Lanthanide luminescent anion sensing: Evidence of multiple anion recognition through hydrogen bonding and metal ion coordination

Article abstract of DOI:10.1039/b705560a

The delayed lanthanide luminescence of the terbium [Tb(iii)] diaryl-urea complex 1·Tb is significantly enhanced upon sensing of dihydrogenphosphate (H₂PO₄^-) in CH ₃CN, which occurs through multiple anion binding

Full text of DOI:10.1039/b705560a

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Lanthanide luminescent anion sensing: evidence of multiple anion
recognition through hydrogen bonding and metal ion coordination{
Cida´lia M. G. dos Santos, Pablo Barrio Ferna´ndez, Sally E. Plush, Joseph P. Leonard and
Thorfinnur Gunnlaugsson*
Received (in Austin, TX, USA) 12th April 2007, Accepted 18th May 2007
First published as an Advance Article on the web 15th June 2007
DOI: 10.1039/b705560a
3,16 in dry CH3CN in the presence of Cs2CO3 and KI at 85 uC for
The delayed lanthanide luminescence of the terbium [Tb(III)]
72 h. This gave 4, which was purified by column chromatography
on alumina. This was followed by the reduction of the nitro group,
using N2H4 in ethanol at 90 uC, in the presence of 10% Pd/C
catalyst, to yield 5, which was reacted with trifluoro-p-tolyl
isocyanide, 6, in dry CHCl3 at room temperature, giving 1 in 80%
yield. The 1H-NMR (CDCl3) spectrum of 1{ showed the presence
of three characteristic N–H resonances at 9.61, 9.38 and 9.01 ppm.
The complex, 1?Tb, was finally formed in 51% yield by refluxing 1
with one equivalent of Tb(CF3SO3)3 in CH3CN under argon,
followed by precipitation from diethyl ether. The ESMS of 1?Tb
showed the expected isotopic distribution pattern and the 1H
NMR (CD3OD) spectrum showed the shifted resonance for the
equatorial and the axial protons of the cyclen and the a-CH2 due
to the presence of the paramagnetic metal ion. Analysis of the
excited state lifetimes of the Tb(III) emission in D2O and H2O,
respectively, gave the hydration state12,13 y1, indicating a single
metal bound water molecule.
diaryl-urea complex 1?Tb is significantly enhanced upon sensing
of dihydrogenphosphate (H2PO42) in CH3CN, which occurs
through multiple anion binding through hydrogen bonding
interactions and potential metal ion coordination to Tb(III).
The luminescent and colorimetric sensing of anions is both a
challenging and highly topical area of research.1–3 The most
common methods for achieving such anion recognition has been
via the use of: (i) transition metal ion coordination complexes;4 (ii)
ammonium or guanidinium moieties,5 and (iii) charge neutral
hydrogen bonding moieties, such as amides, ureas, thiourea, etc.6
Using heptadentate tri-arm cyclen (1,4,7,10-tetraazacyclo-
dodecane) Tb(III) and Eu(III) complexes, we and others have
recently demonstrated the sensing of both aliphatic and aromatic
carboxylates, via the formation of ternary complexes in buffered
solution, where the emission was either enhanced or reduced upon
coordination of these anions to the f-metal ions.7,8 Similarly, using
chromophores, based on charge neutral receptors, we have
developed numerous examples of both fluorescent and colori-
metric anion sensors, which can function in both organic and
aqueous solutions.2,9,10 Nevertheless, the combination of these two
anion binding interactions in a single molecule, with the aim of
achieving maximum luminescent ‘output’ coupled with high anion
affinity, has not been demonstrated to date using members of the
f-metal ion family. However, such sensing has recently been shown
to work well using d-metal ions.11 With the objective of
demonstrating such dual sensing interactions using f-metal ions,
we synthesised the Tb(III) complex 1?Tb from 1. Herein, we
demonstrate the results from this investigation and prove that this
design gives rise to: (a) high anion binding affinity, (b) multiple
binding interactions and (c) large Tb-luminescence enhancements.
The Tb(III) 5D4 transition is Laporte-forbidden and direct
excitation of Tb(III) is often difficult.12 Nevertheless, excitation via
the use of ligand or antennae, gives rise to sensitised emission.12–14
In 1, the aryl-urea has a dual role, it can function as both the
sensitizing antenna12 and the hydrogen bonding anion receptor.2
Therefore, we anticipated that anion binding at the urea moiety
would modulate the sensitization process to the lanthanide excited
state, and hence, the Tb(III) emission.
The absorption spectra of 1?Tb showed a broad band centred
around 280 nm in CH3CN, assigned to the urea antenna moiety.
The changes in the spectra were monitored as a function of added
anions such as AcO2, H2PO42, F2 and Cl2 (as their TBA salt
solutions). The addition of ions such as AcO2 and H2PO42 caused
the absorption maximum to shift towards the red, with
concomitant y20% reduction in the absorption, showing the
The synthesis of 1 (Scheme 1), involved the coupling of the tri-
arm acetamide cyclen 215 and chloro-N-(nitro-phenyl) acetamide,
School of Chemistry, Centre for Synthesis and Chemical Biology, Trinity
College Dublin, Dublin 2, Ireland. E-mail: gunnlaut@tcd.ie;
Fax: +353 1 671 2826; Tel: +353 1 896 3459
{ Electronic supplementary information (ESI) available: Synthesis and
figures 1–14. See DOI: 10.1039/b705560a
Scheme 1 Synthesis of 1 and the Tb(III) sensors 1?Tb.
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Table 1 Results from the binding constant evaluation of H2PO42 and
AcO2, upon binding to 1?Tb in CH3CNa
Anion
H2PO4
Technique
UV-Vis
Stoichiometry LogK Std deviation
7.07b
4.87
5.64
6.79
7.94b
6.86
5.15
5.34
6.84
7.95b
7.04b
4.59
4.90
7.20
8.04b
6.27
5.84
5.78
5.11
6.87
5.12
0.23
2
2
2
G : L
G : L2
G2 : L
G2 : L2
G3 : L
G : L
G : L2
G2 : L
G2 : L2
G3 : L
0.47c
0.95c
0.37
0.21
0.21
H2PO4
H2PO4
Fluorescence
0.25c
0.94c
0.53c
0.20
Fig. 1 The changes observed in the absorption spectra of 1?Tb (4 mM)
2
upon addition of H2PO4 (0–57.3 mM) in MeCN. Arrows indicate the
Tb(III) emission G : L
0.14
changes upon increasing concentration of H2PO42. Insert: The speciation
2
G : L2
G2 : L
G2 : L2
G3 : L
G : L
G2 : L
G : L
G2 : L
0.24c
0.82c
0.18c
0.12
distribution diagram for the binding of H2PO4
.
2
binding of the anions to the receptor, as demonstrated for H2PO4
in Fig. 1. The titration profile obtained from plotting the
absorption changes in Fig. 1, suggested that the anion binding
was more complex than a simple 1 : 1 stoichiometry (see ESI{).
Fitting these changes, using the nonlinear least squares regression
program SPECFIT (see ESI{), gave several binding constants,
reflecting that the anions were bound through the proposed
‘multiple’ binding interactions. These results are summarised in
Table 1, for H2PO42, and AcO2. Here, the stoichiometry (shown
as the guest : 1?Tb ratio Gn : Ln) and binding values (logK) clearly
shows that 1?Tb initially binds both of these anions with large 1 : 1
binding affinity, where H2PO42 is bound marginally stronger than
AcO2. We assign such a strong anion binding affinity to the
neighbouring lanthanide ion, which, being a strong Lewis acid
makes the urea protons stronger hydrogen bonding donors.
From these changes, the speciation distribution diagram was
AcO2
AcO2
AcO2
a
UV-Vis
0.116
0.087
0.302
0.511
0.475
0.497
Fluorescence
Tb(III) emission G : L
G2 : L
Obtained by fitting the spectroscopic data using SPECFIT.
b
Binding constant is large and consequently SPECFIT has
difficulties giving accurate determination for logb. Species present
in less than 10%.
c
7
deactivation of the Tb(III) excited state [5D4 A FJ (J = 6–3)]
2
upon titration with H2PO4 and excitation at 280 nm.{
The overall Tb(III) luminescence changes for titrations of
H2PO42 are shown in Fig. 2. These results were fully reproducible
and clearly demonstrate that the Tb-emission is significantly more
sensitive to the anion recognition than seen in either the ground or
the singlet excited states. Furthermore, the Tb(III) emission, and
hence, the Tb(III) excited state lifetimes (ESI{), are modulated in a
2
also determined (shown as insert for H2PO4 in Fig. 1), which
demonstrated that initially the 1 : 1 binding event (shown as a green
line) dominates, but is then replaced at higher anion concentrations
with higher order stoichiometries such as G3 : L1. Similarly, for
AcO2, both the G1 : L1 and G2 : L1 binding stoichiometries were
observed. The binding of Cl2 was also observed in the absorption
spectra. However, in comparison to the results in Table 1, it was
weak and spectral changes only occurred at high anion concentra-
tions. The titration of F2gave rise to significant changes in the
2
two ‘step’ process, where the initial addition of H2PO4 leads to
ca. 70% quenching in the Tb(III) emission (insert Fig. 2). This is
subsequently followed by over ca. 90% luminescent enhancement
between ca. 1 A 3 eq. of H2PO42. This is, to the best of our
knowledge, the first time that a lanthanide luminescence is
modulated in this manner by an anion.
Moreover, by further analysing the changes in the 546 nm
transition vs. the number of eq. of H2PO42, it was found that the
absorption spectra assigned to multiple F2 binding interactions as
2 17
well as the deprotonation of the urea moiety by F2, to give HF2
.
However, the binding constants for F2 were significantly lower
than those observed for H2PO42 and AcO2 in Table 1 (see ESI{).
With the aim of further investigating these multiple binding
interactions, both the fluorescence and the Tb(III) emission were
monitored upon anion titration. For the former, the emission from
the antenna (lex = 280 nm) gave rise to a broad band centred at
340 nm (see ESI{), that was both weak and significantly polluted
by the contribution from the Tb(III) emission, which occurs at
longer wavelength. However, upon titration with either H2PO42 or
AcO2 the emission was significantly modulated. Even though, the
spectra were of poor quality (see ESI{), we were able to obtain
binding constants from these changes. These are in reasonable
agreement with those obtained for the changes in the ground state,
Table 1, further supporting the determined anion binding
stoichiometries. However, in contrast to these results, the most
significant findings from these investigations were observed in the
lanthanide emission, which showed striking changes for the
Fig. 2 The Tb(III) emission spectra showing the changes in the intensity
of 1? (4 mM) upon titration with H2PO42 (0 A 57.3 mM) in MeCN. Inset:
The changes observed upon addition of 0 A 1 eq. of H2PO42, which
caused quenching in the Tb(III) emission.
3390 | Chem. Commun., 2007, 3389–3391
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Tb(III) emission enhancements also occurred in a two step process;
where the major contribution to these changes took place within
the 2 A 3 eq. range of H2PO42. The changes in the 546 nm
transition were fitted using the SPECFIT programme. This gave
an excellent fit as shown in the ESI,{ from which several binding
constants were determined. Again, the results from the fitting of
the binding isotherm clearly indicated that 1?Tb senses these
anions through multi-step binding interactions and that the
binding constants correlate well with that observed from the
changes in both the absorption and the fluorescence spectra,
Table 1. The speciation distribution diagram obtained from the
use of a combination of hydrogen bonding anion receptors and
f-metal ion coordination for luminescent anion sensing.
We thank Enterprise Ireland, IRCSET and TCD for financial
support and Dr John E. O’Brian for running NMR.
Notes and references
{ The Eu(III) complex of 1, 1?Eu, was also formed. However, the emission
was very weak in comparison to that of 1?Tb, as the Eu(III) excited state is
quenched by photoinduced electron transfer (cf. ESI{).
§ As these measurements are carried out in less competitive CH3CN solvent
the emission enhancement might even be more striking.
2
changes in the Tb(III) emission upon titration with H2PO4 is
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stoichiometry is formed with a high binding constant of logK =
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2
2
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anion recognition? We propose that the initial anion recognition is
due to binding of the anion at the urea moiety of the antenna. This
is then followed by a second binding event between these anions
…
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interactions. We have confirmed this by carrying out UV-vis
titrations of an acetamide analogue of the receptor/antenna part of
1?Tb, which also gave rise to both G1:L1 and G2:L1 binding for
these anions (cf. 7 in ESI{). However, the binding constants are
lower than observed for 1?Tb. In the case of binding of H2PO42 to
1?Tb, the third binding interaction, G3:L1 is more difficult to
determine without the aid of X-ray crystallography. However, this
binding, clearly giving rise to the largest changes in the Tb(III)
emission, is most likely taking place through a more direct
interaction with the metal ion (and possibly via contribution from
one of the aryl protons.18 Contribution from simple electrostatic
interactions between the complex and the anion cannot be ruled
out either). This may be occurring through the displacement of the
aforementioned axial metal bound water molecule. Water is an
effective quencher of the 5D4 excited state and displacement of the
water generally gives rise to luminescence enhancements.7,8§ These
changes are also observed in the ground state, which suggests that
the Tb(III) ion also affects the electronic properties of the antenna.
In summary, we have developed a novel lanthanide luminescent
sensor for anions by incorporating a hydrogen bonding receptor
into a sensitizing antenna. Analysis of the ground state, and the
emission from the singlet and the Tb(III) excited states, clearly
demonstrated the formation of multiple species and hence multiple
binding interactions in solution. It also showed that the Tb(III) is
extremely sensitive to the changes in the local coordination
environment. Furthermore, the selective detection of H2PO42 over
AcO2 by 1?Tb was also observed, with the H2PO42 forming both
1 : 1 and 3 : 1 complexes with 1?Tb, in CH3CN. These results
represent, to the best of our knowledge, the first examples of the
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