Temperature-induced transformation between layered herringbone polymorphs in molecular bilayer organic semiconductors

Herein we investigated the temperature-induced transformation between distinct layered herringbone (LHB) polymorphs in model organic semiconductors (OSCs) of a series of 2-mono-alkylated-benzothieno[3, 2-b][1]benzothiophenes (mono-C-n-BTBTs). The component molecules are composed of a one-sided linkage between the BTBT core and alkyl chains of various lengths. We propose that the polymorphism originates from a unique feature of LHB structures in which two types of T-shaped contacts are possible for BTBT cores that exhibit relatively low symmetry. Based on polarized UV-visible absorption spectra and powder x-ray diffraction analyses, we show that four long-alkylated mono-C-n-BTBTs (n = 8, 9, 10, and 11) undergo mostly irreversible structural phase transformations into short-chain-type polymorphs at elevated temperatures of approximately 85-100 degrees C. These transformations allowed the investigation of the stability of each polymorph in terms of thermal analysis. Based on the chain-length dependent transition entropies, we demonstrate that the polymorphic transformations are triggered by the partial entropy gain of alkyl chains, followed by the total entropy gain at solid-liquid transitions observed at higher temperatures of approximately 110 degrees C. We also analyzed the structure and intermolecular interactions of the four compounds. The results confirm that each molecular packing consists of a balance between the core-core and chain-chain interactions and also that the unique odd-even parity alkyl-chain-length effect of the transitions emerges as a result of the variation in interlayer stacking arrangements. We highlight the critical role of alkyl chains in terms of their large structural degrees of freedom in solution-processable and highly layered crystalline OSCs. These findings lead to a better understanding of molecular packing toward a more rational design of OSCs for efficient carrier transport.

在此，我们研究了一系列2 - 单烷基化 - 苯并噻吩并[3,2 - b][1]苯并噻吩（单 - C - n - BTBT）模型有机半导体（OSC）中不同层状人字形（LHB）多晶型之间的温度诱导转变。组成分子由BTBT核与不同长度的烷基链之间的单侧连接构成。我们提出，多晶型现象源于LHB结构的一个独特特征，即对于对称性相对较低的BTBT核，两种类型的T形接触是可能的。基于偏振紫外 - 可见吸收光谱和粉末X射线衍射分析，我们表明四种长烷基化的单 - C - n - BTBT（n = 8、9、10和11）在约85 - 100℃的高温下大多会发生不可逆的结构相变，转变为短链型多晶型。这些转变使得能够从热分析的角度研究每种多晶型的稳定性。基于与链长相关的转变熵，我们证明多晶型转变是由烷基链的部分熵增引发的，随后在约111℃的更高温度下观察到固 - 液转变时的总熵增。我们还分析了这四种化合物的结构和分子间相互作用。结果证实，每种分子堆积都由核 - 核和链 - 链相互作用之间的平衡构成，并且由于层间堆积排列的变化，出现了转变的独特奇偶烷基链长效应。我们强调了烷基链在可溶液加工且高度层状的结晶OSC中因其较大的结构自由度而起到的关键作用。这些发现有助于更好地理解分子堆积，从而更合理地设计用于高效载流子传输的OSC。