Air-Stable Benzo[ c]thiophene Diimide n-Type π-Electron Core

Article abstract of DOI:10.1021/acs.orglett.9b01239

In this paper, the molecular design of the first deep-lowest unoccupied molecular orbital (LUMO) level diimide π-electron core, benzo[c]thiophene diimide (BTDI), as a novel n-type organic semiconductor was determined. An original synthetic sequence was de

Full text of DOI:10.1021/acs.orglett.9b01239

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Air-Stable Benzo[c]thiophene Diimide n‑Type π‑Electron Core
Craig P. Yu,† Ryoya Kimura,‡ Tadanori Kurosawa,† Eiji Fukuzaki,∥ Tetsuya Watanabe,∥
Hiroyuki Ishii,⊥ Shohei Kumagai,† Masafumi Yano,‡ Jun Takeya,† and Toshihiro Okamoto*,†,§
†Department of Advanced Materials Science, School of Frontier Sciences, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa,
Chiba 277-8561, Japan
‡Chemistry, Materials and Bioengineering Major, Graduate School of Science and Engineering, Kansai University, 3-3-35
Yamate-cho, Suita, Osaka 564-8680, Japan
∥FUJIFILM Corp., 577 Ushijima, Kaisei-machi, Ashigarakami-gun, Kanagawa 258-8577, Japan
⊥Department of Applied Physics, Faculty of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki
305-8573, Japan
§PRESTO, JST, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan
S
* Supporting Information
ABSTRACT: In this paper, the molecular design of the first
deep-lowest unoccupied molecular orbital (LUMO) level
diimide π-electron core, benzo[c]thiophene diimide (BTDI),
as a novel n-type organic semiconductor was determined. An
original synthetic sequence was devised to obtain the target
cyclohexyl-BTDI (Cy6-BTDI) derivative. Cy6-BTDI demon-
strated completely reversible reduction waves and a stable
radical anionic state. Favorable brickwork molecular assembly
and two-dimensional charge transport properties of Cy6-BTDI
were exhibited in the solid state. As a result, air-stable electron mobilities were obtained from the BTDI organic field-effect
transistors under ambient conditions.
rganic field-effect transistors (OFETs) are promising
Ocandidates for lightweight, low-cost, and flexible
electronics,1 such as sensors2 and radio frequency identifier
(RF-ID) tags.3 To achieve these high-performance electronic
devices with organic materials, both hole-transporting (p-type)
and electron-transporting (n-type) components are required to
construct bipolar transistors and complementary logic circuits.4
To date, a large number of p-type organic semiconductors have
been reported such as [1]benzothieno[3,2-b][1]-benzothio-
phene (BTBT),5 dinaphtho[2,3-b:2′,3′-f ]thieno[3,2-b]-
thiophene (DNTT),6 dinaphtho[2,3-b:2′,3′-d]thiophene
(DNT−V),7 dinaphtho[2,3-d:2′,3′-d′]benzo[1,2-b:4,5-b′] di-
thiophene (DNBDT),8 and chryseno[2,1-b:8,7-b′]dithiophene
(ChDT)9 with charge-carrier mobility (μ) values higher than
10 cm2 V−1 s−1, which are promising for sensor and RF-ID tag
applications. However, contemporary n-type organic semi-
conductors, on the other hand, suffer from low-performance
and instability drawbacks due to oxidation of charge carriers by
ambient oxidants such as O2 and H2O upon charge
injections.10−12 As a general design principle suggested by
numerous studies, lowering the lowest unoccupied molecular
orbital (LUMO) energy level below −4.0 eV protects the n-
type organic semiconductors against detrimental oxidation in
the charge-carrying states.13
significant portion of the molecular designs so far has been
based on naphthalene diimide (NDI)15 (Figure 1a) and
perylene diimide (PDI)16 π-cores for their relatively low
LUMO levels (ca. −3.80 eV). However, the parent NDI and
PDI π-cores cannot achieve air-stable OFET operations, and
chemical modifications such as incorporations of highly
electron-withdrawing substituents17 and lateral expansion of
the rylene framework18 are required to obtain LUMO energy
levels below −4.0 eV. However, the total number of distinct π-
cores remains limited, which impedes the overall development
of the urgently demanded n-channel OFETs.
Herein, we report the first deep-LUMO level diimide π-core,
benzo[c]thiophene diimide (BTDI) (Figure 1a), as an air-
stable n-type organic semiconductor without the requirement
of any chemical modifications. In contrast to naphthalene
(ELUMO = −1.33 eV), which is the main π-conjugated moiety
of NDI, the quinoidal-like benzo[c]thiophene possesses a
respectably deeper LUMO energy level (−1.67 eV). Thus, the
relatively deep LUMO energy level of benzo[c]thiophene can
be potentially harnessed to design deep-LUMO level n-type
organic semiconductors for air-stable OFETs. Albeit having
seemingly intriguing optoelectronic properties, benzo[c]-
thiophene has been reported to be unstable and tends to
To date, a large number of electron-deficient molecules have
been synthesized to obtain low-lying LUMO energy levels
required for air-stable n-channel OFET performances,14 and a
Received: April 9, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01239
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
protruding sulfur is likely to participate in intermolecular
orbital overlap for electron transports. Thus, our current BTDI
design features aim to (1) develop a brand-new deep-LUMO
π-core via ring-fusion between benzo[c]thiophene and imide
moieties and obtain air-stable n-channel OFET operations, (2)
achieve effective intermolecular orbital overlaps in the solid-
state and high electron mobility, and (3) obtain highly soluble
semiconducting materials through the large dipole moment of
benzo[c]thiophene (1.38 debye).
The synthetic route to Cy6-BTDI is illustrated in Scheme 1.
Compound 1 was prepared according to the previously
reported procedure in good yield. As previously mentioned,
the unsubstituted benzo[c]thiophene is highly reactive and
prone to self-polymerization. Thus, compound 2 was generated
in situ by the strong reducing agent lithium bis(trimethylsilyl)-
amide (LiHMDS) under argon, and its formation was
1
determined by H NMR. Intermediate 2 was lithiated in the
1,3-positions, and it is worth mentioning that the regiose-
lectivity of lithiation could be significantly improved by the
addition of tetramethylethylenediamine (TMEDA). The
dilithiated benzo[c]thiophene was then trapped by dimethyl-
carbamoyl chloride to yield the diamide compound 3 in 42%
yield. By installing the electron-withdrawing amides at the 1,3-
positions of benzo[c]thiophene, 3 exists as a perfectly stable
compound. Regioselective bromination performed by dibro-
moisocyanuric acid in concentrated H2SO4 gave 4,7-
dibromobenzo[c]thiophene 4 in 83% yield. Manabe and co-
workers previously reported the use of trichlorophenyl formate
to serve as a highly reactive CO surrogate in Heck reactions.20
The electron-deficient formate can later be readily hydrolyzed
under basic conditions. Hence, we adopted this method and
converted the dibromo compound 4 to afford compound 5 via
a Pd-catalyzed cross-coupling reaction using palladium acetate
and Xantphos. A subsequent hydrolysis was done using an
aqueous solution of NaOH to hydrolyze both the amide and
formate groups to give the tetracarboxylic acid 6, followed by a
condensation reaction in acetic anhydride to furnish the key
precursor 7 in an excellent 90% yield.
Figure 1. (a) Molecular structure of BTDI (left), the LUMO
coefficients of BTDI (middle), and the structure of NDI (right) and
(b) molecular orbital energy levels of benzo[c]thiophene, BTDI,
NDI, and naphthalene.
self-polymerize in air due to its highly reactive 1,3-positions,
which makes it challenging to study its structure for functional
materials purposes.19 However, by fusing benzo[c]thiophene
with the electron-deficient imide groups, we stabilize the highly
reactive 1,3-positions, making BTDI a stable compound under
ambient conditions. Furthermore, our initial density functional
theory (DFT) calculation shows a LUMO level of −4.17 eV
for BTDI (Figure 1b), which renders air-stable n-channel
OFET operations as a π-core without the installation of any
substituents. Large LUMO coefficients are observed on the
benzo[c]thiophene sulfur (Figure 1a), which suggests the
We intend to compare our BTDI properties and device
performance with the well-studied N,N′-biscyclohexylnaph-
thalene diimide (Cy6-NDI). Since compound 7 adopts a
Scheme 1. Synthetic Route toward BTDI
B
DOI: 10.1021/acs.orglett.9b01239
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Letter
Figure 2. (a) NICS values of benzo[c]thiophene and Cy6-BTDI in ppm, (b) molecular packing structure of Cy6-BTDI along the a-axis, and (c)
mean-plane intermolecular distances and transfer integrals of the LUMO; alkyl chains are omitted for clarity.
slightly distorted geometry due to the five-membered
thiophene moiety, common imidization methods for six-
membered imides only lead to the ring-opened amide
intermediate. Thus, we attempted the synthesis of Cy6-BTDI
in a stepwise fashion by first treating the anhydride 7 with
cyclohexylamine and using acetic anhydride/pyridine to
catalyze the ring-closing step in one pot. The imidization
was successfully completed using this method and furnished
Cy6-BTDI in 64% yield at room temperature as a bright yellow
solid. Cy6-BTDI demonstrated superior solubility in common
organic solvents such as toluene and anisole compared to the
Cy6-NDI counterpart due to the large internal dipole moment
of the benzo[c]thiophene moiety (Table S2). The high
solubility of BTDI allows for purification with column
chromatography and solution-processable OFET fabrications.
Large platelet-shaped single-crystals of Cy6-BTDI were
obtained by physical vapor transport (PVT). The BTDI π-
core appears to be planar. However, the five-membered
thiophene moiety causes the π-core to slightly bend toward the
sulfur atom at an angle of 3.8°. Using the molecular geometry
obtained from the single-crystal X-ray diffraction, we calculated
the nucleus-independent chemical shifts (NICS)21 values of
the BTDI π-core (Figure 2a). The benzene and thiophene
moieties of BTDI exhibit NICS values of −6.6 and −14.2,
respectively, which are consistent with those of the DFT-
optimized BTDI structure and lower than those of the parent
benzo[c]thiophene. Cy6-BTDI adopts a brickwork packing
motif where the molecules are stacked antiparallel to the b-axis,
and each plane is tilted to the a-axis so the distances to the
upper and lower stacked molecules along the b-axis are
different (Figure 2b). Similar to the reported single-crystal
structure of Cy6-NDI,22 adjacent BTDI molecules are held by
C−H···O interactions, but with a shorter distance of 2.519 Å.
Mean plane intermolecular distances of Cy6-BTDI were
measured to be 3.801, 3.762, and 3.838 Å. Transfer integral
(t) values of the LUMO are calculated at the PBEPBE/6-
31G(d) level of theory. Cy6-BTDI exhibits t value of +14.6
meV in the lateral direction, and large negative t values of
−71.1, −39.0, and −99.3 meV in the vertical directions (Figure
2c). From our effective mass calculations at the PBEPBE/6-
31G(d) level, Cy6-BTDI shows a two-dimensional anisotropic
charge transport behavior in the a and b axes with effective
masses of ma = 2.83 and mb = 4.64.
nm in CHCl3, which is dramatically red-shifted with respect to
Cy6-NDI (382 nm) (Figure 3a). From the absorption onset,
Figure 3. (a) UV−vis absorption spectra of Cy6-NDI and Cy6-BTDI
and (b) time-dependent UV−vis of Cy6-BTDI in CHCl3.
the optical energy gap of Cy6-BTDI was calculated to be 2.58
eV. Cy6-BTDI demonstrates its chemical stability under
ambient conditions as its solution-state time-dependent UV−
vis absorption spectra remain unchanged over 30 days (Figure
3b). In addition, Cy6-BTDI appears to be emissive in the
solution state with a λem at 467 nm, as opposed to the
nonemissive NDI (Figure S13). Cy6-BTDI exhibits two fully
reversible reduction waves in cyclic voltammetry (CV)
experiments with the first half-width reduction potential E1/2
at −0.79 V, which corresponds to a LUMO energy level of
−4.01 eV (Figure 4). Compared to the electrochemical LUMO
energy level of Cy6-NDI (−3.69 eV), Cy6-BTDI shows the
potential for achieving air-stable electron mobility in OFETs.
Thermogravimetry-differential thermal analysis (TG-DTA)
experiment reveals the thermal stability of Cy6-BTDI with its
In contrast to the previously reported Cy6-NDI, Cy6-BTDI
showed a pronounced reduction in the optical HOMO−
LUMO energy gap attributed to the quinoidal characteristic of
the benzo[c]thiophene core.23 Cy6-BTDI exhibits a λmax at 460
Figure 4. Cyclic voltammograms of Cy6-NDI (0.1 mM) and Cy6-
BTDI (0.5 mM) in benzonitrile with 0.1 M of NBu4PF6 at a scan rate
of 100 mV s−1.
C
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Figure 5. (a) Schematic diagram of the Cy6-BTDI single-crystal OFET architecture, (b) transfer characteristic of Cy6-BTDI single-crystal OFET at
VD = 40 V, and (c) output characteristic of Cy6-BTDI single-crystal OFET.
5% weight loss temperature (T95) at 296 °C, and it exhibits an
endothermic phase transition peak higher than its T95 value at
336 °C (Figure S14). We observed no apparent phase
transitions from 20 °C to 260 °C for Cy6-BTDI in the
differential scanning calorimetry (DSC) measurement (Figure
S15). The excellent thermal stability of Cy6-BTDI ensures its
consistent crystalline structures during the high-temperature
device fabrication processes.
phase. Air-stable single-crystal n-channel OFETs of Cy6-BTDI
have been fabricated using lamination and edge-casting
methods, and the highest electron mobility of 0.16 cm2 V−1
s−1 is achieved. From our preliminary results herein, BTDI is a
promising π-core for air-stable n-channel OFETs. Prospective
studies involving BTDI such as tuning of electronic structures
and packing structures via chemical modifications and side-
chain engineering are anticipated. Optimizations of device
architecture and fabrications are likewise expected to further
improve the semiconductor performance of the BTDI π-core.
To evaluate the intrinsic charge-carrier properties of BTDI,
we fabricated bottom-gate/top-contact OFETs with PVT-
grown Cy6-BTDI single crystals as the active semiconductor
layer (Figure 5a). The crystals were carefully picked up and
laminated via electrostatic adhesion on the olefin layer above
the SiO2/Si substrate with gold as the source and drain
electrodes. Cy6-BTDI single-crystal OFETs exhibit negligible
hysteresis between the forward and reverse sweeps and
constant threshold voltages in the transfer characteristic curves
(Figure 5b), which demonstrate reproducible n-channel OFET
operations. Cy6-BTDI transistors achieved an air-stable
electron mobility of μe = 0.16 cm2 V−1 s−1 in the saturation
regime, and the OFET performances remained consistent over
1 week of exposure in air. We examined the X-ray diffraction
patterns of the semiconductor layer in Cy6-BTDI-based OFET
and found the channel direction corresponded to neither the
a* nor b* charge-transporting axis. In fact, the OFET channel
is in either the +2a* + b* or −2a* + b* direction, which
resulted in low electron mobility. Other essential OFET
parameters, device stability data, as well as solution-processed
Cy6-BTDI OFET results via the edge-casting method24 are
summarized in Tables S3 and S4.
Cy6-BTDI OFETs demonstrated superior stability under
ambient conditions compared to Cy6-NDI,22,25 which suggests
BTDI may be a promising π-core for future development of
air-stable n-type semiconductors. It is noteworthy that the
maximum drain current ([|ID|1/2]2 = 2.56 μA) in the transfer
characteristic of the Cy6-BTDI OFET is consistent with that of
the output characteristic (Figure 5c), which indicates Cy6-
BTDI n-channel OFETs are insensitive toward bias-stress from
charge trapping in transistor channels and confirms no
overestimation of the electron mobility.26,27
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01239.
Experimental procedures and spectroscopic data (PDF)
Accession Codes
CCDC 1901392 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: tokamoto@edu.k.u-tokyo.ac.jp.
ORCID
Craig P. Yu: 0000-0002-1423-5244
Shohei Kumagai: 0000-0002-1554-054X
Toshihiro Okamoto: 0000-0002-4783-0621
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
In summary, a novel air-stable electron-deficient BTDI π-
core is reported in the current work, and we have developed an
original synthetic route toward possible BTDI derivatives.
Compared to the well-studied NDI core, BTDI has shown
remarkable improvements in photophysical and electronic
properties. The X-ray single-crystal packing structure of BTDI
reveals strong intermolecular interactions as well as effective
orbital overlaps. Thermal studies also suggest Cy6-BTDI
derivative possesses high thermal stability and stabilized crystal
■
This work was supported by the Japan Science and
Technology Agency (JST) PRESTO “the Scientific Innovation
for Energy Harvesting Technology” programme
(JPMJPR17R2) as well as the Japan Society for the Promotion
of Science (JSPS) KAKENHI Grant-in-Aid for Scientific
Research (B; No. 17H03104). C.P.Y. thanks the Todai
Fellowship for supporting his graduate research at the
University of Tokyo.
D
DOI: 10.1021/acs.orglett.9b01239
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(11) Wu
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rthner, F. Plastic Transistors Reach Maturity for Mass
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