n-Channel Organic Semiconductors Derived from Air-Stable Four-Coordinate Boron Complexes of Substituted Thienylthiazoles

Article abstract of DOI:10.1002/chem.201701922

Three acceptor-π-bridge-acceptor (A-π-A) molecules derived from 2-(3-boryl-2-thienyl)thiazole have been synthesized and thoroughly characterized. Incorporation of a B?N unit into thienylthiazole and attachment of suitable acceptor moieties allowed to obtain ambient-stable A-π-A molecules with low-lying LUMO levels. Their potential for applications in organic electronics was tested in vacuum-deposited organic thin film transistors (OTFT). The OTFT device based on boryl-thienylthiazole and 1,1-dicyanomethylene-3-indanone (DCIND) acceptor moieties showed an electron mobility of ≈1.4×10^?2 cm² V^?1 s^?1 in air, which is the highest electron mobility reported to date for organoboron small molecules. Conversely, the device employing the malononitrile (MAL) derivative as an active layer did not show any charge transport behavior. As suggested by single crystal X-ray analysis of indandione (IND) and MAL derivatives, the enhanced mobility of IND (and DCIND) in comparison to the MAL molecule can be attributed to the effective two-dimensional π-stacking in the solid state imparted by the acceptor moieties with an extended π-surface.

Full text of DOI:10.1002/chem.201701922

A Journal of
Accepted Article
Title: n-Channel Organic Semiconductors Derived from Air-
Stable Four-Coordinate Boron Complexes of Substituted
Thienylthiazoles
Authors: Reinhard Hecht, Juliane Kade, David Schmidt, and Agnieszka
Nowak-Król
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n-Channel Organic Semiconductors Derived from Air-Stable Four-
Coordinate Boron Complexes of Substituted Thienylthiazoles
Reinhard Hecht,[a,b] Juliane Kade,[b] David Schmidt,[a,b] and Agnieszka Nowak-Król*[a,b]
Abstract: Three acceptor-π-bridge-acceptor (A-π-A) molecules
derived from 2-(3-boryl-2-thienyl)thiazole have been synthesized and
thoroughly characterized. Incorporation of
a
B-N unit into
thienylthiazole and attachment of suitable acceptor moieties allowed
to obtain ambient-stable A-π-A molecules with low-lying LUMO levels.
Their potential for applications in organic electronics was tested in
vacuum-deposited organic thin film transistors (OTFT). The OTFT
device based on boryl-thienylthiazole and 1,1-dicyanomethylene-3-
indandione (DCIND) acceptor moieties showed an electron mobility of
~1.4 × 10–2 cm2 V–1 s–1 in air, which is the highest electron mobility
reported to date for organoboron small molecules. Conversely, the
device employing the malononitrile (MAL) derivative as an active layer
did not show any charge transport behavior. As suggested by single
crystal X-ray analysis of indandione (IND) and MAL derivatives, the
enhanced mobility of IND (and DCIND) in comparison to the MAL
molecule can be attributed to the effective two-dimensional π-stacking
in the solid state imparted by the acceptor moieties with an extended
π-surface.
Figure 1. Design strategy towards electron-transport materials.
three-coordinate boron compounds are intrinsically sensitive to
attack of nucleophiles, such as water, which limits the application
of these organoboron materials.[1a,b,9] Common strategies that are
applied to circumvent this problem aim either at protecting the
boron center by bulky substituents[1b,10] or embedding a boron
atom in a rigid π-conjugated scaffold.[11] Nevertheless, in terms of
chemical and air-stability four-coordinate boron compounds are,
in general, superior. In the boron-bridged molecules, this
stabilization is imparted by the donation of a lone pair of an
appropriately located heteroatom to the empty boron orbital,
hence forming an electronically saturated compound. Importantly,
boron-bridging increases rigidity of the molecular framework and
extends the π-conjugated system, often by keeping two subunits
of the molecule in a planar arrangement. A strong conformational
constraint caused by chelation to the boron atom is an essential
tool for tailoring photophysical properties of the parent systems.
To date, a plethora of four-coordinate boron compounds
bearing different ligands and substituents on the boron atoms
have been synthesized. In this group of compounds a choice of
the ligand and boron substituents is, however, also critical for
stability and properties. For instance, some BF2 complexes of
diketonates undergo hydrolysis in contact with moisture,[12] while
sterically demanding mesityl (Mes) substituents on the boron
center can induce stimulus-responsive material behavior.[13] Yet,
there are plenty of molecules whose exciting photophysical
features can be explored in air- and moisture-stable devices,
mainly in OLEDs, or as imaging materials. More specifically, some
BODIPY derivatives have been commercialized and are sold as
fluorescent biological labels (Invitrogen BODIPY® dyes[14] or
CHROMIS 500 N dyes).[15]
Introduction
A strong electron-accepting ability of boron endows organic
aromatic scaffolds with valuable photophysical and electronic
properties, such as low energies of the lowest unoccupied
molecular orbitals (LUMOs), increased electron affinity, and both
bathochromically shifted absorption and emission spectra.[1]
These properties in combination with high fluorescence efficiency
account for the rapid progress in the field of organoboron
materials over the last twenty years. Accordingly, three- and four-
coordinate boron complexes were intensively investigated for
applications in organic light emitting diodes (OLEDs),[2] organic
lasers,[3] nonlinear optics,[1a,4] fluorescence imaging[5] as well as
photoresponsive materials,[6] and memory devices.[7] In addition,
a strong electron affinity of three-coordinate boron with the vacant
p-orbital on the boron center makes them attractive molecules for
anion sensing.[8] On the other hand, due to a strong Lewis acidity,
[a]
R. Hecht, Dr. D. Schmidt, Dr. A. Nowak-Król
Center for Nanosystems Chemistry (CNC) & Bavarian Polymer
Institute (BPI)
Universität Würzburg
Theodor-Boveri-Weg, D-97074 Würzburg (Germany)
E-mail: agnieszka.nowak-krol@uni-wuerzburg.de
R. Hecht, J. Kade, Dr. D. Schmidt, Dr. A. Nowak-Król
Institut für Organische Chemie
[b]
Universität Würzburg
Am Hubland, D-97074 Würzburg (Germany)
Supporting information for this article can be found under
http://dx.doi.org/10.1002/chem.2017xxxxx.
In our studies on electron-transport and acceptor materials
for organic electronics we were looking for interesting platforms
that would display favorable properties, such as low-lying LUMO
1
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Scheme 1. Synthesis of A-π-A dyes with B-N dative bonds.
levels and high tunability of optical and electrochemical properties, structures, high reproducibility and purity without batch to batch
which upon chemical functionalization would provide materials of
suitably adjusted frontier molecular orbital (FMO) levels.
variations.
The properties of this platform in small molecules have been
Previously,
we
investigated
dithienopyrrole
and
elucidated only for dimeric regioisomers described by
Yamaguchi,[17] and very recently, for π-conjugated systems
reported by Wakamiya.[19] Herein, we describe the synthesis,
optical, electrochemical and structural characterization of a set of
A-π-A molecules derived from 2-(3-boryl-2-thienyl)thiazole. In this
work we were particularly interested whether the introduction of
this boron-heterocyclic core into A-π-A systems would provide the
opportunity for fine-tuning FMOs to the level required for ambient
stable n-channel semiconductors or acceptor materials for
photovoltaics. As we will show, by a proper combination of the
acceptor unit and boryl-substituted thienylthiazole, we were able
to obtain an electron-transport material which was successfully
utilized in organic field effect transistors (OFET), showing the
highest electron mobility observed to date for OFET devices
based on organoboron small molecules.
cyclopentadithienyl (CPDT) derivatives and observed a decrease
of HOMO/LUMO levels by ca. 0.3 eV upon replacement of
nitrogen for carbon as a bridging atom (Figure 1).[16] In this work,
we planned to utilize boron to tune photophysical and charge
transport properties of the π-core. Four-coordinate boron
molecules in view of their valuable features appear to be well-
suited molecules for our studies. Accordingly, we selected (3-
boryl-2-thienyl)-2-thiazole for the central unit of A-π-A systems
(see Figure 1). This scaffold, first introduced by Yamaguchi and
co-workers, combines a common tendency of boron complexes
to lower the LUMO level with the intrinsically high electron affinity
of N-heterocyclic aromatic compounds containing C-N double
bonds. Its potential as a building block for n-type organic
semiconductors was verified by showing moderate electron
mobility (μn = 1.5 × 10–4 cm2 V–1 s–1) for one of three dimeric boryl-
thienylthiazoles.[17] Later, Liu and co-workers inserted this
interesting core into long polymer chains with alternating boron-
bridged thienylthiazole and acceptor units.[18] Some of the new
boron-containing polymers were utilized as acceptors in organic
solar cells. Despite these prominent demonstrations, polymeric
materials have some deficiencies when compared to small
molecules. That is, the individuality of the repeating unit is lost in
the polymer. Moreover, polydispersity along with a formation of
various regioisomers (i.e. head-to-tail, and head-to-head bonding
modes) limit the possibility to gain deeper insight into the basic
optical and electrochemical properties of a particular repeating
unit, whereas small molecules benefit from well-defined molecular
Results and Discussion
Design considerations and synthesis. Our molecular design of
A-π-A molecules assumes attachment of two identical acceptor
units to a thienylthiazyl-boron-core serving here as a π-bridge. To
this end, we have selected three acceptor moieties, i.e.
malononitrile, commonly used in organic electronics, and two CH-
acids: indandione (IND) and 1,1-dicyanomethylene-3-indandione
(DCIND) with a rigid framework and extended aromatic surface.
Previously, we utilized the latter methylene compounds in A-π-A
systems based on dithienopyrrole and cyclopentadithienyl
2
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cores.[16] They also proved effective to tune FMO levels and a
packing arrangement in a large variety of merocyanines.[20] An
introduction of electron-withdrawing substituents was performed
by Knoevenagel condensation between dialdehyde boryl-
substituted thienylthiazole and respective methylene compounds.
Thus, in contrast to previous 2-(3-boryl-2-thienyl)thiazole
derivatives,[17-18] the boryl-substituted π-scaffold and acceptor
components are intervened by ethylene linker to ensure full
coplanarity and enhance the conjugation. Lastly, we decided to
introduce mesityl groups on the boron center, which endowed the
synthesized compounds with excellent stability and good
solubility required to pursue these studies.
The synthetic route towards A-π-A molecules with boron-
nitrogen dative bond is illustrated in Scheme 1. 3-Bromo-2-
iodothiophene (1)[21] and 2-(tributylstannyl)thiazole (2)[22] were
prepared according to the reported procedures. Stille reaction of
these coupling partners in toluene afforded thienylthiazole 3 in the
excellent yield of 98%. Subsequent lithiation thereof, followed by
the reaction with Mes2BF was performed in a similar fashion to
the procedure reported by Yamaguchi and co-workers.[17]
Functionalization of dimesitylboryl compound 4 with nBuLi and
DMF produced dialdehyde 5 in 66% yield. Finally, Knoevenagel
condensation between CH-acids 6a-c and dialdehyde 5 afforded
three target molecules 7a-c in moderate to good yields. To ensure
high conversion rate we used 2 eq. of the methylene compounds
per formyl group.
Figure 3. UV/Vis absorption spectra of dyes 7a-c measured at c ~ 10–5 M in
CH2Cl2.
B-C(thiophene) bonds were observed in the crystal of 7a
(1.647(5) Å), while the two B-C(Mes) bonds were 1.643(5) Å and
1.629(5) Å long. Boron atoms in both cases adopt a distorted
tetrahedral geometry to form N,C-chelate five membered rings.
Thienylthiazole ligands display smaller C(thiophene)-B-N angles
than the ideal angle of 109.5°. The corresponding values are
94.2(1)°, and 93.7(3)°in molecules 7b, and 7a, respectively.
Conversely, the C(Mes)-B-C(Mes) angles (115.4(1)° in 7b and
120.2(3)° in 7a) are larger than the ideal angle. The details of the
X-ray measurements and the crystal data are described in the
Supporting Information.
Solid state structural analysis. Single crystals of 7b suitable for
X-ray analysis were obtained by slow evaporation of toluene
(Figure 2c,d) at room temperature. The dihedral angle between
thiophene and thiazole rings is 3.3°, which shows that the two
fragments are nearly co-planar. The dihedral angles between
indandione unit and a boryl core mean plane linked by vinylene
bridge are 2.1° and 2.0° on the side of thiophene and thiazole,
respectively. Hence, the acceptor moieties are effectively
conjugated to the boryl central core. The crystal of 7a was
obtained by slow evaporation of CD2Cl2 at room temperature. The
dihedral angle between thiophene and thiazole ring is 4.4°, which
is comparable to that in 7b. The acceptor units are twisted out of
the mean plane of the thienylthiazole-boron core by 11.1° and
4.3° on the sides of thiophene and thiazole, respectively (Figure
2a,b). The B-N and B-C(thiophene) bond lengths are 1.650(2) and
1.645(2) Å, respectively, and both B-C(Mes) bond lengths are
1.637(2) and 1.638(2) Å in the crystal of 7b. Equal lengths of B-N,
Absorption properties. Optical properties of A-π-A dyes 7a-c
were investigated by steady-state absorption and emission
spectroscopy in methylene chloride. The UV/Vis absorption spectra
are shown in Figure 3 and the data are listed in Table 1. All A-π-A
dyes feature moderate to high absorption coefficients in the range
of 3.5 x 104 to 6.7 x 104 M–1 cm–1. The corresponding square
transition dipole moments μ2eg of dyes 7a, 7b, and 7c are 82, 118,
and 140 D2, respectively. Compound 7a exhibits an orange color in
solution (Figure S1), which corresponds to a broad absorption band
located at 454 nm with a shoulder at ca. 495 nm. In comparison to
Table 1. Optical and electrochemical properties of dyes 7a-c and 8b,c.
ox
red
[d]
[d]
[a]
[b]
[c]
μ2
퐸
퐸
λabs
εmax
eg
1/2
1/2
Dye
[nm]
[104 M–1 cm–1
]
[D2]
[V]
[V]
7a
7b
454
530
499
589
554
584
664
3.5
5.6
5.4
6.7
6.2
82
118
–1.00, –1.25
+0.91[e]
–1.12, –1.49[e]
+0.89,[e]
+1.01,[e] +1.17[e]
+0.90[e]
7c
140
–0.84[e,f]
8b[16]
8c[16]
12.2
13.1
138
167
–1.30
+0.84
+1.08
–0.84[e]
[a] Absorption maximum and vibronic progression in CH2Cl2. [b] Molar absorption
coefficient in CH2Cl2. [c] Square transition dipole moment calculated for the lowest
energy transition of the UV/Vis absorption spectrum in CH2Cl2. [d] Redox
potentials were measured in dry CH2Cl2 (c ~ 10–4 – 10–5 M) at a scan rate of 100
mV s–1 and with 50% of iR compensation; supporting electrolyte Bu4NPF6 (c = 0.1
M). Measurements were calibrated vs. the ferrocenium/ferrocene (Fc+/Fc) redox
couple as an internal standard. [e] Peak potential. [f] Further reduction processes
were observed but peak potentials could not be resolved.
Figure 2. Solid-state molecular structures determined by single-crystal X-ray
diffraction of compound 7a obtained from CD2Cl2 a) front view, b) side view, and
compound 7b obtained from toluene c) front view, d) side view; ellipsoids set at
50% probability. Crystal of 7b contains toluene molecules.
3
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molecule 7a, the lowest-energy absorption bands of 7b and 7c
are bathochromically shifted. This is due to the extension of the
conjugated system in 7b,c. Furthermore, a larger shift observed
for 7c can be attributed to a stronger electron accepting nature of
the DCIND vs. IND moieties. Solution of 7b and 7c exhibit red and
purple colors in CH2Cl2 (Figure S1). The spectra of these
compounds reveal broad absorption bands centered at 530 nm
and 589 nm, respectively, with vibronic progressions at 499 for 7b,
and 554 nm for 7c. The emission of 7a-c was effectively quenched
in CH2Cl2. These results may suggest charge transfer (CT)
interactions as one of the possible deactivation pathways for all
three A-π-A systems.
Figure 4. a) Cyclic voltammograms of 7a-c calibrated versus
ferrocenium/ferrocene (Fc+/Fc) (c ~ 10–4 – 10–5 M in dry CH2Cl2; scan rate = 100
mV s–1, 0.1 M Bu4NPF6 as supporting electrolyte, 50% iR compensated). The
cyclic voltammogram of compound 7c was multiplied by factor four for better
visibility. b) HOMO and LUMO levels and electrochemical band gaps (solid
area) of compounds 7a-c and 8b,c.
Electrochemistry. Electrochemical analysis allowed to assess
frontier molecular orbitals of these A-π-A molecules. Redox
potentials of the compounds were measured by cyclic
voltammetry (CV) in CH2Cl2 in the presence of Bu4NPF6 as a
supporting electrolyte and calibrated vs. ferrocenium/ferrocene
(Fc+/Fc). The cyclic voltrammograms are shown in Figure 4a,
while the electrochemical properties are summarized in Table 1.
Molecule 7a bearing two dicyanomethylene groups undergoes
two reversible reduction processes at −1.00 and −1.25 V. These
potentials are comparable to the reduction potentials of a core-
unsubstituted perylene bisimide (−0.95, −1.15 V) which was
successfully applied as an n-type semiconducting material,[23] and
correspond well with the position of the LUMO of fullerenes (−3.8
to −4.3 eV).[24] Furthermore, the reversible reduction processes of
indandione derivative 7b are cathodically shifted vs. potentials of
7a to −1.12, and −1.49 V, which implies that reduction is more
feasible in less electron-rich 7a. The more pronounced
modulation of electron density in A-π-A compounds was achieved
upon substitution of the boryl core with strongly electron-
accepting DCIND substituents. The first irreversible reduction is
located at −0.84 V, followed by several reduction processes which
could not be resolved satisfactorily. This is due to a very low
solubility of compound 7c and concomitant low measured
currents. Nevertheless, the measurements of redox potentials by
more sensitive square-wave voltammetry confirmed the results
obtained by CV.
The measurements also show that radical anions of 7a and
7b are rather stable, unlike the radical anion of molecule 7c. In
contrast to the impact of the substitution on reduction processes,
a variation of the acceptor units has little effect on the oxidation
processes. The first oxidation potentials of all A-π-A dyes 7a-c
are observed at comparable values of +0.91, +0.89, and +0.90 V
for 7a, 7b, and 7c, respectively, and correspond to irreversible
processes. Further oxidation processes were measured at +1.01,
+1.17 V for 7b. The energy levels of FMOs were determined from
the CV measurements assuming the energy level of Fc+/Fc to be
at −5.15 eV vs. vacuum.[25] Figure 4b illustrates the band gaps
and the positions of the FMOs along with the LUMO level of
PC61BM for comparison. The highest occupied molecular orbital
(HOMO) levels of A-π-A systems 7a-c are virtually equal (~ −6.05
eV), while their lowest unoccupied molecular orbitals (LUMOs)
vary by 0.28 eV and decrease in the following order: 7b (−4.03
eV) > 7a (−4.15 eV) > 7c (−4.31 eV). Therefore, as demonstrated,
we were able to tune significantly the electrochemical behavior of
the parent boryl-substituted thienylthiazole via substitution of the
core with electron-withdrawing groups. Moreover, low-lying FMO
levels indicate high electron affinity of these dyes. The calculated
band gaps are in the range of 1.74-2.01 eV, which makes these
dyes suitable candidates for solar light harvesters. To evaluate the
impact of the boron-bridging on the electrochemical properties of
thienylthiazole molecules, we compared the newly synthesized
boron complexes with CPDT A-π-A compounds 8b and 8c
(Scheme 1). The first reduction potentials (and the corresponding
LUMO levels, see Figure 4b) of DCIND dyes 7c and 8c are equal,
whereas the reduction potential of IND derivative 7b was shifted
anodically by 0.18 eV vs. 8b. This moderate change in LUMO
levels has significant consequences on the utilization of
indandione derivatives in organic field effect transistors, i.e. 7b is
an n-type, while 8b a p-type semiconductor.
Quantum chemical calculations. To elucidate the effect of the
B-N coordinative bond on the properties of the new systems, we
conducted density functional theory (DFT) calculations at the
B3LYP[26]/def2-SVP[27] (solvent CH2Cl2, PCM model) level of
theory for all the synthesized A-π-A molecules. The optimized
geometries of dyes 7a-c and their electrostatic potential maps are
displayed in Figure S2. The energies and contour plots of selected
Kohn-Sham molecular orbitals, including HOMOs and LUMOs,
are illustrated in Figure 5. According to the calculations, all the
studied molecules are endowed with essentially planar molecular
scaffolds. The HOMOs of 7a-c are confined to one electron-rich
mesityl moiety, while the LUMOs are delocalized over the entire
thienylthiazole scaffold and adjacent acceptor units. The next
highest occupied molecular orbital (HOMO–1) is composed
largely of the π orbital of the second mesityl substituent.
Electrostatic potential surfaces show that the electron density is
not equally redistributed over the thienylthiazole core pointing that
thiazole is more electron poor than a thienyl subunit. Furthermore,
indandione derivative features the highest, while malononitrile
counterpart – the lowest electron density in the set of synthesized
A-π-A systems.
To get insight into the nature of the lowest-energy
absorption band we performed TD-DFT calculations (CAM-
B3LYP[28]/def2-SVP, solvent CH2Cl2, PCM model). The results
indicate that S0→S1 transitions of 7b,c bearing structurally similar
acceptor moieties are mostly attributed to HOMO to LUMO (60-
67%) and HOMO–1 to LUMO (18-21%) transitions (Tables
S3,S4), which are accompanied by the charge transfer from
mesityl substituents to the core with adjacent acceptor units. As
suggested by TD-DFT calculations, an experimentally observed
broad and irregular absorption band of 7a in CH2Cl2 is a
superposition of three transitions and should be ascribed to
4
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Figure 5. Kohn–Sham orbital energy level diagram of 7a-c calculated at the B3LYP/def2-SVP (solvent CH2Cl2, PCM model) level of theory and contour plots of
selected orbitals (isovalue = 0.02 a.u.).
a mixture of mainly HOMO to LUMO, HOMO–1 to LUMO, and
HOMO–4 to LUMO transitions (Table S2). In addition to CT from
Mes groups to the core, the lowest-energy transition has also
some contribution from the π-π* transition of the chelate
backbone. The complete TD-DFT data can be found in the
Supporting Information.
featuring a small π-scaffold, was used as an active material, the
OTFTs did not work. Conversely, compounds 7b and 7c, whose
chromophores are much more extended by the IND or DCIND
acceptors, showed semiconducting n-type behavior. Electron
mobility of 5.55 × 10−4 cm2 V–1 s–1 [μmax = 6.05 × 10−4 cm2 V–1 s–1],
a current on/off ratio of 4 × 103 and a threshold voltage of +1 V
could be measured for the devices based on molecule 7b. Even
Device fabrication. To investigate the semiconducting behavior
of the new dyes, organic thin film transistor devices (OTFTs) in a
bottom-gate, top-contact configuration were prepared by vacuum
deposition of the organic materials on Si/SiO2/AlOx substrates,
covered with a monolayer of n-tetradecylphosphonic acid (TPA).
The devices had a channel length L of 100 μm and a channel
width W of 200 μm, defined by gold contacts. The OTFTs were
characterized by measuring transfer and output curves under
ambient conditions. The charge carrier mobility μ was estimated
according to the following equation: μ = 2 IDS L/[W Ci(VGS – VT)2],
where IDS describes the drain-source current, Ci the capacitance
of the gate dielectric, VGS denotes the gate-source voltage, VT the
threshold voltage and L and W are defined as transistor channel
length and width, respectively. Thermal properties of 7a-c were
investigated by differential scanning calorimetry (DSC). DSC
curves for 7a and 7b indicate that these compounds are stable
until melting at 230.3 and 268.8 °C, respectively, whereas 7c is
robust up to ~ 350°C and does not show any isotropization peak
(Figure S6). Thus, the boron compounds 7a-c exhibit high thermal
stability and are suitable for vacuum deposition. When 7a,
better electron mobility as high as 1.39 × 10−2 cm2 V–1 s–1 [μmax
=
1.56 × 10−2 cm2 V–1 s–1], an on/off ratio of 2 ×106 and a VT of +6 V
were measured for devices with compound 7c. The transfer
curves and square root plots of the drain-source current of
devices based on 7b and 7c are shown in Figure 6a. Both yielded
transfer curves with significant hystereses.
The atomic force microscopy images of the corresponding
devices (Figure 6b and c) show that 7b exhibits a densely packed
and uniform surface morphology, while the thin film of 7c shows
much more defined structures, indicating a higher crystallinity.
At the microscopic scale, the efficiency of charge transport
in organic semiconductors is determined by two key parameters,
i.e. the electronic coupling (transfer integral) between adjacent
molecules and the reorganization energy (λ). The charge carrier
mobility is higher if the first factor is maximized and when λ is as
small as possible. To rationalize the observed differences in
electron mobility of the new A-π-A molecules, we calculated the
inner-sphere reorganization energy (λi) (for details, see the
Supporting Information). The obtained λi values for the molecules
5
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We rationalize a better performance of molecules 7b and 7c
in OTFT devices by the increased interchromophoric interactions
imparted by the acceptor units with an extended π-surface. The
behavior of 7b can be related to its packing arrangement. In
contrast to crystals of 7a, crystals of 7b include solvent molecules
(i.e. 2.5 molecules of toluene per one molecule of 7b).
Nevertheless, it is clear that 7b shows a tendency to pack in a
two-dimensional (2D) layer structure. In the molecular packing
structure, the molecules are arranged in a slipped fashion in a 2D
array. That is, parallel-oriented molecules (red color in Figure 6e,f)
form one-dimensional stacks with the interplanar distances of
3.27 Å. The distance between mean planes of antiparallel-
oriented molecules (red and green color in Figure 6e,f) is 3.32 Å.
An acceptor moiety of one molecule is located partially over a
vinylene bridge and an acceptor moiety of an adjacent molecule,
which results in effective π-orbital overlaps (short contacts 3.34-
3.39 Å). These stacks are interacting with neighboring arrays of
antiparallel-oriented molecules via C-H···π interactions between
indandione C-H and mesityl π-surface, as well as π-π interactions
between thiophene ring and a vinylene bridge of one molecule
with an acceptor moiety of an adjacent molecule (short contacts
of 3.34-3.38 Å), and interaction between acceptor units of both
molecules with antiparallel orientation (short contacts of 3.36 Å).
Thus, there is an evident communication between neighboring
layers.
Importantly, both 7b and 7c display n-type semiconducting
behavior, whereas in the structurally related CPDT the charge-
transport behavior is dependent on the type of acceptor moieties.
These results evidence that the strategy based on a) achieving
low LUMO energy levels by applying a boron-bridge combined
with b) the extension of the π-scaffolds by a certain type of
acceptor units proved an effective way to obtain n-type
semiconducting materials. In particular, dye 7c with mobility ca.
1.4 × 10−2 cm2 V−1 s–1 constitutes a potential candidate for further
applications in vacuum processed organic solar cells, e.g. for
replacement of fullerene acceptors.[30] It is worth noting that
electron mobilities of organoboron small molecules reported to
date for OFET devices[12,31] or obtained from time-of-flight (TOF)
carrier-mobility measurements[17,32] are usually at least 10 times
lower than electron mobility observed for 7c. A further increase in
mobility can be most probably achieved upon replacement of
mesityl groups with less sterically demanding substituents on the
boron center and extension of the core.
Figure 6. a) Transfer curves and square root plots of the drain-source current
with corresponding linear fit (black line) for OTFT devices based on molecule
7b (red) and 7c (blue). The black arrows are indicating the relevant y-axis.
Devices were prepared on TPA-modified substrates and measured under
ambient conditions. AFM morphology measured between the contacts of the
corresponding devices of molecules b) 7c, and c) 7b. The packing arrangement
of d) 7a, and e) and f) 7b. Solvent molecules and hydrogen atoms are omitted
for clarity.
increase in the following order: 7c (0.235 eV) < 7a (0.266 eV)
< 7b (0.307 eV). 7c features the smallest reorganization energy
and shows the highest electron mobility. For comparison, the
reported λi for the common electron transport material Alq3 is
0.276 eV.[29] The reorganization energy of the malononitrile
derivative is smaller than that of 7b. However, the devices
employing 7a as an active layer did not show any charge transport
behavior, in contrast to the devices based on 7b. These results
indicate an important role of the packing arrangement and
intramolecular interactions between adjacent molecules.
Conclusions
The newly synthesized A-π-A molecules display attractive
properties along with exceptional stability. We showed that by a
gradual increase in the acceptor strength from IND to DCIND we
were able to systematically lower LUMO levels of the boron-
bridged A-π-A dyes. A variation of acceptor substituents in
combination with a B-N motive in the central π-core afforded
promising molecules with low band gaps and LUMO levels
comparable to PC61BMs LUMO level. To demonstrate the
promising features of A-π-A molecules for organic electronics,
thin film transistors were fabricated. OFET devices using 7c as a
semiconducting layer were found to exhibit electron mobility
under ambient conditions of ca. 1.4 × 10–2 cm2 V–1 s–1, which is
the highest value reported to date for organoboron small
molecules. The superior behavior of DCIND and IND derivatives
The steric hindrance of the bulky mesityl groups in
combination with the small size of the π-scaffold in molecule 7a
could prevent a close π-stacking and a good orbital overlap
between the molecules. In the packing arrangement of derivative
7a only one-dimensional stacks are observed. The molecules are
aligned parallel with the interplanar distance of 3.52 Å in a slipped
arrangement (Figure 6d). This alignment is governed by the
bulkiness of mesityl group and electrostatic interactions between
electron-rich thiophene ring and a dicyanomethylene group of
adjacent molecule, i.e. a short contact of 3.41 Å is observed
between sp2 carbon atom of C(CN)2 and sulfur thiophene atom of
a neighboring molecule. Due to lack of π-overlaps, interactions
between adjacent molecules are effectively diminished.
6
This article is protected by copyright. All rights reserved.

10.1002/chem.201701922
Chemistry - A European Journal
FULL PAPER
next 1.5 h. Afterwards, the reaction was quenched with a saturated
aqueous solution of NH4Cl. A water layer was extracted with CH2Cl2, dried
over Na2SO4, filtered and evaporated. The crude product was purified by
column chromatography (silica; pentane/EtOAc 9:1) and crystallized
(EtOAc/pentane) to give 5 (225 mg, 66%) as orange crystals. M.p. 215-
218 °C (EtOAc/pentane). 1H NMR (400 MHz, CDCl3, 25 °C) δ 9.96 (s, 1H),
9.90 (s, 1H), 8.32 (s, 1H), 7.87 (s, 1H), 6.70 (s, 4H), 2.20 (s, 6H), 1.87 (s,
12H). 13C NMR (100 MHz, CDCl3, 25 °C) δ 184.4, 180.6, 170.2, 152.7,
145.2, 140.1, 138.8, 138.7, 135.8, 135.5, 130.1, 24.6, 20.9 (two carbon
signals corresponding to the C atoms bound to the B atom are not visible
due to the quadrupolar relaxation). MS HR (ESI) m/z calcd for
C27H26BNNaO2S2 [M+Na]+ 494.1390, found 494.1386. Anal. calcd for
C27H26BNO2S2: C, 68.79, H, 5.56, N, 2.97, S, 13.60. Found: C, 68.71, H,
5.69, N, 2.92, S, 13.69.
to a malononitrile derivative can be attributed to a 2D packing
arrangement and stronger interchromophoric interactions which
are effectively enhanced by the presence of the acceptor units
with an extended π-surface. Moreover, a comparison of boron-
bridging with carbon-bridging in structurally related indandione
derivatives indicates that incorporation of boron into an aromatic
scaffold is an effective way towards n-channel materials.
Experimental Section
Experimental.
General. All reagents were purchased from commercial sources and used
as received without further purification, unless otherwise stated. Reagent
grade solvents were distilled prior to use. Column chromatography was
performed on silica (silica gel, 230–400 mesh). 1H, 13C, 11B NMR, and 2D
NMR spectra were recorded on a Bruker Avance 400 spectrometer or a
Synthesis of compound 7a. To dialdehyde 5 (100 mg, 0.21 mmol) and
compound 6a (56 mg, 0.85 mmol) in dichloroethane (8 mL) dry pyridine
(0.17 mL, 2.11 mmol) was added and the reaction mixture was stirred at
60 °C for 1 h 45 min. Then the solution was loaded on a column and
chromatographed (silica, CH2Cl2). The consecutive chromatography (silica,
CH2Cl2/pentane 4:1) afforded pure 7a which was recrystallized
(CH2Cl2/pentane) to give 7a (92 mg, 77%) as a red solid. 1H NMR (400
MHz, CD2Cl2, 25 °C) δ 8.23 (d, J = 0.5 Hz, 1H), 7.91 (d, J = 0.5 Hz, 1H),
7.86 (d, J = 0.5 Hz, 1H), 7.71 (d, J = 0.5 Hz, 1H), 6.69 (s, 4H), 2.18 (s, 6H),
1.86 (s, 12H). 13C NMR (101 MHz, CD2Cl2, 25 °C) δ 168.6, 151.7, 148.2,
148.1, 144.6, 142.3, 140.6, 139.8, 136.8, 131.8, 130.4, 113.9, 113.4, 113.0,
112.8, 83.3, 81.9, 24.6, 21.1 (two carbon signals corresponding to the C
atoms bound to the B atom are not visible due to the quadrupolar
relaxation). 11B NMR (128 MHz, CD2Cl2, 25 °C) δ 3.24. UV/Vis (CH2Cl2, c
= 6.89 × 10−5 M): λmax/nm 454 (ε/M−1 cm−1 35 000), 325 (17 000). MS LR
(MALDI) m/z calcd for C33H26BN5S2 M− 567.2 found 567.1. MS HR (ESI)
m/z calcd for C33H26BN5S2 M− 567.1723, found 567.1609. MS HR (ESI)
m/z calcd for C24H15BN5S2 [M−Mes]+ 448.0856, found 448.0873.
1
Bruker DMX 600 spectrometer H, and 13C NMR spectra were calibrated
to the residual solvent signals. 11B NMR spectra were calibrated to the
signal of boron trifluoride diethyl etherate (BF3·OEt2) as external standard.
J values are given in Hz. The following abbreviations were used to
designate multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet.
High resolution mass spectra were obtained by electrospray ionization
(ESI) and were recorded on an ESI micrOTOF Focus spectrometer from
Bruker Daltonics. Low resolution mass spectra were obtained by matrix-
assisted laser desorption/ionisation (MALDI) and were recorded on an
Autoflex II MALDI-TOF mass spectrometer (Bruker Daltonik GmbH).
Melting points were determined on an optical microscope and are
uncorrected. Malononitrile (6a) and 1,3-indandione (6b) were
commercially available. Compounds 1[21] and 2,[22] and 6c[33] were
prepared according to the reported procedures.
Synthesis of compounds 7b. To dialdehyde 5 (100 mg, 0.21 mmol) and
compound 6b (124 mg, 0.85 mmol) in dichloroethane (8 mL) dry pyridine
(0.17 mL, 2.11 mmol) was added and the reaction mixture was stirred at
60 °C for 2.5 h. Then the solution was loaded on a column and
chromatographed (silica, CH2Cl2) to give product 7b which was crystallized
(CH2Cl2/pentane) to give pure 7b (81 mg, 53%) as a dark red solid. 1H
NMR (400 MHz, CD2Cl2, 25 °C) δ 9.89 (s, 1H), 8.31 (s, 1H), 7.98–7.88 (m,
2H), 7.83–7.77 (m, 3H), 7.75 (s, 1H), 6.59 (s, 4H), 2.09 (s, 6H), 1.79 (s,
12H). 13C NMR (101 MHz, CD2Cl2, 25 °C) δ 190.3, 189.83, 189.76, 188.8,
171.2, 149.0, 148.0, 144.3, 142.8, 142.4, 141.4, 141.2, 140.6, 140.4, 136.5,
136.4, 136.1, 136.0, 135.9, 135.1, 132.9, 131.5, 130.4, 128.1, 127.6,
124.99, 123.96, 123.7, 24.9, 21.0 (two carbon signals corresponding to the
C atoms bound to the B atom are not visible due to the quadrupolar
relaxation). 11B NMR (128 MHz, CD2Cl2, 25 °C) δ 5.17. UV/Vis (CH2Cl2, c
= 1.02 × 10−5 M): λmax/nm 530 (ε/M−1 cm−1 56 000), 493 (54 000), 361 (21
00) with two shoulders. MS HR (ESI) m/z calcd for C45H35BNO4S2 [M+H]+
728.2095, found 728.2087.
UV/Vis absorption and fluorescence spectroscopy. UV/Vis absorption
measurements were performed using Lambda 950 (Perkin-Elmer) or
Jasco V-670 including NCP-706 thermostatted 6-position automatic cell
changer. The spectra were measured in spectroscopic grade solvents
from ACROS at a concentration of about 10–6 - 10–5 M–1 in quartz cuvettes
with path lengths of 1 cm at room temperature. Fluorescence was
investigated on a QM-4/2003 (PTI) by the optical dilution method (OD <
0.05).[34]
Electrochemical analysis. The CV measurements were performed on a
standard, commercial electrochemical analyzer (EC epsilon; BAS
Instruments, UK) in a three electrode single-compartment cell under an
argon atmosphere. Dichloromethane (HPLC grade) was dried over
calcium hydride under an argon atmosphere, distilled, and degassed prior
to use. The supporting electrolyte NBu4PF6 was synthesized according to
the literature,[35] recrystallized from ethanol/water, and dried in a high
vacuum. The measurements were carried out in dichloromethane/0.1 M
NBu4PF6 under the exclusion of air and moisture at a concentration of 2.5
× 10−4 − 1.1 × 10−3 M with the ferrocenium/ferrocene redox couple (−5.15
eV vs. vacuum)[25] as an internal standard for the calibration of the potential.
A cyclic voltammogram of 7c was measured at a lower concentration due
to the low solubility of the compound. Working electrode: Pt disc (Ø 3 mm);
reference electrode: Ag/AgCl; auxiliary electrode: Pt wire. The internal
resistance was compensated by 50 %.
Synthesis of compound 7c. The flask was charged with compound 5
(100.0 mg; 0.21 mmol) and CH-acid 6c (165 mg; 0.85 mmol). Then DCE
(8 mL) and pyridine (0.17 ml; 2.11 mmol) were added and the reaction
mixture was stirred at 60 °C for 2 h. Afterwards, the reaction was cooled
to RT and the precipitate was filtered and washed with acetone (150 mL).
The resulting solid was suspended in the mixture of acetone and pentane
and filtered to give 7c (130 mg, 74%) as dark purple crystals. 1H NMR (400
MHz, C2D2Cl4, 125 °C) δ 8.86 (s, 1H), 8.83–8.74 (m, 2H), 8.72 (s, 1H),
8.37 (s, 1H), 8.07 (t, J = 5.9 Hz, 2H), 7.97–7.77 (m, 5H), 6.76 (s, 4H), 2.27
(s, 6H), 2.06 (s, 12H). 1H NMR (600 MHz, C2D2Cl4, 75 °C) δ 8.88 (s, 1H),
8.82–8.64 (m, 3H), 8.42–8.29 (m, 1H), 8.14–7.99 (m, 2H), 7.97–7.78 (m,
5H), 6.72 (s, 4H), 2.24 (s, 6H), 1.95 (s, 12H). 13C NMR (151 MHz, CD2Cl2,
75 °C) δ 188.0, 187.5, 170.8, 159.5, 158.2, 151.4, 146.5, 145.4, 143.9,
141.0, 140.1, 137.2, 136.9, 136.5, 136.2, 135.5, 135.2, 134.9, 133.0, 132.4,
129.6, 126.5, 125.5, 125.3, 124.5, 124.2, 123.6, 120.2, 113.9, 113.4, 99.5,
79.7, 79.5, 79.4, 24.0, 20.7 (two carbon signals corresponding to the C
Synthetic procedures and product characterization
Synthesis of compound 5. A solution of compound 4 (300 mg; 0.72
mmol) in dry THF (7.2 ml) was cooled to –78 °C and nBuLi (2.5 M in
hexane; 0.66 mL; 1.65 mmol) was added dropwise. The reaction mixture
was stirred at –78 °C for 1 h. Then dry DMF (0.17 ml; 2.20 mmol) was
added dropwise and the stirring was continued at –78 °C. After 40 min the
mixture was allowed to warm to RT and stirred at this temperature for the
7
This article is protected by copyright. All rights reserved.

10.1002/chem.201701922
Chemistry - A European Journal
FULL PAPER
atoms bound to the B atom are not visible due to the quadrupolar
relaxation). Compound 7c features very low solubility. 1H NMR spectra
were measured at 125 °C and 75 °C. The measurement at 125 °C shows
sharper proton signals. However, the 13C NMR spectrum could not be
collected at such a high temperature because a sample partially
decomposed during the required long acquisition time. 11B NMR (128 MHz,
CD2Cl2, 25 °C) δ 3.89. UV/Vis (CH2Cl2, c = 4.10 × 10−6 M): λmax/nm 589
(ε/M−1 cm−1 67 000), 554 (62 000), 305 (43 000) with two shoulders. MS
LR (MALDI) m/z calcd for C51H34BN5O2S2 M+ 823.2, found 823.2. MS HR
(ESI) m/z calcd for C51H35BN5O2S2 [M+H]+ 824.2320, found 824.2319.
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Acknowledgements
We are grateful to Prof. Frank Würthner for his support and many
helpful discussions. We would like to thank Christian Simon for
the technical assistance in the synthesis of the building blocks.
We thank the Bavarian State Ministry of Education, Science, and
the Arts for financial support of this project by the establishment
of the Key Laboratory “Supramolecular Polymers” at the Bavarian
Polymer Institute (BPI) and the University of Würzburg for support
by the Emil Hilb Program.
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Keywords: boron • OTFT • A-π-A molecules • electron transport
• thienylthiazole
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Entry for the Table of Contents
Layout 1:
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Boron-bridge to organic n-type
semiconductors: Introduction of the
boron-bridge into thienylthiazole and
attachment of acceptor moieties
allowed to obtain ambient-stable
A-π-A molecules with low-lying
LUMO levels. The materials were
applied as active layers in OTFT
devices. Enhanced electron mobility,
measured for A-π-A systems bearing
larger aromatic acceptor moieties,
can be attributed to the effective 2D
π-stacking in the solid state.
Reinhard Hecht, Juliane Kade, David
Schmidt, Agnieszka Nowak-Król*
Page No. – Page No.
n-Channel Organic Semiconductors
Derived from Air-Stable Four-
Coordinate Boron Complexes of
Substituted Thienylthiazoles
10
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