Ambipolar organic field-effect transistors based on N-Unsubstituted thienoisoindigo derivatives

Article abstract of DOI:10.1016/j.dyepig.2020.108418

To investigate a hybrid of isoindigo and thienoisoindigo (TIIG), a new series of unsymmetrical TIIG analogs, in which one of the outer thiophene rings of TIIG is replaced by benzene (CS) or pyridine (NS) are prepared. In addition, the π-skeleton extension

Full text of DOI:10.1016/j.dyepig.2020.108418

Dyes and Pigments 180 (2020) 108418
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journal homepage: http://www.elsevier.com/locate/dyepig
Ambipolar organic field-effect transistors based on N-Unsubstituted
thienoisoindigo derivatives
,
1
,
,2
Dongho Yooa *, Tsukasa Hasegawaa, Akihiro Koharaa, Haruki Sugiyamab
,
,
Minoru Ashizawaa **, Tadashi Kawamotoa, Hiroyasu Masunagac, Takaaki Hikimad,
Noboru Ohtac, Hidehiro Uekusab, Hidetoshi Matsumotoa, Takehiko Moria
,
***
a Department of Materials Science and Engineering, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo, 152-8552, Japan
b Department of Chemistry, Tokyo Institute of Technology, O-okayama 2-12-1, Meguro-ku, Tokyo, 152-8551, Japan
c Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5198, Japan
d RIKEN SPring-8 Center, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo, 679-5148, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords:
To investigate a hybrid of isoindigo and thienoisoindigo (TIIG), a new series of unsymmetrical TIIG analogs, in
which one of the outer thiophene rings of TIIG is replaced by benzene (CS) or pyridine (NS) are prepared. In
Thienoisoindigo
Thieno-benzo-isoindigo
Thieno-pyridine-isoindigo
addition, the π-skeleton extension effects are studied by examining α-substituted TIIG derivatives with thienyl
(Dth-TIIG), furyl (Dfu-TIIG), and 1-phenyl-5-pyrazolyl groups (Bis(1-ph-5-py)-TIIG). These materials exhibit
ambipolar transistor properties as expected from the energy levels. Dth-TIIG and Dfu-TIIG show hole mobilities
exceeding 0.05 cm2 VÀ 1 sÀ 1, and electron mobilities about 0.04 cm2 VÀ 1 sÀ 1, which are close to those of the
diphenyl derivative. These molecules are arranged nearly perpendicularly to the substrates in the thin films, and
Dth-TIIG has a brickwork-like structure, whereas Dfu-TIIG has uniform columns. CS and Bis(1-ph-5-py)-TIIG
have uniform stacking structures, but NS has a dimerized stacking structure due to the slightly bent molecular
plane that results from largely unbalanced electron density.
α
-substituted thienoisoindigo derivatives
Organic field-effect transistors
Ambipolar transistors
1. Introduction
transporting properties are integrated into a single component [11,12].
Numerous OSCs have been designed and devised for ambipolar trans-
Ambipolar organic semiconductors (OSC) are capable of conducting
both holes and electrons particularly in organic transistors [1,2]. In
practice, however, unipolar transport dominates when OSCs are applied
in organic transistors because of the charge traps and existence of the
injection barriers between electrodes and molecular frontier orbitals
[3–5]. Therefore, it is necessary that the highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO)
have to fulfill relatively strict conditions to achieve ambipolar transport
[6,7]. For more than a decade, OSCs showing ambipolar transport have
been extensively investigated to identify their carrier-transport mecha-
nism [8–10], and to develop applications to inverters and light-emitting
field-effect transistors. In these semiconductors, p-type and n-type
port [13]; they include small energy-gap organic small molecules [9,
14–21], donor-acceptor (DA) polymers [22–28], mix-stacked DA-type
charge-transfer complexes [29–35], and metal complexes [36–38].
Among them, organic small molecules are composed of a single
component, but others consist of more than two components. In prin-
ciple, organic small molecules that show ambipolar transport can be
designed by engineering the energy levels and energy gaps, but this is a
difficult task. To achieve appropriate energy levels with suitable HOMO
levels and low-lying LUMO levels, it is necessary to introduce both
electron-donating and electron-withdrawing groups. From this point of
view, indigo and isoindigo, which is a structural isomer of indigo, are
promising materials because the amino and carbonyl groups realize
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail addresses: dongho312@postech.ac.kr (D. Yoo), ashizawa.m.aa@m.titech.ac.jp (M. Ashizawa), mori.t.ae@m.titech.ac.jp (T. Mori).
1
Department of Chemical Engineering and Center for Advanced Soft Electronics, Pohang University of Science and Technology, 77 Cheongam-Ro, Nam-Gu,
Pohang 37673, Korea.
2
Research and Education center for Natural Sciences, Keio University, 4-1-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223–8521, Japan.
https://doi.org/10.1016/j.dyepig.2020.108418
Received 17 February 2020; Received in revised form 31 March 2020; Accepted 31 March 2020
Available online 19 April 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
well-balanced ambipolar transport in organic field-effect transistors
(OFET) [14,18,39,40]. Substituting thiophene for the benzene ring of
isoindigo yields thienoisoindigo (TIIG), and this molecule has
oxygen-sulfur (S⋯O) intramolecular interactions that stabilize the mo-
lecular planarity. In addition, because of the sulfur atom, the intermo-
lecular sulfur-sulfur (S⋯S) interactions are expected to lead to efficient
carrier transport [41]. Therefore, TIIG has been extensively investigated
by varying the N-alkyl chains as OFET materials and as repeating units in
conjugated polymers [42–50].
2. Experimental section
2.1. Synthesis
t-Boc-thienoisatin and compound 2 were prepared according to the
previous report [21,52]. CS and NS were prepared by cross coupling of
t-boc-thienoisatin with oxindole or Compound 2 (Scheme 2). These re-
actions are kinds of aldol condensation, and hydrochloric acid was used
as a catalyst. Therefore, the t-boc groups were removed concurrently
during the coupling reactions.
N-unsubstituted TIIGs with diphenyl groups (Dph-TIIG) exhibit
ambipolar transport with hole and electron mobilities exceeding 0.1
cm2 VÀ 1 sÀ 1 [21]. Removal of N-alkyl chains allows S⋯S interactions
and hydrogen bonds, which together tighten the molecular packing.
Actually, Dph-TIIG forms a two-dimensional conduction path due to the
combination of the core-part brickwork and the phenyl-part herring-
bone arrangements [21]; this configuration is advantageous for tran-
sistor properties [9].
t-Boc-dBr-TIIG was prepared according to the previous report [21],
and Dth-TIIG, Dfu-TIIG, and Bis(1-ph-5-py)-TIIG were prepared by the
Stille coupling reaction (Scheme 3), followed by removal of t-boc groups
by using trifluoroacetic acid.
2.2. Device fabrication
Here, to further investigate substitution effects from isoindigo to
TIIG in more detail, we have prepared unsymmetrically-substituted
TIIG analogs in which one of the outer thiophene rings of TIIG is
replaced by benzene (thieno-benzo-isoindigo, CS) or pyridine (thieno-
pyridine-isoindigo, NS) (Scheme 1). We then quantified how the sub-
stitutions affected changes of the energy levels, crystal structures, and
transistor properties. Although these are unsymmetrical molecules, CS
with N-alkyl chains has been previously synthesized as a monomer unit
of a DA copolymer that has fine backbone coplanarity [51]. This
copolymer has a face-on orientation so it is favorable for use in organic
photovoltaics, but CS without the N-alkyl substituent chain has not been
investigated as a small-molecule OSC. Furthermore, Dph-TIIG has
realized a favorable molecular packing, so other substituents are
Thin-film transistors were fabricated onto n-doped Si substrates with
a thermally-grown SiO2 (300 nm, C ¼ 11.5 nF/cm2) dielectric layer. The
SiO2 surface was passivated with 20 nm thick layer of tetratetracontane
(C44H90, TTC,
ε
¼ 2.5), by thermal deposition under a vacuum of 10À 3
Pa [53–55]. The resulting overall capacitance of the gate dielectrics was
10.4 nF/cm2 [56]. A 50 nm thick active layer was formed by thermal
deposition under a vacuum of 10À 3 Pa. Then the top-contact electrodes
were patterned by thermal deposition of Au through a shadow mask; the
channel had width W ¼ 1000 μm and length L ¼ 100 μm. OFET prop-
erties were measured using a Keithley 4200 semiconductor parameter
analyzer under a vacuum of 10À 3 Pa. The field-effect mobility
μ and
threshold voltage VT were calculated in the saturation regime by using
the equation, IDS
¼
μ
(WCi/(2L))(VG -VT)2, where IDS is drain current and
investigated: three
(Dth-TIIG), difuryl-TIIG
pyrazolyl)-TIIG (Bis(1-ph-5-py)-TIIG) are designed for use in extending
α
-substituted TIIG derivatives, dithienyl-TIIG
VG is gate voltage; μ is extracted from the slope where the plot of √IDS vs.
(Dfu-TIIG), and bis(1-phenyl-5--
VG is straight. We attempted to make the single-crystal transistors, but
the characteristics were not observed.
the π-skeleton. In this paper, we report the crystal structures and OFET
properties of these five N-unsubstituted TIIG derivatives.
2.3. DFT calculation
The density functional calculations such as geometry optimization,
calculation of transfer integrals t, and calculation of reorganization en-
ergies were performed with the B3LYP* functional and TZP basis set
using ADF software [57].
Scheme 1. Chemical structures of (a) TIIG [21], (b) CS, (c) NS, (d) Dph-TIIG
[21], (e) Dth-TIIG, (f) Dfu-TIIG, and (g) Bis(1-ph-5-py)-TIIG.
Scheme 2. Synthetic scheme to CS and NS.
2

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
Scheme 3. Synthetic scheme of the α-substituted TIIG derivatives.
3. Results and discussion
Bis(1-ph-5-py)-TIIG has slightly larger Eogpt than Dth-TIIG and Dfu-
TIIG because of the torsion angle of the pyrazolyl planes against the
central TIIG plane. When gold is used as electrodes, hole transport is
expected when the HOMO level is higher than À 5.6 eV, and electron
transport appears when the LUMO level is lower than À 3.15 eV [6,7].
The observed energy levels indicate that these compounds are expected
to exhibit ambipolar OFET properties (Fig. 2). The absorption bands of
the thin film undergo bathochromic shift due to the intermolecular in-
teractions in the solid state (Fig. S1 and Table S1).
3.1. Electronic properties
The HOMO and LUMO levels were estimated by using cyclic vol-
tammetry and ultraviolet visible (UV–vis) absorption spectroscopy. Dth-
TIIG and Bis(1-ph-5-py)-TIIG in DMF solutions exhibited reversible
oxidation waves, whereas CS, NS, and Dfu-TIIG showed irreversible
oxidative peaks (Fig. 1a and b). The HOMO levels were estimated from
the oxidation onset potentials Eonset. The optical energy gaps Eogpt were
estimated from onset wavelength λonset of the DMF solution absorption
spectra (Fig. 1c and d), and the LUMO levels are estimated by adding Egopt
to the HOMO levels. The energy levels (Table 1 and Fig. 2) deepen in the
order of TIIG < CS < NS according to the order of electron-withdrawing
ability of thiophene < benzene < pyridine. The HOMO and LUMO levels
of CS and NS are by about 0.1 eV deeper than those of TIIG. The optical
3.2. Crystal structures
Detailed crystal data were obtained for all five compounds (Table 2).
CS crystallizes in the space group P21/c, in which one molecule is
crystallographically independent (Fig. 3), and CS has an intramolecular
S⋯O interaction with the distance of 2.760(5) Å, which guarantees
planar structure (Fig. 3a). CS forms a uniform stacking structure along
the b axis with interplanar spacing of 3.34 Å, and with adjacent columns
in the c axis direction tilted in opposite directions (88�) (Fig. 3b and c).
An S⋯S short contact with the distance of 3.444(2) Å is observed along
the a þ c axis (Fig. 3d), and hydrogen bonds with length around 2.82 Å
for N(H)⋯O are observed between adjacent columns along the c axis.
Although intermolecular interactions occur, CS constructs a one-
dimensional conduction path along the stacking direction, which is
represented in the values of the transfer integrals in Fig. 3e and Table 3.
The crystal structure of NS (thieno-pyridine-isoindigo) is iso-
structural to benzo-pyridine-isoindigo [18]. NS crystallizes in a triclinic
system (Fig. 4), in which two molecules (1 and 2) are crystallographi-
cally independent (Fig. 4a–d). Molecules 1 and 2 have intramolecular
S⋯O interactions with the distance of 2.682(4) and 2.691(4) Å,
respectively (Fig. 4a). Molecules 1 and 2 construct independent con-
ducting layers (Fig. 4e–h). Within the layers, these molecules have
hydrogen bonds with the adjacent molecules; N(H)⋯O: 2.869(6) Å and
N(H)⋯N: 2.868(6) Å for Molecule 1, and N(H)⋯O: 2.867(6) Å and N
(H)⋯N: 2.884(6) Å for Molecule 2 (Fig. 4c and d). Between these layers,
the molecules are tilted in opposite directions with a mutual dihedral
angle of 88�, between which intermolecular S⋯O interactions with the
distance of 3.197(4) Å are formed (Fig. 4b). Despite the intramolecular
S⋯O interaction, the NS molecules are considerably bent (Fig. 4e and f),
possibly because of largely unbalanced electron density that results from
the remarkably different electron-donating and electron-withdrawing
abilities of the thiophene and pyridine rings. The twist angles are 9.3�
for Molecule 1 and 10.5� for Molecule 2. Therefore, concave and convex
molecules are alternately stacked in a column, so slight dimerization
occurs. The transfer integrals (Fig. 4g and h and Table 4) indicate that
within the intradimer pair (ta1 and tb1), the molecules are slightly slipped
along the molecular short axis, whereas for the interdimer interactions
energy gaps of the three
α
-substituted TIIG derivatives are smaller than
those of CS and NS because of the extended
π
-skeleton. The acceptor
abilities of Dth-TIIG and Dfu-TIIG are comparable to those of CS and
NS, whereas the donor abilities are obviously improved, even in com-
parison with Dph-TIIG. Dth-TIIG and Dfu-TIIG have identical Eogpt, but
Fig. 1. Cyclic voltammograms of (a) CS and NS, and (b)
α
-substituted TIIG
derivatives. UV–vis absorption spectra of (c) CS and NS, and (d)
TIIG derivatives in DMF solutions.
α
-substituted
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D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
Table 1
Summary of electrochemical and optical properties.
Compound
Eonset (V)
EHOMO (eV)
λonset (nm)
λmax (nm)
Eogpt (eV)
ELUMO (eV)
CS
0.45
0.47
À 5.25
À 5.27
À 5.11
À 4.99
À 4.97
À 5.16
613
614
381, 512
368, 511
2.02
2.02
1.79
1.73
1.73
1.82
À 3.23
À 3.25
À 3.32
À 3.26
À 3.24
À 3.34
NS
Dph-TIIG [21]
Dth-TIIG
Dfu-TIIG
Bis(1-ph-5-py)-TIIG
0.19
0.17
0.36
716
715
682
370, 619, 659
328, 617, 662
335, 593
Fig. 2. Energy levels of CS, NS and α-substituted TIIG derivatives with TIIG and Dph-TIIG [21]. Values in the parentheses are obtained using ADF calculation.
Table 2
Crystallographic data of the five compounds tested.
CS
NS
Dth-TIIG
Dfu-TIIG
Bis(1-ph-5-py)-TIIG
Chemical formula
Formula weight
Crystal system
Space group
a (Å)
C
14H8N2O2S
C13H7N3O2S
269.28
C20H10N2O2S4
438.55
C20H10N2O4S2
406.43
C30H18N6O2S2
558.63
orthorhombic
Pna21
268.29
monoclinic
P21/c
triclinic
monoclinic
Cc
triclinic
PÀ 1
PÀ 1
15.7745(4)
4.72502(12)
15.5424(4)
90
7.45322(14)
7.57431(14)
20.1443(4)
100.7500(11)
95.8199(10)
90.6023(10)
1110.98(4)
4
11.2237(2)
11.0897(2)
30.5522(5)
90
4.69727(9)
14.6021(3)
15.1633(3)
71.6590(9)
83.8198(9)
89.0535(8)
981.31(3)
2
32.9717(12)
4.6935(2)
16.1986(7)
90
b (Å)
c (Å)
α
(�)
β (�)
94.2498(15)
90
96.6100(10)
90
90
γ (�)
90
V (Å3)
1155.26(5)
4
3777.48(11)
8
2506.78(18)
4
Z
Total refls.
12212
13173
21225
11377
26882
Unique refls. (Rint
)
2025 (0.0985)
1.542
3982 (0.0529)
1.610
6699 (0.0557)
1.542
3519 (0.0421)
1.375
4589 (0.0885)
1.480
D
caalc (g/cm3)
R1 b
0.0994
0.0665
0.0952
0.0523
0.0777
0.2199
1.068
Rw
0.3130
0.2382
0.2927
0.1653
GOF
T (K)
1.127
1.148
1.174
1.155
104
171
275
104
171
a
b
R1[F2 > 2
σ
(F2)].
wR2(F2) for all observed reflections.
(ta2 and tb2), the molecules are considerably slipped along the molecular
long axis. Consequently, NS constructs a dimerized one-dimensional
conduction path (Fig. 4e and f).
Two crystallographically-independent Molecules 1 and 2 have the same
kind of disorder, in which the ratio of the main and flipped molecules is
3:1. To avoid confusion, only the main molecules are depicted in the
following description. Molecules 1 and 2 have intramolecular S⋯O
interaction with the distance of about 2.8–2.9 Å, which result in a planar
The Dth-TIIG molecules assume two disordered orientations that are
connected by a two-fold axis along the molecular long axis (Fig. 5a).
4

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
Fig. 3. (a) Crystal structure of CS viewed along the b axis. (b) Molecular overlap mode. (c) Crystal structure viewed along the c axis. (d) Molecular network with
intermolecular interactions. (e) Crystal structure viewed along the molecular short axis (Columns 2 and 4 are omitted for clarity). Distances in (a) and (d) are
in angstroms.
associated with the C-centered lattice. These molecular layers are
Table 3
stacked along the c axis, in which the adjacent layers are alternately
Transfer integrals of CS.
tilted in reverse directions, similarly to Dph-TIIG (Fig. 5g) [21]. Con-
t (meV)
tb
tp
tq
tr
trary to the ideal brickwork structure [21,58], the transfer integrals are
not uniform (Table 5), because two kinds of Molecules are present.
Dfu-TIIG crystallizes in a triclinic system (Fig. 6), in which two units
of half molecules are crystallographically independent. This molecule
has similar intramolecular S⋯O interactions with the distance of 2.866
(2) Å for Molecule 1 and 2.802(2) Å for Molecule 2 (Fig. 6a and b),
HOMO
LUMO
À 44.2
2.56
18.5
0.80
1.98
53.6
À 0.82
À 1.07
TIIG skeleton (Fig. 5b). However, the
α-substituted thiophenes are
twisted from the TIIG plane by about 12–14� for Molecule 1 and by
about 8–11� for Molecule 2 (Fig. 5b and c); these observations agree
with the results from ADF calculation (Supplementary data). The outer
thiophene sulfur atom points to the opposite side relative to the inner
thiophene sulfur atom. Hydrogen bonds are formed between the parallel
molecules with the distance of about 2.77(2) ~ 2.87(2) Å for N(H)⋯O
(Fig. 5d and e). The molecules in a column are considerably slipped
along the molecular short axis, and constitute a brickwork structure
(Fig. 5f), where the molecular layers are parallel to the ab plane
(Fig. 5g). Within the same layer, Molecules 1 are aligned in the same
horizontal level, and Molecules 2 construct the next level above and
below the layer of Molecules 1 (Fig. 5f). The repetition of Molecules 1 is
which yield a planar TIIG skeleton. The α-substituted furans in Dfu-TIIG
are arranged almost in the identical planes to the TIIG skeletons, where
the furan oxygen atoms are located on the same side as the thiophene
sulfur atoms (Fig. 6a and b); this arrangement is different from Dth-
TIIG, and is a result of the intramolecular attractive force between the
sulfur and oxygen atoms even though a short contact between these
species is not observed. The stacked molecules are slipped along the
molecular short axis as well as along the molecular long axis, and form a
uniform stacking structure (Fig. 6c). Molecules 1 and 2 construct
respectively independent columns. The adjacent columns along the c
axis are tilted by about 85� (Fig. 6d), in which hydrogen bonds with the
distance of about 2.725(3) ~ 3.015(3) Å are formed between N(H)⋯O of
5

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
Fig. 4. (a) Molecular structure of NS. (b) Molecular network with the intermolecular interactions. (c) Hydrogen bonds in the Molecule 1 and (d) Molecule 2 layers.
Crystal structure viewed along the molecular short axis of (e) Molecule 1 and (f) Molecule 2. Crystal structure viewed along the molecular long axis of (g) Molecule 1
and (h) Molecule 2. All distances are in angstroms.
Molecules 1 and 2 (Fig. 6e). Consequently, Dfu-TIIG forms a relatively
planar molecular structure, and these molecules construct a one-
dimensional conduction path with interplanar spacing of 3.37 Å along
the a axis (Fig. 6f and g). The calculated transfer integrals also indicate
that the conducting path is one-dimensional (Table 6).
space group Pna21, in which one molecule is crystallographically inde-
pendent (Fig. 7). This molecule has intramolecular S⋯O interactions
with distances of 2.796(9) and 2.86(1) Å in the TIIG skeleton, like Dth-
TIIG and Dfu-TIIG (Fig. 7a). The torsion angles of the pyrazolyl planes
against the TIIG plane are 20.0� and 32.2�, respectively, and the phenyl
groups are twisted by 55.4� and 50.5� (Fig. 7b). The bulky substituents
Bis(1-ph-5-py)-TIIG crystallizes in an orthorhombic system with the
6

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
TIIG. The structure of Dth-TIIG is similar to the monomer unit in
polymers composed of TIIG and thiophene, the present finding suggests
Table 4
Transfer integrals of NS.
the possibility that the π-stacking structure is maintained also in such
t (meV)
ta1
ta2
tb
tr1
tr2
polymers despite the disordered main chains. By contrast, CS, Dfu-TIIG,
and Bis(1-ph-5-py)-TIIG have uniform stacks, whereas NS has a
dimerized column due to the nonplanar molecule. In general, unsym-
metrically pyridine-condensed isoindigo analogs adopt a nonplanar
molecular structure owing to the unbalanced electron density, although
symmetrically pyridine-condensed isoindigo forms a planar molecular
structure [59].
Molecule 1
Molecule 2
HOMO
LUMO
t (meV)
HOMO
LUMO
3.50
34.1
tb1
58.6
39.6
tb2
7.51
À 11.4
ta
À 8.75
5.71
tq1
À 1.16
2.47
tq2
8.46
22.8
55.3
38.9
7.50
À 12.9
À 8.37
5.67
À 1.16
2.50
interrupt the intermolecular interactions, so Bis(1-ph-5-py)-TIIG forms
a uniform stacking structure with interplanar spacing of 3.27 Å, in which
the molecules are slipped along the molecular long axis and the short
axis (Fig. 7c and d). A few hydrogen bonds form between the nitrogen of
the pyrazole and the amino groups of the TIIG skeleton (Fig. 7e), but this
molecule constructs a one-dimensional conduction path along the b axis
in the crystal (Fig. 7 caption).
Table 5
Transfer integrals of Dth-TIIG.
t (meV)
ta1
ta2
tb1
tb2
tr1
tr2
In summary, the brickwork structure of Dph-TIIG is maintained in
Dth-TIIG, although with lowered symmetry. Although molecular flip-
ping disorder is observed, the brickwork structure is analogous to Dph-
HOMO
LUMO
À 25.6
À 24.7
À 24.5
À 23.5
8.23
8.18
8.19
5.51
24.1
22.6
25.7
20.6
Fig. 5. (a) Molecular structure of Dth-TIIG; the major molecules are bonded and the minor molecules are non-bonded. (b) Intermolecular hydrogen bonds in Dth-
TIIG, and (c) a view along the molecular short axis. (d) Hydrogen bonds of Molecules 1 and (e) Molecules 2. Crystal structure viewed (f) along the molecular long axis
and (g) along the a axis. For clarity, the minor molecules are omitted except in (a). Distances in (b) are in angstroms.
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D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
Fig. 6. Molecular structure of Dfu-TIIG for (a) Molecule 1 and (b) Molecule 2. (c) Molecular overlap mode. Crystal structure viewed (d) along the c axis and (e) along
the a axis. Crystal structure viewed (f) along the molecular long axis of Molecule 1 and (g) along the molecular long axis of Molecule 2. Distances in (a), (b), and (e)
are in angstroms.
These values correspond to half of |aþc| (10.7 Å); this similarity in-
Table 6
dicates that the (101) plane is aligned parallel to the substrate (Fig. S6).
Transfer integrals of Dfu-TIIG.
Therefore, the molecules are oriented in the side-on arrangement, in
which the molecular tilt angle is 25� from normal to the substrate.
In NS thin films, XRD results estimate the d-spacing as 10.1 Å, which
corresponds to qz ¼ 0.62 ÅÀ 1 (¼ 10.1 Å) in the GIWAXS pattern (Fig. 8d).
t (meV)
ta1
ta2
tc1
tc2
tc3
HOMO
LUMO
À 27.1
À 15.4
À 0.92
0.84
1.43
10.4
16.0
22.5
2.49
À 5.18
This value is very close to (c/2)∙sin(α)∙sin(β) (¼9.8 Å) of the crystal
lattice, and the peak observed in the in-plane GIWAXS pattern at qxy
¼
3.3. Thin film properties
0.84 ÅÀ 1 (¼ 7.45 Å) corresponds to the crystallographic a axis (7.45 Å).
These observations indicate that the crystallographic ab plane is aligned
parallel to the substrate (Fig. S7). Therefore, the molecular tilt angle is
11� from the substrate normal in the edge-on orientation.
To investigate the thin-film morphologies and the molecular orien-
tation, X-ray diffraction (XRD), and grazing-incidence wide-angle X-ray
scattering (GIWAXS) measurements were carried out (Fig. 8). All thin
films except for Bis(1-ph-5-py)-TIIG show sharp XRD peaks (Fig. 8a and
b), and the extracted d values are in good agreement with the out-of-
plane d-spacings of GIWAXS. In the in-plane (qz ¼ 0) GIWAXS, NS ex-
hibits a single peak; the other thin films show only the in-plane peaks of
TTC.
In Dth-TIIG thin films, the d-spacing extracted from XRD is 17.0 Å,
which corresponds to qz ¼ 0.37 ÅÀ 1 (¼ 17.0 Å) in the GIWAXS pattern
(Fig. 8e). However, this value is considerably larger than half (¼15.3 Å)
of the crystallographic c axis. This discrepancy suggests the molecules in
the thin films are less tilted than the molecules in the single crystals. The
thin-film tilt angle β0 is calculated from l ¼ d/cos(β0), where l ¼ 18.2 Å is
the molecule length estimated from the crystal structure, and d ¼ 17.0 Å
In CS thin films, XRD results estimate the d-spacing as 10.7 Å, which
is close to qz ¼ 0.58 ÅÀ 1 (¼ 10.8 Å) in the GIWAXS pattern (Fig. 8c).
8

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
Fig. 7. (a) Molecular structure of Bis(1-ph-5-py)-TIIG and (b) a view along the molecular short axis. (c) Molecular overlap mode. Crystal structure viewed (d) along
the molecular short axis and (e) along the b axis. Transfer integrals of the HOMO-HOMO interactions are tb ¼ À 55.6, tq1 ¼ À 6.45, and tq2 ¼ À 3.41 meV. Transfer
integrals of the LUMO-LUMO interactions are tb ¼ À 29.9, tq1 ¼ À 2.68, and tq2 ¼ 0.16 meV. Distances in (a) and (e) are in angstroms.
is the d-spacing in the XRD pattern. The result is β’ ¼ 21�, which is
remarkably smaller than the single-crystal value, 35�. In the thin films,
the molecules are packed more closely to the perpendicular direction
than the single crystals are (Fig. S8).
film of Bis(1-ph-5-py)-TIIG is not crystalline, because the bulky phenyl
substituents inhibit the crystallization during deposition on the
substrate.
From the XRD pattern of Dfu-TIIG thin films, the d-spacing is esti-
3.4. Transistor properties
mated as 14.6 Å, which is identical to qz ¼ 0.43 ÅÀ 1 (¼ 14.6 Å) in the
GIWAXS pattern (Fig. 8f). This d-spacing is close to c∙sin(α)∙sin(β)
As expected, these molecules exhibit ambipolar transistor charac-
teristics under vacuum (Fig. 9 and Table 7). CS and NS show comparable
carrier mobilities to TIIG because they form one-dimensional stacking
structures [21]. The comparatively large VT is also in common with
TIIG; large VT difference for electron and hole more than 100 V is
characteristic of single-component ambipolar molecules [9]. For CS, the
transfer integrals of LUMO along the diagonal direction (tp in Table 3) as
(¼14.3 Å) of the crystal lattice; this similarity indicates that the ab plane
is aligned parallel to the substrate (Fig. S9). Therefore, the molecules are
arranged with a side-on orientation, and the molecular tilt angles are 4�
for Molecule 1 and 17� for Molecule 2 from the substrate normal.
In Bis(1-ph-5-py)-TIIG thin films, XRD and GIWAXS show no
distinct peaks except for the TTC peaks (Fig. S10). Therefore, the thin
9

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
mobilities than CS. As shown in the thin-film properties, the NS mole-
cules are arranged more closely than CS to the perpendicular direction
to the substrate [58]. The side-on orientation is another factor of the low
mobilities of CS.
Owing to the balanced transfer integrals (Tables 5 and 6), Dth-TIIG
and Dfu-TIIG show well-balanced carrier mobilities, which are higher
by one or two orders of magnitude than are those in CS and NS. The
reduced reorganization energy due to the extended π-skeleton is another
factor of the improved transistor properties (Table S2). In addition, the
two-dimensional conduction path coming from the brickwork structure
is also important for the increased mobilities. Compared to Dph-TIIG
with a similar crystal structure [21], Dth-TIIG exhibits reduced tran-
sistor properties because of the flipping disorder and the reduced sym-
metry, but the carrier mobilities are comparable to those of isoindigo
[18]. In case of Dfu-TIIG, the molecular orientation is very close to the
perpendicular to the substrate; this orientation is favorable for the
transistor properties [58]. However, the use of furyl instead of phenyl as
a substituent, causes side-on orientation and reduced dimensionality of
the conduction path; these changes are the causes of the reduced carrier
Table 7
Summary of the transistor properties.
hole
electron
Compound
μav
[
μmax
]
VT (V)
Ion
/
μav
[
μmax
]
VT
Ion
/
(cm2 VÀ 1
Ioff
(cm2 VÀ 1
(V)
Ioff
sÀ 1
)
sÀ 1
)
TIIG [21]
CS
1.3 � 10À 3
[1.6 � 10À 3
3.5 � 10À 4
[1.1 � 10À 3
5.5 � 10À 4
[1.3 � 10À 3
À 62.4
À 97
104
104
104
106
105
106
103
2.5 � 10À 3
[2.9 � 10À 3
1.5 � 10À 3
[2.8 � 10À 3
6.4 � 10À 3
[7.6 � 10À 3
55.8
61
106
105
105
105
105
105
102
]
]
]
]
]
]
Fig. 8. XRD patterns of (a) CS and NS, and (b) Dth-TIIG and Dfu-TIIG.
GIWAXS patterns of (c) CS, (d) NS, (e) Dth-TIIG, and (f) Dfu-TIIG.
NS
À 79
68
well as the stacking direction (tb in Table 3) are much larger than those
of HOMO. For NS, the transfer integrals of HOMO between the dimer-
ized molecules (ta1 and tb1 in Table 4) are considerably smaller than
those of LUMO, but the transfer integrals of LUMO are well balanced
along the stacking direction (ta1, ta2, tb1, and tb2 in Table 4). These are the
reasons that both CS and NS exhibit slightly n-dominant ambipolar
transistor properties even though reorganization energies for electron
are larger than those for hole (Table S2). NS shows higher carrier
Dph-TIIG
[21]
0.084 [0.12]
À 13.9
À 17
0.102 [0.13]
77.1
70
Dth-TIIG
0.033
0.029
[0.055]
0.031
[0.039]
0.029
Dfu-TIIG
À 31
80
[0.056]
[0.041]
Bis(1-ph-5-
py)-TIIG
5.1 � 10À 5
À 28
2.3 � 10À 6
22
[6.4 � 10À 5
]
[3.1 � 10À 6
]
Fig. 9. Transfer characteristics of (a) CS, (b) NS, (c) Dth-TIIG, (d) Dfu-TIIG, and (e) Bis(1-ph-5-py)-TIIG.
10

D. Yoo et al.
Dyes and Pigments 180 (2020) 108418
mobilities compared to Dph-TIIG [21]. Bis(1-ph-5-py)-TIIG shows
slightly p-dominant ambipolar properties, and well-balanced threshold
voltages in the hole and electron transport. However, thin films
composed of this compound are rather amorphous even though the
molecules form a uniform stacking structure in the single crystal, so
OFETs that use the thin-films exhibit a reduced on/off current ratio and
considerably reduced carrier mobilities.
analysis. Akihiro Kohara: Formal analysis. Haruki Sugiyama: Soft-
ware, Formal analysis. Minoru Ashizawa: Investigation, Data curation,
Writing - review & editing, Supervision. Tadashi Kawamoto: Formal
analysis, Validation, Writing - review & editing. Hiroyasu Masunaga:
Formal analysis. Takaaki Hikima: Formal analysis. Noboru Ohta:
Formal analysis. Hidehiro Uekusa: Supervision. Hidetoshi Matsu-
moto: Supervision. Takehiko Mori: Validation, Writing - review &
editing, Supervision, Project administration.
4. Conclusions
Acknowledgments
In this study, we prepared five compounds: two of them are N-
unsubstituted TIIG analogs, which are unsymmetrically substituted
thieno-benzo-isoindigo (CS) and thieno-pyridine-isoindigo (NS). Other
The authors are grateful to the Center for Advanced Materials
Analysis, Tokyo Institute of Technology, for X-ray diffraction measure-
ments. GIWAXS experiments were performed on BL45XU and BL40B2 at
SPring-8 with the approval of the Japan Synchrotron Radiation Research
Institute (JASRI) (Proposal No. 2016B1004, 2017A1002, and
2019A1004). This work was partly supported by JPSJ KAKENHI Grant
Number 18H02044, and Takahashi Industrial and Economic Research
Foundation.
three compounds are
α-substituted TIIG derivatives, which are
dithienyl-TIIG (Dth-TIIG), difuryl-TIIG (Dfu-TIIG), and bis(1-phenyl-5-
pyrazolyl)-TIIG (Bis(1-ph-5-py)-TIIG). CS and NS show almost the
same energy levels, which are slightly lower than that of TIIG. Dth-TIIG,
Dfu-TIIG, and Bis(1-ph-5-py)-TIIG exhibit increased electron-donating
ability and narrow energy gaps due to the extended π-skeleton. The
HOMO and LUMO levels of these molecules are suitable for the ambi-
polar OFETs. Single crystals of CS, Dfu-TIIG, and Bis(1-ph-5-py)-TIIG
construct uniform stacks of the planar molecular cores, but NS forms
twisted molecular cores with the slight dimerization. Dth-TIIG shows
higher dimensionality of the conduction path due to the brickwork
structure, but this molecule has flipping disorder that impedes charge
transport. CS and NS form many intra- and intermolecular interactions
through S⋯S, S⋯O short contacts, and hydrogen bonds. Dth-TIIG and
Dfu-TIIG also form hydrogen bonds with the adjacent molecules. Such
interactions are favorable for the compact molecular packing and the
effective charge transport.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108418.
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CRediT authorship contribution statement
Dongho Yoo: Conceptualization, Formal analysis, Resources,
Writing - original draft, Visualization. Tsukasa Hasegawa: Formal
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