Control over Structure Formation of Small Molecular Weight Thiophenes in Vacuum Deposited Films

Abstract

Recent structural studies of small-molecular-weight thiophenes are surveyed, with particular focus on naphthyl-end-capped derivatives and comparison to alkyl-capped and unsubstituted analogues. Grazing-incidence wide-angle X-ray scattering of 5,5′-bis(naphth-2-yl)-2,2′-bithiophene (NaT2) on octadecyl-trichlorosilane-passivated Si, graphene, MoS₂, muscovite mica, and in operando thin-film transistors reveals substrate-dependent unit cells, polymorphs, strain fields, and epitaxial orientations. Bulk crystallography exposes multiple polymorphs in ambient conditions and under compression up to the gigapascal regime. In situ vacuum deposition experiments track layer-by-layer nucleation, a wetting-layer–mediated 2D-to-3D transition, and the emergence of bulk packing. High stability permits long measurements, whereas strong crystallinity enables high quality diffraction signals even from monolayers and through diamond-anvil cells and high-background vacuum chambers. Detailed comparisons with other small-molecular-weight thiophenes are made throughout to contextualize and generalize these observations. Together these results establish naphthyl-terminated thiophenes as convenient model systems for probing substrate interactions, growth modes, and strain-coupled polymorphism in organic semiconductors.