Synthesis and photophysical and cation-binding properties of mono- and tetranaphthylcalix[4]arenes as highly sensitive and selective fluorescent sensors for sodium

Article abstract of DOI:10.1002/1521-3765(20011105)7:21<4590::AID-CHEM4590>3.0.CO;2-A

The syntheses and properties of two calixarene-based fluorescent molecular sensors are reported. These comprise tert-butylcalix[4]arene either with one appended fluorophore and three ester groups (Calix-AMN1), or with four appended fluorophores (Calix-AMN4). The fluorophore is 6-acyl-2-methoxynaphthalene (AMN), which contains an electron-donating substituent (methoxy group) conjugated to an electron-withdrawing substituent (carbonyl group); this allows photoinduced charge transfer (PCT) to occur upon excitation. The investigated fluoroionophores thus belong to the family of PCT fluorescent molecular sensors. In addition to the expected red shifts of the absorption and emission spectra upon cation binding, a drastic enhancement of the fluorescence quantum yield-in an off-on fashion comparable to that seen in photoinduced electron transfer (PET) molecular sensors-was observed. For Calix- AMN1, it increases from 10^-3 for the free ligand to 0.68 for the complex with Ca²⁺. This exceptional behaviour can be interpreted in terms of the relative locations of the nπ* and ππ* levels, which depend on the charge density of the bound cation. For Calix-AMN4, in addition to the photophysical effects observed for Calix-AMN1, interactions between the chromophores by complexation with some cations have been found in the ground state (hypochromic effect) and in the excited state (excimer formation). Steady-state fluorescence anisotropy measurements for the system Na⁺ ? Calix-AMN4, show a depolarization effect due to energy transfer (homotransfer) between the fluorophores. Regarding the complexing properties, a high selectivity for Na⁺ over K⁺, Li⁺, Ca²⁺ and Mg²⁺ was observed in ethanol and ethanol-water mixtures. The selectivity (Na⁺/other cations) expressed as the ratio of the stability constants was found to be more than 400.

Full text of DOI:10.1002/1521-3765(20011105)7:21<4590::AID-CHEM4590>3.0.CO;2-A

FULL PAPER
Synthesis and Photophysical and Cation-Binding Properties of Mono- and
Tetranaphthylcalix[4]arenes as Highly Sensitive and Selective Fluorescent
Sensors for Sodium
Isabelle Leray,*[a] Jean-Pierre Lefevre,[a, b] Jean-François Delouis,[a] Jacques Delaire,[a] and
Bernard Valeur*[a, b]
Abstract: The syntheses and properties
of two calixarene-based fluorescent mo-
lecular sensors are reported. These com-
prise tert-butylcalix[4]arene either with
one appended fluorophore and three
ester groups (Calix-AMN1), or with four
appended fluorophores (Calix-AMN4).
The fluorophore is 6-acyl-2-methoxy-
naphthalene (AMN), which contains an
electron-donating substituent (methoxy
group) conjugated to an electron-with-
drawing substituent (carbonyl group);
this allows photoinduced charge transfer
(PCT) to occur upon excitation. The
investigated fluoroionophores thus be-
long to the family of PCT fluorescent
molecular sensors. In addition to the
expected red shifts of the absorption and
emission spectra upon cation binding, a
drastic enhancement of the fluorescence
quantum yieldÐin an ªoff ± onº fashion
comparable to that seen in photoin-
duced electron transfer (PET) molecu-
lar sensorsÐwas observed. For Calix-
addition to the photophysical effects
observed for Calix-AMN1, interactions
between the chromophores by complex-
ation with some cations have been found
in the ground state (hypochromic effect)
and in the excited state (excimer forma-
tion). Steady-state fluorescence aniso-
tropy measurements for the system
3
AMN1, it increases from 10 for the
free ligand to 0.68 for the complex with
Ca2‡. This exceptional behaviour can be
interpreted in terms of the relative
locations of the np* and pp* levels,
which depend on the charge density of
the bound cation. For Calix-AMN4, in
‡
Na ꢀ Calix-AMN4, show a depolariza-
tion effect due to energy transfer (ho-
motransfer) between the fluorophores.
Regarding the complexing properties, a
‡
‡
‡
high selectivity for Na over K , Li ,
Ca2‡ and Mg2‡ was observed in ethanol
and ethanol ± water mixtures. The selec-
Keywords: calixarenes ´ excimers ´
fluorescence spectroscopy ´ sensors ´
supramolecular chemistry
‡
tivity (Na /other cations) expressed as
the ratio of the stability constants was
found to be more than 400.
ionophores. The recognition moiety is responsible for the
selectivity and the efficiency of binding, therefore consider-
able efforts have been made to develop selective sensors for
particular cations. In this respect, calixarenes with appropriate
appended groups exhibit high selectivities[6] and have been
shown to be useful building blocks in the design of fluorescent
sensors for cations.[7±12] The design of such calixarene-based
sensors differs according to the nature of the cation and to the
photoinduced process responsible for the photophysical
changes upon cation binding: photoinduced electron transfer
(PET),[7] photoinduced charge transfer (PCT),[8, 9] excimer
formation[10] or energy transfer.[11]
It is worth bearing in mind that the electron-donor and
electron-acceptor groups in PET sensors are nonconjugated
(transfer of an electron), whereas those in PCT sensors are
conjugated (partial charge transfer).
A distinct feature of PET molecular sensors is the very
large cation-induced change in fluorescence intensity that is
usually observed. The absence of a shift in the fluorescence or
excitation spectra, however, precludes the possibility of
intensity-ratio measurements at two wavelengths. In contrast,
Introduction
The design of selective fluorescent molecular sensors for
cations is a subject of considerable interest because of their
numerous potential applications in analytical chemistry,
biology, medicine (clinical diagnosis), environmental chem-
istry, chemical oceanography etc.[1±5] Most fluorescent molec-
ular sensors consist of a cation recognition unit (ionophore)
linked to a fluorophore; thus they are often called fluoro-
[a] Dr. I. Leray, Prof. B. Valeur, J.-P. Lefevre, J.-F. Delouis,
Prof. J. Delaire
Laboratoire de Photophysique et Photochimie
Â
Â
Supramoleculaires et Macromoleculaires (CNRS UMR 8531)
Â
Departement de Chimie, ENS-Cachan
61 Avenue du President Wilson
94235 Cachan cedex (France)
Â
Fax : (‡33)1-47-40-24-54
E-mail: icmleray@ppsm.ens-cachan.fr, valeur@cnam.fr
[b] Prof. B. Valeur, J.-P. Lefevre
Â
Â
Laboratoire de Chimie Generale
Â
Conservatoire National des Arts et Metiers
292 rue Saint-Martin, 75141 Paris Cedex 3 (France)
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4590±4598
shifts in the fluorescence or excitation spectra of PCT
molecular sensors permit ratiometric measurements to be
made; this allows the determination of cation concentration
independently of various parameters such as sensor concen-
tration, incident-light intensity and photobleaching. However,
the changes in fluorescence quantum yield on cation binding
are generally not very large relative to those observed with
PET sensors. This paper reports the properties of two
calixarene-based PCT molecular sensors that exhibit an
enhancement of fluorescence quantum yieldÐin an ªoff ±
onº fashionÐas large as those seen in PET sensors with the
additional advantage of spectral shifts upon complexation.
In line with our previous works on PCT fluoroiono-
phores,[13] we selected 6-acyl-2-methoxynaphthalene (AMN)
as a fluorophore. This moiety incorporates an electron-
donating substituent (methoxy group) conjugated with an
electron-withdrawing substituent (carbonyl group). As re-
gards the recognition moiety,[14] a tert-butylcalix[4]arene
frame with appended carbonyl groups was chosen, with the
aim of selectivity for sodium. The investigated fluoroiono-
phoresÐCalix-AMN1 and Calix-AMN4, comprising tert-
butylcalix[4]arene with one and four appended acyl naphtha-
lene fluorophores, respectivelyÐare shown in Scheme 1.
Preliminary results on Calix-AMN1 have been reported
previously.[9]
Abstract in French: Cet article prØsente la synth›se et lꢀØtude
des propriØtØs photophysiques et de complexation de deux
nouvelles sondes fluorescentes à base de calixar›ne. Ces
fluoroionophores sont constituØs de tert-butylcalix[4]arenes
liØs de façon covalente à un fluorophore et trois groupes ester
(Calix-AMN1) ou à quatre fluorophores (Calix-AMN4). Le
fluorophore utilisØ est composØ dꢀun groupe methoxy (don-
neur) conjuguØ à un groupe carbonyle (accepteur), participant
à la complexation du cation. Ainsi un transfert de charge
photoinduit se produit lors de lꢀexcitation lumineuse. La
complexation du cation par le calixar›ne sꢀaccompagne non
seulement dꢀun dØplacement bathochrome des spectres dꢀab-
sorption et de fluorescence mais aussi dꢀune tr›s forte
augmentation du rendement quantique de fluorescence aussi
importante que dans le cas de fluorionophores fonctionnant
suivant le principe du transfert dꢀØlectron photoinduit (PET).
Dans le cas du Calix-AMN1, le rendement quantique passe de
Scheme 1. Synthesis of Calix-AMN1 (3) and Calix-AMN4 (4): i) KHCO3,
acetone; ii) BrCH2COOEt, K2CO3, acetone; iii) NaI, K2CO3, acetone.
The new Calix-AMN4 compound, containing four fluoro-
phores, offers additional interesting features with respect to
Calix-AMN1 in terms of: i) sensitivity of detection (molar
absorption coefficient four times larger), ii) additional photo-
physical effects (possibility of excimer formation) and iii) im-
proved photochemical stability. The advantages of multi-
chromophoric sensors have already been outlined, in partic-
ular for sensors based on dendrimers.[15]
In this paper the syntheses of Calix-AMN1 and Calix-
AMN4 are reported, together with their cation-induced
photophysical changes and their complexing properties.
3
ꢁ10 à 0.68 lors de la complexation avec le calcium. Ce
Results and Discussion
phØnom›ne remarquable peut sꢀexpliquer par une modification
des niveaux dꢀØnergie relatifs des Øtats singulets pp* et np*
(dØpendant de la densitØ de charge du cation complexØ). Dans
le cas du Calix-AMN4, des intØractions entre les chromophores
lors de la complexation de cations ont ØtØ mises en Øvidence
aussi bien à lꢀØtat fondamental (effet hypochrome) quꢀà lꢀØtat
excitØ (formation dꢀexcim›res). Dꢀautre part des mesures
Synthesis: Calix-AMN1 (3) was prepared (Scheme 1) from
the parent calix[4]arene. 4-tert-Butylcalix[4]arene was mono-
alkylated with 2-chloroacetyl-6-methoxynaphthalene in the
presence of KHCO3 in acetone. The 2-chloroacetyl-6-meth-
oxynaphthalene was obtained by Friedel ± Crafts acylation of
methoxynaphthalene with chloroacetyl chloride in nitroben-
zene.[16] The remaining three phenolic positions were then
functionalized with ethyl bromoacetate in acetone in the
presence of K2CO3.
Calix-AMN4 (4) was prepared according to Scheme 1, by
condensation with 2-chloroacyl-6-methoxynaphthalene and
K2CO3 in acetone[14] in the presence of sodium iodide to
provide halogen exchange.
Compounds 2, Calix-AMN1 and Calix-AMN4 had 1H NMR
spectra characteristic[6] of a cone conformation.
‡
dꢀanisotropie de fluorescence sur le syst›me Na ꢀ Calix-
Ã
AMN4 ont montrØ un effet de dØpolarisation du à lꢀexistence
dꢀun transfert dꢀØnergie entre les chromophores (homotrans-
fert). En ce qui concerne les propriØtØs de complexation, une
excellente sØlectivitØ de ces syst›mes vis-à-vis du sodium par
rapport aux autres cations a ØtØ obtenue dans des solvants
protiques (Øthanol et mØlanges Øthanol ± eau). La sØlectivitØ du
sodium par rapport aux autres cations (exprimØe par le rapport
des constantes de stabilitØ) est supØrieure à 400.
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I. Leray, B. Valeur et al.
FULL PAPER
Results of the photophysical studies
Absorption spectra: Figure 1 displays the absorption spectra
of Calix-AMN1 and Calix-AMN4 in acetonitrile. In the free
ligands, the molar absorption coefficient of Calix-AMN4 is
about four times larger than that of Calix-AMN1; this implies
‡
Figure 2. Energy-minimized structure of Na ꢀ Calix-AMN4 obtained by
using Hyperchem software with the molecular mechanics subroutine.
between similar pairs of atoms depends on the bound cation.
The average distance between the carbonyl groups of the
naphthalene rings decreases in the following order: 0.57 nm
‡
for the complex of Calix-AMN4 with K , 0.53 nm for Ca2‡,
‡
‡
0.51 nm for Na and 0.48 nm for Li .
Fluorescence spectra and quantum yields: The emission
spectra of Calix-AMN1 and Calix-AMN4 and their com-
plexes in acetonitrile are presented in Figure 3. The two
3
ligands have very low fluorescence quantum yields (ꢁ10 ).
Figure 1. Absorption of a) Calix-AMN1 and b) Calix-AMN4 in acetoni-
trile and their complexes with perchlorate salts.
that there is no significant interaction between the naphtha-
lene fluorophores in the ground state. Upon cation binding,
the observed red shifts in the absorption spectra of Calix-
AMN1 and Calix-AMN4 were to be expected from cation ±
dipole interactions: the interaction between the bound cation
and the electron-withdrawing carbonyl group conjugated to
the electron-donating methoxy group. These effects on
absorption spectra are comparable to those reported with
aminocoumarins linked to crown ethers.[13]
After division of the absorption coefficients of the Calix-
AMN4 complexes by four, for comparison with those of the
Calix-AMN1 complexes, no significant difference was ob-
served except with lithium complexes, for which a hypochro-
mic effect of about 15% was found on going from the complex
‡
‡
Li ꢀ Calix-AMN1 to the complex Li ꢀ Calix-AMN4. This
can be explained by the smaller size of the lithium cation, the
binding of which reduces the distance between the naphtha-
lene rings so that some excitonic coupling takes place. This
was confirmed by energy-minimized structures (Figure 2)
obtained according to the procedure described by Diamond
for similar compounds.[17] The estimated interatomic distances
Figure 3. Corrected emission spectra of a) Calix-AMN1 and b) Calix-
AMN4 and their complexes with perchlorate salts in acetonitrile.
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Calixarene Sensors
4590±4598
Interaction of a cation with the carbonyl group of the
fluorophore produces a red shift in the emission spectrum,
as a result of the enhancement of the photoinduced charge
transfer from the methoxy group to the carbonyl group.
Moreover, an outstanding enhancement of the fluorescence
quantum yield was observed (0.68 for the complex Ca2‡
ꢀ
Calix-AMN1).
For the complexes Ca2‡ ꢀ Calix-AMN4 and Li ꢀ Calix-
AMN4, the fluorescence quantum yields were found to be
smaller than those of the corresponding Calix-AMN1 com-
plexes as a result of self-quenching of the fluorophores. In
addition to this effect, tails at long wavelengths were observed
‡
‡
‡
in the emission spectra of the complexes with Li and Na .
This is likely to be due to excimer formation between adjacent
naphthalene groups, previously observed in the case of b-
cyclodextrins with seven appended naphthalene chromo-
phores.[18] The relative contributions of excimers and mono-
mers to the fluorescence intensities were found to be related
‡
‡
‡
to the size of the complexed cation (Li > Na > K ). The
smaller the size of the cation, the smaller the average distance
between the naphthalene groups and the higher the proba-
bility of excimer formation. This result is in accordance with
the ground-state interaction between the fluorophores in the
‡
complex Li ꢀ Calix-AMN4, as revealed by the absorption
spectrum (hypochromic effect).
In ethanol, the enhancements of the fluorescence quantum
yields of Calix-AMN1 and Calix-AMN4 upon complexation
‡
‡
with Na and K follow the same trend as in acetonitrile.
Measurement of the fluorescence quantum yields of com-
plexes of Calix-AMN4 and Calix-AMN1 with the other
Figure 4. a) Fluorescence decays of Calix-AMN1 complexes in acetoni-
trile. Excitation wavelength 310 nm, emission wavelength 420 nm; b) fluo-
rescence decays of Calix-AMN4 complexes with perchlorate salts in
acetonitrile. Excitation wavelength 310 nm, emission wavelength 420 nm.
‡
cations (Li , Ca2‡) in ethanol was impossible because of the
very small stability constants (see below).
Table 1. Fluorescent decay components [ns] of the various complexes with
perchlorate salts of Calix-AMN1 and Calix-AMN4 in acetonitrile at different
observation wavelengths [lexc ˆ 310 nm]
Time-resolved fluorescence: An attempt to record the fluo-
rescence decay of free Calix-AMN1 failed because of its very
low fluorescence quantum yield. The fluorescence decay
curves of complexes of Calix-AMN1 with various cations are
shown in Figure 4a. Table 1 shows that in most cases a sum of
two or three exponentials was necessary to obtain a satisfac-
tory fit of the decays. Such multiexponential decays can be
interpreted in terms of the existence of different types of
available binding sites for the complex. Indeed, different
binding sites were found by Arnaud-Neu et al.[14] by X-ray
diffraction on samples of tetraethyl p-tert-butylcalix[4]arene-
tetraacetate. Interestingly, in the case of the complex with
Ca2‡, the fluorescence decay was found to be almost mono-
exponential; this led us to conclude that there is close to a
single conformer for this complex. The average decay times
(defined as hti ˆ Sfiti, in which fi and ti are the fractional
intensities and time constants, respectively) for the complexes
follow the variation in the fluorescence quantum yields.
The fluorescent decays for the complexes of Calix-AMN4
with alkali and alkaline earth cations were recorded at two
observation wavelengths: at 420 nm (Figure 4b), where no
emission from the excimers was expected, and at 520 nm,
lem [nm] t1 [ns]
t2 [ns]
t3 [ns]
hti [ns] c2R
(f1)
(f2)
(f3)
Calix-AMN1
‡
K
420
420
420
0.19 Æ 0.02
0.67 Æ 0.02
3.0 Æ 0.5
0.51
0.81
2.8
1.1
(0.52 Æ 0.06) (0.44 Æ 0.05)
(0.04 Æ 0.04)
2.9 Æ 0.5
‡
Na
0.22 Æ 0.01
0.71 Æ 0.01
1.1
(0.28 Æ 0.02) (0.58 Æ 0.03)
(0.14 Æ 0.12)
3.37 Æ 0.02
(0.78 Æ 0.04)
4 Æ 0.15
‡
Li
0.33 Æ 0.05
1.03 Æ 0.06
1.03
1.05
(0.05 Æ 0.02) (0.16 Æ 0.04)
1.0 Æ 0.26
Ca2‡ 420
3.8
(0.05 Æ 0.03)
(0.94 Æ 0.02)
Calix-AMN4
‡
K
420
0.27 Æ 0.03
1.13 Æ 0.06
0.9
1.07
(0.22 Æ 0.06) (0.73 Æ 0.05)
1.48 Æ 0.03
520
420
1.5
1.7
0.96
1.07
‡
Na
0.33 Æ 0.04
1 Æ 0.3
3.4 Æ 0.5
(0.4 Æ 0.1)
3.6 Æ 0.2
(0.31 Æ 0.07) (0.29 Æ 0.1)
1.3 Æ 0.2
520
420
520
3.2
1.2
4.0
1.4
3.3
1.03
1.01
0.97
0.99
1.03
(0.18 Æ 0.07)
(0.81 Æ 0.6)
2.9 Æ 0.5
‡
Li
0.21 Æ 0.02
0.90 Æ 0.10
(0.25 Æ 0.05) (0.36 Æ 0.08)
1.5 Æ 0.2
(0.3 Æ 0.1)
4.6 Æ 0.3
(0.82 Æ 0.04) (0.17 Æ 0.05)
which should provide the larger contribution. A sum of two or Ca2‡ 420
three exponentials was necessary to fit the fluorescence
0.27 Æ 0.02
1.64 Æ 0.05
(0.16 Æ 0.03) (0.84 Æ 0.03)
1.96 Æ 0.01
520
15.2 Æ 1.4
decays. Compared with Calix-AMN1 complexes, the fluores-
cence decays of the Calix-AMN4 counterparts also reflect
(0.9 Æ 0.01)
(0.1 Æ 0.03)
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interactions between the fluorophores. In particular, the
decays are affected by the formation of excimers, especially
for smaller cations. However, it is important to note that no
rise time resulting from excimer formation was detected at
520 nm on the shortest timescale available through our
instrumentation (around 60 ps); this means that the excimers
are preformed, as previously reported for b-cyclodextrins with
seven appended naphthalenic fluorophores.[18]
These observations prompted us to examine the radiative
and nonradiative rate constants (kr and knr ˆ kic
‡
kisc
respectively) for de-excitation of the low-lying S1 states.
These can be calculated from the fluorescence quantum yields
and average excited-state lifetimes for the complexes of Calix-
AMN1 and Calix-AMN4 (Table 2). Since the fluorescence
Table 2. Photophysical properties for Calix-AMN1 and Calix-AMN4 and
their complexes with perchlorate salts in acetonitrile
Interpretation of the fluorescence enhancement upon cation
binding: In order to understand the enhancement of the
fluorescence quantum yield upon complexation, the relative
locations of the singlet np* and pp* states of the naphthalene
fluorophores in the absence and in the presence of a bound
cation should be considered. This fluorophore, belonging to
the family of ketones, would indeed be expected to have a
low-lying np* excited state.[19] The molar absorption coef-
Charge density FF
hti
kr
knr ˆ kic ‡ kisc
1
1
1
[q Š
]
[ns] [108 s
]
[108 s
]
Calix-AMN1
0.001
‡
K
Na
Li
Ca2‡
0.75
1.03
1.47
2.02
0.020 0.5
0.044 0.8
0.32
0.68
0.001
0.4
0.5
1.14
1.78
19.6
12
2.4
‡
‡
2.8
3.8
0.84
Calix-AMN4
‡
K
0.75
1.03
1.47
2.02
0.024 0.9
0.031 1.7
0.059 1.2
0.092 1.4
0.27
0.18
0.49
0.66
10.8
5.7
7.9
1
ficient of an np* band is generally of the order of 102 m 1 cm .
‡
Na
Li
‡
Therefore, in view of the absorption spectrum and the molar
Ca2‡
6.5
1
absorption coefficient (ꢁ104 M 1 cm ), the np* absorption
band is likely to be buried beneath the intense pp* absorption
band generally observed for aromatic ketones. The very low
fluorescence quantum yield of Calix-AMN1 can be explained
in terms of efficient intersystem crossing from the low-lying
np* excited state to the triplet state, which has a pp*
character, as previously determined by ESR for 6-methoxy-2-
acetonaphthalene.[20]
decays deviate strongly from a single exponential in most
cases, the values of these rate constants should be taken with
caution, but the trends provide valuable information. For the
complexes with Calix-AMN1, the nonradiative rate constant
decreases drastically when the charge density of the bound
1
‡
cation increases; it varies from about 2 Â 109 s for K ꢀ
It is worth noting that the fluorescence quantum yield, the
absorption spectra and the emission spectra of Calix-AMN1
are solvent-dependent. In ethanol, the fluorescence quantum
yield is twice as large as in acetonitrile. The greater the
polarity of the solvent, the higher the fluorescence quantum
yield. This observation is consistent with the fact that, in
general, when the polarity and the hydrogen-bonding power
of the solvent increases, the np* state shifts to higher energy
whereas the pp* state shifts to lower energy and can
eventually become the low-lying excited state, with the
consequence of a higher fluorescence quantum yield (see,
for instance, the solvent-dependence of the luminescence of
1
Calix-AMN1 to 8.4  107 s for Ca2‡ ꢀ Calix-AMN1. Such a
drastic change is consistent with a decrease in the efficiency of
intersystem crossing as a de-excitation pathway as the charge
density of the cation increases. This supports our interpreta-
tion in terms of the relative locations of the singlet np* and
pp* states, the latter becoming the low-lying state in the
presence of cations of large charge density.
In order to confirm this interpretation further, the transient
absorption spectra of Calix-AMN1 and its complexes with
alkali and alkaline earth cations were recorded in acetonitrile
after nanosecond pulse excitation at 355 nm (Figure 5). The
triplet absorption spectrum of the free ligand exhibits a
maximum at 447 nm (Table 3). For the complex Ca2‡ ꢀ Calix-
AMN1, the triplet absorption was too low to be determined at
xanthone.[21]
)
Possible changes in the relative locations of the np* and
pp* levels as a function of solvent polarity should be kept in
mind when the effect of cation binding on the fluorescence
quantum yield of Calix-AMN1 is considered. Such an analogy
is indeed supported by the fact that the higher the charge
‡
density of the bound cation (Li and Ca2‡), the larger
the fluorescence quantum yield (Figure 3). Therefore,
it is reasonable to make the assumption that interaction with
a bound cation can reduce the energy difference between
the np* and pp* levels and can even invert these levels
if the charge density on the cation is large enough. When the
np* and pp* levels are close (in cases of interaction with
‡
‡
cations of moderate charge density, such as Na and K ),
there is competition between intersystem crossing from the
np* state and emission from the pp* state (allowed tran-
sition). In the case of cations of greater charge density, de-
excitation mainly occurs from the pp* state through the
allowed S1 ! S0 transition, and the fluorescence quantum
yield is higher.
Figure 5. Transient absorption spectra of Calix-AMN1 and its complexes
in degassed acetonitrile obtained 8 ns after laser pulse excitation at 355 nm.
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Calixarene Sensors
4590±4598
Table 3. Transient absorption maxima and triplet lifetimes of Calix-AMN1
and its complexes with perchlorate salts in acetonitrile.
Cation
Diameter [Š]
Charge
density [q Š
lT [nm]
tT [ms]
1
]
Calix-AMN1
447
450
450
456
1.37
4.14
4.83
10
‡
K
Na
Li
2.66
1.94
1.36
1.98
0.75
1.03
1.47
2.02
‡
‡
Ca2‡
not measurable
each wavelength. For the complexes with the other cations,
the triplet absorption efficiencies were found to be smaller
than those of the ligands. This effect is again related to the
charge density of the bound cation and is in agreement with
less-efficient intersystem crossing to the triplet state when the
calixarene is complexed with a cation. The absence of a
significant shift of the transient absorption spectra upon
complexation (Table 3) means that the nature of the triplet
state is the same whatever the bound cation and should have a
pp* character according to El Sayedꢁs rules.[19] This is
consistent with the ESR measurements on 6-methoxy-2-
acetonaphthalene.[20]
Finally, the single-exponential decays of the transient
absorption show that the triplet lifetime increases when the
charge density of the bound cation increases (Table 3). The
nonradiative de-excitation from the triplet state indeed
becomes slower when the spin ± orbit coupling between the
triplet state (3pp* character, totally symmetrical) and the
ground state (totally symmetrical) is weaker. Such an increase
in the lifetime of the triplet states of aryl ketones on going
from a 3np* to a 3pp* triplet state has been described
previously.[22, 23]
Figure 6. Excitation polarization spectra of the sodium complexes of
Calix-AMN1 and Calix-AMN4 in rigid glasses formed from ethanol-
methanol mixtures (9:1, v/v) at 100 K. Observation wavelength 410 nm.
Experimental Section). The lack of depolarization observed
at the extreme red edge of the absorption spectrum (Weber
red-edge effect) is due to inhomogeneous broadening, as
previously reported with multichromophoric b-cyclodex-
trins.[18, 24] It is interesting to note that the shoulder of the
absorption spectrum at about 345 nm corresponds to a
‡
shoulder in the excitation polarization spectrum of Na ꢀ
Calix-AMN4; this means that excitation at the red edge of
vibronic bands is able to produce a vibronic red-edge effect
(as in the case of multichromophoric cyclodextrins[18, 24]).
Around the absorption maximum (ꢁ315 nm), and thus in a
region where red-edge effects should not be significant, the
‡
anisotropy value of Na ꢀ Calix-AMN1 is about 0.33 and that
‡
of Na ꢀ Calix-AMN4 is about 0.05 ± 0.04. The ratio is thus
about 7. This value should be compared with the value that
would be observed if the fluorophores were oriented at
random. In such a case, under the reasonable assumption that
the energy-transfer process is very fast with respect to the
fluorescence decay, the ratio should be 4, because there are
four fluorophores. In fact, when the fluorophores are ran-
domly oriented, only the initially excited one contributes
significantly to the value of the anisotropy and, since the
probability that this fluorophore emits a photon is 0.25, the
anisotropy should be a quarter of the value in the absence of
transfer. A ratio of 7 instead of 4 means that the fluorophores
In the case of Calix-AMN4, the interpretation of the
calculated values for the nonradiative rate constants is not
straightforward because of the additional excited state
interactions between the fluorophores and excimer forma-
tion.
Steady-state fluorescence anisotropy: Since the fluorescence
quantum yields of the free ligands, Calix-AMN1 and Calix-
AMN4, are too low for it to be possible to carry out
fluorescence polarization experiments, we compared the
excitation polarization spectra of the sodium complexes of
Calix-AMN1 and Calix-AMN4 in rigid glasses formed at
100 K from ethanol ± methanol mixtures (9:1, v/v) (Figure 6).
It turned out to be impossible to perform the same measure-
ments for the complexes of Calix-AMN4 and Calix-AMN1
‡
are not randomly oriented in the Na ꢀ Calix-AMN4 com-
plex; in other words, the bound cation induces some
preferential conformations, as expected.
Cation binding properties of Calix-AMN1 and Calix-AMN4:
The stoichiometries and stability constants of complexes were
determined by analysis of the variations in absorbance or
fluorescence intensity upon addition of metal ion. When the
stoichiometry is 1:1, satisfactory fits must be obtained by using
Equation (1) as previously reported.[13b]
‡
‡
with the other cations (Li , Ca2‡, K ) because of their very
low stability constants in ethanol (see below).
The anisotropy values of Calix-AMN4 are clearly smaller
than those of Calix-AMN1, except upon excitation at the red
edge of the absorption spectrum. Since rotational motions of
the fluorophores are impossible in frozen glasses, this
depolarization effect is solely due to nonradiative energy
transfer between the fluorophores. Energy transfer was
indeed expected, because the average distance between the
fluorophores is about 0.51 nm, which is smaller than the
Förster critical radius, estimated at about 0.8 nm (see
ꢀ
ꢃ
s
ꢁ
ꢂ
2
Xlim X0
1
1
X ˆ X0 ‡
CL ‡ CM
‡
CL ‡ CM
‡
4 CLCM
(1)
2 CL
b
b
Here X is the absorbance or fluorescence intensity of the
solution. X0 and Xlim are the values of X for the free ligand and
the ML complex, respectively. CL and CM are the total
Chem. Eur. J. 2001, 7, No. 21
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0721-4595 $ 17.50+.50/0
4595

I. Leray, B. Valeur et al.
FULL PAPER
concentrations of ligand and metal ion, respectively, and b is
the stability constant. When Xlim cannot be accurately
determined, it can be left as a floating parameter in the
analysis. An example of the spectral evolution and titration
curve is given in Figure 7. Apart from the complexation of
water± ethanol mixtures are reported in Table 4. The values
of the stability constants in acetonitrile are in good agreement
with those previously reported in the case of tetraethyl p-tert-
butylcalix[4]arenetetraacetate and p-tert-butylcalix[4]arene
tetraphenyl tetraketone.[14] In ethanol, the stability constants
‡
‡
‡
Calix-AMN1 with Na in ethanol, satisfactory fits for 1:1
complexes were found.
of the complexes with Na and K are much larger than those
‡
with Li , Ca2‡ and Mg2‡ because the last three cations have
higher charge densities and are thus more solvated than
complexed, in contrast to the situation observed in acetoni-
‡
‡
trile. In ethanol, a good selectivity for Na in preference to K
was observed. With the goal of practical applications, the
stability constants were measured in the presence of water.
Since the compounds Calix-AMN1 and Calix-AMN4 are not
soluble in water, the stability constants were measured in
ethanol ± water mixtures. The maximum water content allow-
ing sufficient solubility for spectroscopic measurements was
40% by volume for Calix-AMN1 and 20% by volume for
Calix-AMN4. This fact is not a drawback because our goal is
to design an optical-fibre device in which the fluoroiono-
phores will be immobilized within a polymer or a sol-gel film.
Insolubility in water is therefore preferable, to minimize
leaching. The stability constants for Calix-AMN4 and Calix-
AMN1 in an ethanol ± water mixture (80:20, v/v) showed the
Figure 7. Evolution of the absorption spectrum of Calix-AMN4 (1.9 Â
10 5 molL 1) upon addition of calcium perchlorate in acetonitrile. Inset:
titration curve at 326 nm.
‡
‡
expected very good selectivity towards sodium: the Na /K
‡
selectivities (expressed by the ratio of the stability constants)
were found to be 500 for Calix-AMN1 and 407 for Calix-
AMN4. For Calix-AMN1, when the amount of water in-
creases (EtOH/H2O 60:40, v/v), a very high selectivity
towards sodium (1300) is still shown.
In the case of the complex of Calix-AMN1 with Na in
ethanol, no satisfactory fits were found by using Equation (1).
Further analysis of the evolution of the whole absorption
spectra (200 wavelengths) by means of the SPECFIT software
(global analysis) showed that the titration curves were
consistent with the formation of two complexes, metal/ligand
1:1 and 1:2; this is consistent with the NMR study undertaken
by Detellier[25] in the case of the complex of sodium with
calix[4]arene tetraester. The constants of the successive
equilibria K11 and K12 were determined.
Conclusion
The fluoroionophores Calix-AMN1 and Calix-AMN4 descri-
bed in the present paper belong to the PCT sensor class. It is
remarkable that, in addition to the spectral shifts upon cation
binding, they exhibit fluorescence enhancements as large as
those found in PET sensors, due to inversion of the low-lying
np* and pp* levels. This is the first time that advantage has
been taken of such a photophysical effect in a fluoroiono-
phore. In addition to these outstanding photophysical re-
sponses, they show very good selectivities for sodium ions in
water± ethanol mixtures. Such selectivities in the presence of
water are very promising for practical applications to aqueous
samples.
‰MLŠ
M ‡ L > ML
K11
K12
ˆ
ˆ
(2)
(3)
‰MŠ‰LŠ
‰ML2Š
ML ‡ L > ML2
‰MLŠ‰LŠ
The global equilibrium for the formation of the complex
ML2 is:
‰ML2Š
‰MŠ‰LŠ
M ‡ 2L > ML2
b12
ˆ
(4)
2
The stability constants obtained for the complexes with
alkali and alkaline-earth cations in acetonitrile, ethanol and
Table 4. Stability constants of Calix-AMN1 and Calix-AMN4 complexes with alkali and alkaline-earth cations in acetonitrile, ethanol and ethanol-water
mixtures. The values given in this table are log10 b.
‡
‡
‡
Solvent
Compound
Li
(1.36 Š)
Na
K
Mg2‡
(1.32 Š)
Ca2‡
(1.98 Š)
(1.94 Š)
(2.66 Š)
acetonitrile
ethanol
Calix-AMN1
Calix-AMN4
Calix-AMN1
6.28 Æ 0.06
6.32 Æ 0.07
4.74 Æ 0.03
4.0 Æ 0.01
2.87 Æ 0.03
2.06 Æ 0.04
5.3 Æ 0.06
6.84 Æ 0.12
1.25 Æ 0.03
7.12 Æ 0.12
0.9 Æ 0.08
7.03 Æ 0.1
2.75 Æ 0.04
1.82 Æ 0.06
logK11 ˆ 4.6 Æ 0.1
logb12 ˆ 9.7 Æ 0.13
logK12 ˆ 5.1 Æ 0.1
3.87 Æ 0.05
EtOH/H2O
(80:20, v/v)
EtOH/H2O
(60:40, v/v)
Calix-AMN1
Calix-AMN4
Calix-AMN1
1.17 Æ 0.05
1.62 Æ 0.03
0
(b ˆ 0.75 Æ 0.3)
< 1
4.23 Æ 0.04
< 1
1.1 Æ 0.03
3.04 Æ 0.07
4596
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0947-6539/01/0721-4596 $ 17.50+.50/0
Chem. Eur. J. 2001, 7, No. 21

Calixarene Sensors
4590±4598
ˆ
ˆ
160.2 (CArC O),195.6 (C O); elemental analysis calcd (%) for
C97H98O12 ´ 3H2O: C 77.16, H 6.94; found C 77.38, H 6.81.
Experimental Section
Solvents and salts: Acetonitrile from Aldrich (spectrometric grade) and
absolute ethanol from SDS (spectrometric grade) were used as solvents for
absorption and fluorescence measurements. Alkali and alkaline-earth
perchlorates and potassium thiocyanate, from Alfa, were of the highest
quality available and vacuum dried over P2O5 prior to use.
Synthesis
General procedures: 1H and 13C NMR spectra were recorded at room
temperature on a Brucker AC400 spectrometer. Elemental analyses were
performed at the Institut de Chimie des Substances Naturelles (France).
2-Chloro-1-(6-methoxynaphthalen-2-yl)ethanone (1): 2-Methoxynaphtha-
lene (3.95 g, 25 mmol) dissolved in nitrobenzene (20 ml) was added to a
mixture of chloroacetyl chloride (2 mL, 25 mmol) and aluminium chloride
(3.6 g, 27 mmol) in nitrobenzene (20 mL). The solution was left stirring at
08C for 20 min and was then heated at 408C for 2 h. Water was carefully
added to the solution mixture. Nitrobenzene was removed by steam
distillation. The product was extracted with ether. The organic layer
obtained was washed with water, dried over MgSO4 and evaporated in
vacuum. Column chromatography on silica (eluent cyclohexane/AcOEt
95:5) gave a white solid, which was recrystallized from toluene/hexane to
give 6-chloroacetyl-2-methoxynaphthalene (1). Yield: 1.28 g, 21%; m.p.
1138C; 1H NMR (400 MHz, CDCl3): d ˆ 3.97 (s, 3H; CH3), 4.84 (s, 2H;
CH2), 7.19 (m, 2H; ArH), 7.84 (m, 3H; ArH), 8.41 (s, 1H; ArH), elemental
analysis calcd (%) for C13H11ClO2: C 66.53, H 4.72; found C 66.53, H 4.73.
Spectroscopic measurements and calculations: UV/vis absorption spectra
were recorded on a Varian Cary5E spectrophotometer. Corrected emission
spectra were obtained on a SLM-Aminco8000C spectrofluorimeter. The
fluorescence quantum yields were determined by using quinine sulfate
dihydrate in H2SO4 (0.1n) as a reference (FF ˆ 0.546)[12]. All solvents used
were of spectroscopic grade.
Steady-state fluorescence anisotropies, defined as rˆ (Ik I?)/(Ik ‡ 2I?)
(where Ik and I? are the fluorescence intensities observed with vertically
polarized excitation light and vertically and horizontally polarized emis-
sions, respectively) were determined by the G-factor method. Low
temperature measurements (100 K) were carried out in specially made
1 cm  1 cm strain-free quartz cuvettes and with an OxfordDN1704
cryostat with quartz windows.
The critical radius for transfer by the dipolar mechanism was evaluated
from the following Equation:
5,11,17,23-Tetra-tert-butyl-25-mono-[(6'-methoxynaphthalen-2'-yl)-carbon-
ylmethoxy]calix[4]arene (2): 4-tert-Butylcalix[4]arene (414 mg, 0.63 mmol)
was suspended in acetone (30 mL) containing KHCO3 (126 mg, 1.26 mmol)
1
ꢀ
ꢃ
Z
1/6
4
R0 ˆ 0.020108 k2f0n
I(l)e(l)l4 dl
(5)
and
2-chloro-1-(6-methoxynaphthalen-2-yl)ethanone
(1)
(300 mg,
1.26 mmol). The reaction mixture was heated under reflux for 2 days. The
cooled mixture was filtrated and evaporated under reduce pressure. The
crude product was chromatographed on silica column (eluent cyclohexane/
AcOEt 95:5) to give 2. Yield: 265 mg, 50.2%; m.p. 2358C; 1H NMR
(400 MHz, CDCl3): d ˆ 1.22 (s, 18H; tBu), 1.24 (s, 18H; tBu), 3.42 (d,
2J(H,H) ˆ 13.6 Hz, 2H; Ar-aCHeq), 3.45 (d, 2J(H,H) ˆ 13 Hz, 2H; Ar-
aCHeq), 3.97 (s, 3H; CH3), 4.32 (d, 2J(H,H) ˆ 13.6 Hz, 2H ; Ar-aCHeq), 4.61
0
with R0 in nm, where k2 is the orientational factor, f0 is the donor
fluorescence quantum yield, n is the average refractive index of the
medium in the wavelength range in which spectral overlap is significant,
I(l) is the normalized fluorescence spectrum of the donor, e(l) is the
acceptor absorption coefficient [dm3 mol 1 cm 1] and l is the wavelength in
nanometers. The orientation factor was taken to be equal to the dynamic
2
(d, J(H,H) ˆ 13 Hz, 2H; Ar-aCHeq), 5.76 (s, 2H; Ar-aCH2), 7.1 (m, 10H;
ArH), 7.84 (d, 2J(H,H) ˆ 9 Hz, 1H; ArH), 7.88 (d, 2J(H,H) ˆ 9.3 Hz, 1H;
ArH), 8.06 (d, 2J(H,H) ˆ 8.5 Hz, 1H; ArH), 8.46 (s, 1H; ArH), 9.55 (s, 2H;
OH), 10.38 (s, 1H; OH); elemental analysis calcd (%) for C57H66O6 ´
1.5 H2O: C 78.32, H 7.96; found C 78.41, H 8.15.
2
average, that is ꢂ3. The refractive index was chosen to be that of ethanol.
Under these conditions, the value of R0 calculated by Equation (5) is
0.8 nm.
Time-resolved fluorescence-intensity decays were obtained by the single-
photon timing method, with picosecond laser excitation with a Spectra-
Physics set-up composed of a Titanium Saphir Tsunami laser pumped by an
argon ion laser, a pulse selector and doubling (LBO) and tripling (5BBO)
crystals. Light pulses were selected by optoacoustic crystals at a repetition
rate of 4 MHz. Fluorescence photons were detected by means of a
5,11,17,23-Tetra-tert-butyl-25-mono-[(6'-methoxynaphthalen-2'-yl)carbonyl-
methyoxy]-26,27,28-tris-(ethoxycarbonylmethoxy)-calix[4]arene (3): Com-
pound 2 (425 mg, 0.4 mmol) and ethyl bromoacetate (177 mL, 1.6 mmol)
were refluxed in the presence of K2CO3 (123 mg, 0.8 mmol) in acetone
(40 mL) for 12 h. The reaction mixture was filtered and concentrated under
vacuum. The crude product was chromatographed on silica column (eluent
CH2Cl2/AcOEt 80:20) and recrystallized from EtOH to give the desired
product. Yield: 305 mg, 67%; m.p. 1448C; 1H NMR (400 MHz, CDCl3):
d ˆ 1.05 (m, 36H; tBu), 1.21 (m, 9H; CH3), 3.21 (m, 4H; CH2), 3.95 (s, 3H;
CH3), 4.1 (t, 2J(H,H) ˆ 6 Hz, 4H), 4.15 (t, 2J(H,H) ˆ 6 Hz, 2H), 4.81 (m,
8H), 5.05 (d, 2J(H,H) ˆ 12 Hz, 2H), 5.72 (s, 2H), 6.82 (m, 8H), 7.15 (m,
2H), 7.72 (d, 2J(H,H) ˆ 7.4 Hz, 1H; ArH), 7.91 (d, 2J(H,H) ˆ 8.1 Hz, 1H;
ArH), 8.07 (d, J ˆ 8 Hz, 1H; ArH ), 8.59 (s, 1H; ArH); elemental analysis
calcd (%) for C69H84O12: C 74.97, H 7.66; found C 74.82, H 7.81.
Hamamatsu MCPR3809U photomultiplier, connected to
a constant-
fraction discriminator. The time-to-amplitude converter was purchased
from Tennelec. In this investigation, the FWHM (full width at half
maximum height) instrument response was 61 ps. Data were analysed by
the maximum entropy method.[26]
Transient absorption measurements were carried out by nanosecond laser
flash photolysis. The instrument uses the third harmonics of a BM Indus-
tries Q-switched Nd-YAG (model BMI 5011 DNS 10), delivering 7 ± 8 ns
pulses at 1064 nm. Q-switching was achieved with a Pockels cell inside the
cavity. The giant pulse was frequency-doubled and -tripled in potassium
dihydrogen phosphate (KDP) crystals. The output energy was 120 mJ at
355 nm. The energy deposited in the sample was lowered to 3 mJ by
interposing a diffusing plate in front of the irradiation cell. The excitation
beam and the probe beam generated by a pulse xenon source were
perpendicular to each other inside the 1 Â 1 cm cell. The analysing beam
was spectrally dispersed by a monochromator and converted into an
electric signal by a Hamamatsu R 928 PM tube. The electrical signal was
recorded by a digital memory oscilloscope (Tektronix TDS 620 B) con-
nected to a PC. The transient signals were analysed by an Igorꢃ procedure-
based in-house routine. The reported decays and rate constants are the
mean values of at least ten different measurements.
5,11,17,23-Tetra-tert-butyl-25,26,27,28-tetra-[(6'-methoxynaphthalen-2'-yl)-
carbonylmethyloxy]calix[4]arene (4): 4-tert-Butylcalix[4]arene (505 mg,
0.77 mmol), K2CO3 (418 mg, 3.3 mmol), NaI (454 mg, 3.3 mmol) and
2-chloro-1-(6-methoxynaphthalen-2-yl)ethanone (710 mg, 3.3 mmol) in dry
acetone were stirred under reflux under argon for 48 h. The reaction
mixture was poured into water (50 mL) and extracted with dichloro-
methane. The extract was washed consecutively with 5% aqueous sodium
metabisulfite, water, 3% sulfuric acid and water. The organic layer was
dried over MgSO4. The crude product was chromatographed on silica
(eluent CH2Cl2/acetone 80:20) and recrystallized from MeOH to give the
desired product. Yield: 280 mg, 34%; m.p. 1528C; 1H NMR (400 MHz,
2
CDCl3): d ˆ 1.21 (s, 36H; tBu), 3.52 (d, J(H,H) ˆ 12 Hz, 4H; Ar-aCHax),
3.96 (s, 12H; OCH3), 4.52 (d, 2J(H,H) ˆ 12 Hz, 4H; Ar-aCHeq), 5.49 (s, 4H;
CH2), 7.12 (d, 2J(H,H) ˆ 8.8 Hz, 4H; ArH), 7.17 (s, 4H; ArH), 7.23 (m, 8H;
ArH), 7.5 (d, 2J(H,H) ˆ 9.2 Hz, 4H; ArH), 7.72 (m, 8H; ArH), 8.16 (s, 4H;
ArH); 13C NMR (100.6 MHz, CDCl3): d ˆ 30.2 (CH2), 31.4 (CH3), 34.3
(C(CH3)3), 55.6 (OCH3), 79.1 (CH2CO), 105.9 (CHAr), 120.1 (CHAr),
123.9 (CHAr), 126 (CHAr), 127.6 (CHAr), 129.2 (CAr), 129.4 (CHAr),
131.1 (CHAr), 134.6 (CAr), 137.9 (CAr), 148.5 (COAr), 150.4 (COAr),
Energy-minimized structures were generated by Hyperchem V5.1 software
with MM ‡ as force-field.
Global analysis of the evolution of the whole absorption spectra was
performed with the Specfit Global Analysis System V3.0 for 32-bit
Windows systems. This software uses singular value decomposition and
nonlinear regression modelling by the Levenberg ± Marquardt method.[27]
Chem. Eur. J. 2001, 7, No. 21
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2001
0947-6539/01/0721-4597 $ 17.50+.50/0
4597

I. Leray, B. Valeur et al.
FULL PAPER
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Acknowledgement
We thank P. Denjean and Dr. R. Pansu for their assistance in tuning the
single-photon timing instrument. F. OꢁReilly, J.-L. Habib-Jiwan and J.-Ph.
Soumillion are acknowledged for their help in the synthesis of Calix-
AMN1.
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Received: March 2, 2001 [F3106]
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