Carbazole-containing push-pull chromophore with viscosity and polarity sensitive emissions: Synthesis and photophysical properties

Article abstract of DOI:10.1016/j.dyepig.2016.02.012

Carbazole based D-π-A extended styryl dyes with intramolecular charge transfer characteristics were synthesized. The intramolecular charge transfers of these D-π-A extended styryls have been examined with the study of photophysical properties like absorption, emission and quantum yield in various solvents of different polarities. All the dyes demonstrated positive solvatochromism. They showed largely improved photophysical properties and large Stokes shifts due to twist geometry. Oscillator strengths and transition state dipole moments have been studied to understand charge transfer within the molecules. The fluorescence molecular rotors properties of the series of extended styryls have been evaluated. The dyes having good charge transfer characteristics showed better FMR properties. Sensitivity of the fluorescence emission towards solvent polarity and viscosity has been investigated using fluorescence emission spectra.

Full text of DOI:10.1016/j.dyepig.2016.02.012

Dyes and Pigments 129 (2016) 1e8
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Carbazole-containing pushepull chromophore with viscosity and
polarity sensitive emissions: Synthesis and photophysical properties
*
Rahul D. Telore, Nagaiyan Sekar
Department of Dyestuff Technology, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 September 2015
Received in revised form
Carbazole based D-
synthesized. The intramolecular charge transfers of these D-
p
-A extended styryl dyes with intramolecular charge transfer characteristics were
-A extended styryls have been examined
p
with the study of photophysical properties like absorption, emission and quantum yield in various sol-
vents of different polarities. All the dyes demonstrated positive solvatochromism. They showed largely
improved photophysical properties and large Stokes shifts due to twist geometry. Oscillator strengths
and transition state dipole moments have been studied to understand charge transfer within the mol-
ecules. The fluorescence molecular rotors properties of the series of extended styryls have been evalu-
ated. The dyes having good charge transfer characteristics showed better FMR properties. Sensitivity of
the fluorescence emission towards solvent polarity and viscosity has been investigated using fluores-
cence emission spectra.
10 February 2016
Accepted 15 February 2016
Available online 18 February 2016
Keywords:
Extended styryl
Photophysical properties
Solid-state emission
Intramolecular charge transfer (ICT)
Fluorescent molecular rotor (FMR)
© 2016 Elsevier Ltd. All rights reserved.
1
. Introduction
The thermal stability and photophysical properties of
gated system are enhanced by the introduction of carbazole moiety
[22]. The stable -conjugated chromophores show positive sol-
vatochromism in various solvents inducing a red shift with the
increase in polarity [23]. The positive solvatochromism results from
the stabilization of the ground and excited states of the fluo-
rophores in the solvent with change in polarity [24]. The molecular
p-conju-
Carbazole-based heteroatomic moieties have been extensively
studied for their unique photophysics and photochemistry [1e6].
Carbazole core containing colorants are very useful in optoelec-
tronic and electroluminescence devices [1], material chemistry [7],
organic light emitting device [8], photoconductors [9,10], non liner
optics [11], thin film transistors [12], photovoltaics [13], metal
sensors [14], and dye sensitized solar cells [15].
p
and optical properties of
p-conjugated carbazole system can be
altered by structural modification at 2-, 3-, 6-, 7- and 9- positions
[25e27]. Triphenylamine based fluorescent styryls show emission
in the red region due to the twisted-intramolecular charge transfer
phenomenon (TICT) [28].
Some conjugated carbazole derivatives are largely typical
pushepull, D-
linked to the strong acceptor (A) through a planar
16e18]. A system containing N-substituted carbazole acts as a
p
-A systems in which a strong electron donor (D) is
p
system
[
Recently, much attention has been paid to viscosity sensitive
emission along with the charge transfer emission, viscosity has a
great influence on the fluorescence intensity [29,30]. In low viscous
solution, the intramolecular rotation is the predominant in exci-
tation pathway, whereas in the high-viscous solutions, the intra-
molecular rotation is hindered, and fluorescence emission intensity
increases as the intramolecular rotation decreases [31e34]. The
fluorophores emit with low intensity in low viscosity solvents due
to the free rotation of the CeC and C]C bonds in the excited state.
Such system with low emission intensity emits with higher in-
tensity in higher viscosity solvents or mixture of high viscosity
solvents. p-(Dialkylamino)benzylidene malononitriles showed
most predominant fluorescent molecular rotor (FMR) properties,
donor and withdrawing groups present at 2-, 3-, 6 and 7 positions
of carbazole are responsible for the intramolecular charge transfer
(
ICT). This type of
p-conjugated system shows ICT characteristics
[19]. The ICT molecules exhibit multi functionality such as multiple
electroactivity or photoactivity with applications in organic elec-
tronics [11,20]. The organic conjugated molecules with large
conjugated system generally exhibit two-photon absorption (TPA)
with ICT [21].
p-
*
Corresponding author.
with donoreacceptor relation through p-conjugation [31,32,35].
E-mail addresses: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com (N. Sekar).
http://dx.doi.org/10.1016/j.dyepig.2016.02.012
143-7208/© 2016 Elsevier Ltd. All rights reserved.
0

2
R.D. Telore, N. Sekar / Dyes and Pigments 129 (2016) 1e8
In this work we have synthesized N-phenylcarbazole based
1H NMR (DMSO,
d
): 7.10 (d, 1H, J ¼ 15.4 Hz), 7.35 (m, 3H), 7.46 (t,
extended styryls with red shifted emissions than the corresponding
N-ethylcarbazole analogs. The extended styryl has twist geometry
due to the presence of N-phenyl group at 9-position of carbazole.
The extended styryl of N-phenylcarbazole exhibit the substitu-
1H), 7.45 (m, 2H), 7.58 (d, 1H, J ¼ 7.4 Hz), 7.63 (m, 6H), 7.69 (t, 2H),
7.82 (d, 1H, J ¼ 8.8 Hz), 8.32 (d, 1H, J ¼ 7.8 Hz), 8.64 (s, 1H) ppm.
13
C NMR (DMSO, d): 79.46, 110.62, 111.10, 113.99, 114.79, 121.54,
121.64, 122.04, 123.05, 124.18, 126.92, 127.23, 129.42, 129.50, 130.79,
131.44, 133.88, 136.49, 141.35, 142.57, 151.01, 171.84 ppm.
tionally tunable features of these simple twisted
p-conjugated
systems. The extended styryl of N-phenylcarbazole behaves
differently as viscosity sensitive emitters. N-Phenylcarbazole was
formylated by VilsmeiereHaack reaction and condensed with two
different active methylene compounds having electron with-
drawing capacity to give range of extended styryls. Study of pho-
tophysical properties of synthesized extended styryls in different
solvents has been carried out.
2
.2.3.2. Synthesis of Ethyl 2-cyano-3-phenyl-5-(9-phenyl-9H-carba-
zol-3-yl)penta-2,4-dienoate 8. Color: Yellow; Yield: 79%; M.P.:
ꢂ
186e188 C.
FT-IR: 2209 (eCN), 1715 (eC]O), 1592 (eC]Ce), 1485(eAr),
1131(eCeO).
CHN Analysis: Found C ¼ 81.99; H ¼ 5.11; N ¼ 5.92. Calculated
for C32
H
24
N
2
O
2
C ¼ 82.03; H ¼ 5.16; N ¼ 5.98%.
2
. Experimental section
Mass: Calculated 468.1 for C32
H NMR (DMSO,
H
24
N
2
O
2
found 469.2 (Mþ1).
): 1.33 (t, 3H), 4.34 (q, 2H), 6.82 (d, 1H,
J ¼ 15.89 Hz), 7.32 (m, 2H), 7.39 (d, 1H, J ¼ 8.6 Hz), 7.44 (m, 3H), 7.60
1
d
2.1. Materials and equipments
(
m, 6H), 7.70 (m, 3H), 8.27 (d,1H, J ¼ 7.9 Hz), 8.41 (s, 1H), 8.61 (d,1H,
All the reagents and solvents were purchased from Sd Fine
J ¼ 15.8 Hz) ppm.
Chemicals Pvt. Ltd. and used without purification. All the solvents
used were of spectroscopic grade. Melting point was recorded by
open capillary on Sunder Industrial Product and is uncorrected. The
reactions were monitored using pre-coated silica gel aluminum
backed TLC plates; Kisel gel 60 F254 Merck (Germany). The FT-IR
spectra were recorded on a Jasco 4100 Fourier Transform IR in-
13
C NMR (DMSO, d): 14.53, 62.16, 100.80, 110.53, 111.10, 117.53,
1
1
1
21.38, 121.48, 123.01, 123.48, 123.65, 123.83, 126.27, 127.2, 127.45,
27.78, 128.66, 129.1, 129.31, 130.25, 130.76, 136.62, 136.94, 141.26,
42.03, 162.68, 167.88 ppm.
1
13
2.2.3.3. Synthesis of 2-(3-(4-(9H-carbazol-9-yl)phenyl)-1-
phenylallylidene)malononitrile 9. Color: Orange; Yield: 61%; M.P.:
strument (ATR accessories). H and C NMR spectra were recorded
on a 500 MHz Varian, USA instrument using TMS as an internal
standard. Mass spectra were recorded on Finnigan Mass spec-
trometer. The absorption spectra of the compounds were recorded
on a Perkin Elmer Lambda 25 UVevisible spectrophotometer;
fluorescence emission spectra were recorded on Varian Cary
Eclipse fluorescence spectrophotometer using freshly prepared
ꢂ
170e172 C.
FT-IR: 2213 (eCN), 1592 (eC]Ce), 1448(eAr).
CHN Analysis: Found C ¼ 85.51; H ¼ 5.51; N ¼ 9.91. Calculated
for C30
H
19
N
3
C ¼ 85.49; H ¼ 4.54; N ¼ 9.97%.
Mass: Calculated 421.1 for C30
H NMR (DMSO, d
H
19
N
3
found 422.2 (Mþ1).
): 6.9 (d, 1H, J ¼ 15.6 Hz), 7.32 (t, 2H), 7.44 (m,
H), 7.60 (m, 3H), 7.67 (m, 3H), 7.77 (d 2H, J ¼ 8.4 Hz), 8.14 (d 2H,
J ¼ 7.8 Hz) ppm.
1
ꢁ
6
ꢁ1
solutions at the concentration of 1 ꢀ 10 mol L . Fluorescence
quantum yield ware measured by using fluorescein as reference
standard (in 0.1 M NaOH) [36].
6
13
C NMR (DMSO,
23.86, 124.98, 120.21, 127.09, 128.91, 129.12, 130.29, 131.29, 132.86,
33.00, 140.10, 140.79, 147.82, 170.89 ppm.
d): 82.78, 109.72, 112.78, 113.30, 120.49, 120.66,
1
1
2
2
.2. Synthesis
.2.1. Synthesis of 3-formyl-9-phenylcarbazole 4, N-(4-
formylphenyl)carbazole 4a and 9-phenyl-9H-carbazole-3,6-
dicarbaldehyde 4b
The compounds 4, 4a and 4b were prepared by the reported
methods [37,38].
2.2.3.4. Synthesis of Ethyl 5-(4-(9H-carbazol-9-yl)phenyl)-2-cyano-
3-phenylpenta-2,4-dienoate 10. Color: Yellow; Yield: 71%; M.P.:
ꢂ
188e190 C.
FT-IR: 2214 (eCN), 1717 (eC]O), 1596 (eC]C), 1447(eAr), 1118
eCeO).
CHN Analysis: Found C ¼ 82.04; H ¼ 5.12; N ¼ 5.93. Calculated
for C32
C ¼ 82.03; H ¼ 5.12; N ¼ 5.93%.
Mass: Calculated 468.1 for C32
found 469.2 (Mþ1).
): 1.43 (t, 3H), 4.41 (q, 2H), 6.74 (d, 1H,
(
2.2.2. Synthesis of (1-phenylethylidene) propanedinitrile 5 and
ethyl-2-cyano-3-phenyl-2-butenoate 7
The compounds 5 and 7 were prepared by the reported
method [39].
24 2 2
H N O
24 2 2
H N O
1
3
H NMR (CDCl , d
J ¼ 15.7 Hz), 7.30 (t, 2H), 7.42 (m 6H), 7.55 (m, 3H), 7.59 (d, 2H,
J ¼ 8.4), 7.73 (d, 2H, J ¼ 8.4), 8.13 (d, 2H, J ¼ 7.7 Hz), 8.75 (d, 1H,
J ¼ 16.4 Hz) ppm.
2
.2.3. General procedure for synthesis of extended styryls 6, 8e12
The carbaldehyde (4, 4a and 4b 1 eq.) and different active
13
3
C NMR (CDCl , d): 14.19, 62.12, 102.89, 109.72, 116.76, 120.41,
methylene compound (5 and 7; 1.2 eq.) were dissolved in absolute
ethanol (10 mL). Piperidine was added in catalytic amount (0.1 mL)
and the reaction mixture was refluxed for 4 h. The corresponding
extended styryls (6, 8, 9, 10, 11 and 12) were obtained, filtered and
recrystallized from methanol.
123.70, 126.10, 126.35, 127.03, 128.72, 128.96, 129.91, 134.25, 136.24,
139.68, 140.33, 145.99, 162.53, 167.42 ppm.
0
.2.3.5. Synthesis of 2,2 -(9-phenyl-9H-carbazole-3,6-diyl)bis(1-
2
phenylprop-2-en-3-yl-1-ylidene)dimalononitrile 11. Color: Red;
ꢂ
2.2.3.1. Synthesis of 2-(1-phenyl-3-(9-phenyl-9H-carbazol-3-yl)ally-
Yield: 48%; M.P.: 190e192 C.
lidene)malanonitrile 6. Color: Orange; Yield: 68%; M.P.:
FT-IR: 2219 (eCN), 1589 (eC]C), 1441(eAr).
ꢂ
2
08e210 C.
CHN Analysis: Found C ¼ 84.06; H ¼ 4.15; N ¼ 11.63. Calculated
FT-IR: 2215 (eCN), 1584 (eC]Ce), 1497(eAr).
CHN Analysis: Found C ¼ 85.41; H ¼ 4.49; N ¼ 9.91. Calculated
for C32
H
24
N
2
O
2
C ¼ 84.12; H ¼ 4.20; N ¼ 11.68%.
Mass: Calculated 622.2 for C42
H
45
N
5
Na found 623.4 (Mþ1).
1
for C30
H
19
N
3
C ¼ 85.47; H ¼ 4.54; N ¼ 9.97%.
H NMR: 7.03 (d, 2H, J ¼ 15.5 Hz), 7.37 (d, 2H, J ¼ 9 Hz), 7.63 (m,
Mass: Calculated 421.1 for C30
H
19
N
3
found 422.2 (Mþ1).
16H), 7.99 (d, 2H, J ¼ 8.5 Hz), 8.70 (s, 1H) ppm.

R.D. Telore, N. Sekar / Dyes and Pigments 129 (2016) 1e8
3
13C NMR (DMSO,
27.22, 127.39, 127.77, 127.95, 128.46, 129.24, 129.48, 130.88, 131.45,
33.81, 135.86, 143.22, 150.62, 171.74 ppm.
d): 80.02, 111.59, 113.87, 113.86, 123.86, 124.18,
1
1
0
.2.3.6. Synthesis of diethyl 5,5 -(9-phenyl-9H-carbazole-3,6-diyl)
2
bis(2-cyano-3-phenylpenta-2,4-dienoate) 12. Color: Yellow; Yield:
ꢂ
4
5%; M.P.: 220e222 C.
FT-IR: 2221 (eCN), 1721 (eC]O), 1594 (eC]C), 1432(eAr), 1122
(
eCeO).
CHN Analysis: Found C ¼ 79.58; H ¼ 5.02; N ¼ 6.01. Calculated
for C32
C ¼ 79.63; H ¼ 5.08; N ¼ 6.06%.
Mass: Calculated 693.2 for C46 found 693.
): 1.41 (t, 3H), 4.37 (q, 2H), 6.84 (d, 1H,
J ¼ 16.1 Hz), 7.37 (m, 7H), 7.59 (m, 9H), 7.62 (m, 4H), 7.98 (s,1H), 8.16
s, 1H), 8.71 (d, 1H, 15.5 Hz).
24 2 2
H N O
35 3 4
H N O
1
3
H NMR (CDCl , d
(
13
C NMR (CDCl
22.07, 123.18, 123.87, 123.37, 123.18, 127.90, 128.02,128.11, 128.59,
28.94, 129.69, 130.00, 134.54, 136.83, 137, 04, 140.20, 142.62,
3
, d): 14.24, 61.76, 100.50, 110.30, 117.36, 120.59,
1
1
149.32, 163.09, 168.41.
Scheme 2.
3
. Results and discussions
The compounds 6 and 8 were synthesized by the condensation
of 9-phenyl-9H-carbazole-3-carbaldehyde 4 with methyl 2-(1-
All the extended styryls were synthesized from carbazole 1. N-
phenylethylidene)malononitrile
5
and
ethyl
2-cyano-3-
Substitution of carbazole followed by VilsmeiereHaack reaction
have been carried to afford 3-formyl-9-phenylcarbazole 4 (Scheme
1
sized by coupling 4-bromobenzaldehyde 2a with carbazole 1 using
Buchwald reaction. The crude compound was purified by column
chromatography.
carbaldehyde 4b (Scheme 3) was synthesized by bis-formylation
of N-phenyl carbazole and purified by using column chromatog-
raphy. The carbaldehydes (1 eq.) were heated with 2-(1-
phenylethylidene)malononitrile 5 (1.2 eq.) and ethyl 2-cyano-3-
phenylbut-2-enoate 7 (1.2 eq.) by Knoevenagel condensation to
get the novel extended styryls.
phenylbut-2-enoate 7 respectively by Knoevenagel condensation.
Similarly 9 and 10 were synthesized by the condensation of 4-(9H-
carbazol-9-yl)benzaldehyde 4a with 5 and 7. The compounds 11
and 12 were synthesized by condensation of 9-(4-formylphenyl)-
). 4-(9H-Carbazol-9-yl)benzaldehyde 4a (Scheme 2) was synthe-
9
H-carbazole-3-carbaldehyde 4b with 5 and 7 respectively. The
1
9-(4-Formylphenyl)-9H-carbazole-3-
styryl dyes were purified by crystallization in methanol. FT-IR, H
NMR, C NMR and CHN spectral analysis were performed to
13
confirm the structures of the extended styryls.
3.1. Photo-physical properties
The extended styryls can be divided in to two groups; one
containing 2-(1-phenylethylidene)malononitrile as an acceptor
unit (6, 9 and 11), second containing 2-cyano-3-phenylbut-2-
enoate as an acceptor unit (8, 10 and 12). The UVevis absorption
and fluorescence emission spectra of the extended styryls in
dichloromethane are shown in Figs. 1 and 2. The experimental
absorption values and computed values of all the extended styryls
in different solvents with varying polarities are tabulated in Table 1,
S1eS5 and Figs. S1eS3. The extended styryls having D-
p-A (6,
8
e10) and A- -D- -A (11, 12) systems consist of an electron
p
p
donating N-phenyl carbazole core and electron withdrawing
dicyano vinylene and cyanoethoxy vinylene groups linked through
p
-bond. The absorption spectra of all the extended styryls showed
two maxima. Shorter maxima was observed at around
00e350 nm due to the * transition with low energy and
3
pep
longer maxima at around 350e450 nm due to ICT between the
donor carbazole unit and the acceptor active methylene units. The
presence of phenyl group on nitrogen atom in carbazole ring is
responsible for more effective conjugation in the molecules. The
molar extinction coefficients (ε) of the extended styryls 6, 8, 9 and
1
0
correspond to the ICT were in the range of
ꢁ
1
ꢁ1
8
800e44,800 Lmol cm while the extended styryls 11 and 12
observed much higher value of in the range of
in different solvents. These two
ε
ꢁ1
ꢁ1
cm
32,000e60,000 Lmol
extended styryls 11 and 12 observed shoulder peak, which may be
due to conjugation from any one side of 3- and 6- position of
carbazole.
Scheme 1.

4
R.D. Telore, N. Sekar / Dyes and Pigments 129 (2016) 1e8
Scheme 3.
6
6
9
Absorption
Emission
4
0
5
0
0
0
0
.18
.16
.14
.12
9
3
11
11
30
25
0
.1
20
15
10
5
0
0
0
0
0
.08
.06
.04
.02
0
460
510
560
610
660
300
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorption and emission graph of compound 6, 9, and 11 in dichloromethane.
8
8
1
Emission
Absorption
0
0
0
0
.18
.16
.14
.12
30
25
20
15
10
5
0
10
0
12
12
0
.1
0
0
0
0
.08
.06
.04
.02
0
480
530
580
630
680
3
00
350
400
450
500
Wavelength (nm)
Wavelength (nm)
Fig. 2. Absorption and emission graph of compound 8, 10, and 12 in dichloromethane.

R.D. Telore, N. Sekar / Dyes and Pigments 129 (2016) 1e8
5
Table 1
Absorption maxima (l
extended styryl 6 in different solvents.
due to the rotation barriers of the cisetrans isomerization at ter-
minal C]C bond and the internal rotations around CeC bond at 3-,
abs, nm), molar extinction coefficient (ε; Lmol 1 cm ) of
ꢁ ꢁ1
6
- and 9- position of carbazole that decreases the band gap in
ground (S ) and excited state (S ) and may quenches the fluores-
cence quantum yield [43,44].
a
ꢁ1
ꢁ1
fb
Solvent
Toluene
l
abs
FWHM
(ε) (Lmol cm
)
0
1
446
450
453
438
438
441
439
447
452
63
63
183
188
74
150
176
72
31,200
44,000
44,800
40,400
31,600
37,200
35,200
26,800
27,600
0.288
0.264
0.476
0.521
0.378
0.363
0.359
0.279
0.321
1,4-Dioxane
The extended styryls 11 and 12 showed positive fluorescence
solvatochromism with the increase in solvent polarity (Fig. S3).
These molecules have more conjugation from involvements of both
the sides of 3- and 6- positions on the carbazole, which is
responsible for higher extinction coefficient, positive sol-
vatochromism, higher Stokes shift and good quantum yield. In
these compounds fluorescence was enhanced due to intra-
molecular charge transfer from carbazole to electron withdrawing
group present at 3- and 6- positions on carbazole.
DCM
Chloroform
Ethyl acetate
Acetone
Acetonitrile
DMF
DMSO
189
a
Absorption maxima experimental.
Oscillator strength experimental.
b
3.2. Fluorescence emission
3.3. Emission in the solid-state
The extended styryls were having similar properties as
The solid-state emission of all the extended styryls in the range
compared to the known styryl [16]. All the extended styryls 6, 8e12
observed red shift in emission maxima from non-polar solvent
toluene to polar solvent DMSO Figs. S1eS3. The compounds 6, 9
and 11 containing two powerful electron withdrawing cyano
groups emit orange light in solid state as well as in solution
whereas the compounds 8, 10 and 12 containing one powerful
electron withdrawing cyano group and one weak electron with-
drawing carboethoxy group were yellow fluorescent in solid as well
as in solution.
The excited states of these molecules in polar solvents are more
stable which might be due to the charge separation in the mole-
cules at the excited state. These red shifts for extended styryls from
non-polar to polar solvents in 6, 8e12 were found to be 80 nm,
of 508e597 nm as shown in Fig. 3. The extended styryl 6 containing
cyano group observed a blue shift as compared to its analog 8
having 1- cyano-1-thoxy carbonyl vinylene group. While the
extended styryls 9 and 11 containing more polar dicyanovinylene
group showed red shift as compared to their respective analogs 10
and 12. Dicyanovinylene extended styryl dyes 6, 9 and 11 showed
better emission intensity as related to their analogs 8, 10 and 12
containing ethoxycarbonylcyano vinylene group. Extended styryl
dyes 11 and 12 observed less emission intensity as they contained
two-acceptor unit and one donor. Two acceptor units may quench
the fluorescence. These extended styryls may lead to H-type of
aggregation that shows blue shift while mono substituted styryls 6,
8e10 shows J-type of aggregation with red shift [45].
7
7 nm, 43 nm, 52 nm, 79 nm and 74 nm respectively. It was
observed that the extended styryls 9 and 10 showed lower shift as
they have less donor effect from N-phenyl substituted carbazole
moiety. This underlines that extension of conjugation due to the
presence of strong electron donating group at 3- and 6- positions
on the carbazole moiety have good effect on the emission values. In
the case of extended styryls 9 and 10, fluorescence quantum yield
decreases with increasing solvent polarity, which is caused by the
radical ion formation in aromatic hydrocarbons with increasing
solvent polarity [40e42] (Table S7eS8).
3
.4. Dipole moment changes of the extended styryls on
photoexcitation
The ICT feature of the extended styryls can be evaluated by a
LipperteMataga plot [Eq. (1)] [46e48]. The common solvent effects
on the molecules were described solvatochromism properties by
LipperteMataga Eq. (1). LipperteMataga equation is used to esti-
mate change in dipole moment on photoexcitation as a function of
the solvent polarity.
The fluorescence quantum yields of these molecules are good in
non-polar solvents. The extended styryl 6 showed
f
¼ 0.14 and
2Df
ꢀ
ꢁ
2
Dn ¼ 4
pε Zca
me ꢁ mg þ b
(1)
f
¼ 0.12 fluorescence quantum yields in toluene and 1,4-dioxane
3
0
respectively. Similar results were obtained in the case of 8, which
showed higher fluorescence quantum yield in non-polar solvents
2
ε ꢁ 1
n ꢁ 1
like toluene (
¼ 0.14) and chloroform (
fluorescence quantum yield was observed in chloroform (
while the extended styryls 11 and 12 showed the highest fluores-
cence quantum yields in polar solvents like DMSO (
¼ 0.16)
Table 2 S6eS10). The lower values of fluorescence quantum yields
f
¼ 0.14), 1,4-dioxane (
¼ 0.15). In the case of 9, the highest
¼ 0.42)
f
¼ 0.11), dichloromethane
Df ¼
ꢁ
2
2
ε þ 1 2n þ 1
(f
f
where,
f
Dn
n
¼
n
abs
ꢁ
n
em stands for Stokes shift,
f
ꢁ1
abs and n
em ¼ absorption and emission (cm ),
(
Z ¼ Planck's constant,
c ¼ velocity of light in vacuum,
a ¼ Onsager cavity radius,
b ¼ constant,
Table 2
f
Fluorescence emission maxima (lem, nm), fluorescence quantum yield (f ) and
ꢁ
1
Stokes shift (cm ) of extended styryl 6 in different solvents.
D
f ¼ orientation polarizability,
Solvent
Toluene
l
em
Stokes shift (cmꢁ1)
f(f)
m
m
g
¼ ground-state dipole in the ground-state geometry,
515
490
549
542
497
511
515
570
521
3004.0
2422.8
4007.2
3624.8
2710.3
3106.2
3361.5
4827.5
2930.03
0.14
0.12
0.035
0.027
0.095
0.12
0.16
0.05
0.03
e
¼ excited-state dipole in the excited-state geometry,
1,4-Dioxane
ε
0
¼ permittivity of the vacuum,
DCM
2
(m e
e
m
g
) ¼ proportional to the slope of the LipperteMataga plot.
Chloroform
Ethyl acetate
Acetone
Acetonitrile
DMF
The LipperteMataga plots of the Stokes shift of the extended
styryl dyes 6, 8, 11 and 12 against orientation polarizability showed
very good linearity as shown by regression coefficient close to unity
DMSO
(
Fig. S4). In case of extended styryl dyes 9 and 10, LipperteMataga

6
R.D. Telore, N. Sekar / Dyes and Pigments 129 (2016) 1e8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
0
6
8
9
1
0
11
1
2
470
500
530
560
590
620
650
680
Wavelength (nm)
Fig. 3. Solid-state emission spectra of extended styryls.
plots were non linear with low regression factor, which concluded
that fluorescence quenching occurred in polar solvents. However,
The extended styryls 6 (5.0e6.9 D), 8 (3.3e4.9 D), 11 (5.3e8.6 D)
and 12 (6.1e7.9) have significant transition dipole moment as
compared to 9 (2.3e3.1 D) and 10 (3.1e4.5 D) due to lack of charge
transfer. Observed results conclude that the conjugation of 3- and
6- position of carbazole gave better charge transfer than the sub-
stitution at the 9- position of carbazole.
in case of the extended
p-conjugation framework such as 11 and
12, the dipole moment changes were much better with the solvent
polarity (up to 8 D). The higher dipole moment changes in these
compounds infer that the ICT feature in the extended styryls was
more significant. These results are attributed to the di-substitution
on the carbazole ring at 3- and 6-positions. Similar observation is
encountered in case of the extended styryls 6 and 8, the dipole
moment differences were up to 6 D due to only mono substitution
on carbazole ring at 3-position.
3.6. Relation between fluorescent molecular rotor (FMR) and
intramolecular charge transfer
The emission intensity of the synthesized extended styryls
increased with increasing solvent viscosity. The compounds 6, 8, 11
and 12 showed good FMR properties than the remaining extended
styryls 9 and 10. These diverse results may be due to the intra-
molecular charge transfer (ICT) between the acceptor and donor.
The extended styryls 6, 8, 11 and 12 have better ICT than 9 and 10.
ICT was disturbed due to the radical ion formation in aromatic
hydrocarbons, which quenched the fluorescence with increasing
solvent polarity. Similar results are observed in case of fluorescence
quantum yield. In highly polar solvents fluorescence of extended
styryl dyes 9 and 10 diminishes. These results prove that lack of ICT
is observed in these molecules. Hence it has been concluded that
extended styryl dyes 9 and 10 were inactive as FMRs. The rotors 6,
8, 11 and 12 were compared with the traditionally reported rotors A
and B. The emission maxima of extended styryl dyes 6, 8, 11 and 12
which can act as FMRs was observed to have a red shift of
3
.5. Oscillator strength and transition state dipole moment
Oscillator strength is dimensionless quantity that expresses the
probability of absorption and emission properties in energy levels,
which helps to understand charge transfer within the molecules. It
simply describes number of electron transition from the ground to
the excited states. Oscillator strength (f) can be calculated using the
following Eq. (2) [49]. Where ε is the extinction coefficient
ꢁ1 ꢁ1 ꢁ1
(
Lmol cm ), and n represents the wavenumber (cm ). From this
equation we have calculated oscillator strength for extended styryls
and tabulated in Table S11.
Z
f ¼ 4:32 ꢀ 10ꢁ
9
εðnÞ dn
(2)
By using the value of f, we have calculated transition dipole
moment that was the differences in electric charge distribution
between the ground and excited state of the molecule. The tran-
30e60 nm as compared to the rotor A (
The ICT is more favorable at 3- and 6- positions of carbazole as
compared to 9-phenyl carbazole.
l
em ¼ 470 nm in toluene).
a
sition dipole moment for absorption (m ) is a measure of the
probability of radiative transitions which have been calculated for
the extended styryls in different solvent environments using the
Eq. (3) [50]. The transition dipole moment increased with increase
in the oscillator strength.
f
2
m ¼
a
(3)
4
:72 ꢀ 10ꢁ7 ꢀ n
where,
m
a
is transition dipole moment (D),
f is oscillator strength,
is wavenumber (cmꢁ1).
n

R.D. Telore, N. Sekar / Dyes and Pigments 129 (2016) 1e8
7
DMF
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
0
6
6
1.98
3
.8
.6
.4
4.24
9
2
2
2
x = 0.3454
1
3
8
8.95
9.59
2.12
169.04
2.2
2
2
7
45.4
00.8
1.8
0
1
2
3
480
530
580
630
680
log
Wavelength (nm)
Fig. 4. Emission spectra of 6 in mixed solvents of DMF and glycerol, and emission intensity on the viscosity of the solvents.
3.7. Sensitivity of emission to viscosity
solvent system for viscosity dependent emission. Synthesized
extended styryl dyes showed good photophysical properties with
The effects of change in viscosity on emission of the synthesized
good intramolecular charge transfer. The strong p conjugation in
FMRs 6, 8, 11 and 12 have been studied. The molecules 6, 8, 11 and
2 have less solubility in the mixture of ethylene glycol and glyc-
erol. Thus, these molecules have been solubilized in highly polar
aprotic solvent DMF and increased their viscosity using glycerol
the molecules leads FMRs, hence photophysical properties can be
significantly enhanced. The ICT effect plays a key role in the vis-
cosity sensitivity emission of the extended styryls. The absorption,
emission, oscillator strength and charge distribution between
acceptor and donor were discussed.
1
(Fig. 4). The emissions intensities of synthesized FMRs were
increased with increase in solvent viscosity. But the emission in-
tensities of extended styryl dyes 9 and 10 were decreased by
increasing solvent polarity.
Acknowledgment
log I ¼ C þ xlogh
(4)
Rahul D. Telore is greatly thankful to University Grant
Commission-SAP, New Delhi, India for providing financial support.
The emission intensity of FMR and effect of viscosity of the
solvent was described by using F o€ rstereHoffmann Eq. (4), in which
h
is viscosity of the solvents, I is the emission intensity of the rotors,
Appendix A. Supplementary data
C is a constant and x is the viscosity sensitivity of the rotors. The
FMR 6 (x value 0.34) showed low value of x as compared to the FMR
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2016.02.012.
8
(x value 0.49). The extended styryl dyes 9 and 10 show
enhancement in emission intensity with the increasing viscosity.
The FMR 9 showed x value 0.39 and 10 showed x value 0.47, which
indicates that those molecules are low emissive in low viscous
system can emit good in higher viscosity. Similar study has been
carried out in DCM: PEG-400 system instead of DMF-Glycerol
system, but such an observation was not evident. No enhance-
ment on fluorescence of extended styryl dyes 9 and 10 has been
observed. According to these two results compound 9 and 10 are
FMR inactive. These results concluded that FMRs required good
intramolecular charge transfer within the system. The FMR 11 (x
value 0.1) showed a low value of x as compared to FMR 12 (x value
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