Rational Design of Azo-Azomethine Receptors for Sensing of Inorganic Fluoride: Construction of Molecular Logic Gates and DFT Study

Article abstract of DOI:10.1071/CH17310

New azo-azomethine receptors, HLn (n≤1-3), have been synthesised via condensation reaction of 5-(4-X-phenyl)-azo-salicylaldehyde (X≤NO2, Cl and CH3) with (4-nitrobenzylidene)hydrazine. The receptor with a p-NO2 substituent on the aromatic ring of the azo moiety (HL1) has excellent sensitivity and selectivity towards basic anions with proper discrimination between F- and AcO- or H2PO4- in DMSO-water (4:1). A Job's plot displays a 1:1 stoichiometry between HL1 and F- alone with a detection limit of 0.737M for fluoride ions. The solvatochromic behaviour of HL1 was probed by studying its UV-vis spectra in four pure organic solvents of different polarities and a meaningful correlation was observed. Furthermore, HL1 was used for detection of inorganic fluoride in toothpaste. The systematic density functional theory (DFT) and time dependent-DFT calculations have been carried out to investigate the mechanism of colourimetric sensing of fluoride ion by HL1 in the gas phase and in solution. Moreover, by using F- and H+ as chemical inputs, and the absorbance as output, a INHIBIT logic gate was constructed, which exhibits 'Write-Read-Erase-Read' ability without obvious degradation in its optical output.

Full text of DOI:10.1071/CH17310

CSIRO PUBLISHING
Aust. J. Chem. 2017, 70, 1254–1262
Full Paper
https://doi.org/10.1071/CH17310
Rational Design of Azo-Azomethine Receptors for Sensing
of Inorganic Fluoride: Construction of Molecular Logic
Gates and DFT Study
A
,
B
A
Hamid Khanmohammadi,
and Nafiseh Shabani
Khatereh Rezaeian,
A
A
Department of Chemistry, Faculty of Science, Arak University, Arak 38156-8-8349,
Iran.
B
Corresponding author. Email: h-khanmohammadi@araku.ac.ir
n
New azo-azomethine receptors, HL (n ¼ 1–3), have been synthesised via condensation reaction of 5-(4-X-phenyl)-
azo-salicylaldehyde (X ¼ NO , Cl and CH ) with (4-nitrobenzylidene)hydrazine. The receptor with a p-NO substituent
2
3
2
1
on the aromatic ring of the azo moiety (HL ) has excellent sensitivity and selectivity towards basic anions with proper
ꢀ
ꢀ
ꢀ
discrimination between F and AcO or H PO in DMSO–water (4 : 1). A Job’s plot displays a 1 : 1 stoichiometry
2
4
1
ꢀ
1
between HL and F alone with a detection limit of 0.737 mM for fluoride ions. The solvatochromic behaviour of HL was
probed by studying its UV-vis spectra in four pure organic solvents of different polarities and a meaningful correlation was
1
observed. Furthermore, HL was used for detection of inorganic fluoride in toothpaste. The systematic density functional
theory (DFT) and time dependent-DFT calculations have been carried out to investigate the mechanism of colourimetric
sensing of fluoride ion by HL in the gas phase and in solution. Moreover, by using F and H as chemical inputs, and the
absorbance as output, a INHIBIT logic gate was constructed, which exhibits ‘Write–Read–Erase–Read’ ability without
obvious degradation in its optical output.
1
ꢀ
þ
Manuscript received: 7 June 2017.
Manuscript accepted: 29 June 2017.
Published online: 10 August 2017.
Introduction
allow naked eye detection of fluoride in an easy way. It is always
preferable to have potential hydrogen bond donor groups, such as
–OH or –NH, in chemosensors, which in turn can increase the
In recent years, the development of artificial receptors for rec-
ognition and colourimetric sensing of biologically important
anions has gained much significance.
several chemoreceptors have been designed and evaluated for
naked eye detection of various anions without resorting to any
expensive spectroscopic instruments.
recently, much attention has been paid to the synthesis and use of
Schiff base receptors, as privileged chemosensors which can be
easily prepared, for naked eye detection of anions in organic,
[
1–3]
ꢀ
For this purpose
attraction of the sensor towards F via highly dissociable pro-
Accordingly, highly sensitive and easy-to-prepare
[19,20]
tons.
salicylaldehyde-based Schiff base colourimetric receptors, which
are regarded as the hydrogen donors to form real hydrogen bonds
[
4,5]
On the other hand,
ꢀ
between fluoride ions and corresponding receptors R–O–H?F ,
have become particularly attractive.
[21,22]
Moreover, chemosensors based on colourimetric receptors
can be further explored for the construction of molecular devices
such as molecular logic gates, switches, diodes, wires, and
[
6–8]
aqueous, and semi-aqueous media.
Among the biologically important anions, F is of particular
interest owing to its established role in biological systems and in
ꢀ
[
23]
molecular keypads.
considered advanced microdevices capable of performing binary
In this context, Boolean logic gates are
[
9–11]
our life.
For instance, fluoride ions are especially useful in
[7,24,25]
arithmetic and logical operations based on molecules.
the treatment of osteoporosis, dental care, drinking water quality
control, and detection of chemical warfare agents. However,
high doses of fluoride ions cause skeletal fluorosis, thyroid
activity depression, bone disorders, and immune system disrup-
These systems are believed to alter the chemically encoded
information as input into optical changes as readable output
signals, leading to the development of molecular logic gates
such as AND, OR, XOR, NAND, NOR, and INHIBIT logic
[
12,13]
tion.
Besides the above emphasis, the detection of fluoride
ions by using simple preparation and minimal instrumental
[
26–28]
gates.
been devoted towards integrated logic gates such as INHIBIT,
During the past years, extensive investigations have
[
14,15]
assistance is desirable towards practical applications.
date several receptors with amide, urea, thiourea, amidourea,
pyrrole, imidazolium, and indole moieties have been reported
which are capable to detect fluoride ions with high affinity.
Nevertheless, in most cases, the reported receptors are consid-
erably expensive and difficult to prepare. Particular interest
in this regard is inexpensive colourimetric sensing, which would
To
[29]
half-adder and half-subtractor with single receptors.
In pursuit of these, we have designed and synthesised new
n
[
16,17]
azo-azomethine based colourimetric receptors, HL (n ¼ 1–3)
(Scheme 1). The prepared receptors were characterised using
[
18]
1
spectroscopic methods (FT-IR, UV-vis, and H NMR) as well as
elemental analysis data. The sensing ability of the receptors
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Journal compilation Ó CSIRO 2017

Azo-Azomethine Receptors for Sensing of Inorganic Fluoride
1255
H
O
H N
N
2
NO2
R
N
N
OH
H
1
–3
H
N
N
H
NO2
R
N
N
OH
HL1 HL2 HL3
R
NO2 Cl CH3
n
Scheme 1. Azo-azomethine receptors, HL (n ¼ 1–3).
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
towards various anions (such as F , Cl , Br , AcO , I ,
ꢀ
4-methylaniline were obtained from either Aldrich or Merck.
Azo-coupled precursors, 1–3, were prepared as described
previously.
ꢀ
ꢀ
ꢀ
ꢀ
H PO , N3 , NO2 , NO3 , and HSO4 as their tetrabutylam-
4
2
[
30]
monium (TBA) salts) has been investigated. In these circum-
ꢀ
stances, all receptors sense the more basic anions such as F ,
ꢀ
ꢀ
1
AcO , and H PO . However, it was observed that HL , with a
Instrumentation
2
4
p-NO substituent on the aromatic ring of the azo moiety, has
2
1
H NMR spectra were recorded on a Bruker Avance 300 MHz
spectrometer. FT-IR spectra were recorded as a pressed KBr
disc, using a Unicom Galaxy Series FT-IR 5000 spectropho-
tometer in the region of 400–4000 cm . Melting points were
determined on an Electrothermal 9200 apparatus. Electronic
spectroscopic measurements were carried out using an Optizen
excellent sensitivity and selectivity towards basic anions with
ꢀ
ꢀ
ꢀ
proper discrimination between F and AcO or H PO in
2
4
–
DMSO–water (4 : 1). Furthermore, the recognition details of F
sensing were evaluated using UV-vis spectroscopy and time-
dependent density functional theory (TD-DFT) methods. The
ꢀ1
1
molecular structure of HL has been optimised at the standard
3220 UV in the range 200–700 nm. C, H, and N, analyses were
performed on a Vario EL III elemental analyzer.
level of theory. The energies of the highest occupied molecular
orbitals and lowest un-occupied molecular orbitals (HOMO and
LUMO) of the energetically stable tautomer, enol–amine, were
also obtained, before and after reaction with fluoride ions, in the
gas phase and in solution. The result indicated that the energy
Computational Study
Computational studies were carried out using the Gaussian 09
[
31]
1
software package.
tions was prepared using the GaussView package.
prepared compound’s structure was fully optimised using the
The starting structure for DFT calcula-
gap between the HOMO and LUMO of HL decreased in the
presence of fluoride ions due to complex formation.
Moreover, the remarkable UV-vis spectroscopic responses
[
32]
Each
[
33,34]
1
of HL in DMSO towards fluoride ions and protons, as external
B3LYP/6–311G (p, d) level of theory.
interactions were calculated using the polarisable continuum
The solute–solvent
chemical stimuli, inspired us to design circuits for electronic
devices (i.e. molecular logic gates). Accordingly, the potential
[35]
model (PCM).
Visualisation of the UV-vis spectra of the
1
circumstances provide opportunities for HL to mimic the
INHIBIT (integrated by combining a NOT, a YES, and an
receptors and their complexes was carried out using the Che-
missian program http://www.chemissian.com.
þ
ꢀ
AND gate) and IMP gates using H and F as chemical inputs,
and monitoring absorbance as output. More interestingly, the
spectroscopic responses as well as the colour change can be
Synthesis
Synthesis of (4-Nitrobenzylidene)hydrazine I
The mono-iminated precursor was prepared according to a
ꢀ
switched back and forth by successive addition of F and
trifluoroacetic acid (TFA), as a proton donor, and which may
be depicted by a complementary ‘IMP/INH’ molecular logic
gate. Furthermore, based on this reversible and reproducible
switching behaviour, a potential ‘Write–Read–Erase–Read’
memory function possessing a ‘Multi-Write’ ability has been
proposed.
[
36]
modified previously reported method. A solution of 4-nitro-
benzaldehyde (1.51 g, 1 mmol) in 10 mL of EtOH was added
dropwise, at room temperature, to a solution of N H (0.10 g,
2
4
3
.12 mmol) in 20 mL of EtOH over a period of 1 h with stirring.
After the addition was complete, the mixture was kept in a
refrigerator for 24 h. The formed solid was collected by filtra-
tion, washed with diethyl ether, recrystallised in EtOH,
and dried at room temperature. Yield: 1.0 g (60 % based on
Experimental
Materials
4
ppm) 7.51 (s, 2H), 7.66 (d, 2H, J 8.50), 7.74 (s, 1H), 8.15 (d, 2H,
-nitrobenzaldehyde), mp 146–1488C. d (DMSO-d , 300 MHz,
6
All of the reagents and solvents involved in the synthesis were of
analytical grade and used as received without further purifica-
tion. 4-Nitrobenzaldehyde, 4-nitroaniline, 4-chloroaniline, and
ꢀ
1
J 8.50). vmax (KBr)/cm 3424 (NH ), 1630 (C=N), 1578 (C=C),
2
1507 (NO ), 1327 (NO ), 1103 and 841.
2
2

1
256
H. Khanmohammadi, K. Rezaeian, and N. Shabani
General Procedure for the Synthesis of Azo-Azomethine
Receptors, HL –HL
Results and Discussion
Synthesis
1
3
A solution of (4-nitrobenzylidene)hydrazine (I) (0.165 g,
mmol) in 10 mL of absolute EtOH was added with stirring to
n
The azo-azomethine receptors HL (n ¼ 1–3), were obtained by
1
the condensation reaction of azo-coupled precursors 1–3 with
(4-nitrobenzylidene)hydrazine (I) in EtOH (Scheme 1). The
compounds were fully characterised using IR and NMR spec-
troscopy as well as microanalytical data (Supplementary
Material). The total absence of the v(C=O) absorption band of
a solution of azo-coupled precursors 1–3 (1 mmol) in 50 mL of
absolute EtOH during a period of 10 min at 608C. The solution
was heated in a water bath for 12 h at 808C with stirring. The
mixture was filtered while hot and the obtained solid was
washed with hot EtOH (three times) and then with diethyl ether.
The resultant product was dried in air.
ꢀ1
the azo-coupled precursors in the IR spectra at 1666–1668 cm
together with the appearance of a new absorption band at 1628–
1
-(4-(Hydrazonomethyl)phenyl)-2-(4-nitrophenyl)diazene
ꢀ1
1
were formed (Table 1).
632 cm obviously indicate that the new azo-azomethines
n
1
HL ). Orange solid. Yield: 95 %, mp 257–2598C. dH
(
[37,38]
1
In the H NMR spectra of HL
(
(
3
1
DMSO-d , 300 MHz) 7.22 (d, 1H, J 8.79), 8.07 (m, 5H), 8.41
6
(n ¼ 1–3), the CH=N protons appear as two singlet resonances at
m, 5H), 8.98 (s, 1H), 9.10 (s, 1H), 11.94 (b, 1H). d (DMSO-d ,
C 6
8.90–9.98 ppm (Supplementary Material). The broad signal at
11.50–12.00 ppm was assigned to the OH proton, as was con-
firmed by deuterium exchange when D O was added to a
00 MHz) 152.2, 124.6, 123.2, 160.8, 144.7, 123.9, 126.7,
16.0, 166.3, 118.4, 147.1, 146.4, 135.9, 128.1, 124.0, 149.2.
2
ꢀ
1
vmax (KBr)/cm 1628 (C=N), 1603 (C=C), 1518 (NO ), 1460
2
n
DMSO-d solution of HL (n ¼ 1–3).
6
(
(
1
[
N=N), 1341 (NO ), 1277, 1107, 854. l
e /M cm ) 280 (26933), 355 (41633), 395 (42233). m/z
(DMSO)/nm
2
max
ꢀ1
ꢀ1
The Electronic Absorption Spectra: Substituent Effect
22.0, 149.1, 269.2 (molecular ion peak was not observed:
þ
The electronic absorption spectra data of the azo-coupled pre-
cursors and their related azo-azomethine receptors, recorded in
DMSO at room temperature, are given in Table 1.
M] ¼ 417.1). Anal. Calc. for C H N O : C 57.42, H 3.37,
2
0 14 6 5
N 20.09. Found: C 57.10, H 3.22, N 20.31 %.
-(4-(Hydrazonomethyl)phenyl)-2-(4-chlorophenyl)diazene
1
2
n
The electronic absorption spectra of HL (n ¼ 1–3), in
(
d6, 300 MHz) 7.18 (m, 2H), 7.67 (m, 1H), 7.88 (d, 4H, J 7.93),
HL ). Yellow solid. Yield: 90 %, mp . 3008C. d (DMSO-
H
DMSO, mainly display two or three bands (Supplementary
Material). The first UV band located at 245–280 nm can be
assigned to the moderate energy (p-p*) transition of the
aromatic rings while the second band at ,350 nm is due to
8
3
1
.16 (d, 4H, J 8.46), 9.11 (s, 2H), 11.80 (br, 1H). d (DMSO-d ,
C 6
00 MHz) 137.1, 126.4, 122.2, 152.1, 141.7, 123.2, 127.1,
16.1, 164.3, 118.3, 145.1, 146.4, 135.9, 128.1, 124.0, 149.2.
ꢀ1
the low energy (p-p*) transition involving the p-electrons of
vmax (KBr)/cm 1630 (C=N), 1576, 1481 (N=N), 1400, 1313
ꢀ
[39]
ꢀ
1
the azo and azomethine groups.
It can be found that the
(
2
NO ), 1277, 1196, 837, 743. lmax (DMSO)/nm (e /M cm )
2
maximum absorption band, at ,340 nm, slightly shifted bath-
ochromically along with the increase in the electron-accepting
2
ability of substituents in the sequence of HL . HL . HL ,
63 (11500), 348 (34366) in DMSO. m/z 111.0, 122.0, 149.1,
þ
2
Anal. Calc. for C H ClN O : C 58.90, H 3.46, N 17.17.
58.7 (molecular ion peak was not observed: [M] ¼ 407.0).
1
3
2
0
14
5 3
consistent with an increase in molecular donor–acceptor polar-
Found: C 58.77, H 3.60, N 17.35 %.
1
[40]
1
isation (Table 1). The electronic absorption spectrum of HL ,
recorded in DMSO at room temperature, displays another intense
band at ,395 nm, due to charge transfer transitions involving the
-(4-(Hydrazonomethyl)phenyl)-2-(4-methylphenyl)diazene
3
HL ). Yellow solid. Yield: 95 %, mp 238–2408C. dH
(
(
(
DMSO-d , 300 MHz) 2.40 (s, 3H), 7.16 (d, 1H, J 8.79), 7.39
6
d, 2H, J 7.93), 7.78 (d, 2H, J 7.93), 7.97 (d, 1H, J 11.65), 8.15 (d,
[40]
whole molecule of the receptor (Supplementary Material).
2
1
1
1
1
H, J 8.43), 8.36 (m, 3H), 8.98 (s, 1H), 9.10 (s, 1H) 11.73 (br,
H). d (DMSO-d , 300 MHz) 24.1, 139.2, 128.4, 122.9, 147.1,
Anion Sensing Study
C
6
40.7, 123.1, 126.1, 116.1, 163.3, 118.2, 145.1, 146.4, 135.9,
To provide fundamental insights into the suitability of the pre-
pared receptors as colourimetric anion sensors, the anion sensing
behaviour was first evaluated qualitatively by visual examination
of the colour changes of a DMSO solution of the receptors after
ꢀ1
28.1, 124.0, 149.2. vmax (KBr)/cm 1632 (C=N), 1603 (C=C),
522 (NO ), 1489 (N=N), 1341 (NO ), 1279, 1109, 843. l
2
2
max
ꢀ
1
ꢀ1
(DMSO)/nm (e /M cm ) 250 (1633), 345 (43100).m/z 911.0,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
addition of anions, such as F , Cl , Br , AcO , I , H PO ,
4
1
22.0, 149.1, 238.0 (molecular ion peak was not observed:
þ
2
ꢀ
ꢀ
ꢀ
ꢀ
[M] ¼ 387.1). Anal. Calc. for C H N O : C 65.11, H 4.42,
N3 , NO2 , NO3 , and HSO4 as their tetrabutylammonium
(TBA) salts. The anion recognition behaviour of the current
2
1 17 5 3
N 18.08. Found: C 64.90, H 4.50, N 18.19 %.
Table 1. Tentative assignments of some selected IR frequencies and UV-vis data of the prepared azo-azomethine dyes and their azo precursors
A
IR frequency [cm
ꢀ1
ꢀ1
ꢀ1
Compound
]
lmax [nm] (e [M cm ]) in DMSO
v(C–O)
v(C=O)
v(C=C)
v(C=N)
v(NO
2
)
v(NH
2
)
v(N=N)
1
2
3
I
1288
1286
1287
—
1666
1668
1668
—
1608
1622
1620
1578
1603
1597
1605
—
—
1525, 1346
—
—
—
—
1479
1477
1481
—
246 (4500), 382(12700), 551 (1850)
256 (6490), 410 (7850), 466(9345)
260 (7580), 344 (23400), 437 (3720)
—
—
—
1630
1628
1630
1632
1507, 1327
1518, 1341
1524, 1346
1522, 1341
3424
—
1
HL
1277
1279
1270
—
1479
1483
1489
280 (26933), 355 (41633), 395 (42233)
263 (11500), 348 (34366)
2
HL
—
—
3
HL
—
—
250 (1633), 345 (43100)
A
KBr discs.

Azo-Azomethine Receptors for Sensing of Inorganic Fluoride
1257
receptors in DMSO was explored using UV-vis spectroscopy
upon the addition of 10 equiv. of the anions. Detailed spectro-
scopic studies revealed that the addition of the relatively basic
centred at ,355 nm, ascribed to the p-p* transition of the
chromophores, decreased and simultaneously the band at
,600 nm, attributed to the charge transfer and interaction
between the fluoride ion and receptor, appeared and developed
remarkably with a distinct isosbestic point at 434 nm. These
observations were in accordance with the dramatic colour
change from yellow to dark blue. Moreover, the Benesi–
Hildebrand plot
1
HL with anionic guests provided a straight line (Fig. 2), corrob-
1
orating the 1 : 1 ratio between HL with a deprotonation constant
ꢀ
ꢀ
ꢀ
anions such as F , AcO , and H PO elicited marked responses
4
2
n
in the electronic spectra of HL (n ¼ 1–3) whereas weakly basic
anions failed to cause any conspicuous spectroscopic changes
(Supplementary Material). It has been postulated that among
[43]
ꢀ
of 1/[A – A ] versus 1/[F ] for titration of
0
three of the commonly basic anions tested, fluoride and acetate
with a higher basicity are anticipated to form strong complexes
with sensors and consequently exhibited remarkable spectro-
scopic changes.
From the viewpoint of practical applications of anion sen-
sors, it is crucial that the probes operate in water-containing
media. This objective can be accomplished utilising the sensor
decorated by electron-withdrawing substituents to detect anions
5
ꢀ1
ꢀ
of 2.74 ꢁ 10 M towards F . The stoichiometry of the host–
guest interaction was also authenticated by use of continuous-
variation plots (Job’s plots) (Fig. 3). Interestingly, the current
ꢀ
ꢀ7
system can detect F at a low limit of ,7.37ꢁ 10 M.
[
22]
through the deprotonation mechanism.
the practicability of the sensors in an aqueous environment, we
Hence, to evaluate
Proton NMR Spectra
In the next step, to shed light on the nature of the interaction
n
monitored the UV-vis spectra of HL (n ¼ 1–3) with increasing
1
between receptor HL and fluoride, H NMR titration experi-
1
1
water content, from 9 : 1 to 4 : 1 DMSO–water solutions. HL
1
ments were conducted. Fig. 4 displays the H NMR spectra of
was found to be tolerant to the presence of water under 4 : 1
DMSO–water conditions and evidenced good selectivity for
fluoride over acetate since it showed a higher extinction coeffi-
1
1
HL with different amounts of TBAF in DMSO-d . The H
6
NMR spectrum of the free receptor exhibited a broad signal at
11.94 ppm, assigned to the phenolic OH proton, as was
authenticated by a D O exchange experiment. Interestingly, as
ꢀ
ꢀ
cient with F than that obtained with AcO . Surprisingly, the
ꢀ
2
addition of H PO , as one of the basic anions, did not induce
2
4
expected, the OH proton of the azo phenol entirely vanished,
ꢀ
colour and spectroscopic changes. Accordingly, it is worth
1
noting that HL is capable of allowing a rough discrimination
reflecting deprotonation when 1 equiv. of F was added into the
1
DMSO-d solution of HL . Simultaneously, all aromatic and
6
ꢀ
ꢀ
ꢀ
of the three common basic anions (F , AcO , and H PO ).
4
2
azomethine proton signals underwent a continuous upfield shift.
The profound upfield shift is attributed to the considerable
These spectroscopic changes also provided colourimetric
changes from light yellow to dark blue which can be visually
1
2
3
recognised. In contrast to HL , the HL and HL receptors
exhibited minor spectroscopic changes on addition of basic
anions in a 4 : 1 DMSO–water solution. The above observation
can be ascribed to the fact that the introduction of another
3.5
3
2
.0
.5
electron-withdrawing (NO ) group into the backbone of the
2
molecular sensor augments the acidity of active sites. Therefore
1
the receptor HL is able to compete with water to sense a trace
amount of the basic anions.
Encouraged by the favourable features of the sensory system,
1
we employed HL as a colourimetric sensor for practical
2.0
1
1
.5
.0
[
41,42]
0.5
0
purposes and to detect fluoride ions in an aqueous environment.
To gain a deeper insight into the binding characteristics of
0
1
2
3
4
5
6
1
sensor HL , UV-vis spectroscopic titrations were performed
1/[F ]) ꢁ 10 [Mꢀ1
ꢀ
ꢀ5
(
]
ꢀ
upon the addition of standard solutions of F as its TBA salt to
1
ꢀ5
ꢀ1
1
Fig. 2. Benesi–Hildebrand plot of sensor HL binding with F anion
ꢀ
HL (3 ꢁ 10 mol L ) in a 4 : 1 DMSO–water solution. As
ꢀ
depicted in Fig. 1, on an incremental increase of F , the band
associated with an absorbance change at 555 nm in 4 : 1 DMSO–water.
1
1
1
0
0
0
0
.4
.2
.0
.8
.6
.4
1.0
3
5
55 nm
55 nm
0.5
0
0
1
2
3
4
5
Equiv. of Fꢀ ions
.2
0
240
340
440
540
640
740
Wavelength [nm]
1
ꢀ5
ꢀ1
Fig. 1. UV-Vis titration of HL (3 ꢁ 10 mol L ) in 4 : 1 DMSO–water solution with incremental addition of TBAF. Inset
shows the binding isotherms at selected wavelengths.

1
258
H. Khanmohammadi, K. Rezaeian, and N. Shabani
augmentation of electron density on to the p-conjugated
framework brought by neat OH deprotonation.
excited state, a charge transfer transition will be established. As
a result, interactions between newly established dipole moments
and solvent dipole moments will be formed. Eventually, the
energy levels of the receptor change with solvent polarity. As the
polarity of the solvent increases, these energy levels decrease.
Hence, with increasing solvent polarity, the bands correspond-
ing to charge transfer (CT) transitions show a bathochromic shift
[
22,44]
In addition,
,121 Hz) appeared which is
1
a new triplet at 16.1 ppm ( J
HF
ꢀ
ꢀ
precisely ascribed to the FHF dimer. The generation of FHF
supports the supposition of the deprotonation event.
Solvatochromic Study
[
45]
ꢀ
1
(positive solvatochromism).
was compared with the dielectric constant of solvents which is
This positive solvatochromism
To our delight, upon the addition of 1 equiv. of F ions to a HL
ꢀ
5
solution in dry DMSO (3 ꢁ 10 M), a colour change from
yellow to dark blue was observed. This dissimilarity in colour
change (yellow to dark blue in DMSO and yellow to red in
dioxane) can be attributed to the solvatochromic effect of HL .
Surprisingly, this study was further extended to other aprotic
[
44]
depicted in Table 2.
The polarity of the solvent is related to
the dielectric constant. The greater the dielectric constant, the
greater the polarity and as a result with the increasing dielectric
constant the red-shift was expected. This colourimetric discrim-
ination in different solvents was successfully applied to deter-
mine the percentage composition of a binary solvent mixture.
1
1
solvents such as DMF and acetonitrile where the receptor HL
revealed different colouration and spectra only in the presence
ꢀ
of F ions with respect to the solvent polarity (Fig. 5).
ꢀ
2
Detection of F in Toothpaste
It is worth mentioning that the addition of F ions to the
1
receptor solutions induced charge separation. Since HL attains
different dipole moments in its ground and lower energy singlet
1
In order to examine the potential applicability of HL , a sample
ꢀ
containing commercial toothpaste was used for F detection.
ꢀ
1
The prepared sample consisted of 0.05 mg mL of toothpaste
(
Crest brand), 1 ꢁ 10ꢀ mol L of F ions (from a standard
5
ꢀ1
ꢀ5
ꢀ
ꢀ1
1
0
0
0
0
.0
.8
.6
.4
.2
0
1
solution of TBAF), and 2 ꢁ 10 mol L of receptor HL in 4 : 1
DMSO–water. The UV-vis absorption spectrum of the resultant
solution was measured and compared with the toothpaste-free F
ꢀ
ꢀ
solution. As is shown in Fig. 6, the signal from the F contami-
nated toothpaste solution was stronger than that from the one
without toothpaste which is ascribed to fluoride in toothpaste
1
confirming that sensor HL can be successfully employed for
ꢀ
qualitative detection of F in toothpaste.
Construction of Logic Gates
0
0.2
0.4
0.6
0.8
1.0
ꢀ
1
1
1
ꢀ
As discussed above, the addition of F (3 equiv.) to HL in
DMSO resulted in a colour change from yellow to dark blue and
emergence of a new absorption band at 600 nm, acting as ON
[
HL ]/[HL ] ꢃ [F ]
1
Fig. 3. Job’s plot for sensor HL with tetrabutylammonium fluoride using
ꢀ
UV-vis spectroscopy.
behaviour. Subsequently, the F -induced chromogenic process
HF2ꢀ
HL ꢃ 3 equiv. of Fꢀ
1
17.0
16.0
15.0
1
ꢀ
HL ꢃ 1 equiv. of F
OH
Free HL1
ꢂ
13.50 13.00 12.50 12.00 11.50 11.00
9.00
8.50
8.00
7.50
7.00
6.50
6.00
[ppm]
1
Fig. 4. H NMR spectra of HL in DMSO-d
1
(2 ꢁ 10ꢀ2 mol L ) in the absence and in the presence of different amounts
ꢀ1
6
of TBAF.

Azo-Azomethine Receptors for Sensing of Inorganic Fluoride
1259
ꢀ
was fully reversed by the addition of trifluoroacetic acid (TFA),
as a proton donor, acting as an OFF switch. Surprisingly, as
illustrated in Fig. 7, these procedures could be repeated over
several cycles which obviously manifest the high degree of
In these circumstances, only when F is present, the absorption
is ‘1’ while the values ofall other functions are ‘0’. Consequently,
a complementary IMP/INH function could be interpreted with a
single molecule and the logic circuit along with corresponding
truth table are shown in Fig. 8a, b.
1
reversibility. Overall, the results disclosed that the receptor HL
could be reused with proper treatment.
Encouraged by these observations, we further endeavoured
1
More interestingly, HL can be offered as a potential ‘Write–
Read–Erase–Read’ memory function through the absorption
output signal at 600 nm as depicted in Fig. 9. In the current
system, the ON state (Out2 ¼ 1) is defined as the strong absorp-
tion at 600 nm, whereas the OFF state (Out2 ¼ 0) corresponds to
the weak absorption at identical wavelength. The two chemical
inputs of F and TFA ions are designated In F and In T for the Set
(S) and Reset (R), respectively. When the Set input is high
(S ¼ 1), the system writes and memorises the binary state ‘1’.
Then, the stored information is erased by the Reset input (R¼ 1),
resulting in the writing and memorisation of binary state ‘0’.
These reversible and reconfigurable sequences of Set/Reset logic
operations can be represented in the form of a feedback loop,
ꢀ
þ
to construct a molecular logic gate by using F and H , from
TFA, as two chemical inputs (In F and In T, respectively) and
monitoring two different absorbances at 355 and 600 nm, as
outputs. To elucidate the design of the logic gate, the inputs and
optical outputs were coded with binary digits ‘0’ and ‘1’. If we
consider the absorbance at 355 nm as the output signal (Out1),
an ‘IMP’ logic gate could be obtained at the molecular level,
where the absorption was diminished only in the presence of
fluoride ion (In F) but in all other circumstances, it remained in
1
the ON state. Also, HL can behave as an INHIBIT gate by
monitoring the absorbance at 600 nm as another output (Out2).
(
a)
2
1
1
1
1
1
0
0
0
0
.0
.8
.6
.4
.2
.0
.8
.6
.4
.2
DMSO
Acetonitrile
DMF
0.4
Dioxane
0.3
0.2
0.1
0
0
3
00
400
500
600
700
800
Wavelength [nm]
HL1 ꢃ TBAF
HL1 ꢃ TBAF ꢃ Toothpaste
(
b)
DMSO
Dioxane DMF
Acetonitrile
Fig. 6. The proof of concept for detection of fluoride in toothpaste: (left)
1
1
HL þ TBAF; (right) HL þ TBAF þ toothpaste.
1
1
1
.5
.3
.1
1
Fig. 5. (a) Changes in the UV-vis spectrum of HL in various aprotic
solvents upon addition of excess TBAF; (b) variable colour changes
ꢀ
1
observed with the addition of F (3 equiv.) to the solutions of HL in
various polar aprotic solvents.
0.9
0
0
0
0
.7
.5
.3
.1
1
Table 2. Change in absorption maxima of HL in different solvents on
adding TBAF
Solvent
,4-Dioxane
Dielectric constant
l
max [nm]
0
1
2
3
4
5
6
7
1
2.21
35.49
36.71
46.8
545
545
595
600
Cycle
Acetonitrile
Dimethylformamide
Dimethyl sulfoxide
1
ꢀ5
ꢀ1
Fig. 7. Reversibility of the sensor HL (3 ꢁ 10 mol L ) with sequential
ꢀ
addition of F and TFA.
(
a)
(b)
INPUTS
OUTPUTS
In F
In T
IMP
INH
Out 1
Out 2
Out 1
Out 2
In F
In T
(355 nm) (600 nm)
0
0
1
1
0
1
0
1
1
1
0
1
0
0
1
0
Fig. 8. (a) The logic circuit and (b) the truth table for the complementary IMP/INH logic gate.

1
260
H. Khanmohammadi, K. Rezaeian, and N. Shabani
Writing
corroborating the memory feature with a ‘Write–Read–Erase–
Read’ function through the output signal at 600 nm (Fig. 9).
Set (In F) ꢂ 1
Calculation Details
Absorption
(Out2)
OFF
Absorption
(Out2)
ON
Reading
Reading
The molecular geometry of the receptors were fully optimised
using the Gaussian 09W package at the B3LYP/6–311G (p, d)
level. Vibrational frequency analysis, obtained at the same level,
indicate that the optimised structures are at the stationary points
corresponding to local minima without any imaginary fre-
quency. Total energy and HOMO/LUMO energies of all the
receptors, in the gas phase and in DMSO, were also examined. It
is found that the enol–imine tautomer is energetically more
Reset (In T) ꢂ 1
Erasing
Fig. 9. The feedback loop exhibiting reversible logic operations with
Write–Read–Erase–Read’ function.
‘
(
a)
(b)
(
c)
1
Fig. 10. The optimised geometry of enol–imine tautomer of the receptors: (a) HL , (b) HL , (c) HL .
2
3
(
a)
(b)
LUMO
LUMO
ΔE ꢂ 1.946 eV
ΔE ꢂ 3.272 eV
HOMO
HOMO
1
Fig. 11. HOMO–LUMO energy levels and the interfacial plots of the orbitals for (a) HL and (b) the L –F complex.
1

Azo-Azomethine Receptors for Sensing of Inorganic Fluoride
1261
1
2
1
˚
stable than the hydrazon form by ,303 (HL ), 309 (HL ), and 3
the formed complex L –F is 1.275 A which is shorter than the
C–O bond length in the free receptor indicating development of
a partial double bond character. This clearly indicated that
3
HL ) kcal mol . The optimised geometry of the enol–imine
tautomer of the molecules and their complexes with F anions,
in DMSO media, has been given in Fig. 10.
The orbital diagrams of the HOMO and LUMO of HL
(n ¼ 1–3) were investigated (Fig. 10) (Supplementary Material).
The energy gap between the HOMO and LUMO of HL , HL ,
and HL and their complexes with F , in DMSO, were 3.272,
.022, and 2.863 eV and 1.946, 1.810 and 1.723 eV, respectively,
reflecting substantial stabilisation due to complexation.
ꢀ1
(
ꢀ
1
ꢀ
[7]
deprotonation of HL by F occurred. The UV-vis frequency
n
1
1
calculation of the L –F complex showed that HL can be
characterised by absorption spectra exhibiting one intense band
at ,600 nm, corresponding to the HOMO-LUMO excitation.
This absorption band agreed with that obtained experimentally
at 595 nm. Visualisation of the UV-vis spectra of HL and the
1
L –F complex was carried out using the Chemissian program,
http://www.chemissian.com (Fig. 12).
The theoretical determination of thermodynamic parameters
1
(DG, DH, and TDS) and structural optimisation of HL with
1
2
3
ꢀ
1
3
ꢀ
The HOMOofthe receptors, beforeand after reactionwithF ,
were more distributed on the 4-X-phenol-azo (X¼ NO , Cl and
2
ꢀ
CH ) moieties. However, after reaction with F the LUMO of the
3
ꢀ
deprotonated species were located on the azo and 4-nitrobenzal-
dehyde moieties (Fig. 11). This indicated the binding of fluoride
1
with HL due to the homogenisation of electron density through
hydrated F anions provide further insight into the source of
ꢀ
interactions between hydrated F ions with receptors in partial
ꢀ
1
aqueous medium. The complexation of HL with F at 298 K
ꢀ
1
ꢀ1
extended p-conjugation, which lead to the homogeneous distri-
bution of electron density throughout the molecule.
gives DG ¼ ꢀ5.42 kcal mol , DH ¼ ꢀ1.36 kcal mol , and
ꢀ
1
TDS ¼ 4.06 kcal mol .The negative enthalpy and positive
ꢀ
1
The selected bonds length of HL and its complex, L –F, are
1
1
entropy changes indicated that binding between HL and F
was created through hydrogen bonding and van der Waals
1
given in Table 3. The O–H bond distance in the L –F complex is
[
46]
˚
.449 A significantly longer than the O–H bond length (0.965 A)
˚
1
of the free receptor. On the other hand, the C–O bond length in
interactions.
Conclusion
n
Table 3. Calculated (B3LYP/6-311G (p, d)) of selected
In the present work three new azo-azomethine receptors, HL
(
1
bond length of HL and its complex L –F
1
n ¼ 1–3), were synthesised via condensation reaction of
5
-(4-X-phenyl)-azo-salicylaldehyde (X ¼ NO , Cl and CH )
2
3
˚
Bond length [A]
Compound
1
–3 with (4-nitrobenzylidene)hydrazine precursors.
The receptor with a p-NO substituent on the aromatic ring of
2
O–H
C–O
1
the azo moiety (HL ) has excellent sensitivity and selectivity
1
HL
1
L –F
0.965
1.449
1.350
1.275
ꢀ
1
towards F in DMSO–water (4 : 1) solution. The H NMR
titration revealed that the colourimetric response was considered
(
a)
1
0
0
0
0
0
0
0
0
0
.0
Calculated
.9
.8
.7
.6
.5
.4
.3
.2
.1
0
Experimental
250
300
350
400
450
500
550
600
650
700
Wavelength [nm]
(
b)
0
0
0
0
0
0
0
0
0
.9
.8
.7
.6
.5
.4
.3
.2
.1
0
Calculated
Experimental
200 250 300 350 400 450 500 550 600 650 700 750 800 850
Wavelength [nm]
1
Fig. 12. Comparisons of (a) experimental and calculated absorption spectra of HL and (b) calculated
1
absorption spectrum of the L –F complex.

1
262
H. Khanmohammadi, K. Rezaeian, and N. Shabani
to be the direct consequence of hydrogen-bond formation
1
between phenolic groups of HL and the fluoride ion followed
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by deprotonation. The mechanism of colourimetric sensing was
corroborated by systematic DFT and TD-DFT calculations.
ꢀ
þ
[23] D. Margulies, C. E. Felder, G. Melman, A. Shanzer, J. Am. Chem. Soc.
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Furthermore, based on the distinct reversibility of F with H
1
as chemical inputs, HL has also shown a reversible switching
behaviour with complementary ‘IMP/INH’ logic function, which
mimics the functions of a logic circuit displaying ‘Write–Read–
Erase–Read’ behaviour possessing ‘Multi-Write’ ability.
[
[
[
24] D. C. Magri, G. J. Brown, G. D. McClean, A. P. de Silva, J. Am. Chem.
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J.TETLET.2014.11.062
Characterisation data of all compounds, NMR spectra, IR and
UV-vis spectrophotometric titrations, stoichiometric determi-
nation by Benesi–Hildebrand method, Job’s plot, and DFT data
are available on the Journal’s website.
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[
[
Conflicts of Interest
6
The authors declare no conflicts of interest.
31] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb,
J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A.
Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F.
Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr, J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N.
Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell,
J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam,
M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo,
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi,
C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G.
Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich,
Acknowledgements
The authors express their gratitude to the Iran National Science Foundation
(INSF) for financial support of this research (Grant No. 94015419).
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