Photophysical properties of asymmetric and water-soluble dinuclear lanthanide complexes of poly glycol chain functionalized-benzoic acid derivative: Experimental and theoretical approaches

Article abstract of DOI:10.1039/c6ra21487h

New water-soluble lanthanide complexes with a poly glycol chain (-OEtOEtOEtOMe) functionalized benzoic acid derivative ligand were synthesized and photophysically characterized. The results indicate a formation of an asymmetric dinuclear lanthanide comple

Full text of DOI:10.1039/c6ra21487h

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DOI: 10.1039/C6RA21487H.
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ARTICLE
Photophysical properties of asymmetric and water-soluble
dinuclear lanthanide complexes of poly glycol chain
functionalized-benzoic acid derivative: experimental and
theoretical approaches
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
G. Z. Bossharda, G. A. Britoa, J. H. S. K. Monteirob, A. de Bettencourt-Diasb, I. O. Mazalia, F. A. Sigolia
www.rsc.org/
New waterꢀsoluble lanthanide complexes with a poly glycol chain (–OEtOEtOEtOMe) functionalized benzoic acid
derivative ligand were synthesized and photophysically characterized. The results indicate a formation of an asymmetric
dinuclear lanthanide complexes. The [Eu2(mee)6(H2O)2] complex shows emission in water solution and it was noted that
after 48 h part of the ligands are replaced by water molecules resulting on quenching of the emission and decreasing of
lifetimes by OꢀH oscillators from water molecules. The average JuddꢀOfelt intensity parameters were determined
experimentally from emission spectra of the europium(III) complex. Additionally, the contribution of each asymmetric
europium(III) center to the JO intensity parameters were calculated independently using time resolved spectroscopy and the
LUMPAC software helping the proposition of the polyhedral coordination sphere determined by the ground state geometry
using the Sparkle/PM3 model. The transfer and back energy transfer rates were also calculated. The values obtained for
energy transfer rates are lower than previous values obtained for europium(III) complexes with benzoic acid derivative
ligands, indicating the role of the poly glycol chain on photophysical properties of the [Eu2(mee)6(H2O)2] complex.
achieved upon functionalizing the ligand with a glycol chain, –
,
Introduction
O(CH2CH2O)nCH3.4 5 The strategy of adding glycol chains into
the ligand structure was successfully pursued by Maury and
,
Lanthanide-based materials have unique photophysical
properties and therefore a broad range of applications, such as
displays, sensors and especially luminescent markers in
biological systems. The latter application has drawn the
attention of the scientific community, as the long emission
co-workers6 7 and also by de Bettencourt-Dias and co-
workers.8 Tuck and co-workers reported the addition of carbon
chains such as 3,5-bis((R)-2,3-dihydroxypropoxy)benzoate and
3,4,5-tris((R)-2,3-dihydroxypropoxy)benzoate) to benzoic acid
ligand to achieve water solubility of the complexes and
demonstrated that these can be used as asymmetric catalysts
for epoxide ring opening reactions.9
lifetimes in the ꢀs – ms range allows the rejection of the short-
lived emission from the biological tissues.1 Strong coordination
of the ligand to trivalent lanthanide ions and water solubility of
Facing the possibility of increasing the water solubility of
lanthanide benzoate complexes, this work reports a facile
synthesis of a poly glycol chain (-OEtOEtOEtOMe) benzoic acid
derivative ligand (Scheme 1) that can be used in the
preparation of luminescent water-soluble lanthanide(III)
complexes and their chemical stability in water solution. The
benzoic acid derivative ligand was characterized by 1H-NMR
and 13C-NMR, FT-IR spectroscopy and mass spectrometry. The
lanthanide(III) complexes (Ln = EuIII, GdIII and TbIII) were
characterized by high resolution mass spectrometry, TGA, FT-
IR and photoluminescence spectroscopies. The photophysical
properties were extensively studied in both solid state, in
water and acetonitrile solutions. Experimental photophysical
parameters were compared to theoretical ones in order to
propose a polyhedral coordination sphere and also to give
insights about the ligands exchange in water solution.
the
complexes are important factors for biological
applications, and for asymmetric catalysis in organic
,2
synthesis.1 Ligands derived from benzoic acid easily
coordinate to lanthanide(III) ions. However, the applications of
resulting lanthanide complexes are usually in the solid state,
due to the low solubility of this class of complexes. Li and co-
workers showed that complexes with the benzoic acid
nitrogen mustard (banm) ligand are soluble in water with 10 %
dimethyl sulfoxide (dmso) and show an inhibitory effect
against K562 and Vero cell lines.3 Bünzli and co-workers showed
that water solubility of lanthanide(III) complexes can be
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123.1, 114.1, 71.9, 70.9, 70.7, 70.6, 69.6, 67.6, 60.6, 59.0, 14.4
ppm (ESI-Figure S2).
Experimental Section
Synthesis of the ligand and complexes
4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzoic acid (Hmee) (3)
All solvents and the p-toluenesulphonyl chloride were dried
and purified, respectively, according to the standard methods.
The synthetic route to obtain the ligand 4-(2-(2-(2-
515 mg (1.65 mmol) of
2 was added to a 4.2 mL solution of
THF:methanol (1:1, in volume) containing 200 mg (5 mmol) of
NaOH and 2.1 mL of water. The system was kept under stirring
at RT for 20 h. The aqueous phase was washed with ethyl
acetate, and acidified (until pH ∼
2) with HCl 1 mol L-1 aqueous
methoxyethoxy)ethoxy)ethoxy)benzoic acid
represented in the Scheme 1.
(Hmee) is
solution and the product extracted with ethyl acetate (3 x 30
mL). The organic phase was dried under Na2SO4 filtered and
the solvent removed under reduced pressure to yield a white
solid. Yield: 374 mg (80 %). 1H-NMR (400 MHz, CDCl3):
δ 8.06
(d, Ar, J = 8.9 Hz, 2H), 6.97 (d, Ar, J = 8.8 Hz, 2H), 4.22 (dd, CH2,
J = 4.0 and 5.6 Hz, 2H), 3.91 (dd, CH2, J = 3.9 and 5.6 Hz, 2H),
3.77 (dd, CH2, J = 3.5 and 6.0 Hz, 2H), 3.66 – 3.74 (m, CH2, 4H),
3.58 (dd, CH2, J = 3.6 and 5.7 Hz, 2H), 3.40 (s, CH3, 3H) ppm.
13C-NMR (100 MHz, CDCl3):
δ 171.4, 163.2, 132.3, 121.9, 114.3,
71.92, 70.9, 70.7, 70.6, 69.5, 67.6, 59.0 ppm (Figure S3). ESI-
MS: [M + H]+ m/z found (calc.): 285 (285) (Figure S4).
Scheme 1. Synthetic route to obtain the ligand Hmee.
Lanthanides complexes - [Ln(mee)(H2O)x] (Ln = Eu3+, Gd3+ and Tb3+)
2-(2-(2-methoxyethoxy)ethoxy)ethyl 4-methylbenzenesulfonate
,
11
(1)10
The ligand was dissolved in ethanol and a NaOH solution was
added in the stoichiometric proportion 1:1 (L:OH-). LnCl3 was
dissolved in a small amount of deionized water and added to
the ethanolic solution containing the deprotonated ligand in
1:3 (Ln:L) stoichiometric ratio. The solution was kept under
stirring at 25 oC for 5 h. The solvent was removed under
reduced pressure, and the precipitate was dissolved in water
and filtered off to remove any unreacted ligand. The solvent
was again removed under reduced pressure and the solid re-
dissolved in chloroform and filtered off to remove any residual
salts. The solvent was removed under reduced pressure to
yield the product as a pale yellow solid. ESI-MS: [EuL3 + H]+
m/Z found (calc.) 1003.22 (1003.28) (ESI-Figure S5). [GdL3 + H]+
m/Z found (calc.) 1008.25 (1008.29) (ESI-Figure S6). [TbL3 + H]+
m/Z found (calc.) 1009.23 (1009.29) (ESI-Figure S7).
6.0 g (31.5 mmol) of p-toluenesulphoynl chloride and 5.04 mL
(31.5 mmol) of 2-(2-(2-methoxyethoxy)ethoxy)ethanol were
dissolved in 81 mL of CH2Cl2 followed by the addition of 5.48
mL (39.4 mmol) of triethylamine. The system was kept under
stirring at 9 °C for 20 h. The reaction was quenched with
sodium bicarbonate solution and extracted with ethyl acetate
(3 x 30 mL), dried under Na2SO4, filtered and the solvent was
removed under reduced pressure to yield a colorless oil that
was used in the next step without further purification. Yield:
9.56 g (95 %).1H-NMR (400 MHz, CDCl3):
δ 7.73 – 7.79 (m, Ar,
2H), 7.31 (d, Ar, J = 8.1 Hz, 2H), 4.16 – 4.09 (m, CH2, 2H), 3.67 –
3.62 (m, CH2, 2H), 3.60 – 3.53 (m, CH2, 2H), 3.51 – 3.46 (m,
CH2, 2H), 3.33 (s, CH3, 3H), 2.41 (s, CH3, 3H) ppm. 13C-NMR (100
MHz, CDCl3):
δ 144.8, 133.0, 129.8, 128.0, 72.5, 71.9, 70.7,
70.5, 69.3, 68.6, 59.0, 21.6 ppm (ESI-Figure S1)
Characterization
Methyl 4-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)benzoate (2)10
Unless otherwise indicated, all data were collected at a
constant temperature of 298
±
1 K. The 1H and 13C-NMR
7.70 g (55.7 mmol) of potassium carbonate, 0.45 g (1.7 mmol)
spectra were obtained in deuterated solvents in a Bruker
Avance 400 or 500 MHz. Low resolution electrospray mass
spectrum (ESI-MS) of the ligand was obtained in the positive
ion mode in a Waters Micromass ZQ quadrupole mass
spectrometer. High resolution electrospray mass spectra (ESI-
MS) of the lanthanide(III) complexes were obtained in the
positive ion mode with methanol as solvent in a Waters Xevo
QTOF mass spectrometer. The thermogravimetric analyses (TA
instruments SDTQ600) were carried out using a synthetic air
flow (100 mL min-1) under a heating rate of 10 °C min-1.
Infrared spectroscopy (Cary 630 FTIR) data were obtained in
attenuated total reflectance mode. The photoluminescence
data were obtained with the samples in the solid state and in
aqueous and acetonitrile solutions (1x10-4 mol L-1) in a
Fluorolog-3 spectrofluorometer (Horiba FL3-22-iHR320), with
double-gratings (1200 gr/mm, 330 nm blazed) in the excitation
of 18-crown-6 and 1.84
hydroxybenzoate were added to a solution containing 4.56 g
(10 mmol) of the freshly prepared in acetone to yield a pale
yellow suspension. The system was kept under stirring and
nitrogen atmosphere at 55 C for 22 h. The reaction was
g (10 mmol) of methyl 4-
1
°
quenched with water and the product extracted with ethyl
acetate and purified through a silica chromatographic column
using ethyl acetate:hexane (1:1) as eluent to yield a colourless
1
oil. Yield: 2.56 g (72 %). H-NMR (400 MHz, CDCl3):
δ 8.06 –
7.94 (m, CH, 2H), 6.99 – 6.88 (m, CH, 2H), 4.35 (q, J = 7.1 Hz,
CH2, 2H), 4.21 – 4.17 (m, CH2, 2H), 3.91 – 3.86 (m, CH2, 2H),
3.75 (dd, J = 5.9 and 3.5 Hz, CH2, 2H), 3.72 – 3.64 (m, CH2, 4H),
3.58 – 3.53 (m, CH2, 2H), 3.38 (s, CH3, 3H), 1.38 (t, J = 7.1 Hz,
3H) ppm. 13C-NMR (100 MHZ, CDCl3):
δ 166.3, 162.5, 131.5,
2 | J. Name., 2012, 00, 1-3
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monochromator and double-gratings (1200 gr/mm, 500 nm The Sparkle/PM3 model was used to determine the complexes
blazed) in the emission monochromator. An ozone-free xenon ground state geometries.18 In this model the lanthanide ion is
lamp of 450 W (Ushio) was used as radiation source. The replaced by a +3e point charge.19 The RHF wave functions were
excitation spectra were obtained between 250-550 nm and optimized using the Broyden–Fletcher–Goldfarb–Shanno
they were corrected in real time according to the lamp (BFGS) procedure with a convergence criterion of 0.15 kcal
intensity and the optical system of the excitation mol-1 Å-1 and the semi empirical PM3 with a convergence
monochromator using a silicon diode as reference detector. criterion of 10-6 kcal mol-1 for the SCF. In Mopac2016 package20
The emission spectra were obtained between 400-720 nm the following keywords were used: PM3, SPARKLE, XYZ,
using the front face mode at 22.5° (for solid samples) or right SCFCRT=1D-10, GEO-OK, BFGS, CHARGE=X, PRECISE,
angle (for solutions). The time-resolved emission spectra of GNORM=0.25 ALLVEC AUX and T=2D. The theoretical JO
europium(III) complexes were obtained with a pulsed 150 W intensity parameters were calculated using the adequate
Xenon lamp using a TCSPC system with successive delay equations and adjusting, in the physical acceptable range,13 the
increments. All emission spectra were corrected according to polarizability (α) and the charge factors (g) of the ligands in
the optical system of the emission monochromator and the order to fit the theoretical JO intensity parameters with the
photomultiplier response (Hamamatsu R928P). The time- experimental ones. The experimental JO intensity parameters
resolved phosphorescence emission spectra of the analogous obtained without delay correspond to the average value
gadolinium(III) complexes were obtained at
∼77 K using a between the two europium(III) sites (Eu1 and Eu2) of dinuclear
TCSPC system with successive delay increments, in order to get complex. The determination of JO experimental parameters,
only the emissions from the triplet level of the ligand. The Arad (radiative emission rate) and Anrad (non-radiative decay
energy values of the ligand triplet level were obtained using rate) of the Eu2 was done using the time-resolved emission
two approaches: (i) fitting a tangent to the highest energy spectra and the experimental parameters of the Eu2 center
edge of the emission spectra or (ii) the maximum of the were estimated using the equation (3).
highest energy vibrational-coupled band (0-0 phonon)
obtained from the deconvolution of the emission spectra.12
ꢐꢑꢒ.ꢓꢓ.ꢔꢕꢖ
ꢎꢏꢅ
=
(3)
ꢒ.ꢗꢘ
The emission decay curves were obtained with a pulsed 150 W
Where ꢎꢏꢅ refers to the Arad or Anrad or JO intensity
parameters of the europium with two coordinated water
molecules; T refers to the Arad or Anrad or JO intensity
parameters determined from europium(III) stead-state
emission spectra and ꢎꢏꢙ refers to the Arad or Anrad or JO
intensity parameters determined from time-resolved
europium(III) emission spectra, that eliminates emission from
Eu2 center, that has shorter emission lifetime than Eu1 center.
The 0.66 and 0.34 values are the contribution ratios of the first
and second coefficient terms of the bi-exponential function
used for fitting the lifetime emission decay curve (ESI-Figure
S8).
xenon lamp using
intensity parameters
parameters (Arad, Atot and
equations described in the literature.13 The number of
coordinated water molecules (q) was determined using the
equation 1, proposed by Horrocks and co-workers, considering
that the O-H oscillator is the only quenching route of the
emitting level.14
a
TCSPC system. The Judd-Ofelt (JO)
ꢁ2 and ꢁ4 and the efficiency
) were calculated using the
(
)
η
1
1
ꢀ = ꢁ ∙ ꢂ
−
− ꢈꢉ
(1)
ꢃꢄꢅꢆ ꢃꢇꢅꢆ
where A = 1.1,
α = 0.31 and τD2O = τ
rad for Eu(III).15
The excited states calculations were performed using ORCA
software21 using the INDO/S-CIS22 with the lanthanide replaced
by a +3e point charge.13,19 The transfer and back transfer
The relative quantum yields Φꢊ of the samples were
determined at 25.0
±
0.1oC using the equation (2).
5
energy rates from the ligand’s triplet level to the D0,1
ꢁꢋꢌ ∙ ꢍꢋꢌ ∙ ꢎꢊ
europium(III) levels as well the theoretical quantum efficiency
and quantum yield were calculated using the adequate kinetics
equations described by Malta and collaborators23 24 25
implemented in the LUMPAC software26 (version 1.2.0).
Φꢊ
=
∙ Φꢋꢌ
(2)
ꢁꢊ ∙ ꢍꢊ ∙ ꢎꢋꢌ
,
,
Ast and Ax are the absorbance at the excitation wavelength of
the standard st and sample x, respectively; Ist and Ix are the
intensities of the excitation source at the excitation
wavelength, Est and Ex are the integrated emission spectra and
φst is the quantum yield of the standard. Standards for
Results and discussion
New lanthanide complexes (EuIII, GdIII and TbIII) with a benzoic
acid derivative ligand, functionalized with the polyglycol chain
-OEtOEtOEtOMe at the para position were synthetized,
isolated and characterized. The stoichiometries of the
complexes, as monomer motifs once they were solubilized in
methanol containing 0.1 % of formic acid, were confirmed by
high resolution ESI-MS (ESI - Figures S5 – S7 and Table S1) and
the amounts of coordinated water molecules were determined
by TGA (ESI-Figure S9) and by emission lifetime using Horrocks
quantum yield measurements were Cs3[Eu(dipic)3] (
and Cs3[Tb(dipic)3] (
22 %) in 0.1 mol L-1 aqueous TRIS/HCl
buffer solution (pH
φ∼24 %)
φ
∼
∼
,
7.4).16 17 The excitation wavelengths were
chosen to ensure a linear relationship between the intensity of
emitted light and the concentration of the absorbing/emitting
species (A
≤ 0.05).
Ground state geometries and theoretical calculations
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equation (1). The coordination mode of the carboxylate moiety geometries have been reported.30 Recently, Malvestiti and
was confirmed by the shift to lower wavenumbers of the collaborators31 have published an interesting extended
ν
a(COO-), Figure 1. The coordination of the carboxylate ion to structure with asymmetrical dinuclear units based on 2-
the lanthanide(III) can be bidentate bridging, and/or bidentate (methylthio)benzoic acid derivate. The asymmetric units of the
chelate, and/or monodentate. The value of ꢂν ꢂν
a(COO-) - terbium(III) complex obtained from crystallographic data,
s(COO-)] provides some indication about the possible indicate that the carboxylate group is coordinated basically by
[
= ν
ν
,
coordination modes of this group.27 28 According to the values bidentate chelate and bridging modes, being one terbium(III)
determined for this series of complexes, Table 1, the ligands ion coordinated by eight oxygens from carboxylate groups and
may be coordinated to the metal ion through the bidentate the other terbium(III) ion is coordinated by four oxygens from
bridging mode (ꢂνligand
mode (ꢂνligand salt
∼ ꢂνcomplex) or chelate bidentated carboxylate group plus four oxygens from coordinated water
salt
>
ꢂνcomplex). Specifically, for the europium(III) molecules.31 The benzoic acid-derivate ligand published by
complex, one may note, that both asymmetric and symmetric Malvestiti and collaborators31 is structurally similar to the one
vibrations mode of carboxylate group are slightly split synthetized in this work. Based on the FT-IR data in addition to
indicating a possible mixture between the bridging and chelate the photoluminescence decay curve of the europium(III)
mode simultaneously, as previously observed.29
complex that was fitted by a bi-exponential decay function
indicating at least two emitting centers and two coordinate
water molecules in one of the europium centers, it is possible
to infer that the structure of the europium(III) complex has
two asymmetric units (Figure 2). In the first one the
europium(III) ion (Eu1) probably is bonded to eight oxygens
from carboxylate groups, where three carboxylate groups
(oxygen atoms numbers 9 to 14) are coordinated in a
bidentate chelate mode and the other two (6 and 8) are
coordinated as
a bidentate bridging and chelate mode
simultaneously. The other europium center (Eu2) probably is
bounded to two oxygen from carboxylate groups (oxygen
atoms 3 and 4 - Figure 2) coordinated as bidentate chelate
mode, four oxygen (oxygen atoms numbers 5 to 8) from the
bidentate bridging and chelate mode from Eu1 and two
oxygens (oxygen atoms numbers 1 and 2) from coordinated
water molecules. The FT-IR spectra of terbium(III) and
gadolinium(III) complexes are comparable to the ones
published by Malvestiti and collaborators31 probably indicating
similar structures and coordination modes.
The ground state geometry of the europium(III) complex
(Figure 2) was calculated using the Sparkle/PM3 model
implemented in the MOPAC2016 software and it will be
Figure 1. FT-IR spectra of the ligand salt and lanthanide(III) complexes. (a)
Na+mee- (b) europium(III) complex (c) gadolinium(III) complex and (d)
terbium(III) complex
discussed
later.
Table 1. Values of
complexes.
ν ν
a(COO-), s(COO-) and ꢂν(COO-) of the ligand sodium salt and the
ν
a(COO-)/
cm-1
ν
s(COO-)/
cm-1
ꢂν(COO-)/
Coordination
Modes
cm-1
Na+
1547
1417
1393
130
ꢀꢀꢀ
EuIII
1539/1506
1542/1508
146/113
mixed bidentate
chelate and
bridging
GdIII
TbIII
1405
1405
137/103
140/103
1545/1508
In the past years, different coordination modes of the
carboxylate ligand toward trivalent lanthanide ions have been
identified and consequently a large number of complex
4 | J. Name., 2012, 00, 1-3
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Figure 2. Ground state geometry of the [Eu2(mee)6(H2O)2] complex obtained using the Sparkle/PM3 model. The theoretical bond distances are in Angstroms.
The triplet energy level (T) of the ligand was determined using
the two approaches described in the Experimental Section. The
time resolved phosphorescence emission spectra of analogous
gadolinium(III) complex (ESI-Figure S10.a) and the
deconvolution of the phosphorescence bands (ESI-Figure
S10.b) are shown in the Electronic Supporting Information. The
energy values of the triplet energy level obtained by the two
approaches were 25,610 cm-1 (edge) and 26,130 cm-1 (0-0
phonon). These values are comparable to the ones (24,641 cm-
1
and 23,202 cm-1) obtained for benzoic acid ligand.32 The
excitation and emission spectra of the europium(III) and
terbium(III) complexes in the solid state and dissolved in
acetonitrile and water are shown in Figures 3A and 3B.
(B)
Figure 3. Excitation and emission spectra of the (A) europium(III) and (B)
terbium(III) complexes. The spectra were obtained at 298 K in (a) the solid state,
(b) acetonitrile and (c) water. The measurements were done after 48 h of the
solution preparation.
The excitation and emission spectra of the europium(III)
complex (Figure 3A) in different solvents as well in the solid
state provide information about the microsymmetry around
the europium(III) ion33 and the degree of covalence of the Eu–L
,
,
bonds.34 35 36 In all cases, Figure 3A, the expected transitions
5D0 7FJ, (J = 0; 1; 2; 3 and 4) are observed. The ratio of the
band intensity attributed to the forced electric dipole
transition (5D0 7F2) and the band intensity attributed to the
magnetic dipole one (5D0 7F1) correlates with the
→
→
(A)
→
microenvironment around the europium(III). If the
microenvironment around the europium(III) lacks an inversion
center, the band attributed to 5D0
→
7F2 transition has higher
7F1 one. Figure 3A shows that the
intensity than the 5D0
→
europium(III) lies in a microenvironment without inversion
center when in the solid state and dissolved in acetonitrile.
However, a decrease in the intensity of the 5D0 7F2 is
→
observed when the europium(III) complex is dissolved in water
(Figure 3A-c) meaning that, in this solvent, the
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microenvironment around the europium(III) ion is more emission lifetime value almost three times higher than Eu2
symmetric leading to an increase of the symmetry also center. When the sample is solubilized in acetonitrile the
illustrated by the intensity of the 5D0
→
7F4 transition band.
emission lifetime values are higher than the ones in the solid
The emission decay curve of the solid state europium(III) sample, suggesting that, in solid state, the poly glycol chain
complex was fitted using a bi-exponential decay function (ESI- may entrap water molecules close to the europium(III) ion
Figure S8) indicating the existence of at least two emitting and/or create other routes for emission quenching. The
centers (Table 2). Using the Horrocks equation (equation 1) it emission lifetime values are comparable to the ones available
was possible to infer that the Eu2 center has two coordinated in the literature for europium(III) complexes obtained from
water molecules and the Eu1 has none. The emission lifetime benzoic acid derivate ligands with no coordinated water
values may be related to emission deactivation of molecules31, and with two coordinated water molecules per
37
,
,
europium(III) ions. In the solid sample, the Eu1 center has europium(III) center.32,
38 39 40
Table 2. Values of the emission lifetime (τ
) and relative quantum yield (ꢚꢛꢛꢜ) of the europium(III) and terbium(III) complexes in the solid state or dissolved in acetonitrile or water.
Ln
τ
/ ms
ꢚꢛꢛꢜ / %
Acetonitrile
2.1
Solid
Acetonitrile
Water
Solid
ꢀꢀꢀ*
Water
< 1
1.15 ± 0.01 (Eu1)
0.41 ± 0.01 (Eu2)
1.60 ± 0.01 (Eu1)
0.77 ± 0.02 (Eu2)
Eu
0.130 ± 0.001
0.413 ± 0.001
Tb
0.986 ± 0.002
1.451 ± 0.002
ꢀꢀꢀ*
11.4
1.4
* the absolute ꢝꢞꢞꢟ of the complexes in the solid state were not measured due to limitations of the experimental setup that allows excitations beyond 350 nm.
experimental JO intensity parameters, the optimized ground
state geometry of dinuclear complex (Figure 2) was taken and
The excitation and emission spectra of the terbium(III)
in sequence the JO intensity parameters were calculated
complex in the solid state or in different solvents (Figure 3B)
considering only the coordinated atoms to Eu1 center. There is
show the characteristics bands of this ion attributed to the
5D4 7Fj transitions. There are no significant differences of the
→
good agreement between the experimental and calculated JO
intensity parameters (Table 3) indicating that the proposed
overall ground state geometry is acceptable. In order to
terbium(III) emission spectra on solid state, in water or
acetonitrile solutions. One may see just slight changes on the
7F6 in water solution (Figure
evaluate the JO intensity parameters of the Eu2 center, the
ꢦꢧꢨ
5
emission band attributed to D4
→
overall JO intensity parameters (ꢠꢡ
) were determined
ꢢꢣꢢꢤꢥ
3B-c) and a different sensitization of the terbium(III) when the
excitation spectra are obtained in solutions (Figure 3B-bc). In
all cases, the emission decay curves of terbium(III) complex
were fitted using single exponential decay functions (ESI-Figure
S12), corroborating with the geometries proposed by
Malvestiti and collaborators.31
using the europium(III) steady-state emission spectrum, then
equation 3 was used to infer the JO values of the Eu2 center
and the experimental values of Ω2 and Ω4 of both Eu1 and Eu2
were used as a reference for the evaluation of the theoretical
ground state geometry (Figure 2). The Ω2 parameter of the Eu2
is larger than Eu1 (Table 3), indicating that the former is
situated in a lower symmetric chemical environment than Eu1
center. One may note that the experimental (ꢠꢡꢦꢩꢧꢨ) and
The Judd-Ofelt (JO) intensity parameters ꢁ2 and ꢁ4 are
strongly correlated to the symmetry of the microenvironment
around the europium(III) ion. A high value for the ꢁ2 intensity
parameter means low local symmetry. The decrease of the ꢁ2
intensity parameter (Table 3) when the complex is dissolved in
water, compared with acetonitrile solutions or the solid state,
indicates that the europium complex is not stable in water and
possibly undergoes to ligand exchange process. As discussed
before, the europium(III) complex has two different chemical
environments (centers) and it is possible to determine the
theoretical JO intensity parameters for each one. In order to
do so, the emission spectra of europium(III) complex were
obtained with successive delay increments (ESI-Figure S13) and
the minimum necessary delay time to eliminate the emission
of the less symmetric Eu2 ion was calculated from the lifetime
emission decay functions. Therefore, the experimental JO
intensity parameters of the Eu1 center were determined using
the emission spectra with a delay of 6.5 ms. To evaluate the
theoretical
( ) values of ꢁ4 and Anrad are significantly
ꢠꢡꢪꢩℎꢫꢬ
different. That discrepancy probably indicates an important
role of the poly glycol chain of the ligands over the
photophysical parameters of the Eu2 center that could not
properly be predict because of geometry distortion and/or
water molecules entrapped in this chain. These entrapped
water molecules may be close enough to Eu2 center promoting
non-radiative routes.
The complex in acetonitrile solution also has two emission
lifetime values, 1.60 ± 0.02 ms (Eu1) and 0.77 ± 0.02 ms (Eu2).
The Horrocks equation (equation 1) indicates that there are no
coordinated water molecules to Eu2 center and, therefore,
probably two acetonitrile molecules are coordinated to Eu2, as
also showed by Kumar and collaborators.41 As it was done in
solid state, the minimum delay time necessary to eliminate the
Eu2 center emission was calculated by adjusting the equations
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from the emission lifetime decay curves (ESI-Figure S8). between experimental and theoretical JO intensity parameters
However, the delay time necessary would be 15.5 ms and in acetonitrile solution.
using this delay it was not possible to obtain a resolved This result corroborates with that obtained in solid state
emission spectrum to proceed with the JO intensity sample, suggesting again that the proposed overall ground
parameters determination for each europium(III) center. state geometry is reasonable.
Therefore, the steady-state emission spectrum was used to
determine the JO parameters and the both europium centers
were adjusted to these values showing a good agreement
Table 3. Experimental and theoretical JO intensity parameters (ꢁ2 and
ꢁ
4), radiative emission rate (Arad), of the europium(III) complexes in the solid state or dissolved in
acetonitrile or water.
Solid State
ꢠꢡꢢꢭꢮꢦꢣ
12.55
Acetonitrile Solution
∗∗
ꢦꢧꢨ
ꢢꢣꢢꢤꢥ
ꢦꢧꢨ
ꢢꢣꢢꢤꢥ
ꢠꢡ
ꢠꢡꢦꢭꢧꢨ∗
12.55
2.61
ꢠꢡꢦꢩꢧꢨ
21.67
12.28
ꢀꢀꢀ
ꢠꢡꢢꢩꢮꢦꢣ
23.26
5.76
ꢠꢡ
ꢠꢡꢢꢭꢮꢦꢣ∗∗∗
17.06
ꢠꢡꢢꢩꢮꢦꢣ∗∗∗
16.12
ꢁ2 x10ꢀ20 cm²
ꢁ4 x10ꢀ20 cm²
ꢁ6 x10ꢀ20 cm²
Arad
15.69
5.94
16.12
8.07
ꢀꢀꢀ
2.61
4.41
8.07
ꢀꢀꢀ
ꢀꢀꢀ
0.1622
465.5
0.443
834.9
1604.1
0.1630
447.7
0.3385
467.2
612.9
1826.1
466.1
411.1
892.4
4519.7
479.5
145.5
Anrad
411.7
177.4
831.5
* The ꢠꢡꢭꢦꢧꢨ was determined from time-resolved emission spectra using a delay time of 6.5 ms. ** The values were calculated using equation 3. *** The Eu1theo and
ꢦꢧꢨ
ꢢꢣꢢꢤꢥ
theo
Eu2
values were adjusted to ꢠꢡ
, that was obtained from steady-state emission spectrum.
When the [Eu2(mee)6(H2O)2] complex is solubilized in water
7F2 transition has almost
5
the emission band attributed to D0
→
the same intensity as the 5D0
→
7F1 one and the band attributed
5
to D0
→
7F4 transition is intensified indicating a modification of
the microsymmetry around europium(III) ion. There is also a
decrease of the emission lifetime when the europium(III)
complex is dissolved in water, compared to solid state or
acetonitrile, Table 2. The modification of the microsymmetry
and the emission lifetime decrease might be an indication of
ligands exchange by water molecules. In order to verify this
indication, the emission spectra were obtained in different
ratios of water:acetonitrile solution (Figure 4).
Figure 4. Emission spectra of the europium(III) complex in different
water:acetonitrile volume ratios indicated in the figure. The measurements were
done after 48 h of the solution preparation.
Another indication of the replacement of the ligand by water
molecules is the appearance of the broad emission band
between 450 – 575 nm that may attributed to the ligand. If the
ligand is non-coordinated or even mono-coordinated to the
europium(III), the energy transfer to the europium(III) is
reduced and the broad band due to ligand phosphorescence
appears. This data agrees with emission lifetime decay curves
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Journal Name
and also with data obtained by ESI-MS (ESI-Figures S5-S7) that
indicates a 1Ln:3mee molar ratio. Therefore, the complex
dissolution in water probably breaks up the dinuclear unit and
subsequently promotes the exchange of part of the ligands by
water molecules. A close examination of the theoretical
coordination polyhedral (Figure 2) shows that the oxygens
from carboxylate group bonded as chelate/bridging modes
(Figure 2: oxygens 6 and 8) have long bond distances (2.512
and 2.519 Å) that are similar to the bond distance (2.512 Å)
between the water oxygen (Figure 2: oxygen 1) and
europium(III) ion. The long distances between these bridging
oxygens and the central ion together with the hydrophilic
nature of the poly glycol chains may explain the instability of
the complex into water solution, being the ligand solvated by
water.
Figure 5. Correlation between the emission lifetime (τ) and the number of
coordinated water molecules (q) of the [Eu(mee)3(H2O)n] complexes as a function
of the amount of the percentage of water into acetonitrile solution.
The data (Figure 5) indicates an increase in the number of
coordinated water molecules in the europium(III) complex,
explaining the low emission lifetime value in water (Table 2),
as the O–H oscillators close to the lanthanide(III) are efficient
emission quenchers. The high number of coordinated water
molecules obtained for 100% water solution indicate that
majority of the ligand molecules were replaced by water.
Kumar and co-workers also observed a decrease in the
emission lifetimes values for the [Eu(ba)3] complex (ba =
benzoic acid) by increasing water concentration in
acetonitrile.41
The energy transfer rates (Table 4) from ligand to europium(III)
ion, were calculated for the europium(III) complex using the
software LUMPAC (version 1.2.0).26 The energy transfer rates
from the ligand to the lanthanide excited levels are higher than
the back-transfer ones. However, the combination of the high
value of the RL parameter (from 5.5393 Å to 7.0475 Å) with the
high energy of the triplet level (25,606 cm-1) leads to low
The increase of the number of coordinated water molecules
(q) as a function of the percentage of water into acetonitrile
solution was calculated using Equation 1. The correlation
between the emission lifetime (
τ) and the coordinated water
molecules (q) is shown in Figure 5.
energy transfer rates ligand
→
EuIII and, consequently a low
relative quantum yields (Table 2). The transfer and back
transfer rates calculated for Eu2 in the solid state or in
acetonitrile solution are smaller than that one calculated to
Eu1 center. This can be justified by the higher values of RL
parameter
related
to
Eu2
than
Eu1
center.
Table 4. Transfer and back energy transfer rates between the ligand triplet level and 5D1 and 5D0 europium(III) levels, calculated from the ground state geometries provided by
Sparkle/PM3.
Solid State
Acetonitrile Solution
[Eu(abz)3(H2O)2].3/2H2O32
Eu1
Eu2
Eu1
Eu2
7.8
T → 5D1 / sꢀ1
5D1 → T / sꢀ1
T → 5D0 / sꢀ1
5D0 → T / sꢀ1
RL / Å
4.8x108
1.2x10ꢀ5
8.3x107
4.6x10ꢀ10
5.5235
4.5x101
3.3x10ꢀ5
3.8x102
5.7 x10ꢀ9
5.5782
1.8x101
1.1x10ꢀ6
1.3x101
2.0x10ꢀ10
6.7098
6.8x102
3.3x10ꢀ5
4.3x102
5.8x10ꢀ9
5.5393
4.3x10ꢀ7
5.6
7.5x10ꢀ11
7.0475
* reference 31 used the first LUMPAC version.
8 | J. Name., 2012, 00, 1-3
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Conclusions
In summary, a new series of water-soluble luminescent
binuclear lanthanide complexes (EuIII, GdIII and TbIII) were
prepared. The ligand coordinates to the lanthanide(III) through
the mixed bridging and bidentate chelate mode, as
determined by FT-IR and indicated by comparing the
experimental and theoretical photophysical parameters
determined from ground state geometry, that was calculated
using the Sparkle/PM3 model. Both europium(III) and
terbium(III) complexes are luminescent in water solution. The
decrease of the band intensity attributed to 5D0 7F2 transition
→
in aqueous solution when compared with the solid, indicates
that there is an increase in the microsymmetry around the
europium(III) when it is dissolved in water. The decrease of the
emission lifetime in aqueous solution and the changes of
emission profile in several ratio of acetonitrile:water solutions
indicates the replacement of the coordinated ligands by water
molecules. The increase of coordinated water molecules (q)
was confirmed using the well-established Horrocks equation.
The relative quantum yield of sensitized emission is higher in
acetonitrile than in water, due to the quenching effect of the
O–H oscillators. The transfer and back energy transfer rates
were calculated using the LUMPAC software and the values
obtained for energy transfer rates are lower than previous
values obtained for europium(III) complexes with benzoic acid
derivative ligands, indicating the role of the poly glycol chain
attached to benzoic acid on photophysical properties of the
[Eu2(mee)6(H2O)2] complex.
Acknowledgements
GZB is indebted to CAPES for the Ph.D. fellowship. GAB and JHSKM
thank to FAPESP (grant: 2015/22426-5) and CNPq (grant:
159249/2014-7) for the Postdoctoral fellowships, respectively. IOM
and FAS are indebted to CNPq, CAPES and FAPESP (grant:
2013/22127-2) for financial support. AdBD and FAS thank to
MEC/MCTI/CNPq/CAPES PEV program (grant: 400214/2014-8).
AdBD also acknowledges financial support through NSF (CHE-
1363325). All authors would like to thank the Multiuser Laboratory
of Advanced Optical Spectroscopy
UNICAMP.
- Institute of Chemistry-
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