A Combined Experimental and Computational Investigation on Spectroscopic and Photophysical Properties of a Coumarinyl Chalcone

Article abstract of DOI:10.1007/s10895-016-1823-2

Here, we synthesized a new coumarinyl chalcone derivative 3-[3-(3-Methyl-thiophen-2-yl)-acryloyl]-chromen-2-one (MTC) by simple and proficient method. A comprehensive study on the photophysics of a coumarinyl chalcone derivative having pi-conjugated potential chromophore system 3-[3-(3-Methyl-thiophen-2-yl)-acryloyl]-chromen-2-one (MTC) has been carried out spectroscopically. The electronic absorption and emission characteristic of MTC were studied in different protic and aprotic solvents using absorption and steady-state fluorescence techniques. The spectral behavior of this compound is found to be extremely sensitive to the polarity and hydrogen bonding nature of the solvent. The compound shows very strong solvent polarity dependent changes in their photophysical characteristics, namely, remarkable red shift in the emission spectra with increasing solvent polarity, change in Stokes shift, significant reduction in the fluorescence quantum yield; indicating that the fluorescence states of these compounds are of intramolecular charge transfer (ICT) character. The solvent effect on the photophysical parameters such as singlet absorption, molar absorptivity, oscillator strength, dipole moment, fluorescence spectra, and fluorescence quantum yield of the compound has been investigated in detail. The difference between the excited and ground state dipole moments (Δμ) were estimated from solvatochromic methods using Lippert–Mataga and Reichardt’s correlations. The prepared compound was also studied by density functional theory (DFT) and time-dependent density functional theory (TDDFT). The results revealed that it could be easily reproduce by computational means.

Full text of DOI:10.1007/s10895-016-1823-2

J Fluoresc
DOI 10.1007/s10895-016-1823-2
ORIGINAL ARTICLE
A Combined Experimental and Computational Investigation
on Spectroscopic and Photophysical Properties
of a Coumarinyl Chalcone
Abdullah G. Al-Sehemi1,2 & Mehboobali Pannipara2 & Abul Kalam1,2
&
Abdullah M. Asiri3,4
Received: 6 January 2016 /Accepted: 9 May 2016
#
Springer Science+Business Media New York 2016
Abstract Here, we synthesized a new coumarinyl chalcone
derivative 3-[3-(3-Methyl-thiophen-2-yl)-acryloyl]-chromen-
2-one (MTC) by simple and proficient method. A comprehen-
sive study on the photophysics of a coumarinyl chalcone de-
rivative having pi-conjugated potential chromophore system
3-[3-(3-Methyl-thiophen-2-yl)-acryloyl]-chromen-2-one
(MTC) has been carried out spectroscopically. The electronic
absorption and emission characteristic of MTC were studied
in different protic and aprotic solvents using absorption and
steady-state fluorescence techniques. The spectral behavior of
this compound is found to be extremely sensitive to the polar-
ity and hydrogen bonding nature of the solvent. The com-
pound shows very strong solvent polarity dependent changes
in their photophysical characteristics, namely, remarkable red
shift in the emission spectra with increasing solvent polarity,
change in Stokes shift, significant reduction in the fluores-
cence quantum yield; indicating that the fluorescence states
of these compounds are of intramolecular charge transfer
(ICT) character. The solvent effect on the photophysical pa-
rameters such as singlet absorption, molar absorptivity, oscil-
lator strength, dipole moment, fluorescence spectra, and fluo-
rescence quantum yield of the compound has been investigat-
ed in detail. The difference between the excited and ground
state dipole moments (Δμ) were estimated from
solvatochromic methods using Lippert–Mataga and
Reichardt’s correlations. The prepared compound was also
studied by density functional theory (DFT) and time-
dependent density functional theory (TDDFT). The results
revealed that it could be easily reproduce by computational
means.
.
.
.
Keywords Coumarin derivative Photophysics ICT Effect
of solvent
Introduction
Coumarins and its derivatives are important class of com-
pounds that possess multiple biological activities including
disease prevention, growth modulation and antioxidant prop-
erties [1]. As these classes of compounds encompass a broad
array of photophysical properties including high molar ab-
sorptivity, quantum yield, large Stokes shift, lifetime and abil-
ity to respond towards the immediate environment [2], it is
widely used in synthesizing laser dyes, fluorescent whiteners
and organic nonlinear optical materials [3]. The optical prop-
erties these compounds are strongly altered by the nature of
substituent, substituted position in the coumarin ring and the
surrounding media [4]. Since the photophysics and photo-
chemistry of coumarin derivatives mainly rely on the effect
of substitution on the ring, structurally modified coumarin
based luminophores have become a subject of intense research
in various aspects of science.
Electronic supplementary material The online version of this article
(doi:10.1007/s10895-016-1823-2) contains supplementary material,
which is available to authorized users.
* Abdullah G. Al-Sehemi
agmasq@gmail.com
1
Department of Chemistry, Faculty of Science, King Khalid
University, P.O. Box 9004, Abha 61413, Saudi Arabia
2
Research Center for Advanced Materials Science (RCAMS), King
Khalid University, P.O. Box 9004, Abha 61413, Saudi Arabia
3
Department of Chemistry, Faculty of Science, King Abdulaziz
University, P.O. Box 80203, Jeddah 21589, Saudi Arabia
4
Center of Excellence for Advanced Materials Research (CEAMR),
King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi
Arabia

J Fluoresc
The dipole moments of a molecule offer an insight to
their ground and excited state and give information about
change in electronic distribution upon excitation. Hence,
the knowledge of dipole moment is useful in designing
non-linear optical materials [5], studying the nature of pho-
tochemical reactions [6], and calculation of molecular po-
larizability [7]. Though there are various methods used for
experimental determination of dipole moment that include
electric dichroism [8], microwave conductivity [9], Stark
splitting [10] and thermochromic shift method [11], the
use of these methods are limited because they are equip-
ment sensitive and the studies have been restricted to rela-
tively very simple molecules. The use of solvent shift meth-
od based on absorption and emission intensity is the most
widely accepted method for the estimation of the dipole
moments of a molecule [12]. Understanding solute–solvent
interactions and measuring them quantitatively is an active
area of research since the solvent affects the equilibria, ab-
sorptions, emissions and the mechanisms of reactions for
many compounds.
Compounds containing electron donor and acceptor
functionality in conjugation exhibit interesting optical and
spectral properties due to intramolecular charge transfer
(ICT) that can be exploited in various fields. The aim of
the present work is to perform a detailed investigation on
the spectral behaviors and photophysical properties of a
donor–acceptor type coumarin derivative in different media
by applying steady state spectroscopy and density function-
al theory (DFT) calculation.
measured using an optically diluted solution of quinine sul-
fate as reference according to Eq. (1):
Iu As n2u
Is Au n2s
ϕu ¼ ϕs Â
Â
Â
ð1Þ
where ϕu, ϕs are the fluorescence quantum yields of the
unknown and standard, respectively, I is the integrated
emission intensity; A is the absorbance at excitation wave-
length, and n is the refractive index of the solvent. The
subscript u and s refers to unknown and standard,
respectively.
Procedure for the Synthesis of 3 –Acetyl Coumarin (2)
To a solution of salicylaldehyde (1 mg, 8.1 mmol) and
ethylacetoacetate (1.06 mg, 8.1 mmol) in ethanol (20 ml),
0.3 mL of piperidine and 2–3 drops of glacial acetic acid were
added. The mixture was refluxed for 4 h. After the completion
of the reaction as identified by TLC, the reaction mixture was
cooled to room temperature and 20 mL of ice cold water was
added. The solid product was collected by filtration, dried and
recrystallized from ethanol to give 2 (0.36 mg, 72 %) as yel-
low solid.
Yield 72 %, yellow solid, Melting point: 154 °C; IR (KBr, v
cm−1): 1640 (CO), 1725 (CO), 1497, 1420, 1232, 792, 648;
1H NMR (600 MHz, CDCl3): δ 7.62 (m, 4H, Ar-H), 8.72 (s,
1H, Ar-H), 2.27 (s, 1H, −CH3); 13C NMR (150 MHz, CDCl3)
δ 196.23, 150.96, 131.27, 165.07, 125.22, 123.90, 127.40,
149.45, 22.18, 124.03, 125.79.
Experimental
Procedure for the Synthesis of 3-[3-(3-Methyl-thiophen-
2-yl)-acryloyl]-chromen-2-one (MTC) (3)
Materials and Methods
To a solution of 3–Acetyl coumarin (0.5 mg, 2.65 mmol) and
3-methyl-2-thiophenecarboxaldehyde (0.38 mg, 2.65 mmol)
in ethanol (50 ml), 0.3 mL of piperidine and 2–3 drops of
glacial acetic acid were added. The mixture was refluxed for
12 h. After the completion of the reaction as identified by TLC
the solvent was removed under vacuum. The residue thus
obtained was triturated with 10 mL of ethanol until the forma-
tion of fine precipitates. The precipitate was filtered off and
recrystallized from methanol to give 3 (0.36 mg, 76 %) as pale
yellow solid.
Yield 76 %, Pale yellow solid; Melting point: 176-178 °C;
IR (KBr, v cm−1): 1640 (CO), 1725 (CO), 1497, 1420, 1232,
792, 648; 1H NMR (600 MHz, CDCl3): δ 7.72 (m, 4H, Ar-H),
8.02 (s, 1H, Ar-H), 5.32 (d, 1H, J = 15.4 Hz), 4.98 (d, 1H,
J = 15.1 Hz) 2.27 (s, 1H, −CH3), 7.02 (d, 1H, J = 8.0 Hz ),
7.42 (d, 1H, J = 8.0 Hz ); 13C NMR (150 MHz, CDCl3) δ
186.0, 196.2, 162.96, 160.27, 150.07, 126.22, 125.90, 121.40,
121.45, 127.18, 134.03, 135.79, 150.82, 132.63, 130.70,
132.07, 137.51, 15.58 (Scheme 1).
All solvents and chemicals used in this work were of spec-
troscopic grade obtained from Sigma Aldrich and used
without further purification. Melting points were deter-
mined on a Gallenkamp melting point apparatus and the
infrared (IR) spectra were recorded on Shimadzu FT-IR
8400S infrared spectrophotometer using the KBr pellet
technique. The NMR (1H and 13C) spectra were recorded
on a Bruker DPX-600 at 600 MHz and 150 MHz, respec-
tively, using tetramethylsilane as the internal standard. The
chemical shift values are recorded on δ scale and coupling
constants (J) in Hertz; Splitting patterns were designated as
follows: s: singlet; d: doublet; m: multiplet. UV-Vis elec-
tronic absorption spectra were recorded on a PG UV-vis
double-beam spectrophotometer, and the steady-state fluo-
rescence spectra were measured Varain Cary Eclipse
spectrofluorophotometer using a rectangular quartz cell of
dimensions 0.2 cm × 1 cm. The emission was monitored at
right angle. The fluorescence quantum yield (ϕf) was

J Fluoresc
Scheme 1 Synthetic route of MTC
Results and Discussion
constituted of a conjugated π-electron system. The red shifted
band in polar solvents arises from the charge transfer state and
short wavelength emission band originate from a locally ex-
cited state (Fig. 2).
Absorption and Emission Spectra of MTC in Different
Solvents
It is concluded that emission spectra is more sensitive to
solvent polarity compared to absorption spectra, which indi-
cates that photoinduced intramolecular charge transfer (ICT)
occurs in the singlet excited state from the electron donating
substituent group to the electron acceptor carbonyl group of
chromophore. Moreover, stabilization of highly dipolar excit-
ed state in polar solvents confirm the presence of π-π* tran-
sitions in MTC. The intensity of the emission spectra of MTC
was found to be highest in polar aprotic solvents and lowest in
polar protic solvents due to solute-solvent interaction such as
hydrogen bonding.
UV-vis absorption spectra of 1 х 10−5 mol L−1 MTC recorded
in solvents of different polarity show slight variation in the
absorption maxima on going non polar solvents to polar protic
solvents. Representative absorption spectra of MTC recorded
in some selective solvents are depicted in Fig. 1 and the cor-
responding spectroscopic, optical band gap and photophysical
properties are collected in Table 1. As seen in Fig. 1, the
compound show narrow absorption band at ~375 nm on going
from aprotic to protic solvents and hence, the absorption spec-
tra and optical band gap are hardly solvent dependent while
the high molar absorption coefficients indicates the π to π*
transition in all the solvents. On excitation at 365 nm, the
emission spectrum show smooth correlation with increasing
polarities of the solvents (Fig. 2; Table 1). A considerable red
shift is observed in the emission spectrum from 445 to 471 nm
on changing the solvent polarity from nonpolar to polar,
whereas no such shift is observed in the absorption spectrum
with varying solvent polarity, suggesting the involvement of
photo induced intramolecular charge transfer (ICT) in the sin-
glet excited state [13, 14]. Only one emission peak is observed
for MTC in non-polar solvents but in polar solvents a red
shifted emission band is observed in addition to the short wave
length band. This kind of dual emission is characteristic of
compounds containing electron donor and acceptor group
Estimation of Dipole Moments Using Solvatochromic
Methods
The extent of charge distribution in a molecule used to elucidate
a variety of physical and chemical properties. The change in
dipole moment between the excited singlet state and ground
state has been calculated using the method proposed by
Lippert-Mataga (2, 3) [15, 16], which is based on the correla-
tion of energy difference between the ground and excited states
(Stokes’ shift) with the solvent orientation polarizability (Δf).
ꢀ
ꢁ
2
2 μe−μg
Δνst ¼
Δf ¼
Δf þ Const:
ð2Þ
ð3Þ
hca3
ε−1
n2−1
−
2ε þ 1 2n2 þ 1
where Δνst is the Stokes shift expressed in wave numbers
(cm−1), h is Planck’s constant, c is the speed of light, a is the
Onsager cavity radius, ε and n are the dielectric constant and
refractive index of the solvent, respectively; μe and μg are the
dipole moments in the excited and ground state, respectively, and
Δƒ is the orientation polarizability of the solvent which measures
both electron mobility and dipole moment of the solvent mole-
cule. The Onsager cavity radii (a) from molecular volume of
molecules is calculated by using Suppan’s Eq. (4) [17]
ꢂ
ꢃ
1=3
3M
Fig. 1 Electronic absorption spectra of 1 × 10−5 mol L−1 of MTC in
different solvents
a ¼
ð4Þ
4πδN

J Fluoresc
Fig. 2 Emission spectra of 1 × 10−5 mol L−1 of MTC in different solvents
(λ ex = 365 nm)
where δ is the density of dye, M is the molecular weight of dye
and N is Avogadro,s number. From the slope of Stokes shift
versus the orientation polarization (Δƒ) plot (Fig. 3), change in
dipole moment calculated to be 5.17 D. Thus, the high value for
dipole moment and the linear correlation of Lippert-Mataga plot
further corroborates the occurrence of photoinduced intramolec-
ular charge transfer.
In addition, the change in dipole moment (Δμ) between the
excited singlet and ground state was also investigated using
solvatochromic shift method [18, 19], making use of the di-
mensionless microscopic solvent polarity parameters ETN
given by the Eq. (5)
ET ðsolventÞ−30:7
ETN
¼
ð5Þ
32:4
where λmax corresponds to the peak wavelength in the red
region of the intramolecular charge transfer absorption of the
betain dye. In this method, change in dipole moment is calcu-
lated by correlating the Stokes shift of the fluorophore to ETN
(Fig. 4) according to Eq. (6).
ꢂ
ꢃ
2
ꢀ
ꢁ
3
Δμ
aD
a
Δ ν ¼ 11307:6
ENT þ Const:
ð6Þ
ΔμD
Fig. 3 Plot of ΔF versus Stokes shift (cm−1
)

J Fluoresc
Fig. 4 Plot of ENT versus Stokes shift (cm−1
)
Fig. 5 Plot of ET(30) versus fluorescence quantum yield
Δμ is the difference between the excited and ground state
dipole moments of the probe molecule and ΔμD is the
change in the dipole moment of the betaine dye; a (taken
as 4.35 Å) and aD are the Onsager cavity radiia of the probe
molecule and betaine molecule respectively. Since the
values of aD and μD are known (6.2 Å and 9 Debye, respec-
tively) the change in dipole moment is calculated using
Eq. (7),
Effect of Solvents on Fluorescence Quantum Yield
The knowledge of fluorescence quantum yield (ϕf) values
gives an insight on the photophysical properties of an un-
known fluorescent molecule. Fluorescence quantum yield of
MTC was measured in solvents of different polarity. The fluo-
rescence quantum yield has been correlated with ET (30) of
the solvent, where ET(30) is the solvent polarity parameter put
forward by Reichardt [18], that deliberates interactions such as
solvent polarizability and hydrogen bonding besides those of a
specific nature. The fluorescence quantum yield (ϕf) of MTC
is strongly affected by the polarity and hydrogen bonding
ability of the solvents as shown in Table 1 and Fig. 5, where
"
#
1=2
81m
Δμ ¼
ð7Þ
3
ð6:2=aÞ Â 11307:6
where, m is the slope of linear plot of ENT vs Stokes shift
(Fig. 4) and the value for Δμ is found to be 2.16 D. The
value of Δμ obtained by Lippert-Mataga’s equation is
greater than that obtained by dimensionless microscopic
solvent polarity parameters ENT , because in Lippert’s plot
only the dipole-dipole interactions are considered and does
not consider the polarizability of solute molecules.
Table 2 The bond length (Å) of ground and excited states at B3LYP/6-
31G* and TD-B3LYP/6-31G* level of theories
Bonds
Bond Length (Å)
S0 S1
Bonds
Bond Length (Å)
S0 S1
The ground to excited state transition dipole moment (μ12)
in various solvents was calculated using Eq. (8) [20]:
C1-C2
C1-C6
C2-C3
C3-C4
C4-C5
C4-O1
C5-C6
C5-C9
C8-C9
C8-C10
C7-C8
C7-O1
C7-O3
1.4065 1.4032 C11-C12
1.3858 1.3880 C12-C13
1.3580 1.3862
1.4353 1.4045
1.3927 1.4236
1.7640 1.7686
1.5045 1.4996
1.4276 1.4034
1.3680 1.3845
1.7294 1.7177
Dipole
f
2
μ12
¼
ð8Þ
4:72 Â 10−7Emax
1.3916 1.400
1.3950 1.3861 C13-S1
1.4071 1.4211 C17-C16
1.3638 1.3690 C16-C15
1.4115 1.4208 C15-C14
C13-C16
where, Emax is the energy maximum absorption in cm−1
and f is the oscillator strength which shows the number of
electrons whose transition from ground to excited state
gives the absorption area of the electronic spectrum. The
experimental oscillator strength values were ascertained
using Eq. (9) [21]
1.4271 1.4130 C14-S1
1.3677 1.4163 Solvents
1.5104 1.4203
S0
S1
ꢀ ꢁ
1.4669 1.4329 Acetonitrile
2.31
9.72
9.64
9.78
9.77
9.72
f ¼ 4:32 Â 10−9 ∫ ε v dv
ð9Þ
1.3920 1.3690 Dichloromethane 2.22
1.2159 1.2404 Propanol
2.29
2.30
2.31
where, ε is the numerical value for molar decadic extinction
coefficient measured in dm mol−1 cm−1 and ν is the numer-
ical value of the wave number measured in cm−1. The
values of f and μ12 are listed in Table 1 and indicate that
the So → S1 transition is strongly allowed.
C10-C11 1.4668 1.5046 Ethanol
C10-O2
H6-O3
H5-O2
1.2378 1.2532 Methanol
2.1498 2.6141
2.3117
2.7596

J Fluoresc
Fig. 6 Optimized geometry of
MTC in EtOH solvent in the
ground and excited state
ϕf values undergo very sharp decline on increasing the solvent
polarity. This kind of change in the ϕf values with the solvent
polarity strongly implies that there is a large structural change
for the MTC following their photo excitation in the fluores-
cent state. The decrease in ϕf (positive solvatokinetic effect)
with charge transfer character was explained by several mech-
anisms such as proximity effect and conformational changes
[22]. The reduction of ϕf in highly protic alcoholic solvents is
caused by the hydrogen bonding between solvent molecules
and the carbonyl group of fluorophore due to enhanced radi-
ationless processes.
Computational Details
Here, we used Gaussian 09 program to achieve all calculation
for the synthesized molecule. The geometries, electronic and
absorption spectra of synthesized molecule were executed by
means of density functional theory (DFT) and time dependent
density functional theory (TDDFT), respectively [23].
All calculations were executed by using 6-31G* basis set
[24] and we used Becke’s three parameter gradient-corrected
exchange potential and the Lee-Yang-Parr gradient-corrected
correlation potential (B3LYP) [25–27] for DFT. TDDFT
Fig. 7 The representations of the
HOMO and LUMO orbital
charge density of MTC at B3LYP/
6-31G* level of theory and their
energy

J Fluoresc
Table 3 Calculated global reactivity descriptors for MTC at B3LYP/6-
31G(d,p) level
Softness (S) was calculated from the following equation:
S ¼ 1=2η
Sample EHOMO ELUMO Eg
η
χ
ω
S
The B3LYP/6-31G* and TD-B3LYP/6-31G* level of the-
ory have been applied to compute the ground state and excited
state geometries, respectively. Their geometrical parameters
are listed in Table 2 and labeling scheme is showed in
Fig. 6. The results revealed that bond length increased from
ground state to excited state in C4–C5, C7–O3, C8–C9, C10–
C11, C10–O2, C11–C12, C13–C16 and C15–C14 as 0.014,
0.024, 0.048, 0.037, 0.015, 0.028, 0.030 and 0.016 Å, respec-
tively. While bond length shortened from ground state to ex-
cited state in C5–C9, C7–C8, C8–C10, C7–O1, C12–C13,
C16–C15 and C14– S1 as 0.014, 0.090, 0.034, 0.023, 0.030,
0.024 and 0.011 Å, respectively. The major change toward
lengthening has been noticed for C8–C9, C10–C11, and
C13–C16, i.e., 0.048, 0.037 A, and 0.030 ° Awhile shortening
has been noticed for C14– S1, i.e., 0.011 A. There are also
intramolecular hydrogen bonding between H5–O2 and H6–
O3 and bond length increased from ground state to excited
state i.e., 0.447 A, and 0.464 A, respectively.
From ground to excited state the dipole moment is also
increased. The effect of change in dipole moment from ground
to excited state studied by Godefroid et al. and determined that
quantum phosphorescence efficiency decreased if change in
dipole moment from S0 → S1 (0.26 D/0.32 D) was lowly
whereas quantum phosphorescence efficiency increased if
change in dipole moment from S0 → S1 (0.38 D/2.76 D)
was huge. In our case, we calculated dipole moment in
MTC −5.87 −2.54 3.33 1.665 4.205 5.309917417 0.3003003
approach is used for the calculation of the absorption spectra
of molecule after the optimization which showed an efficient
method [28, 29] . In last two decades, the polarizable contin-
uum model (PCM) [30–32] has been appeared as the most
actual tools to realize the effects of bulk solvents for both
the ground- and excited-states. In this paper, the absorption
spectra has been computed by using PCM-TDB3LYP/6-31G*
level of theory.
The DFT method can be used to calculate reactivity descrip-
tors like electronegativity (χ), hardness (η), electrophilicity (ω)
and softness (S).
Mulliken electronegativity (χ) was calculated from the fol-
lowing equation:
χ ¼ ðEHOMO þ ELUMOÞ=2
Hardness (η) was calculated from the following equation:
η ¼ ðELUMO−EHOMOÞ=2
Electrophilicity (ω) was calculated from the following
equation:
2
ω ¼ ðEHOMO þ ELUMO=2Þ =2η
Fig. 8 The computed absorption spectra of MTC in different solvents

J Fluoresc
different solvents and the S0 → S1 dipole moment change is
7.41–7.49 D showing that this material would have good
quantum phosphorescence efficiency (Table 2). The calculat-
ed dipoles are in good agreement with experimental
observation.
The density functional theory (DFT) was used to compute
ground state geometries of the molecule. The computed peak
observed between 351–353 nm and 408–415 nm are very
close to the experimental absorption wavelengths.
Acknowledgments Authors are thankful to Research Center for
Advanced Materials Science (RCAMS), King Khalid University, Abha,
Saudi Arabia for the support and facilities to carry out the research work.
Figure 7 shows the highest occupied molecular orbitals
(HOMOs) energies (EHOMO), lowest unoccupied molecular
orbitals (LUMOs) energies (ELUMO) and HOMO–LUMO
energy gaps (Egap). In the target molecule HOMOs are
delocalized toward the right side of the schemes while the
LUMOs are delocalized over the entire molecule. The
HOMO-LUMO energy gap of this compound was calculated
at the B3LYP/6-31G* level (Fig. 7) and orbital energy level
analysis at the B3LYP/6-31G* level of theory. The kinetic
strength of a molecule is determined by the resultant
HOMO–LUMO energy gap (Egap). The calculated energy
gaps are in good agreement with the experimental values
(Table 1).
The DFT method can be used to calculate reactivity de-
scriptors like electronegativity (χ), hardness (η), electrophilic-
ity (ω) and softness (S) at the B3LYP/6-31G (d,p) level of
theory has been presented in Table 3. The value of chemical
hardness is 1.665 eV. In terms of chemical hardness, if
HOMO–LUMO energy gap of a molecule is large then it is
hard. Conversely, if the HOMO–LUMO energy gap is small,
it is soft. One can also relate molecular strength to hardness,
which means that the molecule with smaller HOMO–LUMO
energy gap is more reactive because it’s very easy to add
electron to LUMO and remove electron from LUMO.
The UV–Vis absorption spectra of MTC exhibit two band
of absorption between 351–353 and 408–415 nm, depending
on the solvent (Fig. 8 and Table 1 ). The results revealed that
calculated value increase regularly with an increase in dielec-
tric constant of the solvents. The calculated absorption spectra
are in good agreement with experimental observation.
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