The Effect of Ring Expansion in Thienobenzo[ b]indacenodithiophene Polymers for Organic Field-Effect Transistors

Article abstract of DOI:10.1021/jacs.9b09367

A fused donor, thienobenzo[b]indacenodithiophene (TBIDT), was designed and synthesized using a novel acid-promoted cascade ring closure strategy, and then copolymerized with a benzothiadiazole (BT) monomer. The backbone of TBIDT is an expansion of the well-known indacenodithiophene (IDT) unit and was expected to enhance the charge carrier mobility by improving backbone planarity and facilitating short contacts between polymer chains. However, the optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm² V^-1 s^-1, lower than the performance of IDT-BT (～1.5 cm² V^-1 s^-1). Mobilities extracted from time-resolved microwave conductivity measurements were consistent with the trend in hole mobilities in organic field-effect transistor devices. Scanning tunneling microscopy measurements and computational modeling illustrated that TBIDT-BT exhibits a less ordered microstructure in comparison to IDT-BT. This reveals that a regular side-chain packing density, independent of conformational isomers, is critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations indicated that TBIDT-BT, despite containing a larger, planar unit, showed less stabilization of planar backbone geometries in comparison to IDT-BT. This is due to the reduced electrostatic stabilizing interactions between the peripheral thiophene of the fused core and the BT unit, resulting in a reduction of the barrier to rotation around the single bond. These insights provide a greater understanding of the general structure-property relationships required for semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with IDT-type units, guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect transistors.

Full text of DOI:10.1021/jacs.9b09367

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Article
The Effect of Ring Expansion in Thienobenzo[b]indacenodithiophene
Polymers for Organic Field-Effect Transistors
Hu Chen, Andrew Wadsworth, Chun Ma, Alice Nanni, Weimin Zhang, Mark Nikolka, Alexander M.T.
Luci, Luis M. A. Perdigao, Karl J. Thorley, Camila Cendra, Bryon Larson, Garry Rumbles, Thomas
D. Anthopoulos, Alberto Salleo, Giovanni Costantini, Henning Sirringhaus, and Iain McCulloch
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b09367 • Publication Date (Web): 15 Oct 2019
Downloaded from pubs.acs.org on October 20, 2019
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The Effect of Ring Expansion in Thienobenzo[b]indacenodithiophene Polymers for
Organic Field-Effect Transistors
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Hu Chen*†, Andrew Wadsworth*∥, Chun Ma†, Alice Nanni†, Weimin Zhang†, Mark Nikolka‡,
Alexander M. T. Luci§, Luís M. A. Perdigão§, Karl J. Thorley∥, Camila Cendra⊥, Bryon Larson,
Garry Rumbles, Thomas D. Anthopoulos†, Alberto Salleo⊥, Giovanni Costantini§, Henning
Sirringhaus‡, and Iain McCulloch†, ∥
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† King Abdullah University of Science and Technology (KAUST), KAUST Solar Center (KSC), Thuwal 23955-6900,
Saudi Arabia.
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‡
Optoelectronics Group, Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge, CB3
0HE, UK.
§Department of Chemistry University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK.
∥ Department of Chemistry and Centre for Plastic Electronics, Imperial College London, Exhibition Road, London,
SW7 2AZ, United Kingdom.
 National Renewable Energy Laboratory, Golden, CO, 80401, USA.
⊥ Department of Materials Science and Engineering, Stanford University, 476 Lomita Mall, Stanford, CA 94305,
USA.
KEYWORDS: OFET, Mobility, IDT, TBIDT, STM, DFT.
ABSTRACT: A fused donor, thienobenzo[b]indacenodithiophene (TBIDT), was designed and synthesized using a novel
acid-promoted cascade ring closure strategy, and copolymerized with a benzothiadiazole (BT) monomer. The backbone
of TBIDT is an expansion of the well-known indacenodithiophene (IDT) unit and was expected to enhance the charge
carrier mobility, by improving backbone planarity and facilitating short-contacts between polymer chains. However, the
optimized field-effect transistors demonstrated an average saturation hole mobility of 0.9 cm2 V−1s−1, lower than the
performance of IDT-BT (~1.5 cm2 V−1s−1). Mobilities extracted from time-resolved microwave conductivity (TRMC)
measurements were consistent with the trend in hole mobilities in OFET devices. Scanning Tunneling Microscopy (STM)
measurements and computational modelling illustrated that TBIDT-BT exhibits a less ordered microstructure in
comparison to IDT-BT. This reveals that a regular side chain packing density, independent of conformational isomers, is
critical to avoid local free volume due to irregular packing, which can host trapping impurities. DFT calculations
indicated that TBIDT-BT, despite containing a larger, planar unit, showed less stabilization of planar backbone
geometries, in comparison to IDT-BT. This is due to the reduced electrostatic stabilizing interactions between the
peripheral thiophene of the fused core with the BT unit, resulting in a reduction of the barrier to rotation around the
single bond. These insights provide a greater understanding of the general structure-property relationships required for
semiconducting polymer repeat units to ensure optimal backbone planarization, as illustrated with IDT-type units,
guiding the design of novel semiconducting polymers with extended fused backbones for high-performance field-effect
transistors.
INTRODUCTION
During the past decade, π-conjugated organic
semiconductors have attracted substantial interest owing to
advantages that they possess over their inorganic
counterparts; these include their easily tunable
optoelectronic and structural properties, lightweight nature,
cost-effectiveness, mechanical robustness and the capability
to fabricate large area devices.1-5 The charge carrier mobility
of organic field-effect transistors (OFETs), which is the most
crucial parameter that determines the characteristics in such
devices, has increased from less than 0.1 cm2 V–1 s–1 to exceed
the benchmark of thin-film amorphous silicon devices (0.5–1
cm2 V–1 s–1), up to > 3 cm2 V–1 s–1 for both polymers and small
molecules.6, 7 In addition to developments in device
architecture optimization, the structural design of organic
semiconductors has become a key strategy to achieve
excellent device performance. 8-10
Polymers containing 4,9-dihydro-s-indaceno[1,2-b:5,6-
b’]dithiophene (IDT), a promising molecular design motif,
are characterized by low energetic disorder, excellent
solution processability and exhibit high charge carrier
mobility in p-type devices. The unique IDT unit has a fused
rigid-ring backbone, with two sp3 bridging carbons,
substituted with aliphatic chains to confer its solubility, as
well as acting as a template for molecular packing. It was first
demonstrated as a building block for high-performance
OFET polymers by copolymerization with 2,1,3-
benzothiadiazole (BT), to produce IDT-BT.11This is an
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amorphous p-type polymer with a hole mobility of up to 3.6
cm2 V-1 s-1.10
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Despite its relatively amorphous nature, IDT-BT possesses
impressive p-type transport properties, owing to the high
degree of backbone order along polymer chains.10It has been
suggested that the key interaction that imparts the backbone
order upon IDT-BT is between the nitrogen of the BT group
and the IDT hydrogen α to the IDT-BT linkage, via non-
traditional hydrogen bonding, leading to a high energetic
barrier to backbone torsion. 12 Maintaining the thienyl-BT
linkage in analogous OFET polymers is therefore a desirable
strategy to encourage highly ordered polymer backbones in
IDT polymers. Expansion of the IDT core has previously
been used, with the aim of increasing the short-contact
interactions between polymer chains, in order to improve the
charge carrier mobility. Thienoindenofluorene (TIF) was
copolymerized with BT, resulting in the polymer TIF-BT,
and was able to achieve a hole mobility of 2.9 cm2 V-1 s-1.13
Notably, it was suggested that the extended TIF core
improved the inter-chain charge transport.13,14 Despite this,
only a few examples have been reported that describe
modifications of the IDT unit, owing mainly to the complex
synthesis of extended fused-ring systems.
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Scheme 2 shows the synthesis of the TBIDT-BT polymer.
Firstly, it was necessary to synthesize the key intermediate
(4) via four high yielding steps: the readily available 3-
bromothiophene was selectively deprotonated at the 2
position and quenched by the addition of dimethyl
formamide to produce 3-bromothiophene-2-carbaldehyde
(compound 2), which was then reduced to (3-
bromothiophen-2-yl)methanol (compound 3) by sodium
borohydride. The triisopropylsilane group was found to be
the best protecting group to protect the hydroxy group and
afforded compound 4. It should be noted that other
commonly used protecting groups, such as trimethylsilane
and triethylsilane, were shown to decompose during later
steps of the TBIDT synthesis.
Scheme 1. The synthetic transformation of IDT to
TBIDT
The next key intermediate to be prepared was the
dialdehyde-IDT(compound 7). Alkylation of the known
compound 4,9-dihydro-s-indaceno[1,2-b:5,6-b']dithiophene
(compound 5) afforded the IDT monomer with C16 alkyl
chains (compound 6). Deprotonation of C16-IDT followed by
quenching with dimethylformamide gave dialdehyde-IDT
(compound 7) in high yield.
Having successfully produced the key intermediates (4 and
7), the next step was to construct the cyclization precursor
(11). This first involved the conversion of compound 4 to a
nucleophile, which was carried out by a halogen exchange
step using butyl lithium. The in-situ generated nucleophile
attacked the dialdehyde-IDT and afforded compound 8. To
avoid the direct reaction of butyl lithium with compound 7,
it was necessary to use slightly fewer equivalents of butyl
lithium than of compound 4 and the halogen exchange step
was completed before the addition of the dialdehyde-IDT. In
order to prepare compound 9, exhaustive screening and
optimization of the reaction conditions was required. The
combination of NaCNBH3/ZnI2 was found to be the mildest
condition for the reduction of the hydroxyl groups, in high
yield, without affecting other functional groups.
Herein, we have designed and reported the synthesis of a
polymer comprising of a novel extended IDT unit, namely
thiophenebenzo[b]indacenodithiophene (TBIDT), which
was formed using an acid-promoted cascade ring closure
strategy (Scheme 1). By synthesizing the aldehyde precursor,
a cyclization/dehydration cascade reaction was utilized to
afford the desired TBIDT monomer. This novel synthetic
strategy will prove to be useful in order to access other fused
aromatic units for organic electronic applications. The
TBIDT monomer was copolymerized with BT and the
resulting polymer (TBIDT-BT) exhibited an average mobility
of 0.9 cm2 V−1 s−1 in OFET devices
RESULTS AND DISCUSSION
Scheme 2. Synthetic route to afford TBIDT-BT
Deprotection of 9 using TBAF then afforded compound 10,
which was then oxidized to compound 11 using Dess-Martin
Periodinane in a high yield. Dess-Martin Periodinane
provided the best oxidation for the transformation, while
other stronger oxidants such as chromium oxide also
oxidized the sp3 carbons, affording relatively low yields.
Having successfully synthesized the cyclization precursor 11,
the final step in producing the desired TBIDT motif was a
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cyclization/dehydration of compound 11. Different acids were
The thermal properties of both IDT-BT and TBIDT-BT thin
films were investigated using thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC), see
supplementary information. Both polymers showed good
thermal stability up to 400 °C, while the DSC showed no
discernible thermal transitions at temperatures up to 300 °C.
This is indicative of a low degree of ordering in the films. The
observation of relatively disordered IDT-BT and TBIDT-BT
thin-films is also consistent with the Atomic Force
Microscopy (AFM) measurements and the Grazing Incidence
Wide-Angle X-ray Scattering (GIWAXS) data of both
polymers (Figure 1).
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screened to promote the cascade cyclization/dehydration.
Amberlyst-15 was found to be the most efficient reagent to
afford the desired product in moderate yield, in comparison
to other acids such as trifluoroacetic acid, acetic acid and p-
toluene sulfonic acid. This versatile procedure offers great
potential to be utilized for the synthesis of other novel
bridged donor molecules.
Upon completion of the novel fused aromatic unit,
stannylation of TBIDT afforded the monomer 13. This was
then co-polymerized with dibromo-BT using microwave-
assisted Stille conditions to produce the donor-acceptor (D-
A) copolymer TBIDT-BT. Variation of the polymerization
process resulted in an optimum number average molecular
weight, determined by gel permeation chromatography
(GPC), of ~ 62 kDa (Mn) with a very narrow polydispersity
index (PDI) of 1.4, after purification using Soxhlet extraction
and GPC.
UV−Vis absorption spectra of the TBIDT-BT and its parent
polymer IDT-BT are shown in Supplementary Information
Figure S7. Both the polymers showed two maxima peaks,
with a shoulder close to the second peak, which is the
common dual-band absorption observed for many donor-
acceptor type polymers. TBIDT-BT shows a significant blue-
shift in the onset of its absorption and its absorption
maximum, compared to IDT-BT. This indicates that the
expansion of the backbone has resulted in an increase in the
band gap of TBIDT-BT. The ionization potential (IP) of both
polymers was measured by photoelectron spectroscopy in air
(PESA) and the data shown in Table 1. The IP was found to
be approximately -5.5 eV, which is slightly deeper than IDT-
BT (-5.4 eV) due to the slightly lower electron density of the
unit from a reduced thiophene ratio of aromatic groups.
Conversely, the electron affinity (EA) of TBIDT-BT is less
than IDT-BT due to the lower ratio of the electron
withdrawing BT unit along the backbone.
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Figure 1. 2D GIWAXS patterns of a) TBIDT-BT thin film; b)
IDT-BT thin film.
Tapping mode AFM was used to investigate the surface
morphology of the polymer thin films (Figure 1a and Figure
1b). The surface roughness for the two polymers is
comparable, with a root mean square roughness RMS = 0.486
nm and 0.655 nm extracted for TBIDT-BT and IDT-BT,
respectively, consistent with a lack of crystalline texture.
GIWAXS analysis was performed on as-cast films to provide
insight into the thin film microstructure. Figure 1c and 1d show
the two-dimensional GIWAXS patterns of TBIDT-BT and
IDT-BT, respectively (see SI for the 1D lineouts taken along
the out-of-plane and in-plane scattering directions). A broad
π-π stacking peak is observed out-of-plane at a peak center
position of q = 1.49 Å-1 and q = 1.53 Å-1 for TBIDT-BT and IDT-
BT, respectively. This corresponds to π-π stacking distances of
4.22 Å and 4.10 Å for TBIDT-BT and IDT-BT, respectively.
Both polymers show relatively weak crystalline ordering,
consistent with their predominantly amorphous nature, IDT-
BT shows an in-plane peak at around 0.4 Å-1, corresponding to
the backbone stacking, while TBIDT-BT has no trace of this
peak. This indicates better ordering of IDT-BT and it is in line
with the STM observation.(Figure 3) The slightly shorter π-π
stacking distances in IDT-BT also suggest that it is able to
pack more efficiently than TBIDT-BT.
Table 1. Summary of the Properties of the Polymers
Polyme
rs
Mn/
λmax
Eopt.gap
(eV) c
IP
EA
Eu
μg
Mw(kDa)a
/nmb
(eV) d
(eV)e
(meV)f
IDT-
BT
58.3/95.3
62.1/87.4
666
595
1.7
1.9
-5.4
-5.5
-3.7
-3.6
24
40
1.5
TBIDT
-BT
0.9
a
Solution absorption spectra (in chlorobenzene), Mn: molar
mass averages of the number; Mw: molar mass averages of the
weight.
b Solid state absorption spectra from pristine thin-film.
c Obtained from the onset value of absorption in pristine
thin-film. Eopt.gap: The gap between IP and EA.
d Measured by Photo-Electron Spectroscopy in Air (PESA)
system. IP: ionization potential.
e Calculated from Eopt.gap and IP. EA: electron affinity.
f Urbach Energy: Measured by Photothermal Deflection
Spectroscopy.
Next, we evaluated the electrical performance and charge
transport properties of TBIDT-BT and IDT-BT by fabricating
bottom-contact top-gate (BC/TG) field effect transistors (see
SI). Figure 2a shows the representative transfer characteristics
of TBIDT-BT OFETs with the linear and saturation
g
Mobility: Measured by bottom-contact top-gate (BC/TG)
field effect transistors(cm2 V−1s−1).
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characteristics scaling according to theory. Despite a delayed
shallow sub-bandgap tail with an Urbach Energy of 40 meV,
in comparison to the extremely low 24 meV extracted for IDT-
BT. The higher energetic disorder of TBIDT-BT exceeds the
thermal energy kBT (kB, Boltzmann constant). Hence high
carrier concentrations populate more mobile states in the
density of states, this could be the reason for the observed
gate-voltage dependence of the charge carrier mo-bility. We
also observe an extended charge transfer feature in the sub-
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switching-on of the devices at -3V and a threshold voltage
(Vth) of -15V, they exhibit a steep sub-threshold swing and an
on-off ratio of 106. Also, the output characteristics of a typical
device shown in Figure 2b do not present any pinching at
lower gate- and drain-voltages suggesting that devices are not
limited by poor injection. Subsequent extraction of the
saturation charge carrier mobility of TBIDT-BT devices
resulted in an average hole mobility of 0.9 cm2 V-1 s-1 across 8
devices, compared to 1.5 cm2 V-1 s-1 for IDT-BT measured in the
same device architecture. It is well known that erroneous
charge carrier mobility calculations, extracted from transistor
bandgap absorption of TBIDT-BT,
a feature that has
previously associated with interchain charge transfer states.14
This might also explain why the polymer despite its relatively
large Urbach energy of 40 meV can still achieve respectable
hole mobilities approaching 1 cm2V-1s-1.
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current voltage characteristics, have at times led to an
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overestimate of mobility.15,
All charge carrier mobilities
Scanning Tunneling Microscopy (STM) measurements were
performed under ultrahigh vacuum conditions on sub-
monolayer films of TBIDT-BT and IDT-BT deposited on a
Au(111) surface by electrospray deposition (ESD), according to
a recently developed methodology (see SI).18 Figure 3a shows
a large scan STM image where individual TBIDT-BT polymer
strands appear as worm-like chains with regularly spaced
bright protrusions (occasional even brighter protrusion
appear where polymer strands cross). The image
demonstrates the absence of long-range order in the ESD films
and the high flexibility of the polymers resulting in several
high-curvature bends. On the local scale, the polymer strands
tend, however, to align parallel to each other, as can be seen
in the higher magnification image of Figure 3b. The polymer
backbones are clearly recognized, demonstrating that the
regular bright protrusions are actually double-dot features
periodically separated by 2.7±0.3 nm. This distance is very
close to the length of the repeat unit of TBIDT-BT as
calculated by DFT (2.57 nm). The alkyl side chains can also be
identified as slightly less bright linear features oriented almost
perpendicular to the backbone. By superposing a molecular
model of TBIDT-BT on the STM images (Figure 3b), it
becomes evident that the alkyl chains interdigitate in these
closely packed regions. The model also allows the
identification of the bright double-dot features as being
related to the two non-planar sp3 bridging carbons in the
TBIDT unit (one on each side of the backbone) and the
determination of the orientation of individual TBIDT units
along the backbone.
exhibited a pronounced gate-voltage (VG) dependence, with
the highest values obtained only at VG = -60V (Figure 2c). A
similar gate voltage dependence has been observed also for
other polymers in the IDT-polymer family, such as TIF-BT13
but not in IDT-BT.11This independence of the charge carrier
mobilities on the gate-voltage in IDT-BT has been associated
to an exceptionally low degree of energetic disorder of the
polymer backbone.
Figure 2. Performance of TBIDT-BT polymer transistors. a)
Linear and saturation transfer characteristics as well as b)
output characteristics of a representative TBIDT-BT OFET
(Length (L )= 20 µm and Width (W) = 1 mm); the device
architecture is shown as an inset in a); c) Extracted saturation
charge carrier mobility of a representative TBIDT-BT OFET;
d) Absorption of TBIDT-BT and IDT-BT films, measured by
Photothermal Deflection Spectroscopy. The solid line
represents an exponential tail fit for extraction of the Urbach
Energy (Eu).
Photothermal deflection spectroscopy (PDS) was used to
probe the energetic disorder of TBIDT-BT. It has previously
been shown that the optically measured joint density of states
(DOS), measured by PDS reflects accurately the electronic
sub-bandgap traps and hence, charge transport properties.12, 17
Figure 2d shows the measured PDS spectrum of TBIDT-BT
and IDT-BT as well as a fit to the sub-bandgap tail used to
extract the Urbach Energy. TBIDT-BT has a comparatively
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optimal polymer packing. Figure 4 compares the 2D assembly
1
of TBIDT-BT and IDT-BT oligomers considering the case
where the sidechain-bearing units of the oligomers are
oriented identically to each other (left column in Figure 4) and
the case where these units have a different orientation (right
column). The orientation of the TBIDT and IDT subunits can
be identified according to the red or purple color of the pairs
of dots that have been superposed onto their two non-planar
sp3 bridging carbons. It is evident that, for both polymers, the
highest ordered assemblies are obtained when the sidechain-
bearing units have the same orientation. In such cases the
vdW contact is maximized throughout the assembly; this is
also what has been often observed experimentally for TBIDT-
BT, as shown in the Supplementary Information. However it
should be noted that, because of its higher sidechain
attachment density, IDT-BT is capable of efficient
interdigitation also in cases where the sidechain-bearing units
have different orientations (Figure 4d), managing to achieve
an excellent overall vdW contact, which is seen also
experimentally (Figure 3d). This ability is significantly
reduced for TBIDT-BT (Figure 4b) which has a lower side-
chain attachment density, resulting in some of the sidechains
to be isolated and causing the formation of nanovoids even in
the parallel orientation case (Figure 4a). This can lead to a
number of problems in the packing of TBIDT-BT, in
comparison to IDT-BT, such as a higher chance for possible
assembly errors and a lower energy gain (via vdW contacts)
per unit length. An additional issue of the nanovoids present
in TBIDT-BT is the opportunity for the insertion of
impurities, thereby disrupting the assembly, which can be
seen in the STM images (Figure 3b and in more detail in Figure
S6. While this model is strictly valid in 2D, similar
considerations are expected to hold also for the 3D assembly
of thin films made of these polymers. Both effects – lower
energy gain from polymer assembly and formation of
nanovoids – lead to a less ordered microstructure in TBIDT-
BT than that observed for IDT-BT, which is likely to be
detrimental to the performance of OFET devices based on
such thin films, particularly if the free volume within the voids
is occupied by species capable of trapping charge carriers,
such as water.19
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Figure 3. STM analysis of TBIDT-BT and IDT-BT deposited
by ESD on Au(111). a) Large scale STM image of TBIDT-BT,
sample bias V =  1.4 V, tunneling current I = 70 pA. b)
Corresponding higher magnification image (V=  0.8 V, I = 110
pA). c) Large scale STM image of IDT-BT (V=  1.5 V, I = 30
pA). d) Corresponding higher magnification image (V=  1.3 V,
I = 70 pA). Molecular models of the corresponding polymers
are superposed in b) and d).
The analysis of a larger set of STM data for TBIDT-BT films
(see S5 in the Supplementary Information) revealed that while
the orientation of consecutive TBIDT units within a given
backbone is essentially random, TBIDT units in adjacent
locally parallel backbones generally have the same orientation
as one another. This configuration maximizes van der Waals
(vdW) contact between the side chains but is more difficult to
achieve because it is statistically less frequent. Even in these
more compact assemblies, nanovoids are observed in the alkyl
chain regions in between polymer backbones (e.g. in the
center of Figure 3b).
The STM image in Figure 3c shows that IDT-BT deposited on
Au(111) has a similar appearance to TBIDT-BT when imaged
on a large scale, with polymers strands looking as elongated
chains, dotted with a regular sequence of bright protrusions.
Also in this case we assign these latter to the two non-planar
sp3 bridging carbons in the IDT units. The average distance
between the bright protrusions is 1.6±0.1 nm, in excellent
agreement with the theoretically predicted periodicity IDT-
BT. In higher magnification images such as Figure 3d it is
possible to resolve the alkyl side chains and to recognize that
they interdigitate where the polymer backbones are locally
oriented parallel to each other. At variance with the case of
TBIDT-BT, these closely packed regions are much less
defective for IDT-BT and do not present any nanovoids.
Figure 4. Schematic modelling of the 2D assembly of TBIDT-
BT, top row, and IDT-BT, bottom row, oligomers. The red or
purple pairs of dots identify different orientations of the
TBIDT and IDT subunits.
A simple molecular modelling demonstrates this effect and
highlights the importance of the side-chain density for
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Some insight into torsional disorder of the polymer backbones
weaker for TBIDT-BT than IDT-BT, while the Hδ+-Hδ+
interaction might be marginally more repulsive. At 180°, the
repulsive Sδ+-Hδ+ interaction decreases in TBIDT-BT due to
less positive charge on sulfur, while the stabilizing force
between NBT and HIDT remains comparable to that of IDT-BT.
These observations may help to explain why TBIDT-BT has a
greater preference for the 180° conformer than the 0°
conformer.
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can be gained from the calculated potential energy surface
(PES) for rotation between the thienyl group with the adjacent
BT group (Figure 5). Despite similar non-covalent interactions
between atoms close in space, primarily S-N or H-N
interactions, the profiles of IDT-BT and TBIDT-BT display a
number of differences to one another. Firstly, the barrier to
rotation is lower in TBIDT-BT or, in other words, the planar
conformations are less stabilized than in IDT-BT. The
consequence of this might be a greater degree of disorder
along the polymer chain. Secondly, the energetic difference
between the more stable 180° conformer and the less favored
0° conformer is larger for TBIDT-BT (1.6 kcal/mol) than for
IDT-BT (1.0 kcal/mol).
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Figure 6. a) Mulliken atomic charges on atoms involved in
non-covalent interactions (SIDT, HIDT, NBT, HBT) as a function
of dihedral angle. b) NBO resonance stabilization due to π- π*
donation across the rotatable thieno-BT bond, as a function of
dihedral angle.
Figure 5. B3LYP-D3/6-31G* Torsional PES of thieno-BT
derivatives relative to the energy of the orthogonal conformer,
where conjugation and non-covalent interactions are
minimal. Rotatable bond highlighted in red, where the 0°
conformer is shown.
Information about the difference in barrier height between
IDT-BT and TBIDT-BT can be elucidated from the electron
delocalization in NBO calculations. These calculations
partition electron density into different types of orbitals
based on bonding, anti-bonding, and lone pairs on each
atom. Resonance stabilization energies for the delocalization
of electrons of the π bond at the end of the IDT group into
the π* antibonding orbital of the adjacent BT group are
plotted in Figure 6b. At 90°, there is no orbital overlap and so
the resonance stabilization is zero. Upon rotation in either
direction, the π and π* orbitals start to overlap, resulting in
increasing stabilization. Rotating to the 180° conformer
results in more efficient orbital overlap than rotating to 0°,
due to the trans relationship between the orbitals. There is a
greater stabilization in IDT-BT than in TBIDT-BT of up to
2.5 kcal/mol, resulting in the lower energetic minima in the
torsional PES in Figure 5.
Contributing factors to the differences in torsional PES were
investigated by comparing atomic charges on each of the
atoms contributing to the non-covalent interactions. Mulliken
charges from each of the restrained geometry relaxations were
extracted for the sulfur and proton of the IDT/TBIDT group,
and the nitrogen and proton of the BT group (Figure 6a).
These were seen to vary with dihedral angle as the electronic
structure of the system changes due to increased conjugation
and to the changing of interatomic distances. The main
difference between TBIDT and IDT groups is the partial
charge on sulfur, which is less positive in TBIDT-BT owing to
the more electron rich nature, and greater delocalization, of
the extended TBIDT group. The positive charge on sulfur is in
part due to the delocalization of its lone pair that gives
thiophene its aromaticity. From natural bond orbital (NBO)
calculations, discussed in more detail below, this
delocalization is weaker in the case of TBIDT compared to
IDT (see Figure S8). The more localized sulfur lone pair in
TBIDT results in a smaller sulfur positive charge. This
influences the mostly electrostatic non-covalent interactions
during rotation. At 0°, the attractive Sδ+-Nδ- interaction is
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energetic barriers to rotation about the TBIDT-BT bond.
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BC/TG field-effect transistors fabricated from TBIDT-BT
yielded a p-type mobility of 0.9 cm2V−1 s−1, which is lower
than that of IDT-BT (1.5 cm2 V−1s−1). These results
demonstrate that the regularity of side chain density,
independent of conformational differences along the
backbone, as well as the optimization of non-covalent
interactions for backbone planarity are both critical to
achieve high charge carrier mobility. These findings provide
new insight into the fundamental requirements to design
high charge carrier mobility semiconducting polymers.
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ASSOCIATED CONTENT
Figure 7. a) Peak 휑휮휇 values for IDT-BT and TBIDT-BT
versus absorbed photon flux using excitation wave-lengths of
640 and 580 nm, respectively. In parentheses, the OFET
carrier mobilities are given for reference for each polymer. b)
The corresponding normalized transient data recorded over
500 ns with average carrier lifetimes extracted from bi-
exponential fits (dashed lines) of the data.
This material is available free of charge via the Internet at
http://pubs.acs.org.” Experimental details, synthesis of
polymers, device fabrication, and detailed experimental
procedures.
AUTHOR INFORMATION
Corresponding Authors
Time-resolved microwave conductivity (TRMC) provides
insight into the carrier mobilities measured at the microwave
fre-quency of ~9 GHz. Thin films of the two polymers were
spin-coated onto quartz substrates and excited using a
pulsed laser to generate microwave transients, as shown in
Figure 7b with both exhibiting very similar transient profiles
and average carrier lifetimes. Although the exact mechanism
for carrier generation is not known, the similar transient
profiles suggest that the yield of carriers in both systems is
very small, but similar. Therefore, the peak of the transient
signal can be used as a good comparison of the carrier
mobilities between the two samples. These two peak values
of  are plotted in Figure 7a and exhibit a similar factor of
two difference between the two polymer mobilities found by
the OFET measurements. Unlike the OFET measurements,
however, it should be noted that the mobilities implied by
the microwave data are local mobilities and not transport
mobilities.
* hu.chen@kaust.edu.sa
*andrew.wadsworth11@imperial.ac.uk
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript. H. Chen and A. Wadsworth contributed
equally and share first authorship.
ACKNOWLEDGMENT
The authors thank KAUST and BASF for financial support
and acknowledge EC FP7 Project SC2 (610115) EC H2020
(643791), and Engineering and Physical Sciences Research
Council (EPSRC) Projects EP/G037515/1 and EP/M005143/1.
M.N. and H.S. acknowledge financial support from the
EPSRC though a Programme Grant (EP/M005141/1). G.C.
acknowledges financial support from the University of
Warwick. AS and CC gratefully acknowledge financial
support from the National Science Foundation, Division of
Materials Research Award# 1808401. Use of the Stanford
Synchrotron Radiation Lightsource, SLAC National
Accelerator Laboratory, is supported by the U.S. Department
of Energy, Office of Science, Office of Basic Energy Sciences
under Contract No. DE-AC02-76SF00515. The transient
microwave conductivity work was funded from the through
the Solar Photochemistry Program of Office of Basic Energy
Sciences. This work was authored in part by the National
Renewable Energy Laboratory, operated by Alliance for
Sustainable Energy, LLC, for the U.S. Department of Energy
(DOE) under Contract No. DE-AC36-08GO28308. The views
expressed in the article do not necessarily represent the
views of the DOE or the U.S. Government. The U.S.
Government retains and the publisher, by accepting the
article for publication, acknowledges that the U.S.
CONCLUSIONS
In summary, we have designed and synthesised a novel
TBIDT moiety with linear alkyl chain substitution, using a
novel acid promoted cascade cyclization strategy, which
could be used in the preparation of high-mobility transistor
materials with rigid backbones. Upon copolymerization of
TBIDT with benzothiadiazole, the polymer TBIDT-BT was
prepared. Surprisingly, the energetic disorder of this large
ring polymer was higher than the benchmark IDT-BT. On
further evaluation of the polymer microstructure, defects in
the side chain packing assembly were observed by high
resolution STM, arising from conformational asymmetry in
the side chain distribution and an irregular interdigitated
packing density. Consequently, the lower side-chain density
of TBIDT-BT, in comparison to IDT-BT, leads to fewer vdW
contacts and the presence of nanovoids, thereby disrupting
its packing and resulting in less-ordered thin-films.
Government retains a nonexclusive, paid-up, irrevocable,
worldwide license to publish or reproduce the published
Moreover, torsional PES and atomic charge calculations
further highlighted a further potential cause for lower
polymer backbone order in TBIDT-BT, owing to the lower
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form of this work, or allow others to do so, for U.S.
Government purposes.
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