Sensitized near-infrared emission from ytterbium(III) via direct energy transfer from iridium(III) in a heterometallic neutral complex

Article abstract of DOI:10.1039/b800162f

A tetrametallic iridium-ytterbium complex has been synthesised that shows sensitized near-infrared emission (λ_max = 976 nm) upon excitation of the iridium unit in the visible region (400 nm) due to efficient energy transfer from the iridium uni

Full text of DOI:10.1039/b800162f

View Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/dalton | Dalton Transactions
Sensitized near-infrared emission from ytterbium(III) via direct energy
transfer from iridium(III) in a heterometallic neutral complex†
Marita Mehlsta¨ubl, Gregg S. Kottas,* Silvia Colella and Luisa De Cola*
Received 7th January 2008, Accepted 21st February 2008
First published as an Advance Article on the web 18th March 2008
DOI: 10.1039/b800162f
A tetrametallic iridium–ytterbium complex has been synthe-
sised that shows sensitized near-infrared emission (kmax
active antenna, NIR emission could be achieved via electrical
excitation of Ln(III) ions in a light-emitting diode (LED) type
configuration. In addition, the use of transition metal complexes
as sensitizers for lanthanides extends the ability to photoexcite the
systems throughout the visible and into the near-infrared.5a
Previously, we reported a ligand system wherein three mono-
anionic bipyridine carboxylates coordinate one Ln(III) ion to yield
neutral, nine-coordinate complexes with excellent photophysical
properties.7 By exploiting this binding motif, we can attach
different units to the bipyridine core. Here we report the synthesis,
characterization, and photophysical properties of a tetrametallic
Ir–Yb complex (Ir3Yb), bearing three iridium units around a
central Yb(III) core. The photoinduced electronic energy transfer
from the excited Ir(III) moiety to the Yb(III) unit is also described.
=
976 nm) upon excitation of the iridium unit in the visible
region (400 nm) due to efficient energy transfer from the
iridium units to the Yb(III) ion. The iridium phosphorescence
is quenched nearly quantitatively while the ytterbium ion
emits brightly in the NIR.
Introduction
The luminescent properties of near-infrared (NIR) emitting
lanthanide ions [Ln(III)] are widely studied due to possible
applications for optical amplification in lasers1 as well as in
telecommunications2 and bioimaging.3 Lanthanide complexes
often display narrow emission spectra, long excited-state lifetimes,
and large energy differences between absorption and emission.4
Direct excitation of Ln(III) ions is however difficult because the
optical transitions in the 4f subshell are parity forbidden, yielding
absorption coefficients on the order of 1–10 M−1cm−1.4 Therefore,
lanthanide ions are generally excited via light-harvesting antenna
or sensitizing groups that transfer energy to the excited states
of the emitting ion, typically organic molecules.4 Near-infrared-
emitting lanthanides such as Er(III) and Pr(III) are standards in
telecommunications as dopants into the glass cores of fiber optic
amplifiers.2 The amplifiers are required to ‘regenerate’ the signal
due to intrinsic losses that occur as it propagates through the fiber;
currently, the amplifiers are pumped optically with a laser.2
Recently, a number of complexes with d-block transition metals
have been used as sensitizers for Ln(III) ions.5 However, to the best
of our knowledge, the only example of an Ir(III)–Ln(III) complex
has been published by our group.5b Iridium(III) complexes are
highly tunable over the entire visible and NIR spectra, generally
with relatively small changes to the coordinating ligands, and are
commonly used in electroluminescent applications due to their
excellent redox properties.6 Therefore, we sought to combine the
properties of the lanthanides with those of Ir(III) complexes in
a single molecule. The advantage would be the possibility to
electrically populate the excited states of the lanthanide ions, which
themselves have poor redox properties (limited oxidation state
range), using an iridium compound as a sensitizer. With a redox-
Results and discussion
The synthesis of Ir3Yb is shown in Scheme 1. Ir-COOEt was pre-
pared by a Suzuki coupling of Ir-Br with the pinacolatoboronate
derivative of ethyl phenylbipyridine carboxylate (see ESI†). After
saponification of the ester with NaOH and neutralization of the
solution, the resulting product Ir-COOH was dissolved in H2O–
Et3N, and, upon addition of a solution of YbCl3·6H2O in water,
a precipitate formed that was washed and dried to give Ir3Yb in
80% yield.
High-resolution electrospray ionization mass spectrometry con-
firmed the formation of Ir3Yb (Fig. S1 and S2†), and comparison
of molar absorptivity values proved the tetrametallic nature of
the complex (Fig. 1; vide infra). The precursors were character-
ized by mass spectrometry, elemental analysis and NMR spec-
troscopy (see ESI†). [tBu-COO]3Yb (Fig. 2, inset) was synthesized
(Scheme S1†) and investigated as reference for the multimetallic
Westfa¨lische Wilhelms-Universita¨t Mu¨nster, Department of Physics,
and the Center for Nanotechnology (CeNTech), Heisenbergstrasse 11,
48149, Mu¨nster, Germany. E-mail: kottas@uni-muenster.de, decola@uni-
muenster.de; Fax: +49 251 5340 6102; Tel: +49 251 5340 6840
† Electronic supplementary information (ESI) available: Synthesis of
[tBu-COO]3Yb, experimental details, excitation spectrum of Ir3Yb, mass
spectra of Ir3Yb, isoabsorptive emission spectra of Ir3Yb and [tBu-
COO]3Yb and normalized emission spectra in the visible region. See DOI:
10.1039/b800162f
Fig. 1 Absorption and emission spectra in degassed CH2Cl2 (kexc
400 nm). The intensities of the emissions of Ir-COOH and Ir3Yb are
multiplied by a factor of 5 for clarity.
=
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2385–2388 | 2385
©

View Online
Scheme 1 Synthetic procedure for the preparation of Ir3Yb.
state.6 The degree of mixing depends on the relative energy of
3
the MLCT state with respect to the triplet excited states of the
ligand, which are strongly effected by the extent of conjugation
and of substituents (ref. 8(a)and vide infra). The emission spectra
reported here all have the same shape and maxima (Fig. 1 and
S5†), but the emission quantum yields and excited-state lifetimes
are dramatically different.9a The emission of Ir-COOH (Uvis
=
0.06) is about 7 times lower in intensity compared with Ir-COOEt
(Uvis = 0.39) or Ir-Br (Uvis = 0.42). Ir-COOEt phosphoresces with
a remarkably long excited-state lifetime for an iridium complex
(127 ls) when compared to the typical lifetimes for similar
complexes (∼1–5 ls),6 such as that of Ir-Br (1.5 ls). Ir-COOH
has an excited-state lifetime of 20 ls, and while still long, it is one
order of magnitude less than the ester. As already indicated by
the absorption spectra, the aromatic rings lead to a conjugated
system with an approximate triplet energy (3LC) of 18 900 cm−1.8b
Therefore, strong mixing between the 3MLCT state (20 576 cm−1)8c
and the low-lying 3LC states of the ligands was envisaged.8d
The iridium emission is drastically quenched when complexed
to ytterbium in Ir3Yb (Uvis = 0.006), compared to that of the
iridium precursors (Fig. 1). This indicates that electronic energy
transfer from the iridium fragment to the ytterbium moiety, upon
excitation into the MLCT state of the iridium, is taking place.
The iridium emission in the Ir3Yb complex is quenched by 95%
when compared to Ir-COOEt,10 which was chosen as a model
for the donor system, under identical conditions (Fig. 1).11 In
addition, we find a shortened lifetime of 2 ls, which indicates a
98% quenching.11 Due to the uninterrupted conjugation between
the iridium unit and the ytterbium ion, we propose a through-
bond, Dexter mechanism for the energy transfer, which is fully
Fig. 2 Normalized Yb(III) emission of Ir3Yb (kexc = 400 nm) and
[tBu-COO]3Yb (kexc = 301 nm) in CH2Cl2.
system, in order to understand the role of the iridium moiety in
the Ir3Yb system.
The absorption spectra of compounds Ir-Br, Ir-COOEt, Ir-
COOH, Ir3Yb and [tBu-COO]3Yb are shown in Fig. 1. All
the compounds investigated display bands assigned to p–p*
transitions at energies greater than 320 nm due to the phenylene
and pyridine units. The iridium-containing complexes also show
bands in the visible region that are assigned to singlet metal-to-
ligand charge transfer transitions (1MLCT) and, at lower energy,
3
spin-forbidden MLCT absorptions. It should be noted that at
energies lower than 26 000 cm−1 (380 nm) the absorption is almost
exclusively due to the iridium moiety, and therefore virtually
selective excitation is possible at these energies in this system. The
molar absorptivity for the Ir3Yb complex is about three times
higher than in the iridium ligands, as is expected due to the
presence of three chromophoric moieties in the molecule. All the
compounds possessing the diphenyl-bipyridine-carboxylate show
an intense absorption between 350 and 400 nm. We attribute
these transitions to conjugation in these systems in which the
bipyridine moiety is also involved. The absorption spectrum of
[tBu-COO]3Yb has been used as a reference to understand the
contribution of the iridium moiety to the absorption spectrum of
Ir3Yb.
2
allowed (DJ = 1) with respect to the F7/2 ground state of the
ytterbium ion.5c,d
The energy transfer leads to typical Yb(III) luminescence in
the near infrared with a maximum at 976 nm assigned to the
2F5/2
→
2F7/2 transition and broader vibronic components at
longer wavelengths (Fig. 2). The near-infrared emission spectrum
was recorded after excitation at 400 nm, where the MLCT states
of the iridium moieties absorb, and direct population of the
p-states of the ligand directly bound to the ytterbium ion is
negligible (see Fig. 1; vide supra). We can therefore conclude that
the ytterbium emission is due to an efficient electronic energy
Fig. 1 also shows the room-temperature visible emission of Ir-
Br, Ir-COOEt, Ir-COOH and Ir3Yb in degassed CH2Cl2. For
iridium cyclometallated systems or in heteroleptic complexes,
the emission in the visible region originates from the 3MLCT
state, which can be mixed with the ligand-centred triplet (3LC)
2
transfer from the excited iridium moiety to the F5/2 level of the
2386 | Dalton Trans., 2008, 2385–2388
This journal is
The Royal Society of Chemistry 2008
©

View Online
Yb ion. Further confirmation that energy transfer originates from
the MLCT states of the iridium is found in the excitation spectrum
Conclusions
2
recorded upon monitoring the 2F5/2 → F7/2 emission at 976 nm of
In conclusion, we have prepared an iridium complex containing
a conjugated phenylene bridge able to coordinate lanthanide ions
in a 3 : 1 stoichiometry to give an overall neutral complex. Upon
selective excitation of the iridium to the MLCT states, a strong
quenching (≥95%) of the iridium emission is observed along with
intense infrared emission from the ytterbium ion at 976 nm. The
Ir3Yb complex has a quantum yield of 0.7% (kexc = 300 nm),
which is high for emitters in the near-infrared and likely due to the
exclusion of solvent molecules from the inner coordination sphere
of the lanthanide ion, which is a result of the nine coordination
sites occupied by the bipyridine-carboxylate ligand.
These findings open up interesting possibilities for the sensiti-
zation of lanthanide ions with metal complexes and perhaps with
electrical excitation in systems containing such electroluminescent
metal complexes. In addition, the iridium precursors have interest-
ing photophysical properties such as long (>100 ls) excited-state
lifetimes that are currently being investigated further and will be
published in a forthcoming full paper.
Yb(III) (Fig. S3†). The excitation spectrum overlaps the absorption
spectrum in the MLCT region between ∼350–450 nm, proving that
the energy originates from these states.
Thus far, we have determined that the iridium units in Ir3Yb
are quenched nearly quantitatively (≥95%) and that energy can
be transferred to the Yb(III) ion under selective excitation of the
iridium moiety (400 nm). To determine the efficiency of the energy
transfer and to rule out other possible quenching pathways (e.g.,
electron transfer), we have determined the degree of sensitization
of the ytterbium by measuring the NIR emission spectra of [tBu-
COO]3Yb and Ir3Yb upon excitation at an isoabsorptive point
(370 nm). Unfortunately, we cannot excite both complexes at
400 nm because the [tBu-COO]3Yb complex used as reference does
not absorb at this wavelength (vide supra). However, at 370 nm
most of the excitation light is absorbed by the iridium moiety
(∼95% based on Fig. 1). The integrals of the emission spectra
(Fig. S4†) reveal that the energy is transferred with an efficiency
of ∼65% in Ir3Yb relative to [tBu-COO]3Yb. In other words,
there is a non-radiative loss mechanism upon excitation of the
iridium unit that does not exist when the sensitizer bound to the
ytterbium ion is directly excited. The incomplete energy transfer
upon excitation of the iridium moiety seems to indicate that a
competitive process occurs, leading to quenching of the iridium
emission and to a loss of electronic energy. We do not have a good
explanation for this behavior but suggest that it could be related to
an electron transfer process, which has been postulated for other
ytterbium complexes.12 As discussed above, the emissive states of
the iridium complex have energies near ∼20 000 cm−1, while the
2F5/2 state of Yb(III) lies at 10 200 cm−1—this large gap between
donor and acceptor levels would suggest inefficient energy transfer
by a Dexter mechanism and therefore hint at the possibility of
other processes.5c,d
The ytterbium emission in the Ir3Yb complex has a quantum
yield of 0.7% when excited at 300 nm,9b which is relatively high for
near-infrared emitters, equivalent to [tBu-COO]3Yb (UNIR = 0.7%)
and a related system (UNIR = 0.7%) previously reported.7 This is
likely due to the fact that the three bipyridine-carboxylate ligands
occupy nine coordination sites around the ytterbium ion, leaving
no positions free for vibronically deactivating solvent molecules.4
To the best of our knowledge, the highest quantum yield values for
ytterbium complexes in solution were reported by Imbert et al.13a
with an emission quantum yield of 0.8% in deuterated water and by
Puntus et al.13b who measured a quantum yield in toluene of 1.1%.
The fact that Ir3Yb and [tBu-COO]3Yb have identical quantum
yields (UNIR = 0.7%) upon excitation in the UV (300 nm), further
suggest that there is an additional decay pathway open upon
selective excitation of the iridium units as evidenced by the lower
efficiency upon excitation of Ir3Yb at 400 nm (vide supra).
Acknowledgements
We would like to thank the Deutsche Forschungsgemeinschaft
(SFB 656) and the DAAD Vigoni project (Nr. D/05/54254) for
partial support of this work.
Notes and references
1 R. Reisfeld and C. K. Jorgensen, in Lasers and Excited States of
Rare Earths, Springer, Heidelberg, 1977; K. Kuriki, Y. Koike and Y.
Okamoto, Chem. Rev., 2002, 102, 2347–2356.
2 W. J. Miniscalco, J. Lightwave Technol., 1991, 9, 234–250; J. K. R.
Weber, J. J. Felten and P. C. Nordine, Nature, 1998, 393, 769–771.
3 G. R. Motson, J. S. Fleming and S. Brooker, Potential applications for
the use of lanthanide complexes as luminescent biolabels, in Advances
in Inorganic Chemistry, Elsevier Inc., Amsterdam, 2004, vol. 55, pp.
361–432; S. Pandya, J. Yu and D. Parker, Dalton Trans., 2006, 2757–
2766; S. Faulkner, S. J. A. Pope and B. P. Burton-Pye, Appl. Spectrosc.
Rev., 2005, 40, 1–31; D. Parker, Chem. Soc. Rev., 2004, 33, 156–165.
4 N. Sabbatini and M. Guardigli, Coord. Chem. Rev., 1993, 123, 201–228;
J. P. Leonard, C. B. Nolan, F. Stomeo and T. Gunnlaugsson, Top. Curr.
Chem., 2007, 281, 1–43 and references therein; J.-C. G. Bu¨nzli and C.
Piguet, Chem. Rev., 2002, 102, 1897–1928; D. Parker, R. S. Dickins, H.
Puschmann, C. Crossland and J. A. K. Howard, Chem. Rev., 2002, 102,
1977–2010.
5 (a) For a recent overview, see: M. D. Ward, Coord. Chem. Rev., 2007,
251, 1663–1677; (b) P. Coppo, M. Duati, V. N. Kozhevnikov, J. W.
Hofstraat and L. De Cola, Angew. Chem., Int. Ed., 2005, 44, 1806–
1810; (c) N. M. Shavaleev, L. P. Moorcraft, S. J. A. Pope, Z. R. Bell, S.
Faulkner and M. D. Ward, Chem.–Eur. J., 2003, 9, 5283–5291; (d) T. R.
Ronson, T. Lazarides, H. Adams, S. J. A. Pope, D. Sykes, S. Faulkner,
S. J. Coles, M. B. Hursthouse, W. Clegg, R. W. Harrington and M. D.
Ward, Chem.–Eur. J., 2006, 9299–9313.
6 L. Flamigni, A. Barbieri, C. Sabatini, B. Ventura and F. Barigelletti,
Top. Curr. Chem., 2007, 281, 143–203 and references therein.
7 G. S. Kottas, M. Mehlsta¨ubl, R. Fro¨hlich and L. De Cola, Eur. J. Inorg.
Chem., 2007, 3465–3468.
8 (a) F. Lafolet, S. Welter, Z. Popovic and L. De Cola, J. Mater. Chem.,
2005, 15, 2820–2828; S. Welter, F. Lafolet, E. Cecchetto, F. Vergeer and
L. De Cola, ChemPhysChem, 2005, 6, 2417–2427; (b) we are comparing
Ir–COOR (R = H, CH2CH3) with [Ir-Ph4-Ir]2+ from ref. 8(a) because
both have the same number of phenyl rings in the conjugated segment;
(c) value is taken from the emission maximum at RT of Ir–Br (Fig. 1);
(d) we are currently investigating the photophysics of the iridium
complexes in more detail and will publish the results in a forthcoming
full paper.
The nine coordination in Ir3Yb also leads to a long excited-state
lifetime of 17.7 ls of Yb(III).14 This is also in good agreement
with other ytterbium complexes measured in solution: e.g., the
reference compound [tBu-COO]3Yb has an excited-state lifetime
of 18.3 ls, and in the literature ytterbium complexes with excited-
state lifetimes of 12.0 ls13b up to 23.2 ls13c have been reported. One
outstanding example of a 1.1 ms lifetime for an Yb(III) complex
has been reported recently by Glover et al.15
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2385–2388 | 2387
©

View Online
9 (a) Quantum yields in the visible region were measured versus qui-
nine bisulfate in 1 N H2SO4 (φref = 0.546) as reference compound
(kexc = 400 nm); (b) NIR quantum yields were measured versus
Yb(TTA)3 in toluene (TTA = thenoyltrifluoroacetylacetonate, U =
0.35%) (kexc = 300 nm). It was not possible to excite at 400 nm,
due to the lack of absorbtion of Yb(TTA)3 at this wavelength; S. B.
Meshkova, Z. M. Topilova, D. V. Bolshov, S. V. Beltyukova, M. P.
Tsvirko and V. Y. Venchikov, Acta Phys. Pol., A, 1999, 95, 983–
990.
10 Ir-COOEt was chosen as model for the donor system, assuming that
the binding of the carboxylic group to the ethyl moiety is closer to the
linkage with the ytterbium ion in Ir3Yb. In addition, we have evidence
that the photophysics of Ir-COOH are affected by the presence of the
carboxylic acid proton, which could lead to hydrogen-bonded dimers
or aggregates (see 8(d)).
12 (a) W. D. Horrocks, Jr., J. P. Bolender, W. D. Smith and R. M.
Supkowski, J. Am. Chem. Soc., 1997, 119, 5972–5973; (b) R. M.
Supkowski, J. P. Bolender, W. D. Smith, L. E. L. Reynolds and W. D.
Horrocks, Jr., Coord. Chem. Rev., 1999, 185–186, 307; (c) S. Faulkner,
B. P. Burton-Pye, T. Khan, L. R. Martin, S. D. Wray and P. J. Skabara,
Chem. Commun., 2002, 1669–1669; (d) A. Beeby, S. Faulkner and
J. A. G. Williams, J. Chem. Soc., Dalton Trans., 2002, 1918–1922.
13 (a) D. Imbert, S. Comby, A. S. Chauvin and J.-C. G. Bu¨nzli, Inorg.
Chem., 2006, 45, 732–743; (b) L. N. Puntus, K. J. Schenk and J.-C. G.
Bu¨nzli, Eur. J. Inorg. Chem., 2005, 4739–4744; (c) S. Torelli, D. Imbert,
M. Cantuel, G. Bernardinelli, S. Delahaye, A. Hauser, J.-C. G. Bu¨nzli
and C. Piguet, Chem.–Eur. J., 2005, 11, 3228–3242.
14 Measurement of the ytterbium excited-state lifetime was performed
upon excitation at 337 nm with a N2 laser and monitoring the emission
at 976 nm.
11 B. Valeur, Molecular Fluorescence: Principles and Applications, Wiley-
VCH, Weinheim, 2001.
15 P. B. Glover, A. P. Bassett, P. Nockemann, B. M. Kariuki, R. Van Deun
and Z. Pikramenou, Chem.–Eur. J., 2007, 13, 6308–6320.
2388 | Dalton Trans., 2008, 2385–2388
This journal is
The Royal Society of Chemistry 2008
©