Estimation of ground and excited state dipole moments of synthesized coumarin derivative [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane

Article abstract of DOI:10.1016/j.molliq.2014.09.054

Electronic absorption and fluorescence spectra of coumarin derivative [N-(2-oxo-2H-chromen-4-yl)imino] triphenyl phosphorane have been recorded at room temperature in wide range of solvents of different polarities. The absorption maximum remains almost un

Full text of DOI:10.1016/j.molliq.2014.09.054

Journal of Molecular Liquids 200 (2014) 115–119
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Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Estimation of ground and excited state dipole moments of synthesized
coumarin derivative [N-(2-oxo-2H-chromen-4-yl)imino]
triphenyl-phosphorane
a
b
b
b
a,
Sunita Joshi , Santosh Kumari , Rituparna Bhattacharjee , Rajeev Sakhuja , Debi D. Pant ⁎
a
Department of Physics, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India
Department of Chemistry, Birla Institute of Technology and Science, Pilani 333031, Rajasthan, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Electronic absorption and fluorescence spectra of coumarin derivative [N-(2-oxo-2H-chromen-4-yl)imino]
triphenyl phosphorane have been recorded at room temperature in wide range of solvents of different polarities.
The absorption maximum remains almost unchanged with the increase in solvent polarity, whereas, a red shift in
fluorescence emission maximum was observed. Synthesized compound [N-(2-oxo-2H-chromen-4-yl)imino]
Received 26 August 2014
Accepted 30 September 2014
Available online 1 October 2014
1
13
triphenyl phosphorane was characterized by H and C NMR, and FT-IR spectral studies. The ground and excited
state dipole moments of molecule were obtained from Bakhshiev's and Bilot–Kawski's equations by means of the
solvatochromic shift method. Very high value of dipole moment is observed for excited state as compared to
ground state value and this is attributed to more polar excited state of the molecule. Numerical calculations
are performed using B3LYP/6-31G+(d) and B3LYP/6-31G(d) level of theory for ground state in Gaussian 03.
Studies in different solvents are also done using the CPCM method and UA0 radii with the same level of theory.
A critical analysis between the values of dipole moment in gas phase and various solvents is also carried out for
ground state.
Keywords:
Solvatochromic shift
Dipole moment
Absorption
Fluorescence
[N-(2-oxo-2H-chromen-4-yl)imino] triphenyl
phosphorane
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
to ground state. The dipole moment of an electronically excited state of
a molecule is an important property that provides information on the
electronic and geometrical structure of the molecule in short-lived
state. Knowledge of the excited-state dipole moment of electronically
excited molecules is quite useful in designing nonlinear optical mate-
rials [27], elucidating the nature of the excited states and in determining
the course of photochemical transformation. For a chromophore, the
tunability range of the emission energy as a function of polarity of the
medium is also determined by excited state dipole moment [28].
Solvent affects the equilibria, absorptions, emissions and the mecha-
nisms of reactions for many compounds. Understanding solute–solvent
interactions and measuring them quantitatively are an active area of re-
search. The emission and absorption spectroscopy is the widely used
method for investigating the solvatochromism with the help of various
solvent parameters.
A number of techniques e.g. electronic polarization of fluorescence,
electric-dichroism, microwave conductivity and stark splitting [29–32]
are available for determination of excited-state dipole moment, but
their use is limited because they are considered equipment sensitive
and studies have been related to very simple molecules. As the
solvatochromic method does not use any external field [33,34], it is
experimentally much simpler and widely accepted. The solvatochromic
method is based on the shift of absorption and fluorescence maxima in
different solvents of varying polarity. The solvent dependence of
Coumarins have been extensively studied due to their practical
applications [1–5], which include uses as biological and chemical sen-
sors, fluorescent probes and laser dyes [6–9]. Coumarins possess anti-
inflammatory, antiallergic, hepatoprotective, spasmolitic, antiviral, anti-
carcinogenic and anticoagulant activities [10]. They also constitute an
important group of organic compounds that are used as additives to cos-
metics, as optical brightening agents [11–13]. Due to their inherent
physicochemical and photophysical characteristics, such as reasonable
relative ease of synthesis, coumarin derivatives have been extensively
investigated for electronic and photonic applications such as charge-
transfer complexes, laser dyes, fluorescence whiteners, solar energy col-
lectors, and non-linear optical materials [14–17].
Studies on the photophysical properties of the coumarin and its de-
rivative have been the subject of intense research in photochemistry for
quite a long time [18]. In spite of significant amount of literature reports,
many of the photophysical characteristics are still not explored [19–26].
Excitation of a molecule by photon causes a redistribution of charges
leading to conformational changes in the excited state. This can result in
increase or decrease of dipole moment of the excited state as compared
⁎
Corresponding author.
E-mail address: ddpant@pilani.bits-pilani.ac.in (D.D. Pant).
http://dx.doi.org/10.1016/j.molliq.2014.09.054
167-7322/© 2014 Elsevier B.V. All rights reserved.
0

116
S. Joshi et al. / Journal of Molecular Liquids 200 (2014) 115–119
absorption and fluorescence maxima is used to determine the excited-
state dipole moments of different molecules. Several workers have
made extensive experimental and theoretical studies on ground (μ
and excited state (μ ) dipole moments using different techniques in va-
riety of organic fluorescent compounds like coumarins [35,36], indoles
37,38], purines [39,40], exalite dyes [41,42], quinazolines [43], quinine
g
)
e
[
sulfate [44,45], quinidine [46], 6-methoxyquinoline [47] etc.
In this paper, we have estimated the ground and excited state dipole
moments of synthesized coumarin derivative [N-(2-oxo-2H-chromen-
4
-yl)imino] triphenyl phosphorane by the solvatochromic shift method
using Bakhshiev [48] and Bilot–Kawski [49,50] correlations. Although
this compound is described in the literature, Saito et al. [51] have carried
out some thermal studies on this compound and they have used very
different synthesis methods than ours. To the best of our knowledge
there are no reports available in literature on the determination of
ground and excited state dipole moments of this compound.
Fig. 1. Molecular structure of [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane.
13C NMR (100 MHz, CDCl
): δ 164.1, 161.7, 154.0, 154.0, 132.8, 132.8,
3
1
1
32.7, 132.6, 130.9, 129.2, 129.1, 128.4, 127.4, 125.5, 122.9, 122.5, 122.2,
16.6, 94.6, 94.4.
2
. Experimental
2.3. Measurements
2
.1. Materials
The melting point was found out using the capillary method in a
1
melting point apparatus. The H NMR was recorded on a Bruker AMX
4-hydroxycoumarin, POCl
3
, NaN
3
and triphenylphosphine were pro-
13
4
00 MHz spectrometer and C NMR spectra were recorded on a Bruker
cured from Spectrochem and SRL, India and were used without further
purification. All the solvents used were of HPLC grade or AR (purchased
from Alfa Aesar).
AMX 100 MHz solid state NMR spectrometer. IR spectra (KBr) were
recorded on ABB Bomen MB 3000 FTIR. TLC analysis was carried out
on pre-coated silica gel glass plates using the solvent system ethyl
acetate:hexane (40:60).
Absorption spectra were taken using dual beam Thermo Evolution
01 UV/Vis/NIR spectrophotometer and fluorescence spectra were
recorded using a Shimadzu RF-5301PC spectrofluorometer. The data
were analyzed using a related software. The spectral shifts obtained
with different sets of samples were identical in most of the cases and
values were within ± 1.0 nm. Data were analyzed and were fitted to a
2
2
.2. Synthesis of coumarin derivative
2
.2.1. 4-chlorochromen-2-one (2)
3
A solution of 4-hydroxycoumarin (1) (3.0 g, 1 eq.) in POCl (6 mL)
was refluxed at 90 °C for 2 h. The completion of the reaction was mon-
itored via TLC. The reaction mixture was allowed to attain room temper-
ature and was added drop-wise to an ice bath with continuous stirring.
The precipitated yellow solid was filtered and washed with cold
water (3 × 20 mL). The dried crude solid was washed with hot hexane
0
straight line using Origin 6.1 software. Molecular radius a (Å) of the
spherical cavity is obtained for optimized geometry calculated at the
B3LYP/6-31G(d) level of theory (in liquid phase) using Gaussian 03.
The concentration of compound in all the solutions prepared in dif-
(
3 × 30 mL) and filtered from the remaining solid. Evaporation of
the combined hexane filtrates under reduced pressure yielded 4-
chlorocoumarin (2) as a white solid.
−4
ferent solvents was 10 M and the molecular structure of [N-(2-oxo-
2
H-chromen-4-yl)imino]triphenyl-phosphorane is shown in Fig. 1.
−
1
Yield: 2.2 g, 65%; m.p. 86–88 °C ([52] 87–89 °C); IR (KBr) cm
1
755, 1720, 1601, 1450, 1271, 1176.
3. Method
2
.2.2. 4-azidochromen-2-one (3)
To a solution of 4-chlorocoumarin (2) (2.0 g, 1 eq.) in anhydrous
DMF (10 mL), NaN (2 eq.) was added at room temperature. The stirring
Most theories of solvent effect on the location of absorption and
3
fluorescence spectra, in spite of different assumptions, lead to similar
expressions for the Stokes shift. We have used the following two for-
mulae to determine the ground and excited state dipole moments by
the solvatochromic method. These equations have been obtained by
employing the simplest quantum-mechanical second order perturbation
was continued for about 10 h, until the consumption of the starting ma-
terial as monitored by TLC. After the completion of the reaction, the re-
action mixture was added drop-wise to crushed ice and the resultant
precipitate was filtrated, washed with cold water (3 × 15 mL) and
dried to pure 4-azidocoumarin (3) as a light yellow solid.
−
1
Yield: 2.03 g, 98%; m.p. 161–162 °C ([53] 160 °C); IR (KBr) cm
175 (N ) and 1736 (C_O).
2
3
2
.2.3. [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane (4)
To a solution of 4-azidocoumarin (3) (2.0, 1 eq.) in THF (20 mL) at
room temperature, a solution of triphenylphosphine (2 eq.) in THF
10 mL) was added drop-wise. The reaction mixture was stirred for
h at room temperature and the progress of the reaction was monitored
(
4
via TLC. After the completion of the reaction, solvent was evaporated
under reduced pressure and re-crystallization of the residue using
diethyl ether gave [N-(2-oxo-2H-chromen-4-yl)imino] triphenyl-
phosphorane (4) as a colorless solid.
Yield: 4.4 g, 99%; m.p. 238–239 °C ([51] 239.2–240.5 °C).
1
3
H NMR (400 MHz, CDCl ): δ 8.44 (dd, J = 7.9, 1.6 Hz, 1H), 7.81–7.73
(
(
m, 6H), 7.61 (td, J = 7.2, 1.4 Hz, 3H), 7.55–7.44 (m, 7H), 7.31–7.23
m, 2H), 5.09 (s, 1H).
Fig. 2. Optimized molecular geometry of [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-
phosphorane using B3LYP/6-31G(d) level of theory in gas phase.
IR (KBr) cm−1: 1697 (C_O), 1535, 1384.

S. Joshi et al. / Journal of Molecular Liquids 200 (2014) 115–119
117
Table 1
states are parallel, the following expressions are obtained using
Eqs. (3) and (6) [49]
Ground state dipole moment evaluated at B3LYP/6-31+G(d) level of theory.
Solvent
Dielectric constant
Dipole moment (μ
g
)a
"
#
S −S hca03
1=2
Dimethyl sulfoxide
Acetonitrile
Dichloromethane
Tetrahydrofuran
Diethyl ether
Toluene
46.4
35.9
8.9
7.6
4.3
2.3
2.2
–
12.51
12.48
12.13
12.04
11.65
11.02
10.93
9.20
2
1
μg
¼
ð7Þ
ð8Þ
2
2S1
"
#
3
1=2
S þ S hca0
1
2
Carbon tetrachloride
Gas
μe
¼
2
2S1
a
g
μ is the ground state dipole moment evaluated at B3LYP/6-31+G(d) level of theory
and
for gas phase and also the same level of theory using the CPCM model along with UA0 radii
for solvents.
jS þ S j
1
2
μe
¼
μ :
ð9Þ
g
jS −S j
2
1
theory and taking into account the Onsager reaction field for a polarisable
dipole.
Bakhshiev's formula [48]
3.1. Computational calculations
v −v ¼ S F ðε; ηÞ þ const:
ð1Þ
a
f
1
1
Density functional theory (DFT) [54,55] cuts down the steep rise in
computational cost efficiently compared to ab initio wave function
techniques and it is also proved to be accurate for calculating several
important physical properties of molecules. Systematic and critical
comparisons of DFT theories with experiment and also with HF and
Møller–Plesset (MP2) treatments are carried out by many researchers
over years [56]. In this paper, geometry optimizations were carried out
at B3LYP/6-31G(d) [57–59] level of theory for ground state of coumarin
derivative [N-(2-oxo-2H-chromen-4-yl)imino] triphenyl phosphorane.
Harmonic frequencies are simultaneously checked to ensure the mini-
mum energy structure. Subsequently, single point calculations are done
for different solvents and also for gas phase with higher level of the-
ory, i. e., B3LYP/6-31G+(d) to obtain the ground state dipole mo-
here, v and v are the wavenumbers of the absorption and emission
a
f
maxima respectively.
the bulk solvent polarity function and S
F
1
1
the slope are defined as
ð2Þ
follows:
"
#
2
2
F ðε; ηÞ ¼ 2
η þ 1 ε−1
−
η −1
1
2
2
η þ 2 ε þ 2 η þ 2
and
ꢀ
ꢁ
2
2
μe−μ
g
S1 ¼
ð3Þ
hca03
g
ment μ . The basis set 6-31G+(D) (which has polarization functions
on nonhydrogen atoms and diffuse functions are added to heavy
atoms) can efficiently determine the properties of moderately large or-
ganic molecules. To take care of the solute–solvent interaction, the sol-
ute is placed in a cavity (via a set of overlapping spheres) within the
solvent reaction field using a conductor-like polarizable continuum
model (CPCM) [60,61]. To build the cavity, UA0 radii are used where
the United Atom Topological Model is applied on atomic radii of the
UFF force field for heavy atoms. Hydrogens are enclosed in the sphere
of the heavy atom to which they are bonded. The SCF density is used
for all ground state calculations. All calculations were carried out
using Gaussian 03 suite of program [62]. The optimized structure is
given in Fig. 2.
here, h denotes the Planck's constant, c is the velocity of light in vacuum,
is the dipole moment in the excited singlet state, a is the Onsager
cavity radius, ε is the solvent dielectric constant and η is the solvent re-
fractive index.
μ
g
0
Bilot–Kawski formula [49,50]
v þ v
a
f
¼
−S F ðε; ηÞ þ const:
ð4Þ
2
2
2
here, the meaning of symbols is the same as given above except for F
2
and S
2
which are defined as follows
"
#
"
#
2
2
4
2
2
η þ 1 ε−1
η −1
3
2
η −1
F2ðε; ηÞ ¼
ꢂ
ꢃ
−
þ
ꢂ
ꢃ
ð5Þ
4. Results and discussion
2
2
2
2
η þ 2 ε þ 2 η þ 2
η þ 2
Dipole moment bears a close relation with the degree of charge
distribution and thus it reflects the net molecular polarity. Usually,
it is exploited to determine a variety of physical and chemical prop-
erties. The dipole moment of coumarin derivative [N-(2-oxo-2H-
chromen-4-yl)imino] triphenyl phosphorane is computed at the
B3LYP/6-31+G(d) level of theory in different environments (various
solvents with varying polarities and gas phase) and are shown in
Table 1. A distinct variation in dipole moment is observed while going
and
ꢀ
ꢁ
2
2
2
μe −μg
S2 ¼
:
ð6Þ
hca03
The parameters S
1 2
and S are the slopes which can be calculated
from Eqs. (1) and (4) respectively. Assuming the ground and excited
Scheme 1. Synthesization methodology of [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane.

118
S. Joshi et al. / Journal of Molecular Liquids 200 (2014) 115–119
Table 2
Different solvent parameters and spectral data of [N-(2-oxo-2H-chromen-4-yl)imino]
triphenyl-phosphorane in different solvents.
Solvent
ε
η
F
1
F
2
va−vf
va þv f
2
cm 1)
−
(
cm−1)
(
Dimethyl sulfoxide
Acetonitrile
Dichloromethane
Tetrahydrofuran
Ethylacetate
Diethyl ether
O-xylene
Toluene
46.4
35.9
8.9
7.6
6.1
4.3
2.4
2.3
2.2
1.4770
1.3442
1.4241
1.4070
1.3720
1.3526
1.5054
1.4969
1.4610
0.8410
0.8660
0.5900
0.5450
0.4890
0.3730
0.0270
0.0330
0.0144
0.7440
0.6560
0.5820
0.5490
0.4970
0.4260
0.3540
0.3510
0.3190
9015.44
9360.99
8803.22
9292.04
9034.52
9019.14
8816.31
8628.94
8716.50
26461.6
26452.8
26683.3
26352.1
26500.1
26623.7
26551.7
26502.2
26639.9
Carbon tetrachloride
acetonitrile on excitation at 320 nm. The variation of wavenumber of
absorption and emission maxima with the solvent polarity function, F
ε, η), is shown in Fig. 4. The absorption maximum for different solvents
studied remains constant with the polarity function, whereas, the emis-
sion maximum shifts towards lower frequencies with the increase in po-
larity of the solvent.
1
Fig. 3. Normalized absorption spectrum and fluorescence spectra of [N-(2-oxo-2H-chromen-
-yl)imino]triphenyl-phosphorane in (a) acetonitrile, (b) ethylacetate, and (c) toluene,
d) CCL
(
4
(
4
.
from gaseous to solvent phases. Higher values of dipole moments are
obtained in the solvent phases compared to the gas phase. This is due
to the intermolecular interactions between the solvent molecules and
the studied compound (i. e., the coumarin derivative). The value of
the dipole moment increases with the polarity of the solvent, i. e., the
higher the dielectric constant, the higher is the observed dipole mo-
ment (Table 1).
The red shift in fluorescence spectrum is more in the case of acetoni-
trile aprotic solvent as compared to nonpolar solvents. This trend in the
fluorescence spectra is a bathochromic shift with increase in polarity
and is an indication of π, π* transition. The shift of the fluorescence
wavelengths towards longer wavelengths could be caused, if the excited
state charge distribution in the solute is markedly different from the
ground-state charge distribution, and is as to give rise to stronger inter-
actions with polar solvents in the excited state. Solvent polarity func-
tions F (ε, η) and F (ε, η) have been calculated in order to ascertain
[N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane (4) used
in the present studies was synthesized in a methodology shown in
1
2
Scheme 1. Commercially available 4-hydroxychromen-2-one (1) on re-
action with POCl under refluxing conditions gave 4-chlorochromen-2-
3
the ground and excited state dipole moments of the molecule and are
given in Table 2.
one (2), which on azidation using sodium azide in DMF gave 4-
azidochromen-2-one (3). 3 on reaction with triphenylphosphine in
THF at room temperature gave (4) in pure form as a colorless solid.
The absorption and fluorescence spectra of [N-(2-oxo-2H-chromen-
The spectral shifts v −v versus the solvent polarity function F (ε, η)
2
a
f
1
vaþvf
and
2
versus F (ε, η) are shown in Figs. 5 and 6, respectively. The linear
behavior of Stokes shift versus solvent polarity function indicates general
solvent effects as a function of dielectric constant and refractive index.
Using software origin 6.1, the data were fitted to a straight line. Slopes
4
-yl)imino]triphenyl-phosphorane were recorded in nonpolar and
−
1
−1
aprotic polar solvents at ambient temperature and are shown in Fig. 3.
No significant change in absorption spectrum with solvent polarity for
this coumarin compound is observed while markedly emission maxi-
mum shifts towards lower frequencies with the increase in polarity of
the solvent.
were found to be S1 = 560.78 cm
and S2 = −289.84 cm
from
Figs. 5 and 6, respectively. Using Gaussian 03 software, we calculated
ground state dipole moment for the probe at B3LYP/6-31G+(d) level.
The ground state dipole moment obtained was 9.20 D in gas phase
which matched well with the experimental one.
The emission spectrum has only a structure-less broad band with
maximum around 448 nm in carbon tetrachloride and 460 nm in
Further by making use of Eqs. (7) and (8), we calculated μ = 7.86 D
g
and μ = 24.90 D. The change in dipole moment from excited state to
e
32000
31000
30000
29000
9
400
2
6
3
2
5
9
7
4
9300
4
1
8
9200
9100
9000
5
6
1
9
3
8
6
7
5
8900
8800
22000
21500
21000
1
7
3
4
2
8700
9
8
8600
0
.0
0.2
0.4
F1
0.6
0.8
1.0
0.0
0.2
0.4
0.6
0.8
1.0
F1
Fig. 4. Variation of absorption and emission maxima of [N-(2-oxo-2H-chromen-4-yl)
imino] triphenyl phosphorane with solvent polarity function. (1) DMSO, (2) acetonitrile,
Fig. 5. Plot for Stokes shift versus solvent polarity function F
(3) dichloromethane, (4) THF, (5) ethyl acetate, (6) diethyl ether, (7) O-xylene, (8) toulene,
(9) CCL
1
. (1) DMSO, (2) acetonitrile,
(3) dichloromethane, (4) THF, (5) ethyl acetate, (6) diethyl ether, (7) O-xylene, (8) toulene,
(
9) CCL
4
.
4
.

S. Joshi et al. / Journal of Molecular Liquids 200 (2014) 115–119
119
26700
26650
26600
26550
26500
26450
26400
26350
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Table 3
Dipole moment of [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane molecule
in ground and excited states.
(cm−1)
(cm−1
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a
0
(Å)
S
1
S
2
)
μ
g
(D)
μ
e
(D)
Δμ (D)
μ
e
/μ
g
1
5.96
560.78
289.84
7.86
24.90
17.04
3.16
[
[
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We have calculated ground state and excited state dipole moments of
1
synthesized [N-(2-oxo-2H-chromen-4-yl)imino]triphenyl-phosphorane
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2
tween the ground state and the excited state suggests that fluorescence
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We acknowledge the University Grants Commission (UGC) and DST,
New Delhi, for major research projects and SJ thanks CSIR for senior re-
search fellowship, SK is thankful to UGC for JRF and RB is grateful to Birla
Institute of Technology and Science (BITS Pilani), Pilani campus, for the
computational facilities and financial assistance.
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