Molecular packing analysis of the crystal smectic E phase of a benzothieno-benzothiophene derivative by a combined experimental / computational approach

The molecule 2-decyl-7-phenyl[1]benzothieno[3,2-b][1]benzothiophene has gained a lot of attention since high charge carrier mobility was observed in thin film transistors. Its thermotropic liquid crystalline states may play an important role in the thin film formation since the <i>smectic A</i> and the <i>crystal</i> s<i>mectic E</i> phase (SmE) are claimed to be pre-stages of the final bulk structure. To understand the phase diversity, structural characterisation of solution processed thin films is performed by X-ray diffraction in the complete temperature range up to the isotropic state at 240°C. The diffraction pattern of the SmE phase is analysed in detail. Peak broadening analysis reveals that the crystallographic order across the smectic layers is larger than the order along the smectic layers. A combined experimental and computational approach is used to determine the molecular packing within the SmE phase. It leads to a number of different packing motifs. A comparison of the calculated diffraction pattern with the experimental results reveals that nano-segregation is present within the SmE phase. Energy consideration clearly favours a herringbone arrangement of the aromatic units. The nano-segregation within the SmE phase with herringbone packing of the aromatic units is accompanied with interdigitation of side chains from neighbouringherringbone layers.

化合物2-癸基-7-苯基[1]苯并噻吩并[3,2-b][1]苯并噻吩因在薄膜晶体管中观测到高载流子迁移率而受到广泛关注。其热致液晶态可能在薄膜形成过程中发挥重要作用，因为近晶A相（smectic A）与结晶近晶E相（SmE）被认为是最终块体结构的前驱阶段。为阐明该体系的相多样性，研究人员针对溶液加工制备的薄膜，在直至240℃各向同性态的完整温度区间内开展了X射线衍射结构表征。研究对SmE相的衍射图样进行了详细分析，通过峰宽化分析发现：近晶层间的晶体学有序度高于沿近晶层方向的有序度。采用实验与计算结合的方法确定了SmE相内的分子堆积模式，该方法最终得到多种不同的堆积基元。将计算得到的衍射图样与实验结果对比后发现，SmE相内存在纳米相分离现象。能量学分析结果明确支持芳香单元采用鱼骨形排列的堆积方式。SmE相内芳香单元呈鱼骨形堆积的纳米相分离结构，同时伴随相邻鱼骨形层之间的侧链互穿现象。