Synthesis and characterization of molecules containing thiazole and oxazole moieties and study of ESIPT phenomenon

Article abstract of DOI:10.1007/s10895-012-1064-y

Two novel ESIPT molecules, 2-[4-(1,3-benzothiazol-2-yl)naphtho[1,2-d][1,3] oxazol-2-yl]phenol 9a and 4-[4-(1,3-benzothiazol-2-yl)naphtho[1,2-d][1,3]oxazol- 2-yl]benzene-1,3-diol 9b were synthesized by condensing 1-amino-3-(1,3- benzothiazol-2-yl)naphthale

Full text of DOI:10.1007/s10895-012-1064-y

J Fluoresc (2012) 22:1237–1248
DOI 10.1007/s10895-012-1064-y
ORIGINAL PAPER
Synthesis and Characterization of Molecules Containing
Thiazole and Oxazole Moieties and Study of ESIPT
Phenomenon
Manjaree A. Satam & Rajesh K. Raut &
Abhinav B. Tathe & N Sekar
Received: 10 January 2012 /Accepted: 28 May 2012 /Published online: 10 June 2012
#
Springer Science+Business Media, LLC 2012
Abstract Two novel ESIPT molecules, 2-[4-(1,3-benzo-
thiazol-2-yl)naphtho[1,2-d][1,3]oxazol-2-yl]phenol 9a
and 4-[4-(1,3-benzothiazol-2-yl)naphtho[1,2-d][1,3]oxa-
zol-2-yl]benzene-1,3-diol 9b were synthesized by con-
densing 1-amino-3-(1,3-benzothiazol-2-yl)naphthalen-2-
ol with 2-hydroxybenzoic acid and 2,4-dihydroxyben-
zoic acid respectively. The novel compounds were char-
acterized by FT-IR, 1H NMR, Mass spectral and
elemental analysis. Effect of polarity on photo physical
properties, absorption and emission were studied. Com-
pounds showed single absorption and dual emission due to
ESIPT phenomenon. The structural changes due to ESIPT
phenomenon in terms of bond angle, bond distances and
geometry were investigated by using Gaussian 03 software.
These two novel ESIPT molecules are thermally stable up to
200 °C.
Introduction
Fluorescent molecules find extensive applications in the
investigation of proteins in biological systems [1]. Dual
emission spectroscopy is a strong tool to detect a protein
binding site as well as H-bond donating and accepting
ability. The use of a dual emitter gives a ratio metric re-
sponse independent of the concentration of probe molecules
[2].
Excited State Intramolecular Proton Transfer (ESIPT) mol-
ecules form a very effective basis for the design of probes with
dual fluorescence. In this process, on UV excitation two
isomers (enol E* and keto K*) are formed which are having
different photo physical properties. These tautomeric forms
immediately come at ground state with loss of energy in the
form of fluorescence. This results into highly separated two
emission bands [3–5]. Therefore ESIPT molecules show sin-
gle excitation with dual emissions.
.
.
.
.
Keywords Thiazole Oxazole ESIPT Fluorescence
.
Solvatochromism DFT
Amongst the large range of systems undergoing excited-
state intramolecular proton transfer (ESIPT), compounds of
the azole type, that is oxazole and thiazole, form an impor-
tant class [6–13]. There are many reports describing the
dependence of fluorescence on solvent polarity [14–17] as
well as theoretical calculations regarding the geometry of
the different conformers [18–23]. The azoles exhibit dual
emission in polar solvents [24, 25].
:
:
M. A. Satam A. B. Tathe N. Sekar (*)
Department of Dyestuff Technology,
Institute of Chemical Technology,
Mumbai, India 400 019
e-mail: n.sekar@ictmumbai.edu.in
M. A. Satam
One of the most important features of ESIPT molecules is
the phenolic group, which participates in intramolecular
hydrogen bonding with nitrogen. There are radical changes
in the molecule around acidic and basic centers upon UV
excitation. These changes generally lead to proton transfer
from hydroxyl (enol) to nitrogen, generating the keto form
of the carbonyl group in the process [22]. It has been shown
e-mail: manjareesatam@gmail.com
Present Address:
R. K. Raut
Department of Chemistry and Biomolecular Sciences,
Macquarie University,
Sydney, NSW 2109, Australia
raut.rajesh23@gmail.com

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J Fluoresc (2012) 22:1237–1248
that different species coexist in the ground state [26] and the
closed enol conformer alone is responsible for larger Stokes
shift after undergoing ESIPT [27]. Most of the o-hydroxy
azole type molecules undergoing ESIPT show larger Stokes
shifts [28–32]. In continuation of our work in ESIPT mole-
cules [33, 34], we report here two novel ESIPT molecules,
their photo physical properties and DFT calculations.
This paper describes organic molecules containing
both thiazole as well as oxazole units. The molecules
9a and 9b are synthesized by a synthetic sequence out-
lined in the synthetic scheme (Fig. 1) [35–37]. The
target molecules (9a and 9b) are hitherto unknown
and they are fully characterized by spectral techniques.
These molecules show three emissions in highly polar
aprotic solvent like DMF. Thermal stabilities and solva-
tochromism of these molecules are also reported. All
these molecules undergo ESIPT phenomenon having larger
Stokes shifts.
Preparation of 4-{(E)-[3-(1,3-Benzothiazol-2-yl)-2-Hydrox-
ynaphthalen-1-yl]Diazenyl}Benzenesulfonic Acid 6 [36]
A mixture of sulfanilic acid (10.5 g, 0.055 mol) and sodium
carbonate (2.65 g, 0.025 mol) in water (100 mL) was heated
until a clear solution obtained. The reaction mixture was
then cooled to about 15 °C. A cold solution of sodium nitrite
(3.8 g, 0.055 mol) in water (10 mL) was added to the above
mixture. The resulting solution was slowly poured over
35 % hydrochloric acid (14.2 mL, 14.9 g, d01.2 gL-1,
0.165 mol) at 0–5 °C, with constant stirring. The progress
of the reaction was checked with starch- iodide paper.
3-(1,3-Benzothiazol-2-yl)naphthalen-2-ol (0.01 mol,
2.77 g) was dissolved in a mixture of 80 % methanol in
water (50 mL) containing sodium hydroxide (0.4 g,
0.01 mol). A clear yellow solution was obtained. The reac-
tion mixture was then cooled to 5 °C. The above diazotized
amine 5 was slowly added with constant stirring and main-
taining temperature at 5 °C and pH between 9 and 10. The
acid dye 6 was formed and recrystallized from methanol.
Yield: 80 %; M.p. > 300 °C.
Experimental
Anal. Calcd. for C23H15N3O4S2 (461.52 gmol−1): C,
59.86; H, 3.28; N, 9.10; S, 13.90.
Materials
Found: C, 59.79; H, 3.21; N, 9.09; S, 13.80.
3-Hydroxynaphthalene-2-carboxylic acid (BON acid), 2-
aminothiophenol, 4-aminobenzenesulfonic acid, 2-
hydroxybenzoic acid, 2,4-dihydroxybenzoic acid, phos-
phorus trichloride, sodium nitrite, sodium dithionite,
toluene, xylene, ethyl acetate and methanol were pur-
chased from S.D. Fine Pvt. Ltd. Mumbai. Solid reagents
were characterized by melting point and used without
purification. Liquid reagents were distilled at their boil-
ing points and used immediately. All solvents were used
after necessary purification and drying according to standard
processes.
FT-IR (KBr, cm−1): 3460, 1606, 3040, 1552, 1193.
1H NMR (400 MHz, CD3OD) δ (ppm): 2.15 (s, 1H,
SO3H); 4.62 (s, 1H, OH; D2O exchangeable) 7.76–8.92 (m,
13H, Ph).
m/z: 462(M+1).
Preparation of 1-Amino-3-(1,3-Benzothiazol-2-yl)Naphthalen-
2-ol 7 [37]
Sodium dithionite (19.4 g, 0.11 mol) was added to the above
suspension of dye at pH 8-9. The reaction mixture was
heated to 50 °C and the alkaline pH was maintained
throughout the reaction. After complete discharge of dye
color, compound 7 separated out, and was filtered and
recrystallized from ethyl acetate.
Synthesis
Synthesis of 3-(1,3-Benzothiazol-2-yl)Naphthalen-2-ol 3
[35]
Yield: 90 %; M.p. 162–164 °C.
Anal. Calcd. for C17H12N2OS (292.36 gmol-1): C,
69.86; H, 4.14; N, 9.58; S, 10.97.
Phosphorus trichloride (2.6 mL, 4.13 g, d01.57 gL-1,
0.03 mol) was added drop wise to a solution of the 3-
hydroxynaphthalene-2-carboxylic acid (BON acid) (6.2 g
0.033 mol) and 2-aminothiophenol (3.85 mL, 4.5 g, d0
1.17 gL−1, 0.036 mol) in toluene (50 mL), maintaining the
temperature below 40–45 °C. The mixture was boiled vig-
orously under reflux for 4 h, whereupon the cooled solution
was made alkaline with aqueous sodium carbonate solution
(20 %w/v). Toluene was removed by vacuum distillation
and the solid remained was collected and purified by recrys-
tallization from methanol.
Found: C, 69.56; H, 4.04; N, 9.51; S, 10.79.
FT-IR (KBr, cm−1): 3423, 3319, 3051, 1643, 1494, 1456.
1H NMR (400 MHz, CDCl3) δ (ppm): 1.54 (s, 2H,
NH2), 7.46–8.25 (m, 9H, Ph), 12.28 (s, 1H, OH).
m/z: 293(M+1).
Synthesis of 9a and 9b
Phosphorus trichloride (2.6 mL, 4.13 g, d01.57 gL−1
0.03 mol) was added drop wise to a solution of the hydrox-
,
Yield: 82 %; M.p. 182–184 °C (lit. 183 °C) [35].
ybenzoic acid 8a or 8b (0.033 mol) and 1-amino-3-(1,3-

J Fluoresc (2012) 22:1237–1248
1239
Step-1:
OH
H2N
HS
OH
S
PCl3, Toluene
Reflux
+
N
COOH
[1]
[3]
[2]
Step-2:
SO3Na
N2+Cl-
NH2
Diazotization, NaNO2/ HCl
0-5 °C
Coupling, pH 8 to 9
0-5 °C
[3]
+
N
N
SO3H
[4]
SO3H
[5]
OH
N
Step-3:
[6]
SO3Na
S
NH2
OH
N
Reduction
Na2S2O4, pH 8 to 9, 45-50 °C
N
N
OH
S
S
[7]
N
[6]
OH
Step-4:
NH2
HO
HO
OH
N
PCl3, Xylene
N
N
O
O
COOH
COOH
S
OH
OH
N
N
[7]
S
S
[9a]
[9b]
OH
[8a]
[8b]
Fig. 1 Synthesis of ESIPT benzothiazolyl naphthooxazol phenol derivatives
benzothiazol-2-yl)naphthalen-2-ol 7 (10.51 g, 0.036 mol) in
xylene (50 mL), maintaining the temperature at 40–45 °C. The
mixture was then refluxed for 5 h and allowed to cool to room
temperature and made alkaline with 20 % aqueous sodium
carbonate solution. The reaction mixture was then filtered and
compounds 9a and 9b were recrystallized from methanol.
Anal. calcd. for C24H14N2O2S (394.45 gmol−1) C,
73.09; H, 3.58; N, 7.10; S, 8.13.
Found: C, 72.86; H, 3.48; N, 1.02; S, 8.08.
FT-IR (KBr, cm−1): 3410, 3053, 1653, 1573,
1521.
1H NMR (400 MHz, CDCl3) δ (ppm): 4.88 (s, 1H, OH,
D2O exchangeable), 7.38–8.02 (m, 13H, Ph), 12.12 (s, 1H,
OH, hydrogen bonding).
2-[4-(1,3-benzothiazol-2-yl)naphtho[1,2-d][1,3]oxazol-2-
yl]phenol 9a Yield: 72 %; M.p. 140–142 °C.
m/z: 395 (M+1).

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J Fluoresc (2012) 22:1237–1248
4-[4-(1,3-benzothiazol-2-yl)naphtho[1,2-d][1,3]oxazol-2-
yl]benzene-1,3-diol 9b Yield: 69 %; M.p. 170–172 °C.
Anal. calcd. for C24H14N2O3S (410.45 gmol−1) C,
70.23; H, 3.44; N, 6.83; S, 7.81.
Found: C, 70.15; H, 3.39; N, 6.79; S, 7.79.
FT-IR(KBr, cm−1): 3414, 3049, 1613, 1579, 1527.
1H NMR (400 MHz, DMSO-d6) δ (ppm): 3.40 (s, 2 H,
OH, D2O exchangeable), 7.56–8.88 (m, 12 H, Ph),
11.56 (s, 1H, OH, hydrogen bonding).
acid) was condensed with 2-aminothiophenol in toluene in
the presence of PCl3 as catalyst to obtain benzothiazole 3
(Step 1) [35]. The azo dye 6 (Step 2) [36] was prepared by
reacting diazotized sulfanilic acid 4 and coupling with
compound 3 under alkaline condition. The dye 6 was re-
duced at pH 7-8 by using sodium dithionite to obtain o-
aminohydroxybenzothiazole 7 (Step 3) [37]. Sulfanilic acid
remains in the solution as sodium sulfanilate while the
amine was suspended in the solution and filtered off. The
cake was washed thoroughly with water to remove the
sulfanilic acid. Compound 7 was then condensed with 2-
hydroxybenzoic acid 8a and 2, 4-dihydroxybenzoic acid 8b
to yield oxazoles 9a and 9b (Step 4). Physical character-
istics of compounds 7, 9a and 9b are given in Table 1.
In these molecules –OH group at ortho to azole ring is
acidic as compare to –N 0 of oxazole ring which is strongly
basic. Under UV excitation, these two groups become
strongly acidic and basic respectively. In excited state, pro-
ton is transferred from –OH group to hetero atom i.e. nitro-
gen to generate keto form. Hence ESIPT process occurred in
these molecules. Mechanism of ESIPT occurred in molecule
9a is given in Fig. 2.
m/z: 411.5 (M+1).
Characterization
All melting points were recorded by open capillary on Sunder
Industrial Product and are uncorrected. All reactions were
monitored by using precoated silica gel aluminum backed
plates, Kisel gel 60 F254 Merck (Germany). UV- Visible
absorption measurements were carried out using a Spectronic
Genesys 2 spectrophotometer with 1 cm quartz cells. The
excitation wavelength was taken as λmax of compound. The
scan range was 250 to 650 nm. Fluorescence emission meas-
urements were recorded on Cary Eclipse fluorescence spec-
trophotometer (Varian, Australia) using 1 cm quartz cells.
Quantum yield of these compounds were calculated using
Anthracene and Fluorescein as standards. FT-IR analysis
was performed on a FTIR-8400 S SHIMADZU spectropho-
tometer. Thermo gravimetric analysis was carried out on a
SDT Q600 TA Instruments and elemental analysis was carried
out on a Harieus rapid analyzer. Method for thermo gravimet-
ric analysis: Ramp 10 °C per minute from ambient tempera-
ture to 600 °C under a nitrogen atmosphere. 1H NMR spectra
were recorded on VXR 400-MHz instrument using TMS as an
internal standard and CDCl3, CD3OD or DMSO-d6 as solvent.
Purification of all compounds was generally achieved by
recrystallization. The purity of compounds was generally
ascertained by thin layer chromatography.
DFT calculations were performed on an HP workstation
XW8600 with Xeon processor @ 2.8 GHz, 4 GB RAM and
Windows Vista as operating system. The software package
used was Gaussian 03 W [38] with Gauss View 4 as graphical
program [39]. The ground state geometry was optimized at
B3LYP functional and 6–31 G (d) as basis set. All the geometry
optimizations were confirmed to be true by frequency analysis
and showed no imaginary frequencies. Excited state calcula-
tions were performed by TD-DFT with B3LYP hybrid func-
tional and 6–31 G (d) basis set.
Study of Photo Physical Properties
The absorption and emission of molecular species are influ-
enced by the environment. The solvents used in making
solutions of these molecular species provide a varied environ-
ment due to differences in dielectric constants and refractive
indices of the solvents [14–17]. The ESIPT molecules 7, 9a
and 9b reported here are likely to exist in different conforma-
tional states. The conformer E* (Fig. 2) which forms hydrogen
bonding with hetero atom (nitrogen) or with the polar solvents
gives fluorescence.
Due to ESIPT phenomenon, oxazole derivatives of benzo-
thiazole 9a and 9b showed single absorption having dual
emission in less polar solvents. But these compounds showed
three fluorescence emissions in polar aprotic solvent like DMF.
This third emission peak observed is due to rotamer of enol
form which may stabilise in DMF (Fig. 2). Due to ESIPT, these
molecules show large Stokes shift (Table 1).
The effect of solvents (either as a pure solvent, such as
ethanol, DMF, DMSO or as a mixture of solvents, such as
different proportions of ethanol: water and DMF: ethanol)
were studied in terms of absorption and fluorescence emis-
sion of the molecules (Table 2). In case of ethanol: water
proportions, compounds are soluble minimum in 50 % of
ethanol in water and as concentration of ethanol increases
absorbance of compounds also increases. In case of ethanol:
DMF proportions, there is no any significant change ob-
served in absorbance (Table 2).
Results and Discussions
The synthetic scheme for the synthesis of azoles is described
in Fig. 1, 3-hydroxy-2-naphthalenecarboxylic acid (BON
It is observed that these compounds show two fluores-
cence emission bands in less polar solvent like ethanol, but

J Fluoresc (2012) 22:1237–1248
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Table 1 Physical characteristics of compounds
No. Mol. Formula
Mol.
%
Melting
λmax (nm)
Molar extinction
Fluorescence
Stokes
Relative
weight
Yield point (°C)
in DMF
coefficient (ε)
emission (nm)
shift (nm)
quantum yield
(dm3mol−1 cm−1) in DMF in DMF
7
C17H12N2OS
C24H14N2O2S
C24H14N2O3S
292
394
410
90
72
69
162–164
140–142
170–172
332
46,800
22,400
22,600
372
440
050
108
0.00137
562
230
9a
9b
330
322
384
432
054
132
0.00205
0.00759
558
228
356
454
034
132
528
206
as polarity increases these compounds show three fluores-
cence emission bands. These compounds are found to be
insoluble in non-polar solvents.
all three molecules in three pure solvents have been compared.
It is seen that emission intensity increases with an increase in
polarity, and in a highly polar aprotic solvent like DMF, three
emissions are observed (Table 2).
It has been found that as the polarity of the solvent
increases, the intensity of third emission increases. DMF
being more polar aprotic solvent gives the most intense emis-
sion. In the series of DMSO, DMF and ethanol, ethanol gives
the least intense emission. When the polarity of DMF is
decreased by adding ethanol, the fluorescence intensity of
the third emission decreases progressively and finally dimin-
ishes in a 3:2 mixture of DMF: ethanol. Emission intensities of
Calculation of Relative Fluorescence Quantum Yield
Relative fluorescence quantum yield of compounds 7, 9a and
9b are determined by using standards with known quantum
yields (Fluorescein and Anthracene) and using Formula 1
[40]. All these compounds are found to be dual emitting dyes
O
H
O
H
O
N
N
N
Rotamerisation
ESIPT
H
N
O
O
O
N
N
[E*]
S
[K*]
keto at E. S.
S
S
rotamer of enol at E. S.
enol at E. S.
Emission 2
Absorption
Emission 1
Emission 3
O
N
O
H
H
O
N
N
O
O
H
N
O
N
N
[K]
[E]
enol at G. S.
S
S
S
keto at G. S.
rotamer of enol at G. S.
Fig. 2 Mechanism of ESIPT in molecule 9a

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J Fluoresc (2012) 22:1237–1248
1243
Fig. 3 Protection of –OH
Step-5:
group
R
R1
OH
OC2H5
COOH
(C2H5)2SO4
KOH, Acetone
COOH
[8a-b]
[10a-b]
R1
H5C2O
Step-6:
N
O
PCl3, Xylene
Reflux
[7]
[10a-b]
+
N
R= a: H; b: -OH
R1= a: H; b: -OC2H5
[11a-b]
S
corresponding to their single excitation in less polar solvents.
For compounds 7, 9a and 9b two different quantum yields are
reported at two different emissions. Quantum yields at long
wavelength emission were calculated by using Fluorescein as
a standard. Quantum yields at short wavelength emission were
calculated using Anthracene as a standard. Solutions of dif-
ferent concentration (2 ppm to 10 ppm) in absolute ethanol for
each sample were used for the measurement of absorbance
and emission. Emission intensity values were plotted against
absorption intensity values. These measurements gave a
straight line graph, from which gradients were calculated for
each compound and for standards. All the parameters such as
solvent, cuvette and slit width were kept constant during the
measurements to avoid errors. Relative fluorescence quantum
yields of compounds 7, 9a and 9b are mentioned in Table 1.
Formula 1: Relative Quantum Yield:
Protection of –OH group:
Salicylic acid or Resorcylic acid (0.1 mol) was dissolved
in acetone having (0.1 mol) KOH. Then added (0.12 mol)
diethyl sulphate in it and reaction mass was stirred at room
temperature. Reaction was monitored on TLC. Reaction was
completed after 8 h. After this reaction mass was poured in
a
Emission graph of compound 9a and 11a
0.25
0.2
0.15
9a
0.1
11a
0.05
ꢀ
ꢁꢀ
ꢁ
GradX
ηX2
η2ST
0
ΦX ¼ ΦST
300
400
500
600
GradST
Wavelength (nm)
Where:
b
Emission graph of compound 9b and 11b
ΦX
quantum yield of unknown sample
quantum yield of standard used
1.4
1.2
1
ΦST
GradX gradient of unknown sample
GradST gradient of standard used
2
0.8
0.6
0.4
0.2
0
ηST
ηX
refractive index of solvent for standard sample
refractive index of solvent for unknown sample
2
9b
11b
Confirmation for ESIPT Process
300
400
500
600
Wavelength (nm)
As we know phenolic –OH group plays an important role in
ESIPT process. To confirm ESIPT, protection of this hy-
droxyl group was carried out using conventional method as
shown in Fig. 3 and studied their photo physical properties.
Fig. 4 ESIPT phenomenon confirmed by emission graph of protected
and unprotected compounds. a Emission graph of compound 9a and 11a
in ethanol. b Emission graph of compound 9b and 11b in ethanol

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Fig. 5 a Optimized enol-keto structure of compound 9a in gas phase
at ground state. b Optimized enol-keto structure of compound 9a in
ethanol at ground state. c Optimized enol-keto structure of compound
9b in gas phase at ground state. d Optimized enol-keto structure of
compound 9b in ethanol at ground state
water and extracted in ethyl acetate. Solvent was removed
under reduce pressure to get 10a and 10b. These compounds
10a and 10b were cyclised with compound 7 using PCl3 as a
catalyst in xylene to give corresponding protected compounds
11a and 11b [35].
It was observed that ethylated products showed single
absorption and single emission in ethanol as a solvent.
Compound 11a is having absorption at 328 nm and emission
at 472 nm. Compound 11b is having absorption at 345 nm
and emission at 446 nm in ethanol.
9b free hydroxyl group was taken part in ESIPT process to show
single absorption having dual emissions. When this free hydrox-
yl was protected by ethyl group, there is no free proton available
for tautomerization. Therefore these compounds showed single
absorption with single emission. Longer wavelength emission
band was disappeared in both protected molecules. ESIPT se-
quence gets destroyed in absence of proton at oxygen.
Density Functional Theory Calculations
Figure 4a and b indicate that protected hydroxyl group com-
pounds i.e. 11a and 11b (o-ethylated) showed single emission as
compare to ESIPTcompounds 9a and 9b. In compounds 9a and
The ESIPT phenomenon also can be explained on the basis
of DFT calculations performed on compounds 9a and 9b.
Using Gaussian 03 software package, optimization of

J Fluoresc (2012) 22:1237–1248
1245
Fig. 5 (continued)
molecular enol- keto structures of compounds 9a and 9b at
ground state in gas phase as well as in ethanol were carried
out as shown in Fig. 5a, b, c, d respectively. Structural
parameters like bond length, bond angle and dihedral
Table 3 Energy differences between enol-keto form and vertical excitations
9a
9b
Gas phase
Ethanol
Gas phase
Ethanol
Vertical excitation E-E* (X)
Vertical excitation K-K* (Y)
3.6712 eV (337.72 nm)
3.4056 eV (364.06 nm)
0.59035 eV
3.6192 eV (342.57 nm)
3.5203 eV (352.20 nm)
1.00245 eV
3.7023 eV (334.88 nm)
3.3623 eV (368.74 nm)
0.52701 eV
3.6363 eV (340.96 nm)
3.4379 eV (360.64 nm)
0.4102 eV
Energy difference between
enol-keto form in G. S. (Z eV)

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J Fluoresc (2012) 22:1237–1248
Table 4 Thermo gravi-
metric analysis of com-
pounds 7, 9a and 9b
Compound
TGA (°C)
7
260 (94 %)
9a
9b
205 (86.93 %)
200 (91.43 %)
and hydrogen becomes more positive. Dihedral angles be-
tween hydroxyl-phenyl ring and naphthoxazole, benzthiazole
and naphthoxazole rings are almost zero which shows both the
rings (thiazole and oxazole) are planar to naphthoxazole core.
This facilitates ESIPT phenomenon.
These calculations have revealed that the ground state enol
form of compound 9a and 9b are stabilized by 0.59035 eV
(13.6158 Kcal/mole) and 0.52701 eV (12.1548 Kcal/mole)
energy than its ground state keto form respectively (Table 3).
And these are the only existing tautomers at ground state. Also
practical observation of a single absorption peak supports this.
Energy difference between vertical excitation E-E* is greater
than the energy difference between vertical excitation K-K* in
both gas phase and ethanol (Fig. 6). After undergoing ESIPT
molecules 9a and 9b relax to lower vibronic states, and give
emission with larger Stokes shift. The results of DFT calcula-
tion support the fact of dual emission due to ESIPT.
In the solvent DMF all molecules have shown triple emis-
sion. Though reason for this is not confirmed, this can be
attributed to a stable intramolecular hydrogen bonded rotamer
of enol (rotation of hydroxyl phenyl ring and H-bonding with
oxygen) as which gets stabilized by polar aprotic solvent i.e.
DMF.
Fig. 6 Schematic representation of ESIPT process
angle of compounds 9a and 9b also have been shown in
Fig. 5a, b, c, d.
Bond lengths of C24-O29 in 9a and that of C28-O29 in 9b
in enol form are longer than double bonds and show a single
bond character to respective C-O bonds of compounds 9a
and 9b respectively. The interatomic distances of N22-H39
bond of compound 9a and N20-H43 bond of compound 9b
suggest a strong intramolecular hydrogen bonding between
nitrogen from benzoxazole ring and hydrogen of hydroxyl
group and favor the migration of proton from oxygen of
hydroxyl group to nitrogen of oxazole ring. In case of com-
pound 9b, bond length of hydroxyl group (0.99109 oA) which
participated in ESIPT process is more elongated than that of
another hydroxyl group (0.97032 oA) present on same ring. It
is also observed that the oxygen atom becomes more negative
(−0.677 at excited state as compared to −0.673 at ground state)
and makes the hydrogen more acidic for 9a. Similarly in
compound 9b charge on oxygen becomes more negative
(−0.702 at excited state as compared to −0.680 at ground state)
Thermal Stability Study
Thermo gravimetric analysis of the molecules was carried out
to study their thermal stability. The thermo gravimetric analysis
(Fig. 7) shows that oxazoles derivatives have lower stabilities
as compared with 7. These results indicate that the dyes are
stable up to 200 °C (Table 4).
TGA Plots
7
120
9a
Conclusion
100
9b
80
The present work describes,
1. Simple synthesis and characterization of ESIPT mole-
cules containing both oxazole and thiazole units.
2. The effect of polarity of solvents on ESIPT molecules
and its role in the stabilization of different rotamers and
tautomers in the ground state are studied.
3. Optimization of ESIPT molecules using Density Func-
tional Theory was successfully carried out.
4. ESIPT phenomenon was confirmed by protection of phe-
nolic –OH group which take part in ESIPT process.
60
40
20
0
0
100
200
300
400
500
600
Temperature oC
Fig. 7 Thermo gravimetric analysis

J Fluoresc (2012) 22:1237–1248
1247
Acknowledgement The authors are greatly thankful to I.I.T., Mumbai
for recording the 1H-NMR, Mass spectra and elemental analysis. One of
the authors (Manjaree Satam) is grateful for UGC-CAS for providing
fellowship under SAP. One of the authors (Abhinav Tathe) is grateful for
UGC-CSIR for providing fellowship as well as UGC for financial support
by way of Major Research Project Grant.
17. Klymchenko AS, Pivovarenko VG, Demchenko AP (2003)
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analogues. Int J Mol Sci 4:335–361
19. Santra S, Dogra SK (1998) Excited-state intramolecular proton
transfer in 2-(2′-aminophenyl) benzimidazole. Chem Phys
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