New Indolo[3,2-b]indole based small organic molecules for Organic Thin Film Transistors (OTFTs): A combined experimental and DFT Study

Article abstract of DOI:10.1016/j.molstruc.2020.129491

Synthesis of new indolo[3,2-b]indoles (5a- 5j) in presence of Ag-doped ZnO and (Diacetoxyiodo) benzene system under visible-light have been reported. All the new fused linear heterocyclic indolo[3,2-b]indole systems (5a- 5j) thoroughly characterized by spectroscopic methods like mass, UV-visible, NMR and C, H, N elemental analysis. Further, their photophysical properties were carried out by combined experimental and theoretical studies. Thermogravimetric studies are carried out to confirm the thermal stability of molecules. The frontier molecular orbitals of molecules are characterized with the help of cyclic voltammetry. Additionally, the compounds of series 5 were used for the fabrication of organic thin-film transistors, which indicated the hole mobilities in the range of 0.11 – 0.85 cm²/Vs and with on/off ratio 10⁵ on ODTS-SiO₂ substrate at 50 °C and are also supported by DFT studies.

Full text of DOI:10.1016/j.molstruc.2020.129491

Journal Pre-proof
New Indolo[3,2-b]indole based small organic molecules for Organic
Thin Film Transistors (OTFTs): A Combined Experimental and
DFTStudy
Venkat Swamy Puli , Mahesh Subburu , Bhongiri Yadagiri ,
Anuj Tripathi , K.R.S. Prasad , Anindita Chatterjee ,
Someshwar Pola , Prabhakar Chetti
PII:
S0022-2860(20)31806-8
DOI:
Reference:
https://doi.org/10.1016/j.molstruc.2020.129491
MOLSTR 129491
To appear in:
Journal of Molecular Structure
Received date:
Revised date:
Accepted date:
2 September 2020
17 October 2020
18 October 2020
Please cite this article as: Venkat Swamy Puli , Mahesh Subburu , Bhongiri Yadagiri ,
Anuj Tripathi , K.R.S. Prasad , Anindita Chatterjee , Someshwar Pola , Prabhakar Chetti ,
New Indolo[3,2-b]indole based small organic molecules for Organic Thin Film Transistors
OTFTs): A Combined Experimental and DFTStudy, Journal of Molecular Structure (2020), doi:
(
https://doi.org/10.1016/j.molstruc.2020.129491
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highlights
•
Synthesis of new indolo[3,2-b]indoles via photooxidation and condensation have been
reported.
•
•
Photophysical properties of new indolo[3,2-b]indoles are explored and correlated
experimental results with DFT.
Indolo[3,2-b]indoles exhibited good hole mobility in organic thin-film transistor studies.

New Indolo[3,2-b]indole based small organic molecules for
Organic Thin Film Transistors (OTFTs): A Combined
Experimental and DFTStudy
b
a
a
c
b
Venkat Swamy Puli, Mahesh Subburu, Bhongiri Yadagiri, Anuj Tripathi, K. R. S. Prasad,
b
a
c
Anindita Chatterjee, * Someshwar Pola, * and Prabhakar Chetti *
a
Department of Chemistry, Osmania University, Hyderabad, India.
b
Departmentof Chemistry, Koneru Lakshmaiah Education Foundation, Vaddeswaram, Guntur,
Andhra Pradesh, India.
c
Department of Chemistry, National Institute of Technology, Kurukshetra, India.
Abstract:
Synthesis of new indolo[3,2-b]indoles (5a- 5j) in presence of Ag-doped ZnO and
Diacetoxyiodo) benzene system under visible-light have been reported. All the new fused
(
linear heterocyclic indolo[3,2-b]indole system (5a- 5j) thoroughly characterized by
spectroscopic methods like mass, UV-visible, NMR and C, H, N elemental analysis. Further,
their photophysical properties were carried out by combined experimental and theoretical
studies. Thermogravimetric studies are carried out to confirm the thermal stability of
molecules. The frontier molecular orbitals of molecules are characterized with the help of
cyclic voltammetry. Additionally, the compounds of series 5 were used for the fabrication of
organic thin-film transistors, which indicated the hole mobilities in the range of 0.11 – 0.85
2
5
o
cm /Vs and with on/off ratio 10 on ODTS-SiO
DFT studies.
2
substrate at 50 C and are also supported by
TO WHOM CORRESPONDENCE SHOULD BE ADDRESSED:
*
Email:chetty_prabhakar@yahoo.com (P. Chetti); somesh.pola@gmail.com (S. Pola)

Introduction:
Recent advancement in the field of material chemistry shows that organic materials are the
best alternative for inorganic ones. The major advantages of organic materials as
semiconducting materials are low cost, light weight and they possess possibility of producing
flexible and large area devices [1, 2]. These organic molecules find applications in different
optoelectronic devices majorly in organic field-effect transistors (OFETs), organic
photovoltaics (OPVs) and organic light-emitting diodes (OLEDs) [2-5]. Within organic
molecules, π-conjugated organic framework finds application in organic semiconductors,
some of them are well-reported in literature such as heteroacenes [6-7], polythiophenes [8- 9]
oligoacenes [10-11], hexabenzocoronenes [12] and phthalocyanines [13-14]. Among different
organic molecules mentioned above, acenes and heteroacenes are well explored by many
researchers as they have high mobility, which is the key characteristic of organic
2
-1 -1
semiconductor device. Within acenes, pentacene own high hole mobility of 5.0 cm V s and
is one of the most studied p-type materials [10,15]. However, these acenes i.e. pentacene was
suffering from several shortcomings like air instability and low solubility under ambient
conditions [16]. In order to overcome these shortcomings several strategies have been applied
like substrate variation, functionalization of the molecular structure and introduction of
heteroatom [17-18]. The structure of a molecule plays a key role in the molecular electronic
approaches, since the molecular assemblies are in the solid-state, especially in the thin-film
state, which directs intermolecular orbital overlap and advances the carrier/charge transport
applications. Therefore, researchers are continuously focused on design and synthesis of new
organic frameworks with better charge transport properties which find applications in
optoelectronic devices. In context to this, here in this work it is focused on indolo[3,2-
b]indole (InIn) as a promising π-core for a new type of organic semiconductors [19]. The
rigid π-framework and planarity of InIn can help strong intermolecular interactions that
would be appropriate for developing new organic semiconductor materials. The significant
structural characteristics in InIn molecules is associated with a core structure of pyrrolo[3,2-
b]pyrrole, which is fully π-extended molecular system and it offers high stability to the
molecular system by lowering the HOMO energy levels [19]. These noticeable properties of
pyrrolo[3,2-b]pyrrole have motivated organic chemists to develop new organic
semiconductors with the pyrrolo[3,2-b]pyrrole structure in prolonged π-systems with more
than one fused aromatic rings due to its π-extended structures [20-21].

The prominent π-structures of pyrrolo[3,2-b]pyrrole attracted us as a central core system for
the synthesis of new organic molecules having applications in optoelectronic devices. For
this, we concentrated on two indole systems fused together to give InIn and its derivatives
with various small functional groups (molecule 5a-5j in Figure1). The very first report on the
synthesis of simplest system of InIn (5a in figure 1) was given by W. Treibs in 1961, but not
fully characterized the synthesised molecule and its stability is also not clear [22]. Later,
Hunig et al. in 1976 characterized InIn by Uv-vis and IR spectra [23]. Further, the stability of
InIn was reported by Li Qui et. al. and is very stable in nature [19]. Recently Zeng et al
reported about the stability of InIn and their derivatives, and they showed that the linear
fusion of aromatic rings to the antiaromatic core increases the stability of heteroacenes [24].
By considering all the above facts, here in this paper, we report the synthesis of a series of
InIn derivatives through insitu photooxidation-condensation reactions. The characterization
of the synthesized InIn derivatives are done by means of C, H, N elemental analysis, mass,
other spectral analysis, and their organic thin-film transistor (OTFT) features. The obtained
results also supported with DFT studies.
Experimental Section:
Starting materials are purchased from Sigma-Aldrich such as various substituted
nitrobenzenes, 2-iodo aniline, AgNO
3
, Zn(NO
3
2
) and other solvents and are used without
further purification. 1,2-disubstituted-4-nitrobenzenes are converted into 3,4-disubstituted
anilines using the reported procedure [25] and these anilines were reacted with N-
iodosuccinimide to give 2-iodo-4,5-disubstituted anilines [26]. The detailed procedure for the
synthesis of new photocatalysts and fused linear type heterocyclic compounds are presented
in supporting information. The method for fabrication of the device for OTFTs and specific
techniques are also shown in supporting information. Preparations of Ag-doped ZnO NPs are
carried out by using the reported method [27].

Fig.1. Structures of synthesized InIn and its derivatives (5a-5j)
General synthesis procedure:
The synthesis of new fused linear heteroacenes via photooxidation and condensation
reactions in one-pot operation conditions is reported. Furthermore, the efficiency and
sustainability of catalyst play a significant role in the catalytic properties [28]. Scheme 1
outlines the synthesis of indolo[3,2-b]indole and its derivatives from internal alkynes. The
strategy is to synthesize various substituted 2-amino aryl iodides [29] 1a-j and subsequent
Sonagashira coupling reaction [30], followed by deprotection of the trimethylsilyl group for
the formation of 2a-j. The deprotection of 2a-j yields terminal alkynes 3a-j, which can
further react with 1a-j under Sonagashira coupling to obtain the corresponding internal
alkynes 4a-j. Further, the reaction of 4a-j in the presence of Ag-doped ZnO and PhI(OAc)
2
system under visible light irradiation was performed to get the desired product indolo[3,2-
b]indoles 5a-j (table 1). Detailed synthetic procedure of each step and spectral data is
provided in supporting information).

Si
R1
R1
I
R1
R2
a
b
c
R2
NH2
R2
NH2
NH2
1a-j
2a-j
R1
R1
NH2
R2
R1
R1
R2
N
d
e
R2
R2
NH2
N
3
a-j
NH2
R1
R2
4
a-j
5a-j
a) NIS, acetic acid, rt, 5h; b) ethynyltrimethylsilane, Pd(PPh ) Cl . CuI, triethylamine, toluene, reflux;
3
2
2
c) K CO , methanol, rt; d) compounds (1a-j), Pd(PPh ) Cl , PPh , CuI, triethylamine, toluene, reflux;
2
3
3 2
2
3
e) Ag-doped ZnO NPs, PhI(OAc) , H O and CH CN (1:1), h.
2
2
3
Scheme 1: Synthesis of indolo[3,2-b]indole and its derivatives
Table 1:Synthesis of new indolo[3,2-b]indole derivatives with different functional groups
and their %yield
Substituents
=R = -H
=R = -CH
=R = -Cl
=R = -F
=-OCH ; R
= -OCH ; R
=R = -CN
= -CN; R = -H
= -CH ; R = -Cl
=R = -CF
Product
5a
%Yield
86
R
1
2
R
1
2
3
5b
71
R
1
2
5c
66
R
1
2
5d
59
R
1
3
2
= -F
5e
68
R
1
3
2
=-Cl
5f
65
R
1
2
5g
58
R
1
2
5h
52
R
1
3
2
5i
69
R
1
2
3
5j
54
Organic field effect transistors (OFETs) characteristics:
Bulky-Area Crystalline Films Placed on SiO for compounds 5 series. The silicon wafers,
2
with 200 nm thermally grown oxide layer, were cleaned by Piranha solution and rinsed
thoroughly with ultra-high purity water. The cleaned silicon wafers were soaked into 1% n-

octadecyltrichlorosilane (ODTS) solution in dry toluene for 3 minutes to generate the ODTS-
modified silicon oxide surface (ODTS-SiO /Si). Thin films of compound 5 series were
thermally evaporated under a vacuum of 1×10 torr and deposited on SiO
SiO /Si coatings at different substrate temperatures (Figure 2) [31]. FET devices were
fabricated with the 200 nm-thick SiO surfaces as the gate dielectric and the highly n-doped
2
-5
2
/Si or ODTS-
2
2
silicon wafer as the gate electrode. The 50 nm-thick gold source and drain electrodes were
deposited on the organic films (top contact) through a shadow mask. The channel length (L)
and width (W) are 50 μm and 500 μm respectively.
Fig.2: Schematic device structure
Characterization of Indolo[3,2-b]indole derivatives and device fabrication
All the photoreactions were performed by using a multi-tube photo-reactor system with LED
visible light, Lelesil Innovative Systems, India. Mass spectral data collected by HR-EI
1
13
systems on JMS-700 double-focusing mass spectrometer (JEOL, Tokyo, Japan). H and C-
NMR spectra recorded on a Bruker AV400 MHz Spectrometer with chemical shifts
1
referenced using the H resonance of residual CDCl
3
and d -DMSO. C, H, N, F and Cl
6
elemental analysis have been carried out by using Elemental Analyzer Flash EA 1112.
Melting points verified on a Cintex apparatus. Cyclic voltammetry measurements were
conducted on a PC-controlled CHI 62C electrochemical analyser in a C
H
2 4
Cl solvent at a
2
-1
scan rate of 100mVs using tetrabutylammonium perchlorate (0.1 M) as the supporting
electrolyte. The glassy carbon, standard calomel electrode (SCE) and platinum wire were
used as working, reference and counter electrodes, respectively. The electronic spectra
obtained in chloroform solutions on a JASCO V-650 UV-Vis spectrophotometer. The AFM
imaging was achieved on KeySight 5500 instrument (Agilent Technologies) underneath
tapping mode with silicon nitride tip. Electrical characteristics were recorded under dark in
air using a semiconductor device analyzer (Agilent-B-1500A). The mobility and threshold

voltage were taken out from transfer characteristics in the saturated regime by using the
following equation:
푊
ꢂ
퐼푑 =
휇푠푎푡퐶ꢀ(푉 − 푉 )
ꢁ 푡ℎ
2퐿
Where Wand L are the channel width and length respectively, and C
i
is the capacitance per
unit area of the SiO insulator, V is the gate voltage, and Vth is the threshold voltage.
2
g
Results and Discussions:
Absorption and emission properties:
The InIn derivatives exhibits lemon yellow to bronze colour in their solid states (colour of
the solid compounds shown in figure S1). Initially, the UV-vis absorption spectra of the
compound 5a recorded in dilute dichloroethane (DCE) solution as a function of time, are
revealed in Figure S2. After 24 hours, no transformation in the electronic spectra was
observed. The absorption spectra of all InIn derivatives as verified in dilute DCE solution are
shown in Figure S3a-S3d and associated data are summarized in Table 2. Fused hetero-
aromatic molecules are recognized to have robust, visible absorption and strong emission
with competent quantum yields [32-33]. The absorption maxima for all the molecules are in
the range of 433 nm to 477 nm with high molar extinction coefficients in DCE (Table 2). The
red-shift tendency of the absorption maximum was noticed when the compound substituted
with electron-donating group (5b, 5e, 5f and 5i)as compared with the parental compound 5a
(Figure S3a). With the substitution of electron-withdrawing group on InIn (5c, 5d, 5g, 5h
and 5j), results in blue-shifted absorption maxima (Figure S3b).
The absorption maxima of compounds 5a-5j in the solid phase were significantly shifted to a
higher wavelength region compared to its solution state (Table 2). The absorption maximum
of 5b is shifted from 472 nm in DCE to 582 nm in the solid phase, ensuing in a wavelength
shift of 110 nm. The excitation maximum of 5e is also shifted from 473 nm in DCE to 591
nm in the solid phase, ensuing in a 118 nm shift towards higher wavelength side. Compound
5
b and 5e show the most substantial red-shift in the solid phase, while compounds 5i and 5f
show the smallest wavelength shift in the solid-state (Table 2). Based on these outcomes, it is
presumed that the shift towards higher wavelength region in the excitation maxima in its solid
phase for compounds 5a-5j is because of the compressed molecular arrangement in the solid
phase and non-covalent interactions form its π-π assembly [34]. These π-π interactions

generate a distended system of conjugation in the solid phase, ensuing in the high wavelength
shifts in the solid phase as related to the solution outcomes.
Table 2: Experimental UV-visabsorption (λmax) and molar extinction coefficient (ε) for the
compounds 5a-5j in DCE and absorption in solid-state (λmax-SS
)
ε
Material

max (nm)

max-ss (nm)
−
1
−1
L mol cm
58125
59000
55750
55125
59125
58750
54250
54125
59625
54500
5
a
465
472
446
441
473
470
434
433
477
436
523
582
508
523
591
519
506
510
504
517
5
b
5
c
5
d
5
5
5
e
f
g
5
h
5
5
i
j
To advance understanding into the experimental electronic excitations of the molecules, DFT
and TD- DFT calculations have been carried out for 5a-5j. The ground state molecular
geometries of 5a-5j are fully optimized without any symmetric considerations in gas phase
by using Gaussian 09 software [35] with DFT method at B3LYP level in combination with
the basis set 6-31G (d, p). There is no imaginary frequency in the calculated vibrational
spectrum of all the optimized structures, which indicates that all the optimized structures are
located at the least point of the potential surface. TD-B3LYP method with 6-31G (d, p) basis
set is used to get electronic absorption properties for the optimized geometries, and frontier
molecular orbitals were pictorially visualized from the population analysis at same level of
theory.
Computed excitations are in good agreement with the experimentally determined excitation
energies. The calculated absorption maxima for all the molecules are in the range of 448 nm–
4
98 nm. The calculated lowest three excitation energies, oscillator strengths, significant
transitions, and percentage configuration interactions (% Ci) for molecules 5a-5j are shown
in Table S1. In all the molecules, the lowest excitation energy is with low intensity, the
second and third excitation energies are with high intensities. For all the molecules (except
molecule 5c and 5e), low-intensity absorption is due to the transition among HOMO-2 to
LUMO. In the case of molecules 5c and 5e, the first excitation with low intensity is from
HOMO-3 to LUMO. The corresponding frontier molecular orbitals for molecule 5a, are

shown in Figure 3. The second excitation in all the molecules (except 5e) is due to the
transition between HOMO-1 to LUMO.
Fig. 3: Molecular orbital pictures for molecule 5a
The fluorescence spectra of molecules 5a-5j are measured against tetracene as a typical
reference. Table 3 displays the emission maxima and the quantum yields of all the fused
hetero-aromatic molecules in DCE and the emission maxima in the solid-state. In solution
state, the compounds 5b, 5e, and 5i show emission intensity approximately twice than that of
5
a (Figure S4). However, the quantum yield of all the compounds is in the range of 0.21 to
.51 (Table 3), comparable to that of compound 5a.
0
For all the molecules 5a-5j, the emission maxima were moved to a higher wavelength region
in its solid phase. In the case of compound 5h, exhibited the higher change in emission
maxima from 479 nm in DCE to 608 nm in the solid phase, ensuing in a wavelength
difference is 129 nm (Table 3). The maximum change in wavelength was observed in
compounds 5g and 5h, whereas a marginal shift of wavelength noticed in compounds 5d and
5
i. The more compressed molecular arrangements in the solid phase can outcome in π−π
assemblies and endorse a change of emission maxima [34, 36].

Table 3 Emission λmax and quantum yield for compounds 5a-5j in DCE and in the solid state
Solid-state excitation
Material

max (nm)
Φ
F

max (nm)
655
5
a
536
544
489
485
539
536
489
479
552
480
0.37
0.46
0.49
0.35
0.42
0.44
0.51
0.29
0.55
0.21
5
b
658
607
587
652
654
612
608
655
5
c
5
d
5
e
5
f
5
g
5
h
5
i
5
j
603
HOMO-LUMO Energies and Thermal Properties:
The HOMO and LUMO energy levels are measured by cyclic voltammetry (CV) in DCE
Cyclic voltammograms are shown in Figure 4). The oxidation potentials along with the
(
HOMO and LUMO energy levels obtained from CV of compounds 5a to 5j, presented in
Table 4. The ranges of oxidation potentials for all the reported compounds are from 1.24 to
1
.46 V. The measured HOMO levels of all the molecules are in the range of -5.85 to -5.62 eV;
similarly, the LUMO energies are in the range of -3.09 to -2.81 eV. Further, HOMO-LUMO
energies are also calculated by DFT methodology at the B3LYP method. The calculated
HOMO and LUMO energies are also tabulated in Table 4. All the calculated HOMO, LUMO
energy levels are comparable with the experimentally determined energies (following the
trend). All these HOMO levels are lesser than that of tetracene (-4.85eV) [37]. It may have to
do with the electronegative groups present in the molecular framework, even though indole
rings are fused. The lower HOMO levels suggest that all these compounds are more stable
against oxidation. The optical band gaps of these compounds (5a-5j) are obtained from the
absorption edges of their UV-visible spectra. LUMO energies are evaluated through two
methods, one is subtracting the optical band gap values from HOMO values and another
method is reduction potentials of CV. From both the techniques LUMO energies are almost
similar. With the substituent electron donating / withdrawing groups are responsible for the
-
small variation in observed in LUMO values [38].Compounds 5a-5j showing 1e irreversible
oxidation peaks, which corresponds to the electron-rich indole moiety. These are also
showing multiple oxidation peaks that could originate from indole moiety and the

corresponding voltammograms are shown in Figure 4 [38]. HOMO energy levels of
compound 5a–5j are calculated from the onsets of the oxidation peaks observed in the CV
analysis (Table 4).
Table 4: Measured optical band gap, oxidation potentials, HOMO-LUMO energies and DFT
calculated HOMO-LUMO gap along with HOMO-LUMO energies
opt
opt
E
(
oxd
V)a
E
HOMO
E
LUMO
E
g
E
HOMO
E
LUMO
E
g
Compound
(eV)a
-5.82
-5.79
-5.76
-5.85
-5.62
-5.63
-5.68
-5.80
-5.67
-5.75
(eV)a
-2.89
-2.96
-3.01
-3.09
-3.01
-3.08
-3.01
-3.02
-2.88
-2.81
(eV)a (eV)b
(eV)b
-3.10
-2.82
-3.78
-3.56
-2.99
-3.17
-4.85
-4.06
-3.33
-4.24
(eV)b
2.92
2.78
2.71
2.69
2.59
2.50
2.72
2.84
2.75
2.96
5
a
1.43
1.38
1.37
1.46
1.31
1.24
1.29
1.41
1.28
1.33
2.93
2.83
2.75
2.76
2.61
2.55
2.67
2.78
2.79
2.94
-6.02
-5.60
-6.49
-6.25
-5.58
-5.67
-7.57
-6.90
-6.08
-7.20
5
b
5
c
5
d
5
e
5
f
5
g
5
h
5
i
5
j
a
b
Determined from CV and DFT-Calculated energies
Fig. 4: Cyclic voltammograms for compounds 5a – 5j
All the reported compounds are analyzed for their thermal properties. Measured melting
points and decomposition temperatures are presented in Table 5. Thermo-gravimetric plots of
compounds 5a-5j are shown in Figure S5. All the compounds exhibit high-thermal stability,

o
decomposing at >300 C and the compound 5d with four fluorine atoms retains highest T .
d
Such high thermal stability of these molecules gives a clear indication that these were less
prone to thermal degradation. This particular feature has significance in the utility of these
materials in device properties.
o
o
Table 5 Melting point (M.P in C) and decomposition temperature (T
d
in C) of compound 5
series
Mol
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
M.P
256-
232- 324- 297-
234 326 301
286-
288
291-
293
331-
333
276- 288- 341-
258
278
366
290
378
343
402
T
d
331
318 392 383
376
389
381
Thin film deposition:
All the compounds of 5 series were fabricated for the thin film transistor devices and mobility
data shown in Table 6. The AFM image of 50 nm dense film of compound 5a and 5b are
shown in Figure 5a-d. In the case of the AFM image of 50 nm dense films, the compound 5a
and 5b are placed on ODTS-SiO
substrate temperatures were utilized throughout the process of deposition. Grains size of
molecules on ODTS-modified SiO surface was bigger than placed on un-modifiedr SiO
2
, and SiO surfaces at various temperatures. The same
2
2
2
o
o
surface at a substrate temperature of 25 C and 50 C. Films of compound 5a and 5b placed at
o
a substrate temperature 50 C provides the smaller grain size of SiO
2
surface but on ODTS
modified SiO
the powder X-ray diffraction of thin-films of compound 5a placed on SiO
surfaces at various temperatures. Diffraction pattern has the peaks such as 2θ at 5.9854o,
2
surface largest grain size and fine terrace structure [39]. Figure S6a exhibits
2
and ODTS-SiO
2
o,
o
1
2.042 and 16.337 are obtained at all substrates temperature and are assigned as the
diffractions from (001), (002), (003) planes correspondingly. This designates a similar crystal
phase at various conditions of deposition. The d-spacing value obtained from (001) peak is
1
51.9 A, equated to the determined length of 149.6 A, signifying molecules were vertical
near standing on all surfaces [40]. This alignment will enable charge-transporting facility in a
flat device arrangement subsequently the path of π-π assembling of the brick-wall or co-facial
prearrangement equals the conduction pathway. In the diffractograms of compound 5a the
intensity of the diffracted peaks on different surfaces and substrate temperatures
corresponded with mobility. Peak intensities are higher for the films grown on SiO
ODTS modified surfaces at all substrate temperatures. The thin-film is produced on
theSiO /Si surface with the substrate temperature at 50 °C peak intensity higher than room
2
, and
2

temperature and at 80°C. Figure S6b shows films of compound 5b placed on SiO
2
and
ODTS-SiO at various temperatures. Another time, a single crystalline phase showing
2
o
o
o
diffraction peaks at 2θ of6.488 (001), 12.68 (002), 17.14 (003) correspondingly were
attained at non-identical substrate temperature. The d-spacing value obtained from (001) is
1
45.1 A which also specifies a vertical alignment of compound 5b on the surfaces.
o
Fig.5: AMF images of ODTS on SiO
2
substrate a) compound 5a at 25 C, b) compound 5a at
o
o
o
5
0 C, c) compound 5b at 25 C, and d) compound 5d at 50 C
Characteristics of FET devices [2] prepared from compound 5a, and compound 5b are shown
in Table 6 with the corresponding curves (Figure 6a-d). For compound 5a, the top device
performance was attained for the film placed on ODTS modified surface at a substrate
o
2
temperature of 50 C, the highest mobility 0.49 cm /Vs is on surface-modified (ODTS-SiO )
2
o
at 50 C. The mobility reduced with smaller substrate temperatures. This tendency also
matches the result from AFM analysis, the bigger the grain size, and then the mobility is
2
higher. The film placed on ODTS-SiO
2
surface has the maximum mobility is 0.12 cm /Vs
o
which was attained at 25 C has deposition conditions.
Additionally, enhancing the substrate temperature leads to decrease in intensity of diffraction
peaks and lower device performance. For compound 5b, the foremost execution was captured
o
2
on ODTS-SiO
2
surface at 25 C deposition conditions. The mobility of 0.28 cm /Vs and an
4
on/off ratio up to 10 were reached. For compound 5b, the foremost device execution was
attained for the film placed on ODTS-SiO modified surface at the substrate temperature of
2

o
2
5
0 C [31] and obtained the highest mobility 0.85 cm /Vs. Advanced deposition temperatures
significantly dropped in the mobility of all surfaces. All the molecules show transistor
property and the mobility data presented in Table 6.
Fig. 6: I-V curves of OFET various temperatures on ODTS-SiO substrates a) compound 5a
2
o
o
on ODTS-SiO
2
at 25 C; b) compound 5a on ODTS-SiO at 50 C, c) compound 5b on
2
o
o
ODTS-SiO at 25 C; and d) compound 5b on ODTS-SiO at 50 C
2
2
Table 6: Electric characteristics of compounds 5a–5j OFETs on ODTS-SiO substrate at
2
various temperatures
2
Compound
Mobility (cm /Vs)
I /I current ratio
on off
0
0
0
0
at 25 C (50 C)
at 25 C (50 C)
4
5
5
a
0.12 (0.49)
10 (10 )
4
5
5
b
0.28 (0.85)
0.21 (0.53)
0.14 (0.35)
0.16 (0.30)
0.11 (0.19)
0.13 (0.21)
0.15 (0.24)
0.08 (0.18)
0.06 (0.11)
10 (10 )
4
4
5
c
10 (10 )
4
4
5
d
10 (10 )
4
4
5
e
10 (10 )
4
4
5
f
10 (10 )
4
4
5
g
10 (10 )
4
4
5
h
10 (10 )
4
4
5
i
10 (10 )
4
4
5
j
10 (10 )
Reorganization energies:
Further, to get insight into the mobility in these molecules, we have also calculated
reorganization energies by using DFT methods. It is well known that mobility and

reorganization energy are inversely proportional to each other i.e. smaller reorganization
energy lead to higher mobility of molecule.
According to Marcus Hopping model, the rate constant for charge transfer reaction is given
by [41-42]
4
휋ꢂ
ꢄ
ꢅ 퐻 (4휋휆푘푇)ꢆ1⁄ꢂ푒푥푝ꢇ
−휆
ꢂ
퐾 = ꢃ
⁄
ꢈ … … … .ꢉ
4
푘푇
Here, K and k are rate constant and Boltzmann constant respectively. T, λ and ΔH are
b
absolute temperature, reorganization energy and charge transfer integral matrix element
between neighbouring molecule respectively [43]. It is clear from the above equation that rate
constant (K) is inversely proportional to reorganization energy of the molecule. Further,
reorganization energies are characterized into external and internal 휆 which are mainly
imparted from surrounding medium and intramolecular vibrations respectively. In
comparison to internal reorganization energies external reorganization energies are very little
and hence can be neglected [44]. Here we calculated internal reorganization energy for hole
(λ
h
) and electron (λ ) using equations 2a and 2b respectively [45-46].
e
+
푛
+
푛
+
푛
휆 = (퐸 − 퐸 ) ꢊ (퐸 − 퐸 ) … … … 2ꢋ
ℎ
ꢆ
푛
ꢆ
푛
ꢆ
푛
휆 = (퐸 − 퐸 ) ꢊ (퐸 − 퐸 ) … … … 2푏
ꢌ
ꢍ
−
ꢊ
In the above-mentioned equation, 퐸 , 퐸 and 퐸 represents the energies of neutral, anion and
−
ꢍ
ꢊ
ꢍ
cation molecules in its lowest energy geometries, respectively. 퐸 (퐸 ) represents energies of
ꢍ
−
ꢍ
anion (cation) molecule obtained at optimized ground state of the neutral molecule.퐸 (퐸 )
ꢊ
represents the energy of neutral molecule obtained at optimized anionic (cationic) state.
The calculated hole (λ
molecules like 5a, 5b, 5c, 5g, 5h, 5i, and 5j are showing comparable or smaller λ
the standard hole-transporting material N,N'-diphenyl-N,N'-bis(3-methylphenyl)-(1,1'-
biphenyl)-4,4'-diamine (TPD, λ =290 meV) [47]. Further, the calculated λ values are above
00 meV which is not good for electron transport material. For all the molecules λ <λ ,
h
) and electron (λ
e
) reorganization energies are given in Table 7. The
h
value than
h
e
4
h
e
means studied molecules are better for hole transporting material which is also confirmed by
experimental results. In conclusion these the molecules can be used for hole transporting
material rather than electron transporting material.

Table 7: Calculated hole (λ
h
) and electron (λ
5b 5c 5d
e
) reorganization energy (in meV)
5e 5f 5g 5h 5i
Compound 5a
5j
λ
h
→
282 269 287 361 391 348 206 230 288 398
474 471 491 545 781 748 409 473 492 592
λ
e
→
Conclusions:
The reaction proceeds in the presence of Ag-doped ZnO with PhI(OAc)
2
conditions to
generate polycyclic fused linear heterocycles by using a broad range of substrates under
visible light irradiation. In the process of photooxidation and condensation of various 2,2'-
(ethyne-1,2-diyl)dianilines converted to indolo[3,2-b]indoles. Indolo[3,2-b]indole and its
derivatives have exhibited good optical properties and the shifting of absorption bands
depends upon dipole groups, and also shown good correlation between experimental and
DFT calculations. All the compounds are showing high-thermal stability, decomposing at
o
>
300 C, which gives a clear indication that these were less prone to thermal degradation and
this particular feature is significant in device properties. The compounds 5a-5j used for
organic thin-film transistors studies and preliminary results shows the hole mobility in the
2
4
5
range of 0.11- 0.85 cm /Vs and with on/off ratio 10 -10 respectively. The calculated hole
reorganization energies are smaller than electron reorgamization energies and these are good
for hole transporting material.
Credit author statement
Methodology, Investigation , Formal analysis and Writing – original draft by Venkatswamy Puli,
Mahesh Subburu, Anuj Tripathi, Yadagiri Bhongiri, K. R. S. Prasad, and Anindita Chatterjee.
Conceptualization, Software, Validation, Formal analysis, Resources, Writing – review and editing,
Supervision and Project administration by Dr. Someswar Pola and Dr. Prabhakar Chetti.
Prabhakar Chetti
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.
Prabhakar Chetti

Supporting Information
UV-visble and emission spectra and thermograms of compounds 5a to 5j are presented if
1
13
supporting information part. P-XRD images of thin films, spectral data such as H-NMR, C-
NMR, Mass spectra and CHN elemental analysis are also supported in supporting
information.
Acknowledgement
SP and CP thanks to SERB - New Delhi, India under EMR (EMR/2014/000452) and
CSIR- New Delhi (02(0339)/18/EMR-II) respectively for financial support. Special
appreciations to DST-FIST, New Delhi. We also thankful to Prof. Yu-Tai Tao,
Institute of Chemistry, Academia Sinica, Taipei, Taiwan and Dr. V. J. Rao, CSIR-
IICT, Hyderabad, India.
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