Copper(II)-directed static excimer formation of an anthracene-based highly selective fluorescent receptor

Article abstract of DOI:10.1016/j.tetlet.2014.11.055

A novel 'turn-on' fluorescent receptor methyl 3-((anthracen-9-ylmethylene)amino)benzoate (MAB) with anthracene as the fluorophore and methyl 3-aminobenzoate as a metal ion chelating center has been designed and synthesized. The ability of the prepared receptor to detect metal ions has been evaluated by the changes in its emission intensity. MAB demonstrates high selectivity and sensitivity for Cu²⁺ among the nineteen metal ions examined in MeCN. The interaction of MAB with Cu²⁺ causes a significant enhancement in emission intensity due to the combination of a unique anthracenyl static excimer formation, the restricted CN isomerization and the suppression of highly efficient photoinduced electron transfer (PET) process. The disassociation of the excimer species to monomers is directed by temperature, Cu²⁺ concentration and solvent fraction. Furthermore, the considerable 'off-on' fluorescence response or the conversion of the excimer species to monomers concomitantly led to the apparent color changes, which could also be identified easily by the naked eye using a UV lamp.

Full text of DOI:10.1016/j.tetlet.2014.11.055

Accepted Manuscript
Copper(II)-directed static excimer formation of an anthracene-based highly se-
lective fluorescent receptor
Sait Malkondu, Dilek Turhan, Ahmet Kocak
PII:
S0040-4039(14)01948-0
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.11.055
TETL 45439
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
2 August 2014
5 November 2014
13 November 2014
Please cite this article as: Malkondu, S., Turhan, D., Kocak, A., Copper(II)-directed static excimer formation of an
anthracene-based highly selective fluorescent receptor, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/
j.tetlet.2014.11.055
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Copper(II)-directed static excimer formation
of an anthracene-based highly selective
fluorescent receptor
Sait Malkondu, Dilek Turhan, Ahmet Kocak

1
Tetrahedron Letters
jo u rn al h ome p ag e : w ww .e lse vie r .c om
Copper(II)-directed static excimer formation of an anthracene-based highly selective
fluorescent receptor
a
Sait Malkondu,* Dilek Turhan, Ahmet Kocak
a
a
a
Selcuk University, Science Faculty, Department of Chemistry, Konya 42250, Turkey.
ARTICLE INFO
ABSTRACT
Article history:
Received
A novel “turn-on” fluorescent receptor methylanthracenylimine benzoate (MAB) with
anthracene as the fluorophore and methyl 3-aminobenzoate as a metal ion chelating center has
been designed and synthesized. The ability of the prepared receptor to detect metal ions has been
evaluated by the changes in its emission intensity. MAB demonstrates high selectivity and
Received in revised form
Accepted
2
+
Available online
sensitivity for Cu among the nineteen metal ions examined in MeCN. The interaction of MAB
2
+
with Cu causes a significant enhancement in emission intensity due to the combination of a
unique anthracenyl static excimer formation, the restricted C=N isomerization and the
suppression of highly efficient photoinduced electron transfer (PET) process. The disassociation
Keywords:
Excimer
2
+
of the excimer species to monomers is directed by temperature, Cu concentration and solvent
fraction. Furthermore, the considerable ‘off-on’ fluorescence response or the conversion of the
excimer species to monomers concomitantly led to the apparent colour changes, which could
also be identified easily by the naked eye using a UV lamp.
Anthracene
PET
Copper
Disassociation
2
009 Elsevier Ltd. All rights reserved.
Copper is the third most abundant element, after iron and zinc,
among the heavy metal ions in the human body and, thus, draws
intensity ratio of excimer to monomer emission as an essential
parameter because this ratio depends on the stoichiometry
between guest and host. The possible interferences from the
environment can be excluded by built-in verification of two
1-4
2+
considerable interest for its determination. Cu ions induce the
activation of dioxygen, which is essential to all living organisms
and is an important cofactor for many metalloenzymes. Copper
also plays roles in iron absorption, in redox processes, and
2
9-31
emission bands in ratiometric sensing.
5-7
Anthracene acting as a fluorophore has been utilized
effectively since its emission property varies according to its
various enzyme activities. Although copper is an essential trace
element for living organisms, it is dangerous at high
32-34
8-10
concentration levels.
local environment.
Excellent monomer and excimer emission
Several methods including inductively
11
coupled plasma-atomic emission spectrometry, inductively
changes occur at remarkably different wavelengths based on the
distance between two anthracene moieties. Excimer emissions
are characteristically observed with a broad fluorescence band
with the maxima at lower energy in comparison to monomer
1
2
coupled plasma-mass spectroscopy, atomic absorption/emission
1
spectroscopy,
3
14
and voltammetry have been utilized for
2+
measuring Cu levels. Most of these methods differ in efficiency
and show disadvantages such as low selectivity, low sensitivity
emissions, and this band can be taken into account to examine the
35-38
2
needing expensive instruments for detection of Cu and delayed
+
molecular recognition process in more detail.
Although host-
2
+
directed intramolecular excimer formation in anthracene-
containing compounds as a result π-π interactions between the
responses to Cu , and are not suitable for on-line monitoring. In
contrast, fluorescence detection is a more practical and favorable
39-41
1
5-17
anthracene units is well documented,
examples which form intermolecular excimer species.
there are a few
42,43
method in medicine, biology, and environmental chemistry.
2+
Most Cu sensors work as fluorescence quenchers upon binding
2
with Cu due to its inherent paramagnetic nature, via
+
Considering such unique physical properties of anthracene, we
have designed
1
8-21
a
simple but highly effective methyl
energy/electron transfer or spin-orbital coupling.
There are a
anthracenylimine benzoate (MAB) derivative, which has the
ability to form self-assembled anthracenyl excimer species with a
specific metal ion.
limited number of examples reported wherein enhancement in the
fluorescence intensity has been observed upon complexation with
2
+
22-25
Cu ions.
MAB was readily synthesized by the route outlined in Scheme
. Methyl 3-aminobenzoate was prepared starting from benzoic
acid. Fischer esterification of benzoic acid afforded methyl
benzoate in quantitative yield. The nitration of methyl benzoate
Recently, changes in the photophysical properties of
molecules on complexation have attracted significant attention in
recognition processes. These changes may result from variations
in several processes such as excimer/exciplex formation or
1
26-28
3 2 4
with a mixture of HNO /H SO provided methyl 3-nitrobenzoate
destruction and electron transfer.
To detect different analytes,
in good yield. Reduction of methyl 3-nitrobenzoate with Fe
powder/AcOH gave methyl 3-aminobenzoate, the condensation
reaction of which with 9-anthracene carboxaldehyde provided
excimer/exciplex formation or destruction is usually
advantageous. Some chemosensors manipulate the changes in the

2
Tetrahedron
2+
MAB in 82% yield. The structures of the intermediates and
The fluorogenic binding affinity of MAB with Cu ions has
been examined by fluorescence titration experiments (Figure 2a).
1
final product were verified by H NMR, C NMR and FT-IR
13
2
+
spectroscopy and HRMS (Supporting Information).
On binding of MAB to Cu ions, the anthracene units come
together and form a static excimer, as indicated by an increase in
the excimer emission intensity at 594 nm. The increasing
2
+
concentration of Cu induces a slight disassociation to the
monomeric form, which has an explicitly different emission
wavelength at 471 nm compared to that of the excimer. The
intensity of the excimer emission band reaches a maximum with
2
+
2
.0 equivalents of Cu . After this point, no significant change in
the spectrum was observed. This observation demonstrates that
the disassociation of the excimer species to monomers is slightly
Scheme 1. Synthetic route to MAB
2
+
dependent on the Cu concentration, which is consistent with
4
4,45
intermolecular excimer formation (Figure 2b).
The sensing ability of MAB toward a wide range of metal ions
+
+
+
2+
2+
2+
2+
Li , Na , Cs , Mg , Ca , Sr , Ba , Ag , Mn , Fe , Co , Ni ,
+
2+
2+
2+
2+
(
Cu , Zn , Cd , Hg , Pb , Al and Fe ) was investigated by
2+ 2+ 2+ 2+ 2+ 3+ 3+
o
fluorescence spectroscopy (λex = 388 nm) in MeCN at 20 C
(
Figure 1). In the absence of any metal ion, MAB shows a very
weak emission at 439 nm in MeCN due to the highly efficient
PET (photoinduced electron transfer) process between the methyl
3
fluorophore. Among the nineteen metal ions, only Cu was
-aminobenzoate receptor moiety and the anthracene
2
+
2
+
found to give a distinct spectral change. The addition of Cu (5.0
equiv) induces two new emission bands, a weak monomer
emission band centred at 471 nm and a strong excimer emission
band centred at 594 nm, indicating strong complex formation
2
+
with Cu ions. Under similar conditions, the other tested metal
ions had very little effect on the emission. The value of the
2
+
sensing index (I594/I439) for MAB to Cu is 14.8 whereas it is less
than 0.4 for the other metal ions (Figure 1b). Therefore, MAB
shows high selectivity toward Cu over other competing metal
2
+
ions.
Figure 2. (a) Emission titration spectra of MAB (10.0 µM) with
o
2
+
Cu (1.0 mM) in MeCN at 20 C, (b) change in the excimer
intensities at 594 nm and the monomer intensities at 471 nm during
2
+
the titration of MAB with Cu .
Generation of a dynamic or static excimer is associated with
4
6
the distance between two fluorophores. A practical method to
discriminate a dynamic and static excimer is by examination of
the excitation spectrum. The results indicate that the excitation
2
+
spectra of the MAB-Cu complex at 471 nm (monomer) differ
from that at 594 nm (excimer) (Figure 3). Thus, the excitation
spectra suggest that the chemical species corresponding to the
emissions observed at 471 and 594 nm are of a different chemical
origin. This observation is also clear evidence for the generation
of an intermolecular anthracenyl static excimer of MAB upon
2
+
Cu ion binding. The UV-vis spectra of MAB show a new
2
+
absorption band at 503 nm upon the addition of Cu . This result
2
+
also indicates that the anthracene moiety of MAB-Cu forms a
favourable intermolecular π-π stacked dimer between two
anthracene moieties in the ground state.
Figure 1. (a) Changes in the emission spectra of MAB (10.0 µM)
upon addition of various metal ions (50.0 µM) in MeCN (λex = 388
nm) at 20 C, (b) histogram of the sensing indexes (I594/I439) for
o
MAB to various metal ions.

3
2
+
Figure 3. Excitation spectra (normalized) of the MAB-Cu complex
o
monitored at 471 and 594 nm at 20 C.
In order to gain an insight into the disassociation of the
excimer species to monomers, changes in the fluorescence
2
+
spectra of the MAB-Cu complex were monitored against
temperature (Figure 4). For this purpose, the fluorescence spectra
2
+
of a solution of the MAB-Cu complex at the corresponding
temperature were recorded until no significant changes in the
spectra were observed. The results clearly show that the excimer
emission intensity at 594 nm decreases significantly as the
temperature increases, whereas the monomer emission band at
4
71 nm increased ratiometrically with the formation of an
isoemissive point at 557 nm. Correspondingly, a decrease in the
excimer/monomer intensity ratio (I /I ) was observed. In Figure
b, excimer and monomer emission intensities are plotted as a
E
M
4
function of temperature. As the temperature increases, the
monomer emission is strongly promoted with regard to the
excimer emission, as reflected by the lines, from which average
Figure 4. (a) Fluorescence response of a solution obtained from a
2+
mixture of MAB (10.0 µM) with Cu (2.0 equiv) in MeCN at
varying temperatures, (b) plots of the decreasing excimer emission at
594 nm and the increasing monomer emission at 471 nm as the
temperature increases during the disassociation of the excimer
species to monomers.
o
intensity changes of 20.62/ C for decreasing excimer intensity
o
and 17.13/ C for increasing monomer intensity are obtained. This
observation explicitly suggests that
a 1:2 (metal:ligand
stoichiometry) complex disassociates yielding a new 1:1 species
in solution when the temperature increases in accordance with the
47
following equilibrium.
2+
]
2+
2+
[
Cu(MAB)
2
+ Cu ⇄ 2[Cu(MAB)]
The disassociation process reveals an application of the MAB-
2
+
Cu complex as a temperature sensor. However, monomeric
species do not become excimers as the temperature decreases.
Therefore, the system is said to be irreversible in the temperature
range tested. Moreover, this conversion results in the apparent
color change from orange to green, which could also be identified
by the naked eye easily using a UV lamp.
Scheme 2. Proposed mechanism and PET process for the interaction
2
+
of MAB with Cu .
2
+
To understand the binding affinity of MAB with Cu , an
absorption titration experiment was performed (Figure 5).
Careful analysis of the spectra obtained during the titration
2
+
MAB has been designed as a “fluorophore–spacer–receptor”
experiments with Cu discloses that further small spectral
changes occur, except for a new absorption band at 503 nm after
2
6
format according to the design principle of a PET process. In
MAB, the methyl 3-aminobenzoate unit is connected by the
imine function to the anthracene, which provides azomethine-N
2
+
the addition of 0.5 equivalents of Cu (Figure 5, inset). In
particular, the absorbance at 503 nm, which is increased until 0.5
4
8,49
2+
equivalents of Cu has been added, decreases again slightly until
and carbonyl-O binding sites to interact with metal ions.
probable mechanism of complexation and PET process is shown
The
2
+
1.0 equivalent of Cu had been added. The titration profile
clearly supports the formation of an excimer. The logarithmic
in Scheme 2. The remarkable fluorescence enhancement of MAB
2
+
2+
upon addition of Cu ions is attributed to the combination of the
following factors: the suppression of the highly efficient PET
association constant (logKa) of the MAB-Cu
complex
according to the 1:2 binding model has been determined by the
3
process, unique self-assembled excimer formation
5
37,38
51
Benesi-Hildebrand method using UV-vis titration data and is
and the
5
0
-2
found to be 11.24 M (Figure S1).
restricted C=N isomerization.

4
Tetrahedron
2
+
nm). Upon complexation, the quantum yield of MAB-Cu
reaches ΦMAB-Cu = 0.35 (ca. 65 fold). Hence, the detection process
results in the apparent color change from colorless to orange,
which could also be identified by the naked eye easily using a
UV lamp (Figure 7).
Competitive metal ion interference studies have been carried
2
+
out for the MAB–Cu system in MeCN in the presence of
background metal ions. MAB exhibits significant selectivity
2
+
towards Cu ions as is shown in Figure 8. This indicates that
copper ion detection by MAB is not influenced by the presence
3+
3+
2+
of the other metal ions tested, except for Al , Fe and Fe ,
where a slight interference is observed. The detection limit of
2
+
MAB–Cu is calculated as 0.53 µM based on excimer emission
from fluorescence titrations. This result shows that MAB
2+
possesses a high sensitivity in detecting Cu ions.
Figure 5. Absorption titration spectra of MAB (0.1 mM) upon
2
addition of an increasing concentration of Cu ions (10.0 mM) in
+
o
MeCN at 20 C. Inset: Mole-ratio plot for the interaction of MAB
2
+
with Cu ions at 503 nm.
In order to support a 1:2 stoichiometry determined by the
mole-ratio method, the method of continuous variations (Job’s
method) was also performed (Figure 6). As expected, the
2
+
complex formation between MAB and Cu is in accordance with
the Job plot analysis. The Job’s plot with respect to 503 nm
shows a maximum absorbance change at about 0.66 which
indicates the presence of a 1:2 (M:L) stoichiometry.
Figure 8. Metal-ion selectivity of MAB (10.0 µM) in the presence of
o
different metal ions (10.0 µM) in MeCN at 20 C.
We have already reported that the disassociation of excimer
2
+
species of the MAB-Cu complex in MeCN to monomer species
is temperature-dependent. However, we have also observed that a
change in the solvent fraction induces the disassociation of
excimers to monomer species. As is well known, the emission
5
2
property is considerably sensitive to the nature of the solvent.
We firstly changed the water fraction in a solution of the MAB-
2
+
Figure 6. Job’s plot for MAB with Cu ions (λ = 503 nm) in MeCN
at 20 C. The total concentration of [MAB]+[Cu ] is 0.1 mM.
o
2+
2+
Cu complex in MeCN, and surprisingly found that water
induces the disassociation of the excimer species to monomers
with increasing water content up to 4.8%, with the formation of
an isoemissive point at about 571 nm, and after this point, no
significant change in the spectra were observed. Figure 9 shows
that there is also a blue-shift (13 nm) of the excimer emission at
5
94 nm and a red-shift (32 nm) of the monomer emission at 508
nm as the water content increases from 0% to 4.8% with respect
to that of a water-free example. The fluorescence titrations with
other polar solvents such as MeOH, EG (ethylene glycol), n-
PrOH and EtOH display similar disassociation behaviours to
water with corresponding solvent contents in MeCN of 11.8%,
1
2.8%, 15.7% and 18.0% at different monomer emission
wavelengths of 512, 513, 510 and 508 nm, respectively (Figures
S4-7). The solvent-directed dissociation process reveals another
2
+
Figure 7. The visual fluorescence response of MAB (0.1 mM) upon
addition of various metal ions (5.0 equiv) under a UV lamp at 365
nm.
application of the MAB-Cu complex as a solvent sensor for the
determination of the tested solvents, primarily H O, in MeCN
through the linear increasing monomer intensity. Average
intensity changes of 3.43/H O (µL) for increasing monomer
intensity were found. However, only MeCN, CH Cl and acetone
2
2
MAB displays a very weak emission band at 439 nm with a
very low quantum yield of ΦMAB = 0.0054, which resulted from
the highly efficient PET quenching of the excited state of the
anthracene moiety from the methyl iminobenzoate electron-
2
2
among the other tested solvents did not show significant
disassociation properties (Figure S8). When the dielectric
constants of the added solvents such as H O, MeOH, EG, n-
2
2+
donating receptor moiety. As the Cu concentration increases, a
considerable enhancement in the anthracene excimer emission
intensity at 594 nm is observed with a significant red-shift (155
PrOH and EtOH were compared to the solvent fraction, it was
observed that there was no rational relationship between them.
Therefore, the introduction of another solvent to a solution of the

5
2
MAB-Cu complex in MeCN contributed to the conversion of
+
Acknowledgments
excimer species into monomers according to the solvent type.
We thank SUBAP (Grant Number 14401027) for financial
support.
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2
5.0 µM) with Cu (2.0 equiv) as a function of water content in
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(
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2
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+
+
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3
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enables MAB to be used as a temperature and solvent sensor.
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3
,
3
2
+
for Cu ions. The detection limit is sufficiently low to determine
,
2
+
micromolar levels of Cu ions. The present study is a good
example of a receptor in which the rational conversion of
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7
•
•
•
•
A novel anthracene-based fluorescent “turn on”
receptor (MAB) is reported.
2
+
MAB exhibited a high selectivity for Cu
among the nineteen metal ions in MeCN.
A self-assembled excimer species is generated
2
after the addition of Cu .
+
Disassociation to monomer is induced by
temperature and solvent fraction.