Sensitized emission of luminescent lanthanide complexes based on a phosphane oxide derivative

Article abstract of DOI:10.1021/jp909402k

The photophysical properties of a complex based on diphenylphosphanoethane (DPPE) fluorescent ligands linked to a europium ion have been investigated by different spectroscopic methods. Upon complexation with europium, the interaction of the phosphane oxi

Full text of DOI:10.1021/jp909402k

3264
J. Phys. Chem. A 2010, 114, 3264–3269
Sensitized Emission of Luminescent Lanthanide Complexes Based on a Phosphane Oxide
Derivative†
Minh-Huong Ha-Thi,*,‡ Jacques Alexis Delaire,‡ Ve´ronique Michelet,§ and Isabelle Leray*,‡
PPSM, ENS Cachan, CNRS, UniVerSUD Paris, 61 AV Pre´sident Wilson, F-94230 Cachan, and Laboratoire
Charles Friedel, E.N.S.C.P., UMR7223, 11 rue P. et M. Curie, 75231 Paris, Cedex 05, France
ReceiVed: September 30, 2009; ReVised Manuscript ReceiVed: NoVember 27, 2009
The photophysical properties of a complex based on diphenylphosphanoethane (DPPE) fluorescent ligands
linked to a europium ion have been investigated by different spectroscopic methods. Upon complexation
with europium, the interaction of the phosphane oxide group with europium leads to a red shift of the absorption
spectrum and a strong quenching of the ligand emission. The typical sensitized emission of Eu3+ is observed
upon excitation of the ligand with a fluorescence quantum yield of 1%. Time-resolved absorption and emission
experiments have been performed in order to investigate the photophysical mechanism involved in this complex.
Photophysical studies show that an energy-transfer mechanism occurs from both the first excited singlet and
5
triplet states of the ligand, and the population of the europium ion to the D1 state takes place, from which
5
the D0 state is populated. Additionally, electron transfer from the excited singlet state of the ligand to the
europium ion appears as a very efficient process.
1. Introduction
SCHEME 1: Structure of the Ligand L1
Over the past few decades, considerable attention has been
devoted to the design of lanthanide complexes due to their very
attractive properties and their potential applications as emitters
in organic-light-emitting diodes (OLED) as well as molecular
probes for biological imaging and sensing.1,2 Due to the
forbidden character of the intra-4f transitions, the lanthanides
exhibit very low absorption coefficients, and considerable efforts
have been directed to the design of new chromophores able to
sensitize the lanthanides efficiently.3,4 Among the complexing
units, it has been demonstrated that the skeleton based on a
phosphane oxide group can strongly coordinate europium ions,5
and several europium complexes containing these key functional
groups as ligands have been synthesized.6-9 Moreover phos-
phane oxide based derivatives offer several advantages in terms
of high fluorescence quantum yields.10
2. Experimental Section
Synthesis. The phosphane oxide 3 was prepared according
to previously described procedures.13 To a solution of 3 (100
mg, 0.19 mmol) in toluene (30 mL) and triethylamine (8 mL),
377 mg of 4 (1.23 mmol) was added. The mixture was degassed
under argon, and CuI (8.7 mg, 0.04 mmol) and Pd(PPh3)4 (26
mg, 0.02 mmol) were added. The reaction mixture was then
stirred for 20 h at 50 °C under argon. The volatiles were
evaporated under vacuum, and the residue was dissolved in
dichloromethane (10 mL). The organic layer was washed with
aqueous HCl (10%), dried (Na2SO4), and filtered, and the
solvents were evaporated under reduced pressure. The crude
product was then purified by flash chromatography (Silica gel:
CH2Cl2 100% to CH2Cl2-acetone: 1/1) to afford the desired
We have recently been involved in the design of new
fluorescent probes and have described the synthesis and the
photophysical properties of new fluorophores and fluorescent
molecular sensors bearing phosphane oxide11-13 and phosphane
sulfide groups.14-16 These fluorophores show very high absorp-
tion coefficients and efficiently complex cations such as heavy
metal ions. Taking advantages of the complexing abilities of
the PdO group with europium, we decided to synthesize the
ligand L1 consisting of diphenylphosphanoethane derivative
(Scheme 1) and to evaluate the photophysical properties of the
1
complex of L1 with Eu3+ 17,18
Especially, we have been
.
ligand L1 (130 mg, 56% yield) as a white solid. H NMR
(CDCl3, 300 MHz): δ (ppm) ) 1.30 (t, J ) 7.35 Hz, 12H),
2.51 (m, 4H), 4.27 (q, J ) 7 Hz, 8H), 4.64 (s, 8H), 6.88 (d, J
) 8.8 Hz, 8H), 7.47 (d, J ) 8.8 Hz, 8H), 7.5-7.7 (m, 16H).
13C NMR (CDCl3, 75 MHz): δ (ppm) ) 14.3 (CH3), 61.6 (CH2),
65.5 (CH2), 87.5, 92.4 (C′C), 114.9 (CHar.), 116.0 (Car.) 127.9
(Car.), 130.8 (CHar.), 131.9 (CHar.), 133.5 (CHar.), 158.4 (Car.-O),
168.6 (CdO). 31P NMR (CDCl3, 121 MHz): δ (ppm) ) 32.3.
HRMS (MALDI): m/z: calcd for C74H65O14P2: 1239.3844; found:
1239.3800 [M+H]+. The complex ML3 was formed in situ by
interested in investigating by different photophysical methods
(absorption, time-resolved fluorescence, and transient absorption
measurements) the involved mechanism in the energy-transfer
process from the ligand to the europium ion.
† Part of the “Benoît Soep Festschrift”.
* To whom correspondence should be addressed. E-mail: minh-huong.ha-thi@
u-psud.fr (M.-H.H.-T.); Isabelle.Leray@ppsm.ens-cachan.fr (I.L.).
‡ UniverSUD Paris.
§ E.N.S.C.P.
10.1021/jp909402k  2010 American Chemical Society
Published on Web 12/23/2009

Sensitized Emission of Luminescent Lanthanide Complexes
J. Phys. Chem. A, Vol. 114, No. 9, 2010 3265
SCHEME 2: Synthesis of Ligand L1a
mixing typically 4 × 10-6 mol·L-1 of Eu3+ with 1.2 × 10-5
mol·L-1 of ligand.
Solvents and Salts. Acetonitrile and chloroform from Aldrich
(spectrometric grade) were used as solvents for the absorption
and fluorescence measurements. Europium nitrate or terbium
nitrate from Aldrich or Alfa Aesar was of the highest quality
available and vacuum-dried over P2O5 prior to use.
Spectroscopic Measurements. UV/vis absorption spectra
were recorded on a Varian Cary5E spectrophotometer, and
corrected emission spectra were obtained on a Jobin-Yvon Spex
Fluorolog 1681 spectrofluorimeter. The fluorescence quantum
yields were determined by using either quinine sulfate dihydrate
in sulfuric acid, 0.5 N (ΦF ) 0.54619), or [Ru(bipy)3]Cl2 in water
(ΦF ) 0.02820) as standards. The complexation equilibrium
constants were determined by global analysis of the evolution
of all absorption and/or emission spectra by using the Specfit
Global Analysis System V3.0 for a 32 bit Windows system.
This software uses singular value decomposition and nonlinear
regression modeling by the Levenberg-Marquardt method.21
Fluorescence intensity decays were obtained by the single-
photon timing method with picosecond laser excitation with a
Spectra-Physics setup composed of a titanium sapphire Tsu-
nami laser pumped by an argon ion laser, a pulse detector, and
doubling (LBO) and tripling (BBO) crystals. Light pulses were
selected by an optoacoustic crystal at a repetition rate of 4 MHz.
Fluorescence photons were detected through a long-pass filter
(375 nm) by means of a Hamamatsu MCP R3809U photomul-
tiplier, connected to a constant-fraction discriminator. The time-
to-amplitude converter was purchased from Tennelec. Data were
analyzed by a nonlinear least-squares method with the aid of
Globals software (Globals Unlimited, University of Illinois at
Urbana-Champaign, Laboratory of Fluorescence Dynamics).
Transient absorption measurements were carried out by
nanosecond laser flash photolysis. The setup used the third
harmonics of a Q-switched Nd:YAG laser by BM Industries
(model BMI 5011 DNS 10), delivering 7-8 ns pulses at 1064
nm. Q-switching was achieved with a Pockels cell inside of
the cavity. The giant pulse was frequency-doubled and -tripled
in KDP crystals. The output energy was 50 mJ at 355 nm. The
energy deposited in the sample was lowered to 3 mJ by
interposing a half-wave plate between cross polarizers. The
excitation beam and the probe beam generated by a pulsed xenon
source were perpendicular to each other inside of the 1 × 1 cm
cell. The analyzing beam was spectrally dispersed by a
monochromator and converted to an electric signal by a
Hamamatsu R 928 PM tube. The electric signal was recorded
by a digital memory oscilloscope (Tektronix TDS 620 B)
connected to a PC computer. The transient signals were analyzed
by an in-house routine using Igor procedure. The reported decays
and rate constants are the mean values of at least 10 different
measurements.
a (a) Mg, THF, 50°C, 1.5 h then Cl2PCH2CH2PCl2, THF, RT, 20 h,
26%; (b) H2O2, CH2Cl2/MeOH, RT, 100%; (c) K2CO3, MeOH/CH2Cl2,
RT, 4h, 80%; (d)
3 mol % Pd(PPh3)4, 6 mol % CuI,
I-C6H4-O(CH2)CO2Et 4, Et3N, toluene, 50°C, 56%.
Figure 1. Absorption of ligand L1 (7.01 × 0-6 M) in the presence of
increasing concentration of Eu3+ in CH3CN/CHCl3 8:2 v/v. Inset:
Variation of the absorbance at 300 and 330 nm upon europium addition.
reacting iodophenol and bromoethylethanoate under basic
conditions) was then performed in the presence of 3 mol % of
Pd catalyst and 6 mol % of copper iodide. This reaction allowed
the introduction of the desired aryl group, leading to the
phosphane oxide ligand L1 in 56% isolated yield. Having in
hand such a bidentate derivative, we next studied complexation
and photophysical properties of the europium complex.
Complexing Properties. The complexation studies of eu-
ropium and terbium with L were performed in CH3CN/CHCl3
(8:2 v/v) in order to have a good dissociation of the rare earth
salt and a good solubility of the complexes. The evolution of
the absorption upon europium complexation is shown in Figure
1. Europium complexation induces a bathochromic shift of the
absorption, which may be rationalized by an enhancement of
the electron-withdrawing character of the phosphane oxide group
due to the interaction of the oxygen atom with the cation. Careful
analysis of the absorption spectra upon increasing europium
addition by means of the SPECFIT program reveals that
different complexes are formed with the following stoichiom-
etry, ML, ML2, ML3. Analyses of the corresponding data gave
the following values for the global association constants: log
ꢀ13 ) 20.80 ( 0.2; log ꢀ12 ) 15.60 ( 0.2; and log ꢀ11 ) 8.4
( 0.13, which are defined by the following equations
3. Results and Discussion
Synthesis. Scheme 2 outlines the synthesis of the ligand L1.
We based our strategy on the classical reaction between 1,2-
bis(dichlorophosphano)ethane and the Grignard derivative of
the commercially available 4-bromotrimethylsilylphenylacetylene,
2. The oxidation step of phosphorus atoms was performed in
the presence of H2O2 and was followed by the desilylation
reaction. The phosphane oxide, 3, was isolated in 26% yield
over three steps. This intermediate key had already been
prepared by our group and used for the preparation of other
fluorophores.13 The Sonogashira22 cross-coupling between the
phosphano product 3 and the iodide 4 (obtained easily by

3266 J. Phys. Chem. A, Vol. 114, No. 9, 2010
Ha-Thi et al.
[ML3]
[M][L]3
M + 3L h ML3
ꢀ13
)
)
[ML2]
[M][L]2
M + 2L h ML2
M + L h ML
ꢀ12
ꢀ11
[ML]
[M][L]
)
We then turned our attention to the characterization of the
photophysical properties of the europium complex. We have
chosen the concentration conditions in which EuL3 is the
predominant species. As shown in Figure 2, under an excitation
at 324 nm, the wavelength where Eu3+ is not absorbing, a strong
quenching of the ligand emission was observed upon europium
complexation together with the appearance of sharp peaks
associated with 5D0 f 7FJ and 5D1 f 7FJ (J ) 0-4) transitions
characteristic of the europium luminescence.23 The fluorescence
quantum yield of the lanthanide ion was found to be 1%. The
fluorescence of the ligand in the europium complex still
remained, and the fluorescence quantum yield was found to be
2.5%. Such a value must be compared with the fluorescence
quantum yield of the free ligand (ΦF ) 20%). We have also
examined the complexation with the terbium ion. A quenching
of the ligand fluorescence is observed, but no specific emission
of the terbium is observed, which excludes the energy-transfer
process from the ligand to the cation. Since electron transfer
from the ligand to the terbium ion is thermodynamically
forbidden, the observed quenching is due to the heavy atom
effect, which enhances intersystem crossing. We have therefore
used this complex TbL3 as a reference compound to evaluate
the heavy atom effect in the europium complex.
Figure 3. Fluorescence decays of L1 (1.4 × 10-6 M) and its complex
ML3 with europium in CH3CN/CHCl3 8:2 v/v. Channel width: 4.5 ps.
Excitation wavelength: 323 nm. Emission wavelength: 400 nm.
In order to further evaluate the photophysical properties of
this new complex, time-resolved fluorescence measurements
were performed by the single-photon counting method with
picosecond laser excitation. The fluorescence decays of the
ligand and its complex ML3 with europium are shown in Figure
3. The decays are satisfactorily fitted by considering three decay
times (τ1 ) 4.5 ns, R1 ) 0.02; τ2 ) 0.45 ns, R2 ) 0.5; and τ3
) 0.01 ns, R3 ) 0.48) for L1 ligand. As previously observed
for the multichromophoric phosphane oxide ligand analogues,13
this effect can be explained by the presence of two different
conformers; one may envisage the presence of an expanded
structure leading to monomer emission (τ2 ) 0.45 ns) and a
near-sandwich geometry which allows the formation of an
Figure 4. Luminescence of L1 (1.4 × 10-6 M) in EtOH/MeOH 4/1
(v/v) at room temperature (dotted line) and at 77 K (solid line) (λexc
)
325 nm).
excimer due to interactions between the chromophores in the
excited state (τ1 ) 4.5 ns, τ3 ) 0.01 ns). By complexation with
europium, a strong decrease of the fluorescence decay time at
430 nm was observed. For the ML3 complex, satisfactory
analysis of the fluorescence decay can be obtained by consider-
ing the following decay times (τ1 ) 0.02 ns, R1 ) 0.84; and τ2
) 0.45 ns, R2 ) 0.16). The average decay time, which is defined
as 〈τ〉 ) ∑i Riτi, decreases from 0.31 to 0.088 ns upon
complexation with Eu3+. Such an effect is a consequence of
either an energy transfer to the europium ion because of the
spectral overlap between the absorption of the europium ion
and the emission spectrum of the ligand or an electron transfer
from the excited fluorophore to the Eu3+. The electrochemical
potential of the excited state of L1 was determined (E(L•+/L*)
) -2 V/SCE) and clearly showed that the excited state of the
ligand allows a reduction of the Eu3+ cation (E ) -0.35
V/SCE). Such an electron transfer has already been reported
for the interaction of Eu3+ with other fluorophores.24
In order to evaluate the energy level of the triplet state of the
ligand, we next examined its luminescence in a rigid matrix
(EtOH/MeOH, 4:1 v/v) at 77 K. As observed in Figure 4, a
structured and sharp band is observed with a maximum at 346
nm; this can be easily attributed to the 1LE band instead of the
CT band.25 In addition, a weak structured band is observed
between 460 and 650 nm, which can be attributed to the
Figure 2. Corrected emission spectra of ligand L1 (1.4 × 10-6 M)
and its complexes EuL3 and TbL3; λexc ) 324 nm.

Sensitized Emission of Luminescent Lanthanide Complexes
J. Phys. Chem. A, Vol. 114, No. 9, 2010 3267
Figure 5. Normalized transient absorption spectra of the complexes
of L1 with Eu3+ and Tb3+ (Eu3+L3 and Tb3+L3, respectively) in CH3CN/
CHCl3 8:2 v/v (nitrogen-saturated, obtained 10 ns after nanosecond
laser excitation at 355 nm).
Figure 6. Time-resolved luminescence decays of the complex Eu3+L3
at λem ) 585 nm (λexc ) 355 nm) in nitrogen-saturated CHCl3/CH3CN
80/20 v:v.
coupling in the presence of a heavy atom. However, this
decrease is more pronounced for europium complexation than
for terbium complexation, which can be explained by an efficient
energy transfer between the triplet state of the ligand and
europium ion.
TABLE 1: Photophysical Data
L1a
EuL3
2.5
0.088
7
TbL3
15
a
a
compound
ΦF (%)b
〈τS〉 (ns)c
ΦT (%)b
τT (µs)
20
0.31
We then examined the kinetics of the europium luminescence
in the complex by using a nanosecond laser and by excitation
of the ML3 complex at 355 nm. On the submillisecond time
4.5 ( 0.5
0.5 ( 0.1
1.7 ( 0.2
a Measured in CHCl3/CH3CN 20/80. b Errors: (10%.
c Fluorescence average decay time of the ligand defined as 〈τ〉 )
∑i Riτi.
5
scale, by observing the D1 emission at 585 nm, a decay time
of 15 ( 2 µs was observed (Figure 6). In order to improve the
accuracy of the fast rise time, the fluorescence signal of the
complex (EuL3) was recorded on a shorter time scale (inset of
Figure 6). A typical rise of the europium luminescence was
observed, which was strongly disturbed by the ligand fluores-
cence. In the analysis, the longer decay time was fixed at 15
µs, and a rise time of 0.5 ( 0.1 µs has been found. This rise
time corresponds exactly to the lifetime of the triplet state of
the EuL3 complex in a N2-saturated solution, which corroborates
an efficient energy transfer between the triplet state of the ligand
to the 5D1 level of the lanthanide. It should be pointed out that,
as the triplet quantum yield of L in the europium complex is
low (7%), the proportion of excited states of europium populated
by energy transfer from this triplet state is obviously small.
In order to obtain further information on this energy-transfer
process, the resulting rise time in the temporal evolution of the
europium luminescence was examined under different concen-
trations of oxygen (a solution saturated with nitrogen, a solution
saturated with oxygen, and a solution under air atmosphere).
The observed luminescence signals are displayed in Figure 7,
and after fitting these signals with biexponentials, the corre-
sponding rise times are equal to 0.096 ( 0.005 µs for an O2-
saturated solution, 0.28 ( 0.03 µs for an aerated solution, and
0.5 ( 0.1 µs for a N2-saturated solution. The rise time, which
becomes shorter as the oxygen concentration increases, is
evidence of the triplet energy-transfer process.
phosphorescence of the triplet state, and from the threshold of
this band, the energy of this state has been estimated to be 21000
cm-1. Thus, energy transfer is possible from the triplet state of
5
the ligand to the D states of the europium.
Further experiments have been made in order to characterize
the energy transfer between the ligand and the europium.
Transient absorption and emission experiments have been
performed by using the third harmonic of a nanosecond Nd/
YAG laser. The absorption spectrum of the triplet of ligand L1
cannot be determined because of the low absorption of the
fundamental at 355 nm. The triplet absorption spectra of the
complex with Eu3+ and Tb3+ are displayed in Figure 5, and
they exhibit a maximum at 505 and 540 nm, respectively. It
should be noticed that the absorbance of the triplet of the ligand
in the europium complex is significantly lower than that in the
terbium complex. In fact, in both of these complexes, the
intersystem conversion S1-T1 is accelerated due to a spin-orbit
coupling enhancement produced by a heavy atom effect. The
lower absorbance observed in the case of the europium complex
is the consequence of an efficient quenching process from the
excited singlet state by energy or electron transfer. The
intersystem-crossing quantum yield Φisc was determined by
studying the triplet energy transfer between the europium
complex and benzophenone.26 The Φisc for the ligand in the
europium complex was found to be 0.07, which means that the
most efficient nonradiative pathway from the singlet excited state
occurs via electron or energy transfer to the europium.
Furthermore, it should be noticed that the fits of the different
luminescence signals start at the same point, which is different
from zero. This can be rationalized by the fact that the energy
transfer from the ligand to the europium may also occur via
the singlet state of the ligand, which has been already described
by Wang in the case of europium complexes.27 The proportion
of the different energy-transfer pathways can be determined from
the interpolation of the luminescence curves when t is equal to
The triplet lifetimes in a degassed and nitrogen-saturated
solution have also been determined, and all of the photophysical
data are gathered in Table 1. Upon complexation, a significant
decrease of the triplet decay time of the ligand was observed
for these two complexes, which may be rationalized again by
an enhancement of the conversion T1-S0 by the spin-orbit
0 (I0). In a first approximation, we can estimate that ΦSET/ΦT
ET

3268 J. Phys. Chem. A, Vol. 114, No. 9, 2010
Ha-Thi et al.
Figure 7. Time-resolved luminescence signals of the complex EuL3
at λem ) 585 nm (λexc ) 355 nm) in CHCl3/CH3CN 80/20 v:v of a
nitrogen-saturated solution, an oxygen-saturated solution, and an aerated
solution.
Figure 9. Emission spectra of 5D1 and 5D0 states of the Eu3+ complex;
5
the solid line is associated with the D1 curve (delay: 1 µs), and the
5
dashed line is associated with the D0 emission (delay: 80 µs) (λexc
)
355 nm). The involved transitions are indicated.
SCHEME 3: Schematic Diagram of the Photophysical
Processes Leading to Sensitized Luminescence of the
Ligand L
5
5
Figure 8. Time-resolved luminescence decays of the complex Eu3+L3
at λem ) 617 nm (λexc ) 355 nm) in CHCl3/CH3CN 80/20 v:v. Inset:
time-resolved decay obtained with a smaller time scale.
spectra of the D1 and D0 states are shown in Figure 9. The
5D1 state has a maximum at 585 nm, whereas the D0 state
presents a maximum at 617 nm. The shapes of these emission
spectra are in perfect agreement with those reported by Van
Veggel for similar europium complexes.28
5
) 0.24 (where ΦSET and ΦTET are the energy-transfer quantum
yields from the first excited singlet and triplet states, respec-
tively), which means that the energy transfer is four time more
efficient from the triplet state than that from the singlet excited
state. Going back to the different quenching processes of the
first excited singlet state of the ligand, we may conclude that
the energy-transfer process has a quantum yield which is lower
than 2% (one-fourth of Φisc), the electron-transfer process being
therefore largely dominant.
4. Conclusion
In conclusion, we have synthesized a new phosphane oxide
derivative able to efficiently sensitize the europium emission.
It was shown that this ligand may interact with the trivalent
europium ion, the stability constant of the resulting ML3
complex being log ꢀ13 ) 20.80 ( 0.2. Photophysical investiga-
tions of the complex were performed by stationary and time-
resolved luminescence studies. The different pathways involved
in the sensitization of the europium ion by the excited state of
the ligand L1 are shown in Scheme 3. The energy-transfer
process occurs from both the singlet and the triplet states of
the ligand L1. Additionally, the electron-transfer process leading
to a charge separation (L+• Eu2+) appears to be the most efficient
deactivating pathway of the first excited singlet state of the
ligand, which partially explains the relatively low luminescence
quantum yield of the europium ion in the complex (ΦF ) 1%).
The photophysical studies have shown that there is no direct
We then focused on the study of the luminescence signals of
the europium ion in the complex by exciting the complex at
355 nm. The populations of the 5D1 and 5D0 states were studied
at around 600 nm (Figure 8). The obtained decay at 617 nm
can be satisfactorily fitted by considering a decay time of 3.08
( 0.03 ms and a rise time of 10 ( 1 µs, whereas the signal at
584 nm shows a decay time of 15 ( 2 µs (inset of Figure 8).
5
The rise time of the D0 state is therefore in good agreement
with the decay time of the 5D1 state. Such an effect is completely
consistent with a population of the 5D0 state from the 5D1 state
as previously described by Van Veggel for other europium
complexes.28 From the luminescence decays obtained at different
emission wavelengths, it has been possible to rebuild the
luminescence spectrum of both states 5D1 and 5D0. The emission
5
energy transfer from the antenna to the D0 state, but instead
the D0 state is populated via the D1 state.
5
5
Acknowledgment. This work was supported by the Ministe`re
de la Recherche program “ACI Jeunes Chercheurs 2004-2006”

Sensitized Emission of Luminescent Lanthanide Complexes
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