Dual colorimetric receptor with logic gate operations: Anion induced solvatochromism

Article abstract of DOI:10.1039/c3nj01229h

A receptor R1 was designed and synthesised for colorimetric detection of F^- ions as well as Cu²⁺ ions via intramolecular charge transfer mechanism. Upon addition of F^- ions in dry DMSO, the color of the receptor R1 changed

Full text of DOI:10.1039/c3nj01229h

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Dual colorimetric receptor with logic gate
operations: anion induced solvatochromism†
Cite this: New J. Chem., 2014,
38, 1484
Madhuprasad,a N. Swathi,a J. R. Manjunatha,b Uttam Kumar Das,c
A. Nityananda Shettya and Darshak R. Trivedi*a
A receptor R1 was designed and synthesised for colorimetric detection of Fꢀ ions as well as Cu2+ ions via
intramolecular charge transfer mechanism. Upon addition of Fꢀ ions in dry DMSO, the color of the receptor R1
changed from pale yellow to blue. The receptor showed a unique property of solvatochromism by displaying
different coloration with different solvents only in the presence of Fꢀ ions, which were applied to determine
the percentage composition of binary solvent mixtures. The receptor R1 was able to detect Cu2+ ions
colorimetrically where it exhibited a color change from pale yellow to orange-red. In addition, the receptor was
subjected to molecular logic gate applications, wherein it showed ‘ON–OFF’ switching operations.
Received (in Montpellier, France)
8th October 2013,
Accepted 12th December 2013
DOI: 10.1039/c3nj01229h
www.rsc.org/njc
Similarly, researchers have devoted their efforts towards the
design and synthesis of various receptors for the detection of
Introduction
The design and synthesis of organic chemoreceptors for selective hazardous metal ions.5 Amidst the widespread transition metal
detection of anions and metal ions has gained the attention of ions, copper is distinctive as it is essential to the normal growth
scientists due to their prospective applications in the fields of of living organisms.6 However, at high concentration copper is
biochemistry and environmental science.1 In addition, the toxic to various organisms including humans.7 Generally, the
anions are associated with many other important aspects of detection of copper ion is carried out using complicated
supramolecular chemistry such as supramolecular gels, controlling instrumental methods such as inductively coupled plasma
pharmaceutical crystal growth and polymer templating in a simple mass spectroscopy,8 atomic absorption spectroscopy9 and X-ray
organogelator.2 Among all the anions, fluoride plays a vital role in photoelectronic spectrometry.10 On the other hand, recently the
clinical applications although there are disadvantages associated detection of copper ions by colorimetric methods have attained
with it.3 Therefore, the detection of fluoride has significance. prominence11 due to various advantages such as selectivity,
Though instrumental methods such as fluoride ion monitor sensitivity and rapid detection over conventional methods of
probes are available for the detection of fluoride ions, it analysis using instruments. In addition, the quest for effective
consumes much time and requires skilled efforts to operate. On dual receptors that can detect both anions and cations are
the other hand, colorimetric method of detection is instantaneous, emerging and extensive efforts have been put forth to achieve
and is equally selective and sensitive. Consequently, for the last the same.12 Nevertheless, the colorimetric detection of both Fꢀ
couple of decades colorimetric anion receptor chemistry has been and Cu2+ using the same chemoreceptor is a challenging task
studied extensively.4
due to their different binding phenomena and needs to be
explored. To introduce a dual sensing property to a receptor,
the molecule should contain different binding sites that can
accommodate Fꢀ ions or/and Cu2+ ions.13
a Department of Chemistry, National Institute of Technology Karnataka (NITK),
Srinivasnagar, Surathkal, Mangalore-575025, Karnataka, India.
E-mail: darshak_rtrivedi@yahoo.co.in; Fax: +91-824-2474033;
Tel: +91-824-2474000 ext. no: 3205
Being an essential component of molecular computing
operations and molecular devices, molecular logic gates for
converting chemical input signals into measurable output
signals has become an exciting research area in supramolecular
chemistry. Subsequently, increasing attention has been paid to
logic gate applications based on anion/cation detecting receptors.14
Various types of logic gate functions can be achieved by
applying two-input systems. Even though molecular analogues
of many logic gate functions (based on two-input systems) are
reported in the literature,15,16 only a few of them are based on
colorimetric receptors.17 Accordingly, the colorimetric receptor
b PPSFT Department, Central Food Technological Research Institute (CFTRI),
Mysore, Karnataka, India
c Department of Organic Chemistry, Indian Association for the Cultivation of
Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700032,
West Bengal, India
† Electronic supplementary information (ESI) available: 1H NMR spectra of R1–
R3, photographs, UV-vis spectroscopic titration studies of receptors R1, R2 with
Fꢀ ions and Cu2+ ions. UV-vis changes of R1, R2 and R3 with different anions and
different cations. CCDC 912057 and 939305. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c3nj01229h
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Fig. 1 Change in color of receptor R1 after addition of 1 equiv. of different
anions as TBA salt to the receptor solution in DMSO (5 ꢁ 10ꢀ5 M). (a)
ꢀ
Receptor R1, (b) Fꢀ, (c) Clꢀ, (d) Brꢀ (e) Iꢀ, (f) NO3ꢀ, (g) HSO4ꢀ, (h) H2PO4
and (i) AcOꢀ.
Scheme 1 Molecular structure of receptors.
blue (receptor R2) was observed instantaneously with the addition
of Fꢀ ions. A slight color variation from pale yellow to greenish
yellow in the case of receptor R1 and pale yellow to dark yellow in
the case of receptor R2 was observed upon adding the acetate ion.
On the other hand, no color change was noticed upon addition of
other anions (Fig. 1 and Fig. S9, ESI†). This colorimetric selectivity
of receptors R1 and R2 for the detection of Fꢀ ion was further
confirmed using UV-vis spectroscopy (Fig. S25 and S26, ESI†).
In order to examine the selectivity towards Fꢀ ion over other
anions, 1 equiv. of Fꢀ ion was added to the receptor R1 in the
presence of other anions such as Clꢀ, Brꢀ, Iꢀ, NO3ꢀ, HSO4ꢀ and
H2PO4ꢀ ions. The presence of any other anions makes virtually no
difference on the colorimetric detection of the Fꢀ ion (Fig. S15,
ESI†). Thus, the receptor R1 could selectively detect the fluoride ion
even in the presence of other competing anions.
based molecular logic gates developed interest among supra-
molecular chemists.
As a part of our on-going research in designing the colorimetric
receptors,18 herein we report a receptor R1 (Scheme 1), which is
capable of detecting anions as well as cations, to be specific Fꢀ ions
and Cu2+ ions with the naked eye instantaneously.
The receptor R1 encompasses a hydroxyl (–OH) functional
group, which detects the Fꢀ ion by a deprotonation mechanism,
and phenolic oxygen (–O) along with imine nitrogen (QN) for the
detection of Cu2+ via complex formation.11e R1 served as an
effective receptor for the detection of the Fꢀ ion as well as Cu2+
with the logic gate function. In addition, this receptor showed
remarkable solvatochromic property by displaying different colors
in different solvents only in the presence of Fꢀ ions. This solvato-
chromic property of the receptor molecule has indispensable
applications in many fields of chemistry including characterization
of mixed solvent systems, solid surfaces and polymers, chemical
sensors to detect the presence of water in polymers and to study the
behaviour of solute–solvent interactions in supercritical fluids.19
The receptor R2 was synthesised to verify the substitution position
of –OH functional group in detection process and receptor R3 was
synthesised to examine the role of –OH as a binding site.
UV-vis studies and binding mechanism
As part of the quantitative analysis, the UV-vis spectrophoto-
metric titration of receptor R1 with TBAF in dry DMSO was
carried out. Upon gradually increasing the concentration of Fꢀ
ions, the intensity of the peaks at 282 nm, corresponding to
imine (–NQC–) linkage, and 385 nm, corresponding to –OH
group, decreased gradually. This was followed by the appearance of
new bathochromic bands at 339 nm and 555 nm (Fig. 2). The
development of the new band at 339 nm was attributed to the
formation of an exocyclic imine when the receptor undergoes enol
to keto tautomerisation (due to the deprotonation of 2-hydroxy
Results and discussion
All the synthesised molecules were characterized by standard
techniques. The single crystal of the receptors R1 suitable for X-ray
difraction analysis was obtained by slow evaporation of dichloro-
methane solution at room temperature and the single crystal of
receptor R3 was obtained directly from reaction media (ethanol
used as solvent). The ORTEP diagrams (50% probability) of the
receptor R1 and R3 are given in Fig. S7 (ESI†). The receptors R1 and
R3 were crystallised in a monoclinic lattice. Detailed crystallographic
data of receptor R1 and R3 are given in Table S1 (ESI†).
Colorimetric study of anions
Initially, the receptors R1 and R2 were evaluated for colorimetric
detection of Fꢀ ions over other anions (chloride, bromide, iodide,
nitrate, hydrogensulfate, dihydrogenphosphate and acetate) in
dry DMSO solvent. Both the receptors have hydroxyl (–OH)
functionality as electron donors and nitro (–NO2) functionality
as electron acceptors. Due to this donor–acceptor (D–A) inter-
actions these receptors are pale yellow in color. These receptors
(5 ꢁ 10ꢀ5 M) were treated with tetrabutylammonium (TBA) salt
of different anions (1 equiv.) in DMSO. A significant color
change from pale yellow to blue (receptor R1) and greenish
Fig. 2 UV-vis titration spectra of R1 (5 ꢁ 10ꢀ5 M) with increasing concen-
tration of TBAF (0–25 equiv.) in dry DMSO. Inset: Job’s plot for R1 with
Fꢀ ion at 555 nm in dry DMSO.
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group) and the new band at 555 nm was ascribed to the develop- double bond and aromatic protons of 4-nitrophenyl group at d
ment of an intramolecular charge transfer (ICT) transition. This 8.3 and d 8.33 shifted upfield to d 8.1 and d 7.98, respectively
constant decrease of two absorption bands and generation of two (Hc). These upfield observations represent the increase in
new absorption bands resulted in the formation of three clear electron density over the 4-nitrophenyl group of receptor molecule
isosbestic points at 309 nm, 361 nm and 433 nm, which evidently upon Fꢀ ion binding. In contrast, the protons of the 2-aminophenol
suggests the generation of two species in the system during the group shifted downfield from the d range 6.86–7.29 (Hd) to d range
detection process.
7.30–8.87. This shift was due to the decrease in electron density over
To ensure that the detection process follows deprotonation the 4-aminophenol group of the receptor R1, which resulted in the
and not complex formation, receptor R1 was titrated with TBA deshielding of the corresponding protons upon Fꢀ ion binding.
hydroxide (Fig. S18, ESI†). Upon incremental addition of Therefore the hydroxyl phenyl group acts as an electron donor and
TBAOH a constant decrease in the peaks at 282 nm and the nitrophenyl group acts as an electron acceptor, thus forming an
385 nm was seen along with a gradual increment in the peaks ICT transition. Furthermore, at 0.5 equiv. and 1.0 equiv., the
at 351 nm and 572 nm being observed. This confirms that the 1H NMR illustrates both structures R1 and R10 (Scheme 2), wherein
deprotonation of –OH proton is responsible for the colorimetric the receptor undergoes keto–enol tautomerism. Finally, the keto
change and not complex formation.
tautomer is stabilized at 2 equiv. of Fꢀ ion (Fig. 3).
In case of receptor R2, the titration resulted in a constant
decrease in the intensity of the absorption band at 388 nm due
to the deprotonation of the hydroxyl group. Simultaneously, a
new bathochromic band at 558 nm appeared, which was
attributed to the generation of ICT transition (Fig. S19, ESI†).
The stoichiometry of the receptor–Fꢀ ion system was determined
by a Job’s plot obtained from UV-vis spectrophotometric titration
data at 555 nm for receptor R1 in DMSO solution (Fig. 2, inset).
A maximum absorbance was observed when the mole fraction
was 0.66 and this confirms the formation of 1 : 2 (host/guest)
stoichiometric ratio between R1 and Fꢀ ions. Hence, it is
evident that Fꢀ ion detection using receptor R1 is a two-step
process as shown in Scheme 2. In the first instance, a Fꢀ ion binds
through hydrogen bonding to the hydroxyl proton of receptor R1,
resulting in a 1 : 1 adduct to give a R1ꢂꢂꢂFꢀ complex.20 In the next
step, the second Fꢀ ion induces deprotonation of the phenolic –OH
proton of receptor R1 and as a result, the electron density over
the deprotonated receptor R1 was increased. Thus, the charge
separation in the receptor was introduced, which resulted in
ICT transition between the electron deficient –NO2 group at the
p-position and the electron rich –Oꢀ, which leads to intense
colorimetric change.21
Solvatochromic study
To our surprise, when 1 equiv. of Fꢀ ion was added to the
receptor R1 solution in dry ACN (5 ꢁ 10ꢀ5 M), a color change
from pale yellow to pink was observed. This dissimilarity in
color change (in DMSO pale yellow to blue and in ACN pale
yellow to pink) was due to the solvatochromic effect of receptor R1.
This study was further extended to other aprotic solvents such
1H NMR titration was carried out in dry DMSO-d6 to confirm
the binding mechanism.
Upon addition of 0.5 equiv. TBAF, the –OH peak at d 9.26
(Ha) disappeared completely, thus confirming deprotonation of
phenolic –OH.22 The peak Hb at d 8.88 corresponding to imine
–CH– shifted upfield to d 8.45 due to the formation of exocyclic
Fig. 3 Partial 1H NMR titration spectra of receptor R1 with Fꢀ ion
at different concentrations in DMSO-d6. (a) 0, (b) 0.5, (c) 1.0 and
(d) 2.0 equiv. TBAF.
Scheme 2 Proposed mechanism for the fluoride ion binding to receptor R1.
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Fig. 5 The color change of receptor R1 with varying composition of
DMSO in DCM on adding Fꢀ ions (10 equiv.) (a) 0% i.e. 100% DCM,
(b) 20%, (c) 40%, (d) 60%, (e) 80% DMSO in DCM and (f) 100% DMSO.
Fig. 4 Solvatochromism in receptor R1 upon addition of 1 equiv. of Fꢀ
ions in different solvents. Top row: R1 solution (5 ꢁ 10ꢀ5 M) in different
solvents. Bottom row: R1 + Fꢀ. (a) 1,4-dioxane, (b) THF, (c) DCM, (d)
acetone, (e) ACN and (f) DMSO.
viscosity, refractive index or density. On the other hand, photo-
physical properties such as solvatochromism are not well
explored to determine the composition of the solvent mixture.
The receptor R1, being solvatochromic in presence of Fꢀ ions,
was applied to determine the percentage composition of two
different solvents. This experiment was demonstrated by
inspecting the color change of receptor R1 upon adding Fꢀ
ions in proportionate mixtures of DCM and DMSO. The recep-
tor R1 was dissolved in a mixture of two solvents (by varying
compositions between 0% and 100% of DMSO in DCM) to
prepare 5 ꢁ 10ꢀ5 M solution. Upon adding Fꢀ ions (10 equiv.)
to each solution, a color change between brown and blue was
observed (Fig. 5).
This colorimetric change was confirmed with UV-vis spectro-
scopy wherein a bathochromic shift of lmax was observed with
increasing the percentage of DMSO in DCM (Fig. 6). A shift of
approximately 7.5 nm in the lmax was observed for each 20%
increase of DMSO in the solvent mixture. A calibration curve
was plotted with this bathochromic shift and the percentage
composition of DMSO in DCM (Fig. 6, inset).
as 1,4-dioxane, THF, DCM, acetone and DMSO where the receptor
R1 showed different coloration (Fig. 4) only in the presence of Fꢀ
ion with respect to the solvent polarity. The color of the receptor R1
was changed from pale yellow to red in 1,4-dioxane, brown in THF
and DCM, purple in acetone, pink in ACN and finally blue in
DMSO. As the receptor solution does not have any charge
separation and charge transfer (CT) transitions, it did not show
any solvatochromism in the absence of Fꢀ ions.
However, addition of Fꢀ ions to the receptor solutions
induced charge separation. Hence, a charge transfer transition
was established, due to which the receptor attains different
dipole moments in its ground and lower energy singlet excited
states. These newly established dipole moments interacted with
the solvent dipole moments to change the energy levels of the
receptor and as the solvent polarity increased, these energy
levels decreased. As a result the peaks corresponding to CT
transitions red shifted (positive solvatochromism)23 with
increasing solvent polarity. This solvatochromism was compared
with the dielectric constant of solvents, which are compiled in
Table 1. The polarity of any solvent is dependent on its dielectric
constant. Greater the dielectric constant, the greater the polarity
and as a result, with increasing dielectric constant the red shift
increased.
A linear graph was obtained with the increasing percentage
of DMSO in DCM. The percentage composition of unknown
solvent mixture (DMSO/DCM) can be easily determined using
this calibration curve.
The same trend was observed in case of R2 where the color
changed from pale yellow to orange in 1,4-dioxane, dark yellow
in THF and DCM, dark brown in acetone, light brown in
ACN and blue in DMSO upon addition of 1 equiv. of Fꢀ ions
(Fig. S17, ESI†).
The percentage composition of a solvent mixture is generally
determined by measuring its physical properties such as
Table 1 Change in absorption maxima (lmax) of receptor R1–Fꢀ complex
in different solvents on adding TBAF
Solvent
Dielectric constant
lmax (nm)
1,4-Dioxane
2.21
7.58
8.93
20.70
37.5
46.8
500
504
505
510
511
555
Tetrahydrofuran
Dichloromethane
Acetone
Acetonitrile
Dimethylsulfoxide
Fig. 6 UV-vis spectral changes of receptor R1 (5 ꢁ 10ꢀ5 M) on adding
10 equiv. of Fꢀ ions with varying compositions of DMSO in DCM.
Inset: calibration curve plotted with % composition of DMSO in DCM vs.
Shift in lmax
.
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Table 2 Change in absorption (Dlmax) of receptors R1–R3 (5 ꢁ 10ꢀ5 M) in
the presence of TBAF
lmax
lmax
Dlmax
Dlmax
Receptor (ACN)a (nm) (DMSO)b (nm) (ACN) (nm) (DMSO) (nm)
R1
R2
R3
511
500
293
555
558
294
136
129
0
170
170
0
a
10 equiv. of TBAF was added to receptor solutions (5 ꢁ 10ꢀ5 M) in dry
ACN. 10 equiv. of TBAF was added to receptor solutions (5 ꢁ 10ꢀ5 M)
b
in dry DMSO.
Colorimetric study of cations
Further, the receptor R1 was evaluated for its binding ability
towards cations in ACN. A colorimetric detection experiment
was carried out to check the selectivity of the Cu2+ ion over
other cations such as Mg2+, Ca2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+
and Pb2+ (dissolved in water) in the form of their nitrates. The
receptor R1 was able to bind selectively to Cu2+ ions, which
resulted in a remarkable color change from pale yellow to
orange-red (Fig. 8), whereas for other cations either yellow color
was disappeared or no color change was observed.
Fig. 7 UV-vis titration spectra of R1 (5 ꢁ 10ꢀ5 M) with the increasing
concentration of TBAF (0–25 equiv.) in ACN. Inset: Job’s plot for R1 with Fꢀ
ion at 511 nm in dry ACN.
The selectivity of receptor R1 towards Fꢀ ions in ACN follows
same trend as in DMSO solution. When the ACN solution of
receptor R1 (5 ꢁ 10ꢀ5 M) was treated with TBA salts of different
anions (1 equiv.), only the addition of Fꢀ ions induced an
instantaneous color change from pale yellow to pink (Fig. S8,
ESI†), whereas the other anions did not show any color change.
The UV-vis titration was carried out for receptor R1 in dry
ACN solution. Upon increasing the concentration of Fꢀ ions,
the peak at 284 nm increased slightly due to the change in the
p - p* transition. On the other hand, the peak at 375 nm
decreased constantly and a new bathochromic band centred at
511 nm was developed, thus forming two clear isosbestic points
at 350 nm and 400 nm (Fig. 7). This new peak that developed at
511 nm was ascribed to the formation of ICT transition. The
gradual addition of Fꢀ ions to receptor R2 (5 ꢁ 10ꢀ5 M) in ACN
resulted in the bathochromic shift of absorption band from
371 to 393 nm with a broadened band centred at 500 nm, along
with the formation of two isosbestic points at 299 nm and
385 nm (Fig. S20, ESI†). In contrast, the molecule R3, which
lacks the hydroxyl (–OH) functional group, showed neither any
color change (Fig. S11 and S12, ESI†) nor any shift in absorption
maxima even after adding 20 equiv. of Fꢀ ions (Fig. S21 and S22,
ESI†) in DMSO as well as in ACN. This clearly indicates that the
phenolic –OH is the binding site for the Fꢀ ion and hence
responsible for colorimetric detection.
The colorimetric selectivity of receptor R1 towards Cu2+ ions
in the presence of other cations was studied by adding 3 equiv.
of Cu2+ ions to the receptor R1 (5 ꢁ 10ꢀ5 M) in the presence of
other cations (3 equiv.), such as Mg2+, Ca2+, Co2+, Ni2+, Zn2+,
Cd2+, Hg2+ and Pb2+. The presence of other cations did not have any
effect on the colorimetric detection of the Cu2+ ions (Fig. S16, ESI†).
Accordingly, it is clear that receptor R1 prefers only Cu2+ ions for
colorimetric detection over other cations.
The complexation mechanism and the spectral changes
associated with the detection process were studied by means of a
UV-vis titration experiment of R1 with Cu2+. The UV-vis titration of
receptor R1 (5 ꢁ 10ꢀ5 M) in ACN was carried out by gradually
increasing the addition of Cu2+ ions in the form of Cu(NO3)2.
Fig. 9 depicts the UV-vis titration curve of R1 with increasing
concentrations of Cu2+ ions. Upon increasing the addition of
Cu2+ ions, the characteristic peak at 375 nm, which corresponds
to phenolic –OH, decreased gradually. This observation confirms
the participation of phenolic –OH in the complex formation.
Along with decrease in the absorption band at 375 nm, a new
band centred at 427 nm was observed, which was due to
establishment of a charge transfer complex between receptor
R1 and the Cu2+ ion. The peak at 284 nm, which corresponds
to imine (–CQN–), showed a hypsochromic shift to 261 nm.
Development of new bathochromic and hypsochromic bands
indicates the formation of a new CT stabilized Cu2+ complex
with the receptor R1.
The shift in absorption maxima after addition of Fꢀ ions to
the receptor solutions in dry ACN and dry DMSO are summarised
in Table 2.
The 170 nm shift in DMSO compared with 136 nm in ACN
for receptor R1 confirms the stronger binding of the Fꢀ ion
in DMSO due to the establishment of a more stable ICT
transition. The binding constant was calculated using the
Benesi–Hildebrand equation24 and was found to be 7.15 ꢃ
0.31 ꢁ 106 Mꢀ2 for DMSO and 2.13 ꢃ 0.13 ꢁ 106 Mꢀ2 for ACN.
This indicates the binding of the Fꢀ ion to the receptor is much
stronger in the case of DMSO than ACN.
Fig. 8 Change in color after addition of 3 equiv. of different cations to the
receptor solution in ACN (5 ꢁ 10ꢀ5 M). (a) Receptor R1, (b) Mg2+, (c) Ca2+
,
(d) Co2+, (e) Ni2+, (f) Cu2+, (g) Zn2+, (h) Cd2+, (i) Hg2+ and (j) Pb2+
.
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Fig. 10 (i) Reversal of the UV-vis spectral pattern of receptor R1 + 2 equiv.
of Fꢀ upon addition of Cu2+ ions. (ii) Color changes of a 5 ꢁ 10ꢀ5 M ACN
solution of receptor R1. (a) Receptor R1, (b) R1 + 2 equiv. of Fꢀ and (c) R1 +
Fig. 9 UV-vis titration spectra of R1 (5 ꢁ 10ꢀ5 M) with increasing concen-
tration of Cu2+ ions (0–25 equiv.) in ACN. Inset: Job’s plot for R1 with Cu2+
ions at 427 nm.
2 equiv. of Fꢀ + 1 equiv. Cu2+
.
of Cu2+ ions to this R1–Fꢀ ion complex resulted in the disap-
pearance of the pink color and the original pale yellow color of
receptor R1 was restored (Fig. 10ii). This revertion of the color
was confirmed by UV-vis spectroscopy where the band at
511 nm (due to ICT) disappeared and the band at 375 nm
corresponding to receptor R1 was restored (Fig. 10i). This
property of receptor R1 was further applied for the molecular
logic gate and ‘ON–OFF’ switching operation.
The system is said to be ‘switched ON’ when receptor R1 shows
an optical output from pale yellow to pink in the presence of Fꢀ
ions and system is said to be ‘switched OFF’ when Cu2+ ions were
added to the same solution and the pale yellow color of the receptor
R1 reappears. These observations can be correlated to demonstrate
a logic gate operation, where the output signals (change in color)
could be controlled by input signals (addition of Fꢀ ions and Cu2+
ions). The receptor R1 showed no absorbance at 511 nm, which is
accounted for by the Boolean value of ‘‘0’’ and with the addition of
Fꢀ ions it showed an absorbance of 0.13, which is accounted for by
the Boolean value of ‘‘1’’. The process of ‘switch ON’ and ‘switch
OFF’ represented in the molecular logic gate is shown through the
truth table as shown in Table 3.
Scheme 3 Schematic representation of complexation process of Cu2+
with receptor R1.
The stoichiometry of the R1–Cu2+ system was determined
using a Job’s plot where the absorbance at 427 nm was plotted
against mole fraction of Cu2+, which showed a maximum
absorbance when the mole fraction was 0.33 (Fig. 9, inset).
This confirms the formation of a 2 : 1 (host/guest) stoichio-
metric complex and it could be represented as shown in
Scheme 3.25
In contrast, for other metal ions the extent of CT was
diminished, which resulted in the disappearance of the pale yellow
color of the receptor.26 In the case of R2, the –OH group present at
the p-position of phenyl ring was unable to form any complex with
Cu2+. Therefore no color change was observed in the case of R2
upon addition of Cu2+ ions. However, the pale yellow color of R2
disappeared (Fig. S13, ESI†) upon addition of Cu2+ due to dimin-
ished D–A interactions. This result was confirmed by UV-vis spectro-
scopy where the peak at 371 nm corresponding to the phenolic –OH
disappeared (Fig. S23, ESI†) upon addition of Cu2+ ions.
Output signals in the form of spectral change at 511 nm
were observed upon supplying input signals in the form of Fꢀ
ions and Cu2+ ions represents an INHIBIT (INH) logic gate.27
The outputs of the receptor R1 are in agreement with comple-
mentary INH logic function (Fig. 11).
Table 3 Truth table for INHIBIT (INH) logic function. Inputs: addition of 2
equiv. Fꢀ ions and 1 equiv. Cu2+ ions to a 5 ꢁ 10ꢀ5 M ACN solution of R1.
Outputs: absorbance at 511 nm (high absorbance: 1; low absorbance: 0)
Logic gate applications
Input A
Input B
Output
(absorbance at 511 nm)
(Fꢀ ion addition)
(Cu2+ ion addition)
As stated previously, the receptor R1 changed color from pale
yellow to pink upon addition of Fꢀ ions (2 equiv.) in dry ACN
and as a result a new band cantered at 511 nm with an
absorbance of 0.13 was observed. Surprisingly, adding 1 equiv.
0
1
0
1
0
0
1
1
0
1
0
0
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Fig. 11 A schematic representation of a complementary output INH
circuit.
Scheme 5 Syntheses of receptors R1, R2 and R3.
were procured from SD Fine, India, and were of HPLC grade
and used without further distillation.
1
The H NMR spectra were recorded on a Bruker Avance II
(500 MHz) instrument using TMS as internal reference and
DMSO-d6 as solvent. Resonance multiplicities are described as s
(singlet), br s (broad singlet), d (doublet), t (triplet), q (quartet)
and m (multiplet). Melting points were measured on a Stuart-
SMP3 melting-point apparatus in open capillaries. Infrared
spectra were recorded on a Thermo Nicolet Avatar-330 FT-IR
spectrometer; signal designations: s (strong), m (medium), w
(weak), br m (broad medium) and br w (broad weak). UV-vis
spectroscopy was carried out with an Ocean Optics SD2000-
Fibre Optics Spectrometer in standard 3.5 mL quartz cells
(2 optical windows) with 10 mm path length. Elemental analyses
were done using a Flash EA1112 CHNS analyzer (Thermo
Electron Corporation).
Scheme 4 Reversable process of receptor R1 upon addition of Cu2+ to
the R1 + Fꢀ in ACN solution.
Similar color change was observed upon the addition of
other metal ions such as Co2+, Ni2+, Zn2+, Cd2+ and Hg2+ (Mg2+,
Ca2+and Pb2+ salts were not soluble in organic solvents like
ACN) instead of Cu2+. This confirms formation of stable MF2
salts (metal–fluoride salt). Thus, receptor R1 becomes free after
the addition of 1 equiv. of M2+ ions to the R1 + 2 equiv. Fꢀ
solution as demonstrated in Scheme 4.17e
However, the addition of 10 equiv. of Fꢀ ion to the R1–Cu2+
complex did not show any color change. This confirms the
stable complex formation between receptor R1 and Cu2+ ion.
General synthetic procedure for the target compounds R1, R2
and R3 (Scheme 5)
A
mixture of 4-nitrobenzaldehyde (1.32 mmol) and 1
(2.69 mmol) was reacted in ethanol under reflux for 5 h. The
reaction was catalyzed by a drop of acetic acid. The solid
product was obtained after cooling and was filtered and washed
with ethanol to obtain the target compounds (R1, R2 and R3).
All the synthesized compounds were solid and characterized
Conclusion
To conclude, receptor R1 was designed and synthesised for the
colorimetric dual ion sensing application. The receptor R1 was able
to distinguish Fꢀ ions colorimetrically over other anions. This
receptor showed this unique solvatochromic property only in the
presence of Fꢀ ions as it establishes charge separation in the
receptor. The receptor displayed different colorations in different
solvents upon adding Fꢀ ions. This colorimetric discrimination
in different solvents was successfully applied to determine the
percentage composition of binary solvent mixtures. The receptor R1
colorimetrically discriminated Cu2+ ions over other cations by
showing a color change from pale yellow to orange-red. It acts as
a molecular switch which is said to be ‘switched ON’ in presence of
Fꢀ ions and ‘switched OFF’ when Cu2+ ions were added. This
switch ‘ON–OFF’ process was demonstrated by logic gate functions.
The receptor R1 gave output signals corresponding to the INH
circuit with input signals which can be implemented to molecular
computing operations and molecular devices.
1
using H NMR, IR, elemental analysis and mass spectrometry.
(E)-2-(4-Nitrobenzylideneamino)phenol, R1. Yield: 79%. Ele-
mental analysis: calculated for C13H10N2O3 (%): C 64.46, H 4.16,
1
N 11.56; experimental (%): C 64.26, H 4.30, N 11.60. H NMR
(500 MHz, DMSO-d6): d 9.27 (s, 1H, –OH), d 8.88 (s, 1H,QCH), d
8.34 (d, 2H, Ar-H, J = 8 Hz), d 8.30 (d, 2H, Ar-H, J = 8 Hz), d 7.30
(d, 1H, Ar-H, J = 7.5 Hz), d 7.16 (t, 1H, Ar-H, J = 7.25 Hz), d 6.95
(d, 1H, Ar-H, J = 8 Hz), d 6.86 (t, 1H, Ar-H, J = 7 Hz). 13C NMR
(100 MHz, DMSO-d6): d 157.45, d 152.23, d 149.11, d 142.48, d
137.40, d 130.24, d 129.01, d 124.30, d 119.98, d 119.82, d 116.85.
m.p. 165–166 1C. FT-IR in cmꢀ1: 3367.3 (m), 1586.2 (m), 1508.8
(m), 1331.7 (s), 1230.2 (m), 1096.4 (w), 1027.5 (w), 779.9 (m). MS
(ESI): m/z: calculated: 243.2375 [M
+
H]+; experimental:
243.2590 [M + H]+.
(E)-4-(4-Nitrobenzylideneamino)phenol, R2. Yield: 80%.
Elemental analysis: calculated for C13H10N2O3 (%): C 64.46, H
4.16, N 11.56, experimental (%): C 64.32, H 4.28, N 11.48.
1H NMR (500 MHz, DMSO-d6): d 9.72 (s, 1H, –OH), d 8.77
(s, 1H,QCH), d 8.32 (d, 2H, Ar-H, J = 8 Hz), d 8.12 (d, 2H, Ar-H,
Experimental section
General information
All chemicals were purchased from Sigma-Aldrich, Alfa Aesar or J = 8.5 Hz), d 7.32 (d, 2H, Ar-H, J = 8.5 Hz), d 6.85 (d, 2H, Ar-H,
Spectrochem and used without further purification. All solvents J = 8 Hz). 13C NMR (100 MHz, DMSO-d6): d 157.79, d 155.11,
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1500.6 (s), 1328.4 (s), 1252.5 (m), 1197.9 (m), 1195.7 (w).
(E)-N-(4-Nitrobenzylidene)benzenamine, R3. Yield: 63%.
Elemental analysis: calculated for C13H10N2O2 (%): C 69.02, H
4.46, N 12.38; experimental (%): C 68.98, H 4.50, N 12.39.
1H NMR (500 MHz, DMSO-d6): d 8.79 (s, 1H, QCH), d 8.35
(d, 2H, Ar-H, J = 8.5 Hz), d 8.19 (d, 2H, Ar-H, J = 8.5 Hz), d 7.47
(t, 2H, Ar-H, J = 7.5 Hz), d 7.29–d 7.35 (m, 3H, Ar-H). 13C NMR
(100 MHz, DMSO-d6): d 159.27, d 151.02, d 149.27, d 141.95,
d 130.08, d 129.76, d 127.38, d 124.44, d 121.71. m.p. 94–95 1C.
FT-IR in cmꢀ1: 3063.5 (w), 1588.1 (m), 1510.3 (s), 1330.6 (s).
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Acknowledgements
MP, SN and DRT acknowledge the Director and the HOD
(Department of Chemistry), NITK, for providing research infrastruc-
ture. DRT acknowledges the Department of Science and Technology
(DST, Government of India, New Delhi) for financial support. MP is
thankful to NITK for the research fellowship. MP and SN thank the
Optoelectronics Laboratory, Department of Physics, NITK for spec-
tral analysis and Prof. P. Srinivas, HOD, PPSFT Department, CFTRI,
Mysore for his kind help in NMR analysis.
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