Spectroscopic and TD-DFT studies on the turn-off fluorescent chemosensor based on anthraldehyde N(4) cyclohexyl thiosemicarbazone for the selective recognition of fluoride and copper ions

Article abstract of DOI:10.1016/j.poly.2016.01.021

The copper and fluoride ions sensing mechanism of a chemosensor based on anthraldehyde N(4) cyclohexyl thiosemicarbazone (AntCy) was investigated via colorimetric, fluorescence, electrochemical and NMR titration studies. Detailed investigations on their s

Full text of DOI:10.1016/j.poly.2016.01.021

Accepted Manuscript
Spectroscopic and TD- DFT Studies on the Turn-off Fluorescent Chemosensor
based on Anthraldehyde N(4) Cyclohexyl Thiosemicarbazone for the Selective
Recognition of Fluoride and Copper Ions
Sabeel M. Basheer, Anthony C. Willis, Ron J. Pace, Anandaram Sreekanth
PII:
S0277-5387(16)00035-8
DOI:
http://dx.doi.org/10.1016/j.poly.2016.01.021
POLY 11783
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
21 October 2015
20 January 2016
Please cite this article as: S.M. Basheer, A.C. Willis, R.J. Pace, A. Sreekanth, Spectroscopic and TD- DFT Studies
on the Turn-off Fluorescent Chemosensor based on Anthraldehyde N(4) Cyclohexyl Thiosemicarbazone for the
Selective Recognition of Fluoride and Copper Ions, Polyhedron (2016), doi: http://dx.doi.org/10.1016/j.poly.
2016.01.021
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Spectroscopic and TD- DFT Studies on the Turn-off Fluorescent
Chemosensor based on Anthraldehyde N(4) Cyclohexyl Thiosemicarbazone
for the Selective Recognition of Fluoride and Copper Ions
Sabeel M. Basheera, Anthony C. Willisb, Ron J. Paceb, Anandaram Sreekanth *a
Abstract: The copper and fluoride ions sensing mechanism of a chemosensor based on
anthraldehyde N(4) cyclohexyl thiosemicarbazone (AntCy) was investigated via colorimetric,
fluorescence, electrochemical and NMR titration studies. Detailed investigations on their
1
sensing mechanism was done using DFT and TD-DFT studies. H NMR titration shows
deprotonation of NH protons by fluoride ion is a prominent step in naked eye recognition.
The Gibbs free energy of overall sensing reaction has moderate transition barrier with 18.19
Kcal mol-1. Using the vibrational frequency analysis, all the local minima of ground state and
excited state were confirmed. Due to the small size and strong electronegativity of fluoride,
an intramolecular hydrogen bonding interaction with N(3)-H, which is closer to anthracene
moiety was found to be preferentially formed. The excited state proton transfer mechanism
was further confirmed with donor–acceptor interactions using Natural Bond Orbital (NBO)
analysis and Potential Energy Surface (PES) analysis. The ICT mechanism for copper ion
sensing was also confirmed with TD-DFT calculations.
Key Words: Fluoride sensor, Thiosemicarbazone, PET, TD-DFT, Fluorescence, 1H-NMR
a Department of Chemistry, National Institute of Technology, Tiruchirappalli - 620 015, India.
*Corresponding Author: E-mail address: sreekanth@nitt.edu; Tel: +91 431 2503642
bResearch School of Chemistry, The Australian National University, Canberra, ACT 2601,
Australia
CCDC: 1431947 contains the supplementary crystallographic data for AntCy. These 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 e mail: deposit@ccdc.cam.ac.uk.

1. Introduction
Chemosensors with high selectivity and sensitivity through optical response appear to
be particularly attractive due to their simplicity in recognition methods and accuracy [1-4].
Detection and recognition of fluoride is of unique interest owing to its roles in a broad range
of biological, chemical and medicinal processes such as preventing dental caries and
osteoporosis, fluorination of drinking water and chemical and nuclear warfare agents [5,6].
The fluoride toxicity is exposed on a less salubrious level to result in fluorosis in terms of
increasing bone density. Among the cations, the copper ion is important in enzyme catalysed
reactions because of its distinct effects in neurologic system and results pathological disease
prevention protein misfolding and in several neurodegenerative diseases [59-62]. The sensing
of F- and Cu2+ ion give apparent advantages such as operational simplicity, high selectivity-
sensitivity and rapid implementation [7-15].
Fluoride with its high electron density and large electronegativity, prefers to form
stronger hydrogen bonds with receptors [16,17]. N-H---F- interactions are well reported
[18,19]. Chemosensors based on thio-urea derivatives have gained more attention due to the
presence of strong hydrogen donors to evoke potential hydrogen bond formation. According
to Irving-Williams stability series, the presence of sulphur atom increases the affinity for soft
metal ion such as Cu2+. The sensing mechanisms are mainly classified into either electron
transfer (ET) or charge transfer (CT) or energy transfer (ET) or excimer/exciplex interaction
[20-23]. The intramolecular charge transfer (ICT), metal to ligand charge transfer (MLCT)
and twisted intramolecular charge transfer (TICT) are included in the charge transfer (CT)
mechanism, while Photo-Induced electron transfer (PET) is the most widely seen electron
transfer mechanism [24,25]. Fluorescence resonance energy transfer (FRET) and excited state
proton transfer (ESPT) are the other two important mechanisms [26,27]. According to PET,
chemosensor’s electron pair is coordinated to external ions, then the electron transfer will be
prevented and the fluorescence is switch on. Most of the benzenoid compounds [28] show
this type of mechanism. When a receptor as an electron donor, is bound with the anion it
alters the electron density distribution in the receptor, hence blue shift occurs. If the
chemosensor is an electron receptor then the same electron density push-pull effect will result
into red shift [24,25]. However, the theoretical studies on the mechanism for sensing anions
are rare, especially for PET mechanism [29,33].
Previously, we have reported the experimental studies on selective anion sensing using the
thiocarbohydrazone and thiosemicarbazones based Chemosensors [34-37]. Present work deal
with the experimental studies and theoretical mechanism of chemosensing behaviour of
thiosemicarbazone derivative towards selective fluoride and copper ion.
2. Experimental section
2.1 Preparation of AntCy
Ethanolic solutions of cyclohexyl isothiocyanate (0.706 g, 5 mmol) and hydrazine
hydrate (0.250 g, 5 mmol) were mixed with constant stirring. The stirring was continued for 1
hr and then the white product, N(4)-cyclohexyl thiosemicarbazide formed was filtered,
washed and dried (Yield: 91%, 0.788 g). This product (0.346 g, 2 mmol) was dissolved in
methanol (30 mL) and was added to the 9-anthraldehyde (0.43 g, 2 mmol) dissolved in
methanol (5 mL), and the reaction mixture was continuously reflux for 4 hrs after adding a
drop of acetic acid. The reaction mixture was kept aside for slow evaporation at room
temperature. After one week time, the product which has been formed was isolated. Further

the product isolated was recrystallized from acetonitrile and methanol. Yield: 84%, melting
point: 196-198˚C. The scheme is as shown in S1. Colour: Pale Yellow, Yield: 81%, M.P 196-
1
1980 C, IR Data (cm-1): 1530 (C=N), 1216 (C=S), H NMR (400 MHz, DMSO-d6, ppm):
11.55 (1H, s, N-NH), 9.30 (1H, s, N-NH), 8.89 (1H, d, N=CH), 8.31 (9H, m, Ar-H), 2.57
(1H, m, N-CH) 13C NMR (101 MHz, DMSO-d6, ppm): 32.31 (2C, m, C-C), ESI Mass: 362.2
(100%, m+1), 384.2 (Na+m, 50%), HRMS: 384.1508(Calculated), 384.1510 (Experimental)
(Na+m, 100%). The detailed spectra were given in supporting data (S2-S6).
Fig 1 Structure of AntCy
2.2 Computational methods:
All calculations were performed using Gaussian 09 program [38]. The hybrid density
functional theory (DFT) [39,40] and Time Dependent DFT (TD-DFT) calculations [41-44]
were run for excited state as well as electronic spectra. The geometry optimisation of the
chemosensor for the ground state (S0) was obtained at the B3LYP using 6-31G+(d,p) basis
set. The geometry optimisation of receptor-fluoride complex was also carried out with same
method. Handy and co-workers’ long range corrected version of B3LYP was used as
Coulomb-attenuating method (CAM) hybrid function with long-range corrections (CAM-
B3LYP) has been used to calculate the excitation energy (S1) for more accuracy [45-49]. 6-
31G+(d,p) basis set is moderate and suitable for such large organic compounds which is a
proper basis set for ionic compound [50-52]. The transition states of hydrogen transfer
located using Transition State (TS) calculations, with B3LYP/6-31G(d) basis set [53,54]. The
receptor-copper complex was optimized with B3LYP/LanL2DZ method. In order to verify
the nature of the transition state, intrinsic reaction coordinate calculations were carried out.
The local minima of the ground state, Transition state, intermediate and excited state were
confirmed with the vibrational frequency analysis. All electronic structure calculations were
completed with no constrains for symmetry. To investigate the solvent effect of DMSO
(Dielectric constant=46.826), PCM (Polarized Continuum Model) calculation was performed
throughout the steps. The charge distribution, occupancy and energy of bonding orbitals were
calculated using Natural Bond Orbital (NBO) approach at the same theoretical level as for the
ground state and excited state geometry optimised structures [55]. The second order Fock
matrix calculation was carried out to evaluate the donor–acceptor interactions in the NBO
analysis. The mechanism was further confirmed with Potential energy Surface (PES)
calculations.
2.3 X-ray crystallography
The crystal structure data collection of AntCy was done on a CrysAlias Pro., Agilent
Technologies, Version 1.171.37.33d and the program used to solve the structure was SIR92,
and the CRYSTALs was the program software used to refine the crystal structure at
Australian National University, Canberra. The receptor A, C22H23N3S crystallizes in
monoclinic P21/n form with two independent molecules. Crystals of the compound were
examined under a microscope and one was chosen which appeared to be single, of good
quality and was of suitable size. It was mounted on X-ray diffractometer and its diffraction
pattern was examined and found to be suitable for a single-crystal diffraction structure
determination. A full intensity data set was collected on it. All data was collected with the
graphite-monochromatic radiation of Mo Kα (λ=0.71073 Å). All hydrogen atoms were
geometrically fixed and allowed to refine using a riding model. Absorption corrections were
employed using multi ω scan with empirical absorption correction using spherical harmonics,

implemented in SCALE3 ABSPACK scaling algorithm. The crystallographic data along with
details of structure solution refinements are given in Table 1.
Table 1. Crystal data and structure refinement parameters
3. Results & Discussions
The structural formula of AntCy is shown in Figure 1. It was well-characterized by
1
elemental analyses, FT-IR, H and 13C NMR spectra and single crystal XRD studies. IR
spectrum of the compound show the characteristic stretching frequencies at 1530 cm-1
attributed to ν(C=N) indicative of the newly formed azomethine group and bands at 1216 cm-
1 assigned to ν(C=S) indicating that the thione form is dominating in the solid state. 1H NMR
of the AntCy shows singlet at 11.55 and 9.30 ppm for two different N-H protons which are
found to disappear upon D2O exchange.
3.1 Single crystal data of AntCy
The molecular structure of AntCy is shown in Fig. 2, and the important bond
parameters were listed in Table 2. There are two crystallographically independent molecules
in the asymmetric unit with bond lengths and angles agree to each other are within normal
ranges of other thiosemicarbazone derivatives. The hydrogen atoms were all located in a
different map, but those bonded to carbon were repositioned geographically. The hydrogen
atoms were initially refined with soft restrains on the bond lengths and angles to regularize
their geometry (C-H in the range 0.93-0.98 and N-H=0.87 Å) and with Uiso(H) in the range
1.2-1.5 times Ueq of the parent atom, after which the positions were refined with riding
constrains except the Hydrogen on Nitrogen which were allowed to refine freely [58].
Significant hydrogen bonding interactions are listed in table 2.
Fig. 2: Structure of AntCy with labelling of selected atoms. Anisotropic displacement
ellipsoids exhibit 30% probability levels. Hydrogen atoms are drawn as circles with small
radii.
Table 2. Selected bond length, bond angle and dihedral angles of AntCy
3.2 Colorimetric analysis and UV-Visible spectral studies:
The spectroscopic investigations of AntCy (1x10-5 M) was carried out in DMSO as a
solvent, AntCy showed broad absorption bands centred at 260, 312 and 403 nm which are
attributed to π- π* and n- π* transitions. The interaction of AntCy with various anions (F-, Cl-,
-
-
-
I-, Br-, OH-, AcO-, CN-, ClO4 , HSO4 and H2PO4 ) and cations (such as Cr3+, Mn2+, Fe2+,
Fe3+, Co2+, Ni2+, Cu2+ and Zn2+ ) were investigated through colorimetric and UV-Vis spectral
analysis. No change in the spectra was observed for other anions, except fluoride and for
cations except cupric ion. Upon the addition of fluoride, a new band appeared with a
bathochromic shift at 480 nm imparting an immediate orange colour from the colourless
solution. To further quantification of the sensing, titrations were carried out with the addition
of 20 equivalent of fluoride solution to the 1 x 10-5 M receptor. The spectral profile recorded
thus is given in supporting data S7. Excess addition of fluoride results a hyperchromic shift
along with isosbestic points of this red shifted π-π* band at 480 nm. This indicates the
deprotonation of receptor and an increase in the conjugation of the system. The n-π*
transition bands also show a bathochromic shift, which is due to the strong hydrogen-bonding
interactions between the fluoride ion with receptor compound [15,56]. While the addition of

copper ion to the AntCy results the formation of a broad band in the range of 298 nm with
high absorbance. A defined isobistic point was found at 383 nm, which attributes the multi-
complex formation of AntCy-Cu. The UV-vis titration graph of fluoride and copper are as
shown in Fig 4.
Fig. 3: UV Vis spectra of receptor AntCy with fluoride ion
UV Vis spectra of receptor AntCy with copper ion
Fig. 4: Benesi-Hildebrand and Job’s plot of AntCy with fluoride and copper ions
Job’s plot method was used to determine the stoichiometry ratio of the compound to the
fluoride ion and copper ion, which give the receptor bind with fluoride ion in 1:1 ratio, and
2:1 for copper ion. The binding constant was calculated using Benesi-Hildebrand equation
[57] which is as below,



1
A− A
1
1
=
+1

A∞ − A0 K[C]


0
Where, ‘A, A0 and A∞’ is the absorbance with a specific fluoride concentration, free
compound and excess amount of ion respectively. ‘K’ is the association constant, and ‘C’ is
the concentration of fluoride or copper ion added. The plot of 1/[C] Vs 1/(A -A0) shows (Fig.
5) a linear relationship (R = 0.98). The association constant (K) was calculated by the ratio of
intercept/slope and which got as 1.329x106 and 4.58x106 M-1 for fluoride and copper
respectively. From the binding constant value, it is clear that the AntCy bind stronger towards
copper ion than fluoride ion.
3.3 Fluorescence spectral studies
To know the selectivity of the receptor, fluorescence measurements were carried out
-
-
with 10 equivalent of different anions (such as F-, Cl-, I-, Br-, OH-, AcO-, ClO4 , HSO4 and
-
H2PO4 ) and cations (such as Cr3+, Mn2+, Fe2+, Fe3+, Co2+, Ni2+, Cu2+ and Zn2+) with 1x10-5
M concentration of AntCy (S8). AntCy shows high emission intensity at 455 nm, when it
excited at 400 nm. The addition of fluoride anion cause quenching of the emission with a
blue shift. However, the addition of copper ion to AntCy results the enhancement of emission
intensity at 455 nm, whereas no noticeable emission was observed with other cations.
Further, while increasing the copper ion concentration, the emission intensity also increased.
The observed enhancement in emission may be attributed due to the formation of new
geometrically restricted five membered ring. Upon the addition of copper ion, the
conformational restriction occurs due to delocalization of the charge created with in the
molecule.
To confirm the stoichiometric ratio of receptor and fluoride, Job’s plot were drawn in
between [C]/([C]+[R]) Vs I0/I, where [C], [R], I0 and I are the concentration of fluoride or
copper ion, concentration of receptor, intensity of free receptor and intensity with addition of
ions respectively. It reveal the binding ratio between the receptor with fluoride and copper
ion were 1:1 and 2:1 respectively. The fluorescence was quenched as disturbing the
conjugation behaviour of receptor AntCy. The detailed spectra and graph as shown in Fig. 5
and 6.
Fig. 5 : (a) Emission Spectra of AntCy with fluoride anions
(b) Emission Spectra of AntCy with copper cations

Fig. 6 : Job’s Plot for complexation of AntCy with fluoride and copper ions
3.4 Electrochemical analysis
The Fig 7 shows the behaviour of AntCy with fluoride and copper ion via electron
transfer intercalation. The receptor shows less current density at 0.35mV, while adding the
fluoride ion the current density increases with a voltage shift. Upon the addition of copper ion
to the AntCy, a new reduction peak was observed at 0.61 V, where the current density was
increased with increasing the concentration of copper ion. The newly formed peak
corresponds to the Cu2+/Cu+ [61] reduction. The isosbestic points were found in both
oxidative curve and reductive curve at 0.50 and 0.40 V respectively. Based on the increase
in peak current of compounds by the addition of different concentration of fluoride ion, the
binding constants, Kb were calculated according to the following equation,
1
ip2 =
i2 −i2 + i2 −[C]
(
)
p0
p
p0
K [C]
b
Where Kb is the binding constant, ip and ipo are the peak currents with and without ion. A plot
(ip2 −i2p )
0
ꢀꢁꢂ v/s.
gave a straight line, and the binding constants of the receptor with fluoride
[C]
and copper ions were calculated as 0.270 x 106 and 11.63 x 106 M-1 respectively, where
AntCy with copper ion have more association constant than with fluoride ion, corroborated to
the same tendency of UV-Vis spectra calculations. From the binding constant value, the
Gibbs free energy for the sensing processes was calculated using the equation, ∆G = - RT ln
Kb, and was found to be -30.98 and -28.14 KJ mol-1 for fluoride and copper respectively. The
negative value shows the spontaneity of the sensing process.
Fig. 7: (a) CV of receptor AntCy with fluoride ion
(b) CV of receptor AntCy with copper ion
Fig. 8: Binding energy graph of receptors AntCy for (a) fluoride ion and (b) copper ion
3.5 1H NMR titration studies
1
To investigate the molecular interaction between receptors and fluoride anion, H
NMR titration study in DMSO-D6 solvent were done with receptors AntCy in presence and
1
absence of fluoride anion. The H spectra shows a singlet at δ=11.55 and 9.30 ppm for the
N-H protons nearby the anthracene moiety and cyclohexyl group respectively. Upon the
addition, of fluoride intensity of the signals started decreasing eventually with the
disappearance of the peaks with the excess addition. With 3 equivalents of fluoride addition
two new peaks started appearing at δ 7.60 and 6.88. This indicates the interaction of F…H…N
protons. Excess fluoride aids the deprotonation of the moiety. The multiplet at 4.23 ppm
belonging for the N-C-H cyclohexyl proton. This signal also undergoes significant reduction
in the presence of fluoride ion. This peak also found to disappear completely with the excess
addition of fluoride. This may be due to the effect of conjugation in the deprotonated species.
Fig. 9: 1H NMR titration data of AntCy with F-

3.6 Computational studies
3.6.1 Geometry Optimization
The differences in geometries between S0 and S1 states and fluorescence spectra for
Chemosensor (AntCy), fluoride ion complex (AntCy-F) and copper complex (AntCy-Cu)
have been investigated in detail after the geometry optimization. The ground state (S0) and
excited state (S1) structures of the receptors have been optimised, and corresponding
frequencies were calculated. The ground state and excited state SCF energy of AntCy (SA0) is
-8.87x105 and -8.86x105 Kcal/mol respectively. In the ground state, the cyclohexyl group and
anthracene moiety are not coplanar. The dihedral angle between anthracene moiety and
thionyl group is 28.890 and the dihedral angle between boat formed cyclohexane group and
semicarbazone moiety is 40.740. A subsequent vibrational frequency analysis further
confirmed that the structure are at a global minima. The ground state optimised spatial
coordinates and IR frequencies were given in the supporting documents. The ground state
optimised energy of AntCy-F complex is less than that of AntCy, which comes around -
9.46x105 Kcal/mol. In the ground state of AntCy, the angle between anthracene moiety and
semicarbazone groups are at 1200 and the dihedral angle between cyclohexyl group and se
micarbazone moiety is 360.
In the excited state (SA1) of AntCy, the dihedral angle between cyclohexane group and
thiosemicarbazone group is 93.910 and the same for anthracene moiety and thionyl group is
27.470. But, after bonding with fluoride ion, the dihedral angles twisted to 1770 for
cyclohexyl-thiosemicarbazone group and 140 for anthracene-thionyl group. The thiocarbonyl
group (C=S) bond distance remains same in both states. The photoirradiation makes the
nearly planar structure of H1-N-C-N in ground state (0.010 Dihedral angle) twist to 1760 for
the receptor excited structure. For the ground state structure of the complex (AntCy-F), the
same dihedral angle is 0.50 and for the excited state it is twist to 1770 and destroying their
planarity. The bond distance between N1-H1 in receptor’s ground state optimised structure
and excited structure are almost equal (nearly 1.02 Å). The AntCy shows same N2-H2 bond
distance in ground and excited state and even after the formation of fluoride complex. In
excited state, the hydrogen bond is only between H1 which was more than that of ground
state (0.999 Å). From this it is clear that the hydrogen is removed from N25, which is nearby
anthracene moiety and not from the N27 which is nearby cyclohexyl group and the removal
of the hydrogen takes place at the excited state. Whereas in the case of AntCy-Cu complex,
the Cu-S and Cu-N bond distance are 2.41 and 2.06 Å respectively. The C=S bond distance
found to be 1.78 Å, which is 1.60 Å in AntCy., which attributes the bond between ‘C’ and ‘S’
is elongated during complexation. The dominant structural parameters in the optimised
structures before and after sensing is listed in Table 3. There is no notable change in bond
parameters at ground state and excited state. Which attributes the binding was takes place at
the ground state. As the speculation on the decreased distance between Fluoride ion and
upper proton H1 and increased distance between Fluoride ion and H2 in S1 state, the
intermolecular excited-state proton transfer (ESPT) will take place in S1 state in the presence
of fluoride anion. From this it concluded that the binding energy of the fluoride ion to H1
hydrogen is more than of H2.

Fig. 10: (a) Ground state and (b) Excited state structure of AntCy with fluoride
Table 3: Bond characters of AntCy, AntCy-F and AntCy-Cu at ground state and excited state
3.6.2 UV–Vis spectra and molecular orbital analysis:
In order to investigate the absorption behaviour of AntCy in sensing of fluoride,
molecular excitation study was carried out. AntCy shows (as shown in S8) an intense
absorption at 366 nm with 0.2108 oscillating strength as a dominant πꢀπ* type transition,
which is from the highest occupied molecular orbital (HOMO, 96) to the lowest unoccupied
molecular orbital (LUMO, 97). The local HOMO electron is delocalized through anthracene
moiety, carbazonyl and cyclohexyl groups. In the local transition, this delocalized electrons
(HOMO) were transited to only anthracene moiety (LUMO). The second dominant transition
also follows same root, which is at 265 nm wavelength and the transition is from second
highest occupied molecular orbital (HOMO-1) to LUMO with 3.48 eV energy gap. Another
less energy transition, HOMO-2 to LUMO, was in between carbazyl group and anthracene
moiety.
The absorption wave length for strongest πꢀπ* transition is shifted in to 373 nm for
receptor-fluoride complex, where the transition from HOMO(101) to LUMO(102) orbitals,
which is in between carbonyl group and anthracene moiety. The other main transitions are H-
1ꢀL and H-2ꢀL. In the HOMO-1, the electron delocalized on anthracene, cyclohexyl and
carbazone groups. But, in HOMO-2 orbital, the electrons are located on carbazonyl and
cyclohexyl group not in the anthracene moiety. In the case of AntCy-Cu complex, the
excitation wavelength at 409 nm, which was very closer to the experimental value. The
transition with high oscillating strength is at 523 nm, where the transition is between HOMO-
3 to LUMO. The other major transitions are at 571 nm, where the transition is between
HOMO to LUMO with 22 % abundance. Fig 11 shows all the major electronic transitions
before and after sensing respectively.
The mulliken charge analysis was done for the further clarification of ESPT process.
For this, the ground state and excited state mulliken charges of complexes were compared.
By comparing the mulliken atomic charge on N(25) and N(27) at ground state and excited
state, the excited state N(25) had high increase than N(27). This further confirms that the
Hydrogen removal was at N(25). In the ground state of AntCy-F complex, the charges on
N(25), H(31) and Fluoride were -0.1518, 0.1347 and -0.6149 e respectively. The photo-
excitation can induces large influence in the charges for the three atoms, and it seen as -
0.3088, 0.2623 and -0.3638 e in excited state corresponding N(25), H(31) and F respectively.
This result supported the ESPT process such a way that, in the excited state movement of the
proton from nitrogen atom to fluoride anion.
Fig. 11: The major molecular orbitals contributed to the transition for AntCy and AntCy-F
Table 4. Absorption details of receptor AntCy, AntCy-F and AntCy-Cu complexes
3.6.3 Transition state, IRC and Gibbs free energy calculations
In order to capture the dynamics feature of the hydrogen transfer reaction, the profile
of Gibb’s free energy in solution for the sensing mechanism is calculated. The ground state
Gibb’s free energy of the AntCy was found to be -8.86x105 Kcal/mol. The formation of

transition state, the complex between Fluoride and receptor AntCy is endergonic by 239.81
Kcal/mol. In the intermediate and transition state complexes, the fluoride anion was interact
with receptor’s carbazonyl hydrogen, through hydrogen bonding. Intrinsic Reaction
Coordination (IRC) calculation, for the optimised structure of the TS, to verify the correct
transition state connecting the minima of the reactants and products of interest of transition
state was performed. Relaxation of transition state towards the products by IRC calculations
detect an intermediate which was further confirmed with frequency calculation. The Gibbs
free energy profile of the mechanism was as shown in the Fig. 12.
The total change in Gibbs free energy in the reactions are 174.01 Kcal/mol. The less Gibbs
free energy difference in between reactant and product says that the reaction was in
reversible, and best supporting for the explanation of Chemosensing mechanism. The
calculated entropic change (△ s = △ꢃꢄ△ꢅ) in the formation process of the hydrogen bridge
ꢆ
involving fluoride ion is -0.0435 KJ/mol, which indicates that the formation process of the
hydrogen bonded complex is thermodynamically allowed. The binding constant of the
fluoride was calculated from the Gibbs free energy value [ln(ꢇꢈ) = ꢄꢊ△ꢆꢉ] which was found at
1.45x105 M-1 which was close to experimental value.
Fig 12: Gibbs free energy profiles for the Receptor AntCy- F sensing mechanism. R: reactant;
IM: intermediate; TS: transition state; P: product. All Gibbs free energies are in kcal mol-1; all
bond lengths are in Å
3.6.4 Emission spectral studies
The emission spectra were calculated by the TD-DFT/B3LYP method. There are two
paths when S1 state relaxes to S0 state corresponding to the excited process, the spectra as
shown in supplementary data S9. In AntCy, The single electron in 97th orbital (LUMO),
descends to the single occupied molecular orbital (SOMO) of 96th orbital, where there are
large overlap of the electrons over anthracene segment. There the emission wavelength falls
on 468 nm with a less energy gap (2.64 eV) and maximum oscillating strength (0.366).
Another major emission transition is seen at 325 nm, in which the electron emitted from
LUMO+1 to HOMO, and the electrons in these two orbitals present almost overlap over the
anthracene moiety and one side of carbazonyl groups. The formation of the intra-molecular
hydrogen bond with fluoride ion induces the fluorescence spectra by 229 nm. The oscillating
strengths of receptor and complex were 0.366 and 0.2552 respectively. The excitation energy
of the receptor’s excitation and emission energies are -38453.14 eV and -38450.81 eV
respectively. For AntCy-Cu complex, the emission with higher oscillating strength was at 563
nm with 2.19 eV energy, which was from LUMO to HOMO orbital transition. Other major
transition at this wavelength was from LUMO to HOMO-1 orbital. The energy for the
emission process is -1251.86 eV. The fluorescence quenching effect is matching with
experimental values and theoretical predictions. The detailed emission values were tabled
(Table 5).
Table 5: The fluorescence emissions for the receptors AntCy and AntCy-F
3.6.5 Potential energy curves (PES)

To reveal the more features of ESPT process in the S1 state for the fluoride-complex,
the potential-energy surface (PES) of ground state and excited state had been calculated with
only by varying N25–H38 bond length from 0.90 to 1.80 Å in steps of 0.05 Å, which can
provide qualitative energetic pathways for the ESIPT process. The energy of excited state and
ground state were decreased when increasing N-H bond length and it increases through a
stable point at the bond length 1.00 Å and the same was for ground state is 1.15 Å, which are
just the stable optimized geometry of S1 and S0 states. From this data it is clear that the
fluoride anion prefer to form an intermolecular hydrogen bond where the hydrogen
transferred from nitrogen (N2) to fluoride ion. Thus the sensing mechanism was of the
receptor was first react with added fluoride ion and forms hydrogen bond, not in free H+ ion.
Hence it had the red shift in its fluorescence emission spectra and UV absorption spectra and
the fluorescence colour signal directly detect with the naked eye. The potential energy graph
for the AntCy-F complex with varying N(25)-H(38) at both ground state and excited state
were given in the Fig 13.
Fig 13: Potential-energy curves of excited state (S1) and ground state for complex A–F,
which is the function of the N25– H42 bond length and corresponding N(27)-H(43) bond
length
3.6.6 NBO analysis
NBO method gives information about interactions in both filled and virtual orbital
spaces that could enhance the analysis of intra- and inter-molecular interactions. The change
in bond length between N2–H2 and H2-F of optimised geometries in S0 and S1 states and the
emission studies predicts that the proton nearby anthracene moiety has a preference for
fluoride than the N4 proton nearby cyclohexyl group for both reversible binding, which takes
place in S1 state via excited state proton transfer (ESPT) process. In order to support this data,
the calculation of binding energy between two kinds of binding sites of F-H and the residue
for fluoride complex were calculated.. The binding energies △E can be calculated as △E =
EAB – (EA+EB)+BSSE, where BSSE is the Basis Set Superposition Error, EA and EB are the
energies for the fragments A and B localized in the compound AB. For the fluoride complex,
the binding energies between the two segments were separated as H1-F and H2-F and
calculated [32].
Picturesque the priority of proton, which is near to anthracene moiety, in attaching to
Fluoride ion, the natural bond orbital (NBO) analysis for ground state and excited state
fluoride complex compounds have been performed. NBO analysis provides the most accurate
possible ‘natural Lewis structure’ picture, because all orbital details are mathematically
chosen to include the highest possible percentage of the electron density. The receptor
AntCy-F, the N1-H1 and N2-H2 bonds in the S0 state is one single σ bond with the Wiberg
Bond Index (WBI) are 0.61 and 0.5 respectively. And the same for fluoride and hydrogen is
much lower and corresponding bonding character shows there is no chemical bonds between
F and H atoms in S0 state. The natural atomic hybrids where calculated by giving the
percentage of the NBO on each hybrid atoms. The single bond hybridisation in between N25-
H1 and N27-H2 were found. In excited structure, the N25-H32 bond was vanished, it shows
there is no N1-H1 bond in excited state. In ground state sp2 Nitrogen was hybridised to s
Hydrogen and it vanish at excited state. Due to this, the strong donor–acceptor interaction

between fluoride and the AntCy can be treated by the second-perturbation energy E(2) which
has been calculated from electron donor orbital, acceptor orbital and the interacting
stabilization energy at ground state and excited state and using this we can be explain the
weak interaction [52]. The second order perturbation equation as below,
qi F 2
ij
E 2 = ∆E =
( )
ε j − εi
Where E(2) is the second perturbation energy, Fij is the off-diagonal element in the NBO
Fock matrix, qi is the donor orbital occupancy, and εi and εj are orbital energies. There were
two type of interaction σ -σ* and n-σ*, in between N25 – H32 and N27 – H31 towards
Fluoride ion at ground state. But in excited state the N25-H32 bond vanishes and only shows
N27 –H31 with fluoride ion. The second order perturbation energy varies from 59 to 0.1
Kcal/mol and 29 to 1 Kcal/mol for N25-H32 and N27-H31 towards F(50) respectively. All
weak interactions have generated from lone-pair electron (n) and core pair electron (σ) of F,
in which the LP F(50) ꢀ BD*(1) N25–H32 pairs induce larger interaction with fluorine atom
rather than LP F(50) ꢀ BD*(1) N27-H31 pairs. In the case of AntCy-Cu complex, the
nitrogen and sulphur atoms act as donor and copper ion act as acceptor. The highest second
order energy was shown the transition between LP N(24) ꢀ LP* Cu(50), where the energy is
24.35 Kcal/mol, and n ꢀ n* transition, the non-bonding electron of nitrogen to non-bonding
electron of copper ion. The higher second order perturbation energy transition of S(29) to
Cu(50) is n ꢀ σ* transition, where the non-bonding electron of sulphur to core pair electron
of copper atom.
The highest second order perturbation energy at excited state is LP F(50) ꢀ LP* H 32 with
the energy of 356.30 Kcal/mol where the interaction generated from lone pair electron n of
Fluoride to n* of hydrogen. The weak interaction of N25 towards H32 is core pair electron
of Nitrogen to Hydrogen’s lone pair electron, which is less energy than fluorine-hydrogen
interactions. The NBO characters in ground state and excited state were given in the table 6.
The large overlap of the electron density for the interaction confirms the strong donor–
acceptor interaction, indicating the H32 protons prefer to interact with Fluoride ion rather
than N25. The weak interaction in complex a-F is assigned from lone-pair electron n of F or
N2 atoms to n* of H2, well illuminating that the proton at neighbouring anthracene moiety
(H2) tends to transfer to F and forms N…H...F strong hydrogen bond. The tables were listed in
supplementary data S11 and S12.
Table 6: Second perturbation energy of donor- acceptor interaction at ground and excited
states of AntCy-F complex
4. Sensing mechanism
From the experimental excitation spectra the fluorescence quenching of the
chemosensor AntCy, due to the presence of fluoride ion is evident. Where as in the presence
of copper ion, the fluorescence enhancement was takes place. From the NMR titration, the
dehydrogenation takes place at N(25), which was nearby the anthracene moiety. But, the
excess addition of fluoride ion will cause the removal of both N-H protons and for a
hydrogen bonded fluoride complex. The proton transfer takes place at the excited state of
receptor-fluoride complex, which was confirmed with NBO analysis and potential energy

surface study. While the electronic excitation transition from occupied orbital to unoccupied
orbitals, more possible and allowed transition are to delocalised electron in Anthracene
moiety. This infer that the anthracene moiety plays an important role to provide binding site
of fluoride ion. While, the copper binding was happened at ground state with intermolecular
charge transfer (ICT).
Fig 14: The Fluorescence sensing binding mode and mechanism of receptors
Acknowledgment
SMB is thankful for the financial support from the MHRD (India) and Endeavour Research
Fellowship (Australia). Special thanks to Prof. Rob Stranger to give the access to the
supercomputer facilities of the National Computational Infrastructure.
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Fig 1 Structure of AntCy
Fig 2. Fig. 2: Structure of AntCy with labelling of selected atoms. Anisotropic displacement
ellipsoids exhibit 30% probability levels. Hydrogen atoms are drawn as circles with small
radii.

Fig. 3: UV Vis spectra of Receptor AntCy with different concentration of fluoride ion and
copper ion

Fig. 4: Benesi-Hildebrand graph of receptor AntCy with (a) fluoride ion (b) copper ion
(a)
(b)
Fig. 5: Emission Spectra of receptor AntCy with (a) fluoride anions and (b) copper cation

(a)
(b)
Fig. 6: Job’s Plot for complexation of AntCy with (a) fluoride anion and (b) copper cation
(a)

Fig. 7 : CV of Receptor AntCy with (a) fluoride ion and (b) copper ion
(b)
(a)
Fig. 8: Binding energy calculation graph of receptor AntCy with (a) fluoride and (b) copper using CV data

Fig. 9: 1H NMR titration of AntCy with fluoride ion

Fig. 10: Ground state and excited state structure of receptors with fluoride ion

Fig 11: The major molecular orbitals contributed to the transition for AntCy and AntCy–F
Fig 12: Gibbs free energy profiles for the Receptor AntCy- Fluoride sensing mechanism. R:
reactant; IM: intermediate; TS: transition state; P: product. All Gibbs free energies are in kcal
mol-1; all bond lengths are in Å

Fig 13: Potential-energy curves of excited state (S1) and ground state for complex AntCy–F,
which is the function of the N25– H42 bond length and corresponding N(27)-H(43) bond
length

Fig 14: The sensing binding mode and mechanism of receptor towards fluoride and copper
ions

Table 1. Crystal data and structure refinement parameters
Empirical formula
C22H23N3S
Formula weight (M)
Temperature (T), K
Wave length (Mo Kα), A0
Crystal system/Space group
Hall symbol
361.51
150 K
λ=0.71073 Å
Monoclinic P21/n
-P2yn
Unit cell dimensions
a=19.8559, b=8.4480, c=22.7155,
α=90 β=90.1792, γ=90
3810.34(15)
Volume (V), 0A3
Z, Calculated density (ρ), mg m-3
Absorption coefficient (m), mm-1
F000 , F000’
8, 1.260
0.180 mm-1
1536.00, 1537.44
0.40x0.28x0.18 mm
3-280
Crystal size
θ Range for data collection (0)
Limiting indices
→11, l= →29
-31
h=-26 →27, k=-11
Completeness to 2θ
2θmax = 58.8o , 94%
Max. and min. transmission
Goodness-of-fit on F2
R indices (all data)
Tmin= 0.890, Tmax= 0.969
1.02
Rint=0.055, wR(F2)=0.1080, R[F2>2σ(F2)]=0.050
∆ρmax=0.42, ∆ρmin= -0.40
Largest diff. peak and hole (e Å-3)
Table 2. (a) Selected bond length, bond angle and dihedral angles of AntCy
S(1)-C(8)
N(7)-C(8)
C(8)-N(9)
N(9)-N(10)
N(10)-C(11)
C(11)-C(12)
C(6)-N(7)
C(1)-C(6)-N(7)
C(11)-N(10)-N(9)
C(6)-N(7)-C(8)
1.6981(15)
1.333(2)
1.3495(19)
1.3827(18)
1.2854(19)
1.466(2)
1.4633(19)
111.02(12)
114.48(13)
125.46(13)
C(8)-N(9)-N(10)
N(7)-C(8)-N(9)
N(7)-C(8)-S(1)
119.45(13)
115.87(13)
124.79(12)
119.33(12)
-79.01(19)
-8.7(2)
N(9)-C(8)-S(1)
C(1)-C(6)-N(7)-C(8)
C(6)-N(7)-C(8)-S(1)
N(9)-N(10)- C(11)-C(12)
N(10)- C(11)-C(12)-C(13) 25.8(2)
N(10)-N(9)-C(8)-S(1) -177.71(11)
-176.40(14)
(b) Hydrogen bond geometry
D-H…A
D-H (Å)
0.829(18)
0.860(17)
H…A (Å)
D…A (Å)
2.6112(19)
3.3218(14)
2.6375(19)
3.4377(14)
D…H-A (0)
110.6(14)
157.2(15)
110.0(14)
160.7(15)
N7-H71…N10
N9-H91…S2
2.201(19)
2.512(18)
2.2226(18)
2.61(2)
N32-H321…N35 0.846(18)
N34-H341…S1
0.86(2)

Table 3: Bond Characters (at ground and excited state) of AntCy and its complex of fluoride
and copper
N1-
H1
H1-N1-C
N2-H2
H2-N2-C
C=S
N-N-C-
S
N-N-C-
N
H2-N-C-
N
F-H1
F-H2
F-H2-N2
Cu-S
Cu-N
S-Cu-N
A
1.02
35.65
1.01
35.79
1.68
3.30
0.01
0.01
--
--
--
--
--
--
A*
1.02
1.09
1.52
1.02
1.02
118.3
112.9
164.1
35.66
35.66
1.01
1.05
1.02
1.01
1.01
118.14
38.02
37.30
35.78
35.78
1.67
1.70
1.68
1.77
1.76
7.26
0.38
2.36
1.05
0.07
3.55
3.59
0.59
2.82
2.11
2.09
--
1.44
0.99
--
--
1.57
1.98
--
--
153.2
23.82
--
--
--
--
2.41
2.37
--
--
--
2.06
2.06
--
--
--
83.55
83.75
AF
179.7
177.2
179.4
179.7
AF*
ACu
ACu*
--
--
--
Table 4. Absorption details of AntCy and its complex with fluoride ion and copper ion
E
eV
λ
nm
Transition
%
E
eV
λ
nm
Transition
E
eV
λ
nm
Transition
%
%
A : -1413.11583147 A.U
AF: -1513.00338034 A.U
ACu: -1251.86778511 A.U
3.38
3.99
4.04
4.48
4.67
4.86
366.55 HꢀL
310.37 H-3ꢀL
306.58 H-1ꢀL+2
276.48 H-2ꢀL
265.00 H-1ꢀL
254.95 HꢀL+1
HꢀL+2
100
3.32 373.17 H ꢀ L
32.46 3.95 313.60 H-3 ꢀ L
33.97 3.99 310.51 H-4 –ꢀL
100
24.18
1.42 571.0 H ꢀ L
H-1 ꢀ L
22.82
22.77
22.01
27.78 1.50 523.8 H-3 ꢀ L
24.85
25.88 1.60 408.1 H-3 ꢀ L
35.00
H ꢀ L+1
50.62 4.09 302.42 H-1 ꢀ L
32.23
46.78 4.54 273.02 H-2 ꢀ L
11.11
H-1ꢀL+2 25.22
25.08
Table 5: The fluorescence emissions for the AntCy and F-complexes
E
eV
λ
nm
f
Transiti
on
%
E
eV
λ
nm
f
Transiti
on
%
A= -1413.13021575 A.U
AF= -1513.03124466 A.U
2.646
3.671
3.810
468.53
337.71
325.40
0.3660
0.0266
0.0339
100
LꢀH
1.775 698.23 0.255 LꢀH
2.884 429.90 0.168 LꢀH-1
3.315 374.00 0.000 LꢀH-2
85.02
37.47
35.90
35.93
57.56
42.43
25.03
59.30
22.38
24.58
LꢀH-1
LꢀH-4
L+2ꢀH
LꢀH-2
L+1ꢀH
LꢀH-3
LꢀH-2
L ꢀH-1 32.09
3.904
4.411
4.433
317.54
281.07
279.69
0.0857
0.1187
0.0203
3.617 342.69 0.043 L+1ꢀH 31.37
3.697 335.34 0.077 LꢀH-4
3.862 321.01 0.072 LꢀH-2
23.87
47.79

Table 6: Second perturbation energy of donor- acceptor interaction at ground and excited
states of AntCy-F and ground state of AntCy-Cu complex
Interaction
Interaction
Donor
(i)
Acceptor
(j)
E2
kcal/
mol
F(i,j)
a.u.
Donor
(i)
Acceptor
(j)
E2
kcal/
mol
F(i,j)
a.u.
ε(j)- ε (i)
a.u.
ε(j)-ε
(i) a.u.
AF_S0: Ground State
AF_S1: Excited state
LP*(1) H
32
CR (1)
F 50
BD*(1) N
25 -H 32
BD*(1) N
27-H 31
BD*(1) N
25 -H 32
BD*(1) N
27 -H 31
BD*(1) N
27 -H 31
BD*(1) N
25 -H 32
BD*(1) N
27 -H 31
σ -σ*
σ -σ*
n-σ*
n-σ*
n-σ*
n-σ*
n-σ*
2.85
1.43
6.65
3.77
29.4
59.2
12.2
24.74
24.77
1.19
1.23
0.72
0.92
0.95
0.245 CR (1)
F 50
σ -σ*
n-σ*
n-n*
n-n*
n-n*
σ -n*
n-n*
11.80
14.15
14.61
3.41
24.62
0.86
0.84
0.63
0.86
14.40
1.59
0.547
0.112
0.112
0.046
0.513
0.152
0.103
CR (1)
F 50
0.172 LP (1)
F 50
LP*(1) H
32
LP (1)
F 50
0.082 LP (2)
F 50
LP*(1) H
32
LP (1)
F 50
0.062 LP (3)
F 50
LP*(1) H
32
LP (3)
F 50
0.130 LP (4)
F 50
LP*(1) H
32
356.3
0
1.56
LP (4)
F 50
0.208 CR (1)
N 25
LP*(1) H
32
LP (4)
F 50
0.097 LP*(1)
H 32
RY*(1) F
50
1.91
18.67
20.78
A-Cu_Ground state
0.120 LP (1)
N 24
LP (2) S LP*(7)Cu
n-n*
n-n*
14.6
24.3
0.61
0.75
LP*(8)Cu
50
n-n*
0.62
0.61
0.136
0.144
29
50
LP (1)
N 24
LP*(7)Cu
50
0.173 LP (2)
S 29
BD*(1) S
29 -Cu 50
n- σ*