An efficient triazole-pyridine-bistetrazolate platform for highly luminescent lanthanide complexes

Article abstract of DOI:10.1039/c4cc04060k

Two new triazole-pyridine-bistetrazolate ligands were synthesized via a versatile procedure that allows for further derivatization; their corresponding homoleptic tris-ligand nona-coordinated lanthanide complexes are highly luminescent in the solid state and in a PVA polymeric matrix with measured values for the luminescence quantum yield of 70(7) and 98(9)% for Eu ^III and Tb^III, respectively. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04060k

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An efficient triazole-pyridine-bistetrazolate
platform for highly luminescent lanthanide
complexes†
S. Di Pietro,ab D. Imbert*ab and M. Mazzanti*ab
Cite this: Chem. Commun., 2014,
50, 10323
Received 27th May 2014,
Accepted 15th July 2014
DOI: 10.1039/c4cc04060k
www.rsc.org/chemcomm
Two new triazole-pyridine-bistetrazolate ligands were synthesized
via a versatile procedure that allows for further derivatization; their
corresponding homoleptic tris-ligand nona-coordinated lanthanide
complexes are highly luminescent in the solid state and in a PVA
polymeric matrix with measured values for the luminescence quantum
yield of 70(7) and 98(9)% for EuIII and TbIII, respectively.
Due to their remarkable intrinsic photophysical properties (narrow
emission lines, large quantum yields and high resistance to
photobleaching), lanthanide ions have attracted increasing atten-
tion for a broad range of applications ranging from materials
science to bioanalysis.1 However, due to their low absorption
coefficients, the stable complexation of lanthanide ions with
sensitizing ligands such as b-diketonates, aromatic carboxylates
or heterocyclic compounds, capable of efficient energy transfer
to the lanthanide excited state, is essential for the development
of such applications.2 Numerous efforts have been directed
towards the understanding of the energy transfer between
Scheme 1 (a) NaN3/NH4Cl, DMF, (b, c) n-octyl-N3/K2CO3/CuSO4/sodium
ascorbate/t-BuOH/H2O and NaN3/NH4Cl/DMF, (d) Et3N, 0.33 eq. Ln(OTf)3.
ligands and lanthanide ions with the aim of obtaining high such as dpa2À for both the Eu and Tb ions while maintaining
luminescence quantum yields. Indeed high photoluminescence comparable stability with respect to dissociation.
quantum yields and reasonable stability are crucial for the
Here we report two new triazole-pyridine-bistetrazolate ligands
technological application of lanthanide complexes in the areas prepared via a versatile synthetic procedure (Scheme 1) which
of energy conversion (luminescent dyes or solar concentrators) show that the derivatization of the pyridine-ditetrazolate scaffold
or in devices such as light emitting diodes.3 However, lanthanide with a 1,2,3-triazole ring using click chemistry leads to an
complexes showing a quantitative ligand-to-metal energy transfer optimized energy transfer, affording lanthanide complexes with
leading to luminescence quantum yields higher than 90% remain very high luminescent quantum yields, up to 70% for Eu and
extremely rare.4 Recently we have reported a new class of ditetra- 98% for Tb.
zolate ligands containing bipyridine, terpyridine, hydroxyquinoline
The ligands H3L1 and H2L2 were synthesized in six and seven
and pyridine chromophores.5 These complexes demonstrated an steps from chelidamic acid with global yields of 38 and 22%,
extension of the absorption windows and higher luminescence respectively. The chosen synthetic route allows the synthesis of
quantum yields compared to analogous dicarboxylate derivatives both ligands from the same alkyne intermediate in just one or two
steps based on click chemistry. We anticipate that this versatile
route can be used for the synthesis of a wide range of N-substituted
a CEA, INAC-SCIB, F-38000 Grenoble, France. E-mail: Daniel.imbert@cea.Fr,
triazole derivatives. The 1H NMR spectrum in d6-DMSO shows, for
Marinella.mazzanti@cea.fr; Fax: +33 04 38 78 50 90; Tel: +33 04 38 78 38 87
b Univ. Grenoble Alpes, INAC-SCIB, F-38000 Grenoble, France
both the ligands, the labile tetrazole protons and in the case of L1
also the triazole proton (Fig. S1, ESI†). The pKa’s of L1 (pKa1 = 7.7(1)
† Electronic supplementary information (ESI) available: General methods, synth-
pKa2 + pKa3 = 9.2(1)) were determined in water by UV absorption
spectroscopy. These values indicate that the tetrazole group can
esis, equipment, absorption, emission and excitation spectra. See DOI: 10.1039/
c4cc04060k
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be deprotonated using triethylamine as a base while under
these conditions the triazole remains protonated.
The UV-visible absorption spectra of H2L1 and H2L2 feature
two main bands located at around 227 and 303 nm assigned to
the p - p* transitions mainly located on the pyridine. Upon
ligand deprotonation these bands are slightly shifted with the
appearance of an additional absorption band at 250 nm.
The absorption bands of the deprotonated ligands are slightly
red-shifted by ca. 15–20 nm upon complexation to EuIII (Fig. S3,
ESI†). The complexation of EuIII was monitored in MeOH (2.5 Â
10À5 M) by UV spectroscopy and the titration data could be
fitted to the following model with Li = L1 and L2 (charges
omitted for the sake of simplicity):
Fig. 1 Diagram of the PERSEUS optimized solution structure of [Eu(L1)3]3À
(colour code: europium, red; nitrogen, blue; carbon grey; hydrogen,
white). View along the threefold symmetry axis (left) and perpendicular
to it (right).
Eu + Li 2 Eu(Li)n
(1)
(2)
n
This structure (Fig. 1) compares well (see ESI†) with the
solid state structure of the closely related [Ln(pytz)3](Et3NH)3
complexes reported by our group in 2012.5b The coordination
polyhedron around the Eu cation is best described as a slightly
distorted tricapped trigonal prism. The similarity of the chemical
shifts of the paramagnetic [Ln(Li)3](Et3NH)3 complexes indicates
that, as anticipated, the presence of the n-octyl substituent on the
triazole does not significantly affect the solution structure.
Photophysical data have been collected both in the solid
state and in methanol solution for [Ln(Li)3](Et3NH)3 (Ln = Eu,
Tb, Nd, Yb, Fig. 2a and ESI†) and they show that ligands L1 and L2
efficiently sensitize the lanthanide emission both in the visible
and NIR ranges. In solution, the ligand emission levels were
determined for L1 and L2 through UV excitation in the (n,p) - p*
absorption bands. For L2 the emission of the 1pp* and 3pp* states
occur at 28 600 and 24 800 cmÀ1, respectively. For L1 they are
found at 28 450 and 23 900 cmÀ1 (23 100 cmÀ1 in the solid state),
log bn = [Eu(Li)n]/[Eu]Á[Li]n
They are consistent with the presence of four absorbing species
(Li, [Ln(Li)3], [Ln(Li)2] and [Ln(Li)]). The lower values of log b1,
log b2, and log b3 determined for L1 (6.3(2), 10.2(5), 17.2(3))
compared to L2 (7.2(3), 11.8(3), 18.6(4)) indicate slightly reduced
stability of the complexes formed by L1. These values are similar to
those reported for the trianionic homoleptic Eu(III) complexes of
the dipicolinate ligand (dpa2À).6 These results are in line with those
observed for terpyridine-based tetrazolate ligands5a and indicate
that tetrazolate- and carboxylate-based ligands afford lanthanide
complexes of comparable stability. A sizeable increase of both the
absorbance red shift and the absorption coefficient is also observed
for the triazole-substituted L1 and L2 compounds compared to the
pyridine-bistetrazolate analogues (Fig. S3, ESI†).5b
The homoleptic complexes [Ln(Li)3](Et3NH)3 (Ln = La, Pr, Eu,
Tb, Li = L1, L2) have been prepared by reacting three equiva-
lents of L1 or L2 with one equivalent of lanthanide triflate in
methanol solution in the presence of triethylamine as shown in
Scheme 1. Both complexes are soluble in methanol, those of L1 are
also soluble in water while the n-octyl group in L2 renders its
complexes highly soluble in CH2Cl2. The complexes have been
characterized by proton NMR spectroscopy and mass spectro-
metry. The 1H spectra of all complexes in MeOD (Fig. S4–S8, ESI†)
show the presence of only one set of signals with two and six
resonances, respectively, for L1 and L2. These features are consis-
tent with the presence of the undissociated, rigid, D3-symmetric
[Ln(Li)3]3À solution species on the NMR timescale. Similar features
were found in the closely related homoleptic complex
[Ln(pytz)3](Et3NH)3 (H2pytz = 2,6-bis-tetrazolyl-pyridine).5b
The solution structure of the [Eu(L1)3]3À anion was deter-
mined by paramagnetic NMR spectroscopy.7 The proton and
carbon resonances of the [Ln(L1)3](Et3NH)3 (Ln = La, Pr, Eu)
complexes were assigned using 1D- and 2D-NMR experiments
(HSQC, HMBC, 13C-NMR) with Ln = La, Pr and Eu. The separa-
tion of the PCS terms from the FC term, crucial for the
structural determination, was achieved using the NMR data
of the Pr and Eu complexes by the ‘‘two lanthanide method’’
developed by Di Bari et al.8 These PCS values were used together
with the relaxation rates for the structural optimization by
means of the PERSEUS program.9
Fig. 2 (a) Absorption, excitation, singlet, triplet and emission spectra of
[Ln(L2)3](Et3NH)3 in MeOH (Ln = Eu, Tb, Nd, Yb, Gd); (b) flexible PVA films
doped with [Ln(L2)3](Et3NH)3 complexes excited or not using a UV lamp
(Tb left and Eu right).
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Table 1 Lifetimes (ms for Eu–Tb and s for Gd complexes) and absolute
quantum yields (%) measured at 298 K in the solid state, MeOH (lex = 325 nm)
and in PVA films (lex = 335 nm)
respectively, in agreement with the increased conjugation. The
3pp* mono-exponential luminescence decay is unusually long
at 3.1(4) s, close to that obtained for benzoic acid.10
The luminescence emission spectra of the LnIII complexes of
L1 and L2 are consistent with the empirical rules defined for an
optimal ligand-to-metal transfer process,2b,11 since emissions
from both the singlet and triplet states are not observed. It is worth
noting that the metal-centered luminescence is efficiently sensi-
tized, in spite of the relatively large energy gaps between the ligand
triplet state and the EuIII accepting level with DE(3pp*–5D0) E
6600 for L1 and 7560 cmÀ1 for L2. The energy gap between the
TbIII accepting level and the L1 and L2 triplet state is optimum
for a quantitative energy transfer with DE(3pp*–5D4) E 3410 and
4310 cmÀ1, respectively.2b The emission spectra of [Eu(Li)3](Et3NH)3
in MeOH (see Fig. 2a and ESI†), through direct excitation of the
ligand, exhibit the characteristic D0 - FJ transitions and in
spite of the low resolution of the emission spectrum, the crystal
field splitting can be interpreted in terms of an average D3
symmetry point group (in which 7F0 is forbidden), in agreement
with the solution structure determined by NMR. The solid state
emission spectra are identical to those observed in solution
both in shape and intensity, indicating the presence of the
same structure for all the complexes.
Li
Ln
tsolid
tPVA
tMeOH
Fsolid FPVA
FMeOH
L2 Gd(3pp*)
Eu(5D0)
—
—
1.85(1)
—
—
3.4(1)
2.68(4) 2.80(6) 2.98(9) 70(7)
1.29(2) 1.29(3) 1.61(5) 98(9)
—
2.21(3)
0.93(2) 0.70(2) 1.62(2)
70(3) 43(4)
96(4) 94(9)
Tb(5D4)
L1 Gd(3pp*)
Eu(5D0)
—
—
2.05(2)
3.02(5) 42(4)
9(1)
—
—
—
3.4(1)
41(5)
Tb(5D4)
18(1) 79(8)
Preliminary studies show that these ligands can also sensitize
the NdIII- and YbIII-centered NIR luminescence emission in
MeOH (Fig. 2a) with sizeable measured luminescence quantum
yields of 0.023 and 0.13%.
The [Ln(L2)3](Et3NH)3 complexes (Ln = Eu and Tb) are easily
incorporated within flexible PVA films affording doped polymers
(Fig. 2b) with photophysical properties matching those obtained
for the pure complexes in the solid state (Table 1). Thus, the
excellent brightness of these systems is preserved in the polymer
and could be useful for photonic device applications.
In conclusion, we have shown that L1 and L2 form soluble and
stable homoleptic 3 : 1 complexes and sensitize very efficiently the
emission of EuIII and TbIII. For the Tb complex, the measured value
of the absolute quantum yield of B100% indicates the presence of
a quantitative energy transfer from the ligand to the metal which
has only been observed once before. The versatile procedure
developed for the synthesis of L1 and L2 allows access to complexes
with different solubilities and provides a facile route for grafting
or encapsulating these complexes in different substrates. Thus,
the triazole-pyridine-bistetrazolate motif provides a very attractive
platform with optimum energy transfer which is crucial for the
application of lanthanide complexes in optical devices.
The authors thank Colette Lebrun and Lydia Plassais (CEA-
Grenoble) for the support in synthesis and ES-MS spectroscopy
and Prof. L. Di Bari for the fruitful discussion. The authors
gratefully acknowledge financial support from the ‘‘French Agence
Nationale de la Recherche’’, grant ANR-13-BS08-0011-01 and
from Labex Arcane, ANR-11-LABX-003-01.
5
7
The luminescence decays for [Eu(Li)3](Et3NH)3 are mono-
exponential in the 2.21–3.02 ms range and confirm the absence
of solvent in the first coordination sphere of the lanthanide ion.
Eu
tot
The values of the absolute luminescence quantum yield F
measured in MeOH amount to 40–42% and increase from 41 to
70% passing from L1 to L2 in the solid state. The latter value is
among the highest reported in the literature for europium
Eu
complexes (60–76%).5b,12 Since F = nsensÁFEEuu, we have deter-
tot
mined the intrinsic quantum yield F of EuIII upon direct f–f
Eu
Eu
excitation, in order to better understand the origin of the high
measured absolute quantum yields.6a,13,14
The obtained values of nsens at 0.9 and 0.73 in the solid state
and MeOH, respectively, for L2 (0.65 and 0.71 for L1) are in
perfect agreement with a very efficient metal-centred emission.
The [Tb(Li)3](Et3NH)3 complexes display the typical lanthanide
emission spectrum 5D4 - 7FJ transitions (Fig. 2a and ESI†) and
monoexponential luminescence decays (1.6 ms in MeOH and
0.93–1.29 ms in the solid state for L1 and L2, respectively).
The values of the absolute luminescence quantum yield of
[Tb(L2)3](Et3NH)3 both in MeOH and in the solid state are very
high at 98(9)%. Only one example of a ligand architecture
leading to a quantitative quantum yield for terbium emission
has been reported to date.4 The value of the luminescence
quantum yield measured for [Tb(L1)3](Et3NH)3 is very high at
79% in MeOH but is dramatically reduced to 9% in the solid
state. This can be explained by the decrease of the DE(3pp*–5D4)
energy gaps going from solution to the solid state, allowing the
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