Dramatic increases in the lifetime of the Er3+ ion in a molecular complex using a perfluorinated imidodiphosphinate sensitizing ligand

Article abstract of DOI:10.1021/ja0441864

Dramatic increases in the luminescent lifetime of the Er³⁺ ion in a molecular complex have been observed by chelating the rare-earth ion with a perfluorinated imidodiphosphinate sensitizing ligand, F-tpip. For solution, powder, and evaporated t

Full text of DOI:10.1021/ja0441864

Published on Web 12/21/2004
Dramatic Increases in the Lifetime of the Er3+ Ion in a Molecular Complex
Using a Perfluorinated Imidodiphosphinate Sensitizing Ligand
Gaetano Mancino,† Andrew J. Ferguson,† Andrew Beeby,‡ Nicholas J. Long,*,† and Tim S. Jones*,†
Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London, SW7 2AZ, U.K.,
and Department of Chemistry, UniVersity of Durham, South Road, Durham, DH1 3LE, U.K.
Received September 24, 2004; E-mail: t.jones@imperial.ac.uk (T.S.J.); n.long@imperial.ac.uk (N.J.L.)
Scheme 1. Synthesis of HF-tpip and Er(F-tpip)3
There is a growing demand for low cost near infrared (NIR)
light sources for communications, sensing, and analytical detection.
Organic semiconductors are especially attractive due to their
potential for low-cost fabrication and their proven capabilities as
visible light sources. However, progress toward efficient NIR-
emitting organic light-emitting devices (OLEDs) has, to date, been
limited due to the relative scarcity of organic lumophores with
efficient NIR emission. Several rare-earth ions do exist that emit
in the NIR region, in particular, Er3+, Nd3+, and Yb3+, and there
have been several attempts to utilize the emission from these ions
in organolanthanide complexes.1
Luminescence from trivalent lanthanide ions (Ln3+) arises from
electronic transitions between the 4f orbitals. These transitions are
forbidden on symmetry grounds, leading to poor absorption cross
sections and relatively long-lived excited states.2 Consequently,
population of the emitting levels of the Ln3+ ion is best achieved
by employing light-harvesting ligands (antenna chromophores) that
can sensitize the metal ion by intramolecular energy transfer.2,3
It is well established that NIR-emitting Ln3+ ions are particularly
prone to vibrational deactivation. Species containing high-energy
oscillators, such as C-H and O-H bonds (typically found in the
ligand, coordinated solvent and moisture), are able to quench the
metal excited states nonradiatively, leading to decreased lumines-
cence intensities and shorter excited-state lifetimes.4 For example,
photoluminescent (PL) studies of erbium tris(8-hydroxyquinolate),
ErQ3, shows lifetimes in the region of 0.2-2 µs depending on the
ErQ3 environment.5 These observed lifetimes are much shorter than
the natural radiative lifetime of the Er3+ I13/2 f I15/2 transition,
implying that the quinolate ligand is unable to protect the metal
ion from deleterious solvent/moisture coordination. This problem
can be reduced by using sterically demanding ligands that encap-
sulate the metal within a hydrophobic shield. One such ligand is
tetraphenylimidodiphosphinate (tpip)6 and its O-methylated ana-
logue, Metpip, which have been used as the basis for several Ln-
(tpip)3 complexes (Ln ) Dy, Eu, Sm, and Tb).7,8
ligand, tetrapentafluorophenylimidodiphosphinate (HF-tpip), has
been synthesized, first by preparing the appropriate fluorinated
phosphine precursor, (C6F5)2PCl,12 followed by coupling this to
hexamethyldisilazane in a method analogous to that of Pikramenou
et al.8 The full synthetic scheme for HF-tpip is shown in Scheme
1. After formation of the potassium salt of the ligand (KF-tpip),
the metal complex, Er(F-tpip)3, was formed and purified by vacuum
sublimation between 260 and 280 °C at 10-6 mBar, which is
approximately 50-60 °C lower than the temperature required to
sublime the nonfluorinated Er(tpip)3 derivative. Fluorination in-
creases the volatility of this material, and the lower sublimation
temperature is advantageous with respect to fabricating films and
devices using vapor deposition techniques.
4
4
Time-resolved PL13 measurements were carried out on the Er-
(F-tpip)3 complex as a powder, a 500 nm evaporated thin film on
quartz, and in CDCl3 solution. These were compared with similar
measurements performed on the Er(tpip)3 complex.
Hasegawa et al. have shown that replacement of C-H bonds in
â-diketonate ligands with lower-energy C-F oscillators leads to
longer luminescence lifetimes for the Nd3+ ion in solution.9 These
studies have mainly focused on direct excitation into the metal
excited states and describe only one example where a pentafluo-
rophenyl moiety is used as an antenna chromophore.10 Similar work
has also been performed by Zheng et al., who report increased
luminescent intensities for Tb3+-based OLEDs when the metal ion
is coordinated by fluorinated â-diketonate ligands.11
In this communication, we show how the encapsulation properties
of the tpip moiety coupled with the advantages afforded by
fluorination lead to dramatic improvements in the PL properties of
Er3+-based organolanthanide complexes. A new bidentate chelating
For each sample, the characteristic emission around 1530 nm,
due to the intraconfigurational 4I13/2 f 4I15/2 transition, is observed
upon excitation at 266 nm into the phenyl absorption band of the
ligand. A representative PL spectrum and decay profile (shown as
inset) for Er(F-tpip)3 in CDCl3 are shown in Figure 1.
The Er3+ emission observed for the Er(F-tpip)3 complexes is
sharp and shows distinct fine structure, which is in contrast to many
reported emission spectra of other erbium chelates, notably, ErQ3.5
The time-dependence of the Er3+ emission was examined,
following excitation at 266 nm, and the results are summarized in
Table 1, along with a comparison between the decay kinetics of
Er(F-tpip)3 and the nonfluorinated analogue, Er(tpip)3.
For both complexes and all different sample types, the decay is
best described by a biexponential process and a nanosecond-scale
rise-time component, both of which have been observed for other
† Imperial College London.
‡ University of Durham.
9
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J. AM. CHEM. SOC. 2005, 127, 524-525
10.1021/ja0441864 CCC: $30.25 © 2005 American Chemical Society

C O M M U N I C A T I O N S
inorganic glass matrices (typically on the order of milliseconds),14
our results demonstrate that striking improvements in luminescence
efficiency can be achieved by relatively straightforward synthetic
tuning of the ligand, that is, perfluorination of all the aromatic
antenna chromophores. Further work in this area will be geared
toward gaining a better understanding of the decay channels
operating in this perfluorinated system and, in turn, lead to further
developments in the design of NIR-emitting devices for com-
munications, sensing, and analytical detection.
Acknowledgment. We thank the EPSRC U.K. for funding this
work, and Dr. Graham Saunders (The Queens University of Belfast)
for helpful advice on the adapted synthesis of (C6F5)2PCl.
Supporting Information Available: Experimental procedure for
the synthesis of the ligand and complex, the UV-vis absorption
spectrum of Er(F-tpip)3 in solution, and the procedure for the vapor
deposition of thin films. This material is available free of charge via
the Internet at http://pubs.acs.org.
Figure 1. PL spectrum of Er(F-tpip)3 in CDCl3 (∼10-4 M) (decay profile
shown as inset).
Table 1. PL Lifetime Data for Er(tpip)3 and Er(F-tpip)3 as a
Powder, in Solution, and as a Thin Film
Lifetimes (µ
s)b
References
Sample type
and complexa
τ1
τ2
PLQYc
(1) (a) Sloof, L. H.; van Blaaderen, A.; Polman, A.; Hebbink, G. A.; Klink,
S. I.; Van Veggel, F. C. J. M.; Reinhoudt, D. N.; Hofstraat, J. W. J. Appl.
Phys. 2002, 91, 3955-3980. (b) Suzuki, H.; Yokoo, A.; Notomi, M.
Polym. AdV. Technol. 2004, 15, 75-80.
powder
Er(tpip)3
Er(F-tpip)3
6.0 (77%)
164 (95%)
1.5 (23%)
46 (5%)
0.05%
1.8%
(2) Sabbatini, N.; Guardigli, M.; Lehn, J.-M. Coord. Chem. ReV. 1993, 123,
201-228.
CDCl3 solution
Er(tpip)3
Er(F-tpip)3
(3) Shavaleev, N. M.; Pope, S. J. A.; Bell, Z. R.; Faulkner, S.; Ward, M. D.
J. Chem. Soc., Dalton Trans. 2003, 808-814.
5.0 (87%)
145 (27%)
2.4 (13%)
28 (73%)
0.06%
0.4%
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500 nm film
Er(tpip)3
Er(F-tpip)3
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Samuel, I. D. W. Synth. Met. 2003, 138, 463-469.
4.8 (84%)
224 (92%)
1.0 (16%)
27 (8%)
0.04%
1.71%
(6) Ly, T. Q.; Woollins, J. D. Coord. Chem. ReV. 1998, 176, 451-481.
(7) Magennis, S. W.; Parsons, S.; Corval, A.; Woollins, J. D.; Pikramenou,
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Nakashima, N.; Yanagida, S. Angew. Chem., Int. Ed. 2000, 39, 357-
360. (b) Hasegawa, Y.; Kimura, Y.; Murakoshi, K.; Wada, Y.; Kim, J.-
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J.-H.; Nakashima, N.; Yamanaka, T.; Yanagida, S. Chem. Phys. Lett. 1996,
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a Excitation wavelength ) 266 nm. b Fractional intensities in parentheses.
c Photoluminescent quantum yield (PLQY) calculated by using the equation:
(A1τ1 + A2τ2)/τr, where A1 and A2 are the pre-exponential factors, normalized
such that A1 + A2 ) 1, and τr is the average radiative lifetime (8 ms) of the
Er3+ ion.5
Er3+ complexes.5 As can be seen in Table 1, the lifetimes of the
Er3+ ion are substantially longer in the perfluorinated complex
relative to those of the nonfluorinated analogue. The solution-state
kinetic behavior of Er(F-tpip)3 shows some deviation relative to
the solid-state; specifically, the short-lived species dominates. This
result suggests that the C-D bond on the solvent molecule may
provide the dominant nonradiative pathway from the Er3+ excited
state. This implies that the ligand system is sufficiently mobile in
solution to allow solvent molecules to occupy gaps between the
F-tpip ligands. On average, we observe a 10-fold increase in lifetime
in solution, a 30-fold increase for the powder, and a remarkable
50-fold increase for the evaporated thin films.
It would appear that eliminating all of the C-H bonds serves to
significantly reduce the degree of vibrational quenching, which in
turn leads to longer lived Er3+ excited states and enhanced emission
intensity. Structural studies are currently being performed in order
to determine if perfluorination serves to increase the degree of
shielding afforded to the Er3+ ion. Furthermore, studies are also
being directed at understanding the specific energy transfer pro-
cesses that are operational in the Er(F-tpip)3 complex and whether
fluorination serves to improve the degree of energy transfer between
the ligand and metal ion.
(10) Hasegawa, Y.; Wada, Y.; Murakoshi, K.; Nakashima, N.; Yamanaka, T.;
Yanagida, S. J. Lumin. 1998, 79, 29-38.
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(12) Magnelli, D. D.; Tesi, G.; Lowe, J. U.; McQuiston, W. E. Inorg. Chem.
1966, 5, 457-461.
(13) Time-resolved PL measurements were made by illuminating the samples
with a circular beam of ∼5 mm using the 266 nm Q-switched output of
an Nd:YAG laser (Spectra Physics) operating at 10 Hz. Pulse energies
were typically in the region of 3-4 mJ with a pulse fwhm of 8 ns. The
PL was collected at right angles and focused onto the entrance slit of a
Jobin-Yvon Triax 320 monochromator with a grating blazed at 1 µm.
The emitted light was detected with a nitrogen-cooled germanium
photodiode/amplifier (North Coast EO-817P) operating in high-sensitivity
mode. The signal was captured and averaged by a digital storage
oscilloscope (Tektronix TDS320) and transferred to a PC for data analysis.
The PL decays were analyzed by iterative reconvolution and nonlinear
least-squares analysis of the instrument response profile with biexponential
functions. The quality of the fits was assessed by the randomness of the
residuals and a satisfactory reduced ø-squared. The fractional intensities
given in Table 1 for each lifetime (fi) were determined from the values of
pre-exponential factors (Ai) and the lifetimes (τi) as follows: f1 ) A1τ1/
(A1τ1 + A2τ2) and f2 ) A2τ2/(A1τ1 + A2τ2). PL spectra were generated by
measuring the decay of the luminescence intensity at 2.5 nm wavelength
intervals followed by integration of the area under the decay to give the
total emission intensity at each wavelength.
(14) Miniscalo, W. J. J. LightwaVe Technol. 1991, 9, 234-250.
While it is clear that the lifetimes observed for Er(F-tpip)3 still
fall some way short of the natural radiative lifetimes of Er3+ in
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