Influence of Symmetry on the Luminescence and Radiative Lifetime of Nine-Coordinate Europium Complexes

Article abstract of DOI:10.1021/acs.inorgchem.5b01580

Homoleptic mononuclear nine-coordinate lanthanum(III) and europium(III) tris-complexes [Ln(N^N^O)₃]·nH₂O with two tridentate N-benzylbenzimidazole pyridine-2-carboxylates exhibit a rare C₃-symmetry of the lanthanide coordi

Full text of DOI:10.1021/acs.inorgchem.5b01580

Article
pubs.acs.org/IC
Influence of Symmetry on the Luminescence and Radiative Lifetime
of Nine-Coordinate Europium Complexes
Nail M. Shavaleev,* Svetlana V. Eliseeva, Rosario Scopelliti, and Jean-Claude G. Bunzli*
̈
́
Ecole Polytechnique Fed
́ ́ ́
erale de Lausanne, Institut des Sciences et Ingenierie Chimiques, Avenue Forel 2, BCH, CH-1015 Lausanne,
Switzerland
S
* Supporting Information
ABSTRACT: Homoleptic mononuclear nine-coordinate lanthanum(III) and europium-
(III) tris-complexes [Ln(N∧N∧O)3]·nH2O with two tridentate N-benzylbenzimidazole
pyridine-2-carboxylates exhibit a rare C3-symmetry of the lanthanide coordination
polyhedron in the solid state, as confirmed by luminescence spectroscopy and by X-ray
crystallography (the three N∧N∧O ligands are arranged “up−up−up” around the
lanthanide ion). The symmetry, however, is changed to the more common C1 upon
dissolution of the complexes in dichloromethane, as revealed by luminescence
spectroscopy (the three ligands are likely to be arranged “up−up−down”). The new europium complexes
emit efficient ligand-sensitized metal-centered luminescence with excited-state lifetimes of 1.56−2.18 ms and quantum yields
of 25−41% in the solid and in solution. The change of the symmetry from (a higher) C3 to (a lower) C1 alters the luminescence
spectrum, shortens the radiative lifetime, and increases the luminescence efficiency of the europium complexes.
INTRODUCTION
RESULTS AND DISCUSSION
Synthesis. Two new tridentate monoanionic N∧N∧O ligands
HL1 and HL2 have been prepared from 2-carboxaldehyde-6-
hydroxymethylpyridine (Scheme 1).16,17 The formation of a
benzimidazole heterocycle18 was followed by selective oxida-
tion of pyridine-2-methanol first to carboxaldehyde with
■
■
Most trivalent lanthanide ions exhibit characteristic metal-
centered line-like f−f luminescence with high color purity in
the UV, visible, and near-infrared spectral ranges.1−8 Although
the f−f luminescence is long-lived, with a lifetime of up to
milliseconds, it is usually not quenched by oxygen.9
The brightness of lanthanide luminescence, that is, the
product of the luminescence quantum yield and the molar absorp-
tion coefficient (at the excitation wavelength), can be increased
by many orders of magnitude by coordinating the lanthanide ion
with chromophore ligands.5,10−13 The ligands provide efficient
light harvesting and ligand-to-lanthanide energy transfer and
protect the lanthanide(III) from nonradiative deactivation.5
Upon coordination, mixing of ligand and lanthanide orbitals
enhances the intensity of electric-dipole metal-centered
Laporte-forbidden f−f transitions, especially when the coordi-
nation lowers the symmetry around the lanthanide.1−3 Thus,
the symmetry that the ligands define around the lanthanide ion
in the complex determines the relative intensity of the emission
bands and, therefore, both the luminescence color and the rate
of radiative f−f transitions.1−3,14,15
a
Scheme 1. Synthesis of Ligands and Complexes
The emissive lanthanide ions, in particular europium(III), are
used as luminescent probes and sensors,5 because the efficiency,
the lifetime, and the fine structure of the f−f luminescence
spectra are sensitive to the symmetry and the composition of the
coordination sphere of the lanthanide.1−5,11−15 Here, we report
on rare mononuclear nine-coordinate lanthanide(III) complexes
[Ln(N∧N∧O)3] that exhibit C3-symmetry in the solid state but
C1-symmetry in solution and on the effect that this change of
symmetry has on the luminescence of the europium(III)
complexes.
a
Reaction conditions: (a) Na2S2O4, 2-methoxyethanol/water or
DMF/water, under nitrogen, 100−110 °C; (b) SeO2, dioxane, under
nitrogen, 110 °C; (c) H2O2, formic acid, under air, 0 °C; (d) LnCl3·
nH2O, NaOH, ethanol/water, under air, heating.
Received: July 16, 2015
© XXXX American Chemical Society
A
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SeO2 and then to carboxylic acid with H2O2/formic acid.19 The
homoleptic lanthanum(III) and europium(III) tris-complexes
[Ln(Ligand)3]·nH2O (n = 4, 5; Ln = La, Eu; Ligand = L1−,
L2−; LnL1 and LnL2) have been obtained by reacting a 3:3:1
molar ratio of ligand, NaOH, and LnCl3·nH2O in ethanol/
water mixtures. All of the products are air- and moisture-stable
solids. They have been characterized by elemental analysis,
NMR spectroscopy, ESI TOF mass spectrometry, and X-ray
crystallography. The n-octyloxy group in the benzimidazole
heterocycle increases the solubility of ligand HL2 and its
complexes in organic solvents.
Molecular Structure of the Complexes LnL1. Figure 1
and Table 1 report the X-ray structure of [La(L1)3]·CH3CN.
Figure 2. Coordination environment of LaIII in [La(L1)3]·CH3CN
viewed down the square face of the tricapped trigonal prism.
Heteroatoms: O, red; N, blue; La(1), black.
The coordinated ligands are not planar, with the dihedral
angles between pyridine and benzimidazole in the range 16.4−
26.3°. The ligands are not equally strongly bonded to the metal
ion. For a given ligand, the lanthanum−benzimidazole bond
is the longest one with the widest variation. The bonding of
the ligands was quantified by the bond-valence method,20
wherein a donor atom j at a distance dLn,j from the metal ion is
characterized by a bond-valence contribution νLn,j
:
(RLn,j−dLn,j)/b
νLn,j = e
(1)
where RLn,j are the bond-valence parameters for the interacting
atoms (La−O, 2.148 Å; La−N, 2.261 Å)21 and b is a constant
(0.37 Å). The bond-valence sum (BVS) of the metal ion VLn is
supposed to match its oxidation state,20 if average bonds are
standard:
Figure 1. Structure of [La(L1)3]·CH3CN viewed down the triangular
face (C3 axis) of the tricapped trigonal prism (50% probability
ellipsoids; one of the two independent molecules of the complex, the
Λ isomer La(1), is shown; H atoms, benzyl groups, and cocrystallized
acetonitrile molecule omitted; ORTEP). Heteroatoms: O, red; N,
blue; La(1), black.
VLn
=
ν
∑
Ln,j
j
(2)
The BVS for the [La(L1)3]·CH3CN structure (3.03 and
3.07) is close to the expected value for LaIII (3.00
0.25)
(Table 2).20,21 The average contributions from the coordinating
a
Table 1. Structure of [La(L1)3]·CH3CN
a
b
bond lengths (Å)
angles (deg)
Table 2. Bond Valence Parameters of [La(L1)3]·CH3CN
b
complex
La−O
La−N(py)
La−N(b)
py−b
νLn,j
[La(L1)3] (1)
2.448(3)
2.452(3)
2.459(3)
2.449(3)
2.457(3)
2.457(3)
2.454(8)
0.011
2.695(3)
2.693(3)
2.690(4)
2.705(3)
2.688(4)
2.705(3)
2.696(13)
0.017
2.737(3)
2.735(4)
2.755(3)
2.722(4)
2.782(3)
2.745(3)
2.746(38)
0.060
24.4
21.5
24.5
16.4
26.3
22.2
23(6)
9.9
complex
VLn
O
N(py)
N(b)
[La(L1)3] (1)
[La(L1)3] (2)
all data
3.07
3.03
0.44(1)
0.44(1)
0.44(1)
0.31(0)
0.31(1)
0.31(1)
0.27(1)
0.27(4)
[La(L1)3] (2)
0.27(3)
b
a
Two independent molecules are present in the unit cell. Bond-
valence contribution with standard deviation 2σ averaged over the
three ligands.
average (2σ)
max − min
a
Two independent molecules are present in the unit cell. Each row
groups are in the expected order of affinity O, 0.44(1) > N(py),
0.31(1) > N(b), 0.27(3) (Table 2) and are within experi-
mental error of those reported for the reference complexes R1
(Chart 1).17
The X-ray structure of [Eu(L1)3] was of low quality (not
shown), but it confirmed the connectivity, the presence of two
independent molecules, and the C3-symmetry of the complex.
The long Eu−Eu distance in the structure (>9 Å) may
minimize concentration quenching and may favor efficient
luminescence of EuL1.
A review of the literature reveals that C3-symmetry is rare
for mononuclear tris-complexes [Ln(N∧N∧O)3]0/3−. Only two
examples of C3-symmetry but 32 examples of C1-symmetry
(where the three ligands are arranged “up−up−down”) were
b
corresponds to one ligand. py = pyridine. b = benzimidazole. The
dihedral angles between the planes of pyridine and benzimidazole.
The structure contains two slightly different molecules (both
are present as Λ and Δ isomers). The lanthanum ions are nine-
coordinated by three deprotonated tridentate N∧N∧O ligands.
Their coordination polyhedron is a distorted tricapped trigonal
prism, with the N(py = pyridine) atoms in capping positions
and in-plane with LaIII (Figure 2). The La−La distance is long,
9.142(2) Å. The triangular faces of the prism are defined by
O−O−O and N(b)−N(b)−N(b) atoms (b = benzimidazole).
The three ligands are arranged “up−up−up” around the LaIII,
resulting in a complex with a distorted C3-symmetry.
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a
a
Chart 1. Reference Ligands and Complexes
Table 3. Absorption Spectra of Ligands and Complexes
compound
HL1·H2O
λmax/nm (ε/103 M−1 cm−1
)
315 (22)
[La(L1)3]·4H2O
[Eu(L1)3]·4H2O
HL2·0.5H2O
315 (62)
316 (59)
334 (23)
b
[La(L2)3]·4H2O
[Eu(L2)3]·5H2O
340 (48), 256 (24)
342 (50), 254 (27)
b
a
See Figure 3. In DMSO at 298 K. Errors: λmax
In dichloromethane.
,
1 nm; ε, 5%.
b
its maximum red-shifts by up to 8 nm and its molar absorp-
tion coefficient increases from (22−23) × 103 M−1 cm−1 to
(48−62) × 103 M−1 cm−1 (Table 3).
The ligand-to-lanthanide energy transfer often occurs mainly
from the triplet state of the ligand.1,2,27,28 Its efficiency depends
on the overlap integral between the emission spectrum of the
donor and the absorption spectrum of the acceptor and may be
related to the energy gap between the ligand triplet state and
the lanthanide receiving level(s).1,28 The triplet-state energies
of the coordinated ligands in lanthanum complexes (ET) were
determined from the zero-phonon transition in the phos-
phorescence spectra at 77 K to be 20 400 cm−1 for LaL1 and
19 700 cm−1 for LaL2 (Figure 4 and Table 4). These ligand
a
The ligands bond by the set of N∧N∧O atoms with deprotonated
carboxylate, phenolate, and tetrazolate groups. The composition and
symmetry of the complexes are indicated. From the references: R1,17
R2,2 R3,22 R4,23 R5,24 R6,2 R7,2 R8,25 R9,26 R10.36
Figure 4. Phosphorescence spectra (corrected and normalized) of
polycrystalline solid lanthanum complexes at 77 K.
a
Table 4. Phosphorescence of Lanthanum Complexes
E/103 cm−1
reported for the structures of the reference complexes R1−
R52,17,22−24 and R7−R92,25,26 (Chart 1).
complex
0−0
0−1
Δ
Electronic States of the Ligands. The new ligands and
their complexes exhibit electronic absorption transitions at
wavelengths of less than 410 nm in organic solvents (Figure 3).
The main composite absorption band with a maximum at
315−342 nm is assigned to the π → π* transitions of the
benzimidazole chromophore. From the ligands to the complexes,
[La(L1)3]·4H2O
[La(L2)3]·4H2O
20.4
19.7
19.3
18.2
1.1
1.5
a
See Figure 4. In polycrystalline solid at 77 K. Error: 200 cm−1.
triplet states have sufficiently high energy for exothermic energy
transfer to the EuIII excited states29 Eu(5D1) at 19 000 cm−1 and
Eu(5D0) at ∼17 230 cm−1.
The electron-donor n-octyloxy group in the benzimidazole
heterocycle red-shifts both the absorption maximum by 1800−
2400 cm−1 (19−26 nm) and the triplet state by 700 cm−1 from
HL1/LnL1 to HL2/LnL2 by generating intraligand benzimi-
dazole-centered “alkoxy”-to-imine charge-transfer transition.
Europium Luminescence. Upon excitation with UV and
visible light, the new europium complexes emit characteristic
red luminescence with a line-like spectrum in the range 575−
710 nm due to the metal-centered 5D0 → 7FJ (0 → J, J = 0−4)
transitions (Figure 5).3 The emission spectra are independent
of the excitation wavelength. The excitation spectra correspond
to the ligand absorption transitions, thereby confirming
Figure 3. Absorption spectra of ligands (in DMSO) and complexes
(LnL1 in DMSO; LnL2 in dichloromethane).
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groups of the) organic ligands, and from defects in the
microcrystals.3
The X-ray structures reveal that LnL1 in single crystals have
C3-symmetry (Figures 1 and 2). Luminescence spectroscopy
indicates that EuL1 and EuL2 in polycrystalline solids have
C3-symmetry also. The corrected emission spectra of the poly-
crystalline complexes EuL1 and EuL2 are similar (Figure 5).
The emission intensity is fairly equally distributed among
5
three main transitions (Table 5), the magnetic-dipole D0 →
5
7
7F1 (22−24%), the hypersensitive D0 → F2 (33−35%), and
5D0 → F4 (38%), a situation typical of C3-symmetry (see
7
Figures 1 and 2).3,23,30
Figure 5. Luminescence spectra (corrected and normalized) of the
europium complexes displaying the 5D0 → 7FJ (J = 0−4) transitions at
298 K in the polycrystalline solid and in dichloromethane solution;
λexc = 330 nm; emission slit: 0.2 nm.
The crystal field splitting (Table 6) reveals 1, 2, 2 (or 2 + 1
shoulder), and at least 5 (or 4 + 1 shoulder) components for
5
7
the D0 → FJ transitions for J = 0, 1, 2, and 4. It is nearly in
line with predictions for C3-symmetry (1, 2, 3, and 6
components for J = 0, 1, 2, and 4),3,30 except for the very
weak intensity of the 0 → 0 transition, which is allowed in C3
by symmetry-governed selection rules. The emission spectra of
the polycrystalline EuL1 and EuL2 are almost consistent with
a higher than C3-symmetry in the same trigonal class, that is,
with D3-symmetry (0, 2, 2, and 4 transitions predicted for J = 0,
1, 2, and 4).3
ligand-to-europium energy transfer (Figure S1 in the Supporting
Information). The new europium complexes do not exhibit
ligand-centered fluorescence and phosphorescence.
High-resolution excitation scans over the 5D0
←
7F0
transition of the polycrystalline solid complexes at 298 K
exhibit one asymmetrical line with full width at half height
(fwhh) of 9 cm−1 and maximum at 17 236 cm−1 for EuL1 or
17 232 cm−1 for EuL2 (Figure 6). Upon cooling to 10 K, the
High-resolution emission spectra of 5D0 → 7F1 and 5D0 → 7F2
transitions of polycrystalline EuL1 at 10 K exhibit more com-
ponents than would be expected for C3-symmetry (Figure S2 in
the Supporting Information), because of the reasons given
above to explain the asymmetric 5D0 ← 7F0 excitation transition
(Figure 6).
The emission spectrum of EuL2 radically changes in going
from the solid state to dichloromethane solution, indicat-
ing a change in the coordination environment of europium
(Figure 5). In solution, the 0 → 0 transition gains intensity and
the 0 → 2 one becomes dominant, representing 48% of the
total emission intensity (Table 5). In contrast, the contributions
of the 0 → 1 and the 0 → 4 transitions decrease to 19% and
30%, respectively. The number of components of the 0 → 1
transition increases from two to three, and its total crystal-field
splitting increases by about 100 cm−1 to become 278 cm−1
(Table 6). The distribution of intensity and the fine structure of
the luminescence spectrum of EuL2 in the solution (1, 3, >3,
and >5 components for J = 0, 1, 2, 4) are not consistent
with C3-symmerty (which is observed in the solid), but point
to a low C1-symmetry around the metal ion (1, 3, 5, and
9 components predicted for J = 0, 1, 2, and 4).3,17,23
The luminescence spectrum, however, reflects the presence of
only one emitting EuIII center in the solution (Figure 5 and
Table 6).
Figure 6. High-resolution excitation (exc) and emission (em) spectra
of the 5D0
→
7F0 transition of the polycrystalline solid europium
complexes with 1 Å bandpass at 298 K and 10 K. The emission (for
5
7
the excitation spectra) was monitored at the D0 → F2 transition at
610−620 nm.
7
5D0 ← F0 excitation transition for EuL1 becomes narrower
(fwhh = 4.4 cm−1) and shifts to 17 226 cm−1, but it remains
5
7
asymmetrical, while the D0 → F0 emission transition at 10 K
displays a symmetrical broad line at 17 226 cm−1 with a fwhh of
9.8 cm−1 (Figure 6). A single sharp symmetrical line is expected
5
7
for the electronic transition between the D0 and F0 levels of
EuIII in each unique coordination environment, provided that
the transition is allowed by symmetry. The observed asymmetry
The luminescence decays of EuL1 and EuL2 (τobs, Table 7)
in the polycrystalline solid and in dichloromethane solution are
single-exponential functions, again confirming the presence of
one main emissive europium center in each case. The long
luminescence lifetimes of 1.56−2.67 ms and the high ligand-
sensitized luminescence quantum yields of 25−41% (Table 7)
5
7
of the D0 ← F0 excitation transition for the polycrystalline
solids may arise from the presence of two nearly identical
coordination environments of EuIII (as confirmed by the X-ray
analysis), from the variation in the conformation of (benzyl
a
5
7
Table 5. Relative Luminescence Intensity of D0 → FJ Transitions of Europium Complexes
complex
0→0
0→1
0→2
0→3
0→4
total/0→1
[Eu(L1)3]·4H2O
[Eu(L2)3]·5H2O
solid
0.005
0.005
0.02
1.00
1.00
1.00
1.36
1.56
2.60
0.16
0.19
0.15
1.57
1.71
1.63
4.10
4.47
5.40
solid
CH2Cl2
a
From Figure 5. Normalized to the 5D0 → 7F1 transition. Integrated from the corrected luminescence spectra (photon/s) versus wavelength (nm) at
298 K at λexc = 355 nm. Estimated error: 5%.
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a
7
5
Table 6. Energies of FJ and D0 Levels of Europium Complexes
E/cm−1
complex
7F0
0
7F1
7F2
997
7F3
7F4
5D0
[Eu(L1)3]·4H2O
solid
275
446
1842
2718
2760
2897
2950
3023
2714
2901
2967
3027
2697
2773
2832
2902
3053
17236
1034 (sh)
1128
1941
2015 (w)
[Eu(L2)3]·5H2O
solid
0
0
266
442
982
1833
17232
17253
1129
1932
2021 (w)
CH2Cl2
275
373
553
988
1883 (br)
1067
1186 (sh)
a
From Figure 5. At 298 K. Estimated error: 6 cm−1.
a
Table 7. Luminescence Parameters of Europium Complexes
τobs/ms
b
ν0−0/cm−1
Q ELu/%
298 K
10 K
τrad/ms
Q
/%
Eu
ηsens/%
Eu
complex
[Eu(L1)3]·4H2O
[Eu(L2)3]·5H2O
solid
17236(9)
17232(9)
41
25
32
1.94
1.56
2.18
2.53
2.67
4.93
4.52
4.38
39
35
50
100
71
solid
CH2Cl2
64
a
b
Eu
Eu
At 298 K, unless stated otherwise. λexc = 355 nm. Relative errors: τobs
,
2%; Q ELu
,
10%; τrad
,
10%; Q
,
12%; ηsens
,
22%. From Figure 6.
Full-width at half-height in parentheses.
indicate that water molecules are not coordinated to the
europium in EuL1 and EuL2 in the solid and in solution.17,31
The luminescence lifetimes of the polycrystalline complexes
increase significantly on cooling from 298 K to 10 K (but the
luminescence decays remain single-exponential functions),
especially for EuL2, pointing to the presence of a thermally
activated nonradiative deactivation pathway. It may be caused
by the europium-to-ligand back-transfer of energy,28 because
the energy gaps from the ligand triplet to the receiving levels
Eu(5D1) at 19 000 cm−1 and Eu(5D0) at ∼17 230 cm−1 are only
700−3170 cm−1. Alternatively, it may be caused by quenching
of EuIII by a ligand-to-metal charge transfer (LMCT, a
frequently observed process for electron-donating ligands32),
especially for complex EuL2, with an electron-rich n-octyloxy-
substituted ligand.
Table 7 reports the photophysical parameters. The radiative
lifetimes of the solid complexes are 4.93 ms for EuL1 and
4.52 ms for EuL2. When EuL2 is dissolved in dichloromethane,
one anticipates a lengthening of its τrad from 4.52 ms (in the
solid) to 5.28 ms (in solution) due to the decrease in refractive
index (eq 4),1,2 provided that the coordination sphere of the
europium ion remains the same. In the experiment, however,
we observe a shortening of the radiative lifetime to 4.38 ms
in dichloromethane, which indicates that the coordination
sphere of the europium ion changes upon dissolution of EuL2,
probably to give a lower symmetry environment.
The intrinsic quantum yield of europium could not be
measured because of the low intensity of the f−f absorption.
Eu
Eu
Instead, it was calculated from the ratio Q = τobs/τrad to be
35−39% for the solid complexes and 50% for the EuL2
Eu
Eu
We have analyzed the photophysics of the complexes in
solution. The higher Q in solution for EuL2 results both
Eu
terms of eq 3, where Q ELu and Q are ligand-sensitized and
Eu
from the longer τobs and from the shorter τrad.
intrinsic Eu(5D0) luminescence quantum yields, ηsens is the
efficiency of ligand-to-europium energy transfer, and τobs and
τrad are the observed and radiative lifetimes of Eu(5D0):
In the solid complexes, the calculated efficiency of ligand-to-
Eu
europium energy transfer, ηsens = Q ELu/Q , decreases from
Eu
100% in EuL1 to 71% in EuL2, because of the more facile
nonradiative deactivation of EuL2 both by energy back-transfer
and by LMCT (see above). The sensitization efficiency for
EuL2 in solution (64%) is comparable to that in the solid
(71%), indicating that the ligands remain bound to the
europium in dichloromethane, a noncoordinating solvent.
The luminescence quantum yield and observed lifetime of
EuL2 increase in going from the solid to the solution, probably,
in part, because the five cocrystallized water molecules in the
solid EuL2 quench the emissive state of europium via a second-
sphere interaction. In contrast, in dichloromethane, these water
molecules move into the bulk solution and are less likely to
quench the europium ion.
Eu
Q LEu = η × Q = η × (τ /τrad)
(3)
Eu
obs
sens
sens
The radiative lifetime of Eu(5D0) was calculated from eq 4,33
where n is the refractive index (1.5 for the solid complexes or
1.4242 for the CH2Cl2 solution), A0→1 is the spontaneous
5
7
emission probability for the D0 → F1 transition in vacuo
(14.65 s−1), and Itot/I0→1 is the ratio of the integrated emission
intensity of the total corrected europium spectrum to that of
5
7
the magnetic-dipole D0 → F1 transition (Table 5):
1/τrad = A0→1 × n3 × (Itot/I0→1
)
(4)
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Symmetry and Luminescence. The crystal structures
reveal that the solid complexes LnL1 have a rare C3-symmetry
of the lanthanide coordination polyhedron (Figure 2). The
nearly identical luminescence spectra of the polycrystalline
EuL1 and EuL2 suggest that the LnL2 complexes in the solid
have C3-symmetry also (Figure 5). In fact, the luminescence
spectra of the polycrystalline EuL1 and EuL2 are similar in
intensity distribution and fine structure to those of the
reference EuIII complexes R423 and R62 (Chart 1) of C3- or
D3-symmetry (in which the intensity of the 0 → 1 transition is
comparable to that of the 0 → 2, while the 0 → 4 transition is
the most intense), but are different from those of complexes
R117 of C1-symmetry (in which the 0 → 2 transition is the most
intense).
The lanthanide−ligand bonds have predominant ionic
character and, therefore, are nondirectional and labile, even
with polydentate ligands.2,34 The significant changes in the
luminescence spectrum as well as the shortening of the
radiative lifetime of EuL2 in going from the solid state to
dichloromethane solution indicate a modification in the
coordination environment of the europium ion. For example,
in sharp contrast to EuL2, the radiative lifetime of R6, which
have pseudo-D3-symmetry in all media, increases from 4.9 ms
in the solid to 6.2−6.9 ms in aqueous solution due to the
change in refractive index.2
We consider that in dichloromethane solution the labile
lanthanide−ligand bonding lowers the symmetry of EuL2 from
C3 to C1 by modifying the arrangement of the three ligands
from “up−up−up” to “up−up−down” (for example, see the
C1-crystal structures of R117). Taking into account the
spectroscopic data, a mixture of C1- and C3-species in solution
can most probably be ruled out. The photophysical properties
of EuL2 are different in the solid but are similar in solution to
those of the reference EuIII complexes with C1-symmetry. For
example, for 16 complexes R1, which have C1-symmetry in all
media, the radiative lifetimes are 3.22−4.7 ms in the solid and
often increase to 3.91−4.40 ms in dichloromethane solution,
while the luminescence spectra do not change (apart from
becoming broader in solution) and have three components for
the 0 → 1 transition.17 Moreover, the luminescence spectra of
the C1-symmetry complexes R117 and R3−R522−24 are
dominated by the 0 → 2 transition.
complexes were crystallized for [Ln(β-diketonate O∧O)3(H2O)2]37
and [Ln(β-diketonate O∧O)3(diimine N∧N)]38 and were
identified by X-ray structure analysis and by luminescence
spectroscopy of the europium complexes.
Conclusions. We report on mononuclear nine-coordinate
lanthanum(III) and europium(III) complexes [Ln(N∧N∧O)3]
with a rare C3-symmetry of the lanthanide coordination poly-
hedron in the solid. These complexes, however, exhibit a lower
C1-symmetry in dichloromethane solution.
The change of the luminescence spectrum and the shorten-
ing of the Eu(5D0) radiative lifetime in going from the (higher)
C3- to the (lower) C1-symmetry species of EuL2 (Table 5)
suggests that the lower is the symmetry, the more allowed are
the forbidden f−f transitions, which is in line with the predic-
tions made from group-theoretical considerations.1−3
The shorter is the radiative lifetime, in other words, the faster
is the rate of the radiative transition, the better the nonradiative
processes can compete with the nonradiative ones (provided
that they are not changed), and, therefore, the higher is the
luminescence efficiency,1,2,32,39 which is what we observe for
EuL2 in going from the solid to the solution (Table 5).
Therefore, care has to be taken when comparing photo-
physical properties of a labile lanthanide complex in various
media, because the coordination environment of a lanthanide
ion can change its symmetry even without a change in its
composition.
EXPERIMENTAL SECTION
■
General Information. Elemental analyses were performed by
Dr. E. Solari, Service for Elemental Analysis, Institute of Chemical
Sciences and Engineering (EPFL). 1H NMR spectra were recorded on
Bruker Avance DRX 400 MHz spectrometer. Absorption spectra were
measured on a PerkinElmer Lambda 900 UV/vis/NIR spectrometer.
Luminescence spectra were recorded on a Horiba-Jobin Yvon
Fluorolog FL 3-22 spectrometer and were corrected for the instru-
mental function. Quantum yields were determined on the same
instrument by an absolute method with a modified homemade
integrating sphere. Luminescence lifetimes were measured with a
previously described instrumental setup.17 Reported luminescence
quantum yields and lifetimes are an average of 3−6 independent
determinations. Spectroscopic studies were conducted in optical cells
of 2 mm path length or in 2 mm i.d. quartz capillaries under air. The
solutions in CH2Cl2 (Fisher Scientific, analytical reagent grade) were
freshly prepared before each experiment.
It appears that the C3- and C1-symmetry species (isomers)
for EuL2 are close in energy and, because of the labile
lanthanide−ligand bonding, easily interconvert. The isolation of a
rare C3-species in the solid state for LnL1 and LnL2 is probably
driven by crystal packing of the N-benzyl groups (in contrast,
the N-alkyl and N-aryl analogues R1 have C1-symmetry in all
media17). The cocrystallized solvent does not seem to play a
crucial role in determining the C3-symmetry of the complex in
the solid; for example, a molecule of acetonitrile is cocrystallized
in the single crystal of [La(L1)3]·CH3CN, but water molecules
are cocrystallized in the polycrystalline [Eu(L1)3]·4H2O and
[Eu(L2)3]·5H2O. The conversion of C3- to C1-species of EuL2
in going from the solid to the solution is probably driven by a
gain in entropy on lowering of the symmetry.
We note previous reports on the isomerization of the fac and mer
six-coordinate complexes [LnCl3(hexamethylphosphoramide)3]35
and of the C3 and C1 nine-coordinate complexes [Ln(8-
hydroxyquinolinate-2-carboxylate)3]3− and R926 (Chart 1). The
thermodynamics of the interconversion of C3- and C1-species
in acetonitrile solution were studied for R10 (Chart 1).36
Two symmetry/structure isomers of eight-coordinate lanthanide
Commercial reagents were used without purification. Chromatog-
raphy was performed on a column with an i.d. of 30 mm on silica gel
60 (Fluka, Nr 60752). The progress of reactions and the elution of
products were followed on TLC plates (silica gel 60 F254 on aluminum
sheets, Merck).
Synthesis of Ligands. The reaction was performed under air.17,19
Substituted pyridine-2-carboxaldehyde (its synthesis is described in
the Supporting Information) was dissolved in formic acid (Merck,
98−100%) at room temperature to give a yellow solution, which
sometimes appeared cloudy because of the presence of a red solid,
probably residual Se from the previous synthetic step. The amount of
formic acid was a minimum of 3−5 molar equiv relative to aldehyde
or the minimum volume necessary to dissolve the aldehyde at 0 °C.
The solution was cooled to 0 °C and stirred for 10 min. Then, cold
H2O2 (30% wt aqueous solution) was added in excess. The solution
was stirred for 6 h at 0 °C and kept overnight at 0 °C. Addition of ice-
cold water to the solution precipitated the product. The suspension
was stirred for 1 h at 0 °C and filtered. The product was washed with
water and organic solvent, and then it was dried under vacuum.
Further details are provided below.
HL1·H2O. Aldehyde L1-CHO (Supporting Information, 445 mg,
1.42 mmol), formic acid (3 mL, 3.66 g, 0.08 mol), and H2O2 (0.85 mL
of a 30% wt aqueous solution containing 283 mg of H2O2, 8.32 mmol)
F
DOI: 10.1021/acs.inorgchem.5b01580
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
gave after precipitation with 10 mL of water and after washing with
water, hexane, and a small volume of 1:1 hexane/ether a white solid:
436 mg (1.26 mmol, 88%). Anal. Calcd for C20H15N3O2·H2O (MW
347.37): C, 69.15; H, 4.93; N, 12.10. Found: C, 69.14; H, 4.89; N,
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on
the ACS Publications website at DOI: 10.1021/acs.inorg-
chem.5b01580.
■
S
1
11.77. H NMR (400 MHz, DMSO-d6): δ = 8.55 (dd, J = 7.6, 1.2 Hz,
1H), 8.19−8.08 (m, 2H), 7.78 (d, J = 7.2 Hz, 1H), 7.71 (d, J = 7.2 Hz,
1H), 7.36−7.25 (m, 2H), 7.24−7.12 (m, 5H), 6.40 (s, 2H) ppm; CO2H
proton was not observed. ESI− TOF MS: m/z 328.1 {M − H}−.
HL2·0.5H2O. Aldehyde L2-CHO (Supporting Information, 627 mg,
1.42 mmol), formic acid (4 mL, 4.88 g, 0.11 mol), and H2O2 (0.85 mL
of a 30% wt aqueous solution containing 283 mg of H2O2, 8.32 mmol)
gave after precipitation with 15 mL of water and after washing with
water and hexane a pale pink solid: 565 mg (1.21 mmol, 85%). Anal.
Calcd for C28H31N3O3·0.5H2O (MW 466.57): C, 72.08; H, 6.91; N,
9.01. Found: C, 72.16; H, 6.89; N, 8.94. 1H NMR (400 MHz, DMSO-
d6): δ = 8.46 (dd, J = 7.6, 1.6 Hz, 1H), 8.12−8.02 (m, 2H), 7.62 (d, J =
8.8 Hz, 1H), 7.22 (d, J = 2.0 Hz, 1H), 7.21−7.10 (m, 5H), 6.89 (dd,
J = 8.8, 2.4 Hz, 1H), 6.34 (s, 2H), 3.99 (t, J = 6.8 Hz, 2H), 1.75−1.65
(m, 2H), 1.46−1.18 (m, 10H), 0.85 (t, J = 6.8 Hz, 3H) ppm; CO2H
proton not observed. ESI− TOF MS: m/z 456.2 {M − H}−.
Synthesis of Complexes. The reaction was performed under air.
The ligand was suspended in ethanol (5 mL), followed by addition of
NaOH dissolved in water (used as a stock solution with approximately
100 mg of NaOH per 10 mL of water) and stirring for 5 min to give a
solution. The solution was warmed to 70−80 °C. After 5 min of
stirring, a solution of LnCl3·nH2O in water (2 mL) was added drop-
wise over 5 min (a white precipitate may form on addition). Water
(1 mL for LnL1 and EuL2; 2 mL for LaL2) was added to induce and
complete precipitation of the complex. The suspension was stirred for
10 min at 70−80 °C, cooled to 40−50 °C, and filtered while warm
(the strict control of time and temperature is not required in this
synthesis). The product was washed with ethanol/water (1:1) and
either ether (LnL1) or hexane (LnL2) in that order. The complexes
were dried under vacuum at room temperature. All of the complexes
are soluble in DMSO, hot ethanol, and hot acetonitrile and are
insoluble in hexane, ether, and water. The complexes LnL2 (but not
LnL1) are soluble in dichloromethane. Further details are provided
below.
[La(L1)3]·4H2O: white solid; 48 mg (0.040 mmol, 84%) from HL1·
H2O (50 mg, 0.144 mmol), NaOH (5.76 mg, 0.144 mmol), and LaCl3·
7H2O (17.8 mg, 0.048 mmol). Anal. Calcd for C60H42LaN9O6·4H2O
(MW 1196.00): C, 60.25; H, 4.21; N, 10.54. Found: C, 60.32; H, 4.24;
N, 10.26. 1H NMR (400 MHz, DMSO-d6, 373 K, [C] = 0.79 ×
10−3 M; the complex dissociates in DMSO, a coordinating solvent):
δ = 8.17 (d, J = 7.6 Hz, 3H), 8.00 (t, J = 7.6 Hz, 3H), 7.95 (d, J =
7.6 Hz, 3H), 7.87 (d, J = 8.0 Hz, 3H), 7.61 (d, J = 8.0 Hz, 3H), 7.29
(t, J = 7.6 Hz, 3H), 7.27−7.07 (m, 18H), 6.18 (s, 6H) ppm.
[Eu(L1)3]·4H2O: white solid; 49 mg (0.041 mmol, 84%) from HL1·
H2O (50 mg, 0.144 mmol), NaOH (5.76 mg, 0.144 mmol), and
EuCl3·6H2O (17.5 mg, 0.048 mmol). Anal. Calcd for C60H42EuN9O6·
4H2O (MW 1209.06): C, 59.60; H, 4.17; N, 10.43. Found: C, 59.66;
H, 4.34; N, 10.16.
Synthesis of precursors; excitation and emission spectra;
1H NMR spectra; crystallographic data (PDF)
CIF for the crystal structure CCDC 1409902 (CIF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail (N. M. Shavaleev): shava@mail.ru.
■
*E-mail (J.-C. G. Bunzli): jean-claude.bunzli@epfl.ch. Tel:
̈
+41 21 693 9821. Fax: +41 21 693 5550.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This project was supported by the Swiss National Science
Foundation (grant 200020_119866/1).
■
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