Synthesis of Novel D-π-A chromophores: Effect of structural manipulations on photophysical properties, viscosity and DFT study

Article abstract of DOI:10.1016/j.molstruc.2021.130086

Four novel push-pull D-π-A type chromophores namely (E)-2-cyano-3-(4-(di(pyridin-2-yl)amino)phenyl)acrylic acid (BECA), (E)-2-(5-(4-(di(pyridin-2-yl)amino)benzylidene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (BERA), (E)-2-cyano-3-(5-(di(pyridin-2-yl)amino)thiophen-2-yl)acrylic acid (THCA) and (E)-2-(5-((5-(di(pyridin-2-yl)amino)thiophen-2-yl)methylene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid (THRA) were synthesized. The dyes synthesize were characterized by HR-MS, ¹H NMR, and ¹³C NMR spectroscopies. The synthesized dyes show absorption and emission wavelength in the range of 362–499 nm and 471–557 nm respectively. The chromophores BECA and BERA show a large stroke shift (63–130 nm) as compared to chromophores THCA and THRA (38–62 nm). Oscillator strengths (f), transition dipole moments (μ_eg), and different solvent polarity plots have been investigated for understanding intramolecular charge transfer (ICT) characteristics from donor to the acceptor. Interestingly, the viscosity induced emission for all four chromophores were observed in ethanol: polyethylene - 400 solvent mixtures. The computational study of these chromophores was carried out by using density functional theory (DFT) along with time-dependent density functional theory (TD-DFT).

Full text of DOI:10.1016/j.molstruc.2021.130086

Journal of Molecular Structure 1233 (2021) 130086
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Synthesis of Novel d-π-A chromophores: Effect of structural
manipulations on photophysical properties, viscosity and DFT study
Mahesh Jachak, Sushil Khopkar, Khushbu Patel, Yogesh Patil, Ganapati Shankarling∗
Department of Dyestuff Technology, Institute of Chemical Technology, N. P. Marg, Matunga, Mumbai - 400019, Maharashtra, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Four novel push-pull d-π-A type chromophores namely (E)-2-cyano-3-(4-(di(pyridin-2-yl)amino)
phenyl)acrylic acid (BECA), (E)-2-(5-(4-(di(pyridin-2-yl)amino)benzylidene)-4-oxo-2-thioxothiazolidin-
3-yl)acetic acid (BERA), (E)-2-cyano-3-(5-(di(pyridin-2-yl)amino)thiophen-2-yl)acrylic acid (THCA) and
(E)-2-(5-((5-(di(pyridin-2-yl)amino)thiophen-2-yl)methylene)-4-oxo-2-thioxothiazolidin-3-yl)acetic acid
(THRA) were synthesized. The dyes synthesize were characterized by HR-MS, 1H NMR, and 13 C NMR
spectroscopies. The synthesized dyes show absorption and emission wavelength in the range of 362–
499 nm and 471–557 nm respectively. The chromophores BECA and BERA show a large stroke shift
(63–130 nm) as compared to chromophores THCA and THRA (38–62 nm). Oscillator strengths (f), transi-
tion dipole moments (μeg), and different solvent polarity plots have been investigated for understanding
intramolecular charge transfer (ICT) characteristics from donor to the acceptor. Interestingly, the viscosity
induced emission for all four chromophores were observed in ethanol: polyethylene - 400 solvent
mixtures. The computational study of these chromophores was carried out by using density functional
theory (DFT) along with time-dependent density functional theory (TD-DFT).
Received 1 December 2020
Revised 28 January 2021
Accepted 4 February 2021
Available online 8 February 2021
Keywords:
Fluorescent styryl dyes
N-pyridin-(2-ylpyridin)-2-amine
Fluorescence molecular rotors (fmrs)
Positive emission solvatochromism
DFT
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
electron donors such as carbazole, phenothiazine, triphenylamine,
etc., as well as electron acceptors such as quinaxalines, cyanovinyl,
Organic dyes containing donor-acceptor with extended π-
conjugation compounds attracted attention in current years. As the
donor-acceptor form π-conjugated compounds possess a good in-
tramolecular charge transfer process from donor to acceptor via
π-bridge. The photophysical properties of these compounds can
be optimized by changing their electron donor to acceptor moiety
and extend the length of the π-conjugated bridge [1–4]. Due to
easy modification at donor, acceptor, and π-spacers, many donor-
acceptor compounds have been reported for various applications
including dye-sensitized solar cells (DSSC’s) [5,6], biological imag-
ing [7], metal sensors [8], luminescent polymer [9], organic field-
effect transistors devices (OFET’s) [10], optical data processing and
storage [11], organic electronics [12] and organic photovoltaics [13].
Styryl dyes are widely used in donor-acceptor type compounds.
As the styryl dyes show extended π-conjugated structures as well
as good intramolecular charge transfer (ICT) characteristics. The ad-
vantages of styryl dyes over other classes of dyes are ease of syn-
thesis, the scope of different donor and acceptors which can cover
the spectrum from the UV to near-infrared region, and good pho-
tostability [14,15]. Up to date, various styryl dyes with different
pyridine, etc. were reported for various properties and applications
[16]. Styryl dyes are extensively used as additives in photographic
industry [17], laser dyes [18], flexible dyes [19], viscosity sensor
[20] and optical sensitizers [21].
FMRs are π-conjugated fluorescent molecules, in which fluo-
rescence intensity changes as the degree of intramolecular rota-
tion of the rotators affected by environmental viscosity therefore
its act as viscosity and temperature probes in different media. As
the viscosity of medium is low, the fast rotation of the rotators
causing non-radiative thermal relaxation of the excitation energy
resulting in quenching of the fluorescence lifetime but as the vis-
cosity of medium increase, the rotation of rotators get inhibited
resulting in increases in fluorescence intensity as decreases in non-
radiative thermal relaxation of the excitation energy. It has appli-
cations such as in biomedical imaging [22] and measuring micro-
viscosity of specific cellular organelles and biomolecules [23]. It
has also been used in measuring intra channel viscosity [24] and
measuring temperature-sensitive responses in microfluidic devices
[25].
The chromophores containing
a
N-pyridin-(2-ylpyridin)−2-
amine as donor and chelating group used in many areas such
as metal sensor [26,27], metal complex formation [28], homoge-
neous catalysis [29], and photoluminescent materials [30]. As well
∗
Corresponding author.
E-mail address: gsshankarling@gmail.com (G. Shankarling).
https://doi.org/10.1016/j.molstruc.2021.130086
0022-2860/© 2021 Elsevier B.V. All rights reserved.

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
as, the chromophores containing a carboxylic acid or 2-(4-oxo-
2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid group usage as an
electron-withdrawing group and anchoring group in DSSC [31,32].
Also carboxylic acid substituted BODIPYs are used for bioimag-
ing [33] and coumarins as profluorophores for nucleic acid de-
tection [34]. Reactive dyes containing carboxylic acid group uses
for dying on cellulose fibers [35]. The 2-(4-oxo-2-sulfanylidene-1,3-
thiazolidin-3-yl)acetic acid group uses in ratiometric fluorescent
chemosensor for Ag+ [36], also use in anticancer and antibacterial
activity derivatives [37,38].
The fluorescein (ɸ = 0.79) in 0.1 mol/L NaOH solution was used as
a standard to determine the quantum yield of the dye.
2.3. Synthesis and characterization of intermediates and styryl dyes
2.3.1. N-pyridin-(2-ylpyridin)−2-amine (3)
In a THF (50 mL), 2-amino pyridine (1) (1.88 g, 0.02 mole)
was dissolved completely. After the dissolution of starting reagents,
sodium hydride (60% dispersion in mineral oil) (1.2 g, 0.03 mole)
was put in portion-wise to it and the blend was mixed for 30 min
at 30 °C. Later, 2-bromopyridine (2) (2.10 g, 0.022 mole) was added
and further heated to 70 °C for 18 h in a nitrogen atmosphere.
Once completion of the reaction, the brown-colored solution was
obtained. Then, the solvent was evaporated on a rotary evaporator
and the resulting crude sticky gel mass obtained was rinse with
water and further 2–3 times extracted with ethyl acetate. The col-
lected organic phase was dried with anhydrous sodium sulfate and
further evaporator on rotavap. The purification of sticky residue
was done by column chromatography on silica gel (n-hexane: ethyl
acetate = 9:1). Off white colored solid. Yield: 2.291 g (67%), Melt-
ing point = 90 °C, 1H NMR (500 MHz, CDCl3, TMS) δ 8.28 (ddd,
J = 5.0, 1.8, 0.8 Hz, 2H), 8.01 (s, 1H), 7.62 – 7.55 (m, 4H), 6.86 –
6.83 (m, 2H).
N-pyridin-(2-ylpyridin)−2-amine as
a
common electron-
donating with a 2-cyanoacetic acid or 2-(4-oxo-2-sulfanylidene-
1,3-thiazolidin-3-yl)acetic acid group as a strong acceptor group
with different π-bridge make good intramolecular charge transfer
(ICT) and twisted intramolecular charge transfer (TICT) acceptor-
donor (A- π -D) system. As a result, the system affects HOMO-
LUMO energy band gap changes in spectroscopic properties [39].
The changes in different groups with different π-bridge also
affect thermal properties and heat of fusion or heat of crystal-
lization [40]. Therefore it is creating more interest to study the
effect of N-pyridin-(2-ylpyridin)−2-amine as
a common donor
with 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid or 2-
cyanoacetic acid group as a strong acceptor baring chromophores
with different π-bridge.
13C NMR (126 MHz, CDCl3) δ 154.1, 147.7, 137.7, 116.3, 111.7.
In this research work, we designed and synthesized four
d-π-A styryl dyes having N-pyridin-(2-ylpyridin)−2-amine as a
common donor with 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-
yl)acetic acid or 2-cyanoacetic acid as a common acceptor with
phenyl or thiophene as π-bridge. Also, we did a comparison study
of different groups on their photophysical as well as thermal prop-
erties. Finally, we also study the effect of viscosity induced emis-
sion for all four chromophores in ethanol: polyethylene - 400 sys-
tem and results of a DFT study.
2.3.2. 4-(di(pyridin-2-yl)amino)benzaldehyde (4)
In a microwave reaction vial, N-pyridin-(2-ylpyridin)−2-amine
(3) (0.291 g, 0.0017 mole) and 4-bromobenzaldehyde (0.368 g,
0.0020 mole) were dissolved in N-methyl-2-pyrrolidone (NMP)
(12 mL) and after that potassium carbonate (0.276 g, 0.0020 mole)
and copper (I) iodide (0.065 g, 0.00034 mole) were added. This
reaction mixture was then exposed to microwave radiation for
1 hour at 10 bar pressure at 200 °C. When the reaction completion
occurs, the yellow-colored precipitation was obtained. Once the re-
action was completed, it was drowned in ice water and the solid
product collected, further solid dissolved in ethyl acetate. The col-
lected organic phase was dried with anhydrous sodium sulfate and
further evaporator on rotavap. The purification of sticky residue
was done by column chromatography on silica gel (n-hexane: ethyl
acetate = 9:1). Light yellow colored solid. Yield: 0.258 g (55%),
Melting point = 105 °C, 1H NMR (500 MHz, CDCl3, TMS) δ 9.92
(s, 1H), 8.39 (dd, J = 5.0, 2.0 Hz, 2H), 7.83 (d, J = 8.5 Hz, 2H), 7.65
(td, J = 7.8, 2.0 Hz, 2H), 7.23 (d, J = 8.5 Hz, 2H), 7.07 – 7.04 (m,
4H).
2. Experimental section
2.1. Materials and instruments
The synthetic grade chemicals were procured from Sigma-
Aldrich Ltd., Spectrochem Pvt. Ltd., and Oxford Lab Fine Chem LLP.
All the chemical reactions were monitored on a Merck aluminum
TLC plate (coated with silica gel 60 with fluorescent indicator F254).
The NMR 1H (400 MHz) and 13C (126 MHz) analysis of synthe-
sized dyes was performed on an Agilent instrument with TMS as
an internal standard and CDCl3 or DMSO–d6 as a solvent (Support-
ing Information S1-S14). The High-resolution mass spectra (HR-MS)
were recorded on Agilent technologies quadrupole time of flight
instrument (Supporting Information S15-S18). A Jasco V-750 Uv-
vis spectrophotometer was used to record the absorption spectra
of all the chromophores and a Jasco FP-8200 spectrofluorometer
was used to record the fluorescence spectra of all chromophores
with freshly prepared solutions. Thermogravimetric and differen-
tial scanning calorimetric (DSC-TGA) analyses of synthesized dyes
were recorded on a Perkin Elmer instrument.
13C NMR (126 MHz, CDCl3) δ 190.9, 157.5, 150.7, 149.1, 138.1,
132.2, 131.1, 124.7, 119.7, 118.4.
2.3.3. 5-(di(pyridin-2-yl)amino)thiophene-2-carbaldehyde (5)
In a 20 mL microwave reaction vial, N-pyridin-(2-ylpyridin)−2-
amine (3) (0.291 g, 0.0017 mole) with 5–bromo thiophene-2-
carboxaldehyde (0.380 g, 0.0020 mole) were dissolved in N-
methyl-2-pyrrolidone (NMP) (12 mL) and after that potassium
carbonate (0.276 g, 0.0020 mole) and copper(I) iodide (0.065 g,
0.00034 mole) were added. This reaction mixture was then ex-
posed to microwave radiation for 90 min at 10 bar pressure at
200 °C. As the reaction was completed, the yellow-colored pre-
cipitate was obtained. The reaction mixture drowned into ice wa-
ter and the solid product was obtained, further solid dissolved in
ethyl acetate. The collected organic phase was dried with anhy-
drous sodium sulfate and further evaporator on rotavap. The pu-
rification of sticky residue was done by column chromatography on
silica gel (n-hexane: ethyl acetate = 9:1). Dark brown colored solid.
Yield: 0.215 g (45%), Melting point = 135 °C, 1H NMR (500 MHz,
DMSO–d6, TMS) δ 9.73 (s, 1H), 8.60 (dd, J = 5.0, 1.8 Hz, 2H), 7.91
(td, J = 7.8, 2.0 Hz, 2H), 7.68 (d, J = 4.5 Hz, 1H), 7.36 (dd, J = 7.2,
5.0 Hz, 2H), 6.97 (d, J = 8.2 Hz, 2H), 6.02 (d, J = 4.5 Hz, 1H).
2.2. Experimental methods
All the photophysical studies were carried with quartz cuvettes
of 1 cm path length. The slit width of 5 nm is used for the
measurement of excitation and emission spectra. The experimental
conditions used for analysis were kept continuous in all the exper-
iments. All dyes were excited at their maximum absorbance value
(λmax) in corresponding solvents to found out the emission value.
All dyes stock solution formulated in chloroform. The photophysi-
cal data of dyes were measured in seven different solvents by an
increase in polarities with the concentration of ~5 × 10−6 mol/L.
2

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
13C NMR (126 MHz, DMSO–d6) δ 184.1, 154.8, 153.9, 149.2,
140.1, 137.3, 133.4, 121.7, 118.1, 114.0.
13C NMR (126 MHz, DMSO–d6) δ 161.7, 157.5, 153.1, 149.2, 140.5,
126.4, 122.5, 121.7, 118.4, 116.4, 115.9, 114.6, 67.3.,
+ ESI-MS: m/z calcd for C18 H12N4O2S (M+): 349.0411, found:
349.0415.
2.3.4. (E)−2-cyano-3-(4-(di(pyridin-2-yl)amino)phenyl)acrylic acid
(BECA)
2.3.7.(E)−2-(5-((5-(di(pyridin-2-yl)amino)thiophen-2-yl)methylene)−
4-oxo-2-thioxothiazolidin-3-yl)acetic acid (THRA)
In acetonitrile (50 mL), 2-cyanoacetic acid (0.132 mL, 0.0020
mol) was added and mix. To this reaction mixture, 2–3 drops of
piperidine were added and stirred for 10 min in a nitrogen atmo-
sphere. After 10 min, added 4-(di(pyridin-2-yl)amino)benzaldehyde
(4) (0.275 g, 0.0010 mole) and stirred for 4 h at 85 °C. As the re-
action was completed, the pale yellow colored precipitation was
obtained, which was extracted with ethyl acetate. The collected
organic phase was dried with anhydrous sodium sulfate and fur-
ther evaporator on rotavap. The purification of sticky residue was
done by column chromatography on silica gel (n-hexane: ethyl ac-
etate = 9:1). Pale yellow-colored solid. Yield: 0.281 g (82%), Melt-
ing point = 180 °C., 1H NMR (500 MHz, DMSO–d6, TMS) δ 8.31
(dd, J = 4.8, 1.2 Hz, 2H), 8.23 (s, 1H), 7.98 (d, J = 8.8 Hz, 2H), 7.76
(td, J = 8.2, 1.9, 2H), 7.12–7.16 (m, 4H), 7.04 (d, J = 8.2 Hz, 2H).
13C NMR (126 MHz, DMSO–d6) δ 164.1, 157.3, 153.8, 149.7, 149.1,
139.0, 138.4, 132.6, 126.8, 124.8, 120.4, 118.8, 117.0.,
In acetic acid (60 mL), 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-
3-yl)acetic acid (0.161 g, 0.00084 mole) was added. Further ammo-
nium ethanoate (0.054 g, 0.0007 mole) was added and stirred for
5 min in a nitrogen atmosphere. After 5 min, added 5-(di(pyridin-
2-yl)amino)thiophene-2-carbaldehyde (5) (0.197 g, 0.0007 mole)
and stirred for 3 h at 120 °C. As completion of the reaction,
the yellow-colored precipitation was obtained. Then, the obtained
mass was extracted using ethyl acetate. The collected organic
phase was dried with anhydrous sodium sulfate and further evap-
orator on rotavap. The purification of sticky residue was done
by column chromatography on silica gel (n-hexane: ethyl ac-
etate = 9:1). Orange colored solid. Yield: 0.254 g (80%), Melting
point = 210 °C.
1H NMR (400 MHz, DMSO–d6, TMS) δ 8.60 (d, J = 3.2 Hz, 2H),
8.03 (s, 1H), 7.90 (t, J = 7.3 Hz, 2H), 7.54 (d, J = 3.7 Hz, 1H), 7.38 –
7.28 (m, 2H), 6.98 (d, J = 7.9 Hz, 2H), 6.11 (d, J = 4.1 Hz, 1H), 4.69
(s, 2H).
+ ESI-MS: m/z calcd for C20H14 N4O2 (M+): 343.1117, found:
343.1120.
13C NMR (126 MHz, DMSO–d6) δ 192.2, 167.9, 166.3, 154.9,
153.7, 149.2, 140.1, 136.4, 129.3, 128.6, 121.7, 118.0, 115.7, 113.6,
45.6.,
2.3.5. (E)−2-(5-(4-(di(pyridin-2-yl)amino)benzylidene)−4-oxo-2-
thioxothiazolidin-3-yl)acetic acid (BERA)
In acetic acid (60 mL), 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-
3-yl)acetic acid (0.161 g, 0.00084 mole) was added and mix. Fur-
ther ammonium ethanoate (0.054 g, 0.0007 mole) was added and
stirred for 5 min in a nitrogen atmosphere. After 5 min, added
4-(di(pyridin-2-yl)amino) benzaldehyde (4) (0.193 g, 0.0007 mole)
and stirred for 3 h at 120 °C. As completion of the reaction, the
yellow-colored precipitate was obtained. After the completion of
the reaction, the reaction mass was extracted with ethyl acetate.
The collected organic phase was dried with anhydrous sodium sul-
fate and further evaporator on rotavap. The purification of sticky
residue was done by column chromatography on silica gel (n-
hexane: ethyl acetate = 9:1). Yellow-colored solid. Yield: 0.263 g
(84%), Melting point = 245 °C. 1H NMR (500 MHz, DMSO–d6, TMS)
δ 8.30 (d, J = 3.6 Hz, 2H), 7.83 (s, 1H), 7.75 (td, J = 8.2, 1.7 Hz, 2H),
7.62 (d, J = 8.6 Hz, 2H), 7.19 – 7.10 (m, 4H), 7.03 (d, J = 8.2 Hz, 2H),
4.72 (s, 2H).
+ ESI-MS: m/z calcd for C20H14 N4O3S3 (M+): 455.0428, found:
455.0441.
3. Results and discussion
3.1. Design and synthesis
The synthesis protocol for four d-π-A structured styryl dyes
coded as BECA, BERA, THCA, and THRA having common N-
pyridin-(2-ylpyridin)−2-amine donor, different π-bridge and dif-
ferent acceptor units and intermediate represented in Scheme 1.
The nucleophilic substitution reaction between 2-amino pyridine
(1) with 2–bromo pyridine (2) in presence of NaH (60%) in THF
at 70 °C for 18 h gave N-pyridin-(2-ylpyridin)−2-amine (3). Fur-
ther intermediate 4-(di(pyridin-2-yl)amino)benzaldehyde (4) or 5-
(di(pyridin-2-yl)amino)thiophene-2-carbaldehyde (5) were synthe-
sized by Ullmann-type nucleophilic substitution reaction between
N-pyridin-(2-ylpyridin)−2-amine (3) and 4–bromo benzaldehyde
or 5–bromo thiophene-2-carboxaldehyde respectively in N-methyl-
2-pyrrolidone (NMP) in presence of potassium carbonate and cop-
per(I) iodide irradiated with a microwave at 200 °C. The chro-
mophores BECA and THCA were synthesized by Knoevenagel con-
densation of intermediate 4-(di(pyridin-2-yl)amino)benzaldehyde
(4) and 5-(di(pyridin-2-yl)amino)thiophene-2-carbaldehyde (5) re-
spectively with 2-cyanoacetic acid in acetonitrile in presence
of piperidine for 4 h at 85 °C. The chromophores BERA and
THRA were synthesized by Knoevenagel condensation of interme-
diate 4-(di(pyridin-2-yl)amino)benzaldehyde (4) and 5-(di(pyridin-
2-yl)amino)thiophene-2-carbaldehyde (5) respectively with 2-(4-
oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid in acetic acid in
presence of ammonium ethanoate for 3 h at 120 °C. The syn-
thesized dyes were characterized by standard spectroscopic tech-
niques. (Supporting Information S1- S18)
13C NMR (126 MHz, DMSO–d6) δ 193.5, 167.8, 166.9, 157.4,
149.1, 148.0, 138.9, 134.0, 132.6, 128.5, 125.7, 120.2, 118.6, 45.5.,
+ ESI-MS: m/z calcd for C22H16 N4O3S2 (M+): 449.0864, found:
449.0885.
2.3.6. (E)−2-cyano-3-(5-(di(pyridin-2-yl)amino)thiophen-2-yl)acrylic
acid (THCA)
In acetonitrile (50 mL), 2-cyanoacetic acid (0.132 mL,
0.0020 mole) was added. To this reaction mixture, 2–3 drops of
piperidine were added and stirred for 10 min in a nitrogen atmo-
sphere. After 10 min, added 5-(di(pyridin-2-yl)amino)thiophene-
2-carbaldehyde (5) (0.281 g, 0.0010 mole) and stirred for 4 h
at 85 °C. As completion of the reaction, the pale orange colored
precipitate was obtained. After the completion of the reaction,
extracted with ethyl acetate. The collected organic phase was
dried with anhydrous sodium sulfate and further evaporator on
rotavap. The purification of sticky residue was done by column
chromatography on silica gel (n-hexane: ethyl acetate = 9:1). Light
orange colored solid. Yield: 0.271 g (78%), Melting point = 145 °C.
1H NMR (500 MHz, DMSO–d6, TMS) δ 8.61 (dd, J = 4.7, 1.0 Hz,
2H), 8.33 (s, 1H), 7.95 (td, J = 8.2, 1.8 Hz, 2H), 7.69 (d, J = 4.7 Hz,
1H), 7.42 (dd, J = 7.0, 5.2 Hz, 2H), 7.01 (d, J = 8.2 Hz, 2H), 6.07 (d,
J = 4.7 Hz, 1H).
3.2. Absorption and fluorescence properties
To correlate the impact of different π spacers with common
donor and acceptor, we compared the photophysical properties of
BECA against THCA and BERA against THRA. Also, to correlate the
3

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Scheme 1. Synthetic route of synthesized dyes BECA, THCA, BERA and TERA.
impact of different acceptor with common donor and π spacer, we
compared BECA against BERA and THCA against THRA Fig. 1.
From non-polar solvents to polar solvents, the polarity of sol-
vent increases, which can affect the absorption and emission wave-
lengths of molecules, thus we carried out photophysical experi-
ments of four chromophores in seven various solvents of different
polarities. Fig. 2, shown the absorption as well as emission spec-
tra of four dyes whereas, In Table 1, the obtained experimental
photophysical data in various solvents is compiled. From Fig. 2, it
is observed that dyes show a bathochromic shift in the absorp-
tion and emission maxima as the polarity of aprotic solvents in-
creases. In the case of methanol, a polar protic solvent they show
a hypsochromic shift in absorption maxima as compared to apro-
tic solvent (Toluene) in all four chromophores. The chromophores
BECA and BERA show different absorption and emission properties
as they have the same donor and π-bridge but different acceptor
based dye, BERA show red-shifted absorption maxima (432 nm)
and emission maxima (495 nm) as compared to BECA absorption
4

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Fig. 1. Structures of synthesized styryl dyes.
Table 1
Experimental photophysical data of BECA, BERA, THCA, and THRA in different solvents.
a
d
e
f
g
h
k
λmax
(nm)
εb × 105
FWHMc
(nm)
λonset
(nm)
࢞Eopt
λexc
(nm)
λem
(nm)
ꢀλs
(nm)
μeg
(Debye)
−
j
Comp.
BECA
solvent
(M 1cm−1
)
(eV)
ꢁi
f
Toluene
THF
394
403
410
362
374
374
367
451
451
457
457
460
452
450
432
436
438
431
440
434
418
487
492
494
489
499
493
479
0.687
0.780
0.649
0.714
0.726
0.741
0.641
0.771
0.864
0.866
0.861
0.833
0.910
0.825
0.494
0.566
0.549
0.475
0.483
0.516
0.546
0.572
0.605
0.635
0.602
0.583
0.646
0.650
62.3
61.6
60.6
59.5
64.3
61.0
60.8
50.7
51.1
49.7
53.3
54.1
53.4
54.7
70.6
73.4
70.2
75.9
79.1
75.8
74.6
76.0
70.7
78.6
78.6
79.4
75.9
88.8
443
450
457
431
431
427
431
482
485
490
495
498
490
490
482
491
490
492
504
496
480
533
540
547
543
553
548
541
2.80
2.76
2.71
2.88
2.88
2.90
2.88
2.57
2.56
2.53
2.51
2.49
2.53
2.53
2.57
2.53
2.53
2.52
2.47
2.08
2.58
2.33
2.30
2.27
2.28
2.24
2.26
2.29
396
405
413
365
376
377
369
453
454
459
460
463
455
452
435
439
440
433
443
437
421
490
495
496
492
503
496
482
471
487
497
493
500
502
497
489
493
497
502
505
500
497
495
513
516
538
548
546
541
530
539
548
550
557
554
541
77
0.028
0.022
0.015
0.016
0.010
0.008
0.010
0.026
0.019
0.011
0.011
0.016
0.006
0.006
0.034
0.027
0.023
0.015
0.012
0.005
0.005
0.032
0.029
0.023
0.019
0.014
0.010
0.011
1.206
1.474
1.262
1.614
1.591
1.682
1.761
1.163
1.299
1.181
1.369
1.343
1.480
1.386
1.449
1.557
1.559
1.629
1.617
1.717
1.749
1.438
1.463
1.333
1.518
1.505
1.510
1.523
10.060
11.247
10.496
11.156
11.256
11.576
11.733
10.570
11.171
10.720
11.542
11.470
11.937
11.526
11.545
12.025
12.060
12.229
12.310
12.597
12.479
12.213
12.380
11.841
12.572
12.648
12.590
12.463
84
Chloroform
DMF
87
131
126
128
130
38
DMSO
Acetonitrile
Methanol
Toluene
THF
THCA
BERA
THRA
42
Chloroform
DMF
40
45
DMSO
45
Acetonitrile
Methanol
Toluene
THF
48
47
63
77
Chloroform
DMF
78
107
108
112
123
43
DMSO
Acetonitrile
Methanol
Toluene
THF
47
Chloroform
DMF
54
61
DMSO
58
Acetonitrile
Methanol
61
62
a
absorption maxima at molar concentration of c = ~5 × 10−6 mol/L.
b
c
d
e
f
molar extinction coefficient calculated from the absorbance at c = ~5 × 10−6 mol/L.
full width at half maximum (FWHM).
onset absorption edge.
optical band gap calculated from the equation, ࢞Eopt = 1240/λonset
.
excitation maxima.
g
h
i
emission maxima at molar concentration of c = ~5 × 10−6 mol/L.
Stokes shift.
fluorescence quantum yield calculated using fluorescein as a standard (ɸ = 0.79 in 0.1 mol/ L NaOH.) [44].
j
oscillator strength.
k
transition dipole moment.
maxima (394 nm) and emission maxima (471 nm) in toluene.
This is attributed due to 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-
3-yl)acetic acid as a strong acceptor than 2-cyanoacetic acid. The
same trend was observed in the case of THCA and THRA, THRA
show red-shifted absorption maxima (487 nm) and emission max-
ima (530 nm) in toluene as compared to THCA absorption max-
ima (451 nm) and emission maxima (489 nm) in toluene, due
to presence of 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic
acid as strong acceptor than 2-cyanoacetic acid. As compared to
chromophores BECA and THCA, they have the same donor and
acceptor but different π-bridge. THCA show red-shifted absorp-
tion maxima (451 nm) and emission maxima (489 nm) as com-
pared to BECA absorption maxima (394 nm) and emission maxima
(471 nm) in toluene due to the presence of thiophene as π-bridge.
5

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Fig. 2. Absorption spectra of BECA (A), BERA (C), THCA (E), THRA (G) and emission spectra of BECA (B), BERA (D), THCA (F), THRA (H) in various solvents
(c = ~5 × 10−6 mol/L).
The same trend was observed with chromophores BERA and THRA,
as they have the same donor and acceptor but different π-bridge.
THRA shows red-shifted absorption maxima (487 nm) and emis-
sion maxima (530 nm) as compared to BERA absorption maxima
(432 nm) and emission maxima (495 nm) in toluene, as it con-
tains thiophene as π-bridge. Experimental results show a red shift
in absorption maxima, from BECA<BERA<THCA<THRA in all stud-
ied solvents, from which it can be concluded that there is a good
charge transfer within the dyes from BECA<BERA<THCA<THRA.
As with the increase in solvent polarity from non-polar to a polar
solvent, in the excited state stabilization energy increase in the po-
lar solvent which showed a red-shifted emission wavelength from
BECA<THCA<BERA<THRA. The dyes BERA and THRA show full-
width Half-maximum (FWHM) from 70–88 nm which provides a
scope for the application of dyes in the field of photonics. [41]. The
large Stokes shift observed in dyes BECA and BERA (63–130 nm)
6

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
as compared to THCA and THRA (38–62 nm) from non-polar sol-
vent to polar solvent which indicates a fast relaxation from the
initial to emissive state, which may due to good intramolecular
energy transfer within the molecule when phenyl used as a π-
bridge as compared to thiophene in a molecule. The obtained ex-
perimental data, from Table 1, the highest quantum yield was ob-
served in a non-polar solvent in toluene (0.028 for BECA, 0.026 for
THCA, 0.034 for BERA, and 0.032 for THRA) as compared to po-
lar solvent (methanol). Moreover, it was observed that low quan-
tum yield in methanol as compared to toluene which leads to an
assumption that the presence of intramolecular and twisted in-
tramolecular charge transfer [42].
state in polar solvents of the chromophores as compared to the
ground state in polar solvents (positive solvatochromism). The ex-
cited state of dyes was stabilized in polar solvents by polarizability
of the surrounding medium, dipole-dipole interactions, and hydro-
gen bonding.
The solvatochromic behavior of the chromophores was stud-
ied using the aid of Lippert-Mataga, ET (30), and McRae solvent
polarity plots. Lippert-Mataga function describes the relationship
between Stokes shift of the dye with solvent parameters such
as refractive index, a radius of the solvent cage, and dipole mo-
ment [45]. Fig. 3A denotes the Lippert-Mataga solvent polarity
plot which consists of Stokes shift of the BECA, THCA, BERA, and
THRA dyes against orientation polarizability (࢞fLM) of the solvents.
From Fig. 3A, linear regression coefficient (R2 = 0.6594 for BECA,
R2 = 0.7584 for BERA, R2 = 0.8348 for THCA and R2 = 0.5445
for THRA) are obtained, suggesting that redshift in emission spec-
tra is due to influences of the refractive index of solvents, Onsager
radius of dyes and dielectric constant. The McRae solvent func-
tion is the developed form of the Lippert-Mataga function, where
the solute together with solvent polarizability factors has been
taken into consideration [46]. Fig. 3B graph denote McRae function
which consists of Stokes shift of the BECA, THCA, BERA, and THRA
dyes against McRae polarity parameter (࢞fMR). From Fig. 3B, a lin-
ear regression coefficients were as follows, R2 = 0.7404 for BECA,
R2 = 0.8116 for BERA, R2 = 0.8434 for THCA and R2 = 0.5493 for
THRA. The linear relationship in Fig. 3B indicates the solute po-
larity originating in between N-pyridin-(2-ylpyridin)−2-amine as a
donor and 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid
or 2-cyanoacetic acid as an acceptor of the dyes as well added to
the positive emissive solvatochromic shift. The Dimroth-Reichardt
presented solvent polarity parameters which introduced the acidic
character of solvent as well as negative solvatochromic dye [47].
Fig. 3C graph denotes Dimroth-Reichardt ET (30) plot which con-
sists of Stokes shift of the BECA, THCA, BERA, and THRA dyes
against ET (30) polarity parameter [48]. For Fig. 3C, a line re-
gression coefficients (R2 = 0.6290 for BECA, R2 = 0.7594 for
BERA, R2 = 0.7639 for THCA and R2 = 0.8938 for THRA) were
observed, which signifies that observed positive emissive solva-
tochromic shift of dyes in the polar solvent has due to the influ-
ence of dielectric solute-solvent interaction.
With the help of transition dipole moments (μeg) and oscil-
lator strengths (f), we can study the charge transfer characteris-
tic of dyes towards the emissive state. As “oscillator strength” is
a dimensionless quantity which states that the possibility of sev-
eral electromagnetic radiation transitions from the ground state to
the excited state in energy levels during the absorption to emis-
sion process in atom or molecule. The transition dipole moment
is the transient dipole moment induced in the molecule during
the transition. It evaluates the possibility of radiative transitions
in molecules between the initial state and final state in various
solvents. By knowing the oscillator strength of molecule in vari-
ous solvents from the Eq. (1), we further find out the transition
dipole moment of molecule in various solvents from the Eq. (2),
[43]
f = 4.32 × 10−9 ∫ ε v dv
(1)
( )
f
μ2eg = ꢀ
ꢁ
(2)
4.32 × 10−7 × v
In Eqs. (1 and 2), ε is molar absorptivity and ν is wave number
in cm−1
.
From Table 1, The oscillator strengths as well as transition
dipole moments were found in toluene for all four dyes (f = 1.206,
μeg = 10.060 for BECA, f = 1.163, μeg = 10.570 for THCA,
f = 1.449, μeg = 11.545 for BERA and f = 1.438, μeg = 12.213
for THRA). From the obtained data, it was observed that BECA and
BERA show high oscillator strengths as compared to THCA and
THRA, which shows that donor and acceptor π-bridge by phenyl
ring created high oscillator strengths as compared to thiophene
π-bridge. Similar to this BERA and THRA show high oscillator
strengths as compared to BECA and THCA, which shows that 2-
(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid created high
oscillator strengths as compared to 2-cyanoacetic acid.
3.4. Relationship between intramolecular charge transfer
characteristics and dipole moment
From the photo-physical data, the fast relaxation of dye
molecule results in bathochromic shift obtained in emission spec-
tra in a polar solvent which may due to increases in ICT character-
istic in the dye. For the BECA and THCA chromophores, the charge
transfer occurs from N-pyridin-(2-ylpyridin)−2-amine donor to the
2-cyanoacetic acid acceptor and for BERA and THRA from N-
pyridin-(2-ylpyridin)−2-amine donor to 2-(4-oxo-2-sulfanylidene-
1,3-thiazolidin-3-yl)acetic acid acceptor. As there some limitations
of the Lippert-mataga function as it considers only local excitation,
so with the help of Rettig as well as Weller’s plots, we can explain
the relationship between bathochromic shift in emission state with
ICT characteristic. As in Weller’s plot, excited state charge transfer
characteristics of the chromophores were determined. It consists of
the maximum emissive wave number of the chromophore in a par-
ticular solvent in cm−1 against Weller’s function (࢞fw). Fig. 4A, de-
note Weller’s plot, which consists of the maximum emissive wave
number of BECA, BERA, THCA, and THRA against Weller’s function
values in respective solvents. In the Fig. 4A, a linear regression co-
efficient (R2 = 0.7179 for BECA, R2 = 0.7409 for BERA, R2 = 0.5417
for THCA and R2 = 0.6945 for THRA) was obtained, which signifies
that ICT characteristics influence the bathochromic emissive shift
obtained from non-polar to polar solvents [49]. As in polar protic
Similarly, BERA and THRA show high transition dipole mo-
ments as compared to BECA and THCA, which shows that 2-
(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid created high
transition dipole moments as compared to 2-cyanoacetic acid. Sim-
ilar to this THCA and THRA show high transition dipole moments
as compared to BECA and BERA, which shows that donor and ac-
ceptor π-bridge by thiophene ring created high transition dipole
moment as compared to phenyl π-bridge.
From the data observed from Table 1, it appeared that the os-
cillator strength and transition dipole moment of all four dyes
were increased as solvent polarity increased, which specifies
that from non-polar to polar solvent, the intramolecular charge
transfer increases. For example (dye BECA showed
f
=
1.206
and μeg = 10.060 in toluene, upon increase of solvent polarity,
f = 1.761 and μeg = 11.733 increased in methanol)
3.3. Solvent polarity functions plots
The optical properties such as Stokes shift and emission wave-
length of the synthesized chromophores increased towards the po-
lar solvents due to the increase in the stabilization of the excited
7

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Fig. 3. (A) Lippert-Mataga solvent polarity plots (B) McRae solvent polarity plots and (C) ET (30) function of BECA, BERA, THCA and THRA.
Fig. 4. (A) Weller’s plot and (B) Rettig plot of BECA, BERA, THCA and THRA.
8

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Table 2
Relationship between intramolecular charge transfer and dipole moment.
a
b
f
g
Compound
BECA
Solvent
f
μeg
ac (cm−1) × 10−8
m(LM)d
(μe-μg)e Debye
μg (Debye)
μe (Debye)
Toluene
THF
1.206
1.474
1.262
1.614
1.591
1.682
1.761
1.163
1.299
1.181
1.369
1.343
1.480
1.386
1.449
1.557
1.559
1.629
1.617
1.717
1.749
1.438
1.463
1.333
1.518
1.505
1.510
1.523
10.060
11.247
10.496
11.156
11.256
11.576
11.733
10.570
11.171
10.720
11.542
11.470
11.937
11.526
11.545
12.025
12.060
12.229
12.310
12.597
12.479
12.213
12.380
11.841
12.572
12.648
12.590
12.463
10.980
10.972
10.973
10.972
10.972
10.972
10.972
10.910
10.907
10.908
10.904
10.906
10.904
10.908
11.077
11.075
11.077
11.073
11.073
11.073
11.073
11.007
11.005
11.008
11.003
11.002
11.003
11.002
10,718.64
3.756
3.752
3.753
3.752
3.752
3.752
3.752
1.235
1.234
1.234
1.234
1.234
1.234
1.234
1.695
1.695
1.695
1.694
1.694
1.694
1.694
1.272
1.271
1.272
1.271
1.271
1.271
1.271
12.051
14.006
13.352
15.070
15.079
15.083
15.029
10.512
12.675
11.963
13.937
13.897
13.745
13.631
4.499
15.807
17.758
17.105
18.822
18.831
18.835
18.781
11.747
13.909
13.197
15.171
15.131
14.978
14.866
6.194
Chloroform
DMF
DMSO
Acetonitrile
Methanol
Toluene
THF
THCA
BERA
THRA
1180.39
2125.35
1200.27
Chloroform
DMF
DMSO
Acetonitrile
Methanol
Toluene
THF
5.210
6.904
Chloroform
DMF
4.975
6.670
5.620
7.314
DMSO
5.650
7.344
Acetonitrile
Methanol
Toluene
THF
5.615
7.309
5.573
7.267
5.543
6.815
6.944
8.215
Chloroform
DMF
6.301
7.572
7.710
8.981
DMSO
7.641
8.912
Acetonitrile
Methanol
7.773
9.044
7.680
8.951
a
oscillator strength.
b
c
d
e
f
transition dipole moment.
Onsager radius determined from DFT.
slope determined from Lippert-Mataga plot.
ground and excited state dipole moment difference determined by Lippert-Mataga plot.
ground state dipole moment determined from DFT.
excited state dipole moment.
g
solvents, blue shift observed in absorption spectra as compared to
polar aprotic solvent. The observed bathochromic shift is due to
the TICT phenomena also playing the role along with ICT phenom-
ena. Fig. 4B, denote the Rettig plot, which consists of the maximum
emissive wave number of BECA, BERA, THCA, and THRA against
Rettig function values in respective solvents. In Fig. 4B, a linear re-
gression coefficient (R2 = 0.9275 for BECA, R2 = 0.6232 for BERA,
R2 = 0.6592 for THCA and R2 = 0.7054 for THRA) was obtained,
which signifies that TICT phenomena also influence with the aid
of ICT phenomena in existing chromophores BECA, BERA, THCA,
and THRA [50].
μg) (Table 2). The obtained values of excited-state dipole moment
(μe) and ground-state dipole moment (μg), Table 2, it is indicated
that ground-state dipole moment (μe) are lower than excited state
dipole moment (μg), which indicate that ground state (S0) is less
polar than excited-state (S1). Also, the value of excited-state dipole
moment (μe) of all the chromophores increases from non-polar
solvent to the polar solvent, which signifies that the ICT phenom-
ena increase in the excited-state from non-polar to polar solvent
[52].
3.5. Viscosity sensitivity study
It is renowned that the charge distribution characteristics, as
well as induced dipole moment, generates in a molecule during the
transfer of electron charge while absorption and emission of pho-
tons in the molecule were elucidated from dipole moments gener-
ate in the molecule [51]. Table 2 contains the slope found from the
Lippert-Mataga equation (m) and the oscillator strength as well as
transition dipole moment (μeg) found from photo-physical data. To
find out the dipole moment (μe-μg) generates during the absorp-
tion and emission process, we find out the Onsager radius (a) of
all four chromophores in seven different solvents from DFT opti-
mized geometry. It was calculated from N-pyridin-(2-ylpyridin)−2-
amine as a donor to conjugated 2-cyanoacetic acid or 2-(4-oxo-
2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid ring as an acceptor.
By knowing the Onsager radius (a) and Lippert-Mataga slope (m),
the value of the difference between emission and absorption dipole
moment (μe-μg) was calculated.
Viscosity sensitivity study of our synthesized chromophores
was carried out in ethanol: PEG 400 system. From Fig. 5, fluores-
cence spectra, as the viscosity of the system increases, the fluo-
rescence intensity of the four chromospheres in the medium also
gradually increases. In a medium with a composition of 99% PEG
400 and 1% ethanol, THRA shows more enhancement in fluores-
cence intensity as compared to other chromospheres. In the com-
position of 99% PEG 400 and 1% ethanol, THRA, BECA, BERA, and
THCA shows 16.23, 9.42, 7.30, 5.34 fold increase in emission in-
tensity respectively in comparison with pure ethanol. These chro-
mophores show a remarkable viscosity dependent enhanced emis-
sion as compared to reported styryl dyes [53]. The restriction of
TICT state in a more viscous medium as compare to low vis-
cous medium results in maximum fluorescence enhancement in
a more viscous medium. (Fig. 5) [54]. By using Forster-Hoffmann
Eq. (3) [55], the quantitative measurement of viscosity sensitivity
in a high viscous system of fluorescence molecular rotors (FMR)
was explained.
The ground state dipole (μg) of chromophores were calcu-
lated from DFT carried out by using B3-LYP as a function with
triple zeta def2-TZVP as the basis set. The excited state dipole
moment (μe) found out from ground state dipole (μg) and the
difference between emission and absorption dipole moment (μe-
log I = C + x log η
(3)
9

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Fig. 5. Fluorescence spectra of (A) (B)BECA, (C) (D) BERA, (E) (F) THCA, and (G) (H) THRA in Ethanol: PEG 400 (100: 0) to and Ethanol: PEG 400 (1:99) system at molar
concentration of ~1 × 10−5 mol/L.
10

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Fig. 6. Frontier molecular orbitals of BECA, BERA, THCA and THRA in chloroform.
Where, I = emission intensity of the rotors, x = viscosity sensi-
tivity of the rotors, ŋ = viscosity of the system, C = constant.
From Fig. 5, the viscosity sensitivity parameter (x) values for
FMRs BECA, BERA, THCA, and THRA were as follows 0.28, 0.29,
0.25, and 0.42 respectively. From the above-observed data, dye
THRA shows good FMR properties than the other three synthesized
FMRs BECA, BERA, and THCA.
The frontier molecular orbital energy gap amongst HOMO and
LUMO level in chloroform are shown in Fig. 6 to elaborate the
electronic transitions and charge distribution within the molecules.
The non-planar conformations were found from the DFT optimized
geometry of dyes. The optimized geometries of BECA, BERA, THCA,
and THRA are shown in Figure S19.
The HOMO-LUMO band gap, transition dipole moment, oscilla-
tor strengths, vertical electronic excitation (VEE), and orbital con-
tribution of BECA, BERA, THCA, and THRA dyes in different polar-
ity solvents were performed with the aid of TD-DFT and the ob-
tained results are tabulated in Table 3.
3.6. Theoretical study
From the theoretical study, we can find a relationship be-
tween photophysical properties and molecular structures of dye.
The theoretical study was done by using Turbomole software (Ver-
sion 6.5) [56]. Zych et al. [57] have studied similar molecules,
with cyanoacrylic acid as the acceptor group and substituted 1,2,3-
triazole motif as the donor group, in B3LYP, CAM-B3LYP, M06, and
wb97XD exchange-correlation functional in combination with the
6–31G(d,p) basis set. The theoretical results were in good agree-
ment with experimental results when the computations were car-
ried out with B3LYP exchange-correlation functional in combina-
tion with 6–31G(d,p) basis set. Therefore, we have used B3LYP
[58,59] as functional and def2-TZVP as the basis set [60] for ge-
ometry optimization of our synthesized molecules. The vertical
electronic excitation (VEE), oscillator strengths (f), and molecular
orbital contributions aimed at the first five singlet transitions in
seven different solvents of all four dyes were determined by the
aid of time dependent-density functional theory (TD-DFT), where
the same method was used which we had previously used in ge-
ometry optimization.
From Table 3, The BERA show red-shifted vertical electronic ex-
citation maxima (474 nm) as compared to BECA vertical electronic
excitation maxima (424 nm) in toluene. This is attributed due to
2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid as a strong
acceptor than 2-cyanoacetic acid. The same trend was observed in
the case of THCA and THRA, THRA show red-shifted vertical elec-
tronic excitation maxima (494 nm) as compared to THCA vertical
electronic excitation maxima (444 nm) in toluene, due to pres-
ence of 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid as
strong acceptor than 2-cyanoacetic acid.
As compared to chromophores BECA and THCA, THCA show
red-shifted vertical electronic excitation maxima (444 nm) as com-
pared to BECA vertical electronic excitation maxima (424 nm)
in toluene due to the presence of thiophene as π-bridge. The
same trend was observed with chromophores BERA and THRA,
THRA show red-shifted vertical electronic excitation maxima
(494 nm) as compared to BERA vertical electronic excitation max-
ima (474 nm) in toluene, as it contains thiophene as π-bridge.
11

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Table 3
Comparative experimental and computational data of BECA, BERA, THCA, and THRA.
a
d
f
Dye
solvent
λmax (nm)
EHOMO (eV)
ELUMO (eV)
Egb (eV)
VEEc (nm)
f
Orbital contributione
μeg (Debye)
BECA
Toluene
THF
394
403
410
362
374
374
367
432
436
438
431
440
434
418
451
451
457
457
460
452
450
487
492
494
489
499
493
479
−5.84
−5.82
−5.83
−5.82
−5.82
−5.82
−5.82
−5.66
−5.65
−5.66
−5.65
−5.65
−5.65
−5.65
−5.74
−5.74
−5.73
−5.75
−5.76
−5.76
−5.76
−5.55
−5.54
−5.55
−5.54
−5.55
−5.54
−5.54
−2.54
−2.60
−2.58
−2.64
−2.64
−2.64
−2.64
−2.70
−2.75
−2.74
−2.78
−2.78
−2.78
−2.78
−2.60
−2.61
−2.60
−2.64
−2.66
−2.67
−2.67
−2.72
−2.76
−2.75
−2.78
−2.79
−2.78
−2.78
3.30
3.22
3.25
3.18
3.18
3.18
3.18
2.96
2.90
2.92
2.87
2.87
2.87
2.87
3.14
3.13
3.13
3.11
3.10
3.09
3.09
2.83
2.78
2.80
2.76
2.76
2.76
2.76
424
430
428
435
438
433
432
474
481
479
486
488
483
483
444
442
443
444
449
446
447
494
498
497
503
505
500
499
0.872
0.876
0.875
0.887
0.890
0.866
0.861
1.024
0.988
1.002
0.988
0.998
0.965
0.959
0.774
0.817
0.812
0.834
0.822
0.778
0.759
0.924
0.934
0.917
0.947
0.949
0.932
0.923
H→L (98.4)
H→L (98.4)
H→L (98.4)
H→L (98.5)
H→L (98.5)
H→L (98.4)
H→L (98.4)
H→L (98.8)
H→L (98.9)
H→L (98.9)
H→L (98.9)
H→L (98.9)
H→L (98.9)
H→L (98.9)
H→L (97.7)
H→L (97.6)
H→L (97.6)
H→L (97.7)
H→L (97.8)
H→L (97.6)
H→L (97.6)
H→L (98.1)
H→L (98.1)
H→L (98.1)
H→L (98.2)
H→L (98.2)
H→L (98.1)
H→L (98.0)
8.867
8.946
8.926
9.060
9.099
8.928
8.896
10.159
10.048
10.103
10.106
10.177
9.962
9.925
8.548
8.761
8.742
8.877
8.858
8.591
8.496
9.847
9.943
9.848
10.063
10.099
9.949
9.895
Chloroform
DMF
DMSO
Acetonitrile
MeOH
BERA
THCA
THRA
Toluene
THF
Chloroform
DMF
DMSO
Acetonitrile
MeOH
Toluene
THF
Chloroform
DMF
DMSO
Acetonitrile
MeOH
Toluene
THF
Chloroform
DMF
DMSO
Acetonitrile
MeOH
a
experimental absorption.
b
c
d
e
f
HOMO-LUMO energy band gap obtained from DFT.
computational vertical electronic excitation.
computational oscillator strength.
major electronic transition.
computed transition dipole moment.
Theoretical results show a red shift in absorption maxima, from
BECA<THCA<BERA<THRA in all studied solvents, from which it
can be concluded that there is a good charge transfer within the
dyes from BECA<THCA<BERA<THRA. As with the increase in sol-
vent polarity from non-polar to a polar solvent, in the excited state
stabilization energy increase in the polar solvent which showed
From Fig. 6, it is observed that HOMOs of BECA, BERA, THCA,
and THRA dyes are generally placed on N-pyridin-(2-ylpyridin)−2-
amine donor and π-bridge as compared to the acceptor group.
When the excitation of chromophores occurs, the electron density
from the donor system is transferred to 2-(4-oxo-2-sulfanylidene-
1,3-thiazolidin-3-yl)acetic acid or 2-cyanoacetic acid acceptor. The
electron cloud of LUMO +1 of BECA and THCA chromophores
are more distributed on N-pyridin-(2-ylpyridin)−2-amine donor
and in the case of BERA and THRA chromophores were mostly
distributed on 2-(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic
acid. From Fig. 6, In the transfer of electron density from HOMO
to LUMO from N-pyridin-(2-ylpyridin)−2-amine to 2-(4-oxo-2-
sulfanylidene-1,3-thiazolidin-3-yl)acetic acid or 2-cyanoacetic acid
acceptors through the conjugated π-bridge, as outcomes in a
strong interface of intramolecular charge transfer from acceptor
to the donor. Moreover, results in a decrease in the HOMO-LUMO
bandgap. The theoretically observed band gaps of HOMO-LUMO or-
bitals for BECA, BERA, THCA, and THRA in different solvents are
plotted in Fig. 7.
a
red-shifted emission wavelength from BECA<THCA<BERA<
THRA.
From Table 3, the theoretically obtained oscillator strengths and
transition dipole moment were found in toluene for all four dyes
(f = 0.872, μeg = 8.867 for BECA, f = 0.774, μeg = 8.548 for THCA,
f = 1.024, μeg = 10.159 for BERA and f = 0.924, μeg = 9.847 for
THRA). From the obtained data, it was observed that BECA and
BERA show high oscillator strengths as compared to THCA and
THRA, which shows that donor and acceptor π-bridge by phenyl
ring created high oscillator strengths as compared to thiophene
π-bridge. Similar to this BERA and THRA show high oscillator
strengths as compared to BECA and THCA, which shows that 2-
(4-oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid created high
oscillator strengths as compared to 2-cyanoacetic acid.
The BERA shows a lower HOMO-LUMO band gap (2.93 eV)
as compared to BECA (3.25 eV). Similarly, THRA shows a lower
HOMO-LUMO band gap (2.80 eV) as compared to THCA (3.13 eV).
This is because of the strong association between donors and 2-(4-
oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid acceptor com-
pared to 2-cyanoacetic acid as an acceptor via π-bridge. Moreover,
the energy band gap observed in HOMO-LUMO orbitals for THCA
and THRA was lower as compared to BECA and BERA, due to the
thiophene ring as π-bridge, which decreases the bandgap by good
transport of electron density from donor to acceptor. From Table 3,
it was observed that the maximum orbital contribution (97–99%)
to electronic transitions was attributed to the HOMO to LUMO en-
ergy bandgap.
Similarly, BERA and THRA show high transition dipole mo-
ments as compared to BECA and THCA, which shows that 2-(4-
oxo-2-sulfanylidene-1,3-thiazolidin-3-yl)acetic acid created a high
transition dipole moment as compared to 2-cyanoacetic acid. Simi-
lar to this BECA and BERA shows high transition dipole moment as
compared to THCA and THRA, which shows that donor and accep-
tor π-bridge by phenyl ring created high transition dipole moment
as compared to thiophene π-bridge.
From Table 3, it shows that obtained theoretical results
show good congruence experimental results, which strengthen
the results obtained in practical experiments with theoretical
study.
12

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
Fig. 7. HOMO-LUMO energy level band gap plots of BECA, BERA, THCA and THRA in different solvents.
ture 250 °C. The dyes BECA, THCA, BERA, and THRA showed ther-
mal decomposition temperature (Td) at 250 °C by a reduction in
2.5% of total weight, at 271 °C by a reduction in 2% of total weight,
at 295 °C by a reduction in 4% of total weight and at 262 °C by a
reduction in 2.5% of total weight respectively. These results demon-
strated that all the four chromospheres are thermally stable over
250 °C. From Fig. 8, the decomposition temperatures of dyes are
high which indicates that these dyes possess good thermal sta-
bility [61]. In DSC analysis, for THCA, BECA, THRA, and BERA, a
small endothermic peak was observed at 151 °C, 184 °C, 225 °C,
Fig. 8. Thermogravimetric analysis (TGA) of synthesized dyes.
3.7. Thermal properties
The changes in physical properties of BECA, BERA, THCA, and
THRA dyes as a function of temperature were analyzed by ther-
mogravimetric analysis (TGA) and differential scanning calorimetry
(DSC) from the temperature range of 30 to 600 °C with a purge
of nitrogen gas and linear controlled heating rate of 20 °C/minute.
Fig. 8, denotes the thermogravimetric analysis graph, from it was
observed that less amount of weight loss in dyes below tempera-
Fig. 9. Differential scanning calorimetric analysis (DSC) of synthesized dyes.
13

M. Jachak, S. Khopkar, K. Patel et al.
Journal of Molecular Structure 1233 (2021) 130086
and 270 °C respectively, which is typically for the fusion point [62].
Fig. 9, shows that THCA has a low melting point than BECA and
THRA has a low melting point than BERA due to the presence of
the thiophene ring as a π-bridge, which shows that donor and ac-
ceptor π-bridge by phenyl ring increases the latent heat of melting
than the π-bridge by thiophene ring. Also, the rhodamine-3-acetic
acid group increases the fusion point of dyes. For example, THRA
has a high melting point than THCA and BERA has a high melting
point than BECA, which shows that 2-(4-oxo-2-sulfanylidene-1,3-
thiazolidin-3-yl)acetic acid as an acceptor increases the latent heat
of melting than the 2-cyanoacetic acid as an acceptor.
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of BECA, BERA, THCA, and THRA novel styryl chromophores. The
above chromophores exhibited a comprehensive π-π∗ system with
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wavelength from a range of 362–499 nm and 471–557 nm respec-
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CRediT authorship contribution statement
Mahesh Jachak: Conceptualization, Methodology, Software.
Sushil Khopkar: Visualization, Formal analysis. Khushbu Patel: In-
vestigation, Writing – original draft, Writing – review & editing.
Yogesh Patil: Writing – original draft, Writing – review & editing.
Ganapati Shankarling: Supervision, Funding acquisition.
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Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130086.
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