Estimation of ground state and excited state dipole moments of a novel Schiff base derivative containing 1, 2, 4-triazole nucleus by solvatochromic method

Article abstract of DOI:10.1016/j.molliq.2015.12.050

A novel schiff base derivative containing 1, 2, 4-triazole moiety (NBTMPA) has been synthesized from 4- [1, 2, 4] triazol-1-ylmethyl-phenylamine and 4-nitrobenzaldehyde in the presence of glacial acetic acid in an ethanolic medium. The absorbance and fluo

Full text of DOI:10.1016/j.molliq.2015.12.050

Journal of Molecular Liquids 215 (2016) 387–395
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Estimation of ground state and excited state dipole moments of a novel
Schiff base derivative containing 1, 2, 4-triazole nucleus by
solvatochromic method
⁎
Roshmy Alphonse, Anitha Varghese , Louis George, Aatika Nizam
Department of Chemistry, Christ University, Hosur Road, Bangalore 560029, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 May 2015
Received in revised form 7 November 2015
Accepted 14 December 2015
Available online xxxx
A novel schiff base derivative containing 1, 2, 4-triazole moiety (NBTMPA) has been synthesized from 4-
[1, 2, 4] triazol-1-ylmethyl-phenylamine and 4-nitrobenzaldehyde in the presence of glacial acetic acid
in an ethanolic medium. The absorbance and fluorescence spectra of (4-nitro-benzylidene)-(4- [1, 2, 4]
triazol-1-ylmethyl-phenyl)-amine (NBTMPA) were recorded in various solvents to investigate their
solvatochromic behaviour. Dipole moments of the two electronic states of NBTMPA were calculated
from solvatochromic spectral shifts. These were correlated with the refractive index (n) and dielectric
constant (ε) of various solvents. Theoretical calculations were performed to estimate the excited state di-
pole moment on the basis of different solvent correlation methods, like the Bilot–Kawski, Bakhshiev,
Lippert–Mataga, Kawski–Chamma–Viallet and Reichardt methods. The dipole moment in the excited
state was found to be higher than that in the ground state due to a substantial redistribution of electron
densities and charges. Using a multiple regression analysis, the solvent–solute interactions were deter-
mined by means of Kamlet Taft parameters (α, β, π*). Computational studies were performed by Gaussian
09 W software using a time-dependent density functional theory (TDDFT) in order to calculate the atomic
charges and frontier molecular orbital energies in the solvent phase. The calculations indicated that the di-
pole moment of the molecule in an excited state is much higher than that in a ground state. The chemical
stability of NBTMPA was determined by means of chemical hardness (η) using HOMO–LUMO energies. The
reactive centres in the molecule were also identified by molecular electrostatic potential (MESP) 3D plots
as a result of a TDDFT computational analysis.
Keywords:
Solvatochromic effect
TD–DFT calculation
Solvent polarity correlation method
Dipole moment
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
attack in photochemical reactions [6]. Various methods are available
to determine the dipole moments of the solute molecules in different
In recent years the studies on photophysical properties of several or-
ganic donor–acceptor molecules which undergo photon induced intra-
molecular charge transfer (PICT) have been an area of discussion in
research [1–2]. When a molecule gets promoted to an excited state, a
highly polar excited state is formed due to intramolecular charge trans-
fer. Donor–acceptor (D–A) molecules can undergo rotations through D–
A bonds and this makes a fluorophore very helpful in analysing the
properties of the microenvironment that surrounds the molecule
[3–5]. It is well known that a fluorophore containing donor–acceptor
nucleus is accompanied by substantial charge redistribution upon pho-
toexcitation. This phenomenon is a fundamental physico-chemical pro-
cess that takes place in bulk solutions. Ground and excited state dipole
moments of organic molecules are used to study the physico-chemical
processes and to evaluate the site of nucleophilic and electrophilic
electronic states [7–9]. Among the existing methods, the most promi-
nent is based on the change in position and intensity of a band in the
electronic spectrum with solvents of varying polarity (Solvatochromic
shift method) [10–11].
Numerous methods have been proposed for describing solvato-
chromism in terms of solvent characteristics and spectral parame-
ters. The most commonly employed theoretical relations of the
solvatochromic method have been given by Bilot, Lippert, Bakhshiev,
Kawski, Chamma and Reichardt [12–18]. All the theories are based
on the Onsager description of non-specific electrostatic solute–sol-
vent interactions. In Onsager's reaction-field theory, the solvent is
described as a dielectric continuum hosting solvent molecules with-
in Onsager type cavities. The dipole moment of the solute polarizes
the solvent so that the solute itself experiences an electric field,
and the reaction field is proportional to the dipole moment of the
solute in the electronic states.
In designing nonlinear optical materials, the elucidation of con-
formational orientations, distribution of charges and the information
⁎
Corresponding author.
E-mail address: anitha.varghese@christuniversity.in (A. Varghese).
http://dx.doi.org/10.1016/j.molliq.2015.12.050
0167-7322/© 2015 Elsevier B.V. All rights reserved.

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R. Alphonse et al. / Journal of Molecular Liquids 215 (2016) 387–395
on dipole moments in two electronic states are of great importance.
A literature survey revealed that triazole and its derivatives are ex-
tensively studied for non-linear optical properties and are developed
into organic light emitting diodes due to intra molecular charge
transfer [19–20]. A triazole Schiff base (azomethine) derivative bear-
ing a coumarin nucleus was used as a fluorescent sensor for Cu2+
[21]. Triazolyl Schiff bases have received great attention in the phar-
maceutical field due to their diverse biological behaviour [22–24].
Various triazolequinoline derivatives and their Schiff bases were re-
ported to have antibacterial, antimicrobial and anticancer properties
[25,26].
where ‘h’ is the Planck's constant, ‘c’ is the velocity of light, ‘a’ is the
Onsager radius and μg and μe are the dipole moments in electronic
ground and excited states of the molecule. The solvent polarity
functions are given by the following expressions.
2n2 þ 1 ε ꢀ 1 n2 ꢀ 1
f ðn; εÞ ¼
–
ð5Þ
ð6Þ
n2 þ 2 ε þ 2 n2 þ 2
"
#
ꢂ
ꢃ
n4 ꢀ 1
3
gðnÞ ¼
2
ðn2 þ 2Þ2
In the present work, we reported the synthesis of a novel
azomethine derivative of triazole methyl aniline bearing an electron
withdrawing substituent (Fig. 1). It is well established that the ab-
sorption and fluorescence spectra of the fluorophore are influenced
by solvent polarity. The solvatochromic studies of the synthesized
molecule were carried out by using different solvents with varying
polarity. The ground and excited state dipole moments were deter-
mined using both theoretical and experimental studies. The effect
of solvents on excitation and emission spectra along with
solvatochromism was investigated using a multiple linear regression
(MLR) analysis. In order to study the charge transfer, nucleophilic
and electrophilic sites of NBTMPA with solvent, a TDDFT computa-
tional analysis were performed. The ground and excited state opti-
mized geometries of the molecule were also performed using
Gaussian software (Figs. 2 and 3).
The dipole moment in the excited state can be determined experi-
mentally by three independent equations, proposed by Lippert–Mataga
(Eq. (7)) [28,14], Bakhshiev (Eq. (8)) [29] and Kawski–Chamma–Viallet
(Eq. (9)) [30,17], are given below.
υA‐υF ¼ m1 F1 þ constant
υA‐υF ¼ m2 F2 þ constant
ð7Þ
ð8Þ
ðυA þ υF Þ
¼ m3 F3 þ constant
ð9Þ
2
where F1, F2 and F3 are solvent polarity functions; m1, m2 and m3 are the
slopes of linear relationships which are given as follows.
ε ꢀ 1
n2 ꢀ 1
2. Materials and methods
F1
¼
ꢀ
ð10Þ
ð11Þ
2ε þ 1 2n2 þ 1
2.1. Theory
ꢄ
ꢅ
2n2 þ 1 ε ꢀ 1 n2 ꢀ 1
F2
¼
–
n2 þ 2 ε þ 2 n2 þ 2
Most theories of solvatochromic shift in excitation and emission
spectra are based on the Onsager description of non-specific elec-
trostatic solute–solvent interactions. On the basis of quantum-
mechanical second order perturbation theory, Bilot and Kawski
[12,27] derived two expressions for solvatochromism as a function
of refractive index and permittivity of various solvents which are
given as
(
ꢄ
)
ꢆ
ꢇ
ꢅ
2n2 þ 1 ε ꢀ 1 n2 ꢀ 1
3 n4 ꢀ 1
F3
¼
–
þ
ð12Þ
2½n2 þ 2ꢁ ε þ 2 n2 þ 2
2½n2 þ 2ꢁ2
‘n’ and ‘ε’ are the refractive index and dielectric constant for selected
organic solvents in the present study. The sum and difference of maxi-
mum absorbance and fluorescence wave number (cm−1) is plotted
against solvent polarity functions F1, F2 and F3 which gives a linear
plot with gradient m1, m2 and m3 respectively, given by the following
expressions.
υA‐υF ¼ mð1Þ f ðn; εÞþconstant
ð1Þ
ð2Þ
υA þ υF ¼ ‐mð2Þφðn; εÞþconstant
ꢀ
ꢁ
where [φ(n, ε)] = [f(n, ε)+ 2g(n)], the variables m(1) and m(2) are
the slopes of the linear graphs from Eqs. (1) and (2).
2
2 μeꢀμg
m1
¼
ð13Þ
hca3
ꢀ
ꢁ
2
ꢀ
ꢁ
2 μeꢀμg
2
mð1Þ
¼
ð3Þ
ð4Þ
2 μeꢀμg
hca3
m2
m3
¼
¼
ð14Þ
ð15Þ
hca3
ꢀ
ꢁ
2
2
ꢀ
ꢁ
2
μe ꢀ μg
2
2
mð2Þ
¼
2
μe ꢀ μg
hca3
hca3
The ground and excited state dipole moments are considered as
parallel [27], provided that the molecular symmetry remains the same
in both the electronic states. The following expressions were derived
on the basis of Eqs. (3) and (4).
"
#
1
ꢂꢂꢆ
ꢇꢃꢃ
=
2
hca3
½2mꢁðð1ÞÞ
m
ð2Þ ꢀ mð1Þ
μg
μe
¼
¼
ð16Þ
ð17Þ
2
"
#
1
ꢂꢂꢆ
ꢇꢃꢃ
=
2
hca3
½2mꢁðð1ÞÞ
m
ð2Þ þ mð1Þ
Fig. 1. Molecular structure of (4-nitro-benzylidene)-(4- [1, 2, 4] triazol-1-ylmethyl-phe-
nyl)-amine.
2

R. Alphonse et al. / Journal of Molecular Liquids 215 (2016) 387–395
389
Fig. 2. Ground state optimized geometry of (4-nitro-benzylidene)-(4- [1, 2, 4] triazol-1-ylmethyl-phenyl)-amine.
In the above expression, δμ and δμB are the changes in dipole mo-
ment, ‘a’ and ‘aB’ are the Onsager radius of the solute molecule and be-
taine dye respectively. From Eq. (21) the excited state dipole moment
can be calculated and it is given as.
"
#
1
=
2
m4 81
μe
¼
þ μg
ð22Þ
ꢂ
ꢃ
3
6:2
11307:6
=
a
The Onsager description of non-specific electrostatic solute–solvent
interactions of the molecule can be explained on the basis of linear sol-
vation energy relationship (LSER), which is also known as multiple-
linear regression analysis. This concept was proposed by Kamlet–
Abboud–Taft and Katritzky [32] and was used to correlate excitation,
fluorescence wavelength in cm−1, stoke's shift with solvent dipolarity/
polarizability (π*) [33], hydrogen bond donor (α) [34] and hydrogen
bond acceptor (β) ability [35]. The relationship can be expressed as:
Fig. 3. Excited state optimized geometry of (4-nitro-benzylidene)-(4- [1, 2, 4] triazol-1-
ylmethyl-phenyl)-amine.
ꢄ
ꢅ
μe
μg
m
ð2Þ þ mð1Þ
¼
ð18Þ
mð2Þ ꢀ mð1Þ
ðυA‐υF Þ ¼ ðυA‐υFÞ 0 þ aa þ bβ þ cπꢂ
ð23Þ
ð24Þ
Similarly based on Eqs. (13), (14) and (15) the ground and excited
ꢊ
ꢋ
ꢊ
ꢋ
ε−1
2ε þ 1
n2−1
2n2 þ 1
state dipole moments are calculated depending on the slopes obtained
from respective correlation methods. μg and μe are not parallel to each
other but, they make an angle φ that can be determined by the following
equation. [31].
ðυA‐υF Þ ¼ ðυA‐υFÞ 0 þ a
þ b
þ c ET ð30Þ
υA and υF are the maximum absorption and fluorescence wave num-
ber (cm−1) respectively. ε, n and ET (30) are relative permittivity, refrac-
tive index and microscopic solvent polarity parameter respectively.
ꢈ
ꢉ
ꢀ
ꢁ
ꢀ
ꢁ
1
mð1Þ
mð2Þ
2
2
2
2
Cos φ ¼
μe þ μg
ꢀ
μe ꢀ μg
ð19Þ
2μe μg
2.2. Experimental
Reichardt proposed a method based on microscopic polarity param-
eter, ENT [18] which was used to determine μe and also for estimating the
change in dipole moment (δμ). This method provides better results than
the frequently used bulk solvent polarity functions.
All the chemicals used were purchased from the Sigma Aldrich
Chemical Company. The synthesized Schiff base derivative was puri-
fied by recrystallization using ethanol. The reaction completion and
purity of the compounds were checked by a thin layer chromatogra-
phy (TLC) using silica gel coated aluminium plates (Merck). A
solvatochromic investigation was performed using different solvents
like n-hexane, toluene, diethyl ether, 1, 4-dioxane, tetrahydrofuran,
ethyl acetate, dichloromethane, acetone, N, N-dimethyl formamide,
dimethyl sulphoxide, acetonitrile, isopropanol, butanol, ethanol
and methanol. All of these commercially available solvents were
υA‐υF ¼ m4 EN þ constant
ð20Þ
ð21Þ
T
"
#
ꢊ
ꢋ
2
ꢀ
ꢁ
3
δμ
δμB
aB
a
m4 ¼ 11307:6
Scheme 1. Synthesis of (4-nitro-benzylidene)-(4- [1, 2, 4] triazol-1-ylmethyl-phenyl)-amine.

390
R. Alphonse et al. / Journal of Molecular Liquids 215 (2016) 387–395
Table 1.
Analysis of solvatochromic effect for various solvents.
Solvents a
υA
υF
Δυ = (υA − υF)
(υA + υF)
(υA + υF) / 2
Hexane
36,496
36,101
36,231
35,211
34,364
33,557
33,783
34,246
33,557
34,482
32,573
33,444
33,557
33,112
31,948
33,557
32,894
32,467
30,120
29,069
27,932
28,248
27,777
26,954
27,624
25,252
24,875
26,737
25,773
25,641
2939
3207
3764
5091
5295
5625
5535
6469
6603
6858
7321
8569
6820
7339
6307
70,053
68,995
68,699
65,331
63,434
61,490
62,032
62,024
60,511
62,107
57,825
58,320
60,295
58,885
57,589
35,026
34,497
34,349
32,665
31,717
30,745
31,016
31,012
30,255
31,053
28,912
29,160
30,147
29,442
28,794
1,4Dioxane
Toluene
Diethylether
Ethylacetate
THF
DCM
Butanol
Isopropanol
Acetone
Ethanol
Methanol
DMF
Acetonitrile
DMSO
a
Solvents are listed in the ascending order of dielectric constants.
(m, 4 H, Ar–H), 5.455 (s, 2 H, −CH2). 13C NMR (400 MHz, DMSO) δ:
51.66, 121.48, 124.00, 128.90, 129.66, 134.87, 141.40, 148.88, 150.23,
151.75, 159.22 ppm.
Fig. 4. Excitation spectra of NBTMPA in different organic solvents.
procured from Merck and were of spectroscopic grade. The absorp-
tion and fluorescence spectral measurements were recorded in
10−5 M concentration using Shimadzu UV–Visible Spectrophotome-
ter (UV/Visible 1800) and Spectrofluorometer (RF5301PC). The anal-
ysis of solvatochromic studies was carried out using the Origin 8.0
Professional programme. Theoretical studies were performed by
the TDDFT method using the Gaussian 09 W software programme.
3. Results and discussion
3.1. Effect of solvents on excitation and emission spectra
The excitation and emission spectra were recorded in fifteen sol-
vents ranging from non-polar to polar aprotic and to polar protic sol-
vents at room temperature (Figs. 4 and 5). The electronic spectra of
the synthesized compound were recorded in 10−5 M concentration.
The maximum excitation and emission wavelength in cm−1, the differ-
ence and sum of the spectral shifts (stokes shift) and the average of
stokes shift values of a novel Schiff base derivative bearing an electron
withdrawing group (NBTMPA) are given in Table 1. The solvent polarity
functions F1 (ε,n), F2 (ε,n), F3(ε,n) and ENT for fifteen solvents are tabulat-
ed in Table 2.
From Table 1 it is clear that for NBTMPA, when the solvent polarity
increases from non-polar to polar solvent (n-hexane to methanol) the
maximum absorption wavelength shifts from 3 nm to 25 nm. This is
due to π–π* transitions along with the charge transfer of the conjugated
core. The emission spectrum of NBTMPA shows a larger red spectral
shifts ranging from 6 nm to 104 nm. This enhancement in the spectral
shift indicates that the energy of the ground state is less influenced by
increasing the solvent polarity compared to the energy of the excited
2.2.1. Synthesis and characterization of (4-nitro-benzylidene)-(4- [1, 2, 4]
triazol-1-ylmethyl-phenyl)-amine (NBTMPA)
An equal amount of 4-nitro benzaldehyde, 1(1 mmol, 0.151 g), 4-(1,
2, 4- triazol-1-yl methyl) aniline, 2 (1 mmol, 0.174 g) and 3–4 drops of
glacial acetic acid was added and refluxed in an ethanolic medium
(15 mL) at 65–70 °C for 6 h (Scheme 1).The progress of the reaction
was monitored using TLC with a solvent system of 3:1 ethyl acetate:n-
hexane. The obtained crude solid product was filtered, washed with
cold ethanol and purified by recrystallization using ethanol.
Yellow orange solid, yield—72%, m.p. 154–156 °C IR (KBr cm−1):
3109 (Ar C–H str), 1372 (C–N str), 1593 (CN str) 1134 (C–N–C str),
1018 (N–N str), 1510 and 1338 (Ar–NO2). 1H NMR (DMSO, δ, ppm):
8.821 (s, 1 H, CHN), 8.67 (1 H, s, triazole-H) 8.376–8.354 (2 H, d, Ar–
H), 8.194–8.172 (2 H, d, Ar–H), 7.997 (1 H, s, triazole–H) 7.383–7.329
Table 2.
Values of solvent polarity functions using different correlation methods.
Solvent
F1 (ε,n)1
F2 (ε,n)2
F3 (ε,n)3
ENT 4
n-Hexane
1,4-Dioxane
Toluene
Diethylether
Ethylacetate
THF
−0.00136
0.02451
0.01323
0.16674
0.19963
0.20957
0.21713
0.26347
0.27217
0.28430
0.28874
0.30855
0.27438
0.30659
0.26308
−0.00252
0.04990
0.02907
0.37658
0.48908
0.54907
0.59033
0.75042
0.76160
0.79037
0.81293
0.85460
0.83556
0.86391
0.84038
0.25378
0.31162
0.34990
0.42823
0.49791
0.55113
0.58296
0.64655
0.63739
0.63936
0.65245
0.65104
0.70980
0.66421
0.74416
0.009
0.164
0.099
0.117
0.228
0.207
0.321
0.602
0.546
0.355
0.654
0.762
0.386
0.460
0.444
DCM
Butanol
Isopropanol
Acetone
Ethanol
Methanol
DMF
Acetonitrile
DMSO
1
Lippert–Mataga bulk solvent polarity function.
Bakhshiev bulk solvent polarity function.
Kawski–Chamma–Viallet bulk solvent polarity function.
Reichardt microscopic solvent polarity function.
2
3
4
Fig. 5. Emission spectra of NBTMPA in different organic solvents.

R. Alphonse et al. / Journal of Molecular Liquids 215 (2016) 387–395
391
Fig. 6. Linear plots of Bilot–Kawski (1) and (2), Lippert–Mataga (3), Bakhshiev (4), Kawski–Chamma–Viallet (5) and Reichardt (6) correlation methods of solvent polarity functions f(ε,n),
φ(ε,n), F1 (ε,n), F2(ε,n), F3(ε,n) and ENT against (υA − υF), (υA + υF), (υA + υF) / 2.
state. Increasing the solvent polarity enhances the stoke shift values
from 2939 cm−1 to 8569 cm−1 for n-hexane to methanol respectively.
For a polar aprotic solvent, dimethyl sulfoxide and a polar protic solvent,
methanol the stoke shift values were found to be 6307 cm−1 to
8569 cm−1 respectively. These data implies that the molecule is influ-
enced by solvent parameters such as polarity, hydrogen bond donor
and hydrogen bond acceptor strength. It also reveals that in polar sol-
vents the molecule is more stabilized in the excited state than in the
ground state and the expected value for the excited state dipole mo-
ment would be higher. The weaker spectral red shifts of the excitation
spectrum with increasing solvent polarity are due to the fact that the
Schiff base derivative are more polarized in the excited state than the

392
R. Alphonse et al. / Journal of Molecular Liquids 215 (2016) 387–395
Table 3.
Statistical data of linear plots for different correlation methods of NBTMPA.
Compound
NBTMPA
Method
Slope (m)
Intercept (c)
Correlation Coefficient (R2)
Nb
Bilot–Kawski (1)
Bilot–Kawski (2)
Lippert–Mataga
Bakhshiev
Kawski–Chamma–Viallet
Reichardt
m
m
(1) = 4618.44
3156.13
77331.5
3023.57
3192.42
38503.99
3403.63
0.9698
0.9384
0.9598
0.9790
0.9135
0.9280
13
13
14
11
13
9
(2) = 13,635.42
m1 = 13,240.22
m2 = 4533.2
m3 = 13,149.57
m4 = 7770.343
b
Number of data.
ground state. Thus the excitation spectra are less sensitive to the effect
of solvent polarity as compared to the emission spectra.
volume using Edward's atomic increment method. The Onsager cavity
radius (a) of NBTMPA is calculated by the following expression [37].
ꢄ
ꢅ
1
3.2. Experimental and theoretical calculations of dipole moments
=
3
3M
4πδNA
a ¼
ð25Þ
To study the solvatochromism of NBTMPA, the spectral parameters
are found to be correlated with different solvent polarity methods. By
employing Bilot–Kawski, Lippert–Mataga, Bakhshiev, Kawski–
Chamma–Viallet and Reichardt linear correlations, linear graphs of (υ
A −υF) and (υA +υF) were plotted against f(ε,n) and φ(ε,n) respectively,
the stokes shift and its mean {υA − υF and (υA + υF) / 2} Table (1) of
NBTMPA (Fig. 6) are plotted against polarity functions F1(ε,n), F2(ε,n),
F3(ε,n) and ENT (Table 2).
The slopes, intercepts, correlation coefficients and the data of differ-
ent solvent correlation methods from Fig. 6 are summarized in Table 3.
The higher correlation coefficient values (greater than 0.90) suggest
better linearity for these methods. The ground state and excited state di-
pole moments are calculated from the slopes of the linear graphs of
Bilot–Kawski method using Eqs. (16) and (17). By applying Eqs. (13)
to (15) and (22) of Lippert–Mataga, Bakhshiev, Kawski–Chamma–
Viallet and Reichardt solvatochromic shift methods, the excited dipole
moment can be estimated from the respective slopes. The quantum
yield of NBTMPA was calculated using single point method by compar-
ing the integrated intensity and optical density of the sample to that of a
reference compound, anthracence in ethanol at room temperature. The
quantum yield (Ф) of NBTMPA was determined by the following rela-
tion and is expressed as.
where M is the molecular weight of solute, δ is the density of the
solute and NA is the Avagadro's number.
From Table 4, it is observed that the excited state dipole moments
(μe) of NBTMPA are higher than the ground state dipole moments
(μg). The calculated values for excited state dipole moments using
Bilot–Kawski, Bakhshiev and Reichardt correlation methods are com-
paratively in good agreement. The excited state dipole moment calcu-
lated by Lippert –Mataga and Kawski–Chamma–Viallet method is
slightly higher than the values obtained by the earlier correlation
methods, as these does not consider the effect of solute polarizability.
The difference in dipole moment can be explained on the basis of charge
transfer (CT) and nature of the emitting state. By using Eqs. (16) and
(17) the ground and excited state dipole moments of NBTMPA were cal-
culated by assuming that the dipole moments are almost parallel [38].
So we have estimated the angle between the dipole moments using
Eq. (19) which was found to be 0° implying that they are parallel. The
ground state dipole moment was calculated using theoretical ab initio
calculations by Gaussian 09 W software and Bilot–Kawski correlation
method. From Table 4, μg calculated from both methods are fairly in
good agreement. Using TD–DFT analysis the ground and excited dipole
moments were estimated as μg = 3.7 D and μe = 6.9 D indicating that
the molecule is in a polar excited state. Also, the solvent used was
found to be in a more stabilized state. The calculated values for the di-
IS ODR
IR ODS
η2
η2
S
Φ ¼ Φstd
ꢃ
ꢃ
ꢃ
R
pole moments in the two electronic states are μg = 5.7 D and μe
=
Where Φstd is the quantum yield of reference standard (0.27 from lit-
erature), IS and IR are the integrated fluorescence intensity of sample
and reference standard, ODS and ODR are the optical densities of sample
and standard, and η2S and η2R are the square of refractive indices of sol-
vent respectively. The ratio of square of the refractive indices was found
to be 1. The calculated quantum yield (Ф) of NBTMPA was found to be
0.205 [36]. The spectral properties like fluorescence maxima, quantum
yield are controlled by inter molecular hydrogen bonding. The Onsager
cavity radius of NBTMPA was calculated by means of Vander Waal's
11.5 D suggesting a more polar excited state for the molecule. This
also implies that the interactions between solute and solvent are stron-
ger in the excited state which leads to a change in the distribution of
charge densities. Using Eq. (18) the ratio of dipole moment in the excit-
ed state to the dipole moment in the ground state was found to be 2.02.
PICT has been used to explain the charge separation in NBTMPA. 1, 2,
4-Triazole nucleus does not produce any significant change in the
movement of π electrons. The intramolecular charge transfer state is
Table 4.
Dipole moments in two electronic states (Debye) of NBTMPA.
Compound Onsager radiusa μg
μg
μe
μe
μe
μe
μe
μe
b
c
d
e
f
g
h
i
NBTMPA
4.06
3.7 5.7 6.9 11.5 13.6 9.48 13.55 7.64
a
Onsager radius of NBTMPA using Edward's atomic increment method.
Dipole moment in the ground state calculated by Gaussian software.
Dipole moment in the ground state calculated using Bilot–Kawski method (Eq. (16)).
Dipole moment in the excited state calculated by Gaussian software.
Dipole moment in the excited state calculated using Bilot–Kawski method (Eq. (17)).
Dipole moment calculated using Lippert Mataga method (Eq. (13)).
Dipole moment calculated using Bakhshiev method (Eq. (14)).
b
c
d
e
f
g
h
i
Dipole moment calculated using Kawski–Chamma–Viallet method (Eq. (15)).
Dipole moment calculated using Reichardt method (Eq. (22)).
Fig. 7. Canonical structures of NBTMPA.

R. Alphonse et al. / Journal of Molecular Liquids 215 (2016) 387–395
393
Fig. 8. Photon induced intramolecular charge transfer (PICT) of NBTMPA as donor–accep-
tor (D–A) molecule.
formed when a substantial amount of charge is transferred from benzyl
ring to nitro group through an azomethine linkage upon excitation. This
makes the nitro group a strong electron donor and benzylic ring a strong
electron acceptor (Fig. 7). This also indicates that the molecule becomes
more polarized in the excited state than in the ground state.
Fig. 10. Molecular electrostatic potential (MESP) plot of NBTMPA. (For interpretation of
the references to colour in this figure, the reader is referred to the web version of this
article.)
NBTMPA is an example for donor–acceptor molecule, which consists
of both electron donating and acceptor groups within a molecule. When
NBTMPA absorbs a photon and gets promoted into the excited state the
electron from the benzylic ring (donor) migrates towards the nitro
group (acceptor). This results in a highly polar state or CT state making
the benzyl ring a strong electron acceptor and the nitro group a strong
electron donor in the excited state. The dipole moment of NBTMPA at
the excited state is twice as much as it is when in the ground state.
This difference in the dipole moments across the two electronic states
can be explained on the basis of intramolecular charge transfer (ICT).
It indicates that the charge transfer in excited state is accompanied by
photon induced intramolecular charge transfer (PICT). Thus due to
PICT (Fig. 8) the planarity of the molecule decreases on excitation,
which also renders the molecule more polar in the excited state as com-
pared to the ground state [39].
The molecules possessing a large energy gap between HOMO–LUMO
energies are known as hard, whereas molecules possessing a small en-
ergy gap are known as soft molecules. Soft molecules require very
little energy for excitation and thus they are more polarized than hard
molecules. The chemical hardness (η) can be estimated by the following
expression [40].
½ELꢀEHꢁ
η ¼
ð26Þ
2
Where EH and EL are the HOMO and LUMO energies of respective
frontier molecular orbitals. The hardness of the molecule was found to
be — 0.133 a.u. Thus we can conclude that from the estimated value
the molecule has comparatively small energy gap which indicates that
the molecule, NBTMPA is soft.
The molecular electrostatic potential (MESP) plot given in Fig. 10
provides the information for determining a suitable position for nucleo-
philic and electrophilic attack along with the hydrogen bonding interac-
tions of solvent. Different colours in the MESP plot symbolize the
different values of electrostatic potential at the surface. The negative
phase in the MESP plot is represented in red colour which can be related
to the electrophilic site, and the positive phase is represented in blue
colour corresponds to the nucleophilic site. From the MESP plot, the
negative phases are localized in nitro group and 1, 2, 4-triazole nucleus
and positive phases are enclosed by benzylic ring, azomethine linkage
and all hydrogen atoms.
Theoretical calculations by Gaussian 09 W software were used to de-
termine the ground state dipole moment of NBTMPA. The information
regarding the charge distribution was obtained from the atomic charges
of each atoms of NBTMPA and the analysis was performed in ethanol. In
solvent medium, O atoms (35,36), nitrogen atoms except (N-34) and
most of the carbon atoms carry a significant negative charge, which
act as donor atoms; while C (1), C (2), C (11), C (18), C (22), C (24), C
(30) and N (34) atoms carry a positive charge, which act as acceptor
atoms. In solvent medium the distribution of charge directs from N
(34) to C (14) atoms having magnitude +0.388 eV to −0133 eV respec-
tively. Due to this charge separation of the molecule, NBTMPA will be
more polarized in the excited state. This charge transfer phenomena
3.3. Computational analysis
The direction of charge transfer can be estimated on the basis of
frontier molecular orbitals. HOMO and LUMO energies were calculated
in an ethanolic medium which provides the information regarding the
charge transfer within the molecule. The electron donating ability is
characterized by HOMO, whereas electron acceptor ability is character-
ized by LUMO. The difference in the HOMO and LUMO energies gives in-
formation about the chemical stability of the molecule. If the HOMO–
LUMO energy gap is small, the HOMO electrons can be easily excited
to the LUMO orbitals. The highest occupied and lowest unoccupied mo-
lecular orbitals energies were computed in ethanol as −0.239 and
−0.106 a.u. respectively. The HOMO and LUMO 3D plots of NBTMPA
are shown in Fig. 9. From HOMO–LUMO plots, the red region corre-
sponds to the positive phase and the blue region corresponds to the
negative phase. It was observed that HOMO is localized on azomethine
linkage and benzylic ring, whereas LUMO is localized over the entire
molecule except for methylene group and 1, 2, 4-triazole nucleus.
The chemical stability of a molecule gives significant information
about the chemical hardness (η) of a molecule. The hardness of the mol-
ecule can be estimated by using highest occupied molecular orbital
(HOMO) and lowest unoccupied molecular orbital (LUMO) energies.
Fig. 9. HOMO (a) and LUMO (b) 3D plots computed by TDDFT studies for NBTMPA. (For interpretation of the references to colour in this figure, the reader is referred to the web version of
this article.)

394
R. Alphonse et al. / Journal of Molecular Liquids 215 (2016) 387–395
Table 5.
state. These interpretations of multiple linear regression analysis can be
confirmed on the basis of results tabulated in Table 4.
Kamlet–Taft parameters values for multiple linear regression analysis.
Solvents
Α
Β
π*
4. Conclusion
Hexane
0
0
0
0
0
0
0
0
0.3
0.79
0.76
0.08
0.83
0.93
0.19
0
−0.081
0.55
0.54
0.27
0.55
0.58
1
0.88
0.82
0.47
0.48
0.71
0.54
0.6
1,4-Dioxane
Toluene
Diethyl ether
Ethyl acetate
THF
DMSO
DMF
DCM
Butanol
Isopropanol
Acetone
Ethanol
Methanol
Acetonitrile
0.37
0.11
0.47
0.45
0.55
0.76
0.69
0
0.88
0.84
0.48
0.77
0.62
0.31
A novel azomethine derivative containing 1, 2, 4-triazole moiety
bearing an electron withdrawing substituent was synthesized and char-
acterized by 1H NMR, IR and UV spectral studies. The effect of solvents in
excitation and emission spectra was analysed using different solvent
polarity correlation methods. We have estimated the dipole moment
in the ground state by using both Gaussian 09 W Software and Bilot–
Kawski method. The dipole moment in the excited state was deter-
mined by Bilot–Kawski, Lippert–Mataga, Bakhshiev, Kawski–
Chamma–Viallet, and Reichardt correlation methods. In all the methods
the dipole moment in the excited state for NBTMPA were found to be
higher than the ground state dipole moment. This implies that
NBTMPA is more polar or stable in the excited state than in the ground
state. This also indicates that the azomethine derivative has a more re-
laxed excited state due to photon induced intramolecular charge trans-
fer (PICT). Computational studies were performed using TDDFT
calculations which imply that the molecule, NBTMPA has both nucleo-
philic and electrophilic sites. Thus NBTMPA can serve as a promising
candidate for luminescence materials, fluorescent probes and non-line-
ar optical materials. Multiple linear regression analysis was performed
to investigate the interaction between solvent and solute. Both the exci-
tation and emission spectra were controlled by polarity/dipolarizability
0.75
gives rise to a difference in ground state and excited state dipole mo-
ments. The atomic charges of all hydrogen atoms exhibit a positive
charge in ethanol, indicating that it can donate electrons from hydrogen
atom to other atoms. These investigations confirm the results from mul-
tiple linear regression analysis.
3.4. Multiple linear regression analysis
Multiple linear regression analysis provides the information on dif-
ferent types of interactions between solute and solvent. It is also used
to explain the mechanism of electronic transition. From the existing lit-
erature, it is well known that the intensity, position and shape of the ab-
sorption spectrum can explain the spectral shifts with the nature of
solvent and solute. The spectral shifts induced by solvents were calcu-
⁎
of non-specific interactions (π ) of solvent.
Acknowledgements
The authors are grateful to D Sravan Kumar Perumalla (IISc, Banga-
lore, India) for computational analysis. Author would also like to thank
NMR Research Centre (IISC, Bangalore, India) for the spectral analysis.
⁎
lated using Kamlet–Taft parameters such as α, β and π . In order to
get more information on the solvatochromic properties of NBTMPA,
the spectral properties were correlated with Kamlet–Taft parameters.
The wave numbers of absorption (υA), fluorescence (υF) and stokes
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