Synthesis and luminescence properties of a kinetically stable dinuclear ytterbium complex with differentiated binding sites

Article abstract of DOI:10.1039/b303012a

The complex Yb₂L contains two DO3A units separated by a phenol bridging group and gives time-resolved luminescence spectra in solution consistent with the presence of two types of binding site.

Full text of DOI:10.1039/b303012a

Synthesis and luminescence properties of a kinetically stable dinuclear
ytterbium complex with differentiated binding sites†
Simon J. A. Pope,a Alan M. Kenwright,b Sarah L. Heath*a and Stephen Faulkner*a
a Department of Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL.
E-mail: Stephen.Faulkner@man.ac.uk; Fax: +44 161 275 4616; Tel: +44 161 275 4659
b Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
Received (in Cambridge, UK) 18th March 2003, Accepted 2nd May 2003
First published as an Advance Article on the web 30th May 2003
The complex Yb2L contains two DO3A units separated by a
phenol bridging group and gives time-resolved luminescence
spectra in solution consistent with the presence of two types
of binding site.
not yield a satisfactory fit, as judged by residual squared and
reduced chi squared. By contrast, fitting to two exponential
decay components yielded very good fits in all cases. A typical
fitted decay is shown in Fig. 2, while the lifetimes obtained are
shown in Table 1. It is worth noting that the relative weighting
of the short and long components is the same in protiated and
deuteriated solvents. This implies that there are two distinct
lanthanide containing environments. In all cases, energy
transfer occurs within the envelope of uncertainty (i.e. < 25 ns),
which is consistent with an energy transfer process mediated by
Luminescence from lanthanide complexes is of considerable
interest from the viewpoint of synthesising complexes for
bioassay and time-gated imaging applications as well as being
relevant to the preparation of new luminescent materials.1 In
such applications, the problems associated with the inherently
low extinction coefficients of lanthanide ions can be circum-
vented by using sensitising chromophores or ‘antenna
groups’.
In recent years, considerable interest has centred on lantha-
nide ions which are emissive in the near-IR, particularly
neodymium2 and ytterbium3 complexes. These absorb and emit
outside the range associated with biological molecules, making
them ideal for high sensitivity bioassays.3 We and others have
devoted considerable effort to understanding the mechanisms
associated with sensitised luminescence from these ions,4 and
observed that ytterbium ions can be sensitised through the
LMCT state as well as via the ligand centred triplet state,
meaning that a wide range of chromophores can be used as
antennae.
Multinuclear complexes containing more than one lanthanide
ion are comparatively rare in macrocyclic chemistry.5,6 Nearly
all the systems reported are based on kinetically unstable
complexes derived from acyclic ligands, and only a handful of
studies have been carried out on complexes that are emissive in
the near IR.7
We now report the synthesis of a well-defined, kinetically
stable, binuclear ytterbium complex, Yb2L, whose lumines-
cence properties demonstrate that there are two distinct
lanthanide environments on the luminescence timescale.
The complex was prepared as shown in Scheme 1.‡
Trialkylation of cyclen with tert-butyl bromoacetate by estab-
lished procedures yielded the triester 1, which was reacted
further with the dichloride, 2, to yield the bis macrocycle 3.
Hydrolysis using trifluoroacetic acid yielded L. Complexation
was carried out in methanol using ytterbium triflate, to yield
Yb2L.
Luminescence spectroscopy was used to probe the local
environment around the lanthanide ion in aqueous solution.§
Excitation at 337 nm gave rise to a typical ytterbium centred
time resolved emission spectrum (Fig. 1). This indicates that
energy transfer must occur from the ligand chromophore to the
metal, as ytterbium has no absorption bands at this wavelength,
while the ligand chromophore absorbs strongly.
Scheme 1 Synthesis of Yb2L.
Time resolved luminescence spectroscopy gives more in-
formation about the local environment around the lanthanide
ions. Luminescence lifetimes in H2O and D2O solution were
obtained from the time resolved emission spectra by iterative
reconvolution with the detector response, which is comparable
with the luminescence lifetimes. Unusually for ytterbium
containing systems, reconvolution with a single exponential did
† Electronic supplementary information (ESI) available: 1H NMR spectrum
of Yb2L. See http://www.rsc.org/suppdata/cc/b3/b303012a/
Fig. 1 Time resolved emission spectrum of Yb2L in solution in D2O.
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CHEM. COMMUN., 2003, 1550–1551
This journal is © The Royal Society of Chemistry 2003

Society for equipment and a University Research Fellowship (S.
L. H.).
Notes and references
‡
Analytical data for all compounds was consistent with the proposed
structures. Sample data: L UV(H2O) lmax (e) 205 (17500), 221 (6400), 288
(2000 dm3 mol21 cm21) nm; IR (solid) nmax 3410 (br), 2981, 2856, 1666
(br), 1460, 1398, 1354, 1179, 1124, 1087 cm21. ES2 MS (MeCN/H2O): m/z
823 {M 2 H}2; ES+ MS (MeCN/H2O): m/z 869 {M 2 H + 2Na}+, 847 {M
1
+ Na}+, 825 {M + H}+. H NMR (400 MHz, D2O, 300 K) dH 2.2 (3H, s,
ArCH3), 2.7–3.9 (48H, m, NCH2), 7.2 (2H, br, s, ArH). 13C{1H} NMR (100
MHz, D2O, 300 K) dC 42.8, 48.2, 48.6, 49.3, 49.5, 50.6, 51.1, 52.2, 53.6,
56.2, 63.2 (br NCH2), 116.8 (q, CF3CO2H), 133.6, 142.0 (br Ar), 163.6 (q,
CF3CO2H), 169.9, 174.9 (CO). Found C, 41.82; H, 5.09; N, 8.95; calc. for
C37H60N8O13·4CF3CO2H: C, 42.19; H, 5.0; N, 8.75 %.
Fig. 2 Time resolved profile of the luminescence from a solution of Yb2L
in H2O. The graph shows changes in intensity of the luminescence at 980
nm with time and is fitted by reconvolution of the detector response with
two exponential components corresponding to t = 0.51 and 1.67 ms.
Yb2L IR (solid) nmax 2860, 1591 (br), 1431, 1415, 1241, 1165, 1083,
1027 cm21; UV(H2O) lmax (e) 202 (14500), 245 (2600), 307 (1300), 977
(20 dm3 mol21 cm21) nm; ES2 MS (MeCN): m/z 1162 {M 2 H}2. 1H
NMR (500 MHz, CD3OD, 300 K) Peaks consistent with the proposed
structure were observed (the spectrum is available as ESI. A relatively sharp
peak was observed for each proton environment, together with a broader
peak of roughly equal intensity somewhat shifted and in some cases
partially overlapping, suggesting either inequivalent binding of the two
ytterbiums, or the existence of two distinct forms of the complex in a 1 : 1
ratio, each having equivalent binding of the two ytterbiums. Selected peak
positions for the sharp peaks are reported for the characteristic dipolar
shifted regions. dH 156.9, 134.4, 118.0, 72.5, 57.8, 39.4, 35.2, 215.4,
221.0, 235.0, 247.5, 260.4, 265.9, 281.8, 2126.5.
Table 1 Luminescence lifetimes and inner sphere hydration numbers (q) for
Yb2L
tH O/ms
1.67 (67%), 0.51 (33%)
4.95 (67%), 1.17 (33%)
2
tD O/ms
2
q
0.3,
1.0
Luminescence lifetimes have errors of ±10%, relative weightings are quoted
in brackets.
§ The sample was irradiated at 337 nm using the output from a pulsed
nitrogen laser (PTI GL3300). Luminescence from the sample was collected
at right angles to the incident beam and focused onto the slits of a
monochromator (PTI-120). The growth and decay of the luminescence at
selected wavelengths was detected using a germanium photodiode (Edin-
burgh Instruments, EI-P) and recorded using a digital oscilloscope
(Tektronix TDS220).
the LMCT state, as might be expected with an electron donating
phenolate antenna group. Systems containing very similar
chromophores do not act as sensitisers for europium,6 where the
LMCT state is lower in energy than the excited state. In the case
of ytterbium this situation is reversed, making LMCT a suitable
pathway to sensitising the emissive state of the metal.4
For a lanthanide complex, the inner sphere hydration number,
q, can be established using the equation
1 D. Parker, R. S. Dickins, H. Puschmann, C. Crossland and J. A. K.
Howard, Chem. Rev., 2002, 102, 1977; D. Parker and J. A. G. Williams,
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q = ALn(1/tH O 2 1/tD O 2 B)
2
2
where ALn is a proportionality constant unique to a given
lanthanide (AYb = 1.0 µs), tH O and tD O are the luminescence
3 A. Beeby, R. S. Dickins, S. Faulkner, D. Parker and J. A. G. Williams,
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2
2
lifetimes in water and D2O measured in microseconds and B is
a correction term which accounts for the presence of outer
sphere water molecules (B = 0.1 µs for ytterbium).8 In this case
we can obtain two q values. It is reasonable to pair the
components with similar weightings with one another, partic-
ularly in the case of ytterbium, where significant quenching by
O–D is possible. Such a treatment gives inner sphere hydration
numbers of 0.3 and 1.0 (Table 1).
Taken together, the evidence implies the formation, either of
a binuclear complex with two distinct binding sites, or of the co-
existence of two forms of the complexes in solution. Both of
these are consistent with the luminescence data, and with the
complexity of the axial resonances in the proton spectrum of
Yb2L. One binding site is eight coordinate, while the other is
seven coordinate since the bulk of the DO3A unit prevents both
lanthanides from sharing the phenolate oxygen donor. The inner
sphere hydration numbers are lower than might be expected,
probably as a result of the lipophilicity of the linking phenolate
unit preventing the close approach of additional water mole-
cules. We are currently engaged in carrying out a systematic
study of related Ln2L complexes, the results of which will be
reported in due course.
6 N. M. Shaveleev, S. J. A. Pope, Z. R. Bell, S. Faulkner and M. D. Ward,
Dalton Trans., 2003, 808; F. R. Gonçalves e Silva, O. L. Malta, C.
Reinhard, H. U. Gudel, C. Piguet, J. E. Moser and J. C. G. Bunzli, J. Phys.
Chem. A, 2002, 106, 1670.
7 P. L. Timmins, M. Lunn and S. L. Heath, manuscript in preparation.
8 A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner, D. Parker, L. Royle,
A. S. de Sousa, J. A. G. Williams and M. Woods, J. Chem. Soc., Perkin
Trans. 2, 1999, 493.
The authors would like to thank the Universities of
Manchester and Durham and EPSRC for support, and the Royal
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