Multispectroscopic and theoretical studies on rapid, selective and sensitive visual sensing of cyanide ion in aqueous solution by receptors possessing varying HBD property

Article abstract of DOI:10.1007/s11164-018-3262-y

Three new benzoquinone–imidazole ensembles possessing varying number (0, 1 and 2) of electron-withdrawing bromo substituents on the quinone ring have been designed, synthesized, characterized and employed as receptors in cyanide sensing. These receptors c

Full text of DOI:10.1007/s11164-018-3262-y

Res Chem Intermed
https://doi.org/10.1007/s11164-018-3262-y
Multispectroscopic and theoretical studies on rapid,
selective and sensitive visual sensing of cyanide ion
in aqueous solution by receptors possessing varying
HBD property
P. Raja Lakshmi1 · R. Manivannan1 · P. Jayasudha1 ·
Kuppanagounder P. Elango1
Received: 24 September 2017 / Accepted: 8 January 2018
© Springer Science+Business Media B.V., part of Springer Nature 2018
Abstract Three new benzoquinone–imidazole ensembles possessing varying
number (0, 1 and 2) of electron-withdrawing bromo substituents on the quinone
ring have been designed, synthesized, characterized and employed as receptors in
cyanide sensing. These receptors colorimetrically sense cyanide ion selectively and
sensitively in aq. 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid (HEPES)
bufer–acetonitrile (ACN) (8:2 v/v) medium with striking colour change that can
easily be seen visually. Fluorescence spectral studies revealed that the receptors
form strong complex at 1:1 stoichiometry with cyanide ion. The binding constants
for the complexes were found to be in the order of 105–107 M−1 and increase with
increasing number of bromo substituents on the quinone ring. The detection limit
of cyanide by these receptors lay in the range of 10−8 M. The efects of the number
of bromo substituents on the absorption maximum, reduction potential and H-bond
donor (HBD) property of imidazole N–H are discussed. The results of spectral stud-
ies corroborate well with those of electrochemical and theoretical studies. An easy-
to-use test strip was prepared to test cyanide in aqueous solution of sodium cyanide
prepared using deep-well water.
Keywords Cyanide · Sensor · Colorimetry · Quinone · H-bonding · Visual
detection
Electronic supplementary material The online version of this article (https://doi.org/10.1007/
s11164-018-3262-y) contains supplementary material, which is available to authorized users.
*
Kuppanagounder P. Elango
drkpelango@redifmail.com
1
Department of Chemistry, Gandhigram Rural Institute (Deemed to be University),
Gandhigram 624302, India
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P. Raja Lakshmi et al.
Introduction
Cyanide ion is extremely toxic to mammals because it binds strongly to the active
sites of cytochrome c oxidase even at very low concentrations, thereby inhibiting the
mitochondrial electron transport chain, leading to vomiting, loss of consciousness
and ultimately death [1–3]. On the other hand, cyanide is used in many industrial
processes, like electroplating, gold mining, chemical processing etc. [4–8]. Accord-
ing to the World Health Organization (WHO), the permissible limit of cyanide ion
in drinking water is 1.9 μM [9, 10]. Therefore, selective and sensitive detection of
cyanide in water has attracted considerable attention recently. Although a variety of
sensors that work via diferent mechanisms have been documented [11], it is essen-
tial to develop feasible and efcient colorimetric sensors for cyanide detection in
aqueous solution under mild conditions. Among the various chemical sensors, col-
orimetric chemosensors have attracted more attention due to benefts including vis-
ual detection, operational simplicity, low cost and high sensitivity.
We recently designed quinone-based receptors for selective cyanide sens-
ing, demonstrating that naphthoquinone–imidazole ensemble (II) can sense cya-
nide and fuoride ions colorimetrically in dimethylsulphoxide (DMSO), with the
H-atom (N–H) being relatively more acidic (compared with benzimidazole, I) when
directly attached to quinone as a result of enhanced intramolecular charge-transfer
(ICT) transition [12]. Such an acidic H-atom would behave as a relatively better
H-bond donor towards the anion, and II exhibited cyanide ion binding constant of
5.9 × 103 M−1 in DMSO.
Later, on the basis that receptors with greater number of binding sites would
show selectivity and also could accommodate a greater amount of water from the
sensing medium, we designed III, which contains imidazole on either side of the
quinone ring but still possesses a relatively more acidic H-atom (than I). We found
that III binds strongly to cyanide ion selectively in aq. HEPES bufer–dimethylfor-
mamide (DMF) (9:1 v/v) medium with binding constant of 1 × 106 M−1 [13]. Hav-
ing shown the satisfactory design and performance of III in aqueous medium, it
was presumed that a structurally similar receptor (R) containing imidazole ring on
one side of the quinone ring but leaving the other side free for a substituent of our
choice would also exhibit parallel cyanide ion sensing behaviour. Also, introduction
of electron-withdrawing substituents at X and Y positions could further enhance
the acidity of the imidazole H-atom and hence its HBD property towards the anion.
With this intention, in the present study, we designed, synthesized and screened the
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Multispectroscopic and theoretical studies on rapid, selective…
anion sensing behaviour of three receptors (R1: X = Y = H; R2: X = Br, Y = H;
R3: X = Y = Br). Interestingly these receptors colorimetrically sense cyanide ion
selectively in aqueous solution, and increase in the number of bromo substituents
enhanced the HBD property of the H-atom towards cyanide ion. Though many
receptors, wherein benzimidazole moiety acts as the anion binding unit, have been
reported in literature [14–17], the present receptor molecules were designed to have
enhanced HBD property toward the analyte as described above.
Therefore, the main objectives of the present endeavour are synthesis, characteri-
zation and investigation of the anion sensing behaviour of these three new benzo-
quinone–imidazole ensembles (R1–R3). The receptors were characterized using 1H
and 13C nuclear magnetic resonance (NMR), mass and Fourier-transform infrared
(FTIR) spectral techniques. The cyanide ion sensing properties of these receptors
were explored using ultraviolet–visible (UV–Vis), fuorescence and NMR spectral
studies. Electrochemical and theoretical studies were also carried out to provide
further insight into the mechanism of the sensing process. The efect of increasing
the number of bromo substituents on the HBD property of these receptors is also
discussed.
Experimental
Chemicals and apparatus
All reagents for synthesis of the receptors were obtained commercially and used
without further purifcation. Spectroscopic-grade solvents were used as received.
Double-distilled water was used throughout the work, with the second distillation
carried out using alkaline permanganate. UV–Vis spectral studies were carried out
using a double-beam spectrophotometer. Steady-state fuorescence spectra were
obtained using a spectrofuorimeter. The excitation and emission slit width (5 nm)
and scan rate (250 mV s−1) were kept constant for all experiments. Nuclear mag-
1
netic resonance spectra were recorded in DMSO-d6 (600 MHz). H NMR spectral
data are expressed in the form: chemical shift in units of ppm (normalized integra-
tion, multiplicity, and value of J in Hz). Electrospray ion mass spectra (m/z) were
recorded using an LC/MSD TRAP XCT Plus (1200 Agilent). Diferential pulse vol-
tammetric (DPV) experiments using 1 mmol solutions of the compounds were car-
ried out using glassy carbon (GC) working electrode, Pt wire reference electrode,
and Ag wire auxiliary electrode in acetonitrile containing 0.1 M tetrabutylammo-
nium perchlorate as supporting electrolyte at scan rate of 100 mV s−1.
Synthesis and characterization of intermediates
Synthesis of intermediates 1 and 2: Intermediates 1,2-dinitro-3,6-dimethoxybenzene
(1) and 1,2-diamino-3,6-dimethoxybenzene (2) were prepared from commercially
available 1,4-dimethoxybenzene as reported earlier [18, 19] (Scheme 1). Intermedi-
1
ate 2 was characterized using H and 13C NMR and LC–MS spectral techniques:
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P. Raja Lakshmi et al.
Scheme 1 Synthesis of receptors R1–R3
Brown solid (3.36 g, yield 91%); δH (600 MHz; DMSO-d6; Me4Si/ppm): 3.67 (6H,
s), 4.10 (4H, s), 6.15 (2H, s), (Fig. S1). δC (150 MHz; DMSO-d6; Me4Si): 56.16,
99.85, 124.42, 142.58 (Fig. S2). LC–MS (ESI-APCI) m/z: cacld. for C8H12N2O2,
168.19, found, 169.20 [M + H]+ (Fig. S3).
Synthesis of intermediate 3: Intermediate 3 was prepared using the procedure for
preparation of similar compounds reported earlier [12]. A mixture of 2 and isobu-
tyraldehyde (1 eqv.) in DMSO (5 mL) was heated at 100 °C with stirring for 6 h.
After cooling to room temperature, the precipitate obtained from the reaction mix-
ture was fltered through flter paper and washed with cold ethanol to obtain pure
product 2-isopropyl-4,7-dimethoxybenzimidazole (3 ). Intermediate 3 was charac-
1
terized using H and 13C NMR and LC–MS spectral techniques: White solid (3 g,
yield 77%); δH (600 MHz; DMSO-d6; Me4Si/ppm): 1.30–1.31 (6H, d, J = 6.6 Hz),
3.07–3.11 (1H, m), 3.84–3.85 (6H, d, J = 5.4 Hz), 6.53 (2H, s), 12.27 (1H, s)
(Fig. S4). δC (150 MHz; DMSO-d6; Me4Si): 21.95, 28.71, 56.04, 102.51, 134.57,
141.42, 158.71 (Fig. S5). LC–MS (ESI-APCI) m/z: cacld. for C12H16N2O2, 220.27,
found, 221.30 [M + H]+ (Fig. S6).
Synthesis of intermediate 4: Intermediate 2-isopropyl-4,7-dimethoxy benzi-
midazole (3) was refuxed with 48% hydrobromic acid (3 eqv.) for 6 h. The reac-
tion mixture was cooled and extracted using ethyl acetate and water to yield crude
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Multispectroscopic and theoretical studies on rapid, selective…
2-isopropyl-4,7-dihydroxybenzimidazole (4), which was used without further purif-
cation [18, 19].
Synthesis of receptor R1 (2‑isopropyl‑1H‑benzo[d]imidazole‑4,7‑dione)
2-Isopropyl-4,7-dihydroxybenzimidazole (intermediate 4) was treated with 2 eqv.
ferric chloride dissolved in 10 mL water and stirred at RT for 3 h. After reaction
completion, the reaction mixture was partitioned with ethyl acetate. The organic
layer was concentrated and dried using sodium sulphate, then purifed by column
chromatography to yield the pure product. The receptor was characterized using 1H
and 13C NMR and LC–MS spectral techniques: Yellow solid (2.1 g, yield 81%). δH
(600 MHz; DMSO-d6; Me4Si): 1.27–1.29 (d, 6H, J = 7.2 Hz), 3.05–3.09 (m, 1H),
6.67 (s, 2H), 13.49 (s, 1H) (Fig. S7); δC (150 MHz; DMSO-d6; Me4Si): 21.55, 28.44,
132.64, 135.91, 136.86, 140.87, 160.27, 178.19, 181.75 (Fig. S8). LC–MS (ESI-
APCI) m/z: Calcd. for C10H10N2O2: 190.20; Found: 191.20 [M + H]+ (Fig. S9).
Synthesis of receptor R2 (5‑bromo‑2‑isopropyl‑1H‑benzo[d]
imidazole‑4,7‑dione)
Compound R1 was treated with 48% hydrobromic acid (2 eqv.) and refuxed for
6 h. The reaction mixture was cooled and extracted using ethyl acetate and water
to yield crude 5(6)-bromo-4,7-dihydroxy-2-isopropylbenzimidazole, which was
used without further purifcation. The obtained crude was treated with 2 eqv. fer-
ric chloride dissolved in 10 mL water and stirred at RT for 3 h. The reaction mix-
ture was extracted with ethyl acetate and dried using sodium sulphate, then purifed
by column chromatography. The receptor was characterized using 1H and 13C NMR
and LC–MS spectral techniques: Orange solid (151 mg, yield 53%); δH (600 MHz;
DMSO-d6; Me4Si): 1.27–1.29 (d, 6H, J = 7.2 Hz), 3.05–3.10 (m, 1H), 7.31 (s, 1H),
13.71 (s, 1H) (Fig. S10). δC (150 MHz; DMSO-d6; Me4Si): 21.51, 28.48, 133.02,
137.30, 139.16, 142.05, 160.99, 179.14, 182.37 (Fig. S11). LC–MS (ESI-APCI)
m/z: Calcd. for C10H9BrN2O2: 269.09; Found: 270.20 [M + H]+ (Fig. S12).
Synthesis of receptor R3 (5,6‑dibromo‑2‑isopropyl‑1H‑benzo[d]
imidazole‑4,7‑dione)
Compound R1 was dissolved in dichloromethane (5 mL), cooled to 0 °C, then bro-
mine (2 eqv.) was added dropwise and stirred at RT for 2 h. The reaction mixture
was extracted with ethyl acetate and water, then purifed by column chromatography
1
to yield the pure product. The receptor was characterized using H and 13C NMR
and LC–MS spectral techniques: Orange solid (138 mg, yield 50%). δH (600 MHz;
DMSO-d6; Me4Si): 1.28–1.29 (d, 6H, J = 7.2 Hz), 3.06–3.10 (m, 1H), 13.79 (s, 1H)
(Fig. S13); δC (150 MHz; DMSO-d6; Me4Si): 21.49, 28.47, 128.06, 129.31, 132.27,
139.16, 161.24, 181.35, 183.12 (Fig. S14); LC–MS (ESI-APCI) m/z: Calcd. for
C10H8Br2N2O2: 347.99; Found: 349.30 [M + H]+ (Fig. S15).
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P. Raja Lakshmi et al.
Results and discussion
The three new receptors (R1–R3) were prepared from commercially available start-
1
ing materials as depicted in Scheme 1. The receptors were characterized using H
and 13C NMR, FTIR and mass spectral techniques. The FTIR spectrum of free R1
exhibited an intense single peak at 1665 cm−1 corresponding ν(C=O) of quinone
ring (Fig. 1). In the case of free R3 (Fig. S16–S18), this peak appeared at 1679 cm−1,
also as a single peak. However, in the case of free R2, the peak due to quinone car-
bonyl stretching underwent splitting and appeared at 1683 and 1647 cm−1; That is,
in receptors R1 and R3, both carbonyl groups are nearly similar and their lengths
equal. This may be due to the presence of symmetrical substituents at X and Y posi-
tions in R1 (both H) and R3 (both Br) and unsymmetrical substitution in R2 (H and
Br). Further, the shift in this stretching frequency from 1665 to 1679 cm−1 from R1
to R3 suggests that, as the number of bromo substituents was increased, the elec-
tron-withdrawing power of the quinone moiety increased, shortening the carbonyl
bond [20]. The anion-sensing properties of these receptors were investigated using
spectral, electrochemical and theoretical studies.
Visual detection
As a preliminary investigation, the change in colour of these receptors in absence
and presence of various anions such as F−, Cl−, Br−, CN−, H2PO4−, OAC−, I−,
NO−3 , NO−2 , S2− and NCS− was observed visually in aq. HEPES bufer–ACN (8:2
v/v) medium. The results obtained are shown in Figs. 2 and S19. It is evident
from these fgures that these receptors instantaneously changed their colour only
R1
R2
R3
ν
c=o 1683.55
ν
c=o 1679.69
ν
c=o 1665.23
ν
c=o 1647.87
Fig. 1 Partial FTIR spectra of receptors, showing carbonyl stretching peak
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Multispectroscopic and theoretical studies on rapid, selective…
Fig. 2 Colour changes observed in aq. HEPES bufer–ACN (8:2 v/v) solution of R1 (7.5 × 10−4 M)
upon addition of one equivalent of various anions
in presence of cyanide ion; with all other chosen anions, no observable change in
colour was noticed. This indicates the selectivity of the receptors toward cyanide
ion. The change in colour of receptor R1, as a representative case, with addition of
one equivalent of common metal ions is also shown in Fig. S20. As seen from this
fgure, the colour of the receptor remained unchanged upon addition of the chosen
metal ions, confrming the selectivity of the receptors towards cyanide ion. As a rep-
resentative case, the change in colour of R1 in absence and presence of cyanide at
diferent pH values is collected in Fig. S21. The results indicate that the receptor can
sense cyanide in neutral medium. The results of these visual detection experiment
are well supported by the electronic spectral data (Fig. S22). The change in colour
of the receptor itself in basic medium suggested that the mechanism of sensing may
involve H-bond formation between N and H moiety and cyanide ion, as hydroxide
ion is a well-known deprotonating agent [21].
Electronic spectral studies
The electronic spectra of 7.5 × 10−6 M solution of these receptors in aq.
HEPES–ACN (8:2 v/v) showed a band at 399 (log ε 4.68), 417 (log ε 4.67) and
426 nm (log ε 4.59) for R1, R2, and R3, respectively (Figs. 3, S23, S24). This
absorption band corresponds to the characteristic intramolecular charge-transfer
(ICT) transition from imidazole nitrogen atoms to the electron-defcient quinone
ring [12]. It is seen that, as the number of electron-withdrawing bromo substituents
increased from R1 to R3, the ICT transition was found to occur energetically more
easily, as expected. This observation corroborates the FTIR spectral results. Further-
more, as the ICT transition becomes easier, the H-atom (N–H) will become more
acidic and thus act as a relatively better H-bond donor towards cyanide ion. The
selectivity towards cyanide ion of these receptors was further confrmed by elec-
tronic spectral studies in absence and presence of one equivalent of chosen anions
such as F−, Cl−, Br−, CN−, H2PO4−, OAC−, I−, NO3−, NO−2 , S2− and NCS−. It is evi-
dent from Fig. S25 that the ICT transition band of R1 (as a representative case) at
399 nm exhibited a red-shift only in presence of cyanide ion, confrming the selec-
tivity of these receptors towards cyanide.
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P. Raja Lakshmi et al.
Fig. 3 UV–Vis spectra of R1 (7.5 × 10−6 M) with incremental addition of cyanide (0–7.5 × 10−6 M) in
aq. HEPES bufer–ACN (8:2 v/v) medium
The nature of the binding between the receptors and cyanide ion was investigated
using electronic spectral titration studies (Figs. 3, S23, S24). It is evident from these
fgures that, upon addition of incremental amounts of cyanide to solution of these
receptors, the absorbance of the ICT transition band decreased gradually with con-
comitant increase in the absorbance of a new band at relatively higher wavelength.
Such electronic spectral behavior suggests that addition of cyanide makes the ICT
transition energetically easier. This may be due to formation of H-bond between imi-
dazole N–H group and cyanide ion. Such a H-bond would increase the electron den-
sity on the N-atom (N–H) and thus make the ICT transition energetically easier [21].
The red-shifts observed in λICT were found to be 96, 90 and 97 nm for R1, R2 and
R3, respectively. Such a large shift (ΔλICT) indicates strong interaction between the
receptors and cyanide ion. Furthermore, the appearance of a clear isosbestic point
(at 431, 445 and 448 nm for R1, R2 and R3, respectively) suggests presence of two
species in equilibrium in the solution, viz. free receptor (N–H) and H-bonded recep-
tor (N–H…CN−) [12]. The stoichiometry of the receptor–CN− complexes was deter-
mined by Job’s continuous variation method (Fig. S26) [22]. In all the three cases,
Job’s curve showed a maximum at mole fraction of 0.5, indicating 1:1 stoichiometry.
Fluorescence spectral studies
The intensity of binding between the receptors and cyanide ion was assessed by fu-
orescence spectral titration studies. The fuorescence spectra of the receptors with
addition of incremental amounts of cyanide are shown in Figs. 4, S27 and S28.
As seen from these spectra, the free receptors showed strong emission (λex/λem at
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Multispectroscopic and theoretical studies on rapid, selective…
Fig. 4 Fluorescence spectra of R1 (7.5
×
10−5 M) with incremental addition of cyanide
(0–7.5 × 10−5 M) in aq. HEPES bufer–ACN (8:2 v/v) medium
302/351, 330/426 and 330/428 nm for R1, R2 and R3, respectively), which was
quenched upon addition of cyanide ion, indicating formation of a complex between
them (receptor–CN− complex). The binding constants of the complexes were deter-
mined from the fuorescence spectral data using the Benesi–Hildeband equation
[23]. The plots for these receptors were found to be linear (r > 0.99). The bind-
ing constants were found to be 5.6 × 105, 1.5 × 106 and 5.2 × 107 M−1 for R1,
R2 and R3, respectively. The magnitude of the binding constant indicates formation
of strong receptor–CN− complexes in the present study. The steady increase in the
binding constant value from R1 to R3 is due to the fact that increasing the number
of electron-withdrawing bromo substituents on the quinone ring increased the inten-
sity of the ICT transition and thus the acidity (HBD property) of the H-atom (N–H),
strengthening its binding with cyanide ion.
The fuorescence quenching in these receptors caused by cyanide ion was quan-
tifed by measuring the quantum yields of the receptors in absence and presence
of cyanide ion. The quantum yields, measured using quinine sulphate as standard
[24], were found to be 0.525, 0.647 and 0.789 for free receptor R1, R2 and R3,
respectively. The quantum yields for the receptor–CN− complexes, after addition
of one equivalent of cyanide, were observed to be 0.346, 0.409 and 0.412, respec-
tively (Fig. S29–S35). This reduction in the quantum yield values in the recep-
tor–CN− complexes compared with corresponding free receptors strongly supports
the quenching of fuorescence of the receptors by cyanide through complex forma-
tion. The detection limits, at S/N = 3, of cyanide by these receptors were computed
as reported elsewhere [25–28]. The results thus obtained were 64 × 10−8, 8 × 10−8
and 5 × 10−8 M for R1, R2 and R3, respectively (Fig. S36–S38). The observed
limits of detection of cyanide using these receptors are far below the level recom-
mended by WHO (1.9 × 10−6 M).
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P. Raja Lakshmi et al.
NMR spectral studies
1
To delineate the mechanism of interaction of cyanide with the receptors, H NMR
spectral titration studies were carried out in DMSO-d6. The spectra obtained in
absence and presence of various amounts of cyanide are depicted in Figs. 5, S39,
S40. The signal corresponding to N–H proton in R1 appeared at 13.497 ppm, higher
than in simple benzimidazole (12.531 ppm). This is due to the fact that, when imida-
zole N–H is directly attached to electron-defcient quinone ring, due to the existence
of ICT transition, the N–H proton becomes more acidic. This signal (δNH) in R2 and
R3 appeared at 13.708 and 13.787 ppm, respectively. It is evident from these spectra
that increasing the number of bromo substituents progressively increased the acidity
of the N–H proton and hence its HBD property. This is in line with our perception
spelt out in “Introduction” section.
As seen from Fig. 5, upon addition of 0.5 equivalent cyanide to the solution
of R1, the signal due to N–H proton was found to be deshielded from 13.497 to
13.553 ppm with broadening. Furthermore, this signal was found to disappear after
addition of one equivalent of cyanide. The other two receptors showed similar spec-
tral behaviour on addition of cyanide (Fig. S39, S40). These observations indicate
that the cyanide sensing mechanism of these receptors involves formation of H-bond
between N–H and cyanide ion followed by deprotonation (Scheme 2) [12]. This
proposed mechanism accounts for the results obtained in the electronic and fuores-
cence spectral studies. The observed increase in the binding constants of the recep-
tor–CN− complexes, from R1 to R3, is due to the increase in the HBD property of
the N–H group with increase in the number of bromo substituents in these receptors.
Fig. 5 1H NMR spectrum of R1 with addition of a 0 eqv., b 0.5 eqv., and c 1.0 eqv. cyanide in DMSO-
d6
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Multispectroscopic and theoretical studies on rapid, selective…
Scheme 2 Cyanide ion sensing
mechanism of receptors
The N-atom (of N–H group), after deprotonation, becomes relatively electron rich,
which makes the ICT transition from such a N-atom to quinone energetically easier,
hence the receptor–CN− complex absorbs at higher wavelength [13].
Electrochemical studies
To support the observations made in spectral studies, regarding the efect of increas-
ing the number of bromo substituents on the behaviour of the receptors, electro-
chemical study was also carried out. The diferential pulse voltammograms of the
receptors recorded in ACN with tetrabutylammonium perchlorate as supporting
electrolyte are shown in Figs. 6, S41, S42. The voltammogram for electroreduction
of quinone ring showed two characteristic reduction peaks (at Epc = R1: − 0.512,
−0.864; R2: − 0.484, − 0.840; R3: − 0.444, − 0.796 V). The frst peak at lower
negative potential is due to formation of radical anion (Q·−), while the second peak
at relatively higher negative potential corresponds to formation of dianion (Q2−)
[29]. Furthermore, the reduction potential of both peaks decreased from R1 to R3,
Fig. 6 Changes in redox properties of R1 upon addition of cyanide in acetonitrile
1 3

P. Raja Lakshmi et al.
making electroreduction of the quinone easier, due to increase in the number of
electron-withdrawing bromo substituents. It is evident from these voltammograms
that, upon addition of incremental amounts of cyanide ion to the receptor solution,
both reduction peaks shifted to higher negative potential with a concurrent decrease
in current density. This shift to relatively higher potential suggests that electroreduc-
tion of the quinone becomes difcult upon addition of cyanide ion. This is due to the
fact that addition of cyanide ion makes the ICT transition easier and thus increased
the electron density on the quinone ring, making electroreduction difcult [13, 21].
Theoretical studies
Density functional theory calculations at B3LYP/3-21G basis level set were car-
ried out using the Gaussian 03 package to estimate the electronic properties of these
receptors before and after addition of cyanide ion [30]. The optimized geometries
and the orientation of the highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) in the free receptors and their receptor–CN−
complexes are shown in Figs. S43, S44, S45. The energies of the MOs are collected
in Table S1. In the free receptors, the HOMO was localized on the imidazole ring
and the LUMO concentrated on the quinone moiety. Such a MO distribution is a
prerequisite for a compound to be an ICT probe [31]. The energy corresponding to
the ICT transition (∆E) is the diference in energy between these MOs. The results
in Table S1 show that the ∆E values were relatively lower in the receptor–CN−
complex than in the corresponding free receptor, indicating an energetically easier
ICT transition in the complex. Thus, the results of this theoretical study are in good
agreement with the red-shifts observed in the electronic spectra of the receptors
after addition of cyanide ion.
Practical application
Since these receptors colorimetrically sense cyanide with a striking colour change
in aqueous solution, easy-to-use inexpensive test strips were prepared by immersing
flter paper into solution of R1 (as a representative case) and drying in air (Fig. 7).
Sodium cyanide solutions of various concentrations were prepared using deep-well
water. The composition of the deep-well water employed in the present study is (all
in mg/L except pH): pH 8.02, total alkalinity 358, total hardness 271, total dissolved
solids 1403, Cl− 198, F− 1.02, SO24− 46. As seen from the fgure, when the test strips
were immersed in variation concentrations of cyanide in water, the colour of the
strips immediately changed, being easily seen visually. Therefore, the prepared
strips are an efective test-kit for detection of cyanide ion in water with high selec-
tivity for use in real-time applications.
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Multispectroscopic and theoretical studies on rapid, selective…
Fig. 7 Colour change of test strips upon dipping in cyanide ion solution in water
Conclusions
Three new benzoquinone–imidazole ensembles containing varying numbers of
bromo substituents were designed, synthesized and characterized. These receptors
colorimetrically sense cyanide ion selectively and sensitively in aqueous solution.
Increasing the number of bromo substituents on the quinone ring had a marked
infuence on the spectral, electrochemical and cyanide ion binding properties of the
receptors, as shown below:
The cyanide sensing mechanism of these receptors involves formation of H-bond
between imidazole N–H group and cyanide ion. The detection limit of cyanide by
the receptors was in the order of 10−8 M, much lower than the permissible limit
of cyanide ion in drinking water set by the WHO (1.9 × 10−6 M). Electrochemi-
cal and theoretical studies were carried out to substantiate the proposed sensing
mechanism. Easy-to-use test strips were developed as a useful cyanide test-kit for
1 3

P. Raja Lakshmi et al.
feld measurements. It is interesting to compare the results of the present study with
those for structurally analogous quinone–imidazole ensembles II and III. Recep-
tors R1–R3 sensed cyanide selectively in aqueous solution, similar to III, while II
exhibited colour change with both cyanide and fuoride in DMSO. The novelty of
the present receptors is that one can tune the HBD property by varying the number
of bromo substituents on the quinone ring, which is possible with the present struc-
tural design, in contrast to III.
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