Lowerature Cross-Linking Benzocyclobutene Based Polymer Dielectric for Organic Thin Film Transistors on Plastic Substrates

Article abstract of DOI:10.1021/acs.joc.9b02981

The synthesis of a new benzocyclobutene based polymer, PSBBB, designed as a dielectric material for use in organic thin film transistors was reported. Compared to conventional benzocyclobutene-based materials, the introduction of a butoxide substituent at the 7-position of the benzocyclobutene pendant unit on the polymer allowed PSBBB to be cross-linked at temperatures of 120 °C, thus rendering it compatible with the processing requirements of flexible plastic substrates. The cross-linking behavior of PSBBB was investigated by Fourier transform infrared spectroscopy and differential scanning calorimetry, demonstrating cross-linking of the polymer after curing at 120 °C. Bottom-gate bottom-contact organic thin film transistors were fabricated using PSBBB as dielectric, affording a performance comparable to that of other dielectric polymeric materials.

Full text of DOI:10.1021/acs.joc.9b02981

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Article
A low temperature crosslinking benzocyclobutene based polymer
dielectric for organic thin film transistors on plastic substrates
Rawad Kamal Hallani, Maximilian Moser, Helen Bristow, Maud V. C. Jenart, Hendrik Faber, Marios
Neophytou, Emre Yarali, Alexandra F. Paterson, Thomas D. Anthopoulos, and Iain McCulloch
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02981 • Publication Date (Web): 29 Nov 2019
Downloaded from pubs.acs.org on December 7, 2019
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A low temperature crosslinking benzocyclobutene based
polymer dielectric for organic thin film transistors on plastic
substrates
Rawad K. Hallani,1* Maximilian Moser,2 Helen Bristow,2 Maud V. C. Jenart,2 Hendrik
Faber,1 Marios Neophytou,1 Emre Yarali,1 Alexandra F. Paterson,1 Thomas D.
Anthopoulos,1 Iain McCulloch1,2
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Affiliations
1King Abdullah University of Science and Technology (KAUST), KAUST Solar Center
(KSC), Thuwal 23955-6900, Saudi Arabia
2Imperial College London, Department of Chemistry and Centre for Plastic Electronics,
London, W12 0BZ, United Kingdom
Rawad.hallani@kaust.edu.sa
Abstract
The synthesis of a new benzocyclobutene based polymer, PSBBB, designed as a
dielectric material for use in organic thin film transistors was reported. Compared to
conventional benzocyclobutene based materials, the introduction of a butoxide substituent
at the 7-position of the benzocyclobutene pendant unit on the polymer allowed PSBBB to
be crosslinked at temperatures of 120 °C, thus rendering it compatible with the processing
requirements of flexible plastic substrates. The crosslinking behavior of PSBBB was
investigated by Fourier transform-infrared spectroscopy and differential scanning
calorimetry, demonstrating crosslinking of the polymer after curing at 120 °C. Bottom-gate
bottom-contact organic thin film transistors were fabricated using PSBBB as dielectric,
affording a comparable performance to other dielectric polymeric materials.
Introduction
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High performance organic thin film transistors (OTFTs) are essential components
in the fabrication of next generation electronic devices.1-5 Over the past few decades,
significant research effort has been dedicated to improving the performance of OTFTs by
optimization of the organic semiconducting materials, with respect to their charge carrier
mobilities, ambient stability and compatibility with environmentally friendly and large
area, high throughput processing.6-8
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The use of OTFT semiconductors and dielectrics that are compatible with flexible
plastic substrates is particularly desirable, as these can enable new technological
applications (e.g. flexible displays and electronic skins) and are compatible with roll-to-
roll processing techniques, thus contributing to lower manufacturing costs and higher
throughput. Although OTFT performance is strongly dependent on the semiconductor used
as the active layer,9-12 the material employed as the dielectric in the transistor architecture
can also affect the device operation.13-15 Dielectric materials should be compatible with
solution processing methods, have the versatility to be used in both top and bottom gate
transistor devices, provide a trap-free, well defined interface with the semiconductor,
possess the required dielectric constant, and be processable at temperatures suitable for use
with plastic substrates, such as polyethylene terephthalate (PET), polyethylene naphthalate
(PEN), etc.16
Despite organic semiconductors being typically fabricated with a top-gate bottom-
contact (TGBC) OTFT architecture, OTFTs with a bottom-gate bottom-contact (BGBC)
structure enable the organic semiconductor acting as channel material to be deposited last,
thus facilitating the fabrication and patterning of micron sized electrodes and interconnects
prior to application of the semiconductor. In BGBC OTFTs, the dielectric layer is usually
a metal oxide (e.g. SiO2, Al2O3, etc.), however these oxides are often incompatible with
flexible plastic substrates due to their brittleness and necessity for high temperature
processing.17, 18 Organic polymer dielectrics (e.g. polystyrene, poly(methyl methacrylate),
etc.) however are compatible with low temperature solution processing techniques, thus
making them attractive candidates as dielectric materials. The requirements for top and
bottom gate dielectric materials have some significant differences. Top gate dielectrics are
required to be deposited on top of the organic semiconductor surface, and therefore the
dielectric solvent needs to be completely orthogonal to the semiconductor solubility in
order to ensure that it does not dissolve or swell the semiconductor on application, ensuring
a clean interface. As a result, solvent choice is restricted to the extremes of polarity, i.e.
either water based, or more commonly, fluorocarbon based. One example of the latter is
the dielectric, CYTOP, a fluoropolymer resin, applied from a fluorosolvent. Although this
approach ensures a relatively trap free interface, the subsequent low energy fluoropolymer
dielectric surface is difficult to print on top of, which complicates deposition of the gate
electrode. Additionally, the low adhesive forces at the fluoropolymer interfaces can
facilitate delamination. In the case of bottom gate dielectrics, the solution choice is much
wider, as the dielectric/substrate interface does not have such exact electrical requirements,
and processed substrates are typically more robust and inert to solvent exposure. On the
other hand, the dielectric thin film has to be completely inert to the solution deposition of
the organic semiconductor. This requirement has proved extremely difficult to satisfy,
particularly as the semiconductor is typically deposited from aggressive solvents that
dissolve most common organic dielectric polymers. The obvious way to circumvent this
problem is that the dielectric undergoes an in-situ crosslinking reaction, thus preventing
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further dissolution. Common crosslinking chemistries include epoxide ring opening
reactions, such as the one used for the SU-8 dielectric, and condensation reactions, such as
the one occurring between polyvinylphenol and melamine.19-23 A key drawback of both of
these crosslinking chemistries is that they give rise to products with polar functionalities,
which have been demonstrated to function as charge traps at the semiconductor interface,
thus also lowering the charge carrier mobility in the semiconductor accumulation layer.24
Moreover, although high- κ dielectrics are commonly used in the fabrication of OTFT that
requires lower turn on voltage, it is found that some organic semiconductors operate better
in combination with low-κ dielectrics.25 This is attributed to the low dipole disorder of the
dielectric. Low dipole disorder results in a narrowing of the density of states of the
semiconductor at the interface, and as a result, a lower thermal activation is required to
activate charge hopping, in comparison to higher dielectric constants, and thus an
enhancement in charge carrier mobility is achieved.14, 25 A solution to this issue is offered
by benzocyclobutene (BCB) based resins, a low- κ dielectric, whose crosslinking reaction
does not generate any products with polar functionalities.26 The crosslinking reaction of
BCB proceeds by application of thermal energy, which results in the strained cyclobutene
moiety to go through an electrocyclic ring opening rearrangement shifting the equilibrium
toward the formation of the highly reactive o-xylylene intermediate. This intermediate can
then undergo a cycloaddition reaction with unsaturated functionalities, such as vinyl units
to afford a tetrahydronaphthalene linkage, or with another o-xylylene unit, by converting
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the -bonds at the termini of the acyclic conjugated -system to bonds, to yield the
휋
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reactive spirodimer intermediate that can rearrange to the energetically lower 1,2,5,6-
dibenzocyclooctadiene linkage.26, 27
One key drawback of common BCB dielectrics however, is that their crosslinking
reaction necessitates temperatures of over 240 C, which is unsuitable for flexible OTFTs
fabricated on plastic substrates (e.g. PET) that typically can only tolerate processing
temperatures up to 130 °C.16, 26 Various research efforts have thus been targeted at
foregoing this temperature limitation, whereby the introduction of electron donating alkoxy
groups to the cyclobutene ring of the BCB moiety has shown promising results.28-32 The
electrocyclic opening of the alkoxy substituted BCB occurs via a conrotatory process
through an outward rotation that leads to a more reactive transition state with comparison
to the less preferable inward rotation.33-35 Consequently, efficient crosslinking can be
achieved at lower temperatures.30-32
Despite these advances, the application of lower crosslinking temperature BCB
dielectrics in electronic devices has to the best of our knowledge not yet been reported in
the literature. In this work we thus present the synthesis of a new, low-κ BCB based
polymer with a pendant butoxide group on the sp3 carbon of the cyclobutene functionality,
and its subsequent use as polymer dielectric in OTFTs. We employ the
indacenodithiophene-benzothiadiazole based polymer, C16IDT-BT, as the channel
semiconducting polymer in a BGBC device architecture, and demonstrate similar OTFT
performances in comparison with state of the art dielectrics.
Results and Discussion
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Synthesis of PSBBB. Poly(Styrene-co-7-butoxybenzocyclobutene) (PSBBB) was
prepared as demonstrated in Scheme 1, starting with the iodination of 4-bromophenol,
which was synthesized using N-iodosuccinimide to generate 1 in 71% yield.36 Phenol
protection of 1 with HDMS to afford 2 was achieved quantitatively. N-BuLi addition to
compound 2 at -78 °C provided compound 3 via an intramolecular rearrangement in 92%
yield. Compound 3 was converted to the triflate 4 using triflic anhydride and triethylamine,
which was subsequently mixed with cesium fluoride to generate a reactive benzyne
intermediate that was cyclized with butyl vinyl ether via a [2+2] cycloaddition reaction to
afford compound 5 as an isomeric mixture in 77% yield. The Suzuki coupling of 5 with 4-
vinylphenylboronic acid gave compound 6 in 40% yield, which was then polymerized via
a free radical polymerization in a 1:1 ratio with a styrene co-monomer using benzoyl
peroxide initiator to produce PSBBB in 46% yield.
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Scheme 1. Synthetic scheme depicting the formation of PSBBB.
Curing behavior of PSBBB. The crosslinking kinetics of the PSBBB polymer
developed herein were monitored by a combination of differential scanning calorimetry
(DSC), Fourier transform infrared spectroscopy (FTIR) and solvent resistivity
measurements.
DSC was employed as a starting analytical technique to evaluate the crosslinking
kinetics. The crosslinking of the BCB moiety is an exothermic process due to the release
of ring strain from the four-membered ring on the BCB unit.37 For unsubstituted BCB, this
process generally necessitates temperatures >240 °C, as curing onsets around 200 °C with
the peak exotherm occurring around 250 °C.37-39 The inclusion of electron-donating
substituents, such as alkoxy groups, on the four-membered ring of the cyclobutene unit has
been shown to reduce the curing temperature of BCB analogues.29, 40 Similar findings were
obtained in this study, see Figure 1, as incorporation of a butoxy group on the cyclobutene
ring of the BCB unit resulted in an onset of curing at 110 °C, and a peak exotherm at 160
°C for the neat PSBBB sample. Consequently, a curing temperature of 120 °C was deemed
as appropriate in order to cure PSBBB samples.
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Crosslinking Degradation
exotherm
exotherm
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Figure 1. DSC traces obtained for the PSBBB samples cured for different time periods
at 120 °C and employing a scan rate of 10 K s-1.
As highlighted in Figure 1, curing the samples at 120 °C prior to DSC analysis
progressively supressed the exotherm associated with the crosslinking reaction of the BCB
unit at 160 °C, with the exotherm being absent after 180 min of curing. During the DSC
measurements, a second exotherm at 208°C was noted that progressively became more
intense upon increasing the curing time while maintaining a constant temperature of 120
°C. As reported elsewhere,41 the crosslinked products of the alkoxy substituted BCB based
moieties can undergo several reaction pathways, including 1,3-H transfer and transannular
bond cleavage, to afford both BCB and alkene containing species.41 Importantly, while
these pathways only occur to a limited extent at 130 °C and 160 °C, the further reaction
of the crosslinked products is significantly enhanced at 190 °C. We therefore speculate
that the second exotherm recorded for the PSBBB samples may be caused by further
reaction of the crosslinked products resulting in the formation of several elimination
products containing reactive groups, which can then further react in an exothermic
crosslinking reaction and generate additional degradation products.41 The degradation of
the samples at elevated temperatures (~200 °C) is also consistent with the results obtained
from thermogravimetric analysis (TGA), see Figure S13 in the supporting information,
which indicates a change in mass from temperatures around 200 °C. Moreover, curing the
PSBBB samples at 200 °C, even for short time periods (<5 min), resulted in a colour
change from colourless to yellowish-brown and a decrease in film thickness, thus further
indicating sample degradation.
The thermal polymerization of PSBBB was also monitored with FTIR spectroscopy
by annealing the PSBBB polymer samples at 120 °C for the same time periods used in
DSC. The ring opening process of BCB can be monitored by probing the disappearance of
the aromatic ring bending mode of the BCB moiety, which occurs around 1470 cm-1. This
process is typically accompanied by the formation of a new vibrational peak that
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corresponds to the ring bending mode of the newly formed cyclooctadiene. This ring
bending mode occurs at a higher wavenumber, typically around 1495 cm-1.39
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Figure 2. FTIR spectra close-up between 1510 and 1460 cm-1 recorded for PSBBB
samples following curing times between 0-180 min at 120 °C.
The FTIR spectra recorded for PSBBB as a function of different curing times at
120 °C in the region of interest between 1510 cm-1 and 1460 cm-1 are depicted in Figure 2,
while the full FTIR spectra can be found in Figure S14 in the supporting information. As
demonstrated in Figure 2, the as cast PSBBB has characteristic vibrational modes ascribed
to the BCB moiety, with a pronounced absorption at 1466 cm-1.39 Upon increasing the
curing time of the sample at 120 °C the peak at 1466 cm-1 progressively decreases in
intensity, illustrating the consumption of the BCB moiety, while the vibrational feature
arising from the cyclooctadiene unit at 1490 cm-1 progressively increases in intensity.
Together, these changes give rise to an isosbestic point centered around 1475 cm-1, thus
indicating the interconversion between the two species.
Evidence of crosslinking of the PSBBB samples were also provided by solvent
resistivity tests, which demonstrated that already after just 5 min of curing at 120 °C, a
crosslinked material was formed that was no longer soluble in a range of organic solvents
(toluene, chlorobenzene and dichlorobenzene) that are commonly employed for active
layer processing during OTFT fabrication.
Device fabrication. For this study, a C16-IDTBT active layer was employed with
gold source and drain electrodes modified with pentafluorothiophenol (PFBT), see Figure
3. Notably, in BGBC transistors the performance of C16-IDTBT has been reported to be
significantly worse than in TGBC transistors. In BGBC transistors employing a 300 nm
thick polyvinyphenol dielectric C16-IDTBT has previously been shown to exhibit a
mobility of ~0.02 cm2 V-1 s-1.42 whereas, in TGBC transistors, much higher mobilities of
more than 1 cm2 V-1 s-1 are expected. The BGBC transistors fabricated in this work
incorporating a 350 nm thick PSBBB dielectric layer exhibit a linear regime mobility of
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0.05 cm2 V-1 s-1 and a saturation regime mobility of 0.08 cm2 V-1 s-1, thus exceeding those
of the previously reported BGBC transistors with C16-IDTBT.42
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Figure 3. (a) Bottom-gate bottom-contact thin-film transistor architecture of the PSBBB
based OTFTs. Representative (b) saturation transfer plot, (c) output plot and (d) linear
transfer plot for transistors fabricated with each PSBBB layer annealed at 120 °C for 5
minutes.
Conclusion
In conclusion, the synthesis of a novel benzocyclobutene based dielectric material,
PSBBB, and its application as a bottom gate dielectric in organic thin film transistors was
demonstrated. The incorporation of a butoxide substituent on the benzocyclobutene moiety
was shown by DSC and FTIR to enable crosslinking at 120 °C, a temperature significantly
lower than for unsubstituted BCB based materials and compatible with the processing
conditions incurred by flexible plastic substrates. Notably, curing PSBBB for 5 minutes at
120 °C was shown to also incur a good OTFT performance, yielding a saturation mobility
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of 0.08 cm2 V-1 s-1 in C16-IDTBT BGBC OTFTs, which exceeds the mobility reported in
this transistor architecture with other dielectrics.
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Experimental Section
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General Procedures. The H and 13C NMR spectra were acquired in either a Bruker
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Avance III 400 MHz NMR spectrometer equipped with a BBFO SmartProbe or a Bruker Avance
III 500 MHz NMR spectrometer equipped with a BBFO CryoProbe. Gas chromatography-mass
spectrometry (GC-MS) was carried out on an Agilent Technologies 7693. Gel permeation
chromatography (GPC) was measured using a PLgel 10 um Mixed-B column on an Agilent
Technologies 1260 Infinity system with chlorobenzene as the mobile phase at 80 °C . Fourier
transform infrared spectroscopy (FTIR) was conducted on an attenuated total reflectance (ATR)
Agilent Cary 630 FTIR spectrometer. Samples for ATR-FTIR measurements were prepared by
drop-casting 80 µL of a 20 mg mL-1 polymer in toluene solution onto glass substrates and curing
the samples on a pre-heated hotplate for either 0 min, 5 min, 10 min, 15 min, 30 min, 60 min or
180 min at 120 °C. The cured polymers were then scraped off the glass substrates and placed on
the ATR-FTIR crystal and subsequently analysed. Differential scanning calorimetry (DSC) was
performed on a TA Instruments Q20 equipped with a RCS40 refrigerated cooling system. DSC
samples were prepared by casting 100 µL of a 20 mg mL-1 polymer in toluene solution into
weighed DSC pans. Samples were left to dry overnight to ensure complete evaporation of the
solvent. The samples were successively sealed with a lid and cured in an analogous fashion as the
samples prepared for FTIR measurements. DSC traces of each sample were obtained by heating
across a temperature range of 40-300 °C at a heating rate of 10 K min-1. A Bruker Dimension Icon
atomic force microscope operating in tapping mode was utilized to characterize the morphology.
Micrographs were analyzed with an open source software (Gwyddion).
Thin-film Transistor Fabrication and Characterisation. Bottom gate bottom contact
thin film transistors were fabricated on glass substrates cleaned by sequential sonication in acetone
and propan-2-ol for 10 minutes each. Onto these one-inch squared glass substrates, gold gate
electrodes (45 nm) were deposited by thermal evaporation through a shadow mask. To form the
dielectric layer, first PSBBB (30 mg mL-1 in toluene) was spin coated at 1000 rpm for 45 seconds
and annealed for 5 minutes at 120 °C. Next a second layer of PSBBB (30 mg mL-1 in toluene) was
spin-coated at 1000 rpm for 45 seconds and annealed again for 5 minutes at 120 °C. Overall this
processing resulted in a 350 nm thick dielectric layer. On top of the dielectric, gold source and
drain electrodes (35 nm) were thermally evaporated through a shadow mask. A surface
modification was applied by soaking the substrates in pentafluorobenzenethiol (PFBT) (0.005 mol
dm-3 in propan-2-ol) for 2 minutes before rinsing with propan-2-ol and annealing at 70 °C for
5 minutes. Finally, C16-IDTBT (10 mg mL-1 in dichlorobenzene) was spin coated at 1200 rpm for
45 seconds and annealed at 100 °C for 5 minutes.Transistors with a channel length of 40 µm and
width of 1000 µm were electrically characterised under a N2 atmosphere using a Keysight B2900A
Precision Source/Measure Unit.
Dielectric constant calculation of PSBBB. Glass substrates with pre-patterned ITO
bottom electrodes were employed to determine the relative permittivity of PSBBB. Substrates
were initially cleaned by subsequent ultrasonication in de-ionized water, acetone, and 2-propanol
for 10 min, respectively. A 10 min UV/O3 treatment was applied directly before the spin coating
deposition of PSBBB using the same deposition and annealing parameters as for the TFT
fabrication. The thermal evaporation of 40 nm Al top electrodes concluded the metal-insulator-
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metal test structure with a defined area A = 0.1 cm2. Capacitance-frequency measurements were
carried out using a Solartron Si 1260 impedance analyzer, within the frequency range of 100 Hz
to 100 kHz, at an AC level of 20 mV and 0 V DC component. The relative permittivity ϵr was
є푟 = 퐶 푡
퐴 є0
calculated according to the equation:
, with C = Capacitance, t = thickness, and ϵ0 = vacuum
permittivity. Both, device fabrication as well as measurements, were carried out inside a nitrogen
filled glove box. The dielectric constant in Figure S15 was found to be 2.82.
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Synthetic Procedures. 4-bromo-2-iodophenol (1). The synthesis of this molecule was
achieved following literature procedure.36 The characterization data are in agreement with the
published data.
(4-bromo-2-iodophenoxy)trimethylsilane (2). In a flame dried round bottom flask equipped
with a stirring bar 1 (11.4 g, 38.2 mmol, 1.00 eq.) was dissolved in 60 mL of dry THF. To this
solution, Hexamethyldisilazane (16 mL, 76 mmol, 2.0 eq) was added dropwise and the resulting
reaction mixture was heated in an oil bath to reflux. The reaction was allowed to proceed for 30
min before checking the reaction progress with GC-MS. Once the reaction reached completion,
excess solvent was removed under reduced pressure. Due to its poor stability, the crude material
was used in the next step without any purification. MS (EI): m/z 372 [M+]
4-bromo-2-(trimethylsilyl)phenol (3). In a flame dried round bottom flask equipped with a
stirring bar, 2 was dissolved (14.8 g, 39.8 mmol, 1.00 eq.) in 200 mL of dry THF. The solution
was placed in a dry ice bath, then a 2.5 M n-BuLi solution in hexane (17.5 mL, 1.10 eq) was added
drop wise and the reaction left stirring at -78 C for 1 hour. The reaction progress was monitored
with GC-MS. The reaction was quenched by addition of a saturated aqueous solution of sodium
bicarbonate. The aqueous layer was extracted with DCM and the combined organic layers were
dried over magnesium sulfate. Excess solvent was removed under reduced pressure. The crude
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product was employed in the next step without any further purification. H NMR (400 MHz,
CDCl3): δ (ppm) 7.42 (d, J = 2.5 Hz, 1H), 7.32 – 7.28 (m, 1H), 6.63 (d, J = 8.5 Hz, 1H), 0.32 (S,
9H). MS (EI): m/z 246 [M+]
4-bromo-2-(trimethylsilyl)phenyl trifluoromethanesulfonate (4). In a flame dried round
bottom flask equipped with a stirring bar 3 (9.00 g, 36.5 mmol, 1.00 eq.) was dissolved in 80 mL
dry DCM, then triethylamine (7.77 mL, 76.8 mmol, 2.10 eq.) was added drop wise. After 15 min
the flask was placed in an ice bath before adding triflic anhydride (7.44 mL, 43.9 mmol, 1.20 eq.).
The reaction progress was monitored by GC-MS and was deemed complete after stirring for 2 h
at 0 C. The reaction was quenched by addition of water. The aqueous layer was extracted with
DCM. The combined organic phases were dried over magnesium sulfate, filtered and concentrated
under vacuum. The crude product was purified via silica column chromatography using a 1:1 v/v
mixture of PE:DCM to obtain the final product as a colorless oil (8.00 g, 22.0 mmol, 58% yield).
1H NMR (400 MHz, CDCl3): δ (ppm) 7.64 (d, J = 2.6 Hz, 1H), 7.57 (dd, J = 8.8, 2.6 Hz, 1H), 7.25
(d, J = 8.8 Hz, 1H), 0.41 (S, 9H). 13C{1H}NMR (100 MHz, CDCl3): δ (ppm) 153.7, 138.8, 135.8,
134, 121.6, 121.3, 120.3. HRMS (ESI-TOF) m/z: [M+H-CH3]+ Calcd for C9H9BrF3O3SSi 360.9171
Da; Found 360.9177 Da.
3-bromo-7-butoxybenzocyclobutene (5). In a flame dried round bottom flask equipped
with a stirring bar cesium fluoride (11.4 g, 75.0 mmol, 6.00 eq.) was added and flask heated to
150C under high vacuum. After 3 hours, the flask was cooled down and placed under a nitrogen
atmosphere. 100 mL of dry acetonitrile were added to generate a slurry to which butyl vinyl ether
(4.9 mL, 38 mmol, 3.0 eq.) and 4 (5.70 g, 15.7 mmol, 1.00 eq.) were added dropwise. The mixture
was allowed to react at room temperature, after 2 hours the reaction was deemed completed after
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checking with GC-MS. The reaction was quenched with water and the aqueous phase extracted
with DCM. The combined organic layers were dried over magnesium sulfate, filtered and
concentrated under reduced pressure. The crude liquid was purified using silica column
chromatography employing a 4:1 v/v mixture of PE:DCM as the eluent. Excess solvent was
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removed under reduced pressure to obtain 5 as a colorless oil (3.01 g, 11.7 mmol, 77% yield). H
NMR (500 MHz, CDCl3): δ (ppm) 7.50 (d, J = 8.8 Hz, 1H), 7.25 (d, J = 7.6 Hz, 1H), 6.89 (s, 1H),
5.31 (d, J = 10.8 Hz, 1H), 3.72 (m, 1H), 3.66 (m, 1H), 3.53 (m, 1H), 3.2 (m, 1H), 1.7 (m, 2H), 1.48
(m, 2H), 0.99 (t, J = 7.4 Hz, 3H). MS (EI): m/z 256 [M+].
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3-styrene-7-butoxybenzocyclobutene (6). In a 100 mL round bottom flask equipped with
a stirring bar potassium carbonate (3.27 g, 23.5mmol, 3.00 eq) was dissolved in 12 mL of distilled
water. The solution was purged with nitrogen for 30 min, before 30 mL of THF were added. 5 (2.0
g, 7.8 mmol, 1.0 eq.) and 4-vinylphenylboronic acid (1.75 g, 11.8 mmol, 1.50 eq.) were then added.
The solution was purged for an additional 10 min before Pd(PPh3)4 (270 mg, 0.235 mmol, 0.0300
eq.) was added. The reaction was then heated in an oil bath to 70 C overnight. The reaction was
quenched with water and the aqueous layer extracted with DCM. The organic phases were dried
over magnesium sulfate and concentrated under vacuum. The crude product was purified via silica
column chromatography using a 95:5 v/v mixture of PE:EA as the eluent. Excess solvent was
removed under reduced pressure to obtain the final product as a colourless oil (2.52 g, 9.09 mmol,
77% yield). 1H NMR (500 MHz, CDCl3): δ (ppm) 7.61 – 7.53 (m, 3H), 7.50 (d, J = 8.8 Hz, 3H),
7.25 (d, J = 7.6 Hz, 1H), 6.79 (dd, J = 17.6, 10.9 Hz, 1H), 5.82 (dd, J = 17.6, 1.0 Hz, 1H), 5.31 (d,
J = 10.8 Hz, 1H), 5.12 (dd, J = 4.4, 2.0 Hz, 1H), 3.72 (m, 1H), 3.66 (m, 1H), 3.53 (m,1H), 3.2 (m,
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1H), 1.7 (m, 2H), 1.48 (m, 2H), 0.99 (t, J = 7.4 Hz, 3H). C{1H} NMR (125 MHz, CDCl3): δ
(ppm) 146.8, 141.7, 141.4, 141.2, 136.4, 128.5, 127.4, 126.6, 123.8, 123, 122.3, 121.5, 113.8, 68.8,
38.4, 32, 19.4, 14.0. HRMS (ESI-TOF) m/z: [M+H]+ Calcd for C20H23O 279.1743 Da; Found
279.1746 Da.
Polymerization method. 2 mL of dry benzene was added to an oven dried microwave vial
and purged for 30 min. Styrene (131.1 mg, 1.259 mmol, 1.000 eq.) and 6 (350.0 mg, 1.259 mmol,
1.000 eq.) were added. The resulting solution was purged for 5 min then the reaction was heated
in an oil bath to 65C for 5 min. Benzoyl peroxide (10 mg in 0.1 mL of dry benzene) was then
added drop wise and the reaction allowed to run for 16 hours. Excess solvent was removed under
reduced pressure and the crude product redissolved in 2 mL of chloroform. The product was
precipitated by pouring the chloroform solution into 20 mL of MeOH. The white precipitate was
collected by suction filtration and this process was repeated for three additional times. PSBBB was
obtained as a white powder (220 mg, 46% yield). GPC (Chlorobenzene, 80C) Mn = 46 kDa, Mw
= 91 kDa. 1H NMR (500 MHz, CDCl3): δ (ppm) 7.07 (m, 8H), 6.58(m, 3H), 5.06(s,1H), 3.6 (m,
3H), 3.15 (s, 1H), 1.48 (m, 9H), 0.95 (s, 2H).
Supporting Information
The Supporting Information is available free of charge.
1H and 13C{1H} NMR spectra, GC-MS measurements, TGA traces, FTIR spectra, dielectric constant
measurements, AFM data and OFET characterization plots can be found in the supporting
information.
Acknowledgments
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The research reported in this publication was supported by funding from King Abdullah
University of Science and Technology (KAUST).
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