Synthesis and photoluminescence properties of novel europium complexes of 2′-hydroxyacetophenone and 4,6-diacetylresorcinol

Article abstract of DOI:10.1016/j.saa.2007.05.019

Novel europium (III) complexes of the formulae Eu(OHAP)₃·2H₂O, Eu(OHAP)₃Phen, Eu₂(DAR)₃·4H₂O and Eu₂(DAR)₃Phen₂ (HOHAP = 2′-hydroxyacetophenone, H₂DAR = 4,6-diacetylresorcinol, Phen = 1,10-phenanthroline) have been designed and synthesized in this paper. These complexes were characterized by elemental analysis, FT-IR, and UV-vis. Based on these observations, the ligands are coordinated to Eu(III) via the acetyl and phenolic oxygens, and H₂DAR is concluded to be bis-bidentate donor. Photoluminescence studies showed that the several complexes emitted red luminescence. Thermo-gravimetric analysis showed that the complexes possess good thermal stability. Also, it was found that Phen as a synergic ligand, coordinated to Eu(III) in a composite system like 2′-hydroxyacetophenone and 4,6-diacetylresorcinol, could enhance the complexes luminescence intensity, quantum yield and lifetime.

Full text of DOI:10.1016/j.saa.2007.05.019

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 69 (2008) 664–669
Synthesis and photoluminescence properties of novel europium
ꢀ
complexes of 2 -hydroxyacetophenone and 4,6-diacetylresorcinol
∗
Liu Sheng-Li , Wen Chun-Lin, Qi Su-Su, Liang En-Xiang
School of Chemistry and Chemical Engineering, Hunan University of Science and Technology, Xiangtan 411201, China
Received 17 January 2007; received in revised form 7 May 2007; accepted 8 May 2007
Abstract
Novel europium (III) complexes of the formulae Eu(OHAP)
ꢀ
2 2 3 2
O and Eu (DAR) Phen (HOHAP = 2 -
3
·2H
2
O, Eu(OHAP)
3
Phen, Eu
2
(DAR)
3
·4H
hydroxyacetophenone, H DAR = 4,6-diacetylresorcinol, Phen = 1,10-phenanthroline) have been designed and synthesized in this paper. These
complexes were characterized by elemental analysis, FT-IR, and UV–vis. Based on these observations, the ligands are coordinated to Eu(III) via
the acetyl and phenolic oxygens, and H DAR is concluded to be bis-bidentate donor. Photoluminescence studies showed that the several complexes
2
2
emitted red luminescence. Thermo-gravimetric analysis showed that the complexes possess good thermal stability. Also, it was found that Phen
ꢀ
as a synergic ligand, coordinated to Eu(III) in a composite system like 2 -hydroxyacetophenone and 4,6-diacetylresorcinol, could enhance the
complexes luminescence intensity, quantum yield and lifetime.
©
2007 Elsevier B.V. All rights reserved.
ꢀ
Keywords: Europium (III) complexes; Luminescence; Quantum yield; 2 -Hydroxyacetophenone; 4,6-Diacetylresorcinol; 1,10-Phenanthroline
1
. Introduction
and specially the Phen can enhance the luminescence intensity,
quantum yield and lifetime of the Eu(III) complexes.
Lanthanide complexes have been studied by many resear-
chers as potential emitting materials and typically have been
found to possess narrow emission bands and high efficiency
2. Experimental
[
1–4]. Since the emissions of lanthanide complexes originate
2
.1. Materials and physical techniques
from transitions between f levels, which are effectively field
from the influence of external forces by filling 5s and 5p orbi-
tal, the various states arising from f configurations are splited
by external fields only to the extent of 100 nm . In particu-
lar, Eu(III) has five narrow emission bands corresponding to
2
6
ꢀ
The 2 -hydroxyacetophenone (98%) and Eu2O3 (99.99%)
n
were purchased from Aldrich and used as received, while the
other reagents used were of analar grade. H2DAR was prepared
following the method reported in Ref. [7], and was characte-
rized by C, H microanalysis, melting point, IR and UV–vis
spectroscopic analysis.
Elemental analyses were measured using an Elementar vario
EL III elemental analyzer, and europium ion contents were
determined by EDTA complexometry. IR spectra were recorded
over the range 4000–400 cm using Perkin-Elmer Spectrum
One spectrophotometer by the KBr pellet technique. UV was
performed with a Perkin-Elmer UV/Vis Lambda 35 spectrome-
ter. Fluorescence excitation and emission spectra were recorded
using an F-4500 FL spectrophotometer.
−
1
5
7
the D0 → FJ transitions, where J = 0, 1, 2, 3, 4. The strongest
5
7
transition, D0 → F2, usually occurs at approximately 613 nm
with a narrow emission band which produces a red color in
ꢀ
Eu(III) complexes [5,6]. In this manuscript, we find that 2 -
hydroxyacetophenone and some of its derivatives have the same
character.
−
1
Also, novel Eu(III) complexes of the formulae, Eu(OHAP)3·
2
H2O, Eu(OHAP)3Phen, Eu2(DAR)3·4H2O and Eu2(DAR)3
Phen2 have been designed and synthesized. The emission data
shows narrow and strong bands from the D0 → F2 transition,
5
7
Fluorescence lifetime was measured using an EMG 201 MSC
5
7
∗
quasi -molecular 308 nm laser by monitoring D → F emis-
Corresponding author.
E-mail address: slliu@hnust.edu.cn (S.-L. Liu).
0
2
sion line of Eu(III) at room temperature.
1
386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.05.019

S.-L. Liu et al. / Spectrochimica Acta Part A 69 (2008) 664–669
665
The emission quantum yield q for the Eu(OHAP)3·2H2O,
Eu(OHAP)3·Phen, Eu2(DAR)3·4H2O and Eu2(DAR)3·2Phen
were obtained following the procedure described in Ref. [8].
The q value is defined as the ratio between the number of pho-
tons emitted by the Eu(III) and the number of photons absorbed
by the ligands. According to the method developed by Bril [9] at
Philips Research Laboratories, the q value for a given sample can
be determined by comparison with standard phosphors, whose
quantum yields have been previously determined by absolute
measurements. This method provides absolute yields while avoi-
ding absolute measurements, which are in general complicated.
The quantum yield qx of a sample is thus determined as follows:
Eu2(DAR)3Phen2, Gd2(DAR)3·4H2O, Gd(OHAP)32H2O)
were prepared in the same way.
3. Results and discussions
All the complexes are yellow, and slightly soluble in DMF
and DMSO. The elemental analytical data (Table 1) agree well
with their formula as expected. Thermal data of the complexes
showthatthereisnoendothermicpeakbeforedecompose, which
indicatesthatthecomplexeshavenomeltingpointduetodecom-
posed when the temperature was increased to some degree at
atmospheric pressure, all these complexes start decomposing
◦
around 300 C as indicated by the exotherms.
ꢀ
ꢁ ꢀ
ꢁ
1
− rST
− rx
ꢀφx
qx =
qST
1
ꢀφST
3.1. FT-IR analysis
where rST and rx are the amount of exciting radiation reflec-
ted by the standard and by the sample, respectively, and qST
is the quantum yield of the standard phosphor. The terms ꢀΦx
In Eu(OHAP) ·2H O and Eu (DAR) ·4H O, the broad band
3
2
2
3
2
−
1
in the region 3500–3100 cm is due to the coordination with
water. The rocking mode of the coordinated water was obser-
−
1
and ꢀΦST give the integrated photon flux (photons s ) for
the sample and the standard phosphor, respectively. The stan-
dard in our case was sodium salicylate (Aldrich), which has a
broad emission band with a maximum at 450 nm and q = 60%
at room temperature [9]. But our experiment show that the best
excited wavelength of the complexes are not the same as that
of sodium salicylate, so the q is not an absolutely quantum
yield, it only can be used for making a comparison between each
other.
−
1
−1
ved around 800 cm . The bands at 1312 and 1323 cm are
assigned to the phenolic C–O stretching mode of the ligands.
Compared with the ligand, these shift to higher frequencies in
the spectra of the chelated complexes is due to its coordination
to europium ion through the oxygen atom of the phenolic group
and it appeared as a singlet or a doublet [11]. Furthermore, the
−1
absorptionintheregion1182and1160 cm duetoO–Hwasnot
observedintheinfraredspectraofthecomplexes, suggestingthat
the deprotonation of the ligands occurs before their coordination
to europium metal. Likewise, carbonyls’ stretching frequencies
2
.2. Preparation of the complexes
−1
in the ligands appearing at 1659 and 1643 cm have a small
−
1
Europium chloride solution was prepared as follows:
.5 mmol of europium oxide was dissolved in 4 ml of concentra-
decrease (by 30 and 40 cm , respectively) in all the complexes.
This indicates that the carbonyl oxygen is involved in the coor-
dination with the metal ion. A characteristic feature of the IR
spectra of the complexes is the appearance of a prominent band
0
ted hydrochloric acid (36%) in a beaker and digested on a steam
bath until completely dissolved. Surplus HCl was removed by
evaporation and the residue was diluted with 10 ml anhydrous
alcohol.
Eu(OHAP)3Phen was prepared as follows [10]: 3 mmol of
HOHAP and 1 mmol of Phen were dissolved in 95% ethanol
and a proper NaOH solution with ethanol (0.5 mol/l) was
added in the solution above. The europium chloride solution
−
1
in the region 480–530 cm for all the complexes. From a com-
parison with the ligand spectra, we have tentatively attributed
this prominent band to the M–O vibrational mode. Such bands
have been observed in acetylacetonate complexes [12] and other
o-hydroxyarylcarbonyl compounds [13]. Based on these obser-
vations the ligands are chelated to the europium ions via the
acetyl oxygen and the phenolic oxygen, H2DAR is concluded
to be bis-bidentate donor, the similar structure that coordina-
ted with metal ions had been confirmed by Shyamala [14] and
Takano [15]. The C–H bending vibration peaks of Phen shifted
(
1 mmol of EuCl3) was added dropwise to the mixed solution
◦
of HOHAP and 1,10-phen with stirring at 50–60 C, and
then stirred for another 4 h. The precipitation appeared imme-
diately, then filtered and washed with 80% ethanol solution
−
−1
−1
(
V(H O):V(EtOH) = 1:4) until no Cl was found in the water.
from 854 and 738 cm to 845 and 729 cm , respectively in
Eu2(DAR)3Phen2, which indicates that the bonds are formed
between europium ions and nitrogen atoms of Phen [16–19].
2
◦
The product was vacuum dried for 6 h at 80 C. The other
rare earth complexes with the ligands (Eu2(DAR)3·4H2O,
Table 1
Elemental analysis results of complexes
Compound
ExpC (Cal) (%)
ExpH (Cal) (%)
ExpN (Cal) (%)
ExpEu (Cal) (%)
Eu(OHAP)3·2H2O
Eu(OHAP)3Phen
Eu2(DAR)3·4H2O
Eu2(DAR)3Phen2
47.95 (48.58)
58.60 (58.62)
37.80 (37.83)
52.50 (52.27)
4.20 (4.25)
3.89 (3.96)
3.19 (3.39)
3.60 (3.25)
–
25.48 (25.61)
20.70 (20.60)
31.86 (31.91)
24.50 (24.49)
3.82 (3.80)
–
4.58 (4.52)

6
66
S.-L. Liu et al. / Spectrochimica Acta Part A 69 (2008) 664–669
Fig. 1. The chemical structure of the complexes (a): Eu(OHAP)3·2H2O; (b): Eu(OHAP)3Phen; 2a: Eu2(DAR)3·4H2O; 2b: Eu2(DAR)3Phen2).
Table 2
Infrared spectral bands in cm
−1
Compound
VH–O–H
VO–H
VC=O
VC–O phenolic
δC–H phen
δM–O
Phen
HOHAP
Eu(OHAP)3·2H2O
Eu(OHAP)3Phen
H2DAR
–
–
–
–
1643
1610
1612
1659
1621
1620
–
1323
1338
1340
1312
1325
1326
854, 739
–
–
846, 732
–
–
–
–
525
521
–
3011, 3061
–
–
2850, 2920
–
–
3500–3200
–
–
Eu2(DAR)3·4H2O
3450–3200
–
482
478
Eu2(DAR)3Phen2
845, 729
The possible chemical structure of the complexes is given in
Fig. 1.
The main absorption peaks of different complexes are listed
in Table 2.
(2) give the absorption spectra of the ligands H2DAR, HOHAP
and their europium complexes in dimethylsulfoxide (DMSO,
10 mol/l) and in the 250–500 nm range.
−
5
For example, it can be seen that the main absorption peaks
of H2DAR are at 288.6 and 321.7 nm. The weak peak at
288.6 nm are attributed to ␲ → ␲* transitions in the aromatic
rings, and the peak at 321.0 nm is the maximum absorp-
tion (kmax), which can be attributed to n → ␲* transitions of
the carbonyl oxygen [18–20]. The main absorption spectra of
H2DAR in Eu2(DAR)3·4H2O are red-shifted to around 293.3
and 324.9 nm. These also indicate that the ligands are chelated
to the europium ions via the carbonyl and the phenolic oxygens
3
.2. UV absorption spectra
The UV–vis spectra of synthesized compounds also confir-
med the formation of the europium complexes. Fig. 2(1) and
Table 3
Electronic absorption data for Eu (III) complexes
(Table 3).
Compound
UV
4
λabs. (nm)
εmax. (×10 )
3
.3. Luminescence properties
HOHAP
322.8, 266.5
332.8, 269.0
321.7, 288.6
324.9, 293.3
1.2623
1.7051
0.9725
1.2803
Eu(OHAP)3·2H2O
TheexcitationspectrumoftheEu(OHAP)3·2H2Ocomplexin
H2DAR
the solid was recorded in the spectral range from 250 to 550 nm
with the emission monitored on the hypersensitive D0 → F2
Eu2(DAR)3·4H2O
5
7

S.-L. Liu et al. / Spectrochimica Acta Part A 69 (2008) 664–669
667
−
5
Fig. 2. (1) Electronic absorption spectra of (a) Eu2(DAR)3·4H2O (solid line) and (b) H2DAR (dotted line); 1 × 10 M (DMSO); (2) electronic absorption spectra
−
5
of (a) Eu(OHAP)3·2H2O (solid line) and(b) HOHAP (dotted line); 1 × 10 M (DMSO).
transition at 612 nm (Fig. 3(1) and (2)). This spectrum displays a
The excitation and emission spectrum of others complexes
strong band from 300 to 450 nm that is attributed to the HOHAP.
In addition, several narrow bands are also observed which are
were measured as the same way, the result indicates that HOHAP
and H2DAR are good sensitizer for the Eu(III) luminescence and
Phen as a synergic ligand can enhance the complexes lumines-
cence intensity.
6
assigned to 4f -intraconfigurational transitions from the ground
7
state F0 to the following excited state of the europium ion (in
5
5
5
nm): D2 (464) and D1 (532).
The lifetimes of D0 excited state for the complexes were
The emission spectrum of the Eu(OHAP)3·2H2O complex
measured by time-resolved spectroscopy, the lifetime valves
were calculated through the mono-exponential decay method
[22], and the results are given in Table 4, too.
in solid state was recorded in the range of 500–750 nm. The
5
7
emission data show intense narrow bands from the D0 → FJ
3+
transitions (where J = 0–4) and are dominated by the hypersen-
Gd complex was selected as model complex for deter-
mination of the triplet state energies of the organic ligands
owing to their high phosphorescence–fluorescence ratio com-
5
7
sitive D0 → F2 transition around 610 nm. The presence of the
5
7
D0 → F0 of Eu(III) transition indicates that the europium ion
pared to those of other Ln3 complexes and Gd can sensitize
the phosphorescence emission of ligands. The phosphorescence
spectra of the gadolinium complexes in ethanol with HOHAP
and H2DAR were measured (Fig. 4(1) and (2)), and the triplet
+
3+
is located in a symmetry site of the type Cs, Cn or Cnv [21]. The
most outstanding feature in the emission spectra is the enhan-
cement of the Eu(III) emissions in Eu(OHAP)3Phen excited at
3
92 nm.
Fig. 3. (1) Emission spectra of (a) Eu(OHAP)3Phen (dotted line, λex = 376.5 nm) and (b) Eu(OHAP)3·2H2O (dotted line, λex = 376.0 nm) in the solid state; (2)
emission spectra of (a) Eu2(DAR)3Phen2 (solid line, λex = 392.1 nm) and (b) Eu2(DAR)3·4H2O (solid line, λex = 391.5 nm) in the solid state.

6
68
S.-L. Liu et al. / Spectrochimica Acta Part A 69 (2008) 664–669
Table 4
The luminescence properties of the Eu (III) complexes in the solid state
Complexes
λmax.(ex) (nm)
λmax.(em) (nm)
Luminescence intensity (a.u.)
Lifetimes (␶/ms)
q (%)
5
7
5
7
5
7
5
7
D0 → F0
D0 → F1
D0 → F2
D0 → F3
Eu(OHAP)3·2H2O
Eu(OHAP)3Phen
Eu2(DAR)3·4H2O
Eu2(DAR)3Phen2
391.5
392.1
376.0
376.5
610.6
610.0
610.4
611.6
84.5
85.5
847.5
4354
1837.0
7780.5
73.2
60.8
34.41
71.3
0.954
1.012
0.269
0.482
5.0
7.5
4.8
5.4
247.5
30.0
95.9
362.5
152.6
412.3
−
1
−1
−1
−1
state of HOHAP(H2DAR) can be determined to be 23,095 cm
(
band at 433 nm(460 nm).
17,265 cm ) can be calculated to be 5830 cm (4474 cm ),
−1
5
21,739 cm ) based on the both maximum phosphorescence
they are all indeed sufficient to efficiently populate the D0
3+
luminescent state of Eu . Theoretically, if the differences bet-
ween the triplet state of A (for example: HOAHP) and the
resonance energy level of Eu3 is larger than that of B (the
other), the luminescent intensity of A should be smaller than
B, but the luminescence intensity of Eu(OHAP)3·2H2O is lar-
ger than that of Eu2(DAR)3·4H2O. There may be two reasons.
Firstly, comparedwithHOHAP, thedifferencebetweenthetriple
state of H2DAR and the excited states of Eu(III) is too small
to transfer the energy efficiently from the ligand to europium
ion, so HOHAP is more suitable than H2DAR. Secondly, it
should be an explanation that the complex Eu(OHAP)3·2H2O
contains three chromophores/Eu, and the Eu2(DAR)3·4H2O
only contains 1.5/Eu (i.e. about one half of the more luminescent
complex).
Phen as a synergic ligand enhances the complexes lumi-
nescence intensity. The main reason should be replaced the
quenching water molecules in the coordination sphere of Eu(III).
According to the theory of nonradiative transition in lanthanide
complexes, the nonradiative relaxation between various J states
may occur by interaction of the electronic levels of the lantha-
nide ion with suitable vibrational modes of the environment. The
efficiency of these processes depends on the energy gap between
the group and the excited states of the lanthanide ions and the
vibrational energy of the oscillators. When solvents containing
O–H groups are coordinated to lanthanide ions, efficient nonra-
diative deactivations take place via vibronic coupling with the
The luminescence properties of the europium complexes in
the solid state are summarized in Table 4.
Luminescence in rare earth complexes is produced by the f–f
transitions of the metal ion, and the complexes display a bright
and narrow lanthanide ion emission. For Eu(III) complexes,
the emission spectra are dominated by the D0 → F2 transi-
tion around 610 nm. During luminescence, the excited energy of
the organic ligand undergoes an intramolecular energy transfer
from the triplet state of the ligand to an excited state of Eu(III),
and then the emission occurs when the energy transfer from the
lowest excited state ( D0) to the ground state ( F2) of Eu(III). In
order for energy transfer to be effective, the triplet state of the
ligand must be higher in energy than the emissive excited state
of the metal ion, and for Eu(III) complexes the most efficient
ligands of energy transfer are those in which the triplet state is a
little higher (too much and too small should decrease the lumi-
+
5
7
5
7
5
nescence intensity) in energy than the D0 level of Eu(III). Thus,
although the emitting color of a rare earth complex depends on
the metal ion, its luminescence properties are affected by the
ligands, especially their energy transfer properties. Thus it can
be seen that HOHAP and H2DAR are two ligands that can trans-
fer the energy to the Eu(III) efficiently and emit characteristic
3+
red luminescence emission of Eu strongly.
The energy differences between the triplet state of HOHAP
5
(
H2DAR) and the resonance energy level of Eu3+ ( D0,
Fig. 4. (1) Phosphorescence spectra of Gd(OHAP)3·2H2O (λem = 433 nm, λex = 310 nm); (2) phosphorescence spectra of Gd2(DAR)3·4H2O (λem = 460 nm,
λex = 320 nm).

S.-L. Liu et al. / Spectrochimica Acta Part A 69 (2008) 664–669
669
vibrationalstatesoftheO–Hoscillators. Toreducethenumberof
O–Hoscillators(watermolecular)inthecoordinationsphereand
increase the intensity of the lanthanide luminescence exploiting
the antenna effect, complexes with strongly absorbing synergic
ligands have been designed, for example, some synergic ligands
with aromatic segment, e.g. 1,10-phenanthroline, which show
intense absorption bands in the UV region due ␲ → ␲* tran-
sitions. And if the coordination sphere is supplemented with
Provincial Key Discipline and Hunan Provincial General Col-
lege Leader of Learning of China.
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6
2
H2O, Eu(OHAP)3Phen, Eu2(DAR)3·4H2O and Eu2(DAR)3
ꢀ
Phen2 (HOHAP = 2 -hydroxyacetophenone, H2DAR = 4,6-
diacetylresorcinol, Phen = 1,10-phenanthroline) have been
designed and synthesized. The characterizations of the com-
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carbonyl and the phenolic oxygens. The emission data show
9
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2
[
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Acknowledgements
The authors acknowledge the financial support from the Edu-
cation Committee of Hunan Province of China (02C465), Hunan