Preparation and photoluminescence of a novel β-diketone ligand containing electro-transporting group and its europium(III) ternary complex

Article abstract of DOI:10.1016/j.saa.2006.01.030

A novel β-diketone with an electro-transporting oxadiazole group, 1-(4′-(5-(4-methylphenyl)-1,3,4-oxadiazol-2-yl)biphenyl-4-yl)-4,4,4-trifluorobutane-1,3-dione (MPBDTFA), was prepared with high yield. With this synthesized ligand as the first ligand and 1

Full text of DOI:10.1016/j.saa.2006.01.030

Spectrochimica Acta Part A 65 (2006) 907–911
Preparation and photoluminescence of a novel ␤-diketone
ligand containing electro-transporting group and
its europium(III) ternary complex
Neng-Jun Xianga,b, Louis M. Leungb, Shu-Kong Sob, Meng-Lian Gonga,∗
a State Key Laboratory of Optoelectronic Materials and Technologies, School of Chemistry and Chemical Engineering,
Sun Yat-sen University, Guangzhou 510275, China
b Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Kowloon Tong, Kowloon, Hong Kong, China
Received 27 September 2005; received in revised form 22 January 2006; accepted 23 January 2006
Abstract
A novel ␤-diketone with an electro-transporting oxadiazole group, 1-(4ꢀ-(5-(4-methylphenyl)-1,3,4-oxadiazol-2-yl)biphenyl-4-yl)-4,4,4-
trifluorobutane-1,3-dione (MPBDTFA), was prepared with high yield. With this synthesized ligand as the first ligand and 1,10-phenanthroline (Phen)
as the secondary ligand, a new europium(III) ternary complex, Eu(MPBDTFA)3Phen, was synthesized. The new ␤-diketone and its europium(III)
ternary complex were characteristized by elemental analysis, thermo-gravimetric analysis, IR and UV–visible spectroscopies. Photoluminescence
measurements indicated that the energy absorbed by the organic ligands was efficiently transfered to the central Eu3+ ions, and the complex showed
5
7
intensely and characteristically red emissions due to the D0 → Fj transitions of the central Eu3+ ions. With an electro-transporting group in
molecule and highly thermal stability, the synthesized Eu(III) ternary complex is expected as a red-emitting candidate material for fabrication of
organic light-emitting diodes (OLEDs).
© 2006 Elsevier B.V. All rights reserved.
Keywords: Optical material; Europium organic complex; Electro-transporting group; Luminescence
1. Introduction
one Eu(III) complex molecule [5–8]. The other way of improv-
ing EL performance is to develop multilayer EL devices by using
TPBD or Alq3 as electro-transporting layer. However, this is a
tedious process and is difficult to select the appropriate thick-
nesses of different layers to optimize the devices. Therefore,
highly efficient and pure red EL devices based on Eu(III) organic
complexes are facing a new challenge.
In this work, a novel ␤-diketone with an electro-transporting
group (oxadiazole moiety), 1-(4ꢀ-(5-(4-methylphenyl)-1,3,4-
oxadiazol-2-yl)biphenyl-4-yl)-4,4,4-trifluorobutane-1,3-dione
(MPBDTFA), was designed and synthesized. With this ␤-
diketone as the first ligand and 1,10-phenanthroline (Phen)
as the second ligand, a new europium(III) ternary complex,
Eu(MPBDTFA)3Phen, was synthesized. The functional moiety
oxadiazole and Eu3+ ion are expected to respectively retain
their own electron-transporting and emission properties. The
photoluminescence and thermal stability of the oxadiazole
functionalized europium(III) complex were investigated. The
photoluminescence of the complex Eu(MPBDTFA)3Phen was
compared with that of the complex Eu(TTA)3Phen, where
TTA− is thenoyltrifluoroacetonate.
Europium(III) complex-based electroluminescence (EL)
devices were pioneered by Kido et al. [1] in 1991 and recently
attracted great interest due to their sharp emission peak with
typical bandwidths of 5–10 nm at ∼610 nm. This made Eu(III)
complexes one of the promising candidates for full color flat-
panel displays which require pure red, green and blue emis-
sions. Since subtle changes in the molecular structures of the
Eu(III) complexes could dramatically tune the photolumines-
cence (PL) and electroluminescence (EL) properties, the devel-
opment of new Eu(III)–␤-diketonate mixed ligand complexes
for the light-emitting layers of the EL devices is an attract
research activity [2–4]. To fabricate an organic electrolumines-
cence device (OLED), the research efforts include designs and
synthesis of new ␤-diketones and neutral second ligands, incor-
porating electron-transporting and hole-transporting groups into
∗
Corresponding author. Tel.: +86 20 84112830; fax: +86 20 84112245.
E-mail address: cesgml@zsu.edu.cn (M.-L. Gong).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.01.030

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N.-J. Xiang et al. / Spectrochimica Acta Part A 65 (2006) 907–911
2. Experimental
zinc chloride were added to a three-necked 500 mL round bot-
tomed flask. The reaction solution was refluxed for 24 h with
vigorous stirring. After the mixture was cooled to room temper-
ature, the pH value was adjusted to 1.0 with concentrated HCl,
and the solution was stirred for 30 min to form a solid precipitate.
The new precipitate was then filtered, washed with 1 mol/L HCl,
and dried in a drying oven at 90 ◦C overnight to give 33.6 g of
5-p-tolyl-2H-tetrazole as a white powder, the crude product was
recrystallized in ethanol (82% yield). MS(FAB): 161 (M+ + 1).
2.1. Apparatus and measurements
All the starting materials are analytical reagent grade with-
out further purification. Solvents were freshly distilled and dried
by standard methods. Elemental analysis for the synthesized
ligand and complex were carried out with an Elementar vario
EL elemental analyzer. IR spectra in the region 4000–400 cm−1
were recorded on a Bruker infrared spectrophotometer using
conventional KBr method. FAB-MS spectra were performed on
2.2.2. Synthesis of 2-(4-biphenyl)-5-(4-methyl-phenyl)-
1,3,4-oxadiazole (MPBD, 3)
1
a VG ZAB-HS spectrometer. H NMR spectra were recorded
on a JEOL FT-NMR270 spectrometer. Electronic absorption
spectra were recorded on a Varlan UV–vis spectrophotome-
ter. Thermo-gravimetric analysis (TGA) was carried out up to
900 ◦C in N2 atmosphere on a Perkin-Elmer TGA6 thermo-
gravimetric analyzer. Excitation spectra, emission spectra, flu-
orescence lifetimes and quantum yield were measured with a
Photo Technology International Fluorescence Life time Scan-
ning spectrometer. The relative emission intensity was recorded
from the highest emission peak under the optimum excitation
wavelength.
Thirty milliliter thionyl chloride was added to 10 g (50 mmol)
4-biphenylcarboxylic acid and the mixture was refluxed for 24 h.
The solution was concentrated to 5 mL under reduced pressure
and then dry benzene (20 mL) was added. The benzene was
removed also under reduced pressure later. Fifty milliliter dry
pyridinewasaddedtotheresidueandthenstirreduntildissolved.
5-p-tolyl-2H-tetrazole (8.53 g, 52 mmol) in 50 mL dry pyridine
was added drop by drop into the mixture, and then refluxed for 2
days. The crude product was concentrated under reduced pres-
sure, then was recrystallized in ethanol, giving 14.2 g product
(90% yield). MS (FAB): 313 (M+ + 1).
2.2. Syntheses of MPBDTFA and its europium(III) ternary
complex Eu(MPBDTFA)3Phen
2.2.3. Synthesis of 2-[4-(4ꢀ-acetylbiphenyl)]-5-(4-methyl-
phenyl)-1,3,4-oxadiazole (4)
The synthetic routines of MPBDTFA and the europium(III)
ternary complex Eu(MPBDTFA)3Phen are shown in Fig. 1.
Acetyl chloride (9 mL, 131.6 mmol) and aluminum chloride
(13 g, 98 mmol) were mixed with 100 mL of dry methylene chlo-
ride in a 250 mL Schlenk flask. The solution was stirred at room
temperature for 10 min, then was droped into a 500 mL Schlenk
flask containing 10 g (32 mmol) MPBD and 100 mL methylene
chloride equipped with a magnetic stirring bar and a nitrogen
2.2.1. Synthesis of 5-p-tolyl-2H-tetrazole (1)
According to the method described in literature [9], 30 g 4-
methylbenzonitrile, 200 mL of water, 20 g sodium azide and 20 g
Fig. 1. Synthetic routines of the compounds.

N.-J. Xiang et al. / Spectrochimica Acta Part A 65 (2006) 907–911
909
outlet leading to an oil bubbler. The mixture was stirred at ice-
water bath for 1 h and refluxed 5 h, then was hydrolyzed by slow
addition of 50 mL of water containing 20 mL of concentrated
hydrochloric acid. Two layers, aqueous and organic, appeared,
then the aqueous layer was extracted with methylene chloride
and the combined organic layer was washed with water. The sol-
vent was removed under reduced pressure and the crude product
was recrystallized in ethanol, giving 9.8 g product (87% yield).
1H NMR(CDCl3): δ (ppm) 8.203–8.234 (d, 2H), 8.016–8.072
(t, 4H), 7.718–7.792 (t, 4H), 7.240–7.349 (d, 2H), 2.647 (s, 3H,
COCH3), 2.439 (s, 3H, Φ CH3). MS(FAB): 355 (M+ + 1).
for Eu(TTA)3Phen (EuC36H20F9N2O6S3) were: found (calcu-
lated)/%: C 43.65 (43.45), H 2.16 (2.04), and N 2.92 (2.81).
3. Results and discussion
3.1. IR absorption spectrum of Eu(MPBDTFA)3Phen
The main IR absorption bands of the europium(III) ternary
complex were assigned according to reference [10]. One thou-
sand five hundred and twenty-two per centimeter vibration band
is characteristic for MPBDTFA− ligand coordinated to Eu3+
central ion. This shows that the C O band in the free ligand
was weaken by C O Eu and C O Eu resonance structure in
the complex. New weak absorption bands at 518 and 436 cm−1
were ascribed to Eu N and Eu O vibrations, respectively. One
thousand five hundred and fifty per centimeter was ascribed
to the skeleton vibration of the phenanthroline ring. All these
evidences indicate that the MPBDTFA− and Phen ligands coor-
dinated to the central Eu3+ ion via the C O and N N groups.
2.2.4. Synthesis of 1-(4ꢀ-(5-(4-methylphenyl)-1,3,4-
oxadiazol-2-yl)biphenyl-4-yl)-4,4,4-trifluorobutane-
1,3,-dione (MPBDTFA, 5)
2-[4-(4ꢀ-acetylbiphenyl)]-5-(4-methyl-phenyl)-1,3,4-oxadia-
zole (5 g, 15 mmol), CF3COOC2H5 (7.95 g, 28 mmol), and
dry benzene (150 mL) was added into a 250 mL flask. 1.9 g
(16.8 mmol) potassium tert-butoxide was added to the flask and
the reaction mixture was heated at 45 ◦C for 4 h. After cooling,
the reaction solution was poured into a mixture of 150 mL
ice-water and 20 mL of concentrated hydrochloric acid. The
organic layer was washed twice with water, then the solvent
was removed under reduced pressure. The crude product was
recrystallized in toluene, giving 5.40 g product (85% yield).
The elemental analysis data for MPBDTFA (C25H17F3N2O3)
were: found (calculated)/%: C 66.53 (66.67), H 3.96 (3.80), and
N 6.17 (6.22). 1H NMR(CDCl3): δ (ppm) 8.240–8.270 (d, 2H),
8.038–8.088 (t, 4H), 7.786–7.817 (t, 4H), 7.343–7.373 (d, 2H),
6.633 (s, 1H), 2.461 (s, 3H, CH3). MS (FAB): 451 (M+ + 1).
3.2. Thermal stability of Eu(MPBDTFA)3Phen
Materials applied in fabrication of OLEDs are required to
have high thermal stability (high melting point and high decom-
position temperature). Thermo-gravimetric analysis (TGA)
showed that the decomposition temperature of the complex is
363.4 ◦C. Rare earth organic complexes with such high thermal
stability are seldom found, which is favourable to fabrication
of OLED by vacuum-coating with the Eu3+ complex as emitter
material.
2.2.5. Preparation of the complex, tri (1-(4ꢀ-(5-(4-
methylphenyl)-1,3,4-oxadiazol-2-yl)biphenyl-4-yl)-4,4,4-
trifluorobutane-1,3,-dione)(1,10-phenanthroline)-
3.3. UV absorption spectra
The electronic absorption spectra for 1.0 × 10−5 mol/L
MPBDTFA, Phen and Eu(MPBDTFA)3Phen in dichloro-
methane solution were shown in Fig. 2. The absorption band for
Eu(MPBDTFA)3Phencomplexshiftedtolongerwavelengthand
europium(III) (Eu(MPBDTFA)3Phen, 6)
1.6 g (3.6 mmol) MPBDTFA dissolved in 50 mL
toluene/ethanol (1:1) was added into an aqueous solution
containing 1.2 mmol EuCl3, which was obtained by reaction of
Eu2O3 (99.5%, Zhu-jiang smeltery Co.) and HCl (6.0 mol/L).
The pH value of the mixture was adjusted to 6.0–7.0 by
adding an aqueous solution of sodium hydroxide and then
0.22 g 1,10-phenanthroline (Phen, 1.2 mmol) was added into
the reaction mixture. The reaction mixture was stirred 24 h at
60 ◦C. Twelve hour later, a white precipitate deposited from
the solution, and was filtered out and washed with deionized
water and toluene/ethanol (1:1). Eu(MPBDTFA)3Phen solid
complex was obtained and dried in vacuum at 80 ◦C for 24 h,
giving 1.58 g product (85% yield). The elemental analysis data
for Eu(MPBDTFA)3Phen (EuC87H56F9N8O9) were: found
(calculated)/%: C 62.35 (62.23), H 3.56 (3.36), and N 6.52
(6.67).
2.3. Preparation of the complex, tri(thenoyltrifluoro-
acetonate) (1,10-phenanthroline) europium, Eu(TTA)3Phen
Fig. 2. Absorption spectra of the ligands and the Eu3+ complex in
dichloromethane (1 × 10−5 mol/L) (a) Eu(MPBDTFA)3Phen; (b) MPBDTFA;
(c) Phen.
Eu(TTA)3Phen was synthesized according to the same
method described in Section 2.2.5. The elemental analysis data

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N.-J. Xiang et al. / Spectrochimica Acta Part A 65 (2006) 907–911
was broader than that for MPBDTFA and Phen ligands due to
theextensionoftheconjugatedsystemaftercoordination. Strong
and broad absorption bands appear at about 265–380 nm for the
complex. Comparison of the three absorption curves showed that
both the ligands contribute to the absorption of the complex, and
the absorbed energy of the Eu(III) ternary complex mainly orig-
inates from the first ligand, MPBDTFA−, and the absorption
from the secondary ligand Phen only plays a secondary role.
However, existence of the secondary ligand satisfies the high
coordination number of 8 of the central Eu3+ ion and improves
the coordination stability of the complex.
3.4. The lowest triplet state energy level of the ligand
MPBDTFA
Fig. 3. Excitation (Ex) and emission (Em) spectra. (a) Ex of the
complex Eu(MPBDTFA)3Phen (λem = 612 nm); (b) Em of the complex
Eu(MPBDTFA)3Phen (λex = 389 nm); (c) Em of the MPBDTFA (λex = 389 nm);
(d) Ex of the complex Eu(TTA)3Phen (λem = 610 nm); (e) Em of the complex
Eu(TTA)3Phen (λex = 365 nm).
Phosphorescence spectrum for 1 × 10−5 mol/L ethanol:
CHCl3 (1:1) solution of the corresponding Gd(III) binary com-
plex with the ligand MPBDTFA was measured at 77 K. The
excitation wavelength used to obtain the phosphorescence spec-
trum was set at the maximum UV-absorption wavelength of
the Gd(III) complex (λex = 346 nm). The lowest excited state
energy level of Gd3+ ion (6P7/2, ∼32000 cm−1) is much higher
than the T1 (L) of MPBDTFA ligand, and the energy can-
not be transferred to the central gadolinium(III) ion from the
ligand. So phosphorescence emission from MPBDTFA ligand
was observed for Gd(III)–MPBDTFA binary complex while
no Gd3+ ion luminescence was found. The lowest triplet state
energy of the ligand MPBDTFA was determined by the short-
est wavelength transition in the phosphorescence spectrum to be
24277 cm−1 (411.9 nm), which is higher by 6977 cm−1 than the
lowest excited state of Eu3+, 5D0 (17300 cm−1), and is matched
with the latter, so in the corresponding europium(III) complex
the ligand strongly sensitizes the luminescence of Eu3+ [11] (see
Section 3.5).
located at 265–300 nm mainly due to phenanthroline (the sec-
ondary ligand) absorption, and another centered at 365 nm is
very intense and is contributed by the first ligand TTA−. Under
365 nm light excitation, Eu(TTA)3Phen powder showed multi-
7
ple emission bands (Fig. 3e) due to Eu3+ 5D0 → Fj transitions
with the strongest peak at 610 nm (5D0 → F2). Comparison
7
of the emission spectra of the Eu(MPBDTFA)3Phen (Fig. 3b)
and the Eu(TTA)3Phen (Fig. 3e) powder showed that the flu-
orescence intensity for Eu(TTA)3Phen is a little stronger than
that of Eu(MPBDTFA)3Phen under the optimum near-UV light
excitation. However, existence of an electro-transporting oxadi-
azole group in Eu(MPBDTFA)3Phen molecule is favourable to
fabrication of OLED.
The photoluminescence mechanism for an Eu3+ organic com-
plex can be explained as follows: (1) energy of exciting UV light
is absorbed by the ligands; (2) the excited ligands decay rapidly
to their lowest triplet state; (3) the energy is transferred from
the triplet state of the ligands to the quasi-resonant energy state
of the Eu3+ ion (the Dexter process); (4) the excited europium
ion decays to the ground state via photo emissions in the visible
region. The step (3) is crucial in determining the 4f–4f emis-
sion quantum yield. The match relationship between the lowest
triplet state energy level of the ligand MPBDTFA− and the low-
est excited state level of Eu3+, 5D0 (Section 3.4) is favourable to
the energy transfer from the ligand to Eu3+. The overall lumines-
cence quantum yield (ϕlum) is in principle determined not only
by the luminescence efficiency of europium ion itself (ϕEu), but
3.5. Fluorescence spectra and quantum yield
The excited spectrum (Fig. 3a) monitored at 612 nm for the
Eu(MPBDTFA)3Phenpowdershowedtwoexcitationbands:one
is located at 260–320 nm mainly due to phenanthroline (the sec-
ondary ligand) absorption, and another centered at 389 nm is
very intense and is contributed by the first ligand MPBDTFA
since its molecule has of an expanded ␲-conjugated system
(see Fig. 1). Under 389 nm UV light excitation, the emission
spectrum for ligand MPBDTFA (Fig. 3c) showed one blue
emission band centered at 450 nm while the emission spec-
trum for the Eu3+ complex (Fig. 3b) showed multiple emission
7
bands due to Eu3+ 5D0 → Fj transitions with the strongest peak
also by the quantum yields of the intersystem crossing (ϕISC
)
7
at 612 nm (5D0 → F2). The sharp peak suggests high color
and the energy transfer steps (ϕET). Thus, get an equation [12],
namely
purity for Eu(MPBDTFA)3Phen complex. Comparison of the
emission of the MPBDTFA ligand (Fig. 3c) with that of the
Eu(MPBDTFA)3Phen complex (Fig. 3b) indicated that the lig-
and emission completely disappeared in the complex, and the
energy transfer from the MPBDTFA ligand to the central Eu3+
ion was very efficient.
ϕlum = ϕISCϕETϕEu
The luminescence quantum yield for the Eu3+ ternary com-
plexes Eu(MPBDTFA)3Phen and Eu(TTA)3Phen were deter-
minedtobeϕ = 0.14and0.17withareferencesolutionofquinine
sulfate in 0.5 mol/L sulfuric acid (ϕ = 0.546) and were corrected
by the refractive index of the solvent at 298 K. The luminescence
The excited spectrum (Fig. 3d) monitored at 610 nm for the
Eu(TTA)3Phen powder showed two excitation bands: one is

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911
cated that the energy absorbed by the organic ligands was
efficiently transfered to the central Eu3+ ions, and the com-
plex showed intensely and characteristically red emission due
5
to the D0–7Fj transitions of the central Eu3+ ions. With an
electro-transporting group in molecule and highly thermal sta-
bility, the synthesized Eu(III) ternary complex is expected as a
red-emitting candidate material for fabrication of organic light-
emitting diodes (OLEDs).
Acknowledgements
The authors would like to acknowledge the support from the
Scientific and Technical Projects of Guangdong Province (No.
B10502) and the Research Grant Council of Hong Kong (grant
No. HKBU/22037/02P).
Fig. 4. The decay curve of Eu3+ 5D0 excited state for Eu(MPBDTFA)3Phen.
References
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3.6. Excited state lifetime of the Eu3+ complex
5
The lifetime measurements for the excited state D0 of the
Eu(III) ion were recorded for the complex Eu(TPBDTFA)3Phen
(Fig. 4). The decay curves for these complexes can be fit with a
single exponential [13], consistent also with only one site sym-
metry for the Eu(III) ion. Fig. 4 shows that the luminescence
5
lifetime of the emitting D0 level in the complex. Since the
luminescence lifetime of this Eu(III) complex is relatively long
(341 ␮s), the probability of quenching by charge carriers is rel-
atively slow, this is advantageous to luminescence.
4. Conclusions
A novel europium(III) ternary complex, Eu(MPBDTFA)3-
Phen, was synthesized. Photoluminescence investigation indi-