Mechanochemical grinding-enhanced olefin-olefin [2+2] cycloaddition in cyanostilbene derivatives: mechanistic dependence on halogen-based bonding.

Abstract

Solid-state [2+2] photocycloaddition reactions are significant in crystal engineering and photo-responsive materials, yet are often constrained by the Schmidt topological rule. Herein, we designed a series of brominated cyanostilbene derivatives with aggregation-induced emission (AIE) properties. Remarkably, the dibromo-substituted compound Z-BBBA underwent [2+2] cycloaddition reactions through strong aggregation in aqueous solution and solid state under 365 nm ultraviolet irradiation. The reaction mechanism of Z-BBBA was revealed through UV-vis absorption, nuclear magnetic resonance (NMR), single crystal X-ray diffraction (SC-XRD), and scanning tunneling microscopy (STM), combined with theoretical calculations. Z-BBBA formed antiparallel dimers via dynamic Br-mediated interactions, enabling ideal π-stacking and reversible olefin spacing regulation to satisfy Schmidt geometry under UV light. Mechanical grinding enhanced the reactivity by introducing crystal defects, increasing surface areas, and promoting molecular sliding. This work elucidates the regulatory rules of halogen substitution on solid-state photochemical reactions and unveils the synergistic effect mechanism of molecular packing, mechanical force, and photoactivation, offering design principles for AIE materials with mechano-optical responsiveness.