Probing anion and cation with novel salicylidene Schiff base receptor appended with 1, 10-phenanthroline: Mimicking INHIBIT molecular logic gate

Article abstract of DOI:10.1016/j.ica.2018.05.010

A novel tailor-made 1, 10-phenanthroline based Schiff base, 2-((E)-(1, 10-phenanthroline -5-ylimino) methyl) phenol, (1) receptor has been designed, synthesized and reported as multi-ion responsive scaffolds for the sensing of both anions and cations via chromogenic methods. In CH₃CN, receptor 1 displayed excellent selectivity for F^? ions amongst anions and for Al³⁺ and Hg²⁺ ions amongst cations. Moreover, the colorless solution of 1 turned yellow only with F^? ions due to new absorption band in visible region. Receptor 1 formed 1:1 complex with F^? ions while 1:2 with the Hg²⁺/Al³⁺ ions as revealed by the separate Job's plot analysis. Receptor 1 can also mimic the functioning of “TRANSFER” and INHIBIT molecular logic gate with chemical inputs from Al³⁺ and F^? ions.

Full text of DOI:10.1016/j.ica.2018.05.010

Accepted Manuscript
Research paper
Probing anion and cation with novel salicylidene Schiff base receptor appended
with 1, 10-phenanthroline: mimicking INHIBIT molecular logic gate
Priya Alreja, Navneet Kaur
PII:
S0020-1693(18)30303-7
https://doi.org/10.1016/j.ica.2018.05.010
ICA 18261
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
23 February 2018
8 May 2018
9 May 2018
Please cite this article as: P. Alreja, N. Kaur, Probing anion and cation with novel salicylidene Schiff base receptor
appended with 1, 10-phenanthroline: mimicking INHIBIT molecular logic gate, Inorganica Chimica Acta (2018),
doi: https://doi.org/10.1016/j.ica.2018.05.010
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Probing anion and cation with novel salicylidene Schiff base receptor
appended with 1, 10-phenanthroline: mimicking INHIBIT molecular logic
gate
Priya Alreja and Navneet Kaur*
Department of Chemistry, Panjab University, Chandigarh 160014, India
Tel.: +91 172 2534430; fax: +91 172 2545074;
e-mail: neet_chem@yahoo.co.in; neet_chem@pu.ac.in (N. Kaur)
Abstract: A novel tailor-made 1, 10-phenanthroline based Schiff base, 2-((E)-(1, 10-
phenanthroline -5-ylimino) methyl) phenol, (1) receptor has been designed, synthesized and
reported as multi-ion responsive scaffolds for the sensing of both anions and cations via
chromogenic methods. In CH3CN, receptor 1 displayed excellent selectivity for F- ions amongst
anions and for Al3+ and Hg2+ ions amongst cations. Moreover, the colorless solution of 1 turned
yellow only with F- ions due to new absorption band in visible region. Receptor 1 formed 1:1
complex with F- ions while 1:2 with the Hg2+/Al3+ ions as revealed by the separate Job’s plot
analysis. Receptor 1 can also mimic the functioning of “TRANSFER” and INHIBIT molecular
logic gate with chemical inputs from Al3+ and F- ions.
Keywords: 1, 10-phenanthroline, colorimetric, cation /anion recognition, Schiff base, molecular
logic gates
1. Introduction
Schiff bases constitute a major category in coordination chemistry as their coordinating ability
containing various donor atoms is widely reported. They have several applications in analytical
chemistry, food and dye industry, catalysis and in many biological aspects [1-3]. The spectacular
characteristics of Schiff bases like low cost simple synthesis coupled with synthetic tailorability,
biodegradability, pronounced photophysical properties and ability to coordinate to metal ions tag
Schiff bases as one of the most widely explored molecular chemosensors [4-6]. Schiff bases can
be used for heavy metal sensing and removal of pollutant in the environment. They are good
spectrophotometric and fluorimetric agents [2, 7]. The complexing ability of Schiff base has
been attributed to the presence of azomethine linkage, which has a tendency to donate the lone
pair of electrons present on the nitrogen atom of the azomethine moiety (C=N-). The structure of
Schiff bases is responsible for geometric and cavity control of host guest complexation and

produce remarkable selectivity and sensitivity for specific ion. Consequently, Schiff base
complexes have attracted increasing attention in the area of ionic binding. [8-11].
Development of 2-Hydroxy Schiff base receptors for anion recognition process is
commonly based on the fact that strongly basic anions can easily deprotonate the –OH proton or
promote the formation of keto-tautomer or inhibit the excited state proton transfer (ESIPT)
phenomenon, leading to modification of spectral properties and inducing the desired
chemoselectivity in the sensor [12-16]. The presence of an acidic group like a phenolic OH or
another donor group close to the azomethine group increases the coordinating effect of the lone
pair of electrons, thereby increasing the stability of the metal complexes.
More recently, interfacing of chemistry with electronics has attained immense significance.
This interfacing is achieved by connecting microcontroller services with chemosensors [17]. The
tailor-made chemosensors can be viewed as computational devices that use physical or chemical
inputs to generate outputs based on a set of logical operators. The development of molecular
logic gates [18-20] and their integration into working automation [21] and arithmetic systems
[22-23] has brought chemists closer to the realization of a molecular scale calculator i.e.
moleculator. These molecular logic gates operate in a wireless mode and thus, they have the
potential for computation on a nanometer scale that silicon-based devices cannot address [24]. In
addition to Boolean molecular logic gates, Fuzzy logic is playing key roles in the development of
a machine intelligent quotient. Fuzzy logic is an “infinite-valued” logic that helps in making
decisions based upon incomplete and vague information tainted by environmental noise i.e. in
soft computing [25].
Based on the above known facts, we report the recognition studies on cation and anion
sensing by novel 5-substituted-1, 10-phenanthroline based azomethine receptor 1 possessing a
phenolic –OH group that helps in binding to F- ion. The position of azo-methine (-CH=N) group
in this Schiff base allows it to act as a ligand to bind with cations (Al3+ / Hg2+). It is well
documented in literature that presence of a functional group (–OH) close to the azomethine
group makes the Schiff bases to bear an excellent coordinating ability to act as multidentate
ligands. [26-27]. To the best of our knowledge, 1, 10-phenanthroline based Schiff bases are
remotely explored for chemosensing properties. The receptor 1 displayed selectivity for most
basic F- ions accompanied by colorimetric response of 1 for F- ions; while UV-vis spectral
changes in 1 were observed with only Al3+ and Hg2+ ions.

2. Experimental
2.1 Methods and Instrumentation
All reagents and chemicals were purchased from Aldrich and used without further purification.
1
Solvents used for spectroscopic studies were purified by standard procedures before use. H
NMR spectra were recorded on a Bruker Advance II 400 at 400 MHz from solutions in DMSO-
d6. 13C NMR spectra were recorded at 100 MHz and values are reported relative to CDCl3 signal
at  77.0. Absorption spectra were recorded on Shimadzu UV-240 spectrophotometer.
2.2 General procedure for Spectroscopic experiments
Stock solutions of the receptor 1 (10-2 M), different anions and metal ions (both 10-1 M) were
prepared in CH3CN. These stock solutions were further diluted with CH3CN and used further for
-
different spectroscopic experiments (UV-vis). All the anions (F-, Cl-, Br-, I-, AcO-, H2PO4 and
HSO4 ) were added as their tetrabutylammonium salts; while cations (Na+, K+, Mg2+, Al3+, Mn2+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+ and Hg2+) were added as their perchlorate salts for the absorption
experiments. Aliquots of ions under investigation were then injected into the sample solution
through a rubber septum in the cap. The solutions were allowed to get stabilized after each
addition, and were then scanned.
2.3 1H NMR Titrations
To the 4 × 10-2 M solution of 1 in DMSO-d6, the varying equivalents of TBAF were added and
1H NMR spectra were recorded after each addition.
2.4 Synthesis of 2-((E)-(1, 10-phenanthroline -5-ylimino) methyl) phenol (1)
5-amino-1, 10-phenanthroline has been synthesized from 1, 10-phenathroline using the known
literature methods [28]. 5-amino-1, 10-phenanthroline (0.5 g, 2.56 mmol) was dissolved in 5 ml
of hot absolute ethanol. To this hot stirring solution of 5-amino-1, 10-phenanthroline,
salicyldehyde (0.312 g, 2.56 mmol) was added and the contents we e heated at C till the TLC
indicated completion of the reaction [29]. Yellow colored solid separated at the completion of
the reaction. The yellow residue was filtered and collected as crude product. Recrystallization of
crude solid from CHCl3 / CH3OH resulted in the formation of pure product 1.Yellow solid, 85%

Yield; M.P: 196 ºC. Selected FT-IR peaks (cm-1): 3411 (υO-H), 3 6 (υC–Hstr., aromatic), 1617
(υC=NH) 1H NMR (DMSO, 4 MHz) δ: . 7-7.01 (m, 2H, Ar-H), 7.49 (t, J = 7.08 Hz, 1H, Ar-
H), 7.89-7.75 (m, 4H, Ar-H), 8.48 (d, J = 7.92, 2H, Ar-H), 8.67 (d, J = 7.08 Hz, 1H, Ar-H), 9.05
(s, 1H, Ar-H), 9.17 (s, 2H, Ar-H);13C NMR (DMSO, 1 MHz) δ: 112.61, 11 .45, 119.16,
119.59, 123.29, 123.49, 125.21, 128.57, 132.06, 132.82, 134.18, 135.88, 145.07, 145.38, 146.34,
149.87, 150.94, 161.25, 164.99. ESI-MS: m/z= 300.14 (M+1) +.
3. Results and Discussions
1
Scheme-1 outlines the synthesis of receptor 1, whose structure has been characterized by H
NMR, 13C NMR and Mass Spectra (Figs. S1-S3).
HNO3 / H2SO4
Reflux
N
N
N
N
NH2
SnCl2.2H2O
NO2
N
N
N2 atm
Abs. ethanol
CHO
Abs. ethanol
Reflux
HO
N
N
N
OH
1
Scheme 1: Synthesis of receptor 1
3.1. Optical changes of 1 with addition of various anions
The recognition and sensing ability of receptor 1 towards various anions (F-, Cl-, Br-, I-, AcO-,
-
-
H2PO4 and HSO4 (tetrabutyl ammonium as a counterion) was determined by naked eye and
UV-vis titration spectra. With immediate addition of 100 equiv. of all the anions, the colorless
solution of 1 turned to yellow with only F- ions in CH3CN. All the other added anions did not
produce any color change that could be detected with naked eye.
In the absence of anions, receptor 1 (2 μM in CH3CN) displayed absorption bands at 270
nm and 343 nm. Here, the low energy band at 343 nm may occur due to intramolecular charge
transfer (ICT) transitions within the whole structure of the Schiff base [30-31]. The ICT band at
343 nm indicates the presence of strong intramolecular hydrogen bond between the hydroxyl
group and the azomethine nitrogen that causes planarity of the molecules and facilitates the
charge transfer to take place within the whole molecule [32]. Amongst all the added anions,

significant changes in the absorption spectra of 1 were observed only with the addition of F- ions,
where a new band at longer wavelength was observed in case of 1, responsible for naked eye
color change. The UV-vis spectrum of 1 was unaltered with addition of all other anions (Fig. 1).
Along with CH3CN, the photophysical properties of receptor 1 has also been evaluated in
other solvents like pure MeOH, DMSO as well as in pure aqueous media. The receptor 1 showed
maximum selectivity and sensitivity towards F-, Al3+ and Hg2+ ions only in CH3CN. In all other
solvents, non-selective behavior of the receptor as well as precipitation in case of pure H2O
system was observed.
Fig. 1: UV-vis spectral changes of 1 (2 μM) upon addition of 1 equiv. of va ious anions.
To have a better insight into the response and recognition phenomenon, UV-vis
titrations were performed with F- ions. With the addition of F- to solution of 1 (20 μM), the
absorbance of 343 nm band decreased; while a new band emerged in the visible region at 436
nm. With the progressive addition of F-, the intensity of 436 nm band increased with concomitant
color change from colorless to yellow. At the same time, 343 nm band was blue shifted to 332
nm. With the addition of F-, the intramolecular hydrogen bond between the –OH group and the –
HC=N nitrogen might have broken, leading to new hydrogen bonding interactions between the –
OH groups of the phenyl ring of salicylaldimine with F- ions and causing the color transition
from colorless to yellow.

Fig. 2: Family of UV-vis spectra of 1 with F- ions in CH3CN recorded during the titration Inset: Plot of
absorbances at 343 and 436 nm versus the concentration of F - ions added along with naked eye color
change in 1 with F- ions.
The Job’s plot experiment was performed which pointed towards 1:1 binding
stoichiometry of 1 with F- ions (Fig. S4). The binding constant of 1 with F- was calculated based
on the UV-vis titration through the Benesi-Hildebrand equation [33], which was given as
follows;
1
1
1
=
+
A-Ao
Amax-Ao
[Amax-Ao] K [C]
Here, Ao, A, and Amax is the absorbance of free 1, measured with F- and measured with excess
amount of F- at 436 nm, respectively and K is the association constant and [C] is the
concentration of F- ion added. Plotting of 1/(A-A0) versus 1/[F-] showed a linear relationship,
which indicates that 1 associates with F- in a 1:1 stoichiometry. The binding constant (K)
between 1 and F- is determined from the ratio of intercept/slope to be 5.4 x 104 M-1.
From the changes in F- dependent absorption intensity, the detection limit was calculated
using equation LOD = 3σ/ρ. So, to calculate limit of detection, standard deviation is divided by
slope of line followed by multiply it with 3 [34]. The detection limit is reasonably estimated to
be 5.2 µM.
3.2 Optical changes of 1 with addition of various metal ions
The binding interaction studies of 1 were conducted in CH3CN against various cations like Na+,
K+, Mg2+, Al3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+ (perchlorate as the counter ion) using

colorimetric and UV-vis experiments. Upon adding 100 equiv. of said cations to solution of 1, no
obvious color changes were observed in the colorless solution of 1, excluding the possibility of
colorimetric detection. However, the absorption spectrum of 1 was perturbed mainly with Hg2+
and Al3+ ions (Fig. 3). Different literature reports highlight similar type of absorption changes
with addition of Al3+ to azomethine receptors, where low absorption intensity changes have been
observed [8, 35]. All the other cations exhibited negligible changes in the absorption spectrum.
The increased absorbance due to Cu2+ is because of its own absorbance in this region, so any
actual changes on interaction of 1 with Cu2+ cannot be measured in this region (Fig. S5).
Fig. 3: UV-vis spectral response of 1 (20 µM) with 100 equiv. of various cations in CH3CN.
The UV-vis titrations of 1 were carried out with Al3+ and Hg2+ ions in CH3CN to deduce
the detailed complexation characteristics (Fig. 4). With immediate addition of Al3+ ions, 270 and
343 nm bands were red shifted to 294 and 348 nm, respectively, accompanied by gradual
decrease in the absorbance (Fig. 4a). The absorbance at both the wavelengths reached the
limiting values when 70 equiv. of Al3+ ions have been added.

Fig. 4: Family of UV-vis spectra of 1 (2 μM) (a) with Al3+ ions (b) Hg2+ ions during the course of
titration in CH3CN. Inset: Plot of absorbances at 348 nm versus the concentration of Al3+ and Hg2+ ions.
Gradual addition of Hg2+ ions displayed the similar behavior as Al3+ ions at both the
wavelengths except that the absorbance value reached saturation at 80 equiv. of Hg2+ ions (Fig.
4b). The Job’s plot analysis pointed towards 1:2 (1:Mn+) stoichiometry of 1 with Mn+ (Mn+
=
Al3+, Hg2+) ions (Fig. S6). The phenanth oline N’s a e known to offe good binding affinity
towards cations [36]. Moreover, the presence of hydroxyl and imine groups in the same receptor
molecule confers the coordination capacity required to coordinate Al3+ and Hg2+ ions [37].
Therefore, one Mn+ ion may bind to the two phenanth oline N’s; while the othe to the hyd oxyl
and imine group simultaneously.
The respective binding constants calculated using Benesi-Hildebrand equation (K) [33]
and LOD values calculated using equation LOD = 3σ/ρ [34] have been given in table 1. The
higher binding constant and lower detection limit for Al3+ ions are indicative of better binding of
1 with Al3+ ions. According to United Nations Environment Report (UNEP), exposure to
me cu y vapou of level ~ 2 μM o highe have subtle effects on the cent al ne vous system and
this lower value is still higher than the LOD value calculated for Hg2+ by this method. Moreover,
the literature reports reflect that Al3+ levels highe than 2.4 μM may cause encephalopathy [38],
which is comparable to the LOD value for Al3+ calculated by this method.
Table 1. Binding constants (K in M-2) of receptor 1 with Al3+ and Hg2+ and corresponding limit of
detection (LOD in µM) values in CH3CN.
Receptor Metal ion
K (M-2)
8.6 × 109
4.14 × 109
LOD (µM)
3.2
Al3+
Hg2+
1
4.9

3.3 1H NMR Titration studies for the complexation of 1 with F-
To further understand the binding behavior of receptor 1 with F-, 1H NMR titration experiment of
1 (4 × 10-2 M) was performed in DMSO-d6 with varying equivalents of TBAF (Fig. 5). The
1
distinct signal of OH did not appear for 1 in its H NMR spectrum in DMSO-d6. With the
addition of F-, the protons of benzene ring adjacent to imine showed spectral pattern change and
an upfield shift, probably due to increased electron density owing to H-bonding interactions of F-
ion with OH [39-40]. Since the signal of OH did not appear for receptor 1, deprotonation
followed by hydrogen bonding could not be ascertained.
Fig. 5: 1H–NMR titration of 1 (4 × 10-2 M) with incremental addition of TBAF in DMSO.
3.4 Plausible sensing mechanism
Scheme 2 demonstrates the plausible sensing mechanism of receptor 1 with F- and Al3+/Hg2+
1
1
ions on the basis of Job’s plot and H NMR titrations. As suggested by the H NMR titration
experiments, the -OH of receptor 1 might be due to hydrogen bonding interactions with F- ion.
The selectivity of the receptor 1 for F- ions may be justified on the basis of its characteristics like
high basicity, small size and high charge density which makes it better fit for hydrogen bonding
and dep otonation inte actions. On the othe hand, the active pa ticipation of phen N’s and
cooperative participation of salicylaldimine oxygen and imine nitrogen might be contributing in
Al3+/Hg2+ ion binding corresponding to 1:2 stoichiometry (Scheme 2).

Scheme 2. Plausible sensing mechanism of interaction of receptor 1 with F-, Al3+/Hg2+ ions.
3.5 Elaboration of molecular photonic logic systems
Careful analysis of absorption changes observed with individual addition of Al3+ and F- ions and
their binding constant values reveals that different logic operations can be viewed in receptor 1
by observing different output values. Recently, the conversion of chemically encoded
information into optical (fluorescence / absorbance) signals for the development of sequential
logic devices, has attracted tremendous attention for unconvential computing.
The molecular switching behavior of receptor 1 with two chemical inputs (Al3+ and F-) can
be demonstrated with the help of binary logic. Depending upon the two chemical inputs Al3+ and
F- (400 μM), the receptor 1 gave different absorption bands that helps in elaboration of multi
logic operations within single molecule. There are two input signals, viz. In1 (Al3+) and In2 (F-),
whereas the output signals are, Out1 (A300 nm) and out2 (A436 nm). Figure 6A displayed different
absorption spectra observed with addition of two inputs. Monitoring the absorption changes at
300 nm and 436 nm i.e. the addition of Al3+, F- and an equimolar mixture of Al3+ and F-, led to
“TRANSFER” and “INHIBIT” logic gates, respectively as represented in truth table (fig. 6B).
(A)
(B)
In1
(Al3+) (F- (A300
)
In2
Out1
Out2
(A436)
)
0
1
0
1
0
0
1
1
0
1
0
1
0
0
1
0
INHIBIT
Figure 4. (A) Molecular scale implementation of different logic gates; (a).blank; (b).1 + Al3+; (c).1 + F-; (d).1+ F- +
Al3+ and (e). 1 + Al3++ F- (B) T uth table fo “NOR” and “TRANSFER” gates.

“INHIBIT” is neither communicative nor associative and it can be viewed as two input
AND gates, one of whose input lines contain an inverter [41]. Here, addition of F- induced the
appearance of the absorption band at 436 nm, acting as ON behavior, whereas Hg2+ addition led
to only small absorption intensity at 436 nm, acting as the OFF switch. Receptor 1 (20 µM) gave
“ ” output at 436 nm with addition of Al3+ also, which on addition of F- was converted to high
output at 436 nm. But addition of both Al3+ (4 μM) and F- (4 μM) resulted in low output at
436 nm and thus Al3+ inhibited the effect of F- and is te med as ‘INHIBIT’ logic gate (Fig. 6).
Moreover, by changing the observation wavelength from 436 nm to 300 nm, the spectral
changes led to “TRANSFER” logic gate [42]. The output of TRANSFER gate is high when
either one of the inputs or both of the inputs are high.
Conclusions:
The present work reports the new salicylidene Schiff base (1) receptor possessing 1, 10-
phenanthroline for selective recognition of F- ions and sensing of Al3+ and Hg2+ ions. The
hydrogen bonding interactions between the –OH groups of the phenyl ring of salicylaldimine and
F- ions are responsible for the recognition event. On the contrary, the phenanth oline N’s and the
availability of hydroxyl and imine groups in the same receptor molecule (1) account for the 1:2
stoichiometry with Al3+ and Hg2+ ions. The differential outputs observed by sequential addition
of Al3+ and F- ions to solution of 1 led to elabo ation of “TRANSFER” and “INHIBIT”
molecular logic gates at 300 nm and 436 nm, respectively.
Acknowledgements
The authors are greatly thankful to SAIF, Panjab University Chandigarh for recording the NMR
and Mass spectra and are grateful to DST (Grant no. SR/FT/CS-36/2011) and DST PURSE-II
(Grant no. 48/RPC) for the financial assistance.
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 Novel salicylidene Schiff base (1) receptor appended with 1, 10-phenanthroline for
selectively sensing F- ions.
 Colorless solution of 1 turns yellow with F- ions.
 Receptor 1 recognizes Al3+ and Hg2+ ions with 1:2 stoichiometry.
 Receptor 1 mimics the functioning of “TRANSFER” and “INHIBIT” molecula logic
gates.