pH responsive Eu(III)-phenanthroline supramolecular conjugate: Novel off-on-off luminescent signaling in the physiological pH range

Article abstract of DOI:10.1021/ja035425a

The formation of luminescent supramolecular ternary complexes in water: delayed luminescence sensing of aromatic carboxylates using coordinated unsaturated cationic heptadenatate lanthanide ion complexes. Copyright

Full text of DOI:10.1021/ja035425a

Published on Web 09/12/2003
pH Responsive Eu(III)-Phenanthroline Supramolecular Conjugate: Novel
“Off-On-Off” Luminescent Signaling in the Physiological pH Range
Thorfinnur Gunnlaugsson,*,† Joseph P. Leonard,† Katell Se´ne´chal,†,‡ and Andrew J. Harte†
Department of Chemistry, Trinity College Dublin, Dublin 2, Ireland
Received April 2, 2003; E-mail: gunnlaut@tcd.ie
Scheme 1. Synthesis of the Eu(III)-Based Cyclen Derivative 1a
The development of luminescent signaling devices is an active
area of supramolecular chemistry.1 Those where the emission is
modulated by single or several external sources (inputs) such as
light, ions, and molecules are of particular interest.2 In this regard
luminescent switches, sensors, and logic gate mimics have recently
been reported.2,3 Many life processes, such as enzymes, operate
within a very narrow pH window, where their function or activity
can be described as being “switched on” or “switched off” as a
function of pH.4 Attempts to mimic such “off-on-off” or “on-
off-on” behavior by constructing luminescence devices that are
modulated by a single input, e.g., pH, have recently been achieved
a Reaction conditions: (i) Cs2CO3, 2 and 3,7 DMF reflux; (ii) Eu-
by employing organic fluorophores.5 Lately, Pallavicini et. al. have
(SO3CF3)3, CH3CN, reflux.
extended this kind of work and reported “on-off-on” fluorescent
indicators for H+ using a tetraaza ligand, Cu(II) and Coumarin 343
as a fluorescent “chemosensing ensemble”.6
We have been particularly interested in designing robust multi-
functional lanthanide complexes from cyclen (1,4,7,10-tetraaza-
cyclododecane) as kinetically and thermodynamically stable lumi-
nescent chemosensors, switches, and logic gate mimics, as well as
ribonuclease mimics.7-9 Herein we present the synthesis and the
photophysical evaluation of the cationic tetraamide 1,10-phenan-
throline (phen)-based Eu(III) complex 1.Eu, which is the first
example of a fully reversible pH controlled “off-on-off” signaling
system that employs lanthanide luminescence rather than fluores-
Figure 1. Changes in the Eu(III) emission as a function of pH between
12.5 (bottom spectrum) and 5.5 (top spectrum) upon excitation at 266 nm.
cence, where the “off-on-off” process is due to pH modulation
of the phen ligand.10 In addition to being purely metal-based
emission, the Eu(III) emission changes occur in the physiological
pH range, in a pH window between ca. 4-8. Moreover, the
emission of 1.Eu is long-lived (ms range), emitting at long
wavelengths (between 500 and 750 nm) with line-like emission
bands under ambient conditions. As such, 1.Eu holds greater
advantages over fluorescence systems that can be seriously affected
by autofluorescence and light scattering, for instance from the
physiological environment.
The synthesis of 1.Eu (ESI) was achieved as shown in Scheme
1. 1.Eu was purified by multiple precipitation; first from ether and
then from CH2Cl2, giving the desired complex in 80% yield from
1. 1.Eu was characterized by conventional methods (calculated for
1.Eu: C35H51N10O7F3SEu: 965.2876. Found: 965.2827). The
ESMS showed peaks at 406.9 and 481.9 (m/z) for the M+2 and
(M + triflate)/2 respectively, whereas the 1H NMR (400 MHz, D2O)
indicated a typical mono-capped square antiprism geometry11 as a
major isomer (ca. 95%) in solution, with resonances at 28.4, 0.28,
-3.3, -8.8, -12.1, and -15.7 ppm for the equatorial and axial
ring, and the methylene protons of the pendent arms.
absorptivities of Eu(III) the population of the Eu(III) excited state
(5D0) is achieved indirectly by the phen antenna via sensitiza-
tion.8,9,11 We predicted that the ability of the phen ligand to engage
in such sensitization would be pH dependent since both the amide
and the nitrogens of the phen ligand are sensitive to protonation.
Indeed, this was found to be the case. In alkaline solution between
pH 12-8.5 only a weak Eu(III) emission was observed at 581,
593, 615, 624, 654, 686, and 702 nm for the deactivation of the
5D0 to the ground states 7FJ (J ) 0, 1, 2, 3, and 4) after excitation
at 266 nm. This signifies the rather inefficient population of the
5D0 by the phen ligand in this pH range. The hydration number (q)
for 1.Eu was determined by measuring the Eu(III) luminescent
lifetimes [τEu(III)] in H2O and D2O. This gave τEu(III) ) 0.271 ms
and 0.371 ms for H2O and D2O, respectively (at pH 11), giving q
≈ 1, and as such an overall nine-coordinated complex.11 Upon
further acidification, this sensitization process was greatly enhanced,
and the Eu(III) emission was “switched on” between pH 8.5-5.5,
with sharp luminescence enhancements as demonstrated in Figure
5
7
1; e.g. LE ≈ 20 for D0 f F1, with τEu(III) ) 0.395 and 0.952 ms
in H2O and D2O respectively, which gives q ≈ 1.3, again indicating
that the complex was monohydrated. Further acidification gave rise
to two effects; notably between pH 5-6.5 the Eu(III) emission was
almost constant, whereas between pH 5-3 the emission was
gradually “switched off”, being ca. 70% quenched at pH 2.5 vs
that at pH 6 (Supporting Information). Similarly, using I ) 0.1 M
The pH dependence of the Eu(III) emission was evaluated in
H2O in the presence of 0.1 M tetramethylammonium perchlorate
to maintain constant ionic strength (I).8 Due to the low molar
† Department of Chemistry, Trinity College Dublin.
‡ Laboratorie de Chimie de Coordination et Catalyse, UMR 6509 CNRS
Universite´ Rennes I, Institut de Chimie, Campus de Beaulieu.
9
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J. AM. CHEM. SOC. 2003, 125, 12062-12063
10.1021/ja035425a CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Figure 2. pH-dependence in the luminescent profile for 1.Eu, showing
the “off-on-off” pH-dependent changes in the Eu(III) emission at
592 nm. This pH dependence was fully reversible: in blue the changes
from pH 12 f 1.5, and in red the back-titration of the same sample.
Figure 3. pH dependence in the absorption spectra of 1.Eu, at 266 nm
blue circles, and at 278 nm in red.
nation of the phen nitrogen moiety, the oxidation potential is
increased in conjunction with modification of the ICT excited state
of the phen-amide ligand,9 which reduces the ability of the ligand
to populate the 5D0 state efficiently. We are currently investigating
these features in greater detail.
In summary we have developed a novel lanthanide luminescence
device that displays a clear and complete bell-shaped luminescent
“off-on-off” switching as a function of pH in water. This dual
switching takes place over 4-5 pH units, being centered at pH ≈
6, as well as being fully reversible.
of tetramethylammonium chloride showed identical luminescence
behavior, i.e. these changes were not anion dependent.7
By plotting the changes in the intensity of the 5D0
f
7FJ
transitions as a function of pH gave, for all of these emission bands,
a bell shaped curve, consisting of two sigmoidal slopes, between
pH ca. 3-5 and 6.5-8.5, with maximum intensity being reached
at pH 5.8-6.0. Figure 2 shows these changes for the 5D0 f 7F1 in
blue for the titration of the alkaline solution with HCl, and the back-
titration of this same solution in red, indicating that this pH
dependence is fully reversible. It also clearly shows that the changes
of these “off-on-off” pH-dependent emissions clearly transpire
over the physiological pH range, and, as such, mimic the pH
dependences of many enzymatic processes.4,9 From these lumines-
cence changes two pKas were determined as 3.8 ((0.1) and 8.1
((0.1), the latter being assigned to the deprotonation of the aryl
amide proton in alkaline solution, whereas the former was assigned
to the protonation of one of the phen nitrogen moieties. In
comparison, potentiometric titrations of 1.Eu gave pKas of 3.6
((0.1) and 7.6 ((0.1), respectively, which is similar to those
observed spectroscopically. Furthermore, 1.Eu was found to be
stable to Eu(III) dissociation in this solution over a period of months,
without any loss of its luminescence properties. To the best of our
knowledge 1.Eu is the first example of a lanthanide luminescent
system capable of successfully mimicking such pH dependence by
modulating the sensitization process from the antenna. Furthermore,
these changes cannot be attributed to changes in q as it was pH
independent, with q ) 1.7
Acknowledgment. We thank Enterprise Ireland and TCD for
financial support, and Drs. Hazel M. Moncrieff and Dr. John E.
O’Brien for valuable discussion.
Supporting Information Available: Synthesis and experimental
details for 1.Eu, UV-vis, fluorescence, lanthanide luminescence spectra
and q as a function of pH. Potentimetic titration of 1.Eu (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
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