Synthesis and spectral investigation of colorimetric receptors for the dual detection of copper and acetate ions: application in molecular logic gates

Article abstract of DOI:10.1080/10610278.2017.1298764

Colorimetric receptors L1, L2 and L3 possessing –OH functionality as binding site and –NO₂ as signalling unit with varied positional substitution of –OMe functionality has been designed and synthesised by simple Schiff base condensation reaction. Receptors L1, L2 and L3 showed selective response towards AcO^?ion among other interfering monovalent anions viz., F^?, Cl^?, Br^?, I^?, NO₃^?, HSO^?₄ and H₂PO^?₄. L2 imparts high selectivity towards AcO^? ion assisted by push-pull effect of –NO₂ and –OMe functionality in conjugation with imine linkage. The binding constant for L2–OAc complex was found to be 9.04?×?10⁴?M^?2. L2 exhibited a detection limit of 0.61?ppm towards NaOAc. The selectivity of L1 towards Cu²⁺ ions with a lower detection limit of 1.81?ppm implies the role of electron donating –OMe in favouring the coordinative interaction of hetero atoms of L1 with Cu²⁺ ions. The selective detection of AcO^? and Cu²⁺ ions is further established with OR and INHIBIT logic gate application of receptors L1, L2 and L3 correspondingly. The colorimetric studies reveal the role of OMe functionality in the selective detection of AcO^? and Cu ²⁺ ions in aqueous media.

Full text of DOI:10.1080/10610278.2017.1298764

Supramolecular Chemistry
ISSN: 1061-0278 (Print) 1029-0478 (Online) Journal homepage: http://www.tandfonline.com/loi/gsch20
Synthesis and spectral investigation of
colorimetric receptors for the dual detection of
copper and acetate ions: application in molecular
logic gates
Srikala Pangannaya, Arshiya Kaur, Makesh Mohan, Keyur Raval, Dillip
Kumar Chand & Darshak R. Trivedi
To cite this article: Srikala Pangannaya, Arshiya Kaur, Makesh Mohan, Keyur Raval, Dillip
Kumar Chand & Darshak R. Trivedi (2017) Synthesis and spectral investigation of colorimetric
receptors for the dual detection of copper and acetate ions: application in molecular logic gates,
Supramolecular Chemistry, 29:8, 561-574, DOI: 10.1080/10610278.2017.1298764
To link to this article: http://dx.doi.org/10.1080/10610278.2017.1298764
Published online: 23 Mar 2017.
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Date: 27 March 2017, At: 02:27

Supramolecular chemiStry, 2017
Vol. 29, No. 8, 561–574
http://dx.doi.org/10.1080/10610278.2017.1298764
Synthesis and spectral investigation of colorimetric receptors for the dual
detection of copper and acetate ions: application in molecular logic gates
Srikala Pangannayaa, Arshiya Kaurb, Makesh Mohanc, Keyur Ravald, Dillip Kumar Chandb and Darshak R. Trivedia
b
aSupramolecular chemistry laboratory, National institute of technology Karnataka (NitK), Surathkal, india; Department of chemistry, indian
c
institute of technology madras (iitm), chennai, india; Department of physics, National institute of technology Karnataka (NitK), Surathkal,
india; dDepartment of chemical engineering, National institute of technology Karnataka (NitK), Surathkal, india
ABSTRACT
ARTICLE HISTORY
received 20 october 2016
accepted 19 February 2017
Colorimetric receptors L1, L2 and L3 possessing –OH functionality as binding site and –NO2 as
signalling unit with varied positional substitution of –OMe functionality has been designed and
synthesised by simple Schiff base condensation reaction. Receptors L1, L2 and L3 showed selective
KEYWORDS
−
response towards AcO−ion among other interfering monovalent anions viz., F−, Cl−, Br−, I−, NO3
,
Schiff base; substitution
effect; colorimetric; dual
sensor; logic gate
HSO−4 and H2PO4−. L2 imparts high selectivity towards AcO− ion assisted by push-pull effect of –
NO2 and –OMe functionality in conjugation with imine linkage. The binding constant for L2–OAc
complex was found to be 9.04 × 104 M−2. L2 exhibited a detection limit of 0.61 ppm towards NaOAc.
The selectivity of L1 towards Cu2+ ions with a lower detection limit of 1.81 ppm implies the role of
electron donating –OMe in favouring the coordinative interaction of hetero atoms of L1 with Cu2+
ions. The selective detection of AcO− and Cu2+ ions is further established with OR and INHIBIT logic
gate application of receptors L1, L2 and L3 correspondingly. The colorimetric studies reveal the role
of OMe functionality in the selective detection of AcO− and Cu 2+ ions in aqueous media.
1. Introduction
Supramolecular chemistry of host–guest interaction is a
vibrant area of research owing to its applicability in bio-
chemistry and environmental science (1) .The design and
synthesis of new artificial receptors have seen progressive
path unveiling their utility in the detection of physiolog-
ically important anions and cations (2). Acetate ion has
been one of the intriguing targets due to its significance
in metabolic processes. The rate of acetate production and
sedimentation has been an indicator of organic decompo-
sition in marine sediments. Moreover, utility of acetate ion
is realised in the manufacture of paper, plastics, dyes and
paints (3). Correspondingly, past few years have witnessed
a number of receptors for the detection of biologically
and environmentally relevant transition metal cations.
Copper being the third most abundant essential trace
element is required in optimum amount for regulation
CONTACT Darshak r. trivedi
darshakrtrivedi@nitk.ac.in
Supplemental data for this paper is available online at http://dx.doi.org/10.1080/10610278.2017.1298764
© 2017 informa uK limited, trading as taylor & Francis Group

562
S. PANGANNAYA ET AL.
of fundamental physiological process (4). Copper ion is
known to promote bone development, cellular respiration,
nerve function regulation and haemoglobin synthesis (5).
Numerous technologies such as atomic absorption spec-
troscopy, X-ray photoelectron spectroscopy, voltammetry,
inductively coupled plasma emission or mass spectrometry
have been developed for the detection of trace amount of
cations and anions (6). In reality, sophisticated instrumenta-
tion, time factor and cost are the reasons that strictly limit the
usage of these techniques on the real time basis (7). Thus far,
numerous colorimetric receptors have been developed by
researchers by virtue of the detection of anions and cations
at ease without resorting to sophisticated instrumentation
(8–14). In this direction, numerous reports are available in
the literature wherein researchers have developed artificial
receptors which exhibit excellent anion binding property
both in solution and solid states (15–21). Simple urea-based
cleft-type receptors have been designed and synthesised by
researchers involving structural manipulation of receptors
appended to a signalling unit which drives selective detec-
tion of anions (22). Most of the receptors reported so far
for the detection of acetate ions exhibit interference from
fluoride and phosphate ions (23–25). Selective detection
of anions has been one of the thrust area of research in
anion receptor chemistry and development of new design
strategies involving effective binding sites and signalling
unit is gaining prime importance.
In the last few decades, the colorimetric detection of
anions and cations based on molecular devices has been
a subject of extensive research interest (26). The integra-
tion of molecular logic gates into working automation and
arithmetic systems has revealed the clear idea of a molec-
ular scale calculator (27–29). Molecular logic gates involve
chemical inputs (30) and generate output signals (31) that
operate in a wireless mode. This highlights the advantages
and potential of logic gate devices in computation on a
nanometre scale over the silicon-based devices eventu-
ally leading to the possibility to load an array of Boolean
functions onto a single molecule.
used as received without further purification. All the sol-
vents were purchased from SD Fine, India, were of HPLC
grade and used without further distillation.
Melting point was measured on Stuart SMP3 melt-
ing-point apparatus in open capillaries. Infrared spectra
were recorded on Bruker FTIR spectrometer. UV/Vis spec-
troscopy was performed with Thermo Scientific Genysys
10S spectrophotometer in standard 3.0 mL quartz cell with
1 cm path length. The 1H NMR spectra were recorded on
Bruker Ascend (400 MHz) instrument using TMS as internal
reference and DMSO-d6 as solvent. Resonance multiplici-
ties are described as s (singlet), d (doublet), t (triplet) and
m (multiplet).
2.2. Synthesis of L1
To 197.44 mg (1.29 mmol) of the 2-hydroxy-5-methoxyben-
zaldehyde, 200 mg (1.29 mmol) of 2-amino-5-nitrophenol
was added. To this mixture, approximately 10 mL of meth-
anol and a drop of glacial acetic acid (catalyst) were added
and the reaction mixture was sonicated at room temper-
ature for 5 min. The beaker was covered with aluminium
foil with pinholes for slow evaporation of solvent. The
solid product was recrystallised in methanol in 75% yield.
Melting point: 192 °C
1H NMR (DMSO- d6, 400 MHz, ppm): δ 12.35 (OH, s),
10.25 (OH, s), 8.98 (HC=N, s), 7.79–7.77 (Ar CH, d), 7.49–
7.29 (Ar CH, s), 6.94–6.91 (Ar CH, d), 6.62–6.61 (Ar CH, d),
6.20(Ar CH, s), 3.76 (OMe, s). 13C NMR (DMSO-d6, 100 MHz,
ppm): 56.05, 109.10, 111.43, 115.42, 118.5, 119.8, 121.02,
121.95, 142.92, 146.16, 151.57, 152.34, 155.24, 164.72.
FT-IR (cm−1): 3372 (OH), 2998 (Ar C-H), 1607 (CH=N), 1506
(aromatic C=C), 1231 (C–O, stretching). Mass: calculated:
288.07, obtained: 289.08 (M + H)+.
2.3. Synthesis of L2
To 197.44 mg (1.29 mmol) of the 2-hydroxy-4-methoxyben-
zaldehyde, 200 mg (1.29 mmol) of 2-amino-5-nitrophenol
was added. To this mixture approximately 10 mL of metha-
nol and a drop of glacial acetic acid (catalyst) were added
and the reaction mixture was sonicated at room temper-
ature for 5 min. The beaker was covered with aluminium
foil with pinholes for slow evaporation of solvent. The
solid product was recrystallised in methanol in 75% yield.
Melting point: 195 °C
1H NMR (DMSO- d6, 400 MHz, ppm): δ 13.78 (OH, s),
10.73 (OH, s), 8.94 (HC=N, s), 7.79 (Ar CH, s), 7.77 (Ar CH, s),
7.56 (Ar CH, s), 7.53 (Ar CH, s), 6.56 (Ar CH, s), 6.47 (Ar CH, s),
6.46 (Ar CH, s), 3.82 (OMe, m). 13C NMR (DMSO-d6, 100 MHz,
ppm): 56.03, 101.45, 107.82, 111.13, 113.56, 115.70, 120.30,
134.91, 141.81, 145.78, 151.20, 163.45, 165.06, 165.53.
FT-IR (cm−1): 3389 (OH), 3082 (Ar C-H), 1585 (CH=N), 1521
In this regard, finding the utility of both Cu2+ and AcO−
ions, we report three new isomeric receptors with varied
positional substitution of –OMe functionality following
binding site-signalling unit approach dedicated to bind
both anions and cations. Furthermore, the dual ion sensing
property could be an impetus to study molecular logic
gate applications.
2. Experimental section
2.1. Materials and methods
All the chemicals used in the present study were procured
from Sigma-Aldrich, Alfa Aesar or Spectrochem and were

SUPRAMOLECULAR CHEMISTRY
563
(aromatic C = C), 1205 (C–O, stretching). Mass: calculated:
288.07, obtained: 289.08 (M + H)+.
such as FT-IR and 1H-NMR in order to confirm the structural
aspects.
2.4. Synthesis of L3
3.1. UV–vis spectrophotometric studies
To 197.44 mg (1.29 mmol) of the 2-hydroxy-3-methoxy-
benzaldehyde, 200 mg (1.29 mmol) of 2-amino-5-nitro-
phenol was added. To this mixture approximately 10 mL
of methanol and a drop of glacial acetic acid (catalyst) was
added and the reaction mixture was sonicated at room
temperature for 5 min. The beaker was covered with alu-
minium foil with pinholes for slow evaporation of solvent.
The solid product was recrystallised in methanol in 75%
yield. Melting point: 198 °C
1H NMR (DMSO- d6, 400 MHz, ppm): δ 13.26 (OH, s),
10.76 (OH, s), 9.03 (HC=N, s), 7.8–7.6 (Ar CH, d), 7.57 (Ar
CH, s), 7.17–6.94 (Ar CH, s), 6.92 (Ar CH, s), 6.90 (Ar CH, s),
3.83 (OMe, m). 13C NMR (DMSO-d6, 100 MHz, ppm): 56.37,
111.23, 115.57, 116.48, 119.02, 119.70, 120.97, 120.97,
124.41, 142.12, 146.33, 148.61, 151.93, 151.54, 165.30.
FT-IR (cm−1): 3438 (OH), 2830 (Ar C-H), 1631 (CH=N), 1528
(aromatic C=C), 1256 (C–O, stretching). Mass: calculated:
288.07, obtained: 289.08 (M + H)+.
Receptors L1, L2 and L3 by virtue of varied positional sub-
stitution of –OMe functionality at para, meta and ortho
position to the –OH substitution is expected to exhibit sig-
nificant colorimetric response in the presence of anions.
The presence of –NO2 group as signalling unit is known to
produce a colour change visible to the naked eye in the
anion binding process.
The anion binding properties of receptors L1, L2 and L3
were investigated by UV–vis spectroscopic studies. In the
colorimetric analysis, 1 equiv. of 10−2 M solution of tetrabu-
tylammonium (TBA) salts of anions such as F−, Cl−, Br−, I−,
−
−
−
NO3 , HSO4 H2PO4 and AcO− (10−2 M DMSO) were added
to 2 mL of 10−4 M solutions of L1, L2 and L3 in acetoni-
trile. In the visual investigation, the pale yellow solution of
receptor L1 turned deep orange on addition of 1 equiv. of
AcO –. Receptor L2 and L3 exhibited colour change from
yellow to deep red and bright pink on addition of 1equiv.
of AcO –.ion. Mild colour change was observed with the
addition of F– ions indicating weak interaction of F– with
receptors L1, L2 and L3. In contrast, no obvious colour
changes were observed upon addition of other anions.
In order to confirm the observed colour changes, UV–vis
spectra has been recorded with the addition of 1 eq. of ani-
ons to 2 ml of ACN solution of receptors L1, L2 and L3. The
colour change is represented in Figures 1–3.The unbound
receptor L1 and L2 displayed absorption maxima centred
at ~ 381 nm whereas L3 exhibited two absorption bands
at 304 nm and 367 nm. The higher energy band at 304 nm
3. Results and discussion
Receptors L1, L2 and L3 have been synthesised in good
yield by simple Schiff base condensation reaction of
methoxy substituted salicylaldehyde with 2-amino,5-ni-
trophenol using acetic acid as catalyst and methanol as
solvent (Scheme 1). TLC has been performed to confirm the
product formation and the receptors L1, L2 and L3 have
been characterised by standard spectroscopic techniques
Scheme 1. Synthesis of L1, L2 and L3.

564
S. PANGANNAYA ET AL.
Figure 1. colour change of L1 in acN (10−4 m) before and after addition of 1 equiv. of anions.
Figure 2. colour change of L2 in acN (10−4 m) before and after addition of 1 equiv. of anions.
Figure 3. colour change of L3 in acN (10−4 m) before and after addition of 1 equiv. of anions.
To gain further insight into the anion binding inter-
action with the receptor, UV–vis spectrophotometric
titration studies has been performed. On successive addi-
tion of 0.1 eq. of TBAOAc (10−2 M in DMSO) to receptor
L1 (10−4 M in ACN), the band at 381 nm decreased in its
intensity with the appearance of a new band at 496 nm as
shown in Figure 4. The red shift observed corresponds to
the bifurcated hydrogen bond interaction of AcO−.ion with
the two OH groups present on the receptor. Yet, the posi-
tion of OMe being para to one of the OH moiety indicates
the weaker acidity of –OH proton and relative stronger
bond exiting between oxygen and hydrogen. The possi-
bility of intramolecular hydrogen bonding of second –OH
functionality with imine nitrogen is relatively weaker as
evidenced in the 1H-NMR of unbound receptor L1. The ten-
dency of OH proton to bind AcO−.ion is further enhanced
owing to the greater basicity of the anion. Two isosbestic
points at 425 and 340 nm were observed during the titra-
tion, indicating that the presence of two reacting species
in the anion binding process. Benesi Hildebrand (B–H) plot
indicates the 1:1 complexation for L1-acetate complex as
indicated in Figure S10 (Supporting information). The
binding ratio could be correlated to the ability of AcO−
ion to bind and deprotonate the one of OH functionality
forming CH3COOH by counteracting the intramolecular
hydrogen bonding interactions within the receptor. The
possibility of CH3COOH to abstract proton from second
–OH functionality generates ionic species [CH3COOH2]+.
Figure 4. uV–vis absorption spectra of L1 in acN (10−4 m) upon
titration with tetrabutylammonium acetate (10−2 m in DmSo).
Note: inset plot represents the binding isotherm at 496 nm.
corresponds to the π–π* transitions of azomethine group
and lower energy band at 381 nm and 367 nm in L2 and
L3 represent the overall intramolecular charge transfer
transitions within the receptor molecule.(32) Appearance
of red shift band upon addition of 1 eq. of AcO−.ion to
receptors L1, L2 and L3 at 496 nm, 518 nm and 520 nm
correspondingly indicate the formation of receptor–anion
complex. The UV–vis spectra are represented in Figure S7,
S8 and S9 (Supporting information).

SUPRAMOLECULAR CHEMISTRY
565
Figure 6. uV–vis absorption spectra of L3 in acN (10−4 m) upon
titration with tetrabutylammonium acetate (10−2 m in DmSo).
Note: inset plot represents the binding isotherm at 520 nm.
Figure 5. uV–vis absorption spectra of L2 in acN (10−4 m) upon
titration with tetrabutylammonium acetate (10−2 m in DmSo).
Note: inset plot represents the binding isotherm at 518 nm.
UV–vis spectra of Receptor L3 exhibited significant col
-
The above mechanism could be an evidence for the hydro-
gen bonding ability of both –OH functionality.
orimetric response with the incremental addition of 0.1 eq
of AcO−.ion. Unlike L1 and L2, the receptor L3 exhibited
a varied colour change with a decrease in the bands cen-
tred at 304 and 367 nm with the emergence of new red
shift band at 520 nm and a blue shift band at 350 nm as
shown in Figure 6. The presence of –OMe at meta position
relative to the imine linkage and at ortho position to the
OH functionality of salicylaldimine represents a unique
intramolecular hydrogen bond preferences of the –OH
functionalities. The competitive nature of OMe and imine
nitrogen to involve in hydrogen bond interaction with the
OH functionality leads to downfield shift of –OH proton to
δ 13.26 ppm. The relative decrease in the downfield shift of
OH proton of L3 in comparison with L2 indicates relative
weak nature of intramolecular hydrogen bond interaction
facilitating strong binding of anions. As a consequence,
NO2 functionality imparts strong influence on the OH
linked to imine functionality leading to deprotonation of
receptor with CH3COOH and [L3]− as end products. Thus
formed CH3COOH can further abstract another proton
Receptor L2 (10−4 M in ACN) showed remarkable colori-
metric response with the incremental addition of 0.1 eq. of
AcO−.ion. Successive decrease in the original band centred
at 381 nm with the growth of new red shift band at 518 nm
is indicative of the strong anion-receptor interaction as
shown in Figure 5. The presence of electron donating –
OMe functionality para to the imine linkage, allows the
–OH functionality to involve in intramolecular hydrogen
bond interaction. The presence of intramolecular hydro-
gen bond is confirmed from the 1HNMR spectrum which
exhibits signal for –OH proton at δ 13.78 ppm (downfield).
The presence of electron withdrawing –NO2 functionality
imparts the formation of strong intramolecular hydrogen
bond of –OH of nitrophenyl group with the imine func-
tionality. The conformational change of the receptor L2 in
the presence of AcO−.ions is more preferred in the anion
binding followed by deprotonation of receptor L2. The
preorganisation of receptor allows bifurcated hydrogen
bond interaction between AcO− and two OH functional-
ities of L2.
+
from hydroxyl functionality generating [CH3COOH2 ] and
[L3]2− as final products. Four isosbestic points are observed
at 415, 343, 313 and 284 nm indicating the involvement of
interacting species in a complex equilibrium process (33)
The B–H plot indicates the coexistence of two species with
1:1 and 1:2 binding ratio of L3- AcO− ion complex as rep-
resented in Figure S13 and S14 (Supporting information).
Anion recognition has overwhelming applications
in the medical, environmental, biological and industrial
arenas which typically require aqueous media inorder to
mimick the physiological conditions. The design of selec-
tive anion receptors that function in aqueous solution has
been a key challenge to supramolecular chemists. In this
The stoichiometry of L2- AcO− complex was deter-
mined by B–H plot. The linear plot obtained with the first
power and second power of concentration of AcO− ion
clearly revealed the existence of at least two species of
complexes (1:1 and 1:2 for L2:AcO−. (33) (Figure S11 and
S12 (Supporting information)). This could be the proba-
ble reason for the appearance of four isobestic points in
the titration spectra. Binding ratio indicates the formation
+
of dimer, [(CH3COO)H2 ] and [L2]2−. The presence of four
isosbestic points observed at 424, 345, 314 and 286 nm
indicates the presence of complex equilibrium process in
the formation of L2- AcO− ion complex.

566
S. PANGANNAYA ET AL.
regard, we have aimed to check the binding ability of L1,
L2 and L3 towards sodium salt of AcO− ion in aqueous
media. It was observed that addition of 1 equiv. of 10−2
M
solution of NaOAc (10−2 M in H2O) changed the pale yel-
low colour of the receptor L1 to orange as represented
in Figure 7. L2 exhibited significant colour change in the
presence of 1 equiv. of 10−2 M solution of NaOAc (10−2
M
Figure 7. colour change of L1 in acetonitrile (10−4 m) before and
after addition of 1 equiv. of NaF and Naoac.
in H2O) from pale yellow to red as shown in Figure 8. L3
displayed a colour change from pale yellow to bright pink
implying the efficiency of the receptor in combating the
solvent interferences as given in Figure 9.
The titration of L1 with incremental addition of 0.1 eq.
of NaOAc resulted in red shift of the original absorption
band to 490 nm with isobestic point at 429 nm as seen
in Figure S15(Supporting information). B–H plot revealed
the 1:1 binding ratio of L1- AcO− complex as shown in
Figure S16 (Supporting information). Titration studies per-
formed with incremental addition of 0.1 eq. of NaOAc to
L2 indicated red shift of the band to 512 nm with isobes-
tic points at 426, 346, 313 and 284 nm as given in Figure
S17(Supporting information). B–H plot indicated the for-
mation of 1:1 and 1:2 complex in the binding process as
represented in Figure S18 and S19 (Supporting informa-
tion). Titration studies with the successive addition of AcO−
ion to solution of L3 indicated shift in absorption band to
519 nm with isobestic points at 415, 343, 314 and 271 nm
reflective of the complex formation. Titration profile is
shown in Figure S20 (Supporting information). B–H plot
indicated the formation of 1:1 and 1:2 complex as shown
in Figure S21 and S22 (Supporting information). L1, L2
and L3 could be considered as synthetic anion receptor
that can operate effectively in water, surpassing solvent
interferences.
Figure 8. colour change of L2 in acetonitrile (10−4 m) before and
after addition of 1 equiv. of NaF and Naoac.
Figure 9. colour change of L3 in acetonitrile (10−4 m) before and
after addition of 1 equiv. of NaF and Naoac.
The presence of hetero atoms in the receptors L1, L2
and L3 has been the driving force to check the cation sens-
ing ability. Upon addition of 1 eq. of nitrate salt of cations
(Na+, K+,Ca2+,Mg2+, Al3+, Co2+,Ni2+, Cu2+, Zn2+, Cd2+, Hg2+
and Pb2+) to L1, L2 and L3, significant colour change was
observed in the presence of Cu2+ ion. The colour change
is shown in Figure S23, S24 and S25 (Supporting infor-
mation). In naked-eye inspection, the receptor L1 turns
from pale yellow to pale orange on addition of 1 eq. of
Cu2+ ion. L2 and L3 exhibited a colour change from pale
yellow to orange-yellow on addition of 1 eq. of Cu2+ ion.
In contrast, no significant changes were witnessed upon
addition of other cations. In order to confirm the observed
colour changes, UV–vis spectra have been recorded with
the addition of 1 eq. of cations to 2 ml of ACN solution
of receptors L1, L2 and L3. The UV–vis spectra are repre-
sented in Figures 10–12.
Figure 10. uV–vis absorption spectra of L1 in acN (10−4 m) upon
addition of 1 equiv. of nitrate salts of a variety of cations.
of receptor with Cu2+ ion in 1:1, 1:1 and 1:1 binding ratio,
respectively. Titration profile of L1, L2 and L3 with incre-
mental addition of Cu2+ ion is shown in Figure S26, S28
and S30 (Supporting information). The corresponding
The photophysical properties of L1, L2 and L3 have
been studied to confirm the binding process. The red shift
of band to 490, 477 and 467 nm indicates complexation

SUPRAMOLECULAR CHEMISTRY
567
Figure 13. uV–vis absorption spectra of L2 in acN (10−4 m) upon
titration with cu(oac)2 in h2o (10−2 m). inset plot represents
binding isotherm at 453 nm.
Figure 11. uV–vis absorption spectra of L2 in acN (10−4 m) upon
addition of 1 equiv. of nitrate salts of a variety of cations.
the receptor to bind metal acetates, in particular Cu(OAc)2.
With an aim to envisage the simultaneous sensing of AcO−
and Cu2+ ion, UV–vis titration studies has been performed
with the incremental addition of Cu(OAc)2 to receptors L1,
L2 and L3. The addition of Cu(OAc)2 to 10−4 M DMSO solu-
tion of L1, L2 and L3 resulted in visual colour change form
pale yellow to pale red, yellow and deep yellow, respec-
tively, as shown in Figure S32 (Supporting information).
Receptor L1 exhibited a red shift of original absorption
band to 502 nm which is approximately closer to the titri-
metric response observed for TBAOAc and Cu(NO3)2 indi-
vidually. The binding of Cu(OAc)2 by L1 was confirmed by
the presence of clear isosbestic points at 340 and 421 nm
as seen in Figure S33 (Supporting information). Receptor
L2 produced a new red shift band at 453 nm with the grad-
ual disappearance of band at 380 nm at the expense of
new blue shift band at 330 nm indicative of diminishing
intramolecular hydrogen bonding interactions within the
receptor. This is shown in Figure 13. The red shift band is
in good agreement with the titrimetric response observed
with Cu(NO3)2 alone nullifying the role of AcO− ion in the
binding process. The absence of well-defined isosbestic
points indicates the presence of significant side reactions.
Titration of Cu(OAc)2 with receptor L3 revealed a new red
shift band at 466 nm, being similar to the bare Cu(NO3)2
implies the strong binding of Cu2+ ion. The appearance of
clear isobestic points at 281, 318, 340 and 405 nm signifies
the complex equilibrium process involved in the binding
process as presented in Figure 14. B–H plot of L1, L2 and
L3 with Cu(OAc)2 exhibited stoichiometric ratio of 1:1 as
represented in Figure S34, S35 (Supporting information)
and Figure 15 and the proposed binding mechanism is
shown in Scheme 2.
Figure 12. uV–vis absorption spectra of L3 in acN (10−4 m) upon
addition of 1 equiv. of nitrate salts of a variety of cations.
B–H plot is shown in Figure S27, S29 and S31 (Supporting
information).
Mechanistically, maximum red shift in L1 can be attrib-
uted to the change in structural conformation of receptor
on interaction with Cu2+ indicating the coordinative inter-
action between the lone π pair of the donor hetero-atoms
OH and imine nitrogen which are conjugated to the π sys-
tem of the receptor and the cation. Consequently, there
is an increase in conjugation of the receptor in a tightly
bound complex with Cu2+, resulting in planarisation of
complex and red-shift (34). However, no such considerable
change was induced by the addition of other metal ions.
The dual responsive nature of receptors L1, L2 and L3
towards AcO− ion and Cu2+ ion throws light on the ability of

568
S. PANGANNAYA ET AL.
Figure 14. uV–vis absorption spectra of L3 in acN (10−4 m) upon
titration with cu(oac)2 in h2o (10−2 m). inset plot represents
binding isotherm at 466 nm.
Figure 15. B–h plot for 1:1 L3-cu(oac)2 complex.
Scheme 2. proposed binding mechanism of L1, L2 and L3 with cu(oac)2 and cu(No3)2.
Table 1. Binding constant and detection for L1, L2 and L3 in the presence of tBaoac, Naoac and cu2+ ions.
Binding constant (102)
Detection limit (ppm)
L1(M−1
4.07
)
L2(M−2)a and (M−1 b
)
L3(M−2)a and (M−1 b
)
L1
3.76
L2
5.27
L3
4.5
Ions
tBaoac
336.93a
208.88a
3.36b
2.08b
Naoac
cu2+
4.82
904.08a
9.04b
300.92a
3.00b
0.82
0.61
0.82
4.6
1.55
1.29
0.86
1.8
3.28
a1:2 receptor–acetate complex.
b1:1 receptor–acetate complex.
The binding constant has been calculated using B–H
equation (35) and the values are tabulated in Table 1.
Lower detection limit (LOD) of 0.61 ppm towards AcO−
ion has been achieved with L2 indicating its practical
utility. LOD of 1.8 ppm with Cu2+ reflects the role of L1 as
a colorimetric receptor in practical applications. The lower
detection limit of L2 towards NaAcO in comparison with
TBAOAc could be correlated with the nature of counter
cation in the stabilising the negative charge on the recep-
tor. TBA ion having larger size in comparison with Na+ ion
renders meagre influence on stabilising the deprotonated
receptor. Further, a comparison of the present sensors with
the previously reported ones has been tabulated inTable 2.
3.2. 1H-NMR titration studies
With a view to unveil the nature of reaction between recep-
1
tors L1, L2 and L3 with AcO− ion, H-NMR titration stud-
ies have been performed. With the incremental addition
of 0.5, 1.0 eq. of TBAOAc to receptors L1, L2 and L3 the
disappearance of both OH protons was observed indicat-
ing the formation of strong hydrogen bond interaction

SUPRAMOLECULAR CHEMISTRY
569
Table 2. Data of comparison (medium, detection limit and binding constant) with other reported receptors.
Sl. No.
Receptor
Analytes Binding constant (M−1
)
Detection limit (M)
Medium
DmSo
Ref.
1
cu(oac)2
7.38 × 103
0.19 × 10−6
36
2
3
cu2+
3.69 × 106 ( 0.00208)
Not reported
ch3cN
37
34
cu2+
1.56 × 104
1.15 × 10−6
ch3cN/h2o (9:1, v/v)
4
cu2+
1.96 × 106
9.72 × 10−7
ch3cN
38
5
aco ‒
3.12 × 104
Not reported
DmSo/DmSo/h2o
(88:12, v/v).
39
6
cu2+
oac
1.55
4.82
2.8 × 10−5
2.9 × 10−5
acN
this work
ch3cN/h2o (9:1, v/v)
cu2+
(oac)
1.29
904.08
9.6 × 10−6
acN
1.17 × 10−4
ch3cN/h2o (9:1, v/v)
cu2+
(oac)
0.86
300.92
7.2 × 10−5
2.9 × 10−5
acN
ch3cN/h2o (9 : 1, v/v)
between AcO− and receptor (34, 40). The aromatic proton
signals underwent an upfield shift indicating the presence
of through-bond effect which tends to increase the elec-
tron density on the aromatic ring. Correspondingly, L1,
L2 and L3 exhibited a distinct downfield shift for imine
proton from δ 8.96, 8.94, 9.04 to 9.3 ppm, representing the
through-space interactions which polarise the C–H bonds
in proximity of OH- AcO− complex (41). The titration profile
for L1, L2 and L3 is provided in Figures 16–18. The binding
mechanism has been proposed based on the above con-
siderations and represented in Scheme 3.
The various possibilities involved in the intramolecular
hydrogen bond interactions (42) in L1, L2 and L3 is pro-
vided in Scheme S4 (Supporting information). The binding
mechanism has been proposed based on the UV–vis titra-
1
tion and H-NMR titration studies. L1 exhibits hydrogen
bonding interaction with AcO− ion through –OH func-
tionalities followed by single step deprotonation process

570
S. PANGANNAYA ET AL.
Figure 16. 1h-Nmr titration spectra of L1 with incremental addition of tBaoac.
Figure 17. 1h-Nmr titration spectra of L2 with incremental addition of tBaoac.

SUPRAMOLECULAR CHEMISTRY
571
Figure 18. 1h-Nmr titration spectra of L3 with incremental addition of tBaoac.
Scheme 3. proposed binding mechanism of receptors L1, L2 and L3 with aco – ion.

572
S. PANGANNAYA ET AL.
Table 3. truth table of cu2+ and aco− ionic interaction with
the L1.
Table 4. truth table of cu2+ and aco− ionic interaction with
the L2.
Input
Cu2+
Input
AcO−
Output
496 nm
Input
Cu2+
Input
AcO−
Output
477 nm
Output
518 nm
0
0
1
1
0
1
0
1
0
1
1
1
0
0
1
1
0
1
0
1
0
0
1
1
0
1
0
0
Table 5. truth table of cu2+ and aco− ionic interaction with
the L3.
Input
Cu2+
Input
AcO−
Output
467 nm
Output
520 nm
Figure 19. representation of or logic gate for receptor L1.
0
0
1
1
0
1
0
1
0
0
1
1
0
1
0
0
forming CH3COOH. L2 and L3 undergo two step deproto-
nation process yielding [(CH3COO)2H2]+ species confirming
the binding ratio of 1:2 for L2 / L3: AcO− ion complex.
3.3. Logic gate applications
There is substantial interest in emerging molecular systems
that imitates the behaviour of digital logic gates (43–46).
The introduction of different inputs and their rule as logic
gates is represented in the form of truth table. Receptor
L1, L2 and L3 was explored for the design and construc-
tion of Boolean logic gates by the introduction of Cu2+
and AcO− ionic inputs. These inputs and the corresponding
optical outputs were coded with binary digits (01), which
represents the‘on’and‘off’states. The ionic inputs for Cu2+
and AcO− in the truth table were regarded as‘0’when they
are absent and‘1’if they are present. Receptor L1 exhibited
a system response as that of a two input OR gate, while
that of L2 and L3 exhibited an INHIBIT logic gate response.
Absorbance value at 496 nm regarded as the output
signal upon the action of the Cu2+ and AcO− ionic input.
Receptor L1 mimics the function of an OR gate which is one
of the basic logic gates that implement logical disjunction.
The truth table of Cu2+ and AcO− ionic interaction with the
L1 clearly demonstrate the OR logical behaviour as given
in Table 3. It is seen that the output of this molecular gate
shoots to logic high state 1, if any of the input is 1. (Figure 19)
The inhibit gate consisting of integration of two NOR
gate is important because of its non-commutative behav-
iour wherein the output signal is inhibited by one powerful
input. In the present study, Cu2+ mainly decides the state of
the signal output. Realisation of INHIBIT logical behaviour
of receptor L2 and L3 for Cu2+ and AcO− ions is as given in
the truth table (Tables 4 and 5). The circuit is realised with
two 2 input NOR gate wherein one of the input of NOR gate
is set to 0 as shown in the circuit diagram Figures 20 and
21. L2 and L3 take in two inputs and exhibits two different
Figure 20. representation of iNhiBit logic gate for receptor L2.
Figure 21. representation of iNhiBit logic gate for receptor L3.
response of absorbance at 477 and 518 nm. It is clear from
the truth table that output 2 gives a value 1 upon binding
of AcO− ion alone. INHIBIT response of the gate could be
realised with addition of Cu(OAc)2 ion to receptor L2 and
L3. The observed output value of 1 corresponds to the
wavelength shift of receptor L2 and L3 to 477 and 467 nm
found in the presence of Cu(NO3)2. The output value 0 sig-
nifies the absence of AcO− ion in the signalling process.
Conclusions
In conclusion, we have presented a series of simple col-
orimetric receptors L1, L2 and L3 synthesised by Schiff
base condensation reaction and characterised by stand-
ard spectroscopic techniques. UV–vis and 1H-NMR titration
studies confirm the hydrogen bond interaction between
phenolic proton and acetate ion which is mirrored in the
higher binding constant of the order 9.04 × 104 M−2 for
NaOAc and 1.55 x 102 M−1 for Cu2+ ion. Selectivity of L1 for
Cu2+ ion and L2 for AcO− ions with spectroscopic detection

SUPRAMOLECULAR CHEMISTRY
573
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Acknowledgements
Authors express their gratitude to the Director and the HOD
(Department of Chemistry) NITK Surathkal for the providing the
research infrastructure. DRT thanks DST (SB/FT/CS-137/2012)
for the financial support of this work. SP is thankful to NITK for
the Research fellowship. We thank IITM, Chennai and MIT Mani-
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Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by Science and Engineering Research
Board [SB/FT/CS-137/2012].
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