Synthesis of asymmetrically substituted cyclen-based ligands for the controlled sensitisation of lanthanides

Article abstract of DOI:10.1039/b705757a

A series of unsymmetrical cyclen-based ligands incorporating an antenna and a quencher have been prepared for the complexation of the visible- (Eu, Tb) and near IR-emitting (Nd, Yb) lanthanides. Eu and Tb were sensitised with coumarin 2, and Nd and Yb with rhodamine. Both antennae were paired with nucleoside (uridine and adenosine) quenchers. The interaction between the quencher and the antenna can be regulated by the addition of the complementary base or DNA to the complexes, resulting in changes in the lanthanide luminescence intensity and lifetime. The Royal Society of Chemistry.

Full text of DOI:10.1039/b705757a

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Synthesis of asymmetrically substituted cyclen-based ligands for the
controlled sensitisation of lanthanides†
K. Eszter Borbas‡ and James I. Bruce*
Received 16th April 2007, Accepted 23rd May 2007
First published as an Advance Article on the web 11th June 2007
DOI: 10.1039/b705757a
A series of unsymmetrical cyclen-based ligands incorporating an antenna and a quencher have been
prepared for the complexation of the visible- (Eu, Tb) and near IR-emitting (Nd, Yb) lanthanides. Eu
and Tb were sensitised with coumarin 2, and Nd and Yb with rhodamine. Both antennae were paired
with nucleoside (uridine and adenosine) quenchers. The interaction between the quencher and the
antenna can be regulated by the addition of the complementary base or DNA to the complexes,
resulting in changes in the lanthanide luminescence intensity and lifetime.
Introduction
by an increase or decrease in luminescence intensity, making
unambiguous detection difficult. There are notable exceptions,
a particularly elegant one being the Cu(I)-probe developed by
The luminescent lanthanide ions have been receiving considerable
attention from the scientific community for a number of years.
This interest stems from their excellent properties, namely their
long emission wavelengths (400–1300 nm), well-defined, line-like
spectra, and long emission lifetimes (ls to ms range), which
make them suitable for life sciences applications. As the f–f
transitions involved in lanthanide luminescence are Laporte-
forbidden, absorption coefficients are typically <1. Effective
excitation can be performed via a light-harvesting antenna capable
of transferring the excitation energy to the metal centre. This is
well established and europium and terbium have been sensitised
with, among others, benzyloxyquinoline, acridone derivatives,
phenanthridine, pyridines and dansyl groups. Sensitisation of
Nd(III), Yb(III) and Er(III) is possible with fluorescein, pyrene,
Pd-porphyrins, eosin, Tb(III)-complexes, rhenium(II)- and
ruthenium(II) bipyridyl moieties, and quinoline. However,
efforts to control the degree and extent of sensitisation by the
antenna are more limited. Attention has focused mainly on the
7
Viguier and Hulme. We have been interested in developing
lanthanide-based luminescent sensors that are non-emissive in the
absence of their substrates, and become emissive upon binding.
We have sought to control the energy transfer between the
excited state of the antenna and the lanthanide. Recently we have
reported the synthesis of a series of Nd(III) and Yb(III) complexes
that incorporated a quencher moiety in close proximity of a
1
2
27
rhodamine antenna. The quencher (the nucleotides adenosine
and uridine) possesses a recognition unit (the hydrogen-binding
motif), which enables substrate binding, i.e. Watson–Crick base
pairing or p–p stacking. The formation of base pairs impairs the
quencher’s ability to disrupt energy transfer from the antenna to
the lanthanide, thus turning the emission ‘on’. Here we report
the full characterisation of the Nd(III) and Yb(III) complexes, as
well as the preparation of a series of Eu(III) and Tb(III) complexes
designed according to the same principle.
3
4
5
6
7,8
9
10
1
1
12
13
14
15
16
The general complex design is shown in Chart 1. Cyclen was
chosen as the scaffold to which the antenna, the quencher and
the additional stabilising arms could be attached via a series
of N-alkylations. Many traditional cyclen based ligands have
identical coordinating arms on the nitrogens or are only partially
substituted. Some examples exist with two types of arms. The
key requirement for the successful strategy was to develop a
method for three different types of pendant arms to be sequentially
added to the cyclen ring. The ligands for europium and terbium
complexation would be equipped with a coumarin 2 antenna,
which, with an emission maximum of 432 nm, is ideally suited
1
7,18
visible-emitting europium and terbium complexes,
which have
19
20
found applications in fields as diverse as pH-sensing, DNA-,
21
22
7
oxygen-, organophosphate- and Cu(I)- detection. Recently,
complexes of the near infrared-emitting neodymium, ytterbium
23–25
and erbium
have been gaining attention. Excitation of the latter
group can be performed with visible light (>400 nm), avoiding
interference from the medium, for example biological samples.
Although a number of examples of luminescent detection with
lanthanide fluorophores have been reported,
of the sensors developed to date signal the substrate presence
7
,19–22,26
the majority
28
for the sensitisation of these lanthanides. Adenosine or uridine
The Department of Chemistry, The Open University, Walton Hall, Milton
Keynes, UK MK7 6AA. E-mail: j.i.bruce@open.ac.uk; Fax: +44(0)1908
8
58327; Tel: +44 (0)1908 654171
†
Electronic supplementary information (ESI) available: General experi-
mental procedures for selected compounds. Excitation spectrum for 1ATb,
emission spectra for 2AYb and 2UYb with and without added substrates,
changes in emission spectra of 1UEu and 1ATb with complementary bases,
and the spectrum of 1UEu with dsDNA. MMX calculated geometries for
synthetic intermediates and an NMR titration curve for the free ligand.
See DOI: 10.1039/b705757a
‡
Present address: Department of Chemistry, North Carolina State Uni-
versity, Raleigh, NC 27695-8204.
Chart 1
2
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serve as the quencher as it was shown that the redox couples
conditions yields almost exclusively N1,N7-alkylated products.
Thus alkylation of the third nitrogen in the cyclen ring with the
second ethylbromoacetate will always leave one of the nitrogens
of these nucleosides make them ideal quenchers via an electron
29
transfer process. These quenchers were chosen both for their
29
31
known abilities to quench both rhodamine and coumarins, and
for their synthetic accessibility. The synthesis of the neodymium
and ytterbium complexes ([Nd(2U)], [Yb(2U)], [Nd(2A)], [Yb(2A)])
cis to the antenna free. The fourth substituent can then be
introduced in the final step. An alternative approach branches off
at the monoalkylated cyclen derivative 3. This route closely mimics
our synthesis of the cyclen-based ligands for the sensitisation
of Nd and Yb, and we were eager to test the strategy on
27
have been described previously.
To maximise the antenna–quencher interaction it was deemed
essential to introduce these functionalities at adjacent nitrogens of
the cyclen. The other two nitrogens would bear carbonyl-donor
substituents, thus yielding a ligand with eight donor atoms. The
coordination sphere is expected to be completed with a solvent
molecule.
27
different antenna–quencher pairs. Briefly, 3 can be cis-alkylated
in chloroform in the presence of triethylamine with L-X to give 6.
Simultaneous alkylation of the remaining two secondary nitrogens
with ethyl bromoacetate introduces the stabilising arms. In the final
step the quenchers can be attached via amide bonds through the
primary amino group of the linker, yielding 1 (1U or 1A).
We have considered a second approach to the desired ligands,
starting from a trifold-protected cyclen derivative, bearing two
orthogonal protecting groups (Route B). In Route B the stabilising
arms are attached at the final stage of the synthesis to a cyclen
already equipped with both the quencher and the antenna. The
two secondary nitrogens (N7 and N10) are revealed after removal
Results and discussion
1
. Synthesis
The retrosynthetic analysis is outlined in Scheme 1. Two distinct
routes were considered. Route A does not require protection
of the cyclen nitrogens, and is therefore particularly attractive.
Monoalkylation of cyclen (4) with A-X or L-X (where X is a
halogen or a pseudohalogen, A is the antenna, L is a linker)
is possible by a careful choice of conditions, giving 3 as the
32,33
of the bifunctional oxalamide
protecting group in 7 or a
suitable derivative. Stepwise attachment of the quencher and the
antenna are possible by selective protection of one of the non-
amidified nitrogens (N1 or N4). The forward sequence starts with
27,30
33
major product.
Dialkylation of the second and third nitrogen
monoprotection of the known cyclen oxamide 10 to furnish 9.
positions with ethyl bromoacetate at high temperature and/or
polar and protic solvents was expected to furnish mainly 2. It is
known from the literature that the second N-alkylation under such
Alkylation of the fourth (unprotected) nitrogen in 9 with either
a suitably modified antenna, or a linker gives 8. Removal of the
protecting group is followed by the second alkylation step, this
Scheme 1
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time with the quencher, or a linker to which the quencher can be
attached at a later stage to yield 7. Cleavage of the oxalamide with
concentrated sodium hydroxide or hot hydrochloric acid, followed
by dialkylation with ethyl bromoacetate would afford the desired
N1,N4-functionalised ligand 1.
basic alumina. Alternatively, the alkylation could be carried out
in warm DMF with potassium carbonate base. The latter method
yielded sufficiently pure 3 for use in the following synthetic step
after aqueous–organic work-up and trituration with diethyl ether.
2
The major side product in this reaction was (5) . Dialkylation of
The synthesis of the precursors is shown in Scheme 2a.
Coumarin 2 (11) was coupled to chloro- or bromoacetic acid
in EDCI-mediated amide bond-formations giving 5Cl or 5Br in
excellent yield. Chloroacetic acid coupled to coumarin 2 without
the formation of side-products. Unreacted coumarin 2 (33%)
could be recovered by column chromatography. Bromoacetic
acid reacted faster than chloroacetic acid and with quantitative
consumption of coumarin 2, but this reaction gave rise to a small
3 with ethyl bromoacetate yielded 13, with the formation of some
isomeric 13a (<5%) and fully alkylated 13b (30–35%). Reactions
on the secondary nitrogen in 13 with a number of alkylating agents
gave either low yields of the desired product, or no reaction at all
(e. g. 12–15% with N-Boc bromoethylamine). Treatment of 13 with
ꢀ
Ms-5 -O-adenosine yielded a fully equipped ligand 14A in 3.5%
yield after extensive chromatography, but the strategy could not be
extended to the synthesis of 14U. Therefore, the alternative route,
based on our previous synthesis of 2U and 2A was implemented
(Scheme 3).
cis-Alkylation on N4 of 3 was possible in chloroform with
N-Boc-bromoethylamine (12Br). The N1,N4-alkylated 6Boc was
isolated after column chromatography. An analogous reaction
with N-Boc-O-Ms-ethanolamine (12Ms) yielded the same dialky-
lated product in low yield (14%), presumably because of the
increased steric demand of the mesityl group as compared to
1
amount (13% by H NMR spectroscopy) of another compound,
tentatively identified as (5)
difficult to remove. As (5)
2
(see box in Scheme 2), which was
could be removed in the successive
2
steps, this was not a significant problem. Two linkers, 12Ms and
2Br were prepared from ethanolamine and bromoethylamine
1
hydrochloride, respectively, by protection of the primary amino
groups with a Boc protecting group under standard conditions.
Two alkylating quenchers, AMs and UMs were also prepared by
ꢀ
ꢀ
treating 2 ,3 -isopropylidene adenosine or -uridine with Ms-Cl in
the bromide in what is likely to be an S
same reasons, 5 -O-Ms nucleosides (AMs, UMs) also gave the
corresponding N1,N4-functionalised cyclen derivatives in only
N
2 reaction. For the
ꢀ
the presence of TEA (see ESI†). Treatment of cyclen with 5Cl or
5
Br in hot ethanol in the presence of triethylamine yielded mostly
ꢀ
the monoalkylated derivative 3 after column chromatography on
moderate yields. Furthermore, quenchers introduced via 5 -O-Ms
Scheme 2
2
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Scheme 3
nucleosides would lack a carbonyl moiety necessary for an
octadentate ligand. Treatment of 6Boc with ethyl bromoacetate in
TEA. After column chromatography 9Boc was isolated in 70%
yield. Treatment of 9Boc with 5Cl or 5Br in the presence of
potassium carbonate and sodium iodide in DMF afforded the
desired product 8Boc after standard aqueous–organic work-up
and column chromatography in only moderate yields. Two side-
reactions were identified: (1) decomposition of 5Cl and 5Br to
the presence of K
2
CO and NaI in acetonitrile furnished the fully
3
N-alkylated cyclen derivative 15Boc in excellent yield. Cleavage of
the Boc protection revealed the primary amino group to which
ꢀ
34
5
-carboxynucleotides 16 and 17 could be coupled in an EDCI-
mediated amidation in 84% and 47% yields, respectively. This short
5-step) synthesis could be applied to the preparation of several
2
coumarin 2 and (2) dimerisation of 5Cl and 5Br to yield (5) .
(
A model reaction using N-methylaniline chloroacetate instead
of 5Cl under the same conditions proceeded with 51% yield,
further supporting the theory that the low yields in the case of the
coumarin 2 antenna were at least partially due to the instability of
5Cl and 5Br, in addition to the low reactivity of 9Boc. Acidolysis
of the Boc protecting group of 8Boc, followed by neutralisation
afforded secondary amine 18. Treatment of 18 with either of the
linkers (12Br or 12Ms) gave derivatives of 19 in prohibitively low
yields (10–12% yield after extensive chromatography). Therefore,
Route B was abandoned at this point. The failure of this strategy
is most likely due to a combination of several factors, among
them the strongly electron-withdrawing nature of the oxalamide
hundreds of milligrams of 1U or 1A. Complexation of Eu(III) and
Tb(III) was accomplished by treatment of methanolic solutions
of 1U and 1A with the corresponding anhydrous chlorides.
The complexes ([Eu(1U)], [Tb(1U)], [Eu(1A)] and [Tb(1A)]) were
isolated by precipitation from cold ether, followed by freeze-
drying from water. A similar reaction sequence yielded [Nd(2U)],
[Yb(2U)], [Nd(2A)] and [Yb(2A)].
Route B. The attempted route to synthesise the ligands 1U,A
from trifold-protected cyclen derivative 9Boc is shown in Scheme 4.
33
The known oxalamide 10 could be protected on one sec-
ondary nitrogen with Boc CO in chloroform in the presence of
2
Scheme 4
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protection resulting in low reactivity of 9Boc and 18, possible
steric hindrance caused by the coumarin 2, and the sensitivity of
the reactants to the reaction conditions. The oxalamide is also
locking the cyclen ring in a conformation that may render the lone
pair of the fourth nitrogen less accessible (see MMX-calculated
geometry in ESI†).
3. Luminescent properties
Neodymium and ytterbium complexes. The absorption spectra
of the ligands 2U and 2A displayed maxima at 355 and 545 nm,
27
indicative of the rhodamine antenna. Excitation of [Nd(2U)],
Yb(2U)] and [Yb(2A)] at 355 nm resulted in characteristic
[
near IR emission (Fig. 1 and ESI†). On the other hand, only
very short-lived luminescence was observed for [Nd(2A)]. For
all complexes, steady-state emission spectra were calculated by
integration of each decay and plotting the values as the function
of the wavelength. For 2ANd without added substrate this was
not possible due to the low signal-to-noise ratio, but a typical
Nd-spectrum was obtained with emission lines centred at 890 nm
2
. Characterisation
1
The intermediates and the ligands were characterised by H and
1
3
1
C NMR spectroscopy. In most cases, to aid the assignment, H-
1
1
13
H COSY, H- C correlation, DEPT and NOESY experiments
were performed. Elemental compositions were confirmed by high
resolution ESI-MS spectroscopy.
4
4
4
4
( F3/2 → I9/2) and 1064 nm ( F3/2 → I11/2) upon plotting the decay
values for the adenosine- or uridine-treated [Nd(2A)]. Quenching
of the antenna by the built-in quencher (adenosine) was almost
complete. The quenching could be disrupted by engaging the
quencher in base pairing with either uridine or adenosine. Thus,
as proposed in the original design for these types of molecules,
addition of the substrate ‘turns on’ the non-luminescent probe.
The lifetimes for the NIR emitting complexes were measured
35
using a known method, and were comparable to previously
reported complexes.
The luminescent lifetimes of the europium and terbium
complexes were determined following established procedures
3
6
(
Table 1). The lifetimes are in good agreement with data obtained
3,18,36
for similar compounds.
The number of co-ordinated solvent
molecules (OH-oscillators, n) was calculated from the differences
in the luminescent lifetimes of the complexes in H O and D
using eqn (1), where q stands for the number of water molecules
2
2
O
−
1
bound to the central atom, A is an experimental value (0.525 ms
−
1
in the case of Eu per OH oscillator, 2.1 ms for Tb). The rate
constants, assuming first-order decays, are the reciprocals of the
experimentally determined lifetimes. As expected, q is >1. The q
values obtained for compounds [Eu(1U)], [Tb(1U)], [Eu(1A)] and
[Tb(1A)] are somewhat higher than for similar octadentate ligands
while being less than two as has been observed for heptadentate
1
ligands. The carbonyl group on the fourth arm bearing the
quencher forms a seven-membered ring when coordinated the
lanthanide ion and may not be as closely bound to the metal ion.
It is probable that a second water molecule is partially coordinated
at a longer distance as a result. It should be noted that the less
polarisable carbonyl oxygen atoms on the ester groups makes them
poorer donors for lanthanides compared to the amide groups.
While we observed no decomplexation in solution, it is possible
that a slight increase in lability due to the donor ester groups also
allows a second water molecule to partially coordinate.
Fig. 1 Emission spectra for [Nd(2A)].
Compound [Nd(2U)] was only slightly luminescent, and was
further quenched upon the addition of either its complementary
base (adenosine) or its own base (uridine). The 890 nm emission of
Nd(2A)] + A, + U and [Nd(2U)] consisted of two peaks, at 875 nm
and at 900 nm. A similar splitting was observed for the F3/2
11/2 transition (1055 nm and 1065 nm). The Yb-complexes were
weakly luminescent with emission maxima at 980 nm ( F5/2
7/2). Addition of the complementary bases to either [Yb(2U)]
or [Yb(2A)] resulted in further decrease in the emission intensity.
The changes in the luminescence lifetimes of [Nd(2U)], [Nd(2A)],
[
4
→
q = A(kH2O − kD2O
)
(1)
4
I
2
→
2
I
Table 1 The lifetimes and the hydration states of the Eu, Tb complexes,
and the lifetimes of the Nd and Yb complexes. Luminescence decay
measured at 592 nm (Eu), 545 nm (Tb), 890 nm (Nd) and 980 nm (Yb)
[
Yb(2U)] and [Yb(2A)] upon addition of complementary bases and
[
Eu(1U)]
[Tb(1U)]
[Eu(1A)]
[Tb(1A)]
27
solutions of the quenchers has already been reported. Briefly, the
Nd-complex [Nd(2A)] had a very short-lived luminescence, which
increased considerably upon the addition of the complementary
base uridine. In the cases of the other three compounds ([Nd(2U)],
s(D
s(H
q
2
2
O)
O)
2.70 ms
0.61 ms
1.34
0.35 ms
0.27 ms
1.72
3.11 ms
0.49 ms
1.67
0.56 ms
0.40 ms
1.47
[
Yb(2U)] and [Yb(2A)]), the lifetimes were comparable to those
[
Nd(2U)]
[Yb(2U)]
[Nd(2A)]
[Yb(2A)]
37
reported for similar octadentate complexes. Addition of the
complementary bases to [Nd(2U)], [Yb(2U)] and [Yb(2A)] resulted
in a decrease of the lifetimes.
a
s/lsb
s/ls
0.09
0.47
0.81
2.61
0.89
—
2.23
1.17
0.23
0.16
c
a
a
s/ls
—
—
Europium and terbium complexes. The excitation spectrum of
[Tb(1A)] in water (pH 7.4) displayed a maximum at 328 nm,
which is consistent with the presence of the coumarin 2 antenna
a
c
b
Lifetime could not be calculated. 1 equiv. complementary base added.
1
equiv. own base added.
2
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(
ESI†). Excitation of [Eu(1U)], [Tb(1U)], [Eu(1A)] and [Tb(1A)]
is possible at 328 nm without interference from the metals or
the quenchers. The emission spectra of [Eu(1U)] and [Eu(1A)]
(
Fig. 2) are indicative of highly asymmetrical species, with the high
intensity of the hypersensitive DJ = 2 transition at 618 nm being
particularly indicative of the asymmetrical environment around
the europium ion. Furthermore the magnetic-dipole allowed
DJ = 1 transition shows the components allowed by the low
symmetry environment. The emission spectra have similar splitting
patterns and intensities to heptadentate complexes reported by
1
Parker and the intensity and splitting pattern of the DJ = 4
is further indication of the unsymmetrical environment. It is
also clear that the coumarin is not completely quenched by the
nucleotides and is able to serve as the sensitiser. Addition of
the complementary base adenosine to a solution of [Eu(1U)] in
methanol, acetonitrile, or pH 7.4 MOPS buffer resulted in a
modest increase in the luminescence intensity at DJ = 1 (588 nm)
and DJ = 2 (613 nm) (ESI†). Consistent data could not be
obtained for the adenosine quencher-equipped europium complex
Fig. 3 Increase in luminescent lifetimes for [Eu(1U)] (upper) and [Tb(1U)]
(
lower) upon addition of adenosine.
Table 2 Stern–Volmer constants (K) of the quenching of the lanthanide
complexes by DNA and the complementary bases
[
Eu(1A)]. The Tb(III)-complexes were quenched by the addition of
[
Eu(1U)]
[Tb(1U)]
[Eu(1A)]
[Tb(1A)]
their substrates (ESI†). One equivalent of complementary base
quenched [Tb(1U)] and [Tb(1A)] by approximately 37% and 20%,
respectively. The quenching is presumably the result of direct
quenching of the antenna by the added complementary base.
b
a
a
3
a
3
c.b.
DNA
6.8 × 10
1.0 × 10
8.4 × 10
6.8 × 10
4
3
3
1.8 × 10
a
b
No consistent data were obtained and K could not be calculated. c.b.:
complementary base.
the Stern–Volmer constants from fitting I(0)/I = 1 + K(SV)[Q] (I
0
:
initial intensity, I: actual intensity, [Q]: quencher concentration)
38
to the initial (linear) sections of the Stern–Volmer plots. The
Stern–Volmer constants are summarised in Table 2. These are
consistent with a dynamic quenching process occurring up until
one equivalent of the substrate is added in the case of nucleosides.
In this case the SV constants are a measure of K and are
d
of an order for Watson–Crick interactions between nucleosides.
This was confirmed by titrating the ligands with nucleoside and
1
monitoring their H NMR spectra in DMSO. Monitoring shifts
in the NH protons of the nucleosides gave binding constants
comparable to the binding of nucleoside base pairs in DMSO. The
interaction with dsDNA is more complex with static quenching
likely to play a role if intercalation is occurring.
Fig. 2 Emission spectrum of [Eu(1A)].
Thus for the all the europium and terbium complexes reported
in this paper the lanthanide sensitisation could not be completely
switched off and intensity and lifetime changes were modest upon
substrate addition. This reflects that the rate of energy transfer
from the antenna is faster than the electron transfer quenching
processes. The changes upon binding are therefore minimal.
Adenosine, being more readily oxidised, is a superior quencher
to uridine. This is similar to the behaviour reported by Seidel
When the luminescent lifetimes (s) of [Eu(1U)] or [Eu(1A)]
were monitored at 588 nm, slight (approximately 0.05 ms for
both complexes) increases were observed upon addition of 1
equivalent of complementary base (Fig. 3) and this could be
observed reproducibly. The s of [Tb(1U)] increased and that of
[
Tb(1A)] slightly decreased upon addition of the complementary
bases.
Treatment of Eu(III)-complexes [Eu(1U)] and [Eu(1A)] with
29
et al. Thus when added as substrate it is also able to quench the
antenna directly.
dsDNA quenched the lanthanide emission, possibly due to
intercalation of the coumarin 2 antenna with the DNA strands.
A similar trend was observed for [Tb(1A)], while no consistent
data could be obtained for [Tb(1U)]. The lifetimes did not show
a consistent variation for [Eu(1U)] and [Eu(1A)], presumably due
to the presence of a number of competing processes. A decrease
was observed for the lifetime changes of [Tb(1A)], and a minor
increase for [Tb(1U)] (data not shown).
Conclusions
A series of donor–acceptor–quencher (D–A–Q) triads were
synthesised and characterised. The donors are coumarin 2 or
rhodamine, and the acceptors are the luminescent lanthanides
Eu(III) and Tb(III), or Nd(III) and Yb(III), respectively. The D–
A–Q moieties are held together by a cyclen scaffold. Disruption
The quenching of [Eu(1U)], [Tb(1U)], [Eu(1A)] and [Tb(1A)] by
complementary bases and dsDNA were analysed by calculating
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of the lanthanide sensitisation by the antennae was attempted
by the incorporation of nucleotide quenchers into the molecules.
In one instance, ([Nd(2A)]), a very short-lived luminescence was
observed that could be extended by the addition of the substrate
to [Nd(2A)]. This highly efficient and flexible synthesis enables the
straightforward introduction of other antenna–quencher pairs to
fine-tune the photophysical and recognition behaviour of similar
lanthanide complexes.
(d, J = 0.9 Hz, 1H), 7.03 (d, J = 4.0 Hz, 1H), 7.48 (s, 1H), 7.66 (m,
1
3
1H), 7.80 (br s, 1H); C NMR 12.62, 13.93, 14.10, 17.19, 17.30,
18.55, 25.00, 26.94, 37.00, 43.00, 51.82, 52.57, 53.35, 54.86, 55.48,
59.94, 60.02, 82.99, 83.97, 102.44, 113.81, 115.48, 117.20, 119.84,
127.11, 131.72, 142.27, 151.54, 151.85, 160.09, 169.00, 169.64,
◦
169.64, 169.76, 171.27; mp: turns glassy at 87 C, gas evolution at
◦
◦
−1
110 C, complete melting at 122 C; IR (mmax/(cm ), KBr): 2983,
2938, 2826, 1729, 1697, 1669, 1627, 1614, 1558, 1526, 1455.
1
A. (B = A): Yield: 0.45 g, 47%; HR-ESI-MS (C46
calcd 948.4938 [M + H] , obsd 948.4947 [M + H] ; H NMR d 1.05
t, J = 7.1 Hz, 3H), 1.12–1.21 (m, 6H), 1.33 (s, 3H), 1.54 (s, 3H),
.22 (s, 3H), 2.40 (s, 3H), 2.15–3.34 (m, 33H), 4.01–4.07 (m, 4H),
H
66
N
11
O11):
Experimental
+
+ 1
(
2
General procedures
1
13
H NMR (300 MHz) and C NMR (75 MHz) spectra were
recorded in CDCl unless noted otherwise. Absorption spectra
and fluorescence spectra were collected at room temperature in
O or D O, unless noted otherwise. Infrared absorption spectra
5.34–5.36 (m, 1H), 5.40 (m, 1H), 5.84 (br s, 2H), 6.15 (m, 1H), 6.25
(s, 1H), 7.10 (d, J = 11.3 Hz, 1H), 7.47 (s, 1H), 8.03 (d, J = 1.5 Hz,
3
1
3
1H), 8.22 (d, J = 1.5 Hz, 1H); C NMR 12.79, 14.24, 14.27, 17.35,
18.67, 25.04, 26.83, 43.15, 51.45, 51.58, 51.80, 52.41, 55.15, 56.32,
56.51, 60.21, 83.56, 84.07, 86.78, 91.43, 113.85, 115.70, 117.48,
117.61, 119.75, 119.98, 127.05, 131.86, 140.19, 149.50, 151.54,
151.60, 152.03, 153.09, 155.54, 160.24, 168.77, 171.48; mp: turns
H
2
2
were recorded as neat or paraffin-admixed thin films, or as KBr
pellets. Low resolution ESI-MS spectra were recorded on VG
Quattro equipment in the positive ion mode by direct infusion of
methanolic solution. High resolution mass spectra were recorded
at the EPSRC National Mass Spectrometry Service at Swansea.
Melting points are uncorrected. Solvents were dried according to
standard procedures. Preparative chromatography was performed
using silica or alumina (80–200 mesh). Thin layer chromatography
was performed on silica or alumina. Samples were visualized by
◦
◦
glassy at 105 C, gas evolution at 114 C, complete melting at
137 C; IR (mmax/(cm ), KBr): 2981, 2828, 2361, 2341, 1654, 1577,
1559, 1541, 1522, 1507, 1498.
◦
−1
[
Eu(1U)] and [Eu(1A)]. A solution of 1U or 1A (74 mg or
1 mg, 0.074 mmol) in HPLC-grade methanol (0.5 mL) was treated
with anhydrous EuCl (17.5 mg, 0.067 mmol). The solution was
7
3
UV-light (254 nm and 365 nm), I
2
-vapor or KMnO
4
–K
2
CO .
3
refluxed under Ar for 12 h. The reaction mixture was allowed to
cool back to room temperature. The solution was poured into cold
diethyl ether (5 mL). The mixture was centrifuged and the ether
was poured off the white precipitate. The solid was suspended
in ether (5 mL), and centrifuged again. The ether was decanted
again. The solid residue was dissolved in distilled water (1 mL)
and was filtered through a plug of cotton wool. The sample was
freeze-dried. White solids were obtained:
The Eu and Tb complexes were titrated as solutions in pH 7.4
MOPS buffer, in HPLC-grade acetonitrile or methanol. Lumi-
nescent lifetimes were determined in water and D O following
2
36
standard procedures.
Emission spectra and excited state lifetimes were recorded on a
FluoromaxP or a Fluorolog 3 Tau spectrofluorimeter. Emission
intensities and luminescent lifetimes of the Nd and Yb complexes
35
were determined analogously to a reported method. Steady state
luminescence spectra were obtained by recording the luminescence
decay from 800 nm to 1200 nm (Nd, 5 nm intervals) and from
[
Eu(1U)]. (B = U): Yield: 60 mg, 74%; ESI-MS: 897 [1U − 2
Et] ; H NMR (CD OD) d 9.23, 9.97, 12.02, 13.42, 15.81, 17.28,
8.53, 19.70, 20.94, 21.41, 22.58.
+
1
3
8
00 nm to 1100 nm (Yb, 2.5 nm intervals). The decays were
1
integrated, and plotted as the function of the wavelength to give
the total emission spectra.
[Eu(1A)]. (B = A): Yield: 63 mg, 76%; ESI-MS: 920 [1A −
+
+
1
The synthesis of the Nd and Yb complexes and ligands has been
Et] , 1043 [[Eu(1A)] − 2 Et] ; H NMR (CD OD) d several peaks
3
27
reported previously.
between 1.63 and 8.40, 10.84, 18.01.
ꢀ
1
U and 1A. A solution of 5 -carboxylic nucleotide (2.00 mmol,
[
Tb(1U)] and [Tb(1A)]. A solution of 1U or 1A (74 mg or
1 mg, 0.074 mmol) in HPLC-grade methanol (0.5 mL) was treated
with anhydrous TbCl (19 mg, 0.67 mmol). The solution was
2
eq) and EDCI (0.43 g, 2.2 mmol, 2.2 eq) in DMF (5 mL) was
7
treated with a sample of 15 (0.65 g, 1.0 mmol) in DMF (1 mL).
The reaction mixture was stirred at room temperature for 24 h.
The DMF was removed at reduced pressure. The sample was
3
refluxed under Ar for 12 h. The reaction mixture was allowed to
cool back to room temperature. The solution was poured into cold
diethyl ether (5 mL). The mixture was centrifuged and the ether
was poured off the white precipitate. The solid was suspended
in ether (5 mL), and centrifuged again. The ether was decanted
again. The solid residue was dissolved in distilled water (1 mL)
and was filtered through a plug of cotton wool. The sample was
freeze-dried. White solids were obtained:
dissolved in a mixture of CH
separated and the aqueous phase was extracted with CH
The combined organic phases were washed with water and dried
MgSO ). Purification of the sample by column chromatography
silica, CH
2
Cl
2
and water. The phases were
2
Cl
2
.
(
[
4
i
2
Cl
2
– PrNH
2
(0 → 20%)] gave pale yellow solids:
1
U. (B = U): Yield: 0.78 g, 84%; HR-ESI-MS (C45
H
64
8
N O13):
+
+
1
calcd 947.4485 [M + Na] , obsd 947.4477 [M + Na] ; H NMR
[
Tb(1U)]. (B = U): Yield: 63 mg, 78%; ESI-MS: 1101 [M +
d 1.06 (t, J = 7.1 Hz, 3H), 1.17 (t, J = 7.1 Hz, 6H), 1.28 (s, 3H),
+
+
H
2
O] , 897 [1U − Et] .
1
3
4
.51 (s, 3H), 2.23 (d, J = 1.7 Hz, 3H), 2.35 (s, 3H), 2.17–3.35 (m,
3H), 3.22 (s, 4H), 4.05 (q, J = 7.1 Hz, 4H), 4.01–4.15 (m, 1H),
.53 (m, 1H), 4.92–4.98 (m, 2H), 5.66 (m, 1H), 5.74 (m, 1H), 6.26
[Tb(1A)]. (B = A): Yield: 66 mg, 80%; ESI-MS: 920 [1A −
Et] ; H NMR (CD OD) d 10.76, 13.85, 16.05, 18.04, 20.91.
+
1
3
2
280 | Org. Biomol. Chem., 2007, 5, 2274–2282
This journal is © The Royal Society of Chemistry 2007

View Article Online
1
3
.
Method A
shown by H NMR. This material was of good enough quality
to use in the step. Analytically pure samples could be obtained
Cyclen (4, 2.52 g, 14.7 mmol) and 5Cl (2.87 g, 9.78 mmol) were
by column chromatography [silica, EtOAc–CH
ESI-MS (C15
2
Cl
2
(1 : 9)]: HR-
dissolved in dry DMF (30 mL). K CO (3 g, 22 mmol) and NaI
3 g, 20 mmol) were added to the solution. The reaction mixture
2
3
+
H
16NO
3
Br) calcd 338.0386 [M + H] , obsd 338.0389
(
+
1
[
M + H] ; H NMR d 1.10 (t, J = 7.0 Hz, 3H), 2.27 (s, 3H),
◦
was stirred at 80 C under an Ar atmosphere for 24 h. The DMF
was removed at reduced pressure. The residue was dissolved in
a mixture of water and CH
the aqueous phase was extracted with 2 × 30 mL CH
combined organic phases were washed with 30 mL water, dried
over MgSO , filtered, and evaporated to dryness. The dark yellow
oily residue was dissolved in a minimum amount of methanol and
triturated with ether. The crystals were filtered and washed with
small portions of cold diethyl ether. The product was isolated as
pale yellow crystals in 87% yield (3.65 g). For characterisation see
Method B.
2
6
1
1
1
.40 (s, 3H), 3.13–3.23 (m, 1H), 3.61 (s, 2H), 4.08–4.18 (m, 1H),
13
.28 (s, 1H), 7.09 (s, 1H), 7.49 (s, 1H); C NMR 12.49, 17.38,
8.64, 41.54, 43.99, 116.04, 117.58, 120.38, 127.24, 131.97, 142.17,
51.29, 152.02, 159.96, 165.45; mp: 171 C; IR (mmax/(cm ), KBr):
723, 1676, 1626, 1614, 1558, 1502, 1444.
2
Cl
2
. The phases were separated and
2
2
Cl . The
◦
−1
4
+
1
(5) . (Limited characterisation) ESI-MS: 513 [M + H] ; H
2
NMR d 1.11 (t, J = 7.1 Hz, 6H), 2.23 (s, 6H), 2.39 (s, 6H), 3.20–
3.31 (m, 2H), 3.45 (d, J = 15.8 Hz, 2H), 3.74 (d, J = 15.8 Hz,
2H), 4.07–4.19 (m, 2H), 6.28 (s, 2H), 7.03 (s, 2H), 7.53 (s, 2H);
C NMR 12.76, 17.12, 18.65, 30.29, 43.67, 60.66, 116.18, 117.58,
20.56, 132.01, 140.39, 151.24, 152.20, 159.79, 171.14; mp: 133–
1
3
1
1
1
◦
−1
34 C; IR (mmax/(cm ), KBr): 3450, 3066, 2983, 2930, 1728, 1662,
611, 1557, 1499, 1448.
Method B
Cyclen (4, 2.52 g, 14.7 mmol) and 5Cl (2.87 g, 9.78 mmol)
were dissolved in 100% EtOH (25 mL), in the presence of TEA
6
Boc. A sample of 3 (3.10 g, 7.23 mmol) was dissolved in
CHCl (100 mL) and TEA (10.20 mL, 7.40 g, 72.3 mmol) was
added to the solution. The solution was flushed with Ar for
0 min. N-Boc-Bromoethylamine (1.77 g, 7.95 mmol), dissolved in
CHCl (20 mL) was added dropwise (1 h). Stirring was continued
for 24 h. Water was added. The phases were separated and
the aqueous phase was extracted with CH Cl . The combined
organic phases were washed with water and dried (MgSO ). The
solution was concentrated. The sample was subjected to column
3
(
2
4.12 mL, 29.34 mmol, 3 eq). The solution was heated at reflux for
4 h under an inert atmosphere. The solvents were evaporated.
Chromatography on basic alumina [pH 9.5, CH Cl –MeOH
0 → 10%)] yielded a pale yellow foam (1.89 g, 45%): HR-ESI-MS
): calcd 430.2813 obsd 430.2814 [M + H] ; H NMR
1
2
2
3
(
(
+
1
C
23
H
35
N
5
O
3
2
2
d 1.11–1.16 (t, J = 6.8 Hz, 3H), 2.27 (s, 3H), 2.40 (s, 3H), 2.66–3.32
4
(
m, 17H), 3.41 (q, J = 6.8 Hz, 2H), 3.99–4.11 (m, 1H), 6.26 (s,
13
1
4
1
H), 7.04 (s, 1H), 7.50 (s, 1H); C NMR 12.80, 15.22, 17.53, 18.69,
3.36, 45.33, 46.40, 47.65, 51.87, 65.79, 115.81, 117.27, 120.21,
27.48, 131.95, 142.23, 151.58, 152.07, 160.10, 169.88.
i
chromatography [silica, CH
pale yellow solid (1.21 g, 29%): HR-ESI-MS (C30
2
Cl
2
– PrNH
2
(0 → 20%)] to afford a
H
48
N
6
O
5
): calcd
+
1
5
3
4
1
5
1
5
1
73.3759 [M + H] , obsd 573.3765; H NMR d 1.06 (t, J = 7.1 Hz,
H), 1.22 (s, 9H), 2.24 (s, 3H), 2.39 (s, 3H), 2.38–3.23 (m, 24H),
5
Cl. Coumarin 2 (11, 210 mg, 1.00 mmol) was dissolved in
CH Cl (10 mL). Chloroacetic acid (0.13 g, 1.30 mmol) and EDCI
0.19 g, 1.00 mmol) were added and the solution was stirred at
room temperature for three days. The solution was diluted with
CH Cl and water, the phases were separated, the aqueous phase
was extracted with CH Cl . The combined organic phases were
washed with dilute NaOH and water. The organic layer was dried
MgSO ), filtered and concentrated. Chromatography [silica, ethyl
acetate–hexane (1 : 1)] yielded recovered coumarin 2 (70 mg,
3%) and a white, crystalline solid (200 mg, 67%): HR-ESI-MS
1
3
.02 (m, 1H), 6.26 (s, 1H), 7.00 (s, 1H), 7.48 (s, 1H); C NMR
2.72, 17.22, 18.55, 28.19, 38.79, 43.09, 45.96, 47.41, 47.63, 51.49,
1.75, 51.98, 53.16, 53.35, 77.98, 115.67, 117.07, 120.02, 127.25,
31.65, 142.70, 151.44, 152.05, 156.07, 159.96, 169.56, 172.73; mp:
2
2
(
2
2
◦
−1
5 C; IR (mmax/(cm ), KBr): 3368, 2975, 2933, 2824, 1703, 1664,
2
2
626, 1614, 1559, 1502.
(
4
1
5Boc. A sample of 6Boc (1.09 g, 1.91 mmol) was dissolved
in CH CN (6 mL). K CO (4.25 g, 30.6 mmol), NaI (4.58 g,
0.6 mmol) and ethyl bromoacetate (2.12 mL, 3.19 g, 19.1 mmol)
3
2
3
3
3
+
+
(
C
15
H
16NO
3
Cl): calcd 294.0891 [M + H] , obsd 294.0891 [M + H] ;
were added to the solution. The mixture was refluxed under Ar
for 24 h. The sample was concentrated. The residue was dissolved
in a mixture of water and CH
the aqueous phase was extracted with CH
organic phases were washed with water and dried (MgSO
The solution was concentrated. Column chromatography [silica,
CH
0%): ESI-MS: 743 [M + H] , 765 [M + Na] ; H NMR d 1.12
m, 3H), 1.25 (m, 6H), 1.38 (s, 9H), 2.32 (s, 3H), 2.45 (s, 3H),
.00–3.21 (m, 33H), 4.15 (m, 1H), 6.29 (s, 1H), 6.95 (s, 1H), 7.62
1
H NMR d 1.11 (t, J = 7.0 Hz, 3H), 2.27 (s, 3H), 2.39 (s, 3H), 3.14–
3
1
1
1
1
.26 (m, 1H), 3.67 (s, 2H), 4.04–4.18 (m, 1H), 6.28 (s, 1H), 7.08 (s,
H), 7.48 (s, 1H); C NMR 13.04, 17.94, 19.17, 42.05, 44.54,
16.62, 118.16, 120.88, 127.75, 132.51, 142.55, 151.96, 152.78,
60.55, 165.86; mp: 172 C; IR (mmax/(cm ), KBr): 1722, 1678,
626, 1615, 1558, 1502, 1444, 1402.
2
Cl
2
. The phases were separated and
Cl . The combined
).
1
3
2
2
4
◦
−1
i
2
Cl
2
– PrNH
2
(0 → 5%)] yielded an off-white solid (0.99 g,
+ + 1
7
(
5
Br. Coumarin 2 (11, 4.06 g, 18.70 mmol) and bromoacetic
acid (10.40 g, 74.80 mmol, 4 eq) were dissolved in CH Cl
2
2
.
2
(
EDCI (14.36 g, 74.80 mmol, 4 eq) was added in small portions
to the vigorously stirred solution. The reaction was allowed to
proceed for 24 h at room temperature. The reaction mixture was
1
3
s, 1H); C NMR 12.58, 14.06, 14.12, 17.53, 18.87, 28.38, 43.48,
4
1
1
3.71, 50.52, 53.40, 55.10, 55.73, 56.27, 61.23, 61.36, 67.74, 79.98,
05.01, 115.84, 116.78, 117.10, 120.49, 127.99, 141.48, 141.55,
51.86, 160.06, 170.64, 173.24.
diluted with water and CH
the aqueous layer was extracted with CH
organic phases were washed with water and NaOH (2 M), dried
MgSO ), and filtered. The sample was concentrated to dryness
2
Cl
2
. The phases were separated and
2
2
Cl . The combined
15. A solution of 15Boc (0.99 g, 1.33 mmol) in CH
was treated with TFA (5 mL). The reaction mixture was stirred
at room temperature for 30 min. The volatile components were
2
2
Cl (10 mL)
(
4
to yield a white solid (5.28 g, 84%), which was ∼87% pure, as
This journal is © The Royal Society of Chemistry 2007
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View Article Online
evaporated. The residue was redissolved in CH
aqueous KHCO was added in small portions until pH 8 was
reached. The aqueous phase was extracted with CH Cl , and the
combined organic phases were washed with water. The organic
phase was dried (MgSO ). A pale yellow solid was obtained
2
Cl
2
. Concentrated
13 S. Faulkner and S. J. A. Pope, J. Am. Chem. Soc., 2003, 125, 10526.
1
4 M. R. Sambrook, D. Curiel, E. J. Hayes, P. D. Beer, S. J. A. Pope and
S. Faulkner, New J. Chem., 2006, 30, 1133.
5 K. S e´ n e´ chal-David, S. J. A. Pope, S. Quinn, S. Faulkner and T.
Gunnlaugsson, Inorg. Chem., 2006, 45, 10040.
16 A. Casnati, F. Sansone, A. Sartori, L. Prodi, M. Montalti, N.
3
2
2
1
4
Zaccheroni, F. Ugozzoli and R. Ungaro, Eur. J. Org. Chem., 2003,
after evaporation of the solvent, which was used without further
1
475.
+
purification (0.72 g, 84%): ESI-MS: 643 [M + H] , 686 unassigned,
1
7 C. Yang, L.-M. Fu, Y. Wang, J.-P. Zhang, W.-T. Wong, X.-C. Ai, Y.-F.
Qiao, B.-S. Zou and L.-L. Gui, Angew. Chem., Int. Ed., 2004, 43, 5010;
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8 P. J. Skinner, A. Beeby, R. S. Dickins, D. Parker, S. Aime and M. Botta,
J. Chem. Soc., Perkin Trans. 2, 2000, 1329.
19 S. Blair, M. P. Lowe, C. E. Mathieu, D. Parker, P. K. Senanayake and
1
H NMR d 1.07 (t, J = 6.9 Hz, 3H), 1.13–1.26 (m, 6H), 2.25 (s,
3
1
H), 2.39 (s, 3H), 2.32–3.74 (m, 27H), 4.04–4.17 (m, 5H), 6.26 (s,
1
3
H), 7.00 (s, 1H), 7.50 (s, 1H), 7.95 (s, 1H), 8.01 (br s, 1H);
C
1
NMR 12.63, 14.00, 17.20, 18.64, 43.22, 49.16, 49.82, 50.95, 51.88,
5
1
3.39, 54.95, 60.42, 63.99, 115.74, 117.11, 127.47, 151.99, 160.09,
62.44, 164.67, 166.44, 171.38.
R. Kataky, Inorg. Chem., 2001, 40, 5860.
2
2
0 G. Bobba, J. C. Frias and D. Parker, Chem. Commun., 2002, 890.
1 B. Song, G. Wang, M. Tan and J. Yuan, J. Am. Chem. Soc., 2006, 128,
1
3442.
2 J. R. Schwierking, L. W. Menzel and E. R. Menzel, TheScientificWorld,
004, 4, 948.
3 B. P. Burton-Pye, S. L. Heath and S. Faulkner, Dalton Trans., 2005,
46.
Acknowledgements
2
2
2
We would like to thank Dr Andrew Beeby at the University
of Durham for assistance with the measurements of the NIR
spectra. This work was supported by the Nuffield Foundation,
the Open University and EPSRC. High resolution mass spectra
were obtained at the EPSRC National Mass Spectrometry Service
at Swansea. K.E.B. would like to thank Universities UK for an
ORS award.
1
24 A. Beeby, S. Faulkner and J. A. G. Williams, J. Chem. Soc., Dalton
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2
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