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 pi-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-C-n 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 cm(2) V-1 s(-1). In contrast, PE-BTBT-Cn with longer alkyl chains (n = 8, 10, 12) exhibits higher mobility of approximately 7 cm(2) 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.

对于π-共轭有机分子而言，二维层状晶相和/或液晶相的控制对于拓展有机电子材料和器件的潜力至关重要。在这项工作中，我们开发了独特的可溶液加工的有机半导体，其基于[1]苯并噻吩并[3,2 - b][1]苯并噻吩（BTBT）被两种不同的取代基非对称取代，即苯乙炔基（PE）和具有不同链长n（-CₙH₂ₙ₊₁）的正烷基，这两种取代基在保持整个分子呈棒状的同时都表现出结构灵活性。在n = 6时的PE - BTBT - C - n中可获得一种独特的层状固体晶相，类似于近晶液晶相，此时取代基长度几乎相同。BTBT部分保持刚性的层状人字形（LHB）堆积，而分子长轴呈现完全的取向无序。我们将这种堆积称为无序LHB（d - LHB），其独特的几何结构可通过新兴的微晶电子衍射晶体学技术进行分析。分子间的核 - 核相互作用稳定了d - LHB堆积，使得场效应迁移率相对较高，约为3 cm² V⁻¹ s⁻¹。相比之下，具有较长烷基链（n = 8、10、12）的PE - BTBT - Cₙ通过形成双层型LHB（b - LHB）表现出更高的迁移率，约为7 cm² V⁻¹ s⁻¹，这与取代基的不对称长度有关。我们基于结构、热学和晶体管特性讨论了d - LHB、b - LHB和近晶液晶相之间的相关性和竞争关系。这些发现证明了层状有机半导体中各种相的可控性。