Novel chemosensor for alkaline earth metal ion based on 9-anthryl aromatic amide using a naphthalene as a TICT control site and intramolecular energy transfer donor

Article abstract of DOI:10.1016/j.tetlet.2008.01.085

Novel fluorescent chemosensors 1 and 2 with two different fluorophores (naphthalene and anthracene) at the both ends of polyether was synthesized. These compounds based on 9-anthryl aromatic amide adopted a naphthalene as a TICT controller and an intramol

Full text of DOI:10.1016/j.tetlet.2008.01.085

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Tetrahedron Letters 49 (2008) 1984–1987
Novel chemosensor for alkaline earth metal ion based on
9-anthryl aromatic amide using a naphthalene as a TICT
control site and intramolecular energy transfer donor
Jeongsik Kim a, Tatsuya Morozumi b, Namiko Kurumatani b, Hiroshi Nakamura b,*
a Division of Environmental Material 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
Received 30 November 2007; revised 15 January 2008; accepted 18 January 2008
Available online 26 January 2008
Abstract
Novel fluorescent chemosensors 1 and 2 with two different fluorophores (naphthalene and anthracene) at the both ends of polyether
was synthesized. These compounds based on 9-anthryl aromatic amide adopted a naphthalene as a TICT controller and an intramole-
cular energy transfer source. Compound 1 shows high fluorescence efficiency upon complexation with metal ion, and the fluorescence
efficiency of 2 is regulated by metal ionic size.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Intramolecular energy transfer; Twisted intramolecular charge transfer; Fluorescent sensor
Intramolecular energy transfer (intraET) between chromo-
phores has been a subject of considerable interest because
this phenomenon is widely applicable in photochemistry
and photobiology.1 In recent years, Chattoraj et al.2 and
Speiser et al.3 have studied intraET systems using bichromo-
phoric molecules comprising two different chromophore
units connected by a methylene bridge. In particular,
9-anthryl-10-naphthyl compounds4–7 have been noted for
their efficiency of energy transfer by various numbers of
methylene chain. Until now most frequently reported stud-
ies of intraET were in a rigid or semirigid system which
defines a distance between donor and acceptor.8–10 If a
donor–acceptor pair is bound by a flexible chain and their
distance is controlled by a guest molecule or metal ion, it
is expected that the dynamic conformation can be moni-
tored by an intraET in the system.
compounds with pyrene, anthracene, or naphthalene moie-
ties.11–14 This TICT-quenching process was controlled
effectively by a formation of the complex with alkaline
earth metal ions. This encouraged us to develop chemo-
sensors based on polyether, which exhibit controlling of
TICT and intraET. With this in mind, we synthesized
chemosensors 1 and 2, which have a naphthalene and an
anthracene group at both terminals of a polyether
(Fig. 1). Naphthalene moiety of these sensors will be
employed as an obstructor of TICT relaxation process
and an intraET donor.
11
11
NH10
O C
CH29
O
O
O
O
NH10
O C
CH29
O
O
O
O
NH13
C O
NH13
C O
12
8
5
12
Recently, we have investigated behaviors of twisted
intramolecular charge transfer (TICT) on linear polyether
1
2
7
6
3
8
3
4
4
7
5
6
1
2
*
Corresponding author. Tel.: +81 11 706 2259; fax: +81 11 706 4863.
Fig. 1. Structures of 1 and 2.
E-mail address: nakamura@ees.hokudai.ac.jp (H. Nakamura).
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.01.085

J. Kim et al. / Tetrahedron Letters 49 (2008) 1984–1987
1985
Compounds 1 and 2 were synthesized in a usual manner
(see Supplementary data). Their structures and purities
were confirmed using 1H NMR spectra and elemental anal-
yses (see Supplementary data). Fluorescence and UV–vis
spectra were recorded at 25 °C using respective equipment
(RF-5300PC and UV-2400PC; Shimadzu Corp.). Concen-
trations of the reagents were 1 ꢀ 10ꢁ5 mol/dm3 in purified
acetonitrile. For complex formation experiments, alkaline
earth metal cations were added to the solution as perchlo-
rate salts.
Figure 2 shows UV spectra of 2 and its Ca2+ complexes
as a typical result. Almost no UV spectral changes of all
complexes on the naphthalene and anthracene moieties
were observed compared with that of the free compound.
This observation suggests that naphthalene and anthracene
rings did not interact with each other in the ground state.
This phenomenon supports that the metal ions did not
induce the interaction between naphthalene and anthra-
cene. The same results were also observed in the UV spec-
tra of 1 and its metal complexes.
Figure 3 depicts fluorescence spectra of 1 and 2, and
their Ca2+ complexes when naphthalene moieties were
excited. The excitation wavelength was set to 281 nm,
which is the absorption maximum of the compounds and
attributed to a naphthalene moiety. In the absence of metal
ion, weak fluorescence from naphthalene moiety was
observed at around 330 nm, and also weak fluorescence
from anthracene moiety was observed at 440 nm. After
the addition of Ca2+, a structureless broad emission at
around 440 nm increased remarkably, but the emission at
around 330 nm, which was attributed to naphthalene, did
not change. These results show that an intraET from the
naphthalene to the anthracene occurred. The fluorescence
maxima (kmax = 440 nm) of 1 and 2 are similar to that of
9-anthracenecarboxamide. Therefore, anthracene and
naphthalene groups did not show any p–p interaction such
as an exciplex.13 These observations underline that the
TICT quenching processes of free 1 and 2 occurred in the
anthracenecarboxamide, as reported previously,13,15 and
were controlled by the formation of Ca2+ complexes. Sim-
Fig. 3. Fluorescence spectra of 1 (a) and 2 (b) and its Ca2+ complexes in
acetonitrile at 25 °C. The excitation wavelength is 281 nm.
[1] = [2] = 1 ꢀ 10ꢁ5 M.
ilar ‘Off–On’ behavior was observed in the presence of
Mg2+, Sr2+, and Ba2+. On the other hand, intermolecular
energy transfer was ruled out by the observation of fluores-
cence spectra of model solution, which is a mixture of
1-naphthaleneacetamide and 9-anthryl aromatic amide
(see Supplementary data).
The analytical usefulness is assessed, respectively, using
low and high emission intensities of the free reagent and the
complexes. The ‘Off–On’ responses were evaluated as Imax
/
I0 values. Here, I0 is the fluorescence intensity of free 1 or 2,
and Imax is that of the complex 1 or 2 at the same condition.
The fluorescence intensity will also change according to the
absorbance at the excitation wavelength, if it is changed by
the complexation. This effect does not come from the
change of quenching process. To distinguish this effect,
the emission intensity should be normalized by the absor-
bance; alternatively, an isosbestic point should be excited.
In these experiments, the excitation wavelengths were
adjusted to the isosbestic points (Fig. 2).
The Imax/I0 values are independent on the excitation
wavelength if the ‘Off–On’ phenomenon is controlled by
TICT alone. However, the values of 1 on excitation at
281 nm are 10–20% larger than those of anthracene excita-
tion for all metal cations (Table 1). This result indicates
Fig. 2. UV spectra of 2 and its Ca2+ complex in acetonitrile at 25 °C.
[2] = 1 ꢀ 10ꢁ5 M. [Ca2+] = 0, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5.

1986
J. Kim et al. / Tetrahedron Letters 49 (2008) 1984–1987
Table 1
Fluorescence ‘Off–On’ response (Imax/I0) and excitation wavelength dependence
Fluoroionophores
Imax/I0 (kmax)
1
Mg2+
Ca2+
Sr2+
Ba2+
Excitation wavelength
281 nm
363 nm
15.2 (413)
13.8 (413)
1.10
32.7 (439)
28.1 (439)
1.16
10.6 (439)
8.9 (439)
1.19
5.1 (440)
4.6 (440)
1.10
{Imax/I0 (281 nm)/Imax/I0 (363 nm)}
2
Mg2+
Ca2+
Sr2+
Ba2+
Excitation wavelength
281 nm
363 nm
{Imax/I0 (281 nm)/Imax/I0 (363 nm)}
12.7 (413)
14.3 (413)
0.88
36.5 (439)
37.8 (439)
0.96
9.8 (439)
13.6 (439)
0.72
3.9 (440)
6.3 (440)
0.62
that the efficiency of intraET from naphthalene to anthra-
cene in 1 was increased by complexation with metal ions.
This effect was also observed in excitation spectra. Excita-
tion spectra of 1 and 1ꢂCa2+ complex normalized at 363 nm
is shown in Figure 4. The excitation intensity of the com-
plex around at 281 nm is ca. 16% larger than that of 1.
On the other hand, Imax/I0 values of 2 are of the order of
Ca2+ (96%) > Mg2+ (88%) > Sr2+ (72%) > Ba2+ (62%)
(Table 1). Moreover, they are smaller than 100%. The
Imax/I0 values of 2 can be ordered by the metal ionic size,
except for Mg2+, but they are smaller than those of 1. This
finding manifested that the efficiency of energy transfer in 2
is governed by the fitness between the pseudo-polyether
cavity and the sizes of metal ions. These phenomena of 1
and 2 occur because of the geometric structure of naphtha-
lene moiety by the difference of substitution position.
Fluorescence spectral data suggested a structural change
in 1 and 2 upon complexation with metal cations. To clar-
ify the effect of complexation with metal cations, 1H NMR
measurements of 1 and 2 were carried out in the absence
and the presence of metal cations in acetonitrile-d3 at
30 °C. The chemical shifts of 1, 2 and their changes at
the formation of complexes with various metal ions are
shown in Table 2. The peaks were assigned using H–H
COSY and NOESY spectra. The addition of Ca2+ to the
solution of 1 or 2 induced low magnetic field shifts for pro-
tons 13 and 10 (Dd = 0.61–0.67 ppm and 0.74–0.77 ppm,
respectively). This indicates that 1 or 2 formed Ca2+ com-
plexes cooperatively with the carbonyl oxygen atom in
both amide groups. Before the addition of Ca2+, proton
12 of 1 or 2 exhibited an unusual lower chemical shift
(8.47 and 8.45 ppm, respectively). However, a large higher
magnetic chemical shift change (Dd = ꢁ1.06 and
ꢁ1.08 ppm, respectively) was observed in complexation
with Ca2+. This fact indicates that the benzene proton 12
existed on a deshielding zone of the carbonyl group, and
that the carbonyl moiety is reversed by complexation with
Fig. 4. Excitation spectra of 1 and its Ca2+ complex normalized at 363 nm
in acetonitrile-d3. [1] = 1 ꢀ 10ꢁ5 mol/dm3.
Table 2
Chemical shifts (d, ppm) of 1 and 2, and their changes upon complexation with various cationsa
Metal ion
1
2
3
4
5
6
7
8
9
10
11
3.12
0.02
ꢁ0.57
ꢁ0.33
ꢁ0.09
12
8.47
ꢁ0.83
ꢁ1.06
ꢁ0.93
ꢁ0.97
13
1
Blank
Mg2+
Ca2+
Sr2+
7.35
7.40
7.77
0.02
7.86
7.47
7.48
7.94
3.84
0.01
6.35
1.18
0.77
0.68
0.63
8.81
0.68
0.61
0.44
0.33
ꢁ0.05
ꢁ0.02
ꢁ0.08
ꢁ0.05
ꢁ0.01
ꢁ0.08
ꢁ0.06
ꢁ0.04
ꢁ0.02
ꢁ0.04
ꢁ0.07
ꢁ0.03
ꢁ0.01
ꢁ0.10
ꢁ0.06
ꢁ0.02
ꢁ0.02
ꢁ0.11
ꢁ0.07
ꢁ0.03
ꢁ0.21
ꢁ0.35
ꢁ0.22
ꢁ0.11
ꢁ0.03
ꢁ0.01
0.00
ꢁ0.54
ꢁ0.39
ꢁ0.09
Ba2+
2
Blank
Mg2+
Ca2+
Sr2+
7.67
7.36
0.02
ꢁ0.21
ꢁ0.16
0.00
7.78
0.01
0.01
0.00
0.02
7.80
0.00
0.02
7.44
0.01
0.02
0.02
0.01
7.44
0.01
0.02
0.02
0.01
7.81
0.02
ꢁ0.03
ꢁ0.02
ꢁ0.07
3.53
0.02
ꢁ0.34
ꢁ0.27
ꢁ0.05
6.51
1.05
0.74
0.69
0.63
3.17
0.01
ꢁ0.47
ꢁ0.25
ꢁ0.06
8.45
ꢁ0.88
ꢁ1.08
ꢁ1.07
ꢁ1.00
8.86
0.69
0.67
0.52
0.37
ꢁ0.05
ꢁ0.12
ꢁ0.09
ꢁ0.07
ꢁ0.01
Ba2+
ꢁ0.06
a
Positive values show lower field shifts, negative values show higher field shifts. The ‘blank’ values indicate chemical shifts (d, ppm, from TMS) of
protons in acetonitrile-d3 at 30 °C. Other protons were omitted because significant chemical shift changes were not observed.

J. Kim et al. / Tetrahedron Letters 49 (2008) 1984–1987
1987
O
O
O
281 nm
O
O
O
O
NH
O C
CH2
O
C
NH
hv1
O
C
O
steric
NH
metal ion
H
O
HN
repulsion
CH2
C
TICT
hv1
energy transfer
281 nm
energy
transfer
No Fluorescence
Fluorescence
Fig. 5. Fluorescence emissions mechanism of 1 before and after addition of M2+ in the ground state.
(Marquardt’s method),15 as summarized in Table 3. The
complex formation constants (logK) of chemosensors 1
and 2 showed the best affinity for Ca2+, which shows
that the ionic diameter of Ca2+ resembles the size of the
pseudocavity formed by polyoxyethylene group and
carbonyl oxygens. In both cases, the order of logK is
Table 3
Complex formation constants (logK)
Mg2+
Ca2+
Sr2+
Ba2+
1
2
4.74
4.71
5.75
5.96
5.22
5.43
5.11
4.99
Ca2+ > Sr2+ > Ba2+ > Mg2+
.
Ca2+, and that the chemical shift returned to a normal
value through complexation.
In conclusion, 1 and 2 exhibited different fluorescence
responses to metal ions by excitation moieties (281 and
363 nm). However, the mechanism of the difference of the
responses in intraET is not elucidated. Further studies
are in progress.
It is noteworthy that two methylene proton peaks 11
and 9 of 1 or 2 shifted to a higher magnetic field (proton
11: Dd = ꢁ0.57 and ꢁ0.47 ppm; proton 9: Dd = ꢁ0.54 and
ꢁ0.34 ppm, respectively) at complexation with Ca2+. In
addition, the proton 8 in 1 showed high-magnetic chemi-
cal shift changes (Dd = ꢁ0.35 to ꢁ0.11 ppm), although
that in 2 showed almost no chemical shift changes
(Dd = ꢁ0.07 to 0.02 ppm). These chemical shift changes
are explained as follows: In the naphthaleneacetamide
group, methylene moiety is located in an opposite direc-
tion to the carbonyl group that will bind to the metal
ion. The naphthalene ring is, therefore, positioned toward
the anthracene ring because the methylene moiety induces
a steric hindrance for the hydrogen atom (e.g., proton 8
for 1) of the peri-position of the naphthalene ring. The
phenomenon in the free rotation range of naphthalene
ring should be more restricted in 1 on the geometric
structure.
Protons 8, 9, and 11 of 1 showed higher chemical shift
changes than in 2. This phenomenon showed that they
are attributable to a shielding effect of the anthracene ring
in the complex. This result shows that the complex struc-
ture of 1 takes an effective obstacle conformation to the
TICT relaxation process, which is a reason for higher fluo-
rescence intensity. No other protons of anthracene and
naphthalene indicated significant chemical shift changes
before or after complexation with all metal ions. This result
shows that anthracene and naphthalene moieties did not
take a so-closed structure, and did not give p–p interac-
Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/j.tetlet.
2008.01.085.
References and notes
1. Speiser, S. J. Photochem. 1983, 22, 195–211.
2. Chattoraj, M.; Chung, D. D.; Bal, B.; Closs, G. L.; Levy, D. H. J.
Phys. Chem. 1994, 98, 3361–3368.
3. Speiser, S. Chem. Rev. 1996, 96, 1953–1976.
4. Wang, X.; Levy, D.; Rubin, M.; Speiser, S. J. Phys. Chem. A 2000,
104, 6558–6565.
5. Jolibois, F.; Bearpark, M. J.; Klein, S.; Olivucci, M.; Robb, M. A. J.
Am. Chem. Soc. 2000, 122, 5801–5810.
6. Schael, F.; Rubin, M. B.; Speiser, S. J. Photochem. Photobiol. A 1998,
115, 99–108.
7. Nakabayashi, T.; Wu, B.; Morikawa, T.; Iimori, T.; Rubin, M. B.;
Speiser, S.; Ohta, N. J. Photochem. Photobiol. A 2006, 178, 236–
241.
8. Hasegawa, M.; Enomoto, S.; Hoshi, T.; Igarashi, K.; Yamazaki, T.;
Nishimura, Y.; Speiser, S., et al J. Phys. Chem. B 2002, 106, 4925.
9. Jin, T. Chem. Commun. 1999, 2491–2492.
10. Wan, C. W.; Burghart, A.; Chen, J.; Bergstrom, F.; Johansson, L. B.
A.; Wolford, M. F.; Kim, T. G.; Topp, M. R.; Hochstrasser, R. M.;
Burgess, K. Chem. Eur. J. 2003, 9, 4430–4441.
11. Morozumi, T.; Hiraga, H.; Nakamura, H. Chem. Lett. 2003, 32, 146–
147.
1
tions. On the bases of fluorescence and H NMR studies,
a plausible complex structure of 1ꢂM2+ on the ground state
12. Morozumi, T.; Anada, T.; Nakamura, H. J. Phys. Chem. B 2001, 105,
2923–2931.
13. Morozumi, T.; Hama, H.; Nakamura, H. Anal. Sci. 2006, 22, 659–
661.
14. Kim, J.; Morozumi, T.; Nakamura, H. Org. Lett. 2007, 9, 4419–4422.
15. Marquardt, D. W. J. Soc. Ind. Appl. Math. 1963, 11, 431–441.
is depicted in Figure 5.
The complex formation constants (logK) were evaluated
using the titration curve of fluorescence intensity versus
[M2+] with a nonlinear least-squares curve fitting method