Preparation, photo- and electro-luminescent properties of a novel complex of Tb (III) with a tripod ligand

Article abstract of DOI:10.1016/j.saa.2007.05.017

A novel Tb(III) complex TbL (L = tris[2-(2-carboxyphenoxy)ethyl]amine, H₃L) was synthesized and characterized by means of elemental analyses, IR spectra, thermal analyses, and molar conductivity measurement. The photoluminescent properties of the complex were investigated. In addition, PVK doping Tb(III) complex was fabricated as the emissive layer by spin-coating and its electroluminescent properties were studied, in which the structure of the device was ITO (indium tin oxide)/PVK (polyvinylcarbazole)/PVK: TbL/PBD (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole)/LiF/Al. It was indicated that pure green and narrow bandwidth emission at 545 nm from photoluminescence of TbL complex film and the organic electroluminescent device is the characteristic emission of Tb(III) ion, and the electroluminescence spectrum of the device was very similar to that of the photoluminescence of TbL complex film. The lowest triplet level of the ligand was calculated from the phosphorescence spectrum of GdL in N,N-dimethyl formamide (DMF) dilute solution determined at 77 K, and the energy transfer mechanisms in TbL complex were discussed.

Full text of DOI:10.1016/j.saa.2007.05.017

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 654–658
Preparation, photo- and electro-luminescent properties
of a novel complex of Tb (III) with a tripod ligand
Tianzhi Yua,∗, Yuling Zhaob, Duowang Fana, Ziruo Hongc, Wenming Suc
a Key Laboratory of Opto-Electronic Technology and Intelligent Control (Lanzhou Jiaotong University), Ministry of Education, Lanzhou 730070, China
b School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou 730070, China
c Key Laboratory of Excited State Processes, Changchun Institute of Optics, Fine Mechanics and Physics,
Chinese Academy of Sciences, Changchun 130033, China
Received 2 March 2007; received in revised form 20 April 2007; accepted 7 May 2007
Abstract
A novel Tb(III) complex TbL (L = tris[2-(2-carboxyphenoxy)ethyl]amine, H3L) was synthesized and characterized by means of elemental
analyses, IR spectra, thermal analyses, and molar conductivity measurement. The photoluminescent properties of the complex were investigated.
In addition, PVK doping Tb(III) complex was fabricated as the emissive layer by spin-coating and its electroluminescent properties were studied, in
which the structure of the device was ITO (indium tin oxide)/PVK (polyvinylcarbazole)/PVK: TbL/PBD (2-(4-biphenylyl)-5-(4-tert-butylphenyl)-
1,3,4-oxadiazole)/LiF/Al. It was indicated that pure green and narrow bandwidth emission at 545 nm from photoluminescence of TbL complex
film and the organic electroluminescent device is the characteristic emission of Tb(III) ion, and the electroluminescence spectrum of the device was
very similar to that of the photoluminescence of TbL complex film. The lowest triplet level of the ligand was calculated from the phosphorescence
spectrum of GdL in N,N-dimethyl formamide (DMF) dilute solution determined at 77 K, and the energy transfer mechanisms in TbL complex were
discussed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Tripod ligand; Photoluminescence; Electroluminescence; Tb(III) complex
1. Introduction
that most triplet state energies of aromatic carboxylic acids are
more suitable for the resonant energy level of Tb3+ and there
may exist an efficient ligand-to-metal energy transfer process
(antenna effect) [25,26]. These complexes have not only good
luminescence properties, but also possess better photo- and ther-
mal stability compared with that ␤-diketonates.
In this paper, we report the details of synthesis, characteriza-
tion, and photo- and electro-luminescent properties of a novel
Tb(III) complex with tripodal ligand containing salicylic acid,
tris[2-(2-carboxyphenoxy)ethyl]amine (H3L).
The photophysical properties of lanthanide complexes with
organic ligands have been extensively studied for their poten-
tial applications as luminescent materials and as a probe of
chemical and biological molecules [1–3]. It is well-known that
the chelating agents commonly used for the sensitization of
lanthanide ions luminescence include ␤-diketones, aromatic car-
boxylic acids, cryptands, and several other macrocyclic ligands
[4–11]. In recent 20 years, lanthanide chelates with organic
ligands have been successfully developed as the organic elec-
troluminescent (EL) materials due to their sharp emission bands
with colors ranging from blue to red in thin film electrolumines-
cent devices [12–20]. However, the focus of the work has mainly
been upon ternary lanthanide complexes with ␤-diketones, lit-
tle attention has been paid to the lanthanide complexes with
aromatic carboxylic acids [21–24]. Research results indicated
2. Experimental details
2.1. Material and methods
Tris(2-hydroxyethyl)amine was purchased from Alfa Aesar.
Salicylic acid was analytical grade reagents from Sichuan
Chengdu Chemical Reagent Factory (PR China). Thionyl chlo-
ride was freshly distilled prior to use. Tb4O7 was converted to
its nitrate by treating with concentrated nitric acid. The other
solvents were analytical grade reagents. Tris(2-chloroethyl)
∗
Corresponding author. Tel.: +86 931 4956935; fax: +86 931 4938756.
E-mail address: ytz823@hotmail.com (T. Yu).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.05.017

T. Yu et al. / Spectrochimica Acta Part A 69 (2008) 654–658
655
lute methanol, and the pH of the reaction mixture was adjusted
to about 6.5 using triethylamine. Then the reaction mixture was
stirred for 24 h at room temperature, resulting in a white precip-
itate. The precipitation was filtered off, washed with ethanol
and water. The product was dried under reduced pressure,
and purified with recrystallization from methanol. Yield: 75%.
m.p. > 250 ◦C. Anal. Calc. for C27H26NO10Tb [TbL·H2O] (%):
C, 47.44; H, 3.81; N, 2.05; Tb, 23.28. Found: C, 47.33; H, 4.01;
N, 2.21; Tb, 23.48. FT-IR (KBr) (cm−1): 3400 (νO H, H2O);
3040 (νC H); 1690 (νC O); 1603−, 1584, 1491 (benzene ring);
Scheme 1. Synthetic route to tris[2-(2-carboxyphenoxy)ethyl]amine hydrochlo-
ride.
amine hydrochloride was prepared according to literature meth-
ods [27].
IR spectra (400–4000 cm−1) were measured on a Shimadzu
IRPrestige-21 FT-IR spectrophotometer. C, H, and N analyses
were obtained using an Elemental Vario-EL automatic elemental
analysis instrument. The metal ion concentration of the com-
plex was determined by EDTA titration method, and xylenol
orange used as indicator. The molar conductivity of the com-
plex in DMSO solution was measured by using a DDST-308
conductivity instrument made in China. The fluorescent spectra
were recorded on a HITACHI F-4500 spectrometer. The thermo-
gravimetry (TG) and differential thermal analysis (DTA) were
recorded on a Shimadzu DT-40 thermal analysis instrument.
Melting points were measured by using a X-4 microscopic melt-
ing point apparatus made in Beijing Taike instrument limited
company, and the thermometer was uncorrected.
−
1541 (νas (COO )); 1452 (νs (COO )); 1238, 1049 (νAr O C).
2.4. Preparation of EL devices with the Tb(III) complex
The EL devices with the structure of ITO (indium tin
oxide)/PVK (polyvinylcarbazole)/PVK: TbL/PBD (2-(4-
biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole)/LiF/Al
were fabricated to study the electroluminescence of the TbL
complex. The emissive layer was formed by spin-casting PVK
doped with TbL complex from methanol/chloroform solution.
The host transfer layer PBD, LiF and Al were sequentially
deposited at a rate of 0.1–0.4 nm/s by thermal evaporation in
a vacuum chamber (2 × 10−4 Pa). The emitting area of the EL
devices was 3 mm × 4 mm. A quartz crystal oscillator placed
near the substrate was used to measure the thickness of the
thin film. The PL spectra of the Tb(III) complex film on a
quartz substrates were studied. The EL spectra were measured
with a Hitachi MPF-4 Fluorescence spectrophotometer. The
luminance–current–voltage characteristics were measured
simultaneously by the EL test system made in Beijing Normal
University, PR China. All the performance measurements were
carried out under ambient atmosphere without encapsulation.
2.2. Synthesis of tris[2-(2-carboxyphenoxy)ethyl]amine
hydrochloride (H3L·HCl)
Tris[2-(2-carboxyphenoxy)ethyl]amine
hydrochloride
(H3L·HCl) was synthesized by the synthetic route shown in
Scheme 1.
Salicylic acid (13.8 g, 0.10 mol) and potassium hydroxide
(12.9 g, 0.23 mol) were dissolved in 200 mL of dimethyl-
formamide. After stirring for 1 h at room temperature,
tris(2-chloroethyl)amine hydrochloride (7.2 g, 0.03 mol) was
added and the reaction mixture was then heated to reflux for
24 h. The solvent was removed from the reaction mixture in
vacuo and the residue was stirred with 50 mL of 6N HCl for
1 h at room temperature, then a sticky mass was formed from
the solution. The supernatant was decanted and the residue
was partitioned between chloroform and water. The chloroform
phase was retained, and the aqueous phase was re-extracted two
times. The combined chloroform extracts were dried over anhy-
drous sodium sulfate and concentrated in vacuo. Crystallization
occurred upon standing to afford 7.9 g (48.5%) of white needles.
m.p. 191–192 ◦C. Anal. Found C, 59.50(59.40); N, 2.71(2.57);
H, 5.51(5.13); C27H28NO9Cl requires C, 59.40; N, 2.57; H,
5.13. FT-IR (KBr) (cm−1): 3424 (ν COOH); 2959 (νC H, benzene
ring); 1699 (νC O); 1603, 1581, 1491 (benzene ring); 1286, 748
3. Results and discussion
3.1. IR spectra
Comparing the IR spectra of the free ligand with the com-
plex, the characteristic absorption bands of C O and C O bands
of the free ligand disappear, while the characteristic absorption
peaks of carboxylic group COO− appear, which indicate that the
oxygen atoms of carboxyl groups of the ligand are coordinated
to Tb3+. The group C O stretching vibration of the free ligand
is shifted from 1699 to 1690 cm−1 upon forming the complex.
Besides, it can be observed that the absorption frequencies of
Ar O C bond of the ligand are shifted from 1240 and 1053 to
1238 and 1049 cm−1, respectively. It is shown that the oxygen
atoms of C O and Ar O C groups in ligand are coordinated
with Tb3+. In addition, from the IR spectrum of the complex
we can observe a new absorption band at 415 cm−1 with weak
intensity, which is absent in the spectrum of the free ligand, so it
is concluded that the band presumably was ascribed to vibration
of the Tb O bond [28]. The strong asymmetric and symmetric
stretching absorptions of carboxylic group COO− in the com-
plex appear at 1541 (νas) and 1452 (νs) cm−1, respectively. The
ꢀν (ꢀν = νas − νs) of the complex is 89 cm−1 (<116 cm−1, the
ꢀν of Na3L), and the result indicates that the carboxylic groups
1
(δ COO H); 1240, 1053 (νAr O C). H NMR (CDCl3, 80 MHz)
δH (ppm): 10.3 (3H, S, COOH); 6.9–7.7 (12H, m, –C6H4); 4.2
(6H, t, NCH2–); 3.3 (6H, t, –CH2–O–Ar). FAB-MS m/z (%):
510 (M–Cl)+.
2.3. Synthesis of Tb(III) complex
The ligand (H3L·HCl) (0.2728 g, 0.5 mmol) and terbium
nitrate (0.2718 g, 0.6 mmol) were dissolved in 25 mL of abso-

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T. Yu et al. / Spectrochimica Acta Part A 69 (2008) 654–658
Fig. 2. Suggested structure of the Tb(III) complex.
DTA curve, which were accompanied by a weight loss in the
TG curve, it shows that the ligand undergoes decomposition.
From Fig. 1(b), at 73 ◦C an endothermic peak can be observed
in the DTA curve, and the weight loss is about 2.5% of the
complex in the TG curve, which corresponds to the loss of one
molecule of water of the composition TbL·H2O, and the result
suggests that the water is lattice water. With rising temperature,
three exothermic peaks at 171, 352 and 447 ◦C were observed in
the DTA curve, and correspond to sharp weight losses in the TG
curve, it shows that the complex undergoes decomposition. The
final residue was found to be Tb4O7. The total loss of weight
agrees with the corresponding calculated data.
The results corroborate some of the assumptions made on
the basis of infrared spectral and elemental analyses studies. In
addition, the molar conductivity of the complex in DMSO solu-
tion was 6.1 s cm2 mol−1, which indicates that the complex is a
non-electrolyte [29]. So the structure proposed for the complex
is shown in Fig. 2.
3.3. Photo- and electro-luminescent properties of the
Tb(III) complex
Fig. 3 shows the excitation and emission spectra of the Tb(III)
complex in thin film. The sample for photoluminescent spec-
troscopy was made by spin-coating the methanol solution onto
clean quartz substrates. From Fig. 3, it is shown that an intense
excitation peak was observed at 281 nm. Photoexcitation of the
thin film of the complex at 281 nm resulted in photolumines-
cence (PL) emission characteristic of a terbium ion: λ = 489 nm
Fig. 1. TG–DTA of the ligand (a) and the TbL complex (b).
7
7
7
(5D4 → F6), 545 nm (5D4 → F5), 585 nm (5D4 → F4), and
of the ligand coordinate to Tb3+ through bidentate models in
the complex. It is noted that there is a broad band at 3400 cm−1
in IR spectrum of the complex, which is assigned to stretching
vibration of the water molecule.
3.2. Thermal property
The ligand and the complex were characterized by the ther-
mogravimetry and differential thermal analysis (TG–DTA), the
results of TG and DTA measurements of the ligand and the com-
plex are shown in Fig. 1(a and b). It can be seen from Fig. 1(a)
that the DTA curve of the ligand shows an endothermic peak at
192 ◦C, but without mass loss in the TG curve. It is the melt-
ing point of the ligand, which is in agreement with the result
obtained by capillary tube method. At 207 ◦C an endothermic
peak was observed in the DTA curve, and corresponds to a sharp
weight loss in the TG curve. It should be the sublimation temper-
ature, shown that the ligand undergoes sublimation. By further
heating two exothermic peaks at 520 and 593 ◦C appear in the
Fig. 3. Excitation (– – –) and emission (—) spectra of the Tb(III) complex in
thin film.

T. Yu et al. / Spectrochimica Acta Part A 69 (2008) 654–658
657
Fig. 4. Phosphorescence spectrum of the GdL complex at 77 K.
Fig. 5. EL spectra of OLED device with ITO/PVK (30 nm)/PVK: TbL (25 wt%,
45 nm)/PBD (30 nm)/LiF (1 nm)/Al (100 nm) at different applied voltages.
620 nm (5D4 → F3). The most efficient emission originates
7
7
7
transitions: 490 nm (5D4 → F6), 545 nm (5D4 → F5), 586 nm
from the electron transition from 5D4 state to 7F5 level at 545 nm
with a half peak width of less than 10 nm, so the complex exhibits
intensive green emission in the thin film.
(5D4 → F4), 620 nm(5D4 → F3). Itdemonstratesthattheelec-
troluminescence is also from trivalent Tb3+ ion of the TbL in the
blend emissive layer, no electroluminescence is observed from
PVK and PBD. As shown in Fig. 5, the intensity of EL emission
increases gradually with increasing the driving voltages, but the
emission spectral features do not change.
The EL process based on the multilayer structure device is
considered as follows: under forward bias, holes injected from
ITO through PVK and electrons injected from Al through PBD
migrate and recombinate to form excitons in the PVK doped
with TbL film. Then the energy of exciton caused by raditive
decay is absorbed by the ligand and finally transferred to Tb3+
ion of the complex by intersystem crossing from the triplet state
of the ligand.
The current–voltage and luminance–voltage relationships of
the device are depicted in Fig. 6. The device shows a turn-on
voltage of about 10 V. When the forward bias voltage is lower
than 10 V, the current increase slowly with increasing the for-
ward bias voltage. When the forward bias voltage is larger than
10 V, the current increases intensively. It is shown that the device
has the rectification characteristic. The luminance increases with
7
7
Molecular phosphorescence can reflect the excited state char-
acteristic of organic molecules and different phosphorescent
emission bands correspond to the different ligands, so the
excited triplet state energies of the ligand can be determined
by their phosphorescence spectra. In general, Gd(III) complex
was selected as model complex for the determination of the
triplet state energies of the organic ligands due to their high
phosphorescence–fluorescence ratio compared with those of the
other Ln(III) complex and Gd(III) ion can sensitize the phospho-
rescence emission of ligands. Fig. 4 shows the phosphorescence
spectrum of the GdL complex at nitrogen atmosphere (77 K),
and two apparent phosphorescence bands appear at 448 and
510 nm, respectively. The triplet state energy of the ligand based
on the first phosphorescence band (448 nm) was found to be
22,321 cm−1, which is 1821 cm−1 above the resonant energy
5
level of D4 of Tb(III) (20,500 cm−1). To efficiently sensitize
Tb(III) fluorescence, the triplet state of the ligand has to be
2400 300 cm−1 above the 5D4 of Tb(III) [30]. The triplet
energy level of the ligand is slightly lower than the ideal energy
level, so it can be predicated that the intramolecular energy
transfer from the ligand to Tb(III) occurred in the complex.
The TbL complex cannot be sublimed, so it cannot be used
as an emitter in OLEDs by vacuum deposition. To examine
the EL properties of the TbL complex in OLED devices, we
chose PVK as the host material, and doped the complex into
PVK matrix (25 wt%), and then the mixture was deposited to
the substrate by spin cast. The electroluminescent device con-
sisted of a layer of spin-casting PVK (30 nm), a layer of the
TbL complex doped PVK film (25 wt%, ∼45 nm) and a layer
of vacuum deposited PBD (30 nm) sandwiched between the
ITO anode and the LiF (1 nm)/Al (100 nm) cathode, in which
PBD was used as the electron-transporting material. The EL
emission was observed at different driving voltages (Fig. 5).
The EL spectra had sharp peaks with the maxima at 490, 546,
586 and 620 nm, which correspond closely to the Tb3+ ion
Fig. 6. Current–voltage and luminance–voltage characteristics of the device.

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T. Yu et al. / Spectrochimica Acta Part A 69 (2008) 654–658
increasing the forward bias voltage. The turn-on voltage of the
device is observed at 8 V, the maximum brightness is about
135 cd m−2 at an operating voltage of 19 V.
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A novel terbium carboxylate complex with a tripodal lig-
and was synthesized and characterized by means of elemental
analyses, IR spectra, thermal analyses, and molar conductivity
measurement. Preliminary study on electroluminescent property
of this complex shows that it is a green emitter for light-emitting
device. Further optimization of the EL devices is being carried
out.
Acknowledgements
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This work was supported by ‘Qing Lan’ talent engineering
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was also supported by Program for Changjiang Scholars and
Innovative Research Team in University (IRT0629).
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