Green Synthesis, Characterization and Electrochemical Behavior of New Thiazole Based Coumarinyl Scaffolds

Article abstract of DOI:10.1007/s10895-016-1795-2

The present paper deals with the synthesis of thiazolo-coumarin derivatives, prepared starting from naphthaldehyde, ethyl acetoacetate and hydrazine in three steps and characterized by spectroscopic analysis. UV-Visible spectra of the compounds was carrie

Full text of DOI:10.1007/s10895-016-1795-2

J Fluoresc
DOI 10.1007/s10895-016-1795-2
ORIGINAL ARTICLE
Green Synthesis, Characterization and Electrochemical Behavior
of New Thiazole Based Coumarinyl Scaffolds
Aamer Saeed1 & Pervaiz Ali Channar1 & Ghulam Shabir1 & Fayaz Ali Larik1
Received: 4 December 2015 /Accepted: 27 March 2016
#
Springer Science+Business Media New York 2016
Abstract The present paper deals with the synthesis of
thiazolo-coumarin derivatives, prepared starting from
naphthaldehyde, ethyl acetoacetate and hydrazine in three
steps and characterized by spectroscopic analysis. UV-
Visible spectra of the compounds was carried out in different
solvents DMF, ethanol, methanol, ethyl acetate and acetone
and the absorption was observed in the range 338–390 nm.
Electrochemical study of thiazoles was conducted in DMF
and redox behavior was examined. Fluorescence carried out
in ethanol showed sharp emission in the range 440–505 nm.
which furnishes strong fluorescence and high fluorescence
quantum yield and photostability to coumarin derivatives
[11] with the view to highlight this behavior, some thiazole
based coumarin scaffolds has been synthesized and they have
been passed through the shower of studies (i.e. Cyclic volt-
ammetry, Ultraviolet, and Fluorescence) [12]. It was indicated
in the late 1950s that substitutions on the coumarin structure
shifted the fluorescence band, electron donating groups
when attached at 4 or 7 position bring a bathochromic shift.
Addition of electron-repelling groups at the 4-, 6-, or 7-position
or electron-attracting groups in the 3-position shift the fluo-
rescence band to longer wavelengths. Changing the solvent
or the solution pH also affected the fluorescence spectra
[10].
.
.
.
Keywords Thiazole Coumarin Absoption Emisssion
A survey of literature reveals that thiazole based
coumarinyl scaffolds show various biological activities [7, 8,
9]. All the synthesized compounds were passed through intel-
ligent eyes having tools to visualize the electrochemical be-
havior and florescence phenomena and results obtained were
highly encouraging. Herein we report a green reaction of
thiosemicarbazone (2a–j) with 3-bromoacetyl-(6-
substituted)coumarin (1) in dry ethanol using silica sulfuric
acid catalyst to afford a series of thiazole derivatives appended
to coumarin.
Introduction
A plethora of literature is available regarding the bioactive
nature of coumarins derivatives [1–5]. Installation of thiazole
motif to coumarins nucleus further leads to various bioactiv-
ities such as antiinflammatory, analgesic, antimicrobial,
anti-HIV, antihypertensive and herbicidal activity [6–9].
Coumarins possess a double bond in trans conformation
Research Highlights:
• Facile synthesis of new thiazole based coumarinyl derivatives has been
accomplished.
• Thiazole based coumarinyl derivatives have been found to be highly
fluorescent.
Experimental
• Coumarinyl derivatives exhibited absorptions in the range 338–390 nm.
Apparatus, Reagents and Chemicals
* Aamer Saeed
All commercial products were purchased from Sigma-
Aldrich. Solvents used were of analytical grade and, when
necessary, were purified and dried by the standard methods.
Melting points were determined in open capillary tubes on a
Stuart melting point apparatus. The IR spectra were run on the
aamersaeed@yahoo.com
1
Department of Chemistry, Quaid-I-Azam University,
Islamabad 45320, Pakistan

J Fluoresc
single beam Nicolet IR 100 (Fourier-Transform); while UVof
all the samples were run in water using UV-Genesys spectro-
photometer. The 1H-NMR and 13C-NMR spectra were record-
ed in DMSO-d6 using NMR Bruker DPX 300 spectrophotom-
eter operating at 300 MHz TMS was used as internal standard
with the deuterium signal of the solvent as the lock and
chemical shifts δ recorded in ppm. The elemental analysis
(C, H, N, S) of the compounds were performed using Flash
EA 1112 elemental analyzer. Compounds were routinely
checked by TLC on silica gel G plates using eluting solvents,
pet ether: ethyl acetate (7: 3, v/v). Also, the developed plates
were visualized using a UV lamp for the presence of spots and
Rf values were duly calculated.
2-(2-(2,4-Dimethylphenylamino)thiazol-4-yl)-3H-benzo[f]
chromen-3-one
Yellow Solid M.P = 268 °C, Yield =73 %, Rf = 0.65 (Pet
Ether: Ethyl Acetate 7: 3). FTIR (KBr) cm−1: 3370 (N-H),
1715, 1738 (C = O), 3060, 2962, 2952(CH str). 2951 (methyl
1
C–H str), 1512 (C = N str), 1495 (C = C str); H NMR
(300 MHz, DMSO-d6): δ (ppm): 7.23(s, 1H, CH), 7.22–
8.42 (m, 6H, Ar-H), 6.9–7.39 (m, 3H Ar-H), 9.23(s, 1H,
benzocoumarin H-4). 11.72 (s, 1H, NH), 2.49 (s, 3H), 2.21
(s, 6H Ar 2× CH3); 13C-NMR (75 MHz, DMSO-d6): δ (ppm):
168.3, (C = O), 158.18, (C = N), 153.2 (C = C), 144.8, 137.1,
136.4, 135.8, 134.1, 133.2, 130.5, 129.7, 129.5, 129.1, 128.9,
126.5, 121.9, 120.1, 116.9, 113.6111.9, 110.5109.1 (Ar-C),
21.54 (CH3), 18.24 (CH3),
General Procedure of Synthesis
To a cold mixture of 2-hydroxy-1-naphthalaldehyde (0.1 mol)
and ethylacetoacetate (0.1 mol, 12.63 ml) was added 1 g
(1.12 ml) of piperdine by the rapid shaking. The solid sepa-
rated was filtered and washed with ethanol. Crystallization of
the solid from pure water provided pure 2-acetyl-3H-
benzo[f]chromen-3-one. A solution of bromine (0.01 mol,
1.72 g) in chloroform was added by rapidly shaking to a so-
lution 2-acetyl-3H-benzo[f]chromen-3-one (0.01 mol, 2.38)
in chloroform. The mixture was heated under reflux
for 2–3 h and then slowly cooled. The solid separated
was crystallized from chloroform–ethanol (2:1) to get
pure 2-(2-bromoacetyl)-3H-benzo[f]chromen-3-one (3)
(0.001 m). The latter was dissolved in 30 mL of ethanol
in a round bottom flask fitted with a reflux condenser,
and added 0.001 m of KSCN and stirred further at 45–
50 °C. After 1 h, suitably substituted aniline (0.0012 m)
was added and the reaction mixture was refluxed and
the progress of the reaction was monitored by TLC.
After 4–5 h, the solid product was obtained either by cooling
the reaction mixture or by pouring it into ice-cold water. The
solid separated was purified by recrystallization in ethanol. In
this way a series of derivatives 5a-j were synthesized.
2-(2-(2-Chlorophenylamino)thiazol-4-yl)-3H-benzo[f]
chromen-3-one
Yellow Solid M.P = 234 °C, Yield =72 %, Rf = 0.67 (Pet
Ether: Ethyl Acetate 7: 3). FTIR (KBr) cm−1: 3372 (N-H),
1729 (C = O), 3070, 2962, 2942(CH str). 1511 (C = N str),
1494 (C = C str); 1H NMR (300 MHz, DMSO-d6): δ (ppm):
7.24 (s, 1H, CH), 7.34–8.42 (m, 6H, Ar-H), 7.75–6.92 (m, 4H,
ArH), 9.53(s, 1H, benzocoumarin H-4). 10.92 (s, 1H, NH),
13C-NMR (75 MHz, DMSO-d6): δ (ppm): 167.3, (C = O),
157.18, (C = N), 150.2 (C = C), 145.8, 143.8, 142.8, 136.1,
135.4, 134.8, 134.2, 133.5, 130.2, 129.5, 129.4, 129.2, 128.7,
124.5, 122.9, 121.1, 117.9, 114.6, 108.1 (Ar-C).
2-(2-(3-Nitrophenylamino)thiazol-4-yl)-3H-benzo[f]
chromen-3-one
Yellow Solid M.P = 248 °C, Yield =77 %, Rf = 0.63 (Pet
Ether: Ethyl Acetate 7: 3). FTIR (KBr) cm−1: 3378 (N-H),
1727 (C = O), 3071, 2962, 2922(CH str). 1511 (C = N str),
1494 (C = C str); 1H NMR (300 MHz, DMSO-d6): δ
(ppm): 7.20(s, 1H, CH), 7.32–8.32 (m, 6H, Ar-H),
7.89–7.32 (m, 4H, ArH), 9.33(s, 1H, benzocoumarin
H-4). 10.55 (s, 1H, NH), 13C-NMR (75 MHz, DMSO-
d6): δ (ppm): 166.3, (C = O), 158.18, (C = N), 152.2
(C = C), 150.8, 144.8, 143.8, 137.1, 134.4, 133.8, 132.2,
131.5, 130.4, 128.5, 128.7, 128.4, 127.7, 123.5, 122.4,
120.1, 114.9, 114.6, 107.1 (Ar-C),
2-(2-(Mesitylamino)thiazol-4-yl)-3H-benzo[f]chromen-3-one
Yellow Solid M.P = 238 °C, Yield =69 %, Rf = 0.68 (Pet
Ether: Ethyl Acetate 7: 3). FTIR (KBr) cm−1: 3380 (N-H),
1712, 1728 (C = O), 3080, 2972, 2952(CH str). 2851 (methyl
1
C–H str), 1510 (C = N str), 1492 (C = C str); H NMR
(300 MHz, DMSO-d6): δ (ppm): 7.22(s, 1H, CH), 7.32–8.44
(m, 6H, Ar-H), 7.32 (s, 2H, Ar-H), 9.33(s, 1H, benzocoumarin
H-4). 11.92 (s, 1H, NH), 2.39 (s, 3H), 2.21 (s, 6H Ar 2× CH3);
13C-NMR (75 MHz, DMSO-d6): δ (ppm): 169.3, (C = O),
159.18, (C = N), 152.2 (C = C), 144.8, 137.1, 136.4, 135.8,
134.1, 133.2, 130.5, 129.7, 129.5, 129.1, 128.9, 126.5, 121.9,
120.1, 116.9, 113.6109.16.} (Ar-C), 21.58 (CH3), 18.27
(2CH3),
2-(2-(4-Bromophenylamino)thiazol-4-yl)-3H-benzo[f]
chromen-3-one
Yellow Solid M.P = 229 °C, Yield =76 %, Rf = 0.60 (Pet
Ether: Ethyl Acetate 7: 3). FTIR (KBr) cm−1: 3388 (N-H),
1737 (C = O), 3071, 2972, 2932(CH stretch-ing). 1531
(C = N str), 1493 (C = C str); 1H NMR (300 MHz, DMSO-
d6): δ (ppm): 7.22(s, 1H, CH), 7.32–8.32 (m, 6H, Ar-H), 7.66

J Fluoresc
(d, 2H, J = 7.7 Hz ArH), 7.43(d,2H, J = 7.7 Hz ArH)
9.30(s, 1H, benzocoumarin H-4). 11.55 (s, 1H, NH),
13C-NMR (75 MHz, DMSO-d6): δ (ppm): 167.3,
(C = O), 159.18, (C = N), 154.2 (C = C), 152.8,
145.8, 144.8, 138.1, 135.4, 134.4, 133.2, 131.5, 130.3,
129.4, 129.3, 128.2, 127.8, 123.6, 122.5, 121.1, 116.9,
114.6, 109.1 (Ar-C).
2-(2-(Benzylamino)thiazol-4-yl)-3H-benzo[f]chromen-3-one
Yellow Solid M.P = 235 °C, Yield = 78 %, Rf = 0.68 (Pet
Ether: Ethyl Acetate 7: 3) FTIR (KBr) cm−1: 3199.8 (N-H,
str), 2979.7 (Aromatic C-H, str), 1687.5 (C = O, str), 1511.8
1
(C-C, str), 1421.5 (C-H, bending), 1289.3 (C = S, str). H
NMR (300 MHz, DMSO-d6): δ (ppm): 7.28 (s, 1H, CH),
7.37–8.67 (m, 6H, Ar-H), 7.26–6.9 (dd, 3H, ArH),3.95(s,
2H), 9.43(s, 1H, benzocoumarin H-4). 13.05 (s, 1H, NH),
13C-NMR (75 MHz, DMSO-d6): δ (ppm): 169.5, (C = O),
160.3, 154.7 (C = N), 155.4, 154.5 (C = C), 157.6, 145.7,
146.8, 138.5, 135.1, 134.4, 133.5, 131.7, 130.1, 129.7,
127.8, 125.7, 123.9 (Ar-C), 46.5
2-(2-(4-Chlorophenylamino) thiazol-4-yl)-3H-benzo[f]
chromen-3-one
Yellow Solid M.P = 245 °C, Yield =72 %, Rf = 0.67 (Pet
Ether: Ethyl Acetate 7: 3). FTIR (KBr) cm−1: 3378 (N-H),
1739 (C = O), 3081, 2972, 2922(CH stretch-ing). 1531
(C = N str), 1493 (C = C str); 1H NMR (300 MHz, DMSO-
d6): δ (ppm): 7.23 (s, 1H, CH), 7.34–8.36 (m, 6H, Ar-H), 7.66
(d, 2H, J = 7.60 Hz Ar-H), 7.42(d,2H, J = 7.7 Hz ArH) 9.33(s,
1H, benzocoumarin H-4). 11.55 (s, 1H, NH), 13C-NMR
(75 MHz, DMSO-d6): δ (ppm): 169.3, (C = O), 160.18,
(C = N), 155.2 (C = C), 153.8, 146.8, 145.8, 138.6, 135.6,
134.6, 133.4, 131.6, 130.4, 129.5, 129.3, 128.6, 127.6, 123.7,
122.7, 121.2, 116.9, 114.8, 109.5 (Ar-C),
2-(2-(p-toluidino) thiazol-4-yl)-3H-benzo[f]chromen-3-one
Yellow Solid M.P = 250 °C, Yield =77 %, Rf = 0.63 (Pet
Ether: Ethyl Acetate 7: 3) FTIR (KBr) cm−1: 3367 (N-H),
1759 (C = O), 3171, 2973, 2943 (CH str). 1534 (C = N str),
1495 (C = C str); 1H NMR (300 MHz, DMSO-d6): δ (ppm):
7.24 (s, 1H, CH), 7.36–8.38 (m, 6H, Ar-H), 7.63 (d, 2H,
J = 7.60 Hz ArH), 7.24(d,2H, J = 7.7 Hz ArH)
9.44(s, 1H, benzocoumarin H-4). 12.05 (s, 1H, NH),
2.88 (s, 3H CH3), 13C-NMR (75 MHz, DMSO-d6): δ
(ppm): 167.3, (C = O), 162.18, (C = N), 156.2 (C = C),
154.8, 148.8, 145.8, 139.6, 136.4, 135.6, 133.4, 131.7,
130.4, 129.7, 129.8, 128.8, 127.7, 123.9, 122.5, 121.4,
114.8, 114.7, 109.6 (Ar-C), 27.5 (Me).
2-(2-(4-Methoxyphenylamino)thiazol-4-yl)-3H-benzo[f]
chromen-3-one
Light Yellow Solid M.P = 236 °C, Yield =76 %, Rf = 0.77 (Pet
Ether: Ethyl Acetate 7: 3). FTIR (KBr) cm−1: 3377 (N-H),
1749 (C = O), 3071, 2973, 2942 (CH stretch-ing). 1532
(C = N str), 1494 (C = C str); 1H NMR (300 MHz, DMSO-
d6): δ (ppm): 7.24 (s, 1H, CH), 7.35–8.37 (m, 6H, Ar-H), 7.63
(d, 2H, J = 7.60 Hz Ar-H), 7.22(d,2H, J = 7.7 Hz ArH) 9.43(s,
1H, benzocoumarin H-4). 11.05 (s, 1H, NH), 3.88 (s, 3H
OCH3), 13C-NMR (75 MHz, DMSO-d6): δ (ppm): 169.3,
(C = O), 160.18, (C = N), 155.2 (C = C), 155.8, 147.8,
146.8, 138.6, 135.4, 134.6, 133.4, 131.7, 130.4, 129.7,
129.8, 128.8, 127.7, 123.9, 122.5, 121.4, 114.8, 114.7, 109.6
(Ar-C), 60.5 (OMe)
Results and Discussion
Synthesis
Coumarinyl derivatives 5a-j have been synthesized well in
excellent yields and high purity through synthetic route
depicted in Scheme 1. These derivatives were synthesized
in order to obtain coumarin derivatives potentially applica-
ble as drugs and bioimaging materials. Thus, 3-acetyl cou-
marin was (2) synthesized by reacting naphthaldehyde and
ethyl acetoacetate which was further converted to target
thiazolocoumarin derivatives (5a-j) by bromination and
condensation of intermediate (2) with different substituted
aniline derivatives 4a-j at reflux (Scheme 1).
2-(2-(Thiazol-5-ylamino)thiazol-4-yl)-3H-benzo[f]
chromen-3-one
Yellow Solid, M.P = 224 °C, Yield = 65 %, Rf = 0.70 (Pet
Ether: Ethyl Acetate 7: 3) FTIR (KBr) cm−1: 3189.8 (N-H,
str), 2989.7 (Aromatic C-H, str), 1686.5 (C = O, str), 1511.8
The physical data of these coumarinyl derivatives is shown
in Table 1.
1
(C-C, str), 1421.5 (C-H, bending), 1289.3 (C = S, str). H
NMR (300 MHz, DMSO-d6): δ (ppm): 7.27 (s, 1H, CH),
7.38–8.67 (m, 6H, Ar-H), 7.69–7.26 (d, 2H, ArH 9.43(s,
1H, benzocoumarin H-4). 13.05 (s, 1H, NH), 13C-NMR
(75 MHz, DMSO-d6): δ (ppm): 169.3, (C = O), 160.18,
154.6 (C = N), 155.2, 154.2 (C = C), 157.8, 145.8, 146.6,
138.4, 135.4, 134.4133.4, 131.8, 130.1, 129.7, 126.8, 125.7,
123.9, 113.3 (Ar-C),
Spectral Characterization of Coumarinyl Thiazoles (5a-j)
The structures of newly synthesized coumarinyl deriva-
tives 5a-j were elucidated by UV, FTIR and NMR stud-
ies. The UV-visible absorption spectra of the coumarins
5a-j (1 × 10−6 M) were obtained at room temperature in

J Fluoresc
Scheme 1 Synthetic pathway to
thiazole coumarin derivatives
(5a-j)
different solvents (Fig. 1) and the selected spectral data is
summarized in Table 2.
UV-visible spectra of all coumarinyl derivatives 5a-j were
taken in DMF, ethanol, methanol, ethyl acetate and acetone. It
was observed from their UV-visible spectra of coumarin 5a-j,
that all compounds exhibited the only absorption maximum at
338–390 nm (Fig. 1) due to π-π* transition of naphthalene
and benzene rings common in all derivatives [13]. The λmax
for 5b is 390 nm is due to π-π* transitions of benzene and
heterocyclic rings present in the molecule and this is largest
among all the coumarinyl derivatives. There is more and more
availability of electrons causing the bathochromic shift within
the molecule. Thereby it has lager λmax. In case of 5 h the λmax
is 338 nm while that of 5e, 5c-g and 5i-j is in the range 360–
384 nm in given solvents. All this is due to π-π* and n-π*
transitions of lone pairs and π-bonded electrons. The lowest
λmax of 5 h is attributed to the intervening methylene func-
tionality between the aryl and heterocyclic coumarin motif
which vanishes the conjugation between the rings and causes
hypsochromic shift. The UV-visible study of these coumarinyl
compounds is different in different solvents due to the nature
of solvents as it was conducted in different solvents and there
is no larger change in λmax for a given compound with respect
to solvent polarity. This shows that the π-π* transitions
Table 1 Physical characteristics of coumarinyl derivatives 5a-j
Compound
Colour
Melting Point (°C)
Rf Value
5a
5b
5c
5d
5e
5f
Yellow Solid
Yellow Solid
Yellow Solid
Yellow Solid
Yellow Solid
Yellow Solid
Light Yellow Solid
Yellow Solid
Yellow Solid
Yellow Solid
238
268
234
248
229
245
236
224
235
250
0.68
0.65
0.67
0.63
0.60
0.67
0.77
0.70
0.68
0.63
5 g
5 h
5i
5j

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DMF
Ethanol
Ethylacetate
Acetone
Methanol
Fig. 1 UV-visible spectra of thiazole coumarin derivatives (5a-j) in DMF, ethanol,methanol, ethyl acetate and acetone
occurring over here is of B-type bands which are independent
of the nature of solvent. So these compounds are not showing
the phenomenon of solvatochromism.
The FTIR spectra of thiazole coumarin derivatives (5a-j)
exhibited absorption bands due to N-H, Ar-H, C = O of lac-
tone, C = N of imine, C = C, C = S and C-O, stretchings and
bending vibrations at 3370–3399, 2980–3170, 1727–1749,
1620–1652, 1510–1533, 1411–1431, 1280–1295, 1105–
1155, 836–860 and 793–818 cm−1respectively. In case of cou-
marin 5d, ester motif was confirmed by the appearance of
peak at 1727 due to the carbonyl group of cyclic six mem-
bered esters. Presence of peak at 1149 cm−1 was result of C-O-
C functionality. The absorption bands at 1630, 1533 and
859 cm−1 depicted the presence of C = C stretching and bend-
ing vibrations respectively. FTIR spectrum of dye 5e, showed
NH group peak at 3380 cm−1 and C = C-H stretching vibra-
tions at 3168 cm−1.
1
The H-NMR spectrum of compound 5a (Fig. 2) showed
6H singlet at 2.21 ppm and 3H singlet at 2.39 ppm due to three
CH3 substituents attached to benzene. Singlet peak at

J Fluoresc
Table 2 Wavelength of maximum absorption (λmax/nm) of coumarinyl
derivatives 5a-j in different solvents
The carbonyl and imine carbons were observed are at
169.3 and 159.1 ppm respectively. Three methyl car-
bons at benzene ring in 5a appeared at at 21.5 and
18.2 ppm. Similarly the rest of compounds 5b-j were
Compound DMF
Ethanol Methanol Ethyl
Acetone
λmax
λmax
λmax
acetate λmax λmax
1
confirmed by H-NMR and 13C-NMR spectra. The 1H-
5a
5b
5c
5d
5e
5f
378
390
377
379
379
368
366
342
384
364
379
388
377
381
377
376
369
344
376
363
384
385
376
377
375
373
377
343
387
362
376
386
377
375
384
371
390
340
386
363
383
388
396
375
381
369
384
338
385
360
and 13C-NMR spectrum of coumarin derivative 5a are shown
in (Figs. 2 and 3) respectively.
Electrochemical Properties
The electrochemical characterization of these coumarinyl
derivatives (5a-j) was conducted by cyclic voltammetry
(Fig. 4) using ethanol having 0.1 M TBAPF6 as a
supporting electrolyte. All redox potentials, HOMO
(highest occupied molecular orbital) and LUMO (lowest
unoccupied molecular orbital) were calculated from cy-
clic voltammograms (Fig. 4).
5 g
5 h
5i
5j
11.92 ppm is assigned to N-H substituted by heterocyclic and
phenyl rings of coumarin. The multiplets at 7.32–8.44
ppm are due to six mutually coupled aromatic protons at-
tached to coumarin part of molecule. The distinguishing peak
for all coumarins is the presence of coumarin H-4 at 9.33 ppm.
Difference among 5a-j series is due to condensed substituted
aromatic moieties. The 13C-NMR of 5a showed peaks for
different carbon atoms present within the molecule (Fig. 3).
Redox Potentials (E1/2
)
Synthesized coumarin derivatives (5a-j) showed oxidation
and reduction potentials on conducting their cyclic
voltammetric analysis. From the cyclic voltammograms redox
potentials was observed shown in Table 3. The lowest redox
Fig. 2 1H-NMR of coumarin derivative (5a)

J Fluoresc
Fig. 3 13C-NMR of coumarin derivative 5a
potential was observed for compounds 5a having 2,4,6-
trimethyl ligands on the chromophore and highest redox po-
tential was observed for 5 h and 5i containing different benzyl
and heterocyclic moieties attached at thiazole ring [14]. It can
be visualized from the data in Table 3 that coumarinyl
derivatives bearing electron withdrawing groups have low re-
dox potentials while those having electron donating groups
have high redox potential values [15]. Behavior of derivative
5 h based on benzyl functionality was different from coumarin
containing methyl groups.
0.00006
0.00004
0.00002
0.00000
-0.00002
-0.00004
-0.00006
Fig. 4 Combined cyclic
voltammogram of coumarin
derivatives (5a-j) in ethanol
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Potential (v)

J Fluoresc
Table 4 HOMO/eV
energy levels and band
gap energy coumarinnyl
derivatives (5a-j)
Lowest Unoccupied Molecular Orbital (LUMO)
Compound
Eg(V)
HOMO (eV)
In order to calculate the absolute energies of LUMO
level with respect to the vacuum level, the redox data
are standardized to the ferrocene/ferricenium couple
which has a calculated absolute energy of – 4.8 eV. The data
related to LUMO level energies of dyes are presented in Table
3.
It is inferred from LUMO energy levels, which varies from
−4.71 to −5.92 eV that electron donating groups on the
coumarinyl motives decrease the energy of LUMO levels
while electron withdrawing groups increase the energy of
LUMO energy levels. The energy of LUMO levels is varied
only by increasing the delocalization of electrons through al-
ternating single and double bonds and it is noticed that energy
difference decreases with increasing conjugation and vice
versa.
5a
5b
5c
5d
5e
5f
3.28
3.17
3.28
3.27
3.27
3.36
3.38
3.68
3.20
3.4
−8.08
−7.97
−8.08
−8.07
−8.07
−8.16
−8.18
8.48
5 g
5 h
5i
8.00
5j
8.20
Highest Occupied Molecular Orbital (HOMO)
Table 4 depicts the highest occupied molecular orbital energy
levels, which are calculated using the standard reported pro-
cedure. Considering the energy range from −7.97 to −8.48 eV
for coumarin derivatives 5a-j, it was observed that for couma-
rin 5 h the HOMO energy levels are at very low energy be-
cause the benzyl group is present in the chromophore while
5 hhas high HOMO energy levels due to inductive effect of
methyl groups present in the molecule. It is observed that there
is the effect of electron donating groups on the HOMO energy
levels is to increase their energy while electron withdrawing
groups definitely decrease the energy of HOMO levels by
increasing the energy gap between HOMO and LUMO.
Band Gap Energy (Eg)
The optical band gap values were calculated using the
standard procedure. The band gap energy is the span of
energies that lies between the valence and conduction
bands for insulators and semiconductors. Every solid
has its own characteristic energy-band structure. This
variation in band structure is responsible for the wide
range of electrical characteristics observed in various
materials. Band gap energy of these coumarinyl deriv-
atives is given in Table 3.The band gap energy varied
from 3.17 to 3.68 eV which is highest for dyes 5 g,
5 h, 5j and minimum for dyes 5b and 5i which depend
upon substituents attached to chromophore as well as
on the precursor utilized for synthesis of coumarins.
In case of compound5bmethyl groups are attached to
coumarin motif while 5i has a thiazole ring which in-
creases the electron density of chromophore and energy
levels get close to each other and decrease the band
gap energy.
Fluorescence Studies
Fluorescence study of all the coumarin derivatives was made
by preparing the ethanolic solution of compounds (5a-j) and
all the compounds were found to highly fluorescent (Table 5).
Table 5 Fluorescence data of thiazole coumarin derivatives (5a-j) in
ethanol
Compound Excitation
Emission
Stoke shift FLQ
Table 3 LUMO/eV
Compound.
E
1/2 (V)
LUMO (eV)
wavelength (nm) wavelength (nm)
energy levels of thiazole
coumarin derivatives
(5a-j)
5a
5b
5c
5d
5e
5f
−0.012
0.241
−4.788
−5.04
−4.75
−4.87
−5.4
−4.71
−4.76
−5.81
−5.92
−5.01
5a
5b
5c
5d
5e
5f
445
445
450
450
450
436
440
460
440
440
456
463
465
460
461
450
456
505
455
456
16
18
15
10
11
14
16
45
15
16
0.56
0.65
0.76
0.81
0.66
0.79
0.73
0.48
0.78
0.61
−0.049
0.075
0.690
−0.087
−0.031
1.015
5 g
5 h
5i
5 g
5 h
5i
1.127
5j
0.218
5j

J Fluoresc
Fig. 5 Fluorescence spectrum of
thiazole coumarin derivatives (5a-
j) in ethanol
5a
5b
5c
5d
5e
5f
2000
1500
1000
500
0
5g
5h
5i
5j
100
200
300
400
500
600
700
800
900 1000
Wavelength(nm)
These coumarin derivatives fulfilled the premier require-
ment of fluorescence as these molecules were highly
conjugated and were devoid of rotational or vibrational
motions as a whole molecule [16]. Fluorescence spec-
trum of all derivatives 5a-j is shown in Fig. 5, which
was recorded by selecting different excitation wave-
lengths of the source. Fluorescence spectrum provides
the intensity contribution to the observed emission at a
given wavelength by different excitation wavelengths
for which the samples were exposed. The fluorescence
spectrum showed only one emission for all dyes. The
emission peak of highest emission wavelength was not-
ed for 5 h at 505 nm on excitation of ethanolic solution
at a concentration 10−7 to 10−8 M.
exhibit absorptions in the range 370–395 nm. The
of all the dyes was found to be maximum in
max
DMF, which was in accordance with the general rule
that polar solvents shift the π-π* transitions to higher
wavelength. Solvatochromic behavior was observed for
the derivatives and the absorption maxima underwent a
bathochromic shift on increasing the polarity of sol-
vents. Fluorescence study of these derivatives was con-
ducted in ethanol and high stoke shift values were ob-
served and maximum emission was noted for com-
pound 5 h. The coumarinyl derivatives bearing
methoxy group have lowest band gaps and high energy
HOMO levels.
High fluorescence of these coumarinyl derivatives en-
ables them to be used for bio labeling and bio imaging
[17]. Therefore these derivatives can be applied where
usual fluorescent compounds are being used.
Fluorescence quantum (FLQ) yield of coumarinyl deriv-
atives 5a-j was determined by preparing an equimolar
solution (1 × 10−5 M) of synthesized dyes and fluorescein
and comparing the emission intensity of dyes with reference
compound. FLQ value of these coumarinyl derivatives is ap-
proximately half of the fluorescein [18].
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