An Air‐Stable Semiconducting Polymer Containing Dithieno[3,2‐b:2′,3′‐d]arsole

Abstract Arsole‐containing conjugated polymers are a practically unexplored class of materials despite the high interest in their phosphole analogues. Herein we report the synthesis of the first dithieno[3,2‐b;2′,3′‐d]arsole derivative, and demonstrate that it is stable to ambient oxidation in its +3 oxidation state. A soluble copolymer is obtained by a palladium‐catalyzed Stille polymerization and demonstrated to be a p‐type semiconductor with promising hole mobility, which was evaluated by field‐effect transistor measurements.


Experimental details
General techniques 3,3'-Dibromo-4,4'-didodecyl-[2,2'-bithiophene]-5,5'-bis(trimethylsilane) (1) was synthesized by a previously published route. [1] Reagents were purchased from Sigma Aldrich and VWR and used with no additional purification, and all reactions were performed using oven-dried glassware under argon using standard Schlenk techniques. Thin layer chromatography (TLC) was performed on silica plates (Merck Kieselgel 60 F254 aluminum sheets) and visualized using UV light (254 and 365 nm). Column chromatograph was performed on silica gel (Merck Kieselgel 60 230-400 mesh). 1 H and 13 C NMR spectra were recorded using Bruker AV-400 spectrometers ( 1 H = 400 MHz, 13 C = 101 MHz) at 298 K. Microwave experiments were carried out in a Biotage Initiator (v. 2.3). UV-visible spectroscopy was carried out using a UV-1800 Shimadzu UV-vis spectrometer for both solution and thin-film samples. Thin films of the polymer were prepared by spin coating a 5 mg ml -1 solution of PDTAsV in chlorobenzene at 1000 rpm for 60 s. Molecular weight (Mn, Mw, PDI) was measured using an Agilent Technologies 1200 series GPC at 80 °C, using chlorobenzene as an eluent with two PL mixed B columns in series. The column was calibrated using narrow polydispersity polystyrene standards. Xray diffraction (XRD) measurements were carried out with a PANalytical X′Pert-pro MRD diffractometer equipped with a nickel-filtered Cu Kα source (1.54 Å) and X′Celerator detector, using current I = 40 mA and accelerating voltage V = 40 kV. Samples were prepared by drop casting onto silicon substrates (100) and annealing was carried out for 30 min at 200 °C under argon. Samples were rotated at 60 rpm during measurements. Cyclic voltammetry (CV) was performed in a standard three-electrode setup with polymer film on FTO as the working electrode, a Pt mesh as the counter electrode and an Ag wire as the reference electrode. The working electrode was prepared by spin coating a 10 mg/ml solution of PDTAsV in chlorobenzene onto FTO-coated glass (supplied by Sigma Aldrich, ~7 Ω/sq) at 1000 rpm for 60 s. Voltammograms were measured using a 0.1 M tetrabutylammonium hexafluorophosphate ((TBA)PF6) solution in acetonitrile as an electrolyte, at a scanning rate of 0.05 V/s. Redox potentials were calibrated against a standard ferrocene/ferrocenium (FOC) system, assuming the energy level of FOC to be 4.8 eV below vacuum. Geometries and frontier molecular orbitals were calculated using density functional theory at the B3LYP level of theory with the 6311G(d,p) basis set, using the GAUSSIAN09 software package. [2] OFET device fabrication Top gate/bottom contact devices were fabricated on glass substrates using Au (60 nm) source-drain electrodes and CYTOP dielectric. Au electrodes were treated with pentafluorobenzene thiol (PFBT) SAM to increase the work function. Polymer was dissolved in chlorobenzene at a concentration of 10 mg/ ml, and spun cast at 2000 rpm for 60 s. The obtained polymer film was annealed at 200 °C for 30 min before spin coating of CYTOP dielectric. The channel width and length of the transistors were 1000 µm and 40 µm, respectively.

Synthesis of 3,3'-dibromo-4,4'-didodecyl-[2,2'-bithiophene] (2)
To a solution of 1 (11.11 g, 13.8 mmol) in anhydrous THF (120 ml) at 0 °C was added a solution of TBAF (1M in THF, 32 ml, 2.3 eq) in three portions, and the resulting solution was stirred for 5 min. Hexane (250 ml) was added, followed by a further 10 min of stirring. The resulting precipitate was removed by filtration through a short silica plug, and the solvent removed under reduced pressure to afford 2 (8.76 g, 13.3 mmol, 96%) as a yellow solid (m.p. 59-60 °C). 1 [3] (4) Caution: PhAsCl2 is highly toxic and a vesicant, and as such should be handled with extreme care. Proper safety equipment should be used at all times, and reactions should be carried out in a wellventilated fumehood. Treat any spillages and all glassware with dilute bleach to prevent accidental contamination and injury.

Synthesis of dichlorophenylarsine
Phenylarsonic acid (3) (4.99 g, 24.7 mmol) was placed in a flask, and HCl (37%, 50 ml) was added, followed by stirring until 3 was completely dissolved. Trace I2 was added, and the reaction was heated to 55 °C while SO2 was bubbled through the reaction for 6 h. The reaction was then left undisturbed overnight to allow phase separation to occur, after which the dense bottom layer (containing product) was removed, and the top layer extracted with CHCl3. The combined organic fractions and initial bottom layer were concentrated in vacuo, after which the product was purified by vacuum distillation (~71 °C at 0.5 mbar) to yield the red liquid, 4 (4.94 g, 22.2 mmol, 89%). 1

Stille polymerization of 6 with trans-1,2-bis(tributylstannyl)ethene
An oven-dried high-pressure microwave tube was charged with 6 (82.2 mg, 0.101 mmol), trans-1,2bis(tributylstannyl)ethene (61.4 mg, 0.101 mmol), and Pd(PPh3)4 (2.3 mg, 0.002 mmol). The tube was sealed with a septum and flushed with Ar, after which degassed chlorobenzene was added (0.5 ml). The mixture was thoroughly degassed with Ar, after which the inlet was removed. The vial was then placed in a microwave reactor and heated as follows: 4 min at 100 °C, 120 °C, and 140 °C, 20 min at 160 °C, and 120 min at 180 °C. After cooling to room temperature, the resulting polymer was precipitated into methanol and filtered into a Soxhlet thimble. The polymer was extracted (Soxhlet) with methanol, acetone, and hexane in that order. The hexane fraction was concentrated under reduced pressure and the resulting polymer was dissolved in chloroform, warmed to 50 °C and stirred in the presence of diethylammonium diethyldithiocarbamate [4] (ca. 20 mg) for 1 hr. The solution was concentrated in vacuo, dissolved in a minimal amount of hot chlorobenzene and the polymer collected by re-precipitation into methanol followed by vacuum filtration. After drying under vacuum, polymer PDTAs-V was collected (54 mg, 79%). GPC (chlorobenzene): Mn 12,300 g/mol, Mw 23,700 g/mol, Ð 1.92. 1 H NMR (500 MHz, 323 K, CDCl3) δ 7.31 (broad, 2H), 7.21 (broad,

Frontier molecular orbitals of DTAs
HOMOs are shown below LUMOs.