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

Abstract

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-<i>b</i>]­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 (<i>R</i>_wp = 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