Fluorescent Styryl Dyes from 4-Chloro-2-(Diphenylamino)-1, 3-Thiazole-5-Carbaldehyde - Synthesis, Optical Properties and TDDFT Computations

Article abstract of DOI:10.1007/s10895-015-1668-0

4-Chloro-2-(diphenylamino)-1,3-thiazole-5-carbaldehyde was reacted with an active methylene compounds, cyanomethyl benzimidazole, cyanomethyl benzothiazole, barbituric acid and Meldrum's acid under Knoevenagel conditions to give novel push-pull styryl chromophores 8a-8d. The synthesized styryl chromophores were characterized by FT-IR, Mass and ¹H NMR spectral analysis. The photophysical characteristics of these styryl chromophores were evaluated. The effect of solvent polarity and viscosity on the absorption and emission properties of these chromophores was studied. The structural, molecular, electronic and photophysical parameters of the push-pull dyes were studied by using density functional theory (DFT) and time dependent density functional theory (TDDFT) computations. The ratio of the ground to the excited state dipole moment of the synthesized novel styryl dyes were calculated by Bakhshiev and Bilot-Kawski correlations.

Full text of DOI:10.1007/s10895-015-1668-0

J Fluoresc
DOI 10.1007/s10895-015-1668-0
ORIGINAL ARTICLE
Fluorescent Styryl Dyes from 4-Chloro-2-(Diphenylamino)-1,
3
-Thiazole-5-Carbaldehyde—Synthesis, Optical Properties
and TDDFT Computations
1
1
1
Nagaiyan Sekar & Prashant G. Umape & Sharad R. Patil
Received: 8 July 2015 /Accepted: 14 September 2015
#
Springer Science+Business Media New York 2015
Abstract 4-Chloro-2-(diphenylamino)-1,3-thiazole-5-
carbaldehyde was reacted with an active methylene com-
pounds, cyanomethyl benzimidazole, cyanomethyl
benzothiazole, barbituric acid and Meldrum’s acid under
Knoevenagel conditions to give novel push-pull styryl chro-
mophores 8a-8d. The synthesized styryl chromophores were
Introduction
The styryl class of dyes has some of the brightest hues and
they are the lightfast dyes with a compact molecular structure.
These properties lead them as ideal dyes for acetate and poly-
ester fibers [1]. Due to their low molecular mass they are also
applied for thermal transfer printing on polyester fabrics [2].
Monostyryl dyes are especially used as yellow colorants for
reproducing electrically recorded color photographs [3]. The
styryl molecules are characterized by the presence of a styryl
group as the chromophoric system. In the commercially im-
portant dyes of this class the styryl core is usually accompa-
nied by a suitable donor (D) and acceptor (A) groups. The first
electronically excited state with positive and negative charges
has the positive charge on the donor residue and the negative
charge on the acceptor residue consequently generating a large
dipole moment in the excited state. Such a phenomenon in-
duces a large dipole moment difference on laser irradiation, a
necessary requirement for nonlinear optical materials [4].
Thus the light absorption results as depicted in the Chart 1.
Styryl dyes due to their bright hue, high photostability and
excellent light fastness as well as endearing photophysical
properties find applications in textile dyeing and in hi-tech
areas like NLO materials [5, 6], optical data storage devices
and laser dyes [7, 8], probe materials for biological applica-
tions [9, 10], dye sensitized solar cells [11] and metal ion
sensors [12, 13]. Dyes containing styryl chromophoric groups
in conjugation with a heterocyclic ring imparts bright colors.
Also several publications report that the styryl colorants with
diphenyl amino substitution pattern have appreciable proper-
ties for hi-tech applications [14, 15].
1
characterized by FT-IR, Mass and H NMR spectral analysis.
The photophysical characteristics of these styryl chromo-
phores were evaluated. The effect of solvent polarity and vis-
cosity on the absorption and emission properties of these chro-
mophores was studied. The structural, molecular, electronic
and photophysical parameters of the push-pull dyes were stud-
ied by using density functional theory (DFT) and time depen-
dent density functional theory (TDDFT) computations. The
ratio of the ground to the excited state dipole moment of the
synthesized novel styryl dyes were calculated by Bakhshiev
and Bilot-Kawski correlations.
.
.
.
Keywords Styryl dyes Fluorescence Solvatochromism
Computational studies
Electronic supplementary material The online version of this article
doi:10.1007/s10895-015-1668-0) contains supplementary material,
which is available to authorized users.
(
*
Nagaiyan Sekar
n.sekar@ictmumbai.edu.in; nethi.sekar@gmail.com
We have extended our contribution to synthesis of different
heterocyclic colorants [16–19]. Here, we have reported an ef-
ficient synthesis of novel fluorescent styryl dyes 8a-8d
(Table 1) and further experimental investigation on
1
Department of Dyestuff Technology, Institute of Chemical
Technology, (Formerly UDCT), Nathalal Parekh Marg, Matunga,
Mumbai 400 019, India

J Fluoresc
300 MHz instrument respectively using TMS as an internal
standard. The visible absorption spectra of the compounds
were recorded on a Spectronic Genesys 2 UV-Visible spectro-
photometer. Simultaneous DSC-TGA measurements were
performed out on SDT Q600 v8.2 Build 100 model of TA
instruments Waters (India) Pvt. Ltd. The Quantum yields were
determined in different solvents by using Rhodamine B
(Φ=0.97 in ethanol) [30] as a reference standard using the
comparative method.
The photophysical properties were investigated using
solvatochromic and solvatofluoric behaviors of the dyes.
The solvatochromic data was used to determine the ground
and excited state dipole moments of the dyes 8a-8d by using
Lippert-Mataga, Bakhshiev and Bilot-Kawski correlations.
Chart 1 Charge transfer mechanism in donor-acceptor styryl molecules
solvatochromism, solvatofluorism, and quantum yields of the
synthesized dyes in various solvents. Lippert-Mataga and re-
lated models [20–29] were used to derive the ground and ex-
cited states dipole moments. The DFT computations [B3LYP/
6
-31G(d)] were carried out to study the geometrical and elec-
tronic properties of the synthesized molecules. TD-DFT based
approach has been used to have more understanding of the
geometry in the ground and excited states, photophysical and
dipole moment properties of these four dyes.
Computational Details
Experimental Section
All the computations were performed using the Gaussian 09
package [31]. The ground state (S0) geometry of the synthe-
sized dyes in their C1 symmetry was optimized in vacuum
using DFT [32]. The functional used was B3LYP. The
B3LYP method combines Becke’s three parameter exchange
functional (B3) [31] with the nonlocal correlation functional
by Lee, Yang and Parr (LYP) [33]. The basis set used for all
the atoms was 6-31G(d). The vibrational analysis was per-
formed using the same method to verify that the optimized
structures correspond to local minima on the energy surface.
The vertical excitation energies and oscillator strengths were
obtained for the lowest 10 singlet-singlet transitions at the
Materials and Equipments
All the commercial reagents and solvents were procured from
S.D. fine chemicals (India). The reactions were monitored by
TLC using 0.25 mm E-Merck silica gel 60 F254 precoated
plates, which were visualized with UV light. Melting points
were measured on standard melting point apparatus from
Sunder industrial product Mumbai, and are uncorrected. The
FT-IR spectra were recorded on a Perkin-Elmer Spectrum 100
1
FT-IR Spectrometer. H NMR spectra were recorded on VXR
Table 1 Synthesized 4-chloro-2-(diphenylamino)-1, 3-thiazole-5-carbaldehyde based styryl derivatives
Dye
Structure (synthesized)
Dye
Structure (synthesized)
8
a
8c
8b
8d

J Fluoresc
optimized ground state equilibrium geometries by using
TDDFTwith the same hybrid functional and basis set [34–36].
The low-lying first singlet excited states (S1) of the dyes
were relaxed using the TDDFT to obtain their minimum en-
ergy geometries. All the computations in solvents of different
polarities were carried out using the Self-Consistent Reaction
Field (SCRF) under the Polarizable Continuum Model (PCM)
26.18 mmol) dissolved in DMF was added slowly at 0–5 °C
and stirred for 30 min. The reaction mixture was then heated
to 70–75 °C for 2–3 h and the progress of the reaction was
monitored by TLC. The reaction mass was poured into ice,
stirred, neutralized with sodium bicarbonate, filtered and
dried. The crude aldehyde was recrystallized from ethanol.
Yield=95 %, Melting point=146–148 °C (144–145 °C l)
[43].
[37, 38]. The electronic absorption spectra, including wave-
lengths, oscillators strengths, and main configuration assign-
ment, were systematically investigated using TDDFT with
PCM model on the basis of the optimized ground state
geometries.
Synthesis of (E)-2-(1H-Benzo[d]Imidazol-2-yl)
-3-(4-Chloro-2-(Diphenylamino) Thiazol-5-yl)Acrylonitrile
(8a)
Synthesis and Characterization
4-Chloro-2-(diphenylamino) thiazole-5-carbaldehyde (1.0 g,
3
.18 mmol) 6 and 2-cyanomethyl benzimidazole (0.50 g,
Synthesis of N, N-Diphenyl-N′-Benzoyl Thiourea (4)
3.18 mmol) were stirred in ethanol, a drop of piperidine was
added and refluxed for 2 h. The progress of the reaction was
monitored by TLC. After the completion of the reaction the
product was filtered, washed with water and dried. The crude
product was purified by column chromatography using silica
gel 100–200 mesh and toluene as eluent system.
To a stirred solution potassium thiocyanate (5.0 g, 51.4 mmol)
in 50 ml acetone, a solution of benzoyl chloride 1 (5.8 ml,
5
0.0 mmol) in 30 ml of acetone was added slowly. The reac-
tion mass was stirred at reflux for 1 h to give benzoyl
isothoicyanate 2 and allowed to attain room temperature. To
this, a solution of diphenyl amine 3 (8.46 g 50.0 mmol) in
Yield=76 %; Melting point=236–238 °C.
−1
FT-IR (KBr, cm )=3278 (NH), 2211 (CN), 1576 (Ar),
3
0 ml acetone was added slowly in 30 min. The resultant
1247 (C-N).
reaction mixture was stirred for 2 h and the reaction mass
was filtered to remove potassium chloride (KCl) and the fil-
trate was poured into water with constant stirring. The yellow
precipitate obtained was filtered and washed well with water
and dried. The crude product obtained was purified from
ethanol.
Mass=m/z 454 (M+1).
1
H NMR (CDCl , 300 MHz)=δ 7.30–7.49 (m, 12H),
3
7.74–7.84 (s, 2H), 8.52 (s, 1H), 9.32 (bs, 1H).
Synthesis of (E)-2-(Benzo[d]Thiazol-2-yl)
-3-(4-Chloro-2-(Diphenylamino)Thiazol-5-yl) Acrylonitrile
Yield=85 %, Melting point=121–123 °C (122–124 °C l)
(8b)
[39, 40].
4
-Chloro-2-(diphenylamino)thiazole-5-carbaldehyde (1.0 g,
3.18 mmol) 6 and 2-cyanomethyl benzothiazole (0.62 g,
.18 mmol) were stirred in ethanol, a drop of piperidine was
Synthesis of 2-(Diphenylamino)Thiazole-4(5H)-One (5)
3
N, N-Diphenyl-N′-benzoyl thiourea 4 (10.0 g, 30.12 mmol)
and chloroacetyl chloride (2.64 ml, 33.13 mmol) are
mixed thoroughly and heated over a water bath at 60–
added and refluxed for 2 h. The progress of the reaction was
monitored by TLC. After the completion of the reaction the
product was filtered, washed with water and dried. The crude
product was purified by column chromatography using silica
gel 100–200 mesh and toluene as eluent system.
7
0 °C until a brown sticky mass was obtained, heating
continued for next 30 min. The reaction was monitored
by TLC, hot acetone was poured into the gummy mass
and stirred. The reaction mixture was allowed to cool.
Yield=80 %; Melting point=decomposes at 200 °C.
−1
FT-IR (KBr, cm )=2210 (CN), 1569 (Ar), 1313 (C-N).
2
-(Diphenylamino)thiazole-4(5H)-one 5 crystallized out
Mass=m/z 471 (M+1).
1
was filtered and recrystallized from ethanol.
H NMR (CDCl , 300 MHz)=δ 7.35–7.49 (m, 12 H),
3
Yield=88 %, Melting point=192–194 °C (198 °C l)
7.86 (d, 1H, J=7.7 Hz), 8.03 (d, 1H, J=7.7).
[41, 42].
Synthesis of 5-((4-Chloro-2-(Diphenylamino)Thiazol-5-yl)
Synthesis of 4-Chloro-2-(Diphenylamino)
Methylene)-2,2-Dimethyl-1,3-Dioxane-4,6-Dione (8c)
Thiazole-5-Carbaldehyde (6)
4-Chloro-2-(diphenylamino)thiazole-5-carbaldehyde (1.0 g,
POCl (5.19 ml, 54.98 mmol) was added to DMF (20.27 ml,
3.18 mmol) 6 and Meldrum’s acid (0.53 g, 3.18 mmol) were
stirred in ethanol, a drop of piperidine was added and refluxed
for 2 h. The progress of the reaction was monitored by TLC.
3
2
0
61.8 mmol) at 0 °C within 15 min and stirred for 30 min at
°C. 2-(Diphenylamino)thiazole-4(5H)-one 5 (7 g,

J Fluoresc
Scheme 1 Synthesis of styryl
dyes from 4-chloro-2-
(diphenylamino)-1, 3-thiazole-5-
carbaldehyde 6
1
After the completion of the reaction the product was filtered,
washed with water and dried. The crude product was purified
by column chromatography using silica gel 100–200 mesh
and toluene as eluent system.
H NMR (CDCl , 300 MHz)=δ 1.25(s, 6H), 7.36–7.40
3
(m, 6H), 7.45–7.48 (m, 4H).
Yield=67 %; Melting point=224–226 °C.
FT-IR (KBr, cm )=1726, 1692 (C=O),
Mass=m/z 441 (M+1).
Table 2 Photo-physical properties of the synthesized dye 8a in
different solvents
−1
maxa
abs
maxb
ems
c
Solvents
λ
(
λ
Stokes shift
(nm)
Φ
f
nm)
(nm)
(%)
THF
437
437
440
434
428
437
437
434
437
520
521
526
521
522
518
521
521
523
83
84
86
87
94
81
84
87
86
0.3
0.3
0.4
1.1
0.1
0.3
0.5
0.2
0.5
DCM
CHCl
3
EtOAc
Acetone
MeOH
EtOH
ACN
DMF
a
Absorption wavelength maxima
Fluorescence emission maxima
Fluorescence quantum yield
b
c
Fig. 1 Solid state emission spectra of the dyes 8a-8c

J Fluoresc
Table 3 Photo-physical properties of the synthesized dye 8b in
Table 5 Photo-physical properties of the synthesized dye 8d in
different solvents
different solvents
maxa
abs
maxb
ems
c
maxa
abs
max b
ems
c
Solvents
λ
(
λ
Stokes shift
(nm)
Φ
f
Solvents
λ
λ
Stokes shift
(nm)
Φ
f
nm)
(nm)
(%)
(nm)
(nm)
(%)
THF
455
455
455
449
449
449
452
449
455
549
541
539
535
543
546
542
545
551
94
86
84
86
94
97
90
96
96
0.14
0.69
0.59
0.31
0.31
0.20
0.04
0.23
0.62
THF
458
464
467
458
458
461
458
458
461
543
547
552
542
543
548
539
537
525
85
83
85
84
85
87
81
79
64
1.13
1.99
0.98
0.66
0.29
0.34
0.12
0.19
0.17
DCM
DCM
CHCl
3
CHCl
3
EtOAc
Acetone
MeOH
EtOH
EtOAc
Acetone
EtOH
MeOH
ACN
ACN
DMF
DMF
a
a
Absorption wavelength maxima
Fluorescence emission maxima
Fluorescence quantum yield
Absorption wavelength maxima
Fluorescence emission maxima
Fluorescence quantum yield
b
c
b
c
Synthesis of 5-((4-Chloro-2-(Diphenylamino)Thiazol-5-yl)
Results and Discussion
Methylene)Pyrimidine 2,4,6(1H,3H,5H)-Trione (8d)
Synthesis and Characterization of Synthesized Dyes 8a-8d
4
3
-Chloro-2-(diphenylamino)thiazole-5-carbaldehyde (1.0 g,
.18 mmol) 6 and barbituric acid (0.48 g, 3.18 mmol) were
The styryl dyes 8a-8d are prepared from the key intermediate
4-chloro-2-(diphenylamino)-1, 3-thiazole-5-carbaldehyde 6
by the Knoevenagel condensation [26] with different active
methylene moieties.
stirred in ethanol, a drop of piperidine was added and refluxed
for 2 h. The progress of the reaction was monitored by TLC.
After the completion of the reaction the product was filtered,
washed with water and dried. The crude product was purified
by heating in hot water and then recrystallized from DMF.
Yield=84 %; Melting point=246–248 °C.
Synthesis of 6 was carried out as follows. Potassium thio-
cyanate was first refluxed with benzoyl chloride 1 in acetone
to yield benzoyl isothoicyanate 2 which was further reacted
with diphenyl amine 3 in refluxing acetone to give
N-(diphenylcarbamothioyl)benzamide 4 [39, 40]. The com-
pound 4 was heated with chloroacetyl chloride to yield
2-(diphenylamino)-1, 3-thiazol-4(5H)-one 5 [41, 42]. The thi-
azole intermediate 5 was subjected to Vilsmeier formylation to
obtain 6 [43]. The aldehyde 6 was subsequently reacted with
different active methylene compounds by the classical
Knoevenagel condensation to get desired styryl dyes 8a-8d
(Table 1). The synthetic route is well described in Scheme 1.
−1
FT-IR (KBr, cm )=3029 (NH), 1743, 1700 (C=O), 1644
N-H).
(
Mass=m/z 346 (loss of phenyl ring).
1
H NMR (CDCl , 300 MHz)=δ 7.29–7.48 (m, 11 H),
3
1
0.19 (s, 2 H).
Table 4 Photo-physical properties of the synthesized dye 8c in
different solvents
maxa
abs
maxb
ems
c
Solvents
λ
(
λ
Stokes shift
(nm)
Φ
f
nm)
(nm)
(%)
THF
449
449
452
446
446
446
449
443
446
546
541
552
543
528
531
537
517
503
97
92
0.54
2.14
0.93
0.45
0.21
0.03
0.25
0.09
0.07
DCM
CHCl
3
100
97
EtOAc
Acetone
MeOH
EtOH
82
85
88
ACN
74
DMF
57
a
Absorption wavelength maxima
Fluorescence emission maxima
Fluorescence quantum yield
b
c
Fig. 2 Absorption spectra of the dyes 8a-8d in DMF

J Fluoresc
Fig. 5 Absorption spectra of the dye 8c in different solvents
Fig. 3 Absorption spectra of the dye 8a in different solvents
The photophysical properties of a chromophoric system is
greatly affected by the nature of substituent, extent of charge
transfer, and microenvironment such as solvent polarity, vis-
cosity, pH, temperature, hydrogen bonding etc. Here, we have
investigated effect of solvent polarity and viscosity on the
absorption, emission and quantum yield of the synthesized
chromophores. Further we have calculated the ratio of the
excited state dipole moment to the ground state dipole mo-
ment by solvatochromic method.
The dyes were purified by column chromatography. The syn-
thesized dyes were characterized by using FT-IR, H NMR,
and mass spectral analysis. DSC-TGA analysis showed these
dyes are having good thermal stability. Photophysical proper-
ties of the synthesized molecules are also evaluated.
1
Optical Properties
The synthesized styryl dyes 8a-8c show solid state fluores-
cence while the compound 8d with barbituric acid incorpora-
tion does not fluoresce in solid state (Fig. 1). The dye 8a
shows solid state absorption maxima at 437 nm and emission
maxima at 591 nm. In the case of dye 8b solid state absorption
maxima observed at 449 nm and emission maxima at 578 nm.
The absorption maxima of the dye 8c in solid state is 446 nm
and it shows emission at 592 nm. The emission intensity of the
dyes 8a and 8b is higher than dye 8c (Table 6). The absorption
maxima of the dyes 8a-8d (N, N-diphenyl amino donor) is 2–
Effect of Solvent Polarities on Absorption Properties
The effect of solvent polarity on the photophysical properties
of the synthesized styryl dyes 8a-8d were analyzed by record-
ing the UV–Vis absorption, fluorescence emission and fluo-
−6
−1
rescence excitation spectra at concentration 1×10 mol L
in different solvents of varying polarities. The data are sum-
marized in Tables 2, 3, 4, and 5 and the spectra are shown in
Figs. 2, 3, 4, 5, and 6.
The synthesized styryl colorants has donor-π-acceptor
(D-π-A) structural system. 4-Chloro-2-(diphenylamino)-1,3-
thiazole ring acts as the electron donating unit, while cyano,
benzimidazole, benzothiazole, barbituric or meldrum acid act
as the electron withdrawing units conjugated through the
2
0 nm red shifted than our previous reported morpholine do-
nor dyes [19]. The emission intensity of the dyes 8a-8d (N,
N-diphenyl amino donor) is 15–40 nm red shifted than the
morpholine donor dyes (Table 7).
Fig. 4 Absorption spectra of the dye 8b in different solvents
Fig. 6 Absorption spectra of dye 8d in different solvents

J Fluoresc
Table 6 Solid state fluorescence data of dye 8a, 8b and 8c
Table 8 Observed UV-Visible absorption, emission, computed vertical
excitation of the dye 8a in different solvents
maxa
maxb
Dye
λ
abs (nm)
λ
ems (nm)
Ems. intensity
TD-B3LYP/6-31G(d)
8a
8b
8c
437
449
446
591
578
592
85.22
87.80
50.74
maxa
abs
c
maxb
λ
ems
Solvent
λ
Vertical excitation
f
Major
contribution
(
nm) nm eV
% D
(nm)
a
Absorption wavelength maxima
Fluorescence emission maxima
d
e
Gas
–
441 2.811
–
1.0776 H →L (0.7025)
–
b
d
e
THF
DCM
437
440
440
437
459 2.701 −5.03 1.2126 H →L (0.7037) 520
d
e
459 2.701 −4.32 1.2162 H →L (0.7038) 521
d
e
CHCl
3
460 2.695 −4.55 1.2208 H →L (0.7038) 526
olefinic bond. The styryl molecules 8a-8d absorb in 428–
67 nm range (Tables 6 and 7). The dye 8a has a blue shifted
d
e
EtOAc
458 2.707 −4.81 1.2059 H →L (0.7037) 521
4
d
e
Acetone 428
457 2.713 −6.78 1.2014 H →L (0.7037) 522
absorption compared to the dye 8b indicating that the
benzothiazolyl ring is a better acceptor than the
benzimidazolyl ring. These experimental results are supported
by TD-DFT and the results are in good agreement (Tables 8
and 9) with the computed values. The dye 8a shows red
d
e
MeOH
EtOH
ACN
431
431
431
428
456 2.719 −5.80 1.1938 H →L (0.7037) 518
d
e
457 2.713 −6.03 1.2017 H →L (0.7037) 521
d
e
456 2.719 −5.80 1.1972 H →L (0.7037) 521
d
e
DMF
459 2.699 −7.24 1.215
H →L (0.7038) 523
a
b
c
d
e
max
abs
shifted absorption in non-polar solvent as CHCl (440 nm)
λ
(Experimental absorption wavelength in nm)
(Experimental emission wavelength in nm)
3
max
than polar protic solvent as MeOH (431) and polar aprotic
solvent as DMF (428 nm). But, in the case of morpholine
donor dye do not shifted significantly. Similar results were
obtained for the dyes 8b, 8c and 8d. The experimental absorp-
tion maxima are not close to the vertical excitation data ob-
tained from TD-DFT computations. The emission maxima of
the synthesized molecules were in the range 503–552 nm. The
dyes 8a-8d in different solvents of increasing polarity do not
show any abrupt change in stokes shift. But, in the case of
morpholine donor dyes shows sudden change in stokes shift.
The dye 9d as shorter Stokes shift of 29–39 nm (in different
solvents) while 9c has higher Stokes shift in the range of 112–
λ
ems
Electronic transition (CI expansion coefficient for given excitation)
H HOMO
L LUMO, f is oscillator strength, (% D) % Deviation between experi-
mental absorption and vertical excitation computed by DFT
THF, CHCl , DCM and DMF with absorption at 455 nm,
3
while in acetonitrile, ethyl acetate, acetone and MeOH maxi-
mum absorption was observed at 449 nm with a blue shift of
6 nm (Table 3). In the case of the compound 8c, blue shift is
observed in DMF and red shifted in CHCl by 16 nm
(Table 4). The dye 8d has shown blue shift in ethyl acetate,
3
1
95 nm (in different solvents) (Table 6). Absorption spectra of
acetone, MeOH and acetonitrile compared to the CHCl by
3
the dyes 8a-8d in different solvents of increasing polarity do
not show any abrupt change in the absorption wavelength
9 nm (Table 5). The compound 8c absorbs with higher inten-
sity in EtOH (Fig. 5) while 8d absorbs with higher intensity
acetone (Fig. 6).
(
Tables 2, 3, 4, and 5). But it has been observed that 8a has
maximum absorption intensity in chloroform, while in DMF it
absorbs with the lowest absorption intensity (Fig. 3). In the
case of the compound 8b there is a slight red shift observed in
The chromophores 8a-8d absorb in the range 428–461 nm
(Fig. 2), while the emission is observed in the range 503–
551 nm in DMF (Fig. 7). Here, Fig. 2 clearly indicates that
Table 7 Comparative
Solvent
Dye 8a (9a)
Dye 8b (9b)
Dye 8d (9d)
photophysical properties of the
substituting a diphenylamino
group and substituting a
morpholine group (given in
bracket) based dyes in different
solvents
maxa
maxb
maxa
maxb
maxa
maxb
λ
abs (nm)
λ
ems (nm)
λ
abs (nm)
λ
ems (nm)
λ
abs (nm)
λems (nm)
THF
437 (428)
437 (430)
440 (430)
434 (430)
428 (430)
437 (427)
437 (428)
434 (427)
437 (430)
520 (505)
521 (503)
526 (511)
521 (505)
522 (502)
518 (495)
521 (506)
521 (484)
523 (495)
455 (449)
455 (446)
455 (446)
449 (446)
449 (446)
449 (445)
452 (446)
449 (430)
455 (451)
549 (507)
541 (507)
539 (508)
535 (506)
543 (510)
546 (504)
542 (501)
545 (506)
551 (521)
458 (428)
464 (430)
467 (430)
458 (427)
458 (430)
458 (430)
461 (428)
458 (421)
461 (430)
543 (556)
547 (577)
552 (566)
542 (541)
543 (544)
539 (542)
548 (542)
537 (616)
525 (613)
DCM
CHCl
3
EtOAc
Acetone
MeOH
EtOH
ACN
DMF
a
Absorption wavelength maxima
Fluorescence emission maxima
b

J Fluoresc
Table 9 Observed UV-Visible absorption, emission, computed vertical
excitation of the dye 8b in different solvents
TD-B3LYP/6-31G(d)
maxa
c
max b
Solvent λabs Vertical excitation
f
Major contribution λems
(
nm) nm eV
% D
(nm)
d
e
Gas
–
438 2.831
–
1.148 H →L (0.7026)
–
d
e
THF
DCM
455
455
455
449
462 2.683 −1.54 1.262 H →L (0.7038)
549
541
539
535
543
546
542
545
551
d
e
463 2.677 −1.76 1.265 H →L (0.7037)
d
e
CHCl
3
463 2.677 −1.76 1.273 H →L (0.7037)
d
e
EtOAc
461 2.689 −2.67 1.257 H →L (0.7036)
d
e
Acetone 449
MeOH 449
460 2.695 −2.45 1.249 H →L (0.7036)
Fig. 8 Emission spectra of the dye 8a in different solvents
d
e
459 2.701 −2.23 1.242 H →L (0.7036)
d
e
EtOH
ACN
DMF
452
449
455
460 2.695 −1.77 1.249 H →L (0.7036)
the evaluated solvatochromism data for the dye 8b, it is clear
that solvent polarity does not affect the absorption maxima of
the synthesized dye, the absorption is observed either at 455 or
449 nm (Table 3), and here the maximum absorption intensity
is observed in ethanol (Fig. 4). Similarly the dye 8c does not
show any significant change in the absorption maxima with
the change in polarity of the solvents except in chloroform a
bathochromic shift of 9 nm, the absorption is seen (Table 4).
The absorption overlay spectra of dye 8c shown in Fig. 5
which illustrates that no significant absorption intensity
change is observed with the polarity of solvent. The dye 8d
shows bathochromic absorption in chloroform 452 nm while
hypsochromic shift is observed in DMF at 436 nm and it
absorbs at 446–449 nm in other solvents (Table 5, Fig. 6).
d
e
460 2.695 −2.45 1.245 H →L (0.7036)
d
e
462 2.679 −1.54 1.263 H →L (0.7037)
a
Absorption wavelength maxima
Fluorescence emission maxima
Fluorescence quantum yield
H HOMO
b
c
d
e
L LUMO, f is oscillator strength, (% D) % Deviation between experi-
mental absorption and vertical excitation computed by DFT
the styryl derivatives show bathochromic shift when benz-
imidazole unit is replaced by benzothiazole unit. The longest
absorption is observed in the styryl derivative with barbituric
group. This absorption and emission characteristics are ob-
served because of the intramolecular charge transfer from do-
nor to acceptor ends. Also in DMF, Stokes shift observed in
the range of 64 to 96 nm.
Effect of Solvent Polarity on Fluorescence Emission
The solvatochromism study of chromophore 8a shows that
there is a blue shift in the absorption maxima (428 to 440 nm)
from non-polar to polar solvents. The red shifted absorption in
chloroform and dichloromethane is observed at 440 nm, while
for ethyl acetate and THF it is at 437 nm. In the rest of the
solvents it is observed at 428 and 431 nm (Table 2). In chlo-
roform 8a absorbs with maximum absorption intensity and
lowest absorption intensity is seen in DMF (Fig. 3). From
Solvatofluorism data tabulated in Tables 2, 3, 4, and 5 illus-
trate that the synthesized dyes 8a-8d emits in the range of
503–552 nm. The overlay emission spectra of these dyes in
DMF indicate that a bathochromic shift of 28 nm occurs when
benzimidazole unit is replaced by benzothiazole unit. The
barbituric acid incorporated derivative also has bathochromic
shift in emission maxima than the styryl derivative with
meldrum’s acid incorporation with very low emission
Fig. 7 Emission spectra of the dyes 8a-8d in DMF
Fig. 9 Emission spectra of the dye 8b in different solvents

J Fluoresc
Table 10 Observed UV-Visible absorption, emission, computed verti-
cal excitation and computed emission of the dye 8c in different solvents
TD-B3LYP/6-31G(d)
maxa
c
maxb
Solvent
λ
abs Vertical excitation
f
Major contribution λems
(nm) nm eV
% D
(nm)
d
e
Gas
–
406 3.054
–
0.676 H →L (0.6993)
–
d
e
THF
DCM
449
449
452
426 2.910 5.12 0.781 H →L (0.7020)
546
541
552
543
528
531
537
517
503
d
e
426 2.910 5.12 0.784 H →L (0.7021)
d
e
CHCl
3
426 2.910 5.75 0.791 H →L (0.7022)
d
e
EtOAc 446
Acetone 446
MeOH 446
425 2.917 4.71 0.775 H →L (0.7019)
d
e
425 2.917 4.71 0.769 H →L (0.7017)
Fig. 10 Emission spectra of the dye 8c in different solvents
d
e
424 2.924 4.93 0.762 H →L (0.7016)
d
e
EtOH
ACN
DMF
449
443
446
425 2.917 5.35 0.769 H →L (0.7017)
intensity as compared to benzimidazole and benzothiazole
substituted derivatives (Figs. 7, 8 and 9).
d
e
424 2.924 4.29 0.766 H →L (0.7016)
d
e
426 2.907 2.29 0.783 H →L (0.7020)
The emission pattern of the dye 8c in the solvent media of
different polarity shows that the blue shifted emission is ob-
served in polar solvents such as DMF with emission maxima
a
Absorption wavelength maxima
Fluorescence emission maxima
Fluorescence quantum yield
H HOMO
b
c
d
e
5
03 nm and bathochromic emission maxima of 552 nm is
observed in non-polar solvent chloroform, it clearly indicates
the negative solvatofluorism property of the dye 8c (Table 4).
Also it emits with stronger emission intensity in dichlorometh-
ane at 541 nm (Fig. 10). Stokes shift data also follows the
same pattern as seen in the emission maxima. Largest Stokes
shift of 100 nm is observed in chloroform and smallest value
L LUMO, f is oscillator strength, (% D) % Deviation between experi-
mental absorption and vertical excitation computed by DFT
respectively, hence it has positive solvatofluoric effect with
increasing solvent polarity (Table 3). Also strong emission
6
7 nm is observed in DMF for dye 8c (Table 4). Similarly, 8d
also shows negative solvatofluorism, it has bathochromic ab-
sorption maxima of 552 nm in chloroform, while in DMF 8d
has blue shifted emission maxima at 525 nm (Table 5). The
strongest emission intensity was observed in chloroform at
Table 11 Observed UV-Visible absorption, emission, computed verti-
cal excitation and computed emission of the dye 8d in different solvents
5
47 nm (Fig. 11). Stokes shift of 87 nm has been observed
in ethanol; and in chloroform, THF and acetone Stokes shift of
5 nm was observed. The lowest value of stokes shift is found
TD-B3LYP/6-31G(d)
maxa
abs
c
maxb
λ
ems
Solvent
λ
Vertical
f
Major
contribution
8
excitation
in DMF 64 nm (Table 5).
For the dye 8b bathochromic emission maxima observed in
DMF at 551 nm, while blue shifted emission spectra are ob-
served for ethyl acetate and chloroform at 535 and 539 nm
(nm) nm eV
% D
(nm)
d
e
Gas
–
417 2.973
–
0.6818 H →L (0.6994)
–
d
e
THF
DCM
458
464
467
458
437 2.837 4.59 0.7992 H →L (0.7025) 546
d
e
437 2.837 5.82 0.8029 H →L (0.7025) 541
d
e
CHCl
3
437 2.837 6.42 0.8088 H →L (0.7026) 552
d
e
EtOAc
436 2.844 4.8
0.7924 H →L (0.7024) 543
d
e
Acetone 458
435 2.850 5.02 0.7874 H →L (0.7023) 528
d
e
MeOH
EtOH
ACN
458
461
458
461
435 2.850 5.02 0.7799 H →L (0.7021) 531
d
e
436 2.844 5.42 0.7878 H →L (0.7023) 537
d
e
435 2.850 5.02 0.7836 H →L (0.7022) 517
d
e
DMF
437 2.835 5.21 0.803
H →L (0.7025) 503
a
Absorption wavelength maxima
Fluorescence emission maxima
Fluorescence quantum yield
H HOMO
b
c
d
e
L LUMO, f is oscillator strength, (% D) % Deviation between experi-
Fig. 11 Emission spectra of the dye 8d in different solvents
mental absorption and vertical excitation computed by DFT

J Fluoresc
intensity is observed in DMF dye for the dye 8b (Fig. 9). The
dye 8b shows the largest Stokes shift in methanol 97 nm
followed by DMF and acetonitrile 96 nm, lowest value of
with an increase in the solvent polarity (80 nm in chloroform
and 95 nm in DMF). Here stronger emission intensity was
observed in chloroform and DMF while lower emission inten-
sity is observed in acetonitrile (Fig. 8).
8
4 nm is seen in chloroform (Table 3). In case of the dye 8a,
there is no such significant change in the emission maxima
with the change in the solvent polarity; it emits in the range
Fluorescence excitation maxima of the dyes 8a-8d were
observed from 417 to 469 nm. The dye 8a has average exci-
tation at 434 nm the excitation maxima are very close to ab-
sorption maxima except excitation maxima observed in
5
18 nm in methanol and 526 nm in chloroform. Stokes shift
values given in the Table 2 indicate that Stokes shift increases
Table 12 Frontier molecular orbitals and its energy of the dyes 8a-8d in the ground and excited state in DMF solvent
Compound
HOMO
LUMO

J Fluoresc
methanol where a difference of 16 nm is obtained (Table 2).
For the dye 8b the emission maxima in dichloromethane differ
by 14 nm otherwise it is almost same as absorption maxima
and 0.0066 (0.66 %) in ethyl acetate. Here a lower value of
quantum yields are estimated in polar solvents such as 0.0012
(0.12) in methanol, 0.0017 (0.17 %) in DMF, 0.0019 (0.19) in
acetonitrile (Table 5). The dye 8a has highest quantum yield in
ethyl acetate, 0.0114 (1.14 %) and lower value is obtained in
acetone, 0.0015 (0.15 %) (Table 2). Similarly for the dye 8b
appreciable quantum efficiency is observed in dichlorometh-
ane, 0.0069 (0.69 %), DMF 0.0062 (0.62 %), and chloroform
0.0059 (0.59 %) while lowest value of 0.0004 (0.04 %) is
evaluated in ethanol (Table 3).
(
1
Table 3). The dye 8c has a considerable difference of 15 and
9 nm in acetone and DMF respectively, here the excitation
was observed at lower wavelength than the absorption wave-
length (Table 4). The dye 8d has significant difference in the
excitation and absorption but in dichloromethane a difference
of 26 nm is observed (Table 5).
In benzimidazole derivative 8a substituting morpholine
group by diphenyl group there is red shift in emission wave-
length as well as in stokes shift also stokes shift increases with
polarity reverse is observed in morpholino derivatives. The
red shift in emission wavelength is seen from 15 nm in chlo-
roform to 37 nm in acetonitrile, there is rise in red shift with
increasing polarity. Similar trend is seen in stokes shift, in
chloroform 5 nm rise in stokes shift while 33 nm rise in stokes
shift is observed in Acetonitrile. In case of Benzthiazole de-
rivative 8b also there is red shift in emission wavelength as
well as in stokes shift. The observed red shift in emission
wavelength is in 29 (Ethyl acetate) to 42 nm (THF and meth-
anol) range and that of stokes shift is 20 (in acetonitrile) to
Effect of Viscosity on Photophysical Properties of Dyes
8a-8d
The molecules has number of molecular motions, these mo-
tions are responsible for diminishing fluorescence of
fluorophores by collisions and other non-radiative pathways.
The viscosity of solvent media plays a crucial role in enhanc-
ing the fluorescence intensity by restricting the molecular mo-
tions and with that loss of energy responsible for non-radiative
emission. Glycerin is a viscous media hence increased con-
centration of glycerin increases the viscosity of solvent media.
Such enhancement in the viscosity confines the molecular
motion, hence lowers the loss of excitation energy of excited
species by non-radiative ways.
Here, we have verified the effect of increasing viscosity on
fluorescence intensity of synthesized novel fluorophores. The
absorbance and emission spectra were recorded in an in-
creased concentration of glycerin from 0 to 50 %. In order to
verify the linearity of change in fluorescence intensity with
viscosity graph of absorption intensity and emission intensity
were plotted against increasing concentration of glycerin in
methanol (Figs. 12, 13, 14, 15, 16, and 17).
3
8 nm (in methanol) range. As well as in case for barbituric
acid derivative 8d the rise in emission wavelength is observed
ranging from 32 nm (in DMF) to 61 nm (Chloroform and
Ethanol), While stokes shift increased in the range 29 nm
(
DMF) to 58 (Ethanol) (Table 7). The red shift in the emission
wavelength and rise in stokes shift may be due to the presence
of two benzene rings instead of aliphatic chain, also here the
benzene rings are twisted and thiazole ring is in the plane of
nitrogen hence there is sufficient intermolecular charge trans-
fer from nitrogen to electron withdrawing group present in
thiazole ring.
The overlay fluorescence emission spectra of the dye 8b
clearly show that as viscosity increases fluorescence intensity
increases (Fig. 12). The graph of absorption intensity and
fluorescence emission intensity vs. percentage of glycerol in
Quantum Yield
The relative fluorescence quantum yields of the synthesized
chromophores in different solvents of different polarity are
calculated and summarized in Tables 2, 3, 4, and 5. As
discussed earlier, the photophysical properties of the
fluorophores are largely affected by polarity of solvent. Here
we have investigated the effect of solvent polarity on the quan-
tum yield of the synthesized fluorophore. Fluorescein is used
as a reference standard (Tables 10, 11, and 12).
The dye 8c has largest quantum yield of 0.0214 (2.14 %) in
dichloromethane among all the synthesized styryl derivatives.
Its quantum yield decreases with the increasing solvent polar-
ity. The quantum efficiency in chloroform is 0.0093 while in
polar solvents methanol, DMF and acetonitrile quantum
yields of 0.0003, 0.0007 and 0.0009 are obtained respectively
(
Table 4). The dye 8d also has high quantum efficiency in
non-polar solvents 0.0119 (1.19 %) in dichloromethane,
.0113 (1.13 %) in THF, 0.0098 (0.98 %) in chloroform,
Fig. 12 Fluorescence emission spectra of dye 8b at different % of
glycerol in methanol
0

J Fluoresc
Fig. 15 Graph of absorption intensity and fluorescence emission
intensity vs. percentage of glycerol in methanol solvent for dye 8c
Fig. 13 Graph of absorption intensity and fluorescence emission
intensity vs. percentage of glycerol in methanol solvent for dye 8a
relationship. A detailed theory for the calculation of dipole
moment is described in supporting information [29]. All the
solvatochromism and solvatofluorism details of the dyes 8a-
methanol solvent for dye 8b (Fig. 14) shows that enhancement
in the absorption value with a slope of 0.0049, with that the
emission intensity increases with the slope of 0.0322. The
linear regression for absorption is 0.59 and for emission
−1
8
b such as absorption wavelength (cm ), emission wave-
−1
−1
length (cm ), Stokes shift (cm ) and arithmetic mean of
0
.8892. From Fig. 13 for dye 8a, we have obtained increment
−1
absorption and emission wavelengths (cm ) with solvent pa-
rameter and Bakhshiev and Bilot-Kawski functions of sol-
vents are tabulated in Table S1. The graphs of Stokes shift
in the emission intensity with a slope of 0.0329 and absorption
intensity enhances with 0.0014 slope value, linear regression
was found to be 0.9025 and 0.0792 respectively. Fluorophore
(
Δν) of dyes 8a-8d verses Bakhshiev’s polarity parameter f
8
d has slope value of 0.0247 and 0.0034 for emission and
absorption intensity with a linear regression of 0.8944 and
.1676 respectively (Fig. 16), while emission and absorption
1
À
Á
(ε, η) (Figure S1) and of ( νa þ νf =2 ) with Bilot-Kawski
0
solvent polarity parameter f (ε,η) (Figure S2) were plotted
2
intensity of dye 8c has slope of 0.0505 and 0.0017 and the
linear regression of 0.4761 and 0.413 (Fig. 15).
and their slopes are calculated as m and m respectively.
1
2
The calculated slope values, their constants and correlation
coefficients are compiled in Table S2, and finally ratio of
ground sate to the excited state dipole moment are computed
and tabulated in Table S3.
From Table S3, it is evident that dyes 8a and 8b have
higher dipole moment in exited state than the ground state.
In other words we can say that excited state of dyes 8a and 8b
is more polarized than the ground state, indicating good extent
of charge transfer in excited state. However dipole moment
ratio of dyes 8c and 8d indicates that they have lower dipole
moment in exited state than ground state.
From all the above mentioned values of slope, it is evident
that there is effective enhancement in fluorescent intensity
with viscosity as compared with absorption intensity which
is direct indication of enhancement in quantum yield of
fluorophores with increasing viscosity of solvent medium.
Here, we evaluated the ratio of ground sate to the excited
state dipole moment of the synthesized novel styryl dyes 8a-d
using Bakhshiev [27], and Bilot-Kawski correlations [29, 44].
It is to be noted that the formation of molecular complexes and
hydrogen bonding does not account in the Lippert–Mataga
Fig. 14 Graph of absorption intensity and fluorescence emission
intensity vs. percentage of glycerol in methanol solvent for dye 8b
Fig. 16 Graph of absorption intensity and fluorescence emission
intensity vs. percentage of glycerol in methanol solvent for dye 8d

J Fluoresc
Acknowledgments The authors Prashant Umape is thankful to UGC
for providing fellowship under UGC-SAP programme and Sharad R.
Patil is thankful to University Grant Commission (UGC-NET SRF) for
providing Senior research fellowship.
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In summary, we have developed an efficient and simple pro-
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1
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5
03 to 551 nm with the Stokes shift ranging from 67–100 nm.
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1
1
8
a and 8b are polar in excited state than the ground state,
while dyes 8c and 8d have polar ground state than the excited
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1
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