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 $2\text{\ensuremath{-}}\mathrm{mono}\text{\ensuremath{-}}\mathrm{alkylated}\text{\ensuremath{-}}\mathrm{benzothieno}[3,2\text{\ensuremath{-}}b][1]\mathrm{benzothiophenes} (mono\text{\ensuremath{-}}{\mathrm{C}}_{n}\text{\ensuremath{-}}\mathrm{BTBTs})$. 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 $mono\text{\ensuremath{-}}{\mathrm{C}}_{n}\text{\ensuremath{-}}\mathrm{BTBTs}$ ($n=8$, 9, 10, and 11) undergo mostly irreversible structural phase transformations into short-chain-type polymorphs at elevated temperatures of approximately $85\ensuremath{-}100{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. 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 $110{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$. 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.