Definition of an intramolecular Eu-to-Eu energy transfer within a discrete [Eu2L] complex in solution

Article abstract of DOI:10.1002/chem.201200087

Ligand L, based on two do3a moieties linked by the methylene groups of 6,6'-dimethyl-2,2'-bipyridine, was synthesized and characterized. The addition of Ln salts to an aqueous solution of L (0.01 M Tris-HCl, pH 7.4) led to the successive formation of [LnL] and [Ln₂L] complexes, as evidenced by UV/Vis and fluorescence titration experiments. Homodinuclear [Ln₂L] complexes (Ln=Eu, Gd, Tb, Yb, and Lu) were prepared and characterized. The ¹H and ¹³C NMR spectra of the Lu and Yb complexes in D₂O solution (pD=7.0) showed C₁ symmetry of these species in solution, pointing to two different chemical environments for the two lanthanide cations. The analysis of the chemical shifts of the Yb complex indicated that the two coordination sites present square antiprismatic (SAP) coordination environments around the metal ions. The spectroscopic properties of the [Tb₂L] complex upon ligand excitation revealed conventional behavior with τ_H2O=2.05(1) ms and φ_H2O=51 %, except for the calculation of the hydration number obtained from the luminescent lifetimes in H₂O and D₂O, which pointed to a non-integer value of 0.6 water molecules per Tb^III ion. In contrast, the Eu complex revealed surprising features such as: 1) the presence of two and up to five components in the ⁵D₀→⁷F₀ and ⁵D₀→⁷F₁ emission bands, respectively; 2) marked differences between the normalized spectra obtained in H₂O and D₂O solutions; and 3) unconventional temporal evolution of the luminescence intensity at certain wavelengths, the intensity profile first displaying a rising step before the occurrence of the expected decay. Additional spectroscopic experiments performed on [Gd _2-xEu_xL] complexes (x=0.1 and 1.9) confirmed the presence of two distinct Eu sites with hydration numbers of 0 (site I) and 2 (site II), and showed that the unconventional temporal evolution of the emission intensity is the result of an unprecedented intramolecular Eu-to-Eu energy-transfer process. A mathematical model was developed to interpret the experimental data, leading to energy-transfer rates of 0.98 ms^-1 for the transfer from the site with q=0 to that with q=2 and vice versa. Hartree-Fock (HF) and density functional theory (DFT) calculations performed at the B3LYP level were used to investigate the conformation of the complex in solution, and to estimate the intermetallic distance, which provided Foerster radii (R₀) values of 8.1 A for the energy transfer from site I to site II, and 6.8 A for the reverse energy transfer. These results represent the first evidence of an intramolecular energy-transfer equilibrium between two identical lanthanide cations within a discrete molecular complex in solution. Copyright

Full text of DOI:10.1002/chem.201200087

FULL PAPER
DOI: 10.1002/chem.201200087
Definition of an Intramolecular Eu-to-Eu Energy Transfer within a Discrete
[
Eu L] Complex in Solution
2
[
a]
[b]
[b]
[b]
Aline Nonat, Martꢀn Regueiro-Figueroa, David Esteban-Gꢁmez, Andrꢂs de Blas,
[
b]
[b]
[a]
Teresa Rodrꢀguez-Blas, Carlos Platas-Iglesias,* and Loꢃc J. Charbonniꢄre*
Abstract: Ligand L, based on two do3a
moieties linked by the methylene
groups of 6,6’-dimethyl-2,2’-bipyridine,
was synthesized and characterized. The
addition of Ln salts to an aqueous solu-
tion of L (0.01m Tris-HCl, pH 7.4) led
to the successive formation of [LnL]
fH2O =51%, except for the calculation
of the hydration number obtained from
with hydration numbers of 0 (site I)
and 2 (site II), and showed that the un-
conventional temporal evolution of the
emission intensity is the result of an
unprecedented intramolecular Eu-to-
Eu energy-transfer process. A mathe-
matical model was developed to inter-
pret the experimental data, leading to
the luminescent lifetimes in H O and
2
D O, which pointed to a non-integer
2
III
value of 0.6 water molecules per Tb
ion. In contrast, the Eu complex re-
vealed surprising features such as:
1) the presence of two and up to five
and [Ln L] complexes, as evidenced by
2
5
7
5
ꢀ1
UV/Vis and fluorescence titration ex-
components in the D ! F and D !
energy-transfer rates of 0.98 ms for
0
0
0
7
periments. Homodinuclear [Ln L] com-
F
1
emission bands, respectively;
the transfer from the site with q=0 to
that with q=2 and vice versa. Hartree–
Fock (HF) and density functional
theory (DFT) calculations performed
at the B3LYP level were used to inves-
tigate the conformation of the complex
in solution, and to estimate the inter-
metallic distance, which provided Fçr-
2
plexes (Ln=Eu, Gd, Tb, Yb, and Lu)
2) marked differences between the nor-
were prepared and characterized. The
malized spectra obtained in H O and
2
1
13
H and C NMR spectra of the Lu and
Yb complexes in D O solution (pD=
D O solutions; and 3) unconventional
2
temporal evolution of the luminescence
intensity at certain wavelengths, the in-
tensity profile first displaying a rising
step before the occurrence of the ex-
pected decay. Additional spectroscopic
2
7
.0) showed C symmetry of these spe-
1
cies in solution, pointing to two differ-
ent chemical environments for the two
lanthanide cations. The analysis of the
chemical shifts of the Yb complex indi-
cated that the two coordination sites
present square antiprismatic (SAP) co-
ordination environments around the
metal ions. The spectroscopic proper-
ster radii (R ) values of 8.1 ꢀ for the
0
experiments performed on [Gd2 Eu L]
energy transfer from site I to site II,
and 6.8 ꢀ for the reverse energy trans-
fer. These results represent the first
evidence of an intramolecular energy-
transfer equilibrium between two iden-
tical lanthanide cations within a discrete
molecular complex in solution.
ꢀx
x
complexes (x=0.1 and 1.9) confirmed
the presence of two distinct Eu sites
Keywords: energy transfer · europi-
um · lanthanides · luminescence ·
macrocyclic ligands
ties of the [Tb L] complex upon ligand
2
excitation revealed conventional be-
havior with tH2O =2.05(1) ms and
Introduction
Energy-transfer (ET) processes are of fundamental impor-
tance both for the understanding of natural phenomena
such as the photon funneling of the photosynthetic systems
[
1,2]
of some bacteria,
and for the development of synthetic
[
a] A. Nonat, L. J. Charbonniꢁre
Laboratoire dꢂIngꢃnierie Molꢃculaire Appliquꢃe ꢄ lꢂAnalyse, IPHC,
UMR 7178 CNRS/UdS, ECPM Bꢅtiment R1N0, 25 rue Becquerel,
[3,4]
scaffolds such as photoactive polymer-based sensors,
solar concentrators, or fluorescence-based immunogenic
[5]
6
7087 Strasbourg Cedex (France)
[6,7]
assays.
E-mail: l.charbonn@unistra.fr
ET processes are generally resonant processes between
isoenergetic levels of a donor and an acceptor, sometimes
assisted by interaction with phonons, and are usually ac-
companied by energy losses that result in a bathochromic
shift of the energy output. Such processes have mainly been
studied within the framework of dipole–dipole (Fçrster) or
exchange (Dexter) formalisms, to account for ET process-
es within organic or inorganic molecules.
cases, particularly those where the high photon flux gener-
[
b] M. Regueiro-Figueroa, D. Esteban-Gꢆmez, A. de Blas,
T. Rodrꢇguez-Blas, C. Platas-Iglesias
[8]
Departamento de Quimica Fundamental, Universidade da CoruÇa,
campus da Zapateira-Rffla da Fraga 10, 15008 A Coruna (Spain)
Supporting information for this article is available on the WWW
1
under http://dx.doi.org/10.1002/chem.201200087, and contains: H and
[9]
1
3
+
C NMR spectra of 2 and H
complexes, the mathematical treatment of the energy transfer in
Eu L], complete Ref. [47], and the optimized Cartesian coordinates
of [Ln L] complexes (Ln=Eu, Lu).
6 2
L, HR-ESI mass spectra of [Ln L]
[10]
[11,12,13]
In a few
[
2
2
Chem. Eur. J. 2012, 18, 8163 – 8173
ꢈ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8163

[
19]
ates a large population of excited states, ET processes can
occur directly between excited states, leading to an output
signal at a higher energy than the input, i.e., up-converting
A
H
U
G
R
N
U
(tBuO)
with 1 in acetonitrile under reflux conditions in
3
the presence of Na CO gave compound 2 in good yield
(85%). Full deprotection of the tert-butyl esters of 2 was
achieved cleanly with a CF COOH:H O (1:1) mixture to
2
3
[14,15]
processes.
3
2
Cases in which the ET process occurs between two isoe-
nergetic levels are rather rare, not only because they require
photoactive entities deprived of a Stokesꢂ shift (no energy
difference between the absorption and emission of photons),
but also because it becomes difficult to discriminate the in-
formation arising from each entity when their spectral char-
acteristics (such as absorption and emission spectra) are sim-
ilar. Lanthanide complexes appear to be good candidates
for possible isoenergetic ET. Their electronic transitions in-
give the desired ligand as the hexatrifluoroacetate salt (75%
yield). Reaction of H L·6CF COOH with lanthanide tri-
6
3
flates in the presence of an excess of triethylamine resulted
in the formation of compounds of the formula [Ln L]·4H O
2
2
(Ln=Eu, Gd, Tb, Yb, or Lu), which were isolated in 77–
+
85% yields. The high-resolution mass spectra (ESI )
2
+
+
showed peaks due to the [Ln L+2H] and [Ln L+H] en-
2
2
tities, thereby confirming the formation of the desired com-
plexes (Figures S3–S7, Supporting Information).
n
volve 4f orbitals (n=1–13), which are shielded from inter-
6
2
[16]
1
action with the environment by the filled 5p 6s sub-shells.
Structures of the complexes in solution: The H spectrum of
Their corresponding excited states are only weakly affected
by the ligand field or by interaction with solvent molecules,
resulting in narrow emission bands that are characteristic of
each lanthanide cation. In contrast, the luminescence life-
times of lanthanide complexes are very sensitive to quench-
ing processes arising from vibrational deactivation through
the diamagnetic [Lu L] complex recorded in D O solution
(500 MHz, 298 K, pD 7.0) shows six signals in the range
2
2
8.16–7.59 ppm, together with relatively broad signals in the
III
region 5.32–2.27 ppm, which are typical of Ln complexes
[
20]
with N-alkylated do3a derivatives.
The corresponding
1
3
C NMR spectrum, however, is well resolved, and consists
[17]
NH, OH, and CH oscillators. Hence, despite the similar
spectral signatures of two isoelemental Ln complexes, they
may be distinguished on the basis of their luminescence life-
of 46 signals: 6 signals in the range 183.3–180.5 ppm due to
the carbonyl groups, 10 signals between 161.3 and 124.5 ppm
arising from the carbon nuclei of the bipyridyl unit
(Figure 1), and 24 signals in the region 68.2–46.8 ppm due to
the ꢀCH ꢀ groups of the ligand. This pattern points to a C
III
times. ET processes between identical Ln cations in differ-
ent environments have been reported for solid-state com-
2
1
[18]
pounds, but no such example has yet been found for dis-
crete molecular entities in solution.
symmetry of the complex in solution, and suggests that the
III
two Lu ions present different coordination environments.
III
1
We report here that the complexation of Eu cations by
The H NMR spectrum of the paramagnetic [Yb L] complex
2
a symmetrical ligand based on two do3a moieties linked by
(300 MHz, 298 K, pD 7.0) is also in agreement with C sym-
1
a 6,6’-dimethyl-2,2’-bipyridine unit resulted in a [Eu L] com-
metry in solution. It shows 54 paramagnetically shifted sig-
nals ranging between 160.1 and ꢀ122.5 ppm (Figure 1). The
eight non-equivalent resonances in the range 110–161 ppm
may be assigned to pseudo-axial protons on the cyclen rings.
The chemical shifts of these signals are characteristic of
2
plex with two different coordination sites. Within this com-
plex, an unprecedented example of Eu-to-Eu back-and-forth
energy transfer was observed in aqueous solution. The pro-
cess was established unambiguously by a combination of
photophysical experiments, and
a mathematical treatment was
developed to account for the
ET phenomenon.
Results and Discussion
Synthesis and characterization
III
of the ligand and Ln com-
plexes: The synthetic procedure
used for the preparation of L
III
and its Ln complexes is out-
lined in Scheme 1. The synthe-
sis of the ligand was achieved
in three steps from 6,6’-dimeth-
yl-2,2’-bipyridine, which was
converted to the bromomethyl
derivative 1 by free-radical bro-
mination with dibenzoylperox-
ide and N-bromosuccinimide
NBS). N-alkylation of do3a- iv) Ln
Scheme 1. i) NBS, dibenzoyl peroxide, CCl
(CF SO , Et N, 2-propanol.
4 3 2 3 3 3 2
; ii) do3a ACHTUNTGRENNUNG( tBuO) , Na CO , CH CN; iii) CF COOH:H O (1:1);
(
A
H
U
G
R
N
U
G
3
3
)
3
3
8164
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Chem. Eur. J. 2012, 18, 8163 – 8173

Intramolecular Eu-to-Eu Energy Transfer
FULL PAPER
Figure 2. Evolution of the UV/Vis absorption spectra of an aqueous solu-
ꢀ
5
tion of L (c=4.99”10 m, TRIS-HCl 0.01m at pH 7.0) upon addition of
increasing amounts of EuCl ·6H O (uncorrected for dilution). Inset: Evo-
lution of the absorbances at 285 and 310 nm as a function of the [Eu ]/
L] ratio and the corresponding fit (see text).
3
2
III
[
which the species are formed according to Equations (a)
and (b) (charges are omitted for the sake of clarity):
EuþL $ ½EuLꢁ K11 ¼ ½EuLꢁ=½Euꢁ½Lꢁ
ðaÞ
ðbÞ
1
3
Figure 1. Top: Partial C NMR spectrum of [Lu
2
L] recorded in D
tion (125.8 MHz, 298 K, pD 7.0). Bottom: H NMR spectrum of [Yb
recorded in D O solution (300 MHz, 298 K, pD 7.0).
2
O solu-
1
2
L]
Euþ½EuLꢁ $ ½Eu Lꢁ K21 ¼ ½Eu Lꢁ=½Euꢁ½EuLꢁ
2
2
2
Figure 3 displays the calculated spectra of the species
formed during the titration and the evolution of the concen-
trations of the species observed.
square antiprismatic (SAP) coordination geometries around
the two metal ions, as determined by comparison with relat-
ed compounds.
[21]
III
Spectrophotometric titration of the ligand by Eu : To gain
insights into the complexation behavior of ligand L toward
the lanthanide cations, we performed spectrophotometric ti-
trations in which we monitored the changes in the absorp-
tion and emission spectra of L upon addition of EuCl . To
3
avoid any trouble inherent to the slow complexation kinetics
of the macrocyclic ligand, we added aliquots of a EuCl3
stock solution to solutions containing a constant concentra-
ꢀ
5
tion of ligand L (final concentration 5”10 m in 0.01m Tris-
HCl buffer at pH 7.0), and the mixtures were heated for 5 h
at 608C and 64 h at 458C so that they reached thermody-
namic equilibrium. The changes observed in the UV/Vis ab-
sorption spectra are presented in Figure 2.
Figure 3. Calculated UV/Vis absorption spectra of the species formed
during the titration of L by EuCl ·6H O in 0.01m Tris-HCl at pH 7.0.
3 2
Inset: evolution of the concentration of the species during the titration.
III
Upon addition of Eu , the strong absorption band cen-
tered at 287 nm was bathochromically shifted, with its maxi-
mum observed at 306 nm upon addition of 1.1 equivalents
of EuCl ·6H O. These changes are indicative of the forma-
The same titration experiment was followed by fluores-
3
2
III
tion of a mononuclear [EuL] species, which is accompanied
cence spectroscopy, performed by monitoring the Eu emis-
by the isomerization of the bipyridine unit from the trans
sion in the 550–750 nm region upon excitation at 300 nm.
From 0 to 2 equivalents, the Eu emission intensity gradual-
[22]
III
conformation for L to a cis conformation in [EuL]. Upon
III
addition of the second equivalent of Eu , a small hypso-
ly increased. The data were similarly fitted with the regres-
sion analysis software Specfit, and the results of the fitting
were in good agreement with the data obtained by absorp-
tion spectroscopy. Both titrations were in agreement with
particularly high binding constants, as expected for such
a cyclen-based ligand. As a comparison, a stability constant
chromic shift from 306 nm to 300 nm was observed, pointing
to the formation of a [Eu L] complex with a distorted cis
2
conformation of the bipyridyl unit. These changes could be
studied by an evolving factor analysis with the Specfit soft-
[23]
ware, and were satisfactorily fitted to a binding model in
Chem. Eur. J. 2012, 18, 8163 – 8173
ꢈ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8165

L. J. Charbonniꢁre, C. Platas-Iglesias et al.
of log K =23.5 was previously measured for [Eu-
measured in the solid state, and the corresponding quantum
1
ꢀ
1
[24]
[27]
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(DOTA)] .
Although the formation constant of the 1:1
yield determined using an absolute method
24%.
was fsolid =
complex is too high to be determined by using direct titra-
tion experiments (log K >6), the apparent stepwise forma-
1
1
tion constant of log K =6.1(6) could be determined for the
Spectroscopic properties of the [EuL] and [Eu L] com-
2
1
2
second complexation step.
plexes: The spectroscopic characterization of [EuL] was per-
formed for an equilibrated solution containing ligand L and
0.1 equivalents of EuCl ·6H O. Under these conditions, the
Spectroscopic properties of the [Tb L] complex: The photo-
2
3
2
physical properties of the binuclear [Tb L] complex were
studied in solution and in the solid state. The UV/Vis ab-
mononuclear species represents more than 99% of the com-
2
III
plexed Eu in solution according to the spectrophotometric
III
sorption spectrum of [Tb L] in a 0.01m Tris-buffered aque-
titration experiments (see above). The Eu emission spec-
2
ous solution (pH 7.4) displays a broad absorption band with
trum was measured upon excitation at 300 nm, and is in
good agreement with the calculated spectrum obtained from
the titration data (Figure 5). The emission spectrum displays
ꢀ
1
ꢀ1
a maximum at 301 nm (e=10 920m cm ), which is typical
of p!p* transitions centered on the bipyridine units, and
ꢀ
1
ꢀ1
a shoulder at approximately 240 nm (e=6 880m cm at
2
38 nm, Figure 4). As stated before, the intense p!p* tran-
sition centered at 301 nm is indicative of a distorted cis con-
formation of the two pyridine rings.
[22]
Figure 5. Normalized experimental (—) and calculated (···) emission spec-
tra of [EuL] upon excitation at 300 nm in 0.01m Tris-HCl, pH 7.0.
[
Arrows and dotted arrows refer to TSAP and SAP spectral signatures,
respectively (see text and ref. [27a])].
5
7
the characteristic bands associated with the D ! F (J=0–
0
J
4
) transitions. At least five components are observed for the
Figure 4. UV/Vis absorption (solid line, left part), excitation (dotted
lines, lem =545 nm) and high-resolution emission spectrum (lex =300 nm,
5
7
D ! F transitions between 582 and 602 nm, which is clear-
0
1
right part) recorded for [Tb
buffer, pH 7.4, c=1.5”10 m).
2
L] in aqueous solutions (0.01m Tris-HCl
ly indicative of the presence of two species in solution. Re-
gardless of the emitting wavelength, monoexponential
decays were measured with lifetimes in the range 1.06 to
ꢀ
5
1
.16 ms. This behavior was attributed to the presence in so-
Upon excitation into the absorption band in the UV/Vis
domain, the complex displays an emission pattern character-
lution of a mixture of two distinct coordination environ-
ments associated with the square antiprismatic (SAP) and
twisted square antiprismatic (TSAP) isomers, which was
5
7
III
[28]
istic of the D ! F (J=6–3) transitions of Tb ions
4
J
[25]
(
Figure 4).
No residual fluorescence of the ligand could
further confirmed by the observation of characteristic spec-
tral signatures of both species, at 586.5 and 688.0 nm for the
be observed. The excitation and UV absorption spectra are
very similar, which strongly supports efficient ligand-to-
metal energy transfer.
[27a]
SAP isomer, and 689.0 and 681 nm for the TSAP isomer.
The number of coordinated water molecules in [EuL] was
determined by using the equation of Beeby and co-work-
The luminescence decay profiles of [Tb L] were recorded
2
[17a]
in phosphorescence mode, and could be fitted conveniently
with monoexponential decays. From the measured lifetimes
in H O (t =2.05 ms) and D O (t =3.18 ms), a hydra-
A
H
U
G
R
N
N
ers
and the luminescence decays in H O and D O [t
=
H2O
2
2
1.11(5) and tD2O =1.66(2) ms], pointing to a non-hydrated
III
species (q=0.06) in which the Eu coordination sphere is
2
H2O
2
D2O
[17a]
tion number of q=0.6ꢂ0.2 was determined.
This value
filled by the four nitrogen atoms of the macrocycle, two ni-
trogen atoms of the bipyridine unit, and the three oxygen
atoms of the acetate arms.
strongly suggests a mixture of species with different hydra-
tion states in solution. Sensitized emission quantum yields of
fH2O =51% and fD2O =57% were determined in 0.01m Tris-
The UV/Vis absorption, excitation, and high-resolution
HCl buffered aqueous solutions (pH 7.4) and in D O, re-
emission spectra of [Eu L] in 0.01m Tris-buffered aqueous
2
2
spectively, using rhodamine 6G (f =76%) in water as
solutions (pH 7.4) are presented in Figure 6a. The UV/Vis
H2O
[26]
a reference.
The emission spectrum of [Tb L] was also
absorption spectrum of [Eu L] is very similar to that of
2
2
8166
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Chem. Eur. J. 2012, 18, 8163 – 8173

Intramolecular Eu-to-Eu Energy Transfer
FULL PAPER
[30]
Horrocks and Frey are more important for N atoms (d=
12.1 for CN=9) than for coordinated oxygen atoms of
ꢀ
water molecules (d=ꢀ10.4 for CN=9). A second remark-
able feature of the emission spectrum is the presence of at
least five components in the region corresponding to the
5
7
D ! F emission band (583 to 600 nm). This region is com-
0
1
posed of a maximum of three sublevels for a single species
[31]
of low symmetry,
presence of two distinct Eu sites.
and this observation corroborates the
III
A striking result was obtained when water was replaced
by D O, with the pattern of the emission spectrum of the
2
complex found to be very different (Figure 7). In heavy
Figure 6. a) UV/Vis absorption (solid line, left part), excitation (dotted
lines, lem =613 nm) and high-resolution emission spectrum (lex =300 nm
solid line, right part) recorded for [Eu
aqueous solutions (pH 7.4, 1.5”10 m). b) Enlargement of the
2
L] in 0.01m Tris-HCl buffered
ꢀ5
5
7
D
0
! F
0
region with experimental data [*, fitting to a sum of two Lorentzian
2
peaks (—, R =0.998)], and the corresponding individual Lorentzian sig-
nals (dashed and dashed–dotted lines).
ꢀ
1
ꢀ1
[
Tb L] (Figure 4), with maxima at 301 (e=10 140m cm )
2
ꢀ
1
ꢀ1
and 236 nm (e=7 220m cm ). Upon excitation into the
pp* absorption bands of the bipyridyl unit, the characteristic
emission pattern of Eu is observed between 575 and
III
720 nm. The sensitized emission quantum yields were deter-
mined in 0.01m Tris-HCl buffered aqueous solutions at
pH 7.4 (fH2O =7.0%) and in D O (f =16%), using [Ru-
2
D2O
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(bipy) ]Cl in non-degassed water as a reference (f=
3
29]
2
[
5
0
.04). Interestingly, a thorough examination of the D !
0
7
Figure 7. a) Emission spectra (lex =300 nm, emission slits=0.5 nm) re-
F transition (Figure 6b) clearly indicates the presence of
0
corded for [Eu
2
L] in 0.01m Tris-HCl aqueous solution (black, pH 7.4,
two components: a maximum at 578.90 nm and a marked
shoulder at 578.55 nm. Since this transition is non-degener-
ate, this pattern is typical of the presence of at least two
ꢀ5
ꢀ5
1
.5”10 m) and in D
2
O solution (gray, 1.5”10 m), normalized at their
5
7
maxima. Inset: Zoom on the D ! F emission band in 0.01m Tris-HCl
0
1
2
aqueous solution (black), in D O (gray), and in 0.01m Tris-HCl upon
III
direct metal excitation at lex =396 nm (dotted line). b) Calculated spec-
trum of the hydrated species.
Eu sites in solution. The emission spectrum corresponding
5
7
to the D ! F transitions can be deconvoluted by using
0
0
two pure Lorentzian functions, which allow the calculation
of the emission spectra of each coordination site individually
(
Figure 6b). The strongest emissive site gives rise to a transi-
water, the relative intensities of some of the transitions were
greatly increased (see, for example, the transitions at 587.5
and 592.2 nm), while others remained constant (peaks at
585.2 or 680.0 nm).
tion centered at 578.90 nm (63% of the emitted intensity)
and the other site presents a maximum at 578.45 nm (37%).
Considering that the coordination spheres of the two sites
differ only by the coordination of the bipyridyl unit for site
I or two water molecules for site II (see below), the high-
This behavior can be attributed to the different hydration
III
states for the two distinct Eu sites (I and II), for which the
5
7
energy component of the D ! F transition can be related
reduction in vibrational quenching from water to heavy
water was translated into a more important luminescence
0
0
to site II, as the nephelauxetic parameters calculated by
Chem. Eur. J. 2012, 18, 8163 – 8173
ꢈ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8167

L. J. Charbonniꢁre, C. Platas-Iglesias et al.
enhancement for the most hydrated Eu site (site II). With
the assumption that the thin emission band at 585.2 nm (Fig-
ure 7a) can be attributed to one of the Eu sites, and that its
relative intensity was unchanged in water and D O, this
2
transition was set as a reference transition for site I, and
used to determine the shape of the emission pattern of site
II, by subtracting the contribution of site I from the spec-
trum obtained in D O (Figure 7b). Considering the non-
2
classical behavior of this complex (see below), it was not
possible to estimate the hydration states of each site by clas-
sical measurement of the luminescence lifetimes in H O and
2
[
17a]
D O.
2
To circumvent this problem, we turned our atten-
tion to aqueous mixtures containing the ligand L and Eu
and Gd cations (as chloride salts) in 1:1.9:0.1 stoichiome-
III
III
try. Assuming that the small ionic contraction has a negligi-
ble impact on the conformation of the [Ln L] complexes in
2
solution, and that the stability constants for the Eu and Gd
complexes are similar, this composition leads to a mixture
of [Gd L] (90.25%), [GdEuL] (9.5% overall, half of it cor-
2
responding to Eu in site I) and a minor amount of [Eu L]
2
(
0.25%), which could be neglected. The emission spectrum
obtained for this mixture of complexes could be superim-
Figure 8. Luminescence intensity profiles (gray) of [Eu
HCl at pH 7.4 upon excitation at 300 nm and emission at lem =585 nm
top) and 698 nm (bottom), and the corresponding fits (dashed lines) and
2
L] in 0.01m Tris-
posed perfectly on that of the [Eu L] complex, indicating
2
that the conditions given above are fulfilled. The lumines-
cence decays were then measured in H O and D O at differ-
(
2
2
residues (black points).
ent wavelengths to obtain information on the hydration
number of each site. At 585 nm, a strictly monoexponential
decay was obtained with lifetimes of 1.21(3) ms in H O and
ligand excitation (lexc =300 nm, Figure 8). Analysis of the
data revealed that none of the emission decay profiles were
strictly mono- or biexponential, and that two different pat-
terns were obtained depending on the site responsible for
the observed transition. The most striking pattern was ob-
served at lem =698 nm (Figure 8), for which there was
a major contribution of the hydrated site (Figure 7). Indeed,
this emission decay profile was characterized by an increase
in the emission intensity in the first few hundred microsec-
onds, followed by the expected decay of the luminescence.
On the contrary, at lem =585 nm (site I), where the emission
intensity is mainly due to the non-hydrated species, a very
rapid decay was observed, which was faster than that mea-
sured previously for the [Gd Eu L] species (see above).
2
1
.9(1) ms in D O, pointing to the absence of inner-sphere
2
water molecules for site I (q=0.1ꢂ0.2). These emission life-
times are very similar to those determined for other coordi-
III
[32]
natively saturated Eu complexes,
particularly for the
case of one based on a do3a moiety functionalized with a 6-
[33]
methyl-bipyridine moiety (t=1.15 ms).
At 698 nm, the
decay could only be fitted with a biexponential behavior, be-
cause the contribution of site I could not be disregarded. By
fixing the lifetime corresponding to site I to the value ob-
tained at 585 nm, a lifetime of 0.44(1) ms was obtained for
[
34]
site II. This value lies between those obtained for mono-
[35]
III
and bis-hydrated Eu complexes. Unfortunately, in D O,
2
the admixture of emission signals arising from the two sites
at 698 nm did not allow unambiguous decomposition into
two exponential components, probably as a result of the
similar values of the lifetimes for the two sites under these
1
.9
0.1
All these data strongly suggest energy transfer between the
two sites, with the increase in intensity observed at 698 nm
(Site II) being assigned to the delayed population of the ex-
cited state of the bis-hydrated site, probably due to energy
transfer from the non-hydrated site.
conditions. Assuming that the lifetime in D O is close to
2
that obtained for site I, a q value of 1.8ꢂ0.2 is obtained, in-
dicating that site II is bis-hydrated. As a convention, sites I
and II will be denoted with subscripts 0 and 2 with reference
to their hydration states in the following sections.
The energy transfer between the excited state of the non-
hydrated species, Eu *, and the excited state of the bis-hy-
0
drated species, Eu *, can be modeled by the kinetic scheme
2
shown in Scheme 2.
Intramolecular energy transfer in [Eu L]: To gain additional
information on the two different coordination sites present
in aqueous solution, time-correlated single-photon counting
The mathematical treatment of this model leads to the
system of differential equations given by Equations (1) and
(2), which can be solved to give the time-dependent concen-
trations of the excited species [Eu *] and [Eu *] [Eqs. (3)
2
(
TCSPC) lifetime measurements were performed at various
characteristic emission wavelengths [i.e., 585 nm (Site I),
13 nm (Site I and II), 698 nm (mostly site II), and 702 nm
mostly site I)] in 0.1m Tris-HCl at pH 7.4 and in D O upon
0
2
and (4)], in which l , l , n , and n can be expressed as poly-
1
2
1
2
6
(
nomial combinations of the luminescence rate constants of
III
the two Eu centers (k and k for q=0 and q=2, respec-
2
0
2
8168
www.chemeurj.org
ꢈ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8163 – 8173

Intramolecular Eu-to-Eu Energy Transfer
FULL PAPER
of these complexes, we turned our attention to theory. Be-
cause of the relatively large sizes of the complexes of inter-
est, our calculations were performed at the HF level. The
lanthanides were described by using the quasi-relativistic ef-
[36]
fective core potential (ECP) of Dolg et al. and the related
5s4p3d]-GTO valence basis set. This ECP includes the 4f
[
electrons in the core, as they are not expected to make a sub-
stantial contribution to chemical bonding.
Scheme 2. Kinetic model for the deactivation and energy transfers be-
tween the excited levels of Eu * (q=0) and Eu * (q=2) in [Eu L].
0 2 2
The minimum energy conformation calculated for the
[
Ln L ACHTUNGTERNNUG( H O) ] (Ln=Eu or Lu) complexes presents a cis con-
2 2 2
[37]
tively) and of the rate constants of the energy-transfer pro-
cesses (kET0 and kET2, respectively) for the transfer from site
I to site II and vice versa [Eqs. (5) to (8)]. The full mathe-
matical treatment of this system is presented in the Support-
ing Information.
formation of the 6,6’-substituted bipyridyl unit (Figure 9),
in agreement with the UV spectra described above. Accord-
ing to our calculations, the geometry of the [Eu (L)(H O) ]
ACHTUNGTRENNUNG
2
2
2
complex with SAP coordination around the metal ions is
ꢀ
1
17.0 kJmol more stable than that corresponding to the
TSAP isomer. The stability of the SAP isomer with respect
ꢀ
ꢁ
ꢁ
ꢀ
1
III
d Eu*ðtÞ
ꢀ
ꢁ
ꢀ
ꢀ
ꢁ
ꢁ
2
to the TSAP one increases to 26.5 kJmol for the Lu
complex, in line with the progressive stabilization of the
SAP coordination upon moving to the right across the lan-
ð1Þ
¼
¼
ꢀðkET2 þ k Þ Eu*ðtÞ þ k
Eu*ðtÞ
2
2
ET0
0
dt
ꢀ
d Eu*ðtÞ
ꢀ
ꢁ
0
[38]
ꢀðkET0 þ k Þ Eu*ðtÞ þ k
Eu*ðtÞ
ð2Þ
thanide series.
An SAP coordination environment for
0
0
ET2
2
dt
1
these complexes is in perfect agreement with the H NMR
n ½Eu*ꢁ þ n ½Eu*ꢁ
n2
ꢂ
ꢃ
spectrum of [Yb L] described above (Figure 1). One of the
2
1
2
0
2
0
0
l t
2
l t
1
½
Eu*ðtÞꢁ ¼
e
ꢀ
½Eu*ꢁ þ ½Eu*ꢁ e
III
2
2
0
0
0
Ln ions is nine-coordinated, being directly bound to the
n ꢀ n
1
n1 ꢀ n2
2
four nitrogen atoms of the cyclen unit, three oxygen atoms
of the acetate groups of the do3a cage, and two inner-sphere
water molecules. The second metal ion presents a coordina-
tion number of eight, which is provided by the four nitrogen
atoms of the macrocycle, three oxygen atoms of the acetate
groups, and one nitrogen atom of the 6,6’-substituted bipyr-
ð3Þ
ꢂ
ꢃ
l2t
½
Eu*ðtÞꢁ ¼ ½Eu*ꢁ þ ½Eu*ꢁ e ꢀ ½Eu*ðtÞꢁ
ð4Þ
0
2
0
0
0
2
ꢄ
ꢄ
ꢅ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
2
2
l1 ¼
l2 ¼
ꢀ
ðk þ k þ k þ k Þ ꢀ4ðk k þ k k þ k k Þ ꢀ ðk þ k þ k þ kET2
Þ
ð5Þ
ð6Þ
ð7Þ
ð8Þ
2
0
ET0
ET2
0
2
2
ET0
0
ET2
0
2
ET0
ꢅ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
2
2
ðk þ k þ k þ k Þ ꢀ4ðk k þ k k þ k k Þ ꢀ ðk þ k þ k þ kET2
Þ
2
0
ET0
ET2
0
2
2
ET0
0
ET2
0
2
ET0
ꢄ ꢄ
ꢅ
ꢅ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
1
2
2
n1 ¼ ꢀ
n2 ¼ ꢀ
ðk þ k þ k þ k Þ ꢀ4ðk k þ k k þ k k Þ þ k þ k þ k þ kET2 ꢀ k ꢀ k
0
2
ET0
ET2
0
2
0
ET2
2
ET0
0
2
ET0
0
ET0
kET2
ꢄ ꢄ
ꢅ
ꢅ
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
1
2
ꢀ
ðk þ k þ k þ k Þ ꢀ4ðk k þ k k þ k k Þ þ k þ k þ k þ kET2 ꢀ k ꢀ k
0
2
ET0
ET2
0
2
0
ET2
2
ET0
0
2
ET0
0
ET0
kET2
2
The measured intensity profiles at 585 and 698 nm were
then fitted to Equations (3) and (4) assuming that: 1) the lu-
minescence decays for sites I and II (k and k , respectively)
III
idyl unit. The distances between the Ln ion and the second
nitrogen atom of the bipy fragment (2.93 and 3.02 ꢀ for the
0
2
are those obtained for the [Gd Eu L] complex (1.21 and
1
.9
0.1
III
III
Eu and Lu derivatives, respectively) are too long to be
considered as bond lengths, but short enough to prevent the
possible coordination of a water molecule.
The distances between the Eu ion and the nitrogen
atoms of the macrocycle fall within the range 2.70–2.76 ꢀ
for site I, and 2.65–2.83 ꢀ for the Eu center of site II. The
coordinated nitrogen atom of the bipy unit provides a stron-
ger interaction with the Eu ion than the amine nitrogen
0.44 ms, respectively); and 2) the emission at 585 nm arises
purely from site I. Through this procedure, the evolution of
the emitted intensities at 585 and 698 nm could be fitted
well to the sum of two exponential functions, affording
III
values of l and l (Figure 8), from which energy-transfer
1
2
III
ꢀ
1
rate constants kET0 =kET2 =0.98 ms were obtained.
III
Ab initio modeling of the [Eu L] complex and Fçrster radii:
Attempts to grow single crystals of the [Ln L ACHTUNGRTNEUNG( H O) ] com-
2 2 2
plexes presented in this work were unsuccessful. Thus, with
the aim of obtaining information concerning the structures
atoms (2.60 ꢀ). The distances to the oxygen atoms of car-
boxylate groups fall within a relatively narrow range (2.29–
2.37 ꢀ), whereas the distances between Eu and the oxygen
2
III
atoms of inner-sphere water molecules are 2.59 and 2.45 ꢀ.
Chem. Eur. J. 2012, 18, 8163 – 8173
ꢈ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8169

L. J. Charbonniꢁre, C. Platas-Iglesias et al.
spectroscopic data (ꢃ6 ꢀ for Eu-to-Nd energy transfer in
[
42]
bis-dota-type complexes,
8.5 and 8.9 ꢀ, respectively, for
[43]
Eu-to-Pr and Eu-to-Nd in protein systems), or by lifetime
analysis (10.7 ꢀ for Tb-to-Eu transfer in heterodinuclear
[44]
triple helices).
Conclusion
The successive complexation of two lanthanide cations to
ligand L resulted in the formation of a dissymmetric dinu-
clear [Ln L] complex, in which the two metal ions possess
2
different coordination environments. Both metal ions are
bound directly to seven heteroatoms per do3a unit, their co-
ordination spheres being completed by a nitrogen atom of
the bipyridyl unit for the first site or by two water molecules
for the second site. The close proximity of the two lantha-
nide cations allowed the observation of an unprecedented
Eu-to-Eu energy-transfer phenomenon within a discrete
complex in solution. The energy transfer is translated into
unconventional behavior of the time-dependent lumines-
cence intensity, which, for the hydrated site, shows a rising
step of the luminescence intensity during the very first hun-
dreds of microseconds after the pulsed excitation. It should
be emphasized that, although such a process is probably also
2 2 2
Figure 9. Optimized geometry of the [Eu L ACHTUNGTRNNEG(U H O) ] complex obtained
from HF calculations. Hydrogen atoms are omitted for the sake of sim-
plicity.
The relatively short value of the latter EuꢀOwater distance is
attributed to the hydrogen-bonding interaction established
between one of the hydrogen atoms of the water molecule
and a non-coordinated oxygen atom of one of the acetate
groups of the neighboring do3a cage. The calculated EuꢀO
present in the case of the [Tb L] complex, the weaker influ-
2
bond lengths are very close to those observed in the solid
state for complexes with do3a and dota derivatives, whereas
the calculated EuꢀN distances are typically 0.05–0.10 ꢀ
ence of the coordinated water molecules on the emission in-
[17]
tensity and the complicated emission pattern characteris-
III
tics of Tb did not allow the unequivocal establishment of
[39]
longer than those observed in the solid state.
For the
the occurrence of intramolecular ET. However, in the case
III
[
Lu L] complex, our calculations provide very similar coor-
of the [Eu L] complex, our results confirm that a Eu -cen-
2
2
dination environments around the metal ions, although the
bond lengths of the metal coordination sphere are obviously
tered excited state can be populated either by direct photo-
sensitization through the antenna unit or by energy transfer
from a proximal Eu site, which opens very interesting op-
III
shorter than for the [Eu L] complex as a consequence of the
2
[40]
lanthanide contraction.
Ln ···Ln distances are 7.05 and 7.16 ꢀ for the Eu and Lu
complexes, respectively.
The calculated intramolecular
portunities such as the possibility to bring excess energy to
an excited state, potentially resulting in energy up-conver-
sion processes within discrete molecular entities in solu-
III
III
III
III
[15a]
The calculated intramolecular Eu ···Eu distance, r, was
tion.
used to calculate the Fçrster radii, R , the distance at which
0
50% of the energy of the donor is transferred to the accept-
or for the two donor/acceptor pairs (depending whether Eu0
or Eu2 is considered as the donor), according to Equa-
tion (9), in which tD is the lifetime of the donor in the ab-
Experimental Section
General methods: Elemental analyses were carried out on a Carlo Erba
1108 elemental analyzer. High-resolution ESI-TOF mass spectra were re-
corded using an LC-Q- TOF Applied Biosystems QSTAR Elite spec-
trometer in the positive mode. IR spectra were recorded using a Bruker
Vector 22 spectrophotometer equipped with a Golden Gate Attenuated
[41,13e]
sence of the acceptor.
ꢄR
ꢅ
6
1
tD
0
kET
¼
ð9Þ
1
13
r
Total Reflectance (ATR) accessory (Specac). H and C NMR spectra
were recorded at 258C on Bruker Avance 300 and Bruker Avance
5
00 MHz spectrometers. For measurements in D
2
O, tert-butyl alcohol
Values of R0
=8.1 ꢀ and R
=6.8 ꢀ were calculat-
0(2!0)
(0!2)
was used as an internal standard with the methyl signal calibrated at d=
1
ed, highlighting the larger efficiency of the energy transfer
from site I to site II compared to the reverse situation, and
explaining why it was possible to observe a rise in intensity
at certain wavelengths for which the contribution of site II
was predominant. It should also be noted that these values
are in good agreement with values reported in the literature
for energy transfer between different lanthanide cations ob-
tained by direct calculation of the integral overlap from
1
13
.2 ( H) and 31.2 ppm ( C).
UV/Vis absorption spectra were recorded on a Specord 205 (Analytik
Jena) spectrometer. Steady-state emission and excitation spectra were re-
corded on a Horiba Jobin Yvon Fluorolog 3 spectrometer working with
a continuous 450 W Xe lamp. For the measurements in the solid state,
the spectrometer was fitted with an integrating sphere Quanta-F from
Horiba. Detection was performed with a Hamamatsu R928 photomulti-
plier. All spectra were corrected for the instrumental functions. When
necessary, a 399 nm cutoff filter was used to eliminate second-order arti-
8170
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ꢈ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 8163 – 8173

Intramolecular Eu-to-Eu Energy Transfer
FULL PAPER
facts. Phosphorescence lifetimes were measured on the same instrument
working in phosphorescence mode, with a 50 ms delay time and a 100 ms
integration window, or in the TCSPC lifetime spectroscopy mode, both
using a Xenon flash lamp as the excitation source.
the desired compound. Yield: 75%. Elemental analysis calcd (%) for
C
40
H
60
N
10
O
12·6CF
3
COOH·3H
2
O: C 38.76, H 4.50, N 8.69; found: C
+
+
39.04, H 4.37, N 8.86; MS (ESI ): m/z 873 ([C40
([C40
H
61
N
10
O
12] ) and 437
2
+
62 10 12
H N O ]
); IR (ATR): n˜ =3436 (O-H), 1673 and 1678 (C=O),
ꢀ
1
1
1584 cm (C=N); H NMR (D
8.68–7.78 (m, 6H), 4.40–2.86 ppm (m, 48H); C NMR (D
125.8 MHz, 258C, TMS): d=176.6, 176.0, 170.8, 156.4, 149.7, 142.7, 129.8,
2
O, pD 2.1, 500 MHz, 258C, TMS): d=
Monoexponential and biexponential emission decay profiles were fitted
with the FAST program from Edinburgh Instruments or with the Data-
station software from Jobin Yvon. Hydration numbers, q, were obtained
1
3
2
O, pD 2.1,
[
17a]
125.6, 59.8, 57.2, 56.1, 55.3, 54.6, 53.6, 53.3, 52.6, 50.8, 50.5, 50.1,
9.2 ppm.
General procedure for the preparation of [Ln
mixture of L·6CF COOH·3H (0.100 g, 0.062 mmol), triethylamine
0.075 g, 0.744 mmol), and Ln(OTf) (0.124 mmol, Ln=Eu, Gd, Tb, Yb,
using Equation (10),
in which tH2O and tD2O refer to the measured lu-
4
minescence decay lifetimes (in ms) in water and deuterated water, re-
III
spectively, using AEu =1.2 and aEu =0.25 for Eu , and ATb =5.0 and aTb
=
2 2
L]·4H O complexes: A
III
0
.06 for Tb
.
3
2
O
(
A
H
U
G
R
N
U
G
3
or Lu) in 2-propanol (10 mL) was heated to reflux for 24 h. The reaction
was allowed to cool to room temperature, resulting in the formation of
a white precipitate that was washed with MeOH and diethyl ether. The
mother liquor was stored at 48C for several days, resulting in the forma-
tion of a second batch of complex, which was again collected by filtration
and washed with MeOH and diethyl ether.
q ¼ ALnð1=tH2
O
ꢀ 1=tD2
O
ꢀ aLn
Þ
ð10Þ
Luminescence quantum yields were measured according to conventional
procedures, with diluted solutions (optical density < 0.05), using [Ru-
ACHTUNGTRENNUNG( bipy) ]Cl in non-degassed water (F=4.0%), and rhodamine 6G in
3 2
water (F=76.0%) as references. The estimated errors were ꢂ15%.
Solid-state studies and quantum yields were determined using an abso-
lute method with the integrating sphere. The quantum yield in the abso-
lute method can be calculated using Equation (11).
[
29]
[
26]
A
H
U
G
R
N
U
G
2
L]·4H
2
O: Yield: 0.062 g, 81%; elemental analysis calcd (%) for
C H54Eu N O12·4H O: C 38.65, H 5.03, N 11.27; found: C 38.78, H 5.26,
40 2 10 2
+
2+
2 10 12
N 10.98; HS-MS (ESI ): m/z 587.1257; calcd for [C40H56Eu N O ]
+
5
(
87.1246; m/z 1173.2373; calcd for [C40
H
55Eu
2
N
10
O
12
]
1173.2420; IR
ꢀ
1
ATR): n˜ =1579 cm (C=O).
[Gd L]·4H O: Yield: 0.065 g, 83%; elemental analysis calcd (%) for
C H Gd N O ·4H O: C 38.33, H 4.99, N 11.17; found: C 38.18, H 4.81,
R
l
hc
l
sam
em
ref
ref
em
N
emission
fI ðlÞ ꢀ I ðlÞgdl
A
H
U
G
R
N
N
2
2
ꢀ
¼ N
¼ R
ð11Þ
sam
ex
absorption
fIex ðlÞ ꢀ I ðlÞgdl
hc
40 54
2
10 12
2
+
2+
N 10.98; HS-MS (ESI ): m/z 592.1284; calcd for [C40H56Gd N O ]
2 10 12
+
5
(
92.1275; m/z 1183.2457; calcd for [C40
H
55Gd
2
N
10
O
12
]
1183.2477; IR
Here, Nabsorption is the number of photons absorbed by the sample and
ꢀ
1
ATR): n˜ =1582 cm (C=O).
[Tb L]·4H O: Yield: 0.062 g, 79%; elemental analysis calcd (%) for
·4H O: C 38.23, H 4.97, N 11.14; found: C 38.39, H 4.90,
N 11.33; HS-MS (ESI ): m/z 593.1279; calcd for [C40
N
emission is the number of photons emitted from the sample, l is the wave-
sam
ref
length, h is Planckꢂs constant, c is the velocity of light, Iex and Iex are
the integrated intensities of the excitation light with and without the
sample, respectively, and Iem
tensities with and without the sample, respectively. This method has been
described in detail by L. S. Rohwer and J. E. Martin.
Chemicals and starting materials: 6,6’-Bis(bromomethyl)-2,2’-bipyridine
A
H
N
T
E
N
N
2
2
C
40
H
54
N
10
O
12Tb
2
2
sam
ref
+
2+
and Iem are the photoluminescence in-
56 10 2
H N O12Tb ]
+
593.1287; m/z 1185.2535; calcd for [C40
55 10
H N O12Tb] 1185.2502; IR
[
45]
ꢀ1
(ATR): n˜ =1580 cm (C=O).
A
H
U
G
R
N
U
G
2
L]·4H
2
O: Yield: 0.068 g, 85%; elemental analysis calcd for
·4H O: C 37.39, H 4.86, N 10.90; found: C 37.48, H 5.03,
N 11.05; HS-MS (ESI ): m/z 608.1441; calcd for [C40
[
46]
(
1) was prepared according to the literature method. All other chemi-
cals were purchased from commercial sources and used without further
purification, unless otherwise stated. Neutral Al (Fluka, 0.05–
.15 mm) was used for preparative column chromatography.
,6’-Bis(4,7,10-tris(tert-butoxycarbonylmethyl)-1,4,7,10-tetraazacyclodo-
mixture of do3a(tBuO)
(0.211 g, 1.99 mmol) in acetonitrile
25 mL) was stirred for 30 min, and then 6,6’-bis(bromomethyl)-2,2’-bi-
C
54
N
10
O
12Yb
2
2
+
2+
56 10 2
H N O12Yb ]
+
2
O
3
608.1422; m/z 1215.2717; calcd for [C40
H
55
N
10
O
12Yb
2
]
1215.2772; IR
ꢀ
1
0
(ATR): n˜ =1589 cm (C=O).
[Lu L]·4H O: Yield: 0.062 g, 77%; elemental analysis calcd (%) for
40 2 10 2
C H54Lu N O12·4H O: C 37.27, H 4.85, N 10.87; found: C 37.44, H 4.73,
6
A
H
U
G
R
N
N
2
decane-1-ylmethyl)-2,2’-bipyridine (2):
(
(
A
A
T
N
T
E
N
N
3
+
2+
0.500 g, 0.971 mmol) and Na
2
CO
3
N 10.65; HS-MS (ESI ): m/z 609.1457; calcd for [C40H56Lu N O ]
609.1441; m/z 1217.2792; calcd for [C40 1217.2811; IR
(ATR): n˜ =1589 cm (C=O); C NMR (D
O, pD 7.0, 125.8 MHz, 258C,
2
2 10 12
+
H
55Lu
2
N
10
O
12
]
ꢀ
1
13
pyridine (0.166 g, 0.486 mmol) and a catalytic amount of KI were added.
The mixture was heated to reflux with stirring under an inert atmosphere
TMS): d=183.3, 182.8, 182.7, 182.0, 181.5, 180.5, 161.3, 159.5, 157.3,
156.3, 143.8, 140.5, 130.2, 126.6, 124.9, 124.5, 68.2, 68.1, 68.0, 67.8, 67.6,
67.5, 67.1, 66.4, 59.4, 58.6, 58.1, 57.8, 57.7, 57.5, 57.3, 57.2, 56.9, 56.7, 56.6,
56.5, 55.0, 53.8, 47.4, 46.8 ppm.
(
Ar) for a period of 24 h, and then the excess Na
The filtrate was concentrated to dryness and the yellow oil was extracted
with a 1:3 mixture of H O and CH Cl (100 mL). The organic phase was
evaporated to dryness to give an oily residue that was purified by column
chromatography on Al with a CH Cl /MeOH 5% mixture as the
eluent to give 1 (0.535 g) as a yellow foam. Yield: 85%. Elemental analy-
sis calcd (%) for C64 Cl : C 60.31, H 8.56, N 10.82; found:
C 60.32, H 8.42, N 10.67; MS (ESI ): m/z 1210 ([C64
2 3
CO was filtered off.
2
2
2
Computational methods: All calculations were performed employing the
2
O
3
2
2
Gaussian 09 package (Revision C.01). Full geometry optimizations of
[47]
2 2 q
the [Ln L AHCTUNGTRENNG(U H O) ] systems (q=1, 2; Ln=Eu, Gd, or Lu) were performed
H
108
N
10
O
12·CH
2
2
at the HF level by using the effective core potential (ECP) of Dolg et al.
+
+
H
109
N
10
O
12] ) and
[36]
and the related [5s4p3d]-GTO valence basis set for the lanthanides,
2
+
ꢀ1
6
05 ([C64
H
110
N
10
O
12
]
); IR (ATR): n˜ =1721 (C=O), 1579 cm (C=N);
and the 3–21G basis set for C, H, N, and O atoms. Although small, HF
calculations employing this basis set in combination with the f-in-core
1
H NMR (CDCl
3
, 500 MHz, 258C, TMS): d=8.74 (s, 2H), 7.73 (t, 2H,
3
3
J=7.6 Hz), 7.38 (d, 2H, J=7.5 Hz), 3.75–1.89 (m, 48H, -NCH
2
), 1.46–
III
ECP of Dolg were shown to provide molecular geometries of Ln dota-
1
3
1
.42 ppm (m, 54H, tBuO-); C NMR (CDCl , 125.8 MHz, 258C, TMS):
3
like complexes, in good agreement with the experimental structures ob-
d=172.9, 172.4, 156.7, 156.1, 137.4, 125.8, 120.1, 82.1, 81.2, 60.1, 56.6,
[49]
served by single-crystal X-ray diffraction studies.
No symmetry con-
5
6.4, 56.0, 53.4, 51.7, 49.8, 48.8, 48.5, 31.8, 28.1, 28.0, 27.8 ppm.
straints were imposed during the optimizations. The stationary points
found on the potential energy surfaces as a result of the geometry optimi-
zations were tested to represent energy minima rather than saddle points
using frequency analysis. The relative free energies of the different con-
formations obtained from geometry optimizations were calculated at the
same computational level, and they include non-potential-energy contri-
butions (zero-point energies and thermal terms) obtained through fre-
quency analysis. Selected geometries optimized at the HF level were sub-
sequently fully optimized by using hybrid DFT with the B3LYP ex-
6
,6’-Bis(4,7,10-tris(carboxymethyl)-1,4,7,10-tetraazacyclododecane-1-yl-
methyl)-2,2’-bipyridine hexatrifluoroacetate (H
6
L): Compound 2 (0.535 g,
0
.413 mmol) was dissolved in a 1:1 mixture of water and trifluoroacetic
acid (10 mL). The mixture was heated to reflux with stirring for 24 h, and
then the solvents were removed in a rotary evaporator to give a brown
oil. This was dissolved in MeOH (10 mL) and the solvent was evaporat-
2 2
ed. This process was repeated twice, and then three times with CH Cl .
The oily residue was dissolved in MeOH (1 mL), and diethyl ether was
added until the precipitation of a white solid was complete. The white
solid was isolated by filtration and dried under vacuum to give 0.498 g of
[
48,50]
change-correlation functional,
and the standard 6–31G(d) basis set
for the ligand atoms. Because of the considerable computational effort
Chem. Eur. J. 2012, 18, 8163 – 8173
ꢈ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8171

L. J. Charbonniꢁre, C. Platas-Iglesias et al.
involved in the calculation of second derivatives at this level, the opti-
mized geometries were not characterized by using frequency analysis. A
comparison of the molecular geometries optimized at the HF and B3LYP
levels shows that both models provide very similar coordination environ-
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Acknowledgements
We gratefully acknowledge Prof. David Parker for fruitful discussions
and comments on this work. M. R.-F., D. E.-G., A. de B., T. R.-B., and
C. P.-I. thank Ministerio de Educaciꢆn
y
Ciencia (MEC, CTQ2009–
0721), Fondo Europeo de Desarrollo Regional (FEDER, CTQ2009–
0721) and Xunta de Galicia(IN845B-2010/063) for financial support.
1
1
This research was performed in the framework of the EU COST Ac-
tionD38 “Metal-Based Systems for Molecular Imaging Applications”.
The authors are indebted to Centro de Supercomputaciꢆn de Galicia
CESGA) for providing the computer facilities. A. N. and L. J. C. grate-
fully acknowledge the financial support of the French Centre National de
la Recherche Scientifique and the University of Strasbourg.
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Received: January 9, 2012
Published online: May 21, 2012
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www.chemeurj.org
8173