Benzoquinone-imidazole hybrids as selective colorimetric sensors for cyanide in aqueous, solid and gas phases

Article abstract of DOI:10.1039/c5ra13597d

Five new chemosensors (R1-R5), possessing benzoquinone as the signaling unit and imidazole as the H-bond donor unit, for cyanide sensing have been rationally designed, synthesized and characterized by NMR and mass spectroscopy. The structure of R5 was confirmed by single crystal XRD studies. These receptors exhibited a prominent visual colour change toward the cyanide ion over other common anions in an aqueous HEPES buffer-DMF (9:1 v/v) medium. The complexation of receptor-CN^- has been addressed by UV-Vis, fluorescence and ¹H NMR spectra and was supported by electrochemical and DFT studies. The mechanism of sensing involves formation of H-bonds between imidazole N-H and CN^- ions. The stoichiometry of the receptor-CN^- complexes was found to be 1:2 (receptor-CN^-) and the detection limit was observed to be in the range of 1.1-3 nM. The test strips based on R5 were fabricated and could act as convenient and efficient CN^- test kits. Notably, the novelty of the present investigation is that the receptor R5 selectively senses CN^- ions in solid, aqueous and gas phases i.e. 'a complete receptor'.

Full text of DOI:10.1039/c5ra13597d

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Benzoquinone-imidazole hybrids as selective colorimetric sensors for
cyanide in aqueous, solid and gas phases
a
b
b
Ramalingam Manivannan , Samuele Ciattini , Laura Chelazzi , and Kuppanagounder P.
a*
Elango
a
Department of Chemistry, Gandhigram Rural Institute- Deemed University, Gandhigram-
624302, India.
b
Centro di Cristallografia Strutturale, Università degli studi di Firenze, 50019, Sesto
Fiorentino (Fi), Italy
Abstract
Five new chemosensors (R1-R5), possessing benzoquinone as signaling unit and
imidazole as Hꢀbond donor unit, for cyanide sensing have been rationally designed,
synthesized and characterized by NMR and mass spectroscopy. The structure of R5 was
confirmed by single crystal XRD studies. These receptors exhibited prominent visual colour
change toward cyanide ion over other common anions in aqueous HEPES bufferꢀDMF (9:1
ꢀ
v/v) medium. The complexation of receptorꢀCN has been addressed by UVꢀVis, fluorescence
1
and H NMR spectra and was supported by electrochemical and DFT studies. The mechanism
ꢀ
of sensing involves formation of Hꢀbond between imidazole NꢀH and CN ion. The
ꢀ
ꢀ
stoichiometry of the receptorꢀ CN complexes was found to be 1:2 (receptorꢀ CN ) and the
detection limit was observed to be in the range of 1.1ꢀ3 nM. The test strips based on R5 were
ꢀ
fabricated, which could act as convenient and efficient CN test kits. Notably, the novelty of
ꢀ
the present investigation is that the receptor R5 selectively senses CN ion in solid, aqueous
and gas phases i.e. ‘a complete receptor’.
*
Corresponding author. Tel.: +91 451 245 2371; Fax: +91 451 2454466
E-mail address: drkpelango@rediffmail.com (Dr. K.P. Elango)
1

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Introduction
The development of molecular sensors for anions such as cyanide ion has been a
subject of intense research interest because cyanide ion is a wellꢀknown hazardous chemical
1
,2
both in biology and the environment due to its extreme toxicity to physiological system. It
3
is known that 0.5ꢀ3.5 mg of cyanide per kilogram of body weight is fatal for human.
According to the World Health Organization (WHO) maximum permissible level of cyanide
4
in drinking water is 1.9 µg. Review of literature revealed that a number of analytical
5
ꢀ10
11
techniques such as colorimetry, and fluorescent method,
ion chromatography,
1
2
13
14
voltammetry, flow injection analysis and electrochemical analysis have been developed
for the detection of cyanide ion. The molecular probes reported for sensing cyanide ions are
15ꢀ18
19ꢀ22
mainly based on Hꢀbonding interaction,
nucleophilic addition,
complex
23,24
25ꢀ27
28ꢀ30
formation,
coordination with Cu(II) ion,
supraꢀmolecular selfꢀassembly
and
31
rearrangement reaction. Although, so far a wide variety of sensors has been reported for the
detection of cyanide ion, improving the detection selectivity and sensitivity in the context of
interference from coexisting anions such as fluoride, acetate and dihyrogen phosphate and
3
2ꢀ34
compatibility within an aqueous environment have been still challenging.
Therefore, the
search for receptors which can sense cyanide ion sensitively and selectively in aqueous
solution is still on. It is well known that the main disadvantage, with most of the traditional
chemosensors which depend on the Hꢀbonding moieties, is the lack of selectivity. Other
32ꢀ34
anions such as fluoride, acetate and phosphate ions often interfere with such assays.
Recently we have reported few anion receptors based on quinone as signaling unit in
which the Hꢀbond donor moiety is directly attached to the quinone moiety so as to have
35ꢀ37
enhanced intramolecular charge transfer (ICT) transition.
Such an assembly will exhibit
striking colour changes as and when form Hꢀbond with the anions. One such report involves
2

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35
naphthoquinoneꢀimidazole hybrid structures of the type I. It was found that the Hꢀbond
donor property of the imidazole NꢀH moiety can be tuned by varying the substituent (R).
However, the receptors are not selective towards a particular anion and impart colour changes
with cyanide and fluoride ions in DMSO.
3
8,39
Similarly, Batista et al.
have reported anthraquinoneꢀimidazole based colorimetric
and fluorimetric receptors of the type II. These receptors are also not selective and showed
colour change from yellow to pink with cyanide, fluoride and hydroxide ions in acetonitrile
medium. Hence, it is presumed that the following quinoneꢀimidazole type receptors with a
relatively weaker quinone i.e. benzoquinone in them may exhibit different selectivity and
sensitivity behaviour towards anions.
Also, in the case of receptors that works via the formation of Hꢀbond with the anions,
compared to the single Hꢀbond donor moiety (like NꢀH), the receptors with multiple Hꢀbond
donor moieties can both significantly increase the anion affinity and allow the receptors to
4
0
tolerate a substantial amount of water from the solvent. Here again, in the case of receptors
having urea (III), thiourea (IV) and guanidine (V) as Hꢀbond donor moieties, as the two NꢀH
groups are close to each other, they can make Hꢀbonds with anions like acetate and phosphate
in a bidentate manner leading to lack of selectivity.
3

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Under these circumstances, the proposed benzoquinoneꢀimidazole hybrids are expected
to exhibit higher affinity towards anions, higher selectivity and can accommodate higher
amount of water in the medium, as they possess two NꢀH moieties that are away from one
another. The main objective, therefore, of the present endeavour is to synthesize
benzoquinoneꢀimidazole hybrids with different substituents and to investigate their cyanide
ion sensing properties using spectral (UVꢀVis, fluorescence, NMR), electrochemical and
theoretical studies in aqueous solution.
Results and discussion
The following five new benzoquinoneꢀimidazole hybrids possessing different
substituents (R1-R5) were synthesized via the condensation of 2,3,5,6ꢀtetramineꢀ1,4ꢀ
benzoquinone with corresponding aldehydes (Scheme 1).
Scheme 1. Synthesis of R1ꢀR5.
4

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1
The receptors were characterized using H NMR, LCꢀMS and UVꢀVis spectral
techniques (See Experimental section). Owing to limited solubility of the receptors, we are
1
3
not able to record the C NMR spectra of the compounds. The structure of R5 was
confirmed using single crystal Xꢀray diffraction study. Xꢀray quality crystals of R5 were
developed by diffusing chloroform and acetone to a methanolic solution of R5. The ORTEP
diagram of R5 is shown in Figure 1 and the crystal data are collected in Table 1. The packing
diagram of R5 is shown in Figure S1. As seen from the Figure 1 that in R5 the two imidazole
NꢀH groups that are trans to each other. The NꢀH bond length was observed to be 1.02 Å,
41
which longer than that in similar benzimidazole compound (0.86 Å).
Figure 1. Crystal structures of R5.
This observation suggested that the NꢀH proton in R5 is relatively more acidic than
simple benzimidazole NꢀH proton and thus can acts as a better Hꢀbond donor towards anions.
The anion sensing property of the receptors R1-R5 was investigated by spectral (UVꢀVis,
1
fluorescence and H NMR), electrochemical and theoretical studies in aqueous solution.
5

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-
Table 1. Crystal data and structure refinement for R5 and R5-CN
-
Molecule
R5
R5+CN
Empirical formula
C
14
H
16
N
4
O
2
14 4 2 36 1
(C14H N O )1, (C16H N )1,
(H
2
O
1
)2
548.78
100(2) K
1.54184 A
Monoclinic, P21/n
a = 7.8417(2) Åa= 90°.
Formula weight
Temperature
Wavelength
Crystal system, space group
Unit cell dimensions
272.31
293(2) K
0.71073 A
monoclinic, P21/c
a = 14.7426(13) Åa= 90°.
b = 9.9228(7) Åb= 92.030(7)° b = 24.0400(8) Åb= 98.963(3)°
c = 9.8994(7) Åg = 90°.
c = 17.3481(5) Åg = 90°.
3
3
Volume
1447.25(19) Å
3230.43(17) Å
3
3
Z, Calculated density
Absorption coefficient
F(000)
Crystal size
Theta range for data
collection
4, 1.250 Mg/m
4, 1.128 Mg/m
ꢀ1
ꢀ1
0.087 mm
576
0.596 mm
1204
3
3
0.350 x 0.300 x 0.300 mm
2.475 to 25.253°
0.5 x 0.4 x 0.35 mm
4.475 to 72.145°
Index ranges
ꢀ17<=h<=17, ꢀ11<=k<=11,
11<=l<=11
16234
16234 [R(int) = ?]
99.1 %
ꢀ9<=h<=9, ꢀ28<=k<=29,
ꢀ
ꢀ19<=l<=21
17250
0.06
Reflections collected
Independent reflections
Completeness to theta =
99.9
25.242°
Absorption correction
Semiꢀempirical from
equivalents
Semiꢀempirical from
equivalents
Max. and min. transmission
Refinement method
0.9799 and 0.9710
Fullꢀmatrix leastꢀsquares on
0.812 and 0.751
2
Fullꢀmatrix leastꢀsquares on F
2
F
Data / restraints / parameters 16234 / 108 / 210
1.020
Final R indices [I>2sigma(I)] R1 = 0.0694, wR2 = 0.1537
6373 / 0 / 350
1.096
2
Goodnessꢀofꢀfit on F
R1 = 0.0769
R indices (all data)
Extinction coefficient
Largest diff. peak and hole
CCDC No.
R1 = 0.1291, wR2 = 0.1837
n/a
0.247 and ꢀ0.423 e.Å
R1 = 0.0899, wR2 = 0.242
0.0005(3)
1.248 and ꢀ0.626 e.Å
ꢀ3
ꢀ3
1039004
1423570
6

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Visual detection
To investigate the anion sensing abilities of the receptors, visual inspection of R1-R5
ꢀ4
(
6.25x10 M) in aq. HEPES bufferꢀDMF (9:1 v/v) medium (pH = 7.26) before and after the
ꢀ
ꢀ
ꢀ
addition of one equivalent of tetrabutylammonium salts of various anions such as F , Cl , Br ,
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
3 2 4
I , AcO , NO , H PO and CN were carried out. As a representative case the colour change
observed for R5 is shown in Figure 2 and that for R1-R4 is given in Figure S2. As shown in
the Figure 2, the receptor R5 immediately responded with obvious colour change from
ꢀ
yellow to pink when CN ion was added to the aqueous solution of R5 at room temperature.
However, the colour remained unchanged after the addition of the other chosen common
ꢀ
anions. All the other receptors R1-R4 also exhibited similar colour changes only with CN
ions and not with other anions. This observation indicated that the receptors R1-R5 can serve
ꢀ
as selective ‘nakedꢀeye’ colorimetric sensors for CN ion in aqueous solution. The effect of
pH on the sensing behaviour of R5, as a representative case, has also been investigated and
the colour change observed is shown in Figure S3. The results indicated that the receptor
exhibited a clear colour change with cyanide ions only around pH 7. However, at other pH
values (both acidic and basic) similar colour change has been observed by bringing the pH of
the solution to nearly neutral before adding cyanide ions.
Figure 2. Colour changes observed in aq. HEPES buffer/DMF (9:1 v/v) solution of R5
ꢀ4
(
6.25x10 M) upon addition of various anions.
7

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UV-Vis spectral studies
ꢀ
After establishing the selectivity of the receptors R1-R5 towards CN ion, to further
investigate the sensing properties of the receptors, UVꢀVis spectral studies were performed.
ꢀ4
All the receptors R1-R5 (6.25x10 M) exhibited an intense absorption in the 433ꢀ500 nm
range (log ε 2.99ꢀ3.32) in aq. HEPES bufferꢀDMF (9:1 v/v) medium (Table 2). This intense
absorption peak in the visible region corresponds to the intramolecular charge transfer (ICT)
transition from imidazole Nꢀatoms (donor) to the electron deficient quinone moiety
42,43
(acceptor).
The existence of the ICT transition is supported by the fact that the NꢀH bond
length in R5 is relatively longer in length than that in benzimidazole (by 0.16 Å). That is
charge transfer from donor NꢀH group to acceptor quinone render the Nꢀatom relatively
electron deficient, thus makes the NꢀH bond longer and consequently can allow the NꢀH
ꢀ
group to act as a relatively better Hꢀbond donor towards CN ions.
ꢀ
Table 2. Data obtained from the UVꢀVis spectra upon titration of R1-R5 with CN in aq.
HEPES bufferꢀDMF (9:1 v/v) medium.
ꢀ
1
Receptor
In Receptor
λICT (nm)
log ε
In complex Red shift
Isosbestic
point (nm)
K
A
(M )
λICT (nm)
ꢁλICT
nm)
(
6
R1
R2
R3
R4
R5
495
500
470
460
433
3.28
3.30
3.26
3.32
2.99
601
106
70
62
80
92
530
445
517
523
481
5.4x10
3.4x10
2.7x10
2.3x10
4.3x10
6
6
6
5
570
532
540
525
ꢀ
On gradual addition of CN ion to the solution of the receptors R1-R5, the absorbance
of the ICT transition band decreased and a new band at higher wavelength emerged and the
8

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absorbance of which increased gradually with a distinct isosbestic point (Table 2). As a
representative case, the UVꢀVis spectral changes of R5 is given in Figure 3. The electronic
spectral titration studies of R1-R4 are collected in Figures S4ꢀS7. The appearance of an
ꢀ
isosbestic point indicated the formation of only one species between the receptors and CN
ion. The bathochromic shift (ꢁλICT = 62ꢀ106 nm) observed in the ICT transition band after the
ꢀ
addition of CN ion is due to the fact that the formation of Hꢀbond between NꢀH group and
ꢀ
ꢀ
CN ions (NꢀH…CN ) render the Nꢀatom relatively electron rich and thus makes the ICT
35
transition relatively energetically easier.
ꢀ4
Figure 3. UVꢀVis spectra of R5 (6.25x10 M) with the incremental addition of TBACN (0 ꢀ
ꢀ6
6
.25x10 M) in aq. HEPES bufferꢀDMF (9:1 v/v) medium (pH = 7.26).
As a representative case, the UVꢀVis spectra of R5 in the presence of one equivalent
of tetrabutylammonium salts of the chosen anions were also recorded in aq. HEPES bufferꢀ
DMF (9:1 v/v) solution. The results are shown in Figure 4. The results indicated that only
ꢀ
addition of CN ion caused a bathochromic shift in the λICT and all other chosen anions
ꢀ
showed no significant influence on the UVꢀVis spectra of R5. The absorbance of R5ꢀCN ion
solution in the presence of other anions is collected in Figure S8. Likewise, the absorbance of
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ꢀ
the R5ꢀCN ion solution in the presence of cations is also recorded (Fig. S9) and the results
indicated that the chosen cations exhibited no appreciable influence on the electronic spectra
of the receptor. The results of the effect of other anions and cations on the UVꢀVis spectra of
ꢀ
R5 indicated that the receptor exhibited high selectivity towards CN ion. The Job’s plots
ꢀ
indicated that a 1:2 (receptorꢀCN ) Hꢀbonded complex was formed between the receptors R1-
ꢀ
44
ꢀ
R5 and CN ions (Figure S10). The association constants K for the RꢀCN complexes,
A
45,46
6
ꢀ
calculated using Scott plot,
are also collected in Table 2. The observed magnitude (10 M
1
)
A
of the K values indicated the formation of strong complexes between the receptors and
ꢀ
36
CN ions. The high selectivity and the striking colour changes observed in these receptors
ꢀ
on adding CN ion observed in the visual detection experiments is well supported by the
results of UVꢀVis spectral studies.
ꢀ4
Figure 4. UVꢀVis absorption changes of R5 (6.25x10 M) upon addition of 1 eqv. of
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
2ꢀ
3ꢀ
ꢀ
ꢀ
3 2 4
tetrabutylammonium salts of F , Cl , Br , I , NO , H PO , AcO , S , N , SCN and CN .
Fluorescence spectral studies
The cyanide ion binding properties of the receptors R1-R5 were also investigated using
fluorescence spectral studies. Figure 5 shows a group of fluorescence emission spectra
1
0

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ꢀ
relating to the titration of R5 with CN ions (as a representative case). It can be observed that
ꢀ
with the addition of incremental amounts of CN ions, the fluorescence emission of the
ꢀ
receptor R5 is quenched indicating the formation of a complex between R5 and CN ions.
ꢀ
Also, there observed no shift in the emission wavelength, on adding CN ion, suggesting the
ꢀ
36
receptorꢀCN complex quenching its intrinsic fluorescence. All the other receptors R1-R4
ꢀ
exhibited similar fluorescence emission spectral changes on adding CN ions (Fig. S11ꢀS14).
ꢀ
From the decrease in the emission intensity, the binding constant of the receptorꢀCN
4
7
complexes was calculated as reported earlier. The binding constants thus determined are
collected in Table 3. The results indicated that the magnitude of the binding constants is in
6
ꢀ1
ꢀ
the order of 10 M suggesting strong binding between the receptors and CN ions. The
48
detection limit of the receptors was determined at S/N=3 (Table 3). The detection limit was
much lower than the maximum permissible level for cyanide ion in drinking water (1.9 ꢂM)
4
set by the World Health Organization (WHO).
ꢀ4
Figure 5. Fluorescence emission spectra of R5 (6.25x10 M) with incremental addition of
ꢀ6
TBACN (0 ꢀ 6.25x10 M) in aq. HEPES bufferꢀDMF (9:1 v/v) medium (pH = 7.26).
ꢀ
Table 3. Fluorescence spectral data for the interaction of the receptors with CN in aq.
HEPES bufferꢀDMF (9:1 v/v) medium.
1
1

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Receptor
λex(nm)
λ (nm)
Stokes shift
nm)
Binding
Detection
limit (nM)
em
ꢀ1
(
constant (M )
7
R1
R2
R3
R4
R5
495
500
470
460
433
629
134
1.5x10
1.1
1.5
2.2
2.6
3
6
576
626
610
564
76
5.6x10
6
156
150
131
3.6x10
6
1.3x10
6
1x10
1
H NMR spectral studies
ꢀ
The mechanism of the intermolecular interaction of the receptors with CN ions was
1
studied using H NMR spectroscopy in DMSOꢀd
6
. The imidazole NꢀH protons of R1-R5
initially appeared at 13.400ꢀ14.324 ppm (Table S1). In all the receptors the two NꢀH protons
appeared as a singlet indicating that these two protons are in chemically same environment.
1
ꢀ
The H NMR spectral changes caused by the addition of CN ion as tetrabutylammonium salt
1
in DMSOꢀd containing R1-R5 were studied. As a representative case, the H NMR spectra
6
ꢀ
ꢀ
of R5 before and after addition of CN ion is shown in Figure 6. Upon the addition of CN
(0.5 equiv.) the singlet due to NꢀH protons was downshifted from 13.400 ppm to 13.534 ppm
ꢀ
49
ꢀ
(
ꢁδ = 0.134 ppm) due to Hꢀbonding with CN ion. After the further addition of CN ion (2
equiv.), the NꢀH protons disappeared immediately. These results clearly showed that the
mechanism of the sensing involves the formation of Hꢀbond between the imidazole NꢀH
ꢀ
1
protons and CN ions. All other receptors exhibited similar H NMR spectral changes upon
ꢀ
addition of two equivalents of CN ions (Fig. S15ꢀS22).
The downfield shift observed in the signal of the NꢀH protons (ꢁδ) after the addition of
ꢀ
0
.5 equivalent of CN ion to the receptors is in the range of 0.103ꢀ0.142 ppm (Table S1). The
high ꢁδ values observed in the present study indicated stronger interaction between the
1
2

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ꢀ
receptors and CN ions. Also, there exists a good correlation (r = 0.99) between log K and
A
ꢁδ values with unit slope (Fig. S23). Thus, stronger is the interaction between the receptor
ꢀ
ꢀ
and CN ions, higher is the association constant of the receptorꢀ CN complex and higher is
the downfield shift experienced by the NꢀH protons.
1
ꢀ
Figure 6. H NMR spectrum of R5 with (a) 0, (b) 0.5 (c) 1.0 and (d) 2.0 eqv. of CN ion in
DMSOꢀd
6
.
The results of the spectral studies indicated that the mechanism of sensing of cyanide
ion by the receptors is via Hꢀbond formation between the imidazole NꢀH and the cyanide ion
followed by deprotonation. This proposed mechanism is further confirmed by the single
ꢀ
ꢀ
crystal XRD study of the R5ꢀCN complex. The ORTEP diagram of R5ꢀCN complex is
shown in Figure 7 and the crystal data are also collected in Table 1. The packing diagram of
1
3

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ꢀ
R5 CN complex is shown in Figure S1. The crystal structure clearly indicated that the
imidazole NꢀH exists as tetrabutylammonium salt after deprotonation. It is to note that the
results of the NMR spectral and Job’s method studies indicated that the receptorꢀcyanide
ꢀ
complex possesses 1:2 (receptor:CN ) stoichiometry. However, the single crystal structure
given in Figure 7 showed that while crystallization only deprotonation of one NꢀH group has
occurred.
ꢀ
Figure 7. Crystal structure of R5ꢀCN complex.
Electrochemical studies
Since the receptors (R1-R5) under investigation contain redox active quinone moiety,
it would of interest to study the electrochemical behaviour of the receptors before and after
the addition of cyanide ions. Such a study would substantiate the proposed mechanism of
sensing based on spectral investigations. The cyclic voltammograms of the free receptors and
ꢀ
that upon addition of incremental amounts of CN ions are shown in Figures 8 and S24ꢀS27.
The electrochemical data obtained are collected in Table 4. It is evident from the
voltammograms that all the free receptors exhibit reversible redox peaks in the potential (E1/2
)
5
0
range ꢀ0.360 to ꢀ0.480 V, characteristic of the quinone redox couple. Also, with the addition
ꢀ
of incremental amounts of CN ions, an anodic shift towards more negative potential with a
concomitant lowering in current density was observed suggesting that the electro reduction of
ꢀ
the quinone becomes relatively difficult. This is due to the fact that the added CN ion to the
1
4

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receptor solution forms Hꢀbond with the imidazole NꢀH proton (which leads to
deprotonation) increases the electron density on the Nꢀatom and consequently render the ICT
transition relatively easier, as explained in the spectral studies. Such an enhanced ICT
transition makes to quinone relatively electron rich and thus its electro reduction becomes
5
1,52
difficult.
ꢀ
Figure 8. CV response of receptor R5 with CN ion.
1
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ꢀ
Table 4. Electrochemical data of the receptors with the addition of CN ion.
Receptor
R1
E
pa (V)
E
pc (V)
E
1/2 (V)
∆E1/2(V)
0.112
ꢀ0.430
ꢀ0.538
ꢀ0.530
ꢀ0.646
ꢀ0.480
ꢀ0.592
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
R1+CN
R2
ꢀ0.423
ꢀ0.538
ꢀ0.408
ꢀ0.496
ꢀ0.336
ꢀ0.443
ꢀ0.326
ꢀ0.433
ꢀ0.518
ꢀ0.650
ꢀ0.514
ꢀ0.602
ꢀ0.412
ꢀ0.521
ꢀ0.395
ꢀ0.517
ꢀ0.470
ꢀ0.594
ꢀ0.461
ꢀ0.549
ꢀ0.374
ꢀ0.482
ꢀ0.360
ꢀ0.490
R2+CN
R3
0.124
0.088
0.108
0.130
R3+CN
R4
R4+CN
R5
R5+CN
Theoretical studies
ꢀ
With an aim to shed more light on the CN ion sensing behaviour of the receptors R1-
R5, the structural and electronic properties of the free receptors and their cyanide complexes
were studied using Density Functional Theory. The geometry optimization was carried out
53
using B3LYP exchange function with 6311G basic sets using Gaussian 03 package. The
ꢀ
optimized geometries of the receptors and receptorꢀCN complexes are shown in Figure S28.
The relevant frontier molecular orbitals (HOMO and LUMO) of the receptors and their
cyanide complexes are depicted in Figures S29 and S30, respectively. As seen from the
1
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optimized geometry of R5, the lengths of imidazole NꢀH and quinone C=O bonds were found
to be 1.007 and 1.24 Å, respectively. These bond lengths are in good agreement with those
obtained from single crystal XRD study (1.02 and 1.21 Å, respectively).
It is evident from Figure S29 that the HOMO orbital of the free receptors is delocalized
on the entire molecule symmetrically on both sides of the quinone ring, indicating the
existence of conjugation between the imidazole ring and the substituent attached to it. The
LUMO orbital is mainly localized on the quinone moiety. This is consistent with a strong
54,55
stabilization of this LUMO due to the presence of electron withdrawing quinone moiety.
Such a distribution of the MOs resulted in a charge transfer from imidazole moiety to
quinone moiety (i.e. ICT transition). The energies of the molecular orbitals and the energy
correspond to the ICT transition (ꢁE) in the free receptors and their cyanide complexes are
given in Table S2. The results indicated that the energy required for ICT transition (ꢁE) is
ꢀ
relatively low in the receptorꢀCN complexes, indicating that the occurrence of ICT transition
is relatively easier in the complex when compared to the free receptor. And thus the λICT
ꢀ
experienced a bathochromic shift upon addition of CN ions to the receptor solutions as
shown in UVꢀVis spectral studies. The calculated electronic nature (and structural features of
R5) of the receptors complies nicely with the experimental observations.
The foregoing results and discussion indicated that the receptors, under investigation,
have sense cyanide ion colorimetrically with high selectivity and sensitivity. The overall
experimental observations and their corresponding scientific inferences derived are collected
in Scheme 2.
1
7

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O
H
N
N
R
R
N
N
Where R= Five different alkyl,
aryl or heterocyclic substituents
H
O
CNꢀ
aq. HEPES buffer ꢀ DMF (9:1 v/v)
CNꢀ
(vi) Difficult to reduce electrochemically
O
H
N
N
R
R
ꢀ
N
N
H
(ii) Increase in e density on Nꢀatom
O
CNꢀ
(iii) Relatively easier ICT
(
i) Hꢀbond formation followed by deprotonation
(
iv) Bathochromic shift in
and change of colour
ICT
(1
HꢀNMR)
(UVꢀVis)
ꢀ
(
v) 1:2 (receptor : CN ) complex
ꢀ
Scheme 2. Overall experimental observations and inferences upon addition of CN ion to the
receptors.
Practical application
To investigate the practical application of the receptors, test strips were prepared by
immersing filter papers into a DMF solution of R5 (as a representative case) and then drying
ꢀ
them in air. The test strips were utilized to sense CN ions of different concentrations in
ꢀ
water. As shown in Figure 9, when the test strips were immersed in aqueous solution of CN
ions, clear colour changes from yellow to pink was observed instantaneously. Development
1
8

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of such test strips approach is extremely attractive for ‘inꢀthe field’ qualitative identification
ꢀ
of toxic CN ions as it doesn’t require any additional equipment.
ꢀ
Figure 9. Colour changes of the test papers (coated with R5) for detecting CN of different
concentrations in water.
Encouraged by the selective response observed with the receptors for the sensing of
ꢀ
CN ions in aqueous solution, we have tested the applicability of the receptors for the
ꢀ
chromogenic detection of CN in solid state also. To our delight, the receptor R5 exhibited an
excellent colour change from yellow to pink (Fig. 10) on simple grinding with solid
tetrabutylammonium cyanide in a mortar for few minutes (< 5 min). In the solid phase also
the receptor R5 showed the same selectivity. However, other receptors R1-R4 exhibited only
a feeble colour change in the solid phase. To the best of our knowledge R5 is the first
ꢀ
receptor which selectively senses CN ion colorimetrically in solid phase. Such a receptor will
no doubt be useful in the detection of cyanide in many solid materials.
ꢀ
Figure 10. Colour change of the receptor R5 with CN in solid state.
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9

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Obviously, it is quite natural (when the receptor R5 can act as selective chemosensor
ꢀ
for the detection of CN ion solid and aqueous phases) to test the applicability of the receptor
R5 to sense cyanide in gas phase also. As a preliminary attempt one of the test strip
(mentioned above) was kept on the top of a reaction vial containing tetrabutylammonium
cyanide (150 mg) and Con. HCl was added to it. Here again, the test strip showed noticeable
colour change upon exposure to HCN gas for 2 min (Fig. 11). [CAUTION: HCN gas is
extremely toxic and the experiment should be carried out in a wellꢀventilated fume hood].
ꢀ
Figure 11. Colour change of the receptor R5 with CN in gaseous state.
Conclusion
Five new receptors were designed, synthesized and tested for their utility as
ꢀ
chemosensors for CN ions. The receptors exhibited selective, sensitive and rapid
ꢀ
colorimetric response toward CN ion in aqueous solution. As spelt in the introduction
section, selection of benzoquinone as the signalling unit along with good HBD unit
(imidazole) improved the selectivity of sensing when compared to similar naphthoquinone
and anthraquinone based receptors. Also, the receptors with multiple Hꢀbond donor moieties
have accommodated substantial amount of water in the medium. Though the hydration
ꢀ
ꢀ1
56
energy of CN (ꢁHhyd = ꢀ67 kJ mol ) is relatively lower than that of other common anions,
the cyanide sensor which work via Hꢀbond formation are less in number than
chemodosimeters. Here again majority of the receptors can tolerate only <75% of water in the
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35,39,40,57,58
ꢀ
medium.
But in the present case the receptors developed sense CN ion in a
medium containing 90% water. The most interesting result of the present investigation is that
ꢀ
one of the receptor R5 selectively senses CN ions colorimetrically in solid, aqueous and gas
phases i.e. ‘a complete receptor’
.
Experimental section
Chemical and apparatus
All the reagents for synthesis of the receptors R1ꢀR5 were obtained commercially and
were used without further purification. Spectroscopic grade solvents were used as received.
UVꢀVis spectral studies were carried out in a double beam spectrophotometer. Steady state
fluorescence spectra were obtained on a spectrofluorimeter. The excitation and emission slit
width (5 nm) and the scan rate (250 nm) was kept constant for all of the experiments. Nuclear
1
1
magnetic resonance spectra were recorded in DMSOꢀd
6
( H NMR 300 MHz). The HꢀNMR
spectral data is expressed in the form: Chemical shift in units of ppm (normalized integration,
13
multiplicity, and the value of J in Hz). The C NMR spectra of the compounds were not
recorded due to their low solubility. The cyclic voltammetric (CV) 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 DMF containing 0.1 M tetrabutylammonium perchlorate as
ꢀ1
a supporting electrolyte at a scan rate of 100 mVs .
Synthesis and characterization of 2,3,5,6-tetraaminocyclohexa-2,5-diene-1,4-dione.(1)
One of the starting compounds (for the synthesis of R1-R5) was prepared as reported
59
earlier (Scheme 3).
Step1: To a stirred solution of tetrachloroꢀpꢀbenzoquinone (50 g, 0.20 mol) in ACN (500
mL), potassium phthalimide (150 g, 0.80 mol) was added. The reaction mixture was refluxed
for 12 h under nitrogen atmosphere. Afterwards the reaction mixture was cooled to room
2
1

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temperature and filtered through a filter paper and the filtered solid was redissolved in DMF
200 mL) and stirred at 100 °C for 10 min and the hot solution was filtered to get white solid.
(
Step2: The white solid thus obtained was transferred to a 1L round bottom flask and then 500
mL of hydrazine hydrate was added. The reaction mixture was heated to 65ºC for 2 h. Finally
the reaction mixture was cooled to roomtemperature and the crystal formed in the reaction
mixture was filtered to get the pure product as a purple crystalline solid (10 g, Yield= 29%).
δ
H
(300 MHz; DMSOꢀd6; Me
4
Si): 4.55 (s, 8H) (Fig. S31). δ
C
4
(75 MHz; DMSOꢀd6; Me Si):
+
121.36, 178.81 (Fig. S32). LCMS (ESIꢀAPCI) m/z: Calcd for C
5
H
4
ClN
3
O
2
[MꢀH] : 168.0;
Found: 167.0 (Fig. S33).
O
O
O
O
O
O
O
N
N
N
o
Cl
Cl
Cl
Cl
ACN/80 C/12 h
O
O
O
+
O
N K
Step1
N
O
O
O
Hydrazine.hydrate/
o
6
5 C/2 h
O
H N
NH2
NH2
2
H N
2
O
1
Scheme 3. Synthesis of 2, 3, 5, 6ꢀtetraaminocyclohexaꢀ2, 5ꢀdieneꢀ1, 4 – dione.
2
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General procedure for the synthesis of receptors (R1-R5)
A mixture of compound 1 (1 mmol) and corresponding aldehyde (2 mmol) in DMSO
(5 mL) was heated at 90º C with stirring for 12 h. After cooling to room temperature, the
precipitate obtained from the reaction mixture was filtered through a filter paper and washed
with cold ethanol to obtain the pure product. The receptors R1-R5 were characterized using
1
H NMR and LCMS spectral techniques. The results obtained are:
Receptor R1: Brown solid (0.4 g, yield =42.8%). δ
H
(300 MHz; DMSOꢀ
6 4
d ; Me Si): 7.09ꢀ7.23
(
(
(
m, 8H), 7.42ꢀ7.56 (m, 6H), 7.78 (s, 2H), 7.93ꢀ7.95 (d, 2H, J=7.5Hz), 14.32 (s, 2H)
+
Fig.S34). LCMS (ESIꢀAPCI) m/z: Calcd for C32
Fig. S35).
20 4 4
H N O [MꢀH] : 524.15; Found: 523.20
Receptor R2: Reddish brown solid (0.3 g, yield =52.5%). δ
H 6 4
(300 MHz; DMSOꢀd ; Me Si):
6.71ꢀ6.72 (t, 2H, J=1.5Hz, J=1.8Hz), 7.23ꢀ7.24 (d, 2H, J=3Hz), 7.93 (s, 2H), 14.28 (s, 2H)
+
(
Fig. S36). LCMS (ESIꢀAPCI) m/z: Calcd for C16
Fig. S37).
8 4 4
H N O [MꢀH] : 320.05; Found: 319.0
(
Receptor R3: Yellowish orange solid (0.35 g, yield = 44.5%). δ
H 6
(300 MHz; DMSOꢀd ;
Me Si): 7.62ꢀ7.70 (m, 6H), 8.02ꢀ8.14 (m, 6H), 8.89ꢀ8.92 (d, 2H, J=7.5Hz), 14.21 (s, 2H)
4
+
(
Fig. S38). LCMS (ESIꢀAPCI) m/z: Calcd for C28
Fig.S39).
16 4 2
H N O [MꢀH] : 440.13; Found: 439.10
(
Receptor R4: Yellowish orange solid (0.35 g, yield =57.6%). δ (300 MHz; DMSOꢀd6
;
H
Me
4
Si): 7.52ꢀ7.68 (m, 6H), 8.17ꢀ8.19 (m, 4H), 14.13 (s, 2H) (Fig. S40). LCMS (ESIꢀAPCI)
+
m/z: Calcd for C20
12 4
H N O
2
[MꢀH] : 340.10; Found: 339.10 (Fig. S41).
2
3

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Receptor R5: Yellow solid (0.35 g, yield =72%). δ
H
(300 MHz; DMSOꢀ
d
6
; Me
4
Si): 1.26ꢀ1.28
(d,12H, J=6.9Hz), 2.98ꢀ3.08 (m,2H), 13.40 (s, 2H) (Fig. S42).LCMS (ESIꢀAPCI) m/z: Calcd
+
for C14
H
16
N
4
O
2
[MꢀH] : 272.13; Found: 271.10 (Fig.S43).
Appendix A
CCDC numbers 1039004 and 1423570 contain the supplementary crystallographic
ꢀ
data for the receptor R5 and R5ꢀCN complex, respectively given as CIF file.
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 336 033; or eꢀmail:
deposit@ccdc.cam.ac.uk.
Acknowledgement
One of the authors (R.M.) is thankful to the UGC, New Delhi for the award of UGCꢀ
BSR Fellowship in Sciences for Meritorious Students.
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