Luminescence and structural studies of yttrium and heavier lanthanide-picrate complexes with pentaethylene glycol

Article abstract of DOI:10.1016/j.ica.2009.05.045

The monoclinic structural isomers with molecular formula of [M(Pic)₂(EO5)](Pic) where EO5 = pentaethylene glycol, Pic = picrate anion, and M = Gd, Tb, Er, Tm, Yb, and Y have been synthesized and characterized. The current study was conducted in

Full text of DOI:10.1016/j.ica.2009.05.045

Inorganica Chimica Acta 362 (2009) 4025–4030
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Inorganica Chimica Acta
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Luminescence and structural studies of yttrium and heavier lanthanide–picrate
complexes with pentaethylene glycol
b,
Eny Kusrini a, , Muhammad Idiris Saleh
*
*
a Department of Chemical Sciences, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Terengganu, Malaysia
b School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 24 June 2008
Received in revised form 17 May 2009
Accepted 22 May 2009
Available online 29 May 2009
The monoclinic structural isomers with molecular formula of [M(Pic)2(EO5)](Pic) where EO5 = pentaeth-
ylene glycol, Pic = picrate anion, and M = Gd, Tb, Er, Tm, Yb, and Y have been synthesized and character-
ized. The current study was conducted in the solid state to evaluate the coordination pattern of the
central metal ion with the EO5 ligand in the presence of Pic anion. The photoluminescence (PL) spectra
of the Tb and Yb complexes have the typical 4f–4f transition emissions of Tb(III) and Yb(III) ions. The Gd,
Er, Tm, and Y complexes had broad bands resulting from the ligands. The counteranion factor influenced
the emission intensity in the Tb–Pic–EO5 and Tb–NO3–EO5 complexes in several solvents also were stud-
ied. The Pic anion acts as a quencher in the [Tb(Pic)2(EO5)](Pic) complex due to the nitro withdrawing
groups was clearly observed both in solution and the solid state.
Keywords:
Heavier lanthanide
Pentaethylene glycol
Photoluminescence
X-ray crystal structures
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
P97% purity) was purchased from Fluka (Buchs, Switzerland). HPic
((NO2)3 C6H2OH, >98% purity) was purchased from BDH (Poole,
The coordination behavior of lanthanide ion with the acyclic
EO5 ligand has attracted interest due to the strong coordination
capability with the metal ion, flexible structure and its donor oxy-
gen atoms can be coordinated to the metal ion in a hexadentate
mode effects for luminescence properties and their ability to adopt
different structural diversity [1–5]. The picric acid (HPic) may dis-
place the coordinated water molecule [4,6] as well as increase the
molecular rigidity to attain the high efficiency luminescence prop-
erty. Scheme 1a and b shows the molecular structure of the EO5
and HPic molecules [3].
In this article, we report the preparation, spectral characteriza-
tion, molar conductance, luminescence, and crystal structures
determination of the yttrium and heavier lanthanide complexes,
[M(Pic)2(EO5)](Pic). The nitro withdrawing group from the Pic an-
ion plays an important role in modifying of the luminescence prop-
erties of the prepared compounds.
England). M(NO3)3Á5H2O (99.9% purity) for M = Tb, Er, Tm, Yb,
and were purchased from Aldrich (Wisconsin, USA).
Y
Gd(NO3)3Á6H2O (99.9% purity) was obtained from Johnson Matthey
Electronic (New Jersey, USA).
2.2. Preparation of the complexes
The complexes were prepared as previously described [1–5]. A
mixture of EO5 (0.6 g, 2.52 mmol), HPic (0.91 g, 3.97 mmol), and
[M(NO3)3Á7H2O] (0.434 g, 1 mmol) for each M = Gd, Tb, Er, Tm,
Yb, and Y was dissolved in 20 mL acetonitrile:methanol (3:1, v/
v). The solution mixture was stirred for 5 min. The mixture was left
to stand for 1–5 days at room temperature and single crystals for
Gd (60%), Tb (80%), Er (60%), and Y (75%) suitable for X-ray diffrac-
tion determination were collected. However, the reactions of EO5
and HPic with the Tm and Yb nitrate salts in acetonitrile:methanol
(3:1, v/v) did not result in any single crystal products.
2. Experimental
2.3. Physical measurements
2.1. Materials
The percentages of carbon, hydrogen, and nitrogen were deter-
mined by using a Perkin–Elmer 2400II elemental analyzer. Conduc-
tivity measurements were made in a DMSO solution at 25.95
0.01 °C using a Scan500 conductivity meter. IR spectra were re-
corded on a Perkin–Elmer 2000 FTIR spectrophotometer in the re-
gion of 4000–400 cmÀ1 by using the conventional KBr pellet
method for solid samples. For liquid sample, e.g. the EO5 ligand,
All chemicals and solvents were of analytical grade and used
without further purification. Pentaethylene glycol (EO5, C10H22O6,
* Corresponding authors. Tel.: +609 6683676; fax: +609 6683326.
E-mail addresses: e.kusrini@umt.edu.my (E. Kusrini), midiris@usm.my (M.I.
Saleh).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.05.045

4026
E. Kusrini, M.I. Saleh / Inorganica Chimica Acta 362 (2009) 4025–4030
O
O
OH
HO
O
O
(a)
O
N+ O-
O
N+
-O
HO
N+ O-
O
(b)
Scheme 1.
a thin layer of sample was applied to the surface of a KRS-5 (thal-
lium bromoiodide).
2.4. X-ray crystallographic study
Photoluminescence (PL) measurements were made at room
temperature by using a Jobin Yvon HR800UV system. The data
were collected and processed with Labspec Version 4 software.
An HeCd laser was used for excitation at 325 nm, and the emission
spectra were scanned from 330 to 1000 nm. An incident laser
(20 mW) was used as the excitation source. A microscope objective
lens (UV40Â) was used to focus the laser on the sample surface.
The emitted light was dispersed by a double grating monochroma-
tor (0.8 m focal length) equipped with an 1800 groove/mm holo-
graphic plane grating. Signals were detected with a Peltier-cooled
CCD4 array detector.
X-ray diffraction data were collected from single crystals by
using a Bruker APEX2 area-detector diffractometer with a graphite
monochromated Mo Ka radiation source and a detector distance of
5 cm. Data were processed using APEX2 software [7]. The collected
data were reduced by using the SAINT program and the empirical
absorption corrections were applied with the SADABS program [7].
The structures were solved by direct methods and refined by
least-squares method on F2obs using the SHELXTL program [8]. All
non-hydrogen atoms were refined anisotropically. Hydrogen atoms
were located from different Fourier maps and were isotropically re-
fined. The final refinement converged well. Data for publication were
prepared with SHELXTL [8] and PLATON [9]. Crystallographic data and
refinement of the [M(Pic)2(EO5)](Pic) complexes are summarized
in Table 1. Selected bond lengths, and bond angles of the complexes
studies are listed in Table 2. One oxygen atom (O20) in the Gd com-
plex is disordered over two positions with refined occupancies of
Fluorescence measurements were made with a Jasco Spectroflu-
orometer Model FP-750 at room temperature with a xenon light
source, and the excitation and emission slits used were both
5 nm. The emission spectra were scanned from 220 to 730 nm at
500 nm/min. The concentrations for samples were 80 lmol/L.
Table 1
Summary of crystallographic data and refinement for the [M(Pic)2(EO5)](Pic) complexes.
Parameter
Compound
Gd
Tb
Er
Y
Formula
C28H28N9O27Gd
1079.84
3702.68(14)
monoclinic
P21/c
C28H28N9O27Tb
1081.51
3829.23(15)
monoclinic
P21/c
C28H28N9O28Er
1089.85
3818.5(3)
monoclinic
P21/c
C28H28N9O27
1011.50
3818.1(4)
monoclinic
P21/c
Y
Formula weight
Volume (Å3)
Crystal system
Space group
Z
4
4
4
4
Unit cell dimensions
a (Å)
b (Å)
18.4972(4)
8.9107(2)
23.7748(5)
90
18.7250(3)
8.9956(2)
24.0786(7)
90
18.707(8)
8.980(4)
24.077(1)
90
18.708(1)
8.985(5)
24.063(1)
90
c (Å)
a
=
c
(°)
b (°)
Dcalc (g/cm3)
(mmÀ1
F(0 0 0)
109.1100(10)
1.937
1.911
109.2440(10)
1.876
1.963
109.252(1)
1.896
2.314
2172
109.272(1)
1.760
1.644
l
)
2156
2160
2056
Crystal size (mm)
h Range (°)
0.50 Â 0.39 Â 0.11
0.43 Â 0.30 Â 0.25
0.50 Â 0.34 Â 0.34
0.24 Â 0.20 Â 0.20
1.17–30.00
1.15–26.41
2.41–28.29
2.41–28.28
h, k, l
26/À25, À12/12, À33/33
112 256/10 805 (0.0473)
10 805/3/598
R1 = 0.0502, wR2 = 0.1259
R1 = 0.0540, wR2 = 0.1317
1.213
0/23, 0/12, À31/30
7620/7620 (0.0000)
6187/0/586
R1 = 0.0512, wR2 = 0.1291
R1 = 0.0720, wR2 = 0.1656
1.184
À24/21, À11/11, À20/31
22 884/9329 (0.021)
9329/0/586
R1 = 0.0322, wR2 = 0.081
R1 = 0.040, wR2 = 0.086
1.071
À24/24, À11/11, À31/22
23 312/9310 (0.037)
9437/0/596
R1 = 0.054, wR2 = 0.148
R1 = 0.087, wR2 = 0.175
1.036
Reflections collected/unique (Rint
Data/restraints/parameter
Final R indices [I > 2
R indices (all data)
Goodness-of-fit
)
r(I)]

E. Kusrini, M.I. Saleh / Inorganica Chimica Acta 362 (2009) 4025–4030
4027
Table 2
Selected bond lengths (Å) and bond angles (°) in the [M(Pic)2(EO5)](Pic) complexes.
Bond
Length (Å)
Gd
Bond
Angle (°)
Tb
Er
Y
Gd
Tb
Er
Y
M1–O1
M1–O2
M1–O3
M1–O4
M1–O5
M1–O6
M1–O7
M1–O14
M1–O8
2.479(3)
2.541(3)
2.438(3)
2.515(3)
2.528(4)
2.441(3)
2.302(3)
2.308(3)
2.476(3)
2.441(4)
2.530(5)
2.511(4)
2.443(4)
2.527(4)
2.476(4)
2.296(4)
2.288(4)
2.484(4)
2.419(2)
2.507(3)
2.478(3)
2.424(3)
2.510(2)
2.450(2)
2.270(2)
2.268(2)
2.445(3)
2.414(3)
2.506(3)
2.483(3)
2.428(3)
2.509(3)
2.451(3)
2.262(3)
2.259(3)
2.451(3)
O1–M1–O2
O3–M1–O2
O3–M1–O4
O4–M1–O5
O6–M1–O5
O1–M1–O6
63.88(9)
62.72(15)
62.16(18)
63.68(17)
61.82(14)
63.37(13)
72.0(1)
63.1(9)
62.9(1)
63.3(1)
61.9(9)
63.7(8)
71.9(8)
63.2(1)
62.9(1)
63.2(1)
61.9(1)
63.8(9)
71.8(9)
61.59(10)
63.42(10)
62.67(11)
63.03(11)
71.30(10)
71:29. Four oxygen atoms (O20, O23, O25, and O26) in the Tb com-
plex are disordered over two positions with refined occupancies of
77:23, 75:25, 73:27, and 44:66.
3400 cmÀ1 that may be attributed to O–HÁ Á ÁO hydrogen bonding
[5].
3.2. X-ray studies
3. Results and discussion
The structural isomers of [M(Pic)2(EO5)](Pic) complexes are
crystallized in a monoclinic with space group P21/c (Table 1). The
volume of the complexes is reduced due to the heavier lanthanide
ions. The Y complex is like the heavier lanthanide complexes. The
lighter and heavier lanthanide ions control the geometry of the in-
ner coordination spheres in the Ln–picrate complexes [6]. The cal-
culated density of the Y complex is the smallest (1.760 g/cm3).
Meanwhile, the calculated density of the Gd complex is the highest
(1.937 g/cm3), thus suggesting that the crystal packing in the Gd
complex is more efficient than that in the Er, Tb, and Y complexes.
The asymmetric unit of the complexes studied, contain two
crystallographically independent [M(Pic)2(EO5)]+ and a Pic anion.
The most favorable site of the Pic anion is the negatively charged
phenolic oxygen due to the ion–ion interaction [11]. The oxygen
atom of ortho-nitro group is also favorable coordination site due
to the charge localization on this oxygen atom. Between the phe-
nolic and ortho-nitro oxygen atoms had the short distance about
of 2.683 Å that provide an effective bidentate mode with both
charge and dipole binding ability [12].
The central metal ion was coordinated to nine donor oxygen
atoms and the positive charge was balanced by one Pic anion acts
as a counteranion. Six oxygen atoms belong to the EO5 ligand in
hexadentate mode and the remaining three from each bidentate
and monodentate Pic anions (Fig. 1). The slightly distorted tri-
capped trigonal prismatic geometry has the O1, O3, and O5 atoms
at the peak of the capping position of the three square planes for
the Tb, Er, and Y complexes. The oxygen atoms as a peak of the cap-
ping position, namely O1, O3, and O5 form the trigonal plane
which is the Tb1, Er1, and Y1 atoms lie on the center of this trigonal
plane (Fig. 2). Because of the O1, O3, and O5 atoms are coplanar
with the central metal ion with maximum deviation of À0.004(1)
Å for the Tb1 atom, À0.001(1) Å for the Er1 atom, and 0.000(4) Å
for the O3 atom. As in the other EO5 complexes, the inner coordi-
nation sphere of [Gd(Pic)2(EO5)]+ has a slightly tricapped trigonal
prismatic geometry with the O2, O4, and O6 atoms coplanar and
3.1. Physical properties and spectral analysis
Analytical data for the newly synthesized complexes indicate
that the complexes are consistent with a 1:1:3 (metal-to-ligand-
to-picrate) compositions, [M(Pic)2(EO5)](Pic) (Table 3). For com-
parison studies, these complexes can be formed following by slow
evaporation in THF, CH3CN, CH3OH, and H2O solutions within a rel-
atively short time (few days). However, the complex can be ob-
tained in the DMSO solution after slow evaporation for a long
time (seven months). This slow formation is due to the tight com-
petition between the coordination oxygen donor atoms from the
DMSO molecule and the Pic anion.
The solubility of the complexes in common solvents is low,
which made solution studies difficult and prevented the measure-
ment of conductivity [5,6]. Conductivity measurement has pro-
vided
a method of testing the degree of ionization of the
complexes, the molecular ion that a complex liberates in solution,
the higher will be its molar conductivity and vice versa [10]. The
molar conductivity values for the [M(Pic)2(EO5)](Pic) complexes
were in the range 113–189
X
À1 molÀ1 cm3 indicating that the
complexes to be electrolytes nature in a 1:3 ratio [5] (Table 3). This
means that the DMSO molecules have replaced the two Pic anions
from the structures of cation in solution. Thus, the actual solution
species that is the [M(DMSO)3(EO5]3+ and three Pic anions. From
this results are in a well agreement and prove the suggested that
the formation of [M(Pic)2(EO5)](Pic) complexes in DMSO solution
is slowly.
Upon complexation, the typically absorption stretching bands
of m(O–H), m(C–H), m(C–C), m(C–O–C), ma(NO2), ms(NO2), and m(C–N)
were shifted towards the free EO5 and HPic molecules. On the basis
of the similarity of their IR spectra, we assume that the complexes
studied have similar structures with the lighter lanthanide EO5
complexes have been reported by Saleh et al. [1–4] and Kusrini
et al. [5]. The IR spectra of the complexes had broad bands over
Table 3
Elemental analysis of the [M(Pic)2(EO5)](Pic) complexes.
Compound
Decomposition (°C)
Found (Calc.) (%)
C
Yield molar conductivity (
X
À1 molÀ1 cm3)
Colour
H
N
Gd
Tb
Er
Tm
Yb
Y
260.5–276.8
254.4–273.1
237.6–256.2
148.6–229.7
226.0–243.9
247.5–256.0
31.23 (31.04)
31.17 (31.01)
30.84 (30.74)
29.82 (30.71)
29.82 (30.69)
33.06 (33.13)
2.47 (2.52)
2.57 (2.51)
2.61 (2.47)
2.26 (2.56)
2.26 (2.55)
2.78 (2.86)
11.64 (11.69)
11.66 (11.62)
10.97 (11.53)
11.66 (11.52)
11.66 (11.50)
12.17 (12.42)
60
80
60
50
40
75
182
125
139
189
113
113
yellow
yellow
yellow
yellow
yellow
Yellow

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E. Kusrini, M.I. Saleh / Inorganica Chimica Acta 362 (2009) 4025–4030
Fig. 1. Molecular structure of [Gd(Pic)2(EO5)](Pic) complex with 50% probability ellipsoids. All the hydrogen atoms and the Pic counteranion have been omitted for clarity.
Fig. 2. The inner coordination sphere of [Y(Pic)2(EO5)]+ shows the trigonal plane
from the O1–O3–O5 atoms.
Fig. 3. Coordination geometry around the Gd1 atom as
a tricapped trigonal
prismatic.
at the top in the capping position (Fig. 3). The Gd1 atom also lies on
the trigonal plane from the O2–O4–O6 atoms. This is due the O2,
O4, and O6 atoms are coplanar with the Gd1 atom with maximum
deviation of À0.001(1) Å for the Gd1 atom.
nine-coordination number was found [13]. The M–Oether bond
lengths are longer than the M–Oalcohol bond lengths. The average
M–Ophenol bond lengths are the shortest, i.e. 2.305(3), 2.292(4),
2.269(2), and 2.261(3) Å, respectively, for the Gd, Tb, Er, and Y
complexes. The shorter of bond length is caused by the higher elec-
tron density of the phenolic oxygen of the Pic anion [1–6,14]. The
M–Onitro bond length for the heavier lanthanide–picrate complexes
is shorter than those found in the lighter lanthanide–picrate com-
plexes [1–5]. These lengths are influenced by the free position of
the ortho-nitro oxygen atoms before this oxygen is coordinated
with the metal ion. In the Gd complex, both of the nitro groups
of the bidentate Pic anion are rotated with respect to the aromatic
part with torsion angles of O8–N1–C12–C11 [4.4(6)°]; O9–N1–
C12–C11 [À174.7(4)°] and O12–N2–C16–C11 [À49.8(7)°]; O13–
N2–C16–C11 [130.9(6)°]. While, both of the nitro groups of the
monodentate Pic anion also are rotated with respect to the aro-
Both aromatic rings of the coordinated Pic anions were planar
and made dihedral angles with the structures of cation, i.e.
88.03(18)°, 89.3(2)°, 89.2(2)°, and 89.4(2)° for the Gd, Tb, Er, and
Y complexes, respectively. Fig. 2 shows the acyclic EO5 ligand
was coordinated to the central metal ion in the complexes which
exhibited pseudo-cyclic behavior [1–5]. The Pic anion can prevent
the water molecules involved in the inner coordination sphere of
these compounds was clearly observed.
All of the M–O bond lengths were reduced from the Gd, Tb, Er,
and Y complexes with respect to increasing of the atomic number
or lanthanide contraction effect (Table 2). The effective ionic radii
of Gd(III), Tb(III), Er(III), and Y(III) ions were quite similar, i.e.
1.107, 1.095, 1.062, and 1.075 Å, respectively [13]. In this case a

E. Kusrini, M.I. Saleh / Inorganica Chimica Acta 362 (2009) 4025–4030
4029
matic part with torsion angles of O12–N2–C16–C11 [À49.8(7)°];
O13–N2–C16–C11 [130.9(6)°] and O19–N6–C22–C17 [20.0(11)°];
O20A–N6–C22–C17 [148.2(7)°]. For the Tb, Er, and Y complexes,
the behavior of the nitro groups of the coordinated Pic anion also
are similar to those in the Gd complex. These values are compara-
ble to those reported in the acid [15].
molecule units by N9C–O26CÁ Á ÁH8AB–C8B [2.572 Å] were ob-
served in the Tb, Er, and Y complexes (Fig. 4).
The intermolecular
anion and the monodentate Pic anion via face-to-face mode with
centroid distance of 4.458(2), 4.484(3), 4.482(2), and
p–p interactions between the Pic counter-
a
4.485(3) Å, respectively, were observed in the Gd, Tb, Er, and Y
complexes. The relative orientation of the two adjacent aromatic
rings of the Pic anion is determined by the electrostatic repulsions
The O–M–O bond angle between the adjacent oxygen atoms in
the inner coordination sphere of all of the complexes was approx-
imately the same, and slightly larger than 60° (Table 2). The O6–
M1–O1 bond angles in the heavier lanthanide complexes were
slightly larger than the O6–M1–O1 bond angles in the lighter lan-
thanide complex [1–5]. The Tb, Er, and Y complexes showed the
same a geometric conformational pattern in the EO5 ligand of g+
gÀ g+ gÀ gÀ with following average O–C–C–O torsion angles of
40.06(8)°, 37.3(8)°, and 37.0(9)°, respectively. The Gd complex
showed the geometric conformational pattern in the EO5 ligand
of g+ g+ gÀ g+ gÀ with average torsion angle of 48.48(5)°. The O–
C–C–O torsion angle patterns in the yttrium and heavier lantha-
nide complexes were different from those measured in the other
derivative complexes involving La [4], Pr [1], Sm, and Dy [3].
Among the different lanthanide ion complexes, the structures of
cation, and geometrical arrangement also influence the torsion an-
gle sequence. All the C–O–C–C torsion angles are anti, except C2–
O2–C3–C4 (for the Gd complex) and C9–O5–C8–C7 (for the Tb,
Er, and Y complexes) is close to the gauche conformation (gÀ).
There are different types of hydrogen bonding in the complexes
that give rise to different supramolecular architectures. The com-
plexes show one-dimensional (1D) architecture with symmetry
direction [001] that is connected through intra- and intermolecular
O–HÁ Á ÁO and C–HÁ Á ÁO hydrogen bonding (Fig. 4). The presence of
the terminal alcoholic group provides the possibility of the forma-
tion of strong and weak hydrogen bonds to confer high stability on
the Tb, Er, and Y complexes. However, both the terminal alcoholic
groups only contribute to form the weak intra- and intermolecular
hydrogen bonds were observed in the Gd complex (S1). The O–
HÁ Á ÁO, O–HÁ Á ÁN, and C–HÁ Á ÁO hydrogen bonds have stabilized in
the crystal packing of the Gd complex. Only intermolecular O–
HÁ Á ÁO and C–HÁ Á ÁO hydrogen bonds were stabilized the crystal
packing of the Tb complex. Two bifurcated hydrogen bonds were
observed for the Tb, Er, and Y complexes namely O6–H6CÁ Á ÁO21
and O6–H6CÁ Á ÁO27 and C8–H8AÁ Á ÁO26 and C8–H8BÁ Á ÁO7. The Pic
counteranion is linked with [M(Pic)2(EO5)]+ by two weak hydrogen
bonds, i.e. N8C–O25CÁ Á ÁH13C–C13C [2.553 Å] and C27–
H27CÁ Á ÁO10C–N2C [2.424 Å], and then with the other adjacent
between the two negatively charged p-systems
3.3. Photoluminescence (PL) studies
The ability of the EO5 and Pic ligands to satisfy the coordination
requirements of the central Ln(III) with a high coordination num-
ber is an important criterion in the design of supramolecular pho-
tonic devices. However, the EO5 ligand also contains two terminal
alcohol groups that may quench the lanthanide luminescence. The
efficient non-radiative deactivations of the O–H and C–H groups,
enables their participation in vibrionic coupling with the vibra-
tional state of the O–H and C–H oscillator [16]. The electron-with-
drawing nitro and phenolic groups in the Pic ligand also influence
the distribution of p-electronic density.
From the PL spectrum at room temperature, the HPic ligand had
a
broad emission band with the center peak at 537 nm
(18 622 cmÀ1) based on D2 filter measurement. The weak band as-
signed to the nitro group by n-p* transition with an admixture of
intra-ligand charge transfer (ILCT) is seen in the region of 350–
425 nm. At excitation in transitions of aromatic anions and in the
short-wavelength Ln(III) transitions the absorbed energy is trans-
ferred to the excited state of the nitro group. The direct excitation
of the NO2 group through n-p* transition also occurs. The excita-
tion energy can be dissipated in the lattice, transferred to the low-
est excited ligand triplet level, where it can generate ligand
fluorescence and Ln(III) luminescence. A dissipation channel for
the excitation energy through the excited state of the NO2 group
from the Pic anion by
curs in the Tb complex.
p
*-n transition with participation of ILCT oc-
The free EO5 ligand had also broad emission bands with the
highest intensity at 492.8 nm based on D2 filter measurements.
The lowest triplet state energy level of the EO5 ligand [T1(L)] is
ꢀ20 292 cmÀ1. This energy level is above the lowest excited reso-
2
nance level 5D4 (ꢀ20 500 cmÀ1) of Tb(III), but lower than the F5/
2 (ꢀ10 300 cmÀ1). The T1 (L) should be nearly equal to or lie above
a resonance level of the central ion concerned. If the Ln(III) ion is
Fig. 4. View of the packing arrangement of the [Er(Pic)2(EO5)](Pic) complex along b-axis. The hydrogen bonds are indicated by dashed lines.

4030
E. Kusrini, M.I. Saleh / Inorganica Chimica Acta 362 (2009) 4025–4030
excited by transfer of energy from the excited chelate molecule to
the central ion resulting in line emission. Of the six complexes
examined, only the Tb and Yb complexes have the typical 4f–4f
transition emissions of Tb(III) and Yb(III). The PL spectra of the
Gd, Er, Tm, and Y complexes had broad bands with the center peak
at 535 nm due to the EO5 and Pic ligands. For the Gd complex, the
metal-centered 4f–4f states are located at high energy about
313 nm, outside the visible spectral region due to the stability of
the half-filled 4f7 shell configuration in the Gd(III) ion.
of the EO5 ligand surrounding the Tb(III), or (iii) the equivalent dis-
tances between the Tb(III) ion and the donor oxygen atoms. In
CH3CN solution, the [Tb(NO3)2(EO5)](NO3) complex has the stron-
gest fluorescence followed by CH3OH, DMSO, H2O and THF. This or-
der suggests that the coordinating effects of the solvent in the
formation of complexes, i.e. solvent effect where vibrational
quenching of the complexes excited state may occur through a
high energy oscillator on the solvent molecule [16,19,20]. For the
[Tb(NO3)2(EO5)](NO3) complex, the broad band due to the
5D4 ? 7F3 transition also was observed in H2O, THF, and CH3OH
solution. That the Pic anion acts as a quencher in the [Tb(Pi-
c)2(EO5)](Pic) complex due to the nitro withdrawing groups was
clearly observed both in solution and the solid state. In addition,
the [Tb(Pic)2(EO5)](Pic) complex is not suitable for organic light
emitting diode devices because of its ionic character and low
volatility.
The T1(L) of the EO5 ligand is nearly equal with the lowest ex-
cited resonance level of Tb(III), therefore the intramolecular energy
transfer is quite efficient and the emission intensity is increased. In
the Tb complex, energy is primarily transferred from ligand triplet
to the 5D4 level from which luminescent transition to the ground
state manifold 7FJ (J = 0, 2, 3, 4, 5, 6) are observed in the green spec-
tral region (Table 4). The most intense transition for Tb(III) is the
5D4 ? 7F5 transition, corresponding to a green emission band at
542.9 nm. The 5D4 ? 7F5 transition is magnet dipole and is less af-
fected by environment than the 5D4 ? 7F6 transition. In the solid
state, the hypersensitive peak of the 5D4 ? 7F5 transition for the
Tb complex [542.6 nm; 4749 Â 102 a.u.] is higher than that found
in Tb(NO3)3Á6H2O [541.4 nm; 298.7 Â 102 a.u.] and Tb4O7
[542.1 nm; 41.1 Â 102 a.u.] [6].
4. Conclusion
The structural isomers [M(Pic)2(EO5)](Pic) complexes were
crystallized in monoclinic with space group P21/c. The central me-
tal ion had nine-coordinates with six oxygen atoms from the EO5
ligand and three oxygen atoms from the two Pic anions. The Tb
and Yb complexes have the typical 4f–4f emission transitions cor-
responding to the Tb(III) and Yb(III) ions. The Tb complex is not
suitable for organic light emitting diode devices because of its ionic
character and low volatility.
The Yb complex has one weak peak emission at 978.2 nm with
2
an emission intensity of ꢀ220 a.u. that is assigned to the F5/2
?
2F7/2 transition in the near infrared (NIR). The electron configura-
tion of 4f13 for the Yb(III) ion only allows only the ground state
of 2F7/2 level and the excited state of 7F5/2 level [17]. Due to the en-
ergy gap between the lowest triplet state energy level of the EO5
and Pic ligands, the lowest excited resonance level of the Yb(III)
ion is very large. Thus, the energy could not be transferred from
the T1(L) to the Yb(III) ion. The broad lines due to structural disor-
der and coordinated terminal alcohol groups were observed in the
Tb and Yb complexes [2].
Acknowledgements
We thank the Malaysian Government for funding research
grants FRGS and SAGA to support this research.
Appendix A. Supplementary material
For further study, we prepared the Tb complex with the EO5 li-
gand in the presence of the nitrate anion. The crystal structure of
the [Tb(NO3)2(EO5)](NO3) complex has been reported by Roger
et al. [18], but not its luminescence property. We studied lumines-
cence properties of the [Tb(NO3)2(EO5)](NO3) complex relative to
the [Tb(Pic)2(EO5)](Pic) complex in various solvents at an excita-
tion wavelength of 275 nm (Table 4). The crystal solid of the [Tb(Pi-
c)2(EO5)](Pic) complex in DMSO solution had three peaks at 488,
549, and 615 nm. However, the [Tb(Pic)2(EO5)](Pic) complex from
direct mixtures of the EO5 ligand, terbium nitrate salt and HPic in
each CH3CN, CH3OH, THF, and H2O solution had only two broad
weak peaks at 486 and 547 nm.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2009.05.045.
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Table 4
The photoluminescence data of the [Tb(Pic)2(EO5)](Pic) complex in the solid state.
Compound
k (nm)
Intensity  102 (a.u.)
Ligand region
414.7
493.1
542.9
584.3
620.4
645.5
687.1
1123
2873.6
4692
2335
1350
841
5D4
5D4
5D4
5D4
5D4
5D4
?
?
?
?
?
?
7F6
7F5
7F4
7F3
7F2
7F0
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468