Crystal Structures of β-Methylchalcogenated Tetrathienoacenes: From One-Dimensional π-Stacking to Sandwich Pitched π-Stacking Structure

The control and prediction of crystal structures of molecular semiconductors is critical to optimize such solid-state properties as carrier mobility, although it still presents a challenge owing to the complicated interplay of various intermolecular interactions in the solid state. We recently reported that methylthio and methoxy groups can act as a “controller” of crystal structure from the herringbone to the rubrene-like pitched π-stacking in a series of acenedithiophenes and simple acenes, such as naphthalene and anthracene. In this study, we focused on tetrathienoacene (4TA), which affords a one-dimensional π-stacking structure, and investigated the effect of methylchalcogenation on its crystal structure. The crystal structures of newly synthesized 4TA derivatives were altered significantly; the methoxy and methylthio derivatives crystallized into a “sandwiched pitched π-stacking” structure, whereas the methylseleno 4TA crystallized into a “pseudo π-stacking” structure. These structural changes were explained by the analysis of intermolecular interactions estimated by the symmetry-adapted perturbation theory method. The interplay between the CH-π interactions induced by the methylchalcogenation and the inherent nature of the π-π interactions of the 4TA cores determined the crystal structures of the 4TA derivatives. We conclude that the mutual effects of the parent π-conjugated system and the methylchalcogeno groups are important to define the packing structure.