A water-soluble Pybox derivative and its highly luminescent lanthanide ion complexes

Article abstract of DOI:10.1021/ja209572m

A new water-soluble Pybox ligand, 1, has been synthesized and found to crystallize in the monoclinic P2₁/n space group with unit cell parameters a = 6.0936(1) A, b = 20.5265(4) A, c = 12.0548(2) A, and β = 90.614(1)°. In the crystal, a water molecule is bound through hydrogen-bonding interactions to the nitrogen atoms of the oxazoline rings. This ligand was used to complex a variety of lanthanide ions, opening up new avenues for luminescence and catalysis in aqueous environment. These complexes are highly luminescent in aqueous solutions, in acetonitrile, and in the solid state. Aqueous quantum yields are high at 30.4% for Eu(III), 26.4% for Tb(III), 0.32% for Yb(III), and 0.11% for Nd(III). Er(III) did not luminesce in water, but an emission efficiency of 0.20% could be measured in D₂O. Aqueous emission lifetimes were also determined for the visible emitting lanthanide ions and are 1.61 ms for Eu(III) and 1.78 ms for Tb(III). Comparing emission lifetimes in deuterated and nondeuterated water indicates that no water molecules are coordinated to the metal ion. Speciation studies show that three species form successively in solution and the log β values are 5.3, 9.6, and 13.8 for Eu(III) and 5.3, 9.2, and 12.7 for Tb(III) for 1:1, 2:1, and 3:1 ligand to metal ratios, respectively.

Full text of DOI:10.1021/ja209572m

Article
pubs.acs.org/JACS
A Water-Soluble Pybox Derivative and Its Highly Luminescent
Lanthanide Ion Complexes
Ana de Bettencourt-Dias,* Patrick S. Barber, and Sebastian Bauer
Department of Chemistry, University of Nevada, Reno, Nevada 89557, United States
S
* Supporting Information
ABSTRACT: A new water-soluble Pybox ligand, 1, has been
synthesized and found to crystallize in the monoclinic P21/n space
group with unit cell parameters a = 6.0936(1) Å, b = 20.5265(4) Å, c =
12.0548(2) Å, and β = 90.614(1)°. In the crystal, a water molecule is
bound through hydrogen-bonding interactions to the nitrogen atoms of
the oxazoline rings. This ligand was used to complex a variety of
lanthanide ions, opening up new avenues for luminescence and catalysis
in aqueous environment. These complexes are highly luminescent in
aqueous solutions, in acetonitrile, and in the solid state. Aqueous
quantum yields are high at 30.4% for Eu(III), 26.4% for Tb(III), 0.32%
for Yb(III), and 0.11% for Nd(III). Er(III) did not luminesce in water,
but an emission efficiency of 0.20% could be measured in D2O. Aqueous
emission lifetimes were also determined for the visible emitting
lanthanide ions and are 1.61 ms for Eu(III) and 1.78 ms for Tb(III).
Comparing emission lifetimes in deuterated and nondeuterated water indicates that no water molecules are coordinated to the
metal ion. Speciation studies show that three species form successively in solution and the log β values are 5.3, 9.6, and 13.8 for
Eu(III) and 5.3, 9.2, and 12.7 for Tb(III) for 1:1, 2:1, and 3:1 ligand to metal ratios, respectively.
dichroism spectrum of chiral assemblies of quinone-hydro-
quinone systems with a Sc(III)-Pybox complex37 and circularly
polarized luminescence of Ln(III) complexes of a chiral Pybox
complex were reported.38 While its use as a sensitizer of Ln(III)
ion luminescence remains scarce, since Pybox was first reported
by Nishiyama in 1989, it has been extensively utilized in
catalysis.39−41 Nishiyama et al. showed that Pybox can be
synthesized with substituted oxazoline rings and used as a
ligand in a Ru(IIII) catalyst for the asymmetric hydrosilylation
of ketones.42 Recent reports of enantioselective catalysis using
transition-metal complexes with oxazoline-derivatized Pybox
ligands include cross-coupling reactions on racemic halides with
aryl zinc reagents,43−45 Nazarov cyclizations,46 asymmetric [3 +
2] cycloadditions of racemic cyclopropanes and aldehydes47 or
alkynes with N-tosylaziridines,48 carbonyl-ene reactions,49
aminations,50,51 Negishi cross-coupling,52,53 Mukaiyama-aldol
reactions,54 allylic substitutions with N or O nucleophiles,55
and copper-catalyzed reduction of secondary amines.56 Pybox
complexes of transition metals have also successfully been
immobilized on solid supports to be used in catalytic
processes.57−59 Pybox complexes of the rare earths have been
used in Friedel−Crafts alkylations,60 Mukaiyama-aldol reac-
tions,61,62 and silylcyanation of aldehydes.63 The lanthanide-
Pybox-based systems studied thus far displayed luminescence
only in organic solvents, including methanol, and were not
INTRODUCTION
■
The luminescence of lanthanide (Ln(III)) ions, of interest for
applications such as lighting and bioimaging, is usually
sensitized by coordinated ligands.1−4 Sensitization was first
reported by Weissman with β-diketonate and salicylate ligands.5
In addition to β-diketonates,6−9 a large variety of additional
ligands has since been screened, from simple chelating ligands
such as dipicolinates10−13 and oligopyridines14,15 to macro-
cycles such as cyclen16,17 and crown ether18,19 derivatives and
porphyrins,20,21 as well as cryptands22 and calixarenes.23−25
Water solubility is an important property of the complexes in
applications such as bioimaging and anion sensing.26
Luminescent and water-soluble complexes of Eu(III) and
Tb(III) have been reported as binding tags for studying protein
interactions,27,28 for bacterial spore29 or singlet oxygen30
detection, for signaling of carbonate chelation,31 as labels for
prolactinin human serum,32 and as luminescent tags on
magnetic nanoparticles for cell imaging applications,33 among
others. Our group recently reported the use of pyridine-
bis(oxazoline) as a sensitizer in highly luminescent lanthanide
ion complexes in organic solvents.34−36 We showed that in
para-derivatized Pybox ligands, when the oxazoline rings are
unsubstituted, the ligands completely saturate the coordination
sphere of the Ln(III) ion in solution and in the solid state,
when the complex is prepared in a 3:1 ligand-to-metal ratio.34,35
The coordination of luminescence quenching solvent molecules
containing O−H oscillators, such as water, is therefore avoided
and radiative decay is promoted. Subsequently, the circular
Received: October 11, 2011
Published: April 13, 2012
© 2012 American Chemical Society
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chromatography (2% NH4OH in MeCN, silica) to give 0.7412 g of
pure material. Yield: 48.6%. H NMR (400 MHz, CD3CN, TMS): δ
water-soluble. To date, only two types of water-soluble Pybox
derivatives have been reported; the first has hydroxymethyl
groups64,65 and the second carboxylate methyl ester at the
oxazoline rings.66 The synthesis of the simplest representative
of the first type occurs via two unstable intermediates, and it
has been characterized only by NMR of a derivative, as this
water-soluble compound is hard to purify.64,65 However, it was
used in cyclopropanation reactions,67 in cycloaddition reactions
of nitrones and activated alkenes,68 and in Mukaiyama-aldol
reactions.54 The carboxylate derivative was initially isolated as
an intermediate and only recently reported to catalyze catalytic
H/D exchange in water.69 Water solubility is a criterion that is
also highly desirable for greener manufacturing procedures,
such as in catalysis, as there is an increasing drive for more
environmentally benign processes, which use water as a
preferred, nontoxic solvent.70,71 Here we present a new, stable,
and easily synthesized water-soluble Pybox ligand with an
ethylene glycol ethyl ether at the para position of the pyridine
ring and its highly luminescent complexes of lanthanide ions.
Although not demonstrated here, the synthetic steps for
isolation of this Pybox derivative can be easily followed to
develop new, chiral, and water-soluble Pybox ligands for
aqueous organic transformations.
1
7.63 (s, 2H), 4.48 (t, J = 9.0 Hz, 4H), 4.28 (m, 2H), 4.03 (t, J = 9.0
Hz, 4H), 3.77 (m, 2H), 3.56 (q, J = 8.0 Hz, 2H), 1.17 (t, J = 8.0 Hz,
3H). 13C NMR (126 MHz, CD3CN, TMS): δ 166.06, 163.27, 148.62,
1
111.96, 68.60, 68.32, 68.26, 66.38, 54.97, 14.66. H NMR (400 MHz,
D2O, DSS): δ 7.54 (s, 2H), 4.59 (t, J = 9.8, 4H), 4.30 (m, 2H), 4.07 (t,
J = 9.8, 4H), 3.91 (m, 2H), 3.68 (q, J = 7.1, 2H), 1.22 (t, J = 7.1, 3H).
13C NMR (101 MHz, D2O, DSS): δ 168.79, 166.34, 149.72, 115.19,
71.70, 70.63, 70.57, 69.60, 56.41, 16.81. ESI-MS: experimental [M +
H] m/z 306.1454, calculated 306.145.
Synthesis of Metal Complexes. All metal complexes were
prepared in a similar manner in air by mixing stoichiometric amounts
of ligand and Ln(NO3)3 or Ln(CF3SO3)3 (Ln = Eu, Tb, Tm, Dy, Pr,
Sm, Yb, Nd, Er) in acetonitrile. After the mixture was heated for
several hours, the solvent was removed under reduced pressure and
the resulting material was dried in a vacuum oven overnight to give the
metal complexes. Complexes were dissolved in an appropriate solvent
prior to quantum yield and lifetime measurements. Since quantitative
data were collected only on the complexes of Eu, Tb, Nd, Er, and Yb,
characterization by ESI-MS and elemental analysis was carried out only
for these complexes. Attempts to isolate X-ray-quality crystals of the
metal complexes are ongoing.
[Eu(1)3](NO3)3. ESI-MS: [Eu(1)33+] m/z found (calculated)
356.1117 (356.111). Anal. Calcd (found) for Eu(1)](NO3)3·3H2O:
C, 41.32 (41.14); H, 4.85 (4.84); N, 12.85 (12.74).
[Tb(1)3](NO3)3. ESI-MS [Tb(1)33+] m/z found (calculated)
358.1111 (358.113). Anal. Calcd (found for Tb(1)](NO3)3·3H2O:
C, 41.10 (40.37); H, 4.83 (4.75); N, 12.78 (12.52).
EXPERIMENTAL SECTION
■
All commercially obtained reagents were of analytical grade and were
used as received. Solvents were dried by standard methods. Lanthanide
salts were dried under reduced pressure and heating and kept in a
glovebox under a controlled atmosphere (O2 <0.2 ppm, H2O <2
ppm). Unless otherwise indicated, all data were collected at a constant
temperature of 25.0 0.1 °C. NMR spectra were recorded on Varian
400 and 500 MHz spectrometers with chemical shifts reported (δ,
ppm) against tetramethylsilane (TMS) or sodium 2,2-dimethyl-2-
silapentane-5-sulfonate (DSS). Electrospray ionization mass spectra
(ESI-MS) were collected in positive ion mode on a Waters Micromass
ZQ quadrupole mass spectrometer. The samples were prepared by
diluting solutions to a concentration of ∼1 mg/mL with acetonitrile.
All samples were filtered through a 0.2 μm syringe filter before
injecting into the mass spectrometer.
[Nd(1)3](NO3)3. ESI-MS [Nd(1)33+] m/z found (calculated)
353.1104 (353.115). Anal. Calcd (found) for Nd(1)](NO3)3·3H2O:
C, 41.57 (41.53); H, 4.88 (4.80); N, 13.03 (12.92).
[Yb(1)3](NO3)3. ESI-MS [Yb(1)33+] m/z found (calculated)
363.1159 (363.123). Anal. Calcd (found) for Yb(1)](NO3)3·3H2O:
C, 40.67 (39.93); H, 4.77 (4.73); N, 12.48 (12.61).
[Er(1)3](NO3)3. ESI-MS [Er(1)33+] m/z found (calculated) 361.1115
(361.122). Anal. Calcd (found) for Er(1)](NO3)3·3H2O: C, 40.84
(39.84); H, 4.80 (4.56); N, 12.70 (12.49).
Photophysical Characterization. Solutions for spectroscopic
studies were prepared by dissolving the 3:1 metal to ligand complex in
acetonitrile, in D2O or in 0.1 M Tris-HCl buffer (pH 7.4). The
solutions were diluted to a final concentration of 1 × 10−4 M.
Absorption spectra were measured on a Perkin-Elmer Lambda 35
spectrometer and emission/excitation spectra on a Perkin-Elmer LS-55
fluorescence spectrometer or a Horiba Jobin Yvon Fluoro-3
spectrofluorometer. Slit widths for emission and excitation measure-
ments were 5−10 nm, and the scan rate was 250 nm/s. On the Perkin-
Elmer fluorimeter the data were collected in phosphorescence mode
with a delay of 0 ms, a cycle time of 16 ms, and a gate time of 0.05 ms.
The near-IR measurements and all lifetimes were recorded on a
Horiba Jobin Yvon Fluoro-3 spectrofluorometer equipped with a
Hamamatsu PMT NIR detector. Solutions were allowed to equilibrate
for 1−2 h before photophysical measurements. For quantum yield
measurements, both the absorption and emission/excitation spectra
were measured using 0.2 cm path length cells, and while measuring the
spectra the emitted light was at a right angle and along the long path
length (1 cm). Quantum yields were measured for concentrations in
the range 1 × 10−5−5 × 10−4 M. All measurements, except the triplet-
state measurements, were performed at 25.0 0.1 °C. The triplet-state
measurements were performed at 77 K, as proposed by Crosby.74
Singlet and triplet state energies are indicated as the 0−0 transitions,
after deconvolution of the fluorescence and phosphorescence spectra
into their Franck−Condon progressions.
The metal ion concentration of all stock solutions was determined
by titration with standardized ethylenediaminetetraacetic acid (EDTA;
0.01 M) with hexamine buffer and using xylenol orange as indicator.72
All reported photophysical data are the average of at least three
independent measurements.
Ligand Synthesis. The ligand was synthesized according to
modified literature procedures.42,73
4-Chloro-N2,N6-bis(2-chloroethyl)pyridine-2,6-dicarboxamide (a).
A 5.00 g portion (27 mmol) of chelidamic acid and 20 μL of DMF
were refluxed in 20 mL of SOCl2 for 24 h. The excess thionyl chloride
was removed to give 4-chloropyridine-2,6-dicarbonyl dichloride, which
was used without further purification. A solution of the acid chloride in
100 mL of chloroform was added dropwise to the vigorously stirred
solution of 6.95 g (60 mmol) of 2-chloroethylamine hydrochloride and
6.40 g (114 mmol) of KOH in minimal H2O at 0 °C. The resulting
mixture was stirred for another 2 h. The organic phase was diluted
with 100 mL of CHCl3, separated, washed with water (2 × 100 mL)
and saturated NaHCO3 (2 × 100 mL), and dried over MgSO4. The
solvent was removed under reduced pressure to give a bright white
1
solid. Yield: 6.70 g (76.4%, from chelidamic acid). H NMR (400
MHz, CDCl3): δ 8.34 (s, 2H), 3.86 (t, J = 6 Hz, 4H), 3.75 (t, J = 6 Hz,
4H) ppm. 13C NMR (126 MHz, CDCl3, TMS): δ 162.47, 149.84,
148.00, 125.62, 43.69, 41.30 ppm.
2,2′-(4-(2-Ethoxyethoxy)pyridine-2,6-diyl)bis(4,5-dihydrooxazole)
(1). A 2.00 g portion (5 mmol) of 4-chloro-N2,N6-bis(2-chloroethyl)-
pyridine-2,6-dicarboxamide and 1.46 g (26 mmol) of KOH were
heated at 75 °C in 150 mL of 2-ethoxyethanol for 2 days. The solvent
was then removed, and the crude material was purified by flash column
Quantum yields were calculated using the equation
2
nx Aref IrefE
Φ =
x Φ
x
ref
nref 2AxIxEref
where Φ is the quantum yield of sample x and reference ref, n is the
refractive index (1.343 in acetonitrile, 1.332 87 in water), A is the
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absorbance at the excitation wavelength, I is the intensity of the
corrected excitation spectrum at the excitation wavelength, and E is the
integrated corrected emission spectrum. The spectra are always
corrected for instrumental functions. Quantum yields for the reported
strategy that combines ω and ψ scans of 0.3° per frame and an
acquisition time of 10 s per frame. Multiscan absorption corrections
were applied. Cell parameters were retrieved using SMART82 software
and refined using SAINTPlus83 on all observed reflections. Data
reduction and correction for Lp and decay were performed using the
SAINTPlus83 software. Absorption corrections were applied using
SADABS.84 The structure was solved by direct methods and refined by
least-squares methods on F2 using the SHELXTL85 program package.
All non-hydrogen atoms were refined anisotropically. The hydrogen
atoms were located on the difference map and their parameters
constrained to the parent site. The X-ray crystallographic information
file can either be found on the same Web site or can be obtained free
of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; fax +44 1223
336033; e-mail @ccdc.cam.ac.uk). CCDC 868880 contains the
supplementary crystallographic data for 1·H2O.
solutions were measured against Cs3[Eu(dipic)3] (Φref = 24.0%, A279
≈
0.15, 7.5 × 10−5 M) and Cs3[Tb(dipic)3] (Φref = 22.0%, A279 ≈ 0.15,
6.5 × 10−5 M) in Tris buffer (0.1 M) as reference standards75,76 for the
visible emitting species and [Yb(TTA)3(H2O)2] (Φref = 0.35%, 1 ×
10−4 M, anhydrous toluene)77 for the near-IR emitting complexes. The
excitation wavelengths of the samples were chosen to ensure that there
is a linear relationship between the intensity of emitted light and the
concentration of the absorbing/emitting species (A ≤ 0.05).
Ln
Ln
The intrinsic luminescence efficiency Φ is given by
τ
τR
Ln
Ln
obs
Φ
=
where τobs is the experimentally determined emission lifetime and τR
the radiative lifetime. τR can be obtained from the integrated emission
spectrum, ETOT, and the integrated emission band of a magnetic dipole
transition, EMD, if AMD, the spontaneous emission probability of the
RESULTS AND DISCUSSION
■
PyboxO(CH2)2OEt (1) was synthesized following a procedure
analogous to other para-derivatized Pybox ligands reported by
our group35 with an overall yield of 37% from chelidamic acid
(Scheme 1). It was characterized by NMR spectroscopy and
high-resolution ESI-MS.
5
7
magnetic dipole transition (e.g., D0 → F1 in Eu3+), is known, by
utilizing the equation78
1
τR
= AMDn3(ETOT/EMD
)
Scheme 1. Synthetic Scheme for PyboxO(CH2)2OEt
In this equation n is the refractive index of the solution. Klink et al.
have calculated and confirmed experimentally that AMD is 14.65 s−1 for
Eu(III).78 The efficiency of sensitization Φsens for the Eu(III)
complexes can then be estimated, as
Eu
Eu
Φ = ΦEu = Φ × Φ
sens
The number of coordinated water molecules q was determined
through comparison of the emission lifetimes of Eu(III) and Tb(III) in
water and deuterated water, using the equation below, proposed by
Horrocks et al.79
−1
q = A(τH O−1 − τD O
)
X-ray-quality single crystals of 1·H2O were obtained after a
few weeks from a saturated solution of acetonitrile. The
structure of this molecule is shown in Figure 1. 1 displays a
2
2
where A = 1.05 for Eu and 4.2 for Tb.
Determination of Stability Constants. All the solutions were
prepared in air in Nanopure (18.2 MΩ) water at constant ionic
strength (I = 0.1 M) using Et4NCl and 0.1 M Tris-HCl buffer (pH
7.4). Stock solutions of lanthanide ions at 0.005 M were prepared by
dissolving appropriate quantities of Ln(CF3SO3)3 (Ln = Eu, Tb) in
Nanopure water (18.2 MΩ). A stock solution of the ligand was also
prepared at 0.01 M concentration in Nanopure water (18.2 MΩ) and
was diluted as needed. In a typical experiment 50 mL of 1 × 10−4
M
ligand was titrated against metal triflate solution. After each metal ion
addition and a delay of 5 min, the emission spectrum was measured.
Each titration run had at least 30 data points so that there was a good
fit. A wide range of ligand to metal ion stoichiometric ratios was
considered. At least three repeat titrations were performed for each
system to account for experimental error. Stability constants, log β,
were refined using the HYPERQUAD2006 software package,80 and
speciation diagrams were drawn with the program HySS,81 assuming
the three equilibria
Figure 1. Thermal ellipsoid plot of 1·H2O with atom labeling.
Ellipsoids are shown at the 50% probability level. Hydrogen atoms on
1 are omitted for clarity.
Ln3+ + 1 ⇌ [Ln(1)]3+ β11
Ln3+ + 2 1 ⇌ [Ln(1)2]3+ β21
Ln3+ + 3 1 ⇌ [Ln(1)3]3+ β31
pyridine ring with two oxazoline substituents on the 2- and 6-
positions as well as an O(CH2)2OCH2CH3 chain at the para
position. The two oxazoline rings are almost planar with the
pyridine ring, with torsion angles N1−C9−C8−N2 = 3.2(1)°
and N2−C4−C3−N3 = −7.3(1)°. The nitrogen atoms of the
oxazoline rings, which usually point in the opposite direction of
the pyridine nitrogen atom due to stabilizing hydrogen-bonding
interactions between these atoms and the aromatic C−H,86
point in 1 toward the center of the cavity formed by the three
X-ray Crystallographic Characterization. Crystal data, data
collection, and refinement details for 1·H2O are given in Table S1
(Supporting Information). A suitable crystal was mounted on a glass
fiber and placed in the low-temperature nitrogen stream. Data were
collected on a Bruker SMART CCD area detector diffractometer
equipped with a low-temperature device, using graphite-monochro-
mated Mo Kα radiation (λ = 0.710 73 Å). Data were measured using a
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rings. This position is favored due to the presence of a solvent
water molecule in the cavity, which hydrogen bonds with the
nitrogen atoms of the oxazoline rings. The hydrogen-bonding
distances and angles are N1····H5A−O5 = 2.157(17) Å,
174.9(2)° and N3····H5B−O5 = 2.170(17) Å, 174.5(2)°.
This water molecule further interacts with neighboring
molecules of 1, through weak hydrogen bonding to C−H
groups of the oxazoline rings and the glycol moiety, as shown in
Figure S1 in the Supporting Information.
In addition to common organic solvents such as dichloro-
methane, chloroform, and acetonitrile, 1 is also soluble in water.
In aqueous solution the compound displays absorption maxima
at 210 and 236 nm, with a shoulder at 295 nm, as shown in
Figure 2. Excitation in the 236−300 nm range yields a broad
emission with a maximum at 351 nm.
Figure 3. Absorption (dotted), excitation (dashed), and emission
spectra (solid) of [Ln(1)3]3+ (Ln = Tb (green), Eu (red)) in H2O.
5
7
The inset shows the D0 → F0 transition of Eu(III) in more detail.
[complex] = 1 × 10−4 M, λexc 290 nm.
narrow full width at half-maximum (fwhm) of 45 cm−1, shown
in detail in the inset of Figure 3, are indicative of the presence
of only one species in solution. This value compares well with
reported fwhm of 15−20 cm−1 for single species measured at
1211 and 77 K.87 The large intensity of the hypersensitive D0
5
7
→ F2 transition with respect to the magnetic dipole allowed
7
5D0 → F1 transition indicates a low symmetry environment
around the metal ion. The quantum yields of emission and
excited state lifetimes were measured in different solvents and
are summarized in Table 1. In acetonitrile, the emission
efficiencies of Eu(III) and Tb(III) are 31.1 and 19.6%,
respectively. Other para-derivatized Pybox sensitizers, pre-
viously reported by our group, display emission efficiencies in
the range 26−76% for Eu(III) and 23−59% for Tb(III), which
compare well with the data presented here. Similar high
efficiencies were obtained in water with PyboxO(CH2)2OEt as
the sensitizer, with values of 30.4 and 26.4% for Eu(III) and
Tb(III), respectively. These values compare favorably with
emission efficiencies of 16−24% reported for Eu(III) and 22−
40% for Tb(III) in aqueous solution.11,88,89 These data, along
with data collected in degassed solutions and Tris-HCl buffered
aqueous solutions, show that different solvents do not influence
the emission process appreciably, as the lanthanide ion’s
coordination sphere is adequately shielded by the three
coordinated ligands.
The dependence of the emission efficiencies on solution
concentrations was also investigated. It was observed that, in
aqueous solution, within the concentration range 1 × 10−5−5 ×
10−4 M, the efficiencies, within experimental error, do not
change appreciably (Figure S2, Supporting Information). This
signifies that the complexes are stable within the concentration
range studied.90
While the shielding is important, the location of the ligand’s
singlet and triplet states plays an important role in determining
the efficiencies of intersystem crossing and energy transfer from
ligand to lanthanide ion. Their energies were therefore
determined using the Gd(III) complex91 of 1 and are
summarized in Table 1. The triplet state energy is 26 200
cm−1 in acetonitrile and slightly higher, at 26 720 cm−1, in
water, consistent with a small degree of solvatochromism, likely
Figure 2. Absorption (dotted), excitation (dashed), and emission
(solid) spectra of 1 in water.
Stirring the ligand with Ln(NO3)3 (Ln = La, Eu, Tb, Nd, Yb,
Er) in a 3:1 ligand to metal ratio in acetonitrile yielded the
corresponding complexes [Ln(PyboxO(CH2)2OEt)3](NO3)3,
which were characterized by high-resolution ESI-MS (Table S1
and Figures S3−S13, Supporting Information) and 1H NMR of
a complex with the diamagnetic La(III) (Figure S14,
Supporting Information). The formation and characterization
of these complexes confirm that the complexes with Pybox-type
ligands are stable in a 3:1 ligand to metal ratio, as shown
previously.34,35
The Eu(III) and Tb(III) complexes are highly luminescent in
solution, in solvents such as acetonitrile and water, and in the
solid state, showing the characteristic metal-centered emission.
The absorption spectra of the complexes, shown in Figure 3,
are featureless in comparison with the free ligand, with a
shoulder that can be discerned at 240 nm. Excitation maxima
for the Eu(III) and Tb(III) complexes are at 290 nm, which
corresponds to a second shoulder in the absorption spectra,
consistent with ligand-centered sensitization. Excitation of the
complexes at this wavelength yields the characteristic emission
peaks for each Ln(III) ion, as shown in Figure 3. The emission
spectrum of Tb(III) displays five distinct peaks at 489, 543,
582, 620, and 646 nm corresponding to the 5D4 → 7FJ (J = 6, 5,
4, 3, 2) transitions. Eu(III) also displays five distinct peaks at
5
579, 592, 615, 649, and 693 nm corresponding to the D0 →
7FJ (J = 0, 1, 2, 3, 4) transitions. The absence of splitting in the
7
5D0 → F0 transition of Eu(III), its symmetrical shape, and its
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a
Table 1. Quantum Yields of Luminescence (Φ) and Singlet (1S) and Triplet (3T) States of [Ln(1)3]3+ in Different Solvents
ΦEu (%)
ΦTb (%)
ΦYb (%)
ΦNd (%)
ΦEr (%)
b
MeCN
H2O
1S 33 000 160 cm−1
3T 26 200 40 cm−1
0.24 0.02
b
31.1 2.3
19.6 1.3
0.23 0.07
0.19 0.03
Φ
sens = 59.9 6.8
ΦEuEu = 51.7 4.4
bc
,
1S 33 900 120 cm−1
3T 26 720 220 cm−1
0.32 0.16
bc
,
30.4 5.9
26.4 7.1
0.11 0.03
no emission
Φ
sens = 70.6 14.0
ΦEuEu = 43.0 2.2
28.6 6.5
H2O w/0.1 Tris-HCl
H2O w/0.1 Tris-HCl (degassed)
D2O
25.9 4.9
25.0 1.5
29.1 5.6
0.30 0.06
0.15 0.04
0.20 0.04
a
b
Data are reported with standard deviations. All complexes as the nitrate salts with a concentration of 1 × 10−4 M. Reported as the 0−0 transition
c
from Gd(III) complexes. Complexes were dissolved in a 50:50 mixture of ethylene glycol and water.
due to variable hydrogen-bonding interactions between the
oligo(ethylene) ligand moiety and the two different sol-
vents.92,93 The singlet state shows a similar solvent-dependent
behavior and is located approximately 7000 cm−1 higher than
the triplet state, a gap conducive to good intersystem
crossing.25 The triplet state-lanthanide emissive states gaps
are approximately 9000 cm−1 for Eu(III) and 6000 cm−1 for
Tb(III), slightly larger than the suggested ranges for optimum
energy transfer.94,95 The intrinsic quantum yield of emission of
Eu(III) Φ ,78 can be determined to be 51.7 in acetonitrile and
Eu
Eu
43.0% in water (Table 1), with the latter value being slightly
lower most likely due to luminescence quenching through the
diffusion of outer-sphere water molecules.96 Since the overall
emission efficiency is a product of the sensitization efficiency,
Eu
Eu
Φ
sens, and of Φ , Φsens is 59.9% in acetonitrile and 70.6% in
water. These values show that exclusion of solvent molecules
from the first coordination sphere of the lanthanide ions and an
overall combination of efficient intersystem crossing process
with a good energy transfer between triplet state and excited
state for the lanthanide ion are responsible for the high
quantum yields of emission observed.
Figure 4. Emission spectra of the complexes [Ln(1)3]3+ (Ln = Dy
(yellow), Pr (green), Sm (orange)). [complex] = 1 × 10−4 M, λexc 290
nm.
Other Ln(III) ions, which emit in the visible region of the
spectrum, were also sensitized in aqueous solution. The
emission spectra of the complexes of Dy(III), Pr(III), and
Sm(III) with 1 are shown in Figure 4, while the spectrum of
Tm(III) with 1 is shown in Figure 5. Absorption and excitation
spectra and emission peak assignments are shown in Figures
S15−S18 (Supporting Information). In the case of Dy(III) and
Sm(III) no residual ligand emission is seen at the onset of the
emission spectra of the metal ions, indicating efficient energy
transfer. For Tm and Pr, with their emissive states 1G4 situated
3
at approximately 21 300 cm−1 and P0 at approximately 21 390
cm−1,97 respectively, significant residual ligand emission is seen,
as a result of either inefficient ligand to metal energy transfer or
due to back-transfer from the emissive state to the ligand.
Sensitization of the near-infrared-emitting Ln(III) ions
Yb(III), Nd(III), and Er(III) was also achieved and is shown
in Figure 6. Emission spectra with peak assignments along with
absorption and excitation spectra for these three ions are shown
in Figures S19−S21 (Supporting Information). Exciting the
Yb(III) complexes at 290 nm resulted in its characteristic
2
emission at 991 nm, corresponding to the F5/2
→
2F7/2
Figure 5. Absorption (dotted), excitation (dashed), and emission
spectra (solid) of [Tm(1)3]3+. [complex] = 1 × 10−4 M, λexc 290 nm.
transition. Similarly, the Nd(III) complex displayed emission
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Journal of the American Chemical Society
Article
from which water molecules are excluded. This is similar to
what was previously observed for other Pybox derivatives,
where it was shown that moderate amounts of water present in
organic solvents did not influence the emission behavior.35
Since previous studies on other para-derivatized Pybox-
containing systems showed the selective formation of three
species with 1:1, 2:1 and 3:1 ligand to metal ion stoichiometry
in solution and in the solid state depending on the
stoichiometry of the reagents used, we also performed
speciation studies by emission spectroscopy on this new ligand
and its complexes. Example titrations can be seen in Figures
S22 and S23 (Supporting Information).
Table 3. Speciation Data of Ln(CF3SO3)3 with
PyboxO(CH2)2OEt in Aqueous Solution with 0.1 M Tris
Buffer Obtained by Emission Titrations (I = 0.1 M Et4NCl)
Ln(III)
log β11
log β21
log β31
Eu(III)
Tb(III)
5.3 0.5
5.3 0.3
9.6 0.3
9.2 0.1
13.8 0.4
12.7 0.3
Figure 6. Absorption (dotted), excitation (dashed), and emission
spectra (solid) of [Ln(1)3]3+ (Ln = Yb (gold), Nd (purple), Er (blue))
in D2O. [complex] = 1 × 10−4 M, λexc 290 nm.
bands at 876, 909, 1060, and 1336 nm, which correspond to the
4
9
4F3/2 → FJ (J = /2, 11/2, 13/2) transitions.
The stability constants log β, summarized in Table 3, are
consistent with the stepwise formation of 1:1, 2:1, and 3:1
ligand to metal complexes and as shown in the speciation
diagrams shown in Figure 7 for the titration of 1 with Tb(III)
and Figure S24 (Supporting Information) for the titration of 1
with Eu(III). The stability constants are higher than those for
previously reported Pybox complexes with either hydrogen or a
methoxy group at the para position of the pyridine ring. For
those complexes stability constants were in the range log β31
12.0−12.8 for Eu(III) and 11.7−12.2 for Tb(III). They are,
however, not as high as for complexes with bromo- and
thiophen-3-yl-derivatized Pybox, which showed stability con-
stants in the range log β31 14.9−15.5 for Eu(III) and 12.7−15.4
for Tb(III). Nonetheless, the complexes’ stability prevents the
coordination of water molecules in the first coordination sphere
and resulting concentration quenching, leading to very efficient
luminescence in aqueous solution.
For the Er(III) complex the emission maximum is located at
4
1497 nm, corresponding to the I13/2
→
4I15/2 transition.
Emission was observed for all solvents with all near-IR-emitting
ions, except for Er(III), whose emission is not observed in
water but is in D2O. This is due to the fact that water has strong
absorption bands located in the 1500 nm region of the
spectrum, while the absorption bands of D2O are at 1900 nm.98
The quantum yields of emission for the near-IR-emitting
Ln(III) ions are summarized in Table 1 and are high in
comparison with literature values, which are in the range 0.02−
0.18% and 0.1−0.45% for Yb(III) and 0.004−0.02% and 0.02−
0.038 for Nd(III) in H2O and D2O, respectively.98−100 The
quantum yields for Er(III) are up to 10 times greater than the
literature values of 0.004−0.019% in D2O.98,99 The emission
efficiencies for these ions are also fairly independent of the
concentration chosen (Figure S2, Supporting Information).
The excited-state lifetimes for Eu(III) and Tb(III) were
determined in water and deuterium oxide, confirming the
absence of inner-sphere water molecules (q),79 as summarized
in Table 2, along with lifetimes in acetonitrile. The emission
intensity decay curves could be fit to a single exponential,
indicating that only one emissive species exists in solution.
Table 2. Excited State Lifetimes with Standard Deviations
for the 3:1 Ligand to Metal Species in MeCN, H2O, and D2O
Solutions and the Corresponding Number of Inner-Sphere
Water Molecules
Ln(III)
τMeCN (ms)
τH2O (ms)
τD2O (ms)
q
Eu(III)
Tb(III)
2.87 0.15
2.03 0.05
1.61 0.10
1.78 0.10
2.72 0.02
2.10 0.08
0.24 0.04
0.30 0.18
Both Eu(III) and Tb(III) display long emission lifetimes in
acetonitrile at 2.87 and 2.03 ms, respectively. These are longer
than lifetimes for similar complexes with thiophen-3-yl-
derivatized and methoxy-derivatized Pybox reported by our
group.34,35 The data in acetonitrile are mirrored by exception-
ally long emission lifetimes in water and deuterated water,
which are again consistent with an inner coordination sphere
Figure 7. Speciation diagram of the titration of 1 with Tb(III) at
constant 0.1 M ionic strength and pH 7.4.
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Journal of the American Chemical Society
Article
(18) Bunzli, J. C. G.; Klein, B.; Chapuis, G.; Schenk, K. J. Inorg. Chem.
̈
CONCLUSIONS
■
1982, 21, 808−812.
In summary, we have successfully synthesized a water-soluble
Pybox ligand that efficiently sensitizes emission of several
Ln(III), in both the visible and near-IR regions, in organic
solvents and also in aqueous solution. We have shown that the
ligand successfully shields the coordination environment of the
lanthanide ions from water molecules. Along with appropriately
located singlet and triplet energy levels, high emission
efficiencies could be measured in the visible and near-infrared
regions of the spectrum, which are fairly independent of the
solvent and complex concentration. These complexes are
promising for luminescence applications in aqueous environ-
ments, and the new water-soluble ligand opens up new
possibilities for aqueous catalysis.
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ASSOCIATED CONTENT
* Supporting Information
Figures, tables, and the CIF file giving ESI mass spectra, NMR
spectra, and additional emission, absorption, and excitation
spectra and crystal data for 1·H2O. This material is available
free of charge via the Internet at http://pubs.acs.org.
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B.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 9408−9414.
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■
S
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AUTHOR INFORMATION
Corresponding Author
*E-mail: abd@unr.edu.
Notes
■
(30) Song, B.; Wang, G.; Tan, M.; Yuan, J. J. Am. Chem. Soc. 2006,
128, 13442−13450.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Science Foundation
(Grant Nos. NSF-CHE0733458 and NSF-CHE1058805) and
the University of Nevada, Reno, for supporting this work
financially.
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