Perfluorinated nitrosopyrazolone-based erbium chelates: A new efficient solution processable NIR emitter

Article abstract of DOI:10.1039/b906494j

We show the design and synthesis of new perfluorinated nitrosopyrazolone- based ligands and the original method employed for their complexation of erbium ions in the presence of the co-ligand perfluorotriphenylphosphine oxide; the resulting chelate is non

Full text of DOI:10.1039/b906494j

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Perfluorinated nitrosopyrazolone-based erbium chelates: a new efficient
solution processable NIR emitterw
Luca Beverina,* Maurizio Crippa, Mauro Sassi, Angelo Monguzzi, Francesco Meinardi,
Riccardo Tubino and Giorgio A. Pagani*
Received (in Cambridge, UK) 1st April 2009, Accepted 9th July 2009
First published as an Advance Article on the web 24th July 2009
DOI: 10.1039/b906494j
We show the design and synthesis of new perfluorinated nitroso-
pyrazolone-based ligands and the original method employed for their
complexation of erbium ions in the presence of the co-ligand
perfluorotriphenylphosphine oxide; the resulting chelate is non-
hygroscopic, solution processable and possesses a NIR emission with
lifetimes as long as 16 ls.
coordinating solvent molecules. In this respect the presence
of O–H, N–H and even C–H groups in close proximity
to erbium is a well known and a very efficient relaxation
pathway.5 In the case of erbium tri(hydroxyquinolinate)
(ErQ3), the most studied erbium chelate, the excited state
lifetime drops from the milliseconds commonly observed
in erbium doped glass-based devices to a mere 0.2–2 ms
depending on the environment.6
Interest in the development of efficient NIR emitting organic
derivatives is connected with the recent development of plastic
optical fibers.1 While in fact it is accepted that silica-based optical
technologies will remain unchallenged for long distance informa-
tion transmission, short distance telecommunications, such as
LAN, are expected to profit from alternative, low cost, plastic-
based solutions. However, the more the absorption (and
emission) of an organic compound is shifted towards low
energies, the less the compound becomes able to stand long term
exposure to ambient atmosphere, especially under the device
operating conditions (illumination, temperature, moisture...).
This observation indicates how challenging the task of designing
efficiently NIR operating organic materials may be.
Recently, the use of fluorinated ligands afforded a viable
solution to the vibrational quenching issue.7 The most notable
drawback of this approach is the relatively high absorption
energy of the ligands employed so far, whose absorption
cut-off scarcely reaches the 400 nm wavelength.8 In view of
practical applications it would be highly profitable to employ
ligands having absorption spectra compatible with broad-
band, low cost light sources such as high-power LED now
available in the near UV region, i.e. 407 nm GaN laser diodes.
Moreover most of the known perfluorinated chelates have
been processed by high temperature vacuum evaporation, a
relatively high cost process especially when compared with
spin or blade coating.
The development of lanthanide-based emitting chelates
represents a valuable solution. These particular compounds
profit from the favorable properties of a conjugated organic
ligand (efficient absorption in the UV-Vis spectrum, stability,
solubility, compatibility with plastic fibers and polymers
in general) and from the peculiar emission capabilities of lanthanide
We here propose a new class of perfluorinated ligands,
pertaining to the class of nitrosopyrazolone derivatives,
possessing efficient erbium chelating capabilities, relatively
low band gaps and facile preparation procedures. Moreover,
the compounds we obtained are highly soluble in common
organic solvents, thus enabling the preparation of films by spin
coating, and they are completely hydrophobic.
4
ions, in this case the Er31 4I13/2 - I15/2 transition at 1550 nm,
matching well the POF (plastic optical fiber) transmission–loss
dispersion minima.2
The new ligand 2 was prepared according to the classic
pyrazolone synthetic scheme. Pentafluorophenyl hydrazine
was thus reacted with ethyl 4,4,4-trifluoroacetoacetate in
refluxing acetic acid and under microwave irradiation to give
the pyrazolone derivative 1, nitrosation of which with
isoamylnitrite in THF afforded the pure ligand 2 in an overall
yield of 30%. See Scheme 1.
Lanthanide ions cannot be efficiently pumped directly; their
absorption transitions are in fact prohibited on symmetry
grounds. The population of the rare-earth excited states is
best achieved through energy transfer from suitable organic
ligands.3 This strategy has been so far successfully applied to
several lanthanides, including europium, whose complexes are
widely employed as emitting materials in LED technologies.4
The case of erbium chelates is however more complex.
Erbium emission is in fact particularly prone to non-radiative
relaxation due to dipole coupling of the ion excited states with
the high energy vibrational modes of the ligands and the
A slight modification of the same procedure afforded ligand
3, the hydrogenated analog of ligand 2 which was made for
comparison (see the ESIw).
University of Milano-Bicocca, Department of Materials Science,
20215, Milan, Italy. E-mail: luca.beverina@mater.unimib.it
w Electronic
supplementary
information
(ESI)
available:
Synthetic procedures, IR-spectra and DSC traces for ligands and
complexes. Details on the photophysical characterization. See
DOI: 10.1039/b906494j
Scheme 1
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The structure of the nitrosopyrazolone ligands resembles
that of a b-diketone, whose efficient chelating and excitation
transfer capabilities towards lanthanides are well known.9
Also, in a recent paper Zhang and co-workers demonstrated
the favorable chelating capabilities of acylated pyrazolone
derivatives towards erbium ions in the framework of NIR
electroluminescent organic devices.10 When compared to
either standard or perfluorinated b-diketonates, ligand 2
shows a more extended conjugated structure, a design feature
intended to enhance the ligand visible region absorption
capabilities.
Scheme 3
Hydrogenated and fluorinated ligands were converted to the
corresponding potassium salt and reacted with a stoichiometric
amount of ErCl3, in water in the case of 3 and in a water–
chloroform biphasic mixture in the case of ligand 2. To the best of
our knowledge the latter is an original, and resourceful, synthetic
procedure.11 In all of the attempts we made to react ligand 2
under homogeneous conditions we recovered a light pink, very
insoluble precipitate that was identified as Er(OH)3.
Interestingly, while in the case of 3 we obtained the
trihydrated chelate 4, ligand 2 gave, under the described
biphasic conditions, the tetrakis compound 5. In both cases
the Er coordination number is 9, a common value observed in the
lanthanide series. We speculate that the strong hydrophobic
character imparted to the chelate by the presence of the
fluorinated ligand 2 prevents more than one molecule of water
from entering the coordination sphere. See Scheme 2.
The residual water molecule was successfully removed from
the coordination sphere of complex 5 by reaction with an
excess of perfluorotriphenylphosphine oxide 6 in refluxing
hexane. The hydrated complex, insoluble in hot hexane,
gradually dissolves upon the addition of the phosphine oxide.
See Scheme 3.
As shown in Fig. S1 of the ESIw, the final complex was
isolated in a completely anhydrous form. In particular the IR
spectrum of 8 shows the complete disappearance of any band
in the 4000–2000 cmÀ1 region, in contrast to the spectrum of 5
where the O–H stretching mode of both the coordinating H2O
molecules and of the ligand 2 are clearly visible. We stored 7
and 8 under ambient atmosphere without taking any particular
precautions and did not observe any water uptake even 6
months after the original preparation.
Fig. 1 shows the UV-Vis absorption spectrum of chelate 8 in
a 1.2 Â 10À5 M CDCl3 solution. The inset shows a snapshot of
the 500–700 nm region absorption spectrum of a 10À4
M
chelate solution. The H11/2 and F9/2 Er31 absorption lines,
although overlapping with the ligand onset absorption, are
clearly distinguishable.13
2
4
The relatively low energy gap of the chelates enabled us
to carry out time resolved luminescence measurements at the
355 nm excitation wavelength. Then, we characterized chelates
4, 5, 7 and 8 in solution as well as in powder form and as cast
films. The characterization of hydrogenated complex 4 is
reported in the ESIw. For each sample we observed the
expected characteristic emission around 1530 nm, due to the
The complex 7 separated, after a few minutes of reflux, as a
light orange solid and was immediately hot filtered. Cooling of
the filtrate afforded a second light yellow precipitate that was
identified as derivative 8.
4
Er31 4I13/2 - I15/2 transition. We did not observe any direct
emission from the ligand. This feature suggests a complete
energy transfer to the erbium ion and enables the evaluation of
the emission efficiency on the basis of the measured lifetime.7
Fig. 2 shows a representative PL (photoluminescence) spectrum
and decay profile of a powder sample.
Both chelates are soluble in common organic solvents,
derivative 8 in particular is highly soluble in hexane and
CCl4. All chelates were characterized by FT-IR (ATR mode)
and elemental analysis (C, H, N).12
Fig. 1 UV-Vis absorption (CDCl3 solution, 1.2 Â 10À5 M, 1 cm
optical path) of derivative 8. Inset: snapshot of a 10À4 M solution
absorption.
Scheme 2
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In conclusion, we believe that the exploitation of both the
ligand 2, providing an efficient antenna effect, and the
co-ligand 6, having a crucial role in the complete removal of
water from the erbium coordination sphere in both 7 and 8,
represents an important entry in the growing field of NIR
emitting organic materials. This is in accordance with the three
most favorable features they possess: ease of preparation, high
solubility providing processability, low optical gap enabling
visible region pumping by commercially available LEDs. We
expect to be able to further increase the performance of the
complexes through the development of different functionaliza-
tion patterns around the nitrosopyrazolone structure.
This work has been supported by the INSTM Consortium
(PRISMA 2007), Ministero Istruzione, Universita e Ricerca
(PNR-FIRB RBNE033KMA) and by the Fondazione Cariplo
(2005, research title: Compositi polimerici a base di innovativi
complessi organolantanidi per amplificatori ottici usati nelle
telecomunicazioni).
Fig. 2 PL spectrum (red line) and decay profile (inset, green dots) of a
powder sample of chelate 7 excited at 355 nm. The red straight line is
the fit of the luminescence decay with a single exponential function.
Table 1 PL lifetime data for 4, 5, 7 and 8 for various sample types
Sample type
and complex
PL lifetime
t1/ms
Sample type
and complex
PL lifetime
t1/ms
Notes and references
4
Solution (CCl4)
7
—
1 K. Kuriki, Y. Koike and Y. Okamoto, Chem. Rev., 2002, 102,
2347; K. Kuriki, S. Nishihara, Y. Nishizawa, A. Tagaya, Y. Koike
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3 N. Sabbatini, M. Guardigli and J. M. Lehn, Coord. Chem. Rev.,
1993, 123, 201.
5.4
Solution (CCl4)
Solution (CDCl3)
Solid
Film
8
16.0
14.4
15.7
15.2
—
5
Solution (CCl4)
Powder
5.4
5.1
Solution (CCl4)
Powder
14.7
14.5
4 P. P. Sun, J. P. Duan, H. T. Shih and C. H. Cheng, Appl. Phys.
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5 V. L. Ermolaev and E. B. Sveshnikova, Russ. Chem. Rev. (Engl.
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The luminescence decay of the emission signal at 1530 nm
was shown to be best described by a single exponential process
(powder samples). This demonstrates that even in the solid
state erbium ions do not interact strongly, thus avoiding
additional deactivation channels for the excitation energy.
Table 1 summarizes the results obtained with the various
samples we measured. The results clearly indicate that the
lifetimes are significantly enhanced by ligand fluorination and
complete removal of any hydrogen containing molecule.
Chelate 7 in particular shows a solid powder emission lifetime
of 15.7 ms which is substantially preserved upon deposition as
a spin coated film.
As we mentioned above, the PL natural lifetimes for erbium
ions are of the order of milliseconds. We speculate that the still
somewhat short lifetime of the anhydrous complexes 7 and 8
could be due to the presence of trace amounts of hydrogen
containing impurities coming from the employed starting
materials.
10 Z. Li, J. Yu, L. Zhou, H. Zhang, R. Deng and Z. Guo, Org.
Electron., 2008, 9, 487.
11 For a comprehensive review of the synthetic methods so far
employed for the preparation of rare-earth chelates see:
R. Van Deun, P. Fias, P. Nockemann, A. Schepers, T. N. Parac-Vogt,
K. Van Hecke, L. Van Meervelt and K. Binnemans, Inorg. Chem.,
2004, 43, 8461.
12 Strongly electron deficient nitroso compounds could possess
low thermal stability and a tendency to explosive decomposition.
We thus submitted ligand 2 and the corresponding chelate 8 to
DSC characterization. The DSC traces reported in the ESIw show
that both compounds are stable well above the corresponding
melting point (the decomposition temperature onset for both
derivatives is around 170 1C). Their exothermic decomposition
peak is broad, thus excluding any possible explosive behavior.
13 G. A. Kunmar and R. E. Riman, Chem. Mater., 2005, 17, 5130.
Structural studies are currently being performed in order
to resolve the complex crystal structure and the possible
occurrence of polymorphism. Moreover, studies will be
directed at the complete characterization of the specific energy
transfer processes taking place between ligand and ion, with
particular focus on the role of perfluorination in the overall
process efficiency.
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This journal is The Royal Society of Chemistry 2009
Chem. Commun., 2009, 5103–5105 | 5105