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.