Light-Driven Crystal Actuation of Fluorinated Acylhydrazones Enabled by [2+2] Photocycloaddition

Graphical Three isomeric fluorinated acylhydrazone derivatives (DFBP-IND, DFBP-BHD, DFBP-3-FBHD) were successfully synthesized and demonstrated dynamic photo-responsive behavior ranging from crack initiation to crystal fragmentation under 365 nm UV irradiation. The photochemical mechanism was explored in detail and highlighted the essential role of the C═N bond in the acylhydrazone framework for enabling [2+2] cycloaddition-driven actuation. Our structural engineering strategy established a design paradigm for photoresponsive materials through rational selection of hydrazide precursors, enabling precise modulation of optomechanical properties. Crystalline materials with photomechanical responsiveness have garnered significant attention in flexible actuators and micro/nano-electromechanical systems due to their unique capability to directly convert light into mechanical energy. In this work, four novel fluorinated acylhydrazone compounds were successfully synthesized through direct condensation reactions between 4,4′-difluorobenzophenone (DFBP) and acylhydrazine compounds, including benzoylhydrazine (BHD), 3-fluorobenzoylhydrazine (3-FBHD), isoniazid (IND), and acetylhydrazine (AED). Systematic investigation of their photomechanical responses revealed distinct molecular mechanisms. Experimental results demonstrate that isomeric fluorinated acylhydrazone derivatives (DFBP-BHD, DFBP-3-FBHD, DFBP-IND) exhibit dynamic photo-responsive behavior ranging from crack initiation to crystal fragmentation under 365 nm UV irradiation, whereas the hydrazone derivative DFBP-AED remains inert. This confirms the essential role of [2 + 2] photocycloaddition-dependent C = N double bonds in photomechanical activity. Comprehensive characterization through DSC, TG, XRD, ¹H/¹ 3 C NMR, and MS elucidated the formation pathways and structural characteristics of photocycloaddition products. Density functional theory calculations revealed that energy disparities between (0 0 1) and (1 0 0) crystallographic planes govern longitudinal fragmentation patterns. This study establishes a “molecular structure-crystal arrangement-macroscopic response” multi-scale regulation strategy, offering novel insights for developing high-performance light-driven materials.