Photophysical study of 6-amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile and estimation of ground-state and singlet excited-state dipole moments by solvatochromic approaches

Article abstract of DOI:10.1016/j.molliq.2016.07.133

An investigation of absorption and fluorescence behaviour of 6-amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (NDPPC) at room temperature was carried out using a series of twelve organic solvents with different polarities. Ground and excited state dipole moments were determined experimentally by using Lippert–Mataga polarity function, Bakhshiev solvent polarity parameter, Kawski-Chamma-Viallet solvent polarity parameter, Reichardt's microscopic solvent polarity parameter, Kamlet-Abboud-Taft multiple linear regression and Catalan dipolar polarizibility approach. Due to considerable π- electron density redistribution, the value of excited state dipole moment was found to be greater than that of the ground state. The ground state dipole moment value was determined by quantum chemical method which was used to estimate excited state dipole moment using solvatochromic correlations. Both non-specific solute-solvent interactions and hydrogen bonding interactions were investigated by using Kamlet-Abboud-Taft and Catalan method. TD-DFT (B3LYP/6-311G (d,p)) was used for the determination of HOMO-LUMO energies.

Full text of DOI:10.1016/j.molliq.2016.07.133

Photophysical
study
of
6-amino-3-methyl-4-(4-nitrophenyl)-1,4-
dihydropyrano[2,3-c]pyrazole-5-carbonitrile and estimation of ground-state
and singlet excited-state dipole moments by solvatochromic approaches
Rekha Kumari, Anitha Varghese, Louis George, Akshaya K. B.
PII:
DOI:
Reference:
S0167-7322(16)30675-4
doi: 10.1016/j.molliq.2016.07.133
MOLLIQ 6147
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
18 March 2016
27 July 2016
28 July 2016
Please cite this article as: Rekha Kumari, Anitha Varghese, Louis George, Akshaya K.
B., Photophysical study of 6-amino-3-methyl-4-(4-nitrophenyl)-1,4-dihydropyrano[2,3-
c]pyrazole-5-carbonitrile and estimation of ground-state and singlet excited-state dipole
moments by solvatochromic approaches, Journal of Molecular Liquids (2016), doi:
1
0.1016/j.molliq.2016.07.133
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ACCEPTED MANUSCRIPT
Photophysical
study
of
6-amino-3-methyl-4-(4-nitrophenyl)-1,4-
dihydropyrano[2,3-c]pyrazole-5-carbonitrile and estimation of ground-
state and singlet excited-state dipole moments by solvatochromic
approaches
Rekha Kumari, AnithaVarghese*, Louis George, Akshaya K. B.
Department of Chemistry, Christ University, Bangalore – 560 029, India.
ABSTRACT
An investigation of absorption and fluorescence behaviour of 6-amino-3-methyl-4-(4-
nitrophenyl)-1,4-dihydropyrano[2,3-c]pyrazole-5-carbonitrile (NDPPC) at room temperature
was carried out using a series of twelve organic solvents with different polarities. Ground and
excited state dipole moments were determined experimentally by using Lippert–Mataga
polarity function, Bakhshiev solvent polarity parameter, Kawski-Chamma-Viallet solvent
polarity parameter, Reichardt's microscopic solvent polarity parameter, Kamlet-Abboud-Taft
multiple linear regression and Catalan dipolar polarizibility approach. Due to considerable π-
electron density redistribution, the value of excited state dipole moment was found to be
greater than that of the ground state. The ground state dipole moment value was determined
by quantum chemical method which was used to estimate excited state dipole moment using
solvatochromic correlations. Both non-specific solute-solvent interactions and hydrogen
bonding interactions were investigated by using Kamlet-Abboud-Taft and Catalan method.
TD-DFT (B3LYP/6-311G (d,p)) was used for the determination of HOMO-LUMO energies.
Keywords: Dipole moment, Solvatochromic method, Stokes shift, Intra-molecular charge
transfer
*
Corresponding author. Tel: +91 80 4012 9313; Fax: +918040129000
E-mail address: anitha.varghese@christuniversity.in
Introduction
Photophysical properties of organic molecules have been the subject of intensive
investigation due to their ability to undergo intramolecular charge transfer (ICT) which
results in the formation of highly polar excited state. Upon photoexcitation, charge
redistribution occurs in a molecule containing donor-acceptor nucleus. In order to study such
a physico-chemical process taking place in bulk solutions, determination of ground and
excited state dipole moments are of great importance. This helps us to examine the attack of
1

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nucleophilic and electrophilic sites in photochemical reactions [1]. The sensitivity of solvents
to UV-visible absorption technique has played a major role in probing solute-solvent
interactions. Differential stabilization of ground and excited states are observed when the
polarities of the solvents are varied. Therefore, the determination of the ground and excited
state dipole moments are important in developing new molecules and non-linear optical
materials. Various methods have been reported for the determination of dipole moments of
the solute molecule in different electronic states [2-6]. Among the existing methods, the most
widely accepted one is based on solvatochromism due to the high linear correlation between
spectroscopic parameters and solvent polarity functions. Solvatochromic method has been
used for the determination of dipole moments of ground and excited state of various
fluorescent molecules such as coumarins [7-10], substituted anthraquinones [11-13],
fluorescein [14], quinazolines [15], curcuminoid dyes [16] and some laser dyes [17-19].
The pyranopyrazole moiety is a prolific resource of biologically important molecules.
Compounds containing this moiety have various pharmacological properties such as anti-
inflammatory [20], antimicrobial [21], anticancer, analgesic [22] and hypoglycemic [23].
Moreover these compounds are also used in the development of photoactive materials [24].
The photophysical properties of pyranopyrazoles are of great interest because of the
molecular structure which contains a 4-pyrone ring fused on to a pyrazole ring. There are
considerable reports on 4-pyrones based on photochemical reactions which involve
photodimerization, photoisomerization to 2-pyrones and photo ring contraction to yield 4-
hydroxy- cyclopentenone solvent adducts [24].
The present investigation focuses on the estimation of ground and excited state dipole
moments
of
6-amino-2,4-dihydro-3-methyl-4-nitrophenylpyrano[2,3-c]pyrazole-5-
carbonitrile (NDPPC) molecule using solvents with different polarities (Fig. 1). The effects of
pure solvents on spectral properties of the molecule were studied by employing Bilot-Kawski
[
25], Lippert-Mataga bulk solvent polarity parameter [26], Bakhshiev [27] and Kawski-
Chamma-Viallet [28] and Reichardt [29] solvatochromic shift methods.
Further, the solvatochromic study of NDPPC has been done using Kamlet-Abboud-Taft
and Catalan solvent polarity parameters to study about specific and nonspecific interactions
between the solute and solvents. In fact, to the best of our knowledge there have been no
reports on the determination of ground and excited state dipole moment values of NDPPC.
2

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NO2
H3C
CN
N
N
O
NH2
H
Fig. 1. Structure of NDPPC.
Experimental
Reagents and apparatus
All chemicals and solvents used in this study were of spectroscopic grade obtained from
Sigma-Aldrich Chemical Company and S.D. Fine Chemicals Ltd., India respectively.
Absorption and fluorescence spectra were recorded by using Shimadzu UV–visible
spectrophotometer (UV/VIS1800) and Shimadzu spectrofluorophotometer (RF5301PC)
respectively at room temperature. All sample solutions were prepared in various solvents
-5
with different polarities at low concentrations (10 M) to avoid self-absorption and
aggregation formation. An ultrasonic bath (SIDILU, Indian make) working at 35 kHz
°
maintained at 25-30 C was used for sonication. Melting point of the compound was recorded
using a melting point apparatus (Vego, model-VMPD). IR spectrum was recorded on Bruker
1
13
Alpha‐T ATR/FTIR spectrometer. H NMR and C NMR spectra were recorded on 400
MHz Bruker NMR spectrometer using DMSO as the solvent and chemical shift values were
recorded in units (ppm) relative to tetramethylsilane (Me Si) as an internal standard. Mass
4
spectra were recorded by LC-MSD-Trap-XCT Plus mass spectrophotometer.
Synthesis and characterization of NDPPC
A mixture of hydrazine hydrate (2 mmol), ethyl acetoacetate (2 mmol), 4-
nitrobenzaldehyde (2 mmol), malononitrile (2 mmol) in water (10 mL), 10 mol% nano ceria
as a catalyst was sonicated at room temperature for 8-10 min (Scheme 1). Completion of the
reaction was monitored by TLC. The crude product was separated by filtration, washed with
water to remove the catalyst and air-dried. The product was purified by column
chromatography on silica gel (60 -120 mesh, Merck) using hexane : ethyl acetate (4:1) eluent
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system. In presence of ceria nanoparticles, the reaction proceeds with short reaction time and
results in high yield compared to other reported methods [30,31].
°
−1
Brown solid; Yield = 96%; m.p. 250–251 C: IR (KBr,cm ): 3477 ν(NH), 3228 ν(NH ),
2
1
2
3
8
1
1
195 ν (CN), 1648 ν(C=N), 1595 ν(C=C); HNMR (400 MHz, DMSO-d , ppm): δ 1.79 (s,
6
H, CH ), 4.81 (s, 1H, C-H), 7.44 (d, J = 4.4 Hz, 1H, Ar-H), 7.46 (d, J = 4.4 Hz, 1H, Ar-H),
3
.17(d, J = 8.8 Hz, 1H, Ar-H), 8.20(d, J = 4.4 Hz, 1H, Ar-H), 7.01 (s, 2H, NH ), 12.17 (s,
2
13
H, NH); C NMR (400 MHz, DMSO): δ= 9.67, 29.4, 59.18, 96.51, 120.39, 128.78, 135.81,
46.35, 152.04, 161.09; Mass (m/z): 298.7(M+H).
NO2
NO2
O
CN
CN
Nano Ceria, Water
8-10 min, 80-90%
CN
H N NH H O
2
2. 2
N
O
2
O
)
))
N
O
5
NH2
H
O
H
1
3
4
Scheme 1. Synthesis of NDPPC.
Theoretical calculations of ground-state dipole moment
Quantum chemical calculations were employed for theoretical determination of ground
state dipole moment (μ ) of the molecule. Gaussian 09W program [32] was used to carry out
g
the computational study. B3LYP level with the 6-311G (d, p) basis set was used for the
structure optimization and calculation of HOMO-LUMO energies.
Experimental calculations of ground and excited singlet-state dipole moments
Various theories have been applied for the estimation of ground and excited singlet-state
dipole moments in the solvatochromic analysis. Eq. (1) proposed by Lippert-Mataga is given
below [26]
(1)
-1
Where ῡ and ῡ are absorption and fluorescence maxima wave number (in cm ) respectively,
a
f
represents the Lippert-Mataga solvent polarity function which was calculated from
Eq. (2) using dielectric constant ( ) and refractive index (n) of the solvents taken from
literature [33].
Most theories of Solvatochromic shift in excitation and emission spectra are based on the
Onsager description of non-specific electrostatic solute solvent interactions [25]. Lippert-
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Mataga equation follows Onsager’s reaction-field theory which considers that the
surrounding solvent molecule forms a spherical cavity of radius (a) around the solute
molecule in which the dipole moment exists at the centre of the cavity.
Lippert-Mataga plot was constructed by plotting Stokes shift (ῡ -ῡ ) values against
a
f
orientation polarizability F (ε,n) of the solvent. In case of non-polar and aprotic solvents,
1
interactions are dominated by dipole-dipole and dipole-induced dipole forces.
The expression for Lippert-Mataga solvent polarity function has been given below:
(2)
The equations proposed by Bakhshiev (Eq. 3) [27] and Kawski-Chamma-Viallet (Eq. 4) [28]
can be used for the experimental determination of ground and excited state dipole moments
and are given as follows:
(
3)
(4)
Where F , F are Bakhshiev and Kawski-Chamma-Viallet solvent polarity functions
+
2
3
determined using Eq. (5) and Eq. (6) respectively.
(5)
(6)
The linear plot with slopes m (Eq. 7), m (Eq. 8) and m (Eq. 9) respectively are given by the
1
2
3
following expressions:
(
7)
8)
(
(9)
where h, c and a represent Planck's constant, velocity of light in vacuum and Onsager cavity
radius (calculated by atomic increment method) of solute molecule respectively. μ and μ
g
e
indicates the ground and excited state dipole moments of the solute molecule respectively.
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The values of ground and excited state dipole moments and their ratio were calculated
using Eqs. (10), (11) and (12) respectively, assuming that the symmetry of investigated solute
molecule has not changed upon excitation and both the dipole moments remain parallel.
(10)
(11)
µe =
µ for m >m
g 3 2
(12)
Generally, the ground and excited state dipole moments are not parallel to each other but they
make an angle φ which can be determined by Eq. (13).
(13)
The microscopic solvent polarity scale
, proposed by Reichardt [29] overcomes the
also
error in estimation of Onsager cavity due to the involvement of Onsager radii ratio.
considers intermolecular solute/solvent, hydrogen bond donor/acceptor interactions along
with the solvent polarity. According to Eq. (14), the spectral shifts were correlated with
on the basis of theoretical calculations.
(14)
m =
(15)
(16)
4
∆µ = (
=
Where μ and a are the change in dipole moment and Onsager radius respectively of the
B
B
Betaine dye, and Δμ and a are the change in dipole moment and Onsager radius of the
molecule under study. Using Eq. (15), slope m for Reichardt method can be calculated.
4
Change in dipole moment Δμ (Eq. 16) can be calculated from the slope obtained by plotting
Stokes shift values against
using reported values μ = 9D of Betaine dye and its Onsager
B
radius a = 6.2 Å. Theoretical ground state dipole moment value was used for the calculation
B
of excited state dipole moment.
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The microscopic solvent polarity parameter
literature [34].
values of solvents were taken from the
Based on linear solvation energy relationship (LSER), also known as multiple linear
regression analysis, the Onsager description of non-specific electrostatic solute-solvent
interactions of the molecule can be explained. This multiparameters solvation energy
relationship proposed by Kamlet-Abboud-Taft and Katritzky [35] are expressed in Eq. (17).
*
(17)
Where
value indicate the physiochemical property of corresponding shift which is
independent of solvent (in the gas phase) and can be taken as absorption, fluorescence
maxima or Stokes shift in different solvents. This method gives insight into individual
contributions of hydrogen bond donor and hydrogen bond acceptor (HBD and HBA
respectively) abilities of the solvents on spectroscopic properties of the molecule. The above
expression correlates excitation, fluorescence wavelength (in cm ), Stokes shift with solvent
-1
dipolarity or polarizability (π*), hydrogen bond donor (α) and hydrogen bond acceptor (β)
ability [35]. The coefficients a, b and c are susceptible to their corresponding spectroscopic
parameters α, β and π* respectively.
The coefficients a, b and c represent changes in their corresponding parameters with
spectroscopic parameters
In order to differentiate and investigate polarizability effects of two related solvents by
solvatochromic method, Catalan method was used [36].
Catalan solvatochromic parameters
=
+ aSA+ bSB + cSP + dSdP
(18)
Catalan method was also used multiple regression analysis to get information about hydrogen
bonding interaction by using two additional terms SP and SdP through polarity and dipolarity
interactions. SA, SB, SP and SdP represent hydrogen bond donor, hydrogen bond acceptor,
polarizability and dipolarity of solvents respectively, where a, b, c and d are their coefficients
as shown in Eq. (18). The coefficients a, b and c are prone to spectroscopic parameters for
any change in their corresponding parameters.
Results and discussion
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Effect of solvents on absorption and fluorescence spectra of NDPPC
The absorption and fluorescence spectra of NDPPC were recorded in different solvents in
the increasing order of their polarity at room temperature as shown in Fig. 2 and Fig. 3
respectively. Maximum absorption and fluorescence wavelength of NDPPC was found to be
in the range of 259-268 nm and 387-417 nm respectively in the chosen solvents. Absorption
maxima (ῡ ), emission maxima (ῡ ), Stokes shifts (ῡ -ῡ ) and the arithmetic mean of Stokes
a
f
a
f
-1
shift values (in cm ) were calculated and tabulated in Table 1. On increasing the solvent
polarity upon going from non-polar to polar solvents, appreciable red-shifts were observed
for the absorption and fluorescence spectra i.e, absorption spectra (∆λ = 7 nm) when
a
compared to emission spectra (∆λ =22 nm). This implies that excited state is more stabilized
f
than the ground state. On increasing the solvent polarity from non-polar (toluene) to polar
-1
(
methanol), an increase in Stokes shift was observed from 12,770 to 13,332 cm . This
indicates a bathochromic shift and confirms π → π* transition. The shift in fluorescence band
to longer wavelength region arises from a strong intermolecular interaction of the excited
state of solute with polar solvents. This results in a significant difference in charge
distribution of the solute in ground and excited state. With increase in solvent polarity, the
bathochromic shift produced in the emission indicates a large charge transfer character of the
molecule. It also indicates that relaxed excited state S is energetically more stabilized than
1
ground state S0.
The quantum yield of NDPPC in ethanol was calculated using single point method by
comparing the integrated intensity and optical density of the sample to that of a reference
compound (anthracene). Anthracene in ethanol was used as a standard solution. The quantum
yield (Φ) of NDPPC was found to be 0.26 and was determined by Eq. (19).
(19)
Where Φ is quantum yield of reference standard 0.27 (anthracene) [37], I and I are the
std
S
R
integrated fluorescence intensity of sample and reference standard, OD and OD are the
R
S
2
2
optical densities of reference standard and sample, and η
and η
are the square of
R
S
refractive indices of solvent respectively. The ratio of square of the refractive indices was
found to be 1. The Onsager cavity radius of NDPPC was calculated by means of Van der
Waal’s volume using Edward’s atomic increment method [38]. The Onsager cavity radius (a)
of NDPPC was calculated by Eq. (20).
8

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(20)
Where, M is the molecular weight of the solute, δ is the density of the solute and N is the
A
Avagadro’s number.
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
toluene
dioxane
acetonitrile
butanol
ethanol
220
240
260
280
300
320
340
Wavelength () nm
Fig. 2. Normalized absorption spectra of NDPPC in different solvents.
9

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1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
toluene
dioxane
acetonitrile
butanol
ethanol
360
380
400
420
440
460
Wavelength () nm
Fig. 3. Normalized emission spectra for NDPPC in different solvents.
Experimental and theoretical calculations of dipole moments
To investigate the solvatochromic behaviour of NDPPC, spectroscopic properties were
correlated with relevant solvent polarity scales. Lippert-Mataga, Bakhshiev, Kawski-
Chamma-Viallet and Reichardt linear correlation methods [34] have been used for the
solvatochromic analysis. The convenient NDPPC wavenumbers (Table 1) were plotted
against polarity functions F (ε,n), F (ε,n) , F (ε,n) and
(Table 2). The correlation
1
2
3
coefficients and slopes m (Lippert-Mataga in Fig. 8), m (Bakhshiev) and m (Kawaski-
1
2
3
Chamma-Viallet) are summarized in Table 5. The plot of Stokes shift with
for NDPPC
has been shown in Fig. 9. Correlation coefficient (found to be 0.9760), indicates good
linearity and existence of non-specific solute-solvent interaction. The value of µ and µ ,
g
e
calculated using Eq. (10) and (11) were found to be 5.41 and 7.97 respectively.
As observed in Table 6, the dipole moment value of the singlet excited-state was found to
be higher than that of the ground state. Calculated excited state dipole moment values
obtained by Lippert-Mataga and Kawski-Chamma-Viallet method were found to be higher
1
0

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than those obtained by Bakhshiev and Reichardt correlation methods. This was observed as
the former methods do not consider the effect of solute polarizability. The difference in
dipole moment can be attributed to the charge transfer and the nature of emitting state.
Assuming that the dipole moments are parallel, the ground and excited state dipole moments
were calculated using Eq. 10 and 11 respectively. Therefore, the angle between the dipole
moments (µ = 5.41D and µ = 7.97D) was found to be 0 ꢀ using Eq. 13 which indicates that
g
e
they are parallel and this is in good agreement with the theoretical value. The theoretical
ground state dipole moment calculations were done by Gaussian 09 W software using basis
set B3LYP/6-311G. Theoretical value of ground and excited state dipole moments for
NDPPC were estimated as 8.0 D and 9.1 D respectively which makes an angle of 0.6 ꢀ. Using
Eq. 12, the ratio of excited state dipole moment to ground state dipole moment was found to
be 1.47.
Computational Analysis
The optimized geometry of NDPPC molecule has been shown in Fig. 4. The frontier
molecular orbitals explain the direction of intramolecular charge transfer. The energy of
HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular
orbital) frontier orbitals of NDPPC molecule were calculated in ethanolic medium which
explains the charge transfer within the molecule. HOMO depicts the electron donating ability
whereas LUMO has the electron accepting ability. Smaller the energy gap between HOMO
and LUMO, easier will be the excitation of electrons from HOMO to LUMO energy levels.
The energies of HOMO and LUMO were computed as -6.09 eV and -2.60 eV respectively.
The energy gap between HOMO and LUMO was found to be very small and the value was
3
.49 eV. Hence the transition of electrons from HOMO to LUMO was easier. In Fig. 5, the
red and green region corresponds to the positive and negative phase respectively. It was
observed that HOMO is confined to the pyrazole and dihydropyrano moiety whereas LUMO
is localized on the nitrophenyl ring.
Chemical stability of a molecule can be estimated by using HOMO and LUMO energies.
This gives significant information on chemical hardness (η) of a molecule. A molecule is
considered as hard if they possess large energy gap between HOMO and LUMO whereas, it
is considered as soft if the energy gap is small [39]. In case of lower excitation energy, soft
molecules are said to be more polarized than hard molecules. Therefore, the chemical
stability (η) can be calculated by the following Eq. (21).
1
1

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2
1
Where, E and E are HOMO and LUMO energies respectively. The hardness of the
H
L
molecule was calculated and found to be 1.74 eV. This value implies that NDPPC molecule
soft as the energy gap between respective frontier orbitals is smaller.
Molecular electrostatic potential (MESP) surface is used to determine electron density at
each point on the surface of molecule which gives information about sites for H-bonding
interaction, electrophilic and nucleophilic reactions. From the mapped MESP surface, the
blue and red region corresponds to highly electropositive (0.132 eV) and electronegative
(-0.132 eV) electrostatic potential value respectively. Therefore, the acidic H atom of NH
group in pyrrole ring is electropositive and O atom of NO group in nitrobenzene ring is
2
electronegative, with low electron density and high electron density respectively (Fig. 6).
This indicates that charge transfer occurs from acidic H atom of NH in pyrrole ring to O atom
of NO group in nitrobenzene ring.
2
The ground state dipole moment of NDPPC was determined theoretically by using
Gaussian 09 W software. Insights into the charge distribution were obtained from Mulliken
atomic charges given in Fig. 5, which were calculated in ethanolic medium (Fig. 7). The
results indicate that C(1), C(2), C(4), C(5), O(12), O(13), C(14), C(16), C(17), O(20), N(22),
N(23), C(24), N(30, N(31) atoms contain negative charge, which act as donor atoms, whereas
C(3), C(6), N(11), C(18), C(19), C(21) and C(29) contain positive charge, which behave as
acceptor atoms. This directs charge distribution from H (28) to O (12) atoms with magnitude
+
0.455 eV to -0.625 eV respectively. Due to this large charge separation, NDPPC molecule
will be more polarized in the excited state. Therefore, charge transfer results in difference
between ground and excited state dipole moments. All hydrogen atoms exhibit positive
values in ethanol as it can donate electrons from hydrogen atom to other atoms. This confirms
the charge transfer from electropositive H (28) atom of NH of the pyrrole ring to
electronegative O (12) atom of NO group of the nitrobenzene ring. These analyses confirm
2
the results obtained using multiple linear regression method.
1
2

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Fig. 4. Optimized ground state molecular geometry of NDPPC.
Fig. 5. HOMO (a) and LUMO (b) orbital of NDPPC molecule.
1
3

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Fig. 6. Molecular electrostatic potential surface of NDPPC molecule.
Fig. 7. Mulliken charge representation of molecular geometry of NDPPC.
Table 1.
Solvatochromic data of NDPPC in different solvents.
-
1
-1
-1
-1
Solvents
Toluene
ʎ (nm) ʎ (nm) ῡ (cm ) ῡ (cm ) Δῡ ( cm )
ῡ + ῡ (cm )/2
a
f
a
f
a
f
0.09
0.16
0.21
0.22
259
260
261
262
263
387
38610 25839.8
12770.2
12820.5
12868.9
12915.4
13147.2
32224.9160
32051.2821
31879.7344
31710.2321
31449.2178
1
,4-Dioxane
390 38461.5 25641.0
393 38314.2 25445.3
396 38167.9 25252.5
402 38022.8 24875.6
Tetrahydrofuran
Ethyl acetate
Methelene dichloride 0.31
1
4

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Dimethylformamide
Dimethyl sulphoxide
Acetonitrile
Butanol
Propanol
Ethanol
0.39
0.44
0.46
0.59
0.62
0.65
0.76
263
264
265
266
266
267
268
405 38022.8 24691.4
404 37878.8 24752.5
407 37735.8 24570.0
410 37594.0 24390.2
411 37594.0 24330.9
414 37453.2 24154.6
417 37313.4 23980.8
13331.5
13126.3
13165.8
13203.7
13263.1
13298.6
13332.6
31357.0859
31315.6316
31152.9368
30992.1144
30962.4426
30803.8864
30647.1240
Methanol
Table 2.
Solvent functions F , F and F used in the Lippert-Mataga, Bakhshiev and Kawski-Chamma-
1
2
3
Viallet equations respectively.
a
b
c
d
e
Solvents
Toluene
F1(ε,n)
F2(ε,n)
F3(ε,n)
ε
n
0.01531 0.03339 0.34568 2.37 1.487
0.02150 0.04361 0.30848 2.22 1.422
0.20836 0.54463 0.54878 7.47 1.407
0.20072 0.49254 0.49958 6.08 1.372
0.21835 0.59496 0.58534 9.08 1.424
0.27519 0.83933 0.71168 38.25 1.430
0.26398 0.84210 0.74349 47.24 1.477
0.30538 0.86303 0.66593 37.5 1.344
0.25856 0.74787 0.65637 17.84 1.416
0.26249 0.77075 0.67112 20.8 1.421
0.28976 0.81696 0.65427 25.33 1.361
0.30870 0.85580 0.65191 33.1 1.328
1
,4-Dioxane
Tetrahydrofuran
Ethyl acetate
Methylene Dichloride
Dimethylformamide
Dimethyl sulphoxide
Acetonitrile
Butanol
Propanol
Ethanol
Methanol
a
Lippert-Mataga solvent function.
b
Bakhshiev solvent function.
c
Kawski-Chamma-Viallet solvent function.
ε = dielectric constant and n = refractive index [33].
d
e
1
5

ACCEPTED MANUSCRIPT
13400
13300
13200
13100
13000
12900
12800
12700
Methanol
Ethanol
Propanol
y = 1820x + 12757.31
DMF
Butanol
2
R = 0.93
DCM
DMSO
Dioxane
Toluene
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
F ( n)
1
Fig. 8. The variation of Stoke’s shift with F (ε, n) using Lippert-Mataga equation for
1
NDPPC.
13400
13300
13200
13100
13000
12900
12800
12700
Methanol
Ethanol
y = 890.23x + 12702.62
Propanol
Butanol
2
R = 0.97
Acetonitrile
DMSO
Ethyl acetate
THF
Dioxane
Toluene
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
N
E
T
Fig. 9. The variation of Stoke’s shift with
for NDPPC.
1
6

ACCEPTED MANUSCRIPT
Table 3.
Kamlet-Abboud-Taft solvatochromic parameters.
Solvents
Toluene
ᵦ
ᴨ*
0
0
0
0
0.11
0.37
0.55
0.45
0
0.69
0.76
0.31
0.88
0.84
0.77
0.62
0.54
0.55
0.58
0.55
0.82
0.88
1
0.75
0.47
0.48
0.54
0.6
1
,4-Dioxane
Tetrahydrofuran
Ethyl acetate
Methylene Dichloride 0.3
Dimethylformamide
Dimethyl sulphoxide
Acetonitrile
Butanol
Propanol
Ethanol
Methanol
0
0
0.19
0.79
0.76
0.83
0.93
α = hydrogen bond donor ability of solvent
β = hydrogen bond acceptor ability of solvent
π* = dipolarity or polarizability of solvent [35]
Multiple linear regression analysis
Different types of solute-solvent interactions can be studied by Multiple linear regression
analysis which also explains the mechanism of electronic transition. Multiple linear
regression analysis (MLR) proposed by Kamlet-Abboud-Taft [35] explains various
interactions between solute and solvent. Wave number of absorption (ῡ ) and emission (ῡ ),
a
f
and Stokes shift (ῡ - ῡ ) were correlated with Kamlet-Abboud-Taft parameters α, β and π*
a
f
[
35]. The literature values of α, β and π* have been tabulated in Table 3. The regression
analysis with correlation coefficient is given below in Eq. (22).
-1
ῡ (cm ) = 4111.97 - 542.05α – 118.2β – 698.42π* r = 0.87;
a
-1
ῡ (cm ) = 26939.43 - 1247.59α - 208.77β – 1381.27π* r = 0.89;
22
f
-1
ῡ - ῡ (cm ) = 12504.47 + 542.96α + 160.46β + 957.42π* r = 0.81;
a
f
MLR analysis reveals that the value of π* coefficient is significantly more important than that
of α and β coefficients in the above spectroscopic properties. Hence, the contribution of π* is
more towards solute-solvent interactions. This implies that the influence of π* is greater than
that of α (HBD) and β (HBA) solvent abilities. Therefore, the polarity/dipolarizability of non-
specific interactions should mainly control the excitation and emission spectral shifts.
1
7

ACCEPTED MANUSCRIPT
Moreover, the coefficient value of α (HBD) was found to be greater than that of β (HBA) in
all cases. This indicates that the hydrogen bond donor ability of the solvent was higher. Based
on MLR analysis, it can be observed that ῡ and ῡ have relatively good correlation
a
f
coefficients. In comparison to the excitation spectrum, the molecule undergoes a large red-
shift of emission spectrum, which can be justified by the higher values of π*, α and β. These
results confirm that NDPPC is more polarized in the singlet excited-state than in the ground-
state. Therefore, excited state dipole moment of NDPPC will be greater than the ground state
dipole moment.
Table 4.
Catalan solvent dipolarity and polarizibility parameters.
Solvents
Toluene
SA
0
0
0
0
SB
SP
SdP
0.284
0.312
0.634
0.603
0.769
0.977
0.647
0.9745
0.655
0.748
0.783
0.904
0.128
0.444
0.591
0.542
0.178
0.782
0.737
0.714
0.656
0.761
0.759
0.072
0.645
0.674
0.658
0.633
0.608
1
,4-Dioxane
Tetrahydrofuran
Ethyl acetate
Methylene Dichloride
Dimethylformamide
Dimethyl sulphoxide
Acetonitrile
Butanol
Propanol
Ethanol
Methanol
0.04
0.031 0.613
0.83
0.044 0.286
0.341 0.809
0.367 0.782
1
0.4
0.658
0.605 0.545
SA = hydrogen bond donor ability of solvent
SB = hydrogen bond acceptor ability of solvent
SP = polarizability of solvent
SdP = dipolarity of solvent [36]
Multiple regression analysis was also done using Catalan solvent dipolarity and
polarizability parameters (Table 4) to quantify the effect of solvents on absorption,
fluorescence and Stokes shift energies using Eq. (23).
-1
ῡ (cm ) = 40190.67 – 1043.55SA + 169.57SB - 789.93SP – 728SdP r = 0.82;
a
-1
ῡ (cm ) = 28100.21 -1502.83SA - 105.02SB – 1288.51SP – 1355.95SdP r = 0.90;
23
f
-1
ῡ - ῡ (cm ) = 12307.46 + 640.03SA + 215.06SB + 750.22SP + 755.68SdP r = 0.88
a
f
From the analysis, large coefficient of SdP value indicates positive solvatochromism.
Dipolarity interactions (SdP value) have major influence on solute-solvent interactions in
1
8

ACCEPTED MANUSCRIPT
case of emission. This confirms that the singlet excited-state has higher dipole moment than
the ground state. Dipolarity parameter of the solvents is responsible for ICT from H (28)
atom of NH of the pyrrole ring to O (12) atom of NO group of the nitrobenzene ring.
2
Analysis of specific interactions reveals that the hydrogen bond donor ability was more
important than the hydrogen bond acceptor ability of the solvent. Based on the above results,
it can be concluded that solvent influences ICT from H (28) atom of NH of the pyrrole ring to
O atom of NO group belonging to the nitrobenzene ring during solute-solvent interactions.
2
Table 5.
Statistical treatment of the correlations of solvents spectral shifts of NDPPC.
Number
Correlations
Lippert-Mataga
Bakhshiev
Compound Slope
Correlation factor ‘r’ of data
NDPPC
NDPPC
1820.00 0.9356
655.83 0.9242
9
9
Kawaski-Chamma-Viallet NDPPC
-3422.44 0.9237
890.23 0.9760
8
10
Reichardt
NDPPC
Table 6.
Ground and excited state dipole moments of NDPPC.
0
a
b
c
d
e
f
g
h
i
j
k
Radius 'a' (A )
µ (D)
µ (D)
µ (D) µ (D)
µ (D)
µ (D) µ (D)
∆µ (D) ∆µ (D) (µ /µ )
ᶲ
g
g
e
e
e
e
e
e
g
0
4
.82
8.083
5.41
7.97
12.35
10.64
13.94
9.81
2.56
1.73
1.47
0
-30
-18
Debye (D) = 3.33564 x 10 cm = 10 esu cm.
a
The theoretically calculated ground-state dipole moment by Guassian software.
The ground state dipole moment calculated from Eq. (10).
The excited state dipole moment calculated from Eq. (11).
The excited-state dipole moment calculated from Lippert-Mataga equation.
The excited-state dipole moment calculated from Bakhshiev's equation.
The excited-state dipole moment calculated from Kawski–Chamma–Viallet's equation.
b
c
d
e
f
g
The excited-state dipole moment calculated from microscopic solvent polarity parameter
equation.
h
i
The change in dipole moments for µ and µ .
e
g
The change in dipole moments calculated from Eq. (16).
j
The ratio of excited-state and ground state dipole moment calculated from equation (12).
The angle between ground state dipole moment and excited state dipole moment.
k
Conclusion
Photophysical properties of NDPPC molecule have been investigated and their ground
and excited state dipole moments were determined as a function of solvent polarity. Different
solvent polarity correlation methods were used to analyse solvent effects on the excitation
and emission spectra. Gaussian 09 W software was used to theoretically determine the ground
state dipole moment. The experimental excited state dipole moment of NDPPC was
1
9

ACCEPTED MANUSCRIPT
determined by Lippert-Mataga, Bakhshiev, Kawski-Chamma-Viallet and Reichardt
correlation methods. The excited-state dipole moment value was found to be greater than the
ground-state one, using all the above methods. The higher value of excited-state dipole
moment suggests that the NDPPC molecule is more polar or stable in the singlet excited-state
than in the ground state. This indicates that electronic charge transfer takes place from acidic
H atom of NH in pyrrole ring to O atom of NO group in nitrobenzene ring. From the
2
computational studies, it was also found that NDPPC molecule had both nucleophilic and
electrophilic sites. Therefore, NDPPC is an ideal candidate for non-linear optical materials,
fluorescent probes and luminescence materials. The correlation of spectral properties with
Kamlet-Abboud-Taft and Catalan solvatochromic parameters reveals that the influence of
non-specific dipolar interactions was more important than that of the hydrogen bond
interactions between solute and solvent molecules. The excited state is mainly stabilized by
the dipolar interactions and hydrogen bond donor ability of the solvents.
Acknowledgements
We wish to thank to Sravan Perumalla, Department of IPC, IISc, Bangalore, for providing
the facility of Guassian studies. We thank NMR Centre, IISc, Bangalore, India and Anthem
Biosciences, Bangalore, India for the spectral analysis.
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Photophysical
study
of
6-amino-3-methyl-4-(4-nitrophenyl)-1,4-
dihydropyrano[2,3-c]pyrazole-5-carbonitrile and estimation of ground-
state and singlet excited-state dipole moments by solvatochromic
approaches
Rekha Kumari, AnithaVarghese*, Louis George, Akshaya K. B.
Department of Chemistry, Christ University, Bangalore – 560 029, India.
Graphical abstract
2
3

ACCEPTED MANUSCRIPT
Highlights



The photophysical properties were studied in different organic solvents.
Dipole moments for NDPPC were estimated by Solvatochromic method.
The ICT in NDPPC controlled by dipolarity/polarizability and hydrogen bonding.

Excited state is more polar than the ground state.
2
4