3D Electron Diffraction Structure of an Organic Semiconductor Reveals Conformational Polymorphism.

Area of Science:

• Materials Science
• Crystallography
• Organic Chemistry

Background:

• Crystal and conformational polymorphism significantly influence material properties like stability and solubility.
• Analyzing microcrystalline variants, especially conformational polymorphisms, is challenging due to limited high-resolution techniques.
• Three-dimensional electron diffraction (3D ED) is a promising technique for microcrystal structure elucidation, yet its application to low-symmetry small molecules is scarce.

Purpose of the Study:

• To determine the crystal structure of the novel organic semiconductor Ph-anti-benzothieno[5,6-b]benzothieno[3,2-b]thiophene-C10 (antiC10) using 3D ED.
• To investigate the conformational polymorphism present in the antiC10 crystal structure.
• To demonstrate the capability of advanced 3D ED techniques for analyzing challenging small molecules.

Main Methods:

• Utilized three-dimensional electron diffraction (3D ED) for structural analysis.
• Employed a sequential molecular replacement approach with an *ab initio*-generated search model to overcome preferred orientation and missing cone issues.
• Analyzed the obtained crystal structure to identify packing arrangements and conformational variations.

Main Results:

• Successfully determined the 3D ED structure of antiC10, a molecule with the lowest crystallographic symmetry (space group *P*1).
• Revealed a two-monolayer architecture with antiparallel alkyl-interdigitated herringbone packing, distinct from its isomer.
• Identified conformational polymorphism in both herringbone packing and intramolecular conformations, suggesting a metastable state stabilized by twinning.

Conclusions:

• Advanced 3D ED and phasing methodologies enable the study of previously inaccessible crystal structures.
• The detailed structural insights into antiC10 provide a basis for understanding crystallization mechanisms.
• Findings facilitate the rational design of organic semiconductors with tailored properties.