Re(I) sensitised near-infrared lanthanide luminescence from a hetero-trinuclear Re2Ln array

Article abstract of DOI:10.1039/b402270j

The trinuclear complexes Re₂Ln (Ln = Nd, Yb or Er) contain two Re^I tricarbonyl units linked to a DTPA binding site via 2,2′-bipyridyl ligands; Ln^III-centred emission is sensitised by the Re^I MLCT excited states.

Full text of DOI:10.1039/b402270j

Re( ) sensitised near-infrared lanthanide luminescence from a
I
hetero-trinuclear Re Ln array†
2
Simon J. A. Pope, Benjamin J. Coe* and Stephen Faulkner*
Department of Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL.
E-mail: Stephen.Faulkner@man.ac.uk; Ben.Coe@man.ac.uk; Fax: +44 (0) 161 275 4616; Tel: +44 (0) 161
275 4659
Received (in Cambridge, UK) 13th February 2004, Accepted 27th April 2004
First published as an Advance Article on the web 21st May 2004
The trinuclear complexes Re
2
Ln (Ln = Nd, Yb or Er) contain
of ReI tricarbonyl 2,2A-bipyridyl complexes. On addition of
I
III
two Re tricarbonyl units linked to a DTPA binding site via 2,2A-
3
Ln (OTf) , this emission is attenuated by ca. 30%, indicative of
III
I
III
I
III
bipyridyl ligands; Ln -centred emission is sensitised by the Re
3 2
quenching by the Ln component in {fac-Re Cl(CO) } L–Ln .
MLCT excited states.
Furthermore, time-resolved luminescence measurements on the
mixed-metal neodymium complex show that the lifetime of the
MLCT emission is reduced to ca. 10 ns. Whilst such lifetimes are
close to the limit of reliability for reconvolution-based methods,
the trend upon complexation is clear.
The synthesis and photophysical investigation of multi-chromo-
phoric, often luminescent, metal complexes is an area of intense
4
1
current activity. Such studies have been dominated by bipyridyl
II
II
I
complexes of d-block metal ions such as Ru , Os and Re .
However, interest in luminescence from lanthanide complexes has
grown considerably, particularly due to applications in time-
resolved immunoassays. The long-lived emissions associated with
lanthanide luminescence can be used to improve the sensitivity of
It is reasonable to assume that the observed quenching arises
I 3
from energy transfer from the Re MLCT states to the proximate
lanthanides. This conclusion, and the assignment of the structures
2
III
of the complexes, is borne out by a study of the Ln -centred
I
III
emission from {fac-Re Cl(CO)
3 2
} L–Ln (Ln = Nd, Er, Yb) in
3
assays or imaging experiments.
CH OH and CD OD. A typical fitted decay is shown in Fig. 1 and
3
3
Lanthanides such as neodymium, ytterbium5 and erbium,6
which emit near-IR light outside the range of biological absorption,
can be sensitised by well-chosen d-block complexes. The literature
contains a few examples of such an approach: notably using
4
the luminescence lifetimes for the complexes are shown in Table 1.
Time-resolved emission spectra (Fig. 2) show the short-lived
III
MLCT emission superimposed on the Ln spectrum. For the Nd
II
2+ 7
II 8
7
II
9
[
3
Ru (2,2A-bipyridyl) ] , Pt , ferrocenyl and Pd -porphyrin
complexes as sensitising chromophores in covalently-linked d–f
I
hybrids that are stable in protic solvents. The use of Re complexes
as lanthanide sensitisers via bridging ligands has also been
demonstrated, but these species dissociate in protic solvents, and
their application to assays is thus severely restricted.1 Helicates
have also been used to form d–f hybrids, most notably in the case
0
III
11
of a heterobinuclear Cr –Nd assembly, in which rate-determin-
ing energy transfer prolongs the luminescence lifetime of the Nd-
centred emission by several orders of magnitude.
We now report the synthesis of a kinetically stable complex
I
III
incorporating two Re ions and one Ln ion. In {fac-Re-
I
III
III
3 2
Cl(CO) } L–Ln , the Ln ion is bound in a diethylenetriamine
pentaacetic acid (DTPA)-derived coordination environment with
I
the Re tricarbonyl moieties held in close proximity. Our synthetic
approach (Scheme 1) utilises kinetically stable complexes in
combination with orthogonal protecting groups, and builds on our
recent application of such methodologies to the formation of
1
2
heteronuclear f–f hybrids. Treatment of 4A-methyl-2,2A-bipyridyl-
-carboxylic acid (1) with thionyl chloride in dichloromethane
4
yields the corresponding acid chloride, which is used without
further purification. Subsequent reaction with 2, affords the
functionalised 2,2A-bipyridyl ligand 3, which is purified by column
chromatography on silica [dichloromethane/methanol (95:5)].
Attempted deprotection of 3 with trifluoroacetic acid (TFA) results
I
L–LnIII and its precursors.
Scheme 1 Synthesis of {fac-Re Cl(CO)
}
3 2
in unacceptably low product yields and very long reaction times.
I
However, reaction of 3 with Re Cl(CO)
5
in chloroform under reflux
I
gives the complex fac-Re Cl(CO)
3
(3). Cleavage of the BOC
protecting group is then accomplished using TFA at room
temperature, with the reaction proceeding to completion within 30
minutes. Treatment of the deprotected complex with 0.5 equiva-
lents of DTPA dianhydride at room temperature in the presence of
I
triethylamine yields the dinuclear complex {fac-Re Cl(CO)
Excitation of {fac-Re Cl(CO)
3
}
2
L.
I
3 2
} L at 337 nm produces an
intense, short-lived (tem = 13 ns) emission peak at 625 nm, typical
I
L–ErIII in CD
†
Electronic supplementary information (ESI) available: experimental
Fig. 1 Fitted decay for {fac-Re Cl(CO)
3
}
2
3
OD (lex = 337 nm,
protocols and characterisation, time-resolved emission spectra of the
heteronuclear complexes, illustrative spectra, showing signal separation by
time gating. See http://www.rsc.org/suppdata/cc/b4/b402270j/
lem = 1530 nm). The fit was obtained by iterative reconvolution with the
detector response by minimisation of residuals squared. The fitted curve and
the observed decay are almost exactly superimposed.
1
550
C h e m . C o m m u n . , 2 0 0 4 , 1 5 5 0 – 1 5 5 1
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Table 1 Photophysical data for the complexes
works well with aminocarboxylate ligands.14 Application of this
equation to our system gives q = 0.3 for the Nd complex. This
l
(nm)
em(Re)
l
em(Ln)
t
(ns)
CH3OH
t
CD3OD
(ns)
result is entirely consistent with the octadentate coordination
Compound
t
Re (ns) (nm)
q
III
environment binding the Nd ion.
In the case of Er, quenching by C–H oscillators is even more
pronounced and energy matching with O–H oscillators is extremely
effective, to the point where the low intensity of the luminescence
{
{
{
{
ReCl(CO)
ReCl(CO)
ReCl(CO)
ReCl(CO)
3
3
3
3
}
}
}
}
2
2
2
2
L
625
13
10
12
13
—
237
—
1882
—
512
540
8303
—
0.3
—
L–Nd 625
L–Er 625
L–Yb 625
1340
1530
980
0.7
3
in CH OH precludes analysis of the luminescence lifetime.
However, the lifetime of 540 ns obtained in CD
that of the Nd complex.
As well as confirming the structures of the complexes, time-
resolved spectroscopy can be used to assess the effectiveness of the
3
OD is similar to
Luminescence lifetimes were obtained by iterative reconvolution of the
detector response with exponential components for growth and decay of the
luminescence. Errors are ±10% for lifetimes and ±20% for calculated inner
sphere hydration numbers.
III
energy transfer processes. In all cases, the rise-time of the Ln -
centred signal is short, suggesting efficient energy transfer from the
III
and Er complexes, LnIII emissions dominate the tail of the MLCT
emissions (at 1340 and 1530 nm, respectively), and the observed
decays exhibit single exponential behaviour, with no improvement
in fit on fitting to two or more exponentials.
MLCT state with rate-determining Ln -centred emission.
I
In summary, our results show the potential of Re sensitisers for
near-IR luminescence from lanthanide ions. We are currently
investigating the utility of these and related systems as biological
probes.
In contrast, the Yb emission spectrum (and the higher energy
bands of the Nd emission spectrum) is itself convoluted with the
MLCT emission. In such cases, the luminescence lifetimes for
The authors wish to acknowledge support from the EPRSC
(grant GR/S04949) and the University of Manchester.
III
Ln -centred processes can be obtained by time-gating, or by fitting
to a double exponential decay where the decay constant for one, and
in each case the shorter, component is defined by the lifetime of the
Notes and references
1
V. Balzani and F. Scandola, Supramolecular Photochemistry, Ellis
Horwood, Chichester, 1991; C. A. Bignozzi, R. Argazzi and C. J.
Kleverlaan, Chem. Soc. Rev., 2000, 29, 87; L.-C. Sun, L. Hammarström,
B. Åkermark and S. Styring, Chem. Soc. Rev., 2001, 30, 36; V. Balzani,
P. Ceroni, A. Juris, M. Venturi, S. Campagna, F. Puntoriero and S.
Serroni, Coord. Chem. Rev., 2001, 219–221, 545; A. Prodi, M. T.
Indelli, C. J. Kleverlaan, E. Alessio and F. Scandola, Coord. Chem.
Rev., 2002, 229, 51.
MLCT state.
Differences in the efficiency of non-radiative quenching of LnIII
luminescence by O–H and O–D oscillators permit the inner sphere
hydration state, q, to be deduced for Yb in methanolic solution
using the relation q = 2(kCH3OH 2 kCD3OD 2 0.05) where k
H
and
2
1
k
D
are the observed rate constants for luminescence in ms
,
13
combined with a correction term for outer sphere contributions. In
this case the DTPA-like site is expected to be octadentate and
previous studies have shown that when bound in this environment
the degree of solvation, q, for Yb is approximately zero. The
calculated value of q in the Yb complex is slightly higher, but still
2 S. Faulkner and J. L. Matthews, Fluorescent and Luminescent
Complexes for Biomedical Applications, in Volume 9 of Comprehensive
Coordination Chemistry, 2nd Edition, ed. M. D. Ward, Elsevier,
Oxford, 2003.
3
A. Beeby, S. W. Botchway, I. M. Clarkson, S. Faulkner, A. W. Parker,
<
1, possibly indicative of the weakly coordinating amide
moieties.
Further structural evidence can be obtained by study of the Nd
D. Parker and J. A. G. Williams, J. Photochem. Photobiol., B, 2000, 57,
8
3.
4
5
A. Beeby and S. Faulkner, Chem. Phys. Lett., 1997, 266, 116.
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Trans., 2002, 1918.
complex. In this case, the relation between q and the observed rate
constants is less well established, owing to the ease of non-radiative
quenching of the excited state by C–H oscillators and the
consequent dependence of quenching efficiency on ligand struc-
ture. However, the relation q = 290(kCH3OH 2 kCD3OD) 2 0.4
6 M. H. V. Werts, J. W. Verhoeven and J. W. Hofstraat, J. Chem. Soc.,
Perkin Trans. 2, 2000, 433.
7
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9
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1
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1
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1
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I
L–NdIII in
Fig. 2 Time-resolved emission spectrum of {fac-Re Cl(CO)
}
3 2
14 S. Faulkner, A. Beeby, M. C. Carrie, A. Dadabhoy, A. M. Kenwright
3
CD OD (lex = 337 nm).
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C h e m . C o m m u n . , 2 0 0 4 , 1 5 5 0 – 1 5 5 1
1551