An indolium ion functionalized naphtha imide chemodosimeter for detection of cyanide in aqueous medium

Article abstract of DOI:10.1016/j.ica.2018.08.040

A naphthalimide platform based indolium ion functionalized colorimetric as well as fluorometric chemodosimeter (L) has been synthesized and characterized. This can selectively sense cyanide (CN^?) in aqueous medium with a low limit of detection (approximately 0.5 μM), which is around four times lower than the value of 1.9 μM set by WHO. The UV–vis and PL studies have been carried out in 40% aqueous-acetonitrile medium which shows a significant change in the visible region allowing naked eye colorimetric detection of CNˉ. The mass spectrometry and ¹H NMR spectroscopy are used to characterize the corresponding cyanide adduct which has also been corroborated by time-dependent density functional theory (TD-DFT) during the establishment of the sensing mechanism.

Full text of DOI:10.1016/j.ica.2018.08.040

Accepted Manuscript
Research paper
An Indolium Ion Functionalized Naphthalimide Chemodosimeter for Detection
of Cyanide in Aqueous Medium
Siddhartha Maji, Bijit Chowdhury, Sanchari Pal, Pradyut Ghosh
PII:
S0020-1693(18)31026-0
https://doi.org/10.1016/j.ica.2018.08.040
ICA 18440
DOI:
Reference:
Inorganica Chimica Acta
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Received Date:
Revised Date:
Accepted Date:
5 July 2018
24 August 2018
24 August 2018
Please cite this article as: S. Maji, B. Chowdhury, S. Pal, P. Ghosh, An Indolium Ion Functionalized Naphthalimide
Chemodosimeter for Detection of Cyanide in Aqueous Medium, Inorganica Chimica Acta (2018), doi: https://
doi.org/10.1016/j.ica.2018.08.040
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An Indolium Ion Functionalized Naphthalimide Chemodosimeter
for Detection of Cyanide in Aqueous Medium
Siddhartha Maji,a,b Bijit Chowdhury,a Sanchari Pala and Pradyut Ghosh* a
aDepartment of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A &
2B, Raja S. C. Mullick Road, Jadavpur, Kolkata, 700032 (India)
b Department of Chemistry, Katwa College, Purba Bardhaman 713130 (India)
E-mail: icpg@iacs.res.in.
Abstract
A naphthalimide platform based indolium ion functionalized colorimetric as well as fluorometric
chemodosimeter (L) has been synthesized and characterized. This can selectively sense cyanide
(CN-) in aqueous medium with a low limit of detection (approximately 0.5 μM), which is around
four times lower than the value of 1.9 μM set by WHO. The UV-vis and PL studies have been
carried out in 40% aqueous-acetonitrile medium which shows a significant change in the visible
1
region allowing naked eye colorimetric detection of CN¯. The mass spectrometry and H-NMR
spectroscopy are used to characterize the corresponding cyanide adduct which has also been
corroborated by time-dependent density functional theory (TD-DFT) during the establishment of
the sensing mechanism.
Keywords: Chemodosimeter, Cyanide sensor, Aqueous media, Colorimetric, Fluorometric,
NMR titration, DFT.

1. Introduction
Cyanide is one of the most toxic species to the living systems. It occurs naturally in some of the
common foods such as peach, cassava, almond and apricot [1]. It is also released artificially from
several industrial sources such as plastic industries, gold mining, etc. [2]. The adverse
physiological effect [3] of cyanide is based upon its strong and irreversible binding with metal
ions originating from its strong π-acidity [4]. When cyanide enters in biological systems, it binds
with mitochondrial cytochrome c oxidase and inhibits the respiratory chain resulting in rapid
suffocation death [5]. Cyanide contamination also affects the central nervous system [6]. Hence
detection of cyanide in an aqueous environment is of prime importance. Although various
analytical techniques are being practically utilized to detect and quantify cyanide [7] however,
simple organic molecule-based [8-16] chemosensors are found to be the most convenient and
popular method to detect such ion presently. Metal ions, mainly Cu2+ based sensors have been
utilized for the detection of cyanide due to the strong binding affinity of Cu+/Cu2+ with cyanide
[17-18]. Imidazole complexed with Cu2+ appended from polyacetylene backbones can trigger
fluorescence response on interaction with cyanide [19]. Again, mimicking vitamin B12 as a
reliable platform for sensing cyanide in millimolar as well as micromolar concentrations in water
can be achieved through the binding of cyanide with Co3+ [20]. Sensing of cyanide over the gold
surface is also known [21]. The more environmental friendly metal zinc based ditopic receptors
where Zn2+-porphyrin complex is coupled with a crown or aza-crown ethers are used for binding
with NaCN or KCN [22]. So far we have a glimpse of metal-based chemosensors for cyanide
where co-ordination of cyanide with metal ions attributes the sensing process. There are
examples in the literature where a chemical reaction forming a covalent bond between the
chemosensors and cyanide is the key phenomenon which triggers the sensing mechanism [23].

Boronic acid [24], borane [25, 26], coumarin [27-29], BODIPY [30, 31], naphthalimide [25, 32],
calix[4]pyrrole [33] platform based sensors have been used more conveniently for sensing
cyanide considering the very good nucleophilicity of cyanide [34]. Based on the well-established
fact that cyanide forms cyanohydrins on reaction with carbonyl functional groups (>C=O),
several chemodosimeters are known [35, 36]. Few polymeric sensors for cyanide have also been
reported, mainly containing pyrylium cation, fluorene and borane functionalities [33, 37-41].
Further, indolium ion functionalized chemodosimeters have also been explored earlier where
cyanide adds to the activated [>C=N<]+ bond [31, 42, 43]. Detection of cyanide produced from
various cyanogenic glycosides [44] due to enzymatic hydrolysis has also been accomplished by
indolium moiety [42]. Herein, we report a new chemodosimeter L (Scheme 1) based on long
chain incorporated naphthalimide platform functionalized with an indolium ion to detect CN-
-
-
-
-
-
over other anions such as F-, Cl-, Br-, I-, NO2 , NO3 , ClO4 , H2PO4 , HSO4 and OAc- in the
aqueous medium.
Scheme 1. Synthetic route for L: (a) Br2, KOH; (b) 3-Amino-1-propanol, ethanol, 70oC, 2h; (c) 4-
Formylphenylboronic acid, Pd(PPh3)2Cl2, Cs2CO3, 80oC, 6h; (d) 1,2,3,3-tetramethyl-3H-indolium iodide, piperidine,
ethanol, 70oC, 4h.

2. Experimental
1.1. Materials and instrumentation
Most of the chemicals were bought from Sigma-Aldrich Co. and were used without
further purification. Other chemicals were supplied by Spectrochem, India. HPLC grade
acetonitrile and double distilled water were used for absorption and emission studies.
Whereas, acetonitrile-d3, CDCl3, and D2O were used as were available in Sigma-Aldrich
Co.
HRMS analyses were performed with a QToF-Micro YA 263 mass spectrometer where
1
13
ESI (+ve) as well as (-ve) modes were used. H and C-NMR experiments were carried
1
out using an FT-NMR Bruker DPX 300 MHz NMR spectrometer. Chemical shifts for H
and 13C-NMR were reported in parts per million (ppm), calibrated to the residual solvent
peak set. The absorption and emission studies were performed with a PerkinElmer
Lambda 900 UV−vis−NIR spectrometer (NIR = near-infrared) (with a quartz cuvette of
path length 1 cm) and a FluoroMax-3 spectrophotometer from Horiba Jobin Yvon,
respectively. Solution electronic spectra (single/time-dependent) were measured on an
Agilent 8454 diode array spectrophotometer.
The following equation was used to analyze the time-resolved emission decays:
P(t) is decay, i is the number of discrete emissive species, B is the baseline correction, αi
is the pre-exponential factor, and τi is the excited state lifetime associated with the ith
component. In the case of multi-exponential decays the following equation was used to
calculate an average lifetime:

where ai is the contribution of the ith decay component, and ai = αi/Σαi.
A diffractive size crystal was collected from the mother liquor, dipped in paratone oil and
using epoxy resin it was cemented on the tip of a glass fiber. The intensity data of the
crystals were collected using Mo Kα (λ = 0.7107 Å) radiation on a Bruker SMART
APEX diffractometer, equipped with a CCD area detector at 100 K. CCDC 1863558
contains the supplementary crystallographic data for this paper.
1.2. Synthesis
Synthesis of 3. Compound
1
and
2
were synthesized following previously reported
compound (388 mg, 1 mmol) was taken in a 100
procedure [45, 46]. For synthesis of
3
2
ml two neck round-bottomed flask along with 4-formylphenylboronic acid (180 mg, 1.2
mmol), caesium carbonate (1.3 g, 4 mmol) in degassed DMF/Water (2:1, v/v) mixture.
Bis(triphenylphosphine)palladium(II) dichloride (30 mg, 0.04 mmol) was added to the
heterogeneous mixture under inert condition. The whole system was allowed to be heated
at 80°C for 6 hours. After the completion of the reaction the crude product was extracted
with ethyl acetate, the organic layer was dried over anhydrous Na2SO4 and concentrated
in rotary evaporator. Pure compound was isolated through column chromatography (using
230-400 mesh silica) using 2.2% EtOAc-Hexane as eluent. Yield: 250 mg (60%). ESI-
1
MS: m/z calculated for [M]+ 413.1991, found 413.9969 (Fig. S3); H-NMR (300 MHz,
CDCl3): δ (ppm) = 0.84 (t, 3H, J=6 Hz), 1.24-1.40 (m, 10H), 1.66-1.76 (m, 2H), 4.12 (t,
2H, J=7.5 Hz), 7.66–7.72 (m, 4H, Ar-H), 8.05 (d, 2H, J=9 Hz, Ar-H), 8.14 (d, 1H, J=9
13
Hz, Ar-H), 8.58–8.63 (m, 2H, Ar-H), 10.12 (s, 1H) (Fig. S1). C-NMR (75 MHz,
CDCl3): δ (ppm) = 14.1, 22.6, 27.2, 28.1, 29.2, 29.3, 31.8, 40.6, 122.6, 123.1, 127.3,

127.8, 128.6, 129.7, 130.0, 130.6, 130.6, 131.3, 131.9, 136.1, 144.9, 145.1, 163.8, 164.0,
191.6 (Fig. S2).
Synthesis of L. Compound
3 (250 mg, 0.6 mmol), 1,2,3,3-tetramethyl-3H-indolium
iodide (192 mg, 0.63 mmol) were taken in a 100 ml round-bottomed flask. 10 ml of dry
ethanol was added to the mixture under N2 atmosphere. Then 10 μL of piperidine was
added to it. The whole suspension was heated at 70°C for 4 hours. The suspension
became solubilised under heating condition. Upon slow cooling
L got precipitated from
the solution. The solid was filtered, washed with cold ethanol and dried in vacuum to get
the pure compound. Yield: 230 mg (55%). ESI-MS: m/z calculated for [L-I]+ 569.3163
found 569.4860 (Fig. S6); Elemental analysis for C39H41IN2O2: found: C, 67.24; H,
1
5.93; N, 4.02. Calculated: C, 67.37; H, 5.98; N, 4.15; H-NMR (300 MHz, ACN-d3): δ
(ppm) = 0.88 (t, 3H, J=7.5 Hz), 1.30-1.39 (m, 10H), 1.72 (m, 2H), 1.85 (s, 6H), 4.11-4.15
(m, 5H), 7.58 (d, 1H, J=15 Hz), 7.65–7.84 (m, 8H, Ar-H), 8.21 (d, 2H, J=9 Hz, Ar-H),
8.29 (d, 1H J=9 Hz, Ar-H), 8.38 (d, 1H, J=15 Hz), 8.59–8.63 (m, 2H, Ar-H) (Fig. S4);
13C-NMR (75 MHz, ACN-d3): δ (ppm) = 13.3, 22.3, 24.9, 26.8, 27.7, 28.9, 29.0, 31.5,
34.8, 40.1, 52.8, 113.5, 115.1, 122.6, 122.8, 123.2, 127.5, 128.1, 128.5, 129.3, 129.5,
130.1, 130.1, 130.3, 130.8, 131.0, 131.9, 134.2, 141.8, 143.6, 144.9, 152.6, 163.7, 163.9,
182.6 (Fig. S5).
3. Results and discussion
Designing aspect: Our basic idea is to exploit the facile nucleophilic addition of cyanide
into an activated double bond of indolium moiety which can reduce the extent of
conjugation in the chemodosimeter
L. Thus the produced output signal can be monitored
spectroscopically, as well as, through visual color change. A long chain alkyl (octyl)

group has been strategically incorporated to make the molecule more lipophilic, so that
the molecule might be suitable for interaction/sensing studies, even in biological
systems/membrane. On the other hand, the polarity of the indolium ion functionality can
be utilized to enhance its aqueous solubility.
The synthetic approach for
some steps in reasonably good yield (55%).
spectroscopic as well as spectrometric methods (Fig. S4-S7). When
L
is outlined in Scheme 1. Thus
has been well characterized by
was subjected to
L can be synthesized through
L
L
ESI-MS (+ve) in acetonitrile (ACN) a peak at m/z = 569.4860 appeared corresponding to
the monocationic species, [L-I]+ of the ligand (Fig. S6). The experimental isotopic
distribution pattern of the mass of the ligand matches quite well with the simulated
pattern and indicates the presence of the mono charged species of
L (Fig. S7). Various
spectroscopic and colorimetric studies with different anions as their tetrabutylammonium
salts have been performed to establish the selectivity and sensitivity of
acetonitrile/aqueous-acetonitrile binary solution.
L in
Detection of Cyanide via Colorimetric, UV-vis, and PhotoLuminescence (PL)
methods:
The anion sensing ability of
acetonitrile or 40% aqueous-acetonitrile at room temperature. Firstly, the
chemodosimetric property of towards cyanide over other anions was examined
colorimetrically through visual colour change by adding 1.0 eqv. of each of the anion
L was tested by Colorimetric, UV−vis and PL spectroscopy
L
-
-
-
-
-
solutions (F-, Cl-, Br-, I-, CN-, NO2 , NO3 , ClO4 , H2PO4 , HSO4 and OAc- as their
tetrabutylammonium salts) into a solution of in acetonitrile or 40% aqueous-acetonitrile
(Fig. 1). Addition of cyanide (TBACN) solution triggers the visual color change of
L
L

from yellow to colorless, whereas, in the presence of other anions no appreciable visual
change is observed. Such color change can be employed to distinguish cyanide from other
anions by the naked eye.
Fig. 1. Naked eye color changes upon addition of 1.0 eqv. of all anions as their TBA salts to L (40 μM) in
40% H2O/ACN binary solvent mixture; The numbers in the inset indicate different anions; color code: 1: F-,
-
-
-
-
-
2: Cl-, 3: Br-, 4: I-, 5: CN-, 6: NO2 , 7: NO3 , 8: ClO4 , 9: H2PO4 , 10: HSO4 , 11: OAc-.
Spectroscopic proof for cyanide sensing property of
L
was studied by absorption
in
spectroscopic measurements. In the UV-vis channel, the absorption spectrum of
L
ACN exhibits maximum at about 400 nm which undergoes significant absorption
changes, i.e., the blue shift of λmax (Δλ = 30 nm) with CN− which can be correlated with
the naked eye color changes from yellow to color less. Interestingly, no significant
changes of λmax was observed in the presence of other anions, such as F−, Cl−, Br−, I−,
−
−
−
−
−
NO2 , NO3 , AcO−, HCO3 , H2PO4 , and HSO4 (Fig 2a), indicating a selective reaction
of CN− with the conjugated indolium double bond of
is further monitored by UV-vis titration. Addition of increasing amounts of CN− (0−1.5
L
. This selective reaction of cyanide
equiv) to the ACN solution of
L (40 μM) at room temperature reveals a gradual decrease
of the absorbance at 400 nm with concomitant appearance of band at 370 nm and no

further change is observed after addition of more than 1.0 equiv of CN− ions (Fig. 2b).
During the
Fig. 2. (a) Qualitative UV-vis studies upon addition of 1.0 eqv. of other anions and cyanide (as their TBA
salts) to L (40 μM) in 40% H2O/ACN binary solvent mixture; (b) UV-vis titration of
L (40 μM) with
TBACN (1mM) in 40% H2O/ACN solvent mixture.
course of the UV−vis titration one isosbestic point was clearly observed at
during the addition of 0−1 equiv of CN− (Figure 2b) indicating the existence of one
and CN-. A plot of the absorbance at 400 nm as a
∼380 nm
equilibrium between the probe
L
function of the amount of cyanide clearly shows that it decreases linearly up to 1.0 equiv
of CN− and then saturation takes place which clearly indicates 1:1 reaction stoichiometry
between the probe
L
and CN- (Fig. 3b) which is further verified by Job’s plot
analysis(Fig. 3a). The observed relationship should allow for a quantitative determination
of cyanide in the UV−vis channel. The CN- sensing property of L is further studied in
ACN (100%) solvent system. Upon addition of 1.0 equiv TBACN solution to the 40 μM

Fig. 3. (a) Job’s plot of absorbance changes with the different ratio of L (40 μM) and TBACN (40 μM) in
40% H2O/ACN binary solvent mixture; (b) Changes in absorbance of (40 μM) with increasing amount of
TBACN(1mM) added during UV-vis titration) in 40% H2O/ACN solvent mixture.
L
solution of
L
, ~30 nm blue shift of the absorption maximum is observed (Fig. S7,
can detect cyanide both in aqueous as well as non-
Supporting Information). Hence
L
aqueous media, indicating that similar mechanistic phenomena might be operative in both
the solvent systems.
Probe
L shows emission at ~550 nm in acetonitrile at room temperature. Upon treatment
of
L
with various anions show a notable behavior only with CN− resulting in a
∼100 nm
blue shift of the emission maximum (Fig. 4a), whereas other anions remain silent,
indicating the selective sensing of CN− through PL channel. The CN− detection ability of
L
was thoroughly investigated via PL titration experiment. During titration of L
with CN−
intensity of the PL band at ~550 nm gradually decreases with the concomitant increase of
band ~470 nm with a clear isosbestic point at ~500 nm, indicating the existence of one
equilibrium between
L
and CN− as obtained from UV-vis titration experiment.

Fig. 4. (a) Qualitative PL studies upon addition of 1.0 eqv. of other anions and cyanide (as their TBA salts) to L (8
μM) in 40% H2O/ACN binary solvent mixture; (b) PL titration of L (8 μM) with TBACN (1mM) in 40% H2O/ACN
solvent mixture.
This PL spectroscopic change continues till the addition of the one equiv CN− at which
the saturation occurs. From the PL titration profile the host: guest stoichiometry is
calculated as 1:1 (Fig. 4b inset). The blue shift of the PL spectra must be due to the
conjugate addition of the cyanide to the indolium double bond and its truncation in the
conjugation after that. From the PL titration, it is thus reasonable to assume that one step
reaction is taking place between L and CN− to form the 1:1 adduct. When the PL intensity
of
L
at 550 nm is plotted against the amount of added CN−, the graph shows a good linear
relationship, as in the case of UV−vis titration during the course of 0 to 1.0 equiv of CN−
addition. No more intensity change is observed after more than 1.0 equiv of CN− has been
added, confirming the formation of 1:1 adduct (Fig. 4b inset).

Fig. 5. (a) Competetive Fluorescence graph of L with TBACN in the presence of other anions; (b) Linear
fitting of Fluorescence intensity vs. Cyanide concentration calibration curve.
To confirm the high selectivity of L toward cyanide a competition study was performed
as done in UV-vis experiment. The selectivity plot is displayed in Figure 5a, which shows
that no remarkable PL intensity change of
L was observed upon addition of excess (> 10
equiv) of other possibly competing anions to the solution of
L in the absence of cyanide.
However, upon addition of CN− the PL spectra change drastically. Therefore,
L
may be
considered as a highly selective probe for CN−. This PL intensity change of L when
plotted against the increasing concentration of CN− a good linear relationship is obtained
and the corresponding detection limit is calculated as 0.476 μM (Fig. 5b) which is lower
than 1.9 μM set By WHO [47]. This selectivity and sensitivity plots are well reproducible,
and error limits are within 10%.The plot of the PL intensity change (I0-I) at 550 nm for
L
as a function of the cyanide concentration shows a good linear relationship in the range of
could be used to quantitatively detect the CN−
0−1 equiv of cyanide, indicating
L
concentration in an unknown sample (Fig. 5b).
Reaction kinetics of CN− with
H2O/ACN medium by adding 1.0 equiv. of CN− (1.0 mM) solution to
L
was performed through UV-vis spectroscopy in 40%
L
(40 μM), and the

absorption maximum (max) at 400 nm was monitored (Fig. 6a). From this kinetics, it is
observed that with time the absorbance at 400 nm decreases and the reaction between
and CN− is found to follow a pseudo first-order rate equation possessing the first order
10-3 s-1. The plot of absorption spectral changes with time shows
L
rate constant of 4.78
×
Fig. 6. Time course of response of
L
with the addition of 1 equiv CN- in (a) UV-vis and (b) PL channel.
that the reaction takes 20 min to complete (Fig. 6a). This same reaction kinetics is
monitored via the PL channel also. The time course of the PL response of L upon addition
of 1.0 equiv of cyanide at room temperature was monitored (Fig. 6b), showing that the
reaction is saturated ~30 min. The little slow PL response compared to the UV-vis one
can be realized from the point that PL experiment was done with lower host-guest
concentration (10µM) as compared to that taken in UV-vis experiment (20µM).
Apart from this steady state PL experiment, the CN− sensing behavior of the probe
also established via time-correlated single photon counting (TCSPC) measurements
(Figure 7). The decay profile as well as the lifetime of the probe is changed
L was
L
significantly upon addition of the CN−. Interestingly the probe shows lifetime of 1.574 ns
whereas upon addition of CN− the life time of the probe monitored at 550 nm is decreased

to 0.083 ns (Table S1), indicating that the probe has changed its structure/ property upon
addition of the CN−. Moreover, when these lifetime measurements are monitored at 460
nm, it is observed that the lifetime of the probe
L at 460 nm was 0.22 ns which increase
upon addition of CN− to 0.54 ns (Table S1). Thus the lifetime data also supports the
ratiometric behavior of probe
L towards cyanide sensing as that is established via UV-vis
and PL titration experiments. Thus PL and TCSPC data suggest a selective reaction based
sensing of CN− through the conjugate addition of the analyte in the ligand structure.
Further the sensing study via PL experiment in semi-aqueous (40% aqueous acetonitrile)
environment leading to same results as obtained in 100% ACN medium.
Fig. 7. Decay profile of
L
(20 µM) in the presence of TBACN (0.2 mM) in 40% H2O/ACN monitored at
, green: L+CN-
550 nm; Color code: red:
L
.
Thus probe
L can be utilized as a selective and sensitive reaction based stoichiometric
probe for CN− in aqueous and semi-aqueous medium even in the presence of large excess
of other competitive anions.

Mechanistic Investigation of Cyanide Sensing by NMR, Mass and TD-DFT Study:
1
The reaction based cyanide sensing property of
L
was finally confirmed by the H NMR
experiments in D2O/ACN-d3 binary solvent mixture, and similar observations were found
in both the media (Fig. 8 and Fig. S8). We were able to increase the percentage of D2O up
to 15%. The selectivity of
L
towards CN− as obtained from spectroscopic studies (UV-vis
1
and PL) was also confirmed by the H-NMR experiments whereas no noticeable change
was observed with other competitive anions of same concentration (Fig. S9). In the
1
presence of CN−, some signals in the H-NMR spectrum get vanished, and some new
signals are generated in the aromatic region as well as in the aliphatic region, indicating
the formation of a new species in solution. But other anions do not show any change in
spectra which confirms the chemodosimetric selectivity of
L
towards CN− (Fig. S9). The
was monitored via 1H-
stoichiometry of this reaction based cyanide sensing property of
L
NMR titration studies in D2O/ACN-d3 (15% D2O, v/v).
Fig. 8. 1H-NMR titration of L (8.25 mM) with TBACN (38.17 mM) in 15% D2O/ACN-d3 solvent mixture exhibiting
that saturation attains on the addition of 1.0 equiv. of TBACN (enlarged figure).

Upon gradual addition of 38.17 mM solution of cyanide to 8.25 mM solution of
L, the
peaks corresponding to is diminished with the concomitant appearance of new peaks
L
related to [L-CN] adduct. This phenomenon gets saturated upon addition of 1.0 equiv. of
TBACN solution (Fig. 8). The same observation is found in case of ACN-d3 alone where
50.49 mM solution of cyanide in ACN-d3 was gradually added to 8.97 mM solution of
in ACN-d3 (Fig. S8).
L
To have an insight into the mechanistic pathway of CN− sensing by
in the presence of excess CN− are compared. It is observed that the
L, both the NMR
spectra of
L and L
peaks corresponding to both the vinylic protons Ha and Hb, which appear as a doublet due
to trans-coupling with J = 15 Hz, show upfield shift from 8.38 and 7.58 ppm to 7.22 and
6.51 ppm respectively, in the presence of excess CN− (Fig. 9). On the other hand, the
signal corresponding to N-CH3, i.e., Hc protons also exhibits an upfield shift from 4.11 to
1
2.81 ppm. Again, from H-NMR titration studies of
L with TBACN, it is observed that
the reaction gets saturated after the addition of 1.0 equiv. of CN− (Fig. 8). From these
findings, a plausible mechanism of the reaction can be drawn as depicted in Fig. 9, as
reported earlier by another group [48]. Since CN− is a good nucleophile compared to all
other considered anions, it can add to the electrophilic [>C=N<]+ bond of the indolium-
ring in
L
forming 1:1 addition product rendering the positively charged nitrogen atom
and
neutral. This phenomenon is further supported by the ESI-MS experiment. When
L
TBACN are mixed in 1:1 ratio in acetonitrile and subjected to ESI-MS (-ve), a peak at
m/z = 721.1680 corresponding to [(L-CN)-H˙]− species appears (Fig. S10) indicating the
formation of a new [L-CN] compound whose isotropic distribution pattern of mass
exactly matches with the simulated pattern (Inset of Fig. S10).

1
Fig. 9. H-NMR spectrum of L in 15% D2O/ACN-d3 (bottom) and 1H-NMR spectrum of L with CN- (1.2 equiv.) in
15% D2O/ACN-d3 (top) showing the upfield shift of Ha, Hb and Hc on addition of TBACN solution and the plausible
mechanism for binding of CN- with L.
Here it is to be noticed that since CN− is a good nucleophile compared to all other
considered anions when a solution of TBACN is added to a solution of
electrophilic [>C=N<]+ group, forming a covalent bond. In
L, it adds to the
L
, the activated as well as an
electrophilic double bond is conjugated with the naphthalimide bound benzene moiety.
Hence the whole molecule has a π-π* absorption band near 390-400 nm. When all other
anions except cyanide are added to
add to the activated double bond as a result of which absorption band near 390-400 nm
does not show any kind of shift. But whenever cyanide is added to , due to its greater
L, due to their less nucleophilic character they do not
L
nucleophilic character, it adds to the electrophilic double bond rendering the positively
charged nitrogen atom neutral. Thus the extent of conjugation in the molecule is
decreased, resulting in the shift of the absorption band towards the shorter wavelength
that means the cyanide adduct of
spectrum is observed in UV-vis experiment. To account for the decrease in the emission
intensity of on the addition of cyanide, excited state electron transfer phenomenon can
be considered. When cyanide adds to indolium nitrogen atom becomes neutral leaving
the molecule neutral as a whole. Now there is a possibility of excited state photoinduced
L absorbs at shorter wavelength, and a blue shifted
L
L

electron transfer (PET) from the neutral nitrogen atom of the indolium moiety to the
electron deficient naphthalimide ring. Due to this phenomenon, the emission intensity of
L
gets quenched by the addition of cyanide.
To support the mechanistic pathway further and explain the experimental findings
1
obtained from UV-vis, fluorescence and H-NMR studies, DFT calculations have been
performed both on
L and the ligand-cyanide adduct (L-CN) with B3LYP functional [49]
using 6-311G(d) [50] basis set for all the atoms with conductor-like polarizable
continuum model (CPCM) [51] for acetonitrile solvent in Gaussian 09 package [52].
The Hartree−Fock-type exchange was mixed with Becke’s exchange functional [53] and
that proposed by Lee−Yang−Parr for the correlation contribution [54]. The optimized
energy structure of
L shows that the indolium moiety and the benzene ring attached to the
naphthalimide moiety lie in the same plane (Figure S11). But in the energy optimized
structure of the L-CN adduct, they come out of planarity hampering the conjugated π-
framework (Fig. S11). This observation supports the occurrence of a blue shift in the UV-
vis studies on the addition of TBACN. For the probe
L, it is expected that the lower
energy band at 400 nm is due to intraligand charge transfer. The electron with drawing
∼
character of the indolium unit and its π conjugation should efficiently decrease the LUMO
energy. A significant enhancement of the LUMO energy level is observed after cyanide
addition to the indolium double bond due to the much less electron-withdrawing character
of the corresponding neutral analogue and concomitant loss of extended π conjugation.
Time-dependent DFT calculated UV-vis spectra of the cyanide adduct of
L matches very
well with the one obtained experimentally which also supports the formation of the
adduct (Figure S12).

We can also get some idea about the quenching phenomenon in the PL studies from TD-
DFT calculations. In , the HOMO is distributed both on the naphthalimide moiety, the
L
adjacent benzene ring and the indolium unit (Fig. 10). Whereas, in the L-CN adduct the
HOMO is mostly concentrated on the indolium moiety and the LUMO is concentrated on
the naphthalimide unit. Hence HOMO to LUMO charge transfer in the excited state may
cause the quenching process.
Fig. 10. Frontier Molecular Orbitals (MOs) of (a) L and (b) L-CN complex generated from DFT
calculations (isovalence = 0.02).
Single Crystal X-ray Structural Evidence:
The cyanide binding mechanism as established by NMR, ESI-Ms and TD-DFT studies is
finally corroborated by the single crystal X-ray structure of the cyanide adduct of the
probe
L. Slow layer diffusion of stoichiometric (1:1) mixture of probe L
and CN− in
acetonitrile into benzene results in colorless crystals of the [L-CN] adduct. The crystal
structure of the L-CN adduct is demonstrated in Fig. 11, which shows that the cyanide

Fig. 11. Single crystal X-ray structure of the cyanide adduct of L (L-CN). Thermal ellipsoids are drawn at
30% probability level; Color code: Carbon-grey, hydrogen-white, nitrogen-blue, oxygen-red.
adduct is formed through the attack of the cyanide anion into the indolium ring as
discussed in the UV-vis, PL and NMR experimental section. The crystal structure of the
cyanide adduct clearly evidences in favor of the fact that the conjugation in the probe
truncated upon addition of the cyanide which is reflected in the spectroscopic properties
of the probe
L is
L
.
Conclusion
In conclusion, we report a new naphthalimide-indolium ion conjugate molecule which
can selectively sense cyanide in the presence of other anions in an aqueous medium. The
facile nucleophilic attack of CN− to the [>C=N<]+ group makes it an effective
chemodosimeter to sense CN− with a low limit of detection. The ease of synthesis of the
molecule and its ability to detect CN− through a visual color change in aqueous medium,
make it imperative. Currently, we are interested to explore the possibility of interactions
of the sensor having long chain alkyl functionality with synthetic
membranes/biomembranes, as this molecule has both polar and non-polar ends which
exist together just like a surfactant molecule. Moreover, functionalization of the molecule
with appropriate side chain in the imide part instead of long chain alkyl can lead to its

immobilization onto polymeric backbone, on which we are working on presently in our
laboratory extensively.
Acknowledgments
P.G. gratefully acknowledges the Science and Engineering Research Board (SERB;
EMR/2016/000900), India, for financial support. S.M. would like to acknowledge the
Council of Scientific and Industrial Research (CSIR), New Delhi, India for SRF. B.C. and
S.P. acknowledge the Science and Engineering Research Board (SERB) for RA-I and
National Postdoctoral Fellowship (NPDF) respectively.
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An Indolium Ion Functionalized Naphthalimide Chemodosimeter
for Detection of Cyanide in Aqueous Medium
Siddhartha Maji,a,b Bijit Chowdhury,a Sanchari Pala and Pradyut Ghosh* a
aDepartment of Inorganic Chemistry, Indian Association for the Cultivation of Science, 2A &
2B,
Raja
S.
C.
Mullick
Road,
Jadavpur,
Kolkata,
700032
(India)
b Department of Chemistry, Katwa College, Purba Bardhaman 713130 (India)
E-mail: icpg@iacs.res.in.
Graphical Abstract
A new naphthalimide platform based indolium ion functionalized colorimetric as well as
fluorometric chemodosimeter (L) has been synthesized which selectively sense CN- ion in
aqueous media through nucleophilic addition of CN- in L with low detection limit.