NLOphoric 3,6-di(substituted quinoxalin) Carbazoles – Synthesis, Photophysical Properties and DFT Studies

Article abstract of DOI:10.1007/s10895-017-2092-4

Synthesis of novel 3,6-di(substituted quinoxalin) carbazole fluorophores by the condensation of 1,1′-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-bromoethanone) with methyl, chloro and unsubstituted o-phenylenediamine is presented. Synthesized derivatives are well characterized by ¹H NMR, ¹³C NMR, FTIR and Mass spectroscopy. Photophysical studies are carried out using solvents of varying polarities revealed positive solvatochromism and intramolecular charge transfer from carbazole (Donor) to quinoxalin (Acceptor). Intramolecular charge transfer properties are correlated by dipole moment changes and different polarity functions like Lippert–Mataga, Bilot-Kawski, Bakhshiev and Liptay plots with very good regression factors. Mulliken hush-analysis further support charge transfer characteristic. Linear and Nonlinear optical properties are explained by solvatochromic data using two-level quantum mechanical model and are correlated with computational calculations using density functional theory at B3LYP/6-31G(d) level. First hyperpolarizability value of all the synthesized compounds is found to be greater than urea by >333 times. Moreover, increase of hyperpolarizability values from non-polar to polar solvents are in good correlation with the significant charge transfer characteristic in polar solvents.

Full text of DOI:10.1007/s10895-017-2092-4

J Fluoresc
DOI 10.1007/s10895-017-2092-4
ORIGINAL ARTICLE
NLOphoric 3,6-di(substituted quinoxalin) Carbazoles – Synthesis,
Photophysical Properties and DFT Studies
1
1
1
Rahul D. Telore & Amol G. Jadhav & Nagaiyan Sekar
Received: 10 January 2017 /Accepted: 4 April 2017
#
Springer Science+Business Media New York 2017
Abstract Synthesis of novel 3,6-di(substituted quinoxalin)
carbazole fluorophores by the condensation of 1,1′-(9-ethyl-
Introduction
9
H-carbazole-3,6-diyl)bis(2-bromoethanone) with methyl,
In the designing of electro-optical materials carbazole core
plays a key role acting as the donor part of D- π-A system
with BODIPY [1], coumarin [2], styryl [3], quinoxaline [4]
and cyanine [5] as acceptors. The carbazole moiety can be
easily functionalized at 3-, 6- or 9- positions and covalently
linked to other molecular moieties [6]. Substituted carbazoles
are known to possess desirable photophysical properties [7, 8]
as the requirement for functional applications.
chloro and unsubstituted o-phenylenediamine is presented.
1
Synthesized derivatives are well characterized by H NMR,
1
3
C NMR, FTIR and Mass spectroscopy. Photophysical
studies are carried out using solvents of varying polar-
ities revealed positive solvatochromism and intramolecular
charge transfer from carbazole (Donor) to quinoxalin
(
Acceptor). Intramolecular charge transfer properties are cor-
related by dipole moment changes and different polarity func-
tions like Lippert–Mataga, Bilot-Kawski, Bakhshiev and
Liptay plots with very good regression factors. Mulliken
hush-analysis further support charge transfer characteristic.
Linear and Nonlinear optical properties are explained by
solvatochromic data using two-level quantum mechanical
model and are correlated with computational calculations
using density functional theory at B3LYP/6-31G(d) level.
First hyperpolarizability value of all the synthesized
compounds is found to be greater than urea by >333 times.
Moreover, increase of hyperpolarizability values from non-
polar to polar solvents are in good correlation with the signif-
icant charge transfer characteristic in polar solvents.
The quinoxaline ring at 3 and 6 positions of carbazole acts
as an acceptor through covalent bond or π bridge which leads
to D-π-A system with fluorescent properties [9]. The mole-
cules having D- π-A bridge are good candidates for electron-
ics as well as biological applications such as dye sensitizer
solar cell (DSSC) with light harvesting properties [10], organ-
ic light emitting diode OLED [11], lasers [12], optoelectronics
[13], semiconductors [14], sensors [15–18] and bio-imaging
[19, 20]. Electron accepting quinoxaline moieties have been
exploited for the construction of electron transporting mate-
rials suitable for OLED fabrication [21–23]. Exceptionally,
carbazole and oxadiazoles, quinoxaline or dicyanovinyl con-
jugates have been demonstrated to function as dual transport
materials with promising emission characteristics [24, 25].
Such small arrangement of molecules, respond to photoexci-
tation with an intramolecular charge transfer from the donor to
the acceptor moiety through π-conjugation [26].
.
.
.
Keywords Carbazoles ICT NLOphoric Solvent polarity
.
functions Substituted quinoxaline
Electronic supplementary material The online version of this article
A simple experimental method used to determine optical
second order polarizability of the molecules on the basis of
second harmonic generation (SHG) was developed by using a
two-level quantum mechanical model [27–30]. We have se-
lected this method which is based on solvatochromism, and
solvent dependence of the UV-visible absorption and emission
spectrum of the synthesized quinoxaline derivatives [28, 31,
32]. The solvatochromic method for determination of first
(doi:10.1007/s10895-017-2092-4) contains supplementary material,
which is available to authorized users.
*
Nagaiyan Sekar
n.sekar@ictmumbai.edu.in; nethi.sekar@gmail.com
1
Department of Dyestuff Technology, Institute of Chemical
Technology, Matunga, Mumbai 400 019, India

J Fluoresc
order hyperpolarizability βijk or βxxx (βCT) was successfully
applied first by Paley et al. [27].
Relative Quantum Yield Calculations
The geometry of synthesized compound was optimized
with density functional theory (DFT) [33]. The time dependent
density functional theory (TD-DFT) computation has been uti-
lized to predict photophysical properties and polarizability
values [34]. The DFT and solvatochromism based on conven-
tional correlation between the NLO properties and their param-
Quantum yields (ϕ) of compounds 6a–6c in various solvents
were evaluated. Relative quantum yields were measured using
quinine sulfate in 0.1 N H2SO4 as standard (ϕ = 0.546).
Quantum yields were calculated using the comparative meth-
od [41, 42]. Absorption and emission characteristics of the
standard (in 0.1 N H2SO4) and the compounds in various
solvents were measured at different concentrations (1, 2, 3,
4, and 5 μM). Emission intensity values were plotted against
absorbance values and linear plots were obtained. Gradients
were calculated for the standard and the compounds in each
solvent keeping slit width constant at 5. Relative quantum
yields of the synthesized compounds in various solvents were
calculated by using the Eq. 1 [41, 42].
eters like the linear polarizability (α), and first (β ) and second
0
(γ) hyperpolarizabilities of π-conjugated system.
In this paper we are reporting synthesis of 9-ethyl-3,6-
di(quinoxalin-2-yl)-9H–carbazole derivatives with orderly
change in substituents on quinoxaline ring. The geometry of
all the carbazole based quinoxalines were optimized with DFT
using B3LYP method and 6-31G(d) basis set. TD-DFT com-
putation has been utilized to predict photophysical properties
and nonlinear optical properties of the synthesized
quinoxaline and the theoretical values are correlated with ex-
perimental values.
Gradx
ηx
ϕ ¼ ϕ X
X
ð1Þ
x
st
Gradst ηst
where,
ϕx
ϕst
Gradx
Gradst
ηx
Quantum yield of compound
Quantum yield of standard sample
Gradient of compound
Gradient of standard sample
Refractive index of solvent used for Compound
Refractive index of solvent used for standard sample
Experimental Section
Materials and Equipment
All the reagents and solvents were purchased from the S. D.
Fine Chemicals Pvt. Ltd. and used without purification. All
the solvents used were of spectroscopic grade. Melting points
were recorded by open capillary on Sunder Industrial Product
and are uncorrected. UV-Visible absorption measurements
were carried out using a Perkin Elmer spectrophotometer with
ηst
Quantum yields of all the dyes in various solvents are
tabulated in Tables 1, S1 and S2.
Experimental
1
cm quartz cells. The excitation wavelengths were taken as
the absorption maxima (λmax) of the compounds. The scan
Preparation of 9-ethyl-9H-carbazole 2
range was 250 to 650 nm. Fluorescence emission spectra were
recorded on Cary Eclipse fluorescence spectrophotometer
The compounds 5 and 7 were prepared by the reported method
[12].
1
(
Varian, Australia) using 1 cm quartz cells. H NMR spectra
were recorded on VNMR 500-MHz instrument using TMS as
an internal standard in CDCl and DMSO as a solvent.
Preparation of 1,1′-(9-ethyl-9H-carbazole-3,6-diyl)
bis(2-bromoethanone) 4
3
Computational Methods
The compounds 4 were prepared by the reported method [43].
The ground state (S ) geometry of the 6a-6c in their C sym-
o
1
metry were optimized using the tight criteria in vacuum as
well as in different solvents using DFT [35]. The vibrational
frequencies at the optimized structures were computed using
the B3LYP/6-31G(d) method to verify that the optimized
structures correspond to local minima on the energy surface
General Procedure for Synthesis of Carbazole Substituted
Quinoxaline 6a-6c
A mixture of 1,1′-(9-ethyl-9H-carbazole-3,6-diyl)bis(2-
bromoethanone) 4 (0.5 g, 0.0014 mol), substituted ortho-
phenylenediamine 5a (0.308 g, 0.0028 mol) and potassium
carbonate (0.579 g, 0.0042 mol) in DMF (15 ml) was stirred
at 90 °C for 3–4 h. After completion of the reaction, the mass
was poured into the 50 mL ice cold water. The resulting solids
6a-6c were filtered; washed with water and purified by col-
umn chromatography on 100–200 mesh silica with
[36–38]. The vertical excitation energies at the ground-state
equilibrium geometries were calculated with TD-DFT. All the
computations were carried out in vacuum phase and also in
solvent of different polarities using the Polarizable Continuum
Model (PCM) [39, 40]. All the electronic structure computa-
tions were carried out using the Gaussian 09 program [33].

J Fluoresc
Table 1 Effect of solvent polarity on photophysical properties of 6a
ε (Lmol− cm )
1
−1
λemi (nm)
Stokes shift
ϕ
f (Oscil. Strngth.)
6
a
λabs (nm)
−
1
(cm )
Experimental
Computed
Experimental
Computed
Experimental
Computed
Toluene
Dioxane
DCM
389
387
386
387
389
384
362
390
394
405.81
404.7
24,400
10,000
18,800
28,000
22,000
9600
432
468
468
452
442
470
490
480
498
434.8
2558.7
4472.2
4539.2
3715.8
3082.5
4765.0
7216.1
4807.6
5300.3
0.120
0.020
0.059
0.055
0.018
0.074
0.025
0.026
0.033
0.348
0.289
0.111
0.267
0.404
0.108
0.430
0.338
0.310
0.785
0.766
0.750
0.761
0.737
0.728
0.722
0.748
0.744
432.59
441.4
411.21
409.23
409.51
412.09
412.46
413.3
CHCl
3
438.41
439.16
442.43
442.85
444.08
444.08
EtOAc
Acetone
ACN
36,800
22,800
19,600
DMF
DMSO
413.32
ε: Molar extinction coefficient, ϕ: Quantum yiled, f: Oscillator strength
chloroform/methanol as eluent. The synthesized compounds
are well characterized by FT-IR, H NMR, C NMR and
Mass (Supporting Information).
FT-IR: 1598 (-C = N-), 1539 (-C = C-), 1483 (-Ar), 829
(-C-Cl).
1
13
1
H NMR (500 MHz, CDCl ): 1.56 (t, 3H), 4.51 (q, 2H),
3
7
.63 (d, 2H, J = 8.5 Hz), 7.68 (d, 1H, J = 7.1 Hz), 7.71 (d, 1H,
J = 6.8 Hz), 8.14 (d, 2H J = 2 Hz), 8.16 (d, 2H, J = 9.4 Hz),
.42 (t, 2H), 9.10 (s. 2H), 9.51 (d, 2H, J = 6.6 Hz).
1
. 9-Ethyl-3,6-di(quinoxalin-2-yl)-9H-carbazole 6a
8
1
3
Color: Yellow solid, Yield: 72%, M.P.: 222-224 °C.
C NMR (125 MHz, CDCl ): 13.94, 38.17, 109.60,
3
−
1
FT-IR (cm ): 1593 (-C = N-), 1541 (-C = C-), 1486 (-Ar).
120.47, 123.95, 125.95, 126.048, 128.02, 128.21, 130.02,
130.25, 130.49, 131.36, 136.16, 139.60, 142.08, 142.75,
143.42, 144.20, 152.93.
1
H NMR (500 MHz, DMSO-d ): 1.41 (t, 3H), 4.60 (q, 2H),
.81 (td, 2H), 7.88 (m, 4H), 8.12 (d, 2H, J = 7.1 Hz), 8.17 (d, 2H,
6
7
J = 7.3 Hz), 8.56 (d, 2H, J = 6.9 Hz), 9.40 (s, 2H), 9.79 (s, 2H).
Mass: 521 (M + 1).
13
C NMR (125 MHz, DMSO-d ): 14.37, 38.00, 110.81,
6
1
1
20.90, 123.62, 126.26, 128.05, 129.34, 131.00, 141.86,
42.10, 144.41, 152.18.
Mass: 455 (M + 1).
Results and Discussions
Synthesis
2
.
9-Ethyl-3-(6-methylquinoxalin-2-yl)-6-(7-
methylquinoxalin-2-yl)-9H carbazole 6b:
Compounds 6a-6c were synthesized by using carbazole (1) as
starting material (Scheme 1). The first step involved N-
substitution of carbazole 1 by the diethylsulphate resulted into
intermediate 2. Friedel-Crafts reaction was carried out on in-
termediate 2 at the 3- and 6- positions by using bromoacetyl
bromide to give intermediate 4. This intermediate 4 was treat-
ed with different substituted ortho-phenylenediamine deriva-
tives in presence of potassium carbonate in DMF at room
temperature to yield 6a-6c. All the synthesized compounds
were purified by column chromatography on silica with
chloroform/methanol as eluent.
Color: Yellow solid. Yield: 56%.
M.P.: 202-204 °C.
−1
FT-IR (cm ): 1600 (-C = N-), 1540 (-C = C-), 1487 (-Ar).
1
H NMR (500 MHz, CDCl ): 1.51 (t, 3H), 2.62 (s, 6H),
3
4
.44 (q, 2H), 7.56 (m, 4H), 7.96 (s, 1H), 8.00 (d, 2H,
J = 8.6 Hz), 8.07 (d, 1H, J = 8.3 Hz), 8.38 (d, 2H,
J = 7.9 Hz), 9.05 (s, 2H), 9.42 (s, 2H).
13
C NMR (125 MHz, CDCl ): 13.91, 21.87, 38.08, 109.37,
3
1
1
1
20.26, 125.66, 127.99, 128.52, 131.23, 132.45, 139.41,
39.72, 140.65, 140.89, 141.27, 141.64, 141.70, 142.51,
42.59, 143.39, 151.57, 152.24.
Absorption and Emission Spectra in Different Solutions
Mass: 480 (M + 1).
Absorption and emission spectra of compounds 6a-6c were
recorded in varying polarity solvents (Figs. S1-S3) and the
relevant parameters were compiled in Tables 1, S1 and S2.
Compounds 6a-6c showed a broad and relatively intense ab-
sorption band around 390–410 nm attributable to the
3
. 3-(6-Chloroquinoxalin-2-yl)-6-(7-chloroquinoxalin-2-
yl)-9-ethyl-9H-carbazole 6c:
Color: Greenish yellow solid, Yield: 52%, M.P.: 218–220 °C.

J Fluoresc
Scheme 1 Synthesis of carbazole
based quinoxaline derivatives
electronic transition originating from the π-molecular orbitals.
But in acetonitrile they showed hypsochromic shift of lower
degree around the 360–370 nm. The red-shifted absorption
was observed for the arylamine derivatives (6b and 6c) with
expressive involvement of conjugative delocalization in the
ring from donor (carbazole) to acceptor (quinoxaline) [44].
Different substitution at 6th position of aryl ring of the
arylamine showed effect on absorption maxima. Hydrogen
atom substitution showed hypsochromic shift as compared
to chloro and methyl substitution on 6th positions. This may
be due to good +I (inductive effect) of methyl group [45] and
The compounds 6a-6c exhibited broad emission bands
with maxima at 6a 498 nm, 6b 488 nm and 6c 514 nm in
DMSO, which can be indicative of the intramolecular
charge-transfer (ICT) transition due to the dipolar interaction.
All the compounds showed moderate quantum efficiencies in
nonpolar solvents but in case of polar solvents, the quantum
yield decreased which may due to the dipolar interaction be-
tween molecule and solvents.
Dipole Moment Changes of the 6a-6c on Photoexcitation
+
M (mesomeric effect) of chloro group [46] as compared to
The ICT feature of the compounds 6a-6c can be evaluated by
Lippert–Mataga plots [Eq. (1)] [38–40]. The positive
solvatochromism indicates that excited states of molecules
are more stable in polar solvents. This can happen due to the
ICT. These common solvent effects on the molecules were
described solvatochromism properties by Lippert-Mataga
Eq. 2. Lippert-Mataga equation (Eq. 2) was used to estimate
change in dipole moment on photoexcitation as a function of
the solvent polarity.
hydrogen. The introduction of methyl and chloro groups as
electron-donating substituents increases the electron density
of quinoxaline ring. On comparing dyes 6b and 6c there
may be reduction of accepting ability of quinoxaline core,
which cause blue shift in absorption for methyl (6b) as com-
pared to chloro-substituted quinoxalines (6c).
All the dyes studied in this work exhibited a slight red shift
in emission maxima from non-polar to polar solvents. This
result indicates that compounds exhibited positive
solvatochromism, leading to gradual increase in the Stokes
shift on increasing the solvent polarity. This mainly originates
from the difference in the dipole moments of the molecule in
the ground state and the excited states, interactions such
as dipole-dipole, hydrogen bonding and solvation. The
dipole moment ratios from ground state to excited state
μ /μ are calculated and tabulated (Table S3). The red-
ꢀ
ꢁ
2
2
Δf
Δν ¼
μ μ
þ b
e−
g
4πε0ℏca3
ð2Þ
2
ε−1
n −1
Δ f ¼ 2
ε þ 1 − 2n þ 1
2
where,
e
g
shifted emission observed for the arylamine derivatives
6b and 6c) expressed the involvement of conjugative
Δν
νabs - νem stands for Stokes shift,
−1
(
νabs and νem are absorption and emission (cm ),
delocalization in the ring. The effects of solvent polar-
ity on the emission are shown in Figs. S1-S3 for the
compounds 6a-6c.
ℏ
c
a
Planck’s constant,
velocity of light in vacuum,
Onsager cavity radius,

J Fluoresc
b
constant,
where,
Δf
μg
μe
orientation polarizability,
μa
f
ν
is transition dipole moment (D),
is oscillator strength,
is wavenumber (cm ).
ground-state dipole in the ground-state geometry,
excited-state dipole in the excited-state geometry,
permittivity of the vacuum,
proportional to the slope of the Lippert-
Mataga plot.
−1
ε0
2
(
μ -μ )
The carbazole-based quinoxaline derivatives 6a-6c
have good transition dipole moment. Observed results
show that the carbazole and quinoxaline moieties have
good charge transfer within the small system. We ob-
tained transition dipole moment up to 6 D for 6a-6c
molecules from the relationship between the Stokes
e
g
The Lippert-Mataga plots of the Stokes shift of compounds
6
a-6c against orientation polarizability showed very good re-
gression (Fig. S4).
−
1
The polarity plots of the compounds 6a-6c show good
regression factors with good charge transfer. In 6a regression
factor was 0.74 and in 6b was 0.77 with good charge transfer.
In 6c very good regression was observed (about 0.81 with
good linearity). The dipole moment changes in compounds
shifts (cm ) and the solvation parameter tabulated in
Table S4.
DFT Calculations
6
a-6c inferred that the ICT features in the molecules are sig-
To investigate the structure-electronic property relationship in
the carbazole based quinoxaline, we have performed DFT
calculations in Gaussian. We optimized the molecular struc-
ture of compounds 6a-6c in maximum solvent using B3LYP
method and 6-31 g(d) as basic set. The lowest energy
transitions resulting from the theoretical study along
with their energy, oscillator strengths, absorption emis-
sion maxima, and the compositions in terms of molecular
orbital contributions.
nificant. The compounds 6a-6c have μ /μ ratio calculat-
e
g
ed by various method like Bilot-Kawski, Bakhshiev and
Liptay. The fact that the values are less than unity im-
plies that the excited state is less polar (Table S3).
These results are due to the electron withdrawing
quinoxaline moiety on carbazole ring at 3rd and 6th
position, which induce ICT.
Oscillator Strength and Transition Dipole Moment
Optimized Geometries of 6a-6c
Oscillator strength is dimensionless quantity that expresses the
probability of absorption and emission properties in energy
levels, which helps to understand charge transfer within the
molecules. It was simply described by number of electron
transition from ground to excited state. Oscillator strength
Ground state geometries of the carbazole based quinoxaline
derivatives were optimized at B3LYP/6-31G(d) basic set. A
small twisting was observed between C15-C14-C44-C58 (6a:
20.3, 6b: 20.18 and 6c: 18.25) (Fig. 1) in toluene. From the
(
f ) can be calculated using the following Eq. 3 [41].
optimized geometries, it is clear that the slight twisting of the
donor carbazole moiety from the quinoxaline ring by an angle
−9
f ¼ 4:32 Â 10 ∫ εðνÞdν
ð3Þ
o
20 in all the derivatives. The distribution of HOMO and
LUMO are well separated within the molecule 6a-6c and a
significant overlap is present.
−
1
−1
where ε is the extinction coefficient (L mol cm ), and ν
−1
represents the wavenumber (cm ). From this equation we
have calculated oscillator strength for the synthesized carba-
zole based quinoxaline derivatives 6a-6c and tabulated in
Table S4.
Electronic Vertical Excitation Spectra (TD-DFT)
The absorption band at lower energy with higher oscil-
lator strength is due to ICT and is characteristic of
donor-π-acceptor molecules. These ICT bands for all
the dyes mainly occurred due to the electronic transition
from HOMO to LUMO. The compound 6a shows blue
shifted absorption in acetonitrile (362 nm) and red
shifted absorption in DMSO (394 nm). The vertical ex-
citation of 6a was computed and it shows blue shift
absorption in dioxane (404 nm) and slightly red shift
in DMSO (413 nm). Similar solvatochromic results were
obtained for compounds 6b and 6c. The trend observed
in vertical excitations of carbazole based quinoxaline
By using the value of f, we have calculated transition dipole
moment, which was the difference in electric charge distribu-
tion between a ground and excited state of the molecule. The
transition dipole moment for absorption (μ ) was the measure-
a
ment of the probability of radiative transitions which have
been calculated in Debye unit in different solvent environ-
ments using the Eq. 4 [47]. The transition dipole moment
was increased with increase in the oscillator strength for all
compounds 6a-6c
f
2
μ ¼
ð4Þ
a
−7
4
:72 Â 10 Â ν

J Fluoresc
Fig. 1 Dihedral angles and bond angles in 6a-6c
compounds were very similar to the absorption spectra
based quinoxaline derivative in all the solvent is well supported
by the vertical excitation theoretical computations and results
are good agreement with the experimental absorption.
(
Table 1 and S1, S2).
Frontier Molecular Orbitals
Nonlinear Optical (NLO) Properties by Solvatochromic
and DFT Method
The frontier molecular orbitals were studied to understand the
electronic transition and charge delocalization within these π-
conjugated systems The energy band gap in 6a 2.29 eV, 6b
The nature and the length of π-conjugated system for NLO
properties were investigated by using DFT method. Optimized
geometries at B3LYP/6-31G(d) levels were used for evaluation
of NLO properties of all dyes. Due to an effective ICT, the
quinoxaline derivatives must show significant NLO properties.
The strong ICT containing quinoxaline derivative and their first
and second order NLO properties were studied.
2
.31 eVand 6c 2.15 eV in toluene.
The electronic distributions in the HOMO and LUMO for
the all compounds (6a-6c) are presented in the (Fig. S5-S7)
with oscillator strength and band gap (eV) in toluene. The
HOMO diagram of 6a and 6b shows that the electron density
is delocalized over the carbazole ring, with maximum compo-
nents arising from the carbazole nitrogen from the π conjuga-
tion to the end of quinoxaline ring. In the LUMO, maximum
electron density localized on quinoxaline ring (Fig. S5-S7). In
case of 6c, HOMO has maximum electron density localized at
chlorine on the quinoxaline ring. The LUMO showed maxi-
mum electron density located on the quinoxaline ring and car-
bazole ring is totally unaffected by electron delocalization (Fig.
S7). The observations indicate that quinoxaline core enhances
the electron-accepting ability and drifts the electron-density
toward it and the effect is highly dependent on the powerful
donor like carbazole ring. Stronger donor such as carbazole
pushes more electron density towards the quinoxaline core.
The trend observed for the absorption maxima of the carbazole
Molecular Nonlinearity
Organic molecule with small π-conjugated system is well
known for NLO property. These systems have strong π-
configuration with ICT characteristics and commonly provide
good arrangement for electrons to excite from ground to ex-
cited state on photo-excitation. NLO properties do not depend
only on the nature of the strong conjugation, but also on the
substituent positions and the geometry of the molecule.
Carbazole linked to quinoxaline possess rotating single bonds
in the π-conjugation resulted into change in the dihedral an-
gles, which enhance the polarizability of these molecules.

J Fluoresc
Linear Optical Properties by Solvatochromism
Calculation of β from the Solvatochromic Data
0
Calculation of αCT and μCT from the Solvatochromic Data
The two level model used to determine solvent dependent
hyperpolarizability is based on the Oudar equation.
The linear polarizability αCT was evaluated experimentally for
the the dyes 6a-6c. The values are obtained by two-level mod-
el using UV-vis absorption/emission spectroscopy. The
solvatochromic method can also be utilized in determination
of the dipole moment of the lowest lying charge transfer ex-
cited state. αCT are calculated by using the Eqs. 5 and 6. And
all αCT values are calculated for quinoxaline derivatives
2
2
3
νegμegΔμCT
βCT ¼ βxxx ¼
 ꢀ
ꢂ
ꢃ
ð7Þ
2
2
ꢁ
2
2
h C
2
2
L
2
ν −ν
ν −4ν
eg
eg
L
where,
(
6a-6c), and are tabulated in (Table 2 S5, S6).
x
ℎ
c
μ
νeg
direction of charge transfer,
Planck’s constant,
speed of light in vacuum,
transition dipole moment,
transition frequency,
2
eg
2
μ
2μ λeg
eg
αCT ¼ αxx ¼ 2
¼
ð5Þ
Eeg
hc
where,
ν
L
frequency of the reference incident radiation to
which the β value would be referred,
difference between the charge transfers excited
state and ground state dipole moment.
x
ℎ
c
direction of charge transfer,
Planks constant,
velocity of light in vacuum,
ΔμCT
λeg
The wavelength of transition from the ground state to
excited state,
The transition dipole moment, that is related to the
oscillator strength f.
The static βCT is obtained from the above Eq. 7 as
under the static conditions the value of ν = 0 The
L
.
2
μ
values for first hyperpolarizability obtained using the
solvatochromic method (Table 2, S5, S6) is based on
several assumptions, and thus allow only approximate
estimate of dominant tensor of total hyperpolarizability
along the direction of charge transfer, which is the ma-
jor contributor to the total hyperpolarizability. Though
the values are approximate it has advantages over the
other well-known expensive method.
eg
2
3
e h
f
2
eg
μ
¼
Â
π2mc
ð6Þ
8
νeg
where,
m
f
νeg
e
mass of electron,
oscillator strength,
Absorption frequency,
charge on electron,
Calculation of solvatochromic descriptor of 〈γ〉SD
from the solvatochromic data
The third order hyperpolarizability 〈 γ〉
at molecular lev-
SD
The oscillator strength can be obtained by integrated
absorption coefficient.
el originating from the electronic polarization in the non-
resonant region can be treated by a three-level model
Table 2 Measured linear optical
−
8
ΔμCT (10−17
(esu)
2
−35
CT (1f 0−30)
−35
6
a
α (10 )
λ
(nm)
max
ν
max
)
μ
eg (10
)
β
(esu)
γ (10g
)
properties and static first
hyperpolarizability of 6a in
different solvent
a
b
−1
c
d
2 e
(cm)
(cm )
(esu cm)
(esu)
Toluene
,4-Dioxane
DCM
5.91
5.95
5.99
6.11
6.19
6.02
6.07
6.08
5.98
389
387
386
387
389
384
362
390
394
25,706
25,839
25,906
25,839
25,706
26,041
27,624
25,641
25,380
2.24931
2.27218
2.29513
2.36445
2.41104
2.3124
2.04701
1.88198
2.73478
1.93689
1.3628
26.4493
24.3123
35.5018
26.0377
18.8747
26.1563
25.234
7.44752
6.9085
1.0000
7.713
1
CHCl
3
EtOAc
Acetone
ACN
5.7978
7.4950
6.8891
3.3737
6.5676
2.02072
2.16657
0.823234
1.66186
2.34127
2.34705
2.28939
DMF
11.1563
22.4208
DMSO

J Fluoresc
30
−
o
Table 3 DFT calculated linear first order hyperpolarizability (β ) for
6
value of compound 6a-6c was ranging from 130 X 10
(
30
a-6c (X10− esu)
−30
−30
6a), 126 X 10 (6b) and 161 X 10 (6c) esu (Table 3)
−30
β
o
6a
6b
6c
in DMSO. These values are greater than urea (0.38 X 10
)
by 343 (6a), 333 (6b) and 425 (6c) times. The value of asym-
metric quinoxaline 6c is larger than the other derivatives,
which proved that quinoxaline with asymmetric geometry
passes the higher polarizability values.
Toluene
Dioxane
DCM
82.04426518
80.55682137
116.4425202
103.4826739
109.0156252
126.0520412
129.5023522
129.6977813
130.6411062
82.04426518
79.68670073
113.5312623
101.3554092
106.5610698
122.5130164
125.7238153
125.9056029
126.7823651
102.7707489
99.71068233
144.3146756
128.2542364
135.1084113
156.1124712
160.2953904
160.5334077
161.6748385
CHCl
3
EA
Second Order Hyperpolarizability (γ)
Acetone
ACN
The quinoxaline derivatives show very high γ values, by DFT
DMF
DMSO
calculation. The carbazole based quinoxaline show 1226.40 X
−35
−35
1
1
0
0
esu. (6a), 1251.57 X 10
esu. (6b) and 1509.53 X
−35
esu. (6c) γ values by computationally (Table S8).
[
48–52]. The quasi-two-level model in place of the three level
model using the density matrix formalism to a simpler Eq. 8
53, 54].
Charge Transfer Characteristics of 6a-6c
[
An efficient charge transfer in a Donor–π–Acceptor chromo-
ꢀ
ꢁ
phore is responsible for the manifestation of NLO properties.
1
2
2
2
eg
<
γ > α
μ
Δμ −Δμ
ð8Þ
Degree of delocalization or fractional degree of localization of
3
eg
E
2
b
eg
the excess charge (C ) and donor–acceptor coupling matrix
ꢀ
ꢁ
value (HDA) was calculated by Eqs. 8 and 9 respectively [55].
The value E1
μ
2
eg
Δμ −Δμ2 appearing in the Eq. 8 may
2
3
eg
eg
sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
!
2
be termed as third order Bsolvatochromic descriptor^. The values
for quinoxaline 6a-6c in various solvents are given in (Table 2,
S5, S6). The values calculated are static dipole moment (μ), the
1
2
Δμ
2
≥
C ¼
1−
ð9Þ
b
2
2
Δμ þ 4μ
≥
≥
mean polarizability (α ), the anisotropy of the polarizability (Δα)
0
ΔE Δμ
ΔE Δμ
≥
≥
≥
≥
the mean first hyperpolarizability (β ) and static second
o
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
HDA ¼
¼
ð10Þ
hyperpolarizability (γ), of the quinoxaline 6a-6c molecule in
different polarity of solvents.
Δμ≥ D
Δμ þ 4μ
≥
2
≥
2
where,
Solvent Dependency of NLO Properties of Carbazole
Based Quinoxaline (6a-6c) by DFT
ΔE≥
Δμ≥
vertical excitation energy
difference between the adiabatic dipole moments
of the ground and excited states
difference in adiabatic state dipole moments
transition dipole moment
D
The linear polarizability (α) value of quinoxaline derivatives
Δμ
μ≥
≥
6
a-6c tabulated in (Table S7). The first hyperpolarizability
Table 4 Degree of delocalization
2
Solvents
6a
6b
6c
b
(C ), donor–acceptor coupling
value (HDA) and donor–acceptor
coupling distance (RDA) for the
dyes 6a–6c in various solvent
2
b
o
2
b
o
2
b
o
C
H
DA
R
DA (A )
C
H
(cm )
DA
R
DA (A )
C
H
(cm )
DA
RDA (A )
−
1
−1
−1
(
cm )
Toluene
Dioxane
DCM
0.327
0.133
0.188
0.270
0.285
0.120
0.163
0.496
0.151
12,057
8787
4.73
2.66
4.35
5.72
4.43
2.67
8.74
4.15
5.01
0.348
0.247
0.246
0.274
0.250
0.219
0.137
0.204
0.179
12,120
11,034
10,993
11,438
11,021
10,582
9485
3.21
5.25
4.60
4.35
2.01
4.86
5.70
5.07
4.90
0.361
0.237
0.186
0.258
0.277
0.203
0.109
0.173
0.178
11,944
10,665
9708
3.48
4.12
2.39
4.04
3.11
4.61
5.02
4.36
5.08
10,131
11,470
11,600
8452
CHCl
3
10,996
11,188
10,111
8601
EtOAc
Acetone
ACN
10,195
12,820
9077
DMF
10,229
9687
9400
DMSO
9449

J Fluoresc
4
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Donor–acceptor coupling matrix distance (RDA) was calcu-
lated by Eq. 11 [56]
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
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ν
ϵ
Δν
12
−2
max max
RDA ¼ 2:06 x 10
ð11Þ
H
DA
6
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Joule JA (1984) Recent advances in the chemistry of 9H-carba-
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molar extinction coefficient at maximum
absorption
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682. doi:10.1021/ma010152u
Δν12
full width at half maximum (FWHM) of charge-
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higher coupling between donor and acceptor in polar solvents
while reverse situation was observed in 6b and 6c (Table 4).
On the other hand RDA values increased from nonpolar to
polar solvents indicated good charge separation in polar sol-
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,6-di(substituted quinoxalin) carbazole fluorophores were de-
signed and synthesized successfully. Photophysical properties
study revealed positive solvatochromism with strong intramo-
lecular charge transfer between the donor carbazole and accep-
tor quinoxaline through the π-conjugation. Computed vertical
excitations using B3LYP/6-31G(d) basic set are in good corre-
lation with the experimental values. Solvent polarity functions
correlated well with the ICT phenomenon in these
fluorophores. Nonlinear optical properties have been evaluated
by solvatochromic method using two-level quantum mechani-
cal model and are found to be in good correlation with density
functional theory calculated values. Due to good charge transfer
in these novel molecules can be applied to OLED or DSSC
applications with suitable molecular structural modifications.
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Acknowledgements Rahul Telore is grateful to UGC-CAS for provid-
ing fellowship under SAP. Amol Jadhav is thankful to UGC for
fellowship.
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