Using Molecular Design to Increase Hole Transport: Backbone Fluorination in the Benchmark Material Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]-thiophene (pBTTT): Backbone Fluorination in the Benchmark Material Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]-thiophene (pBTTT)

The synthesis of a novel 3,3′-difluoro-4,4′-dihexadecyl-2,2′-bithiophene monomer and its copolymerization with thieno[3,2-b]thiophene to afford the fluorinated analogue of the well-known poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]-thiophene) (PBTTT) polymer is reported. Fluorination is found to have a significant influence on the physical properties of the polymer, enhancing aggregation in solution and increasing melting point by over 100 C compared to nonfluorinated polymer. On the basis of DFT calculations these observations are attributed to inter and intramolecular S...F interactions. As a consequence, the fluorinated polymer PFBTTT exhibits a fourfold increase in charge carrier mobility compared to the nonfluorinated polymer and excellent ambient stability for a nonencapsulated transistor device. Backbone fluorination of the benchmark liquid crystalline polymer poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]-thiophene) is shown to significantly influence its physical properties, substantially increasing the melting point and promoting aggregation. Density functional theory calculations suggest that this is related to a more coplanar backbone for the fluorinated polymer. Field effect transistor measurements show a fourfold increase in mobility upon fluorination.