Methoxy supported, deep red emitting mono, bis and tris triphenylamine-isophorone based styryl colorants: Synthesis, photophysical properties, ICT, TICT emission and viscosity sensitivity

Article abstract of DOI:10.1016/j.dyepig.2016.08.025

A novel strategy for the synthesis of triphenylamine based and methoxy supported deep red emitting push-pull chromophores is developed. The methoxy groups though not in conjugation with the acceptor exhibited a red shifted absorption and emissions (670?nm

Full text of DOI:10.1016/j.dyepig.2016.08.025

Accepted Manuscript
Methoxy supported, deep red emitting mono, bis and tris triphenylamine-isophorone
based styryl colorants: Synthesis, photophysical properties, ICT, TICT emission and
viscosity sensitivity
Shantaram Kothavale, Nagaiyan Sekar
PII:
S0143-7208(16)30429-6
10.1016/j.dyepig.2016.08.025
DYPI 5405
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 17 June 2016
Revised Date: 8 August 2016
Accepted Date: 9 August 2016
Please cite this article as: Kothavale S, Sekar N, Methoxy supported, deep red emitting mono, bis and
tris triphenylamine-isophorone based styryl colorants: Synthesis, photophysical properties, ICT, TICT
emission and viscosity sensitivity, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2016.08.025.
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ACCEPTED MANUSCRIPT
Methoxy supported, deep red emitting mono, bis and tris triphenylamine-
isophorone based styryl colorants: Synthesis, Photophysical properties, ICT,
TICT emission and viscosity sensitivity
Shantaram Kothavale, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, Matunga,
Mumbai-400 019, India.
E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel: +91 22 3361 2707; Fax: +91 22 3361 1020.
Graphical abstract

ACCEPTED MANUSCRIPT
Methoxy supported, deep red emitting mono, bis and tris triphenylamine-
isophorone based styryl colorants: Synthesis, Photophysical properties, ICT,
TICT emission and viscosity sensitivity
Shantaram Kothavale, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, Matunga,
Mumbai-400 019, India.
E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Tel: +91 22 3361 2707; Fax: +91 22 3361 1020.
Abstract:
A novel strategy for the synthesis of triphenylamine based and methoxy supported deep red
emitting push-pull chromophores is developed. The methoxy groups though not in conjugation
with the acceptor exhibited a red shifted absorption and emissions (670 nm) as compared to the
unsubstituted analogues due to the improved donating ability of triphenylamine. The dyes show
-1
-1
high molar extinction coefficients (1, 65,800 M cm ) and as the number of withdrawing groups
increases the molar extinction coefficient and emission intensity increase. Positive
solvatochromism (96 nm) was observed which is well supported by linear and multi-linear
regression. The mono styryl dyes showed enhancement of fluorescence while the di and tri-styryl
dyes show quenching of fluorescence in polar solvents, which is correlated with the
intramolecular charge transfer and twisted intramolecular charge transfer. Viscosity induced
emission enhancement has also been observed. The experimental findings were correlated
theoretically by using Density Functional theory computations.

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Keywords: Triphenylamine, Push-pull chromophores, Solvatochromism, ICT, TICT, Viscosity
sensitivity, Multi-linear regression analysis, and Density Function theory.
1
. Introduction:
Triphenylamine, a unique and widely accepted donating group for its very good electron
donating ability and charge transfer characteristic has been applied in a number molecules
designed for opto-electronic devices. It has been widely used as a framework for the design of
two photon absorption (TPA) chromophores[1–7]. The high co-planarity between the central
nitrogen atom and the three adjacent carbon atoms makes it possible to maintain uninterrupted
conjugation between the lone pair of nitrogen and the dendrimer arms [8–13]. Metal-free organic
dyes based on triphenylamine has attracted great attention as the best hole transporting material
due to their inexpensive synthesis, high molar extinction coefficient and tunable absorption
spectral response [14–19]. Different starburst triarylamine derivatives have been used for their
applications in blue to red emitting Organic Light Emitting Diodes (OLEDs) [20–27].
Intramolecular charge transfer (ICT) capability of triphenylamine is also well studied by
attaching it with different strong electron accepting group [28–34]. After the debut of the
aggregation induced emission (AIE) concept in 2001 initially though the hexaphenyl silol (HPS),
tetra-phenyl ethylene (TPE) or plain hydrocarbon based systems were studied for their AIE
properties, and recently triphenylamine based donor-acceptor systems are also widely
investigated for their very good AIE properties [35–43].
The donating ability of triphenylamine is further increased by attaching
supporting/additional donor group at the para position of the two phenyl rings of the

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triphenylamine to get improved photophysical properties as well as better efficiency in Dye-
sensitized solar cell [44–47]. However after the introduction of the sterically bulky moieties,
such as carbazole, diphenylamine, alkoxy substituted phenyl groups and heterocyclic ring as
additional donor has increased steric hindrance and further decreased the amount of dye
adsorbed on TiO surface in Dye-sensitized solar cell. Therefore to minimize this drawback
2
recently few researchers synthesized and used some methoxy supported triphenylamine
derivatives and found very good results where methoxy acts as additional electron donor with
triphenylamine as main donor [48]
Though there are number of reports of mono and di-substituted triphenylamine based donor-
π-acceptor systems the tri-substituted starburst systems remain as the main attraction recently in
the field of Dye-sensitized solar cell due to their better thermal stability, lower aggregation
tendency and high molar extinction coefficients [49–52]. The synthesis of such type of starburst
derivatives requires very drastic conditions and costly catalysts[1,2,4,5]. We developed a novel
synthetic strategy where due to methoxy support the tri-formylation is easily completed by using
3
only up-to 5 equivalent of POCl .
Recently fluorescent molecular rotors [FMRs], a group of fluorescent molecules that consists of
an electron donor group in π-conjugation with an electron acceptor group and which forms
twisted intramolecular charge transfer (TICT) states upon photo excitation have attracted much
attention [53–55]. It exhibit two competing de-excitation pathways: fluorescence emission via
locally excited state (LE) or via an internal non-radiative process which involves molecular
rotation between the donor and the acceptor (TICT). When this rotation is hindered in highly
viscous media, the population of the LE state will increase, correspondingly the fluorescence
emission intensity will be enhanced [56–58]. Though the 9-dicyanovinyl)-julolidine (DCVJ) and

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its different derivatives were extensively studied for their viscosity sensitivity, recently different
triphenylamine derivatives also shown to have very good FMR properties due to the freely
rotating phenyl rings around C-N bond which gets hindered in viscous environment causing
fluorescence enhancement [59]. In general the TICT states are dark states causing quenching of
fluorescence in polar solvents, which further becomes emissive in a highly viscous environment.
Among the four dyes we studied i.e. mono-methoxy mono- styryl dye (TMS), tri-methoxy
mono-styryl dye (TTMS), di-methoxy di-styryl dye (TDS) and tri-methoxy tri-styryl dye (TTS),
we found quenching of fluorescence for TDS and TTS, due to the TICT state while enhancement
of fluorescence for TMS and TTMS in polar solvents may be due to the simply ICT state. When
we studied these dyes for their viscosity effect, as expected we found better enhancement of
fluorescence for TDS and TTS dye as compared to TMS and TTMS dyes.
<
< Please insert Fig. 1. Structures of the synthesized dyes TMS, TTMS, TDS and TTS >>
The methoxy supported dyes showed red shifted absorption as well as emission as compared to
the known triphenylamine styryl dyes from our group [60] as well as isophorone based extended
styryl dyes [61]. All these dyes are fluorescent in solution as well as in solid state.
Solvatochromism study of all these dyes were carried out in non-polar to polar solvents. Density
functional theory computations [B3LYP/6-31G (d)] were used to study the geometrical and
electronic properties of the synthesized molecules.
2
2
. Experimental section
.1 Materials and equipments

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All the reagents and solvents required for the synthesis were purchased from S d Fine
Chemical Limited, Mumbai, India and used without further purification. The reactions were
monitored by TLC using on 0.25 mm E-Merck silica gel 60 F254 precoated plates, which were
1
13
visualized with UV light. H (500 MHz) and C (125 MHz) NMR spectra were recorded on a
Varian Cary Eclipse Australia. The chemical shift values are expressed in δ ppm using CDCl3
and DMSO-d6 as solvents and TMS (Tetramethylsilane) as an internal standard. High resolution
liquid chromatography mass spectra were recorded on 6550 iFunnel QTOF LC/MS spectrometer
from Agilent Technologies. Melting points were measured on standard melting point apparatus
from Sunder Industrial Product Mumbai and are uncorrected. The absorption spectra of the
compounds were recorded on a PerkinElmer UV- visible spectrophotometer Lambda 25.
Fluorescence emission spectra were recorded on Varian Cary Eclipse fluorescence
-6
-1
spectrophotometer using freshly prepared solutions at the concentration of 1x10 mol L .
Fluorescence quantum yields were determined in different solvents by using Rhodamine B (Φ =
0
.97 in ethanol) as a reference standard using the comparative method.
2
.2 Computational Methods
Density Functional Theory (DFT) [62] was used to optimize ground state (S ) geometry of all
0
compounds by using the Gaussian 09 package and the popular hybrid functional B3LYP. The
B3LYP combines Becke’s three parameter exchange functional (B3)[63] with the nonlocal
correlation functional by Lee, Yang, and Parr (LYP) [64]. The basis set used for all atoms was 6-
3
1G (d) in both DFT and time-dependent density functional theory (TD-DFT) method. The low-
lying first singlet excited states (S1) of the dyes were relaxed to obtain their minimum energy
geometries using the TD-DFT in their gas phase. By using TD-DFT at the same hybrid

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functional and basis set, the vertical excitation energies and oscillator strengths were obtained for
the lowest 20 singlet-singlet transitions at the optimized ground state equilibrium geometries
[65]. Frequency computations were also carried out on the optimized geometry of the low-lying
vibronically relaxed first excited state of conformers. Optimized ground state structures were
utilized to obtain the electronic absorption spectra, including maximum absorption and oscillator
strengths.
2
.3 Synthetic strategy
To check out the effect of extra number of additional methoxy groups as well as
withdrawing isophorone ester groups on main triphenylamine donor moiety four different D-π-A
styryl derivatives were successfully synthesized. A novel and easy synthetic route has been
developed to get the mono, di and tri-formylated triphenylamine intermediates (6, 7, and 8).
Mono-methoxy substituted intermediate 3 was successfully synthesized by Buchwald coupling
reaction starting from 3-methoxy aniline and iodobenzene using 1, 10 phenanthroline and CuI as
catalyst. For the synthesis of intermediate 4 and 5, we utilized 3-bromo anisole as reagent instead
of iodobenzene for additional methoxy groups, using the same Buchwald coupling reaction
condition and refluxing the reaction mixture for extra time (48 h). The Buchwald coupling
reaction of 3-bromo anisole with 3-methoxy aniline was giving better yield as compared to that
of aniline as a starting compound.
The formylated intermediate 6 and 9 were synthesized by using regular Vilsmeiere-
Haack formylation at room temperature, and a little harsh reaction condition was required for the
synthesis of intermediate 7 and 8. Initially after generating the Vilsmeiere-Haack adduct at room

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temperature, the methoxy intermediate were dissolved in 1, 2 dichloroethane and added drop
wise to the reaction mixture and then refluxed for 12 h to get the desired formylated product
(
Figure S15). Little excess POCl and extra time was given for the synthesis of intermediate 8.
3
Both the derivatives were purified on column chromatography (100-200 mesh size silica). Final
styryl derivatives (TMS, TDS and TTMS) were obtained by using regular Knoevenagel
condensation reaction condition (i.e. Piperidine/ethanol) to get the pure products after filtration,
while styryl derivative TTS was synthesized by refluxing the reaction mixture for 12 h in
acetonitrile and piperidine as catalyst, crude product obtained after filtration was purified by
column chromatography.(100-200 mesh size silica).
<
< Please insert Scheme 1. Synthesis of TMS, TDS, TTS and TTMS (a) Iodobenzene,
potassium-t-butoxide,1,10 Phenanthroline / CuI, toluene (b)3-bromo anisole, potassium-t-
butoxide,1,10 Phen / CuI, toluene (c) DMF/POCl (d) DMF/POCl , 1,2 dichloroethane
3
3
(
e)11,piperidine,ethanol (f) ethyl cyanoacetate, NH OAc, acetic acid, toluene. >>
4
2
.4 Synthesis and characterization
2
.4.1 Synthesis of 3-methoxy-N, N-diphenylaniline (3)
To a stirred solution of m-anisidine (20 g, 162.4 mmol), iodobenzene (39.8 ml, 357.28 mmol),
and 1,10 phenanthroline (1.17 g, 6.49 mmol) in toluene (350 ml) were added potassium t-
butoxide (54 g, 112.21 mmol) and copper(I) iodide (1.23 g, 6.49 mmol). The reaction mixture
0
was heated under reflux for 24 h at 125 C. On completion of the reaction, the mixture was
cooled to room temperature, and filtered to remove copper metal. The filtrate was extracted by

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adding more ethyl acetate. The combined organic phases were concentrated under vacuum and
purified by column chromatography using 5 % EtOAc in hexane as the eluent to get the pure
1
product (colorless oil) (29.5 g, 66.08 %) H NMR (500 MHz, CDCl ,): δ 7.24 (t, J = 7.9 Hz,
3
4
H), 7.14 (t, J = 8.5 Hz, 1H), 7.09 (d, J = 7.9 Hz, 4H), 7.01 (d, J = 7.3 Hz, 2H) , 6.65 (d, J = 8.5
13
Hz, 1H) ,6.62 (s, 1H) , 6.56 (d, J = 8.5 Hz, 1H), 3.71 (s, 3H); C NMR (125 MHz, CDCl ): δ
3
1
60.4, 149.1, 147.7, 129.8, 129.2, 124.4, 122.8, 116.4, 109.7, 107.9, 55.2 ppm.
2
.4.2 Synthesis of 3-methoxy-N-(3-methoxyphenyl)-N-phenyl aniline (4)
To a stirred solution of aniline (30 g,322.13 mmol), 3-bromo anisole (93 ml, 740.89 mmol), and
,10 phenanthroline (2.32 g, 12.88 mmol) in toluene (300 ml) were added potassium-t-butoxide
1
(108.4 g, 966.39 mmol) and copper(I)iodide (2.45 g, 12.88 mmol). The mixture was heated
o
under reflux for 48 hrs at 130 C under nitrogen atmosphere. The starting material and
intermediate (3-methoxy-N-phenylaniline) formed during reaction were not getting fully
converted into the product even after adding excess potassium-t-butoxide (18.07 g, 161.065
mmol) and 3-bromo anisole (20.21 ml, 161.065 mmol). The reaction mixture was cooled to room
temperature, filtered to remove copper metal. The filtrate was extracted by adding more ethyl
acetate (200 ml x 3). The combined organic phases were concentrated under vacuum and
purified by column chromatography using 5 % EtOAc in hexane as the eluent to get the pure
1
product (colorless oil). (24.5 g, 24.90 %) H NMR (500 MHz, CDCl ,): δ 6.99-7.22 (m, 7H),
3
1
3
6
1
.61-6.65 (m, 4H), 6.53-6.55 (d, J = 8 Hz, 2H), 3.69 (s, 6H); C NMR (125 MHz, CDCl ): δ
3
60.32, 148.87, 147.49, 129.65, 129.08, 124.53, 122.89, 116.61, 109.93, 108.07, 55.14 ppm.
2
.4.3 Synthesis of tris (3-methoxyphenyl) amine (5)

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To a stirred solution of m-anisidine (30 g, 243.60 mmol), 3-bromo anisole (91.73 ml, 730.81
mmol), and 1,10 phenanthroline (1.75 g, 9.74 mmol) in toluene (300 ml) were added potassium-
t-butoxide (82 g, 730.81 mmol) and copper(I)iodide (1.85 g, 9.74 mmol). The mixture was
o
heated under reflux for 48 h at 130 C under nitrogen atmosphere. The starting material and
intermediate (bis(3-methoxyphenyl)amine) formed during reaction was not getting fully
converted into the product even after the addition of excess potassium-t-butoxide (13.66 g, 121.8
mmol) and 3-bromo anisole (15.28 ml, 161.065 mmol). The reaction mixture was cooled to room
temperature, filtered to remove copper metal. The filtrate was extracted by adding more ethyl
acetate (200 ml x 3). The combined organic phases were concentrated under vacuum and
purified by column chromatography using 2 % EtOAc in hexane as the eluent to get the pure
product (colorless oil). (31 g, 37.94 %) 1H NMR (500 MHz, CDCl ,): δ 7.14-7.17 (t, J = 8 Hz,
3
13
3
H), 6.66-6.70 (m, 6H), 6.57-6.59 (dd, J = 8 and 2.5 Hz, 3H), 3.73 (s, 9H); C NMR (125 MHz,
CDCl ): δ 160.38, 148.80, 129.75, 116.92, 110.21, 108.33, 55.24 ppm.
3
2
.4.4 Synthesis of 4-(diphenylamino)-2-methoxybenzaldehyde (6)
POCl ( 2.54 ml, 27.23 mmol) was added drop-wise into ice-cooled N,N-dimethyl formamide (7
3
o
ml, 90.79 mmol) at 0 C under nitrogen atmosphere and the mixture was stirred at room
o
temperature for an hour. The Vilsmeier adduct formed was again cooled to 0 C, compound 3 (5
o
g, 18.15 mmol) dissolved in DMF was added drop-wise and the mixture was stirred at 50 C for
1
hr. On cooling water was added to the reaction mixture, the yellow colored solid precipitated
o
out was filtered and dried well as pure product. (4.6 g, 83.48 %, melting point=128 C)
1
H NMR (500 MHz, CDCl ) δ 3.69 (s, 3H), 6.45 (d, J=1.5, 1H), 6.52-6.54 (dd, J=8.5 and 2, 1H),
3
1
3
7
.16-7.19 (m, 6H) 7.33-7.36 (t, 4H), 7.65-7.67 (dd, J=9 and 0.5, 1H), 10.23 (s, 1H). C NMR

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(125 MHz, CDCl
3
) 55.39, 101.74, 112.22, 118.21, 125.15, 126.44, 129.64, 129.88, 146.13,
1
54.84, 163.09, 187.79.
2
.4.5 Synthesis of 4,4'-(phenylazanediyl)bis(2-methoxybenzaldehyde)(7)
POCl
3
( 7.65 ml, 81.86 mmol) was added drop-wise into ice-cooled N,N-dimethyl formamide
o
(
12.71 ml, 163.73 mmol) at 0 C under nitrogen atmosphere and the mixture was stirred at room
o
temperature for an hour. The Vilsmeier adduct formed was again cooled up-to 10 C, compound
4
(5 g, 16.373 mmol) dissolved in 1, 2 dichloroethane (50 ml) was added drop-wise and the
o
mixture was stirred at 80 C for 5-6 hrs. On cooling to room temperature water was added in the
reaction mixture and extracted with DCM (100 x 3 ml). The combined organic phases were
concentrated under vacuum and purified by column chromatography using 1 % MeOH in DCM
o
as the eluent to get the pure product. (4.3 g, 72.75 %, melting point=148 C).
1
H NMR (500 MHz, CDCl
3
) δ 3.74 (s, 6H), 6.62-6.63 (d, J = 2, 2H), 6.67-6.70 (dd, J = 8.5 and 2,
2
H) 7.17 -7.19 (m, 2H), 7.24-7.27 (m, 1H), 7.37- 7.40 (m, 2H), 7.72- 7.74 (d, J = 8.5, 2H), 10.30
1
3
(
s, 2H). C NMR (125 MHz, CDCl ): δ 55.57, 105.54, 115.46, 120.27, 126.25, 127.05, 129.89,
3
1
29.97, 145.33, 153.36, 162.88, 188.06.
2
.4.6 Synthesis of 4, 4’, 4’’-nitrilotris (2-methoxybenzaldehyde) (8)
POCl ( 13.93 ml, 149.07 mmol) was added drop-wise into ice-cooled N,N dimethyl-formamide
3
o
(
23.18 ml, 298.15 mmol) at 0 C under nitrogen atmosphere and the mixture was stirred at room
temperature for an hour. To this adduct compound 5 (10 g, 29.8 mmol) dissolved in 1.2
o
dichloroethane (100 ml) was added drop-wise and the mixture was stirred at 90 C for 12 hrs.
On cooling to room temperature water was added in the reaction mixture and extracted with

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DCM (150 x 3 ml). The combined organic phases were concentrated under vacuum and purified
by column chromatography using 2 % MeOH in DCM as the eluent to get the pure product. (7.6
o
g, 60.8 %, melting point=231 C).
1
H NMR (500 MHz, CDCl ) δ 3.79 (s, 9H), 6.68-6.69 (d, J = 2, 3H), 6.74-6.76 (dd, J = 8.5 and
3
1
3
1
1
.5, 3H), 7.78-7.80 (d, J = 8.5, 3H), 10.35 (s, 3H). C NMR (125 MHz, CDCl ): δ 55.70,
3
07.28, 116.97, 121.19, 129.81, 152.37, 162.78, 188.12.
2
.4.7 Synthesis of 4-(bis (3-methoxyphenyl) amino)-2-methoxybenzaldehyde (9)
POCl (0.72 ml, 7.75 mmol) was added drop-wise into ice-cooled N,N dimethyl formamide
3
o
(
2.31 ml, 29.83 mmol) at 0 C under nitrogen atmosphere and the mixture was stirred at room
o
temperature for an hour. The Vilsmeier adduct thus formed was again cooled to 0 C,
compound 5 (2 g, 5.96 mmol) dissolved in DMF (10 ml) was added drop-wise and the mixture
o
was stirred at room temperature for 1 hr. On cooling to 10 C, ice-water was added in the
reaction mixture; the solid precipitated out was filtered and dried well as pure product. (1.8 g,
o
8
3.33 %, melting point =143 C)
1
H NMR (500 MHz, CDCl ) δ 3.72 (s, 3H), 3.76 (s, 6H), 6.49 (d, J = 1.5, 1H), 6.54-6.56 (dd, J =
3
8
1
1
.5 and 2, 1H), 6.71-6.73 (m, 4H), 6.76-6.78 (m, 2H), 7.23-7.26 (m, 2H), 7.65-7.67 (d, J=8.5,
1
3
3
H), 10.23 (s, 1H). C NMR (125 MHz, CDCl ): δ 55.34, 55.46, 102.23, 110.66, 112.19,
12.68, 118.37, 118.71, 129.79, 130.21, 147.14, 154.60, 160.64, 163.02, 187.87.
2
.4.8 Synthesis of (Z)-ethyl 2-cyano-2-(3-((E)-4-(diphenylamino)-2-methoxystyryl)-5, 5-
dimethylcyclohex-2-en-1-ylidene) acetate (TMS)
Compound 6 (0.3 g, 0.98 mmol) and compound 11 (0.34 g, 1.48 mmol) were dissolved in
absolute ethanol, 1-2 drops of piperidine was added and the mixture was refluxed for 2 hrs. After

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cooling to room temperature the dark red colored solid separated out was filtered, washed with
small quantity of absolute ethanol and dried well to get the pure product. (0.4 g, 78.43 %,
o
melting
point
=
222
C)
1
H NMR (500 MHz, CDCl ) δ 1.03 (s, 6H), 1.33-1.36 (t, 3H), 1.59 (s, 2H), 2.97 (s, 2H), 3.69 (s,
3
3
H) 4.23-4.28 (q, 2H), 6.55 (d, J = 2, 1H), 6.58-6.59 (d, J = 2.5, 1H), 6.60-6.61 (s, J = 2.5, 1H),
.86 (s, 1H), 6.95 (s, 1H), 6.98 (s, 1H), 7.06-7.14 (m, 5H), 7.25-7.30 (m, 4H). 7.37-7.39 (d, J =
6
8
9
1
1
3
3
.5, 1H). C NMR (126 MHz, CDCl ): δ 14.18, 28.36, 30.89, 31.41, 38.80, 41.01, 55.45, 61.21,
8.16, 104.77, 114.86, 116.94, 118.68, 123.88, 125.34, 127.95, 128.07, 129.37, 129.82, 146.93,
+
50.17, 153.75, 158.41, 163.44, 167.01, 206.87. HRMS (ESI): m/z calculated for (M + H)
C H N O , 519.2569; found 519.2592.
34
34
2
3
2
.4.9 Synthesis of (2Z,2'Z)-diethyl 2,2'-(((1E,1'E)-((phenylazanediyl)bis(2-methoxy-4,1-
phenylene))bis(ethene-2,1-diyl))bis(5,5-dimethylcyclohex-2-en-3-yl-1-ylidene))bis(2-
cyanoacetate)(TDS)
Compound 7 (0.4 g, 1.10 mmol), compound 11 (0.77 g, 3.32 mmol) and 1-2 drop piperidine
were refluxed in anhydrous acetonitrile for 6 hr under nitrogen atmosphere. On cooling to room
temperature the dark red colored solid separated was filtered and dried well to get the pure
o
product ( 0.44 g, 50.57 %, melting point=188-192 C)
1
3
H NMR (500 MHz, CDCl ) δ 0.96 (s, 6H), 0.99 (s, 6H), 1.23-1.26 (t, 6H), 2.42 (s, 2H), 2.43 (s,
2
6
2
H), 2.57 (s, 2H), 2.91 (s, 2H), 3.67 (s, 6H) 4.16-4.22 (q, 4H), 6.52-6.55 (dd, J = 8.5 and 2, 2H),
.65-6.66 (d, J = 2, 2H), 6.79 (s, 1H), 7.06-7.09 (d, J = 16, 1H), 7.14-7.20 (m, 4H), 7.26-7.30 (m,
1
3
H), 7.37-7.40 (t, J = 8, 2H) 7.67-7.68 (d, J = 9, 1H), 7.70-7.72 (d, J = 9, 1H), 7.78 (s, 1H). C
NMR (126 MHz, CDCl ) δ 14.49, 28.07, 28.28, 31.51, 31.96, 38.34, 38.77, 40.93, 44.59, 56.03,
3

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6
1
1.53, 97.25, 97.94, 106.20, 116.07, 116.79, 117.58, 119.81, 123.24, 125.03, 125.50, 126.38,
46.14, 149.20, 153.99, 154.61, 158.45, 162.53, 162.97, 165.40, 167.07. HRMS (ESI): m/z
+
calculated for (M + H) C H N O 792.3934; found 815.3971(+Na).
50
53
3
6,
2
.4.10
Synthesis
of
(2Z,2'Z,2''E)-triethyl
2,2',2''-(((1E,1'E,1''E)-(nitrilotris(2-
methoxybenzene-4,1-diyl))tris(ethene-2,1-diyl))tris(5,5-dimethylcyclohex-2-en-3-yl-1-
ylidene))tris(2-cyanoacetate)(TTS)
Compound 8 (0.3 g, 0.71 mmol), compound 11 (0.83 g, 3.57 mmol) and 1-2 drop piperidine
were refluxed in anhydrous acetonitrile for 12 h under nitrogen atmosphere. On cooling to room
temperature the dark red colored solid separated was filtered, dried and purified by column
chromatography ( 10 % EtOAc in hexane) to get the pure product (0.22 g, 28.94%, melting
o
1
point= 158-162 C )
H NMR
) δ 0.96 (s, 9H), 0.99 (s, 9H), 1.22-1.26 (t, 9H), 2.42 (s, 3H), 2.43 (s, 3H), 2.57
s, 3H), 2.91 (s, 3H), 3.70 (s, 9H) 4.16-4.22 (q, 6H), 6.62-6.64 (d, J = 8.5, 3H), 6.73 (d, J = 1,
(
500 MHz, CDCl
3
(
1
3
3
H), 6.80 (s, 1H), 7.09-7.12 (d, J = 16.5, 2H), 7.20-7.32 (m, 4H), 7.70-7.80 (m, 5H). C NMR
(
126 MHz, CDCl ): δ 14.49, 28.06, 28.27, 31.12, 31.51, 31.93, 38.33, 38.76, 40.93, 44.57,56.16,
3
6
1
1.55, 97.47, 98.13, 107.34, 116.65, 117.16, 117.53, 120.72, 123.76, 125.27, 128.85, 129.41,
+
29.82. HRMS (ESI): m/z calculated for (M + H) C H N O 1065.5299; found 1065.5283.
66
72
4
9,
2
.4.11 Synthesis of (Z)-ethyl 2-(3-((E)-4-(bis (3-methoxyphenyl) amino)-2-methoxystyryl)-5, 5-
dimethylcyclohex-2-en-1-ylidene)-2-cyanoacetate (TTMS)
Compound 9 (0.3 g, 0.82 mmol), compound 11 (0.28 g, 1.23 mmol) and 1-2 drop piperidine
were refluxed in absolute ethanol for 12 h, 1 h under nitrogen atmosphere. On cooling to room

ACCEPTED MANUSCRIPT
temperature the solid separated was filtered and dried and purified by column chromatography
o
(
15 % EtOAc in hexane) to get the pure product (0.16 g, 33.54 %, melting point = 123 C)
1
H NMR (500 MHz, DMSO-d6) δ 0.97 (s, 3H), 1.00 (s, 3H), 1.22-1.26 (t, 3H), 2.42-2.44 (d,
2
9
2
1
1
1
5
H), 2.57 (s, 1H), 2.92 (s, 1H), 3.68 (s, 9H), 4.18-4.22 (q, 2H), 6.46-6.47 (d, 1H), 6.56-6.76 (m,
1
3
H), 7.24-7.31 (m, 3H), 7.62-7.68 (dd, 1H). C NMR (126 MHz, dmso-d ) δ 14.51, 28.08,
6
8.29, 31.52, 31.93, 55.60, 55.93, 61.51, 97.05, 97.70, 101.27, 104.86, 110.14, 111.29, 111.73,
12.71, 114.84, 116.85, 117.86, 118.77, 119.20, 123.00, 124.76, 128.34, 128.63, 128.75, 129.99,
30.28, 130.82, 131.12, 146.85, 147.85, 149.82, 154.78, 158.47, 160.64, 160.80, 162.58, 163.02,
+
65.45, 167.13, 186.88. HRMS (ESI): m/z calculated for (M + H) C H N O 579.2781; found
36
38
2
5,
79.2802.
2
.4.12 Synthesis of (Z)-ethyl 2-cyano-2-(3, 5, 5-trimethylcyclohex-2-en-1-ylidene) acetate (11)
Synthesized by using the reported literature procedure [66]
Isophorone (10 g, 72.35 mmol) and ethyl cyanoacetate (11.5 ml, 108.53 mmol) were dissolved in
anhydrous toluene (100 ml). Sodium acetate (4 g) and acetic acid (8 ml) were added. The
o
reaction mixture was refluxed at 130 C for 48 hrs. Second lot of sodium acetate (2 g) and acetic
acid (4 ml) were added after 12 hrs. Water generated in the reaction mixture was removed by
using dean stark apparatus. On cooling water was added in the reaction mixture and extracted
with ethyl acetate (100 ml x 3). The combined organic phases were concentrated under vacuum
and purified by column chromatography using 5 % EtOAc in hexane as the eluent to get the pure
1
product (8.9 g, 52.72 %). H NMR (500 MHz, CDCl ,): δ 6.65 (m, 1H), 4.24-4.26 (q, 2H), 2.88
3

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1
3
(
s, 2H), 2.53 (s, 2H), 1.96 (s, 3H), 1.32-1.35 (t, 3H), 0.98 (s, 3H), 0.96 (s, 3H); C NMR (125
MHz, CDCl ): δ 165.96, 162.50, 156.58, 120.48, 116.27, 98.15, 61.35, 45.69, 45.28, 31.72,
3
2
3
3
7.92, 25.45, 14.13, ppm.
. Results and discussion
.1 Photophysical properties
Absorption and emission properties of all the four styryl derivatives TMS, TTMS, TDS and
TTS were studied in solvents of different polarities. Table 1 shows absorption and emission
-1
-1
-1
λmax, Stokes shift in cm and molar extinction coefficient in mol cm of all the four dyes in
different solvents. Fig. 2 represents combined absorption and emission spectra for all the four
dyes. The first absorption shoulder peak around 300 nm is due to the π-π* transition of the dye
and the second peak near 500 nm is due to the intramolecular charge transfer transition (ICT)
between donor group and acceptor moiety. Slight red shifted absorption spectra are observed
when the number of withdrawing isophorone carbethoxy groups increased on triphenylamine
moiety. Also with the increasing number of withdrawing groups on triphenylamine group,
remarkable change in the molar extinction coefficient is observed (Fig. 2). The styryl derivative
with three supporting methoxy donating groups and three strong withdrawing isophorone
carbethoxy groups (TTS) shows highest molar extinction coefficient, while the styryl derivative
with three supporting methoxy donating group and only one withdrawing isophorone carbethoxy
group (TTMS) show the lowest molar extinction coefficient.
The absorption maxima λmax as well as molar extinction coefficient increases in the order of
TTMS < TMS < TDS < TTS suggesting that as the number of isophorone carbethoxy groups
increases from TTMS to TTS better intramolecular charge transfer (ICT) occurred from donor

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triphenylamine to withdrawing isophorone carbethoxy groups. In the absorption spectra a
shoulder peak around 375 nm for the isophorone moiety increases as the number of isophorone
carbethoxy groups increases from TTMS to TTS. In the emission spectra same trend is observed
i.e. emission intensity increases from TTMS to TTS, but very slight red shift in emission λmax is
observed even after increasing the number of withdrawing isophorone carbethoxy groups.
Similarly due to the additional methoxy groups in TTMS derivative as compared to the only one
methoxy group in TMS derivative shows slightly red shifted absorption and emission λmax. So it
can be predicted that there is a better intramolecular charge transfer (ICT) after varying the
number of withdrawing groups instead the donating groups on triphenylamine.
Different photophysical parameters calculated from the available absorption and emission data
are represented in Table 1 for all four dyes in chloroform solvent. Table 2 and S1-S3 represents
the photophysical parameters calculated in set of non-polar to polar solvents for dyes TTMS and
TMS to TTS respectively. It is observed that all four dyes exhibited slightly red shifted
absorption spectra and highly red shifted emission spectra from non-polar to polar solvents. All
four dyes show very high value of full width half maximum (FWHM) and hence they can be
treated as very good energy absorbing dyes for their optoelectronic applications, further its value
increases from non-polar to polar solvents. The value of oscillator strength (f) and transition
dipole moment (µ ) were calculated from integrated absorption spectra using well-known
eg
expressions [67]. It is observed that as we go from mono to tri-styryl derivatives both the values
increases and the highest value of oscillator strength (3.27) and transition dipole moment (18.80
Debye) were reported for the tri-styryl derivatives in DCM solvent. Similarly the values of
radiative rate constant (Kr) in all solvents were calculated using Strickler Berg equation
ꢁ
ꢂ ꢄ ꢄ
Kr = 2.88ꢀ ×ꢀ10 ꢃ ꢅ
̅
ꢈ ꢉ ꢋꢅ
(1)
ꢆꢇ
ꢇꢊ ꢀ

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Where ꢅ
̅
is the average wave number corresponding to the 0–0 transition and the integral part
ꢆꢇ
is evaluated from the area under the curve of absorption and emissions in all solvents. Again the
highest Kr values were obtained for TTS dye. We also calculated the values of emission
transition dipole moment, µ (Ems) in all solvents by utilizing the value of Kr and integrated
eg
emission spectra [68] and it is found that these values are slightly lower than the transition dipole
moment calculated from absorption spectra. No much variation in µ (Ems) is observed when
eg
we go from non-polar to polar solvents.
<
< Please insert Fig. 2. Combined Absorption and normalized emission spectra of dyes TTMS,
TMS, TDS and TTS in chloroform solvent >>
<
<
< Please insert Fig. 3. Absorption spectra of dye TTMS in all solvents>>
< Please insert Table 1. Photophysical parameters of TMS, TTMS, TDS and TTS in
chloroform solvent >>
< Please insert Table 2. Photophysical properties of dye TTMS in all solvents >>
<
3
.2 Solvatochromism, Lippert-Mataga correlation and Quantum yield of the dyes
The dyes were checked for their solvatochromism behavior in solvents of different polarity
and red shifted emissions were observed after increasing the solvent polarity suggesting that the
charge separation increases in the excited state which results in a larger dipole moment in the
excited state as compared to the ground state. The absorption and emission spectra of dye TTMS
in different polarity of solvents is shown in Fig.3 and Fig.4 respectively. A very good red shifted
emission, 96 nm was observed for this dye from non-polar to polar solvent (Fig. 5). Similarly,
remaining three dyes TMS, TDS and TTS also show red shifted emission of 95 nm, 88 nm and
9
3 nm respectively from non-polar to polar solvents (Fig.S2-S9). In polar solvents, emission
intensity increases for dye TTMS and TMS, and decreases for dye TDS and TTS. Also for all

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four dyes Stokes shift increases with the increase in the solvent polarity suggesting that these
dyes have polar excited state and it is more stabilized in the polar solvent.
All the dyes emit in the red to deep-red region from 568 to 670 nm and also show red
solid state fluorescence. The dyes TDS and TTS show the highest quantum yields in dioxane and
the lowest one in acetonitrile solvent. The dye TMS show highest quantum yield in chloroform
and lowest one in hexane solvent, while the dye TTMS shows the highest quantum yield in
DMSO and the lowest one in hexane. In general the dyes TDS and TTS showed higher quantum
efficiencies in non-polar solvents and lower quantum efficiencies in polar solvents, while the
trend is opposite for TTMS and TMS dyes. This can be correlated with the observed quenching
of fluorescence in polar solvents for TDS and TTS dyes and in non-polar solvents for TTMS
and TMS dyes. As we predicted that dyes TTMS and TMS showed intramolecular charge
transfer (ICT) while dyes TDS and TTS showed twisted intramolecular charge transfer (TICT),
which was further supported by their viscosity sensitivity study. The twisted charge transfer state
of dyes TDS and TTS is more stabilized by polar solvents as compared to the non-polar solvents
to deliver red shifted emissions and quenching of fluorescence.
<
<
< Please insert Fig 4. Emission spectra of dye TTMS in all solvent >>
< Please insert Fig 5. Normalized emission spectra of dye TTMS in all solvents >>
Solvatochromism observed in these compounds and gradual increase in Stokes shift with
increase in the solvent polarity can be correlated by using Lippert-Mataga equation [69]. Lippert-
Mataga equation is showing direct correlation of physical parameters of solvent i.e. dielectric
constant, refractive index and change in dipole moment of the compound upon excitation with
Stokes shift. It is a very general equation for the effect of solvent on emission properties and
does not account for specific solvent–fluorophore interactions, for e.g. through hydrogen

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bonding, etc. The plot of stokes shift Vs Lippert-Mataga function for all the four dyes exhibited
linear relationship suggesting that solvent parameters such as dielectric constant and refractive
index are collectively responsible for the red shift in emissions (Fig.6). The individual plot of
stokes shift Vs orientation polarizability i.e. Lippert-Mataga function (ꢀf) for all four dyes in
different solvents is also represented in Fig. S10-S13. As Mac Rae is the improved version of
Lippert-Mataga where in addition to the solvent polarizability, solute polarizability is also taken
into account [70]. We found linear relationship between the plots of Mac Rae function (ꢀfMR) Vs
Stokes shift for all four derivatives in different solvent as shown in Fig. S14. This suggests that
the polarity originated from the dipole created by the solute particles i.e. styryl dyes also
contributed for the solvatochromic shift observed in polar solvents.
<
< Please insert Fig. 6. Lipper-Mataga solvent polarity graphs for all four dyes >>
3
.3 Multilinear regression analysis of absorption and emission shift using Kamlet-Taft and
Catalan parameters
N
It is found that the single parameter scale such as Lippert-Mataga, Mac-Rae, and E
T
(30) are
inappropriate to explain exactly which solvent property or solute-solvent interaction is
responsible for the observed solvent dependent physiochemical changes. Hence a multi-
parameter approach proposed by Kamlet-Taft and Catalan is preferred and has been successfully
applied to various physiochemical processes for e.g., UV-Visible absorption spectra, emission
maxima in fluorescence spectra, Stokes shifts, quantum yield, radiative and non-radiative rate
constants, fluorescence life time, etc.
In general the multi-linear expression is given as
y = y + aA + bB + cC + dD
(2)
0

ACCEPTED MANUSCRIPT
0
Where y is the solvent affected physiochemical property, y is the studied physiochemical
property in the gas phase. When all other solvent parameters are absent, then y = y a, b, c and d
0
.
are adjustable coefficients which reflect the dependence of y to the various {A, B, C, D} solvent
parameters. We studied three different physiochemical properties (y) namely absorption maxima
(
ῡ ), emission maxima (ῡ ) and Stokes shift (ꢀῡ). Among various solvent scales available in
abs emi
the literature, those provided by Kamlet and Taft [71,72] is most frequently used and is
expressed as...
y = y + a α + b β + c π*
(Kamlet–Taft) (3)
0
α
β
π*
Where α represents effect of acidity (hydrogen-bond donating ability) of solvent, β represents
effect of basicity (hydrogen-bond accepting ability) of solvent and π* represents the collective
effect of solvent polarity (also known as dipolarity) and polarizability. But if acidity or basicity
of solvents is not affecting on the value of y, then among polarity and polarizability it is difficult
to sort out which factor is exactly influencing the studied physiochemical property y, hence
Catalan[73–75] proposed another expression…
y = y + a SA + b SB + c SP + d SdP (Catalan) (4)
0
SA
SB
SP
SdP
Here total four solvent parameters namely SA (solvent acidity), SB (solvent basicity) SP (solvent
polarizability) and SdP (solvent dipolarity) were introduced and the newly introduced parameter
solvent dipolarity (SdP) can make difference between the exact contribution of solvent
polarizability (SP) and solvent dipolarity (SdP). Also the individual solvatochromic parameters
explained by Catalan are based on well-defined reference processes as opposed to the Kamlet-
Taft parameters which are based on the average of several solvent-dependent processes.

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To study our highly red shifted emission spectra and very high Stokes shift observed for all four
styryl derivatives in polar solvents we used both, the well-established Kamlet-Taft as well as its
improved version by Catalan simultaneously to understand exactly which factor i.e. solvent
acidity, basicity, polarizability or dipolarity is mainly affecting the altered photophysical
properties of these styryl derivatives in solvents of different polarities.
Multi-linear analysis of TMS is carried out in nine different solvents using absorption, emission
and Stokes shift data. For the absorption data of this dye comparatively poor fit by both the
methods is observed but Catalan parameter shows better correlation coefficient (r = 0.65) than
Kamlet-Taft parameters (r = 0.29). Estimated coefficients for TMS dye by Kamlet-Taft and
Catalan method are represented in Table 3. Large standard errors observed on both Kamlet-Taft
and Catalan parameters for solvent acidity and basicity factors suggest that mainly solvent
dipolarity or polarizability are responsible for the slightly red shifted absorption spectra. Also as
a very high standard error is observed on solvent dipolarity (dSP) factor, solvent polarizability
(cSdP) factor obtained by Catalan method can be considered as mainly responsible for the slightly
red shifted absorption spectra. In the case of emission spectra again Catalan parameters show
better correlation coefficient (r = 0.98) than Kamlet-Taft parameters (0.83). The negative value
observed for all four solvent parameters (acidity, basicity, dipolarity and polarizability) both by
Kamlet-Taft and Catalan analysis supports the red shifted emissions observed in polar solvents.
Very small correlation coefficient observed for solvent acidity and basicity factors as well as
very high standard error observed for the collective parameters of solvent polarizability as well
as dipolarity, Kamlet-Taft method hasn’t provide any conclusive result. Also for Catalan
parameters very small correlation coefficient is observed for solvent acidity as well as basicity
parameters when individually plotted with emission spectra, hence they cannot be considered for

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the further analysis of emission spectra. Among solvent polarizability and dipolarity, though by
first observation comparatively high standard error is observed for solvent dipolarity factor, it is
also showing very high correlation coefficient (0.96) when individually plotted with the emission
spectra. Hence solvent dipolarity (c ) can be considered as the main factor responsible for the
SdP
highly red shifted emission spectra of TMS dye. The graph plot of estimated emission values
against experimental emission values shows excellent linear relation by Catalan parameters
(slope = 0.99) (Fig. 7).
<
< Please insert Table 3. Estimated coefficients (y , a, b, c, d), their standard errors and
0
correlation coefficients (r) for the multi-linear analysis of (ῡ ), (ῡ ) and (ꢀῡ) of TMS as a
abs
emi
function of Kamlet-Taft (2) and Catalan (3) solvent scales. Where, α or SA for solvent acidity, β
or SB for solvent basicity, π* for collective parameter of solvent dipolarity and polarizability
according to Kamlet-Taft equation, SdP and SP for solvent dipolarity and polarizability
respectively according to Catalan equation>>
As like TMS dye multilinear regression analysis using Kamlet-Taft and Catalan parameters is
carried out for other three dyes i.e.TDS, TTS and TTMS and it is observed that for TDS and
TTS dyes again Catalan parameters show better correlation coefficient than Kamlet-Taft
parameters, but in the case of TTMS dye both the parameters are not showing very high
correlation coefficient. Estimated coefficients for dyes TDS, TTS and TTMS by Kamlet-Taft
and Catalan method are represented in Table S4. By seeing the sign of the each estimated
coefficient as well as the extent of standard error and correlation coefficient obtained by both the
methods, solvent dipolarity is emerged out as the main factor responsible for the shift in
absorption as well as emission spectra, which was further supported by plotting absorption or
emission data with each individual factor. Hence solvent dipolarity can be considered as the

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main factor responsible for the slightly red shifted absorption as well as highly red shifted
emission spectra of TDS and TTS dye. For red shifted emission spectra of TTMS dye again
solvent dipolarity is the main factor responsible, but no any conclusive results were obtained to
explain the slightly red shifted absorption spectra of this dye. The graph plot of estimated
emission values against experimental emission values for dyes TDS, TTS and TTMS are
represented by Fig. 7 which shows excellent linear relation with slope of 0.9776, 0.9765 and
0
.7614 respectively.
< Please insert Fig. 7. Correlation between experimental and predicted emission wave number
for TMS (a), TDS (b), TTS (c) and TTMS (d) by Catalan method. >>
<
3
.4 Intramolecular and twisted intramolecular charge transfer (ICT and TICT)
We studied the optical properties of all four dyes i.e. TMS, TDS, TTS and TTMS in different
solvents. In general in the case of donor-acceptor compounds the polarity of solvents strongly
affects the excited state of the molecule by stabilizing it through dipole-dipole, hydrogen
bonding, and solvation interactions. In the excited state, such type of compounds instead of
emitting from locally excited (LE) or Franck-Condon state they emit through newly generated
intramolecular charge transfer (ICT) state or twisted intramolecular charge transfer state (TICT)
state which is well-stabilized by polar solvents. In our case, emission spectra of all four dyes
show very good positive solvatochromism from non-polar to polar solvents and all the emission
peaks are very broad suggesting that they are not emitting through locally excited (LE) state but
through ICT or TICT state. There are number of reports in the literature explaining the ICT and
TICT emission. After seeing emission spectra of all four dyes, interestingly we found that TMS
and TTMS dyes show enhancement of fluorescence in polar solvents while TDS and TTS dyes
show quenching of fluorescence in polar solvents (Fig. 8). As all four dyes consisting of same

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donor i.e. triphenylamine and same acceptor i.e. isophorone unit, the only difference is the
number of donating or withdrawing groups. Recently it is proved that the free rotations of phenyl
rings around single bond non-radiatively deactivates the excited state to a large extent through
the formation of TICT state and as the number of such freely rotating group increases the
chances of formation of TICT state is also increases [76]. In the case of styryl dyes as the
number of double bond increases, though there is red shift in emission but at the same time
lowering of fluorescence quantum yield observed due to the increased flexible units. It is also
proved that in the case of styryl dyes, the main non-radiative decay channel is not the rotation
around C-C bond, but the rotations around the dicyanovinyl C=C bond in the excited state [59].
In our case TMS and TTMS dyes having only one withdrawing unit, hence possibly emitting
through the ICT state where no any fluorescence quenching in polar solvents is observed. In the
case of TDS and TTS dyes, two and three withdrawing isophorone units are present respectively
and hence more number of double bonds are present which may forcing the dye to emit through
the TICT state after excitation. It is observed that the compounds emitting through TICT state,
get twisted in the excited state where donor and acceptor moieties are mutually perpendicular to
each other and total charge separation between the donor and acceptor is observed, which further
elevates the HOMO level to narrow the band gap. This TICT state is stabilized by polar solvents
to give bathochromically shifted emission, but at the same time it is susceptible for the various
non-radiative quenching processes and shows quenching of fluorescence in polar solvents. The
TICT emissions can be blocked chemically by restricting the rotations around single bond by
designing structure of compounds having bulkier groups in vicinity.
Now-a-days instead of taking lot of synthetic efforts to make such type of rigid structures
researchers tried to enhance the fluorescence of such TICT fluorophores through AIE effect,

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viscosity study or by studying the fluorescence at lower temperature. To support our above
observations we also studied the viscosity effect on the fluorescence properties of all four styryl
dyes and found better fluorescence enhancement for dyes TDS and TTS as compared to dyes
TMS and TTMS.
<
< Please insert Fig. 8. Emission spectra of compound TMS, TTMS, TDS and TTS in all
solvents. Designed line represents non-polar solvents while plane line represents polar solvents
>
>
3
.5 Viscosity sensitivity and twisted intramolecular charge transfer (TICT)
To correlate our above experimental findings i.e. quenching or enhancement of fluorescence
from non-polar to polar solvents we studied the viscosity sensitivity of these styryl dyes in
ethanol: PEG 400 mixture of solvents. Due to its extra phenyl rings rotating around single bond
the triphenylamine based styryl dyes already reported to show enhanced fluorescence after
hindering the rotation in viscous media [59]. In the case of styryl dyes TDS and TTS, quenching
of fluorescence is observed as polarity of solvent increases and we proposed that these emissions
are due to the TICT state formed in polar solvents. Hence instead of using polar solvent like
glycerol for viscosity study we used polyethylene glycol 400, which is comparatively non-polar
and less toxic commercially used viscous solvent. In 5 µM solutions of these dyes in ethanol
percentage of PEG increased from 10 to 100 % where enhancement of fluorescence was
observed with the increased percentage of PEG. Fig. 9 represents viscosity induced emission of
all four styryl dyes with increasing percentage of PEG 400 in ethanol. We found 2.24 and 2.64
times enhancement of fluorescence for TMS and TTMS dye respectively, which is almost
doubled for TDS and TTS dye and they show 4.03 and 5.02 times fluorescence enhancement
respectively. As we proposed earlier that the very high fluorescence enhancement observed for

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TDS and TTS dye is due to the formation of TICT state in the excited state which get blocked in
viscous media and show increased fluorescence with slightly blue shifted emission spectra. In
between TDS and TTS comparatively better enhancement in fluorescence is observed for TTS
dye which possessing highest number of withdrawing groups in all four dyes. Due to their good
sensitivity to viscous media styryl dyes TDS and TTS can be considered as fluorescent
molecular rotors (FMRs) as they show very higher emission wavelengths (637 and 641nm for
TDS and TTS respectively) as compared to well-known traditional FMR 1 (2-(4-
(dimethylamino)benzylidene)malononitrile, at 460 nm). Similarly they show very high Stokes
shift (128 and 131 nm for TDS and TTS dyes respectively) as compared to 40 nm for FMR 1.
Due to its biological importance viscosity sensitivity above 600 nm is highly essential and there
are very few reports till date for the viscosity sensitivity in this region. As all our styryl dyes
emits above 600 nm due to their methoxy they can be considered as a very good FMRs in the
deep red region.
<
< Please insert Fig. 9. Emission spectra of compound TMS(I), TDS(II), TTS (III) and
TTMS(IV) in mixture of ethanol and PEG 400 >>
3
.6 Electrochemical Properties
To study their redox behavior we performed cyclic voltametry study of these dyes in
dichloromethane using tetrabutylammonium hexafluorophosphate (NBu PF ) as electrolyte at a
4
6
scan rate of 100 mV/s, where ferrocene is used as reference standard. The cyclic voltammograms
are displayed in Fig. 10. The observed main oxidation peak for these styryl derivatives are
mainly due to the removal of electron from central nitrogen atom of triphenylamine and the extra
oxidation peaks observed for dyes TMS, TDS and TTMS are may be due to the removal of

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electron from extra methoxy groups present on triphenylamine donor though they are not in
direct conjugation with the withdrawing isophorone unit. First oxidation peak observed for all
four dyes is at around 0.84 to 0.92 mV/s in which dyes TMS and TTMS shows lowest oxidation
potential at 0.8410 mV/s and dye TTS shows highest oxidation potential at 0.9178 mV/s. Except
dye TTS, remaining three dyes show second oxidation peak at around 1.50 mV/s confirming the
supporting donating ability of extra methoxy groups present on main triphenylamine donor. Due
to the two more extra methoxy groups present on TTMS dye, it shows one more oxidation peak
at 1.1333 mV/s. Also due to its highly symmetric structure and unavailability of electron for
removal due to the more number of withdrawing isophorone unit present on TTS dye it shows
only one oxidation peak at 0.9178 mV/s.
<
< Fig.10 Cyclic voltammograms of dye TMS, TDS, TTS and TTMS in DCM at room
temperature >>
3
.7 Comparative photophysical properties of plain and methoxy supported triphenylamine-
isophorone styryls:
Recently donor capability of triphenylamine is further improved by decorating the extra phenyl
rings of triphenylamine with additional donating groups such as carbazole [19], phenothiazine
[46], etc and such starburst triphenylamine based dyes are utilized for Dye-sensitized solar cell
applications, but due to the steric hindrance they are not reported to show very high efficiency
for dye sensitized solar cell, hence small donating groups such as methoxy are preferred over
bulkier groups [48]. We have synthesized different triphenylamine styryl dyes with extra
methoxy groups as supporting donor. Though these methoxy groups are not in direct conjugation
with the withdrawing groups, they improved total donating ability of triphenylamine and show

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red shifted absorptions as well as emissions than the plane triphenylamine styryl dyes reported
recently [61]. We developed a novel synthetic strategy to get these methoxy triphenylamine
intermediates 3, 4 and 5. Due to the presence of methoxy at ortho position, substitution at para
position of the triphenylamine nitrogen was found easier and we synthesized the tri-formylated
derivative of triphenylamine in a very good yield. We compared the photophysical properties
such as absorption, emission and Stokes shift of these newly synthesized dyes with recently
reported dyes by Zhou et al, where they used same donor-acceptor combination but on plane
triphenylamine [61]. The comparative photophysical properties of mono-substituted (Compound
1
), di-substituted (Compound 3) and tri-substituted (Compound 5) dyes with our methoxy
supported dyes TMS, TTMS, TDS and TTS are shown in Table 4. We found red shifted
absorption and emission values for the methoxy supported dyes. In the case of TTMS dye which
is having two more methoxy groups than TMS dye also shows red shifted absorptions as well as
emissions than compound 1(reported dye) as well as TMS dye which possessing only one extra
methoxy group. But after putting extra methoxy groups, no much more variation in Stokes shift
is observed. Fig. 11 represents the effect of additional methoxy groups on absorption and
emission λmax of these styryl dyes.
<
< Please insert Fig.11: Comparative photophysical properties of Comp 1(reported dye), TMS
and TTMS dye >>
<
< Please insert Table 4. Comparative photo-physical properties of the plain triphenylamine
isophorone styryl (blue color) and methoxy supported triphenylamine based styryl (red color)
dyes in different solvents >>
3
.8 Optimized geometries of the styryl dyes