Dependence of the photophysical properties on the number of 2,2′-bipyridine units in a series of luminescent europium and terbium cryptates

Article abstract of DOI:10.1021/ic3010568

The luminescence properties of a series of lanthanoid cryptates with an increasing number of 2,2′-bipyridine units have been investigated for the lanthanoids Eu and Tb in aqueous solution. The trends in important parameters that influence the photophysics in these complexes have been determined. With increasing bipyridine content, an increase is observed for the intersystem crossing efficiencies and the number of inner-sphere water molecules. In contrast, a decrease is found in the same direction for overall quantum yields, triplet energies, and sensitization efficiencies.

Full text of DOI:10.1021/ic3010568

Article
pubs.acs.org/IC
Dependence of the Photophysical Properties on the Number of 2,2′-
Bipyridine Units in a Series of Luminescent Europium and Terbium
Cryptates
Nicola Alzakhem, Caroline Bischof, and Michael Seitz*
Inorganic Chemistry I, Department of Chemistry and Biochemistry, Ruhr-University Bochum, 44780 Bochum, Germany
S
* Supporting Information
ABSTRACT: The luminescence properties of a series of
lanthanoid cryptates with an increasing number of 2,2′-
bipyridine units have been investigated for the lanthanoids Eu
and Tb in aqueous solution. The trends in important
parameters that influence the photophysics in these complexes
have been determined. With increasing bipyridine content, an
increase is observed for the intersystem crossing efficiencies and
the number of inner-sphere water molecules. In contrast, a
decrease is found in the same direction for overall quantum
yields, triplet energies, and sensitization efficiencies.
behavior was not commented on, and in subsequent
publications, only the less luminescent tris(bipyridine)
cryptates Ln-3 were investigated in further detail3 presumably
due to their superior complex stability in biologically relevant
media4 and the naturally higher absorption coefficients
compared to species with fewer bipyridine arms.
INTRODUCTION
■
Photoluminescence in molecular lanthanoid complexes has
received a great deal of attention over the last decades because
of their unique characteristics, making them vital components
of many modern photonic applications.1 The challenge for such
complexes is the utilization of an appropriate ligand environ-
ment that is capable of populating lanthanoid emissive levels.
This can be achieved through energy transfer from ligand-
centered excited states. In addition, the ligand shields the
lanthanoid center from deactivating, high-frequency oscillators
such as water molecules. In 1987, Lehn et al. introduced a new
class of luminescent lanthanoid complexes with 2,2′-bipyridine-
based macrobicylic cryptands.2 The initial report included the
terbium(III) and europium(III) cryptates Ln-1 with one 2,2′-
bipyridine unit and a complementary nonphotoactive crown
ether moiety, as well as the corresponding tris(2,2′-bipyridine)
species Ln-3 (Figure 1).
For the rational design of new and effective luminescence
probes, it would be very helpful to have a detailed knowledge of
how the different numbers of bipyridine chromophores in these
cryptates change the various photophysical parameters that
determine luminescence performance (intersystem crossing
and energy transfer efficiencies, ligand triplet energies,
lanthanoid radiative lifetimes, quantum yields, etc.). Continuing
our previous work on bipyridine-based cryptates,5 we have
therefore undertaken a study in this context with the cryptates
Ln-1, Ln-2, and Ln-3 (Figure 1; Ln = Eu, Gd, Tb), which show
a systematically increasing number of bipyridine units (from 1
to 3). We report here the synthesis of the previously unknown
cryptates Ln-2 and discuss the impact of the different numbers
of bipyridine arms on the photophysical properties of this
series.
Despite the apparent similarity of the photoactive bipyridine
moieties in all three cryptands, Ln-1 and Ln-3 showed
significant differences in luminescence efficiency.2a For example
in aqueous solution, the terbium complex Tb-1 had a
luminescence lifetime of τobs = 0.72 ms, while Tb-3 exhibited
a greatly reduced emission with τobs = 0.33 ms. At the time, this
RESULTS AND DISCUSSION
■
Synthesis of the Cryptates. All lanthanoid cryptates were
prepared as the chloride species from the corresponding
sodium complexes by metal exchange with the appropriate
lanthanoid trichloride hexahydrates. The synthesis of Na-1 was
carried out following the pathway described in published
articles2a,6 (previously reported without synthetic details)
starting from the commercially available aza-crown-ether 4
Figure 1. Known 2,2′-bipyridine-based lanthanoid cryptates Ln-1 and
Ln-3 and new lanthanoid cryptate Ln-2 with an intermediate number
of bipyridine arms.
Received: May 21, 2012
Published: August 21, 2012
© 2012 American Chemical Society
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and 6,6′-bis(bromomethyl)-2,2′-bipyridine7 (5) (Scheme 1).
This method proved much less cumbersome than an earlier
alternative route.8 Na-3 and the complexes Ln-3 were prepared
by a published procedure.9
Scheme 3. Synthesis of the Lanthanoid Cryptates Ln-2
Scheme 1. Synthesis of the Lanthanoid Cryptates Ln-1
The synthesis of the unknown sodium cryptate Na-2 was first
attempted by the reaction of the bis(bipyridine) macrocycle 610
(Scheme 2) with the triethylene glycol derivative 711 under
Scheme 2. Attempted Synthesis of the Sodium Cryptate Na-2
standard high-dilution and sodium-templating conditions.
Unfortunately, this macrobicyclization reaction, which generally
gives only low to modest yields, did not even provide the
corresponding sodium cryptate Na-2 in trace amounts. Instead,
a different synthetic route involving the novel macrocycle 11
(Scheme 3) proved to be successful. The synthesis of 11 was
achieved starting from 6,6′-bis(hydroxymethyl)-2,2′-bipyridine
(8).12 Swern oxidation13 to the corresponding dialdehyde 914
followed by a magnesium-templated macrocyclization/reduc-
tion sequence (used in the past for a similar compound15) with
commercially available 1,8-diamino-3,6-dioxaoctane (10)
yielded 11 in 53% yield over three steps. Macrobicyclization
with 6,6′-bis(bromomethyl)-2,2′-bipyridine7 (5) under stand-
ard, nonoptimized conditions (Na2CO3, CH3CN, high
dilution) gave Na-2 in low yield (17%).
Single-Crystal X-ray Crystallography. Single-crystal X-
ray analysis was possible on the sodium cryptate Na-1 after
crystallization of its methanolic solutions by slow evaporation.
The asymmetric unit contains two very similar, independent
cryptates, both showing an eight-coordinate sodium center with
all available donor atoms bound to the metal center (Figure 2).
The sodium cations are well separated from the bromide
counteranions (minimal distance Br2−Na2 = 5.88 Å) and do
not bind additional external ligands. This situation is
completely analogous to the one previously described for the
sodium complex with the cryptand [phen·phen·phen], which
consists of three phenanthroline arms and which is structurally
a very good model for the tris(bipyridine) ligand in Na-3.17
Figure 2. Thermal ellipsoid plot for the two independent sodium
cryptate cations Na-1 in the asymmetric unit (Ortep 3 for Windows,16
50% probability level). The hydrogen atoms, the free bromide anions,
and the isolated water molecules are omitted for clarity. Color scheme:
C, gray; N, blue; O, red; Na, orange.
One important difference between the cryptate structures in
Na-1 and the sodium complex with [phen·phen·phen] is the
flexibility of the ligand arms. [phen·phen·phen] is very rigid and
enforces a rather long, average bond length between Na and the
six aromatic nitrogen donors of the phenanthroline units (av
Nar−Na = 2.70 Å).17 In contrast, the crown ether arms in Na-1
are capable of wrapping flexibly around the metal, which
permits a closer approach of the metal to the bipyridine
nitrogens (av Nar−Na = 2.61 Å).
The situation in the corresponding lanthanoid cryptates is
different. The crystal structure of Tb-3 was previously reported
to exhibit external ligands (one chloride and one water
molecule) in the inner coordination sphere of the metal
center.18 This binding of additional ligands seems to be valid
for the entire series Ln-1, Ln-2, and Ln-3 (see luminescence
lifetime measurements in H2O and D2O). This is not
surprising, considering the much higher Lewis acidity of
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trivalent lanthanoid cations compared to monocationic sodium
centers.
Photophysical Properties. The Eu and Tb complexes
with the same cryptand show very similar absorption spectra in
aqueous solution (Figures 3 and 4). The spectral features,
Figure 5. Steady-state emission spectra for Eu-1, Eu-2, and Eu-3 (λexc
= 313 nm, tris buffer, pH 6.8).
Figure 3. Absorption spectra of Eu-1, Eu-2, and Eu-3 (tris buffer, pH
6.8).
Figure 6. Steady-state emission spectra for Tb-1, Tb-2, and Tb-3 (λexc
= 306 nm, tris buffer, pH 6.8) (* second-order excitation peak).
Eu-2 to Eu-1. This trend seems to indicate a decreasing level of
symmetry in the same direction, because this transition is only
allowed in coordination environments with low symmetry
around the europium cation.
The positions of the ligand-centered triplet levels (T1) were
measured by low-temperature emission spectroscopy of the
gadolinium cryptates (Figure 7, Table 1). The zero-phonon
T1→S0 transition energy, E(T1), was determined by fitting the
structured phosphorescence bands with a series of Gaussians
(see Figures S1−S3 in the Supporting Information). In our
Figure 4. Absorption spectra of Tb-1, Tb-2, and Tb-3 (tris buffer, pH
6.8).
however, change considerably with the use of different
cryptands. Ln-1 exhibits a broad unstructured band with a
maximum at 311 nm. With an increasing number of bipyridines
in Ln-2 and Ln-3, the absorption maxima shift to shorter
wavelengths (Ln-2: 309 nm; Ln-3: 304 nm) and show more
pronounced shoulders around 320−325 nm.
The steady-state emission spectra after excitation close to the
absorption maxima in aqueous solutions show the typical bands
for the lanthanoids Eu and Tb (Figures 5 and 6). In the case of
the terbium cryptates, the differences are only marginal. In
contrast, the spectra for the europium species expectedly show
changes along the cryptate series due to the generally higher
sensitivity of europium bands to the coordination environment.
Notably, only one single peak is found in the europium spectra
for each 5D0→7F0 band (which theoretically involves only
nondegenerate levels and is therefore not supposed to be
showing any crystal field splitting), suggesting the presence of
one single species in all cases (at least on the time scale of the
measurement). In addition, the same transitions show a
progressive increase in relative intensity going from Eu-3 via
Figure 7. Low-temperature (77 K) steady-state emission spectra of
Gd-1, Gd-2, and Gd-3 (λexc = 304 nm, MeOH/EtOH, 1:1, v/v).
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very similar ionic radii of Eu and Tb. The q values vary widely
(Table 2: Tb-1, 0.9; Tb-2, 2.0; Tb-3, 4.4). The unrealistically
high value for Tb-3 is most likely due the fact that there is
considerable energy back-transfer because of the low triplet
level (see above). This has been pointed out before in a similar
case.21 In addition, it has been noted before that the empirical
equations, which are usually established using structurally
characterized lanthanoid complexes with DOTA-derived or
aminocarboxylate ligands, do not adequately describe the
situation for lanthanoid bipyridine-based cryptates.22
Table 1. Ligand-Centered Zero-Phonon T1→S0 Transition
Energy, E(T1), in the Gadolinium Complexes
entry
complex
E(T1) [cm−1
]
1
2
3
Gd-1
Gd-2
Gd-3
22 200
22 200
21 600
hands, Gd-3 exhibits a value E(T1) = 21 600 cm−1, which is in
excellent agreement with previous measurements.3d As
concluded in earlier reports, this energy level lies too close to
the emitting level for Tb (5D4 at ca. 20 500 cm−1)19 to prevent
energy back-transfer onto the ligand and concomitant loss of
sensitization efficiency.3d Remarkably, the two other gadoli-
nium cryptates Gd-1 and Gd-2 show a substantially higher
triplet level (both at E(T1) = 22 200 cm−1). This helps to
reduce the detrimental impact of the energy back-transfer
mechanism as in Gd-3 and should therefore yield higher
luminescence efficiencies. In contrast to the situation for Tb, all
three triplet energies are sufficiently above the emitting level of
Eu (mainly 5D0 at ca. 17 300 cm−1)19 to rule out back-transfer,
again a fact that has been established before for Eu-3.3d The
fluorescent transitions from the first excited singlet state S1 to
the ground state S0 (around 29 000 cm−1) show an interesting
trend (Figure 7). With increasing number of bipyridine units
(Gd-1 via Gd-2 to Gd-3), the intensity of this band steadily
decreases in comparison to ligand phosphorescence. This
indicates an enhancement of intersystem crossing (ISC) (S1→
T1) in the same direction. It has been reported before that ISC
efficiencies in Ln-3 are close to unity (e.g., >95% for Tb-33b,d).
The situation, however, that this phenomenon seems to be
connected to the number of bipyridine arms has not been
realized before.
Luminescence lifetimes were determined in H2O and D2O at
ambient temperature. For the terbium complexes, where Tb-3
is known to be susceptible to quenching by O2 due to the low-
lying T1 states,3b,d the measurements were performed on
degassed solutions (sparging with dioxygen-free N2). From the
lifetime differences, the number of inner-sphere water
molecules q was evaluated using the empirical equations
introduced by Beeby et al.20 For the europium cryptates Eu-1
and Eu-2, better shielding of the metal was observed compared
to Eu-3, as indicated by smaller q values (Table 2: 1.9/2.1 vs
2.5). Presumably, this is due to the greater flexibility of the
ethylene ether bridges compared to the rather stiff bipyridine
arms (see crystal structure for Na-1). These occupy much more
space around the europium centers, leading to a decrease of
available free coordination sites for water molecules. This trend
should also be found in the terbium complexes because of the
Quantum yields of the europium and terbium cryptates were
measured in aqueous solutions for Tb and Eu. For both
lanthanoids, the quantum yields for Ln-1 are approximately
twice as high as for the corresponding complexes Ln-2 and Ln-
3 (Table 2; Eu: 5.2% vs 2.4%/2.0% and Tb: 6.8% vs 3.7%/
3.4%). In order to get further insight into the various
parameters that determine luminescence efficiency, the data
for the cryptates Eu-(1−3) were examined in more detail.
Europium is a special case among the lanthanoids because
according to Judd−Ofelt theory it is relatively straightforward
to determine the radiative lifetime τrad from the emission
spectra using the special properties of the magnetic-dipole-
23
5
allowed transition D0→7F1. This approach is probably more
reliable than the previous determination of τrad in Eu-3 by
assuming that the real radiative lifetime is equal to the observed
lifetime in D2O at low temperature (77 K).3c,d The latter
method gave a radiative lifetime τrad = 1.7 ms for Eu-3,3c,d
whereas the Judd−Ofelt approach yields substantially higher
values of τrad = 6.2−8.1 ms for Eu-(1−3) (Table 2). With the
Ln
Ln
calculated radiative lifetimes, the intrinsic quantum yields Φ
for Eu-(1−3) were established using the equation displayed in
Table 2. While a trend toward lower intrinsic quantum yields
appears to be present going from Eu-1 (6.3%) via Eu-2 (5.4%)
to Eu-3 (4.2%), the relatively large uncertainties of the
calculated values preclude conclusive statements. Using the
Ln
Ln
intrinsic quantum yields Φ in combination with the measured
absolute quantum yields ΦLLn, however, gives significant
differences in the sensitization efficiencies, ηsens (Table 2):
Eu-1 (83%) via Eu-2 (44%) to Eu-3 (48%). ηsens is mainly
reflecting ISC and energy transfer from the ligand-centered
triplet state to the lanthanoid. The qualitative result (vide infra)
that ISC efficiencies seem to increase with an increasing
number of bipyridine rings (see Figure 7) would suggest a
concomitant rise in ηsens. Since the opposite effect is actually
observed, i.e., Eu-3 having a low value for ηsens, the energy
transfer efficiencies would consequently have to decrease in the
direction Eu-3 via Eu-2 to Eu-1 in order to be able to explain
the measured data.
Table 2. Luminescence Data for the Lanthanoid Complexes Ln-1, Ln-2, and Ln-3 in H2O and D2O
τ
τ
H O
2
ΦLLn
η
sens
d
Ln
Ln
entry
complex
λabs [nm]
τH O [ms]
0.39
τD O [ms]
1.3
q
τrad [ms]
Φ
=
2
2
rad
g
1
2
3
4
5
6
Eu-1
Eu-2
Eu-3
311
309
304
1.9
2.1
2.5
0.90
2.0
6.2
0.063
0.054
0.042
0.052
0.024
0.020
0.068
0.037
0.034
0.83
0.44
0.48
g
0.34
1.1
6.3
c
c
fg
c
0.34
1.7
8.1 ,
a
b
b
b
b
Tb-1
311
1.3
1.9
a
b
Tb-2
309
0.42
0.44
0.52
0.75
a
b
e
Tb-3
304
(4.4)
a
b
c
d
Degassed solutions, λexc = 306 nm, λem = 542 nm. In 0.5 mM tris buffer (pH 6.8). See refs 3c and 3d. q = number of inner-sphere water
e
f
g
molecules; see ref 20. Equation for the calculation of q not applicable. See ref 3c. Calculated using the equation 1/τrad = AMD,0n3(Itot/IMD); see ref
23.
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Kα radiation. Data were collected at a temperature of 233(2) K.
Frames corresponding to an arbitrary hemisphere of data were
collected using ω scans. The structure was solved within the Wingx24
package using direct methods (SIR9225) and expanded using Fourier
techniques (SHELXL-9726). The hydrogen atoms were included but
not refined. They were positioned geometrically, with distances C−H
= 0.93 Å for aromatic hydrogens and C−H = 0.97 Å in all benzylic and
aliphatic positions. All hydrogens were constrained to ride on their
parent carbon atoms. Uiso(H) values were set at 1.2 times Ueq(C).
Crystallographic data for Na-1: chemical formula C24H34BrN4NaO6,
formula weight 577.45, T = 233(2) K, λ = 0.71073 Å, triclinic, space
In summary, the following trends in important photophysical
parameters are observed when increasing the number of
bipyridine units in the cryptates Ln-(1−3): (i) an increase in
the energy of the first excited singlet state S1; (ii) a lowering of
the T1 triplet energy levels combined with an increase in ISC
efficiencies; (iii) an increase in the number of bound solvent
molecules; and (iv) a decrease in overall quantum yields for
both Eu and Tb, with the cryptates Ln-1 being approximately
twice as emissive as Ln-2 and Ln-3, the latter two being very
similar.
For the europium cryptates, the following additional
phenomena can be seen: While the radiative lifetimes and the
intrinsic quantum yields are similar for all three cryptates Eu-
(1−3) within the limits of the uncertainties, the sensitization
efficiencies, ηsens, deteriorate with increasing number of
bipyridine units. This trend seems to be connected to a
concomitant decrease in energy transfer efficiencies from the
ligand to the metal.
group P1, a = 12.046(5) Å, b = 15.136(6) Å, c = 17.045(7) Å, α =
̅
113.260(8)°, β = 96.789(9)°, γ = 102.264(9)°, V = 2718(2) Å3, Z = 4,
2
μ = 1.572 mm−1, R(Fo) = 0.1160, Rw(Fo ) = 0.3073, GOF = 0.964.
Synthesis of Na-1. Under N2 1,4,7,10-tetraoxa-7,16-diazacyclooc-
tadecane (4) (90 mg, 342 μmol, 1.0 equiv) was dissolved in CH3CN
(200 mL, HPLC grade), and Na2CO3 (363 mg, 3.4 mmol, 10 equiv)
was added. The suspension was heated to reflux, and a solution of 6,6′-
bis(bromomethyl)-2,2′-bipyridine (5) (117 mg, 342 μmol, 1.0 equiv)
in CH3CN (250 mL, HPLC grade) was added under high-dilution
conditions over 16 h. The mixture was heated under reflux for an
additional 18 h, cooled to ambient temperature, and filtered. The
filtrate was concentrated under reduced pressure, and the resulting
residue was subjected to column chromatography (SiO2, gradient:
CH2Cl2/MeOH, 100:1 → 25:1) to yield the title compound as a
colorless solid (54 mg, 29%). Single crystals suitable for X-ray analysis
were grown by slow evaporation of solutions of the title compound in
moist MeOH.
EXPERIMENTAL SECTION
■
General Procedures. 1,8-Diamino-3,6-dioxaoctane (10) and
1,4,7,10-tetraoxa-7,16-diazacyclooctadecane (4) (Kryptofix 22) were
purchased from Merck KGaA (Darmstadt, Germany). Anhydrous
MgCl2 was purchased from Sigma-Aldrich. 6,6′-Bis(hydroxymethyl)-
2,2′-bipyridine (8),12 6,6′-bis(bromomethyl)-2,2′-bipyridine (5),7 and
the cryptates Ln-1/Ln-3 (Ln = Eu, Gd, Tb)6 were prepared according
to previously reported procedures. CH3CN for the synthesis of the
cryptates was HPLC-grade. Other solvents were dried by standard
procedures (MeOH: Mg/I2; NEt3 and CH2Cl2: CaH2). Air-sensitive
reactions were carried out under a dry, dioxygen-free atmosphere of
N2 using Schlenk techniques. Column chromatography was performed
with silica gel 60 (Merck KGaA, 0.063−0.200 mm). Analytical thin
layer chromatography was done on silica gel 60 F254 plates (Merck,
coated on aluminum sheets). ESI mass spectrometry was measured
using Bruker Daltonics Esquire6000. NMR spectra were measured on
a Bruker DPX-250 (1H: 250 MHz, 13C: 62.9 MHz) and DPX-200 (1H:
200 MHz). UV/vis spectra were recorded on a Jasco-670
spectrophotometer using 1.0 cm quartz cuvettes.
1H NMR (250 MHz, CDCl3): δ 7.91−7.79 (m, 4 H), 7.37−7.27
(m, 2 H), 3.78 (s, 4 H), 3.71−3.37 (m, 16 H), 2.88−2.69 (m, 4 H),
2.67−2.48 (m, 4H) ppm. 13C NMR (62.9 MHz, CDCl3): δ 158.7,
155.5, 138.6, 124.2, 120.9, 68.5, 66.4, 59.9, 53.4.
Synthesis of Dialdehyde 9. Under N2 and at −60 °C, oxalyl
chloride (452 mg, 0.30 mL, 3.56 mmol, 2.2 equiv) was dissolved in
CH2Cl2 (20 mL), and a solution of dry DMSO (556 mg, 7.12 mmol,
4.4 equiv) in dry CH2Cl2 (5 mL) was added. The mixture was stirred
for 5 min at this temperature before a solution of 6,6′-bis-
(hydroxymethyl)-2,2′-bipyridine (8) (350 mg, 1.62 mmol, 1.0 equiv)
in CH2Cl2 (50 mL, with a minimum of dry DMSO necessary to
dissolve everything) was added dropwise. After 15 min, dry NEt3 (16.2
mmol, 1.63 g, 2.26 mL, 10 equiv) was added, and the mixture was
allowed to reach room temperature over the course of ca. 5 h. Water
(75 mL) was added, the phases were separated, and the aqueous layer
was extracted with CH2Cl2 (2 × 50 mL). After drying the combined
organic phases (MgSO4), the solvent was removed under reduced
pressure and the remaining light brown solid was subjected to column
chromatography (SiO2, CH2Cl2/MeOH, 24:1, detection: UV). The
product was isolated as a colorless solid (196 mg, 57%). The analytical
data of the product were in agreement with previous reports using
different synthetic procedures.14
Photophysical Measurements. Steady-state emission spectra at
room temperature were acquired on a PTI Quantamaster QM4
spectrofluorimeter using 1.0 cm quartz cuvettes. The excitation light
source was a 75 W continuous xenon short arc lamp. Emission was
monitored at 90° using a PTI P1.7R detector module (Hamamatsu
PMT R5509-72 with a Hamamatsu C9525 power supply operated at
−1500 V and a Hamamatsu C9940 liquid N2 cooling unit set to −80
°C). Where applicable, solutions were degassed by sparging with dry,
dioxgen-free N2. Low-temperature spectra were recorded on frozen
glasses of solutions of the gadolinium complexes (MeOH/EtOH, 1:1,
v/v) in standard NMR tubes using a dewar cuvette filled with liquid N2
(T = 77 K). Spectral selection was achieved by single grating
monochromators (excitation: 1200 grooves/mm, blazed at 300 nm;
visible emission: 1200 grooves/mm, blazed at 400 nm). Luminescence
lifetimes were determined with the same instrumental setup. The light
source for these measurements was a xenon flash lamp (Hamamatsu
L4633: 10 Hz repetition rate, pulse width ca. 1.5 μs fwhm). Lifetime
data analysis (deconvolution, statistical parameters, etc.) was
performed using the software package FeliX32 from PTI. Lifetimes
were determined either by fitting the middle and tail portions of the
decays or by deconvolution of the decay profiles with the instrument
response function, which was determined using a dilute aqueous
dispersion of colloidal silica (Ludox AM-30). The estimated
uncertainties in τobs are 10%. All measured values are averages of
three independent experiments.
1H NMR (200 MHz, CDCl3): δ 10.19 (s, 2 H), 8.85−8.81 (m, 2
H), 8.07−8.02 (m, 4 H) ppm.
Synthesis of Macrocycle 11. Under N2, anhydrous MgCl2 (88
mg, 0.92 mmol, 1.0 equiv) and 2,2′-bipyridine-6,6′-dicarbaldehyde (9)
(196 mg, 0.92 mmol, 1.0 equiv) were suspended in dry MeOH (20
mL), and the mixture was stirred at room temperature for 30 min. 1,8-
Diamino-3,6-dioxaoctane (10) (137 mg, 0.92 mmol, 1.0 equiv) in dry
MeOH (2 mL) was added, stirring was continued for 1 h, and NaBH4
(4.62 mmol, 175 mg, 5.0 equiv) was added in small portions. After 20
h at room temperature, water (20 mL) was added to the clear solution,
MeOH was removed under reduced pressure, and the aqueous phase
was extracted with CH2Cl2 (3 × 35 mL). The combined organic layers
were dried (MgSO4) and concentrated. The crude product (280 mg,
93%) was obtained as a faintly yellow oil that appears to be slightly air-
sensitive. It was therefore stored under inert atmosphere at 4 °C and
was used without further purification.
X-ray Analysis. A colorless block of Na-1 having approximate
dimensions of 0.5 × 0.3 × 0.2 mm3 was mounted on a glass fiber using
perfluoropolyether oil. The measurement was carried out on a Bruker
Smart 1000 CCD diffractometer using graphite-monochromated Mo
1H NMR (250 MHz, CDCl3): δ 8.41−7.06 (m, 4 H), 7.35−7.06
(m, 2 H), 4.04−3.85 (m, 4 H), 3.72−3.52 (m, 8 H), 2.94−2.74 (m, 4
H) ppm. 13C NMR (62.9 MHz, CDCl3): δ 159.7, 159.1, 158.9, 156.6,
137.1, 137.0, 122.2, 122.0, 120.6, 119.3, 70.8, 70.6, 70.3, 64.2, 55.0,
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Inorganic Chemistry
Article
54.9, 49.0, 48.8. MS (ESI+): m/z (%) 351.13 (100, [M + Na]+),
329.18 (13, [M + H]+).
REFERENCES
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(1) Selected reviews: (a) Lanthanide Luminescence (Springer Series
on Fluorescence 7); Hanninen, P.; Harma, H., Eds.; Springer: Berlin,
Synthesis of Na-2. Macrocycle 11 (132 mg, 402 μmol, 1.0 equiv)
was dissolved in CH3CN (200 mL), and anhydrous Na2CO3 (426 mg,
4.02 mmol, 10 equiv) was added. The suspension was heated to reflux,
and a solution of 6,6′-bis(bromomethyl)-2,2′-bipyridine (5) (137 mg,
402 μmol, 1.0 equiv) in freshly distilled CH3CN (300 mL) was added
under high-dilution conditions over 9 h. The mixture was heated
under reflux for an additional 20 h, cooled to ambient temperature,
and filtered. The filtrate was concentrated under reduced pressure.
The residue was subjected to column chromatography (SiO2, gradient:
CH2Cl2/MeOH 24:1 → 9:1, detection: UV and I2 vapor). The
product was isolated as a colorless solid (42 mg, 17%).
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2011. (b) Bunzli, J.-C. G. Chem. Rev. 2010, 110, 2729−2755.
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(2) (a) Alpha, B.; Lehn, J.-M.; Mathis, G. Angew. Chem., Int. Ed. Engl.
1987, 26, 266−267. (b) Alpha, B.; Balzani, V.; Lehn, J.-M.; Perathoner,
S.; Sabbatini, N. Angew. Chem., Int. Ed. Engl. 1987, 26, 1266−1267.
(3) Review: (a) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord.
Chem. Rev. 1993, 123, 201−228. Selected original articles: (b) Blasse,
G.; Dirksen, G. J.; Sabbatini, N.; Perathoner, S.; Lehn, J.-M.; Alpha, B.
J. Phys. Chem. 1988, 92, 2419−2422. (c) Blasse, G.; Dirksen, G. J.; van
der Voort, D.; Sabbatini, N.; Perathoner, S.; Lehn, J.-M.; Alpha, B.
Chem. Phys. Lett. 1988, 146, 347−351. (d) Alpha, B.; Ballardini, R.;
Balzani, V.; Lehn, J.-M.; Perathoner, S.; Sabbatini, N. Photochem.
Photobiol. 1990, 52, 299−306. (e) Prodi, L.; Maestri, M.; Balzani, V.;
Lehn, J.-M.; Roth, C. Chem. Phys. Lett. 1991, 180, 45−50. (f) Cross, J.
P.; Dadabhoy, A.; Sammes, P. G. J. Lumin. 2004, 110, 113−124.
(g) Guillaumont, D.; Bazin, H.; Benech, J.-M.; Boyer, M.; Mathis, G.
ChemPhysChem 2007, 8, 480−488.
1H NMR (250 MHz, CDCl3): δ 8.03−7.81 (m, 8 H), 7.43−7.33
(m, 4 H), 3.97 (d, J = 14.0 Hz, 4 H), 3.82−3.61 (m, 12 H), 2.72 (t, J =
5.2 Hz, 4 H) ppm. 13C NMR (62.9 MHz, CDCl3): δ 158.8, 155.6,
138.7, 124.7, 120.8, 69.0, 66.1, 59.9, 53.3 ppm. MS (ESI+): m/z (%)
531.13 (100, [M]+). Rf = 0.17 (SiO2, CH2Cl2/MeOH, 9:1, detection:
UV and I2 vapor).
Synthesis of Ln-2. General procedure: A solution of Na-2 (1.0
equiv) and LnCl3·6H2O (1.05 equiv) in dry acetonitrile was heated
under reflux overnight. The solvent was removed in vacuo. The
remaining residue was dissolved in a minimum amount of MeOH, and
the solution was layered with Et2O. After storing the suspension at
room temperature overnight, the precipitate was collected on a
membrane filter (nylon, 0.45 μm), washed with Et2O, and dried in
vacuo to yield the lanthanoid complex as a light yellow powder.
Eu-2: 1.4 mg (from 10.8 mg of Na-2). MS (ESI+): m/z (%) 730.93
([M + 2Cl]+). Anal. Calcd (Found) for [C30H32N6O2Eu]-
(Cl)3·EuCl3·H2O·3MeOH·Et2O (Mr = 1213.52): C, 36.6 (36.9); H,
4.65 (4.79); N, 6.93 (7.34).
Gd-2: 5.1 mg (from 10.1 mg of Na-2). MS (ESI+): m/z (%) 735.94
([M + 2Cl]+). Anal. Calcd (Found) for [C30H32N6O2Gd]-
(Cl)3·GdCl3·8H2O (Mr = 1179.95): C, 30.5 (30.0); H, 4.10 (3.75);
N, 7.12 (6.78).
Tb-2: 2.7 mg (from 7.1 mg of Na-2). MS (ESI+): m/z (%) 736.85
([M + 2Cl]+). Anal. Calcd (Found) for [C30H32N6O2Tb]-
(Cl)3·TbCl3·4H2O (Mr = 1111.24): C, 32.4 (32.1); H, 3.63 (4.04);
N, 7.56 (8.16).
(4) Mathis, G.; Bazin, H. In Lanthanide Luminescence (Springer Series
on Fluorescence 7); Hanninen, P.; Harma, H., Eds.; Springer: Berlin,
2011; pp 47−88.
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N.; Molon, M.; Seitz, M. Inorg. Chem. 2012, 51, 4539−4545.
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(9) Rodriguez-Ubis, J.-C.; Alpha, B.; Plancherel, D.; Lehn, J.-M. Helv.
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(10) Newkome, G. R.; Pappalardo, S.; Gupta, V. K.; Fronczek, F. R. J.
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ASSOCIATED CONTENT
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(13) Omura, K.; Swern, D. Tetrahedron 1978, 34, 1651−1660.
(14) 2,2′-Bipyridine-6,6′-dicarbaldehyde (9) was previously prepared
by different routes: (a) Parks, J. E.; Wagner, B. E.; Holm, R. H. J.
Organomet. Chem. 1973, 56, 53−66. (b) Newkome, G. R.; Lee, H.-W.
J. Am. Chem. Soc. 1983, 105, 5956−5957.
S
* Supporting Information
Gaussian fits for the triplet levels in Gd-(1−3). Crystallo-
graphic information file for Na-1. This material is available free
of charge via the Internet at http://pubs.acs.org.
(15) Luning, U.; Muller, M. Liebigs Ann. Chem. 1989, 367−374.
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(16) Ortep-3 for Windows: Farrugia, L. J. J. Appl. Crystallogr. 1997,
30, 565.
AUTHOR INFORMATION
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(17) Caron, A.; Guilhelm, J.; Riche, C.; Pascard, C.; Alpha, B.; Lehn,
J.-M.; Rodriguez-Ubis, J. C. Helv. Chim. Acta 1985, 68, 1577−1582.
(18) Bkouche-Waksman, I.; Guilhem, J.; Pascard, C.; Alpha, B.;
Deschenaux, R.; Lehn, J.-M. Helv. Chim. Acta 1991, 74, 1163−1170.
(19) Energy levels of the trivalent lanthanoid aquo species: (a) Tb:
Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys. 1968, 49, 4447−
4449. (b) Eu: Carnall, W. T.; Fields, P. R.; Rajnak, K. J. Chem. Phys.
1968, 49, 4450−4455.
Corresponding Author
*E-mail: michael.seitz@rub.de.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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(20) Beeby, A.; Clarkson, I. M.; Dickins, R. S.; Faulkner, S.; Parker,
D.; Royle, L.; de Sousa, A. S.; Williams, J. A. G.; Woods, M. J. Chem.
Soc., Perkin Trans. 2 1999, 493−503.
M.S. thanks Prof. Dr. Nils Metzler-Nolte (Ruhr-University
Bochum) for his continued support. The authors thank Dr.
Klaus Merz (Ruhr-University Bochum) for help with the crystal
structure measurement. Financial support is gratefully acknowl-
edged from DFG (Emmy Noether Fellowship to M.S.), Fonds
der Chemischen Industrie (Liebig Fellowship to M.S. and
predoctoral fellowships for N.A.), Int. Max Planck Research
School in Chemical Biology (Predoctoral fellowship for C.B.),
and Research Department Interfacial Systems Chemistry
(Ruhr-University Bochum). C.B. is grateful for support by
the Ruhr-University Research School.
(21) Deiters, E.; Song, B.; Chauvin, A.-S.; Vandevyver, C. D. B.;
Bunzli, J.-C. New J. Chem. 2008, 32, 1140−1152.
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(22) Faulkner, S.; Beeby, A.; Carrie, M.-C.; Dadabhoy, A.; Kenwright,
A. M.; Sammes, P. G. Inorg. Chem. Commun. 2001, 4, 187−190.
(23) Werts, M. H. V.; Jukes, R. T. F.; Verhoeven, J. W. Phys. Chem.
Chem. Phys. 2002, 4, 1542−1548.
(24) Wingx: Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−848.
(25) SIR92: Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi,
A. J. Appl. Crystallogr. 1993, 26, 343−350.
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Inorganic Chemistry
Article
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