Persistent Ambipolar Heptacenes and Their Redox Species

Abstract Sixfold TIPS‐ethynylation combined with fourfold bromination of the armchair edges furnishes a long‐lived, soluble heptacene; π‐extension via Stille coupling accesses a persistent tetrabenzononacene. Both types of acenes were stabilized best by double TIPS‐ethynylation on every other benzene ring. Tetrabromoheptacene is an ambipolar transistor material (up to 0.036 cm2 V−1 s−1 n‐channel), which was corroborated by generation of its monoanion and monocation.

Melting points were determined in open glass capillaries on a Melting Point Apparatus MEL-TEMP (Electrothermal, Rochford, UK) and are uncorrected. X-ray single-crystal structure analyses of 1a, 1b, 1c, and 9a were measured on a STOE Stadivari CCD area detector diffractometer.
Diffraction intensities were corrected for Lorentz and polarization effects. An empirical scaling and absorption correction was applied using X-Area LANA 1.70.0.0 based on the Laue symmetry of reciprocal space, structures were solved with SHELXT-2014 [1] and refined against F 2 with a Full-matrix least-squares algorithm using the SHELXL-2018/3 (Sheldrick, 2018) software. [2] X-ray single-crystal structure analyses of 1a ·were measured on a Bruker APEX-II Quazar area detector. An empirical scaling and absorption correction was applied using SADABS based on the Laue symmetry of the reciprocal space, structure solved with SHELXT-2018/2  and refined against F 2 with a Full-matrix least-squares algorithm using the SHELXL-2018/3 (Sheldrick, 2018) software. [2] NMR spectra were recorded on Bruker Avance III spectrometers using the specified frequency. Chemical shifts (δ) are given in parts per million (ppm) relative to internal solvent signals. [3] Signal multiplicities are described by the following abbreviations: s = singlet, d = doublet, dd = doublet of doublets, m = multiplet, bs = broad signal.

GPS1:
The corresponding silyl acetylene (100 eq.) was dissolved in dry n-hexane under argon atmosphere. n-BuLi (95.0 eq., 2.50 M in n-hexane) was added dropwise at r.t. and the mixture was stirred for 30 min. The respective acenequinone (1.00 eq.) and small volumes of dry THF were added. The suspension was stirred for 12 h at room temperature (rt). The reaction was quenched with saturated aqueous ammonium chloride solution and extracted with DCM. The combined organic layers were dried over anhydrous magnesium sulfate and the solvent was removed under reduced pressure.

GPS2:
The corresponding acenetetraol (1.00 eq.) was dissolved in a mixture of acetonitrile / THF (1:1) under argon atmosphere. Anhydrous SnCl2 (10.0 eq.) was added, and the reaction mixture was stirred at r.t. overnight. The resulting mixture was filtered, and the precipitate was washed with acetonitrile until the filtrate was clear. 4a (500 mg, 434 µmol, 1.00 eq.), diazabicycloundecene (1.33 g, 1.30 mL, 8.69 mmol, 20.0 eq.) and lithium iodide (116 mg, 869 µmol, 2.00 eq.) were dissolved in dry THF (150 mL) under argon atmosphere and the mixture was refluxed overnight. After cooling to rt the reaction mixture the precipitate was collected by filtration and the orange brown residue was washed with THF (100 mL), water (50.0 mL) and methanol (50.0 mL) to yield an orange solid as the pure product (333 mg, 299 µmol, 69 %). 3a was insoluble in common organic solvents impeding characterization via NMR spectroscopy.

Hexakis((triisopropylsilyl)ethynyl)-tetrabenzononacen (9a)
1a (50.0 mg, 28.2 µmol, 1.00 eq.) and 9 (25.4 mg, 84.5 µmol, 3.00 eq.) were added to a Schlenk flask under argon atmosphere. Pd(P(tBu)3)2 (1.4 mg, 2.81 μmol, 0.1 eq.) was added and the solids were dissolved in dry THF (10.0 mL) in a glovebox. The reaction mixture was stirred at 70 °C for 48 h, cooled to rt and the product was precipitated by dropwise adding the mixture to ACN. The suspension was filtered, and the dark green precipitate was washed with acetonitrile to yield the crude compound 9a as a brown solid (15.0 mg, 8.52 µmol, 30%). Single crystal specimen were obtained by evaporation of a concentrated dichloromethane solution of the crude product. The compound was not soluble enough for 13 C NMR analysis. [

Figure S37
: X-ray single crystal structure (top view) including bond lengths (in Å) of 1a (blue) and the anionic form 1a ·-(green). Determined by crystal analysis.

Analysis of Decomposition Products
A freshly prepared solution of 1a in anhydrous THF under inert atmosphere was exposed to ambient conditions (light and air) for several days at rt. Afterwards the solvent was evaporated and the solid was analyzed via MALDI mass spectrometry. Formation of di-oxo-adducts is evident (position of oxidation was assigned via UV/vis spectroscopy as bands generated are characteristic of tetracene fragments).

OFET Fabrication and Characterization
A sliced, highly doped silicon wafer with 100 nm thick thermally grown SiO2 was successively cleaned via ultra-sonication in acetone, isopropanol and ethanol, each for 10 min. It was washed with water and dried in a stream of nitrogen. The wafer was placed in freshly produced Caro´s acid and was heated to 100 °C for 20 min. After cleaning with water and drying, a 150 mM solution of Al(NO3)3 * 9 H2O in ethanol was spin-coated (5000 rpm; 40 s) onto the substrate. Right after that the wafer was heated to 300 °C for 30 min. For the formation of the self-assembled monolayer, the substrate was placed in a 15.0 mM solution of 12-cyclohexyldodecylphosphonic acid (CDPA) [10] in isopropanol at room temperature for 16 h. Then the substrate was cleaned via ultra-sonication in isopropanol for 10 min, rinsed with water and dried in a stream of nitrogen. The capacitance of the dielectric layer amounted to 26 nF cm -2 .
Drop-cast thin-films were prepared by dropping the prepared solution (1a: 1.50 mg/mL in toluene; 1c: 0.75 mg/mL toluene) onto the heated substrate (50 °C) covering the wafer. Electrode formation was achieved by depositing a 40 nm thick layer of gold was deposited through a shadow mask onto the organic layer in a vacuum evaporator at a pressure below 2 x 10 -6 bar. Transistor characteristics were measured with a semiconductor characterization system (Keithley 4200-SCS) in a nitrogen filled glove box. The field effect mobilities were determined in the saturated regime using the equation = ( 2 ) µ( − ℎ ) 2 , where is the source-drain current, W is the channel width, L the channel length, is the capacitance per unit area of the gate dielectric layer, µ is the field effect mobility, and ℎ is the threshold voltage. Figure S40. OFET output characteristics of heptacene 1c Figure S39. OFET output characteristics of heptacene 1a;
Corresponding lattice planes are parallel to the substrate surface, thus the π planes are perpendicular to the surface (1a: 88.4°, 1c: 89.7°).   Figure S43: Left: Normalized UV/vis absorption spectra of the product of the reduction with NaK alloy (red), the reduction with potassium anthracenide (blue) and heptacene 1a (black) in anhydrous THF at RT under inert atmosphere. Right: EPR spectrum of 1a (black) and the reaction product of 1a and sodium potassium alloy in anhydrous THF under inert atmosphere (9.45 GHz, RT)