ICT-Isomerization-Induced Turn-On Fluorescence Probe with a Large Emission Shift for Mercury Ion: Application in Combinational Molecular Logic

Article abstract of DOI:10.1021/acs.inorgchem.7b01304

A unique turn-on fluorescent device based on a ferrocene-aminonaphtholate derivative specific for Hg²⁺ cation was developed. Upon binding with Hg²⁺ ion, the probe shows a dramatic fluorescence enhancement (the fluorescence quantum yield increases 58-fold) along with a large red shift of 68 nm in the emission spectrum. The fluorescence enhancement with a red shift may be ascribed to the combinational effect of C=N isomerization and an extended intramolecular charge transfer (ICT) mechanism. The response was instantaneous with a detection limit of 2.7 × 10^-9 M. Upon Hg²⁺ recognition, the ferrocene/ferrocenium redox peak was anodically shifted by ΔE_1/2 = 72 mV along with a "naked eye" color change from faint yellow to pale orange for this metal cation. Further, upon protonation of the imine nitrogen, the present probe displays a high fluorescence output due to suppression of the C=N isomerization process. Upon deprotonation using strong base, the fluorescence steadily decreases, which indicates that H⁺ and OH^- can be used to regulate the off-on-off fluorescence switching of the present probe. Density functional theory studies revealed that the addition of acid leads to protonation of the imine N (according to natural bond orbital analysis), and the resulting iminium proton forms a strong H-bond (2.307 ?) with one of the triazole N atoms to form a five-membered ring, which makes the molecule rigid; hence, enhancement of the ICT process takes place, thereby leading to a fluorescence enhancement with a red shift. The unprecedented combination of H⁺, OH^-, and Hg²⁺ ions has been used to generate a molecular system exhibiting the INHIBIT-OR combinational logic operation.

Full text of DOI:10.1021/acs.inorgchem.7b01304

Article
pubs.acs.org/IC
ICT−Isomerization-Induced Turn-On Fluorescence Probe with a
Large Emission Shift for Mercury Ion: Application in Combinational
Molecular Logic
Sushil Ranjan Bhatta,† Bijan Mondal,‡ Gonela Vijaykumar,§ and Arunabha Thakur*,†,⊥
†Department of Chemistry, National Institute of Technology Rourkela, Rourkela-769 008, Odisha, India
‡Department of Chemistry, Indian Institute of Technology Madras, Chennai-600 036, India
§Department of Chemical Science, Indian Institute of Science Education and Research Kolkata, Mohanpur-741 246, India
S
* Supporting Information
ABSTRACT: A unique turn-on fluorescent device based on a
ferrocene−aminonaphtholate derivative specific for Hg2+ cation was
developed. Upon binding with Hg2+ ion, the probe shows a dramatic
fluorescence enhancement (the fluorescence quantum yield increases
58-fold) along with a large red shift of 68 nm in the emission
spectrum. The fluorescence enhancement with a red shift may be
ascribed to the combinational effect of CN isomerization and an
extended intramolecular charge transfer (ICT) mechanism. The
response was instantaneous with a detection limit of 2.7 × 10−9 M.
Upon Hg2+ recognition, the ferrocene/ferrocenium redox peak was
anodically shifted by ΔE1/2 = 72 mV along with a “naked eye” color
change from faint yellow to pale orange for this metal cation.
Further, upon protonation of the imine nitrogen, the present probe
displays a high fluorescence output due to suppression of the CN isomerization process. Upon deprotonation using strong
base, the fluorescence steadily decreases, which indicates that H+ and OH− can be used to regulate the off−on−off fluorescence
switching of the present probe. Density functional theory studies revealed that the addition of acid leads to protonation of the
imine N (according to natural bond orbital analysis), and the resulting iminium proton forms a strong H-bond (2.307 Å) with
one of the triazole N atoms to form a five-membered ring, which makes the molecule rigid; hence, enhancement of the ICT
process takes place, thereby leading to a fluorescence enhancement with a red shift. The unprecedented combination of H+,
OH−, and Hg2+ ions has been used to generate a molecular system exhibiting the INHIBIT−OR combinational logic operation.
such as high sensitivity, rapid response, low background, and
the capability of doing recognition in a nondestructive manner.
The development of highly sensitive and selective metal-ion-
responsive fluorescent sensors for the efficient tracking of
biologically and environmentally important metal ions is one of
the vital goals for chemists.20 Hg2+ ion is well-known for
quenching the fluorescence of fluorophores via a spin−orbit
coupling21 and/or energy transfer mechanism.22 Therefore, the
design and synthesis of new “turn-on” fluorescent probes for
Hg2+ ion based on novel interaction mechanisms between
recognition and signal-reporting units is still an active research
field. A variety of different signaling mechanisms, including
photoinduced electron/energy transfer (PET),23 metal-to-
ligand charge transfer (MLCT),24 intramolecular charge
transfer (ICT),25 excimer/exciplex formation,26 excited-state
intra/intermolecular proton transfer (ESPT),27 and fluores-
cence resonance energy transfer (FRET), etc., have been
developed and extensively applied for the optical detection of
INTRODUCTION
■
A wide range of molecular structures, chemical phenomena, and
mechanisms can be marshaled to generate a variety of
molecular logic operations according to the Boolean blueprint.
Molecular logic gates, capable of performing Boolean logic
operations in response to external stimuli such as physical,
chemical, and biological inputs, are becoming an exciting trend
in the forefront of research because of the growing demands of
future information technology.1−4 Following the pioneering
work of de Silva in 1993,5 a large number of molecular and
supramolecular logic gates6−8 based on organic molecules,
nucleic acids, proteins, and micro- and nanomaterials have been
successfully demonstrated over the last two decades.9−11
However, it is still a great challenge to develop next-generation
molecular logic gates based on new functional materials with
potential applicability. Recently a tremendous development
toward the exploitation of multiple and simple logic gates12,13
such as OR, AND, and INHIBIT gates for the design of various
smart materials have been documented.14−16 The use of
fluorescence signals in logic gates17−19 has become a focus of
extensive research interest because of their many advantages,
Received: May 23, 2017
© XXXX American Chemical Society
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Scheme 1. Synthesis of Compound 6
Scheme 2. Mechanism for the Formation of Compounds 4−4c
different species to date. Among these sensing mechanisms, the
CN isomerization process has been developed recently.28
Compounds possessing CN bonds are often nonfluorescent,
since the CN isomerization process is considered to be one
of the major decay processes of excited states. The pioneer
work by Wang and co-workers demonstrated that metal
complexation to the CN group (the complexation approach)
can inhibit this nonradiative process, thereby leading to strong
fluorescence emission.29 Further, Guo and co-workers found
that the intramolecular hydrogen bonding (the hydrogen bond
approach)30 also plays a crucial role in inhibition of the CN
isomerization process. However, fluorescence enhancement due
to inhibition of CN isomerization by metal complexation and
protonation (hydrogen bonding) simultaneously has not been
explored to date.
that it possesses excellent spectroscopic properties, such as
good photostability, high fluorescence quantum yield, and
tunable fluorescence emission. On the other hand, ferrocene is
a very attractive building block because of its reversible redox
property and its high stability in the presence of air and
moisture. The two parts are linked by a triazole ring, a versatile
recognition unit especially for transition metal cations,31,32 and
by a CN bond. Therefore, the combination of amino-
naphtholate and ferrocene units via a triazole ring and a CN
bond generates a molecular system where fluorescence can be
modulated by controlling the CN bond isomerization
process through complexation or through protonation. Further,
fluorescence modulation via protonation/deprotonation and
metal complexation with the CN bond have been judiciously
applied to construct an INHIBIT−OR combinational logic gate
for the first time.
In the present work, we have developed triazole- and imine-
tethered aminonaphtholate−ferrocene conjugate as a potential
fluorescent sensing molecule for Hg2+ ion. Our choice of the
aminonaphthalene unit as the fluorophore arose from the fact
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Table 1
Figure 1. Molecular structure of 6 with thermal ellipsoids drawn at the 50% probability level. Selected bond lengths (Å) and angles (deg): C14−C15
1.457(3), C14−N1 1.277(3), N1−C8 1.422(3), O2−C3 1.206(3), C3−O1 1.350(3), N4−C17 1.472(3), [Triazole ring: C15−C16 1.378(3), C16−
N4 1.342(3), N3−N4 1.352(3), N2−N3 1.311(3), N2−C15 1.367(3)]; N4−C17−C18 110.6(2), N1−C14−C15 121.3(2), C8−N1−C14 118.0(2),
O1−C3−O2 122.7(2), C2−O1−C3 116.0(2), [Triazole ring N2−N3−N4 107.2(2), N3−N4−C16 111.2(2), N4−C16−C15 104.5(2), C16−C15−
N2 108.3(2)].
which acts a linker between the triazole unit and the
naphtholate moiety. Compound 6 was fully characterized by
1H and 13C NMR spectroscopy, electrospray ionization mass
spectrometry (ESI-MS), and elemental analysis. Additionally,
the molecular structure of compound 6 was unambiguously
established by single-crystal X-ray diffraction analysis. After
successfully synthesizing complex 6, we wondered whether the
metal-complexation-inhibited CN isomerization-induced
quenching mechanism could be used in the design of a
fluorescence turn-on probe for metal ions. With this idea in
mind, we investigated the sensing ability of 6 toward various
metal cations. Excellent sensitivity and selectivity as well as a
huge turn-on fluorescence response toward Hg2+ and H+ ions
in CH3CN/H2O solution (2:8 v/v) were exhibited by the
system. The host−guest complexation properties of receptor 6
were systematically investigated by UV−vis and fluorescence
RESULTS AND DISCUSSION
■
Synthesis of Compound 6. Our attempts to synthesize 4-
amino-1,8-naphthalic anhydride from 4-nitro-1,8-naphthalic
anhydride under reducing conditions using SnCl2·2H2O and
HCl led to the formation of compound 4 in good yield
(Scheme 1). We tuned all of the possible reaction conditions to
obtain aminonaphthalic anhydride, but all the reaction
conditions, even at room temperature, afforded the decarboxy-
lated product 4 in good yield. The formation of the product
1
was confirmed by H and 13C NMR spectra, which clearly
showed the presence of the ethyl moiety in compound 4. A
plausible mechanism for the formation of the product might be
nucleophilic attack by ethanol to the carbonyl carbon C8
followed by decarboxylation (Scheme 2). In order to confirm
the role of the solvent, we investigated the reactivity of several
other solvents, including methanol, n-butanol, and isopropanol,
and we indeed found that the alkyl part of the ester moiety
comes from the solvent (Table 1). All of the products were
1
spectroscopy, electrochemistry, and H NMR spectroscopic
titration along with computational studies.
X-ray Structure Analysis of Compound 6. The single-
crystal X-ray analysis showed that compound 6 crystallized in
the monoclinic noncentrosymmetric system with space group
P21/c (Figure 1). All of the bond distances, including the C−N
bond in the five-membered triazole ring (avg ∼1.331 Å), are in
the normal ranges and that the C14−N1 bond is little shorter
because of the presence of a double bond (Figure 1).
UV−Vis Absorption Studies. The UV−vis binding
interaction studies of receptor 6 in CH3CN/H2O (10−4 M)
against Li+, Na+, K+, Ag+, Ca2+, Mg2+, Mn2+, Fe2+, Al3+, Cr3+,
Co2+, Cu2+, Zn2+, Cd2+, Ni2+, Hg2+, and Pb2+ cations (as their
perchlorate salts) showed a selective response to the Hg2+ ion.
1
characterized by H and 13C NMR spectroscopy and mass
spectrometric analysis.
The reaction of compound 4 with K2CO3 in the presence of
propargyl bromide under refluxing conditions in ethanol/water
(4:1) afforded ethyl 5-(prop-2-yn-1-ylamino)-1-naphthoate (5)
in good yield. Furthermore, as shown in Scheme 1, compound
5 underwent the [2 + 3] cycloaddition reaction with
azidomethylferrocene to afford compound 6 in 75% yield.
Interestingly, the presence of 1,8-diazabicyclo[5.4.0]undec-7-
ene (DBU), a non-nucleophilic base, in the reaction mixture
helps the deprotonation of the amine to obtain the imine,
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Figure 2. Changes in the absorption spectra of 6 in CH3CN/H2O (10−5 M) upon addition of increasing amounts of (a) Hg2+ and (b) H+ ions up to
1 equiv. The insets show expanded region.
The solutions of different cations were added separately to the
prepared solution of compound 6, and UV−vis absorption
spectra were recorded. Except for Hg2+ ion, no metal ion
exhibited any kind of significant effect on the absorption
spectrum (Figure S15). Addition of a low concentration of
Hg2+ led to a significant change in the UV−vis spectrum. To
gain insight, an absorption titration of receptor 6 was
performed with Hg2+ ion. The titration experiment was
accomplished through stepwise addition of a solution of the
metal salt (c = 10−5 M) to a solution of receptor 6 (c = 10−5
M). The UV−vis spectrum of receptor 6 shows a high-energy
absorption band with a maximum at λmax ∼ 239 nm (which may
be due to a π−π* excitation) and a low-energy absorption band
at λmax ∼ 338 nm. The UV−vis absorption spectrum of receptor
6 was found to be perturbed significantly upon the gradual
addition of Hg2+ ion (Figure 2a). After the addition of 1 equiv
of Hg2+ to 6, the absorption bands at around 239 nm (33 898
M−1cm−1) and 338 nm (11 461 M−1 cm−1) were decreased
with the concomitant appearance of new low-energy band at
397 nm (530 M−1 cm−1). Two well-defined isosbestic points
were found at ca. 292 and 375 nm, indicating that a net
interconversion between the uncomplexed and complexed
species occurs. In addition to the Hg2+ ion, the stepwise
addition of H+ ion led to significant changes in the UV−vis
absorption spectrum of compound 6 (Figure 2b). After the
addition of 1 equiv of H+ to 6, the absorption bands at around
239 and 338 nm were decreased simultaneously along with the
appearance of a new weak peak at 401 nm. The stoichiometry
of the complex formed between the ligand and Hg2+ is 1:1 as
determined from the Job’s plot (Figure S16). This result was
also supported by ESI-MS, where a peak at m/z 792
corresponds to the 1:1 complex of 6 with Hg2+ ion (Figure
S17). Furthermore, the formation of a 1:1 complex of 6 with
Hg2+ ion was confirmed by elemental analysis of the isolated [6·
Hg2+] species.
enhancement of the fluorescence with a large bathochromic
shift in the emission wavelength from 468 to 536 nm (Δλ = 68
nm). In order to study the sensitivity of receptor 6 toward Hg2+
ion, a fluorescence titration experiment was carried out. As
shown in Figure 3, compound 6 exhibits weak fluorescence (ϕ
Figure 3. Fluorescence emission titration of compound 6 (10−8 M)
upon addition of Hg2+ ion up to 1 equiv in CH3CN/H2O (2:8 v/v)
solution.
= 0.009)33 at 468 nm when excited at 365 nm in CH3CN/H2O
(2:8 v/v) solution. The weakly fluorescent nature of compound
6 is mainly due to the CN bond isomerization-induced
quenching effect. However, upon addition of Hg2+ ion, the
fluorescence intensity remarkably increased and became nearly
constant once the amount of Hg2+ reached 1 equiv. As a result,
an approximately 58-fold enhancement of the fluorescence
quantum yield was observed (ϕ = 0.52 for 6). The huge
fluorescence enhancement of 6 upon addition of Hg2+ strongly
suggests that the CN isomerization-induced quenching is
inhibited in the [6·Hg2+] adduct as a result of the formation of
a rigid five-membered ring that prevents the CN bond
isomerization (Scheme 3). The selective fluorescence enhance-
ment of Hg2+ could be due to more effective coordination of
Hg2+ with the nitrogen atom of the CN bond and the
Fluorescence Studies. The photophysical properties of
compound 6 were investigated by fluorescence spectroscopy
measurements. Upon addition of 1 equiv of Li+, Na+, K+, Ag+,
Ca2+, Mg2+, Mn2+, Fe2+, Cr3+, Al3+, Co2+, Cu2+, Zn2+, Cd2+, Ni2+,
and Pb2+ ions to a CH3CN/H2O (2:8 v/v) solution of
compound 6, no changes in the fluorescence spectrum were
observed except in the case of Hg2+ ion (Figure S18).
Surprisingly, addition of Hg2+ ion showed a remarkable
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Scheme 3. Proposed Mode of Interaction between Receptor 6 and Hg2+
Scheme 4. Schematic Representation of Fluorescence Turn-On upon Hg2+ Coordination
nitrogen atom of the triazole ring of compound 6 in
comparison with other metal ions.
against [Hg2+]−1, the value of K (15%) extracted from the slope
was 7.2 × 107 M−1 for 6 with Hg2+ (Figure S19).
Furthermore, in order to obtain the optimal experimental
conditions and acquire a high signal-to-noise ratio for detection
of Hg2+, the incubation time was investigated. For this
experiment, the net signal ΔF = F − F0 was used as the
standard, where F and F0 are the fluorescence intensities in the
presence and absence of the target Hg2+, respectively.
Moreover, the incubation time of Hg2+ can affect the signal
amplification strongly. As shown in the inset of Figure 3, with
increasing Hg2+ concentration the fluorescence difference
increased rapidly and then attained saturation after addition
of 1 equiv. Therefore, 1 equiv of Hg2+ was found to be
optimum and was selected for further experiment. To
accomplish the optimum conditions for binding, the effect of
the incubation time for Hg2+ was also investigated. As indicated
in Figure 4, the fluorescence intensity difference increased
The energy of the ICT state can be regulated by varying the
strengths of the donor and acceptor groups, which are
distinctive of an ICT chromophore.34−36 Increasing the
strength of the donor group and extending the conjugation in
the molecule lead to an enhancement of the charge separation
and dipole moment. Consequently, the energy of the ICT state
decreases. Spectroscopically, this is evident from a red shift of
the UV−vis and fluorescence emission spectra.37 Addition of
Hg2+ ion makes the molecule rigid through the formation of a
five-membered ring, and as a result, electron delocalization
from the N atom of the triazole ring (donor) to the ester
moiety (acceptor) is much more viable (Scheme 4).
Consequently, an enhancement of dipole moment and charge
separation takes place, thereby leading to a large red shift of 68
nm in the fluorescence emission spectrum.
In order to determine the sensitivity of receptor 6, a
fluorescence titration of 6 (9 × 10−8 M) with Hg2+ (9 × 10−8
M) in CH3CN/H2O (2:8 v/v) was performed. No detectable
change was observed in the fluorescence spectrum up to an
analyte concentration of 0.02 equiv of Hg2+. In the presence of
0.03 equiv of Hg2+, an appreciable enhancement with a red shift
in the signal intensity was observed, whereas the maximum
fluorescence intensity was observed in the presence of 1 equiv
of Hg2+ ion. This experiment shows that the low detection limit
(DL) of 6 is 2.7 × 10−9 M (2.7 ppb) for Hg2+. Alternatively, to
evaluate limit of detection (LOD), we fitted a straight line (data
extracted from Figure 3) and applied the extensively used
criterion LOD = 3.3σ/S, where σ is the standard deviation of
the blank and S is the slope of the calibration curve. The
detection limit (taken as 3.3σ/S) is in the same range as
obtained from other methods also. The quantification limit
(LOQ) for probe 6 (taken as 10σ/S) is 8.1 nM for Hg2+ ion.
The value of the binding constant of Hg2+ with 6 was
determined from the emission intensity data following the
Figure 4. Optimization of the incubation time for Hg2+ ion.
simultaneously with the interval of 5 min and it reached the
highest ΔF value at 30 min. Upon further prolongation of the
incubation time, the fluorescence intensity difference was
saturated, which may be the result of complete complexation of
Hg2+ with probe 6. Hence, an incubation time of 30 min was
selected for the optimization conditions.
Furthermore, we wondered whether the fluorescence
emission of probe 6 could be regulated through inhibition of
the CN isomerization-induced quenching process by an
intramolecular hydrogen bond. Natural bond orbital (NBO)
modified Benesi−Hildebrand equation:38,39 1/ΔI = 1/ΔImax
+
−
(1/K[Hg2+])(1/ΔImax), where ΔI = I − I0 and ΔImax = Imax
I0, in which I, Imax, and I0 are the emission intensities of 6
considered at an intermediate Hg2+ concentration, in the
absence of Hg2+, and at a concentration of complete
interaction, respectively, K is the binding constant, and
[Hg2+] is the Hg2+ concentration. From the plot of ΔImax/ΔI
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charge analysis using density functional theory (DFT)
calculations revealed that the imine nitrogen (−CHN)
accumulated more charge, as can be seen from its natural
valence population of +5.44, compared with the three nitrogen
atoms of the triazole ring. Therefore, addition of acid can
protonate the imine N to form the iminium ion. The iminium
hydrogen atom can form a strong hydrogen bond with one of
the N atoms of the triazole ring (Ntria···H distance = 2.307 Å by
DFT calculations), resulting the formation of a five-membered
heterocyclic ring (Figure 5).40 As a result of this attractive
and the circuits are shown in Figure 7. The values of I/I0 at 536
nm for 6 with different combinations of varying concentrations
Figure 7. Circuits for different logic gates.
of H+ and OH− correspond to an INHIBIT logic gate. A binary
INHIBIT logic operation, which is represented by the situation
in which the output is 1 only if one particular input is present
and the other is absent, can be implemented as a gate with H+
and OH− as double inputs (Figure 7). As shown in Figure 7, an
OR logic gate was designed using Hg2+ and H+ ions as inputs.
After the successful generation of the basic binary logic gates, it
was reasonable to make an attempt to construct some
integrated gates as well. With this idea in mind, a three-input
INHIBIT−OR combinational gate was generated, where H+
and OH− constituted the inputs of an INHIBIT gate, whose
output was cascaded along with Hg2+ into an OR gate.
Figure 5. Formation of a five-membereded heterocyclic ring upon
protonation of ligand 6.
hydrogen-bond-inhibited CN isomerization, probe 6 shows a
drastic (20-fold) enhancement of the fluorescence emission
upon addition of acid (Figure 6a). The reversibility of this
process (deprotonation) was investigated by adding sodium
hydroxide as a base. As shown in Figure 6b, upon addition of
NaOH up to 1 equiv with respect to the protonated solution,
the fluorescence steadily decreases. This experiment clearly
indicates that H+ and OH− can be used to regulate the off−on−
off fluorescence switching of molecule 6. Thus, this concept
could be further utilized to construct a molecular logic gate.
Construction of the Logic Gate. Inspired by the
observations of the sensing capabilities of fluorescence of
probe 6 toward Hg2+, H+, and OH−, we developed a
fluorometric logic system capable of performing individual
logic operations. The presence and absence of each of these
inputs were coded with Boolean logic functions as 1 and 0,
respectively. The enhanced and quenched fluorescence output
values (at 536 nm) were assigned as “1” (the on state, above the
threshold value) and “0” (the off state, below the threshold
value), respectively. The output value was defined as 1 when
the relative fluorescence intensity was higher than 25 and 0
when the relative intensity was lower than 25. The fluorescence
response of receptor 6 exhibits INHIBIT and OR logic gates,
Four different conditions were investigated (Figure 8A): H+
as input 1 at 0 mM (low, 0, bars a and c) or 15 mM (high, 1,
bars b and d) and OH− as input 2 at 0 mM (low, 0, bars a and
b) or 15 mM (high, 1, bars c and d). As shown in Figure 8A,
the output value of I/I0 at 536 nm has a maximum (I/I0 = 88)
in the presence of only H+. On the other hand, the value of I/I0
is at a low level (I/I0 < 25) for any other combination. This
behavior is well-consistent with an INHIBIT logic gate.
Analogously, the different input combinations of Hg2+ and
H+ for 6 can behave like an OR logic gate with the value of I/I0
(536 nm) as the output (Figure 8B). In this OR logic gate,
Hg2+ and H+ have been taken as inputs 1 and 2, respectively,
with 15 mM concentration. On combination of the three inputs
Hg2+, H+, and OH− a combinational INHIBIT−OR logic
circuit can be made (Figure 8C): out of eight possible input
combinations, the output value of I/I0 at 536 nm is high when
Figure 6. (a) Fluorescence titration of compound 6 upon addition of H+ ion up to 1 equiv in CH3CN/H2O solution. (b) Fluorescence spectra of 6
upon the addition of 1 equiv of OH− in the presence of 1 equiv of H+.
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Figure 8. (A) INHIBIT logic gate constructed with H+ and OH− as an input and (B) OR logic gate constructed with Hg2+ and OH− at I/I0 = 536
nm. (C) Combinational logic operation.
Hg2+ and H+ are present. The threshold for all of the logic
operations was set at 25.
no significant change in fluorescence intensity was exhibited
when Hg2+ ion was added to the protonated solution of free 6
(pH range 2−4.5) (Figure S20b). From these experiments it is
evident that no interaction would be possible between 6 and
Hg2+ ion at acidic pH because of the electric charge repulsion
between protonated imine (the imine was protonated in this
acidic solution) and Hg2+ ion. In a similar fashion, H+ ion also
cannot perturb the fluorescence spectrum of the [6·Hg2+]
adduct because the imine N atom is no longer available to be
protonated once it coordinates with Hg2+.
Effect of pH. To elucidate the role of pH in the detection of
Hg2+ ion, the pH titration was carried out with free 6 and in the
presence of 1.0 equiv of Hg2+ ion in CH3CN/H2O (2:8 v/v)
solution. The pH of the free ligand was found to be 6.8. In the
pH range from 2 to 4.5, a drastic increase in fluorescence
intensity (Figure 9) with a red shift of 18 nm was observed for
free 6, whereas no significant change in fluorescence intensity
was observed for the [6·Hg2+] adduct (Figure S20a). Similarly,
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Visual Detection of Hg2+. In colorimetric experiments,
significant color changes of a CH3CN/H2O (2:8 v/v) solution
of 6 were observed upon addition of Hg2+ ion over the other
tested metal cations (Figure 11). A distinct color change of
Figure 11. Visual color changes observed for 6 (10−3 M) in CH3CN/
H2O after addition of 1 equiv of several metal cations.
Figure 9. Influence of pH on the fluorescence emission of 6 and the
[6·Hg2+] complex.
receptor 6 from faint yellow to pale orange was observed in the
presence of Hg2+ ion, which indicates the sensitive and selective
“naked eye” detecting ability of 6 for Hg2+ ion. When 1 equiv of
different metal cations (Li+, Na+, K+, Ag+, Ca2+, Mg2+, Mn2+,
Al3+, Cr3+, Co2+, Cu2+, Zn2+, Cd2+, Ni2+, and Pb2+, as their
perchlorate salts) was added to the solution of ligand 6 in
CH3CN/H2O (2:8 v/v), no significant color changes were
observed. Interestingly, Hg2+ ion showed a significant color
change even at very low concentration (10−5 M) (Figure S22).
Furthermore, to investigate the practical analytical applicability
of probe 6, we investigated the reversibility of 6 (Figure S23)
and its action in a solid support (Figure S24).
With the increase of pH from 6.0 to 12, no significant
changes in fluorescence intensity were observed for free ligand
6. Similar results were obtained when the fluorescence was
measured in the presence of 1 equiv of Hg2+ ion in the pH
range of 6−12. These results suggested that the [6·Hg2+]
adduct was stable enough under strongly basic conditions (pH
∼ 12) that the OH− could not displace the receptor to form
Hg(OH)2. To the best of our knowledge, this is the first
ferrocene-based electrochemical and fluorescent chemosensor
for Hg2+ ion working over such a broad pH range from 6 to 12.
Electrochemical Study. The metal recognition properties
of compound 6 were investigated by an electrochemical (cyclic
voltammetry) study. The electrochemical behavior was
investigated in the presence of several metal cations, such as
Li+, Na+, K+, Ag+, Ca2+, Mg2+, Cr3+, Al3+, Zn2+, Fe3+, Ni2+, Co2+,
Cu2+, Cd2+, Hg2+, and Pb2+ (as their perchlorate salts), in a
CH3CN/H2O (2:8 v/v) solution of 6 containing 0.1 M [(n-
Bu)4N]ClO4 as the supporting electrolyte. The free receptor 6
displayed a reversible one-electron redox potential at the half-
wave potential value E1/2 = 0.584 V versus the ferrocene/
ferrocenium redox couple (Fc/Fc+). No perturbation of the
cyclic voltammogram (CV) of 6 was observed except in the
case of Hg2+ ion (Figure S21). As shown in Figure 10, stepwise
addition of Hg2+ up to 1 equiv induced a significant anodic shift
of ΔE1/2 = 72 mV for 6 due to the formation of a new complex
species.
1H NMR Spectroscopic Titration. In order to confirm the
1
binding mode of the receptor with Hg2+ ion, an H NMR
spectroscopic titration study was carried out in DMSO-d6
(Figure 12). The extensive NMR spectroscopy characterization
was undertaken to assign the two near-conjugated −CH
protons. With the help of the two-dimensional 1H−13C
heteronuclear single-quantum coherence (HSQC) NMR
spectroscopic method, the protons Ha and Hb were
unambiguously assigned as the triazole −CH and the −N
CH protons, respectively (Figure S25). The ‘d’ carbon atom of
the triazole ring is known to resonate at 122−124 ppm,41 which
is correlated with the proton at 8.85 ppm. Thus, on the basis of
this correlation we assigned the peak at 8.85 ppm to the triazole
−CH proton. Upon addition of Hg2+ ion to a solution of
receptor 6, the following significant spectral changes were
observed: (i) Proton Ha attached to the triazole unit was
downfield-shifted by 0.12 ppm and (ii) proton Hb attached to
the CN bond was downfield-shifted by 0.15 ppm with
increasing Hg2+ concentration, along with a decrease in the
peak intensity. A polar aprotic solvent like DMSO may form a
strong hydrogen-bonding interaction with highly acidic hydro-
gen atom (Hb) of the −NCH group, which may lead to the
decrease in the peak height. (iii) Similarly, proton Hc attached
to the NCH2 group was very slightly downfield-shifted by 0.05
ppm. No change in the chemical shifts were observed for other
protons attached to the naphthalene moiety and ester unit.
1
From the H NMR chemical shifts, it is evident that the
probable binding mode of the Hg2+ ion is with the N atom of
the CN linkage and one of the N atoms of the triazole ring
(the one closest to the imine N).
Theoretical (DFT) Studies. Quantum-chemical calcula-
tions were performed to reveal the structural and electronic
parameters that control the aforementioned responses of
receptor 6 toward Hg2+. The ligand-to-metal binding ratio of
6 and Hg2+ was found to be 1:1 on the basis of spectral data
Figure 10. Evolution of the CV of 6 (10−4 M) in CH3CN/H2O (2:8
v/v) using [(n-Bu)4N]ClO4 as the supporting electrolyte as 1 equiv of
Hg2+ was added. The scan rate was 0.05 V s−1.
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Figure 12. Proposed mode of binding between receptor 6 and Hg2+ in DMSO-d6.
Figure 13. Optimized geometries of (a) free 6 and (b) [6·Hg2+](ClO4)2. (c) Contour line diagram of the Laplacian of the electron density, ∇2ρ(r),
of the complex in the N2−N1−Hg plane. Solid red lines indicate areas of charge concentration (∇2ρ(r) < 0), while dashed black lines show areas of
charge depletion (∇2ρ(r) > 0). Solid brown lines connecting the atomic nuclei are the bond paths. The thick blue lines separating the atomic basins
indicate the zero-flux surfaces crossing the molecular plane. Blue, orange, and green dots indicate bond critical points, ring critical points, and cage
critical point, respectively.
(e.g., absorption and emission spectra) as well as electro-
chemical and ESI-MS data. Furthermore, the formation of the
mononuclear complex [6·Hg2+](ClO4)2 was modeled by DFT
calculations at the B3LYP level using the Gaussian 09 package,
as explained in Computational Details. The corresponding
optimized structures are shown in Figure 13.
To analyze the nature of the bonding, we examined the plot
of the Laplacian of the electron density, ∇2ρ(r), of 6 (Figure
13c). The figure shows that the N−Hg bond in 6 has an area of
charge concentration (Δ2ρ(r) < 0, solid red lines) at N while
the Hg end carries a charge depletion area (Δ2ρ(r) > 0, solid
lines), indicating the existence of an electrostatic interaction
between the N and Hg centers. The evidence of this interaction
can also be verified by the presence of a large electron density
over the Hg-centered LUMO (Figure S26) due to electron
delocalization from the N atom. The bond between N and Hg
is relatively weak, as is evident from the electron density values
(Table S1) at the N1−Hg/N2−Hg bond critical points.
Furthermore, the ring critical point of the N1−C14−C15−
N2−Hg five-membered metallacycle ring shows a slight
electron density (0.015 a.u.), which is in the favor of the
formation of the five-membered metallacycle ring. Interestingly,
our attempts to optimize the Hg complex with the cis isomer
(Figure S27) led to an optimized geometry (Figure S28) that
was 29.21 kJ/mol higher in energy compared with the Hg
complex with the trans isomer. This indicates that upon
The geometry optimization of [6·Hg2+](ClO4)2 at the
B3LYP/def2-SVP level of theory resulted in a slight conforma-
tional change of the free ligand. The −CH2−N(triazole) bond
is rotated in such a way that both the imine N and one of the
triazole N atoms can come close to form a binding core for the
Hg2+ ion. In this binding situation, Hg exhibits a distorted
tetrahedral N2Cl2 coordination. The Hg2+ center shows a
distorted geometry that deviates from both the square-planar
and tetrahedral geometries. Among the four coordinates, both
of the Hg−N bonds (dHg−N1 = 2.476 Å and dHg−N2 = 2.390 Å in
[6·Hg2+](ClO4)2 are relatively weak, as evidenced by the low
Wiberg bond index values (WBIHg−N1 = 0.2236 and WBIHg−N2
= 0.2223 in [6·Hg2+](ClO4)2), whereas the other two sites are
occupied by two Cl atoms of the ClO4 units.
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complexation with Hg2+ ion only one isomer (trans) get
stabilized and no cis−trans isomerization of CN is possible,
thereby leading to a large fluorescence enhancement.
TD-DFT calculations of the protonated ligand revealed that the
UV−vis spectrum follows a trend similar to that of the Hg2+
complex. The low-energy band at ∼257 nm is responsible for a
HOMO−9 to LUMO transition with a ∼65% contribution (the
HOMO−9 is mainly based on π orbitals of the naphthalene
unit, whereas the LUMO is based on the π* orbitals with a
major contribution from the triazole and imine units). The
signal at ∼365 nm is majorly due to the HOMO−2 to LUMO
transition with a ∼90% contribution (HOMO−2 is the
ferrocene-based filled lone-pair orbitals) (Table S2).
Frontier molecular orbitals (MOs) of 6 and its Hg complex
[6·Hg2+](ClO4)2 are shown in Figure 14 (and Figures S29 and
The electrostatic potential surface (EPS) diagrams of the
ligand and [6·Hg]2+ were also calculated (Figure S33). It is
evident from the EPS diagram of the Hg complex that there is a
depletion of charge at the N end and an accumulation of charge
(apart from localized electrons on the Cl atoms) at the O end
of the ester moiety. The second-order perturbation energy
analysis obtained from the NBO analysis shows that
delocalization of the lone pairs of electrons from N2 and N3
to the empty Hg orbital (34.4 and 34.8 kcal/mol) and
delocalization of the lone pair of electrons on N4 toward the
ester moiety (36 kcal/mol) are highly favorable. This supports
our proposed mechanism described in Scheme 4 for the
fluorescence enhancement along with a large red shift.
CONCLUSION
Figure 14. Frontier molecular orbitals (isovalue = 0.03) of (left) 6 and
(right) [6·Hg2+](ClO4)2 as obtained from DFT calculations.
■
An optoelectronic molecular device for selective and sensitive
detection of Hg2+ ion has been synthesized. The probe shows a
huge enhancement of emission intensity with a large bath-
ochromic shift upon addition of Hg2+ ion. Furthermore, a
similar trend was also observed in the fluorescence emission
spectra when the probe was treated with a strong acid. The
presence of Hg2+ and H+ causes the inhibition of cis−trans
isomerization around CN by forming a rigid five-membered
metallacycle and hydrogen-bonded framework, respectively,
thereby leading to a fluorescence enhancement of the system.
To the best of our knowledge, molecule 6 is the first example of
a triazole- and imine-conjugated ferrocene−naphtholate system
where the fluorescence signal can be modulated by both metal
complexation and protonation/deprotonation strategies. Thus,
H+ and OH− have been used to regulate the off−on−off
fluorescence switching of molecule 6. On the basis of the on−
off switching of the fluorescence output mechanism, basic OR
and INHIBIT binary logic gates were developed using the Hg2+
and H+ ions and the H+ and OH− ions, respectively. These
basic binary logic gates were further attached to form an
INHIBIT−OR combinational gate, which could be utilized in
multitasking of biosensing of dual targets. The constructed
combinational INHIBIT−OR logic gate functions not only in
organic solution but also via conjugation with silica gel as a
solid support.
S30). In 6, both the HOMO and LUMO were found to be
majorly localized on the naphthalene unit (the π and π*
orbitals, respectively). In [6·Hg2+](ClO4)2, the HOMO is
virtually same as that of the free ligand, but the LUMO is
majorly found in the binding core, which is essentially a Hg-
based vacant orbital. Therefore, the computational studies and
Laplacian electron density plot as well as MO analysis support
our proposed binding mode of ligand 6 with Hg2+ ion.
Furthermore, time-dependent DFT (TD-DFT) calculations
were performed to obtain detailed information about the
absorption of 6 and [6·Hg2+](ClO4)2 in CH3CN as a solvent.
The TD-DFT calculations showed two absorption maxima at
227 and 335 nm for 6 which nicely corroborate the
experimental values (238 and 339 nm). The higher-energy
peak is majorly due to the electronic transition from the
HOMO (naphthalene-based π orbitals) to the LUMO+2,
which is largely localized on ferrocene with a slight contribution
from the triazole ligand. The lower-energy peak is due to the
transition from the naphthalene-based HOMO to the LUMO,
which are essentially π and π* orbitals, respectively. The UV−
vis study showed decreased intensity of both absorption
maxima upon binding with Hg2+ along with the appearance
of a new low-energy peak. The TDDFT results indicate that the
decrease in intensity is due to the less favored transition of the
ferrocene lone pair (HOMO−2) to a vacant Hg orbital
(LUMO). Furthermore, the DFT calculations show that the
HOMO−LUMO gap diminishes upon complex formation
(Figure 14), which satisfactorily corroborates the observed red
shift in UV−vis study.
EXPERIMENTAL SECTION
■
Materials and Methods. The perchlorate salts of Li+, Na+, K+,
Cu2+, and Hg2+ as well as DBU and CuI were procured from Sigma-
Aldrich and used directly without further purification. The perchlorate
salts of Ag+, Cr3+, Ca2+, Mg2+, Al3+, Fe2+, Mn2+, Zn2+, Pb2+, and Co2+
were purchased from Alfa Aesar. Ferrocene, N,N,N′,N′-tetramethyle-
thylenediamine (TMEDA), POCl3, n-butyllithium (2.5 M in hexane),
NaN3, NaBH4, K2CO3, Na2Cr2O7, propargyl bromide, and SnCl2·
2H2O were purchased from local chemicals. DMF and acetonitrile
(HPLC) were purchased from Thermo Fisher Scientific and freshly
distilled prior to use. Chromatography was carried out using 100−200
mesh silica gel and basic alumina in a column of 2.5 cm diameter. All
of the necessary solvents were dried by conventional methods and
In addition, the geometry of the protonated species was
optimized using DFT calculations at the B3LYP level, and the
frontier MOs were also calculated (Figure S31). From the
DFT-optimized geometry it is evident that after protonation,
the iminium hydrogen atom can form a stable five-membered
ring via a hydrogen-bonding interaction (2.307 Å) with the
nearest N atom of the triazole ring (Figure S32). Furthermore,
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distilled under an atmosphere of N2 before use. Azidomethylferrocene
was synthesized as per the literature procedure.42,43 The cyclic
voltammetry study was performed using glassy carbon as the working
electrode, platinum as the auxiliary electrode, and Ag/Ag+ as the
mmol) in dry DMF was made in a Schlenk flask equipped with a
stirrer bar and degassed for 45 min. CuI (0.4 equiv) and DBU (0.6
equiv) were added, and the resulting solution was heated at 60 °C for
4 h. The reaction mixture was diluted with dichloromethane, followed
by excess methanol/water (1:1 v/v) solution. The organic phase was
separated and dried over Na2SO4, and the solvent was removed under
reduced pressure. The crude residue was purified by column
chromatography using silica gel. Elution with EtOAc/hexane (2:8 v/
v) yielded 6 as a yellow solid (0.250 g, 75%).
reference electrode. The experiments were carried out with a 10−4
M
solution of the sample in CH3CN/H2O (2:8 v/v) containing 0.1 M
[(n-Bu)4NClO4] as the supporting electrolyte. The CVs were recorded
at a scan rate of 0.05 V s−1. The working electrode was cleaned after
each run. The UV−vis spectra were measured using CH3CN/H2O
(2:8 v/v) solutions at c = 10−5 M and the fluorescence spectra were
measured at c = 10−8 M, as stated in the corresponding figure captions.
Instrumentation. The 1H and 13C spectra were recorded on
Bruker 400 MHz FT-NMR spectrometers using tetramethylsilane as
an internal reference. The absorption spectra were recorded with a
Shimadzu 2450 UV−vis spectrophotometer at room temperature.
Fluorescence was recorded with a HORIBA Scientific Fluoromax-4
spectrophotometer. Cyclic voltammetry was performed on a CH
Instruments S8/39 electochemical workstation. ESI-MS measurements
were carried out on PerkinElmer Flexar SQ 300 MS detector. A
SYSTRONICS μ pH system 361 with a glass electrode was used for
pH detection. CHN analysis was performed on a vario EL elementar
CHNS analyzer. Melting points were measured in a Labotech melting
point apparatus.
1
6: H NMR (CDCl3, 400 MHz): δ 8.77 (d, 1H, Hnaphthalene, J = 8.4
Hz), 8.71 (s, 1H, CH), 8.48 (d, 1H, Hnaphthalene, J = 8.8 Hz), 8.20−8.18
(m, 1H, Hnaphthalene), 8.13 (s, 1H, Htriazole), 7.60−7.56 (m, 1H,
Hnaphthalene), 7.51−7.47 (m, 1H, Hnaphthalene), 7.10 (d, 1H, Hnaphthalene, J
= 7.6 Hz), 5.41 (s, 2H, NCH2), 4.47 (q, 2H, OCH2ester), 4.36 (t, 2H,
HCp, J = 1.6 Hz), 4.28 (t, 2H, HCp, J = 1.6 Hz), 4.23 (s, 5H, HCp), 1.46
(t, 3H, OCH2CH3). 13C NMR (CDCl3, 100 MHz): δ 167.9, 152.8,
148.9, 131.8, 130.6, 129.2, 128.9, 128.0, 127.6, 124.5, 124.2, 122.8,
114.6, 113.3, 80.2, 69.5, 69.2, 69.1, 61.2, 50.6, 29.8, 14.5. Mp: 106−108
°C. ESI-MS, m/z (relative intensity): 493 (M+ + 1). Anal. Calcd for
C27H24FeN4O2: C, 65.87; H, 4.91; N, 11.38. Found: C, 65.69; H, 5.08;
N, 11.14.
X-ray Crystallographic Analysis. X-ray-quality crystals of 6 were
grown by slow diffusion of a hexane/EtOAc (4:6 v/v) solution. The
intensity data were collected on a Super Nova, Dual, Cu at zero, Eos
diffractometer. The crystal was kept at 100.00(10) K during data
collection. With OLEX2,44 the structure was solved with the
Superflip45 structure solution program using charge flipping and
refined with the SHELXL46 refinement package and least-squares
minimization. The hydrogen atoms were refined isotropically at
calculated positions using a riding model. CCDC 1543246 contains
the supplementary crystallographic data for this paper. These data can
be obtained free of charge from the Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Caution! Metal perchlorate salts are potentially explosive under certain
conditions. All due precautions should be taken while handling perchlorate
salts.
Synthesis of Compound 4. A stirred cloudy solution of
compound 3 (0.240 g, 0.987 mmol) in ethanol (10 mL) was added
dropwise to a solution of SnCl2·2H2O (1.336 g, 5.922 mmol) in
concentrated hydrochloric acid (2 mL) at room temperature. The
reaction mixture was refluxed for 2 h and then cooled to room
temperature, and an aqueous solution of Na2CO3 (10%) was added to
quench the reaction. The reaction mixture was diluted with ethyl
acetate and washed several times with water and then brine. The
combined organic layer was dried over anhydrous Na2SO4 and then
evaporated under reduced pressure. The crude residue was purified by
column chromatography using basic alumina. Elution with EtOAc/
hexane (3:7 v/v) yielded ethyl 5-amino-1-naphthoate (4) as a pale-
yellow oil (0.142 g, 67%).
Crystal Data for 6. Formula, C27H24FeN4O2; crystal system,
monoclinic; space group, P21/c; unit cell dimensions, a = 13.7802(4)
Å, b = 9.5136(3) Å, c = 17.4606(5) Å, β = 90.693(2); Z = 4; density
(calcd) = 1.429 mg/m3; Rint = 0.0217; final R indices [I > 2σ(I)], R1 =
0.0370; wR2 = 0.0791; θ range (deg), 2.33−27.46; total reflection
collected, 4600; independent reflections, 3917; goodness of fit on F2 =
1.098.
1
4: H NMR (CDCl3, 400 MHz): δ 8.29 (d, 1H, Hnaphthalene, J = 8.8
Hz), 8.10 (d, 1H, Hnaphthalene, J = 6.8 Hz), 8.0 (d, 1H, Hnaphthalene, J = 8.4
Hz), 7.43−7.37 (m, 2H, Hnaphthalene), 6.80 (d, 1H, Hnaphthalene, J = 7.2
Hz), 4.47 (q, 2H, OCH2), 4.16 (s, 2H, NH2), 1.45 (t, 3H, CH3, J = 7.2
Hz). 13C NMR (CDCl3, 100 MHz): δ 168.1, 142.5, 132.0, 129.6,
128.1, 125.6, 124.1, 123.2, 116.4, 110.2, 61.1, 14.3. ESI-MS, m/z
(relative intensity): 216 (M++ 1); Anal. Calcd for C13H13O2N: C,
72.54; H, 6.09; N, 6.51. Found: C, 72.47; H, 6.18; N, 6.44.
Computational Details. All of the calculations (DFT and TD-
DFT) were carried out with the Gaussian 09 program package
(revision C.01).47 The ground-state geometries were optimized
without symmetry constraints using the hybrid Becke−Lee−Yang−
Parr (B3LYP) functional48 in combination with the def2-SVP basis
set49 from the EMSL Basis Set Exchange Library. The 60 core
electrons of mercury were replaced by the quasi-relativistic effective
core potential def2-ECP.50 The model compounds were fully
optimized in the gas phase (no solvent effect). The crystallographic
coordinates of 6 were used as a starting geometry for complete
geometry optimization. Frequency calculations were performed in
order to verify the nature of the stationary state as a minimum, which
was confirmed by the absence of any imaginary frequency. The energy
values provided herein contain zero-point energy (ZPE) corrections.
The lowest-energy gas-phase vertical transitions were calculated
(singlets, 15 states) on the structures optimized by TD-DFT using
the Coulomb-attenuated functional cam-B3LYP51 in combination with
the def2-SVP basis set. The cam-B3LYP functional was selected
because it has been shown to be effective for ICT systems.52 With the
optimized ground-state (S0) geometry as the starting coordinate, TD-
DFT geometry optimization of the S1 state of 6 was performed.
Additionally, TD-DFT cam-B3LYP/6-31G(d) S1 optimization of 6
was conducted with inclusion of acetonitrile solvation using the
polarizable continuum model. Natural bonding analyses were
performed with the natural bond orbital (NBO) partitioning scheme53
as implemented in Gaussian 09. Wiberg bond index values54 were
obtained by NBO analysis. To understand the nature of the bonding of
6 and [6·Hg2+] further, the topological properties of the resultant
electron density, ρ, obtained from the wave functions of all of the
optimized structures were analyzed with the quantum theory of atoms
Synthesis of Compound 5. Compound 4 (0.190 g, 0.883 mmol)
was dissolved in a 4:1 mixture of ethanol and water, and the solution
was stirred. Then K2CO3 (0.971 mmol) and propargyl bromide (0.883
mmol) were added to the stirred solution, and the resulting mixture
was refluxed for 5 h. After completion of the reaction, the solvent was
evaporated, and the residue was washed with ethyl acetate and brine.
The solvent was dried with anhydrous Na2SO4 and removed under
reduced pressure. The crude product was purified by column
chromatography using basic alumina. Elution with EtOAc/hexane
(1:9 v/v) yielded ethyl 5-(prop-2-yn-1-ylamino)-1-naphthoate (5) as a
yellow oil (0.120 g, 54%).
1
5: H NMR (CDCl3, 400 MHz): δ 8.29 (d, 1H, Hnaphthalene, J = 8.8
Hz), 8.09 (d, 1H, Hnaphthalene, J = 6.8 Hz), 7.9 (d, 1H, Hnaphthalene, J = 8.4
Hz), 7.49 (t, 1H, Hnaphthalene, J = 8.4 Hz), 7.43−7.39 (m, 1H,
Hnaphthalene), 6.76 (d, 1H, Hnaphthalene, J = 7.6 Hz), 4.57 (s, 1H, NH),
4.47 (q, 2H, OCH2), 4.08 (d, 2H, NCH2, J = 2 Hz), 2.29 (t, 1H,
CHalkyne, J = 2.4 Hz), 1.45 (t, 3H, CH3, J = 7.2 Hz). 13C NMR (CDCl3,
100 MHz): δ 168.1, 142.4, 132.1, 129.6, 128.5, 128.2, 124.7, 124.4,
123.5, 116.3, 106.2, 80.7, 71.8, 61.1, 33.0, 14.4. ESI-MS, m/z (relative
intensity): 254 (M+ + 1). Anal. Calcd for C16H15O2N: C, 75.87; H,
5.97; N, 5.53. Found: C, 75.58; H, 5.78; N, 5.34.
Synthesis of Compound 6. A solution of compound 5 (0.170 g,
0.672 mmol) and 0.9 equiv of azidomethylferrocene (0.146 g, 0.605
K
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in molecules (QTAIM).55 The QTAIM analysis was carried out
utilizing the Multiwfn V.3.3.8 package,56 whereas the wave functions
were generated with Gaussian 09 at the same level of theory as was
used for geometry optimization.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01304.
1H, 13C, and ESI-MS data for compounds 4, 4b, 4c, 5,
and 6; UV−vis spectra and fluorescence spectra upon
titration with different metal ions; ESI-MS spectrum of
[6·Hg2+]; Job plot; detailed computational studies
(PDF)
Accession Codes
CCDC 1543246 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing
data_request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
1EZ, U.K.; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: thakura@nitrkl.ac.in, babuiitm07@gmail.com. Phone:
+918895012676.
■
ORCID
Arunabha Thakur: 0000-0003-4577-3683
Present Address
⊥Department of Chemistry, Jadavpur University, Kolkata-
700032, India
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Generous support from the Department of Science and
Technology (DST), New Delhi (an Inspire Faculty Fellowship)
is gratefully acknowledged.
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