Synthesis and characterization of novel yellow azo dyes from 2-morpholin-4-yl-1,3-thiazol-4(5H)-one and study of their azo-hydrazone tautomerism

Article abstract of DOI:10.1016/j.dyepig.2013.05.002

Novel yellow azo dyes were synthesized by diazotization of aromatic amines followed by coupling with 2-morpholin-4-yl-1,3-thiazol-4(5H)-one and fully characterized. The geometries of the synthesized dyes for azo and hydrazone tautomeric forms were optimized using B3LYP, CAM-B3LYP and M06 functional and 6-31G(d) and 6-311++G (d,p) basis sets, also their electronic excitation properties were evaluated using density functional theory. The optimized geometries reveal that the hydrazone for is more stable than the azo form. Photophysical properties of the synthesized dyes were evaluated by UV-Visible spectroscopy and compared with computed vertical excitation obtained from TDDFT. The results clearly illustrate existence of dye 8a, 8b and 8c in hydrazone tautomeric form, while 8d exist in both azo as well as hydrazone form. Thermal stabilities were estimated by using thermo gravimetric analysis, and results revels that the synthesized dyes have good thermal stability.

Full text of DOI:10.1016/j.dyepig.2013.05.002

Accepted Manuscript
Synthesis and Characterisation of Novel Yellow Azo Dyes from 2-Morpholin-4-yl-1,3-
thiazol-4(5H)-one and Study of Their Azo-Hydrazone Tautomerism
Prashant G. Umape, Vikas S. Patil, Vikas S. Padalkar, Kiran R. Phatangare, Vinod D.
Gupta, Abhinav B. Thate, Nagaiyan Sekar
PII:
S0143-7208(13)00164-2
10.1016/j.dyepig.2013.05.002
DYPI 3936
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 28 August 2012
Revised Date: 26 April 2013
Accepted Date: 1 May 2013
Please cite this article as: Umape PG, Patil VS, Padalkar VS, Phatangare KR, Gupta VD, Thate
AB, Sekar N, Synthesis and Characterisation of Novel Yellow Azo Dyes from 2-Morpholin-4-yl-1,3-
thiazol-4(5H)-one and Study of Their Azo-Hydrazone Tautomerism, Dyes and Pigments (2013), doi:
10.1016/j.dyepig.2013.05.002.
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ACCEPTED MANUSCRIPT
Synthesis and Characterisation of Novel Yellow Azo Dyes from 2-Morpholin-4-yl-1,3-
thiazol-4(5H)-one and Study of Their Azo-Hydrazone Tautomerism.
Prashant G. Umape, Vikas S. Patil, Vikas S. Padalkar, Kiran R. Phatangare,
Vinod D. Gupta, Abhinav B. Thate, Nagaiyan Sekar*
Tinctorial Chemistry Group, Department of Dyestuff Technology,
Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India)
*
Corresponding author. Tel.: +91 22 3361 1111/2222, 2707(direct), Fax.: +91 22 3361 1020.
E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Abstract:
Novel yellow azo dyes were synthesized by diazotization of aromatic amines followed by
coupling with 2-morpholin-4-yl-1,3-thiazol-4(5H)-one and fully characterized. The geometries
of the synthesized dyes for azo and hydrazone tautomeric forms were optimized using B3LYP,
CAM-B3LYP and M06 functional and 6-31G(d) and 6-311++G (d,p) basis sets, also their
electronic excitation properties were evaluated using density functional theory. The optimized
geometries reveal that the hydrazone for is more stable than the azo form. Photophysical
properties of the synthesized dyes were evaluated by UV-Visible spectroscopy and compared
with computed vertical excitation obtained from TDDFT. The results clearly illustrate existence
of dye 8a, 8b and 8c in hydrazone tautomeric form, while 8d exist in both azo as well as
hydrazone form. Thermal stabilities were estimated by using thermo gravimetric analysis, and
results revels that the synthesized dyes have good thermal stability.
Keywords: Yellow azo dyes, 2-(Morpholin-4-yl)-1,3-thiazol-4(5H)-one, Diazotization,
Diazocoupling, Density functional theory, Azo-hydrazone tautomerism.
1

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1
. Introduction
Azo chromophores have versatile applicability ranging from textile dyeing [1], leather dyeing
[
[
2], coloring of plastics and polymer [3] to advanced applications such as liquid crystal displays
4], biological and medical studies and advanced application in organic synthesis [5]. Also, they
contribute greatest production volume of the dyestuff industry due to simplistic mode of their
synthesis with high yield.
Azo colorants derived from aromatic heterocyclic amines or coupling components have excellent
light and sublimation fastness, color strength and bright hue [6]. Thiazole based azo colorants
have been reported as disperse dyes for polyester, with excellent wash fastness and good
sublimation fastness [7]. They find use in spectrophotometric determination of Iron (III) [8]. A
few thiazole containing azo dyes have been reported as antimicrobial agents [9-12].
The existence of azo-hydrazone equilibrium in azo colorants having hydroxy group at ortho or
para to azo linkage was first proved by Kuhn and Bar, which was postulated by Liebermann in
1
983 [13]. Due to the hydrogen bonding in hydrazone tautomeric form it forms a linear and
coplanar six membered ring having higher thermal stability than those with the azo forms [14].
This tautomerism of azo benzene derivatives leads to their applicability as biomarkers [15],
chemical sensors [16]. Also the azo chromophoric system gives carcinogenic amines after
reductive cleavage [17], while the hydrazone tautomer will cleave the C-N bond instead of N=N
bond [18]. Hence, the study of azo-hydrazone tautomerism will give significant information for
2

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proposing efficient process for degradation of azo chromophores with least toxicological effect
on environment.
In this paper, we report synthesis of novel azo chromophores by diazotization of aromatic amines
followed by coupling with 2-morpholin-4-yl-1,3-thiazol-4(5H)-one [19]. The absorption spectra
were measured using UV-Visible spectrophotomerer and thermal stabilities were determined by
using thermogravimetric analysis (TGA). The structures of the synthesized dyes were confirmed
by spectral analysis. Density functional theory was used to study their azo-hydrazone
tautomerism. The structures of dyes in azo and hydrazone forms were optimized in the ground
state using B3LYP, CAM-B3LYP and M06 functional and 6-31G(d) and B3LYP/6-311++
G(d,p) basis sets. Synthesized compounds are summarized (Figure 1).
Insert Figure 1.
Insert Scheme 1.
2
. Experimental
2
.1. Materials and equipments
Ammonium thiocyanate, benzoyl chloride, morpholine, ethyl chloroacetate were
procured from s. d. fine chemicals (India) and used without further purification. All other
chemicals used were of laboratory grade and used without purification. The reaction was
monitored by TLC using on 0.25 mm E-Merck silica gel 60 F254 precoated plates, which were
visualized under UV light. Melting points were measured by open capillary method on standard
melting point apparatus from Sunder industrial product Mumbai, and are uncorrected. The FT-IR
3

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1
spectra were recorded on a Perkin-Elmer Spectrum 100 FT-IR Spectrometer using KBr discs. H
NMR spectra were recorded on VXR 300 MHz instrument using TMS as an internal standard.
The UV Visible absorption spectra of the dyes were recorded on a Spectronic Genesys 2 UV-
Visible spectrophotometer. TGA measurements were performed out on SDT Q600 v8.2 Build
1
00 model of TA instruments Waters (India) Pvt. Ltd.
2
2
.2. Synthesis and characterization
.2.1. Preparation of silica gel-supported ammonium thiocyanate
Silica gel (40 g) was added to a solution of ammonium thiocyanate (5 g, 65.7 mmol) in dry
acetonitrile (125 mL), and the mixture was stirred at room temperature for 30 min. Acetonitrile
o
was removed under reduced pressure at 50 C. The resulting silica gel-supported ammonium
thiocyanate was dried in vacuo (10 mmHg) at room temperature for 3-4 h [20a].
2
.2.2. Synthesis of N-[(morpholin-4-yl)carbonothioyl]benzamide(3)
To a slurry of NH SCN/SiO (45 g, 65.7 mmol) in 1,2-dichloroethane (150mL), benzoyl chloride
4
2
o
(
3.81 mL, 32.8 mmol) was added slowly for 30 min and stirred at 25-30 C for 2 h. Then
morpholine (5.72 mL, 65.7 mmol) was added to it and stirred for an additional 2 h. The reaction
mixture was filtered to remove the supported reagent, residual silica was again washed by 1,2-
dichloroethane and filtered. The combined filtrate was washed with 5% HCl and then with brine
solution, and dried over Na SO . The solvent was evaporated under reduced pressure to leave the
2
4
crude product 3, which was purified by column chromatography silica gel 100-200 mesh and
toluene as eluent system. [20a]
2
0b
Yield: 65%, Melting point: 153-156 °C (155-157 °C lit )
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2
.2.3. Synthesis of morpholine-4-carbothioamide (4)
N-(Morpholin-4-yl-carbonothioyl) benzamide (3) (10.50 g 41.9 mmol )was charged in conc.
°
HCl (20 mL) and the mixture is stirred at 75-80 C for 4 h, completion of the reaction was
monitored by TLC. The reaction mixture then brought to room temperature then neutralized with
liq. ammonia, extracted with 1,2-dichloroethane dried over Na SO and solvent removed under
2
4
reduced pressure. The resulting crude mass was recrystallized from ethanol as a solvent [20b].
2
0b
Yield: 78%, Melting point: 176-177 °C (177-178 °C lit )
2
.2.4. Synthesis of 2-(morpholin-4-yl)-1,3-thiazol-4(5H)-one hydrochloride (5)
Morpholine-4-carbothioamide(4) (5 g, 34.2 mmol) and ethyl chloroacetate (4 mL, 37.6 mmol)
were heated to reflux for 3 h in ethanol, completion of the reaction was monitored by TLC.
Diethyl ether was added to the resulting reaction mass slowly and precipitated product filtered
off dried [19].
1
9
Yield: 61%, Melting point: 178-180 °C (174-178 °C lit )
2
2
.2.5. Preparation of azo dyes (8a-8e)
.2.5.1. General procedure for diazotization of aniline derivatives
Aniline derivative, (22.4 mmol) was dissolved in 5N HCl (30 mL) and the solution was then
°
cooled to 0-5 C. Sodium nitrite (1.7 g, 24.7 mmol) dissolved in H O (3 mL) was added to the
2
above solution drop wise maintaining the temperature 0-5 °C and stirred for 1 h. The resulting
diazonium solution (7a-7e) was further used immediately for coupling.
2
.2.5.2. General procedure for coupling
The coupling component, 2-(morpholin-4-yl)-1,3-thiazol-4(5H)-one hydrochloride (5) (5 g 22.4
°
mmol) were dissolved in acetic acid and then cooled to 0-5 C. The diazonium solution
5

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previously prepared was added drop wise maintaining the temperature 0-5 °C, pH 4-5 maintained
by using sodium acetate. The reaction mixture further stirred for 30-35 h for completion for
coupling. Precipitated product filter and washed with water, crystallized from DMF. Yields,
physical properties and characterization data are summarized in (Tables 1-2).
3
. Results and discussion
3
.1. Synthesis of dyes
A simple and facile synthetic route was adapted for the synthesis of azo dyes. The key
intermediate 2-(morpholin-4-yl)-1,3-thiazol-4(5H)-one 5 was synthesized from morpholine-4-
carbothioamide 4 by refluxing with ethyl chloroacetate in ethanol as a solvent. Morpholine-4-
carbothioamide 4 was obtained from ammonium thiocyanate and benzyl chloride Scheme 1.
Benzoyl isothiocyanate 2 was formed by the reaction of benzoyl chloride 1 with silica gel
supported ammonium thiocyanate (NH SCN/SiO ). Further benzoyl isothiocyanate on reaction
4
2
with morpholine yield N-[(morpholin-4-yl)carbonothioyl]benzamide 3 [20a]. The debenzoylation
of N-[(morpholin-4-yl)carbonothioyl]benzamide 3 gives morpholine-4-carbothioamide 4 in good
yield [20b].
Morpholine-4-carbothioamide 4 with ethyl chloroacetate in refluxing ethanol gives a key
intermediate 2-(morpholin-4-yl)-1,3-thiazol-4(5H)-one 5 [19]. Different aniline derivatives 6
have been diazotized by using NaNO in HCl-water mixture (1:1) and further coupled with the
2
synthesized coupling component 2-(morpholin-4-yl)-1,3-thiazol-4(5H)-one 5 to obtain the
respective azo dyes 8a-8e in good yields (Table 1). Infrared spectra (Table 2) of the synthesized
-1
chromophore 8a show carbonyl band at 1666 cm and absence of broad band for hydroxyl group
1
gives indication that the chromophore may exist in tautomeric form of azo-hydrazone. H NMR
6

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spectra summarised in (Table 2) indicates the dye 8a shows a singlet at δ 10.5 ppm attributed to
N-H proton which is in hydrazone form. The characteristic doublet for aromatic proton appears
at δ 7.36 and 7.24 ppm, triplet at δ 3.9 and 3.6 ppm and multiplate at δ 3.7 ppm was attributed to
morpholine protons.
Insert Table 1.
Insert Table 2.
3
.2. Photo-physical properties
To find out structural correlation with colour properties of synthesized azo dyes, UV-Visible
spectra of the dyes were recorded in DMF and the results are tabulated in (Table 3). The
absorption spectra of these dyes showed absorption band in the region 368-535 nm, with
shoulder peaks ranging from 268-304 nm. The bathochromic shift was observed in the
absorption spectra, when halo substituent altered from chlorine 8a (371 nm) to fluorine 8b (406
nm), while m-nitro derivative 8d has red shifted absorption (425 nm) than p-nitro derivative 8c
(368 nm). The 2,4-dinitro substituted azo chromophore 8e absorbs at 535 nm. These strong
absorption bands are attributed to the π—π* transitions involving the charge transfer (CT)
throughout the whole molecular π-electronic system. Dyes 8a and 8c has shoulder peak at 299
nm, while dyes 8b, 8d and 8e showed absorption spectra with shoulder peaks at 304, 281 and
2
68 nm respectively. The shoulder peak observed may be due to the π—π* transitions within the
aromatic fragment with molar extinction coefficient in the range 8636-13278. The Absorption
7

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spectrum of dye 8d has one more peak at 344 nm, which indicate the existence of both azo as
well as hydrazone form in DMF. The peak at 344 nm can be attributed to azo form, as hydrazone
form absorbs at higher wavelength than azo form with high colour strength [21].
Insert Table 3
3
.3. Computational study
Gaussian 09 program package was employed to optimize geometry and to study profusion of the
synthesized azo dyes in their azo and hydrazone tautomeric forms. [22]. Ground state (S0)
geometry of the reported dyes in gas phase and DMF was optimized in their C symmetry using
1
DFT [23]. The Becke’s three parameter exchange functional (B3) [24] combining with nonlocal
correlation functional by Lee, Yang and Parr (LYP) [25], long range corrected CAM-B3LYP and
highly parameterized empirical M06 were used; the computations were performed using basis
sets 6-31G(d) and 6-311++G(d,p) for all atoms. Further the local minima on the energy surface
of the optimized structure were verified by computing the vibrational frequencies using the same
methods. Time Dependent Density Functional Theory (TDDFT) using the same hybrid
functional and basis sets was carried out to obtain vertical excitation energies and oscillator
strengths at the optimized ground state equilibrium geometries [26]. On the basis of the
optimized ground structures absorption characteristics, oscillator strengths and configuration
were systematically evaluated using TDDFT with polarizable continuum model PCM model.
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3
.3.1. Optimized geometries of azo-hydrazone tautomeric forms
The optimized ground state geometries of the dyes in their azo-hydrazone tautomeric forms are
summarized in (Table 4). Optimized ground state geometries of these azo dyes are planar except
the azo chromophore 8e with dinitro substitution on aniline ring. The dye 8e has dihedral angular
twist by 24.57º at N20-N21-C22-C27 with B3LYP/6-31G(d) and dihedral angular twist by
2
3.22º at N20-N21-C22-C27 with B3LYP/6-311++G(d,p) level (Figure 2). While hydrazone
form of the dye 8e has twisted dihedral angle of 19.87 º and 18.55º at N20-N21-C22-C27 with
B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels respectively (Figure 3). The dihedral
angular twist for the azo dye 8e using CAM-B3LYP and M06 functional are 28.47º at 6-31G(d),
2
6.62º at 6-311++G(d,p) basis set and 25.77º at 6-31G(d), 23.68º at 6-311++G(d,p) basis set
respectively. Also the dihedral angular twist for hydrazone form of azo dye 8e using CAM-
B3LYP and M06 functional are as follows 22.14º at 6-31G(d), 20.29º at 6-311++G(d,p) basis set
and 20.45º at 6-31G(d), 18.35º at 6-311++G(d,p) basis set.
Optimized bond lengths of the synthesized dyes 8a-e are tabulated (Table 5, Supporting
information Table 2-15). The O-H bond lengths of azo forms for the series are 1.011, 1.010,
1
3
.015, 1.012, 1.013 Å at B3LYP/6-31G(d) and 1.006, 1.005, 1.011, 1.007, 1.008 Å at B3LYP/6-
11++G(d,p) levels for dyes 8a-8e. While the O-H bond lengths for azo forms optimized using
CAM-B3LYP functional are found to be 1.005, 1.004, 1.010, 1.007, 1.009 at 6-31G(d) and
.001, 1.000, 1.007, 1.003, 1.006 at 6-311++G(d,p) levels. Also using M06 functional the
optimized O-H bond lengths are 1.002, 1.001, 1.005, 1.003, 1.004 at 6-31G(d) level and 0.997,
.996, 1.000, 0.998, 0.998 at 6-311++G(d,p) level. The increased O-H bond length is due to the
1
0
hydrogen bonding between N21 and H atom. While the hydrogen bonding between N21···H34,
9

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N21···H34, N21···H36, N21···H36 and N21···H38 for 8a-8e chromophores are 1.732, 1.740,
1
3
.711, 1.723, 1.716 at B3LYP/6-31G(d) and 1.729, 1.736, 1.700, 1.717, 1.712 at B3LYP/6-
11++G(d,p) levels of calculations. Similarly with CAM-B3LYP/6-31G(d) level the N···H bond
lengths are 1.734, 1.743, 1.709, 1.723, 1.706 and at CAM-B3LYP/6-311++G(d,p) level the
optimized N···H bond lengths are 1.729, 1.738, 1.696, 1.714, 1.695. While the optimized N···H
bond lengths for the azo chromophores using M06 functional are 1.775, 1.777, 1.758, 1.767,
1
.759 at 6-31G(d) level and 1.773, 1.775, 1.749, 1.759, 1.754 at 6-311++G(d,p) level.
Hydrazone form of the featuring chromophores have optimized geometry with interatomic
distance for O33···H34, O33···H34, O35···H36, O35···H36, O37···H38 at B3LYP/6-31G(d) and
B3LYP/6-311++G(d,p) levels of calculations are 1.904, 1.907, 1.897, 1.896, 1.873 Å and 1.923,
1
3
1
.924, 1.912, 1.913, 1.888 Å. As well as at CAM-B3LYP/6-31G(d) and CAM-B3LYP/6-
11++G(d,p) levels the optimized O···H bond lengths for hydrazone from are 1.895, 1.897,
.888, 1.888, 1.868 and 1.913, 1.913, 1.903, 1.904, 1.882 respectively. Similarly using M06
functional the optimized O···H bond lengths are as follows 1.907, 1.909, 1.902, 1.922, 1.888 at
B3LYP/6-31G(d) level and 1.916, 1.919, 1.907, 1.928, 1.896 B3LYP/6-311++G(d,p) level. The
N-H bond lengths between N21-H34, N21-H34, N21-H36, N21-H36 and N21-H38 for the series
8
a-8e at both the levels of calculations are as follows 1.024, 1.024, 1.025, 1.024, 1.026 Å and
1
.023, 1.024, 1.023, 1.023, 1.025 Å. Subsequently the optimized N-H bond lengths for
hydrazone form using CAM-B3LYP functional at both the levels of computations are 1.022,
.022, 1.022, 1.022, 1.023 and 1.020, 1.020, 1.021, 1.021, 1.022 respectively. While using M06
1
functional the respective N-H bond lengths are 1.026, 1.026, 1.026, 1.025, 1.027 at 6-31G(d)
level and 1.025, 1.024, 1.025, 1.024, 1.026 at 6-311++G(d,p) level. The N-H bond lengths
1
0

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observed are slightly longer than the actual N-H bond length of 1.01 Å which is due to O···H
hydrogen bonding.
The N20-N21 bond lengths for azo forms are 1.294, 1.292, 1.301, 1.297, 1.307 Å and 1.289,
1
.287, 1.297, 1.292, 1.303 Å at B3LYP/6-31G(d) and B3LYP/6-311++G(d,p) levels of
calculations. While using CAM-B3LYP/6-31G(d) and CAM-B3LYP/6-311++G(d,p) levels of
calculations the N-N bond lengths in azo forms are 1.278, 1.276, 1.285, 1.281, 1.293 and 1.272,
1
.270, 1.280, 1.276, 1.290 Å respectively. At M06/6-31G(d) and M06/6-311++G(d,p) levels of
calculations the N-N bond lengths in azo forms are of 1.285, 1.283, 1.292, 1.288, 1.299 and
.279, 1.277, 1.287, 1.282, 1.295 Å. These optimized N-N bond lengths are longer than the usual
value 1.24 Å. While in hydrazone form N20-N21 bond lengths are 1.329, 1.327, 1.386, 1.333,
.342 and 1.327, 1.325, 1.386, 1.331, 1.341 at B3LYP /6-31G(d) and B3LYP/6-311++G(d,p)
1
1
levels of calculations. Similarly the N-N bond lengths at CAM-B3LYP functional in hydrazone
form are 1.323, 1.321, .330, 1.327, 1.338 at 6-31G(d) level and 1.321, 1.320, 1.329, 1.326, 1.337
at 6-311++G(d,p) level . Also at M06/6-31G(d) and M06/6-311++G(d,p) levels of calculations
the N-N bond lengths in hydrazone forms are 1.323, 1.320, 1.330, 1.327, 1.337 and 1.319,
1
.318, 1.327, 1.324, 1.335. These obtained N-N bond lengths in hydrazone form falls shorter to
the model value of 1.40 Å for single bond [27]. The N20-N21 bond length values obtained for
azo and hydrazone form signify the single-double bond character of the bond and confirms the
azo-hydrazone tautomerism in the synthesized molecules.
Insert Figure 2.
1
1

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Insert Figure 3.
Insert Table 5.
3
.3.2. Calculated energies of azo-hydrazone tautomeric forms
Calculated energies (E/hartree), Gibbs free energies (∆G/hartree) and relative energies (∆E/kJ
-1
mol ) of the chromophores 8a-8e in their azo and hydrazone tautomeric forms at both the levels
of calculation so as to check the more stable form and the values are tabulated in (Table 6-7,
supporting info. Table 16-19). From the Table 6 it is clear that the hydrazone form is relatively
more stable than the corresponding azo form by 50.40, 49.06, 53.27, 50.54, 46.77 and 44.98,
-1
4
3.80, 47.86, 45.02, 40.80 kJ mol at B3LYP /6-31G(d) and B3LYP/6-311++G(d,p) levels in
-1
gas phase. The Calculated energies (E/(∆G/hartree) and relative energies (∆E/kJ mol ) in DMF
as a solvent also shows that the hydrazone form is having more stability than azo form by 50.40,
-1
4
9.00, 53.89, 50.66, 47.70, and 44.79, 43.49, 48.62, 45.01, 42.02 kJ mol at B3LYP /6-31G(d)
and B3LYP/6-311++G(d,p) levels of calculation respectively (Table 7). At CAM-B3LYP /6-
-1
3
1G(d) the hydrazone form have stability by 50.73, 50.80, 56.43, 53.06, 50.42 kJ/mol and at
CAM- B3LYP/6-311++G(d,p) levels of calculations hydrazone form is more stable than azo
-1
form by 47.30, 45.64, 51.18, 47.73, 44.90 kJ/mol in gas phase (Supporting info. Table 16).
Similarly in DMF hydrazone form have more stability than azo form by 59.58, 57.97, 63.03,
-1
-
5
9.65, 56.11 kJ/mol at CAM-B3LYP /6-31G(d) and 56.70, 55.22, 59.90, 56.42, 52.28 kJ/mol
1
at CAM- B3LYP/6-311++G(d,p) level (Supporting info. Table 17). The energy data obtained by
calculation carried our using M06 functional shows that the hydrazone form of the dyes are
1
2

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-1
energetically more stable then the azo form by 50.83, 50.24, 55.13, 52.09, 47.80 kJ/mol at
-1
M06/6-31G(d) level and 44.79, 43.49, 48.63, 45.02, 42.02 kJ/mol at M06/6-311++G(d,p) level
of calculation in gas phase (SI Table 18). Similar trend is observed in DMF using M06/6-31G(d)
and M06/6-311++G(d,p) levels of calculations and the stability of hydrazone form over the azo
-1
form is by 58.45, 56.99, 60.73, 58.30, 52.15 and 54.97, 53.81, 57.31, 54.76, 48.44 kJ/mol
respectively(SI Table 19).
Insert Table 6.
Insert Table 7.
3
.3.3. Electronic vertical excitation spectra
The calculated excitation spectra using B3LYP, CAM-B3LYP and M06 functional at 6-31G(d) and
-311++G(d,p) levels in gas phase as well as in DMF, their oscillator strength (f), orbital
6
contribution and band gap are shown in (Supporting info. Tables 20-31) with their experimental
excitation spectra. The chromophore 8a has absorption at 371 nm in DMF, its calculated vertical
excitation at 6-31G(d,) level is 434, 400, 428 nm for azo tautomer and 390, 342, 378 nm for
hydrazon tautomer in DMF using B3LYP, CAM-B3LYP and M06 functional respectively, while
at B3LYP/6-311++G(d,p) level 445, 409, 438 nm for azo and 404, 351, 387 nm for hydrazone
tautomer. In gas phase vertical excitation is 410, 378, 404 nm and 380, 333, 368 nm at 6-31G(d)
level of calculation; for 6-311++G(d,p) level 418, 84, 412 nm and 387, 340, 376nm excitation
was resulted for azo and hydrazone tautomers respectively. The experimental λmax and computed
vertical excitation values at both the levels of calculation for azo as well as hydrazone tautomeric
1
3

ACCEPTED MANUSCRIPT
form gives conformation of the existence of compound 8a in hydrazone form in gas phase and in
DMF. Shoulder peak at 299 nm attributed to HOMO-1 to LUMO transition, also vertical
excitation spectra of optimized hydrazone form in DMF have shoulder peak at 288, 253, 266 nm
for 6-31G(d) and 293, 257, 269 nm for 6-311++G(d,p) levels of calculations. Such shoulder peak
is not observed in optimized azo form. The computed vertical excitation values for chromophore
8
4
6
b at 6-31G(d) is 429,395, 422 nm for azo form and 391, 344, 379 nm for hydrazone form, also
39, 404, 431 nm for azo and 397, 350, 384 for hydrazone form vertical excitation is observed at
-311++G(d,p) level. Experimental absorbance is found to be 406 nm with shoulder peak at 304
nm. The shoulder peak observed at 286, 251, 263 nm and 290, 256, 266 nm only for hydrazone
form at both the levels of optimisation resembles the 304 nm experimental value suggest existing
hydrazone form for dye 8b. While in gas phase 404, 373, 398 nm and 411, 380, 405 nm for azo
form are the closest values with the actual values, but shoulder peak was not observed for both
azo as well as hydrazone form. Dye 8c also concludes their existence in hydrazone tautomeric
form. The shoulder peak observed at 299 nm has not seen in computed excitation pattern in
DMF at both the levels of calculation using B3LYP functional, while it has been observed in
DMF using M06 functional at 291 and 297 nm for hydrazone form. The shoulder peak observed
in gas phase computation at 293, 263, 278 nm and 297, 254, 282 nm corresponding to HOMO-
1
→LUMO excitation. Dye 8d shows absorption with 281 nm shoulder peak and other two peaks
at 344 nm and 425 nm. Computed vertical excitation pattern for dye 8d shows shoulder peak at
91, 254, 277 nm and 294, 258, 280 nm in DMF at 6-31G(d) and 6-311++G(d,p) levels of
2
calculations, while 283, 249, 271nm in gas phase at 6-31G(d) level. Vertical excitation of the dye
optimized at 6-31G(d) and 6-311++G(d,p) levels using B3LYP, CAM-B3LYP and M06
1
4

ACCEPTED MANUSCRIPT
functional are 428, 397, 421 and 438, 407, 431 nm in DMF, while 402, 374, 396 and 456, 381,
4
01 nm in gas phase for azo form corresponding to the experimentally observed peak at 425 nm.
The hydrazone form has 375, 334, 363and 383, 340, 371 nm in DMF. While vertical excitation
observed in gas phase are 363, 324, 351 and 368, 329, 357 nm respectively corresponding to the
actual peak observed at 344 nm. All these values obtained by experimental and computation
method indicates existence of both azo and hydrazone forms for dye 8d. Dye 8e has absorbance
at 535 nm and shoulder peak is observed at 268 nm, In DMF computed vertical excitation values
for optimized azo form show resemblance with the experimental values for B3LYP and M06
functional i.e. at 498, 479 and 517, 492 nm but in gas phase a large difference is observed in
actual and computed values.
3
.4. Thermal stability of dyes
Thermal stabilities of the synthesized azo dyes were investigated by thermogravimetric analysis
TGA). The TGA data obtained are summarized in (Table 1). The overlay TGA analysis curves
for the azo dye 8a-8e under nitrogen atmosphere (Figure 4) showed no weight loss up to 100-
50 °C, indicating absence of water molecules and/or any other adsorptive solvent molecules.
(
1
The TGA curves shown in the (Figure 4) indicate that these dyes are having good thermal
stability and a steep weight loss observed from 255 °C. Dye 8b with p-fluoro substituent in
phenyl ring is having good thermal stability (316 °C) amongst other synthesized dyes.
Insert Figure 4
1
5

ACCEPTED MANUSCRIPT
4
. Conclusions
In summary, we have developed an efficient and simple protocol for the synthesis of novel azo
dyes from 2-morpholin-4-yl-1,3-thiazol-4(5H)-one. Synthesized dyes were confirmed by FT-IR,
1
H NMR and mass spectral analysis. The TGA results show that they are having good thermal
stability. The optimized structure of azo as well as hydrazone form at B3LYP/6-31G(d) and
B3LYP/6-311++G(d,p) levels of calculations and their calculated energies (E/(∆G/hartree),
-1
relative energies (∆E/kJ mol ) shows that the hydrazone form is having more stability than azo
form. The experimental and computed vertical excitation spectrum clearly indicates existence of
dye 8a, 8b and 8c in hydrazone tautomeric form, while 8d exist in both azo as well as hydrazone
form.
Acknowledgements
The author Prashant G. Umape is thankful to UGC for financial support.
1
6

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List of Tables:
Table 1. Experimental data for synthesis of dyes 8a-8e
1
Table 2. FT-IR, Mass, H NMR data for dyes 8a-8e
Table 3. Photo-physical properties of dyes 8a-8e
Table 4. Ground state optimized structures of dyes 8a-8e in their azo and hydrazone tautomeric
forms.
Table 5. Computed interatomic distances of azo-hydrazone tautomers of dye 8a in Å.
Table 6. Computed energy in gas phase for dyes 8a-8e using B3LYP functional.
Table 7. Computed energy in DMF for dyes 8a-8e using B3LYP functional.
1

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Table 1
Experimental data for synthesis of dyes 8a-8e
Thermal
stability
Yield
%
M.P.
Dye
R
R’
R’’
°
( C)
°
C
8
a
Cl
F
H
H
H
H
H
79
26
70
77
45
278-80
>300
267
316
254
266
264
8
b
8
c
NO
2
2
H
276-78
276-78
270-72
8
d
H
H
NO
H
2
8
e
NO
NO
2
2

ACCEPTED MANUSCRIPT
Table 2
1
FT-IR, Mass, H NMR data for dyes 8a-8e
1
Dye
FT-IR
Mass
m/z
H-NMR
υ max
(ppm, DMSO-d6)
-
1
M+1
(
cm , KBr)
1541(N=N),
666 (C=O)
8
a
325.3 10.5 (1H, S, NH), 7.36 (2H, d, ArH, 8.8 Hz), 7.24 (2H, d, ArH,
8.8 Hz), 3.9 (2H, t, CH, 4.4,5.1 Hz), 3.7 (4H, m, CH, 4.8, 4.4
Hz), 3.6 (2H, m, CH)
1
8
b
1558 (N=N),
682 (C=O)
309.4 11.00 (1H, S, NH), 8.21 (2H,d, ArH, 9.5 Hz), 7.35 (2H, d,
ArH, 9.2 Hz), 3.92 (2H, t, CH, 4.0,5.1 Hz), 3.75 (4H, m, CH),
1
3
.62 (2H, t, CH, 5.1,4.0 Hz)
8
c
1560 (N=N),
649 (C=O)
1556 (N=N),
677 (C=O)
336.4 10.4 (1H, s, NH), 7.2 (4H, m, ArH), 3.9 (2H, t, CH, 4.4,5.1
Hz), 3.7 (4H, m, CH), 3.5 (2H, t, CH, 4.8,4.0 Hz)
1
8
d
336.4 14 (1H, s, OH), 10.4(1H, s, NH), 8.17 (1H, dd, ArH, 7.7 Hz),
7.77-7.79 (2H, m, ArH), 7.14 (1H, m, ArH), 3.94 (2H,t, CH,
1
4
.4, 4.8 Hz), 3.75 (4H, m, CH), 3.63 (2H, t, CH, 5.1,4.0 Hz)
8
e
1571 (N=N),
691 (C=O)
381.3 10.74 (1H, S, NH), 8.85 (1H, d, ArH, 1.5 Hz), 8.53 (1H, d,
ArH, 9.5 Hz), 7.93 (1H, d, ArH, 9.1 Hz), 3.95 (2H, t, CH,4.4,
1
5
.1 Hz), 3.74 (4H, m, CH), 3.67 (2H, t, CH, 5.5 Hz)
3

ACCEPTED MANUSCRIPT
Table 3
Photo-physical properties of dyes 8a-8e
Dye
λmax
nm)
Molar Extinction
co-efficient
(
3
-1
-1
(dm mol cm )
8
a
371(299)
22305(8636.)
8
b
406, 565 (304)
368(299)
37723, 14338 (9138)
28328 (13278)
8
c
8
d
344, 425(281)
535(268)
12742, 14348 (9799)
54981 (11597)
8
e
4

ACCEPTED MANUSCRIPT
Table 4
Ground state optimized structures of dyes 8a-8e in their azo and hydrazone tautomeric forms.
Comp Azo Hydrazone
8
a
8
b
8
c
8
d
8
e
5

ACCEPTED MANUSCRIPT
Table 5
Computed interatomic distances of azo-hydrazone tautomers of dye 8a in Å
B3LYP/6-31G(d)
B3LYP/6-311++G(d,p)
Interatomic
distance
Azo Hydrazone
Azo
Hydrazone
1.308
1.796
1.781
1.494
1.374
1.288
1.327
1.397
1.400
1.760
1.229
1.923
1.023
N1-C2
C2-S3
1.323
1.776
1.775
1.410
1.348
1.341
1.294
1.404
1.407
1.309
1.801
1.783
1.494
1.378
1.292
1.329
1.395
1.403
1.760
1.234
1.904
1.024
1.322
1.772
1.773
1.407
1.345
1.339
1.289
1.405
1.405
1.759
1.318
1.006
1.729
S3-C4
C4-C5
C5-N1
C4-N20
N20-N21
N21-C22
C27-C22
C25-Cl32 1.759
C5-O33
1.320
O33-H34 1.011
N21-H34
1.732
6

ACCEPTED MANUSCRIPT
Table 6
Computed energy in gas phase for dyes 8a-8e using B3LYP functional.
Dye
Azo
Hydrazone
a
b
c
a
b
c
6
-31 G(d)
E/Hartree
∆E/kJ
∆G/Hartree E/Hartree ∆E/kJ ∆G/Hartree
-
1
-1
mol
mol
8
a
-1731.00
-1370.64
-1475.91
-1475.91
-1680.39
50.40
49.06
53.27
50.54
46.77
-1730.79
-1370.43
-1475.69
-1475.69
-1680.18
-1731.02
-1370.66
-1475.93
-1475.93
-1680.41
0
0
0
0
0
-1730.81
-1370.45
-1475.71
-1475.71
-1680.19
8
b
8
c
8
d
8
e
6
-311
+
+G(d,p)
8
a
-1731.29
-1370.94
-1476.24
-1476.23
-1680.79
44.98
43.80
47.86
45.02
40.80
-1731.09
-1370.73
-1476.02
-1476.02
-1680.57
-1731.31
-1370.96
-1476.26
-1476.25
-1680.80
0
0
0
0
0
-1731.10
-1370.75
-1476.04
-1476.04
-1680.59
8
b
8
c
8
d
8
e
1
Hartree = 2626 kJ/mol
a
b
c
Calculated energies, Gibbs free energies, relative energies.
7

ACCEPTED MANUSCRIPT
Table 7
Computed energy in DMF for dyes 8a-8e using B3LYP functional.
Dye
Azo
Hydrazone
a
b
c
a
b
c
6
-31G(d)
E/Hartree ∆E/kJ ∆G/Hartree E/Hartree ∆E/kJ ∆G/Hartree
-1
-1
mol
50.4
mol
0
8
a
-1731.00
-1730.79
-1370.43
-1475.69
-1476.00
-1680.18
-1731.02
-1370.66
-1475.93
-1475.93
-1680.41
-1730.81
-1370.45
-1475.71
-1475.71
-1680.19
8
b
-1370.64 49.00
-1475.91 53.89
-1475.69 50.66
-1680.39 47.70
0
0
0
0
8
c
8
d
8
e
6
-311
+
+G(d,p)
8
a
-1731.29 44.79
-1370.94 43.49
-1476.24 48.62
-1476.23 45.01
-1680.78 42.02
-1731.09
-1370.73
-1476.02
-1476.02
-1680.57
-1731.31
-1370.96
-1476.25
-1476.25
-1680.80
0
0
0
0
0
-1731.1
-1370.75
-1476.04
-1476.04
-1680.59
8
b
8
c
8
d
8
e
1
Hartree = 2626 kJ/mol
a
b
c
Calculated energies, Gibbs free energies, relative energies.
8