Constructing fused bis-isatins from pyrroloindoles using direct oxidation approach and re-visiting indophenine reaction

Article abstract of DOI:10.1016/j.polymer.2020.123032

Two novel electron accepting bis-isatins, having fused bis-lactam rings have been synthesized from corresponding pyrroloindoles 7 and 14 by one-pot direct oxidation approach involving oxidant NBS with or without co-oxidant TBHP in DMF/CHCl₃. Synthesized fused bis-isatins 8 and 15 are extensively studied for thermal, photophysical, electrochemical and thin-film morphological properties and are found to have characteristic properties like solid-solid transitions, visible light absorption, lowered FMO energy levels (LUMOs below ?4.1 eV and HOMOs below ?5.9 eV) as well as oriented packing of molecules in the film state. The famous indophenine reaction is re-visited with fused bis-isatins to form conjugated polymers P-1 and P-2, having quinoidal pyrroloindole dione units. The polymers are studied for thermal, photophysical, electrochemical and thin-film morphological properties indicating sufficiently high thermal stability (up to 200 °C), extended absorptions up to near IR region, lowered FMO energy levels (LUMOs below ?4.3 eV and HOMOs below ?5.8 eV) and amorphous nature of polymer films.

Full text of DOI:10.1016/j.polymer.2020.123032

Polymer 210 (2020) 123032
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Polymer
journal homepage: http://www.elsevier.com/locate/polymer
Constructing fused bis-isatins from pyrroloindoles using direct oxidation
approach and re-visiting indophenine reaction
,
c
,
,***
Viraj J. Bhanvadiaa, Anwesha Choudhuryb, Parameswar Krishnan Iyerb **, Sanjio S. Zaded
,
,
*
Arun L. Patela
a Department of Chemistry, Faculty of Science, The Maharaja Sayajirao University of Baroda, Vadodara, 390002, India
b Centre of Nanotechnology, Indian Institute of Technology Guwahati, Guwahati, Assam, 781039, India
c Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam, 781039, India
d Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur, 741246, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two novel electron accepting bis-isatins, having fused bis-lactam rings have been synthesized from corre-
sponding pyrroloindoles 7 and 14 by one-pot direct oxidation approach involving oxidant NBS with or without
co-oxidant TBHP in DMF/CHCl3. Synthesized fused bis-isatins 8 and 15 are extensively studied for thermal,
photophysical, electrochemical and thin-film morphological properties and are found to have characteristic
properties like solid-solid transitions, visible light absorption, lowered FMO energy levels (LUMOs below ꢀ 4.1 eV
and HOMOs below ꢀ 5.9 eV) as well as oriented packing of molecules in the film state. The famous indophenine
reaction is re-visited with fused bis-isatins to form conjugated polymers P-1 and P-2, having quinoidal pyrro-
loindole dione units. The polymers are studied for thermal, photophysical, electrochemical and thin-film
morphological properties indicating sufficiently high thermal stability (up to 200 ◦C), extended absorptions
up to near IR region, lowered FMO energy levels (LUMOs below ꢀ 4.3 eV and HOMOs below ꢀ 5.8 eV) and
amorphous nature of polymer films.
Conjugated polymers
Donor-acceptor systems
Electron-deficient compounds
Fused ring systems
Oxidation
Polymerization
1. Introduction
Fused bis-lactam units act as strong acceptor in donor-acceptor conju-
gated polymers, which include diketopyrrolopyrrole (DPP) [7,8], indigo
Conjugated small molecules and polymers have gained significant
attention in the field of organic electronics [1,2], based on the advan-
tages offered by organic materials, such as solution processability and
compatibility with flexible substrates as well as convenient fabrication
over large areas with high throughput compared to the conventional
inorganic silicon-based devices [3,4]. Over the past decades and espe-
cially within last few years, the development of new materials, improved
material processing and device optimization studies have led to the
remarkable enhancements in the charge carrier mobilities of the organic
semiconductors [4–6].
[9,10], isoindigo (IIG) [11–13], π-extended bis-isoindigo (PBIs-IIG,
NBIs-IIG and BBBTBIs-IIG) [14–16], benzodipyrrolidone (BDPD)
[17–20], dihydropyrroloindoledione (DPID) [21,22], indophenine
[23–25], and thiophene-S,S-dioxidized indophenine (IDTO) [26,27]
(Fig. 1). Particularly, quinoidal molecular building blocks, such as
BDPD, indophenine, IDTO and quinoidal oligothiophenes have garnered
much interest as a result of several advantages offered, like highly planar
structure, efficient delocalization of
π-electrons resulting into the
reduced band-gaps and amphoteric redox behaviour which makes them
good candidates for ambipolar charge transport [28,29]. In this regard,
the design and development of fused bis-isatins could be the stepping
stone towards the development of novel conjugated polymers.
Fused bis-lactam units containing building blocks have been the
materials of choice for researchers for development of organic conju-
gated small molecules and polymers owing to the ability of these con-
jugated molecular building blocks to induce dipole-dipole interactions
The structures of the reported bis-isatins are shown in Fig. 2. The
reported synthesis of phenyl bis-isatin proceeds via two-directional
Pummerer-type cyclization [31,32] and ceric ammonium nitrate
that facilitates close intermolecular packing and shorten
π-π distances.
* Corresponding author.
** Corresponding author. Centre of Nanotechnology & Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati, Assam, 781039, India.
*** Corresponding author.
E-mail addresses: pki@iitg.ac.in (P.K. Iyer), sanjiozade@iiserkol.ac.in (S.S. Zade), arunpatel_5376@yahoo.co.in (A.L. Patel).
https://doi.org/10.1016/j.polymer.2020.123032
Received 24 April 2020; Received in revised form 5 September 2020; Accepted 9 September 2020
Available online 12 September 2020
0032-3861/© 2020 Elsevier Ltd. All rights reserved.

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
(CAN) mediated oxidation [30]. The process involves difficult and
tedious isolation of unstable intermediates while the CAN-mediated
oxidation step has low conversions (phenyl bis-isatin obtained in 15%
yield) and the pure product isolation requires very careful chromato-
graphic purification [30]. Synthesis of naphthalene bis-isatin involves
Martinet isatin synthesis [15] which works only for 1,5-diaminonaph-
thalene and the pure naphthalene bis-isatin is obtained in ~30% yield
after purification by column chromatography followed by recrystalli-
zation from ethylacetate:hexanes (1:3) while the synthesis of thieno[3,
2-b]thiophene bis-isatin proceeds through two-directional con-
densation-cyclization reaction using oxalyl chloride and the pure
product is obtained in very low yields (thieno[3,2-b]thiophene bis-isatin
obtained in 2% yield) after extensive purification by column chroma-
tography and subsequent re-crystallization from DCM/methanol [30].
Another synthetic pathway for fused bis-isatins has been reported very
recently, which involves palladium-catalyzed Suzuki coupling of isatin
with bis(methylsulfonyl)benzene and subsequent annulation/demethy-
lation to afford benzo[1,2-b:4,5-b’]bis[b]benzothiophene bis-isatin in
35% yield after purification by column chromatography [16].
thiophene in presence of concentrated sulphuric acid. This polymeri-
zation route therefore avoids requirements of transition metal catalysts
and organometallic monomers and outlines a reliable design strategy for
conjugated D-A polymers having delocalized frontier molecular orbitals.
2. Experimental
2.1. Materials and instrumentation
All the chemicals purchased are reagent grade and are used as pur-
chased. Moisture-sensitive reactions were performed under an inert at-
mosphere of dry nitrogen with dried solvents. Reactions are monitored
by thin-layer chromatography (TLC) using Merck 60 F254 aluminium-
coated plates and the spots are visualized under ultraviolet (UV) light.
Column chromatography is carried out on silica gel (60–120 mesh).
NMR spectra are recorded on a Bruker Avance-III 400 spectrometer in
CDCl3 and DMSO-D6. The high resolution mass spectra are recorded on
Xevo G2-XS QTOF Mass Spectrometer. Molecular weights of the polymer
samples are measured with Agilent 1260 Infinity GPC instrument,
equipped with RI detector. Polystyrene is used as a calibration standard.
Polymer samples (~5 mg) are dissolved in THF (~5 mL) and are filtered
Motivated by these synthetic challenges, we developed a new syn-
thetic route based on the direct oxidation of corresponding pyrro-
loindole derivatives for the synthesis of fused bis-isatins. Previously, we
have reported the synthesis, characterization and electrochemistry of
polycyclic fused aromatic 1,8-dihydropyrrolo[3,2-g]indole (benzodi-
pyrrole) and 1,10-dihydrobenzo[e]pyrroloindole (naphthobipyrrole)
[33]. Here, we report syntheses of novel fused bis-isatins (compound 8
and 15) from corresponding benzodipyrrole (compound 7, Scheme 1)
and naphthobipyrrole (compound 14, Scheme 2) using direct oxidation
approach involving oxidant NBS with or without co-oxidant TBHP in
DMF/CHCl3. Till date, very few syntheses of fused bis-isatins have been
reported in the literature. The direct oxidation approach towards fused
bis-isatins involves room temperature reactions without transition metal
catalysts and multistep oxidation reactions as well as much easier
isolation of pure product without fussy chromatographic separations
(only few solvent washes!).
through a 0.2
μ filter. The analysis is done using THF as an eluent at a
flow rate of 1–2 mL/min. Thermogravimetric analysis (TGA) is done on
Exstar SII TG/DTA 6300 using N2 as inert gas. Differential scanning
calorimetry (DSC) is performed with Mattler Toledo DSC822e under N2
atmosphere. UV–Visible absorption spectra are recorded on Jasco V-630
spectrophotometer using a quartz cuvette. The steady-state fluorescence
spectra were recorded on Jasco FP 6300 spectrofluorometer using a
quartz cuvette. Electrochemical studies are carried out with a Princeton
Applied Research 263A potentiostat using platinum (Pt) disk electrode
as the working electrode, a platinum wire as counter electrode and an
AgCl-coated Ag wire as the reference electrode. Nonaqueous Ag/Ag+
wire is prepared by dipping silver wire in a solution of FeCl3 and HCl. Pt-
disk electrodes were polished with alumina and washed with water, and
acetone and were dried with nitrogen gas before use to remove any
incipient oxygen. All electrochemical potentials are reported against
Conventional semiconducting polymer syntheses require transition
metal-mediated coupling reactions that link aromatic donor and
acceptor units along the backbone, however, the use of transition metals
presents difficulties for sustainability and application in biological en-
vironments. Here we show that with fused bis-isatins, simple indophe-
nine reaction conditions can be used to develop conjugated polymers (P-
1 and P-2) having quinoidal pyrroloindole dione units attached to the
aromatic bithiophene units by reacting compounds 8 and 15 with
Ag/Ag+ taking ferrocene as external standard; Eonset Fc/Fc = 0.36 V.
+
Thin film morphological studies of fused bis-isatins and conjugated
polymers are carried out using X-ray diffraction (XRD) analysis with
Rigaku Smartlab Xray Diffractometer. Thin films of the compounds 8
and 15, as well as conjugated polymers P-1 and P-2, are obtained by spin
coating the corresponding chlorobenzene solutions on Si/SiO2 substrate.
Fig. 1. Bis-lactam fused ring containing building blocks.
2

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
Fig. 2. Structures of reported phenyl bis-isatin [19,21,30], naphthalene bis-isatin [15,30], thieno[3,2-b]thiophene bis-isatin [30] and benzo[1,2-b:4,5-b’]bis[b]
benzothiophene bis-isatin [16].
Scheme 1. Synthesis of compound 8 using direct oxidation approach.
Scheme 2. Synthesis of compound 15 using direct oxidation approach.
2.2. Synthesis of fused bis-isatins
Synthesis of 1,8-didodecyl-1,8-dihydropyrrolo[3,2-g]indole
(compound 7): Compound 6 (1.64 g, 10.51 mmol) was dissolved in DMF
(50 mL). To this was added sodium hydroxide flakes (4.20 g, 105.12
mmol) and the resulting suspension was stirred at room temperature for
10 min. To this stirred solution was added, n-dodecyl bromide (16.4 mL,
Polycyclic fused aromatic pyrroloindoles (compound 6 and 13) are
synthesized by following the previously reported synthetic procedures
(Schemes 1 and 2) [33].
3

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
68.33 mmol) dropwise over a period of 5 min. After the complete
addition, reaction mixture was stirred at 80 ◦C for 3 h and monitored by
TLC. After completion of the reaction, as indicated by TLC, 200 mL of
water was added to it and extracted with aliquots of dichloromethane (2
× 50 mL). Combined organic layers are washed with saturated brine
solution, finally with water and dried over MgSO4. Crude product is
obtained after evaporation of solvent which is subjected to the column
chromatography over silica gel using petroleum ether as eluent. The
pure compound 7 is separated by column chromatography, yielding
yellowish oil from the later fractions from the column.
tetradecylbromide (8 mL, 26.7 mmol) dropwise over a period of 5 min.
After the complete addition, reaction mixture was stirred at 80 ◦C for 3 h
and monitored by TLC. After completion of the reaction, as indicated by
TLC, 400 mL of water was added to it and extracted with aliquots of
dichloromethane (2 × 50 mL). Combined organic layers are washed
with saturated brine solution, finally with water and dried over MgSO4.
Crude product is obtained after evaporation of solvent which is sub-
jected to the column chromatography over silica gel using 1% ethyl
acetate-petroleum ether as eluent. The pure compound 14 was obtained
as off white crystalline solids.
1,8-didodecyl-1,8-dihydropyrrolo[3,2-g]indole
(compound
7):
1,10-ditetradecyl-1,10-dihydrobenzo[e]pyrrolo[3,2-g]indole (com-
pound 14): Off white solids (1.60 g, 50%); 1H NMR (400 MHz, CDCl3):
8.20–8.23 (dd, J2 = 6.4 Hz, J3 = 3.6 Hz, 1H), 7.44–7.46 (dd, J2 = 6.0 Hz,
J3 = 3.2 Hz, 1H), 7.11–7.12 (d, J2 = 2.8 Hz, 1H), 7.05–7.06 (d, J2 = 3.2
Hz, 1H), 4.36–4.40 (t, J2 = 7.6 Hz, 2H), 1.82–1.89 (m, 2H), 1.22–1.30
(m, 22H), 0.89–0.92 (t, J2 = 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3):
126.6, 125.2, 123.8, 123.5, 123.2, 122.8, 102.6, 51.0, 31.9, 31.2, 29.7,
29.6, 29.6, 29.5, 29.4, 29.4, 29.2, 26.7, 22.7, 14.1. HRMS (ES+):
Yellowish oil (2.02 g, 39%); 1H NMR (400 MHz, CDCl3): 7.37 (s, 1H),
7.01–7.02 (d, J2 = 2.8 Hz, 1H), 6.63–6.64 (d, J2 = 3.2 Hz, 1H),
4.34–4.38 (t, J2 = 7.6 Hz, 2H), 1.85–1.90 (m, 2H), 1.26–1.36 (m, 18H),
0.91–0.94 (t, J2 = 7.2 Hz, 3H). 13C NMR (100 MHz, CDCl3): 127.6,
127.0, 124.3, 114.4, 103.6, 50.9, 32.0, 31.3, 29.7, 29.6, 29.5, 29.4, 29.3,
26.7, 22.8, 14.2. HRMS (ES+): C34H57N2 requires 493.4522, found
493.4528. IR (NaCl plate, cmꢀ 1): 3020.6, 2927.9, 2855.1, 2400.7,
1599.9, 1524.1, 1424.0, 1215.6, 1019.9, 928.5, 759.2, 669.5, 451.8.
Synthesis of 4,5-dibromo-1,8-didodecyl-1,8-dihydropyrrolo
[3,2-g]indole-2,3,6,7-tetraone (compound 8) and 4,5,6,7-tetra-
bromo-1,8-didodecyl-1,8-dihydropyrrolo[3,2-g]indole-2,3-dione
(compound 9): The compound 7 (0.14 g; 0.29 mmol) was dissolved in
the mixture of chloroform (3 mL) and DMF (20 mL) and stirred at room
temperature. To this stirred solution, tert-butyl hydroperoxide (~70%
aqueous solution, 0.6 mL; 2.92 mmol) was added and reaction mixture
was allowed to stir at room temperature for 5 min. NBS (0.52 g; 2.92
mmol) was added to this stirred reaction mixture and up on addition of
NBS, reaction mixture immediately turned into deep red coloured so-
lution. After stirring the reaction mixture for 2 h, additional tert-butyl
hydroperoxide (~70% aqueous solution, 0.3 mL; 1.46 mmol) and NBS
(0.26 g; 1.46 mmol) was added and reaction was allowed to continue for
20 h. After completion, the reaction mixture was poured into the 400 mL
of aqueous sodium thiosulphate solution and was stirred for about 30
min at room temperature. The resulting dark red-purple precipitates
were filtered and washed with water for several times to remove traces
of DMF and tert-butanol. The pure compound 8 was obtained by washing
the crude material with copious amounts of petroleum ether, leaving
behind deep purple-black solid product. The combined petroleum ether
washings were evaporated to dryness and the deep reddish brown solid
product was subjected to the column chromatography over silica gel and
eluting the pure compound 9 as a red coloured band in 15% ethyl
acetate-petroleum ether.
C
42H67N2 requires 599.5304, found 599.5302. IR (KBr, cmꢀ 1): 3433.5,
2950.1, 2916.6, 2850.8, 1580.8, 1471.1, 1375.0, 1353.8, 1289.7, 754.8,
721.7, 599.2.
Synthesis of 1,10-ditetradecyl-1,10-dihydrobenzo[e]pyrrolo
[3,2-g]indole-2,3,8,9-tetraone (compound 15): The compound 14
(0.52 g, 0.87 mmol) was dissolved in the mixture of DMF (240 mL) and
chloroform (80 mL) and was allowed to stir for 5 min. To this stirred
solution, was added NBS (3.09 g, 17.4 mmol) in a single portion and
resulting reddish-brown reaction mixture was allowed to stir at room
temperature for 96 h. After completion of the reaction, the reaction
mixture was poured into 800 mL of de-ionized water and resulting
mixture was stirred at room temperature for 2 h. The crude product was
extracted using chloroform (2 × 50 mL) and combined organic extracts
were washed with water (2 × 100 mL), brine (1 × 100 mL), dried over
anhydrous sodium sulphate and evaporated to dryness under reduced
pressure to yield dark red-brown precipitates. Finally, the crude pre-
cipitates were washed with small amounts of petroleum ether to remove
the by-products and impurities, leaving behind the pure compound 15 as
dark purple-black amorphous solids.
1,10-ditetradecyl-1,10-dihydrobenzo[e]pyrrolo[3,2-g]indole-
2,3,8,9-tetraone (compound 15): Dark purple-black amorphous solids
(0.13 g; 24%); 1H NMR (400 MHz, CDCl3): 8.75–8.78 (dd, J2 = 6.4 Hz,
J3 = 3.2Hz, 1H), 7.64–7.66 (dd, J2 = 6.8 Hz, J3 = 3.6 Hz, 1H), 3.94–3.98
(t, J2 = 7.6 Hz, 2H), 1.74 (br s, 2H), 1.20–1.27 (m, 22H), 0.87–0.90 (t, J2
= 6.8 Hz, 3H). 13C NMR (100 MHz, CDCl3): 183.0, 160.5, 141.1, 130.8,
127.2, 123.7, 120.4, 46.3, 31.9, 29.7, 29.6, 29.6, 29.6, 29.4, 29.4, 29.3,
29.0, 27.8, 26.6, 22.7, 14.2. HRMS (ES+): C42H63N2O4 requires
659.4788, found 659.4787. IR (KBr, cmꢀ 1): 3431.5, 2921.1, 2851.5,
1753.5, 1716.2, 1617.2, 1570.9, 1468.6, 1403.1, 1373.8, 1187.8,
1114.9, 752.9, 601.9, 436.2.
4,5-dibromo-1,8-didodecyl-1,8-dihydropyrrolo[3,2-g]indole-
2,3,6,7-tetraone (compound 8): Dark purple-black solids (0.05 g, 24%);
1H NMR (400 MHz, CDCl3): 3.94–3.97 (t, J2 = 7.2 Hz, 2H), 1.70 (br s,
2H), 1.23–1.28 (m, 18H), 0.87–0.90 (t, J2 = 7.2 Hz, 3H). 13C NMR (100
MHz, CDCl3): 179.8, 158.4, 136.0, 125.5, 122.5, 46.0, 31.9, 29.6, 29.6,
29.5, 29.3, 29.0, 27.6, 26.6, 22.7, 14.2. HRMS (ES+): C34H51N2O4Br2
requires 709.2216, found 709.2210. IR (KBr, cmꢀ 1): 3432.6, 2921.2,
2851.8, 1734.9, 1599.9, 1566.9, 1468.1, 1396.8, 1366.0, 1217.2, 771.9,
433.8.
2.3. General procedure for synthesis of conjugated polymers
Conjugated polymers P-1 and P-2 are synthesized following the
modification of the literature procedure reported by Tormos et al. [34]
and Hwang et al. [35] The fused bi-isatins (0.17 mmol, 1 eq.) are dis-
solved in 20 mL of anhydrous toluene and allowed to stir at room
temperature under nitrogen atmosphere for 5 min. To this stirred solu-
tion, thiophene (0.35 mmol, 2.1 eq.) is added drop-wise and reaction
mixture is stirred vigorously at room temperature for 5 min under ni-
trogen atmosphere. Concentrated sulphuric acid (0.5 mL) is added
dropwise to this vigorously stirred reaction mixture at room temperature
under a nitrogen atmosphere and the resulting reaction mixture is stir-
red at 70 ◦C for 30 h. After completion of the reaction, the reaction
mixture is poured into 200 mL of 0.2 M aqueous sodium hydroxide so-
lution and is allowed to stir vigorously for 1 h. The crude polymer is
extracted using chloroform (3 × 100 mL) and combined organic extracts
are washed with water (2 × 100 mL), brine (1 × 100 mL), dried over the
4,5,6,7-tetrabromo-1,8-didodecyl-1,8-dihydropyrrolo[3,2-g]indole-
2,3-dione (compound 9): Dark red-brown solids (0.04 g, 16%); 1H NMR
(400 MHz, CDCl3): 4.26–4.30 (t, J2 = 7.6 Hz, 2H), 3.92–3.96 (t, J2 = 8.0
Hz, 2H), 1.62–1.67 (m, 2H), 1.46–1.53 (p, J2 = 7.6 Hz, 2H), 1.20–1.30
(m, 34H), 1.09–1.11 (m, 2H), 0.87–0.90 (t, J2 = 7.2 Hz, 6H). 13C NMR
(100 MHz, CDCl3): 178.0, 162.5, 141.3, 135.4, 130.7, 125.1, 118.1,
115.5, 112.6, 100.1, 50.9, 47.4, 31.9, 29.8, 29.6, 29.6, 29.5, 29.4, 29.4,
29.3, 29.3, 28.9, 28.9, 28.0, 26.7, 26.3, 22.7, 14.2. HRMS (ES+):
C
34H51N2O2Br4 requires 835.0684, found 835.0670.
Synthesis of 1,10-ditetradecyl-1,10-dihydrobenzo[e]pyrrolo
[3,2-g]indole (compound 14): Compound 13 (1.10 g, 5.34 mmol) was
dissolved in DMF (107 mL). To this was added sodium hydroxide flakes
(1.49 g, 37.4 mmol) and the resulting suspension was stirred at room
temperature for 10 min. To this stirred solution was added, n-
4

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
anhydrous sodium sulphate and evaporated to dryness under reduced
pressure to afford dark purple-black solids. The sticky solids are tritu-
rated with petroleum ether and are filtered, washed with small amounts
of petroleum ether and then subjected to the Soxhlet extraction using
petroleum ether, methanol and the polymer fraction is eluted using
chloroform. The polymer is obtained by evaporating chloroform under
reduced pressure and further dried under vacuum at 50 ◦C to yield dark
purple-black solids.
synthesize conjugated polymers P-1 and P-2, where synthesized fused
bis-isatins (compounds 8 and 15) are reacted with thiophene in presence
◦
of concentrated sulphuric acid in toluene at 70 C under nitrogen at-
mosphere (Scheme 3). The synthetic procedure followed to synthesize
polymers P-1 and P-2 neither required any costlier transition-metal
catalysts nor harsh temperature and pressure conditions. Moreover,
the concentrated sulphuric acid used for the polymerization can be
easily removed by simple basic aqueous workup procedure. The poly-
mers are further purified by sequential Soxhlet extraction using different
solvents like methanol, acetone, petroleum ether and the polymer
fraction is finally eluted using chloroform. The solvent is evaporated
under reduced pressure and solids are further dried under vacuum at
50 ◦C.
P-1: Dark purple-black solids (0.06 g, 42%); 1H NMR (400 MHz,
CDCl3) δ: 7.32–7.68 (br, 1H), 6.80–7.21 (br, 1H), 4.39–3.48 (br s, 2H),
2.01–2.58 (br s, 2H), 1.52–1.95 (br, 2H), 0.97–1.52 (br, 16H), 0.68–0.95
(br, 3H). IR (KBr, cmꢀ 1): 3405.4, 2956.8, 2925.2, 2856.1, 1726.3,
1685.5, 1596.7, 1555.8, 1456.2, 1304.4, 1179.4, 1118.8, 808.8, 771.2,
724.2.
P-2: Dark purple-black solids (0.07 g, 52%); 1H NMR (400 MHz,
CDCl3) δ: 7.89–8.9 (m, 2H), 7.46–7.80 (m, 1H), 6.83–7.20 (m, 1H),
3.78–4.23 (br, 2H), 1.57–1.78 (br, 2H), 1.14–1.33 (m, 22H), 0.81–0.91
(br, 3H). IR (KBr, cmꢀ 1): 3406.0, 2922.4, 2851.1, 1682.7, 1653.1,
1564.6, 1461.6, 1305.8, 1183.9, 1106.5, 756.7.
3.2. Structural aspects of conjugated polymers
1H NMR spectra of synthesized conjugated polymers P-1 and P-2 are
shown in Fig. 3a and b. The polymer P-1 shows two different peaks for
aromatic thiophene protons Ha and Hb between 7.32 and 7.68 ppm and
6.80–7.21 ppm, respectively. Similarly, polymer P-2 shows two different
peaks for aromatic thiophene protons Ha and Hb between 7.46 and 7.80
ppm and 6.83–7.20 ppm, respectively. Protons Ha of P-1 and P-2 appear
downfield due to the proximity to the electron withdrawing quinoidal
pyrroloindole dione units, while proton Hb appears upfield due to the
proximity of electron donating thiophene unit. It is evident from the 1H
NMR spectra of polymers P-1 and P-2, that the polymers are having
pyrroloindole dione-alt-bithiophene structure rather than pyrroloindole
dione-alt-thiophene structure, in case of which, the 1H NMR spectra of
polymers would have shown only one signal for thiophene protons. Also,
formation of conjugated polymers with pyrroloindole dione-alt-thio-
phene structure would have caused severe steric hindrance, as the two
pyrroloindole dione units would be placed closer to each other. Though,
during polymerization reaction, formation of few oligomers, having
pyrroloindole dione-alt-thiophene structure, cannot be denied, however,
relatively polar nature of these oligomers allowed their removal via
sequential Soxhlet extraction. 13C NMR spectra for conjugated polymers
P-1 and P-2 couldn’t be obtained even after increasing the number of
scans, as in case of conjugated polymers, the relaxation time required for
the magnetic vector of the 13C nuclei is much higher due to the extended
lengths of polymer chains, containing conjugatively linked aromatic
units.
3. Results and discussion
3.1. Synthesis of fused bis-isatins and conjugated polymers
Polycyclic compound 6 is synthesized by fluoride-mediated two-
directional cyclization of compound 5 using literature procedures [33].
Compound 6 is subjected to N-alkylation reaction using an excess of
n-dodecylbromide and NaOH in DMF at 80 ◦C for 3 h (Scheme 1).
Compound 13 is synthesized by the one-pot hydrolysis-decarboxylation
reaction of compound 12 which is obtained from bis-hydrazone 11 via
two-directional Fischer indole cyclization using literature procedures
[33]. Compound 13 is N-alkylated using an excess of n-tetradecyl-
bromide and NaOH in DMF at 80 ◦C for 3 h (Scheme 2).
The N-alkylated compounds 7 and 14 are subjected to the direct
oxidation reaction to obtain desired fused bis-isatins 8 and 15. Com-
pound 7 is oxidized by excess (20 eq.) of N-bromosuccinimide (NBS) in
DMF/chloroform (2:1), using 70% aqueous tert-butylhydroperoxide
(TBHP; 20 eq.) solution as co-oxidant at room temperature (Scheme 1).
The addition of NBS to the vigorously stirred reaction mixture leads to
the immediate colour change from light yellow to red, indicating the
formation of intermediate compound 9 from compound 7, which slowly
gets converted into desired compound 8 by reacting with an excess of
NBS/TBHP. The completion of reaction required 20 h of vigorous stir-
ring at room temperature. However, the complete conversion of com-
pound 9 into the compound 8 is not achieved at the end of the reaction,
and a small amount of the compound 9 (16%) is always present along
with the desired compound 8, no matter for how long the reaction has
been continued or how much excess of oxidizing reagent is used. The
bromine substitution on the 4- and 5-position of the phenyl ring takes
place during the NBS/TBHP-mediated oxidation because both of these
positions, being para to the nitrogen atoms of the fused pyrrole rings, are
susceptible to bromination. Reducing the stoichiometric ratio of NBS
used for the oxidation does not favour the formation of unsubstituted
The polymer structures are expected to have quinoidal thiophene
units along the polymer backbone in accordance to the reported litera-
ture. Rigidity of quinoidal thiophene backbone would give rise to
various geometrical isomers resulting from the orientation of thiophene
units with respect to the adjacent pyrroloindole dione units as well as
thiophene units [23]. However, because of the steric bulk caused by the
bromine substituents at 4- and 5-positions within compound 8 and the
same caused by central naphthalene ring within compound 15, the ex-
pected quinoidal thiophene backbone formation is not preferred.
Instead, twisted polymer backbone should be formed which involves
quinoidal pyrroloindole dione units attached to the aromatic bithio-
phene units as shown in Fig. 4.
bis-isatin,
tetraone.
1,8-didodecyl-1,8-dihydropyrrolo[3,2-g]indole-2,3,6,7-
To get more insights into the structural aspects of polymers P-1 and
P-2, density functional theory (DFT) calculations are carried out at PBC/
B3LYP/6-31G(d) [36] on model polymers (by replacing N-alkyl by
N-ethyl groups) comprising of the repeat unit of each parent copolymer
(Fig. 4). Optimized structure of P-1 and P-2 show that pyrroloindole
dione units are significantly twisted from the bithiophene unit, however,
polymers maintain the conjugation significantly. The dihedral angle
among any consecutive four carbons along the conjugation of the
polymer (unit cell) is less than 25◦. A similar type of polymer backbone
twists have been reported recently [30].
The same oxidation reaction conditions are used for the synthesis of
compound 15 from compound 14, which surprisingly, yield very less
amount of the desired compound 15 (less than 2% yield). So, the
oxidation process for compound 14 is slightly modified, using excess (20
eq.) of NBS in DMF/chloroform (2:1) at room temperature (Scheme 2)
and avoiding the co-oxidant tert-butylhydroperoxide (TBHP; 70%
aqueous solution). The reaction is stirred at room temperature for 96 h
to obtain the desired compound 15. The crude product obtained after
aqueous workup is purified by washing with copious amounts of n-
hexane, yielding the pure bis-isatins (compounds 8 and 15).
Moreover, pyrroloindole dione units are significantly bent with
curvature away from the bithiophene unit. DFT calculation confirms the
quinoidal nature of the pyrroloindole dione units, whereas bithiophene
The indophenine reaction conditions [23,34] are employed to
5

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
Scheme 3. Synthesis of conjugated polymers P-1 and P-2.
polymer P-2.
units are in the non-quinoidal aromatic state, thereby ruling out the
presence of various geometrical conformations. We believe that the
expected quinoidal thiophene backbone formation in P-1 and P-2 is
prevented by the steric hindrance between the thiophene units and the
bromine substituents at 4- and 5-positions within compound 8 and
central naphthalene ring within compound 15. Instead, the twisted
polymer backbone is formed which involves quinoidal pyrroloindole
dione units attached to the aromatic bithiophene units as shown in
Fig. 4.
DSC study of compound 8 shows a sharp differential scanning calo-
rimetric peak at 93 ◦C, along with small endothermic peaks at 78 ◦C and
116 ◦C, followed by melting of crystalline solid to an isotropic liquid at
153 ◦C (Tm) during heating (Fig. 6a). Up on cooling, the exothermic peak
appears at 136 ◦C (Tc) which corresponds to the phase transition from
isotropic liquid to crystalline solid. The polarizing optical microscopic
◦
(POM) visualization of compound 8 suggested that before 153 C, the
material is completely crystalline and no liquid crystalline (LC) phases
are accompanied with either the sharp peak at 93 ◦C or the small peaks
◦
◦
at 78 C and 116 C, suggesting that these endothermic peaks corre-
sponds to the solid-to-solid phase transitions. Similarly, during heating,
DSC of compound 15 (Fig. 6b) exhibits small endothermic peak at 96 ◦C,
followed by melting of crystalline solid to an isotropic liquid at 169 ◦C
(Tm). Up on cooling, the exothermic peak appears at 136 ◦C (Tc) which
corresponds to the phase transition from isotropic liquid to crystalline
solid. The polarizing optical microscope visualization of compound 15
confirmed that below 169 ◦C, the material is completely crystalline and
no liquid crystalline (LC) phases are associated with the endothermic
peak at 96 ◦C. Thus, in case of compound 15, the peak at 96 ◦C corre-
sponds to the solid-to-solid transition. Unfortunately, polarizing optical
microscope (POM) images corresponding to these solid-to-solid transi-
tions couldn’t be captured due to highly coloured natures of compounds
8 and 15. As there are no LC phases associated with any of the differ-
ential scanning calorimetric peaks obtained for compounds 8 and 15,
additional DSC scans are not performed. On the other hand, the DSC
profiles of polymers P-1 and P-2 (Figure S1a and b) do not show well-
resolved transition peaks and no discernible thermal transitions corre-
sponding to glass transition temperature (Tg) or melting temperature
(Tm) are obtained during the heating as well as cooling cycles.
3.3. Molecular weight and thermal properties
Weight average molecular weight (Mw), poly-dispersity index (Đ)
and decomposition temperatures (Td) of the polymers are summarized in
Table 2. Molecular weights obtained from the gel permeation chroma-
tography (GPC) analysis indicate the chain length of 16–18 and 12–15
unit cells for the polymer P-1 and P-2, respectively. The lower weight
average molecular weight (Mw) values obtained for P-1 and P-2 are
arising from lack of solubilising alkyl chain density, which leads to
subsequent precipitation of growing polymer chain, before degree of
polymerization reached an appropriate level. Thermal properties of the
compounds 8, 15 and polymers (P-1 and P-2) are analyzed by ther-
mogravimetric analysis (TGA) at a heating rate of 10 ◦C/min under ni-
trogen atmosphere (Fig. 5a and b) as well as by differential scanning
◦
◦
calorimetry (DSC) at rate of 5 C/min for bis-isatins and 2 C/min for
polymers (Fig. 6a and b as well as Figure S1a and b). The decomposition
temperature (Td) is defined as the temperature at which, the molecule or
polymer loses 5% of its weight. Compounds 8 (Fig. 5a; blue line) and 15
(Fig. 5a; red line) exhibit decomposition temperatures (Td) of 204 ◦C and
◦
270 C, respectively. The improved thermal stability of compound 15
compared to that of compound 8 can be correlated to the presence of
central naphthalene ring unit. Both of the synthesized polymers show
decomposition temperatures above 200 ◦C, which demonstrates their
sufficiently high thermal stabilities for applications of organic electronic
devices. Polymer P-1 (Fig. 5b; blue line) exhibits slightly higher thermal
decomposition temperature (Td) of 229 ◦C, compared to P-2 (Fig. 5b; red
line), whose decomposition temperature (Td) is 214 ◦C. Additionally, P-
2 exhibits lower decomposition temperature (214 ◦C) compared to that
of compound 15 (270 ◦C). The difference in the decomposition tem-
peratures of compound 15 and polymer P-2 might be arising due to
relatively less thermal stability of quinoidal form of compound 15
(quinoidal benzo[e]pyrroloindole dione units), embedded within the
3.4. Photophysical properties
The study of photophysical properties of fused bis-isatins (com-
pounds 8 and 15) is accomplished by UV–visible spectroscopy of 5 ×
10ꢀ 4 M chloroform solutions as well as thin films on quartz glass, cast
from chloroform solutions. Both of compounds 8 and 15 exhibit dual-
band absorption spectra in the solution state. Compound 8 in chloro-
form solution (Fig. 7a; blue line) shows the first absorption band
extending up to 430 nm, having absorption maxima (λmax) at 340 nm
with a weak shoulder peak at 395 nm, which can be ascribed to the
πꢀ π*
transitions. The second broad absorption band ranging from 431 nm to
6

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
Fig. 3. 1H NMR spectra of (a) P-1 and (b) P-2 in CDCl3.
700 nm; having λmax at 570 nm, can be correlated to the charge transfer
(CT) transitions within compound 8 scaffold. However, the solution
to the 0-0 and 0–1 transitions, having λmax at 510 nm and 556 nm,
respectively (Fig. 7b; wine-red line). Vibronic splitting observed in the
film state can be implied to the highly ordered (crystalline) structural
arrangement of compound 15 in solid-state [23]. The optical band-gaps
spectrum of compound 15 (Fig. 7b; red line) exhibits πꢀ π* transition at
λmax at 318 nm extending up to 405 nm. The CT band, ranging from 406
nm to 722 nm and centered at 528 nm is much broader and intense
compared to that of compound 8. The reason behind this may be the
relatively improved charge transfer between the electron-rich fused
naphthalene ring and electron-deficient fused pyrrole-2,3-dione and
pyrrole-8,9-dione moieties.
(Eogpt) of compounds 8 and 15 are calculated from the absorption edges in
the film state and are found to be 1.83 eV and 1.75 eV, respectively
which are lower than the optical band-gap (Eogpt) of 1.94 eV, reported for
benzo[1,2-b:4,5-b’]bis[b]benzothiophene bis-isatin [16]. On the other
hand, the reported linear phenyl bis-isatin and naphthalene bis-isatin
have not been studied for their photophysical properties [15,30].
Photophysical study of polymers P-1 and P-2 is carried out by
UV–visible spectroscopy of 2 × 10ꢀ 4 M chloroform solutions of polymers
as well as thin films on quartz glass, cast from chloroform solutions. The
In the film state, the absorption spectra of compounds 8 and 15
exhibit broader CT bands compared to the absorption spectra in the
solution state. The absorption spectrum of the thin film of compound 15
exhibits multiple absorption bands with vibronic splitting corresponding
7

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
Fig. 4. Expected and actual structures of synthesized polymers, structures of model compounds 16 and 17 for P-1 and P-2 and DFT-calculated optimized structures
(red lines indicate the propagation vectors). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of
this article.)
Table 1
TD-DFT (B3LYP/6-31G(d)) calculated structures, HOMO and LUMO of polymer model of P-1 and P-2.
Table 2
Photophysical properties of fused bis-isatins and conjugated polymers, decomposition temperature Td (obtained from TGA), Molecular weight Mw and poly-dispersity
index (Đ, obtained from GPC analysis) of indophenine polymers.
Compound
λmaxsoln (nm)
λmaxfilm (nm)
λedgefilm (nm)
Td (oC)
Mw (Dalton)
PDI (Đ)
Eogpt(eV)a
8
340, 570
318, 518
566
346, 495
678
708
827
855
1.83
1.75
1.50
1.45
204
270
229
214
–
–
15
P-1
P-2
308, 320, 510, 556
346, 517, 588
325, 450, 555
–
–
13973
10819
2.76
1.30
447, 542
a
Calculated using equation Eogpt = 1240/λedge
.
8

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
Fig. 5. Thermogravimetric analysis (TGA) of (a) compound 8 (blue line) and compound 15 (red line) and (b) conjugated polymers; P-1 (blue line) and P-2 (red line).
(For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 6. Differential scanning calorimetric analysis of (a) compound 8 and (b) compound 15 at heating rate of 5 ◦C/min.
Fig. 7. Absorption spectra of (a) compound 8; chloroform solution (blue line) and film (purple line) state and (b) compound 15; chloroform solution (red line) and
film (wine red line) state. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
chloroform solution of P-1 (Fig. 8a; blue line) show
πꢀ π
* transition into
maxima (λmax) at 566 nm. On the other hand, P-2 (Fig. 8b; red line)
exhibits * transition ranging from the UV region and extending up to
368 nm, while CT band, starting from 369 nm, is extending up to the
the UV region, extending up to 430 nm, while the broad CT band is
ranging from 431 nm and extending up to 850 nm having absorption
πꢀ π
9

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
Fig. 8. Absorption spectra of (a) P-1; chloroform solution (blue line) and film (purple line) state and (b) P-2; chloroform solution (red line) and film (wine red line)
state. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
985 nm in the near IR region, having absorption maxima (λmax) at 542
nm with a shoulder peak at 447 nm.
The photophysical properties of the compounds 8 and 15 and polymers
P-1 and P-2 are summarized in Table 2.
In the thin films of P-1 and P-2 absorption edges are red-shifted
compared to that of solution-state. In the case of P-2, redshifts are
more prominent compared to that of P-1. The thin-film absorption
spectra of P-1 and P-2 (Fig. 8a and b; purple line and wine red line) do
not show any shifting and/or enhancement of vibronic peak positions
compared to the solution absorption spectra, indicating the amorphous
nature of the polymers in solid-state as well as absence of any H-type or
J-type aggregation behaviour in solution or film state [23]. The optical
band-gaps (Eogpt) are in following order: P-2 (1.45 eV) < P-1 (1.50 eV).
PBC/B3LYP/6-31G(d) calculated HOMO and LUMO of P-1 and P-2
are delocalized over the conjugated backbones. However, the electron
density coefficient for HOMO is higher on the bithiophene unit and that
for LUMO is higher on the pyrroloindole dione units (Table 1). Thus,
calculation suggests that there should be a significant contribution from
the charge transfer excitation in the lowest energy absorption band of
the polymers. TD-DFT calculation (B3LYP/6-31G(d)) [36] on the model
compounds 16 and 17 of the P-1 and P-2, respectively, reproduce the
two band absorption spectra (Figure S2a and b). The lowest energy
Fig. 9. (a) Oxidation and (b) reduction curves of P-1; (c) oxidation and (d) reduction curves of P-2, obtained by cyclic voltammetry at 50 mV/s in dry acetonitrile
+
using TBAPF6 as a supporting electrolyte Eonset Fc/Fc = 0.326 V.
10

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
absorption band is of significant intensity arising from the HOMO-LUMO
transition with significant charge transfer characteristic (Table S1 and
S2).
respectively (Fig. 9b and d). The LUMO energy levels were calculated
using the equation: ELUMO = EHOMO + Eogpt and found to be at ꢀ 4.38 eV
for P-1 and P-2. The irreversible oxidation peaks obtained, correspond
to the oxidation of aromatic thiophene units while irreversible reduction
peaks obtained can be correlated to the reduction of quinoidal pyrro-
loindole dione units embedded within the polymer backbone. Delocal-
ization of the electrons due to the quinoidal nature of pyrroloindole
dione units as well as electron donating bithiophene units embedded
within the polymer backbone result into the elevated HOMO and low-
ered LUMO energy levels of conjugated polymers P-1 and P-2 compared
to that of compounds 8 and 15. The electrochemical properties of the
synthesized fused bis-isatins and conjugated polymers are summarized
in Table 3.
The synthesized fused bis-isatins (compounds 8 and 15) as well as
conjugated polymers P-1 and P-2 are non-emissive as indicated by
photoluminescence spectroscopy of 5 × 10ꢀ 5 M chloroform solutions of
compounds 8 and 15 and polymers P-1 and P-2.
3.5. Electrochemical properties
The frontier orbital energy levels of bis-isatins and conjugated
polymers P-1 and P-2 are measured using cyclic voltammetry (CV). CV
experiments are carried out in the dry acetonitrile and dichloromethane
using TBAPF6 as a supporting electrolyte (~50 mM) using a three-
electrode system: a Pt disc electrode as the working electrode, a Pt
wire electrode as the counter electrode and Ag/Ag+ as the reference
electrode. The CV measurements of compound 8 as well as polymers P-1
and P-2, are carried out in dry acetonitrile (~5 mM) while compound 15
is dissolved in dichloromethane (~5 mM). The solutions are degassed
with nitrogen gas prior to the CV measurements.
3.6. Thin film morphology
Thin-film morphological studies of fused bis-isatins and conjugated
polymers are carried out using X-ray diffraction (XRD) analysis
(Figure S4). Thin films of the compounds 8 and 15, as well as polymers
P-1 and P-2, are obtained by spin coating the corresponding chloro-
benzene solutions on glass substrate. The X-ray diffraction of the as-spun
thin film of compound 8 (Figure S4a) show weak (100) diffraction peak
in low angle region at 6.9◦ indicating lamellar packing and the calcu-
lated interlayer d-spacing distance is 12.8 Å. Another broad (010)
diffraction peak at 21.5◦ with interlayer d-spacing distance of 4.1 Å can
All the synthesized compounds and conjugated polymers show
irreversible oxidation and reduction in the CV (Fig. 9 and supporting
information Figure S3). The oxidation of compounds 8 and 15 result into
unresolved oxidation peaks. Corresponding onset oxidation potentials of
+1.52 V and +1.47 V for compounds 8 and 15 are measured from the
oxidation waves (Figure S3a and c). The HOMO energy levels are
calculated from the onset oxidation potentials and found to be at ꢀ 5.99
eV and ꢀ 5.94 eV, respectively. The lowering of HOMO energy levels can
be ascribed to the superior electron-withdrawing properties of fused
pyrrolodione (pyrrole-2,3-dione and pyrrole-6,7-dione in case of com-
pound 8 and pyrrole-2,3-dione and pyrrole-8,9-dione in case of com-
pound 15) rings and absence of the any electron donating groups. The
reduction potentials of compounds 8 and 15 are found to be at ꢀ 0.86 V
and ꢀ 0.78 V, with onset reduction potentials of ꢀ 0.66 V and ꢀ 0.59 V,
respectively (Figure S3b and d), which can be correlated with the
reduction of fused bis-lactam units. The LUMO energy levels are calcu-
be ascribed to the possible π-π stacking aided with some additional non-
bonding interactions. On the other hand, XRD analysis of as-spun thin
film of compound 15 (Figure S4b) show sharp and intense (100) peak at
4.2◦, corresponding to the highly ordered (crystalline) lamellar packing
[26] with interlayer d-spacing distance of 21.0 Å and weak (200) peak at
8.5◦, showing interlayer lamellar d-spacing distance of 10.4 Å. This
observation is also in accordance with the observed enhanced vibronic
splitting in the absorption spectrum of the thin film of compound 15
(Fig. 7b; wine-red line), indicating the formation of highly ordered
(crystalline) structural arrangement of compound 15 in solid-state [23,
26]. Additionally, as a thin film of compound 15 show X-ray diffraction
peaks for interlayer d-spacings only, it can be assumed that molecules of
compound 15 adopt edge-on orientation respective to the substrate
[26].
lated using the equation: ELUMO = EHOMO + Eogpt and are found to be at
ꢀ 4.16 eV and ꢀ 4.19 eV, respectively for compounds 8 and 15. The
calculated HOMO and LUMO energy levels for compounds 8 and 15 are
much lower compared to the reported HOMO and LUMO energy levels
(ꢀ 5.49 eV and ꢀ 3.55 eV, respectively) of benzo[1,2-b:4,5-b’]bis[b]
benzothiophene bis-isatin [16] where as linear phenyl bis-isatin and
naphthalene bis-isatin have not been studied electrochemically [15,30].
The oxidation potentials of conjugated polymers P-1 and P-2 are
measured from the oxidation waves (Fig. 9a and c) and found to be at
+1.70 V and +1.79 V, with onset oxidation potentials of +1.41 V and
+1.36 V, respectively. The corresponding HOMO energy levels are
calculated from the onset oxidation potentials and found to be at ꢀ 5.88
and ꢀ 5.83 eV. The reduction potentials of P-1 and P-2 are measured
from the reduction wave and found to be at ꢀ 1.07 V and ꢀ 0.99 V,
respectively with onset reduction potentials of ꢀ 0.77 V and ꢀ 0.66 V,
For conjugated polymers, highly ordered chain packing with close
π-π stacking distance is desirable for efficient intermolecular charge
transport [27]. However, the XRD analysis of the thin film of P-1 is
showing the absence of any diffraction peaks in the low angle region
(Figure S4c), indicating the orientation of polymer chains in the
solid-state is completely random and no lamellar packing is observed.
On the other hand, thin-film XRD of P-2 is showing very weak (100)
diffraction peak at 4.96◦ indicating weak lamellar packing with inter-
layer d-spacing distance of 17.8 Å (Figure S4d). Additionally, XRD
analysis of P-1 and P-2 is showing very weakly intense broad diffraction
hump at around 30◦ (Figure S4c and d) as an indication of very weak
π-π
stacking between polymer chains. In general, it is evident from XRD
Table 3
Electrochemical properties of bis-isatins and conjugated polymers.
Compound
Eoxi(V)a
Eonset oxi(V)a
EHOMO(eV)b
Ered(V)a
Eonset red(V)a
ELUMO(eV)c
ELUMO(eV)d
Eegle(eV)e
8
(–f)
(+1.52)
(+1.47)
(+1.41)
(+1.36)
(ꢀ 5.99)
(ꢀ 5.94)
(ꢀ 5.88)
(ꢀ 5.83)
(ꢀ 0.86)
(ꢀ 0.78)
(ꢀ 1.07)
(ꢀ 0.99)
(ꢀ 0.66)
(ꢀ 0.59)
(ꢀ 0.77)
(ꢀ 0.66)
ꢀ 3.81
ꢀ 3.88
ꢀ 3.70
ꢀ 3.81
(ꢀ 4.16)
(ꢀ 4.19)
(ꢀ 4.38)
(ꢀ 4.38)
2.18
2.06
2.18
2.02
15
P-1
P-2
(–f)
(+1.70)
(+1.79)
a
b
c
Potential v/s Ag/Ag+.
Calculated from equation EHOMO = ꢀ (Eonset oxi + 4.8 ꢀ Eonset Fc/Fc+ ).
+
Calculated from equation ELUMO = ꢀ (Eonset red + 4.8 ꢀ Eonset Fc/Fc ).
d
Calculated from equation ELUMO = EHOMO + Egopt
.
e
f
Calculated from Eegle = ELUMO ꢀ EHOMO
.
Unresolved peak.
11

V.J. Bhanvadia et al.
Polymer 210 (2020) 123032
analysis, that the synthesized polymers are largely amorphous in nature.
002784). PKI acknowledges DST, India through the project DST/TSG/
PT/2009/23, CRG/2019/002614 and Max- Planck-Gesellschaft IGSTC/
MPG/PG(PKI)/2011A/48.
4. Conclusion
In the present manuscript, we report direct oxidation approach for
the syntheses of novel fused bis-isatins from corresponding pyrro-
loindoles using NBS/TBHP in DMF/CHCl3 for the synthesis of compound
8 and NBS in DMF/CHCl3 for the synthesis of compound 15. Thermal
analysis (TGA and DSC) shows that both compounds 8 and 15 are
thermally stable up to and above 200 ◦C and exhibit few solid-solid
transitions below their melting points. Measured HOMO energy levels
of fused bis-isatins are significantly lowered (below ꢀ 5.9 eV) due to the
presence of strong electron-withdrawing fused pyrrolodione units. Thin-
film morphological studies indicate the possible face-on orientation of
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.polymer.2020.123032.
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The famous indophenine reaction is re-visited with compounds 8 and
15 to form conjugated D-A polymers P-1 and P-2 (with 16–18 repeating
units and 12–15 repeating units for the polymer P-1 and P-2, respec-
tively as indicated by GPC) having a quinoidal backbone. The structural
aspects of synthesized polymers are studied by DFT calculations on
model compounds 16 and 17 of polymer P-1 and P-2, which revealed
existence of quinoidal pyrroloindole dione units and aromatic bithio-
phene units incorporated within the twisted polymer backbone (dihe-
dral angle < 25◦) with HOMO and LUMO concentrated on bithiophene
and pyrroloindole dione units, respectively. The polymers show poor
solubility properties in the common organic solvents, sufficiently high
thermal stabilities, lowered optical band-gaps as well as the random
orientation of polymer chains in the solid-state. Moreover, TD-DFT
calculations on model compounds 16 and 17 confirm that the intense
lowest energy absorption band arises from the HOMO-LUMO charge
transfer transitions. The polymers have lowered FMO energy levels
(HOMO levels below ꢀ 5.8 eV and LUMO levels below ꢀ 4.3 eV), which
can be ascribed to the presence of strong electron-withdrawing quinoi-
dal pyrroloindole dione units. Thin-film morphological studies of P-1
and P-2 indicate largely amorphous nature of the thin films of the
polymers. These properties make conjugated polymers P-1 and P-2 po-
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Acknowledgements
VJB is thankful to the University Grants Commission (UGC), Delhi for
the UGC-BSR fellowship (F.4-1/2006(BSR)/7–303/2010(BSR)). ALP
thanks SERB-DST, New Delhi for the financial support (F.No. EEQ/
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program for funding NMR facility at Department of Chemistry, Faculty
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