Highly Selective Sensing of Li+ in H2O/CH3CN via Fluorescence ‘Turn-on’ Response of a Coumarin-Indole Linked Dyad: an Experimental and Theoretical Study

Article abstract of DOI:10.1007/s10895-016-1913-1

A coumarin-indole dyad, N-((7-hydroxy-2-oxo-2H-chromen-4-yl)methyl)-1H-indole-2-carboxamide has been synthesized and characterized by ¹H-NMR and ¹³C-NMR. Effect of various metal ions on fluorescent behavior was also studied. The synthesized compound showed remarkable specificity towards Li⁺ in organo-aqueous medium over other metal ions. Coordination of the compound with Li⁺ induces a turn-on fluorescence response. The sensor exhibited good binding constant and low detection limit towards Li⁺. Experimental results have been verified with Density Functional Theory and Time Dependent Density Functional Theory calculations.

Full text of DOI:10.1007/s10895-016-1913-1

J Fluoresc
DOI 10.1007/s10895-016-1913-1
ORIGINAL ARTICLE
Highly Selective Sensing of Li+ in H2O/CH3CN via Fluorescence
‘Turn-on’ Response of a Coumarin-Indole Linked Dyad:
an Experimental and Theoretical Study
Santosh Kumari1 & Sunita Joshi2 & Amrit Sarmah3 & Debi Pant4 & Rajeev Sakhuja1
Received: 30 June 2016 /Accepted: 16 August 2016
#
Springer Science+Business Media New York 2016
Abstract A coumarin-indole dyad, N-((7-hydroxy-2-oxo-
2H-chromen-4-yl)methyl)-1H-indole-2-carboxamide has
in solution phase [1–6]. Among the metal ions, lithium ion is
one the most biologically important alkali metal cation.
Lithium-containing drug preparations are frequently used in
clinical applications especially for the treatment of manic-
depressive psychosis [7–10], [11]. Lithium also finds its use
in treatment of skin diseases such as dermatitis and autoim-
mune and immunological diseases. In many cases, patients
are required to take the drug over periods of several months
or even years. The concentration of lithium ions in blood serum
after drug intake varies from person to person and needs to be
monitored in the individual patient routinely. Therefore, the
reliable determination of the lithium ion concentration levels
in blood samples is important for successful and safe therapeu-
tic applications, since too low levels show no effect at all and an
overdose of lithium can lead to life-threatening toxic effects.
From the reported studies concentration of lithium ion in serum
during the treatment should be within the narrow range of 0.6
and 1.2 mM [12, 13].
1
been synthesized and characterized by H-NMR and 13C-
NMR. Effect of various metal ions on fluorescent behavior
was also studied. The synthesized compound showed
remarkable specificity towards Li+ in organo-aqueous
medium over other metal ions. Coordination of the com-
pound with Li+ induces a turn-on fluorescence response.
The sensor exhibited good binding constant and low detection
limit towards Li+. Experimental results have been verified
with Density Functional Theory and Time Dependent
Density Functional Theory calculations.
.
.
.
.
Keywords Coumarin Indole Fluorescence Sensor
.
Lithium DFT
Introduction
Many analytical techniques, such as ion selective elec-
trodes [14, 15], colorimetric sensors [16, 17], voltammetry
[18], atomic absorption and emission spectrometry [19, 20],
chromatography [21] and inductively coupled plasma mass
spectroscopy [22] have been used to determine the presence
of metal ions in different samples. Among the various detec-
tion methods, fluorescence sensing [23–32] has become the
most useful and popular technique in clinical, biology and
environmental chemistry due to its non-destructive nature,
high selectivity and sensitivity, real-time response and possi-
ble naked eye detection. Coumarin has been efficiently used
as a fluorophore due to its excellent photo-physical properties
such as great fluorescent intensity, high quantum yield, high
photostability, biological stability and non-toxicity [33–35]. A
number of examples of coumarin-based fluorescent probes are
reported for sensing of different cations such as Fe3+, Zn2+,
Cu2+, Ni2+ and Hg2+ during the last decade [33–38]. Although
Due to the importance of metal ions in various science sub-
disciplines, great attention has been laid down towards devel-
oping newer molecules capable of sensing different metal ions
* Rajeev Sakhuja
sakhuja.rajeev@gmail.com
1
Department of Chemistry, Birla Institute of Technology and Science,
Pilani, Rajasthan 333 031, India
2
Department of Pathophysiology, Charles University,
Prague, Czech Republic
3
Department of Chemistry, Ben-Gurion University, Negev, Israel
4
Department of Physics, Birla Institute of Technology and Science,
Pilani, Rajasthan 333031, India

J Fluoresc
1
several lithium sensors are known [39–41], however, very few
reports on the development of fluorescent chemosensors for
selective recognition of lithium ion in organic solvents have
appeared [42–47]. One such example was recently reported by
Sakai and Akutagawa, whereby quinoxalinone based fluores-
cent probe senses the cation (M+ = Li+ and Na+) and anion
(X− = F−, Cl−, Br−, and CH3COO−) in organic solvents [48].
However, selectively detecting Li+ at low concentrations in
aqueous medium by fluorescent chemosensors remains a ap-
pealing and necessary field of research [49]. In continuation of
our [50, 51] effort on the the synthesis of coumarin based
chemosensors, we report the synthesis of new
coumarin-indole dyad based on a ‘fluorophore–spacer–
receptor’ model, where the fluorescence emission at the
coumarin site is modulated by Li+ recognition at the
indolic site. This resulted in efficient selective sensing
of Li+ which usually results in the suppression of photoin-
duced electron transfer (PET) quenching operating between
the two moieties.
spectra were recorded on Bruker 400 spectrometer. All H
NMR experiments were reported with TMS as an standard
in δ units, parts per million (ppm), and were measured relative
to residual DMSO (2.5 ppm) in the deuterated solvent. 13C
NMR spectra were reported in ppm relative to ppm [d6]
DMSO (39.5 ppm). All coupling constants J were reported
in Hz. The following abbreviations were used to describe peak
splitting patterns when appropriate: s = singlet, d = doublet,
t = triplet, dd = doublet of doublet, m = multiplet and br
s = broad singlet. Melting points were determined on a capil-
lary point apparatus equipped with a digital thermometer.
Mass spectra were recorded on Waters Synapt G2 high detec-
tion mass spectrometer at Core Instrumentation Centre,
University of Mysore.
Synthesis of N-((7-hydroxy-2-oxo-2H-chromen-4-yl)
methyl)-1H-indole-2-carboxamide (6)
To the stirred solution of 4 (0.300 g, 1.5 mmol, 1 eq.) in DMF,
triethyl amine (2.5 eq.) was added drop-wise at 0 °C and
subsequently EDC.HCl (0.429 g, 2.2 mmol, 1.5 eq.) and
HOBt (0.243 g, 3.7 mmol, 1.2 eq.) was added and the reaction
mixture was stirred for 20 min. at 0 °C. Later 1H-indole-2-
carboxylic acid (0.303 g, 1.8 mmol, 1.2 eq.) was added and
the reaction was stirred at room temperature for 6 h. The
completion of the reaction was monitored by TLC. After the
completion of the reaction, crushed ice was added that resulted
in the precipitation of the product 6, which was filtrated,
washed with cold water and re-crystallized from ethanol.
Experimental
Materials and Methods
All the chemicals were purchased from Sigma-Aldich, Alfa
Aesar, and Spectrochem India Pvt. Ltd. and used without fur-
ther purification. The solvents (HPLC grade) used were pur-
chased from Merck (India) and were distilled and dried before
use. Absorption spectra were taken using dual beam Thermo
Evolution 201 UV/Vis/NIR spectrophotometer and fluores-
cence spectra were recorded using a Shimadzu RF-5301PC
spectrofluorometer. The data were analyzed using related soft-
ware. The concentration of compound in all the solutions pre-
pared was 10−4 M. Nuclear Magnetic Resonance (NMR)
1
Yield: 0.235 g, 90 %; white solid; mp: 232–234 °C; H
NMR (400 MHz, DMSO-d6) δ 12.11 (s, 1H), 9.47 (t,
J = 5.6 Hz, 1H), 7.65 (t, J = 8.2 Hz, 2H), 7.47 (d,
J = 8.2 Hz, 1H), 7.25 (s, 1H), 7.23–7.18 (m, 1H), 7.05 (s,
1H), 6.77 (dd, J = 8.8, 2.2 Hz, 1H), 6.68 (d, J = 2.2 Hz,
1H), 5.93 (s, 1H), 4.67 (d, J = 5.3 Hz, 2H); 13C NMR
Scheme 1 Synthesis of coumarin-Indole Dyad (6)

J Fluoresc
(101 MHz DMSO-d6) δ 166.0, 161.9, 161.4, 156.1, 154.4,
137.2, 131.7, 127.5, 125.8, 124.0, 122.1, 120.3, 115.0,
112.9, 108.4, 106.0, 104.0, 103.1, 24.8; HRMS: Chemical
formula for C19H15N2O4; calcd 335.1024 [M + H]+; found
335.1039[M + H]+.
General Procedure for UV/Fluorescence Measurements
UV/Fluorescence measurements were carried in
CH3CN:water (3:7) with different Metal ions (5 eq.) keeping
the concentration of compound 6 at 10−4 M. UV/Fluorescence
titrations were carried in CH3CN:water (3:7) with gradual
increase in the concentration of Li+ (0 to 100 μM) in a micro
quartz cuvette, For each addition, at least three UV/
fluorescence spectrums were recorded at 298 K to obtain con-
cordant value. The λex was chosen 340 nm for compound 6,
with 3 nm slit width.
Fig. 2 Fluorescence spectra of 6 (10−4 M) upon addition of metal ions:
Blank, Ba2+, Pb2+, Ca2+, Ni2+, Cd2+, Cr3+, Al3+, Zn2+, Li+, Co2+, Hg2+
Cu2+, Fe3+, Na+ and K+ (5 equiv.) in H2O/CH3CN. [λex = 340 nm &
em = 465 nm]
,
λ
Procedure for Quantum Yield of Calculation
Computational Calculations
The fluorescence quantum yield was determined by using
quinine sulfate in 0.1 M sulfuric acid (Φ = 0.55) as the stan-
dard reference [52] The quantum yield is calculated using the
following equation: [53]
All the Density Functional Theory (DFT) based calculations
are performed using the Gaussian09 (G09) [54] program
package. Geometry of the compound 6 along with its different
analogues (after attachment of Li+) are fully optimized at
B3LYP/6-31G(d,p) level [55, 56]. The minimum energy of
the optimized structures is ensured through frequency calcu-
lations. Absence of any negative frequencies indicates the
global minima for the optimized geometry on potential energy
surface. To provide some details insights to the different elec-
tronic transitions associated with UV-VIS spectrum, Time
Dependent Density Functional Theory (TD-DFT) [57, 58]
2
FsArηs
φs ¼ φr
2
FrAsηr
where Ar and As are the absorbance of the ‘reference standard’
and ‘sample’ respectively at the excitation wavelength, Fr and
Fs are the relative integrated fluorescent intensities (area under
the fluorescence curve, peak area) of the reference and sam-
ples respectively. ηr and ηs are respectively the refractive in-
dices of the solvents in which the reference standard and sam-
ples are prepared.
Fig. 3 Column diagrams of the fluorescence intensity of compound +
Metal ions at 465 nm. Dark cyan bars represent the addition of various
metal ions to the blank solution and navy blue bars represent the subsequent
addition of Li+ (5 equiv.) to the above solutions (compound + Mn+ + Li+);
From left to right (i) Blank; (ii) Ba2+; (iii) Pb2+; (iv) Ca2+; (v) Ni2+; (vi)
Cd2+; (vii) Cr3+; (viii) Al3+; (ix) Zn2+; (x) Li+; (xi) Co2+; (xii) Hg2+; (xiii)
Cu2+; (xiv) Fe3+; (xv) Na+; (xvi) K+
Fig. 1 UV–visible spectrum of 6 (10−4 M) upon addition of 5 equiv. of
Li+ and other metal ions

J Fluoresc
Fig. 6 The plot of I465 vs. the concentration of Li+
Fig. 4 UV–visible titration profile of compound 6 in H2O:CH3CN
(7:3, v/v) solution upon concomitant additions of Li+ ion (0 to 100 μμ)
inset – absorption changes at 375 nm
EDC.HCl/HOBt in DMF at room temperature for 6 h yielded
coumarin-indole dyad linked via methylene bridge (6) in 92 %
1
calculations are also done at the same level of theory and basis
sets, i.e., B3LYP/6-31G(d,p). The solvent effect of water has
been incorporated in entire calculations through Self-
Consistent Reaction Field (SCRF) [59] formalism using
IEF-PCM model [60] as implemented in Gaussian09.
yield (Scheme 1). The structure of 6 was confirmed by H
NMR, 13C NMR, and high resolution mass spectrometry.
UV − vis absorption and steady state fluorescence emission
of 6 was measured in organo-aqueous solution in the absence
and presence of metal ions at room temperature. In order to
choose an appropriate solvent, selectivity test was performed
with a number of solvent systems including H2O/CH3CN,
H2O/THF, H2O/DMSO. We established that compound 6
showed good fluorescence efficiency in acetonitrile than other
solvents studied (H2O/THF, H2O/DMSO). The binding ca-
pacity of the coumarin-Indole Dyad (compound 6) towards
various metal ions (5 equiv.) such as Ba2+, Pb2+, Ca2+, Ni2+,
Cd2+, Cr3+, Al3+, Zn2+, Li+, Co2+, Hg2+, Cu2+, Fe3+, Na+ and
K+ was measured by UV-vis absorption studies. UV-vis spec-
trum of compound 6 exhibited absorption bands at 322 nm
and 280 nm in H2O/CH3CN (7:3, v/v) ratio. When Li+ was
added to the compound 6, a new red shifted band emerged at
Results and Discussion
The newly synthesized fluorescent coumarin-indole dyad was
synthesized as shown in Scheme 1. Commercially available
resorcinol (1) on reaction with ethyl chloroacetoacetate (2) in
presence of sulfuric acid at 0–5 °C afforded 4-(chloromethyl)-
7-hydroxy-2H-chromen-2-one [61] (3), which on reaction
with 25 % ammonium hydroxide at 50 °C for 1 h yielded 7-
hydroxy-4-(aminomethyl)coumarin [62] (4) in 80 % yield.
The coupling of 4 with 1H-indole-2-carboxylic acid (5) using
Fig. 5 Fluorescence spectra of compound 6 with addition of various
concentrations of Li+ in H2O/CH3CN (λex = 340 nm)
Fig. 7 Benesi-Hildebrand plot for compound at various concentrations
of Li+

J Fluoresc
Fig. 8 1H NMR of compound 6
(bottom) in the presence of
0.5–2 equiv. of Li + in DMSO-d6
375 nm (Fig. 1). This could be due to delocalization of elec-
trons on formation of complex between Li+ and compound 6.
Moreover, the red-shift can also be explained due to the intra-
molecular charge transfer (ICT) process and the lowering of
the band gap between HOMO and LUMO on complexation
with Li+. Importantly, no distinguishable spectral change in
the UV-vis spectrum of compound 6 was observed in the
presence of other tested metal ions.
To further explore sensing behavior of 6, fluorescence ex-
periments were carried out by mixing it with different metal
ions (5 equiv.) namely Ba2+, Pb2+, Ca2+, Ni2+, Cd2+, Cr3+,
Al3+, Zn2+, Li+, Co2+, Hg2+, Cu2+ and Fe3+ in H2O/CH3CN
(7:3, v/v) ratio. Compound 6 exhibited fluorescence maxima
at 465 nm (λex = 340 nm) in absence of metal ions with very
low quantum yield (Ф = 0.129). Addition of Li+ (5 equiv.) to
the above solution shows an excellent fluorescence enhance-
ment and an increase in quantum yield to 0.262, while metal
ions Ba2+, Pb2+, Ca2+, Ni2+, Cd2+, Cr3+, Al3+, Zn2+, Co2+,
Hg2+, Cu2+ and Fe3+ (5 equiv.) did not affect the fluorescence
spectra of 6 studied (Fig. 2).
To evaluate the selectivity of compound 6 toward Li+ ions
over other metal ions, competitive fluorescence experiments
of the Li+ (5 equiv.) solutions mixed with other common in-
terfering metal ions such as Ba2+, Pb2+, Ca2+, Ni2+, Cd2+,
Cr3+, Al3+, Zn2+, Co2+, Hg2+, Cu2+, Fe3+, Na+ and K+
(5 equiv.) were carried out. In Fig. 3 the navy blue bars rep-
resent the intensity of the emitted radiation in the presence of
Li+ ions in solution together with individual metal ions such as
Fig. 9 Optimized structure of the
compound after complexation
with Li+ and their corresponding
binding energy values. Hydrogen
atoms are omitted for clarity.
Binding Energy is calculated as
BE = Ecomplex - (Ecompound + ELi)
BE = Ecomplex − (Ecompound + ELi)

J Fluoresc
Fig. 10 Proposed binding site for
compound 6 + Li+
Ba2+, Pb2+, Ca2+, Ni2+, Cd2+, Cr3+, Al3+, Zn2+, Li+, Co2+,
Hg2+, Cu2+, Fe3+, Na+ and K+ that shows high impact on the
emission intensity when compared to the dark cyan bars
which correspond to the emission intensity of a solution of
the same metal ion in the absence of Li+. The results showed
that, the fluorescence enhancement of 6 was not affected by
the competing ions. Therefore compound 6 can be used as
highly selective sensor for detection of Li+ ions in the presence
of other common metal ions.
The absorption titration was next performed with an incre-
mental addition of Li+ (Fig. 4) that caused a progressive in-
crease in the absorption band around 375 nm suggesting that
the compound has high sensitivity toward Li+ metal ion. The
spectral changes with the formation of isosbetic point at
340 nm indicate the formation of single complex species be-
tween sensor and Li+.
The fluorescence titration of compound 6 was performed in
the presence of various concentrations of Li+ in H2O/CH3CN
7:3 (v/v) in Fig. 5. Upon addition of increasing amount
of Li+ leads to a continuous fluorescence enhancement around
465 nm.
By using the above mentioned titration results, the detec-
tion limit for Li+ ion was calculated. The emission intensity of
compound 6 without Li+ was measured by ten times and stan-
dard deviation of blank measurements was determined. The
fluorescence intensity increased linearly with an increase in
the concentration of Li+ (Fig. 6). The limit of detection was
measured [63], [64, 65] to be 37.1 nM for compound 6, based
on 3σ/m, where σ corresponds to the standard deviation of the
blank measurements, and m is the slope in the plot of the
intensity versus the sample concentration (Fig. 6). Therefore
the sensor is responsible for nanomolar determination of lith-
ium in various samples.
For investigating the binding stoichiometry and the deter-
mining the association constant, Benesi-Hildebrand plot [66]
was constructed. The linearity of Benesi-Hildebrand plot
(Fig. 7) between 1/(Fi – F0) against 1/[Li+] indicates the 1:1
stoichiometry between sensor and Li+ ions. The association
constant was found to be Ka = 5.5 × 103 M−1 for compound as
obtained from Benesi-Hildebrand plot. The high value of as-
sociation constant Ka indicates that a strong complex forma-
tion takes place between metal ion (Li+) and ligand (6).
We next aim to understand the most probable site of coor-
dination for the Li+ that might be responsible for the Bturn-on^
fluorescence behaviour of compound 6. Different binding
sites are available in the compound 6 where the metal ion
(Li+) could bind, which includes (i) the carbonyl oxygen of
coumarin moiety, (ii) the carbonyl oxygen of amide linkage,
(iii) amidic N-H, and (iv) indolic N-H. To elucidate the most
prominent binding site in compound 6 for Li+, 1H NMR titra-
tion experiments were carried out in DMSO-d6 (Fig. 8). The
addition of 1 equivalent of lithium salt to the solution of com-
pound 6 in DMSO-d6 led to slight down-field shifts in
the position of indolic NH (0.007 ppm) and amidic NH
(0.008 ppm), thereby indicating the binding of Li+ to
indolic and amidic NH. No further change in the 1H
NMR of compound 6 was observed upon addition of
more than 1 equiv. of Li+, thereby confirming 1:1 stoichiom-
etry, as earlier suggested by Benesi-Hildebrand plot. The re-
sults suggested the binding of compound 6 to Li+ by a rigid
conjugation system through interactions with indolic NH and
amide proton [67].
Binding energy calculations are performed to determine the
most possible binding site of Li+ in the compound. The bind-
ing energy values corresponding to the binding of Li+ at the
four possible binding sites (mentioned above) were computed
Table 1 Computed vertical electronic transitions with oscillator strength (f) and other relevant parameters for the compound and Li-compound
complex. Calculated at TD-B3LYP/ 6-31G(d,p)/scrf = (iefpcm, solvent = water) level
System
Origin
CI Contribution
λ nm
E (eV)
F
Assign
Compound
S0 to S1
S0 to S2
S0 to S5
S0 to S1
HOMO → LUMO(98 %)
HOMO-1 → LUMO(94 %)
HOMO-2 → LUMO(82 %)
HOMO → LUMO(98 %)
330
300
277
378
3.74
4.12
4.00
3.28
0.0017
0.423
(n → π*)
(π → π*)
(π → π*)
(π → π*)
0.4898
0.0117
Li-Complex

J Fluoresc
Fig. 11 Possible HOMO-LUMO
transitions for the compound and
its Li-complex
1
and the values are reported below along with the optimized
geometries (Fig. 9).
on the H NMR titration study, Benesi-Hildebrand plot and
DFT calculation results, the possible structure of compound
6 + Li+ is proposed in Fig. 10.
From the computed BE values, it’s clear that the attachment
of Li+ on the amidic NH is thermodynamically most prefera-
ble (BE = −320.67 kcal/mol). Indeed, the indolic NH (BE =
−220.14 kcal/mol) also appears to be a potential binding site
and might be stabilized by some kind of weak interaction
(bridging) with the neighboring oxygen of C = O group.
However, the computed bond distance values show that the
Li is relatively closer to nitrogen compared to the oxygen
atom. The Li-N bond length is 1.69 A0 whereas the Li-O is
1.98 A0. So, it can be argued that the bonding between Li-O
rather appears to be a weak interaction. It is worth mentioning
here that the higher binding affinity of Li+ for amidic NH is
also evident from the experimental spectroscopic data. Based
TD-DFT calculations are performed to understand the vis-
ible changes in the nature of electronic excitations associated
with the UV-vis spectrum of the compound before and after
the attachment of Li+. The ground state optimized geometries
are considered as the starting orientation for TD-DFT calcula-
tions. The findings of TD-DFT calculation are reported in
Table 1. The Frontier Molecular Orbital (FMO) picture of
the compound along with its Li analogue is provided in
Fig. 11. The two intense band in the UV-vis spectrum of the
compound 6 at 300 nm (f = 0.423) (λexp = 322 nm) and
277 nm (f = 0.4898) (λexp = 280 nm) results from the π →
π* type of electronic transitions (Table 1).
Table 2 A comparative study with previous literature reports
Publication
Material used
Detection Limit
Medium Used
Present manuscript
Coumarin derivative
37.1 nM
-
H2O/ CH3CN (v/v = 7:3)
Tetrahedron Lett. 2002, 43, 4989
J. Photochem. Photobiol. A 2004, 162,289
diaza-9-crown-3 derivative
CH3CN
N-(9-methylanthracene)-25,
27-bis(1-propyloxy)-4-tert-butylcalix
[4]arene-azacrown-3
3.2 μM
Dichloromethane/THF
(v/v = 75:25)
Dyes Pigments 1999, 43 , 21
Tetrahedron 2004, 60 5799
Organic Lett. 2004, 6, 4599
RSC Adv. 2016, 6, 1792
squarylium dye
-
-
CH2Cl2/CH3CN (v:v /1:4)
CH3CN
diaza-9-crown-3 derivatives
1-(9-anthryl)-4-ferrocenyl-2-aza-1,3-butadien
Biginelli-based organic nanoparticles
Porphyrin
H2O/ CH3CN (v/v = 7:3)
H2O
122 nM
<1 ppm
0.6 mM
-
Talanta 1998,46, 703
H2O
Anal. Chem. 2007, 79, 1237
Dyes and Pigments 2009, 82, 336
Polymer based fluoroionophore
Polyamidoamine dendrimer
H2O/ CH3OH (v/v = 99:1)
DMF + NaOH

J Fluoresc
Acknowledgments The authors acknowledge the Department of
Science & Technology (DST), New Delhi, for research funding (SB/
FT/CS-033/2012). SK is thankful to University Grants Commission
(UGC), New Delhi for Senior Research Fellowship.
This is basically an intramolecular charge transfer (ICT)
process (from the orientation of FMOs). With the addition of
Li+ to the compound 6, a red shifted absorption band is ap-
peared at 378 nm (λexp = 375 nm) with a comparatively low
oscillator strength nm (f = 0.0117). Subsequently the
TD-DFT based computations correlate well with the ex-
perimental results. With the addition of Li+, a significant de-
crease in the HOMO-LUMO gap is also observed for the
compound (Fig. 11).
As observed above, the coumaryl-indole dyad sensor (6)
showed an increase in fluorescence intensity when co-
ordinated with Li+ ion. The responsible phenomenon for
enhancement of fluorescence intensity with the metal
ion is due to inhibition of PET as previously reported
[68–70]. In absence of Li+, when the compound 6 is in
turn off mode, the lone pair of electrons residing on
amide nitrogen is exclusively delocalized in the couma-
rin ring which leads to non-radiative decay of the ex-
cited state and could be responsible for flourescence
quenching. However, when compound 6 interacts with
metal ion Li+ it coordinates through amide–N and
indolic-N moieties, which in turn introduces planarity
and rigidity in the molecule that further inhibits the non-
radiative decay of the excited state leading to increase in emis-
sion of compound [71–73].
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The sensing ability of the present compound towards Li+ ion
was also compared with other reported in literature, which is
given in (Table 2). It is evident, from the comparison, that the
present compound is a simple sensor for Li+ ion and its LOD
is in nanomolar. The present method involves highly sensitive,
selective method and avoids the need for sophisticated and
costly instruments.
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