Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and cyclic voltammetry of new organic dyes for dye-sensitized solar cells

Article abstract of DOI:10.1016/j.molstruc.2019.127228

In recent years, dye-sensitized solar cells (DSSCs) have regarded as potential solar cells for the next generation of photovoltaic technologies. Many organic compounds are explored and used in DSSCs to produce low-cost devices and improve the cell efficiency. In this work, three new heterocyclic purple dyes are synthesized from the reaction of 3-nitropyrazolo[1,5-a]pyridine with various arylacetonitriles for dye-sensitized solar cells (DSSCs), exhibiting high molar extinction coefficients and a broad absorption range led to the good photovoltaic performance of 6.95–7.18%. Physical spectral, analytical data and optical properties are established the structures of the new dyes. The optimized geometries and relevant frontier orbitals of the dyes are obtained by density functional theory (DFT) at the level of B3LYP/6-311 + G(d,p). Electrostatic potential maps and electron density maps of the dyes were also obtained by atoms in molecules (AIM) analysis. Cyclic voltammetry measurement was performed to evaluate the electrochemical properties of the dyes and reversible oxidation waves were observed for them.

Full text of DOI:10.1016/j.molstruc.2019.127228

Journal of Molecular Structure xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, characterization, optical properties, computational
characterizations, QTAIM analysis and cyclic voltammetry of new
organic dyes for dye-sensitized solar cells
Zahra Agheli, Mehdi Pordel*, Safar Ali Beyramabadi
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
In recent years, dye-sensitized solar cells (DSSCs) have regarded as potential solar cells for the next
generation of photovoltaic technologies. Many organic compounds are explored and used in DSSCs to
produce low-cost devices and improve the cell efficiency. In this work, three new heterocyclic purple dyes
are synthesized from the reaction of 3-nitropyrazolo[1,5-a]pyridine with various arylacetonitriles for dye-
sensitized solar cells (DSSCs), exhibiting high molar extinction coefficients and a broad absorption range
led to the good photovoltaic performance of 6.95e7.18%. Physical spectral, analytical data and optical
properties are established the structures of the new dyes. The optimized geometries and relevant frontier
orbitals of the dyes are obtained by density functional theory (DFT) at the level of B3LYP/6-311 þ G(d,p).
Electrostatic potential maps and electron density maps of the dyes were also obtained by atoms in
molecules (AIM) analysis. Cyclic voltammetry measurement was performed to evaluate the electro-
chemical properties of the dyes and reversible oxidation waves were observed for them.
Received 10 August 2019
Received in revised form
8 October 2019
Accepted 13 October 2019
Available online xxx
Keywords:
Dye-sensitized solar cells
Purple dyes
3-Nitropyrazolo[1,5-a]pyridine
DFT
Cyclic voltammetry
AIM
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
inflammatory [13], anti-tumor [14], anti-fungal [15], anti-
tubercular [16] and anti-viral activities [17]. Also, they possess
Dye sensitized solar cells (DSSCs) are considered as the most
efficient third-generation solar technology available. They are
extensively used in various applications such as rooftop solar col-
lectors. The power production efficiency is around 11%, as
compared to thin-film technology cells which are between 5% and
13%, and traditional commercial silicon panels which operate be-
tween 12% and 15%.
The sensitized dyes are one of the main components of DSSCs,
which correspond to the photo-driven electron pump of the device.
Many organic and inorganic compounds have been examined for
semiconductor sensitization, such as fluorescent dyes [1], phtha-
locyanines [2], porphyrins [3], carboxylated derivatives of anthra-
cene [4], chlorophyll derivatives [5], platinum complexes [6,7] and
polymeric films [8].
interesting optical properties as dye and fluorescent compounds
[18e20]. To the best of our knowledge, no significant study has
been done on pyrazolo[1,5-a]pyridine derivatives for DSSCs. Herein,
three new pyrazolo[1,5-a]pyridine dyes were designed and syn-
thesized for dye-sensitized solar cells. The structures of the com-
pounds were confirmed by their physical spectral and analytical
data. The density functional theory (DFT) at the level of B3LYP/6-
311 þ G(d,p) was employed to obtain the optimized geometries and
relevant frontier orbitals of the dyes. Furthermore, electrostatic
potential maps and electron density maps of the dyes were deter-
mined by atoms in molecules (AIM) analysis. Cyclic voltammetry
measurement was also used to evaluate the electrochemical
properties of the dyes and a reversible oxidation wave was
observed for them. The photovoltaic performance parameters of
the dyes were elucidated by IPCE spectra and the IeV curves.
On the other hand, pyrazolopyrimidines are bicyclic ring sys-
tems which have recently received considerable attention due to
their constructional structure in anticancer agents [9e11]. Among
those pyrazolopyrimidine derivatives, pyrazolo[1,5-a]pyridines
show potent biological effects such as anti-microbial [12], anti-
2. Experimental
2.1. Materials and equipment
1-Aminopyridinium iodide, ethyl propiolate and aryl acetoni-
* Corresponding author.
E-mail address: mehdipordel58@mshdiau.ac.ir (M. Pordel).
triles were purchased from SigmaeAldrich. Compounds 1 (pyrazolo
https://doi.org/10.1016/j.molstruc.2019.127228
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Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
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[1,5-a]pyridine) and 2 (3-nitropyrazolo[1,5-a]pyridine) were syn-
thesized according to the procedures reported in the literatures
[21,22]. Other reagents were purchased from Merck. All solvents
were dried according to standard procedures.
2.2.3. (2E,3E)-2-(Cyano(4-methoxyphenyl)methylene)-3-
(hydroxyimino)-2,3-dihydropyrazolo [1,5-a]pyridin-8-ium-1-ide
(4c)
Compound 4c was obtained as purple powder, yield (84%), mp
Melting points were obtained on an Electrothermaltype-9100
melting-point apparatus. The FT-IR (as KBr discs) spectra were
measured on a Tensor 27 spectrometer and only noteworthy ab-
sorptions are listed. The 13C NMR (75 MHz), the 1H NMR (300 MHz)
and NOESY spectra were recorded on a Bruker Avance DRX-300
spectrometer. Chemical shifts are reported in ppm downfield
from TMS as internal standard; coupling constant J is given in Hz.
The mass spectra were obtained on a Varian Mat, CH-7 at 70 eV.
Elemental analysis was performed on a Thermo Finnigan Flash EA
microanalyzer. Absorption and fluorescence spectra were recorded
on Varian 50-bio UVeVisible spectrophotometer and Varian Cary
Eclipse spectrofluorophotometer. UVevis scans were obtained
from 200 to 800 nm. Cyclic voltammograms were recorded on a
797 V A Computrace Metrohm. Tetrabutylammonium perchlorate
(TBAP) was utilized used as the supporting electrolyte and a plat-
inum wire served as a counter electrode and Ag/AgCl (KCl 3 M) was
used as a reference electrode. The concentrations used to obtain
each voltammogram are given in the corresponding figure caption.
Voltage-Current analyses were done using an EIS-26H model of
Potentiostat-Galvanostat at 5 V.
199e201 ꢀC; 1H NMR (DMSO‑d6)
d 3.83 (s, 3H, OCH3), 6.65 (t,
J ¼ 7.5 Hz, 1H, Ar H), 7.15 (d, J ¼ 8.7 Hz, 1H, Ar H), 7.31 (d, J ¼ 8.4 Hz,
2H, Ar H), 7.65 (t, J ¼ 7.5 Hz, 1H, Ar H), 8.19 (d, J ¼ 8.4 Hz, 2H, Ar H),
8.99 (d, J ¼ 6.9 Hz, 1H, Ar H), 13.49 (br s, 1H, OH); 13C NMR
(DMSO‑d6)
d 57.3, 117.3, 117.9, 125.1, 126.6, 127.7, 129.4, 131.5, 132.8,
134.7, 134.9, 136.2, 145.7, 157.9; IR (KBr): 3452 cmꢁ1 (OH),
2193 cmꢁ1 (CN). MS (m/z) 292 (Mþ). Anal. Calcd for C16H12N4O2
(292.3): C, 65.75; H, 4.14; N, 19.17. Found: C, 66.11; H, 4.16; N, 18.98.
2.3. Computational details
All of the DFT method calculations have been performed by
using the B3LYP functional [23] and 6-311 þ G(d,p) basis sets. The
Gaussian 03 program [24] was used. Geometries of the investigated
species were fully optimized, which were used for the comple-
mentary calculations. The optimized geometries don’t show any
imaginary frequency. In the frequency calculations, the DFT-
calculated vibrational frequencies are higher than the experi-
mental ones. Since, the calculated vibrational frequencies were
improved by applying the scale factor of 0.9614 [25]. The gauge-
including atomic orbital (GIAO) method [26] was used to predict
the 1H NMR chemical shifts of the investigated compounds with
respect to the tetramethylsilane (TMS) in DMSO‑d6 as the solvent.
Also, the NBO analysis was performed to identify highest-occupied-
molecular orbital (HOMO) and the lowest-unoccupied-molecular
orbital (LUMO) frontier orbitals. The Chemcraft 1.7 program was
employed for visualization of structures [27].
2.2. General procedure for the synthesis of 4aec
Compounds 2 (1.63 g, 10 mmol) and arylacetonitriles 2aec
(12 mmol) were added with stirring to a solution of KOH (20 g,
357 mmol) in MeOH (30 mL). The mixture was refluxed with stir-
ring for 3 h, and then poured into water. The precipitate was
collected by filtration, after neutralization with dilute HCl solution.
Then, it was washed with water and then air dried to give crude
4aec. More purification was obtained by crystallization from
acetone.
In addition to the DFT calculations, the QTAIM was employed to
characterize important bonds and rings. The QTAIM calculations
have been performed using the AIMALL package [28]. The QTAIM
calculations are based on the topological analysis of the electron
density, r(r) [29]. On the other hands, the kinetic energy density (Gb
), the potential energy density (Vb), the total energy density (Hb),
the electron density (
point (BCP) are related to the electron density. These quantities are
used to realize nature of the bonds.
r
) and its Laplacian ðV2rÞ at a bond critical
2.2.1. (2E,3E)-2-(Cyano(phenyl)methylene)-3-(hydroxyimino)-2,3-
dihydropyrazolo [1,5-a]pyridin-8-ium-1-ide (4a)
Compound 4a was obtained as purple powder, yield (79%), mp
210e212 ꢀC; 1H NMR (DMSO‑d6)
d
6.56 (t, J ¼ 7.5 Hz, 1H, Ar H), 7.09
2.4. Photoelectrochemical measurements
(d, J ¼ 8.7 Hz, 1H, Ar H), 7.25 (t, J ¼ 7.5 Hz, 1H, Ar H), 7.43 (t,
J ¼ 8.3 Hz, 2H, Ar H), 7.58e7.64 (m, 1H, Ar H), 8.16 (d, J ¼ 8.3 Hz, 2H,
Ar H), 9.01 (d, J ¼ 6.9 Hz, 1H, Ar H), 13.44 (br s, 1H, OH); 13C NMR
To investigate the properties of the solar cells which made by
our dyes (DSSCs), we used titanium dioxide (TiO2) nanoparticles in
rutile phase (p-25), (Iꢁ/I3ꢁ) standard electrolyte, acetic acid (99%
sigma Aldrich), ethanol (99% Merck) and FTO glasses.
For DSSCs fabrication, FTO glasses cleaned 3 times with deion-
ized water, acetone and ethanol in ultrasonic bath, and their
conductive side was determined by Ohm meter. Then TiO2 paste
was deposited on one of them by Doctor Blade’s method [30]. TiO2
paste was prepared using TiO2 nano powder (2 g) which were
grinded in mortar by adding three drops acetic acid.
During grinding process two another drops of acetic acid were
added in order to making colloidal paste with fluid and uniform
consistency. In the next step, 0.2 g polyethylene glycol (2000) and
deionized water were added and mixed with the paste completely
to make that sticky. This process causes paste to stick better to the
slide. After a while, when the paste was dried to some extent, the
slides were heated in oven in 450 ꢀC for 30 min. The dried slides
were placed in dyes for 20 h.
(DMSO‑d6)
d 118.3, 119.7, 125.2, 126.4, 128.6, 128.9, 130.3, 130.8,
133.6, 134.8, 136.3, 145.7, 156.4; IR (KBr): 3455 cmꢁ1 (OH),
2190 cmꢁ1 (CN). MS (m/z) 262 (Mþ). Anal. Calcd for C15H10N4O
(262.3): C, 68.69; H, 3.84; N, 21.36. Found: C, 69.02; H, 3.86; N,
21.12.
2.2.2. (2E,3E)-2-((4-Chlorophenyl)(cyano)methylene)-3-
(hydroxyimino)-2,3-dihydropyrazolo [1,5-a]pyridin-8-ium-1-ide
(4b)
Compound 4b was obtained as purple powder, yield (83%), mp
228e230 ꢀC; 1H NMR (DMSO‑d6)
d
6.63 (t, J ¼ 7.5 Hz, 1H, Ar H), 7.12
(d, J ¼ 8.7 Hz, 1H, Ar H), 7.46 (d, J ¼ 8.6 Hz, 2H, Ar H), 7.67 (t,
J ¼ 7.5 Hz, 1H, Ar H), 8.20 (d, J ¼ 8.6 Hz, 2H, Ar H), 9.05 (d, J ¼ 6.9 Hz,
1H, Ar H), 13.53 (br s, 1H, OH); 13C NMR (DMSO‑d6)
d 117.8, 118.5,
125.3, 126.0, 128.2, 129.3, 130.9, 131.3, 133.4, 134.5, 137.8, 145.0,
159.8; IR (KBr): 3453 cmꢁ1 (OH), 2199 cmꢁ1 (CN). MS (m/z) 296
(Mþ), 298 (Mþ þ 2). Anal. Calcd for C15H9ClN4O (296.7): C, 60.72; H,
3.06; N, 18.88. Found: C, 61.08; H, 3.09; N, 18.59.
Counter electrodes were fabricated depositing Pt paste on FTO.
These deposited FTO were connected to those which TiO2 paste was
deposited on and completely were sealed. Then electrolyte was
Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2019.127228

Z. Agheli et al. / Journal of Molecular Structure xxx (xxxx) xxx
3
injected to empty space between two electrodes on effective sur-
face (0.25 cm2).
absorption band at 2199 cmꢁ1 and a broad absorption band at
3453 cmꢁ1 assignable to CN and OH groups are observed in the FT-
IR spectrum of the compound 4b, respectively. Analytical data and
the mass spectrum are also further confirmed the structure of dye
4b. Furthermore, as can be seen in Scheme 2, no cross-peak be-
A
solar simulator system under 1.5 A M and intensity of
100 mW/cm2 (SIM 1000, Sharif solar) was applied for investigating
of photovoltaic characteristics including short circuit current, open
circuit voltage, fill factor, and cells efficiency.
tween the OH proton (d 13.53, br s) and the aromatic protons is
observed in the data from NOESY experiment of dye 4b which
confirms the 2E, 3E configuration of the new dyes (Scheme 3).
The DFT method calculations by using the B3LYP functional and
6-311 þ G(d,p) basis sets are used to deduce the configuration of
new dyes 4aec. Each of the dyes 4aec has four different isomers.
For example, the DFT-optimized geometries for four isomers of dye
4b are shown in Fig. 1 with labeling of their atoms. These four
isomers are named as the 4b1, 4b2, 4b3 and 4b4 species. The
calculated relative energies of four isomers are listed in Table 1. As
seen, the 4b1 (2E, 3E) isomer is the most stable isomer in MeOH
solution of dye 4b. Similar to dye 4b, the 4a1 and 4c1 isomers are
the most stable isomers in methanol solution of dyes 4a and 4c,
respectively. Optimized geometries of the 4a1 and 4c1 isomers of
dyes 4a and 4c are shown in Fig. 2. As seen in Figs. 1 and 2, all the
4a1, 4b1 and 4c1 species involve the O1H5/N4 intramolecular-H
bond, which increases stability of these isomers rather than the
other isomers of dyes 4aec.
3. Results and discussion
The precursor of 3-nitropyrazolo[1,5-a]pyridine (2) was syn-
thesized according to reported procedures [21,22]. Reaction of 1-
aminopyridinium iodide with ethyl propiolate gave ethyl pyrazolo
[1,5-a]pyridine-3-carboxylate (Scheme 1). Pyrazolo[1,5-a]pyridine
(1) was obtained when ethyl pyrazolo[1,5-a]pyridine-3-carboxylate
was hydrolyzed in aqueous KOH and MeOH and then decarboxyl-
ation by heating in HI [21]. Nitration of compound 1 in HNO3/H2SO4
led to the formation of 3-nitropyrazolo[1,5-a]pyridine (2) in good
yield [22]. Finally, the new heterocyclic purple dyes 4aec were
synthesized from the reaction of compound 2 with aryl acetoni-
triles 3aec in excellent yields (Scheme 1).
A tentative mechanism to clarify the formation of dyes 4aec is
depicted in Scheme 2. As shown, the new dyes 4aec are obtained
by attack of the deprotonated form of compounds 3aec on 3-
nitropyrazolo[1,5-a]pyridine (2) and then subsequent protonation.
Physical spectral (1H, 13C NMR, FT-IR spectra) and analytical data
confirm the structure of new dyes 4aec. For example, there is an
Selected structural parameters of the dyes 4aec are gathered in
Table 2. As seen, their structures are roughly planar, but pyrazolo
[1,5-a]pyridine and benzene rings make a dihedral of about 30ꢀ to
each other. The C8eC9eN4 moiety makes a dihedral angle about 20
and 10ꢀ with pyrazolo[1,5-a]pyridine and benzene rings,
respectively.
exchangeable peak at d 13.53 ppm attributed to OH group proton in
the 1H NMR spectrum of compound 4b. Moreover, in the 13C NMR
spectrum of dye 4b, there are 13 different carbon atoms. A weak
Scheme 1. Synthesis of new purple dyes 4aec.
Scheme 2. A proposed mechanism to explain the formation of dyes 4aec.
Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2019.127228

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Scheme 3. NOESY spectrum of dye 4b.
3.1. NMR spectra
Herein, the NMR spectra of the 4aec compounds have been
identified by using the DFT calculations. The experimental and DFT-
computed 1H NMR chemical shifts (
d) of the compounds are listed
in Table 3. The atom positions of the 4a1, 4b1 and 4c1 species are
numbered as in Figs. 1 and 2.
As seen in Table 3, there is good agreement between the
experimental and DFT-computed chemical shifts, approving val-
idity of 4a1, 4b1 and 4c1 geometry for the optimized geometries of
the compounds 4aec respectively. Especially, a signal at 12.74,
12.90 and 12.63 ppm for 4a, 4b and 4c compounds, respectively, is
related to the NOH moiety, confirm suitability of the 4a1, 4b1 and
4c1 optimized geometries.
3.2. Vibrational spectroscopy
Comparing the experimental and DFT-calculated vibrational
frequencies for the species 4a1, 4b1 and 4c1, important IR vibra-
tional bands of the investigated compounds have been assigned.
The obtained results are gathered in Table 4.
In the 3600-2500 cmꢁ1 spectral region of the IR spectra, over-
lapping of the OeH, NeH, SeH and CeH stretching vibrations
causes to broad bands [31e34]. Observed bands of this region have
been assigned theoretically. The obtained results are given in
Table 4. As seen, the most intensive band is related to the stretching
vibrations of the O1eH5 bond.
3.3. NBO analysis
Many characters of the chemical compounds such as the frontier
orbitals, energy gap, stability, electronic charge transfers, the bond
order, the intra- and intermolecular bonding interactions can be
explored by employing the NBO analysis [32e37]. For example, the
3D-distribution maps of the HOMO and the LUMO frontier orbitals
of the species 4b1 are shown in Fig. 3. As seen, both of the HOMO
orbital is mainly localized on the benzene ring; but the LUMO
orbital is mainly localized on the pyrazolo[1,5-a]pyridine ring.
Fig. 1. Optimized geometries of the four possible isomers (4b1-4b4) of dye 4b together
with their labeling.
The energy difference between the HOMO and LUMO frontier
orbitals is named as the energy gap, which has an essential role in
many characters of the chemical compounds such as the electron
Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
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Table 1
The relative energies (kJ.molꢁ1) of the investigated species 4aec.
Species
relative energies
Species
relative energies
Species
relative energies
4a1
4a2
4a3
4a4
0.0
4b1
4b2
4b3
4b4
0.0
4c1
4c2
4c3
4c4
0.0
23.63
7.88
30.15
23.10
10.97
30.36
22.04
6.73
26.92
example, the molecular graph of the species 4b1 is shown in Fig. 4,
where small green and red spheres correspond to the bond critical
points (BCPs) and the ring critical points (RCPs), respectively.
Considering the H5eN4 and H6eN2 H-bonds, there are five
rings in structure of the investigated compounds (Fig. 4). The r(r)
values of these rings have been computed and reported in Table 6.
As seen, the pyrazole ring involves the highest electron density in
structure of all investigated compounds.
3.4. Photophysical properties of compounds 4aec
The absorption spectra of compounds 4aec were investigated
by UVeVis spectroscopy in 10ꢁ5 M MeOH solution, in the wave-
length range of 200e800 nm (Fig. 5). Table 7 shows numerical
spectral data of dyes 4a-c. Values of extinction coefficient (ε) were
obtained as the slope of the plot of absorbance vs concentration.
Absorption maximums of dyes 4a-c can be observed at 325 and
530 nm in the UVeVis spectra of the dyes. Intramolecular charge
transfer (ICT) states [40] from OH group and endocyclic N2 (as
donor sites) to CN (C^N and eC]Nþ) group (as acceptor moiety)
can be proposed for the appearance of the signals in the visible
region of the investigated dyes 4aec (see Fig. 6).
As depicted in Fig. 5, a relatively sharp peak at 325 nm can be
considered as
p- p* transitions (donor endocyclic nitrogen to the
acceptor CN (C^N and eC]Nþ) group) and a broad band at 530 nm
can be attributed to n-
the acceptor CN group.
p* transitions from the donor OH group to
The solvatochromic properties of dyes 4a-c were studied and
summarized in Table 7. The obtained results revealed that the
visible absorption spectra of the dyes undergo a relatively modest
red shift in polar solvents. Increasing the solvent polarity stabilizes
the ICT excited-state molecule relative to the ground-state mole-
cule and leads to the observed red shift of the absorption maximum
as the experimentally observed result (Table 7). For example, in the
absorption spectra of dye 4a, labs shifts from 530 to 515 nm as the
solvent changes from methanol to n-hexane (Table 7).
Fig. 2. Optimized geometries of the 4a1 and 4c1 isomers of dyes 4a and 4c.
transitions and the photochemical reactions. The calculated energy
gaps for the compounds 4aec are 2.38, 2.35 and 2.25 respectively.
3.5. Electrochemical properties
3.3.1. AIM analysis
Based on the AIM analysis, the nature of important intra-
molecular bonds as well as the electronic density (r(r)) of the ar-
The electrochemical properties of the dyes 4aec were examined
to gain more quantitative insight into the redox properties of the
dyes. Cyclic voltammetry was employed in the presence of tetra-
butylammonium perchlorate (TBAP) as the supporting electrolyte,
using Pt working and counter electrodes and Ag/AgCl as reference
electrode in DMF as the solvent. The potentials were referenced to
ferrocene/ferrocenium (Fc/Fcþ) as the internal reference.
omatic rings have been evaluated for all of the 4a1-c1 species. The
calculated values of
r
ðrÞ; V2rðrÞ; Hb; Gb; Vb and ꢁGb
at BCP are
=
Vb
gathered in Table 5. Optimized geometry for each of the 4a1, 4b1
and 4c1 species involves two intramolecular H-bonds, O1eH5/N4
and C11eH6/N2. As seen in Table 5, for both of these H-bonds
For example, as seen in Fig. 7, cyclic voltammogram of dye 4c
(ꢁ1 to þ1 V) revealed an oxidation wave with an anodic peak po-
tential of ꢁ0.112 V and a cathodic peak potential of ꢁ0.210 V versus
SCE (ipa/ipc z1). The observed peak separation for the reversible
waves was larger than the value of 59/n mV expected for a
reversible system, suggesting that the redox couple has a quasi-
reversible behavior [41]. Scan rate studies of dye 4c showed that
the anodic and cathodic peak currents (ipa, ipc) of the oxidation
V
2rðrÞ and Hb are positive, confirming weakness of these H-bonds.
On the other hands, the ꢁGb
value is greater than 1, which is
=
Vb
correspond to the noncovalent property of these bonds [38]. Also,
1
the hydrogen bond energies can be calculated by EHB
¼
=
2Vb
Ref. [39]. For the 4a1 species, the calculated EHB values for the
O1eH5eN4 and C11eH6eN2 H-bonds are ꢁ23.76 and ꢁ47.43H,
respectively. These values are ꢁ23.76 and ꢁ51.82H for the 4b1
species, ꢁ24.06 and ꢁ48.93H for the 4c1 species, respectively. For
wave were proportional to the square root of the scan rate (n1/2
)
Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2019.127228

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Z. Agheli et al. / Journal of Molecular Structure xxx (xxxx) xxx
Table 2
Selected structural parameters of the 2E, 3E isomer of the investigated compounds.
Species
Species
4a1
4a1
4b1
4c1
4b1
4c1
Bond length (pm)
C1eC2
C2eN1
N1eN2
N2eC7
C7eC6
C6eN3
N3eO1
O1eH5
Angle (ꢀ)
C5eC1eC2
C4eC3eN1
C3eN1eN2
N1eN2eC7
N2eC7eC6
C7eC6eN3
C6eN3eO1
N3eO1eH5
O1eH5eN4
N2eC7eC8
C7eC8eC9
C8eC9eN4
C7eC8eC10
C8eC10eC11
C10eC11eC13
C13eC15eCl
C13eC15eO2
C15eO2eC16
Dihedral angle (ꢀ)
C1eC5eC4eC3
C4eC3e N1eN2
N1eN2eC7eC6
N2eC7eC6eN3
C7eC6eN3eO1
C6eN3eO1eH5
N2eC7eC8eC9
C7eC8eC9eN4
C10eC8eC9eN4
C9eC8eC10eC11
N2eC7eC8eC10
C7eC8eC10eC11
C8eC10eC11eC13
C10eC11eC13eC15
C11eC13eC15eCl
C11eC13eC15eO2
C13eC15eO2eC16
138.6
136.7
135.4
135.9
150.4
130.0
134.2
98.2
196.3
140.0
141.0
116.4
147.6
140.8
108.4
e
138.6
136.7
135.6
135.8
150.4
130.0
134.1
98.2
196.2
140.1
141.0
116.4
147.4
140.9
e
138.6
136.8
135.2
136.2
150.2
130.0
134.5
98.1
196.9
140.0
141.1
116.4
147.2
140.5
e
118.8
118.9
123.1
106.9
109.0
140.6
123.6
113.6
163.9
121.8
116.9
178.3
126.3
122.7
120.7
e
118.8
118.9
123.1
106.9
109.0
140.8
123.7
113.7
163.7
121.8
116.8
178.4
126.5
123.0
121.2
119.7
e
118.9
118.9
123.2
106.8
109.0
140.7
123.4
113.5
164.2
121.6
116.8
178.3
126.6
123.2
121.6
e
H5/N4
C7eC8
C8eC9
C9eN4
C8eC10
C10eC11
C15eH10
C15eCl
C15eO2
O2eC16
175.8
e
e
e
136.2
142.2
e
124.8
118.7
e
e
e
e
0.1
0.1
0.1
ꢁ179.2
ꢁ179.2
ꢁ179.3
4.8
4.6
4.9
160.8
7.6
161.1
7.5
18.2
161.0
7.2
19.1
18.7
162.1
ꢁ36.4
137.8
166.4
ꢁ11.5
ꢁ20.0
ꢁ177.9
ꢁ0.3
e
162.3
ꢁ42.1
132.3
170.4
ꢁ11.4
ꢁ15.8
ꢁ178.2
ꢁ0.3
ꢁ179.6
e
162.3
ꢁ48.3
126.2
169.3
ꢁ11.4
ꢁ17.0
ꢁ178.1
ꢁ0.2
e
e
ꢁ179.8
ꢁ0.8
e
e
Table 3
Experimental and DFT-calculated 1H NMR chemical shifts of the investigated com-
pounds in DMSO solution, )ppm(.
3.6. Photovoltaic properties
d
Experimental details of the photovoltaic measurements can be
found in experimental section. The IPCE spectra and the IeV curves
are depicted in Fig. 9. The photovoltaic performance parameters (a
short circuit photocurrent density (Jsc), an open-circuit photo-
voltage (Voc), a fill factor (ff) and a solar energy-to-electricity con-
version yield (h)) are collected in Table 9. The peak IPCE was found
to be 64% at 505 nm for 4a and 67% at 500 nm for 4b, in contrast to
72% at 500 nm for 4c based devices. The IeV curves show that the Jsc
Species Experimental
4a
Atom position
Theoretical
4a1
4b
4c
4b1
4c1
H1
H2
H3
H4
H5
H6
H7
H8
7.09
7.12
9.05
6.63
7.67
13.53
7.46
7.46
8.20
8.20
e
7.15
8.99
6.65
7.65
13.49
7.31
7.31
8.19
8.19
3.83
3.83
3.83
7.07
9.15
7.18
7.18
12.74
8.12
8.07
7.44
7.44
7.29
e
7.07
9.17
7.23
7.23
12.90
7.92
7.70
8.36
8.49
e
7.02
9.11
7.17
7.12
12.63
7.56
7.47
7.97
8.17
3.62
3.62
4.01
9.01
6.56
7.25
13.44
8.16
8.16
7.43
7.43
7.58e7.64
e
and
h
values are 14.92 mA cmꢁ2 and 6.95%, 16.62 mA cmꢁ2 and
7.06%, 17.60 mA cmꢁ2 and 7.18% for 4aec, respectively. The high
photovoltaic performances of DSSCs based on dyes 4aec are
attributed to the high electron-injection efficiency due to the high
LUMO level and the faster charge recombination due to the BODIPY
core, leading to the low Jsc and Voc values, respectively.
H9
H10
H11
H12
e
e
e
e
e
e
4. Conclusions
indicating diffusion control (Fig. 8). Cyclic voltammetric data of
dyes 4aec were summarized in Table 8. The relatively small dif-
New heterocyclic purple dyes were obtained from the reaction
of 3-nitropyrazolo[1,5-a]pyridine with arylacetonitriles under basic
condition. The structure of the dyes was confirmed by physical
spectral and analytical data. The data obtained from NOESY
experiment revealed that the configuration of the dyes is 2E,3E. The
DFT method calculations by using the B3LYP functional and 6-
311 þ G(d,p) basis sets were used to approve the configuration of
the dyes. There was a good agreement between the experimental
ference of
DEP (mV) in dyes 4aec (Table 8) show that the main
reason for observation of the waves in cyclic voltammogram of the
dyes is the presence of donor sites (OH group and endocyclic N2)
and acceptor moiety (CN group) and the substituent in dyes 4aec
do not have much effect in this case.
Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2019.127228

Z. Agheli et al. / Journal of Molecular Structure xxx (xxxx) xxx
7
Table 4
Selected experimental and the DFT-computed IR vibrational frequencies (cmꢁ1) of the investigated compounds. (Intensity is in term of km.molꢁ1).
4a1
4b1
4c1
Vibrational assignment
Exp.
Calculated
Freq.
Exp.
Calculated
Freq.
Exp.
Calculated
Freq.
Intensity
Intensity
Intensity
53
e
e
e
e
e
1440
1446
dsci(methyl group)
e
1470
1535
1553
1607
2174
1465
1519
1547
1600
2165
e
24
168
22
34
217
e
1455
1515
1573
1595
2174
e
1462
1517
1563
1600
2167
e
92
169
43
30
210
e
1475
1510
1590
1596
2174
2895
1479
1516
1581
1600
2167
2890
68
y
y
y
y
y
(CeC) of the benzene ring
(C6eN3)
(CeC) of the benzene ring
(CeC) of the pyridine ring
(C9eN4)
146
274
30
229
74
ysym(CeH) of the methyl group
e
e
e
e
e
e
2935
2947
36
yasym(CeH) of the methyl group
e
3060e3080
3095e3125
3410
3041e3060
3068e3116
3337
15
24
47
3073e3089
3100e3129
3430
3061e3075
3087e3116
3337
3
13
47
3065e3079
3095e3124
3455
3057e3076
3085e3116
3352
13
7
42
yasym(CeH) of the aromatic rings
ysym(CeH) of the aromatic rings
y(O1eH5)
Table 5
Important topological parameters of the 4a1, 4b1 and 4c1 species in a.u.
and DFT-computed chemical shifts and IR vibrational frequencies.
Moreover, AIM analysis established that the pyrazole ring involves
the highest electron density in structure of the compounds. Pho-
tophysical properties of the dyes were characterized by UVeVis
spectroscopy and ICT states from donor sites to acceptor moiety
was proposed for the appearance of the signals in the visible region
Bond
r
(r)
Vb
Gb
H
ꢁGb/Vb
V
2r
4a1
O1eH5 0.345802 ꢁ2.40724 ꢁ0.7379
0.068043 ꢁ0.66985 0.092212
H5/N4 0.030848 0.090805 ꢁ0.02104 0.021871 0.000831 1.039496
C11eH6 0.288518 ꢁ1.0143
ꢁ0.3269
0.036663 ꢁ0.29024 0.112154
N2/H6 0.016949 0.063666 ꢁ0.01054 0.013229 0.002688 1.255004
4b1
O1eH5 0.345867 ꢁ2.40849 ꢁ0.73808 0.067977 ꢁ0.6701
0.092100
H5/N4 0.030854 0.090922 ꢁ0.02104 0.021887 0.000844 1.040108
C11eH6 0.287679 ꢁ1.00738 ꢁ0.32651 0.037332 ꢁ0.28918 0.114337
N2/H6 0.015708 0.058774 ꢁ0.00965 0.012171 0.002523 1.261505
4c1
O1eH5 0.34697
ꢁ2.41546 ꢁ0.74067 0.0684
ꢁ0.67227 0.092349
H5/N4 0.030544 0.090325 ꢁ0.02078 0.02168
0.000901 1.043361
C11eH6 0.288291 ꢁ1.01239 ꢁ0.32684 0.036872 ꢁ0.28997 0.112813
N2/H6 0.016515 0.061892 ꢁ0.01022 0.012846 0.002627 1.257070
Fig. 4. The QTAIM molecular graphs of the species 4b1.
Fig. 3. The HOMO and LUMO frontier orbitals of the species 4b.
of the dyes. Cyclic voltammetry measurement was performed to
evaluate the electrochemical properties of the dyes and a reversible
oxidation wave was observed for them. The IPCE spectra and the
IeV curves of the dyes were obtained and the photovoltaic per-
formance of 6.95e7.18% was observed. To further increase of
Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2019.127228

8
Z. Agheli et al. / Journal of Molecular Structure xxx (xxxx) xxx
Table 6
The calculated (
r
(r) for aromatic rings in structure of the species 4a1, 4b1 and 4c1 (in term of C.Bobhrꢁ3).
Member of the ring
Number of atoms
r(r)
4a1
4b1
4c1
C1eC2eN1eC3eC4eC5
C2eN1eN2eC7eC6
C6eC7eC8eC9eN4eH5eO1eN3
N2eC7eC8eC10eC11eH6
C10eC11eC13eC15eC14eC12
6
5
8
6
6
þ0.022404
þ0.047429
þ0.008401
þ0.010230
þ0.021538
þ0.022423
þ0.047400
þ0.008375
þ0.010345
þ0.021533
þ0.022369
þ0.047433
þ0.008376
þ0.010286
þ0.021299
Fig. 5. Visible absorption (10ꢁ5 mol Lꢁ1) of dyes 4aec in MeOH.
Fig. 7. Cyclic voltammogram of dye 4c in DMF (10ꢁ4 M) at scan rate 100 mV/s.
Fig. 6. Visible absorption spectra of compound 4a in some solvents.
Table 7
Spectroscopic data of dyes 4aec in different solvents at 298 K.
Dye
Solvent
4a
4a
4b
labs (nm)
4b
ε ꢂ 10
4c
4c
b
ꢁ4
[L molꢁ1 cmꢁ1
labs (nm)a
ε ꢂ 10 ꢁ3 [(mol Lꢁ1
)
ꢁ1 cmꢁ1
]
]
labs (nm)
ε ꢂ 10 ꢁ4 [L molꢁ1 cmꢁ1
]
MeOH
Acetone
EtOAc
325, 530
320, 525, 560
315, 525
28.0
26.0
25.5
20.0
320, 530
315, 525
315, 520
305, 515
31.0
29.5
27.0
21.1
320, 530
315, 525
315, 520
305, 515
30.3
30.5
27.5
20.7
n-hexane
305, 515
a
Wavelengths of maximum absorbance (labs).
Extinction coefficient.
b
Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2019.127228

Z. Agheli et al. / Journal of Molecular Structure xxx (xxxx) xxx
9
Table 9
Photovoltaic parameters of dyes 4aec.
Dye
JSC (mA.cmꢁ2
)
VOC (V)
FF (%)
h (%)
4a
4b
4c
14.92
16.62
17.60
0.73
0.76
0.79
64
71
73
6.95
7.06
7.18
photovoltaic performance in the dyes, a great attempt to func-
tionalization of the dyes is in progress and will soon be published
elsewhere.
Acknowledgment
1/2
We would like to express our sincere gratitude to Research Of-
fice, Mashhad Branch, Islamic Azad University, Mashhad-Iran, for
financial support of this work.
Fig. 8. Plots of anodic and cathodic peak currents vs
y
.
Table 8
Cyclic voltammetric data of dyes 4aec in scan rates 100 mV/s.
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Please cite this article as: Z. Agheli et al., Synthesis, characterization, optical properties, computational characterizations, QTAIM analysis and
cyclic voltammetry of new organic dyes for dye-sensitized solar cells, Journal of Molecular Structure, https://doi.org/10.1016/
j.molstruc.2019.127228