Two-photon absorption properties of Eu3+-DPA-triazolyl complexes and the derived silica nanoparticles embedding these complexes

Article abstract of DOI:10.1002/ejic.201402617

Several complexes and silica-based nanohybrids of rare-earth ions (Eu³⁺, Gd³⁺) have been synthesized from dimethyl 4-azidopyridine-2,6-dicarboxylate (4) following the Click chemistry approach. A complete spectroscopic study indicates that such compounds exhibit strong sensitization by the antenna effect from both UV and NIR excitations. The Gd³⁺-based materials show phosphorescence under ambient conditions, which originates from the lowest-energy intra-ligand triplets. Fine analysis of the NIR excitation spectra using time resolved photoluminescence spectroscopy (TRS) indicates that the spectral repartition of the triplet T₁ state differs notably between the complexes and the NPs embedding the complexes. Moreover the dependence of Eu³⁺ luminescence vs. incident beam power in the NIR region diverges from pure quadratic dependence expected in the framework of the two-photon absorption process. The results are discussed considering the occurrence of a direct singlet-to-triplet optical absorption transition (S₀→T₁) upon NIR excitation.

Full text of DOI:10.1002/ejic.201402617

FULL PAPER
DOI:10.1002/ejic.201402617
Two-Photon Absorption Properties of Eu3+-DPA-Triazolyl
Complexes and the Derived Silica Nanoparticles
Embedding These Complexes
CLUSTER
ISSUE
Pierre Adumeau,[a,b] Claire Gaillard,[c] Damien Boyer,[b,d]
Jean-Louis Canet,[b,d] Arnaud Gautier,[a,b] and Rachid Mahiou*[a,b]
Keywords: Lanthanides / Nanoparticles / Luminescence / Phosphorescence / Two-photon absorption / Europium
Several complexes and silica-based nanohybrids of rare-
earth ions (Eu3+, Gd3+) have been synthesized from dimethyl
4-azidopyridine-2,6-dicarboxylate (4) following the Click
chemistry approach. A complete spectroscopic study indi-
cates that such compounds exhibit strong sensitization by the
resolved photoluminescence spectroscopy (TRS) indicates
that the spectral repartition of the triplet T1 state differs no-
tably between the complexes and the NPs embedding the
complexes. Moreover the dependence of Eu3+ luminescence
vs. incident beam power in the NIR region diverges from
pure quadratic dependence expected in the framework of
the two-photon absorption process. The results are discussed
considering the occurrence of a direct singlet-to-triplet op-
tical absorption transition (S0ǞT1) upon NIR excitation.
antenna effect from both UV and NIR excitations. The Gd3+
-
based materials show phosphorescence under ambient con-
ditions, which originates from the lowest-energy intra-ligand
triplets. Fine analysis of the NIR excitation spectra using time
low-energy ligand-to-metal charge transfer (LMCT) transi-
tion. Both ILCT and LMCT transitions are allowed by La-
porte’s selection rules.[2] In comparison with organic materi-
als, the incorporation of metal ions in coordination com-
plexes introduces more sublevels into the electronic energy
scheme of the material, which is expected to enable more
allowed electronic transitions to take place and can enhance
for example the nonlinear optical (NLO) effects including
the two-photon absorption (TPA).
Introduction
Lanthanide complexes, characterized by strong lumines-
cence, are of great interest because of their potential appli-
cations as one- or two-photon optical materials in several
areas such as fluorescence imaging, photodynamic therapy,
photo-switching systems, electroluminescent devices for
OLED or light-harvesters for enhancing efficiency of silicon
solar cells.[1] So far, Eu3+ and Tb3+ complexes have been
the most studied for such purposes, because the lumines-
cence of these ions is characterized by a large Stokes shift
upon excitation through the ligand states, narrow emission
bands and long luminescence decay times. The most rel-
evant feature arises from the incorporation of the lanthan-
ide ions in organic lattices. They then play the role of a
powerful 3D template gathering simple organic ligands in a
variety of multipolar arrangements showing tailored elec-
tronic and optical properties that can be tuned according
to the coordinated metal centre. This can induce a strong
intra-ligand charge transfer (ILCT) transition as well as a
However, in several applications, the excitation occurs by
a classical one-photon (OPA) antenna effect with the exci-
tation wavelength lying generally below ca. 450 nm. Since
this wavelength range limits the applications of such com-
plexes, notably in the case of biological imaging (low depth
penetration into the tissues, scattering by biological media,
tissue autofluorescence) the challenge to overcome these
drawbacks is to sensitize the lanthanide ions by the NLO
absorption antenna effect, mostly through resonant two-
photon absorption (TPA) with an S1 singlet excited state of
the ligands in the case of either lanthanide complexes or
derived hybrid materials. Although, for the special case of
a molecular lanthanide-containing complex, despite there
being several examples of S1(1π*, ligand)Ǟ4f* direct trans-
fers, very often the S1(1π*)ǞT1(3π*)Ǟ4f* path occurs pre-
dominantly through an intercrossing system (ICS), particu-
[a] Clermont Université, Université Blaise Pascal, Institut de
Chimie de Clermont-Ferrand,
BP 10448, 63000 Clermont-Ferrand, France
E-mail: Rachid.Mahiou@univ-bpclermont.fr
http://iccf.univ-bpclermont.fr
[b] CNRS, UMR 6296, ICCF,
BP 80026, 63171 Aubière, France
larly for Eu3+ and Tb3+ [3,4]
Both one- (OPA) and two-pho-
.
[c] European Commission, DG Joint Research Centre - Unit
JRC.I.04 Nanobiosciences,
ton absorption (TPA) are expected to follow this simple
scheme However, the investigations of resonant TPA sensiti-
zation with the triplet T1 state in organic coordination envi-
ronments remain rather rare.
Via E. Fermi, 2749 TP-12521027 ISPRA (VA), Italy
[d] Clermont Université, ENSCCF, Institut de Chimie de
Clermont-Ferrand,
BP 10448, 63000 Clermont-Ferrand, France
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Among the variety of lanthanide chelates now available, tra of these materials, we have investigated both the steady-
those prepared from dipicolinic acids (DPAs) exhibit rel- state and time-resolved luminescence spectra (TRS) at room
evant optical properties leading to efficient sensitization of temperature for Eu3+- and Gd3+-based complexes and silica
Eu3+ and Tb3+ ions for both OPA and TPA.[5] Moreover NPs in the solid state. The luminescence-based method
we have reported that luminescent tris-dipicolinate LnIII (two-photon excited luminescence) was used by standing
complexes can be directly and easily embedded into silica the laser wavelength in the near infrared range (NIR). The
nanospheres and PEG-NHS appendages can be grafted results are discussed considering the dependence of the
onto this surface.[6]
Eu3+ luminescence intensity on the external pumping
In order to compare the ability of such chromophores to power.
sensitize Eu3+ luminescence, overall OPA quantum yields
have been determined experimentally by considering three
Results and Discussion
europium(III) 4-triazolyl-DPA complexes [Eu1], [Eu2],
[Eu3], bearing phenyl, 1-methylimidazolyl and N,N-di-
methyl-2-ethanolammoniomethyl appendages, respectively,
substituting the position 4 of the triazole group, and the
One Photon Spectroscopy (OPA)
The (Eu3+,Gd3+) samples (Figure 1) were synthesized
derived monodisperse silica nanoparticles (NPs) embedding from dimethyl 4-azidopyridine-2,6-dicarboxylate (4) follow-
them. Recently, with a combination of the advantages of ing the Click chemistry approach developed in the labora-
both lanthanide ions and two-photon scanning microscopy, tory.[5,6,16] For convenience, the samples are named phenyl,
TPA sensitized luminescence of lanthanide complexes has imidazole and choline for the Eu3+ samples [Eu1], [Eu2]
attracted great attention,[7–15] which is in favour of less- and [Eu3], respectively, in the figures.
harmful labelling and deep-penetrating bioimaging applica-
Photoluminescence (PL) quantum yield excitation spec-
tions. Accordingly, as stated above, the most popular sensi- tra recorded for the three complexes in the solid form, con-
tization mechanism is the “antenna effect”,[3,4] i.e. TPA of sidering the overall luminescence of the Eu3+ ion, are re-
organic ligands and energy transfer to the lanthanide ion.
ported in Figure 2. These spectra are recorded using an ab-
The crux of the mechanism, which governs the cross-sec- solute PL quantum yield measurement system Hamamatsu
tion and the energy transfer efficiency from ligand to lan- C9920–02G (see Experimental Section). From this figure
thanide ion for both OPA and TPA, is the relative location we clearly see that the different moieties grafted on the or-
of the triplet T1 state to the near-resonant energy levels of ganic ligands lead to different excitation spectra shapes, in-
the lanthanide metal ions. Indeed the energy gap between dicating that the OPA can be tailored on this basis (i.e. just
the upper T1 state and the lower quasi-resonant energy of by modifying the design of the ligand).
the excited state of the lanthanide metal ions must be
The values reported on the y axis correspond to the
enough to limit the energy back transfer from the metal to absolute quantum yield, which is defined as the internal
the T1 state by a thermal Boltzmann effect. For such pur- quantum yield εin corrected from the absorption coefficient
poses, and to determine the intrisic phosphorescence spec- leading to the εex.[17]
Figure 1. Synthesis of the ligands 1–3, and structure of the related complexes (R1: phenyl, R2: 1-methylimidazolyl, R3: N,N-dimethyl-2-
ethanolammoniomethyl).
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Figure 3. Representative emission spectra of ligand-excited Eu3+-4-
triazolyl-DPA complexes. The insert shows the emission spectrum
of the [Eu1] complex excited at 396 nm (Eu3+: 7F0Ǟ5L6 transition).
Figure 2. Photoluminescence quantum yield excitation spectra of
Eu3+-4-triazolyl-DPA complexes bearing phenyl [Eu1], 1-methyl-
imidazolyl [Eu2] and N,N-dimethyl-2-ethanolammoniomethyl
[Eu3], monitoring the overall luminescence of the Eu3+ ions.
The ligand-centred emission is not detected under such
broad excitation, suggesting an efficient ligand-to-metal en-
ergy transfer process. However, in the case of the [Eu1]
complex, excitation at the 4f:5L6 level of Eu3+ at 396 nm
produces a broad band emission peaking at 499 nm (insert
of Figure 3).
The internal efficiency εin and the external quantum effi-
ciency εex were defined by:
This band corresponds to the phosphorescence arising
from the T1 triplet state. Its observation is an indication
that back transfer occurs in the case of the [Eu1] complex
leading to the filling up of the triplet state from which radi-
ative de-excitation occurs. Such an observation can explain
why the QY of [Eu1] is lower than that of [Eu2] or [Eu3].
In order to enhance the thermal and photo stabilities of
Nem
εin
=
Nabs
and
Nem
Nex
Nabs
Nex
εex
=
= εin(
)
where Nem corresponds to the number of photons of the
emission light of the sample, Nabs corresponds to the these complexes, they were dispersed in silica nanoparticles
number of photons that are absorbed by the sample and (NPs) using classical reverse emulsion processes involving
Nex corresponds to the number of photons of the excitation TEOS, NP-5 surfactant and ammonia as described by us.[6]
light from a Xe source.
The synthesized NPs have a well-defined spherical shape
These spectra are dominated by strong broad bands cor- and an average diameter of 20 nm, with a narrow size distri-
responding to the sensitization of the luminescence via the bution (average standard deviation: 3 nm). As expected,
ligands, while the narrow bands are attributed to the f–f only the NP synthesis in the presence of [Eu3] exhibits a
intra-shell transitions of Eu3+. Because these transitions are strong luminescence indicating a good integration of this
polarity forbidden their intensities are weak and their ener- complex. For the two other complexes [Eu1] and [Eu2], the
getic location is fairly insensitive to the metal-ion environ- incorporation seems to be weak. However, the recorded
ment. As a consequence they are easily recognizable. The emission spectra match well with those of the complexes
[Eu2] and [Eu3] complexes present the highest quantum and do not seem to be affected by the embedding in silica.
yields (QY) that reached 51.6% at 339 nm for [Eu2] and
Figure 4 shows the QY-excitation spectrum of [NP-Eu3]
53.5% at 299 nm for [Eu3], while for [Eu1] the highest value recorded at room temperature, which is compared with that
is 29.5% obtained at 339 nm. The influence of the phenyl of the [Eu3] complex. The broad band related to the ligands
and methylimidazolyl groups in extending the π-conjugated is shifted from 299 nm to 294 nm and the QY measured at
system and in so broadening the excitation spectra of the this wavelength reaches 40% instead of the ca. 53% mea-
corresponding Eu3+ complexes is clearly evidenced. The dif- sured for the complex.
ferences in the excitation spectra, just by modifying the ap-
Generally, when the S1 state is higher than the T1 state,
pendage, can be very useful for extending the excitation the energy position of the triplet state can be determined
windows toward the visible region, which is of importance from the phosphorescence spectra usually recorded below
for biomedical applications or optoelectronic devices.
room temperature. Further insights into the photophysical
The visible luminescence spectra of the complexes re- properties of the samples were obtained for this purpose
corded in the solid state at room temperature are all iden- by analyzing the luminescence spectra of the Gd3+-based
tical upon broad band excitation through the ligand levels. materials. The f–f states of Gd3+ are located at an excep-
The Eu3+ emission (Figure 3) exhibits the characteristic tionally high energy level owing to the extreme stability of
5D0Ǟ7FJ transitions and is dominated by the band related its half-filled f shell (4f7). Accordingly, Gd3+ complexes are
5
to the D0Ǟ7F2 hypersensitive transition.
frequently characterized by emissive intra-ligand states
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6
ingly, no peak corresponding to the lowest P7/2 excited
level of Gd3+ (expected around 2ϫ312 nm) is recorded.
This means that this attribution is puzzling and subject to
further proofs.
The maximum of the broad band is shifted from 506 nm
for [Gd3] to 522 nm for [Gd1] and to 575 nm for [Gd2] and
extends for the latter to beyond 650 nm. Figure 6 reports
the emission spectra of [Gd3] and [NP-Gd3] recorded at
room temperature under pulsed laser excitation using time
resolved spectroscopy (delay: 8 μs, gate: 5 ns). We observed
the same features as in the case of continuous wave (cw)
excitation, but with the emission band of the ligand blue-
shifted by ca. 19 nm in the case of the NPs. The corre-
sponding decay times, measured at room temperature upon
excitation at 290 nm, are depicted in Figure 7.
Figure 4. Photoluminescence quantum yield excitation spectrum of
[Eu3] and of the corresponding NPs [NPs-Eu3] monitoring the
overall luminescence of the Eu3+ ions.
(fluorescence from the S1 singlet state and phosphorescence
from the triplet T1 state with characteristic decays in time
of up to a few μs for S1 and up to several ms for T1).
Owing to its heavy-atom effect and paramagnetism,
Gd3+ induces a strong singlet/triplet mixing in the ligand.[18]
It follows that the intra-ligand fluorescence is largely
quenched while the phosphorescence is facilitated. Figure 5
reports the 290 nm-excited steady-state emission of Gd3+-4-
triazolyl-DPA complexes recorded in the solid state at room
temperature keeping the conditions the same for the three
complexes. The excitation is supplied by a monochro-
matized xenon lamp. The emission spectra consist of a
broad band characteristic of phosphorescence arising from
the triplet state T1 surmounted by two narrow bands peak-
ing at 544 nm and 613.4 nm, independent of the nature of
the complex. These two bands can be attributed to the sec-
Figure 6. Time resolved emission spectra (TRS) of the Gd3+-4-tri-
azolyl-DPA complex bearing N,N-dimethyl-2-ethanolammo-
niomethyl [Gd3] and corresponding NPs [NP-Gd3] recorded at
room temperature under laser pulsed excitation at 290 nm.
6
ond order of the 4f emissions of Gd3+ arising from its I7/2
6
(ca. 272 nm) and P5/2 (ca. 307 nm) excited levels. Surpris-
Figure 7. Luminescence decays of ligands (emission at 470 nm) of
the Gd3+-4-triazolyl-DPA complex bearing N,N-dimethyl-2-ethan-
olammoniomethyl [Gd3] and corresponding NPs [NP-Gd3] re-
corded at room temperature.
Figure 5. Emission spectra of Gd3+-4-triazolyl-DPA complexes
bearing phenyl [Gd1], 1-methylimidazolyl [Gd2] and N,N-dimethyl-
2-ethanolammoniomethyl [Gd3] recorded at room temperature un-
der excitation at 290 nm.
It can be seen that the decay profiles are not single ex-
ponential and the extracted values in the tail of the decays
reach about 18 μs for [Gd3] and about 340 μs for [NP-Gd3].
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Because of the non-exponential character of the decays, the
values of lifetimes were estimated, over the de-excitation
range time, as an effective emission decay time using the
following expression:[19]
where I(t) represents the luminescence intensity at time t
corrected from the background and the integrals are evalu-
ated over the range 0ϽtϽtmax where tmax ϾϾτeff. It was
found that the effective decay time increases (from ca. 2 μs
to ca. 30 μs) significantly when the complex is embedded in
silica NPs. This behaviour can be attributed to the better
dispersion of the complex in the NPs, which limits the en-
ergy transfer between the interacting active luminescent
species.
Figure 8. Room-temperature time-resolved excitation spectra of the
Eu3+-4-triazolyl-DPA complex bearing, 1-methylimidazolyl [Eu2]
and N,N-dimethyl-2-ethanolammoniomethyl [Eu3] monitoring the
5D0Ǟ7F2 emission transition of Eu3+ ions peaking at 616 nm.
Two-Photon Spectroscopy (TPA)
Upon irradiation by intense light, molecules and related
molecular materials can simultaneously absorb two photons
having half the energy (in the case of a one-colour source)
of the energy transition between the fundamental state of
the molecule and one of its first excited states. From Fig-
ure 2 it appears clearly that TPA can occur, depending on
the ligand, approximately in the range between 2ϫ250 nm
and 2ϫ400 nm considering that the ligand-excited state
may be the same in OPA and TPA for non-centrosymmetric
chromophores, which is the case for our materials.
Firstly we excited the Eu3+ complexes in the visible re-
gion under pulsed laser excitation using Rhodamine 590
(Rh590) as the dye solution, which allows excitation, using
our experimental setup, between 552 nm and 584 nm. In
Figure 9. Luminescence intensities as a function of the excitation
density at 563 nm. The points show the data and the line represents
the fit according to quadratic dependence.
this region, the Eu3+ ions have no intrinsic absorption. This infrared (NIR) spectral range corresponding to the bio-
wavelength range corresponds to the 276–292 nm doubled logical transparency window (700–1100 nm).
wavelength range. We measured the TRS excitation spectra
Recently such an experiment has been reported for a do-
of [Eu1], [Eu2], [Eu3] and [NP-Eu3] from 552 nm to 584 nm nor-π-conjugated functionalized tris-dipicolinate Eu3+ com-
5
monitoring the D0Ǟ7F2 transition of Eu3+ (616 nm). For plex for which the luminescence of Eu3+ is observed upon
better clarity, in Figure 8 we only reported the recorded NIR excitation at around 740 nm.[20] For these reasons, we
spectra for [Eu2] and [Eu3]; the excitation spectra of [Eu1] have recorded the luminescence spectra for both cw and
and [NP-Eu3] in this wavelength range are quite similar to time resolved spectroscopy in the NIR range. For the cw
that of [Eu3].
spectroscopy, the Eu3+-red luminescence is very weak and
The TRS emission spectra recorded under excitation over necessitates special care by adding suitable filters to obtain
this wavelength range are similar to those recorded upon unambiguously a non-parasitized emission by the excitation
excitation in the UV region by OPA and fit well with that source. Appreciable signals have been obtained only for
depicted in Figure 3. Figure 9 shows the relationship be- [Eu2] and [Eu3]. The recorded excitation spectra obtained
5
tween luminescence intensity of the D0Ǟ7F2 emission and for the two samples monitoring the 5D0Ǟ7F2 emission tran-
excitation density. Despite the excitation wavelengths lying sition of Eu3+ ions peaking at 616 nm are reported in Fig-
at a higher energy than the emission, the quadratic depen- ure 10.
dence observed for all samples indicates that the converted
luminescence is induced by a TPA antenna process.
They consist of broad bands, one peaking at 731 nm for
[Eu3] and three peaking at 735, 763 and 794 nm for [Eu2].
Nevertheless, using this range of wavelength for generat- The half values of these wavelengths fit the low energy edge
ing TPA is not satisfactory especially with the aim of using of the OPA excitation spectrum of [Eu3] where a shoulder
such compounds as imaging bio-probes. Such wavelengths is observed at around 377 nm and fits the low energy range
are strongly absorbed or scattered by biological media. To of [Eu2] as observed in Figure 2. These bands correspond
overcome this drawback, the better way is to sensitize the unambiguously with the NLO effect as argued hereafter. In
metal-luminescence through excitation lying in the near- the case of the Eu3+ ions that present an easily reducible
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749 nm for [Eu3] and around 727, 743 and 750 nm for
[Eu2]. Excitation of these bands produces the characteristic
emission spectrum of the Eu3+ embedded in these com-
plexes as shown in Figure 12. The same emission spectrum
is recorded for the NPs [NPs-Eu3]. The luminescence decay
of Eu3+ is exponential with a time constant of 1.15 ms, a
value similar to that measured under UV excitation.[6] In-
creasing the delay after the laser pulse produces a modifica-
tion on the shape of the NIR excitation spectra, notably the
bands on the low-energy side disappear completely for de-
lays longer than 100 μs as observed in Figure 13, which de-
picts the NIR excitation spectra of [Eu3] recorded at two
delay times (8 and 100 μs).
Figure 10. Room temperature cw infrared excitation spectra of the
Eu3+-4-triazolyl-DPA complex bearing, 1-methylimidazolyl [Eu2]
and N,N-dimethyl-2-ethanolammoniomethyl [Eu3] monitoring the
5D0Ǟ7F2 emission transition of Eu3+ ions peaking at 616 nm.
configuration, when the LMCT state lies at a high enough
energy, the ILCT, LMCT and ligand triplet state T1 play a
role in the energy transfer processes and act as an antenna
for efficiently sensitizing the metal ion.
Since the two complexes [Eu2] and [Eu3] have a common
excitation band around 730 nm, we have used a laser exci-
tation in the TRS for a short time after the laser pulse (8 μs)
to check this wavelength range value with the Stokes-shifted
Rh590 as the dye solution.
Figure 11 shows clearly that the NIR excitation spectra
consist of broad bands that can be deconvoluted into three
Gaussian contributions peaking around 730, 740 and
Figure 12. Room-temperature time resolved emission spectrum (de-
lay: 100 μs, gate: 5 ns) of the Eu3+-4-triazolyl-DPA complex bearing
1-methylimidazolyl [Eu2] or N,N-dimethyl-2-ethanolammo-
niomethyl [Eu3] upon NIR excitation at 727 nm. The insert shows
the corresponding luminescence decay of the 5D0Ǟ7F2 emission
transition of Eu3+ ions peaking at 616 nm.
Figure 11. Room temperature time resolved excitation spectra (de-
lay: 8 μs, gate: 5 ns) of the Eu3+-4-triazolyl-DPA complex bearing,
1-methylimidazolyl [Eu2] and N,N-dimethyl-2-ethanolammo-
niomethyl [Eu3] monitoring the 5D0Ǟ7F2 emission transition of
Eu3+ ions peaking at 616 nm.
Figure 13. Room-temperature time resolved excitation spectra of
the Eu3+-4-triazolyl-DPA complex bearing N,N-dimethyl-2-ethan-
olammoniomethyl [Eu3] and the corresponding complex embedded
5
in silica NPs [NPs-Eu3] monitoring the D0Ǟ7F2 emission transi-
tion of Eu3+ ions peaking at 616 nm.
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The NIR-TRS of the NPs recorded with a time delay of
100 μs is very different from that of the complex shown in
Figure 13. Two main excitation bands located at 725 and
736 nm are observed (corresponding to 362.5 nm and
368 nm, respectively, in OPA).
At this stage, some conclusions can be drawn. The NIR
spectra are clearly composed of fast and long-time contri-
butions indicating that they originate from different excited
levels of the ligands.
Since the triplet state is expected to have a long decay
time, we have attributed the band peaking at around
730 nm to the electronic transition connecting the singlet S0
to the triplet T1. Such a transition has already been ob-
[21]
served in oxygen-deficient glassy SiO2
but not reported,
to the best of our knowledge, for rare-earth-based com-
plexes. An interesting feature that concerns the intensity de-
Figure 15. Luminescence intensity of the Eu3+-4-triazolyl-DPA
complex bearing N,N-dimethyl-2-ethanolammoniomethyl [Eu3]
embedded in silica NPs [NPs-Eu3] as a function of the excitation
density, upon NIR excitation at 736 nm. The points show that the
data and the line represent the fit as discussed in the text.
5
pendence of the Eu3+: D0Ǟ7F2 emission vs. the excitation
density.
Clearly for both the complex and the NPs the depen-
dence of the europium luminescence intensity is non-linear
as shown in Figure 14 and Figure 15. For the complex this
dependence is lower than the quadratic since the best fit is
obtained with the power function Pα where α = 1.4, a value transfer. Such a purpose is supported by the fact that quali-
close to that estimated from Figure 6 in ref.,[20] which is tatively, two effects are observed in the laser intensity de-
around 1.6. For the NPs the situation is quite puzzling and pendence of the NPs. One for which the slope is 0.4 at low-
needs further studies to clarify this unusual behaviour.
energy pumping and the other for which this slope is de-
creased to 0.18 indicating that the high-energy pumping
produces a population of the T1 triplet state that cannot
empty in sufficient time in comparison to the intrinsic de-
excitation rate of the triplet T1 state. It is worth noting that
the slopes are lower than 1. Resolving the emission spectra
at the shortest time given by our setup (0.5 μs) after the
laser pulse (duration 8 ns), we did not observe any signal
that could correspond to the phosphorescence from the T1
triplet state. This indicates that in addition to the energy
transfer from the T1 triplet state to the Eu3+, the processes
involved in the rapid de-excitation of the T1 state occur at
a minimum within the pulse duration and at a maximum
within the 500 ns following the laser pulse.
Figure 16 shows for comparison purposes the room-tem-
perature NIR time resolved excitation spectrum of NPs
[NPs-Eu3] and the phosphorescence spectrum of NPs [NPs-
Gd3]. For a better understanding, the wavelength scale of
the NIR spectrum has been divided by a factor of two for
simulating the OPA.
Despite a weak band appearing at 362 nm in the emis-
sion spectrum of [Gd3] as indicated in Figure 16 by the ver-
tical arrow, the resulting phosphorescence does not possess
Figure 14. Luminescence intensity of the Eu3+-4-triazolyl-DPA
complex bearing N,N-dimethyl-2-ethanolammoniomethyl [Eu3] as
a function of the excitation density, upon NIR excitation at
730 nm. The points show that the data and the line represent the
fit as discussed in the text.
Of most importance is that these dependencies are not a clear structure that can be attributed to the pure electronic
linear and diverge from quadratic behaviour as expected by 0–0 component of the first excited T1 triplet state. For this
the TPA antenna effect. This means that a large number of reason, we have considered that the bands recorded in the
relaxation channels contribute to the variation in the T1 NIR wavelength range correspond to the pure electronic
luminescence decay time at room temperature. Independent S0(0)ǞT1(0–2). In spite of this assignment remains subject
of the energy mismatch between the triplet T1 and Eu3+ 5D1 to further justification, notably by recording both excitation
excited level, because of its “relatively” long-lived excitation and phosphorescence spectra at low temperature the de-
state character, the T1 state acts as a “reservoir” from which rived experimental results are unambiguous.
several de-excitation mechanisms can occur; non-radiative
Another conclusion can be drawn related to the differ-
energy transfer or thermal assisted, direct or back energy ences observed between the excitation spectra of the com-
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pyridine cycle offers the possibility to modulate and to ex-
tend the optical excitation wavelength range toward the vis-
ible domain as shown for [Eu2] appended by 1-methyl-
imidazolyl. Non-linear effects attributed mainly to the two-
photon absorption process have been experimentally evi-
denced for either continuous or pulsed excitations. Time re-
solved spectroscopy following short pulsed laser excitation
is used to discuss the energy transfer pathways connecting
the de-excitation channels of the ligands and the emitting-
excited level of europium. The obtained results suggest the
occurrence of direct singlet-to-triplet optical absorption
giving rise to efficient excitation of metal-centered lumines-
cence.
Experimental Section
Figure 16. Room-temperature time resolved emission spectrum (de-
lay: 100 μs, gate: 5 ns) of the Gd3+-4-triazolyl-DPA complex bear-
ing N,N-dimethyl-2-ethanolammoniomethyl [Gd3] upon UV exci-
tation at 302 nm superimposed with the NIR excitation spectrum
of the Eu3+-4-triazolyl-DPA complex bearing N,N-dimethyl-2-eth-
anolammoniomethyl [Eu3] embedded in silica NPs.
Synthesis
4-(4-Phenyl-1H-1,2,3-triazol-1-yl)pyridine-2,6-dicarboxylic Acid (1):
Synthesised as described in the literature.[15]
Dimethyl 4-[4-(1-Methyl-1H-imidazol-5-yl)-1H-1,2,3-triazol-1-yl]-
pyridine-2,6-dicarboxylate (6): Compound 4 (synthesised as de-
scribed in the literature,[7] 850 mg, 3.60 mmol, 1 equiv.) was dis-
solved in hot methanol (0.1 molL–1) and 5-ethynyl-1-methyl-1H-
imidazole (440 μL, 4.32 mmol, 1.2 equiv.) was added. The copper
complex (1 mol-%) was then added in one portion
[CuCl(SIMes)(4,7-dichloro-1,10-phenanthroline)]. The resulting
mixture was stirred at room temperature overnight. The triazole
6 precipitated during the reaction and was recovered by filtration
followed by washing with cold methanol. After filtration, the solid
was dissolved into CH2Cl2 and the resulting organic layer was
washed twice with an aqueous EDTA solution (0.1 m) in order to
remove any copper. The organic layer was dried on MgSO4 and
concentrated under reduced pressure to give the expected diester
as a beige solid (1.13 g, yield: 92%). TLC: Rf = 0.67 (CH2Cl2/
plex [Eu3] and the NPs [NPs-Eu3]. The structure of the
complexes can be considered in terms of a local symmetry
of Eu3+ ions close to a C3 point group symmetry. However,
5
the very low D0Ǟ7F0 transition of Eu3+ (Figure 3) sug-
gests that the crystal field can be interpreted in terms of D3
5
symmetry in which the DE (Electric Dipole) D0Ǟ7F0 and
5D0Ǟ7F3 transitions are forbidden. This observation is
more pronounced in the emission spectrum of the com-
plexes [Eu2], [Eu3] or of the NPs [NPs-Eu3] upon NIR exci-
tation (Figure 12) in which no signal that can be attributed
to these two electronic transitions is recorded. Moreover,
the Magnetic Dipole (MD) transitions are expected to split
into two crystal-field components. Despite the fact that the
emission spectra are not resolved enough, two main contri-
MeOH, 80:20). IR (KBr): ν = 3106, 3043, 2955, 1755, 1716, 1597,
˜
1
1447, 1253, 1126, 1049, 1018, 752 cm–1. H NMR (400 MHz, [D6]-
DMSO): δ = 9.59 (s, 1 H, Htriazole), 8.85 (s, 2 H, Hpyr), 7.80 (s, 1
H, NCHN), 7.39 (s, 1 H, C=CHN), 3.99 (s, 6 H, COOCH3), 3.90
(s, 3 H, NCH3) ppm. 13C NMR (100 MHz, [D6]DMSO): δ = 163.8
(2 C, 2ϫ COOCH3), 149.6 (2 C, 2ϫ CpyrCOOCH3), 144.6 (C,
Cpyr), 140.2 (CH, NCHN), 139.2 (C, Ctriazole), 128.7 (CH,
C=CHNimidazole), 122.4 (C, Cimidazole), 119.6 (CH, CHtriazole), 117.0
(2 CH, 2ϫ CpyrH), 53.0 (2 CH3, 2ϫ COOCH3), 33.0 (CH3,
NCH3) ppm. HRMS-ESI: calcd. for C15H15N6O4 [M + H]+
343.1155; found 343.1155. C15H14N6O4 (342.31): calcd. C 52.63, H
4.12, N 24.55; found C 52.33, H 4.08, N 24.53.
5
butions can be observed for the Eu3+: D0Ǟ7F1 transition.
Such a consideration can be applied for the transition con-
necting the ground S0 state and the T1 triplet state (accord-
ing to our attribution), which is MD transition in nature,
for which two contributions are observed, notably for the
NPs, as shown in the excitation spectra depicted in Fig-
ure 13.
4-[4-(1-Methyl-1H-imidazol-5-yl)-1H-1,2,3-triazol-1-yl]pyridine-2,6-
dicarboxylic Acid (2): A solution of aqueous NaOH (1 m, 2 equiv.)
Conclusions
In this paper, we successfully synthesized three europi- was added to a suspension of 6 in water (1.06 g, 3.09 mmol,
0.2 molL–1). The reaction mixture was refluxed until all of the solid
starting material had disappeared (about 2 h). After cooling to
um(III) and gadolinium(III) 4-triazolyl-DPA complexes,
bearing phenyl [Ln1], 1-methylimidazolyl [Ln2] and N,N-
room temperature, aqueous hydrochloric acid (3 m) was added
dimethyl-2-ethanolammoniomethyl [Ln3] appendages sub-
dropwise until pH = 1 was obtained. The desired diacid precipi-
stituting position 4 of the triazole group, and the derived
tated and was isolated by filtration followed by washing with water,
monodisperse silica nanoparticles (NPs) embedding them.
then drying under vacuum, giving compound 2 as a white solid
The ligands form robust tris-chelate lanthanide(III) com-
(1.03 g, yield: 94%). IR (KBr): ν = 3464, 3248, 3132, 1728, 1597,
˜
plexes. The ligands efficiently sensitize the lanthanide ions
like Eu3+ with absolute quantum yields higher than 50%
for the two complexes [Eu2] and [Eu3] upon UV excitation.
1
1419, 1388, 1222, 1049, 864, 744 cm–1. H NMR (400 MHz, D2O/
NaOD): δ = 8.49 (s, 1 H, Htriazole), 8.14 (s, 2 H, Hpyr), 7.39 (s, 1 H,
NCHN), 6.99 (s, 1 H, C=CHN), 3.63 (s, 3 H, NCH3) ppm. 13C
The appending group substituted on the C-4 position of the NMR (100 MHz, D2O/NaOD): δ = 171.2 (2 C, 2ϫ COOH), 155.2
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(2 C, 2ϫ CpyrCOOH), 144.4 (C, Cpyr), 141.4 (CH, NCHN), 139.5 590 used as the dye solution provides energy up to 400 μJ in the
(C, Ctriazole), 128.7 (CH, C=CHNimidazole), 122.4 (C, Cimidazole), doubled selected UV region or 4 mJ in the H2-Raman Stokes
120.0 (CH, CHtriazole), 115.3 (2 CH, 2ϫ CpyrH), 33.5 (CH3, shifted NIR region. The frequency doubled or Raman shifted dye
NCH3) ppm. C13H10N6O4·HCl (350.7): calcd. C 44.52, H 3.16, N
23.96; found C 43.95, H 3.76, N 23.73.
laser beam is spatially isolated from the fundamental dye laser
beam by two Pellin–Brocca prisms associated with an iris dia-
phragm. In addition, an adequate set of low or long pass-band
filters are used to block any parasitic laser radiation. The red
fluorescence of Eu3+ was analyzed through a Jobin–Yvon HR 1000
monochromator (focal length: 1 m, 1200 groove mm–1 grating and
band-pass of 8 Å mm–1 slits). The detector was a Hamamatsu
R1104 photomultiplier tube. Time-Resolved-Spectra (TRS) were
obtained with the help of an analogic Box-Car averager EG&G
(Model 162/164) triggered by the laser, allowing recording of the
spectra with temporal resolution between 0.1 μs and 50 ms after
the laser pulse. Fluorescence decays were measured with a LeCroy
1 GHz-wave Runner digital oscilloscope.
N-{[1-(2,6-Dicarboxypyridin-4-yl)-1H-1,2,3-triazol-4-yl]methyl}-2-
hydroxy-N,N-dimethylethanaminium Chloride (3): Synthesised as
described in the literature.[5]
Complex [Eu1]: The diacid 1 (100 mg, 0.288 mmol, 3 equiv.) and
Na2CO3 (5 equiv.) were dissolved in deionized water (10 mL). A
solution of europium chloride hexahydrate (35.1 mg, 0.096 mmol,
1 equiv. in 0.5 mL of deionized water) was added. The complex
precipitated and was purified by recrystallisation from deionized
water. The complex was isolated by filtration, washed with deion-
ized water and methanol and then dried under vacuum, giving an
off-white powder (70 mg, yield: 64%). IR (ATR): ν = 3361, 3100,
˜
The resolution of the setup is better than 1 nm for both emission
and excitation. The luminescence spectra were corrected for the
spectral response of the apparatus (Grating, PM, spectral repar-
tition of the Xe lamp).
1619, 1590, 1416, 1379, 1237, 1049, 1011, 805, 759, 736, 690 cm–1.
1H NMR: compound was not soluble enough in the common
NMR solvents.
Complex [Eu2]: The diacid 2 (100 mg, 0.285 mmol, 3 equiv.) and
Na2CO3 (5 equiv.) were dissolved in deionized water (10 mL). A
solution of europium chloride hexahydrate (34.8 mg, 0.095 mmol,
1 equiv., in 0.5 mL of deionized water) was added. The complex
precipitated and was purified by recrystallisation from deionized
water. The complex was isolated by filtration, washed with de-ion-
ized water and methanol and then dried under vacuum, giving yel-
Acknowledgments
The authors are grateful for financial support from the Agence
Nationale de la Recherche, ANR), project Hybiotag-P2N, conven-
tion number 2010 NANO 001 01-03).
low needles (95 mg, yield: 86%). IR (ATR): ν = 3369, 3102, 1615,
˜
1588, 1419, 1373, 1343, 1118, 1049, 1013, 919, 807, 736 cm–1. 1H
NMR (400 MHz, D2O): δ = 7.76 (s, 1 H, Htriazole), 7.56 (s, 1 H,
NCHN), 7.04 (s, 1 H, C=CHN), 4.95 (s, 2 H, Hpyr), 3.50 (s, 3 H,
NCH3) ppm.
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Luminescence Measurements: The internal and external quantum
efficiencies were estimated using the C9920-02G PL-QY measure-
ment system from Hamamatsu. The setup comprised a 150 W mo-
nochromatized Xe lamp, an integrating sphere (Spectralon® Coat-
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detecting the whole spectral luminescence (the emission spectra
were integrated from 576 nm to 728 nm for the europium-based
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.
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Horriba) and analyzed by a TRIAX 550 Jobin–Yvon/Horriba
monochromator equipped with either a R928 Hamamatsu photo-
multiplier (PM) or a nitrogen-cooled CCD camera (Jobin–Yvon
Symphony LN2 series) as detector. CW excitation and emission
spectra were obtained by monitoring the detector response while
scanning the appropriate monochromator. The CW-excitation spec-
tra were obtained by scanning the Xe lamp through the TRIAX
180 and monitoring the red fluorescence of Eu3+. To avoid the
scattering beam from the lamp, a set of low pass-babd filters were
added at the output of the TRIAX 180. The luminescence decays
were obtained by pulsed excitation using a pulsed dye laser (Con-
tinuum ND60) pumped by a Continuum Surelite I-SL10 doubled
Nd:YAG laser (10 ns pulse, 0.1 cm–1 band-width, 10 Hz repetition
rate). The dye laser was followed by a H2-Raman cell (for NIR
excitation) or a KDP frequency doubler (for UV excitation). Rh
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Published Online: November 21, 2014
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim