Red Luminescent Eu(III) Coordination Bricks Excited on Blue LED Chip

Article abstract of DOI:10.1021/acs.inorgchem.8b00805

Three types of red luminescent Eu(III) complexes with Schiff base and hfa ligands (hfa: hexafluoroacetylacetonate), mononuclear [Eu(hfa)₂(OAc)(salen)₂] (OAc: acetate anion, salen: N,N′-bis(salicylidene)ethylenediamine), brick-type [Eu₂(hfa)₄(OAc)₂(salbn)₂] (salbn: N,N′-bis(salicylidene)-1,4-butanediamine), and polynuclear [Eu(hfa)₂(OAc)(salhen)]_n (salhen: N,N′-bis(salicylidene)-1,6-hexanediamine) are reported for white light-emitting diode (LED) devices. Among these complexes, brick-type [Eu₂(hfa)₄(OAc)₂(salbn)₂] excited by blue light (460 nm) exhibits the photosensitized quantum yield (φ_π-π? = 47%) and remarkably high efficiency of sensitization (η_sens = 96%). The efficiency of sensitization is caused by the excited state based on ligand-ligand interaction between the Schiff base and hfa ligands in Eu(III) complexes. To fabricate LED devices, the red luminescent [Eu₂(hfa)₄(OAc)₂(salbn)₂] was mounted on an InGaN blue LED chip.

Full text of DOI:10.1021/acs.inorgchem.8b00805

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Red Luminescent Eu(III) Coordination Bricks Excited on Blue LED
Chip
Toru Koizuka,†,‡ Kei Yanagisawa,‡ Yuichi Hirai,‡ Yuichi Kitagawa,§ Takayuki Nakanishi,§
Koji Fushimi,§ and Yasuchika Hasegawa*,§
†Semiconductor Development Division, Nichia Corporation, 1-1 Tatsumi, Anan, Tokushima 774-0001, Japan
‡Graduate School of Chemical Sciences and Engineering and §Faculty of Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku,
Sapporo 060-8628, Hokkaido, Japan
S
* Supporting Information
ABSTRACT: Three types of red luminescent Eu(III)
complexes with Schiff base and hfa ligands (hfa: hexafluor-
oacetylacetonate), mononuclear [Eu(hfa)2(OAc)(salen)2]
(OAc: acetate anion, salen: N,N′-bis(salicylidene)-
ethylenediamine), brick-type [Eu2(hfa)4(OAc)2(salbn)2]
(salbn: N,N′-bis(salicylidene)-1,4-butanediamine), and poly-
nuclear [Eu(hfa)2(OAc)(salhen)]n (salhen: N,N′-bis-
(salicylidene)-1,6-hexanediamine) are reported for white
light-emitting diode (LED) devices. Among these complexes,
brick-type [Eu2(hfa)4(OAc)2(salbn)2] excited by blue light
(460 nm) exhibits the photosensitized quantum yield (Φπ−π*
47%) and remarkably high efficiency of sensitization (ηsens
=
=
96%). The efficiency of sensitization is caused by the excited state based on ligand−ligand interaction between the Schiff base and
hfa ligands in Eu(III) complexes. To fabricate LED devices, the red luminescent [Eu2(hfa)4(OAc)2(salbn)2] was mounted on an
InGaN blue LED chip.
luminescence of lanthanide(III) ions, various organic chromo-
phores such as β-diketonates and pyridine-based ligands in
Eu(III) complexes have been developed.14−21 These organic
chromophores with π-conjugation systems are referred to as
light-harvesting antenna ligands. In a previous study, Gong
reported a red luminescent Eu(III) complex containing β-
diketonate ligands with carbazole units excited by blue LED
(emission quantum yield = 16%).22 Expanded π-conjugation in
β-diketonate ligands promotes energy back transfer from
Eu(III) ion to the triplet state of the ligand, which results in
a decrease of the emission quantum yield.23,24 The triplet state
level of the β-diketonate ligand with carbazole units is estimated
to be 18 800 cm−1, which is close to the emitting levels in
Eu(III) ion (17 000 cm−1).22,25 Photosensitized units with a
higher triplet state (≥19 000 cm−1) should be effective for
suppressing of the energy back transfer. We reported strongly
luminescent Eu(III) complexes and coordination polymers
composed of low vibrational hfa ligands (hfa: hexafluoroace-
tylacetonate, small nonradiatve rate constants = knr, emission
quantum yields > 70%).26,27 The Eu(III) complexes with hfa
ligands suppress the energy back transfer (triplet state ≥ 22 400
cm−1), which promotes efficient photosensitization and high
emission quantum yields. The hfa ligand in Eu(III) complex
absorbs the UV light (absorbance band < 400 nm), which is not
INTRODUCTION
■
White light-emitting diodes (white LEDs) are a key device in
recent industrial applications for general lighting and backlight
displays as an environmentally friendly light source in the 21st
century.1−3 White LEDs are generally fabricated as one of two
types. Type A combines green and red phosphors with a blue
LED, while Type B combines red, green, and blue phosphors
with a UV LED (Figure 1).4−9 At present, photodegradation of
encapsulated and packaging materials (silicone, epoxy,
polyester, and polyamide derivatives) is of concern in the
LED applications.10 Qiu and co-workers reported the
degradation of silicone and epoxy molding compounds under
UV irradiation.11 Lin and co-workers pointed out the
importance of photodegradation of bisphenol-type resin in
UV LEDs.12 For these reasons, white LED combining green
and red phosphors with blue LEDs have been mainly
commercialized as practical light sources. The developments
of brilliant green and red phosphors excited by a blue LED
(wavelength: 450−470 nm) is required for fabrication of high-
efficiency white LED devices.
For the red phosphor, the narrow 4f−4f transition of Eu(III)
ion (5D0 → 7F2: 615 nm, full width at half-maximum (fwhm) =
15 nm) is a promising candidate for backlight display devices,
although broad 4f−5d transitions of Eu(II) ions in inorganic
matrices (e.g, CaAlSiN3:Eu) have been applied for general
lighting. Europium(III) ion, however, has a small absorption
coefficient (ε < 1 M−1 cm−1 at 465 nm).13 To sensitize the
Received: March 26, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b00805
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Inorganic Chemistry
Article
anediamine) to obtain red phosphors under blue LED
excitation (Figure 2). Their geometrical structures were
characterized using X-ray single-crystal analysis. The emission
quantum yie lds of [Eu(hfa)2 (OAc)(salen)2 ],
[Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)(salhen)]n
excited at 460 nm were calculated to be 27%, 47%, and 16%,
respectively. The brick-type [Eu2(hfa)4(OAc)2(salbn)2] showed
bright luminescence excited at 460 nm with a high efficiency of
sensitization (ηsens = 96%). The highly efficient luminescence of
the Eu(III) complex with salbn ligands is based on LLI between
the Schiff base and hfa ligands. Red luminescence was also
successfully observed from [Eu2(hfa)4(OAc)2(salbn)2] on an
InGaN blue chip LED device. In this study, the efficient red
luminescence of the brick-type Eu(III) complex with blue LED
was achieved for the first time.
EXPERIMENTAL SECTION
■
Materials. Europium(III) acetate n-hydrate (99.9%) was purchased
from Wako Pure Chemical Industries Ltd. Hexafluoroacetylacetone,
1,4-butanediamine, 1,6-diaminohexane, salicylaldehyde, and N,N′-
bis(salicylidene)ethylenediamine were obtained from Tokyo Chemical
Industry Co., Ltd. All other solvents were reagent grade and used
without further purification.
Figure 1. Phosphor-converted white LED: Type A blue LED with red
and green phosphors, Type B UV LED with red, green, and blue
phosphors.
1
Apparatus. H NMR (400 MHz) spectra were obtained using a
JEOL ECS400 spectrometer; CD2Cl2 (δH = 5.32 ppm) was used as
internal reference. Elemental analyses were performed on an Exeter
Analytical CE440. Infrared spectra were recorded with a JASCO FTIR-
4600 spectrometer.
excited using the blue light (λ: 450−470 nm). The π-
conjugated antenna ligands with higher triplet states and
absorption bands in the blue light region are thus desirable for
A-type white LEDs (Figure 1).
Preparation of N,N′-Bis(salicylidene)-1,4-butanediamine
(salbn). Salicylaldehyde (2.4 g, 20.0 mmol) and 1,4-diaminobutane
(0.88 g, 10.0 mmol) were dissolved in ethanol (40 mL). The solution
was refluxed for 2 h with stirring. The reaction solution was cooled at 0
°C. The resulting precipitate was separated by filtration, washed with
cool ethanol, and dried under vacuum. The yellow powder was
obtained.
Here, we focus on a new combination of photosensitized
Schiff base and low vibrational hfa ligands to achieve a strong
luminescent Eu(III) complex under blue light irradiation.28−32
The energy level of triplet state in the salen-type Schiff base
ligand (salen: N,N′-bis(salicylidene)ethylenediamine) is esti-
mated to be more than 20 000 cm−1, which is larger than that of
Yield: 2.6 g (88%); 1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 13.47
(broad, OH), 8.37 (s, 2H; imine-H), 7.31−7.26 (m, 4H; Ar), 6.91−
6.87 (m, 4H; Ar), 3.64 (t, 4H; N−CH2), 1.80 (m, 4H; −CH2) ppm.
IR (KBr): 2946−2849, 1634, 1284 cm−1. Anal. Calcd (%) for
C18H20N2O2: C, 72.95; H, 6.80; N, 9.45. Found: C, 73.11; H, 6.80; N,
9.46.
5
the D0 state in the Eu(III) ion (emitting level ≅ 17 000
cm−1).25,32 A specific excited state based on ligand−ligand
interaction (LLI) between the Schiff base and hfa ligands in the
Eu(III) complex is expected to provide bright luminescence
excited at 460 nm (blue light). Three types of Eu(III)
complexes were prepared with Schiff base and hfa ligands,
mononuclear [Eu(hfa)2(OAc)(salen)2] (OAc: acetate anion),
brick-type [Eu2(hfa)4(OAc)2(salbn)2] (salbn: N,N′-bis-
(salicylidene)-1,4-butanediamine), and polynuclear [Eu-
(hfa)2(OAc)(salhen)]n (salhen: N,N′-bis(salicylidene)-1,6-hex-
Preparation of N,N′-Bis(salicylidene)-1,6-hexanediamine
(salhen). Salicylaldehyde (4.9 g, 40.0 mmol) and 1,6-diaminohexane
(2.3 g, 20.0 mmol) were dissolved in ethanol (60 mL). The solution
was refluxed for 2 h with stirring. The reaction solution was cooled at 0
°C. The resulting precipitate was separated by filtration, washed with
cool ethanol, and dried under vacuum. The yellow powder was
obtained.
Figure 2. Chemical structure of (a) [Eu(hfa)2(OAc)(salen)2], (b) [Eu2(hfa)4(OAc)2(salbn)2], and (c) [Eu(hfa)2(OAc)(salhen)]n.
B
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Yield: 6.0 g (92%); 1H NMR (400 MHz, CD2Cl2, 20 °C): δ = 13.57
(broad, OH), 8.35 (s, 2H; imine-H), 7.31−7.25 (m, 4H; Ar), 6.91−
6.86 (m, 4H; Ar), 3.59 (t, 4H; N−CH2), 1.71 (m, 4H; −CH2), 1.44
(m, 4H; −CH2) ppm. IR (KBr): 2934−2859, 1634, 1281 cm−1. Anal.
Calcd (%) for C20H24N2O2: C, 74.05; H, 7.48; N, 8.63. Found: C,
74.14; H, 7.50; N, 8.62.
1
1
τrad
knr
=
−
τ
(3)
obs
kr
kr + knr
τ
τrad
obs
Φ
=
=
4f−4f
(4)
where AMD,0
has a value of 14.65 s−1, which is the spontaneous
Preparation of [Eu(hfa)2(OAc)(salen)2]. Europium acetate n-
hydrate (0.50 g, 1.4 mmol) was dissolved in ethanol (40 mL). A
solution of hfa (0.90 g, 4.3 mmol) was added to the solution. Salen
(0.77 g, 2.9 mmol) was added to the solution and stirred until
dissolved. After the solution was stirred at room temperature for 24 h,
the precipitates were separated by filtration, washed with water several
times, and dried under vacuum. The yellow powder was obtained.
Recrystallization from ethanol (80 vol %) and dichloromethane (20
vol %) gave yellow needle crystals of the europium complex.
[Eu(hfa)2(OAc)(salen)2]. Yield: 0.93 g (56%); Anal. Calcd (%) for
C44H37EuF12N4O10: C, 45.49; H, 3.21; N, 4.82. Found: C, 45.70; H,
3.17; N, 4.79. IR (KBr): 2954−2852, 1653, 1630, 1610, 1538, 1280,
1256, 1218−1202, 1146 cm−1.
Preparation of [Eu2(hfa)4(OAc)2(salbn)2]. Europium acetate n-
hydrate (1.0 g, 2.9 mmol) was dissolved in ethanol (60 mL). A
solution of hfa (1.2 g, 5.8 mmol) was added to the solution. After the
solution was stirred at room temperature for 1 h, salbn (0.85 g 2.9
mmol) was added in the solution. After the solution stirred at room
temperature for 24 h, the precipitates were separated by filtration and
washed with diethyl ether. The precipitates were filtered and dried
under vacuum. The yellow powder was obtained. Recrystallization
from tetrahydrofuran (THF; 40 vol %) and hexane (60 vol %) gave
yellow block crystals of the europium complex.
emission probability for the 5D0−7F1 transition in vacuo, n is the
refractive index of silicone resin (1.5), and (Itot/IMD) is the ratio of the
5
total area of the Eu(III) emission bands to the area of the D0−7F1
band.33
Emission lifetimes (τobs) were determined using a Q-switched
Nd:YAG laser pulsed at 355 nm (Spectra Physics, INDI-50, fwhm = 5
ns, λ = 1064 nm) and a photomultiplier tube (Hamamatsu Photonics,
R5108, response time = 1.1 ns). The Nd:YAG laser response was
displayed on a digital oscilloscope (Sony Tektronix, TDS3052, 500
MHz) synchronized to the single-pulse excitation. Emission lifetimes
were estimated by the slope of logarithmic plots of the decay profiles.
The photosensitized emission quantum yields (Φπ−π*) were
measured using a JASCO F-6300-H spectrometer equipped with an
integrating sphere unit (JASCO ILF-533, φ = 100 mm). The efficiency
of sensitization (ηsens) was given by
Φ
*
π−π
η
=
sens
Φ
(5)
4f−4f
Computational Details. Time-dependent density functional
theory (TD-DFT) calculations were performed with the Gaussian
09 program.34−37 The initial structures of Schiff base and hfa ligands
were determined from the crystallographic data of the Eu(III)
complexes.
Single-Crystal X-ray Crystallography. The crystal of the
complexes was mounted on a MiTeGen micromesh using paraffin
oil. All measurements were performed on a Rigaku R-AXIS RAPID
diffractometer using graphite monochromated Mo Kα radiation at
−150 °C. An empirical absorption correction was applied, and the data
were corrected for Lorentz and polarization effects. The structures
were solved by direct methods using SHELXS-9738 and expanded
using Fourier techniques. Structural refinements were performed by
the full-matrix least-squares techniques. All non-hydrogen atoms were
refined anisotropically, and hydrogen atoms were refined using riding
model.
Fabrication of LED Package. An InGaN blue LED as a side-view
type of LED package (Nichia Corporation) was fabricated for liquid-
crystal display. Silicone resin with 7 wt % [Eu2(hfa)4(OAc)2(salbn)2]
was used for molding resin on LED chip. The resin including Eu(III)
complex was degassed and poured into the LED package. After the
resin was injected, the LED was heated at 150 °C for 5 h to harden the
resins.
[Eu2(hfa)4(OAc)2(salbn)2]. Yield: 1.8 g (34%); Anal. Calcd (%) for
C60H50Eu2F24N4O16: C, 39.10; H, 2.73; N, 3.04. Found: C, 39.25; H,
2.62; N, 2.97. IR (KBr): 2980−2863, 1654, 1610, 1550, 1478, 1255,
1211, 1146 cm−1.
Preparation of [Eu(hfa)2(OAc)(salhen)]n. Europium acetate n-
hydrate (5.0 g, 14 mmol) was dissolved in water (40 mL). A solution
of hfa (9.0 g, 43 mmol) was added to the solution. After the solution
was stirred at room temperature for 4 h, precipitates were separated by
filtration and dried under vacuum. White solid was obtained. Prepared
white solid (0.8 g, 1.0 mmol) was dissolved in diethyl ether (40 mL).
Salhen (0.65 g, 2.0 mmol) was added to the solution. After the
solution was stirred at room temperature for 24 h, the precipitates
were filtered and dried under vacuum. The yellow powder was
obtained. Recrystallization from ethanol (60 vol %) and dichloro-
methane (40 vol %) gave yellow plate crystals of the europium
complex.
[Eu(hfa)2(OAc)(salhen)]n: Yield: 0.90 g (95%); Anal. Calcd (%)
for C32H29EuF12N2O8: C, 40.48; H, 3.08; N, 2.95. Found: C, 40.34; H,
2.96; N, 2.88. IR (KBr): 2950−2860, 1653, 1610, 1539, 1479, 1254,
1202, 1143 cm−1.
Optical Measurements. Kubelka−Munk spectra of the Eu(III)
complexes were measured using a JASCO V-670 spectrometer with an
integrating sphere unit. Kubelka−Munk function is given as
RESULTS AND DISCUSSION
■
Coordination Structures. Three Eu(III) complexes were
synthesized by the chelation of europium acetate (Eu(OAc)3·
nH2O) with hfa and Schiff base ligands (salen, salbn, and
salhen) at room temperature. (Figure 2). Single crystals of
[Eu(hfa)2(OAc)(salen)2] and [Eu(hfa)2(OAc)(salhen)]n were
prepared by recrystallization from ethanol and dichloro-
methane. A single crystal of [Eu2(hfa)4(OAc)2(salbn)2] was
also obtained by recrystallization from THF and hexane.
Crystallographic data and perspective views of Eu(III)
complexes with hfa and Schiff base ligands are shown in
Table S1 and Figure 3.39 The perspective views of each Eu(III)
complexes show eight-coordinated structures. The three
complexes are coordinated by one anionic OAc molecule,
two hfa ligands, and two Schiff base ligands in the Eu(III)
coordination unit. The selected Eu−O coordination bond
lengths of the three Eu(III) complexes are described in Table
S2. The average Eu−O distances of the Eu-Schiff base ligand in
(1 − R∞)2
k
s
=
, R∞ = R∞,sample/R∞,std
2R∞
(1)
where k is the absorption coefficient of sample, s is the scattering
coefficient of sample, and R∞ is the diffuse reflectance of the sample
with respect to BaSO4. The R∞,sample (powder of Eu(III) complexes)
were measured without BaSO4.
Emission and excitation spectra for ligands, Gd(III) complexes, and
Eu(III) complexes were recorded on a Fluorolog-3 spectrofluorometer
and corrected for the detector system (HORIBA). Radiative rate
constant (kr) and nonradiative rate constant (knr) of Eu(III) complexes
were calculated using the following equations:
⎛
⎜
⎝
⎞
⎟
⎠
Itot
1
τrad
kr = AMD,0n3
=
I
(2)
MD
C
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Inorganic Chemistry
Article
Table 1. Average Distance of Eu−O Bond Length
Eu-Schiff (Å) Eu-hfa (Å) Eu-OAc (Å)
[Eu(hfa)2(OAc)(salen)2]
[Eu2(hfa)4(OAc)2(salbn)2]
[Eu(hfa)2(OAc)(salhen)]n
average
2.296
2.281
2.267
2.281
2.426
2.431
2.425
2.427
2.436
2.473
2.447
2.452
complexes, intermolecular CH/F and CH/π interactions were
observed. Characteristic intermolecular π−π interactions were
also identified in the brick-type [Eu2(hfa)4(OAc)2(salbn)2]
complex, which promote a tight packing structure in a solid
(Figure 3d, Figure S1).
On the basis of the crystal data, we calculated the continuous
shape measures factor S to estimate the degree of distortion of
the coordination structure in the first coordination sphere. The
S value is given by
∑kN |Qk − p |2
k
S = min
× 100
∑kN |Qk − Q 0|2
(6)
where N is the number of vertices, Qk is the vertices of an actual
structure, Q0 is the center of mass of an actual structure, and Pk
is the vertices of an ideal structure.40,41 Eight-coordinated
lanthanide complexes generally exhibit trigonal dodecahedron
(TDH), square antiprism (SAP), or biaugmented trigonal
prism (BTP) structures, according to evaluation of the S values
(Table S3). In [Eu2(hfa)4(OAc)2(salbn)2], the STDH value
(1.609) was smaller than that of SSAP (3.653) and SBTP (2.196).
The geometrical structure of [Eu2(hfa)4(OAc)2(salbn)2] is
categorized as a distorted TDH. The S values of mononuclear
[Eu(hfa)2(OAc)(salen)2] are estimated to be 2.571 (SSAP),
2.144 (STDH), and 2.491 (SBTP), respectively. The S values of
polynuclear [Eu(hfa)2(OAc)(salhen)]n were also calculated to
be 2.112 (SSAP), 2.141 (STDH) and 1.861 (SBTP), respectively.
We cannot find a significant difference between SSAP, STDH, and
S
BTP in mononuclear [Eu(hfa)2(OAc)(salen)2] and polynuclear
[Eu(hfa)2(OAc)(salhen)]n.
Photophysical Properties. The emission and excitation
spectra of Eu(III) complexes in solid state are shown in Figure
4. The emission bands at ∼578, 592, 613, 650, and 698 nm
were assigned to be 4f−4f transitions of Eu(III) ions (5D0−7FJ:
J = 0, 1, 2, 3, and 4, respectively). The emission spectra were
normalized with respect to the spectral area of the magnetic-
5
dipole transition at ∼592 nm (Eu: D0−7F1). In the excitation
Figure 3. Perspective views of (a) [Eu(hfa)2(OAc)(salen)2], (b)
[Eu2(hfa)4(OAc)2(salbn)2], (c) [Eu(hfa)2(OAc)(salhen)]n, and (d)
i n t e r m o l e c u l a r π − π i n t e r a c t i o n s ( b l u e l i n e ) o f
[Eu2(hfa)4(OAc)2(salbn)2] with the coordination geometry of each
Eu(III) ion showing 50% probability displacement ellipsoids. Hydro-
gen atoms were omitted for clarity.
mononuclear [Eu(hfa)2(OAc)(salen)2], brick-type
[Eu2(hfa)4(OAc)2(salbn)2], and polynuclear [Eu(hfa)2(OAc)-
(salhen)]n were determined to be 2.296, 2.281, and 2.267 Å,
respectively, which are shorter than those of Eu-hfa (2.427 Å)
and Eu-OAc (2.452 Å) (Table 1). These results indicate that
Eu(III) complexes with OAc, hfa, and Schiff base ligands have
asymmetric coordination geometry with different Eu−O bond
lengths. According to the specific interactions in Eu(III)
Figure 4. Excitation and emission spectra of (a) [Eu(hfa)2(OAc)-
(salen)2] (blue line), (b) [Eu2(hfa)4(OAc)2(salbn)2] (red line), and
(c) [Eu(hfa)2(OAc)(salhen)]n (green line) excited at 460 nm in solid
state.
D
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Table 2. Photophysical Properties of Eu(III) Complexes
a
b
b
b
b
c
Φπ−π* (%)
Φ4f−4f (%)
ηsens (%)
kr (s−1
)
knr (s−1
)
τobs (ms)
[Eu(hfa)2(OAc)(salen)2]
[Eu2(hfa)4(OAc)2(salbn)2]
[Eu(hfa)2(OAc)(salhen)]n
27
47
16
37
49
30
75
96
55
8.4 × 102
8.7 × 102
8.8 × 102
1.4 × 103
9.3 × 102
2.1 × 103
0.44
0.56
0.34
a
b
c
Φπ−π* of the Eu(III) complexes were measured by excitation at 460 nm. From calculation using eqs 2−5. τobs of the Eu(III) complexes were
measured by excitation at 355 nm.
spectra, characteristic broad bands at ∼450 nm were attributed
to the combination band between the Schiff base and hfa
ligands. Significantly broad excitation bands are useful for
efficient luminescence from Eu(III) complexes under blue LED
irradiation. All complexes also exhibit 4f−4f transition of
Eu(III) at ∼532 nm in the excitation spectra. The diffuse
reflectance spectra are shown in Figure S2. We found that the
emission spectral shapes of the Eu(III) complexes are much
different from those of previous reported eight-coordinated
Eu(III) complexes (Figure S3). Generally, emission spectral
shape of Eu(III) complex is dependent on the coordination
geometry. The characteristic spectral shapes of [Eu-
(hfa)2(OAc)(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu-
(hfa)2(OAc)(salhen)]n are caused by asymmetric structures
with different Eu−O bond lengths (Eu-Schiff: 2.28 Å, Eu-hfa:
2.43 Å, and Eu-OAC: 2.45 Å). To clarify packing structure
interaction in crystal, we compared emission spectrum of
[Eu2(hfa)4(OAc)2(salbn)2] in solid with that in solution state
(Figure S4). The emission shape of [Eu2(hfa)4(OAc)2(salbn)2]
in THF is much different from that in solid state. These results
indicate that tight packing interactions in crystal influences the
photophysical properties, directly.
The luminescence decay curves of Eu(III) complexes in solid
state revealed single exponential decays with millisecond-scale
lifetimes. The emission lifetimes were determined from the
slopes of the logarithmic plots of the profiles (Figure S5). The
emission quantum yields of the 4f−4f transitions in Eu(III)
complexes were calculated using the emission lifetime and
radiative rate constant estimated from emission spectra. The
photosensitized quantum yields excited at π−π* transitions
were also measured using an integrating sphere.
Figure 5. (a) Molecular orbitals related to T1 state of hfa and Schiff
base in [Eu2(hfa)4(OAc)2(salbn)2] based on the TD-DFT (CAM-
B3LYP/6-31G+ (d, p)) and (b) LLI image of brick-type
[Eu2(hfa)4(OAc)2(salbn)2].
CAM-B3LYP/6-31G+ (d, p), we found that
[Eu2(hfa)4(OAc)2(salbn)2] shows the lowest triplet state with
LLI (Figure 5a). On the other hand, the orbitals related to
lowest triplet state of [Eu(hfa)2(OAc)(salen)2] and [Eu-
(hfa)2(OAc)(salhen)]n do not exhibit LLI (Figure S6). We
also observed T2 level transition; [Eu(hfa)2(OAc)(salen)2]
shows LLI (Figure S7 and Table S5). To explain LLI
experimentally, we measured UV−vis and phosphorescence
spectra of salen, salbn, salhen, Gd(III) complexes, and Eu(III)
complexes (Figures S8−S12). We observed new absorption
edges at ∼500 nm in UV−vis spectra of Eu(III) complexes.
The different shapes of phosphorescence spectra between
Gd(III) complexes and ligands indicated the presence of triplet
states with LLI. Trivedi and co-workers reported that near-
infrared lanthanide metallacrown complexes exhibited signifi-
cantly high quantum yields based on efficient sensitization due
to the intra- and/or interligand charge transfer.42 Liu and co-
workers achieved high emission quantum yield of tetranuclear
Eu(III) complex with combination of intraligand charge
transfer (ILCT) sensitization.43 The new combination of Schiff
base and hfa ligands in Eu(III) complexes led to the formation
of the LLI, which results in high efficiency of sensitization.
Finally, we demonstrate d red lumine scent
[Eu2(hfa)4(OAc)2(salbn)2] was applied to an LED device
(Figure 6). The LED operating at 20 mA exhibited bright pink
color emission, as shown in Figure 6a. The radiant flux was 1.9
lm. The emission spectrum of the LED package is shown in
Figure 6b. Blue emission (InGaN) and red emission bands
(Eu(III) complex) were observed. The CIE 1931 chromaticity
The photophysical properties (kr, knr, Φ4f‑4f, Φπ−π*, and ηsens
)
of [Eu(hfa)2(OAc)(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and
[Eu(hfa)2(OAc)(salhen)]n are summarized in Table 2. The
emission quantum yields Φ4f−4f value of [Eu(hfa)2(OAc)-
(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)-
(salhen)]n are 37, 49, and 30%, respectively. The knr value of
brick-type [Eu2(hfa)4(OAc)2(salbn)2] is smaller than that of
[Eu(hfa)2(OAc)(salen)2] and [Eu(hfa)2(OAc)(salhen)]n. The
tight packing structure of [Eu2(hfa)4(OAc)2(salbn)2] promotes
suppression of the radiationless transitions via vibrational
relaxation. The small knr in [Eu2(hfa)4(OAc)2(salbn)2] leads to
high Φ4f−4f. The Φπ−π* value of [Eu2(hfa)4(OAc)2(salbn)2]
(47%) is larger than those of [Eu(hfa)2(OAc)(salen)2] and
[Eu(hfa)2(OAc)(salhen)]n. On the basis of the calculation of
Φ4f−4f and Φπ−π*, Eu(III) complexes with the salen, salbn, and
salhen promote highly efficient sensitization. In particular, the
brick-type structure of [Eu2(hfa)4(OAc)2(salbn)2] shows a high
efficiency of sensitization (ηsens = 96%).
To reveal mechanism for the efficient sensitization of Eu(III)
complexes with Schiff base and hfa ligands, we performed TD-
DFT calculations for hfa and Schiff base ligands (CAM-
B3LYP/6-31G+ (d, p), Figures 5, S6, and S7 and Tables S4 and
S5).34−37 On the one hand, according to calculation using
E
DOI: 10.1021/acs.inorgchem.8b00805
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(salhen)]n, respectively. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*Phone/Fax: +81 11 706 7114. E-mail: hasegaway@eng.
Figure 6. (a) Luminescence image of LED package
([Eu2(hfa)4(OAc)2(salbn)2]/InGaN chip) and (b) emission spectrum
of LED package.
hokudai.ac.jp.
ORCID
Yuichi Kitagawa: 0000-0003-1487-2531
Takayuki Nakanishi: 0000-0003-3412-2842
Yasuchika Hasegawa: 0000-0002-6622-8011
xy-coordinates (x, y = 0.29, 0.11) were calculated using the
emission spectrum. Gong and co-worker also fabricated 5 wt %
Eu(III) complex containing β-diketonate ligands with carbazole
units dispersed in silicone gel.22 They observed red luminescent
Eu(III) complex excited by InGaN blue LED operating at 20
mA. The emission quantum yield of Eu(III) complex
containing β-diketonate ligands with carbazole units was
estimated to be 16% excited 460 nm. We consider that Eu(III)
complex with Schiff base and hfa ligand promotes large
emission quantum yields, resulting in successful red lumines-
cence excited by blue LED chip. The new combination of salbn
and hfa in a Eu(III) complex thus provided efficient
luminescence under blue LED irradiation.
Present Address
∥Faculty of Engineering, Hokkaido University.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank A. Suzuki of Frontier Chemistry Center “Labo-
ratories for Future Creation” Project.
REFERENCES
■
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CONCLUSION
■
In this study, novel Eu(III) complexes were successfully
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blue light irradiation (460 nm). With the LLI between the
Schiff base and hfa ligands, high efficiency of sensitization and
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b00805.
■
S
Intermolecular CH/F interactions and CH/π interac-
tions; diffuse reflectance spectra; emission spectra of
[Eu2(hfa)4(OAc)2(salbn)2] and [Eu(hfa)3(tppo)2]; emis-
sion spectra of [Eu2(hfa)4(OAc)2(salbn)2] in solid state
and THF solution; emission decay profiles; molecular
orbitals rerated to the T1 and T2 state of Schiff base and
hfa ligands in Eu(III) complexes; absorption spectra of
[Gd(hfa)3(H2O)2], Eu(III) complexes, and ligands in
THF solution; synthesis and phosphorescence spectra of
Gd(III) complexes and ligands in solid state; calculated
oscillator strengths (PDF)
Accession Codes
CCDC 1824941, 1824944, and 1825354 contain the
supplementary crystallographic data for [Eu(hfa)2(OAc)-
(salen)2], [Eu2(hfa)4(OAc)2(salbn)2], and [Eu(hfa)2(OAc)-
F
DOI: 10.1021/acs.inorgchem.8b00805
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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