Quantum mechanical calculations of different monomeric structures with the same electroactive group to clarify the relationship between structure and ultimate optical and electrochemical properties of their conjugated polymers

Article abstract of DOI:10.1016/j.jpcs.2020.109720

Quantum mechanical calculations can clarify the relationship between structure and ultimate optical and electrochemical properties of conjugated polymers and produce a ground for the design of advanced materials for futuristic applications. Herein, we have examined the structural geometries and electronic properties of different monomeric structures with the same electroactive group to provide a relationship between calculated electronic properties of the monomers with the observed optical and electrical properties of their conjugated polymers. For this purpose, three different amide substituted 2,5-di(2-thienyl)-1H-pyrrole compounds containing a different number of electroactive groups have been synthesized and molecular structure optimizations have been carried out with the Density Functional Theory (DFT) calculations. FT-IR and NMR spectra of optimized geometries have been compared with experimental data. Furthermore, electronic properties of the monomeric structures such as chemical hardness/softness, ionization potential, HOMO-LUMO energy levels, electronegativity have been revealed and molecular electrostatic potential (MEP) surface has been calculated to determine the electrophilic and nucleophilic reactive attack regions of the molecules considered in this study. Finally, Total Density of State (TDOS), Partial Density of State (PDOS) and Mulliken charge analyses of these molecules have been carried out. Our calculations based on ab-initio methods for the monomers have been compared with the optical and electrical properties of their conductive polymers in order to understand interrelationship between monomer structure and polymer properties.

Full text of DOI:10.1016/j.jpcs.2020.109720

Journal of Physics and Chemistry of Solids 149 (2021) 109720
Contents lists available at ScienceDirect
Journal of Physics and Chemistry of Solids
journal homepage: http://www.elsevier.com/locate/jpcs
Quantum mechanical calculations of different monomeric structures with
the same electroactive group to clarify the relationship between structure
and ultimate optical and electrochemical properties of their
conjugated polymers
a,
**
b
a
c
d,*
Pinar Tunay Tasli
, Tugba Soganci , Sevgi Ozdemir Kart , Hasan Huseyin Kart , Metin Ak
a
Pamukkale University, Physics Department, Campus, Denizli, Turkey
b
c
Eskisehir Technical University, Graduate School of Sciences, Department of Advanced Technologies, Eskisehir, Turkey
Aydin Adnan Menderes University, Physics Department, Aydin, Turkey
d
Pamukkale University, Chemistry Department, Campus, Denizli, Turkey
A R T I C L E I N F O
A B S T R A C T
Keywords:
Quantum mechanical calculations can clarify the relationship between structure and ultimate optical and elec-
trochemical properties of conjugated polymers and produce a ground for the design of advanced materials for
futuristic applications. Herein, we have examined the structural geometries and electronic properties of different
monomeric structures with the same electroactive group to provide a relationship between calculated electronic
properties of the monomers with the observed optical and electrical properties of their conjugated polymers. For
this purpose, three different amide substituted 2,5-di(2-thienyl)-1H-pyrrole compounds containing a different
number of electroactive groups have been synthesized and molecular structure optimizations have been carried
out with the Density Functional Theory (DFT) calculations. FT-IR and NMR spectra of optimized geometries have
been compared with experimental data. Furthermore, electronic properties of the monomeric structures such as
chemical hardness/softness, ionization potential, HOMO-LUMO energy levels, electronegativity have been
revealed and molecular electrostatic potential (MEP) surface has been calculated to determine the electrophilic
and nucleophilic reactive attack regions of the molecules considered in this study. Finally, Total Density of State
Conducting polymers
Spectroelectrochemistry
Density functional theory calculations
(
TDOS), Partial Density of State (PDOS) and Mulliken charge analyses of these molecules have been carried out.
Our calculations based on ab-initio methods for the monomers have been compared with the optical and elec-
trical properties of their conductive polymers in order to understand interrelationship between monomer
structure and polymer properties.
1
. Introduction
areas is understanding the tuning parameters of the electrochemical and
optical properties of CPs. It is well known that the parameters that most
In recent years, conjugated polymers have been the focus of interest
in fundamental and practical research due to their promising optical and
electrical properties [1–7]. These -conjugated systems which have
alternating single ( ) and double ( ) bonds exhibit inherently optical
affect the optical and electrochemical properties of conjugated polymers
are their degree of crystallinity, conjugation length and intra-/-
inter-chain interactions [30,31]. These parameters can be controlled
effectively with smart monomer design based on structure-property
relationship approaches. In this point of view, quantum mechanical
calculations have crucial role in designing of monomers and their con-
jugated polymers for desired optical and electrochemical properties
[32–36].
π
σ
π
and electrical properties. Conjugated polymers with stable optical, me-
chanical and electrical properties are key components in many practical
applications such as electrochromic devices [8–13], photovoltaic cells
[
14–16], light emitting diodes [17,18], biosensors [19–27], super-
capacitors [28,29]. The most important factor for development in these
Structure and spectroscopic properties of the chemical compounds
*
Corresponding author.
** Corresponding author.
E-mail addresses: ptunay@pau.edu.tr (P.T. Tasli), metinak@pau.edu.tr (M. Ak).
https://doi.org/10.1016/j.jpcs.2020.109720
Received 14 July 2020; Accepted 16 August 2020
Available online 28 September 2020
0
022-3697/© 2020 Elsevier Ltd. All rights reserved.

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
are theoretically illuminated with quantum physics. The way quantum
physics applying to chemical problems is called quantum chemistry.
Computer-assisted quantum chemical calculations are used to predict
the structural, spectroscopic and electronic properties of chemical ma-
terials without making any experimental studies [37]. The most
important purpose of these programs is to determine the quantized en-
ergy levels of molecules, ions or nuclei by using spectroscopy. Then it is
to calculate structural parameters, electrical-electronic properties and
thermal properties using the forces and interactions between atoms,
molecules or nuclei [38].
(FT-IR) and Nuclear Magnetic Resonance (NMR) spectra of optimized
geometries have been compared with experimental data. Furthermore,
electronic properties of the monomeric structures such as chemical
hardness/softness, ionization potential, highest occupied molecular
orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy
levels, electronegativity have been revealed and molecular electrostatic
potential (ESP) surface has been computed to determine the electro-
philic and nucleophilic reactive attack regions of the investigated mol-
ecules. Finally, TDOS, PDOS and Mulliken charge analyses of the
molecules have been carried out. Our ab-initio calculations of the
monomers have been compared with the optical and electrical proper-
ties of their conductive polymers in order to understand interrelation-
ship between monomer structure and polymer properties. Firstly, in this
study, the structures of the molecules have been optimized by using DFT
at the level of B3LYP with the basis set of 6–311G(d-) via Gaussian 09 W
program [45]. Later, the structural and vibrational properties, NMR
chemical shifts, molecular frontier orbital energies, electronic properties
such as TDOS, PDOS analysis, Mulliken charge analysis, molecular ESP
maps of the optimized molecular structure of title molecules are ob-
tained theoretically. As a result, the experimental data of electrochromic
materials have been supported by ab-initio calculations in this study.
This work confirms that DFT computations allow us to better understand
the nature of electrochromic materials.
The future of conjugated polymers based on the ability of the theo-
retical and organic chemist to understand which synthetic modifications
to existing and yet unknown conjugated polymers will result in the
improvement of their optical and electrical properties [39–41]. Quan-
tum chemistry calculations can clarify the relationship between struc-
ture and ultimate optical and electrochemical properties of conjugated
polymers and produce a ground for the design of advanced materials for
futuristic applications.
In this point of view, in this study we have examined the structural
geometries and electronic properties of different monomeric structures
with the same electroactive group to provide a relationship between the
calculated electronic properties of the monomers and the observed op-
tical and electrical properties of their conjugated polymers. For this
purpose, three different amide substituted 2,5-di(2-thienyl)-1H-pyrrole
compounds containing a different number of electroactive groups have
been synthesized and molecular structure optimizations have been
carried out with the Density Functional Theory (DFT) calculations. 2,5-
Di(2-thienyl)-1H-pyrrole derivatives have been chosen because they
have better optical and electrical properties than thiophene and pyrole-
based electroactive monomers [35,42–44]. Fourier Transform Infrared
2. Experimental
2.1. Synthesis and characterization of monomers
First of all, 1,4-di(2-thienyl)-1,4-butanedione has been synthesized
by Friedel-Crafts Acylation reaction of thiophene and succinyl chloride
Scheme 1. Synthesis routes of M1, M2 and M3.
2

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
in presence of aluminum chloride. Details of this reaction can be found
elsewhere [43,46,47]. The monomers have been obtained by the
Knorr-Paal reaction of 1,4-di(2-thienyl)-1,4-butanedione and corre-
sponding hydrazide derivative in toluene containing a catalytic amount
of pTSA as specified in the literature (Scheme 1) [11,12,43]. Detailed
3. Results and discussion
3.1. Geometrical structure
The three-dimensional approximate geometry of the studied mole-
cules in the ground state has been visualized by the GaussView 5.0.8
molecular viewing program and the simulations of the structural,
spectroscopic and electronic properties of molecules are achieved via
Gaussian 09 package program in the gas phase [45,48]. These calcula-
tions have been succeeded using the Density Function
Theory/Becke-3-Lee-Yang-Parr (DFT/B3LYP) method with the 6-311G
(d) basis set [50,51]. The atom numbering schemes and the optimized
structures of the M1, M2 and M3 molecules obtained from
DFT/B3LYP/6-311G(d) methods are given in Fig. 1. The dihedral angles
of the selected regions (yellow part in Fig. 1) in the optimized molecules
are given in Table 1. The band gap of a conducting polymer can be
expressed by the sum of the five contributions denoted as bond length
alternation, planarity, resonance energy, substitution, intermolecular
effects [52]. Considering that all effects except planarity in the three
molecules tackled are approximately similar each other, the most
important feature in these molecules is the planarity of the 2,5-di(2-thie-
nyl)-1H-pyrrole groups. With the aim of determining the planarity of all
molecules, dihedral angles of the atoms in the 2,5-di(2-thienyl)-1H-pyr-
role groups of all three molecules are calculated. The dihedral angles of
optimized structures show that the molecule M3 is more planar as
compared to the molecules of M1 and M2. In order to characterize the
molecular structures of the M1, M2 and M3 molecules, 41, 70 and 99
bond lengths, 65, 112 and 159 bond angles and 96, 167 and 241 dihedral
angles are necessary, respectively. All these bond lengths, bond angles
and dihedral angles are given in Tables S1, S2 and S3, respectively, as a
supplementary data in ESI.
synthesis
procedures
for
monomers
(N-(2,5-di(thio-
1
4
phen-2-yl)-1H-pyrrol-1-yl)benzamide as M1, N ,N -bis(2,5-di(thio-
1
3
5
phen-2-yl)-1H-pyrrol-1-yl)terephthalamide as M2 and N ,N ,N -tris(2,
-di(thiophen-2-yl)-1H-pyrrol-1-yl)benzene-1,3,5-tricarboxamide as
5
M3) are given in Electronic Supporting Information (ESI). Furthermore,
chemicals used in this research and instruments used for characteriza-
tions of the materials are mentioned in ESI.
2
.2. Electrochemical and spectroelectrochemical processes
Electropolymerizations and electrochemical characterizations of
monomer and corresponding polymers have been performed in three-
electrode electrochemical cell at ambient temperature. Indium tin-
ꢀ
1
+
oxide coated glass slides (R = 4–16 Ωsq , 0.7 × 0.5 × 0.07 cm), Ag/Ag
and platinum wire (% 99.99, 0.01 × 5 cm) have been used as working,
reference and counter electrodes in electrochemical cell. The electrolyte
solution used in the electropolymerization process is 0.1 M LiClO
.01 M related monomer containing acetonitrile solution. Monomer-free
electrolyte solution is used in all other electrochemical processes.
4
and
0
2
.3. Structural characterization
The chemical structures of the monomers (M1-M3) have been
1
13
confirmed by NMR spectral analyses. H NMR and C NMR spectra of
monomers are recorded with Bruker-Instruments-NMR Spectrometer
(
(
DPX-400) operating at 400 MHz in the medium of dimethylsulfoxide
DMSO). PerkinElmer 2000 model an attenuated total reflectance
3.2. Vibrational spectrum analysis
Fourier transform infrared spectroscopy (ATR-FTIR) is used to identify
the vibration modes of the synthesized monomers. The surface mor-
phologies of corresponding polymer films on ITO electrode obtained by
electropolymerization of the synthesized monomers was investigated by
SEM images.
Vibrational spectroscopy is a non-destructive identification method
that measures the vibrational energy of a compound. The binding sites
and the state of bonds in the structure, the functional groups of organic
compounds in the form of solid, liquid and solution can be determined
with the use of FT-IR Spectrometer. The experimental vibrational
spectra of three molecules have been measured by FT-IR Spectrometer.
The vibrational spectra of our molecules are calculated by using DFT/
B3LYP method with 6-311G(d) basis set. It is known that a non-linear
molecule with N atoms has 3N-6 normal modes of vibration. The mol-
ecules of M1, M2 and M3 have 38, 64 and 90 atoms, and 108, 186 and
264 fundamental vibrational modes, respectively. Of the 108 vibrational
normal modes for M1 molecule, 37 modes are stretching vibration, 36
modes are bending and the remaining 35 modes are torsional vibrations.
There are 63 stretching, 62 bending and 61 torsional modes for M2
molecule, while M3 has 89 stretching, 88 bending and 87 torsional of the
fundamental modes. M1, M2 and M3 molecules have 39, 60, and 81 C–H
vibrational modes, respectively. The vibration frequencies of the mole-
cules computed from DFT/B3LYP method with 6-311G(d) basis set are
in better agreement with the experiment when multiplied by the scale
factor of 0.966 [53]. Later, these normal modes of vibration are marked
with Gaussian 5.0.8 and VEDA4 program [49]. The FT-IR spectra
computed from DFT/B3LYP method with 6-311G(d) basis set and those
measured for M1, M2 and M3 are plotted in Fig. 2 (a), (b) and (c),
respectively. The all vibrational wavenumbers and IR intensity calcu-
lated from DFT and the assignments of IR vibration modes to charac-
terize the M1, M2 and M3 molecules are given in Tables S4, S5 and S6 as
supplementary materials provided in ESI. For the M1, M2 and M3
molecules, the N–H stretching vibrations are recorded at range of
2
.4. Theoretical methods
Imaging and calculation procedures of M1, M2 and M3 molecules are
made with Gauss View 5.0.8 and Gaussian 0.9 programs, respectively
45,48]. Ab-initio simulations based on DFT/B3LYP/6-311G(d) method
[
are performed to optimize the geometries of all molecules taking into
account in this work. Then, the vibrational wave numbers of the main
molecules by means of the optimized geometries are calculated using the
same method. The vibration wave numbers calculated are scaled by
0
.966 for 6-311G(d) basis set [49]. The marking of the vibration fre-
quencies of the studied molecules is carried out by using the VEDA4
program (Vibrational Energy Distribution Analysis). Since experimental
chemical shifts are performed in DMSO solution, the 1H and C NMR
chemical shifts of the same molecules in the same solution are computed
by using the Gauge-Independent Atomic Orbital (GIAO) approach
applying DFT/B3LYP method with the basis set of 6-311G(d) [50]. In the
next step, the electronic properties such as chemical hardness, chemical
softness, ionization potential, electron affinity, electronegativity, elec-
trophilicity and chemical potential of the conductive polymers consid-
ered are investigated by utilizing the HOMO-LUMO energies.
Theoretically, the molecular ESP surface is used to determine the elec-
trophilic and nucleophilic reactive attack regions of the molecules.
Then, Mulliken charge analyses of conductive polymers are performed.
Finally, the TDOS and PDOS analyses are evaluated to clarify the elec-
tronic properties.
13
ꢀ 1
3477–3497 cm in theoretical calculations. According to PED analysis,
these modes are almost pure N–H stretching vibrations (PED, 100%) for
–
all molecules. The C
–
O band and aromatic C–H band are in the range of
ꢀ
1
ꢀ 1
1706–1726 cm
and 3070–3106 cm
,
respectively. The
3

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
Fig. 1. The optimized molecular structures obtained from DFT/B3LYP/6-311G(d) method for the molecules M1, M2 and M3.
experimentally FT-IR spectra of monomers are investigated between
Table 1
ꢀ 1
4
000 and 400 cm . Since the functional groups in all three molecules
Dihedral angles of the optimized structures of chemical compounds M1, M2 and
examined are identical, the FT-IR spectrum is also very similar as ex-
M3 at the B3LYP/6-311G(d) level of theory.
pected. The bands of the N–H stretching vibrations of all three molecules
0
Dihedral Angles ( )
DFT/B3LYP/6-311G(d)
ꢀ 1
ꢀ 1
are observed at the wavenumbers of 3876 cm , 3737 cm and 3567
ꢀ 1
–
M1
M2
M3
cm while C
–
O stretching vibrations are observed as very strong band
ꢀ
1
ꢀ 1
at 1679 cm in FT-IR spectra. The peak at 1611 cm is attributed to
N21–C22–C8–S9
N21–C6–C4–S3
N39–C30–C28–S27
N39–C40–C32–S33
N57–C58–C50–S51
N57–C48–C46–S45
47.991
ꢀ 46.276
ꢀ 31.245
ꢀ 31.236
ꢀ 46.263
–
48.102
–
ꢀ 52.033
ꢀ 52.587
ꢀ 50.704
49.416
C
–
C stretching vibration as C–C stretching, out-of plane bending,
–
–
–
–
in-plane-bending and torsion vibrations modes observed at 1611–1453
ꢀ 1
cm . The theoretical vibration frequencies obtained from the DFT
38.850
calculations of the molecules show good agreement with the experi-
mentally observed vibrations. The correlation graphs between the
experimental and the calculated frequencies are shown in Fig. 3 for M1,
–
ꢀ 47.225
Fig. 2. The calculated and experimental infrared spectra (FT-IR) of M1, M2, and M3 molecules, respectively.
Fig. 3. The regression analysis of experimental and theoretical wavenumber predicted from DFT method for a) M1 b) M2 c) M3 molecules, respectively.
4

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
2
M2 and M3 molecules. Respectively, the correlation values (R ) between
the theoretical and experimental vibration bands are found to be
chemical hardness and softness parameters, charge distribution, bond
degree, electrostatic potential and molecular orbital shapes can be
attained by exploring the electron density distributions of molecules.
The ionization potential energy (I = -EHOMO) is the minimum energy
required to remove an electron from the molecule in the gas phase.
When an electron is added to the molecule in the gas phase, the
0
.99979, 0.99968, and 0.99971 for M1, M2 and M3 molecules.
.3. 1H and 13C NMR NMR spectra
3
increasing amount of energy gives the electron affinity (A = -ELUMO).
Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical
I+A
Electronegativity (
χ
=
) is the power of an atom in a molecule to
chemistry technique used to determine the molecular structure, purity
2
Iꢀ A
) is a measure of the in-
2
1
13
attract electrons. Chemical hardness (
η
=
and content of a material in the solvent [54,55]. The NMR ( H and C)
chemical shift values of the synthesized molecules are calculated using
the GIAO approach at the level of DFT/B3LYP with 6-311G(d) basis set
in DMSO solvent. The 1H and C NMR spectra of M1, M2 and M3
hibition of charge transfer in the molecule. Global electrophilicity index
2
(
ω
=
μ
/2 ) can be explained by using Koopmans theorem [57]. If the
η
13
value of chemical hardness is high, the charge transfer within the
molecule is very low [58]. If the HOMO-LUMO energy difference of
molecule is small, the electron distribution can be easily guided and
large polarization is obtained. The electronic structure parameters of the
molecules such as the ionization potential, electron affinity, global
electronegativity, global electrophilicity, chemical hardness, chemical
potential and global softness are calculated by means of DFT/B3LYP
with 6-311G(d) basis set. The calculated electronic structure parameters
of the compounds are given in Table 3. The molecule M3 is the best
acceptor since it has the biggest electronegativity. Moreover, M3 is the
most electrophile since the electrophilic value of M3 molecule is greater
than other molecules. The energy difference of HOMO-LUMO (ΔE) is
1
molecules are recorded in the medium of DMSO. The observed H and
1
3
C NMR spectra of M1, M2 and M3 molecules (with respect to TMS, and
in DMSO solution) are given in Fig. S1 as a supplementary material in
1
13
ESI. The results of both experimental and theoretical H and C NMR
for the molecules of M1, M2 and M3 are given in Table 2.
2
The linear correlation coefficients (R ) are obtained by using the
1
13
values of experimental and computational H and C NMR chemical
shift. The linear correlations graphs of 1H and 13C NMR chemical shift
for M1, M2 and M3 molecules are given in Fig. S2 as a supplementary
material in ESI. The correlation values (R2 = 0.76384, 0.97564 and
1
0
.9093) between the experimental and calculated H chemical shifts for
3
.954; 3.213 and 3.307 eV, respectively, for molecules M1, M2 and M3,
the DFT/B3LYP level are good in the solvent of DMSO for M1, M2 and
M3 molecules. As interested in 1 C chemical shifts, the correlations be-
tween the experimental and calculation are obtained as R2 = 0.97292,
3
which reflects that the values greater than 1.5 eV get the molecules
thermodynamically stable and durable. In addition, the molecule
doesn’t react with itself, does not dimerize and polymerize. The
HOMO-LUMO energy difference of the M1 molecule is higher than the
other molecules. Therefore, M1 molecule becomes harder and less
chemical reactive than other molecules. Low electron flow occurs due to
its high energy difference. This cause M1 molecule to be low reactive.
The electron density increases from green to red according to the elec-
trostatic potential drawing. The atoms located at the HOMO-LUMO or-
bitals are very important since chemical reactions occur in the
HOMO-LUMO orbitals. The HOMO-LUMO representations of M1, M2
and M3 molecules are calculated by DFT/B3LYP/6-311G(d) method and
shown in Fig. 4.
0
.93409 and 0.90207 for M1, M2 and M3 electroactive monomers,
respectively. When the experimental and theoretical NMR results are
compared, it is seen that the DFT/B3LYP method is versatile tool in
examining the NMR properties of the electroactive materials.
3
.4. Analysis of FMOs
The Molecular Orbital (MO) theory is a method used in chemistry to
determine the electronic structures of molecules by utilizing quantum
mechanics. The Frontier Molecular Orbital Theory (FMO) is an appli-
cation of MO theory that determines the HOMO-LUMO interaction.
According to the FMO theory, it is concluded that good approaches to
reactivity can be found by analyzing the HOMO-LUMO interactions of
the molecules. As a result, chemical reactions of molecules can be
managed by examining FMO behaviors. Obtaining the difference be-
tween the HOMO-LUMO energies of the molecules causes the reaction
between the molecules to be easy [56]. By looking at Information about
3.5. ESP analysis
Electrostatic potential maps, also known as molecular electrical po-
tential surface, show the charge distributions of molecules in three-
dimensional structure. This map monitoring the charge distributions
Table 2
1
13
The theoretical and available experimental H NMR (d, ppm, DMSO‑d
6
) and C NMR (d, ppm, DMSO‑d
6
) values for M1, M2, and M3 molecules, respectively.
M1
M2
M3
Atom Label
DFT
Exp.
Atom Label
DFT
Exp
Atom Label
DFT
Exp.
H37 (a)
H28 (b)
H30 (c)
H33 (d)
H29 (e)
H32 (f)
H31 (g)
H36 (h)
C23 (a)
C18 (b)
C7 (c)
6.60
7.25
6.10
H54 (a)
H48 (b)
6.62
7.06
6.22
6.67
H71 (a)
H61 (b)
6.58
7.29
6.82
7.25
6.72
6.64
7.59
7.43
H46 (c)
H47 (d)
H49 (e)
H53 (f)
C24 (a)
C14 (b)
C5 (c)
7.32
6.83
6.80
7.58
7.21
H63 (c)
H65 (d)
H67 (e)
H70 (f)
C23 (a)
C5 (b)
7.39
7.40
7.76
6.86
7.42
7.97
7.53
7.90
7.30
8.17
8.83
8.69
8.03
7.49
12.25
12.48
7.79
9.38
6.62
107.64
124.41
127.22
127.66
127.82
128.64
128.92
132.64
135.48
165.23
111.91
130.48
128.24
129.02
133.51
135.93
141.73
170.29
140.43
133.48
123.63
129.31
130.27
133.88
134.59
140.40
143.83
153.44
191.91
113.50
130.49
129.13
134.45
134.71
137.78
136.50
142.00
169.87
107.95
124.36
127.22
127.66
128.09
128.81
132.15
133.66
143.61
191.46
113.34
131.29
130.92
129.00
134.91
132.08
137.30
136.92
142.18
174.34
C1 (c)
C1 (d)
C14 (d)
C2 (e)
C11 (d)
C10 (e)
C17 (f)
C6 (g)
C10 (e)
C6 (f)
C6 (f)
C4 (g)
C13 (g)
C4 (h)
C12 (i)
C12 (h)
C13 (i)
C17 (k)
C16 (h)
C8 (i)
C12 (j)
Labels of the atoms in this Table are given according to Fig. 1 used in the assignment of the chemical shifts for M1, M2 and M3 molecules.
5

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
Table 3
of molecules conducts how molecules interact with other molecules. The
molecular electrostatic potential provides important information about
the determination of regions where electrophilic and the nucleophilic
reactions may take place in the molecule and the formation of intra-
molecular hydrogen bonds [59,60]. In an ESP map, the electron den-
sity of the molecule appears to change from red to blue. The
electron-rich (red) region where the molecule repels the outer electrons
has low potential energy. On the other hand, the electron-poor (blue)
region where the molecule attracts the outer electrons (this region is
positively charged) has high potential energy. The green regions on the
ESP map has the neutral electrostatic potential is neutral points. In
addition, the orange-yellow and light blue regions on the map show
partial negative or positive charges. ESP maps of M1, M2 and M3 mol-
ecules in the optimized geometries are achieved by utilizing
DFT/B3LYP/6-311G(d) method. The electrophilic and nucleophilic re-
gions obtained from the ESP maps are illustrated in Fig. 5 Electrostatic
potential values on the surface of M1, M2 and M3 molecules vary in the
range of ±0.06073 a.u ., ±0.05778 a.u and ±0.05515 a.u., respectively.
As seen in Fig. 5, while the electron-rich regions are formed around O
atoms, the electron-poor regions are created around N–H groups for the
M1, M2 and M3 molecules. Additionally, the regions where H atoms are
located in are also positive area on the ESP maps. These regions con-
sisting of electron density indicate the position where molecules have
the ability of metallic bonds [61].
Calculated electronic structure parameters and measured optical and electro-
chemical properties of the M1, M2 and M3 molecules.
DFT/B3LYP/6-311G(d)
M1
M2
M3
E
HOMO (eV)*
LUMO (eV)*
ꢀ 5.373
ꢀ 1.419
3.954
ꢀ 5.329
ꢀ 2.116
3.213
ꢀ 5.421
ꢀ 2.114
3.307
E
Eg (eV)*
I (eV)*
5.373
5.329
5.421
A (eV)*
1.419
2.116
2.114
χ
η
μ
(eV)*
(eV)*
(eV)*
3.396
3.722
3.768
1.977
1.606
1.654
ꢀ 3.396
0.253
ꢀ 3.722
0.311
ꢀ 3.768
0.302
ꢀ
1
S (eV )*
ꢀ
1
ω
(eV )*
11.398
High
11.129
Modarate
Modarate
0.72
11.735
Highest
Highest
0.60
Planarity of Group A*
Donor character of Group A*
Emonset (V)**
High
0.62
Emox (V)**
0.93
1.18
0.87
Eponset (V)**
0.22
0.31
0.18
Epox (V)**
0.54
0.65
0.46
2
Qp (mC/cm )**
1.877
1.741
3.265
ΔT
ΔTbipolaron (%)**
Eg (eV)**
π
-
π*
(%)**
15 (430 nm)
32 (906 nm)
2.10
41 (431 nm)
94 (870 nm)
2.28
21 (414 nm)
65 (920 nm)
2.21
p
*
obtained from DFT calculations **obtained from electrochemical and optical
measurements, (Eg): band gap energy (I): ionization potential, (A): electron
affinity, ( ): global electronegativity, ( ): chemical hardness, ( ): chemical po-
tential and (S): global softness, ( ): global electrophilicity, (Emonset): Onset
χ
η
μ
ω
potential (Emox): Monomer oxidation peak potential, (Eponset):Polymer onset
oxidation potential, (Epox): Polymer oxidation peak potential, (Qp):Polymer
3
.6. Investigation of Mulliken charge
charge density, (ΔT
π
-
π
*
):Optical contrast at λmax for
π
-
π
* transitions, (ΔTbipolaron):
Optical contrast at bipolaron transitions Eg
p
: Polymer optical band gap calcu-
Mülliken charge distribution is one of the most used charge analysis
lated from 1240/λmax
methods. Mulliken charge analysis are used to estimate qualitatively the
experimental results [62–64]. Mulliken atomic charges are predicted by
Fig. 4. The schema displaying the frontier molecular orbitals (HOMO and LUMO) computed via DFT/B3LYP method in the gas phase for M1, M2 and M3 molecules.
Fig. 5. The isosurface total electron density mapped with the use of molecular electrostatic potential of M1, M2 and M3 molecules by using the method of DFT at the
level of B3LYP.
6

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
using the DFT/B3LYP/6-311G(d) method to investigate the electron
population of each atom in organic molecules. Mulliken atomic charges
of each atom are obtained for M1, M2 and M3 molecules. Mulliken
atomic charges for the selected atoms groups in each molecule are given
in Fig. 6.
molecules. These values are 97%, 99% and 98% for M1, M2 and M3
molecules, respectively. In addition, the contribution of group A to the
PDOS of LUMO+1 for the M1 molecule is 88% while the contribution of
group B is 12%. For M2, the contribution of group A to the PDOS of
LUMO+1 is 85% while the contribution of group B is 15%. While the
contribution of group A to the PDOS of LUMO+2 for the M3 molecule is
93% while the contribution of group B is 7%. Generally, the TDOS of
LUMO + mostly comes from PDOS of group A of M3 molecule.
It is reported from the Mulliken analysis that C atoms bound to O and
N atoms are positively charged, while C atoms bound to the H atoms are
negatively charged. It is seen that mostly C atoms have a negative atomic
charge as expressed in Ref. [65]. All H and S atoms in the molecules have
a positive charge while O and N atoms have a negative charge.
3
.8. Electrochemical and spectroelectrochemical properties
3
.7. TDOS and PDOS analysis
Electropolymerizations and electrochemical characterizations of
monomer and corresponding polymers have been performed in three-
electrode electrochemical cell at ambient temperature. Indium tin-
The function of density of states describes the number of available
+
states in a system and is essential for determining the carrier concen-
trations and energy distributions of carriers with in compounds. The
partial density of state shows the contribution of the specific atomic
groups or functional groups selected in the compounds to each of mo-
lecular orbital. In this part of our study, the density of states spectra of
three molecules are obtained. The TDOS, PDOS and band structures of
the title molecules are calculated using the GaussSum 3.0 program with
Gaussian curves of unit height and full width at half maximum (FWHM)
of 0.3 eV. The representation of TDOS, PDOS and band are given in
Fig. 7. In the TDOS graphs, the pink colored region shows the HOMO
orbital, while the green colored region shows the LUMO orbital. The
PDOS is used to find bonding, anti-bonding and non-bonding properties
according to certain fragments. While the positive region indicates the
binding interaction in molecules, the anti-bonding interaction indicates
that negative values and non-bonding interactions are closed to zero
oxide coated glass slides, Ag/Ag and platinum wire have been used as
working, reference and counter electrodes in electrochemical cell. The
electrolyte solution used in the electropolymerization process is 0.1 M
4
LiClO and 0.01 M related monomer containing acetonitrile solution.
Monomer-free electrolyte solution is used in spectroelectrochemical
processes. In spectrophotometric analysis, the changes in the optical
properties of the polymer under applied potentials with a potentiostat
were examined simultaneously with an UV spectrophotometer. Elec-
trochemical properties and electropolymerizations of electroactive
molecules called M1, M2 and M3 have been investigated by cyclic vol-
tammetry technique. Fig. 8 shows cyclic voltammetry (CV) plots of all
three molecules. As seen in Fig. 8 a-c, the current increase in each cycle
shows that all three molecules polymerized on the electrode surface and
the coated polymer is conductive. When the first two cycles of the cyclic
voltammetry graphs of the molecules have been examined in order to
further elaborate the electropolymerization process, it has been
observed that all molecules start oxidation at the different onset po-
tentials in the first CV cycle. The onset potential for the M1 molecule has
been observed at 0.62 V, whereas these values are 0.72 V and 0.60 V for
M2 and M3 molecules, respectively. Change in onset potentials can be
explained using the frontier molecular orbitals diagram in Fig. 4. The
[
66,67]. It is seen in the PDOS plots that the orbitals of the determined
atoms in the molecules contribute to the molecular orbitals. The TDOS of
HOMO for the M1, M2 and M3 molecules mostly comes from PDOS of
selected group A in the molecules. These values are 99%, 100% and
1
00% for M1, M2 and M3 molecules, respectively. On the other hand,
the LUMO is mainly composed of PDOS of selected group B in title
Fig. 6. Mulliken atomic charges of M1. M2 and M3 molecules, respectively.
7

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
Fig. 7. Total density of states (TDOS), partial density of states analysis (PDOS) and band structures of a) M1, b) M2 and c) M3 molecules, respectively.
8

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
Fig. 8. Cyclic voltammetry of the a) M1, b) M2, c) M3 and first and second cycle of cyclic voltammograms of d) M1, e) M2 and f) M3 at 100 mV/s in 0.1 M
ACN/LiClO
4
.
energy required in the oxidation and reduction of the examined mole-
cules, is tightly related to the required energy to remove an electron
from the HOMO level and released energy from addition of electron to
the LUMO level, respectively. M3 has most easily oxidized and M2 has
the biggest onset oxidation potential value as compatible with the result
of DFT calculations. The difference between the onset potentials can also
be explained by the three dimensional geometries of the molecules.
Comparing the dihedral angles of optimized structures, it has been
determined that the structure planarity is related with the onset po-
tentials. Having the most planar 2,5-di(2-thienyl)-1H-pyrrole group, M3
has the lowest onset potential, while M2 with the most plane distortion
has the highest onset potential. The reason for difference in onset po-
tentials can be explained as the rotational disorder around single bonds
that means the large dihedral angle between consecutive units limits the
produces charge carriers in the form of polarons and bipolarons on the
backbone of the CP. The charge densities of the examined molecules
have been investigated by the cyclic voltammetry technique. For this
purpose, conductive polymer films were synthesized on the ITO elec-
trode with the oxidative polymerization technique detailed in the
experimental part, and their cyclic voltammetry’s have been taken in the
0.1 M LiClO
4
/ACN electrolyte solution in the range of ꢀ 0.5/1.5 V at 100
mV/s of scan rate (Fig. 9). Charge density values for M1, M2 and M3
2
have been calculated as 1.877, 1.741 and 3.264 mC/cm , respectively.
These values correspond to the area under the cyclic voltammetry curves
and have been obtained by integrating of the curves in Fig. 9. Studies
have shown that molecules with high dihedral angles have very low
backbone planarity. This result probably caused the low charge density
of polymer-based devices. In accordance with the literature, polymers of
more planar molecules with low dihedral angles have a high charge
density [72].
delocalization of -electrons along the conjugated structure. Structure
π
planarity affects not only onset potentials but also other redox proper-
ties. Redox properties such as monomer oxidation peak potentials,
polymer oxidation peak potentials, and polymer oxidation onset po-
tentials have exhibited regular change with respect to the planarity of
the molecular structure.
UV–vis spectroelectrochemical technique is the most suitable tech-
nique used to determine the spectrum of electronic transitions between
the fundamental level and polaronic or bipolaronic levels of conductive
polymers. Spectroelectrochemistry is used for the optical characteriza-
tion of the conducting polymers under applied voltages. In this tech-
nique, a UV–vis spectrophotometer is combined with a potentiostat to
examine changes optical properties of the polymer with applied poten-
tial. The recording of the spectrum during gradual reduction or oxida-
tion provides useful information about optical band gap, polaronic and
bipolaronic states. Spectroelectrochemical measurements have been
made for polymer films coated on ITO quartz electrodes that immersed
in a quartz cuvette containing electrolyte solution.
Furthermore, Partial State Density (PDOS) calculations can provide
excellent solutions for interpreting the redox behavior of molecules. As a
result of PDOS calculations for the examined molecules, almost all the
HOMO levels of each molecule are occupied by the 2,5-di(2-thienyl)-1H-
pyrrole groups (Group A), and the LUMO is occupied by the rest of the
structure (Group B). Thus, the 2,5-di(2-thienyl)-1H-pyrrole groups acts
as an electron donor, while group B acts as an electron acceptor in all
molecules. Since the electron donor property of the Group A in all
molecules shows a tendency as M3 > M1 > M2 from the PDOS calcu-
lations, redox properties such as onset potentials of the molecules have
been affected from this tendency as shown in Table 3. Reversible charge
storage capacity of conducting polymers make them good candidates for
batteries, electrochromic devices, and supercapacitors applications. A
simple explanation of the reason of charge density in CPs is that elec-
trons are extracted from the HOMO of the valence band or transferred to
the LUMO of the conduction band. This oxidation/reduction process
Fig. 10 a, b and c show spectroelectrochemical graphs for conductive
polymers of M1, M2 and M3 molecules, respectively. Intense absorption
corresponds to bipolaron bands have been observed in the spectrum of
the oxidized states of the polymers at about 900 nm. Optical contrast
values of the polymers at the bipolaron bands wavelength have been
calculated as 32%, 94% and 65%, respectively. Optical contrast values
of all polymers at both wavelength (
π ꢀ π∗ transition bands and bipo-
laron bands) have been observed to be compatible with each other as
9

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
Fig. 9. Cyclic voltammetry studies of the conducting polymers obtained from a) M1, b) M2 and M3 to calculate charge density.
Fig. 10. Spectroelectrochemical investigations of the conducting polymers obtained by electrochemical polymerization of a) M1, b) M2 and c) M3.
optical contrast values at both wavelengths exhibited M1<M3<M2
arrangement. From this result, it has been observed that the optical
contrast values of the polymers are related to the magnitude of the band
gap energy of the monomeric molecules. Conductive polymers obtained
from 2,5-di(2-thienyl)-1H-pyrrole molecules with low band gap have
been found to have high optical contrast values. Therefore, for this
study, the optical contrast values of the conductive polymer obtained as
a result of polymerization of molecules containing 2,5-di(2-thienyl)-1H-
pyrrole can be increased by tuning the band gap energy of the mono-
meric molecules.
considering in this work. The fundamental vibrational modes of the
multifunctional conjugated polymers are assigned. The vibrational fre-
quencies computed from DFT method are compatible with the experi-
mental results, since the regression coefficients goes to approximately
unity. It is shown that the computational values of the harmonic fre-
quencies deviate slightly from the experimental data. This disagreement
may be due to ignoring the anharmonic effects and the general trend of
the quantum chemical methods overestimating the force constants at the
equilibrium geometry. The correlation between the calculation and
experimental FT-IR data is provided by fitting our data to linear equa-
tions. The electronic properties of conjugated polymers synthesized in
the present work are revealed by fulfilling frontier molecular analysis,
electrostatic potential maps, Mulliken charge distribution, the total
density of states, partial density of states analysis and band structures of
the title molecules. It is observed that M1 molecule has less chemical
reactivity than other molecules by analyzing the results of FMO. It is also
shown that M3 molecule is the most electrophilic and reactive molecule
than ones of other molecules. Moreover, it is seen that the electron
density of M3 molecule is highest thanks to the electroactive regions by
examining ESP maps and Mulliken charges analyses. The difference
between the onset potentials have been explained by the three dimen-
sional geometries of the molecules. Comparing the dihedral angles of
optimized structures, it has been determined that the structure planarity
is related with the onset potentials. Having the most planar 2,5-di(2-
thienyl)-1H-pyrrole group, M3 has the lowest onset potential, while
M2 with the most plane distortion has the highest onset potential. The
reason for difference in onset potentials can be explained as the rota-
tional disorder around single bonds that means the large dihedral angle
4
. Conclusion
In this study, the structural geometries and electronic properties of
different monomeric structures with the same electroactive group have
been investigated to provide a relationship between calculated elec-
tronic properties of the monomers with the observed optical and elec-
trical properties of their conjugated polymers. For this purpose, three
different amide substituted 2,5-di(2-thienyl)-1H-pyrrole compounds
containing a different number of electroactive groups have been syn-
thesized and molecular structure optimizations have been carried out
with the DFT calculations. FT-IR and NMR spectra of optimized geom-
etries have been compared with experimental data. The structural,
spectroscopic and electronic properties of M1, M2 and M3 conductive
polymers synthesized in this work are identified by performing the
measurements of FT-IR, 1H NMR and C NMR spectra as well as
quantum chemistry calculations based on DFT method. The geometries
of the electroactive molecules titled as M1, M2, and M3 are optimized
through DFT method at the level of B3LYP with basis set of 6-311G(d).
The bond lengths, bond angles and dihedral angles are computed as
structural properties by employing quantum computational methods. It
has been observed that M3 and M1 molecules are more planar than M2
molecules by examining the geometrical structures of molecules
13
between consecutive units limits the delocalization of -electrons along
π
the conjugated structure. Structure planarity affects not only onset po-
tentials but also other redox properties such as monomer oxidation peak
potentials, polymer oxidation peak potentials, and polymer oxidation
onset potentials. Reversible charge storage capacity of conducting
1
0

P.T. Tasli et al.
Journal of Physics and Chemistry of Solids 149 (2021) 109720
polymers make them good candidates for batteries, electrochromic de-
vices, and supercapacitors applications. Charge density studies have
shown that molecules with high dihedral angles have very low backbone
planarity and this probably caused the low charge density of polymer-
based devices. In accordance with the literature, polymers of more
planar molecules with low dihedral angles have a high charge density.
Hui Chua (Eds.), Electrochromic Smart Mater. Fabr. Appl., First, The Royal Society
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Optical contrast values of all polymers at both wavelength (
π
- * tran-
π
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with each other as optical contrast values at both wavelengths exhibited
M1<M3<M2 arrangement. From this result, it has been observed that
the optical contrast values of the polymers are related to the magnitude
of the band gap energy of the monomeric molecules. Conductive poly-
mers obtained from 2,5-di(2-thienyl)-1H-pyrrole molecules with low
band gap have been found to have high optical contrast values. There-
fore, for this study, the optical contrast values of the conductive polymer
obtained as a result of polymerization of molecules containing 2,5-di(2-
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the monomeric molecules. As a result, optical and electrical properties of
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[
[
[
[
[
Acknowledgements
20] T. Soganci, H.C. Soyleyici, M. Ak, Fabrication of multifunctional 2,5-di(2-thienyl)
pyrrole based conducting copolymer for further sensor and optoelectronic
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This study has been supported by Pamukkale University (Grant Nos:
2
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