Click-chemistry-based bis-triazolylpyridine diphosphonate ligand for the sensitized luminescence of lanthanides in the solid state within the layers of γ-zirconium phosphate

Article abstract of DOI:10.1016/j.tetlet.2009.07.027

The synthesis by means of 'click' chemistry of a new ligand bearing the bis-triazolylpyridine motif and pendant phosphonate groups is described. The topotactic phosphate/phosphonate exchange of the ligand into gamma-zirconium phosphate led to an organic-inorganic-layered material which revealed an excellent matrix to achieve the efficient sensitization of emitting lanthanides.

Full text of DOI:10.1016/j.tetlet.2009.07.027

Tetrahedron Letters 50 (2009) 5361–5363
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Click-chemistry-based bis-triazolylpyridine diphosphonate ligand
for the sensitized luminescence of lanthanides in the solid state
within the layers of c-zirconium phosphate
Ernesto Brunet , Olga Juanes, Laura Jiménez, Juan Carlos Rodríguez-Ubis *
*
Departamento de Química Orgánica, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis by means of ‘click’ chemistry of a new ligand bearing the bis-triazolylpyridine motif and
pendant phosphonate groups is described. The topotactic phosphate/phosphonate exchange of the ligand
into gamma-zirconium phosphate led to an organic–inorganic-layered material which revealed an excel-
lent matrix to achieve the efficient sensitization of emitting lanthanides.
Received 17 June 2009
Revised 1 July 2009
Accepted 6 July 2009
Available online 10 July 2009
Ó 2009 Elsevier Ltd. All rights reserved.
The preparation of phosphors or luminescent powders may find
immediate applications in many technological areas (telecommu-
nications, solar energy, artificial photosynthesis, lighting, displays,
photo-signaled molecular recognition, biotechnology, medical
diagnostics, bio-imaging, etc.) related to the broad concept of pho-
tonics, the science and technology for mastering the interaction of
1,2,3-triazol-1,4-diyl)bis(ethane-1,2-diyl) bis-phosphonic acid;
BTP}, synthesized by ‘click-chemistry’ procedures. The resulting
material will be tested as a solid host to efficiently sensitize the
emission of lanthanide ions.
The synthetic route for the BTP ligand is depicted in Scheme 1.
2,6-Diethynylpyridine (1) was synthesized from 2,6-dibromopyri-
dine by Sonogashira coupling according to the literature proce-
1
light with matter. To this effect, the sharp and intense lumines-
6
cence of lanthanides, due to their ff electronic transitions, has lots
of basic and applied research implications,2 yet direct lanthanide
excitation produces weak emission owing to the metal’s low molar
absorptivity. Significantly emission enhancement can in turn result
when the lanthanide metals chelate with organic ligands that effi-
ciently take the light in and effectively transfer the absorbed en-
ergy to the metal. Over the years, our group has succeeded in the
development of a large body of organic molecules based on differ-
ent chromophores which exerted the so-called antenna effect with
dures, followed by hydrolysis with potassium carbonate in THF/
,3
7
methanol. The azido ethylphosphonate derivative 2 was synthe-
sized from commercial diethyl 2-bromoethylphosphonate and
sodium azide in the presence of tetrabutylammonium hexafluoro-
8
phosphate in THF. The coupling of 2,6-diethynylpyridine and the
9
azidophosphonate by means of typical click-chemistry rendered
1
0
the bis-triazolylpyridine derivative 3 which was finally hydro-
lyzed to the bis-phosphonic acid ligand BTP in 71% overall yield
1
1
from 2,6-dibromopyridine.
4
outstanding quantum yields. On the other hand, we are currently
One of the strongest points of layered c-ZrP is its use as 2D tem-
involved in the building of organic–inorganic scaffolds rendering
insoluble, thermally stable solids for a variety of applications.
plate to build 3D materials with aprioristic knowledge of its struc-
ture, following a synthetic rationale similar to that practiced by
The inorganic part is zirconium phosphate in its
c form (c-ZrP)
which has revealed a very versatile carving board where organic
phosphonates and other phosphorous functions can be covalently
attached by topotactic exchange.
i
SiMe
3
N
Br
N
Br
R
R
Previous work published by us evidences that pillared
c
-ZrP
R =TMS
R =H
K CO
96%
2
3
with polyethylenoxygenated chains may constitute a good accom-
modation for the sensitized emission of lanthanides by suitable
chromophores, thus paving the way to the synthesis of highly
luminescent solids for applications in optical devices, the sensitive
detection of lanthanides, or other chemical species among other
1
N
R
iii
N
N
N
R
N
N
N
O
P
O O
3
R = PO
3
Et
2
HCl (37%)
80°C, 24h
97%
ii
P
Br
N
3
O
BTP R = PO
3
H
2
O
2
i
i: Pd(PPh
ii: NaN3, nBu
2
iii: CuSO / 5H O (5%), H
2
O : BuOH (1:4), Na Ascorbate 82%
3
)
4, CuI (4.7%) Toluene,Pr
2
NH, r.t 48h 93%
5
applications. In this Letter we describe the incorporation into
4
NPF6,THF 60°C, 48h 87%
t
´ ´
c-ZrP of a new chromophore, {2,2-[4,4-(pyridine-2,6-diyl)bis(1H-
4
3 4
Scheme 1. Synthetic route for BTP ligand. Reagents and conditions: (i) Pd(PPh ) ,
i
*
Corresponding authors. Tel.: +34 914973926; fax: +34 914973966 (E.B.).
CuI (4.7%), toluene, Pr
2
NH, rt, 48 h 93%; (ii) NaN
3 4 6
, nBu NPF ,THF 60 °C, 48 h, 87%;
E-mail address: ernesto.brunet@uam.es (E. Brunet).
(iii) CuSO /5H O (5%), H
4
2
2
O:tBuOH (1:4), Na ascorbate 82%.
0
040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.07.027

5
362
E. Brunet et al. / Tetrahedron Letters 50 (2009) 5361–5363
organic chemists. In short, laminar
c-ZrP contains two different
kinds of phosphates, one internal which sustains the integrity of
the layers (by being bonded to four different zirconium atoms
through each of its four oxygen atoms) and the other in the lamel-
lae surface pointing to the interlayer region and using only two
oxygen atoms to bond two zirconium atoms. These surface phos-
phates are the ones which can be exchanged by mild hydrothermal
processes with other phosphorous functions topotactically, that is,
maintaining the layered structure intact.12 Layered
c-ZrP was the
first to be exfoliated in order to make its interlayer region accessi-
ble to phosphonic acids, that is, BTP. This can be easily achieved by
colloidally suspending
eral minutes. The suspension was treated with the appropriate
amount of BTP leading to material -ZrP-BTP. Figure 1 shows its
solid state MAS P NMR, which exhibits thee signals at chemical
shifts À27.0 (PO ), À14.5 [O P(OH) ], and 13.1 [O P(OH)-R-
P(OH)O ] ppm. The relative integrals (100:68:30) of these signals
c-ZrP in 1:1 water–acetone at 80 °C for sev-
c
3
1
4
2
2
2
Figure 2. Thermogravimetric profile of material c-ZrP-BTP.
2
in one hand and the weight losses observed in the thermogravi-
metric analysis (TGA; Fig. 2) in the other allowed for the calcula-
Our preliminary results show that the goal of finding ways to
introduce lanthanide ions into a suitable solid material while keep-
ing them brightly luminescent and the solid scaffold intact has
tion of
4 2 4 0.64
c-ZrP-BTP molecular formula as ZrPO (H PO )
(
C
13
H
15
N
7
P
2
O
6
)
0.18Á2H
2
O, in reasonable accordance with the ele-
13
mental analysis (calcd: C, 7.78; H, 2.23; N, 4.89; found: C, 7.32;
H, 2.35; N, 5.33).
been achieved in
acetonitrile solution (10 M) absorption bands at 210 and
c
-ZrP-BTP. The organic ligand BTP showed in
À6
Therefore, experimental data indicates that the topotactic ex-
change occurred by the replacement of ca. 36% of the surface phos-
phonate groups by diphosphonates, that is, there are 18 organic
4
À1
À1
2
90 nm, the latter being quite intense (
probably the most useful one to sensitize the lanthanides. In fact,
the simple suspension of the solid -ZrP-BTP for 48 h at rt in a
.1-M aqueous solution of LnCl (Ln = Tb or Eu ) followed by
e
= 4 Â 10 M cm ) and
c
chains interspersed in the interlayer region of
c
-ZrP per 100 Zr
3+
3+
0
3
atoms or, what is the same, per 100 surface phosphate/phospho-
nate positions. Figure 4 shows an idealized model of a portion of
the material of the aforementioned molecular formula where two
consecutive opposing layers of 25 + 25 surface phosphate/phos-
phonate groups are set at the experimental interlayer distance, ob-
tained from powder DRX (Fig. 3). The model strongly supports that
the inorganic interlayer space is perfectly able to comfortably
house the 9 required BTP chains between the modeled lamellae
containing 50 surface phosphorous sites in opposing faces.
A solid material capable of hosting lanthanides and sensitizing
their emission by the antenna effect should comprise three simple
but stringent conditions: (i) a complex chelating environment to
fulfill the high demanding coordination sphere of lanthanide ions;
centrifugation, water washing, and drying at 100 °C for 3 h ren-
dered a new material containing ca. 20 Ln ions per 100 Zr atoms,
as measured by X-ray fluorescence.
3+
The luminescence measurements of these materials in the solid
3+
state (Fig. 5) did show the typical structured emission of Ln , and
the excitation spectrum displayed bands at 300–310 nm close to
that belonging to the free organic ligand, strongly supporting that
the bis-triazolylpyridine chromophore performs the purported an-
tenna effect when covalently bonded to the galleries of c-ZrP.
With the limited data at hand it is not possible to exactly know
where the lanthanides are located within the material. Yet it is
noteworthy that the number of lanthanide atoms accepted per
1
00 Zr (20) almost matches the number of the organic chains, sug-
(
ii) the presence of a suitable chromophore able to transfer the ab-
sorbed photons to the metal ions; (iii) transparency of the inor-
ganic network at the excitation wavelength. The layered -ZrP
gesting that entering of the metal ions is somewhat conditioned by
the chelating chromophore. In the case of europium, the splitting
c
3+
14
of the ground state levels hints the Eu local symmetry. The
salt is a good candidate for this purpose, since it does not absorb
much visible or UV light. The achievement of the first two condi-
tions may be fulfilled by the afore-synthesized material.
3+
emission spectrum of Eu
ꢀ
c-ZrP-BTP (Fig. 5B) shows single peaks
D ? F and D ? F transitions and low intensity for
0 1 0 2
5
7
5
7
for the
Figure 1. Solid-state MAS (10 kHz) 31P NMR spectrum of material
c
-ZrP-BTP.
Figure 3. Powder X-ray diffraction pattern of material c-ZrP-BTP.

E. Brunet et al. / Tetrahedron Letters 50 (2009) 5361–5363
5363
4 2 4 15 7 6 2
Figure 4. Idealized model for Zr(PO )(H PO )0.64 (C13H N O P )0.18 (see text).
5D
symmetry.
The solid state phosphorescence lifetime of materials Tb
?7F
and 5D
?7F
emissions thus suggesting a distorted D
0
3
0
4
4
Acknowledgments
1
5
3
+
ꢀc
-
Financial support from the Spanish Ministry of Science and Edu-
cation (MAT2006-00570) and indirect funding from ERCROS-
Farmacia S.A. (Aranjuez, Spain) are gratefully acknowledged.
3
+
ZrP-BTP and Eu
time spans which are shorter than those observed for BTP com-
ꢀ
c-ZrP-BTP was 0.7 and 0.3 ms, respectively,
plexes in acetronitrile solution (2.5 and 1 ms for Tb3 and Eu ,
respectively) thus suggesting that one or more non-radiative path-
ways are contributing in the solid state to the deactivation of the
metal-centered excited state, leading to the shortening of lumines-
cence lifetimes. This can be easily explained by the weak vibronic
coupling that should take place among the lanthanides and the OH
oscillators belonging to the phosphate/phosphonate and/or inter-
stitial water.
+
3+
References and notes
1. Escribano, P.; Julian-Lopez, B.; Planelles-Arago, J.; Cordoncillo, E.; Viana, B.;
Sanchez, C. J. Mater. Chem. 2008, 18, 23–40.
2
3
.
.
Werts, M. H. V. Science Prog. 2005, 88, 101–131.
Brunet, E.; Rodriguez-Ubis, J. C.; Juanes, O. Curr. Chem. Biol. 2007, 1, 11–39.
4. (a) Rodríguez-Ubis, J. C.; Sedano, R.; Barroso, G.; Juanes, O.; Brunet, E. Helv.
Chim. Acta 1997, 80, 372–387; (b) Takalo, H.; Mukkala, V. M.; Merio, L.;
Rodríguez-Ubis, J. C.; Sedano, R.; Juanes, O.; Brunet, E. Helv. Chim. Acta 1997, 80,
Summing up, a new ligand based in the bis-triazolylpyridine
motif with pendant phosphonate groups has been synthesized
372–387; (c) Azèma, J.; Galaup, C.; Picard, C.; Tisnès, P.; Ramos, P.; Juanes, O.;
Rodríguez-Ubis, J. C.; Brunet, E. Tetrahedron 2000, 56, 2673; (d) Brunet, E.;
Juanes, O.; Sedano, R.; Rodríguez-Ubis, J. C. Org. Lett. 2002, 4, 213–216; (e)
Brunet, E.; Juanes, O.; Rodríguez-Blasco, M. A.; Garayalde, D.; Rodríguez-Ubis, J.
C. Tetrahedron Lett. 2005, 46, 7801–7805; (f) Brunet, E.; Juanes, O.; Sedano, R.;
Rodriguez-Ubis, J. C. Tetrahedron Lett. 2007, 48, 1091–1094; (g) Brunet, E.;
Juanes, O.; Rodriguez-Blasco, M. A.; Pereira, S.; Rodriguez-Ubis, J. C. Tetrahedron
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and topotactic exchanged into c-ZrP. This new material has dem-
3+
3+
onstrated coordination abilities with Eu and Tb and interesting
luminescent properties. Additional work is in progress to avoid the
shortcoming of the deleterious influence of the OH oscillators and
to measure the quantum yields of emission of c-ZrP-BTP and re-
lated materials in order to improve the properties of these layered
5.
Brunet, E.; Mata, M. J.; Juanes, O.; Rodríguez-Ubis, J. C. Chem. Mater. 2004, 16,
1517–1522.
hybrid materials for its efficient utilization in optical applications.
6. (a) Orita, A.; Nakano, T.; Lie An, D.; Tanikawa, K.; Wakamatsu, K.; Otera, J. J. Am.
Chem. Soc. 2004, 126, 10389–10396; (b) Takahashi, S.; Kuroyama, Y.;
Sonogashira, K.; Hagihara, N. Synthesis 1980, 8, 627–630.
7.
Spectroscopic data of compound 1: Brown solid; 1H NMR (CDCl
, 300 MHz): d
3
13
3
(
.15 (s, 2H), 7.44 (d, J = 7.5 Hz, 2H), 7.64 (dd, J = 0.9, 7.5 Hz, 1H); C NMR
CDCl , 75.5 MHz): d 77.7, 82.1, 127.1, 136.5, 142.7.
Spectroscopic data of compound 2: Yellow oil; H NMR (CDCl
3
1
8
.
3
, 300 MHz): d 1.32
t
= 7.7 Hz, JHP = 18.6 Hz, 2H), 3.53
(
(
1
dt, J = 7.07 Hz, JHP = 0.44 Hz, 6H), 2.04 (dt, J
dt, J = 7.7 Hz, JHP = 12.2 Hz, 2H), 4.11 (m, 4H); C NMR (CDCl
6.3, (d, JCP = 6.1 Hz), 25.91 (d, JCP = 140.8 Hz), 45.32 (d, J = 1.92 Hz), 61.85 (d,
13
t
3
, 75.5 MHz): d
31
J
CP = 6.4 Hz); P NMR (CDCl
Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004–2021.
, 300 MHz): d 1.30
t, J = 7.1 Hz, 12H), 2.47 (dm, J = 18.5 Hz, 4H), 4.10 (m, 8H), 4.71 (dm,
3
, 75.5 MHz): d 26.90.
9
1
.
0. Spectroscopic data of compound 3: Brown oil; 1H NMR (CDCl
3
(
13
J = 12.1 Hz, 4H), 7.85–8.08 (m, 3H), 8.28 (s, 1H); C NMR (CDCl
3
, 75.5 MHz):
d 16.25 (d, JCP = 3.6 Hz), 16.33 (d, JCP = 3.5 Hz), 27.16 (d, JCP = 141.4 Hz), 27.7 (d,
J
J
CP = 141.2 Hz), 44.61 (d,
JCP = 1.9 Hz), 45.31 (d, JCP = 2.1 Hz), 61.89 (d,
CP = 6.6 Hz), 62.16 (d, JCP = 6.6 Hz), 119.17, 122.63, 137.66, 148.26, 149.77;
31
P NMR (CDCl
1. Spectroscopic data of compound BTP: Brown solid; 1H NMR (D
MHz): d 2.24 (dm, J = 18.7 Hz, 4H), 4.52 (m, 4H), 7.94–8.27 (m, 3H), 8.58 (s,
3
, 75.5 MHz): d 25.53.
1
1
2
O–TFA, 300
1
1
(
H); 13C NMR (D
26.61, 138.36, 142.58, 146.95; P NMR (D
: 430.08020 (M+H ), found 430.07884.
2
O–TFA, 75.5 MHz): d 25.91, 27.73, 44.96, 44.99, 123.15,
31
2
O–TFA, 75.5 MHz): d 23.55; HMRS
+
MALDI): m/z calcd for C13
H
18
N
7
O
6
P
2
2. Alberti, G.. In Comprehensive Supramolecular Chemistry; Alberti, G., Bein, T., Eds.;
Pergamon: New York, 1996; Vol. 7, p 151; Clearfield, A.; Costantino, U.. In
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New York, 1996; Vol. 7, p 107.
1
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Sildos, I. Phys. Solid State 2008, 50, 1727–1730; Tong, B. H.; Wang, S. J.; Jiao, J.; Ling,
F. R.; Meng, Y. Z.; Wang, B. J. Photochem. Photobiol., A: Chem. 2007, 191, 74–79.
4. Chandler, B. D.; Yu, J. O.; Cramb, D. T.; Shimizu, G. K. H. Chem. Mater. 2007, 19,
1
4467–4473.
Figure 5. Emission and excitation spectrum of the solid
2
c
-ZrP-BTP containing ca.
15. Luminescence spectra were recorded on a Varian spectro-fluorimeter Cary
Eclypse and they are corrected for emission following internal protocols.
0 ions of Tb3 (green) and Eu (red) per 100 Zr atoms. kexc = 300 nm.
+
3+