New fluorimetric alkali and alkaline earth metal cation sensors based on noncyclic crown ethers by means of intramolecular excimer formation of pyrene

Article abstract of DOI:10.1021/jp981567t

New fluorescent reagents, 2,2′-[oxybis(3-oxapentamethyleneoxy)]-bis[N-(1-pyrenylmethyl)benzamide)] (4) and its analogues (2, 3, and 5) which have two pyrenes at the both terminals of polyoxyethylene compounds, were synthesized, and their complexation behavior with alkaline earth cations was investigated by fluorescence spectrometry, fluorescence lifetimes, and ¹H NMR spectrometry. These reagents (3-5) showed strong intramolecular excimer emissions around at 480 nm in the fluorescence spectra. On the complexation with alkaline earth metal cations, the increase of monomer emission around at 400 nm accompanied by the disappearance of intramolecular excimer emission of free reagents was observed. These reagents formed a 1:1 complex, and the order of complex formation constants was Ca²⁺ ? Sr²⁺ > Ba²⁺ > Mg²⁺ > Li⁺ for all reagents. ¹H NMR spectra of these complexes with alkaline earth cations suggested the helical structures of the complexes. Fluorescence spectral changes at the formation of complexes depended on the chain length of the oxyethylene moiety and metal cations. In all cases, the formation of helical structures at the complexation was supported by the ¹H NMR spectra.

Full text of DOI:10.1021/jp981567t

7910
J. Phys. Chem. B 1998, 102, 7910-7917
New Fluorimetric Alkali and Alkaline Earth Metal Cation Sensors Based on Noncyclic
Crown Ethers by Means of Intramolecular Excimer Formation of Pyrene
Yoshio Suzuki, Tatsuya Morozumi, and Hiroshi Nakamura*
DiVision of Material Science, Graduate School of EnVironmental Earth Science, Hokkaido UniVersity,
Sapporo 060-0810, Japan
Masatsugu Shimomura
Molecular DeVice Laboratory, Research Institute for Electronic Science, Hokkaido UniVersity,
Sapporo 060-0812, Japan
Takashi Hayashita
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku Sendai 980-77, Japan
Richard A. Bartsh
Department of Chemistry and Biochemistry, Texas Tech UniVersity, Lubbock, Texas 79409
ReceiVed: March 19, 1998; In Final Form: July 28, 1998
New fluorescent reagents, 2,2′-[oxybis(3-oxapentamethyleneoxy)]-bis[N-(1-pyrenylmethyl)benzamide)] (4)
and its analogues (2, 3, and 5) which have two pyrenes at the both terminals of polyoxyethylene compounds,
were synthesized, and their complexation behavior with alkaline earth cations was investigated by fluorescence
1
spectrometry, fluorescence lifetimes, and H NMR spectrometry. These reagents (3-5) showed strong
intramolecular excimer emissions around at 480 nm in the fluorescence spectra. On the complexation with
alkaline earth metal cations, the increase of monomer emission around at 400 nm accompanied by the
disappearance of intramolecular excimer emission of free reagents was observed. These reagents formed a
1:1 complex, and the order of complex formation constants was Ca2+ = Sr2+ > Ba2+ > Mg2+ > Li+ for all
reagents. 1H NMR spectra of these complexes with alkaline earth cations suggested the helical structures of
the complexes. Fluorescence spectral changes at the formation of complexes depended on the chain length
of the oxyethylene moiety and metal cations. In all cases, the formation of helical structures at the complexation
1
was supported by the H NMR spectra.
Introduction
demonstrated that oligo-oxyethylene compounds appending
quinoline units at their terminals bind K+ strongly via electro-
static interaction between the ion and oxygen atoms with the
aid of π-π interaction between end-capped quinolines. Their
study also showed that quinoline moieties, which can be strong
ligands themselves, provide an increase in rigidity around the
oxyethylene part and significantly contribute to stabilization of
the complex with K+. Thus, artificial noncyclic polyether
compounds have been used for an ion selective ionophore as
active cation transport and extraction reagents.
Nakahama et al.8-11 demonstrated that a series of synthetic
carboxylic ionophores exhibit high selectivity for K+ over Na+
in the ion transport experiment across the dichloromethane liquid
membrane. This excellent selectivity for K+ was explained by
the high lipophilicity of helical K+ complex. This idea was
further supported by X-ray crystal structure analysis.12,13 Some
podand derivatives incorporating ionophoric functional groups
such as carboxylic acid, hydroxyl group,14 and quinoline15,16
located at the terminals of molecules exhibit an excellent
property as metal ion carriers. These ionophores revealed a
reversible conformational change between the pseudocyclic and
linear structures using interactions between hydroxyl group or
quinoline moiety and carboxylate anion. Recently, a noncyclic
polyether dicarboxylic acid ionophore was employed as an
It is known that tetraethylene glycol dimethyl ether (glyme),
poly(ethylene glycol), and their ethers solvate salts of alkali
metals.1,2 Although these noncyclic ether derivatives do not
show a strong complexation ability compared to that of crown
ethers,3 the conformation of these compound changes drastically
from a linear structure to a pseudocyclic one upon the complex
formation with metal ions. Thus, noncyclic crown ether
derivatives have received much attention, and various attempts
have been reported to improve their low binding ability for metal
ions, e.g., the formation of the pseudocyclic structure through
head-to-tail interaction on noncyclic crown ethers. The pseudocy-
clic structure increases the binding force and plays important
roles not only in artificial ionophores but also in natural
ionophores for the selective binding of metal cations. Monensin
is a naturally occurring noncyclic ion carrier which is con-
structed with three tetrahydrofuran units and two pyran units
having hydroxyl and carboxyl groups at their terminals.4,5 In
alkaline conditions monensin forms the pseudocyclic structure
like a crown ether molecule via intramolecular head-to-tail
hydrogen bonds between the carboxylate anion and hydroxyl
group.
An early study on artificial noncyclic crown ether derivatives,
called podand, was carried out by Vo¨gtle et al.6,7 They
S1089-5647(98)01567-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 09/11/1998

Alkali and Alkaline Earth Metal Cation Sensors
J. Phys. Chem. B, Vol. 102, No. 40, 1998 7911
extraction reagent for the separation of Pb2+ from weakly acidic
aqueous solutions by liquid chloroform membranes.17
SCHEME 1
If the conformational change of a noncyclic crown ether is
converted into physical signals, such as UV absorption and
fluorescence, etc., one can develop metal ion sensors more
sensitive than crown ethers. On the basis of this idea, we
recently synthesized novel fluorescent reagents that have two
fluorescent chromophores (anthracene, pyrene, fluorene, etc.)
symmetrically or asymmetrically placed at both terminals of
1,13-diamino-4,7,10-trioxatridecane, and their complexations
with alkali and alkaline earth cations were examined.18,19 The
complexations brought a shift of the emission maximum shifted
from 400 to 490 nm. This indicated that the two anthracenes
adopted in stack conformation upon ion complexation.
A
fluorescent regent containing two asymmetric fluorescent chro-
mophores (one side was anthracene (electron donor) and the
other was anthraquinone (electron acceptor)) gave quenching
of the emission of the anthracene monomer upon the addition
of alkaline earth cations. Anthracene approaches anthraquinone
upon complexation, and a rapid electron-transfer quenching of
the anthracene excited state occurs. In these cases, the order
of complex formation constants of both complexes was Ca2+
> Sr2+ > Ba2+ > Mg2+. A large fluorescence maximum shift
and the quenching of anthracene fluorescence are a clear
indication of the binding event.
Moreover, one can expect that if the chain length at the
oxyethylene moiety becomes long, a structural change upon the
formation of a complex will be significantly increased. The
large conformational change is favorable for making ion and
molecular sensing systems.20-24 However, the lower complex
formation constants and selectivity against metal cations will
decrease with increased chain length. Thus, we considered two
points in synthesizing a new fluorimetric noncyclic crown ether:
(a) formation of pseudocyclic structure as a binding site and
(b) increase of rigidity at the oxyethylene part.
(a) As is well-known, two pyrene molecules tend to stack
and form an excimer.25,26 This strong interaction enabled us to
monitor dynamics of cyclizations of polymer chains by means
of fluorescence spectroscopies by using R,ω-dipyrenyl reagents
as probes: poly(styrene),27 poly(ethylene oxide),28 poly(tetram-
ethylene oxide),29 poly(dimethylsiloxane),30 poly(bisphenol A-
diethylene glycol carbonate),31 and 1,8-bis[[(1-pyrenylmethoxy)-
carbonyl]ethyl]perfluorooctane.32 A similar method was used
to determine the effect of dissociated carboxylic acid groups
on hydrogen bonding with poly(methacrylic acid).33 Besides,
a detection system of guest molecules and ions was also reported
by use of fluorescence changes20 in intramolecular excimer
emission and fluorescence quenching of pyrene functional-
ized calix[4]arene,34-36 homotrioxacalix[3]arene,37 calix[4]-
resorcinarene,38 and cyclodextrin.39 These observation prompted
us to introduce pyrene at the end of polyether as a reporting
moiety and attachment for constructing the pseudocyclic
structure.
(b) The increase of rigidity at the oxyethylene part will be
expected to increasing binding constants. We thus planned to
introduce two benzenes at both terminals where carbonyl oxygen
atoms were located at the ortho position of the benzene ring.
This molecular design may produce high complex formation
constants through strong coordination of carbonyl oxygen atoms
to metal cations.
alkaline earth cations was investigated with the use of fluores-
cence spectrometry, fluorescence lifetimes, and 1H NMR
spectrometry.
Experimental Section
On the basis of the above, we synthesized a new type of
fluorescent reagents 2-5, bearing two pyrenes connected via
an amide linkage at the ortho position of the benzene ring at
the end of the polyether. Their complexation with alkali and
The synthetic pathway of the compounds 2-5 was sum-
marized in Scheme 1. The preparation of polyether-dicar-
boxylic acids (1) was carried out according to the method
reported previously.40

7912 J. Phys. Chem. B, Vol. 102, No. 40, 1998
Suzuki et al.
General Procedure of Syntheses of 2-5. Forty milliliters
of SOCl2 was added to the dicarboxylic acid (1.2 mmol) and
refluxed for 3 h. Excess SOCl2 was distilled off in vacuo and
evaporated after the addition of 10 mL of benzene four times,
and the corresponding acid chloride was obtained. 1-Pyren-
emethylamine (2.4 mmol) dissolved in 20 mL of chloroform
and 1 mL of triethylamine were added dropwise to this acid
chloride in an ice bath. The mixture was stirred for 1 day,
washed with 30 mL of water three times, and dried over MgSO4.
The solvent was evaporated in reduced pressure, and the residue
was purified by silica gel column chromatography (eluent:
chloroform-ethyl acetate). These compounds were confirmed
Measurement of 1H NMR. 1H NMR spectra were measured
by a JEOL JNM-EX400 at 30 °C. Concentrations of these
reagents were 1 × 10-2 mol/dm3 in acetonitrile-d3. In the case
of measurements of metal complexes, an excess amount of metal
cations was added to these solutions.
Results and Discussion
Parts a and b of Figure 1 show the fluorescence spectra of 4
as functions of concentrations of Li+ and Ca2+ perchlorate in
acetonitrile at 25 °C, respectively. Free 4 gave the strong
intramolecular excimer emission around at 480 nm and a weak
monomer emission around at 400 nm. Fluorescence spectra of
4 (Figure 1a) changed a little even if excess Li+ was added. In
contrast, 4‚Ca2+ (Figure 1b) showed a large decrease of the
intramolecular excimer emission accompanied by a correspond-
ing increase of the monomer emission. This result suggested
that the shape of the molecule in a pyrene moiety varied from
the intramolecular excimer conformation to a monomer con-
formation via binding of Ca2+ to oxyethylene moiety and
carbonyl oxygen atoms. The fluorescence spectra of 3 only
changed by the addition of Li+ (Figure 1c,d). This result also
supported that the same structural change occurred in complex
3‚Li+ due to the complementarity of the pseudocavity of 3 and
ionic diameter of Li+.
1
by H NMR spectra and elemental analyses.
2,2′-[Oxybis(ethyleneoxy)]-bis[N-(1-pyrenylmethyl)benza-
mide] (2): Yield 28.6%. 1H NMR (CDCl3) δ ) 1.76 (C-CH2-
O, t, 4H), 2.20 (-C-CH2-O, t, 4H), 5.16 (Ar-CH2, d, 2H),
5.49 (aromatic, d, 2H), 6.97 (aromatic, t, 2H), 7.07 (aromatic,
t, 2H), 7.85 (aromatic, d, 2H), 7.90 (NH, t, 2H), 7.95-8.23
(aromatic, m, 20H). Found: C, 79.41; H, 5.33; N, 3.56%. Calcd
for C52H40O5N2‚1/2H2O: C,78.90; H, 5.31; N, 3.54%.
2,2′-[(Ethylenedioxy-bis(ethyleneoxy)]-bis[N-(1-pyrenylmeth-
yl)benzamide] (3): Yield 30.5%. 1H NMR (acetonitrile-d3) δ
) 2.20 (C-CH2-O, s, 4H), 2.91 (-C-CH2-O, t, 4H), 3.63
(-C-CH2-O, t, 4H), 5.11 (Ar-CH2, d, 2H), 6.72 (aromatic,
d, 2H), 7.02 (aromatic, t, 2H), 7.34 (aromatic, t, 2H), 8.01
(aromatic, d, 2H), 7.90-8.25 (aromatic, m, 20H), 8.34 (NH,
m, 2H). Found: C, 78.56; H, 5.65; N, 3.28%. Calcd for
C54H44O6N2‚1/2H2O: C, 78.45; H, 5.45; N, 3.38%.
2,2′-[Oxybis(3-oxapentamethyleneoxy)]-bis[N-(1-pyrenylm-
ethyl)benzamide] (4): Yield 25.5%. 1H NMR (acetonitrile-d3)
δ ) 2.42 (-C-CH2-O, t, 4H), 2.60 (-C-CH2-O, t, 4H),
3.19 (-C-CH2-O, t, 4H), 3.86 (-C-CH2-O, t, 4H), 5.18
(Ar-CH2, d, 2H), 6.84 (aromatic, d, 2H), 7.05 (aromatic, t, 2H),
7.38 (aromatic, t, 2H), 8.08 (aromatic, d, 2H), 7.85-8.25
(aromatic, m, 20H), 8.48 (NH, m, 2H). Found: C, 77.53; H,
5.90; N, 3.24%. Calcd for C56H48O7N2‚1/2H2O: C, 77.32; H,
5.63; N, 3.22%.
The spectral changes of fluorescence and binding constants
for various cations are summarized in Table 1. Compounds
2-5 showed no fluorescence spectral changes, and only their
excimer emissions were observed in the presence of an excess
amount of alkali metal ions (except for Li+). New fluorescent
ionophores (2-5) of Table 1 will be classified into three groups:
the first one (compound 2) features no spectral change in the
presence of any metal ions. The second one (compound 3)
features the fluorescence spectral changes in the presence of
only Li+ and Mg2+. The last one (compounds 4 and 5) features
spectral changes in the presence of all alkaline earth metal ions
(except for Mg2+). In the case of group 1, its chain length of
oxyethylene is too short to surround metal cations. Thus, the
complexation ability was too weak to form any complexes with
cations. Compound 3 (group 2) in its free state forms an
intramolecular pseudocyclic structure which was supported by
the excimer fluorescence emission. Conformational changes
will be expected to occur since the oxyethylene part can orient
to the ions more suitably because of its cavity size matching
the ionic radius of Li+ or Mg2+. The cavity size of 3 is too
2,2′-[Ethylenedioxy-bis(3-oxapentamethyleneoxy)]-bis[N-(1-
pyrenylmethyl)benzamide] (5): Yield 20.5%. 1H NMR (aceto-
nitrile-d3) δ ) 2.79 (-C-CH2-O, s, 4H), 2.68 (-C-CH2-O,
t, 4H), 2.79 (-C-CH2-O, t, 4H), 2.79 (-C-CH2-O, t, 4H),
3.96 (-C-CH2-O, t, 4H), 5.23 (Ar-CH2, d, 2H), 6.92
(aromatic, d, 2H), 7.04 (aromatic, t, 2H), 7.37 (aromatic, t, 2H),
7.93-8.35 (aromatic, m, 20H), 8.54 (NH, m. 2H). Found: C,
75.34; H, 6.15; N, 2.71%. Calcd for C58H52O8N2‚1/2H2O: C,
75.40; H, 5.85; N, 3.03%.
small for other alkaline earth metal ions (Ca2+, Sr2+, and Ba2+
)
to form complexes. Compounds 4 and 5 (group 3) have larger
cavities than that of 3 and responded to alkaline earth cations
(Ca2+, Sr2+, and Ba2+) whose ionic radii are larger than those
Measurement of Fluorescence Spectra. Fluorescence spec-
tra were measured by a Shimadzu RF-5300PC at 25 °C.
Concentrations of fluorescent reagents were 1 × 10-6 mol/dm3
in purified acetonitrile. Alkaline earth metal cations were added
into the solution of fluorescent reagent as perchlorate salts. To
prevent nonlinearity of the fluorescence intensities, the excitation
wavelength was set to 341 nm, which was an isosbestic point
in the absorption spectra. The temperature was maintained at
25 °C.
of Li+ and Mg2+
.
The successive increase of monomer emission with addition
of metal ion will finally cause a complete disappearance of the
intramolecular excimer. The fluorescence intensity of the
excimer emission of 4 at 480 nm was plotted against the ratio
of [metal]/[ligand] (Figure 2). The curve obtained clearly shows
the formation of a 1:1 complex with Ca2+
. The complex
formation constant (K) was evaluated from the curve by means
of a nonlinear least-squares curve fitting method (Marquardt’s
method).41 Formation constants for other ligands and various
cations were also determined in the same way and summarized
in Table 1. All the ligands formed 1:1 complexes and preferred
alkaline earth cations to alkali cations. Compound 3 shows Li+
and Mg2+ specificity, which may stem from good complemen-
tarity between its cavity size and their ionic sizes. On the other
hand, compound 4 has about 10 times larger complex formation
Measurement of Fluorescence Lifetime. Fluorescence
lifetimes were measured by a Horiba NAES-500 at room
temperature. Concentrations of fluorescent reagents were 1 ×
10-6 mol/dm3, and those of metal cations were 1 × 10-3 mol/
dm3. The excited wavelength was 350 nm, and the monitored
wavelength was 400 and 480 nm. The sample solutions were
degassed by the freeze-pump-thaw method. All the experi-
ments were carried out at room temperature.

Alkali and Alkaline Earth Metal Cation Sensors
J. Phys. Chem. B, Vol. 102, No. 40, 1998 7913
Figure 1. Fluorescence spectra of 4 and its Li+ complex (a) and Ca2+ complex (b); 3 and its Li+ complex (c) and Ca2+ complex (d) in
acetonitrile at 25 °C. Excitation wavelength ) 341 nm. [3 or 4] ) 1 × 10-6 mol/dm3.
TABLE 1: Summary of Fluorescence Spectral Changes and
Complex Formation Constants (log K) of 2-5 for Various
Metal Ions in Acetonitrile at 25 °C
introduction of benzene rings. Presumably, the pseudocyclic
structure also contributed to the increase of binding constants.
Fluorescence spectral data clearly indicate structural change
of ligands upon complex formation. To clarify the structures
of complexes in detail, a 1H NMR study was carried out in the
absence and presence of metal cations in acetonitrile-d3 at 30
°C. The 1H NMR spectra of 4 before and after addition of Ca2+
are shown in Figure 3 as a typical result. Peak assignments
were made by using H-H COSY and NOESY spectra. The
center methylene protons (b and c) of free 4 (and also 3)
exhibited unusually high (ca. 2.2 ppm) upfield chemical shifts.
The usual peak position of these oxyethylene protons is ca. 3.5
ppm, and this diamagnetic shift should be due to the aromatic
ring current, although the structure was unknown. In the 4‚
Ca2+ complex, oxyethylene proton peaks (b, c, and d) shifted
to low magnetic field (∆δ ) 1.29, 1.11, and 0.21 ppm,
respectively). Because of reducing electron density of ca. 0.2-
0.3 ppm on the oxygen atoms by the coordinated cations, the
low-field shifts of these protons (b and c) accompanied with
complexation were dominantly due to the reduction of the ring
current effects. On the other hand, the oxyethylene proton peak
(e) (∆δ ) 0.68 ppm) nearest to the benzene ring and aromatic
protons (f, g, h, and i) (∆δ ) 0.94, 0.28, 0.31, and 1.29 ppm,
respectively) shifted to high magnetic field. Especially the
aromatic protons (f and i) showed the large high-field shifts
(∆δ ) ca. 0.94 and 1.29 ppm). This result can be explained
by desheilding due to stacking of the benzene and pyrene rings.
Li+
Na+
Mg2+
Ca2+
Sr2+
Ba2+
2
3
4
5
a
3.49
a
a
a
a
a
a
a
a
6.57
6.94
a
a
6.71
6.33
a
a
5.57
5.98
4.61
5.07
3.64
a
a These log K were not able to be determined because changes of
fluorescence spectra were not observed.
Figure 2. Dependence of fluorescence intensity at 480 nm of 4 on
the concentration of Ca2+. Excitation wavelength ) 341 nm. [4] ) 1
× 10-6 mol/dm3.
1
On the basis of H NMR and fluorescence spectral data, an
constants than those of N,N′-(4,7,10-trioxatridecane-1,13-diyl)-
bis(anthracene-9-carbonamide) with alkaline earth cations as
compared with our previous data.18 This was attributed to the
increase of the rigidity of oxyethylene moiety due to the
expected structural change of 4 before and after the addition of
metal cation on the ground state is depicted in Figure 4. The
oxyethylene moiety and amide group were omitted for clarity.

7914 J. Phys. Chem. B, Vol. 102, No. 40, 1998
Suzuki et al.
Figure 3. 1H NMR spectra of 4 before (a) and after the addition of Ca2+(b) in acetonitrile-d3 at 30 °C. [4] ) 1 × 10-2 mol/dm3. [Ca(ClO4)2] )
1 mol/dm3.
Chemical shift changes of 4 in the presence of alkali cations
(Li+, Na+) were also listed in Table 2. Oxyethylene proton
peaks (b, c, and d) shifted to low magnetic field (0.11-0.42
ppm), which strongly indicated that alkali cations were bound
to oxyethylene parts. However, oxyethylene proton peak (d)
and benzene protons shifted to high magnetic fields, although
these amounts (0.17 ppm) were much smaller than those in the
case of alkaline earth cations (0.68 ppm). The same trends were
observed in the case of 3 and 5. This shows that the structural
changes of the complex 4 with alkali metal ions (Li+ and Na+)
are smaller than that upon the complexation with alkaline earth
cations.
Figure 4. Proposed conformational changes before and after complex
formation on the ground state. Amide and oxyethylene moieties are
omitted for clarity.
The 1H NMR data of the complex 4‚Mg2+ indicate a different
behavior from other complexes. The complex 4‚Mg2+ had a
much smaller value of chemical shift changes (a-i) (0.02-0.6
ppm) than those of 4 with alkaline earth cations (0.21-1.3 ppm
for Ca2+), even with strong monomer emission. In addition,
the amide proton peak (j) and methylene proton peak (k) shifted
to low magnetic field (j, 0.29 ppm; k, 0.07 ppm), although these
peaks of other complexes shifted to a high magnetic field. These
results suggest that Mg2+ does not penetrate so deeply into the
oxyethylene moiety but does approach carbonyl oxygen atoms,
allowing the reduction of electron density at carbonyl oxygen
atoms and then the chemical shift to low magnetic field at amide
and methylene proton peaks. The NMR study on the complex
of 4 and 3 with Li+, Na+, Ca2+, and Mg2+ demonstrates that
the fluorescence spectral changes originate from the strong
coordination of the metal ions on the o-carbonyl group and
oxyethylene moiety.
Before the formation of the complex, 4 formed an intramolecular
pseudocyclic structure like a crown ether with the aid of a π-π
interaction of two pyrenes, which was supported by the
intramolecular excimer emission in the fluorescence spectra.
After the addition of metal cations, the complex formed a helical
structure, separating two pyrenes. At this time, the pyrene and
another side of benzene approached each other and then stacked
because of a strong coordination between metal ion and carbonyl
group located at the ortho position on benzene. This structural
change was also supported by the Corey-Pauling-Koltun
(CPK) molecular model study.
Induced chemical shift changes of 3-5 on the formation of
complexes with various metal ions were listed in Table 2. In
the case of 4‚Sr2+, high magnetic field shifts at oxyethylene
protons (d) (∆δ ) 0.53 ppm) and aromatic protons on the
position of (i) (∆δ ) 1.4 ppm) and (g) (∆δ ) 0.49 ppm) were
observed. The aromatic protons (f) to (i) (∆δ ) 0.02-1.11
ppm) and oxyethylene proton (e) (∆δ ) 0.28 ppm) of 5‚Ba2+
indicated high magnetic field shift similar to the chemical shifts
in 4‚Ca2+. Although the chemical shift changes of the pyrene
protons would give further information, the spectra were too
complicated for complete assignments to be made.
The shape of pyrene proton peaks of the complexes in the
NMR spectra was quite different from that before addition of
the salts (see Figure 3), supporting the occurrence of the
conformational changes of two pyrenes. Since it is well-known
that the fluorescence lifetime is sensitive to the environment of
the fluorescent moieties, we carried out fluorescence lifetime

Alkali and Alkaline Earth Metal Cation Sensors
J. Phys. Chem. B, Vol. 102, No. 40, 1998 7915
TABLE 2: Chemical Shifts (δ ppm) of 3-5 and Their Changes at the Complexation with Various Cation Complexesa
metal ion
a
b
c
d
e
f
g
h
i
j
k
3
4
blank
Li+
2.00
1.26
1.64
1.62
2.60
0.36
0.38
1.11
b
0.99
2.79
0.16
b
2.91
0.77
1.09
1.05
3.19
0.06
0.11
0.21
0.53
0.63
3.31
0.0
3.63
0.41
0.68
0.71
3.86
-0.07
-0.17
-0.68
-0.27
0.28
6.72
-0.10
0.43
0.42
6.84
-0.10
-0.27
-0.94
-0.78
0.02
7.34
-0.15
0.07
0.10
7.38
-0.02
-0.14
-0.28
-0.49
-0.14
7.37
7.02
-0.19
-0.23
-0.07
7.05
-0.04
0.16
-0.31
-0.37
-0.19
7.04
-0.05
-0.31
-0.01
8.01
-0.92
-0.78
-0.65
8.08
-0.34
-0.51
-1.29
-1.40
-1.11
8.11
8.34
-0.62
-0.28
-0.43
8.48
-0.14
-0.52
-0.31
-0.66
-0.83
8.54
5.11
-0.14
-0.21
-0.06
5.18
-0.10
-0.10
-0.03
-0.17
-0.23
5.23
Ca2+
Ba2+
blank
Li+
2.42
0.44
0.42
1.29
b
1.01
2.68
0.20
b
Na+
Ca2+
Sr2+
Ba2+
blank
Li+
5
2.79
0.12
0.29
b
3.96
6.92
-0.09
0.09
-0.11
-0.16
-0.48
0.0
0.23
-0.05
-0.37
-1.29
-0.52
-0.49
-0.31
-0.27
-0.10
-0.03
0.09
Ca2+
Ba2+
0.19
-0.16
b
b
-0.44
a Positive values show low field shifts, and negative values show high field shifts. The values in blank show the chemical shifts (δ ppm from
TMS) of the protons in acetonitrile-d3 at 30 °C. b These values could not be assigned due to overlapping of peak of water.
TABLE 3: Fluorescence Lifetimes of 3-5 before and after
In the fluorescence spectra, the existence of both excimer
and monomer emissions in free and complexed compounds may
assume the presence of an intramolecular equilibrium between
monomer (M2*) and excimer (E*) in excited singlet states. As
mentioned above, the transformation from monomer to excimer
(τ ≈ 4 ns) was much faster than the decay of monomer (τ ≈
100 ns) or excimer (τ ≈ 40 ns) itself. However, in these cases
we should observe only two components in the fluorescence
decay. (One corresponds to a transformation from the monomer
to excimer in the excited singlet state which gives a decay of
monomer emission, and another is a decay of excimer emission.)
But three decay components were observed in the present
experimental data for free compounds. We have to assume the
existence of another excited monomer species. The most
plausible one (M1*) will be the species having a molecular
conformation of a rolled-up type where two pyrene moieties
fall apart. The probable dynamics of 3-5 in the absence of
metal ions on the ground and excited states is depicted in Figure
5. From the experimental data, we assign the rate of each step
as follows. Compounds 3-5 have a slow exchange rate
between the rolled-up type (M1) and the pseudocyclic (M2)
conformations in the ground state. In the excited state, the
transition rate between M1* and M2* is still slow(τ > 1 µs)
whereas the process from M2* to E* is fast (τ ≈ 4 ns). Under
these conditions, the excited monomer M2* can be converted
to the excimer E* in a few nanoseconds. The excited rolled-
up monomer M1* shows the long decay component (ca. 120-
170 ns) which is still faster than the conformational change (M1*
f M2*).
Formation of Complexa
monitored wavelength
400 nm (τ/ns)
480 nm (τ/ns)
3
2.5 (74%)
120 (26%)
4 (3%)
2.5 (-9%)
45 (91%)
3‚Li+
60 (97%)
2.1 (18.6%)
153 (81.4%)
142 (100%)
2.7 (8%)
127 (92%)
4 (25%)
4
2.1 (-4.4%)
44 (95.5%)
4‚Ca2+
5
2.2 (-5%)
43 (95%)
5‚Ca2+
14 (38%)
146 (36.5%)
a [3]-[5] ) 1 × 10-6 mol/dm3, [metal ion] ) 1 × 10-3 mol/dm3, in
acetonitrile at room temperature. Negative values in parentheses indicate
rising up of excimer from monomer. Excitation ) 350 nm.
measurements of pyrene which will bring useful information
on the excited-state dynamics of this fluorophore. Table 3
shows the fluorescence lifetimes of 3-5 in the presence and
absence of metal cations. For the free compounds, decays with
two or three exponential components were observed. The results
of Table 3 are discussed as follows. The longer lifetimes of
>100 ns are due to monomer emissions of pyrene.42 Fluores-
cence lifetimes of 10-40 ns correspond to the excimer emissions
of pyrene. The short lifetimes from 2 to 4 ns correspond to the
transition from monomer to excimer. Complexes that did not
give any fluorescence spectral changes had fluorescence life-
times corresponding to those of a free compound. The lifetimes
of both longer-lived and shorter-lived emissions hardly changed
upon the complexation, but the magnitudes of the emissions
changed. This result shows that the environment around the
compounds remained constant upon the complexation.
On the other hand, as shown in the fluorescence spectra of
the complexes, the metal ions clearly enhanced the formation
of the rolled-up type conformation. Thus, the increase in the
amount of monomer M1* species should be observed (Figure
6). In the case of 4‚Ca2+, only one emission around at 400 nm

7916 J. Phys. Chem. B, Vol. 102, No. 40, 1998
Suzuki et al.
Figure 5. Schematic representation of the kinetics of excited singlet state of 3-5 in the absence of metal ions.
Figure 6. Schematic representation of the kinetics of excited singlet state of 3-5 in the presence of metal ions.
was observed, and the transition from monomer to excimer was
not detected. This fact indicated that the rolled-up conformation
of 4‚Ca2+ was almost fixed with a strong binding force through
the good matching sizes between its cavity and ions. Complex
5‚Ca2+ showed the enhancement of the monomer emission due
to increase of the amount of rolled-up conformation as well as
three decay components. This may mean that the conformation
of 5‚Ca2+ is not so rigid compared with that of 4‚Ca2+ and

Alkali and Alkaline Earth Metal Cation Sensors
J. Phys. Chem. B, Vol. 102, No. 40, 1998 7917
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The present paper demonstrated the detection of alkali and
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