Synthesis, characterization and organic field-effect transistors applications of novel tetrathienoacene derivatives

Article abstract of DOI:10.1016/j.dyepig.2020.108911

Two novel organic semiconductors with tetrathienoacene (TTA) as the central core end-capped with 5-hexyl-2-thiophene, (Hex-T)₂-TTA, and 4-hexyl-phenyl, (Hex-Ph)₂-TTA, have been synthesized and investigated for organic field effect transistor (OFET) applications. The novel TTA derivatives were characterized by thermal gravimetric analysis, differential scanning calorimetry, UV–Vis spectroscopy, and cyclic voltammetry as well as studied by density functional theory calculations. Two types of OFETs with the solution and vacuum-deposited active layer were fabricated and characterized. Both TTA-derivatives demonstrated electroluminescence in OFETs, and (Hex-Ph)₂-TTA showed ambipolar charge transport with the hole mobility as high as 0.68 cm² V^?1s^?1.

Full text of DOI:10.1016/j.dyepig.2020.108911

Dyes and Pigments 185 (2021) 108911
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Synthesis, characterization and organic field-effect transistors applications
of novel tetrathienoacene derivatives
a
a
a
,
b
a,b
Oleg V. Borshchev , Maxim S. Skorotetcky , Vasily A. Trukhanov , Roman S. Fedorenko
,
a
a
a
,
b
c,d
Nikolay M. Surin , Evgeniya A. Svidchenko , Andrey Yu. Sosorev , Maxim S. Kazantsev
Dmitry Yu. Paraschuk
,
a,
b,
**
a,*
, Sergei A. Ponomarenko
a
Enikolopov Institute of Synthetic Polymeric Materials of Russian Academy of Sciences, Profsoyuznaya St. 70, Moscow 117393, Russia
Faculty of Physics & International Laser Centre of Lomonosov Moscow State University, Leninskiye gory 1/62, 119991 Moscow, Russia
N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS, Lavrentieva 9, Novosibirsk 630090, Russia
Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russia
b
c
d
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two novel organic semiconductors with tetrathienoacene (TTA) as the central core end-capped with 5-hexyl-2-
thiophene, (Hex-T) -TTA, and 4-hexyl-phenyl, (Hex-Ph) -TTA, have been synthesized and investigated for
Tetrathienothiophene
2
2
Absorption and luminescence spectra
Organic semiconductors
Оrganic field effect transistors
electroluminescence
organic field effect transistor (OFET) applications. The novel TTA derivatives were characterized by thermal
gravimetric analysis, differential scanning calorimetry, UV–Vis spectroscopy, and cyclic voltammetry as well as
studied by density functional theory calculations. Two types of OFETs with the solution and vacuum-deposited
active layer were fabricated and characterized. Both TTA-derivatives demonstrated electroluminescence in
2
OFETs, and (Hex-Ph)
2
-TTA showed ambipolar charge transport with the hole mobility as high as 0.68 cm
ꢀ
1
ꢀ 1
V
s .
1
. Introduction
Organic electronics has been actively developed in recent years due
charge-transfer properties [15,16]. Several tetrathienoacene (TTA) de-
rivatives have been reported to possess decent hole [17,18] and electron
[19,20] mobilities. Usually low solubility of TTA derivatives could be
improved by adding alkyl side chains in the β-position of the
fused-thiophene core [21–24]. Therefore, continued efforts to design
and develop small molecular semiconductors based on the TTA core are
crucial to their potential for OFET and related applications.
to their advantages such as ease of processing, low manufacturing cost,
and mechanical flexibility [1]. Electronic devices made from organic
semiconductor materials have many promising applications, including
organic light-emitting diodes (OLEDs) [2,3], organic photovoltaics
(
OPVs) [4,5], and organic field-effect transistors (OFETs) [6,7]. Among
a large variety of organic semiconductors, fused phenylene-thiophene
[1]benzothieno [3,2-b] [1]benzothiophene (BTBT) derivatives) and
In this study, we report on two novel conjugated oligomers with TTA
as the central core and different terminal groups. Specifically, the TTA
(
core was selected due to its rigid planar structure and enhanced
π-π
thienothiophene-based molecules have been widely investigated as
promising organic semiconductors in high-performance OFETs [8–11].
Among them, 2,7-dioctyl [1]benzothieno [3,2-b] [1]benzothiophene
interactions in the crystal [25,26]. The TTA core was end-capped with
4-hexyl-phenyl (Hex-Ph) and 5-hexyl-2-thienyl (Hex-T) groups to
investigate the effect of different substituents on the molecular proper-
ties and OFET performance. The chemical structures, physicochemical,
optical, and electronic properties of the compounds synthesized were
characterized by NMR, thermogravimetric analysis, differential scan-
ning calorimetry, UV–vis spectroscopy, cyclic voltammetry, and theo-
(
C8-BTBT) is one of the most studied [12]. However, in contrast to BTBT
derivatives, the synthesis of thienothiophene-based compounds is still a
difficult synthetic task and prevents their wide distribution [13,14].
Fused-thiophene based molecules have unique properties, such as
extensive conjugation, strong intermolecular S–S interactions, and rigid
coplanar conjugated core, which contributes to attractive
retical calculations. Based on (Hex-Ph)
2 2
-TTA and (Hex-T) -TTA, OFET
samples of two types were fabricated and studied: vacuum-processed
*
Corresponding author.
** Corresponding author. Enikolopov Institute of Synthetic Polymeric Materials of Russian Academy of Sciences, Profsoyuznaya St. 70, Moscow, 117393, Russia
E-mail addresses: paras@physics.msu.ru (D.Yu. Paraschuk), ponomarenko@ispm.ru (S.A. Ponomarenko).
https://doi.org/10.1016/j.dyepig.2020.108911
Received 14 April 2020; Received in revised form 9 September 2020; Accepted 2 October 2020
Available online 7 October 2020
0
143-7208/© 2020 Elsevier Ltd. All rights reserved.

O.V. Borshchev et al.
Dyes and Pigments 185 (2021) 108911
(
(
thin-film) and solution-processed (ultrathin) ones. We found that
Hex-Ph) -TTA oligomer shows ambipolar charge transport and stronger
-TTA.
was added in the cell after recording the CV curve of the studied com-
pound. The onset oxidation potential evaluated from the CV data was
2
electroluminescence as compared to (Hex-T)
2
used to estimate the HOMO energy, according to the following equation
ox
E
HOMO = ꢀ (E onset+4.8) eV.
2
. Experimental section
.1. Materials
All reagents were obtained from Sigma-Aldrich or ABCR and used as
2
.3. Theoretical calculation
2
Density functional theory (DFT) and time-dependent DFT (TDDFT)
calculations were carry out using GAMESS package [32,33] at
B3LYP/6-31G (d,p) level, which provides a reasonable tradeoff between
the calculation time and accuracy.
received. Solutions of 1.6 M and 2.5 M n-butyllithium and 2 M lithium
diisopropylamide (LDA) (CAUTION: solutions can ignite in contact with
water and carbon dioxide, this compound should be handled under ni-
trogen with special equipment), 3-bromothiophene, potassium and so-
2.4. Device fabrication and measurement
2
dium carbonate, CuCl , quinoline and sulfur were obtained from Acros
organics. 3,4-Dibromothiophene, ethyl 2-thioxoacetate, LiOH and po-
tassium ferricyanide were obtained from ABCR and used as received.
Organoboron derivatives 4-hexyl-1-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)benzene (3) and 2-(5-Hexyl-2-thienyl)-4,4,5,5-tetra-
methyl-1,3,2-dioxaborolane (4) were synthesized as described
elsewhere [27,28]. THF, DMF, NMP, chloroform, toluene, acetic acid
and diethyl ether were dried and purified according to the standard
techniques and then used as the solvents.
Two types of OFET samples were fabricated on precleaned Si/SiO2
substrates: vacuum-processed (thin-film) and solution-processed (ul-
trathin) ones. The first one was based on the thermally vacuum depos-
ited polycrystalline oligomer thin films with metal electrodes and the
second – on the solution-grown ultrathin (2D) single crystals of the
oligomers with organic (PEDOT:PSS) electrodes.
Substrate pretreatment: Silicon substrates coated with a thermally
grown 200-nm-thick layer of SiO were used after the following cleaning
2
steps. First, the substrates were cleaned in an ultrasonic bath for 10 min
2
.2. Characterization
in isopropanol, washed in distilled water, and dried in a stream of argon.
After that the substrates were exposed to ultraviolet radiation for 15 min
(PL16-110, SenLights). For thin-film samples hexamethyldisilazane
(HMDS) self-assembled monolayer was deposited by keeping the sub-
strates in saturated HMDS vapors in a closed Petri dish for 20 h in a
glovebox with the inert atmosphere. All further steps of the thin-film
OFET samples fabrication and measurement were conducted in argon-
filled gloveboxes.
Differential scanning calorimetry (DSC) was carried out on a Mettler
Toledo DSC30 instrument under nitrogen atmosphere at the scanning
◦
-1
◦
rate of 20 C min in the temperature range of +20–370 C. N
2
flow of
5
0 mL/min was used. Thermogravimetric analysis (TGA) was carried
out using Mettler Toledo TG50 system and reported decomposition
temperatures represent the temperature observed at 5% mass loss. New
compounds were studied in air and under nitrogen flow of 200 mL/min.
UV–vis absorption spectroscopy measurements were conducted on a
Shimadzu UV-2501PC (Japan) spectrophotometer in the standard 10
mm photometric quartz cuvette using THF solutions with the concen-
2
Solution-processed films: (Hex-Ph) -TTA was dissolved at a concen-
tration in the range of 0.1–0.8 g/L in ortho-dichlorobenzene with stir-
◦
ring (1200 rpm) and heating (60 C) during 3 h. The solution (<100
μL)
was spin-coated (SCS G3-8, Spincoating Systems) at 600–800 rpm for 30
s or drop-cast on the substrates, which were then placed in a closed Petri
dish for 5–15 h to provide conditions for slow solvent evaporation
[34–36]. The optimal concentration and deposition method were chosen
to maximize the lateral sizes of the thinnest domains of the films.
ꢀ 5
trations of 10 M. Photoluminescence (PL) spectra were obtained with
a scanning spectrofluorimeter ALS01 M (Russia) with registration in
single photon counting mode at successive time intervals and automatic
adjustment of the intensity of the measured emission. Measurements
were carried out for several optical densities in the range from 0.06 to
Vacuum-processed films: (Hex-Ph) -TTA and (Hex-T)
2
2
-TTA were
◦
0
.12 absorbance units in 10 mm cuvette, measurement geometry – 90 .
evaporated onto HMDS-treated Si/SiO substrates by thermal deposition
2
The photoluminescence quantum yield (PLQY) was measured by
using a vacuum system (Univex 300 G, Leybold) at residual gas pressure
ꢀ 6
ꢀ 6
comparing the integral PL intensity of 10 M diluted solutions in THF
with the integral PL intensity of the standard as described elsewhere
of 2.6–4.7 × 10 mbar with deposition rates in the range of 1–7 Å/s,
until the thickness reached 50 nm according to a thickness monitor
(TM400, Maxtek).
[
29]. As the standards in measuring the PLQY a solution of 1,4-bis
5-phenyloxazol-2-yl)benzene (POPOP) in cyclohexane (PLQY = 0.93)
was used.
Thin polycrystalline films for the study of PL properties were ob-
(
Morphological and structural characterization: The grown films were
characterized with the use of differential interference-contrast micro-
scopy (AxioImager A2m, Zeiss) in circularly polarized light (Epi C-DIC)
to determine the lateral dimensions of the films, estimate their crystal-
linity and surface homogeneity. Atomic force microscopy (AFM) was
used to determine the thickness of the films.
tained by deposition from a THF solution on a quartz substrate. Mea-
surements of the PL spectra in polycrystalline thin films were carried out
in an integrating sphere. Fluorescence was register both when the film
was illuminated in a line (through transmission) and under frontal
illumination. The calculated luminescence spectra of polycrystalline
films (Fig. 2 blue curve) was obtained in accordance with the technique
described earlier [30]. The PLQY of polycrystalline films was by com-
parison with the luminescence intensity of a tetraphenylbutadiene
polycrystalline film with a known quantum yield (0.9) [31].
Fabrication and Characterization of OFETs: OFETs were fabricated on
2
Si/SiO substrates in the bottom-gate top-contacts architecture. Ultra-
thin samples were deposited from solution on the precleaned substrates
as described above. As the top electrodes, PEDOT:PSS (PH 1000, Her-
aeus GmbH) with an additive (5% dimethyl sulfoxide) was deposited by
using a microplotter (GIX Micro-plotter Desktop, SonoPlot). As the ratio
of the OFET channel width (W) to its length (L) was about unity, the
active layer was carefully scratched out around the W × L area to avoid
the current flow out of this area and hence overestimation of the charge-
carrier mobility. On the substrates with vacuum-deposited thin films the
source and drain electrodes were also thermally evaporated in vacuum
through shadow masks (Ossila, UK) which led to 20 devices per sub-
2 2
Сyclic voltammetry (CV) measurements were performed in CH Cl
with 0.1 M tetrabutylammonium hexafluorophosphate as supporting
electrolyte using a potentiostat (P-8nano, Elins) in combination with a
three-electrode cell (Gamry). Pt disk (3 mm), Pt wire, and Ag/AgCl were
used as working, counter and reference electrodes, respectively. Because
of low solubility of TTA derivatives the measurements were performed
for a films drop-casted (multiple deposition from CH
2
Cl
2
solution and
strate with different values of L from 10 to 30
μ
m with the step of 5 m
μ
drying in the air) on the surface of working electrode. The measurements
were standardized by measuring the redox potential of ferrocene, which
and W = 1 mm. The two types of source/drain electrodes were used:
calcium (Ca) for electron injection and bilayer of molybdenum oxide
2

O.V. Borshchev et al.
Dyes and Pigments 185 (2021) 108911
and silver (MoO
3
/Ag) for hole injection. MoO
3
/Ag layers were evapo-
solubility, which allowed to obtain only 1H NMR spectroscopy and
elemental analysis data. Completeness of the reaction was controlled by
3D GPC chromatography with a diode array detector.
rated with some shift relatively to the Ca layer due to a shadow effect, in
order to allow both the hole and electron-injecting electrodes have a
contact with the active layer. Electrical measurements were performed
in an argon atmosphere with a source meter (Keithley 2636A, USA)
using a probe station (ProbeStation 100, Printeltech LLC, Russia). The
charge-carrier mobility and the threshold voltages were evaluated ac-
cording to Shockley’s equations in the linear and saturation regimes:
3
.2. Thermal behavior
Thermal properties of the compounds synthesized were explored by
DSC and TGA methods, and the results are summarized in Table 1. The
DSC heating curves indicate that both compounds are crystalline with
two exothermic peaks having rather high enthalpies (Table 1 and Fig. S
3,4 in ESI). Most probably, the first transition correspond to crystal 1 to
crystal 2 phase transition, while the second one – to melting from the
2nd crystal phase to isotropic liquid that was confirmed by polarizing
optical microscopy The melting temperature is higher for (Hex-Ph)2-
W
L
W
2L
2
ID = μlinC(VG ꢀ VT)VD; ID = μsatC(VG ꢀ VT) , where
μ
lin and
μ
sat are
the charge-carrier mobilities in the linear and saturation regime,
ꢀ 2
respectively; C = 17.3 nF cm is the gate insulator capacitance per unit
area for SiO layer calculated using its thickness provided by the sup-
plier (200 nm) and the dielectric constant of SiO (3.9); I is the source-
drain current, V is the source-drain voltage, V is the gate voltage, V is
2
2
D
D
G
T
the threshold voltage. Light emission was captured using a microscope
equipped with a CCD-camera (Infinity 3, Luminera) at an exposition
time of 60 s during the transfer characteristics measurement. The rela-
tive electroluminescence (EL) intensity was calculated in arbitrary units
as a sum of intensities of pixels belonging to the channel area in a
captured light-emission image.
TTA (333◦С) than for (Hex-T) -TTA (259◦С), which can be explained by
2
stronger intermolecular interactions within the crystals of (Hex-Ph)2-
TTA.
Analyzing the dependence of changes in the weight of the samples
with increasing temperature for (Hex-Ph) -TTA and (Hex-T) -TTA
2
2
(Fig. S 5,6 in ESI), we conclude that the lower thermo-oxidative stability
of alkyl substituents leads to different results in an inert atmosphere and
in the presence of atmospheric oxygen. In the air, the TGA curves show
3
. Results and discussion
2 2
that the decomposition temperatures for (Hex-Ph) -TTA and (Hex-T) -
3
.1. Synthesis
TTA are over 350 and 346 ◦C, respectively, with a loss of less than 5% of
their weight. However, in the nitrogen atmosphere at the temperatures
Synthesis of the new TTA derivatives was carried out in three main
of 5% weight loss most likely no decomposition occurs until 398 and
steps. It should be noted that the combination and optimization of
various literature methods allowed to increase the total reaction yield of
TTA from 27% to 45% (Fig. 1) [37].
370 ◦C for (Hex-Ph) -TTA and (Hex-T) -TTA, respectively.
2
2
3
.3. Optical and electrochemical properties
2
Optical properties of (Hex-Ph) -TTA and (Hex-T) -TTA in dilute THF
At the last stage, we carried out the bromination of TTA with NBS in a
mixture of chloroform/acetic acid (Fig. 1). According to GPC, the re-
action is almost quantitative, which for some applications allows the use
of the resulting 2,6-dibromo-tetrathienothiophene without future puri-
fication. A highly pure Br-TTA-Br can be obtained by recrystallization
from toluene.
2
solutions and thin films were investigated by UV–Vis spectroscopy. The
corresponding absorption and photoluminescence spectra are presented
in Fig. 2, and the data obtained are summarized in Table 2.
2 2
The absorption maxima for (Hex-Ph) -TTA and (Hex-T) -TTA in so-
At the last stage, target compounds 1 and 2 were synthesized by the
Suzuki cross-coupling reaction of Br-TTA-Br with organoboron de-
rivatives of hexylbenzene (3) and hexylthiophene (4), respectively. The
crude products were purified by recrystallization from toluene to give
pure compounds 1 and 2 as yellow solids with the yield of 71% and 63%,
respectively. Unfortunately, the compounds obtained had rather low
lutions are located at 387 and 408 nm, respectively. Replacement of the
phenyl rings by thiophene heterocycles leads to a bathochromic shift. In
this case, the energy of the 0–0 transition decreases by 0.17 eV. If we
consider the energy of the 0–0 transition as the width of the optical gap,
then its value is almost the same as those obtained by TD DFT calcula-
tions (vide infra; DFT calculation). The PL (fluorescence) spectra in
Fig. 1. Optimization of the literature scheme for the preparation of TTA and its derivatives 1 and 2.
3

O.V. Borshchev et al.
Dyes and Pigments 185 (2021) 108911
ꢀ
5
Fig. 2. Optical properties of (Hex-Ph)
2
-TTA (a) and (Hex-T)
2
-TTA (b). (1) Absorption spectra in diluted THF solution (10 М) (black curve); (2) PL spectra in diluted
ꢀ 6
THF solution (10 М) (red curve); (3) PL spectra of polycrystalline films (green curve); (4) calculated PL spectra of polycrystalline films (blue curve); (5) long-
wavelength edge of the excitation spectrum of PL of polycrystalline films (black dash curve). (For interpretation of the references to color in this figure legend,
the reader is referred to the Web version of this article.)
the absolute values of the PLQY, the integrated fluorescence intensity of
Table 1
(
2 2
Hex-Ph) -TTA and (Hex-T) -TTA was compared with the integrated
Thermal properties of TTA derivatives.
intensity of a thin polycrystalline films of TPB (1,1,4,4-tetraphenyl-1,3-
butadiene). The PLQY values measured in thin polycrystalline films
Compound
Т
Cr1-Cr2
,
ΔHCr1-Cr2, J
Т
m
,
ΔH
m
ꢀ 1
, J
Т
N2
,
Т
air
,
◦
ꢀ 1
◦
◦
◦
C
g
C
g
C
C
were significantly lower, in particular, 24 and 4% for (Hex-Ph)
and (Hex-T) -TTA, respectively.
2
-TTA
(
(
Hex-Ph)
TTA
2
-
204
157
16
15
333
259
23
24
398
370
350
346
2
In order to better understand the optical properties of both molecules
investigated, DFT and TDDFT calculations have been made. Fig. 3 shows
the equilibrium geometries of the molecules (the alkyl groups were
replaced by Me groups for the simplicity of calculations) and their
HOMO and LUMO patterns obtained at DFT B3LYP/6-31G (d,p) level.
Hex-T)
TTA
2
-
Notes: ТCr1-Cr2 and ΔHCr1-Cr2 are temperature and enthalpy of crystal 1 to crystal
phase transition, respectively; Т aremelting temperature and
and ΔH
enthalpy, respectively; TN2 and Tair are the decomposition temperatures (at 5%
weight loss) according to TGA in the inert atmosphere (N ) and in the air.
2
m
m
From this figure it follows that in (Me-Ph)
2
-TTA, phenyl rings are
2
◦
twisted by ~27 with respect to the TTA core, which is readily explained
by the repulsion between the phenyl and TTA hydrogens previously
observed in thiophene-phenylene co-oligomers [38,39]. On the con-
solution shows a clearly distinguishable vibronic structure with two
maxima at 425, 451 nm and 452, 479 nm for (Hex-Ph) -TTA and (Hex-
T) -TTA, respectively. For (Hex-T) -TTA the photoluminescence quan-
tum yield (PLQY) in diluted THF solution is 67% that slightly greater
than those of (Hex-Ph) -TTA (52%) (Table 2). It may be ascribed by the
2
2
trary, in (Me-T) -TTA, the thiophene rings are coplanar with the TTA
2
2
core since the hindrance mentioned above is absent in this molecule.
Accordingly, while in the former molecule, both HOMO and LUMO are
located mostly at the TTA core, in the latter molecule, HOMO and LUMO
electron densities at the thiophene rings are considerable. Thus, it can be
2
higher co-planarity and better conjugation of the former.
A long-wavelength edge of the fluorescence excitation spectrum of
polycrystalline films is shown on Fig. 2. (curve 5). Study of the fluo-
rescence spectra of the polycrystalline films showed that its short-
wavelength edge is strongly suppressed due to reabsorption (Fig. 2
curve 3). Therefore, we have modeled the PL spectra of the poly-
crystalline films for the case of complete absence of reabsorption (Fig. 2
curve 4). The PL maximum in the polycrystalline films is red shifted by
suggested that the extent of the
larger than that in (Me-Ph) -TTA molecule.
Table 2 summarizes HOMO and LUMO energies calculated at DFT
level, as well as optical bandgaps, E , and oscillators strengths of the first
dipole-allowed transition, fS0-S1, obtained at TDDFT level. As follows
from this Table, the calculated HOMO energy for (Me-Ph)
-TTA (ꢀ 4.91
eV) is lower than that for (m-Me-T)
-TTA (ꢀ 4.76 eV), in line with the
experimental data shown in Table 2. The ~0.3 eV higher HOMO level
π
2
-electron conjugation in (Me-T) -TTA is
2
g
2
2
~
0.5 eV as compared to the PL peak in the solution. In order to obtain
Table 2
Optical characteristics of the diluted THF solutions, theoretically calculated properties and cyclic voltammetry (CV) data of the TTA derivatives.
Optical spectroscopy
DFT
CV
opt
g
1
2
Abs.
max, nm
PL λmax
nm
,
Stokes
PL
nm
s
λ
max
,
PLQY
%
,
E
,
f
S0-
LUMO
eV
E
g
,
f
S0-
HOMO,
eV
LUMO,
eV
E
1/
E
1/
HOMO,
eV
λ
shift, nm
eV
S1
eV
S1
2
,
2
,
V
V
(
Hex-
Ph)
387
408
425,
451
38
44
554
590
52 ±
2.98
2.81
1.69
1.73
ꢀ 2.35
ꢀ 2.30
3.08
2.77
1.61
1.65
ꢀ 4.91
ꢀ 4.76
ꢀ 1.56
ꢀ 1.78
0.49
1.00
ꢀ 5.25
ꢀ 5.11
2
-
4
TTA
(
Hex-T)
TTA
2
-
452,
479
67 ±
0.38
0.76
4
opt
g
Notes: E
s
is the optical bandgap, the experimental LUMOs were calculated from CV and absorption spectrum edge; PL - maximum of PL band in film; PLQY is
1
2
photoluminescence quantum yield; fS0-S1 is the oscillator strength for the lowest-energy dipole-allowed transition; E 1/2 and E 1/2 are the half wave potentials for the
first and the second oxidation processes, respectively.
4

O.V. Borshchev et al.
Dyes and Pigments 185 (2021) 108911
Fig. 3. Equilibrium geometries (a) and patterns of the frontier orbitals HOMO (b) and LUMO (c) for the compounds studied.
estimated theoretically as compared to the experimental data can be
attributed to intermolecular interactions in the films and the solvent
effect [34,35]. The smaller difference between HOMO and LUMO en-
can be readily attributed to the better
molecule (see above). Accordingly, the optical bandgap
(Me-T) -TTA is narrower than that for (Me-Ph) -TTA, which is in a good
agreement with the experimental red-shifted absorption of the former
π
-conjugation in the former
E
g
for
2
2
2 2
ergies for (Me-T) -TTA than that for (Me-Ph) -TTA (2.98 eV vs 3.35 eV)
Fig. 4. Polycrystalline OFET scheme (a), EL images of OFET based on (Hex-Ph)
2 2 2
-TTA (b) and (Hex-T) -TTA (c), typical transfer characteristics for (Hex-Ph) -TTA (d)
and (Hex-T) -TTA (e). EL images are superpositions of the black-and-white images under backlight and colored in blue images in dark during transfer characteristics
2
measurement. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
5

O.V. Borshchev et al.
Dyes and Pigments 185 (2021) 108911
g
compound as compared to the latter (note that E is generally lower than
the energy difference between HOMO and LUMO because the latter does
not account for the interaction between the excited electron and its
“
hole”, i.e. the exciton binding energy). The lowest-energy dipole-al-
lowed transition has the largest oscillator strength and involves mostly
the transition from the HOMO to the LUMO. The dipole moment of this
transition is oriented along the long molecular axes. Calculated oscil-
lator strength of the transition is slightly higher for (Me-T)
Me-Ph) -TTA, in line with the experimental data (see Table 2).
Due to low solubility cyclic voltammetry measurements (CV) were
2
-TTA than for
(
2
performed for a drop-casted thin films of the compounds on the surface
of Pt electrode. The two quasi-reversible oxidation peaks were observed
for both derivatives, whereas the reduction processes were not available
within the electrochemical window of the solvent used (Fig. S7 in ESI).
(
Hex-Ph)
2
-TTA possesses slightly lower half-wave potentials for both
-TTA. The
oxidation events (Table 2) as compared with that of (Hex-T)
2
HOMO energy levels estimated using the onset oxidation potentials of
first peaks in cyclic voltammograms are summarized in Table 2. The
measured values are slightly higher than ꢀ 5.4 eV previously reported
2
for (Ph) -TTA [14] what could be accounted for by the weak electron
donor effect of the hexyl groups and a better conjugation and
co-planarity of thiophene moiety as compared with that of the
phenylene.
2
Fig. 5. C-DIC optical microscopy images of 2D (Hex-Ph) -TTA sample.
Numbers correspond to the different domains of the film.
demonstrate faceted shapes (clearer seen for domains 1, 2, 4), which
indicates that they are 2D single crystals.
3
.3.1. Vacuum-deposited light-emitting devices
Using (Hex-Ph) -TTA and (Hex-T) -TTA as active layers, we fabri-
2
2
Fig. 6 shows optical images and electrical characteristics of 2D (Hex-
cated thin-film (polycrystalline) OFET samples with low and high-work
function electrodes to study ambipolar charge transport and light
emission in devices [39]. Both TTA-derivatives demonstrated EL in
OFETs. Fig. 4 shows schematic of OFET samples, transfer characteristics,
and EL images for the two TTA derivatives. The OFET based on
2
Ph) -TTA OFETs. The devices demonstrate clear p-conductive FET
behavior as expected for the PEDOT:PSS electrodes with relatively high-
work function (5.1 eV). The output characteristics (panel b) are linear
near the zero voltage, which is a sign of negligible contact effects. Panels
c–d show the transfer characteristics in the linear (c) and the saturation
(
2
Hex-Ph) -TTA exhibited ambipolar conductivity with the maximal
2
ꢀ 1 ꢀ 1
(
d) regimes. The hysteresis in the transfer characteristics is very small
electron mobility of 0.03 cm
V
s
(the average value 0.0031 ±
ꢀ 1 ꢀ 1
2
ꢀ 1 ꢀ 1
V s
indicating a stable device operation. The hole mobility values were very
close for the linear and saturation regimes pointing out that the device
performance is closely follows to Shockley’s model. The statistical data
on seven OFET devices are presented in Table S1 in ESI. For 2D OFETs,
0
.0009 cm2
V
s
), the maximal hole mobility of 0.128 cm
2
ꢀ 1 ꢀ 1
(
the average value 0.0746 ± 0.0025 cm V
voltage was 69 ± 17 V for electrons and ꢀ 4±8 V for holes. The OFET
based on (Hex-T) -TTA showed the only unipolar p-type conductivity,
s ). The average threshold
2
2
the maximum hole mobility in the linear regime was 0.66 ± 0.06 cm
with the maximal hole mobility and the average threshold voltage very
ꢀ
1 ꢀ 1
2
ꢀ 1 ꢀ 1
V s in average. In the saturation
similar to those of (Hex-Ph)
2
-TTA: 0.129 cm2
V
ꢀ 1 ꢀ 1
s
and ꢀ 4±11 V,
V
s
with 0.56 ± 0.07 cm
2
ꢀ 1 ꢀ 1
s
regime, the hole mobility reached a value of 0.68 ± 0.06 cm
V
respectively, whereas the average hole mobility was slightly lower and
2
ꢀ 1 ꢀ 1
2
ꢀ 1 ꢀ 1
with 0.51 ± 0.10 cm
between 10 and 40 V.
V
s in average. The threshold voltage was
amounted to 0.067 ± 0.003 cm
charge transport in the (Hex-T)
gas-phase theoretical LUMO energy as compared with (Hex-Ph)
V
s
. The absence of the electron
-TTA does not agree with its lower
-TTA,
2
2
4
. Conclusions
as the lower LUMO energy is expected to result in better electron in-
jection and charge transport. At the moment, we have no clear expla-
nation of this difference in the electron mobility of the TTA-derivatives.
New organic semiconductors with tetrathienoacene (TTA) as the
central core end-capped with 5-hexyl-2-thiophene ((Hex-T) -TTA) or 4-
hexyl-phenyl ((Hex-Ph) -TTA) have been synthesized, characterized by
The lower estimate for EL intensity in the ambipolar OFET based on
2
3
(
(
Hex-Ph)
2
-TTA was 2067 ± 5 × 10 a.u., and the EL intensity in
2
3
different physico-chemical methods, and studied in OFETs. The photo-
physical, electrochemical and electronic properties of these molecules
were compared, and it was shown that both molecules exhibit strong
Hex-T)
2
-TTA based OFET was almost nine times lower, 234 ± 4 × 10 a.
-TTA is attributed to the
u. The much higher EL intensity for (Hex-Ph)
2
presence of ambipolar charge transport. For both oligomers EL occurs
near the electron-injecting Ca electrode (on the right in Fig. 4), because
the electron charge transport is absent or significantly less efficient than
the hole transport.’
fluorescence and high thermal stability. (Hex-T)
2
-TTA exhibits a red-
-TTA that
shift of absorption and PL maxima as compared to (Hex-Ph)
2
could be accounted for by the higher planarity of the former. Both new
TTA derivatives have a relatively high quantum yield of photo-
luminescence in solution, and thin polycrystalline film of (Hex-Ph)
showed the six times higher PLQY values as compared those for (Hex-
T) -TTA. The elucidated structure-property correlations are in agree-
2
-TTA
3
.3.2. Solution-processed devices
2
From the data on the polycrystalline OFETs, (Hex-Ph) -TTA looks
more promising for ambipolar and light emissive OFETs. As a result, we
chose (Hex-Ph) -TTA for further studies in ultrathin solution-processed
2
ment with the theoretical calculations based on the density functional
theory. Both TTA derivatives have shown electroluminescence in OFETs,
2
films and OFETs prepared by drop or spin-casting with subsequent slow
solvent evaporation. This method can provide large-area 2D single
and (Hex-Ph)
port. We have shown that (Hex-Ph)
large-area ultrathin (2D) films sustaining efficient charge transport with
2
-TTA have demonstrated clear ambipolar charge trans-
2
-TTA can be solution-processed in
crystals as shown in Refs. [30]. 2D films of (Hex-Ph)
2
-TTA were obtained
-TTA
on a silicon substrate. Fig. 5 shows the optical images of (Hex-Ph)
2
2
ꢀ 1 ꢀ 1
the hole-mobility exceeding 0.6 cm V
s .
ultrathin (2D) films grown by the drop casting method combined with
subsequent slow solvent evaporation. The films had a domain structure,
and the lateral domain dimensions exceed 800
μm. The domains
6

O.V. Borshchev et al.
Dyes and Pigments 185 (2021) 108911
Fig. 6. OFET data for 2D films: optical image of the device, s/d denotes the source/drain contacts (a); output characteristics (b); transfer characteristics in the linear
c) and saturation (d) regimes.
(
CRediT authorship contribution statement
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