Synthesis, photophysical, electrochemical and computational study of indolocarbazole based donor acceptor type conjugated polymers

Article abstract of DOI:10.1007/s11696-020-01445-2

This research reports the synthesis and characterization of two new donor–acceptor type conjugated polymers carrying indolocarbazole as donor, benzothiadiazole and diketopyrrolopyrrole as acceptor via stille coupling polymerization reactions. The structures of the polymer were established by spectroscopic techniques and polymer molecular weight by gel permeation chromatography. Further the linear optical and electrochemical properties of the polymers have been investigated. Polymers showed broad absorption and good fluorescence behaviour with good quantum yields. Optical bandgaps of the polymers were estimated using UV–visible absorption edge and found to be 2.34 and 2.12?eV respectively. Further, theoretical evaluation of the polymers was carried using density functional theory to investigate their geometrical, electrochemical and optical properties using DFT/B3LYP. The obtained results are in good agreement with the experimental results.

Full text of DOI:10.1007/s11696-020-01445-2

Chemical Papers (2021) 75:1969–1980
https://doi.org/10.1007/s11696-020-01445-2
ORIGINAL PAPER
Synthesis, photophysical, electrochemical and computational study
of indolocarbazole based donor acceptor type conjugated polymers
K. A. Vishnumurthy1 · K. H. Girish2
Received: 11 September 2020 / Accepted: 20 November 2020 / Published online: 5 January 2021
© Institute of Chemistry, Slovak Academy of Sciences 2021
Abstract
This research reports the synthesis and characterization of two new donor–acceptor type conjugated polymers carrying
indolocarbazole as donor, benzothiadiazole and diketopyrrolopyrrole as acceptor via stille coupling polymerization reactions.
The structures of the polymer were established by spectroscopic techniques and polymer molecular weight by gel permeation
chromatography. Further the linear optical and electrochemical properties of the polymers have been investigated. Polymers
showed broad absorption and good fuorescence behaviour with good quantum yields. Optical bandgaps of the polymers were
estimated using UV–visible absorption edge and found to be 2.34 and 2.12 eV respectively. Further, theoretical evaluation
of the polymers was carried using density functional theory to investigate their geometrical, electrochemical and optical
properties using DFT/B3LYP. The obtained results are in good agreement with the experimental results.
Keywords DFT method · HOMO–LUMO · Frontier molecular orbitals
Introduction
transistors (Kim et al. 2020) and chemosensors (Fan et al.
2009; Mahesh et al. 2019). Especially D–A type conju-
gated polymers ofer enhanced fexibility in tuning their
optoelectronic properties through intramolecular charge
transfer (ICT). To alter the bandgap of these polymers,
a remarkable number of donor–acceptor moieties and
their combinations are studied. Among the other donor
moieties, carbazole and its derivatives have been investi-
gated extensively due to their enhanced hole transporta-
tion property and inherent electron-donating nature (Li
and Grimsdale 2010; Janosik et al. 2018). Particularly, the
indolocarbazole (ICZ) based donor moieties(Gokce and
Icli Ozkut 2018) ofer high thermal stability and charge
carrier mobility which is supported by their rigid back-
bone and low HOMO level (Negru and Grigoras 2019).
The fused carbazole ring in ICZ supports the extension
of conjugation length and improves the planarity of the
molecule thereby enhancing the π-π stacking(Suman et al.
2019). Additionally, ICZ derivatives can also act as accep-
tor when coupled with strong donors such as 3,4-propy
lenedioxyhtiophene(Gokce and Icli Ozkut 2018). Many
acceptor moieties have been coupled with the ICZ donor
to enhance the π-conjugation and optoelectronic properties
of the resulting alternating copolymers (Tsai et al. 2009;
Chen et al. 2013; Khetubol et al. 2015). For example, the
incorporation of benzothiadiazole-thiophene core acceptor
Conjugated polymers have become a key player in interdis-
ciplinary research due to their applications across various
felds of science and engineering (Li et al. 2013, 2018;
Zeglio et al. 2019; Jin Cheon et al. 2020). Π-conjugated
polymers show interesting structural, optical, and elec-
tronic properties that can be tuned through various molec-
ular modifcations and synthesizing processes (Chung
et al. 2015; Park et al. 2019). This makes the conjugated
polymers to be promising candidates for various optoe-
lectronic applications such as organic solar cells (OSC)
(Cheng et al. 2009; Zhan and Zhu 2010; Akkuratov et al.
2020), organic light-emitting diode (OLED) (Sekine
et al. 2014; Wong 2017; Zhang et al. 2020), feld-efect
Supplementary Information The online version contains
supplementary material available at https://doi.org/10.1007/s1169
6-020-01445-2.
* K. A. Vishnumurthy
vishnumurthyka@rvce.edu.in
1
Department of Chemistry, RV College of Engineering,
Bangalore, India
2
Department of Mechanical Engineering, RV College
of Engineering, Bangalore, India
Vol.:(0123456789)
1 3

1970
Chemical Papers (2021) 75:1969–1980
with the ICZ donor through the Suzuki cross-coupling
method resulted in a lower bandgap copolymer exhibiting
broad absorption in the range of 300–720 nm (Tong et al.
2015) that is suitable for organic solar cell application.
The benzothiadiazole (BT) acceptor moieties have been
used to obtain low bandgap D–A conjugated polymers for
organic solar cell applications due to their broader solar
light absorption supported from intramolecular charge
transfer (Pei et al. 2011; Ekbote et al. 2018). Additionally,
the diketopyrrolopyrrole (DPP) moiety also ofers greater
thermal stability and photophysical properties that are
essential for organic optoelectronic devices. But, the major
drawback of the DPP core was its insolubility in common
organic solvent due to strong intermolecular hydrogen
bonding. This is overcome by introducing substituents
groups like alkyl and phenyl groups on DPP core (Grzy-
bowski and Gryko 2015; Pop et al. 2019) which enhanced
its solubility in organic solvents. Since this advancement,
DPP has been widely used as acceptor moiety in various
organic solar cells and organic feld-efect transistors for
its high electron-withdrawing efect and excellent charge
carrier mobility facilitated by strong π-π interactions.
In this study, we report the design and synthesis of two
new donor–acceptor copolymers ICBTD and ICDPP through
stille coupling polymerization reactions. The polymers have
indolocarbazole as donor moiety which is incorporated with
benzothiadiazole and diketopyrrolopyrrole acceptor moieties
having a strong electron-withdrawing efect. The obtained
polymers had a fairly planar structure supported by planarity
of the indolocarbazole unit and highly rigid phenyl backbone
which enhanced the conjugation of polymers through π-π
stacking. The optoelectronic properties of polymers such
as HOMO–LUMO, excitation, and emission energies were
characterized using cyclic voltammetry, UV–Vis, and fuo-
rescence spectra analysis. The experimental calculations
were supported by theoretical computations using density
functional theory (DFT) and time-dependant DFT (TDDFT)
studies. The structural and photophysical properties of the
polymers were theoretically evaluated with a monomeric
approximation to limit the computational cost. The TDDFT
based calculations of HOMO–LUMO and excitation ener-
gies showed less sensitivity towards diferent basis sets that
were used for the computation(Majidizadeh Fini et al. 2020).
4,7-dibromo-2,1,3-benzothiadiazole was purchased from
Aldrich.
Instrumentation
1H NMR spectra of compounds were recorded in Bruker
400 MHz NMR spectrometer. UV–Vis absorption and fuo-
rescence emission spectra were recorded with Perkin Elmer
Lambda 25 UV–Vis spectrometer using THF as a solvent.
Autolab-30 was used to carry out electrochemical measure-
ment of polymers where the platinum electrode was used
as a counter electrode, silver–silver chloride electrode was
used as reference electrode and glassy carbon electrode used
as a working electrode. Molecular weight analysis of the
polymer was determined by Waters to make gel permeation
chromatography.
Synthesis and characterization
Indolocarbazole based monomer was synthesized from a
series of reactions. Initially, indole was made to react with
bromo benzaldehyde in presence of hydroiodic acid. Then
aromatization of dihydroindolocarbazole was done using
molecular iodine. Further alkyl chains were introduced to
indolocarbazole to increase the hydrophobicity. In the fnal
step, stannylation was done using tributyltinchloride. Syn-
thesis of indolocarbazole based monomer 4 is depicted in
Fig. 1. Further diketopyrrolopyrole based monomer was
synthesized from the reaction of 2 methyl 2 butanol with
bromobenzonitrile in presence of potassium tert butoxide.
Obtained diketopyrrolopyrrole was alkylated with bromooc-
tane to get soluble monomer 6. Then the obtained monomer
6 was made to react with monomer 4 to get polymer ICDPP.
On similar lines, ICBTD polymer was synthesized from
monomer 4 and dibromobenzothiadiazole.
12‑bis(4‑bromophenyl)‑5,6,11,12‑tetrahydroindolo[3,2‑b]
carbazole 1
5 g of indole(0.0426 mol) and 7.89 g of p-bromo benzal-
dehyde was dissolved in 100 ml of acetonitrile. HI (57%)
(1.08 g, 0.00852 mol) was slowly added into the round bot-
tom fask at room temperature. Then continued the stirring
for about 1 h then reaction temperature was raised to 80 °C
for about 20 h. Completion of the reaction was monitored by
TLC(Thin layer chromatography). Then the reaction mass
was cooled to room temperature and the obtained solid was
fltered and washed with cold acetonitrile and dried under
vacuum. The resulting solid was directly taken for next step
without any further purifcation. During the reactions some
portion of the compound was aromatized to next compound
hence further analysis was not done.
Procedure
Materials and chemicals
All the solvents and chemicals are purchased from
Aldrich and directly used without any further purifica-
tions. One of the monomer for preparation of polymer,
1 3

Chemical Papers (2021) 75:1969–1980
1971
Fig. 1 Synthesis of indolocarbazole monomer 4
12‑bis(4‑bromophenyl)‑5,11‑dihydroindolo[3,2‑b]carba‑
zole 2
then extracted with chloroform and further dried to obtain
the crude product. The crude product was purifed by col-
umn chromatography using hexane: ethyl acetate system
(1:2). Final product was yellowish solid with a yield of 79%.
1H NMR (CDCl3, 400 MHz, ppm): 7.79 (d, 4H (aromatic)),
7.6 (d, 4H, aromatic), 7.4 (t, 2H, aromatic), 7.30 (d, 2H),
6.90 (t, 2H), 6.63 (d, 2H), 3.82(t, 4H, N-CH2), 1.54 (t, 4H),
1.25 (m, 20H, alkyl protons), 0.91 (t, 6H terminal CH3).
The synthesized dihydroindolocarbazole 1 was dissolved
in 60 ml of acetonitrile and 0.453 g of iodine was added
then reaction mixture was heated at 80 °C for 20 h. Comple-
tion of the reaction was ensured by TLC. Then the solvent
was removed under reduced pressure and the obtained pre-
cipitate was washed with cold acetonitrile and dried under
vacuum. This crude product was purifed by column chro-
matography using hexane:ethyl acetate (1:4) system. Yellow
coloured solid was obtained as a product with a yield of
48%. The solid product did not melt upto 300 °C and above
11‑dioctyl‑6,12‑bis(4‑(tributylstannyl)phenyl)‑5,11‑dihydro
indolo[3,2‑b]carbazole 4
1
this temperature it started charring. H NMR (DMSOd6,
Alkylated indolocarbazole was dissolved in dry THF and
then cooled to -78 °C using dry ice and acetone under argon
atmosphere. Slowly n-butyl lithium (1.6 M) was added drop-
wise over a period of 30 min and the reaction was stirred for
1 h and then tributyltin was added at a stretch. Reaction was
continued for about 3 h. After the completion of the reaction
was quenched with crushed ice and then extracted with die-
thyl ether. Solvent was removed under reduced pressure and
the product was washed repeatedly with cold methanol to
remove excess tributyltin. The stannylated manomer was not
characterized because of its instability in room temperature.
Hence it is kept in a refrigerator and used in the reaction.
Also it was observed that decomposition of the sample under
300 MHz) δ 6.98–6.99(m, 2H), 7.34–7.36(m, 6H), 7.66 (d,
4H), 7.82–7.87(m, 6H (including NH proton).
12‑bis(4‑bromophenyl)‑5,11‑dioc‑
tyl‑5,11‑dihydroindolo[3,2‑b]carbazole 3
Indolocarbaozle 2 (3.5 g, 6 mmol) and 1-bromooctane
(4.7 g, 6 mmol) was slowly mixed in 100 ml of dry Dimethyl
formamide (DMF) then powdered NaOH (1.5 g, 35 mmol)
was added into it under vigorous stirring. Reaction tempera-
ture was raised to 100 °C and continued for 16 h in an inert
atmosphere. The reaction mass was poured to crushed ice
1 3

1972
Chemical Papers (2021) 75:1969–1980
air for long exposure. The resulting yellow syrupy product
The crude product was purifed by column chromatography
was taken to polymerization without further purifcation.
using hexane:Ethylacetate (1:5) system with a low yield of
1
35%. H NMR (300 MHz, CDCl3): δ is 7.6 (m, 8H, Aro-
Synthesis of 3,6‑bis(4‑bromophenyl)‑2,5‑dihydropyrrolo[3,
4‑c]pyyol‑1,4‑dione (5)
matic), 3.68 (t, 4H, N-CH2), 1.22 (m, 16H, CH2), 0.9 (t, 6H
CH3) (Fig. 2).
Potassium tert-butoxide (4 g, 35.6 mmol) was completely
dissolved in 2 methyl-2-butanol in inert atmosphere at 0 °C.
4-bromo benzonitrile (7.5 g, 41 mmol) was added slowly
into the reaction mixture and the temperature was slowly
raised to 98 °C for 40 min. Then reaction temperature was
lowered to 60 °C and dimethyl succinate was added drop
wisely into flak with constant stirring over a period of
2 h, and the temperature was again increased to 98 °C for
about 4 h. Then the reaction was cooled to 60 °C and 50 ml
of methanol was added and the resulting suspension was
fltered while it was in hot condition. The flter cake was
washed with methanol to remove impurities. Then the crude
product was recrystallized using hot methanol and the prod-
uct was dried under vacuum. The product observed was a red
coloured solid with a low yield of 62%. FT-IR (KBr pellet,
cm−1): 3145 (Ar‒H stretching), 1644 (C–O), 1608 (aromatic
Synthesis of ICBTD polymer
Indolocarbazole monomer 4 was ( 2 g, 0.00177 mol) dis-
solved in dry toluene and completely purged with argon.
Dibromobenzothiadiazole (0.521 g, 0.00177 mol)was dis-
solved in dry toluene and added dropwise to the above reac-
tion and purging was continued for 30 min. Then a pinch of
palladium tetrakis tryphenylphosphene was added and purg-
ing is continued for 30 min. The colour of the solution tuned
in to dark orange red. Slowly reaction temperature was raised
to 110 °C stirring was continued for 24 h. Completion of the
reaction was ensured by Thin layer chromatography. The
solvent was distilled of and the resulting crude gelly mass
was reprecipitated in methanol. The obtained crude prod-
uct was purifed using Soxhlet extraction using hexane, and
methanol. Finally, chloroform soluble fraction was taken and
reprecipitated in methanol, fltered and dried under vacuum
to obtain ICBTD polymer (Fig. 3). The obtained yield of the
polymer was found to 55%. 1H NMR (300 MHz, CDCl3): δ
δ, 7.42–6.78 (m, 18H (aromatic), 3.89 (m, 4H, (–NCH2–)),
1.95–1.27 (m, 24H (–(CH2)6–), 0.94–0.9 (t, 6H) (supplemen-
tary information). The gel permeation chromatography was
done in THF solvent the weight average molecular weight
was found to be, Mw=12,800 Da with a polydispersity of
1.9.
1
C–C), 1488, 1448, 1327, 1191, 1141, 816, 698 cm−1. H
NMR (400 MHz, ppm): 7.72–7.75 (m, 8 aromatic protons).
Synthesis of 3,6‑bis(4‑bromophenyl)‑2,5‑dihexylpyrrolo[3,
4‑c]pyrrole‑1,4(2H,5H)‑dione 6.
Synthesized diketopyrrolopyrrole 3 (4 g, 0.08969 mol) was
mixed with potassium tertbutoxide (2.2 g, 0.019 mol) in dry
N-methyl pyrolidinone (30 ml). Reaction temperature was
raised to 60 °C for 1 h followed by the addition of bromo
octane drop by drop. Stittring was continued for about 24 h.
After the completion of the reaction, it was poured to water
and was extracted with ethyl acetate and dried with sodium
sulphate. The solvent was removed under reduced pressure.
Synthesis of ICDPP polymer
Monomer 4 (2 g, 0.00177 mol)and diketopyrrolopyrrole
monomer 6 (1.09 g, 0.00177 mol) were dissolved in dry
Fig. 2 Synthesis of diketopyrrolopyrrole monomer 6
1 3

Chemical Papers (2021) 75:1969–1980
1973
Fig. 3 Synthesis of ICBTD polymer
toluene and completely purged with argon atmosphere at
room temperature. Then slowly palladium tetrakis triph-
enylphosphine palladium(0)(4 mol %)added into fak then
purging is continued for another 30 min. The colour of the
polymer was changed to dark red. Then reaction temperature
was raised to 110 °C and continued for 24 h. After comple-
tion of polymerisation fask was cooled to room temperature
solvent was stripped of and resulting solid was purifed in
soxhlet extraction with hexane, methanol solvent (each of
24 cycles) and fnally chloroform fraction was taken and
concentrated and resulting solid was precipitated in metha-
nol and dried under vacuum (Fig. 4). Yield of the polymer
Computational details
The structure and optical properties of the synthesized poly-
mers were examined theoretically utilizing Density Func-
tional Theory (DFT) in ORCA 4.0.1 program (Neese 2012).
To limit the computational cost of the calculation, single
repeating units of the polymers capped with hydrogen atoms
were considered for all computations. The alkyl chains on
the indolocarbazole and diketopyrrolopyrrole unit were
replaced with methyl groups. The structure of the polymers
ICBTD and ICDPP were established through geometry opti-
mizations performed using Becke’s three-parameter hybrid
functional (Becke 1993) with the Lee et al. (1988) correla-
tion functional (B3LYP) with the Pople basis set, 6-31G(d)
(Hehre et al. 1972) in gas phase considering default conver-
gence criteria and gradient techniques. To confrm the min-
ima of the obtained structures, analytical frequencies were
calculated at the same level of theory. Further, the optical
properties of the polymers like the HOMO–LUMO energy
gap, electronic excitations, and oscillator strengths (f) were
evaluated using Time-Dependent Density Functional Theory
1
was observed to be 48%. H NMR (300 MHz, CDCl3): δ
1H NMR (300 MHz, CDCl3): δ, 7.48–6.89 (m, 24H (aro-
matic)), 4.09, (m, 4H, (–NCH2–) diketopyrrolo pyrrole),
-3.89 (m, 8H, (–NCH2–) indolocarbazole), 2.19–1.27 (m,
40H (–(CH2)6–), 0.94–0.90 (m, 12H)(supplementary infor-
mation). The Gel permeation chromatography study to deter-
mine the molecular weight of the polymer in THF solutions,
and it was found to be Mw=12,800 Da with a polydispersity
of 1.9.
Fig. 4 Synthesis of ICDPP polymer
1 3

1974
Chemical Papers (2021) 75:1969–1980
(TDDFT). The calculation of vertical singlet transitions and
corresponding optical absorption simulations were consid-
ered at diferent basis sets such as 6-31G(d), cc-pVTZ and
def2-TZVP in gas phase using the ground state optimized
geometries.
recorded in thin flms were highly emissive with emission
maxima of 565 nm and 486 nm respectively (Fig. 6). Also
both the polymers showed a good stoke shift of 60 nm with
good quantum yields which are further detailed in Table 1.
This suggests that the polymers ICBTD and ICDPP may be
a good candidate for light-emitting applications.
Data, value and validation
Electrochemical study of the polymer
Optical studies of the polymers
The electrochemical studies of the polymer were studied
using cyclic voltammeric techniques using three-electrode
systems. The conductive polymer films were coated on
glassy carbon button electrodes by evaporating solution of
polymer on top of the electrode by drop-casting method.
The cyclic voltametric measurement was done using 0.1 M
Tetrabytylammonium tetrafuoroborate solutions in acetoni-
trile solvent with a scan rate of 50 mV/s(Shivashankar et al.
2013). The argon was purged with acetonitrile prior to meas-
urement to ensure the absence of moisture. The obtained
electrochemical data are represented in Table 2. In oxidation
cycle, polymers ICBTD and ICDPP showed oxidation peaks
around 0.61 eV and 0.64 eV. This indicates that polymer
can act as a good hole transport layer when it is used in
polymer light-emitting material. However, there is no sharp
peak during reduction cycle due to high rigidity of strong
UV–visible spectra and fuorescence emission spectra of
the polymers were recorded in thin flms. Polymers were
dissolved in teterahydrofuran (THF) and then spin coated
on a quartz glass plate and dried under ambient conditions
to prepare respective thin flms of polymers. Absorption
maxima of ICPBTD and ICPDPP polymers were found to
be 498 nm and 427 nm respectively (Fig. 5). The ICDPP
polymer showed broad absorption spectra when compared
to that of ICBTD polymer along with 71 nm redshift in
absorption maxima. This is mainly due to the presence of
strong electron accepting nature of diketopyrrolopyrrole
molecules. The red shift in the absorption maxima of poly-
mer ICPDPP compared to other polymer is mainly attrib-
uted to the incorporation of highly electron accepting nature
of diketopyrrolopyrrole moiety. These results suggest that
diketopyrrolopyrrole molecule is strong electron acceptor
when compared to benzothiadiazole moiety. Also the ICDPP
polymer showed red-shifted absorption maxima with a broad
absorption spectra indicating its suitability in polymer solar
cells. Optical bandgaps were calculated using absorption
band edges and were found to be 2.34 eV and 2.12 eV
respectively for polymers ICBTD and ICDPP. Further, fuo-
rescence emission spectra of a polymers ICBTD and ICDPP
Fig. 6 Fluorescence emission of polymer ICBTD and ICDPP in THF
solvent
Table 1 Optical characterization of polymers
Polymers Absorp-
Emission Stokes
Fluores-
cence
Optical
tion
maxima
(λemi) in
nm
shift in
nm
band gap
maxima
(λmax) in
nm
quantum (eV)
yield (%)
ICBTD
ICDPP
427
498
486
565
59
67
38
42
2.34
2.12
Fig. 5 UV–Vis spectra of polymers ICBTD and ICDPP in Thin flm
1 3

Chemical Papers (2021) 75:1969–1980
1975
Table 2 Electrochemical
Polymers E(V) oxidation E(V) reduction Eonset(V) Eonset(V) HOMO in eV LUMO In eV Eg in eV
characterization of polymers
oxida-
tion
reduc-
tion
ICBTD
ICDPP
0.61
0.64
− 1.93
− 2.07
0.480
0.416
− 1.5
− 1.2
− 4.88
− 4.81
− 2.9
− 3.2
1.98
1.61
electron-accepting benzothiadiazole and diketopyrrolopyr-
role as depicted by cyclic voltammetry curves in Fig. 7.
The onset oxidation and onset reduction potentials
were used to calculate the highest occupied molecular
orbital(HOMO) and lowest unoccupied molecular orbitals
(LUMO). The calculation of HOMO and LUMO energy lev-
els of the polymer was performed according to literature(de
Leeuw et al. 1997) by comparing the results with ferrocine
standard(Udayakumar and Adhikari 2006). HOMO of the
polymers ICBTD and ICDPP were found to be − 4.88 eV
and − 4.81 eV respectively. The close proximity in the
HOMO values is mainly due to the presence of indolocar-
bazole moiety in the polymer backbone. This HOMO val-
ues indicate that the indolocarbazole monomer is a good
donor molecule. Further, the LUMO values of the polymers
ICBTD and ICDPP were found to be − 2.9 eV and − 3.2 eV
due to the presence of benzothiadiazole and diketopyrro-
lopyrrole molecules. Thus, HOMO and LUMO values of
the polymers indicate better electron injection ability when
they are used in LED applications.
to reduce the computational cost. The respective optimized
geometry structures are depicted in Figs. 8 and 9. The
indolocarbazole moiety in both the optimized geometries
showed high planarity illustrating the existence of efec-
tive π-π stacking. In optimized structure ICBTD, the ori-
entation of indolocarbazole and benzothiadiazole moieties
were found to be slightly nonplanar between the phenyl
spacer, having a dihedral angle with the phenyl backbone
as C1–C2–C3–C4 = − 73.4° and C5–C6–C7–C8 = 45.3°
(see Fig. 8 for atom numbering). However, the opti-
mized structure of ICDPP showed fair planarity between
indolocarbazole and diketopyrrolopyrrole hetero moieties
having a dihedral angle of C1–C2–C3–C4 = − 67.60 and
C5–C6–C7–C8 =38.7° with the phenyl spacer (see Fig. 9 for
atom numbering) thereby enhancing the charge transport
in the system. In polymer ICBTD structure the length of
the C–C bond varied from 1.3709–1.4966 Å and the N–C
bond length from 1.3392–1.4537 Å and in ICDPP struc-
ture the C–C bond length varied from 1.3818–1.4937 Å
and the N–C bond length from 1.3863–1.4568 Å. The
additional optimized geometry parameters are detailed in
the supplementary material (Table S1-S2). Further, the
calculation of analytical frequency showed no imaginary
frequencies for both ICBTD and ICDPP, which indicated
that the polymers were at their local minima. Hence these
optimized geometries were used for further optoelectronic
property calculations.
Molecular geometry
The minimum energy state of the polymer ICBTD and
ICDPP were obtained through geometry optimization per-
formed at the B3LYP/6-31G(d) level in the gas phase. The
single monomer units were considered for all computations
Fig. 7 Cyclic voltammetry of polymer ICBTD and ICDPP
1 3

1976
Chemical Papers (2021) 75:1969–1980
Fig. 8 Optimized structure of
ICBTD at B3LYP/6-31G(d)
level of theory
Fig. 9 Optimized structure of ICDPP at B3LYP/6-31G(d) level of theory
three diferent basis set, 6-31G(d), cc-pVTZ and def2-TZVP
using TDDFT. The isosurface plots of HOMO and LUMO of
both the polymers at TD-B3LYP/6-31G(d) level are visual-
ized in Fig. 10 (rest of the isosurfaces are depicted in the
supplementary material, Figure S1–S2). For ICBTD the
HOMO is localized on indolocarbazole donor moiety and
the LUMO involves the benzothiadiazole acceptor moiety.
Both molecular orbitals depict π type thus illustrating the
probable intramolecular charge transfer from donor to accep-
tor upon excitation (Chung et al. 2020). Similarly, for ICDPP
Frontier molecular orbitals
The analysis of the highest occupied and lowest unoccupied
molecular orbitals (HOMO, LUMO) favors the qualitative
indication of the vertical excitations and the resulting optical
properties of the π-conjugated polymer (Sun and Autschbach
2014). Also, the HOMO and LUMO values and correspond-
ing energy gaps give information about the probable charge
transport in the conjugated system. The frontier molecular
orbitals of polymer ICBTD and ICDPP were obtained with
1 3

Chemical Papers (2021) 75:1969–1980
1977
Fig. 10 Frontier Molecular Orbitals of ICBTD and ICDPP calculated using TDDFT at 6-31G(d) level of theory
Table 3 Orbital energy values and bandgaps of ICBTD and ICDPP
Simulated absorption spectra
calculated using TDDFT
The ground state excitation energies of polymer ICBTD
and ICDPP were calculated employing TDDFT with mul-
tiple basis sets. The theoretical absorption spectrum of the
polymers is visualized in Figs. 11 and 12. Although the
Polymer
ICBTD
Orbitals
6-31G(d)
cc-pVTZ
def2-TZVP
HOMO
LUMO
Eg
− 4.604 eV
− 2.326 eV
2.278 eV
− 4.882 eV
− 2.600 eV
2.282 eV
− 4.912 eV
− 2.645 eV
2.267 eV
ICDPP
HOMO
LUMO
Eg
− 4.647 eV
− 2.348 eV
2.299 eV
− 4.930 eV
− 2.651 eV
2.279 eV
− 4.960 eV
− 2.690 eV
2.270 eV
the HOMO is localized on indolocarbazole moiety and the
LUMO located over diketopyrrolopyrrole moiety involv-
ing phenyl rings as shown in Fig. 10. In both polymers, the
HOMO and LUMO orbitals were well separated over donor
and acceptor moieties indicating efcient charge transport
through the system. The isosurface plots obtained from
other basis sets also depicted similar results. The HOMO
and LUMO energy values of both the polymers obtained
with three diferent basis sets are detailed in Table 3. From
Table 3, the HOMO and LUMO energy values of ICBTD at
6-31G(d) level were found to be − 4.604 eV and − 2.326 eV
having an optical energy gap of 2.278 eV which is close to
the experimental value of 2.34 eV. Similarly, the HOMO and
LUMO values of ICDPP were found to be − 4.647 eV and
− 2.348 eV with an optical energy gap of 2.299 eV that was
comparable with the experimental value of 2.12 eV.
Fig. 11 Theoretical UV–Vis absorption spectrum of ICBTD calcu-
lated using TD-B3LYP
1 3

1978
Chemical Papers (2021) 75:1969–1980
peak value of ICDPP showed a red shift of 55.58 nm. This
indicates the strong electron accepting capability of diketo-
pyrrolopyrrole compared to benzothiadiazole acceptor moi-
ety which makes the polymers suitable for organic solar cell
application which correlates with the experimental fndings.
Conclusions
In summary, two new polymers with a donor–acceptor
structure having indolocarbazole as donor moiety, diketo-
pyrrolopyrrole and benzothiadiazole as acceptor moieties
were synthesized through stille coupling polymerization
reactions. The synthesized polymers showed low bandgap
energy values of 2.34 eV (ICBTD) and 2.12 eV (ICDPP)
aided by the efective donor–acceptor nature indolocarba-
zole donor and of BTD and DPP moieties as acceptors. The
linear optical studies indicated that the polymers show broad
absorption and are highly emissive in the visible region with
good quantum yields indicating their potential applica-
tions in light emitting devices. The electrochemical studies
showed that the polymers had an efcient donor–acceptor
system with their HOMO values closely located. This indi-
cated the better electron injection capability of the polymers.
Further, theoretical studies showed that the polymers had a
fairly planar structure in their optimized state thus enhanc-
ing the conjugation and π–π stacking in the system. The
frontier orbital studies showed notable separation between
HOMO and LUMO orbital densities on donor and accep-
tor indicating the efective charge transfer in the system.
Theoretically calculated HOMO of the polymers were very
close to experimental values. There is no sharp peak in the
reduction cycle of cyclic voltammetry was observed due
to rigidity of acceptors hence experimental LUMO values
Fig. 12 Theoretical UV–Vis absorption spectrum of ICDPP calcu-
lated using TD-B3LYP
S0 →S1 excitations of both ICBTD and ICDPP were domi-
nated by one electron transition from HOMO to LUMO, the
absorption peaks resulted from the contribution of diferent
orbital excitations having the highest oscillator strength. The
absorption wavelengths along with oscillator strengths at
diferent levels of theory are detailed in Tables 4 and 5.
The vertical excitation energies calculated using the def2-
TZVP basis set were fairly close to experimental values. For
ICBTD the absorption peak was located at 393.29 nm at
def2-TZVP level, resulting from the transition of H→L+1
(91%) having the highest oscillator strength of 0.1979. The
absorption peak for ICDPP was found to be 448.87 nm
due to the transition from H-1→L (87%) with an oscilla-
tor strength of 1.1486. Compared to ICBTD the absorption
Table 4 Photophysical
properties of ICBTD calculated
using TD-B3LYP
Level of theory Major transitiona Energy (cm−1
)
Osc. strength (ƒ) ꢀabs(nm)
Exp. Cal.
427 381.28 Singlet
Symmetry
6-31G(d)
cc-pVTZ
def2-TZVP
H
H
H
͢
͢
→L+1 (87%)
→L+1 (91%)
͢→L+1 (91%)
26,227.4
25,497.6
25,426.5
0.2321
0.1957
0.1979
392.19 Singlet
393.29 Singlet
aH=HOMO and L=LUMO
Table 5 Photophysical
properties of ICDPP calculated
using TD-B3LYP
Level of theory Major transitiona Energy (cm−1
)
Osc. Strength (ƒ) ꢀabs(nm)
Exp. Cal.
Symmetry
6-31G(d)
cc-pVTZ
def2-TZVP
H
H
H
͢
͢
͢
-→1 L (86%)
-→1 L (87%)
-→1 L (87%)
22,865.7
22,323.9
22,278.0
1.1655
1.1517
1.1486
498
437.34 Singlet
447.95 Singlet
448.87 Singlet
aH=HOMO and L=LUMO
1 3

Chemical Papers (2021) 75:1969–1980
1979
Hehre WJ, Ditchfeld R, Pople JA (1972) Self—consistent molecular
orbital methods. XII. Further extensions of Gaussian—type basis
sets for use in molecular orbital studies of organic molecules.
J Chem Phys 56:2257–2261. https://doi.org/10.1063/1.1677527
Janosik T, Rannug A, Rannug U et al (2018) Chemistry and proper-
ties of indolocarbazoles. Chem Rev 118:9058–9128. https://doi.
org/10.1021/acs.chemrev.8b00186
are showing slight deviation to their simulated values. The
computed absorption peaks showed good correlation with
the experimental values and the broad absorption and high
fuorescence emission of the polymers indicate that these
polymers can fnd potential applications in polymer solar
cells and light-emitting devices.
Jin Cheon H, Li X, Jin Jeong Y et al (2020) A novel design of donor–
acceptor polymer semiconductors for printed electronics: applica-
tion to transistors and gas sensors. J Mater Chem C 8:8410–8419.
https://doi.org/10.1039/D0TC01341B
Acknowledgements Authors are grateful to RV College of Engineering
for providing characterization facility to carry out this research work .
Khetubol A, Van Snick S, Clark ML et al (2015) Improved spectral
coverage and fuorescence quenching in donor-acceptor systems
involving indolo[3-2-b]carbazole and boron-dipyrromethene or
diketopyrrolopyrrole. Photochem Photobiol 91:637–653. https://
doi.org/10.1111/php.12437
Funding This research did not receive any specifc grant from funding
agencies in the public, commercial, or not-for-proft sectors.
Compliance with ethical standards
Kim M, Ryu SU, Park SA et al (2020) Donor–acceptor-conjugated
polymer for high-performance organic feld-efect transistors:
a progress report. Adv Funct Mater 30:1904545. https://doi.
org/10.1002/adfm.201904545
Conflict of interest I certify that there is no actual or potential confict
of interest in relation to this article.
Lee C, Yang W, Parr RG (1988) Development of the Colle–Salvetti
correlation-energy formula into a functional of the electron den-
sity. Phys Rev B 37:785–789. https://doi.org/10.1103/PhysR
evB.37.785
References
Li J, Grimsdale AC (2010) Carbazole-based polymers for organic
photovoltaic devices. Chem Soc Rev 39:2399–2410. https://doi.
org/10.1039/B915995A
Akkuratov AV, Nikitenko SL, Kozlov AS et al (2020) Design of novel
thiazolothiazole-containing conjugated polymers for organic
solar cells and modules. Sol Energy 198:605–611. https://doi.
org/10.1016/j.solener.2020.01.087
Li J, Liu J, Wei C-W et al (2013) Emerging applications of conju-
gated polymers in molecular imaging. Phys Chem Chem Phys
15:17006–17015. https://doi.org/10.1039/C3CP51763B
Li W, Guo Y, Wang Y et al (2018) A “chain–lock” strategy to construct
a conjugated copolymer network for supercapacitor applications. J
Mater Chem A 7:116–123. https://doi.org/10.1039/C8TA08766K
Mahesh K, Karpagam S, Pandian K (2019) How to design donor-accep-
tor based heterocyclic conjugated polymers for applications from
organic electronics to sensors. Top Curr Chem (Z) 377:12. https
://doi.org/10.1007/s41061-019-0237-4
Becke AD (1993) Density-functional thermochemistry. III. The role
of exact exchange. J Chem Phys 98:5648–5652. https://doi.
org/10.1063/1.464913
Chen Q, Zhang L, Ebrahim S et al (2013) Synthesis and structure study
of copolymers from thiadiazole fused indolocarbazole and dith-
ienosilole. Polymer 54:223–229. https://doi.org/10.1016/j.polym
er.2012.11.006
Cheng Y-J, Yang S-H, Hsu C-S (2009) Synthesis of conjugated poly-
mers for organic solar cell applications. Chem Rev 109:5868–
5923. https://doi.org/10.1021/cr900182s
Majidizadeh Fini A, Kalantari Fotooh F, Nateghi MR, Shahi S (2020)
Combined experimental and theoretical investigation of optical
and structural properties of poly aniline derivatives. Chem Pap.
https://doi.org/10.1007/s11696-020-01273-4
Chung K, McAllister A, Bilby D et al (2015) Designing interchain
and intrachain properties of conjugated polymers for latent opti-
cal information encoding. Chem Sci 6:6980–6985. https://doi.
org/10.1039/C5SC02403J
Neese F (2012) The ORCA program system. WIREs Comput Mol Sci
2:73–78. https://doi.org/10.1002/wcms.81
Negru OI, Grigoras M (2019) Synthesis and properties of copoly-
arylenes containing indolo[3,2-b]carbazole moieties in the
backbone. J Polym Res 26:30. https://doi.org/10.1007/s1096
5-018-1693-1
Chung HY, Oh J, Park J-H et al (2020) Spectroscopic studies on
intramolecular charge-transfer characteristics in small-molecule
organic solar cell donors: a case study on ADA and DAD triad
donors. J Phys Chem C. https://doi.org/10.1021/acs.jpcc.0c06741
de Leeuw DM, Simenon MMJ, Brown AR, Einerhand REF (1997)
Stability of n-type doped conducting polymers and consequences
for polymeric microelectronic devices. Synth Met 87:53–59. https
://doi.org/10.1016/S0379-6779(97)80097-5
Park KS, Kwok JJ, Dilmurat R et al (2019) Tuning conformation,
assembly, and charge transport properties of conjugated polymers
by printing fow. Sci Adv 5:eaaw757. https://doi.org/10.1126/sciad
v.aaw7757
Pei J, Wen S, Zhou Y et al (2011) A low band gap donor–acceptor
copolymer containing fuorene and benzothiadiazole units: syn-
thesis and photovoltaic properties. New J Chem 35:385–393. https
://doi.org/10.1039/C0NJ00378F
Ekbote A, Mobin SM, Misra R (2018) Structure–property relationship
in multi-stimuli responsive D-A–A′ benzothiazole functionalized
isomers. J Mater Chem C 6:10888–10901. https://doi.org/10.1039/
C8TC04310H
Pop F, Humphreys J, Schwarz J et al (2019) Towards more sustainable
synthesis of diketopyrrolopyrroles. New J Chem 43:5783–5790.
https://doi.org/10.1039/C9NJ01074B
Fan L-J, Zhang Y, Murphy CB et al (2009) Fluorescent conjugated
polymer molecular wire chemosensors for transition metal ion
recognition and signaling. Coord Chem Rev 253:410–422. https
://doi.org/10.1016/j.ccr.2008.03.008
Sekine C, Tsubata Y, Yamada T et al (2014) Recent progress of high
performance polymer OLED and OPV materials for organic
printed electronics. Sci Technol Adv Mater 15:034203. https://
doi.org/10.1088/1468-6996/15/3/034203
Gokce G, Icli Ozkut M (2018) An indolocarbazole based yellow-to-
cyan soluble electrochromic polymer. Org Electron 52:317–322.
https://doi.org/10.1016/j.orgel.2017.11.017
Shivashankar SM, Anantapadmanabha VK, Adhikari AV (2013)
Optical limiting materials: synthesis, electrochemical and opti-
cal studies of new thiophene based conjugated polymers carrying
Grzybowski M, Gryko DT (2015) Diketopyrrolopyrroles: synthesis,
reactivity, and optical properties. Adv Opt Mater 3:280–320. https
://doi.org/10.1002/adom.201400559
1 3

1980
Chemical Papers (2021) 75:1969–1980
1,3,4-oxadiazole units. Polym Eng Sci 53:1347–1356. https://doi.
org/10.1002/pen.23362
Wong MY (2017) Recent advances in polymer organic light-emitting
diodes (PLED) using non-conjugated polymers as the emitting
layer and contrasting them with conjugated counterparts. J Electr
Mater 46:6246–6281. https://doi.org/10.1007/s11664-017-5702-7
Zeglio E, Rutz AL, Winkler TE et al (2019) Conjugated polymers
for assessing and controlling biological functions. Adv Mater
31:1806712. https://doi.org/10.1002/adma.201806712
Zhan X, Zhu D (2010) Conjugated polymers for high-efciency organic
photovoltaics. Polym Chem 1:409–419. https://doi.org/10.1039/
B9PY00325H
Suman SA, Keshtov ML et al (2019) New indolo carbazole-based non-
fullerene n-type semiconductors for organic solar cell applica-
tions. J Mater Chem C 7:543–552. https://doi.org/10.1039/C8TC0
5318A
Sun H, Autschbach J (2014) Electronic energy gaps for π-conjugated
oligomers and polymers calculated with density functional theory.
J Chem Theory Comput 10:1035–1047. https://doi.org/10.1021/
ct4009975
Tong J, Guo P, Zhang H et al (2015) Synthesis of modifed benzothia-
diazole-thiophene-cored acceptor and carbazole/indolocarbazole
alternating conjugated polymers and their photovoltaic applica-
tions. Polym Bull. https://doi.org/10.1007/s00289-014-1292-1
Tsai J-H, Chueh C-C, Mei-Hsiu L et al (2009) Synthesis of new indolo-
carbazole-acceptor alternating conjugated copolymers and their
applications to thin flm transistors and photovoltaic cells. Mac-
romolecules. https://doi.org/10.1021/ma802720n
Zhang D-W, Li M, Chen C-F (2020) Recent advances in circularly
polarized electroluminescence based on organic light-emitting
diodes. Chem Soc Rev 49:1331–1343. https://doi.org/10.1039/
C9CS00680J
Publisher’s Note Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional afliations.
Udayakumar D, Adhikari AV (2006) Synthesis and characterization
of new light-emitting copolymers containing 3,4-dialkoxythio-
phenes. Synth Met 156:1168–1173. https://doi.org/10.1016/j.
synthmet.2006.07.007
1 3