Emerging Disordered Layered-Herringbone Phase in Organic Semiconductors Unveiled by Electron Crystallography

The control of two-dimensional layered crystalline and/or liquid crystalline phases for π-extended organic molecules is crucial for expanding the potential of organic electronic materials and devices. In this work, we develop unique solution-processable organic semiconductors based on the unsymmetric substitution of [1]benzothieno[3,2-b][1]benzothiophene (BTBT) with two different substituents, namely, phenylethynyl (PE) and normal alkyl with different chain lengths n (-CnH2n+1), both of which exhibit structural flexibility while maintaining the rod-like nature over the entire molecule. A distinctive layered solid crystalline phase, analogous to the smectic liquid crystalline phase, is obtainable in PE-BTBT-Cn at n = 6, where the substituent lengths are almost the same. The BTBT moiety maintains a rigid layered-herringbone (LHB) packing, whereas the molecular long axis exhibits a complete orientational disorder. We refer to this packing as disordered LHB (d-LHB), the unique geometry of which can be analyzed by the emerging technique of microcrystal electron diffraction crystallography. The intermolecular core-core interactions stabilize the d-LHB packing, enabling a relatively high field-effect mobility of approximately 3 cm2 V-1 s-1. In contrast, PE-BTBT-Cn with longer alkyl chains (n = 8, 10, 12) exhibits higher mobility of approximately 7 cm2 V-1 s-1 by constituting bilayer-type LHB (b-LHB), which is associated with the unsymmetrical length of the substituents. We discuss the correlation and competition among the d-LHB, b-LHB, and smectic liquid crystalline phases based on the structural, thermal, and transistor characteristics. These findings demonstrate the controllability of various phases in layered organic semiconductors.