Mechanically induced molecular migrations in molecular crystals

Abstract

The action of mechanical pressure on organic molecular crystals is studied by the new technique of nanoscratching at various faces in different directions in order to induce far-reaching anisotropic long-range molecular migrations. Three-dimensionally interlocked packing as in the chemically unreactive crystals of 2-benzylidenecyclopentanone (1) or cis-1,2-dibenzoylethene (2) cannot experience molecular migrations and only abrasion (sometimes shift of debris in front) is detected upon scratching in any direction. Conversely, chemically reactive crystals of thiohydantoin (3), anthracene (4), thiourea (5), tetraphenylethene (6), and ninhydrin (7) experience mechanically induced anisotropic long-range molecular migrations within the crystal, because they exhibit cleavage planes between mono-, or bi-layers, along which migration occurs. This is shown by detailed analyses of the respective crystal packing. Flat parallel layers give no molecular migrations under pyramidal horizontally advancing tips but abrasion and/or shift of layered segments. Skew cleavage planes give either migration to both sides and in front, or only to the right, or only to the left, or no molecular migration but abrasion, depending on the orientation of the scratching. Vertical cleavage planes give either molecular migrations to both sides and in front, or shift of material in front (probably of extended molecular layer segments for nearly planar molecules), or abrasion. But three-dimensional extended molecules require a push by the tip from their side in order to migrate either to the right or to the left in competition with abrasion if there is some interlocking within the cleavage plane. Polar axes in crystals result in different appearance of the migrations in forward or reverse scratching. The results are discussed in terms of crystal packing.