Enhanced Luminescence of Asymmetrical Seven-Coordinate EuIII Complexes Including LMCT Perturbation

Article abstract of DOI:10.1002/ejic.201700815

Luminescent mononuclear seven-coordinate Eu^III complexes, with monocapped-octahedral (point group: C_3v), monocapped-trigonal-prismatic (C_2v), and pentagonal-bipyramidal (D_5h) coordination structures, are reporte

Full text of DOI:10.1002/ejic.201700815

A Journal of
Accepted Article
Title: Enhanced Luminescence of Asymmetrical Seven-coordinate EuIII
Complexes Including LMCT Perturbation
Authors: Kei Yanagisawa, Yuichi Kitagawa, Takayuki Nakanishi,
Tomoko Akama, Masato Kobayashi, Tomohiro Seki, Koji
Fushimi, Hajime Ito, Tetsuya Taketsugu, and Yasuchika
Hasegawa
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201700815
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10.1002/ejic.201700815
European Journal of Inorganic Chemistry
FULL PAPER
Enhanced Luminescence of Asymmetrical Seven-coordinate EuIII
Complexes Including LMCT Perturbation
Kei Yanagisawa,[b] Yuichi Kitagawa,[a] Takayuki Nakanishi,[a] Tomoko Akama,[c] Masato Kobayashi,[c]
Tomohiro Seki,[a] Koji Fushimi,[a] Hajime Ito,[a] Tetsuya Taketsugu,[c] and Yasuchika Hasegawa*[a]
Abstract: Luminescent mononuclear seven-coordinate EuIII
focus on luminescent lanthanide complexes for study on the
complexes with monocapped-octahedral (point group: C3v),
monocapped-trigonalprismatic (C2v), and pentagonal-bipyramidal
(D5h) coordination structures are reported. The complexes consist of
a EuIII ion, three tetramethylheptanedionates, and a phosphine oxide
derivative. Controlling steric hindrance by means of introducing
methyl groups into phosphine oxide ligands resulted in the formation
of three types of coordination polyhedral structures. The coordination
geometrical structures of the EuIII complexes were evaluated by
single-crystal X-ray analysis and shape-factor calculation. The
radiative rate constant of the EuIII complex with a monocapped-
octahedral structure was larger than those with monocapped-
relationships between photophysical properties and coordination
geometrical structures.
Luminescence of a lanthanide complex arises from 4f-
intraconfigurational transitions. The magnitude of the radiative
rate constant (kr) of 4f-4f transition is influenced by the
coordination polyhedral structures. Since electric dipole
transition of lanthanide complex is forbidden, a mixed parity
state induced by ligand field perturbation is required to observe
4f-4f transition. A lanthanide complex has large coordination
number (generally eight to twelve) and especially tends to form
an eight-coordinate square-antiprismatic structure (8-SAP,
D4d).[13-18] We have reported that the luminescent EuIII complexes
with non-centrosymmetrical coordination structures such as
nine-coordinate monocapped-square-antiprism (9-SAP, C4v) and
eight-coordinate trigonal-dodecahedron (8-TDH, D2d) showed
enhanced 4f-4f transition probability based on the Laporte
rule.[19,20] Considering these photophysical findings, coordination
polyhedral structures of lanthanide complexes should be
strongly correlated with enhancement of luminescence
efficiency.
trigonalprismatic
and
pentagonal-bipyramidal
structures.
Characteristic photophysical properties of the seven-coordinate EuIII
complexes are discussed with TD-DFT calculation and Arrhenius
analysis of ligand-to-metal charge transfer.
Introduction
Luminescent metal complexes have been studied due to
their versatile potential applications as optical materials,[1,2]
OLEDs,[3,4] and fluorescent sensors.[5,6] The photophysical
properties of metal complexes are strongly affected by organic
ligands and the coordination structure. There have been a large
number of studies on control of the MLCT emission wavelengths
of IrIII and RuII complexes.[7,8] The luminescence properties of AuI
and PtII complexes are dependent on the characteristic metal-
metal interaction and metal-metal-to-ligand charge transfer.[9,10]
Intrinsic emission quantum yields of lanthanide (NdIII, SmIII, EuIII,
TbIII, DyIII, YbIII, and so on) complexes are also influenced by the
geometrical structures in the coordination spheres.[11,12] We here
Recently, we reported seven-coordinate EuIII and TbIII
complexes with a monocapped-octahedral structure (7-MCO).[21]
The 7-MCO structure is categorized as C3v symmetry, which is
an asymmetrical structure compared with 8-SAP, 8-TDH, and 9-
SAP structures. The EuIII complex with a 7-MCO structure
accordingly showed larger kr value than that with a 8-SAP
structure. In terms of geometrical structure, representative
seven-coordination polyhedron includes not only a 7-MCO
structure but also monocapped-trigonalprismatic (7-MCTP, C2v)
and pentagonal-bipyramidal structures (7-PBP, D5h) as shown in
Figures 1a-c.[22] Results of photophysical studies on seven-
coordinate lanthanide complexes with 7-MCO, 7-MCTP, and 7-
PBP structures are expected to provide a new design for highly
emissive and monochromatic luminescent materials.
[a]
Dr. Y. Kitagawa, Dr. T. Nakanishi, Dr. T. Seki, Prof. K. Fushimi, Prof.
H. Ito, Prof. Y. Hasegawa
In this study, we synthesized three types of novel seven-
coordinate EuIII complexes composed of three β-diketonate
ligands (tetramethylheptanedionate: tmh) and a phosphine oxide
ligand (diphenyl(p-tolyl)phosphine oxide: L-1, tri-p-tolylphosphine
oxide: L-2, or tri-m-tolylphosphine oxide: L-3) as shown in Figure
1d ([Eu(tmh)3(L-1)]: Eu-1, [Eu(tmh)3(L-2)]: Eu-2, and
[Eu(tmh)3(L-3)]: Eu-3). The geometrical structures of the
prepared EuIII complexes were characterized using single-crystal
X-ray analysis and shape-measure calculation. Emission
properties were evaluated using intrinsic emission quantum yield
Φff, total emission quantum yield Φtot, emission lifetime τobs, and
kr.
Faculty of Engineering
Hokkaido University
Kita-13 Jo, Nishi-8 Chome, Kita-ku, Sapporo, Hokkaido, 060-8628
(Japan)
E-mail: hasegaway@eng.hokudai.ac.jp
Mr. K. Yanagisawa
Graduate School of Chemical Sciences and Engineering
Hokkaido University
Kita-13 Jo, Nishi-8 Chome, Kita-ku, Sapporo, Hokkaido 060-8628
(Japan)
Dr. T. Akama, Dr. M. Kobayashi, Prof. T. Taketsugu
Faculty of Science
Hokkaido University
[b]
[c]
Kita-10 Jo, Nishi-8 Chome, Kita-ku, Sapporo, Hokkaido, 060-0810
(Japan)
In our experiments, we found that the kr value of Eu-1 with
a 7-MCO (C3v) was larger than those of Eu-2 with a 7-MCTP
structure (C2v) and Eu-3 with a 7-PBP structure (D5h), although
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700815
European Journal of Inorganic Chemistry
FULL PAPER
the low-symmetrical coordination geometry are expected to give
rise to large kr because of deviation from centrosymmetric
phosphine oxide ligand (Figures 2a-c). The average bond
lengths between Eu and O (β-diketonate) for Eu-1, Eu-2, and
Eu-3 were 2.33, 2.33, and 2.34 Å, respectively. The bond
lengths are similar to the Eu-O (phosphine oxide) bonds of Eu-1
(2.34 Å), Eu-2 (2.34 Å), and Eu-3 (2.32 Å). The Eu-O (β-
diketonate) bond lengths were shorter than that for previously
geometry[23]
. According to a previous report, considering
perturbation of charge-transfer state into 4f configuration,
oscillator strength of 4f-4f transition is related to the energy level
and dipole strength of charge-transfer transition.[24] Recently,
Hatanaka and Yabushita also reported that hypersensitive
transition probability of lanthanide complexes was theoretically
affected by the ligand-to-metal charge transfer (LMCT) state.[25]
We here discuss enhancement of the kr value related to the
characteristic LMCT state for the seven-coordinate EuIII
complexes. The LMCT states were evaluated using diffuse
reflection spectra, TD-DFT calculations, and Arrhenius analyses
of emission lifetimes. The effect on LMCT perturbation in the
EuIII complexes with 7-MCO, 7-MCTP, and 7-PBP is
demonstrated for the first time.
reported
eight-coordinate
EuIII
complex
with
hexafluoroacetylacetonate (hfa) and triphenylphosphine oxide
(tppo), [Eu(hfa)3(tppo)2] (the average bond length between Eu
and O: 2.42 Å (β-diketonate) and 2.33 Å (phosphine oxide)). [12]
Calculated Mulliken charges at O atoms in diketonate ligands
were slightly smaller than that in phosphine oxide ligand for all
the EuIII complexes. The values of Mulliken charge were
summarized in Table S1.
Table 1. Crystallographic data of Eu-1, Eu-2, and Eu-3.
Eu-1
Eu-2
Eu-3
chemical formula
crystal system
space group
a [Å]
C52H74EuO7P
monoclinic
P 21/n
C54H78EuO7P
orthorhombic
P 212121
16.6636(3)
17.9698(3)
18.5330(3)
—
C54H78EuO7P
monoclinic
P 21/n
11.2648(10)
21.922(2)
22.090(2)
104.8796(17)
5271.8(8)
4
13.5503(3)
24.5056(6)
16.7914(4)
99.7464(7)
5495.3(2)
4
b [Å]
c [Å]
β [deg]
V [Å3]
5549.55(16)
4
Z
dcalcd [g cm-1]
T [K]
1.252
1.223
1.235
Figure 1. Seven-coordinate polyhedral structures, a) monocapped-
octahedron, b) monocapped-trigonalprism, and c) pentagonal-bipyramid. d)
Chemical structure of the EuIII complex.
123
123
123
µ(MoKα) [cm-1]
measured reflections
unique reflections
R[a]
1.266
1.204
1.216
50489
73005
65353
Results and Discussion
12107
12685
12586
Seven-coordinate EuIII complexes (Eu-1, Eu-2, and Eu-3)
were synthesized by complexation of a precursor EuIII complex,
[Eu(tmh)3(MeOH)2][21] with three types of phosphine oxide ligand
(L-1, L-2, and L-3) in methanol under reflux for 6 h. Seven-
coordinate TbIII complexes (Tb-1, Tb-2, and Tb-3) without LMCT
band were prepared by the same method for comparison with
the EuIII complexes including LMCT band (see supporting
information, Figure S1). A GdIII complex, [Gd2(tmh)6], was also
synthesized to estimate energy level of excited triplet state of
tmh ligand. They were identified using IR and elemental
analysis. Single crystals of the lanthanide complexes were
prepared by recrystallization from methanol at room
temperature. The crystal data for EuIII complexes obtained from
single-crystal X-ray analysis are summarized in Table 1. The
ORTEP views of the complexes exhibit seven-coordinate
structures comprised of one EuIII ion, three tmh ligands and one
0.0572
0.0245
0.1146
0.0362
wR[b]
0.1346
0.1024
[a] R = Σ||F0|—|Fc|| / Σ|F0|. [b] wR = [(Σw(|F0|—|Fc|)2 / ΣwF02)]1/2
.
Based on the crystal structures, we calculated shape factor
S in order to determine the coordination geometrical structures
around EuIII ions.[26] The S value is given by
S = min√[(1/m)Σi=1m(δi — θi)2], (1)
where m is the number of possible edges (m = 15 in this study),
δi is the observed dihedral angle between planes along the ith
edge, and θi is the dihedral angle of the ith edge for the ideal
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10.1002/ejic.201700815
European Journal of Inorganic Chemistry
FULL PAPER
structure. The estimated S values of the EuIII complexes are
summarized in Tables S2-S4. From these calculations, we
categorized the pseudo coordination polyhedral structures of Eu-
1, Eu-2, and Eu-3 as 7-MCO, 7-MCTP, and 7-PBP, respectively
(Figures 2d-f). The distance between Eu and O atoms is
dependent on the molecular structure. In this study, the Eu-O
distances for Eu-1, Eu-2, and Eu-3 were found to be 2.33-2.34
Å. The distance of Eu-O are same as an ideal geometry.
values. The values of τobs
,
Φff, Φtot, kr, and estimated
coordination geometrical structures of the EuIII complexes are
summarized in Table 2. The magnitudes of Φtot of the EuIII
complexes were less than 1%, which is due to the inefficient
photosensitized energy transfer from organic ligands to EuIII ion.
The measured Φff values of Eu-1, Eu-2, and Eu-3 were 82%,
85%, and 86%, respectively. These were as high as those of
recently reported lanthanide complexes with efficient
luminescence.[28] The kr value for Eu-1 was larger than those for
Eu-2 and Eu-3.
Table 2. Photophysical properties and coordination geometrical
structures of Eu-1, Eu-2, and Eu-3.
[a]
[b]
[b]
τobs
Φff
Φtot
[%]
kr[c]
geometrical
structure
complex
[ms]
[%]
[s-1]
Eu-1
Eu-2
Eu-3
0.50
0.73
0.61
82
85
86
0.5
0.6
0.6
1.6×103
1.1×103
1.4×103
7-MCO (C3v)
7-MCTP (C2v
7-PBP (D5h
)
)
[a] Emission lifetimes (τobs) were measured by excitation at 355 nm
(Nd:YAG, third harmonics). [b] Emission quantum yields excited at 532
nm (Φff) and 365 nm (Φtot) were measured using an integrating sphere
unit. [c] Radiative rate constants, kr = Φff /τobs
.
Figure 2. ORTEP drawings of a) Eu-1, b) Eu-2, and c) Eu-3. Hydrogen atoms
were omitted for clarity. Thermal ellipsoids were shown at the 50% probability
level. Coordination geometrical structures of d) Eu-1 (7-MCO), e) Eu-2 (7-
MCTP), and f) Eu-3 (7-PBP).
Excitation and emission spectra of the EuIII complexes in
the solid state are shown in Figure 3a. We observed
distinguishable 4f-intraconfigurational excitation bands, 7F0 and
7F1 to 5L6, 5D3, 5D2, 5D1, and 5D0. Emission bands were observed
at 578, 592, 612, 653, and 700 nm, being attributed to 4f-4f
transitions of EuIII ion (5D0→7F0-4). The emission spectra were
normalized with respect to the spectral area of magnetic dipole
[27]
transitions (5D0→7F1).
The inset of Figure 3a shows the
emission spectra of hypersensitive 5D0→7F2 transition of the
complexes. The characteristic Stark splittings and spectral
shapes of 5D0→7F2 transition are attributed to the coordination
structures. The energy width of Stark splitting of 7F2 sublevels for
Eu-1 (430 cm-1) was slightly larger than those for Eu-2 (425 cm-
1) and Eu-3 (417 cm-1). Accurate energy values of the Stark
5
splitting of D0→7F0-2 for the EuIII complexes are given in Figure
S2 and Table S5.
Time-resolved emission profiles of the EuIII complexes
revealed single-exponential decay with a sub-millisecond-scale
lifetime as shown in Figures 3b-d. The emission lifetimes were
determined from the slopes of logarithmic plots of the decay
profiles. The observed emission lifetimes (τobs) were 0.50, 0.73,
and 0.61 ms for Eu-1, Eu-2, and Eu-3, respectively. Emission
quantum yields (Φff) excited at 532 nm (5D1←7FJ) and total
emission quantum yields (Φtot) excited at 365 nm of the EuIII
complexes were measured using an integrating sphere attached
to a photomultiplier. We calculated kr based on the τobs and Φff
Figure 3. a) Excitation and emission spectra of Eu-1 (black line), Eu-2 (blue
line), and Eu-3 (red line) in the solid state at room temperature. Excitation
spectra were recorded with emission at 611 nm (5D0→7F2). Emission spectra
were excited at 464 nm (5D2←7F0). Emission decay profiles of (b) Eu-1, (c) Eu-
2, and (d) Eu-3 in the solid state excited at 355 nm (Nd:YAG laser).
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10.1002/ejic.201700815
European Journal of Inorganic Chemistry
FULL PAPER
The radiative process is affected by the LMCT states of
EuIII complexes.[24] In order to analyze the radiative process of
the seven-coordinate EuIII complexes, we measured diffuse
reflection spectra for observation of their LMCT bands (Figure
4a). Absorption bands at around 320 nm (31250 cm-1) were
assigned to singlet π-π and/or σ-π* transition of tmh ligands,
which was confirmed by the absorption spectrum of
[Gd(tmh)3(MeOH)2] in methanol solution (see supporting
information, Figure S3) and TD-DFT calculation (Tables S6-S8).
We also observed absorption bands at around 370 nm (27000
cm-1) for the EuIII complexes. The absorption band at 370 nm for
TbIII complexes was not observed (Figure 4b). The other
photophysical results of the TbIII complexes were shown in
Figure S4. Considering absorption spectra of EuIII and TbIII
complexes, the absorption bands at around 370 nm are
assumed to be LMCT band. In addition, absorption wavelength
and oscillator strength of LMCT transition of EuIII complexes
were estimated using TD-DFT calculation. (see supporting
information, Tables S6-S8). Estimated excitations of σ→4f (Eu)
and π→4f (Eu) were clearly observed at UV region, which were
categorized as LMCT absorption.
lifetime of EuIII complexes should be due to state transition
(charge transfer) from 5D0 excited state to LMCT state rather
than vibrational relaxation (5D0→7F0) and back-energy transfer
(5D0→T1). In the temperature-dependent emission lifetime of the
EuIII complexes, activation energy Ea and frequency factor A
were estimated from Arrhenius plot of charge-transfer rate kCT
from 5D0 to LMCT state (Figure 6b, Table 3).
Figure 5. Schematic energy diagram of the EuIII complexes
Table 3. Arrhenius parameters of charge-transfer process from 5D0
configuration to LMCT state and estimated wavelength and oscillator strength
of LMCT absorption edge.
[a]
[c]
Ea
A
λCT,edge
[nm]
(ECT)-3PCT
[cm3]
[b]
complex
PCT, edge
[cm-1]
[s-1]
Eu-1
Eu-2
Eu-3
3.5×103
3.3×103
3.9×103
8.0×108
1.3×108
9.8×108
392
423
394
0.0034
0.0005
0.0028
2.0×10-16
3.8×10-17
1.7×10-16
Figure 4. Diffuse reflection spectra of a) the EuIII complexes and b) the TbIII
complexes. Eu-1 and Tb-1 (black lines), Eu-2 and Tb-2 (blue lines) and Eu-3
and Tb-3 (red lines).
[a] Wavelength and [b] oscillator strength of LMCT absorption edge obtained
from TD-DFT calculation. [c] ECT=1/λCT
.
The values of Ea for Eu-1, Eu-2, and Eu-3 were found to be
3.5×103, 3.3×103, and 3.9×103 cm-1, respectively. The Ea is
related to the charge transfer process from 5D0 to LMCT state.
The Ea of Eu-2 was slightly smaller than those of Eu-1 and Eu-3.
The A value of Eu-2 (1.3×108 s-1) was also smaller than those of
Eu-1 (8.0×108 s-1) and Eu-3 (9.8×108 s-1). Since A includes
electronic frequency νel, the value relates to electronic coupling
A schematic energy diagram of the seven-coordinate EuIII
complexes is shown in Figure 5. An excited singlet state (S1)
generally undergoes intersystem crossing to a low-lying excited
triplet state (T1) due to large spin-orbit interaction of lanthanide
complexes. The energy level of T1 (410 nm, ~24400 cm-1) was
also confirmed by a phosphorescence spectrum of [Gd2(tmh)6]
at 100 K (see supporting information, Figure S5). The T1 level is
much higher than 5D0 emitting level. The large energy gap is
expected to suppress back-energy transfer from 5D0
configuration to T1 state.[29] We herein considered that low Φtot is
affected by presence of LMCT state.
5
of D0 configuration with LMCT state.[30] Therefore, the degree of
electronic coupling between 5D0 and LMCT state for Eu-2 is
smaller than those of corresponding Eu-1 and Eu-3.
According to the influence on LMCT in lanthanide complex,
Henrie reported an equation for 4f-4f transition probability Pff
involving perturbation of charge-transfer transition.[24] The
equation is given by
Temperature-dependent emission lifetimes of EuIII
complexes were measured for investigation of photophysical
interaction between 5D0 configuration and LMCT state (Figure
6a). The emission lifetimes of EuIII complexes decreased above
300 K, while we found that the emission lifetimes of TbIII
complexes were constant in the range of 100-350 K (see
supporting information, Figure S6). The decrease in emission
Pff = a2Eff(ECT)-3PCT
,
(2)
where P is oscillator strength and E is energy level. The
subscripts ff and CT represent 4f-4f and charge-transfer
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10.1002/ejic.201700815
European Journal of Inorganic Chemistry
FULL PAPER
Materials: Europium chloride hexahydrate (99.9%), terbium chloride
pentahydrate (99.9%), and gadolinium chloride hexahydrate (99.9%)
were purchased from Kanto Chemical Co., Inc. 2,2,6,6-
Tetramethylheptane-3,5-dion (tmh), diphenyl(p-tolyl)phosphine, tri-p-
tolylphosphine, and tri-m-tolylphosphine were obtained from Tokyo Kasei
Organic Chemicals. Ammonia aqueous solution (28%) and H2O2
aqueous solution (30%) were purchased from Wako Pure Chemical
Industries Ltd. All other chemicals and solvents were reagent-grade and
were used without further purification.
transition, respectively. The a is matrix elements of odd-parity
vibrations and the ligand field that mix the charge-transfer level
-3
with the 4f level. Note that the (ECT
)
P
term is proportional to
CT
the oscillator strength of 4f-4f transition. The values of ECT, PCT
,
and (ECT)-3PCT of LMCT absorption edge obtained from TD-DFT
calculation were also summarized in Table 3. From this
calculation, we found that the value of Eu-1 was larger than
those of Eu-2 and Eu-3. We here assume that the enhanced kr
of Eu-1 was influenced by the large oscillator strength of LMCT
transition. Considering the results described above, the largest kr
of Eu-1 was caused by not only low-symmetrical coordination
geometry (C3v) but also relatively large perturbation of LMCT
state into 4f configuration. We propose that the enhancement of
kr would be influenced by LMCT perturbation rather than
coordination polyhedral structure. In this study, the 7-MCO
structure with large LMCT perturbation provided the highest kr in
the seven-coordinate EuIII complexes.
Apparatus: Elemental analyses were performed with an Exeter
Analytical CE440. Infrared spectra were recorded with a JASCO FT/IR-
4600 spectrometer. 1H NMR (270 and 400 MHz) spectra were recorded
on a JEOL EX270 and ECS400. Chemical shifts were reported in δ ppm,
which is referenced to an internal tetramethylsilane (TMS) standard.
Syntheses of diphenyl(p-tolyl)phosphine oxide (L-1), tri-p-
tolylphosphine oxide (L-2), and tri-m-tolylphosphine oxide(L-3):
Diphenyl(p-tolyl)phosphine (1.93 g, 7.0 mmol), tri-p-tolylphosphine (2.13
g, 7.0 mmol), or tri-m-tolylphosphine (2.13 g, 7.0 mmol) was dissolved
with dichloromethane (30 mL) in a 100 mL flask. The solution was cooled
using an ice bath and then H2O2 solution (5 mL) was added dropwise to
it. The reaction mixture was stirred for 3 h. The product was extracted
with dichloromethane, and the solvent was evaporated to afford a white
solid of titled compounds (diphenyl(p-tolyl)phosphine oxide: L-1, tri-p-
tolylphosphine oxide: L-2, and tri-m-tolylphosphine oxide: L-3).
L-1: Yield 1.99 g (97%); 1H NMR (270 MHz, CDCl3, 25 ℃): δ=7.25-7.70
(m, 14H), 2.41 (s, 3H) ppm.[31] L-2: Yield 2.19 g (97%); 1H NMR (270
MHz, CDCl3, 25 ℃): δ= 7.50–7.57 (m, 6 H), 7.23–7.26 (m, 6 H), 2.40 (s,
1
9 H) ppm.[31] L-3: Yield 2.18 g (97%); H NMR (400 MHz, CDCl3, 25 ℃):
δ=7.54-7.57 (d, 3H), 7.28-7.39 (m, 9H), 2.35 (s, 9H) ppm.
Preparation of [Eu(tmh)3(L-1)] (Eu-1), [Eu(tmh)3(L-2)] (Eu-2), and
[Eu(tmh)3(L-3)] (Eu-3): The precursor complex, [Eu(tmh)3(MeOH)2], was
synthesized as described in our previous report.[21] Europium chloride
hexahydrate (1.0 g, 2.7 mmol) was dissolved in distilled water (5 mL) in a
100 mL flask. An ethanol solution (20 mL) of tmh (1.46 g, 8.1 mmol) was
added to the aqueous solution. An ammonia solution was added
dropwise to the flask until pH 7. After stirring at room temperature for 6 h,
the reaction mixture was poured into cold water (300 mL) in a conical
flask. The produced precipitate, [Eu2(tmh)6][32] was filtered, and the
resulting powder was recrystallized from methanol to afford colorless
block crystals of [Eu(tmh)3(MeOH)2]. The precursor complex
[Eu(tmh)3(MeOH)2] (1.0 g, 1.4 mmol) and L-1 (0.41 g, 1.4 mmol), L-2
(0.45 g, 1.4 mmol), or L-3 (0.45 g, 1.4 mmol) were dissolved with
methanol (30 mL) in a 100 mL flask. The solution was heated under
reflux while stirring for 6 h. The reaction mixture was recrystallized in
methanol to afford colorless crystals of titled compounds ([Eu(tmh)3(L-1)]:
Eu-1, [Eu(tmh)3(L-2)]: Eu-2, and [Eu(tmh)3(L-3)]: Eu-3). Eu-1: Yield 1.1 g
(79%); IR (ATR) ꢀ = 2861-2955 (m, C-H), 1570 (s, C=O), 1173 cm-1 (s,
P=O); elemental analysis calcd (%) for C52H74EuO7P: C 62.83, H 7.50;
found: C 62.66, H 7.31. Eu-2: Yield 1.3 g (91%); IR (ATR) ꢀ = 2860-2960
(m, C-H), 1572 (s, C=O), 1176 cm-1 (s, P=O); elemental analysis calcd
(%) for C54H78EuO7P: C 63.45, H 7.69; found: C 63.14, H 7.69. Eu-3:
Yield 1.3 g (91%); IR (ATR) ꢀ = 2860-2950 (m, C-H), 1572 (s, C=O), 1171
cm-1 (s, P=O); elemental analysis calcd (%) for C54H78EuO7P: C 63.45, H
7.69; found C 63.01, H 7.61. TbIII and GdIII complexes ([Tb(tmh)3(L-1)]:
Tb-1, [Tb(tmh)3(L-2)]: Tb-2, [Tb(tmh)3(L-3)]: Tb-3, [Gd2(tmh)6], and
[Gd2(tmh)3(MeOH)2]) were synthesized by the same method as
described above. Tb-1: IR (ATR) ꢀ = 2860-2950 (m, C-H), 1573 (s, C=O),
1174 cm-1 (s, P=O); elemental analysis calcd (%) for C52H74O7PTb: C
62.39, H 7.45; found C 62.52, H 7.26. Tb-2: IR (ATR) ꢀ = 2860-2960 (m,
C-H), 1573 (s, C=O), 1178 cm-1 (s, P=O); elemental analysis calcd (%) for
C54H78O7PTb: C 63.02, H 7.64; found C 62.68, H 7.31. Tb-3: IR (ATR) ꢀ
Figure 6. a) Thermal dependency of emission lifetimes of Eu-1 (circle), Eu-2
(square), and Eu-3 (rhomboid) in the solid state. b) Arrhenius plots for charge
transfer rate (kCT) of Eu-1 (circle), Eu-2 (square), and Eu-3 (rhomboid).
ln(1/τobs-1/τ300K) = ln(kCT) = lnA-(Ea/kBT).
Conclusions
We successfully synthesized novel seven-coordinate
lanthanide complexes with 7-MCO, 7-MCTP, and 7-PBP
structures. The geometrical structures were dependent on the
steric hindrance of phosphine oxide ligands with additional
methyl groups. The radiative rate constant of Eu-1 with 7-MCO
(C3v) was larger than those of Eu-2 with 7-MCTP (C2v) and Eu-3
with 7-PBP (D5h). We consider that the enhanced radiative rate
constant of Eu-1 is due to synergetic effect between the low-
symmetrical coordination geometry and relatively large
perturbation of the LMCT state into the 4f-excited state. The
large perturbation was estimated by Arrhenius parameters
obtained from temperature-dependent emission lifetime and TD-
DFT calculation. Seven-coordinate EuIII complexes in this study
provided significant photophysical information for investigation of
the radiative process of lanthanide complexes.
Experimental Section
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10.1002/ejic.201700815
European Journal of Inorganic Chemistry
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= 2860-2950 (m, C-H), 1573 (s, C=O), 1173 cm-1 (s, P=O); elemental
analysis calcd (%) for C54H78O7PTb: C 63.02, H 7.64; found C 62.67, H
7.27. [Gd2(tmh)6]: IR (ATR) ꢀ = 2860-2950 (m, C-H), 1572 (s, C=O);
elemental analysis calcd (%) for C66H114Gd2O12: C 56.06, H 8.13; found C
55.89, H 8.15.
Leading Graduate Schools (Hokkaido University “Ambitious
Leader’s Program”). Part of computations were performed using
the computer facilities at Reserch Center for Computational
Science (RCCS), Okazaki, Japan.
Crystallography: Single crystals of the lanthanide complexes were
mounted on micromesh (MiTeGen M3-L19-25L) using paraffin oil. All
measurements were carried out using a Rigaku R-AXIS RAPID imaging
plate area detector with graphite monochromated MoKα radiation. Non-
hydrogen atoms were refined anisotropically. All calculations were
performed using a crystal-structure crystallographic software package.
The quality of CIF data was validated by using the checkCIF/PLATON
service. Crystallographic data of the TbIII complexes and Gd complex
were summarized in Table S9. The CIF data are presented in supporting
information, and also CCDC 1507470 (Eu-1), CCDC 1507471 (Eu-2),
CCDC 1507472 (Eu-3), CCDC 1515208 (Tb-1), CCDC 1515209 (Tb-2),
CCDC 1515210 (Tb-3), and CCDC-1520661 ([Gd(tmh)3(MeOH)2])
contain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
Keywords: Lanthanides • Luminescence • Charge transfer •
Structure elucidation • Density functional calculations
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Entry for the Table of Contents (Please choose one layout)
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Seven-coordinate Lathanide
Complexes: Three types of
coordination geometrical structures,
monocapped-octahedron,
Kei Yanagisawa, Yuichi Kitagawa,
Takayuki Nakanishi, Tomoko Akama,
Masato Kobayashi, Tomohiro Seki, Koji
Fushimi, Hajime Ito, Tetsuya
monocapped-trigonalprism, and
pentagonal-bipyramid were
observed in EuIII complexes
constructed by β-diketonate and
phosphine oxide ligands. The
characteristic photophysical
properties involving LMCT
Taketsugu, Yasuchika Hasegawa*
Page No. – Page No.
Enhanced Luminescence of
Asymmetrical Seven-coordinate EuIII
Complexes Including LMCT
Perturbation
perturbation are discussed.
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