Pitched π‑Stacking Crystal Structure and Two-Dimensional Electronic Structure of Acenaphtho[1,2-k]fluoranthene Analogues with Various Substituents

Electronic properties of organic semiconductors are governed by their crystal structures. Rubrene, a high-mobility organic semiconductor, forms a pitched π-stacking structure. We here focused on 7,14-diphenylacenaphtho[1,2-k]fluoranthene (DPAF), which gives a crystal isomorphic to rubrene. In addition to DPAF, we newly synthesized 7,14-dithienyl-AF (DTAF) and obtained three types of pitched π-stacking structures: the previously reported orthorhombic DPAF (DPAF-O), a new polymorphic monoclinic DPAF (DPAF-M), and DTAF. To investigate factors causing these molecules to form pitched π-stacking structures, the intermolecular interactions of the face-to-face molecular pairs were calculated for these molecules, nonsubstituted AF and butyl-substituted AF molecules. Unsubstituted AF and butyl-substituted AF have the most stable π-stacking structure with a small misalignment, whereas DPAF and DTAF have the most stable π-stacking structure with a large misalignment along the molecular long axis. Such misaligned π-stacking structures are essential for pitched π-stacking structures. Theoretical calculations of the mobility anisotropy based on the hopping model suggested that DPAF-M and DTAF form one-dimensional electronic states while DPAF-O forms two-dimensional electronic states. Actually, isotropic mobility was observed in SC-FETs with DPAF-O. Collectively, our results indicated that molecular design that introduces broad π-conjugated moieties at the ends of molecules is effective for enhancing the two-dimensionality of electronic states of pitched π-stacking structures.