Synthesis, Characterization, Optical, Electrochemical and Current-Voltage Characteristics of Coumarin Dyes

Article abstract of DOI:10.1007/s10895-019-02436-7

A series of coumarin dyes having common acceptor and pi spacer with different donors were synthesized and characterized by spectroscopic techniques. The TiO₂ nanoparticle has been synthesized by sol-gel method and characterized by X-ray diffrac

Full text of DOI:10.1007/s10895-019-02436-7

Journal of Fluorescence
https://doi.org/10.1007/s10895-019-02436-7
ORIGINAL ARTICLE
Synthesis, Characterization, Optical, Electrochemical
and Current-Voltage Characteristics of Coumarin Dyes
1,2
1
1
1
K. V. Basavarajappa & Y. Arthoba Nayaka & R. O. Yathisha & P. Manjunatha
Received: 20 June 2019 /Accepted: 3 September 2019
#
Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
A series of coumarin dyes having common acceptor and pi spacer with different donors were synthesized and characterized by
spectroscopic techniques. The TiO nanoparticle has been synthesized by sol-gel method and characterized by X-ray diffraction,
2
scanning electron microscopy and energy dispersive spectroscopy. Photophysical and electrochemical properties have been
investigated. The band gap, molar absorption coefficient and energies of frontier molecular orbitals (FMO) have been calculated.
The results showed that an electron releasing group helps in tuning the band gap. The geometry optimization and energies of
FMO were obtained by density functional theory calculations. The calculated global chemical parameters help in understanding
the intrinsic donor-acceptor properties of the synthesized dye molecules. The result of current-voltage characteristics showed
good photocurrent response under illumination condition.
.
.
.
Keywords Coumarin Band gap I-V characteristics TiO
2
Introduction
organic molecules helps us to meet the urgent needs for
cheaper and cleaner energy. The dye-sensitized TiO solar
2
Energy is a vital component to our life and the use of energy is
increasing with human advancement and industrial develop-
ment [1]. Most of the present-day energy sources are from non
cells have the advantages of low-cost, eco-friendly, better con-
version efficiency and so on [2].
Small organic molecules have been attracting great atten-
tion in the past few years due to their unique properties of the
low-cost, easy synthetic procedure, flexibility in designing
organic photovoltaics. Among the small organic molecules,
coumarin dyes have been recognized as one of the good or-
ganic dye photosensitizers [3]. Therefore, it was planned to
synthesize the coumarin molecules containing different
electron-donating groups and electron-withdrawing nitro
group. The chemical structures of the synthesized molecules
and the numbering of simple coumarin skeleton are as shown
in Fig. 1.
The above molecules were prepared by a facile novel
method. This novel procedure is found to be a more useful
and convenient method for the preparation of 2,4-
dinitrophenylhydrazones of coumarin than the reported
methods cited elsewhere [4–7]. The proposed method is
fast, simple, avoids the use of corrosive chemicals and
there is no need of column chromatographic separation of
the products. The detailed synthetic procedure is given in
the Experimental Section.
-
renewable energy forms like fossil fuels, thermal energy, etc.
and these sources are going to decline day by day. To over-
come this situation there is a great need for cheaper and clean-
er energy sources. Solar energy is abundant and readily avail-
able, so, one should make solar energy a priority for our future
energy. The development of dye-sensitized solar cells
(
DSSCs), the third generation of photovoltaics using small
Electronic supplementary material The online version of this article
https://doi.org/10.1007/s10895-019-02436-7) contains supplementary
material, which is available to authorized users.
(
*
Y. Arthoba Nayaka
drarthoba@yahoo.co.in; basu.kv2019@gmail.com
K. V. Basavarajappa
basu.kv@gmail.com
1
2
Department of PG Studies and Research in Chemistry, Kuvempu
University, Jnanasahyadri, Shankaraghatta, Karnataka 577451, India
Department of Chemistry, Government First Grade College,
Davanagere, Karnataka 577004, India
Out of many semiconducting metal oxides, TiO nanopar-
2
ticle appears to be more versatile because due to its very high

J Fluoresc
Fig. 1 The chemical structures of
coumarin molecules (2ad-2dd)
and numbering of simple couma-
rin (A)
roughness factor and it provides extremely favorable and ef-
ficiently irreversible entropic and enthalpic driving forces for
charge injection. Low recombination within the semiconduc-
tor matrix as well as between the semiconductor and the hole-
transporting layer (HTL) allows for very efficient electron
transport to the anode. Light scattering within the porous crys-
talline structure increases the radiation path-length and thus
improves photon harvesting [8].
synthesis private limited. 2-hydroxy-1-naphthaldehyde,
4-(diethylamino) salicyladhyde, 3-ethoxysalicylaldehyde
( t e c h n i c a l g r a d e ) a n d t e t r a b u t y l a m m o n i u m
hexaflurophosphate (electrochemical grade) were purchased
from Sigma-Aldrich. All the chemicals were used for the
experiments as such without further purification. Oven-dried
glasswares were used to carry out the reactions. The reactions
were monitored by thin-layer chromatography (TLC) using
silica gel 60 F254 (Merck) and a mixture of ethyl acetate (EA)
and n-hexane (60–80 °C boiling mixtures) as mobile phase.
The TLC spots were visualized under UV light. Melting
points were determined using open-ended capillary tubes
and were uncorrected. FTIR spectra were recorded on KBr
In this work, the target coumarin molecules were synthe-
sized by facile novel procedure and characterized using ana-
lytical methods. The TiO nanoparticles were synthesized by a
2
modified literature procedure using a sol-gel method.
Absorption properties of dyes in solution and dyes coated on
1
TiO nanoparticle were tested. The energies of HOMO and
pellets using Bruker Alpha-T spectrophotometer. H NMR
2
LUMO of the dyes were determined by cyclic voltammetric
studies and compared with theoretical values obtained by DFT
calculations. The current-voltage (I-V) characteristics were
spectra were recorded on a VNMRS–400 Agilent–NMR
spectrometer using Tetramethylsilane (TMS) as an internal
standard. Chemical shift (δ) values were expressed in parts
per million (ppm) units. Mass Spectra (MS) were recorded
using Water’s SYNAPT G2 QTOF LCMS instrument. UV
absorption spectra were recorded using UV-Visible-NIR
spectrometer [USB 4000, Ocean Optics, USA] in anhydrous
solvents. The electrochemical measurements obtained by
CHI660D electrochemical workstation using the three-
performed by dyes coated TiO nanoparticles. The results ob-
2
tained were discussed and concluded.
Experimental Section
Materials and Instruments
electrode system. The crystalline structure of TiO was ob-
2
tained by X-ray diffractometer (XRD) (Rigaku), and the sur-
face morphology and crystalline structure were obtained by
scanning electron microscopy (SEM) (ZEISS EVO LS15).
The analytical grade 2-hydroxybenzaldehyde,
e t h y l a ce t oa c et a t e ( E A A ) , pi pe r i d i n e , N , N -
dimethylformamide (DMF), 1,4-dioxane, ethyl alcohol, 2-
propanol and glacial acetic acid were purchased from SD
Fine- Chem Limited. 2,4-dinitrophenylhydrazine (2,4-DNP)
The elemental analysis of TiO nanoparticle was conducted
2
by energy dispersive analyzer using X-rays (EDAX) (Thermo
Scientific Noran 7) and I-V characterization was performed
by Keithley source meter with USB GRIB adaptor [Model:
2401].
(
Extra pure) was purchased from HiMedia Laboratories Pvt.
Ltd.. Titanium tetrachloride (TiCl ) was obtained from Avra
4

J Fluoresc
Synthesis
3-Acetyl-8-Ethoxy-2H-Chromen-2-One (2c)
General Procedure for the Solvent Free Synthesis of 3-Acetyl
Coumarin Derivatives (2a-2d)
Recrystallization: ethanol; color and appearance: yellow pow-
−1
der. Yield: 86% and mp 122–124 °C. IR (KBr, v, cm ): 1722
lactone C=O), 1677 (CH -C=O), 3095 and 3349 (Ar C-H),
(
3
1
The solvent-free method was selected for the synthesis of 3-
acetyl coumarins via Knoevenagel condensation [9]. This
method involves simple operation, waste minimization and
easier product work-up. The synthetic procedure involves an
equivalent mixture of derivatives of salicylaldehyde (1a-1d,
2930 (CH C-H). H NMR (400 MHz, CDCl ) δ (ppm):
3 3
1.484–1.519 (3H, t, CH CH ), 2.713 (3H, s, COCH ),
2
3
3
4.161–4.214 (2H, q, OCH ) 7.418–98.455 (4H, m, ArH).
2
+
+
MS-AP (m/z) found: 233.0692 (M + H) ; calculated for
+
C H O : 232.0730 (M ).
1
3 12 4
1
0.0 mmol), ethyl acetoacetate (10.0 mmol) and a few drops
of piperidine were mixed and ground well for 5 min at room
temperature (rt). The reaction mixtures were neutralized with
HCl (4 N) and then, the so obtained substituted 3-acetyl
coumarins (2a-2d) were isolated by filtration. The IR,
3-Acetyl-7-(Diethylamino)-2H-Chromen-2-One (2d)
Recrystallization: ethanol; color and appearance: yellow crys-
tals. Yield: 77% and mp 146–148 °C. IR (KBr, v, cm ): 1720
1
−1
H NMR and Mass spectral analysis was used for structural
confirmation of the synthesized molecules. The synthetic
pathway is as shown in Scheme 1. IR, H NMR and Mass
(lactone C=O), 1662 (CH -C=O), 3118 and 2964 (Ar C-H),
3
1
1
2925 (CH C-H). H NMR (400 MHz, CDCl ) δ (ppm):
3
3
spectra of the samples are shown in supplementary material
S1, S2 and S3 respectively.
1.200–1.235 (6H, t, CH CH ), 2.652 (3H, s, CH CO)
3 2 3
3.406–3.460 (4H, CH CH ), 6.437–8.405 (4H, m, ArH).
2
3
+
+
MS-AP (m/z) found: 260.1194 (M + H) ; calculated for
C H NO : 259.1203 (M ).
+
1
5
17
3
3-Acetyl-2H-Chromen-2-One (2a)
Recrystallization: ethanol; color and appearance: colorless
General Procedure for the Synthesis
of 2,4-Dinitrophenylhydrazones of 3-Acetyl
Coumarins (2ad-2dd)
needle-shaped crystals. Yield: 77% and mp 120–124 °C. IR
−
1
(
KBr, v, cm ): 1726 (lactone C=O), 1682 (CH -C=O), 3080
3
1
and 3052 (Ar C-H), 2925.81 (CH C-H). H NMR (400 MHz,
3
CDCl ) δ (ppm): 2.633 (3H, s, COCH ), 7.375–8.637 (5H, m,
3-acetyl coumarins (2a-2d) (2.23 mmole) were added to a
solution of 2,4-dinitrophenylhydrazine (2.52 mmole) in
dimethylformamide (5 mL) in a 50 mL beaker and the mixture
was swirled to get a homogenous solution, added two drops of
HCl (35%) and kept for 30 min at room temperature. The
obtained solid product was filtered and washed with 20 mL
of 2 N aqueous hydrochloric acid, 30 mL of water and 30 mL
of cold 95% aqueous ethanol. The crude product was dried
and recrystallized from 1,4-dioxane to get pure hydrazones
(41–68%) as orange-yellow powder. The structures of newly
synthesized compounds are in agreement with their IR,
3
3
+
+
ArH). MS-AP (m/z) found: 189.0318 (M + H) ; calculated
+
for C H O : 188.0468 (M ).
1
1 8 3
2-Acetyl-3H-Benzo[f]Chromen-3-One (2b)
Recrystallization: ethanol; color and appearance: bright yel-
low crystals. Yield: 67% and mp 174–176 °C. IR (KBr, v,
−
1
cm ): 1737 (lactone C=O), 1673 (CH -C=O), 3058 and
3
3
1
017 (Ar C-H), 2930.06 (CH C-H). H NMR (400 MHz,
3
1
CDCl ) δ (ppm): 2.633 (3H, s, COCH ), 7.598–9.271 (7H,
H NMR and Mass spectral data and are shown in S4, S5
and S6 (supporting information). The synthetic pathway is
as shown in Scheme 2.
3
3
+
+
m, ArH). MS-AP (m/z) found: 239.0558 (M + H) ; calculat-
ed for C H O : 238.0624 (M ).
+
1
5 10 3
Scheme 1 Synthesis of 3-acetyl coumarins (2a-2d)

J Fluoresc
Scheme 2 Synthesis of 2,4-dinitrophenylhydrazones of 3-acetyl coumarins (2ad-2dd)
−1
3-{(1E)-1-[2-(2, 4-Dinitrophenyl)Hydrazinylidene]
(KBr, v, cm ): 3288 (N-H), 3113 and 3072 (Ar C-H), 2921
Ethyl}-2H-Chromen-2-One (2ad)
(CH C-H), 1723 (lactone C=O), 1615 (-C=N-), 1512 and
3
1
1
335 (asy and sy stretch of NO ). H NMR (400 MHz,
2
Recrystallization: 1,4-dioxane; color and appearance: orange
CDCl ) δ (ppm): 1.219–1.255 (6H, t, CH ), 2.488 (3H, s,
3 3
crystal. Isolated yield: 56% and mp 258–260 °C. IR (KBr, v,
COCH ), 3.422–3.475 (4H, q, NCH ), 6.498–9.155 (7H, m,
3 2
−1
+
cm ): 3316 (N-H), 3114 (Ar C-H), 2922 (CH C-H), 1723
ArH) and 11.358 (1H, s, NH). MS-AP (m/z) found: 440.9984
3
+
+
(
lactones C=O), 1610 (-C=N-), 1507 and 1328 (asy and sy
(M + H) ; calculated for C H N O :440.1565 (M + H) .
21 21 5 6
1
stretch of NO ). H NMR (400 MHz, CDCl ) δ (ppm):
2
3
2
.492 (3H, COCH ), 7.247–9.176 (8H, m, ArH) and 11.383
Synthesis of TiO Nanoparticles
2
3
−
+
(
1H, s, NH). MS-AP (m/z) found: 367.0741 (M-H) ; calcu-
+
lated for C H N O : 368.0751 (M ).
The sol-gel method has been used to synthesize the TiO2
nanoparticles using modified reported procedure [10]. A
1
7 12 4 6
2-{(1E)-1-[2-(2,4-Dinitrophenyl)Hydrazinylidene]
4 ml of analytical grade TiCl was added dropwise to 1:1
4
Ethyl}-3H-Benzo[f]Chromen-3-One (2bd)
solution of 2-propanol and glacial acetic acid under vigorous
stirring at room temperature. The resultant solution was stirred
overnight and then dried in a hot air oven for about 6 h. Later
the precipitate formed was calcined at 500 °C for about 2 h
soaking time.
Recrystallization: 1,4-dioxane; color and appearance: orange-
red crystals. Isolated yield: 41% and mp 240–244 °C. IR
−1
(
KBr, v, cm ): 3308 (N-H), (3113 and 3072 (Ar C-H), 2921
(
1
CH C-H), 1618 (-C=N-), 1723 (lactone C=O), 1510 and
3
1
307 (asy and sy str of NO ). H NMR (400 MHz, CDCl )
Dye Coating
2
3
δ (ppm): 2.569 (3H, s, COCH ), 7.510–9.200 (10H, m, ArH)
3
+
and 11.459 (1H s, NH). MS-AP (m/z) found: 418.9489 (M +
Synthesized TiO nanoparticles were ground well to make a
2
+
+
H) ; calculated for C H N O : 418.0908 (M ).
uniform sample of fine nanoparticles. These nanoparticles
were mixed with a chloroform solution of coumarin dyes
(3.5 mM, 2ad-2dd) to prepare a slurry. A thin layer of this
slurry was uniformly deposited on the surface of well-
cleaned glass plates by dip coat method. Dye coated glass
plates were kept aside for 30 min at room temperature for
solvent evaporation, later in a hot air oven for 7 h at 70 °C.
After that, it was cooled down to rt. and taken out for absorp-
tion and I-V studies. The electrical connection was made by
inserting the copper wire.
2
1 14 4 6
3-{(1E)-1-[2-(2,4-Dinitrophenyl)Hydrazinylidene]
Ethyl}-8-Ethoxy-2H-Chromen-2-One (2cd)
Recrystallization: 1,4-dioxane; color and appearance: orange
crystals. Isolated yield: 50% and mp 250–252 °C. IR (KBr, v,
−1
cm ): 3449 (N-H), 3113 and 3072 (Ar C-H), 2921 (CH C-
3
H), 1723 (lactone C=O), 1613 (-C=N-), 1510 and 1333 (asy
1
and sy str of NO ). H NMR (400 MHz, CDCl ) δ (ppm):
2
3
1
4
.506–1.541 (3H, t, OCH CH ), 2.489 (3H, s, COCH ),
2 3 3
.185–4.237 (2H, q, OCH ), 7.123–9.177 (7H, m, ArH) and
2
+
11.380 (1H, s, NH). MS-AP (m/z) found: 413.9733 (M +
Result and Discussion
+
+
H) ; calculated for C H N O : 413.1092 (M + H) .
1
9 16 4 7
Spectral Characterization
7-(Diethylamino)-3-{(1E)-1-[2-(2,4-Dinitrophenyl)
Hydrazinylidene]Ethyl}-2H-Chromen-2-One (2dd)
The target compounds (2ad-2dd) were synthesized by a novel
approach, which involves the condensation of 3-acetyl cou-
marins (2a-2d) with 2,4-DNP in DMF as depicted in Scheme
Recrystallization: 1,4-dioxane; color and appearance: red
shiny crystals. Isolated yield: 68.4% and mp 262-264 °C. IR
1
2. The IR, H NMR and MS techniques were used for

J Fluoresc
Fig. 2 a XRD-Pattern of TiO2,
b
2
EDAX Spectrum of TiO and c
SEM-Image of TiO
2
structural elucidation of the synthesized dyes. The strong IR
absorption band appeared around 1713–1730 cm
(supporting information S4) corresponds to lactone of couma-
rin skeleton. The absence of peaks around 1662–1682 cm in
−
1
−1

J Fluoresc
Table 1 Optical and structural
properties of TiO
opt
g
Entry
λmax (nm)
λedge (nm)
E
(eV)
Average crystalline
size(D) nm
2
a
TiO
2
354.24
448.65
2.76
30.5
a.
opt
g
E
= 1240/λedge eV, [11]
the products 2ad-2dd clearly indicated the conversion of the
ketonic carbonyl group of the starting materials 2a-2d
synthesized TiO nanoparticles. The optical band gap was
calculated and tabulated in Table 1. Debye-Scherrer Eq. (1)
is used to calculate the average crystalline size (D) [12].
2
(
supporting information S1) to the imine group. The new band
−1
appeared around 1610–1618 cm further confirms the imine
C=N) functional group. Moreover, stretching frequencies ap-
D ¼ 0:9λ=β Cosθ
ð1Þ
(
−1
−1
peared around 1507–1512 cm and 1328–1335 cm are re-
sponsible for the asymmetric and symmetric stretch of Ar–
Where, β is the full width at half maxima in radians, λ is the
wavelength of CuKα X-ray radiation used (λ = 0.154 nm) and
θ is the Bragg’s reflection angle in degrees. The calculated
NO group. The proton of N-H for all the molecules appeared
2
around δ ~ 11 as a singlet peak (supporting information S5).
From the mass spectrometry, the calculated molecular weights
of the synthesized compounds were in good agreement with
the experimentally obtained values (supporting information
S6).
data of optical and structural parameters of TiO are given in
Table 1.
2
Photophysical Properties
The absorption spectra of coumarin dyes in DMF (1 × 10− M)
5
XRD, EDAX and SEM Analysis
and the dyes coated on TiO nanoparticles are shown in Fig. 3a
2
opt
Figure 2a shows the powder X-ray diffraction patterns of syn-
and b. The optical band gap (Eg ) and molar absorption coef-
thesized TiO nanoparticles. The Figure indicates phase purity
ficient (Ɛ) are calculated and tabulated in Table 2 [13]. From the
Table, it is cleared that the wavelength maximum of dyes in
DMF has been shifted to higher wavelength for 2dd compared
to other coumarin dyes. This bathochromic shift of 2dd is as-
cribed to the effect of strong electron releasing diethylamine
group and hence decreasing the band gap. The synthesized
compounds have higher molar absorption coefficient (Ɛ) values
than that of typical C343 and NKX series of dyes [14]. The
2
and structural parameters of TiO . The X-ray diffraction pat-
2
terns show one intense peak at (101) plane, three moderately
intense peaks corresponds to (112), (200), and (211) planes
and less intense peak at (204) plane. It suggested that the
synthesized nanomaterial has a tetragonal system with anatase
structure, which is further confirmed from the JCPDS No.84–
1
285 (a = 3.784 and c = 9.512). The chemical elements detec-
tion with their relative abundance can be obtained from
EDAX analysis and is as shown in Fig. 2b, it confirms the
dyes coated TiO nanoparticles show broadening of absorption
maxima in comparison to coumarin dyes in DMF solution.
This broadening clearly shows that there is a good electron
2
purity of the synthesized nanoparticle TiO . Figure 2c, scan-
2
ning electron micrograph reveals the surface morphology of
communication between dye and TiO . In addition, broadening
2
Fig. 3 Absorption spectra of coumarin derivatives. a In DMF and b On TiO
2

J Fluoresc
Table 2 Absorption properties of
coumarin derivatives
opt
g
a
Entry
λmax (nm)
λedge (nm)
E
(eV)
Molar absorption coefficient
5
−1
−1
(Ɛ) 10 M cm
2ad
2bd
2cd
2dd
390.15
406.82
389.75
453.34
479.44
498.19
491.78
549.64
2.59
2.49
2.52
2.26
0.87
1.26
1.08
1.34
a
opt
g
E
(eV) = 1240/λedge(nm)
of the absorption spectrum is advantageous for better harvest-
ing of solar energy and hence to get a large photocurrent [11].
diethylamine and the less negative value in case of 2bd is due
to the extended conjugation of coumarin backbone, and hence
the electron releasing groups/extended conjugation destabi-
lizes the HOMO level [13].
Electrochemical Studies
From the theory of DSSC for the effective injection of
electrons into the conduction band (CB) of TiO , the
LUMO levels of the dye must be sufficiently higher en-
ergy than the conduction band energy of semiconductor
2
Electrochemical properties of synthesized dyes were deter-
mined by cyclic voltammetric studies in DMF using pencil
graphite as the working electrode, Ag/Ag as the reference
electrode, platinum as counter electrode and 0.2 M
tetrabutylammonium hexaflurophosphate as supporting elec-
trolyte. The oxidation onset potentials of dyes were found to
be 0.3727 V, 0.2873 V, 0.3915 V and 0.1788 V for 2ad, 2bd,
+
TiO (−4.21 eV vs. vacuum) [16]. From Table 3, it is clear
2
that all the synthesized dyes have less negative LUMO
values compared to CB of semiconductor TiO and hence
2
they can accomplish the electron injection and thereby
undergo oxidation. This driving force makes the process
thermodynamically more favorable.
2
cd and 2dd respectively. The energies of highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) were calculated using the formula: E
=
HOMO
opt
−
(Eonsetox + 5.1) eV and ELUMO = EHOMO + E , where,
Frontier Molecular Orbital (FMO) Analysis
g
opt
Eonsetox is the onset oxidation potential in V, Eg is the optical
band gap in eV, and 5.1 eV is the assuming electrochemical
potential of ferrocene/ferrocenium (Fc/Fc ) vs. vacuum [15].
To understand the molecular architecture, the electron density
distribution of the compounds and to characterize the global
chemical parameters, FMO analysis was done by DFT calcula-
tions using B3LYP/6-31G (d, p) level software. The geometry
optimized structures and electron density distribution of HOMO
and LUMO orbitals of the compounds are as shown in Fig. 5.
From Fig. 5, it is observed that electron density distribution
in HOMO is concentrated on 2,4-dinitrophenyl moiety and in
LUMO concentrated on coumarin moiety in structures 2ad
and 2cd. For 2bd, the electron density is moderately distribut-
ed between HOMO and LUMO levels. The electron density
distribution in HOMO of 2dd is localized on donor coumarin
+
The obtained values are summarized in Table 3. The cyclic
voltammograms of the compounds 2ad-2dd are as shown in
Fig. 4. Table 3 reveals that HOMO energy levels are increas-
ing in the following order −5.4915 (2cd), −5.4723 (2ad),
−
5.387 (2bd), and − 5.2788 eV (2dd). DFT calculated
HOMO values were also follows the same trend (Table 3).
The observed oxidation peaks in Fig. 4 at A, B, C and D are
respectively due to the electron transfer from HOMO of 2ad,
2
2
bd, 2cd and 2dd to the electrode. The least negative value of
dd is due to the presence of strong electron-donating group
Table 3 Electrochemical
properties of coumarin dyes
a
a
b
b
Entry
The onset oxidation
potential (V)
E
HOMO (eV)
E
LUMO (eV)
E
HOMO (eV)
ELUMO (eV)
2ad
2bd
2cd
2dd
0.3727
0.2873
0.3915
0.1788
−5.47
−5.39
−5.49
−5.28
−3.08
−2.97
−3.08
−3.05
−6.28
−6.20
−6.28
−5.88
−2.80
−2.71
−2.69
−2.57
a
opt
E
HOMO = −(Eonsetox + 5.1) eVand ELUMO = EHOMO + E
g
; where, Eonsetox is the onset oxidation potential in V,
opt
+
E
g
is the optical band gap in eV, and 5.1 eV is the assuming electrochemical potential of Fc/Fc in vacuum
b
DFT calculations

J Fluoresc
Fig. 4 Cyclic voltammogram of 2ad, 2bd, 2cd and 2dd
moiety through a pi bridge and in LUMO it is shifted towards
electron-withdrawing nitro groups of 2,4-DNP and hence it
provides good intramolecular charge transfer (ICT) from do-
nor to acceptor leading to push-pull effect [17].
to donate and accept the electrons by the molecules respec-
tively. The electronegativity (χ) is a measure of the intrinsic
donor-acceptor character of a molecule and is the algebraic
sum of I and A as proposed by Robert S. Mulliken [19] and
is given in Eq. (3).
ꢀ
ꢀ
1
1
Calculations of Global Chemical Parameters
χ ¼ 2ðI þ AÞ or χ ¼
½−EHOMO þ ð−ELUMOފ
ð3Þ
2
Using the values of HOMO and LUMO by FMO analysis, the
global chemical parameters like ionization energy (I), electron
affinity (A), electronegativity (χ), chemical hardness (η) and
chemical softness (σ) have been calculated for the understand-
ing of donor-acceptor interactions. These parameters are ob-
tained by Koopmans theorem and are listed in Tables 4 and 5
Chemical hardness (η) is the resistance to change the elec-
tron distribution or electron density function of a chemical
system. It is given by the Eqs. (4) and (5). The lower chemical
hardness indicates the small HOMO–LUMO gap and it is
desired for better intramolecular charge transfer.
ꢀ
1
η ¼
ðI−AÞ
ð4Þ
ð5Þ
[18]. According to this theorem, the electron affinity (A) and
2
ionization potential (I) are given by Eq. (2).
η ¼ Eg=2
I ¼ −EHOMO and A ¼ −ELUMO
ð2Þ
Chemical softness (σ) is the reverse of the hardness and is
given by Eq. (6).
Ionization energy is defined as the amount of energy re-
quired for the removal of an electron from the highest occu-
pied molecular orbital. Electron affinity is the amount of en-
ergy released when an electron is added to the lowest unoccu-
pied molecular orbital. These parameters determine the ability
σ ¼ ð1=ηÞ
ð6Þ
From Tables 4 and 5, the ionization energy is found to be
smaller for 2dd compared to others in the series 2ad-2dd. This
may be attributed to the presence of strong electron releasing

J Fluoresc
Fig. 5 Molecular architecture and electron density distribution of the compounds calculated by DFT using B3LYP/6-31G (d,p) level software
Table 4 Experimental global chemical parameters of synthesized
Table 5 Theoretical global chemical parameters of synthesized
molecules
molecules at B3LYP/6-31G (d, p) level software
Entry
I (eV)
A (eV)
χ (eV)
η (eV)
σ (eV)
Entry
I (eV)
A (eV)
χ (eV)
η (eV)
σ (eV)
2
2
2
2
ad
bd
cd
dd
5.47
5.39
5.49
5.28
3.08
2.97
3.08
3.05
4.30
4.18
4.29
4.17
1.20
1.21
1.21
1.12
0.83
0.83
0.83
0.89
2ad
2bd
2cd
2dd
6.28
6.20
6.28
5.88
2.80
2.71
2.69
2.57
4.54
4.46
4.49
4.17
1.70
1.75
1.80
1.66
0.59
0.57
0.56
0.60

J Fluoresc
Fig. 6 Current-Voltage (I-V) characteristics of TiO
2
2
and dye coated TiO : a- under dark, b-under light, c, d, e and f: (I-V) characteristics of dyes 2ad, 2bd,
2
cd and 2dd on TiO under dark and light condition respectively
2
group diethyl amine on 7- position of coumarin skeleton.
Smaller ionization energy makes this molecule a strong elec-
tron donor. The order of decrease in the electron affinity
values for series of dyes is as follows 2ad > 2cd > 2bd > 2dd
conjugation of coumarin moiety with benzene ring fusion at 5,
6-position. This shows that the electron donation of 2bd is
higher than that of 2ad and 2cd. Hard and soft nature of the
molecules can be evaluated on the basis of hardness and soft-
ness values. From Table 4, 2dd is found to be a softer molecule
which is further supported with smaller band gap (Table 2)
compared to others in the series. The global chemical param-
eters computed theoretically using DFT calculation follows
(
Table 5). Smaller electronegativity of 2dd is also indicated
that 2dd is a strong electron donor compared to others. The
electronegativity of 2bd is greater than 2dd but smaller than
2
ad and 2cd. This may be explained on the basis of extended

J Fluoresc
3
.
Hara K, Sayama K, Arakawa H, Ohga Y, Shinpo A, Suga S (2001)
A coumarin-derivative dye sensitized nanocrystalline TiO solar
the same trend as that of experimental values except the elec-
tron affinity value of 2bd and 2cd. The order of electron af-
finity values is as follows: 2ad > 2bd > 2cd > 2dd.
2
cell having a high solar-energy conversion efficiency up to 5.6%.
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Current-Voltage (I-V) Characteristics
The current-voltage response of pure TiO and dyes coated
2
TiO were performed under dark and illumination conditions
2
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2
959. https://doi.org/10.1021/ja01370a058
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dark condition. The lower I-V response of dye coated TiO is
attributed to the blocking effect of the dye [20]. The Figs. 6c–f
show the photosensitive behaviour of the dyes 2ad, 2bd, 2cd
2
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and 2dd on TiO nanoparticles, respectively. It is evident that
2
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at the potential of 4.0 V, the current response is higher under
light condition (0.5A) compare to dark condition (~ 0.01A) for
all the dyes. This is probably due to an increase in the number
and mobility of charge carriers (electron-hole pairs) upon il-
lumination. This enhanced photocurrent of dyes significantly
improves the solar cell performance. Hence, these dyes could
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(
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Vinay MM, Basavarjappa KV (2019) Study on the effect of Zn
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Physica E: Low-dimensional Systems and Nanostructures 108:
The coumarin molecules containing electron-withdrawing ni-
tro chromophores have been synthesized using the novel
2
57–268. https://doi.org/10.1016/j.physe.2018.12.021
method. The TiO nanoparticles were synthesized by sol-gel
2
13. Hou J, Guo X (2013) Active layer materials for organic solar cells.
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method. The optical properties of synthesized dye molecules
and dyes coated TiO nanoparticles were studied by measur-
2
1
ing the band gap energy. Energy levels of HOMO and LUMO
were determined using cyclic voltammetric method and com-
pared with theoretical DFT study. The results show that the
energy levels of HOMO and LUMO of the prepared dye mol-
1
1
1
ecules were found to be matched with TiO energy levels. I-V
2
characteristics study shown that the prepared dyes could be
successfully used for photovoltaic applications.
Acknowledgments Authors are thankful to UGC-SWRO and
Department of Collegiate Education-Bangalore for selecting under
UGC-FIP scheme. The authors are also thankful to DST (SERB), New
Delhi, UGC New Delhi, for providing the instrumental facility.
1
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