Lanthanide-sensitized lanthanide luminescence: Terbium-sensitized ytterbium luminescence in a trinuclear complex

Article abstract of DOI:10.1021/ja035634v

A heterotrinuclear lanthanide complex has been prepared which contains two terbium ions in DO3A-derived binding sites and a single ytterbium ion in a DTPA-like site. The luminescence properties of the system have been investigated, showing that the terbiu

Full text of DOI:10.1021/ja035634v

Published on Web 08/08/2003
Lanthanide-Sensitized Lanthanide Luminescence: Terbium-Sensitized
Ytterbium Luminescence in a Trinuclear Complex
Stephen Faulkner* and Simon J. A. Pope
Department of Chemistry, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, UK
Received April 15, 2003; E-mail: stephen.faulkner@man.ac.uk
Luminescence from lanthanide complexes has been of interest
for many years.1 The long luminescence lifetimes of certain
lanthanide complexes (particularly europium and terbium com-
plexes) have been applied widely, particularly in the fields of
bioassay,2 microscopy,3 and sensor development.4 More recently,
interest has developed in lanthanide complexes that luminesce in
the near-IR.5-7 Such complexes contain lanthanide ions with
relatively small energy gaps between the emissive state and the
ground state, including neodymium,5 erbium,6 and ytterbium7
complexes. These near-IR emissive complexes have begun to be
applied in bioassay applications.7c Such complexes can be used with
a much wider range of sensitizing chromophores than conventional
complexes; these include metal complexes8,9 as well a broad range
of organic dye molecules.10 Such species have tended to be d-f
hybrids, since f-f mixed metal complexes are hard to prepare
selectively.
3, which was reduced to the aminobenzyl derivative 4 by catalytic
hydrogenation. Cleavage of the tert-butyl esters was accomplished
using trifluoroacetic acid in dichloromethane, yielding the hepta-
dentate ligand 5 which formed the terbium complex 6 on reaction
with terbium trifluoromethanesulfonate. This complex also contains
an amino group that is well placed to react further; reaction of 2
molar equiv of 6 with DTPA anhydride yielded the dinuclear
complex 7. At this stage, it was deemed necessary to assess the
luminescence properties of the terbium complexes 6 and 7 with a
view to ensuring that there was no migration of terbium to the
central DTPA-like binding site. Differences in the efficiency of
nonradiative quenching of lanthanide luminescence by O-H and
O-D oscillators can be used to calculate of the inner sphere
hydration state, q, using the equation
q ) ALn (kH - kD - B)
We now report the results of a synthetic and luminescence study
on a hetero-trimetallic lanthanide complex containing two terbium
ions and one ytterbium ion (1). The synthetic strategy used (Scheme
1) allows the incorporation of a DTPA-like binding site as a bridge
between two kinetically stable terbium complexes. Such binding
sites are already used for imaging applications in vivo and have
been shown to have high stability in the presence of endogenous
metal ions.1 Our strategy is based on functionalization of the widely
used tert-butyl ester 2,11 prepared by reaction of 1,4,7,10-tetraazcy-
clododecane with tert-butyl bromoacetate. Reaction of 2 with
4-nitrobenzylbromide yielded the nitrobenzyl-substituted derivative
where kH and kD are the observed rate constants for the luminescence
in H2O and D2O, respectively, ALn is a constant for a given
lanthanide, and B is a correction factor for outer sphere solvent
contributions (this is also unique to a given lanthanide).12 For
terbium complexes, A ) 5.0 ms and B ) 0.06 ms-1). Since such
measurements give information about the local environment, they
are an ideal tool to identify the nature of the binding site, particularly
as the DO3A-derived binding site is heptadentate, while the DTPA-
like site is octadentate. Both 6 and 7 gave classical terbium-centered
luminescence spectra, and there is no change in the relative
Scheme 1: Synthesis of 1 and Its Precursorsa
a Reagents and conditions: (a) BrCH2C6H4NO2,NaHCO3, CH3CN; (b) Pd/C, H2, CH3OH; (c) CF3CO2H, CH2Cl2; (d) Tb(OTf)3, CH3OH; (e)
diethylenetriamine pentaacetic dianhydride, NEt3, DMF; (f) Yb(Tf)3, CH3OH.
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10.1021/ja035634v CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Luminescence Lifetimes for the Complexes Studied
ytterbium and the steric demands of the amide substituentssthe
residual hydration probably results from outer sphere effects. We
then studied the interaction between the terbium and ytterbium ions
for a solution of 1 in D2O. Direct excitation of the terbium
absorption band at 488 nm resulted in the observation of ytterbium-
centered emission at 980 nm. Stray light was rejected using a cutoff
filter in front of the monochromator to remove signals below 850
nm and obviate the possibility of 2λ artifacts appearing close to
980 nm. The decay of the emission band at 980 nm fitted to a
lifetime identical to that measured for the ytterbium complex in
D2O when excited at 337 nm. The fitted time-resolved decay is
shown as an inset to Figure 1. It is reasonable to infer energy
transfer from the terbium center, since neither ytterbium nor the
ligand-centered chromophore have any absorption bands at 488 nm.
To our knowledge this represents the first report of lanthanide-
centered near-IR emission sensitized by a lanthanide ion. We are
currently investigating the use of different pairs of lanthanide ions
and more complex arrays with a view to performing a thorough
study of the energy transfer processes involved.
compd
Lna
λex/nm
λem/nm
τH
O/µ
s
τD
O/µs
q
2
2
6
7
1
1
Tb
Tb
Yb
Yb
266
266
337
488
545
545
980
980
860
1140
1.83
1180
1840
4.22
4.22
1.3
1.4
0.2
a Lanthanide ion under examination.
Acknowledgment. The authors wish to acknowledge support
from the University of Manchester and EPSRC (GR/M82608).
Figure 1. Fitted luminescence decays for ligand-sensitized (main graph)
and terbium-sensitized emission (inset) from ytterbium in compound 1
dissolved in D2O. Both fit to a lifetime of 4.2 µs. The observed decay (blue)
and fitted curve (red), superimpose almost exactly, as can be seen from the
residual (dark green). The detector response (black) is also shown.
Supporting Information Available: Synthetic routes to 1 and its
precursors; experimental procedures for luminescence spectroscopy and
deconvolution; energy level diagram showing excited states for terbium
and ytterbium. This material is available free of charge via the Internet
at http://pubs.acs.org.
intensities of the emission maxima, with the strongest transition at
545 nm, when probed directly (λex ) 366 nm) or through the aryl
chromophore of the aminobenzyl group (λex ) 266 nm). This
suggests that the binding site is the same in both cases. Furthermore,
the luminescence lifetimes obtained for 6 and 7 (Table 1) give
similar hydration numbers, showing that the metal environment is
almost certainly the same in both cases. Having thus ascertained
that the terbium remains in a DO3A-like binding site through the
course of the synthesis of 7, it is reasonable to assume that the
complex is kinetically robust. Reaction of 7 with ytterbium
trifluoromethanesulfonate yielded the trinuclear complex 1. Ytter-
bium has been shown to reduce the luminescence intensity of
terbium in kinetically unstable systems.13 In our system the metals
are held in close proximity with one another throughout the
experiment, and the luminescence from the ytterbium center can
be studied without difficulty. Excitation at 337 nm using a nitrogen
laser gave rise to emission from both metal centers. The emission
from the terbium center was reduced in intensity relative to the
terbium-centered luminescence in 7, and a new emission at 980
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