)
2.585(5)
2.604(5)
2.608(5)
2.267(6)
2.277(5)
2.289(5)
2.296(6)
2.506(7)
2.536(7)
2.551(7)
2.568(7)
2.235(4)
2.247(4)
2.247(4)
2.249(4)
2.471(4)
2.515(5)
2.517(5)
2.543(4)
In all the anionic species [M{O2N(C6H4)NNN(O)Ph}4]À (M = Y,
Eu, Ho, Yb), represented for the Eu complex in Fig. 2, the metal cen-
ters present coordination number 8, with square antiprismatic
geometry. The respectively four M–O and M–N bonds present dif-
ferent lengths, and the average distances are: Y–O = 2.2842/
Y–N = 2.5447 (1); Eu–O = 2.3337/Eu–N = 2.5895 (2); Ho–O = 2.2822/
Ho–N = 2.5402 (3); Yb–O = 2.2445/Yb–N = 2.5115 Å (4). The title
compounds do not differ from analoge triazene complexes with re-
spect to the ability of attaining intra and intermolecular secondary
bonds, hence achieving polydimensional molecular aggregates.
The counter ions Et3NH+ and the H2O (solvate) molecules, allied to
the presence of the NO2 groups of the ligands, supply the conditions
for the achievement of effective OÁÁÁH and NÁÁÁH interactions, which
hold the molecules of the four P21/c-type products in three-
dimensional, supramolecular assemblies. These interactions are
already numerous even at relatively low adjustment of the connec-
tivity (2.5 Å) between the atoms O/H and N/H in the bonds delimiter
program [15] (the sum of the H–O/H–N van der Waals radii are 2.61
and 2.64 Å [22], respectively). As we have already shown in previous
reports (see Introduction), the oxygen atoms of the N–(C6H4)–NO2
ligand group are the main promoters of the OÁÁÁH interactions,
although the four metal-bonded oxygens also attain intramolecular
bridges with vicinal phenyl hydrogen atoms, with variable dis-
tances. All compounds show intramolecular OÁÁÁH bonds between
the two oxygen atoms of all the (C6H4)–NO2 groups with their own
phenyl rings. In the crystal lattice of the title compounds the counter
ions Et3NH+ and the solvate molecules (H2O) are paired of through
hydrogen bridges of the type NHÁÁÁHOH . The N–H bond in the cation
Et3NH+ measures 0.910 Å in all compounds. The single ligand 3-(4-
nitrophenyl)-1-phenyltriazene N-oxide shows a strong infrared
band at 3448.7 cmÀ1
des. This band does not appear in the IR spectra of the products, since
full deprotonation of the ligands occurs.
[m
(OÀH)], characteristic for the triazenes oxi-
Fig. 1. Molecular structure of the complex anion [Eu{O2N(C6H4)NNN(O)Ph}4]À (2).
For clarity, the counter ion [Et3NH]+, the solvate molecule (H2O) and the hydrogen
atoms are not shown.
Table 1
Crystal data and structure refinement for 1ÁÁÁ4.
1
2
3
4
Empirical formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
C
54H54N17O13
Y
C54H54N17O13Eu
1301.1
293(2)
monoclinic
P21/c
9.0591(8)
20.2016(13)
30.952(2)
90
C54H54N17O13Ho
1314.07
293(2)
monoclinic
P21/c
9.1328(2)
20.3452(5)
31.1295(7)
90
C55H58N17O14Yb
1354.22
293(2)
monoclinic
P21/c
9.1847(8)
19.8866(17)
32.237(3)
90
1238.05
140(2)
monoclinic
P21/c
9.0240(8)
20.1343(16)
31.055(3)
90
a (°)
b (°)
90.161(5)
90
5642.4(9)
4
1.457
1.114
90.323(3)
90
5664.4(7)
4
1.526
1.187
91.2360(10)
90
5782.8(2)
4
1.509
1.446
96.562(4)
90
5849.6(9)
4
1.538
1.679
g (°)
V (Å3)
Z
qcalc (g cmÀ3
)
l
(Mo K
a
) (mmÀ1
)
k (Å)
F (0 0 0)
0.71073
2560
0.71073
2656
0.71073
2672
0.71073
2756
Collected reflections
Unique reflections
GOF (F2)
46116
11908
0.968
0.0539
0.1139
45783
12489
1.133
0.054
52614
12807
0.963
0.0535
0.17
45743
11607
0.947
0.0421
0.1185
a
R1
b
wR2
0.1427
a
R1 = ||Fo| À |Fc||/|Fo|.
P
wR2 = { w(F2o À Fc2)2/
R .
w(F2o )2}1/2
b

206
G. Manzoni de Oliveira et al. / Inorganica Chimica Acta 366 (2011) 203–208
Table 3
Selected bond angles for 2.
experimental
theoretical
O1–Eu–O2
O1–Eu–O3
O2–Eu–O3
O1–Eu–O4
O2–Eu–O4
O3–Eu–O4
O1–Eu–N43
O2–Eu–N43
O3–Eu–N43
O4–Eu–N43
O1–Eu–N23
O2–Eu–N23
O3–Eu–N23
O4–Eu–N23
127.56(15)
N43–Eu–N23
95.12(16)
87.64(16)
60.91(15)
135.92(15)
80.75(16)
78.51(16)
160.58(16)
60.72(15)
84.06(16)
76.88(16)
139.26(15)
158.64(16)
91.33(16)
101.24(16)
125.37(15)
75.21(15)
78.86(14)
128.77(15)
129.15(15)
140.01(15)
77.16(16)
88.45(16)
62.06(15)
85.69(16)
136.02(16)
61.22(16)
80.07(16)
O1–Eu–N33
O2–Eu–N33
O3–Eu–N33
O4–Eu–N33
N43–Eu–N33
N23–Eu–N33
O1–Eu–N13
O2–Eu–N13
O3–Eu–N13
O4–Eu–N13
N43–Eu–N13
N23–Eu–N13
N33–Eu–N13
150
200
250
300
350
400
450
λ / nm
Fig. 3. Theoretical and experimental absorption spectra of the ligand.
fraction are 2.3337 and 2.5895 Å, respectively. With the Sparkle/
AM1 program the average distances for Eu–O and Eu–N are
2.4303 and 2.5315 Å, respectively. For Eu–O the average error by
the theoretical method is 4.1%, while for Eu–N the error is 2.2%.
Although some differences regarding the coordination polyhedron
are evident by the theoretical method, a good approximation with
the structure obtained by X-ray diffraction was achieved.
Fig. 2. Square antiprismatic structure of the core of the P21/c-type anion
[Eu{O2N(C6H4)NNN(O)Ph}4]À (2). For clarity, the phenyl rings and the NO2 groups,
as well as the cation [Et3NH]+, the solvate molecule (H2O), and the hydrogen atoms,
are not shown.
3.3. Luminescence
3.2. HOMO–LUMO calculations, optical features and ground state
structures
As a first investigation, for determination of the triplet energy
level of the ligand, the emission spectrum of the complex [Gd{O2N-
PhNNN(O)Ph}4](Et3NH)ÁH2O was obtained (Fig. 5). The exception-
ally high energy of the lowest excited level of Gd(III) affords an
efficient method for this kind of measurement, after preparing the
Gd(III) complex with the compound whose triplet energy is the
point in question. The triplet energy is achieved through the cen-
troid of the most intense transition, and for this (triazene-oxide)
ligand, the triplet energy is located in 15086 cmÀ1. The calculated
triplet energy by INDO/S-CIS is 14921 cmÀ1, a value very close to
the experimental one. The energy levels diagram containing the the-
oretical and experimental energy levels for the ligand and Eu3+ is
shown in Fig. 6. The triplet level of the ligand is situated below
the emissor level of the europium ion. This circumstance makes
the metal ? ligand transfer rate more intense than the ligand ?
metal one. Thus, there is a competition between ligand emission
through T ? S0 transition and europium emission through
5D0 ? 7FJ (J = 0, 1, 2, 3 and 4) transitions. The europium emission
is also affected by water molecules present in the complex struc-
ture. When the emission spectrum is performed at room tempera-
ture (ꢀ298 K) both ligand and europium emissions are observed.
However, at low temperature there is a minimization of non-radia-
tive deactivation processes caused by multi-phonon and multi-
vibration due the presence of the water molecule. In this case, at
ꢀ77 K, the europium emission is predominant and the transitions
5D0 ? 7FJ (J = 0, 1, 2, 3 and 4) can be observed, as shown in Fig. 7.
The occurrence of two bands in the 5D0 ? 7F0 region indicates the
presence of two different and non-centrosymetric sites in the unit
cell. These can be related to the Eu–O/Eu–N bonds, justifying the
two bands for the 5D0 ? 7F0 transition. The high intensity of the
hypersensitive 5D0 ? 7F2 transition reveals the high polarizability
of the chemical environment around the europium ion, and the
low split of the 5D0 ? 7F2 transition is a high symmetry indicative.
P21/c is one of the most symmetric space groups of the monoclinic
The HOMO–LUMO orbitals of the ligand 3-(4-nitrophenyl)-
1-phenyltriazene N-oxide, as well as their energies, are shown in
Table 4. The absorption spectra of the ligand are shown in Fig. 3,
and both theoretical and experimental absorption spectra present
the same profile. The utilization of DMSO as solvent has blocked
the visualization of the bands in lower wavelengths. The theoretical
and experimental ground state geometries for [Eu{O2N(C6H4)NN-
N(O)Ph}4](Et3NH)ÁH2O (2) are shown comparatively in Fig. 4. The
average Eu–O/Eu–N distances obtained by monocrystal X-ray dif-
Table 4
Ligand molecular orbital obtained by RM1.
Orbitals
Energy (eV)
À3.8
HOMO
2.2
LUMO

G. Manzoni de Oliveira et al. / Inorganica Chimica Acta 366 (2011) 203–208
207
Fig. 4. Structure of the anion [Eu{O2NPhNNN(O)Ph}4]À: (a) monocrystal X-ray diffraction; (b) sparkle/AM1.
1.5x105
(a)
(b)
1.2x105
8.0x103
4.0x103
0.0
9.0x104
577 578 579 580 581 582
6.0x104
λ / nm
3.0x104
0.0
500
550
600
λ / nm
650
700
750
450
500
550
600
λ / nm
650
700
750
Fig. 7. Emission spectra of [Eu{O2NPhNNN(O)Ph}4](Et3NH)ÁH2O with kex = 365 nm.
Fig. 5. Emission spectrum of the gadolinium compound, kex = 420 nm.
(a) Enlargement of the 5D0
?
7F0 transition; (b) full spectrum.
30000
4. Conclusions
S1
In the previous discussions we have shown that the chelate
25000
20000
compounds [YIII{O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2O (1) and
III
[Ln {O2N(C6H4)NNN(O)Ph}4](Et3NH)ÁH2OÁ{CH3OH } {LnIII = Eu (2),
*
*
5D0
Ho (3), Yb (4)} are structurally equivalent, all of them retaining
one water molecule as solvate. Yttrium resembles the lanthanides
strongly in its chemistry, so that the triazene oxide complex of
yttrium has been also included throughout this work. Although
yttrium itself does not undergo luminescent transitions, its com-
pounds are frequently useful host materials for later activators
Ln3+ ions, like, for example, Eu:Y2O2S (standard material for the
red phosphor in virtually all color and television cathode ray tubes)
and Eu:Y2O3 (used for energy-efficient fluorescent tubes) [23].
Anhydrous salts of the lanthanides are known to exhibit lumi-
nescence which is exploited in their use as solid state laser mate-
rials or phosphors for color screens. By contrast, hydrated
crystals of the lanthanides exhibit much lower luminescence
intensity for the members in the center of the series (Sm, Eu, Gd,
Tb, Dy), and essentially no luminescence at the beginning or end
of the series [24]. The weak luminescence of hydrated Ln(III) ions
15000
10000
5000
0
T
T
7FJ
S0
S0
Experimental
Theoretical
Eu3+
Ligand
Fig. 6. Energy levels diagram for the ligand and europium(III).
system. As commented in the next section, luminescence experi-
ments carried out with complexes 1, 3 and 4 did not deliver men-
tionable results.

208
G. Manzoni de Oliveira et al. / Inorganica Chimica Acta 366 (2011) 203–208
indicates that the high energy vibrations of the O–H groups of the
water molecules embedded in the crystal lattice provide an effi-
cient mechanism for the non-radiative de-excitation of the Ln(III)
excited states. With basis on these statements, it is not surprising
if only the Eu(III) complex presents luminescence, because Ho(III)
and Yb(III) are situated at the end of the lanthanide series, and
these complexes are also hydrated – in the case of Yb(III), with
one additional methanol molecule. Some authors determine the
number of H2O molecules coordinated in the inner sphere of Ln(III)
ions bound in complexes with basis on the luminescence lifetime
of these hydrated complex ions. Haas and Stein [25] demonstrated
that the observed luminescence decay constant for the 5D0 excited
state of Eu(III) in mixtures of water and acetonitrile is proportional
to the number of water molecules in the primary coordination
sphere of the ion. They also proposed that each water molecule
quenches the excited state population independently.
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Appendix A. Supplementary material
CCDC 778915, 778914, 778913 and 779317 contains the sup-
plementary 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.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2010.11.007.
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