Azaborine compounds for organic field-effect transistors: Efficient synthesis, remarkable stability, and BN dipole interactions

Article abstract of DOI:10.1002/anie.201209706

Boron-nitrogen units were incorporated into thiophene-fused polycyclic aromatic compounds. Organic field-effect transistors based on these azaborine compounds were fabricated, demonstrating a novel engineering concept of organic semiconductors and providi

Full text of DOI:10.1002/anie.201209706

Angewandte
Chemie
DOI: 10.1002/anie.201209706
Molecular Electronics
Azaborine Compounds for Organic Field-Effect Transistors: Efficient
Synthesis, Remarkable Stability, and BN Dipole Interactions**
Xiao-Ye Wang, Hao-Ran Lin, Ting Lei, Dong-Chu Yang, Fang-Dong Zhuang, Jie-Yu Wang,* Si-
Chun Yuan,* and Jian Pei*
Organic semiconductors have attracted great attention during
the past few decades for the development of next-generation
(Scheme 1). Four thiophene rings are fused onto a BN-
substituted naphthalene core to extend the p conjugated
plane for intermolecular p–p stacking and charge-carrier
electronics.[1] The incorporation of a B N unit, which is
À
=
isoelectronic to the C C moiety, into p systems provides
a novel approach in the molecular engineering of organic
semiconductors.[2] BN substitution can change the electronic
properties of p systems,[3] and afford additional intermolecu-
lar dipole–dipole interactions.[4] Therefore, BN-incorporated
semiconductors provide new opportunities for organic elec-
tronics. Although significant progress has been made in
azaborine chemistry,[5,6] the construction of azaborine rings in
large p scaffolds remains challenging.[7] Moreover, azaborine
compounds are usually susceptible to moisture and oxygen,
and their thermal decomposition temperatures are around
2008C, thus limiting their promising applications as organic
materials.[7] As a result, the charge-transport properties of
azaborine compounds have rarely been investigated up to
now. Only recently, Nakamura and co-workers reported
a BN-fused polycyclic aromatic compound which exhibited
higher intrinsic hole mobility than its carbon analog by time-
resolved microwave conductivity measurements,[7f] implying
that BN-substituted aromatics might outperform their carbon
analogs in organic electronics. Nonetheless, electronic devices
based on azaborine compounds have not yet been demon-
strated.
Scheme 1. Synthetic route to BN-TTN-C3 and BN-TTN-C6. Reagents
and conditions: a) Ag2CO3, Pd(OAc)2, 2,2’-bipyridine, dioxane, reflux,
=
24 h; b) N-bromosuccinimide (NBS), CHCl3, HOAc, RT, 1 h; c) Ph2C
NH2Cl, Pd2dba3, 2,2’-bis(diphenylphosphino)-1,1’-binaphthyl (BINAP),
tBuONa, toluene, 808C, 15 h; d) HCl (aq. 6m), tetrahydrofuran (THF),
RT, 10 minutes; e) 3, Pd(OAc)2, P(tBu)3, tBuONa, o-xylene, 1208C,
12 h; f) BBr3, NEt3, ortho-dichlorobenzene (o-DCB), 1808C, 12 h.
Herein, we synthesize two novel BN-substituted tetra-
thienonaphthalene derivatives BN-TTN-C3 and BN-TTN-C6
through an efficient one-pot electrophilic borylation method
transport.[8] Alkyl chains are attached to ensure good
solubility and to tune the intermolecular interactions.[9] Our
investigations indicate that the introduction of the fused
thiophene rings effectively improves the aromaticity of the
skeleton, and both molecules show excellent chemical and
thermal stability. Importantly, organic field-effect transistors
(OFETs) based on these two compounds are successfully
fabricated, and high hole mobilities up to 0.15 cm2 VÀ1 sÀ1 and
on/off ratios over 105 are obtained from BN-TTN-C3, which
represents the first example of applying azaborine com-
pounds in organic electronic devices.
[*] X.-Y. Wang, H.-R. Lin, T. Lei, D.-C. Yang, F.-D. Zhuang, Dr. J.-Y. Wang,
Prof. J. Pei
Beijing National Laboratory for Molecular Sciences
Key Laboratory of Bioorganic Chemistry and
Molecular Engineering of Ministry of Education
College of Chemistry and Molecular Engineering
Peking University, Beijing 100871 (P. R. China)
E-mail: wang-jieyu@163.com
jianpei@pku.edu.cn
Scheme 1 illustrates the synthetic route to BN-TTN-C3
and BN-TTN-C6. Commercially available 2-alkylthiophenes
(1) were dimerized through an oxidative coupling reaction.[10]
Monobromination of 2 gave compound 3, which was applied
for the Buchwald–Hartwig coupling reaction. Ketimine 4 was
hydrolyzed with aqueous HCl in THF to afford ammonium
species 5,[11] which is more stable in air than its amino
counterpart. This route is the most efficient one among
various attempts to introduce amino groups onto thiophene
derivatives. Subsequently, another Buchwald–Hartwig ami-
nation between 3 and 5 produced compound 6. The electro-
philic borylation approach was chosen to finish the final
Prof. S.-C. Yuan
Department of Fundamental Science
Beijing University of Agriculture
Beijing 102206 (P. R. China)
E-mail: ysc2007@sina.com
[**] This work was supported by the Major State Basic Research
Development Program (grant numbers 2009CB623601 and
2013CB933501) from the Ministry of Science and Technology and
the National Natural Science Foundation of China. The authors
thank Prof. Xiao-Yu Cao from Xiamen University for helpful
discussions.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201209706.
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cyclization step.[7e,f] Compound 6 was simply heated with BBr3
and NEt3 in o-dichlorobenzene (o-DCB) to afford BN-TTN-
C3 and BN-TTN-C6. Purification by column chromatography
on silica gel and recrystallization from CHCl3/MeOH gave
pure products as white solids in around 80% yields. The
highly efficient one-pot cyclization step involved an electro-
philic borylation of the amino group and subsequent electro-
philic substitutions of the two adjacent thiophene rings. Note
that using BCl3 failed to produce the target compounds,
probably because of its lower reactivity.
Both compounds exhibit excellent stability to ambient
conditions. In consistence with the complexation behavior of
other azaborine derivatives,[7e] no change of the absorption
features was observed when acids (trifluoroacetic acid), or
bases (iPr2NH, Et3N, pyridine, dimethylaminopyridine, OHÀ,
ClÀ, BrÀ, and IÀ) were added to a solution of BN-TTN-C3 or
BN-TTN-C6 in CH2Cl2 (1 ꢀ 10À5 m). Surprisingly, our com-
pounds remained intact in the presence of 0.01m nBu4NF,
whereas other azaborine derivatives exhibited specific bind-
ing to FÀ ions at a low concentration of FÀ ions.[7e] In addition,
thermogravimetric analysis (TGA) revealed decomposition
temperatures (5% weight loss) as high as 3588C for BN-TTN-
C3, and 4008C for BN-TTN-C6 (Figure S1), the highest
values for azaborine compounds reported to date. Such
remarkable thermal stability is crucial for their applications in
materials science.
Figure 1. a) ORTEP drawing of BN-TTN-C3 with thermal ellipsoids
shown at 50% probability. Hydrogen atoms are omitted for clarity.
b) P-helical and M-helical structures of BN-TTN-C3. c) Crystal packing
of BN-TTN-C3 viewed along the c axis. d) Crystal packing of BN-TTN-
C6 with B and N atoms crystallographically undistinguishable.
The molecular structures and packing modes of both
molecules were unambiguously determined by single-crystal
X-ray diffraction (Figure 1).[12] In BN-TTN-C3, the B N bond
À
length (1.474(6) ꢁ) is shorter than the expected value for
[5c]
À
a B N single bond (1.58 ꢁ), but larger than the localized
=
B N double bond (1.403(2) ꢁ), and close to the delocalized
double bond in aromatic 1,2-azaborines (1.446(2) ꢁ) and the
parent BN-naphthalene (1.461(1) ꢁ; Figure 1a).[13] The C B
À
bond (1.52 ꢁ) is slightly shorter than those reported in
À
literature (1.58 ꢁ), so is the C N bond (1.42 ꢁ versus
1.48 ꢁ).[13a] The C2 C3 single bond (1.414(6) ꢁ) is also
À
shorter than that in bithiophenes (1.454(4) ꢁ).[14] The length-
=
ening of the B N double bond and the shortening of the
formal single bonds indicate significant p electron delocaliza-
tion in BN-TTN-C3, as suggested by density functional theory
(DFT) calculations. The molecular orbitals of BN-TTN-C3
are fully delocalized over the entire backbone, similar to
those of CC-TTN-C3 (Figure 2). The HOMO and LUMO
energy levels of BN-TTN-C3 also show little difference to
those of CC-TTN-C3, only with a slightly larger band gap
(3.89 eV versus 3.80 eV). These observations reveal that the
azaborine rings are aromatic, which is further supported by
nucleus-independent chemical shift (NICS) calculations
(Figure 2).[15] The central C4BN rings in BN-TTN-C3 show
a NICS(1) value of À6.0, revealing a moderate yet greater
aromaticity than the reported BN-fused dibenzo[g,p]chrysene
(À2.9).[7f] This increased aromaticity, presumably resulting
from the more planar structure and the electron-rich thio-
phene rings, may be an important reason for the remarkably
high chemical and thermal stability.[16]
Figure 2. Calculated NICS(1) values, molecular orbitals, and corre-
sponding energy levels of BN-TTN-C3 and its isoelectronic carbon
analog CC-TTN-C3.
other (Figure 1b). There are two sets of enantiomers in a unit
cell with different C2-C3-C6-C7 dihedral angles (19.038 for
P1-M1 and 16.608 for P2-M2), which are much smaller than
that of the BN-fused dibenzo[g,p]chrysene (38.878)[7f] because
of the incorporation of thiophene rings. Each enantiomer is
arranged into a sheet along the a axis, which forms slipped
p stacks along the b axis with alternating head-to-tail orien-
tation of the BN dipole moments (P1-M1-P2-M2; Figure 1c).
The unique dipole–dipole interactions, along with the inter-
molecular S···S contacts (3.58 ꢁ) and the close p–p stacking
(3.44 ꢁ), dominate the crystal growth of BN-TTN-C3,
providing a highly ordered packing mode.[17] As revealed by
BN-TTN-C3 and BN-TTN-C6 show different packing
structures in single crystals. BN-TTN-C3 exhibits P- and M-
helical structures with BN dipole moments opposite to each
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the calculated electronic couplings (V) based on the single-
crystal structure of BN-TTN-C3, the adjacent molecules with
good p–p stacking exhibit V values as large as 56.2 meV
(Figure S10). In other words, the major charge-transport
channel in the solid state is along the p–p stacking direction.
In contrast, although BN-TTN-C6 also adopts a layered
packing mode with intermolecular S···S contacts of 3.505 ꢁ
(Figure S5), B and N atoms are not crystallographically
distinguishable, presumably because of the lack of ordered
BN dipole interactions (Figure 1d). Scrutiny of the crystal
structure reveals that the long alkyl chains formed multiple
deposited onto the substrates and gold was patterned through
a shadow mask as the source/drain electrodes. All devices
were measured in ambient conditions, exhibiting p-channel
FET characteristics. Devices based on BN-TTN-C3 reveal the
highest hole mobility up to 0.15 cm2 VÀ1 sÀ1, with a threshold
voltage of À52 V and a current on/off ratio of 5 ꢀ 105. Typical
transfer and output characteristics are shown in Figure 4.
À
C H···p interactions, and the closest intermolecular BN
dipole–dipole distance was enlarged to 9.207 ꢁ. Such a long
distance may weaken the effect of the intermolecular BN
dipole–dipole interactions, resulting in a disordered orienta-
tion of BN dipoles.
The different packing structures of BN-TTN-C3 and BN-
TTN-C6 in the solid state can also be evidenced from the
absorption spectra. The absorption features of BN-TTN-C3
and BN-TTN-C6 in solution are almost identical (lmax
=
325 nm), but they are different in solid state (Figure 3).
Figure 4. a) Transfer and b) output characteristics of the OFET of BN-
TTN-C3 with a hole mobility of 0.15 cm2 VÀ1 sÀ1 (Vsd =source-drain
voltage, Isd =source-drain current, and Vg =gate voltage).
More than 20 devices were measured with an average carrier
mobility of 0.12 cm2 VÀ1 sÀ1. This performance is even better
than the carbon analogs with a larger p conjugated plane,[19]
which can partly be attributed to the low reorganization
energy (l) of 162 meV, the smallest one among reported
polycyclic azaborine compounds (270 and 215 meV).[7e,f]
However, BN-TTN-C6 only shows the highest mobility of
0.03 cm2 VÀ1 sÀ1 and an averaged mobility of 0.01 cm2 VÀ1 sÀ1
(20 devices) under optimized conditions (Figure S6). To
understand the different device performances, atomic force
microscopy (AFM) and X-ray diffraction (XRD) of the thin
films deposited on CYTOPTM-treated Si/SiO2 substrates were
performed. Both thin films showed similar morphologies with
highly crystalline domains in AFM images (Figure S7), which
could not be a key factor determining the difference in carrier
mobilities. However, further XRD analysis indicated distinct
packing modes in the films. Both compounds showed (001)
and higher-order diffraction peaks, which indicates lamellar
packing structures (Figure S8).[20] The interlayer distance (d-
spacing) in BN-TTN-C3 is calculated to be 16.3 ꢁ, which is in
good agreement with the molecular length, indicating an
edge-on orientation. The d-spacing in BN-TTN-C6 is 17.9 ꢁ,
much smaller than the molecular length (about 23 ꢁ), which
may result from the packing of hexyl chains with p-moieties
Figure 3. Normalized absorption spectra of BN-TTN-C3 and BN-TTN-
C6 in CH2Cl2 (dashed line) and in film (solid line). The thin films
(50 nm) were prepared by thermal evaporation under 4ꢀ10À4 Pa.
Both compounds display obvious red-shifts of lmax and
enhanced 0–0 transitions compared to those in solution,
indicating strong intermolecular interactions in solid state.
However, these two compounds show distinct absorption fine
structures in the films. These observations indicate great
differences of the intermolecular interactions and molecular
packings caused by the different length of the alkyl chains.
Cyclic voltammetry (CV) experiments were performed to
determine the energy levels of BN-TTN-C3 and BN-TTN-C6.
These two compounds show almost the same oxidative waves.
The HOMO energy level estimated from the onset value is
about À5.38 eV, which is in good agreement with the
theoretical calculations. The LUMO level, calculated from
the HOMO level and the optical band gap (3.31 eV), is about
À2.07 eV. The low-lying HOMO energy level suggests that
BN-TTN-C3 and BN-TTN-C6 are good candidates as air-
stable p-type semiconductors.[18]
À
due to multiple C H···p interactions. The significant differ-
ence in solid-state packing modes may account for the
different device performances.
In conclusion, two BN-substituted tetrathienonaphtha-
lene derivatives, BN-TTN-C3 and BN-TTN-C6, have been
synthesized through a highly efficient one-pot electrophilic
borylation strategy. The chemical stability of azaborine
compounds to FÀ is reported for the first time. The molecules
exhibit the highest decomposition temperatures up to 4008C
and the smallest reorganization energy of 162 meV among
OFET devices were successfully fabricated in a bottom-
gate/top-contact (BG/TC) configuration. CYTOP (Asahi
Glass Co., Ltd.) was spin-coated onto Si/SiO2 substrates,
providing a 230 nm thick film. The organic layer was vacuum-
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DFT calculations reveal improved aromaticity, which may
contribute to the remarkable stability. The first organic
electronic devices based on azaborine compounds have
been demonstrated with the highest FET mobility of
0.15 cm2 VÀ1 sÀ1. Interestingly, different alkyl chains are
observed to affect the crystal packing structures and result
in different device performances. This work opens the gate of
applying the rapidly developing azaborine chemistry in
electronic science, and demonstrates a novel molecular
engineering concept of organic semiconductors with BN
substitutions, providing a broad class of promising BN-
substituted semiconductors for future organic electronics.
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Received: December 4, 2012
Published online: February 10, 2013
Keywords: azaborine chemistry · boron · electronic devices ·
.
semiconductors · transistors
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