A chromogenic "off-on" azomethine sensor possessing ONNNO receptor site for iron species and its application in the fabrication of INHIBIT type molecular logic gate

Article abstract of DOI:10.1016/j.poly.2016.03.021

A Schiff base "off-on" chromogenic sensor bearing ONNNO receptor site for the encapsulation of iron species have been synthesized. The sensor is capable of detecting Fe²⁺ and Fe³⁺ metal ions, respectively, in organic and mixed organi

Full text of DOI:10.1016/j.poly.2016.03.021

Accepted Manuscript
A chromogenic “off-on” azomethine sensor possessing ONNNO receptor site
for iron species and its application in the fabrication of INHIBIT type molecular
logic gate
Pawanpreet Kaur, Raghubir Singh, Varinder Kaur, Dinesh Talwar
PII:
S0277-5387(16)30001-8
DOI:
http://dx.doi.org/10.1016/j.poly.2016.03.021
POLY 11886
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
21 January 2016
11 March 2016
Please cite this article as: P. Kaur, R. Singh, V. Kaur, D. Talwar, A chromogenic “off-on” azomethine sensor
possessing ONNNO receptor site for iron species and its application in the fabrication of INHIBIT type molecular
logic gate, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.2016.03.021
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A chromogenic “off-on” azomethine sensor possessing ONNNO receptor site for iron
species and its application in the fabrication of INHIBIT type molecular logic gate
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Pawanpreet Kaura, Raghubir Singha*, Varinder Kaurb, Dinesh Talwara
aDepartment of Chemistry, DAV College, Sector 10, Chandigarh 160 011, India
bDepartment of Chemistry, Panjab University, Chandigarh 160 014, India
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Abstract
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A Schiff base “off-on” chromogenic sensor bearing ONNNO receptor site for the encapsulation
of iron species have been synthesized. The sensor is capable of detecting Fe2+and Fe3+ metal
ions, respectively, in organic and mixed organic aqueous media by naked eye. The photophysical
properties of sensor were compared with its structural analogues (with different substituents and
motifs). The sensor was found relatively efficient due to the presence of appropriate binding
cavity formed by the donor set as well as role of the attached substituents in bathochromic and
hyperchromic shift in UV-Vis spectrum. The detection limit for the naked eye sensing of Fe2+
and Fe3+ were 1 µM and 1.1 µM in methanol, respectively. The sensor showed colorimetric ‘off-
on’ switch in the absence and presence of these metal ions (Fe2+ and Fe3+), respectively due to
intermolecular charge transfer from electron rich receptor site to electron deficient metal centre.
Furthermore, it is useful for the fabrication of INHIBIT type logic gate on the basis of
interference caused due to F- and CH3COO- ions. These anions interact with the donor set via
non-covalent interactions and switch off the colorimetric response by reducing the charge
transference. Hence, sensor can be applied for the fabrication of INHIBIT type molecular logic
gate.
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Key words: Chromogenic sensor, Fe3+ Fe2+, INHIBIT logic gate, Schiff base, colorimetric sensor
,
Corresponding author:
raghu_chem2006@yahoo.com, Tel: +919872313583
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Introduction
The importance of cation complexation in biology, chemistry, medicine and environment,
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has played an important role in the field of molecular recognition [1]. A great range of selective
receptors for cationic species has been prepared in the form of powerful chemosensors [2,13].
Amongst various biologically important metal ions, iron is essential trace element for living
beings. It is the most compatible material in enzyme-catalyzed reactions because it helps in rapid
oxidation-reduction reactions, cellular metabolism, oxygen transportation and acts as a mediator
in electron transport [3-6]. Due to variable oxidation states, it forms an important redox pair in
biological systems as well as acts a powerful catalyst in chemical industry [7,8].
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The determination methods of iron in both oxidation states mainly utilize highly
sophisticated instruments such as capillary electrophoresis (CE) [9], inductively coupled plasma-
atomic emission spectrometry (ICP-AES) [10] and flame atomic absorption spectroscopy
(FAAS) [11], which are not always convenient due to their high operational cost, expertise
required to run the instrument, complicated and laborious sample preparation methods and high
maintenance charges. Therefore, chemosensors have gained impetus in many disciplines such as
biomedical and environmental sciences due to simple monitoring method [12-17].
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Amongst different chemosensors, chromogenic “naked eye” sensors have been the
cynosure over other recognition methods for metal ion detection due to their sensitivity,
selectivity, versatility, ability to detect very low concentrations and no need for expensive
instrumentation [18]. In the past, various fluorescent and colorimetric “turn-on” chemosensors
have been developed for the detection of both Fe3+ and Fe2+ [19]. A “turn-on” colorimetric
switch is advantageous because the color inducer (metal ion) can be measured at low
concentration without the matrix interferences which enhances sensitivity of the method. Hence,
a cost effective chromogenic sensor with simple synthetic strategy and high affinity for metal
capturing should be designed for sensing purposes.
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Schiff base derivatives seemed us potentially good sensors because intramolecular charge
transfer (ICT) transitions can be induced easily in such moieties by introducing some active units
in their skeleton [20]. In Schiff bases, ICT bands arise due to transitions from conjugated
aromatic fragment to C=N fragment within the molecule [21]. When a Schiff base compound
captures a metal ion, a complex is formed due to the displacement of electron density from donor
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sites (=N and OH) to metal ion [22]. This further enhances the extent of ICT and/or generates a
new ICT band (due to bathochromic shift or hyperchromic shift of previous ICT band
respectively) and leads to the colorimetric switch ‘on’ for the sensor [23].
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Herein, we report on design and synthesis of a Schiff base “off-on” chromogenic sensor
in which intramolecular charge transfer is stimulated between donor-acceptor fragments, when
Fe3+ and Fe2+, respectively are added in the organic or mixed aqueous systems (Fig. 1). The
sensor consists of; 1) ONNNO receptor site to bind metal ion selectively, 2) a conjugated ring
system linked with azomethine group (active unit in which intramolecular charge transfer
increases after capturing metal ion) and 3) –CCNCC- flexible unit, which helps in effective
binding of metal ions with the receptor. The sensor is capable of naked eye detection and optical
sensing of iron species (Fe2+ and Fe3+) in organic and mixed organic aqueous systems.
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INSERT FIG. 1.
Fig. 1. Illustration showing general structure of sensors 1-3
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Moreover, recent development in the area of signal communication at molecular level has
gained much attention for the integration of molecular logic gates into working automation and
arithmetic systems [24-26]. Various efforts have been made to fabricate molecular system having
capability to execute binary arithmetic and logical operations [27-28]. In colorimetric methods,
usually chemical inputs are given to obtain output signals encoded in the form of absorption. In
the present method, colorimetric sensing of iron species is inhibited in the presence of F- and
CH3OO- ions and remained unaffected in the presence of Cl- and I- ions. Therefore, an INHIBIT
type Boolean molecular logic gate is constructed for the change in the behavior of sensor 1 in the
presence of iron species and with the simultaneous addition of F- and CH3COO- ions,
respectively.
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Moreover, the sensing ability of developed sensor (1) is compared with its analogues (2
and 3) having different substituents (-CH3 and –H) on the C atom (a bridge between donor and
acceptor fragments as shown in Fig. 1). Though synthesis, characterization and their utilization
of 2 and 3 for the synthesis of metal complexes have already been reported in the literature
[29,30], we explored their use as chemosensor to compare the effect of skeletal substituents on
sensing ability. Interestingly, these compounds are less efficient sensors to detect metal ions
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relative to sensor 1. Furthermore, the sensing capability of sensor 1 is also better as compared to
the receptors having half of its skeleton (4 and 5), indicating the role of each fragment in the
efficient detection of iron species.
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Experimental
Materials and Instruments
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All the reagents namely 2-hydroxyacetophenone (Merck), 2-hydroxy-4-methoxy
benzophenone (Sigma-Aldrich), salicylaldehyde (Merck), diethylenetriamine (Merck),
copper(II) chloride dihydrate (Merck), nickel(II) chloride (Merck), iron(III) chloride anhydrous
(Thomas Baker), iron(II) sulphate (Loba Chemie), manganese(II) chloride (Sigma-Aldrich),
chromium(III) chloride (Loba Chemie), cobalt(II) chloride (Loba Chemie), zinc(II) chloride
(s.d.fine), lead(II) nitrate (Fischer Scientific), mercuric(II) chloride (Merck), tin(II) chloride
(Merck), cadmium(II) chloride (CDH), tetraethylammonium fluoride (Acros organics), sodium
chloride (Fischer Scientific), sodium acetate (Qualigens), and sodium iodide ((Loba Chemie)
were purchased and used as such. Different solvents (acetonitrile, methanol, tetrahydrofuran,
dichloromethane, dimethysulphoxide) were purchased from Merck (India) and were dried before
use.
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Sensors 2 and 3 were prepared by following a general procedure reported in literature
[29]. Stock solutions of sensor 1, 2 and 3 with a concentration of 1 mM were prepared by
dissolving 0.525 g L-1, 0.339 g L-1 and 0.311 g L-1 in acetonitrile, respectively. This stock
solution was diluted further to make 0.05 mM solution for absorbance measurements in
optimization experiments. 1 mM stock solutions of metal ions were prepared by dissolving
respective amounts of corresponding salts in distilled water.
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Infrared spectra were routinely obtained as thin films on a Bruker alpha Eco-ATR
spectrometer. Mass spectral measurements (ESI source with capillary voltage, 2500 V) were
carried out on a VG Analytical (70-S) spectrometer. The solution NMR spectra were recorded at
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25°C on BRUKER AVANCE II 400-NMR spectrometer (1H, C). Chemical shifts in ppm are
reported relative to tetramethylsilane (TMS). Electronic spectral measurements were carried out
using ESICO 23750 double beam spectrophotometer in the range 200-800 nm. Effect of pH was
studied with Digital pH meter (ME-962P). Single-crystal X-ray structure analysis was carried out
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on a Bruker KAPPA APEX 2 charge couple device (CCD) Duo diffractometer using graphite
monochromated Mo Kα radiation (λ = 0.71073 Å), The structure was solved by direct methods
(SHELXS-97) and refined with full-matrix least-squares methods (refinement of F2 against all
reflections with SHELXL – 97).
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Synthesis of sensor 1 (N,N'-bis((2-hydroxy-4-methoxyphenyl)(phenyl)methylene)diethyl-
enetriamine)
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A solution of diethylenetriamine (0.47 mL, 4.4 mmol) in 2-propanol was added to a solution of 2-
hydroxy-4-methoxybenzophenone (2 g, 8.8 mmol) in 2-propanol (50 mL). The contents were
refluxed for 6 hours and thereafter, solvent was removed using rotary evaporator. It resulted in
yellow oil, which became solid when kept in refrigerator for few days. Yield: 86 %, M.p.: 98-
100°C; FT-IR (cm-1): 1588 (C=N), 3060 (OH), 3305 (NH); 1H NMR (400 MHz, CDCl3): δ (ppm)
2.85 (t, 4H, CCH2NH, J = 6.2 Hz), 3.36 (t, 4H, CCH2N=C, J = 6.2 Hz), 3.76 (s, 6H, OCH3), 6.10
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(dd, 2Hb, J = 2.5 Hz, J = 9.0 Hz,), 6.35 (d, 2Hd, J = 2.5 Hz,), 6.62 (d, 2Ha, J = 9.0 Hz) 7.19-
7.48 (m, 10H, 2C6H5), 16.24 (s, 2H, Ar–OH); 13C NMR (75.57 MHz, CDCl3): δ (ppm) 45.4
(CCH2N=C), 49.5 (NCH2C), 55.2 (OCH3), 101.7-132.9 (Ar-C), 164.1 (Ce) 170.1 (Cc), 174.2
(C=N), MS (% relative abundance of m/z assignment): 524 (100, M + H)+, 1069 (7.99, M + Na +
H)+. The IR and NMR spectrum of 1 is given in supplementary material for more details (Fig. S1
and S2). Synthesis and characterization of 2 and 3 is given in supplementary material.
Photophysical studies
Colorimetric detection of respective Fe2+and Fe3+ ions using 1-5
To 2 mL of sensor 1 (0.5 mM stock solutions), 1 mL of metal ion solution (1.0 mM Fe2+ and Fe3+
solution, respectively) was added drop wise in 5 mL measuring flask and the total volume of the
solution was made 5 mL by the addition of water. It showed everlasting color change observed
by naked eye due to rapid reaction between sensor and metal ions. UV-Vis spectra of the
solution were recorded in the range 200-800 nm. A similar procedure was adopted for the
colorimetric studies of sensor 2 and 3.
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To study the effect of time on the color observed upon the addition of metal ion to sensor
1, spectra were recorded at different time intervals.
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Titration of Fe2+and Fe3+, respectively with sensor 1
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For the titration of Fe3+, 2 mL of sensor 1 solution (0.5 mM) was added with 0.05 mL of Fe3+
solution (1 mM) and the final volume was adjusted to 4 mL by adding distilled water. The
solution was shaken thoroughly and transferred to a cuvette to record UV-Vis spectrum. A
complete titration was done by increasing the volume of Fe3+ solution (1 mM) by 0.05 mL every
time and data was recorded till the peaks became constant. Similarly, for the titration of Fe2+
ions, 0.025 mL of Fe2+ (1.0 mM) was added in each step to 2 mL of sensor 1 (0.25 mM) till the
appearance of a constant curve.
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The composition of metal-ligand complex was studied by Job’s method. In this operation,
different volumes of 0.5 mM solution of sensor 1 (in the range 4.0, 3.5, 3.0, 2.5, 2.0, 1.5, 1.0, 0.5,
0 mL) were taken in separate 5 mL glass vials and total volume of each vial was scaled to 4 mL
with metal ion solution (1.0 mM). UV-vis spectra were recorded for each solution and graph of
absorption versus mole fraction was plotted.
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Similarly, the effect of solvent system was studied by varying the organic solvent system.
Moreover, composition of acetonitrile was reduced from 100 to 50% (v/v) in the mixed aqueous-
organic system for the same concentration of sensor 1.
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Fabrication of INHIBIT type logic gate
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The effect of anions (F-, Cl-, I- and CH3COO-, respectively) to the chromogenic behavior of
sensor 1 towards Fe3+ and Fe2+ ions was studied. To 2 mL solution of sensor 1 (0.25 mM in
acetonitrile), 1 mL solution of respective Fe3+ and Fe2+ (1 mM in methanol) was added followed
by the addition of varying amounts of tetraethylammonium fluoride (1.0 mM in methanol) and
the final volume was adjusted to 4 mL by acetonitrile. A same procedure was followed to study
the effect of Cl-, I- and CH3COO- ions using same concentrations.
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Results and discussion
Synthesis and characterization of sensor 1
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Herein, a Schiff base sensor 1 has been prepared by the condensation reaction of
diethylenetriamine and 2-hydroxy-4-methoxybenzophenone (Scheme 1). The formation of Schiff
base sensor was confirmed by IR, NMR, mass spectrometry and single crystal X-ray diffraction
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studies. In IR spectrum of 1, strong bands at 1588, 3060 and 3305 cm-1 may be attributed to the
stretching frequency for the azomethine (>C=N-), NH and OH groups, respectively. The NMR
spectrum of 1 clearly indicated the formation of azomethine sensor with ONNNO donor sites
1
embedded with the the aromatic and aliphatic motifs. A signal at 16.24 ppm in H NMR
corroborated to the presence of hydrogen bonding between phenolic OH and azomethine N (O-
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H….N=C). C NMR spectrum of 1 showed a signal at 174.19 (due to C=N) and 164.1 (due to
phenolic C) in addition to aromatic signals. The downfield shifting of the signals confirmed the
presence of C=N and phenolic-OH in the sensor 1.
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INSERT SCHEME 1.
Scheme 1. General scheme for the synthesis of the sensor 1, 2 and 3
Single crystal X-ray diffraction studies
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Single crystal X-ray diffraction data of sensor 1 revealed presence of single molecule in the unit
(Fig. 2). The phenolic proton was found to bridged between azomethine N and phenolic O atom,
which corroborates with presence of downfield signal (16.24 ppm) in 1H NMR of 1. The details
of single crystal X-ray diffraction parameters and some selected bond lengths/bond angles are
given in Table S1 and S2, respectively.
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INSERT FIG. 2
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Fig. 2. Ortep view of sensor 1. Ellipsoids are shown at 40% (non-hydrogen atoms) and 10%
(hydrogens) probability level.
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Photophysical properties of sensor
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Colorimetric changes of sensor 1 were studied by adding different metal ions (Cr3+, Mn2+,
Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Sn2+, Pb2+). Interestingly, only Fe3+ and Fe2+
resulted in an intense color change from faded yellow to wine red which was clearly visible to
naked eye (Fig. 3). However, all the other metal ions failed to show any visible change in the
color of sensor 1. This indicated that both the Fe3+ and Fe2+ can be detected with naked eye even
at lower concentrations.
INSERT FIG. 3
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Fig. 3. Naked eye sensing of Fe3+ and Fe2+ using sensor 1
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Absorption properties of sensor 1 were investigated in different solvents at concentration
of 5.0 x 10-5 M (Fig. S3). On the basis of molecular structure of sensor 1, two types of electronic
transitions are possible; n-π* transition in C=N fragment and π-π* transition in aromatic region.
The extended conjugation in azomethine and aromatic moieties could lead to intramolecular
charge transfer transitions. In this molecule an electron donating group is present at the phenyl
ring, which leads to partial charge transfer in the direction of azomethine group (C=N). UV-Vis
absorption spectrum of sensor 1 in dichloromethane showed four absorption peaks centered at
230 nm, 270 nm, 320 nm and 380 nm. The two absorption peaks in the region 200-300 nm may
be attributed to π-π* transition due to extensive conjugation of benzene rings in polar aprotic
solvents. The shoulder at 320 nm represents the presence of intermolecular hydrogen bonding
between the free hydroxyl group with the solvent [31]. As the polarity increased (except DCM)
the compound showed blue shift which suggests more stability of sensor in the excited state. The
compound showed single absorption around 300 nm in highly polar solvent methanol revealing
the existence of compound in phenolic form [32]. The dependence of absorption band on the
polarity of solvent revealed solvent-solute interactions involved in the solution phase [31b,33].
Furthermore, peak intensity was enhanced in the acetonitrile; water mixture relative to pure
acetonitrile (details are given in next section). Therefore, the photophysical properties were
investigated by monitoring the absorption spectra in acetonitrile or acetonitrile-water mixture at
room temperature.
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Sensor 1 responses to both Fe3+ and Fe2+ ions
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Sensor 1 is structurally divided into two parts; ONNNO set as receptor site for binding
the target, and C=N linked to two phenyl rings via C as chromophoric unit. The presence of
phenyl rings extends the conjugation in the chromophoric fragment. When metal ion solution
(Fe2+ and Fe3+, respectively) is added to the sensor 1, receptor site ONNNO encapsulates the
metal ions very effectively to form a complex and imparts intense red color to the complex.
Moreover, a low energy band is observed in the UV-Vis spectrum of the solution due to charge
transfer transition.
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The sensor 1 showed significant enhancement in the absorption band present in the
region 425-700 nm both for Fe3+ and Fe2+ ions. Two isobestic points were generated at 417 nm
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and 362 nm in case of Fe3+ (Fig. 4), and 414 nm and 355 nm in case of Fe2+ (Fig. 5) indicating
the presence of two species in the solution. In addition to rise in the absorption band centered at
504 nm, subsequent additions of metal ions promoted a gradual decrease of the original
absorption band of sensor centered at 380 nm. These significant changes in the UV-Vis spectrum
indicated the complex formation of sensor 1 with respective Fe3+ and Fe2+ ions.
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INSERT FIG. 4
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Fig. 4. Absorption spectra obtained during the titration of 1 (0.5 mM) with successive additions
of 0.05 mL of Fe3+ (1 mM).
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INSERT FIG. 5
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Fig. 5. Absorption spectra obtained during the titration of 1 (0.25 mM) with successive additions
of 0.025 mL of Fe2+ (1 mM).
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The binding of metal ions (Fe3+ and Fe2+) with the imino nitrogen atom decreases the
electron density of C=N units with subsequent increase in its electron accepting character. This
phenomenon enhanced the charge transfer from donor (conjugated aromatic ring with methoxy
donor) group to azomethine acceptor (C=N) group and accordingly induced spectral changes.
Thereupon, the new wide absorption band at 504 nm is probably attributed to an (ICT)
intramolecular charge transfer band. A similar pattern was observed in case of sensor 2 and 3 but
the ICT band was relatively less intense (Fig. S4). In addition, the intensity of the ICT band in
Fe2+ was also less as compared to Fe3+, which may be attributed its large size and high electron
density in Fe2+ resulting in the less electron flow within the molecule. To further explore the
binding behavior, the stoichiometry of 1 with metal ions was determined by Job’s plot. The
maximum absorbance at 504 nm for Fe3+, and 479 nm for Fe2+ was plotted against mole fraction
of metal ion which revealed 1:1 stoichiometry of metal-sensor binding (Inset Fig. 3 and 4).
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Effect of pH on the detection of Fe3+ and Fe2+
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To investigate the practical application of sensor, absorption of 1 in the presence of Fe3+
and Fe2+ was studied at different pH in the range 2-14. It was observed that the intensity of ICT
absorption band is strongly pH dependent (Fig. 6). The plot showed that absorbance for Fe3+ and
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Fe2+ remained approximately same in the pH range 5.5-8.0 and 6.0-8.0, respectively. Therefore,
pH 5.5 and 6.0 were selected for Fe3+ and Fe2+, respectively for further experiments.
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INSERT FIG. 6.
Fig. 6. The absorbance intensity of sensor 1 in presence of Fe3+ and Fe2+ at various pH values.
Comparison with analogous sensors
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A comparative study of photophysical properties with the structural analogues (2,3) of
sensor 1 was done by varying the substituent present at azomethine C in the sensor (Fig. S5). It
was observed that the phenyl group present on the azomethine C enhanced its sensitivity towards
iron species as compared to 2 and 3. A comparison of sensor 1, 2 and 3 is given in Table 1. In
addition, chelators 4 and 5, both derived from salicylaldehyde derivative and ethanolamine (not
published previously, structures are given in Fig. S6) possessing half donor set i.e. ONO system
was also done. The study affirmed the higher molar absorptivity of sensor 1 towards Fe3+ (ε =
3.42×103, λ=504) as compared to chelators 4 (ε = 2.02×103, λ= 487) and 5 (ε = 2.12×103,
λ=501). A similar trend was observed Fe2+ with sensor 1 (ε = 1.79×103, λ=479) as compared to 4
(ε = 1.21×103, λ=479) and 5 (ε = 1.41×103, λ=479). This could be explained on the basis of more
denticity of sensor 1, which led to increase in the flow of electron density making the transition
more facile. Therefore, the molar absorptivity, absorbance maxima and intensity of the band
showed hyperchromic and red shift. Hence, multiple donor system of sensor 1 with phenyl
substituents makes the sensor 1 better over its analogues 2-5.
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Moreover, a study on the different sensors (1-3) in ACN/Water mixture (1:1 volume
relation) were carried out before investigating the absorption spectral behavior towards metal
ions. It is observed that UV-Vis peak corresponding to sensor 1 remained intense in mixed
solvent system relative to sensor 2 and 3 (Fig. S7). This indicated that sensor 1 can also be used
in the mixed solvent system with water as a co-solvent.
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INSERT TABLE 1.
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Table 1. A comparison of molar absorptivity and absorption maxima of sensors 1-3 for Fe2+ and
Fe3+ complexation.
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Detection limits for naked eye and optical sensing of Fe3+ and Fe2+
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The naked eye detection of Fe2+ and Fe3+, respectively can be achieved (even in ppb
levels) using sensor 1. Additionally, limit of detection was calculated for optical sensing of both
metal ions. As discussed above, sensor 1 showed an ICT band centered at around 504 nm and
479 nm for both the Fe3+ and Fe2+, respectively. The intensity of band increases with the increase
in the concentration of metal ions. The plot of absorbance versus concentration revealed linear
relationship between increase in intensity of ICT band and concentration of metal ions in the
range 0.012 mM to 0.2 mM for Fe3+ and 0.006 mM to 0.1 mM for Fe2+ for the constant
concentration of sensor 1 (i.e 0.25 mM and 0.125 mM, respectively). The regression equations
2+
3+
for Fe2+ and Fe3+ are 3.780CFe - 0.02955, R2 = 0.990 and 2.767CFe + 0.01586, R2 = 0.995
respectively. Moreover, limit of detection (DL) for optical detection of Fe3+ and Fe2+ was
calculated from the regression equation and found to be 12.5 µM and 6.2 µM, respectively. The
naked eye detection of Fe3+ and Fe2+ was 1.5 mM and 1.0 mM, respectively. The sensitivity of
method was found to be comparable and even better than some reported methods. Previously,
limit of detection for Fe2+ was reported 7.4 µM (in DMF, UV-Vis) [34], 0.076 µM (in water,
UV-Vis) [35], 1.3 µM (in water, UV-Vis) [36], and 2 µM (in acetonitrile:water, Fluorescence)
[37], and for Fe3+; 6.8 µM (in DMF, UV-Vis) [34], 0.19 µM (in water, UV-Vis) [38], 0.95 µM
(in DMSO, Fluorescence) [39], 0.080 µM (in water, UV-Vis) [40] and 3.5 µM (in
acetonitrile:water, Fluorescence) [37].
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Time taken by sensor 1 to respond towards the presence of metal ions (Fe3+ and Fe2+) was
studied. For this, UV-Vis absorbance was recorded at 504 nm at different time intervals after the
addition of metal ions. It was observed that the absorbance increases quickly and became
constant (even after 30 min). This indicates that complex formation between metal and sensor 1
completes within seconds and then becomes stable. Therefore, it can be considered as a fast
colorimetric sensor.
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Binding constant (Ka) was calculated using Benesi-Hildebrand equation. The graph was
plotted by recording the absorbance changes of consequent titration (1/A-A0) against
concentration of Mx+ ion 1/[Mx+]. The binding constant Ka of 1 with Fe3+ and Fe2+ was found to
be 5.48 x 103 M-1 and 7.05 x 102 M-1, respectively.
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Fabrication of INHIBIT type logic gate
11

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It was observed that addition of F- and CH3COO- ions in the metal-ligand complex diminished
the red color of the solution. This indicates that anions interfere with the detection of respective
Fe2+ and Fe3+ by colorimetric switch of the sensor. Sensor 1 gave fascinating “on–off”
colorimetric response in the absence and presence of metal ions and anions (F- and CH3COO-),
whereas no significant change was observed for Cl- and I- ions. Therefore, it can be used for the
fabrication of a Boolean type logic gate at molecular level. A yellow solution of sensor became
red only in the presence of either Fe3+ or Fe2+ and absence of F- and/or CH3COO- ions. However,
presence of either these anions and/or iron species as chemical inputs failed to impart red color to
sensor 1. Further, the change in the absorbance at 504 nm of sensor 1 can also be monitored by
adding respective Fe3+ and Fe2+ along with F- and CH3COO- to develop the molecular logic gate.
When sensor was free, it absorbed at 384 nm and after adding Fe3+ or Fe2+ as chemical input,
absorbance maxima was shifted to 504 nm or 479 nm, respectively. Interestingly, on adding F- or
CH3COO- ion to the same solution, formation of metal-sensor complex was disturbed resulting in
hypochromic shift of absorbance band at 504 nm (Fig. S8). When F- or CH3COO- ion was added
in the sensor 1 in the absence of Fe2+ or Fe3+, no new absorption band was observed. Hence, the
change in color and absorbance at 504 nm was used as chemical inputs for the construction of
INHIBIT type logic gate at molecular level as depicted in Fig. 7 A. The chemical inputs can be
represented by 1 for on-state (presence of color) and 0 for off-state (absence of color).
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336
337
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340
On the other hand, no such effect was observed when Cl- and I- were added to the sensor-
metal solution. It is expected that F- and CH3COO- ions formed hydrogen bond with the protic
donor sites i.e. OH and NH and engaged the lone pairs, which was initially involved in the
coordination with metal ion. Therefore, the donor set released the metal ion immediately and
resulted in the disappearance of red color in the solution, while Cl- and I- did not take part in
hydrogen bond formation and hence, showed no significant change. An illustration is given on
the displacement of metal ion by F- ion via forming hydrogen bonds in Fig. 7 B.
341
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INSERT FIG. 7
Fig. 7 (A). INHIBIT type logic gate, (B). Illustration on displacement of iron species by F- ion.
Conclusions
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349
Sensor 1 possesses potential binding sites ONNNO for binding Fe2+ and Fe3+ and the skeletal
moieties enhance the absorption relative to their analogous derivatives. Hence, it proved the
sensor 1 as proficient chromogenic sensor for Fe2+ and Fe3+ in the pH range 6 to 8. Furthermore,
sensor 1 showed interferences towards the sensing of Fe3+/Fe2+ in the presence of F- and
CH3COO- ions and produced a significant on-off colorimetric switch resulting in the INHIBIT
type Boolean logic gate.
350
Acknowledgement
351
352
Authors are thankful to UGC, New Delhi for providing financial support (22/12/2013(ii)EU-V)
and Mr. Shiv Pal from IISER Pune.
353
Appendix A. Supplementary data
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CCDC 1430209 contain the supplementary crytallographic data for 1. This data can be obtained
free of charge
from The
Cambridge Crystallographic Data Centre via
http://www.ccdc.cam.ac.uk/data_request/cif.
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483
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484
485
Figures and Captions:
Fig. 1. Illustration showing general structure of sensors 1-3
486
487
Fig. 2. Ortep view of sensor 1. Ellipsoids are shown at 40% (non-hydrogen atoms) and 10%
(hydrogens) probability level.
488
Fig. 3. Naked eye sensing of Fe3+ and Fe2+ using sensor 1
489
490
Fig. 4. Absorption spectra obtained during the titration of 1 (0.5 mM) with successive additions
of 0.05 mL of Fe3+ (1 mM).
491
492
Fig. 5. Absorption spectra obtained during the titration of 1 (0.25 mM) with successive additions
of 0.025 mL of Fe2+ (1 mM).
493
494
495
496
497
Fig. 6. The absorbance intensity of sensor 1 in presence of Fe3+ and Fe2+ at various pH values.
Fig. 7 (A). INHIBIT type logic gate, (B). Illustration on displacement of iron species by F- ion.
Scheme 1. General scheme for the synthesis of the sensor 1, 2 and 3
498
499
Table 1. A comparison of molar absorptivity and absorption maxima of sensors 1-3 for Fe2+ and
Fe3+ complexation.
500
18

501
502
503
504
Fig. 1. Illustration showing general structure of sensors 1-3
19

505
506
507
Fig. 2. Ortep view of sensor 1. Ellipsoids are shown at 40% (non-hydrogen atoms) and 10%
(hydrogens) probability level.
508
20

509
510
511
512
Fig 3. Naked eye sensing of Fe3+ and Fe2+ using sensor 1
513
514
515
516
21

517
518
519
520
Fig. 4. Absorption spectra obtained during the titration of 1 (0.5 mM) with successive additions
of 0.05 mL of Fe3+ (1 mM).
521
22

522
523
524
525
Fig. 5. Absorption spectra obtained during the titration of 1 (0.25 mM) with successive additions
of 0.025 mL of Fe2+ (1 mM).
526
23

527
528
Fig. 6. The absorbance intensity of sensor 1 in presence of Fe3+ and Fe2+ at various pH values.
529
24

530
531
532
Fig. 7 (A). INHIBIT type logic gate, (B). Illustration on displacement of iron species by F- ion.
533
25

534
535
536
Scheme 1. Synthesis of the sensor 1, 2 and 3
537
26

538
539
Table 1. A comparison of molar absorptivity and absorption maxima of sensors 1-3 for Fe2+ and
Fe3+ complexation
540
Sensor
1
2
3
λ ( nm)
ε ( X 103
mol-1 cm-1)
7.11
λ ( nm)
ε ( X 103
mol-1 cm-1)
3.07
λ ( nm)
ε ( X 103
mol-1 cm-1)
0.697
384
317
253
483
384
315
255
498
384
302
277
504
Absence of
metal ion
22.49
29.2
3.42
8.69
20.69
2.74
7.15
17.91
2.85
Presence of
Fe3+
479
2.40
479
1.86
479
1.79
Presence of
Fe2+
541
542
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543
Table of contents
544
545
A proficient “off-on” chromogenic Schiff base chemosensor for Fe2+ and Fe3+ has been
developed. An INHIBIT type logic gate at molecular level is also fabricated for this sensor.
546
28