Photophysical properties of Schiff's bases from 3-(1,3-benzothiazol-2-yl)- 2-hydroxy naphthalene-1-carbaldehyde

Article abstract of DOI:10.1016/j.saa.2014.05.029

A series of novel Schiff's bases have been synthesized from 3-(1,3-benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde. The presence of hydroxyl group ortho to the benzothiazolyl group as well as the imine linkage lead to the occurrence of excited state intramolecular proton transfer process. The computational strategy was used to study the ESIPT process of the synthesized Schiff's bases, which revealed surprisingly that the keto form predominantly exists in the ground state contradicting the ESIPT process. Density functional theory and time dependent density functional theory have been used to investigate the structural parameters and photophysical properties in different solvents of one of the Schiff's bases. The experimental results correlate well with the computed results. All Schiff's bases show good thermal stability.

Full text of DOI:10.1016/j.saa.2014.05.029

Accepted Manuscript
Photophysical properties of Schiff’s bases from 3-(1,3-benzothiazol-2-yl)-2-
hydroxy naphthalene-1-carbaldehyde
Manjaree A. Satam, Rahul D. Telore, Nagaiyan Sekar
PII:
S1386-1425(14)00808-7
http://dx.doi.org/10.1016/j.saa.2014.05.029
SAA 12191
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomo-
lecular Spectroscopy
To appear in:
Received Date:
Revised Date:
Accepted Date:
9 February 2014
1 May 2014
12 May 2014
Please cite this article as: M.A. Satam, R.D. Telore, N. Sekar, Photophysical properties of Schiff’s bases from 3-
(1,3-benzothiazol-2-yl)-2-hydroxy naphthalene-1-carbaldehyde, Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.05.029
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Photophysical properties of Schiff’s bases from 3-(1,3-benzothiazol-2-yl)-2-hydroxy
naphthalene-1-carbaldehyde
Manjaree A. Satam, Rahul D. Telore, Nagaiyan Sekar*
Tinctorial Chemistry Group, Department of Dyestuff Technology, Institute of Chemical
Technology, Matunga, Mumbai-400 019, India.
E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel: +91 22 3361 2707; Fax: +91 22 3361 1020.
Abstract:
A series of novel Schiff’s bases have been synthesized from 3-(1,3-benzothiazol-2-yl)-2-
hydroxynaphthalene-1-carbaldehyde. The presence of hydroxyl group ortho to the benzothiazolyl
group as well as the imine linkage lead to the occurrence of excited state intramolecular proton
transfer process. The computational strategy was used to study the ESIPT process of the
synthesized Schiff’s bases, which revealed surprisingly that the keto form predominantly exists
in the ground state contradicting the ESIPT process. Density functional theory and time
dependent density functional theory have been used to investigate the structural parameters and
photophysical properties in different solvents of one of the Schiff’s bases. The experimental
results correlate well with the computed results. All Schiff’s bases show good thermal stability.
Keywords: Schiff’s base, benzothiazole, imine linkage, photophysical property, ESIPT, density
functional theory.
1. Introduction:
The reaction between amine and aldehyde functions resulting in the formation of Schiff’s
bases having conjugated carbon-nitrogen double bonds is well known in synthetic organic
chemistry [1]. The formation of Schiff’s bases has great interest in medicinal chemistry and
1

material sciences [2] as they form very stable complexes with the metal ions [3-7]. Mostly the
Schiff’s bases are used as metal sensors [7-10] as well as optical pH sensor [11].
The metal-ion sensing is carried out using fluorescence spectroscopy because of high
sensitivity, low cost and instant response [12-16]. The ratiometric fluorescence sensors have
been used over the conventional method for fluorescence based sensing applications [17-20].The
ratiometric fluorescence sensors with excited state intra molecular proton transfer (ESIPT) have
a great importance as they provide dual emitting channel [21, 22]. Singh N. et al. have
synthesized substituted 1-((phenylimino)methyl)naphthalene-2-ol as a metal-ion sensor
undergoing ESIPT process for the detection of Mg2+ ion [23]. Udhayakumari D. et al. have
synthesized simple imine linked salicylaldimine based Schiff’s base for the detection of Zn2+ ion.
On binding with the metal-ion, this sensor shows a fluorescence enhancement due to the
inhibition of the ESIPT mechanism [24].
In the last decade study of ESIPT has gained considerable attention in terms of the
experimental as well as the theoretical studies [25, 26]. The molecules exhibiting ESIPT are
often used to develop new functional molecules such as laser dyes [27, 28], fluorescent probes in
biology [29] and light-emitting materials for electroluminescent devices [30], higher energy
radiation detectors, polymer protectors [31-32]. The molecules with ESIPT character often show
large Stokes shift [33-35]. Stabilisation of different tautomers of ESIPT molecules are highly
influence by solvent polarity [36, 37]. Guha et al. investigated the proton transfer reaction
occurring in 7-ethylsalicylidenebenzylamine and have concluded that this molecule exists in
more than one structural form in most of the protic solvents [37]. The fluorescence intensity is
influenced by the microenvironment, like viscosity of the solvent, temperature and pH [38-42].
The photophysical properties of Schiff’s base containing an intramolecular hydrogen bond have
2

also been studied by using steady state and time resolved fluorescence spectroscopy [38, 39].
Recently quantum chemical computational studies have been used in structural optimization, and
in understanding the photophysical properties, and nonlinear optical properties of salen type
Schiff’s base [43].
In continuation of our previous work [44], the current study describes the synthesis and
photophysical properties of novel Schiff’s bases, and the effect of the solvent polarity on the
photophysical properties of Schiff’s bases 6a-6f (Scheme 1). Also the structure optimization of
one of the synthesized Schiff’s base 6a was carried out both in the ground and the excited state
by using density functional theory (DFT) to study ESIPT process but anomalous results were
obtained. DFT study reveals that the keto form predominantly exists in the ground state and not
the enol form. Thus there is no existence of ESIPT process in these synthesized Schiff’s bases.
Experimental results also support this as these molecules observed small Stokes shift unlike the
ESIPT molecules.
2. Experimental section:
2.1. Materials and equipments:
All the reagents and solvents were purchased from Sd Fine Chemicals Pvt. Ltd and used
without purification. All solvents used were of spectroscopic grade. Melting point was recorded
by open capillary on Sunder Industrial Product and is uncorrected. The reactions were monitored
using pre-coated silica gel aluminum backed TLC plates; Kisel gel 60 F254 Merck (Germany).
The UV-Visible absorption measurements were carried out using a Spectronic Genesys 2
spectrophotometer with 1 cm length quartz cells. The excitation wavelength was taken as λmax of
compound. The scan range was 250 to 650 nm. The fluorescence emission measurements were
3

recorded on Cary Eclipse fluorescence spectrophotometer (Varian, Australia) using 1 cm length
quartz cells. The quantum yields of these compounds were calculated using fluorescein (Φf= 0.91
in ethanol) [45] as the standard. For this purpose, the absorption and the emission intensities of
the Schiff’s bases 6a-6f in all the solvents were recorded at constant interval of concentrations
(0.5, 1, 1.5, 2, 2.5 ppm). Also absorption and emission intensities of fluorescein were measured
in ethanol (as the quantum yield of fluorescein is reported in ethanol). All the measurements
were recorded at constant parameters such as source of light, slit width, cuvette. The absorption
intensities were plotted against emission intensities for all solvents. From these graphs, slopes
were calculated for each Schiff’s bases and the standard. Using these slopes and Formula 1, the
relative quantum yield of Schiff’s bases in different solvents were calculated. The refractive
indices of the solvents have been taken from literature [46].
Formula 1 Relative quantum yield
ꢅꢁ
ꢅꢂꢃ
ꢈꢉ
ꢆ ꢇ ꢁ ꢊ
ꢈꢉꢂꢃ
ꢀꢁ = ꢀꢂꢃ ꢄ
Where,ꢀꢁ = Quantum yield of analyte
ꢀꢂꢃ = Quantum yield of standard
ꢅꢁ = slope for analyte
ꢅꢂꢃ = slope for standard
ꢈꢉꢂꢃ= Refractive index of solvent for standard
ꢈꢉꢁ= Refractive index of solvent for analyte
The FT-IR analysis was performed on a FTIR-8400S SHIMADZU spectrophotometer.
The thermo gravimetric analyses were carried out on a SDT Q600 TA Instruments. Method for
thermo gravimetric analysis was done with ramp 10°C per minute from ambient temperature to
4

600°C under nitrogen atmosphere. 1H NMR spectra were recorded on VXR 400-MHz instrument
using TMS as an internal standard and DMSO as the solvent. Purification of all the compounds
was generally achieved by recrystallization. The purity of compounds was ascertained by thin
layer chromatography. The analyses data of all Schiff’s bases are given in the supporting
information.
2.2. Computational Methods:
The ground state (So) geometry of the conformers and tautomers of the Schiff’s base 6a
in their C1 symmetry were optimized using the tight criteria in the gas phase as well as in the
different solvents using Density Functional Theory (DFT) [47]. The vibrational frequencies at
the optimized structures were computed using the B3LYP/6-31G(d) method to verify that the
optimized structures correspond to local minima on the energy surface [48-50]. The vertical
excitation energies at the ground-state equilibrium geometries were calculated with Time
Dependent Density Functional Theory (TD-DFT) [51-53]. The low-lying first singlet excited
state (S1) of each conformer was relaxed using the TD-DFT to obtain its minimum energy
geometry. The difference between the energies of the optimized geometries at the first singlet
excited state and the ground state was used in computing the emissions [54, 55]. The frequency
computations were also carried out on the low-lying first excited state of the conformers. All the
computations were carried out in vacuum and also in the solvents of different polarities using the
Polarizable Continuum Model (PCM) [56, 57]. All electronic structure computations were
carried out using the Gaussian 09 program [58].
2.3. Synthesis:
2.3.1. Synthesis of 3-(1,3-benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde 4:
5

Carbaldehyde 4 was synthesized from 3-(1,3-benzothiazol-2-yl)naphthalen-2-ol [59] as
previously reported in ref [44].
2.3.2. General procedure for preparation of Schiff’s bases 6a-6f from 3-(1,3-benzothiazol-2-yl)-
2-hydroxynaphthalene-1-carbaldehyde 4:
3-(1,3-Benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde
4
(0.0033 mol),
substituted aniline 5a-5f (0.0033 mol)and ammonium bromide (0.0033 mol) were stirred in
ethanol (40 mL) for 4 hours. The solid obtained was filtered, washed thoroughly with water, then
with methanol and dried to give the corresponding Schiff’s bases 6a-6f.
2.3.2.1. 1-{(E)-[(2-Aminophenyl)imino]methyl}-3-(1,3-benzothiazol-2-yl)naphthalen-2-ol 6a
Yield: 62%; M.p. 212-216oC.
FT-IR (KBr, cm-1): 1608 (C=O, keto); 1545 (C=N, benzothiazole); 1493 (aromatic ring); 1432
(C=N, imine); 723 (C-S).
1H-NMR ((CD3)2SO, δppm): 15.8 (s, 1H, OH); 9.5 (s, 1H, imine); 9.12 (s, 1H, Ar-H); 8.4 (d,
1H, Ar-H); 8.1 (d, 1H, Ar-H); 8.0 (d, 1H, Ar-H); 7.13 (d, 1H, Ar-H); 7.1 (t, 1H, Ar-H); 7.34 (t,
1H, Ar-H); 7.73 (d, 1H, Ar-H); 7.53 (t, 1H, Ar-H); 7.42 (t, 1H, Ar-H); 7.58 (t, 1H, Ar-H); 7.72
(t, 1H, Ar-H); 6.94 (d, 1H, Ar-H); 5.3 (s, 2H, NH2).
Mass: m/z =396 (M+1).
2.3.2.2. 3-(1,3-Benzothiazol-2-yl)-1-{(E)-[(2-hydroxyphenyl)imino]methyl} naphthalen-2-ol 6b:
Yield: 65%; M.p. 256-260oC.
FT-IR (KBr, cm-1): 3059 (OH); 1610 (C=O, keto); 1541 (C=N, benzothiazole); 1490 (aromatic
ring); 1460 (C=N, imine); 746 (C-S).
6

1H-NMR ((CD3)2SO, δppm): 15.4 (s, 1H, OH); 10.61 (s, 1H, OH); 9.6 (d, 1H, Ar-H); 9.1 (s,
1H, imine); 8.15 (d, 1H, Ar-H); 8.1 (d, 1H, Ar-H); 8.08 (d, 2H, Ar-H); 8.01 (d, 1H, Ar-H); 7.9
(d, 1H, Ar-H); 7.58 (t, 1H, Ar-H); 7.53 (d, 1H, Ar-H); 7.3 (t, 1H, Ar-H); 7.4 (d, 1H, Ar-H); 7.06
(t, 1H, Ar-H); 6.9 (t, 1H, Ar-H).
Mass: m/z = 397 (M+1).
2.3.2.3. 1-{(E)-[(2-Amino-5-nitrophenyl)imino]methyl}-3-(1,3-benzothiazol-2-yl) naphthalen-2-
ol 6c:
Yield: 59%; M.p. 246-248oC.
FT-IR (KBr, cm-1): 3324 (OH); 1610 (C=O, keto); 1545 (C=N, benzothiazole); 1501 (aromatic
ring); 1340 (NO2); 722 (C-S).
1H-NMR ((CD3)2SO, δppm): 15.9 (s, 1H, OH); 9.7 (s, 2H, NH2); 9.1 (s, 1H, Ar-H); 8.4 (s, 1H,
Ar-H); 8.1 (d, 1H, Ar-H); 8.04 (m, 4H, Ar-H); 7.5 (m, 4H, Ar-H); 6.9 (d, 1H, Ar-H).
Mass: m/z = 441 (M+1).
2.3.2.4. 3-(1,3-Benzothiazol-2-yl)-1-{(E)-[(2-hydroxy-5-methylphenyl)imino]methyl} naphthalen-
2-ol 6d:
Yield: 62%; M.p. 262-264oC.
FT-IR (KBr, cm-1): 3057 (-OH); 1610 (C=O, keto); 1541 (C=N, benzothiazole); 1488 (aromatic
ring); 727 (C-S).
1H-NMR ((CD3)2SO, δppm): 15.4 (s, 1H, OH); 10.3 (s, 1H, OH); 9.5 (d, 1H, imine); 9.1 (s, 1H,
Ar-H); 8.4 (d, 1H, Ar-H); 8.1 (s, 1H, Ar-H); 8.0 (d, 1H, Ar-H); 7.98 (t, 1H, Ar-H); 7.94 (s, 1H,
7

Ar-H); 7.59 (t, 1H, Ar-H); 7.53 (t, 1H, Ar-H); 7.42 (t, 1H, Ar-H); 7.3 (t, 1H, Ar-H); 6.9 (q, 2H,
Ar-H); 2.3 (s, 3H, Ar-CH3).
Mass: m/z = 411 (M+1).
2.3.2.5. 3-(1,3-Benzothiazol-2-yl)-1-{(E)-[(2-hydroxy-5-nitrophenyl)imino]methyl} naphthalen-
2-ol 6e:
Yield: 52%; M.p. 282-284oC.
FT-IR (KBr, cm-1): 3010 (OH); 1618 (C=O keto); 1545 (C=N, benzothiazole); 1496 (aromatic
ring); 1334 (NO2); 728 (C-S).
1H-NMR ((CD3)2SO, δppm): 15.4 (s, 1H, OH); 12.5 (s, 1H, OH); 9.7 (d, 1H, Ar-H); 9.1 (s, 1H,
Ar-H); 9.0 (s, 1H, imine); 8.58 (s, 1H, Ar-H); 8.1 (d, 1H, Ar-H); 8.0 (q, 2H, Ar-H); 7.9 (d, 1H,
Ar-H); 7.6 (d, 1H, Ar-H); 7.5 (t, 1H, Ar-H); 7.43 (t, 1H, Ar-H); 7.36 (t, 1H, Ar-H); 7.19 (d, 1H,
Ar-H).
Mass: m/z = 442 (M+1).
2.3.2.6. 3-(1,3-Benzothiazol-2-yl)-1-{(E)-[(2-hydroxy-4-nitrophenyl)imino]methyl} naphthalen-
2-ol 6f:
Yield: 55%; M.p. 226-230oC.
FT-IR (KBr, cm-1): 3024 (OH); 1618 (C=O, keto); 1541 (C=N, benzothiazole); 1490 (aromatic
ring); 1369 (NO2); 739 (C-S).
1H-NMR ((CD3)2SO, δppm): 15.1 (s, 1H, OH); 11.7 (s, 1H, OH); 9.5 (s, 1H, imine); 9.1 (s, 1H,
Ar-H); 8.46-8.32 (d, 2H, Ar-H); 8.2-7.8 (m, 4H, Ar-H); 7.9 (s, 1H, Ar-H); 7.7- 7.2 (m, 4H, Ar-
H).
Mass: m/z = 442 (M+1).
8

3. Results and discussions:
3-(1,3-Benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde 4 was synthesized by
the condensation of 2-aminothiophenol with 3-hydroxynaphthalene-2-carboxylic acid followed
by Riemer-Teimann reaction. This synthesized carbaldehyde 4 was further reacted with the
substituted amines in ethanol in the presence of ammonium bromide to give the corresponding
Schiff’s bases 6a-6f (Figure 1). Literature survey shows that ammonium bromide was used for
cyclization to form bis-azoles [60]. But in this case only Schiff’s bases were formed (Scheme 1).
All these Schiff’s bases were purified by recrystallization using methanol and confirmed by FT-
IR, Mass and 1H-NMR analyses.
< Please insert Scheme 1: Synthetic scheme of Schiff’s bases 6a-6f from 3-(1,3-benzothiazol-2-
yl)-2-hydroxynaphthalene-1-carbaldehyde 4>
< Please insert Figure 1: Structures of the synthesized Schiff’s bases 6a-6f>
These Schiff’s bases are containing -OH group adjacent to the benzothiazole ring as well
as =N of imine linkage which are supposed to be responsible for ESIPT process. Some ESIPT
based Schiff’s bases containing salicylaldimine and naphthaldimine (Figure 2) have been used
for metal-ion sensing, ratiometric sensing as they produce dual channel for the detection [23, 24].
< Please insert Figure 2: ESIPT process observed in known molecules>
The aim of this study is to produce a series of Schiff’s bases 6a-6f undergoing ESIPT
process. Contrary to the expectation reversed ESIPT was not observed owing to the fact that the
keto tautomers are more stable in the ground state as against the fact that the enol tautomer is
more stable in the ground state in the known ESIPT molecules [38, 39]. On UV irradiation, these
molecules showed single emission with small Stokes shift.
9

3.1. Effect of solvent polarity on photophysical properties:
To evaluate the effect of solvent polarity on the photophysical properties of the
synthesized Schiff’s bases 6a-6f, the absorption and the emission properties of all the compounds
were measured in different solvents of differing polarities. These solvents are varying in
polarities, refractive indices, dielectric constants and hydrogen bonding capability which
provides different environment. Solvents of varying polarities generally effects on the absorption
and the emission behavior of the molecules. Analyses were carried out at room temperature with
concentration of the solution 10 ppm. The absorption-excitation-emission data of the Schiff’s
base 6a are given in Figure 3 and Table 1 (Please refer Figure S1-S5, Tables S1-S5 for
photophysical data of remaining Schiff’s bases 6b-6f).
The Schiff’s bases 6b-6f showed dual absorption maxima with almost the same intensity
except 6a, which has a single prominent absorption. The dye 6a showed absorption maxima in
the range of 467 to 482 nm in all solvents. It observed a red shift in DMF as compared to the
other solvents. The dyes 6b-6e showed shorter absorption maxima in the range of 467-479 nm
and longer absorption maxima in the range of 494-504 nm. The presences of electron
withdrawing nitro group as well as the weakly electron realizing methyl group do not bring about
any change in the absorption spectra. The absorbance values of the Schiff’s bases 6a-6d were
almost similar in all the solvents. In the case of dyes 6e and 6f, the absorbance values changed
and it was observed to be lower in methanol and acetonitrile.
The fluorescence emission wavelength and quantum yields of the Schiff’s bases 6a-6f in
different solvents are presented in the Table 1 (Tables S1-S5). The emission wavelength of the
Schiff’s bases experienced a red shift in highly polar aprotic solvents DMF and DMSO. The red
shift in DMF may be attributed to the stabilization of the excited state in DMF than in the other
10

solvents. Also it was observed that the quantum yield of all the Schiff’s bases varied in different
solvents, but independent of the solvent polarity. The Schiff’s base 6a showed the highest
quantum yield in ethyl acetate and the lowest value in ethanol. The dyes 6b, 6c and 6f showed a
higher quantum yield in DMF while the dyes 6d and 6e showed higher quantum yield in
chloroform and tetrahydrofuran respectively as compared to the other solvents.
< Please insert Figure 3: Effect of solvent polarity on photophysical properties of Schiff’s base
6a>
< Please insert Table 1: Absorption- excitation- emission data of Schiff’s base 6a>
3.2. Thermal Stability Study:
To study the thermal stability of Schiff’s bases 6a-6f, thermo gravimetric analyses were
carried out in the temperature range of 50-600oC under nitrogen atmosphere. The TGA results
concluded that the thermal stability of the synthesized Schiff’s bases is range in the range of 235-
285oC (Figure 4). Above this temperature the Schiff’s bases show the major loss in weight.
Furthermore the order of stability was found to be 6d>6a>6e>6b>6c>6f. Though there is no
definite correlation between the structure and thermal stability is found to be slightly more in the
case of Schiff’s bases obtained from 2,5-disubstituted anilines. The comparisons of the Td
(decomposition temperature) of Schiff’s bases 6a-6f are given in Table 2.
< Please insert Figure 4: Thermo gravimetric analysis of Schiff’s bases 6a-6f>
< Please insert Table 2: Thermo gravimetric analysis of Schiff’s bases 6a-6f>
3.3. Density functional theory (DFT) Computations
The optimization of the structural parameters of one of the Schiff’s bases in the ground
and the excited states were carried out using density functional theory (DFT). The hybrid
functional B3LYP was used along with the basis set 6-31G(d). It was assumed that ESIPT
11

process takes place in synthesized Schiff’s bases, as -OH group present adjacent to the =N of
imine linkage as well as benzothiazole ring. To focus on the ESIPT process and photophysical
properties of Schiff’s bases, DFT and TDDFT calculations of one of the Schiff’s bases 6a at the
ground and the excited states were carried out using Gaussian software package [58].
3.3.1. Different possible conformers of Schiff’s base 6a:
< Please insert Figure 5: Different possible conformers of Schiff’s base 6a in gas phase at
ground state>
The Schiff’s base 6a contains the naphthalene core with the benzothiazole ring at 3-
position of the naphthalene ring along with the substituted phenylimine linkage at 1-position and
-OH group at 2-position. In this type of molecules 16 conformers are possible arising out of
single bond rotation. The different conformers and tautomers of the Schiff’s base 6a are
illustrated in Figure 5. The rotation of the free benzothiazole ring as well as the phenyl ring
present on the imine linkage can form different conformers. Also depending upon the direction
of -OH group, the different conformers can also exist.
The energy calculations of these conformers and their tautomers in vacuum ground state
were carried out (Table 3). The computational results show that 6a-Enol 4 has the least energy
as compared to the other conformers. Enol 4 is the predominant species existing among the enol
forms in the ground state.
< Please insert Table 3: B3LYP/6-31G(d) optimized energy (Kcalmol-1) of enol conformers of
Schiff’s base 6a in gas phase at ground state>
Optimization of Enol 4 form and its tautomer Keto 4 in vacuum phase as well as in the
solvents of different polarities in the ground and the excited state were carried out using the
12

method B3LYP/6-31G(d). The optimized structures in highly polar aprotic (DMF) and non polar
(chloroform) solvents in the ground state are shown in Figures 6 and S6.
< Please insert Figure 6: B3LYP/6-31G(d) optimized structure of 6a-Enol and 6a-Keto at
ground state in DMF>
To impose planarity in the structure, dihedral angle between the naphthalene and the
benzothiazole ring as well as that between the phenyl ring on the imine linkage were made to be
zero. Due to this, all conformers are in plane initially but changed a little on optimization cycles.
The values of the bond lengths and Mulliken charge distributions of 6a-Enol and its tautomer
6a-Keto are given in the supporting information (Tables S6-S9). Though, it was observed that -
OH of the enol is little elongated as compared to the standard O-H bond length in the ground
state and is indicative of a strong hydrogen bonding. The hydrogen bonding is also present with a
fair bond length of 1.75 Å between N-H in keto structure. The energy computations show that
6a-Keto form has a lower energy as compared to 6a-Enol form in vacuum and solvent phases
(Table 4). Thus 6a-Keto form exists predominantly in the ground state.
< Please insert Table 4: B3LYP/6-31G(d) optimized energy calculations of 6a-Enol and 6a-
Keto in gas phase and solvent phase at ground state>
3.3.2. Vibrational frequencies:
Also the vibrational frequencies of 6a-Keto in vacuum at the ground state are matched
with that obtained from FT-IR analysis as shown in Table 5. There is no broad peak observed at
around 3500-3600 cm-1 suggests the absence of -OH group and confirms the existence of the keto
form.
< Please insert Table 5: Observed and calculated vibrational frequencies of 6a-Keto>
13

3.3.3. Solvatochromism study:
The structure optimizations studies and energy computations show that the Schiff’s base
6a mainly exists in the keto form. The absorption maxima observed experimentally is much
closer to the value obtained from the vertical excitation of the keto tautomer as compared to that
of the enol one. The experimental values of the absorption and the vertical excitation of both the
enol and the keto forms obtained from TDDFT calculation in different solvents and % deviation
values are tabulated in Table 6. The % deviation values proved that the experimental absorption
wavelength is closer to the computed vertical excitation of 6a-Keto. The % deviation between
the experimental and the computed absorption maxima ranges from 0.8 to 3.7. This observation
is in accordance with previous implication that the keto tautomer is predominant in the ground
state. In all solvents, the prominent absorption can be assigned to HOMO(H)→LUMO(L)
transition (Table 6). The FMO 103 is HOMO (H) and FMO 104 is LUMO (L) in all cases.
< Please insert Table 6: Experimental UV-visible excitation and computed vertical excitation of
6a in different solvents from TD-B3LYP/6-31G(d)>
The electronic transitions in the perspective of the molecular orbital picture, the
molecular HOMOs and molecular LUMOs were generated by using Gauss View 5.0 software
[61]. The HOMO and LUMO graphs of 6a in the keto form at the ground and the excited states
are given in Table 7. It has been observed that in the ground state the electron density is
localized on the benzothiazole ring as it acts as an acceptor. In the ground and the excited
LUMO, the electron density distribution is equal on the entire molecule. Furthermore it transfers
to HOMO of the naphthalene core and the imine linkage in the excited state.
< Please insert Table 7: Frontier molecular orbital of 6a-keto in CHCl3 at ground and excited
states.>
14

3.3.4. Fluorescence emission study:
The experimentally obtained fluorescence emission wavelength and the computed
emission wavelength using TD-B3LYP/6-31G(d) and % deviation values are tabulated in Table
8. It was observed that the experimental emission is closely related to the computed emission
obtained from 6a-Keto*. The emission values computed from the enol form is observed to be
larger than the experimental value. The % deviation of 6a-Keto* from the experimentally
observed emission wavelength is lower than 6a-Enol*. The % deviation between the
experimental and the theoretical emission wavelengths ranges from 4.2 to 8.9. This contradictory
result confirms that the emission of 6a is due to the keto form and not the enol conformer.
All these computational results confirmed that ESIPT process is not observed in
synthesized Schiff’s base 6a.
< Please insert Table 8: Experimental emission and computed emission of 6a in different
solvents from TD-B3LYP/6-31G(d)>
4. Conclusion:
In this paper we report the syntheses of a series of Schiff’s bases from 3-(1,3-
benzothiazol-2-yl)-2-hydroxynaphthalene-1-carbaldehyde by a simple method. Although a -OH
group is present ortho to the benzothiazole ring as well as the imine linkage, these dyes do not
exhibit ESIPT phenomenon. This was confirmed by DFT computation study of one of the
Schiff’s bases 6a. The optimized geometries obtained from B3LYP/6-31G(d) and TD-B3LYP/6-
31G(d) were examined by comparing the ground and the excited state geometries and the charge
distribution. This study reveals that the keto form is more stable than the enol form in the ground
state as against the fact of ESIPT process. The dye 6a showed a prominent absorption due to
HOMO → LUMO transition. The computed absorption and emission wavelengths of the keto
15

form are in good agreement with the experimental results. The solvatochromism study of all the
Schiff’s bases proves that the absorbance and fluorescence quantum yield are influenced by the
solvent polarity. The TGA analyses reveal that all Schiff’s bases are thermally stable.
Acknowledgements:
Manjaree Satam and Rahul Telore are thankful to UGC-CAS for providing research fellowship
under Special Assistance Programme (SAP).
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20

Scheme 1: Synthetic scheme of Schiff’s bases 6a-6f from 3-(1,3-benzothiazol-2-yl)-2-
hydroxynaphthalene-1-carbaldehyde 4.
Step 1:
H2N
HS
OH
OH
Toluene, PCl3
Reflux, 4 Hrs
+
N
COOH
S
3
1
2
Step 2:
CHO
NaOH, CHCl3
OH
OH
Reimer-Tiemann Reaction
70 oC
N
N
S
S
3
4
Step 3:
R2
R1
N
CHO
R2
OH
OH
NH4Br, Ethanol
RT, 4 hrs
+
N
N
R1
NH2
[5]
S
S
[4]
[6]
R1 = -NH2, -OH
R2 = -H, -NO2, -CH3
21

Figure 1: Structures of the synthesized Schiff’s bases 6a-6f.
Figure 2: ESIPT process observed in known molecules.
22

Figure 3: Effect of solvent polarity on photophysical properties of Schiff’s base 6a.
0.5
0.4
0.3
0.2
0.1
0.0
a
b
c
d
e
f
g
h
i
a
b
c
d
e
f
g
h
i
30
25
20
15
10
5
6a
6a
0
500
525
550
575
600
625
650
350
400
450
Wavelength (nm)
500
550
Wavelength (nm)
Where, a = DCM, b = THF, c = Chloroform, d = Ethyl acetate, e = Acetone, f = Methanol, g = Ethanol, h=Acetonitrile, i =
DMF
Figure 4: Thermo gravimetric analysis of Schiff’s bases 6a-6f.
23

6a
6b
6c
6d
6e
6f
120
100
80
60
40
20
0
0
100
200
300
400
500
600
Temperature (oC)
Figure 5: Different possible conformers of Schiff’s base 6a in gas phase at ground state.
24

Figure 6: B3LYP/6-31G(d) optimized structure of 6a-Enol and 6a-Keto at ground state in DMF.
25

26

Table 1: Absorption- excitation- emission data of Schiff’s base 6a.
ꢏꢌꢐ
ꢌꢍꢎ
ꢏꢌꢐ
ꢏꢌꢐ
Solvent
∆λ
∆λ
ꢋ
ꢋ
ꢋ
ꢀ
ε
ꢑꢐꢒ
ꢑꢏꢎ
(nm) (cm-1)
(dm3mol-
1cm-1)
15168
15563
13548
14733
15484
15721
13627
15563
14457
(nm) (nm)
(nm)
470
479
467
473
473
476
476
476
482
490
486
498
484
484
482
492
498
494
534
556
540
554
545
554
554
556
565
2550.0 0.029
2891.2 0.012
2894.7 0.013
3091.1 0.080
2793.0 0.012
2957.8 0.007
2957.8 0.004
3022.7 0.017
3047.7 0.024
DCM
THF
64
77
73
81
72
78
78
80
83
Chloroform
Ethyl acetate
Acetone
Methanol
Ethanol
Acetonitrile
DMF
Table 2: Thermo gravimetric analysis of Schiff’s bases 6a-6f.
Compound
TGA (oC)
Compound
TGA (oC)
280 (90.51%)
270 (92.65%)
250 (95.98%)
285 (94.55%)
270 (94.88%)
235 (81.06%)
6a
6b
6c
6d
6e
6f
27

Table 3: B3LYP/6-31G(d) optimized energy (Kcalmol-1) of enol conformers of Schiff’s base 6a
in gas phase at ground state.
Conformer
6a-Enol 1
6a-Enol 2
6a-Enol 3
6a-Enol 4
6a-Enol 5
6a-Enol 6
6a-Enol 7
6a-Enol 8
Energy
Conformer
6a-Enol 9
Energy
-980472.5
-980475.5
-980477.5
-980480.3
-980473.8
-980473.8
-980464.6
-980464.6
-980459.9
-980463.9
-980463.6
-980467.5
-980473.7
-980477.7
-980463.7
-980467.7
6a-Enol 10
6a-Enol 11
6a-Enol 12
6a-Enol 13
6a-Enol 14
6a-Enol 15
6a-Enol 16
Table 4: B3LYP/6-31G(d) optimized energy calculations of 6a-Enol and 6a-Keto in gas phase
and solvent phase at ground state.
Energy of 6a-Enol
(Kcal mol-1)
-980480.3
Energy of 6a-Keto
(Kcal mol-1)
-980483.1
Phase/ Solvent
Gas
-980491.7
-980496.5
DCM
-980489.6
-980494.0
Chloroform
Ethyl acetate
-980486.3
-980490.4
28

-980493.3
-980494.1
-980488.2
-980498.4
-980499.3
-980492.8
Acetone
Acetonitrile
DMF
Table 5: Observed and calculated vibrational frequencies of 6a-Keto.
Vibrational
frequency
(observed) cm-1
1608
Vibrational
Bond
frequency
(calculated) cm-1
1608
-C=O (keto)
-C=N (benzothiazole)
Aromatic ring
1545
1493
1432
722
1539
1493
1442
718
-C=N (imine)
-C-S (benzothiazole)
Table 6: Experimental UV-visible excitation and computed vertical excitation of 6a in different
solvents from TD-B3LYP/6-31G(d).
%D[e]
TD-B3LYP/6-31G(d)
[a]
λabs
Orbital
contribution
(Keto)
Solvent
[b]
λEnol
[c]
λKeto
(f)[d]
6a-Enol
6a-Keto
470
457
456
452
465
463
460
0.4549
0.4707
0.4644
H→L (0.6607)
2.7
2.3
4.4
1.0
0.8
2.7
DCM
467
473
H→L (0.6599)
H→L (0.6999)
Chloroform
Ethyl acetate
29