Bromo-substituted cibalackrot backbone, a versatile donor or acceptor main core for organic optoelectronic devices

Article abstract of DOI:10.1016/j.molstruc.2018.07.009

Cibalackrot (Ci-I), one of the latest highly conjugated compound possessing bis-lactam structure, was investigated with respect to their brominated derivatives in order to determine their suitable substitution points for the syntheses of new class of small molecules for optoelectronic devices. 7,14-Bis(4-bromophenyl) (Ci-II) and 3,10-dibromo (Ci-III) derivatives of cibalackrot possess moderately narrow band gaps of 2.15 and 2.09 eV, respectively. Notably, Ci-III dye exhibits more red-shifted ultraviolet–visible (UV–vis) absorption and fluorescence emission spectra as compared to that of Ci-II dye because Ci-III shows more prominent intramolecular charge transfer (ICT) complex than that of Ci-II dye. Electron mobilities of the order of 7.0 × 10^?4 cm²/V and 3.1 × 10^?4 cm²/V were measured using Ci-II and Ci-III as active layer, respectively. Charge transfer properties of the molecules were investigated in bulk heterojunction device configuration wherein Ci-III showed p-type behavior against n-type PCBM in photovoltaic device. Photovoltaic performance of Ci-III dye which was used as donor component is 20 times higher than that of the device in which this dye was used as acceptor.

Full text of DOI:10.1016/j.molstruc.2018.07.009

Journal of Molecular Structure 1173 (2018) 512e520
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Bromo-substituted cibalackrot backbone, a versatile donor or acceptor
main core for organic optoelectronic devices
Haluk Dinçalp a , G o€ zde Murat Saltan , Ceylan Zafer , Deniz Aykut Kıymaz
,
*
a
b
b
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:
Received 10 September 2017
Received in revised form
Cibalackrot (Ci-I), one of the latest highly conjugated compound possessing bis-lactam structure, was
investigated with respect to their brominated derivatives in order to determine their suitable substitu-
tion points for the syntheses of new class of small molecules for optoelectronic devices. 7,14-Bis(4-
bromophenyl) (Ci-II) and 3,10-dibromo (Ci-III) derivatives of cibalackrot possess moderately narrow
band gaps of 2.15 and 2.09 eV, respectively. Notably, Ci-III dye exhibits more red-shifted ultraviolet
evisible (UVevis) absorption and fluorescence emission spectra as compared to that of Ci-II dye
because Ci-III shows more prominent intramolecular charge transfer (ICT) complex than that of Ci-II dye.
Electron mobilities of the order of 7.0 ꢀ 10 cm /V and 3.1 ꢀ 10 cm /V were measured using Ci-II and
Ci-III as active layer, respectively. Charge transfer properties of the molecules were investigated in bulk
heterojunction device configuration wherein Ci-III showed p-type behavior against n-type PCBM in
photovoltaic device. Photovoltaic performance of Ci-III dye which was used as donor component is 20
times higher than that of the device in which this dye was used as acceptor.
10 June 2018
Accepted 3 July 2018
Available online 11 July 2018
Keywords:
Cibalackrot
Organic photovoltaic
Electron mobility
Charge transfer
Decay time
ꢁ
4
2
ꢁ4
2
©
2018 Elsevier B.V. All rights reserved.
1
. Introduction
transport property [5,6], high conjugated structure, excellent
electron acceptor property [7], high fluorescence quantum yield [8],
low-lying LUMO energy level, and narrow optical band gap [9].
Cibalackrot dye gives long-wavelength absorption profile with its
maximum absorptions around 540e560 nm [8] due to the
Nowadays, there are increasing efforts on structural variations
of conjugated polymers and small molecules for optoelectronic
devices in order to obtain the most suitable structural design for
managing high performance devices. Such kind of variations
tailored by electron donor (D) and electron-acceptor (A) groups of
the main core have great impact on optoelectronic parameters such
as optical bandgaps, fluorescence characteristics, absorption pro-
files, and ionization potentials [1]. Organic photovoltaics (OPVs)
have gained a considerable attention in the area of solar light-
energy conversion systems owing to their low cost, flexibility and
remarkable lightweight [2]. A considerable tendency of the mo-
lecular design for OPVs are externalized on attachment of different
D or A subunits to the suitable position of the electroactive back-
bone. These provide a wide range of molecular arrangements of
functional molecules for OPVs.
enhanced p-electron resonance localized between the indole and
lactam segments of the structure. Typically, it is noted that
strongly-bound localized excitons in the highly-disordered organic
semiconductors are the result of low dielectric constant. Higher
dielectric constants can be observed in molecules with the higher
degree of order such as cibalackrot dyes (ε ¼ 4.8) as compared to
indigo dyes (ε ¼ 4.3) [7]. Proton transfer pathway through intra-
molecular hydrogen bond is blocked because of highly-annulated
structure of cibalackrot as compared to that of indigo/tyrian pur-
ple type dyes, which exhibit an efficient excited state deactivation
pathway due to the proton-exchange mechanism [7,8].
Noting that annulation pathway through cibalackrot ring opens
up new category of design for acceptor type molecules. In ciba-
lackrot platform, oxindole units of indigo are fixed into a coplanar
geometry where carbonyl groups and nitrogen atoms are annulated
through acetyl chloride group to form bis-lactam structure of
cibalackrot [6,10].
Among the several small organic molecules with electroactive
core, cibalackrot dye [3,4], a bay-annulated indigo derivative, has
received considerable attention for its excellent ambipolar charge
*
Corresponding author.
Despite these advantages of cibalackrot dyes over other organic
E-mail address: haluk.dincalp@cbu.edu.tr (H. Dinçalp).
https://doi.org/10.1016/j.molstruc.2018.07.009
022-2860/© 2018 Elsevier B.V. All rights reserved.
0

H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
513
molecular acceptors, there are limited number of their applications
in OPV devices. To the best of our knowledge, only few attempts
have been made to investigate OPV devices using cibalackrot dye as
the photo-active layer. Bronstein et al. reported charge transport
and photodiode properties of a cibalackrot polymer in organic
electronic devices. This polymer exhibits high ambipolar transport
spectrophotometer and fluorescence emission spectroscopy using a
FLS 920 Edinburg instrument in different solvent of polarities in
1 cm optical path length cuvettes. Single photon counting results
and fluorescence decays were analyzed globally using the Edin-
burgh Instruments F900 exponential tail fit method [13] at the
excitation wavelength of 472.4 nm. The quality of the fits has been
2
2
in OFET device of 0.23 cm /V, and also OPV device efficiencies up to
judged by the fitting parameters such as
c
ꢂ 1.2 [14]. All spectro-
2
.35% [11]. Liu et al. explored the electrochromic efficiency of a
scopic measurements in solution were done at optical density
around 0.1.
cibalakrot polymer giving high optical contrast in the visible and
near-infrared region, good coloration efficiency and long term
stability [12].
2.3. Electrochemical measurements and DFT calculations
In this work, cibalackrot core was brominated to its phenyl or
indole rings (Scheme 1) in order to investigate the relationship
between substitution points of the structure and photovoltaic
performance. Their absorption-emission profiles and fluorescence
decay kinetics were investigated in different solvents of polarities.
Their electrochemical properties were evaluated in electron
mobility measurements and photovoltaic performances.
The electrochemical properties of the dyes were performed in
an acetonitrile solution with 0.1 M tetrabutylammoniumhexa-
fluorophosphate [TBA][PF ] as the supporting electrolyte, glassy
6
carbon as the working electrode, a Pt wire as the counter electrode,
þ
and a Ag/Ag as the reference electrode using a CH instrument
ꢁ
1
(660B-Electrochemical Workstation) at a scan rate of 100 mV s
.
Potentials were referenced to the ferrocenium/ferrocene redox
couple by using ferrocene as an internal standard and its oxidation
potential was detected at þ0.63 V. Onset values of Ered, Eox, and
other parameters were calculated according to the equation [15]:
2
. Experimental
2.1. Materials and reagents
ꢀ
ꢁ
ꢀ
ꢁ
onset
ox
onset
red
EHOMO ¼ ꢁe E
þ 4:8 ; E
¼ ꢁe E
þ 4:8
The main reagents used for the synthesis of cibalackrot dyes, 4-
Bromo-2-nitrobenzaldehyde and phenylacetyl chloride were pur-
chased from Sigma Aldrich and Merck Company, respectively. Tet-
3 4
rakis(triphenylphosphine)palladium(0) (Pd(PPh ) ) catalyst was
purchased from TCI company. All other chemicals and solvents
were purchased from Merck, and used without further purification.
Photovoltaic materials [6,6]-Phenyl C61-butyric acid methyl ester
PCBM) and poly(3-hexylthiophene-2,5-diyl) (P3HT) were pur-
chased from Sigma Aldrich.
LUMO
.
opt
g
opt
¼ 1240
E
onset
abs
; EHOMO ¼ ELUMO ꢁ Eg
l
The electronic structures of Ci-II and Ci-III molecules were
investigated using DFT and TD-SCF [16] with B3LYP/6-31G(d) basis
17], as implemented in the Gaussian 09 package. The UVevis ab-
sorption spectra of Ci-II and Ci-III dyes were also calculated by TD-
SCF. Oscillator strength (f) calculated by time dependent density
functional theory were presented as 0.7063, and 0.5475 for Ci-II
and Ci-III, respectively.
[
(
2
.2. Analytical instruments
1H and 13C NMR spectra were obtained using a Bruker 400 MHz
2.4. Fabrication and characterization of OPV devices
spectrometer. Chemical shifts were reported as ppm relative to the
TMS standard (0 ppm). FT-IR spectra were monitored on a Perkin
Elmer-Spectrum BX spectrophotometer with samples prepared as
KBr pellets. The optical properties of the dyes were investigated by
both UVeVis spectroscopy using a Perkin Elmer Lambda 950
Indium tin oxide (ITO) deposited glass substrates from Delta
Tech. Corp. (2.5 ꢀ 2.5 cm in size) with Rsheet: 10
U/sq conductivity
were cleaned using a deionized water, acetone, and isopropanol for
10 min each by sonication, and dried under a stream of nitrogen.
ꢃ
Scheme 1. Synthesis of (a) Ci-II and (b) Ci-III dyes. (i) Acetone:water (1:1), NaOH, ice bath, then room temperature [4]; (ii) xylene, 140 C, 48 h, reflux [8]; (iii) N-bromosuccinimide,
3
CHCl , room temperature [6].

514
H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
Then, the ITO substrates were treated with UV oxygen plasma with
a 100 W power for 5 min under 10 mbar pressure. The PEDOT:PSS
solution was diluted with pure water to a ratio of 1:1 (by volume)
763 cmꢁ1.1H NMR (400 MHz, CDCl
2H), 7.76e7.73 (m, 4H), 7.61e7.55 (m, 8H), 7.22 (d, J ¼ 7.8 Hz, 4H)
3
d
7.27 ppm)
d
8.52 (d, J ¼ 7.8 Hz,
ꢁ2
13
ppm. C NMR [(100 MHz, CDCl
3
d
77.45 (3 peaks)] 156.8, 149.7,
d
and filtrated through the 0.45
and spin-coated at a speed of 4000 rpm for 30 s, resulting in a layer
with a thickness 40 nm. The PEDOT:PSS-coated ITO substrates were
m
m PTFE syringe filter prior to use
145.6, 135.9, 134.8, 134.3, 132.9, 130.8, 129.9, 129.3, 126.8, 126.3,
126.2, 118.3 ppm.
ꢃ
dried in a vacuum oven at 120 C for 15 min. Solutions of photo-
2.5.2.2. Synthesis of 3,10-dibromo-7,14-diphenyldiindolo[3,2,1-
0
0
0
active components in 1,2-dichlorobenzene:chloroform (7:3 v/v)
were mixed at concentrations of 2 or 4 wt% in different mass ratios
and stirred overnight. The pre-formed solutions of P3HT, PCBM, and
de:3 ,2 ,1 -ij]-1,5- naphtydridine-6,13-dione (Ci-III). Yield 52%, FT-
ꢁ
ꢁ
1
IR (KBr pellets, cm ): 3432, 1735 (amide
n
C]O), 1628, 1597, 1419,
1275, 1075, 699 cm . H NMR (400 MHz, DMSO‑d 2.49 ppm):
¼ 7.31 (d, J ¼ 1.2 Hz, 2H), 7.30e7.28 (m, 6H), 7.24 (d, J ¼ 2.5 Hz, 4H),
11
6
d
Ci-(I-III) dyes were spin-coated (1500 rpm for 1 min), and dried at
d
ꢃ
13
1
20 C for 15 min in a N
2
filled atmosphere, giving in a layer with a
7.22 (d, J ¼ 2.5 Hz, 4H) ppm. C NMR [100 MHz, CDCl
peaks)]:
¼ 176.6, 158.8, 153.4, 141.2, 133.1, 130.4, 129.9, 129.6,
129.5, 129.3, 128.6, 128.4, 127.3, 127.0 ppm.
3
d 76.9 (3
thickness 80 nm. The LiF/Al (0.6 nm/70 nm) metal electrode was
thermally deposited. All of the device fabrication characterization
d
processes were carried out under N
glove box.
2
atmosphere in MBRAUN 200B
2.5.2.3. Synthesis of 7,14-bis(4-bromophenyl)diindole-[3,2,1-
0
0
0
The current density versus voltage (JeV) characteristics were
de:3 ,2 ,1 -ij]-1,5-naphtydridine-6,13-dione (Ci-II). To solution of Ci-
I (112 mg, 0.24 mmol) in chloroform (14 mL) was added N-bromo-
succinimide (NBS) (93 mg, 0.52 mmol) slowly. The mixture was
stirred at room temperature for 16 h. Then, the organic phase was
washed with distilled water (3 ꢀ 50 mL). Solvent was removed by
rotary evaporation under reduced pressure to give Ci-II as a red
recorded on a Keithley model 2400 source meter unit and Lab-
®
VIEW data acquisition software in the dark and under simulated
2
AM1.5G illumination with intensity of 100 mW/cm using ATLAS
Solar Test (75 W) solar simulator integrated to glove box.
For electron mobility measurements, electron-only devices
were fabricated and the mobilities were determined by fitting the
dark current using an equation given below:
ꢁ1
solid, yielding 80%. FT-IR (KBr pellets, cm ): 3441, 2957, 1770 and
1701 (amide C]O), 1631, 1488, 1415, 1321, 1262, 1167, 1080, 1020,
n
ꢁ
11
8
02, 762 cm . H NMR (400 MHz, CDCl
3
d 7.26 ppm) d 7.72 (d,
9
8
V2
L3
J ¼ 7.1 Hz, 2H), 7.55 (d, J ¼ 9.1 Hz, 8H), 7.33 (s, 6H) ppm.
JSCLC
¼
0
r
ε ε m
e
3
. Results and discussion
where ε
stant of the Ret film,
voltage and L is the thickness of the photo-active layer [18,19].
o
is the permittivity of free space, ε
r
is the dielectric con-
m
e
is the electron mobility, V is the applied
3.1. Steady state measurements
Optical absorption profiles of synthesized cibalackrot dyes in
2
2
2
.5. Synthesis procedures
solution and on thin-film states are illustrated in Fig. 1, and corre-
sponding optical data are summarized in Table 1. The visible ab-
sorption spectra of these dyes in benzonitrile were composed of
two main absorption maxima at 548 nm and 512 nm for Ci-II dye,
while that was 557 nm and 518 nm for Ci-III dye, as shown in
Fig. 1a. Concentration experiments have been made for both Ci-II
and Ci-III dyes in chloroform solution in order to understand
whether the aggregates formed or not. Fig. S1a and b shows the
concentration experiments for Ci-II and Ci-III dyes, respectively.
According to the spectra, shapes and the positions of the peaks in
the visible region remained unchanged. This observation indicates
that aggregation does not observed for both dyes. Existence of the
long-wavelength absorption bands of the dyes could not be
explained by the aggregation behavior. The shoulder bands in the
.5.1. General procedures for the synthesis of Indigoid dyes:
Nitrobenzaldehyde derivatives (1 mmol) were dissolved in
0 mL of acetone:water (1:1 v/v). After cooling to ꢁ5 C, 5 mL
ꢃ
aqueous solution of 1 M NaOH was added dropwise to adjust the
pH to 10. The mixture was stirred overnight at room temperature.
The resulting precipitate was collected by suction filtration, and
then precipitates were washed with acetone and distilled water
until washings ran colorless.
2.5.1.1. Synthesis of indigo dye. 2-Nitrobenzaldehyde was used in
the reaction. Indigo was obtained as a blue solid, yielding 48%. FT-IR
ꢁ1
(
KBr pellet, cm ): 3264, 1710 (ketone
nC]O), 1625, 1613, 1585 (ar-
omatic C]C), 1489, 1461, 1392, 1316, 1298, 1197, 1173, 1126, 1095,
n
higher energy region can be attributed to
pꢁp* transition. The
ꢁ
1
1068, 878, 712 cm
.
strongest absorption bands in the lower energy region can be
attributed to intramolecular charge transfer (ICT) complex, previ-
ously described for cibalackrot type dyes in the literature [6,9,12].
From the observational evidence in this work, ICT formed in Ci-III
dye is more prominent than that of Ci-II dye so that the maximum
and the shoulder of the absorption bands of Ci-III are more shifted
to red region in the studied solvents as compared to that of Ci-II.
This red-shifted absorption is also attributed to the auxochrome Br
atoms which can give their non-bonding electrons to the aromatic
core through conjugated structure.
2.5.1.2. Synthesis of tyrian purple dye. 4-Bromo-2-nitrobenzal-
dehyde was used in the reaction. Tyrian purple was obtained as a
ꢁ
1
purplish powder, yielding 42%. FT-IR (KBr pellet, cm ): 3640, 3386
amine C]O), 1634, 1614,
(
n
NꢁH), 2955 (aromatic
n
CꢁH), 1706 (ketone
n
ꢁ
1
1578, 1532, 1439, 1387, 1313, 1231, 1158, 1047, 898, 767 cm
.
2.5.2. General procedures for synthesis of cibalackrot dyes
To a refluxing xylene (30 mL) solution of indigoid dye (1 mol
equiv.) was added phenylacetyl chloride (12 mol equiv.) dropwise
under an argon flow. The mixture was refluxed for 48 h, and then
cooled down to room temperature. The precipitate was collected by
suction filtration, and washed with ethanol and ether, respectively,
to give cibalackrot derivative as a red solid.
Fig.1b gives a comparison visible absorption spectra of Ci-III dye
in different solvent of polarities. Notably, there is a marked red-shift
of 29 nm in the long-wavelength absorption maximum of Ci-III dye
as compared to that of Ci-II dye in ethyl acetate solution. Also, when
compared to other studied solvents, absorption profile of Ci-III dye
shows more bathochromic shifts in ethyl acetate. These results are
more consistent with much higher polarizability of the excited dye
structure with respect to its ground state in ethyl acetate solution,
2
.5.2.1. Synthesis of cibalackrot dye (Ci-I). Yield 45%, FT-IR (KBr
ꢁ
1
pellets, cm ): 3431, 2921, 1632, 1480, 1415, 1313, 1267, 1080, 984,

H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
515
Fig. 1. (a) Comparison of the normalized UVevis absorption spectra of Ci-II and Ci-III dyes in BzCN solution. (b) Normalized UVevis absorption spectra of Ci-III dye in different
solvents. (c) UVevis absorption spectra of Ci-II and Ci-III dyes calculated by TD-SCF. (d) Optical absorption of Ci-II and Ci-III films on ITO-coated substrate.
emission spectra of Ci-II and Ci-III dyes in benzonitrile solution at
Table 1
the excitation wavelength of 485 nm. Ci-II dye exhibits emission
maximum at 572 nm with a shoulder at 615 nm in benzonitrile. Ci-
III dye gives a red shift of 4e6 nm with respect to Ci-II dye. The
most red-shifted emission maximum for Ci-II dye was observed at
Long-wavelength absorption (
and Ci-III dyes in different solvents (
l
(nm)) and emission wavelengths (
exc ¼ 485 nm).
lem (nm)) of Ci-II
l
Dyes/Solvents Ci-II
Ci-III
l
1
l
2
l
em1
l
em2
l
1
l
2
l
em1
l
em2
6
18 nm in chlorobenzene solution, as shown in Fig. 2b. In aceto-
nitrile solution, both dyes gave more blue shifts in their emission
maxima, monitoring at wavelengths of em1:600, em2:557 nm for
Ci-II dye and em1:605, em2:562 nm for Ci-III dye. Fig. 2c gives the
CHCl
BzCl
EtAc
Me CO
2
BzCN
3
538
552
540
540
548
535
503
514
506
502
512
500
600
618
606
602
615
600
560
575
562
560
572
557
545
561
569
546
557
541
507
520
523
510
518
505
609
625
615
610
619
605
568
580
570
567
578
562
l
l
l
l
excitation spectra of Ci-II and Ci-III dyes in chlorobenzene solution
at the collected emission wavelength of 575 nm. It is seen that
excitation spectra of these dyes were almost similar to their ab-
sorption profiles. Noting that contaminants or any other reactive
reagents are not existed in the excited state of the dyes.
MeCN
which then lowers the LUMO energy level of Ci-III dye. Both dyes
give the lowest absorption wavelength numbers in more polar
MeCN solution due to the existence of much lower polarizabilities
of the dyes in the excited state. TDDFT calculations of the dyes
exhibit one broad absorption band which corresponds to the
Fluorescence decay time values and fluorescence quantum
yields for both dyes were given in Table 2. Fig. 3 compares the time-
resolved emission spectra for Ci-II dye in chloroform with benzo-
nitrile solution in 450 ns time window. While the fluorescence
decay times of Ci-II dye can be analyzed as mono-exponential
decays around 4.29e6.78 ns, Ci-III dye gives bi-exponential de-
cays with short decay times around 1.57e3.84 ns and long decay
times around 6.36e10.37 ns in the studied solvents. A more
detailed investigation of the time-decay profiles of cibalackrot dye
was studied by Melo group in 2006. They reported the long-lived
decay component of 6.10 ns for cibalackrot dye in dioxane [8]. In
our measurements, the short decay times can be attributed to
quench of direct emission of cibalackrot core for Ci-III dye. The long
decay times between 6.36 and 10.37 ns with low amplitudes of
combination of
p p* and ICT absorption for each dye, as shown in
ꢁ
Fig. 1c. TDDFT calculations on UVevis spectra did not show the
charge transfer shoulder, and finally computed spectra showed a
little red shift as compared to the experimental measurements. The
absorption bands of Ci-II and Ci-III dyes in the film are significantly
broadened, and are slightly red shift with a huge shoulder below
4
00 nm, as illustrated in Fig. 1d. These observations indicate the
strong molecular stacking of the molecules in solid state.
Fig. 2a gives a comparison of the normalized fluorescence

516
H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
Fig. 2. (a) Comparison of the normalized fluorescence emission spectra of Ci-II and Ci-III dyes in BzCN solution. (b) Normalized fluorescence emission spectra of Ci-II dye in
different solvents (
l
exc ¼ 485 nm). (c) Excitation spectrum of Ci-II and Ci-III dyes in BzCl solution at the collected emission wavelength of 575 nm.
Table 2
a
Fluorescence quantum yields (
F
F) and fluorescence decay times (
t
f (ns)) with amplitudes of Ci-II and Ci-III dyes in different solvents (
lexc ¼ 485 nm,
l
detection ¼ 535 nm) .
Dyes/Solvents
c2
t
f(1)
a
1
t
f(2)
a
2
F
F
Ci-II
CHCl
BzCl
EtAc
3
0.90
0.92
0.99
1.11
0.73
0.76
0.84
0.73
4.29
6.78
6.03
6.12
1.57
3.84
2.68
3.15
100
100
100
100
86.1
79.3
60.2
71.2
e
e
0.64
0.66
0.40
0.71
0.16
0.16
0.034
0.003
e
e
e
e
BzCN
e
e
Ci-III
CHCl
BzCl
EtAc
BzCN
3
6.36
8.30
8.44
10.37
13.9
20.7
49.8
28.8
a
Fluorescence quantum yields have been determined using perylene-3,4,9,10-tetracarboxylic-bis-N,Nʹ-dodecyl diimide (N-DODEPER) (
F
F
¼ 1.0 in chloroform) [20].
1
3.9%e49.8% may belong to the ICT complex of cibalackrot back-
emission wavelength of 750 nm. This observation is taken as the
evidence for the formation of ICT state which is formed at the ex-
penses of the locally excited (LE) state. Following excitation, ICT
state becomes the lowest energy state for both dyes. Interestingly,
the more red-shifted absorption and emission bands for Ci-III dye
as compared to Ci-II dye may be attributed to electron acceptor
strength of bromo-substituted phenyl side. It was noted that
inductive effects of Br atoms slightly increase the formation of ICT
complex between the donor group and fused phenyl side in Ci-III
dye by enhancing the electron-withdrawing capacity of cibalackrot
backbone.
bone in the excited state, which is generated by the possible charge
transfer between the central donor nitrogens to the phenyl side on
the whole surface of the dyes. We took additional measurements
using acetone and acetonitrile solvents to span the polarity range of
the used solvents (Table S1). There is no obvious change on emis-
sion spectra for ICT state as the solvent polarity increases. Similar
results were obtained and both solvents gave mono-exponential
decays for Ci-II dye and bi-exponential decays for Ci-III dye at
collected data around 535 nm.
In order to clarify the ICT decays at long times, we have analyzed
the decay spectra at different detected emission wavelengths of
As illustrated in Table 2, fluorescence quantum yields of Ci-III
4
00, 575, 615 and 750 nm (Table S2). Single photon timing mea-
F F
dye (F :0.003e0.16) are lower than that of Ci-II dye (F :0.40e0.71)
surements for both dyes gave bi-exponential decays only at longest
in the studied solvents. Cibalackrot cannot give any intra- or

H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
517
Both Ci-II and Ci-III dyes show semi-reversible reduction and
oxidation waves, which are corresponded to reduction of ketone
groups and oxidation of tertiary amines, respectively. The oxidation
and reduction potentials are 1.27 V and ꢁ1.04 V, respectively, vs.
þ
0
Ag/Ag for Ci-II dye, 0.90 V (E :1.40 V), and ꢁ0.97 V, respectively,
ox2
þ
vs. Ag/Ag for Ci-III dye, which correspond to HOMO and LUMO
energy levels of ꢁ5.28 and ꢁ3.13 eV, respectively, for Ci-II dye
and ꢁ5.29 and ꢁ3.20 eV, respectively, for Ci-III dye.
Changing the position of the Br-moiety from the phenyl to the
indole group effects the oxidation and reduction behaviours of
cibalackrot backbone. While electron withdrawing ability of Br fa-
cilitates the reduction of carboxyl group inductively in Ci-III dye,
oxidation of the amine centre in this structure becomes more
difficult due to reduced electron density. Therefore, oxidation of Ci-
III dye was observed at higher potential (1.40 V), and its reduction
(
ꢁ0.97 V) was measured at lower potentials compared to Ci-II dye.
In the case of Ci-II, inductive effect of the Br on the p-position of
rotating phenyl moiety is not effective as much as being substituted
directly to the conjugated indole centre.
The molecular orbital contours (Fig. 5) for the HOMO orbitals of
both Ci-II and Ci-III dyes clearly show that the electron clouds lie
mainly along with the axis of peripheral phenyl-lactam core, while
for their LUMO orbitals, the electron density is concentrated
equivalently on whole surface of the molecule. There is a sub-
stantial contribution to the LUMO from the fused phenyl rings of
the structure. Substitution of this position will enable resulting the
change of the LUMO energy level of the cibalackrot dye. As shown
in Fig. 5, there is a little bit of energy gap between the LUMO level of
Ci-II and Ci-III dyes (~0.07 eV).
Fig. 3. Fluorescence decay graphs of Ci-II dye in CHCl
3
and BzCN solution
(
l
detection ¼ 535 nm).
intermolecular proton transfer reaction due to its rigid planar
structure. This leads to high fluorescence quantum yields for ciba-
lackrot core (F :0.76, in dioxane) [8]. However, Ci-III dye may not
F
be completely rigid because of its possible conformational relaxa-
tion existed from the delocalization between bromine atoms and
carbonyl groups in Ci-III dye whereas this is inhibited in Ci-II dye.
This is responsible for much lower fluorescence quantum yields for
Ci-III dye. A considerable ICT formation for Ci-III dye due to the
inductive effect of Br atoms may also lead to the lower fluorescence
quantum yields for Ci-III dye. Also, the lowest fluorescence quan-
tum yields for Ci-III dye in ethyl acetate and benzonitrile solutions
may be attributed to the self-quenching because of much possible
aggregation behavior of the dye in these solutions.
3.3. Charge transfer performances
In order to analyze the electron only mobility characteristic of
the dyes, we have used the devices for space-charge-limited cur-
rent (SCLC) measurements like to solar cell with the structures of
2
FTO/TiO (40 nm)/Ci-(II-III) (60 nm)/LiF(0.6 nm)/Al(70 nm). Fig. 6
gives the variation in the capacitance-frequency (C-F) character-
3.2. Electrochemical properties
istic of FTO/TiO /Ci-II/LiCl/Al device. Also, Fig. 7a and b represent
2
the dark current density-effective voltage characteristics of SCLC
Fig. 4 shows the cyclic voltammograms of Ci-II and Ci-III dyes on
mobilities in the device of FTO/TiO /Ci-II/LiCl/Al and FTO/TiO /Ci-
2
2
glassy carbon working electrode in 0.1 M [TBA][PF
some important electrochemical data are summarized in Table 3.
6
]/Me-CN, and
III/LiCl/Al, respectively.
Ci-II and Ci-III dyes give the electron mobility values of
ꢁ
4
2
ꢁ4
2
7.0 ꢀ 10 cm /V and 3.1 ꢀ 10 cm /V, respectively. Compared to
reported FET values in the literature, we observed an order of
magnitude lower transport rates [7]. Possible explanation of much
lower mobilities can be a different transport mechanism in the bulk
structure compared to surface transport. Degree of ordering and
molecular packaging in crystalline structure is strongly effective on
charge transport rates. Electron mobility values of Ci-II and Ci-III
were found to be a nearly identical.
Performance capabilities of the synthesized dyes as acceptor or
donor components in OPV devices have been evaluated on
Table 3
Electrochemical values and HOMO-LUMO energies of Ci-II and Ci-III dyes with
respect to the vacuum level.
Dyes
0
0
0
LUMO (eV)
HOMO (eV)
E
(
0-0a(eV)
abs)
E
(
E
E
red1
ox1
ox2
V)
(V)
(V)
Ci-I
Ci-II
Ci-III
e0.70
e1.04
e0.97
1.20
1.27
0.90
e
e3.47
e3.13
e3.20
e5.59
e5.28
e5.29
2.12
2.15
2.09
e
1.40
a
The zerothezeroth transition E0e0 values were estimated from the intersection
of the apsis and the straight line which is drawn from the red side of the UVevis
absorption band in chlorobenzene solution.
Fig. 4. Cyclic voltammograms of Ci-II and Ci-III dyes on glassy carbon working elec-
trode in 0.1 M [TBA][PF6]/Me-CN (Scan rate: 100 mV s ).
e1

518
H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
conventional devices using spin-coated films of P3HT as donor or
fullerene as acceptor component, given below:
ITO/PEDOT:PSS/P3HT:Ci-I/LiF(0.6 nm)/Al(70 nm) (%2 w/w)
ITO/PEDOT:PSS/P3HT:Ci-III/LiF(0.6 nm)/Al(70 nm) (%4 w/w)
ITO/PEDOT:PSS/Ci-III:PCBM/LiF(0.6 nm)/Al(70 nm) (%4 w/w)
According to alignment of LUMO energy levels of Ci-(I-III) dyes
with P3HT and PCBM, photovoltaic performances obtained from
P3HT:Ci-(I-III) and Ci-(I-III):PCBM donor-acceptor couples were
predictable. There is an enough threshold for possible hole transfer
between the HOMO levels of the dyes (ꢁ5.59 eV for Ci-I, ꢁ5.28 eV
for Ci-II, and ꢁ5.29 eV for Ci-III) and P3HT donor (ꢁ5.1 eV) for the
active layer of P3HT/dye in device configuration. On the other hand,
LUMO energy level alignments of the synthesized dyes are favor-
able for an efficient electron transfer from their excited states to
PCBM acceptor. Therefore, charge transfers between these donor-
acceptor couples yield in higher efficiencies.
Fig. 8a, b and c give the J-V curves of P3HT:Ci-I, P3HT:Ci-III and
Ci-III:PCBM blends as active layer in solar cell configurations.
Table 4 summarizes the photovoltaic data obtained from the
measurements. We couldn't obtain any photovoltaic signal with Ci-
II dye in OPV device neither as p-type nor as n-type semiconductor.
With the photovoltaic device based on P3HT:Ci-I (1:1) bulk heter-
Fig. 5. Optimized ground-state geometries and frontier orbital energies of compounds
Ci-II and Ci-III dyes obtained by TDDFT.
ojunction, efficiency of 0.003% with 220 mV open circuit voltage,
2
0
.005 mA/cm short circuit current and 0.5 fill factor was achieved,
wherein the device based on P3HT:Ci-III (1:3 w/w) gave a Voc of
2
4
0
0 mV, a Jsc of 0.060 mA/cm , a fill factor of 0.35 and a PCE of
.001%. Remarkably, Ci-III:PCBM (1:4 w/w) based device presented
an improved photovoltaic performance with a Voc of 200 mV, a Jsc of
2
0
.41 mA/cm , a fill factor of 0.28 and an increased PCE of 0.02%.
These performances are below the previously reported OPV effi-
ciency (2.35%) for a cibalackrot structure in the literature. Bronstein
et al. used a conjugated thiophene polymer incorporated with
cibalackrot backbone in conventional and inverted configuration of
OPV device as donor component [11]. In our study, we examined
the only pristine cibalackrot-bromine cores in OPV devices as
acceptor or donor components without functionalizing them any
polymeric fractions. These dyes without any polymeric groups can
give a limited contribution to photocurrent due to their little en-
ergetic offset in IPCE spectra. Fig. 9 shows the IPCE graph for
photovoltaic device using Ci-III:PCBM (1:4) blend as active layer.
Blend exhibits a broad IPCE spectrum in accordance with the
Fig. 6. The variation in the capacitance-frequency (CeF) characteristic of FTO/TiO
II/LiCl/Al device.
2
/Ci-
2 2
Fig. 7. Dark current density-effective voltage characteristics of SCLC mobilities in (a) FTO/TiO /Ci-II/LiCl/Al and (b) FTO/TiO /Ci-III/LiCl/Al devices.

H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
519
Fig. 8. J-V curves of photovoltaic devices using (a) P3HT:Ci-I, (b) P3HT:Ci-III and (c) Ci-III:PCBM blends with different mixing ratios as active layer.
Table 4
OPV parameters of normal BHJ devices prepared from cibalackrot dyes.
Active Layer
V
oc (mV)
I
sc (mA/cm2)
FF
h
Binary Structure
w:w
P3HT:Ci-I
P3HT:Ci-I
1:1
1:3
1:1
1:3
1:1
1:3
1:4
220
160
180
40
560
220
200
0.005
0.005
0.020
0.060
0.010
0.170
0.410
0.50
0.30
0.26
0.35
0.35
0.30
0.28
0.003
0.002
0.001
0.001
0.002
0.010
0.020
P3HT:Ci-III
P3HT:Ci-III
Ci-III:PCBM
Ci-III:PCBM
Ci-III:PCBM
UVevis results in the range of 400e700 nm with a maximum of
4.3% at 500 nm. Photovoltaic conversion efficiencies achieved
~
with these used dyes were lower compared to relative analogues
like indigo dyes, which is attributed to their much higher dielectric
constant.
Fig. 9. IPCE graph for photovoltaic device using Ci-III:PCBM (1:4) blend as active layer.
4
. Conclusions
We synthesized and investigated cibalackrot type donor or
UVevis absorbance gave a remarkable red shift in visible maximum
of Ci-III dye because of the combination effect of both bromine
auxochrome contribution and ICT possibility. Ci-III dye gave an
increased current in conventional OPV device either as p-type or as
acceptor main cores with variations in the position of bromine
group for charge transfer process. By changing the bromine posi-
tion from the peripheral phenyl side to fused phenyl ring, the

520
H. Dinçalp et al. / Journal of Molecular Structure 1173 (2018) 512e520
n-type semiconductor, while there is no current for Ci-II dye in this
type of device. These findings demonstrated the potential use of
small molecule cibalackrot dyes substituted with 3,10 positions of
fused phenyl ring for promising photovoltaic devices.
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Acknowledgements
[
This study was financed by the Scientific and Technological
Research Council of Turkey with the project number of 113Z250.
We also thank to Ege University for the support of the use of
Gaussian 09 W program for theoretical DFT calculations.
[
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