Thiophene fluorination to enhance photovoltaic performance in low band gap donor –acceptor polymers. Zhuping Fei ‡a , Munazza Shahid ‡a , Nir Yaacobi-Gross b , Stephan Rossbauer b , Hongliang Zhong a , Scott E. Watkins c , Thomas D. Anthopoulos b , Martin Heeney* a Experimental section: General: Reagents and chemicals were purchased from Aldrich and Acros unless otherwise noted. 9-(Bromomethyl)nonadecane (1) 1 , 3,3’-Dibromo-5,5’-bis(trimethylsilyl)-2,2’-bithiophene (4) 2 , (5-Bromo-3,4-difluoro-thiophen-2-yl)-trimethyl-silane (7) 3 and 4,7-Bis(5-bromo-2-thienyl)- 2,1,3-benzothiadiazole (11) 4 were synthesized by the reported method. Commercial 2,1,3- benzothiadiazole-4,7-bis(boronic acid pinacol ester) (8) was purified by chromatography over silica gel (eluent: 3% ethyl acetate in hexane), followed by recrystallisation from hexane before use. All reactions were carried out under Ar using solvents and reagents as commercially supplied, unless otherwise stated. 1 H and 13 C NMR spectra were recorded on a Bruker AV-400 (400 MHz), using the residual solvent resonance of CDCl 3 or d-1,1,2,2-tetrachloroethane and are given in ppm. Number-average (Mn) and Weight-average (Mw) were determined by Agilent Technologies 1200 series GPC running in chlorobenzene at 80°C, using two PL mixed B columns in series, and calibrated against narrow polydispersity polystyrene standards. Preparative GPC utilised a Shimadzu recycling GPC system running in hexane at 40°C or chlorobenzene at 80°C, using Agilent PLgel 10µm 50A or MIXED-D column, DGU-20A3 Degasser, LC-20A Pump, CTO-20A Column Oven andSPD-20A UV Detector. Electrospray mass spectrometry was performed with a Thermo Electron Corporation DSQII mass spectrometer. UV-vis spectra were recorded on a UV-1601 Shimadzu UV-vis spectrometer. Flash chromatography (FC) was performed on silica gel (Merck Kieselgel 60 F254 230-400 mesh). Photo Electron Spectroscopy in Air (PESA) measurements were recorded with a Riken Keiki AC-2 PESA spectrometer with a power setting of 5nW and a power number of 0.5. Samples for PESA were prepared on glass substrates by spin-coating. Thermogravimetric analysis (TGA) was carried out using a Perkin Elmer Pyris 1 TGA machine, with heating from 100 to 700°C, in dry air, at a heating rate of 10°C/min. Differential scanning calorimetry (DSC) measurements: ~4 mg material was used for the DSC experiments, which was conducted under nitrogen at scan rate of 20 ˚C min -1 with a TA DSC-Q20 instrument. OFET (organic field effect transistors) devices fabrication: Bottom-gate, bottom-contact OFETs were fabricated on Si/SiO 2 substrates with photolithographically patterned gold electrodes. The electrodes were again treated with the pentafluorobenzene thiol SAM and the SiO 2 was treated with an octadecyltrichlorosilane (OTS) SAM. The channel width and length of the transistors are 1.5 mm and 20 µm, respectively. The polymer was dissolved in dichlorobenzene (5 mg/ml) at 100°C and spin cast to 100 °C Electronic Supplementary Material (ESI) for Chemical Communications This journal is © The Royal Society of Chemistry 2012