Synthesis and Fluorescence Properties of Novel indol-3yl-thiazolo[3,2-a][1,3,5]triazines and indole-3-carbaldehyde Schiff Bases

Article abstract of DOI:10.1007/s10895-015-1659-1

Novel photoactive 4-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-substituted phenyl-2H-thiazolo[3,2-a][1,3,5]triazines were synthesized by the conjugate addition of ammonia to the indole-3-carbaldehyde Schiff bases followed by the condensation with 4-chlorobenzaldehyde. All the synthesized compounds were characterized by FT-IR, NMR, mass spectra and elemental analyses. Their antioxidant property, electrochemical and photophysical properties in different organic solvents were investigated. Comparative discussion on the photophysical properties of indole-3-carbaldehyde Schiff bases and 4-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-substituted phenyl-2H-thiazolo[3,2-a][1,3,5]triazines has been described. The fluorescence quantum yield of Schiff bases (Φ_f = 0.66-0.70 in DMSO) found to be interestingly higher. High fluorescence quantum yield, large molar extinction coefficient, high stokes shift and smaller optical band gap positioning these new derivatives as an efficient metal free organic fluorescent and semiconductor material.

Full text of DOI:10.1007/s10895-015-1659-1

J Fluoresc
DOI 10.1007/s10895-015-1659-1
ORIGINAL ARTICLE
Synthesis and Fluorescence Properties of Novel
indol-3yl-thiazolo[3,2-a][1,3,5]triazines
and indole-3-carbaldehyde Schiff Bases
1
1
T. V. Sravanthi & S. L. Manju
Received: 5 May 2015 /Accepted: 14 September 2015
Springer Science+Business Media New York 2015
#
Abstract Novel photoactive 4-(4-chlorophenyl)-2-(1H-indol-
-yl)-6-substituted phenyl-2H-thiazolo[3,2-a][1,3,5]triazines
efficiency of converting solar energy into electrical energy.
Recent reports focused on metal-free organic dye based
DSSC’s because of their strong molar absorption co-efficient,
intermolecular interactions, low material cost, lightweight and
reduced toxicity [1]. This prompted organic chemists to de-
sign and develop photosensitizers with promising chromo-
phores and substituents to meet the performances of DSSC’s.
Fluorescent materials also appear to be promising sensors for
chemical sensing [2]. Thus, the interest on the fluorescent
materials has steadily increased in recent years. The versatile
structure, Stokes shifts, high molar extinction co-efficient and
the LUMO-HUMO energy levels of metal-free organic mole-
cules have been explored to study the performances of
DSSC’s [1, 3, 4]. Heterocyclic dyes such as coumarin dyes,
thiophene-based dyes, indoline dyes, heteropolycyclic dyes,
xanthene dyes, porphyrin dyes, merocyanine dyes, squaraine
dyes, cyanine dyes and phthalocyanine dyes have been report-
ed for DSSC’s performances [5]. Also the materials with
strong fluorescence, high photostability and molar extinction
co-efficient have been used in industries as paints, inks and
plastics [6]. In addition, materials with antioxidative property
influence their photoconductance [7]. On the other hand, N-
heterocycles also have been used as conducting materials,
electrochemical sensors and photonics [8–12]. Their behavior
has been determined by physiochemical characteristic studies
such as solvatochromic, piezochromic, oscillator strength, di-
pole moment, fluorescent quantum yield and photostability.
The photostability and polarity studies of metal free Schiff
base dyes were reported by Marwani et al. [13].
3
were synthesized by the conjugate addition of ammonia to the
indole-3-carbaldehyde Schiff bases followed by the condensa-
tion with 4-chlorobenzaldehyde. All the synthesized compounds
were characterized by FT-IR, NMR, mass spectra and elemental
analyses. Their antioxidant property, electrochemical and
photophysical properties in different organic solvents were inves-
tigated. Comparative discussion on the photophysical properties
of indole-3-carbaldehyde Schiff bases and 4-(4-chlorophenyl)-
2-(1H-indol-3-yl)-6-substituted phenyl-2H-thiazolo[3,2-a][1,3,
5]triazines has been described. The fluorescence quantum yield
of Schiff bases (Φ =0.66−0.70 in DMSO) found to be interest-
f
ingly higher. High fluorescence quantum yield, large molar ex-
tinction coefficient, high stokes shift and smaller optical band
gap positioning these new derivatives as an efficient metal free
organic fluorescent and semiconductor material.
.
Keywords Indole-3-carbaldehyde Schiff bases
.
.
Indol-3-yl-thiazolo-s-trazines Fluorescence spectroscopy
.
.
Physiochemical parameters Antioxidant property Cyclic
voltammetry
Introduction
Over the past two decades, dye-sensitized solar cells (DSSC’s)
have gained much considerable interest owing to their high
Recently, the thiazolo[5,4-d]thiazoles containing both elec-
tron donor N and S atoms and electron acceptor aromatic rings
have been receiving much attention in the field of electro-
chemistry because of their electronic, spectroscopic and dying
properties [14]. Annulated thiazolo[3, 2-a][1, 3, 5] triazines
were first synthesized by Rathke in 1887, and their
*
S. L. Manju
slmanju@vit.ac.in
1
Organic Chemistry Division, School of Advanced Sciences, VIT
University, Vellore 632014, Tamilnadu, India

J Fluoresc
Table 1 Reaction time, yield and melting point of 2-amino-4-substituted phenyl-1,3-thiazoles (1b–7b)
Compound
R
R′
X
Color and appearance
Reaction
Yield (%)
Melting point(°C)
GC-MS (Daltons)
time (min)
Found
Lit.
1b
2b
3b
4b
5b
6b
7b
H
H
H
White solid
20
22
20
20
25
35
35
100
96
152
162
132
204
198
134
82
152–153 [19]
164–165 [19]
126–127 [19]
206–207 [19]
–
176.24
210.13
190.19
207.17
237.24
193.2
Cl
H
H
White solid
CH
OCH
OCH
H
3
H
H
Golden yellow solid
White crystalline solid
Pale yellow solid
Yellow crystalline solid
Off white solid
96
3
3
H
H
100
72
OCH
3
H
H
OH
H
82
–
F
H
95
102–103 [19]
194.1
bioactivities have been widely discussed. Beyond their bio-
logical properties, various substituted thiazolo[3, 2-a][1, 3,
NMR spectrometer. Chemical shifts in NMR spectra were
reported in parts per million (ppm) from trimethylsilane
(TMS) using as an internal standard and CDCl /DMSO-d
5
]triazines were found to have wide applications in industries
3
6
as dye stuffs, optical bleaches, explosives, in agriculture as
pesticides and also to activate the surface of the materials
as a solvent. Splitting patterns were designated as follows: s:
singlet, d: doublet, t: triplet, q: quartet and m: multiplet. The
reactions were monitored by TLC on silica gel protected
on aluminium sheets and the compound spots were de-
tected by exposure to UV-lamp at 256 nm. The purifi-
cation of the compounds was done by silica gel column
chromatography using petroleum ether and ethyl acetate.
UV–Vis electronic absorption spectra were recorded on
a JASCO V-670 spectrophotometer, by using 2 mL rect-
angular quartz cell. Steady state fluorescence spectra
were measured using Hitachi F7000 spectrophotometer
using rectangular quartz cell. All absorption and fluores-
cence spectra were recorded after subtracting the blank
in data analysis. The cyclic voltammograms were re-
corded in the CH electrochemical work station instru-
ment of model CHI6003D.
[15, 16]. During the past several years, synthesis, UVabsorp-
tion and fluorescence studies of various substituted N-triazinyl
derivatives were investigated [17]. As part our research
interest to develop functional organic materials, in this
paper, we have reported the three component one pot
reaction of indole-3-carbaldehyde Schiff bases, ammoni-
um acetate and p-chloro benzaldehyde to obtain hitherto
unreported 4-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-
substituted phenyl-2H-thiazolo[3,2-a][1,3,5]triazines.
Spectral characterization, fluorescence, electrochemical
and antioxidant studies of Schiff bases and indol-3-yl-
thiazolo-s-trazines were performed. The photochemical
parameters, optical band gap and fluorescence quantum
yield were calculated. We found that compounds (1c–
7
c) have excellent antioxidant property and moderate to
high photoconductivity in organic solvents.
Synthesis of 2-amino-4-substituted phenyl-1,3-thiazoles
(1b-7b)
Experimental
α-Bromoarylethanones (deliquescent) were prepared accord-
ing to the reported method [18]. The ethanolic mixture of α-
bromoarylethanone (1a, 1 mmol) and thiourea in the boiling
tube was ultrasonicated at 45 °C in an ultrasonic bath. The
reaction was monitored by TLC at every 5 min. It was found
that the heterocyclization was completed within 20 min. The
same procedure was followed for the synthesis of 2-amino-4-
substituted phenyl-1,3-thiazoles listed in Table 1.
Melting points were determined in open capillary tube method
and were uncorrected. Mass spectra were recorded on GC-MS
spectrophotometer of GC model Clarus 680 and MS model
Clarus 600(EI) using helium as a carrier gas with qualitative
results. The infrared spectra were recorded on a Perkin-Elmer
1
FT-IR spectrophotometer using the KBr pellet technique. H
1
3
&
C NMR was recorded on a Bruker 400 and 100 MHz FT-
Scheme 1 Preparation of α-
bromoarylethanones and 2-
amino-4-substituted phenyl-1,3-
thiazoles (1b–7b)

J Fluoresc
Scheme 2 Preparation of
thiazole Schiff bases (1c–7c) and
indol-3-yl-thiazolo-s-triazines
(1d–7d)
1
3
Synthesis of the indole-3-carbaldehyde Schiff base dyes
1c–7c)
C NMR(DMSO-d ,100 MHz, δ ppm): 112.38, 118.14,
6
(
120.79, 122.08, 123.42, 123.78, 124.09, 126.24, 129.53,
37.02, 138.41, 141.26, 149.90, 197.13 (C-Cl); GC-MS:
1
+
An ethanolic mixture of indole-3-carbaldehyde (1 mmol) and
corresponding 2-amino-4-substituted phenyl-1,3-thiazoles
339.39 (M +2 peak).
N-((1H-indol-3-yl)methylene)-4-(p-tolyl)thiazol-2-amine
(3c): C H N S: Pale yellow solid; Yield: 90 %; Melting
point: >200 °C; FT-IR(υ in cm ): 3213.41 (N-H), 1666.50
(C=N); H NMR(DMSO-d , 400 MHz, δ ppm): 2.061 (s, 3H,
-CH3), 6.734–7.201 (m, 9H, C5′-H, Ar-H), 7.597 (m, 2H, Ar-
H), 8.008 (s, 1H, Indole C2-H), 9.750 (s, 1H, imino-H),
11.043 (s, 1H, N-H); C NMR(DMSO-d , 100 MHz, δ
(
1b–7b, 1 mmol) was boiled on the water bath. The reaction
1
9 15 3
−1
progress was monitored by TLC, after completion the reaction
mixture was allowed to cool. On cooling, yellow solid Schiff
bases separated out, which was then filtered, washed with cold
ethanol and dried.
1
6
1
3
N-((1H-indol-3-yl)methylene)-4-phenylthiazol-2-amine
6
(
2
(
1c): C H N S: Yellow solid; Yield: 98 %; Melting point:
ppm): 20.55, 111.45, 118.27, 120.90, 121.83, 123.12,
123.89, 127.67, 128.40, 128.61, 128.77, 130.19, 136.03,
136.63, 161.31; GC-MS: 318.04 (M +1 peak).
1
8 13 3
−
1
02–204 °C; FT-IR(υ in cm ): 3207.62 (-N-H), 1728.22
1
+
C=N); H NMR(DMSO-d , 400 MHz, δ ppm): 6.838 (s,
6
1
H, Indole C -H), 6.902 (s, 1H, C -H), 7.086–7.123 (m, 4H,
N - ( ( 1 H - i n d o l - 3 - y l ) m e t h y l e n e ) - 4 - ( 4 -
methoxyphenyl)thiazol-2-amine (4c): C H N OS: Dirty
2
5
Ar-H), 7.172–7.208 (t, 1H, J=7.599Hz, Ar-H), 7.284–7.302
m, 4H, Ar-H), 7.383 (s, 1H, imino-H), 11.02 (s, 1H, -
1
9 15 3
(
white solid; Yield: 86 %; Melting point: 120–124 °C; FT-
1
3
−1
1
NH); C NMR(DMSO-d , 100 MHz, δ ppm): 114.58,
IR(υ in cm ): 2929.87 (N-H), 1672.28 (C=N); H
6
1
1
1
18.88, 121.47, 124.70, 125.99, 127.70, 127.99, 131.38,
33.53, 135.15, 138.05, 139.34, 140.42, 143.00, 145.15,
65.87; LC-MS: 305 (M+2Hpeak).
NMR(DMSO-d , 400 MHz, δ ppm): 3.951 (s,3H, -OCH3),
6
7.209–7.290 (m, 5H, C5′-H, Ar-H), 7.512–7.532 (d, 2H, J=
7.84Hz, Ar-H), 8.093 (s, 1H, Indole C2-H), 8.112–8.126 (d,
2H, J=6.10Hz, Ar-H), 9.946 (s, 1H, imino-H), 12.147 (s, 1H,
N-((1H-indol-3-yl)methylene)-4-(4-chlorophenyl)thiazol-
-amine (2c): C H N SCl : Yellow solid; Yield: 94 %; Melt-
1
3
2
N-H); C NMR(DMSO-d , 100 MHz, δ ppm): 56.71
1
8
12
3
6
−1
ing point: 206–208 °C; FT-IR(υ in cm ): 3232.70 (N-H),
1
7
(OCH3), 110.81, 112.24, 112.39, 118.14, 120.79, 122.09,
123.43, 124.09, 129.91, 130.91, 132.88, 137.02, 138.41,
159.05, 195.53 (Ar C-OCH3); GC-MS: 335.04 (M+2 peak).
N - ( ( 1 H - i n d o l - 3 - y l ) m e t h y l e n e ) - 4 - ( 3 , 4 -
dimethoxyphenyl)thiazol-2-amine (5c): C H N O S: Pale
1
737.86 (C=N); H NMR(DMSO-d , 400 MHz, δ ppm):
6
.206–7.287 (m, 3H, Ar-H), 7.510–7.529 (d, 1H, J=9.11Hz,
Ar-H), 8.090–8.194 (m, 3H, Ar-H), 8.290–8.356 (t, 2H, J=
2.01Hz, Ar-H), 9.943 (s, 1H, imino-H), 12.145 (s, 1H, N-H);
1
2
0 17 3 2
Fig. 1 Graphical representation of antioxidant results of compounds (1c–7c & 1d–7d)

J Fluoresc
f
Table 2 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (1c) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvent
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
327.5
325.9
362.2
326.2
320.8
325.8
326
425.9
429
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
88.40
88.83
79.93
88.75
90.25
88.86
88.81
9000
9500
0.00012
0.00013
0.00014
0.00016
0.00014
0.00011
0.00008
0.09
0.09
0.10
0.10
0.10
0.09
0.07
7054.66
7374.24
4304.46
6925.62
4519.62
3303.92
2584.96
0.03
0.07
0.07
0.02
0.04
0.13
0.06
429.1
421.4
375.2
365.1
356
11,500
12,000
10,500
8500
DMSO
DMF
6000
1
3
yellow solid; Yield: 90 %; Melting point: 140–142 °C; FT-
imino-H), 9.948(s, 1H, N-H); C NMR(DMSO-d6,
100 MHz, δ ppm): 112.40, 114.30, 115.03, 118.13, 120.79,
122.11, 123.45, 124.08, 125.72, 127.45, 128.90, 137.01,
138.44, 171.14 (C-F); LC-MS: 322 (M+H peak).
IR(υ in cm− ): 3367.26 (N-H), 1624.06 (C=N);
1
1
H
NMR(DMSO-d , 400 MHz, δ ppm): 3.816 (s, 3H, -OCH3),
6
3
8
.844 (s, 3H, OCH3), 7.040–7.633 (m, 8H, C5-H, Ar-H),
.289 (s, 1H, C2-H), 8.295 (s, 1H, C2′-H), 9.947 (s, 1H, im-
1
3
ino-H), 12.143(s, 1H, N-H); C NMR(DMSO-d , 100 MHz,
6
δ ppm): 55.44 (-OCH3), 55.70 (-OCH3),110.13, 110.76,
Synthesis of 4-(4-chlorophenyl)-2-(1H-indol-3-yl)
-6-substituted phenyl-2H-thiazolo[3,2-a][1,3,5]triazines
(1d–7d)
112.39, 118.14, 120.79, 122.10, 123.08, 123.44, 124.09,
129.82, 137.02, 138.41, 148.49, 153.01, 183.81 (-OCH3),
184.97 (-OCH3).
2
-(2-(((1H-indol-3-yl)methylene)amino)thiazol-4-
To the ethanolic mixture of the appropriate Schiff bases (1c–
7c, 1 mmol) and p-chlorobenzaldehyde (1 mmol), ammonium
acetate (1 mmol) was added and allowed to reflux in an oil
bath at 90 °C. The reaction was monitored by TLC; it was
observed that the cyclization was achieved within 3 h. The
reaction mixture was poured into crushed ice, filtered, washed
with water and dried.
yl)phenol (6c): C H N OS: Yellowish orange solid; Yield:
0 %; Melting point: 162–164 °C; FT-IR(υ in cm ): 3307.92
O-H), 3167.12 (N-H), 1610.56 (C=N); H NMR(DMSO-d ,
6
00 MHz, δ ppm): 7.289–7.307 (m, 2H, Ar-H),7.532–7.549
t, 1H, J =6.83Hz, Ar-H), 7.749–7.789 (t, 1H, J=7.82Hz, Ar-
H), 8.191–8.224 (m, 2H, Ar-H), 8.433–8.452 (d, 1H, J=
1
8 13 3
−1
9
(
4
(
1
7
1
.81Hz, Ar-H), 8.790 (s, 1H, -OH), 9.250 (s, 1H, imino-H),
4-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-phenyl-2H-
thiazolo[3,2-a][1,3,5]triazine (1d): C H ClN S; Brown sol-
1
3
2.210 (s, 1H, -NH); C NMR (DMSO-d , 100 MHz, δ
6
25 19
4
1
ppm): 112.39, 112.46, 113.65, 114.22, 120.16, 122.43,
id; Yield: 62 %; Melting point: 116–118 °C;
H
1
1
24.54, 130.36, 131.96, 135.95, 137.48, 137.96, 148.37,
49.44, 159.35, 174.67 (C-OH); LC-MS: 319.07 (M ion
NMR(DMSO-d , δ ppm): 7.258–7.556 (m, 10H, Ar-H),
6
+
7.871(s, 1H, C -H), 7.991–8.009 (d, 3H, J=7.34 Hz, Ar-H),
6
peak).
N-((1H-indol-3-yl)methylene)-4-(4-fluorophenyl)thiazol-
-amine (7c): C H FN S: Pale yellow solid; Yield: 87 %;
8.319 (s, 1H, N -H), 8.360–8.381 (m, 2H, Ar-H), 9.237 (1H,
5
1
3
C -H), 12.169 (1H, indole-NH). C MMR (DMSO-d ,
4
6
2
100 MHz, δ ppm): 101.45, 111.04, 112.42, 114.23, 120.79,
121.82, 121.96,122.11, 123.49, 124.56, 125.49, 125.84,
127.14, 127.72, 127.91, 128.42, 128.71, 134.43, 134.89,
137.46, 137.51, 138.44, 151.94, 158.82. GC-MS: 442.62
(M+ ion peak); Elemental analysis: Calculated: C, 68.10; H,
1
8
12
3
−1
Melting point: 98–102 °C; FT-IR(υ in cm ): 3140.11 (N-H),
1
1
7
8
627.92 (C=N); H NMR(DMSO-d , 400 MHz, δ ppm):
6
.077–7.247 (m, 9H, Ar-H), 7.526 (s, 1H, indole C -H),
2
.104 (s, 1H, C -H), 8.294 (s, 1H, Ar-H), 8.910 (s, 1H,
5
f
Table 3 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (2c) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvent
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
326.1
325.8
325.7
325.9
319.1
329.8
330.1
402.8
406.1
422.1
397.5
406.1
383.6
375.5
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
88.78
88.86
88.89
88.83
90.73
87.78
87.70
7500
10,500
7000
0.00010
0.00014
0.00009
0.00011
0.00008
0.00011
0.00017
0.08
0.10
0.08
0.09
0.07
0.09
0.11
5839.22
6069.20
7012.03
5527.03
6713.66
4252.59
3662.69
0.33
0.25
0.20
0.25
0.51
0.72
0.29
8500
6000
DMSO
8500
DMF
13,000

J Fluoresc
f
Table 4 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (4c) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvent
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
326.1
325.7
325.9
325.3
319.8
329.2
330
389.7
385.7
377.3
379
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
88.78
88.89
88.83
89.00
90.53
87.94
87.73
7500
8500
0.00010
0.00011
0.00013
0.00012
0.00007
0.00012
0.00012
0.08
0.09
0.10
0.09
0.07
0.09
0.09
5004.68
4776.21
4180.15
4355.63
5516.26
4150.60
3798.92
0.33
0.21
0.36
0.18
0.54
0.70
0.33
10,000
9000
388.3
381.3
377.3
5000
DMSO
9000
DMF
9000
3
8
.89; Cl, 8.04; N, 12.71; S, 7.27; Found: C, 67.80; H, 4.32; Cl,
.01, N, 12.65; S, 7.22.
, 6-bis(4-chlorophenyl)-2-(1H-indol-3-yl)-2H-thiazolo[3,
-a][1,3,5]triazine (2d): C H Cl N S; Yellow solid; Yield:
C H ClN OS; Off white solid; Yield: 32 %; Melting point:
26 19 4
96–98 °C; LC-MS: 474.87 (M+2H peak); Elemental Analy-
sis: Calculated: C, 66.30; H, 4.07; Cl, 7.53; N, 11.90; O, 3.40;
S, 6.81; Found: C, 66.04; H, 4.48; Cl, 7.50; N, 11.84; O, 3.38;
S, 6.76.
4
2
5
4
5
7
8
2
5
18
2 4
1
9 %; Melting point: 110–112 °C; H NMR (DMSO-d ,
6
00 MHz, δ ppm): 7.065 (s, 1H, C -H), 7.184–7.283 (m,
4-(4-chlorophenyl)-6-(3,4-dimethoxyphenyl)-2-(1H-indol-
3-yl)-2H-thiazolo[3,2-a][1,3,5]triazine (5d): C H ClN O S;
4
′
H, Ar-H), 7.396–7.417 (d, 2H, J=8.62Hz, Ar-H), 7.487–
.513(d, 2H, J=8.73Hz, Ar-H), 7.774–7.796 (d, 2H, J=
2
7
23
4 2
1
semisolid; Yield: 62 %; H NMR (DMSO-d , 400 MHz, δ
6
.908Hz, Ar-H), 7.915 (s, 1H, N -H), 7.987–8.085 (d, 2H,
ppm): 3.939(s, 6H, 2-OCH ), 6.905 (s, 1H, C -H), 7.254–
5
3
4′
J=7.508Hz, Ar-H), 8.266–8.301 (m, 2H, Ar-H), 9.918 (s,
7.601 (m, 10H, Ar-H), 7.849 (s, 2H, N -H & C -H), 8.252–
5 6
1
3
1
1
1
1
H, C -H). C NMR (DMSO-d , 100 MHz, δ ppm): 63.53,
8.264 (d, 2H, J=5.509Hz, Ar-H), 9.995 (s, 1H, C4-H), 11.276
4
6
1
3
02.35, 112.40, 118.13, 120.79, 122.10, 123.44, 124.08,
27.21, 128.47, 131.62, 133.40, 134.39, 137.02, 138.45,
47.96, 168.42. LC-MS: 474.3 (M-2H peak); Elemental anal-
(s, 1H, indole-NH). C NMR (DMSO-d , 100 MHz, δ ppm):
6
55.95, 56.04, 110.86, 112.08, 118.91, 121.52, 121.85, 122.11,
122.46, 123.02, 123.77, 124.03, 124.51, 124.73, 128.48,
129.13, 129.53, 131.15, 136.67, 137.26, 138.68, 184.94,
185.10. LC-MS: 503.0 (M+H peak).
ysis: Calculated: C, 63.16; H, 3.39; Cl, 14.92; N, 11.79; S,
.74; Found: C, 62.91; H, 3.80; Cl, 14.86; N, 11.74; S, 6.70.
-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-(p-tolyl)-2H-
thiazolo[3,2-a][1,3,5]triazine (3d): C H ClN S; Light
6
4
2-(4-(4-chlorophenyl)-2-(1H-indol-3-yl)-2H-thiazolo[3,2-
a][1,3,5]triazin-6-yl)phenol (6d): C H ClN OS; Yellow sol-
2
6
21
4
25 19
4
brown solid; Yield: 49 %; Melting point: 148–150 °C; Yield:
id; Yield: 58 %; Melting point: 92 °C (decomposes); Yield:
1
1
4
9 %; H NMR (DMSO-d , 400 MHz, δ ppm): 2.301 (s, 3H, -
54 %; H NMR (DMSO-d , 400 MHz, δ ppm): 6.835–7.597
6
6
CH ), 6.803–7.509 (m, 10H, Ar-H), 7.736 (s, N -H), 7.802–
(m, 14H, Ar-H & C -H), 7.946 (s, 1H, N -H), 8.225 (s, 1H,
C -H), 10.010 (s, 1H, indole-NH). C NMR (DMSO-d ,
4 6
100 MHz, δ ppm): 99.82, 117.08, 123.46, 126.15, 127.15,
3
5
4′
5
1
3
7
1
1
1
1
4
.851 (m, 2H, Ar-H), 8.266–8.333 (m, 2H, Ar-H), 9.187 (s,
1
3
H, C -H), 12.125 (s, 1H, indole-NH). C NMR (DMSO-d ,
4
6
00 MHz, δ ppm): 21.18, 112.08, 118.90, 121.53, 122.46,
23.75, 124.52, 128.30, 129.03, 129.24, 129.40, 130.82,
36.66, 137.26, 150.54, 161.96, 185.10. GC-MS:
58.64(M+2H peak).
128.48, 129.24, 133.17, 133.50, 135.25, 141.97, 142.11,
+
149.93, 152.66, 163.35, 189.34. LC-MS: 458.5 (M ion
peak).
4-(4-chlorophenyl)-6-(4-fluorophenyl)-2-(1H-indol-3-yl)-
2H-thiazolo[3,2-a][1,3,5]triazine (7d): C H ClFN S; Light
4
-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-(4-
2
5
16
4
1
methoxyphenyl)-2H-thiazolo[3,2-a][1,3,5]triazine (4d):
brown solid; Yield: 52 %; Melting point: 86–88 °C; H NMR
f
Table 5 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (5c) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvent
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
326.5
326.6
326.8
325.9
321.8
330
401.9
394.6
397.9
391.4
406.6
432.6
435.9
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
88.67
88.64
88.59
88.83
89.97
87.73
87.44
4000
2000
3000
5000
2500
5000
2000
0.00005
0.00003
0.00004
0.00007
0.00003
0.00007
0.00003
0.06
0.04
0.05
0.07
0.05
0.07
0.04
5746.06
5276.37
5467.81
5134.95
6481.01
7186.99
7261.31
I
I
I
I
I
abs>1
abs>1
abs>1
abs>1
abs>1
DMSO
0.68
DMF
331.1
Iabs>1

J Fluoresc
f
Table 6 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (6c) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvent
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
326.7
325.7
326
388.3
385.4
374.7
379
0.3054 88.62
0.1483 88.89
0.2171 88.81
0.1996 88.81
0.3085 90.13
0.2634 87.68
0.2744 87.68
7000
7000
8000
9000
3500
9000
8500
0.00009
0.00009
0.00011
0.00012
0.00005
0.00012
0.00011
0.08
0.08
0.09
0.09
0.06
0.09
0.09
4855.84
4756.03
3986.83
4289.62
5596.99
27918.82
6816.01
0.66
0.46
I
ems>1000
326
0.34
321.2
330.2
330.2
391.6
4226.8
426.1
0.36
0.66
0.57
DMSO
DMF
(
8
DMSO-d , 400 MHz, δ ppm): 7.747–8.394 (m, 15H, Ar-H),
ultrasonic bath at 45 °C, which resulted in better yield and
purity without the use of any base catalyst within 20–
35 min. Their physical data are represented in Table 1.
6
.730 (s, 1H, N -H), 10.489 (s, 1H, C -H), 11.946(s, 1H,
5
4
1
3
indole-NH). C NMR (DMSO-d , 100 MHz, δ ppm): 94.57,
6
1
1
1
06.84, 109.67, 109.88, 113.53, 116.18, 117.10, 118.25,
18.41, 119.21, 123.41, 123.96, 124.10, 124.60, 125.00,
25.58, 131.48, 131.99, 179.92. LC-MS: 460.73 (M+ ion
The (1H-indol-3-yl)methylene-4-substituted phenyl-
thiazol-2-amines (1c–7c) were prepared by the condensation
of indole-3-carbaldehyde with corresponding 2-amino-4-
substituted phenyl-1,3-thiazoles (1b–7b) in ethanol on
water bath within 30 min. Indol-3-yl-thiazolo[3,2-a][1,
3,5]triazines(1d–7d) were synthesized by the conjugate
addition of ammonium acetate with Schiff bases (1c–
peak); Elemental Analysis: Calculated: C, 65.43; H, 3.51;
Cl, 7.73; F, 4.14; N, 12.21; S, 6.99; Found: C, 65.35; H,
4
.03; Cl, 7.76; N, 12.26; S, 7.00.
7
c), followed by the condensation of adducts with p-
Results and Discussion
Synthesis
chlorobenzaldehyde as shown in Scheme 2. The better
yield of the compounds (35–62 %) was achieved by
using an oil bath at 90–110 °C using ethanol as the
solvent. Schiff bases bearing unsubstituted phenyl ring
and with electron withdrawing substituents (1c, 2c and
7c) at the phenyl ring were found to be more reactive,
when compared to other compounds.
Retrosynthetically, 2-amino-4-substituted phenyl-1,3-thia-
zoles was identified as one of the precursors to obtain novel
4
2
-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-substituted phenyl-
H-thiazolo[3,2-a][1,3,5]triazines. Preparation of α-
The structure of the resulting compounds was confirmed by
their spectral analyses. FT-IR spectra of the α-
bromoarylethanones (1a–7a) exhibit absorption band at
bromoarylethanones and 2-amino-4-substituted phenyl-1,3-
thiazole derivatives is shown in Scheme 1. To minimize the
use of the hazardous brominating agents, potassium
bromide-potassium bromate mixture was used for the
regioselective bromination of acetophenones [18]. And,
we found that the rate of bromination of acetophenone,
p-chloro, p-fluoro, p-methoxyand o-hydroxyacetophenones
were faster. 2-Amino-4-substituted phenyl-1,3-thiazoles (1b–
−
1
−1
1660–1703 cm for CO stretching, 2943–2951 cm for
−1
CH stretching of CH Br and 775–812 cm for CH wagging
of CH Br respectively. In H NMR spectra of α-
2
1
2
bromoarylethanones (1a–7a), the singlet peak at around
4.49–4.78 ppm corresponds to CH Br confirmed the success-
2
ful regioselective bromination of acetophenones using the
non-hazardous bromide-bromate mixture. Whereas, the disap-
pearance of the singlet peak at around4.49–4.78 ppm and the
7
b) were achieved by the subsequent treatment of appropriate
α-bromoarylethanones (1a–7a) with thiourea in ethanol under
f
Table 7 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (7c) in different solvents:
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvent
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
326
388.1
384.8
374.8
376.5
375.1
383
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
88.81
88.89
88.81
88.92
90.70
87.76
87.73
8000
7500
8500
20500
8000
8500
6000
0.00011
0.00010
0.00011
0.00027
0.00011
0.00011
0.00008
0.09
0.08
0.09
0.14
0.09
0.09
0.07
4908.29
4715.58
3993.95
4152.11
4668.76
4202.56
3114.34
0.24
0.22
0.32
0.06
0.05
0.66
0.53
325.7
326
325.6
319.2
329.9
330
DMSO
DMF
367.8

J Fluoresc
f
Table 8 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (1d) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvents
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
234.8
241.8
238
352.5
355.2
340.3
333.9
352.9
340.5
336.8
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
123.30
119.73
121.64
112.91
118.94
109.29
108.19
173,500
111,000
96,500
0.00319
0.00198
0.00175
0.00170
0.00264
0.00144
0.00156
0.41
0.32
0.30
0.30
0.37
0.28
0.30
14,220.64
13,203.34
12,630.97
9052.47
0.01
0.01
0.01
0.09
0.02
0.13
0.04
256.4
243.4
264.9
267.6
101,000
148,500
88,000
12,748.00
8381.52
DMSO
DMF
96,500
7678.00
appearance of the singlet peak at around 6.9–7.2 ppm corre-
contains 0.4 mL of 1 mmol freshly prepared DPPH, different
volume (80, 160, 240, 320 and 400 μL) of a 1 mg/mL solution
of the compounds and the required volume of ethanol to make
the whole mixture to 4 mL. A blank was prepared without the
addition of the samples. The reaction mixtures were kept in
the dark at room temperature for 30 min. The change in color
(from violet to yellow) was observed, and their absorbance
was measured at 517 nm by using UV–Vis spectrophotometer.
Lower the absorbance of the mixture indicates the higher rad-
ical scavenging activity. Radical scavenging activity was cal-
culated by using the following equation:
1
sponding to C -H in the H NMR spectra of 2-amino-4-
5
substituted phenyl-1,3-thiazoles (1b–7b) evidenced the
thiazolization under ultrasonication. In FT-IR spectra of Schiff
bases (1c–7c), the absorption bands at around 1627–
1
1
666 cm− corresponding to –C=N clearly indicates the con-
densation had taken place. The formation of Schiff bases (1c–
c) was also confirmed by the observation of singlet peak of
imino-H at around 8.91–9.24 ppmand the imino-C peak at
7
1
13
around 159–174 ppm in their H NMR and C NMR respec-
tively. The addition of ammonium acetate and p-
chlorobenzaldehyde with corresponding Schiff bases (1c–7c)
yield the final products (1d–7d) as shown in Scheme 2. The
formation of thiazolo-s-triazines was highly diastereoselective
in favor of the cisisomers [20]. The corresponding molecular
ion peaks in the mass spectra, NMR and elemental analyses
results of compounds (1d–7d) were confirmed the formation
of indol-3-yl-thiazolo-s-triazines. The maximum absorption
wavelengths at 235–265 nm of compounds (1d–7d) also stood
as an additional support for the confirmation of cis isomers,
which are of high energy.
ðAB−ASÞ
%
Inhibition
¼
 100
AB
Where AB – Absorbance of blank sample (t=0 min); AS –
Absorbance of test samples (t=30 min).
The antioxidant property of 2-amino-4-substituted phenyl-
1
(
7
,3-thiazoles (1b–7b), indole-3-carbaldehyde Schiff bases
1c–7c) and indol-3-yl-thiazolo[3,2-a][1,3,5]triazines (1d–
d) were carried out against DPPH radical. The results indi-
cating that Schiff bases (1c–7c) have good antioxidant prop-
erty, the thiazoles (1b–7b) and indol-3-ylthaizolo-s-triazines
(1d–7d) have moderate. Out of all these, compounds (4c–6c
DPPH Radical Scavenging Assay
&
4d–6d) with electron donating substituents have shown
better activity. The order of reactivity of these antioxidants
based on their substituents is OH>OCH >F>CH >Cl>H.
The radical scavenging activity of synthesized compounds
against 2, 2-diphenyl-2-picyrl hydrazyl hydrate (DPPH) was
determined by using Brand-Williams et al. 1995 method [21].
Ascorbic acid was used as a standard. The reaction mixture
3
3
The graphical representation of antioxidant results is repre-
sented in Fig. 1.
f
Table 9 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (2d) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvents
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
234.5
239.8
235.9
256.5
243.9
295.7
297.9
360.9
348.8
340.8
339.4
366
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
123.46
120.73
122.73
112.87
118.70
97.91
173,500
175,500
152,500
100,500
149,500
88,500
0.00320
0.00316
0.00279
0.00169
0.00265
0.00129
0.00140
0.40
0.40
0.37
0.30
0.37
0.28
0.30
14935.42
13031.69
13048.12
9522.60
13678.01
4603.92
4052.37
0.01
0.00
0.00
0.09
0.03
0.19
0.07
DMSO
342.3
338.8
DMF
97.18
96,500

J Fluoresc
f
Table 10 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (3d) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvents
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
230.5
258.3
254.5
258.8
268.7
293.1
299.1
345.3
335.4
335.4
327.2
345.3
338.3
332.2
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
125.60
112.08
113.76
111.87
107.74
98.78
189,500
182,500
152,000
99,000
0.00355
0.00305
0.00258
0.00165
0.00240
0.00212
0.00104
0.42
0.41
0.37
0.30
0.37
0.36
0.26
14,423.62
8,899.53
9,477.58
8077.53
8255.90
4558.49
3331.29
0.00
0.00
0.00
0.06
0.00
0.05
0.04
149,500
143,500
72,000
DMSO
DMF
96.79
Absorption and Emission Behavior of Compounds (1c–7c
1d–7d) in Different Solvents
tabulated in (Tables 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 and
14). It was found that indol-3-yl-thiazolo[3,2-a][1,3,5]tri-
azines (1d–7d) have higher molar extinction coefficients than
Schiff bases (1c–7c). The spectral data indicate that the polar-
ity of the solvent has a significant effect on absorption maxi-
ma. As the polarity of the solvent increases, the fluorescence
emission of the compounds shifted to the longer wavelength
(red shift) due to the intramolecular charge transfer (ICT) in
the singlet excited state from electron donating groups to the
electron acceptor groups in the compounds [22]. This indi-
cates that the polarity of the compounds was raised at their
singlet excited state as the polarity of the solvent increases.
The red shift of the compounds in different solvents de-
pends upon the difference in the dipole moment between the
excited singlet and the ground state of the chromophores [23].
This difference in the dipole moment can be determined using
the simplified Lippert-Mataga equation [24] as follows:
&
As a preliminary work, absorption and fluorescence spectra
were recorded for 2-amino-4-substituted phenyl-1,3-thiazoles
(
3
1b–7b), Schiff bases (1c–7c) and indol-3-ylthiazolo[3,2-a][1,
,5]triazines (1d–7d). Among them, Schiff bases(1c–7c) and
indoly-3-ylthiazolo[3,2-a][1,3,5]triazines (1d–7d) were excit-
ed and studied the absorption and emission behavior of these
−5
−3
compounds (1×10 mol/dm ) in different solvents such as
acetonitrile, chloroform, dichloromethane, methanol, ethyl ac-
etate, dimethylsulfoxide (DMSO) and dimethylformamide
(
DMF). Maximum wavelength absorption peaks (λmax
)
)
around 234–360 nm and fluorescence emission peaks (λmax
around 330–435 nm were observed respectively. The conju-
gation in these compounds caused the bathochromic shifts in
the emission spectra. Due to the strong electron-donating abil-
ity of the hydroxyl group at the ortho position, compound (6c)
has shown high fluorescence intensity. Compound (3c) with
methyl group in the phenyl ring was not shown fluorescence
emission, whereas other Schiff bases (1c–7c) were shown bet-
ter fluorescence emission than indol-3-ylthiazolo[3,2-a][1,3,
À
Á
2
2
μ −μ
e
g
Δvst ¼
Δf þ constant
ð1Þ
ð2Þ
3
hca
2
ε−1
n −1
Δf ¼ 2
ε þ 1 − 2n þ 1
2
5
]triazines (1d–7d). All compounds were red shifted due to
the better stabilization of the molecules in the first excites state
with increasing the polarity of the solvents. The maximum
emission wavelength of compounds (1c–7c) was observed in
dichloromethane at 374.1–429.1 nm and compounds (1d–7d)
in methanol at 354–391 nm. The calculated physiochemical
parameters from the absorption and emission spectra were
Where Δv is the Stokes shift, which increases with in-
creasing the polarity of the solvent counted to stronger stabil-
st
ity in the excited state in polar solvents, h is Planck’s constant
(h=6.6256×10−27 ergs), c is the velocity of light in vacuum
8
(c=2.9979×10 m/s) and a is the radius of the cavity in which
the fluorophore resides. Parameters n and ε in (Eq. 2) denotes
f
Table 11 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (4d) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvents
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
246.4
254.8
249.8
262.1
249.2
261.1
265.4
354.1
354.9
339.7
330.5
354.9
340
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
117.50
113.62
115.90
110.46
116.18
110.88
109.08
178,500
195,000
171,000
143,500
171,000
90,000
0.00313
0.00331
0.00296
0.00237
0.00296
0.00149
0.00177
0.40
0.42
0.40
0.36
0.40
0.29
0.31
12,343.81
11,069.52
10,594.29
7896.19
0.01
0.00
0.00
0.05
0.00
0.13
0.05
11,951.46
8887.74
DMSO
DMF
333.7
108,500
7711.94

J Fluoresc
f
Table 12 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (5d) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvents
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
230.2
237.8
235.8
262
354.8
356.8
342.1
333.3
356.6
340
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
125.76
121.75
122.78
110.50
119.73
97.28
167,500
171,500
154,000
144,500
131,500
107,500
242,000
0.00314
0.00312
0.00282
0.00238
0.00235
0.00156
0.00356
0.39
0.40
0.38
0.36
0.35
0.31
0.47
15,255.59
14,025.24
13,177.60
8164.94
0.02
0.00
0.01
0.04
0.00
0.16
0.02
241.8
297.6
294
13,313.87
4190.39
DMSO
DMF
336.1
98.47
4260.56
−
1
the refractive index and the dielectric constant of the solvent
respectively.
From the absorption and emission spectra, Stokes shift was
calculated and tabulated in (Tables 2, 3, 4, 5, 6, 7, 8, 9, 10, 11,
where Emax is the maximum energy of absorption in cm .It
has been obviously observed the polarity of the solvent have a
significant effect on the emission spectra indicating the strong
polarity of the compounds in the ground state than in the singlet
excited site with increasing the polarity of the solvents. The red
shifts in the emission spectra increases as the polarity of the
solvent increases from DCM to DMSO. The empirical Dimroth
polarity value was calculated based on the below relation [26].
1
2, 13 and 14) using the (Eq. 3) [24].
Δvst ¼ vex−vem
ð3Þ
where vex ex and vem em are the wavenumbers of absorption and
−1
emission maxima (cm ) respectively. The negative value (in
Tables 15 and 16) of the difference in the dipole moment (Δμ)
for the compounds indicates the high polarity of the com-
pounds in the ground state than in the singlet excited state.
The oscillator strength (f) provides the absorption area of the
electronic spectrum defines the effective number of dispersion
electrons between ground and excited states oscillating with
the corresponding frequency (υ). The oscillator strength of the
compounds in different solvents was calculated using the fol-
lowing (Eq. 4):
28; 591
ET
¼
ð6Þ
λmax
where λmax is the peak wavelength of the compounds in the
absorption spectra. Optical band gap (E ) was calculated from
the longest absorption wavelength (λonset) of their UV absorp-
g
tion spectra according to the following equation [27].
1242
Eg ¼
ð7Þ
λonset
Z
ꢀ
ꢁ
−
9
Fluorescence quantum yield (Φ ) of the compounds (1c–7c &
d–7d) was measured by the relative comparison procedure
f ¼ 4:32
Â
10
ε v dv
ð4Þ
f
1
−1
−1
using quinine sulfate (Φ =0.546, in 0.5 M H SO ) as reference
f 2 4
where ε is the extinction coefficient (L Mol cm ) and v
−
1
[28]. Relative quantum yield was determined using the equation:
represents the wavenumber in cm . In addition, the transition
dipole moment (μ in Debye) of the compounds in different
solvents was calculated using the following equation [25].
ꢂ
ꢃ
ꢂ
ꢃ
ꢂ
ꢃ
Iref
Asample
Aref
Rsample
Rref
Φf ¼ 0:546 Â
Â
Â
ð8Þ
Isample
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
f
Where, ref and sample corresponds to reference and sample
μ ¼
ð5Þ
−7
4
:72 Â 10 Â E
max
respectively. Φ is the fluorescence quantum yield, I is the
f
f
Table 13 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (6d) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvents
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
248.7
254.3
251.2
257.9
246
357.8
357.7
391.7
339
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
116.41
113.85
115.25
112.26
117.69
111.35
102.26
185,500
214,500
183,000
130,000
166,000
68,000
0.00322
0.00364
0.00315
0.00218
0.00292
0.00113
0.00185
0.41
0.44
0.41
0.34
0.39
0.25
0.33
12,260.51
11,367.25
14,279.18
9276.19
0.01
0.00
0.01
0.05
0.01
0.25
0.11
391.7
341.2
341.1
15,120.66
9153.21
DMSO
260
DMF
283.1
121,000
6006.29

J Fluoresc
f
Table 14 Physiochemical parameters and fluorescence quantum yield (Φ ) of compound (7d) in different solvents
−
1
ε (M−1 cm−1)
Δ v (cm−1)
Solvents
λab (nm)
λem (nm)
Δf
E
T
(kcal mol )
f
μ (Debye)
Φ
f
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
229.8
243
356.2
359.2
341.2
342
0.3054
0.1483
0.2171
0.1996
0.3085
0.2634
0.2744
125.98
119.14
121.44
112.96
118.60
107.95
102.01
184,000
158,500
177,000
110,000
166,500
106,000
185,000
0.00346
0.00282
0.00321
0.00185
0.00295
0.00171
0.00282
0.41
0.38
0.40
0.32
0.39
0.31
0.41
15,441.99
13,312.62
12,637.99
9777.01
0.01
0.00
0.00
0.05
0.00
0.06
0.01
238.4
256.3
244.1
268.2
283.8
354.2
343
12,734.18
8131.09
DMSO
DMF
342.5
6039.00
Table 15 Change in Δμ of compounds (1c–7c) in different solvents
Compound
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
DMSO
DMF
Average Δμ
1c
2c
4c
5c
6c
7c
−1.25
−4.33
−3.87
−4.28
−6.21
−3.35
−1.50
−4.02
−3.21
−4.28
−4.95
−3.07
−1.86
−2.92
−4.72
−4.21
−8.13
−3.81
−1.04
−4.08
−3.63
−4.22
−5.48
−2.93
−1.98
−3.78
−3.86
−4.31
−3.33
−2.03
−3.44
−6.79
−6.43
−4.48
−5.83
−6.28
−1.79
−4.23
−4.25
−4.60
−6.62
−4.21
−1.84
−4.31
−4.28
−4.34
−5.79
−3.67
absorbance intensity of UV–Vis spectra, A is the area under
the emission peak and R is the refractive index of the solvent.
The dilute solutions of the compounds with absorbance below
excited state intramolecular proton transfer (ESIPT) leads to
small fluorescence quantum yield. On the other hand, the
fluorescence quantum yield is directly proportional to the in-
tensity of the emission spectra. It is concluded that the low
fluorescence quantum yield of indoly-3-yl-thiazolo[3,2-a][1,
3,5]triazines (1d–7d) was due to their low intense fluores-
cence emission peaks.
1
at and above excitation wavelength of intensity below 1000
were used. Quinine sulfate in 0.1 M H SO was used as a
2
4
reference. The fluorescence quantum yield of Schiff bases
2c–7c) was comparatively higher than indol-3-yl-thiazolo[3,
-a][1,3,5]triazines (1d–7d) and varied with solvents. Schiff
bases (2c–7c) have shown fluorescence quantum yield of
.20–0.36 in case of acetonitrile, chloroform and dichloro-
methane and 0.33–0.72 in case of DMSO and DMF. Indol-
-yl-thiazolo[3,2-a][1,3,5]triazines have shown 0.01–0.13
(
2
Cyclic Voltammetry
0
The electrochemical behavior of the Schiff bases (1c–7c) and
indoly-3-yl-thiazolo[3,2-a][1,3,5]triazines (1d–7d) was deter-
mined by cyclic voltammetry (CV) using a standard three-
electrode cell in HPLC acetonitrile containing 0.1 M
3
fluorescence quantum yield in all organic solvents. Though
indol-3-yl-thiazolo[3,2-a][1,3,5]triazines have large molar ex-
tinction co-efficient, but the reason for their lower quantum
yield cannot be resolved. On excitation, the strong basic (–
N=) and strong acidic (p-chlorophenyl) moieties involved in
tetrabutylammonium hexafluorophosphate (TBAPF ) as the
6
supporting electrolyte. The working electrode was glassy car-
bon, the counter electrode used a platinum wire and the
Table 16 Change in Δμ of compounds (1d–7d) in different solvents
Compound
Acetonitrile
Chloroform
Dichloromethane
Ethylacetate
Methanol
DMSO
DMF
Average Δμ
1d
2d
3d
4d
5d
6d
7d
−0.98
−0.88
−0.52
−0.53
−1.35
−0.49
−0.58
−0.54
−0.43
−0.07
−0.07
−0.26
0.09
−0.71
−0.52
−0.10
−0.14
−0.72
−0.59
−0.47
−2.47
−2.48
−2.17
−1.97
−1.89
−1.57
−1.67
−1.12
−1.20
−0.34
−0.11
−0.31
−0.64
−0.23
−2.76
−3.20
−2.22
−2.72
−3.60
−2.99
−1.87
−1.69
−1.94
−1.42
−2.00
−1.82
−2.45
−0.71
−1.47
−1.52
−0.98
−1.08
−1.42
−1.23
−0.82
−0.17

J Fluoresc
of −4.4 eV. The energy levels were calculated using the fol-
lowing equations [29].
Â
Ã
E ðHOMOÞ ¼ − E
E ðLUMOÞ ¼ − E
þ 4:4 eV
ð9Þ
ð10Þ
oxðonsetÞ
Â
Ã
þ 4:4 eV
redðonsetÞ
The onset reduction potential of the Schiff bases around
0.80 to −0.87 V gives the LUMO energies of −3.43 to
3.63 eV as summarized in the Table 17. The irreversible
−
−
reduction peak suggests that the least possibility of the regen-
eration of the Schiff bases.
Conclusion
In summary, we have successfully achieved the synthesis of
4
2
-(4-chlorophenyl)-2-(1H-indol-3-yl)-6-substituted phenyl-
H-thiazolo[3,2-a][1,3,5]triazines (1d–7d) starting from vari-
Fig. 2 Cyclic Voltammetry plots of Schiff bases (1c–7c)
ous substituted acetophenones (1–7) through the intermediate
indole-3-carbaldehyde Schiff bases (1c–7c). Higher yields,
less reaction time and the purity of the compounds were at-
tractive in the whole synthesis. Their structures were con-
firmed by FT-IR, mass spectrum, NMR and elemental analy-
ses. The compounds (1c–7c & 1d–7d) were identified as good
antioxidants. The UV–Vis and fluorescence spectra were re-
corded for the Schiff bases (1c–7c) and indol-3-ylthiazolo[3,
2-a][1,3,5]triazines(1d–7d) in seven solvents of differing po-
larities. Comparison on the fluorescence spectra of Schiff ba-
ses (1c–7c) and indol-3-yl-thiazolo[3,2-a][1,3,5]triazines (1d–
7d) revealed that Schiff bases have shown high intense fluo-
rescence emission peaks. The physiochemical parameters and
fluorescence quantum yield of compounds (1c–7c & 1d–7d)
in different solvents were calculated. The indol-3-yl-
thiazolo[3,2-a][1,3,5]triazines (1d–7d) possessed higher mo-
lar extinction coefficient than Schiff bases (1c–7c). The high
intensity peak in the fluorescence emission spectra of Schiff
bases (1c–7c) featuring for a high fluorescence quantum yield.
Among all these, compound (6c) has emerged as the best
reference electrode was a standard calomel electrode (SCE).
The solutions were purged with nitrogen gas for 5 min before
recording the electrochemical data at a scan rate of 100 mV/s.
From the cyclic voltammograms Fig. 2, irreversible wave
in the reduction side was observed in the case of Schiff bases
and the indol-3-yl-thiazolo[3,2-a][1,3,5]triazines (1d–7d)
have not shown considerable peaks. The irreversible reduction
peak of Schiff bases (1c–7c) demonstrates the instability of the
indolyl radical after one electron transfer, which evolves the
dimerization reaction. From Fig. 2, it was found that the pres-
ence of the electron donating hydroxyl group at the ortho
position of the phenyl ring in compound (6c) shown the strong
reduction peak and the electron withdrawing chloro
group at the para position of the phenyl ring in com-
pound (3c) shown weak reduction peak. According to
CV measurements, the Schiff bases showed cathodic
peaks at around -1.08 to–1.18 V. Ferrocene was used as refer-
ence to calculate the HOMO and LUMO energy levels by
measuring the redox potentials, including the ferrocene value
Table 17 Electrochemical properties of Schiff bases (1c–7c)
a
b
c
Compound
Ered(V)
Eoxd(V)
Ered onset (V)
Eoxd onset (V)
LUMO (eV)
HOMO (eV)
E
g
(eV) from CV
g
E (eV)
1c
2c
3c
4c
5c
6c
7c
−1.11
−1.09
−1.11
−1.12
−1.30
−1.08
−1.06
−0.47
−0.47
−0.43
−0.34
–
−1.27
−1.31
−1.29
−1.38
−1.46
−1.30
−1.22
−0.16
−0.14
−0.19
−0.20
–
−3.13
−3.09
−3.11
−3.02
−2.94
−3.10
−3.18
−4.24
−4.26
−4.21
−4.20
–
1.11
1.17
1.10
1.18
–
3.62
3.64
3.62
3.63
3.65
3.64
3.65
−0.48
−0.44
−0.31
−0.21
−4.09
−4.19
0.99
1.01
a
b
c
Estimated from the onset reduction potential
Estimated from the oxidation potential
Estimated from UV absorption spectra

J Fluoresc
1
2. Khan SA, Asiri AM, Al-Thaqafy SH, Faidallah HM, El-Daly SA
antioxidant and fluorescent material with high quantum yield
0.72). The conjugation in the Schiff bases influences their
(
2014) Synthesis, characterization and spectroscopic behavior of
novel 2-oxo-1,4-disubstituted-1,2,5,6-tetrahydrobenzo[h]quinoline-
-carbonitrile dyes. Spectrochim Acta Part A 133:141–148
(
optical and electrochemical properties. The cathodic values
of the reduction potential wave in the cyclic voltammetry ev-
idences the Donor-π-Acceptor (D-π-A) systems in the Schiff
bases. These results would be helpful to design a new type of
donor-acceptor systems, which influences their photoconduc-
tivity towards the performances in dye-sensitized solar cells
3
13. Marwani HM, Asiri AM, Khan SA (2012) Green-synthesis, char-
acterization, photostability and polarity studies of novel Schiff base
dyes using spectroscopic methods. Bioorg Khim 38(5):604–609
1
4. Olgun U, Gulfen M (2014) Synthesis of fluorescence
poly(phenylenethiazolo[5,4-d]thiazole) copolymer dye:
Spectroscopy, cyclic voltammetry and thermal analysis. Dyes
Pigments 102:189–195
(
DCSS’s) and other photoelectronic applications.
1
1
1
5. Freeman HS, Hinks D, Esancy JF (1996) Physico-chemical princi-
ples of color chemistry. Adv Color Chem Ser 4:254–292
6. Smolin EM, Rapopret L (1954) S-triazine and derivatives.
Interscience publications, New York, pp 6–8
7. El-Sedika M, Almonasy N, Nepras M, Bures F, Dvorak M, Michl
M, Cermak J, Hrdina R (2012) Synthesis, absorption and fluores-
cence properties of N-triazinyl derivatives of 2-aminoanthracene.
Dyes Pigments 92:1126–1131
Acknowledgments T V Sravanthi thanks Vellore Institute of Technol-
ogy, Vellore, Tamilnadu, India, for providing Research Associateship and
Dr. A Sivakumar, Professor, VIT University, Vellore for cyclic voltamm-
etry analysis. The DST-FIST NMR facility, VIT-TBI for FT-IR and UV–
Vis spectral characterization at VIT University are greatly acknowledged.
1
8. Adimurthy S, Ghosh S, Patoliya PU, Ramachandraiah G, Agrawal
M, Mahesh RG, Upadhyaya SC, Ghosh PK, Ranu BC (2008) An
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