High lasing oscillation efficiency of Eu(III) complexes having remarkably sharp emission band

Article abstract of DOI:10.1016/j.jallcom.2004.12.144

Luminescent Eu(III) complexes with bis-phosphine oxide ligands were synthesized. Geometric structures of novel Eu(III) complexes were determined by single-crystal X-ray diffraction. One of the Eu(III) complexes showed remarkably sharp red emission and lar

Full text of DOI:10.1016/j.jallcom.2004.12.144

Journal of Alloys and Compounds 408–412 (2006) 771–775
High lasing oscillation efficiency of Eu(III) complexes
having remarkably sharp emission band
Kazuki Nakamuraa, Yasuchika Hasegawad, Hideki Kawaib, Naoki Yasudac,
Yuji Wadaa,∗, Shozo Yanagidae,∗∗
a
Material and Life Science, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan
b
Research Institute of Electronics, Shizuoka University, Japan
Advanced Technology Research and Development Center, Mitsubishi Electric Corporation, Japan
c
d
Research and Education Center for Materials Science, Nara Institute of Science and Technology, Japan
e
Center for Advanced Science and Innovation, Osaka University, Japan
Received 2 August 2004; received in revised form 1 December 2004; accepted 7 December 2004
Available online 24 June 2005
Abstract
Luminescent Eu(III) complexes with bis-phosphine oxide ligands were synthesized. Geometric structures of novel Eu(III) complexes were
determined by single-crystal X-ray diffraction. One of the Eu(III) complexes showed remarkably sharp red emission and large stimulated
emission cross-section (full width at half maximum (FWHM) = 2.1 nm, stimulated emission cross-section = 1.80 × 10−20 cm2). The sharpness
of the emission bands of the Eu(III) complexes was discussed in terms of the symmetry of the geometrical structure.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Chemical synthesis; Crystal structure and symmetry; Luminescence; X-ray diffraction
1. Introduction
vibrational structures (Fig. 1). In order to discuss the rela-
tion between the FWHM and structure in liquid media, the
o,m-phenylene and alkyl chains with different length were
introduced as linker moieties between bis-phosphine oxide
groups. In this work, we have successfully synthesized a
Eu(III) complex having remarkably sharp red emission band.
The lasing oscillation efficiencies of the Eu(III) complexes
were estimated by the calculation of stimulated emission
cross-sections. The symmetry of Eu(III) complexes was dis-
cussed in terms of the geometrical structures, related to the
emission spectra and the stimulated emission cross-sections.
Lanthanide(III) ions have been regarded as attractive lumi-
nescent centers for optical devices, luminescence sensors of
chemical species and biomedical assays [1–5]. The photo-
physical properties depending on the geometrical structure
of the Eu(III) complexes was reported by Richardson, Hor-
rocks and co-workers [6,7]. These reports mainly discussed
the transition probability or the emission quantum yields of
the Eu(III) complexes. However, the full width at half maxi-
mum (FWHM) of the emission bands was not directly linked
to the geometrical structures of the Eu(III) complexes, though
the control of the sharpness of the emission bands is one of
the most important factors for laser application [8].
2. Experimental
Here, we report the series of novel strong-luminescent
Eu(III) complexes which have anti-symmetrical and low
2.1. Apparatus
∗
∗∗
Corresponding author. Tel.: +81 6 6879 7926; fax: +81 6 6879 7875.
Co-corresponding author.
E-mail address: ywada@mls.eng.osaka-u.ac.jp (Y. Wada).
1H NMR and 19F NMR data were obtained with a
JEOL EX-270 spectrometer. 19F NMR chemical shifts were
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2004.12.144

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K. Nakamura et al. / Journal of Alloys and Compounds 408–412 (2006) 771–775
Fig. 1. Chemical structures of 1–7.
pounds were identified by 1H NMR, 19F NMR and elemental
analysis.
determined using hexafluorobenzene as an external standard
(δ = −162.0 (s, Ar–F) ppm). Elemental analyses were per-
formed with Perkin-Elmer 240C.
For 2: 1H NMR (acetone-d6, 270 MHz, 298 K), δ
(ppm) 9.7–9.4 (8H, br), 8.2–8.0 (32H, br), 6.200 (8H,
s). 19F NMR (acetone-d6, 270 MHz, 298 K) δ (ppm)
−75.3 (CF3, s). Anal. found: C, 51.20; H, 3.11; calc. for
C75H57EuF18O10P4 + 0.80MeOH: C, 51.89; H, 3.31%. For
3: 1H NMR (acetone-d6, 270 MHz, 298 K) δ (ppm) 8.2–7.5
(20H, br), 7.262–7.154 (4H, m). 19F NMR (acetone-d6,
270 MHz, 298 K) δ (ppm) −77.29 (CF3, s). Anal. found:
C, 43.10; H, 2.21; calc. for C45H33EuF18O8P2: C, 42.98;
H, 2.64%. For 4: 1H NMR (acetone-d6, 270 MHz, 298 K) δ
(ppm) 8.22 (8H, br), 7.58–7.55 (4H, t), 7.38 (8H, s). 19F NMR
(acetone-d6, 270 MHz, 298 K) δ (ppm) −76.07 (CF3, s).
Anal. found: C, 48.83; H, 3.12; calc. for C65H53EuF18O10P4:
C, 48.43; H, 3.31%. For 5: 1H NMR (acetone-d6, 270 MHz,
298 K) δ (ppm) 8.99–8.91 (8H, br), 7.78–7.69 (12H, m),
5.38 (2H, s), 0.78 (2H, br). 19F NMR (acetone-d6, 270 MHz,
298 K) δ (ppm) −76.94 (CF3, s). Anal. found: C, 40.46; H,
2.37; calc. for C41H33EuF18O8P2: C, 40.75; H, 2.75%. For
6: 1H NMR (acetone-d6, 270 MHz, 298 K) δ (ppm) 9.31 (8H,
br), 7.81 (12H, br), 5.35 (2H, br), 0.248 (4H, br). 19F NMR
(acetone-d6, 270 MHz, 298 K) δ (ppm) −76.98 (CF3, s).
Anal. found: C, 41.05; H, 2.39; calc. for C42H35EuF18O8P2:
C, 41.23; H, 2.88%. For 7: 1H NMR (acetone-d6, 270 MHz,
298 K) δ (ppm) 9.20 (8H, br), 7.65 (12H, br), 5.55 (2H,
br), 0.25 (4H, br), 0.18 (2H, br). 19F NMR (acetone-d6,
270 MHz, 298 K) δ (ppm) −76.75 (CF3, s). Anal. found:
C, 41.9; H, 2.71; calc. for C43H37EuF18O8P2: C, 41.73; H,
3.01%.
2.2. Materials
Europium acetate tetrahydrate (99.9%), 1,1,1,5,5,5-hexaf-
luoro-2,4-pentanedione (hfa-H2) and triphenylphosphine
oxide (TPPO) were purchased from Wako Pure Chemical
Industries Ltd. 1,1-bis(diphenylphosphino) methane, 1,2-
bis(diphenylphosphino) ethane, 1,3-bis(diphenylphosphino)
propane and 1,4-bis(diphenylphosphino) butane were
purchased from KANTO Chemical Co. Inc. Methanol-d4
and acetone-d6 were obtained from Aldrich Chemical Co.
Inc. All other chemicals were reagent grade and were used
as received.
2.3. Synthesis of novel Eu(III) complexes with
bis-phosphine oxide ligands
Tris(hexafluoroacetylacetonato) europium(III) bis(triphe-
nylphospohine oxide) (1) was prepared by the reaction of
Eu(hfa)3(H2O)2 and triphenylphosphine oxide in methanol
[9]. Tris(hexafluoroacetylacetonato) europium(III), 1,2-phe-
nylenebis(diphenylphosphine oxide) (2) was prepared by
reaction between Eu(hfa)3(H2O)2 and 1,2-phenylenebis
(diphenylphosphine oxide) in methanol under reflux for 8 h.
The reaction mixture was concentrated using rotary evapora-
tor. Recrystallization from methanol gave white crystals (2).
Yield: 56%.
Tris(hexafluoroacetylacetonato) europium(III) 1,3-phen-
ylenebis(diphenylphosphine oxide): 3, tris(hexafluoroacety-
lacetonato) europium(III) methylenebis(diphenylphosphine
oxide): 4, tris(hexafluoroacetylacetonato) europium(III) 1,2-
ethylenebis(diphenylphosphine oxide): 5, tris(hexafluoroac-
etylacetonato) europium(III) 1,3-propylenebis(diphenylpho-
sphine oxide): 6, tris(hexafluoroacetylacetonato) europium
(III) 1,4-butylenebis(diphenylphosphine oxide): 7 were also
prepared by the same method as for 1 and 2. These com-
2.4. Preparation of deuterated Eu(III) complexes in
deuterated solvent
Deuterated Eu(III) complexes were obtained by the
exchange reaction via keto–enol tautomerism of 1–7 with
CD3OD for 6 h under vacuum [9]. Samples for measure-
ments of the luminescence of Eu(III) complexes in organic
media were prepared under deoxygenated conditions. Solu-

K. Nakamura et al. / Journal of Alloys and Compounds 408–412 (2006) 771–775
773
eters Uλ of Eu(III) are referred in the following literatures
[11–13].
tions (0.05 M) of the Eu(III) complexes were prepared in
2.0 mL of acetone-d6 under 10−3 torr and then transferred to
a quartz cell for optical measurements (optical path length,
5 mm) [9,10].
2.7. Crystallography
2.5. Optical measurements
Colorless single-crystals of complexes 2–4 were mounted
on a glass fiber using epoxy resin. X-ray diffraction inten-
sities were collected with a Rigaku RAPID imaging plate
diffractometer at 223 K (for 2 and 4) and a Rigaku AFC-
5R four circle diffractometer (for 3) at 293 K using Mo
K␣ radiation in w–2θ scan mode. Corrections for decay
and Lorentz-polarization effects were made, with empir-
ical absorption corrections solved by direct methods and
expanded using Fourier techniques. Non-hydrogen atoms
were refined anisotoropically. Hydrogen atoms were placed
Emission spectra were measured at room tempera-
ture using a Spex Fluorolog system. The samples were
excited at 465 nm (7F0 → D2). The emission lifetimes
5
were determined by using Q-switched Nd:YAG laser (Spec-
tra Physics INDI-50, fwhm = 5 ns, λ = 1064 nm) and pho-
tomultiplier (Hamamatsu Photonics R7400U-03, response
time ≤ 0.78 ns). Samples were excited by the third harmonic
(355 nm) of the fundamental nanosecond pulse. Nd:YAG
response was monitored with a digital oscilloscope (Sony
Tektronix, TDS3052, 500 MHz) synchronized to the single-
pulse excitation. The emission quantum yields were deter-
mined by the standard procedures with an integrating sphere
(diameter 6 cm) [9,10].
˚
in calculated positions (C–H, 0.95 A) but not refined. All
calculations were performed using the TEXSAN crystallo-
graphic software package.
3. Results and discussion
2.6. Judd–Ofelt analysis
3.1. Geometrical structures
Absorption spectra of Eu(III) complexes were measured
at room temperature using a HITACHI U-3300 system.
Judd–Ofelt analysis of Eu(III) complexes was carried out
The crystal structures of Eu(III) complexes 2–4 were
determined by single-crystal X-ray diffraction. The ORTEP
diagrams of complexes 1–4 and crystallographic data are
shown in Fig. 2 and Table 1 [9], respectively. The com-
5
using the 7F0 → D2 transition (465 nm). Concentrations of
the Eu(III) complexes in solvent were 0.05 M. Tensor param-
Fig. 2. ORTEP drawing of the complexes 1–4.

774
K. Nakamura et al. / Journal of Alloys and Compounds 408–412 (2006) 771–775
Table 1
Summary of crystal data of 2–4, data collection and refinement details
2
3
4
Empirical formula
Formula weight
Crystal system
Space group
C
51H37EuF18O8P2
C51
H
37EuF18O8P2
C
65H47F18EuO10P4
1251.58
Monoclinic
P21/n
1605.91
Monoclinic
P21/n
Monoclinic
P21/c
˚
a (A)
13.2759(6)
22.4970(8)
24.894(1)
90
102.653(2)
90
13.913(3)
22.288(2)
16.510(2)
90
92.60(1)
90
14.9145(4)
31.8787(9)
15.8722(4)
90
116.0074(6)
90
˚
b (A)
˚
c (A)
α (◦)
β (◦)
χ (◦)
3
˚
V (A )
7254.5(6)
4
5114(1)
4
6782.3(3)
4
Z
Reflections collected
Independent reflections
T (K)
37269
25028
213
14899
8535
296
14784
10712
296
Rint
0.111
0.090
0.054
Final R1, wR2
0.1628, 0.3681
0.0476, 0.2358
0.0723, 0.1887
plex 3 formed a column structure because the neighboring
Eu(III) ions were connected by bis-phosphine ligands. From
thecoordinationsiteangles, thegeometricalstructureofcom-
plexes 1–4weredeterminedtobeananti-symmetricalsquare-
antiprism. This result indicates that those Eu(III) complexes
have no inverted center in crystal field, resulting in an increase
in electron transitions in the 4f orbitals due to odd parity. The
two P O groups in the complex 3 occupied the upper square
and lower square, respectively. The coordination structure of
3 was similar to that of 1 [9]. The coordination structures
of 5–7 would be similar coordination structures to those of
1 and 3. On the other hand, the coordination structures of 2
and 4 were different from those of corresponding 1 and 3.
The central Eu(III) ions in the complexes 2 and 4 were coor-
dinated by two bis-phosphine oxide ligands. One of the two
bis-phosphine oxide ligands of the complex 4 bridged the
two squares. In contrast, the two bis-phosphine oxide ligands
of the complex 2 did not bridge the two squares but simply
occupied the two coordination sites of the upper and lower
square. The geometrical structure of Eu(III) complex should
affect the emission spectrum based on the changes of Stark
splitting on the radiative transition of Eu(III) ions.
3.2. Emission properties in liquid media
The emission spectra of 1–7 in acetone-d6, which
were synthesized in this study were shown in Fig. 3.
The emission spectral shape of the 5D0 → F2 (electric
7
dipole) transition of 2 and 4 was quite different from those
7
of corresponding 1, 3–7. The FWHM of the 5D0 → F2
transition band of 2 was found to be 2.1 nm, and was
much smaller than those of 1, 3–7 (FWHM were shown in
Table 2). The narrowing of the emission band of the Eu(III)
complex was achieved in 2. Stark splitting and transition
probability in the f-orbitals of Eu(III) ion was dependent on
symmetry of Eu(III) complexes [14]. Generally, degenerated
7FJ states of Eu(III) complexes split into some Stark levels
(maximum: 2J + 1) according to decreasing of symmetry
7
Fig. 3. Emission spectra of Eu(III) complexes in acetone-d6 (excitation at 465 nm). All spectra were normalized with respect to the 5D0 → F1 (magnetic
dipole) transition. (a) 1 (᭹), 2 (ꢀ) and 3 (ꢀ); (b) 4 (+), 5 (ꢁ), 6 (ꢂ) and 7 (ꢀ).

K. Nakamura et al. / Journal of Alloys and Compounds 408–412 (2006) 771–775
775
Table 2
Photophysical properties of 1–7 (emission lifetimes, τobs, emission quantum yieldsa, Φ, Judd–Ofelt parametersb, Ω2, Einstein coefficientsc, A and emission
cross-sectionsd, σp, in acetone-d6)
τobs(ms)
Φ
Ω2 × 10−20(cm2)
A(×102s−1
)
ꢀλeff(nm)
σp (×10−20 cm2)
1
2
3
4
5
6
7
0.93
1.28
0.96
0.95
0.89
0.98
0.93
0.90
0.48
0.91
0.55
0.82
>0.95
0.40
26.2
9.68
3.75
9.47
5.79
9.21
9.69
4.30
7.0
2.1
7.1
6.0
6.6
6.9
7.1
1.42
1.80
1.39
0.97
1.41
1.42
0.55
14.2
e
–
17.8
23.6
35.6
31.8
a
Emission lifetimes of the Eu(III) complexes were measured by the excitation at 355 nm (Nd:YAG 3ω). Emission quantum yields of the Eu(III) complexes
were measured by the excitation at 465 nm (7F0 → D2) in acetone-d6. Concentrations of Eu(III) complexes were 0.05 M.
5
b
Judd–Ofelt parameter Ω2 is sensitive to the symmetry and sequence of ligand field of lanthanide(III) complexes.
c
Einstein coefficient was determined using A =1/τR = Φ/τobs
.
d
e
Emission cross-sections were determined by Eq. (1).
The Ω2 parameter of 3 was not estimated because of their low solubility. The low solubility of the complex 3 is considered to come from the columnar
structure of the complex 3.
of Eu(III) complexes. The transition from 5D0 state to
degenerated 7F2 state leads to narrower emission band than
those of the normal transition. The coordination structure
of the complex 2 would be much different from those of
corresponding 1, 3–7. From X-ray crystallography and
the low Ω2 value of the Judd–Ofelt analysis (Table 2),
we concluded that the symmetry of the complex 2 was
higher than those of corresponding 1, 3–7. The symmetrical
structures of 2 induce the degeneration of the Stark levels of
the 7F2 state in Eu(III) ion. Degeneration of the Stark levels
of the complex 2 would give the unique sharp emission
(Fig. 3). The high symmetry in the structure of Eu(III)
complex decreases transition probability of the f–f transi-
tions, leading to the longer emission lifetime for complex 2
(Table 2).
4. Conclusion
In conclusion, we had successfully synthesized the series
of Eu(III) complexes with bis-phosphine oxide ligands, and
observed that the novel Eu(III) complex 2 achieved remark-
ably sharp emission and larger stimulated emission cross-
section than those of corresponding other Eu(III) complexes
withbis-phosphineoxide. Wearenowexamininglasingprop-
erties of those series of Eu(III) complexes in relation with
stimulated emission cross-section [16]. Since the stimulated
emission cross-section of 2 is the same order as the values of
Nd–glass laser for practical use ((1.6–4.5) × 10−20 cm2) [8],
the luminescent materials using 2 are promising for applica-
tion such as the high-power laser medium.
In order to construct a high power laser system, a lan-
thanide(III) complex should show the emission property with
large stimulated emission cross-section, which is one of the
most important factors for laser amplification. The stimulated
emission cross-section can be expressed by:
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λp
σp(λp) =
A(bJꢁ : aJ)
(1)
8πcn2ꢀλeff
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