Facile synthesis of novel fluorescent thiazole coumarinyl compounds: Electrochemical, time resolve fluorescence, and solvatochromic study

Article abstract of DOI:10.1016/j.molstruc.2020.129422

In this study, Benzocoumarin-Thiazoles-Azomethine derivatives with bioactive scaffolds were synthesized and characterized. The present investigation is concerned with the multistep synthesis of thiazole coumarinyl derivatives (5a-k), which were accomplished from naphthaldehyde, ethyl acetoacetate, and thiosemicarbazide. The formation of newly synthesized derivatives was confirmed by ¹H NMR and ¹³C NMR spectroscopic studies. Thiazole coumarinyl derivatives were subjected to UV-Visible studies in different solvents such as ethanol, ethyl acetate, and DMF for solvatochromic studies. The synthesized coumarinyl thiazole compounds showed absorption in the range of 332-390 nm. Electrochemical studies were performed in DMSO and redox behavior was offered by thiazoles. Fluorescence of coumarinyl thiazole compounds were examined in ethanol, ethyl acetate, and DMF to visualize the solvent effect on the emitting ability of compounds. Fluorescence spectra of coumarinyl thiazoles expressed a sharp emission in the range 436-550 nm.

Full text of DOI:10.1016/j.molstruc.2020.129422

Journal Pre-proof
Facile synthesis of novel fluorescent thiazole coumarinyl compounds:
Electrochemical, time resolve fluorescence, and solvatochromic study
Rabail Ujan , Ali Bahadur , Ghulam Shabir , Shahid Iqbal ,
Aamer Saeed , Pervaiz Ali Channar , Qaiser Mahmood ,
Muhammad Shoaib , Ifzan Arshad , Muhammad Saifullah ,
Guocong Liu , Rana Muhammad Irfan , Zahoor Ahmad ,
Mohsin Javed , Muhammad Raheel , Muhammad Abdul Qayyum ,
Bilal Khalid , Komal Rizwan
PII:
S0022-2860(20)31737-3
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129422
MOLSTR 129422
To appear in:
Journal of Molecular Structure
Received date:
Revised date:
Accepted date:
4 June 2020
12 September 2020
7 October 2020
Please cite this article as: Rabail Ujan , Ali Bahadur , Ghulam Shabir , Shahid Iqbal ,
Aamer Saeed , Pervaiz Ali Channar , Qaiser Mahmood , Muhammad Shoaib , Ifzan Arshad ,
Muhammad Saifullah , Guocong Liu , Rana Muhammad Irfan , Zahoor Ahmad , Mohsin Javed ,
Muhammad Raheel ,
Muhammad Abdul Qayyum ,
Bilal Khalid ,
Komal Rizwan ,
Facile
synthesis of novel fluorescent thiazole coumarinyl compounds: Electrochemical, time re-
solve fluorescence, and solvatochromic study, Journal of Molecular Structure (2020), doi:
https://doi.org/10.1016/j.molstruc.2020.129422
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Highlights



Synthesis of new thiazole coumarinyl compounds is investigated.
Thiazole coumarinyl compounds exhibited high fluorescence.
Thiazole coumarinyl compounds showed a solvatochromic effect in absorbance and
fluorescence.
1

Facile synthesis of novel fluorescent thiazole coumarinyl compounds: Electrochemical, time
resolve fluorescence, and solvatochromic study
Rabail Ujan1‡, Ali Bahadur2‡, Ghulam Shabir3, Shahid Iqbal4*, Aamer Saeed3*, Pervaiz Ali Channar3,
Qaiser Mahmood5, Muhammad Shoaib3, Ifzan Arshad3, Muhammad Saifullah3, Guocong Liu4, Rana
Muhammad Irfan6, Zahoor Ahmad7*, Mohsin Javed8, Muhammad Raheel9, Muhammad Abdul Qayyum10,
Bilal Khalid11, Komal Rizwan12
Graphical Abstract:
Uv-vis spectras of thiazole coumarinyl Derivatives
5a
5b
5c
5d
5e
5f
5g
5h
5i
Flourescence spectra of thiazole coumarinyl Derivatives
1.5
1.2
0.9
0.6
0.3
0.0
300
200
100
0
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5j
5k
400
500
600
300
325
350
375
400
425
450
475
Wavelength (nm)
Wavelength (nm)
Research Highlights:



The synthesis of new thiazole coumarinyl compounds is investigated.
Thiazole coumarinyl compounds exhibited high fluorescence.
Thiazole coumarinyl compounds showed a solvatochromic effect in absorbance and
fluorescence.
2

Facile synthesis of novel fluorescent thiazole coumarinyl compounds: Electrochemical, time
resolve fluorescence, and solvatochromic study
‡
‡
Rabail Ujan1 , Ali Bahadur2 , Ghulam Shabir3, Shahid Iqbal4*, Aamer Saeed3*, Pervaiz Ali Channar3,
Qaiser Mahmood5, Muhammad Shoaib3, Ifzan Arshad3, Muhammad Saifullah3, Guocong Liu4, Rana
Muhammad Irfan6, Zahoor Ahmad7*, Mohsin Javed8, Muhammad Raheel9, Muhammad Abdul Qayyum10,
Bilal Khalid11, Komal Rizwan12
1Dr. M. A. Kazi Institute of Chemistry, University of Sindh, Jamshoro, Pakistan.
2Department of Transdisciplinary Studies, Graduate School of Convergence Science and Technology, Seoul
National University, Seoul, 16229, South Korea.
3Department of Chemistry, Quaid-i-Azam University, 45320 Islamabad, Pakistan.
4School of Chemistry and Materials Engineering, Huizhou University, Huizhou 516007, Guangdong China.
5Institute of Chemistry, Chinese Academy of Sciences, Beijing, China.
6School of Energy, Soochow University, Suzhou, China.
7Department of Chemistry, University of Engineering and Technology, Lahore-54890, Pakistan.
8Department of Chemistry, School of Science, University of Management & Technology, Lahore-54770, Pakistan.
9Department of Chemistry, Balochistan University of Information Technology, Engineering and Management
Sciences, Quetta, Pakistan.
10Department of Chemistry Division of Science and Technology University of Education Lahore Pakistan.
11Department of Chemistry, University of Okara, Renala Khurd, Okara, Punjab, Pakistan.
12Department of Chemistry, University of Sahiwal, Sahiwal 57000, Pakistan.
* Corresponding Should be Addressed to:
shahidiqbal@hzu.edu.cn (Shahid Iqbal), aamersaeed@yahoo.com (Aamer Saeed),
zahoorchem@yahoo.com (Zahoor Ahmad)
‡ Authors contributed equally
Abstract: In this study, Benzocoumarin-Thiazoles-Azomethine derivatives with bioactive
scaffolds were synthesized and characterized. The present investigation is concerned with the
multistep synthesis of thiazole coumarinyl derivatives (5a-k), which were accomplished from
naphthaldehyde, ethyl acetoacetate, and thiosemicarbazide. The formation of newly synthesized
derivatives was confirmed by 1H NMR and 13C NMR spectroscopic studies. Thiazole coumarinyl
derivatives were subjected to UV-Visible studies in different solvents such as ethanol, ethyl
acetate, and DMF for solvatochromic studies. The synthesized coumarinyl thiazole compounds
3

showed absorption in the range of 332-390 nm. Electrochemical studies were performed in
DMSO and redox behavior was offered by thiazoles. Fluorescence of coumarinyl thiazole
compounds were examined in ethanol, ethyl acetate, and DMF to visualize the solvent effect on
the emitting ability of compounds. Fluorescence spectra of coumarinyl thiazoles expressed a
sharp emission in the range 436-550 nm.
Keywords: Coumarin derivatives; Fluorescence studies; Electrochemical study; Time-resolved
Photoluminescence; CV
1. Introduction
Pharmacological potential of natural and synthetic heterocyclic compounds in increasing
exponentially from the past two decades. Therefore, the syntheses of sulfur and nitrogen
heterocycles appending coumarin moieties, have attracted significant importance from organic
and medicinal chemists. Heterocycles probably constitute the biggest class of organic
compounds that play a vital role in several biologically active pharmaceutical and fine chemicals
[1-5].
Coumarin and its related analogs are the heterocycles which are found to occur naturally
as secondary metabolites [6]. The compounds belonging to this chemical class, are considered as
a backbone to obtain diverse biological and chemical agents. Moreover, it has already been
reported that coumarin derivatives having thiazole, azetidinone, and oxazole ring exhibit
remarkable antifungal and antibacterial activities [7-9]. The coumarin derivatives are the
intermediates for the synthesis of pharmaceutical agents having anti-HIV, anti-cancer, and anti-
bacterial potential [10-13]. Furthermore, these can be also be used as photoluminescent materials
due to fluorescence [14-16].
Thiazoles are extensively used in different chemical reactions as parent nuclei,
intermediates as well as a substituent [17]. These heterocycles have also been reported for the
synthesis of various thiazolyl coumarin derivatives [18]. A clear majority of microbial and
marine originated thiazoles have been reported for various pharmacological activities like an
antibiotic, insecticidal, antioxidant, anticonvulsant, anti-Parkinson, antitumor, antiproliferative,
antitubercular, anti-Alzheimer, anti-inflammatory, antifungal antidegenerative, and larvicidal
4

activities [19-25]. Moreover, these compounds have also been successfully employed in various
fields of organic chemistry, agriculture, and medicines, and are being employed as valuable
intermediates for synthetic dyes, medicines, drugs, and fungicides [26-30].
Furthermore, an increase in coumarin derivatives, and their screening for biological
activity has made it necessary to study this chemical class as such scaffolds possess significant
therapeutic capabilities [31-33]. Keeping in view the above-mentioned broad range of
applications of triazoles and chromogens, novel mixed heterocyclic compounds are synthesized
in this work. Here in this manuscript, we reported the green synthesis of thiazole derivatives (5a-
k) by the reaction of thiosemicarbazone (3a-k), with 3-bromoacetyl-(6-substituted)-coumarin
(1).
2. Experimental
2.1 Materials
Naphthaldehyde, ethyl acetoacetate and thiosemicarbazide, ethanol, N,N-
Dimethylformamide, and ethyl acetate, dimethyl sulfoxide, acetic acid, sodium acetate, bromine,
piperidine, chloroform, purchased from Sigma-Aldrich and used for the synthesis of coumarinyl
thiazole derivatives.
2.2 Characterizations
An NMR (Bruker DPX 300) of 300 MHz frequency was used for obtaining 1H-NMR and
13C-NMR spectra by using DMSO as solvent and TMS as an internal standard. Nicolet IR 100
was used for obtaining FTIR spectra, UV studies were performed by using UV-Genesys
spectrophotometer. The elemental analysis was done on a Flash EA 1112 elemental analyzer.
The melting point was determined with the Stuart melting point apparatus. Purification was
confirmed by pet ether and ethyl acetate (7: 3, v/v) as a mobile phase on the silica gel G plates.
Fluoromax-4 spectrofluorometer (Horiba Scientific Japan) was used for emission spectra and
FL-980 Steady/Transient state fluorescence spectrometer (Edinburgh Instruments, UK) for time-
resolved PL spectroscopy. IME6X electrochemical workstation at voltage -1.5 to 1.5 V and
scanning rate of 0.1 mVs-1 was used for cyclic voltammetry.
2.3 Coumarinyl Thiazole Compounds Synthesis:
5

Synthesis of coumarinyl thiazole compounds involve a four-step procedure involving the
synthesis of coumarins, its bromination, Schiff base formation of thiosemicarbazide with
aromatic aldehydes (3a-k) to achieve targeted thiazoles. In the first step, piperidine (1 g, 1.12 ml)
was added to the mixture of ethyl acetoacetate (0.1 mol, 12.63 ml) and 2-hydroxy-1-1
naphthalaldehyde (0.1 mol) and stirred the reaction mixture. The solid obtained was washed with
ethanol and crystallized in water to get pure 2-acetyl-3H-benzo[f]chromen-3-one (1). In the
second step, a solution 2-acetyl-3H-benzo[f]chromen-3-one (0.01mol, 2.38) in chloroform was
vigorously shake and mixed with bromine solution (1.72g, 0.01 mol). The mixture was refluxed
(2-3 h), cooled and pure 2-(2-bromoacetyl)-3H-benzo[f]chromen-3-one (2) was obtained by
crystallization in ethanol: chloroform (1:2). In the third step, substituted aldehydes (3a-k) (1.0
mM) were added to thiosemicarbazide (0.138 g, 1.0 mM) along with 1-2 drops of acetic acid in
25 mL absolute ethanol. The reaction mixture was refluxed for 6 h, cooled the mixture, and solid
was recrystallized in ethanol to produce compounds (4a-k). In 4th and last step, target
compounds (5a-k) were prepared by stirring and refluxing the mixture of 2-(2-bromoacetyl)-3H-
benzo[f]chromen-3-one (2) (0.0051 mol), and 4a (0.0061 mol) in 25 mL of sodium
acetate/ethanol. At the end reaction mixture was cooled and the solid product obtained was
filtered and crystallized in ethanol. In a similar way series of coumarinyl thiazoles (5a-k) were
synthesized. Supporting information contains complete characterization data.
6

Scheme 1. Synthesis of coumarinyl thiazole compouds (5a-k)
3. Results and Discussion
3.1. Synthesis
Coumarinyl thiazoles 5a-k were prepared as sketched in Scheme 1 and all the products
were in high purity and high yields (Fig. 1). The 3-bromoacetylcoumarin (3) was refluxed with
thiosemicarbazones (4a-k) in ethanol to produce the title compounds (5a-k) [34].
7

Fig. 1. synthesized coumarinyl thiazole (5a-k) derivatives
3.2 Spectral Characterization of Coumarin derivatives:
UV, FTIR, and NMR were used to confirm the structures of the coumarinyl derivatives
5a-k. The UV/Vis absorption spectra are shown in Fig. 2 and the particular spectral data are
given in Table 1. UV-visible spectra of these compounds showed the maximum absorption in the
range of 332-390 nm (Fig. 2(a-c)). This absorption might be due to the π-π* transition of
electrons in aromatic rings (benzene and naphthalene) present in all derivatives. There is an
abundance of electrons that produced a bathochromic shift on conjugation with aromatic rings.
(b)
5a
5b
5c
5d
5e
5f
5g
5h
5i
(a)
0.8
0.6
0.4
0.2
0.0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5j
5k
300
325
350
375
400
425
450
475
300
325
350
375
400
425
450
475
Wavelength (nm)
Wavelength (nm)
(c)
1.5
5a
5b
5c
5d
5e
5f
5g
5h
5i
1.2
0.9
0.6
0.3
5j
5k
0.0
300
325
350
375
400
425
450
475
Wavelength (nm)
Fig. 2. UV/Vis spectra of 5a-k in (a) ethanol, (b) ethyl acetate and (c) DMF
8

The higher λmax (360-384 nm) for 5e, 5c-g, and 5i-j is due to n-π* of lone pair and π-π*
transitions of π electrons. The values of λmax for these compounds depend on the nature of the
solvents and are changed with changing solvent polarity and the largest λmax was observed for
any given compound in DMF having high polarity among the given set of three solvents. This
demonstrates that the π-π* transitions are of B-type bands and do not depend upon the solvent.
Therefore, no phenomenon of solvatochromism was observed.
Table 1: λmax (nm) of coumarinyl derivatives (5a-k)
Sr. No.
5a
Ethanol (λmax
)
Ethyl acetate (λmax
)
DMF (λmax
353
)
336
332
351
365
365
371
360
363
372
366
372
375
363
366
5b
5c
381
390
381
388
5d
5e
375
381
372
379
5f
370
375
5g
376
390
5h
5i
375
389
375
380
5j
378
385
5k
The FTIR spectra of coumarinyl thiazoles 5a-k showed an absorption band due to C=N of the
imine, C=O, Ar-H, and N-H of lactone. The bands at 3370-3399, 2985-3180, 1727-1760, 1625-
1650, 1515-1535, 1420-1440, 1275-1290, 1110-1150, 840-865, and 790-825 cm-1 are due to the
stretching and bending vibration frequencies of C=C, C=S, and C-O, C=C, C=S, and C-O bonds.
In coumarinyl thiazole 5a, the bands at 3370 cm-1 and 3168 cm-1 are due to N-H and C=C-H
stretching [35]. A peak at 1727cm-1 confirmed the ester linkage present in carbonyl of cyclic
esters. All the coumarinyl thiazoles have common peaks for stretching vibrations of N-H, C=C-
H, and C=O. The presence of a band at 1149 cm-1 is due to the C-O stretching, at 1630, 1533 due
to C=C stretching and 859 cm-1 is due to the C=C stretching bending. These are mainly due to
9

the benzene rings in 5a-k while other peaks also confirm the various functionalities present and
agree with the structure.
1
The H NMR of 5a shows a singlet at 2.32 ppm for 3H of methyl group substituted at benzene
ring. A singlet at 12.25ppm is due to the N-H substituted heterocyclic and phenyl rings of
coumarin. The six aromatic protons on coumarin show a multiplet at 7.80-8.34 ppm. All
coumarins are differentiated by the presence of coumarin H-4 at 9.251 ppm. p-Disubstituted
benzene is confirmed by doublet signals at 7.562 and 7.663 ppm (Fig. 3a). Different aromatic
carbons in 5a are confirmed by the peaks at 152.7, 144.8, 137.2, 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, 115.6, 114.1 and 112.1 ppm in 13C-
NMR spectra. The carbonyl carbon appeared at 169.3 ppm, imine carbon at 159.1 ppm and
methyl carbon at benzene ring is at 21.06 ppm. Similarly, other compounds 5a-k have been
confirmed from their respective 1H-NMR and 13C-NMR spectra (Fig. 3b).
Fig. 3. (a) 1H-NMR of coumarin derivative (5a) (b) 13C-NMR of coumarin derivative (5a)
3.3 Electrochemical Properties
Electrochemical properties of coumarinyl thiazole compounds 5a-k were studied by
cyclic voltammetry using DMF containing 0.1 M TBAPF6 as an electrolyte and a glassy carbon
electrode. Cyclic voltammograms as shown in Fig. 4 were used to calculate redox potential, and
energies of HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied
molecular orbital) values as shown in table 2.
10

3.3.1 Redox Potentials (E1/2)
The compound 5f having thiophene ring on the chromophore possesses lowest redox
potential and 5b and 5d having chlorophenyl and fluorophenyl rings attached to thiazole possess
the highest values as shown in Table 2. This higher redox potential might be due to the dual
behavior of halogens (electron donor and acceptor). The lowest value of redox potential of 5f is
due to sulfur atom present in the thiophene ring which has variable oxidation states and electron-
donating effect.
5a
5b
0.000015
5c
5d
5e
0.000010
5f
5g
5h
0.000005
5i
5j
5k
0.000000
-0.000005
-0.000010
-0.000015
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
E (V)
Fig. 4, Combined cyclic voltammogram of coumarin derivatives (5a-k) in ethanol
3.3.2 Lowest unoccupied molecular orbital (LUMO)
To determine the energy of LUMO concerning the vacuum level, redox data were standardized
with ferrocene/ferricenium couple having absolute energy of –4.8 eV. Energies of LUMO given
in Table 2 are in the range of -4.94 eV to -5.16 eV, and it is witnessed that electron-donating
groups (EDG) on coumarinyl part lower the energy of LUMO and vice versa. The energy of
LUMO changes by increasing the delocalization of electrons and energy difference decreases by
increasing conjugation and vice versa. The lowest energy LUMO is 5k having bromothiophene,
in which sulfur donates electrons and decreases the energy of LUMO.
3.3.3 Band Gap Energy (Eg)
11

The standard procedure given in the literature was used to calculate the optical band gap values.
Every solid has energy-band which is responsible for its electrical properties. The bandgap energy
of coumarinyl derivatives is shown in Table 2 and varies from 3.25 to 3.71 eV. The value is
highest for 5d, 5e, 5h, and minimum for 5c, 5g, and 5j which depends upon substituents as well as
on the precursor used. Chloro and nitro groups attached on 5c and 5g are at m and o to each other
and act as donor and acceptor atoms, increase the electron density, and energy levels get closer,
thus band gap energy is decreased.
Table 2: LUMO/ eV energy levels of coumarinyl thiazoles (3a-k)
S. No.
5a
5b
5c
E1/2 (V)
-0.7232
-0.83
LUMO (eV)
-4.07
-3.97
-0.817
-0.806
-0.465
-0.01
-3.98
-3.99
5d
5e
-3.94
-4.39
5f
-0.601
-0.488
-0.807
-0.469
0.361
-4.20
5g
5h
5i
-3.99
-3.99
-4.86
5j
-5.16
5k
Table 3: HOMO/eV energy levels and bandgap energy coumarin derivatives
S. No.
5a
Eg (V)
3.51
3.38
3.25
3.60
3.71
3.44
3.29
HOMO (eV)
-7.58
5b
-7.35
5c
-7.23
5d
-7.59
5e
-7.65
5f
-7.83
5g
-7.49
12

5h
5i
3.68
3.30
3.28
3.35
-7.67
-7.29
-8.14
-8.54
5j
5k
3.3.4 Highest occupied molecular orbital (HOMO)
Table 3 shows the HOMO energy levels which are in the range of -7.23 to -8.54 eV for 5a-
k. For 5j and 5k energy levels of HOMO are low due to the thiophene and pyridine rings
attached to chromophores, while 5c has the highest energy levels due to the inductive effect of
nitro. EWG increases their energy while EDG decreases energy as the energy gap between
HOMO and LUMO is increased.
4. Fluorescence Studies
All the coumarin thiazoles (5a-k) exhibited moderate fluorescence as given in Table 4.
All the synthesized derivatives fulfill the requirements of fluorescence as these derivatives have
no vibrational or rotational signals and are highly conjugated. The fluorescence spectra were
recorded at 385 nm of the source in chronicle order as shown in Fig. 5(a-b) and Fig. 3S
(Normalized spectra), at different excitation wavelengths. The spectra provide the intensity
influence at a certain wavelength by changed excitation wavelengths. All the fluorescent dyes
show only a single emission peak. The 5h dye shows the highest fluorescence at 505 nm on the
excitation of the solution at concentration 1x 10-5 M (Fig. 5d). Ethanolic solutions exhibit the
highest fluorescence, while DMF and ethyl acetate show lesser fluorescence which might be due
to the association of thiazoles in these solvents which causes a reduction in fluorescence. Higher
fluorescence makes them highly useful compounds to bio-imaging and bio-labeling. To study the
excited-state intramolecular proton transfer (ESIPT) mechanism either involved in the emission
behavior of thiazoles or not, the emission spectra were conducted in THF/water mixtures of
compound 5g at 1 x 10-5 M solution, but with different volume fractions of water (fw) from 0-
90% (Fig. 5d). THF and water were selected as the suitable solvent and non-solvent pair
considering that they are miscible. Thus, increasing the water fractions can change the dissolved
compounds in pure THF solutions into aggregated particles which cause changes in emission
spectra. It was observed that in pure THF probe solution was nonfluorescent due to free rotations
of the molecule. On adding water in solution in THF, fluorescence continuously increased and
was found maximum at 90% fw..
13

5. Time Resolved Fluorescence Study
Time-resolved photoluminescence (TRPL) spectroscopic measurements were used to
study the radiative emission and for this analysis all samples were prepared in absolute ethanol.
Time-correlated single-photon counting technique (TCSPT) was applied for a time-resolved
(TR) analysis of the manufactured material. A laser beam of 305 nm was used as an excitation
source to study the PL lifetime of samples. Fluorescence decay behavior of the samples was
explored by TRPL and the measurements were performed with a time resolution of 25 ps at room
temperature. For the comparison of the decay times of samples, the time at which the SSPL
strength has reduced to 1/e of its original value (τ 1/e) has been used as a parameter in TRPL
measurements and PL decay kinetics approved to be biexponential in our system. Fig. 5(c)
describes a comparison of the decay time of all samples under consideration. The samples were
excited using a laser pulse of 305 nm at room temperature. Zero time signifies the measurement
nearly after the laser pulse. PL from several decay time gates has been observed instantly after
excitation from the laser pulse.
The radiative period is the average time that the hole-electron pair takes to recombine,
after the formation of an exciton. The experimental data from the decay profile was fitted with a
bi-exponential decay function. This shows that the lifetime of exciton decreases but to a very
limited extent and appears almost the same except sample 5d. This sample has a huge lifetime as
compared to other family members the rate of the energy transference is reduced which results in
both a longer lifetime and a less nonexponential decay for the exciton PL. The exciton life period
turns out to be short for other sample and the decay curve becomes further nonexponential as the
detection is moved to higher energies. This decrease in the lifetime of is probably due to the
creation of additional discrete energy levels for depopulation of the excited state.
Table 4: Fluorescence data of Coumarinyl thiazoles (5a-k) in ethanol and ethyl acetate
Dye Excitation wavelength (nm)
Emission wavelength (nm)
Stoke Shift
Ethanol
456
495
465
515
Ethylacetate
Ethanol Ethylacetate
445
445
450
450
450
453
525
464
550
500
11
50
15
65
65
08
80
14
100
50
5a
5b
5c
5d
5e
515
14

436
440
460
440
440
445
450
500
505
505
485
525
453
490
490
500
505
525
14
60
45
65
45
80
17
50
30
60
65
80
5f
5g
5h
5i
5j
5k
20
(a)
(b)
5a
5b
5c
5d
5e
5f
5a
5b
5c
5d
5e
5f
5g
5h
5i
300
5g
5h
5i
5j
5k
200
100
0
10
5j
5k
0
400
400
500
600
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
(c)
(d)
0% Water
2000
1500
1000
500
0
5a
1.0
10% Water
20% Water
30% Water
40% Water
50% Water
60%Water
70% Water
80% Water
90% Water
5b
5c
5d
5e
5f
5g
5h
5i
0.8
0.6
0.4
0.2
0.0
5j
5k
450
480
510
540
570
600
630
36
38
40
42
44
46
48
Decay time (ns)
Wavelength (nm)
Fig. 5. The fluorescence spectrum of coumarin derivatives (5a-k) in (a) ethanol, (b) ethyl acetate, (c) Combined
fitted decays in TRPL plot of coumarin derivatives (5a-k), (d) Effect of water on the emission intensity of compound
5g.
6. Conclusion
In this work, a series of coumarinyl derivatives were synthesized via a facile route. These
compounds having yellow color and showed absorptions in 370-395 nm range. The λmax was
15

determined in different solvents and the highest value was obtained in DMF because the polar
solvents shift π-π* wavelength to the higher value. Solvatochromic behavior was also observed
and the bathochromic shift was observed by increasing the solvent polarity. The fluorescence
study showed the higher value of stoke shift and maximum emission was observed for 5h. The
coumarinyl derivatives containing methoxy groups have the lowest bandgap and high energy
HOMO. Higher fluorescence makes them highly useful compounds to be used for bio labeling
and bioimaging applications.
Conflict of interest
Authors declare no conflict of interest
Acknowledgments
The authors gratefully acknowledge the support received for this research work from the
National Natural Science Foundation of China (51162026), the Major Project of Fundamental
and Application Research of the Department of Education of Guangdong Province
(2015KTSCX132 and 2017KZDXM080), the Science and Technology Foundation of Huizhou
(2019X0705016) and Korean Research Fellowship Program through the National Research
Foundation of Korea (NRF), Korea, funded by the Ministry of Science and ICT (Grant No. NRF-
2019H1D3A1A01102931).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the online version, at doi:
Credit Author Statement
Rabail Ujan: Performed major experimental works, writing-original draft preparation. Ali
Bahadur: Conception, design of study, conducted photoluminescence analysis, writing-original
draft preparation. Ghulam Shabir: Material synthesis, visualization of data. Shahid Iqbal:
Conception, design of study, performed some experimental works, writing-original draft
preparation, supervision. Aamer Saeed: Conception, design of study, Interpret the data.
supervision. Pervaiz Ali Channar: Writing-original draft preparation and editing. Qaiser
Mahmood: Writing-original draft preparation and editing. Muhammad Shoaib: Visualization
of data, writing-reviewing and editing. Ifzan Arshad: Visualization of data, writing-reviewing
and editing. Muhammad Saifullah: Reviewed revised manuscript and critical revision.
16

Guocong Liu: Interpret the revised data, help a lot in revising the manuscript, supervision. Rana
Muhammad Irfan: Reviewed revised manuscript and critical revision. Mohsin Javed:
Drafting the revised manuscript and critical revision. Muhammad Raheel: Reviewed revised
manuscript and critical revision. Muhammad Abdul Qayyum: Visualization of data, writing-
reviewing and editing. Bilal Khalid: Visualization of data, writing-reviewing and editing.
Komal Rizwan: Drafting the revised manuscript and critical revision.
Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
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