Effect of fluorine on optoelectronic properties in DI-A-DII-A-DI type organic molecules: A combined experimental and DFT study

Article abstract of DOI:10.1016/j.molstruc.2020.128019

The impact of the substitution of fluorine atom/atoms on the optoelectronic features of organic molecules having D_I-A-D_II-A-D_I type architecture is examined in the current work. The three synthesized organic molecules (SMD1, SMD2 and SMD3) comprise of a dithienopyrrole (DTP) derivative as a central donor (D_II), which is flanked between two benzothiadiazole (BT) moieties (electron acceptors, A). The BT core on each of two ends is joined to an electron-donating benzodithiophene (BDT) derivative (D_I). The SMD1, SMD2 and SMD3 are substituted with 0, 2 and 4 fluorine atoms on their BT moiety respectively. The assistance of DFT methods is taken to evaluate the influence of fluorine on reorganization energies, ionizing potential and electron affinity of molecules. The thermal stability of molecules is mapped by TGA studies. Cyclic voltammetry studies are carried out to comprehend the characteristics of highest molecular orbital, lowest unoccupied molecular orbital and the bandgap of molecules, which are also supported by DFT methods. The molecules displayed better absorption properties in the near-infrared (NIR) region, excellent solution processability in a variety of organic solvents, low bandgap and optimum thermal toughness to make them applicable in the construction of OBHJSCs to play the role of donor materials when connected with acceptors like fullerene derivatives.

Full text of DOI:10.1016/j.molstruc.2020.128019

Journal of Molecular Structure 1210 (2020) 128019
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Effect of fluorine on optoelectronic properties in D -A-D -A-D type
I
II
I
organic molecules: A combined experimental and DFT study
a
b
a
Ejjurothu Appalanaidu , V.M. Vidya , Manohar Reddy Busireddy ,
a
,
*
b, **
Jayathirtha Rao Vaidya , Prabhakar Chetti
a
Fluoro & Agrochemicals Division and AcSIR, CSIR-IICT Indian Institute of Chemical Technology, Tarnaka, Hyderabad, 500 007, India
Department of Chemistry, National Institute of Technology, Kurukshetra, 136119, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 November 2019
Received in revised form
The impact of the substitution of fluorine atom/atoms on the optoelectronic features of organic mole-
cules having D -A-DII-A-D type architecture is examined in the current work. The three synthesized
organic molecules (SMD1, SMD2 and SMD3) comprise of a dithienopyrrole (DTP) derivative as a central
donor (DII), which is flanked between two benzothiadiazole (BT) moieties (electron acceptors, A). The BT
I
I
19 February 2020
Accepted 5 March 2020
Available online 7 March 2020
I
core on each of two ends is joined to an electron-donating benzodithiophene (BDT) derivative (D ). The
SMD1, SMD2 and SMD3 are substituted with 0, 2 and 4 fluorine atoms on their BT moiety respectively.
The assistance of DFT methods is taken to evaluate the influence of fluorine on reorganization energies,
ionizing potential and electron affinity of molecules. The thermal stability of molecules is mapped by TGA
studies. Cyclic voltammetry studies are carried out to comprehend the characteristics of highest mo-
lecular orbital, lowest unoccupied molecular orbital and the bandgap of molecules, which are also
supported by DFT methods. The molecules displayed better absorption properties in the near-infrared
Keywords:
D -A-DII-A-D type Architecture
I I
Photo-physical properties
Near-infrared absorption
Reorganization energies
(
NIR) region, excellent solution processability in a variety of organic solvents, low bandgap and opti-
mum thermal toughness to make them applicable in the construction of OBHJSCs to play the role of
donor materials when connected with acceptors like fullerene derivatives.
©
2020 Elsevier B.V. All rights reserved.
1
. Introduction
In particular, designing of organic molecules with substitution of
the electron-donating groups on D and inclusion of electron-
attracting groups on A plays a critical role in tuning optoelec-
tronics (and hence altering PCE) of a material. In the recent past,
there have been numerous research articles, which illustrate the
impact of substituting A with electron-withdrawing groups
(majorly fluorine atom) [7e20] on the optoelectronic properties of
a material having various types of donor-acceptor materials. Fluo-
rination of acceptor core is an effective approach to obtain the
materials with good PCE since, i) it fine-tunes the energetics of
frontier molecular orbitals (by lowering energies of highest occu-
pied molecular orbital) without compromising bandgap [21], ii)
improves molecular ordering by dipole moment induced along
carbon-fluorine bond [21], iii) imposes no steric hindrance due to
its smaller size and hence helps in planarizing the molecules [14],
iv) triggers non-covalent interactions through F/S and CeF/H
Recently, the enhancement of power conversion efficiency (PCE)
of organic bulk heterojunction solar cells (OBHJSCs) has been the
center of focus in the field of organic photovoltaics (OPV) and has
gathered a number of scientific reports, which elucidate the sig-
nificance of meticulous molecular design, morphology organization
and appropriate molecular orientation in achieving better PCEs for
OBHJCs [1e4]. Generally, increased amounts of short-circuit current
density (JSC) and open-circuit voltage (VOC) contribute to the
elevation of PCE of OBHJSCs along with a contribution from the
elevated magnitude of fill factor (FF) of the devices. In this regard,
organic molecules having interconnected donor (D) and acceptor
(
A) moieties offer to produce better OBHJSCs since they can fulfill
the conditions to have good PCE [5,6].
bonds [12,22], v) strong electron-attracting nature of
F
*
Corresponding author.
causes material to be hydrophobic, which provides stability
to material in environmental conditions [23e28], vi) provide
high fill factor, open-circuit voltage and short circuit current
** Corresponding author.
E-mail addresses: vaidya.opv@gmail.com (J.R. Vaidya), Chetty_prabhakar@
yahoo.com (P. Chetti).
https://doi.org/10.1016/j.molstruc.2020.128019
0
022-2860/© 2020 Elsevier B.V. All rights reserved.

2
E. Appalanaidu et al. / Journal of Molecular Structure 1210 (2020) 128019
density, which are requisite conditions for a good OPV device
Synthesized materials are characterized by NMR spectroscopy
(BrukerAvance, 500 MHz) and mass spectroscopy (Thermofinngan
mass spectrometer) techniques. The trends of absorption and
fluorescence are measured with the help of Cary 5000 UVeViseNIR
and Cary eclipse fluorescence spectrophotometers respectively. A
PC controlled CHI 62C electrochemical analyzer was used for cyclic
voltammetry studies (solvent: dichloromethane, scan rate:
100 mV/s) by taking a supporting electrolyte (0.1 M tetrabuty-
lammonium perchlorate). The platinum wire and glassy carbon are
employed as counter and working electrodes while considering the
standard calomel electrode as a reference electrode (the potential
of which is calibrated by adopting ferrocene as internal standard)
and all the potentials are measured against standard calomel
electrode. All of the above measurements are performed at room
temperature (RT). Thermogravimetric experiments are carried out
[
8e13,15e17,19,20,29e36].
Though incorporation of fluorine bestows a material with
several benefits; it poses some drawbacks like reduced solubility in
organic solvents, energy levels that are not suitable for charge
transport and higher aggregation of polymers in active layers of
polymer solar cells [21,37,38]. Hence, the extent of fluorination or
the number of fluorine atoms substituted needs to be optimized in
order to get the optimal performance of an OPV, which widens the
scope of research in this regard.
Keeping all the above facts in mind, our present work is
designed to synthesize three D -A-DII-A-D type molecules, each
I I
having a varying number of fluorine atoms in the molecular back-
bone. In this work, the synthesis of three organic molecules
designated as SMD1, SMD2 and SMD3 (Fig. 1) having zero, two and
four fluorine atoms respectively at selective places is reported. The
molecule 4,8-bis{(2-ethylhexyl)oxy}-4,8-dihydrobenzo{1,2-b:4,5-
under nitrogen atmosphere (Exstar TG or TGA 7200, heating rate:
ꢀ
10 C/min).
b’}dithiophene (OBDT) is employed as a terminal donor (D
I
) at
two ends and 4-{2-ethylhexyl}-4H-dithieno{3,2-b:2 ,3 -d}pyrrole
DTP) is used as a central donor (DII) for all synthesized molecules.
0
0
(
2
.2. Synthetic routes
The molecule benzo{c}{1,2,5}thiadiazole (BT), which acts as an
electron acceptor (A) is placed between OBDT and DTP at each of
the two ends of all the molecules. The number of fluorine atoms is
varied on the BT core. The architecture of these molecules is based
on the previous literature, which suggests that BDT with its three
planar rings joined to each other can initiate considerable inter-
molecular interactions to develop BHJs having good charge carrier
dynamics [39e42]. The BT moiety offers ease of synthesis and
tuneable substitution. The DTP donor can influence the shifting of
absorption maxima towards visible or NIR region [43,44].
Hence, the current work presents the synthesis of SMD1, SMD2
and SMD3 along with the interpretation of the influence of fluorine
atoms on their photo-physical and electrochemical features. DFT
studies are also carried out to support the experimental absorption
properties. Further, we have also looked for the potential suitability
of molecules SMD1, SMD2 and SMD3 to be applicable in opto-
electronic devices such as OBHJSCs by means of studies of their
reorganization energies, ionization potential and electron affinities
by DFT methods.
The synthetic route of SMD1, SMD2 and SMD3 are depicted in
Scheme 1 and their detailed procedures are provided in the
experimental section. All the molecules were prepared by following
procedure from previous literature [43e46]. The target molecules
of SMD1, SMD2, and SMD3 were prepared through the Stille cross-
coupling reaction from distannyl intermediate 3 and their corre-
sponding monobromo derivatives (SMD1, SMD2 and SMD3 ob-
tained from BT-OBDT, FBT-OBDT and F2BT-OBDT respectively). The
monobromo derivatives were prepared by using the Stille cross-
coupling reaction pathway from their corresponding dibromo-BT
derivatives (BT-OBDT, FBT-OBDT and F2BT-OBDT obtained from
2 2 2
BTBr , FBTBr and F2BTBr respectively) by treating with mono-
stannyl intermediate 7. Intermediate 7 was obtained from 5 by
reductive alkylation with 2-ethylhexylbromide under alkali con-
dition followed by stannilation by using trimethylchloride in the
presence of n-BuLi. The compound 5 was synthesized from
commercially available thiophene-3-carboxylic acid by reacting
with oxalyl chloride and 2-ethylamine followed by dimerization in
the presence of n-BuLi. Intermediate 3 was synthesized from lith-
iation of compound 2 by using n-BuLi followed by quenching with
trimethyltin chloride. Compound 2 was synthesized through
dimerization of 3-bromo thiophene followed by the Buchwald-
Hartwig amination pathway using 2-ethylhexylamine. All the
2
. Experiments
2.1. Materials and instruments
1
13
The chemicals utilized for the synthesis are all procured from
structures of compounds were confirmed by using H NMR,
C
commercial vendors and are employed without any purification
processes. Experiments are performed with dry solvents (dry
dichloromethane, dry toluene and dry tetrahydrofuran).
NMR, ESI-MS and MALDI-TOF analysis (ESI; Figs. S1eS39). The
synthetic procedures of SMD1, SMD2 and SMD3 are illustrated in
Scheme 1.
Fig. 1. Chemical structures of SMD1, SMD2 and SMD3.

E. Appalanaidu et al. / Journal of Molecular Structure 1210 (2020) 128019
3
Scheme 1. Synthetic routes for the synthesis of SMD1, SMD2 and SMD3.
þ
97 5 4 8
MS (m/z): Calc. for C80H N O S is [M] : m/z 1447.53 Found:
2.2.1. 2,6-bis(7-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b’]
dithiophen-2-yl)benzo[c] [1,2,5]thiadiazol-4-yl)-4-(2-ethylhexyl)-
1447.63.
0
0
4H-dithieno[3,2-b:2 ,3 -d]pyrrole (SMD1)
A 50 mL round bottom flask was charged with 3 (0.117 g,
.19 mmol), BT-OBDT (0.375 g, 0.56 mmol) and tetrakis(-
2.2.2. 2,6-bis(7-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b’]
dithiophen-2-yl)-5-fluorobenzo[c] [1,2,5]thiadiazol-4-yl)-4-(2-
ethylhexyl)-4H-dithieno[3,2-b:2 ,3 -d]pyrrole (SMD2)
0
0
0
triphenylphosphine)palladium (22 mg, 0.019 mmol) followed by
the addition of 20 mL of dry toluene. The reaction mixture was kept
under nitrogen atmosphere for 20 min and then refluxed overnight.
When the complete consumption of 3 was indicated by TLC, the
reaction mixture was cooled to room temperature followed by the
addition of 20 mL of chloroform. The resulting solution was passed
through Celite bed and filtrate was given a workup with water and
chloroform to extract compound with chloroform (2 ꢁ 20 mL). The
It is synthesized as per the procedure described for SMD1 using
3 and FBT-OBDT as synthetic precursors (yield 70%).
1
H NMR (400 MHz, CDCl3, d ppm): 1.01e1.06 (m, 18H), 1.08e1.14
(m, 12H), 1.44e1.52 (m, 24H), 1.60e1.74 (m, 14H), 1.77e1.83, (m,
6H), 1.95e2.01, (m, 1H), 3.92e4.04 (m, 2H), 4.08e4.14 (m, 8H), 7.04
(d, J ¼ 5.2 Hz, 2H), 7.14 (d, J ¼ 5.2 Hz, 2H),7.33 (d, J ¼ 13.0 Hz, 2H),
13
7.91 (s, 2H), 8.48 (s, 2H); C NMR (100 MHz, CDCl ,
3
d ppm): 11.33,
collected organic phase was dried by adding anhydrous Na
2
SO
4
and
11.19, 14.10, 14.03, 22.96, 23.02, 23.59, 23.66, 29.04, 29.15, 30.15,
30.39, 40.41, 40.46, 75.02, 111.90, 117.42, 119.55, 121.88, 122.48,
125.46, 127.83, 129.38, 131.68, 136.24, 143.24, 144.39, 146.10, 148.97,
152.06, 156.26, 158.28; MALDI-MS (m/z): Calc. for C80H95F N O S is
then filtered. The filtrate was rota-vaporized and the residue was
purified utilizing a column of silica gel (eluent: chloroform/petro-
leum ether, 4:6 v/v). SMD1 is obtained as a blue-black solid (yield
2
5 4 8
þ
[M] : m/z 1483.51 Found: 1483.77.
7
3%).
1H NMR (400 MHz, CDCl3,
m,12H),1.40e1.52 (m, 22H),1.56e1.77 (m,16H),1.77e1.86, (m, 6H),
.89e1.95 (m, 1H), 3.90e4.06 (m, 2H), 4.11e4.19 (m, 8H), 7.16 (d,
J ¼ 5.2 Hz, 2H), 7.24 (d, J ¼ 5.5 Hz, 2H),7.43 (d, J ¼ 6.4 Hz, 2H), 7.54
d
ppm): 0.97e1.04 (m, 18H), 1.06e1.13
(
2.2.3. 2,6-bis(7-(4,8-bis((2-ethylhexyl)oxy)benzo[1,2-b:4,5-b’]
dithiophen-2-yl)-5,6-difluorobenzo[c] [1,2,5]thiadiazol-4-yl)-4-(2-
1
0
0
ethylhexyl)-4H-dithieno[3,2-b:2 ,3 -d]pyrrole (SMD3)
13
(
d, J ¼ 7.2 Hz, 2H), 7.81 (s, 2H), 8.59 (s, 2H); C NMR (100 MHz,
CDCl ppm): 11.24, 11.36, 14.07, 14.13, 23.01, 23.08, 23.71, 23.79,
9.09, 29.20, 30.29, 30.49, 40.51, 40.58, 75.410, 75.54, 111.69, 115.44,
19.93, 121.17, 123.23, 123.90, 125.61, 126.66, 128.36, 129.81, 131.77,
32.32, 138.30, 143.72, 144.69, 146.8, 146.66, 151.77, 152.32; MALDI-
It is synthesized following the procedure for SMD1 synthesis,
3
,
d
but by using 3 and F2BT-OBDT as precursors (yield 76%).
1
2
1
1
H NMR (400 MHz, CDCl3, d ppm): 1.01e1.05 (m, 18H), 1.06e1.10
(m, 12H), 1.42e1.51 (m, 26H), 1.57e1.66 (m, 10H), 1.65e1.73 (m, 8H),
1.87e1.94 (m, 1H), 3.76e3.91 (m, 2H), 3.93e3.99 (m, 8H), 6.78 (d,

4
E. Appalanaidu et al. / Journal of Molecular Structure 1210 (2020) 128019
J ¼ 50.8 Hz, 4H), 7.57 (s, 2H), 8.14 (s, 2H): 13C NMR (100 MHz, CDCl
,
3
d
ppm): 10.81, 11.42, 11.57, 14.31, 23.18, 23.22, 23.87, 23.97, 28.81,
2
7
9.31, 29.4, 29.71, 30.41, 30.66, 30.70, 40.70, 40.77, 75.12, 75.30,
7.51, 109.41, 112.11, 112.20, 112.61, 112.72, 118.10, 118.27, 119.61,
123.25, 123.36, 125.71, 129.10, 129.84, 130.71, 131.10,131.33, 131.84,
1
C
43.37, 144.67, 146.68, 147.81; MALDI-MS (m/z): Calc. for
þ
þ
80 93 4 5 4 8
H F N O S is [M] : m/z 1519.49 Found: 1519.34 [M] , 1520.39
þ
[MþH] .
3
. Results and discussion
3.1. Solution processability and thermal stability
The compounds SMD1, SMD2 and SMD3 are soluble (~25 mg/
mL) in a number of organic solvents like o-dichlorobenzene, chlo-
roform, dichloromethane (DCM), tetrahydrofuran (THF), acetoni-
trile and toluene. This shows the ease of their solution
processability.
Tough resistance to decomposition at high temperatures maps
the functioning of a material to be employable in actual applica-
tions. Hence, the thermal stability of SMD1, SMD2 and SMD3 are
scrutinized by TGA (thermogravimetric) analysis. The molecules
SMD1, SMD2 and SMD3 showed optimum thermal toughness with
Fig. 2. TGA curve of molecules SMD1, SMD2 and SMD3.
absorption blue shifts as the number of fluorine atoms increase
from SMD1 to SMD3 and it is consistent with earlier observations
7,8,21,47]. The amount of blue shift from SMD1 to SMD2 is 41 nm,
[
d
T (decomposition temperature; at which 5% weight loss occurs) of
ꢀ
ꢀ
ꢀ
while that from SMD1 to SMD3 is 54 nm. As the number of fluorine
atom increases; they withdraw more electron density from highest
occupied molecular orbital (HOMO) and stabilize HOMO, while
lowest unoccupied molecular orbital (LUMO) levels are minimally
affected by the presence/increased number of fluorine atoms. The
HOMO of molecule SMD2 (having two fluorine atoms) is stabilized
by 0.01 eV (Table 1) as compared to that of SMD1. The HOMO of
molecule SMD3 (having four fluorine atoms) is stabilized by 0.07 eV
as compared to that of SMD1. Also, the LUMO levels of SMD2 and
SMD3 are stabilized by 0.01 eV and 0.03 eV respectively as
compared to that of SMD1. Hence, it is clear that the increased
number of fluorine atom has lowered the energies of HOMOs to a
higher extent as compared to that of LUMOs. This has caused the
widening of the energy gap as the number of fluorine atom in-
creases (Table 1). This accounts for the occurrence of blue-shifted
absorption maxima.
3
29 C, 320 C and 244 C respectively (Table 1). TGA curves for
SMD1, SMD2 and SMD3 are given in Fig. 2.
3.2. Photo-physical and electrochemical traits
The absorption and fluorescence features of SMD1, SMD2 and
SMD3 are measured in acetonitrile. The spectra of
ultravioletevisible absorption and emission are presented in Fig. 3a
and Fig. 3b respectively and the spectral data has been shown in
Table 1. In these molecules, the terminal OBDT donor is conjugated
to the acceptor BT which in turn is conjugated to secondary donor
DTP. This conjugation strengthens electronic coupling amid
electron-donating and electron-receiving fragments of a molecule.
This gives way to broad as well as high-intensity optical absorption.
Absorption spectra of all three donors are alike in the way they
show two broad characteristic bands with good intensity; one band
The emission properties of the molecules are measured in
acetonitrile (excitation wavelength at 600 nm) and are presented in
Table 1, from which it can be observed that emission maxima of the
molecules SMD1, SMD2 and SMD3 are 730 nm, 730 nm and 710 nm
respectively and are least affected by fluorine substitution. The
quantum yields for these compounds are also measured in cyclo-
in the 300e425 nm region, which arises as a result of
p/p*
excitation of electrons in the conjugated molecular framework and
another in 450e750 nm region, which arises because of charge
transfer within molecule (intramolecular charge transfer among
the electron-donating and electron-receiving fragments of
molecule).
hexane by using anthracene as standard (
quantum yields for the molecules SMD1, SMD2 and SMD3 are
.052, 0.107 and 0.072 respectively (excitation wavelength for
F
f
¼ 0.9). The measured
From Table 1, it can be noticed that the wavelength of maximum
0
Table 1
quantum yields for the molecules SMD1, SMD2 and SMD3 are
524 nm, 511 nm and 512 nm respectively).
Cyclic voltammetry (CV) experiments are performed in order to
examine electrochemical properties of SMD1, SMD2 and SMD3 and
the resulting CV plots are provided in Fig. 4. The energies of frontier
ꢀ
The decomposition temperature (T
in nm), molar extinction coefficient (ε in L M cm ) and emission maximum (
nm) of molecules SMD1, SMD2 and SMD3 recorded in CH
tration. Cyclic voltammetry measurements are performed in 1 mM dichloromethane
solution with 0.1 M of Bu NClO as a supporting electrolyte.
d
in C), experimental absorption maximum (
l
exp
ꢂ1
ꢂ1
l
flu in
ꢂ5
3
CN at 10 M concen-
4
4
g
molecular orbitals (HOMO and LUMO) and bandgap (E ) of SMD1,
a
Molecule
T
d
Photo-physical
properties
Electrochemical properties
F
f
SMD2 and SMD3, which are derived from CV experiments can be
found in Table 1. In the light of data obtained in Table 1, the energy
gap of molecules vary as follows; SMD3>SMD2 ¼ SMD1.
y
z
z
E
g
l
exp
V
l
flu
E
ox
*
E
lumo
E
homo
5
SMD1
SMD2
SMD3
329 598 0.25 ꢁ 10 730 0.46
ꢂ3.28 ꢂ5.16 1.88 0.052
5
320 557 0.17 ꢁ 10 730 0.447 ꢂ3.29 ꢂ5.17 1.88 0.107
3.3. Density functional theory (DFT) studies
5
244 544 0.16 ꢁ 10 710 0.53
ꢂ3.31 ꢂ5.23 1.92 0.072
z
y
*
(
E
oxis the onset potential of oxidation in Volts. Ehomo ¼ e (5.1 þ Eox) in eV. Elumo
¼
The DFTand TD-DFT (time dependent density functional theory)
ɸ
E
g
þ Ehomo) ineV. E
g
is the optical band gap in eV, which was estimated from the
a
methods are utilized for all the theoretical studies. Calculations for
SMD1, SMD2 and SMD3 molecules are carried out with the help of
the Gaussian16 W package [48].
intersection between the absorption and emission spectra.
f
F Fluorescence quantum
yield measured in cyclo hexane, relative to 9,10-diphenyl anthracene (
cyclohexane).
F
f
¼ 0.9 in

E. Appalanaidu et al. / Journal of Molecular Structure 1210 (2020) 128019
5
Fig. 3. (a) The UVeVisible spectrum of SMD1, SMD2 and SMD3 measured at 10
SMD3 measured in acetonitrile at 10 M concentration.
mM concentration in acetonitrile, (b) The fluorescence spectrum of molecules SMD1, SMD2 and
ꢂ4
CAM-B3LYP [51] is the range separated functional with short-range
HF exchange [Coulomb attenuated version of B3LYP (CAM-B3LYP)
consisting of 19% HF exchange at short range with 65% HF exchange
at long range]. The calculated absorption maxima (
strength (f ), major electronic transitions [(MT) ] and mixing co-
efficient [(%C ] are presented in Table 2. Fig. 6 presents the
G
l ), oscillator
G
G
i G
)
normalized theoretical absorption spectrum of SMD1, SMD2 and
SMD3 in the gas phase.
The experimental absorption (lexp) of molecules SMD1, SMD2
and SMD3 are 598 nm, 557 nm and 544 nm (Table 1) respectively;
while their theoretical absorptions are 571 nm, 532 nm and 520 nm
(Table 2) respectively, which indicates a fair agreement of theo-
retical results with experimental ones within the limitation. The
experimentally observed trend of increasing blue shift as we move
from SMD1 to SMD3 is also followed in theoretical absorption
properties.
The theoretical absorption spectrum of SMD1, SMD2 and SMD3
in the gas phase shows two absorption peaks similar to their
experimental absorption spectrum. The absorption maxima with
the highest oscillator strength in each molecule correspond to the
major electronic transition from HOMO / LUMO. With respect to
this absorption peak, one can notice that the theoretical blue shift
from SMD1 to SMD2 is 39 nm and that from SMD1 to SMD3 is
Fig. 4. Cyclic voltammetry plots of molecules SMD1, SMD2 and SMD3 measured in
1
4 4
mM dichloromethane solution with 0.1 M of Bu NClO as a supporting electrolyte.
3.3.1. Geometry optimization
51 nm, which closely mimics the trend observed experimentally as
The ground-state structure optimization of molecules SMD1,
SMD2 and SMD3 is accomplished by employing B3LYP functional
49,50] with a 6-31G (d, p) basis set in the gas phase. The presence
discussed in section 3.2. The absorption maxima (with lower
oscillator strength) found between 300 nm and 350 nm in each
molecule corresponds to electronic transitions between different
energy levels (mentioned in Table 2).
Further, computations of the wavelength of maximum absorp-
tion of SMD1, SMD2 and SMD3 in acetonitrile (Table S1) are done at
the same level of theory using the IEFPCM method (Integral
Equation Formalism Variant of the Polarisable Continuum Model)
[
of local minima on potential energy curves for each molecule as
well as the absence of imaginary frequencies confirmed the sta-
bility of molecules in their ground states. Replacement of lengthy
and branched alkyl groups with a simple methyl group is done in
order to facilitate computational methods. The molecules SMD1,
SMD2 and SMD3 are planar. The dihedral angle between each
donor and acceptor is calculated to be less than 13⁰ (Fig. S40). The
front view and side view of the planar structures are provided in
Fig. 5.
[52]. The calculated absorption in the acetonitrile solvent is in
agreement with the experimental results (Table S1).
3.3.3. Frontier molecular orbitals (FMOs)
The examination of frontier molecular orbitals is conducted to
3
.3.2. Theoretical absorption properties
UVeVis characteristics of molecules SMD1, SMD2 and SMD3 are
comprehend the distribution of electron density in HOMO and
LUMO of molecules. The frontier molecular orbitals of the mole-
cules SMD1, SMD2 and SMD3 are pictured at CAM-B3LYP/6-31G (d,
p) level of theory (Fig. 7). From FMO analysis, it is very clear that the
electron density is spread majorly on DTP donor and to a lesser
extent on BDT donor in the ground state (in HOMO). Upon elec-
tronic excitation, one can see that electron density shifts majorly on
acceptor BT core (in LUMO).
screened by treating molecules with TD-CAM-B3LYP/6-31G (d, p)
method in the gas phase. The TD-CAM-B3LYP/6-31G (d, p) method
is applied on molecules optimized in their ground state in the gas
phase by B3LYP functional using 6-31G (d, p) basis set. CAM-B3LYP
is a hybrid functional, which is believed to yield good results of
properties of organic molecules with elongated
p-conjugation.

6
E. Appalanaidu et al. / Journal of Molecular Structure 1210 (2020) 128019
Fig. 5. Optimized ground state structures consisting of both front view and side view of molecules SMD1, SMD2 and SMD3 computed at B3LYP/6e31 G(d,p) level of theory in gas
phase.
3
.3.4. Reorganization energy, ionizing potential and electron
affinity
Reorganization energy (lre) screens the applicability of mole-
Table 2
The theoretical absorption maximum (
electronic transitions [ (MT)
molecules SMD1, SMD2 and SMD3 calculated at CAM-B3LYP/6-31G (d, p) level of
theory in gas phase.
l
G
in nm), oscillator strength (G
G
), major
] of
G
] and mixing co-efficient in gas phase [ (%C
i
)
G
cules as hole or electron transport materials and also the ease of
achieving a balance between electron and hole injections. In simple
Molecule
l
G
f
G
(MT)
G
(%C
i
)
G
terms, lre quantifies the energy changes occurring in a system after
a molecular structure relaxes on account of exceeding negative and
positive charges. The Marcus theory [53,54] shows the dependence
of the charge transfer rate (Kct) on reorganization energy as in
equation (1).
SMD1
571
2.35
0.86
0.25
2.20
0.82
0.54
2.33
0.99
0.24
H / L
84.94
41.43
22.01
86.05
26.77
37.97
83.17
39.35
65.45
3
3
27
42
H / Lþ2
H-3 / Lþ1
H / L
H-5 / L
H / Lþ2
H / L
SMD2
SMD3
532
3
3
18
28
!
ꢀ
ꢁ
1
2
ꢀ
ꢁ
V2
h
p
ꢂ
l
re
520
K
ct
¼
exp
(1)
3
3
16
25
H / Lþ2
l
reTkB
4Tk
B
H-3 / Lþ1
In equation (1), k , T, lre and V are Boltzmann constant, tem-
B
perature, reorganization energy and integral of intermolecular
charge transfer respectively. It is evident from equation (1) that

E. Appalanaidu et al. / Journal of Molecular Structure 1210 (2020) 128019
7
Table 3
The hole reorganization energy (
l
H
, in meV), electron reorganization energy (
E
l , in
a a
meV), adiabatic ionization potential (IP , in eV) and adiabatic electron affinity (EA ,
in eV) of SMD1, SMD2 and SMD3 calculated at B3LYP/6-31G(d,p) level of theory.
Molecule
l
H
l
E
IP
a
EA
a
SMD1
SMD2
SMD3
184
190
175
167
171
187
5.54
5.54
5.66
2.10
2.13
2.22
n
n
l
l
l
¼ E ðg
c
Þ ꢂ E ðg
n
Þ
Þ
Þ
2
a
a
¼ E ðg
n
Þ ꢂ E ðg
a
3
4
n
n
¼ E ðg
a
Þ ꢂ E ðg
n
n
where E (g
n
) is the energy of the neutral molecule in neutral ge-
c/a
ometry, E (gc/a) is energy of ion (cation/anion) in ionic(cation/
anion) geometry, E (g ) is energy of ion (cation/anion) in neutral
n
geometry and E (gc/a) is the energy of the neutral molecule in ionic
(cation/anion) geometry.
c/a
n
Fig. 6. Calculated absorption spectrum of molecules SMD1, SMD2 and SMD3 calcu-
lated at CAM-B3LYP/6-31G(d,p) level in gas phase.
From, equation (1) it is obvious that charge transfer rate and
reorganization energy bear inverse relationship. Hence molecules
having lesser reorganization energy possess superior charge
charges transfer rate is majorly dependent on lre and V. Reorgani-
zation energy is composed of internal and external reorganization
energies. Internal reorganization energy is influenced by intra-
molecular vibrations, while external reorganization energy in-
volves the impact of the surrounding medium. One can ignore the
values of external reorganization energy as they are pretty lower
than that of internal reorganization energies [55]. Hence, we have
computed internal reorganization energies [complete internal hole
transfer characteristics. The
SMD3 are calculated and are presented in Table 3.
As compared to the standard hole transport material TPD [N,N -
lre for molecules SMD1, SMD2 and
0
0
0
0
diphenyl- N,N -bis(3-methylphenyl)-(1,1 -biphenyl)-4,4 -diamine],
which exhibits
¼ 290 meV [56]; the molecules SMD1, SMD2 and
SMD3 display lower values implying that they can be good hole
transport materials. The order of variation of is as follows:
l
H
l
H
l
H
H E
reorganization energy (l ) and electron reorganization energy (l )]
SMD1 ySMD2 > SMD3. Further evaluation of electron transport
energies of SMD1, SMD2 and SMD3 reveals that they show lower
as shown in equation (2) and equation (3).
values of
minum(III),
material. Hence SMD1, SMD2 and SMD3 can also be better electron
transport materials. The trend of is as follows:
SMD1 ySMD2 < SMD3. It is concluded that the reported molecules
act as both hole and electron transport materials.
l
E
as compared to Alq3 [tris(8-hydroxyquinolinato)alu-
l
E
¼ 276 meV] [57], which is a typical electron transport
l
l
¼
¼
l
þ
þ
l
2
(2)
(3)
H
1
l
E
l
3
l
E
4
c
c
where
l
1
¼ E ðg
n
Þ ꢂ E ðg
c
Þ
The impact of fluorine substitution on hole and electron
Fig. 7. The frontier molecular orbitals of SMD1, SMD2 and SMD3 molecules as calculated at CAM-B3LYP/6-31G(d,p) level.

8
E. Appalanaidu et al. / Journal of Molecular Structure 1210 (2020) 128019
transport properties can be comprehended as: i) the absence of
strong electron-withdrawing fluorine substitution has caused
SMD1 to show the better electron-transport property, and ii) the
four fluorine atoms substituted SMD3 molecule majorly transports
charge by hole mechanism.
Hence, this combined experimental and DFT study points to-
wards the possibilities of applicability of molecules SMD1, SMD2
and SMD3 in the construction of OBHJSCs and in organic electronic
materials.
Since the difference between
l
H
and
l
E
of each of SMD1, SMD2
Funding
and SMD3 is less than 24 meV, one can expect them to strike a very
good balance between the hole and electron transfer rates and act
as ambipolar molecules.
Funding was received for this work.
Declaration of competing interest
To construct a competent optoelectronic device, it is necessary
to verify the efficiency of the injection of electron and hole into the
organic material. The factors such as ionization potential (IP) and
electron affinity (EA) influence the barrier of energy for the injec-
tion of electrons and holes. Ionization potential is the energy pro-
vided to the system to take its electron away (lesser the IP greater
will be the ease of injection of the hole into HOMO of the molecule).
Electron affinity is the energy unleashed upon the addition of an
electron to the system (higher the EA better will be the injection of
an electron into LUMO of the molecule). Hence, we have calculated
IP (adiabatic) and EA (adiabatic) of SMD1, SMD2 and SMD3 at
B3LYP/6-31G (d, p) level of theory and the result are shown in
Table 3, from which we can comprehend that SMD1, SMD2 and
SMD3 have sufficiently lower values of IP and higher values of EA;
so that they can be employed in organic electronic materials.
The trend of variation of IP and EA both, which are influenced by
fluorine atom inclusion, is as follows: SMD1 ySMD2 < SMD3. The
incorporation of four strongly electron-accepting fluorine atoms
has increased the difficulty of removing an electron from SMD3 by
No conflict of interest exists. We wish to confirm that there are
no known conflicts of interest associated with this publication and
there has been no significant financial support for this work that
could have influenced its outcome.
CRediT authorship contribution statement
Ejjurothu Appalanaidu: Methodology, Data curation, Formal
analysis, Writing - review & editing. V.M. Vidya: Formal analysis,
Writing - original draft, Writing - review & editing. Manohar
Reddy Busireddy: Methodology, Data curation, Formal analysis,
Writing - review & editing. Jayathirtha Rao Vaidya: Conceptuali-
zation, Formal analysis, Resources, Writing - review & editing.
Prabhakar Chetti: Software, Supervision, Validation, Visualization,
Formal analysis, Writing - original draft, Writing - review & editing.
Acknowledgments
0
.12 eV as compared to SMD1 (which has no fluorine substitution),
while the same electron attracting tendency of SMD3 has resulted
in increasing values of EA by 0.12 eV as compared to SMD1. The
influence of two fluorine atoms in SMD2 seems to be minimum and
its EA and IP values are very close to those of SMD1.
EA & BM Reddy acknowledges the CSIR & UGC India respec-
tively, for grant of the research fellowship. VJR thankful to CSIR-
New Delhi for Emeritus Scientist Honour.VVM thanks DST under
WOS-A (SR/WOS-A/CS-46/2017) for the financial assistance. CP is
grateful to SERB - New Delhi, India for financial support (SB/FT/CS-
4
. Conclusions
101/2014).
I
Three organic molecules SMD1, SMD2 and SMD3 having D -A-
II-A-D type architecture are synthesized. The molecules SMD1,
I
Appendix A. Supplementary data
D
SMD2 and SMD3 bear 0, 2 and 4 fluorine atoms respectively, which
are substituted on their acceptor core. The influence casted by
fluorine atoms on the photo-physical and electrochemical traits of
molecules is investigated by means studies of electronic excitations
and cyclic voltammetry. And also, DFT methods helped in appre-
ciating the influence of fluorine atoms on reorganization energies,
ionization potential and electron affinity of molecules. The major
influence of fluorine atoms on properties of molecules presented in
this work can be pointed out as: (a) as number of fluorine atom
increases, there will be more withdrawal of electron density from
HOMOs and hence HOMO levels of molecules go on deepening as
number of fluorine atom increases, (b) the LUMOs of molecules are
not affected significantly by fluorine substitution, (c) increasing
fluorine substitution has increased the HOMO-LUMO gap
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.molstruc.2020.128019.
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