Synergistic complexation of Eu3+ by a polydentate ligand and a bidentate antenna to obtain ternary complexes with high luminescence quantum yields

Article abstract of DOI:10.1021/jp012617o

A series of ternary Eu³⁺ complexes are presented consisting of a polydentate m-terphenyl-based Eu³⁺ complex (Eu)1 and different antenna chromophores possessing lanthanide(III) ion coordinating properties. The series of investigated a

Full text of DOI:10.1021/jp012617o

J. Phys. Chem. A 2002, 106, 3681-3689
3681
Synergistic Complexation of Eu3+ by a Polydentate Ligand and a Bidentate Antenna to
Obtain Ternary Complexes with High Luminescence Quantum Yields
Stephen I. Klink, Gerald A. Hebbink, Lennart Grave, Patrick G. B. Oude Alink, and
Frank C. J. M. van Veggel*
Laboratory of Supramolecular Chemistry and Technology and MESA+ Research Institute,
UniVersity of Twente, 7500 AE Enschede, P.O. Box 217, The Netherlands
Martinus H. V. Werts
Laboratory of Organic Chemistry, UniVersity of Amsterdam, Nieuwe Achtergracht 129,
1018 WS Amsterdam, The Netherlands
ReceiVed: July 10, 2001; In Final Form: January 22, 2002
A series of ternary Eu3+ complexes are presented consisting of a polydentate m-terphenyl-based Eu3+ complex
(Eu)1 and different antenna chromophores possessing lanthanide(III) ion coordinating properties. The series
of investigated antenna chromophores consist of 1,10-phenanthroline, tetraazatriphenylene, and three
â-diketonates, namely dibenzoylmethane, benzoyltrifluoroacetylacetonate, and hexafluoroacetylacetonate. As
a result of the synergistic complexation of Eu3+ by the polydentate ligand and the bidentate antenna, the
distance between the antenna and lanthanide ion has been minimized and the Eu3+ ion has been shielded
completely from the solvent. These are two important requirements to obtain efficiently emitting lanthanide-
(III) complexes. The formation of the ternary complexes and their photophysical properties, in particular the
population of the Eu3+ excited states and the efficiency of the sensitization process, have been studied in
detail. Based on these measurements, it can be concluded that the aforementioned strategy of synergistic
complexation has indeed led to the construction of efficiently emitting Eu3+ complexes. The â-diketonate
ternary Eu3+ complexes combine a high stability (K ) 3.8 ( 0.2 × 107 M-1) with high overall luminescence
5
quantum yields of up to 0.29. The energy transfer from the sensitizer to the Eu3+ is exclusively to the D1
5
level, from which the D0 level is populated.
Introduction
sensitization process of other lanthanide ions, in particular the
near-infrared emitting lanthanide ions.7,8 For example, it is
The design of molecular systems that combine binding
abilities and useful photophysical properties for the construction
of efficient photoluminescent lanthanide(III) complexes con-
tinues to be an active area of research. The long lifetimes of
the excited states of lanthanide ions and their distinct narrow
emission bands ranging from the UV-visible to the near-
infrared region are ideally suited for applications as fluorescent
probes1,2 and as optical signal amplifiers.3 The long luminescent
lifetimes of lanthanide ions are due to the forbidden character
of their intra-4f transitions, which unfortunately also result in
low absorption coefficients (typically 1-10 M-1cm-1).4 For this
reason, the excited state of a luminescent lanthanide(III) ion is
generally populated by energy transfer from the triplet state of
an organic antenna chromophore (the sensitizer).5,6 The quantum
yield of the overall process, which involves the excitation of
the antenna chromophore and intersystem crossing to the triplet
state, energy transfer to the lanthanide ion and subsequent
lanthanide emission (see Figure 1), depends on the efficiency
of these individual steps: the intersystem crossing efficiency
(ηISC), the energy transfer efficiency (ηET), and the lanthanide-
(III) luminescence quantum yield (φLn). Eu3+ is one of the best
studied lanthanide ions, and the research of the sensitization
process of this ion has resulted in a better understanding of the
obvious that ideally ηISC should be (close to) unity. However,
it has been shown that by positioning the antenna chromophore
in close proximity of the lanthanide ion, ηISC is enhanced as a
result of an external heavy atom effect.9,10,11 This understanding
has led to the application of fluorescent dyes, which have very
low intrinsic intersystem crossing quantum yields, as efficient
sensitizers for near-infrared Nd3+, Er3+, and Yb3+ lumines-
cence.11,12
Based on the knowledge of antenna-functionalized Eu3+
complexes, the antenna triplet state should be higher in energy
than the receiving lanthanide 4f state by approximately 2000
cm-1 6 and the antenna should be coordinated to the lanthanide
ion in order to obtain the maximum heavy atom effect and a
fast energy transfer process, which are both strongly distance
dependent. In general, the energy transfer process is an electron-
exchange mechanism.13,14 At the same time, the lanthanide ion
should be shielded by the ligand from their environment because
high-frequency oscillators, like the O-H and C-H vibrations
of the solvent, play an important role in removing energy
nonradiatively from the lanthanide excited state.15
Previously, we have reported the synthesis and photophysical
properties of a series of neutral m-terphenyl-based polydentate
lanthanide complexes.16,17 In the well-defined and stable
complex (Ln)1, the lanthanide ion is coordinated by eight hard
oxygen donor atoms, whereas the ninth coordination site of the
* To whom correspondence should be addressed. Tel: +31-53-4892987,
Fax: +31-53-4894645,E-mail: fcjm.vanveggel@ct.utwente.nl.
10.1021/jp012617o CCC: $22.00 © 2002 American Chemical Society
Published on Web 03/20/2002

3682 J. Phys. Chem. A, Vol. 106, No. 15, 2002
Klink et al.
Figure 1. Left: Schematic representation of the photophysical pathway of the sensitization process. The solid arrows indicate the pathway of the
sensitization process, whereas the dashed arrows indicate the competing processes. Right: The relevant 4f energy levels of Eu3+ as well as the
triplet state energies of hexafluoroacetylacetonate (hfa, 22,000 cm-1), benzoyltrifluoroacetylacetonate (bfa, 21,400 cm-1), dibenzoylmethane (dbm,
20,600 cm-1), 1,10-phenanthroline (phen, 21,480 cm-1) and tetraazatriphenylene (aza, 23,800 cm-1).
CHART 1
instead the 5D0 state is populated via the 5D1 state. Furthermore,
we demonstrate a principle that has not been applied consciously
until now to obtain efficiently emitting lanthanide complexes.
The long-lived excited states of lanthanide(III) ions, make them
very sensitive to quenching processes. By increasing the
radiatiVe rate of Eu3+ as a result of the composition of its
coordination sphere, the radiative lifetime of the excited state
is shortened making it less sensitive to nonradiative deactivation
processes and the overall effect is an increased quantum yield.
â-Diketonates are a proper choice in this respect, because the
emission of complexes with Eu3+ are characterized by a very
strong 5D0-7F2 transition (see “Results and Discussion” for more
details).
Experimental Section
lanthanide ion is occupied by a solvent molecule (depicted in
Chart 1).17,18
Mass spectra were recorded with a Finnigan MAT 90
spectrometer using a mixture of dithiothreitol and dithioerythritol
(5:1 (v/v)), known as Magic Bullet, as the matrix. IR spectra
were obtained using a Perkin-Elmer Spectrum BX FT-IR
System. CH2Cl2 was of spectroscopic grade and used without
further purification. The â-diketones, 1,10-phenanthroline and
NBu4OH (1 M solution in methanol) were purchased from
Aldrich and used without further purification. Tetraazatri-
phenylene and the (Eu)1 complex were synthesized according
to literature procedures.17,22
General Procedure for the Preparation of the m-terphe-
nyl-based Complexes. The â-diketonate complexes were
prepared by mixing a dichloromethane solution (10 mL) of (Eu)-
1 (30 mg, 0.026 mmol), 0.026 mmol of the appropriate
â-diketone, and 27 µL of a 1 M aqueous NBu4OH solution.
After stirring for 0.5 h, the solvent was removed in vacuo. The
neutral tetraazatriphenylene and phenanthroline complexes were
prepared in a similar way. The complexes were characterized
by FAB-MS spectrometry (see Table 1) and IR spectroscopy.
The IR spectra of the â-diketonate complexes show a peak at
1650-1640 cm-1 (νNCdO and νCdO,â-diketone), with a shoulder
around 1610-1600 cm-1 (νCOO), whereas the IR spectra of
[(Ln)1]aza and [(Ln)1]phen show a peak at 1635-1630 cm-1
(νNCdO) with a shoulder around 1590-1600 cm-1 (νCOO).
Photophysical Studies. Steady-state luminescence measure-
ments were performed with a Spex Fluorolog 3-22 instrument
or an Edinburgh FS900 spectrofluorimeter. In the Spex Fluo-
rolog 3-22 instrument the samples were excited with a 450 W
Xe lamp via a double-grating monochromator. The emitted light
In this article, ternary complexes are reported in which the
ninth coordination site of (Eu)1 is occupied by an antenna
chromophore. In this way the antenna-Eu3+ distance has been
minimized and at the same time the lanthanide ion is completely
shielded from the solvent. The coordinating antenna chro-
mophores are a series of three â-diketonates (dibenzoylmethane
(dbm), benzoyltrifluoroacetylacetonate (bfa), and hexafluoro-
acetylacetonate (hfa)), 1,10-phenanthroline (phen), and tetra-
azatriphenylene (aza). The triplet-state energies of these sensi-
tizers and the relevant 4f energy levels of Eu3+ are depicted in
Figure 1.
The 1:3 Eu3+/â-diketonate,5,6,19,20 the 1:2 Eu3+/1,10-phenan-
throline,21 and the 1:2 Eu3+/tetra-azatriphenylene22 complexes
are well-known and have been studied extensively. The
sensitization process in these complexes is very efficient.
However, a drawback of these complexes is their limited
stability and the incomplete shielding of the Eu3+ ion from the
solvent. In the present ternary complexes the excellent sensitiz-
ing capabilities of these bidentate antennas have been combined
with the stability and shielding properties of polydentate
complexes. Besides the preparation and characterization of the
ternary complexes, we have determined the luminescence
quantum yields and studied in detail the time-evolution of the
luminescence of the two emissive states of Eu3+, namely the
5
5D1 (at ∼19,000 cm-1) and D0 (at ∼17,500 cm-1) state. We
have found that in these complexes the energy transfer from
5
the antenna triplet to Eu3+ exclusively takes place to the D1
5
state. There is no direct energy transfer to the D0 state, but

Synergistic Complexation of Eu3+
J. Phys. Chem. A, Vol. 106, No. 15, 2002 3683
CHART 2
TABLE 1: Identification of the Ternary Complexes by Mass
Spectrometry (FAB)
the quantum yield is unknown. nref, Iref, and Aref are the same
observables for the reference sample (quinine sulfate in 1 M
H2SO4).
complex
measured m/z
calcd m/z
The ternary complexes were dissolved to a concentration of
10-5 M in dichloromethane, to keep the absorption of samples
below 0.2 at the excitation wavelength. At this concentration,
the three ternary â-diketonate complexes are not dissociated.
Since the formation constant of [(Eu)1]aza is 1.4 ( 0.1 × 105
M-1 (vide infra), the fraction of this ternary complex at 10-5
M is only 44%. Based on the structural similarity of phen and
aza, it is assumed that [(Eu)1]phen has a similar formation
constant. The emission, excitation spectra, and lifetimes repre-
sent the photophysical properties of the ternary complex, because
only the ternary complex exhibits sensitized luminescence upon
excitation above 300 nm. The measured luminescence quantum
yields of [(Eu)1]aza and [(Eu)1]phen were corrected for the
amount of ternary complex in solution. This was done by
multiplying the measured absorbance at the excitation wave-
length of the [(Eu)1]aza and [(Eu)1]phen samples with 0.44
(since only this fraction of antennas is coordinated to the Eu3+
ion), and to use this value as Au in eq 1.
N(Bu)4[(Eu)1]hfa
N(Bu)4[(Eu)1]bfa
N(Bu)4[(Eu)1]dbm
[(Eu)1]phen
[(Eu)1]aza
[(Tb)1]aza
1343.0 (M-N(Bu)4)-
1351.3 (M-N(Bu)4)-
1359.4 (M-N(Bu)4)-
1261.9 (M-CH2COO)+a)
1371.0 (M+H)+
1343.4
1351.4
1359.4
1261.4
1371.4
1377.5
1377.4 (M+H)+
a This type of fragmentation is commonly observed in FAB MS
experiments, see for instance ref 16a.
was detected at an angle of 90° by a Peltier cooled Hamamatsu
R636-10 photomultiplier operating in the photon counting
mode. In the Edinburgh FS900 spectrofluorimeter, the samples
were excited by a 450 W xenon lamp via a single grating
monochromator. The emitted light was detected at an angle of
90° by a Peltier cooled Hamamatsu R928 photomultiplier, and
subsequently fed to a photon-counting interface. The spectra
were corrected for the wavelength dependence of the detection
sensitivity.
Titration Experiments. To 2.5 mL of a 1.05 × 10-5
M
The time-resolved measurements were performed using an
LTB MSG 400 nitrogen laser (λexc. 337 nm) as the pulsed laser
excitation source and a streak camera system (Hamamatsu) as
the detector to simultaneously probe wavelength and time-
dependence of the luminescence signals in the millisecond and
microsecond domain. The streak images were analyzed with a
mathematical procedure involving singular value decomposi-
tion.32,33
solution of tetraazatriphenylene in CH2Cl2, 25-µL aliquots of a
(Eu)1 solution (5.2 × 10-4 M) were added. The antenna was
excited at 330 nm and the intensity of Eu3+ 5D0f7F2 emission
around 615 nm was monitored as a function of added amounts
of (Eu)1. In a similar way the formation of [(Eu)1]bfa was
monitored. In this experiment the antenna was excited at 340
nm. To 2.6 mL of a 3.05 × 10-5 M solution of (deprotonated)
bfa in CH2Cl2, 25-µL aliquots of a (Eu)1 solution (5.08 × 10-4
M) were added. The bfa stock solution was prepared by mixing
in CH2Cl2 (10 mL), bfa (13.18 mg, 6.10 × 10-2 mmol) with
65 µL (6.5 × 10-2 mmol) of a 1 M solution of NBu4OH in
methanol. The solvents were removed in vacuo, subsequently
dichloromethane was added (10 mL) and the mixture was
concentrated to dryness. This was repeated twice. Finally, the
deprotonated bfa antenna was redissolved in CH2Cl2 to a
concentration of 1.85 × 10-6 M.
All photophysical measurements were performed on aerated
samples at room temperature, unless otherwise stated. The
overall quantum yields of the ternary complexes were deter-
mined relative to a reference solution of quinine sulfate in 1 M
H2SO4 (φ ) 0.546),23 and corrected for the refractive index of
the solvent according to Equation 1.24
2
nu Aref Iu
nref2 Au Iref
Φu )
Φref
(1)
Results and Discussion
In Equation 1, nu, Iu, and Au are the refractive index, the area
of the corrected emission spectrum, and the absorbance at the
excitation wavelength, respectively, for the sample of which
Nature of the Complex. The formation of the ternary
complexes was studied by monitoring the sensitized lumines-
cence intensity as a function of the concentration of the Eu3+

3684 J. Phys. Chem. A, Vol. 106, No. 15, 2002
Klink et al.
Figure 2. The increase in the sensitized 5D0f7F2 emission band around 615 nm upon the addition of increasing concentrations of (Eu)1 to the aza
(a) and bfa (b) antenna chromophores in dichloromethane. The insets show the formation of the ternary complexes as a function of the sensitized
luminescence intensity. The solid lines are the theoretical curves for a 1:1 complex with K ) 1.4 ( 0.1 × 105 M-1 for [(Eu)1]aza and K ) 3.8 (
0.2 × 107 M-1 for [(Eu)1]bfa, respectively.
Figure 3. The corrected (and normalized) excitation (λem 615 nm) and emission spectra (λexc 330 nm) of [(Eu)1]bfa, [(Eu)1]hfa, [(Eu)1]dbm, and
5
[(Eu)1]aza in dichloromethane (10-5 M). The inset shows a magnification of the D0f7F0 emission at 579 nm. The spectra were taken with a
spectral bandwidth of 0.2 nm.
complex in solution during a titration experiment. In this way,
the formation of the two ternary complexes [(Eu)1]aza and
[(Eu)1]bfa in dichloromethane was examined in more detail.
Figure 2 shows the titration experiments, in which small amounts
of a (Eu)1 stock solution were added to a solution of tetraaza-
triphenylene or (deprotonated) bfa. The complexes were excited
in the absorption bands of the antenna chromophores, at a
wavelength (i.e. λexc. 330 nm) where the (Eu)1 complex itself
has no absorption bands.25,26 Upon addition of increasing
concentrations of (Eu)1 to the antenna chromophores, the
sensitized Eu3+ emission increased (see Figure 2).
During the titration experiment, the absorption bands of the
antenna chromophores above 300 nm did not change, indicating
that upon coordination to (Eu)1 the singlet excited states of the
antenna chromophores are not affected significantly by the Eu3+
ion. Analysis of the titration data yields formation constants of
the 1:1 ((Eu)1:antenna) complexes, K ) 1.4 ( 0.1 × 105 M-1
for [(Eu)1]aza and K ) 3.8 ( 0.2 × 107 M-1 for [(Eu)1]bfa,
respectively.
Preparation of the Complexes. The [(Eu)1]â-diketonate
complexes were prepared by mixing one equivalent of the
tetrabutylammonium salt of the appropriate â-diketonate and
one equivalent of (Eu)1 in dichloromethane, and subsequent
removal of the solvent. The neutral tetraazatriphenylene and
phenanthroline complexes were prepared in a similar way. The
ternary complexes have been characterized by mass spectrom-
etry (FAB) and IR spectroscopy (see the Experimental Section).
The mass spectra show the expected 1:1:1 stoichiometry (ligand:
Eu3+ : sensitizer) of the ternary complexes.
The Steady-State Luminescence Spectra of the Ternary
Complexes. Upon excitation of the antennas (λexc. 330 nm) of
the corresponding ternary complexes in dichloromethane (10-5
M), the steady-state luminescence spectra show the narrow Eu3+
emission bands that correspond to the D0f7FJ transitions (see
5
Figure 3 ). The insets of Figure 3 show a magnification of the
580 nm emission band corresponding to the 5D0f7F0 transition.
The excitation spectra of the complexes closely follow the
absorption spectra of the coordinated sensitizers (not depicted),

Synergistic Complexation of Eu3+
J. Phys. Chem. A, Vol. 106, No. 15, 2002 3685
TABLE 2: The Photophysical Data of the Ternary Eu3+
Complexes in Dichloromethane (10-5 M)
5D0 τ 5D1 τ 5D0 τ0
a
b
f
complex
I7F2/I7F1
φSE
(ms)c (µs)d (ms)e 5D0 τ/τ0
[(Eu)1]hfa
[(Eu)1]bfa
[(Eu)1]dbm
[(Eu)1]phen
[(Eu)1]aza
9.0
11.4
10.1
4.9
0.26
0.29
0.034 0.38
0.10 0.93
0.062 0.71
1.27
1.08
1.51
1.53
0.36
n.d.
2.0
1.6
1.7
2.9
2.7
0.6
0.7
0.2
0.3
0.3
5.4
2.97
5
5
a Intensity ratio of the D0f7F2 emission and the D0f7F1 emission
(error ( 0.5, from triplo measurements). b The overall quantum yield
of the sensitized Eu3+ emission (error ( 5%). c Luminescence lifetime
of the D0 emission (error ( 5%). d Luminescence lifetime of the D1
emission (error ( 5%); b-dErrors were determined by duplo measure-
ments (see also ref 23). e Calculated radiative lifetimes of the 5D0
emission using eq 3. f Calculated luminescence quantum yield of the
Eu3+ ion.
5
5
intensity of the MD D0f7F1 transition is independent of the
5
coordination sphere and the 5D0f7F2 transition is a hypersensi-
tive transition, the intensity ratio of the 5D0f7F2 transition and
the 5D0f7F1 transition (I7F2/I7F1) is a good measure of the nature
and symmetry of the first coordination sphere.28,27 In a cen-
Figure 4. Magnification of the normalized emission spectrum (λexc
330 nm) of [(Eu)1]bfa in dichloromethane (10-5 M). The emission
bands have been assigned to the appropriate D0f7Fn transitions. The
5
spectrum was recorded with a spectral bandwidth of 0.2 nm.
5
trosymmetric environment the MD D0f7F1 transition of Eu3+
is dominating, whereas distortion of the symmetry around the
ion causes an intensity enhancement of the hypersensitive
5D0f7F2 transition. For the ‘bare’ (Eu)1 complex in dichlo-
romethane, I7F2/I7F1 is 3.5, which is comparable to the ratio of
4 that was found for this complex in methanol. For the
â-diketonate ternary complexes [(Eu)1]bfa, [(Eu)1]hfa, and
[(Eu)1]dbm, these ratios are significantly higher with values
around 10 (see Table 2 ). This is the photophysical confirmation
of the difference in the structure of the first coordination sphere
of (Eu)1 and the ternary complexes. The domination of the
5D0f7F2 transition in the emission spectrum is a typical feature
of Eu-tris(â-diketonate) complexes and the high I7F2/I7F1 ratios
of the ternary complexes are in line with reported literature
values for Eu-tris(â-diketonate) complexes.28 As we will discuss
later, by introducing a â-diketonate in the first coordination
sphere of (Eu)1 the probability of the Eu3+ the 5D0-7F2 transition
has been increased significantly and consequently the radiative
rate of the complexed Eu3+ ion. By increasing the radiative rate,
the Eu excited state will become less sensitive to deativation
processes, ultimately resulting in a more efficiently emitting
Eu3+ complex.
and thus prove that the Eu3+ luminescent state is populated via
the antenna. From the excitation spectra it can be seen that the
dbm antenna allows the longest wavelength excitation of up to
425 nm.
The emission spectra of the ternary complexes will be
discussed in more detail using the spectrum of [(Eu)1]bfa as an
example. Figure 4 shows an enlarged spectrum of the [(Eu)1]-
bfa complex in which the emission bands have been assigned.
The spectrum has been recorded using a bandwidth of 0.2 nm.
The relatively high resolution was used to make sure that the
bands of the individual D0f7FJ transitions are well separated.
5
The radiative transitions within the [Xe]4f6 configuration of
Eu3+ are parity forbidden and consist mainly of weak magnetic
dipole (MD) and induced electric dipole (ED) transitions. The
emission bands of Eu3+ remain narrow even in an organic matrix
and in solution due to the fact that the partially filled 4f orbitals
are shielded from the environment by the filled 5s and 5p
orbitals. The probabilities of MD transitions are independent
of the chemical environment of the ion, in contrast to those of
the ED transitions. The Judd-Ofelt theory has been very
successful in understanding and predicting the spectral intensities
of the induced ED transitions, especially for ions in glasses and
crystals.27
The coordination of aza and phen causes a much smaller
increase of the hypersensitive transition, with I7F2/I7F1 ratios of
approximately 5. Judd already reported that the symmetry of
the coordination sphere and the polarizability of the coordinating
groups play a role in the hypersensitivity of certain lanthanide-
(III) transitions.29 The coordinating carbonyl groups of â-dike-
tonates are more polarizable30 than the nitrogen donor atoms
of phen and aza resulting in a larger increase in the intensity of
5
7
The D0f F0,3 transitions at 579 and 650 nm, respectively,
are very weak (see Figure 4). In fact these transitions cannot
be accounted for by either the MD mechanism or the Judd-
Ofelt theory. A more detailed analysis has indicated that these
transitions ‘borrow’ intensity from the 5D0f7F2 transition through
higher order perturbations by the ligand field.28 The D0f7F1
the hypersensitive D0f7F2 transition.
5
5
emission around 593 nm is a pure MD transition. The strongest
emission is observed around 615 nm corresponding to the
5D0f7F2 transition. The intensities of some ED transitions are
extremely sensitive to the nature and symmetry of the coordinat-
Time-Evolution of the Sensitized Eu3+ Luminescence. By
monitoring the early stages of the lanthanide luminescence using
pulsed laser excitation (λexc. ) 337 nm) and a streak camera,31
5
5
the population of the D1 and D0 states of Eu3+ was studied.
Upon pulsed excitation of the antenna, the lanthanide lumines-
cence intensity is expected to rise with a rate equal to the decay
rate of the donating state, assuming an immediate population
of the donating state (for example the triplet state) and an energy
transfer rate that exceeds the decay rate of the lanthanide
luminescent state (kLn<kET). This latter assumption is easily valid
for Eu3+, because of its long-lived excited state, which is in
the order of milliseconds.
5
ing environment, and the D0f7F2 transition is an example of
such a hypersensitiVe transition. The spectrum shows splitting
of the 5D0f7F1 and the 5D0f7F2 emission bands in the order of
7
100-200 cm-1 caused by the ligand field. The F0 state is
5
nondegenerate, and therefore the D0f7F0 emission band does
not exhibit ligand field splitting. The single peak at 579 nm in
the emission spectrum therefore indicates that there is only one
(time-averaged) luminescent Eu3+ species in solution. Since the

3686 J. Phys. Chem. A, Vol. 106, No. 15, 2002
Klink et al.
Figure 5. The temporal and spectral curves belonging to the two emissive states of Eu3+ in [(Eu)1]bfa, reconstructed from the streak image
recorded upon pulsed laser excitation. Left: Time-evolution of the 5D1 and 5D0 states. The solid lines are the reconstructed profiles, the dashed lines
are the fitted curves. Right: The emission spectra associated with the two states, the solid line corresponds to the 5D1 emission and the dashed line
5
corresponds to the D0 emission.
Since the streak camera enables the simultaneous detection
of the wavelength and time-dependence of the luminescence
signals, the two time-resolved spectra corresponding to the
5D1f7FJ and5D0f7FJ emissions could be deconvoluted by using
a mathematical procedure involving singular value decomposi-
tion.32 For example, Figure 5 shows the reconstructed temporal
(left) and spectral (right) curves33 belonging to the two emissive
states of Eu3+ in [(Eu)1]bfa. It should be noted that the temporal
lifetime of 0.38 ms (vide infra). These lifetimes are comparable
to the lifetime of (Eu)1 in methanol (0.80 ms).
A striking observation of the time-resolved spectra is that in
all of the investigated ternary complexes there is no direct
5
energy transfer from the antenna triplet to the D0 level: the
5
Eu3+ 5D0 state is only populated via the D1 state. This does
not seem to be in agreement with what has previously been
reported in the literature concerning the sensitization of Eu3+
by energy transfer from â-diketonates. For example, Watson et
al.35 reported that in Eu(dbm)3 the energy transfer takes place
to both the 5D1 and 5D0 states. They have based the conclusion
5
curve of the D0 emission does not represent the behavior of
5
the D0 emission at a particular wavelength, but in fact that it
represents the relative concentration of all Eu3+ species emitting
5
5
from its D0 state. In other words, all the observed D0f7FJ
5
that the D0 state is populated by a fast direct and a slower
indirect energy transfer (via the 5D1 state), on their observation
emissions of [(Eu)1]bfa exhibit the same rise and decay kinetics.
Although the emission spectra corresponding to the D0f7FJ
5
5
that the D0f7F2 emission exhibited a rapid initial rise (faster
transitions are well documented, hardly any spectra correspond-
ing to the 5D1f7FJ transitions of organic Eu3+ complexes have
been reported to date. Comparison of the spectral curves with
than the instrument response time) followed by a slow rise with
rise time in the microsecond region. An explanation for the
discrepancy with our findings may be that Watson et al. have
5
the available D1 emission spectra of Eu(bfa)4 in acetonitrile,5
5
5
overlooked the overlap of the D0f7F2 and D1f7F4 emission
5
bands around 615 nm, and that, in fact, this ‘hidden’ D1f7F4
shows that the assignment of the bands are in agreement. It is
apparent from Figure 5 that the 5D0f7FJ and 5D1f7FJ emissions
have significant overlap, which is important to realize when
emission is responsible for the observed rapid initial rise.
Recently, de Sa et al.30 derived some selection rules for the
energy transfer from an antenna triplet to the excited states of
Eu3+ via electron-exchange and multipolar interactions. Ac-
cording to these rules, direct energy transfer to the 5D0 level is
not allowed, while the 5D1 level is an excellent receiving energy
level for energy transfer via an electron-exchange mechanism.
Our experimental observations are in agreement with these rules.
However, De Sa et al. further reported that the rule that forbids
a direct energy transfer to the 5D0 state is relaxed due to J-mixing
effects (which obscures the assignment of the J-states) and
5
one measures the rise-time of the D0 emission (vide infra).
In the measurements it was found that immediately after the
laser pulse, the Eu3+ complexes exhibited emission from the
5D1 state as is illustrated by the temporal spectrum of [(Eu)1]-
bfa in Figure 5. The rise-time of the 5D1 luminescence is shorter
than the instrument response time, i.e., τrise < 50 ns, indicative
of a fast energy transfer (kET > 108 s-1). The 5D1 luminescence
is expected to rise with a rate equal to the decay rate of the
triplet state, because the triplet state is immediately populated
(kISC ) ∼1011 s-1).34 Furthermore, since it is likely that the
energy transfer process will be dominating the triplet state decay,
the rise-time represents the energy transfer rate.10
7
thermal population of the F1 level (an energy transfer to the
5D0 state is allowed when it involves a 5D0r7F1 transition instead
5
of the ‘forbidden’ D0r7F0 transition). This is in line with the
5
observation that in systems where the antenna triplet energy
Subsequently, the D1 state decayed with a time-constant in
lies between the 5D0 and 5D1 state, a direct energy transfer takes
the order of microseconds (see Table 2), being converted
5
place to the D0 state.36
radiatively to the 7FJ manifold, e.g. the 530 nm emission
5
(5D1f7F1 transition), and nonradiatively to the D0 state. The
The Quantum Yield of the Overall Process. The overall
quantum yields of the ternary complexes were determined
relative to a reference solution of quinine sulfate in 1 M H2SO4
(φ ) 0.546) and corrected for the refractive index of the
solvent.37 All quantum yields were measured with aerated
solutions. The results have been summarized in Table 2 and
show that the bfa antenna is clearly the most efficient sensitizer
for Eu3+ with a quantum yield of 0.29, followed by the hfa
antenna with a quantum yield of 0.26. These values compare
favorably with quantum yields of Eu3+ complexes reported in
the literature.5,22,36 The process is less efficient in [(Eu)1]dbm
5D0 luminescence exhibited a rise-time in the order of micro-
seconds, after which it decayed with a much slower time-
5
constant of ms. The rise-time of the D0 emission is monoex-
5
ponential and corresponds to the decay time of the D1 state,
indicating that the 5D0 state is populated via the 5D1 state. The
5
subsequent decay times of the D0 luminescence were also
monoexponential with lifetimes of 1.27 ms for [(Eu)1]hfa and
1.08 ms for [(Eu)1]bfa, and slightly lower lifetimes of 0.93 ms
for [(Eu)1]phen and 0.71 ms for [(Eu)1]aza (see Table 2).
Compared to these complexes, [(Eu)1]dbm has a relatively short

Synergistic Complexation of Eu3+
J. Phys. Chem. A, Vol. 106, No. 15, 2002 3687
of [(Tb)1]aza is moderate (φSE ) 0.15). The luminescence
lifetimes of [(Eu)1]aza (0.73 ms), [(Tb)1]aza (2.08 ms), and
[(Eu)1]phen (0.93 ms) are similar to the lifetimes of the ‘bare’
complexes in methanol (∼0.80 ms), and therefore a deactivation
process of the lanthanide luminescent state is apparently not
the problem. Dissolved oxygen may be competing with the
lanthanide ion as an alternative acceptor of the excitation energy
from the antenna triplet, if the energy transfer rate is slower or
in the same order of magnitude as the oxygen quenching rate.38
Deoxygenation of the samples did not increase the sensitized
luminescence intensity of both [(Eu)1]aza and [(Tb)1]aza.39 This
is in agreement with the time-resolved measurements which
show that the energy transfer rate exceeds 108 s-1. A competing
process that has frequently been encountered in the search of
efficient sensitizers for Eu3+ is deactivation via a low-lying Eu3+
to antenna charge-transfer state.2,40 However, such a process
does not occur in the case of Tb3+, and this mechanism does
not explain the relatively low luminescence quantum yield of
[(Tb)1]aza.
Figure 6. Increase of the luminescence intensity of [(Eu)1]dbm upon
lowering the temperature from 25 °C to 5 °C.
and rather unexpectedly, also in [(Eu)1]aza with luminescence
quantum yields of only 0.034 and 0.062, respectively.
Influencing the Radiative Rate of Eu3+. Besides the good
sensitizing capabilities of the â-diketonates (in this case bfa and
hfa), these antennas also contribute positively to the overall
luminescence quantum yield (φSE) by increasing the intrinsic
luminescence quantum yield of the Eu3+ ion (φLn). The Eu3+
luminescence quantum yield (φLn) is the ratio of the observed
lifetime (τ) and the radiative lifetime (τ0) of the Eu3+ lumines-
cence (eq 2).
The differences in the luminescence quantum yields for the
â-diketonate complexes can be explained qualitatively in terms
of the antenna triplet state energies, since the intersystem
crossing yields as well as the antenna-lanthanide distances in
5
these three complexes are comparable. Since the D1 level is
the receiving energy level, we will focus on the difference in
energy between the antenna triplet and this level. Sato and Wada
found that in Eu-tris(â-diketonate) complexes, the triplet energy
level of the donating â-diketonate ligand should be ap-
5
proximately 2000 cm-1 above the D1 level of Eu3+ (at ∼19,-
φLn ) τ/τ0
(2)
000 cm-1) in order to obtain the highest luminescence quantum
yields.5 A larger energy difference decreases the luminescence
quantum yield, because the overlap between the energy levels
of the donor and acceptor becomes smaller. On the other hand,
a smaller energy difference reduces the quantum yield because
of a thermally activated energy back-transfer process. For the
hfa and bfa antenna chromophores the energy difference with
the 5D1 state is approximately 3000 and 2400 cm-1, respectively,
resulting in a slightly higher quantum yield for [(Eu)1]bfa than
for [(Eu)1]hfa. The much lower luminescence quantum yield
of [(Eu)1]dbm is explained by a thermally activated energy
back-transfer process. The energy gap is only 1600 cm-1, which
allows for a significant energy back-transfer within the mil-
lisecond lifetime of the lanthanide luminescence. This is
corroborated by the observation that the luminescence quantum
yield of [(Eu)1]dbm increases from 0.03 to 0.06 by decreasing
the temperature from 25 °C to 5 °C (see Figure 6), whereas the
luminescence quantum yield of [(Eu)1]bfa is constant in this
temperature region. Such behavior has also been reported for
the corresponding Eu(dbm)4 and Eu(bfa)4 complexes.5,35 The
In the previous section it was shown that upon coordination to
the Eu3+ ion, the probability (oscillator strength) of the
5
hypersensitive D0f7F2 transition increases. Since the pure
radiative lifetime of the Eu3+ luminescence is related to the
weighted sum of the oscillator strengths of the individual
5D0f7FJ transitions, this increase will influence the radiative
lifetime. Assuming that both the energy of the MD D0f7F1
5
transition and its dipole strength are constant, Equation 3
provides a means to calculate the radiative lifetime directly from
the corrected emission spectrum without the intervention of the
Judd-Ofelt theory.41
1/τ0 ) AMD,0 n3(Itot/IMD
)
(3)
In this equation, n is the refractive index of the solvent, AMD,0
is the spontaneous emission probability for the 5D0f7F1 transition
in vacuo, and Itot/IMD the total area of the (corrected) Eu3+
5
emission spectrum to the area of the D0f7F1 band. It was
theoretically calculated and experimentally verified that AMD,0
has a value of 14.65 s-1 41
. Using Equation 3, the radiative
5
5
relatively short lifetimes of the D0 and D1 states of [(Eu)1]-
dbm further indicate the occurrence of an energy back-transfer
(see Table 2). Since the 5D0 state does not have a strong
lifetimes of the Eu3+ complexes in dichloromethane (n )
1.4242) have been calculated from the corresponding emission
spectra (see Figure 4) and they are tabulated in Table 2. The
ternary â-diketonate complexes have significantly shorter radia-
tive lifetimes than the ternary aza and phen complexes. As a
result, the balance between the radiative and nonradiative decay
of the ternary â-diketonate complexes is shifted in favor of the
radiative decay, resulting in a value of φLn that is twice as high
compared to the ternary aza and phen complexes.
5
interaction with the dbm triplet state, the D0 is depopulated
5
via the D1 state and subsequent energy back-transfer to the
dbm antenna (see Figure 6).
The efficiency of the sensitization process is quite moderate
for the [(Eu)1]aza (φSE ) 0.07) and [(Eu)1]phen complexes (φSE
) 0.09), especially when these results are compared to other
systems involving the same tetraazatriphenylene antenna with
quantum yields of 0.41 and 0.67 for the Eu3+ and Tb3+ complex,
respectively.22 To obtain more insight into the factors governing
the efficiency of tetraazatriphenylene as an antenna chromophore
in our complexes, we have also synthesized the [(Tb)1]aza
complex and have determined its luminescence lifetime and
quantum yield. Also, the overall luminescence quantum yield
The calculated values of τ0 and φLn of Eu3+ in the ternary
â-diketonate complexes deserve two additional comments. First,
although in aqueous solution τ0 has been reported as 5.3 ms
for Eu3+ ions,42 τ0 values for Eu3+ ions in organic ligands as
low as 2 ms are not unprecedented. Some upper limits on τ0,
implied by quantum yields and lifetimes reported in recent

3688 J. Phys. Chem. A, Vol. 106, No. 15, 2002
Klink et al.
literature for organic complexes of Eu3+, include 2.3 ms with a
4-(phenylethynyl)pyridine-2,6-dicarboxylic acid as a ligand43
and 2.0 ms with a bisisoquinoline-N-oxide based cryptate.44
Second, if we consider the overall quantum yield of [(Eu)1]bfa
and [(Eu)1]hfa of approximately 0.3, then the calculated φLn
value of these complexes of approximately 0.7 implies that the
product of the efficiencies of the two preceding steps, i.e.
intersystem crossing from the antenna singlet to the triplet state
and energy transfer from the triplet state to the Eu3+ ion, is
only approximately 0.4. At first sight this is rather unexpected:
a reported value for the intersystem crossing quantum yield of
bfa in Gd(bfa)3 is 0.84,9 whereas the short antenna-Eu3+ distance
would ensure a fast and complete energy transfer. As was
mentioned earlier, a problem that has often been encountered
in the sensitization of Eu3+ is a competing photon-induced
efficiency of the sensitized emission is not only determined by
the distance from the antenna to the lanthanide ion, and by the
requirements of the antenna triplet energy level, but also by
the influence of the coordinated antenna on the radiative lifetime
of the complexed Eu3+ ion.
Acknowledgment. The authors express their gratitude to
Professor Jan Verhoeven (University of Amsterdam, The
Netherlands) for his valuable comments. This research has been
supported financially by the Council for Chemical Sciences of
The Netherlands Organization for Scientific Research (CW-
NWO).
References and Notes
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H. HelV. Chim. Acta 1993, 76, 1361.
(2) (a) Steemers, F. J.; Verboom, W.; Reinhoudt, D. N.; van der Tol,
E. B.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 9408. (b) Steemers,
F. J.; Meuris, H. G.; Verboom, W.; Reinhoudt, D. N. J. Org. Chem. 1997,
62, 4229.
(3) (a) Slooff, L. H.; Polman, A.; Oude Wolbers, M. P.; van Veggel,
F. C. J. M.; Reinhoudt, D. N.; Hofstraat, J. W. J. Appl. Phys. 1997, 83,
497. (b) Slooff, L. H.; Polman, A.; Klink, S. I.; Hebbink, G. A.; Grave, L.;
van Veggel, F. C. J. M.; Reinhoudt, D. N.; Hofstraat, J. W. Optical Mater.
2000, 14, 101.
(4) (a) Gschneidner, K. A.; Eyring, L. R. Handbook on the Physics
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Chem. ReV. 1993, 123, 201, and references therein.
electron-transfer process from the antenna to Eu3+ 45
, due to the
low reduction potential of Eu3+ in comparison with other
trivalent lanthanide ions. In the case of [(Eu)1]bfa and [(Eu)-
1]hfa the singlet and the triplet excited states of the antenna
may be partially deactivated by an electron transfer to Eu3+ 46
,
instead of populating the triplet excited state and the Eu3+ ion
excited states, respectively.47
Based on Equation 448 the driving force can be estimated for
the charge transfer of the excited sensitizer to Eu3+
.
∆GCT ) E(sens./sens-1) - E(sens-1*) - E(Eu3+/Eu2+) (4)
(5) (a) Sato, S.; Wada, M. Bull. Chem. Soc. Jpn. 1970, 43, 1955. (b)
Crosby, G. A.; Whan, R. E.; Alire, R. M. J. Chem. Phys. 1961, 34, 743.
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Chem. Phys. 1982, 76, 114.
With ∆GCT the change in free energy of the electron transfer,
E(sens./sens-1) the oxidation potential of the sensitizer, E(sens-1*)
the singlet or triplet energy of the sensitizer, and E(Eu3+/Eu2+
)
(7) Klink, S. I. Keizer, H.; van Veggel, F. C. J. M. Angew. Chem., Int.
the reduction potential. Please note that in this paragraph we
use sens-1 for the â-diketonate and sens. for the oxidized
sensitizer, whereas in the rest of the article we follow the
conventions in this field. With E(sens./sens-1) of 1.60 and 1.40
V (vs NHE) for bfa/bfa-1 and hfa/hfa-1, respectively,49 and the
1-electron reduction potential of Eu3+ equal to -0.36 V50 this
gives a driving force for bfa of -1.51 eV and -0.69 eV for
energy transfer from the singlet and triplet excited state,
respectively, and -1.83 eV and -0.97 eV for hfa. These
estimates show that deactivation of the antenna seems thermo-
dynamically feasible.51
Although the strategy discussed in this paragraph is not
limited to Eu3+, the radiative rate of Eu3+ is relatively easily
influenced, because its emission spectrum contains a hypersensi-
tive transition. Whereas the luminescence intensities of ‘ordi-
nary’ ED transitions can vary by a factor of 2-3 depending on
the coordination sphere, a hypersensitive ED transition can
increase by a factor of up to 200.27 It is through this transition,
that we have influenced the radiative rate of the complexed Eu3+
ion and thus φLn.
Ed. 2000, 39, 4319.
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Conclusion
(17) Klink, S. I.; Hebbink, G. A.; Grave, L.; Peters, F. G. A.; Van
Veggel, F. C. J. M.; Reinhoudt, D. N.; Hofstraat, J. W. Eur. J. Org. Chem.
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The synergistic complexation of Eu3+ by the bidentate
antenna and the polydentate ligand has led to the construction
of efficiently emitting Eu3+ complexes. In these complexes the
antenna-lanthanide ion distance has been minimized and the
Eu3+ ion is completely shielded from the solvent. The ternary
â-diketonate complexes combine high association constants (K)
and high overall luminescence quantum yields (φSE), for
example K ) 3.8 ( 0.2 × 107 M-1 and φSE ) 0.29 for [(Eu)-
1]bfa. The photophysical studies of the ternary complexes have
shown that there is no direct energy transfer from the antenna
triplet to the 5D0 state, but that instead the 5D0 state is populated
(18) Typical structure of (Eu)1 obtained from a molecular modeling
simulation in a box of OPLS methanol using the CHARMM force field. In
the simulation, the n-butoxypropyl moieties have been replaced by methyl
groups. The hydrogen atoms have been removed for clarity. For the
simulation the same procedure was followed as reported in: van Veggel,
F. J. C. M.; Reinhoudt, D. N. Recl. TraV. Chim. Pays-Bas 1995, 114, 387.
The Lennard-Jones parameter for Eu3+ was taken from: van Veggel, F. C.
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5
via the D1 state. It has been demonstrated that the overall

Synergistic Complexation of Eu3+
J. Phys. Chem. A, Vol. 106, No. 15, 2002 3689
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(23) In principle the quantum yields should have been measured against
a few standards, including one that emits in the red. However, it is our
experience that quantum yields determined for similar complexes against
quinine sulfate, ruthenium trisbipyridine, and cresyl violet are the same
within 5%.
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(25) The Eu3+ has absorption bands in this region (e.g. at 393 nm) but
the concentration was so low (10-5 M) that no effective absorption of Eu3+
took place. Without sensitizer, no Eu3+ emission was observed at these
conditions.
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the diffusion-controlled quenching rate constant and the oxygen concentra-
tion (kdiff[O2]); kdiff is taken as 1010 M-1s-1, [O2] in dichloromethane is 2.2
mM (Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993). Since kox ) kdiff[O2] ) 2 ×
107 s-1
.
(39) Deoxygenation was performed by purging the solutions with argon
for a duration of 10 min.
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nm. If the excitation wavelength is above 300 nm, then the sensitization
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(33) Singular value decomposition (SVD) of the data is readily achieved
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ligand. No data on the complexes are available but it is expected that still
a driving force remains for the charge transfer.