Alkyl Substitution Strategy for Tuning Mechanical Deformability and Photomechanical Behavior in Organic Crystals

Abstract

Achieving tunable mechanical deformation and photobending behavior in organic crystals through simple molecular modification remains a key challenge. Herein, we report a series of structurally analogous acylhydrazone derivatives bearing various alkyl substituents (methyl to tert-butyl) at the para-position of the phenyl ring to systematically investigate how substitution modulates crystal properties. The unsubstituted crystal (BPH) shows weak mechanical deformation and photobending. The methyl-substituted MBPH is brittle but still photoresponsive. Remarkably, introducing an ethyl (EBPH) or butyl group (BBPH) significantly enhances both the mechanical deformability and the photobending performance. Crystals with propyl (PBPH) and isopropyl (i-PBPH) exhibit slightly improved mechanical deformation compared to BPH, while the bulky tert-butyl group in t-BBPH induces plastic deformability with light-driven motion. Structural analysis reveals that peripheral alkyl substitution on the phenyl ring influences intermolecular interactions and packing arrangements, providing a molecular-level handle to tailor the deformability and photoactuation. This study presents a generalizable alkyl substitution strategy to fine-tune the mechanical and photomechanical properties of organic crystals, offering valuable insights into the design of next-generation flexible and responsive crystalline materials.