Large Scale Crystallinity in Kinetically Stable Polythiophene-Based Langmuir-Blodgett Films

The first direct observation of large scale molecular level ordering in a complex poly(3-a lkylthiophene)-based thin film is reported. The fabrication and characterization of highly ordered thin films made from the regioregular, amphiphilic polythiophene derivative poly(3-(11-(2-tetrahydropyranyloxy)undecyl)thiophene) (PTHPUDT) are described. PTHPUDT affords kinetically stable, high optical quality films deposited layer-by-layer using the Langmuir-Blodgett (LB) method. X-ray diffraction studies confirm the deposition of a film with long-range order normal to the surface, characterized by bilayer separations of similar to 30 angstrom. The films display in-plane anisotropy, associated with the preferential alignment of the polymer main chain in the dipping direction. Molecular resolution atomic force microscopy shows the presence of highly ordered crystalline domains within the plane, consistent with an ordered array of parallel, closed-packed, polythiophene chains. Polarized optical microscopy confirms the formation of large scale domains that display uniform optical retardation across macroscopic length scales. FeCl3-doped films yield anisotropic conduction behavior, suggesting higher rates of transport perpendicular to the main chain direction. The kinetic stability of these films is distinct from films of other polythiophenes, other deposition methods, and simpler amphiphiles, all of which tend toward their bulk, thermodynamically stable structures. This stability and long-range order are attributed to the amphiphilic nature of the polymer and the dimensional restrictions of the polymer's main chain, which limits the extent of structural defect-based reorganization, and the limited number of local structures of the alkyl chains. The degree and nature of the ordering in these semicrystalline films make them an ideal model system in which to elucidate the connection between morphology and physical property in complex pi-conjugated polymers.