Synthesis of 6,13-difluoropentacene

6,13-Difluoropentacene was synthesized from 1,4-difluorobenzene. Friedel–Crafts annulation of the latter with phthalic anhydride and subsequent reduction of the anthraquinone gave 1,4-difluoroanthracene. After ortho-lithiation and reaction with phthalic anhydride a carboxylic acid was obtained whose Friedel–Crafts acylation and subsequent reductive removal of the oxygen-functionalities resulted in the formation of the target compound. The HOMO–LUMO gap of 6,13-difluoropentacene was determined via UV–vis spectroscopy and compared to other fluorinated pentacenes.


Methods and materials General information
All anhydrous reactions were carried out using flame-dried glassware under argon atmosphere. All solvents were distilled by rotary evaporation. THF for anhydrous reactions was dried with KOH and subsequently distilled from sodium/benzophenone and from Solvona® respectively. All other solvents employed under anhydrous and/or anaerobic conditions were bought in anhydrous form. C6D6 for NMR-measurements was degassed three times by freeze-pump-thaw cycles prior to use. All commercially available reagents and reactants were used without further purification unless otherwise noted. Reactions were monitored by thin layer chromatography (TLC) using Merck Silica Gel 60 F254 and visualized by fluorescence quenching under UV-light. In addition, TLC-plates were stained using a cerium sulfate/phosphomolybdic acid stain or a potassium permanganate stain. Chromatographic purification of products was performed on Macherey-Nagel Silica Gel 60 (230-400 mesh) using a forced flow of eluents. All crude products were adsorbed onto silica by dissolving in an appropriate solvent and removing the solvent under reduced pressure. Concentration under reduced pressure was performed by rotary evaporation at 40 °C and appropriate pressure and by exposing to high vacuum at room temperature if necessary.
Chemical shifts are reported in ppm with the solvent resonance as internal standard.
All reported 19 F-NMR spectra are proton decoupled 19 F{ 1 H}-measurements and referenced to external CFCl3. Data are reported as follows: s = singlet, d = doublet, t = triplet, q = quartet, quin = quintet, m = multiplet and combination thereof. Due to the low solubility of compounds 5 & 16 no 13 C NMR spectra were obtained. All correlations of atoms from NMR spectra of new compounds could be achieved via additional 2D-NMR data (HSQC-and HMBC-spectra) which is not shown within this Supporting Information.

High resolution mass spectrometry
HR-ESI and APCI mass spectra were acquired with a Finnigan LTQ-FT Ultra mass spectrometer (Thermo Fischer Scientific). EI mass spectra were acquired with an AccuTOF GCv (Jeol) mass spectrometer.

Infrared spectroscopy
FTIR spectra were recorded on a BRUKER IFS 200 spectrometer. Intensities are reported as follows: s = strong, m = medium, w = weak.

Melting points
Melting points were determined on a MP70 (Mettler Toledo) using one end closed capillary tubes.

UV-vis spectroscopy
UV-vis-spectra have been acquired with a Thermo Scientific Multiskan Go using quartz rectangular cuvettes (Teflon cover, 10 mm light path, 3.5 mL) by Aldrich.
The crude 2-(2,5-difluorobenzoyl)benzoic acid was suspended in polyphosphoric acid (41.2 g) and the suspension was stirred at 140 °C for 2 h. The mixture was cooled to room temperature and poured into ice-water (200 mL) and CH2Cl2 (200 mL) and neutralized to pH = 7 using K2CO3. The layers were separated and the aqueous layer was extracted with CH2Cl2 (3 × 150 mL). The combined organic layers were dried over MgSO4 and filtered over Alox (basic). The crude product was adsorbed onto silica and purified via column chromatography (n-hexane/CHCl3 3:7) to obtain anthraquinone 11 (3.72 g, 15.2 mmol, 53% over two steps) as yellow solid.