Modeling of Grazing-Incidence X-ray Diffraction from Naphthyl End-Capped Oligothiophenes in Organic Field-Effect Transistors

The structure of two naphthylene-capped oligothiophene, 5,5'-bis(naphth-2-yl)-2,2'- bi- and tri- thiophene, thin-film field-effect transistor assemblies has been studied using modeling in conjunction with grazing incidence X-ray diffraction. Although the well-known herringbone molecular packing motif is observed in these films for both compounds, density functional calculations and molecular mechanics modeling give evidence for a local polymorphic ordering in which these molecules can be flipped 180 degrees about the long axis. In one case, that of the oligothiophene trimer, a disordered surface induced phase is observed. Prospective structural models are tested and refined using various supercell constructions optimized by molecular mechanics prior to structure refinements of the thin-film scattering data.