Mechanically Coupled Supramolecular Scissors Govern Photomechanical and Elastic Responses in Molecular Crystals

Abstract

Mechanically responsive molecular crystals offer a powerful platform for transducing molecular motion into macroscopic mechanical work, yet molecular-level understanding of how crystal structures govern molecular motion and, consequently, mechanical behavior remains limited. A notable example is the anthracene-pentiptycene-semifluorinated alkyl crystal 1-F4, recently reported to exhibit large elastic bending (ε ≈ 12%) and >15% photoinduced crystal expansion without fracture. Here, we investigate a homologous crystal series (1-Fn, n = 2, 3, 4, and 6) and reveal a mechanically coupled supramolecular scissoring motion that governs their diverse mechanical responses. The balance between scissor stretchability and compressibility, tuned by semifluorinated alkyl substituents, dictates the deformation mode, elasticity, and crystal integrity. An optimally balanced scissor state in 1-F4 enables exceptional elastic and photomechanical deformation while preserving crystal integrity, whereas fully-extended (1-F2), over-contracted (1-F6), or mechanically unbalanced (1-F3) scissor states lead to brittle failure or explosive photosalient behavior. These results establish a direct structure-motion-property relationship within this homologous series, demonstrating that key mechanical properties, including deformation mode and elastic limit, are encoded in the initial scissor geometry within the crystal packing arrangement.