NLOphoric Triphenylamine Derived Donor-π-Acceptor-π-Donor Based Colorants: Synthesis, Spectroscopic, Density Functional Theory and Z-scan Studies

Article abstract of DOI:10.1111/php.13089

Three Donor-π-Acceptor-π-Donor type styryl dyes (5a-c) with different secondary donors are synthesized and characterized to study their nonlinear and linear optical properties. The structure–property relationships of the dyes are described in the light of systematic photophysical and theoretical investigations. The photophysical characteristics of 5a-c are influenced by the polarity of the medium, with an appreciable bathochromic shift in emission (5b = 81 nm) and large Stoke shifts (5b = 104–173 nm) in polar solvents. 5a-c showed intramolecular charge transfer characteristics recognized with the help of emission solvatochromism, solvent polarity graphs, natural bond orbital analysis and HOMO–LUMO energy difference. The optimized geometry and frontier molecular orbitals reveal that the electron donation takes place from secondary donors and not from a fixed donor (triphenylamine) which is more twisted. The nonlinear optical properties obtained using solvent induced spectral shift and computational methods are found within the limiting values. Z-scan results reveal saturable kind of behavior for 5a, 5b and 5c, whereas 5a and 5b show reverse saturable kind of behavior in acetone and ethanol and hence give optical limiting values. The two-photon absorption cross section described by two-level approximation is highest for 5b (251–300 GM).

Full text of DOI:10.1111/php.13089

DR. SEKAR NAGAIYAN NETHI (Orcid ID : 0000-0001-7568-5129)
Article type : Research Article
NLOphoric Triphenylamine Derived Donor-π-Acceptor-π-Donor Based
Colorants: Synthesis, Spectroscopic, Density Functional Theory and Z-scan
Studies
1
2
2
2
Manish M. Raikwar, Elizabeth Mathew, Manu Varghese, Issac H. Joe, and Nagaiyan
Sekar*
1
Department of Dyestuff Technology, Institute of Chemical Technology, (Formerly UDCT), Nathalal
Parekh Marg, Matunga, Mumbai, 400019, India.
2
Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College,
Thiruvananthapuram, Kerala 695015, India.
Corresponding author’s email: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Abstract
Three Donor-π-Acceptor-π-Donor type styryl dyes (5a-c) with different secondary donors are
synthesized and characterized to study their non-linear and linear optical properties. The
structure-property relationships of the dyes are described in the light of systematic
photophysical and theoretical investigations. The photophysical characteristics of 5a-c are
influenced by the polarity of the medium, with an appreciable bathochromic shift in emission
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been through the copyediting, typesetting, pagination and proofreading process, which may
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doi: 10.1111/php.13089
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(
5b = 81 nm) and large Stoke shifts (5b = 104-173 nm) in polar solvents. 5a-c showed
intramolecular charge transfer characteristics recognized with the help of emission
solvatochromism, solvent polarity graphs, natural bond orbital analysis, and HOMO-LUMO
energy difference. The optimized geometry and frontier molecular orbitals reveal that the
electron donation takes place from secondary donors and not from a fixed donor
(triphenylamine) which is more twisted. The non-linear optical properties obtained using
solvent induced spectral shift and computational methods are found within the limiting
values. Z-scan results reveal saturable kind of behavior for 5a, 5b, and 5c, whereas 5a and
5
b show reverse saturable kind of behavior in acetone and ethanol and hence give optical
limiting values. The two photon absorption cross-section described by two level
approximation is highest for 5b (251-300 GM).
Keywords: Donor, Acceptor, Triphenylamine, Dicyano vinylene, Solvatochromism, DFT,
NLO, Z-scan
Introduction
In past few decades, there is notable progress in the design and development of functional
materials as promising candidate for applications in bio-sensing fluorescence technology (1),
electroluminescence devices (2), non-linear optics (NLO) (3, 4), organic light emitting
diodes (OLED) (5, 6), photoconductors (7, 8), photovoltaic (9, 10), solid-state lasers (11, 12),
thin film transistors (13–15), two photon absorption materials (TPA). Push-pull dyes are
organic molecules containing Donor-π-Acceptor (D-π-A) framework where donor and
acceptor are linked through conjugated π-bonds. They are easy to synthesize and their
properties can be improved based on the nature of the donor, acceptor or π-linker used (16).
Besides the typical linear D-π-A arrangement, the recent trend in the design includes
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quadrupolar (D-π-A-π-D or A-π-D-π-A) (17, 18) and octupolar (tripodal) (D-π-)
D (19–21). Recently organic fluorophores with D-π-A-π-D received interest in organic
electronics, on account of intramolecular charge transfer (ICT) improved light absorbance
22–24). As these molecules give broad range absorption spectra resulting in the reduced
3
A or (A-π-
)
3
(
energy difference between the HUMO and LUMO. This can be achieved by tuning the
HOMO and LUMO via suitable choice of electron donor and electron acceptor units in the
organic fluorophores (25, 26). Further, the relationship between the different donor and
acceptor groups and their influence on photophysical and electrochemical properties can be
studied (27, 28).
Over the years, push-pull organic dyes with highly resonating and polarizable π-
electron framework are used as second and third-order non-linear materials (29–31). This is
because organic molecules employed in NLO applications unlike inorganic molecules offer
numerous advantages such as well-defined structure, ease of synthesis and tenability, and
large optical non-linearities and responses. The donor-acceptor units attached to the π-
conjugated system are known to play a crucial role in terms of NLO activity. Thus, by
increasing the donor-acceptor strength of a system can lead to enhanced non-linearity (32).
The non-linear optical responses of the push-pull molecular system is related to ICT from
donor to acceptor through a π-conjugated network. Designing new systems with excellent
charge transfer (CT) plays a vital role as improved ICT will further enhance the NLO
susceptibility for the designed molecule (33). Dipolar (D-π-A)(34, 35), quadrupolar (D-π-A-
π-D or A-π-D-π-A) (32, 36–38) or octupolar systems (39–41) are widely investigated for the
applications as NLO materials. As the CT in such systems describes the low energy physics
of these molecules and is also responsible for low-energy excitations with the large transition
and/or mesomeric dipole moments (42). Quadrupolar molecules with the D-π-A-π-D
framework are also known to exhibit excellent third order NLO properties (43–45). Second-
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order materials NLO materials are being used in green lasers acquired through frequency
doubling of the red sources, second-harmonic generation microscopy and tetrahertz wave
generation (44, 46, 47).
Herein, we report the design and synthesis of fluorescent three styryl dyes with the D-
π-A-π-D framework. The strategy involved in this study is to keep one donor fixed as
triphenylamine whereas varying the secondary donor having different π-conjugated network
and acceptor as dicyano vinylene group so as to ascertain the effect of varying secondary
donor on the photophysics of the dyes. Triphenylamine due to its excellent electron donating
ability and CT characteristic is utilized in numerous molecular design for a variety of
applications. The coplanarity between the central nitrogen atom and the three rings allows an
uninterrupted conjugation of the lone pair electrons with the electrons in the aromatic ring
thereby increasing the π-electron density of the system. Hence, triphenylamine not only helps
in increasing the solubility but also the electron donating ability and the extent of π-
delocalization (48–51). This outstanding property of triphenylamine marks it as a suitable
fixed primary donor for our study. Introduction of an accepting group has found to enhance
the ICT capabilities of triphenylamine (52–55). Here dicyanovinylene group is used as an
acceptor as it offers several advantages like providing highly polar structures and helps in
tuning the photophysical and electrochemical properties as well (56–58). We expect these
dyes to show enhanced photophysical and NLO properties as compared to previously
reported analogous 1 (59). As 1 is a dipolar molecule with only one donor whereas these
synthesis styryl dyes have a D-π-A-π-D framework with the secondary donor, which we
expect will help to enhance the optical and NLO properties of the synthesized styryl dyes in
comparison with the known NIR emitting 1. These styryl dyes were prepared by the
condensation of 2-(1-(4-(diphenylamino)phenyl)ethylidene)malononitrile with different
1
13
aldehydes in ethanol using piperidine as base and were characterized by H NMR and C
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NMR. Photophysical properties of these dyes were investigated in solvents of increasing
polarity. Further, their structural, photophysical and NLO properties were investigated using
DFT and TD-DFT computations.
<
FIGURE 1>
Materials and Methods
Material and equipments. All the reagents were obtained from Sigma Aldrich, Alfa Aesar,
Spectrochem and S. D. Fine Chemicals and used as supplied without any further purification.
All the solvents used were of spectroscopic grade procured from Merck. The reactions were
1
monitored by TLC using 0.25mm E-Merck silica gel 60 F254 precoated plates. H NMR and
1
3
3
C NMR spectra were recorded on Varian 500-MHz instrument (USA) using CDCl solvent
o
and TMS as an internal 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-Visible spectrophotometer and
emission spectra on Varian Inc. Cary Eclipse spectrofluorometer.
Computational details. The molecular geometries of 5a-c were optimized using DFT
0
computations using the Gaussian 09 program (60). The ground state (S ) geometry of these
dyes was optimized in the gas phase as well as in the solvent environment of varying polarity
using self-consistent reaction field (SCRF) incorporated in polarizable continuum model
(PCM) (61, 62) as implemented in Gaussian 09 (63). The functional used was B3LYP, it is
the method that combines Becke’s three parameter exchange functional (B3) (64) with the
nonlocal correlation functional by Lee, Yang and Parr (LYP) (65). The basis set used for all
atoms was 6-311++G(d,p) (66). The excited state optimization was carried out using time-
dependent density functional theory (TD-DFT). The excitation energies, oscillator strengths
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and orbital contribution for the lowest 10 singlet-singlet transitions at the optimized geometry
were obtained by TD-DFT (67, 68) calculations using the same basis set as for the geometry
minimization. The solvents used were toluene, dichloromethane (DCM), 1,4-dioxane, ethyl
acetate (EtOAc), acetone, N, N-dimethyl formamide (DMF), and dimethyl sulfoxide
(DMSO). The NLO parameters were calculated using global hybrid (B3LYP) and range
separated hybrid functional (CAM-B3LYP) with the basis set 6-311++G(d,p) for comparison.
Z-scan measurements. The third-order NLO properties of the synthesized styryl dyes were
measured using the Z-scan technique. It is an effective and popular technique for the accurate
2
measurements of nonlinear refractive index (n ), nonlinear absorption coefficient (β) and
(3)
third-order nonlinear optical susceptibility (χ ). The open and closed aperture Z-scan
measurements were carried out using a pulsed neodymium-doped yttrium aluminium garnet
(Nd:YAG) laser having 5 ns pulses at a repetition rate of 10 Hz giving second harmonic at
5
32 nm. The sample was moved in the direction of propagation of light focused with a lens of
focal length 10 cm. The radius of the beam waist was calculated to be 35 mm. The Rayleigh
2
length, z
0
= πω
0
/λ was calculated as 7.42 mm which is greater than thickness of the sample
cuvette (1 mm), an essential requirement for Z-scan experiments. The transmitted beam
energy, reference beam energy and their ratio are measured simultaneously by an energy ratio
meter having two identical detector heads. The optical limiting behaviour was studied
without aperture under Nd:YAG laser (532 nm) illumination in acetone and ethanol solvent
media. In this experiment, the sample probe is placed at the focal plane of the lens. A neutral
density filter is used to vary the input fluence (or energy) and the transmitted fluence after
passing through the sample is recorded using a photo-detector. The laser damage threshold
values were measured using Nd:YAG laser (532 nm) with a repetition rate of 10 Hz and a
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pulse width (t) of 5 ns. The dyes were placed at the focus of a plano-convex lens of focal
length (f) 10 cm.
Two-photon absorption (2PA) cross section (σ2PA) is described by two-level model using
solvatochromic data, the details of which is described in the supporting information.
Design and Synthesis. The synthetic scheme for the synthesis of the dyes 5a-c with the D-π-
A-π-D framework is illustrated in Scheme 1. The parent chromophore, 1-(4-
(
diphenylamino)phenyl)ethan-1-one (3) was synthesized by the acylation of triphenylamine
by slightly modifying the procedure as described in the literature (69). The Knoevenagel
condensation of 3 with malononitrile using a catalytic amount of ammonium acetate yielded
2
a b c
-(1-(4-(diphenylamino)phenyl)ethylidene)malononitrile (4) (70). Aldehydes R , R , and R
were procured from the commercial sources. Final condensation of the respective aldehydes
with intermediate 4 resulted in styryl dyes 5a, 5b, and 5c. The dyes were fully characterized
1
13
by nuclear magnetic resonance ( H and C NMR) spectroscopy as well as the elemental
analysis and the spectral data are consistent with the proposed structure. The dyes 5a, 5b, and
5
c possessed brick red, orange and greenish golden color respectively, and are soluble in
common organic solvents like toluene (TOL), dichloromethane (DCM), ethyl acetate
EtOAc), acetone, N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO) but
(
sparingly soluble in ethanol (EtOH) and methanol (MeOH). The modified procedures for the
intermediates along with the analytical characterization is given in supporting information.
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Scheme 1. Synthesis route for the styryl dyes 5a-c from the intermediate 4.
Result and discussion
Photophysical properties
Absorption and fluorescence spectroscopy was used to investigate the photophysical
properties of the dyes. Figure 2(a) displays the absorption spectra of the dyes recorded in
toluene and the relevant data are presented in Table 1. All the dyes exhibit two distinct
absorption maxima in the range 280−525 nm. The shorter wavelength absorption maxima
seen below ∼340 nm are allocated to the various localized π−π* electron transitions arising
from chromophores, in particular, triphenylamine, phenyl, and julolidine, (71, 72) which fuse
together in the majority of dyes. The lower energy absorption peak observed in the
wavelength range, 472−523 nm is inevitably seen in all the dyes and are assigned to ICT
2
transition from the attached chromophores (D donor) to the dicyanovinylene acceptor.
Nature of the chromophores, its electron donating tendency and the position of their
attachment have considerable influence on the absorption wavelength and molar absorptivity
of the dyes. Therefore, the absorption maximum for 5a-c in toluene is observed at around 472
nm, 479 nm, and 523 nm respectively. The dyes show a red shifted absorption maxima, with
the escalation in the electron donating tendency of a D-A fragment in the order 5c > 5b > 5a
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(
Table 1) which is a characteristic behaviour of ICT bands. Dyes 5a and 5b have a freely
rotating N-alkyl/aryl donors that directly affect the planarity of the system. On the contrary,
c with a rigid donor (julolidine) has higher planarity in the system. So, we observe here that
5
the peak wavelengths of the absorption band are in order 5c > 5b > 5a. Thus, for 5c, the red
shifted absorption observed is accredited to the rigid planar framework resulting in effective
delocalization of the π – electron between the D
dicyano vinylene group) and +I effect of julolidine.
2
donor (julolidine group) and acceptor
(
<
FIGURE 2>
[
a]
Table 1. Observed photophysical parameters of 5a-c in toluene.
[b]
[c]
4
[d]
[e]
[f]
λ
abs
ε
max x 10
FWHM
λ
em
Δν
[
g]
[h]
Dyes
f
Φ
F
-
1
-1
-1
-1
(
nm)
(M cm )
6.72
nm(cm )
88(4054)
86(3843)
106(4271)
(nm)
561
nm(cm )
89(3361)
104(3724)
5
a
472
479
523
1.2354
1.1688
1.039
0.019
0.118
0.055
5
b
6.62
583
5
c
4.84
591
68(2199)
[
a]
O
[b]
[c]
Analysis was carried out at room temperature (25 C),
Absorption maxima (nm),
-1
-1 [d]
Molar extinction coefficient maxima (mol cm ), Full-width half maxima of the absorption
band, Emission maxima, Stokes Shift (nm) cm , [ Oscillator strength, Fluorescence
quantum yield.
[
e]
[f]
-1 g]
[h]
Figure 2(b) displays the emission spectra obtained for the dyes in toluene and
corresponding data are provided in Table 1. The emission maxima for 5a, 5b, and 5c are 561,
5
83 and 591 nm respectively. Thus, the emission maxima of the dyes in toluene followed a
similar trend to that observed for the absorption spectra. The rigid planar framework and ICT
nature of 5c are reflected in the largely red shifted emission spectrum compared to the other
-1
dyes (73, 74). For 5a-c, the Stokes shift observed are 3361, 3724 and 2199 cm respectively
and are in order 5b > 5a > 5c. Planarity and steric hindrance in the push-pull system plays a
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vital role in amplifying the Stokes shift (75). In the ground state, these effects twist the
substituent out of the core plane of the fluorophore, forcing the substituent to undergo
rotation in the excited state resulting in geometry relaxation. While the substituent rotates in
the excited state, a strong interaction between the substituent and the adjoining atoms results
in large amount of torsional twist, which is transformed into the Stokes shift. As mentioned
above, 5a and 5b with freely rotating N-alkyl/aryl donors affect the planarity of the system,
while 5c with a julolidine group as a rigid donor induces planarity in the push-pull system.
So, we observe here that the absorption and emission bands are in order 5c > 5b > 5a, while
the Stokes shifts follow the order 5b > 5a > 5c. The largest Stokes shift is possessed by 5b in
the series, which probably undergoes significant structural reorganization in the excited state
2
due to ICT from D donor to dicyano vinylene unit. (76)
Further, a solvatochromic study was accomplished with solvents of varying polarities
so as to perceive the interaction of dyes with solvent both in the ground and excited states.
Figure S1 displays the changes observed in the emission and absorption spectra of the dyes
because of the polar environment provided by the solvent. Among the dyes, 5b displays a
solvent insensitive absorption spectra which undoubtedly indicates that the ground state of 5b
is nonpolar in nature and displays insignificant solvent selective association/interaction. A
moderate absorption solvatochromism was observed for 5a and 5c. Conversely, the emission
maxima of the dyes gradually increase on increasing the solvent polarity and hence show
positive solvatochromism (see Figure S1). This indicates that the dyes are selectively
stabilized in an excited state by polar solvents as a result of improved polarization, which is a
common phenomenon in dyes substituted with an amino group.(77) In the polar environment,
the bathochromically shifted emission spectra of the dyes are as a result of following two
aspects, (i) structural reorganization and/or (ii) stabilization in polar solvents due to
photoinduced ICT. Comparing the Stokes shifts in nonpolar (toleune) and a polar solvent
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(
DMSO) further confirms this for the synthesized dyes. For 5a-c, the calculated shift in
emission (Δλ) between the solvents (λDMSO-λTOL) lie in the range of 55-81 nm and follows
the order 5b (81 nm) > 5a (62 nm) > 5c (55 nm). Fluorescence quantum yields (Φ ) were
calculated in different solvents using the reference standard Rhodanine 6G. The Φ decreased
F
F
with the increase in the polarity of the solvent indicating that non-radiative relaxation
pathway is more preferred in polar solvents than in non-polar solvents. Large Stokes shift and
decreased quantum yield in the polar environment cannot originate from geometry relaxation
alone, ICT may be responsible for low quantum yields.(78, 79) More drastic effects for 5c
support the above hypothesis where planarization may smoothen CT between julolidine and
dicyanovinylene. Furthermore, we compared the reported analogue 1 with this work, as seen
from the Table 4, substituting different donors at the opposite end resulted in the enhanced
optical properties of these dyes. On comparing 1 with 5b which differs only in the substituent
1
at D position, the absorption displays a small blue shift of 4-8 nm while, in emission a red
shift of ~10 nm was observed (Table 4). However, in case of 5c, with rigid donor the effect
on the photophysical properties was found to be significant. Thus, keeping the rotor/donor
2
fixed and varying the D with different N-substituents enhances the CT characteristic in these
dyes thereby giving bathochromically shifted behaviour in absorption and emission.
Solvent polarity plots and dipole moment
The influence of polar environment of the solvents on the optical properties of the dyes in the
ground and excited sates was further confirmed by the relationship of emission maxima
intensities ( ꢃꢀ ꢄ ) in cm with the orientation polarizability (Δf) of the molecule using Weller
-1
ꢁꢂ
(80) and Rettig (81, 82) plots. The Weller and Rettig plots for the dyes are shown in Figure
3
, which displays a stronger correlations, indicating that stabilization of the excited state is
governed by general dye–solvent interactions. For 5a, 5b, and 5c the regression coefficient
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obtained from the Weller’s plot are 0.88, 0.93, and 0.91 respectively whereas, from the
Rettig’s plot, they are 0.95, 0.96, and 0.97 respectively. The 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.
<
FIGURE 3>
The concept arising from the dielectric polarization (83) can be used to express the
solvent polarity dependent emission property, which indicates that the spectral differences of
the emission upon increasing the solvent polarity are influenced by the change in permanent
dipole moments between ground (μ
g
) and excited (μ
e
) states. The difference in dipole moment
between the ground and excited states i.e. Δμ = (μ
e
- μ
g
) for the dyes were evaluated using the
equations (S7) and (S8) shown in the supporting information and are listed in Table S2.
Onsager radii (ꢅ ) an essential parameter required to solve the above equations were
ꢆ
evaluated using B3LYP/6-311++G(d,p) level of theory and are listed in Table S3. The Δμ for
5
a-c was found to be in the range of 3-14 Debye (see Table S2). Dye 5b (8.67 Debye)
showed the largest values of Δμ as compared to 5a (3.82 Debye) and 5c (4.22 Debye). This is
due to the triphenylamine units at the opposite ends of the π-conjugated framework in 5b
which results in large Δμ. The rotating phenyl substituents of triphenylamine in 5b is attached
to the central nitrogen atom through a single bond. The rotation of these phenyl rings during
the geometry relaxation of the excited state can be explained by two opposing effects (i) the
steric hindrance, which forces the phenyl substituent to twist out of the main plane of the
fluorophore; (ii) the resonance effect, which provides an inclination to align with the
molecular plane to accomplish an improved electron delocalization. In other words, larger
perturbation in the molecular structure of 5b due to its free rotating phenyl rings in both the
donors and acceptor results in a vibronically disconcerted structure (75). Thus, the CT
characteristic in 5a-c follows the order as 5b > 5c > 5a (see Table S2). Hence, considerable
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ICT in 5a-c is supported by the major difference in dipole moment from the ground to excited
state.
e g
Furthermore, the ratio of excited to ground state dipole moment (μ /μ ) was calculated
using the Bilot-Kawshi (84, 85), Bakshiev (86) and Liptay (87) plots and the values obtained
are listed in Table S4. For 5a-c, this ratio was found to be higher than unity in each case
which clearly rationalizes that the excited state is more polar than the ground state accounting
e g
for CT in the excited state in the dyes (see Table S4). The μ /μ obtained by the above
mentioned plots follows the same order as the order of Δμ as 5b > 5c > 5a. Moreover, the
dyes with larger values of Δμ are known to have potentially large non-linearity and hence 5a-
c were further investigated for the NLO properties.
DFT Study
Optimized geometries of 5a-c
The structural and electronic properties of 5a-c were studied based on the geometries
obtained using DFT computation, employing B3LYP/6-311++G(d,p) level of theory. The
optimized ground state geometries of 5a-c in DMSO are displayed in Figure 4 whereas the
relative bond length and bond angle are tabulated in Table S5. The bond lengths between
donor nitrogen and the adjacent carbon of aryl group in toluene are 5a (C40 – N43) = 1.3672,
5
b (C40 – N43) =1.3978 and 5c (C40 – N41) = 1.3742 Å whereas, the bond lengths in
DMSO are 5a (C40 – N43) = 1.3656, 5b (C40 – N43) = 1.3976 and 5c (C40 – N41) =
.3719. Moreover, as observed from the Table S5 we see a successive increase and decrease
1
in the bond length of 5a-c suggesting CT characteristics. Furthermore, in 5a-c as we move
from non-polar to polar solvent the bond shortening observed suggesting CT characteristics
to be more prominent in a polar solvent in these dyes. Similarly, other bond lengths between
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2
the secondary D and A groups as seen from the Table S5 showed bond shortening in DMSO
as compared to toluene.
<
FIGURE 4>
Further, from Table S6, we see that the optimized structures assume a nonplanar
arrangement of the auxiliary chromophore at the D
1
donor segment, as depicted in Figure 4.
The nonplanarity of the D
1
segments in the dyes can be highlighted in terms of the large
°
dihedral angle (37–38 ) between the triphenylamine and the dicyanovinylene planes (see
Table S6). For the dyes, the dihedral angles between the peripheral chromophore (D
2
o
segment), π-spacer and the dicyanovinylene are in the range of 4 to 6 , which suggests no
major twisting in this segment of the conjugation framework. Indicating that the electron
2 1
donation takes place from the D donor segment rather than the fixed D donor segments, as
this segment is more twisted.
Electronic transitions, Frontier molecular orbitals (FMO) and Molecular electrostatic
potential surface (MEPS)
Theoretically calculated excitation energies, vertical excitation, oscillator strength and their
orbital contributions for 5a-c estimated using CAM-B3LYP/6-311++G(d,p) are summarized
in Table 2, whereas with B3LYP/6-311++G(d,p) are listed in Table S7. Figure 5 displays
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 dyes. The HOMOs of 5a-c are largely delocalized over the D
the dicyano vinylene unit, while the LUMOs are mainly constructed of the molecular orbitals
MO) of the acceptor group i.e. dicyano vinylene. This denotes that there is a substantial CT
from the D donor to the acceptor for the visible wavelength absorption with a reasonable
2
segment and slightly over
(
2
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oscillator strength. The LUMO+1 in 5a and 5c appears to be solely resided over the D
segment, while for 5b it is scattered over the entire molecule. The HOMO-1 for the dyes are
localized over the D segment and slightly over the dicyano vinylene unit. Thus, as observed
1
1
the prominent absorption in the longer-wavelength region had major contributions from the
HOMOLUMO and slightly from HOMO-1LUMO electron excitations. The
HOMOLUMO electron excitation would result in migration of the charge from the donor
to the while the HOMO-1LUMO transition indicated a π–π* character (88, 89).
Table 2. Computed electronic parameters for the dyes in DCM by TD-DFT (CAM-
B3LYP/6-311++G(d,p) level of theory).
CAM-B3LYP
[
f]
[a]
[b]
[c]
[d]
[e]
OC
%)
Dyes
λ
abs
λ
abs
E
f
T
(
(
nm)
(nm)
(eV)
2.77
3.09
4.89
4.72
5
a
489
488
537
447
1.27
0.55
0.12
0.26
HOMO  LUMO
HOMO-1  LUMO
HOMO  LUMO+1
HOMO-1  LUMO+1
92.38
88.08
67.23
62.72
4
2
2
01
53
63
5
b
448
2.77
3.04
4.62
4.65
1.39
0.50
0.24
0.19
HOMO  LUMO
HOMO-1  LUMO
HOMO  LUMO+1
HOMO-1  LUMO+1
85.90
87.35
46.82
45.91
4
2
2
08
68
67
5
c
470
2.64
3.11
4.70
4.72
1.30
0.55
0.07
0.24
HOMO  LUMO
HOMO-1  LUMO
HOMO  LUMO+1
93.15
87.62
49.29
65.91
3
2
2
99
64
63
HOMO-1  LUMO+1
[
a]
[b]
[c]
Experimental absorption (nm),
vertical excitations (eV),
Theoretical absorption (nm), Energy associated with
[d]
[e]
[f]
Oscillator strength for theoretical absorption, Transitions,
Orbital contribution for absorption.
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Moreover, the electronic energy band gap between HOMO to LUMO levels for 5a-c
were found to be 1.63 eV, 1.65 eV, and 1.56 eV respectively (see Figure S2), which further
establishes the trend in CT characteristics for 5a-c. The donor-acceptor substitution pattern
determines the energy band gap of the conjugated dyes system. For example, an increase in
the donor-acceptor strength of the dyes leads to a decrease in the energy band gap (90).
Therefore, an appropriate choice of donor and acceptor groups can successfully lower the
energy band gap resulting in efficient CT characteristics within the dyes. The CT
characteristics were further succored with the aid of MEPS contour plots for 5a-c (see Figure
S3). MEPS plots give a qualitative indication of electrophilic or nucleophilic sites in the dyes
by virtue of positive or negative electrostatic potential surfaces (91). From Figure S3, the
dicyano vinylene segment in the dyes comprises of red color indicating negative electrostatic
potential, while the N-substituted donor and auxiliary donor/rotor (triphenylamine) segment
display blue color indicating positive electrostatic potential surface. From these observations,
it is seen that 5a-c possess a strong electronic transfer from donor to acceptor, resulting in
increased ICT character within the dyes. Above observations are suggestive of strong
electronic transfer from D-A units indicating an enhanced ICT characteristics within the dyes.
<
FIGURE 5>
Natural charges and natural bond order (NBO) analysis from Fock Matrix
The natural charges on the atoms for 5a-c were determined using NBO analysis which helps
to understand the intramolecular charge distribution and interactions. As previous reports
suggest there is no distinct difference in the natural charges obtained at the ground and the
excited state (90, 92), so here the natural charges were determined at the ground state only
using NBO analysis using the NBO version 3.1 (93) as implemented in the Gaussian 09
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program and are summarized in Table S8. As expected and found from the result, the
nitrogen atom due to its electronegative bear a large negative charge. Further, from Table S8,
5
a-c possess alternate negative and positive charges on the atoms within the selected D-A
range indicating significant CT characteristics within the dyes. The second order Fock matrix
analysis for 5a-c is constructed so as to investigate the D-A interaction and stabilization
energy at B3LYP/6-311++G(d,p) level of theory based on the previous reports. A higher
(2)
value of E indicates enhanced D-A interactions resulting in a greater extent of conjugation
within the molecule. Increased electron density on C=C bond to anti-bonding orbitals of C=C
bond results in such interactions. 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. Tables S9–S11 list the D-A orbital interaction for 5a-c.
(2)
-1
For 5a-c the largest value of E was found to be 4399.34, 397.24 and 2534.58 kJ mol
-1
respectively, whereas the lowest values were found to be 20.18, 26.64 and 20.26 kJ mol
respectively. Table S9 represents D-A orbital interaction for 5a, in this, the π-electrons of the
*
lone pair (LP) of N1 interacts with the π of C2 – C3 increasing the electron density by
-
1
0
.0241 and leading to stabilization energy of ~ 38.38 kJ/mol . This nitrogen was also found
*
to conjugate with π of C23 – C24 and C25 – N28 having stabilization energy 20.18 and
-1
3
9.70 kJ/mol , which is the most energetic interaction with the later one. Further, the π-
*
electrons of C34 – C36 were found to be conjugated with π C25 – N28 having stabilization
-1
*
energy 292.67 kJ/mol . It was also found to conjugate with π of C52 – C53 having
-
1
stabilization energy 1440.04 kJ/mol which is the most energetic interaction. Another major
*
interaction was observed with C33 – C34 which interacts with π C52 – C53 having
-1
stabilization energy 4399.34 kJ/mol . Further, the planarity between the dicyno vinylene
acceptor and the secondary D increases the conjugation which suggests delocalization from
2
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*
the donor to acceptor. These intramolecular interactions from π to π results in CT from D
2
–A
in 5a -c subsequently leading to polarization and optical non-linearity in the molecule.
Polarizability and hyperpolarizability of 5a-c by spectroscopic and DFT methods.
Push-pull π-conjugated systems are known to possess excellent non-linear optical properties,
which makes them potentially useful in various electronic and optoelectronic devices
application (94). In this section, we have investigated the NLO responses of 5a-c using
spectroscopic and DFT methods. The values of polarizability and hyperpolarizability
estimated using these methods are compared with with the standard reference NLO material
i.e. urea (95). The NLO parameters such as transition dipole moment (μCT), linear
polarizability (αCT), first hyperpolarizability (βCT) and second hyperpolarizability (γCT) were
calculated using spectroscopic method following the two-level model (96) and are listed in
Table S12. μCT of the lowest lying CT excited state for 5a, 5b, and 5c determined using the
solvatochromic method are found to be 12.59, 12.17, and 9.82 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 (96, 97). The solvatochromically
derived αCT values gradually increased with the increase in the solvent polarity for all the
-24
dyes. 5a (60-97 x 10 e.s.u.) exhibits slightly higher values of αCT compared to 5b (58-78 x
-24
-24
1
0
e.s.u.) and 5c (53-8 x 10 e.s.u.) by the solvatochromic method.
The solvent dependent βCT were determined using the Ouder equation, based on the
two-level microscopic model (98, 99). 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 5a, 5b, and 5c are in the range 70-120, 144-190, and 87-124 x
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-30
1
0
e.s.u. respectively, and highly solvent dependent. 5b with triphenylamine as both the
donors displays 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 region (100–104). The second order
hyperpolarizability regarded as the ‘solvatochromic descriptor’ for the dyes are in the range
-36
1
39-467, 44-133, and 89-303 x 10 e.s.u. respectively. Thus, from the above observation,
γCT values for 5a and 5c are found to be higher than that of 5b by the solvatochromic method.
Table 3. Computationally measured first order hyperpolarizability of 5a-c using various
functionals with 6-311++G(d,p) basis set.
0
First order hyperpolarizability (β )
-30
(
x 10 e.s.u.)
Dyes Solvent B3LYP BHHLYP CAM-B3LYP ωB97 ωB97X ωB97XD
DCM
DMSO
DCM
402
481
659
779
437
524
299
364
368
426
384
476
331
406
387
449
427
533
294
362
269
310
397
505
300
370
289
333
402
509
317
390
342
396
415
522
5
a
5
b
DMSO
DCM
5
c
DMSO
Further, the theoretical NLO parameters for 5a-c such as Onsager radii (a
dipole moment (μ ), mean polarizability (α
second order hyperpolarizability (γ ) were calculated using two global hybrid functionals
B3LYP and BHHLYP) and four range-separated hybrid fucntionals (CAM-B3LYP, ωB97,
0
), static
0
0 0
), static first order hyperpolarizability (β ) and
0
(
ωB97X, ωB97XD) so as to compare the results obtained by the global hybrid (GH) and
range-separated hybrid (RSH) functionals (see Table 3 and S13-16). B3LYP and BHHLYP a
GH functional have a constant percentage of HF exchange i.e. 20 and 50% respectively,
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whereas CAM-B3LYP (19-65%), ωB97 (0-100%), ωB97 (15.77-100%), and ωB97 (22.2-
00%) an improved range separated hybrid functional known for their better performance
1
than that of hybrid functional. These NLO parameters were calculated theoretically and
experimentally based on the theory described in our previous reports (105, 106). The basis set
used was the standard triple zeta basis set as 6-311++G(d,p). Here, we have compared the
NLO parameters obtained using GH and RSH functionals, the results obtained are in good
agreement with both the functionals used. For 5a-c, the calculated values of polarizability and
hyperpolarizability are found to be solvent dependent. Further, altering the transition dipole
moment results in tailored NLO responses for the particular dye (107, 108). The molecular
hyperpolarizability for 5a-c is found to increase with the increase in solvent polarity (Table
3
). Moreover, varying donor and acceptor strength accounts for the altered polarizability and
hyperpolarizability values of 5a-c. For 5a-c, the theoretically derived values of α, β, and γ are
-24
-30
-36
in the range 100-118 × 10 e.s.u., 299-476 × 10 e.s.u., and 1,630-2,989 × 10 e.s.u.
respectively with BHHLYP functional, while with ωB97XD these values are in the range
-24
-30
-36
1
00-119 × 10 e.s.u., 317-522 × 10 e.s.u., and 1,558-2,299 × 10 e.s.u. respectively (see
Table 3, S14 and 16). These values are found to be larger compared with that of urea (αurea
=
-24
-30
-36
6
.2 × 10 e.s.u., βurea=0.31× 10 e.s.u., γurea= 4.46 × 10 e.s.u.), suggesting enhanced NLO
response in 5a-c. The values of polarizability and hyperpolarizability obtained with BHHLYP
functional was found to be comparable with the RSH funcionals. Further, on comparing the
0
β values with that of known analogue 1 the dyes (5a-c) showed higher values (Table 4).
This is due to stronger D-A strength, π-conjugation length as well as the non-centrosymmetry
resulting from triphenylamine group (fixed rotor/donor) which increases the non-linearity in
5
a-c leading to improved NLO properties compared to reported analogue 1. Thus, the optical
and NLO properties of 5a-c were greatly influenced by substituting various donor groups at
the D position and keeping the primary rotor/ donor fixed.
2
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Theoretical limits for the hyperpolarizability values were calculated using the
equations (S3) and (S4) based on the limiting theory proposed by Kuzyk (109) described in
the supporting information, and the limiting values are summarized in Table S17. The γ
values have two fundamental limits - a negative centrosymmetric limit and the positive limit
for an asymmetric NLO molecule. From the equation (S3) and (S4) the optical limiting
values obtained for β and γ were compared with the values obtained from the solvatochromic
shift and theoretical methods for 5a-c. From the results, it is observed that the values of β and
γ lie within the estimated limiting values.
Table 4. Comparison of reported analogue 1 with the 5a-c.
[
a]
[b]
[c]
Donor/Rotor
Donor
λ
abs
λ
emi
β
0
Reference
-
30
D
1
D
2
(nm)
(nm)
(x 10 e.s.u.)
Toluene DMF Toluene DMF
1
25
-
-
439
ND
449
ND
(110)
(
Toluene)
1
96
4
4
87
72
490
499
574
561
654
623
(59)
(gas)
3
31
This Work
(DCM)
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3
87
4
5
79
23
486
557
583
591
664
646
This Work
(
(
DCM)
4
27
This Work
DCM)
[a]
[b]
[c]
Experimental absorption (nm),
hyperpolarizability.
Experimental emission (nm),
First order
Bond length alternation (BLA) and Bond order alternation (BOA): Correlation with
NLO properties.
BLA and BOA are regarded as the geometric and electronic parameters governed by the D-A
strength, length of π-conjugation and the surrounding media. The BLA and BOA for 5a-c
were calculated from the optimized geometry at the ground state. These parameters for all the
dyes were correlated to β
0
values Figure 6 and the results are summarized in Table S18.
values with those of BLA and BOA values. The BLA
Figure 6 shows a linear relation of β
0
values are found to decrease in the order 5b > 5a > 5c, while BOA values decrease in the
order 5b > 5c > 5a. As seen from the Table S18, the BLA and BOA values are approaching
towards the zero suggesting a high degree of polarization in the synthesized dyes. Greater the
polarization for the dyes higher is the μ, α
92). Moreover, the magnitude of μ, α0, and β
and the length of π-conjugation as observed for 5c. From the above implications, it is
0
0
and β values leading to decreased BLA and BOA
(
0
increases with the increase in the D-A strength
0 0
observed that the trends obtained in μ, α and β are found to be in good agreement with
observed trends of BLA and BOA.
<
FIGURE 6>
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Z-scan
The third order non-linear optical properties of the dyes were analyzed using the Z-scan
analysis (111). Wavelength dependent macroscopic parameter viz. the magnitude of non-
linear absorption coefficient (β), the magnitude of non-linear refraction (n
2
) and third order
(3)
non-linear susceptibility (χ ) were estimated from the Z-scan analysis. The macroscopic
(3)
parameter like third order non-linear susceptibility (χ ) were determined using the values of
β and n . In the Z-scan analysis, the β was evaluated from the open aperture (OA) Z-scan
measurements, while n was determined by the closed aperture (CA) Z-scan measurement
111, 112).
The magnitude of non-linear absorption coefficient (β) and the magnitude of non-
linear refraction (n ) for the dyes was calculated using measurements of closed (CA) and
open aperture (OA) of Z-scan techniques in solvents such as acetone, ethanol, and DMSO at
2
2
(
2
-3
0
.3 x10 M concentration. The normalized open aperture and close aperture Z-scan curves
for the dyes in DMSO solvent are shown in Figure 7 and for remaining solvents in Figure
S4-S6 in ESI. OA curves of the dyes in DMSO shows a saturable absorption thereby
displaying a negative type of absorption non-linearity resulting in negative β values.
However, the OA curves of 5a (in acetone) and 5b (in acetone and ethanol) shows a reverse
saturable absorption (see Figure S4 and S5) displaying a positive type of absorption non-
linearity resulting in positive values of β (Table 5). Further, closed aperture z-scan
experiment was performed to measure the non-linear index of refraction (n
2
). In this process
of non-linear refraction, the refractive index of the medium fluctuates as a result of high
(3)
intensity laser beam. Moreover, the real part of χ can be obtained by evaluating the n
2
values. As can be seen from the Figure 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 of behaviour. This alternate prefocal and post focal
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transmittance can be attributed to the characteristic behaviour of chromophores with the
positive non-linear refractive index as a result of self-focusing. Hence, the dyes possess
2
positive n values (Table 5. Division of closed aperture data by open aperture data gives the
pure non-linear refraction curves.
<
FIGURE 7>
The values of β, n
the imaginary part of the third order non-linear susceptibility (Im χ ) and should display
(3)
2
, and χ calculated are listed in Table 5. The β values contribute to
(3)
(3)
minimum values whereas n
2
values pay 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
(3)
as small as possible. Consequently, these two criteria collectively lead to higher values of χ
(3)
for the synthesized dyes. From Table 5 it is seen that the values of Re χ are found to be
(3)
(3)
larger as compared to the Im χ which collectively produces the large values of χ for all the
(3)
-13
dyes. Comparatively, 5b exhibits a higher values of χ in the range 2.94 – 6.10 x 10 e.s.u.,
-13
whereas 5a and 5c show lower values, 1.71 – 4.63 x 10 e.s.u in studied solvents with 5b
-13
showing the highest value in ethanol (6.10 x 10 e.s.u).
Table 5. Third-order non-linear optical parameters of dyes 5a-c.
[c]
(3)
[d]
(3)
[a]
[b]
Re χ
Im χ
[e] (3)
β
n
2
χ
-
13
-13
Dyes Solvent
-12
-19
(x10 )
(x10 )
-13
(
(
x10 )
(x10 )
(x10 )
2
m/W)
(m /W)
(e.s.u.)
(
e.s.u.)
(e.s.u.)
Acetone
8.0206
-4.6908
-7.3464
2.8271
10.9237
9.9227
1.0932
4.5569
3.9605
1.3137
-0.8288
-1.2420
1.7091
4.6317
4.1507
5
a
Ethanol
DMSO
Acetone
Ethanol
12.3288
30.7582
8.2781
0.1144
3.2260
0.0536
2.0351
6.0990
3.8143
6.0992
5
b
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DMSO
-7.6930
6.1407
2.5981
1.3787
2.9413
Acetone
Ethanol
DMSO
-8.7152
-6.8325
-6.7968
3.7832
8.1432
3.7808
1.2770
3.3843
1.4717
-1.2461
-0.9399
-1.1206
1.7842
3.5124
1.8498
5
c
Optical limiting measurements
The characteristic optical limiting curve as a function of incident fluence varying from 0 Jcm
-
2
-2
to 40 Jcm for 5a and 5b in two solvents is displayed in Figure 8. At low incident fluence,
-αL
the output changes linearly i.e. the linear transmission obeyed Beer’s law, I = I
0
e
where, I
0
is output intensity, I is input intensity, α is absorption coefficient and L is the thickness of the
sample. Furthermore, it is observed that (Figure 8), as the input intensity increases, the linear
transmission starts to deviate from linearity and one can observe the optical limiting curve.
The deviation from the linearity is observed both the dyes. The optical limiting curve can be
plotted as input fluence vs. transmittance (Figure 9). Optical limiting values for 5a and 5b in
-2
acetone was found to be 17.20 and 9.55 Jcm respectively, whereas in ethanol it was 6.43
-2
Jcm for 5b.
<
<
FIGURE 8>
FIGURE 9>
Laser damage threshold study
Important information on the maximum permissible laser power which an NLO crystal can
withstand without getting damaged are obtained through laser damage study (113). Several
factors like specific heat, thermal conductivity, optical absorption, etc. govern the laser
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damage threshold of the NLO crystal. The following expression was used to calculate the
surface damage threshold of the crystal:
ꢒꢓ
ꢇꢈꢉꢊꢋꢌꢍꢌꢎꢏꢇꢈꢉꢊꢋꢐꢍꢐꢎꢑ ꢕꢖ
ꢒꢔ
ꢗꢓ
ꢗꢔ
where,
τ
1
2
is the laser pulse length and λ
1
is the laser wavelength for the given LDT and
τ
is the laser pulse length and λ
2
is the laser wavelength with unknown LDT.
ꢧ
ꢨꢜꢓ
ꢘꢙꢚꢛꢜꢝꢞꢛꢟꢠꢡꢢꢣꢝꢊꢤ ꢎꢦꢝ
ꢥ
where
E is the energy in (mJ),
t is the pulse width (ns) and
r is the radius of the beam spot in mm.
Laser damage threshold values for 5a, 5b, and 5c were found to be 4.60, 4.37, and 3.37
-2
GW cm respectively. The values of laser damage threshold for the dyes when compared to
the reported value of the standard inorganic material, β-BBO crystals at same specification
(114) were found to be several times higher than.
Two photon absorption (2PA) cross section from the spectroscopic method
The 2PA cross-section for 5a-c are calculated using the equation (S7) described in the
supporting information for the two-level model and results are summarized in Table 6. The
2
PA cross-section values obtained in toluene for 5a, 5b, and 5c are 47.51, 250.63, and 44.00
GM respectively. Similarly, these values obtained in DMSO for 5a, 5b, and 5c are 68.91,
99.56, and 65.69 GM respectively. 5a and 5c showed the lowest values of the 2PA cross-
2
section whereas, 5b showed the largest value, as a result of efficient CT character within the
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dye. Interestingly, the 2PA cross-section for 5a-c was found to follow the same trend as of
Stokes shift in the order 5b > 5a > 5c (115). Thus, increasing the π-conjugation with enforced
co-planarity leads to higher 2PA cross-section. From the above results, we observed that the
styryl dyes with greater values of dipole moment (∆μeg), molar extinction coefficient (εmax
)
and larger 1PA values showed higher 2PA cross-section. The values for dipole moment
difference and 2PA cross-section obtained by the solvatochromic method are
overemphasized, as we have considered several assumptions in the two level model.
However, to understand the 2PA cross-section trend of the styryl dyes in cost effective way
this solvatochromic two level model serves as a primary tool.
Table 6. Two photon cross section in GM for 5a-c in different solvents using the
solvatochromic method.
σ
2PA
Solvents
(GM)
5b
5
a
5c
Toluene
DCM
47.51
46.16
52.81
51.03
58.76
68.91
250.63
245.71
288.54
266.49
200.28
299.56
44.00
48.35
36.46
48.93
57.54
65.69
EtOAc
Acetone
DMF
DMSO
Conclusion
In summary, three triphenylamine (fixed rotor/ donor) based D-π-A-π-D hybrid dyes (5a-c)
with varying N-substituted secondary donors and dicyano vinylene as an acceptor were
designed and successfully synthesized. The dyes displayed positive solvatochromism
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behavior from non-polar (toluene) to polar (DMSO) environment, more preferably emission
solvatochromism (55-81 nm) which is due to the charge transfer from D donor to the
2
acceptor. The dyes 5a-c showed reasonable sign of CT characteristics as observed by the
solvent polarity plots drawn on the basis of Weller and Rettig models. The molecular
geometries of the synthesized dyes were optimized and theoretically investigated using DFT
computations to gain significant insight into the fundamental structural and electronic
properties of 5a-c. TD-DFT computations were also performed to establish the correlations
between electronic structure and photophysical properties of the dyes. The polarizability and
hyperpolarizability values for 5a-c were estimated using spectroscopic, theoretical and Z-
scan methods and were found be superior compared to that of reference material, making
them suitable for the non-linear optical materials. The values obtained from Z-scan for non-
-12
linear absorption coefficient (β) in DMSO are -7.34, -7.69, and -6.80 x 10 (m/W)
(3)
-13
respectively and for third order non-linear susceptibility (χ ) are 4.15, 2.94, and 1.85 x 10
e.s.u.) respectively. The two-photon absorption cross section values obtained for 5b (251-
00 GM) were found to be larger than 5a and 5c.
(
3
Acknowledgment-The author is thankful to the University Grant Commission (U.G.C.), New
Delhi, India for the award of Junior and Senior Research Fellowship.
Supporting Information
Additional supporting information may be found online in the Supporting Information section
at the end of the article.
Figure S1. Normalized absorption and emission spectra of the dyes 5a-5d in different
solvents.
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Figure S2. HOMO-LUMO energy level diagram of dyes 5a-c.
Figure S3. Molecular electrostatic potential (MEP) surface of the dyes 5a-c.
Figure S4-S6. Normalized closed aperture and closed aperture Z-scan curves for dyes 5a-c in
different solvents.
Table S1. Photophysical parameters of 5a-c in solvents of different polarities.
Table S2. Difference in the dipole moments of 5a-c.
Table S3. Onsager radii (ꢅ ) in Å for the dyes 5a-c.
ꢆ
Table S4. Ratio of dipole moment between ground and excited state of 5a-c by various
method.
Table S5. Selected bond length (Å) for 5a-c in toluene and DMSO.
o
Table S6. Selected dihedral angle ( ) for 5a-c.
Table S7. Computed electronic parameters for the dyes in DCM by TD-DFT (B3LYP/6-
3
11++G(d,p) level of theory).
0
Table S8. Natural charges on the selected atoms within dyes 5a-c in the ground state (S ).
Table S9-S11. Second order perturbation theory analysis of the Fock matrix in NBO basis for
a-c.
5
Table S12-S16. Solvatochromic and theoretically calculated linear and non-linear optical
properties of dyes 5a-c.
Table S17. Comparison of spectroscopic and theoretical values with the limiting values of
hyperpolarizability for 5a-c.
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0
Table S18. BLA and BOA index of 5a-c at the ground state (S ).
1
13
And H and C NMR spectra of the dyes 5a-c.
References
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