Colorimetric and fluorimetric response of salicylaldehyde dithiosemicarbazone towards fluoride, cyanide and copper ions: Spectroscopic and TD-DFT studies

Article abstract of DOI:10.1016/j.saa.2017.04.007

The sensing mechanism of salicylaldehyde phenyldithiosemicarbazone (SDTSC) chemosensor has been investigated by spectroscopic and TD-DFT methods. The SDTSC shows colourimetric and spectral changes towards fluoride, cyanide and copper ions. The interaction

Full text of DOI:10.1016/j.saa.2017.04.007

Accepted Manuscript
Colorimetric and fluorimetric response of salicylaldehyde
dithiosemicarbazone towards fluoride, cyanide and copper ions:
Spectroscopic and TD-DFT studies
Vengayil K. Harikrishnan, Sabeel M. Basheer, Nithin Joseph,
Anandaram Sreekanth
PII:
DOI:
Reference:
S1386-1425(17)30274-3
doi: 10.1016/j.saa.2017.04.007
SAA 15060
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date:
Revised date:
Accepted date:
29 September 2016
6 March 2017
9 April 2017
Please cite this article as: Vengayil K. Harikrishnan, Sabeel M. Basheer, Nithin Joseph,
Anandaram Sreekanth , Colorimetric and fluorimetric response of salicylaldehyde
dithiosemicarbazone towards fluoride, cyanide and copper ions: Spectroscopic and TD-
DFT studies. The address for the corresponding author was captured as affiliation for all
authors. Please check if appropriate. Saa(2017), doi: 10.1016/j.saa.2017.04.007
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ACCEPTED MANUSCRIPT
Colorimetric and fluorimetric response of salicylaldehyde dithiosemicarbazone
towards fluoride, cyanide and copper ions: Spectroscopic and TD-DFT studies
Vengayil K Harikrishnan, Sabeel M Basheer, Nithin Joseph and Anandaram Sreekanth*
Abstract
The sensing mechanism of salicyladehyde phenyldithiosemicarbazone (SDTSC)
chemosensor has been investigated by spectroscopic and TD-DFT methods. The SDTSC
shows colourimetric and spectral changes towards fluoride, cyanide and copper ions. The
interaction between SDTSC with fluoride, cyanide and copper ions was examined through
their absorption and fluorescence behaviour, and found that SDTSC has more sensing ability
1
towards Cu2+ ion than CN- and F- ions. The H NMR titration with SDTSC and F- gives the
structural changes in the sensing process. The reversibility of SDTSC was also evaluated and
thus it is confirmed as a reusable chemosensor which can be clarified by the “Read-Erase-
Read-Write” logic system. The DFT and TD-DFT calculations give the detailed sensing
mechanism of SDTSC towards fluoride ion. The potential energy surface (PES) analysis
confirms the excited state electron transfer mechanism.
Keywords: Dithiosemicarbazone; Spectroscopy; Chemosensor; Fluorescence; PES; TD-DFT
Department of Chemistry, National Institute of Technology, Tiruchirappalli - 620 015,
India.
*Corresponding Author: E-mail address: sreekanth@nitt.edu; Tel: +91 431 2503642

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1. Introduction
There is a growing interest in the development of appropriate sensors for the selective
recognition of anions and cations, which play a vital role in the biological balance of the
environment [1]. There is a high chance for human and other living beings to get exposed to
the high level of these anions due to their immense use in the environment, which may lead to
adverse or even fatal effects. Major challenges faced in this area of study is the development
of structurally simple and stable receptor which is capable of achieving selective recognition
of the anions and cations under consideration from the vicinity of a group of other anions and
cations [2].
Fluoride ion having the smallest size and highest charge density attained much
attention due to its importance in physiological processes and its presence in many
environmental, food, and clinical samples. Beyond the Maximum Contaminant Level (MCL)
established by the environmental protection agency >1.5mg/L, fluoride ions can cause many
serious diseases such as dental and skeletal fluorosis, nephrotoxic changes and urolithiasis
[3]. The detection of cyanide ion is also very important due to its potent toxicity. Cyanide ion
is employed in many industrial processes such as fiber synthesis, resin synthesis, herbicide
synthesis and gold extraction process [4]. This increased concentration of CN¯ will also
cause lethal effects in living beings. Transition metals are very important for the smooth
functioning of biological activities. Among transition metals, Cu2+ ion is an essential trace
element required for metabolism, respiration and enzymatic activities however in excess Cu2+
ions cause toxicity and degenerative memory related diseases. Due to its enormous use in all
scientific fields’ concentration of Cu2+ can go high which may lead to the destruction of
biologically important microorganisms [5-6].
Most of the recent analytical techniques adopted for monitoring anions and cations
are uneconomical, less sensitive, time consuming and tedious. The development of highly
selective and sensitive chemosensors for the effective sensing of anions and cations will
return high consistent results. The ability of the chemosensor to impart a visible colour
change when in contact with the anions and cations will aid for the easy detection. UV-Vis
and fluorescent analyses will provide apt supporting results with low cost and high
sensitivity. The current work emphasises on the selective chemosensing activity of

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salicylaldehyde dithiosemicarbazone towards F-, CN- and Cu2+ and studying its mechanism
both experimentally and computationally.
2. Experimental section
2.1 Materials and Methods
Phenyl dithiocyanate (Alfa), Salicylaldehyde (Aldrich) and tetra-n-butylammonium
(TBA) salt of fluoride, chloride, bromide, iodide, perchlorate, phosphate, and acetate
(Aldrich) were stored in vacuum desiccators containing self-indicating silica-gel and were
used as received. Analytical grade DMSO and other solvents (Merck) were used without
further purification. 1H, and 13C NMR spectra were recorded on a Bruker AVANCE III,
500 MHz in DMSO-d6 at 298 K with TMS as an internal standard. FT-IR spectra were
measured on a Perkin-Elmer FT-IR spectrometer using KBr pellets. UV–visible and
fluorescence spectra were recorded in 1 cm path length quartz cell on PG Instrument T90 +
spectrophotometer and JASCO FP-8300 spectrofluorophotometer, respectively.
2.2 Synthesis of receptor (SDTSC)
Ethanolic solutions (10 ml) of Phenyl dithiocyanate (0.250 g, 5mmol) and hydrazine
hydrate (0.50 g, 10 mmol) were mixed with constant stirring. The Stirring was continued for
one more hour. The product thus obtained was filtered, dried and recrystallized from ethanol.
The obtained product (0.5 mmol) was mixed with salicylaldehyde (1 mmol) in DMF (15 ml).
The mixture was stirred at 1200C for 5 hours. The cold water was added to induce the
precipitation of the product. The resulting precipitate was collected by filtration and washed
with cold water. The resultant product was dried in an oven at 110 0C.
SDTSC: Light green; Yield: 89 %, M.P.: 180-188o C; Elemental analysis: C: 56.53 (56.88),
H: 4.89 (4.34), N: 17.84 (18.09), S: 13.57 (13.80); IR (cm-1): 1542 (C=N), 1271(C=S),
1
3153(NH), 3383(OH); H NMR (500 MHz, DMSO-d6, δ ppm): 11.778 (1H, s, OH), (11.134
(1H, s, N-NH), 10.07 (d, J = 24.5 Hz, 2H), 9.982 (1H, s, N=CH), 8.50-6.85(12H, m, Ar-H),
13C NMR (125 MHz, DMSO-d6, δ ppm): 162.77(1C,C=S), 139.78 (C=N), 130.82(C=C),
99.49 (C-OH); ESI MASS: 464.56 (Calculated), 464.10 (Experimental) (M+, 100%), 487.10
(base peak-M+Na)

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Scheme 1: Synthesis of salicylaldehyde based phenyldithiosemicarbazone ligand
2.3 UV-visible and fluorescence titrations
The binding ability of the SDTSC for anions and cations was investigated through
UV-visible and fluorescence titration methods. The SDTSC (1×10-6 mol. L-1) as well as salts
-
-
-
of anions (such as F-, Cl-, Br-, I-, CN-, H3PO4 , AcO-, NO3 , HSO4 and OH-) and cations (such
as Mn2+, Co2+, Cu2+, Cr3+, Fe2+, Ni2+ and Zn2+) were prepared in DMSO. Two equivalents of
anions and cations were added to the receptor solution and their corresponding UV-visible
and fluorescence spectra were recorded at 298 K.
2.4 Computational Methods
All computational calculations were executed using Gaussian 09 program. The Time
Dependent DFT (TD-DFT) and hybrid density functional theory (DFT) calculations were
performed to optimize the ground state and excited state structures of SDTSC and its
complex with F- and Cu2+ [7-10]. The ground state geometry of SDTSC and SDTSC–F
complex was optimized using B3LYP/6-31G+(d,p) basis set, which helped to evaluate the
hydrogen transfer transition state calculations. The excited state energy calculations were
performed using Coulomb-attenuating method (CAM) with a long-range corrected version of
B3LYP for more accuracy. The calculations of SDTSC–Cu complex were carried out with
B3LYP/LanL2DZ method [10-13]. The UV-visible transition studies were examined using
TD-DFT method starting with ground state geometries optimized in DMSO solution phase
with PCM model. The potential energy surface (PES) gives information about the distribution
of electron density in atoms and in bonds between atoms.
3. Results and Discussions
The structural formula of SDTSC is as shown in Scheme 1. It was well-characterized
1
by elemental analyses, FT-IR, H NMR, 13C NMR and ESI MASS spectral studies. The
electronic spectra of the compound exhibited two bands around 303 and 345 nm, which were
characteristic of →* and n→* transitions respectively. The FT-IR spectrum of the

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compound express the characteristic stretching frequencies at 1271 and 1542 cm-1 attributed
to ν(C=S) and ν(C=N) groups respectively, which indicates the newly formed azomethine
group. Also SDTSC showed a characteristic band at 3383 cm-1 which were assigned to
1
(N−H) group. The H NMR of the SDTSC displays singlet at 11.134 and 9.932 ppm for
two different N-H protons respectively. The peak at 11.778 ppm related to the hydroxyl
13
proton. The aromatic protons were in the expected range of 8.50-6.85 ppm. The C NMR
spectra of the compound shows the thiocarbonyl (C=S) and imine (C=N) carbon peaks at
162.77 and 139.78 ppm respectively.
3.1 Colorimetric analysis and UV-visible spectral studies
The SDTSC shows two principal absorption peaks at 303 and 345 nm which
corresponds to π→π* and n→π* transitions respectively. The absorption properties of
SDTSC significantly vary due to the interaction with selective anions and transition metals.
When a solution of SDTSC is treated with various anions ( such as F-, Cl-, Br-, I-, CN-,
-
-
H2PO4 , AcO-, and NO3 as tetrabutylammonium salts) and transition metals (such as Mn2+,
Ca2+, Cu2+,Co2+ , Cr3+, Fe2+, Ni2+, and Zn2+), the absorption profile as in Figure 1 was
obtained [15-17] . On addition of F- and CN- ions to the SDTSC, the colour of the solution
changes from colourless to light mustard yellow and deep mustard yellow respectively. The
addition of Cu2+ produces a deep yellow colour, but with other anions and cations there is no
observable colour change even on excess addition (S1). The absorption spectral changes of
SDTSC towards F-, CN- and Cu2+ ions were observed through spectrometric titrations. The
presence of F- and CN- ions result in the formation of new peaks at 394 and 465nm, the
absorbance of the bands gradually increased due to the incremental addition of the ions. The
generation of the new peak is due to the complexation between the host-guest molecules via
mere hydrogen bonding. Simultaneously, the hypochromic shift happened at 394 nm, along
with a red shift and a 49 nm difference. This infers that the SDTSC–F complex has more π-
conjugation than SDTSC alone. The isosbestic point was observed at 372 nm, which implies
the presence of multi-ions in the solution. The presence of Cu2+ in the SDTSC results in the
formation of a new peak at 410 nm, the absorbance of this peak proportionally increased with
increase in the concentration of the copper ion [18-21]. The UV-Vis spectral profile of F-,
CN- and Cu2+ with SDTSC is as shown in Figure 2.
Job's Method also known as the method of continuous variation, is a simple and
efficient method to measure the chemical reaction stoichiometry. From the Job’s plot, it is

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clear that the receptor was bonded with fluoride, cyanide ions in 1:2 ratio and copper ions are
in 1:1 ratio (see S2). The association constant of SDTSC with F-, CN- and Cu2+ ions was
calculated using Benesi-Hilderband equation, which is as below [18], [22],
1
1


1


1

A A A  A K[ion]


0

0
where ‘A0’ is the absorbance of the free receptor , ‘A’ is the absorbance with a specific
fluoride ion concentration , ‘A∞’ is the absorbance with an excess amount of ions (F-, CN- or
Cu2+), ‘K’ is the association constant (M-1). S3 shows a linear relationship between 1/(A -A0)
vs 1/[ion], and association constant (Ka) found to be 2.007 x 104, 4.006 x 104 and 6.792 x 104
M-1 for F-, CN- and Cu2+ respectively. From that, it is clear that the SDTSC has a more
binding affinity towards Cu2+ than F- and CN- ions.
3.2 Fluorescence spectral studies
To study the fluorescence behaviour of SDTSC towards anions and transition metals, the
fluorescent emission titrations were carried out with anions (such as F-, Cl-, I-, Br-, AcO-,
-
-
-
-
NO3 ,ClO4 , HSO4 and H2PO4 ) and transition metals (such as Mn2+, Fe3+, Co2+, Ni2+, Cu2+
and Zn2+). The SDTSC shows an emission peak at 458 nm, when it is excited at 400 nm. As
shown in Figure 3 and Figure 4, SDTSC exhibited a remarkable alteration in the emission
band at 394 and 465 nm in the presence of F- and CN- respectively. Addition of Cu2+ to
SDTSC gives continuous quenching of emission intensity at 458 nm. The emission intensity
decreased due to the gradual increase of copper ion concentration. This experimental change
in emission causes quenching, which is due to the formation of a new Cu-complex. There is
no other fluorescence was observed from the receptor with the other anions rather than F- and
CN-. In the case of cations the quenching is maximum with Cu2+ while comparing with the
other metal ions and it. Also a similar quenching has also occurred with Co2+ and Fe3+. So in
that logic the quenching behaviour is not selective to Cu2+. The detection limit was calculated
based on the fluorescence titration. To determine the S/N ratio, the emission intensity of
receptor without any analyte was measured 10 times. The equation [(LOD)=K x (δ/S)] was
used to calculate the limit of detection of receptor towards F-, Cu2+ and CN- ions, where
(F  F)2

0
 
, (F0, F are fluorescence intensity of receptor alone and addition of ion
N 1
respectively. ‘N’ is the number of equivalence). The constant ‘K’ = 3 in this case and σ =

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(Sb1)/(S); Sb1 is the standard deviation of the blank solution; S is the slope of the calibration
curve. The limit of detection of receptor towards fluoride, cyanide and copper ions were
0.974x10-7, 0.348x10-7 and 0.7394x10-7 M respectively. Hence, the SDTSC shows more
sensing towards fluoride ion than copper and cyanide ions. This value is far below the
guideline (211 μM) of the U.S Environment Protection Agency. The limit of detection graphs
were as shown in S4.
3.3 1H NMR titration studies
1
The mechanism of the interaction between SDTSC and F- was further studied by H
1
NMR spectroscopy in DMSO-d6 The H NMR titration leads to the studies in structural
.
changes during the complexation between SDTSC and fluorine ion and thus showing
observable changes in the spectra. The O–H, N1–H1 and N2–H2 protons of SDTSC appeared
as a sharp singlet at 11.01, 8.8 and 6.7 ppm respectively. With the addition of F- ion to the
SDTSC, the peak at 11.01 ppm completely disappeared. While, the intensity of the signals at
8.8 and 6.7 ppm was decreased, and disappeared with the excess addition of F- ion. The
results show, the first deprotonation occurs from the hydroxyl group of SDTSC. The upfield
shifting is due to the aromatic protons which implies that the negative charges generated from
the deprotonation of the receptor by fluoride ion was delocalized over the SDTSC. Two
stepwise mechanism happens during the formation of hydrogen bond between the hydroxyl
group (O–H) and F- and deprotonation of O–H by excess addition of F-. This suggests the
…
interaction of F1 H…O protons and excess fluoride leads to the deprotonation of the moiety.
The broadness of the peaks at 8.8 and 6.7 ppm indicates the formation of hydrogen bonding
…
…
…
…
via N1 H1 F2 and N2 H2 F2. Hence resulting in the sensing mechanism of F- and SDTSC
which is due to intermolecular hydrogen bond, deprotonation, and π-conjugation effect. The
1H NMR spectra shown in Figure 5.
3.4 Reversibility of SDTSC
The reversible property is an important aspect for a chemosensor and to be broadly
hired in the recognition of specific analyses, so the reversibility in the response of the
SDTSC has been further confirmed during several cycles of titration by the alternate addition
of F- and Ca2+ to the SDTSC solution. The addition of F- results in significant fluorescence
change by displaying an ‘ON’ behaviour through SDTSC–F complex formation. The
addition of Ca2+ to the SDTSC–F mixture results in the quenching of fluorescence intensity,
which refers to ‘OFF’ behaviour. A few efficiency loss of fluorescence intensity cause the

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“OFF-ON-OFF” switching process which repeats for several times [27]. This repeated
behaviour of the scheme by fluorescence change evidently proposes that SDTSC is reversible
and thus reusable. The conceivable scheme as presented in Figure 6. The results attribute to
the “Read-Erase-Write-Read” character with the support of logic operations. Thus a model of
the truth table input and output of the suggested logic circuit has been conceded effectively
and summarized as shown in Figure 7 [27].
3.5 Computational Studies
3.5.1 Geometry optimizations
The comparison of the optimized excited structures with ground state structures give
the idea of the excited state proton transfer process (ESPT) and inter charge transfer process
(ICT). S5 displays the optimized structures of the SDTSC, fluoride ion complex (SDTSC–F),
copper ion complex (SDTSC–Cu) and deprotonated receptor (de-SDTSC). The observed
bond parameters were listed in S7. In the ground state of SDTSC, the dihedral angle between
cyclohexane group and thiosemicarbazone group (C3–N3–C2–S1) is 8.20, which attributes that
SDTSC has no coplanarity in the ground state. But in SDTSC–F complex, the dihedral angle
reduces to 5.67o and the deprotonated receptor (de-SDTSC) has a coplanar structure (dihedral
angle is zero).The planarity gives photoirradiation when the fluoride ion binds with the
receptor. The O–H bond in SDTSC and SDTSC–F are 0.96 and 1.41 Å respectively, which
implies that the deprotonation happened in the ground state of the R1-F complex. The
calculated bond distance of N1–N2, C–N and C2=S1 in SDTSC are 1.35, 1.38 and 1.66 Å
respectively. While in the optimized structure in the ground state and the excited state of
SDTSC–F, the bond lengths slightly increase when compared to SDTSC (see Table 1). The
formation of fluoride ion complex is an intermediate stage and fluoride anion interrelates
through hydrogen bond interaction. In the ground state of SDTSC–F, the dihedral angle
between N1–N2–C2–S1 and cyclohexyl-thiosemicarbazone groups are 6.64 and 5.6720
respectively. The thiocarbonyl group (C=S) bond distance remains same in ground state and
excited states of SDTSC and SDTSC–F. The dihedral angle between thiosemicarbazone
moiety and phenyl moiety creates the planar structure of C3-N3-C2-S1 in the ground state,
which twists to 5.670 for the excited state structure of SDTSC and thus the photoirradiation
happened. The dihedral angle is twisted to 5.6720 in the excited state of SDTSC–F, and their
planarity is lost. In SDTSC–F, the bond distance between N2-H5 and N3-H6 are higher than
SDTSC, which is further increasing in the excited state of SDTSC–F. The bond distance of
O1–H14 and N2-H5 in the excited state of SDTSC–F is not enough to form the bond, hence it

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clear that the deprotonation happened in the excited state. So it is strongly verified that the
hydrogen is detached from N2, which is close to thiosemicarbazone moiety and not from the
nitrogen neighbouring to the cyclohexyl group (N3). The deprotonation causes the exclusion
of the hydrogen at the excited state. Finally, the results concluded that the intermolecular
excited-state proton transfer (ESPT) will happen in an excited state in the presence of fluoride
anion. In the SDTSC–Cu complex, the prominent difference of bond parameters in the
ground state was evaluated, which exposes that the copper binding took space in the ground
state. The Cu-O, Cu-S and Cu-N bond distances are 1.93, 2.30 and 2.07 Å respectively. The
C=S bond distance in SDTSC and SDTSC–Cu were found to be 1.66 and 1.81 Å
respectively, which implies that a single bonded C–S in SDTSC–Cu complex. There is no
observable variation in the bond parameters at ground state and excited state and so the
binding took place at the ground state.
3.5.2 UV-visible spectra and molecular orbital analysis
The absorption behaviour of the receptor in sensing of fluoride ions was investigated
with the help of molecular excitation study. The SDTSC gives a strong absorption peak at
329 nm with 1.0425 oscillating strength, which attributes the π→π* transition from the
highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital
(LUMO).The comparison of absorption spectra for SDTSC and SDTSC–F complex have
been conducted to recognize the sensing mechanism. The molecular orbitals involved in the
main electronic transitions have been shown in Figure 8 and S8. The intense transition for
SDTSC–F is at 305 nm, and it is assigned to the π→π* transition from the highest occupied
molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) and the second
highest occupied molecular orbital (HOMO-1) to LUMO with 4.06 ev energy gap. The
SDTSC–Cu complex shows π→π* transition between HOMO to LUMO at 559 nm. The
major electronic transitions are HOMO-2 to LUMO at wavelength 726 nm, and the larger
oscillating strength is at 559 nm, which is HOMO to LUMO transition.
3.5.3 Potential energy curves (PES)
To further clarify the excited state proton transfer mechanism (ESPT) process in the
fluoride sensing mechanism of SDTSC, the potential energy surface calculations were carried
out for the ground state and excited state of SDTSC–F and SDTSC–Cu complexes. In
SDTSC–F, the bond distance between F–H and O–H varies from 0.90 to 1.8 Å with 0.05 Å
increment. This speculate the energetic pathway for ESPT process that is described from this
calculation. It is clearly interpreted from S6 that the energy of excited state for SDTSC–F

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decreases on elongation of the O–H bond length, and it increases through a stable point about
1.2 Å, and leads to a geometry for the stable excited state. Consequently, the fluoride atom
chooses to capture proton from O–H donor to form an intermolecular hydrogen bond.
Therefore the addition of the fluoride anion enhances the spontaneous ESPT process in the
excited state of the proton transferring from oxygen atom to fluoride atom [17], [18].
4. Conclusion
The SDTSC was synthesized systematically and the fluorescence sensing properties of
dithiosemicarbazide for anions and cations were studied by various optical and spectroscopic
techniques. SDTSC showed visible colour changes and observable fluorescence change
towards fluoride, cyanide and copper ions. The deviations in absorption and fluorescence
1
spectra were observed, H NMR studies established the sensing mechanism. The NMR
titration showed that the hydroxyl group underwent deprotonation. The NH proton next to the
hydroxybenzylidene, was deprotonated by further addition of F-. It was seen that a hydrogen
bond was formed between them. The evaluated binding constants confirms that SDTSC has
more binding affinity towards fluoride, cyanide and copper ion. The sensing process was
investigated with computational calculations using DFT and TDDFT methods. The outcomes
of computational studies led to the important aspects of the interaction between SDTSC and
F-/Cu2+. This also predicted the mechanism of sensing. The fluoride sensing process took
place at the excited state of SDTSC–F complex, via deprotonation followed by the hydrogen
bond formation. The PES calculations confirm the excited state proton transfer of SDTSC.
The reversibility of SDTSC was checked with the alternative addition of Ca2+ and F-, which
was seen to be reversible. Hence making SDTSC a reusable chemosensor. The computational
calculations well replicated the experimental UV-vis and fluorescence spectra results.

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Figure 1: UV Vis spectra of SDTSC with different concentration of (a) anions and (b)
cations.
Figure 2: UV Vis titration spectra of receptor SDTSC with (a) fluoride and (b) cyanide (c)
copper ion.

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Figure 3: Emission spectra of receptor SDTSC with different (a) anions and (b) cations.
Figure 4: Emission titration spectra of receptor SDTSC with (a) fluoride and (b) cyanide ion.

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Figure 5:1H NMR titration data of SDTSC with F- ion.

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Figure 6: Reversible switching cycles of SDTSC with fluorescence intensity (λex=458 nm)
by alternate addition of F− ions and Ca2+.
Figure 7: Feedback loop showing reversible logic operations for memory element with
“Read-Erase-Write-Read” functions.

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Figure 8: The energy gap between HOMO to LUMO orbitals contributed to the transition for
SDTSC and SDTSC-F.
Figure 9: The sensing binding mode and mechanism of SDTSC towards fluoride and copper
ions

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Table 1: Association constant of SDTSC with Fluoride. Cyanide and Copper
Ions
UV-vis spectra
Ka
Fluoride ion
Cyanide ion
Copper ion
2.007 x 104
4.006 x 104
6.792 x 104

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Graphical Abstract:
A schematic representation of molecular switching mode of SDTSC towards copper and
fluoride/cyanide ions with the alternative addition of corresponding ions.

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Highlights
 Synthesis and characterization of dithiosemicarbazone derivative
 Experimental evolution of selective fluoride, cyanide and copper sensing via both
colorimetric and spectroscopic studies
 Fluoride ion sensing mechanism was proposed using experimental and DFT/TDDFT
methods
 The multi-detection of receptor was used to describe the molecular switching
behavior.