Crystallographic evidence for resonance assisted H-Bonding effect in selective colorimetric detection of cyanide by arylamino-naphthoquinones

Article abstract of DOI:10.1016/j.molstruc.2019.05.124

Five new chemo-receptors, based on arylamino-naphthoquinone, possessing electron donating and electron withdrawing substituents have been prepared by single step green method. These receptors are found to be highly selective and sensitive towards cyanide

Full text of DOI:10.1016/j.molstruc.2019.05.124

Accepted Manuscript
Crystallographic evidence for resonance assisted H-Bonding effect in selective
colorimetric detection of cyanide by arylamino-naphthoquinones
P. Rajalakshmi, P. Jayasudha, Samuele Ciattini, Laura Chelazzi, Kuppanagounder P.
Elango
PII:
S0022-2860(19)30696-9
DOI:
https://doi.org/10.1016/j.molstruc.2019.05.124
MOLSTR 26631
Reference:
To appear in: Journal of Molecular Structure
Received Date: 14 February 2019
Revised Date: 9 May 2019
Accepted Date: 28 May 2019
Please cite this article as: P. Rajalakshmi, P. Jayasudha, S. Ciattini, L. Chelazzi, K.P. Elango,
Crystallographic evidence for resonance assisted H-Bonding effect in selective colorimetric detection
of cyanide by arylamino-naphthoquinones, Journal of Molecular Structure (2019), doi: https://
doi.org/10.1016/j.molstruc.2019.05.124.
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ACCEPTED MANUSCRIPT
Crystallographic Evidence for Resonance Assisted H-Bonding Effect in Selective
Colorimetric Detection of Cyanide by Arylamino-Naphthoquinones
P. Rajalakshmi, P. Jayasudha and Kuppanagounder P. Elango*

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1
Crystallographic Evidence for Resonance Assisted H-Bonding Effect in Selective
Colorimetric Detection of Cyanide by Arylamino-Naphthoquinones
a
a
b
b
P. Rajalakshmi , P. Jayasudha , Samuele Ciattini , Laura Chelazzi and Kuppanagounder P.
Elango *
a
a
Department of Chemistry, Gandhigram Rural Institute (Deemed to be University), Gandhigram
6
24302, India
b
Centro di Cristallografia Strutturale, Università degli studi di Firenze, 50019, Sesto Fiorentino
Fi), Italy
(
Abstract
Five new chemo-receptors, based on arylamino-naphthoquinone, possessing electron donating
and electron withdrawing substituents have been prepared by single step green method. These
receptors are found to be highly selective and sensitive towards cyanide ions in aqueous HEPES
1
buffer solution (pH 7.2). The mechanism, as evidenced by H NMR, electrochemical and
theoretical studies, is based on instantaneous deprotonation of –NH moiety by cyanide, which is
enabled by substituent dependent resonance effect of the phenyl ring. This is confirmed by single
-8
crystal XRD data. The detection limit is calculated to be in the order of 10 M, which is much
-6
lower than the permissible limit of cyanide in drinking water set by WHO (1.9x10 M).
Keywords: Cyanide; Colorimetry; Quinone; Resonance; Sensing
*
Corresponding author. Tel.: +91 451 245 2371; Fax: +91 451 2454466
E-mail: drkpelango@rediffmail.com (Prof. K.P. Elango)

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1
.
Introduction
During recent past selective and sensitive methods to detect anions have received
substantial attention due to the fundamental and important roles of anions in environmental,
industrial and biological processes. Among the anions, cyanide ion sensing has attracted
relatively more attention due to its high toxicity [1]. Water soluble cyanide salts are one of the
most lethal chemicals and the permissible limit of cyanide in drinking water is 1.9 ꢀM, according
to World Health Organization (WHO) [2]. Review of literature revealed that the mechanism of
interaction between cyanide ion and chemoreceptors include predominantly H-bonding [3-6],
reaction-based sensors [7-14], electrostatic interaction [15-17] or metal-ligand interactions [18-
2
0]. From the practical application view-point, a good receptor should not only be highly
selective and sensitive but also easy to prepare and simple to operate. Thus, the developmental of
new chemosensors for colorimetric detection of cyanide in aqueous solution is essential.
Recently we are in the process of developing quinone based chemosensors for selective
colorimetric sensing of cyanide and fluoride ions in aqueous solution, that work via modulation
of intramolecular charge transfer (ICT) transition. We have reiterated the selection of quinone as
the signaling unit in all these reports. In continuation of our efforts to develop new chemo-
receptors for colorimetric detection of anions in aqueous solution, here in this article we report
the colorimetric sensing of cyanide by five arylamino-naphthoquinones (R1-R5) in aqueous
solution. The anion sensing behaviour of these receptors has been investigated using spectral
1
(
UV-Vis, Fluorescence, H NMR), electrochemical (Differential Pulse Voltammetry, DPV) and
theoretical (DFT) studies.

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The selection of these chemo-receptors for the present study is due to the following
reasons: i) we have reported an easy green protocol for the preparation of these molecules in
solid state from commercially available starting materials, ii) the molecules themselves absorbs
in the visible region and thus are coloured due to the existence of ICT transition from –NH
moiety to electron deficient quinone ring, iii) further, any small perturbation in the electron
density in the –NH receptor unit as a result of interaction with anion, would significantly alter
the easiness of ICT transition and consequently would change the colour, and iv) furthermore, by
changing the substituent R in the molecule, we can tune the acidity of the –NH proton and hence
can very its intensity of interaction with the anion.
For comparison purpose, we have selected the following molecules I-III and studied their
anion sensing behaviour also.
Interestingly, the chemo-receptors R1-R5 selectively detect cyanide ion in aqueous solution with
visible colour change, but not the other molecules (I-III). Based on the experimental results a

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plausible resonance enabled mechanism for the sensing process has been proposed and
discussed.
2
2
.
Experimental section
.1 Chemicals and apparatus. All the reagents for the preparation of the receptors were
obtained commercially and were used without further purification. Spectroscopic grade solvents
were used as received. Doubly distilled water was used throughout the work and the second
distillation was carried out using alkaline permanganate. UV-Vis spectral studies were carried
out on a double beam spectrophotometer. Steady state fluorescence spectra were obtained on a
-1
spectrofluorimeter. The excitation and emission slit width (5 nm) and the scan rate (250 mVs )
was kept constant for all of the experiments. Nuclear magnetic resonance spectra were recorded
1
in DMSO-d (400 MHz). The H NMR spectral data is expressed in the form: Chemical shift in
6
units of ppm (normalized integration, multiplicity, and the value of J in Hz). The differential
pulse voltammetry (DPV) experiments, of 1 mmol solutions of the compounds, were carried out
using GC as working, Pt wire as reference and Ag wire as auxiliary electrodes in
dimethylformamide containing 0.1 M tetrabutylammonium perchlorate as supporting electrolyte
-1
at a scan rate of 100 mVs .
.2 Preparation and characterization of receptors. The receptors were prepared as reported,
2
by us earlier, from commercially available 2,3-dichloro-1,4-naphthoquinone and para-
substituted anilines via solid state reaction [21]. The overall reaction is given in Scheme 1.

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Scheme 1. Preparation of receptors (R1-R5)
1
The receptors were characterized using H NMR and HRMS-ESI spectral techniques. The results
matched well with those reported earlier [21]. Receptor R1: Red solid (yield 85%); Melting
o
1
Point: 220 C; H (400 MHz; DMSO-d ; Me Si/ppm): 7.13-7.15 (m, 3H), 7.30-7.34 (t, J=8 Hz, 8
6
4
1
3
Hz, 2H), 7.79-7.89 (m, 2H), 8.03-8.05 (d, J=8Hz, 2H), 9.32 (s, 1H) (Fig. S1); C NMR (100
MHz, DMSO-d ) δ 114.71,124.43, 124.85, 126.55, 127.00, 128.39, 130.74, 132.43, 133.66,
6
1
35.26, 139.30, 143.62, 177.16, 180.59 (Fig. S2); LCMS (ESI-APCI) m/z: Calculated for
+
-1
C
16 2
H10ClNO [M+H] 283.7 found 284.1 (Fig. S3); FT-IR (KBr), cm : 3245 (N-H), 1675, 1637
o
1
(
C=O), 1331 (C-N) (Fig. S4); Receptor R2: Red solid (yield 85%); Melting Point: 240 C; H
(
400 MHz; DMSO-d ; Me Si/ppm): 7.08-7.10 (d, J=8 Hz, 2H), 7.48-7.50 (d, J=8 Hz, 2H), 7.80-
6
4
1
3
7
1
1
3
.90 (m, 2H), 8.04-8.06 (m, 2H) 9.35 (s, 1H) (Fig. S5); C NMR (100 MHz, DMSO-d ) δ
6
20.59, 121.45, 130.79, 131.33, 131.76, 135.55, 135.93, 137. 09, 138.52, 139.99, 143.62, 148.20,
+
82.01, 185. 23 (Fig. S6); LC-MS (ESI-APCI) m/z: Calculated for C H BrClNO [M+H]
1
6
9
2
-1
62.61 found 364. 1 (Fig. S7); FT-IR (KBr), cm : 3242 (N-H), 1675, 1638 (C=O), 1329 (C-N)
o
1
(
Fig. S8); Receptor R3: Red solid (yield 81%); Melting Point: 260 C; H (400 MHz; DMSO-d
6
;
Me Si/ppm): 7.13-7.15 (d, J=8 Hz, 2H), 7.35-7.38 (d, J=12 Hz, 2H), 7.80-7.89 (m, 2H), 8.03-
4

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1
3
8
1
.05 (d, J=8 Hz, 2H), 9.37 (s, 1H) (Fig. S9); C NMR (400 MHz, DMSO-d
6
) δ 115.65, 125.75,
26.58, 127.01, 128.28, 128.60, 130.80, 132.36, 133.75, 135.24, 138.44, 143.53, 177.25, 180.48
+
(
(
Fig. S10); LC-MS (ESI-APCI) m/z: Calculated for C H Cl NO [M+H] 318.15 found 318.1
1
6
9
2
2
-1
Fig. S11); FT-IR (KBr), cm : 3234 (N-H), 1675, 1639 (C=O), 1329 (C-N) (Fig. S12); Receptor
o
1
R4: Red solid (yield 83%); Melting Point: 202 C; H (400 MHz; DMSO-d ; Me Si/ppm): 2.30
6
4
(s, 3H), 7.02-7.04(d, J=8 Hz, 2H), 7.11-7.14 (d, J=12 Hz, 2H), 7.80-7.89 (m, 2H), 8.03-8.04 (d,
1
3
J= 4 Hz, 2H), 9.26 (s, 1H) (Fig. S13); C NMR (400 MHz, DMSO-d
6
) δ 21.03,113.79, 124.61,
1
26.52, 126.97, 128.86, 130.66, 132.49, 133.56, 134.20, 135.27, 136.65, 143.65, 177.03, 180.62
+
(
(
Fig. S14); LC-MS (ESI-APCI) m/z: Calculated for C17
H12ClNO
2
[M+H] 297.74 found 298.1
-1
Fig. S15); FT-IR (KBr), cm : 3226 (N-H), 1676, 1635 (C=O), 1329 (C-N) (Fig. S16); Receptor
o
1
R5: Red solid (yield 87%); Melting Point: 198 C; H (400 MHz; DMSO-d ; Me Si/ppm): 2.30
6
4
(s, 3H), 7.02-7.04 (d, J=8 Hz, 2H), 7.12-7.14 (d, J=8 Hz, 2H), 7.81-7.89 ( m, 2H), 8.03-8.04 (d,
1
3
J= 4 Hz, 2H), 9.26 (s, 1H) (Fig. S17); C NMR (400 MHz, DMSO-d ) δ 21.04, 113.79, 124.61,
6
1
26.53, 126.98,128.87, 130.67, 132.50, 133.58, 134.21, 135.28, 136.66, 143.67, 177.05, 180.63
+
(
(
Fig. S18); LC-MS (ESI-APCI) m/z: Calculated for C17
H
12ClNO
3
[M+H] 313.74 found 314.2
-1
Fig. S19); FT-IR (KBr), cm : 3228 (N-H); 1675, 1639 (C=O), 1329 (C-N) (Fig. S20).
The compound I is reported by us [22], II is commercially available and III is prepared
by adopting the reported procedure [23].
3
.
Results and discussion
Five new arylamino-naphthoquinone derivatives (R1-R5) were prepared and
characterized using NMR and mass spectral techniques. The anion sensing behaviour of these

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chemo-receptors has been investigated using various spectral, electrochemical and theoretical
studies. Based on the results a plausible mechanism has been proposed and discussed.
3
.1 Visual detection experiment. As a preliminary screening the change in colour of the
receptors in the absence and presence of one equivalent of various anions has been observed in
aqueous HEPES buffer:DMF (2:8 v/v) solution. As seen from Figures 1 and S21, the colour of
the receptor solutions changed instantaneously from red-orange to blue with the addition of
cyanide ion. However, addition of other chosen anions didn’t produce any colour change
indicating high selectivity of these receptors towards cyanide ion. Likewise, the change in colour
of the solution of R1, as a representative case, upon addition of cyanide ion at different pH
values was also observed (Fig. S22). The receptor R1 was found to produce similar colour
change in 7-11 pH range. Surprisingly the compounds I-III were not able to produce any colour
change with these anions, including cyanide ion in aqueous HEPES buffer:DMF (2:8 v/v)
solution (pH 7.2).
3
.2 UV-Vis spectral studies. With an aim to confirm the selectivity of these receptors towards
cyanide ion, the UV-Vis spectra of R1 were recorded, in aqueous HEPES buffer:DMF (2:8 v/v)
solution, with addition of one equivalent of various anions and the spectra obtained is depicted in
Figure 2. As evidenced from the spectra, addition of cyanide ion red-shifted the absorption
maximum of R1 from 481 nm to 589 nm (ꢁλ 109 nm). While all the other anions caused no
observable change in the spectrum of R1, confirming high selectivity of the receptor towards
cyanide ion. The UV-Vis spectra of these receptors with addition of incremental amounts of
cyanide ions were also recorded aqueous HEPES buffer:DMF (2:8 v/v) solution (Fig. 3 and S23-
2
6). In all these cases, upon addition of incremental amounts of cyanide ions, the absorption
maximum around 480 nm decreased with a concomitant increase in absorption maximum of a

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new peak around 590 nm with a single isosbestic point (Table 1). The appearance of single
isosbestic point indicated the presence of only two species in equilibrium [24]. Further, the high
magnitude of the molar extinction coefficient values of the new peak made the visual detection
of cyanide ion easy (Table 1).
3
.3 Fluorescence spectral studies. The binding constants for the interaction of these receptors
with cyanide ion were determined using fluorescence spectral titration data. The fluorescence
spectra of R1-R5 with incremental addition of cyanide in aqueous HEPES buffer:DMF (2:8 v/v)
solution have been recorded and the spectra obtained are given in Figures 4 and S27-30. It is
evident from the spectra that addition of incremental amounts of cyanide was found to
significantly quench the fluorescence of these receptors, suggesting strong interaction between
them. From the fluorescence quenching data the binding constants for the interaction between the
receptors and cyanide were calculated using the following equation [25, 26].
0 A
log (F -F)/F = log K + n log [Q]
Where F is the emission intensity in the absence of quencher [Q], F is the emission intensity at
0
-
the quencher concentration [Q] and K is the binding constant for the receptor-CN ion complex.
A
In all the cases, a plot of log (F -F) versus log [Q] was found to be linear (r > 0.99) and the
0
binding constants calculated are also collected in Table 1. The order of binding constant values
3 3
was observed to be: R2 (Br) > R3 (Cl) > R1 (H) > R4 (CH ) > R5 (OCH ). The observed order
indicated that the receptors with electron withdrawing substituents bind strongly with cyanide
than the unsubstituted compound, while those with electron donating substituents bind relatively
weakly with cyanide ion. The stoichiometry of the binding between these receptors and cyanide
was determined using fluorescence data and the Job’s plot obtained is shown in Figure S31 [27].

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In the Job’s plots of all these cases curves with a maximum at mole fraction 0.5 suggested a 1:1
stoichiometry between the receptors and cyanide ion. The fluorescence spectral data were used to
calculate the detection limit of these receptors as described elsewhere [28-30]. The results
-8
collected in Table 1 indicated that the detection limit was observed to be in the order of 10 M,
which is lower than the maximum level of cyanide in drinking water that the WHO permits
-6
(
1.9x10 M).
1
3
.4 H NMR spectral studies. In order to delineated the mechanism of detection of cyanide by
1
these receptors, H NMR titration studies were carried out in DMSO-d . As a representative case
6
1
the H NMR spectra of R1 with addition of 0-1 equivalent of cyanide were shown in Figure 5
1
and the spectra of other receptors are collected in Figures S32-41. The H NMR spectrum of free
R1 exhibited a signal at δ 9.32 ppm corresponding to –NH proton. In the absence of cyanide ion,
the aromatic protons appeared at δ 7.13-8.05 ppm. As shown in Figure 5, upon addition of
incremental amounts of cyanide, the signal at 9.32 ppm decreased gradually and showed upfield
shift and after adding one equivalent of cyanide this signal disappeared completely suggesting
deprotonation of –NH moiety by the added cyanide ion. The signal for the –NH proton in free
R1 appeared at 9.32 ppm, which is higher than that of free aniline in DMSO-d (δ 4.99 ppm).
6
This may be due to the existence of ICT transition from –NH moiety to the electron deficient
quinone ring. However, if ICT transition is the only contributing factor for the enhanced acidity
of the –NH proton, compounds I and II should also exhibited nearly similar signal for the –NH
protons. But, it is not so. Therefore, in the case of R1 resonance effect of the phenyl ring might
also contributed significantly to the acidity of the proton. This presumption is further supported
by the fact that the chemical shift of the –NH proton in the receptors under investigation (R1-R5)
was found to be substituent dependent. The receptors with electron withdrawing substituents

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showed δNH greater than 9.32 ppm, while that with electron donating substituents had chemical
shifts less than 9.32 ppm for the –NH proton. It is also evident from Figures 5 and S32-41 that
upon addition of one equivalent of cyanide, the proton peak of phenyl ring showed substantial
upfield shift. This may be due to the fact that the electron density generated on N-atom might
have been distributed in the phenyl ring through resonance [31]. It is noteworthy to understand
that the compounds I-III failed to detect cyanide under the experimental conditions, even if
present in large excess. They showed no colour and spectral changes upon addition of cyanide
ion in aqueous HEPES buffer:DMF (2:8 v/v) solution. Based on these findings, we propose a
deprotonation mechanism for the detection of cyanide by these receptors and this is enabled by
resonance effect.
It is well established that resonance effect greatly influences the basicity of aniline (and
consequently the acidity of the –NH proton). Electron withdrawing substituents through
resonance effect would increase the acidity of the –NH proton when compared to the
unsubstituted compound, while electron donating substituents would exhibit opposite effect.

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Hence, it would be relatively easy to deprotonate the –NH proton from receptors possessing
electron withdrawing substituents when compared to the unsubstituted compound. In other
words, the electron density generated on N-atom, upon deprotonation by cyanide ion, would be
stabilized by electron with drawing substituents through resonance. Consequently such receptors
would exhibit relatively higher binding constants than that of the unsubstituted one. While those
with electron donating substituents would show relatively lower binding constant values than the
unsubstituted compound. This is in line with the order of binding constants observed in
fluorescence spectral studies. Hence, it may be concluded that resonance effect plays a
significant role in deciding the easiness of deprotonation of the –NH proton by the added cyanide
ion.

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3
.5 Single crystal XRD study. The molecular structures of R3 and R4 were confirmed by single
crystal XRD study. The structures with atom numbering are given in Figure 6 and the crystal
data are collected in Table T1. The bond lengths and bond angles are given in Tables T2-T5.
Bertolasi et al. [32] have discussed the strength of intermolecular H-bonds of N-H moiety in
heteroconjugated systems (including receptor R1) using single crystal XRD data. In these
enaminonic O=C−C=C−NHR groups C=O distances have been chosen as the indicators of π-
delocalization within the fragment. They concluded that there is a direct relationship between the
strength of the H-bond and lengthening of the C=O bond distance. The selected bond lengths in
R1, R3 and R4 are depicted below.
The bond lengths for R1 were taken from Ref. [32]. It is evident from the data that in receptor
with chloro-substituent (R3) the relatively longer C(11)=O(2) bond distance (1.233 Å), when
compared to the unsubstituted compound, is indicative of stronger H-bonding ability of the N−H
moiety. This is possible due to the resonance assisted H-bonding i.e. the charge density on the
N(1)-atom is removed through resonance by the C(11)=O(2) group. It is also evident that the
charge density on the N-atom is also reduced by the resonance effect of the chloro-substituent
leading to relatively shorter N(1)−C(4) distance (1.407 Å). Such resonance assisted removal of
the charge density from the N(1)-atom would make the H-atom (of N−H) more acidic and could
interact with cyanide ions strongly, as seen above. Thus, the bond distance data strongly

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supported the proposed role resonance effect on the cyanide sensing behaviour of these
receptors.
1
3
.6 Electrochemical study. The foregoing H NMR spectral studies suggested that the
resonance effect dominates over ICT transition influence on the cyanide detection mechanism. In
1
order to substantiate the H NMR spectral studies, electrochemical behaviour of these receptors
in the absence and presence of increasing concentration of cyanide has been investigated. The
DP voltammograms of these receptors recorded in DMF are depicted in Figures 7 and S42-45. It
is evident from the figures that, all the receptors showed a typical voltammogram for the electro-
reduction of the quinone ring with two reduction peaks (at E = R1: -0.6207, -0.9981; R2: -
pc
0
.6267, -0.9744; R3: -0.6440, -1.0079; R4: -0.5654, -1.0730 and R5: -0.6188, -1.0098 V). It is
well established that in the case of quinones the first reduction peak at a lower negative potential
.
-
corresponds to the formation of radical anion (Q ) and the second reduction peak at a relatively
2
-
higher negative potential corresponds to the formation of dianion (Q ) [33]. The electro-
reduction of 2,3-dichloro-1,4-naphthoquinone, under similar experimental conditions in DMF,
showed two reduction peaks at -0.3103 and -1.1320 V [34]. From these values it evident that the
electro-reduction of the quinone ring in R1 (substituted with arylamine) is relatively difficult
when compared to that of 2,3-dichloro-1,4-naphthoquinone (containing two electron
withdrawing Cl-atoms), suggesting existence of ICT transition in R1. As shown in the
voltammograms, with addition of incremental amounts of cyanide to the solution of these
receptors, both the reduction peaks are shifted to lesser negative potential with a concurrent
decrease in current density. That is upon addition of cyanide the electro-reduction of the quinone
becomes relatively easier [35]. If the negative charge generated on N-atom, upon deprotonation
of –NH moiety, dissipates into the quinone ring, the electro-reduction would have become

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14
increasingly difficult [24]. But in the present study, electro-reduction of the quinone ring
becomes increasingly easier with an increase in concentration of cyanide ion. This observation
suggested that the negative charge generated on N-atom is stabilized by resonance effect of the
1
phenyl ring, which is in line with the observations made in H NMR spectral studies.
3
.7 Theoretical study. DFT computations were used to further understand the cyanide detection
process by the receptors under investigation. The calculations were performed at B3LYP/6-311G
level basis set using Gaussian 03 package [36]. The frontier molecular orbitals (HOMO and
-
LUMO) generated for R and R+CN species are shown in Figures 8 and S46-49. As seen from
the figures, in free receptors HOMO is concentrated on the arylamino moiety and LUMO is
localized on the quinone moiety as expected. Such a distribution of MO’s is prerequisite for a
compound to be a good ICT probe [37]. This confirms that the electronic transition (ICT) occurs
from arylamino moiety to quinone ring. Further, decrease in energy gap between HOMO and
-
LUMO from R to R+CN is responsible for the red-shift observed in the UV-Vis spectra of the
-
receptor upon addition of cyanide ion. The electron distribution in R+CN is quite different from
that of R both in HOMO and LUMO. Both these MO’s have almost identical charge distribution
-
in R+CN species, suggesting that the transition is assigned to the π→π* transition of the
depropotated arylamino moiety. Therefore, a charge separation cannot be expected to be
-
generated while an electron is promoted from HOMO to LUMO in R+CN . Thus, the ICT
-
process is less obvious in R+CN than that in free R, which may be due to the significant
resonance contribution of the phenyl ring to stabilize the charge generated upon deprotonation
[
38].
.8 Practical application. The practical applicability of these receptors has been explored by
means of preparing easy to use paper test strips. These strips were prepared by immersing filter
3

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15
paper into an aqueous solution of R1 (as a representative case) and drying in air (Fig. 9). The
cyanide solutions of various concentrations were prepared in naturally occurring deep well water
by dissolving required quantity of sodium cyanide. The composition of the water samples used in
the present study is: (all in mg/L except pH) pH 7.98; Total alkalinity 403; Total hardness 244;
−
−
2−
Total dissolved solids 1416; Cl 197; F 1.02; SO 36. As seen from the figure, when the test
4
strips were immersed in aqueous solutions of cyanide, the colour of the strips immediately
changed, which can be seen visually. Therefore, these paper strips can effectively been used as a
test-kit for the detection of cyanide ion in natural water.
4
.
Conclusion
Chemo-receptors based on arylamino-naphthoquinone for colorimetric detection of
cyanide have been developed and reported in this article. These receptors showed excellent
colour change selectively and sensitively in aqueous solution via deprotonation mechanism as
1
evidenced from H NMR titration studies. The deprotonated N-atom was found to be stabilized
by resonance effect of the phenyl ring. Also, the binding of cyanide with these receptors and
resonance contribution was found to depend on the nature of the substituent attached to the
phenyl ring in the para-position. Electrochemical and DFT studies supported the proposed
mechanism.
Appendix A
CCDC numbers 1892810 and 1892811contain the supplementary crystallographic data for the
receptor R3 and R4, respectively, given as CIF files. Crystallographic data can be obtained free
of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336
0
33; or e-mail: deposit@ccdc.cam.ac.uk.

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1] C. Arivazhagan, B. Rosmita, R. Jagan, S. Ghosh, Benzoindolium-Triarylborane
[
Conjugates: Ratiometric Fluorescent Chemodosimeter for the Detection of Cyanide ions in
Aqueous Medium, Dalton Trans. 45 (2016) 5014-5020.
[2] Guidelines for Drinking-Water Quality, World Health Organization, Genova, 12 (2011)
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Figure Captions
-4
Fig. 1 Colour changes observed in aqueous HEPES buffer: DMF (2:8 v/v) solution of R1 (5x10
M) upon addition of one equivalent of various anions.
-4
Fig. 2 UV-Vis absorption changes of R1 [5x10 M] in presence of various anions
-4
-3
Fig. 3 UV-Vis spectra of R1 (5x10 M) with incremental addition of cyanide (0-5x10 M) in
aqueous HEPES buffer: DMF (2:8 v/v) medium
-4
-3
Fig. 4 Fluorescence spectra of R1 (5x10 M) with incremental addition of cyanide (0-5x10 M)
in aqueous HEPES buffer: DMF (2:8 v/v) medium
1
Fig. 5 H NMR spectrum of R1 with addition of (a) 0 equiv. (b) 0.25 equiv. (c) 0.5 equiv. (d) 1
equiv. of cyanide in DMSO-d
6
Fig. 6 ORTEP view and atom numbering for R3 (Top) and R4 (Bottom)
Fig. 7 Changes in redox properties of R1 upon addition of cyanide in DMF
Fig. 8 HOMO-LUMO energy level diagrams for the receptor R and R+CN
-
Fig. 9 Colour changes of test strips upon dipping in cyanide ion solution in water

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Fig. 7