Photosensitized luminescence of highly thermostable mononuclear Eu(III) complexes with π-expanded β-diketonate ligands

Article abstract of DOI:10.1246/bcsj.20170254

Thermostable mononuclear Eu(III) complexes with a π- expanded system, [Eu(btfa)3(DPEPO)] and [Eu(ntfa)3(DPEPO)] (DPEPO: bis[2-(diphenylphosphino)phenyl] ether oxide, btfa: benzoyltrifluoroacetonate, ntfa: 3-(2-naphthoyl)-1,1,1-trifluoroacetonate), are reported. Decomposition temperature (dp) of [Eu(btfa)3(DPEPO)] and that of [Eu(ntfa)3(DPEPO)] are estimated to be 320 °C and 318 °C, respectively. These values are higher than that of the previous [Eu(hfa)3(DPEPO)] (hfa: hexafluoroacetylacetonate, dp = 228 °C). The photosensitized emission quantum yield Φππ? and photosensitized energy transfer efficiency ηsens of [Eu(ntfa)3(DPEPO)] (Φππ? = 45%, ηsens = 77%) are larger than those of [Eu(btfa)3(DPEPO)] (Φππ? = 38%, ηsens = 55%). The thermostable Eu(III) complex with a π-expanded system is expected to be useful for fabrication of LED devices.

Full text of DOI:10.1246/bcsj.20170254

Advance Publication Cover Page
Photosensitized luminescence of highly thermostable mononuclear
Eu (III) complexes with π-expanded β-diketonate ligands
Toru Koizuka, Masanori Yamamoto, Yuichi Kitagawa, Takayuki Nakanishi,
Koji Fushimi, and Yasuchika Hasegawa*
Advance Publication on the web September 11, 2017
doi:10.1246/bcsj.20170254
© 2017 The Chemical Society of Japan

Photosensitized luminescence of highly thermostable mononuclear Eu (III)
complexes with π-expanded β-diketonate ligands
3
Toru Koizuka,1, 2 Masanori Yamamoto,2 Yuichi Kitagawa,3 Takayuki Nakanishi,3 Koji Fushimi, and Yasuchika
Hasegawa*3
1 Semiconductor Development Division, Nichia Corporation, 1-1 Tatsumi, Anan, Tokushima 774-0001
2 Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita13, Nihi8, Kita-ku, Sapporo, Hokkaido 060-8628
3 Faculty of Engineering, Hokkaido University, Kita13, Nihi8, Kita-ku, Sapporo, Hokkaido 060-8628
E-mail: < hasegaway@eng.hokudai.ac.jp >
Abstract
Thermostable mononuclear Eu(III) complexes with a
π-expanded system, [Eu(btfa)3(DPEPO)] and [Eu(ntfa)3
(DPEPO)] (DPEPO: bis[2-(diphenylphosphino)phenyl] ether
oxide, btfa: benzoyltrifluoroacetonate, ntfa: 3-(2-naphthoyl)-
despersed.2 Densities of typical silicone resins (ρ ≃ 1.20 g
cm-3) are similar to those of lanthanide complexes (ρ =
1.28-1.69 g cm-3), which show good solubility in organic
solvents and polymers, though an inorganic phosphor such as
YAG (ρ ≃ 4.8 g cm-3) is not homogeneously dispersed in
20, 21
silicone resins.7,
From an industrial view point, a
1,1,1-trifluoroacetonate),
are
reported.
Decomposition
temperature (dp) of [Eu(btfa)3(DPEPO)] and that of
[Eu(ntfa)3(DPEPO)] are estimated to be 320oC and 318oC,
respectively. These values are higher than that of the previous
lanthanide complex is an ideal wavelength conversion material
for LED devices.
A thermostable wavelength convertor such as a lanthanide
complex is required for reflow soldering process (260oC) in
fabrication of LED devices. We recently reported a luminescent
lanthanide coordination polymer with high thermostability.21
The use of thermostable lanthanide coordination polymers
composed of phosphine oxide and β-diketonate ligands,
however, leads to the formation of insoluble compounds in
organic media. A thermostable and resin-soluble lanthanide
complex with photosensitized luminescence should be useful
for fabrication of LED devices. In order to achieve
thermostability, we focused on the introduction of π-expanded
systems to β-diketonate ligands. Versatility of β-diketonate and
its derivatives was reviewed in the recent work.22 Honjo
reported that binding constants of lanthanide complexes with
benzoylacetonate and β-naphthoylacetonate ligands are larger
than that with an acetylacetonate ligand.23 The β-diketonate
ligands with π-expanded systems lead to also increase of
boiling points related to the decomposition temperatures (dp) of
lanthanide complexes.
In this study, Eu(III) complexes attached with
Bis[2-(diphenylphosphino)phenyl] ether oxide (DPEPO) and
β-diketonate containing phenyl and napthyl groups,
[Eu(btfa)3(DPEPO)] (btfa: benzoyltrifluoroacetonate) and
[Eu(ntfa)3(DPEPO)] (ntfa: 3-(2-naphthoyl)-1,1,1-trifluoro-
acetonate)) were prepared (Figure 1). The phosphine oxide
ligand DPEPO promotes the formation of an asymmetric
coordination geometry of a lanthanide complex related to
enhancement of the emission quantum yields and the radiative
rate constant kr.7 The dp values of [Eu(btfa)3(DPEPO)] and
[Eu(ntfa)3(DPEPO)] are estimated to be 320oC and 318oC,
respectively. These values are higher than that of the previous
strong luminescent [Eu(hfa)3(DPEPO)] (hfa: hexafluoro
acetylacetonate, dp = 228oC). The photosensitized emission
quantum yield Φπ-π* and photosensitized energy transfer
[Eu(hfa)3(DPEPO)] (hfa: hexafluoroacetylacetonate, dp
=
228oC). The photosensitized emission quantum yield Φπ-π* and
photosensitized energy transfer efficiency ηsens of
[Eu(ntfa)3(DPEPO)] (Φπ-π* = 45%, ηsens = 77%) are larger than
those of [Eu(btfa)3(DPEPO)] (Φπ-π* = 38%, ηsens = 55%). The
thermostable Eu(III) complex with a π-expanded system is
expected to be useful for fabrication of LED devices.
1. Introduction
Lanthanide complexes have attracted considerable
attention due to their highly efficient luminescence for use in
lighting, displays and lasers.1-5
A lanthanide complex
provides highly monochromatic and clear emission with small
full width at half maximum (fwhm) of less than 15 nm in the
visible region. Photosensitized luminescence of lanthanide
complexes with characteristic organic ligands has been
reported.6-15 Moudam and Miyata reported that Eu(III) and
Sm(III) complexes with DPEPO (DPEPO: DPEPO:
Bis[2-(diphenylphosphino)phenyl] ether oxide) show strong red
7
luminescence and deep-red luminescence, respectively.6,
Nakanishi and coworkers reported brilliant green luminescence
of nine Tb(III) clusters.8 Blue luminescence of Tm(III)
metallopolymer with a Pybox (pyridine-bis(oxazoline)) ligand
was also observed.9 Bünzli summarized the design of highly
luminescent lanthanide complexes and discussed all aspects
needing optimization.16 Luminescent lanthanide complexes
have been extensively studied in the fields of photophysical,
coordination and material chemistry.
For recent lanthanide complexes, monochromatic
emission excited by ultraviolet light-emitting diodes
(UV-LEDs) has been investigated. Various types of red, green
and blue luminescent lanthanide complexes based on UV-LED
excitation have been investigated as wavelength conversion
materials for fabrication of a white luminescent LED.17-19 For
fabrication of such an LED, a GaN chip should be covered with
silicone resin in which a wavelength conversion material is
efficiency ηsens of [Eu(ntfa)3(DPEPO)] (Φπ-π* = 45%, ηsens
=
77%) are larger than those of [Eu(btfa)3(DPEPO)] (Φπ-π* = 38%,
ηsens = 55%). High thermostability and good luminescence
properties of mononuclear Eu(III)complexes with π-expanded
systems in β-diketonate ligands are demonstrated.

of water and 40 mL of methanol.
A solution of
4,4,4-Trifluoro-1-phenyl-1,3-butanedione (4.0 g, 19 mmol) was
added to the solution. An ammonia solution was added
dropwise to the solution until it approximately reached pH 7.
After stirring at room temperature for 4 h, the reaction mixture
was poured into water. The mixture solution stirred 1day to
obtain white precipitate. The precipitate was filtered, washed
with hexane several times, and dried in vacuo.
[Eu(btfa)3(H2O)2]. Yield: 4.7 g (94%); 1H NMR (400
MHz, CD3OD, 20oC): δ = 7.02-6.98 (t, 9H; Ar), 6.60-6.50 ppm
(d, 6H; Ar). IR (KBr): 3652 (st, O-H), 1608 (st, C=O), 1283 (st,
C−F) cm−1. Elemental analysis calcd (%) for C30H22EuF9O8: C,
43.23; H, 2.66. Found: C, 43.18; H, 2.50.
Prepared DPEPO (0.57 g, 1.0 mmol) and the [Eu(btfa)3
(H2O)2] (0.83 g, 1.0 mmol) were dissolved in 30 mL of
methanol. The solution was heated at reflux while stirring for 8
h. The reaction mixture was cooled at room temperature. The
produced precipitate was filtered, washed with cool methanol
and dried in vacuo. The white powder was produced.
Figure 1. Chemical structures of a) [Eu(hfa)3(DPEPO)],
b) [Eu(btfa)3(DPEPO)] and c) [Eu(ntfa)3(DPEPO)].
1
[Eu(btfa)3(DPEPO)]. Yield: 1.1 g (83%); H NMR (400
MHz, C3D6O, 20oC): δ = 8.95 (dd, 4H; Ar), 8.45 (dd, 4H; Ar),
7.75 (m, 8H; Ar), 7.55 (s, 4H; Ar), 7.45 (t, 2H; Ar), 7.3 (t, 2H;
Ar), 7.05 (s, 4H; Ar), 7.0 (t, 3H; Ar), 6.9 (t, 6H; Ar), 6.85 (d,
6H; Ar), 4.83 ppm (s, 3H; β-diketone-H). IR (KBr): 1616 (st,
C=O), 1290 (st, C−F), 1171 (st, P=O) cm−1. Elemental analysis
calcd (%) for C66H46EuF9O9P2: C, 57.95; H, 3.39. Found: C,
57.98; H, 3.28.
2. Experimental
Materials. Bis[2-(diphenylphosphino)phenyl]ether and
1,1,1,5,5,5-Hexafluoro-2,4-pentane dione and 3-(2-naphthoyl)
-1,1,1-trifluoroacetone were obtained from Tokyo Chemical
Industry Co., Ltd. Europium chloride hexahydrate was
purchased from Kanto Chemical Co., Inc. 4,4,4-trifluoro
-1-phenyl-1,3-butanedione were obtained from Aldrich
Chemical Company Inc. Europium(III) acetate n-hydrate and
H2O2 aqueous solution (30%), Ammonia aqueous solution
(28%) were purchased from Wako Pure Chemical Industries
Ltd. All other chemicals and solvents were reagent grade and
were used without further purification.
Apparatus. Elemental analyses were performed on a
J-Science Lab JM 10 Micro Corder and an Exeter Analytical
CE440. H-NMR (400 MHz) spectra were recorded on a JEOL
ECS400. Infrared spectra were recorded with a JASCO
FTIR-4600 spectrometer. Thermogravimetric Analysis (TGA)
was performed by a Seiko Instruments Inc. EXSTAR 6000
TG/DTA 6300.
Preparation of [Eu(hfa)3(DPEPO)]. DPEPO (bis[2-
(diphenylphosphory)phenyl]ether) was prepared by the same
method previous report.7 Europium acetate n-hydrate (5.0 g, 13
mmol) was dissolved in 60mL of distilled water. A solution of
1,1,1,5,5,5-Hexafluoro-2,4-pentanedione (8.1 g, 39 mmol) was
added dropwise to the solution. The reaction mixture produced
a precipitation of white yellow powder after stirring for 3 h at
room temperature. The reaction mixture was filtered.
Reprecipitation from water and methanol gave a white powder.
Prepared DPEPO (1.4 g, 2.8 mmol) and the [Eu(hfa)3
(H2O)2] (2.4 g, 3.0 mmol) was dissolved in 40 ml of methanol.
The solution was heated at reflux while stirring for 7 h. The
residue was washed with chloroform several times. The
insoluble material was removed by filtration, and the filtrate
was concentrated. The obtained powder was dissolved in 10
mL of hot methanol solution (50oC), and then permitted to
stand at room temperature. Recrystallization from methanol
gave colorless block crystals of the title complex.
Preparation of [Eu(ntfa)3(DPEPO)]. Europium chloride
hexahydrate (1.7 g, 4.5 mmol) was dissolved in distilled 5 mL
of water and 40 mL of methanol.
A solution of
4,4,4-Trifluoro-1-(2-naphthyl)-1,3-butanedione (3.6 g, 13.5
mmol) was added to the solution. An ammonia solution was
added dropwise to the solution until it approximately reached
pH 7. After stirring at room temperature for 4 h, the reaction
mixture was poured into water. The mixture was solution
stirred 1 day to obtain brown precipitates. The precipitates were
filtered, washed with hexane several times, and dried in vacuo.
The cream powder was obtained.
1
[Eu(ntfa)3(H2O)2]. Yield: 3.4 g (77%); 1H NMR (400
MHz, CD3OD, 20oC): δ=7.88-7.82 (m, 3H; Ar), 7.67-7.60 (d,
3H; Ar), 7.52-7.30 (m, 15H; Ar) ppm. IR (KBr): 3648 (st, O-H),
1607 (st, C=O), 1288 (st, C−F) cm−1. Elemental analysis calcd
(%) for C42H28EuF9O8: C, 51.29; H, 2.87. Found: C, 51.73; H,
2.96.
Prepared DPEPO (0.57 g 1.0 mmol) and [Eu(ntfa)3
(H2O)2] (0.98 g 1.0 mmol) were dissolved in 30 ml methanol.
The solution was heated at reflux while stirring for 8 h. The
reaction mixture was cooled at room temperature. The
produced precipitate was filtered, washed with cool methanol
and dried in vacuo. The brown powder was obtained.
1
[Eu(ntfa)3(DPEPO)]. Yield: 0.89 g (59%); H NMR (400
MHz, C3D6O, 20oC): δ=9.05 (t, 4H; Ar), 8.6 (t, 4H; Ar), 7.8 (m,
3H; Ar), 7.65 (m, 8H; Ar), 7.55 (t, 7H; Ar), 7.32-7.44 (m, 14H;
Ar), 7.25 (t, 2H; Ar), 7.1-7.2 (m, 7H; Ar), 5.27 ppm (s, 3H;
β-diketone-H). IR (KBr): 1617 (st, C=O), 1294 (st, C−F), 1133
(st, P=O) cm−1. Elemental analysis calcd (%) for
C78H52EuF9O9P2: C, 61.71; H, 3.45. Found: C, 61.69; H, 3.34.
Optical Measurements. UV−vis absorption spectra of
the Eu(III) complexes were recorded on a JASCO V-670
spectrometer. Those of emission and excitation spectra were
measured with a HORIBA Fluorolog-3 spectrofluorometer and
corrected for the response of the detector system. The
photosensitized emission quantum yields were obtained by
using a JASCO F-6300-H spectrometer attached to a JASCO
ILF-533 integrating sphere unit (φ = 100 mm). The wavelength
dependence of the detector response and the beam intensity of
[Eu(hfa)3(DPEPO)]: Yield: 2.0g (50%); 1H NMR (400
MHz, C3D6O, 20oC): δ = 8.65 (dd, 4H; Ar), 8.4 (dd, 4H; Ar),
7.9 (t, 2H; Ar), 7.75 (s, 4H; Ar), 7.6 (t, 2H; Ar), 7.4-7.5 (m, 6H;
Ar), 7.1 (m, 4H; Ar), 6.85 (m, 2H; Ar), 5.2 ppm (s, 3H;
β-diketone-H). IR (KBr): 1655 (st, C=O), 1255 (st, C−F), 1242
(st, P=O) cm−1. Elemental analysis calcd (%) for
C51H31EuF18O9P2: C, 45.59; H, 2.33. Found: C, 45.61; H, 2.59.
Preparation of [Eu(btfa)3(DPEPO)]. Europium chloride
hexahydrate (2.2 g, 6.0 mmol) was dissolved in distilled 5 mL

β-diketonate ligand bonded to a lithium ion.8 Calculated
Mulliken charges are summarized in Table 1. The negative
charge of densities in btfa and ntfa (-0.6395 and -0.6363,
respectively) are larger than that in hfa (-0.6015). We consider
that the coordination abilities of ntfa and btfa ligands are
superior to that of an hfa ligand. The boiling point (bp) of btfa
with π-expanded system (bp = 224oC) is much higher than that
of hfa (bp = 70oC). We speculate that β-diketonate ligands with
π-expanded systems caused an increase of decomposition
temperatures in Eu(III) complexes.
Xe light source for each spectrum were calibrated by using a
standard light source. Emission lifetimes (τobs) were measured
by using the third harmonics (355 nm) of a Q-switched
Nd:YAG laser (Spectra Physics, INDI-50, fwhm = 5 ns, λ =
1064 nm) and a photomultiplier (Hamamatsu Photonics, R5108,
response time = 1.1 ns). The Nd:YAG laser response was
monitored with
a digital oscilloscope (Sony Tektonix,
TDS3052, 500 MHz) synchronized to the single-pulse
excitation. Emission lifetimes were determined from the slope
of logarithmic plots of the decay profiles.
Density Functional Theory Calculations. DFT
geometry optimizations and molecular orbital (MO)
calculations of simplified clusters were performed with
Gaussian 09, revision D.01, employing the B3LYP hybrid
functional consisting of the three-parameter Becke exchange
functional and the correlation functional of Lee, Yang, and Parr.
The 6-31G(d) basis set was used for all atoms. 24, 25
Absorption spectra of Eu(III) complexes in acetone (1.0 ×
10-3 M) are shown in Figure 3. The absorption band-edges of
π-π* transition at around 400 nm show effective red shifts
depending on expansion of the π-conjugated system in
β-diketonate ligands. The wavelengths of the band edges in
[Eu(hfa)3(DPEPO)], [Eu(btfa)3(DPEPO)] and [Eu(ntfa)3
(DPEPO)] were found to be 361, 380 and 394 nm, respectively.
3. Results and Discussion
[Eu(btfa)3(DPEPO)] and [Eu(ntfa)3(DPEPO)] were
prepared by chelation of [Eu(btfa)3(H2O)2] and [Eu(ntfa)3
(H2O)2], respectively, with DPEPO in methanol under reflux.
Their identifications were performed using NMR, IR and
elemental analyses. Thermogravimetric analysis (TGA) curves
of Eu(III) complexes are shown in Figure 2. The dp values of
[Eu(hfa)3(DPEPO)], [Eu(btfa)3(DPEPO)] and [Eu(ntfa)3
(DPEPO)] were found to be 228oC, 320oC and 318oC,
respectively. The dp values of [Eu(btfa)3(DPEPO)] and
[Eu(ntfa)3(DPEPO)] with π-expanded systems were higher than
that of [Eu(hfa)3(DPEPO)] without a π-expanded system.
Eu(III) complexes with high dp values (> 300oC) is suitable for
a reflow soldering process (260oC) in fabrication of LED
devices. The dp of an Eu(III) complex is dependent on its
coordination ability.
[Eu(ntfa)3(DPEPO)] is a candidate for an effective luminophore
under UV-LED irradiattion at 390 nm.
Figure 3. Absorption spectra of a) [Eu(hfa)3(DPEPO)], b)
[Eu(btfa)3(DPEPO)] and c) [Eu(ntfa)3(DPEPO)] in 1.0 × 10-3
M
acetone at room temperature. Absorption edges were estimated
by drawing tangents of the spectra.
Emission spectra of Eu(III) complexes are shown in
Figure 4. The emission spectra at around 578, 592, 613, 650,
and 698 nm were attributed to the 4f−4f transitions of Eu(III)
(5D0−7FJ: J = 0, 1, 2, 3, and 4, respectively). The spectra were
normalized with respect to the magnetic dipole transition
intensity at 592 nm (Eu: 5D0−7F1), which is known to be
insensitive to the environment surrounding lanthanide ions. The
5
relative spectral integration at D0−7F2 is generally dependent
on the coordination geometry around Eu(III) ions. The relative
Figure 2. Thermogravimetric analysis (TGA) curves of a)
[Eu(hfa)3(DPEPO)], b) [Eu(btfa)3(DPEPO)] and c) [Eu(ntfa)3
(DPEPO)] in an argon flow atmosphere (100 ml min-1) at a
heating rate of 5oC min-1.
spectral
integration
values
of
[Eu(hfa)3(DPEPO)],
[Eu(btfa)3(DPEPO)] and [Eu(ntfa)3(DPEPO)] are 18.9, 20.4
and 21.9, respectively. The differences of relative spectral
integration values are caused by introduction of asymmetric
geometry linked to enhancement of the electric dipole
transition (5D0−7F2). Expansion of the π-conjugated system in
β-diketonate ligands leads to asymmetric coordination
geometry of the their Eu(III) complexes.
In order to evaluate the coordination abilities of
β-diketonate ligands, Mulliken charges of their oxygen atoms
were estimated by DFT calculation (B3LYP 6-31G (d))
performed on simplified models of an Eu(III) complex, a

Table 1. Carbonyl bond lengths for β-diketonate ligands
Mulliken Charge
X side
Carbonyl bond length (Å)
Bond 1
1.2632
1.2704
1.2704
Bond 2
1.2632
1.2734
1.2739
Δd
0
0.0030
0.0035
[Eu(hfa)3(DPEPO)]
[Eu(btfa)3(DPEPO)]
[Eu(ntfa)3(DPEPO)]
-0.6015
-0.6395
-0.6363
The time-resolved emission profiles of Eu(III) complexes
in acetone revealed single exponential decays with
millisecond-scale lifetimes. The emission lifetimes were
determined from the slopes in logarithmic plots of the profiles
(Figure 5). A decrease in emission lifetime was related to
expansion of the π-conjugated system. The intrinsic emission
an increase in the value of kr. The increase of kr is related to the
coordination geometry of the Eu(III) complex. We here focused
on the bond length of coordination sites (dC=O) in each
β-diketonate ligand. Selected bond lengths (dC=O) that were
estimated by DFT calculation (B3LYP 6-31G (d)) are shown in
Table 1. The difference in length (Δd) between bonds 1 and 2 in
β-diketonate ligands increased with expansion of the
π-conjugated system. Asymmetric coordination sites with large
Δd values promote a large kr value.
quantum yield Φ4f-4f
,
radiative rate constant kr and
non-radiative rate constant knr were estimated by using the
following equations:
ꢄꢅ
ꢈꢉꢊꢋ
ꢈꢅꢌꢍ
ꢀꢁꢂꢃꢁꢂ
=
=
(1)
(2)
(3)
ꢄꢅꢆꢄꢇꢅ
ꢖꢗꢉꢗ
ꢖꢘꢙ
ꢚ
ꢎꢏ = ꢐꢑꢒ,ꢓꢔꢕ
=
ꢈꢅꢌꢍ
ꢚ
ꢚ
ꢎꢛꢏ
=
−
,
ꢈꢉꢊꢋ
ꢈꢅꢌꢍ
where AMD,0 is the spontaneous emission probability for the
5D0−7F1 transition in vacuo (14.65 s−1), n is the refractive index
of acetone (1.365), and (Itot / IMD) is the ratio of the total area of
the corrected Eu(III) emission spectrum to the area of the
5D0−7F1 band. The photosensitized quantum yields Φπ-π* were
measured using an emission spectrometer with an integrated
sphere unit (φ = 100 mm). The photosensitized energy transfer
efficiency ηsens was calculated by
ꢟꢠꢡꢠ
∗
ꢜꢝꢞꢛꢝ
=
.
(4)
ꢟꢢꢣꢡꢢꢣ
Figure
5.
Emission
decay
profiles
of
a)
[Eu(hfa)3(DPEPO)] (5.0 × 10-3 M), b) [Eu(btfa)3(DPEPO)] (1.0
× 10-3 M) and c) [Eu(ntfa)3(DPEPO)] (1.0 × 10-4 M) in acetone
at room temperature.
We also found that expansion of the π-conjugated system
leads to an increase of the knr value. Generally, the knr value is
linked to vibrational relaxation of organic ligands and energy
5
back transfer from the D0 state of Eu(III) ions to the lowest
triplet energy state (T1) of the ligand. The T1 level and the
energy gap between the triplet states of Gd complexes and
emitting level of Eu(III) ion with hfa, btfa and ntfa ligands are
summarized in Table 2. The energy gap (EG) between the
triplet states of Gd complexes and emitting level of Eu(III) ions
with hfa, btfa and ntfa ligands are estimated to be 4900, 4100
and 2300 cm-1, respectively (Gd complex being a standard
compound for estimation of T1 energy level of the ligand).26-28
These energy gaps were larger than 2000 cm-1, which is the
critical energy for prevention of energy back transfer.29 We
consider that energy back transfer from Eu(III) ions to T1 of
ligands is not generated. Note that the number of C-H bonds in
the π-conjugated system directly affects the vibrational
relaxation of an excited Eu(III) complex. The numbers of C-H
bonds in hfa, btfa and ntfa were found to be one, six and eight,
respectively, as shown in Figure 6. We suppose that the
π-expanded system in a β-diketonate ligand is related to
magnitude of the vibrational relaxation in an Eu(III) complex.
Figure 4. Emission spectra of a) [Eu(hfa)3(DPEPO)] (5.0
× 10-3 M), b) [Eu(btfa)3(DPEPO)] (1.0 × 10-3 M) and c)
[Eu(ntfa)3(DPEPO)] (1.0 × 10-4 M) in acetone at room
temperature. Emission spectra were normalized at the 5D0→7F1
transition.
The photophysical properties of [Eu(hfa)3(DPEPO)],
[Eu(btfa)3(DPEPO)] and [Eu(ntfa)3 (DPEPO)] are summarized
in Table 2. Expansion of the π-conjugated system contributes to

Table 2. Photophysical properties of Eu(III) complexes in acetone a
Φπ-π* Φ4f-4f ηsens
τobs
(ms)e
1.0
0.91
0.71
kr
knr
T1
EG
(cm-1)f
4900
4100
2300
(%)b
57
(%)c (%)d
(s-1)e
(s-1)e
(cm-1)f
22200
21400
19600
[Eu(hfa)3(DPEPO)]
[Eu(btfa)3(DPEPO)]
[Eu(ntfa)3(DPEPO)]
71
69
58
79
55
77
7.0 × 102
7.6 × 102
8.2 × 102
2.8 × 102
3.4 × 102
5.9 × 102
38
45
a) Concentrations of [Eu(hfa)3(DPEPO)] (5.0 × 10-3 M), [Eu(btfa)3(DPEPO)] (1.0 × 10-3 M) and [Eu(ntfa)3(DPEPO)] (1.0 ×
10-4 M). b) Photosensitized emission quantum yield (excited at 380 nm). c) Φ4f-4f = kr / (kr + knr) (excited at 380 nm). d)
From calculation using Equation (4). e) Emission lifetimes (τobs) of the Eu(III) complexes were measured by excitation at
355 nm (Nd:YAG 3ω). From calculations using Equations (1) – (3). f) See references 26-28.
Photosensitized energy transfer efficiency ηsens of
5
[Eu(ntfa)3(DPEPO)] (ηsens = 77%) is larger than that of
[Eu(btfa)3(DPEPO)] (ηsens = 55%). The value of ηsens is also
influenced by the energy levels of T1 in a β-diketonate ligand
with the π-expanded system. According to the literature, an
Eu(III) ion is able to accept energy with 5D0 (17227 cm-1), 5D1
Figure 7. Energy diagram of Eu ions at the levels of DJ
(J = 0, 1 and 2) and T1 levels of hfa, btfa and ntfa ligands.
4. Conclusion
A lanthanide complex with highly thermostability and
high photosensitized emission quantum yield Φπ-π* is expected
to be an effective luminophore for LED devices. In this study,
[Eu(ntfa)3(DPEPO)] showed a large decomposition temperature
5
(19028 cm-1), and D2 (21519 cm-1).30 An energy diagram of
5DJ (J = 0, 1 and 2) and T1 levels of the ligands are shown in
Figure 7. Efficient energy transfer is generated by location of
5
5
T1 levels on upper DJ. On the other hand, location under DJ
does not promote efficient energy transfer. The ntfa and hfa
ligands are suitable for accepting energy with 5D1 and 5D2,
respectively. Based on the energy transfer process, high energy
transfer efficiencies of [Eu(ntfa)3(DPEPO)] (ηsens = 77%) and
[Eu(hfa)3(DPEPO)] (ηsens = 79%) are achieved. Taking these
structural and photophysical findings into consideration,
[Eu(ntfa)3(DPEPO)] shows excellent thermo-stability and
energy transfer efficiency.
(dp =
318oC) and high photosensitized energy transfer
efficiency (ηsens = 77%). Suitable expansion of the π-conjugated
system promotes high thermostability. This study has shown a
possible wavelength conversion material for LED devices.
Acknowledgement
This work was partly supported by Matching Planer
Program of Japan Science and Technology Agency (JST).
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Graphical Abstract
<Title>
Photosensitized luminescence of highly thermostable mononuclear Eu (III) complexes with π-expanded β-diketonate ligands
<Authors' names>
Toru Koizuka,1, 2 Masanori Yamamoto,2 Yuichi Kitagawa,3 Takayuki Nakanishi,3 Koji Fushimi, 3 and Yasuchika Hasegawa*3
1 Semiconductor Development Division, Nichia Corporation, 1-1 Tatsumi, Anan, Tokushima 774-0001
2
Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita13, Nihi8, Kita-ku, Sapporo, Hokkaido
060-8628
3 Faculty of Engineering, Hokkaido University, Kita13, Nihi8, Kita-ku, Sapporo, Hokkaido 060-8628
<Summary>
Thermostable mononuclear Eu(III) complexes with a π-expanded system, [Eu(btfa)3(DPEPO)] and [Eu(ntfa)3(DPEPO)], are reported
(decomposition temperatures = 320oC and 318oC). The photosensitized emission quantum yield Φπ-π* and photosensitized energy
transfer efficiency ηsens of [Eu(ntfa)3(DPEPO)] (Φπ-π* = 45%, ηsens = 77%) are larger than those of [Eu(btfa)3(DPEPO)] (Φπ-π* = 38%,
η
sens = 55%). [Eu(ntfa)3(DPEPO)] shows excellent thermostability and high ηsens
.
<Diagram>