Synthesis, photophysical and electrochemical properties of donor-acceptor type hydrazinyl thiazolyl coumarins

Article abstract of DOI:10.1039/d0nj00082e

A water-mediated MCR strategy has been developed for the synthesis of donor (D)-acceptor(A) type hydrazinyl thiazolyl coumarins (HTCs) (4) in excellent isolated yields (90-98%) from a three %component reaction of aromatic aldehydes/ketones (1), thiosemicarbazide (2) and 3-(2-bromoacetyl)-2H-chromen-2-one (3) catalyzed by an environmentally compatible montmorillonite (MMT) K10 clay at RT for 20-40 min. The present MCR strategy has several advantages that include its wide-spread substrate scope, eco-compatibility, short reaction times and products that do not require chromatographic purification. Besides, the method is simple to perform, it is easy to construct C-N, C N and C-S bonds in one pot and the reaction can be scaled up to a gram level. Moreover, the catalyst can be reused 7 times without significant change of activity. The photophysical properties of the synthesized D-A type HTCs are also studied and it was noticed that the fluorescence properties can be varied with the position of electron donating group on the aromatic ring of aldehyde/ketone of HTCs. Most of the compounds exhibited bright fluorescence in chloroform (1.0 × 10^?5M) with emission maxima ranging from 409 to 511 nm and large Stokes shifts. Further, the HOMO and LUMO energy levels of the HTCs are found in the range from ?5.65 eV to ?5.22 eV and ?2.67 eV to ?2.17 eV, respectively, and in good agreement with the reported hole transporting materials.

Full text of DOI:10.1039/d0nj00082e

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DOI: 10.1039/D0NJ00082E
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Synthesis, Photophysical and Electrochemical Properties of
Donor-Acceptor Type Hydrazinyl Thiazolyl Coumarins
Received 00th January 20xx,
Accepted 00th January 20xx
G. Kumar, a G. Trivikram Reddy,a B. Ramakrishna,b Y.V. Divyasri,a L. Subramanyam Sarmaa and
N. C. Gangi Reddy*a
DOI: 10.1039/x0xx00000x
A water mediated MCR strategy has been developed for the synthesis of donor (D)-acceptor(A) type hydrazinyl
thiazolyl coumarins (HTCs) (4) in excellent isolated yields (90–98%) from a three component reaction of aromatic
aldehydes/ketones (1), thiosemicarbazide (2) and 3-(2-bromoacetyl)-2H-chromen-2-one (3) catalyzed by an
environmentally compatible montmorillonite (MMT) K10 clay at RT for 20-40 min. The present MCR has several
advantages which include its wide-spread substrate scope, eco-compatibility, short reaction times and the
products does not require chromatographic purification. Besides, the method is simple to perform, easy to
construct C-N, C=N and C-S bonds in one-pot and can be scaled up to a gram level. Moreover, the catalyst is
reused for 7 times without significant change of activity. Photo physical properties of the synthesized D-A type
HTCs are also studied and noticed that the fluorescence properties are varied with the position of electron
donating group on the aromatic ring of aldehyde/ketone of HTCs. Most of the compounds exhibited bright
fluorescence in chloroform (1.0 × 10−5 M) with emission maxima ranging from 409 to 511 nm and large Stokes
shifts. Further, The HOMO and LUMO energy levels of HTCs are found in the range from -5.65 eV to -5.22 eV
and -2.67 eV to -2.17 eV, respectively and in good agreement with the reported hole transporting materials.
current reseach.5 Among the heterocycles, coumarins and
thiazoles are considered as privileged heterocyclic compounds
because of their potential optoelectronic and biological
Introduction
Design and development of multi-component reactions (MCRs)
in water have received considerable attention, particularly in
the areas of organic synthesis, combinatorial synthesis, drug
discovery, agrochemistry and material science.1-3 Water is
universally accepted benign solvent for synthetic organic
chemistry, because of its natural abundance, non-toxicity,
large dielectric constant, high heat capacity, cost-efficiency,
non-flammability, extensive hydrogen bonding, environmental
compatibility etc.3a-d Further, the search and identification of
water-compatible catalysts for MCRs is one of the most active
areas of research in organic synthesis.3
Fluorescent heterocyclic compounds are an important class of
organic materials and received great attention because of their
wide variety of applications in the fields of organic electronics
and biomedical research.4 Therefore, the design and
development of donor-acceptor type fluorescent heterocyclic
molecules by incorporating both hole–transport and electron–
transport units is one of the continuing objectives in the
properties.6-9 In particular, the hydrazinyl thiazolyl coumarin
(HTC) derivatives exhibit promising biological activities which
includes antimicrobial, antioxidant, antibacterial, anti-
tubercular, antitumor activities etc.10 Till now, both two step
and one-pot synthetic protocols are available for the synthesis
of HTCs. The reported two-step methods proceed through the
formation of thiosemicabazone by the reaction of carbonyl
compounds with thiosemicarbazide followed by cyclization
with 3-bromoacetylcoumarin.10a-d,i,j However, the major
margins of the reported methods are the involvement of
multiple steps, use of expensive techniques and are unsuitable
for large-scale applications. Furthermore, these protocols have
narrow substrate scope, procedural complexity and lengthy
reaction times. To date, very limited one-pot methods have
been reported for the synthesis of hydrazinyl thiazolyl
coumarins from ketones, thiosemicarbazide and phenacyl
bromides
or
3-(2-bromoacetyl)-2H-chromen-2-ones.10k,l
Though the reported one-pot methods are able to provide the
desired products, but there are still some drawbacks like the
usage of acetic acid as catalyst under harsh reaction
conditions, lengthy reaction times, (Ibrar et al.,),10k limited
substrate scope (Rao et al., applied the method only on three
liquid alicyclic ketones)10l and lack of gram scale feasibility. To
overcome the aforesaid shortcomings, the development of
facile, scalable, green and sustainable MCR strategy is still
required for the synthesis of pharmacologically important
a. Department of Chemistry, School of Physical Sciences, Yogi Vemana University,
Kadapa - 516 003, Andhra Pradesh, India.
b. School of Chemistry, University of Hyderabad, Central University P.O.,
Hyderabad 500 046, India.
E-mail: ncgreddy@yogivemanauniversity.ac.in
Electronic Supplementary Information (ESI) available: [General Experimental,
Characterization data, copies of 1H NMR, HRMS and MS spectra of title
compounds and crystallographic data of 4b in CIF format (CCDC 1952520).
See DOI: 10.1039/x0xx00000x
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HTCs. Moreover, no examples of fluorescent HTC compounds Table 1. Optimization of reaction conditions for the synthesis
have been reported so far.
In the present paper, we describe a practical and green MCR 2-one (4a).a
View Article Online
of(E)-3-(2-(2-benzylidenehydrazinyl)thiazDoOl-I4: -1y0l.)1-023H9/-Dch0NroJ0m0e0n82-E
O
strategy for the synthesis of HTC (4) derivatives via a three-
component reaction of aromatic aldehydes/ketones (1),
thiosemicarbazide (2) and 3-bromoacetylcoumarin in the
presence of montmorillonite (MMT) K10 clay11 as a water
compatible and reusable heterogeneous green solid acid
catalyst under ambient reaction conditions (Scheme 1).
Further, the photophysical and electrochemical properties of
the synthesized donor (D) - acceptor (A) type HTC compounds
were studied.
O
O
S
O
3
O
O
Catalyst
H
H2N
+
+
NH2
N
N
Solvent
RT
H
NH
N
Br
1a
S
2
4a
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
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45
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48
49
50
51
52
53
54
55
56
57
58
59
60
Entry Catalysts
Solvent
Time
(min)
Product
4a
Yieldb
(%)
(2.5 wt-%)
1c
2
-
H2O
H2O
EtOH
180
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
-
30
35
30
-
AlCl3
FeCl3
TiCl4
ZnCl2
Isopropanol
EtOAc
Acetonitrile
4a
4a
4a
4a
O
S
O
O
O
O
R''
R'
MMT K10 Clay
Water, RT
R''
H2
N
R
+
+
NH2
Ar/Het
N
Br
A r / H e t
N
H
5
3
NH
2
20-40 min
1
O
R'
N
S
4a - 4ak
R= H/Me/Ph
R'= Me, OMe, OH, OPh, NH2, NO2, CN, Br, F; R''=H
R'= OH, R''= Cl; R'= OH, R''= Br; R'= OH, R''= OMe; R'= OMe, R''=OH;
R'=R''= OMe; R'= OEt, R''= OH; R'=R''= Br; R'= R''= Cl
H2O
EtOH
25
20
20
-
10
35
25
20
-
R
Hydrazinyl Thiazolyl Coumarins
(37 examples)
3
4
5
Isopropanol
EtOAc
Acetonitrile
Scheme 1. MMT K10 clay catalyzed multi-component synthesis of
diversified HTC derivatives (4a-4ak) in water.
H2O
EtOH
Results and discussion
Isopropanol
EtOAc
Acetonitrile
5
In the present study, we aimed to (i) develop a water-
mediated 3-component reaction for the synthesis of HTCs by
incorporation of three important pharmacophores i.e.
coumarin, thiazole and imine group in a single step operation
ii) identify the water-compatible, environmentally benign and
reusable catalyst for the above 3-component reaction iii) apply
the optimized method for the synthesis of a series of HTCs and
iv) study the photophysical and electrochemical properties of
the synthesized D-A type HTCs. To achieve the targets, initially,
a control experiment was conducted for the 3-component
synthesis of (E)-3-(2-(2-benzylidenehydrazinyl)thiazol-4-yl)-2H-
chromen-2-one (4a) by using benzaldehyde (1a),
thiosemicarbazide (2) and 3-(2-bromoacetyl)-2H-chromen-2-
one (3) as model substrates. Unfortunately, no product was
formed in the absence of catalyst in water at RT (Table 1, entry
1). Further, the same reaction was repeated by employing a
variety of Lewis acid catalysts such as AlCl3, FeCl3, TiCl4, ZnCl2,
SnCl4, ZrOCl2, MMT K10 clay, Sc(OTf)3 and Yb(OTf)3 in various
solvents at RT (Table 1, entries 2-10). After examining these
catalysts and solvents, it was found that the MMT K10 clay (2.5
wt %) in water was the best option to obtain better yield (75%)
of the product (4a) (entry 8, Table 1) whereas the other
catalysts afforded moderate to low yields of 4a (entries 2-7, 9
& 10, Table 1). From this study, it was also observed that the
3-component reaction proceeded well in polar protic solvents
as compared with polar aprotic solvents (Table 1). Further, the
amount of MMT K10 clay was varied (5.0, 7.5, 10.0, 12.5, 15.0,
17.5 and 20.0 wt%) to improve the yield of 4a (Table 2). The
study revealed that the 12.5 wt % of MMT K10 clay was the
best option to get the highest yield of product 4a (98%) in
water at RT within a short reaction time (20 min) (Table 2,
entry 5). Further, it was also found that the same yields were
obtained despite the increase in the amount of loading of
catalyst i.e. 15.0, 17.5 and 20.0 wt% (Table 2, entries 6-8).
H2O
EtOH
60
60
60
60
60
30
30
25
-
Isopropanol
EtOAc
Acetonitrile
10
H2O
EtOH
60
60
60
60
25
20
20
-
Isopropanol
EtOAc
6
SnCl4
4a
Acetonitrile
60
-
H2O
EtOH
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
60
40
35
40
-
10
75
65
55
-
15
55
60
55
-
20
60
65
50
-
7
8
ZrOCl2
Isopropanol
EtOAc
Acetonitrile
4a
4a
H2O
EtOH
MMT K10
clay
Isopropanol
EtOAc
Acetonitrile
H2O
EtOH
9
Sc(OTf)3
Isopropanol
EtOAc
Acetonitrile
4a
4a
H2O
EtOH
10
Yb(OTf)3
Isopropanol
EtOAc
Acetonitrile
20
aReagents and conditions: Benzaldehyde (1a) (10.0 mmol),
thiosemicarbazide (2) (10.0 mmol), 3-(2-bromoacetyl)-2H-chromen-2-
one (3a) (10.0 mmol), catalyst (2.5 wt-%) in various solvents at room
temperature; bIsolated yield. cReaction did not proceed in the absence
of catalyst.
2 | J. Name., 2012, 00, 1-3
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O
S
Table 2. Screening of catalyst loading for the betterment of
O
O
O
Montmorillonite
K-10 Clay
R''
O
O
RV'' iewRA' rticle Online
+ H2N
+
R
Ar/Het
N
H
NH2
yield of (4a).a
Water, RT
20-40 min
A r / H e t
Br
DOI: 10.1039/D0NJ00082E
4a - 4ak
N
3
2
NH
1
R'
N
S
R= H/Me/Ph
Entry MMT K10 clay Solvent Time Product Yieldb
R'= Me, OMe, OH, OPh, NH2, NO2, CN, Br, F; R''=H
R'= OH, R''= Cl; R'= OH, R''= Br; R'= OH, R''= OMe; R'=OMe, R''= OH;
R'=R''= OMe; R'= OEt, R''=OH;, R'=R''= Br; R'= R''= Cl
R
Hydrazinyl Thiazolyl Coumarins
(37 examples)
(wt-%)
2.5
5.0
(min)
60
60
40
40
20
20
20
20
(%)
75
83
88
92
98
98
98
98
O
O
O
H
N
H
O
O
O
S
1
2
3
4
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
4a
4a
4a
4a
4a
4a
4a
4a
N
N
N
N
N
N
N
N
H
S
S
O
OH
O
4c, 96%
4a, 98%
4b, 98%
7.5
O
O
O
H
N
O
H
O
H
N
OPh
N
N
10.0
12.5
15.0
17.5
20.0
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
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49
50
51
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59
60
N
N
N
N
N
S
S
S
HO
5
4f, 95%
4e, 94%
4d, 96%
OH
O
H
O
6
7
8
O
N
O
N
N
H
O
H
N
O
N
N
N
N
S
N
OH
S
S
4i, 93%
Cl
NO2
4h, 94%
N
4g, 96%
O
O
O
aReagents and conditions: Benzaldehyde(1a) (10.0 mmol),
thiosemicarbazide (2) (10.0 mmol), 3-(2-bromoacetyl)-2H-chromen-2-
one (3a)(10.0 mmol), MMT K10 clay (2.5% to 20.0 wt-%) in water at
room temperature; bIsolated yield.
H
H
H
O
O
O
N
N
N
S
N
N
N
O
N
O
N
N
S
S
OH
HO
OH
4l, 93%
4k, 96%
Br
4j, 92%
H
O
O
O
O
O
H
O
O
O
O
H
O
N
N
N
N
N
N
N
N
N
S
S
S
HO
With the established optimum reaction conditions, the
substrate scope and generality of the present MCR was
explored by using a wide variety of aromatic/hetero aromatic
aldehydes (1a-x), thiosemicarbazide (2) and 3-(2-bromoacetyl)-
2H-chromen-2-one (3) (Table 3). The parent benzaldehyde (1a)
reacts with thiosemicarbazide (2), 3-(2-bromoacetyl)-2H-
chromen-2-one (3) to form the corresponding product (4a) in
98% isolated yield. Benzaldehyde bearing electron-releasing
groups (4-OMe (1b), 4-OH (1c), 3-OH (1d), 2-OH (1e) and 3-
Oph (1f) groups) at different positions on the ring reacted
efficiently with thiosemicarbazide (2) and 3-(2-bromoacetyl)-
2H-chromen-2-one(3) to give the desired products (4b, 4c, 4d,
4e and 4f) in excellent isolated yields in the range of 94 to
98%. To our delight, the structure of 4b was confirmed by X-
ray crystallography (Fig. 1). Benzaldehyde with electron-
accepting groups (4-NO2 (1g) and 4-CN (1h)) present at
different positions on the ring showed good reactivity with
thiosemicarbazide (2) and 3-(2-bromoacetyl)-2H-chromen-2-
one(3) to afford the products 4g and 4h in good isolated yields
96 and 94%, respectively. Furthermore, the di substituted
benzaldehydes (3-Cl-4-OH (1i), 3-Br-4-OH (1j), 4-OH-3-OMe
(1k), 2-OH-5-OMe (1l), 2-OH-3-OMe (1m), 2,4-diOMe (1n) 2,5-
diOMe(1o), 3-OEt-2-OH(1p), 3-OEt-4-OH (1q), 3,5-di bromo
(1r) and 2,6-di chloro (1s)) were also well-tolerated, providing
the desired products 4i-4s in good isolated yields that ranged
from 90 to 96%. Further, cinnamaldehyde (1t) underwent the
reaction to give the corresponding product 4t in 96% isolated
yield. 2-napthaldehyde (1u) afforded the corresponding
product (4u) in 95% isolated yield. Interestingly, substituted
O
4m, 94%
4o, 95%
4n, 96%
O
O
O
O
H
H
H
O
O
N
Br
N
N
N
O
N
N
N
N
N
N
S
S
S
OH
HO
4q, 96%
4p, 93%
4r, 90%
Br
O
O
O
Cl
O
H
H
O
O
O
H
N
N
O
O
N
N
N
N
N
N
N
S
S
S
Cl
4t, 96%
4u, 95%
4s, 91%
O
N
O
O
O
H
N
H
O
H
N
N
N
N
S
N
N
N
S
S
S
H2N
Cl
N
N
4v, 95%
4w, 93%
4x, 92%
O
O
H
O
O
H
O
H
O
N
N
N
N
N
N
N
N
N
N
S
S
S
4y, 97%
OMe
4z, 95%
4aa, 96%
O
O
H
N
O
H
N
O
N
S
H
N
O
N
S
N
N
N
N
S
4ab, 95%
OH
4ac,96%
4ad, 94%
O
O
OH
O
N
H
O
O
H
N
O
N
N
H
O
N
N
N
N
S
S
S
Br
F
4af, 95%
NO2
4ae, 93%
4ag, 94%
O
O
H
O
O
O
H
N
N
N
H
N
N
O
N
N
N
N
S
S
S
4aj, 93%
4ah, 94%
4ai, 90%
O
H
O
N
N
N
S
O
O
4ak, 91%
aReagents and conditions: Aldehydes/ketones (1) (10.0 mmol),
thiosemicarbazide (2) (10.0 mmol), 3-(2-bromoacetyl)-2H-chromen-2-one
(3) (10.0 mmol), MMT K10 clay (12.5 wt-%) in water at RT.
Inspired by the above results, the scope of the present MCR
strategy was extended to a variety of aromatic ketones (Table 3). To
check the feasibility, a model reaction was conducted by employing
acetophenone (1y), thiosemicarbazide (2) and 3-(2-bromoacetyl)-
2H-chromen-2-one (3) under the optimized reaction conditions for
30 min at RT. It was found that the yield of the desired product, 4y
was 97 %. Acetophenone with electron-donating groups (4-Me (1z),
4-OMe (1aa), 3-OMe (1ab), 4-OH (1ac) and 3-OH (1ad)) at different
positions on the ring showed good reactivity towards this type of
transformation to produce the corresponding products (4z-4ad) in
good to excellent isolated yields that ranged from 94 to 96%.
Acetophenone bearing electron-withdrawing groups (4-Br (1ae), 4-F
(1af) and 4-NO2 (1ag)) at para position on the aromatic ring were
also converted into the corresponding products (4ae-4ag) in good
hetero-aromatic
aldehydes
like
4-methylthiazole-5-
carboxaldehyde (1v), 2-aminonicotinaldehyde (1w) and 2-
chloro-8-methylquinoline-3-carbaldehyde (1x) reacted well
with thiosemicarbazide (2), 3-(2-bromoacetyl)-2H-chromen-2-
one(3) to afford the desired products, 4v, 4w and 4x in good
isolated yields, 95, 93 and 92%, respectively.
Table 3. Synthesis of HTC derivatives of aldehydes/ketones
(4a-4ak) using various aldehydes/ketones, thiosemicarbazide
and 3-(2-bromoacetyl)-2H-chromen-2-one.[a]
This journal is © The Royal Society of Chemistry 20xx
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Scheme 2. A plausible mechanism for the formation of HTCs V(4ie)wcaAtrtaiclylezeOdnlbinye
isolated yields. Benzophenone (1ah) also undergoes the reaction to
obtain the corresponding product 4ah in 94% isolated yield.
Further, 1-acetylnapthalene (1ai) and 2-acetylnapthalene (1aj)
reacted well with thiosemicarbazide (2) and 3-(2-bromoacetyl)-2H-
chromen-2-one (3) to afford the corresponding products, 4ai and
4aj in good isolated yields of 90% and 93%. Notably, the
heterocyclic compound 3-acetylcoumarin (1ak) also reacted well
under the optimized reaction conditions to give the desired product
(4ak) in good isolated yields. From this study, it was concluded that
MMT K10 clay in water.
DOI: 10.1039/D0NJ00082E
Reusability of the catalyst
The reusability of MMT K10 clay catalyst was investigated in the
reaction of 4-methoxy benzaldehyde (1b) (10.0 mmol),
thiosemicarbazide (2) (10.0 mmol) and 3-(2-bromoacetyl)-2H-
chromen-2-one (3) (10.0 mmol) under the optimized reaction
conditions. After completion of the reaction, 10 mL of ethyl acetate
was added to the reaction mixture and the spent catalyst was
separated by filtration under vacuum. The recovered catalyst was
washed with ethyl acetate (2.0 mL) and then 1.0 M HCl (5.0 mL)
after each use. Finally, the recovered catalyst was dried under
vacuum. To check the catalytic activity, the recovered catalyst was
reused for 7 times under the optimized reaction conditions. The
study revealed that the obtained yields of the product, 4b were 98,
98, 98, 98, 97, 97 and 97% for first, second, third, fourth, fifth, sixth
and seventh cycles, respectively (Fig. 2). It is worthy to mention that
the catalytic efficiency of the recovered catalyst is almost same as
compared with the fresh catalyst.
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the developed method was suitable for
aldehydes/ketones (mono substituted (both electron-deficient and
electron-rich), di substituted and hetero aromatic
a wide variety of
aldehydes/ketones) under the established reaction conditions.
Single Crystal XRD Analysis
Golden
yellow
single
crystals
of
(E)-3-(2-(2-(4-
methoxybenzylidene)hydrazinyl)thiazol-4-yl)-2H-chromen-2-one
(4b) were grown in a mixture of methanol and DMSO (a few drops)
by slow evaporation method. Molecular structure of 4b was
determined by X-ray crystallography (Fig. 1). From this study, it was
identified that the two independent molecules (molecule A and B)
were oriented in the same way with a monoclinic P21/c space
group.
Molecule B
Molecule A
Figure 1. Molecular structures of (E)-3-(2-(2-(4-methoxybenzylidene)
hydrazinyl)thiazol-4-yl)-2H-chromen-2-one (4b) (50% probability).
Mechanism
Figure 2. Efficiency of MMT K10 clay catalyst.
A plausible mechanism for the formation of HTCs (4) in the
presence of MMT K10 clay is depicted in Scheme 2. The two NH2
groups of thiosemicarbazide (2) react with the K10 clay activated
carbonyl group of both aldehyde/ketone (1) and 3-(2-bromoacetyl)-
2H-chromen-2-one (3) in water to obtain diimine (I). Subsequently,
clay promoted internal SN2 displacement of bromide ion of diimine
(I) to form dihydro HTCs (II) followed by a proton shift to obtain
HTCs (4).
Scalability of the method
The scalability of the process was then examined by the reaction of
1b, 2 and 3 in the presence of MMT K10 clay (12.5 wt%) in H2O at
RT in different gram scale reactions, 2.5, 5.0, 7.5 and 10 grams. The
yields of product 4b were obtained as 98%, 97%, 96% and 96%.
Hence, it could be concluded that the developed procedure is
efficient, practical and greener alternative for the gram-scale
production of HTCs (4).
Photophysical Properties
Based on recent report on structurally similar systems, it is assumed
that the synthesized HTCs of aromatic aldehydes/ketones exhibit an
intra molecular charge transfer (ICT) transitions due to their donor-
acceptor (D-A) type structure.12a The ICT transitions are highly sensitive
to solvent polarity; hence the solvatochromic studies were conducted
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on 4b. The solvatochromic effect was studied by recording the
View Article Online
DOI: 10.1039/D0NJ00082E
absorption and emission spectra of 4b in different solvents like toluene,
chloroform (non-polar solvents), ethyl acetate (EtOAc), acetonitrile,
DMSO (polar aprotic solvents) and MeOH (polar protic solvent) at the
concentration of 1.0 x10-5M (Fig. 3). A 10 nm red shift was found in
absorption and 121 nm red shift was observed in emission bands of the
luminophore, 4b with increasing the polarity of solvents from toluene
to CH3OH. Hence, the study revealed that the emission of 4b was much
influenced by the polarity of solvent (Fig. 3). Interestingly, longer shifts
towards to longer wavelength (bathochromic shift) from non-polar
solvents, toluene (479 nm) and CHCl3 (511 nm) to polar aprotic & polar
protic solvents, EtOAc (532 nm), ACN (570 nm), DMSO (600 nm) and
MeOH (596 nm) were noticed in the emission spectrum of 4b. Further,
Stokes shift of 4b was also increased from non-polar to polar solvents,
toluene ( = 8535 cm-1) < CHCl3 ( = 10016 cm-1) < EtOAc ( = 10877
cm-1) > ACN ( = 12042 cm-1) < DMSO  = 12152 cm-1) < MeOH
( = 12461 cm-1). This might be attributed to the occurrence of ICT
transitions in 4b and the effect solvent polarity. Hence, 4b is a D-A type
structure.12 Further, the Lippert-Mataga plot of Stokes shifts  )
versus solvent polarity function (f) was examined for 4b which showed
good linear correlation with the solvent polarity (Table 4 & Fig. 4).
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Figure 4. The relationship between Stokes shifts () and solvent polarity
function (f) for the compound 4b in different solvents.
The photophysical properties of a series of D-A type HTCs of
aromatic aldehydes/ketones (4) were recorded in chloroform at the
concentration of 1.0 x10-5M (Fig. 5). The measured absorption and
emission maxima positions, molar extinction coefficients (ε), Stoke
shifts and optical band gaps (Egopt) are presented in Table 5. The
study disclosed that the electron releasing groups (OMe, -OH, -OPh,
di-OMe, -OMe-OH, -OEt-OH, 3-Cl-4-OH etc.) at different positions
on the aromatic ring of aldehydes/ketones of HTC moiety have
shown unfordable influence on the absorption and emission
properties of the studied HTC luminophores (4). The electron
donating groups at para and meta positions (4-OMe (1b), 4-OH(1c),
3-OH(1d), 3-OPh(1f)) on the aromatic ring of aldehydes were
bathochromically shifted with large Stokes shift as compared with
electron donating hydroxyl group at ortho position (2-OH, (1e)).
Further, the di substituted benzaldehydes, 3-Cl-4-OH (1i), 2,4-
diOMe (1n), 2,5-diOMe (1o), 4-OH-3-OMe and 4-OH-3-OEt (1q),
were red shifted with large Stokes shift as compared with 2-OH-5-
OMe (1l) and 2-OH-3-OEt (1p) benzaldehydes. Cinnamaldehyde (1t)
was red shifted with large Stokes shift than the benzaldehyde (1a)
and naphthaldehyde (1u). It was also noticed that the
acetophenone (1y) was red shifted with large Stokes shift than the
2-acetylnapthalene (1aj). p-Hydroxyacetophenone (1ac) was
bathochromically shifted with large Stokes shift as compared with
p-Me (1z), p-OMe (1aa) and m-OH (1ad) acetophenones. Further,
the study revealed that the HTCs of ketones were brightly
fluorescent as compared with HTCs of aldehydes. To our delight,
the examined HTCs of aromatic aldehydes and ketones (4) exhibited
bright blue, violet and yellowish green emission with large Stokes
shift from the corresponding absorption maxima (Fig. 5 and Table
5). Hence, the synthesized HTCs (4) may have potential applications
in various fields like biomedical research and organic electronics.
Figure 3. Normalized absorption (solid lines) and emission spectra (dotted
lines) of 4b in different solvents at 1.0 x10-5M.
Table 4. Lippert-Mataga Plot data for 4b.
 (cm-1)
Solvent λabs(nm) λem(nm) Δƒ
(ν = ν abs- ν em
)
PhMe
CHCl3
E A
ACN
DMSO
MeOH
340
338
337
338
347
342
479
511
532
570
600
596
0.01
0.15
0.18
0.31
0.27
0.31
8535
10016
10877
12042
12152
12461
Further, the quantum yield of HTC compounds (4) were determined
by using quinine sulphate as a standard reference compound (φF =
0.545 in 1.0N H2SO4). The obtained results are summarized in Table
5. From this study, it was noticed that the quantum yields of HTCs
(4) were ranged from 1.90 to 24.17 % in chloroform.
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taken at a scan rate of 100 mV/s. Some of the represVeinewtaAtritvicelecOynclilniec
DOI: 10.1039/D0NJ00082E
voltammograms (CVs) of HTC compounds (4) are shown in Fig. 6.
Other CVs of HTCs (4) are shown in Supplementary information (Fig.
S4). Most of the cyclic voltammograms of HTCs (4) were in quasi
reversible nature (Fig. 6 & Fig. S4). The HOMO energy levels were
calculated from the onset of oxidation potential and the LUMO
energy levels were calculated from the HOMO energy levels and the
optical band gap (Egopt) (UV-Vis absorption). The obtained data is
presented in Table 5. The study revealed that the HOMO and LUMO
energy levels of HTCs that ranged from -5.65 eV to -5.22 eV and -
2.67 eV to -2.17 eV, respectively. To our delight, most of the HOMO
and LUMO levels of the studied HTC compounds were comparable
with the reported hole transport materials (HTMs) like N,N’-di-
(naphthalen-1-yl)-N,N’-diphenylbenzidine (NPB) (HOMO/LUMO
-5.5/-2.4 eV),13b N, N-di(naphthaien-1-yl)-N,N,10-triphenyl-10H-
spiro(acridine-9,9’-fluorene)-2,7-diamine (STNPB) (HOMO/LUMO
5
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-5.35/-2.45
eV),13c
4,4,4’’-tris[(2,3,4,5-
tetraphenyl)phenyl]phenylamine (TTPPPA) (HOMO/LUMO -5.46/-
2.36 eV),13d N,N’-bis-(1-naphthyl)-N,N’-diphenyl benzidine (-
NPD)(HOMO/LUMO -5.43/-2.33 eV),13e
4,4’-[bis-((4-di-n-
hexylamino)benzylideneamino)]stilbene (DHABS) (HOMO/LUMO
-5.57/-2.73 eV)13f and 2-[1{4-(N-phenylnaphthalen-1-amino)phenyl-
3,4-,5,6-tetraphenylbenzene-2-yl]-5-(N-phenyl naphthalen-2-amino)
pyridine (2-PNAP-TB) (HOMO/LUMO -5.25/-2.16 eV). Hence, the
HTC compounds are capable to act as hole transporting materials.13
Further, most of the HTC compounds were equipped with low-lying
HOMO energy levels than the threshold for air oxidation
(approximately -5.2 eV)14 leading to a good stability under ambient
conditions. From this study, it is concluded that the electrochemical
behaviour of HTCs are significantly influenced by the electron
donating groups present at different positions on the aromatic ring
of aldehydes/ketones.
Figure 5. a) Photographs of HTCs of aromatic aldehydes in CHCl3under the
UV light at 365 nm; b) Normalized absorption (solid lines) and emission
spectra (dotted lines) of HTCs of aromatic aldehydes in CHCl3 (1.0 × 10-5 M);
c) Photographs of HTCs of aromatic ketones in CHCl3 under the UV light at
365 nm; d) Normalized absorption (solid lines) and emission spectra
(dotted lines) of HTCs of aromatic ketones in CHCl3 (1.0 × 10-5 M).
Electrochemical properties
The electrochemical properties of HTCs were determined by cyclic
voltammetry using 0.5mM chloroform solution of HTCs along with
0.1 M tetrabutyl ammonium hexaflourophosphate as a supporting
electrolyte. Platinum disk, platinum wire and Ag/Ag+ (satd. KCl)
were used as working electrode, counter electrode and reference
electrode, respectively. All the CV scans were recorded in the
potential range −0.2 to 1.6 V vs Ag/AgCl (satd. KCl) readings were
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Experimental
View Article Online
DOI: 10.1039/D0NJ00082E
General procedure for the synthesis of hydrazinyl thiazolyl
coumarins (HTCs) (4):
A
round-bottomed flask (25 mL) was charged with
aldehydes/ketones (1) (10.0 mmol), thiosemicarbazide (2) (10.0
mmol), 3-(2-bromoacetyl)-2H-chromen-2-one (3) (10.0 mmol),
MMT K10 clay (12.5 wt.-%) and H2O (10.0 mL). The reaction mixture
of was stirred at room temperature for 20-40 min. After completion
of the reaction as indicated by TLC, 5.0 mL of ethyl acetate was
added to the reaction mass and the catalyst was recovered by
filtration followed by washing with ethyl acetate (2.0 mL) and 1.0 M
HCl (5.0 mL) under vacuum. The organic layer was separated, dried
over anhydrous Na2SO4 and concentrated under reduced pressure
to afford the pure product (4). The structures of the synthesized
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1
compounds were confirmed by H NMR, HRMS and mass spectral
analysis and the structure of 4b was confirmed by X-ray
crystallography.
Figure 6. a) Representative cyclic voltammograms of HTCs of aromatic
aldehydes (4a-4d) in CHCl3; b) Representative cyclic voltammograms of HTCs
of aromatic ketones (4y, 4z, 4aa & 4ac) in CHCl3 (0.1 M Bu4NPF6 in CHCl3) at a
scan rate of 100 mV/s.
General procedure for gram scale (10 g scale) synthesis of (E)-3-(2-
(2-(4-methoxybenzylidene)hydrazinyl)thiazol-4-yl)-2H-chromen-2-
one (4b): A mixture of 4-methoxybenzaldehyde (1b) (0.0735 moles),
thiosemicarbazide (2) (0.0735) and 3-(2-bromoacetyl)-2H-chromen-
2-one (3) (0.0735 moles) in H2O (40 mL) was stirred in the presence
of MMT K10 clay (12.5 wt.-%) at RT for 20–40 min. After completion
of the reaction as indicated by TLC, 30 mL of ethyl acetate was
added to the reaction mass and the catalyst was recovered by
filtration followed by washing with ethyl acetate (10.0 mL) and 1.0
M HCl (30 mL) under vacuum. The organic layer was separated,
dried over anhydrous Na2SO4 and concentrated under reduced
pressure to afford the pure product (4b).
Table 5. Photophysical and electrochemical data of HTCs of various aromatic
aldehydes/ketones (4).
e
opt
λedge
(nm)
φem Eg
Eox
EHOMO ELUMO
(eV)g (eV)h
Compd λmax
λemis
ɛ
Stokes
Shift
(onset)
(nm)a
(nm)b (L m-1
(eV)f
(%)
cm-1)c (cm-1)d
4a
4b
4c
342 416 439 30000 6460 17.98 2.98 0.95 -5.37 -2.39
338 419 511 26000 10016 11.37 2.96 1.10 -5.52 -2.56
349 408 479 32000 7776 17.02 3.04 1.15 -5.57 -2.53
346 411 476 28000 7893 17.33 3.02 1.03 -5.45 -2.43
347 402 437 24000 5935 8.02 3.08 0.93 -5.35 -2.27
347 417 476 22000 7893 9.57 2.97 1.14 -5.56 -2.59
339 385 491 20000 9131 1.90 3.22 0.97 -5.39 -2.17
355 412 435 24000 5180 2.06 3.01 1.13 -5.55 -2.54
353 413 474 26000 7231 7.37 3.00 0.89 -5.31 -2.31
359 417 497 30000 7734 8.25 2.97 1.05 -5.47 -2.50
344 391 409 20000 4619 6.36 3.17 0.92 -5.34 -2.17
351 408 474 35000 7392 8.03 3.04 0.80 -5.22 -2.18
358 422 505 53000 8130 2.74 2.94 0.96 -5.38 -2.44
344 408 431 30000 5867 6.08 3.04 0.91 -5.33 -2.29
336 415 506 34000 9999 21.36 2.99 0.92 -5.34 -2.35
347 416 475 20000 7765 24.17 2.98 1.23 -5.65 -2.67
357 412 510 43000 8403 11.49 3.01 0.94 -5.36 -2.35
346 421 474 20000 7804 7.00 2.94 0.91 -5.33 -2.39
337 419 507 13000 9949 12.56 2.96 1.14 -5.56 -2.60
336 419 492 13000 9436 10.76 2.96 1.09 -5.51 -2.55
350 419 472 36000 7384 3.94 2.96 0.97 -5.39 -2.43
4d
4e
4f
Conclusions
A practical green MCR strategy has been demonstrated for the
4k
synthesis of
aldehydes/ketones (4) in good to excellent isolated yields (90-98%)
by 3-component reaction of readily available aromatic
a
series of fluorescent HTCs of aromatic
4l
4n
4o
4p
4q
4t
a
aldehydes/ketones (1) thiosemicarbazide (2) 3-(2-bromoacetyl)-2H-
chromen-2-one (3) catalyzed by MMT K10 clay in water at RT for
20-40 min. Construction of C-N, C=N and C-S bonds in a single step
operation, simple workup procedure, high product yields in short
reaction times with no need of further purification and scaled up to
a gram level are the notable features of the present MCR. Besides,
the developed MCR provides a convenient greener access to HTCs
as it reduces the steps and waste generation and employs an
environmentally benign, inexpensive, readily available and reusable
MMT K10 clay catalyst in water. Hence, the established MCR
procedure is sustainable and greener alternative to the existing
procedures for the synthesis of wide range of HTCs. Photo physical
properties of the synthesized D-A type HTCs aldehydes/ketones
were also examined. Solvent polarity had more impact on the
emission maxima of 4b. Most of the HTC compounds exhibited
bright fluorescence in chloroform with emission maxima ranging
from 409 to 511 nm. The title compounds with electron donating
groups at para/meta positions (p-OMe, p-OH, m-OH, m-OPh) on the
aromatic ring of aldehydes/ketones were bathochromically shifted
with large Stokes shifts. Further, the HOMO and LUMO energy
4u
4y
4z
4aa
4ab
4ac
4ad
4aj
[a] Absorption maxima measured in CHCl3 at 1.0 x10-5M. [b] Emission
maximum in CHCl3 at 1.0 x10-5M. [c] Molar extinction coefficient at 1.0
x10-5M. [d] Stokes shift  = abs-emis. [e] Quantum yield with standard
opt
reference to quinine sulphate (φem = 0.545 in 1 N H2SO4). [f] Eg
=
1240/λedge. [g] EHOMO = -[Eox (onset) -EFOC + 4.8] (EFOC = 0.38 eV (FOC/FOC+ vs.
opt
Ag/AgCl)). [h] ELUMO = EHOMO + Eg
.
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Y. S. Jung, R. A. Marcus, J. Am.Chem. Soc. 2007,V1ie2w9A,rt5ic4le9O2n;linke)
DOI: 10.1039/D0NJ00082E
U. M. Lindstrom, Organic Reactions in Water, Principles,
Strategies and Applications, Blackwell, Oxford, 2007.
levels of HTC compounds are well matched with the HOMO and
LUMO levels of the reported hole transporting materials. Therefore,
these compounds can be used as hole transporting materials.
Because of these attractive features, HTC compounds might have
wide variety of applications in organic electronics. Deeper studies
into the fluorescence properties and further application of the
synthesized fluorophores (4) are underway in our laboratory.
4
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors gratefully acknowledge the financial support for this
work from Council of Scientific and Industrial Research (CSIR), New
Delhi, Government of India under a major research project (No. 01
(2391)/10/EMR-II) and Department of Atomic Energy-Board of
Research in Nuclear Sciences (Bhabha Atomic Research Centre),
Mumbai, Government of India through a major research project
(No. 2011/37C/52/BRNS/2264). Mr. G Kumar is thankful to Prof.
Samar K. Das, School of Chemistry and University Grants
Commission Networking Resource Centre (UGC-NRC), School of
Chemistry, University of Hyderabad for providing infrastructural
facilities.
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DOI: 10.1039/D0NJ00082E
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Synthesis, Photophysical and Electrochemical Properties of Donor-
Acceptor Type Hydrazinyl Thiazolyl Coumarins
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G. Kumar, G. Trivikram Reddy, B. Ramakrishna, Y.V. Divyasri, L. Subramanyam Sarma and N. C. Gangi Reddy*
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A practical green MCR strategy has been developed for the synthesis of D-A type hydrazinyl
thiazolyl coumarins catalyzed by an environmentally compatible MMT K10 clay in water at RT.
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