Melt-mixed superlayer cocrystal formation using symmetric and unsymmetric organic semiconductors

Organic molecules with a rigid, π-conjugated core (π-core) and flexible alkyl chains (C<i>_n</i>) naturally exhibit liquid crystal (LC) phases, promoting self-assembly of quasi-two-dimensional semiconducting layered crystals. However, particular roles of rigid and flexible parts in layer formations remain elusive. Here, we demonstrate formation of an unprecedented superlayer cocrystal phase via a unique smectic LC phase in the equimolar melt mixture of symmetrically distinct molecules. The molecules used are a monoalkylated [(π-core)-C<i>_n</i>] using 2-octyl[1]benzothieno[3,2-<i>b</i>][1]benzothiophene (mono-C₈-BTBT) and a dialkylated [C<i>_n</i>-(π-core)-C<i>_n</i>] using 2,7-dioctyl[1]benzothieno[3,2-<i>b</i>][1]benzothiophene (di-C₈-BTBT). Thermal analyses show that the superlayer cocrystal is exclusively induced at the equimolar mixture via melt crystallization from the LC phase. X-ray structure analysis reveals a reversible C<i>_n</i>-(π-core)-C<i>_n</i>···(π-core)-C<i>_n</i> stacking arrangement in the superlayer cocrystal, where π-cores and alkyl chains form nearly independent layers. Notably, this melt crystallization allows solvent-free fabrication of semiconductive polycrystalline films for excellent thin-film transistors. These findings pave the way for tailoring a quasi-two-dimensional structure in LC materials toward molecular electronics.