Bulky Phenylalkyl Substitutions to Bisthienoisatins and Thienoisoindigos

Article abstract of DOI:10.1021/acs.cgd.0c00087

In order to investigate the effects of bulky substituents on the crystal structures and packing modes, N-benzyl (Bn) and N-2-phenylethyl (EtPh) substituents are introduced in bisthienoisatin (BTI), thienoisoindigo (TIIG), and dibenzothienoisoindigo (DBTII). These molecules maintain uniform stacking structures, though EtPh-BTI has a two-dimensional slipped-herringbone structure. The benzyl groups are largely distorted from the molecular core, and thin films of Bn-TIIG and Bn-DBTII show poor quality due to the two kinds of molecular orientations. In contrast, the 2-phenylethyl-substituted molecules enable suitable molecular packing owing to the ethylene spacer and show relatively good thin-film qualities as well as much improved transistor properties. The BTI derivatives show only electron transport, but other compounds exhibit ambipolar transistor properties. In particular, EtPh-TIIG and EtPh-DBTII show maximum hole mobilities of about 0.04-0.05 cm² V^-1 s^-1 together with moderate electron mobilities.

Full text of DOI:10.1021/acs.cgd.0c00087

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Article
Bulky Phenylalkyl Substitutions to Bisthienoisatins and
Thienoisoindigos
Dongho Yoo,* Akihiro Kohara, Minoru Ashizawa,* Tadashi Kawamoto, Hiroyasu Masunaga,
Noboru Ohta, Hidetoshi Matsumoto, and Takehiko Mori*
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ABSTRACT: In order to investigate the effects of bulky substituents on the crystal structures and packing modes, N-benzyl (Bn)
and N-2-phenylethyl (EtPh) substituents are introduced in bisthienoisatin (BTI), thienoisoindigo (TIIG), and dibenzothienoi-
soindigo (DBTII). These molecules maintain uniform stacking structures, though EtPh-BTI has a two-dimensional slipped-
herringbone structure. The benzyl groups are largely distorted from the molecular core, and thin films of Bn-TIIG and Bn-DBTII
show poor quality due to the two kinds of molecular orientations. In contrast, the 2-phenylethyl-substituted molecules enable
suitable molecular packing owing to the ethylene spacer and show relatively good thin-film qualities as well as much improved
transistor properties. The BTI derivatives show only electron transport, but other compounds exhibit ambipolar transistor
properties. In particular, EtPh-TIIG and EtPh-DBTII show maximum hole mobilities of about 0.04−0.05 cm2 V−1 s−1 together with
moderate electron mobilities.
been attempted regardless of the transporting carrier types.15
One of the ways to achieve high mobility is to control the
INTRODUCTION
The development of organic semiconductors (OSCs) has
■
molecular packing by introducing bulky substituents. In some
previous works, the introduction of bulky substituents has been
found to be effective in improving the OFET properties in
OSCs such as pentacene,16,17 [1]benzothieno[3,2-b][1]-
benzothiophene (BTBT),18,19 naphthalenediimide
(NDI),20,21 and 5,5′-bithiazolidinylidene-2,4,2′,4′-tetra-
thione.22 The bulky substituents ordinarily produce steric
hindrance to destroy the intermolecular interaction, but in
some cases, bulky groups realize molecular packings favorable
to charge transport such as herringbone and brickwork
structures.
drastically progressed in the recent 30 years for applications to
organic field-effect transistors (OFET),1 organic photo-
voltaics,2 and organic light-emitting diodes.3 Among them,
OFETs have been investigated continuously for a long period
of time.4 From the beginning of research in OFETs, p-type
OSCs have been extensively designed starting from penta-
cene,5 and the mobility has been improved from an order of
10−5 to over 10 cm2 V−1 s−1.6 The development of n-type and
ambipolar OSCs is lagging in comparison to p-type OSCs, but
the research trend has largely shifted to these OSCs, and many
promising small molecules and novel polymer semiconductors
have achieved electron mobilities of up to 10 cm2 V−1 s−1.7−12
Recently, a great number of studies have been devoted to
process and interface engineering in order to improve the
mobility through the device fabrication method.13 In addition,
molecular engineering has been investigated by modifying
molecular structures through the substitution of functional
groups and the extension of π skeletons of promising OSCs
showing high mobility.13,14 An exploration of high mobility has
Received: January 20, 2020
Revised: April 8, 2020
Published: April 8, 2020
https://dx.doi.org/10.1021/acs.cgd.0c00087
Cryst. Growth Des. XXXX, XXX, XXX−XXX
© XXXX American Chemical Society
A

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Scheme 1. Chemical Structures of (a) BTI, (b) TIIG, and (c) DBTII, Where R = Benzyl (Bn) and 2-Phenylethyl (EtPh)
Previously, we have prepared bisthienoisatin (BTI; Scheme
1a) by the oxidative coupling of thienoisatins. This molecule
has a linear planar structure and shows n-type OFET
properties owing to the four electron-withdrawing carbonyl
groups.23 Also, we have prepared N-unsubstituted thienoi-
soindigo (TIIG; Scheme 1b) by the dimerization of
thienoisatins, which has a planar molecular structure due to
an intramolecular oxygen−sulfur (S···O) interaction.24 In
addition, TIIG forms intermolecular sulfur−sulfur (S···S)
interactions and hydrogen bonds with adjacent molecules, so
that this molecule shows moderate ambipolar OFET proper-
ties.24 The derivatives with N-alkyl groups also have been
investigated extensively as a repeating unit of polymers and
small-molecule OSCs because the TIIG backbone is a
promising skeleton.25−33 Dibenzothienoisoindigo (DBTII;
Scheme 1c), in which benzene rings are condensed with
TIIG, is also a planar molecule and exhibits p-dominant
ambipolar OFET properties on a tetratetracontane (TTC)
modified substrate, showing much improved mobilities due to
the extended π skeleton.25 However, the above three kinds of
compounds have been reported only in the form with straight
and branched N-alkyl chains except for N-unsubstituted TIIG.
This has prompted us to introduce bulky phenylalkyls to BTI,
TIIG, and DBTII cores for the purpose of investigating the
effects of bulky substituents on the crystal structures and
packing modes of molecules. In previous reports, a 2-
phenylethyl group bearing an ethylene spacer has been
effective in constructing a preferable molecular packing for
carrier transport.21,22 Thus, we have prepared N-2-phenylethyl
(EtPh)-substituted BTI, TIIG, and DBTII and investigated
the correlation between the crystal structures and the OFET
properties. For comparison with the 2-phenylethyl-substituted
molecules, N-benzyl (Bn)-substituted molecules have also
been prepared (Scheme 1).
Scheme 2. Synthetic Scheme to BTI and TIIG
diffractometer with Cu Kα radiation from a rotating anode source
with a confocal multilayer X-ray mirror (Rigaku VM-spider, λ =
1.54187 Å). The structures were solved by the dual-space method
(SHELXT)34 and refined by the full-matrix least-squares method by
applying anisotropic temperature factors for all non-hydrogen atoms
using the SHELXL programs.35 The hydrogen atoms were placed at
geometrically calculated positions. A twin analysis of Bn-BTI was
conducted by PLATON because crystals of Bn-BTI have non-
merohedral twins.36
The thin-film X-ray diffraction (XRD) patterns were taken by using
a Philips X’Pert-Pro-MRD instrument with Cu Kα radiation (λ =
1.540598 Å). Two-dimensional grazing incidence wide-angle X-ray
scattering (GIWAXS) patterns were obtained at SPring-8 on beamline
BL40B2.37 The samples were irradiated at a fixed incidence angle on
the order of 0.125°, and the GIWAXS patterns were recorded with a
2-D image detector (PILATUS3 S 2M). The wavelength of the X-ray
beam was 1 Å (energy of 12.39 keV), and the camera length was
350.4 mm.
EXPERIMENTAL SECTION
■
Syntheses. BTI was synthesized from 3-bromothiophene via
introduction of phenylalkyl (1), cyclization to thienoisatin (2), and
the AgF-mediated oxidative coupling of 2 with Pd(OAc)2 as a catalyst
(Scheme 2).23 TIIG was synthesized from 2 by a homocoupling
reaction using Lawesson’s reagent (Scheme 2). DBTII was prepared
by the same route as for TIIG from 3-bromobenzo[b]thiophene
(Scheme 3).
RESULTS AND DISCUSSION
■
Crystal Preparation. Single crystals of EtPh-BTI (260 °C), Bn-
TIIG (230 °C), Bn-DBTII (320 °C), and EtPh-DBTII (310 °C)
were obtained from sublimation under an N2 atmosphere (flow rate,
50 mL/min; pressure, 10 Pa). Crystals of Bn-BTI were obtained from
a chloroform solution, and single crystals of EtPh-TIIG were obtained
from a toluene solution.
Electronic Properties. The highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital
(LUMO) levels are estimated by using cyclic voltammetry and
ultraviolet visible (UV−vis) absorption spectroscopy. Bn-BTI
and EtPh-BTI show two reversible reduction waves similar to
those of the previous alkyl-BTI (Figure 1a),23 whereas Bn-
DBTII and EtPh-DBTII exhibit two reversible oxidation
X-ray Diffraction. Single-crystal X-ray structure analyses were
carried out by a Rigaku R-AXIS RAPID II imaging plate
B
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Scheme 3. Synthetic Scheme to DBTII
levels are estimated from the onset potentials (Eonset). In other
molecules, the HOMO or LUMO levels are estimated from the
first half-wave oxidation (DBTIIs) or reduction (BTIs)
potentials (E1/2) as given in Table 1. The HOMO−LUMO
gaps (Egopt) are extracted from λonset values of the solution
absorption spectra. The LUMO levels of BTIs are about −3.8
eV, which denotes relatively low lying LUMO levels due to the
four electron-withdrawing carbonyl groups. TIIGs and DBTIIs
are expected to show ambipolar OFET properties because the
LUMO levels are lower than −3.15 eV and the HOMO levels
are higher than −5.6 eV.38 The absorption spectra of the thin
films are red-shifted owing to the intermolecular interactions in
the solid state (Figures 1b,d,f). In the thin films of TIIGs and
DBTIIs, the absorption bands are almost identical regardless
of the substituents (Table 2). However, the absorption band of
Table 2. Optical Properties of the Thin Films
compd
R
λonset (nm)
λmax (nm)
359, 569
343, 607
388, 412, 635
388, 409, 636
306, 439, 684, 747
306, 443, 690, 754
Egopt (eV)
BTI
Bn
EtPh
Bn
EtPh
Bn
EtPh
646
687
685
679
798
799
1.92
1.80
1.81
1.82
1.55
1.55
TIIG
DBTII
Figure 1. Cyclic voltammograms of (a) BTIs, (c) TIIGs, and (e)
DBTIIs. UV−vis absorption spectra of (b) BTIs, (d) TIIGs, and (f)
DBTIIs.
EtPh-BTI is shifted to long wavelength much more than for
Bn-BTI. This means intermolecular interactions are strong in
EtPh-BTI in comparison with Bn-BTI (see Crystal Struc-
tures). Here, the excited energy levels split greatly, resulting in
a narrow energy gap in the solid state.
waves (Figure 1e). However, Bn-TIIG and EtPh-TIIG show
two irreversible oxidation peaks (Figure 1c); thus the HOMO
Table 1. Summary of Electrochemical and Optical Properties in the Solutions
compd
R
E1/2 (V)
Eonset (V)
ELUMO (eV)
λonset (nm)
λmax (nm)
360, 518
364, 521
380, 400, 548
381, 400, 551
306, 421, 441, 647
306, 418, 438, 649
Egopt (eV)
EHOMO (eV)
a
BTI
Bn
EtPh
Bn
EtPh
Bn
EtPh
−0.97
−1.00
−3.83
−3.80
594
600
639
643
758
756
2.09
2.06
1.94
1.93
1.63
1.64
−5.92
a
−5.86
b
TIIG
0.57
0.52
−3.43
−5.37
−5.32
−5.28
−5.24
b
−3.39
b
DBTII
0.48
0.44
−3.65
b
−3.60
a
b
Estimated by subtracting Egopt from ELUMO. Estimated by adding Egopt to EHOMO
.
C
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Crystal Structures. In all crystals, the molecules are
centrosymmetric, and the half-molecule is crystallographically
independent. Bn-DBTII crystallizes in a triclinic system, and
the others crystallize in monoclinic systems. All molecules in
the monoclinic structures form stacks in which the molecules
are oppositely tilted from the molecules in the adjacent
columns (EtPh-BTI, Bn-TIIG, and EtPh-DBTII) or from the
adjacent layers (Bn-BTI and EtPh-TIIG), although all
molecules are parallel in Bn-DBTII. In general, the molecular
cores are perfectly planar, but the phenyl rings are tilted,
exceeding 70° in Bn and less than 22° in EtPh (Figure 2a,e,
Figure 3a,e, Figure 4a,e, and Table 7). Crystallographic data
are given in Table 3.
Figure 3. (a) Molecular structure of Bn-TIIG and (b) molecular
overlap mode. (c) Molecular packing of Bn-TIIG and (d) crystal
structure viewed along the b axis. (e) Molecular structure of EtPh-
TIIG and (f) molecular overlap mode. Crystal structure of EtPh-
TIIG viewed (g) along the molecular long axis and (h) along the b
axis. For clarity, the substituents are omitted in (b), (c), (f), and (g).
of 2.66 Å are formed in CO···H along the c axis (Figure 2g).
These interactions are the origins of the largely red shifted
solid-state absorption bands in comparison with Bn-BTI. The
dihedral angle between the adjacent columns is nearly
perpendicular (84.8°) (Figure 2f). Nonetheless, the large
displacement along the long axis of the molecular core is
disadvantageous to charge transport in comparison to the ideal
herringbone structure,39 because the transfer integrals are
comparatively small (Table 4).
The crystal structure of Bn-TIIG is isostructural with N-
unsubstituted TIIG.24 Bn-TIIG forms a uniform stacking
structure along the b axis with an interplanar spacing of 3.37 Å
(Figure 3c). In Bn-TIIG and EtPh-TIIG, the molecular long
axis is defined by the substituents and is parallel to the N−N
direction. This is the perpendicular direction in Scheme 1b,
and the intercolumnar direction corresponds to the horizontal
direction in Scheme 1b (molecular short axis). In Bn-TIIG,
molecules in the adjacent columns along the a + c axis are
tilted to opposite directions (73.0°) (Figure 3c). In this
direction, S···O intermolecular interactions with a distance of
3.105(1) Å are formed (Figure 3d); this is similar to the
hydrogen bonds of N-unsubstituted TIIG.24
Figure 2. (a) Molecular structure of Bn-BTI. (b) Crystal structure of
Bn-BTI viewed along the c axis. (c) Molecular overlap mode. (d)
Crystal structure of Bn-BTI viewed along the long axis of the
molecular core. (e) Molecular structure of EtPh-BTI. Crystal
structure of EtPh-BTI viewed (f) along the long axis of the molecular
core and (g) along the b axis. For clarity, the substituents are omitted
in (c), (d), (f), and (g).
Bn-BTI forms a uniform stacking structure along the a axis
with an interplanar spacing of 3.40 Å (Figure 2b). The stacked
molecules are slipped mainly along the long axis of the
molecular core (Figure 2c). Molecules in the same ac layer are
parallel, but molecules in the adjacent layers are alternately
tilted (86.0°); these layers are stacked along the b axis (Figure
2b). An S···S interaction with a distance of 3.511(2) Å and an
S···O intermolecular interaction with a distance of 3.263(4) Å
are observed along the c axis (Figure S5c). Accordingly, the
transfer integrals (tc and tp) in the transverse direction are
comparable to ta along the stacking axis (Figure 2d and Table
4); these transfer integrals suggest a comparatively isotropic
conduction, though the magnitude is not large.
The crystal structure of EtPh-BTI looks like a herringbone
structure (Figure 2f), but the molecules are largely slipped
along the long axis of the molecular core (Figure 2g and Figure
S6b). This structure involves side by side intermolecular
interactions (Figure 2f), and hydrogen bonds with a distance
EtPh-TIIG forms a uniform stacking structure along the b
axis with an interplanar spacing of 3.42 Å (Figure 3g). The
adjacent columns along the a axis are parallel (Figure 3h),
whereas the molecules in the adjacent layers are alternately
tilted by about 72.0° in opposite directions; the layers are
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Table 4. Electron Transfer Integrals (meV) of BTIs
R
ta
tb
tc
tp
tr
Bn
EtPh
−32.9
−15.3
21.8
8.3
27.8
stacked along the c axis (Figure 3g). EtPh-TIIG has a large
intramolecular torsion angle (21.3°) between the TIIG plane
and the phenyl ring of EtPh in comparison to other EtPh-
substituted molecules (Figure 3e). Between CO of the
molecular core and a phenyl hydrogen of EtPh, a hydrogen
bond is formed (distance 2.41 Å) (Figure 3h); thus, this
interaction indicates a relatively large intramolecular torsion
angle.
In Bn-TIIG, the transfer integral along the stacking axis is
larger for the LUMO than for the HOMO (Table 5). In
Table 5. Transfer Integrals (meV) of TIIGs
R
ta
tb
tq
4.0
−14.3
Bn
HOMO
LUMO
HOMO
LUMO
−28.7
42.9
−118.1
67.2
EtPh
−3.7
−3.4
contrast, EtPh-TIIG has a particularly large transfer integral
for the HOMO. This difference comes from the slip distance
along the molecular short axis, though the slip distance along
the molecular long axis is the same (Figure 3b,f and Figures S7
and S8). The large displacement in Bn-TIIG results in a ring-
overatom structure (Figure 3b), though the comparatively
small displacement in EtPh-TIIG results in a ring-overbond
structure (Figure 3f).
In Bn-DBTII and EtPh-DBTII, the molecular long axis is
defined by the molecular core and the substituents block the
molecular sides. These molecules form uniform stacking
structures with interplanar spacings of 3.41 Å along the a
and b axes, respectively (Figure 4). In Bn-DBTII, all molecules
are parallel, and an intercolumnar interaction exists along the b
Figure 4. (a) Molecular structure of Bn-DBTII and (b) molecular
overlap mode. Crystal structure of Bn-DBTII viewed (c) along the a
axis and (d) along the long axis of the molecular core. (e) Molecular
structure of EtPh-DBTII and (f) molecular overlap mode. (g)
Molecular packing of EtPh-DBTII and (h) crystal structure viewed
along the b axis. For clarity, the substituents are omitted in (b), (d),
(f), and (g).
Table 3. Crystallographic Data
Bn-BTI
EtPh-BTI
Bn-TIIG
EtPh-TIIG
Bn-DBTII
EtPh-DBTII
chem formula
formula wt
cryst syst
space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
C26H16N2O4S2
484.54
monoclinic
P21/n
4.91789(9)
30.6056(6)
7.66360(14)
90
100.2339(9)
90
C28H20N2O4S2
512.60
monoclinic
P21/c
13.6380(3)
7.18687(16)
13.0238(3)
90
108.7680(13)
90
C26H18N2O2S2
454.56
monoclinic
P21/n
12.6820(3)
5.66961(13)
15.4096(4)
90
110.6364(12)
90
C28H22N2O2S2
482.61
monoclinic
C2/c
19.5477(6)
5.60960(19)
22.1929(7)
90
104.1203(17)
90
C34H22N2O2S2
554.68
triclinic
C36H26N2O2S2
582.73
monoclinic
P21/c
14.2197(3)
6.28460(11)
15.7764(3)
90
95.9370(7)
90
P1
̅
6.37399(12)
8.36881(15)
12.9434(2)
104.1072(7)
98.5380(7)
100.3249(7)
645.27(2)
1
γ (deg)
V (Å3)
Z
1135.13(4)
2
1208.65(5)
2
1036.89(4)
2
2360.02(13)
4
1402.30(4)
2
total no. of rflns
no. of unique rflns (Rint)
Dcalcd (g/cm3)
13117
2062 (0.0576)
1.418
0.0671
0.2239
13557
2214 (0.0353)
1.408
0.0486
0.1711
11296
1899 (0.0552)
1.456
0.0445
0.1194
12872
2162 (0.0328)
1.358
0.0516
0.1767
7637
2332 (0.0498)
1.427
0.0476
0.1318
15470
2575 (0.0407)
1.380
0.0403
0.1133
a
R1
b
wR2
GOF
1.130
1.140
1.081
1.124
1.175
1.153
T (K)
298
275
275
275
298
298
a
b
R1(F2 > 2σ(F2)). wR2(F2) for all observed reflections.
E
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axis (Figure 4c). The phenyl group of Bn-DBTII stretches
toward the c axis facing the phenyl group of the next layer
molecule (Figure 4c). The molecular cores construct a one-
dimensional conduction path along the stacking axis because
there are no obvious intercolumnar short contacts (Figure 4d).
In EtPh-DBTII, molecules in the adjacent columns aligned
along the c axis are alternately tilted by about 65.8° in opposite
directions (Figure 4g). Along this axis, intermolecular
hydrogen bonds are formed between CO of the molecular
core and the hydrogen of the condensed benzene ring (Figure
4h). The overlap modes of Bn-DBTII and EtPh-DBTII are
approximately the same (Figure 4b,f). Along the short axis of
the DBTII plane, Bn-DBTII is slightly slipped while EtPh-
DBTII is not slipped (Figures S9 and S10). Consequently, as
reflected in the transfer integrals, these molecules show the
same tendency that overlaps in the HOMO are much larger
than overlaps in the LUMO along the stacking direction
(Table 6).
ene spacer is more effective in forming an undistorted
molecular structure.
Only Bn-BTI forms an S···S interaction, and Bn-BTI and
Bn-TIIG have intermolecular S···O interactions. In TIIGs and
DBTIIs, intramolecular S···O interactions which guarantee the
molecular core planarity are observed (Figures S7a−S10a).
Thin-Film Properties. By measurement of thin-film XRD
and GIWAXS, molecular orientations in the thin films are
investigated (Figure 5). Bn-BTI and EtPh-substituted
molecules show sharp XRD peaks, but weak XRD peaks are
observed in Bn-TIIG and Bn-DBTII. The extracted d values
agree well with the out-of-plane peaks of GIWAXS (Table 8).
In some cases, in-plane peaks are observed as well. Many sharp
peaks are observed in EtPh-TIIG and EtPh-DBTII, indicating
good crystallinity of these thin films.
In Bn-BTI, the d spacing is estimated to be 15.3 Å from
XRD and GIWAXS (qz = 0.41 Å−1). This value corresponds to
half of the b axis. In addition, in the in-plane GIWAXS, qxy
=
0.82 Å−1 (=7.65 Å) and qxy = 1.30 Å−1 (=4.82 Å) are observed,
which are very close to the values for c sin α sin β (=7.54 Å)
and a sin β sin γ (=4.84 Å), respectively. Therefore, the
crystallographic ac plane is aligned parallel to the substrate,
where the tilt angle of the BTI plane from the substrate normal
is 43° (Figure S12).
Table 6. Transfer Integrals (meV) of DBTIIs
R
ta
tb
tc
tr
Bn
HOMO
LUMO
HOMO
LUMO
57.6
−33.1
−4.3
−6.3
78.7
−1.2
1.0
EtPh
−9.0
−3.1
In EtPh-BTI, the d spacing is estimated to be 18.9 Å from
XRD and GIWAXS (qz = 0.33 Å−1). Although this value is
longer than any axes of the unit cell, the length of the
molecular long axis including EtPh is 18.6 Å and is close to the
d spacing. In addition, in the in-plane GIWAXS, the same peak
is observed at qxy = 0.33 Å−1 (=18.9 Å), indicating that the
molecular long axis is parallel to the substrate. Therefore, with
retainment of the herringbone structure, the face-on and side-
on orientations are mixed on the substrate (Figure S13). In the
side-on orientation, the tilt angle of the BTI plane from the
substrate normal is nearly 90° (84.8°). This orientation is
superposed with the face-on orientation, in which the BTI
plane is parallel to the substrate.
The XRD of Bn-TIIG shows two kinds of d spacings, 11.8
and 7.7 Å, which correspond to qz = 0.53 Å−1 (=11.8 Å) and qz
= 0.82 Å−1 (=7.65 Å) in the out-of-plane GIWAXS. The
former corresponds to the value for a sin β sin γ (=11.87 Å),
and the latter is close to half of the value for c sin α sin β (=7.3
Å). Therefore, two types of orientations coexist in the thin
films, in which the crystallographic bc and ab planes are aligned
parallel to the substrate. In the former case, the tilt angle of the
TIIG plane is 22° from the substrate normal; this is basically
the side-on orientation (Figure S14). In the latter case, the tilt
angle of the TIIG plane is 32° from the substrate normal; this
is the edge-on orientation (Figure S14). In the in-plane
GIWAXS, peaks at qxy = 0.54 Å−1 (=11.7 Å) and qxy = 0.82 Å−1
(=7.65 Å) are observed. This observation also supports the
coexistence of two kinds of orientations.
−42.8
In summary, only EtPh-BTI constructs a two-dimensional
conduction path with a herringbone structure, while the other
molecules form uniform stacking structures (Table 7). Among
Table 7. Summary of Torsion Angle and Short Contacts
with π−π Distances
hydrogen
between
bond
distance
CO···H
(Å)
interplanar
π−π
cores and
S···O
phenyl rings interaction
distance
(Å)
compd
R
(deg)
(Å)
BTI
Bn
82.3
3.263(4)
(inter)
3.40
3.37
EtPh
Bn
4.6
72.0
2.66
TIIG
3.105(1)
(inter),
2.859(1)
(intra)
EtPh
Bn
21.3
79.0
2.7
2.859(2)
(intra)
2.810(1)
(intra)
2.806(1)
(intra)
2.41
2.68
3.42
3.41
3.41
DBTII
EtPh
them, all molecules are parallel in Bn-DBTII, whereas the
adjacent columns are alternately tilted in Bn-TIIG and EtPh-
DBTII. In Bn-BTI and EtPh-TIIG, the adjacent columns are
parallel but the adjacent layers are alternately tilted in the
opposite directions. The phenyl rings of the benzyl and 2-
phenylethyl groups are not oriented parallel to the molecular
core, but the tilt angles of the phenyl rings in the 2-phenylethyl
groups are considerably smaller than those in the benzyl
groups because the ethylene spacer in the 2-phenylethyl group
enables a more appropriate orientation of the phenyl ring
(Table 7). In addition, hydrogen bonds are observed only in
the 2-phenylethyl-substituted molecules. Therefore, the ethyl-
The XRD and out-of-plane GIWAXS in EtPh-TIIG show
the same d spacing of 13.8 Å (qz = 0.456 Å−1), but this value is
larger than half of the c axis (11.1 Å). This implies that the
molecules in the thin films are less tilted than the molecules in
the single crystals. The thin-film tilt angle from the vertical
direction to the substrate (β′) is calculated from l = d/cos β′,
where l is the molecule length estimated from the crystal
structure (19.9 Å) and d is the d spacing (13.8 Å). This leads
to β′ = 46°, which is slightly smaller than the single-crystal
value of 48°. In the thin films, the molecules are oriented more
F
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Article
Figure 5. XRD patterns of (a) BTIs, (b) TIIGs, and (c) DBTIIs. GIWAXS patterns of (d) Bn-BTI, (e) EtPh-BTI, (f) Bn-TIIG, (g) EtPh-TIIG,
(h) Bn-DBTII, and (i) EtPh-DBTII.
Table 8. XRD and GIWAXS d Values (Å)
GIWAXS (hkl)
compd
R
XRD (hkl)
out of plane
in plane
BTI
Bn
EtPh
Bn
EtPh
Bn
EtPh
15.3 (020), 5.1 (060)
15.3 (020)
7.65 (001), 4.82 (100)
18.9 (molecular long axis)
11.7 (100), 7.65 (002)
18.9 (molecular long axis)
11.8 (100), 7.7 (002), 3.85 (004)
13.8 (002), 6.9 (004), 3.45 (006)
12.3 (001), 7.9 (010), 3.95 (020)
14.2 (100), 7.14 (200)
18.9 (molecular long axis)
11.8 (100), 7.65 (002), 3.85 (004)
13.8 (002)
12.3 (001), 7.91 (010), 3.95 (020)
14.2 (100), 7.13 (200)
TIIG
DBTII
12.3 (001), 7.91 (010)
closely to the perpendicular direction in comparison to the
single crystals (Figure S15).
parallel to the substrate. The tilt angle of the DBTII plane is
55° from the substrate normal in the edge-on orientation and
26° in the side-on orientation (Figure S16).
From the out-of-plane GIWAXS and XRD of EtPh-DBTII,
the d spacing is estimated to be 14.2 Å (qz = 0.44 Å−1). This
value is close to that for a sin β sin γ (=14.14 Å), indicating
that the bc plane is aligned parallel to the substrate. Therefore,
the molecules are arranged in an edge-on orientation, and the
tilt angle of the DBTII plane is 47° from the substrate normal
(Figure S17).
The XRD of Bn-DBTII shows two d spacings, 12.3 and 7.9
Å, which correspond to qz = 0.51 Å−1 (=12.3 Å) and qz = 0.79
Å−1 (=7.91 Å) in the out-of-plane GIWAXS, respectively. In
the single crystal, c sin α sin β is 12.3 Å and b sin α sin γ is 7.9
Å; thus, these values are in good agreement with the values of
extracted d spacings. In addition, in-plane GIWAXS peaks are
observed at qxy = 0.51 Å−1 (=12.3 Å) and qxy = 0.79 Å−1 (=7.91
Å). Similarly to Bn-TIIG, two types of orientations coexist in
the thin films; the crystallographic ab and ac planes are aligned
G
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Cryst. Growth Des. XXXX, XXX, XXX−XXX

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Article
Figure 6. Transfer characteristics of (a) Bn-BTI, (b) EtPh-BTI, (c) Bn-TIIG, (d) EtPh-TIIG, (e) Bn-DBTII, and (f) EtPh-DBTII.
Table 9. OFET Properties of BTIs, TIIGs, and DBTIIs
hole
)
electron
compd
Bn-BTI
measurement
μav [μmax] (cm2 V−1 s−1
VT (V)
Ion/Ioff
μav [μmax] (cm2 V−1 s−1
)
VT (V)
Ion/Ioff
under vacuum
in air
under vacuum
in air
under vacuum
under vacuum
under vacuum
under vacuum
5.4 × 10−4 [7.2 × 10−4
2.9 × 10−4 [3.7 × 10−4
6.4 × 10−3 [0.01]
]
]
60
63
67
50
44
64
35
36
105
104
105
104
104
104
104
103
EtPh-BTI
9.6 × 10−4 [1.8 × 10−3
9.4 × 10−5 [2.3 × 10−4
1.3 × 10−3 [3.5 × 10−3
5.3 × 10−5 [8.3 × 10−5
9.1 × 10−5 [1.7 × 10−4
]
]
]
]
]
Bn-TIIG
6.3 × 10−7 [1.2 × 10−6
0.017 [0.039]
2.4 × 10−3 [4.7 × 10−3
0.027 [0.051]
]
]
−41
−38
−70
−68
102
105
103
105
EtPh-TIIG
Bn-DBTII
EtPh-DBTII
Transistor Properties. BTIs show n-type transistor
characteristics under vacuum and in air because the HOMO
levels are too low to show hole transport (Table 1 and Figure
6). EtPh-BTI exhibits improved mobility of more than 1 order
of magnitude in comparison to Bn-BTI. This is attributable to
the tilt angles from the substrate normal in the thin films. In
general, when the molecules are arranged more closely to the
substrate normal, the mobility is improved.40 Even though
face-on-arranged molecules coexist in EtPh-BTI, the side-on-
arranged molecules are oriented more perpendicularly to the
substrate in comparison to Bn-BTI. However, the mobilities of
these two molecules decrease in comparison to the previously
reported alkyl-BTIs.23 The reason is roughness and many grain
boundaries in the thin films, as confirmed by AFM (Figure
S11a,b). In EtPh-BTI, a two-dimensional conduction path is
constructed in the herringbone structure, but one of the
conduction paths faces the direction vertical to the substrate in
the thin films (Figure S13). Consequently, a one-dimensional
conduction path is formed between the electrodes, leading to
the reduced mobility. Upon air exposure, BTIs show slightly
decreased OFET properties and the off current increases.
These phenomena are consistent with the relatively high
LUMO levels in comparison with the air-stable n-type
molecules, which have LUMO levels below −4.0 eV.39
However, owing to the passivation effect of the bulky
substituents,41−44 the air stability is improved in comparison
to the previously reported alkyl-BTIs.23 OFET properties of
BTIs, TIIGs, and DBTIIs are given in Table 9.
TIIGs and DBTIIs exhibit ambipolar transistor character-
istics under vacuum (Figure 6). Bn-TIIG shows n-dominant
ambipolar properties, which is consistent with the transfer
integrals (Table 5). Other molecules exhibit p-dominant
ambipolar properties, which are attributable to the large
transfer integrals between the HOMO and the better donor
abilities of those molecules as shown in the relatively high
HOMO levels (−5.2 to − 5.4 eV, Table 1), which are closer to
the work function of the gold electrodes.45 Among the four
molecules, the Bn-substituted molecules exhibit lower OFET
H
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Cryst. Growth Des. XXXX, XXX, XXX−XXX

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pubs.acs.org/crystal
Article
properties. The Bn-substituted molecules have two kinds of
orientations in the thin films, and the randomness is the origin
of the ineffective carrier transport. In comparison, improved
OFET properties are observed in the EtPh-substituted
molecules. In particular, EtPh-TIIG exhibits a maximum
hole mobility of about 0.04 cm2 V−1 s−1, which is higher than
that of N-unsubstituted TIIG.24 This comes from the
considerably large transfer integral between the HOMOs of
the stacking molecules. The electron mobility is comparable to
that of N-unsubstituted TIIG. EtPh-DBTII shows much
higher hole mobility due to the extended π skeleton with a
reduced reorganization energy for holes (Table S1). However,
in comparison to hexyl-DBTII,25 hole and electron mobilities
are reduced because of the large tilt angle of the molecular
cores from the substrate normal (47°) in the thin films (Figure
S17).
ASSOCIATED CONTENT
* Supporting Information
Crystal data (CIF) The Supporting Information is available
free of charge at https://pubs.acs.org/doi/10.1021/
acs.cgd.0c00087.
■
sı
Additional information on material synthesis, electro-
chemical properties, and optical properties, DFT
calculations, thermal properties, crystal structures, thin-
film microstructures and morphologies, and thin-film
transistor properties (PDF)
Accession Codes
CCDC 1974945−1974950 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
AUTHOR INFORMATION
Corresponding Authors
■
In this paper, we have investigated benzyl (Bn)- and 2-
phenylethyl (EtPh)-substituted bisthienoisatin (BTI), thienoi-
soindigo (TIIG), and dibenzothienoisoindigo (DBTII)
molecules. Since the energy levels depend only on the
molecular core, these molecules show HOMO and LUMO
levels similar to those of the alkyl-substituted BTI, TIIG, and
DBTII. The HOMO levels decrease in the order DBTIIs <
TIIGs < BTIs, and the LUMO levels decrease in the order
TIIGs < DBTIIs < BTIs. Therefore, BTIs show n-type OFET
properties and TIIGs and DBTIIs exhibit ambipolar OFET
properties, which are in good agreement with the expectations
from the energy levels. These molecules form uniform stacking
structures, though EtPh-BTI has a slipped herringbone
structure, and a few intermolecular interactions are observed
owing to the bulky substituents. The benzyl parts have large
torsion angles from the molecular core, while the 2-phenylethyl
parts have small torsion angles from the molecular core. This is
because the latter has an ethylene spacer that is adequate to
enable suitable molecular packing. Therefore, the 2-phenyl-
ethyl substitution is more advantageous to the carrier
transport, and this is evident from the OFET mobilities,
which are about 0.01 cm2 V−1 s−1 for electrons in EtPh-BTI
and 0.04 and 0.05 cm2 V−1 s−1 for holes in EtPh-TIIG and
EtPh-DBTII, respectively. In contrast, from the thin film XRD
and GIWAXS, it is indicated that two kinds of molecular
arrangements coexist in the thin films of the benzyl-substituted
TIIG and DBTII. This is unfavorable for the carrier transport,
which is confirmed by the OFET mobilities of Bn-TIIG and
Bn-DBTII. In TIIGs, the dominant carrier polarity in OFET is
opposite; Bn-TIIG shows n-dominant properties but EtPh-
TIIG shows p-dominant properties. This is caused by the
difference in slip distance along the molecular short axis, as
reflected in the transfer integrals.
Dongho Yoo − Department of Materials Science and
Engineering, Tokyo Institute of Technology, Tokyo 152-8552,
Japan; orcid.org/0000-0003-0886-7533;
Email: dongho312@postech.ac.kr
Minoru Ashizawa − Department of Materials Science and
Engineering, Tokyo Institute of Technology, Tokyo 152-8552,
Japan; orcid.org/0000-0002-6810-256X;
Email: ashizawa.m.aa@m.titech.ac.jp
Takehiko Mori − Department of Materials Science and
Engineering, Tokyo Institute of Technology, Tokyo 152-8552,
Japan; orcid.org/0000-0002-0578-5885;
Email: mori.t.ae@m.titech.ac.jp
Authors
Akihiro Kohara − Department of Materials Science and
Engineering, Tokyo Institute of Technology, Tokyo 152-8552,
Japan
Tadashi Kawamoto − Department of Materials Science and
Engineering, Tokyo Institute of Technology, Tokyo 152-8552,
Japan; orcid.org/0000-0002-5676-4013
Hiroyasu Masunaga − Japan Synchrotron Radiation Research
Institute (JASRI/SPring-8), Hyogo 679-5198, Japan
Noboru Ohta − Japan Synchrotron Radiation Research Institute
(JASRI/SPring-8), Hyogo 679-5198, Japan
Hidetoshi Matsumoto − Department of Materials Science and
Engineering, Tokyo Institute of Technology, Tokyo 152-8552,
Japan; orcid.org/0000-0002-4949-1184
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.cgd.0c00087
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The present work demonstrates how benzyl and 2-
phenylethyl substitutions affect the crystal structures and
OFET properties in BTI, TIIG, and DBTII. It is confirmed
that one-dimensional stacking structures are maintained in
almost all cases, even though the bulky substituents are
attached at the nitrogen face to the short-axis side of the
molecular core. These results give us a hint for designing side
substituents of small-molecule and polymer OSCs.
The authors are grateful to the Center for Advanced Materials
Analysis, Tokyo Institute of Technology, for X-ray diffraction
measurements. The authors also thank Professor Hidehiro
Uekusa for use of the Rigaku VM-spider. GIWAXS experi-
ments were performed on beamline BL40B2 at SPring-8 with
the approval of the Japan Synchrotron Radiation Research
Institute (JASRI) (Proposal No. 2019B1006). This work was
partially supported by JPSJ KAKENHI Grant Number
I
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Crystal Growth & Design
pubs.acs.org/crystal
Article
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K
https://dx.doi.org/10.1021/acs.cgd.0c00087
Cryst. Growth Des. XXXX, XXX, XXX−XXX