Synthesis and photophysical characterization of isoindigo building blocks as molecular acceptors for organic photovoltaics

Article abstract of DOI:10.1016/j.saa.2018.05.048

Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to investigate their intramolecular charge transfer characteristics. UV–vis absorption of these dyes exhibits a wide absorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72 and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices, the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.

Full text of DOI:10.1016/j.saa.2018.05.048

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and photophysical characterization of isoindigo building blocks
as molecular acceptors for organic photovoltaics
a
b
b
Haluk Dinçalp a, , Gözde Murat Saltan , Ceylan Zafer , Adem Mutlu
⁎
a
Department of Chemistry, Faculty of Arts and Science, Manisa Celal Bayar University, Yunus Emre, 45140 Manisa, Turkey
Solar Energy Institute, Ege University, Bornova, 35100, Izmir, Turkey
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Five isoindigo-based donor-acceptor-donor (D-A-D) type small molecules have been synthesized in order to in-
vestigate their intramolecular charge transfer characteristics. UV–vis absorption of these dyes exhibits a wide ab-
sorption band ranging from 300 to 650 nm with two distinct bands, giving the narrow bandgaps between 1.72
and 1.85 eV. Taking into account their HOMO-LUMO energy levels and bandgaps, isoindigo dyes have been
used in the active layer of organic solar cell (OSC) devices. When these small molecule semiconductors were
used as acceptors with the donor poly(3-hexylthiophene-2,5-diyl (P3HT) polymer in the inverted OSC devices,
the highest power conversion efficiency (PCE) was obtained as 0.10% for pyrene-substituted isoindigo derivative.
© 2018 Elsevier B.V. All rights reserved.
Received 15 January 2018
Received in revised form 11 May 2018
Accepted 13 May 2018
Available online xxxx
Keywords:
Isoindigo
Pyrene
Intramolecular charge transfer
Acceptor
Inverted organic solar cell
1. Introduction
intramolecular charge transfer (ICT) process between the donor and ac-
ceptor side of the molecule reduces the bandgap and lowers the LUMO
Recently, bulk heterojunction organic solar cells (BHJ-OSCs) which
use conjugated polymers and fullerene derivatives as donor and accep-
tor components, respectively, are frequently encountered in solar en-
ergy conversion technologies. These types of OSCs give the highly
efficient performances with the power conversion efficiency (PCE) of
over 10% [1,2]. Even though polymer OSCs exhibit highly efficient
solar cell performance, some difficulties of polymers in their synthetic
reproducibility, purification of structure, and molecular weight distribu-
tion of main structure are major problems to overcome in the field of
commercialization for this kind of polymeric donors [3,4]. Small mole-
cule donors for OSC applications with efficient PCE value over 10% [5]
have gained as an alternative structure to their polymeric counterparts
because of their high purities, well-defined chemical compositions and
good solution processabilities [6–8]. Although most OSCs have used ful-
lerene derivatives in the active layers, there have been some undesir-
able properties for fullerene acceptors including their weak absorption
in the vis-NIR regions, limited tunability of their electron affinities, ther-
mal and photochemical instabilities [9,10]. During the last years non-
fullerene small molecule organic acceptors have been extensively used
and given the much higher PCEs in the range of 11–13% [11–13].
Researchers have designed and developed conjugated small mole-
cules with electron donor-acceptor (D-A) forms in order to obtain
highly effective component for OSC devices. It is known that
energy level of the molecule in addition improves charge transport abil-
ity of small molecules [14–16]. These are considered some of the main
strategies to overcome high efficiency in OSC devices.
Among promising small molecule organic semiconductors, isoindigo
dyes get much higher attention because of their outstanding properties
such as good electron-accepting capability, quite planar structure, and
great availability from natural resources, and easy functionalization by
side chain reactions [17,18]. Isoindigo is naturally found in the leaves
of Isatis tinctoria as a minor isomer of indigo dye [19]. The isoindigo
structure is virtually insoluble in common organic solvents due to the
intermolecular hydrogen bonding among the lactam rings and due to
their strong π–π stacking. Generally, branched alkyl groups can be at-
tached to nitrogen atom of the lactam ring to render them soluble in
common organic solvents [17,18]. After the first introduction of
isoindigo dyes in OSC applications reported by Reynolds et al. in 2010
[20], researches are mainly focused on the synthesis of conjugated poly-
mer structures of isoindigo derivatives giving high cell efficiencies in the
literature [21–27]. In addition, isoindigo-based small molecules have
been used in OSC devices giving moderately high cell efficiencies
[18,28–33]. Elsawy et al. in 2013 investigated electron-donating effect
of different thiophene chains on OSC device performance of isoindigo
dye and reported 3.2% cell efficiency for D-A-D type isoindigo dye con-
taining three conjugated thiophene rings. They proved that enhanced
ICT in isoindigo dye shifts the absorption to red region of solar spectra
[28]. Relatively much lower PCE of isoindigo-based small molecules is
generally ascribed to charge transport inefficiencies and morphology
⁎
Corresponding author.
E-mail address: haluk.dincalp@cbu.edu.tr (H. Dinçalp).
https://doi.org/10.1016/j.saa.2018.05.048
1386-1425/© 2018 Elsevier B.V. All rights reserved.

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
197
problems [34]. Different electron donor groups were attached to
isoindigo-like molecules in order to get better cell efficiency by chang-
ing the optoelectronic properties, solution-processabilities, and solid
state morphology [34].
In this work, we have synthesized five different isoindigo-based D-
A-D type dyes with different donor functional groups including pyrene,
indole, benzimidazole, dibenzothiophene, and carbazole donors.
Scheme 1 illustrates the molecular structure of the synthesized dyes.
Structure-optical property relationship was investigated in different
solvents, especially taking into account ICT possibilities. Effect of the dif-
ferent electron donating strengths on photovoltaic performance of
inverted BHJ-OSC devices using isoindigo core as acceptor component
was compared.
Cyclic voltammetry (CV) measurements of compounds Isoi-(IV–
VIII) dyes in MeCN solution were measured using 100 mM [TBA][PF6]
as the electrolyte in a CH instruments 660B-Electrochemical Worksta-
tion at a scan rate of 100 mV/s. Ferrocene was used as the internal stan-
dard and its oxidation potential was detected at +0.57 V. A standard
three-electrode configuration cell composed of a platinum wire auxil-
iary electrode, a glassy carbon working electrode, and an Ag/Ag+ refer-
ence electrode (SCE) was used to measure the oxidation and reduction
potentials of the target compounds. The HOMO and LUMO energy levels
were calculated from the CV measurements using the given equations
below [40]:
À
Á
À
Á
onset
red
EHOMO ¼ −e Eooxnset þ 4:8 ; ELUMO ¼ −e E
þ 4:8
ꢀ
Eogpt
¼
onset ; EHOMO ¼ ELUMO−Egopt
1240
2. Experimental
λabs
2.1. Materials and Reagents
Theoretical calculation was carried out by using density functional
theory (DFT) [41] at the B3LYP/6-31G(d) [37] basis with the Gaussian
09 package in order to provide an insight into the disparity in molecular
conformation of synthesized small molecules.
Zinc powder, 2,5-dibromonitrobenzene, diethylmalonate, 6-
bromoisatin, 2-ethylhexyl bromide, tetrakis(triphenylphosphine)palla-
dium (0) (Pd(PPh3)4), pyrene-1-boronic acid, 5-formyl-2-thienyl bo-
ronic acid, 5-indole boronic acid, dibenzothiophene-2-boronic acid
and 9-ethylcarbazole-3-boronic acid were obtained from Sigma Aldrich,
and they were used without further purification. Other reagents and
solvents were purchased commercially as analytical-grade quality and
used without further purification.
2.3. Fabrication and Characterization of OPV Devices
The inverted solar cells were fabricated as follows: ITO/TiO2/Active
layer/MoO3/Ag common sandwich structure. Scheme 2 shows the sche-
matic illustration of inverted solar cells. The indium tin oxide (ITO)
coated glass substrates (sheet resistance = 15 Ω/sq) were cleaned in
an ultrasonic bath with deionized water, acetone and isopropyl alcohol
for 20 min at 50 °C in each step, respectively. Titanium dioxide (TiO2)
solution was synthesized with sol-gel method [42]. Prior to coated,
TiO2 solution was filtered through a 0.45 μm filter. Then, TiO2 as a cath-
ode buffer layer was coated on the ITO at 1500 rpm for 1 min with spin
coating technique. Next, each sample was transferred to a muffle fur-
nace and annealed at 300 °C for 5 min, then 500 °C for 1 h. The
photoactive layer was deposited on the TiO2 layer at 1500 rpm for
1 min as a solution of P3HT and Isoi-(IV–VIII) (1:1 w/w) in chloroben-
zene (with concentrations of 20 mg/mL) with spin coater at room tem-
perature in ambient condition. Then, the photoactive layers were
thermally annealed at 110 °C for 10 min in glove box. To complete de-
vice fabrication, MoO3/Ag (10 nm/55 nm) top electrodes were depos-
ited by thermal evaporation high vacuum at 2 × 10−6 Torr through a
shadow mask with a device area of 0.15 cm2. The deposited rate for
MoO3 and Ag were 0.3 and 0.7 Å/s, respectively.
2.2. Analytical Instruments
1H NMR and 13C NMR spectra were recorded using a Bruker
400 MHz spectrometer, with tetramethylsilane (TMS) as the internal
reference, chemical shifts were recorded in ppm. FT-IR spectra were re-
corded on a Perkin Elmer-Spectrum BX spectrophotometer preparing
KBr pellets. UV–Vis absorption spectra were recorded on a Perkin
Elmer Lambda 950 spectrophotometer. The fluorescence emission and
time-resolved fluorescence spectra of Isoi-(IV-VIII) dyes have been
measured by using FLS 920 Edinburg instrument. Single photon
counting measurements have been taken at the excitation wavelength
of 472.4 nm on an apparatus described elsewhere [37]. Fluorescence de-
cays were analyzed globally using exponential tail fit method [38]. Fluo-
rescence decay data were collected at λdetection = 560 nm for the dyes in
toluene solution. The quality of the fits has been judged by the fitting pa-
rameters such as χ2 ≤ 1.2 [39].
Scheme 1. The synthetic route to Isoi-(IV–VIII) dyes. (i) K2CO3, dried DMF, 50 °C, 18 h, Zn(s) H2SO4, H20, EtOH (1:1:3) [35], (ii) acetic acid, HCl, 26 h, reflux [35], (iii) 2-ethylhexyl bromide,
DMF, K2CO3, 100 °C [35], (iv) boronic acid derivative, 2 M Na2CO3, Pd(PPh3)4, EtOH/benzene, 85 °C [36].

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H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
Scheme 2. The schematic illustration of inverted type solar cells.
The dark and light current density-voltage curves of solar cells (J-V)
599 cm−1 1H NMR (400 MHz, DMSO-d6 δ 2.49 ppm): δ = 11.06 (2H,
.
were measured with a Keithley 2400 source meter as shown. AM1.5 fil-
ter was used as the light source and a light power of a 100 mW/cm2 was
used in all solar J-V measurements. Incident photon to current conver-
sion efficiency (IPCE) was detected with Enlitech QE-R instrument.
s), 8.97 (2H, d, J = 8.6 Hz), 7.15 (2H, d, J = 8.6 Hz), 6.96 (2H, s) ppm.
13C NMR [100 MHz, DMSO-d6 δ 39.9 (7 peaks)]: δ = 170.2, 146.6,
133.7, 132.0, 126.7, 125.0, 121.7, 113.3 ppm.
2.4.3. (3E)-6,6′-Dibromo-1,1′-bis(2-ethylhexyl)-3,3′-biindole-2,2′(1H,1′
H)-dione (Isoi-III)
2.4. Synthesis Procedures
2-Ethylhexyl bromide (47 μL, 0.26 mmol) was added to a suspension
of Isoi-II (50 mg, 0.12 mmol) and potassium carbonate (83 mg,
0.60 mmol) in dimethylformamide (DMF) (3 mL) under nitrogen atmo-
sphere. The resulting dark brown solution was left to react for 100 °C for
15 h. After cooling to room temperature, the reaction mixture was
poured into water and extracted with chloroform. The combined or-
ganic phases were washed with brine, and then dried over MgSO4. Re-
moval of the solvent gave a crude product, which was purified by
silica-gel column chromatography using a mixture of n-hexane:dichlo-
romethane (v/v 1:1) as eluent to obtain compound, yielding 60%. FT-IR
(KBr pellet, cm−1): 3128 (aromatic νC\\H), 2957, 2928, 2857 (aliphatic
2.4.1. Synthesis of 6-Bromo-1,3-dihydro-2H-indole-2-one (Isoi-I)
To a suspension of 2,5-dibromonitrobenzene (1 g, 3.55 mmol) in
DMSO (2.6 mL) was added anhydrous K2CO3 (4.92 g, 35.6 mmol) in
one portion under an argon atmosphere. The mixture was heated to
50 °C, and then diethylmalonate (4.92 g, 30.7 mmol) in DMSO (5 mL)
was added in small portions. The reaction mixture was left to react for
18 h after which it was extracted with diethyl ether and the combined
organic phases were washed with water. After removal of the solvents
under reduced pressure, this core product was used without further pu-
rification in the next step and was dissolved in a mixture of 22 mL of
ethanol, 7.3 mL of water and 7.3 mL of conc. Sulfuric acid, and heated
to reflux. Zinc powder (2.3 g, 35.2 mmol) was added to the reaction ves-
sel slowly. After 1 h later, the second portion of zinc powder (2.3 g,
35.2 mmol) was added. The reaction was refluxed for 2 h. Then, the so-
lution was poured into 100 mL of water. The product was left to crystal-
lize overnight after which it was filtered. The precipitate was washed
with water, yielding 65%. FT-IR (KBr pellet, cm−1): 3108, 3064, 3023
(aromatic νC\\H), 1727 (νC_O), 1664, 1618 (aromatic νC_C), 1483,
ν
C\\H), 1694 (νC_O), 1595 (aromatic νC_C), 1473, 1447, 1352, 1192,
1111, 1075, 869, 811, 588 cm−1 1H NMR (400 MHz, CDCl3
.
δ
7.26 ppm): δ = 9.05 (2H, d, J = 8.6 Hz), 7.16 (2H, dd, J = 8.6, 1.8 Hz),
6.89 (2H, d, J = 1.8 Hz), 3.62 (4H, t, J = 7.5 Hz), 1.87–1.79 (2H, m),
1.38–1.28 (16H, m), 0.95–0.88 (12H, m) ppm. 13C NMR [100 MHz,
CDCl3 δ 76.9 (3 peaks)]: δ = 168.0, 146.1, 132.4, 131.0, 126.5, 125.1,
120.3, 111.5, 44.3, 37.4, 30.6, 28.5, 23.9, 23.1, 14.0, 10.5 ppm.
1459, 1333, 1307, 1254, 1232, 1202, 1053, 924, 801, 696, 552 cm−1
.
2.4.4. {5-[4,7-Bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]-2-thienyl}
Boronic Acid
1H NMR (400 MHz, DMSO d6 δ 2.49 ppm): δ = 10.48 (1H, s), 7.14
(1H, d, J = 7.8 Hz), 7.09 (1H, dd, J = 7.8 Hz, 1.7 Hz), 6.93 (1H, d, J =
1.7 Hz), 3.42 (2H, s) ppm. 13C NMR [100 MHz, DMSO-d6 δ 39.9 (7
peaks)]: δ = 177.7, 146.5, 127.3, 126.0, 124.6, 120.6, 112.7, 35.7 ppm.
4,7-Dibromo-2,1,3-benzothiadiazole starting compound, 4,7-bis(3-
methoxyphenyl)-2,1,3-benzothiadiazole and bisamine (3,3″-
dimethoxy-1,1′:4′,1″-terphenyl-2′,3′-diamine) were synthesized ac-
cording to literature [43,44]. Bisamine (50 mg, 0.16 mmol) solution in
3 mL of acetonitrile was mixed with 5-formyl-2-thienyl boronic acid
(23 mg, 0.15 mmol). The mixture was stirred at room temperature over-
night, and then the resulting orange precipitates were collected by fil-
tration, washed with methanol, and dried to give {5-[4,7-bis(3-
2.4.2. (3E)-6,6′-Dibromo-3,3′-biindole-2,2′(1H,1′H)-dione (Isoi-II)
Isoi-I (90 mg, 0.42 mmol) and 6-bromoisatin (96.4 mg, 0.43 mmol)
were dissolved in the mixture of acetic acid (2.8 mL) and conc. HCl
(22 μL), and heated to reflux for 26 h. Then, the mixture was cooled to
room temperature and filtered as a dark brown solid. The solid was
washed with water, ethanol and ethylacetate respectively. The solids
were dried under vacuum, yielding 42%. FT-IR (KBr pellet, cm−1):
3172, 3135, 3047 (aromatic νC\\H), 1686 (νC_O), 1610 (aromatic
methoxyphenyl)-1H-benzimidazole-2-yl]-2-thienyl}boronic
acid,
yielding 43%. FT-IR (KBr pellet, cm−1): 3436 (O\\H bending), 2935,
2832 (aliphatic νC–H), 1636, 1607, 1578, 1420, 1319, 1265, 1228, 1108,
1043,779, 697 cm−1 1H NMR (400 MHz, DMSO-d6 δ 2.47 ppm): δ =
.
ν
C_C), 1452, 1320, 1255, 1197, 1124, 1066, 884, 848, 816, 686,
12.80 (1H, s), 8.38 (2H, s), 8.11 (1H, d, J = 3.7 Hz), 7.68 (1H, d, J =

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
199
3.7 Hz), 7.51–7.39 (4H, m), 7.31–7.25 (4H, m), 6.94 (2H, s), 3.85 (6H,
6.8 Hz), 0.91 (6H, d, J = 7.0 Hz) ppm. 13C NMR [100 MHz, CDCl3
δ
76.9 (3 peaks)]: δ = 168.7, 145.7, 135.8, 133.8, 133.4, 128.4, 125.5,
124.6, 121.4, 120.5, 119.3, 119.1, 111.5, 108.1, 106.8, 103.3, 44.1, 37.6,
29.6, 28.8, 24.2, 23.0, 14.1, 10.7 ppm.
s) ppm. 13C NMR [100 MHz, CDCl3 δ 39.7 (7 peaks)]: δ = 159.9, 159.6,
148.5, 139.8, 139.6, 138.8, 137.1, 137.0, 130.4, 130.2, 130.0, 129.7,
129.0, 121.1, 115.1, 114.0, 113.3, 55.4 ppm.
2.4.4.1. General Procedure A for the Suzuki Cross Coupling Reactions. In the
typical procedure, an oven-dried Schlenk flask, a mixture of aryl halide
(1 mol equiv.), aryl boronic acid (2.2 mol equiv.), Na2CO3 (2 M, ~4 mL
aqueous solution), and tetrakis(triphenylphosphine)palladium (0) (Pd
(PPh3)4) (0.1 mol equiv.) were added into the solution of benzene:eth-
anol mixture. The reaction mixture was stirred for 15–24 h at 85 °C.
After completion of the reaction as monitored by TLC, the mixture was
cooled down to room temperature and poured into 100 mL of water,
and then extracted with chloroform (3 × 50 mL). The organic phase
was separated with separatory funnel, dried over anhydrous MgSO4, fil-
tered, concentrated in vacuum and purified by column chromatography
over silica gel.
2.4.4.2. General Procedure B for the Suzuki Cross Coupling Reaction. To a so-
lution of aryl halide (1 mol equiv.) in THF was added ~2 mL aqueous so-
lution of 2 M Na2CO3. Subsequently, tetrakis(triphenylphosphine)
palladium (0) (Pd(PPh3)4) (0.1 mol equiv.) was added to the mixture
at 45 °C under an argon atmosphere, and the mixture was stirred at
45 °C for half an hour. Then, aryl boronic acid (2.2 mol equiv.) in THF
was added in small portions, and the mixture was heated to 75 °C
under an argon atmosphere for 24 h. The reaction mixture was poured
into 100 mL of water and extracted with chloroform (3 × 50 mL). The
organic layer was dried over anhydrous MgSO4, and evaporated in vac-
uum to dryness. The residue was separated by column chromatography
using silica gel.
2.4.5. (3E)-1,1′-bis(2-ethylhexyl)-6,6′-dipyren-1-yl-3,3′-biindole-2,2′
(1H,1′H)-dione (Isoi-IV)
The general procedure A above was followed using Isoi-III (18 mg,
28 μmol), pyrene-1-boronic acid (15 mg, 61 μmol), aqueous solution
of Na2CO3, catalyst tetrakis(triphenylphosphine)palladium (0) (Pd
(PPh3)4), ethanol (1.5 mL) and benzene (8 mL). The crude product
was subjected to the standard workup and chromatography using di-
chloromethane/n-hexane (v/v 1:1). The product was isolated as a dark
brown solid, yielding 60%. FT-IR (KBr pellet, cm−1): 3551, 3478, 3414,
3232, 2925 (aliphatic νC\\H), 1692 (νC_O), 1639 (aromatic νC_C),
1615, 1454, 1353, 1194, 1108, 829, 627, 479 cm−1. 1H NMR (400 MHz,
CDCl3 δ 7.26 ppm): δ = 9.42 (2H, d, J = 8.5 Hz), 8.35 (2H, d, J =
9.3 Hz), 8.27 (2H, d, J = 7.8 Hz), 8.22 (4H, t, J = 7.0 Hz), 8.12 (4H, s),
8.10–8.04 (6H, m), 7.40 (2H, d, J = 8.5 Hz), 7.11 (2H, s), 3.76 (4H, m),
2.00 (2H, m), 1.28–1.44 (16H, m), 0.97 (6H, t, J = 7.3 Hz), 0.86 (6H, t,
J = 6.8 Hz) ppm. 13C NMR [100 MHz, CDCl3 δ 77.0 (3 peaks)]: δ =
168.7, 145.4, 145.3, 137.1, 133.0, 131.4, 131.0, 130.9, 129.7, 128.4,
127.8, 127.3, 127.1, 126.1, 125.4, 125.0, 125.0, 124.8, 124.7, 124.6,
120.8, 110.5, 44.4, 37.8, 30.7, 28.8, 24.1, 23.1, 14.1, 10.7 ppm.
2.4.6. (3′E)-1,1‴-diethyl-1′,1″-bis(2-ethylhexyl)-1H,1‴H-5,6′:3′,3″:6″,5‴-
quaterindole-2′,2″(1′H,1″H)-dione (Isoi-V)
The general procedure B above was followed using Isoi-III (60 mg, 90
μmol), 5-indole boronic acid (30 mg, 0.19 mmol), aqueous solution of
Na2CO3, catalyst tetrakis(triphenylphosphine)palladium (0) (Pd(PPh3)
4) and THF (6 mL). The crude product was subjected to the standard
workup and chromatography using dichloromethane:n-hexane (v/v
3:1). The product was isolated as a dark red solid, yielding 55%, FT-IR
(KBr pellet, cm−1): 3439, 2952, 2925, 2845 (aliphatic νC\\H), 1684
(νC_O), 1613 (aromatic νC_C), 1594, 1454, 1382, 1345, 1230, 1160,
1107, 794, 744, 626 cm−1 1H NMR (400 MHz, CDCl3 δ 7.26 ppm): δ
.
= 9.23–9.16 (2H, m), 9.07 (2H, d, J = 8.6 Hz), 8.36 (2H, s), 7.92 (2H,
s), 7.48 (2H, d, J = 3.9 Hz), 7.34 (2H, d, J = 7.8 Hz), 7.04 (2H, d, J =
8.6 Hz), 6.78 (2H, d, J = 7.8 Hz), 6.64 (2H, s), 3.74–3.62 (4H, m), 1.90
(2H, dd, J = 11.6, 5.0 Hz), 1.43–1.26 (16H, m), 0.95 (6H, d, J =
Fig. 1. The normalized UV–vis absorption spectra of (a) Isoi-(IV–VI), and (b) Isoi-(VII, VIII)
dyes in toluene solution. (c) Comparison of the normalized UV–vis absorption spectra of
Isoi-VI dye in different solvents and, also on thin film.

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2.4.7. (3E)-6,6′-bis{5-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-
yl]-2-thienyl}-1-[(2R)-2-ethylhexyl]-1′-[(2S)-2-ethylhexyl]-3,3′-biindole-
2,2′(1H,1′H)-dione (Isoi-VI)
(2H, d, J = 7.7 Hz), 7.79 (2H, dd, J = 8.5, 1.6 Hz), 7.51 (4H, dd, J =
13.7, 8.5 Hz), 7.47–7.41 (4H, m), 7.28 (2H, dd, J = 13.7, 6.6 Hz), 7.09
(2H, s), 4.40 (4H, q, J = 6.9 Hz), 3.84–3.66 (4H, m), 2.01–1.91 (2H, m),
1.54–1.25 (22H, m), 0.99 (6H, t, J = 6.8 Hz), 0.92 (6H, t, J = 6.9 Hz)
ppm. 13C NMR [100 MHz, CDCl3 δ 77.0 (3 peaks)]: δ = 168.9, 145.7,
145.6, 140.4, 139.9, 132.0, 131.5, 130.0, 126.0, 124.9, 123.5, 123.0,
120.6, 120.5, 120.2, 119.1, 118.8, 108.8, 108.7, 106.4, 44.1, 37.8, 37.7,
30.8, 28.9, 24.2, 23.1, 14.1, 13.8, 10.8 ppm.
The general procedure B above was followed using Isoi-III (25 mg,
38.8 μmol), {5-[4,7-bis(3-methoxyphenyl)-1H-benzimidazole-2-yl]-2-
thienyl}boronic acid (35 mg, 76.7 μmol), aqueous solution of Na2CO3,
catalyst tetrakis(triphenylphosphine)palladium (0) (Pd(PPh3)4) and
THF (6 mL). The crude product was subjected to the standard workup
and chromatography using n-hexane:ethyl acetate (v/v 4:1). The prod-
uct was isolated as a dark red solid, yielding 40%. FT-IR (KBr pellet,
cm−1): 3415, 2953, 2926 (aliphatic νC\\H), 1697 (νC_O), 1618 (aromatic
3. Results and Discussion
ν
C_C), 1598, 1474, 1353, 1225, 1108, 871, 614 cm−1. 1H NMR (400 MHz,
3.1. Steady State Measurements
CDCl3 δ 7.26 ppm): δ = 9.57 (2H, s), 9.18 (2H, d, J = 8.6 Hz), 9.05 (2H, d,
J = 8.6 Hz), 7.83 (2H, s), 7.71 (2H, d, J = 7.5 Hz), 7.59 (2H, d, J = 3.8 Hz),
7.56 (2H, d, J = 7.7 Hz), 7.48–7.35 (6H, m), 7.31 (2H, dd, J = 8.5, 1.7 Hz),
7.20 (2H, s), 7.16 (2H, dd, J = 8.6, 1.8 Hz), 7.01–6.95 (4H, m), 6.89 (2H,
d, J = 1.8 Hz), 3.95 (6H, s), 3.90 (6H, s), 3.75–3.58 (4H, m), 1.94–1.77
(2H, m), 1.45–1.24 (16H, m), 0.97–0.86 (12H, m) ppm. 13C NMR
[100 MHz, CDCl3 δ 77.0 (3 peaks)]: δ = 168.2, 165.8, 163.2, 160.4,
155.9, 148.2, 146.5, 145.7, 141.4, 139.3, 137.2, 132.9, 131.7, 130.8,
130.4, 124.9, 122.1, 116.8, 115.5, 110.8, 106.9, 105.0, 55.2, 52.4, 44.1,
30.5, 29.5, 28.6, 23.0, 14.0, 10.7 ppm.
The UV―visible absorption spectra of synthesized isoindigo dyes in
toluene solution and on thin-films are shown in Fig. 1., and correspond-
ing optical information are summarized in Table 1. As shown in Fig. 1a
and b, the higher energy bands around 422–456 nm are attributed to
π–π* transitions on isoindigo backbone. These bands are resulted from
the additional π–π overlapping of donor segments. The lower energy
bands around 527–547 nm are assigned to ICT between donor parts of
the structure and isoindigo acceptor [21–24,45]. Also, the shortest UV ab-
sorption bands below 400 nm are corresponded to π–π* transitions of
each donor fragments (pyrene, indole, benzimidazole, dibenzothiophene
and carbazole) attached to isoindigo core. In the studied solvents, long-
wavelength absorption shifts are more pronounced for Isoi-VI and Isoi-
VIII dyes containing benzimidazole and carbazole segments, respec-
tively. These observations are attributed to the formation of highly effi-
cient ICT complex for these dyes. These segments slightly increase the
ICT complex between the donor side and isoindigo by enhancing the
electron-withdrawing capacity of isoindigo backbone. This was not ob-
served for Isoi-(IV, V, VII) dyes giving much higher energy absorption
bands for their ICT complexes. This may be ascribed to the electron
donor strength of fragments.
Thin film absorption spectrum of Isoi-VI dye is depicted in Fig. 1c.
The absorption spectra of thin films show a little red shift in their ab-
sorption maxima as compared to their absorption spectra in solution
because of aggregate formation initiated by π–π stacking of aromatic
rings. Thin film absorption of Isoi-V dye gives the highest value at
592 nm. This may be attributed to the intermolecular stacking resulted
from hydrogen bond between N − H group of indole structure and non-
bonding electrons of heteroatoms. For solutions, Isoi-VIII dye gave the
most bathochromic shifts in visible spectra. More electron delocaliza-
tion between two phenyl rings supported by non-bonding electrons of
central-bridged nitrogen atom of carbazole structure in Isoi-VIII dye ex-
plained the red-shifted absorption maxima of the dye in the studied so-
lutions. All dyes give noteworthy red shift around 4–14 nm at their
long-wavelength absorption maxima in chloroform compared to the
corresponding spectra in other solvents even though used solvents
have similar dielectric constant values. This observation is ascribed to
higher polarizabilities of excited states for the studied dyes in chloro-
form solution.
2.4.8. (3E)-6,6′-bis(dibenzo[b,d]thien-4-yl)-1,1′-bis(2-ethylhexyl)-3,3′-
biindole-2,2′(1H,1′H)-dione (Isoi-VII)
The general procedure A above was followed using Isoi-III (20 mg,
31 μmol), dibenzothiophene-2-boronic acid (16 mg, 7 μmol), aqueous
solution of Na2CO3, catalyst tetrakis(triphenylphosphine)palladium
(0) (Pd(PPh3)4), ethanol (1.5 mL) and benzene (5 mL). The crude prod-
uct was subjected to the standard workup and chromatography using n-
hexane:dichloromethane (v/v 1:1). The product was isolated as a dark
red solid, yielding 35%, FT-IR (KBr pellet, cm−1): 3557, 3478, 3414,
3238, 2924 (aliphatic νC\\H), 1692 (νC_O), 1639 (aromatic νC_C),
1616, 1457, 1440, 1108, 732, 623 cm−1
.
1H NMR (400 MHz, CDCl3
δ
7.26 ppm): δ = 9.34 (2H, d, J = 8.3 Hz), 8.21–8.18 (4H, m) 7.88–7.84
(2H, m), 7.59–7.56 (4H, m), 7.49 (4H, dd, J = 5.4, 3.7 Hz), 7.42 (2H,
dd, J = 8.3, 1.6 Hz), 7.27 (2H, d, J = 1.6 Hz), 3.84–3.66 (4H, m),
2.05–1.96 (2H, m), 1.51–1.27 (16H, m), 0.97 (6H, t, J = 7.4 Hz), 0.87
(6H, t, J = 7.0 Hz) ppm. 13C NMR [100 MHz, CDCl3 δ 77.0 (3 peaks)]: δ
= 168.6, 145.6, 144.5, 139.4, 138.2, 136.5, 136.3, 135.5, 132.9, 130.3,
126.9, 126.7, 125.2, 124.5, 122.6, 122.2, 121.7, 121.4, 121.1, 107.8, 44.4,
37.6, 30.8, 28.8, 24.1, 23.1, 14.1, 10.7 ppm.
2.4.9. (3E)-6,6′-bis(9-ethyl-9H-carbazole-3-yl)-1-[(2R)-2-ethylhexyl]-1′-
[(2S)-2-ethylhexyl]-3,3′-biindole-2,2′(1H,1′H)-dione (Isoi-VIII)
The general procedure A above was followed using Isoi-III (40 mg,
62 μmol), 9-ethylcarbazole-3-boronic acid (33 mg, 0.14 mmol), aqueous
solution of Na2CO3, catalyst tetrakis(triphenylphosphine)palladium
(0) (Pd(PPh3)4), ethanol (3 mL) and benzene (10 mL). The crude prod-
uct was subjected to the standard workup and chromatography using
only dichloromethane. The product was isolated as a dark brown
solid, yielding 55%, FT-IR (KBr pellet, cm−1): 3439, 2952, 2925, 2845 (al-
iphatic νC\\H), 1684 (νC_O), 1613 (aromatic νC_C), 1594, 1454, 1382,
The fluorescence emission spectra of isoindigo dyes in toluene and
other studied solvents are shown in Fig. 2. As shown in Fig. 2a and b,
dyes show mainly two emission regions when excited at 485 nm, the
peaks in the short-wavelength region are located around 516–575 nm,
1345, 1230, 1160, 1107, 794 cm−1 1H NMR (400 MHz, CDCl3
. δ
7.26 ppm): δ = 9.27 (2H, d, J = 8.3 Hz), 8.38 (2H, d, J = 1.6 Hz), 8.19
Table 1
UV–visible absorption (λ (nm)) and emission (λem (nm)) wavelengths of Isoi-(IV–VIII) dyes in different solvents and, also on thin films (λexc = 485 nm).
Dyes/solvents
PhCH3
CHCl3
THF
Film
λabs
λem
λabs
λem
λabs
λem
λabs
Isoi-IV
Isoi-V
Isoi-VI
Isoi-VII
Isoi-VIII
528, 453, 345
527, 422, 284
539, 447, 368
528, 422, 332
547, 456, 304
629, 575, 534
736, 567, 525
709, 551
722, 566, 532
722, 566, 527
537, 458, 345
531, 425, 285
550, 465, 363
539, 424, 329
559, 464, 351
619, 568, 534
746, 566, 525
715, 568, 544
598, 559
527, 437, 343
524, 425, 286
545, 467, 368
525, 418, 330
551, 455, 351
618, 568, 527
615, 565, 526
734, 563, 532
564, 531
561, 485, 353
592
558, 378
538, 431
591, 435
747, 560, 516
721, 563, 524

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
201
and the peaks in the long-wavelength region lie along with 598–747 nm
emission scale. The short-wavelength emission is originated from ex-
cited state of isoindigo core. The long-wavelength emission is assigned
to ICT complex formed in the ground state. Long-wavelength emissions
up to 700 nm for different D-A-D type isoindigo derivatives in the liter-
ature are taken as evidence for observation of this unusual emission
Fig. 3. (a) The normalized fluorescence emission spectra of Isoi-(IV–VIII) dyes in
chloroform solution (λexc = 430 nm). (b) Excitation spectrum of Isoi-(IV, VI, VIII) dyes
in chloroform solution collected at long emission wavelength of each dye (λem
=
780 nm for Isoi-IV, 730 nm for Isoi-VI, and 750 nm for Isoi-VIII).
character for the studied isoindigo molecules [28,46]. Fig. 2c gives a
comparison of the normalized fluorescence emission spectra of Isoi-VI
dye in different solvents. Emissions of ICT complexes for Isoi-VI dye in
Fig. 2. The normalized fluorescence emission spectra (a) Isoi-(IV–VI) (λexc = 485 nm), and
(b) Isoi-(VII, VIII) dyes in toluene solution (λexc = 485 nm). (c) Comparison of the
normalized fluorescence emission spectra of Isoi-VI dye in different solvents (λexc
=
Fig. 4. Fluorescence decay analysis of Isoi-VI dye in toluene solution (λdetection = 560 nm).
485 nm).
Inset shows the weighted residuals of the measured decay.

202
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PhCH3, CHCl3 and THF are clearly seen at wavelengths of 709, 715 and
734 nm, respectively. On the other hand, long-wavelength emission of
Isoi-V dye gave remarkable red shift of 121 nm in toluene as compared
to that of in THF solution. Also, Isoi-VII dye gave the highest red shift of
124 nm in toluene as compared to that of in chloroform solution. Inter-
estingly, Isoi-VII dye did not give any ICT emission signal in THF solution
although all dyes gave ICT emission signals properly under the same
conditions. Noting that the high sensitivity to solvents is due to charge
shift away from donor segments to central part of isoindigo backbone.
This result is an indicative for the formation of large dipole moments
in the excited state. This dipole moment interacts with the used solvent
molecules specifically to reduce the energy of the excited state [47].
3.2. Excited State Complex Formation
Fluorescence characteristics of isoindigo dyes are generally studied
in lower polarity of solvents because of their suitable fluorescence
quantum yields in there. Upon photoexcitation of isoindigo derivatives,
firstly photoinduced electron transfer occurs, then non-radiative
deactivation pathways were dominated in the excited state [17]. Triplet
excited state form of some specific isoindigo dyes such as
cyclometallated platinum(II) complexes was observed. This was attrib-
uted to enhanced conjugation length through 6,6′-position of isoindigo
core [48]. In the other study, 7,7′-diazaisoindigo derivatives gave rela-
tively long fluorescence lifetime as compared to pristine isoindigo [49].
More experiments have been planned in order to understand ex-
cited state dynamics of D-A-D type isoindigo derivatives. When the
samples are excited at isoindigo π–π* absorption at 430 nm, as shown
in Fig. 3a, only characteristic emission peaks of isoindigo backbone
were observed between peak maxima of 500 and 560 nm. There is no
marked ICT emission at the excitation of this wavelength. We recorded
the excitation spectra to see the origin of the long emissions for each
isoindigo derivative. Fig. 3b gives the excitation spectrum of Isoi-(IV,
VI, VIII) dyes in chloroform solution. We observed that excitation spec-
tra of these dyes were almost similar to their absorption profiles. How-
ever, Isoi-(V, VII) dyes did not gave marked excitation spectra due to
their low signals which were collected emissions around 700–800 nm.
As a result, these much longer-wavelength of emission peaks are attrib-
uted to the formation of ICT complex formed between the ground state
donors and excited state isoindigo backbone.
The fluorescence lifetimes of isoindigo derivatives were measured
using time-correlated single-photon-counting system on an apparatus
at isoindigo emission around 560 nm in solutions as shown in Fig. 4,
and summarized in Table 2. Isoi-VI dye displays time profile with
biexponential decay consisting of a major contribution of 61.9% at
5.89 ns and a minor contribution of 38.1% of the long decay component
at 9.36 ns for in toluene solution. Generally, similar contributions were
Fig. 5. Cyclic voltammograms of (a) Isoi-(IV–VI), and (b) Isoi-(VII, VIII) dyes on glassy
carbon working electrode in 0.1 M [TBA][PF6]/Me-CN (Scan rate: 100 mV s−1).
detected for other dyes around 3–5 ns and also 8–21 ns in the studied
solvents. The decay components around 3–5 ns are attributed to the
fluorescence decay time of isoindigo backbone, the longer ones define
the decay profile of ICT complex. Some of the measurements are ana-
lyzed as triexponential decays consisting of additional much faster
Table 2
Fluorescence decay times (τf(n) (ns)) and residual values (α(n)) of Isoi-(IV–VIII) dyes in dif-
ferent solvents (λdetect = 560 nm).
Dyes/solvent
χ2
τf(1)
α1
τf(2)
α2
τf(3)
α3
Isoi-IV
PhCH3
CHCl3
THF
PhCH3
CHCl3
THF
PhCH3
CHCl3
THF
PhCH3
CHCl3
THF
PhCH3
CHCl3
THF
1.02
1.20
1.18
1.16
1.06
1.19
1.04
1.03
1.14
1.10
1.18
1.08
1.15
1.03
1.11
4.14
1.68
4.04
0.62
0.30
0.32
5.89
0.28
1.92
1.08
2.94
0.86
3.49
0.27
1.70
81.2
22.4
100
6.3
57.9
0.7
61.9
35.6
6.5
6.8
60.6
4.5
73.2
6.7
30.6
8.48
4.30
–
18.8
77.6
–
–
–
–
10.7
–
10.1
–
10.7
–
9.70
–
8.83
–
–
–
–
14.9
–
13.0
–
34.0
–
Table 3
Electrochemical values and HOMO-LUMO energies of Isoi-(IV–VIII) dyes with respect to
the vacuum level.
Isoi-V
4.27
3.49
4.52
9.36
3.72
9.43
3.86
6.81
3.72
6.37
3.20
4.30
78.8
42.1
86.3
38.1
30.4
93.5
78.1
39.4
70.3
26.8
79.4
69.4
Dyes
E0red2
(V)
E0red1
(V)
E0ox1
(V)
E0ox2
(V)
LUMO
(eV)
HOMO
(eV)
E0-0a (eV)
(abs)
Isoi-VI
Isoi-VII
Isoi-VIII
Isoi-IV
Isoi-V
Isoi-VI
Isoi-VII
Isoi-VIII
−1.05
−1.04
−0.90
−0.98
−1.06
−0.65
−0.64
−0.53
−0.58
−0.70
0.72
1.39
1.38
1.58
1.35
1.83
–
–
1.75
–
−3.58
−3.59
−3.70
−3.65
−3.53
−5.36
−5.33
−5.42
−5.50
−5.25
1.78
1.74
1.72
1.85
1.72
15.1
25.2
–
13.9
–
a
The zeroth–zeroth transition E0–0 values were estimated from the intersection of the
apsis and the straight line which is drawn from the red side of the UV–vis absorption band
on thin film.
21.4
–

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
203
decay times around 0.3–1.0 ns, indicating the short-lived species of
isoindigo acceptor.
reduction process, indicating the formation of stable isoindigo anion
radicals. These potentials are familiar to previously studied isoindigo
dyes [29,50,51]. The HOMO levels of isoindigo derivatives are around
between −5.25 and −5.50 eV. After adding a carbazole group instead
of dibenzothiophene rings, Isoi-VIII dye shows relatively narrower
band gap of 1.72 eV and a higher HOMO level of −5.25 as compared
to that of Isoi-VII dye.
Fig. 6 exhibits the electronic distribution on HOMO and LUMO or-
bitals for the geometrically optimized structure of isoindigo derivatives.
Electronic distribution of HOMO levels of all dyes has a substantial con-
tribution from the donor segments along with the axis of the molecules.
LUMO level of the dyes are entirely located on the central part of
isoindigo backbone. This supports the ICT process between the donor
and acceptor side of the molecules. When the molecules are excited
3.3. Electrochemical Properties
Fig. 5a and b give the cyclic voltammograms of Isoi-(IV–VIII) dyes on
glassy carbon working electrode in 0.1 M [TBA][PF6]/Me-CN, and elec-
trochemical measurements data are summarized in Table 3. Isoi-V,
Isoi-VI, and Isoi-VIII dyes give only one oxidation potentials at Eo0x
=
1.39, 1.38, and 1.35 V, respectively. Isoi-IV, and Isoi-VII dyes show addi-
tional oxidation potentials at 1.83 and 1.75 V, respectively. The lower
oxidation potentials are assigned to the donor substituents. All dyes
give two reduction waves around between −0.53 and −0.70V for the
first reduction, and between −0.90 and −1.06 V for the second
Fig. 6. Electronic distribution of first HOMO and LUMO energy orbitals of Isoi-(IV–VIII) dyes.

204
H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
Table 4
from HOMO to LUMO level, electron transfer occurs from the end-
capping donor to the central isoindigo acceptor.
Photovoltaic parameters and cell efficiency (η) values of inverted OSC devices using
(P3HT:Isoi) as active layer. Last row indicates the IPCE values of ITO/TiO2/P3HT:Isoi-(IV-
VIII)/MoO3/Ag device.
3.4. Photovoltaic Performances for Inverted Organic Solar Cells
Dyes parameters
Voc (mV)
Isoi-IV
Isoi-V
Isoi-VI
Isoi-VII
Isoi-VIII
Zhu et al. investigated the photovoltaic properties of two isoindigo-
pyrene type small molecules, which contain extended acetylene or thio-
phene π-bridge units, and reported that these acetylene- and
thiophene-based dyes gave PCEs of 0.80% and 1.88% as donor compo-
nents in OSC devices, respectively [52]. Another OSC device using
carbazole-isoindigo type dyes along with PC71BM as photoactive mate-
rials gave PCE of 0.38% [53]. When paired with PC61BM, isoindigo struc-
ture with hexyl bithiophene and a centered dithienylsilole core gave
PCE as high as 3.2%, among the best efficiencies for small molecule
isoindigo type dyes [54]. McAfee et al. focused on the development of
new isoindigo alternatives to fullerene acceptors in OSC devices. They
reported solution processable BHJ devices fabricated in inverted archi-
tectures, reaching the PCE of 1.9% [55]. In the other study, A-D-A′-D-A
type small molecule isoindigo acceptor was used in OSC device giving
the PCE of 2.82%. This array for isoindigo derivative was reported as an
effective strategy to design acceptors with superior sunlight harvesting
capacity [56]. In our study, we have used similar strategy for synthesiz-
ing Isoi-(IV–VIII) such dyes D-A-D type small molecules as acceptor
components and investigated their current-voltage characteristics in
BHJ-OSC devices fabricated with inverted architecture, as shown in
Fig. 7.
Devices based on P3HT:Isoi-IV active layer gives an open-circuit
voltage (Voc) of 0.54 V, a short-circuit current (Jsc) of 0.54 mA/cm2, a
fill factor (FF) of 33.1%, and a PCE of 0.10% at (1:1 w/w) donor/acceptor
weight ratio. Also, other results from the active layers of synthesized
isoindigo series are summarized in Table 4. Efficiencies are
corresponded to the related IPCE values, as compared in Fig. 8. The
best IPCE value among the studied dyes with enhanced ICT complex
contribution may account for the increase in Jsc (0.54 mA/cm2) in Isoi-
IV dye-based solar cell device. Absorption bands of P3HT:Isoi-(IV-VIII)
(1:1 w/w) active layers almost cover the visible region which is impor-
tant for photon-flux efficiency in OSC devices. Noting that synthesized
isoindigo acceptors can be good candidates to solve the problem related
with insufficient absorption for fullerene derivatives in visible region. As
shown in Fig. 1a and b, Isoi-(IV-VIII) dyes seem to be good choice in-
stead of fullerene due to their enhanced visible absorption bands
existed from their efficient ICT complexes.
540
270
460
360
480
J
sc (mA/cm2)
0.54
33.1
0.10
3.46
0.20
31.0
0.02
1.62
0.41
33.2
0.07
3.34
0.37
40.7
0.06
1.85
0.26
37.8
0.05
1.59
FF (%)
η (%)
IPCE (%)
for isoindigo derivatives were reported due to their inadequate phase-
separation or the presence of large crystalline domains inhibiting cell
performance [57]. Inverted OSC devices using perylene diimide-
isoindigo conjugates as acceptors gave good efficiencies depending on
their thin film re-organization upon the use of 1,8-diiodooctane (DIO)
additive. It was reported that addition of DIO was favorable on active
layer roughness in the re-organization of some thin films giving moder-
ately high cell efficiencies [58]. Low cell efficiencies for Isoi-(IV–VIII)
dyes may also be related with electron accepting capacity and backbone
planarity for the dyes described for isoindigo derivatives with different
electron affinities [55].
However, Isoi-V and Isoi-VIII dyes had the poorest photovoltaic per-
formances with PCE of 0.02% and 0.05%, respectively. It was noted that
Isoi-V and Isoi-VIII dyes had much longer absorptions on film state as
compared to the other dyes due to their aggregated state. These may
generate intermolecular recombination points and some defects be-
tween isoindigo rings which inhibit the charge injection. Fig. 9 illus-
trates energy relevance between the components of inverted type
BHJ-OSC devices using Isoi-IV dye as acceptor in the active area of the
device. The deep LUMO energy level of Isoi-IV is desirable for electron
transfer between the P3HT and TiO2 which results a higher open circuit
voltage of the device.
4. Conclusions
In summary, five novel small molecules based on isoindigo with lin-
ear D-A-D structures composed of different aryl electroactive segments
have been designed and synthesized, and their optical, excited state dy-
namics, electrochemical, and photovoltaic properties have been investi-
gated. All molecules exhibit broad and wide absorption bands ranging
from 300 to 650 nm including ICT complex absorption, a relatively
low bandgaps around 1.72–1.85 eV. These ICT species contribute to
the total quantum efficiencies. The five small molecules were used as
electron acceptor components in the active layer of inverted BHJ-OSC
In general, these low efficiencies may be assigned to intermolecular
interactions or π − π stacking of aromatic rings which promote low
charge mobilities due to their aggregated forms. Similar low efficiencies
Fig. 8. IPCE graphs for P3HT:Isoi-(IV–VIII) (1:1 w/w) active layers obtained from inverted
Fig. 7. J-V curves of inverted OSC devices plotted in the illuminated conditions.
OSC devices.

H. Dinçalp et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 202 (2018) 196–206
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Acknowledgements
Financial support for this work was provided by the Research Coun-
cil of Manisa Celal Bayar University with the project number of 2015-
114. The authors would like to thank to Ege University for giving the op-
portunity to use Gaussian 09W program for DFT calculations.
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