Synthesis and Semiconducting Characteristics of the BF2 Complexes of Bisbenzothiophene-Fused Azadipyrromethenes

Article abstract of DOI:10.1021/acs.orglett.9b04142

A novel type of dibenzothiophene [b]-fused core-expanded azaBODIPYs were obtained through an efficient post-functionalization of tetrabrominated azadipyrromethenes, using CuI-catalyzed cyclization, followed by BF₂ complexation. These dyes show

Full text of DOI:10.1021/acs.orglett.9b04142

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Synthesis and Semiconducting Characteristics of the BF2 Complexes
of Bisbenzothiophene-Fused Azadipyrromethenes
Wanle Sheng,†,§ Fei Chang,† Qinghua Wu,† Erhong Hao,† Lijuan Jiao,*,† Jie-Yu Wang,*,‡
and Jan Pei*,‡
†Laboratory of Functional Molecular Solids, Ministry of Education; School of Chemistry and Materials Science, Anhui Normal
University, Wuhu 241002, China
‡Beijing National Laboratory for Molecular Sciences, College of Chemistry, Peking University, Peking 100871, China
§Department of Chemistry, BengBu Medical College, Bengbu 233030, China
S
* Supporting Information
ABSTRACT: A novel type of dibenzothiophene [b]-fused
core-expanded azaBODIPYs were obtained through an
efficient post-functionalization of tetrabrominated azadipyrro-
methenes, using CuI-catalyzed cyclization, followed by BF2
complexation. These dyes show nearly planar skeletons,
strong NIR absorption with maximum peaks up to 733 nm,
and remarkable low-lying LUMO level of −4.15 eV. The field-
effect transistor based on 1b exhibits bipolar transport
properties, with the highest electron and hole mobilities up
to 0.012 and 0.046 cm2 V−1 s−1, respectively.
oron-complexed azadipyrromethenes (azaBODIPYs, Fig-
1
Bure 1),
as an emerging class of near-infrared (NIR)
organic dyes, play important roles in the traditional
applications of dyes including chemosensors,2 noninvasive
bioimaging reagents,3 and photosensitizers.4 On the other
hand, their strong NIR absorption, low optical band gaps, high
thermal stability/photostability, low-lying lowest unoccupied
molecular orbital (LUMO), and the large planar and highly
electron-deficient conjugation skeleton with large dipole
moment are ideal features for the organic optoelectronics
application.5 However, the semiconducting properties of
BODIPYs, especially azaBODIPYs, have rarely been studied
and they generally exhibited very low carrier mobilities, despite
the above favorable features.6−8
Lately, the fusion of aromatic ring(s) onto the pyrrolic
position(s) of the (aza)BODIPYs to form the so-called a-, or b-
fused (aza)BODIPYs (Figure 1) has successfully formed large
and extended π-structures with red-shifted absorptions and
large absorption coefficients.9,10 However, synthetic methods
to annulated azaBODIPYs with aromatic rings at their
peripheral positions are limited. One elegant example is the
synthesis of [a]-ring-fused azaBODIPYs using phthalonitrile as
the starting material (see Figure 1). The product, thiophene-
substituted benzo-fused azaBODIPY A, showed strong
intermolecular π−π interactions in the solid state, and was
used as π-functional materials with good electron mobility.8b
Compared with a-fusion, b-fusion is more effective to narrow
the HOMO−LUMO gap, while maintaining a low-lying
LUMO. The latter is the key to the stability toward air
oxidation.11 Thus, [b]-ring-fused azaBODIPYs could be
Figure 1. (Top) Chemical structures of parent (aza)BODIPYs, [a]-
and [b]-fused (aza)BODIPYs, and thiophene substituted benzo-[a]-
fused azaBODIPYs A. (Bottom) The construction of bisbenzothio-
phene [b]-fused azaBODIPY in this work.
promising π-architectures for organic optoelectronic applica-
tion. However, the [b]-ring-fused azaBODIPYs were very
limited, because of the synthetic challenges. In this paper,
dibenzothiophene [b]-fused azaBODIPYs were successfully
developed for the first time, using tetrabrominated azadipyrro-
methenes. We report the characterizations via X-ray analysis,
optical spectroscopy, and density functional theory (DFT)
Received: November 19, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b04142
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Letter
calculations, as well as the preliminary investigation of their
electronic properties by organic field-effect transistors
(OFETs).
The molecular structure of 1a was also determined via
single-crystal X-ray diffraction (Figure 2). The C4S five-
Organic conjugated molecular-containing benzothiophene
units have wide applications in organic electronics.12 Recently,
the Yoshikai group has successfully synthesized sulfur-
embedded π-conjugated systems by using 2,2′-diiobiaryls and
sulfur powder under the catalysis of CuI.13 Inspired by this, the
tetrabrominated BF2-complexed azadipyrromethene 4b has
been chosen first to try the reaction with sulfur powder by
using CuI as a catalyst in dimethylformamide (DMF).
Compound 4b was prepared by direct bromination of
compound 4a with bromine (see the Supporting Information
for details). This coupling reaction under 130 °C for 24 h
indeed realized the desired ring fusion; however, only BF2
removed product of the expected 1a was isolated in a low yield.
Thus, tetrabrominated azadipyrromethene 2a was used directly
in this coupling reaction, and the desired product 1a was
isolated in 77% yield after the BF2 complexation of
intermediate 7 (see Scheme 1). Azadipyrromethene 2 was
Scheme 1. Synthesis of Dibenzothiophene[b]-Fused
AzaBODIPYs 1a and 1b
Figure 2. Single-crystal structure of 1a, with thermal ellipsoids shown
at 30% probability: (a) top view, (b) dimer in one unit, and (c) crystal
packing structure. The hydrogen atoms in the structure were omitted
for clarity.
membered rings were constructed by the formation of the
sulfur bridge between the pyrrole and the phenyls at 3,5-
positions. The C−S bond lengths (1.669−1.737 Å) in 1a are
shorter than the typical C−S single bond length (1.82 Å),
which probably denotes that the S atoms are conjugated with
both the azaBODIPY core and the phenyl rings. The small
dihedral angle (1.5°−4.3°) between the two pyrroles and the
central six-membered ring indicated that the parent skeleton
has only been slightly distorted, which may be affected by the
steric and crystal packing factors. After incorporation into the
benzothiophene group, the pyrrolic rings still maintain a weak
aromaticity with a NICS(0) value of −2.1. The newly formed
thiophene rings exhibit a moderate aromaticity with a NICS(0)
value of −5.0. The molecular orbitals of 1a are fully delocalized
over the entire backbone, similar to those of compound 4c, but
their energy levels are completely different (Figure 3). The
HOMO and LUMO energy levels of 1a are −5.36 eV and
−3.31 eV, which are 0.09 and 0.23 eV lower than those of 4c,
respectively. The band gap of compound 1a is 0.14 eV less
than that of 4c, mainly due to the remarkably lowered LUMO
energy level.
The optical spectra of the target compounds 1a, 1b, and
azaBODIPY 4c have been studied in toluene (Figure 4, as well
as Table S1 in the Supporting Information). Compound 4c
exhibits typical spectral properties of azaBODIPY, with a
maximum absorption peak at 657 nm and the corresponding
emission maximum at 683 nm (ϕ = 0.25). After the
cyclization, the absorption bands of compounds 1a and 1b
undergo significant red shifts to 600−800 nm, with the
maximum absorption peaks at 715 and 733 nm for 1a and 1b,
respectively. The extinction coeffient values of the ring-fusion
compounds 1a and 1b reach 73 300 and 84 700 M−1 cm−1,
which are greater than that of 4c (56 300 M−1 cm−1). Similar
synthesized in high yields by bromination of azadipyrrome-
thene 3 with 2 equiv of bromine. Compound 1b containing
didodecyloxy groups on the aryl groups at the 1,7-positions
was also synthesized by the same method in 80% yield, which
showed enhanced solubility in common organic solvents.
1
Compounds 1a and 1b were characterized by H NMR, 13C
NMR, and high-resolution matrix-assisted laser desorption
ionization−time of flight mass spectroscopy (MALDI-TOF
MS).
B
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measured in CH2Cl2 with n-Bu4NPF6 as the supporting
electrolyte and saturated calomel electrode (SCE) as the
reference electrode. In comparison with the nonfused
azaBODIPY 4c, the [b]-fused 1a showed a reversible oxidation
wave at only a slightly more positive potential (1.22 and 1.26 V
vs SCE, respectively). However, the reversible reduction wave
exhibited a much less negative potential by 0.20 V (reduction
waves are at −0.24 and −0.44 V vs SCE, respectively). The
HOMO and LUMO energy levels of 4c were thus estimated by
redox waves to be −5.62 and −3.96 eV, respectively. In
contrast, the HOMO and LUMO energy levels of 1a reduced
to −5.66 and −4.15 eV, respectively. The electrochemical band
gap of 1a narrows to 1.51 eV, mainly resulting from the
diminution of the LUMO energy level. These results are in
good agreement with the results from theoretical calculations.
The LUMO levels (−4.15 eV for 1a) of these
dibenzothiophene[b]-fused azaBODIPYs observed above are
even lower than those of many perylene bisimides, which are
representative n-type semiconducting materials.14 Further-
more, in the crystal packing structure (Figure 2), the two
molecules of 1a are arranged into a reverse face-to-face packing
in a unit due to the dipole−dipole interaction. The center and
the vertical distances between the two intermolecular faces are
4.40 and 3.75 Å, respectively, indicating that there are strong
intermolecular interactions between two molecules. The axes
of the two molecules are not parallel, with a dihedral angle
(9.5°) of B−N−B′−N′. Therefore, the dipole interaction
between these two molecules cannot be canceled in the unit,
which will increase the interaction between the units to form a
more-ordered arrangement. Each unit is arranged in a
“herringbone” packing model with slipped π stacks along the
b-axis. Overall, it shows a double glyph pattern. This packing
model is rather rare and may have potential applications in
organic electronics.15
Figure 3. DFT-calculated NICS(0) values (in ppm), molecular
orbitals and energy diagrams of compound 4c (left) and 1a (right). H
= HOMO; L = LUMO.
The semiconducting properties for compound 1b were thus
investigated using a field-effect transistor (FET). A top-gate/
bottom-contact device architecture (TG/BC) was used to
fabricate OFETs. The conducting layer was deposited by spin-
coating the toluene solution (4 mg/mL) of 1b onto a
patterned Au/SiO2/Si substrate. After thermal annealing of the
film at 140 °C for 5 min, a well-ordered film was obtained with
an interlayer distance of ∼4.0 nm (see Figure 5, as well as
Figures S7 and S8 in the Supporting Information). The
perfluorinated CYTOP polymer was used as a dielectric layer
by spin-coating, and an aluminum layer was thermally
evaporated as the gate electrode.
Interestingly, the OFET devices exhibited bipolar charge
transport characteristics when tested under ambient con-
ditions, (Figure 5), which were not found previously for
BODIPY- and azaBODIPY-based small molecular semi-
conductors. The highest electron mobility of 0.012 cm2 V−1
s−1 with current on/off ratios of ∼10 and threshold voltage
(VT) values of 73 V, and the highest hole mobility of 0.046 cm2
V−1 s−1 with current on/off ratios of ∼70 and threshold
voltages of VT = −55 V were observed. This charge transport
property was better than most of the reported BODIPY- or
azaBODIPY-derived small molecular semiconductors.6−8 The
performance of the devices may be further improved by
optimization of the device fabrication.
Figure 4. Absorption (Abs) and fluorescence (FL) spectra of
compounds 1a, 1b, and 4c in toluene (5 × 10−6 M).
red shifts in the fluorescence spectra were also observed. In
toluene, 1a and 1b showed the maximum emission peaks at
796 and 804 nm, respectively. Similar photophysical properties
were obtained in chloroform and THF (see Figures S2−S4 and
Table S1 in the Supporting Information). Slight blue shifts of
absorption maxima and increased Stokes shifts were found in a
polar solvent (THF). Although the fusion increased the rigidity
of molecular skeleton, both compounds 1a and 1b show low
fluorescence (ϕ < 0.01) in the test solvents. In further study,
under irradiation with broadband light (>600 nm), both
BODIPYs 1a and 1b can efficiently generate singlet oxygen,
which was trapped by 1,3-diphenylisobenzofuran (DPBF) (see
Figure S5 in the Supporting Information), indicating that their
singlet excited states may be transformed to triplet states, to
some extent, through intersystem crossing.
In summary, we report a novel type of core-expanded
azaBODIPY dyes, annulated with five-membered thiophene
rings between their 3,5-aryls and 2,6-positions. These
dibenzothiophene[b]-fused azaBODIPYs were obtained
Cyclic voltammetry (CV) was used to investigate the
electrochemical properties of 1a and the reference compound
4c (Figure S6 in the Supporting Information), which were
C
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Letter
*E-mail: jieyuwang@pku.edu.cn (J.-Y. Wang).
*E-mail: jianpei@pku.edu.cn (J. Pei).
ORCID
Erhong Hao: 0000-0001-7234-4994
Lijuan Jiao: 0000-0002-3895-9642
Jie-Yu Wang: 0000-0002-1903-8928
Jan Pei: 0000-0002-2222-5361
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the National Nature Science
Foundation of China (No. 21672007, 21871006, 21722201,
21790360), and National Key R&D Program of China (No.
2017YFA0204701). The DFT calculations have been done on
the supercomputing system in the Supercomputing Center of
USTC.
Figure 5. (a) AFM images for 1b films prepared from toluene
solution. Panel (b) shows the height profile for the red line
horizontally traversing the micrograph in panel (a). (c) Transfer
characteristics of the thin-film FET device.
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AUTHOR INFORMATION
Corresponding Authors
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*E-mail: jiao421@ahnu.edu.cn (L. Jiao).
D
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