Nonlinear Optical Chromophores with Aggregation Induced Emission Enhancement Based on 2-N,N-Dibutylamino-4-Phenyl Thiazole with FMR Characteristics

Article abstract of DOI:10.1007/s10895-018-2311-7

The synthesis, photophysical, linear and nonlinear optical (NLO) properties of novel fluorescent styryls based on 2-N,N-dibutylamino-4-phenyl thiazole as donor attached to different acceptor groups via conjugated π-bridge were studied with the help of solvatochromic model. The NLO properties of extended styryls based on benzylidene malononitrile and isophorone were more enhanced than the simple styryls based on malononitile and ethylcyano acetate which was attributed to the longer conjugation path. The benzylidene based extended styryl dye showed maximum value of two-photon absorption cross section σ_2PA 98.85GM in dichloromethane. The 2-amino thiazole derivatives were responsive towards the viscosity of the medium hence investigated for the fluorescent molecular rotor (FMR) properties wherein almost 2.07 to 3.65 fold increase in emission intensity was observed. Also, the aggregates of these dyes in N,N-dimethyl formamide-water mixture showed enhancement in emission on aggregates formation like 3.73 fold increase in emission intensity was observed for benzylidene based extended styryl dye. Thus NLOphoric AIEgens with FMR properties emitting in the red region of the electromagnetic radiation were developed.

Full text of DOI:10.1007/s10895-018-2311-7

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2311-7
ORIGINAL ARTICLE
Nonlinear Optical Chromophores with Aggregation Induced Emission
Enhancement Based on 2-N,N-Dibutylamino-4-Phenyl Thiazole
with FMR Characteristics
Manali Rajeshirke1 & Dharit Shah1 & Nagaiyan Sekar1
Received: 28 August 2018 /Accepted: 14 October 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
The synthesis, photophysical, linear and nonlinear optical (NLO) properties of novel fluorescent styryls based on 2-N,N-
dibutylamino-4-phenyl thiazole as donor attached to different acceptor groups via conjugated π-bridge were studied with the
help of solvatochromic model. The NLO properties of extended styryls based on benzylidene malononitrile and isophorone were
more enhanced than the simple styryls based on malononitile and ethylcyano acetate which was attributed to the longer conju-
gation path. The benzylidene based extended styryl dye showed maximum value of two-photon absorption cross section σ2PA
98.85GM in dichloromethane. The 2-amino thiazole derivatives were responsive towards the viscosity of the medium hence
investigated for the fluorescent molecular rotor (FMR) properties wherein almost 2.07 to 3.65 fold increase in emission intensity
was observed. Also, the aggregates of these dyes in N,N-dimethyl formamide-water mixture showed enhancement in emission on
aggregates formation like 3.73 fold increase in emission intensity was observed for benzylidene based extended styryl dye. Thus
NLOphoric AIEgens with FMR properties emitting in the red region of the electromagnetic radiation were developed.
.
.
.
Keywords 2-N,N-dibutylamino-4-phenyl thiazole Aggregation induced emission (AIE) Florescent molecular rotors (FMR)
Two-photon absorption
Introduction
nitrothiazole and is widely studied [7, 8]. Substituted 2-
aminothiazoles serve as important intermediate in synthesis of
various drugs, as a coupler in azo dyes [9–17]. In our present
research, we focus on novel styryls obtained from 2-N,N-
dibutylamino-4-phenyl thiazole acting as donor attached via
π-bridge of varying length with different acceptors. The
fluorophore with long alkyl chain are sensitive towards the
microenvironment especially the viscosity of the medium as
the molecular rotations are directly dependent on the viscosity
[18, 19]. Such fluorophores which are sensitive towards viscos-
ity are known as Fluorescent molecular rotors (FMRs) and are
used as viscosity measuring probes in several biological sys-
tems, and polymerization processes [20, 21]. Especially the
FMRs with red emission are of current interest to have back-
ground free detection of biomolecules or viscosity of biofluids
in biological systems. Also, another phenomena known as ag-
gregation induced enhancement in emission (AIEE) observed
in molecules with low or minimal fluorescence in dilute solu-
tion emit strongly when they aggregate [22]. The AIEE could
be because of several reasons like restriction of intramolecular
rotation (RIR), planarization, E/Z isomerization [23], twisted
intramolecular charge transfer (TICT), restriction of
Styryl dyes usually comprise of D-π-A design with variety of
applications as sensitizers in photographic industry [1, 2], and
the fluorescent ones are used in high tech applications like
electroluminescence devices [3], optical signal processing [2,
4], nonlinear optics [5, 6]. The ease of synthesis, structure flex-
ibility make them popular. NLO behavior of styryls is studied
widely by substituting different donor and acceptor groups. 2-
Aminothiazoles are important intermediates for disperse dyes,
for example in the synthesis of commercially available
monoazo C.I. Disperse Blue 339, a dye based on 2-amino-5-
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-018-2311-7) contains supplementary
material, which is available to authorized users.
* Nagaiyan Sekar
n.sekar@ictmumbai.edu.in
1
Department of Dyestuff Technology, Institute of Chemical
Technology, Nathalal Parekh Marg, Matunga, Mumbai 400 019,
India

J Fluoresc
intramolecular vibration (RIV) and restriction of intramolecular
motion (RIM) [24]. Thus AIEE is one of the contemporary
topics of interest.
dropwise in N,N-dimethyl formamide (4 ml) at 0 °C, to which
the solution of 2-N,N-dibutylamino-4-phenyl thiazole
(36 mmol, 1 eq) in N,N-dimethyl formamide at 0 °C
was added further. This reaction mixture was stirred for
30 min at room temperature and then refluxed for 4 h.
TLC was used to check the progress of the reaction. On
completion of the reaction water is added to the mixture
and neutralized with 2 N sodium bicarbonate solution.
The product was extracted in chloroform and concen-
trated to obtain the green viscous liquid which solidified
on cooling. Yield 60% M.pt. 38–40 °C. The aldehyde is
condensed with the active methylenes to obtain the 2-
amino thiazole styryls STD1–4.
In our research here we explore the synthesis, characteriza-
tion, viscosity sensitivity, AIEE property and NLO properties
of the novel styryls from 2-N,N-dibutylamino-4-phenyl thia-
zole. The aldehyde of 2-N,N-dibutylamino-4-phenyl thiazole
on Knoevenagel condensation with different active methylene
resulted in formation of STD 1–4. The FMR, AIEE and NLO
properties are studied for these dyes with help of simple spec-
troscopic data. The extended styryls STD 3–4 show enhanced
properties compared to the simple styryls STD 1–2 and serve
as NLOphore with additional functionality.
Experimental
General Procedure for the Synthesis of STD 1–4
Materials and Methods
A mixture of 2-(dibutylamino) 4-phenylthiazole 5-
carbaldehyde Stdcarb (1.3 ml, 4 mmol) and correspond-
ing active methylene (5.5 mmol) in anhydrous ethanol
(10 ml), and catalytic amount of piperidine was refluxed
with constant stirring. The reaction progress was
checked by TLC. The pure product was obtained by
column chromatography using Petroleum Ether: Ethyl
Acetate as eluent.
The chemicals reagents were procured from Merck and SD
Fine Chemical Limited, India. Progress of the reaction were
monitor by thin layer chromatography (TLC) precoated plates
of 0.25 mm E-Merck silica gel 60 F254. Recrystallization and
column chromatography were used to purify the synthesized
dyes. Standard melting point apparatus from Sunder Industrial
Products, Mumbai was used to measure the melting points. 1H
NMR and 13C NMR spectra were recorded on Agilent
500 MHz NMR instrument where TMS was used as an inter-
nal standard. The Perkin Elmer Lambda 25 UV visible spec-
trophotometer was used to record the absorption and further
Varian Cary Eclipse fluorescence spectrophotometer was used
to measure the emission of the synthesized compounds.
2-((2-(Dibutylamino)-4-Phenylthiazol-5-Yl) Methylene)
Malononitrile STD 1
Color: Bright Yellow solid, Yield: 78%, M.pt. 87–90 °C. 1H
NMR (500 MHz, CDCl3) δ 7.62 (s, 1H), 7.55–7.49 (m, 5H),
3.76 (t, 2H), 3.51–3.38 (m, 2H), 1.71 (dd, 4H), 1.41 (s, 4H),
0.99 (t, 6H). 13C NMR (126 MHz, CDCl3) δ 173.42 (s),
169.04 (s), 149.36 (s), 133.09 (s), 130.54 (s), 129.81 (s),
128.87 (s), 116.24 (d, J = 17.7 Hz), 115.02 (s), 67.54 (s),
20.03 (s), 13.79 (s). Anal. Calcd. for C21H24N4S: C, 69.20;
H, 6.64; N, 15.37%. Found: C, 69.23; H, 6.63; N, 15.39%.
Synthesis and Characterization
The raw material 2-N,N-dibutylamino-4-phenyl thiazole was
produced by known sequence of reactions [12, 25–29]. The
purity of the compound was >80% and accordingly all the reac-
tions were carried out. The 2-aminothiazole styryls STD 1–4
were synthesized via Knoevenagel condensation of
2-(dibutylamino) 4-phenylthiazole 5-carbaldehyde with active
methylene compounds. The malononitrile, ethylcyanoacetate
were commercially available whereas the 1a and 1b
(Scheme 1) were obtained by the reported method [30–32].
The dyes were purified by column chromatography and
characterized by melting point, 1H, 13C NMR spectroscopy,
and elemental analysis.
Ethyl (E)-2-Cyano-3-(2-(Dibutylamino)-4-Phenylthiazol-5-Yl)
Acrylate STD 2
Colour: Light Orange solid, Yield: 64%, M.pt. 62–65 °C. 1H
NMR (500 MHz, CDCl3) δ 8.26 (s, 1H), 7.59 (dt, 2H), 7.50–
7.46 (m, 3H), 4.27 (q, 2H), 3.64 (dd, 4H), 1.71 (dt, 4H), 1.44–
1.36 (m, 4H), 1.32 (t, 3H), 0.98 (t, 26H). 13C NMR (126 MHz,
CDCl3) δ 173.02 (s), 167.81 (s), 164.46 (s), 146.34 (s), 133.76
(s), 129.94 (d, J = 11.6 Hz), 128.64 (s), 117.45 (s), 115.93 (s),
90.52 (s), 77.34 (s), 77.08 (s), 76.83 (s), 62.98 (s), 61.63 (s),
29.34 (s), 24.74 (s), 20.05 (s), 14.30 (s), 13.83 (s). Anal.
Calcd. for C23H29N3O2S: C, 67.12; H, 7.10; N, 10.21%.
Found: C, 67.14; H, 7.11; N, 10.23%.
Synthesis of 2-(Dibutylamino) 4-Phenylthiazole
5-Carbaldehyde Stdcarb
Stdcarb aldehyde was obtained from Vilsmeier–Haack
formylation reaction. POCl3 (41.9 mmol, 3.9 ml) was added

J Fluoresc
NC
NC
CN
Scheme 1 Synthesis Strategy
S
N(Bu)2
NC
NC
CN
S
STD 1
STD 2
N(Bu)2
N(Bu)2
N(Bu)2
N
N
3
COOEt
COOEt
S
N
O
S
CN
CN
N(Bu)2
N
NC
CN
S
N
Stdcarb
STD 3
(1a)
CN
CN
NC
S
(1b)
STD 4
N(Bu)2
CN
N
(E)-2-(3-(2-(Dibutylamino)-4-Phenylthiazol-5-Yl)
-1-Phenylallylidene) Malononitrile STD 3
(s), 38.96 (s), 31.96 (s), 29.43 (s), 27.97 (s), 20.10 (s), 13.88
(s). Anal. Calcd. for C30H36N4S: C, 74.34; H, 7.49; N,
11.56%. Found: C, 74.35; H, 7.48; N, 11.57%.
Colour: Grey solid, Yield: 41%, M.pt. 140–144 °C. 1H NMR
(500 MHz, CDCl3) δ 7.46 (dd, 3H), 7.41–7.38 (m, 2H), 7.34–
7.27 (m, 5H), 7.10 (d, 1H), 6.79 (d, J = 14.5 Hz, 1H), 3.64–
3.47 (m, 4H), 1.77–1.66 (m, 4H), 1.47–1.36 (m, 4H), 1.00 (t,
6H). 13C NMR (126 MHz, CDCl3) δ 171.04 (s), 170.65 (s),
162.54 (s), 141.37 (s), 133.88 (d, J = 6.2 Hz), 130.44 (s),
129.37 (d, J = 16.6 Hz), 128.68 (d, J = 4.1 Hz), 128.28 (s),
119.77 (s), 119.41 (s), 114.93 (s), 114.37 (s), 29.38 (s),
20.10 (s), 13.85 (s). Anal. Calcd. for C29H30N4S: C, 74.64;
H, 6.48; N, 12.01%. Found: C, 74.67; H, 6.47; N, 12.02%.
Results and Discussion
Design of Styryls STD1–4
2-N,N-Dibutylamino-4-phenyl thiazole styryl derivatives with
D-π-A framework having various electron acceptor groups are
synthesized by Knovenagel condensation and are characterized
by 1H NMR, 13C NMR, CHN analysis and UV-Vis spectros-
copy. The five membered thiazole ring plays a vital role while
designing series of styryl molecules by adding planarity to the
molecule. We have synthesized STD 1–2 which are simple
styryls whereas STD 3–4 are extended styryls. As expected
the extended styryls show better optical properties than the
simple ones. Also these styryls have low melting point due to
the presence of butyl chain in the structure.
2-(3-((E)-2-(2-(Dibutylamino)-4-Phenylthiazol-5-Yl) Vinyl)-5,
5-Dimethylcyclohex-2-Enylidene)-2-Isocyanoacetonitrile STD
4
Colour: Dark Grey solid, Yield: 35%, M.pt. 175–182 °C. 1H
NMR (500 MHz, CDCl3) δ 7.60 (d, J = 7.4 Hz, 2H), 7.43 (dt,
J = 26.3, 7.3 Hz, 3H), 7.25–7.19 (m, 2H), 6.66 (s, 1H), 6.33
(d, J = 15.2 Hz, 1H), 3.54–3.44 (m, 4H), 2.51 (s, 2H), 2.26 (s,
2H), 1.72–1.64 (m, 4H), 1.38 (dq, J = 14.8, 7.4 Hz, 4H), 1.03–
0.94 (m, 12H). 13C NMR (126 MHz, CDCl3) δ 168.60 (d, J =
10.2 Hz), 157.05 (s), 154.91 (s), 135.03 (s), 129.63 (s), 129.29
(s), 128.65 (d, J = 25.1 Hz), 128.50–128.38 (m), 126.00 (s),
121.12 (s), 120.50 (s), 114.38 (s), 113.61 (s), 51.30 (s), 42.88
Absorption and Emission Properties
The optical properties of 5 μM solution of all the styryls were
examined in various microenvironment at 25 °C and values
were charted in Table 1. All the styryls absorb in the wave-
length range of 442 to 544 nm. The order of absorption is STD

J Fluoresc
Table 1 Experimental optical properties of STD 1–4 in various solvents
λabs(a)
λemi(b)
Δυ(c)
εmax
Φf(e)
f(f)
Kr(g)
Knr
σ1(υ) (i) (cm2)
(d)
(h)
Dye
Solvent
cm−1
nm
STD 1
Toluene
DCM
Acetonitrile
Methanol
DMSO
DMF
Toluene
DCM
Acetonitrile
Methanol
DMSO
DMF
Toluene
DCM
Acetonitrile
Methanol
DMSO
DMF
Toluene
DCM
Acetonitrile
Methanol
DMSO
DMF
442
446
442
442
450
446
438
442
438
492
444
442
510
524
514
512
530
524
530
540
526
530
544
540
503
506
508
502
509
510
499
503
502
502
498
497
579
592
592
588
595
598
623
651
654
657
663
667
2743.72
2658.68
2939.40
2704.12
2575.86
2813.68
2790.97
2743.72
2910.73
404.88
2442.20
2503.71
2336.69
2192.08
2563.36
2524.45
2061.20
2361.56
2816.56
3157.54
3720.89
3647.22
3299.40
3526.01
61
60
66
60
59
64
61
61
64
10
54
55
69
68
78
76
65
74
93
111
128
127
119
127
16340
19100
20000
19280
17980
18300
8020
11380
12720
787.34
14160
13240
5960
0.0005
0.0046
0.0032
0.0030
0.0059
0.0004
0.0330
0.0032
0.0210
0.0260
0.0390
0.0630
0.0080
0.0130
0.0100
0.0130
0.0250
0.0350
0.0350
0.0990
0.0800
0.0910
0.1310
0.2190
2.27
2.50
2.63
2.53
2.35
2.45
1.10
1.52
1.72
0.13
1.89
1.78
0.89
1.20
1.20
1.21
0.41
0.41
0.32
0.20
0.23
0.27
0.26
0.25
9.80
9.7
19,600.00
2100.00
2900.00
2880.00
1850.00
27,800.00
147.00
1870.00
272.00
19.60
211.00
111.00
521.00
406.00
365.00
277.00
57.70
43.50
27.00
4.17
4.83
5.04
2.42
1.87
6.24
7.30
7.64
7.37
6.87
6.99
3.06
4.35
4.86
3.01
5.41
5.06
2.27
3.07
2.99
2.94
1.10
1.01
5.63
4.11
4.20
4.87
5.06
4.87
9.30
8.70
0.11
0.11
5.00
6
5.80
52.00
8.6
7.50
4.10
5.40
3.70
3.70
1.50
1.60
98.00
46.00
42.00
50.00
36.00
53.00
STD 2
STD 3
STD 4
8040
7840
7700
2880
2660
14750
10750
11000
12750
13250
12750
a the wavelength with maximum absorption in nm
b the wavelength with maximum emission intensity in nm
c Stokes Shift in cm−1 and nm
d absorptivity in L M−1 cm−1
e quantum yield
f oscillator strength
g radiative rate constant (×10−8 s−1
)
h non-radiative rate constant (×10−8 s−1 ), and
i absorption cross section in (× 10−20 cm2 )
2(442 nm) > STD 1(446 nm) > STD 3 (524 nm) > STD 4
(540 nm) in dichloromethane. The dyes STD 1–4 have D-π-
A framework with different acceptor groups and π conjuga-
tion length which tunes the photophysical properties of the
dyes. The absorption spectra and emission spectra of the dyes
are presented in Figs. 1 and 2. As observed in Table 1 there is
not much difference in the absorption maxima of dyes STD 1
and STD 2, whereas it significantly increases in case of ex-
tended styryls STD 3 and STD 4 which is attributed to in-
crease in conjugation chain and better intramolecular charge
transfer. The STD 1 shows little bathochromic compared to
STD 2 due to presence of stronger dicynovinyl acceptor than
ethyl cyanoethoxycarbonyl. Similarly, for STD 3 and STD 4
as the π system increases red shift is absorption maxima is
observed. The absorption spectra for simple styryls STD 1 and
STD 2 are least affected by the solvent polarity, whereas for
the extended styryls STD 3 and STD 4 increase of 14-20 nm is
observed with rise in polarity of solvent from toluene to N,N-
dimethyl formamide (DMF) (Table 1).
Red shift in fluorescence emission is observed in order:
STD 2(503 nm) > STD 1(506 nm) > STD 3 (592 nm) > STD
4 (651 nm) in dichloromethane. As observed in absorption
spectra the solvatochromism is more enhanced in case of ex-
tended styryls STD 3 and STD 4 than simple styryls STD 1
and STD 2. In extended styryls, a 20-44 nm bathochromic
shift is observed from non-polar toluene to polar DMF
(Table 1). In extended styryls, the Stokes shift increases with
rise in solvent polarity, for example in STD 4 Stokes shift is
93 nm in toluene which increases to 127 nm in methanol. Also
a blue shift in absorption and emission spectra in polar protic
solvent like methanol was observed for all the styryls STD 1–
4 compared to polar aprotic solvents like DMF due to the
hydrogen bond interactions of solvent with the N, N, dibutyl
amine donor of solute thereby decreasing the donor capacity
and decrease in ICT characteristics leading to blue shifted
absorption and emission. Thus from the results, it is predicted
that strong acceptor and increase in π conjugation resulted in
bathochromic shift in both the absorption and emission

J Fluoresc
Fig. 1 Absorption plot of STD 1–4 in different solvents
Fig. 2 Emission plot of STD 1–4 in different solvents

J Fluoresc
0
1
ꢂ
ꢃ
2
ꢀ
ꢁ
properties. The quantum yield in solvents were measured for
all the dyes using Rhodamine 6G as reference standard and
presented in Table 1. Higher quantum yield is seen in polar
solvents like DMF than in non-polar solvent for all the dyes.
All the dyes show overall low quantum yield indicative of
more non-radiative relaxation than radiative relaxation as best
explained by the study of radiative and non-radiative radiative
rate constant of the dyes.
2 μe−μg
1
B
@
C
A
ꢀ
ꢀ
υabs−υemn
¼
f
þ Constant
ð3Þ
LM
4πϵO
hca3
where Lippert-Mataga (LM) function is given as;
ε−1
n2−1
f LM ðε; nÞ ¼
−
ð4Þ
2ε þ 1 2n2 þ 1
where ῡabs − ῡemn is the Stokes shift (in cm−1), h represents
Planck’s constant (in erg x s), c is the speed of light in vacuum
(in cm s−1), Ba^ as Onsager radius (in cm), ε represents the
relative dielectric constant, and n as static refractive index of
solvent. The LM function also known as orientation polarizabil-
ity behaves linearly against the Stokes shift with good regression
coefficient for the synthesized dyes in studied solvents indicates
the influence of that polarity and polarizability factors of solvent
onto the absorption and emission of STD 3–4 (Fig. 3).
McRae equation
Radiative and Non-radiative Rate Constant
According to Stickler Berg the radiative (Kr) and non-
radiative rate constants in all the studied solvent is given by
the Eqs. (1) and (2);
ꢀ
∫
f ðυÞ
Δυe
Kr ¼ 2:88 Â 10−9n2
1−ϕF
∫∈ðυÞdυ
ꢀ
ð1Þ
ð2Þ
ꢀ
ꢀ
ꢀ ꢀ−3
f ðυÞυ dυ
∫
Δυa
0
1
ꢂ
ꢃ
2
Knr
¼
ꢀ
ꢁ
2 μe−μg
τ
1
B
@
C
A
ꢀ
ꢀ
υabs−υemn
¼
f
McRae þ Constant
ð5Þ
4πϵO
hca3
Wherein the Δυa and Δυe represents the experimental limits
of absorption and emission bands (S0–S1 transitions) respective-
ly, the molar extinction coefficient as ϵ, the wavenumber (cm−1)
as υ¯ and f(υ¯) as the maximum fluorescence intensity mea-
sured in atomic unit (a.u.) and τ fluorescence lifetime. The Kr,
and Knr values for STD1–4 were analyzed using Eqs. 1 and 2
and charted in Table 1. It is clearly observed that the Knr values
are greater than the Kr resulting into lower quantum yields as the
decay of excited state occur via non-radiative pathways. As
observed in Table 1 the extended styryls STD 3–4 are more
responsive towards solvents. For STD 3–4 the Knr values are
higher in nonpolar solvent like toluene than in polar solvents
whereas the Kr and ΦF values are higher in polar solvents than in
nonpolar solvents (Table 1). This behavior in extended styryls is
attributed to strong solvation of the solute molecules by the polar
solvent molecules thus restricting the non-radiative relaxation
pathways and increasing the Kr and ΦF values in polar solvent.
where McRae function is given as;
ε−1
n2−1
f McRaeðε; nÞ ¼
−
ð6Þ
ε þ 2 n2 þ 2
Furthermore improved version of LM is McRae model is
studied wherein the McRae function (ƒMcRae(ε,η)) is given by
Eq. 6, the plot of ƒMcRae versus Stokes shift also showed a good
linear relation for STD 3–4. Thus, the linearity in LM and
MacRae plots for STD 3–4 shows that the CT occur from local
excited state.
Weller’s equation
2
2ðμeÞ
ϑemn
¼
f w þ constant
ð7Þ
hca3
where Weller’s function fw(ε, n) is given as;
ε−1
n2−1
Solvatochromism and Charge Transfer (CT)
Characteristics
f wðε; nÞ ¼
−
ð8Þ
2ε þ 1 4n2 þ 2
Along with the LM and McRae model, the Weller’s model
was also implied in order to study the charge transfer from in
the D-π-A styryls. The Weller’s plot considers only the emis-
sion maxima vs the fw(ε, n) which was linear for STD 3–4
with good regression coefficient indicative of ICT character-
istics at excited state.
The extended thiazole styryls STD 3–4 showed more emis-
sion solvatochromism than absorption solvatochromism,
whereas very minimum to negligible solvatochromism is seen
for simple styryls STD 1–2. Thus the CT characteristics with
the help of polarity functions is studied for STD 3 and 4 only.
As observed in Table 1 there is increase in Stokes shift of the
STD 3–4 in polar solvent compared to nonpolar solvents due
to polar excited state in polar solvent. Thus the CT character-
istics of STD 3–4 can be better understood from the study
excess dipole moment obtained from the polarity function
plots Lippert-Mataga [33], McRae [34], and Bakhshiev [35].
The Lippert-Mataga (LM) theory states CT characteristic as;
Dipole Moment Ratio
Further, the CT characteristics of STD 3–4 were studied from
the excited to ground state dipole moment ratio and by the
generalized Mulliken Hush (GMH) charge transfer analysis.

J Fluoresc
Fig. 3 Lippert-Mataga and McRae plot for STD 3–4
The excited to ground state dipole moment of the dye is re-
sponsive towards the microenvironment of the dye. The rela-
tions between the polarity functions based on the relative per-
mittivity (ε) and refractive index (η) of the solvents with the
spectroscopic data help in the study the excited state dipole
moment of the dyes. Thus by using Bilot-Kawski [36],
Bakhshiev [35] and Liptay [37, 38] polarity functions plots
the ratio of excited state to the ground state dipole moment μe/
μg for STD 3–4 is calculated and listed in Table 2, the detailed
equations are provided in the supporting information. The
ratio of μe/ μg is larger than unity in some cases while near
to unity in a few other cases, suggestive of a more polar ex-
cited state than the ground state in STD 3–4.
with the Btransferring electron^ localized on the acceptor con-
stitutes the adiabatic states, which is studied well in CT sys-
tems [42, 43]. The center to center separation distance (RDA
)
in Å is calculated using following Eq. (10) from the molar
absorptivity (εmax), bandwidth (Δυ¯1/2) and ΔEeg as;
À
!
Á
1=2
ꢀ
ΔEeg  εmax  Δυ1=2
RDA ¼ 2:06 Â 10−2
ð10Þ
ð11Þ
HDA
1
0
1
=
2
2
1
2
Δμ2
C2b ¼
1−
@
A
ΔμDeg
Similarly, the eq. (11) shows the degree of delocalization or
2
fractional degree of localization of the excess charge (Cb ).
The values of HDA, RDA, and Cb2 for STD 3–4 were calculated
Mulliken-Hush Charge Transfer Analysis
2
using Eqs. (9–11) and listed in Table 3. When Cb is zero it
The two-state model based Generalized Mulliken-Hush
(GMH) analysis is also studied for STD 3–4 to understand
the CT characteristic. In this method the strength of electronic
coupling (HDA) is calculated using spectroscopic data and
expressed by Eq. 9 [39–41];
represents the state with total delocalization and total localiza-
tion of charge is seen for Cb approaches to unity. The Cb
2
2
values of STD 3–4 indicates a good operative ICT character-
istics in the dye (Table 3). Also, the RDA for both the dyes is
less in polar solvent which is even supported by the
bathochromic shift on polarity rise (Table 3). These dyes along
with good charge transfer characteristics were also responsive
towards environment, thus viscosity sensitivity and AIEE
study was employed.
μgeΔEeg
ΔμDeg
μgeΔEeg
HDA
¼
¼
ð9Þ
ꢂ
ꢃ
1=2
Δμ2 þ 4Δμ2ge
which relates the electronic coupling HDA strength between
the ground (S0) and CT excited states with the vertical excita-
tion energy (ΔEeg). Δμeg denotes the difference between the
adiabatic dipole moments of the ground and excited states and
ΔμDeg as the difference in diabatic state dipole moments.
Three diabatic states – a donor ground state (GS), a donor
locally excited state (LE), and a charge-transfer state (CT)
Table 3 Generalized Mulliken-Hush charge transfer analysis values for
STD 3–4 in different solvents
Solvent
STD 3
STD 4
2
2
Cb
HDA
RDA
(0A)
Cb
HDA
RDA
(0A)
(cm−1
)
(cm−1
)
ꢂ
ꢃ
μe
μg
Table 2 Excited to ground state dipole moment ratio
of STD 3–4
Toluene
DCM
0.4903 9638
0.4918 9483
2.32
2.25
2.45
2.35
2.39
2.33
0.4754 9166
0.4699 8944
0.4718 9208
0.4746 9172
0.4729 8905
0.4731 8981
2.93
2.91
2.81
2.87
2.60
2.81
STD 3
STD 4
Acetonitrile 0.4918 9669
Bilot-Kawski
Bakhshiev
Liptay
0.83
3.18
4.14
3.48
0.87
0.98
Methanol
DMSO
DMF
0.4917 9705
0.4861 9462
0.4860 9442

J Fluoresc
Viscosity Sensitivity of STD 1–4 in Non-polar Aprotic
(Toluene-Paraffin) Solvent Mixtures
intramolecular rotations (RIR) affect the AEE effect. In con-
densed phase the fluorophores experience RIR which lead to
enhancement in emission intensity. The 50 μM solution of
STD 1–4 in N, N-dimethyl formamide/water solvent mixtures
with varying compositions ranging from 0% to 95% water
with same dye concentration were examined by fluorescence
spectroscopy (Fig. 5). Water is a bad solvent and thus induces
the aggregates formation. The dyes in pure solvent have low
emission intensity but as we increase the fraction of water the
aggregates formation enhances the fluorescence intensity due
to RIR factor. But further increase after certain fraction of
water, however, the fluorescence intensity decreases gradually
this could be attributed to increase in intermolecular stacking
of aggregates. From Fig. 5 for STD 1 and 2 the emission
intensity enhances till 90% water fraction further increase
led to decrease in emission intensity. Similarly, for STD 3,
the fluorescence intensity increases till 80% water fraction
and for STD 4 it was till 70% water fraction, further decrease
in emission intensity was observed with increase in water
fraction (Fig. 5). Thus the STD 1–4 show AEE phenomena
in DMF/water mixture.
The emission quantum yield in non-polar solvent for STD 1–4
was very low (Table 1) indicative of radiationless relaxation
pathway more preferable in coming back from excited state by
the molecule. This can be due to molecular rotations occurring
within the molecule which favors the relaxation of excited
state in non-radiative way like by diminishing the fluores-
cence by collisions. Thus viscosity of the medium plays a vital
role in enhancing the fluorescence intensity by obstructing the
molecular rotations indirectly decreasing the possibility of
non-radiative relaxation. Herein effect of viscosity is studied
in toluene and paraffin solvent mixtures where the viscosity of
the medium is increased by increasing the paraffin liquid vol-
ume and its effect on fluorescence emission intensity is ob-
served. The Fig. 4 represents the enhancement of fluorescence
intensity of 10 μM solution of STD1–4 in different solvent
mixtures 10 to 100% of paraffin. The increase in emission
intensity for STD 1 is 3.65 fold, STD 2 is 2.07, STD 3 is
3.40 and STD 4 is 3.0 fold (Fig. 4). Thus the synthesized
styryls act as FMR especially the extended styryls can act
red emitting FMR which could be useful for biological
applications.
NLO Properties Computed by Solvatochromic Method
The push-pull styryls with D-π-A design usually possess
NLO properties owing to their efficient CT characteristics op-
erative between the end groups [44, 45]. Sophisticated tech-
niques are used to measure the NLO properties like Electric
Field Induced Second Harmonic Generation (EFISH)
Aggregation Enhanced Emission Study
The dyes STD1–4 were subjected to AEE experiments. The
molecular structure, presence of chromophore, and restricted
Fig. 4 Viscosity sensitivity of 10 μM solution of STD 1–4 in Toluene-Paraffin mixture of varying viscosity

J Fluoresc
Fig. 5 AEE of 50 μM solution of STD 1–4 in DMF-Water mixture of varying % of water
technique at 1907 nm fundamental wavelength, hyper-
Rayleigh scattering (HRS) often at 1064 nm which are expen-
sive [2]. So herein we have employed simple solvatochromic
method which uses the spectroscopic data to evaluate the
NLO properties which help in primary screening of
NLOphores before approaching to the sophisticated tech-
niques. The solvatochromic model is grounded on two-level
microscopic model wherein the linear polariizabilty (αCT),
αCT among the dyes is STD1 > STD2 > STD3 > STD4 which
is directly proportional to the donor-acceptor strength and
conjugation length. As the conjugation length increase as in
case of extended styryls, there is decrease in the polarizability
values compared to the simple styryls. Also among the simple
styryls, the STD 1 has stronger acceptor dicynovinyl group
compare to STD 2 thus αCT of STD 1 is higher than STD 2.
hyperpolarizability (βCT), second hyperpolarizability (γSD
)
Hyperpolarizability (βCT)
and two photon absorption cross section (σ2PA) is obtained
for STD 1–4 in the studied solvents.
The Oudar equation founded on two level model for evalua-
tion of hyperpolarizability (βCT) in different microenviron-
ments is given as [46–48],
Linear Polarizability (αCT)
υeg μeg2ΔμCT
2
3
The linear polarizability αCT from two level model is
expressed by Eq. (12), where the direction of CT is given by
x, the Plank’s constant is given by h, c as the velocity of light
in vacuum, λeg is the wavelength of transition from the ground
state to excited state, transition dipole moment,μeg which is
calculated from the oscillator strength obtained from spectro-
scopic data, and thus the Eq. (12)is as;
À
ÁÀ
Á
βCT
¼
ð13Þ
2h2c2
υeg2−υL υeg2−4υL
2
2
where transition frequency denoted as veg, ΔμCT which is
the excited and ground state dipole moment difference is
obtained from the McRae’s theory, the reference incident
radiation frequency to which β is referred is denoted as
vL, h as the Planck’s constant, speed of light as c in cm
s−1, and transition dipole moment as μeg. As the
solvatochromism is seen only for the extended styryls
STD 3–4, linearity in McRae plot is seen only for extend-
ed styryls, and thus ΔμCT and hyperpolarizability are cal-
culated for STD 3–4 only.
2
2
μge
2μeg λeg
αCT ¼ αxx ¼ 2
¼
ð12Þ
ΔEeg
hc
Thus the αCT values for STD 1–4 were evaluated using Eq.
(12) in different solvents and listed in Table 4. From Table 4
for STD 1–4 the αCT values are less in non-polar solvents
toluene, while higher in polar solvents DMF. The order of
The hyperpolarizability calculated from Eq. (13) is with
dominant tensor component in the CT direction thus also

J Fluoresc
Table 4 Linear Polarizability αCT of STD 1–4 in different solvents obtained by solvatochromic model
Property
CT (cm−1
Solvent
STD 1
STD 2
STD 3
STD 4
α
)
Toluene
DCM
9.48923E-23
1.06782E-22
1.0992E-22
1.05994E-22
1.02174E-22
1.04651E-22
4.53173E-23
6.38002E-23
7.07692E-23
6.76154E-24
7.99875E-23
7.43475E-23
4.96103E-23
7.03228E-23
6.65398E-23
6.69741E-23
2.41806E-23
2.41583E-23
1.94391E-23
1.27368E-23
1.34929E-23
1.62535E-23
1.63439E-23
1.54329E-23
Acetonitrile
Methanol
DMSO
DMF
referred as βxxx or βCT. When excitation vL = 0 the static
hyperpolarizability is presented as;
it helps in understanding the trend of hyperpolarizability be-
fore approaching to other expensive sophisticated methods.
μ
2ΔμCT
Eeg
3
2
Second Order Hyperpolarizability 〈γ〉SD
eg
À
Á
βxxx ¼ βCT
¼
ð14Þ
2
The second order hyperpolarizability 〈γ〉SD for STD 3–4 is
evaluated at molecular level with the help of three-level model
[47, 49–51] using transition dipole moment μeg, difference
between ground- and excited-state dipole moments, which in
simpler form presented by Eq. (15) [49, 52];
Thus by substituting the values of ΔμCT and μeg obtained
from the spectroscopic data in the above equation the values
of βCT or βxxx is obtained in different solvents and listed in
Table 5. The value of βCT for STD 3 is 32.93 × 10−30esu and
21.86 × 10−30esu for STD 4 in toluene. Also, the value of βCT
for both the dyes decreases with increase solvent polarity. The
CT characteristics of STD 3 is more than that of STD 4 as
evident from the higher βCT value which is due to the pres-
ence of phenyl ring adding to aromaticity of the dye while in
case of STD 4 though the length of conjugation is more the
aromatic character is less than in STD 3 resulting in lowering
of CT and low βCT. The βCT values calculated by
solvatochromic model though based on several assumptions
allowing only approximate estimation of dominant tensor βCT,
!
ꢂ
ꢃ
1
2
2
< γ > ∝
μeg Δμeg2−μge
ð15Þ
3
ΔEeg
The value,
ꢀ
ꢁ
ꢂ
ꢃ
1
2
2
μeg Δμ2−μeg
ð16Þ
3
Eeg
Symbolizes the trend in the third order hyperpolarizability
and also designated as the Bsolvatochromic descriptor^. The
Table 5 NLO properties of STD 3–4 in different solvents obtained by solvatochromic model
(a)
μ2
ge
(b)
(c)
(d)
Dye
Solvent
σ2PA
βCT
ɣSD
STD 3
Toluene
DCM
9.68828E-35
1.33385E-34
1.31218E-34
1.32074E-34
4.59524E-35
4.58223E-35
3.64539E-35
2.34428E-35
2.54955E-35
3.04798E-35
2.98606E-35
2.84052E-35
94.14
98.85
96.25
94.80
44.24
39.03
65.67
43.52
38.21
42.01
60.84
51.68
3.2926E-29
4.94665E-29
4.29957E-29
4.4182E-29
1.72277E-29
1.69531E-29
2.18619E-29
1.43583E-29
1.44704E-29
1.72716E-29
1.88227E-29
1.7113E-29
1.98351E-35
3.09085E-35
2.51682E-35
2.63011E-35
1.07588E-35
1.05678E-35
2.18399E-35
1.4378E-35
1.37851E-35
1.63031E-35
1.92557E-35
1.68561E-35
Acetonitrile
Methanol
DMSO
DMF
STD 4
Toluene
DCM
Acetonitrile
Methanol
DMSO
DMF
a Square of Transition dipole moment in esu.cm
b Two photon absorption cross-section in GM
c first hyperpolarizability βCT in esu, and
d second hyperpolarizability γ SD in esu

J Fluoresc
calculated values for STD 3–4 in studied solvents is presented
in Table 5. The 〈γ〉SD for STD 3 is 10.75 × 10−36esu and
19.25 × 10−36esu for STD 4 in DMSO which is many fold
higher than the value for urea (0.68 × 10−36 esu).
length is more which is attributed to the presence of non-
aromatic isophorone ring. Thus the density of electron cloud
is less in STD 4 than STD 3 leading to overall decrease in the
CTcharacteristics of the dye and hence the NLO properties of
the same. The σ2PA values for STD 3–4 obtained from the two
state model though overestimated as many assumptions are
considered in the model [53], the solvatochromic two level
model serves as good tool to understand the basic trend in
the NLO properties at lower cost.
Two Photon Absorption Cross-Section (σ2PA
)
The two-level system can also be employed for polar unsym-
metrical molecules with either parallel or antiparallel dipole mo-
ments to get the two-photon absorption cross-section (σ2PA) as
given by Eq. (17) as;
Conclusion
4
ꢂ
ꢃ
2
2ð2πLÞ
σ2PA
¼
Δμeg μ2gegð2υÞ
ð17Þ
We have successfully synthesized novel group of 2-N,N-
dibutylamino-4-phenyl thiazole styryls via Knoevenagel con-
2
5ðhcnÞ
1
densation and were well characterized by H NMR, 13C
n2þ2
ð
Þ
L ¼
, where L is known as the Lorentzian local field
3
NMR, CHN analysis and UV-Vis spectroscopy. The extended
styryls STD 3–4 showed superior optical properties than the
simple styryls STD 1–2 owing to the strong acceptor and
increase in conjugation length. The overall quantum yields of
STD1–4 is low. In particular the quantum yield in extended
styryls STD 3–4 is more in polar aprotic solvents DMF than in
non-polar solvents which is due to the strong solvation of the
solute molecules by the polar solvent molecules thus restricting
the non-radiative relaxation pathways and increasing the Kr and
ɸF values in polar solvent. The extended styryls STD 3–4
showed good charge transfer characteristics as evident form the
dipole moment ratio and Mulliken-Hush charge transfer analysis.
The dyes were responsive towards environment, thus viscosity
sensitivity and AIEE study was employed. The STD 1–4 showed
2–3.65 fold increase in emission intensity, thus these styryls can
be used as red emitting FMR which could be useful for biological
applications. Also the STD 1–4 show AEE phenomena in DMF/
water mixture. The linear and NLO properties of the dyes were
studied with the help of solvatochromic model where the extend-
ed styryls STD 3–4 showed enhanced properties than the STD
1–2. Among the STD 3–4, the STD 3 showed better NLO prop-
erties than STD 4 due to more charge transfer characteristic. Thus
we have successfully developed multi-functional NLOphoric 2-
aminothiazole based styryls.
factor, n represents the refractive index of the solvent used.
The difference between excited and ground state dipole mo-
ments Δμeg and transition dipole μegis obtained from spectro-
scopic data. The term μ210gð2υÞ wherein g(2υ) is the normal-
ized TPAb line shape function can be expressed as;
ꢀ
ꢁ
3 Â 103ln10h εabs
μ210gð2υÞ ¼ μ120gðυ10Þ ¼
3
υemi
ð2πÞ NA
TPAb line shape is assumed to be approximated by the
OPA line shape using spectroscopic properties like the molar
extinction coefficient, dipole moment as follow [53];
ꢀ
ꢁ
εabs
υemi
ꢂ
ꢃ
12ln10 π103L4
15NAhc2n2
εabs
υemi
ꢀ
2
σ2PA
¼
Δμeg
ð18Þ
ð19Þ
ꢁ
ꢂ
ꢃ
L4
2
σ2PA ¼ 4:84 Â 10−15
Δμeg
n2
Thus σ2PA can be obtained from steady state absorption and
emission measurements wherein the Δμeg is obtained from the
slope of Onsager’s polarity function (fOF) plot against Stokes
shift. Onsagar polarity equation is given as
0
1
ꢂ
ꢃ
2
ꢀ
ꢁ
2 μe−μg
1
B
@
C
A
ꢀ
ꢀ
υabs−υemn
¼
f
þ Constant ð20Þ
OF
hca3
Acknowledgements The author Manali Rajeshirke is thankful to
University Grants Commission and Dharit Shah to Technical Education
Quality Improvement Program, Ministry of Human Resource
Development, Government of India for financial support.
4πϵO
f OF
¼
2ððϵ−1Þ, where ϵ is the Dielectric Constant of the
2ϵþ1
solvent.
The σ2PA values of STD 3–4 are evaluated using Eq. (19)
and presented in Table 5. As observed the dye with greater
molar extinction coefficient, and larger one-photon cross sec-
tion (OPA) has larger σ2PA. The dye STD 3 has larger σ2PA
than STD 4 which is due to more aromatic character of STD 3
attributable to the presence of phenyl ring. The aromaticity is
hindered a bit in case of STD 4 even though π-conjugation
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