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.
We report on the control of π-stacking modes (herringbone vs slipped-stack) and photophysical properties of 9,10-bis(( E)-2-(pyridin-4-yl)vinyl)anthracene (BP4VA), an anthracene-based organic semiconductor (OSC), by isosteric cocrystallization (i.e., the replacement of one functional group in a coformer with another of “similar” electronic structure) with 2,4,6-trihalophenols (3X-ph-OH, where X = Cl, Br, and I). Specifically, BP4VA organizes as slipped-stacks when cocrystallized with 3Cl-ph-OH and 3Br-ph-OH, while cocrystallization with 3I-ph-OH results in a herringbone mode. The photoluminescence and molecular frontier orbital energy levels of BP4VA were effectively modulated by the presence of 3X-ph-OH through cocrystallization. We envisage that the cocrystallization of OSCs with minimal changes in cocrystal formers can provide access to convenient structural and property diversification for advanced single-crystal electronics.
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