Influence of thiophene spacer and auxiliary acceptor on the optical properties of 4-(Diethylamino)-2-hydroxybenzaldehyde based D-π-A-π-D Colorants with N-alkyl donors: Experimental, DFT and Z-scan study

Article abstract of DOI:10.1016/j.jphotochem.2018.12.035

We report here design and synthesis of four new D-π-A-π-D type dyes (4 and 5) with triphenylamine/N-ethyl carbazole as fixed rotor/donor and N, N-diethyl amine as a secondary donor having dicyanovinylene as a common acceptor. The Influence of π-linker (thiophene) along with an auxillary cyano group on the nonlinear and linear optical properties is investigated. Molecules with thiophene spacer (4b and 5b) exhibit progressive variation in their absorption and emission profiles reflecting the effect of π-linker, compared to those devoid of thiophene spacer (4a and 5a). The dyes display solvent dependent emission [e.g. 4b (toluene = 689 nm) and 5b (DMSO = 795 nm)] suggesting a more polarized photo-excited state arising as a result of enhanced intramolecular charge transfer in the excited state. The solvatochromic nature is supportive of a general solvent effect, which is confirmed by solvent polarity plots (Weller's and Rettig's plot). The trends in the optical properties were also validated by time dependent density functional theory computations which further reveals charge transfer (from secondary donor to acceptor) for the long wavelength absorption. Dye with the highest charge transfer dipole moment relatively has the maximum two-photon absorption cross-section area (4b = 246–399 GM) which was established using theoretical two-level model. Non-linear optical properties were investigated using solvatochromic two-level model and computationally using global and range separated hybrid functionals. The dyes show high first-order hyperpolarizability (β₀) in the range of 191–1162 × 10^?30 e.s.u. and second-order hyperpolarizability (γ₀) in the range of 491–6,704 × 10^?36 e.s.u. Z-scan results show 4a (3.79 × 10^?13 e.s.u.) possesses a higher value of third order nonlinear susceptibility in DMSO. Thus, the dyes can be considered as promising candidates for NLO application in material science.

Full text of DOI:10.1016/j.jphotochem.2018.12.035

Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Influence of thiophene spacer and auxiliary acceptor on the optical
properties of 4-(Diethylamino)-2-hydroxybenzaldehyde based D-π-A-π-D
Colorants with N-alkyl donors: Experimental, DFT and Z-scan study
a
a
b
b
b
Manish M. Raikwar , Dinesh S. Patil , Elizabeth Mathew , Manu Varghese , Issac H. Joe ,
Nagaiyan Sekar
a
a,⁎
Department of Dyestuff Technology, Institute of Chemical Technology, (Formerly UDCT), Nathalal Parekh Marg, Matunga, Mumbai, 400019, India
Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruvananthapuram, Kerala 695015, India
b
A R T I C L E I N F O
A B S T R A C T
Keywords:
We report here design and synthesis of four new D-π-A-π-D type dyes (4 and 5) with triphenylamine/N-ethyl
carbazole as fixed rotor/donor and N, N-diethyl amine as a secondary donor having dicyanovinylene as a
common acceptor. The Influence of π-linker (thiophene) along with an auxillary cyano group on the nonlinear
and linear optical properties is investigated. Molecules with thiophene spacer (4b and 5b) exhibit progressive
variation in their absorption and emission profiles reflecting the effect of π-linker, compared to those devoid of
thiophene spacer (4a and 5a). The dyes display solvent dependent emission [e.g. 4b (toluene = 689 nm) and 5b
Thiophene
N-ethyl carbazole
Triphenylamine
Dicyano vinylene
Cyano group
DFT
(DMSO = 795 nm)] suggesting a more polarized photo-excited state arising as a result of enhanced in-
NLO
tramolecular charge transfer in the excited state. The solvatochromic nature is supportive of a general solvent
effect, which is confirmed by solvent polarity plots (Weller’s and Rettig’s plot). The trends in the optical
properties were also validated by time dependent density functional theory computations which further reveals
charge transfer (from secondary donor to acceptor) for the long wavelength absorption. Dye with the highest
charge transfer dipole moment relatively has the maximum two-photon absorption cross-section area
(
4b = 246–399 GM) which was established using theoretical two-level model. Non-linear optical properties
were investigated using solvatochromic two-level model and computationally using global and range separated
−
30
hybrid functionals. The dyes show high first-order hyperpolarizability (β
e.s.u. and second-order hyperpolarizability (γ ) in the range of 491–6,704 × 10
e.s.u.) possesses a higher value of third order nonlinear susceptibility in DMSO. Thus, the dyes
can be considered as promising candidates for NLO application in material science.
0
) in the range of 191–1162 × 10
−
36
0
e.s.u. Z-scan results show 4a
−
13
(3.79 × 10
1
. Introduction
A arrangement, recent trend includes quadrupolar (D-π-A-π-D or A-π-D-
π-A) [9–12] and octupolar (tripodal) (D-π-) A or (A-π-) D [13–15]
3
3
“
Push-pull” donor-acceptor (D-A) organic molecules with red-shifted
systems. Molecules with D-π-A-π-D framework receive interest as or-
ganic electronic materials, as a result of effective ICT and improved
light absorbance [16–20]. Thus, materials possessing ICT give a broad
range of absorption spectra as a result of the reduced band gap. This can
be achieved by tuning the HOMO and LUMO via a suitable choice of
donor and acceptor units [21,22]. In this way, tuning the donor or
acceptor in the push-pull system would modify the physical and che-
mical properties [1,23,24].
emission are the most important conjugated systems and attract sig-
nificant academic and industrial research interest [1]. The electron
donating and withdrawing groups in push-pull systems are connected
through a π-conjugated linker. The interaction between the donor and
acceptor through the π-conjugation on photoexcitation facilitate in-
tramolecular charge transfer (ICT) in such systems [2]. Thus, experi-
mental and theoretical investigation of photophysical properties of such
push-pull systems accompanying ICT is of prime focus [3–5]. Factors
such as rigidity of the media, polarity, pH, viscosity, and voltage alter
the optical properties of push-pull systems [6–8]. Beside the linear D-π-
Molecules with push-pull framework serve as electroactive and
photoactive materials in molecular electronics, such as nonlinear op-
tical (NLO) material [25–27], organic light-emitting diodes (OLED’s)
⁎
Corresponding author.
E-mail address: n.sekar@ictmumbai.edu.in (N. Sekar).
https://doi.org/10.1016/j.jphotochem.2018.12.035
Received 12 November 2018; Received in revised form 27 December 2018; Accepted 28 December 2018
Available online 29 December 2018
1010-6030/ © 2018 Elsevier B.V. All rights reserved.

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
[
[
28–30], organic thin film transistors [31–33], chemical sensors
34,35] and dye-sensitized solar cells (DSSC) [36–38]. Over the years,
triphenylamine [74–79]. On the other hand carbazole, a rigid hetero-
cyclic donor unit occurring in numerous fluorescent colorants is iso-
electronic to diphenylamine, but it has a planar structure and can be
regarded as bonded diphenylamine [80]. Molecules containing carba-
zole and its N-alkylated derivatives unit are electron rich and exhibit
excellent CT [81,82]. This outstanding property of triphenylamine and
N-ethyl carbazole marks it as a suitable fixed primary donor for our
study. Dicyanovinylene group is used as an acceptor unit, and offers
several advantages like providing high structural anisotropy and im-
proved photophysical and electrochemical properties [83–87]. We ex-
pect these dyes to show enhanced photophysical and NLO properties
when replaced with the secondary donor. Photophysical properties of
these dyes were investigated in solvents of increasing polarity. Further,
their geometrical, photophysical and NLO properties were investigated
using DFT and TD-DFT computations.
push-pull organic molecules with highly delocalized and polarizable π-
electrons are utilized as second and third-order NLO materials [39–43].
This is because organic molecules used for NLO applications unlike
inorganic molecules offer numerous advantages like well-defined
structure, ease of synthesis, easily tunability, and large nonlinear op-
tical responses [9]. Based on the symmetry of the molecule they can be
classified as dipolar (D-π-A) [44–47], quadrupolar (D-π-A-π-D or A-π-D-
π-A) [48–53] or octupolar systems [54–58] and are widely investigated
for their applications in nonlinear optics. The NLO activity of such
push-pull system is related to ICT arising from the transfer of electron
cloud from donor to acceptor through a π-conjugated network. Thus,
increasing the donor-acceptor strengths result in enhanced nonlinearity
[
51]. Therefore, designing new systems with excellent CT plays a vital
role [59]. Also, the CT in such systems describes low energy physics of
these molecules and is also responsible for low-energy excitations with
the large transition and/or mesomeric dipole moments [60]. So the
push-pull molecular system with high first hyperpolarizability (β)
possess increased dipole moment [61]. Hence, quadrupolar molecules
with D-π-A-π-D framework exhibit third-order NLO properties
2. Experimental
2.1. Materials and equipments
[
9,62,63] as well as second-order NLO properties [62,64–67].
The materials were procured from commercial sources; N-ethyl
carbazole (Sigma Aldrich) and the remaining from S.D. Fine Chemicals
(India) and used as supplied without any further purification. The sol-
vents were of spectroscopic grade procured from Merck. Refer the
Supporting Information for the details of experiments. The reactions
were monitored by thin layered chromatography TLC using 0.25 mm E-
Merck silica gel 60 F254 percolated plates, which were visualized under
Herein, taking into consideration all the above-mentioned facts, we
report the design and synthesis of four fluorescent styryl dyes with the
D-π-A-π-D framework (Fig. 1). The strategy involved in this study is to
keep one rotor/donor fixed as triphenylamine/N-ethyl carbazole while
varying the secondary donor having different π-conjugated network
and acceptor as dicyano vinylene group so as to ascertain the effect of
1
13
the varying secondary donor (D
2
) on the photophysics and photo-
UV light. H NMR and C NMR spectra were recorded on Varian 500-
MHz instrument (USA) using CDCl solvent and TMS as an internal
chemistry of the dyes. Triphenylamine has electron donating ability and
CT characteristic arising from the co-planarity between the central ni-
trogen atom and the three phenyl rings which allow an uninterrupted
flow of the lone pair with the ring thereby increasing the π-electron
density of the system. Hence, triphenylamine enhances the electron
donating ability and the extent of π-delocalization [68–73]. Introduc-
tion of an electron accepting group enhances the ICT capabilities of
3
standard at 25 °C. The chemical shifts were reported in parts per million
(ppm) relative to the internal standard as tetramethylsilane (TMS). The
absorption spectra were recorded on Perkin Elmer Lambda 25 UV–vis
spectrophotometer and emission spectra on Varian Inc. Cary Eclipse
spectrofluorometer.
Fig. 1. Chemical structures of the dyes investigated.
46

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
2.2. Computational methods
with malononitrile in the presence of ammonium acetate yielded in
a b
intermediates 3a and 3b [89]. Aldehydes R and R were prepared
The molecular geometries of the dyes were optimized using DFT
according to the procedure described in the literature [90,91]. Final
computations utilizing the Gaussian 09 program [1]. The ground state
) geometry of the dyes was optimized in the solvent environment of
condensation of aldehydes with intermediates 3a and 3b resulted in
1
(
S
0
dyes 4a, 4b, 5a, and 5b. The dyes were thoroughly characterized by H
varying polarity using self-consistent reaction field (SCRF) incorporated
in polarizable continuum model (PCM) [2,3] as implemented in Gaus-
sian 09 [4]. The functional used was B3LYP, it is the method that
combines Becke’s three parameter exchange functional (B3) [5] with
the nonlocal correlation functional by Lee, Yang and Parr (LYP) [6].
Basis set with both diffuse and polarization functions, 6-311++G(d,p)
was used for all atoms [7]. The excited state optimization was carried
out using time-dependent density functional theory (TD-DFT). The ex-
citation energies, oscillator strengths and orbital contribution for the
lowest 10 singlet-singlet transitions at the optimized geometry were
obtained by TD-DFT [8–10] calculations using the same basis set as for
the geometry minimization. The solvents used were toluene, di-
chloromethane (DCM), ethyl acetate (EtOAc), acetone, N, N-dimethyl
formamide (DMF), dimethyl sulfoxide (DMSO). The NLO parameters
were calculated using two global hybrid (B3LYP and BHHLYP) and four
range separated hybrid functional (CAM-B3LYP, ωB97, ωB97X and
ωB97XD) with the basis set 6-311++G(d,p) for comparison.
and 13C nuclear magnetic resonance (NMR) spectroscopy, elemental
analysis and HRMS, and the spectral data are consistent with the pro-
posed structure. The modified procedures for the intermediates along
with the analytical characterization is given in supporting information.
3.2. Photophysical properties
Fig. 2 displays the absorption spectra of the four hybrid dyes (4 and
5
) recorded in DCM and the relevant data are listed in Table 1. All the
dyes exhibit two distinctive absorption bands, one at shorter wave-
length arising as a result of localized π–π* transition from triphenyla-
mine/carbazole and other assignable to the delocalized π–π* transition
or ICT from the conjugated segment comprising D donor, π-linker and
2
dicyano vinylene units. As the π-conjugation length and electron rich-
ness within the molecular skeleton is strengthened from 4a (512 nm) to
4b (557 nm) a bathochromic shift is observed in absorption wave-
length. Introduction of the thiophene spacer (π-linker) in the conjuga-
tion pathway is found to red shift the absorption profile of 4b due to its
coplanar arrangement [92]. Likewise, incorporating an additional –CN
3. Results and discussion
(
electron withdrawing) group into the conjugation bridge can be ben-
3
.1. Design and synthesis
eficial to the ICT [93]. Hence, a bathochromic shift of 45 nm is observed
for 4b when compared to 4a. Similarly, the introduction of thiophene
spacer in 5b (570 nm), compared to 5a (504 nm), results in a bath-
ochromic shift of ca. 66 nm attributable to the increase in the electron
richness of the π-linker and auxochromic nature of cyano group. Thus,
dyes 4b and 5b owing to its electron rich and rigid π-linker displays
absorption band at 557 and 570 nm with a red shift of 45 and 66 nm,
respectively, compared to 4a and 5a. The influence of electron rich and
rigidified π-linker is also displayed in the molar extinction coefficients
of the absorption (Table 1).
Four dyes: 4a, 4b, 5a, and 5b were synthesized to study the effect of
the different secondary donor (with and without the thiophene spacer
in the conjugation pathway having an –CN group) on their photo-
physical and nonlinear optical properties. Scheme 1 represents the
synthetic approach for the synthesis of dyes. The intermediates 2a and
2
b were synthesized by the acylation of triphenylamine and N-ethyl
carbazole by slightly modifying the procedure as described in the lit-
erature [88]. Knoevenagel condensation of intermediates 2a and 2b
Scheme 1. Synthesis of dyes 4 and 5: (a) Acetyl chloride, ZnCl
a-b), EtOH, piperidine, RT overnight.
2
, DCM, reflux, 5 h. (b) Malononitrile, acetic acid, ammonium acetate, 110 °C, overnight. (c) Aldehydes
(R
47

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
Fig. 2. Normalized absorption and emission spectra of the dyes 4 and 5 in DCM.
Table 1
moderate effect of the solvent polarity, representing inappreciable ICT
transitions in their ground state. On the other hand, the dyes displayed
positive solvatochromism in their emission profiles with the increasing
solvent polarity from toluene to DMSO. The vibronic feature observed
for 4a and 5a in a non-polar solvent like toluene become broad and red-
shifted in polar solvents, indicating the transformation of primarily
formed non-polar local excited (LE) state into a polar excited state in a
polar medium. The red shifted featureless emission profiles observed for
the dyes in polar solvents is accredited to the structural reorganization
or CT induced electronic perturbations in the excited state [94]. In
polar solvents, the large Stokes shifts observed further suggest the
presence of a polar excited state. Thus, from the above results, the
positive solvatochromism observed for the dyes is accredited to the
a
Spectroscopic data and photophysical properties of the dyes 4 and 5 in DCM. .
b
c
max x 104
dFWHM
e
f
gf
h
Dyes
λ
abs
ε
λ
em
Δν
Φ
F
(M cm−1)
−
1
nm(cm−1)
(nm)
nm(cm−1)
(nm)
4
4
5
5
a
b
a
b
512
557
504
570
8.01
6.58
10.80
3.05
95(3812)
193(7038)
74(3191)
165(4756)
595
750
583
742
83(2725)
193(4619)
79(2689)
172(4067)
1.6267
1.3213
0.7928
0.3742
0.178
0.027
0.121
0.019
a
Analysis was carried out at room temperature (25°C).
b
c
d
e
f
Absorption maxima (nm).
Molar extinction coefficient maxima (mol−1 cm−1).
Full-width half maxima of the absorption band.
Emission maxima.
Stokes Shift (nm) cm−1.
2
photoinduced ICT from D donors to the dicyanovinylene acceptor. The
g
h
CT in push-pull organic molecules can be determined taking into ac-
count the degree of bathochromic shift in emission profiles from non-
polar to polar solvents (ΔλTOL–DMSO) [94]. 4b and 5b containing thio-
phene unit in the conjugation pathway along with –CN group exhibited
a large degree of bathochromic shift (∼106 − 110 nm) compared to 4a
and 5a (∼52 nm) (Table S1), signifying pronounced ICT character in
the former dyes.
Oscillator strength.
Fluorescence quantum yield.
The emission spectra of the dyes recorded in DCM are displayed in
Fig. 2 and the relevant data collected in Table 1. The emission maxima
of the dyes followed the trend similar to that of absorption spectra
within the set. The emission maxima for 4a is observed at 595 nm,
whereas for 4b containing a thiophene spacer in the conjugation
pathway along with –CN group is observed at 750 nm. Similarly, the
emission maxima for 5a and 5b are observed at 583 and 742 nm re-
spectively. The emission maxima of 4b and 5b are red shifted by ca.
As observed from Table S1, the molar extinction coefficient values
rely on the donor-acceptor strength, thus 4b and 5b (4.10 and
4
−1
−1
2
ε
.80 × 10 M cm ) with strong donor-acceptor strength have lower
max compared to 4a and 5a (7.21 and 9.45 × 104
M
−1
cm ) in to-
−1
luene. Dyes 4b and 5b show the larger value of FWHM than the re-
maining dyes. FWHM for the dyes increases with increasing solvent
polarity. This higher molar extinction coefficients (ε) for 4 and 5 than
the parent compound (triphenylamine and N-ethyl carbazole) in each
case along with the large Stoke Shifts, make them ideal molecular
1
55 and 159 nm respectively. The comparatively broad emission ob-
served for 4b and 5b, suggesting a more pronounced redistribution of
electron density in the excited state, than in 4a and 5a. Thus, the ad-
dition of thiophene spacer in the conjugation pathway along with –CN
group into the conjugation bridges, was found to have a significant
effect on the emission profile of 4b and 5b when compared to 4a and
F
probes. Fluorescence quantum yields (Φ ) were calculated in different
solvents using the reference standard Fluorescein for 4a and 5a,
whereas for 4b and 5b cresyl violet was used. The quantum yields for
the dyes decreases with the increase in the polarity solvents indicating
that non-radiative relaxation pathway is more preferred in polar sol-
vents than in non-polar solvents.
5
a. Thereby illustrating the involvement of auxiliary chromophores in
excited state properties. The excited state is speculated to assume a
more planar structure which is also supported by the optimized excited
state geometries of the dyes (Fig. 4), thus giving room for an extension
of π-conjugation throughout the molecule. The large Stokes shift ob-
−1
served for 4b and 5b (4619 − 4067 cm ) when compared to that of 4a
−
1
and 5a (2725 − 2689 cm ) further supports this observation. A large
Stokes shift, in general, is indicative of significant structural reor-
ientation or polarized excited state compared to its ground [94].
Solvatochromism study was performed in different solvents of
varying polarity ranging from non-polar toluene to polar DMSO so as to
explicate the response of ground and excited states of the dyes [95]. The
absorption and emission spectra of the dyes recorded in different sol-
vents are displayed in Fig. S1 and the corresponding photophysical data
are listed in Tables S1. The absorption profile of the dyes shows less to
3
.3. Solvent polarity plots and dipole moment
The effect of solvent polarity on the optical properties of the dyes in
the ground and excited sates was further confirmed by correlation of
emission maxima intensities (λ ) in cm
larizability (Δf) of the molecule using Weller [96] and Rettig [96,97]
plots. The Weller and Rettig plots for the dyes are shown in Fig. 3,
which displayed a stronger correlations, indicating that stabilization of
the excited state is governed by general dye–solvent interactions. The
em
−1
versus the orientation po-
max
48

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
Fig. 3. Weller and Rettig’s plots for 4 and 5 in different solvents. The line corresponds to the best linear fit to the data.
linear relationship obtained for the dyes over the entire solvent range
with the regression coefficient values adjacent to unity, indicates a CT
excited state for these dyes.
2
the peripheral chromophore (D segment), π-spacer and the dicyano-
vinylene are in the range of 2 to 7°, which suggests no major twisting in
this segment of the conjugation framework. Indicating that the electron
Table S2 lists the ratio of excited state dipole moment to the ground
state dipole moment for the dyes calculated using Bilot-Kawshi [98,99],
Bakshiev [100] and Liptay [101] plots to examine the effect of solvent
polarity on the electronic structures of the synthesized dyes. From Table
S2, the ratio of excited state dipole moment to the ground state dipole
donation takes place from the D
2
donor segment rather than the fixed
D
1
donor segments, as it is more twisted. The bond lengths between
donor nitrogen and the adjacent carbon of aryl group in toluene are 4a
(N46-C27) = 1.3688, 4b (N54-C50) = 1.3695, 5a (N46-C43) = 1.3740
and 5b (N59-C56) = 1.3709 Å, whereas the bond lengths in DMSO are
4a (N46-C27) = 1.3667, 4b (N54-C50) = 1.36782, 5a (N46-
C43) = 1.3672 and 5b (N59-C56) = 1.3646 Å. Moreover, from Table
S6 we see that there are consecutive increase and decrease in the bond
lengths of all the dyes suggesting CT characteristics from donor to the
acceptor. Also, in the dyes, as we move from non-polar to polar solvent
the bond shortening observed suggesting CT characteristics to be more
prominent in a polar solvent. Similarly, other bond lengths between the
secondary donor and acceptor also show band shortening in DMSO as
compared to toluene (Table S6).
moment (μ
e g
/μ ) for 4a, 4b, 5a, and 5b (1.02, 1.10, 1.23 and 3.76 re-
spectively) in each case is found to be more than unity. This explains
that the excited state is more polar than the ground state accounting for
CT in the excited state in each dye. This is also supported by the red-
shifted absorption and emission with the increase in solvent polarity.
Furthermore, the concept resulting from the dielectric polarization
[
102] can be used to quantitatively express the solvent polarity de-
pendent emission property, indicating that the spectral deviations of the
fluorescence on increasing the solvent polarity are influenced by the
change in permanent dipole moments between ground (μ
) states. The difference in the dipole moment between the ground
and excited states i.e. Δμ = (μ - μ ) were evaluated by the equations
S1) and (S2) are listed in Table S3. Onsager radii ( ) an essential
g
) and excited
The lowest-energy transitions derived from the theoretical calcula-
tions using CAM-B3LYP/6-311++G(d,p) level of theory, along with
their oscillator strengths and the compositions in terms of molecular
orbitals are listed in Table 2, whereas for B3LYP/6-311++G(d,p) are
listed in Table S7. Fig. 5 shows the electronic distribution of the frontier
molecular orbitals [highest occupied molecular orbital (HOMO, HOMO-
1) and lowest unoccupied molecular orbital (LUMO, LUMO + 1)] of the
(
e
μ
e
g
(
a
o
parameter required to solve the above equations were evaluated using
B3LYP/6-311++G(d,p) level of theory and are listed in Table S4. As
seen from Table S3, Δμ for the dyes are in the range of 3–16 Debye, with
4b and 5b displaying larger dipole moment difference in both the cases
dyes. The HOMOs of 4a and 5a largely comprises of the D
slightly of dicyano vinylene unit. However, the HOMOs of 4b and 5b
solely comprises of the D segment. Further, the introduction of thio-
2
segment and
when compared to 4a and 5a. The Δμ for the dyes follows the order 4b
16.23) > 5b (12.16) > 4a (9.19) > 5a (4.96) in both the cases. The
larger perturbation in the molecular structure of 4b and 5b due to its
free rotating phenyl ring, as well as the fixed D donors and acceptor,
(
2
phene unit in the conjugation pathway along with –CN group in dyes 4b
and 5b results in an extension of the HOMO into these units. The LUMO
in 4a and 5a largely resides over the acceptor group i.e. dicyano vi-
nylene, whereas in 4b and 5b it mainly resides over the acceptor. The
1
makes the molecule vibronically disconcerted [103] leading to a large
difference in dipole moment. Thus, ICT in 4 and 5 is supported by the
major difference in dipole moment from the ground to excited state.
LUMO + 1 in 4a and 5a appears to reside solely over the D
whereas in 4b and 5b it is largely scattered over the D segment. The
HOMO-1 for 4a, 4b, and 5a is localized over the D segment, whereas
1
segments,
2
1
3.4. Electronic transitions, Frontier molecular orbitals (FMO) and
for 5b it is largely scattered over the thiophene and dicyano vinylene
units. The prominent absorption in the longer-wavelength region had
major contributions from the HOMO→LUMO electronic excitations and
negligibly from and HOMO-1→LUMO electronic excitations. The
HOMO→LUMO electronic excitation would result in migration of the
charge from the donor to the acceptor, [104,105] while the HOMO-1→
LUMO transition indicated a π–π* character. Delocalization of the
LUMO over the thiophene unit in the conjugation pathway along with
Molecular electrostatic potential (MEP) surface plots
In order to have an insight into the electronic structure of the dyes,
the geometries of the dyes were first optimized by density functional
theory (DFT) using the B3LYP hybrid functional and the 6-311+
+G(d,p) basis set. The optimized geometries of 4 and 5 in DMSO are
shown in Fig. 4, while their dihedral angle and the relative bond length
are given in Table S5 and S6. The optimized structures assume a non-
–CN group in 4b and 5b justifies their longer-wavelength absorptions.
planar arrangement of the auxiliary chromophore at the D
segment, as depicted in Fig. 4. The nonplanarity of the D segments in
the dyes can be highlighted in terms of the large dihedral angle
38–52°) between the triphenylamine/N-ethylcarbazole and the dicya-
novinylene planes (Table S5). For the dyes, the dihedral angles between
1
donor
The virtual HOMO – LUMO band gap further supports the trend ob-
tained in CT for these dyes.
1
The trend in the CT characteristic is further supported by the HOMO
(
–
LUMO energy level diagram (Fig. S2), which gives a quantitative
49

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
Fig. 4. Optimized S
0
(a) and S
1
(b) geometries of 4 and 5 in DMSO at the B3LYP/6-311++G(d,p) level. Note: C atoms = Black color, H atoms = green color, N
atoms = Blue color, O atoms = Red color and S atoms = Yellow color (For interpretation of the references to colour in this figure legend, the reader is referred to the
web version of this article).
insight into the electronic structure and excitation properties of such D-
A system. From Fig. S2, the energy band gap increases in the order
evaluated from the total SCF density [isovalue = 0.001 a.u.; (mapped
with ESP)] and the total densities observed are ± 0.102, ± 0.097, ±
0.109 and ± 0.105 respectively. The red color represents the acceptor
group (nucleophilic character) and blue color represents the donor
group (electrophilic character). Thus, from the above implications, it is
established that these dyes have significant CT characteristics.
4b < 5b < 4a < 5a, with dyes 4a and 5a showing a large energy gap
(
HOMO – LUMO energy difference) while, 4b and 5b showing a small
energy gap which can be accredited to increased D-A strength via ex-
tensive π-conjugation resulting in excellent CT characteristics [106].
The lowering of energy band gap can be accomplished by the proper
substitution of donor and acceptor for efficient CT within the dyes.
MEPS is another tool which gives a qualitative indication of CT from
donor to acceptor [107]. The MEPS plots for the dyes (Fig. S3) were
50

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
Table 2
Table 3
Computed electronic parameters for dyes in DCM by TD-DFT (CAM-B3LYP/6-
Computationally measured static first hyperpolarizability of all the dyes in non-
polar and polar solvents using various functionals with 6–311++G(d,p) basis
set. .
311++G(d,p) level of theory).
a
CAM-B3LYP
Dyes Solvent
B3LYP BHHLYP CAM-B3LYP ωB97 ωB97X ωB97XD
a
b
cE
(eV)
df
eT
fOC (%)
Dyes
λ
abs
λ
abs
(nm)
(nm)
4a
4b
5a
Toluene 351
DMSO 418
Toluene 1,481
DMSO 1,816
Toluene 197
DMSO 388
Toluene 905
DMSO 2,193
289
329
404
996
1192
195
401
650
1470
345
417
727
859
188
405
502
963
330
412
771
912
188
401
527
1,017
312
352
405
4a
4b
5a
5b
512
557
504
570
465
2.67
3.16
4.69
4.66
1.28
0.52
0.26
0.24
HOMO → LUMO
93.13
87.07
60.75
52.14
953
884
3
2
2
93
65
66
HOMO-1 → LUMO
HOMO-1 → LUMO + 1
HOMO-1 → LUMO+3
1,140
174
1,052
191
349
398
5
b
673
589
528
2.35
2.95
4.42
4.67
2.13
0.46
0.32
0.21
HOMO → LUMO
82.29
84.04
29.51
76.49
1,093
1162
4
2
2
21
81
65
HOMO-1 → LUMO
HOMO-1 → LUMO + 1
HOMO-1 → LUMO+4
a
First order hyperpolarizability (x 10−30 esu).
428
2.89
3.53
5.36
5.71
1.42
0.28
0.56
0.36
HOMO → LUMO
94.51
83.50
45.68
23.18
charge distribution and interactions. Previous reports suggest there is
no distinct difference in the natural charges obtained at the ground and
the excited state [106,109], therefore, the natural charges were de-
termined at the ground state using NBO analysis as implemented in
Gaussian 09 program and are summarized in Table S8. As expected and
found from the result the nitrogen atom due to its electronegativity
bears a high negative charge. For all the dyes, it is observed that there
are alternate negative and positive charges on the atoms within the
selected D-A range suggesting considerable CT characteristics (Table
S8). The second order Fock matrix analysis for the dyes has been con-
structed so as to investigate the D-A interaction and stabilization energy
using B3LYP/6-311++G(d,p) level of the theory [110,111]. A higher
3
2
2
51
31
17
HOMO-1 → LUMO
HOMO → LUMO + 1
HOMO-1 → LUMO + 1
527
2.35
3.20
4.87
5.03
2.17
0.36
0.33
0.27
HOMO → LUMO
83.77
78.48
21.80
33.34
3
2
2
88
55
47
HOMO-1 → LUMO
HOMO-1 → LUMO+3
HOMO-2 → LUMO+5
a
Experimental absorption (nm).
Theoretical absorption (nm).
Energy associated with vertical excitations (eV).
Oscillator strength for theoretical absorption.
Transitions.
b
c
d
e
f
(2)
Orbital contribution for absorption.
value of E indicates enhanced D-A interactions leading to a greater
extent of conjugation in the molecule. The increased electron density on
C]C bond to anti-bonding orbitals of C]C bond results in such in-
teractions. The π-electrons of C]C bond undergo strong intramolecular
hyper conjugative interaction with the C]C anti-bonding orbital in the
conjugated system leading to stabilization of molecule. The Tables
S9–S12 list the D-A orbital interaction for 4 and 5. For the styryl dyes
3
.5. Natural charges and natural bond order (NBO) analysis from Fock
Matrix
Natural charges on the atoms of the dyes were determined using
NBO analysis [108] which helps to understand the intramolecular
Fig. 5. Electronic distribution of the selected molecular orbitals for dyes for 4 and 5.
51

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
Fig. 6. BLA and BOA at the ground state vs hyperpolarizability (RSH functional) for 4 and 5.
(
2)
4
4
a, 4b, 5a, and 5b the largest value of E is found to be 15,694.72,
The triphenylamine based hybrid dyes show higher values of first order
hyperpolarizability by the solvatochromic method.
A three-level model was used to determine γCT at the molecular
level, arising as a result of electronic polarization in the non-resonant
−1
55.23, 938.69 and 2000.85 kJ mol respectively, whereas the lowest
value is 21.07, 20.63, 27.30 and 28.78 kJ mol respectively. Table S12
−1
lists the D-A orbital interaction for 5b, in this, the π-electrons of N59
*
(
LP) interacts with the π of C43 - C45 increasing the electron density
region [117–121]. The second order hyperpolarizability regarded as the
−1
−36
by 0.0309 and leading to stabilization energy of ˜ 39.78 kJ/mol . This
‘solvatochromic descriptor’ is highest for 4b (365 × 10
e.s.u.) as
*
nitrogen is also found to interact with π C53 - C56 having stabilization
compared to the other dyes. This is attributed to the extended π-con-
jugation length provided by the thiophene spacer and excellent ICT
−1
energy 2000.85 kJ/mol
which is the most energetic interaction.
Another major interaction is observed with C52 – C54 which conjugates
characteristics. Thus, from the above observation solvatochromically
*
−1
−36
with π C53 – C56 having stabilization energy 339.87 kJ/mol . Fur-
ther, the planarity between the dicyano vinylene acceptor group and
the secondary donor group increases the conjugation which implies
delocalization from the donor to acceptor. These intramolecular inter-
derived hyperpolarizability values for 4a (294 × 10
e.s.u.) and 5a
−36
−36
(87 × 10
e.s.u.) and 5b (152 × 10
Further, the Onsager radii (a
ability (α ), polarizability anisotropy (Δα
ability (β ) and second hyperpolarizability (γ
e.s.u.) are found to be lower than that of 4b (295 × 10
−
36
e.s.u.) within a given set.
), dipole moment (μ
0
0
), mean polariz-
*
actions from π to π results in CT from donor to acceptor for 4 and 5
0
0
), static first hyperpolariz-
) values for the dyes were
subsequently leading to polarization and optical nonlinearity in the
molecule.
0
0
also calculated using two global hybrid (GH) functionals (B3LYP and
BHHLYP) and four range-separated hybrid (RSH) functionals (CAM-
B3LYP, ωB97, ωB97X, ωB97XD) with basis set 6-311+G(d,p) on the
basis of finite field approach and the resulting data are summarized in
Tables 3 and S14–S17. B3LYP and BHHLYP functionals have a constant
percentage of HF exchange i.e. 20 and 50% respectively, whereas CAM-
B3LYP (19–65%), ωB97 (0–100%), ωB97X (15.77–100%), and ωB97XD
3.6. Polarizability and hyperpolarizability of 4 and 5 by spectroscopic and
DFT methods
In this study, we have explored the trend in polarizability and hy-
perpolarizability of 4 and 5 using spectroscopical and DFT approach
and the results obtained are compared with reference NLO material
urea [112]. The values obtained spectroscopically for transition dipole
(
22.2–100%) an improved range separated hybrid functionals are
known for their better performance than that of hybrid functionals. The
detailed theory explaining the theoretical and experimental calcula-
tions to obtain the above mentioned NLO properties are well described
in the previous reports [122–125].
moment (μCT), linear polarizability (αCT), first hyperpolarizability (βCT
)
and second hyperpolarizability (γCT) were calculated as described in the
previous reports [113] and are summarized in Table S13. μCT of the
lowest lying CT excited state for 4a, 4b, 5a, and 5b determined using
the solvatochromic method is found to be 13.32, 12.52, 9.22, and 6.74
Debye respectively in DCM. Assuming that the direction of the CT in the
dyes coincides with the molecular axis, αCT were calculated by the
equation as described in the literature [113,114]. The solvatochromi-
cally derived αCT values gradually increase with the increase in the
solvent polarity for all the dyes. The lower values of αCT are observed
The computed values of polarizability and hyperpolarizability ob-
tained using various functionals are also found to differ with respect to
the donor to acceptor strength and π-conjugation length in particular
dye. The lower values of polarizability and hyperpolarizability are ob-
served for 4a and 5a and the highest for 4b and 5b which can be ac-
credited to increased D-A strength via extended π-conjugation provided
by the co-planar arrangement of thiophene spacer resulting in excellent
ICT characteristics. Also, these values are found to be higher in a polar
solvent like DMSO than in non-polar solvent like toluene. Further, the
NLO responses of the particular dye can be tailored by altering the
transition dipole moment [79,80]. For 4 and 5 the theoretically cal-
−
24
for N-ethyl carbazole based hybrid dyes 5a (30–42 × 10
e.s.u.) and
−
24
5
b (16–21 × 10
e.s.u.), whereas the triphenylamine based dyes 4a
−
24
−24
(
74–92 × 10
e.s.u.) and 4b (72–90 × 10
e.s.u.) give higher va-
lues.
culated β
0
values using BHHLYP functional are 352, 1,140, 349, and
The solvent dependent βCT were determined using the Ouder
equation, based on the two-level microscopic model [115,116]. As this
model is based on several assumptions, it allows only an approximate
estimate of the dominant tensor of total hyperpolarizability along the
direction of charge transfer, which is the major contributor to the total
hyperpolarizability. However, it is an inexpensive method that gives an
insight into the NLO properties of a molecule. The βCT values for 4a and
−30
1
093 (x 10
e.s.u.) respectively, whereas using ωB97XD functional
−30
are 405, 1,052, 398, and 1162 (x 10
e.s.u.) respectively. For 4a and
5
a, comparison of the computationally calculated β
values obtained with B3LYP and RSH functionals are comparable
Table 3). While for 4b and 5b, the values are comparable with
0
suggests that the
(
BHHLYP and RSH functionals. A similar trend is observed with polar-
izability and second-order hyperpolarizability of 4 and 5, where the
values obtained with BHHLYP are comparable with those of RSH
−
30
4
b are in the range 123–162 and 234–304 × 10
e.s.u. respectively,
e.s.u. respectively.
−30
while for 5a and 5b in 36–55 and 58–77 × 10
52

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
Fig. 7. (a) Normalized closed aperture Z-scan curve and (b) normalized open aperture Z-scan for dyes 4a, 4b, 5a, and 5b in DMSO.
functionals (Tables S14–S17). The NLO parameters of the dyes when
A strength and extent of π-conjugation are the key factors in de-
termining the polarizability and hyperpolarizability of a molecule and
thus dyes with such high non-linear behavior can be suitably used in
various optoelectronics and photonic devices.
compared with the standard reference NLO material i.e. urea
−
24
−30
(
μ
urea = 4.24 Debye, αurea = 6.2 × 10
e.s.u., βurea = 0.31 × 10
e.s.u.) are found to be relatively larger,
suggesting these styryl dyes to have excellent NLO properties. Thus, D-
−36
e.s.u., γurea = 4.46 × 10
Fundamental limits for the NLO values were obtained by the
53

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
trends of BLA and BOA.
Table 4
Third-order nonlinear optical parameters of dyes 4a, 4b, 5a, and 5b.
a
bn
c
(3)
d
(3)
e
(3)
3.8. Z-scan analysis
Dyes Solvent
β
2
Re χ
Im χ
χ
(
(
x10−11)
m/W)
(x10−18)
(x10−13) (x10−13) (x10−13)
(e.s.u.)
2
(m /W)
(e.s.u.)
(e.s.u.)
The third order nonlinear optical properties of the dyes were ana-
lyzed using the Z-scan analysis [127]. Wavelength dependent macro-
scopic parameter viz. the magnitude of nonlinear absorption coefficient
(β), the magnitude of nonlinear refraction (n
linear susceptibility (χ ) were estimated from the Z-scan analysis. The
4a
4b
5a
5b
Acetone −0.2861
Ethanol −1.3018
0.5409
0.5968
1.0313
2.0097
1.9500
3.4944
−0.4503 2.0595
−1.8017 2.6549
−1.4716 3.7916
DMSO
−1.0254
2
) and third order non-
(3)
Acetone −0.3049
0.3225
–
0.2051
1.1676
–
0.8403
−0.4675 1.2577
–
−0.9248 1.2495
(3)
Ethanol
DMSO
–
–
macroscopic parameter like third order nonlinear susceptibility (χ
were determined using the values of β and n . In the Z-scan analysis, β
was evaluated from the open aperture (OA) Z-scan measurements,
while n was determined by the closed aperture (CA) Z-scan measure-
)
−0.5329
2
Acetone −0.4251
Ethanol −0.6571
0.9027
0.4608
0.5205
3.0265
1.6129
1.9269
−0.6037 3.0861
−0.9742 1.8843
−1.1926 2.2661
2
DMSO
−0.7606
ment [127–129].
Acetone −0.2061
0.2750
–
0.5349
1.0681
–
2.4685
−0.3390 1.1206
–
−0.7084 2.5681
The magnitude of nonlinear absorption coefficient (β) and the
magnitude of nonlinear refraction (n ) for the dyes was calculated using
2
measurements of open aperture (OA) and closed (CA) of Z-scan tech-
niques in solvents such as acetone, ethanol, and DMSO at 0.3 × 10
concentration. The normalized open aperture and close aperture Z-scan
curves for 4a, 4b, 5a, and 5b in DMSO are shown in Fig. 7 and for
remaining solvents in Figs. S4–S7 in ESI. OA curves of 4a and 4b show
saturable absorption in all solvetns thereby displaying a negative type
of absorption nonlinearity resulting in negative β values. A similar
trend is observed for 5a and 5b (Table 4). Further, closed aperture z-
scan experiment was performed to measure the nonlinear index of re-
Ethanol
DMSO
–
–
−0.3624
−3
a
M
the magnitude of nonlinear absorption coefficient (β).
b
c
2
the magnitude of nonlinear refraction (n ).
real part of the third order nonlinear susceptibility (Re χ(3)).
d
e
imaginary part of the third order nonlinear susceptibility (Im χ(3)).
(3)
third order nonlinear susceptibility (χ ).
Table 5
The two-photon cross-section in GM for 4 and 5 in different solvents using the
solvatochromic method.
fraction (n
2
). In this process of non-linear refraction, the refractive
index of the medium fluctuates as a result of the high intensity laser
Solvents
σ
2PA
(
3)
(
GM)
beam. Moreover, the real part of χ can be obtained by evaluating the
values. As can be seen from the Fig. 7, the plot of closed aperture z-
scan analysis for the dyes exhibit the prefocal transmittance minimum
valley) followed by the post focal transmittance maximum (peak) type
n
2
4
a
4b
5a
5b
(
Toluene
DCM
EtOAc
Acetone
DMF
120.16
130.16
95.12
109.75
145.89
137.50
246.48
387.57
266.05
273.03
364.43
399.39
42.65
44.58
40.30
44.61
47.72
58.75
110.78
109.15
88.04
94.65
109.76
105.75
of behaviour. This alternate prefocal and post focal transmittance can
be attributed to the characteristic behaviour of chromophores with the
positive nonlinear refractive index as a result of self-focusing. Hence,
the dyes possess positive n
2
values (Table 4). Division of closed aperture
DMSO
data by open aperture data gives the pure nonlinear refraction curves.
(
3)
The values of β, n
2
, and χ calculated are listed in Table 4. The β
values contribute to the imaginary part of the third order nonlinear
limiting theory proposed by Kuzyk [126] and are summarized in Table
S18. The γ values have two fundamental limits - a negative cen-
trosymmetric limit and the positive limit for an asymmetric NLO mo-
lecule. Using the equations (S3) and (S4) the optical limiting values
obtained for β and γ are compared with the values obtained from the
solvatochromic shift and theoretical methods for 4 and 5. From Table
S18, it is observed that the values of β and γ lie within the estimated
limiting values for the dyes in all solvents.
(3)
susceptibility (Im χ ) and should display minimum values whereas n
2
(3)
values contribute to the real part of (Re χ ) and value should be high
in order to have practical applications. At the same time, the non-linear
absorption should be as small as possible. Consequently, these two
(
3)
criteria collectively lead to higher values of χ
for the synthesized
(3)
dyes. From Table 4, the values of Re χ are found to be larger as
(
3)
compared to the Im χ which collectively produces the large values of
(
3)
χ
for all the dyes. Comparatively, 4b and 5b show a lower values in
−
13
the range 1.12–2.57 × 10
e.s.u., whereas 4a and 5a show higher
e.s.u. in studied solvents with 4a showing
−
13
3
.7. Bond length alternation (BLA) and Bond order alternation (BOA)
correlation with NLO properties
values, 1.88–3.79 × 10
the highest value in acetone (3.79 × 10
−
13
e.s.u).
BLA and BOA are regarded as the geometric and electronic para-
meters governed by the D-A strength, length of π-conjugation and the
surrounding media. The BLA and BOA for the dyes were calculated from
the optimized geometry at the ground state. These parameters for all
3
.9. Laser damage threshold study
Important information on the maximum permissible laser power
which an NLO crystal can withstand without getting damaged are ob-
tained through laser damage study [130]. Several factors like specific
heat, thermal conductivity, optical absorption, etc. govern the laser
damage threshold of the NLO crystal. For the dyes, the laser damage
threshold values were measured using Nd:YAG laser (532 nm) with a
repetition rate of 10 Hz and a pulse width (t) of 5 ns. The dyes were
placed at the focus of a plano-convex lens of focal length (f) 10 cm. The
following expression was used to calculate the surface damage
threshold of the crystal:
the dyes are correlated with β
summarized in Table S19. Fig. 6 displays a linear relation of β
with those of BLA and BOA values. The BLA values decrease in order
0
values (Fig. 6) and the results are
0
values
5
5
b > 4b > 5a > 4a, whereas BOA values decrease in order
b > 4b > 4a > 5b. As seen from the Table S19, the BLA and BOA
values are approaching towards zero suggesting a high degree of po-
larization in the synthesized dyes. Greater the polarization in the dyes
higher is the μ, α
0
and β
0
values leading to decreased BLA and BOA
increases with
values [109]. Moreover, the magnitude of μ, α0, and β
0
E
πr
the increase in the D-A strength and the length of π-conjugation as
observed for 5b. From the above implications, it is observed that the
power density (P ) =
d
2
trends obtained in μ, α
0
and β
0
are in good agreement with observed
where
54

M.M. Raikwar et al.
Journal of Photochemistry & Photobiology A: Chemistry 373 (2019) 45–58
E is the energy in (mJ),
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