X‑ray Structural Investigation of Nonsymmetrically and Symmetrically Alkylated [1]Benzothieno[3,2‑b]benzothiophene Derivatives in Bulk and Thin Films

A detailed structural study of the bulk and thin film phases observed for two potential high-performance organic semiconductors has been carried out. The molecules are based on [1]­benzothieno­[3,2-b]­benzothiophene (BTBT) as conjugated core and octyl side groups, which are anchored either symmetrically at both sides of the BTBT core (C8–BTBT–C8) or nonsymmetrically at one side only (C8–BTBT). Thin films of different thickness (8–85 nm) have been prepared by spin-coating for both systems and analyzed by combining specular and grazing incidence X-ray diffraction. In the case of C8–BTBT–C8, the known crystal structure obtained from single-crystal investigations is observed within all thin films, down to a film thickness of 9 nm. In the case of C8–BTBT, the crystal structure of the bulk phase has been determined from X-ray powder diffraction data with a consistent matching of experimental and calculated X-ray diffraction patterns (Rwp = 5.8%). The packing arrangement of C8–BTBT is similar to that of C8–BTBT–C8, that is, consisting of a lamellar structure with molecules arranged in a “herringbone” fashion, yet with lamellae composed of two head-to-head (or tail-to-tail as the structure is periodic) superimposed molecules instead of only one molecule for C8–BTBT–C8. As for C8–BTBT–C8, we demonstrate that the same phase is observed in bulk and thin films for C8–BTBT whatever the film thickness investigated.