New fluorescent "off-on" behavior of 9-anthryl aromatic amides through controlling the twisted intramolecular charge transfer relaxation process by complexation with metal ions

Article abstract of DOI:10.1021/jp0030600

New fluorophores based on linear polyether N,N′-[oxybis(3-oxapentamethylenoxy)-2-phenyl]bis(9-anthracenecarbonamide) (3) and its analogues (2 and 4) have been synthesized, and their complexation properties with various alkali-metal and alkaline-earth-metal ions were investigated by fluorescence, UV, and ¹H NMR spectroscopies. In the absence of metal ion, 2-4 showed almost no fluorescence emission (fluorescence quantum yield φ = 0.0003, fluorescence "off state) since twisted intramolecular charge transfer (TICT) occurred through the amide bond. However, 2-4 demonstrated a significant increase in fluorescence intensity around 430 nm upon complexation with alkaline-earth-metal ions. In the case of 4·Ca²⁺, a large enhancement effect on the fluorescence quantum yield (φ = 0.014, [Ca²⁺]/[4] = 5, φ4-ca²⁺/φ_free4 -42, fluorescence "on" state) was observed. This "off-on" fluorescence characteristic was originated from the cooperative strong binding mode between the carbonyl group and the ethylenoxy moiety for alkaline-earth-metal ions, resulting in effective inhibition of photoinduced TICT relaxation. Fluorophores 2-4 formed a 1:1 complex (the order of the complex formation constants was Ca²⁺ > Sr²⁺ ≈ Ba²⁺) and showed no considerable spectral changes upon complexation with alkali-metal ions and Mg²⁺. The ¹H NMR study on 2-4 and their complexes indicated that free 2-4 formed a helical structure. After complexation, the conformational change of 2 from a helical structure to a semicircular structure was observed. The pseudocyclic form was supported as the complex structure of 3, whereas the large conformational change of 4 was not observed after the addition of metal ions.

Full text of DOI:10.1021/jp0030600

J. Phys. Chem. B 2001, 105, 2923-2931
2923
New Fluorescent “Off-On” Behavior of 9-Anthryl Aromatic Amides through Controlling
the Twisted Intramolecular Charge Transfer Relaxation Process by Complexation with
Metal Ions
Tatsuya Morozumi, Takahisa Anada, and Hiroshi Nakamura*
DiVision of Material Science, Graduate School of EnVironmental Earth Science, Hokkaido UniVersity,
Sapporo 060-0810, Japan
ReceiVed: August 23, 2000; In Final Form: December 22, 2000
New fluorophores based on linear polyether N,N′-[oxybis(3-oxapentamethylenoxy)-2-phenyl]bis(9-anthracene-
carbonamide) (3) and its analogues (2 and 4) have been synthesized, and their complexation properties with
1
various alkali-metal and alkaline-earth-metal ions were investigated by fluorescence, UV, and H NMR
spectroscopies. In the absence of metal ion, 2-4 showed almost no fluorescence emission (fluorescence
quantum yield Φ ) 0.0003, fluorescence “off” state) since twisted intramolecular charge transfer (TICT)
occurred through the amide bond. However, 2-4 demonstrated a significant increase in fluorescence intensity
around 430 nm upon complexation with alkaline-earth-metal ions. In the case of 4‚Ca2+, a large enhancement
effect on the fluorescence quantum yield (Φ ) 0.014, [Ca2+]/[4] ) 5, Φ4‚Ca /Φfree 4 ) 42, fluorescence “on”
2+
state) was observed. This “off-on” fluorescence characteristic was originated from the cooperative strong
binding mode between the carbonyl group and the ethylenoxy moiety for alkaline-earth-metal ions, resulting
in effective inhibition of photoinduced TICT relaxation. Fluorophores 2-4 formed a 1:1 complex (the order
of the complex formation constants was Ca2+ > Sr2+ ≈ Ba2+) and showed no considerable spectral changes
upon complexation with alkali-metal ions and Mg2+. The 1H NMR study on 2-4 and their complexes indicated
that free 2-4 formed a helical structure. After complexation, the conformational change of 2 from a helical
structure to a semicircular structure was observed. The pseudocyclic form was supported as the complex
structure of 3, whereas the large conformational change of 4 was not observed after the addition of metal
ions.
Introduction
It is also known that proton-dissociable and -ionizable groups
often tend to make the analytical system complicated because
free proton dissociation could also be occurring and cannot be
neglected. To overcome this shortcoming, we focused on
conformational changes of noncyclic crown ethers from linear
to helical, such as glyme and poly(ethylene glycol) deriva-
tives,20,21 as an alternative method since it does not require the
introduction of proton-ionizable or -dissociable chromophores.
Noncyclic crown ethers and related compounds have also been
known as complexing reagents22,23 for alkali-metal and alkaline-
earth-metal ions and surfactants.24-26 Most researchers, however,
focus mainly on their ability as ion carriers27-36 as in naturally
occurring ionophores such as monensin.37,38 The application of
noncyclic crown ethers in oriented analytical utilization has been
relatively limited. In this context, recently, we have been
studying the complexation behavior of noncyclic crown ether
derivatives bearing several symmetric and asymmetric fluoro-
phores at their terminals.39-44 This large conformational change
upon complexation can transduce into a fluorometric signal. The
binding event can be monitored via fluorescence emission
changes from monomer to excimer and vice versa.
Over the last two decades, the search for new materials as
chemical sensors1-5 for metal ions, anions, and neutral small
molecules has been an area of rapid development. Although
these chemical sensors may consist of crown ethers,6-8 cyclo-
dextrins,9,10 and calix[n]arenes,11-13 significant attention has
been on crown ethers since they have binding ability toward
alkali-metal and alkaline-earth-metal ions that play various
important roles in biochemistry and environmental science. In
view of this trend, there is a real need to establish a simple and
reliable analytical method to study these important ions. The
well-known high sensitivity of various absorption and fluores-
cence spectroscopies led to the design and development of crown
ethers functionalized by UV and fluorescence active groups for
analytical use. In 1977, Takagi et al.14 developed the first
example of these crown ethers, which are also called chro-
mogenic crown ethers. These molecules contained both crown
ether for the binding site with metal ions and the ionic
chromophore moiety that transduced chemical information
produced by the metal binding event into an optical signal. Since
then, many kinds of chromogenic15-17 and fluorimetric5,18,19
crown ethers have been developed. Basically, chromogenic
crown ethers consist of a crown ether moiety attached with
proton-ionizable or proton-dissociable chromophore groups. In
this state, chromogenic crown ethers exhibit a distinct color
change upon complexation in the organic medium; this fact has
been utilized photometrically in the determination of alkali-metal
and alkaline-earth-metal ions.
We have also studied the effect of varying the number of
ethylenoxy units on the binding ability of metal ions in the
presence of 9-anthracenecarbonamide.44 Since Bazilevskaya et
al.45,46 reported anomalous photochemical behavior of 9-an-
thracenecarboxylic acid (9-ACOOH), this subject has been the
topic of discussion for many years.47-49 Despite the interesting
behavior shown by 9-ACOOH and its amide derivatives,50 the
10.1021/jp0030600 CCC: $20.00 © 2001 American Chemical Society
Published on Web 03/27/2001

2924 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Morozumi et al.
utilization as chemical sensors of 9-ACOOH derivatives is rather
limited.51,52 With this in mind, we examined the photophysical
properties of the 9-ACOOH amide derivatives with the hope
of widening this field of study. As a result, we recognized that
fluorescence emission of 9-anthracene aromatic amide (9-AA)
derivatives was almost quenched in solution whereas their
crystal showed green-yellow-colored emission under UV ir-
radiation. This observation suggested that 9-AA derivatives
show a twisted intramolecular (internal) charge transfer (TICT)
quenching relaxation in solution.
Recently, the TICT phenomenon has been a subject of interest
to many photochemical researchers for years. The TICT model
was first suggested by pioneering work of Rotkiewicz and
Grabowski when they explained the photophysical property of
p-(N,N-dimethylamino)benzonitrile, indicating the dual fluo-
rescence and the large Stokes shifted emission in a polar
solvent.53 Since then, explanation using the TICT model has
been extended to other structurally related compounds showing
anomalous dual fluorescence.54,55 These molecules have an
aromatic ring with acceptor and donor groups at the para
positions. The electron donor group is usually a dialkylamine,
and the electron acceptor groups are several electron-withdraw-
ing residues, such as cyano, formyl, carboxyl, ester, and
sulfonamide groups. Aromatic groups which are able to bend
along a single bond, for example, bianthryl,56,57 p-(9-anthryl)-
N,N-dimethylaniline,58,59 2-(4-(N,N-dimethylamino)phenyl)benz-
imidazole, benzothiazole,60 and 9-(N,N-dimethylamino)an-
thracene,61 were also reported to function as either donor or
acceptor groups. The molecular geometry of these para-
substituted N,N-dimethylanilines is flat in the ground state,
whereas it changes to be completely twisted in its excited state
before emitting. Next, we looked at another compound which
involves anomalous dual fluorescence of benzanilide deriva-
tives62,63 and p-(N,N-dimethylamino)benzanilide64 that functions
as the TICT model. Most researchers, however, have focused
mainly on the excited-state behavior of TICT molecules. If the
degrees of electron transfer and molecular geometry can be
controlled by using the methodology of molecular recognition
chemistry, TICT molecules could be used as excellent tools for
detecting ions and molecules, and examining microscopic
molecular environments.
Figure 1. Fluorescence spectra of 9-AA in various solvents at 25 °C:
glycerol (dotted line), cyclohexane (dashed line), diethylene glycol
(dashed-dotted line), acetonitrile or methanol (solid line). Excitation
wavelength: 362 nm. [9-AA] ) 1 × 10-5 mol/dm3.
suggest that the molecular design has good ability for free
rotation of the anilide moiety of 9-AA to be inhibited upon
complexation. It can be expected that the noncyclic ether
connecting the ortho position of benzene in 9-AA via an ether
linkage at both terminals will be exhibited dominantly through
the TICT quench relaxation process in the absence of metal
ions (no fluorescence, “off” state). Upon complex formation,
photoinduced rotation of the 9-AA moiety will be interrupted
by the strong cooperative binding ability between the carbonyl
and ethylenoxy moieties for metal ions. Hence, the TICT
relaxation pathway should be controlled, and fluorescence
emission of 9-AA will be observed (fluorescence, “on” state).
On the basis of this consideration, introduction of 9-AA at the
end of a noncyclic crown ether is one of the favorable ways to
fix the molecular geometry of 9-AA in the ground state upon
complexation with metal ions.
In this paper, we report the results of the fluorescent “off-
on” behavior of a new noncyclic crown ether, N,N′-[oxybis(3-oxa-
pentamethyleneoxy)-2-phenyl]bis(9-anthracenecarbonamide) (3)
1
and its derivatives (2 and 4) using UV, fluorescence, and H
NMR spectroscopies in acetonitrile solution.
The photophysical behavior of 9-AA showed a different
charge-transfer mode compared to N,N-dialkylaminoanthracene
derivatives that display a photoinduced electron transfer (PET)
mode as shown by de Silva et al.,65-68 Czarnick et al.,69-71 and
Fabbrizzi et al.72-74 This fact could be attributed to the absence
of the nitrogen atom of the amide group, which would otherwise
function as the electron-donating moiety. The behavior of 9-AA
also prompted us to contemplate that a novel “on-off” signaling
chemical sensor for metal ions could be developed on the basis
of our knowledge as described below. Recently, we reported
the complexation and fluorescence behavior of 2,2′-[oxybis(3-
oxapentamethyleneoxy)]bis[N-(1-pyrenylmethyl)benzamide] (4-
PPAM)43 with various metal ions. The ionophore 4-PPAM has
two benzenes at both ends of the ethylenoxy chain, and
1-pyrenylmethylamine was introduced via an amide bond at the
ortho position against the ethyleneoxy chain on the benzene
ring. The free 4-PPAM gave mainly a defused emission at 490
nm due to excimer emission of pyrene, whereas the complexes
4-PPAM‚alkaline-earth-metal ions exhibited increasing mono-
mer emission accompanied by a decrease of excimer emission.
Results and Discussion
Relaxation Process of N-Phenyl-9-anthracenecarbonamide
from the Photoexcited State. Prior to a study on the complex-
ation behavior of 2-4 for metal ions, photochemical examina-
tion using a model compound, 9-AA, was carried out. Figure 1
shows fluorescence spectra of 9-AA as a function of solvent.
The model compound 9-AA showed only weak fluorescence
emission in acetonitrile or methanol (solid line). On the other
hand, in glycerol, 9-AA showed emission with a vibronic
structure (dotted line), which is characteristic of a parent
anthracene ring, and a strong fluorescence intensity. Diethylene
glycol solution of 9-AA gave a vibronic fluorescence spectrum
with a moderate intensity (dashed-dotted line). In cyclohexane
(dashed line), a diffused fluorescence spectrum of 9-AA showing
an emission maximum with longer wavelength was observed.
The dependency of the fluorescence emission of 9-AA on
various solvents could be summarized by saying that 9-AA
showed no fluorescence emission in polar and low viscous
solvent such as acetonitrile and methanol, whereas it indicated
fluorescence emission in polar and viscous solvent (i.e., glycerol
and diethylene glycol) and in nonpolar media (i.e., cyclohexane).
It is known that 9-ACOOH shows a strong concentration-
1
The detailed H NMR studies indicated that this molecular
design induced the stronger binding force as well as broken
down π-π interactions of two pyrenes. These results also

New Fluorescent Behavior of 9-Anthryl Amides
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2925
weaker than that of glycerol and diethylene glycol. In methanol
the structure of the anthracene and anilide moieties in 9-AA
was not able to freeze out because of its loose hydrogen bond
network (low viscosity); therefore, the TICT quenching oc-
curred. On the other hand, in low viscous media such as
cyclohexane and acetonitrile, the rotation of functional groups
along the amide linkage axis is not prevented. When the
anthracene moiety of 9-AA is photoexcited, the rotation of the
anthracene-CO bond into a position approaching coplanarity
occurs. In this molecular configuration, a significant large
resonance interaction between benzene and anthracene will be
generated through the amide bond (excited state II). It is well-
known that the nitrogen atom in the amide linkage has sp2 orbital
character, and the amide bond forms a coplanar structure. Thus,
there is a possibility of conjugation interaction of both an-
thracene and benzene rings through the amide bond. The
relaxation path from excited state II separates in two ways by
the difference of solvent polarity. Through the coplanar mo-
lecular geometry at the excited state, the charge delocalization
should have occurred over the whole molecular plane, hence
generating an intramolecular exciplex of 9-AA. This relaxation
path from excited state II showed a diffuse fluorescence
spectrum at longer wavelength in cyclohexane. Due to low
cyclohexane polarity, a stabilization effect for charge separation
species is relatively small. Therefore, the complete charge
separation species (radical-ion) will not be generated, and a
fluorescent chemical species will be obtained in cyclohexane.
In high polarity and low viscous solvent such as acetonitrile
and methanol, the OC-N bond in the amide linkage rotates
spontaneously from excited state II, and the OC-N bond must
be in a bent conformation after the rotation. The highly polarized
TICT structure of 9-AA (excited state III) will be generated
successively, and should be effectively stabilized by the high
polarity of acetonitrile and methanol. Since complete charge
separation will occur, the resulting radical-ion pair species of
9-AA will show no fluorescence emission.
Figure 2. Schematic representation of a mechanism for the relaxation
process of photoexcited 9-AA in various solvents.
dependent, broad, and structureless fluorescence emission band
with an unusually large Stokes shift.45-49 Werner et al.47 pointed
out that anomalous photochemical behavior of 9-ACOOH
originated from a charge-transfer effect at the excited state. Their
interpretation involved a rotation of the 9-carboxyl group in
the excited state into a position approaching coplanarity with
the anthracene ring. In this molecular configuration, a significant
large resonance interaction between the carboxyl group and the
aromatic ring was observed.
Besides it is known that benzanilide undergoes an anomalous
fluorescence with a large Stokes shift which appeared at longer
wavelength compared with that of its phosphorescence.75
Azumaya and co-workers62 reported the mechanism of dual
fluorescence of benzanilide (BA) and N-methylbenzanilide
(MBA) in methylcyclohexane. They concluded that the amide
bond must be rotated for the emission of longer wavelength,
and these emissions were originated from excited TICT species
with a twisted amide bond.
On the basis of these two interpretations for the photochemical
behavior of 9-ACOOH, BA, and MBA, the relaxation mecha-
nism of 9-AA from the excited state can be considered as shown
below. Figure 2 demonstrates a schematic representation of
relaxation pathways of photoexcited 9-AA in various solutions.
The molecular geometry of 9-AA in the ground state is
perpendicular between the anilide moiety and the anthracene
ring. If this configuration of 9-AA is maintained even at the
excited state, the fluorescence spectrum will be similar to that
of anthracene (excited state I). Glycerol and diethylene glycol
have a large viscosity, which originates from strong hydrogen-
bonding ability. When 9-AA was included in the viscous media
and/or the strong hydrogen bond network of glycerol, the
structure of 9-AA in the ground state was almost fixed. Even
the photo-driven rotation at the anilide moiety should be
prevented, and the relaxation pathway from excited state I
showed the anthracene-like emission. The fluorescence intensity
of glycerol solution was larger than that of diethylene glycol.
This shows that the more viscous media can effectively prevent
photoinduced rotation of 9-AA. Although methanol has the
ability of making a hydrogen-bonding network, it is clearly
Fluorescence Spectra and Lifetime of 2-4 and Their
Complexes with Alkaline-Earth-Metal Ions. Figure 3 shows
the fluorescence spectra of 2-4 as a function of the concentra-
tion of Ca2+ in acetonitrile at 25 °C. Fluorescence emissions of
the anthracene moiety in 2-4 were quite weak in the absence
of metal ions. On the other hand, in the case of 2 (Figure 3a)
and 3 (Figure 3b) structureless broad spectra having a fluores-
cence emission maximum (λmax) around 440 nm were observed,
and the fluorescence intensity increased with the addition of
Ca2+. These observations indicated that the TICT process of 2
and 3 was inhibited by coordination of metal ion at the
ethylenoxy moieties and carbonyl group. The fluorescence
emission bands and shapes also indicated that two anthracene
fluorophores did not stack on each other. In the case of N,N′-
(4,7,10-trioxatridecane-1,13-diyl)bis(9-anthracenecarbon-
amide), which showed a well-defined face to face conformation
upon complexation with alkaline-earth-metal ion in the ground
state, the excimer emission was observed at 490 nm.39,44 This
difference of the two λmax values suggested that the emission
of the complex originated from monomer-like chemical species.
The λmax of 2‚Ca2+ and 3‚Ca2+ shifted to relatively longer
wavelength compared with that of a parent hydrocarbon. This
is due to the fact that the structures of the excited state of 2‚
Ca2+ and 3‚Ca2+ included charge transfer character. The shapes
of the spectra of 2‚Ca2+ and 3‚Ca2+ are quite similar to that of
a solution of 9-AA in cyclohexane. This strongly suggests that
the structures of 2‚Ca2+ and 3‚Ca2+ have coplanar molecular
geometry (see Figure 1) at the excited state. The same

2926 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Morozumi et al.
Figure 4. UV spectra of 4 and its Ca2+ complex in acetonitrile at 25
°C. [4] ) 1 × 10-5 mol/dm3.
of the 9-AA moiety of 2-4, attempts of fluorescence lifetime
measurement were carried out. Unfortunately, the fluorescence
lifetime of 4‚Ca2+ was too short and could not be obtained by
the present apparatus. The fluorescence lifetime of 4‚Ca2+ was
shorter than nanosecond order.
UV Absorption Measurement. Figure 4 shows UV spectra
of 4 and its Ca2+ complex as a typical result. UV spectral
changes of all complexes on the anthracene moieties were not
observed compared with that of free compounds. This observa-
tion suggests that two anthracene moieties did not interact with
each other in the ground state significantly. Werner et al.47
pointed out that the carbonyl group was placed perpendicularly
on anthracene ring. Compounds 2-4 also kept their molecular
geometries perpendicular. Hence, even if 4 strongly bound Ca2+
at carbonyl groups, the UV spectral change would be small or
undetectable since the coordination effect on the anthracene ring
gives relatively small electronic perturbation. The UV absor-
bance of 4‚Ca2+ around 275 nm, which originated from the
benzene moiety, was decreased with the increase in Ca2+
concentration. This may reflect that the microenvironment of
the benzene moiety changed electronically and sterically upon
complexation. The same results were also observed using 2 and
3 in the presence of Ca2+, Sr2+, and Ba2+
.
Complex Formation Constants. The successive increase in
monomer-like emission with the addition of metal ions will
finally cause an inhibition of TICT quenching of the fluoro-
phores. The degree of increase of monomer emission clearly
depended on the concentrations of metal ions. The fluorescence
intensity of 4 at 430 nm was plotted against the ratio [metal]:
[ligand] (Figure 5). The obtained curve clearly indicates the
formation of a 1:1 complex with Ca2+. The complex formation
constant (K) was determined from the curve by means of a
nonlinear least-squares curve-fitting method (Marquardt’s
method).76 The other formation constants were evaluated in the
same way and are summarized in Table 1. All ligands formed
1:1 complexes and preferred alkaline-earth-metal ions to alkali-
metal ions. To quantify the effect on complexation for fluores-
cence intensity, fluorescence quantum yields (Φ) were also
determined (Table 1).
The clear dependency of the chain length of ethylenoxy units
and metal ions on complex formation constants was not
observed. In all cases, Ca2+ complexes of 2-4 gave the largest
complex formation constant (log K), whereas the value of log
K for Sr2+ was almost equal to that of Ba2+. This shows large
affinity of the ethylenoxy moiety for Ca2+ compared with that
of Sr2+ and Ba2+. The order of log K was 4 > 3 > 2 for all
Figure 3. Fluorescence spectra of 2 and its Ca2+ complex (a), 3 and
its Ca2+ complex (b), and 4 and its Ca2+ complex (c) in acetonitrile at
25 °C. Excitation wavelength: 362 nm. [2]-[4] ) 1 × 10-5 mol/dm3.
observations were also obtained using 2 and 3 in the presence
of Sr2+ and Ba2+, respectively.
In the case of 4‚Ca2+ as shown in Figure 3c, structured
fluorescence spectra with the strongest intensity were obtained.
This showed that the structure of 4‚Ca2+ provided an effective
prohibition mode for the TICT relaxation process, and the
excited electrons were still localized on the anthracene ring
compared with that of 2‚Ca2+ and 3‚Ca2+. The considerable
similarity of the fluorescence emission maximum and its shape
between 4‚Ca2+ and 9-AA in glycerol strongly suggests that
the structure of 4‚Ca2+ at the excited state is similar to that of
the ground state. The same results were also observed in the
case of Sr2+and Ba2+. Compounds 2-4 did not respond for Na+,
K+, or Mg2+. On the basis of our findings in Figure 3 and the
model study (Figure 1), it can be inferred that the effect of
complexation with metal ions involved an inhibition in the
rotation of the 9-AA moiety in 2-4 at the excited state.
To determine the structures of fluorescent chemical species

New Fluorescent Behavior of 9-Anthryl Amides
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2927
Figure 5. Dependence of the fluorescence intensity at 420 nm of 4
on the concentration of Ca2+. Excitation wavelength: 362 nm. [4] )
1 × 10-5 mol/dm3.
Figure 6. 1H NMR spectra of 4 before (a) and after (b) the addition
of various metal ions in acetonitrile-d3 at 30 °C. [4] ) 1 × 10-2 mol/
dm3. [metal‚(ClO4)2] ) 1 mol/dm3.
TABLE 1: Summary of Complex Formation Constants and
Fluorescence Quantum Yields of Metal Complexes of 2-4 in
Acetonitrile at 25 °C
of Ca2+ are depicted in Figure 6 as a typical result. Peak
assignments were made by 1H-1H COSY and NOESY spectra.
The chemical shift of 2-4 and their induced chemical shift
changes on the formation of complexes with various metal ions
are listed in Table 2.
Three significant characteristic results of Table 2 can be
pointed out: (1) The centers of ethylene protons a-c of free
2-4 showed unexpected high-field chemical shifts (ca. 2.4-
2.7 ppm). (2) Proton i in the benzene moiety exhibited an
unusual lower chemical shift (8.48 ppm). This proton i induced
a large higher magnetic chemical shift change upon complex-
ation with all metal ions. (3) All protons of the anthracene ring
indicated no significant chemical shift changes before and after
complexation with all metal ions.
guest
ion
fluorescence quantum yield
([ligand]:[metal ion])a
ligand
log K
2
none
Ca2+
Sr2+
Ba2+
none
Ca2+
Sr2+
Ba2+
none
Ca2+
Sr2+
Ba2+
0.00030
3.63
3.44
3.42
0.0039 (1:33.3)
0.0026 (1:33.3)
0.0019 (1:33.3)
0.00026
3
4
5.67
4.35
4.53
0.0042 (1:15)
0.0092 (1:25)
0.0079 (1:20)
0.00033
0.014 (1:5)
0.013 (1:7.5)
0.012 (1:7.5)
6.57
5.71
5.81
a The fluorescence quantum yields of all complexes were determined
at these concentration ratios.
Unusually high field chemical shifts (ca. 2.4-2.7 ppm) for
the centers of ethylene protons a-c of free 2-4 were obtained,
whereas the usual peak position of these oxyethylene protons
would be ca. 3.5 ppm. This diamagnetic shift should be due to
the aromatic ring current. On the basis of the present NMR data,
it is obvious that these fluorophores 2-4 mainly exist as helical
conformers. In this conformation, these protons are covered with
a shielding area of the anthracene moiety, resulting in unusually
high field chemical shifts. This was also supported by the
Corey-Pauling-Koltun (CPK) molecular model study.
To account for the NMR behavior of proton i, we carried
out an NMR study using model compounds. Figure 7 shows
1H NMR chemical shifts of 9-AA and N-(2-methoxyphenyl)-
9-anthracenecarbonamide (2M9-AA) in acetonitrile-d3 solution.
The ortho proton of 2M9-AA against the amide nitrogen atom
also indicated the unusually low magnetic field chemical shift
(8.49 ppm). On the other hand, the ortho proton of 9-AA showed
the usual value (7.78 ppm). This suggests that the unusually
low magnetic field shift should be caused by steric repulsion
between the methoxy group and carbonyl oxygen as shown in
Figure 8a. On the basis of the model study, the characteristic
NMR behavior of proton i in 2-4 can be explained as follows.
The molecular geometry of the 9-AA moiety in 2-4 is the same
as that of 2M9-AA in the absence of metal ions. After the
coordination of alkaline-earth-metal ions, the carbonyl group
can stand opposite the ethylenoxy moiety via the metal ion
through the aid of electrostatic interaction (Figure 8b). In this
configuration change, proton i passes through the deshielding
area of the carbonyl group, and the anomalous chemical shift
is reversed to the normal value. This may reflect the decrease
metal ions. It is obvious that the number of ethylenoxy units
become larger the more stable the complexes formed.
It is also reasonable that the fluorescence quantum yield (Φ)
of the complexes is related to the log K (except for 3‚Ca2+). In
the absence of metal ions the values of Φ for 2-4 were obtained
as about 0.0003, which shows them to be nonfluorescent and
difficult to detect. On the other hand, the value of Φ for 4‚
Ca2+ was 0.014, and this was 42.4-fold larger than that of 4
only. The enhancement effects on two Φ values for 4‚Sr2+ and
4‚Ba2+ was 39.4- and 36.4-fold, respectively. In the case of 3,
considerably large enhancement effects of both Φ values (for
3‚Sr2+ 35.4-fold and for 3‚Ba2+ 30.4-fold) were obtained
compared with that of free 3. The rate of increase in Φ for 2
was also determined to be 13- for Ca2+, 8.7- for Sr2+, and 6.3-
fold for Ba2+, respectively. Despite small enhancement effects
on Φ of 2, all Φ values were sufficiently large for detection by
fluorescence spectroscopy.
Although 3‚Ca2+ gave the largest binding constant in this
series, the value of Φ for 3‚Ca2+ (0.0042) was about half of
that of 3‚Sr2+ (0.0092) and 3‚Ba2+ (0.0079). This may reflect
the difference of the structure of the complex between 3‚Ca2+
and 3‚Sr2+,Ba2+ in both the ground and excited states.
1H NMR Measurement of the Complexes. Fluorescence
spectral data evidently showed structural and electronical change
of 2-4 upon complexation with metal ions. To clarify these
changes of the complexes in detail, a 1H NMR study was carried
out in the absence and presence of metal ions in acetonitrile-d3
1
at 30 °C. The H NMR spectra of 4 before and after addition

2928 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Morozumi et al.
TABLE 2: Chemical Shifts (δ, ppm) of 2-4 and Their Changes upon Complexation with Various Cationsa
metal
ion
a
b
c
d
e
f
g
h
i
j
k
l
m
n
2
blank
Ca2+
Sr2+
2.41
0.57
0.59
0.96
0.47
2.78
0.50
0.49
0.53
2.97
0.50
b
3.06
0.29
0.30
0.50
0.14
3.40
0.48
0.37
0.35
3.49
0.13
0.30
0.41
0.20
0.04
3.74
0.06
0.10
0.15
-0.15
3.97
0.50
0.32
0.28
4.02
b
6.94
-0.15
-0.15
-0.25
-0.40
7.04
7.14
0.04
7.14
-0.10
-0.11
-0.09
-0.16
7.17
8.46
-0.82
-0.82
-1.11
-0.81
8.51
-1.35
-1.19
-1.06
8.48
-0.86
-0.87
-1.01
-0.74
-0.65
8.63
0.54
0.41
0.47
0.07
8.76
0.83
0.33
0.21
8.76
0.56
0.37
0.29
0.70
-0.04
8.00
-0.0 3
0.00
7.48
0.02
0.01
0.10
0.01
7.53
0.17
0.02
0.03
7.53
-0.0 1
0.01
0.04
0.01
-0.04
8.07
-0.03
-0.04
0.06
8.53
0.07
0.06
0.14
0.05
8.58
0.21
0.16
0.06
8.59
0.08
0.07
0.08
0.11
0.00
0.02
Ba2+
-0.01
-0.05
7.17
-0.08
0.06
0.11
Na+
0.02
-0.03
3
4
blank
Ca2+
Sr2+
2.42
0.64
0.50
0.56
2.69
0.20
0.30
0.23
0.31
b
8.06
8.12
0.07
0.22
0.14
0.08
0.07
-0.14
-0.0 7
-0.0 6
8.06
0.04
Ba2+
blank
Ca2+
Sr2+
0.11
-0.09
7.17
0.11
-0.04
2.77
-0.06
0.07
-0.15
-0.04
-0.27
7.06
7.15
8.11
-0.31
-0.19
-0.2 4
0.06
0.05
-0.08
-0.08
-0.08
-0.03
-0.08
-0.03
-0.02
-0.01
0.01
0.05
0.16
0.13
-0.06
0.07
0.02
Ba2+
Mg2+
Na+
0.51
0.37
0.14
0.02
0.03
0.21
0.02
0.01
-0.02
-0.10
0.08
-0.01
a Positive values show lower field shifts, and negative values show higher field shifts. The values in “blank” indicate 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 the peak of water.
Figure 8. (a) Schematic representation of the stablest molecular
configuration of 2M9-AA and (b) molecular configuration change of
the 9-AA moiety in 2-4 before and after the addition of metal ions.
the oxygen atoms by the coordinated cations. On the other hand,
protons a and e showed no significant chemical shift change.
A relatively large low magnetic shift change of amide proton j
Figure 7. 1H NMR chemical shift assignment of 9-AA and 2M9-AA
in acetonitrile-d3 at 30 °C.
was observed (∆δ ) 0.56 ppm). These observations indicated
of UV absorbance of 4‚Ca2+ around 275 nm (see Figure 4).
that Ca2+ was mainly bound by the carbonyl group and the two
Chemical shift changes of the other benzene protons (f-h)
exhibited complicated behavior. Relatively small higher (2 and
4) and lower (3) chemical shift changes were obtained. The 1H
NMR behavior of benzene protons f-h can be inferred in terms
of a mixture of electronical perturbations and steric effects at
the binding event. These may reflect the dependency on the
difference of metal ions and chain length of the ethylenoxy unit.
oxygen atoms attached to carbons b and c, and the two oxygen
atoms attached to carbons a and e did not play an important
role upon complexation. The later observation also reflects that
a large conformational change of 4 was not induced during
complexation with Ca2+. On the basis of the fluorescence and
1H NMR study, an expected structural change of 4 before and
after addition of Ca2+ in the ground state is illustrated in Figure
9. When the anthracene moiety of 4‚Ca2+ was excited, photo-
driven rotations of OC-N and N-benzene bonds were inter-
The considerable chemical shift changes of protons on the
anthracene ring (k-n) were not observed upon complexation
with all metal ions. These results indicated that there are no
interactions between the two anthracene rings. This interpretation
was also suggested by UV spectra of all complexes in their
ground state.
In the 4‚Ca2+ complex, oxyethylene proton peaks b-d shifted
to low magnetic field (∆δ ) 0.20, 0.50, and 0.13 ppm,
respectively) because of reduction of the electron density on
rupted through the strong cooperative coordination by Ca2+
.
Furthermore, the existence of proton a can provide an obstacle
to the excited rotation along the anthracene-CO bond. The same
1
trends in H NMR studies on 4‚Sr2+ and 4‚Ba2+ were also
obtained.
The ethylene proton peaks b-d of 3 shifted to low magnetic
field (∆δ ) ca. 0.64-0.35 ppm) in the presence of Ca2+, Sr2+
,

New Fluorescent Behavior of 9-Anthryl Amides
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2929
Figure 9. Schematic representation of the proposed structural change
of 4 after the addition of metal ions in the ground state.
Figure 10. Schematic representation of the proposed structural
difference between (a) 3‚Ca2+ and (b) 3‚Sr2+ and 3‚Ba2+ in the ground
state.
and Ba2+ in their 1H NMR spectra. Proton e and amide proton
j of 3‚Ca2+ exhibited considerably large low magnetic shift
changes (∆δ ) 0.50 ppm (e) and ∆δ ) 0.83 ppm (j)) compared
with those of 3‚Sr2+ (∆δ ) 0.32 ppm (e) and ∆δ ) 0.33 ppm
(j)) and 3‚Ba2+ (∆δ ) 0.28 ppm (e) and ∆δ ) 0.21 ppm (j)).
The present 1H NMR data showed that 3 bound Ca2+ with the
carbonyl oxygen and all of the oxygen in the oxyethylene moiety
since four units of oxyethylene had the best fit for Ca2+. Because
Ca2+ penetrates so deeply into the oxyethylene moiety, structural
crowdedness around the 9-AA moiety is relatively small (Figure
1
10a). On the other hand, the H NMR spectra of 3‚Sr2+ and
3‚Ba2+ suggested that these metal ions cannot interact strongly
with four units of the oxyethylene site since their ionic radii
are too large. This binding mode of 3‚Sr2+ and 3‚Ba2+ should
induce steric crowdedness between the two 9-AA residues
(Figure 10b). This provides the relatively effective prohibition
for the rotation of the anthracene-CO bond at the excited state.
Through the difference of the complex structures, the 3‚Ca2+
showed a relatively small value of the fluorescence quantum
yield.
In the 1H NMR spectrum of free 2, the center of oxyethylene
proton c also gave an unusually high chemical shift (2.41 ppm).
Upon complexation, proton c showed an induced large lower
magnetic field shift (∆δ ) 0.64 ppm for Ca2+) whereas proton
e showed no induced chemical shift change. Amide proton j
also showed a low magnetic field shift change (∆δ ) 0.54 ppm
for Ca2+). These NMR observations indicated that 2 mainly
bound Ca2+ with the carbonyl group and the oxygen atom
attached between carbons c and d. The formation of 2‚alkaline-
earth-metal ions induced the conformational change from helical
to semicircular as illustrated in Figure 11.
Figure 11. Schematic representation of the proposed structural change
of 2 after the addition of metal ions in the ground state.
a-e (0.04-0.37 ppm) were much smaller than those of 4 with
alkaline-earth-metal ions (0.06-0.5 ppm for Ca2+). Furthermore,
the amide proton peak j shifted largely to lower magnetic field
(0.70 ppm) compared with those of 4‚Ca2+(0.56 ppm), 4‚Sr2+
(0.37 ppm), and 4‚Ba2+(0.29 ppm). From these results, Mg2+
did not penetrate the oxyethylene moiety so deeply but did
penetrate the carbonyl oxygen atoms. This limited coordination
of 4‚Mg2+ could not freeze the rotation of anthracene-CO,
OC-NH, and N-benzene bonds at the excited and ground
states.
Chemical shift changes of 2 and 4 in the presence of alkali-
metal ion (Na+) are also listed in Table 2. Oxyethylene proton
peaks c and d of 2‚Na+ were observed at low magnetic field
((c) 0.47 ppm and (d) 0.14 ppm). Proton i of 2‚Na+ and 4‚Na+
The H NMR data of 2‚Mg2+ indicated behavior different
1
from that of other complexes. The peak assignment of complex
2‚Mg2+ was not possible since its peaks were too broad. The
1H NMR observation of 4‚Mg2+ also gave a somewhat broad
spectrum. This suggests that the Mg2+ complex of 2 and 4
showed a slow molecular motion on the NMR time scale (∼10
ms). In the case of 4‚Mg2+, the values of chemical shift changes

2930 J. Phys. Chem. B, Vol. 105, No. 15, 2001
Morozumi et al.
also exhibited higher magnetic shifts (∆δ ) 0.81 ppm and ∆δ
) 0.65 ppm, respectively). However, amide proton peak j of
both 2‚Na+ and 4‚Na+ did not shift to high and low magnetic
fields. This shows that the Na+ ion was bound mainly at the
ethylenoxy site, and the coordination of Na+ onto the carbonyl
oxygen was weaker than that of alkaline-earth-metal ions.
Although this binding mode of Na+ may prevent the rotation
of the N-benzene bond, the rotation around the anthracene-
CO and OC-NH bonds of 2-4 is not controlled at the excited
state.
SCHEME 1
Conclusion
The 9-AA moiety in 2-4 exhibited TICT quenching through
the rotation around the anthracene-CO, OC-NH, and N-ben-
zene bonds at the excited state in acetonitrile solution. Rotations
of three bonds in 2-4 were controlled by the formation of
complexes with Ca2+, Sr2+, and Ba2+. These metal ion
complexes showed large enhancement effects on fluorescence
intensity with the off-on fluorescence signal. The present
fluorophores 2-4 will be available as a photodetecting system
for analytical use with off-on fluorescent signaling character.
Experimental Section
Materials. General Procedure of the Syntheses of 2-4. The
synthetic pathway of compounds 2-4 is shown in Scheme 1.
A solution of 9-anthracenecarboxylic acid (9-anthroic acid) (0.01
mol) in 30 mL of SOCl2 was refluxed for 1.5 h. Excess SOCl2
was distilled off in vacuo, and completely evaporated after
addition of 10 mL of benzene four times. This acid chloride
was dissolved in 80 mL of THF. To this solution was added
2-aminophenol (0.02 mol) dissolved in 20 mL of THF dropwise
at room temperature. The mixture was stirred for 1 day, and
N-(2-hydroxyphenyl)-9-anthrylamide (1) and a small amount
of 2-aminophenol hydrochloride salt were precipitated. The
solution was filtered, and the precipitate was washed with 30
mL of water, EtOH, and CHCl3, successively, and dried in
vacuo. This compound 1 was used without further purification.
A mixture of 1 (0.01 mol) and 0.01 mol of t-BuOK was
dissolved in 50 mL of DMF, and 0.005 mol each of tri-, tetra-,
and pentaethylene glycol p-ditosylate was added under N2
conditions. The mixture was heated for 12 h at 75-80 °C. The
solvent was evaporated under reduced pressure, and the residue
was purified by recrystallization and/or silica gel column
chromatography (Wakogel C-200). Compound 3 was purified
by recrystallization from CH3COOH several times. Reagent 4
was purified by silica gel column chromatography (eluent
CHCl3), and then recrystallized from EtOH. In the case of 5,
the purification was carried out using column chromatography
(eluent benzene/AcOEt). Their structures and purities were
(aromatic, dd, 8H), 8.51 (aromatic, d, 2H), 8.58 (aromatic, s,
2H), 8.76 (NH, m, 2H). Anal. Found: C, 75.81; H, 5.76; N,
3.54. Calcd for C50H44O7N2‚1/2H2O: C, 75.65; H, 5.71; N, 3.53.
N,N′-[Ethylenedioxybis(3-oxaoctamethyleneoxy)-2-phenyl]-
bis(9-anthracenecarbonamide) (4): yield 58.3%; pale yellow
solid; 1H NMR (acetonitrile-d3) δ ) 2.77 (-C-CH2-O, s, 4H),
2.69 (-C-CH2-O, t, 4H), 2.97 (-C-CH2-O, t, 4H), 3.49
(-C-CH2-O, t, 4H), 4.02 (-C-CH2-O, t, 4H), 7.04 (aro-
matic, d, 2H), 7.17 (aromatic, t, 4H), 7.50-7.56 (aromatic, m,
8H), 8.06-8.13 (aromatic, dd, 8H), 8.51 (aromatic, d, 2H), 8.58
(aromatic, s, 2H), 8.76 (NH, m, 2H). Anal. Found: C, 73.71;
H, 5.67; N, 3.26. Calcd for C52H48O8N2·H2O: C, 73.74; H, 5.95;
N, 3.30.
Measurement of Fluorescence and UV Spectra. Fluores-
cence spectra were measured using a Shimadzu RF-5300PC at
25 °C. The concentrations of the fluorescent reagents were 1 ×
10-5 mol/dm3 in purified acetonitrile. Alkaline-earth-metal
cations were added to the solution of fluorescent reagent as
perchlorate salts. Temperature was maintained at 25 °C.
Fluorescence quantum yields were determined by the integra-
tion of corrected fluorescence spectra. Quinine sulfate in 0.5
mol/dm3 H2SO4 was used for correction of fluorescence spectra
as a fluorescent standard (Φ ) 0.54).
Because the fluorescence lifetimes of free and metal-
complexed 2-4 were below 10 ns, the sample solutions were
used without degassing.
1
confirmed by H NMR spectra and elemental analyses.
N,N′-[Ethylenedioxybis(ethyleneoxy)-2-phenyl]bis(9-an-
1
thracenecarbonamide) (2): yield 72.5%; mp 132-136 °C; H
NMR (acetonitrile-d3) δ ) 2.41 (C-CH2-O, s, 4H), 3.06 (-C-
CH2-O, t, 4H), 3.74 (-C-CH2-O, t, 4H), 6.94 (aromatic, d,
2H), 7.17 (aromatic, t, 4H), 7.48-7.56 (aromatic, m, 8H), 8.06-
8.13 (aromatic, dd, 8H), 8.51 (aromatic, d, 2H), 8.58 (aromatic,
s, 2H), 8.63 (NH, m, 2H). Anal. Found: C, 77.00; H, 5.59; N,
3.64. Calcd for C48H40O6N2·1/2H2O: C, 76.89; H, 5.51; N, 3.73.
N,N′-[Oxybis(3-oxapentamethyleneoxy)-2-phenyl]bis(9-an-
thracenecarbonamide) (3): yield 66.0%; mp 83-85 °C (as 3‚
1
UV spectra were recorded on a Shimadzu UV-2400 with an
equipment temperature controller in spectral grade acetonitrile.
Measurement of Fluorescence Lifetime. Fluorescence life-
EtOH); H NMR (acetonitrile-d3) δ ) 2.42 (-C-CH2-O, t,
4H), 2.78 (-C-CH2-O, t, 4H), 3.40 (-C-CH2-O, t, 4H),
3.97 (-C-CH2-O, t, 4H), 7.04 (aromatic, d, 2H), 7.17
(aromatic, t, 4H), 7.50-7.56 (aromatic, m, 8H), 8.06-8.13
times were measured using a Horiba NAES-500 at room

New Fluorescent Behavior of 9-Anthryl Amides
J. Phys. Chem. B, Vol. 105, No. 15, 2001 2931
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Acknowledgment. We gratefully acknowledge the financial
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Ministry of Education, Sport and Culture, Japan.
Supporting Information Available: 1H NMR of 4 and its
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