Multimodal metal cation sensing with bis(macrocyclic) dye

Article abstract of DOI:10.1002/chem.201100998

The synthesis, electrochemical, optical, and cation sensing properties of a bis(macrocyclic) dye 1, in which the benzo-15-crown-5 and phenylazathia-15- crown-5 subunits are linked through a styryl pyridinium moiety, are reported. In this new ditopic recep

Full text of DOI:10.1002/chem.201100998

DOI: 10.1002/chem.201100998
Multimodal Metal Cation Sensing with Bis(macrocyclic) Dye
Elena Tulyakova,[a] Stephanie Delbaere,[a] Yuri Fedorov,[b] Gedaminas Jonusauskas,[c]
Anna Moiseeva,[d] and Olga Fedorova*[b]
Abstract: The synthesis, electrochemi-
cal, optical, and cation sensing proper-
ties of a bis(macrocyclic) dye 1, in
which the benzo-15-crown-5 and phe-
nylazathia-15-crown-5 subunits are
ing site for alkaline earth metal cations
(MgII and BaII), whereas the phenylaza-
thia-15-crown-5 displays a strong bind-
ing affinity towards soft heavy-metal
cations (HgII and AgI). The pro-
nounced changes of the absorption and
fluorescence spectra of this bichromo-
phoric dye observed upon different
metal cation addition make the dye
suitable for dual-wavelength analysis
and offer an enticing potential for
multitasking sensors.
linked through
a styryl pyridinium
Keywords: cations · chromophores ·
crown compounds · dyes/pigments ·
heterometallic complexes
moiety, are reported. In this new ditop-
ic receptor, the benzo-15-crown-5 mac-
rocycle acts as a highly selective bind-
Introduction
necting the above-mentioned components, to maintain a
structural integrity and to allow proper communication be-
tween them.[4] The design of new sensors for cooperative
recognition of different types of metal ions (for instance,
transition-metal cation along with alkaline earth metal) re-
quires the introduction of two recognition units and this is a
topic of primordial importance.[5,6] Naturally, a straightfor-
ward approach would be the design of bifunctional fluores-
cent probes equipped with two receptors that vary either in
cation selectivity or in the complexation/protonation con-
stant and generate coordination-site-specific, spectroscopi-
cally clearly distinguishable signals, for example, strong al-
terations in intensity and/or wavelength as well as fluores-
cence lifetime depending on the underlying operation princi-
ple. At present, the majority of bifunctional molecules that
respond to combinations of protons and alkali or alkaline
earth metal ions are energy-transfer operated. Up to now,
there have been only a few examples reported for bifunc-
tional charge-transfer-operated molecules as well as for sys-
tems signaling transition-metal ions.[4f,7] Such multitasking
sensors could find numerous applications in clinical medi-
cine and biochemistry, for instance, in monitoring bidirec-
tional ion fluxes (e.g., CaII and H+) or the presence of com-
peting, beneficial, or harmful metal ions (e.g., CaII/ZnII
versus CdII).[8]
Molecular signaling systems relying on spectacular changes
in fluorescence intensity and/or absorption band position
upon binding of metal cations have recently received much
attention as photonic molecular devices and are of major in-
terest for (bio)chemical or environmental analysis.[1,2] Many
attempts in the design and construction of chemosensors
employing optical absorption and luminescence changes as
output signals have been reported and some comprehensive
reviews have recently appeared in the literature.[3] An opti-
cal sensor is a molecular device consisting of several inde-
pendent components that have to be properly assembled on
a superstructure with a specific function. Such systems are
constructed by bringing together at least three main subu-
nits: 1) a photoactive fragment displaying a physical proper-
ty that could be easily tuned; 2) a molecular receptor or
pocket that is sensitive to external stimulation and affects
the property of the photoactive unit; and 3) a spacer con-
[a] Dr. E. Tulyakova, Prof. S. Delbaere
CNRS UMR 8516, Universitꢀ Lille Nord de France
3 rue du Professeur Laguesse BP83, 59006 Lille (France)
[b] Dr. Y. Fedorov, Prof. Dr. O. Fedorova
A. N. Nesmeyanov Institute of Organoelement Compounds
28 Vavilova street, 119991 Moscow (Russian Federation)
E-mail: fedorova@ineos.ac.ru
Our strategy towards multimodal metal cation sensing is
based on two well-established binding motifs with different
[c] Dr. G. Jonusauskas
Laboratoire Ondes et Matiꢁre d’Aquitaine
Bordeaux University I, UMR CNRS 5798
351 Cours de la Libiration, 33405 Talence (France)
[d] Dr. A. Moiseeva
Chemistry Department
M. V. Lomonosov Moscow State University
Lenin Hills 1/3, 119991 Moscow (Russian Federation)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100998.
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FULL PAPER
absorption and emission band positions: 1) the HgII or AgI
binding of phenylazathia-15-crown-5 styryl pyridinium dye
as developed recently; and 2) the MgII or Ba(II) benzo-15-
crown-5 coordination.[9,10] Strong differences in the cation
selectivity and basicity of both ligating sites are the chemical
prerequisite for the simultaneous recognition of two differ-
ent metal guests.
Results and Discussion
Synthetic approach and characterization of bichromophore
1 and its metal complexes by NMR spectroscopy: Bichromo-
phoric ligand 1 was obtained in three steps from 2,4-dime-
thylpyridine (Scheme 1). The 4’-formylbenzo-15-crown-5
and 4’-formylphenyl-azathia-15-crown-5 ether precursors
were prepared according to reported procedures.[11] Treat-
ment of 2,4-dimethylpyridine at the 4-position by the 4’-for-
mylbenzo-15-crown-5 ether derivative in the presence of
tBuOK, followed by alkylation with methyl 4-toluene sulfo-
nate and coupling of the 2-methyl function with 4’-formyl-
benzo-15-crown-5 ether afforded bisACTHNUTRGNEUNG(styryl) dye 1 in a 19%
overall yield as a pure E,E isomer.
Figure 1. ESI mass spectrum of bisACTHNUTRGENUGN(styryl) dye 1 showing the mass peak
The elemental analysis data of 1 were in agreement with
the structure. Further characterization of this bichromo-
phore 1 was achieved by electrospray ionization mass spec-
trometry (ESI-MS), which provided the expected mass peak
(m/z 737.3) for 1 and (m/z 796.3) for 1·ACTHUNGTRENNU(G MeCN)·ACHTUNGTREN(UNGN H2O).
1
The H NMR spectrum of bichromophore 1 was fully as-
signed in CD3CN by means of appropriate 2D NMR spec-
troscopy (1H, 13C, 1H–1H COSY, 1H–13C HSQC, 1H–13C
HMBC). A selection and comparison of the most significant
resonances for the compound described here are shown in
Table S1 (Supporting Information). The host ligand 1 is
of singly charged bisACHTUNGTRENNUG
tributed to
(Figure 1).
a
N
ACHTUNGTRENNUNG
composed of a flexible bis
down on the NMR scale of the different aryl–aryl, CH–
PhOCH2, CH–PhN(CH2)2, and Ph–N(CH2)2 signal bond ro-
ACHTUNGTRNE(NUNG styryl) system, and the slowing
A
ACHTUNGTRENNUNG
tation would yield a family of the conformations causing ad-
ditional splitting of the aromatic signals. Thus, two different
families of the proton resonances were observed for protons
NCH3 and H-a’, H-b’, H-2’, and H-3’ of the styryl phenylaza-
thia-15-crown-5 entity. This fact could be attributed to the
coexistence of at least two different conformers in the
ground state, one planar (I) and the second with slightly
twisted geometry (II; see Figure 2) due to the sterically hin-
dered rotation of the styryl phenylazathia-15-crown-5 entity.
1
In contrast, the H NMR spectrum of the bis
N
does not show any broadening or splitting associated with
the H-a and H-b protons, which appear as two sharp dou-
blets at d=7.03 and 7.68 ppm, respectively. This finding is in
accordance with fast conformational equilibrium on the
NMR timescale, presumably CH–PhOCH2 rotation around
the single bond in the benzo-15-crown-5 ether entity or a
stable geometry of one of two possible conformers (most
probably the anti conformer).
Metal-ion-sensing properties: The interactions of 1 with
metal ions were further studied by NMR spectroscopy in
CD3CN to provide some insights into the interaction mecha-
Scheme 1. Synthesis of the bichromophoric dye 1. Reagents and condi-
tions: 1) tBuOK/DMF; 2) MeOTs/D; 3) i) pyrrolidine/BuOH, ii) NaClO4/
MeOH. Ts: tosyl.
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10753

O. Fedorova et al.
result in any spectral changes. The chemical shift changes
are more pronounced for the methylene protons of the
crown ether signals located more closely to the complexed
magnesium(II). For example, the resonances corresponding
to H-a protons are split and spectacularly shielded up to
Dd=0.37 ppm and there is a downfield shift of the proton
H-2’ from 7.38 to 7.65 ppm. Numerous dipolar correlations
between H-a$H-2 and H-b$H-2, H-a$H-6 and H-b$H-
6, and H-a’$H-2’, H-a’$H-3’, H-b’$H-2’, and H-b’$H-3’
found in the ROESY spectrum of 1·Mg2+ indicate the mobi-
lity of the structure and the coexistence of several conform-
ers.
The ionic diameter of the small MgII ion (1.56 ꢃ) fits well
with the size of the internal cavity of the benzo-15-crown-5
ether unit, and is expected to be inserted within the cavity,
surrounded by the oxygen atoms. In contrast, the BaII ion
(2.86 ꢃ) is much larger than the crown ether moiety and it
is known that this cation prefers forming sandwichlike struc-
tures in which the metal cation is held between two cofacial-
ly assembled crown units. After the addition of an equiva-
lent amount of barium(II) at 295 K, the well-resolved reso-
nance signals of the host ligand 1 (Figure 4c) corresponding
to the styryl benzo-5-crown-5 ether entity become broad
and weak, whereas the proton resonances of the styryl phe-
nylazathia-15-crown-5 entity stay unchanged. The proton
signal shift is varied linearly until the mole ratio of 2:1 is
reached; further addition of the cation does not change the
Figure 2. Space-filling representation of the computationally optimized
molecular structures of conformer I (left) and conformer II (right). See
text for details.
nism. The ditopic bisACTHNUTRGNE(NUG styryl) ligand 1 corresponds to a com-
plex receptor, because the two coordination cavities are dif-
ferent. In all cases, considerable changes to the resonance
signals could be noticed upon addition of the alkaline earth
metals as well as silver and mercury perchlorates. The char-
acteristic H-a NMR signals of the benzo-15-crown-5 entity
of the host ligand during the addition of magnesium(II) in
ratios from 1:0 to 1:1 are displayed in Figure 3 and Fig-
ure 4b. The formation of complex 1·Mg2+ in the ratio 1:1 is
clearly indicated by the decrease of the initial group of host
1 signals and the appearance of new downfield resonances
corresponding to the styryl benzo-15-crown-5 ether unit, as
a result of the decrease in the electron density upon magne-
sium(II) binding to the crown cavity. Further addition of
magnesium(II) to 1:2 (ligand/metal) ratio complex does not
1
resonance frequencies. Recording of the H NMR spectrum
of the barium(II)–host 1 complex at 273 K made it possible
to increase the spectral resolution. Only a downfield shift of
the styryl benzo-15-crown-5 proton resonances was ob-
served, which suggests the absence of the intermolecular
“head-tail-head-tail” sandwich structure with parallel ar-
rangement of two host molecules. Furthermore, additional
intermolecular rotational nuclear Overhauser effects were
observed between protons H-b$H-2, H-g$H-5, H-g$H-6,
H-d$H-5, and H-d$H-6, thereby demonstrating the inter-
molecular assistance of the second bisACTHNUTRGNEUNG(styryl) dye molecule
1 in the binding process and the presence of a “head-tail-
tail-head” arrangement based on a size–fit effect as illustrat-
ed in Figure 5.
The one equimolar addition of silver(I) or mercury(II) to
1 results in downfield shift of the signal family correspond-
ing to the styryl phenylazathia-15-crown-5 ether unit (Fig-
ure 4d).[6] The most pronounced downfield shift of this unit
after addition of the mercury(II) cation was found for the
H-a’ methylene protons and aromatic protons H-2’,3’ of the
aniline moiety.
A particular effect was observed upon the addition of
perchloric acid to ligand 1. In this case, the formation of
1·H+ complex in the ratio 1:1 accomplished a decrease of
the initial group of host 1 signals, their broadening, and the
appearance of new downfield resonances corresponding to
the styryl phenylazathia-15-crown-5 ether unit. The chemical
shift changes are more pronounced for the H-a’, d’, e’ meth-
ylene and H-2’,3’ aniline proton resonances (see Table 1),
whereas the other proton signals H-b’’, g’ are found to un-
Figure 3. Stacked plot 1H NMR spectra of the H-a proton resonances re-
corded at 295 K in CD3CN during the titration of the bichromophoric
ligand 1 (C=1 mm) with magnesium perchlorate.
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Multimodal Metal Cation Sensing with Bis(macrocyclic) Dye
FULL PAPER
group of the phenylazathia-15-
crown-5 ether cavity through a
hydrogen-bonding interaction.
Further addition of perchloric
acid to 1:2 (ligand/proton) ratio
complex results in a downfield
shift of the resonance signals
corresponding to the benzo-15-
crown-5 ether unit. Commonly,
aromatic amines such as aniline
and phenylazathia-15-crown-5
ether react with strong acids to
form anilinium, and are stron-
ger bases than benzo-15-crown-
5 ether complexes formed by
hydrogen bonding to the ether
oxygen atoms in addition to
ion–dipole interaction.[12]
In general, from measure-
ments of the peak intensities of
the host 1 signals and of the
new resonances that appear
upon addition of metal cations,
the titration curves could be
Figure 4. 1H NMR spectra of free ligand 1 (a), 1·Mg2+ (b), 12·Ba2+ (c), and 1·Mg2+·Ag+ (d) in CD3CN at
295 K.
plotted to determine the binding constant values. However,
due to the initial bichromophoric ligand 1 signal broadening
that appeared during the titration experiment with metal
guests, it was possible to establish the binding constant
value only for silver(I) and magnesium(II) complexes.
The four equilibria displayed in Scheme 2 represent the
multimodal binding of bichromophoric dye 1. As mentioned
above, the receptor has only four possible states: free 1, par-
tially bound 1·A or 1·B, and fully bound 1·A·B. The equili-
bria are characterized by two microscopic association con-
stants KA1 and KB1, which are defined by [Eq. (1)]:
Figure 5. Representation of the average conformation of 12·Ba2+ sand-
wich-type complex in CD3CN based on NMR studies and modeled with
AM1 calculations.
KA1
A
mþ þ L
AL
ƒƒ!ƒƒ
KB1
B
B
nþ þ L
BL
ƒƒ!ƒƒ
ð1Þ
KBA1
nþ þ AL
BAL
ABL
ƒƒƒ!ƒ
Table 1. Chemical shift changes (Dd)[a] for the bis
ACHTNUTRGNE(NUG styryl) dye 1.
KAB1
A
mþ þ BL
ƒƒƒ!ƒ
H-a
H-a’
H-2
H-6
H-3’
1·Mg2+
0.32, 0.37
0.10
0.01
0.02
0.03
0.33, 0.36
0.22
0.40
0.25
0.27, 0.36
0.22
0.01
0.00
0.25
0.41
0.30
0.26
0.25
0.35
0.30
0.44
0.40
0.26
b.s.[b]
0.01
0.04
0.03
0.26
0.18
0.29
0.25
0.24
0.20
0.21
b.s.[b]
0.01
0.03
0.04
0.21
0.19
0.21
0.20
0.22
0.18
0.10
0.00
0.33
0.79
0.69
0.34
0.33
0.77
0.70
0.82
0.80
12·Ba2+
in which L is the bichromophoric ligand, and Am+ and Bn+
are the metal cations. The constants KAB1 and KBA1 may be
attributed to the cooperativity criteria, depending on wheth-
er one metal cation interaction with the receptor favors or
disfavors another. Cooperativity is also fundamental to un-
derstanding the operation of biological systems, in which co-
ordinated switching of molecular species between discrete
states is the key to managing complexity.[13] At the molecu-
lar level, the cooperativity of the system is described by the
interaction parameters aA and aB, which are defined by the
equations aB ¼ KBA1=KB1andaA ¼ KAB1=KA1, in agreement
with recent thermodynamic models.[13,14] In the absence of
cooperativity, the microscopic association constants for
partly populated receptor 1 are identical to the value for the
1·Ag+
1·Hg2+
1·H+
1·Mg2+·Ag+
1·Ba2+·Ag+[c]
1·Mg2+·H+
1·Ba2+·H+[c]
1·Mg2+·Hg2+
1·Ba2+·Hg2+[c]
[a] Change in the host 1 chemical shift [ppm] after addition of 1 mm
cation salt, in CD3CN at 295 K; initial [1]=1 mm. [b] Broad signal.
[c] Spectrum was recorded at 273 K.
dergo essentially negligible changes, which suggests that the
PhNH+ ion would bind to the oxygen atoms of the CH2O
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O. Fedorova et al.
with respect to the first one. This behavior can be assigned
to the coulomb electrostatic intermetallic repulsions be-
tween two charged species Mg2+ and 1·Ag+ (contrary to
MgII binding to uncomplexed dye 1).
Cyclic voltammetric measurements: The cyclic voltammetric
data for 1 and its parent compounds M1 and M2 in acetoni-
trile, with tetrabutylammonium perchlorate as the support-
ing electrolyte and a scan speed of 200 mVsꢁ1, are shown in
Table 2. For the styryl benzo-15-crown-5 pyridinium dye M1
only one irreversible oxidation process is observed, whereas
the oxidative electrochemical behavior of the 1 and its
parent molecule M2 is more complicated, with each com-
pound showing three irreversible waves. It is remarkable
that the first oxidation potentials of the disubstituted bis-
Scheme 2. Equilibria of the multimodal binding of bichromophoric dye 1.
corresponding second binding event in bimetallic complexes,
that is, KBA1/KB1 and KAB1/KA1 and a=1. We performed
stepwise titration in CD3CN of the host 1 separately by mag-
nesium(II) and silver(I), 1·Ag+ complex by magnesium(II),
and 1·Mg2+ complex by silver(I) (Figure 6). The values of
the microscopic binding constants are logKA1 =4.5ꢀ0.2 for
1·Mg2+ and logKB1 =2.9ꢀ0.2 for 1·Ag+ at 295 K. It is note-
AHCTUNGTREG(NNNU styryl) dye 1 and its parent compound M2 do not differ sig-
nificantly, whereas the oxidation potential of the parent dye
M1 is much higher. Oxidation becomes easier for more elec-
tron rich ligands 1 and M2 compared with M1. The first re-
duction potential values are close to each other for all li-
gands under study.
It is known that cation complexation by redox-active
crown ethers containing reversibly oxidizable probes, such
as ferrocene or tetrathiafulvalene, generally induces a posi-
tive shift of the oxidation potential due to electrostatic re-
pulsion between the positive charges of the metal cation and
the oxidized probe. Moreover, for the macrocycles with two-
redox-state probes such as tetrathiafulvalene, the second ox-
idation step is unaffected in the presence of metal cations.
This invariance is generally attributed to the expulsion of
the cation after the first probe oxidation.[15]
According to the NMR data, large chemical shift changes
for host ligand 1 were obtained in the presence of magnesi-
um and mercury perchlorates. So, the corresponding metal
guests were chosen to study the influence of complex forma-
tion on the electrochemical potential values of the initial
Figure 6. Concentration profiles of the different species in the 1H NMR
titration experiment with
CD3CN at 295 K.
1 (C=1 mm), MgCAHTUNGTRNUEGN(ClO4)2, and AgClO4 in
worthy that the fully complexed
bichromophoric ligand be-
haves differently from expecta-
tions based on the properties of
Table 2. Electrochemical data for the chromophore 1, M1, M2, their metal complexes in acetonitrile and cal-
culated HOMO–LUMO energies.
1
Compound
M1
1.43
M1·Mg2+
M2
1.02
1.43
1.70
M2·Hg2+
1
1·Hg2+
1.55 (540)
1.69
E [V] vs. Ag/AgCl Epa, oxidations
(DEp, mV)
1.45 (20)
1.48 (460)
1.69
1.01
1.32
1.66
the
individual
interactions
acting in isolation. This obser-
vation is in agreement with a
Epc, reductions ꢁ1.02 ꢁ0.96 (60) ꢁ1.17 ꢁ0.93 (240) ꢁ1.01 ꢁ0.82 (190)
ꢁ1.69 ꢁ1.17
ꢁ1.60
ꢁ1.08
classical
cooperativity
test,
which directly compares the
successive stability constants.
The ratios KAB1/KA1 =0.64 and
KBA1/KB1 =0.61 show a nega-
tively heterotopic cooperative
EHOMO [eV]
ELUMO [eV]
ꢁ6.07 ꢁ6.09
ꢁ3.62 ꢁ3.68
ꢁ5.66 ꢁ6.12
ꢁ3.47 ꢁ3.71
ꢁ5.65 ꢁ6.19
ꢁ3.63 ꢁ3.82
elec
DELꢁH [eV][a]
2.45
2.75
2.41
3.03
2.19
2.36
2.41
3.07
2.02
2.21
2.37
2.28
DELꢁHabs [eV][b]
[a] Electrochemical bandgap calculated as the difference between the two peak potentials; [b] HOMO–
binding of the second cation LUMO gap calculated from the onset of the UV/Vis absorption.
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Chem. Eur. J. 2011, 17, 10752 – 10762

Multimodal Metal Cation Sensing with Bis(macrocyclic) Dye
FULL PAPER
host 1 and the results were compared with those for parent
dye complexes M1 and M2. It was found that the maximum
potential shift is reached after addition of one equivalent of
cation. Further addition of the metal salts to all chromo-
phores M1, M2, and 1 did not affect the oxidation and re-
duction potential values and resulted only in increasing the
reduction peak intensities of the uncomplexed metal salts.
Addition of HgACHTUNGTRENNUNG(ClO4)2 to a solution of 1 and its parent com-
pound M2 caused the shift of the redox potential to more
positive values of 540 and 460 mV, respectively. This result
is attributed to the coordination of the nitrogen lone pair of
electrons with mercury(II), which decreases the electron-do-
nating nature of the amine and as a consequence the
charge-transfer character of the bichromophore 1 and its
parent chromophore M2. Unusually, no changes in any of
the peak potentials were found on addition of the mag-
ACHTUNGTRENNUNG
chemical activity and to measure the reduction and oxida-
tion potential values of ligands 1, M1, M2, and their com-
plexes with magnesium and mercury cations. The HOMO
and LUMO energies of these ligands and their complexes
can be obtained from the first oxidation and reduction po-
tential, respectively. The energy level of the normal hydro-
gen electrode (NHE) is situated 4.44 eV below the zero
vacuum energy level.[16] From this energy level of the NHE
and the redox potential of the reference Ag/AgCl electrode
used in the present work (0.197 V vs. NHE), a simple rela-
tion can be written which allows estimation of both energy
values [Eqs. (2), (3)]:
ELUMO ¼ ðꢁ4:44ꢁ0:197ꢁEredÞ eV
EHOMO ¼ ðꢁ4:44ꢁ0:197ꢁEoxÞ eV
ð2Þ
ð3Þ
Figure 7. Illustrations of the contour surface diagrams of the molecular
orbitals of 1 involved in the lowest-energy transition.
The potential difference Eg =ELUMOꢁEHOMO was used to es-
timate the energy gap of ligands 1, M1, M2, and their com-
plexes (Table 2).
The HOMO–LUMO gap Eg was also measured from the
optical absorption curve. Eg is related to the absorbance A
by Equation (4):[17]
LUMO+2 are concentrated on the electron-accepting pyri-
dinium fragment, with the LUMO+2 having especially lim-
ited contributions from the rest of the molecule. In qualita-
tive terms, the lowest-energy electronic transition of the bis-
AHCTUNGTREG(NNNU styryl) dye 1 is determined mainly by its parent compound
M2. Thus, the measurements show only a small increase for
the lowest transition energy on moving from the parent
1=2
compound M2 to bis
G
ð4Þ
Ahn ꢂ ðhn ꢁ EgÞ
when compared with its parent compound M1 (DE=
0.43ꢁ0.54 eV).
in which hn is the photon energy. The value of the direct op-
tical bandgap Eg was determined by extrapolation of the
linear region of the plots of A2 versus the photon energy to
zero absorption (Table 2).
Figure 7 shows the topologies of the molecular orbitals in-
volved for bichromophore 1 obtained using AM1 calcula-
When mercury(II) cations are added to the host 1 and its
parent compound M2, the HOMO energies are affected sig-
nificantly whereas smaller changes were observed for
LUMO energies (see Figure 8). The decrease of HOMO
energy for the complexes 1·Hg2+ and M2·Hg2+ in compari-
son with the corresponding free ligands suggests that the
HOMO in complexes is located on the complexed phenyl-
tions. The parent dyes M1 and M2 and bisACHTNUTRGNEUNG(styryl) dye 1 re-
semble stilbazolium species, with lowest-energy electronic
transitions of HOMO!LUMO character.[18] The HOMO
and HOMO-1 are always mostly located on the electron-
rich phenylazathia-15-crown-5 or benzo-15-crown-5 groups,
but with significant contributions from the ethenyl bridges
and the pyridinium unit. The LUMO, LUMO+1, and
AHCTUNGTREGaNNNU za-15-crown-5 ether unit. When magnesium(II) ions are
added to 1, the relative position of the HOMO located on
the electron-rich phenylazathia-15-crown-5 is not changed,
since only deeper molecular orbitals located on benzo-15-
crown-5 are affected.
Chem. Eur. J. 2011, 17, 10752 – 10762
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O. Fedorova et al.
Figure 8. Qualitative assessment of relative energy levels of frontier mo-
lecular orbitals in 1, its parent molecules M1 and M2, and the corre-
sponding magnesium(II) and mercury(II) complexes.
Steady-state absorption measurements: The UV/Vis absorp-
tion spectra of ligand 1 and its parent molecules M1 and M2
are illustrated in Figure 9. The bichromophoric dye 1 shows
an intense broad absorption band in the visible region, at-
tributable to intermolecular charge-transfer (ICT) transi-
tions: 4-(dimethylamino) phenyl group/benzo-15-crown-5
unit ! N-methylpyridinium group. The less intense UV ab-
sorption is due to higher-lying ICT and p!p* excitations.
The bathochromic shift (19 nm) of the absorption maximum
in 1 indicates that the styryl-extended N-methylpyridinium
group is a stronger p-electron acceptor than its parent com-
pound M2.[9b]
Figure 10. Spectrophotometric titration of 1 with MgACTHUNTRGNE(UNG ClO4)2 (top) and
AgClO4 (bottom) in acetonitrile at 295 K, CL =2.0ꢄ10ꢁ5 m.
To further investigate the binding process of the bichro-
mophoric dye 1, the steady-state absorption and fluores-
cence spectra of the host were recorded in the presence of
MgII, BaII, HI, HgII, and AgI perchlorates. Figure 10 (top)
shows the absorption spectra of 1 in acetonitrile upon suc-
cessive addition of magnesium perchlorate. During the titra-
tion, the large hypsochromic shift of the band at 396 nm cor-
responding to the benzo-15-crown-5 unit ! N-methylpyridi-
nium ICT transition was observed. The absorption band of 1
undergoes similar changes upon barium(II) addition. On the
contrary, addition of AgI, HgII, and HI to the host 1 solution
results in blueshift of the band at 490 nm, which corresponds
to the 4-(dimethylamino) phenyl group ! N-methylpyridi-
nium ICT transition (Figure 10, bottom).
The values of the binding constants obtained from UV/
Vis titration are in good agreement with our previously re-
ported data for the parent M1 and M2 molecules.[9b,11a] Fur-
thermore, the binding constants obtained from UV/Vis titra-
tion of metal complexes 1·Ag+, 1·Mg2+, and 1·Mg2+·Ag+
are similar to those calculated (see above) from NMR titra-
tion experiments (see Table 3). As expected, the negative
heterotopic cooperative binding of the second cation with
respect to the first one was observed in bimetallic com-
plexes.
Steady-state and time-resolved fluorescence measurements:
The two parent compounds M1 and M2 and dye 1 show
weak fluorescence emission, with quantum yields ranging at
10ꢁ2. Interestingly, the emission quantum yield of bichromo-
phore 1 is wavelength dependent. On increasing the excita-
Figure 9. Absorption and normalized fluorescence spectra of 1 (c) and
its parent compounds M2 (a) and M1 (g), CL =2.0ꢄ10ꢁ5 m, in aceto-
nitrile at 295 K. [a] Excitation at 407 nm. [b] Excitation at 480 nm.
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Multimodal Metal Cation Sensing with Bis(macrocyclic) Dye
FULL PAPER
Table 3. Optical properties of 1 and its complexes in acetonitrile at 295 K.
fluorescence spectrum and caused slight fluores-
cence quenching as compared to the free ligand.
The wavelength dependence of the fluorescence
decay profiles is clearly evident from Figure 11a.
As can be seen, when monitored at the red side of
the fluorescence spectrum, the profiles consist of a
steady decay of the fluorescence intensity with
time. However, when monitored at shorter wave-
lengths, the time profiles consist of a slow growth
(rise) followed by normal decay. The decay profiles
were fitted to a multiexponential decay function.
For the dye 1, the fluorescence decay profiles could
be fitted to at least three exponentials. Based on
previous studies, we attribute the components in
the decay kinetics as follows: the short component
predominantly arises from the twist around the
single bond in the CH–PhN unit analogous to its
parent molecule M2, thereby implying a “loose
bolt” relaxation pathway;[19] the second component
arises due to “anti–syn” population equilibration of
the benzo-15-crown-5 moiety in the exited state;
and the longer component is due to isomerization
of the M1 subunit. Such a presence of temporal–
spectral signatures related to different relaxation
processes occurring at the same time in the solution
of 1 suggests that two distinct ICTs, which do not
interfere with each other and are located on differ-
ent parts of the compound, can be excited despite
the fact that the HOMO is involved only when ICT
occurs between the 4-(dimethylamino) phenyl
group and N-methylpyridinium and the lower-lying
occupied orbital participates when ICT is observed
between the benzo-15-crown-5 unit and N-methyl-
pyridinium. This behavior is quite unusual because
the singly occupied, and after excitation, lower-
lying molecular orbital remains empty during all
the fluorescence lifetime (900 ps). This ICT case
should normally involve an electron-transfer pro-
cess from the doubly occupied HOMO located on
dimethylaniline to the singly occupied lower molec-
ular orbital situated on dimethoxyphenyl. Yet,
probably due to a high energetic barrier in the
Marcus process, such an electron transfer has low
Compound
Emax ꢄ104
labs
logK[a]
lfl
Ffl ꢄ10ꢁ2 Lifetime
max
max
[nm][b]
[Lmolꢁ1 cmꢁ1
]
[nm]
[ps]
1
1.7
1.6
3.5
0.9
1.1
1.5
396,[c]
480
608
(407)
644
(480)
631
(360)
638
(480)
613
(370)
644
(480)
437
(335)
594
1.8
1.7
1.1
1.6
1.4
1.7
1.5
37, 270,
900
1·Mg2+
12·Ba2+
1·H+
361,[c]
482
4.7ꢀ0.1
9.9ꢀ0.1
5.2ꢀ0.1
37, 230,
900
367,[c]
481
37, 350,
990
335,
82, 200
428[c]
(420)
1·Hg2+
1·Ag+
361,
14.4ꢀ0.2[d] 611
1.4
1.8
1.4
1.8
2.0
1.8
6.0
8.0
1.4
1.8
1.6
2.0
22, 100,
400
410[c]
(350)
598
(410)
396,[c]
468
3.08ꢀ0.03 611
28, 270,
1000
(370)
628
(470)
598
(360)
614
1·Hg2+·Mg2+ 1.1
362,[c]
466
22, 300,
1200
[e]
(470)
1·H+·Mg2+
0.9
343,
4.4ꢀ0.1[f] 542
290, 1300
362[c]
(340)
4.7ꢀ0.1[g] 568
(370)
1·Ag+·Mg2+ 1.5
12·(H+)2·Ba2+ 3.6
361,[c]
472
4.5ꢀ0.1[f] 604
29, 290,
1200
(360)
2.8ꢀ0.1[h] 626
(470)
583
(340)
340,
9.6ꢀ0.1[i]
400, 1500
368[c]
4.8ꢀ0.1[g] 574
(380)
[a] Values obtained from UV/Vis titration by SpecFit calculations. [b] Excitation wave-
lengths [nm] are given in parentheses. [c] ICT absorption band corresponding to the
benzo-15-crown-5 unit ! N-methylpyridinium p!p* transition. [d] The constant sta-
bility value was determined by using competing reference ligand (see Experimental
Section).[9b] [e] Not possible to determine. [f–i] Binding constant value for MgII [f],
HI [g], AgI [h], and BaII [i] cation in bicomplexed host 1.
tion photon energy, the fluorescence yield decreases. Emis-
sion maxima and quantum yields at different excitation
wavelengths for 1 are reported in Table 3. The emission
bands for all compounds are relatively broad and exhibit
large Stokes shifts. Maxima of the fluorescence spectra of 1
are also shifted bathochromically relative to those of mono-
chromophoric model dyes M1 and M2.[9b,11a] It is noteworthy
that a strong decrease of the fluorescence quantum yield at
both excitation wavelengths (400 and 480 nm) observed for
1 relative to the parent M1 and M2 molecules is justified by
an increase of the conformational flexibility, and as a conse-
quence increase of the twisted intramolecular charge-trans-
fer (TICT) nonradiative relaxation rate.[4f] Addition of dif-
ferent metal guests to the host 1 resulted in blueshift of the
probability and has negligible influence on the relaxation
from excited states.
Addition of the corresponding metal perchlorates affects
the fluorescence lifetime values of bichromophoric ligand 1
(see Table 3). Notably, various metal cations change differ-
ently the relaxation mechanism of the excited state 1. For
instance, addition of the alkaline earth cations shifts the
higher-lying transition of the benzo-15-crown-5 chromo-
phore styryl entity to higher energies (from 410 to 360 nm),
and thus the lowest transition related to the M1 entity of 1
is not strongly affected: all excited-state properties (fluores-
cence) remain similar. Addition of HgII or AgI strongly re-
duces the charge transfer in the phenylazathia-15-crown-5
chromophore styryl unit. Once excited, the decoordination
Chem. Eur. J. 2011, 17, 10752 – 10762
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10759

O. Fedorova et al.
tributed to isomerization of the protonated phenylazathia-
15-crown-5 part of the molecule in this case.
Remarkably, the lifetime increase of the longest fluores-
cence decay component was found for all bimetallic com-
plexes of the host dye 1. This observation could be a result
of an increase in the host 1 conformational rigidity upon
binding of two different cations and prohibition of the TICT
nonradiative relaxation pathways upon bimetallic complex
excitation.
Conclusion
A multitasking receptor 1, in which the benzo-15-crown-5
and phenylazathia-15-crown-5 subunits are linked through
vinyl to a pyridinium moiety, has been synthesized. Taking
together all the results of the present electrochemical, opti-
cal, and NMR investigations, it can be concluded that two
different crown ether styryl subunits form the joint conjugat-
ed system, although each subunit conserves its proper spec-
tral characteristics, which are similar to those of the crown-
contained monostyryl parent compounds M1 and M2. Coor-
dination studies with alkaline earth cations showed that a
blueshift of the ICT band from 396 to 360 nm occurs upon
coordination of the benzo-15-crown-5 styryl subunit. On the
other hand, hypsochromically shifted ICT bands (480 nm) at
335, 360, and 470 nm are formed if a proton, mercury(II), or
silver(I), respectively, binds to the phenylazathia-15-crown-5
vinyl subunit. The spectroscopically determined complexa-
tion constants suggest that two binding modes in the host 1
exhibit a small negative cooperative influence in the forma-
tion of the bimetallic complexes due to a coulomb electro-
static intermetallic repulsion effect. The emission behavior
shows that addition of different metal cations to the dye 1
results in a blueshift of the fluorescence spectrum and
causes slight fluorescence quenching towards the free
ligand. In spite of the complexity and wavelength depend-
ence of the ligand emission spectrum in the presence of
metal cations, the fluorescence kinetic decay parameters can
be attributed to each specific crown-contained styryl subu-
nit. The results from cyclic voltammetry demonstrate that
the addition of magnesium(II) cations does not affect the
oxidation and reduction potential values of dye 1, whereas
addition of mercury(II) results in a positive shift of the
ligand redox potential values.
Figure 11. Time-spectrum resolved fluorescence maps of the free host 1
at lexc. =410 nm (a), 1·H+ at lexc. =350 nm (b), and 1·Mg2+·H+, lexc.
=
340 nm (c), in acetonitrile at 295 K.
of HgII or AgI from the nitrogen lone pair of the phenylaza-
thia-15-crown-5 chromophore styryl entity takes place simi-
larly to its parent molecule M2, whereas the fluorescence
decay components of the M1 unit remain unaffected.[20] The
lowest excited state in the 1·H+ complex remains lying on
the benzo-15-crown-5 chromophore entity; the relaxation in-
volves fast rotation around one or two single bonds in the
Such multichromophoric dyes incorporating different
chromophore units are still rare in supramolecular chemis-
try. Given the fact that the synthetic access to this class of
compounds is very easy, the aforementioned characteristics
make them potentially well suited for applications in analyt-
ical chemistry. Moreover, the bimetallic complexes of the
host 1 can be used as structural elements to build up anion
receptors.[21] Additionally, the possibility to adsorb or anchor
the multitasking dye on a suitable surface may in the future
enable the realization of regenerative sensor molecules for
rapid screening of the target metal guests.
ꢁ
ꢁ
C C=C Ph fragment. Yet, the excitation at 350 nm to a
higher-lying transition corresponding to the protonated phe-
nylazathia-15-crown-5 chromophore styryl unit does not
lead to complete vibrational relaxation towards the lowest-
lying excited state located on the M1 subunit, and short-
lived blueshifted (450 nm) fluorescence is observed (see Fig-
ure 11b). The relaxation mechanism could be reasonably at-
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Chem. Eur. J. 2011, 17, 10752 – 10762

Multimodal Metal Cation Sensing with Bis(macrocyclic) Dye
FULL PAPER
each addition. The quantities of the different species were calculated
from the integrated areas of the NMR signals.
Experimental Section
Electrochemistry: Electrochemical measurements were carried out at am-
bient temperature with an IPC-Pro M potentiostat. Cyclic voltammetric
experiments were performed in a 1.0 mL cell equipped with a glassy
carbon (GC) electrode (d=2 mm), Ag/AgCl/KCl (aq saturated; refer-
ence electrode), and platinum electrode (counter electrode). The com-
plexes were dissolved in degassed dry CH3CN containing tetrabutylam-
monium perchlorate (Fluka) as the supporting electrolyte (0.10m). The
concentration of the complexes was 1.0 mm. Dry argon gas was bubbled
through the solutions for 30 min before cyclic voltammetric experiments.
General: Solvents and reagents were obtained from commercial suppliers
and were used without further purification. ESI-TOF mass spectrometry
was carried out on a microTOF focus instrument (JEOL JMS-T100 LC
AccuTOF LC) in positive mode. The styryl derivative a (see Scheme 1)
was prepared according to a known literature procedure.[22]
1,2-Dimethyl-4-[(E)-2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzopen-
taoxacyclo-pentadecin-15-yl)vinyl]pyridinium tosylate (b): A mixture of
compound a (0.60 g, 1.56 mmol) and methyl 4-methylbenzenesulfonate
(0.27 g, 1.56 mmol) was heated for 2 h at 1258C. After cooling to room
temperature, the product was washed with toluene and dried in vacuum.
The scan rate was 200 mVsꢁ1
.
1
Time-resolved fluorescence studies: The fluorescence excitation light
pulses were obtained by using an optical parametric generator (OPG)
pumped by a Ti:sapphire femtosecond laser system (Femtopower Com-
pact Pro) output pulse. Typically, we tuned the OPG to the wavelength
of maximum absorption of the sample or to the wavelength of some spe-
cific absorption feature. All excited-state lifetimes were obtained by
using depolarized excitation light. The highest pulse energies used to
excite fluorescence did not exceed 100 nJ and the average power of the
excitation beam was 0.1 mW at a pulse repetition rate of 1 kHz focused
into a spot with a diameter of 0.1 mm in the 10 mm long fused silica cell.
The fluorescence emitted in the forward direction was collected by re-
flective optics and focused with a spherical mirror onto the input slit of a
spectrograph (Chromex 250) coupled to a streak camera (Hamamatsu
5680 equipped with a fast single-sweep unit M5676, temporal resolution
2 ps). The convolution of a rectangular streak camera slit in the sweep
range of 250 ps with an electronic jitter of the streak camera trigger pulse
provided a Gaussian (over four decades) temporal apparatus function
with a full width at half maximum of 20 ps. The fluorescence kinetics
were later fitted by using the Levenberg–Marquardt least-squares curve-
fitting method using a solution of the differential equation describing the
evolution in time of a single excited state and neglecting depopulation of
the ground state [Eq. (5)]:
Yield 89%; H NMR ([D6]DMSO): d=2.66 (s, 3H), 2.71 (s, 3H), 3.61 (s,
8H), 3.79 (m, 4H), 4.10 (m, 4H), 4.13 (s, 3H), 7.04 (d, 3J=8.6 Hz, 1H),
7.10 (d, 3J=8.2 Hz, 2H), 7.25 (dd, 3J=8.6, 4J=1.8 Hz, 1H), 7.31 (d,
3Jtrans =15.9 Hz, 1H), 7.36 (d, 4J=1.8 Hz, 1H), 7.46 (d, 3J=8.2 Hz, 2H),
7.86 (d, 3Jtrans =15.9 Hz, 1H), 7.95 (dd, 3J=6.9, 4J=1.4 Hz, 1H), 8.06 (d,
3
4J=1.4 Hz, 1H), 8.76 ppm (d, J=6.8 Hz, 1H); ESI-MS (in MeCN): m/z:
400.2 [b+]; elemental analysis calcd (%) for C30H37NO8S: C 63.03, H
6.52, N 2.45; found: C 62.95, H 6.47, N 2.51.
2-{(E)-2-[4-(1,4-Dioxa-7,13-dithia-10-azacyclopentadecan-10-yl)phenyl]-
vinyl}-1-methyl-4-[(E)-2-(2,3,5,6,8,9,11,12-octahydro-1,4,7,10,13-benzo-
pentaoxacyclopentadecin-15-yl)vinyl]pyridinium perchlorate (1): A mix-
ture of styryl dye b (0.68 g, 1.2 mmol), 4’-formylphenyl-azathia-15-crown-
5 ether precursor (0.42 g, 1.2 mmol), and pyrrolidine (0.5 mL) in nBuOH
(30 mL) was heated at reflux for 30 min. After evaporation of solvent,
the residue was washed with boiled toluene (3ꢄ10 mL) and dissolved in
dry MeOH. Sodium perchlorate (0.14 g, 1.2 mmol) in MeOH (5 mL) was
added dropwise to the residue. The precipitated solid was separated,
washed with Et2O, and purified by column chromatography (gradient elu-
tion, Silicagel 100 C18, toluene/MeCN). After evaporation of the solvent,
1 was reprecipitated from MeOH as a dark red solid. Yield 67%; m.p.
187–1898C; ESI-MS (1 in MeCN): m/z: 737.3 [1+] (for NMR spectrum,
consult the Supporting Information); elemental analysis calcd (%) for
C40H53ClN2O11S2: C 57.37, H 6.38, N 3.35; found: C 57.41, H 6.42, N 3.32.
dIðtÞ
dt
IðtÞ
ð5Þ
¼ Gaussðt0; Dt; AÞ ꢁ
UV/Vis absorption: UV/Vis spectra were measured by a Cary 50 UV/Vis
t
spectrophotometer in conventional quartz cells of 10 mm path length.
The spectral bandwidth and the scan rate were 2 nm and 140 nmminꢁ1
,
in which I(t) is the fluorescence intensity, Gauss is the Gaussian profile
of the excitation pulse, t0 stands for the excitation pulse arrival delay, Dt
is the excitation pulse width, and A is the amplitude. The parameter t
represents the lifetime of the excited state. The initial condition for the
equation is I(ꢁ1)=0. Typically, the fit shows a c2 value better than 10ꢁ4
and a correlation coefficient R>0.999. The uncertainty of the lifetime
was better than 0.1 ps. Routinely, the fluorescence accumulation time in
our 35 measurements did not exceed 90 s.
respectively. Stock solutions of each compound were accurately prepared,
and dilutions of these stock solutions were used for absorption measure-
ments over a concentration range obtained by taking into account the
solubility and absorbance of the particular compound. The temperature
was controlled with a single-cell Peltier element (Varian). The solvents
for UV/Vis absorption were of spectroscopic grade and were used as re-
ceived. Titrations were performed as constant host titrations (0.05 mm in
acetonitrile) at 295 K by addition of aliquots (5–20 mL) of the respective
metal perchlorate hexahydrate solution (0.5 mm in acetonitrile), by using
a microsyringe, to quartz cuvettes containing host solution (2.000 mL).
To keep the ionic strength of the solutions constant (0.010–
0.011 moldmꢁ3), the supporting electrolyte Et4NClO4 was added. UV/Vis
spectra were recorded after each addition. The binding constant and the
stoichiometry were obtained from the titration curve by fitting these
changes by using the nonlinear regression analysis program SPEC-
FIT32.[23] In the case of the mercury(II) complex, the constant stability
value was determined by using competing reference ligand 3-methyl-2-
[(E)-2-(2,3,5,6,8,9,11,12-octahydro-1,7,13,4,10-benzotrioxadithia-cyclopen-
tadecin-15-yl)-1-ethenyl]-1,3-benzothiazol-3-ium perchlorate.[9b,24] Sam-
ples (1ꢄ10ꢁ5 m in CH3CN) were irradiated in quartz cells (10 mm) at
295 K in in-house-constructed apparatus and attached to the UV/Vis
spectrometer.
In some cases, the kinetics were fitted using a system of differential equa-
tions describing the evolution in time of two excited populations in pre-
cursor–successor relationship [Eq. (6)]:
dN1ðtÞ
dt
N1ðtÞ
¼ Gaussðt0; Dt; AÞ ꢁ
t1
ð6Þ
dN2ðtÞ N1ðtÞ N2ðtÞ
¼
ꢁ
t2
dt
t1
in which N1 and N2 represent the populations of two excited states, and
t1 and t2 are the lifetimes of the corresponding populations. Other pa-
rameters as well as initial conditions are similarly defined as described
above.
NMR experiments: All NMR experiments were recorded on a Bruker
Avance DRX 500 MHz spectrometer with a TXI Bruker 5 mm probe.
The measurements were performed by using the CD3CN signal as inter-
nal reference (1.94 ppm at 295 K).[25] Data acquisition and processing
were performed with Topspin 2.1 software (Bruker). For 1H NMR titra-
tion experiments with the respective metal perchlorates, aliquots of a
metal perchlorate solution (0.01–0.1m in CD3CN) were added to a 1 solu-
tion (0.8–1.0 mm in CD3CN) and 1H NMR spectra were recorded after
Acknowledgements
E.T. thanks the Centre National de la Recherche Scientifique (CNRS)
for the postdoctoral fellowship. The 500 MHz NMR facilities were
funded by the Rꢀgion Nord-Pas de Calais (France), the Ministꢁre de la
Jeunesse de l’Education Nationale et de la Recherche (MJENR), and the
Fonds Europꢀens de Dꢀveloppement Rꢀgional (FEDER). Part of this
Chem. Eur. J. 2011, 17, 10752 – 10762
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
10761

O. Fedorova et al.
collaborative work was realized within the framework GDRI CNRS 93
“Phenics ”. G.J. thanks the Rꢀgion Aquitaine for the financial support.
The Russian group thanks RFBR 09-03-0283 and 10-03-93106 for finan-
cial support.
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Received: April 1, 2011
Published online: August 16, 2011
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