Control between TICT and PET using chemical modification of N-phenyl-9-anthracenecarboxamide and its application to a crown ether type chemosensor

Article abstract of DOI:10.1016/j.tet.2008.08.092

Both twisted intramolecular charge transfer (TICT) and photoinduced electron transfer (PET) relaxation processes of N-phenyl-9-anthrylcarboxamide derivatives can be characterized by modified substitution of the phenyl group. Introduction of a methoxy group to phenyl moiety quenched fluorescence of the anthracene using TICT or PET process, and was not retrieved even using highly viscous media. The introduction of a methylene unit induced fluorescence emissions using a solvent with both viscosity and polarity. This phenomenon demonstrates that the effects of both TICT and PET are involved in this system. Based on these data, we synthesized a novel crown ether derivative 7: its analytical usefulness as a fluorescent chemosensor for alkaline earth metal ions is reported herein.

Full text of DOI:10.1016/j.tet.2008.08.092

Tetrahedron 64 (2008) 10735–10740
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Control between TICT and PET using chemical modification of
N-phenyl-9-anthracenecarboxamide and its application to a crown
ether type chemosensor
,
Jeongsik Kim a, Tatsuya Morozumi b, Hiroshi Nakamura b
*
a Division of Environmental Materials Science, Graduate School of Environmental Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
b Section of Materials Science, Research Faculty of Environmental Earth Science, Hokkaido University, Sapporo, Hokkaido 060-0810, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 July 2008
Received in revised form 28 August 2008
Accepted 28 August 2008
Available online 4 September 2008
Both twisted intramolecular charge transfer (TICT) and photoinduced electron transfer (PET) relaxation
processes of N-phenyl-9-anthrylcarboxamide derivatives can be characterized by modified substitution
of the phenyl group. Introduction of a methoxy group to phenyl moiety quenched fluorescence of the
anthracene using TICT or PET process, and was not retrieved even using highly viscous media. The in-
troduction of a methylene unit induced fluorescence emissions using a solvent with both viscosity and
polarity. This phenomenon demonstrates that the effects of both TICT and PET are involved in this
system. Based on these data, we synthesized a novel crown ether derivative 7: its analytical usefulness as
a fluorescent chemosensor for alkaline earth metal ions is reported herein.
Keywords:
Intramolecular charge transfer
Donor–acceptor system
Fluorescent chemosensor
Modification
Ó 2008 Published by Elsevier Ltd.
1. Introduction
fluorescence emission. The TICT model is useful to describe fluo-
rescence behavior of N,N0-dimethyl-4-aminobenzonitrile. Although
An electron donor and acceptor system in one molecule en-
genders an intramolecular charge transfer state by photo-irradia-
tion. From the perspective of the modes of charge transfer in a short
range, they are classifiable into two categories: photoinduced
electron transfer (PET)1–4 and twisted intramolecular charge
transfer (TICT).5–11 The PET process depends on the distance and
redox levels between the donor and acceptor. In contrast, the oc-
currence of TICT is expected to be governed only by redox levels.
Moreover, the TICT process is expected to require the twisted mo-
tion of the donor or acceptor at the charge transfer event.
Fluorescent PET sensors have received much attention for their
wide application to various analytical and chemical processes;12
they have been used as a tool in the field of molecular switches and
photonic device.13–17 A PET sensor generally comprises a guest
binding site as the receptor and fluorophore moiety. Consequently,
the sensitivity is directly related to the strength of the emission
signal produced after complexing of the guest molecule by the
receptor.
numerous studies of TICT family molecules have been reported, in
contrast to the PET, the use of TICT for chemosensors and signaling
purposes remains limited.19,20
Recently, the present author reported that N-phenyl-9-anthra-
cene-21 and N-phenyl-1-pyrene-22 carboxamide derivatives
showed weak fluorescence emission in an acetonitrile solution
through intramolecular charge transfer pathway, which is similar
to TICT. Based on these results, fluoroionophores having these flu-
orophores and linear polyether have been synthesized. Upon
complexing with alkaline earth metal ions, the absorption spec-
trum did not change, but a large enhancement of fluorescence in-
tensity was obtained as a result of freezing of the twisted motion of
amide bond and fluorophores by the coordination of ions. Here, the
anthracene or pyrene unit acts as an acceptor; the benzene unit acts
as a donor. To extend this idea, an introduction of an electron do-
nating group, such as the methoxy unit, on the benzene ring will
enhance the charge transfer characteristics.
We also reported chemosensors based on linear polyether
possessing N-phenyl-1-naphthalenacetamide23 or benzo-crown
ether.24 These sensors show that binding of alkaline earth metal
ions signals a switching-on of the fluorescence of naphthalene.
From spectroscopic data, the hybrid quenching process of TICT and
PET on N-phenyl-1-naphthylacetamide was strongly evidenced
through switching ‘Off–On’ monitoring. However, unfortunately,
The TICT concept, as presented by Grabowski et al.18 has become
widely recognized as an explanation of anomalous dual
* Corresponding author. Tel.: þ81 11 706 2259; fax: þ81 11 706 4863.
E-mail address: nakamura@ees.hokudai.ac.jp (H. Nakamura).
0040-4020/$ – see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tet.2008.08.092

10736
J. Kim et al. / Tetrahedron 64 (2008) 10735–10740
COOH
photochemical properties on its signal ‘Off–On’ were not elucidated
in detail using spectroscopic data with model compounds.
Bearing these in mind, we checked effects of a modifying sub-
stitution, which are the number and position of electron donating
group (–OMe) on the benzene ring. We also report the influence of
the spacer between the donor and acceptor for N-phenyl-9-
anthraceneacetamide derivatives (Scheme 1).
COOH
SOCl2
O
O
O
O
COCl
O
H2N
2. Experimental section
2.1. Materials
DCC
/ DMF
H2N
2.1.1. N-(2-Methoxy or 4-methoxyphenyl)-9-anthracene-
carboxmide (1 or 2)
OCH3
or
O
O
O
O
O
O
A solution of 9-anthracenecarboxylic acid (9-anthroic acid)
(2.22 g, 0.01 mol) in 30 mL of SOCl2 was refluxed for 1.5 h. Ex-
cess SOCl2 was distilled off in vacuo and completely evaporated
after addition of 10 mL of benzene three times. This obtained
acid chloride was dissolved in 80 mL of THF. To this solution was
added each 2-anisidine (0.28 g, 0.02 mol) or 4-anisidine (0.28 g,
0.02 mol) dissolved dropwise in 20 mL of THF at room temper-
ature. The mixture was stirred for 1 day; a small amount of
2-aminophenol hydrochloride salt was precipitated. The solution
was filtered. Then the precipitate was washed with 30 mL of
water, EtOH, and CHCl3, successively, and dried in vacuo. Com-
pounds 1 and 2 were purified by recrystallization from ethanol
(Scheme 2).
N
H
OCH3
H2N
O
6
R2
N
H
R1
1:R1=OCH3,R2=H
2:R1=H, R2=OCH3
Scheme 2.
2.1.2. N-(2-Methoxy-, 4-methoxy-, or 3,4-dimethoxy-phenyl)-9-
anthraceneacetamide (3, 4, or 5)
stirring for 1 day at 0 ꢀC. After the solvent was evaporated under
reduced pressure, a crude compound was obtained. Compound
3, 4, or 5 was purified by recrystallization from acetic acid.
9-Anthracenemethanol (5 g, 0.024 mol) in 1,4-dioxane (50 mL)
was refluxed for 2 h with the addition of SOCl2 (1.15 mL,
0.026 mol). The solution was cooled to room temperature. The
crude chlorinated methyl anthracene was precipitated in H2O and
dried at room temperature. The residue was dissolved in aceto-
nitrile (50 mL) and refluxed with the addition of NaCN (2.45 g,
0.05 mol). The reaction mixture was filtered. Then, the acetoni-
trile mixture was added to water (100 mL). After the precipitate
was filtered, the residue (4.17 g, 0.02 mol) in acetic acid (150 mL)
was refluxed carefully for 5 h with the addition of HCl (50 mL).
The solution was cooled and precipitated in water. The crude
product was dissolved in 20% NaOH aqueous solution and fil-
tered. Subsequently, HCl (20 mL) was added to the solution and
a yellow solid (9-anthraceneacetic acid) was precipitated. The
filtered 9-anthraceneacetic acid (4.28 g, 0.018 mol) and HOBt (2.4 g,
0.018 mol) were dissolved in 50 mL of DMF; then 2-anisidine
(2.21 g, 0.018 mol), 4-anisidine (2.21 g, 0.018 mol), or 3,4-dime-
thoxyaniline (2.75 g, 0.018 mol) was added. The solution was
treated with dicyclohexylcarbodiimide (3.71 g, 0.018 mol) under
2.1.3. 40-(9-Anthracencarboxamido)benzo-15-crown-5 (6)
9-Anthracenecarboxylic acid (9-anthroic acid) (2.22 g, 0.01 mol)
and HOBt (1.53 g, 0.01 mol) were dissolved in 50 mL of DMF, and 40-
aminobenzo-15-crown-5-ether (3.1 g, 0.01 mol) was added. The
solution was treated with dicyclohexylcarbodiimide (2.6 g,
0.01 mol) under stirring for 1 day at 0 ꢀC. The solvent was evapo-
rated under reduced pressure, and the crude compound was
obtained. Compound 6 was recrystallized from acetic acid.
2.1.4. 40-(9-Anthraceneacetamido)benzo-15-crown-5 (7)
9-Anthraceneacetic acid (2.35 g, 0.01 mol) and HOBt (1.53 g,
0.01 mol) were dissolved in 50 mL of DMF; then 40-aminobenzo-
15-crown-5-ether (5.1 g, 0.018 mol) was added. The solution was
treated using dicyclohexylcarbodiimide (3.4 g, 0.018 mol) under
stirring for 1 day at 0 ꢀC. The solvent was evaporated under reduced
electron donating group
OCH3
OCH3
H3CO
OCH3
H3CO
H3CO
N
N
C
N
C
H
H
H
C
O
N
N
O
O
O
O
H
H
H2C
H2C
H2C
C
spacer
C
4
5
3
2
1
Scheme 1.

J. Kim et al. / Tetrahedron 64 (2008) 10735–10740
10737
pressure. Thereby, a crude compound was obtained. Compound 7
was recrystallized from acetic acid (Scheme 3).
2.1.8. N-(4-Methoxyphenyl)-9-anthraceneacetamide (4)
Yield 83%; pale yellow solid; mp 185–187 ꢀC; 1H NMR (CDCl3)
d
¼3.71 (–OCH3, s, 3H), 4.73 (–C–CH2, s, 2H), 6.70 (aromatic, d, 2H),
6.76 (NH, s, 1H), 7.08 (aromatic, d, 2H), 7.56–7.62 (aromatic, m, 4H),
8.13 (aromatic, d, 2H), 8.35 (aromatic, d, 2H), 8.57 (aromatic, s, 1H).
Anal. Found: C, 80.01; H, 5.64; N, 4.03. Calcd for C23H19O2N$1/4H2O:
C, 79.86; H, 5.68; N, 4.05.
OH
or
H2N
OCH3
OCH3
or
2.1.9. N-(3,4-Methoxyphenyl)-9-anthraceneacetamide (5)
Yield 81%; pale yellow solid; mp 178–180 ꢀC; 1H NMR (CDCl3)
d
¼3.76 (–OCH3, s, 6H), 4.73 (–C–CH2, s, 2H), 6.45 (aromatic, d, 1H),
H2N
H2N
SOCl2
6.61 (aromatic, d, 2H), 6.77 (NH, s, 1H), 7.07 (aromatic, s, 1H), 7.51–
7.57 (aromatic, m, 4H), 8.08 (aromatic, d, 2H), 8.26 (aromatic, d, 2H),
8.52 (aromatic, s, 1H). Anal. Found: C, 77.26; H, 5.70; N, 3.77. Calcd
for C24H21NO3: C, 77.16; H, 5.71; N, 3.78.
/ 1,4-dioxane
OCH3
Cl
or
O
OCH3
2.1.10. 40-(9-Anthracenecarboxamido)benzo-15-crown-5 (6)
O
Yield 78%; pale yellow solid; mp 167–169 ꢀC; 1H NMR (CDCl3)
O
d
¼3.72 (–O–CH2, m, 8H), 3.88 (–O–CH2, m, 4H), 4.12 (–O–CH2, t, 2H),
O
H2N
4.19 (–O–CH2, t, 2H), 6.85 (aromatic, d, 1H), 7.04 (aromatic, d, 1H),
7.47–51 (aromatic, m, 4H), 7.60 (aromatic, s, 1H), 7.73 (NH, s,
1H), 8.01 (aromatic, d, 2H), 8.12 (aromatic, d, 2H), 8.49 (aromatic, s,
1H). Anal. Found: C, 70.55; H, 6.03; N, 2.80. Calcd for C29H29O6N$1/
4H2O: C, 70.79; H, 6.04; N, 2.85.
NaCN
/ DMF
O
DCC
/ DMF
CN
R2
O
2.1.11. 40-(9-Anthraceneacetamido)benzo-15-crown-5 (7)
Yield 74%; pale yellow solid; mp 163–165 ꢀC; 1H NMR (CDCl3)
N
H
3:R1=OCH3,R2=H
R1
CH3COOH
/ HCl
d
¼3.70 (–O–CH2, m, 8H), 3.83 (–O–CH2, m, 4H), 4.02 (–O–CH2, m,
4:R1=H, R2=OCH3
5:R1=OCH3, R2=OCH3
4H), 4.71 (C–CH2, s, 2H), 6.45 (aromatic, d, 1H), 6.63 (aromatic, d,
1H), 6.76 (NH, s, 1H), 7.03 (aromatic, s, 1H), 7.52 (aromatic, t, 2H),
7.59 (aromatic, t, 2H), 8.07 (aromatic, d, 2H), 8.23 (aromatic, d, 2H),
8.51 (aromatic, s, 1H). Anal. Found: C, 71.53; H, 6.22; N, 2.79. Calcd
for C30H31O6N: C, 71.84; H, 6.23; N, 2.79.
COOH
O
O
O
O
O
O
N
H
2.2. Measurement of fluorescence and UV spectra
Fluorescence spectra were measured using a spectrometer (RF-
5300PC; Shimadzu Corp.) at 25 ꢀC. The excitation wavelength was
363 nm. Concentrations of the fluorescent reagents were
1ꢁ10ꢂ5 mol/dm3 in purified acetonitrile. Alkaline earth metal cat-
ions were added to the solution of fluorescent reagent as perch-
lorate salts. The UV spectra were recorded using a spectrometer
(UV-2400; Shimadzu Corp.) with an equipment of temperature
controller in spectral grade acetonitrile.
7
Scheme 3.
2.1.5. N-(2-Methoxyphenyl)-9-anthracenecarboxamide (1)
Yield 73%; pale yellow solid; mp 164–166 ꢀC; 1H NMR (CDCl3)
d
¼3.74 (–OCH3, s, 3H), 6.92 (aromatic, d, 1H), 7.14 (aromatic, m, 2H),
7.49 (aromatic, m, 4H), 8.03 (aromatic, d, 2H), 8.18 (aromatic, d, 2H),
8.31 (NH, s, 1H), 8.53 (aromatic, s, 1H), 8.85 (aromatic, d, 1H). Anal.
Found: C, 80.14; H, 5.23; N, 4.23. Calcd for C22H17NO2$1/5H2O: C,
80.27; H, 5.27; N, 4.26.
3. Results and discussion
3.1. Photochemical properties of model compounds 1–5
To study effects of molecular motions on charge transfer (CT) at
an excited state, the solvent dependence on fluorescence intensities
was assessed using model compounds 1–5, as shown in Scheme 1.
Compounds 1 and 2, respectively, show 2-methoxy and 4-methoxy
groups on anthracene aromatic amide; compounds 3 and 4 have
a methylene group between anthracene and amide moieties based
on compounds 1 and 2. Compound 5 possesses two methoxy
groups at 3 and 4 substitution positions of the benzene ring.
Modification studies of the methoxy group on anthracene aromatic
amide donate a foundation of the benzo-crown ether compound, as
explained below.
2.1.6. N-(4-Methoxyphenyl)-9-anthracenecarboxamide (2)
Yield 75%; pale yellow solid; mp 168–170 ꢀC; 1H NMR (CDCl3)
d
¼3.84 (–OCH3, s, 3H), 6.96 (aromatic, d, 2H), 7.48–52 (aromatic,
m, 4H), 7.59 (NH, s, 1H), 7.67 (aromatic, d, 2H), 8.01 (aromatic, d,
2H), 8.16 (aromatic, d, 2H), 8.52 (aromatic, s, 1H). Anal. Found: C,
80.56; H, 5.25; N, 4.29. Calcd for C22H17NO2: C, 80.71; H, 5.23; N,
4.28.
2.1.7. N-(2-Methoxyphenyl)-9-anthraceneacetamide (3)
Yield 81%; pale yellow solid; mp 182–184 ꢀC; 1H NMR (CDCl3)
d
¼3.70 (–OCH3, s, 3H), 4.76 (–CO–CH2, s, 2H), 6.65 (aromatic, d,
Three polar solvents were used: acetonitrile, methanol, and
glycerin. Polarities (dielectric constants) of these solvents are al-
most equal (30¼37.5 for acetonitrile, 33.7 for methanol, and 42.5
1H), 6.72 (NH, s, 1H), 6.79 (aromatic, d, 1H), 7.01 (aromatic, m,
2H), 7.56–7.60 (aromatic, m, 4H), 8.09 (aromatic, d, 2H), 8.30
(aromatic, d, 2H), 8.65 (aromatic, d, 1H). Anal. Found: C, 80.69;
H, 5.73; N, 4.04. Calcd for C23H19O2N$1/5H2O: C, 80.49; H, 5.64;
N, 4.08.
for glycerin), but their viscosities (
m
) mutually differ (
m¼0.34 mPa s
for acetonitrile, 0.58 mPa s for methanol and 1490 mPa s for glyc-
erin at 20 ꢀC).25

10738
J. Kim et al. / Tetrahedron 64 (2008) 10735–10740
Figure 1 presents fluorescence spectra of 1 and 2 as a function of
using a steric repulsion of amide moiety in contrast to that of para
position. The difference of the charge transfer property should re-
sult from a geometric structure of the substituent if the electron
donating capabilities of OMe group in 1 and 2 are almost equal. The
occurrence of CT requires a change of conformation to be a suitable
configuration (e.g., all-planar molecular geometry). The fluores-
cence quenching properties of 1 and 2 revealed the two mecha-
nisms, TICT and PET, by the substitution position of OMe group.
We investigated the effects on the CT of an introduction of
a spacer, –CH2–, between the donor (methoxy-benzene and amide
groups) and the acceptor (anthracene moiety). Figure 2 portrays
solvents. In acetonitrile or methanol, the fluorescence emissions
from 1 or 2 are weak. These spectroscopic data indicate that fluo-
rescence emissions 1 and 2 were quenched by charge separation at
the excited state, where high polarity of the solvent stabilized the
charge-separated state. A glycerin solution of 1 provided a stronger
fluorescence emission than other solvents, whereas that of 2 gave
weak emission equal to that of acetonitrile and methanol solution.
A clear difference existed in the charge transfer mode between 1
and 2. Glycerin has three hydroxyl groups. For that reason, it forms
a strong hydrogen bond network in a solution, which causes high
viscosity. The molecular motion is expected to be suppressed when
substances are dissolved in such a viscous environment. The mo-
lecular motion of 1, which includes a rotation of the benzene ring
around C–N, was frozen by the high viscosity of glycerin. This
phenomenon indicates that fluorescence properties are governed
by the TICT relaxation process. A glycerin solution of 2 did not
impart a considerable fluorescence enhancement as that in the
acetonitrile and methanol solution did, which indicates that the
quenching of 2 occurs in any molecular geometry such as that of
PET. In other words, the formation of the charge-separated state of
1 necessitates a change of molecular motion, whereas that of 2 does
not. Consequently, it can be concluded that an excited state of 1
decays through the TICT process, whereas that of 2 reverts to the
ground state via the PET process rather than TICT.
According to the OMe group position (ortho and para), the
HOMO energy level of phenyl moiety might be disparate because
free rotation of benzene ring with ortho-OMe group is restrained
Figure 1. Fluorescence spectra of 1 and 2 in various solvents at 25 ꢀC; [L]¼1ꢁ10ꢂ5 mol/
Figure 2. Fluorescence spectra of 3, 4, and 5
in various solvents at 25 ꢀC;
dm3; excitation wavelength: 364 nm; slit width: 3 nm.
[L]¼1ꢁ10ꢂ5 mol/dm3; excitation wavelength: 364 nm; slit width: 3 nm.

J. Kim et al. / Tetrahedron 64 (2008) 10735–10740
10739
fluorescence spectra of 3, 4, and 5 as a function of solvents. Com-
pound 3 gave strong emissions from anthracene moiety in all sol-
vents, with almost identical fluorescence intensities (Fig. 2), which
shows that the CH2 group in 3 inhibited the charge transfer in-
teraction between a donor and an acceptor. Figure 2 shows that
quenching efficiencies of these compounds were markedly lower
than those of 1 and 2. The fluorescence spectra of 4 exhibited
weaker emission than that of 3 in acetonitrile, which is expected to
result from charge transfer beyond the CH2 group. In methanol,
compound 4 formed a hydrogen bond between the amide moiety of
4 and the solvent. These conditions induced the suppression of
internal molecular motion of 4; moreover, a slight fluorescence
enhancement effect was observed. In glycerin, a large enhancement
of fluorescence intensity of 4 was obtained (Fig. 2). This solvent
dependence of 4 closely resembled that of 1, and can be interpreted
by the relaxation mechanism as TICT. The fluorescence intensity of
5, which has two OMe groups, was faint compared to that of 4 in all
solvents (Fig. 2), which indicated that the addition of an OMe group
showed increased electron donating capability, and the PET process
became predominant. Model studies using 3–5 are summarized as
follows. (1) The introduction of a spacer greatly decreased of the CT
efficiency; the electron donating ability of the benzene moiety,
which is adjustable through introduction of OMe group, played an
important role in both TICT and PET over the spacer. (2) These CT
behaviors were determined by the substituent position and num-
ber of the OMe group. (3) The para-OMe group enhanced TICT of 4,
although the ortho-OMe substituent in 3 did not increase the CT
character. (4) Two OMe groups emphasized PET character in 5. As
described above, controlling TICT and PET on N-phenyl-9-anthra-
cenecarboxamide derivatives can be achieved through appropriate
selection for introduction of OMe group and the spacer. Based on
the model study, we synthesized novel benzo-crown ethers having
9-anthracencarboxamide (6) and 9-anthraceneacetamide (7)
(Scheme 4).
Figure 3. Fluorescence spectra of 7 and its Mg2þ complex in acetonitrile at 25 ꢀC;
[L]¼1ꢁ10ꢂ5 mol/dm3; excitation wavelength: 364 nm; slit width: 3 nm.
The addition of Mg2þ induced considerably large enhancement of
the fluorescence intensity of 7. This fluorescence enhancement is
clearly induced by complexation. The shape and region of fluores-
cence of 7$Mg2þ were featurized by anthracene emission, which
indicated that 7$Mg2þ had no CT interaction between the anthra-
cene and benzo-crown moieties. It is acceptable that cooperative
binding of Mg2þ with a crown ether moiety and a carbonyl group in
7 formed a bending structure.25
The bending motion around the amide bond will engender the
breakdown of
p-conjugation between the benzene ring and the
carbonyl group. The shutting off of the
p-conjugation corresponds
with the great enhancement of fluorescence intensity. A similar
result was also obtained with a similar fluorescence enhancement
effect for 7$Ca2þ. Regarding 7$Sr2þ, a unique feature in the fluo-
rescence spectra was observed, as portrayed in Figure 4. A broad
excimer-like emission at 450–550 nm was obtained. With in-
creasing Sr2þ concentration, the fluorescence emission intensity
increased to an approximately 1:1 molar ratio (L/M2þ), and then
decreased at greater ratios. This phenomenon is explained using
the formation of a 2:1 (L/M2þ) complex prior to a 1:1 complex. The
same result was observed for 7$Ba2þ. However, these observations
were not obtained in Mg2þ and Ca2þ. This result is explainable by
the fact that Mg2þ and Ca2þ, which have small ionic size, form only
1:1 complex in the inside of the crown ether cavity. However, large
O
O
O
O
O
O
O
O
O
O
H
N
C
O
N
O
H2C
H
C
6
7
Scheme 4.
3.2. Photochemical properties of chemosensors 6 and 7
Although it is identifiable from 1H NMR data that the crown
ether moiety of 6 formed a complexation with metal ions, the
fluorescence emission was not increased by the presence of any
alkaline earth metal ion. This phenomenon indicates that excited 6
is relaxed to the ground state with the PET mechanism just as 2,
which could not break the charge transfer pathway. These results
suggest that the coordination of cations on the benzo-crown ether
moiety and the carboxyl group connected to anthracene ring was
insufficient to intercept a conjugation system between the donor
and the acceptor.
Figure 3 displays fluorescence spectra of 7 and its Mg2þ com-
plexes in acetonitrile at 25 ꢀC. The weak emission of free 7 was
attributed to the PET action as an analogy of model study using 5.
Figure 4. Fluorescence spectra of 7 and its Sr2þ complex in acetonitrile at 25 ꢀC;
[L]¼1ꢁ10ꢂ5 mol/dm3; excitation wavelength: 364 nm; slit width: 3 nm.

10740
J. Kim et al. / Tetrahedron 64 (2008) 10735–10740
Table 1
photochemical behavior of donor–acceptor systems linked by
amide bonds and develop the use of CT action for fluorescent
chemosensors, molecular switches, photonic devices, and other
photo-active materials.
Wavelength of fluorescence maxima (lmax), fluorescence intensity ratios (Imax/I0) at
lmax and complex formation constants (log K) of 7a
Mg2þ
Ca2þ
Sr2þ
Ba2þ
lmax (nm)
Imax/I0
log K (1:1)
(1:2)
391
37.1
5.86
391
34.5
5.79
391
391
25.8
4.25
6.20
30.8
4.21
6.09
References and notes
d
d
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a
Fluorescence spectra of
7
was measured in acetonitrile at 25 ꢀC;
[7]¼1ꢁ10ꢂ5 mol/dm3; excitation wavelength: 364 nm; K (1:1)¼[ML2þ]/[M2þ][L], K
(1:2)¼[ML22þ]/[ML2þ][L].
Sr2þ and Ba2þ formed 2:1 complexes with a sandwich-type struc-
ture.26,27 In general, benzo-15-crown-5 derivatives28–33 having
fluorophores are known to have high sensitivity and selectivity for
alkali metal cations. However, fluorescence responses to Liþ, Naþ,
and Kþ were not observed in this work.
The fluorescence response ratios of 7, expressed as Imax/I0 values
for metal cations, are presented in Table 1. The order of Imax/I0 was
Mg2þ (37.1)>Ca2þ (34.5)>Sr2þ (30.8)>Ba2þ (25.8). This shows large
PET control ability of the crown ether moiety for Mg2þ compared to
that of other metal cations. The binding constant (log K) of the
complex was also evaluated from the fluorescence intensities using
a nonlinear least-squares curve fitting method.34 For complexes
formed with 1:1 stoichiometry, Mg2þ and Ca2þ showed larger af-
finity than either Sr2þ or Ba2þ (Table 1); 1:2 complexes in Sr2þ and
Ba2þ gave larger log K values than 1:1 complexes.
The introduction of a spacer in 7 decreased the CT efficiency and
increased molecular flexibility compared to that of 6. The addition
of flexibility for the molecular motion induced breakage of
p-conjugation in 7 upon complexation with alkaline earth metal
ions, which results in a large fluorescence enhancement.
27. Schmittel, M.; Lin, H.; Thiel, E.; AlfredMeixner, J.; Ammon, H. Dalton Trans. 2006,
4020.
4. Conclusions
28. de Silva, A. P.; Gunaratne, H. Q. N.; McCoy, C. P. J. Am. Chem. Soc. 1997, 119, 7891.
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Chem. Soc. 1999, 121, 2319.
32. Yamauchi, A.; Hayashita, T.; Kato, A.; Nishizawa, S.; Watanabe, M.; Teramae, N.
Anal. Chem. 2000, 72, 5841.
33. Hifumi, H.; Tanimoto, A.; Citterio, D.; Komatsu, H.; Suzuki, K. Analyst 2007, 132,
1153.
The control of TICT and PET on N-phenyl-9-anthracenecarbox-
amide derivatives can be carried out by an appropriate introduction
of combination of OMe group and the spacer. The substitution
position of the electron donating OMe group played an important
role for both TICT and PET. The CT behavior can be controlled
through formation of complexation with metal ions. The novel
fluorescent crown ether 7 exhibits a high response ability (Imax
/
I0¼37.1) for Mg2þ. Our future efforts will specifically elucidate
34. Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431.