Advantage of arch-shaped structure on transistor performances over linear-shaped structure in dibenzothienopyrrole semiconductors

Article abstract of DOI:10.1016/j.orgel.2018.06.044

To understand the influence of molecular shape on the optoelectronic properties of heteroacenes, we report here the syntheses and characterizations of two linear- and two arch-shaped dibenzothienopyrrole (DBTP) derivatives, namely, N-hexyldibenzothieno[3,2-b:2′,3′-d]pyrrole (l-HDBTP), N-phenyldibenzothieno[3,2-b:2′,3′-d]pyrrole (l-PDBTP), N-hexyldibenzothieno[2,3-b:3′,2′-d]pyrrole (a-HDBTP) and N-phenyldibenzothieno[2,3-b:3′,2′-d]pyrrole (a-PDBTP). Their structure-property relationships have been systematically studied by optical absorption, cyclic voltammetry, single-crystal X-ray diffraction analyses and the single-crystal-based and thin-film-based field-effect transistors (FETs). The results demonstrate that the arch-shaped DBTP derivatives can modify their electronic structures and molecular arrangements by tuning of substituents, to obtain a better charge-transporting ability relative to the linear-shaped ones. The arch-shaped N-phenyl substituted a-PDBTP exhibits optimum molecular π-stacking arrangement and charge transport properties, with the hole mobility of 0.75 cm²V^?1s^?1 and 0.058 cm²V^?1s^?1 for the single-crystal and the thin-film OFETs, respectively. These results indicate that the arch-shaped DBTP core is a promising building block in the area of organic semiconducting materials.

Full text of DOI:10.1016/j.orgel.2018.06.044

Accepted Manuscript
Advantage of arch-shaped structure on transistor performances over linear-shaped
structure in dibenzothienopyrrole semiconductors
Wenjing Mu, Su Sun, Jing Zhang, Mingyang Jiao, Wentao Wang, Yanwei Liu,
Xiangnan Sun, Lang Jiang, Bozhen Chen, Ting Qi
PII:
S1566-1199(18)30335-5
10.1016/j.orgel.2018.06.044
ORGELE 4767
DOI:
Reference:
To appear in: Organic Electronics
Received Date: 19 April 2018
Revised Date: 22 May 2018
Accepted Date: 28 June 2018
Please cite this article as: W. Mu, S. Sun, J. Zhang, M. Jiao, W. Wang, Y. Liu, X. Sun, L. Jiang, B. Chen,
T. Qi, Advantage of arch-shaped structure on transistor performances over linear-shaped structure in
dibenzothienopyrrole semiconductors, Organic Electronics (2018), doi: 10.1016/j.orgel.2018.06.044.
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ACCEPTED MANUSCRIPT

ACCEPTED MANUSCRIPT
Advantage of Arch-Shaped Structure on Transistor Performances over
Linear-Shaped Structure in Dibenzothienopyrrole Semiconductors
Wenjing Mu,a Su Sun,b,d Jing Zhang,c Mingyang Jiao,a Wentao Wang,a Yanwei Liu,a Xiangnan
Sun,b Lang Jiang,c Bozhen Chen,a Ting Qi*,a
aSchool of Chemical Sciences, University of Chinese Academy of Sciences, Beijing 100049, PR
China
bKey Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in
Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, PR China
cCAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences,
Beijing 100190, PR China
dDepartment of Materials Science and Engineering, College of Science, China University of
Petroleum Beijing, Beijing 102249, PR China
* Corresponding author. E-mail address: qiting@ucas.ac.cn (T. Qi)
Keywords: Heteroacenes, Arch-shaped, Organic field-effect transistors, Organic semiconductors,
Dibenzothienopyrrole
ABSTRACT
To understand the influence of molecular shape on the optoelectronic properties of heteroacenes,
we report here the syntheses and characterizations of two linear- and two arch- shaped
dibenzothienopyrrole (DBTP) derivatives, namely, N-hexyldibenzothieno[3,2-b:2’,3’-d]pyrrole
(l-HDBTP),
N-phenyldibenzothieno[3,2-b:2’,3’-d]pyrrole
(l-PDBTP),
N-
hexyldibenzothieno[2,3-b:3’,2’-d]pyrrole (a-HDBTP) and N-phenyldibenzothieno[2,3-b:3’,2’-
d]pyrrole (a-PDBTP). Their structure-property relationships have been systematically studied by
optical absorption, cyclic voltammetry, single-crystal X-ray diffraction analyses and the single-
crystal-based and thin-film-based field-effect transistors (FETs). The results demonstrate that the
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arch-shaped DBTP derivatives can modify their electronic structures and molecular
arrangements by tuning of substituents, to obtain a better charge-transporting ability relative to
the linear-shaped ones. The arch-shaped N-phenyl substituted a-PDBTP exhibits optimum
molecular π-stacking arrangement and charge transport properties, with the hole mobility of 0.75
cm2V−1s−1 and 0.058 cm2V−1s−1 for the single-crystal and the thin-film OFETs, respectively.
These results indicate that the arch-shaped DBTP core is a promising building block in the area
of organic semiconducting materials.
1. Introduction
Over the past decades, acenes have attracted a great deal of attentions as organic
semiconductors because of their potential applications in electronic devices, such as organic
field-effect transistors (OFETs) [1−6]. Pentacene, as a typical representative of acenes, is a
landmark material due to its outstanding mobility [7−8]. However, its shortcomings are fatal, in
that pentacene is insoluble in common organic solvents and degraded rapidly in ambient
conditions, thus to limit its further development in OFETs [9−10]. Therefore, heteroacenes with
the better solubility and stability have aroused extensive attentions. Various novel heteroacenes
containing sulfur [11−16] or nitrogen [17−24] atoms have been developed. Thienoacenes
consisting of fused thiophene rings have been commonly studied, getting closer to the good
mobilities of pentacene [25−27]. Nitrogenated heteroacenes consisting of fused pyrrole rings
such as indolocarbazole and carbazolocarbazole have also offered promising results in OFETs
[28−30]. Nevertheless, with a few exceptions [31−37], heteroacenes consisting of both sulfur and
nitrogen atoms have not been commonly studied and applied in OFETs by far.
Another to be noted is that the impact of regioisomerism for heteroacenes on semiconductor
properties captures increasing attention [38−39]. Many studies focusing on the isomeric
structural adjustment indicated that a simple modification of isomers would drastically affect the
electrochemical properties [30,40−41], the optical properties [30,40−41] and the intermolecular
packing patterns [30,42], which determine the device performance. What is more,
regioisomerism can also bring change of shape about extended π-conjugated molecular backbone.
Recently, researchers gradually discovered critical roles of “shape” on the packing in the solid
state with a positive effect on the charge mobility [41]. In terms of the “shape”, to date
heteroacenes were mainly studied on two types, namely, linear shape [43−48] and angular shape
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[30,49−56]. For the angular-shaped molecules, the S-shaped was extensively researched
[30,49−50], but the arch-shaped has been just reported in recent years (Fig. 1) [51−53]. For
instance, thiophene fused dinaphthothiophene (DNT) [51] and bisbenzothienothiophene (BBTT)
[52] applied in solution-crystallized OFETs showed a hole mobility as 9.5 cm−2 V−1 s−1 and 0.6
cm−2 V−1 s−1, respectively, attributing to an almost optimized molecular arrangement. Pyrrole
fused dinaphthocarbazole (DNC) [53] as a p-type organic semconductor has a mobility of 0.055
cm−2 V−1 s−1 for its solution-processed OFETs, presenting good stability resulting from a
stabilization of the highest occupied molecular orbital (HOMO) level. In general, the linear-
shaped fused acenes exhibit superior charge mobilities in OFETs when compared with the
corresponding angular-shaped isomers [41], such as naphthodithiophene (NDT) [49] and
dibenzotetrathienoacene (DBTTA) [50] isomers. However, the angular-shaped carbazolo[4,3-
c]carbazole (CC), reported by Marks group, exhibited the higher charge mobility than the linear-
shaped isomers did [30]. This reason is that the angular-shaped molecules present a highly
ordered structure with a very close π-stacking, whereas the linear-shaped molecules have a very
weak or no π-stacking order. Currently, only a tiny minority of angular-shaped molecules
reported have the more satisfactory results relative to linear-shaped ones [30,57], which is mainly
caused by two facets. For one thing, it is limited by the diversity of existing molecular structure
for heteroacenes, for another, the influence of linear- and angular-shaped core isomerism on the
performance of semiconductor devices is still seldom studied systematically and compared
together.
linear-shape
S-shape
arch-shape
S
R
S
R1
R2
R1
R2
S
R
R
S
S
R
NDT
NDT
DNT
S
S
S
S
S
S
S
S
S
S
S
DBTTA
DBTTA
BBTT
R
R
R
N
N
N
N
N
R
R
CC
CC
DNC
Fig. 1. Known typical angular heteroacenes [30,49−53].
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In 2008, we first attempted the related research about the heteroacenes containing both sulfur
and nitrogen with the arch-shaped backbone [58]. A novel aromatic fused system, the linear-
shaped isomer (anti-1) and the arch-shaped isomer (syn-1) of dibenzothienopyrroles (DBTPs)
(Fig. 2) have been designed to investigate the relationships of their structures and properties.
Unfortunately, the conjugated structure of syn-1 is unstable and not obtained. Only the
unsubstituted linear-shaped anti-1 showed the optimal OFET performance. Thus, we cannot
evaluate that which shape of this heteroacene core will take the advantage on transistor
performances. Based on this, we improved the synthetic method and introduced substituents on
nitrogen atom by Buchwald-Hartwig amination to form N-functionalized DBTP derivatives
directly. In this facile and effective way, we synthesized the N-hexyl substituted a-HDBTP and
the N-phenyl substituted a-PDBTP with the stable arch-shaped conjugated backbone structures.
Meanwhile, the corresponding linear-shaped isomers, the N-hexyl substituted l-HDBTP and the
N-phenyl substituted l-PDBTP (Fig. 2), were also synthesized. Herein, we focused the interest
on their structure-property relationships about the linear- and arch-shaped molecules with
different substituents in the thin-film and single-crystal transistors, to discuss the influence of
molecular shape on the optoelectronic properties. Their structural and physical properties were
characterized and discussed in details in this article.
Fig. 2. Molecular structures of linear- and arch-shaped DBTP isomers in previous work (left)
and DBTP derivatives in this work (right).
2. Results and discussion
2.1. Synthesis and thermal properties
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Synthetic routes of the linear-shaped DBTP derivatives, l-HDBTP and l-PDBTP are depicted
in Scheme 1. According to the literature, the N-hexyl substituted l-HDBTP can be achieved by
alkylation of anti-1 by employing 5 equivalent of KOH in DMF [58]. On this basis, we improved
the alkylated reaction by using 1.9 equivalent of KOH in acetone that is easier for the post-
processing. Differently, the phenyl group was introduced onto the nitrogen atom of anti-1 by
using bromobenzene in the presence of potassium carbonate and Cu power to give the N-arylated
product l-PDBTP.
As to the arch-shaped DBTP derivatives, because the unsubstituented syn-1 cannot be
obtained via Cadogan reaction like anti-1 [58], in order to construct this arch-shaped conjugated
backbone, introducing substituents on nitrogen atom is an effective solution. As shown in
Scheme 1, the N-hexyl substituted a-HDBTP and N-phenyl substituted a-PDBTP were
synthesized via Buchwald-Hartwig amination of 2,2’-dibromo-3,3’-dibenzothiophene, 4 [59], in
the presence of tris(diphenylideneacetone)dipalladium, sodium tert-butoxide and 2,2’-
bis(diphenylphosphino)-1,1’-binaphthyl (BINAP), using hexylamine and aniline as an aminating
agent, respectively. Their detailed syntheses are described in the Supporting Information (SI).
All new compounds were characterized by NMR spectroscopy, mass spectrometry and elemental
analyses.
Scheme 1. Synthetic routes of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP.
The thermal stability of the four compounds was determined by thermogravimetric analysis
(TGA). l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP showed that the decomposition
temperature at 5% weight loss (Td) occurred at 297, 325, 300 and 331 oC on heating (Fig. S1, SI),
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respectively, indicating that the thermal stability of these compounds is good enough for their
application in OFETs. In particular, for the same DBTP cores, the N-arylated compounds have
much better thermal stability than the corresponding N-alkylated compounds. Isomerism of core
does not bring obvious changes whether the substituted group on nitrogen atom is hexyl or
phenyl. The thermal stability of the arch-shaped molecules is just a little better than the
corresponding same N-functionalized linear-shaped molecules. Thus, substituents have a striking
influence on the thermal stability for this type of isomers.
2.2. Optical Properties
UV-vis absorption spectra of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP in dilute CH2Cl2
solutions and thin films at ambient condition are shown in Fig. 3 and the data of their
photophysical properties are listed in Table 1.
Table 1
Photophysical data of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP.
solutionsa
filmsc
λabs
(nm)
b
compound
λabs
Eg
(eV)
Eg
(nm)
(eV)
270, 328, 345 3.48 270, 332, 349 3.33
266, 329, 344 3.46 266, 336, 353 3.33
280, 307, 320 3.71 280, 312, 327 3.64
l-HDBTP
l-PDBTP
a-HDBTP
a-PDBTP
267
3.67
273
3.36
a Mearsured in a dilute CH2Cl2 solution (1×10–5 M).
b Estimated from the onset of absorption (Eg = 1240/λonset).
c
Films were spin-coated from CH2Cl2 solutions at the
concentration of 10 mg/mL (thickness of 80 nm).
Absorption spectra of the linear-shaped N-alkylated l-HDBTP and N-arylated l-PDBTP in the
solution are very similar to each other with fine absorption bands between 260 and 360 nm,
signifying their similar electronic structures and the high rigidity of their π-conjugated linear-
shaped DBTP backbone. Thus, for the linear-shaped DBTP derivatives, different substituents
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can hardly affect their electronic structures. The maxima absorption wavelengths at 345 and 344
nm for l-HDBTP and l-PDBTP, respectively, are characteristic of π–π* transitions that are
assigned to HOMO/LUMO transitions. As shown in Fig. 4, the two linear-shaped molecules
have extremely resembled delocalization of the electron density about HOMO and LUMO in
which the electronic clouds mainly focus on the DBTP core. Because the HOMO/LUMO
difference is clearly involved in the optical energy gaps (Eg) [55], Eg of 3.48 eV for the N-hexyl
substituted l-HDBTP and 3.46 eV for the N-phenyl substituted l-PDBTP are unsurprisingly very
close to each other only with little difference of 0.02 eV, coincident with molecular modeling
(0.04 eV).
Fig. 3. Normalized absorption spectra of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP (a) in
CH2Cl2 solutions and (b) in thin films.
For the arch-shaped molecules, both the N-alkylated a-HDBTP and the N-arylated a-PDBTP
present narrower and less resolved absorption bands than those observed for the linear-shaped
molecules. Meanwhile, the profiles of their UV-visible spectra are exceedingly different. The
spectrum of the N-hexyl substituted a-HDBTP exhibited three absorption peaks at 280, 307 and
320 nm that are a little more resolved than that observed for the N-phenyl substituted a-PDBTP.
By comparison, a-PDBTP gave a single peak at 267 nm and a weak shoulder at the longer
wavelength region near the onset of the low-energy absorption band of a-HDBTP. Thus, the
structure of the linear- and arch-shaped backbones has an obvious effect on the electronic
structures of the isomers, and the arch-shaped molecular electronic structure can be intensely
affected by different substituents. According to the Density Functional Theory (DFT)
calculations, the HOMO and LUMO of the N-alkylated a-HDBTP are both fully localized on the
DBTP core like the linear-shaped isomers, whereas the HOMO of the N-arylated a-PDBTP is
fully localized on the DBTP core and the LUMO mainly on the phenyl moiety (Fig. 4). The
difference of their electronic structures defines the different properties of their optical spectra.
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Moreover, l-HDBTP and l-PDBTP with the linear-shaped core appear to have narrower
bandgaps as compared to a-HDBTP and a-PDBTP with the arch-shaped core, in agreement with
the theoretical calculations. This result can be qualitatively understood by considering that the
linear-shaped and arch-shaped DBTPs are isoelectronic with pentacene and
dibenzo[d,d’]phenanthrene, respectively [48,60].
The absorption spectra of thin films for l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP all
present bathochromic shifts relative to their spectra in solutions. The accompanying bandgap
contraction from solution to solid state is from 3.48 to 3.33 eV for l-HDBTP, from 3.46 to 3.33
eV for l-PDBTP, from 3.71 to 3.64 eV for a-HDBTP and from 3.67 to 3.36 eV for a-PDBTP,
respectively, which suggests a more compact molecular arrangement in solid state. Furthermore,
the arch-shaped N-arylated a-PDBTP shows the maximum bathochromic shift among them
leading to its bandgap approaching the bandgap of linear-shaped molecules, demonstrating its
significantly strong intermolecular interaction in solid state.
Fig. 4. Calculated frontier orbitals, the HOMO/LUMO energies and energy gaps for l-HDBTP,
l-PDBTP, a-HDBTP and a-PDBTP by DFT.
2.3. Electrochemical properties
Electrochemical characterizations of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP were
investigated in Bu4NPF6 0.1 M/CH2Cl2 solutions at a concentration of 1×10−3 M for the four
compounds. Their cyclic voltammetries (CVs) are shown in Fig. 5 and Table 2.
The linear- and arch-shaped molecules all display one reversible oxidation wave, suggesting
the electrochemically stability of their oxidized species. Their potentials could be related to the
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HOMO energy. Due to the reduction peaks are out of our scan range (Fig. S2, SI), the LUMO
energy levels of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP can be calculated to be −1.91,
−1.98, −1.61 and −1.75 eV, respectively, from their optical band gaps and HOMO energy levels.
For the linear-shaped DBTP derivatives, the HOMO energy level of the N-arylated l-PDBTP
(−5.44 eV) is slightly more stabilized by 0.05 eV than that of the N-alkylated l-HDBTP (−5.39
eV), indicating little effect of substituents on HOMO energy level. This feature also confirms
that the first electron transfer in oxidation occurs on the π-conjugated backbone with no or very
weak influence of the substituents.
Fig. 5. Cyclic voltammograms of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP recorded in
CH2Cl2 (10−3 M) containing ferrocene (10−3 M) as an internal standard and Bu4NPF6 (0.1 M) as a
supporting electrolyte at room temperature with a scan rate of 100 mV/s; Ag/AgCl as the
reference electrode; glassy carbon as the working electrode; Pt wire as the counter electrode.
For the arch-shaped DBTP derivatives, a-HDBTP and a-PDBTP also exhibit a similar trend
that the N-arylated a-PDBTP has a more stable HOMO energy level than the N-alkylated a-
HDBTP does, stabilized by 0.10 eV as illustrated in Table 2. Moreover, for the same N-alkyl
substituent, the linear-shaped isomer has a more stable HOMO energy level relative to the
corresponding arch-shaped isomer, stabilized by 0.07 eV greater than that resulted from the
effect of different substituents as previously described (0.05 eV). When attaching the aryl group
with positive inductive effect onto the nitrogen atom instead of the alkyl group, the HOMO
energy level of the arch-shaped N-phenyl substituted a-PDBTP (−5.42 eV) becomes more stable
than that of the linear-shaped N-hexyl substituted l-HDBTP does (−5.39 eV), even getting closer
to the HOMO energy level of the linear-shaped N-phenyl substituted l-PDBTP (−5.44 eV).
Finally, a-PDBTP with a low-lying HOMO energy level would confer good stability against
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oxygen under ambient conditions. Thus, the HOMO energy level of the arch-shaped isomer can
be well tuned by N-functionalized substitutions to a greater extend relative to the linear-shaped
isomer. These data are basically in accordance with the theoretical calculations, which clearly
manifest the similarity and difference of the energy levels of the frontier orbitals (Fig. 4).
Table 2
Electronic properties of l-HDBTP, l-PDBTP, a-PDBTP and a-PDBTP.
optical electrochemical
Theoretical
calculations
studies
studies
compound
Eg
E1/2 HOMO
(eV)b (eV)c
Eg
HOMO
(eV)a
3.48
3.46
3.71
3.67
(eV)d
4.14
4.10
4.58
4.52
(eV)d
−5.24
−5.27
−5.18
−5.28
1.21
1.27
1.17
1.23
−5.39
−5.44
−5.32
−5.42
l-HDBTP
l-PDBTP
a-HDBTP
a-PDBTP
a Estimated from the onset of the absorption spectra in
CH2Cl2 solutions (1×10–5 M).
b V vs Ag/AgCl. All the potentials were calibrated with
the Fc/Fc+ as the internal standard.
c
Calculated from a following equation: EHOMO (eV) =
+
−[E1/2 − E(Fc /Fc)1/2 + 4.8].
d Obtained from DFT calculations.
2.4. Crystallographic characterization
In order to further understand the effects of molecular structure on electronic performance in
organic molecular semiconductors, analysis of single crystal structure is a most effective and
direct countermeasure. Fortunately, besides the reported l-HDBTP with the single crystal
structure [58], the molecular structure and packing pattern of other three compounds were also
all determined by X-ray diffraction analyses. The colorless block single crystals of l-PDBTP and
a-HDBTP were obtained by slow evaporation of mixed hexane and CH2Cl2 solutions (2/1, v/v),
and the colorless needle-like crystals of a-PDBTP were obtained by vacuum sublimation.
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The linear-shaped DBTP core basically reveals a coplanar structure in light of the single
crystal structure. As far as the linear-shaped N-hexyl substituted l-HDBTP, according to the
reported molecular structure [58], although its molecular packing contains strong S–S contacts
(3.56, 3.21 and 3.56 Å, Fig. 6a) to form dimmers, it still constructs a herringbone packing pattern
between dimmers in an alkyl C–H···π contact manner (Fig. 6b). Thus, no π–π stacking exists
because the substituted alkyl chains prevent π–π interactions between the adjacent stacks,
implying a very poor charge transport.
Fig. 6. Crystal packing of l-HDBTP [58]: (a) S–S contacts in the dimmer and (b) packing
structure, hydrogen atoms omitted for clarity.
The crystal structure of l-PDBTP shows that the N-phenyl substituents twisted 62.31o with
respect to the DBTP plane (Fig. 7a). The molecular packing reveals a sandwiched herringbone
arrangement (Fig. 7d and 7e). According to the DFT calculations, the dipole moment of l-
PDBTP is 4.76 D. Thus, possibly due to minimize the dipolar interactions, the adjacent two
molecules trend to form a dimmer [61], in which they are π-stacked in a face-to-face manner (π–
π separation of 3.54 Å) accompanying N-phenyl C–H···S contacts (2.99 Å, Fig. 7a). These π-
stacked dimmers construct a two-dimensional contacting network further by core C–H···S
interactions (2.82 Å) and C–H···H–C contacts between cores (2.37 Å, Fig. 7c). However, this
interactional two-dimensional system is disturbed by the sandwiched molecules (column A, Fig.
7d and 7e). The molecules in column A only constitute one-dimensional contacting channels in
an edge-to-face manner together with adjacent molecules in column B or C that form π-stacked
dimmers, in which the substituted phenyl and adjacent core plane display short C–Hphenyl···πcore
contacts with each other (Fig. 7b, 2.76 Å and 2.89 Å). Consequently, this sandwiched
herringbone pattern is not very favorable for electronic transport.
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Fig. 7. Crystal structure of l-PDBTP: (a) a π-stacked dimmer; (b) C–H···π contacts between
column A and column B or C; (c) interactions between π-stacked dimers; (d) packing structure
along the a-axis direction without phenyl substituents and hydrogen atoms and (e) packing
structure along the b-axis direction without hydrogen atoms.
Different from the linear-shaped isomers, the arch-shaped DBTP core presents a little twisted
configuration. According to the single crystal structure, the arch-shaped N-hexyl substituted a-
HDBTP has a non-planar backbone with a twisted angle of 3.23° (Fig. 8b), presumably resulting
from the H–H interaction (the distance between H1 and H2 is 2.44 Å, as shown in Fig. 8a) [62].
The dipole moment of a-HDBTP is 1.32D calculated by the DFT calculations, so the adjacent
two molecules trend to form a π-stacked dimmer (π–π contact of 3.40 Å) in order to minimize
the dipolar interactions [61]. Nevertheless, due to alkyl C–H···π contacts (2.63, 2.81 and 2.84 Å)
between molecules (Fig. 8c), a-HDBTP still forms a herringbone arrangement (Fig. 8d). These
alkyl substutuents completely orient out of the heteroacene plane, which interferes with the
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packing of the overall aromatic system. Thus, whether the molecule is linear-shaped or arch-
shaped, the substituted alkyl chains both affect the π–π interactions that are more helpful in
OFET applications. Nevertheless, owing to the difference of the shape of molecular backbone,
the arch-shaped isomer is more inclined to form π-stacked dimmers than the linear-shaped one,
which is a little advantageous for charge transport.
Fig. 8. Crystal structure of a-HDBTP: (a) top (up) and side (bottom) views; (b) a π-stacked
dimmer; (c) C–H···π contacts between alkyl chains and cores and (d) packing structure without
alkyl chains and hydrogen atoms.
Fig. 9. Crystal structure of a-PDBTP: (a) H–H and S–H contacts in one molecule; (b) and (c)
crystal packing with hydrogen omitted for clarity.
For the arch-shaped N-phenyl substituted a-PDBTP, the twisted angle of the conjugated
backbone is 3.70° (Fig. 9b) greater than that of the N-hexyl substituted a-HDBTP, most
probably ascribed to the strong repulsion between the H atoms (the distance between the H1 and
H2 is 2.30 Å as shown in Fig. 9a) [62]. The phenyl substituent has a twisted angle of 41.81°
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relative to the core (Fig. 9c) much smaller than that of 62.31° for the linear-shaped N-phenyl
substituted l-PDBTP. This result is presumably attributed to the reduced inter-ring steric
interactions between hydrogen atoms, when CH groups at conjugated core near to N-phenyl are
replaced with S atoms leading to inter-ring Score–Hphenyl interactions (2.87 Å as shown in Fig. 9a)
[63]. The crystal structure of a-PDBTP shows a highly ordered structure with a π-stacking in a
face-to-face manner contacting at 3.52 Å, indicating the good spatial overlap between the
stacking molecules, thus resulting in the strong intermolecular π–π interactions which are
extremely favorable for the charge transport.
These results demonstrate that the shape of core backbone and the differences of the
substituents both have apparent influences on molecular packing pattern, thus resulting in
different ability of charge transfer in OFETs. As to the same substituent whether it is alkyl or
phenyl, the arch-shaped DBTP core reveals better π-stacking pattern than the corresponding
linear-shaped one. Meanwhile, the arch-shaped molecule can be modified by different
substituents to modulate its molecular stacking mode, eventually conducing to perfect interlayer
π-stacking pattern that is overwhelmingly beneficial to charge transfer, and becoming a
promising organic semiconductor.
2.5. Field-effect transistor characterizations
Single-crystal devices are advantageous in exhibiting the intrinsic performance of organic
semiconductors due to avoiding grain boundaries [64]. Therefore, OFET devices based on single
crystals of l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP were fabricated via a solution-
crystallization process. During growing single crystals of the four compounds by a solution
process, we found the poor crystalline of l-HDBTP, l-PDBTP and a-HDBTP on Si/SiO2
substrates. No crystals obtained from them except a-PDBTP met the quality able to be applied in
single-crystal devices (Fig. S4, SI). Consequently, only the arch-shaped N-phenyl substituted a-
PDBTP presents a good performance as a semiconductor, whereas others have no FET
performance exhibiting. Thus, their FET characterizations can not be compared with each other.
The typical output and transfer characteristics of the device for a-PDBTP are shown in Figure 10.
a-PDBTP displayed typical p-channel FET responses with the highest mobility up to 0.75
cm2V−1s−1 with the low threshold voltage Vth of 11.02 V and the on/off ratio of 4.02Õ104. This
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ACCEPTED MANUSCRIPT
hole mobility is not up to the order of magnitude of 1 cm2V−1s−1, which is probably ascribed to
the undesirable single crystal quality and crystal boundary contact with the electrode.
Fig. 10. (a) Transfer and (b) output characteristics for single-crystal OFETs for a-PDBTP.
To better evaluate their structure-property relationships, bottom-contact/bottom-gate thin-film
transistors as references were fabricated by vacuum evaporation on Si/SiO2 (300 nm) substrates
which were modified by octadecyltrichlorosilane (OTS), perfluorinateddecyltriethoxy-silane
(FDTS) and nothing, respectively. Tens of devices have been examined carefully, and their field-
effect characteristics are summarized in Table 3. The typical output and transfer characteristics
of the devices for l-PDBTP, a-HDBTP and a-PDBTP are depicted in Fig. S6, S7 (SI) and 11,
respectively. Unsurprisingly, the device fabricated using the arch-shaped N-phenyl substituted a-
PDBTP revealed the best FET performance. Its IDS-VGS curves with fluctuation in Fig. 11a
display a unipolar p-channel feature. a-PDBTP on the FDTS-modified devices showed the
highest mobility of 0.058 cm2V−1s−1 with the low threshold voltage Vth of 3.0 V and the on/off
ratio of 105. However, the arch-shaped N-hexyl substituted a-HDBTP only on the OTS-modified
devices and linear-shaped N-phenyl substituted l-PDBTP only on the unmodified devices
showed lower FET performances with the modest hole mobility in the order of magnitude of
10−6 cm2V−1s−1, and the linear-shaped N-hexyl substituted l-HDBTP on the devices all showed
no field effects (Table 3). These results further suggest that the arch-shaped DBTP core has more
excellent transport properties governed by better solid-state electronic structures than the linear-
shaped one does when the same group substituted, which has been confirmed by their crystal
packing patterns. The π–π stacking pattern of a-PDBTP can be observed between all adjacent
molecules, whereas the others are packed by herringbone arrangement that is not so favorable to
the charge transport. Specifically, a-HDBTP and l-PDBTP both exhibit weak π-stacking order
only containing π-stacked dimers to cause week charge transport, and l-HDBTP has no π-
stacking existing to cause no field effect. In addition, we can conclude that the arch-shaped
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ACCEPTED MANUSCRIPT
molecule can obtain better transport properties in OFETs further by tuning of substituents.
However, the mobility of a-PDBTP film is not desirable, which is presumably due to strong
molecular aggregation on substrates resulting in inhomogeneity of thin films, and this
phenomenon for other three molecules (l-HDBTP, l-PDBTP and a-HDBTP) appears more
evident (Fig. S8, SI). Meanwhile, the solubility of a-PDBTP is not excellent in some common
solvents, such as CH2Cl2. These problems could be improved by introducing long alkyl chains on
a-PDBTP molecule with best semiconductor performance, being carried on in our lab.
Fig. 11. (a) Transfer and (b) output characteristics for bottom-contact OFETs for a-PDBTP,
measured under vacuum.
Table 3
FET characteristics of devices fabricated on OTS-modified, FDTS-modified and unmodified
Si/SiO2 substrates for l-HDBTP, l-PDBTP, a-HDBTP and a-PDBTP.
OTSa
Vth
(cm2v–1s–1) (V)
FDTSb
Vth
(cm2v–1s–1) (V)
UNc
Vth
(cm2v–1s–1) (V)
compound
µh
µh
µh
Ion/off
Ion/off
Ion/off
nd
nd
7.8×10–6
nd
nd
nd
nd
nd
nd
ndd
nd
nd
nd
3.0
nd
nd
nd
1.3×10–6
nd
nd
nd
l-HDBTP
l-PDBTP
a-HDBTP
a-PDBTP
14.3 104
nd nd
11.3 104
16.0 106
13.1 105
nd
105
0.026
0.058
0.037
a OTS-modified Si/SiO2 substrates.
b FDTS-modified Si/SiO2 substrates.
c Unmodified Si/SiO2 substrates.
d Not determined.
3. Conclusions
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In summary, the linear- and arch-shaped DBTP derivatives have been synthesized and
characterized. Physicochemical properties studied reveal that the shape of DBTP core has an
obvious influence on the electronic structures of the isomers. Compared with linear-shaped l-
HDBTP and l-PDBTP, the electronic structures of the arch-shaped a-HDBTP and a-PDBTP
can be tuned better by different substituents. Their single-crystal XRD analyses indicate that the
arch-shaped N-phenyl substituted a-PDBTP forms a highly ordered structure with a π-stacking
in a face-to-face manner which is extremely favorable to the charge mobility. In contrast, the
other compounds all form a herringbone arrangement, in which both the linear-shaped N-phenyl
substituted l-PDBTP and the arch-shaped N-hexyl substituted a-HDBTP exhibit weak π-
stacking order only containing π-stacked dimers, and the linear-shaped N-hexyl substituted l-
HDBTP has no π-stacking existing. Consequently, a-PDBTP on both the single-crystal and the
thin-film transistors showed optimum results with the hole mobility of up to 0.75 cm2V−1s−1 and
0.058 cm2V−1s−1, respectively. Relatively, based on the thin-film transistors, l-PDBTP and a-
HDBTP showed lower FET performances with the hole mobility in the order of magnitude of
10−6 cm2V−1s−1, and l-HDBTP showed no FET performance. These results demonstrate that the
arch-shaped DBTP derivatives can modify their electronic structures and molecular
arrangements by tuning of substituents to optimize the charge-transporting ability, and eventually
to obtain a better semiconductor performance relative to the linear-shaped ones. Thus, the arch-
shaped DBTP is a promising core structure for developing new organic semiconducting
materials. Meanwhile, this result is a strong complement to effect of the molecular shape on the
performance of semiconductors. Further material developments based on a-PDBTP, by
introducing different groups, are currently investigated in our group.
Conflicts of interest
The authors declare no competing financial interests.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China
(Grant No. 21402193, 21673059), the Special Fund for Scientific Research and Cooperation of
University and Institutes of Chinese Academy of Sciences, the Ministry of Science and
17

ACCEPTED MANUSCRIPT
Technology of China (2016YFA0200700, 2017YFA0206600), and the CAS Pioneer Hundred
Talents Program.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/...
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Highlights
•
•
Linear- and arch- shaped dibenzothienopyrrole derivatives were synthesized.
The arch-shaped N-phenyl substituted molecule exhibited the optimum π−π
stacking.
•
The influence of molecular shape on transistor performances is discussed.