Indigo-Based Acceptor Type Small Molecules: Synthesis, Electrochemical and Optoelectronic Characterizations

Article abstract of DOI:10.1007/s10895-018-2287-3

In this paper, we report design and synthesis of novel low bandgap small molecules, indigo-benzimidazole (Tyr-3) and indigo-schiff base (Tyr-4) type acceptors. In these structures, tert-butoxycarbonyl (t-BOC) group has been attached to indigo nitrogen ato

Full text of DOI:10.1007/s10895-018-2287-3

Journal of Fluorescence
https://doi.org/10.1007/s10895-018-2287-3
ORIGINAL ARTICLE
Indigo-Based Acceptor Type Small Molecules: Synthesis, Electrochemical
and Optoelectronic Characterizations
Gözde Murat Saltan1 & Deniz Aykut Kıymaz2 & Ceylan Zafer2 & Haluk Dinçalp1
Received: 12 May 2018 /Accepted: 20 August 2018
#
Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
In this paper, we report design and synthesis of novel low bandgap small molecules, indigo-benzimidazole (Tyr-3) and indigo-
schiff base (Tyr-4) type acceptors. In these structures, tert-butoxycarbonyl (t-BOC) group has been attached to indigo nitrogen
atom in order to increase the solubility. UV-vis absorption spectra of Tyr-3 and Tyr-4 dyes exhibit wide absorption bands ranging
from 350 to 600 nm, indicating the relatively low bandgap giving around 2.07 eV for each. Excitation of both Tyr-3 and Tyr-4
dyes at 485 nm displays characteristic emission features of indigo moiety and also intramolecular charge transfer complex (ICT)
related with their subunits. Besides increasing fluorescence quantum yields as compared to model compound, biexponential
decay times for fluorescence life times were also obtained for Tyr-3 and Tyr-4 dyes. Their appropriate energy levels along with
low HOMO levels are desired for light harvesting acceptors for organic solar cells when blended with poly[N-9′-heptadecanyl-
2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole] (PCDTBT) as donor polymer. Photovoltaic behavior of the
synthesized dyes were examined in bulk heterojunction concept and achieved photovoltaic conversion efficiencies were
discussed.
.
.
.
.
Keywords Indigo Benzimidazole Fluorescent Intramolecular charge transfer complex Bulk heterojunction solar cell
Introduction
is produced by extraction of the secretions of a gland for
gastropod mollusks living in the Mediterranean basin since
ancient times [4]. Indigo has an extraordinarily low solubility
in common organic solvents and a high melting point (~390-
392 °C), arising from stabilization with strong inter- and in-
tramolecular hydrogen bonding. Their usages are restricted in
synthetic organic chemistry because of their low solubilities
[3, 5]. As a technique to make indigos processable for chem-
ical synthesis, Glowacki et al. reported that thermolabile t-
BOC protecting group is attached to indigo nitrogen. Finally,
indigo pigment can be suitable for chemical manipulation and
processing [3, 6]. However, N,N′-disubstitution with alkyl or
acyl units eliminates H-bonding and causes a large deviation
from coplanarity of indigo ring, causing blue shift in absorp-
tion wavelength of indigo [6, 7].
Indigo, considerably known for thousands of years, is crucial
and popular pigment which has been widely used in textile
industries [1, 2]. Natural indigo precursor is extracted from the
plants Indigofera tinctoria and Isatis tinctoria which involve a
glucoside called indicant, giving unstable indoxyl molecule in
the presence of special enzymes, then oxidized into stable
indigo pigment in the presence of atmospheric oxygen
[3]. Tyrian purple, the 6,6′-dibromo derivative of indigo com-
pound, is well-known in valuable pigment production. It
Electronic supplementary material The online version of this article
(https://doi.org/10.1007/s10895-018-2287-3) contains supplementary
material, which is available to authorized users.
Indigo and its derivatives are used in many applications for
optoelectronic technologies such as organic field-effect tran-
sistors (OFETs) [6, 8, 9], organic photodiodes [10], and bulk
heterojunction solar cells (BHJ-SCs) [11–13]. Indigo and tyr-
ian purple are intrinsically ambipolar organic semiconductors
with electrochemical bandgap values of 1.7 [9, 14] and 1.8 eV
[15], respectively, possessing high and well-balanced electron
and hole mobilities μe/μh = 1.0 × 10−2/0.5-1.0 × 10−2 [14] and
* Haluk Dinçalp
haluk.dincalp@cbu.edu.tr
1
Department of Chemistry, Faculty of Arts and Science, Manisa Celal
Bayar University, Yunus Emre, 45140 Manisa, Turkey
2
Solar Energy Institute, Ege University, Bornova,
35100 Izmir, Turkey

J Fluoresc
μe/μh = 3.0 × 10−2/22 × 10−2 cm2/V·s [15], respectively. Guo
et al. reported that a new n-type semiconducting indigo poly-
mer with thermocleavable t-BOC group gave electron mobil-
ity around 6.0 × 10−3 cm2/V·s in OFET device after thermal
treatment at 170 °C to remove the t-BOC groups [7]. Among
the limited number of photovoltaic studies for indigo dyes, Lu
et al. reported that indigo-doped poly(3-hexylthiophene-2,5-
diyl) (P3HT):[6, 6]-phenyl C61-butyric acid methyl ester
(PCBM) device gave cell efficiency of 2.55 whereas the de-
vice without indigo pigment gave only cell efficiency of 2.14
[12]. Liu et al. fabricated a BHJ-SC device with an inverted
structure wherein the photoactive layer of benzodithiophene-
indigo type polymer with t-BOC group and [6, 6]-phenyl C71-
butyric acid methyl ester (PC71BM) at 1:4 weight ratio and
reported the cell efficiency of 0.75 for this device without
annealing process [13].
Herein, we have synthesized novel two t-BOC functional-
ized indigo dyes which contain benzimidazole unit for Tyr-3
and Schiff base unit for Tyr-4 in their 6,6′ positions in order to
get a closer look at these conjugated indigos electronic nature
in charge transfer processes in addition to optoelectronic per-
formance, specifically in photovoltaics. Benzimidazole and its
derivatives constitute an important family of dyes due to their
large Stokes shift values and also high fluorescence quantum
yields [16]. Optoelectronic properties of the benzimidazoles
can be easily tuned by introducing diverse functional groups
[17, 18]. In this study, we have investigated electronic contri-
bution of benzimidazole on the optoelectronic performance of
indigo dye. We have compared photophysical properties of
Tyr-3 and Tyr-4 dyes with model compound Tyr-1 in different
solvents (Scheme 1). We have also investigated their electron-
acceptor behaviors as n-type semiconductor in the active layer
of BHJ-SC device with p–type PCDTBT donor low band gap
conjugated polymer and examined the effect of benzimidazole
and Schiff base group on photovoltaic efficiency.
Experimental
Materials and Reagents
Used as the main reagents, 4-Bromo-2-nitrobenzaldehyde, di-
tert-butyl dicarbonate, 4-formylphenylboronic acid, 4-
dimethylaminopyridine, and tetrakis(triphenylphosphine)pal-
ladium (0) (Pd(PPh3)4) were purchased from Sigma Aldrich.
Benzene, bromine, hydrogen bromide solution (47%), sodium
borohydride, tetrahydrofuran, methanol, and ethanol were
purchased from Merck Company. Sodium carbonate (from
Carlo Erba) and CoCl2.6H2O, N,N-dimethyl-1,3-phenylene
diamine dihydrochloride, N,N-dimethyl phenyl boronic acid,
and magnesium sulphate (from Alfa Aesar) were used as re-
ceived. Other reagents and solvents were purchased commer-
cially as analytical-grade quality and used without further pu-
rification. Tyr-3-benzimidazole precursors, 4,7-Dibromo-
2,1,3-benzothiadiazole, (3-{7-[3-(dimethylamino)phenyl]-
2,1,3-benzothiadiazole-4- yl}phenyl)dimethylamine, and bis
amine N3,N3,N3′′,N3′′-tetramethyl-1,1′:4′,1′′-terphenyl-
2′,3,3′,3′′-tetramine were synthesized according to reference
[21]. PEDOT:PSS (CLEVIOS™ P VP AI 4083, Heraeus
Deutschland GmbH & Co. KG) was purchased from
Scheme 1 Synthetic route to target Tyr-1, Tyr-3, and Tyr-4 dyes. (i)
Acetone:water (1:1), 1 M NaOH, room temperature [6, 19]; (ii) di-tert-
butyl-dicarbonate, 4-dimethylaminopyridine (DMAP), THF, room
temperature [6]; (iii) THF, 2 M Na2CO3, Pd(PPh3)4, 4-
formylphenylboronic acid, 65 °C [6, 20]; (iv) (4-{4,7-bis[3-
(dimethylamino)phenyl]-1H-benzimidazole-2-yl}phenyl)boronic acid,
benzene, 2 M Na2CO3, Pd(PPh3)4, 85 °C [6, 20]; (v) N,N-dimethyl-1,3-
phenylenediamine dihydrochloride, CHCl3:pyridine solution, addition at
−20 °C, then stirring at room temperature.

J Fluoresc
Aldrich, PCDTBT was purchased from Luminescence
Technology Corp. (Lumtec), Taiwan and used as received
without further purification.
drying with nitrogen. Then, the substrates were etched with
oxygen plasma for 5 min. PEDOT:PSS (AL4083) suspen-
sion was filtered and, then spun cast at 4000 rpm for 30 s,
followed by annealing at 120 °C for 30 min. Tyr-1, Tyr-3
and Tyr-4 dyes were dissolved in o-dichlorobenzene
(20 mg/mL). The photoactive layers containing blends of
PCDTBT and indigo dye were coated at 1500 rpm for 1 min
and, then annealed at 120 °C for 15 min in an inert-
atmosphere glovebox. As a result, thickness of the obtained
active layers were measured as 80 nm. Finally, Ca (30 nm)/
Al (70 nm) cathode layer was deposited by thermal evapo-
ration with shadow mask giving an active area of 0.16 cm2
in thermal evaporator integrated to inert atmosphere glove
box. Photovoltaic evaluation of fabricated devices were
performed under standard conditions (100 mW.cm−2 light
intensity and AM1.5G irradiation) in inert atmosphere with
solar simulator integrated glove box. Current density-
Voltage (J-V) measurement were done by using Keithley
2400 source-measure unit and LabView® data acquisition
software.
Analytical Instruments
FT-IR spectra were recorded on a Perkin Elmer-Spectrum BX
spectrophotometer with samples prepared as KBr pellets. 1H
NMR (400 MHz) and 13C NMR (100 MHz) spectra were
recorded using a Bruker spectrometer. UV-Vis absorption
and fluorescence emission spectra were recorded on a Perkin
Elmer Lambda 950 and FLS 920 Edinburg spectrophotome-
ters, respectively. Fluorescence quantum yields of the com-
pounds were determined using perylene-3,4,9,10-
tetracarboxylic-bis-N,N′-dodecyl diimide (N-DODEPER)
(ΦF = 1.0 in CHCl3) [22]. Cyclic voltammetry (CV) measure-
ments on compounds Tyr-1, Tyr-3 and Tyr-4 dyes in MeCN
were measured using 100 mM [TBA][PF6] as the electrolyte
in a CH instruments 660B-Electrochemical Workstation at a
scan rate of 100 mV/s. Ferrocene was used as the internal
standard and its oxidation potential detected at +0.61 V. A
glassy carbon working electrode, a platinum wire auxiliary
electrode, and an Ag/Ag+ reference electrode (SCE) were used
in a standard three-electrode configuration cell. The HOMO
and LUMO energy levels were calculated according to the
following equations [23]:
Space-charge-limited current (SCLC) measurements
were applied to measure electron mobility of Tyr-1 in de-
vice configuration of FTO/TiO2(40 nm)/Tyr-1(60 nm)/
LiF(0.6 nm)/Al(70 nm). Tyr-1 mobility was determined
by fitting the dark current using an equation given below:
9
8
V2
L3
À
Á
À
Á
EHOMO ¼ −e Eoonxset þ 4:8
; ELUMO ¼ −e Eorendset þ 4:8
JSCLC
¼
ε0εrμe
ꢀ
1240
Eogpt
¼
onset ; EHOMO ¼ ELUMO−Egopt
λabs
where εo is the permittivity of free space, εr is the dielectric
constant of Tyr-1 film, μe is the electron mobility, V is the
applied voltage, and L is the thickness of the photo-active
layer [28, 29].
The fluorescence decays of the compounds were obtained
with nanosecond resolution at excitation wavelength of
472.4 nm on an apparatus described elsewhere [21] and were
analyzed using a nonlinear, exponential tail fit method [24].
The goodness-of-fit was judged from the reduced χ2 (≤ 1.2)
[25]. Non-contact mode atomic force microscopy (AFM) was
performed with an Ambious Technology Q-Scope 250 Model
instrument.
Synthesis Procedures
Synthesis of Tyrian Purple
The minimum energy of geometry was performed in the
framework of density functional theory (DFT) [26] by means
of the B3LYP using the Gaussian 09 W program. HOMO and
LUMO orbitals of the dyes were illustrated by the 6-31G(d)
level [27] for stable structures.
Solution of 1 M NaOH (5 mL) was added to a mixture of 4-
Bromo-2-nitrobenzaldehyde in 20 mL of acetone:water (v:v,
1:1) volume percent. The mixture was stirred overnight. The
resulting precipitate was collected by suction filtration and, then
washed with acetone and distilled water, respectively, yielding
42%. FT-IR (KBr pellet, cm−1): 3640, 3386 (amine νN − H), 2955
(aromatic νC − H), 1706 (ketone νC=O), 1634, 1614, 1578, 1532,
1439, 1387, 1313, 1231, 1158, 1047, 898, 767 cm−1.
Fabrication and Characterization of OPV Devices
BHJ-SCs were fabricated in a regular BHJ arrangement by
sandwiching 1:1 or 1:4 (w/w) for PCDTBT:indigo (mixing
at concentrations of 2 wt%) film between a transparent
ITO/PEDOT:PSS and Al-Ca electrodes. ITO glass sub-
strates were 2.5 × 2.5 cm in size and 10 Ω/sq in conductiv-
ity. They were cleaned before the coating using the follow-
ing sequential steps: rinsing with deionized water; sonica-
tion in acetone, and isopropanol for 15 min each; and
Synthesis of Di-tert-butyl (2E)
-6,6′-dibromo-3,3′-dioxo-2,2′-biindole-1,1’(3H,3’H)
-dicarboxylate (Tyr-1)
A mixture of di-tert-butyl dicarbonate (0.12 g, 604 μmol) and
4-dimethylaminopyridine (DMAP) (24 mg, 196 μmol) in

J Fluoresc
5 mL of THF were stirred for 15 min at room temperature and,
then tyrian purple (30 mg, 71 μmol) was added to the reaction
mixture. The mixture was stirred at room temperature for an
hour. After completion, the solvent was evaporated and the
crude product was purified by column chromatography on
silica gel using n-hexane:ethyl acetate 4:1 (v:v) as an eluent,
yielding 98%. FT-IR (KBr pellet, cm−1): 2963, 2921, 2854
(aliphatic νC − H), 1742 (ester νC=O), 1677 (ketone νC=O),
1597, 1422, 1367, 1273, 1185, 1142, 1085, 1054, 1004,
1
872, 843 cm−1. H NMR (400 MHz, CDCl3 δ 7.27 ppm) δ
8.27 (s, 2H), 7.63 (d, J = 8.1 Hz, 2H), 7.38 (d, J = 8.1 Hz, 2H),
1.61 (s, 18H) ppm. 13C NMR [100 MHz, CDCl3 δ 77.41 (3
peaks)] δ 183.3, 150.2, 150.0, 131.8, 128.4, 125.6, 122.3,
120.8, 85.7, 28.2 ppm.
Synthesis of Di-tert-butyl (2E)-6,6′-bis(4-formylphenyl)
-3,3′-dioxo-2,2′-biindole-1,1’(3H,3’H)-dicarboxylate (Tyr-2)
To a solution of Tyr-1 (30 mg, 48.4 μmol) in 2 mL of THF
was added solution of 1 mL 2 M Na2CO3. Subsequently,
tetrakis(triphenylphosphine)palladium (0) (Pd(PPh3)4) which
was used as a catalyst, 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, 4-formylphenylboronic acid (16 mg,
0.11 mmol) in 1 mL of THF was added to the solution in
portions and the mixture was heated at 65 °C overnight under
an argon atmosphere. Then, the reaction mixture was poured
into water and extracted with chloroform (3 × 50 mL). The
organic layer was dried over anhydrous magnesium sulfate
and evaporated in vacuum to dryness. The residue was sepa-
rated by column chromatography on silica gel using n-
hexane:ethyl acetate 4:1 (v:v), yielding 40%. FT-IR (KBr pel-
let, cm−1): 3374, 2976, 2926, 2853 (aliphatic νC − H), 1738
(ester νC=O), 1704 (ketone νC=O), 1681 (aldehyde νC=O),
1604, 1462, 1436, 1370, 1301, 1188, 1150, 1079, 1005,
1
836, 759 cm−1. H NMR (400 MHz, CDCl3 δ 7.26 ppm) δ
10.09 (s, 2H), 8.37 (s, 2H), 8.00 (d, J = 8.1 Hz, 4H), 7.86 (d,
J = 7.9 Hz, 2H), 7.83 (d, J = 8.1 Hz, 4H), 7.48 (d, J = 7.9 Hz,
2H), 1.62 (s, 18H) ppm. 13CNMR [100 MHz, CDCl3 δ 77.43
(3 peaks)] δ 192.6, 183.9, 150.7, 150.3, 148.1, 146.7, 136.9,
131.0, 128.8, 125.2, 124.3, 123.1, 116.3, 85.2, 28.3 ppm.
Fig. 1 a Normalized UV–vis absorption spectra of Tyr-1, Tyr-3, and
Tyr-4 dyes in toluene solution. b Comparison of normalized UV–vis
absorption and fluorescence emission spectra of Tyr-4 in
tetrahydrofuran solution (λexc = 485 nm). c UV–vis absorption spectra
of Tyr-1, Tyr-3, and Tyr-4 films on ITO-coated glass
Synthesis of (4-{4,7-bis[3-(dimethylamino)phenyl]
-1H-benzimidazole-2-yl}phenyl)boronic acid
N3,N3,N3′′,N3′′-tetramethyl-1,1′:4′,1′′-terphenyl-2′,3,3′,3′′-
tetramine (50 mg, 0.14 mmol) was dissolved in 2 mL of
MeCN and 4-formylphenylboronic acid (30 mg, 0.2 mmol)
was added to this solution. The mixture was stirred at room
temperature overnight and, then the resulting orange precipi-
tates were collected by filtration, washed with methanol, and
dried to give target compound, yielding 80%. FT-IR
(KBr pellet, cm-1): 3432 (boronic acid νO − H), 2923 (aliphatic
1
ν
C − H), 1609, 1384, 1265, 670 cm−1. H NMR (400 MHz,
DMSO-d6 δ 2.49 ppm) δ 10.04 (s, 1H), 9.79 (s, 1H), 8.29
(d, J = 8.4 Hz, 2H), 8.14 (d, J = 10.6 Hz, 2H), 7.98 (d, J =
10.6 Hz, 1H), 7.92 (d, J = 8.4 Hz, 1H), 7.86 (d, J = 7.9 Hz,
2H), 7.76 (d, J = 3.4 Hz, 2H), 7.34 (s, 2H), 6.93 (d, J =
10.6 Hz, 2H), 6.49 (s, 1H), 3.00 (s, 12H) ppm. 13C NMR

J Fluoresc
Table 1 Long-wavelength absorption (λ (nm)) and emission wavelengths (λem (nm)) of Tyr-1, Tyr-3, and Tyr-4 dyes in different solvents
(λexc = 485 nm)
Dyes / Solvents
PhCH3
CHCl3
THF
BzCN
λ1
λ2
λem1
λem2
λ1
λ2
λem1
λem2
λ1
λ2
λem1
λem2
λ1
λ2
λem1
λem2
Tyr-1
Tyr-3
Tyr-4
545
557
553
337
359
371
597
614
596
-
546
558
556
341
360
376
605
606
582
-
544
555
552
335
357
371
596
608
591
-
547
559
556
334
358
373
593
610
531
531
576
511
582
536
561
534
572
528
[100 MHz, CDCl3 δ 40.22 (7 peaks)] δ 195.0, 192.4, 164.7,
160.3, 153.3, 151.9, 138.4, 135.7, 135.4, 133.2, 129.6, 129.4,
127.4, 116.9, 64.8 ppm.
solution was added slowly into the Tyr-2 solution. The reac-
tion mixture was stirred at room temperature overnight. After
the complementation, the reaction mixture was extracted with
purified water, followed by HCl-acidified water. The organic
layer was dried over anhydrous magnesium sulfate and
Synthesis of Di-tert-butyl (2E)
-5,5′-bis(4-{4,7-bis[3-(dimethylamino)phenyl]
-1H-benzimidazole-2-yl}phenyl)
-3,3′-dioxo-2,2′-biindole-1,1’(3H,3’H)-dicarboxylate (Tyr-3)
To a 25 ml two-necked round-bottomed flask were added Tyr-
1 (38 mg, 61.3 μmol) in 8 mL of benzene and Na2CO3 (2 M,
4 mL). A solution of (4-{4,7-bis[3-(dimethylamino)phenyl]-
1H-benzimidazole-2-yl}phenyl)boronic acid (65 mg,
0.14 mmol) in 3 mL of ethanol was added to two-necked flask
in small portions at 65 °C under an argon atmosphere.
Catalytic amount of Pd(PPh3)4 was added to the solution
and the solution was stirred at 85 °C for 7 h. Small amount
of water was poured into the solution. The reaction mixture
was extracted with chloroform and concentrated under vacu-
um. The resulting solid was purified by preparative layer chro-
matography using n-hexane:ethyl acetate 35:15 (v:v) as an
eluent, yielding 8%. FT-IR (KBr pellet, cm−1): 2923, 2854
(aliphatic νC − H), 1732 (ester νC=O), 1703 (ketone νC=O),
1674, 1605, 1434, 1367, 1269, 1189, 1149, 1082, 1003,
824 cm−1. 1H NMR (400 MHz, CDCl3 δ 7.28 ppm) δ 10.12
(s, 2H), 8.40 (s, 2H), 8.05–8.01 (m, 6H), 7.90–7.81 (m, 8H),
7.51 (d, J = 9.4 Hz, 2H), 7.31 (t, J = 8.3 Hz, 4H), 6.96 (d, J =
8.3 Hz, 8H), 6.75 (dd, J = 8.3, 2.7 Hz, 4H), 3.00 (s, 24H), 1.64
(s, 9H), 1.26 (s, 9H) ppm. 13C NMR [100 MHz, CDCl3 δ
77.40 (3 peaks)] δ 192.6, 150.7, 150.4, 146.7, 143.7, 136.9,
136.0, 135.2, 134.7, 134.3, 131.8, 131.0, 130.4, 128.9, 125.2,
124.3, 123.2, 120.2, 116.4, 85.2, 29.8, 28.2, 22.7 ppm.
Synthesis of Di-tert-butyl (2E)-6,6′-bis[4-((E)
-{[3-(dimethylamino)phenyl]imino}methyl)phenyl]
-3,3′-dioxo-2,2′-biindole-1,1’(3H,3’H)-dicarboxylate (Tyr-4)
A solution of Tyr-2 (30 mg, 44.7 μmol) in 0.6 mL of chloro-
form was cooled to ―20 °C. Moreover, N,N-dimethyl-1,3-
phenylenediamine dihydrochloride (21 mg, 0.10 mmol) was
dissolved in 1 mL of chloroform:pyridine (v:v, 1:1) and this
Fig. 2 a Normalized fluorescence emission spectra of Tyr-1, Tyr-3, and
Tyr-4 dyes in toluene solution. b Comparison of normalized fluorescence
emission spectra of Tyr-3 in different solvents (λexc = 485 nm)

J Fluoresc
Table 2 Fluorescence quantum yields (ΦF), fluorescence decay times
(τf (ns)) with amplitudes, and mean fluorescence lifetimes (τm) for Tyr-1,
Tyr-3, and Tyr-4 dyes in different solvents (λexc = 485 nm)
Results and Discussion
Optical Properties
Dyes / Solvents
χ2
τf(1) α1
τf(2)
α2
τm
ΦF
The UV―visible absorption spectra of the synthesized indigo
dyes in toluene solution are shown in Fig. 1a, and all optical
data are summarized in Table 1. Model dye Tyr-1 gave two
major absorption bands, the first absorption band is peaked at
545 nm, and the second band is observed at 337 nm in toluene
solution. Indeed, tyrian purple has absorption maximum at
601 nm in DMF solution [30]. Striking blue shift in Tyr-1
absorption is attributed to the loss of the H-chromophore na-
ture of tyrian purple because of the distorted planarity of indi-
go ring initiated by N-substitution with t-BOC group. Tyr-3
with benzimidazole donor showed similar optical behavior
but its low energy absorption was further red-shifted by
12 nm and, also gave a new absorption band at 359 nm.
Tyr-4 with Schiff base donor showed similar absorption pro-
file giving a red-shifted indigo band with maximum at
553 nm, and a new band at 371 nm. Both renewed bands
observed at 359 and 371 nm for Tyr-3 and Tyr-4, respectively,
were assigned to the formation of charge transfer state, which
will be discussed in the following statements. UV―visible
absorption spectra of Tyr-3 and Tyr-4 dyes in other studied
solvents were given in Fig. S1(a-b).
Issa et al. reported the formation of intramolecular charge
transfer complex (ICT) for 3-amino-1,2,4-triazole Schiff bases
around 370 nm as a result of the π-delocalized electron cloud
of Schiff group [31]. Also, Gan et al. explained the outstand-
ing optical properties of carbazole-Schiff base derivatives by
ICT between carbazole donor and acceptor Schiff base [32].
In our measurements, when excited in THF solution at 485 nm
(Fig. 1b), Tyr-4 dye displays characteristic emission band of
indigo main core at 591 nm and, also gives a new emission
Tyr-1 Ph-CH3 1.08 4.81
-
-
-
-
-
-
-
-
-
-
-
-
0.0137
0.0081
0.0170
CHCl3
THF
0.67 4.45
0.84 4.16
Bz-CN 0.76 3.29 38.9 10.05 61.1 7.42 0.0365
Tyr-3 Ph-CH3 0.78 3.47 62.6 7.73
37.4 5.06 0.0980
30.3 4.38 0.0633
77.2 4.86 0.1307
56.1 5.40 0.3050
CHCl3
THF
0.87 3.39 69.7 6.68
0.86 2.12 22.8 5.67
Bz-CN 1.10 2.50 43.9 7.67
Tyr-4 Ph-CH3 0.84 2.98 24.2 10.24 75.8 8.48 0.0147
CHCl3
THF
0.87 3.86 65.8 12.45 34.2 6.79 0.0287
1.12 4.36 42.2 11.02 57.8 8.20 0.0052
Bz-CN 0.94 5.54 83.2 10.72 16.8 6.41 0.2274
evaporated in vacuum to dryness. Tyr-4 was precipitated with
chloroform:n-hexane binary solvent system, yielding 90%.
FT-IR (KBr pellet, cm−1): 3440, 2969, 2923, 2845 (aliphatic
ν
C − H), 1717 (ester νC=O), 1681 (ketone νC=O), 1606 (νC=N),
1516, 1478, 1435, 1369, 1276, 1150, 1082, 847 cm−1. H
NMR (400 MHz, CDCl3 δ 7.27 ppm) δ 8.34 (s, 2H), 7.83
(d, J = 8.3 Hz, 4H), 7.63 (d, J = 7.1 Hz, 2H), 7.59–7.36 (m,
6H), 7.32 (d, J = 8.3 Hz, 2H), 6.60 (d, J = 8.3 Hz, 2H), 6.22–
6.03 (m, 6H), 2.91 (s, 12H), 1.64 (s, 18H) ppm. 13C NMR
[100 MHz, CDCl3 δ 77.43 (3 peaks)] δ 181.7, 161.8, 131.1,
131.0, 130.9, 130.8, 130.4, 128.2, 122.8, 118.6, 117.3, 114.9,
114.6, 111.9, 107.1, 104.3, 102.6, 101.2, 101.1, 98.3, 40.7,
28.3 ppm.
1
Fig. 4 Cyclic voltammograms of Tyr-1, Tyr-3, and Tyr-4 dyes on glassy
Fig. 3 Fluorescence decay graph of Tyr-4 dye in tetrahydrofuran solution
(λdetection = 600 nm)
carbon working electrode in 0.1 M [TBA][PF6]/Me-CN (Scan rate:
100 mV s─1
)

J Fluoresc
Table 3 Electrochemical values and HOMO-LUMO energies of Tyr-1, Tyr-3, and Tyr-4 dyes with respect to the vacuum level
Dyes
E0red2 (V)
E0red1 (V)
E0ox1 (V)
E0ox2 (V)
E0ox3 (V)
LUMO (eV)
HOMO (eV)
E0-
a(eV)(abs)
0
Tyr-1
Tyr-3
Tyr-4
-
─0.29
─0.28
─0.40
0.13
0.11
0.05
1.82
1.80
0.79
-
─3.90
─3.91
─3.79
─6.01
─5.98
─5.86
2.11
2.07
2.07
─0.62
─0.74
-
1.73
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 in toluene solution
band around at 528 nm, most probably indicating a new specie
caused by the Schiff base-related charge transfer complex.
Figure 1c depicts the UV―visible absorption spectra of
Tyr-1, Tyr-3 and Tyr-4 in film state, showing similar vision
as given in solution phase, but giving red-shifted indigo ab-
sorptions with maxima at 567, 573, and 557 nm, respectively.
Figure 2a illustrates the comparison of normalized fluores-
cence emission spectra of Tyr-1, Tyr-3, and Tyr-4 dyes in
toluene solution. At the excitation wavelength of 485 nm,
while Tyr-1 brings about only one emission band located at
597 nm, Tyr-3 dye like Tyr-4 shows interestingly two emis-
sion bands, one of which is observed around 614 nm, and the
other one, minor band, is observed at 582 nm. The low energy
emission band of Tyr-3 is more red-shifted as compared to
that of Tyr-1 because of the strongly intermolecular hydrogen
bonding between benzimidazole N-H hydrogen and t-BOC
carbonyl group. More importantly, high energy emission band
of Tyr-3 does not belong to indigo nature. Figure 2b monitors
the uncommon emission bands for indigo nature in the struc-
ture of Tyr-3 in different solvents. When Tyr-3 is excited at
485 nm, in which is mostly indigo but also benzimidazole/
indigo ground state complex-related absorption band, the dye
displays characteristic emission features of benzimidazole in
high energy and indigo core emission in low energy region.
This observation is assigned to ICT complex between
the benzimidazole and indigo core. Similar ICT possibilities
of benzimidazole for different structures are reported in
the literature [21, 33, 34]. Fluorescence emission spectra of
Tyr-3 and Tyr-4 dyes in other studied solvents were given in
Fig. S2(a-b).
Table 2 summarizes fluorescence decay times and fluores-
cence quantum yields of synthesized indigo series. Also, we
have calculated the mean fluorescence lifetimes (τm = α1τ1 +
α2τ2) for bi-exponential decays. Single photon timing mea-
surement for one of the selected indigo dye, Tyr-4 dye, in
THF solution is shown in Fig. 3. The decays collected in
indigo emission at 600 nm were found to fit to a mono-
exponential for Tyr-1 dye around 4 ns in the studied solvents,
except Bz-CN solution. Tyr-1 dye gave an extra emission
band at 531 nm in addition to observed bi-exponential decay
fit, indicating the formation of ICTcomplex in more polar Bz-
CN solution cages. The lifetimes of Tyr-3 dye can be analyzed
as bi-exponential with short decay times around 2.12–3.47 ns
and long decay times around 5.67–7.73 ns. Long decay times
with moderately high amplitude values were attributed to the
delayed fluorescence caused by the existence of ICT complex
Fig. 5 Electronic structures of the
HOMO and LUMO for Tyr-1,
Tyr-3, and Tyr-4 dyes and
comparison of their energy levels
with PCDTBT energy alignment

J Fluoresc
Fig. 7 The variation in the capacitance-frequency (C-F) characteristic of
FTO/TiO2/Tyr-1/LiCl/Al devices
to indigo moiety when the copolymer is excited at fluorine
absorption region [35]. Similar results were analyzed for Tyr-
4 dye with Schiff base derivative. Tyr-4 dye gave bi-
exponential decay times, one of which is short decay times
around 2.98–5.54 ns, and the long decay times are observed
around 10.24–12.45 ns. Short decay times belong to indigo
lifetime value, but long decay times are more probably related
to existence of complex structure.
Fluorescence quantum yields (ΦF) of Tyr-1 dye was calcu-
lated to be about between 0.0081–0.0365 in different solvent
of polarities. It is reported that ΦF value of indigo is quite low
(ΦF:0.0023 in DMF) due to its efficient internal conversion
radiationless process which may be related to intra- or inter-
molecular transfer reactions in H-chromophores of indigo
Fig. 6 AFM images (5 μm × 5 μm) of a Tyr-1, b Tyr-3, and c Tyr-4
films spin-coated from chloroform solution
between the benzimidazole donor and indigo acceptor. ICT
possibilities of indigo nature was known in the literature for
different substituents. Pina et al. reported that charge transfer
occurs in indigo-fluorene type copolymers from the fluorene
Fig. 8 Dark current density-effective voltage characteristic of SCLC
mobility in FTO/TiO2/Tyr-1/LiCl/Al device

J Fluoresc
at −0.40 and − 0.74 V for Tyr-4 dyes. In the oxidation region,
while two oxidation peaks appear at 0.11, and 1.80 V for Tyr-
3, an extra peak at 0.05 V was observed in addition to 0.79,
and 1.73 V for Tyr-4 dye. This additional peak at 0.05 V is an
accordance with the oxidation state of Schiff base group
which may be observed independently. The much easier oxi-
dation of Tyr-3 at 0.11 V is attributed to the presence of benz-
imidazole group decorated with four electron donor N(CH3)2
substituents as compared to the other dyes. The LUMO and
HOMO energies of Tyr-1, Tyr-3, and Tyr-4, estimated by CV,
are −3.90, −3.91, and − 3.79 eV, and − 6.01, −5.98, and −
5.86 eV, respectively. After introducing the Schiff base unit
to the indigo core, Tyr-4 dye gave a higher LUMO and
HOMO level as compared to the other dyes.
Figure 5 compares the energy level of PCDTBT donor
polymer with HOMO and LUMO levels of the dye structures
which is illustrated by DFT-calculated orbital contours. For
Tyr-1 dye, the HOMO is mainly contributed by the peripheral
t-BOC groups while the LUMO is delocalized over the mainly
indigo region. From the electronic distributions in the molec-
ular orbital contours of Tyr-3 dye a certain amount of charge
may be transferred from the benzimidazole side to the accep-
tor indigo core. However, while there is little contribution to
the HOMO from the peripheral Schiff base groups in Tyr-4
molecule, LUMO delocalization mainly occurs onto both in-
digo core and one of the t-BOC groups. The LUMO levels of
these indigo dyes were well-matched with the LUMO level of
donor PCDTBT polymer (−3.6 eV), which is enough level for
electron injection and charge separation processes.
Fig. 9 J-V curves of photovoltaic devices using PCDTBT:Tyr-1,
PCDTBT:Tyr-3 blends as active layer
nature [36]. This kind of reactions are inhibited by the N-
substitution of indigo with t-BOC group in Tyr-1 structure
so that ΦF value increases. ΦF values for both Tyr-3 and
Tyr-4 dyes are quite higher than those presented by Tyr-1.
Nothing that our synthesized Tyr-3 and Tyr-4 dyes gained an
extra emission band in high energy region because of the
contribution of benzimidazole or Schiff base moieties
resulting much higher ΦF values as compared to that of Tyr-
1 dye. Those higher ΦF values for Tyr-3 and Tyr-4 dyes may
also be referred to ICT complex formation. Other striking
results are obtained in Bz-CN solution giving somewhat
higher ΦF values as compared to that of other studied solu-
tions. This may be related to existence of ICT complex in
more polar Bz-CN solution.
Optoelectronic Performance and Morphological
Studies
The morphology of thin films spin-coated from chloroform
solution of dyes was investigated by AFM images (Fig. 6a-
c). The root-mean-square (rms) of surface roughness was
15.4, 8.8 and 2.2 nm for Tyr-1, Tyr-3, and Tyr-4, dyes, re-
spectively. It is noted that only Tyr-4 layer depicted smooth
and uniform film formation.
Electron-only diode were fabricated from thin films of Tyr-
1 dye to understand the electron transport behavior of the
material. Capacitance-frequency (C-F) and dark current
density-effective voltage characteristics for FTO/TiO2/Tyr-1/
LiCl/Al devices were illustrated in Figs. 7 and 8, respectively.
SCLC electron mobility of 3.4 × 10− 3 cm2/Vs for Tyr-1 dye
was measured. Wherein electron mobilities of other dyes
could not be measured because of poor film formation quali-
ties. Obtained electron mobility value for Tyr-1 is much lower
compared to reported values for pristine indigo structure (1 ×
10−2 cm2/Vs) [14]. This may be assigned to the formation of
more disordered films of Tyr-1 dye caused by the branched t-
BOC groups that inhibit the molecular reorganization. Similar
indigo derivative in the literature with branched long alkyl
Electrochemical Properties
Figure 4 shows the cyclic voltammetry (CV) diagram of syn-
thesized indigo type small molecules in order to estimate their
energy levels. Table 3 summarizes the electrochemical data of
the CV measurements. The oxidation potentials of Tyr-1 are
0.13 and 1.82 V versus Ag/Ag+. The reduction potential of
Tyr-1 is −0.29 V, indicating the formation of stable indigo
anion radical. Both new indigo derivatives present two con-
secutive reduction peaks at −0.28 and − 0.62 V for Tyr-3, and
Table 4 OPV parameters of conventional devices prepared from the
synthesized indigo dyes as acceptor
Active Layer
Voc (mV) Isc (mA/cm2) FF
η
Binary Structure w:w
PCDTBT:Tyr-1
PCDTBT:Tyr-3
1:4
1:1
460
20
0.10
0.01
0.21 0.010
0.86 0.002

J Fluoresc
2. Fukumoto H, Nakajima H, Kojima T, Yamamoto T (2014)
Preparation and chemical properties of π-conjugated polymers con-
taining indigo unit in the main chain. Mater 7:2030–2043
3. Głowacki ED, Voss G, Leonat L, Irimia-Vladu M, Bauer S,
Sariciftci NS (2012) Indigo and tyrian purple - from ancient natural
dyes to modern organic semiconductors. Isr J Chem 52:540–551
4. Imming P, Imhof I, Zentgraf M (2006) An improved synthetic pro-
cedure for 6,6′-dibromoindigo (tyrian purple). Synth Commun 31:
3721–3727
5. Pitayatanakul O, Iijima K, Kadoya T, Ashizawa M, Kawamoto T,
Matsumoto H, Mori T (2015) Ambipolar organic field-effect tran-
sistors based on indigo derivatives. Eng J 19:61–74
6. Głowacki ED, Apaydin DH, Bozkurt Z, Monkowius U, Demirak
K, Tordin E, Himmelsbach M, Schwarzinger C, Burian M, Lechner
RT, Demitri N, Voss G, Sariciftci NS (2014) Air-stable organic
semiconductors based on 6,6′-dithienylindigo and polymers there-
of. J Mater Chem C 2:8089–8097
surface group gave the lowest electron mobility value around
5 × 10−2 cm2/Vs because of its unsymmetrical structure in
which reduces the intermolecular interactions for neat film
formation [10].
BHJ-SCs were fabricated with a conventional structures of
ITO/PEDOT:PSS/PCDTBT:dyes/LiCl/Al (70 nm)/Ca
(30 nm), wherein the photoactive layer with synthesized indi-
go series as acceptor. Figure 9 shows the J-V curves of pho-
tovoltaic devices using PCDTBT:Tyr-1, PCDTBT:Tyr-3
blends as active layer, and extracted photovoltaic data are
summarized in Table 4. The optimal device of Tyr-1 dye
was fabricated by the 20:80 donor:acceptor ratio, giving a
short circuit current (Isc) of 0.1 mA/cm2, an open circuit volt-
age (Voc) of 460 mV, a fill factor (FF) of 0.21 and a power
conversion efficency (PCE) of 0.010%. While Tyr-4 dye did
not give any photovoltaic response from the measurements,
Tyr-3 dye showed the much lower device performance ob-
tained with a 50:50 donor:acceptor ratio, giving a Isc of
0.01 mA/cm2, a Voc of 20 mV, a FF of 0.86 and a PCE of
0.002% as compared to Tyr-1 dye.
7. Guo C, Quinn J, Sun B, Li Y (2015) An indigo-based polymer
bearing thermocleavable side chains for n-type organic thin film
transistors. J Mater Chem C 3:5226–5232
8. Pitayatanakul O, Higashino T, Kadoya T, Tanaka M, Kojima H,
Ashizawa M, Kawamoto T, Matsumoto H, Ishikawa K, Mori T
(2014) High performance ambipolar organic field-effect transistors
based on indigo derivatives. J Mater Chem C 2:9311–9317
9. Pitayatanakul O, Iijima K, Ashizawa M, Kawamoto T, Matsumoto
H, Mori T (2015) An iodine effect in ambipolar organic field-effect
transistors based on indigo derivatives. J Mater Chem C 3:8612–
8617
10. Kim IK, Li X, Ullah M, Shaw PE, Wawrzinek R, Namdas EB, Lo
SC (2015) High-performance, fullerene-free organic photodiodes
based on a solution-processable indigo. Adv Mater 27:6390–6395
11. Glowacki ED, Voss G, Demirak K, Havlicek M, Sunger N, Okur
AC, Monkowius U, Gasiorowski J, Leonat L, Sariciftci NS (2013)
A facile protection-deprotection route for obtaining indigo pig-
ments as thin films and their applications in organic bulk
heterojunctions. Chem Commun (Camb) 49:6063–6065
12. Lu Y-M, Wong J-S, Hsu L-C (2013) Characterization of indigo-
doped poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid meth-
yl ester bulk heterojunction solar cells. Thin Solid Films 529:58–61
13. Liu C, Dong S, Cai P, Liu P, Liu S, Chen J, Liu F, Ying L, Russell
TP, Huang F, Cao Y (2015) Donor-acceptor copolymers based on
thermally cleavable indigo, isoindigo, and DPP units: synthesis,
field effect transistors, and polymer solar cells. ACS Appl Mater
Interfaces 7:9038–9051
14. Irimia-Vladu M, Glowacki ED, Troshin PA, Schwabegger G,
Leonat L, Susarova DK, Krystal O, Ullah M, Kanbur Y, Bodea
MA, Razumov VF, Sitter H, Bauer S, Sariciftci NS (2012)
Indigo–a natural pigment for high performance ambipolar organic
field effect transistors and circuits. Adv Mater 24:375–380
15. Głowacki ED, Leonat L, Voss G, Bodea M-A, Bozkurt Z, Ramil
AM, Irimia-Vladu M, Bauer S, Sariciftci NS (2011) Ambipolar
organic field effect transistors and inverters with the natural material
tyrian purple. AIP Adv 1(1–6):042132
Conclusions
In summary, we have synthesized a novel class of t-BOC
functionalized indigo type small molecules which contain
benzimidazole or Schiff base unit in 6,6′ position in the struc-
ture and evaluated the charge transfer possibilities by compar-
ing their optical and electrochemical properties. New generat-
ed UV-vis and uncommon emission bands of Tyr-3 and Tyr-4
dyes were assigned to the formation of charge transfer state.
Interestingly, long decay times for the dye solutions support to
the delayed fluorescence caused by the ICT complex between
the subunits and acceptor indigo core. We have also utilized
them as electron acceptors in molecular BHJ-SC devices for
the first time. Although tested indigo dyes did not exhibited
superior solar cell efficiencies, indigo derivative dyes will be
considered as promising materials for photovoltaic devices if
their structures can be improved for better charge transfer
abilities.
Acknowledgements This work was supported by the Scientific and
Technological Research Council of Turkey (TUBITAK) with the project
number of 113Z250. The authors acknowledge the Ege University for
theoretical DFT calculations.
16. Çimen O, Dinçalp H, Varlıklı C (2015) Studies on UV–vis and
fluorescence changements in Co2+ and Cu2+ recognition by a new
benzimidazole–benzothiadiazole derivative. Sensor Actuat B Chem
209:853–863
17. Harris JD, Liu J, Carter KR (2015) Synthesis of π-bridged dually-
dopable conjugated polymers from benzimidazole and fluorene:
separating sterics from electronics. Macromolecules 48:6970–6977
18. Ku J, Song S, Park SH, Lee K, Suh H, Lansac Y, Jang YH (2015)
Palladium-assisted reaction of 2,2-dialkylbenzimidazole and its im-
plication on organic solar cell performances. J Phys Chem C 119:
14063–14075
References
1. Robb MJ, Ku S-Y, Brunetti FG, Hawker CJ (2013) A renaissance of
color: New structures and building blocks for organic electronics. J
Polym Sci Pol Chem 51:1263–1271
19. Schmidt H (1997) Indigo–100 jahre industrielle synthese. Chem
Unserer Zeit 31:121–128

J Fluoresc
20. Kotha S, Lahiri K, Kashinath D (2002) Recent applications of the
Suzuki-Miyaura cross-coupling reaction in organic synthesis.
Tetrahedron 58:9633–9695
21. Saltan GM, Dinçalp H, Kıran M, Zafer C, Erbaş SÇ (2015) Novel
organic dyes based on phenyl-substituted benzimidazole for dye
sensitized solar cells. Mater Chem Phys 163:387–393
22. Icli S, Icil H (1996) AThermal and photostable reference probe for
Qf measurements: chloroform soluble perylene 3,4,9,10-
tetracarboxylic acid-bis-N,N-dodecyl diimide. Spectrosc Lett 29:
1253–1257
23. Pommerehne J, Vestweber H, Guss W, Mahrt RF, Bassler H, Porsch
M, Daub J (1995) Efficient 2-layer LEDs on a polymer blend basis.
Adv Mater 7:551–554
24. Knutson JR, Beechem JM, Brand L (1983) Simultaneous analysis
of multiple fluorescence decay curves - a global approach. Chem
Phys Lett 102:501–507
25. Zuker M, Szabo AG, Bramall L, Krajcarski DT, Selinger B (1985)
Delta-function convolution method (DFCM) for fluorescence de-
cay experiments. Rev Sci Instrum 56:14–22
26. Frisch MJ, Trucks GW, Schlegel HB, Scuseria GE, Robb MA,
Cheeseman JR, Scalmani G, Barone V, Mennucci B, Petersson
GA, Nakatsuji H, Caricato M, Li X, Hratchian HP, Izmaylov AF,
Bloino J, Zheng G, Sonnenberg JL, Hada M, Ehara M, Toyota K,
Fukuda R, Hasegawa J, Ishida M, Nakajima T, Honda Y, Kitao O,
Nakai H, Vreven T, Montgomery JA Jr, Peralta JE, Ogliaro F,
Bearpark MJ, Heyd J, Brothers EN, Kudin KN, Staroverov VN,
Kobayashi R, Normand J, Raghavachari K, Rendell AP, Burant JC,
Iyengar SS, Tomasi J, Cossi M, Rega N, Millam NJ, Klene M,
Knox JE, Cross JB, Bakken V, Adamo C, Jaramillo J, Gomperts
R, Stratmann RE, Yazyev O, Austin AJ, Cammi R, Pomelli C,
Ochterski JW, Martin RL, Morokuma K, Zakrzewski VG, Voth
GA, Salvador P, Dannenberg JJ, Dapprich S, Daniels AD, Farkas
Ö, Foresman JB, Ortiz JV, Cioslowski J, Fox DJ (2009) Gaussian
09. Gaussian, Inc., Wallingford
27. Kohn W, Sham LJ (1965) Self-consistent equations including ex-
change and correlation effects. Phys Rev 140:A1133–A1138
28. Goh C, Kline RJ, Mcgehee MD, Kadnikova EN, Frechet JMJ
(2005) Molecular-weight-dependent mobilities in regioregular
poly(3-hexyl-thiophene) diodes. Appl Phys Lett 86(1–3):122110
29. Carbone A, Kotowska BK, Kotowski D (2005) Space-charge-
limited current fluctuations in organic semiconductors. Phys Rev
Lett 95(1–4):236601
30. De Melo JS, Rondao R, Burrows HD, Melo MJ, Navaratnam S,
Edge R, Voss G (2006) Spectral and photophysical studies of
substituted indigo derivatives in their keto forms. Chemphyschem
7:2303–2311
31. Issa YM, Hassib HB, Abdelaal HE, Kenawi IM (2011) Spectral
investigation of the intramolecular charge-transfer in some
aminotriazole Schiff bases. Spectrochim Acta A Mol Biomol
Spectrosc 79:1364–1374
32. Gan X, Liu G, Chu M, Xi W, Ren Z, Zhang X, Tian Y, Zhou H
(2016) Intermolecular interactions boost aggregation induced emis-
sion in carbazole Schiff base derivatives. Org Biomol Chem 15:
256–264
33. Kulhanek J, Bures F (2012) Imidazole as a parent pi-conjugated
backbone in charge-transfer chromophores. Beilstein J Org Chem
8:25–49
34. Thanikachalam V, Arunpandiyan A, Jayabharathi J, Ramanathan P
(2014) Photophysical properties of the intramolecular excited
charge-transfer states of π-expanded styryl phenanthrimidazoles–
effect of solvent polarity. RSC Adv 4:6790–6806
35. Pina J, Seixas De Melo JS, Eckert A, Scherf U (2015) Unusual
photophysical properties of conjugated, alternating indigo-
fluorene copolymers. J Mater Chem A 3:6373–6382
36. Rondao R, Seixas De Melo J, Schaberle FA, Voss G (2012) Excited
state characterization of a polymeric indigo. Phys Chem Chem Phys
14:1778–1783