Indole-Based NLOphoric Donor-π-Acceptor Styryl Dyes: Synthesis, Spectral Properties and Computational Studies

Article abstract of DOI:10.1007/s10895-016-1901-5

Donor-π- acceptor styryl chromophores based on indole core were synthesized by Knoevenagel condensation of N-ethyl indole-3-carbaldehyde and different active methylene compounds. The absorption and emission properties of these dyes in different solvents were studied. The dyes displayed a broad absorption maximum in the UV and visible region between 397 and 469?nm with FWHM, 50 to 75?nm. Due to the extended π – conjugated systems this styryl chromophores shows strong intramolecular charge transfer characteristics. The dyes showed solid state emission and emission in solid state was red shifted as compared to their emission in less polar solvents. Density Functional Theory [B3LYP/6–311?+?G(d)] computations were used to correlate the structural, molecular, electronic and photo physical parameters of styryl dye with experimental study. Synthesized dyes were confirmed by using FT-IR, ¹H NMR, ¹³C NMR and HRMS spectral analysis.

Full text of DOI:10.1007/s10895-016-1901-5

J Fluoresc
DOI 10.1007/s10895-016-1901-5
ORIGINAL ARTICLE
Indole-Based NLOphoric Donor-π-Acceptor Styryl Dyes:
Synthesis, Spectral Properties and Computational Studies
1
1
Santosh Chemate & Nagaiyan Sekar
Received: 31 January 2016 /Accepted: 4 August 2016
Springer Science+Business Media New York 2016
#
Abstract Donor-π- acceptor styryl chromophores based on
indole core were synthesized by Knoevenagel condensation of
N-ethyl indole-3-carbaldehyde and different active methylene
compounds. The absorption and emission properties of these
dyes in different solvents were studied. The dyes displayed a
broad absorption maximum in the UV and visible region be-
tween 397 and 469 nm with FWHM, 50 to 75 nm. Due to the
extended π – conjugated systems this styryl chromophores
shows strong intramolecular charge transfer characteristics.
The dyes showed solid state emission and emission in solid
state was red shifted as compared to their emission in less
polar solvents. Density Functional Theory [B3LYP/6–311 +
G(d)] computations were used to correlate the structural, mo-
lecular, electronic and photo physical parameters of styryl dye
with experimental study. Synthesized dyes were confirmed by
potential applications in electro/photoactive materials, such as
dye sensitized solar cells [1, 2] laser dyes [3], nonlinear optical
(NLO) materials [4, 5], organic light-emitting diodes
(OLEDs) [4] and organic field-effect transistors (OFETs).
Owing to their excellent phtotophysical properties. D–π-A
chromophores are widely used in sensitizers [6, 7], photo-
graphic materials [8], optical recording materials [9–11], pho-
to initiators for free radical polymerization [12] and fluores-
cent probes [13]. The styryl chromophores derived from in-
dole heterocycle constitute a part of various dye materials
[14–19] and have been used for dyeing of keratin fibers
[20], lightning keratin materials [21] and dyeing of synthetic
fibres. Organic fluorophores exhibiting solid state fluores-
cence efficiency have received increasing attention owing to
their potential application in OLEDs [22–24], organic field-
effect transistors [25], in the field of lasers [26], photovoltaic
cells [27], sensors for biological molecule [10, 28–31] and
bio-cell imaging [32].
1
13
using FT-IR, H NMR, C NMR and HRMS spectral
analysis.
.
.
Keywords Indole-3-carbaldehyde Styryl Solid state
The donor-π-acceptor styryl chromophore shows strong
intramolecular charge-transfer (ICT) characteristics because
the donor group (D) is an electron-rich unit, linked through a
π bridge spacer to the electron-acceptor group (A). It is easy to
modify their physical and chemical characteristics by tuning
different donor or acceptor units. In D-π-A molecules the
electron-rich nitrogen or sulphur atom usually acts as electron
donor. The absorption and emission of the molecules are sol-
vent dependent and the styryl derivatives show aggregation
induced emission [32, 33]. The emission of organic dyes is
effectively suppressed in solid state due to aggregation in pla-
nar -π- conjugated system based on π-π interaction or the
formation of eximers [34–36]. There has been considerable
interest in preparing indole based donor–π-acceptor styryl
dyes which retain their photophysical properties in the solid
state [37, 38]. The absorption and emission mainly depend on
conjugation, planarity and rigidity in the molecule as well as
.
fluorescence DFT
Introduction
During the past few decades, the donor–π-acceptor (D–π-A)
chromophores have attracted considerable interest due to their
Electronic supplementary material The online version of this article
doi:10.1007/s10895-016-1901-5) 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, Mumbai 400 019, India

J Fluoresc
on solvent polarity. It is our endeavor to look into extensively
conjugated fluorescent dyes in this chromophoric system. The
indole based D-π-A styryl chromophore reported in this paper
shows excellent solid state emission between 487 nm to
USA instrument using TMS as an internal standard. Mass
spectra were recorded on Finnigan Mass spectrometer. The
chemical shift values are expressed in δ ppm using DMSO-
d6 as a solvent and TMS as an internal standard. DFT calcu-
lations were performed on a HP workstation XW 8600 with
Xeon processor, 4 GB RAM and Windows Vista as operating
system. The software package used was Gaussian 09 W [39].
The ground state geometry was optimized at B3LYP level of
theory and 6–311 + G (d) as a basis set (Scheme 1).
5
51 nm. DFT study has been carried out using B3LYP/6–
3
11 + G(d) to understand photophysical properties of synthe-
sized molecules.
Experimental Section
Materials and Equipments
Computational Methods
All the reagents and solvents were purchased from S. D. Fine
Chemicals Ltd. and they were used without further purifica-
tion and all the solvents were used of spectroscopic grade. The
absorption spectra of these dyes were recorded on Perkin
Elmer UV-visible spectrophotometer, and emission spectra
were recorded on Varian Cary Eclipse fluorescence spectro-
photometer (Varian, Australia) using 1 cm length quartz cells.
The emission spectra was recorded by exciting the molecules
at their maximum wavelength of absorption in the respective
solvents. The melting points were recorded on instrument
from Sunder Industrial Product Mumbai by using open capil-
lary and are uncorrected. The FT-IR spectra were recorded on
a Jasco 4100 Fourier Transform IR instrument (ATR accesso-
ries). 1H NMR spectra were recorded on a Varian 500 MHz
Gaussian 09 program package was used for geometry optimi-
zation of synthesized styryl dyes [39]. Geometry optimization
of molecules in their ground state was carried out in both
vacuum and solvent by using polarizable continuum model
(PCM) [40]. Ground state (S ) geometry of the dyes in gas
0
and solvent was optimized in their C1 symmetry using DFT
[41]. The Becke’s three parameter exchange functional (B3)
[42] combining with nonlocal correlation functional by Lee,
Yang and Parr (LYP) [43] and basis set 6–311 + G(d) was used
for all the atoms. Same method was used for vibrational anal-
ysis to verify that the optimized structures correspond to local
minima on the energy surface. Time Dependent Density
Functional Theory (TDDFT) computations were used to ob-
tain the vertical excitation energies and oscillator strengths at
Scheme 1 Synthesis of 3a-e

J Fluoresc
the optimized ground state equilibrium geometries at the same
hybrid functional and basis set [41].
oil) (0.44 g, 0.011 mol) was added and stirred for 10 min under
N atmosphere. Then ethyl bromide (1.2 g, 0.011 mol) was
2
added dropwise and stirred at room temperature for 2 h. The
reaction mixture was filtered to remove salt formed and filtrate
was concentrated to get solid product. The product was recrys-
tallized from 95 % ethanol to get pure product. Yield: 95 %,
Light yellow solid, m.p. 62 °C (60–65 °C) [47].
Relative Quantum Yield Calculations
The quantum yields of compounds 3a to 3e in different sol-
vents were evaluated. The fluorescence quantum yields (Φ ) at
f
room temperature were measured relative to quinine sulfate in
General Procedure for Synthesis of Compounds 3a-3e
0
.1 N H SO (Φ = 0.51) [44]. The absorption and emission
2 4 f
characteristics of the standards and the compounds in polar as
well as non-polar solvents were measured at different concen-
trations. The emission intensity values were plotted against
absorbance values and linear plots were obtained. Gradients
were calculated for the compounds in each solvent and for the
standards. All the measurements were done by keeping the
parameters such as solvent and slit width constant. The rela-
tive quantum yields of synthesized compounds in different
solvents were calculated by using Eq. (1). The refractive indi-
ces of the solvents have been taken from literature [45].
N-Ethyl indole-3-aldehyde (1.0 mmol) was dissolved in 10 ml
of dry methanol and active methylene compound (1.0 mmol)
were added at room temperature. Then piperidine (0.1 mmol)
was added as a catalyst and the reaction mixture was refluxed
for 5 h. The progress of the reaction was monitored on TLC.
The reaction mixture was filtered and solid obtained was
dried. The crude products were recrystalised form aq. ethanol.
Structural Confirmations
ηx2
Gradx
Gradst ηst2
((1-Ethyl-1H-Indol-3-Yl)Methylene)Malononitrile (3a)
Yield: 95 %, Melting Point:148 °C, H NMR (DMSO-d6,
Φ ¼ Φ Â
Â
ð1Þ
x
st
1
5
00 MHz) δ ppm: 1.41 (t, 3H), 4.41 (q, 2H), 7.33 (dd,
Where:
J = 8 Hz & 16 Hz, 1H), 7.36 (dd, J = 8 Hz & 16 Hz, 1H),
Φ = Quantum yield of compound.
x
7
8
1
.68 (d, J = 8 Hz, 1H), 8.06 (d, J = 13 Hz, 1H), 8.54 (s, 1H),
.65 (s, 1H,), C NMR (DMSO-d6, 125 MHz) δ ppm: 152.2,
36.3, 135.2, 127.9, 124.5, 123.4, 119.8, 116.4, 116.3, 112.0,
Φ = Quantum yield of standard sample
13
st
Grad = Gradient of compound.
x
Grad = Gradient of standard sample.
−1
st
110.6, 69.5, 43.5, 15.4, FT-IR (cm ): 2213 (−CN), 1585
ηx = Refractive index of solvent used for synthesized
compound.
ηst = Refractive index of solvent used for standard sample.
(−C = C-, olefinic), 1566, 1512 (−C = C-, aromatic) HRMS
+
(
EI): calcd for C H N , 222.1031 [M + H] ; found:
1
4
12
3
222.1024.
Experimental Details
(
(
Z)-Ethyl 2-Cyano-3-(1-Ethyl-1H-Indol-3-Yl)Acrylate
3b) Yield: 96 %, Melting Point: 98 °C, H NMR (DMSO-
1
Synthesis of Compound 1
d6, 500 MHz) δ ppm: 1.42 (t, 3H), 3.30 (s, 3H), 3.81 (s, 2H),
.39 (q, 2H), 7.97 (d, J = 7.0 Hz, 1H), 7.29 (dd, J = 7.0 Hz &
13 Hz, 1H), 7.67 (d, J = 8 Hz, 1H), 7.35 (dd, J = 7.9 Hz &
4
Phosphorous oxychloride (POCl ) (7.65 g, 0.05 mol) was
added drop wise to N, N-dimethyl formamide (15.4 ml,
3
1
3
1
1
1
3 Hz, 1H), 8.51 (s,1H), 8.58 (s, 1H), C NMR (DMSO-d6,
25 MHz) δ ppm: 164.3, 146.5, 136.4, 134.8, 128.1, 124.2,
23.0, 119.4, 118.4, 111.9, 109.6, 92.2, 79.6, 53.1, 42.3, 15.4,
0
.20 mol) at 5–10 °C with constant stirring under nitrogen
atmosphere. The complex formed was stirred for 30 min.
The solution of indole (5.3 g, 0.0453 mol) in DMF (20 ml)
was slowly added to complex and stirred at 35 °C for 1 h, ice
was added, followed by 20 % aq. sodium hydroxide (NaOH),
and the mixture was refluxed for 6 h. The reaction mixture
was cooled to room temperature and then poured into cold ice
water, and the precipitated product was collected, washed with
water and dried. Yield: 95 %, Light yellow solid, m.p. 190–
−
1
FT-IR (cm ): 2212 (CN), 1709 (carbonyl), 1617, 1589
(−C = C-, olefinic), 1589, 1516 (−C = C-, aromatic) HRMS
+
(
2
EI): calcd for C H N O , 269.1290 [M + H] ; found:
1
6 16 2 2
69.1284.
(
Z)-2-(1H-Benzo[d]Imidazol-2-Yl)-3-(1-Ethyl-1H-Indol-
yl)Acrylonitrile (3c) Yield: 94 %,, Melting Point: 230 °C,
H NMR (DMSO-d6, 500 MHz) δ ppm: 1.43 (t, 3H), 4.39 (q,
3
1
1
92 °C (190–192 °C) [46].
2H), 7.2–7.3 (m, 4H), 7.51 (d, 1H), 7.64 (dd, J = 1.5 Hz,
Synthesis of Compound 2
7.0 Hz,2H), 7.99 (dd, J = 1.5 Hz, 7.0 Hz, 1H), 8.50 (s, 1H),
1
3
8.58 (s, 1H), 12.8 (s,1H), C NMR (DMSO-d6, 125 MHz) δ
Indole-3-carbaldehyde (1.45 g, 0.01 mol) dissolved in dry
acetonitrile (30 ml) and cooled to 0 °C and NaH (60 % in
ppm: 149.3, 144.0, 137.2, 136.0, 131.1, 128.3, 123.7, 123.3,
122.0, 119.0, 118.7, 111.6, 110.1, 94.5,79.6, 42.0, 15.6, FT-

J Fluoresc
Table 1 The UV-Vis absorption and fluorescence emission of compounds 3a-3e in solvents of varying polarities
Stokes Shift cm−
1
Є( Mole−1 cm−1)
FWHM (nm)
f
σ(1 × 10−14
)
Ф
Entry
Solvent
λabsnm
λemsnm
3
a
DMSO
DMF
398
398
394
395
398
397
400
390
391
443
442
444
444
435
436
436
442
440
2552
2501
2858
2793
2137
2253
2064
3016
25,772
24,905
34,227
26,917
26,145
29,776
38,734
27,796
27,695
54
52
52
52
52
52
50
52
52
0.4156
0.3891
0.5319
0.4249
0.4116
0.4554
0.6073
0.4419
0.4257
0.9857
0.9526
1.0295
1.3091
1.0000
1.1389
1.4815
1.0632
1.0593
0.0184
0.0242
0.0163
0.0071
0.0041
0.0142
0.0128
0.0128
0.0201
ACN
MeOH
Acetone
DCM
CHCl
3
EA
THF
2848
6849
2678
7047
2552
5441
2923
5
34
433
34
443
08
1
,4 dioxane
388
398
27,177
23,986
51
50
0.4287
0.3666
1.0394
0.9175
0.0184
0.0313
5
Toluene
5
3
b
DMSO
DMF
393
389
387
391
387
392
393
386
384
444
441
439
443
433
436
437
434
432
25,718
25,191
31,187
26,334
25,712
34,484
35,140
27,326
27,329
54
55
51
53
53
51
53
51
51
0.4144
0.4149
0.4853
0.4217
0.4076
0.5317
0.5626
0.4197
0.4242
0.9837
0.9636
1.0072
1.1929
0.9835
1.3190
1.3441
1.0452
1.0533
0.0174
0.0205
0.0183
0.0093
0.0057
0.0191
0.0149
0.0186
0.0174
3031
3060
3002
2745
2574
2562
2865
ACN
MeOH
Acetone
DCM
CHCl
3
EA
THF
2893
7350
2947
7315
2957
5
35
433
34
1
,4 dioxane
384
26,216
51
0.4109
1.0028
0.0164
5
Toluene
DMSO
DMF
387
408
406
401
407
402
412
417
402
402
402
422
437
463
461
457
464
459
469
480
460
459
462
467
26,966
37,166
35,002
29,688
34,804
40,285
29,826
28,046
44,055
40,057
41,338
36,142
53
66
66
68
64
99
69
71
66
65
66
67
0.4301
0.6344
0.6123
0.5182
0.6138
0.7077
0.4996
0.4541
0.7845
0.7031
0.53258
0.5326
1.0314
1.4216
1.3388
1.3312
1.1355
1.5409
1.1389
1.0728
1.6851
1.5322
1.5811
1.3824
0.0304
0.0191
0.0211
0.0179
0.0074
0.0064
0.0171
0.0142
0.0179
0.0193
0.0184
0.0231
3
c
2911
2938
3055
3018
3089
2950
3147
3136
3089
3230
ACN
MeOH
Acetone
DCM
CHCl
3
EA
THF
1
,4 dioxane
Toluene
2283
5178
3035
5
40
3
d
DMSO
DMF
427
423
418
422
419
420
421
416
420
416
422
469
493
485
483
486
481
492
491
478
482
481
483
540
32,411
34,634
50,269
35,844
40,192
52,549
54,212
38,738
36,611
331,515
36,311
20,589
69
68
68
67
68
66
68
66
67
67
67
72
0.4425
0.5046
0.7772
0.5193
0.6331
0.7629
0.7891
0.5916
0.5416
0.5046
0.5298
0.3166
1.2397
1.3247
1.3710
1.9227
1.5373
2.0100
2.0736
1.4817
1.4004
1.2680
1.3889
0.7875
0.0194
0.0231
0.0167
0.0059
0.0077
0.0171
0.0182
0.0165
0.0192
0.0194
0.0315
0.0171
3022
3219
3120
3076
3484
3386
3118
3063
3248
2993
2803
ACN
MeOH
Acetone
DCM
CHCl
3
EA
THF
1
,4 dioxane
Toluene
DMSO
3
e

J Fluoresc
Table 1 (continued)
Stokes Shift cm−
1
Є( Mole−1 cm−1)
FWHM (nm)
f
σ(1 × 10−14
)
Ф
Entry
Solvent
λabsnm
λemsnm
DMF
463
453
454
452
455
457
447
452
445
453
535
528
531
524
533
526
518
525
514
517
2906
3136
3194
3039
3216
2870
3066
3076
3017
3233
20,402
43,562
42,646
41,909
49,894
67,631
42,682
37,446
40,056
39,977
71
70
71
69
68
67
67
67
69
68
0.3146
0.6909
0.6657
0.6509
0.7602
1.0259
0.6521
0.5673
0.6289
0.6215
0.7804
1.6313
1.6665
1.6030
1.9084
2.5869
1.6326
1.4323
1.5321
1.5291
0.0214
0.0196
0.0060
0.0068
0.0691
0.0152
0.0174
0.0195
0.0179
0.0279
ACN
MeOH
Acetone
DCM
CHCl
3
EA
THF
1
,4 dioxane
Toluene
FWHM Full width at half maximum, σ = Absorption cross-section, Є = molar extinction coefficient, f = Oscillator strength, Ф = Quantum yield
Where, MeOH Methanol, EtOH Ethanol, EA Ethyl Acetate, DMF Dimethyl formamide, DMSO Dimethyl sulfoxide, DCM Dichloromethane, CHCl
Chloroform, THF Tetrahydrofuran
3
−
1
IR (cm ): 2213 (−CN), 1637, 1620 (−C = C-, olefinic), 1592,
J = 9.0 Hz & 3.0 Hz, 1H, Ar-H), 7.66 (dd, 7.0 Hz & 2.0
1H), 7.80 (dd, J = 7.0 Hz & 2.0 Hz, 1H, Ar-H), 8.21 (s,
1H), C NMR (DMSO-d6, 125 MHz) δ ppm: 172.5, 144.6,
1
531 (−C = C-, aromatic) HRMS (EI): calcd for C H N ,
13.1453 [M + H] , found: 313.1442.
2
0 17 4
+
13
3
138.8, 138.1, 133.9, 131.1, 129.4, 129.3, 125.5, 124.1, 123.1,
(
Z)-2-(Benzo[d]Thiazol-2-Yl)-3-(1-Ethyl-1H-Indol-3-
120.4, 118.1, 115.0, 113.5, 112.2, 79.6, 41.7, 15.3, FT-IR
1
−1
Yl)Acrylonitrile (3d) Yield: 95 %, Melting Point: 169 °C, H
NMR (DMSO-d6, 500 MHz) δ ppm: 1.44 (t, 3H), 4.40 (q,
(cm ): 2215 (−CN), 1592, 1504 (−C = C-, olefinic),
+
HRMS (EI): calcd for C22
H
N
, 324.1501 [M + H] , found:
18
3
2
7
H), 7.28–7.36 (m, 2H), 7.43 (dd, J = 8.5 Hz, 16.5 Hz, 1H),
.54 (dd, J = 7.0, 15.5 Hz, 1H), 7.67 (d, J = 8.5 Hz, 1H), 8.01
324.1498.
(
d, J = 8.0 Hz, 1H), 8.07 (dd, J = 8 Hz & 17 Hz, 1H), 8.12 (dd,
1
3
J = 8 Hz & 17 Hz, 1H), 8,49 (s, 1H), 8.57 (s, 1H), C NMR
DMSO-d6, 125 MHz) δ ppm: 164.5, 153.6, 139.8, 136.3,
Result and Discussion
Photophysical Study
(
1
1
34.4, 133.2, 128.0, 127.3, 125.8, 124.0, 122.8, 122.6,
22.5, 119.6, 111.7, 110.2, 97.0, 79.6, 42.1, 15.5. FT-IR
−1
(
cm ): 2985 (aromatic, −CH), (−CN), 1620 (−C = C-, olefin-
The UV-visible absorption emission spectra of the compounds
3a-3e were evaluated in polar as well as in less polar solvents
(Table 1). The styryl derivatives of indole showed red and blue
shifted absorption and emission depending upon the solvent
polarities. These 3-vinylindole derivatives showed more red
shifted emission in polar solvents than in less polar solvents.
The compound 3a exhibited red shifted emission in polar
solvents (methanol, ethanol, DMSO and acetonitrile) and blue
shifted emission in less polar solvents (1, 4 dioxane, THF and
ic), 1580, 1517 (−C = C-, aromatic) HRMS (EI): calcd for
+
C H N S, 330.1065 [M + H] , found: 330.1058.
2
0 16 3
(
Z ) - 2 - ( 3 - ( 1 - E t h y l - 1 H - I n d o l - 3 - Y l ) - 1 -
Phenylallylidene)Malononitrile (3e) Yield: 91 %,, Melting
1
Point: 203 °C, H NMR (DMSO-d6, 500 MHz) δ ppm:1.37
(
(
t, 3H), 4.23 (q, 2H), 7.14 (d, J = 15, 1H), 7.34 (m, 2H), 7.45
d, J = 15.5 Hz, 1H), 7.47–7.48 (m, 2H, Ar-H), 7.59 (dd,
Fig. 1 Absorption and emission
spectra of compound 3a in
different solvents
120
DMSO
DMF
MeOH
Acetonitrile
Acetone
DCM
CHCl3
EA
THF
DMSO
DMF
MeOH
Acetonitrile
Acetone
DCM
CHCl3
EA
THF
0.4
3
a
3a
1
00
80
0
0
0
.3
.2
.1
0
6
0
40
20
0
1,4-dioxane
Toluene
1,4-dioxane
Toluene
3
30 355 380 405 430 455
4
10
460
510
560
610
Wavelength (nm)
Wavelength (nm)

J Fluoresc
0
0
0
0
.4
.3
.2
.1
0
Fig. 2 Absorption and emission
spectra of compound 3b in
different solvents
3
b
DMSO
DMF
MeOH
Acetonitrile
Acetone
DCM
CHCl3
EA
THF
180
3b
DMSO
DMF
MeOH
Acetonitrile
Acetone
DCM
CHCl3
EA
THF
1
1
50
20
90
1,4-dioxane
6
3
0
0
0
Toluene
1,4-dioxane
Toluene
3
30 360 390 420 450
4
10
460
510
560
610
Wavelength (nm)
Wavelength (nm)
ethyl acetate) (Fig. 1). The similar trend was observed for the
dyes 3b, 3c and 3d (Figs. 2, 3 and 4). A more red shifted in
absorption and emission was observed in case of compound
chlorinated solvent for the dyes 3a, 3b, 3d and 3e. In the case
of dye 3c it is higher in ethyl acetate, THF and 1,4 dioxane.
The values for the σ falls in the range of 0.7804–
−
14
2
3
e due to extended conjugation and intramolecular charge
2.5869 × 10
cm . The molar extinction co-efficient at
transfer from the donor to the acceptor moiety (Fig. 5). The
red shifted absorption and emission was observed for 3e with
increasing solvent polarity. All the dyes showed large Stokes
shift in all the solvents. The fluorescence quantum yields of
the synthesized 3-vinylindole derivatives were determined in
different solvents and are tabulated in Table 1. The result
showed that the fluorescence quantum yield is depen-
dent on the solvent polarity. The compounds 3a-3c
showed two emissions peaks in nonpolar solvent like
λmax is the highest in chlorinated solvents for the dyes 3a,
3b, 3d and 3e. In the case of dye 3c it is higher in less polar
solvents such as ethyl acetate, THF and 1,4 dioxane (Table 1).
The molar extinction coefficient of the dyes varied from
−1
−1
24,905 to 67,631 mol cm .
Solid State Emission
1
, 4-dioxane, toluene and THF. The quantum yield
The dyes 3a, 3b and 3d showed solid state fluorescence max-
ima at 487, 486, and 561 nm respectively (Fig. 6). The mol-
ecule with effective intramolecular π-π interactions shows
strong solid state emission intensity so that dye 3b shows three
times more intensity than the dye 3a. The introduction of
bulky substituent at the periphery of the π - conjugation sys-
tem, decreases its solid state emission efficiency because it
inhibits aggregation. Thus the dye 3c and 3e shows poor solid
state fluorescence efficiency. The dyes 3c and 3d have benz-
imidazole and benzothiazole unit in acceptor part but only the
dye 3d shows a weak solid state fluorescence, and this is
because in case of the dye 3d effective charge transfer is pos-
sible because of vacant d-orbital of sulphur and such a charge
transfer is not possible for the dye 3c (Table 2).
values for 3a-3e increases with an increase in solvent
polarity, but for 3a and 3b in toluene it showed high
quantum yield (Ф = ~0.03) due to twisted intramolec-
ular charge transfer (TICT) process. As compared with
the fluorescence spectra in solution, the dyes 3a, 3b and
f
3
d showed slightly red shifted solid state fluorescence
emission revealing intramolecular π-π interactions pres-
ent in the D-π-A molecules. The dyes 3a, 3b and 3e
showed more FWHM in polar solvents than in less po-
lar solvent while no correlation in FWHM with respect
to solvent polarity was observed for the dyes 3c and 3d
(
Table 1). The molar absorption cross section (σ) were
determined in all the solvents and found higher in
DMSO
DMF
DMSO
DMF
Fig. 3 Absorption and emission
spectra of compound 3c in
different solvent
0.5
0.4
1
1
50
20
90
60
30
0
3
c
3c
MeOH
Acetonitrile
Acetone
DCM
MeOH
Acetonitrile
Acetone
DCM
0
0
0
.3
.2
.1
0
CHCl3
EA
CHCl3
EA
THF
THF
1,4-dioxane
1,4-dioxane
Toluene
Toluene
3
30
380
430
480
530
430
480
530
580
630
Wavelength (nm)
Wavelength (nm)

J Fluoresc
Fig. 4 Absorption and emission
spectra of compound 3d in
different solvents
80
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
0
DMSO
DMF
MeOH
Acetonitrile
Acetone
DCM
CHCl3
EA
THF
DMSO
DMF
MeOH
Acetonitrile
Acetone
DCM
CHCl3
EA
THF
3d
3d
6
0
40
1,4-dioxane
1,4-dioxane
Toluene
2
0
0
Toluene
3
30
380
430
480
530
438 488 538 588 638 688
Wavelength (nm)
Wavelength (nm)
Dipole Moment Determination by Solvatochromic
Method
Calculation of Dipole Moment
The solvent polarity functions f (ε, η) and f (ε, η) for
1
2
The properties of solvents such as refractive index are dielectric
constant are very important for study the physicochemical pro-
cess of the dye molecules. These properties of solvent induce
changes in the electronic transitions of solutes
each solvent were calculated by substituting the respec-
tive values for dielectric constant and refractive index in
Eqs. (7) and (10) (SI) the values obtained are tabulated
in Table S1. Graph of f (ε, η) verses Stokes shift in
1
−
1
cm is plotted and calculated m as slope 1, as well
(solvatochromism) depending on the nature and the degree of
1
−
1
interaction between the solute solvent in ground and first excit-
ed states of solutes (developed locally in the immediate vicin-
ity) [48, 49]. The dipole moment of electronically excited spe-
cies is of great importance in revelation of the nature of excited
state, designing nonlinear optical material and parameterization
of semi-empirical quantum chemical procedures of these states
as graph of f
2
(ε, η) verses ðv þ vf Þ in cm were plot-
a
ted to get values of function m as slope 2, the obtained
2
values are tabulated in Table S1. Further ratio of excited
state dipole moment to the ground state dipole moment
is calculated from the Eq. 12 by substituting the values
of m
and m
2
and summarized in Table S1. The com-
/μ than 3b, 3c, 3d and 3e
1
[50]. A most accepted technique for dipole moment calculation
pounds 3a has higher μ
e
g
is based on Lippert-Mataga equation [51–54]. This technique is
based on the absorption and emission maxima with polarity
functions of solvents, expressed by the relative permittivity
which suggest that the dyes with two electron withdraw-
ing cyano group at acceptor part has more polar excited
state. The dye 3d has higher μ /μ ratio than 3c be-
e g
(ε) and refractive index (η) of the solvent medium [55, 56].
cause of effective charge transfer in the dye 3d because
Hence solvatochromism and solvatofluorism are found to be
excellent tools to evaluate the ground state and excited state
dipole moments of the short-lived species [57–66].
of vacant d-orbitals of sulphur atom supports the charge trans-
fer in the molecule. In the case of dye 3e, higher μ /μ ratio
e g
was observed than 3b, 3c, 3d because the extended conjuga-
tion present in the molecule facilitates charge transfer from
donor to acceptor.
This observation is supported by DFT computations. We
have calculated the dipole moment ratio computationally and
detail values are given in the table (Table S2). The dipole
moments of the molecules in the excited state were observed
to be higher than in the ground state (Table S2).
The solvatochromic behaviour of the molecules was stud-
ied with the help of Lippert plot [51]. Lippert’s plot is the plot
−
1
of stokes shift in cm vs orientation polarizability. The
Lippert function is constituted of the polarity function f (ε)
and polarizability function f (η) showed good correlation in
less polar solvents which supports the charge transfer in the
dyes 3a-3e (Fig. S1).
0
.8
Fig. 5 Absorption and emission
spectra of compound 3e in
different solvent
DMSO
DMF
DMSO
DMF
250
200
3
e
3e
0.7
MeOH
Acetonitrile
Acetone
DCM
CHCl3
EA
MeOH
Acetonitrile
Acetone
DCM
0
0
0
0
0
0
.6
.5
.4
.3
.2
.1
0
1
50
CHCl3
EA
100
THF
THF
1,4-dioxane
1,4-dioxane
Toluene
5
0
0
Toluene
4
70 520
570 620 670
720
3
60
410
460
510
560
Wavelength (nm)
Wavelength (nm)

J Fluoresc
5
4
3
2
1
00
00
00
00
00
0
calculated for the dyes different solvent environments using
the Eq. (3) [70].
3
3
3
3
a
b
c
d
f
2
μa ¼
ð3Þ
−7
4:72 Â 10 Â v
3e
The values of oscillator strength ( f ) and transition dipole
moment (μ ) for compound for each solvent is given in
4
50
500
550
600
650
a
Wavelength(nm)
Table S3.
The observed higher values of transition dipole moments
Fig. 6 Solid-state fluorescence spectra (normalized) of dyes 3a-3e in
powder form
(μ ) in less polar solvents indicates effective D-π-A charge
a
transfer in the dyes 3a-3e (Table S3).
Transition Dipole Moment
Optimized Geometries of Dyes 3a-3e
In addition to this the dipole moments between the excited and
ground states the charge transfer character of the fluorophore
can be understood from the oscillator strength ( f ) and transition
The ground state geometries of the dyes 3a to 3e were opti-
mized at B3LYP/6–311 + G(d) level. In the case of dye 3a the
dihedral angle between indole moiety and acceptor part (C3-
dipole moment of the dyes (μ ). The effective number of elec-
a
trons transition from the ground to excited state is usually de-
scribed by the oscillator strength, which provides the absorption
area in the electronic spectrum. The oscillator strength ( f ) can
be calculated using the following Eq. (2) [67–69].
o
C11-C22-C23) in ground state is 4.37 whereas twisting was
o
observed in excited state with dihedral angle 88.8 which sug-
gest that the compound has almost planar geometry in ground
state while nonplanar in excited state. This may be due to two
strong electron withdrawing cyano groups present at the accep-
tor part (Fig. 7, Table S4, S5). The dihedral angle for 3a, be-
tween double bond and electron withdrawing cyano group
Z
ꢀ
ꢁ
−9
f ¼ 4:32 Â 10
ε v dv
ð2Þ
o
Where,
(C11-C22-C23-C27) in ground state is 2.2 and in excited state
−1
−1
o
ε is the extinction coefficient (L mol cm ).
is 0.6 . This suggest that double bond C22-C23 and electron
−1
v represents the numerical value of wavenumber (cm ).
acceptor part constitutes planar geometry. The similar observa-
tion was observed for compound 3b (Fig. S2, Table S4, S5)). In
the case of dye 3c the dihedral angle between the indole unit
Transition dipole moments for absorption (μ ) which is a
a
measure of the probability of radiative transitions have been
Table 2 Solid state emission photographs for dyes 3a–3e
λmax ems
Dye
Day Light
UV Light
3
a
491 nm
489 nm
3
b
3
c
Not emissive
3
d
570 nm
3
e
Not emissive

J Fluoresc
Fig. 7 Optimized geometry
parameters of dye 3a in
acetonitrile solvent in the ground
state and excited state (bond
length are in Å, dihedral angles
are in degree)
o
and acceptor part in ground state was −6.4 and in excited state
correlation between the experimentally obtained absorption
wavelength maxima and TD-B3LYP/6–311 + G(d) computed
vertical excitation of the synthesized dyes. The dye 3a showed
the blue shifted absorption in 1,4 dioxane (388 nm) and red
shifted absorption in chloroform (400 nm). In the case com-
pound 3a the deviation between experimental absorption and
vertical excitation in different solvents was ~30 nm. Similar
observations were observed for compound 3b, whereas less
deviation is observed between experimental absorption and
vertical excitation of the compounds 3c, 3d and 3e in ethyl
acetate, chloroform and 1,4 dioxane. In less polar solvent ethyl
acetate, the compounds 3c, 3d show 4 nm deviation whereas
3e shows 3 nm deviation in DMF, ACN, chloroform and tol-
uene (Table S6). The experimentally obtained fluorescence
emission spectral data and emission computed from TD-
B3LYP/6–311 + G(d) computations are shown in Tables 3
and Table S6. All the dyes showed an increase in the emission
wavelength increase in solvent polarity. A good correlation is
seen between experimentally recorded emissions against the
emissions computed from TD-B3LYP/6–311 + G(d) for the
synthesized dyes in the different solvents.
o
it was 0.8 , suggests the planar geometry of the compound
Fig. S3). The similar observation was observed for dye 3d
Fig. S4, Table S4, S5). In the case of dye 3e the dihedral angle
between the indole core and the acceptor part (C3-C11-C22-
C23) in GS and ES was 6.4 and 0.9 respectively which sup-
ports the planar geometry. The dihedral angle between the C22-
C23-C25-C27 was in GS and ES, −157.4 and −179.9° respec-
tively which supports the trans geometry of the double bond of
(
(
o
3
e (Fig. S5). In the excited state geometry, the major bond
lengthening was observed between the bonds C12-C11, C11-
C22, C22-C23, C25-C24 by 0.03, 0.05, 0.05 and 0.01 Å and
bond length shorten for the bonds N14-C12, C23-C25, C23-
C27 by 0.01, 0.02, 0.02 Å for the dye 3a (Fig. 6). Similar
observations are found for the dyes 3b-3e (Table S4). Such
lengthening and shortening of the bond lengths were due to
the donor π acceptor charge transfer present in the molecules.
Electronic Vertical Excitation Spectra (TD-DFT)
The computed vertical excitation spectra associated with their
oscillator strengths, composition, and assignments of the dyes
as well as the corresponding experimental absorption wave-
lengths of the dyes 3a to 3e are shown in Table 3 and Table S6.
The charge transfer band for donor π acceptor dyes are mainly
due to the electronic transition from highest occupied molec-
ular orbital (HOMO) to the lowest unoccupied molecular or-
bital (LUMO) Table 3 and Table S6 show that there is a
Frontier Molecular Orbitals
The contribution of electronic transition from HOMO to
LUMO is about 95 to 96 % in 3a and 97 % for 3b. The
contribution of electronic transition from HOMO to LUMO
is about 98 to 100 % in 3c, 3d and 3e. Therefore, we confirm

J Fluoresc
Table 3 Observed UV-visible absorption and vertical excitation of compound 3a-3b in different solvents
Solvent
Experimental
TD-DFT
a
b
c
d
e
f
λ
nm
λ nm
Vertical Exc nm
ev
f
Assignment
% D abs
TD-DFT Emission
% D ems
3
a
DMSO
DMF
398
398
394
395
398
397
400
390
391
388
398
393
389
387
391
387
392
393
386
384
384
387
443
442
444
444
443
436
436
442
440
433
443
444
441
439
443
433
436
437
434
432
433
437
360
361
359
358
359
360
360
358
359
357
359
358
358
357
356
357
357
357
356
357
354
356
3.4400
3.4376
3.4559
3.4595
3.4538
3.4444
3.4473
3.4589
3.4498
3.4697
3.4538
3.4624
3.4604
3.4778
3.4813
3.4768
3.4701
3.4759
3.4856
3.4760
3.5016
3.4862
0.6949
0.6979
0.6751
0.6705
0.6778
0.6912
0.6913
0.6749
0.6851
0.6767
0.6946
0.7398
0.7422
0.7228
0.7189
0.7248
0.7354
0.7343
0.7209
0.7299
0.7200
0.7354
H-L = 96 %
H-L = 96 %
H-L = 95 %
H-L = 95 %
H-L = 95 %
H-L = 96 %
H-L = 96 %
H-L = 96 %
H-L = 96 %
H-L = 96 %
H-L = 96 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
H-L = 97 %
9.5
9.3
8.9
9.4
9.8
9.3
10.0
8.2
8.2
8.0
9.8
8.9
8.0
7.8
8.9
7.8
9.3
9.2
7.8
7.0
7.8
8.0
423
422
422
423
422
422
423
423
423
428
427
409
409
409
409
409
409
408
408
408
405
405
4.5
4.5
4.9
4.7
4.7
3.2
3.0
4.3
3.8
1.2
3.6
7.9
7.2
6.8
7.6
5.5
6.2
6.6
6.0
5.6
6.5
7.2
ACN
MeOH
Acetone
DCM
CHCl
3
EA
THF
1
,4 dioxane
Toluene
DMSO
DMF
ACN
MeOH
Acetone
DCM
3
b
CHCl
3
EA
THF
1
,4 dioxane
Toluene
a
b
c
d
e
f
Experimental absorption wavelength
Experimental emission wavelength
Computed absorption wavelength
Oscillator strength
%
Deviation between experimental absorption and vertical excitation computed by DFT
Deviation between experimental emission and computed (TD-DFT) emission
%
that the electronic transitions of dyes originate from HOMO to
with the 6-31G(d) basis set. For completeness, we will here
briefly summarize the essential formulas used in our work,
highlighting the quantities in which we are interested.
Here, second-order NLO properties of the carbazole
based push pull chromophore 5a, 5b, 6a and 6b were
calculated by using density functional theory (DFT).
The static first hyperpolarizability (βo) and second
hyperpolarizability (γ) were calculated using B3LYP/6-
31G(d) on the basis of the finite field approach [68,
LUMO. As shown by molecular orbital diagrams (Table 4) of
the dyes, the electron densities in the HOMO of all these dyes
were largely located on donor indole unit and electron
densities on the LUMO were found localised on the acceptor
through carbon-carbon double bond. The HOMO levels of the
dyes are mainly dominated by π-orbital contribution of the
electron donor indole unit, while the LUMO levels are largely
delocalized at the electron acceptor part (Table 4).
7
1–73]. In the presence of an applied field, the energy
Linear Polarizability from DFT
of the system is a function of the electric field, and the
first hyperpolarizability is a third rank tensor that can be
described by a 3 × 3 × 3 matrix. The 27 components of
the 3D matrix can be reduced to 10 components because of the
Kleinman symmetry [74]. The matrix can be given in the
lower tetrahedral format. It is obvious that the lower part of
the 3 × 3 × 3 matrix is a tetrahedral. The components of β are
defined as the co-efficient in the Taylor series expansion of the
Theory to Calculate Nonlinear Optical (NLO) Properties
by Theoretical Method
In this work we have also calculated dipole moment (μ), elec-
tronic polarizability (α), mean first hyperpolarizability (β0)
and second hyperpolarizability (γ) using B3LYP functional

J Fluoresc
Table 4 Frontier molecular orbital’s of the compounds 3a to 3e in Acetonitrile
Dye
HOMO
LUMO
3a
3b
3c
3d
3e

J Fluoresc
energy in the external electric field. When the external
electric field is weak and homogeneous, this expansion
becomes.
The isotropic polarizability can be calculated from the trace
of the polarization tensor,
À
Á
a þ a þ a
xx
yy
zz
α0 ¼
ð6Þ
1
1
0
3
E ¼ E −μ F −
−
þ ⋅⋅⋅
ð4Þ
a
a
2
aaβ Fa Fβ 6βaβγ Fa Fβ Fγ
Anisotropy of the polarizability Δα is expressed by
Where E° is the energy of the unperturbed molecules, F is
α
.
h
i
the field at the origin, μ , α and βαβγ are the components of
−1 2 À
Á
α
αβ
2
2
2
xx
Δα ¼ 2
axx þ ayy þ ðazz þ axxÞ þ 6α
ð7Þ
dipole moment, polarizability and the first hyperpolarizabilities,
respectively.
The total static dipole moment μ is expressed by following
equation
The mean first polarizability (β ) is expressed by
0
.
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢀ
ꢁ
1
2
2
2
2
2
x
2
y
2
z
μ ¼ μ þ μ þ μ
ð5Þ
β ¼ β þ β þ β
ð8Þ
ð9Þ
x
y
z
0
h
i
1
2
À
Á
À
Á
À
Á
2
2
2
β ¼ βxxx þ βxyy þ βxzz þ β þ βyyy þ βyzz þ β þ βzyy þ βzzz
0
yxx
zxx
Where, β , β , and β are the components of the second-
Second Hyperpolarizability (γ) by DFT
x
y
z
order polarizability tensor along the x, y, and z axes.
The mean second hyperpolarazibility (γ) is expressed by
The donor-π-acceptor styryls show very high γ values, both in
DFTas well as the values obtained from solvatochromic meth-
hꢀ
ꢁ
ꢀ
ꢁi
1
5
−36
γ ¼
γxxxx þ γyyyy þ γzzzz þ 2 γ
þ γyyzz þ γzzxx
od. The styryl shows γ values in the range of 33.62 × 10 to
xxyy
−36
4
50.35 × 10 esu. by DFT (Table S8).
ð10Þ
Experimental Hyperpolarizability
Thus, these donor-π-acceptor chromophores have been
studied due to their good linear and nonlinear optical proper-
ties. Second-order NLO properties of the donor-π-acceptor
chromophores were calculated by using B3LYP/6–311 +
G(d) on the basis of the finite field approach [75]. The com-
The solvent dependent NLO properties of donor-π-acceptor
styryls dyes were calculated [73, 76] The linear polarizability
(a) value of extended styryls 3a-e are tabulated in (Table S3).
The first hyperpolarizability value of 3a - 3e are found in the
puted mean polarizability (α ), of the dyes 3a-e were found to
0
−
30
−30
−
24
−24
range of 5.68 × 10 to 23.0 × 10 e.s.u. These values are
be ranging from 29.86 × 10
Table S7).
to 67.73 × 10
e.s.u
−30
greater than urea (0.38 × 10 esu).
(
The third order hyperpolarizability 〈γ〉SD at molecular level
originating from the electronic polarization in the non-
First Hyperpolarizability (β ) by DFT
o
Table 5 Total first order hyperpolarizability (β
B3LYP/6–311 + G(d) for compound 3a-e
o
) calculated with
The donor –π- acceptor chromophores are expected to have
good non-linear properties and their first hyperpolarizability
Solvent
3a
3b
3c
3d
3e
(βo) value can be enhanced due to their relative orientation.
The computed first hyperpolarizability (β ), of the dyes 3a-e
o
DCM
–
41.19
36.86
38.72
40.14
–
25.88
20.59
22.82
24.56
–
77.88
66.64
110.31
74.33
168.41
144.31
154.45
162.42
104.59
–
−
30
were found to be ranging from 12.64 × 10
to
CHCl
3
–
30
1
68.41 × 10− e.s.u in the solvents (Table 5). These values
EA
32.33
33.74
22.73
23.57
12.83
−30
are greater than urea 0.38 × 10 e.s.u. Thus these dyes show
a large hyperpolarizability, suggesting considerable charge
transfer characteristics from the donor to acceptor part. In
polar solvents more hyperpolarability was observed then less
polar solvents may be due to the effective π-π* transition in
polar solvents.
THF
1
,4 dioxane
Toluene
Gas
–
12.64
22.23
50.57
29.77
18.60
58.92
30
Values in × 10− esu

J Fluoresc
5
.
.
Huang JY, Lewis a, Loew L (1988) Nonlinear optical properties of
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resonant region can be treated by a three-level model and is
expressed in literature [77–81]. The Quasi-two-level model in
place of three level model using the density matrix formalism
to a simpler Eq. (11) [82, 83].
/S0006–3495(88)83147–3
6
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doi:10.1016/0143-7208(85)80018-8
ꢀ
ꢁ
1
2
eg
2
2
eg
<
γ >¼ E
μ
Δμ −Δμ
ð11Þ
3
7. Hagberg DP, Yum JH, Lee H, et al. (2008) Novel near-infrared
cyanine dyes for fluorescence imaging in biological systems. J
Am Chem Soc 130:6259–6266. doi:10.1021/ja800066y
eg
The third order hyperpolarizability that may be termed as
8
.
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Lee CC, Hu AT (2003) Synthesis and optical recording properties
of some novel styryl dyes for DVD-R. Dyes Pigments 59:63–69.
doi:10.1016/S0143-7208(03)00098-6
the Bsolvatochromic descriptor^ and the values calculated are
tabulated in Table S9. The styryl dye 3e shows more third
order hyperpolarizability 〈γ〉 than other indole styryl dye
may be due to extended conjugation present in the molecule.
9
.
1
0. Yashchuk VM, Yarmoluk SM, Kudrya VY, et al. (2008) Optical
Biomedical Diagnostics: Sensors with Optical Response Based on
Two-Photon Excited Luminescent Dyes for Biomolecules
Detection. Adv Opt Technol 2008:1–11. doi:10.1155/2008/908246
Conclusion
1
1. Park KH, Lee CJ, Song D, et al. (2001) Optical recording properties
In this paper we have designed and synthesized novel
donor-π- acceptor styryl dyes containing indole as electron
donor and –CN as an electron acceptor group. The
photophysical properties are depends on solvent polarity.
The absorption of the prepared dyes were in the region of
of styryl derivatives for digital versatile disc-recordable (DVD-R).
Mol Cryst Liq Cryst Sci Technol Sect A Mol Cryst Liq Cryst 370:
1
65–168. doi:10.1080/10587250108030062
1
2. Sokołowska J, Podsiadły R, Stoczkiewicz J (2008) Styryl dyes as
new photoinitiators for free radical polymerization. Dyes Pigments
77:510–514. doi:10.1016/j.dyepig.2006.11.011
3
97–469 nm and emission in region of 432 nm to 554 nm.
13. Haidekker M, Ling T, Anglo M, et al. (2001) New fluorescent
probes for the measurement of cell membrane viscosity. Chem
Biol 8:123–131. doi:10.1016/S1074-5521(00)90061-9
Due to the extended conjugation present in the dye 3e, red
shifted absorption and emission was observed as compare to
dye 3a, 3b, 3c and 3d. The fluorescence spectra of indole
styryl particularly 3a, 3b and 3d in solid state was red shifted
as compared to their fluorescence emission in less polar sol-
vents. From the theoretical study it is observed that, the dyes
1
4. Metsov S, Simov D, Stoyanov S, Nikolov P (1990) Photophysical
characteristics of some 2-styrylindolium dyes. Dyes Pigments 13:
11–19. doi:10.1016/0143-7208(90)80009-E
1
5. Kim SD, Park EJ (2001) Relation between chemical structure of
yellow disperse dyes and their lightfastness. Fibers Polym 2:159–
1
63. doi:10.1007/BF02875330
showed more first hyperpolarability (β ) in polar solvents
o
16. Chung S-J, Zheng S, Odani T, et al. (2006) Extended squaraine dyes
with large two-photon absorption cross-sections. J Am Chem Soc
than less polar solvents may be due to the effective π-π*
transition in polar solvents. The vertical excitation spectra
were computed at B3LYP/6–311 + G(d) level and compared
with the experimental values. The calculated results are in
good accordance with the experimental values.
1
28:14444–14445. doi:10.1021/ja065556m
1
1
1
2
7. Lee H-S, Kim S-K, Lee W-J, et al. (2001) Synthesis and properties
of organic light-emitting diodes using new emissive materials. Mol
Cryst Liq Cryst Sci Technol Sect A Mol Cryst Liq Cryst 370:39–42.
doi:10.1080/10587250108030034
8. Fei X, Gu Y, Li C, et al. (2013) Synthesis and spectra of a kind of
novel longer-wavelength benzoxazole indole styryl cyanine dye
with a carbazole-bridged chain. J Fluoresc 23:221–226.
doi:10.1007/s10895-012-1137-y
Acknowledgments Santosh Chemate (JRF/SRF) is thankful for fel-
lowship from the Principal Scientific Adviser (PSA), Government of
India.
9. Babür B, Seferoğlu N, Aktan E, et al. (2014) Phenylazoindole dyes
2
: the molecular structure characterizations of new phenylazo in-
doles derived from 1,2-dimethylindole. Dyes Pigments 103:62–70.
doi:10.1016/j.dyepig.2013.11.019
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