Novel functionalized indigo derivatives for organic electronics

Article abstract of DOI:10.1016/j.dyepig.2020.108966

A series of nine novel indigo derivatives, including diiodoindigo, octahalogenated indigoids and compounds with extended π-conjugated system, were synthesized, characterized and investigated as semiconductor materials in organic field-effect transistors (OFETs). Among them, 6,6′-diiodoindigo demonstrated the ambipolar behavior with balanced p-type and n-type mobilities. The complete substitution of hydrogens at the indigo core with halogen atoms led to low electron mobilities in OFETs. An extension of the conjugated system through the introduction of small aromatic substituents (thiophene and phenyl) resulted in predominant p-type behavior. Fusion of aromatic rings resulted in z-shaped dibenzoindigo, which showed poor charge transport properties due to the non-optimal arrangement of molecules along each other in the crystal lattice. The acquired data fulfilled the previously reported model based on the relationship between the chemical nature of substituents and their positions at the indigo core, optoelectronic properties of materials and their performance in OFETs. The results of this study will be useful for rational design of a new generation of the indigo-based semiconductors for biocompatible organic electronics.

Full text of DOI:10.1016/j.dyepig.2020.108966

Journal Pre-proof
Novel functionalized indigo derivatives for organic electronics
Irina V. Klimovich, Alexander V. Zhilenkov, Lidiya I. Кuznetsova, Lubov A. Frolova,
Olga R. Yamilova, Sergey I. Troyanov, Konstantin A. Lyssenko, Pavel A. Troshin
PII:
S0143-7208(20)31663-6
DOI:
https://doi.org/10.1016/j.dyepig.2020.108966
DYPI 108966
Reference:
To appear in: Dyes and Pigments
Received Date: 7 July 2020
Revised Date: 6 October 2020
Accepted Date: 24 October 2020
Please cite this article as: Klimovich IV, Zhilenkov AV, Кuznetsova LI, Frolova LA, Yamilova OR,
Troyanov SI, Lyssenko KA, Troshin PA, Novel functionalized indigo derivatives for organic electronics,
Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2020.108966.
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Irina V. Klimovich Writing - Original Draft, Investigation; Alexander V. Zhilenkov
Investigation; Lidiya I. Кuznetsova Investigation; Lubov A. Frolova Investigation; Olga
R. Yamilova Investigation; Sergey I. Troyanov Investigation; Konstantin A. Lyssenko
Investigation, Data Curation; Pavel A. Troshin Project administration, Supervision,
Writing - Review & Editing

1
Novel functionalized indigo derivatives for organic electronics
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3
Irina V. Klimovich,a,b Alexander V. Zhilenkov,b Lidiya I. Кuznetsova,b Lubov A.
Frolova,a,b Olga R. Yamilova,a,b Sergey I. Troyanov,c Konstantin A. Lyssenko,c,d and
Pavel A. Troshina,b
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a Skoltech, CEST, Bolshoy Boulevard, 30/1, Moscow, 121205, Russia
b IPCP RAS, Semenov Prospect 1, Chernogolovka, 141432, Russia.
c Department of Chemistry, Moscow State University, Leninskie gory 1, Moscow, 119991,
Russia
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d G.V. Plekhanov Russian University of Economics, 36 Stremyanny Per., 117997 Moscow, Russia
E-mail: I.Klimovich@skoltech.ru; Tel: +7 9057274584
12 ABSTRACT
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A series of nine novel indigo derivatives, including diiodoindigo, octahalogenated indigoids
and compounds with extended π-conjugated system, were synthesized, characterized and
investigated as semiconductor materials in organic field-effect transistors (OFETs). Among them,
6,6'-diiodoindigo demonstrated the ambipolar behavior with balanced p-type and n-type
mobilities. The complete substitution of hydrogens at the indigo core with halogen atoms led to
low electron mobilities in OFETs. An extension of the conjugated system through the introduction
of small aromatic substituents (thiophene and phenyl) resulted in predominant p-type behavior.
Fusion of aromatic rings resulted in z-shaped dibenzoindigo, which showed poor charge transport
properties due to the non-optimal arrangement of molecules along each other in the crystal lattice.
The acquired data fulfilled the previously reported model based on the relationship between the
chemical nature of substituents and their positions at the indigo core, optoelectronic properties of
materials and their performance in OFETs. The results of this study will be useful for rational
design of a new generation of the indigo-based semiconductors for biocompatible organic
electronics.
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Keywords: indigo, indigoids, organic field effect transistors, semiconductors, OFET
3
INTRODUCTION
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There is a constantly increasing demand for various electronic devices. However, their
components based on inorganic compounds are at reasonable risk to once confront the limited
availability of key elements (such as Li, In, and Ga), in addition to environmental issues related to
their production and utilization.[1] In contrast, organic materials consist of widely distributed
elements, many of their precursors can be obtained from biomass, and their preparation and
purification are less energy-consuming. The continuous search for new organic semiconductor
materials has already led to ongoing improvements of their performance in solar panels, light-
emitting diodes, transistors, etc.[2],[3] Current efforts are aimed at improving stability and safety
of developed materials and developing technologies for growth of high-quality semiconductor
films.[4],[5],[6],[7],[8] Searches for new materials with desired properties have also covered the
well-known natural dyes and pigments.[9],[10],[11] For instance, indigo and its derivatives are
widely used in cosmetics, textile, food, printing inks, etc. as the colorants with low acute
toxicity.[12],[13] Thus, some novel applications of these derivatives have been explored, viz., as
photoinitiators for the polymerization,[14] electrochemical doping of nanoporous anodic
aluminum oxide,[15] in photodiodes,[16] as electroactive materials for Na-ion[17] and flow[18]
batteries, as sensor materials for ozone[19],[18] and fluorine ions[20]. Indigo and its
functionalized derivatives possess optimal optoelectronic and physicochemical properties to be
applied as semiconductor materials in organic field effect transistors. Semiconducting properties
of the parent compound (indigo) have been discovered in 2010s,[21],[22],[23] giving rise to
chemical functionalization of indigo molecule as the method for design of novel materials with
improved charge transport characteristics. Several works reported the application of 6,6'-
dibromoindigo, 6,6'-dichloroindigo as semiconductors for OFETs.[24] Complementary-like
inverters [25] and organic photodetectors [26] have been also designed and investigated. Our
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group has previously explored indigoids bearing halogen, cyano and trifluoromethyl
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1
2
substituents.[27] It was found that the introduction of strong electron-withdrawing groups can
significantly affect HOMO and LUMO energy levels as well as the crystal structure of indigoids.
Those observations could be helpful in the design of new indigoids with superior charge-transport
properties and ambient stability.
3
4
5
Extension of π-conjugated system is an efficient approach to increase the charge carrier
mobilities. As compared to pristine indigo, dibenzoindigo with two fused benzene rings
demonstrates superior p-type transport properties. [28] Both 5,5'-diphenyl and 6,6'-dithienyl
substituted indigoids exhibit well-balanced ambipolar behavior in OFETs.[29][30] However, the
fabrication of devices using these compounds involves energy-consuming vacuum evaporation
processing techniques, while inexpensive printing technologies require the solution-processible
materials. Unfortunately, only a limited set of indigo-based soluble small molecules is currently
known. Bisalkoxy-substituted derivatives have been synthesized, but demonstrated poor
characteristics in OFETs.[31] Although bisalkyloxyphenyl indigo derivatives are capable of
forming liquid crystalline phase, the charge carrier mobility has been never measured for these
compounds.[32]
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Thus, a very limited number of indigo derivatives was explored as potential components for
the organic electronics, while the testing conditions were not unified, which does not allow
making any clear comparison of the characteristics of transistors based on different compounds.
No reliable relationships between their structure and performance in transistors have been
established yet due to the lack of systematic research.
Herein we report on the design, synthesis, and investigation of a diverse set of indigo-based
molecules (Figure 1) as promising materials for organic electronic devices. Common approach to
their design is based on the introduction of electron-withdrawing groups in the case of n-type
materials and on the expansion of conjugated electronic system for the p-type materials.
Therefore, we have started from the introduction of halogen atoms, up to the complete substitution
at benzene rings of the indigo core, followed by the evaluation of their performance as
semiconductors in transistors. Novel derivatives incorporating aromatic substituents and fused
structure 9 were also designed as the p-type materials.
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1
2
Figure 1. Chemical structures of the designed and investigated indigo derivatives.
3
RESULTS AND DISCUSSION
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5
6
7
8
Molecular design and synthesis. Although the effect of iodine substituents at the indigo core on
the material performance in OFETs has been already demonstrated for 5,5'-diiodoindigo,[33] our
previous work has revealed that 6,6'-dihalogen isomers of indigoids possess superior
semiconductor characteristics as compared to their 5,5'-analogues. [27] Thus, the number of
considered indigo derivatives was extended with the corresponding 6,6'-diiodoindigo.
9
10
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13
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15
Synthetic route towards 6,6'-diiodoindigo is shown on Scheme 1. Commercially available 4-
iodotoluene was nitrated in glacial acetic acid to give 2-nitrotoluene. Then it was treated with
N,N'-dimethylformamide dimethyl acetal and subsequently oxidized by NaIO4, resulting in 4-iodo-
2-nitrobenzaldehyde. Target 6,6'-diiodoindigo 1 was synthesized in the yield of 54% via Harley–
Mason reaction consisting of two steps: (1) 2-nitrobenzaldehyde was treated with nitromethane in
the presence of sodium methylate (Henry reaction) and (2) consequently reduced with sodium
dithionite.
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18
Scheme 1. Synthesis of 6,6'-diiodoindigo 1.
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1
2
The introduction of four fluorine atoms increases ambient stability of n-type OFETs due to the
decreased LUMO energy level, [27] so the full substitution of hydrogens by halogen atoms should
shift energy levels even further. Different synthetic routes were used for the synthesis of indigo
derivatives 2 and 3 bearing 8 halogen atoms (Scheme 2). The limiting step in the synthesis of
octafluoroindigo was the alkylation of corresponding antranilic acid with bromoacetic acid (30%
yield even after the optimization of reaction conditions). Intramolecular Claisen condensation of
obtained carboxyaryl glycine led to diacetylindoxyle that was hydrolyzed with alkali to give
indigo derivative 2.
3
4
5
6
7
8
9
Similar alkylation did not occur in the case of tetrachloroantranilic acid due probably to the
mesomeric effect of chlorine atoms. Target compound 3 was obtained via the treatment of
tetrachloroantranilic acid with formalin. The subsequent ring opening of obtained formalide by
potassium cyanide (CAUTION: highly toxic substance) led to 2,3,4,5-tetrachloro-6-
((cyanomethyl)amino)benzoic acid. Alkali hydrolysis of nitrile and consequent intramolecular
Claisen condensation resulted in tetrachloroindoxyl, the precursor of octachloroindigo 3.
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17
Scheme 2. Synthesis of indigo derivatives 2 and 3.
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1
2
It has been previously shown that the introduction of phenyl group in indigo results in high-
performance and air-stable ambipolar semiconductors,[30] so we have explored the possibility of
substituting indigo at the 6,6' positions with phenyl, thienyl, benzothienyl and benzothiazolyl
groups.
3
4
5
Synthetic routes towards indigo derivatives bearing phenyl, thienyl, benzothienyl and
benzothiazolyl substituents are shown in Scheme 3. Commercially available dibromonitrobenzene
was lithiated with phenyl lithium at -110 °С, followed by treatment with dimethylformamide
resulting in 4-bromo-2-nitrobenzaldehyde. The Suzuki and Stille cross-coupling reactions with
corresponding boronic acids and their ethers or with stannanes led to o-nitrobenzaldehydes that
were converted into target compounds 4-7.
6
7
8
9
10
11
12
Scheme 3. Synthesis of indigo derivatives 4–7.
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14
Soluble indigo derivative 8 comprising thiophene rings was obtained via the reaction of 6,6'-
dibromoindigo with tributyl(5-(2-ethylhexyl)thien-2-yl)stannane according to Scheme 4.
6

1
2
Scheme 4. Synthesis of soluble indigo derivative 8.
3
4
Extension of π-conjugated system of indigo in novel fused derivatives is the efficient approach
to design new semiconductor materials with enhanced properties. [28],[34],[35] Thus, another
isomer of dibenzoindigo with the fused structure was designed. Synthesis of dibenzo[g,g']indigo is
shown in Scheme 5. 2-Methylnaphtalene was nitrated with fuming nitric acid in glacial acetic acid
to give 2-methyl-1-nitronaphtalene. It was subsequently oxidized with dimethylformamide
dimethyl acetal and NaIO4 to give 1-nitronaphthyl carbaldehyde. Its treatment with nitromethane
and sodium methylate followed by the reduction with sodium dithionite led to target
dibenzo[g,g']indigo 9.
5
6
7
8
9
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13
Scheme 5. Synthesis of dibenzo[g,g’]indigo 9.
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19
All the obtained indigo derivatives (except for 8) exhibited an extremely low solubility in
organic solvents, making their NMR characterization quite challenging. However, all these
compounds were characterized with mass spectroscopy, FTIR spectroscopy, and elemental
analysis (ESI), while structures of some of them were completely confirmed by the single crystal
X-ray diffraction analysis.
20
21
22
23
Single crystal X-ray diffraction analysis was carried out for compounds 1, 8 and 9 (CCDC:
2013267, 2010617 and 2013266; See Table S1 for crystal data, data collection and structure
refinement details). In all crystals studied molecules participate in the stacking interactions but the
plane-to-plane distance as well as type of molecules overlap are different. (Fig. 2) The shortest
7

1
2
3
4
5
plane-to-plane distance 3.26 is observed for 9, the longest one 3.49 for 8, the corresponding
value for 1 is 3.398 . As compared to parent indigo (intermolecular spacing of 3.40 Å),
diiodoindigo 1 and dibenzoindigo 9 demonstrate the same or shorter π-π distances, thus suggesting
the possibility for formation of efficient intermolecular contacts. In the case of alkylthienyl indigo
8, its branched 2-ethylhexyl chains hinder close intermolecular contacts.
6
7
In addition to the different character of overlap of π-systems in stacks, a significant difference is
observed for the mutual disposition of the π-stacked columns. Each molecule of 8 participates in
the formation of four intermolecular N-H…O hydrogen bonds (N…O is 3.07Å) with two
neighbouring molecules (Figure S1). These hydrogen bonds assemble molecules in slightly
corrugated H-bonded layers and stacking-bonded columns, which are parallel to each other. In the
diiodoindigo (1), each molecule also forms four N-H…O bonds (N….O 2.903 ) but with four
neighbouring molecules in perpendicular dimensions (Figure S2). Such type of H-bonding
assembles molecules into the three-dimensional framework and stacked-bonded columns are
almost perpendicular (81°). Therefore, isotropic charge transport properties can be expected for
diiodoindigo featuring it as a promising organic semiconductor material. The molecules of 9 do
not participate in any intermolecular H-bonds that is rare for indigo derivatives and the angles
between the stacking-bonded columns is equal to 62.6°. Despite of the differences in the H-
bonding in 1, 8 and 9 the strengths of the intramolecular N-H….O bonds in this derivatives are
identical. Thus, one can conclude that the N…O distance for intramolecular interaction is solely
governed by the conjugation.
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
It should be noted that the molecules are shifted along each other in the stacks, whereas the angle
between the straight line connecting centers and planes of the molecules (see Figure S3 for
definition of angle) is not 90° (expected for perfect H-type molecular stack), but is equal to 126,
147 and 154° for 1, 8, and 9, respectively. It is known that shifting of the molecules with respect
to each other reduces the charge transport mobility along the π-stack. [36], [37]
26
8

1
2
3
Figure 2. The molecules overlap in the stacked-bonded columns in 1 (a), 8 (b), and 9 (c). The
4
alkyl substituents in 8 are omitted for clarity.
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7
8
Optoelectronic properties of the obtained compounds. Figure 3a,b shows the absorption
spectra of indigo derivatives dissolved in a hot 1,2-dichlorobenzene, wherein compounds 1–9
exhibit peaks, very similar to those of indigo in long-wavelength range. Compounds 3–8 afforded
very small red shift as compared to indigo (600nm).
9
9

a)
1
2
3
9
1,0
0,8
0,6
0,4
0,2
0,0
indigo
450
500
550
600
650
700
750
Wavelength, nm
1
2
b)
4
5
6
7
8
1,0
0,8
0,6
0,4
0,2
0,0
indigo
450 500 550 600 650 700 750 800
Wavelength, nm
3
4
Figure 3. Normalized UV-Vis spectra of indigo and its derivatives 1-9 in hot 1,2-dichlorobenzene.
5
6
Optical and electrochemical properties of indigo derivatives were also investigated in thin
films, except for compound 6 due to its insufficient thermal stability and low solubility preventing
film deposition by either of the vapor phase and solution techniques. All the other compounds
demonstrated quite different optical spectra recorded for their solutions in hot 1,2-dichlorobenzene
and thin films (ESI, Figures S4–S10). The spectra of compounds 2–6, 8, and 9 in thin films are
red-shifted with respect to those of the solutions. This suggests the formation of J-aggregates [38]
in thin films, that does not facilitate the charge-transport properties. On the contrary, the spectra of
7
8
9
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2
thin films of 1 and 7 are blue-shifted in comparison with the solutions, suggesting the formation of
H-aggregates promoting the efficient charge transport.
3
Cyclic voltammetry measurements were performed for thin films deposited by the thermal
evaporation on glassy carbon disc electrodes in order to reveal onsets of the oxidation and
reduction waves for these compounds (ESI, Figure S11). Assuming that the Fermi energy for the
Fc+/Fc redox couple is - 5.1 eV, the HOMO and LUMO energies of the compounds were estimated
according to the standard approach.[39] Table 1 summarizes the acquired data. Almost all the
indigoids demonstrated close values of their reduction potentials, defining LUMO energies of ca.
–3.9 eV. The introduction of fluorine or chlorine atoms has significantly decreased the LUMO
levels below -4.0 eV, which is favoring ambient stability of the anion species. The HOMO energy
levels were less affected by the substitution in indigo core.
4
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6
7
8
9
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12
13
14
15
16
It is known that indigo is a fairly stable pigment, which does not decompose up to 380 oC. [40]
Most of the studied indigo derivatives also demonstrated comparably good thermal stability
o
showing no weight loss below 400 C as revealed by thermal gravimetry (ESI, Figure S12). An
exception is compound 8 containing alkyl solubilizing chains, which substantially decrease the
thermal stability of the material and it starts to decompose at the temperature as low as 200 oC.
11

1
Table 1. Optoelectronic properties of indigo (I) and its derivatives 1–9.
Solution
Film
Film
Eoxons vs.
Fc/Fc+ (V)
0.72
HOMO
(eV)
Eredons vs.
Fc/Fc+ (V)
-1.45
LUMO
(eV)
Compound
λmax
λmax
Eg
(nm)
(nm)
(eV)
1.77
307, 359,
593
-5.82
-3.88
311, 375, 539
1
304, 584
311, 607
334, 417,
609
330, 645
311, 621
327, 410, 560,
635
1.76
1.80
1.73
0.75
0.67
0.69
-5.85
-5.77
-5.79
-0.91
-1.02
-1.15
-4.19
-4.08
-3.95
2
3
4
5
311, 373,
602
1.66
-1.16
-3.94
315, 378, 620
334, 394,
628
n/a*
n/a
n/a
0.59
0.8
n/a
n/a
n/a
6
7
8
340, 606
332, 436,
610
348, 566
1.72
1.71
1.72
-5.69
-5.90
-5.83
-1.16
-1.18
-1.20
-3.94
-3.92
-3.90
330, 415, 628
303, 374,
398, 600
300, 600
322, 380, 400,
533, 664
0.73
9
I
335, 665
1.71
0.67
-5.77
-1.20
-3.90
2
* It was impossible to obtain thin films of compound 6 due to its decomposition during evaporation.
3
4
Electrical properties of materials in field effect transistors. The synthesized indigo derivatives
were investigated as semiconductor materials in OFET devices with the architecture shown in
Figure 4a. It is known that dielectric material can affect the ordering of semiconductor molecules
in thin films [41], whereas using additional passivation coatings can eliminate the surface trap
states, improve charge carrier mobility and reduce hysteresis. [42],[43] Therefore, bare alumina
AlOx and its combinations with cross-linked benzocyclobutene derivative (BCB) or normal-chain
hydrocarbon tetracontane (C40H82, TC) passivation coatings were used as dielectrics to investigate
the electron and hole transport properties of the materials (See ESI for detailed procedure of the
5
6
7
8
9
10
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2
device fabrication). Some of the investigated compounds have exhibited no any semiconductor
behavior in OFETs. Table 2 shows major characteristics of the assembled devices.
3
4
5
6
7
Diiodoindigo 1 demonstrated ambipolar behavior with balanced mobility in both channels
in the case of TC used as the dielectric (Table 2, Figure 4b). Octafluoroindigo 2 and
octachloroindigo 3 did not exhibit semiconductor properties in combination with all the three
selected dielectrics, while only very low electron mobility was detected for octafluoroindigo 2
in combination with plain AlOx.
a)
b)
V
= -7 V
V
= 10 V
DS
1E-7
1E-8
1E-9
DS
I
I
DS
GS
-14 -12 -10 -8
6
8 10 12 14 16
V
, V
GS
I
1E-7
1E-8
c)
d)
GS
V
= -3 V
I
DS
GS
I
1E-8
DS
I
DS
VDS = 4 V
1E-9
1E-10
1E-11
1E-9
1E-10
1E-11
-6
-5
-4
-3
-2
3
4
5
6
7
8
9
10
V
, V
V
, V
GS
GS
8
9
Figure 4. Configuration of OFET devices (a) and transfer characteristics of indigoids 1 (b), 4 (c),
and 8 (d) in these devices.
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2
Table 2. Characteristics of OFETs incorporating semiconductor layers based on the designed
indigo derivatives.
Compound
Type of
Dielectric
Ion/Ioff
μ
charge
VTH (V)*
(cm2 V-1 s-1)**
μe/μh
carriers
n
n
p
n
n
p
p
n
n
n
n
p
BCB
TC
15
7
10
1.1
4.0
4.7
3.1
×
×
×
×
10-4
10-3
10-3
10-5
n/a
~1
1
9
1
×
102
TC
-7
10
10
10
AlOx
TC
10
-3
n/a
2
4
~10-6
~0.001
TC
4
×
103
3.8
×
×
×
×
10-3
TC
-4.5
7.2
2.1
3.8
4.9
-5.0
4
×10
3.8
2.8
9.1
10-4
10-4
10-5
n/a
n/a
n/a
n/a
~4
5
TC
1
1
3
4
2
×
×
×
×
×
102
102
102
103
102
8
9
TC
BCB
TC
(0.8-1.5)
(1.0-1.3)
(1.0-3.0)
×
×
×
10-4
10-3
10-4
Indigo
TC
3
4
* Threshold voltage. ** Charge carrier mobility.
It was expected that the presence of thienyl, phenyl, or naphthyl rings in compounds 4–9
would lead to an effective ordering of the molecules in thin films resulting in improved charge
transport properties. However, the acquired experimental data have not supported this suggestion.
In particular, thin films of compounds 6 and 7 did not demonstrate any semiconductor properties
with all the studied dielectrics. Derivative 5 exhibited low hole mobilities in OFETs (Table 2). On
the contrary, 6,6'-dithienylindigo 4 turned out to be the ambipolar semiconductor on hydrocarbon
dielectrics (TC), but the electron mobilities were lower by 3 orders of magnitude than the hole
mobilities (Table 2, Figure 4c). Alkylditienylindigo 8 and dibenzoindigo 9 demonstrated only a
poor electron mobility in combination with TC dielectric (Figure 4d), while known isomer of
compound 9 showed excellent p-type behavior in OFETs with the mobility higher by few orders
5
6
7
8
9
10
11
12
13
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1
2
of magnitude than that of parent indigo.[28] Such poor OFET characteristics for new compound 9
are caused by its crystal structure (see above).
3
The films of indigoids 1, 3, 4, 5 and 7 grown on TC and BCB dielectric coatings were
investigated using atomic force microscopy (AFM) (ESI, Figure S13). However, the surface
topography of the films failed to explain the observed differences in the electrical performance of
OFETs. For instance, indigoid 1 shows more crystalline films when it is deposited on BCB rather
than on TC. However, the OFETs assembled with 1 and TC dielectric show better performance
presumably due to the molecular templating effect revealed previously [41]. Surface topography
of thin films of compounds 1, 3 and 7 grown on TC dielectric looks very similar. However, no
semiconductor properties were revealed for 3 and 7, whereas 1 showed good performance in
OFETs. The obtained results match our previous findings [27] suggesting that supramolecular
ordering of the molecules in the films, i.e. crystal packing, is the most important factor defining
the semiconductor performance of indigo derivatives in OFETs.
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
It should be also noted that the chemical functionalization of indigo allows one to tune
electronic properties of indigo derivatives (frontier orbitals energies are given in Figure S14, ESI)
and their electrical performance in OFETs. The ambipolar behavior with balanced electron and
hole mobilities was clearly observed in the case of 6,6'-diiodoindigo (Figure 5). Increase in
electronegativity of halogen atoms at the 6 and 6' positions as well as introduction of fluorine
atoms at the 5 and 5' positions (tetrafluoroindigo) or implementation of strong electron acceptor
substituents (CF3) cause a decrease in the hole mobility down to its complete disappearance. [27]
In contrast, the electron mobility only slightly depends on the chemical nature in the case of
derivatives bearing two halogen substituents, but it decreases upon increasing the number of
electron-withdrawing substituents (F atoms). In the case of four fluorine atoms (i.e.,
tetrafluoroindigo), the electron mobility fell just to one half of that of 6,6'-difluoroindigo, but for
the eight fluorine atoms (octafluoroindigo 2) the mobility was lower by few orders of magnitude.
The obtained results are clearly indicating the opportunities for using chemical design to tune
properties of indigo-based organic semiconductor materials.
15

µh
µe
0,01
1E-3
1E-4
1E-5
1
2
1
2
Figure 5. Effect of substituents in indigo core on electron and hole mobilities in OFETs
3
CONCLUSIONS
4
5
The set of novel indigo derivatives was designed following two major strategies based on: (1)
considering the effects of halogen substituents used to tailor optoelectronic properties of the
materials and their performance in OFET devices and (2) increasing the molecular π-conjugated
system with additional aromatic rings. The first approach has revealed a well-balanced ambipolar
behavior for 6,6'-diiodoindigo, which can be considered as a highly promising organic
semiconductor. On the contrary, the introduction of multiple halogen atoms with stronger
electron-withdrawing properties suppresses hole transport and, unfortunately, deteriorates also
electron mobility lie observed in case of octafluoroindigo and octachloroindigo. The second
approach provided a more diverse series of the indigo derivatives. The introduction of thiophene
and phenyl substituents at positions 6 and 6' of indigo led to the predominance of hole transport
due to the donor properties of these groups. Another compound, z-shaped dibenzoindigo
demonstrated a poor performance in OFET devices as compared to both parent indigo and linear
dibenzoindigo due to the strong displacement of the molecules with respect to each other and a
reduced intermolecular overlap of the boundary orbitals. These revealed structure-properties
6
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9
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12
13
14
15
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2
3
relationships will be beneficial for the future design of novel indigo-based semiconductors with
improved electronic properties for sustainable and biocompatible organic electronics.
4
ACKNOWLEDGEMENTS
5
6
7
8
This work was supported by the Ministry of Science and Higher Education of the Russian
Federation (Grant No. 075-15-2020-779). We acknowledge support of Mr. A. Maskaev with
preparation of the final version of this article.
17

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HIGHLIGHTS
1. Nine novel indigo derivatives are designed, synthesized and chracterized.
2. Single crystal structures are obtained for three indigo derivatives
3. A well-balanced ambipolar behavior in OFETs is found for 6,6’-diiodoindigo.
4. Semiconductor performance of halogenated indigoids correlates with their structures

Declaration of interests
☒ The authors declare that they have no known competing financial interests or
personal relationships that could have appeared to influence the work reported in
this paper.
☐The authors declare the following financial interests/personal relationships
which may be considered as potential competing interests: