Unraveling Molecular Design Principle of Ferroelasticity in Organic Semiconductor Crystals with Two-Dimensional Brickwork Packing

Ferroelasticity of organic single crystals has recently attracted great research interest. It is a reversible twinning transition in response to mechanical stress that imparts remarkable deformability to crystalline materials while allowing materials to retain their inherent functional properties. These appealing attributes of ferroelasticity promise high-performance ultraflexible, stretchable single-crystalline (opto-) electronics. In this work, we unravel structural criteria for ferroelastic transition of trialkylsilyl-acene (TAS-acene) crystals, which are known as high-performance organic semiconductor materials owing to two-dimensional electronic coupling. This study unveils that ferroelastic transitions are achievable only if two-dimensional brickwork packing is absent from both neighboring aromatic core and TAS side-chain interlocking. This is because aromatic core interlocking prevents cooperative molecular gliding and rotation during structural transition, while side-chain interlocking prevents TAS side-chain reconfiguration necessary for relieving steric strain occurring upon the cooperative molecular motions. The correlation of molecular arrangement and ferroelastic transition capability revealed herein will provide insight into the material design principle of inherently flexible organic semiconductor crystals.