Synthesis and Processing of Monodisperse Oligo(fluorene-co-bithiophene)s into Oriented Films by Thermal and Solvent Annealing

A series of oligo(fluorene-co-bithiophene)s, OF2TS, have been synthesized and characterized to investigate the effects of oligomer length and pendant aliphatic structure on glassy-nematic mesomorphism. The OF2Ts comprising more than one repeat unit and their polymer analogue, PF2T, carrying 52 number-average repeat units, possess the highest occupied molecular orbital energy level at -5.3 +/- 0.2 eV, but the anisotropic field-effect mobilities increase with the oligomer length. Spin coating from high-boiling chlorobenzene with and without subsequent exposure to saturated chlorobenzene vapor constitute solvent-vapor annealing and quasi-solvent annealing, respectively. Solvent-vapor annealing yields monodomain glassy-nematic films in which OF2Ts are aligned as well as with thermal annealing across a 2 cm diameter. Quasi-solvent annealing, however, amounts to kinetically trapping a lower orientational order than solvent-vapor or thermal annealing. While amenable to thermal annealing at elevated temperatures, PF2T shows no alignment at all following either strategy of solvent annealing.