Synthesis and photophysical properties of Eu(III) complexes with phosphine oxide ligands including metal ions

Article abstract of DOI:10.1246/bcsj.20170241

Lanthanide (Ln³⁺) complexes composed of luminescent Eu³⁺ complex and joint metal blocks (Al³⁺, Zn²⁺ and Pd²⁺ complexes) are reported. The Eu³⁺ complexes [Eu(hfa)_3- (dppy)₂

Full text of DOI:10.1246/bcsj.20170241

Synthesis and Photophysical Properties of Eu(III) Complexes with
Phosphine Oxide Ligands including Metal Ions
Masanori Yamamoto,1 Takayuki Nakanishi,2 Yuichi Kitagawa,2 Tomohiro Seki,2 Hajime Ito,2
Koji Fushimi,2 and Yasuchika Hasegawa*2
1Graduate School of Chemical Sciences and Engineering, Hokkaido University,
Kita 13-Jo, Nishi 8-Chome, Kita-ku, Sapporo, Hokkaido, 060-8628
2Faculty of Engineering, Hokkaido University, Kita-13 Jo, Nishi-8 Chome, Kita-ku, Sapporo, Hokkaido, 060-8628
E-mail: hasegaway@eng.hokudai.ac.jp
Received: July 26, 2017; Accepted: September 22, 2017; Web Released: October 12, 2017
Abstract
Lanthanide (Ln3+) complexes composed of luminescent
Eu3+ complex and joint metal blocks (Al3+, Zn2+ and Pd2+
complexes) are reported. The Eu3+ complexes [Eu(hfa)3-
(dppy)2PdCl2]n (Eu-Pd), [Eu(hfa)3(dppy)2ZnCl2]n (Eu-Zn) and
[Eu(hfa)3(dppy)4AlCl3]n (Eu-Al) (hfa: hexafluoroacetylaceto-
nato, dppy: 4-pyridyldiphenylphosphine oxide) were synthe-
sized by the complexation of [Eu(hfa)3(H2O)2] with [MCln-
(dppy)m] (M = Pd2+, Zn2+ and Al3+). These predicted struc-
tures were estimated using single-crystal X-ray structural ana-
lyses and DFT calculations. Photophysical properties of these
complexes were evaluated based on the emission lifetimes and
emission quantum yields. The emission quantum yields are
dependent on the moiety of the joint metal blocks. Eu-Al com-
plex shows the largest emission quantum yield (Φff = 72%).
In contrast, the emission quantum yield of Eu-Pd is found to
be 1.3%. The structures and photophysical properties of Eu3+
complexes linked with joint metal blocks are demonstrated.
Figure 1. (a) General images of coordination polymers or
MOFs. (b) Structural image of the Eu3+ complex (M =
Pd2+, Zn2+ and Al3+).
1. Introduction
performances for functional molecular materials. Here, we
present luminescent coordination polymers composed of lumi-
nescent and joint metal blocks (Figure 1b). The structural and
physical properties of these coordination polymers can be tuned
by changing the composition of luminescent and joint metal
blocks.
For the luminescent metal block, we selected lanthanide
complexes with 4f-4f transitions. The 4f-4f transitions of lanth-
anide complexes lead to narrow emission bands (FWHM < 10
nm) and long emission lifetime (¸ > 1 ¯s), resulting in the for-
mation of remarkable luminescent materials for various appli-
cations such as display devices and bio-imaging applications.8,9
Strong and characteristic luminescence of lanthanide com-
plexes including Eu3+, Tb3+, Sm3+, Nd3+, Er3+, Yb3+ and Pr3+
ions have been reported.10-17 In earlier studies, the luminescent
properties of lanthanide complexes were shown to depend on
two parameters, radiative and non-radiative rate constants
(kr and knr).18,19 The radiative rate constant depends on the
Three-dimensional building blocks composed of metal ions
and organic bridged ligands are called “coordination poly-
mers” or “metal-organic frameworks”. Transition metal-organic
frameworks (MOFs) with catalytic and ion-sensing prop-
erties have been reported.1-3 Various types of metalloporphyrin
aggregates such as coordination polymers for study of artificial
photosynthesis have also been reported.4,5 Thermostable lumi-
nescent coordination polymers with lanthanide ions have been
presented for future photonic materials.6,7 MOFs and metal
coordination polymers with characteristic chemical and phys-
ical properties can be synthesized by suitable selection of metal
ions and bridged organic ligands (Figure 1a).
Generally, three-dimensional structures of metal coordina-
tion polymers and MOFs are dominated by the coordination
number and geometric structure of the metal ions. The coordi-
nation number and geometric structure in metal coordination
polymers are directly linked to their chemical and physical
6 | Bull. Chem. Soc. Jpn. 2018, 91, 6–11 | doi:10.1246/bcsj.20170241
© 2018 The Chemical Society of Japan

geometric symmetry of the coordination structure in lanthanide
complexes. It has been widely accepted that the radiative tran-
sition probability between 4f orbitals is enhanced by struc-
tures.18 The non-radiative rate constant is influenced by the
vibrational structures of lanthanide complexes. Previously, we
prepared a Eu3+ complex with low-vibrational frequency (LVF)
hexafluoroacetylacetonato (hfa) and phosphine oxide ligands.
The coordination structure composed of LVF hfa (C-F: 1200
cm¹1) and phosphine oxide (P=O: 1125 cm¹1) ligands pro-
vides a luminescent Eu3+ complex with a high emission quan-
tum yield (>65%) and a relatively small non-radiative rate
constant.20,21
In order to construct joint metal blocks, we considered the
use of p- and d-block metal complexes with various coordina-
tion structures. The p- and d-block metal complexes generally
provide four, five and six coordination structures.22-24 Based on
the characteristic coordination structures of p- and d-block
metal complexes, one-, two- and three-dimensional networks
as a joint metal block are promoted for construction of novel
coordination polymer. We here designed some joint metal
blocks composed of metal ions and 4-pyridyldiphenylphos-
phine oxide (dppy). The dppy molecule in joint metal blocks
possesses a phosphine oxide group, which plays an important
role in effective connection with luminescent lanthanide blocks
such as an LVF phosphine oxide. The pyridyl group in dppy
is linked to the p- and d-block metal ions for formation of
joint metal blocks. By using dppy molecules, Pd2+ complexes
[trans-PdCl2(dppy)2], Zn2+ complexes [ZnCl2(dppy)2], and
Al3+ complexes [AlCl3(dppy)4] were prepared as joint metal
blocks.
In this study, we prepared [Eu(hfa)3(dppy)2PdCl2]n (Eu-Pd),
[Eu(hfa)3(dppy)2ZnCl2]n (Eu-Zn), and [Eu(hfa)3(dppy)4AlCl3]n
(Eu-Al). According to the Eu-Zn polymer, we have already
reported.25 We here focus on the heavy element such as Pd2+
ions, and the light element, Al3+ ions, which have higher coor-
dination number than that of Zn2+ ions. We report the effects
on photophysical properties of these Eu3+ coordination poly-
mers by using different metal ions. The predicted structures
were estimated by single-crystal X-ray structural analyses and
DFT calculations. Photophysical properties were evaluated
based on the emission lifetimes and emission quantum yields.
The preparation, structures and photophysical properties of new
Eu3+ coordination polymers linked with joint metal blocks are
described.
formed using a JEOL JMS-T100LP and a JEOL JMS-700TZ,
respectively.
Preparation of 4-Pyridyl Diphenyl Phosphine Oxide
(dppy) C17H14NOP. Dppy was prepared according to litera-
ture references.25 1H NMR (400 MHz, CDCl3/TMS): δ8.74-
8.79 (t, 2H, py), 7.48-7.71 (m, 12H, Ar) ppm. ESI-MS calcd.
for [M + H]+: 280.09 Found, 280.09. Elemental analysis calcd
(%) for C17H14NOP: C 73.11, H 5.05, N 5.02; found: C 73.03,
H 5.07, N 4.95.
Preparation of Pd2+ Complex [trans-PdCl2(dppy)2].
Palladium(II) chloride (88.6 mg, 0.50 mmol) and 4-pyridyl
diphenyl phosphine oxide (0.28 g, 1.00 mmol) were dissolved
in toluene (40 mL). The solution was heated at 100 °C and
refluxed while stirring for 48 h. The solvent was evaporated to
afford a yellow powder. Recrystallization from methanol gave
yellow crystals of the titled compound. (0.27 g, 72%) 1H NMR
(400 MHz, CDCl3/TMS): δ8.92-8.97 (t, 4H, py), 7.49-7.68
(m, 24H, Ar) ppm. ESI-MS calcd. for [M + Cl]+: 770.97
Found, 770.94. Elemental analysis calcd (%) for C34H28Cl2N2-
O8P2Pd+CH3OH: C 54.74, H 4.20, N 3.65; found: C 54.62,
H 3.84, N 3.77.
Preparation of Zn2+ Complex [ZnCl2(dppy)2]. [ZnCl2-
(dppy)2] was prepared according to literature references.25
1H NMR (400 MHz, CDCl3/TMS): δ = 8.84-8.89 (t, 4H, py),
7.71-7.77 (d, 4H, py), 7.62-7.70 (m, 12H, Ar), 7.50-7.57 (m,
¹
8H, Ar) ppm. ESI-MS calcd. for [ZnCl(dppy)2] : 692.03 Found,
692.03. Elemental analysis calcd (%) for C34H28Cl2N2O2P2Zn:
C 58.77, H 4.06, N 4.03; found: C 59.27, H 4.31, N 3.73.
Preparation of Al3+ Complex [AlCl3(dppy)4]. Alumi-
num(III) chloride (74.4 mg, 0.56 mmol) and 4-pyridyl diphenyl
phosphine oxide (0.50 g, 1.81 mmol) were dissolved in chloro-
form (40 mL). The solution was heated at 60 °C and refluxed
while stirring for 24 h. The solvent was evaporated to afford a
white powder of the titled compound. (91.0 mg, 13%) 1H NMR
(400 MHz, MeOD/TMS): δ8.84-8.89 (t, 8H, py), 7.89-7.96
(d, 8H, py), 7.68-7.76 (m, 24H, py), 7.58-7.65 (m, 16H, Ar)
ppm. ESI-MS calcd. for [AlCl3(dppy)4¢5H2O + H]+: 1339.27
Found, 1339.16.
Preparation of Eu-Pd Complex [Eu(hfa)3(dppy)2PdCl2]n.
Pd2+ complex [trans-PdCl2(dppy)2] (1 equiv.) and [Eu(hfa)3-
(H2O)2] (1 equiv.) were dissolved in THF. The solution was
refluxed while stirring for 5 h, and the reaction mixture was
concentrated to dryness. A single crystal suitable for X-ray
structural determination of Eu-Pd complex was obtained by
diffusion of a methanol-chloroform solution at room temper-
ature. FAB-MS calcd. for [Eu(hfa)3(dppy)PdCl2]+: 1300.90
Found, 1300.9. Elemental analysis calcd (%) for C49H31Cl2-
EuF18N2O8P2Pd: C 39.00, H 2.07, N 1.86, Cl 4.70; found:
C 38.83, H 2.41, N 1.80, Cl 4.73.
Preparation of Eu-Zn Complex [Eu(hfa)3(dppy)2ZnCl2]n.
[Eu(hfa)3(dppy)2ZnCl2]n was prepared according to litera-
ture references.25 FAB-MS calcd. for [Eu(hfa)3(dppy)ZnCl2]+:
1258.93 Found, 1259.9. Elemental analysis calcd (%) for
C49H31Cl2EuF18N2O2P2Zn+2H2O: C 39.13, H 2.35, N 1.86;
found: C 38.73, H 2.25, N 1.88.
Preparation of Eu-Al Complex [Eu(hfa)3(dppy)4AlCl3]n.
Al3+ complex [AlCl3(dppy)4] (1 equiv.) and [Eu(hfa)3(H2O)2]
(2 equiv.) were dissolved in methanol. The solution was
refluxed while stirring for 5 h, and the reaction mixture was
2. Experimental
Materials. 4-Bromopyridine hydrochloride (98%), tetrakis-
(triphenylphosphine)palladium (97%), diphenylphosphine
(90%) and hexafluoroacetylacetone (95%) was purchased from
Tokyo Chemical Industry Co., Ltd. Europium(III) acetate n-
hydrate (99.9%), palladium(II) chloride (98%), zinc(II) chlo-
ride (99.9%) and aluminum(III) chloride (99%) were obtained
from Wako Pure Chemical Industries, Ltd. All other chemicals
were reagent grade and were used without further purification.
Apparatus. 1H NMR spectra were recorded on a JEOL
JNM-ECS400 (400 MHz). 1H NMR chemical shifts were deter-
mined with tetramethylsilane (TMS) as an internal standard.
Elemental analyses were performed using a J-SCIENCE
MICRO CORDER JM10. ESI-MS and FAB-MS were per-
Bull. Chem. Soc. Jpn. 2018, 91, 6–11 | doi:10.1246/bcsj.20170241
© 2018 The Chemical Society of Japan | 7

concentrated to dryness. 1H NMR (400 MHz, CDCl3/TMS):
δ8.91-8.73 (br, 8H, py), 8.10-7.36 (br, 48H, Ar), 5.05 (s,
3H, hfa-H) ppm. FAB-MS calcd. for [Eu(hfa)2(dppy)4AlCl2]+:
1952.16 Found, 1952.5.
Computational Details. Density functional theory (DFT)
geometry optimizations of joint Zn2+ block [ZnCl2(dppy)2]
and joint Al3+ block [AlCl3(dppy)4] were carried out with
Gaussian09 Rev D.01 by employing the three-parameter hybrid
functional of Becke based on the correlation functional of Lee,
Yang, and Parr (B3LYP).26,27 The 6-31G(d) basis set was used
for all other atoms.
Crystallography. Colorless single crystals of Eu3+ and
Pd2+ complex were mounted on the MiTeGen micromesh using
paraffin oil. All measurements were made by using a Rigaku
RAXIS RAPID imaging-plate area detector with graphite-
monochromated Mo-Kα radiation. Non-hydrogen atoms were
refined anisotropically. All calculations were performed by
using the crystal-structure crystallographic software package.
CIF data was confirmed by using the checkCIF/PLATON
service. CCDC-1507442 for [Eu(hfa)3(dppy)2] and CCDC-
1510581 for [trans-PdCl2(dppy)2] contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Scheme 1. Synthetic schemes of phosphine oxide ligand and
Eu3+ complexes.
Optical Measurements. Emission and excitation spectra of
Eu3+ complexes were measured with a HORIBA Fluorolog-3
spectrofluorometer and corrected for the response of the detec-
tor system. Emission lifetimes (¸obs) of Eu3+ complexes were
measured 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 Tektronix,
TDS3052, 500 MHz) synchronized to the single-pulse excita-
tion. Emission lifetimes were determined from the slope of
logarithmic plots of the decay profiles.
3. Results and Discussion
Network Structures. 4-pyridyldiphenylphosphine oxide
(dppy) contains selective coordination sites for lanthanide ions
(P=O) and metal ions (N atoms). The dppy was synthesized by
the coupling reaction of 4-bromopyridine hydrochloride with
diphenylphosphine and Pd0 complex as a catalyst, and then
oxidized using hydrogen peroxide. Joint metal blocks were
prepared by the complexation of metal chlorides (palladium,
zinc and aluminum chlorides) with dppy in an organic solvent
under reflux conditions. Finally, [Eu(hfa)3(dppy)2PdCl2]n (Eu-
Pd), [Eu(hfa)3(dppy)2ZnCl2]n (Eu-Zn) and [Eu(hfa)3(dppy)4-
AlCl3]n (Eu-Al) were prepared by the complexation of [Eu-
(hfa)3(H2O)2] with a p- or d-metal joint block (Scheme 1). The
chemical structures were confirmed by MS (ESI- and FAB-MS)
and elemental analyses. Single crystals of [Eu(hfa)3(dppy)2]
and [trans-PdCl2(dppy)2] for X-ray structural analyses were
successfully prepared by recrystallization from solutions in
diethyl ether/hexane and methanol, respectively. However,
single crystals of Eu-Zn and Eu-Al were not obtained. The
structures of [ZnCl2(dppy)2] and [AlCl3(dppy)4] were estimated
using DFT calculation (B3LYP/6-31G(d)) based on previous
X-ray crystal data for a pyridyl metal complex derivative.28,29
Figure 2. Structures of the luminescent Eu3+ block and
joint metal blocks. ORTEP views of (a) luminescent Eu3+
block: [Eu(hfa)3(dppy)2] and (b) joint Pd2+ block: [trans-
PdCl2(dppy)2]. Hydrogen atoms have been omitted for
clarity and thermal ellipsoids are shown at the 50% proba-
bility level. Structure optimizations of (c) joint Zn2+ block:
[ZnCl2(dppy)2] and (d) joint Al3+ block: [AlCl3(dppy)4]
using DFT calculations based on previous X-ray crystal
data for a pyridyl metal complex derivative.28,29
Structural images and crystal data are shown in Figure 2 and
Table S1, respectively. From the result of single-crystal X-ray
diffraction, the coordination site of the luminescent Eu3+ block
[Eu(hfa)3(dppy)2] is comprised of three hexafluoroacetylaceto-
nato (hfa) ligands and two phosphine oxide (dppy) ligands
(Figure 2a). We carried out calculations of the shape measure
factor S to estimate the degree of distortion of the coordination
structure in the Eu3+ coordination sphere. The S value is given
by eq (1):
8 | Bull. Chem. Soc. Jpn. 2018, 91, 6–11 | doi:10.1246/bcsj.20170241
© 2018 The Chemical Society of Japan

sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
ꢁ
X
1
m
2
S ¼ min
ð¤i ꢀ ªiÞ ;
ð1Þ
i¼1
m
where m, ¤i, and ªi are the number of possible edges (m = 18 in
this study), the observed dihedral angle between planes along
the i th edge, and the dihedral angle for the ideal structure,
respectively. According to the crystals data of the luminescent
Eu3+ block [Eu(hfa)3(dppy)2], the S values for 8-SAP (point
group D4d, S = 7.92) are smaller than those for 8-TDH (point
group D2d, S = 9.37), suggesting that the 8-SAP structure
is less distorted than the 8-TDH structure in the systems.
Thus, the coordination geometry of the luminescent Eu3+ block
[Eu(hfa)3(dppy)2] was determined to be 8-SAP like structures
(see Figure S1 and Table S2 in supporting information).
From the result of single-crystal X-ray diffraction, the coor-
dination site of the joint Pd2+ block [trans-PdCl2(dppy)2] is
composed of two chloride ions and two phosphine oxide
ligands (Figure 2b). The Pd2+ complex shows four planar coor-
dination structures without structural distortion, being similar
to those of a previously reported Pd2+ complex.30 From the
structural analysis, we determined that the Pd complex formed
trans-isomer.
Concerning the Zn2+ complex with pyridyl groups, Englert
and co-workers have reported [ZnCl2(3,5-Me2py)2] (3,5-
Me2py: 3,5-dimethylpyridine) with tetrahedral structure.28 We
estimated the structure of our joint Zn2+ block using DFT
calculation based on the previous X-ray single crystal data. The
results of DFT calculation showed that the joint Zn2+ block was
also composed of chloride ions and phosphine oxide ligands
(Figure 2c). The coordination geometry of the joint Zn2+ block
provides a tetrahedral structure.
Figure 3. (a) Emission spectra of Eu3+ complexes (i) Eu-Al
and (ii) Eu-Pd in solid state at room temperature excited
at 350 nm. (b) Emission lifetime decays of Eu3+ complexes
(i) Eu-Al and (ii) Eu-Pd in solid state at room temperature
excited at 355 nm and detected at 610 nm (third harmonics
of Q-switched Nd:YAG laser. FWHM = 5 ns, - = 1064
nm).
Kemniz and co-workers have also described octahedral Al3+
complex [AlBr3(py)4] (py: pyridine). They also reported that
the bromide counter anion existed around Al3+ complex.29
From these results, we assumed that the chloride counter anion
existed around the joint Al3+ block. Thus, the structural opti-
mization of the joint Al3+ block was evaluated by DFT cal-
culation based on the [AlCl2(dppy)4]+. The joint Al3+ block
shows an octahedral structure with three chloride ions and four
phosphine oxide ligands (Figure 2d).
We also consider that the joint Zn2+ block in Eu-Zn is a
tetrahedral structure formed by two chloride ions and two
phosphine oxide ligands, resulting in the formation of a one-
dimensional Eu-Zn polymer structure. The ESI-MS fragment of
Eu-Zn ([Eu(hfa)2(dppy)2ZnCl2]+) is similar to that of the one-
dimensional structure of Eu-Pd ([Eu(hfa)2(dppy)2ZnCl2]+).
From these findings, we consider that Eu-Pd, Eu-Zn and Eu-
Al complexes have a one-dimensional network structure.
Luminescence Properties. The emission spectra for Eu-Al
and Eu-Pd complex in a solid state are shown in Figure 3a.
Emission bands were observed at around 578, 592, 613, 650,
and 698 nm, and they are attributed to 4f-4f transitions of Eu3+
(5D0¼7FJ: J = 0, 1, 2, 3, and 4), respectively. The spectra were
normalized with respect to the magnetic dipole transition
intensities (MD) at 592 nm (5D0-7F1), which are insensitive to
the surrounding environment of the Eu3+ ions. The emission
bands at around 613 nm are due to the electric dipole (ED)
transitions, which are strongly dependent on the coordination
geometry.18 The spectral shapes of emission bands at around
613 nm of Eu-Al and Eu-Pd complex are related to their
coordination geometry. The ED transition intensity of Eu-Al
is larger than those of Eu-Pd. The large ED transition intensity
is linked to the asymmetric structure of Eu-Al and enhanced
kr constant.
Emission lifetimes observed from the 5D0 excited level (¸obs
)
were determined from the slope of the logarithmic decay pro-
files (Figure 3b). The time-resolved emission profile of Eu-Al
shows a single exponential decay with millisecond-scale life-
time. The emission lifetime for Eu-Al was determined to be
0.80 ms. In contrast, the time-resolved emission profile of Eu-
Pd also shows a single exponential decay with microsecond-
scale lifetime. The emission lifetime for Eu-Pd was determined
to be 22 ¯s.
The values of kr, knr, and Φff were calculated using ¸obs and
emission spectra by the following equations.31,32
kr
¸
¸
obs
ꢀff
¼
¼
ð2Þ
ð3Þ
ð4Þ
ð5Þ
kr þ knr
¼ AMDn3
rad
ꢀ
ꢁ
1
Itot
¸
IMD
rad
1
kr ¼
¸
rad
1
1
knr
¼
ꢀ
;
¸
¸
rad
obs
where AMD is the spontaneous emission probability for
5D0¼7F1 transition in vacuo (14.65 s¹1), n is the refractive
index of the medium (An average index of refraction equal to
Bull. Chem. Soc. Jpn. 2018, 91, 6–11 | doi:10.1246/bcsj.20170241
© 2018 The Chemical Society of Japan | 9

Table 1. Photophysical properties of Eu3+ complexes in the
solid state
[a]
[b]
[c]
[d]
obs
[f]
Φff
%
Φππ
%
©
¸
kr[e]
knr
sens
¹1
¹1
%
¯s
s
s
Eu-Al
Eu-Pd
Eu-Zn[h] 59
72
1.3
52
®
28
72
®
48
800 9.0 © 102 3.5 © 102
22 6.0 © 102 4.5 © 104
850 7.0 © 102 4.8 © 102
[g]
[a] The intrinsic emission quantum yields were calculated from
Φff = kr/(kr + knr) = ¸obs/¸rad. [b] The total emission quantum
yields Φtot were measured by integrating sphere (excitation at
370 nm). [c] The photo-sensitization efficiencies were calcu-
lated from ©sens = Φtot/Φff. [d] Emission lifetime (¸obs) of Eu3+
complexes were measured by excitation at 355 nm (Nd:YAG
3ω). [e] Radiative rate constants (kr) for Eu3+ complexes
estimated from 1/¸rad = AMDn3(Itot/IMD).31,32 [f] Non-radiative
rate constant knr = 1/¸obs ¹ 1/¸rad. [g] We could not observe.
Emission was very weak. [h] Ref. 25.
Figure 4. Energy diagram of Eu3+ complexes linked with
joint Pd2+ blocks (EnT: energy transfer).
complex is caused by the energy transfer from luminescent
Eu3+ blocks to joint Pd2+ blocks.
It is well known that the radiative and non-radiative rate
constants of a lanthanide complex are dependent on the organic
ligand or the solvent molecules.35 In this study, all of the Eu3+
coordination sites in Eu3+ complexes were attached to three
hfa and two dppy ligands. The photophysical properties of the
Eu3+ complexes could be linked to moieties of metal ions
(steric and electronic properties) in the joint metal blocks.
We also calculated the photo-sensitized energy transfer effi-
ciency, ©sens based on the Φff and Φππ*. In Table 1, ©sens of Eu-
1.5 was employed.),33 and (Itot/IMD) is the ratio of the total
area of the corrected Eu3+ emission spectrum to the area of
the 5D0¼7F1 band. The photophysical properties of Eu3+
complexes linked with joint metal blocks are summarized in
Table 1. The emission quantum yield of Eu-Al and Eu-Pd were
estimated to be 72% and 1.3%, respectively. The Eu-Al com-
plex shows a large emission quantum yield (Φff = 72%), which
is based on the large kr and small knr. The large kr of Eu-Al
is most likely caused by the enhancement of the ED transition
based on the asymmetric coordination geometry. In the pre-
vious study, we found that the radiative rate constant kr of
luminescent Eu3+ complexes was enhanced by the asymmetric
coordination geometry with various types of phosphine ligands,
and these Stark splittings in emission spectra are dependent on
the steric structures of lanthanide complexes with phosphine
ligands.21 It should be noted that the coordination site of Eu3+
ions of Eu-Al, Eu-Zn, and Eu-Pd are similar structure, which
are comprised of three hfa ligands and two dppy ligands. Thus,
the radiative rate constants kr of these Eu3+ complexes were
expected to be similar value. We observed that kr of Eu-Al is
larger than that of Eu-Pd, although the Stark splitting shape of
Eu-Al is similar to that of Eu-Pd (see Figure S2 in supporting
information). From these photophysical findings, the large kr
of Eu-Al seems to be affected by not only the asymmetric
coordination geometry but also electronic structure of joint
Al3+ parts. The small knr of Eu-Al is probably caused by the
rigid network structure. The emission quantum yield of Eu-Al
is larger than those of Eu-Zn and Eu-Pd complex.
Al (©sens = 72%) is much larger than those of Eu-Zn (©sens
=
48%). The ©sens of Eu-Al is also much larger from that of
previous [Eu(hfa)3(dpbp)2]n (©sens = 40%).36 We consider that
the polymeric structure effect from rigid zig-zag structure led to
37
enhanced ©sens
.
4. Conclusion
We have synthesized Eu3+ complex composed of lumines-
cent Eu3+ and joint Al3+ and trans-Pd2+ complexes. The struc-
tures and photophysical properties of these Eu3+ complexes are
dependent on the joint metal blocks. The emission quantum
yield of the Eu-Al complex is the largest in these Eu3+ complex
linked with joint metal blocks.
We consider that the network structure of the Eu-Al complex
based on the coordination structure of joint Al3+ blocks leads
to a large radiative rate constant and small non-radiative rate
constant, resulting in a high emission quantum yield (Φff =
72%). The high triplet energy level of joint metal blocks such
as Al3+ ions might promote effective luminescence. In contrast,
the emission quantum yields of Eu-Pd was found to be 1.3%.
The small emission quantum yield of the Eu-Pd complex is
caused by the energy transfer from luminescent Eu3+ ions to
joint Pd2+ complexes. In this study, we provide a new design
of Eu3+ complex linked with joint metal blocks for control of
geometric structure and photophysical properties.
The Eu-Pd complex shows a much smaller emission quan-
tum yield. We propose that both the characteristic emission
lifetime profile and smaller quantum yield of Eu-Pd are caused
by the excited quenching from the emitting level of Eu3+ ions.
The weak emission spectrum of joint Pd2+ block [trans-
PdCl2(dppy)2] at 88 K was observed at around 760 nm, which is
attributed to MLCT transitions from the Pd2+ ion to phosphine
oxide ligands (see Figure S3 in supporting information).34 The
energy transfer diagram of Eu3+ complex linked with Pd2+
complexes is shown in Figure 4. The energy level of MLCT
This work was supported by a Grant-in-Aid for Scientific
Research on Innovative Area of “New Polymeric Materials
Based on Element-Blocks” from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT), Japan (grant
number 2401). This present research was supported by the
Ministry of Education, Culture, Sports, Science and Technol-
5
of the Pd2+ complex is lower than that of the excited D0 state
of Eu3+ ions. The small emission quantum yield of the Eu-Pd
10 | Bull. Chem. Soc. Jpn. 2018, 91, 6–11 | doi:10.1246/bcsj.20170241
© 2018 The Chemical Society of Japan

J.-C. Bünzli, S. V. Eliseeva, O. Guillou, Eur. J. Inorg. Chem. 2013,
3464.
ogy through Program for Leading Graduate Schools (Hokkaido
University “Ambitious Leader’s Program”).
16 L. Song, J. Wang, J. Hu, X. Liu, Z. Zhen, J. Alloys Compd.
2009, 473, 201.
Supporting Information
17 A. Ishii, S. Kishi, H. Ohtsu, T. Iimori, T. Nakabayashi, N.
Ohta, N. Tamai, M. Melnik, M. Hasegawa, Y. Shigesato,
ChemPhysChem 2007, 8, 1345.
18 Y. Hasegawa, Y. Wada, S. Yanagida, J. Photochem.
Photobiol., C 2004, 5, 183.
An additional experimental results including single-crystal
X-ray structure analyses, shape-measure calculations, emission
spectra, and diffuse reflection spectra. This material is available
on http://dx.doi.org/10.1246/bcsj.20170241.
19 A. F. Kirby, F. S. Richardson, J. Phys. Chem. 1983, 87,
2544.
References
1
H.-J. Son, S. Jin, S. Patwardhan, S. J. Wezenberg, N. C.
20 K. Nakamura, Y. Hasegawa, H. Kawai, N. Yasuda, N.
Kanehisa, Y. Kai, T. Nagamura, S. Yanagida, Y. Wada, J. Phys.
Chem. A 2007, 111, 3029.
Jeong, M. So, C. E. Wilmer, A. A. Sarjeant, G. C. Schatz, R. Q.
Snurr, O. K. Farha, G. P. Wiederrecht, J. T. Hupp, J. Am. Chem.
Soc. 2013, 135, 862.
21 K. Miyata, T. Nakagawa, R. Kawakami, Y. Kita, K.
Sugimoto, T. Nakashima, T. Harada, T. Kawai, Y. Hasegawa,
Chem.®Eur. J. 2011, 17, 521.
2
3
T. Zhang, W. Lin, Chem. Soc. Rev. 2014, 43, 5982.
J. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen,
J. T. Hupp, Chem. Soc. Rev. 2009, 38, 1450.
B. Chen, L. Wang, F. Zapata, G. Qian, E. B. Lobkovsky,
J. Am. Chem. Soc. 2008, 130, 6718.
22 J. Ni, Y.-G. Wang, H.-H. Wang, L. Xu, Y.-Q. Zhao, Y.-Z.
Pan, J.-J. Zhang, Dalton Trans. 2014, 43, 352.
23 A. Rana, P. K. Panda, Chem. Commun. 2015, 51, 12239.
24 M. Sjödin, J. Gätjens, L. C. Tabares, P. Thuéry, V. L.
Pecoraro, S. Un, Inorg. Chem. 2008, 47, 2897.
25 M. Yamamoto, T. Nakanishi, Y. Kitagawa, K. Fushimi, Y.
Hasegwa, Mater. Lett. 2016, 167, 183.
4
5
9232.
6
J. Heine, K. Müller-Buschbaum, Chem. Soc. Rev. 2013, 42,
H. Zhang, L. Zhou, J. Wei, Z. Li, P. Lin, S. Du, J. Mater.
Chem. 2012, 22, 21210.
7
K. Miyata, Y. Konno, T. Nakanishi, A. Kobayashi, M.
26 C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785.
27 A. D. Becke, J. Chem. Phys. 1993, 98, 5648.
28 C. Hu, U. Englert, CrystEngComm 2002, 4, 20.
29 A. Dimitrov, D. Heidemann, K. I. Khallow, E. Kemnitz,
Inorg. Chem. 2012, 51, 11612.
Kato, K. Fushimi, Y. Hasegawa, Angew. Chem., Int. Ed. 2013, 52,
6413.
8
G. Shao, H. Yu, N. Zhang, Y. He, K. Feng, X. Yang, R.
Cao, M. Gong, Phys. Chem. Chem. Phys. 2014, 16, 695.
S. J. Butler, L. Lamarque, R. Pal, D. Parker, Chem. Sci.
9
30 G. Dahm, E. Borré, C. Fu, S. Bellemin-Laponnaz, M.
Mauro, Chem.®Asian J. 2015, 10, 2368.
2014, 5, 1750.
10 N. M. Shavaleev, S. V. Eliseeva, R. Scopelliti, J.-C. G.
Bünzli, Chem.®Eur. J. 2009, 15, 10790.
31 L. Prodi, M. Maestri, R. Ziessel, V. Balzani, Inorg. Chem.
1991, 30, 3798.
11 W.-S. Lo, J. Zhang, W.-T. Wong, G.-L. Law, Inorg. Chem.
2015, 54, 3725.
32 M. H. V. Werts, R. T. F. Jukes, J. W. Verhoeven, Phys.
Chem. Chem. Phys. 2002, 4, 1542.
12 O. A. Blackburn, N. F. Chilton, K. Keller, C. E. Tait, W. K.
Myers, E. J. Mclnnes, A. M. Kenwright, P. D. Beer, C. R. Timmel,
S. Faulkner, Angew. Chem., Int. Ed. 2015, 127, 10933.
13 L. J. Daumann, D. S. Tatum, B. E. R. Snyder, C. Ni, G.
Law, E. I. Solomon, K. N. Reymond, J. Am. Chem. Soc. 2015, 137,
2816.
33 R. Pavithran, N. S. Saleesh Kumar, S. Biju, M. L. P. Reddy,
S. A. Junior, R. O. Freire, Inorg. Chem. 2006, 45, 2184.
34 A. Krogul, J. Cedrowski, K. Wiktorska, W. P. Ozimiński, J.
Skupińska, G. Litwinienko, Dalton Trans. 2012, 41, 658.
35 M. Morikawa, S. Tsunofuri, N. Kimizuka, Langmuir 2013,
29, 12930.
14 M. M. Nolasco, P. M. Vaz, V. T. Freitas, P. P. Lima, P. S.
André, R. A. S. Ferreira, P. D. Vaz, P. Ribeiro-Claro, L. D. Carlos,
J. Mater. Chem. A 2013, 1, 7339.
15 V. Haquin, M. Etienne, C. Daiguebonne, S. Freslon, G.
Calvez, K. Bernot, L. L. Pollès, S. E. Ashbrook, M. R. Mitchell,
36 K. Miyata, T. Ohba, A. Kobayashi, M. Kato, T. Nakanishi,
K. Fushimi, Y. Hasegawa, ChemPlusChem 2012, 77, 277.
37 Y. Hirai, T. Nakanishi, Y. Kitagawa, K. Fushimi, T. Seki, H.
Ito, Y. Hasegawa, Angew. Chem., Int. Ed. 2016, 55, 12059.
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